Sources of mercury varied in the Mariana Trench during the Last Glacial Maximum to the Holocene

Sources of mercury varied in the Mariana Trench during the Last Glacial Maximum to the Holocene | 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 Article Sources of mercury varied in the Mariana Trench during the Last Glacial Maximum to the Holocene Zhengwen Zhou, Huiling Wang, Yu Xin, Yingjun Wang, Xiting Liu, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4518189/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 Mariana Trench, is one of the ultimate sinks of the earth’ system, providing unique insights to matter cycling and environmental evolution. Trench sediments receive mercury (Hg) from the upper ocean and constitute a global Hg sink. However, little is known about the variation in the Hg cycle that have been driven by geological or environmental changes prior to human activity. Here we present results covering concentrations and isotopic signatures of Hg in the deepest trench system to identify the evolution of Hg cycling in trenches before the Anthropocene. Sediment cores collected from the Mariana Trench showed values for mass independent fractionation (Δ 199 Hg) of > 0 with ratios of Δ 199 Hg/Δ 201 Hg close to 1.0, suggesting that Hg in this system was primarily subjected to atmospheric or water column photochemical processes prior to deposition. Geological proxies and isotopic compositions (δ 202 Hg: -4.2‰ to -4.5‰, Δ 199 Hg: 0.28‰ to 0.29‰) comparable only in volcanoes reveal that Hg contents coinciding with the transition from the last glacial termination to the early Holocene can predominantly be attributed to volcanic activity. During the Holocene, atmospheric Hg constituted the main source of Hg in the Mariana Trench, while the last glacial maximum was characterized by an accumulation of both atmospheric and biogenic Hg. Earth and environmental sciences/Biogeochemistry/Element cycles Earth and environmental sciences/Solid Earth sciences/Geochemistry Earth and environmental sciences/Environmental sciences/Environmental chemistry/Geochemistry Earth and environmental sciences/Ocean sciences/Marine chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Mercury (Hg) is a significant global environmental pollutant that can undergo long-range transport in the atmosphere and even reach oceanic trenches through carrion or particles 1, 2 . Trenches in the ocean are integral components of the earth system and play a crucial role in matter transport and energy exchange between the shallow and deep layers 3 . Trench sediments encompass abundant sedimentary information concerning environmental geological history, including events such as volcanic eruptions, biotic surges, and seismic activity, and are often referred to as the “ultimate sink” of surface earth sediment 4, 5 . Based on Hg concentration and sedimentation rates in trench sediment, recent studies found that although hadal trenches occupy a small portion of the ocean, they are significant hotspots for oceanic Hg burial 6, 7 . However, little is known about how the temporal variability of hadal Hg cycling, particularly those modifications driven by extensive geological or environmental transformations before the Anthropocene. Mercury sources in marine environments that predate anthropogenic industrial activities primarily originate from natural processes, such as river transport, hydrothermal venting, forest fires, or volcanic eruptions 8 . Global and regional climatic factors such as productivity, sea ice cover, river discharge, sea-level changes, and precipitation patterns, also can control its dynamic in different environments 9 . A similar change in paleo-production rates and Hg sources was recorded in sediment cores of the tropical Western Pacific 10 , characterized by glacial-interglacial variation. Volcanic eruptions and other geological events are considered the main reason responsible for significant and anomalous increases in Hg concentration in sediment before the industrial revolution 11–13 . Marine dissolved Hg from different input sources is adsorbed by particulate organic matter within the seawater column and subsequently transported to deeper sediment systems through ocean currents and organic matter transportation. Mercury is strongly bound to organic matter and typically exhibits a positive correlation with total organic carbon (TOC), limiting post-depositional Hg mobility and isotopic fractionation, which facilitates the use of isotopic tools for investigating the Hg sources in sediments 14, 15 . Mercury isotopic fractionation can be categorized into mass-independent fractionation (MIF, indicated by Δ 199 Hg and Δ 201 Hg) and mass-dependent fractionation (MDF, indicated by δ 202 Hg). Hg-MDF is widely observed during Hg biotic, chemical and physical transport and transformation processes 16 . Hg-MIF is mainly induced by the magnetic isotope effect in photochemical processes 17 . Hg isotope variations have been used to trace Hg environmental processes and sources in the earth systems, given the large differences in the Hg isotopic fingerprint characteristics from different sources 18 . For example, Hg collected in the open ocean atmosphere presents more positive δ 202 Hg than biogenic foliage and litter Hg, and terrestrial soil Hg shows a more negative Δ 199 Hg than atmospheric and volcanic input Hg 19–22 . Furthermore, recent studies have established a correlation between environmental temperature, sea ice, and Hg isotopic fractionation, highlighting Hg-MIF as a promising indicator of sea ice changes in polar environments 23–26 . Thus, Hg isotopes provide unique insights into the Hg sources and the effects of environmental changes on isotope fractionation before sediment deposition. Marine sediments are critical archive in the study of material cycles, playing an essential role in unraveling the complex Hg cycling processes in hadal trench environments 27, 28 . Here, we collected a sediment core from the Mariana Trench (MT) in the North Pacific Ocean (NPO, 11.07°N, 142.19°E) in 2019 (Fig. 1 ). These sediment samples were analyzed for total mercury (THg) concentrations, and Hg stable isotopic compositions to evaluate the Hg cycle evolution in the deepest trenches over time. Mercury isotopes were utilized to determine the main Hg sources, while the Hg/TOC ratio served as a proxy for geological activity. We find that the trench Hg sources have been dominated by atmospheric input since the Last Glacial, with the exception of a significant increase in volcanic Hg input during the Last Glacial Termination into the early Holocene. Results and Discussion Mercury concentration and isotopes composition during last glacial maximum to the Holocene Deep-sea sediments receive particulate Hg deposited from the upper ocean after transformation by physical, chemical, and biological processes. As shown in Fig. 2 , a sharp increase in Hg content was observed in the 215–240 cm layer, reaching a peak of 299 ng/g, which is six-fold higher than the average concentration of 49 ± 20 ng/g (1SD, n = 19) in the profile. In view of Hg/TOC ratio also significantly increased around the same time; therefore, the spike is likely caused by an anomalous Hg influx during sedimentation. At the depth of anomalous high Hg concentration, δ 202 Hg values presented a large negative shift (decrease to -4.48‰), falling well below the average MDF (-1.79 ± 1.05‰, 1SD, n = 23) of the profile. This variation in isotopic compositions was likely caused by process or source differences during Hg deposition. The Δ 199 Hg value exhibits initially a descending fluctuation trend from the surface down to a depth of 215 cm (from 0.24‰ to 0.10‰, mean 0.22 ± 0.08‰, 1SD, n = 15), followed by more variable values between 215–325 cm (0.29‰ − 0.21‰, mean 0.29 ± 0.08‰, 1SD, n = 8). It is worth noting that the MIF trend is in line with the turning point of the Hg concentrations at the same depth of 210 cm. We hypothesized that the variation in Δ 199 Hg values is influenced by photochemical dynamics in the upper atmosphere and water column during its transport and migration 18 . Despite limitations in establishing precise chronological frameworks, an approximate timeline for the cores can be inferred from the MT sedimentation rates and geological core data. It has been estimated that sedimentation rates in the MT were between 0.02 and 0.04 cm yr − 1 based on Pb-210 data from sediment cores retrieved from the southern slope and the axis of the MT 31 . This suggests that the sediment core MT03 may contain sedimentary age information spanning the past 5 to 20 thousand years 32 . Laminated diatom mats (LDMs) are present at the base of the MT03 sediment core (Fig. 1 C), which could be a result of silicon diatom accumulation during the last glacial maximum (LGM) 33 . This extensive diatom blooming was likely caused by an increase in nutrients and minerals source from aeolian input during the LGM 34 . Thus, our date suggest that the deepest trenches showed anomalous increases Hg input during the LGM due to possible environmental or geologic evolution, which also affected the Hg isotopic composition on this profile. For comparison, the geo-environmental Hg levels reported for Arctic ice cores and shelf peat 35, 36 , likely suggests there was global sediment Hg anomaly from the Last Glacial Termination into the early Holocene. The possible reasons for the anomalous increase in Hg level and isotopic variations are discussed further below. Mercury source variability revealed from Hg isotopic composition The ratios of Δ 199 Hg/Δ 201 Hg in the Mariana Trench sediment, as shown in Fig. 3 , was approximately 0.95 ± 0.12 (R 2 = 0.76, p < 0.001), closely aligning with the Δ 199 Hg/Δ 201 Hg slope (1.00 ± 0.01) typically seen in studies on photochemical reduction of Hg 2+ to Hg 0 37 . In addition, MIF varies very little with depth, implying that the Hg found at different depths was similarly affected by photochemistry. The consistent MIF > 0 across the sedimentary profile suggests that buried Hg was constantly influenced by atmospheric processes 38 . However, an atmospheric Hg source alone seems insufficient to explain the large variations in MDF across the sedimentary profile, because existing data on atmospheric Hg (particulate Hg and gaseous elemental Hg) do not exhibit such a substantial negative MDF. Therefore, we hypothesize that the sediment core MT03 may contain Hg from additional sources (sediment rock/aquatic plant) 39 , where Hg undergoes photochemical processes similar to Hg in the upper atmosphere or water column before being deposited into the trench sediments. The average δ 202 Hg isotopic composition in 240–325 cm profile was − 2.25 ± 0.62 (n = 6, 1 SD), significantly more negative (t-test, p < 0.01) than that in the 0-215 cm profile (-1.26 ± 0.52, 1 SD, n = 15). Although enhanced particle adsorption or microbial methylation of Hg 2+ may result in a negative MDF shift in 240–325 cm profile, they would be insufficient to explain the large negative δ 202 Hg observed for the period 18 . Specifically, dissolved Hg 2+ is adsorbed by Fe-Mn oxides or thiol ligand particles, with fractionation (enrichment) ranging from 0.30‰ to 0.62‰ 40 . Additionally, microbial-controlled methylation of Hg 2+ under anoxic conditions can lead to significant isotopic fractionation in produced methylmercury (~ -2‰) 41 . However, these processes typically represent the maximum fractionation between methylmercury and total dissolved Hg 2+ observed in natural environment matrices (e.g., sediment/porewater). The extent of fractionation is also influenced by the Hg bioavailability and the methylation rate 42 . Methylation rates are usually less than 2% 43 , suggesting that microbial methylation would not generate sufficient methylmercury to significantly alter the isotope ratio of the total Hg and therefore is not the primary contributor to the significant negative MDF observed in the profile. The isotopic composition of residual Hg, which is enriched in Hg 2+ by algae, is heavier than the atmospheric mercury isotopic composition. The positive MDF of the isotopic composition of residual Hg outside of algae is also inconsistent with the isotopic trend of the sediment profile. Consequently, it is reasonable to argue that the main differences in Hg concentration and isotope ratios between the sedimentary profile are caused by a different Hg source. Previous reports on short sediment cores (MT1) from the MT indicate an OC/TN ratio of 7.0 ± 0.93 and δ 13 C of -19.9 ± 0.40‰ (1 SD, n = 13) 29 , suggesting that the MT is primarily influenced by atmospheric Hg input from marine sources (average THg concentration of 45 ± 6.7 ng/g). The MT03 core at the depth of 0-215 cm is consistent with the Hg concentrations and stable isotopic compositions (Δ 199 Hg and δ 202 Hg) of MT sediments influenced by atmospheric Hg inputs (t-tests, p > > 0.05) 2 . This finding suggests that the 0-215 cm sediment profile of the deposited Hg source is associated with atmospheric Hg. Mercury from atmospheric dry and wet deposition enters the upper ocean and subsequently undergoes gravitational settling into the trench sediment 16 . The main Hg sources in marine sediments, including background rock weathering, volcanoes, hydrothermal fluids, biological tissue decomposition, and the transport of particle deposition from the atmosphere, could be potential explanations for the abnormal increase in Hg concentrations between 220–240 cm depth 8 . This coincides with a significant negative shift in δ 202 Hg, reaching values as low as -4.48‰. This value is considerably lower than the isotopic MDF composition of potential source for Hg inputs, such as gaseous elemental Hg/ particulate bound Hg and Hg transported by hydrothermal vents or river (t-test, p < 0.01). Comparable values are only recorded in sedimentary igneous rocks 44 (Fig. 4 ). Furthermore, the reported TOC content is greater than 0.2% in MT03 sediment core 30 , and the proxy of Hg/TOC for volcanic emissions in the geologic record exhibits a significant increase within the MT03 sediment core in 220–240 cm profile (Fig. 2 B) 45, 46 , corroborating the input of volcanic Hg during the period of LGM 47 . Non-zero MIF values at this stage are attributed to particles released by volcanoes adsorbing gaseous oxide Hg from the atmosphere to form particle Hg 2+ (positive MIF) 48 or emitted gaseous elemental Hg undergoing photochemical oxidation-reduction transformation during atmospheric transport 49 . During the Late Glacial and Holocene, climatic and volcanic signals were evident in the Hg record in ombrotrophic peat bog 36 , suggesting that the accumulation of Hg during this period may have been caused by a global volcanic event. Role of diatom in Hg cycling in the LGM period The suggested sources for Hg found at depths ranging from 0-240 cm (Holocene to the LGM) cannot fully explain the differences in δ 202 Hg observed at deeper depths of 240–325 cm. The pronounced global environmental shifts during the LGM, which is manifested at depths of 215–325 cm and associated with an enhanced flux of Asian eolian dust, contributed to a marked increase in marine primary productivity in the western Pacific Ocean 34, 55 . Recent research highlights the role of algae and algal-derived organic matter in aqueous-phase mercury removal, underscoring the significance of diatom laminations in Hg sequestration within marine environments 56, 57 . The continuous existence of LDMs in the 215–325 cm sediment layer indicates that diatoms may be involved in the Hg cycle. Despite the absence of stable isotopic data for marine diatom laminae, available evidence from marine macroalgae (δ 202 Hg: -3.22 ± 0.81‰, Δ 199 Hg: 0.16 ± 0.08‰) indicates a distinct isotopic signature 58 . It is reported that isotopic compositions of vegetation Hg suggest moderately negative MDF values (δ 202 Hg: -2.56 ± 0.64‰) 19 . This finding aligns with our observation of similar δ 202 Hg values in selected sediment layers (δ 202 Hg values of -2.53‰, -3.15‰, and − 2.64‰ at 250, 295, and 310 cm depth, respectively). Algae may continuously enrich lighter Hg isotopes and deplete odd mass isotopes during the Hg 0 re-emission process, resulting in the retention of lighter isotopes (negative MDF) and positive MIF within their structure 59, 60 . This phenomenon is consistent with the observation that isotopic compositions in the 215–325 cm layer (-2.25 ± 0.57‰, 1 SD, n = 6) are notably lighter than those atmospheric sources contributing to Hg deposited in the 0-215 cm range (-1.26 ± 0.52‰, 1 SD, n = 14). In addition, Vandal et al. 61 inferred that the oceanic productivity may have been higher during the period of LGM (18,000 years ago) based on changes in Hg concentrations in Antarctica ice cores. Our findings advance the understanding of the hadal Hg cycle in the process of geological historical evolution (Fig. 5 ), suggesting that during the LGM period, diatoms functioned as the conveyor for the sequestration of Hg, facilitating the Hg transportation from the atmosphere to hadal trenches. In this study, we reconstructed the geochemical cycle of Hg in the trench environment during the LGM period. Our findings reveal variations in the geochemical cycling of trench Hg, shedding light on the source and environmental drivers of Hg cycling prior to human influence. Geological activities (e.g. volcanism and shelf weathering) transported the sources of Hg input to the trench. The primary source shifted from atmospheric and algal deposits during the LGM to predominantly atmospheric deposition in the Holocene. Furthermore, volcanic activities during the transition may have been a principal cause of sudden increases in trench Hg deposition, potentially indicative of a global phenomenon. Methods Sediment sampling and sample preparation The MT originates from the subduction of the Pacific plate beneath the eastern boundary of the Philippine Sea plate. It stretches approximately 2500 km in length and maintains an average width of around 70 km 62 . The studied core (MT03) was collected from trench sediment at a depth of 8300 meters on the southern slope of the “Challenger Deep” in the MT in 2019. The length of the MT03 core is 3.2 m, and the collection site coordinates are 142° 19.5797' E, 11° 07.6804' N (Fig. 1 A). The complete core displays distinct color stratification. In the lower portion of the core (215–325 cm), the sediment exhibits a shade of gray, grayish-green, or green and contain notable quantities of diatoms (detailed description for MT03 core see Lai et.al. 30 ) (Fig. 1 B). In the laboratory, sediment cores were subdivided into segments and sampled at approximately 15 cm intervals, resulting in a total of 24 samples. Subsequently, these samples were subjected to freeze-drying and homogenized in preparation for THg concentration and isotopic ratio analysis. Hg concentration analyses Concentrations of THg in sediment samples were measured using a method modified following EPA method 7474 63 . Approximately 0.5 g of homogenized sediment were weighed into a 10 mL ampule vial. After adding 2 mL of HNO 3 and 1 mL of deionized water, the ampule was sealed and the sample digested at 105°C in an autoclave for 1 h. Next, 0.1 mL of the digested sample was added to 25 mL of 1% (v/v) HCl and THg in the samples were determined by a MERX automated THg analytical system (Brooks Rand Laboratories, USA) after the addition of SnCl 2 (20% (w/v)). Standard quality assurance and control procedures were followed during the analysis of Hg. For each batch analysis (20 samples), two method blanks, two certified reference materials (ERM-CC580 for THg in sediment), and triplicates of one randomly chosen sample were included. ERM-CC580 estuarine sediment reference material was measured with each batch (20 samples) and the THg recoveries were in the range of 91–95%, which is within the acceptable range of the EPA method (70–130%). Three parallel samples randomly selected gave RSD’s from 1–10%, which was also within the acceptable range (< 15%). Method blanks for THg in sediment were between 0.2 to 0.3 ng g − 1 , which is within the acceptable range of the EPA method. Hg isotope ratio analysis All sediment samples were measured for Hg isotope composition by a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS; Thermo Scientific Neptune Plus, USA). According to the THg concentrations measured above, the digested sample solutions were diluted to 1.0 ng/mL, with acid concentrations of < 20% (v/v). The procedure consisted of Hg reduction employing an in-line cold-vapor system (HGX-200, CETAC Technologies, USA) using a reducing agent of SnCl 2 (5% SnCl 2 in 10% HCl), followed by stripping of Hg 0 with Ar and mixing in cold-vapor system with thallium (Tl) aerosols produced with a desolvation nebulizer (CETAC Aridus 3, USA) and then introduced into the MC-ICPMS. The mass bias of MC-ICP-MS was corrected with the Tl standard solution applying the exponential law. A matrix-matched NIST-3133 Hg isotope standard solution was used for standard-sample bracketing 64 . Hg isotope ratios are reported in δ xxx Hg (‰) notation, referring to the bracketed NIST-3133: δ xxx Hg(‰) = ([( xxx Hg/ 198 Hg) sample /( xxx Hg/ 198 Hg) SRM 3133 ]- 1) × 1000 ( 1 ) Where the superscript xxx represents Hg isotope mass between 199 and 204. Here, δ 202 Hg is reported as Hg-MDF. MIF is expressed by Δ xxx Hg (‰) notation, representing the difference between the measured δ xxx Hg value and that theoretically predicted δ 202 Hg using the MDF law: Δ xxx Hg(‰) = δ xxx Hg − (β xxx × δ 202 Hg(‰)) ( 2 ) Where β xxx is the obtained mass dependent scaling factor according to the mass fractionation law, and its values are 0.2520 and 0.7520 for 199 and 201, respectively. Δ 199 Hg and Δ 201 Hg are referred to as odd MIF. To check system performance and reproducibility, a secondary standard solution of NIST-3177, and estuarine sediment Reference Material (ERM-CC580) were measured regularly. The NIST-3177 had mean values of δ 202 Hg = -0.53 ± 0.06‰, Δ 199 Hg = -0.02 ± 0.04‰ (2SD, n = 3), ERM-CC580 (n = 3) had mean values of δ 202 Hg = -0.49 ± 0.10‰, Δ 199 Hg = -0.05 ± 0.006‰ (2SD, n = 3), consistent with values reported by others 65–67 . Statistical Analysis Station maps were drawn using Ocean Data View (AWI, Germany). The vertical variations in THg concentrations and Hg stable isotopic values were drawn using Origin (OriginLab Co. Ltd., USA). The Hg stable isotope composition of different natural samples was presented using Sigmaplot (Systat Software Inc., USA). The t-test and Pearson correlation analysis between Δ 199 Hg and Δ 201 Hg were performed using SPSS 19.0 (SPSS Inc., USA). Declarations Data availability Data used in this study can be found in the Supplementary Index. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (42373076) and Fundamental Research Funds for the Central Universities (202072001, 202372001). Contributions Z.Z. was responsible for the analysis of mercury concentration and isotopes, drafting the manuscript, and participating in its revision. H.W. analyzed the mercury stable isotopes. Y.X. and J.T. provided environmental samples, contributing essential data for the research. X.L. assisted in manuscript revision, providing TOC data. H.H., Y.W., Y.G.Y., G.L., and Y.C. reviewed and revised the manuscript, offering significant help in isotope analysis. Y.L. designed the study and led the writing and revision of the manuscript. All authors discussed and interpreted the results and contributed to the manuscript. All authors reviewed, edited, and contributed to the final version. Competing interests The authors declare no competing interests. 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Anal Bioanal Chem 388:353–359 Janssen SE, Johnson MW, Blum JD, Barkay T, Reinfelder JR (2015) Separation of monomethylmercury from estuarine sediments for mercury isotope analysis. Chem Geol 411:19–25 Reinfelder JR, Janssen SE (2019) Tracking legacy mercury in the Hackensack River estuary using mercury stable isotopes. J Hazard Mater 375:121–129 Zhou Z et al (2024) Spatial and temporal variations in the pollution status and sources of mercury in the Jiaozhou bay. Environ Pollut 346:123554 Additional Declarations There is NO Competing Interest. 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-4518189","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":310110645,"identity":"b308433f-debe-4fe4-867b-af284d9f3f56","order_by":0,"name":"Zhengwen Zhou","email":"","orcid":"","institution":"Frontiers Science Center for Deep Ocean Multispheres and Earth System, and Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, China; Chemistry Department, Trent University, Peterborough, Ontario K9J 7B8, Canada","correspondingAuthor":false,"prefix":"","firstName":"Zhengwen","middleName":"","lastName":"Zhou","suffix":""},{"id":310110646,"identity":"c8f01e80-4086-4ece-ac10-3b7750a8af27","order_by":1,"name":"Huiling Wang","email":"","orcid":"","institution":"Frontiers Science Center for Deep Ocean Multispheres and Earth System, and Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, China","correspondingAuthor":false,"prefix":"","firstName":"Huiling","middleName":"","lastName":"Wang","suffix":""},{"id":310110647,"identity":"dae80231-670c-4ea7-9bad-b2b960008d97","order_by":2,"name":"Yu Xin","email":"","orcid":"","institution":"Frontiers Science Center for Deep Ocean Multispheres and Earth System, and Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, China","correspondingAuthor":false,"prefix":"","firstName":"Yu","middleName":"","lastName":"Xin","suffix":""},{"id":310110648,"identity":"b73879ab-2f05-4dcd-9b69-0fdf2b5e82e4","order_by":3,"name":"Yingjun Wang","email":"","orcid":"","institution":"School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China","correspondingAuthor":false,"prefix":"","firstName":"Yingjun","middleName":"","lastName":"Wang","suffix":""},{"id":310110649,"identity":"728726a6-20b3-446e-b6d6-470b222c36f6","order_by":4,"name":"Xiting Liu","email":"","orcid":"https://orcid.org/0000-0002-0188-5504","institution":"Key Laboratory of Submarine Geosciences and Prospecting Technology, Ministry of Education, College of Marine Geosciences, Ocean University of China, Qingdao 266100, China","correspondingAuthor":false,"prefix":"","firstName":"Xiting","middleName":"","lastName":"Liu","suffix":""},{"id":310110650,"identity":"0f39de73-f272-40ef-882f-9086d1fb93d4","order_by":5,"name":"Jiwei Tian","email":"","orcid":"https://orcid.org/0000-0002-3634-8688","institution":"Frontier Science Center for Deep Ocean Multispheres and Earth System (FDOMES) and Physical Oceanography Laboratory/Key Laboratory of Ocean Observation and Information of Hainan Province, Sanya Oceanographic Institution, Ocean University of China, Qingdao/Sanya, China; Laoshan Laboratory, Qingdao, China","correspondingAuthor":false,"prefix":"","firstName":"Jiwei","middleName":"","lastName":"Tian","suffix":""},{"id":310110651,"identity":"35555474-5dbd-4dd7-9ff0-bcd28b41f6be","order_by":6,"name":"Holger Hintelmann","email":"","orcid":"https://orcid.org/0000-0002-5287-483X","institution":"Chemistry Department, Trent University, Peterborough, Ontario K9J 7B8, Canada","correspondingAuthor":false,"prefix":"","firstName":"Holger","middleName":"","lastName":"Hintelmann","suffix":""},{"id":310110652,"identity":"9282571d-e40d-43fa-a827-a34981e7a74a","order_by":7,"name":"Yongguang Yin","email":"","orcid":"https://orcid.org/0000-0002-7287-8598","institution":"Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China","correspondingAuthor":false,"prefix":"","firstName":"Yongguang","middleName":"","lastName":"Yin","suffix":""},{"id":310110653,"identity":"1f342dcf-d5e3-4b96-8d91-fb92d3469909","order_by":8,"name":"Guangliang Liu","email":"","orcid":"","institution":"Department of Chemistry \u0026 Biochemistry, Florida International University, Miami, FL 33199, United States","correspondingAuthor":false,"prefix":"","firstName":"Guangliang","middleName":"","lastName":"Liu","suffix":""},{"id":310110654,"identity":"63cc71dd-a454-49b3-917e-88f1f29128c8","order_by":9,"name":"Yong Cai","email":"","orcid":"","institution":"Department of Chemistry \u0026 Biochemistry, Florida International University, Miami, FL 33199, United States","correspondingAuthor":false,"prefix":"","firstName":"Yong","middleName":"","lastName":"Cai","suffix":""},{"id":310110644,"identity":"2039c345-6fd7-423c-a137-521265c5ba9b","order_by":10,"name":"Yanbin Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAz0lEQVRIiWNgGAWjYBACPmYgwdggwcAP4TMT1sIG0yLZQLQWBrAWBgaDA0RrYecx/Fy4wyLP+HiP4QeGCuvEBvazBwg4jMdYeuYZiWKzM2eMJRjOpCc28OQlENJiIM3bJpG47UaOGQNj2+HEBgkeA4K2/AZp2TwDpOUfcVrMwLZskABpaSBKC1uZNe8ZicQZZ44VSyQcSzdu48nBr4Wf//Dm27w76hL725s3fvhQYy3bz34GvxYGBg4kBQkM0JjCD9gfEFYzCkbBKBgFIxsAAC+LOipUCXW3AAAAAElFTkSuQmCC","orcid":"","institution":"Frontiers Science Center for Deep Ocean Multispheres and Earth System, and Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, China","correspondingAuthor":true,"prefix":"","firstName":"Yanbin","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2024-06-02 19:45:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4518189/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4518189/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57759149,"identity":"841c22b2-bafb-41d9-ab3e-484becbfa2f2","added_by":"auto","created_at":"2024-06-05 09:03:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":514596,"visible":true,"origin":"","legend":"\u003cp\u003eStudy site and depth profile of MT03 core in the MT. (A) The location of the MT03 core in MT. (B) Sampling station of MT03 was indicated by the red star (the surface sediment \u003csup\u003e2\u003c/sup\u003e and MT1 core \u003csup\u003e29\u003c/sup\u003e with a depth of 14 cm was indicated by blue dot and yellow dot, respectively). (B) the depth profile of MT03 sediment core (total 325 cm) \u003csup\u003e30\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-4518189/v1/11e4b2d9766cb5aed29b523d.png"},{"id":57759144,"identity":"56b312ab-c1eb-4bf3-b9f2-901bf5c40f57","added_by":"auto","created_at":"2024-06-05 09:03:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":54546,"visible":true,"origin":"","legend":"\u003cp\u003eVariations of THg content (A), Hg/TOC (B), δ\u003csup\u003e202\u003c/sup\u003eHg (C), Δ\u003csup\u003e199\u003c/sup\u003eHg (D), TOC (E) in the MT03 sediment core (The TOC was cited from Lai et al.\u003csup\u003e30\u003c/sup\u003e). The 0-215 cm and 215-325 cm profiles were marked by different background color.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4518189/v1/dee61058689d939887fb0d7e.png"},{"id":57759145,"identity":"59d7c0f0-2316-4980-94bc-7e0f964c3276","added_by":"auto","created_at":"2024-06-05 09:03:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":79051,"visible":true,"origin":"","legend":"\u003cp\u003eThe relationship of Δ\u003csup\u003e199\u003c/sup\u003eHg/Δ\u003csup\u003e201\u003c/sup\u003eHg in MT03 trench sediments. The colors indicate the depth of each subsample.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4518189/v1/9b06df818ac970fda6e88667.png"},{"id":57759706,"identity":"500cad07-fec4-4026-974e-a507dfbe40c6","added_by":"auto","created_at":"2024-06-05 09:11:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":38257,"visible":true,"origin":"","legend":"\u003cp\u003eHg isotope ratio signatures of potential sources (continental shelf soil \u003csup\u003e50, 51\u003c/sup\u003e, hydrothermal vent \u003csup\u003e52, 53\u003c/sup\u003e, open ocean atmosphere \u003csup\u003e21, 54\u003c/sup\u003e, sedimentary rock/volcanic emissions \u003csup\u003e44, 45\u003c/sup\u003e in trenches and previously reported isotopic signatures of surface sediment in the Mariana \u003csup\u003e2\u003c/sup\u003e and New Britain Trench\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4518189/v1/6034dd4f052b114816f83d59.png"},{"id":57759146,"identity":"2f8c2570-5abe-40f9-b75d-92de8e862708","added_by":"auto","created_at":"2024-06-05 09:03:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":97547,"visible":true,"origin":"","legend":"\u003cp\u003eThe Hg input patterns in the MT during the LGM to the Holocene. A, B, and C depict three different Hg sources and potential modes, which include atmospheric Hg (A), volcanic Hg (B), and biogenic Hg (C). Main Hg source variations during different periods from the LGM to the Holocene (D).\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-4518189/v1/1563ae3feafe1f586c778144.png"},{"id":60087428,"identity":"525c76bd-5880-4b00-b98a-5cf679521d48","added_by":"auto","created_at":"2024-07-11 15:14:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1251316,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4518189/v1/857992dd-f67f-4557-9a6d-1b5bf7513da9.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Sources of mercury varied in the Mariana Trench during the Last Glacial Maximum to the Holocene","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMercury (Hg) is a significant global environmental pollutant that can undergo long-range transport in the atmosphere and even reach oceanic trenches through carrion or particles \u003csup\u003e1, 2\u003c/sup\u003e. Trenches in the ocean are integral components of the earth system and play a crucial role in matter transport and energy exchange between the shallow and deep layers \u003csup\u003e3\u003c/sup\u003e. Trench sediments encompass abundant sedimentary information concerning environmental geological history, including events such as volcanic eruptions, biotic surges, and seismic activity, and are often referred to as the \u0026ldquo;ultimate sink\u0026rdquo; of surface earth sediment \u003csup\u003e4, 5\u003c/sup\u003e. Based on Hg concentration and sedimentation rates in trench sediment, recent studies found that although hadal trenches occupy a small portion of the ocean, they are significant hotspots for oceanic Hg burial \u003csup\u003e6, 7\u003c/sup\u003e. However, little is known about how the temporal variability of hadal Hg cycling, particularly those modifications driven by extensive geological or environmental transformations before the Anthropocene.\u003c/p\u003e \u003cp\u003eMercury sources in marine environments that predate anthropogenic industrial activities primarily originate from natural processes, such as river transport, hydrothermal venting, forest fires, or volcanic eruptions \u003csup\u003e8\u003c/sup\u003e. Global and regional climatic factors such as productivity, sea ice cover, river discharge, sea-level changes, and precipitation patterns, also can control its dynamic in different environments \u003csup\u003e9\u003c/sup\u003e. A similar change in paleo-production rates and Hg sources was recorded in sediment cores of the tropical Western Pacific \u003csup\u003e10\u003c/sup\u003e, characterized by glacial-interglacial variation. Volcanic eruptions and other geological events are considered the main reason responsible for significant and anomalous increases in Hg concentration in sediment before the industrial revolution \u003csup\u003e11\u0026ndash;13\u003c/sup\u003e. Marine dissolved Hg from different input sources is adsorbed by particulate organic matter within the seawater column and subsequently transported to deeper sediment systems through ocean currents and organic matter transportation. Mercury is strongly bound to organic matter and typically exhibits a positive correlation with total organic carbon (TOC), limiting post-depositional Hg mobility and isotopic fractionation, which facilitates the use of isotopic tools for investigating the Hg sources in sediments \u003csup\u003e14, 15\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eMercury isotopic fractionation can be categorized into mass-independent fractionation (MIF, indicated by Δ\u003csup\u003e199\u003c/sup\u003eHg and Δ\u003csup\u003e201\u003c/sup\u003eHg) and mass-dependent fractionation (MDF, indicated by δ\u003csup\u003e202\u003c/sup\u003eHg). Hg-MDF is widely observed during Hg biotic, chemical and physical transport and transformation processes \u003csup\u003e16\u003c/sup\u003e. Hg-MIF is mainly induced by the magnetic isotope effect in photochemical processes \u003csup\u003e17\u003c/sup\u003e. Hg isotope variations have been used to trace Hg environmental processes and sources in the earth systems, given the large differences in the Hg isotopic fingerprint characteristics from different sources \u003csup\u003e18\u003c/sup\u003e. For example, Hg collected in the open ocean atmosphere presents more positive δ\u003csup\u003e202\u003c/sup\u003eHg than biogenic foliage and litter Hg, and terrestrial soil Hg shows a more negative Δ\u003csup\u003e199\u003c/sup\u003eHg than atmospheric and volcanic input Hg \u003csup\u003e19\u0026ndash;22\u003c/sup\u003e. Furthermore, recent studies have established a correlation between environmental temperature, sea ice, and Hg isotopic fractionation, highlighting Hg-MIF as a promising indicator of sea ice changes in polar environments \u003csup\u003e23\u0026ndash;26\u003c/sup\u003e. Thus, Hg isotopes provide unique insights into the Hg sources and the effects of environmental changes on isotope fractionation before sediment deposition.\u003c/p\u003e \u003cp\u003eMarine sediments are critical archive in the study of material cycles, playing an essential role in unraveling the complex Hg cycling processes in hadal trench environments \u003csup\u003e27, 28\u003c/sup\u003e. Here, we collected a sediment core from the Mariana Trench (MT) in the North Pacific Ocean (NPO, 11.07\u0026deg;N, 142.19\u0026deg;E) in 2019 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These sediment samples were analyzed for total mercury (THg) concentrations, and Hg stable isotopic compositions to evaluate the Hg cycle evolution in the deepest trenches over time. Mercury isotopes were utilized to determine the main Hg sources, while the Hg/TOC ratio served as a proxy for geological activity. We find that the trench Hg sources have been dominated by atmospheric input since the Last Glacial, with the exception of a significant increase in volcanic Hg input during the Last Glacial Termination into the early Holocene.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMercury concentration and isotopes composition during last glacial maximum to the Holocene\u003c/h2\u003e \u003cp\u003eDeep-sea sediments receive particulate Hg deposited from the upper ocean after transformation by physical, chemical, and biological processes. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, a sharp increase in Hg content was observed in the 215\u0026ndash;240 cm layer, reaching a peak of 299 ng/g, which is six-fold higher than the average concentration of 49\u0026thinsp;\u0026plusmn;\u0026thinsp;20 ng/g (1SD, n\u0026thinsp;=\u0026thinsp;19) in the profile. In view of Hg/TOC ratio also significantly increased around the same time; therefore, the spike is likely caused by an anomalous Hg influx during sedimentation. At the depth of anomalous high Hg concentration, δ\u003csup\u003e202\u003c/sup\u003eHg values presented a large negative shift (decrease to -4.48\u0026permil;), falling well below the average MDF (-1.79\u0026thinsp;\u0026plusmn;\u0026thinsp;1.05\u0026permil;, 1SD, n\u0026thinsp;=\u0026thinsp;23) of the profile. This variation in isotopic compositions was likely caused by process or source differences during Hg deposition. The Δ\u003csup\u003e199\u003c/sup\u003eHg value exhibits initially a descending fluctuation trend from the surface down to a depth of 215 cm (from 0.24\u0026permil; to 0.10\u0026permil;, mean 0.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u0026permil;, 1SD, n\u0026thinsp;=\u0026thinsp;15), followed by more variable values between 215\u0026ndash;325 cm (0.29\u0026permil; \u0026minus;\u0026thinsp;0.21\u0026permil;, mean 0.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u0026permil;, 1SD, n\u0026thinsp;=\u0026thinsp;8). It is worth noting that the MIF trend is in line with the turning point of the Hg concentrations at the same depth of 210 cm. We hypothesized that the variation in Δ\u003csup\u003e199\u003c/sup\u003eHg values is influenced by photochemical dynamics in the upper atmosphere and water column during its transport and migration \u003csup\u003e18\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDespite limitations in establishing precise chronological frameworks, an approximate timeline for the cores can be inferred from the MT sedimentation rates and geological core data. It has been estimated that sedimentation rates in the MT were between 0.02 and 0.04 cm yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e based on Pb-210 data from sediment cores retrieved from the southern slope and the axis of the MT \u003csup\u003e31\u003c/sup\u003e. This suggests that the sediment core MT03 may contain sedimentary age information spanning the past 5 to 20 thousand years \u003csup\u003e32\u003c/sup\u003e. Laminated diatom mats (LDMs) are present at the base of the MT03 sediment core (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC), which could be a result of silicon diatom accumulation during the last glacial maximum (LGM) \u003csup\u003e33\u003c/sup\u003e. This extensive diatom blooming was likely caused by an increase in nutrients and minerals source from aeolian input during the LGM \u003csup\u003e34\u003c/sup\u003e. Thus, our date suggest that the deepest trenches showed anomalous increases Hg input during the LGM due to possible environmental or geologic evolution, which also affected the Hg isotopic composition on this profile. For comparison, the geo-environmental Hg levels reported for Arctic ice cores and shelf peat \u003csup\u003e35, 36\u003c/sup\u003e, likely suggests there was global sediment Hg anomaly from the Last Glacial Termination into the early Holocene. The possible reasons for the anomalous increase in Hg level and isotopic variations are discussed further below.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eMercury source variability revealed from Hg isotopic composition\u003c/h2\u003e \u003cp\u003eThe ratios of Δ\u003csup\u003e199\u003c/sup\u003eHg/Δ\u003csup\u003e201\u003c/sup\u003eHg in the Mariana Trench sediment, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, was approximately 0.95\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12 (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.76, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), closely aligning with the Δ\u003csup\u003e199\u003c/sup\u003eHg/Δ\u003csup\u003e201\u003c/sup\u003eHg slope (1.00\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01) typically seen in studies on photochemical reduction of Hg\u003csup\u003e2+\u003c/sup\u003e to Hg\u003csup\u003e0 37\u003c/sup\u003e. In addition, MIF varies very little with depth, implying that the Hg found at different depths was similarly affected by photochemistry. The consistent MIF\u0026thinsp;\u0026gt;\u0026thinsp;0 across the sedimentary profile suggests that buried Hg was constantly influenced by atmospheric processes \u003csup\u003e38\u003c/sup\u003e. However, an atmospheric Hg source alone seems insufficient to explain the large variations in MDF across the sedimentary profile, because existing data on atmospheric Hg (particulate Hg and gaseous elemental Hg) do not exhibit such a substantial negative MDF. Therefore, we hypothesize that the sediment core MT03 may contain Hg from additional sources (sediment rock/aquatic plant) \u003csup\u003e39\u003c/sup\u003e, where Hg undergoes photochemical processes similar to Hg in the upper atmosphere or water column before being deposited into the trench sediments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe average δ\u003csup\u003e202\u003c/sup\u003eHg isotopic composition in 240\u0026ndash;325 cm profile was \u0026minus;\u0026thinsp;2.25\u0026thinsp;\u0026plusmn;\u0026thinsp;0.62 (n\u0026thinsp;=\u0026thinsp;6, 1 SD), significantly more negative (t-test, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) than that in the 0-215 cm profile (-1.26\u0026thinsp;\u0026plusmn;\u0026thinsp;0.52, 1 SD, n\u0026thinsp;=\u0026thinsp;15). Although enhanced particle adsorption or microbial methylation of Hg\u003csup\u003e2+\u003c/sup\u003e may result in a negative MDF shift in 240\u0026ndash;325 cm profile, they would be insufficient to explain the large negative δ\u003csup\u003e202\u003c/sup\u003eHg observed for the period \u003csup\u003e18\u003c/sup\u003e. Specifically, dissolved Hg\u003csup\u003e2+\u003c/sup\u003e is adsorbed by Fe-Mn oxides or thiol ligand particles, with fractionation (enrichment) ranging from 0.30\u0026permil; to 0.62\u0026permil; \u003csup\u003e40\u003c/sup\u003e. Additionally, microbial-controlled methylation of Hg\u003csup\u003e2+\u003c/sup\u003e under anoxic conditions can lead to significant isotopic fractionation in produced methylmercury (~ -2\u0026permil;) \u003csup\u003e41\u003c/sup\u003e. However, these processes typically represent the maximum fractionation between methylmercury and total dissolved Hg\u003csup\u003e2+\u003c/sup\u003e observed in natural environment matrices (e.g., sediment/porewater). The extent of fractionation is also influenced by the Hg bioavailability and the methylation rate \u003csup\u003e42\u003c/sup\u003e. Methylation rates are usually less than 2% \u003csup\u003e43\u003c/sup\u003e, suggesting that microbial methylation would not generate sufficient methylmercury to significantly alter the isotope ratio of the total Hg and therefore is not the primary contributor to the significant negative MDF observed in the profile. The isotopic composition of residual Hg, which is enriched in Hg\u003csup\u003e2+\u003c/sup\u003e by algae, is heavier than the atmospheric mercury isotopic composition. The positive MDF of the isotopic composition of residual Hg outside of algae is also inconsistent with the isotopic trend of the sediment profile. Consequently, it is reasonable to argue that the main differences in Hg concentration and isotope ratios between the sedimentary profile are caused by a different Hg source.\u003c/p\u003e \u003cp\u003ePrevious reports on short sediment cores (MT1) from the MT indicate an OC/TN ratio of 7.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.93 and δ\u003csup\u003e13\u003c/sup\u003eC of -19.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40\u0026permil; (1 SD, n\u0026thinsp;=\u0026thinsp;13) \u003csup\u003e29\u003c/sup\u003e, suggesting that the MT is primarily influenced by atmospheric Hg input from marine sources (average THg concentration of 45\u0026thinsp;\u0026plusmn;\u0026thinsp;6.7 ng/g). The MT03 core at the depth of 0-215 cm is consistent with the Hg concentrations and stable isotopic compositions (Δ\u003csup\u003e199\u003c/sup\u003eHg and δ\u003csup\u003e202\u003c/sup\u003eHg) of MT sediments influenced by atmospheric Hg inputs (t-tests, p\u0026thinsp;\u0026gt;\u0026thinsp;\u0026gt;\u0026thinsp;0.05) \u003csup\u003e2\u003c/sup\u003e. This finding suggests that the 0-215 cm sediment profile of the deposited Hg source is associated with atmospheric Hg. Mercury from atmospheric dry and wet deposition enters the upper ocean and subsequently undergoes gravitational settling into the trench sediment \u003csup\u003e16\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe main Hg sources in marine sediments, including background rock weathering, volcanoes, hydrothermal fluids, biological tissue decomposition, and the transport of particle deposition from the atmosphere, could be potential explanations for the abnormal increase in Hg concentrations between 220\u0026ndash;240 cm depth \u003csup\u003e8\u003c/sup\u003e. This coincides with a significant negative shift in δ\u003csup\u003e202\u003c/sup\u003eHg, reaching values as low as -4.48\u0026permil;. This value is considerably lower than the isotopic MDF composition of potential source for Hg inputs, such as gaseous elemental Hg/ particulate bound Hg and Hg transported by hydrothermal vents or river (t-test, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). Comparable values are only recorded in sedimentary igneous rocks \u003csup\u003e44\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Furthermore, the reported TOC content is greater than 0.2% in MT03 sediment core \u003csup\u003e30\u003c/sup\u003e, and the proxy of Hg/TOC for volcanic emissions in the geologic record exhibits a significant increase within the MT03 sediment core in 220\u0026ndash;240 cm profile (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB) \u003csup\u003e45, 46\u003c/sup\u003e, corroborating the input of volcanic Hg during the period of LGM \u003csup\u003e47\u003c/sup\u003e. Non-zero MIF values at this stage are attributed to particles released by volcanoes adsorbing gaseous oxide Hg from the atmosphere to form particle Hg\u003csup\u003e2+\u003c/sup\u003e (positive MIF) \u003csup\u003e48\u003c/sup\u003e or emitted gaseous elemental Hg undergoing photochemical oxidation-reduction transformation during atmospheric transport \u003csup\u003e49\u003c/sup\u003e. During the Late Glacial and Holocene, climatic and volcanic signals were evident in the Hg record in ombrotrophic peat bog \u003csup\u003e36\u003c/sup\u003e, suggesting that the accumulation of Hg during this period may have been caused by a global volcanic event.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eRole of diatom in Hg cycling in the LGM period\u003c/h2\u003e \u003cp\u003eThe suggested sources for Hg found at depths ranging from 0-240 cm (Holocene to the LGM) cannot fully explain the differences in δ\u003csup\u003e202\u003c/sup\u003eHg observed at deeper depths of 240\u0026ndash;325 cm. The pronounced global environmental shifts during the LGM, which is manifested at depths of 215\u0026ndash;325 cm and associated with an enhanced flux of Asian eolian dust, contributed to a marked increase in marine primary productivity in the western Pacific Ocean \u003csup\u003e34, 55\u003c/sup\u003e. Recent research highlights the role of algae and algal-derived organic matter in aqueous-phase mercury removal, underscoring the significance of diatom laminations in Hg sequestration within marine environments \u003csup\u003e56, 57\u003c/sup\u003e. The continuous existence of LDMs in the 215\u0026ndash;325 cm sediment layer indicates that diatoms may be involved in the Hg cycle. Despite the absence of stable isotopic data for marine diatom laminae, available evidence from marine macroalgae (δ\u003csup\u003e202\u003c/sup\u003eHg: -3.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.81\u0026permil;, Δ\u003csup\u003e199\u003c/sup\u003eHg: 0.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u0026permil;) indicates a distinct isotopic signature \u003csup\u003e58\u003c/sup\u003e. It is reported that isotopic compositions of vegetation Hg suggest moderately negative MDF values (δ\u003csup\u003e202\u003c/sup\u003eHg: -2.56\u0026thinsp;\u0026plusmn;\u0026thinsp;0.64\u0026permil;) \u003csup\u003e19\u003c/sup\u003e. This finding aligns with our observation of similar δ\u003csup\u003e202\u003c/sup\u003eHg values in selected sediment layers (δ\u003csup\u003e202\u003c/sup\u003eHg values of -2.53\u0026permil;, -3.15\u0026permil;, and \u0026minus;\u0026thinsp;2.64\u0026permil; at 250, 295, and 310 cm depth, respectively). Algae may continuously enrich lighter Hg isotopes and deplete odd mass isotopes during the Hg\u003csup\u003e0\u003c/sup\u003e re-emission process, resulting in the retention of lighter isotopes (negative MDF) and positive MIF within their structure \u003csup\u003e59, 60\u003c/sup\u003e. This phenomenon is consistent with the observation that isotopic compositions in the 215\u0026ndash;325 cm layer (-2.25\u0026thinsp;\u0026plusmn;\u0026thinsp;0.57\u0026permil;, 1 SD, n\u0026thinsp;=\u0026thinsp;6) are notably lighter than those atmospheric sources contributing to Hg deposited in the 0-215 cm range (-1.26\u0026thinsp;\u0026plusmn;\u0026thinsp;0.52\u0026permil;, 1 SD, n\u0026thinsp;=\u0026thinsp;14). In addition, Vandal et al. \u003csup\u003e61\u003c/sup\u003e inferred that the oceanic productivity may have been higher during the period of LGM (18,000 years ago) based on changes in Hg concentrations in Antarctica ice cores. Our findings advance the understanding of the hadal Hg cycle in the process of geological historical evolution (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), suggesting that during the LGM period, diatoms functioned as the conveyor for the sequestration of Hg, facilitating the Hg transportation from the atmosphere to hadal trenches.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn this study, we reconstructed the geochemical cycle of Hg in the trench environment during the LGM period. Our findings reveal variations in the geochemical cycling of trench Hg, shedding light on the source and environmental drivers of Hg cycling prior to human influence. Geological activities (e.g. volcanism and shelf weathering) transported the sources of Hg input to the trench. The primary source shifted from atmospheric and algal deposits during the LGM to predominantly atmospheric deposition in the Holocene. Furthermore, volcanic activities during the transition may have been a principal cause of sudden increases in trench Hg deposition, potentially indicative of a global phenomenon.\u003c/p\u003e \u003c/div\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eSediment sampling and sample preparation\u003c/h2\u003e \u003cp\u003eThe MT originates from the subduction of the Pacific plate beneath the eastern boundary of the Philippine Sea plate. It stretches approximately 2500 km in length and maintains an average width of around 70 km \u003csup\u003e62\u003c/sup\u003e. The studied core (MT03) was collected from trench sediment at a depth of 8300 meters on the southern slope of the \u0026ldquo;Challenger Deep\u0026rdquo; in the MT in 2019. The length of the MT03 core is 3.2 m, and the collection site coordinates are 142\u0026deg; 19.5797' E, 11\u0026deg; 07.6804' N (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). The complete core displays distinct color stratification. In the lower portion of the core (215\u0026ndash;325 cm), the sediment exhibits a shade of gray, grayish-green, or green and contain notable quantities of diatoms (detailed description for MT03 core see Lai et.al. \u003csup\u003e30\u003c/sup\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). In the laboratory, sediment cores were subdivided into segments and sampled at approximately 15 cm intervals, resulting in a total of 24 samples. Subsequently, these samples were subjected to freeze-drying and homogenized in preparation for THg concentration and isotopic ratio analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eHg concentration analyses\u003c/h2\u003e \u003cp\u003eConcentrations of THg in sediment samples were measured using a method modified following EPA method 7474 \u003csup\u003e63\u003c/sup\u003e. Approximately 0.5 g of homogenized sediment were weighed into a 10 mL ampule vial. After adding 2 mL of HNO\u003csub\u003e3\u003c/sub\u003e and 1 mL of deionized water, the ampule was sealed and the sample digested at 105\u0026deg;C in an autoclave for 1 h. Next, 0.1 mL of the digested sample was added to 25 mL of 1% (v/v) HCl and THg in the samples were determined by a MERX automated THg analytical system (Brooks Rand Laboratories, USA) after the addition of SnCl\u003csub\u003e2\u003c/sub\u003e (20% (w/v)).\u003c/p\u003e \u003cp\u003eStandard quality assurance and control procedures were followed during the analysis of Hg. For each batch analysis (20 samples), two method blanks, two certified reference materials (ERM-CC580 for THg in sediment), and triplicates of one randomly chosen sample were included. ERM-CC580 estuarine sediment reference material was measured with each batch (20 samples) and the THg recoveries were in the range of 91\u0026ndash;95%, which is within the acceptable range of the EPA method (70\u0026ndash;130%). Three parallel samples randomly selected gave RSD\u0026rsquo;s from 1\u0026ndash;10%, which was also within the acceptable range (\u0026lt;\u0026thinsp;15%). Method blanks for THg in sediment were between 0.2 to 0.3 ng g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is within the acceptable range of the EPA method.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eHg isotope ratio analysis\u003c/h2\u003e \u003cp\u003eAll sediment samples were measured for Hg isotope composition by a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS; Thermo Scientific Neptune Plus, USA). According to the THg concentrations measured above, the digested sample solutions were diluted to 1.0 ng/mL, with acid concentrations of \u0026lt;\u0026thinsp;20% (v/v).\u003c/p\u003e \u003cp\u003eThe procedure consisted of Hg reduction employing an in-line cold-vapor system (HGX-200, CETAC Technologies, USA) using a reducing agent of SnCl\u003csub\u003e2\u003c/sub\u003e (5% SnCl\u003csub\u003e2\u003c/sub\u003e in 10% HCl), followed by stripping of Hg\u003csup\u003e0\u003c/sup\u003e with Ar and mixing in cold-vapor system with thallium (Tl) aerosols produced with a desolvation nebulizer (CETAC Aridus 3, USA) and then introduced into the MC-ICPMS. The mass bias of MC-ICP-MS was corrected with the Tl standard solution applying the exponential law. A matrix-matched NIST-3133 Hg isotope standard solution was used for standard-sample bracketing \u003csup\u003e64\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHg isotope ratios are reported in δ\u003csup\u003exxx\u003c/sup\u003eHg (\u0026permil;) notation, referring to the bracketed NIST-3133:\u003c/p\u003e \u003cp\u003eδ\u003csup\u003exxx\u003c/sup\u003eHg(\u0026permil;) = ([(\u003csup\u003exxx\u003c/sup\u003eHg/\u003csup\u003e198\u003c/sup\u003eHg) \u003csub\u003esample\u003c/sub\u003e/(\u003csup\u003exxx\u003c/sup\u003eHg/\u003csup\u003e198\u003c/sup\u003eHg)\u003csub\u003eSRM 3133\u003c/sub\u003e]- 1) \u0026times; 1000 (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eWhere the superscript xxx represents Hg isotope mass between 199 and 204. Here, δ\u003csup\u003e202\u003c/sup\u003eHg is reported as Hg-MDF. MIF is expressed by Δ\u003csup\u003exxx\u003c/sup\u003eHg (\u0026permil;) notation, representing the difference between the measured δ\u003csup\u003exxx\u003c/sup\u003eHg value and that theoretically predicted δ\u003csup\u003e202\u003c/sup\u003eHg using the MDF law:\u003c/p\u003e \u003cp\u003eΔ\u003csup\u003exxx\u003c/sup\u003eHg(\u0026permil;) = δ\u003csup\u003exxx\u003c/sup\u003eHg \u0026minus; (β\u003csup\u003exxx\u003c/sup\u003e\u0026thinsp;\u0026times;\u0026thinsp;δ\u003csup\u003e202\u003c/sup\u003eHg(\u0026permil;)) (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eWhere β\u003csup\u003exxx\u003c/sup\u003e is the obtained mass dependent scaling factor according to the mass fractionation law, and its values are 0.2520 and 0.7520 for 199 and 201, respectively. Δ\u003csup\u003e199\u003c/sup\u003eHg and Δ\u003csup\u003e201\u003c/sup\u003eHg are referred to as odd MIF.\u003c/p\u003e \u003cp\u003eTo check system performance and reproducibility, a secondary standard solution of NIST-3177, and estuarine sediment Reference Material (ERM-CC580) were measured regularly. The NIST-3177 had mean values of δ\u003csup\u003e202\u003c/sup\u003eHg = -0.53\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u0026permil;, Δ\u003csup\u003e199\u003c/sup\u003eHg = -0.02\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u0026permil; (2SD, n\u0026thinsp;=\u0026thinsp;3), ERM-CC580 (n\u0026thinsp;=\u0026thinsp;3) had mean values of δ\u003csup\u003e202\u003c/sup\u003eHg = -0.49\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u0026permil;, Δ\u003csup\u003e199\u003c/sup\u003eHg = -0.05\u0026thinsp;\u0026plusmn;\u0026thinsp;0.006\u0026permil; (2SD, n\u0026thinsp;=\u0026thinsp;3), consistent with values reported by others \u003csup\u003e65\u0026ndash;67\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eStatistical Analysis\u003c/h2\u003e \u003cp\u003eStation maps were drawn using Ocean Data View (AWI, Germany). The vertical variations in THg concentrations and Hg stable isotopic values were drawn using Origin (OriginLab Co. Ltd., USA). The Hg stable isotope composition of different natural samples was presented using Sigmaplot (Systat Software Inc., USA). The t-test and Pearson correlation analysis between Δ\u003csup\u003e199\u003c/sup\u003eHg and Δ\u003csup\u003e201\u003c/sup\u003eHg were performed using SPSS 19.0 (SPSS Inc., USA).\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData used in this study can be found in the Supplementary Index.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (42373076) and Fundamental Research Funds for the Central Universities (202072001, 202372001).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eZ.Z. was responsible for the analysis of mercury concentration and isotopes, drafting the manuscript, and participating in its revision. H.W. analyzed the mercury stable isotopes. Y.X. and J.T. provided environmental samples, contributing essential data for the research. X.L. assisted in manuscript revision, providing TOC data. H.H., Y.W., Y.G.Y., G.L., and Y.C. reviewed and revised the manuscript, offering significant help in isotope analysis. Y.L. designed the study and led the writing and revision of the manuscript. All authors discussed and interpreted the results and contributed to the manuscript. All authors reviewed, edited, and contributed to the final version.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBlum JD et al (2020) Mercury isotopes identify near-surface marine mercury in deep-sea trench biota. 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Environ Pollut 346:123554\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","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":"","lastPublishedDoi":"10.21203/rs.3.rs-4518189/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4518189/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe Mariana Trench, is one of the ultimate sinks of the earth\u0026rsquo; system, providing unique insights to matter cycling and environmental evolution. Trench sediments receive mercury (Hg) from the upper ocean and constitute a global Hg sink. However, little is known about the variation in the Hg cycle that have been driven by geological or environmental changes prior to human activity. Here we present results covering concentrations and isotopic signatures of Hg in the deepest trench system to identify the evolution of Hg cycling in trenches before the Anthropocene. Sediment cores collected from the Mariana Trench showed values for mass independent fractionation (Δ\u003csup\u003e199\u003c/sup\u003eHg) of \u0026gt;\u0026thinsp;0 with ratios of Δ\u003csup\u003e199\u003c/sup\u003eHg/Δ\u003csup\u003e201\u003c/sup\u003eHg close to 1.0, suggesting that Hg in this system was primarily subjected to atmospheric or water column photochemical processes prior to deposition. Geological proxies and isotopic compositions (δ\u003csup\u003e202\u003c/sup\u003eHg: -4.2\u0026permil; to -4.5\u0026permil;, Δ\u003csup\u003e199\u003c/sup\u003eHg: 0.28\u0026permil; to 0.29\u0026permil;) comparable only in volcanoes reveal that Hg contents coinciding with the transition from the last glacial termination to the early Holocene can predominantly be attributed to volcanic activity. During the Holocene, atmospheric Hg constituted the main source of Hg in the Mariana Trench, while the last glacial maximum was characterized by an accumulation of both atmospheric and biogenic Hg.\u003c/p\u003e","manuscriptTitle":"Sources of mercury varied in the Mariana Trench during the Last Glacial Maximum to the Holocene","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-05 09:03:18","doi":"10.21203/rs.3.rs-4518189/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","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":"9adf4feb-a8fb-4f81-b8ae-015fcbe41d0c","owner":[],"postedDate":"June 5th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":32769601,"name":"Earth and environmental sciences/Biogeochemistry/Element cycles"},{"id":32769602,"name":"Earth and environmental sciences/Solid Earth sciences/Geochemistry"},{"id":32769603,"name":"Earth and environmental sciences/Environmental sciences/Environmental chemistry/Geochemistry"},{"id":32769604,"name":"Earth and environmental sciences/Ocean sciences/Marine chemistry"}],"tags":[],"updatedAt":"2024-07-11T15:06:13+00:00","versionOfRecord":[],"versionCreatedAt":"2024-06-05 09:03:18","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4518189","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4518189","identity":"rs-4518189","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}