{"paper_id":"995937f2-c81a-4768-9583-364c3c8da004","body_text":"Discovery of α-PtO2 nanoparticles in deep-sea ferromanganese deposits: a hidden platinum sink | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Discovery of α-PtO 2 nanoparticles in deep-sea ferromanganese deposits: a hidden platinum sink Jianlin Liao, Yan Li, Julien Léger, Andrea Koschinsky, Catherine Dejoie, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9606997/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The enrichment of platinum (Pt) in marine ferromanganese (FeMn) deposits has attracted persistent interest for over half a century; yet the chemical form of Pt remains unclear. Here, we collected Pt-enriched FeMn crusts and nodules from the world’s oceans and used high-energy-resolution X-ray absorption spectroscopy (XAS) to decipher how Pt is sequestered at the atomic level. Platinum occurs in its tetravalent form, resulting from the oxidation of divalent Pt in seawater upon contact with Mn oxides. Tetravalent platinum is precipitated as α-PtO 2 nanoparticles with longer Pt-Pt distances than well-crystallized α-PtO 2 . Density functional theory (DFT) shows that the local structure of Pt is well represented by α-PtO 2 layers topotactically stacked on vernadite phyllomanganates. Evidence of α-PtO 2 nanoparticles challenges previous hypotheses that Pt exists as discrete metallic particles or within phyllomanganate MnO 2 layers replacing Mn. Since α-PtO 2 is the most thermodynamically stable Pt oxide, this Pt form may represent the ultimate sedimentary sink of Pt in oceans. Geochemistry Planetary Science Economic Geology Platinum Marine FeMn deposits XAS XANES EXAFS DFT Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Teaser Evidence of α-PtO 2 nanoparticles in deep-sea ferromanganese deposits sheds light on the Pt sink in marine environments and aids resource exploration. Introduction Platinum (Pt), a precious metal in jewelry and a versatile critical metal in catalysis, has long been central to scientific and industrial progress. The industrial demand for Pt has increased in the past thirty years ( 1 ), due to its uses in fuel cells ( 2 ), carbon-neutral technologies ( 3 ), and cancer treatments ( 4 ). The low natural abundance and high cost of Pt motivate global prospecting for new resources beyond traditional land deposits ( 5 – 7 ). Deep-sea polymetallic ferromanganese (FeMn) deposits encompass FeMn nodules and crusts that accrete via hydrogenetic, diagenetic, or hydrothermal processes at low growth rates (millimetres to a few centimeters per million years), with growth histories occasionally interrupted by phosphatization events ( 8 – 12 ). FeMn deposits can concentrate Pt at levels (e.g., 3.2 µg/g) ( 13 ) up to eight orders of magnitude greater than seawater (picomolar level) ( 14 – 17 ), making them valuable repositories for Pt ( 8 – 10 , 12 , 18 , 19 ). Although more than a century and a half have passed since deep-sea FeMn crusts and nodules were first discovered in 1873 ( 20 – 22 ), the chemical form and enrichment process of Pt remain a mystery. Current understanding of Pt mobilization and accumulation behaviors in the marine environment largely relies on indirect methods, such as thermodynamic modeling ( 23 – 29 ) and laboratory adsorption experiments ( 13 , 30 – 34 ). Three oxidation states of Pt in marine FeMn deposits have been proposed, metallic [Pt(0)] ( 35 – 37 ), divalent [Pt(Ⅱ)] ( 13 , 32 ), and tetravalent [Pt(IV)] ( 13 , 30 – 32 ), which have led to conflicting hypotheses: 1) deposition of extraterrestrial Pt(0) spherules ( 35 , 36 ), 2) reductive immobilization of Pt(Ⅱ) to Pt(0) by Mn 2+ ( 36 , 37 ), 3) Pt(Ⅱ) adsorption onto Fe (oxyhydr)oxide surfaces ( 13 , 32 ), and 4) oxidative adsorption of Pt(Ⅱ) on Mn oxides followed by isomorphic Pt(IV) for Mn(IV) substitution in the crystal structure ( 13 , 30 – 32 ). The lack of direct experimental evidence from natural samples has kept this controversy ongoing. Resolving this question requires direct analysis of the Pt chemical form in marine FeMn substrates, which is difficult due to 1) their Pt content that falls below the detection limit of standard spectroscopy techniques, and 2) their complex multielement compositions. In this study, we addressed the two difficulties as follows. Firstly, we selected three Pt-rich FeMn crusts and nodules from the Indian Ocean, Pacific Ocean, and Atlantic Ocean (Fig. 1 , Table S1) from a collection of 182 samples ( 13 ). Using microscale elemental analysis, we identified the Pt-richest FeMn layers within the selected crusts and nodules. Subsequently, we determined the chemical form of Pt in these layers using high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) to eliminate matrix effects. Then, the structural relationship between Pt and the FeMn layers was modeled by density functional theory (DFT). Results Growth structure and chemical composition The Indian FeMn crust consists of black growth layers with high Mn (24.65 wt.% on average) and Fe contents (14.15 wt.% on average, Data S1, Fig. 2 A). The phosphorous content is low (0.27 wt.% on average), and there are no phosphatized layers (Fig. S1A–B). The black FeMn growth layers of the Pacific nodule contain phosphorous-rich veins from apatite [Ca 5 (CO 3 ) x (PO 4 ) 3−x F 1+x ] in the middle and basal parts (25–40 mm, Fig. 2 B, Fig. S1C–D), indicating past phosphatization events. Their P content reaches 10.12 wt.% (Data S1). The FeMn layers of the Atlantic crust are interbedded with several, thick phosphorite layers at 2–3 mm, 8–11 mm, and 41–44 mm from the crust surface (Fig. 2 C, P content up to 13.99 wt.%, Data S1), resulting from intense and prolonged phosphatization events. The elemental profiles across the growth layers of the three FeMn crusts and nodules show Pt contents up to 4–6 µg/g (Fig. 2 , Data S2–4). The Pt-richest layers are located at 17–19 mm in the Indian crust, 26–28 mm and 37–39 mm in the Pacific nodule, and 32–34 mm and 64–66 mm in the Atlantic crust (Fig. 2 ). All Pt-rich FeMn layers commonly exhibit a spheroidal growth structure that differs from the wave-layered and stromatolithic columnar structures of the non-Pt-rich layers (Fig. 3 ). The geochemical data were plotted on ternary diagrams (Fig. 4 A–C), which differentiate hydrogenetic, diagenetic, and hydrothermal FeMn deposits based on their chemical compositions ( 38 ). The Indian FeMn crust is entirely hydrogenetic in origin (Fig. 4 A and D). The Pacific FeMn nodule comprises both hydrogenetic and diagenetic layers (Fig. 4 B), with the Pt-rich layers being of hydrogenetic origin (Fig. 4 E). The Atlantic FeMn crust exhibits a complex growth history, consisting of alternating layers of hydrogenetic and diagenetic, and some layers pointing towards a certain hydrothermal influence (Fig. 4 C). Pt-rich layers occur in both hydrogenetic and diagenetic layers (Fig. 4 F). Mineral composition The mineral compositions determined by X-ray diffraction (XRD) are shown in Fig. 5 and Fig. S2–S4. The FeMn layers of the Indian crust consist of Fe-vernadite (FeOOH + δ-MnO 2 ) with main diffraction peaks at 2.45 Å and 1.42 Å and no basal reflection (Fig. 5 A and Fig. S2). The Pt-richest layers of the Pacific nodule at 26–28 mm and 37–39 mm consist of Fe-vernadite intergrown with apatite and calcite (CaCO 3 , Fig. 5 B and Fig. S3). In contrast, the younger FeMn layer on the rim of the nodule at 5–7 mm from the surface consists of Fe-vernadite only (Fig. S3). The Pt-rich layer at 64–66 mm in the Atlantic crust is also made up of Fe-vernadite, whereas the other Pt-rich FeMn layer at 32–34 mm contains apatite and calcite in addition to Fe-vernadite (Fig. 5 C–D). The three mineral phases also co-occur at 5–7 mm, 21–23 mm, and 32–34 mm, and apparently at 64–66 mm, independent of the Pt content (Fig. S4). The XRD patterns of the 5–7 mm, 32–34 mm, and 64–66 mm depth layers have a peak at 9.56 Å from the 001 reflection of 10 Å manganates, which can be from 10 Å-vernadite or the todorokite tectomanganate (Fig. S4A, C, and D). Its shift to 9.2 Å after heating to 105°C identified todorokite ( 39 ). The 002 reflection at 4.73 Å consistently behaved similarly to the 001 reflection at 9.56 Å after heating (Fig. S4A, C, D). Mn average oxidation state The FeMn layers analyzed by XRD were measured by Mn K-edge X-ray absorption near-edge structure (XANES) spectroscopy (Fig. S5) to determine the average manganese oxidation state (AMOS), which is a proxy of redox condition. The AMOS values vary only slightly across all layers of the Indian FeMn crust (3.89–3.91) and the Pacific nodule (3.93–3.94, Table S2). The older layers of the Atlantic FeMn crust at 64–66 mm also have high AMOS (3.90), whereas the younger layers (5–7 mm, 21–23 mm, 32–34 mm), which are phosphatized, have lower AMOS ranging from 3.91 to 3.83, with higher Mn 3+ proportions (9%–15%, Table S2). Oxidation state and coordination environment of Pt The oxidation state of Pt was determined by HERFD-XANES spectroscopy. The Pt L 3 -edge XANES spectra are nearly identical across the three FeMn crusts and nodules (Fig. 6 A–B), with energy position and shape closely matching the PtO 2 reference (tetravalent, Fig. 6 C–D). The structural form of Pt was identified using extended X-ray absorption fine structure spectroscopy in HERFD mode (HERFD-EXAFS). The EXAFS spectra for the three FeMn crusts and nodules are statistically identical, meaning that the chemical form of Pt is invariant (Fig. S6). Therefore, the three EXAFS spectra were averaged to generate a representative spectrum of higher quality for Pt in marine FeMn deposits (Pt FeMn , Fig. 7 A–B). The spectra of Pt FeMn and the α-PtO 2 reference have essentially the same frequency, but the Pt FeMn spectrum is less structured (Fig. 7 A and C). In real space, after Fourier transform of the EXAFS spectra, this distinction is seen mainly as a reduction in amplitude of the nearest Pt-Pt atomic pair at R + Δ R = 2.6–3.6 Å (Fig. 7 B). The α-PtO 2 and Pt FeMn spectra were least-squares fitted in the common 3.2 Å –1 ≤ k ≤ 12.1 Å –1 interval, giving a distance resolution of Δ d = π/2 k max = 0.13 Å ( 40 ) (Fig. 7 C–D). The best-fit EXAFS parameters are 6 O1 at 2.02 Å, 6 Pt1 at 3.10 Å, and 12 O2 at 3.70 Å for the α-PtO 2 reference, and 6 O1 at 2.00 Å, 4.6 Pt1 at 3.17 Å, and 12 O2 at 3.66 Å for Pt FeMn (Table S3). The Pt-Pt1 distance of Pt FeMn is 0.07 Å longer than that of α-PtO 2 . Another noticeable difference is a peak at R + Δ R = 6.0 Å observed in the Fourier transform of α-PtO 2 (Fig. 7 D and F). This peak corresponds to multiple scattering paths between collinear Pt-Pt1-Pt4 atoms in the layered α-PtO 2 structure (Fig. 7 F). Modeling of the Pt local structure DFT modeling was undertaken to understand the 0.07 Å increase in Pt-Pt distance of Pt FeMn compared to α-PtO 2 . Two structures were geometrically optimized: α-PtO 2 nanoclusters incorporated into the δ-MnO 2 layers (Fig. 8 A), and α-PtO 2 nanolayers topotactically stacked on δ-MnO 2 layers (Fig. 8 B). In the first model, seven Mn atoms were replaced with Pt atoms in each MnO 2 layer (Fig. 8 A). The average Pt-Pt distance is 3.04 Å, which is 0.13 Å shorter than the Pt-Pt distance in Pt FeMn (i.e., 3.17 Å). In the second model, incorporating a Pt 19 O 38 nanolayer of 12.6 Å in diameter between two MnO 2 layers yielded Pt-Pt distances between 3.12 Å and 3.22 Å, with a mean of 3.17 Å (Fig. 8 B), consistent with EXAFS results. Discussion Formation of the Pt-rich FeMn layers The Pt contents in the enriched layers were quantified using single-spot laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). All depth profiles of the Pt signal measured in the three FeMn crusts/nodules were uniform during ablation, indicating constant Pt concentration within the analyzed FeMn spots (Fig. S7). Thus, Pt is likely dispersed in the FeMn layers, rather than concentrated in discrete metallic nuggets. Several micrometric nuggets of platinum-group elements, perhaps of cosmic origin, have been observed in a deep-sea sediment, using scanning electron microscopy ( 35 ). This possible Pt form is insufficient to explain Pt enrichment in FeMn deposits ( 36 , 37 ). The two Pt-rich FeMn layers at 17–19 mm in the Indian FeMn crust and at 64–66 mm in the Atlantic FeMn crust mainly contain Fe-vernadite (Fig. 5 ). Vernadite (δ-MnO 2 ), a nanocrystalline turbostratic phyllomanganate ( 41 ), is the main constitutive Mn oxide in hydrogenetic FeMn deposits, which form under oxic marine conditions ( 42 – 44 ). The hydrogenetic origin of the two FeMn layers is confirmed by geochemical (Fig. 4 ) and spectroscopic (AMOS ≥ 3.9) analyses (Fig. S5 and Table S2). Observation of the occurrence of Pt-rich FeMn layers formed under hydrogenetic conditions contradicts the previously-held hypothesis that Pt enrichment is linked to phosphatization ( 10 , 13 ). Apatite and calcite coprecipitate with the Fe and Mn oxides during phosphatization events ( 11 , 12 , 45 ). Neither apatite nor calcite hosts Pt within their structures ( 46 ), so the presence of these minerals does not enhance Pt enrichment in the FeMn layers. In addition, phosphatization is usually accompanied by an oxic-to-suboxic transition in the depositional setting, as attested by the transformation of vernadite into todorokite, which contains more Mn(III) and less Mn(IV) than vernadite ( 39 , 47 ). Some of the FeMn layers in the phosphatized Atlantic and Pacific crusts indeed contain todorokite, yet Pt contents vary independently of the presence of todorokite (Fig. 2 , Fig. S3 and S4). This observation aligns with our statistical analysis of available literature data ( 12 , 13 ), which show that Pt and P concentrations are not correlated (Fig. S8). Pt accumulation most likely occurs at the seawater-seafloor interface during hydrogenetic deposition in both phosphatized and non-phosphatized FeMn deposits across the global oceans (Fig. S8). Speciation and redox potential of Pt in seawater conditions To understand the enrichment of Pt in hydrogenetic FeMn layers, which form in direct contact with seawater, one needs to know the solubility and speciation of Pt in seawater. Direct measurement is hindered by its extremely low concentration ( 14 , 15 ), therefore, its solubility and possible complexation were estimated by thermodynamic calculations. The concentration of Pt in seawater is at the picomolar level (10⁻ 12 M) ( 14 , 17 ), much lower than its calculated solubility limit under seawater conditions (10⁻ 1.24 M, Fig. 9 A). This large difference suggests that Pt predominantly exists as soluble Pt(Ⅱ) rather than in equilibrium with a solid phase. The speciation of Pt in seawater was evaluated in the Pt-O-H-Cl system from the known equilibrium reaction constants (Table S4). Our calculation shows that the neutral hydroxyl species, Pt(OH)₂ (aq) , predominates at pH 8 (Fig. 9 B). However, previous studies ( 31 , 32 , 36 ) suggested that dissolved Pt(Ⅱ) can form negatively charged complexes [PtCl 4− n (OH) n ] 2− (where n = 0–4), given the high chloride concentration and alkaline conditions of seawater. The formation and stability field of these complexes remains uncertain due to the lack of thermodynamic complexation constants for these species. Divalent Pt can be oxidized to tetravalent Pt through 1) a disproportionation reaction, 2) oxidation driven by dissolved oxygen in seawater, and 3) oxidative uptake driven by δ-MnO 2 . The disproportionation reaction is observed in high-temperature experiments (e.g., 152℃) ( 26 , 48 ). This pathway requires significant energy input ( 26 , 48 ) and leads to the formation of metallic Pt, which has been identified in marine hydrothermal fields ( 49 , 50 ). Given that hydrogenetic FeMn crusts and nodules precipitate under low-temperature conditions in direct contact with seawater, and that no metallic Pt has been detected, this redox reaction can be dismissed. Oxidation of Pt(Ⅱ) by dissolved oxygen in seawater can be evaluated through thermodynamic modeling ( 51 ). Because the thermodynamic complexation constants for the [PtCl 4− n (OH) n ] 2− complexes are unknown, redox reactions were modeled within the Pt-O-H system excluding Cl (Fig. 9 C). Calculations indicate that oxidizing Pt(Ⅱ) to Pt(IV) is not thermodynamically feasible because the oxygen fugacity in seawater does not reach the required oxidation potential ( 51 , 52 ). Therefore, Pt enrichment in marine FeMn deposits most likely occurs through Pt(II) oxidation by δ-MnO 2 (i.e., vernadite). This pathway is supported by sorption experiments, which showed that Pt(II) is oxidized by δ-MnO 2 ( 13 , 30 , 31 ), not by Fe (oxyhydr)oxides ( 13 ). Some studies suggest that Pt(IV) is adsorbed on δ-MnO 2 ( 10 , 32 ), while others suggest its incorporation within the mineral structure through Pt(VI) for Mn(IV) substitution ( 13 , 19 , 30 , 31 ). This question is answered below by HERFD-XAS. Atomic-level chemical form of Pt in FeMn layers The structural form of Pt(IV) was identified using EXAFS spectroscopy. Although this method has been previously used to probe the local atomic environment of Pt adsorbed onto Mn minerals under controlled laboratory conditions ( 13 , 30 , 31 , 53 ), it has never been applied to the original marine FeMn oxide materials because of the chemical complexity of their polymetallic matrix and their Pt content (usually < 1 µg/g), which is below the detection limit of standard EXAFS. In these previous studies, Pt was artificially sorbed from seawater solution to natural or artificial (Fe)Mn oxide substrates to reach detectable Pt concentrations. These difficulties were overcome in our study by measuring EXAFS at high-energy-resolution ( 54 , 55 ). The Pt(Lα1) fluorescence line was selected with a spectrometer (Fig. S9) ( 55 ). The sharp parasitic As(Kα) white line occurring at k = 9 Å –1 in the Pt-EXAFS spectra was almost completely extinguished with the spectrometer (Supplementary Text, Fig. 7 A and S6). Platinum in FeMn deposits has an α-PtO 2 -type local structure that differs from the α-PtO 2 reference by a reduction in the number of Pt atoms in the first (Pt1) and fourth (Pt4) Pt shells (Table S3). Thus, Pt is likely nanoparticulate in marine FeMn deposits. Since Pt(Ⅱ) is oxidized to Pt(IV) by Mn(III) or Mn(IV) cations from vernadite (δ-MnO 2 ) in the FeMn crusts and nodules, one might expect to detect Pt-Mn pairs. However, they are absent in our best-fit EXAFS model (Fig. 7 A–B). EXAFS spectroscopy can effectively differentiate between Pt-Pt and Pt-Mn pairs due to a phase shift difference of ∣Φ Pt-Pt – Φ Pt-Mn ∣ = 4.9 rd between their electronic waves. In addition, the backscattering amplitude of the Pt-Mn wave peaks at k ≈ 6 Å –1 , whereas the Pt-Pt wave is at a minimum at this k value (Fig. S10). The high sensitivity of EXAFS to Pt-Pt and Pt-Mn pairs and the close match of the modulus and the imaginary parts for the Pt-Pt1 peak between fit and experiment confirm the reliability of the fit model (Fig. 7 B). This provides strong evidence for Pt clustering. Based on the average number of Pt-Pt1 pairs (i.e., 4.6, Table S3), the α-PtO 2 -type layers of Pt FeMn have a diameter of approximately 15 Å, on average. The α-PtO 2 -type nanoclusters are either incorporated into the MnO 2 layers or topotactically stacked on the δ-MnO 2 layers of the FeMn crusts and nodules. Both possibilities were assessed using DFT calculations. Structural incorporation was simulated by replacing seven Mn atoms with Pt atoms within three MnO 2 layers (Fig. 8 A). The average Pt-Pt distance is 3.04 Å, 0.19 Å longer than the Mn-Mn distance, and 0.13 Å shorter than the Pt-Pt distance in Pt FeMn . Therefore, a substitutional incorporation of Pt into the MnO 2 layers is unlikely. Conversely, depositing a Pt 19 O 38 nanolayer of 12.6 Å in diameter in between the MnO 2 layers produced Pt-Pt distances between 3.12 and 3.22 Å, with a mean of 3.17 Å (Fig. 8 B), the same as the EXAFS distance of 3.17 Å. These findings strongly support the occurrence of α-PtO 2 nanolayers intergrown between the MnO 2 layers of the FeMn crusts and nodules (Fig. 8 B). Furthermore, analysis of the second and third O shells confirms the layered structure of the α-PtO 2 nanoparticles. Well-crystallized α-PtO 2 has a second O shell (O2) at 3.60 Å and a third O shell (O3) at 3.73 Å ( 56 ). The two O shells are not resolved in the EXAFS analysis of the α-PtO 2 reference when the fit is performed to k max = 12.1 Å –1 , as Δ d = π/2 k max = 0.13 Å ( 40 ). However, they can be distinguished by increasing k max to 16.0 Å –1 , yielding Δ d = 0.10 Å ( 40 ) (Fig. 7 E–F). The EXAFS parameters of the fit for the α-PtO 2 reference up to k max = 16 Å –1 are 6 O1 at 2.02 Å, 6 Pt1 at 3.10 Å, 6 O2 at 3.64 Å, and 6 O3 at 3.77 Å (Table S3), in good agreement with the crystal structure (2.07 Å, 3.10 Å, 3.60 Å, 3.73 Å) ( 56 ). In the α-PtO 2 structure, the O3 shell at 3.73 Å is located within the PtO 2 layer, while the O2 shell at 3.60 Å is in the adjacent upper and lower PtO 2 layers (Fig. 7 F). Therefore, the Pt-O distance of 3.66 Å for Pt FeMn is likely an average distance from O2 and O3 shells (Fig. 7 B). The O2 shell of Pt FeMn may belong to a MnO 2 layer or to another PtO 2 layer. If the α-PtO 2 nanolayers are sandwiched in between MnO 2 layers, then the local mineral structure is akin to that of Ni-Co asbolane ( 57 – 59 ), and can be described as a Pt-asbolane. The PtO 2 nanoparticles were undetected in transmission electron microscopy (TEM, Fig. S11), likely because their amount is vanishingly small ([Pt] = 4–6 µg/g). Implications The α-PtO 2 nanoparticles within FeMn deposits differ from the Pt species adsorbed on the surface and substituted isomorphically into the crystal structure of synthetic δ-MnO 2 , as documented in previous laboratory studies ( 13 , 30 , 31 , 53 ). The difference in Pt uptake between natural settings and laboratory surrogates is due, at least in part, to the large differences in 1) Pt concentration and 2) in reaction time between short-term experimental conditions and long-term natural processes. Firstly, the Pt concentrations used in experimental studies, typically at micromolar levels ( 13 , 30 , 31 , 53 ), greatly exceeded the picomolar concentrations of Pt observed in seawater ( 14 , 17 ). As a result, the Pt/Cl molar ratio in experimental systems was approximately six orders of magnitude higher than in seawater. Platinum mainly exists as Pt 2+ at the high experimental Pt/Cl ratios ( 24 ), while anionic Pt-Cl complexes, such as [PtCl 4− n (OH) n ] 2− , are thermodynamically favored at low Pt/Cl ratios in seawater ( 24 ). Those complexes likely interact differently than Pt 2+ on the negatively charged δ-MnO 2 surface. Secondly, the time scales of the two types of Pt uptake differ greatly: laboratory experiments usually last hours to days ( 13 , 30 , 31 , 53 ), whereas the formation of marine FeMn deposits including enrichment of trace metals such as Pt occurs over millions of years ( 9 , 10 , 13 , 60 , 61 ). This extended time period allows processes such as phase transformation, Pt atom clustering, and formation of thermodynamically stable α-PtO 2 precipitates. Our results providing insight into the chemical form of Pt in marine FeMn deposits will be instrumental in more realistic experimental studies and modeling aimed at advancing understanding of the oxidation of Pt(Ⅱ) and the subsequent formation of α-PtO 2 nanoparticles. They underscore the importance of accounting for metal concentration and reaction time when extrapolating laboratory-derived chemical and structural data to natural systems. The occurrence of α-PtO 2 nanoparticles in FeMn oxide deposits from three oceans illuminates a previously unrecognized but widespread pathway for Pt accumulation in the marine environment. The formation of α-PtO 2 nanoparticles begins with the oxidative uptake of Pt(II) onto Mn oxides, followed by nucleation and crystal growth. In this process, Mn oxides serve as adsorbents and electron acceptors. Surface-catalyzed redox reactions lower energy barriers and lead to strong metal partitioning. In seawater, the oxidation of Pt(II) on vernadite results in eight orders of magnitude enrichment. Because α-PtO 2 is the most thermodynamically stable Pt oxide ( 52 ), it likely represents the ultimate sedimentary sink in the oceanic Pt cycle. Lastly, this study establishes a basis for exploring and recovering Pt from deep-sea FeMn deposits. The Pt-rich FeMn layers exhibit a characteristic spheroidal structure (Fig. 3 ), which was previously observed in slow-growing FeMn crusts ( 60 , 62 ). Pt enrichment has also been reported in FeMn crusts exhibiting a growth hiatus ( 63 ). A slower accretion rate of the FeMn layers provides more time for Pt to accumulate from seawater. This relationship has also previously been shown for other trace metals enriched via a similar surface enrichment and oxidation process, such as Co ( 64 ) and Te ( 65 ). Therefore, when searching for Pt-rich resources, hydrogenetic FeMn layers with slow growth rates should be prioritized. The identification of Pt as α-PtO 2 nanoparticles structurally bound to the MnO 2 layers implies that it cannot be effectively desorbed by cation exchange, but that its recovery requires dissolving the Mn oxide matrix. Materials and Methods Sample information The FeMn crusts and nodules were collected from the Ninety East Ridge in the Indian Ocean, the Manihiki Plateau in the Western South Pacific Ocean, and the Tropic Seamount in the Eastern North Atlantic Ocean (Fig. 1 ). Detailed sampling information is provided in Table S1. The samples were sectioned across their layer growth direction and then polished to expose a flat cross-sectional surface for in situ elemental and spectroscopic measurements. Scanning Electron Microscope The microscale structures of the FeMn crusts and nodules were imaged using a Zeiss Gemini LEO 1530 field emission scanning electron microscope (FE-SEM) coupled with X-MaxN 20 (Oxford instruments) energy dispersive spectroscopy (EDS). Images were collected in SE2 mode at 20 kV and 1 nA. Electron probe micro-analyzer The major elements were analyzed using an electron probe micro-analyzer (EPMA, JEOL, FEG, JXA-iHP200F) at the ISTerre MicroAnalytical Platform, Université Grenoble Alpes (UGA), France ( 66 ). The EPMA operated with an acceleration voltage of 15 kV, a beam current of 5 nA, and a probe diameter of 1 µm. Signal processing included a matrix correction using the ZAF method as implemented in the EPMA-PC JEOL software. The following elements and X-ray lines were measured using five spectrometers: 1) WDS1, TAPL: Na-Kα (peak counting time 10 s / background counting time 10 s), Si-Kα (30/30), Sr-Lα (60/60); 2) WDS2, PETH: Ti-Kα (30/30), Ba-Lα (30/30), Cl-Kα (30/30); 3) WDS3, LIFH: Zn-Kα (20/20), Ni-Kα (20/20), Co-Kα (20/20), Fe-Kα (20/40), Mn-Kα (20/20); 4) WDS 4, TAPL: Mg-Kα (50/50), Al-Kα (50/50); 5) WDS5, PETH: K-Kα (10/10), Ca-Kα (20/20), S-Kα (30/30), P-Kα (20/20). Standardization was performed before analysis with the following materials: Na (albite), Si (wollastonite), Sr (strontium sulfate), Ti (titanium dioxide), Ba (barium oxide), Cl (cancrinite), Zn (sphalerite), Ni (nickel oxide), Co (cobalt oxide), Fe (hematite), Mn (rhodonite), Mg (magnesium oxide), Al (corundum), K (orthoclase), Ca (wollastonite), S (sphalerite), P (apatite). LA-ICP-MS The LA-ICP-MS measurements were conducted at the ISTerre MicroAnalytical Platform, UGA. The system includes a laser ablation system (RESOlution SE, Applied Spectra, 193 nm excimer) paired with an ICP-MS (Agilent 8900, triple quadrupole, operated in no gas mode). For single-spot ablation, the settings were adjusted to a circular spot size of 50 µm in diameter, a repetition rate of 5 Hz, an energy density of 3 J/cm², and an ablation time of 30 s. A SQUID signal smoothing device was used. For line-scanning analysis, the samples were ablated with a square beam (20 µm × 20 µm) at a repetition rate of 10 Hz, an energy density of 3 J/cm², and a scan speed of 10 µm/s. The SQUID device was not used, in order to decrease aerosol transport duration between the laser and ICP-MS. Before each analysis, a pre-ablation was performed using a larger beam size to remove any surface contamination. Standard reference materials, NISTSRM-614 ( 67 ) and FeMnOx-1 ( 68 ), were included in the measuring queue for drift assessment, calibration, and quality control. Data reduction was conducted using the LADR software (Norris Scientific, www.norsci.com ). Quantification was performed by normalizing the chemical compositions to the total metal oxide amounts ( 69 ) and by using the Mn contents as an internal standard. The total metal oxide amounts and the Mn contents were previously determined by EPMA (Data S1). XRD FeMn growth layers were sampled with a micro-drill, finely ground, and loaded into quartz-glass capillaries. Two subsamples were prepared for each sample. One set was dehydrated in an oven at 105°C for 8 hours, and immediately sealed afterward to prevent rehydration. Synchrotron-based XRD measurements were performed on the ID22 beamline at the European Synchrotron Radiation Facility (ESRF) under a beam energy of 35 keV ( 70 ). Data were collected using a Perkin Elmer XRD 1611CP3 medical imaging detector positioned at a distance of 1400 mm from the sample. The wavelength and the detector parameters were calibrated using a diffraction pattern collected on a LaB6 NIST standard. Azimuthal integration was carried out using the pyFAI library ( 71 ). XAS Mn K-edge XANES spectra were acquired at the ESRF on beamline BM23. Samples from the FeMn layers within FeMn crusts and nodules were extracted using a micro-drill, ground into fine powders, and pressed into pellets. Data were collected at room temperature in transmission mode, with the photon energy calibrated to the first inflection point of metallic Mn at 6537.7 eV, as referenced by Kraft et al. (1996) ( 72 ). The average Mn oxidation states were determined using the Combo method ( 73 ), implemented in the Athena software ( 74 ), with an updated spectral dataset ( 75 ). The Combo method, as reported in its original development study, has a standard deviation of 0.04 valence units ( 73 ). Pt L 3 -edge XANES and EXAFS spectra were acquired at the ESRF on beamline ID24-DCM. The incident X-ray flux, sourced from an undulator, was approximately 5 × 10 12 photons/s. The Pt L 3 -M 5 fluorescence line was selected using a five-crystal analyzer with Ge (110) crystals bent to a 0.5 m radius and aligned at a Bragg angle of 79.95° in a vertical Rowland geometry ( 55 ). The diffracted intensity was measured using a silicon drift detector. The sample thin sections, previously analyzed by LA-ICP-MS, were mounted on the sample stage. The Pt-richest FeMn layers of interest were relocated by rastering the samples in an XY pattern with an X-ray beam of 1 × 1 mm 2 in size. Four XANES spectra were collected over an energy range of 11500 to 11750 eV with a step size of 0.3 eV, each scan lasting 375 seconds. Additionally, approximately 200 EXAFS scans were acquired from 11500 eV to 12136 eV with a step size of 1 eV, each with a scanning time of 318 seconds. No changes in spectral features were observed during data collection that would indicate radiation damage. All scans were averaged to increase the signal-to-noise ratio and subsequently processed using Athena ( 74 ). EXAFS fits were performed with WinXAS ( 76 ) and theoretical amplitude and phase shift functions generated by FEFF 8.2 ( 77 ), using α-PtO 2 as a structure model ( 56 ). DFT Two cell sizes were geometrically optimized: 1) a smaller cell consisting of three MnO 2 layers, each containing 35 Mn atoms and 7 Pt atoms, used to study the incorporation of Pt (Fig. 8 A), and 2) a larger cell composed of two MnO 2 layers, each containing 81 Mn atoms, and a third MnO 2 layer replaced with a Pt 19 O 38 nanolayer (Fig. 8 B). Periodic DFT computations were performed with the Vienna Ab-initio Simulation Package (VASP) ( 78 , 79 ). The projector augmented wave (PAW) pseudopotentials ( 80 , 81 ) distributed in 2019 were used in combination with an energy cutoff of 400 eV for the plane-wave basis set. The energies and forces were evaluated with the Perdew-Burke-Ernzerhof (PBE) ( 82 ) exchange-correlation functional together with the density-dependent dispersion correction (dDsC) ( 83 , 84 ). The gamma-point was found to be sufficient for the Brillouin zone integration. A Fermi-smearing of 0.025 eV (~ 300 K) was applied. The wave functions were optimized to an energy change below 10 –6 eV in consecutive cycles. A maximum force below 0.05 eV/Å on all atoms was used as a criterion for geometry optimization, where the cell parameters, together with the position of all atoms, were relaxed. Optimized geometries can be retrieved from doi.org/10.17172/NOMAD/2025.11.19-1 . TEM TEM investigations were performed with a Themis Z G3 Cs-probe corrected microscope (Thermo Fisher Scientific) operated at 80kV and equipped with a GATAN 4K OneView camera. The FeMn sample was gently crushed in pure ethanol, dispersed onto a lacey-C-coated copper grid, and treated with a 20 Ar plasma just before analysis to reduce C contamination under the focused electron beam. Elemental analyses were performed in scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) mode using a Super-X emission energy-dispersive X-ray spectrometer (EDS) consisting of four windowless silicon-drift detectors providing a large 0.7 srad collection solid angle. EDS mapping was acquired at 180 pA current, a 50 µs dwell time per pixel, and a 1.6 Å pixel size. Declarations Acknowledgments We thank Thomas Kuhn for providing sample SO193_TVG71-5, James Hein and Kira Mizell for providing sample KNOX06RR_D20-1, Alexander Sobolev and Valentina Batanova for EPMA and LA-ICP-MS measurements. Eric Gautron is thanked for the access and assistance with the Nant’Themis TEM of the IMN’s characterization platform, PLASSMAT. Computational resources were provided by the “Centre Blaise Pascal de simulation et modélisation numérique (CBPSMN)”, which runs with the SIDUS solution ( 85 ). Funding: Financial support was provided by the European Union (ERC, Advanced Grant DEEP-SEE, 101052913). The EPMA and LA-ICP-MS have been funded by the ERC Synergy 856555 MEET project and the Isère Department, France. The views and opinions expressed are solely those of the authors and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. The European Union and the granting authority accept no responsibility for the views expressed herein. Author contributions: Conceptualization: A.M., O.M., A.K., and J.L. Methodology: O.M., A.M., J.L., Y.L., Julien L., S.S., A.G., C.D., A-C.G., and I. S. Investigation: J.L., A.M., A.K., S.S., and Y.L. Visualization: J.L., A.M., Y.L., S.S., and A.G. Supervision: A.M., O.M., and A.K. Writing—original draft: J.L., and Y.L. 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J Chem Theory Comput 7:3567–3577 Quemener E, Corvellec M (2013) SIDUS—the solution for extreme deduplication of an operating system. Linux J 2013 3:3 Manceau A, Lanson M, Geoffroy N (2007) Natural speciation of Ni, Zn, Ba, and As in ferromanganese coatings on quartz using X-ray fluorescence, absorption, and diffraction. Geochim Cosmochim Acta 71:95–128 Additional Declarations The authors declare no competing interests. Supplementary Files 3.SupplementaryPtinFeMnSA20240421.docx Supplementary Materials for \"Discovery of α-PtO 2 nanoparticles in deep-sea ferromanganese deposits: a hidden platinum sink\" Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-9606997\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":true,\"archivedVersions\":[],\"articleType\":\"Research Article\",\"associatedPublications\":[],\"authors\":[{\"id\":634066167,\"identity\":\"60d93307-e38b-4d6e-b627-2d2db94b347c\",\"order_by\":0,\"name\":\"Jianlin Liao\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4ElEQVRIie2PvwrCMBCHLxwkS7BroaCvEBEqguCr5AEUBUEcRAoFXQTXvoaLcyFgFx/BoSJ07lQcRDxRcWscHfLB5S8f9zsAh+MPYTEtuQJoIB1KKhGx3K5oUjgpLKGLTFHZW2kqToXyFwXXWOR6cgIu8Hzpz5djCcjL+mC8q7QqKBjvdIZH09sAYmKZJfS1Ms9ZwmC0StUAPFMbjMWieiuiCnr3paJgaFHkp4sMAxbhT8rsqUhSpu3NwSiJFqW9zfZ+eTNNz8t2+XVBwURsUaLXLr9P9QJAy/LvcDgcDoAHI842cKoZw3cAAAAASUVORK5CYII=\",\"orcid\":\"https://orcid.org/0000-0001-5504-3487\",\"institution\":\"European Synchrotron Radiation Facility\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Jianlin\",\"middleName\":\"\",\"lastName\":\"Liao\",\"suffix\":\"\"},{\"id\":634066168,\"identity\":\"930a88df-86b6-4cc9-bdb9-1607e29a3bee\",\"order_by\":1,\"name\":\"Yan Li\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0001-8456-0352\",\"institution\":\"European Synchrotron Radiation Facility\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yan\",\"middleName\":\"\",\"lastName\":\"Li\",\"suffix\":\"\"},{\"id\":634066169,\"identity\":\"b27b851b-c24a-4610-b69b-f7347a5db1af\",\"order_by\":2,\"name\":\"Julien Léger\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0001-8282-3040\",\"institution\":\"ISTerre, Université Grenoble Alpes, CNRS\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Julien\",\"middleName\":\"\",\"lastName\":\"Léger\",\"suffix\":\"\"},{\"id\":634066170,\"identity\":\"baec32ed-54b6-444d-b046-132bafba2946\",\"order_by\":3,\"name\":\"Andrea Koschinsky\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-9224-0663\",\"institution\":\"Department of Physics and Earth Sciences, School of Science, Constructor University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Andrea\",\"middleName\":\"\",\"lastName\":\"Koschinsky\",\"suffix\":\"\"},{\"id\":634066171,\"identity\":\"557fb586-f3c5-4e16-bc1a-4c487eff78aa\",\"order_by\":4,\"name\":\"Catherine Dejoie\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0003-3313-3515\",\"institution\":\"European Synchrotron Radiation Facility\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Catherine\",\"middleName\":\"\",\"lastName\":\"Dejoie\",\"suffix\":\"\"},{\"id\":634066172,\"identity\":\"9442e1fd-d99e-4ea3-8426-295191083e72\",\"order_by\":5,\"name\":\"Anne-Claire Gaillot\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-2694-6756\",\"institution\":\"Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Anne-Claire\",\"middleName\":\"\",\"lastName\":\"Gaillot\",\"suffix\":\"\"},{\"id\":634066173,\"identity\":\"b4edb066-4879-4dce-82b9-a41d4fc19c89\",\"order_by\":6,\"name\":\"Irina Snigireva\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0003-3481-2202\",\"institution\":\"European Synchrotron Radiation Facility\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Irina\",\"middleName\":\"\",\"lastName\":\"Snigireva\",\"suffix\":\"\"},{\"id\":634066174,\"identity\":\"1e66387f-6a59-4bd3-9e7b-80d8f79c85a3\",\"order_by\":7,\"name\":\"Andrea Giacomelli\",\"email\":\"\",\"orcid\":\"https://orcid.org/0009-0005-2115-6164\",\"institution\":\"ENS de Lyon, CNRS, Laboratoire de Chimie\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Andrea\",\"middleName\":\"\",\"lastName\":\"Giacomelli\",\"suffix\":\"\"},{\"id\":634066175,\"identity\":\"5e9e8ca2-12d3-4d80-abde-554fe3d7982f\",\"order_by\":8,\"name\":\"Stephan N. Steinmann\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-2777-356X\",\"institution\":\"ENS de Lyon, CNRS, Laboratoire de Chimie\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Stephan\",\"middleName\":\"N.\",\"lastName\":\"Steinmann\",\"suffix\":\"\"},{\"id\":634066176,\"identity\":\"9442baa0-cf70-435c-af19-7b9572f4bc30\",\"order_by\":9,\"name\":\"Olivier Mathon\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-5487-0226\",\"institution\":\"European Synchrotron Radiation Facility\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Olivier\",\"middleName\":\"\",\"lastName\":\"Mathon\",\"suffix\":\"\"},{\"id\":634066177,\"identity\":\"4f60dca8-543f-4fad-91b5-cb8761a8f6f5\",\"order_by\":10,\"name\":\"Alain Manceau\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABDUlEQVRIiWNgGAWjYDACdgaGAwheBYhgbgCR/Di1MKNoOQMiGMFaJBvwaEEAxjYitPA3Mz88XMCwTd6c/fDBz7zzbPLkGxjbJD7uYJAwx6FH4jCbweEZDLcNd/akJUvzbksrNjjA2CY58wyDhMwB7FoYDjMYHOZhuM244QaPAVDL4cQN8g/bpHnbGOokcOiQP8z+AaTFfsMN/s+/eef8T5wPdBhIiwQuLUArwLYkAm1hk+ZtOJDYcICAFsPDPAVAXbeTN5xJM7Occyw5ccMBxmbLmW0SOLXIHW/f/Jmn4rbthuOHH994U2MHdBjzwRsf22xwaoE6D5XLAlSNXwMGYP5AmvpRMApGwSgY5gAAii9XztzyUSkAAAAASUVORK5CYII=\",\"orcid\":\"https://orcid.org/0000-0003-0845-611X\",\"institution\":\"European Synchrotron Radiation Facility\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Alain\",\"middleName\":\"\",\"lastName\":\"Manceau\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2026-05-04 10:08:58\",\"currentVersionCode\":1,\"declarations\":{\"humanSubjects\":false,\"vertebrateSubjects\":false,\"conflictsOfInterestStatement\":false,\"humanSubjectEthicalGuidelines\":false,\"humanSubjectConsent\":false,\"humanSubjectClinicalTrial\":false,\"humanSubjectCaseReport\":false,\"vertebrateSubjectEthicalGuidelines\":false},\"doi\":\"10.21203/rs.3.rs-9606997/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-9606997/v1\",\"draftVersion\":[],\"editorialEvents\":[],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":108470362,\"identity\":\"1c7b64a4-ffe4-459a-922d-e1427a778f0a\",\"added_by\":\"auto\",\"created_at\":\"2026-05-05 05:21:40\",\"extension\":\"jpeg\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":675612,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eSample locations and description.\\u003c/strong\\u003e (\\u003cstrong\\u003eA\\u003c/strong\\u003e) The sampling sites are the Ninety East Ridge in the Indian Ocean, the Mannihiki Plateau in the Western South Pacific Ocean, and the Tropic Seamount in the Eastern North Atlantic Ocean. (\\u003cstrong\\u003eB\\u003c/strong\\u003e) A specimen of the non-phosphatized FeMn crust from the Indian Ocean. (\\u003cstrong\\u003eC\\u003c/strong\\u003e) A specimen of the phosphatized FeMn nodule from the Pacific Ocean. (\\u003cstrong\\u003eD\\u003c/strong\\u003e) A specimen of the phosphatized FeMn crust from the Atlantic Ocean. The white arrows indicate the growth direction of the FeMn layers. The yellow dashed lines indicate the locations of the cut sections.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage1.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/db8b058e5ff1f8ad57d99280.jpeg\"},{\"id\":108804186,\"identity\":\"8eaeb262-6477-47d7-976c-b94e73ea7386\",\"added_by\":\"auto\",\"created_at\":\"2026-05-08 15:17:28\",\"extension\":\"jpeg\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":413314,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eThin sections and Pt contents.\\u003c/strong\\u003e (\\u003cstrong\\u003eA\\u003c/strong\\u003e) Non-phosphatized FeMn crust from the Indian Ocean. (\\u003cstrong\\u003eB\\u003c/strong\\u003e) Phosphatized FeMn nodule from the Pacific Ocean. The white veins at 40 mm depth are phosphorite. (\\u003cstrong\\u003eC\\u003c/strong\\u003e) Strongly phosphatized FeMn crust from the Atlantic Ocean. The orange layers are phosphorite. The yellow boxes indicate the positions of the Pt-rich layers.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage2.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/b218e28a9df77de5f7304807.jpeg\"},{\"id\":108803863,\"identity\":\"4285931c-547e-40c7-903a-dc284e9e3b02\",\"added_by\":\"auto\",\"created_at\":\"2026-05-08 15:09:45\",\"extension\":\"jpeg\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":941055,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003ePt contents of some spheroidal and waved growth layers. \\u003c/strong\\u003e(\\u003cstrong\\u003eA\\u003c/strong\\u003e–\\u003cstrong\\u003eC\\u003c/strong\\u003e) Micrographs of the Indian FeMn crust. (\\u003cstrong\\u003eD\\u003c/strong\\u003e–\\u003cstrong\\u003eF\\u003c/strong\\u003e) Micrographs of the Pacific FeMn nodule. (\\u003cstrong\\u003eG\\u003c/strong\\u003e–\\u003cstrong\\u003eI\\u003c/strong\\u003e) Micrographs of the Atlantic FeMn crust. The Pt contents of the LA-ICP-MS spots are indicated. The element contents of the laser-ablated spots are provided in Data S5.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage3.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/c0d225cb89b658a28d2f9d51.jpeg\"},{\"id\":108803766,\"identity\":\"452a4718-fd75-43fb-aed2-647ba71db2f6\",\"added_by\":\"auto\",\"created_at\":\"2026-05-08 15:06:09\",\"extension\":\"jpeg\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":454614,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eTernary compositional diagrams for genetic classification.\\u003c/strong\\u003e (\\u003cstrong\\u003eA\\u003c/strong\\u003e–\\u003cstrong\\u003eC\\u003c/strong\\u003e) Ternary genetic diagrams (\\u003cem\\u003e38\\u003c/em\\u003e) of the Indian FeMn crust, Pacific FeMn nodule, and Atlantic FeMn crust, using element contents obtained by LA-ICP-MS line-scanning analysis. (\\u003cstrong\\u003eD\\u003c/strong\\u003e–\\u003cstrong\\u003eF\\u003c/strong\\u003e) Selection of spots from (\\u003cstrong\\u003eA\\u003c/strong\\u003e), (\\u003cstrong\\u003eB\\u003c/strong\\u003e), and (\\u003cstrong\\u003eC\\u003c/strong\\u003e), respectively, corresponding to the Pt-rich FeMn layers shown in Fig. 2. Hydro: hydrogenetic, Dia: diagenetic. The data of the LA-ICP-MS line-scanning are provided in Data S2–4.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage4.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/557c67fae4035eca30009ad4.jpeg\"},{\"id\":108470364,\"identity\":\"48b8f577-b8ca-4978-adc6-307dd792a0f4\",\"added_by\":\"auto\",\"created_at\":\"2026-05-05 05:21:40\",\"extension\":\"jpeg\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":555262,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eXRD patterns of Pt-rich layers in FeMn crusts/nodules.\\u003c/strong\\u003e (\\u003cstrong\\u003eA\\u003c/strong\\u003e) XRD pattern of the Pt-richest layer (17–19 mm) in the non-phosphatized FeMn crust from the Indian Ocean. (\\u003cstrong\\u003eB\\u003c/strong\\u003e) XRD pattern of the Pt-richest layer (26–28 mm) in the phosphatized FeMn nodule from the Pacific Ocean. (\\u003cstrong\\u003eC\\u003c/strong\\u003e–\\u003cstrong\\u003eD\\u003c/strong\\u003e) XRD patterns of the Pt-rich layers (32–34 mm and 64–66 mm) in the phosphatized FeMn crust from the Atlantic Ocean. Vrn: vernadite. Tdr: todorokite. Additional XRD patterns of the three samples are shown in Fig. S2–S4.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage5.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/c9b43e72026597c83e809fd8.jpeg\"},{\"id\":108470366,\"identity\":\"d1b9947a-63be-47b8-a76c-02b5a5f05dc1\",\"added_by\":\"auto\",\"created_at\":\"2026-05-05 05:21:40\",\"extension\":\"jpeg\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":221003,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003ePt L\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e3\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003e-edge XANES spectra.\\u003c/strong\\u003e (\\u003cstrong\\u003eA\\u003c/strong\\u003e–\\u003cstrong\\u003eB\\u003c/strong\\u003e) Normalized XANES spectra (\\u003cstrong\\u003eA\\u003c/strong\\u003e) and first derivatives (\\u003cstrong\\u003eB\\u003c/strong\\u003e) of the three marine FeMn deposits. (\\u003cstrong\\u003eC\\u003c/strong\\u003e–\\u003cstrong\\u003eD\\u003c/strong\\u003e) Normalized XANES spectra (\\u003cstrong\\u003eC\\u003c/strong\\u003e) and first derivatives (\\u003cstrong\\u003eD\\u003c/strong\\u003e) of Pt references. The maximum of the first derivative is at ~11564 eV for Pt(Ⅱ) and ~11567 eV for Pt(Ⅳ).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage6.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/7a1086ed109aa8c968135dab.jpeg\"},{\"id\":108470370,\"identity\":\"de7086c9-f1a9-4ac7-8b7a-0577a696bd9e\",\"added_by\":\"auto\",\"created_at\":\"2026-05-05 05:21:40\",\"extension\":\"jpeg\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":479785,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eBest-fits of the Pt L\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e3\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003e-edge EXAFS spectra.\\u003c/strong\\u003e (\\u003cstrong\\u003eA\\u003c/strong\\u003e–\\u003cstrong\\u003eB\\u003c/strong\\u003e) EXAFS spectrum (\\u003cstrong\\u003eA\\u003c/strong\\u003e) and Fourier transform (FT, \\u003cstrong\\u003eB\\u003c/strong\\u003e) of the marine FeMn deposits (Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e) fitted in the 3.2 Å\\u003csup\\u003e–1\\u003c/sup\\u003e ≤ \\u003cem\\u003ek\\u003c/em\\u003e ≤ 12.1 Å\\u003csup\\u003e–1\\u003c/sup\\u003e interval. (\\u003cstrong\\u003eC\\u003c/strong\\u003e–\\u003cstrong\\u003eD\\u003c/strong\\u003e) EXAFS spectrum (\\u003cstrong\\u003eC\\u003c/strong\\u003e) and FT (\\u003cstrong\\u003eD\\u003c/strong\\u003e) of the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference fitted in the same \\u003cem\\u003ek\\u003c/em\\u003e-interval as Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e. (\\u003cstrong\\u003eE\\u003c/strong\\u003e–\\u003cstrong\\u003eF\\u003c/strong\\u003e) EXAFS spectrum (\\u003cstrong\\u003eE\\u003c/strong\\u003e) and FT (\\u003cstrong\\u003eF\\u003c/strong\\u003e) of the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference fitted in a longer \\u003cem\\u003ek\\u003c/em\\u003e-interval (3.2 Å\\u003csup\\u003e–1\\u003c/sup\\u003e ≤ \\u003cem\\u003ek\\u003c/em\\u003e ≤ 16.0 Å\\u003csup\\u003e–1\\u003c/sup\\u003e). Inserted in (\\u003cstrong\\u003eF\\u003c/strong\\u003e) is the layered structure of α-PtO\\u003csub\\u003e2 \\u003c/sub\\u003e(\\u003cem\\u003e56\\u003c/em\\u003e), constituted of edge-sharing PtO\\u003csub\\u003e6\\u003c/sub\\u003e octahedra, similar to the shared-edge MnO\\u003csub\\u003e6\\u003c/sub\\u003e octahedral structure of vernadite.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage7.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/b9d11ce27db710278feeef5d.jpeg\"},{\"id\":108470368,\"identity\":\"52330f8a-d101-4c4b-b1b6-dd11d5ee63bd\",\"added_by\":\"auto\",\"created_at\":\"2026-05-05 05:21:40\",\"extension\":\"jpeg\",\"order_by\":8,\"title\":\"Figure 8\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":254744,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eDFT models. \\u003c/strong\\u003e(\\u003cstrong\\u003eA\\u003c/strong\\u003e) DFT model 1: incorporation of Pt within δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers. This model comprises three layers (supercell), each containing 42 Mn atoms, of which seven Mn atoms are replaced with Pt atoms. (\\u003cstrong\\u003eB\\u003c/strong\\u003e) DFT model 2: an α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanolayer is topotactically stacked between two δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers (asbolane-type structure). The edges of the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanolayer are saturated with OH/H\\u003csub\\u003e2\\u003c/sub\\u003eO groups. The black frame delineates the supercell. Color code: Mn, black; Pt, orange; O, gray; H, green. The coordinates of the DFT models in CIF format are provided in Data S9–10.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage8.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/f5fe9ba30c060c6b45183f6e.jpeg\"},{\"id\":108470369,\"identity\":\"3994ca11-e0b0-4b17-9c9a-92ce57587f7f\",\"added_by\":\"auto\",\"created_at\":\"2026-05-05 05:21:40\",\"extension\":\"jpeg\",\"order_by\":9,\"title\":\"Figure 9\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":190617,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003ePt solubility and speciation. \\u003c/strong\\u003e(\\u003cstrong\\u003eA\\u003c/strong\\u003e) Solubility of Pt in pure water and seawater (NaCl 0.6 M). (\\u003cstrong\\u003eB\\u003c/strong\\u003e) Percentage of Pt(Ⅱ) species in the Pt-O-H-Cl system (0.6 M NaCl) as a function of pH. The thermodynamic constants (\\u003cem\\u003e28\\u003c/em\\u003e) used in (\\u003cstrong\\u003eA\\u003c/strong\\u003e) and (\\u003cstrong\\u003eB\\u003c/strong\\u003e) are listed in Table S4. (\\u003cstrong\\u003eC\\u003c/strong\\u003e) pe-pH diagram of 10\\u003csup\\u003e–12\\u003c/sup\\u003e M Pt at atmospheric conditions (25℃, 1 bar) in the Pt-O-H system. The Gibbs free energies of the Pt species are from the literature (\\u003cem\\u003e51\\u003c/em\\u003e). The vertical gray lines represent the seawater pH. aq: aqueous.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage9.jpeg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/b463a6dcddfc0a6a34013c74.jpeg\"},{\"id\":108811694,\"identity\":\"011ce0d4-3dbf-4a4b-b6fd-6eff1c8d5388\",\"added_by\":\"auto\",\"created_at\":\"2026-05-08 16:06:44\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":4681985,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/5dd86372-bf53-4af0-ba6d-0d5c682c73e3.pdf\"},{\"id\":108470361,\"identity\":\"fdf5479d-caa3-4e9a-9db8-11498bcdf7f5\",\"added_by\":\"auto\",\"created_at\":\"2026-05-05 05:21:40\",\"extension\":\"docx\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":3752782,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSupplementary Materials for \\\"Discovery of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles in deep-sea ferromanganese deposits: a hidden platinum sink\\\"\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"3.SupplementaryPtinFeMnSA20240421.docx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-9606997/v1/96b757c8594714fb677a79d8.docx\"}],\"financialInterests\":\"The authors declare no competing interests.\",\"formattedTitle\":\"\\u003cp\\u003e\\u003cstrong\\u003eDiscovery of α-PtO\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e2\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003e nanoparticles in deep-sea ferromanganese deposits: a hidden platinum sink\\u003c/strong\\u003e\\u003c/p\\u003e\",\"fulltext\":[{\"header\":\"Teaser\",\"content\":\"\\u003cp\\u003eEvidence of \\u0026alpha;-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles in deep-sea ferromanganese deposits sheds light on the Pt sink in marine environments and aids resource exploration.\\u003c/p\\u003e\"},{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003ePlatinum (Pt), a precious metal in jewelry and a versatile critical metal in catalysis, has long been central to scientific and industrial progress. The industrial demand for Pt has increased in the past thirty years (\\u003cspan citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e), due to its uses in fuel cells (\\u003cspan citationid=\\\"CR2\\\" class=\\\"CitationRef\\\"\\u003e2\\u003c/span\\u003e), carbon-neutral technologies (\\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e3\\u003c/span\\u003e), and cancer treatments (\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e). The low natural abundance and high cost of Pt motivate global prospecting for new resources beyond traditional land deposits (\\u003cspan additionalcitationids=\\\"CR6\\\" citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e7\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003eDeep-sea polymetallic ferromanganese (FeMn) deposits encompass FeMn nodules and crusts that accrete via hydrogenetic, diagenetic, or hydrothermal processes at low growth rates (millimetres to a few centimeters per million years), with growth histories occasionally interrupted by phosphatization events (\\u003cspan additionalcitationids=\\\"CR9 CR10 CR11\\\" citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e). FeMn deposits can concentrate Pt at levels (e.g., 3.2 \\u0026micro;g/g) (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e) up to eight orders of magnitude greater than seawater (picomolar level) (\\u003cspan additionalcitationids=\\\"CR15 CR16\\\" citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e), making them valuable repositories for Pt (\\u003cspan additionalcitationids=\\\"CR9\\\" citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e). Although more than a century and a half have passed since deep-sea FeMn crusts and nodules were first discovered in 1873 (\\u003cspan additionalcitationids=\\\"CR21\\\" citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e), the chemical form and enrichment process of Pt remain a mystery.\\u003c/p\\u003e \\u003cp\\u003eCurrent understanding of Pt mobilization and accumulation behaviors in the marine environment largely relies on indirect methods, such as thermodynamic modeling (\\u003cspan additionalcitationids=\\\"CR24 CR25 CR26 CR27 CR28\\\" citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR29\\\" class=\\\"CitationRef\\\"\\u003e29\\u003c/span\\u003e) and laboratory adsorption experiments (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan additionalcitationids=\\\"CR31 CR32 CR33\\\" citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR34\\\" class=\\\"CitationRef\\\"\\u003e34\\u003c/span\\u003e). Three oxidation states of Pt in marine FeMn deposits have been proposed, metallic [Pt(0)] (\\u003cspan additionalcitationids=\\\"CR36\\\" citationid=\\\"CR35\\\" class=\\\"CitationRef\\\"\\u003e35\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR37\\\" class=\\\"CitationRef\\\"\\u003e37\\u003c/span\\u003e), divalent [Pt(Ⅱ)] (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e32\\u003c/span\\u003e), and tetravalent [Pt(IV)] (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan additionalcitationids=\\\"CR31\\\" citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e32\\u003c/span\\u003e), which have led to conflicting hypotheses: 1) deposition of extraterrestrial Pt(0) spherules (\\u003cspan citationid=\\\"CR35\\\" class=\\\"CitationRef\\\"\\u003e35\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR36\\\" class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e), 2) reductive immobilization of Pt(Ⅱ) to Pt(0) by Mn\\u003csup\\u003e2+\\u003c/sup\\u003e (\\u003cspan citationid=\\\"CR36\\\" class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR37\\\" class=\\\"CitationRef\\\"\\u003e37\\u003c/span\\u003e), 3) Pt(Ⅱ) adsorption onto Fe (oxyhydr)oxide surfaces (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e32\\u003c/span\\u003e), and 4) oxidative adsorption of Pt(Ⅱ) on Mn oxides followed by isomorphic Pt(IV) for Mn(IV) substitution in the crystal structure (\\u003cem\\u003e13\\u003c/em\\u003e, \\u003cem\\u003e30\\u003c/em\\u003e\\u0026ndash;\\u003cem\\u003e32\\u003c/em\\u003e). The lack of direct experimental evidence from natural samples has kept this controversy ongoing. Resolving this question requires direct analysis of the Pt chemical form in marine FeMn substrates, which is difficult due to 1) their Pt content that falls below the detection limit of standard spectroscopy techniques, and 2) their complex multielement compositions.\\u003c/p\\u003e \\u003cp\\u003eIn this study, we addressed the two difficulties as follows. Firstly, we selected three Pt-rich FeMn crusts and nodules from the Indian Ocean, Pacific Ocean, and Atlantic Ocean (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e, Table S1) from a collection of 182 samples (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e). Using microscale elemental analysis, we identified the Pt-richest FeMn layers within the selected crusts and nodules. Subsequently, we determined the chemical form of Pt in these layers using high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) to eliminate matrix effects. Then, the structural relationship between Pt and the FeMn layers was modeled by density functional theory (DFT).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e\"},{\"header\":\"Results\",\"content\":\"\\u003cdiv id=\\\"Sec3\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eGrowth structure and chemical composition\\u003c/h2\\u003e \\u003cp\\u003eThe Indian FeMn crust consists of black growth layers with high Mn (24.65 wt.% on average) and Fe contents (14.15 wt.% on average, Data S1, Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eA). The phosphorous content is low (0.27 wt.% on average), and there are no phosphatized layers (Fig. S1A\\u0026ndash;B). The black FeMn growth layers of the Pacific nodule contain phosphorous-rich veins from apatite [Ca\\u003csub\\u003e5\\u003c/sub\\u003e(CO\\u003csub\\u003e3\\u003c/sub\\u003e)\\u003csub\\u003ex\\u003c/sub\\u003e(PO\\u003csub\\u003e4\\u003c/sub\\u003e)\\u003csub\\u003e3\\u0026minus;x\\u003c/sub\\u003eF\\u003csub\\u003e1+x\\u003c/sub\\u003e] in the middle and basal parts (25\\u0026ndash;40 mm, Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eB, Fig. S1C\\u0026ndash;D), indicating past phosphatization events. Their P content reaches 10.12 wt.% (Data S1). The FeMn layers of the Atlantic crust are interbedded with several, thick phosphorite layers at 2\\u0026ndash;3 mm, 8\\u0026ndash;11 mm, and 41\\u0026ndash;44 mm from the crust surface (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eC, P content up to 13.99 wt.%, Data S1), resulting from intense and prolonged phosphatization events.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eThe elemental profiles across the growth layers of the three FeMn crusts and nodules show Pt contents up to 4\\u0026ndash;6 \\u0026micro;g/g (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e, Data S2\\u0026ndash;4). The Pt-richest layers are located at 17\\u0026ndash;19 mm in the Indian crust, 26\\u0026ndash;28 mm and 37\\u0026ndash;39 mm in the Pacific nodule, and 32\\u0026ndash;34 mm and 64\\u0026ndash;66 mm in the Atlantic crust (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e). All Pt-rich FeMn layers commonly exhibit a spheroidal growth structure that differs from the wave-layered and stromatolithic columnar structures of the non-Pt-rich layers (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eThe geochemical data were plotted on ternary diagrams (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eA\\u0026ndash;C), which differentiate hydrogenetic, diagenetic, and hydrothermal FeMn deposits based on their chemical compositions (\\u003cspan citationid=\\\"CR38\\\" class=\\\"CitationRef\\\"\\u003e38\\u003c/span\\u003e). The Indian FeMn crust is entirely hydrogenetic in origin (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eA and D). The Pacific FeMn nodule comprises both hydrogenetic and diagenetic layers (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eB), with the Pt-rich layers being of hydrogenetic origin (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eE). The Atlantic FeMn crust exhibits a complex growth history, consisting of alternating layers of hydrogenetic and diagenetic, and some layers pointing towards a certain hydrothermal influence (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eC). Pt-rich layers occur in both hydrogenetic and diagenetic layers (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eF).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003c/div\\u003e\\n\\u003ch3\\u003eMineral composition\\u003c/h3\\u003e\\n\\u003cp\\u003eThe mineral compositions determined by X-ray diffraction (XRD) are shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e and Fig. S2\\u0026ndash;S4. The FeMn layers of the Indian crust consist of Fe-vernadite (FeOOH\\u0026thinsp;+\\u0026thinsp;δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e) with main diffraction peaks at 2.45 \\u0026Aring; and 1.42 \\u0026Aring; and no basal reflection (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003eA and Fig. S2). The Pt-richest layers of the Pacific nodule at 26\\u0026ndash;28 mm and 37\\u0026ndash;39 mm consist of Fe-vernadite intergrown with apatite and calcite (CaCO\\u003csub\\u003e3\\u003c/sub\\u003e, Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003eB and Fig. S3). In contrast, the younger FeMn layer on the rim of the nodule at 5\\u0026ndash;7 mm from the surface consists of Fe-vernadite only (Fig. S3). The Pt-rich layer at 64\\u0026ndash;66 mm in the Atlantic crust is also made up of Fe-vernadite, whereas the other Pt-rich FeMn layer at 32\\u0026ndash;34 mm contains apatite and calcite in addition to Fe-vernadite (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003eC\\u0026ndash;D). The three mineral phases also co-occur at 5\\u0026ndash;7 mm, 21\\u0026ndash;23 mm, and 32\\u0026ndash;34 mm, and apparently at 64\\u0026ndash;66 mm, independent of the Pt content (Fig. S4). The XRD patterns of the 5\\u0026ndash;7 mm, 32\\u0026ndash;34 mm, and 64\\u0026ndash;66 mm depth layers have a peak at 9.56 \\u0026Aring; from the 001 reflection of 10 \\u0026Aring; manganates, which can be from 10 \\u0026Aring;-vernadite or the todorokite tectomanganate (Fig. S4A, C, and D). Its shift to 9.2 \\u0026Aring; after heating to 105\\u0026deg;C identified todorokite (\\u003cspan citationid=\\\"CR39\\\" class=\\\"CitationRef\\\"\\u003e39\\u003c/span\\u003e). The 002 reflection at 4.73 \\u0026Aring; consistently behaved similarly to the 001 reflection at 9.56 \\u0026Aring; after heating (Fig. S4A, C, D).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e\\n\\u003ch3\\u003eMn average oxidation state\\u003c/h3\\u003e\\n\\u003cp\\u003eThe FeMn layers analyzed by XRD were measured by Mn K-edge X-ray absorption near-edge structure (XANES) spectroscopy (Fig. S5) to determine the average manganese oxidation state (AMOS), which is a proxy of redox condition. The AMOS values vary only slightly across all layers of the Indian FeMn crust (3.89\\u0026ndash;3.91) and the Pacific nodule (3.93\\u0026ndash;3.94, Table S2). The older layers of the Atlantic FeMn crust at 64\\u0026ndash;66 mm also have high AMOS (3.90), whereas the younger layers (5\\u0026ndash;7 mm, 21\\u0026ndash;23 mm, 32\\u0026ndash;34 mm), which are phosphatized, have lower AMOS ranging from 3.91 to 3.83, with higher Mn\\u003csup\\u003e3+\\u003c/sup\\u003e proportions (9%\\u0026ndash;15%, Table S2).\\u003c/p\\u003e\\n\\u003ch3\\u003eOxidation state and coordination environment of Pt\\u003c/h3\\u003e\\n\\u003cp\\u003eThe oxidation state of Pt was determined by HERFD-XANES spectroscopy. The Pt L\\u003csub\\u003e3\\u003c/sub\\u003e-edge XANES spectra are nearly identical across the three FeMn crusts and nodules (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003eA\\u0026ndash;B), with energy position and shape closely matching the PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference (tetravalent, Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003eC\\u0026ndash;D).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eThe structural form of Pt was identified using extended X-ray absorption fine structure spectroscopy in HERFD mode (HERFD-EXAFS). The EXAFS spectra for the three FeMn crusts and nodules are statistically identical, meaning that the chemical form of Pt is invariant (Fig. S6). Therefore, the three EXAFS spectra were averaged to generate a representative spectrum of higher quality for Pt in marine FeMn deposits (Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e, Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eA\\u0026ndash;B). The spectra of Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e and the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference have essentially the same frequency, but the Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e spectrum is less structured (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eA and C). In real space, after Fourier transform of the EXAFS spectra, this distinction is seen mainly as a reduction in amplitude of the nearest Pt-Pt atomic pair at \\u003cem\\u003eR\\u003c/em\\u003e\\u0026thinsp;+\\u0026thinsp;Δ\\u003cem\\u003eR\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;2.6\\u0026ndash;3.6 \\u0026Aring; (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eB).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eThe α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e and Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e spectra were least-squares fitted in the common 3.2 \\u0026Aring;\\u003csup\\u003e\\u0026ndash;1\\u003c/sup\\u003e \\u0026le; \\u003cem\\u003ek\\u003c/em\\u003e\\u0026thinsp;\\u0026le;\\u0026thinsp;12.1 \\u0026Aring;\\u003csup\\u003e\\u0026ndash;1\\u003c/sup\\u003e interval, giving a distance resolution of Δ\\u003cem\\u003ed\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;π/2\\u003cem\\u003ek\\u003c/em\\u003e\\u003csub\\u003emax\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;0.13 \\u0026Aring; (\\u003cspan citationid=\\\"CR40\\\" class=\\\"CitationRef\\\"\\u003e40\\u003c/span\\u003e) (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eC\\u0026ndash;D). The best-fit EXAFS parameters are 6 O1 at 2.02 \\u0026Aring;, 6 Pt1 at 3.10 \\u0026Aring;, and 12 O2 at 3.70 \\u0026Aring; for the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference, and 6 O1 at 2.00 \\u0026Aring;, 4.6 Pt1 at 3.17 \\u0026Aring;, and 12 O2 at 3.66 \\u0026Aring; for Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e (Table S3). The Pt-Pt1 distance of Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e is 0.07 \\u0026Aring; longer than that of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e. Another noticeable difference is a peak at \\u003cem\\u003eR\\u003c/em\\u003e\\u0026thinsp;+\\u0026thinsp;Δ\\u003cem\\u003eR\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;6.0 \\u0026Aring; observed in the Fourier transform of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eD and F). This peak corresponds to multiple scattering paths between collinear Pt-Pt1-Pt4 atoms in the layered α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e structure (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eF).\\u003c/p\\u003e\\n\\u003ch3\\u003eModeling of the Pt local structure\\u003c/h3\\u003e\\n\\u003cp\\u003eDFT modeling was undertaken to understand the 0.07 \\u0026Aring; increase in Pt-Pt distance of Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e compared to α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e. Two structures were geometrically optimized: α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoclusters incorporated into the δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eA), and α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanolayers topotactically stacked on δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eB). In the first model, seven Mn atoms were replaced with Pt atoms in each MnO\\u003csub\\u003e2\\u003c/sub\\u003e layer (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eA). The average Pt-Pt distance is 3.04 \\u0026Aring;, which is 0.13 \\u0026Aring; shorter than the Pt-Pt distance in Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e (i.e., 3.17 \\u0026Aring;). In the second model, incorporating a Pt\\u003csub\\u003e19\\u003c/sub\\u003eO\\u003csub\\u003e38\\u003c/sub\\u003e nanolayer of 12.6 \\u0026Aring; in diameter between two MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers yielded Pt-Pt distances between 3.12 \\u0026Aring; and 3.22 \\u0026Aring;, with a mean of 3.17 \\u0026Aring; (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eB), consistent with EXAFS results.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e\"},{\"header\":\"Discussion\",\"content\":\"\\u003cdiv id=\\\"Sec9\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eFormation of the Pt-rich FeMn layers\\u003c/h2\\u003e \\u003cp\\u003eThe Pt contents in the enriched layers were quantified using single-spot laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). All depth profiles of the Pt signal measured in the three FeMn crusts/nodules were uniform during ablation, indicating constant Pt concentration within the analyzed FeMn spots (Fig. S7). Thus, Pt is likely dispersed in the FeMn layers, rather than concentrated in discrete metallic nuggets. Several micrometric nuggets of platinum-group elements, perhaps of cosmic origin, have been observed in a deep-sea sediment, using scanning electron microscopy (\\u003cspan citationid=\\\"CR35\\\" class=\\\"CitationRef\\\"\\u003e35\\u003c/span\\u003e). This possible Pt form is insufficient to explain Pt enrichment in FeMn deposits (\\u003cspan citationid=\\\"CR36\\\" class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR37\\\" class=\\\"CitationRef\\\"\\u003e37\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003eThe two Pt-rich FeMn layers at 17\\u0026ndash;19 mm in the Indian FeMn crust and at 64\\u0026ndash;66 mm in the Atlantic FeMn crust mainly contain Fe-vernadite (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e). Vernadite (δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e), a nanocrystalline turbostratic phyllomanganate (\\u003cspan citationid=\\\"CR41\\\" class=\\\"CitationRef\\\"\\u003e41\\u003c/span\\u003e), is the main constitutive Mn oxide in hydrogenetic FeMn deposits, which form under oxic marine conditions (\\u003cspan additionalcitationids=\\\"CR43\\\" citationid=\\\"CR42\\\" class=\\\"CitationRef\\\"\\u003e42\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR44\\\" class=\\\"CitationRef\\\"\\u003e44\\u003c/span\\u003e). The hydrogenetic origin of the two FeMn layers is confirmed by geochemical (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e) and spectroscopic (AMOS\\u0026thinsp;\\u0026ge;\\u0026thinsp;3.9) analyses (Fig. S5 and Table S2). Observation of the occurrence of Pt-rich FeMn layers formed under hydrogenetic conditions contradicts the previously-held hypothesis that Pt enrichment is linked to phosphatization (\\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003eApatite and calcite coprecipitate with the Fe and Mn oxides during phosphatization events (\\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR45\\\" class=\\\"CitationRef\\\"\\u003e45\\u003c/span\\u003e). Neither apatite nor calcite hosts Pt within their structures (\\u003cem\\u003e46\\u003c/em\\u003e), so the presence of these minerals does not enhance Pt enrichment in the FeMn layers. In addition, phosphatization is usually accompanied by an oxic-to-suboxic transition in the depositional setting, as attested by the transformation of vernadite into todorokite, which contains more Mn(III) and less Mn(IV) than vernadite (\\u003cspan citationid=\\\"CR39\\\" class=\\\"CitationRef\\\"\\u003e39\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR47\\\" class=\\\"CitationRef\\\"\\u003e47\\u003c/span\\u003e). Some of the FeMn layers in the phosphatized Atlantic and Pacific crusts indeed contain todorokite, yet Pt contents vary independently of the presence of todorokite (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e, Fig. S3 and S4). This observation aligns with our statistical analysis of available literature data (\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e), which show that Pt and P concentrations are not correlated (Fig. S8). Pt accumulation most likely occurs at the seawater-seafloor interface during hydrogenetic deposition in both phosphatized and non-phosphatized FeMn deposits across the global oceans (Fig. S8).\\u003c/p\\u003e \\u003c/div\\u003e\\n\\u003ch3\\u003eSpeciation and redox potential of Pt in seawater conditions\\u003c/h3\\u003e\\n\\u003cp\\u003eTo understand the enrichment of Pt in hydrogenetic FeMn layers, which form in direct contact with seawater, one needs to know the solubility and speciation of Pt in seawater. Direct measurement is hindered by its extremely low concentration (\\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR15\\\" class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e), therefore, its solubility and possible complexation were estimated by thermodynamic calculations.\\u003c/p\\u003e \\u003cp\\u003eThe concentration of Pt in seawater is at the picomolar level (10⁻\\u003csup\\u003e12\\u003c/sup\\u003e M) (\\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e), much lower than its calculated solubility limit under seawater conditions (10⁻\\u003csup\\u003e1.24\\u003c/sup\\u003e M, Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig9\\\" class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003eA). This large difference suggests that Pt predominantly exists as soluble Pt(Ⅱ) rather than in equilibrium with a solid phase. The speciation of Pt in seawater was evaluated in the Pt-O-H-Cl system from the known equilibrium reaction constants (Table S4). Our calculation shows that the neutral hydroxyl species, Pt(OH)₂\\u003csub\\u003e(aq)\\u003c/sub\\u003e, predominates at pH 8 (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig9\\\" class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003eB). However, previous studies (\\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e32\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR36\\\" class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e) suggested that dissolved Pt(Ⅱ) can form negatively charged complexes [PtCl\\u003csub\\u003e4\\u0026minus;\\u003cem\\u003en\\u003c/em\\u003e\\u003c/sub\\u003e(OH)\\u003csub\\u003e\\u003cem\\u003en\\u003c/em\\u003e\\u003c/sub\\u003e]\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e (where \\u003cem\\u003en\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0\\u0026ndash;4), given the high chloride concentration and alkaline conditions of seawater. The formation and stability field of these complexes remains uncertain due to the lack of thermodynamic complexation constants for these species.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eDivalent Pt can be oxidized to tetravalent Pt through 1) a disproportionation reaction, 2) oxidation driven by dissolved oxygen in seawater, and 3) oxidative uptake driven by δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e. The disproportionation reaction \\u003cimg src=\\\"data:image/png;base64,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\\\" width=\\\"231\\\" height=\\\"25\\\"\\u003eis observed in high-temperature experiments (e.g., 152℃) (\\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR48\\\" class=\\\"CitationRef\\\"\\u003e48\\u003c/span\\u003e). This pathway requires significant energy input (\\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR48\\\" class=\\\"CitationRef\\\"\\u003e48\\u003c/span\\u003e) and leads to the formation of metallic Pt, which has been identified in marine hydrothermal fields (\\u003cspan citationid=\\\"CR49\\\" class=\\\"CitationRef\\\"\\u003e49\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR50\\\" class=\\\"CitationRef\\\"\\u003e50\\u003c/span\\u003e). Given that hydrogenetic FeMn crusts and nodules precipitate under low-temperature conditions in direct contact with seawater, and that no metallic Pt has been detected, this redox reaction can be dismissed. Oxidation of Pt(Ⅱ) by dissolved oxygen in seawater can be evaluated through thermodynamic modeling (\\u003cspan citationid=\\\"CR51\\\" class=\\\"CitationRef\\\"\\u003e51\\u003c/span\\u003e). Because the thermodynamic complexation constants for the [PtCl\\u003csub\\u003e4\\u0026minus;\\u003cem\\u003en\\u003c/em\\u003e\\u003c/sub\\u003e(OH)\\u003csub\\u003e\\u003cem\\u003en\\u003c/em\\u003e\\u003c/sub\\u003e]\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e complexes are unknown, redox reactions were modeled within the Pt-O-H system excluding Cl (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig9\\\" class=\\\"InternalRef\\\"\\u003e9\\u003c/span\\u003eC). Calculations indicate that oxidizing Pt(Ⅱ) to Pt(IV) is not thermodynamically feasible because the oxygen fugacity in seawater does not reach the required oxidation potential (\\u003cspan citationid=\\\"CR51\\\" class=\\\"CitationRef\\\"\\u003e51\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR52\\\" class=\\\"CitationRef\\\"\\u003e52\\u003c/span\\u003e). Therefore, Pt enrichment in marine FeMn deposits most likely occurs through Pt(II) oxidation by δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e (i.e., vernadite). This pathway is supported by sorption experiments, which showed that Pt(II) is oxidized by δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e), not by Fe (oxyhydr)oxides (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e). Some studies suggest that Pt(IV) is adsorbed on δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e (\\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e32\\u003c/span\\u003e), while others suggest its incorporation within the mineral structure through Pt(VI) for Mn(IV) substitution (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e). This question is answered below by HERFD-XAS.\\u003c/p\\u003e \\u003cdiv id=\\\"Sec11\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eAtomic-level chemical form of Pt in FeMn layers\\u003c/h2\\u003e \\u003cp\\u003eThe structural form of Pt(IV) was identified using EXAFS spectroscopy. Although this method has been previously used to probe the local atomic environment of Pt adsorbed onto Mn minerals under controlled laboratory conditions (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR53\\\" class=\\\"CitationRef\\\"\\u003e53\\u003c/span\\u003e), it has never been applied to the original marine FeMn oxide materials because of the chemical complexity of their polymetallic matrix and their Pt content (usually\\u0026thinsp;\\u0026lt;\\u0026thinsp;1 \\u0026micro;g/g), which is below the detection limit of standard EXAFS. In these previous studies, Pt was artificially sorbed from seawater solution to natural or artificial (Fe)Mn oxide substrates to reach detectable Pt concentrations. These difficulties were overcome in our study by measuring EXAFS at high-energy-resolution (\\u003cspan citationid=\\\"CR54\\\" class=\\\"CitationRef\\\"\\u003e54\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR55\\\" class=\\\"CitationRef\\\"\\u003e55\\u003c/span\\u003e). The Pt(Lα1) fluorescence line was selected with a spectrometer (Fig. S9) (\\u003cspan citationid=\\\"CR55\\\" class=\\\"CitationRef\\\"\\u003e55\\u003c/span\\u003e). The sharp parasitic As(Kα) white line occurring at \\u003cem\\u003ek\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;9 \\u0026Aring;\\u003csup\\u003e\\u0026ndash;1\\u003c/sup\\u003e in the Pt-EXAFS spectra was almost completely extinguished with the spectrometer (Supplementary Text, Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eA and S6).\\u003c/p\\u003e \\u003cp\\u003ePlatinum in FeMn deposits has an α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e-type local structure that differs from the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference by a reduction in the number of Pt atoms in the first (Pt1) and fourth (Pt4) Pt shells (Table S3). Thus, Pt is likely nanoparticulate in marine FeMn deposits. Since Pt(Ⅱ) is oxidized to Pt(IV) by Mn(III) or Mn(IV) cations from vernadite (δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e) in the FeMn crusts and nodules, one might expect to detect Pt-Mn pairs. However, they are absent in our best-fit EXAFS model (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eA\\u0026ndash;B). EXAFS spectroscopy can effectively differentiate between Pt-Pt and Pt-Mn pairs due to a phase shift difference of ∣Φ\\u003csub\\u003ePt-Pt\\u003c/sub\\u003e \\u0026ndash; Φ\\u003csub\\u003ePt-Mn\\u003c/sub\\u003e∣ = 4.9 rd between their electronic waves. In addition, the backscattering amplitude of the Pt-Mn wave peaks at \\u003cem\\u003ek\\u003c/em\\u003e\\u0026thinsp;\\u0026asymp;\\u0026thinsp;6 \\u0026Aring;\\u003csup\\u003e\\u0026ndash;1\\u003c/sup\\u003e, whereas the Pt-Pt wave is at a minimum at this \\u003cem\\u003ek\\u003c/em\\u003e value (Fig. S10). The high sensitivity of EXAFS to Pt-Pt and Pt-Mn pairs and the close match of the modulus and the imaginary parts for the Pt-Pt1 peak between fit and experiment confirm the reliability of the fit model (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eB). This provides strong evidence for Pt clustering. Based on the average number of Pt-Pt1 pairs (i.e., 4.6, Table S3), the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e-type layers of Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e have a diameter of approximately 15 \\u0026Aring;, on average.\\u003c/p\\u003e \\u003cp\\u003eThe α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e-type nanoclusters are either incorporated into the MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers or topotactically stacked on the δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers of the FeMn crusts and nodules. Both possibilities were assessed using DFT calculations. Structural incorporation was simulated by replacing seven Mn atoms with Pt atoms within three MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eA). The average Pt-Pt distance is 3.04 \\u0026Aring;, 0.19 \\u0026Aring; longer than the Mn-Mn distance, and 0.13 \\u0026Aring; shorter than the Pt-Pt distance in Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e. Therefore, a substitutional incorporation of Pt into the MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers is unlikely. Conversely, depositing a Pt\\u003csub\\u003e19\\u003c/sub\\u003eO\\u003csub\\u003e38\\u003c/sub\\u003e nanolayer of 12.6 \\u0026Aring; in diameter in between the MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers produced Pt-Pt distances between 3.12 and 3.22 \\u0026Aring;, with a mean of 3.17 \\u0026Aring; (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eB), the same as the EXAFS distance of 3.17 \\u0026Aring;. These findings strongly support the occurrence of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanolayers intergrown between the MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers of the FeMn crusts and nodules (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eB).\\u003c/p\\u003e \\u003cp\\u003eFurthermore, analysis of the second and third O shells confirms the layered structure of the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles. Well-crystallized α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e has a second O shell (O2) at 3.60 \\u0026Aring; and a third O shell (O3) at 3.73 \\u0026Aring; (\\u003cspan citationid=\\\"CR56\\\" class=\\\"CitationRef\\\"\\u003e56\\u003c/span\\u003e). The two O shells are not resolved in the EXAFS analysis of the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference when the fit is performed to \\u003cem\\u003ek\\u003c/em\\u003e\\u003csub\\u003emax\\u003c/sub\\u003e = 12.1 \\u0026Aring;\\u003csup\\u003e\\u0026ndash;1\\u003c/sup\\u003e, as Δ\\u003cem\\u003ed\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;π/2\\u003cem\\u003ek\\u003c/em\\u003e\\u003csub\\u003emax\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;0.13 \\u0026Aring; (\\u003cspan citationid=\\\"CR40\\\" class=\\\"CitationRef\\\"\\u003e40\\u003c/span\\u003e). However, they can be distinguished by increasing \\u003cem\\u003ek\\u003c/em\\u003e\\u003csub\\u003emax\\u003c/sub\\u003e to 16.0 \\u0026Aring;\\u003csup\\u003e\\u0026ndash;1\\u003c/sup\\u003e, yielding Δ\\u003cem\\u003ed\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0.10 \\u0026Aring; (\\u003cspan citationid=\\\"CR40\\\" class=\\\"CitationRef\\\"\\u003e40\\u003c/span\\u003e) (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eE\\u0026ndash;F). The EXAFS parameters of the fit for the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e reference up to \\u003cem\\u003ek\\u003c/em\\u003e\\u003csub\\u003emax\\u003c/sub\\u003e = 16 \\u0026Aring;\\u003csup\\u003e\\u0026ndash;1\\u003c/sup\\u003e are 6 O1 at 2.02 \\u0026Aring;, 6 Pt1 at 3.10 \\u0026Aring;, 6 O2 at 3.64 \\u0026Aring;, and 6 O3 at 3.77 \\u0026Aring; (Table S3), in good agreement with the crystal structure (2.07 \\u0026Aring;, 3.10 \\u0026Aring;, 3.60 \\u0026Aring;, 3.73 \\u0026Aring;) (\\u003cspan citationid=\\\"CR56\\\" class=\\\"CitationRef\\\"\\u003e56\\u003c/span\\u003e). In the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e structure, the O3 shell at 3.73 \\u0026Aring; is located within the PtO\\u003csub\\u003e2\\u003c/sub\\u003e layer, while the O2 shell at 3.60 \\u0026Aring; is in the adjacent upper and lower PtO\\u003csub\\u003e2\\u003c/sub\\u003e layers (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eF). Therefore, the Pt-O distance of 3.66 \\u0026Aring; for Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e is likely an average distance from O2 and O3 shells (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003eB). The O2 shell of Pt\\u003csub\\u003eFeMn\\u003c/sub\\u003e may belong to a MnO\\u003csub\\u003e2\\u003c/sub\\u003e layer or to another PtO\\u003csub\\u003e2\\u003c/sub\\u003e layer. If the α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanolayers are sandwiched in between MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers, then the local mineral structure is akin to that of Ni-Co asbolane (\\u003cspan additionalcitationids=\\\"CR58\\\" citationid=\\\"CR57\\\" class=\\\"CitationRef\\\"\\u003e57\\u003c/span\\u003e\\u0026ndash;\\u003cspan citationid=\\\"CR59\\\" class=\\\"CitationRef\\\"\\u003e59\\u003c/span\\u003e), and can be described as a Pt-asbolane. The PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles were undetected in transmission electron microscopy (TEM, Fig. S11), likely because their amount is vanishingly small ([Pt]\\u0026thinsp;=\\u0026thinsp;4\\u0026ndash;6 \\u0026micro;g/g).\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec12\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eImplications\\u003c/h2\\u003e \\u003cp\\u003eThe α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles within FeMn deposits differ from the Pt species adsorbed on the surface and substituted isomorphically into the crystal structure of synthetic δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e, as documented in previous laboratory studies (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR53\\\" class=\\\"CitationRef\\\"\\u003e53\\u003c/span\\u003e). The difference in Pt uptake between natural settings and laboratory surrogates is due, at least in part, to the large differences in 1) Pt concentration and 2) in reaction time between short-term experimental conditions and long-term natural processes. Firstly, the Pt concentrations used in experimental studies, typically at micromolar levels (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR53\\\" class=\\\"CitationRef\\\"\\u003e53\\u003c/span\\u003e), greatly exceeded the picomolar concentrations of Pt observed in seawater (\\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e). As a result, the Pt/Cl molar ratio in experimental systems was approximately six orders of magnitude higher than in seawater. Platinum mainly exists as Pt\\u003csup\\u003e2+\\u003c/sup\\u003e at the high experimental Pt/Cl ratios (\\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e), while anionic Pt-Cl complexes, such as [PtCl\\u003csub\\u003e4\\u0026minus;\\u003cem\\u003en\\u003c/em\\u003e\\u003c/sub\\u003e(OH)\\u003csub\\u003e\\u003cem\\u003en\\u003c/em\\u003e\\u003c/sub\\u003e]\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e, are thermodynamically favored at low Pt/Cl ratios in seawater (\\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e). Those complexes likely interact differently than Pt\\u003csup\\u003e2+\\u003c/sup\\u003e on the negatively charged δ-MnO\\u003csub\\u003e2\\u003c/sub\\u003e surface. Secondly, the time scales of the two types of Pt uptake differ greatly: laboratory experiments usually last hours to days (\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR53\\\" class=\\\"CitationRef\\\"\\u003e53\\u003c/span\\u003e), whereas the formation of marine FeMn deposits including enrichment of trace metals such as Pt occurs over millions of years (\\u003cspan citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR60\\\" class=\\\"CitationRef\\\"\\u003e60\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR61\\\" class=\\\"CitationRef\\\"\\u003e61\\u003c/span\\u003e). This extended time period allows processes such as phase transformation, Pt atom clustering, and formation of thermodynamically stable α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e precipitates. Our results providing insight into the chemical form of Pt in marine FeMn deposits will be instrumental in more realistic experimental studies and modeling aimed at advancing understanding of the oxidation of Pt(Ⅱ) and the subsequent formation of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles. They underscore the importance of accounting for metal concentration and reaction time when extrapolating laboratory-derived chemical and structural data to natural systems.\\u003c/p\\u003e \\u003cp\\u003eThe occurrence of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles in FeMn oxide deposits from three oceans illuminates a previously unrecognized but widespread pathway for Pt accumulation in the marine environment. The formation of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles begins with the oxidative uptake of Pt(II) onto Mn oxides, followed by nucleation and crystal growth. In this process, Mn oxides serve as adsorbents and electron acceptors. Surface-catalyzed redox reactions lower energy barriers and lead to strong metal partitioning. In seawater, the oxidation of Pt(II) on vernadite results in eight orders of magnitude enrichment. Because α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e is the most thermodynamically stable Pt oxide (\\u003cspan citationid=\\\"CR52\\\" class=\\\"CitationRef\\\"\\u003e52\\u003c/span\\u003e), it likely represents the ultimate sedimentary sink in the oceanic Pt cycle.\\u003c/p\\u003e \\u003cp\\u003eLastly, this study establishes a basis for exploring and recovering Pt from deep-sea FeMn deposits. The Pt-rich FeMn layers exhibit a characteristic spheroidal structure (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e), which was previously observed in slow-growing FeMn crusts (\\u003cspan citationid=\\\"CR60\\\" class=\\\"CitationRef\\\"\\u003e60\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR62\\\" class=\\\"CitationRef\\\"\\u003e62\\u003c/span\\u003e). Pt enrichment has also been reported in FeMn crusts exhibiting a growth hiatus (\\u003cspan citationid=\\\"CR63\\\" class=\\\"CitationRef\\\"\\u003e63\\u003c/span\\u003e). A slower accretion rate of the FeMn layers provides more time for Pt to accumulate from seawater. This relationship has also previously been shown for other trace metals enriched via a similar surface enrichment and oxidation process, such as Co (\\u003cspan citationid=\\\"CR64\\\" class=\\\"CitationRef\\\"\\u003e64\\u003c/span\\u003e) and Te (\\u003cspan citationid=\\\"CR65\\\" class=\\\"CitationRef\\\"\\u003e65\\u003c/span\\u003e). Therefore, when searching for Pt-rich resources, hydrogenetic FeMn layers with slow growth rates should be prioritized. The identification of Pt as α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles structurally bound to the MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers implies that it cannot be effectively desorbed by cation exchange, but that its recovery requires dissolving the Mn oxide matrix.\\u003c/p\\u003e \\u003c/div\\u003e\"},{\"header\":\"Materials and Methods\",\"content\":\"\\u003cdiv id=\\\"Sec14\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eSample information\\u003c/h2\\u003e \\u003cp\\u003eThe FeMn crusts and nodules were collected from the Ninety East Ridge in the Indian Ocean, the Manihiki Plateau in the Western South Pacific Ocean, and the Tropic Seamount in the Eastern North Atlantic Ocean (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e). Detailed sampling information is provided in Table S1. The samples were sectioned across their layer growth direction and then polished to expose a flat cross-sectional surface for \\u003cem\\u003ein situ\\u003c/em\\u003e elemental and spectroscopic measurements.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec15\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eScanning Electron Microscope\\u003c/h2\\u003e \\u003cp\\u003eThe microscale structures of the FeMn crusts and nodules were imaged using a Zeiss Gemini LEO 1530 field emission scanning electron microscope (FE-SEM) coupled with X-MaxN 20 (Oxford instruments) energy dispersive spectroscopy (EDS). Images were collected in SE2 mode at 20 kV and 1 nA.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec16\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eElectron probe micro-analyzer\\u003c/h2\\u003e \\u003cp\\u003eThe major elements were analyzed using an electron probe micro-analyzer (EPMA, JEOL, FEG, JXA-iHP200F) at the ISTerre MicroAnalytical Platform, Universit\\u0026eacute; Grenoble Alpes (UGA), France (\\u003cspan citationid=\\\"CR66\\\" class=\\\"CitationRef\\\"\\u003e66\\u003c/span\\u003e). The EPMA operated with an acceleration voltage of 15 kV, a beam current of 5 nA, and a probe diameter of 1 \\u0026micro;m. Signal processing included a matrix correction using the ZAF method as implemented in the EPMA-PC JEOL software.\\u003c/p\\u003e \\u003cp\\u003eThe following elements and X-ray lines were measured using five spectrometers: 1) WDS1, TAPL: Na-Kα (peak counting time 10 s / background counting time 10 s), Si-Kα (30/30), Sr-Lα (60/60); 2) WDS2, PETH: Ti-Kα (30/30), Ba-Lα (30/30), Cl-Kα (30/30); 3) WDS3, LIFH: Zn-Kα (20/20), Ni-Kα (20/20), Co-Kα (20/20), Fe-Kα (20/40), Mn-Kα (20/20); 4) WDS 4, TAPL: Mg-Kα (50/50), Al-Kα (50/50); 5) WDS5, PETH: K-Kα (10/10), Ca-Kα (20/20), S-Kα (30/30), P-Kα (20/20). Standardization was performed before analysis with the following materials: Na (albite), Si (wollastonite), Sr (strontium sulfate), Ti (titanium dioxide), Ba (barium oxide), Cl (cancrinite), Zn (sphalerite), Ni (nickel oxide), Co (cobalt oxide), Fe (hematite), Mn (rhodonite), Mg (magnesium oxide), Al (corundum), K (orthoclase), Ca (wollastonite), S (sphalerite), P (apatite).\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec17\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eLA-ICP-MS\\u003c/h2\\u003e \\u003cp\\u003eThe LA-ICP-MS measurements were conducted at the ISTerre MicroAnalytical Platform, UGA. The system includes a laser ablation system (RESOlution SE, Applied Spectra, 193 nm excimer) paired with an ICP-MS (Agilent 8900, triple quadrupole, operated in no gas mode). For single-spot ablation, the settings were adjusted to a circular spot size of 50 \\u0026micro;m in diameter, a repetition rate of 5 Hz, an energy density of 3 J/cm\\u0026sup2;, and an ablation time of 30 s. A SQUID signal smoothing device was used. For line-scanning analysis, the samples were ablated with a square beam (20 \\u0026micro;m \\u0026times; 20 \\u0026micro;m) at a repetition rate of 10 Hz, an energy density of 3 J/cm\\u0026sup2;, and a scan speed of 10 \\u0026micro;m/s. The SQUID device was not used, in order to decrease aerosol transport duration between the laser and ICP-MS. Before each analysis, a pre-ablation was performed using a larger beam size to remove any surface contamination. Standard reference materials, NISTSRM-614 (\\u003cspan citationid=\\\"CR67\\\" class=\\\"CitationRef\\\"\\u003e67\\u003c/span\\u003e) and FeMnOx-1 (\\u003cspan citationid=\\\"CR68\\\" class=\\\"CitationRef\\\"\\u003e68\\u003c/span\\u003e), were included in the measuring queue for drift assessment, calibration, and quality control. Data reduction was conducted using the LADR software (Norris Scientific, \\u003cspan class=\\\"ExternalRef\\\"\\u003e\\u003cspan class=\\\"RefSource\\\"\\u003e\\u003ca href=\\\"http://www.norsci.com\\\" target=\\\"_blank\\\"\\u003ewww.norsci.com\\u003c/a\\u003e\\u003c/span\\u003e\\u003cspan address=\\\"http://www.norsci.com\\\" targettype=\\\"URL\\\" class=\\\"RefTarget\\\"\\u003e\\u003c/span\\u003e\\u003c/span\\u003e). Quantification was performed by normalizing the chemical compositions to the total metal oxide amounts (\\u003cspan citationid=\\\"CR69\\\" class=\\\"CitationRef\\\"\\u003e69\\u003c/span\\u003e) and by using the Mn contents as an internal standard. The total metal oxide amounts and the Mn contents were previously determined by EPMA (Data S1).\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec18\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eXRD\\u003c/h2\\u003e \\u003cp\\u003eFeMn growth layers were sampled with a micro-drill, finely ground, and loaded into quartz-glass capillaries. Two subsamples were prepared for each sample. One set was dehydrated in an oven at 105\\u0026deg;C for 8 hours, and immediately sealed afterward to prevent rehydration. Synchrotron-based XRD measurements were performed on the ID22 beamline at the European Synchrotron Radiation Facility (ESRF) under a beam energy of 35 keV (\\u003cspan citationid=\\\"CR70\\\" class=\\\"CitationRef\\\"\\u003e70\\u003c/span\\u003e). Data were collected using a Perkin Elmer XRD 1611CP3 medical imaging detector positioned at a distance of 1400 mm from the sample. The wavelength and the detector parameters were calibrated using a diffraction pattern collected on a LaB6 NIST standard. Azimuthal integration was carried out using the pyFAI library (\\u003cspan citationid=\\\"CR71\\\" class=\\\"CitationRef\\\"\\u003e71\\u003c/span\\u003e).\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec19\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eXAS\\u003c/h2\\u003e \\u003cp\\u003eMn K-edge XANES spectra were acquired at the ESRF on beamline BM23. Samples from the FeMn layers within FeMn crusts and nodules were extracted using a micro-drill, ground into fine powders, and pressed into pellets. Data were collected at room temperature in transmission mode, with the photon energy calibrated to the first inflection point of metallic Mn at 6537.7 eV, as referenced by Kraft et al. (1996) (\\u003cspan citationid=\\\"CR72\\\" class=\\\"CitationRef\\\"\\u003e72\\u003c/span\\u003e). The average Mn oxidation states were determined using the Combo method (\\u003cspan citationid=\\\"CR73\\\" class=\\\"CitationRef\\\"\\u003e73\\u003c/span\\u003e), implemented in the Athena software (\\u003cspan citationid=\\\"CR74\\\" class=\\\"CitationRef\\\"\\u003e74\\u003c/span\\u003e), with an updated spectral dataset (\\u003cspan citationid=\\\"CR75\\\" class=\\\"CitationRef\\\"\\u003e75\\u003c/span\\u003e). The Combo method, as reported in its original development study, has a standard deviation of 0.04 valence units (\\u003cspan citationid=\\\"CR73\\\" class=\\\"CitationRef\\\"\\u003e73\\u003c/span\\u003e).\\u003c/p\\u003e \\u003cp\\u003ePt L\\u003csub\\u003e3\\u003c/sub\\u003e-edge XANES and EXAFS spectra were acquired at the ESRF on beamline ID24-DCM. The incident X-ray flux, sourced from an undulator, was approximately 5 \\u0026times; 10\\u003csup\\u003e12\\u003c/sup\\u003e photons/s. The Pt L\\u003csub\\u003e3\\u003c/sub\\u003e-M\\u003csub\\u003e5\\u003c/sub\\u003e fluorescence line was selected using a five-crystal analyzer with Ge (110) crystals bent to a 0.5 m radius and aligned at a Bragg angle of 79.95\\u0026deg; in a vertical Rowland geometry (\\u003cspan citationid=\\\"CR55\\\" class=\\\"CitationRef\\\"\\u003e55\\u003c/span\\u003e). The diffracted intensity was measured using a silicon drift detector. The sample thin sections, previously analyzed by LA-ICP-MS, were mounted on the sample stage. The Pt-richest FeMn layers of interest were relocated by rastering the samples in an XY pattern with an X-ray beam of 1 \\u0026times; 1 mm\\u003csup\\u003e2\\u003c/sup\\u003e in size. Four XANES spectra were collected over an energy range of 11500 to 11750 eV with a step size of 0.3 eV, each scan lasting 375 seconds. Additionally, approximately 200 EXAFS scans were acquired from 11500 eV to 12136 eV with a step size of 1 eV, each with a scanning time of 318 seconds. No changes in spectral features were observed during data collection that would indicate radiation damage. All scans were averaged to increase the signal-to-noise ratio and subsequently processed using Athena (\\u003cspan citationid=\\\"CR74\\\" class=\\\"CitationRef\\\"\\u003e74\\u003c/span\\u003e). EXAFS fits were performed with WinXAS (\\u003cspan citationid=\\\"CR76\\\" class=\\\"CitationRef\\\"\\u003e76\\u003c/span\\u003e) and theoretical amplitude and phase shift functions generated by FEFF 8.2 (\\u003cspan citationid=\\\"CR77\\\" class=\\\"CitationRef\\\"\\u003e77\\u003c/span\\u003e), using α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e as a structure model (\\u003cspan citationid=\\\"CR56\\\" class=\\\"CitationRef\\\"\\u003e56\\u003c/span\\u003e).\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec20\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eDFT\\u003c/h2\\u003e \\u003cp\\u003eTwo cell sizes were geometrically optimized: 1) a smaller cell consisting of three MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers, each containing 35 Mn atoms and 7 Pt atoms, used to study the incorporation of Pt (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eA), and 2) a larger cell composed of two MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers, each containing 81 Mn atoms, and a third MnO\\u003csub\\u003e2\\u003c/sub\\u003e layer replaced with a Pt\\u003csub\\u003e19\\u003c/sub\\u003eO\\u003csub\\u003e38\\u003c/sub\\u003e nanolayer (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003eB). Periodic DFT computations were performed with the Vienna Ab-initio Simulation Package (VASP) (\\u003cspan citationid=\\\"CR78\\\" class=\\\"CitationRef\\\"\\u003e78\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR79\\\" class=\\\"CitationRef\\\"\\u003e79\\u003c/span\\u003e). The projector augmented wave (PAW) pseudopotentials (\\u003cspan citationid=\\\"CR80\\\" class=\\\"CitationRef\\\"\\u003e80\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR81\\\" class=\\\"CitationRef\\\"\\u003e81\\u003c/span\\u003e) distributed in 2019 were used in combination with an energy cutoff of 400 eV for the plane-wave basis set. The energies and forces were evaluated with the Perdew-Burke-Ernzerhof (PBE) (\\u003cspan citationid=\\\"CR82\\\" class=\\\"CitationRef\\\"\\u003e82\\u003c/span\\u003e) exchange-correlation functional together with the density-dependent dispersion correction (dDsC) (\\u003cspan citationid=\\\"CR83\\\" class=\\\"CitationRef\\\"\\u003e83\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR84\\\" class=\\\"CitationRef\\\"\\u003e84\\u003c/span\\u003e). The gamma-point was found to be sufficient for the Brillouin zone integration. A Fermi-smearing of 0.025 eV (~\\u0026thinsp;300 K) was applied. The wave functions were optimized to an energy change below 10\\u003csup\\u003e\\u0026ndash;6\\u003c/sup\\u003e eV in consecutive cycles. A maximum force below 0.05 eV/\\u0026Aring; on all atoms was used as a criterion for geometry optimization, where the cell parameters, together with the position of all atoms, were relaxed. Optimized geometries can be retrieved from \\u003cspan class=\\\"ExternalRef\\\"\\u003e\\u003cspan class=\\\"RefSource\\\"\\u003edoi.org/10.17172/NOMAD/2025.11.19-1\\u003c/span\\u003e\\u003cspan address=\\\"10.17172/NOMAD/2025.11.19-1\\\" targettype=\\\"DOI\\\" class=\\\"RefTarget\\\"\\u003e\\u003c/span\\u003e\\u003c/span\\u003e.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec21\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eTEM\\u003c/h2\\u003e \\u003cp\\u003eTEM investigations were performed with a Themis Z G3 Cs-probe corrected microscope (Thermo Fisher Scientific) operated at 80kV and equipped with a GATAN 4K OneView camera. The FeMn sample was gently crushed in pure ethanol, dispersed onto a lacey-C-coated copper grid, and treated with a 20 Ar plasma just before analysis to reduce C contamination under the focused electron beam. Elemental analyses were performed in scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) mode using a Super-X emission energy-dispersive X-ray spectrometer (EDS) consisting of four windowless silicon-drift detectors providing a large 0.7 srad collection solid angle. EDS mapping was acquired at 180 pA current, a 50 \\u0026micro;s dwell time per pixel, and a 1.6 \\u0026Aring; pixel size.\\u003c/p\\u003e \\u003c/div\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgments\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eWe thank Thomas Kuhn for providing sample SO193_TVG71-5, James Hein and Kira Mizell for providing sample KNOX06RR_D20-1, Alexander Sobolev and Valentina Batanova for EPMA and LA-ICP-MS measurements. Eric Gautron is thanked for the access and assistance with the Nant\\u0026rsquo;Themis TEM of the IMN\\u0026rsquo;s characterization platform, PLASSMAT. Computational resources were provided by the \\u0026ldquo;Centre Blaise Pascal de simulation et mod\\u0026eacute;lisation num\\u0026eacute;rique (CBPSMN)\\u0026rdquo;, which runs with the SIDUS solution (\\u003cem\\u003e85\\u003c/em\\u003e).\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFunding:\\u0026nbsp;\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eFinancial support was provided by the European Union (ERC, Advanced Grant DEEP-SEE, 101052913). The EPMA and LA-ICP-MS have been funded by the ERC Synergy 856555 MEET project and the Is\\u0026egrave;re Department, France. The views and opinions expressed are solely those of the authors and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. The European Union and the granting authority accept no responsibility for the views expressed herein.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAuthor contributions:\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eConceptualization: A.M., O.M., A.K., and J.L.\\u003c/p\\u003e\\n\\u003cp\\u003eMethodology: O.M., A.M., J.L., Y.L., Julien L., S.S., A.G., C.D., A-C.G., and I. S.\\u003c/p\\u003e\\n\\u003cp\\u003eInvestigation: J.L., A.M., A.K., S.S., and Y.L.\\u003c/p\\u003e\\n\\u003cp\\u003eVisualization: J.L., A.M., Y.L., S.S., and A.G.\\u003c/p\\u003e\\n\\u003cp\\u003eSupervision: A.M., O.M., and A.K.\\u003c/p\\u003e\\n\\u003cp\\u003eWriting\\u0026mdash;original draft: J.L., and Y.L.\\u003c/p\\u003e\\n\\u003cp\\u003eWriting\\u0026mdash;review \\u0026amp; editing: A.M., A.K., A.G., and S.S.\\u003c/p\\u003e\\n\\u003cp\\u003eFunding acquisition: A.M.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCompeting interests:\\u003c/strong\\u003e\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors declare no competing financial interests.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eData and materials availability:\\u003c/strong\\u003e\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe data needed to evaluate the conclusions are included in the article, the Supplementary Materials, and the ESRF data repository. https://doi.esrf.fr/10.15151/ESRF-ES-1883564635; https://doi.esrf.fr/10.15151/ESRF-ES-1758857501.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\u003cli\\u003e\\u003cspan\\u003eCowley A (2024) PGM Market Report 2024. 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Geochim Cosmochim Acta 71:95\\u0026ndash;128\\u003c/span\\u003e\\u003c/li\\u003e\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[{\"identity\":\"a180fefa-9404-40fc-ba7f-9f398f06278e\",\"identifier\":\"10.13039/501100000781\",\"name\":\"European Research Council\",\"awardNumber\":\"101052913\",\"order_by\":0},{\"identity\":\"98244d8d-ce47-489c-89fb-adfcd9ef6085\",\"identifier\":\"10.13039/501100000781\",\"name\":\"European Research Council\",\"awardNumber\":\"856555\",\"order_by\":1}],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":true,\"hideJournal\":true,\"highlight\":\"\",\"institution\":\"European Synchrotron Radiation Facility\",\"isAcceptedByJournal\":false,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true},\"keywords\":\"Platinum, Marine FeMn deposits, XAS, XANES, EXAFS, DFT\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-9606997/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-9606997/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eThe enrichment of platinum (Pt) in marine ferromanganese (FeMn) deposits has attracted persistent interest for over half a century; yet the chemical form of Pt remains unclear. Here, we collected Pt-enriched FeMn crusts and nodules from the world’s oceans and used high-energy-resolution X-ray absorption spectroscopy (XAS) to decipher how Pt is sequestered at the atomic level. Platinum occurs in its tetravalent form, resulting from the oxidation of divalent Pt in seawater upon contact with Mn oxides. Tetravalent platinum is precipitated as α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles with longer Pt-Pt distances than well-crystallized α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e. Density functional theory (DFT) shows that the local structure of Pt is well represented by α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e layers topotactically stacked on vernadite phyllomanganates. Evidence of α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e nanoparticles challenges previous hypotheses that Pt exists as discrete metallic particles or within phyllomanganate MnO\\u003csub\\u003e2\\u003c/sub\\u003e layers replacing Mn. Since α-PtO\\u003csub\\u003e2\\u003c/sub\\u003e is the most thermodynamically stable Pt oxide, this Pt form may represent the ultimate sedimentary sink of Pt in oceans.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Discovery of α-PtO2 nanoparticles in deep-sea ferromanganese deposits: a hidden platinum sink\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2026-05-05 05:21:29\",\"doi\":\"10.21203/rs.3.rs-9606997/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"c2b8e600-acc2-4c88-b252-f90b7bb6f51b\",\"owner\":[],\"postedDate\":\"May 5th, 2026\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"posted\",\"subjectAreas\":[{\"id\":67475166,\"name\":\"Geochemistry\"},{\"id\":67475167,\"name\":\"Planetary Science\"},{\"id\":67475168,\"name\":\"Economic Geology\"}],\"tags\":[],\"updatedAt\":\"2026-05-05T05:21:29+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2026-05-05 05:21:29\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-9606997\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-9606997\",\"identity\":\"rs-9606997\",\"version\":[\"v1\"]},\"buildId\":\"XKTyCvWXoU3ODBz1xrDgd\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}