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by admin | Mar 25, 2022 | mainpost, vol21

Y. Lv, S.-A. Liu, H. Wu, Z. Sun, C. Li, J.X. Fan

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Enhanced organic carbon burial intensified the end-Ordovician glaciation

Y. Lv1,

1State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China

S.-A. Liu1,

1State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China

H. Wu2,

2School of Marine Sciences, China University of Geosciences, Beijing 100083, China

Z. Sun3,

3Institute of Sedimentary Geology, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China

C. Li4,

4Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing 210008, China

J.X. Fan5

5State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

Affiliations | Corresponding Author | Cite as | Funding information

S.-A. Liu
Email: lsa@cugb.edu.cn

1State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
2School of Marine Sciences, China University of Geosciences, Beijing 100083, China
3Institute of Sedimentary Geology, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
4Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing 210008, China
5State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

Lv, Y., Liu, S.-A., Wu, H., Sun, Z., Li, C., Fan, J.X. (2022) Enhanced organic carbon burial intensified the end-Ordovician glaciation. Geochem. Persp. Let. 21, 13–17. https://doi.org/10.7185/geochemlet.2210

National Key R&D Program of China (Grant No. 2019YFA0708400), the National Natural Science Foundation of China (Grant Nos. 41730214 and 41725007) and the Fundamental Research Funds for the Central Universities (Grant No. 2-9-2020-042).

Geochemical Perspectives Letters v21 | https://doi.org/10.7185/geochemlet.2210
Received 14 October 2021 | Accepted 24 February 2022 | Published 25 March 2022

Copyright © 2022 The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0

Keywords: zinc isotopes, organic carbon burial, end-Ordovician glaciation

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Abstract

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information

The end-Ordovician (Hirnantian) glaciation, causally linked with the first of five Phanerozoic mass extinction events, is the first major Phanerozoic-glaciation with short duration and paradoxically happened under a relative greenhouse-condition. Here, we present the first zinc isotope study on both carbonate and shale successions that span the Ordovician–Silurian boundary interval in South China. Two positive shifts in Zn isotope composition are observed during two main periods of glacial maxima, indicative of two pulses of extensive carbon burial. The enhanced organic carbon burial during glacial maxima intervals might be causatively linked to cooling-induced elevation in organic carbon burial efficiency. This implies that a large oceanic organic carbon reservoir played a vital role in regulating the atmospheric pCO2, causing the Hirnantian glaciation and producing the pattern of glacial-to-deglacial change, sensitive to the temperature effect.

Figures

Figure 1 Palaeogeographic map for (a) South China and (b) the Yangtze Shelf Sea at ∼445 Ma (Zhang et al., 2016; Zou et al., 2018). Orange circle and text represents the Yihuang-1 (YH-1) section located at outer shelf in the Upper Yangtze platform of South China and connected to open sea (Li et al., 2020). Black circle and pink text represents the Wanhe (WH) carbonate section deposited on platform (Tang et al., 2017).

Figure 2 Zinc isotopic (a) mass balance and (b) major fluxes in modern oceans, modified from Little et al. (2016) and Isson et al. (2018), respectively. (c) Stratigraphy, δ66Zn and δ13C records from the YH-1 drill core and the WH section. The Late Ordovician mass extinction (LOME) includes two pulses. The durations (in kyr) of positive Zn isotope excursions during the glaciation were determined by astronomical time scale (Zhong et al., 2020). Ocean redox condition in Yihuang-1 section was reported in Li et al. (2020) based on Fe-speciation and Mo concentration data. Low δ13Ccarb value at the KYQ Bed in the WH section is considered a result of diagenetic alteration. ‘Glacial maxima’ in the carbonate section is defined based on the positive δ13Ccarb excursions.

Figure 3 (a) Modelling results of organic Zn burial fluxes based on Zn isotopic mass balance. (b) The estimated organic carbon burial flux in this study and Δ47 temperatures of seawater recorded in brachiopods (Finnegan et al., 2011). Modelling details are listed in Supplementary Information for methods.

Figure 1 Figure 2 Figure 3

View all figures and tables





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Introduction

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


The end-Ordovician (Hirnantian) glaciation (∼445 Ma) was the culmination of long-term climate cooling that had begun in the Early or Middle Ordovician, and was coeval and causally linked with the Late Ordovician mass extinction (LOME), the first of the ‘Big Five’ Phanerozoic catastrophic events (Finnegan et al., 2011

Finnegan, S., Bergmann, K., Eiler, J., Jones, D.S., Fike, D.A., Eisenman, I., Hughes, N., Tripati, A., Fischer, W.W. (2011) The magnitude and duration of Late Ordovician–Early Silurian glaciation. Science 331, 903–906. https://doi.org/10.1126/science.1200803

; Melchin et al., 2013

Melchin, M.J., Mitchell, C.E., Holmden, C., Štorch, P. (2013) Environmental changes in the Late Ordovician–Early Silurian: Review and new insights from black shales and nitrogen isotopes. GSA Bulletin 125, 1635–1670. https://doi.org/10.1130/B30812.1

). This glaciation happened abruptly within a short duration of ∼1 Myr at greenhouse conditions with a high atmospheric partial pressure of greenhouse gas (pCO2), up to 3–16 times higher than present levels (Finnegan et al., 2011

Finnegan, S., Bergmann, K., Eiler, J., Jones, D.S., Fike, D.A., Eisenman, I., Hughes, N., Tripati, A., Fischer, W.W. (2011) The magnitude and duration of Late Ordovician–Early Silurian glaciation. Science 331, 903–906. https://doi.org/10.1126/science.1200803

; Pohl et al., 2016

Pohl, A., Donnadieu, Y., Le Hir, G., Ladant, J.-B., Dumas, C., Alvarez-Solas, J., Vandenbroucke, T.R.A. (2016) Glacial onset predated Late Ordovician climate cooling. Paleoceanography 31, 800–821. https://doi.org/10.1002/2016PA002928

). The causal mechanism has been connected to the drawdown of atmospheric pCO2 caused by an increasing rate of silicate weathering, a mounting sink of organic matter burial, or a combination of both (Melchin et al., 2013

Melchin, M.J., Mitchell, C.E., Holmden, C., Štorch, P. (2013) Environmental changes in the Late Ordovician–Early Silurian: Review and new insights from black shales and nitrogen isotopes. GSA Bulletin 125, 1635–1670. https://doi.org/10.1130/B30812.1

and references therein). A global positive carbon isotopic excursion (Hirnantian Isotopic Curve Excursion, HICE; Fig. S-1) has been interpreted to indicate an enhanced burial of organic carbon and consequent drawdown of atmospheric pCO2, even though it is at odds with stratigraphical observations of the disappearance of the black shales during the Hirnantian (Fig. S-2; Melchin et al., 2013

Melchin, M.J., Mitchell, C.E., Holmden, C., Štorch, P. (2013) Environmental changes in the Late Ordovician–Early Silurian: Review and new insights from black shales and nitrogen isotopes. GSA Bulletin 125, 1635–1670. https://doi.org/10.1130/B30812.1

). The HICE may be alternatively explained by enhanced carbonate weathering during glacial regression, which would not draw down the atmospheric pCO2, and the magnitude of HICE can also be influenced by local carbon cycling (e.g., Kump et al., 1999

Kump, L.R., Arthur, M.A., Patzkowsky, M.E., Gibbs, M.T., Pinkus, D.S., Sheehan, P.M. (1999) A weathering hypothesis for glaciation at high atmospheric pCO2 during the Late Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology 152, 173–187. https://doi.org/10.1016/S0031-0182(99)00046-2

). These uncertainties call for additional geochemical tools to confirm the “missing” sink of carbon during the Hirnantian glaciation.

Zinc is a micronutrient in phytoplankton biomass (Morel and Price, 2003

Morel, F.M.M., Price, N.M. (2003) The Biogeochemical Cycles of Trace Metals in the Oceans. Science 300, 944–947. https://doi.org/10.1126/science.1083545

) and shows strong nutrient-like behaviour in the surface ocean (Bruland, 1980

Bruland, K.W. (1980) Oceanographic distributions of cadmium, zinc, nickel, and copper in the north pacific. Earth and Planetary Science Letters 47, 176–198. https://doi.org/10.1016/0012-821X(80)90035-7

). Organic Zn dominates on shelves (92 %; Weber et al., 2018

Weber, T., John, S., Tagliabue, A., DeVries, T. (2018) Biological uptake and reversible scavenging of zinc in the global ocean. Science 361, 72–76. https://doi.org/10.1126/science.aap8532

) where about 80 % of global marine organic matter (Burdige, 2007

Burdige, D.J. (2007) Preservation of organic matter in marine sediments: controls, mechanisms, and an imbalance in sediment organic carbon budgets? Chemical Reviews 107, 467–485. https://doi.org/10.1021/cr050347q

) is accumulated and is characterised by lighter Zn isotopic compositions (δ66Zn) around −0.1 ‰ in comparison with the average seawater (0.5 ‰) and other oxic output flux, including Fe-Mn crusts and nodules (0.9 ‰; Little et al., 2016

Little, S.H., Vance, D., McManus, J., Severmann, S. (2016) Key role of continental margin sediments in the oceanic mass balance of Zn and Zn isotopes. Geology 44, 207–210. https://doi.org/10.1130/G37493.1

). Therefore, Zn isotope record could be a promising tracer for change in organic carbon fluxes in past oceans (e.g., Isson et al., 2018

Isson, T.T., Love, G.D., Dupont, C.L., Reinhard, C.T., Zumberge, A.J., Asael, D., Gueguen, B., McCrow, J., Gill, B.C., Owens, J., Rainbird, R.H., Rooney, A.D., Zhao, M.-Y., Stueeken, E.E., Konhauser, K.O., John, S.G., Lyons, T.W., Planavsky, N.J. (2018) Tracking the rise of eukaryotes to ecological dominance with zinc isotopes. Geobiology 16, 341–352. https://doi.org/10.1111/gbi.12289

; Sweere et al., 2018

Sweere, T.C., Dickson, A.J., Jenkyns, H.C., Porcelli, D., Elrick, M., van den Boorn, S.H., Henderson, G.M. (2018) Isotopic evidence for changes in the zinc cycle during Oceanic Anoxic Event 2 (Late Cretaceous). Geology 46, 463–466. https://doi.org/10.1130/G40226.1

), with increasing δ66Zn values as more organic-rich sediments with isotopically light Zn are buried. Further, the shorter residence time (∼11 kyr) of Zn than dissolved inorganic carbon (∼83 kyr; De La Rocha, 2006

De La Rocha, C.L. (2006) 6.04. The biological pump. In: Elderfield, H., Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry, Volume 6: The Oceans and Marine Geochemistry. First Edition, Elsevier, Amsterdam, 83–111. https://doi.org/10.1016/B0-08-043751-6/06107-7

) in the oceans makes Zn isotopes an appropriate proxy to identifying multiple pulses of enhanced organic carbon burial. Here, we decipher the δ66Zn record and quantify organic carbon burial during the Hirnantian glaciation, relying on reproducible stratigraphic trends in δ66Zn of two carbonate and shale successions at the Ordovician–Silurian transition.

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Geological Settings and Samples

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


The Yangtze Block was attached to the margin of Gondwana located in a subtropical to tropical area during the Late Ordovician to Early Silurian (Chen et al., 2010

Chen, X., Zhou, Z., Fan, J. (2010) Ordovician paleogeography and tectonics of the major paleoplates of China. In: Finney, S.C., Berry, W.B.N. (Eds.) Special Paper 466: The Ordovician Earth System. Geological Society of America, Boulder, 85–104. https://doi.org/10.1130/2010.2466(06)

; see Supplementary Information). The Yangtze Platform was primarily covered by the broad epeiric sea connected to the open ocean in the Late Ordovician and then uplifted in the earliest Silurian (Fig. 1). Two well-preserved successions spanning the Ordovician–Silurian boundary were investigated in this study, including the Wanhe (WH) carbonate section and the Yihuang-1 (YH-1) shale drill core (Fig. 2; Zhang et al., 2016

Zhang, L.N., Fan, J.X., Chen, Q. (2016) Geographic distribution and palaeogeographic reconstruction of the Upper Ordovician Kuanyinchiao Bed in South China. Chinese Science Bulletin 61, 2053–2063. https://doi.org/10.1360/N972015-00981

; Tang et al., 2017

Tang, P., Huang, B., Wu, R.C., Fan, J., Yan, K., Wang, G.X., Liu, J.B., Wang, Y., Zhan, R.B., Rong, J.Y. (2017) On the upper Ordovician Daduhe Formation of the upper Yangtze region. Journal of Stratigraphy 41, 119–133. https://doi.org/10.19839/j.cnki.dcxzz.2017.02.001

). Two glacial cycles of the Hirnantian glaciation in the YH-1 section have been identified by weathering intensities reflected by the chemical index of alteration (Li et al., 2020

Li, N., Li, C., Algeo, T.J., Cheng, M., Jin, C., Zhu, G., Fan, J. Sun, Z. (2020) Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis. Earth-Science Reviews 212, 103443. https://doi.org/10.1016/j.earscirev.2020.103443

). The Hirnantian glacial interval in the WH section was determined by the results of magnetic susceptibility (Zhong et al., 2020

Zhong, Y., Wu, H., Fan, J., Fang, Q., Shi, M., Zhang, S., Yang, T., Li, H., Cao L. (2020) Late Ordovician obliquity-forced glacio-eustasy recorded in the Yangtze Block, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 540, 109520. https://doi.org/10.1016/j.palaeo.2019.109520

), whereas within the Hirnantian ice age the multiple, shorter term periods of glaciation have not yet been identified in detail.


Figure 1 Palaeogeographic map for (a) South China and (b) the Yangtze Shelf Sea at ∼445 Ma (Zhang et al., 2016

Zhang, L.N., Fan, J.X., Chen, Q. (2016) Geographic distribution and palaeogeographic reconstruction of the Upper Ordovician Kuanyinchiao Bed in South China. Chinese Science Bulletin 61, 2053–2063. https://doi.org/10.1360/N972015-00981

; Zou et al., 2018

Zou, C., Qiu, Z., Poulton, S.W., Dong, D., Wang, H., Chen, D., Lu, B., Shi, Z., Tao, H. (2018) Ocean euxinia and climate change “double whammy” drove the Late Ordovician mass extinction. Geology 46, 535–538. https://doi.org/10.1130/G40121.1

). Orange circle and text represents the Yihuang-1 (YH-1) section located at outer shelf in the Upper Yangtze platform of South China and connected to open sea (Li et al., 2020

Li, N., Li, C., Algeo, T.J., Cheng, M., Jin, C., Zhu, G., Fan, J. Sun, Z. (2020) Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis. Earth-Science Reviews 212, 103443. https://doi.org/10.1016/j.earscirev.2020.103443

). Black circle and pink text represents the Wanhe (WH) carbonate section deposited on platform (Tang et al., 2017

Tang, P., Huang, B., Wu, R.C., Fan, J., Yan, K., Wang, G.X., Liu, J.B., Wang, Y., Zhan, R.B., Rong, J.Y. (2017) On the upper Ordovician Daduhe Formation of the upper Yangtze region. Journal of Stratigraphy 41, 119–133. https://doi.org/10.19839/j.cnki.dcxzz.2017.02.001

).
Full size image



Figure 2 Zinc isotopic (a) mass balance and (b) major fluxes in modern oceans, modified from Little et al. (2016)

Little, S.H., Vance, D., McManus, J., Severmann, S. (2016) Key role of continental margin sediments in the oceanic mass balance of Zn and Zn isotopes. Geology 44, 207–210. https://doi.org/10.1130/G37493.1

and Isson et al. (2018)

Isson, T.T., Love, G.D., Dupont, C.L., Reinhard, C.T., Zumberge, A.J., Asael, D., Gueguen, B., McCrow, J., Gill, B.C., Owens, J., Rainbird, R.H., Rooney, A.D., Zhao, M.-Y., Stueeken, E.E., Konhauser, K.O., John, S.G., Lyons, T.W., Planavsky, N.J. (2018) Tracking the rise of eukaryotes to ecological dominance with zinc isotopes. Geobiology 16, 341–352. https://doi.org/10.1111/gbi.12289

, respectively. (c) Stratigraphy, δ66Zn and δ13C records from the YH-1 drill core and the WH section. The Late Ordovician mass extinction (LOME) includes two pulses. The durations (in kyr) of positive Zn isotope excursions during the glaciation were determined by astronomical time scale (Zhong et al., 2020

Zhong, Y., Wu, H., Fan, J., Fang, Q., Shi, M., Zhang, S., Yang, T., Li, H., Cao L. (2020) Late Ordovician obliquity-forced glacio-eustasy recorded in the Yangtze Block, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 540, 109520. https://doi.org/10.1016/j.palaeo.2019.109520

). Ocean redox condition in Yihuang-1 section was reported in Li et al. (2020)

Li, N., Li, C., Algeo, T.J., Cheng, M., Jin, C., Zhu, G., Fan, J. Sun, Z. (2020) Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis. Earth-Science Reviews 212, 103443. https://doi.org/10.1016/j.earscirev.2020.103443

based on Fe-speciation and Mo concentration data. Low δ13Ccarb value at the KYQ Bed in the WH section is considered a result of diagenetic alteration. ‘Glacial maxima’ in the carbonate section is defined based on the positive δ13Ccarb excursions.
Full size image


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Results

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


In the shaly succession, two pulses of δ66Zn (bulk rock digestion) increase were identified, ranging from 0.47 ‰ to 0.91 ‰ at the first glacial cycle and from 0.65 ‰ to 0.81 ‰ at the Kuanyinchiao (KYQ). The authigenic Zn component here is defined as the excess Zn in shales above the clastic level, a combination of the bio-authigenic fraction associated with particulate organic matter and authigenic sulfides. The normalisation of metal to aluminium was used to estimate and subtract the lithogenic component in order to calculate the fraction of authigenic Zn (Xauth). Only strongly Zn-enriched shales (Xauth > 70 %) were used to calculate the δ66Zn of authigenic fractions (δ66Znauth) in order to reduce calculation error. For euxinic shales with FeHR/FeT > 0.38 and FePy/FeHR > 0.7–0.8, the calculated δ66Znauth values mostly agree well within ±0.1 ‰ with the value obtained by the leaching extraction (a partial 2 M HNO3 digestion) of sulfide and partial Zn bound to organic matter (see Supplementary Information for methods; Table S-2). The high δ66Znauth value coincides with two intervals of glacial maximum (Fig. 2). In the carbonate succession, authigenic Zn in the carbonate phase was obtained by a weak (0.1 M) acetic acid leaching, and its δ66Zn was relatively steady before the glaciation (0.80 ‰), followed by a positive excursion of ∼0.2 ‰ reaching 0.99 ‰, and then drawing back to ∼0.8 ‰. The second increase of δ66Zn, up to 1.11 ‰, occurs at the KYQ Bed supposed to be the peak of the Hirnantian glaciation.

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Heavy Zinc Isotope Signature of the Glacial Maximum

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


Zinc isotopic ratios of the authigenic fractions (sulfides and organic matter) in euxinic sediments can record the coeval seawater δ66Zn value due to the near quantitative removal of water column Zn to sediments (Little et al., 2016

Little, S.H., Vance, D., McManus, J., Severmann, S. (2016) Key role of continental margin sediments in the oceanic mass balance of Zn and Zn isotopes. Geology 44, 207–210. https://doi.org/10.1130/G37493.1

; Vance et al., 2016

Vance, D., Little, S.H., Archer, C., Cameron, V., Andersen, M.B., Rijkenberg, M.J., Lyons, T.W. (2016) The oceanic budgets of nickel and zinc isotopes: the importance of sulfidic environments as illustrated by the Black Sea. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374, 20150294. https://doi.org/10.1098/rsta.2015.0294

; Isson et al., 2018

Isson, T.T., Love, G.D., Dupont, C.L., Reinhard, C.T., Zumberge, A.J., Asael, D., Gueguen, B., McCrow, J., Gill, B.C., Owens, J., Rainbird, R.H., Rooney, A.D., Zhao, M.-Y., Stueeken, E.E., Konhauser, K.O., John, S.G., Lyons, T.W., Planavsky, N.J. (2018) Tracking the rise of eukaryotes to ecological dominance with zinc isotopes. Geobiology 16, 341–352. https://doi.org/10.1111/gbi.12289

). Thus, the Zn isotope data of euxinic shales that have been identified by Fe speciation data (Li et al., 2020

Li, N., Li, C., Algeo, T.J., Cheng, M., Jin, C., Zhu, G., Fan, J. Sun, Z. (2020) Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis. Earth-Science Reviews 212, 103443. https://doi.org/10.1016/j.earscirev.2020.103443

) could reflect the δ66Zn variation of seawater during the Hirnantian glaciation (Fig. S-3). The δ66Zn values do not vary with local changes in depositional setting, mineralogy, redox condition and primary productivity (Supplementary Information). Two positive δ66Zn shifts also are observed in the carbonate WH section after inspecting possible diagenetic effects (Supplementary Information). Volcanic activities were identified in the Katian strata in South China (Melchin et al., 2013

Melchin, M.J., Mitchell, C.E., Holmden, C., Štorch, P. (2013) Environmental changes in the Late Ordovician–Early Silurian: Review and new insights from black shales and nitrogen isotopes. GSA Bulletin 125, 1635–1670. https://doi.org/10.1130/B30812.1

), which may have resulted in a decline of seawater pH value. However, Zn isotope fractionation between carbonate and seawater (Δ66Zncarb–SW) becomes larger as seawater pH decreases (Mavromatis et al., 2019

Mavromatis, V., González, A.G., Dietzel, M., Schott, J. (2019) Zinc isotope fractionation during the inorganic precipitation of calcite – Towards a new pH proxy. Geochimica et Cosmochimica Acta 244, 99–112. https://doi.org/10.1016/j.gca.2018.09.005

), which should result in elevated δ66Zn in carbonates. This contrasts to the relatively low δ66Znauth values during the Katian (Fig. 2). Thus, the high δ66Zn values of two glacial maximum intervals most likely reflect global changes instead of local changes.

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Flux Estimate of Increasing Organic Burial

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


Organic-rich shelf sediment is the major sink of isotopically light Zn in modern oceans (Weber et al., 2018

Weber, T., John, S., Tagliabue, A., DeVries, T. (2018) Biological uptake and reversible scavenging of zinc in the global ocean. Science 361, 72–76. https://doi.org/10.1126/science.aap8532

), although the mechanism is not well understood yet (probably ascribed to bio-uptake/scavenging of light Zn isotopes or the early diagenetic processes; Weber et al., 2018

Weber, T., John, S., Tagliabue, A., DeVries, T. (2018) Biological uptake and reversible scavenging of zinc in the global ocean. Science 361, 72–76. https://doi.org/10.1126/science.aap8532

; Köbberich and Vance, 2019

Köbberich, M., Vance, D. (2019) Zn isotope fractionation during uptake into marine phytoplankton: Implications for oceanic zinc isotopes. Chemical Geology 523, 154–161. https://doi.org/10.1016/j.chemgeo.2019.04.004

; Horner et al., 2021

Horner, T.J., Little, S.H., Conway, T.M., Farmer, J.R., Hertzberg, J.E., Janssen, D.J., Lough, A.J.M., McKay, J.L., Tessin, A., Galer, S.J.G., Jaccard, S.L., Lacan, F., Paytan, A., Wuttig, K., GEOTRACES–PAGES Biological Productivity Working Group Members (2021) Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES-Era Data. Global Biogeochemmical Cycles 35, e2020GB006814. https://doi.org/10.1029/2020GB006814

). The high δ66Zn value of the Hirnantian ocean suggests an increasing removal flux of Zn into continental margin sediments compared to burial flux of Zn into oxic sediments. Burial of organic carbon in margin sediments is the major sink of the global organic carbon cycle (Burdige, 2007

Burdige, D.J. (2007) Preservation of organic matter in marine sediments: controls, mechanisms, and an imbalance in sediment organic carbon budgets? Chemical Reviews 107, 467–485. https://doi.org/10.1021/cr050347q

). Thus, the positive δ66Znauth shifts might reflect two pulses of massive organic carbon burial during two main glacial cycles. More speculatively, the organic burial fluxes are quantitatively estimated from Zn isotopic mass balance and evaluated by means of Monte Carlo, using zinc isotopic data of the YH-1 shales deposited in euxinic environments (see the Supplementary Information for methods). The major Zn output fluxes in modern oceans include organic-rich sediments (FOrg, mainly continental margin sediments in mildly reducing (suboxic-anoxic) conditions), oxic sediments (FOx, mainly of Fe-Mn oxides), and euxinic sinks (FEux) (Little et al., 2016

Little, S.H., Vance, D., McManus, J., Severmann, S. (2016) Key role of continental margin sediments in the oceanic mass balance of Zn and Zn isotopes. Geology 44, 207–210. https://doi.org/10.1130/G37493.1

). When solving for FOrg, the influx and other outfluxes, and their Zn isotopic ratios are forced by a uniformly distributed random number within their given ranges, considering their uncertainties in the oceanic zinc cycle (e.g., Li et al., 2021

Li, Z., Cole, D.B., Newby, S.M., Owens J.D, Kendall B., Reinhard C.T. (2021) New constraints on mid-Proterozoic ocean redox from stable thallium isotope systematics of black shales. Geochimica et Cosmochimica Acta 315, 185–206. https://doi.org/10.1016/j.gca.2021.09.006

; Table S-5). The results show that to produce the largest observed increase in seawater δ66Zn from 0.47 ± 0.03 ‰ to 0.88 ± 0.06 ‰, a nearly double increase in organic Zn burial from ∼3.5 × 108 to 5.5 × 108 mol/yr is required, regardless of whether the euxinia expanded or shrank (Fig. 3). Considering a mean Zn/C ratio of 0.036 (mmol/mol) of plankton (Little et al., 2015

Little, S.H., Vance, D., Lyons, T.W., McManus, J. (2015) Controls on trace metal authigenic enrichment in reducing sediments: Insights from modern oxygen-deficient settings. American Journal of Science 315, 77–119. https://doi.org/10.2475/02.2015.01

), our estimate suggests that the flux of organic carbon deposition has increased from 9.7 × 1012 mol/yr to 15.2 × 1012 mol/yr before and during the glaciation. LaPorte et al. (2009)

LaPorte, D.F., Holmden, C., Patterson, W.P., Loxton, J.D., Melchin, M.J., Mitchell, C.E., Finney, S.C., Sheets, H.D. (2009) Local and global perspectives on carbon and nitrogen cycling during the Hirnantian glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology 276, 182–195. https://doi.org/10.1016/j.palaeo.2009.03.009

reported a positive δ13Ccarb shift of 2.7 ‰ from the Nevada section during the Hirnantian glaciation and suggested a doubling organic carbon burial flux. The estimated flux of organic carbon burial based on carbon isotopic data is of the same order of magnitude as the independent estimate from zinc isotopes, supporting an enhanced organic carbon burial during the Hirnantian glaciation.


Figure 3 (a) Modelling results of organic Zn burial fluxes based on Zn isotopic mass balance. (b) The estimated organic carbon burial flux in this study and Δ47 temperatures of seawater recorded in brachiopods (Finnegan et al., 2011

Finnegan, S., Bergmann, K., Eiler, J., Jones, D.S., Fike, D.A., Eisenman, I., Hughes, N., Tripati, A., Fischer, W.W. (2011) The magnitude and duration of Late Ordovician–Early Silurian glaciation. Science 331, 903–906. https://doi.org/10.1126/science.1200803

). Modelling details are listed in Supplementary Information for methods.
Full size image


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Temperature-Controlled Organic Carbon Burial

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


The sudden global climate cooling during the Hirnantian has been attributed to the enhanced silicate weathering and organic carbon burial (Melchin et al., 2013

Melchin, M.J., Mitchell, C.E., Holmden, C., Štorch, P. (2013) Environmental changes in the Late Ordovician–Early Silurian: Review and new insights from black shales and nitrogen isotopes. GSA Bulletin 125, 1635–1670. https://doi.org/10.1130/B30812.1

and references therein). In this study, we emphasise the important role of enhanced organic carbon burial on CO2 sequestration during glacial intervals. The accumulation of organic matter in sediments is generally related to improved organic carbon preservation and enhanced primary production. In terms of organic carbon preservation, no more expanded dysoxia-anoxia that can increase the burial efficiency of organic carbon has been globally observed within the Hirnantian glaciation, in comparison with the subsequent early Rhuddanian oceanic anoxic event (Stockey et al., 2020

Stockey, R.G., Cole, D.B., Planavsky, N.J., Loydell, D.K., Frýda, J., Sperling, E.A. (2020) Persistent global marine euxinia in the early Silurian. Nature communications 11, 1804. https://doi.org/10.1038/s41467-020-15400-y

). Furthermore, the glacial pulse is supposed to promote a more oxygenated ocean, at least a surface ocean, with a better ventilation (Pohl et al., 2021

Pohl, A., Lu, Z., Lu, W., Stockey, R.G., Elrick, M., Li, M., Desrochers, A., Shen Y., He, R., Finnegan, S., Ridgwell, A. (2021). Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation. Nature Geoscience 14, 868–873. https://doi.org/10.1038/s41561-021-00843-9

). The input of additional nutrients from increased continental weathering is observed before the glaciation (Finlay et al., 2010

Finlay, A.J., Selby, D., Gröcke, D.R. (2010) Tracking the Hirnantian glaciation using Os isotopes. Earth and Planetary Science Letters 293, 339–348. https://doi.org/10.1016/j.epsl.2010.02.049

) which can promote primary productivity and organic carbon burial, although it is not significant during the glaciation in a recent study of osmium and lithium isotope record (Sproson et al., 2022

Sproson, A.D., von Strandmann, P.A.P., Selby, D., Jarochowska, E., Frýda, J., Hladil, J., Loydell, D.K., Slavík, L., Calner, M., Maier, G., Munnecke, A., Lenton, T.M. (2022) Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations. Earth and Planetary Science Letters 577, 117260. https://doi.org/10.1016/j.epsl.2021.117260

). Here, we speculate about a temperature-dependent control on the massive organic carbon burial since it significantly postdates the glacial onset, that is cooling-induced elevation in the burial efficiency of organic carbon. However, other factors of organic carbon burial, such as astronomical forcing, are not ruled out here (Sproson et al., 2022

Sproson, A.D., von Strandmann, P.A.P., Selby, D., Jarochowska, E., Frýda, J., Hladil, J., Loydell, D.K., Slavík, L., Calner, M., Maier, G., Munnecke, A., Lenton, T.M. (2022) Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations. Earth and Planetary Science Letters 577, 117260. https://doi.org/10.1016/j.epsl.2021.117260

). In the early Phanerozoic Ocean with relatively low dissolved oxygen, cooling, which could have been underestimated before, constitutes the dominant control on enhanced organic carbon burial through an elevation in organic carbon burial efficiency with rates of organic carbon degradation (Fakhraee et al., 2020

Fakhraee, M., Planavsky, N.J., Reinhard, C.T. (2020) The role of environmental factors in the long-term evolution of the marine biological pump. Nature Geoscience 13, 812–816. https://doi.org/10.1038/s41561-020-00660-6

), compared with high eukaryotic export before the glacial maximum (Shen et al., 2018

Shen, J., Pearson, A., Henkes, G.A., Zhang, Y.G., Chen, K., Li, D., Wankel, S.D., Finney, S.C., Shen, Y. (2018) Improved efficiency of the biological pump as a trigger for the Late Ordovician glaciation. Nature Geoscience, 11, 510–514. https://doi.org/10.1038/s41561-018-0141-5

). This scenario can also shed light on the several glacial cycles with short duration during the Hirnantian. The temperature-dependent, biological CO2 fixation can be more fluctuant than other ways to consume atmosphere CO2, such as changes in chemical weathering of silicate rocks. Compared with two other major Phanerozoic glaciations (the Karoo and the Cenozoic glaciations) with high atmospheric O2 condition, the biological carbon pumping efficiency in the Hirnantian glaciation is sensitive to a cooling climate, serving as positive feedback to encourage the organic carbon fixation after glacial onset and, consequently, the extreme icehouse climate.

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Acknowledgements

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


We would like to thank Bin Zhou for carbon isotope analysis. This work is supported by the National Key R&D Program of China (Grant No. 2019YFA0708400), the National Natural Science Foundation of China (Grant Nos. 41730214, 42003013 and 41725007) and the Fundamental Research Funds for the Central Universities (Grant No. 2-9-2020-042). We thank Editor Claudine Stirling and three anonymous reviewers for their constructive comments which improved this manuscript.

Editor: Claudine Stirling

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References

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information

Bruland, K.W. (1980) Oceanographic distributions of cadmium, zinc, nickel, and copper in the north pacific. Earth and Planetary Science Letters 47, 176–198. https://doi.org/10.1016/0012-821X(80)90035-7
Show in context

Zinc is a micronutrient in phytoplankton biomass (Morel and Price, 2003) and shows strong nutrient-like behaviour in the surface ocean (Bruland, 1980). Organic Zn dominates on shelves (92 %; Weber et al., 2018) where about 80 % of global marine organic matter (Burdige, 2007) is accumulated and is characterised by lighter Zn isotopic compositions (δ66Zn) around −0.1 ‰ in comparison with the average seawater (0.5 ‰) and other oxic output flux, including Fe-Mn crusts and nodules (0.9 ‰; Little et al., 2016).
View in article


Burdige, D.J. (2007) Preservation of organic matter in marine sediments: controls, mechanisms, and an imbalance in sediment organic carbon budgets? Chemical Reviews 107, 467–485. https://doi.org/10.1021/cr050347q
Show in context

Burial of organic carbon in margin sediments is the major sink of the global organic carbon cycle (Burdige, 2007).
View in article
Zinc is a micronutrient in phytoplankton biomass (Morel and Price, 2003) and shows strong nutrient-like behaviour in the surface ocean (Bruland, 1980). Organic Zn dominates on shelves (92 %; Weber et al., 2018) where about 80 % of global marine organic matter (Burdige, 2007) is accumulated and is characterised by lighter Zn isotopic compositions (δ66Zn) around −0.1 ‰ in comparison with the average seawater (0.5 ‰) and other oxic output flux, including Fe-Mn crusts and nodules (0.9 ‰; Little et al., 2016).
View in article


Chen, X., Zhou, Z., Fan, J. (2010) Ordovician paleogeography and tectonics of the major paleoplates of China. In: Finney, S.C., Berry, W.B.N. (Eds.) Special Paper 466: The Ordovician Earth System. Geological Society of America, Boulder, 85–104. https://doi.org/10.1130/2010.2466(06)
Show in context

The Yangtze Block was attached to the margin of Gondwana located in a subtropical to tropical area during the Late Ordovician to Early Silurian (Chen et al., 2010; see Supplementary Information).
View in article


De La Rocha, C.L. (2006) 6.04. The biological pump. In: Elderfield, H., Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry, Volume 6: The Oceans and Marine Geochemistry. First Edition, Elsevier, Amsterdam, 83–111. https://doi.org/10.1016/B0-08-043751-6/06107-7
Show in context

Further, the shorter residence time (∼11 kyr) of Zn than dissolved inorganic carbon (∼83 kyr; De La Rocha, 2006) in the oceans makes Zn isotopes an appropriate proxy to identifying multiple pulses of enhanced organic carbon burial.
View in article


Fakhraee, M., Planavsky, N.J., Reinhard, C.T. (2020) The role of environmental factors in the long-term evolution of the marine biological pump. Nature Geoscience 13, 812–816. https://doi.org/10.1038/s41561-020-00660-6
Show in context

In the early Phanerozoic Ocean with relatively low dissolved oxygen, cooling, which could have been underestimated before, constitutes the dominant control on enhanced organic carbon burial through an elevation in organic carbon burial efficiency with rates of organic carbon degradation (Fakhraee et al., 2020), compared with high eukaryotic export before the glacial maximum (Shen et al., 2018).
View in article


Finlay, A.J., Selby, D., Gröcke, D.R. (2010) Tracking the Hirnantian glaciation using Os isotopes. Earth and Planetary Science Letters 293, 339–348. https://doi.org/10.1016/j.epsl.2010.02.049
Show in context

The input of additional nutrients from increased continental weathering is observed before the glaciation (Finlay et al., 2010) which can promote primary productivity and organic carbon burial, although it is not significant during the glaciation in a recent study of osmium and lithium isotope record (Sproson et al., 2022).
View in article


Finnegan, S., Bergmann, K., Eiler, J., Jones, D.S., Fike, D.A., Eisenman, I., Hughes, N., Tripati, A., Fischer, W.W. (2011) The magnitude and duration of Late Ordovician–Early Silurian glaciation. Science 331, 903–906. https://doi.org/10.1126/science.1200803
Show in context

The end-Ordovician (Hirnantian) glaciation (∼445 Ma) was the culmination of long-term climate cooling that had begun in the Early or Middle Ordovician, and was coeval and causally linked with the Late Ordovician mass extinction (LOME), the first of the ‘Big Five’ Phanerozoic catastrophic events (Finnegan et al., 2011; Melchin et al., 2013).
View in article
(a) Modelling results of organic Zn burial fluxes based on Zn isotopic mass balance. (b) The estimated organic carbon burial flux in this study and Δ47 temperatures of seawater recorded in brachiopods (Finnegan et al., 2011).
View in article
This glaciation happened abruptly within a short duration of ∼1 Myr at greenhouse conditions with a high atmospheric partial pressure of greenhouse gas (pCO2), up to 3–16 times higher than present levels (Finnegan et al., 2011; Pohl et al., 2016).
View in article


Horner, T.J., Little, S.H., Conway, T.M., Farmer, J.R., Hertzberg, J.E., Janssen, D.J., Lough, A.J.M., McKay, J.L., Tessin, A., Galer, S.J.G., Jaccard, S.L., Lacan, F., Paytan, A., Wuttig, K., GEOTRACES–PAGES Biological Productivity Working Group Members (2021) Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES-Era Data. Global Biogeochemmical Cycles 35, e2020GB006814. https://doi.org/10.1029/2020GB006814
Show in context

Organic-rich shelf sediment is the major sink of isotopically light Zn in modern oceans (Weber et al., 2018), although the mechanism is not well understood yet (probably ascribed to bio-uptake/scavenging of light Zn isotopes or the early diagenetic processes; Weber et al., 2018; Köbberich and Vance, 2019; Horner et al., 2021).
View in article


Isson, T.T., Love, G.D., Dupont, C.L., Reinhard, C.T., Zumberge, A.J., Asael, D., Gueguen, B., McCrow, J., Gill, B.C., Owens, J., Rainbird, R.H., Rooney, A.D., Zhao, M.-Y., Stueeken, E.E., Konhauser, K.O., John, S.G., Lyons, T.W., Planavsky, N.J. (2018) Tracking the rise of eukaryotes to ecological dominance with zinc isotopes. Geobiology 16, 341–352. https://doi.org/10.1111/gbi.12289
Show in context

Therefore, Zn isotope record could be a promising tracer for change in organic carbon fluxes in past oceans (e.g., Isson et al., 2018; Sweere et al., 2018), with increasing δ66Zn values as more organic-rich sediments with isotopically light Zn are buried.
View in article
Zinc isotopic (a) mass balance and (b) major fluxes in modern oceans, modified from Little et al. (2016) and Isson et al. (2018), respectively.
View in article
Zinc isotopic ratios of the authigenic fractions (sulfides and organic matter) in euxinic sediments can record the coeval seawater δ66Zn value due to the near quantitative removal of water column Zn to sediments (Little et al., 2016; Vance et al., 2016; Isson et al., 2018).
View in article


Köbberich, M., Vance, D. (2019) Zn isotope fractionation during uptake into marine phytoplankton: Implications for oceanic zinc isotopes. Chemical Geology 523, 154–161. https://doi.org/10.1016/j.chemgeo.2019.04.004
Show in context

Organic-rich shelf sediment is the major sink of isotopically light Zn in modern oceans (Weber et al., 2018), although the mechanism is not well understood yet (probably ascribed to bio-uptake/scavenging of light Zn isotopes or the early diagenetic processes; Weber et al., 2018; Köbberich and Vance, 2019; Horner et al., 2021).
View in article


Kump, L.R., Arthur, M.A., Patzkowsky, M.E., Gibbs, M.T., Pinkus, D.S., Sheehan, P.M. (1999) A weathering hypothesis for glaciation at high atmospheric pCO2 during the Late Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology 152, 173–187. https://doi.org/10.1016/S0031-0182(99)00046-2
Show in context

The HICE may be alternatively explained by enhanced carbonate weathering during glacial regression, which would not draw down the atmospheric pCO2, and the magnitude of HICE can also be influenced by local carbon cycling (e.g., Kump et al., 1999).
View in article


LaPorte, D.F., Holmden, C., Patterson, W.P., Loxton, J.D., Melchin, M.J., Mitchell, C.E., Finney, S.C., Sheets, H.D. (2009) Local and global perspectives on carbon and nitrogen cycling during the Hirnantian glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology 276, 182–195. https://doi.org/10.1016/j.palaeo.2009.03.009
Show in context

LaPorte et al. (2009) reported a positive δ13Ccarb shift of 2.7 ‰ from the Nevada section during the Hirnantian glaciation and suggested a doubling organic carbon burial flux.
View in article


Li, N., Li, C., Algeo, T.J., Cheng, M., Jin, C., Zhu, G., Fan, J. Sun, Z. (2020) Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis. Earth-Science Reviews 212, 103443. https://doi.org/10.1016/j.earscirev.2020.103443
Show in context

Orange circle and text represents the Yihuang-1 (YH-1) section located at outer shelf in the Upper Yangtze platform of South China and connected to open sea (Li et al., 2020).
View in article
Two glacial cycles of the Hirnantian glaciation in the YH-1 section have been identified by weathering intensities reflected by the chemical index of alteration (Li et al., 2020).
View in article
Thus, the Zn isotope data of euxinic shales that have been identified by Fe speciation data (Li et al., 2020) could reflect the δ66Zn variation of seawater during the Hirnantian glaciation (Fig. S-3).
View in article
The durations (in kyr) of positive Zn isotope excursions during the glaciation were determined by astronomical time scale (Zhong et al., 2020). Ocean redox condition in Yihuang-1 section was reported in Li et al. (2020) based on Fe-speciation and Mo concentration data.
View in article


Li, Z., Cole, D.B., Newby, S.M., Owens J.D, Kendall B., Reinhard C.T. (2021) New constraints on mid-Proterozoic ocean redox from stable thallium isotope systematics of black shales. Geochimica et Cosmochimica Acta 315, 185–206. https://doi.org/10.1016/j.gca.2021.09.006
Show in context

When solving for FOrg, the influx and other outfluxes, and their Zn isotopic ratios are forced by a uniformly distributed random number within their given ranges, considering their uncertainties in the oceanic zinc cycle (e.g., Li et al., 2021; Table S-5).
View in article


Little, S.H., Vance, D., Lyons, T.W., McManus, J. (2015) Controls on trace metal authigenic enrichment in reducing sediments: Insights from modern oxygen-deficient settings. American Journal of Science 315, 77–119. https://doi.org/10.2475/02.2015.01
Show in context

Considering a mean Zn/C ratio of 0.036 (mmol/mol) of plankton (Little et al., 2015), our estimate suggests that the flux of organic carbon deposition has increased from 9.7 × 1012 mol/yr to 15.2 × 1012 mol/yr before and during the glaciation.
View in article


Little, S.H., Vance, D., McManus, J., Severmann, S. (2016) Key role of continental margin sediments in the oceanic mass balance of Zn and Zn isotopes. Geology 44, 207–210. https://doi.org/10.1130/G37493.1
Show in context

Zinc isotopic (a) mass balance and (b) major fluxes in modern oceans, modified from Little et al. (2016) and Isson et al. (2018), respectively.
View in article
Zinc isotopic ratios of the authigenic fractions (sulfides and organic matter) in euxinic sediments can record the coeval seawater δ66Zn value due to the near quantitative removal of water column Zn to sediments (Little et al., 2016; Vance et al., 2016; Isson et al., 2018).
View in article
The major Zn output fluxes in modern oceans include organic-rich sediments (FOrg, mainly continental margin sediments in mildly reducing (suboxic-anoxic) conditions), oxic sediments (FOx, mainly of Fe-Mn oxides), and euxinic sinks (FEux) (Little et al., 2016).
View in article
Zinc is a micronutrient in phytoplankton biomass (Morel and Price, 2003) and shows strong nutrient-like behaviour in the surface ocean (Bruland, 1980). Organic Zn dominates on shelves (92 %; Weber et al., 2018) where about 80 % of global marine organic matter (Burdige, 2007) is accumulated and is characterised by lighter Zn isotopic compositions (δ66Zn) around −0.1 ‰ in comparison with the average seawater (0.5 ‰) and other oxic output flux, including Fe-Mn crusts and nodules (0.9 ‰; Little et al., 2016).
View in article


Mavromatis, V., González, A.G., Dietzel, M., Schott, J. (2019) Zinc isotope fractionation during the inorganic precipitation of calcite – Towards a new pH proxy. Geochimica et Cosmochimica Acta 244, 99–112. https://doi.org/10.1016/j.gca.2018.09.005
Show in context

However, Zn isotope fractionation between carbonate and seawater (Δ66Zncarb–SW) becomes larger as seawater pH decreases (Mavromatis et al., 2019), which should result in elevated δ66Zn in carbonates.
View in article


Melchin, M.J., Mitchell, C.E., Holmden, C., Štorch, P. (2013) Environmental changes in the Late Ordovician–Early Silurian: Review and new insights from black shales and nitrogen isotopes. GSA Bulletin 125, 1635–1670. https://doi.org/10.1130/B30812.1
Show in context

The causal mechanism has been connected to the drawdown of atmospheric pCO2 caused by an increasing rate of silicate weathering, a mounting sink of organic matter burial, or a combination of both (Melchin et al., 2013 and references therein).
View in article
A global positive carbon isotopic excursion (Hirnantian Isotopic Curve Excursion, HICE; Fig. S-1) has been interpreted to indicate an enhanced burial of organic carbon and consequent drawdown of atmospheric pCO2, even though it is at odds with stratigraphical observations of the disappearance of the black shales during the Hirnantian (Fig. S-2; Melchin et al., 2013).
View in article
Volcanic activities were identified in the Katian strata in South China (Melchin et al., 2013), which may have resulted in a decline of seawater pH value.
View in article
The sudden global climate cooling during the Hirnantian has been attributed to the enhanced silicate weathering and organic carbon burial (Melchin et al., 2013 and references therein).
View in article
The end-Ordovician (Hirnantian) glaciation (∼445 Ma) was the culmination of long-term climate cooling that had begun in the Early or Middle Ordovician, and was coeval and causally linked with the Late Ordovician mass extinction (LOME), the first of the ‘Big Five’ Phanerozoic catastrophic events (Finnegan et al., 2011; Melchin et al., 2013).
View in article


Morel, F.M.M., Price, N.M. (2003) The Biogeochemical Cycles of Trace Metals in the Oceans. Science 300, 944–947. https://doi.org/10.1126/science.1083545
Show in context

Zinc is a micronutrient in phytoplankton biomass (Morel and Price, 2003) and shows strong nutrient-like behaviour in the surface ocean (Bruland, 1980). Organic Zn dominates on shelves (92 %; Weber et al., 2018) where about 80 % of global marine organic matter (Burdige, 2007) is accumulated and is characterised by lighter Zn isotopic compositions (δ66Zn) around −0.1 ‰ in comparison with the average seawater (0.5 ‰) and other oxic output flux, including Fe-Mn crusts and nodules (0.9 ‰; Little et al., 2016).
View in article


Pohl, A., Donnadieu, Y., Le Hir, G., Ladant, J.-B., Dumas, C., Alvarez-Solas, J., Vandenbroucke, T.R.A. (2016) Glacial onset predated Late Ordovician climate cooling. Paleoceanography 31, 800–821. https://doi.org/10.1002/2016PA002928
Show in context

This glaciation happened abruptly within a short duration of ∼1 Myr at greenhouse conditions with a high atmospheric partial pressure of greenhouse gas (pCO2), up to 3–16 times higher than present levels (Finnegan et al., 2011; Pohl et al., 2016).
View in article


Pohl, A., Lu, Z., Lu, W., Stockey, R.G., Elrick, M., Li, M., Desrochers, A., Shen Y., He, R., Finnegan, S., Ridgwell, A. (2021). Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation. Nature Geoscience 14, 868–873. https://doi.org/10.1038/s41561-021-00843-9
Show in context

Furthermore, the glacial pulse is supposed to promote a more oxygenated ocean, at least a surface ocean, with a better ventilation (Pohl et al., 2021).
View in article


Shen, J., Pearson, A., Henkes, G.A., Zhang, Y.G., Chen, K., Li, D., Wankel, S.D., Finney, S.C., Shen, Y. (2018) Improved efficiency of the biological pump as a trigger for the Late Ordovician glaciation. Nature Geoscience, 11, 510–514. https://doi.org/10.1038/s41561-018-0141-5
Show in context

In the early Phanerozoic Ocean with relatively low dissolved oxygen, cooling, which could have been underestimated before, constitutes the dominant control on enhanced organic carbon burial through an elevation in organic carbon burial efficiency with rates of organic carbon degradation (Fakhraee et al., 2020), compared with high eukaryotic export before the glacial maximum (Shen et al., 2018).
View in article


Sproson, A.D., von Strandmann, P.A.P., Selby, D., Jarochowska, E., Frýda, J., Hladil, J., Loydell, D.K., Slavík, L., Calner, M., Maier, G., Munnecke, A., Lenton, T.M. (2022) Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations. Earth and Planetary Science Letters 577, 117260. https://doi.org/10.1016/j.epsl.2021.117260
Show in context

However, other factors of organic carbon burial, such as astronomical forcing, are not ruled out here (Sproson et al., 2022).
View in article
The input of additional nutrients from increased continental weathering is observed before the glaciation (Finlay et al., 2010) which can promote primary productivity and organic carbon burial, although it is not significant during the glaciation in a recent study of osmium and lithium isotope record (Sproson et al., 2022).
View in article


Stockey, R.G., Cole, D.B., Planavsky, N.J., Loydell, D.K., Frýda, J., Sperling, E.A. (2020) Persistent global marine euxinia in the early Silurian. Nature communications 11, 1804. https://doi.org/10.1038/s41467-020-15400-y
Show in context

In terms of organic carbon preservation, no more expanded dysoxia-anoxia that can increase the burial efficiency of organic carbon has been globally observed within the Hirnantian glaciation, in comparison with the subsequent early Rhuddanian oceanic anoxic event (Stockey et al., 2020).
View in article


Sweere, T.C., Dickson, A.J., Jenkyns, H.C., Porcelli, D., Elrick, M., van den Boorn, S.H., Henderson, G.M. (2018) Isotopic evidence for changes in the zinc cycle during Oceanic Anoxic Event 2 (Late Cretaceous). Geology 46, 463–466. https://doi.org/10.1130/G40226.1
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Therefore, Zn isotope record could be a promising tracer for change in organic carbon fluxes in past oceans (e.g., Isson et al., 2018; Sweere et al., 2018), with increasing δ66Zn values as more organic-rich sediments with isotopically light Zn are buried.
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Tang, P., Huang, B., Wu, R.C., Fan, J., Yan, K., Wang, G.X., Liu, J.B., Wang, Y., Zhan, R.B., Rong, J.Y. (2017) On the upper Ordovician Daduhe Formation of the upper Yangtze region. Journal of Stratigraphy 41, 119–133. https://doi.org/10.19839/j.cnki.dcxzz.2017.02.001
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Black circle and pink text represents the Wanhe (WH) carbonate section deposited on platform (Tang et al., 2017).
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Two well-preserved successions spanning the Ordovician–Silurian boundary were investigated in this study, including the Wanhe (WH) carbonate section and the Yihuang-1 (YH-1) shale drill core (Fig. 2; Zhang et al., 2016; Tang et al., 2017).
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Vance, D., Little, S.H., Archer, C., Cameron, V., Andersen, M.B., Rijkenberg, M.J., Lyons, T.W. (2016) The oceanic budgets of nickel and zinc isotopes: the importance of sulfidic environments as illustrated by the Black Sea. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374, 20150294. https://doi.org/10.1098/rsta.2015.0294
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Zinc isotopic ratios of the authigenic fractions (sulfides and organic matter) in euxinic sediments can record the coeval seawater δ66Zn value due to the near quantitative removal of water column Zn to sediments (Little et al., 2016; Vance et al., 2016; Isson et al., 2018).
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Weber, T., John, S., Tagliabue, A., DeVries, T. (2018) Biological uptake and reversible scavenging of zinc in the global ocean. Science 361, 72–76. https://doi.org/10.1126/science.aap8532
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Organic-rich shelf sediment is the major sink of isotopically light Zn in modern oceans (Weber et al., 2018), although the mechanism is not well understood yet (probably ascribed to bio-uptake/scavenging of light Zn isotopes or the early diagenetic processes; Weber et al., 2018; Köbberich and Vance, 2019; Horner et al., 2021).
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Organic-rich shelf sediment is the major sink of isotopically light Zn in modern oceans (Weber et al., 2018), although the mechanism is not well understood yet (probably ascribed to bio-uptake/scavenging of light Zn isotopes or the early diagenetic processes; Weber et al., 2018; Köbberich and Vance, 2019; Horner et al., 2021).
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Zinc is a micronutrient in phytoplankton biomass (Morel and Price, 2003) and shows strong nutrient-like behaviour in the surface ocean (Bruland, 1980). Organic Zn dominates on shelves (92 %; Weber et al., 2018) where about 80 % of global marine organic matter (Burdige, 2007) is accumulated and is characterised by lighter Zn isotopic compositions (δ66Zn) around −0.1 ‰ in comparison with the average seawater (0.5 ‰) and other oxic output flux, including Fe-Mn crusts and nodules (0.9 ‰; Little et al., 2016).
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Zhang, L.N., Fan, J.X., Chen, Q. (2016) Geographic distribution and palaeogeographic reconstruction of the Upper Ordovician Kuanyinchiao Bed in South China. Chinese Science Bulletin 61, 2053–2063. https://doi.org/10.1360/N972015-00981
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Palaeogeographic map for (a) South China and (b) the Yangtze Shelf Sea at ∼445 Ma (Zhang et al., 2016; Zou et al., 2018).
View in article
Two well-preserved successions spanning the Ordovician–Silurian boundary were investigated in this study, including the Wanhe (WH) carbonate section and the Yihuang-1 (YH-1) shale drill core (Fig. 2; Zhang et al., 2016; Tang et al., 2017).
View in article


Zhong, Y., Wu, H., Fan, J., Fang, Q., Shi, M., Zhang, S., Yang, T., Li, H., Cao L. (2020) Late Ordovician obliquity-forced glacio-eustasy recorded in the Yangtze Block, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 540, 109520. https://doi.org/10.1016/j.palaeo.2019.109520
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The Hirnantian glacial interval in the WH section was determined by the results of magnetic susceptibility (Zhong et al., 2020), whereas within the Hirnantian ice age the multiple, shorter term periods of glaciation have not yet been identified in detail.
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The durations (in kyr) of positive Zn isotope excursions during the glaciation were determined by astronomical time scale (Zhong et al., 2020). Ocean redox condition in Yihuang-1 section was reported in Li et al. (2020) based on Fe-speciation and Mo concentration data.
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Zou, C., Qiu, Z., Poulton, S.W., Dong, D., Wang, H., Chen, D., Lu, B., Shi, Z., Tao, H. (2018) Ocean euxinia and climate change “double whammy” drove the Late Ordovician mass extinction. Geology 46, 535–538. https://doi.org/10.1130/G40121.1
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Palaeogeographic map for (a) South China and (b) the Yangtze Shelf Sea at ∼445 Ma (Zhang et al., 2016; Zou et al., 2018).
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Supplementary Information

Abstract | Introduction | Geological Settings and Samples | Results | Heavy Zinc Isotope Signature of the Glacial Maximum | Flux Estimate of Increasing Organic Burial | Temperature-Controlled Organic Carbon Burial | Acknowledgements | References | Supplementary Information


The Supplementary Information includes:
  • Geologic Background and Sample Description
  • Methods
  • Tables S-1 to S-5
  • Figures S-1 to S-7
  • Supplementary Information References


  • Download the Supplementary Information (PDF).
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    Figures



    Figure 1 Palaeogeographic map for (a) South China and (b) the Yangtze Shelf Sea at ∼445 Ma (Zhang et al., 2016

    Zhang, L.N., Fan, J.X., Chen, Q. (2016) Geographic distribution and palaeogeographic reconstruction of the Upper Ordovician Kuanyinchiao Bed in South China. Chinese Science Bulletin 61, 2053–2063. https://doi.org/10.1360/N972015-00981

    ; Zou et al., 2018

    Zou, C., Qiu, Z., Poulton, S.W., Dong, D., Wang, H., Chen, D., Lu, B., Shi, Z., Tao, H. (2018) Ocean euxinia and climate change “double whammy” drove the Late Ordovician mass extinction. Geology 46, 535–538. https://doi.org/10.1130/G40121.1

    ). Orange circle and text represents the Yihuang-1 (YH-1) section located at outer shelf in the Upper Yangtze platform of South China and connected to open sea (Li et al., 2020

    Li, N., Li, C., Algeo, T.J., Cheng, M., Jin, C., Zhu, G., Fan, J. Sun, Z. (2020) Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis. Earth-Science Reviews 212, 103443. https://doi.org/10.1016/j.earscirev.2020.103443

    ). Black circle and pink text represents the Wanhe (WH) carbonate section deposited on platform (Tang et al., 2017

    Tang, P., Huang, B., Wu, R.C., Fan, J., Yan, K., Wang, G.X., Liu, J.B., Wang, Y., Zhan, R.B., Rong, J.Y. (2017) On the upper Ordovician Daduhe Formation of the upper Yangtze region. Journal of Stratigraphy 41, 119–133. https://doi.org/10.19839/j.cnki.dcxzz.2017.02.001

    ).
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    Figure 2 Zinc isotopic (a) mass balance and (b) major fluxes in modern oceans, modified from Little et al. (2016)

    Little, S.H., Vance, D., McManus, J., Severmann, S. (2016) Key role of continental margin sediments in the oceanic mass balance of Zn and Zn isotopes. Geology 44, 207–210. https://doi.org/10.1130/G37493.1

    and Isson et al. (2018)

    Isson, T.T., Love, G.D., Dupont, C.L., Reinhard, C.T., Zumberge, A.J., Asael, D., Gueguen, B., McCrow, J., Gill, B.C., Owens, J., Rainbird, R.H., Rooney, A.D., Zhao, M.-Y., Stueeken, E.E., Konhauser, K.O., John, S.G., Lyons, T.W., Planavsky, N.J. (2018) Tracking the rise of eukaryotes to ecological dominance with zinc isotopes. Geobiology 16, 341–352. https://doi.org/10.1111/gbi.12289

    , respectively. (c) Stratigraphy, δ66Zn and δ13C records from the YH-1 drill core and the WH section. The Late Ordovician mass extinction (LOME) includes two pulses. The durations (in kyr) of positive Zn isotope excursions during the glaciation were determined by astronomical time scale (Zhong et al., 2020

    Zhong, Y., Wu, H., Fan, J., Fang, Q., Shi, M., Zhang, S., Yang, T., Li, H., Cao L. (2020) Late Ordovician obliquity-forced glacio-eustasy recorded in the Yangtze Block, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 540, 109520. https://doi.org/10.1016/j.palaeo.2019.109520

    ). Ocean redox condition in Yihuang-1 section was reported in Li et al. (2020)

    Li, N., Li, C., Algeo, T.J., Cheng, M., Jin, C., Zhu, G., Fan, J. Sun, Z. (2020) Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis. Earth-Science Reviews 212, 103443. https://doi.org/10.1016/j.earscirev.2020.103443

    based on Fe-speciation and Mo concentration data. Low δ13Ccarb value at the KYQ Bed in the WH section is considered a result of diagenetic alteration. ‘Glacial maxima’ in the carbonate section is defined based on the positive δ13Ccarb excursions.
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    Figure 3 (a) Modelling results of organic Zn burial fluxes based on Zn isotopic mass balance. (b) The estimated organic carbon burial flux in this study and Δ47 temperatures of seawater recorded in brachiopods (Finnegan et al., 2011

    Finnegan, S., Bergmann, K., Eiler, J., Jones, D.S., Fike, D.A., Eisenman, I., Hughes, N., Tripati, A., Fischer, W.W. (2011) The magnitude and duration of Late Ordovician–Early Silurian glaciation. Science 331, 903–906. https://doi.org/10.1126/science.1200803

    ). Modelling details are listed in Supplementary Information for methods.
    Back to article

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