Unstable climate for 50,000 years after Chicxulub impact: evidence from Antarctica

Unstable climate for 50,000 years after Chicxulub impact: evidence from Antarctica | 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 Unstable climate for 50,000 years after Chicxulub impact: evidence from Antarctica Sha Li, Paul Wignall, James Witts, Meng Wang, Zhiliang He, Nan Wang, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8071084/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The ~ 66 Ma Cretaceous/Paleogene (K/Pg) boundary coincides with a severe mass extinction widely attributed to the Chicxulub impact and consequent abrupt climatic changes. However, it is unclear how long the subsequent climatic disruptions affected terrestrial and marine environments and what caused them. Here, we present magnesium isotope (δ²⁶Mg) and Chemical Index of Alteration data from an expanded section on Seymour Island, Antarctica, where a new cyclostratigraphic age model provides millennial-scale resolution. Pre-boundary weathering indices, including δ²⁶Mg values, record a stable, mild weathering regime. Post-boundary weathering saw a transition to a substantially variable regime with intervals of intense weathering revealed. This environmental instability persisted for at least 43.89 kyr after impact, and may reflect the climatic influence of Deccan volcanism. Thus, whilst the Chicxulub impact can be directly implicated in mass extinction, the recovery interval was substantially affected by Deccan volcanism. Earth and environmental sciences/Solid Earth sciences/Geochemistry Earth and environmental sciences/Climate sciences/Palaeoclimate Figures Figure 1 Figure 2 Figure 3 Main The trigger of the ~66 Ma Cretaceous/Paleocene (K/Pg) boundary crisis is widely thought to have been linked with the Chicxulub impact 1, 2, 3 and/or Deccan Traps eruptions 4, 5, 6 The former likely caused a global winter at the K/Pg boundary, resulting from the combined effects of impact-generated silicate dust, sulfur, and soot from wildfires on the post-impact climate 3 . The finest component of the ejected dust may have persisted in the atmosphere for up to 15 years, causing a decrease in global average surface temperatures by up to 15°C, and a period of global darkness that resulted in a shutdown or reduction of primary productivity 3 . However, the high-resolution changes in chemical weathering following the Chicxulub impact on a decadal to millennial scale are poorly studied owing to the poor temporal resolution of most sedimentary archives. Previous research has primarily focused on continental weathering at the boundary. Thus, Martin and Macdougall 7 suggested there was enhanced weathering caused by acid rain that resulted in the rapid addition of radiogenic Sr to the oceans (Fig. 1), although the duration of this “rapid” pulse is not quantified. To investigate environmental changes following the Chicxulub impact, we conducted cyclostratigraphic analysis to obtain a high-resolution age model for the Seymour Island K/Pg succession. Then we have assessed chemical weathering changes based on high-resolution analysis of the chemical index of alteration (CIA) and Mg isotopes in the expanded K/Pg boundary succession on Seymour Island, Antarctica. The CIA is a widely applied proxy, because chemical weathering of the upper crust is dominated by feldspar degradation and the concomitant formation of clay minerals; Ca, Na, and K are removed from feldspars by aggressive soil solutions 8 . This process increases the proportion of alumina relative to alkalis in the weathered product, a change that is recorded by higher CIA values 8 . The Mg isotopic composition of the primary components of the upper continental crust falls within a narrow range from −0.3‰ to −0.1‰ 9 , thereby minimising source-related biases that could otherwise influence CIA values. By contrast, chemical weathering induces substantial Mg isotope fractionation, with variations of up to ~2‰ 10 . Sedimentary rocks display large isotopic heterogeneity: clastic rocks range from −0.75 to +0.92‰ 11 . Consequently, Mg isotopes are regarded as a reliable tracer of chemical weathering processe 10, 11, 12 . Fractionation of Mg isotopes results in the preferential retention of heavy 26 Mg in weathering residues, resulting in the δ 26 Mg values of weathering residues increasing progressively with greater chemical weathering intensity 13 . Geological Setting Seymour Island lies within the Weddell Sea, to the northeast of the Antarctic Peninsula (Fig. 1). The James Ross Basin contains a 6–7 km-thick sedimentary succession, spanning the Upper Cretaceous to the lowest Oligocene, that records a range of shallow marine palaeoenvironments 14, 15, 16 . Within this succession, the Upper Cretaceous–Paleocene Marambio Group—comprising the Haslum Crag, López de Bertodano, and Sobral formations—consists primarily of fine-grained clastic sediments deposited in a marine shelf environment 17 . The López de Bertodano Formation (Maastrichtian to lower Paleocene) 18 reaches over 1,000 m in thickness, and is composed of grey to tan, friable, sandy, muddy siltstone, informally subdivided into 10 informal units on palaeontological and sedimentological grounds 19 . The lower six units, known as the Rotularia Units, have a sparse macrofauna other than the abundant calcareous tubes of the eponymous annelid, and formed in a shallow marine setting near a delta or estuary 19 . Units 7–10, known as the Molluscan Units, contain a rich marine macrofauna 19 . The depositional environment deepened progressively from Units 7–9, transitioning from middle (Units 7–8) to outer shelf facies (Unit 9) before Unit 10 records a return to a middle to inner shelf setting 19 . The age of the López de Bertodano Formation has been determined using both biostratigraphy 20 and magnetostratigraphy 21 . Seymour Island is an excellent location for studying the K/Pg boundary due to the preservation of a complete and expanded stratigraphic record, and the abundantly fossiliferous succession 22 . Filo Negro Section and the K/Pg Boundary The Filo Negro section of this study (Fig. 1, Supplementary Fig. 1) encompasses the upper Maastrichtian and lowermost Danian portion of the López de Bertodano Formation, where an iridium anomaly 23 , serves as the a marker for the K/Pg boundary 24 . The succession consists of muddy to very fine-grained sandstones that exhibit slight variations in glauconite content and colour, along with carbonate concretions 25 that record continuous deposition 26, 27 , in a shallow marine platform setting 17 . The iridium anomaly that marks the K/Pg transition occurs within a 20–30 cm thick interval coinciding with the first appearance of Paleocene dinoflagellate cysts 23 . Dinocyst biostratigraphy, integrated with geochemical proxies (e.g., Ir anomaly and siderophile element enrichment), refines the K/Pg boundary to a narrow interval between 9.5 and 9.6 m at Filo Negro 25 . Signatures of weathering We conducted detailed measurements of stratigraphic profiles and a systematic collection of samples at the Filo Negro Section (James Ross Basin). Platinum group element (PGE) analysis confirmed the iridium spike at a height of 9.300 to 9.600 m (Fig. 2) whilst the Danian dinocyst Danea californica is at 9.600 m height 23 . Time series analysis of the untuned thorium/uranium (Th/U) ratios from the López de Bertodano Formation indicates a dominant cyclicity with a wavelength of ~7.2 m. Existing magnetochronological data place the K/Pg boundary within this section in Magnetic Chron 29r, with the base positioned ~45 m below the K/Pg 21, 22 , yielding an average sedimentation rate of 17.8 cm/kyr 21 . Using this value, the ~7.2 m cycles are considered to record the 41-kyr obliquity cycle (see Materials and Methods), with our measured Filo Negro section recording 99.3 kyr (Fig. 2). We calculated CIA values and divided the entire section into five intervals (Fig. 2), each reflecting different intensities of chemical weathering. In interval I, from 0 m to 9.60 m, the CIA values are stable within a narrow range between 56.3 and 61.1. In interval II (9.60–11.30 m) immediately above the iridium spike, CIA values decrease to 51.2 to 55.6. In the short interval III, from 11.30 m to 11.65 m, CIA values slightly increase, ranging from 55.4 to 60.1. The interval IV, between 11.65 m and 14.90 m, shows a more substantial decline, with CIA values ranging from 49.5 to 54.5, suggesting a continued reduction in weathering intensity. Finally, in interval V, from 14.90 m to 17.20 m, CIA values rise again, fluctuating between 55.9 and 71.3, with a significant peak value of 71.3 at 15.20 m. This peak indicates a period of more intense chemical weathering, which then gradually decreases toward the top of the section at 17.2 m, where the CIA value is 58.4, values close to those at the base of the section. As additional evidence, we calculated the illite chemistry index 29 (Fig. 2) to assess the intensity of chemical weathering 30 . The interval I is characterised by a relatively high illite chemistry index, ranging from 0.19 to 0.51, below the K/Pg boundary. In interval II, the index narrows and declines to a range of 0.10 to 0.30 before increasing slightly in interval III: values vary between 0.19 and 0.51, and they are similar in interval IV (0.11 to 0.25). Finally, the interval V is marked by initially high values with a peak of 1.07 at 15.20 m height. There is then a gradual decrease toward the top of the section at 17.2 m, where the illite chemistry index is 0.30, similar to values seen in the Cretaceous. A total of fifty-one Mg isotope values were obtained (Fig. 2). These exhibit trends consistent with the CIA and illite chemistry index values, with the exception of interval I. Here, the Mg isotope values are rather variable, ranging from 0.01‰ to 0.22‰. In interval II, there is a decrease in Mg isotope values, which drop from -0.07‰ to 0.11‰. The interval III sees an increase in Mg isotope values, ranging from -0.10‰ to 0.34‰. The interval IV shows a more significant decline, with values ranging from -0.11‰ to 0.02‰. Finally, in interval V, Mg isotope values rise once again, fluctuating between 0.02‰ and 0.40‰, with a notable peak of 0.40‰ at 15.20 m. Four main clay minerals (illite, kaolinite, chlorite, and smectite) were identified by X-ray diffraction analyses of the clay fraction (< 2 μm) (Fig. 2). In interval I, the illite, kaolinite, chlorite, and smectite content remains relatively stable. In interval II, the clay mineral composition becomes more variable, with a noticeable increase in illite content and a corresponding decrease in smectite. In interval III, this trend culminates in a mineral assemblage overwhelmingly dominated by smectite, while the illite content declines to its lowest levels. The interval IV is characterised by a reversal of the previous trend, with a sharp decline in smectite content and a renewed increase in illite. Finally, in interval V, there is a sharp increase in smectite content again, which dominates the clay mineral assemblage. The δ 13 C org data exhibit distinct variations (Fig. 2). The interval I shows stable δ 13 C org values, ranging from -26.01‰ to -24.71‰, with a minor negative excursion observed just before the K/Pg boundary. Interval II has lower values, ranging from -25.63‰ to -24.39‰, with a minor negative excursion occurring after the K/Pg boundary. Interval III shows consistently declining values as δ 13 C org values drop from -25.78‰ to -24.89‰, a trend that continues into interval IV, where values range from -26.83‰ to -25.99‰. Finally, in interval V, there is a recovery of δ 13 C org values to values from -27.38‰ to -25.95‰. The total organic carbon (TOC) profile shows notable variations (Fig. 2). The interval I exhibits moderately stable TOC values, ranging from 0.36% to 0.59%. In contrast, the interval II demonstrates a significant depletion in TOC values, varying from 0.15–0.43%. The interval III marks recovery of values to 0.32–0.50% before they decline again in the interval IV to 0.23–0.39%. Finally, the interval V is characterised by the lowest TOC values of the profile, with values ranging from 0.18–0.35‰. Weathering fluctuations The intensity of chemical weathering, assessed by the CIA proxy, was relatively stable in the mainly latest Cretaceous (interval I) before declining above the K/Pg Ir spike (interval II), suggesting that weathering reduced after the Chicxulub impact. The brief interval III sees a slight recovery to Cretaceous values, but is followed by a substantial decline in interval IV. During interval V, CIA values rise sharply, peaking at 15.20 m in the section before gradually declining. The fluctuations in CIA are closely matched by those shown by the other weathering proxies (illite chemistry index and δ²⁶Mg values), providing good corroboration of these trends. Thus, the stable weathering regime of the latest Cretaceous was replaced by dynamic and fluctuating conditions in the first ~43.89 kyr of the Paleocene. A lack of correspondence between smectite/illite ratios (indicative of volcanic input) and the consistent REE (rare earth elements) patterns (see Supplementary Figs. 2–3) indicates that the chemical weathering proxies record climatic rather than provenance changes. Palaeontological and palaeotemperature data from Seymour Island also support the inferred climate record at Filo Negro. During the Maastrichtian, a relatively humid climate prevailed, as indicated by fossil plant evidence that suggests a lowland to mid-altitude vegetation of mixed podocarp-southern beech temperate rainforest was growing on the eastern flank of the Antarctic Peninsula arc 31 . Mean annual air temperatures during this time were 16.0–16.5°C based on MBT’/CBT (methylation index of branched tetraethers/cyclisation ratio of branched tetraethers) data 32 . There is a transition to cooler and drier climates in the Danian recorded by increased prevalence of the high-latitude pollen Nothofagidites spp. and appearance of the cold-water dinocyst Impletosphaeridium clavus 32 . There was also an abrupt decrease in Humidity Index values (this weighted averaging function uses spore and pollen taxa with affinities to known hydrological groups 31 ). The MBT’/CBT palaeothermometer and clumped isotope data from marine bivalves indicate a ~2-degree drop in air temperatures during interval II 32, 33 . Thus, the decreased weathering intensity of interval II corresponds to cooling and decreased humidity. No temperature data is available for younger levels, but our study shows that climatic oscillations continued to persist for at least a further 43.89 kyrs, indicating a long term disruption to climate stability after the K/Pg boundary. Disruption of weathering regimes after the K/Pg boundary is also observed in marine sediments from NE India, where stable CIA values (70–80) in the uppermost Maastrichtian decline sharply above the K/Pg boundary 28 to persistently low values (30–40), during the early Danian including the P0 zone (ca. the first 30 kyr 34 ) and part of P1a (1)) 28 . This corresponds to the low CIA values seen in intervals II and IV on Seymour Island. Delayed recovery after Chicxulub impact There has been considerable study of the aftermath of the Chicxulub impact and its effects in the oceanic realm. During the initial phase of global darkness and cooling, marine productivity was very low for the first 3 years and was likely attributable to the development of sulphate aerosols post-impact 35 . Productivity levels recovered rapidly after this, with the appearance of calcareous nannofossils and dinoflagellate blooms that all point to eutrophic conditions 36, 37, 38 . These conditions persisted for ~50 ky in shelf seas, but open ocean sites record the persistence of more oligotrophic conditions at this time 39 . The millennial-scale aftermath of the Chicxulub impact on land is less well documented, but recovery appears to have been substantially more rapid than in the oceans. Palynological data indicate that the persistence of the post-impact, opportunistic-dominated fern ecosystems was regionally variable 40 . In the New Zealand record, recovery is well underway within 30 ky of the boundary with angiosperm pollen becoming a significant part of palynological assemblages by this time 41 . In Antarctica, fern spore abundances peak within a ~0.5 m interval (probably ~3 ky) after the K/Pg boundary 32 . These, and other observations 42 , suggest that diverse floral communities were becoming re-established within a few thousand years of impact, although full recovery took longer 43 . In contrast, the mammalian rebound was more delayed, with diversification only increasing notably after 100 ky 43 . Marine and terrestrial ecosystems display different response times, but the role of continuing effects of the Chicxulub impact versus those of the Deccan eruptions has not been clarified. Volatile releases from impact are unlikely to have directly affected the climate on more than a decadal scale 2 . Oxygen isotope data suggest a gradual warming for the first 200 kyr of the Paleogene 2 , but our data show that this gradual trend was subject to higher-order fluctuations of weathering, which were likely climate driven for at least the first ~50 ky of the Paleogene. This pattern contrasts with the relative stability of the last ~50 kyr of late Maastrichtian. These trends show a close match with the eruption history of the Deccan Traps, recorded by Renne and colleagues 44 , in which frequent, relatively small-scale eruptions in the Maastrichtian were followed by rarer, but larger-scale eruptions in the first half-million years of the Paleogene. The increase of weathering intensity in phases III and V on Seymour Island could reflect large-scale Deccan eruptions and volcanic-CO 2 -driven warming. Therefore, the post-Chicxulub recovery interval was likely influenced by large-scale Deccan volcanism. The longer recovery in the marine realm may reflect this Deccan influence on climate, whilst the more rapid floral recovery suggests that terrestrial ecosystems were either less affected by the climate changes or the extinction losses were less consequential. However, this dichotomy of response could be further tested by reconstructing the weathering regime after the first ~50 ky of the Paleogene to see if the link with large-scale Deccan eruptions persisted. Materials and Methods Our samples were collected from the López de Bertodano Formation at the Filo Negro section (coordinates: 64° 17’ 24” S; 56° 44’ 30” W), James Ross Basin, on Seymour Island, Antarctica, in the austral summer of 2024. Platinum group element content About 0.2 ~ 0.5 g of powders for each sample were weighed together with a mixed PGE spike ( 99 Ru, 106 Pd, 185 Re, 190 Os, 191 Ir, and 194 Pt), then sealed along with inverse aqua regia in Carius tubes and subsequently heated to 240°C for 24 h. Ir, Ru, Pt, Pd, and Re were first purified by an AG50-X8 resin to remove matrix elements 45 . The LN resin was then used to separate Zr and Hf 46 . The analysis of PGE abundance was performed on a secondary electron multiplier (SEM) on a Thermo Element 2 ICP-MS at the China University of Geosciences, Beijing. The uncertainties of Re-PGE contents are reported as 2SD, ranging from 0.0005 to 0.0188 ppb, based on 100 cycles conducted by the mass spectrometer for each sample. Major and trace element analysis The Inductively Coupled Plasma (ICP) Sample Digestion Procedure had the following steps: 1. 50.00 mg of rock sample were placed in a PTFE digestion vessel. A few drops of deionised water wetted the sample. A second vessel containing deionised water was prepared as a blank, with standards and duplicates. 2. Each vessel received 1.5 mL of high-purity HF and was heated on a hot plate at 120°C until the HF evaporated to a wet salt state to remove most of the silicon matrix. Subsequently, 1.0 mL of HNO₃ and 1.5 mL of HF were added, the vessels were sealed with lids and steel jackets, and they were digested in an oven at 190°C for 72 hours. 3. After cooling, the vessels were heated on a hot plate and again evaporated to a wet salt state; 1.0 mL of HNO₃ was added, and the sample was evaporated twice to eliminate residual HF. Then, 1.5 mL of HNO₃ and approximately 2 mL of water were added, the vessels were sealed, and they were held in an oven at 120°C for 12 hours. 4. Following cooling, the extracts were transferred to clean 50 mL centrifuge tubes, 1 mL of a 500 µg/L Rh internal standard solution was added, and the solutions were diluted with deionised water to 50 mL, yielding a Rh concentration of 10 µg/L. Major elements were measured by Agilent Technologies 710 ICP-OES (Optical Emission Spectroscopy), and trace and rare earth elements were measured by Agilent Technologies 7700x ICP-MS (Mass Spectrometry) at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Precision (RSD) for major and trace elements was mainly below 1% and did not exceed 3% for any element. Magnesium isotope analyses The analyses were performed at Northwest University in Xi’an, China. Approximately 50 mg of sample was decarbonated, and ~ 5 mg of decarbonated powders were used to extract silicates, and potassium (K) was removed by coprecipitation. Magnesium was purified through two sets of columns. The first column, used to elute Ca, was packed with 2 mL of Bio-Rad AG50W-X12 resin (200–400 mesh) 47 . The second column, packed with 0.5 mL of Bio-Rad AG50W-X12 resin (200–400 mesh), was used to elute sodium (Na), aluminium (Al), iron (Fe), and titanium (Ti). Magnesium isotope measurements were performed on the Nu Plasma multi-collector inductively coupled plasma mass-spectrometer (MC-ICP-MS). Magnesium isotopic data are presented as per mil (‰) relative to a pure Mg standard solution DSM3: δ X Mg (‰) = [( X Mg/ 24 Mg) sample /( X Mg/ 24 Mg) DSM3 −1] × 1000 where x refers to mass 26. Four standards (SW, alfa Mg, AGV-2, BCR-2) were analysed. All samples and standards were each analysed four times during an analytical session, producing 2SD values between 0.01‰ and 0.07‰. Clay minerals Clay mineralogy was determined by X-ray diffraction (XRD) using a PANalytical X’Pert PRO diffractometer housed at the State Key Laboratory of Marine Geology, Tongji University. The detailed analytical procedure was described in Liu et al. 48 . In summary, after decarbonization (20% acetic acid) and removal of organic matter (H 2 O 2 ), the clay fraction (< 2 µm) was extracted. The XRD measurements were performed on oriented thin sections that had been successively air dried, saturated with ethylene glycol under vacuum for 24 hr, and heated at 490°C for 2 hr. Semi-quantitative estimates of the peak areas of the basal reflections for the main clay mineral groups (smectite: 15–17 Å; illite: 10 Å; and kaolinite/chlorite: 7 Å) were carried out on the ethylene glycol saturation diffractogram using MacDiff software. Kaolinite was discriminated from chlorite via the 3.57/3.54 Å peak area comparison, and illite chemistry was calculated using 10 Å/5 Å peak areas 29 . Replicate measurements produced results with a relative error of ± 2% (2SD). Time series analysis The investigation of astronomical periodicities utilised Acycle software v2.8 49 , following established cyclostratigraphic protocols 50 . The Th/U ratio has been suggested as a variable proxy that combines the influence of climatic and marine redox fluctuations 51 . Recognising that raw Th/U can contain significant low-frequency, non-periodic trends capable of distorting spectral estimates via power leakage, all raw data were detrended by removing linear trends to eliminate long-term variations unrelated to orbital forcing. We analysed both untuned and tuned Th/U ratios for cyclical patterns using the multi-taper method (MTM) of spectral analysis 8 . To reveal the dominant wavelength of the proxy series and search for potential astronomical cycles, MTM spectra were statistically evaluated using the robust red noise modelling procedure 52 . Specific frequency bands potentially corresponding to Milankovitch cycles were isolated using Gaussian and Taner bandpass filters 50 . Astronomical tuning was subsequently performed by aligning the ~ 7.2 m cycles to 41-kyr obliquity periods using the “Age Scale” module in Acycle , establishing a robust astrochronological framework for the studied interval (Fig. 3 ). Declarations ACKNOWLEDGMENTS The National Natural Science Foundation of China (grants 42472053, 42322607, 42402030), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (grant 2022310), and the Fundamental Research Funds for NIGPAS (NGBS202423) supported this work. References Schulte P et al (2010) The Chicxulub asteroid impact and mass extinction at the Cretaceous–Paleogene boundary. Science 327:1214–1218 Hull PM et al (2020) On impact and volcanism across the Cretaceous–Paleogene boundary. Science 367:266–272 Senel CB et al (2023) Chicxulub impact winter sustained by fine silicate dust. Nat Geosci 16:1033–1040 Font E, Chen J, Regelous M, Regelous A, Adatte T (2022) Volcanic origin of the mercury anomalies at the Cretaceous–Paleogene transition of Bidart, France. Geology 50:142–146 Callegaro S et al (2023) Recurring volcanic winters during the latest Cretaceous: sulfur and fluorine budgets of Deccan Traps lavas. 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Geostand Geoanal Res 39:151–169 Huang K-J et al (2015) Magnesium isotopic compositions of the Mesoproterozoic dolostones: implications for Mg isotopic systematics of marine carbonates. Geochim Cosmochim Acta 164:333–351 Liu Z et al (2004) Erosional history of the eastern Tibetan Plateau since 190 kyr ago: clay mineralogical and geochemical investigations from the southwestern South China Sea. Mar Geol 209:1–18 Li M, Hinnov L, Kump L (2019) Acycle: time-series analysis software for paleoclimate research and education. Comput Geosci 127:12–22 Hinnov LA (2018) In: Montenari M (ed) Stratigraphy & Timescales. Academic, pp 1–80 Li M et al (2019) Paleoclimate proxies for cyclostratigraphy: comparative analysis using a Lower Triassic marine section in South China. Earth Sci Rev 189:125–146 Mann ME, Lees JM (1996) Robust estimation of background noise and signal detection in climatic time series. Clim Change 33:409–445 Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryFig.2.xlsx Dataset 1 SupplementaryInformation.docx Supplementary Information Cite Share Download PDF Status: Under Review 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. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-8071084","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":548284472,"identity":"030712d9-2b83-48cc-a1c9-713ba8f8ab39","order_by":0,"name":"Sha Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzElEQVRIiWNgGAWjYDACCQbGAwkVDDwkaWE4kHCGZC2MbaS4S35284MDD+fdkZGPbn78gaHGjoF/dgN+LQZ3jhkcSNz2jMfwzjEzCYZjyQwSdw4Q0CKRANJymMdwRoIZAwPbAZAIAYfNSP9wIHEOSEv65w8M/4jQwnAjB2hLw2EeeYkcAwnGNiK0GNzIKTiQcOwwj4HMmTKJxL5kHokbhB228eGPmsP28rPbN3/48M1Ojn8GIYchrAMSQMUkxKn8DOLVjoJRMApGwQgDAHVxRoXhaCzpAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-1268-5364","institution":"State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences","correspondingAuthor":true,"prefix":"","firstName":"Sha","middleName":"","lastName":"Li","suffix":""},{"id":548284473,"identity":"aa098f6d-fe7d-4a9e-a7e0-87c3ac776524","order_by":1,"name":"Paul Wignall","email":"","orcid":"","institution":"University of Leeds","correspondingAuthor":false,"prefix":"","firstName":"Paul","middleName":"","lastName":"Wignall","suffix":""},{"id":548284474,"identity":"e137d8fe-39f7-4333-a58c-1fa46e73b8b0","order_by":2,"name":"James Witts","email":"","orcid":"","institution":"Natural History Museum","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Witts","suffix":""},{"id":548284475,"identity":"e3af3700-c335-4480-ae69-7a99f8a8160c","order_by":3,"name":"Meng Wang","email":"","orcid":"https://orcid.org/0000-0003-2538-0284","institution":"Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Meng","middleName":"","lastName":"Wang","suffix":""},{"id":548284476,"identity":"4cc5a18c-7b47-489d-a719-1ddff4bddff1","order_by":4,"name":"Zhiliang He","email":"","orcid":"","institution":"Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan","correspondingAuthor":false,"prefix":"","firstName":"Zhiliang","middleName":"","lastName":"He","suffix":""},{"id":548284477,"identity":"e08f8f04-7500-45f2-848b-b0e1782c18d5","order_by":5,"name":"Nan Wang","email":"","orcid":"","institution":"State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Nan","middleName":"","lastName":"Wang","suffix":""},{"id":548284478,"identity":"0277a191-2fca-40c8-9c84-812e751d11e3","order_by":6,"name":"Dongxv Zhang","email":"","orcid":"","institution":"State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Dongxv","middleName":"","lastName":"Zhang","suffix":""},{"id":548284479,"identity":"e0ce7cc8-20e1-432a-af21-38bb103e3df1","order_by":7,"name":"Tian Jiang","email":"","orcid":"","institution":"China University of Geosciences, 29 Xueyuan Road, Beijing","correspondingAuthor":false,"prefix":"","firstName":"Tian","middleName":"","lastName":"Jiang","suffix":""},{"id":548284480,"identity":"8cdd48c0-c73a-4aed-bc89-cd4c3f964945","order_by":8,"name":"Liang Gao","email":"","orcid":"","institution":"China University of Geosciences (Beijing)","correspondingAuthor":false,"prefix":"","firstName":"Liang","middleName":"","lastName":"Gao","suffix":""},{"id":548284481,"identity":"82c4ece4-04ce-4ec7-924c-3dbeb479f346","order_by":9,"name":"Qifei Wang","email":"","orcid":"","institution":"State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Qifei","middleName":"","lastName":"Wang","suffix":""},{"id":548284482,"identity":"e4fa9936-91d6-4257-beee-dbbd9735e888","order_by":10,"name":"Haichun Zhang","email":"","orcid":"https://orcid.org/0000-0003-4553-8317","institution":"Nanjing Institute of Geology and Paleontology","correspondingAuthor":false,"prefix":"","firstName":"Haichun","middleName":"","lastName":"Zhang","suffix":""},{"id":548284483,"identity":"b75574d4-c707-400f-81c8-648b82444a9c","order_by":11,"name":"Xiaoqiao Wan","email":"","orcid":"","institution":"China University of Geosciences","correspondingAuthor":false,"prefix":"","firstName":"Xiaoqiao","middleName":"","lastName":"Wan","suffix":""},{"id":548284484,"identity":"52159e8d-0c35-438d-823e-1688e2b54bfc","order_by":12,"name":"Bo Wang","email":"","orcid":"https://orcid.org/0000-0002-8001-9937","institution":"Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Bo","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2025-11-09 20:10:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8071084/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8071084/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":96805629,"identity":"55710b6b-ce10-4272-8dcb-d7f8e4d43811","added_by":"auto","created_at":"2025-11-26 09:10:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":494035,"visible":true,"origin":"","legend":"\u003cp\u003ePalaeogeographic map of the K/Pg boundary, indicating the locations of Seymour Island, the Um Sohryngkew section of Meghalaya, NE India (where CIA studies have been documented\u003csup\u003e28\u003c/sup\u003e), as well as Sites 384, 577, 356, and 528, which have been analysed using Sr isotope data\u003csup\u003e7\u003c/sup\u003e. The locations of the Deccan Traps (orange area) and the Chicxulub Crater (red star) are also marked. The global palaeogeography Mollweide projection base map of the K/Pg boundary is used with permission from the license holder ©2016 Colorado Plateau Geosystems, Inc.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8071084/v1/7d3b5e232b494ec9930afe21.png"},{"id":96805620,"identity":"f8ea5c91-c474-445a-bf46-0ab6b2fd2a52","added_by":"auto","created_at":"2025-11-26 09:10:34","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":381493,"visible":true,"origin":"","legend":"\u003cp\u003eGeochemical profiles at Filo Negro, James Ross Island. (A) Astronomical time scale with 41-kyr obliquity cycles, with the K/Pg boundary anchored at 0 kyr. (B) Ir concentrations. (C) CIA index and five intervals. (D) δ\u003csup\u003e26\u003c/sup\u003eMg values. (E) Organic carbon isotope (δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eorg\u003c/sub\u003e) values. (F) TOC values. (G) Illite chemistry index (Ill chem). (H) Smectite/illite ratio. (I) Clay mineral percentages. The purple band indicates the stratigraphic layer with the iridium spike.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8071084/v1/ffcbea2966b05ab2eddbb6be.png"},{"id":96805634,"identity":"769a9363-ddc6-45e6-b14f-8db9f4a3c254","added_by":"auto","created_at":"2025-11-26 09:10:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":72756,"visible":true,"origin":"","legend":"\u003cp\u003eUntuned Th/U series from the Filo Negro section, with interpreted 41-kyr obliquity cycles (red). The corresponding 2π MTM power spectrum illustrates the dominant ~7 m cyclicity (Gaussian passband: 0.143 ± 0.03 cycles/m).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8071084/v1/98f4c885bc61464296f4f44a.png"},{"id":96922961,"identity":"17a1eb92-68c3-406a-9fb5-4d01ee96dbfa","added_by":"auto","created_at":"2025-11-27 14:20:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1472320,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8071084/v1/27cd7adf-9c00-4a10-8ca6-0eaccbd08758.pdf"},{"id":96805622,"identity":"4d613e13-166b-4c34-8e43-9770c5fd04c2","added_by":"auto","created_at":"2025-11-26 09:10:34","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":69167,"visible":true,"origin":"","legend":"Dataset 1","description":"","filename":"SupplementaryFig.2.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-8071084/v1/e59654befd0e31b809af15dc.xlsx"},{"id":96917097,"identity":"4afef0e1-89dc-46e7-8175-f8998e9dcb96","added_by":"auto","created_at":"2025-11-27 14:09:16","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":740535,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8071084/v1/703520ede3ce143df61bf332.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Unstable climate for 50,000 years after Chicxulub impact: evidence from Antarctica","fulltext":[{"header":"Main","content":"\u003cp\u003eThe trigger of the ~66\u0026thinsp;Ma Cretaceous/Paleocene (K/Pg) boundary crisis is widely thought to have been linked with the Chicxulub impact\u003csup\u003e1, 2, 3\u003c/sup\u003e and/or Deccan Traps eruptions\u003csup\u003e4, 5, 6\u003c/sup\u003e The former likely caused a global winter at the K/Pg boundary, resulting from the combined effects of impact-generated silicate dust, sulfur, and soot from wildfires on the post-impact climate\u003csup\u003e3\u003c/sup\u003e. The finest component of the ejected dust may have persisted in the atmosphere for up to 15 years, causing a decrease in global average surface temperatures by up to 15\u0026deg;C, and a period of global darkness that resulted in a shutdown or reduction of primary productivity\u003csup\u003e3\u003c/sup\u003e. However, the high-resolution changes in chemical weathering following the Chicxulub impact on a decadal to millennial scale are poorly studied owing to the poor temporal resolution of most sedimentary archives. Previous research has primarily focused on continental weathering at the boundary. Thus, Martin and Macdougall\u003csup\u003e7\u003c/sup\u003e suggested there was enhanced weathering caused by acid rain that resulted in the rapid addition of radiogenic Sr to the oceans (Fig. 1), although the duration of this \u0026ldquo;rapid\u0026rdquo; pulse is not quantified.\u003c/p\u003e\n\u003cp\u003eTo investigate environmental changes following the Chicxulub impact, we conducted cyclostratigraphic analysis to obtain a high-resolution age model for the Seymour Island K/Pg succession. Then we have assessed chemical weathering changes based on high-resolution analysis of the chemical index of alteration (CIA) and Mg isotopes in the expanded K/Pg boundary succession on Seymour Island, Antarctica. The CIA is a widely applied proxy, because chemical weathering of the upper crust is dominated by feldspar degradation and the concomitant formation of clay minerals; Ca, Na, and K are removed from feldspars by aggressive soil solutions\u003csup\u003e8\u003c/sup\u003e. This process increases the proportion of alumina relative to alkalis in the weathered product, a change that is recorded by higher CIA values\u003csup\u003e8\u003c/sup\u003e.\u0026nbsp;The Mg isotopic composition of the primary components of the upper continental crust falls within a narrow range from \u0026minus;0.3\u0026permil; to \u0026minus;0.1\u0026permil;\u003csup\u003e9\u003c/sup\u003e, thereby minimising source-related biases that could otherwise influence CIA values.\u0026nbsp;By contrast, chemical weathering induces substantial Mg isotope fractionation, with variations of up to ~2\u0026permil;\u003csup\u003e10\u003c/sup\u003e.\u0026nbsp;Sedimentary rocks display large isotopic heterogeneity: clastic rocks range from \u0026minus;0.75 to +0.92\u0026permil;\u003csup\u003e11\u003c/sup\u003e. Consequently, Mg isotopes are regarded as a\u0026nbsp;reliable\u0026nbsp;tracer of chemical weathering processe\u003csup\u003e10, 11, 12\u003c/sup\u003e. Fractionation of Mg isotopes results in the preferential retention of heavy \u003csup\u003e26\u003c/sup\u003eMg in weathering residues, resulting in the \u0026delta;\u003csup\u003e26\u003c/sup\u003eMg values of weathering residues increasing\u0026nbsp;progressively with greater chemical weathering intensity\u003csup\u003e13\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGeological Setting\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSeymour Island lies within the Weddell Sea, to the northeast of the Antarctic Peninsula (Fig. 1). The James Ross Basin contains a 6\u0026ndash;7 km-thick sedimentary succession, spanning the Upper Cretaceous to the lowest Oligocene, that records a range of shallow marine palaeoenvironments\u003csup\u003e14, 15, 16\u003c/sup\u003e. Within this succession, the\u0026nbsp;Upper Cretaceous\u0026ndash;Paleocene Marambio Group\u0026mdash;comprising the Haslum Crag, L\u0026oacute;pez de Bertodano, and Sobral formations\u0026mdash;consists primarily of fine-grained clastic sediments deposited in a marine shelf environment\u003csup\u003e17\u003c/sup\u003e. The L\u0026oacute;pez de Bertodano Formation (Maastrichtian to\u0026nbsp;lower\u0026nbsp;Paleocene)\u003csup\u003e18\u003c/sup\u003e reaches over 1,000 m in thickness, and is composed of grey to tan, friable, sandy, muddy siltstone, informally subdivided into 10 informal units on palaeontological and sedimentological grounds\u003csup\u003e19\u003c/sup\u003e. The lower six units, known as the Rotularia Units, have a sparse macrofauna other than the abundant calcareous tubes of the eponymous annelid, and formed in a shallow marine setting near a delta or estuary\u003csup\u003e19\u003c/sup\u003e. Units 7\u0026ndash;10, known as the Molluscan Units, contain a rich marine macrofauna\u003csup\u003e19\u003c/sup\u003e. The depositional environment deepened progressively from Units 7\u0026ndash;9, transitioning from middle (Units 7\u0026ndash;8) to outer shelf facies (Unit 9) before Unit 10 records a return to a middle to inner shelf setting\u003csup\u003e19\u003c/sup\u003e. The age of the L\u0026oacute;pez de Bertodano Formation has been determined using both biostratigraphy\u003csup\u003e20\u003c/sup\u003e and magnetostratigraphy\u003csup\u003e21\u003c/sup\u003e. Seymour Island is an excellent location for studying the K/Pg boundary due to the preservation of a complete and expanded stratigraphic record, and the abundantly fossiliferous succession\u003csup\u003e22\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFilo Negro Section and the K/Pg Boundary\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Filo Negro section of this study (Fig. 1, Supplementary Fig. 1) encompasses the upper Maastrichtian and lowermost Danian portion of the L\u0026oacute;pez de Bertodano Formation, where an iridium anomaly\u003csup\u003e23\u003c/sup\u003e, serves as the a marker for the K/Pg boundary\u003csup\u003e24\u003c/sup\u003e. The succession consists of muddy to very fine-grained sandstones that exhibit slight variations in glauconite content and colour, along with carbonate concretions\u003csup\u003e25\u003c/sup\u003e that record continuous deposition\u003csup\u003e26, 27\u003c/sup\u003e, in a shallow marine platform setting\u003csup\u003e17\u003c/sup\u003e. The\u0026nbsp;iridium anomaly that marks the K/Pg transition occurs\u0026nbsp;within a 20\u0026ndash;30 cm thick\u0026nbsp;interval coinciding with the first\u0026nbsp;appearance of Paleocene dinoflagellate cysts\u003csup\u003e23\u003c/sup\u003e. Dinocyst biostratigraphy, integrated with geochemical proxies (e.g., Ir anomaly and siderophile element enrichment), refines the K/Pg boundary to a narrow interval between 9.5 and 9.6 m at Filo Negro\u003csup\u003e25\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSignatures of weathering\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe conducted detailed measurements of stratigraphic profiles and a systematic collection of samples at the Filo Negro Section (James Ross Basin). Platinum group element (PGE) analysis confirmed the iridium spike at a height of 9.300 to 9.600 m (Fig. 2) whilst the Danian dinocyst \u003cem\u003eDanea californica\u003c/em\u003e is at 9.600 m height\u003csup\u003e23\u003c/sup\u003e. Time series analysis of the untuned thorium/uranium (Th/U) ratios from the L\u0026oacute;pez de Bertodano Formation indicates a dominant cyclicity with a wavelength of ~7.2 m. Existing magnetochronological data place the K/Pg boundary within this section in Magnetic Chron 29r, with the base positioned ~45 m below the K/Pg\u003csup\u003e21, 22\u003c/sup\u003e, yielding an average sedimentation rate of 17.8 cm/kyr\u003csup\u003e21\u003c/sup\u003e. Using this value, the ~7.2 m cycles are considered to record the 41-kyr obliquity cycle (see Materials and Methods), with our measured Filo Negro section recording 99.3 kyr (Fig. 2).\u003c/p\u003e\n\u003cp\u003eWe calculated CIA values and divided the entire section into five intervals (Fig. 2), each reflecting different intensities of chemical weathering. In interval I, from 0 m to 9.60 m, the CIA values are stable within a narrow range between 56.3 and 61.1. In interval II (9.60\u0026ndash;11.30 m) immediately above the iridium spike, CIA values decrease to 51.2 to 55.6. In the short interval III, from 11.30 m to 11.65 m, CIA values slightly increase, ranging from 55.4 to 60.1. The interval IV, between 11.65 m and 14.90 m, shows a more substantial decline, with CIA values ranging from 49.5 to 54.5, suggesting a continued reduction in weathering intensity. Finally, in interval V, from 14.90 m to 17.20 m, CIA values rise again, fluctuating between 55.9 and 71.3, with a significant peak value of 71.3 at 15.20 m. This peak indicates a period of more intense chemical weathering, which then gradually decreases toward the top of the section at 17.2 m, where the CIA value is 58.4, values close to those at the base of the section.\u003c/p\u003e\n\u003cp\u003eAs additional evidence, we calculated the illite chemistry index\u003csup\u003e29\u003c/sup\u003e (Fig. 2) to assess the intensity of chemical weathering\u003csup\u003e30\u003c/sup\u003e. The interval I is characterised by a relatively high illite chemistry index, ranging from 0.19 to 0.51, below the K/Pg boundary. In interval II, the index narrows and declines to a range of 0.10 to 0.30 before increasing slightly in interval III: values vary between 0.19 and 0.51, and they are similar in interval IV (0.11 to 0.25). Finally, the interval V is marked by initially high values with a peak of 1.07 at 15.20 m height. There is then a gradual decrease toward the top of the section at 17.2 m, where the illite chemistry index is 0.30, similar to values seen in the Cretaceous.\u003c/p\u003e\n\u003cp\u003eA total of fifty-one Mg isotope values were obtained (Fig. 2). These exhibit trends consistent with the CIA and illite chemistry index values, with the exception of interval I. Here, the Mg isotope values are rather variable, ranging from 0.01\u0026permil; to 0.22\u0026permil;. In interval II, there is a decrease in Mg isotope values, which drop from -0.07\u0026permil; to 0.11\u0026permil;. The interval III sees an increase in Mg isotope values, ranging from -0.10\u0026permil; to 0.34\u0026permil;. The interval IV shows a more significant decline, with values ranging from -0.11\u0026permil; to 0.02\u0026permil;. Finally, in interval V, Mg isotope values rise once again, fluctuating between 0.02\u0026permil; and 0.40\u0026permil;, with a notable peak of 0.40\u0026permil; at 15.20 m.\u003c/p\u003e\n\u003cp\u003eFour main clay minerals (illite, kaolinite, chlorite, and smectite) were identified by X-ray diffraction analyses of the clay fraction (\u0026lt; 2 \u0026mu;m) (Fig. 2). In interval I, the illite, kaolinite, chlorite, and smectite content remains relatively stable. In interval II, the clay mineral composition becomes more variable, with a noticeable increase in illite content and a corresponding decrease in smectite. In interval III, this trend culminates in a mineral assemblage overwhelmingly dominated by smectite, while the illite content declines to its lowest levels. The interval IV is characterised by a reversal of the previous trend, with a sharp decline in smectite content and a renewed increase in illite. Finally, in interval V, there is a sharp increase in smectite content again, which dominates the clay mineral assemblage.\u003c/p\u003e\n\u003cp\u003eThe \u0026delta;\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eorg\u003c/sub\u003e data exhibit distinct variations (Fig. 2). The interval I shows stable \u0026delta;\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eorg\u003c/sub\u003e values, ranging from -26.01\u0026permil; to -24.71\u0026permil;, with a minor negative excursion observed just before the K/Pg boundary. Interval II has lower values, ranging from -25.63\u0026permil; to -24.39\u0026permil;, with a minor negative excursion occurring after the K/Pg boundary. Interval III shows consistently declining values as \u0026delta;\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eorg\u003c/sub\u003e values drop from -25.78\u0026permil; to -24.89\u0026permil;, a trend that continues into interval IV, where values range from -26.83\u0026permil; to -25.99\u0026permil;. Finally, in interval V, there is a recovery of \u0026delta;\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eorg\u003c/sub\u003e values to values from -27.38\u0026permil; to -25.95\u0026permil;.\u003c/p\u003e\n\u003cp\u003eThe total organic carbon (TOC) profile shows notable variations (Fig. 2). The interval I exhibits moderately stable TOC values, ranging from 0.36% to 0.59%. In contrast, the interval II demonstrates a significant depletion in TOC values, varying from 0.15\u0026ndash;0.43%. The interval III marks recovery of values to 0.32\u0026ndash;0.50% before they decline again in the interval IV to 0.23\u0026ndash;0.39%. Finally, the interval V is characterised by the lowest TOC values of the profile, with values ranging from 0.18\u0026ndash;0.35\u0026permil;.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eWeathering fluctuations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe intensity of chemical weathering, assessed by the CIA proxy, was relatively stable in the mainly latest Cretaceous (interval I) before declining above the K/Pg Ir spike (interval II), suggesting that weathering reduced after the Chicxulub impact. The brief interval III sees a slight recovery to Cretaceous values, but is followed by a substantial decline in interval IV. During interval V, CIA values rise sharply, peaking at 15.20 m in the section before gradually declining. The fluctuations in CIA are closely matched by those shown by the other weathering proxies (illite chemistry index and \u0026delta;\u0026sup2;⁶Mg values), providing good corroboration of these trends. Thus, the stable weathering regime of the latest Cretaceous was replaced by dynamic and fluctuating conditions in the first ~43.89 kyr of the Paleocene. A lack of correspondence between smectite/illite ratios (indicative of volcanic input) and the consistent REE (rare earth elements) patterns (see Supplementary Figs. 2\u0026ndash;3) indicates that the chemical weathering proxies record climatic rather than provenance changes.\u003c/p\u003e\n\u003cp\u003ePalaeontological and palaeotemperature data from Seymour Island also support the inferred climate record at Filo Negro. During the Maastrichtian, a relatively humid climate prevailed, as indicated by fossil plant evidence that suggests a lowland to mid-altitude vegetation of mixed podocarp-southern beech temperate rainforest was growing on the eastern flank of the Antarctic Peninsula arc\u003csup\u003e31\u003c/sup\u003e. Mean annual air temperatures during this time were 16.0\u0026ndash;16.5\u0026deg;C based on MBT\u0026rsquo;/CBT (methylation index of branched tetraethers/cyclisation ratio of branched tetraethers) data\u003csup\u003e32\u003c/sup\u003e. There is a transition to cooler and drier climates in the Danian recorded by increased prevalence of the high-latitude pollen \u003cem\u003eNothofagidites\u003c/em\u003e spp. and appearance of the cold-water dinocyst \u003cem\u003eImpletosphaeridium clavus\u003c/em\u003e\u003csup\u003e32\u003c/sup\u003e. There was also an abrupt decrease in Humidity Index values (this weighted averaging function uses spore and pollen taxa with affinities to known hydrological groups\u003csup\u003e31\u003c/sup\u003e). The MBT\u0026rsquo;/CBT palaeothermometer and clumped isotope data from marine bivalves indicate a ~2-degree drop in air temperatures during interval II\u003csup\u003e32, 33\u003c/sup\u003e. Thus, the decreased weathering intensity of interval II corresponds to cooling and decreased humidity. No temperature data is available for younger levels, but our study shows that climatic oscillations continued to persist for at least a further 43.89 kyrs, indicating a long term disruption to climate stability after the K/Pg boundary.\u003c/p\u003e\n\u003cp\u003eDisruption of weathering regimes after the K/Pg boundary is also observed in marine sediments from NE India, where stable CIA values (70\u0026ndash;80) in the uppermost Maastrichtian decline sharply above the K/Pg boundary\u003csup\u003e28\u003c/sup\u003e to persistently low values (30\u0026ndash;40), during the early Danian including the P0 zone (ca. the first 30 kyr\u003csup\u003e34\u003c/sup\u003e) and part of P1a (1))\u003csup\u003e28\u003c/sup\u003e. This corresponds to the low CIA values seen in intervals II and IV on Seymour Island.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDelayed recovery\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;after Chicxulub impact\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere has been considerable study of the aftermath of the Chicxulub impact and its effects in the oceanic realm. During the initial phase of global darkness and cooling, marine productivity was very low for the first 3 years and was likely attributable to the development of sulphate aerosols post-impact\u003csup\u003e35\u003c/sup\u003e. Productivity levels recovered rapidly after this, with the appearance of calcareous nannofossils and dinoflagellate blooms that all point to eutrophic conditions\u003csup\u003e36, 37, 38\u003c/sup\u003e. These conditions persisted for ~50 ky in shelf seas, but open ocean sites record the persistence of more oligotrophic conditions at this time\u003csup\u003e39\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe millennial-scale aftermath of the Chicxulub impact on land is less well documented, but recovery appears to have been substantially more rapid than in the oceans. Palynological data indicate that the persistence of the post-impact, opportunistic-dominated fern ecosystems was regionally variable\u003csup\u003e40\u003c/sup\u003e. In the New Zealand record, recovery is well underway within 30 ky of the boundary with angiosperm pollen becoming a significant part of palynological assemblages by this time\u003csup\u003e41\u003c/sup\u003e.\u0026nbsp;In Antarctica, fern spore abundances peak within a ~0.5 m interval (probably ~3 ky) after the K/Pg boundary\u003csup\u003e32\u003c/sup\u003e. These, and other observations\u003csup\u003e42\u003c/sup\u003e, suggest that diverse floral communities were becoming re-established within a few thousand years of impact, although full recovery took longer\u003csup\u003e43\u003c/sup\u003e. In contrast, the mammalian rebound was more delayed, with diversification only increasing notably after 100 ky\u003csup\u003e43\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eMarine and terrestrial ecosystems display different response times, but the role of continuing effects of the Chicxulub impact versus those of the Deccan eruptions has not been clarified. Volatile releases from impact are unlikely to have directly affected the climate on more than a decadal scale\u003csup\u003e2\u003c/sup\u003e. Oxygen isotope data suggest a gradual warming for the first 200 kyr of the Paleogene\u003csup\u003e2\u003c/sup\u003e, but our data show that this gradual trend was subject to higher-order fluctuations of weathering, which were likely climate driven for at least the first ~50 ky of the Paleogene. This pattern contrasts with the relative stability of the last ~50 kyr of late Maastrichtian. These trends show a close match with the eruption history of the Deccan Traps, recorded by Renne and colleagues\u003csup\u003e44\u003c/sup\u003e, in which frequent, relatively small-scale eruptions in the Maastrichtian were followed by rarer, but larger-scale eruptions in the first half-million years of the Paleogene. The increase of weathering intensity in phases III and V on Seymour Island could reflect large-scale Deccan eruptions and volcanic-CO\u003csub\u003e2\u003c/sub\u003e-driven warming. Therefore, the post-Chicxulub recovery interval was likely influenced by large-scale Deccan volcanism. The longer recovery in the marine realm may reflect this Deccan influence on climate, whilst the more rapid floral recovery suggests that terrestrial ecosystems were either less affected by the climate changes or the extinction losses were less consequential. However, this dichotomy of response could be further tested by reconstructing the weathering regime after the first ~50 ky of the Paleogene to see if the link with large-scale Deccan eruptions persisted.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003eOur samples were collected from the L\u0026oacute;pez de Bertodano Formation at the Filo Negro section (coordinates: 64\u0026deg; 17\u0026rsquo; 24\u0026rdquo; S; 56\u0026deg; 44\u0026rsquo; 30\u0026rdquo; W), James Ross Basin, on Seymour Island, Antarctica, in the austral summer of 2024.\u003c/p\u003e\u003cp\u003ePlatinum group element content\u003c/p\u003e\u003cp\u003eAbout 0.2\u0026thinsp;~\u0026thinsp;0.5 g of powders for each sample were weighed together with a mixed PGE spike (\u003csup\u003e99\u003c/sup\u003eRu, \u003csup\u003e106\u003c/sup\u003ePd, \u003csup\u003e185\u003c/sup\u003eRe, \u003csup\u003e190\u003c/sup\u003eOs, \u003csup\u003e191\u003c/sup\u003eIr, and \u003csup\u003e194\u003c/sup\u003ePt), then sealed along with inverse aqua regia in Carius tubes and subsequently heated to 240\u0026deg;C for 24 h. Ir, Ru, Pt, Pd, and Re were first purified by an AG50-X8 resin to remove matrix elements\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. The LN resin was then used to separate Zr and Hf\u003csup\u003e46\u003c/sup\u003e. The analysis of PGE abundance was performed on a secondary electron multiplier (SEM) on a Thermo Element 2 ICP-MS at the China University of Geosciences, Beijing. The uncertainties of Re-PGE contents are reported as 2SD, ranging from 0.0005 to 0.0188 ppb, based on 100 cycles conducted by the mass spectrometer for each sample.\u003c/p\u003e\u003cp\u003eMajor and trace element analysis\u003c/p\u003e\u003cp\u003eThe Inductively Coupled Plasma (ICP) Sample Digestion Procedure had the following steps: 1. 50.00 mg of rock sample were placed in a PTFE digestion vessel. A few drops of deionised water wetted the sample. A second vessel containing deionised water was prepared as a blank, with standards and duplicates. 2. Each vessel received 1.5 mL of high-purity HF and was heated on a hot plate at 120\u0026deg;C until the HF evaporated to a wet salt state to remove most of the silicon matrix. Subsequently, 1.0 mL of HNO₃ and 1.5 mL of HF were added, the vessels were sealed with lids and steel jackets, and they were digested in an oven at 190\u0026deg;C for 72 hours. 3. After cooling, the vessels were heated on a hot plate and again evaporated to a wet salt state; 1.0 mL of HNO₃ was added, and the sample was evaporated twice to eliminate residual HF. Then, 1.5 mL of HNO₃ and approximately 2 mL of water were added, the vessels were sealed, and they were held in an oven at 120\u0026deg;C for 12 hours. 4. Following cooling, the extracts were transferred to clean 50 mL centrifuge tubes, 1 mL of a 500 \u0026micro;g/L Rh internal standard solution was added, and the solutions were diluted with deionised water to 50 mL, yielding a Rh concentration of 10 \u0026micro;g/L. Major elements were measured by Agilent Technologies 710 ICP-OES (Optical Emission Spectroscopy), and trace and rare earth elements were measured by Agilent Technologies 7700x ICP-MS (Mass Spectrometry) at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Precision (RSD) for major and trace elements was mainly below 1% and did not exceed 3% for any element.\u003c/p\u003e\u003cp\u003eMagnesium isotope analyses\u003c/p\u003e\u003cp\u003eThe analyses were performed at Northwest University in Xi\u0026rsquo;an, China. Approximately 50 mg of sample was decarbonated, and ~\u0026thinsp;5 mg of decarbonated powders were used to extract silicates, and potassium (K) was removed by coprecipitation. Magnesium was purified through two sets of columns. The first column, used to elute Ca, was packed with 2 mL of Bio-Rad AG50W-X12 resin (200\u0026ndash;400 mesh)\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. The second column, packed with 0.5 mL of Bio-Rad AG50W-X12 resin (200\u0026ndash;400 mesh), was used to elute sodium (Na), aluminium (Al), iron (Fe), and titanium (Ti). Magnesium isotope measurements were performed on the Nu Plasma multi-collector inductively coupled plasma mass-spectrometer (MC-ICP-MS). Magnesium isotopic data are presented as per mil (\u0026permil;) relative to a pure Mg standard solution DSM3:\u003c/p\u003e\u003cp\u003eδ\u003csup\u003eX\u003c/sup\u003eMg (\u0026permil;) = [(\u003csup\u003eX\u003c/sup\u003eMg/\u003csup\u003e24\u003c/sup\u003eMg)\u003csub\u003esample\u003c/sub\u003e/(\u003csup\u003eX\u003c/sup\u003eMg/\u003csup\u003e24\u003c/sup\u003eMg)\u003csub\u003eDSM3\u003c/sub\u003e\u0026minus;1] \u0026times; 1000\u003c/p\u003e\u003cp\u003ewhere x refers to mass 26. Four standards (SW, alfa Mg, AGV-2, BCR-2) were analysed. All samples and standards were each analysed four times during an analytical session, producing 2SD values between 0.01\u0026permil; and 0.07\u0026permil;.\u003c/p\u003e\u003cp\u003eClay minerals\u003c/p\u003e\u003cp\u003eClay mineralogy was determined by X-ray diffraction (XRD) using a PANalytical X\u0026rsquo;Pert PRO diffractometer housed at the State Key Laboratory of Marine Geology, Tongji University. The detailed analytical procedure was described in Liu et al.\u003csup\u003e48\u003c/sup\u003e. In summary, after decarbonization (20% acetic acid) and removal of organic matter (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e), the clay fraction (\u0026lt;\u0026thinsp;2 \u0026micro;m) was extracted. The XRD measurements were performed on oriented thin sections that had been successively air dried, saturated with ethylene glycol under vacuum for 24 hr, and heated at 490\u0026deg;C for 2 hr. Semi-quantitative estimates of the peak areas of the basal reflections for the main clay mineral groups (smectite: 15\u0026ndash;17 \u0026Aring;; illite: 10 \u0026Aring;; and kaolinite/chlorite: 7 \u0026Aring;) were carried out on the ethylene glycol saturation diffractogram using MacDiff software. Kaolinite was discriminated from chlorite via the 3.57/3.54 \u0026Aring; peak area comparison, and illite chemistry was calculated using 10 \u0026Aring;/5 \u0026Aring; peak areas\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Replicate measurements produced results with a relative error of \u0026plusmn;\u0026thinsp;2% (2SD).\u003c/p\u003e\u003cp\u003eTime series analysis\u003c/p\u003e\u003cp\u003eThe investigation of astronomical periodicities utilised Acycle software v2.8\u003csup\u003e49\u003c/sup\u003e, following established cyclostratigraphic protocols\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. The Th/U ratio has been suggested as a variable proxy that combines the influence of climatic and marine redox fluctuations\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. Recognising that raw Th/U can contain significant low-frequency, non-periodic trends capable of distorting spectral estimates via power leakage, all raw data were detrended by removing linear trends to eliminate long-term variations unrelated to orbital forcing. We analysed both untuned and tuned Th/U ratios for cyclical patterns using the multi-taper method (MTM) of spectral analysis\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. To reveal the dominant wavelength of the proxy series and search for potential astronomical cycles, MTM spectra were statistically evaluated using the robust red noise modelling procedure\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. Specific frequency bands potentially corresponding to Milankovitch cycles were isolated using Gaussian and Taner bandpass filters\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. Astronomical tuning was subsequently performed by aligning the ~\u0026thinsp;7.2 m cycles to 41-kyr obliquity periods using the \u0026ldquo;Age Scale\u0026rdquo; module in \u003cem\u003eAcycle\u003c/em\u003e, establishing a robust astrochronological framework for the studied interval (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe National Natural Science Foundation of China (grants 42472053, 42322607, 42402030), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (grant 2022310), and the Fundamental Research Funds for NIGPAS (NGBS202423) supported this work.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSchulte P et al (2010) The Chicxulub asteroid impact and mass extinction at the Cretaceous\u0026ndash;Paleogene boundary. 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Clim Change 33:409\u0026ndash;445\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"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":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8071084/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8071084/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe ~\u0026thinsp;66 Ma Cretaceous/Paleogene (K/Pg) boundary coincides with a severe mass extinction widely attributed to the Chicxulub impact and consequent abrupt climatic changes. However, it is unclear how long the subsequent climatic disruptions affected terrestrial and marine environments and what caused them. Here, we present magnesium isotope (δ\u0026sup2;⁶Mg) and Chemical Index of Alteration data from an expanded section on Seymour Island, Antarctica, where a new cyclostratigraphic age model provides millennial-scale resolution. Pre-boundary weathering indices, including δ\u0026sup2;⁶Mg values, record a stable, mild weathering regime. Post-boundary weathering saw a transition to a substantially variable regime with intervals of intense weathering revealed. This environmental instability persisted for at least 43.89 kyr after impact, and may reflect the climatic influence of Deccan volcanism. Thus, whilst the Chicxulub impact can be directly implicated in mass extinction, the recovery interval was substantially affected by Deccan volcanism.\u003c/p\u003e","manuscriptTitle":"Unstable climate for 50,000 years after Chicxulub impact: evidence from Antarctica","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-26 09:10:29","doi":"10.21203/rs.3.rs-8071084/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d541b5d8-9e9a-401a-aa49-8be08c3f64dd","owner":[],"postedDate":"November 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":58341484,"name":"Earth and environmental sciences/Solid Earth sciences/Geochemistry"},{"id":58341485,"name":"Earth and environmental sciences/Climate sciences/Palaeoclimate"}],"tags":[],"updatedAt":"2025-11-26T09:10:29+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-26 09:10:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8071084","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8071084","identity":"rs-8071084","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}