Prolonged ^187Os/^188Os excursion implies hydrothermal influence after the Chicxulub impact in the Gulf of Mexico

Domain: earth_science
Published: 2025-04-08
Editorial outcome: minor_revision

Abstract
The Cretaceous/Paleogene boundary asteroid impact is recorded globally as a negative 187Os/188Os excursion, including in sediments recovered from the IODP-ICDP drilling within the peak ring of the Chicxulub structure in the Gulf of Mexico. The reconstructed marine 187Os/188Os curves can be used for global age correlations on the ~10 kyr scale. However, the versatility of Os isotope clock between the proximal and distal sites remains unclear. This paper presents 187Os/188Os records from early Paleocene sediments deposited in the Chicxulub impact basin and Mexican sites with biochronological scales. The results for these proximal sites show a recovery timescale of ~700 kyr, which is significantly longer than that of the distal sites (~200 kyr). The interval showing the 187Os/188Os decline coincides with the enrichment of hydrothermally-derived Mn, implying that hydrothermal venting at the Chicxulub structure may have played a role in the marine chemistry and ecosystem of the Gulf of Mexico.

Introduction
The impact event at the Cretaceous-Paleogene (K/Pg) boundary, ~66 million years ago, formed the ~200-km diameter Chicxulub impact structure on the Yucatán Platform of the Gulf of Mexico 1–4, causing a mass extinction of over 70% of the fossil species 5–7. The decline and subsequent recovery of productivity in the marine ecosystem has been extensively studied in the K/Pg boundary sediments on a global scale 8–11. It has been proposed that the recovery of the global marine ecosystem, measured as primary productivity, was heterogeneous 12–14. Recent micropaleontological and geochemical studies of the impact site show a rapid recovery of both the local population and export production 15–18. To understand post-impact changes in marine environments and ecosystems on a global scale, geochemical markers of the impact event, particularly highly siderophile element concentrations (HSEs: Os, Ir, Ru, Pt, Pd, and Re) and Os isotope ratios, can be used to accurately correlate K/Pg boundary sediments 5,19,20. The anomalous concentration of HSEs provides a key temporal horizon that precisely links Chicxulub to K/Pg boundary sections worldwide 7,21,22. Osmium isotope (187Os/188Os) records across the K/Pg boundary also constrain the time scale of the post-impact environmental recovery. Since the residence time of seawater Os (10–50 thousand years (kyr) 23,24) is longer than the time scale of modern oceanic circulation, seawater 187Os/188Os values are dominated by changes in whole-ocean chemistry and relatively constant throughout the global oceans. Seawater 187Os/188Os ratios reflect contributions to the global ocean from weathered continental crust (187Os/188Os=~1.4), mantle and extraterrestrial inputs (187Os/188Os=~0.12–0.13) 25. Detailed profiles of seawater 187Os/188Os ratios from the pelagic carbonate sections in the previous studies show similar Os isotope curves that drop sharply (187Os/188Os=~0.17 to 0.2) at the K/Pg boundary and then recover to their original seawater values (187Os/188Os=~0.4) over ~200 kyr 19,20,26. A model for the 200 kyr recovery of seawater 187Os/188Os in the distal sites was directly compared with age models based on biostratigraphy, magnetostratigraphy, and cyclostratigraphy, allowing independent estimates of stratigraphic time based on Os isotope analysis 20. The development of this Os isotope based geochemical clock 20 has broad implications for paleoceanographic research, providing a tool for global correlation of pelagic carbonate sequences across the K/Pg boundary, which makes it possible to elucidate the onset of biotic crisis and subsequent recovery. However, the seawater 187Os/188Os curve in the Gulf of Mexico has not been reported, and correlations using the K/Pg Os isotope clock between the proximal and distal sites remain unclear.

Here, Os isotope ratios are measured in the early Paleocene limestone samples obtained from the IODP-ICDP Expedition 364 at Site M0077 in the Chicxulub impact basin in the Gulf of Mexico and early Paleocene limestone, marl, and clay samples from several sections in northeastern and southeastern Mexico (El Mulato, La Lajilla, Bochil, and Guayal sections) to reconstruct the Os isotope reference curve in the Gulf of Mexico (Figs. 1, Supplementary Note 1) 22,27,28. Biostratigraphic correlations and calibrated ages between the Chicxulub site and the Mexican sections are calculated based on a biochronological scale relying on planktic foraminifera and astrochronological calibrations 29,30. The obtained HSE concentrations confirm the contribution of extraterrestrial materials in proximal sediments all around the Gulf of Mexico region. For comparison, Os isotope ratios and HSE concentrations are also analyzed in suevite (polymict melt-rich impact breccia), impact melt rock, and granitic samples underlying the Paleocene limestones at Site M0077 (Supplementary Figs. 1–3).

Results
Planktic foraminiferal biochronology
Planktic foraminiferal biostratigraphy of the Paleocene white limestone for Site M0077 is updated from that published by ref. 16, which was based on P zones 31. The planktic foraminiferal age model for Site M0077 relies on Dan zones 29. The lowest and highest occurrence data (LOD and HOD) of key taxa occur in the correct stratigraphic order (from Dan 2 to Dan 4c; Supplementary Figs. 4, Supplementary Data 1). However, the LOD of Eoglobigerina simplicissima, which defines the base of Dan 3b, could not be recognized in Site M0077, and the base of Dan 3b was inferred to be at 616.23 mbsf (Supplementary Figs. 4, Supplementary Data 1).

Calibrated ages are assigned to each datum in accordance with ref. 29. The age of each sample in the Paleocene white limestone is determined by average sedimentation rates, which are constrained by the stratigraphic thickness and the calibrated time duration of the planktic foraminiferal biozones (Supplementary Figs. 4, Supplementary Data 1) 29. Calculated sedimentation rates from each planktic foraminiferal biozone from the lower to the upper part of the white limestone are 0.667 cm kyr–1, 2.6 cm kyr–1, 0.125 cm kyr–1, 0.669 cm kyr–1, 0.256 cm kyr–1, 0.068 cm kyr–1, and 0.198 cm kyr–1 in ascending order (Supplementary Figs. 4, Supplementary Data 1). The high sedimentation rate calculated for Biozone Dan 3a (2.6 cm kyr–1) is anomalous, probably resulting from assuming that there is no hiatus affecting Biozone Dan 3b.

Rhenium-osmium isotopes and highly siderophile elements
Age-corrected initial Os isotope ratios (187Os/188Os) in Site M0077 are low (187Os/188Os ~0.19) in the ~3-cm-thick green-gray marlstone that transitions into the lowermost Paleocene white limestone 22 and then gradually increase in the Paleocene white limestone (Fig. 2a). 187Os/188Os recover to pre-impact value (~0.4; ref. 20) at 614.32 mbsf in the lower part of Biozone Dan 4c (Figs. 2a, Supplementary Data 2). In Biozone Dan 4c, 187Os/188Os remain constant at steady state and subsequently increase to ~0.45. Based on the sedimentation rates calculated by the planktic foraminiferal age model, the depth of 614.32 mbsf corresponds to ~65.265 Ma 29, indicating that the recovery time of the Os isotope ratio at Site M0077 occurs more than ~700 kyr after the K/Pg impact. Measured 187Os/188Os ratios of suevite and granite underlying the Paleocene white limestone are generally high, ranging from 1.1 to 4.0 (Supplementary Figs. 5, Supplementary Data 2). In contrast, the measured 187Os/188Os ratios for the impact melt rocks display significant variations, ranging from unradiogenic values of 0.18–0.19 for the lower impact melt rocks found in the granite unit (917.18 to 917.25 mbsf) to radiogenic values of 0.9–1.5 for the upper impact melt rocks, which overlies shocked granitoid target rock of the peak ring (721.01 to 744.20 mbsf; Supplementary Figs. 1 and 5, Supplementary Data 2).

Osmium isotope ratios from marl samples in the La Lajilla section of northeastern Mexico show a minimum value (187Os/188Os ~0.241) just above the K/Pg impact-induced Complex Clastic Unit (CCU) 32 and recover to ~0.3 in about 70 kyr (Figs. 3d; Supplementary Data 3). The marl samples in the El Mulato section located in northeastern Mexico show relatively high 187Os/188Os of ~0.320 at 1 cm above the CCU, and its lowest value about 15 cm above the CCU (Fig. 3c). Subsequently, the isotope ratios gradually increase to ~0.33 about 200 kyr after the impact. The Bochil section in southeastern Mexico shows the lowest 187Os/188Os values (~0.167) in the clay sample about 3 cm above the CCU and then gradually increases in the limestone and marl samples, although the Os isotopic ratios are less than 0.3 at 200 kyr after the impact (Fig. 3a). The 187Os/188Os ratios in the Guayal section located in southeastern Mexico show the lowest value just above the CCU (~0.192) in the clay sample and increase to ~0.27 in the uppermost marly limestone layer of the measured sample (~110 cm above the CCU; 80 kyr after the impact; Fig. 3b).

The Os concentrations of the Paleocene white limestone at Site M0077 gradually decrease upward throughout the limestone (Fig. 2b). The high Ir concentrations at the base of the limestone (~0.49 ppb) decrease abruptly in the lowest part of the limestone, and then remain almost constant throughout the limestone (Fig. 2b). As a result, the Os/Ir ratios of the limestones in the lower part of the study section are high compared to the ratios of upper continental crust (UCC) and CI chondrite (Fig. 2e). The concentrations of HSE show high values for Os-Ir-Ru at the base of limestone (Fig. 2), and their CI chondrite-normalized HSE patterns exhibit a relatively flat pattern (Fig. 4a) 22. Other limestone samples between 616.48-610.33 mbsf exhibit low HSE concentrations (Fig. 2). These samples are depleted in Os-Ir-Ru and enriched in Pt-Pd-Re and characterized by Os/Ir > 1 (Figs. 2 and 4a).

The Ir and Os concentrations of the impact melt rocks are generally low, similar to the average UCC values (Supplementary Fig. 6). Only two samples, 163R3_35.5–41.0 and 163R3_47.0–48.0 from the lower impact melt rocks show slight enrichments in Os and Ir concentrations (Supplementary Fig. 6). The suevite samples are relatively similar in Os and Ir concentrations to the upper impact melt rock samples. The granite sample shows significantly lower concentrations of HSE compared to UCC (Supplementary Fig. 6).

In other Gulf of Mexico sections, Os and Ir concentrations remain high up to a few cm above the CCU (Fig. 3). The Ir concentration decrease abruptly, followed by constant low values in each section (Fig. 3). In all four sections, Os/Ir ratios show high values (Os/Ir > 1) relative to CI chondrite and UCC similar to the concentration/pattern observed at Site M0077 (Supplementary Fig. 7). The CI chondrite-normalized HSE patterns show relatively flat patterns for samples immediately above the CCU, while samples at higher stratigraphic levels display coherent patterns characterized by depletion in Os-Ir-Ru relative to Pt-Pd-Re (Fig. 4b, c).

Methods
Planktic foraminiferal analyzes
The age models for Site M0077 and Mexican sections (El Mulato, La Lajilla, Bochil, and Guayal) are based on planktic foraminifera high-resolution biostratigraphy (see refs. 16,28,29; and references herein). The studied rock samples were disaggregated using two techniques; 1) using a H2O2 solution for 6 to 24 h (to disaggregate marl and clay samples), and 2) using a solution with 80% acetic acid for 4 to 6 h (to disaggregate very lithified marly limestone and limestone samples). The residual fractions were then washed through a 63 μm sieve and dried at 50 °C.

All correlations and ages were obtained following the same taxonomic and biostratigraphic criterion (Supplementary Data 1 and 3). The planktic foraminiferal biochronological scale used was magnetochronologically calibrated by refs. 29,64, and astrochronologically by ref. 30. This scale is based on the Dan biozonation of ref. 29, which used the following key-biohorizons: HOD of Abathomphalus mayaroensis (= K/Pg mass extinction event; 66.001 Ma; base of Dan 1), LOD of Parvularugoglobigerina longiapertura (65.995 Ma; base of Dan 2), LOD of Parvularugoglobigerina eugubina (65.983 Ma; base of Dan 3a), LOD of Eoglobigerina simplicissima (65.973 Ma; base of Dan 3b), LOD of Parasubbotina pseudobulloides (65.933 Ma; base of Dan 4a), LOD of Subbotina triloculinoides (65.797 Ma; base of Dan 4b), and LOD of Globanomalina compressa (65.485 Ma; base of Dan 4c). The biostratigraphic study of Site M0077 also includes the following key-biohorizons: LOD of Acarinina trinidadensis (base of homonym biozone), and LOD of Acarinina uncinata (base of homonym biozone), magnetochronologically calibrated at 63.888 and 62.902 Ma by ref. 64.

Re-Os isotope and highly siderophile element analyzes
Rhenium-osmium (Re-Os) isotope compositions and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) concentrations for 23 samples of Paleocene limestone and 16 samples (with 1 replicate) of suevite, impact melt rock, and granite within the Chicxulub impact basin, and 36 Paleocene samples from other K/Pg sites around the Gulf of Mexico (Bochil, Guayal, El Mulato, and La Lajilla sections) were determined by isotope dilution mass spectrometry (ID-MS) after quartz glass tube digestion. The methods used for sample digestion, chemical purification, and mass spectrometry are based on procedures outlined in refs. 59,65. Powdered samples (~0.25–0.5 g) and spike solutions enriched in 185Re and mixed 190Os-191Ir-99Ru-196Pt-105Pd were transferred into a quartz glass tube. After adding 3–4 mL of inverse aqua regia, each quartz glass tube was frozen in dry ice, sealed with an oxygen-propane torch, and placed in an oven at 240 °C for 72 h. Subsequently, 1–3 mL of 38% hydrofluoric acid (HF) was added to the residual solids after Os extraction for desilicification.

Isotope ratio measurements of HSE were conducted using two types of mass spectrometers. Osmium concentrations and isotopic compositions were measured by negative thermal ionization mass spectrometry (N-TIMS; Triton Plus, Thermo Fisher Scientific) at JAMSTEC, Japan. Osmium purified after CCl4 solvent extraction and microdistillation 66 was loaded in HBr on baked 99.997% Alfa Aesar Pt wire and covered with a NaOH-Ba(OH)2 activator solution. The average total procedural blank for Os was 0.32 ± 0.46 pg (n = 9, 1 SD) with a 187Os/188Os ratio of 0.152 ± 0.015. Blank corrections were applied to all analyzes, which relies on the blank associated with each batch of digestion rather than using a long-term average. The uncertainties for 187Re/188Os and 187Os/188Os were estimated by error propagation of the blank uncertainties. Blank contributions for the measured Os concentrations and 187Os/188Os ratios of the samples from the Paleocene limestone in Site M007 were mostly insignificant: less than 1.4% and 0.9%, respectively. The blank contributions for the suevite, impact melt rocks and granite samples were variable depending on their Os concentrations (0.2–4.7% for Os and 0.03–4.8% for 187Os/188Os). The blank contributions for the measured Os concentrations and 187Os/188Os ratios for the samples from other Gulf of Mexico regions were less than 0.7% and 0.3%, respectively.

Purification of Ir, Pt, Ru, Pd, and Re was accomplished in three steps: (1) anion exchange chromatography; (2) cation exchange chromatography; and (3) solvent extraction using N-benzoyl-N-phenylhydroxylamine (BPHA). HSE concentrations were measured by high-resolution ICP-MS (HR-ICP-MS; Element XR, Thermo Fisher Scientific) housed in the University of Tokyo at Komaba. For sample introduction, a combination of 100 μ/min PFA self-aspirating nebulizer and dual cyclonic/Scott double-pass spray chamber was used during all measurements, and oxide level (HfO+/Hf) was set to 1%. Sample and standard solutions were interspersed throughout the analytical sessions to monitor and correct for instrumental fractionation. The monitored masses of analyzes and interferences are 89Y, 90Zr, 95Mo, 97Mo, 99Ru, 100Ru, 101Ru, 105Pd, 106Pd, 108Pd, 111Cd, 178Hf, 185Re, 187Re, 191Ir, 193Ir, 194Pt, 195Pt, 196Pt, and 202Hg. Although all raw signal intensities of samples were mathematically corrected for the measured isobaric oxide interferences, contributions of interferences to analyte signals are mostly insignificant (<0.1%). The average total procedural blanks for the analyzed elements are 2.3 ± 1.7 pg for Ir, 1.4 ± 0.1 pg for Ru, 22 ± 7 pg for Pt, 4.0 ± 2.6 pg for Pd, and 1.9 ± 0.8 pg for Re (n = 9, 1 SD). As in the case of Os, all analyzes were blank-corrected. Blank contributions to samples from Site M0077 for Ir, Ru, Pt, Pd, and Re were <37%, <21%, <22%, <2.2%, and <20%, respectively, except for granite sample 95R1_91.0-93.0 (42% for Ir, 22% for Pt, 52% for Pd, and 0.4% for Re). The blank contributions to samples from other Gulf of Mexico regions for Ir, Ru, Pt, Pd, and Re were <3.0%, <12%, <32%, <1.1%, and <29%, respectively. The uncertainties on each sample were estimated by error propagation of the analytical uncertainties during ICP-MS measurement (2 SE) and blank correction. The accuracy of the analytical methods was evaluated by measuring basaltic (BIR-1a) and sedimentary limestone (JLs-1) reference materials (Supplementary Data 2).

Nine replicates of BIR-1a display good reproducibility (0.3% RSD for 187Os/188Os, 12% RSD for Os, 12% RSD for Ir, 4.2% RSD for Ru, 5.4% RSD for Pt, 4.5% RSD for Pd, and 1.3% RSD for Re in the ~0.5 g powdered samples; n = 9) and are in good agreement with data for larger powdered samples (~1 g; ref. 65). JLs-1 also displays good reproducibility for 187Os/188Os (0.5%) and Os (0.3%) and Re (1.5%) concentrations, whereas other HSE concentrations show larger variability (33% RSD for Ir, 53% RSD for Ru, 64% RSD for Pt, and 31% RSD for Pd in the ~0.5 g powdered samples; n = 4) due to their very low HSE concentrations.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I'm not a geochemist, but enjoyed reading this paper. The authors have provided very compelling evidence of hydrothermal activity at the Chicxulub site initiated by the massive force of the impact. What caught my attention in reading through the manuscript was the quantification of Phosphorous (P), a limiting nutrient in primary productivity. In my reading and assessment of Figures 2 and 6, the record of P concentration above the K/Pg boundary, and the declining values at 614 mbsf and further at 613.2 mbsf coincide with major assemblage changes in the calcareous plankton: calcareous nannofossils and planktic forams (see red rectangles added to these two figures on the annotated ms). Maybe this is coincidental, but maybe it suggests that recovery of high productivity after the extinction event at the Chicxulub site was in fact driven by this hydrothermal activity, the latter being the major premise of the paper. In other words, I wonder if there is biosphere recovery story here as well. There is some discussion about \"hydrothermal supply and marine ecosystem change\", but I would argue that there is some compelling parallels between the hydrothermal data presented here and the productivity record reconstructed by Lowery et al. 2021.\nI have provided some editorial suggestions for the authors to consider. Awesome science; I recommend publishing with some consideration of the arguments I've advanced about hydrothermal activity driving the early recovery of productivity at the Chicxulub impact site.\nSpecific comments:\nLine 358: is the limestone discolored, especially in the lower portion, or does it display a color change from the base to the top?\nLine 405: and maybe facilitated the very high productivity documented by Lowery et al. 2021.\nLine 406: red rectangle (~614-613.2 mbsf) = interval of fundamental changes in calcareous plankton assemblages (nannos and forams). P has a 2-step decline at the bottom and top of the red rectangle, and Mn flat-lines above the red rectangle at ~613.2 mbsf. I wonder if this decline in hydrothermal activity and decline in P facilitated the decline in productivity and observed changes in calc plankton assemblages. And it was the hydrothermal activity triggered by the impact that drove high productivity at this location, compared with the sharp decline observed at other locations. This scenario was not presented in Lowery et al. 2021, but may be impactful and relevant here in the context of how variable the productivity record (recovery of the biological pump) is across the globe as documented by a number of authors (e.g., Hull et al.; Bralower et al.).\nMark Leckie, Univ. of Massachusetts Amherst", "reviewer_2": "The K-Pg impact event caused an anomaly in the concentration of Os and a decrease in Os isotope ratios. The reason why it took ~700 kyrs for the isotope ratios to return to their pre-impact values is discussed in the article. While other HSEs, such as Ru and Ir, which have the same chemical properties, quickly returned to their pre-impact values, only Os took a longer time to recover. Also, the duration for the recovery was significantly longer than that of distal pelagic sites, where similar Os isotopic variations were reported by other researchers. The authors carefully considered the reasons for the deviations, and one possible scenario is the continuous supply of unradiogenic Os by post-impact hydrothermal activity. This scenario seems plausible, because it is consistent with many of the geochemical records presented here, and is reinforced by consideration of previous studies. I do not reject this scenario, but I feel strongly that some reinforcement is needed to support the authors' conclusions.\nOf all the HSEs in the hydrothermal scenario, only Os appears to have been supplied to Chicxulub and the Gulf of Mexico sites by hydrothermal activity. The other HSEs quickly returned to pre-impact levels after the impact, and the hydrothermalism did not cause any enrichment for the other HSEs. Why and how was only Os supplied by hydrothermal activity? The processes that produced selective enrichments of Os need to be discussed in terms of differences in their chemical behavior from the other HSEs.\nIn relation to the above comment, I am confused by the explanations for the difference in the post-impact decreasing patterns of Os and Ir, which are not consistent: in Line 322-330, the authors assumed that Os was supplied from airborne microscopic dust and impact vapor condensates, which partially dissolved into seawater, and that the subsequent weathering of these materials caused the different supply process for Os and Ir. That is, the processes in the atmosphere and oceans, and the subsequent weathering are the cause for selective supply of Os. On the other hand, (Line 343 and later) the authors also argues that Os was supplied by hydrothermal fluids for ~700 kyrs after the impact. How are these two different scenarios related? Which is more plausible should be explained.\nRelated to the above comment, I am not sure what the authors want to say in the sentence (Line 325-328) “In contrast, lager fragments of melt droplets or sheets enriched in projectile components are distributed and deposited concentrically around the impact site, possibly with larger amounts of downrange direction to the southwest”. Are they trying to explain the process of Ir supply? If so, a more careful explanation is needed as to what in this process leads to the supply of “negligible amounts of Ir” in the next sentence.\nIn the discussion in Line 320-330, it seems that the supply process of Os and other HSEs (represented by Ir) during the post-impact period was different. If so, does this mean that Os and other HSEs are contained in different mineral phases? The consideration and verification of what mineral phase each of HSEs is contained in is needed. My greatest interest is that the selective dissolution of the mineral phases by diagenesis, especially the dissolution of Os-bearing mineral, and the release and remobilization of Os, may explain the observed depth profiles of Os isotopic ratios and Os concentrations presented in this paper. The authors discuss the possibility of remobilization and reworking of HSEs (Lines 244-250). They suggested the possibility of post-depositional contamination of fine meteoritic dust particles, but other possibilities were not discussed. The possibility of selective dissolution and remobilization of Os-bearing mineral phases should be clearly rejected with evidence.\nThe main scientific question of this study, whether there is versatility of Os isotope clocks between the distal and proximal sites (Line 39-40), has not been finally judged. The authors' conclusion can be read as a denial of versatility. This point should be clearly stated in the concluding remarks.\nSpecific comments\nLine 162, Caption of Fig. 2 “HSE concentrations stratigraphy” should be re-considered.\nFig. 2, It may be better to add an axis that shows numerical ages (Ma)."}, "author_response": "Response to reviewers\nWe appreciate the time and effort that you dedicated to providing feedback on our manuscript and are grateful\nfor the insightful comments on and valuable improvements to our paper. We have incorporated/addressed all\nthe suggestions/comments. Please see below for a point-by-point response to the comments and concerns. The\ncomments are in black, and the responses are in blue.\nReviewer #1 (Remarks to the Author):\nI'm not a geochemist, but enjoyed reading this paper. The authors have provided very compelling evidence of\nhydrothermal activity at the Chicxulub site initiated by the massive force of the impact. What caught my\nattention in reading through the manuscript was the quantification of Phosphorous (P), a limiting nutrient in\nprimary productivity. In my reading and assessment of Figures 2 and 6, the record of P concentration above the\nK/Pg boundary, and the declining values at 614 mbsf and further at 613.2 mbsf coincide with major assemblage\nchanges in the calcareous plankton: calcareous nannofossils and planktic forams (see red rectangles added to\nthese two figures on the annotated ms). Maybe this is coincidental, but maybe it suggests that recovery of high\nproductivity after the extinction event at the Chicxulub site was in fact driven by this hydrothermal activity, the\nlatter being the major premise of the paper. In other words, I wonder if there is biosphere recovery story here\nas well. There is some discussion about \"hydrothermal supply and marine ecosystem change\", but I would argue\nthat there is some compelling parallels between the hydrothermal data presented here and the productivity record\nreconstructed by Lowery et al. 2021.\nWe thank the reviewer for providing useful comments and suggestions to improve our manuscript. We agree\nwith your suggestions, and we have added the discussion of the linkage between hydrothermal activity and\nrecovery of high productivity within the Chicxulub impact basin in the revised manuscript. Please refer our\nresponses to your specific comments below.\nI have provided some editorial suggestions for the authors to consider. Awesome science; I recommend\npublishing with some consideration of the arguments I've advanced about hydrothermal activity driving the\nearly recovery of productivity at the Chicxulub impact site.\nWe thank the reviewer for his constructive comments and suggestions. We have added a discussion of\nhydrothermal activity driving the early recovery of high productivity within the impact basin in the revised\nmanuscript (pages 29-30, lines 437-469), referring to Bralower et al. (2020; AGU advances). We have also\nrevised our manuscript following your editorial suggestions in the attached file.\nSpecific comments:\nLine 358: is the limestone discolored, especially in the lower portion, or does it display a color change from the\nbase to the top?\nThere is no color change from the base to the top of the section at Site M0077. Therefore, we revised manuscript\n(pages 24-25, lines 368-370) as follows:\n“Although early Paleocene limestones from Site M0077 do not show a significant change in color\n(Supplementary Fig. S2), a strong positive correlation between Os/Al and Mn/Al (R2 = 0.90) was observed for\nour data during the delayed recovery of 187Os/188Os values (614.32 to 616.57 mbsf).”\nLine 405: and maybe facilitated the very high productivity documented by Lowery et al. 2021.\nWe have added this possibility in the revised manuscript (pages 29-30, lines 437-469). Please also refer to the\nreply to the next comment.\nLine 406 (Fig. 6): red rectangle (~614-613.2 mbsf) = interval of fundamental changes in calcareous plankton\nassemblages (nannos and forams). P has a 2-step decline at the bottom and top of the red rectangle, and Mn flat-\nlines above the red rectangle at ~613.2 mbsf. I wonder if this decline in hydrothermal activity and decline in P\nfacilitated the decline in productivity and observed changes in calc plankton assemblages. And it was the\nhydrothermal activity triggered by the impact that drove high productivity at this location, compared with the\nsharp decline observed at other locations. This scenario was not presented in Lowery et al. 2021, but may be\nimpactful and relevant here in the context of how variable the productivity record (recovery of the biological\npump) is across the globe as documented by a number of authors (e.g., Hull et al.; Bralower et al.).\nRegarding the relationship between hydrothermal activity and the early recovery of productivity at the\nChicxulub impact site, we have revised the manuscript (pages 29-30, lines 437-446) in accordance with the\nreviewer's comments as follows:\n“Interestingly, the time scale for complete recovery from the impact induced Os isotope excursion coincides\nwith a transition in the marine ecosystem from eutrophic to oligotrophic conditions (Fig. 6e, f). … Indeed, the\ndeclining pattern of P concentration in early Paleocene limestone at Site M0077 at 614 mbsf and further at 613.2\nmbsf coincides with major assemblage changes in the calcareous nannoplankton and planktic foraminifera\ntrophic group (Fig. 6c, e, f). While this timing may be coincidental, these covariations suggest that the\nhydrothermal activity in the impact basin led to large-scale shifts in the dominant nannoplankton and planktic\nforaminiferal trophic groups, and that the hydrothermal supply of phosphorus, a limiting nutrient in primary\nproductivity facilitated the early recovery of diverse life, as speculated by Ref.17.\nOur study infers that there are some intimate relationships between the impact-induced hydrothermal activity\nand the biosphere recovery within the Chicxulub impact basin. However, in order to test the scenario that\nhydrothermal supply of nutrients and energy was a key driver for changes in the biota of the marine surface\nenvironment, it is necessary to clarify how these nutrients were supplied by the hydrothermal system and entered\nthe euphotic zone to drive enhanced primary production. Because there are no large-scale hydrothermal systems\nin the oceans today comparable to that formed by the Chicxulub impact, sedimentary records from variable sites\nin nearby oceans are critical to examining the extents of hydrothermal effects on the marine surface environment.\nFor example, there is the problem of why similar trends in export productivity (e.g., Ba/Ti ratio) are observed\nin the Caribbean Sea63, far from the hydrothermal system, which cannot be explained if we assume that the\nsupply of nutrients from the hydrothermal system was limited to the Gulf of Mexico. Thus, further research on\nOs isotope and other geochemical profiles in different sites such as the Caribbean Sea and the Pacific Ocean are\nclearly required to confirm the versatility of the Os isotope clocks and to reconstruct the recovery patterns of\nthe marine ecosystem.”\nComments and suggestions in the attached file:\nLine 1: and driver of high productivity??\nWe added a description of the possibility of hydrothermal influence as a driver of high productivity within the\nimpact basin, but we kept the title of our manuscript because the title is required to be 15 words or fewer.\nLine 42: biogeochronological scales?\nIn our manuscript, we used the term “biochronological scales” based on planktic foraminifera and\nastrochronological calibrations following Arenillas et al. (2022).\nLine 57: of the impact site?\nWe modified “for the impact site” to “of the impact site” in the revised manuscript (line 57).\nLine 110: the\nWe modified “based on the P zones” to “based on P zones” in the revised manuscript (line 110).\nLine 111: the\nWe modified “relies on the Dan zones” to “relies on Dan zones” in the revised manuscript (line 111).\nLine 121: biozones\nWe modified “biozones” to “biozone” in the revised manuscript (line 121).\nLine 123: 0.125 cm/kyr\nWe modified “0.125cm/kyr” to “0.125 cm kyr–1” in the revised manuscript (lines 122-123).\nLine 125: the\nWe modified “for the Biozone Dan3a” to “for Biozone Dan3a” in the revised manuscript (line 125). Following\nthis suggestion, we also corrected “the Biozone” to “Biozone” in lines 126, 435, and 440 of the revised\nmanuscript.\nLine 133: the\nWe modified “in the lower part of the Biozone Dan4c” to “in the lower part of Biozone Dan4c” in the revised\nmanuscript (line 133).\nLine 134: the\nWe modified “In the Biozone Dan4c” to “In Biozone Dan4c” in the revised manuscript (line 134).\nLine 146: the\nWe modified “of the northeastern Mexico” to “of northeastern Mexico” in the revised manuscript (lines 146-\n147).\nLine 150: the\nWe modified “in the northeastern Mexico” to “in northeastern Mexico” in the revised manuscript (line 150).\nLine 153: the\nWe modified “in the southeastern Mexico” to “in southeastern Mexico” in the revised manuscript (line 153).\nLine 156: the\nWe modified “in the southeastern Mexico” to “in southeastern Mexico” in the revised manuscript (line 156).\nFig. 2: red rectangle = interval of fundamental changes in calcareous plankton composition (nannos and forams).\nFollowing your comment, we modified the manuscript as described in our response to your specific comment.\nLine 165: A21 (Arenillas et al., 2021) and W11 (Wade et al., 2011)\nWe presented references for planktic foraminiferal biozones in the revised manuscript (page 10, line 165) as\nfollows:\n“Planktic foraminiferal zonations A21 and W11 are from Ref.29 and Ref.31, respectively.”\nLine 166: please define UCC here\nWe modified “UCC” to “upper continental crust (UCC)” in the revised manuscript (line 166).\nLine 192: In other Gulf of Mexico sections,\nWe modified “In other sections of Gulf of Mexico” to “In other Gulf of Mexico sections” in the revised\nmanuscript (line 192).\nLine 203: please define CCU here\nWe modified “above the CCU” to “above the K/Pg impact-induced Complex Clastic Unit (CCU)” in the revised\nmanuscript (line 203).\nLines 216, 219, and 229: Hole 1262B\nWe modified “ODP Site 1262B” to “ODP Hole 1262B” in the revised manuscript (lines 217, 220, and 229).\nWe also corrected “ODP Site 1262B” to “ODP Hole 1262B” in the revised Fig. 1 and Fig. 5.\nLine 217: ODP Hole 690C (Maud Rise, Southern Ocean)\nWe modified “ODP Hole 690C (Southern Ocean)” to “ODP Hole 690C (Maud Rise, Southern Ocean)” in the\nrevised manuscript (line 217).\nFig.5: Hole 690C is in the Southern Ocean (not South Ocean)\nWe modified “ODP Hole 690C (South Ocean)” to “ODP Hole 690C (Southern Ocean)” in the revised Fig. 5.\nLine 427: please see comments in Figure 6 related to the red rectangle...\nPlease see our response to your specific comment for Fig. 6.\nLine 472: 65.983 Ma\nWe modified “65,983 Ma” to “65.983 Ma” in the revised manuscript (line 487).\nMark Leckie, Univ. of Massachusetts Amherst\nReviewer #2 (Remarks to the Author):\nThe K-Pg impact event caused an anomaly in the concentration of Os and a decrease in Os isotope ratios. The\nreason why it took ~700 kyrs for the isotope ratios to return to their pre-impact values is discussed in the article.\nWhile other HSEs, such as Ru and Ir, which have the same chemical properties, quickly returned to their pre-\nimpact values, only Os took a longer time to recover. Also, the duration for the recovery was significantly longer\nthan that of distal pelagic sites, where similar Os isotopic variations were reported by other researchers. The\nauthors carefully considered the reasons for the deviations, and one possible scenario is the continuous supply\nof unradiogenic Os by post-impact hydrothermal activity. This scenario seems plausible, because it is consistent\nwith many of the geochemical records presented here, and is reinforced by consideration of previous studies. I\ndo not reject this scenario, but I feel strongly that some reinforcement is needed to support the authors'\nconclusions.\nWe thank the reviewer for very useful and constructive comments and suggestions to improve our manuscript.\nRegarding the chemical properties of HSEs noted by the reviewer, it is known that Os, Ir, and Ru have similar\nchemical properties in high-temperature processes such as mantle melting (>1300˚C). However, in low-\ntemperature processes occurring at the Earth’s surface environment, they show quite different chemical\nbehavior. For example, the estimated residence times of Os and Ir in the modern ocean are ~40 kyr and ~2 kyr,\nrespectively (Levasseur et al., 1999; Lee et al., 2003). There is thus a clear fractionation of Os from Ir in\nprecipitation out of seawater in sedimentary sections. Since our Os and Ir profiles are consistent with their\nresidence times, we have added the below sentence in the revised manuscript (pages 18-19, lines 261-264):\n“Indeed, distinct declining trends between Os and Ir abundances throughout the Paleocene sediments are\nbroadly consistent with their contrasting residence times estimated from the modern ocean of ~40 kyr for Os\nand ~2 kyr for Ir23,36 (Figs. 2 and 3), indicating that the hydrogenous component dominates the sedimentary\nbudget.”\nOf all the HSEs in the hydrothermal scenario, only Os appears to have been supplied to Chicxulub and the Gulf\nof Mexico sites by hydrothermal activity. The other HSEs quickly returned to pre-impact levels after the impact,\nand the hydrothermalism did not cause any enrichment for the other HSEs. Why and how was only Os supplied\nby hydrothermal activity? The processes that produced selective enrichments of Os need to be discussed in\nterms of differences in their chemical behavior from the other HSEs.\nIn the original manuscript, we invoked that alteration and weathering processes resulted in the selective\nenrichment of Os, since Os is known to be highly mobile during these processes relative to Ir and Ru (e.g.,\nWallace et al., 1990; Jaffe et al., 2002; Wimpenny et al., 2007; Aiglsperger et al., 2021; Feignon et al., 2022).\nTherefore, we have modified the descriptions in the revised manuscript (page 22, lines 322-323 and page 27,\nlines 409-410) as follows:\n“Considering that Os is known to be highly mobile during alteration and weathering processes44,45, there are\ntwo possible mechanisms to acquire a large but continuous input of meteoritic Os with unradiogenic\ncomposition into the Gulf of Mexico: ...”\n“These deviations can be attributed to the selective hydrothermal remobilization of Os but not for Ir.”\nIn relation to the above comment, I am confused by the explanations for the difference in the post-impact\ndecreasing patterns of Os and Ir, which are not consistent: in Line 322-330, the authors assumed that Os was\nsupplied from airborne microscopic dust and impact vapor condensates, which partially dissolved into seawater,\nand that the subsequent weathering of these materials caused the different supply process for Os and Ir. That is,\nthe processes in the atmosphere and oceans, and the subsequent weathering are the cause for selective supply\nof Os. On the other hand, (Line 343 and later) the authors also argues that Os was supplied by hydrothermal\nfluids for ~700 kyrs after the impact. How are these two different scenarios related? Which is more plausible\nshould be explained.\nIn the original manuscript, we do not claim that airborne microscopic dust and impact vapor condensates\nenriched in HSEs were the source for the selective supply of unradiogenic Os only to the Gulf of Mexico\nseawater. This is simply because they were transported globally after the K/Pg impact. In addition, we\nmentioned that the stratigraphic variations in HSE concentrations for the Gulf of Mexico sites do not support\nthe idea that the reduced Os isotope ratios during ~700 kyr were caused by reworking of the K/Pg deposits\nenriched in chondritic dust particles. As a result, we hypothesized that Os was supplied from larger fragments\nof melt droplets deposited concentrically around the impact site, or from the melt sheets or melt-rock deposits\nformed within the impact crater. For the process of selective enrichment of Os, please refer to the reply to the\ncomment above.\nFor possible mechanisms for the continuous supply of unradiogenic Os into the Gulf of Mexico, we discussed\n(1) weathering of impact ejecta deposited on the continents, or (2) venting of hydrothermal fluids beneath the\nChicxulub impact basin. We concluded that the first mechanism was not significant because the impact melt\nrocks enriched in HSE have only been discovered within from the Chicxulub crater (Koeberl et al., 1994).\nFurthermore, the larger 187Os/188Os excursion detected in the limestones from Site M0077 (within the basin)\ncompared to those from other Mexican sites (nearby continents) seems likely the result of regional differences\nin the effects of hydrothermal fluids generated beneath the impact basin. The explanations for this interpretation\nwere not clear in the original manuscript, we have modified and added the descriptions in the revised manuscript\n(pages 22-23, lines 333-346) as follows:\n“Although their compositions and constituent minerals/phases remain unknown, it is possible that the\nincongruent weathering of these materials could have released significant and negligible amounts of\nunradiogenic Os and Ir, respectively. If such Os were transported with some time lag by riverine flow to the\nGulf of Mexico47, this could explain the delayed recovery of 187Os/188Os after the K/Pg impact observed in our\nstudied sections. However, this mechanism may not be effective because the impact melt rocks enriched in HSE\nhave rarely been observed on the continents surrounding the impact site, in contrast to their occurrence within\nthe Chicxulub impact basin45,48. Furthermore, our data demonstrate that the Mexican sites distributed on the\ncontinental margin show smaller 187Os/188Os excursions than that of Site M0077, located in the impact basin\n(Fig. 5b). In other words, the largest 187Os/188Os excursion for Site M0077 among the study sites likely indicates\nthat the crater hydrothermal mechanism is the dominant process for introducing the unradiogenic Os into the\nGulf of Mexico seawater.”\nRelated to the above comment, I am not sure what the authors want to say in the sentence (Line 325-328) “In\ncontrast, lager fragments of melt droplets or sheets enriched in projectile components are distributed and\ndeposited concentrically around the impact site, possibly with larger amounts of downrange direction to the\nsouthwest”. Are they trying to explain the process of Ir supply? If so, a more careful explanation is needed as\nto what in this process leads to the supply of “negligible amounts of Ir” in the next sentence.\nLarge fragments of melt droplets are estimated to have been deposited around the impact site (Artemieva and\nMorgan, 2020). Thus, greater amounts of HSE would have been deposited proximally on the continental shelf\nand central Mexican continental block, and Os leached out from these sources could have been delivered to the\nGulf of Mexico seawater. However, as described above, HSE-enriched impact melt rocks have rarely been\nobserved in the K/Pg boundary sections of nearby continents.\nWe have added the explanation for the selective enrichment of Os in the discussion of the hydrothermal process.\nPlease refer to the reply to your comment above.\nIn the discussion in Line 320-330, it seems that the supply process of Os and other HSEs (represented by Ir)\nduring the post-impact period was different. If so, does this mean that Os and other HSEs are contained in\ndifferent mineral phases? The consideration and verification of what mineral phase each of HSEs is contained\nin is needed. My greatest interest is that the selective dissolution of the mineral phases by diagenesis, especially\nthe dissolution of Os-bearing mineral, and the release and remobilization of Os, may explain the observed depth\nprofiles of Os isotopic ratios and Os concentrations presented in this paper. The authors discuss the possibility\nof remobilization and reworking of HSEs (Lines 244-250). They suggested the possibility of post-depositional\ncontamination of fine meteoritic dust particles, but other possibilities were not discussed. The possibility of\nselective dissolution and remobilization of Os-bearing mineral phases should be clearly rejected with evidence.\nIt is difficult to hypothesize which mineral phases host the HSE in impact melt rocks because HSE-enriched\nmelt rock samples are rarely recovered from the K-Pg boundary sites. However, it should be emphasized here\nthat selective leaching of Os from silicate matrix is common process as described in Feignon et al. (2022).\nTherefore, this question is beyond the scope of this paper.\nThe main scientific question of this study, whether there is versatility of Os isotope clocks between the distal\nand proximal sites (Line 39-40), has not been finally judged. The authors' conclusion can be read as a denial of\nversatility. This point should be clearly stated in the concluding remarks.\nTo assess the versatility of the Os isotope clocks between the distal and proximal sites, we need more data for\nstratigraphic variations of 187Os/188Os ratios after the K/Pg impact in other sites such as the Caribbean Sea and\nthe Pacific Ocean. Therefore, we have added the following sentence in the revised manuscript (page 30, lines\n466-469):\n“Thus, further research on Os isotope and other geochemical profiles in different sites such as the Caribbean\nSea and the Pacific Ocean are clearly required to confirm the versatility of the Os isotope clocks and to\nreconstruct the recovery patterns of the marine ecosystem.”\nSpecific comments\nLine 162, Caption of Fig. 2 “HSE concentrations stratigraphy” should be re-considered.\nWe modified the title of Fig. 2 to “Stratigraphic profiles of Re-Os isotopes, HSE concentrations, and Os/Ir ratios\nof the Paleocene white limestone at Site M0077.” in the revised manuscript (lines 161-162).\nFig. 2, It may be better to add an axis that shows numerical ages (Ma).\nWe added an axis showing numerical ages (Ma) in the revised Fig. 2 based on new biochronological scales\n(Arenillas et al., 2021; Gilabert et al., 2022)."}, {"round": 2, "reviewer_comments": {"reviewer_2": "I have read the revised paper, and find that most of the problems have been corrected and improved. I concur in accepting this manuscript in this version."}, "author_response": "Response to reviewers\nPlease see below for a response to the comments. The comments are in black, and the responses are in blue.\nReviewer #2 (Remarks to the Author):\nI have read the revised paper, and find that most of the problems have been corrected and improved. I concur\nin accepting this manuscript in this version.\nOnce again, we appreciate the time you dedicated to reviewing our revised manuscript. We also thank you for\nyour concurrence with the acceptance of the revised version of our manuscript."}]

Decision letter