Enhanced organic carbon burial intensified the end-Ordovician glaciation
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Keywords: zinc isotopes, organic carbon burial, end-Ordovician glaciation
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Abstract
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 |
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Introduction
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., 2013Melchin, 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., 2011Finnegan, 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., 2016Pohl, 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., 2013Melchin, 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., 2013Melchin, 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., 1999Kump, 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, 1980Bruland, 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., 2018Weber, 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, 2007Burdige, 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., 2016Little, 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., 2018Isson, 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., 2018Sweere, 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, 2006De 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.top
Geological Settings and Samples
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., 2016Zhang, 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., 2017Tang, 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., 2020Li, 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., 2020Zhong, 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., 2018Zou, 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., 2020Li, 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., 2017Tang, 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
).
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., 2020Zhong, 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.top
Results
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
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., 2016Vance, 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., 2018Isson, 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., 2020Li, 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., 2013Melchin, 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., 2019Mavromatis, 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.top
Flux Estimate of Increasing Organic Burial
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., 2018Weber, 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, 2019Köbberich, 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., 2021Horner, 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, 2007Burdige, 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., 2016Little, 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., 2021Li, 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., 2015Little, 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.top
Temperature-Controlled Organic Carbon Burial
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., 2020Stockey, 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., 2021Pohl, 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., 2010Finlay, 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., 2022Sproson, 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., 2022Sproson, 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., 2020Fakhraee, 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., 2018Shen, 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.top
Acknowledgements
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
top
References
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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
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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
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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).
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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
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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
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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
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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
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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
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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
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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
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
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
Show in context
Black circle and pink text represents the Wanhe (WH) carbonate section deposited on platform (Tang et al., 2017).
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
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
Show in context
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
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
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
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
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
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
Show in context
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.
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
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
Show in context
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
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Supplementary Information
The Supplementary Information includes:
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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., 2018Zou, 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., 2020Li, 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., 2017Tang, 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
).
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., 2020Zhong, 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.
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.