Geochemical redox proxies indicate that seafloor anoxia occurred during the latest Ordovician glacial maximum, coincident with the second pulse of the Late Ordovician mass extinction. However, expanded anoxia in a glacial climate strikingly contrasts with the warming-associated Mesozoic anoxic events and raises questions as to both the causal mechanism of ocean deoxygenation and its relationship with extinction. Here we firstly report iodine-to-calcium ratio (I/Ca) data that document increased upper-ocean oxygenation despite the concurrent expansion of seafloor anoxia. We then resolve these apparently conflicting observations as well as their relationship to global climate by means of a series of Earth system model simulations. Applying available Late Ordovician (Hirnantian) sea-surface temperature estimates from oxygen isotope studies as constraints, alongside our I/Ca data, leads us to identify a scenario in which Hirnantian glacial conditions permit both the spread of seafloor anoxia and increased upper-ocean oxygenation. Our simulated mechanism of a reorganization of global ocean circulation, with reduced importance of northern-sourced waters and a poorer ventilated and deoxygenated deep ocean has parallels with Pleistocene state transitions in Atlantic meridional overturning (despite a very different continental configuration) and suggests that no simple and predictable relationship between past climate state and oxygenation may exist.
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The I/Ca data can be downloaded from Zenodo (https://zenodo.org/record/5136966#.YP5vClMzbu6).
The code for the version of the ‘muffin’ release of the cGENIE Earth system model used in this paper is tagged as v0.9.20 and available at https://doi.org/10.5281/zenodo.4618203.
Configuration files for the specific experiments presented in the paper can be found in the directory: genie-userconfigs/MS/pohletal.NatGeo.2020. Details on the experiments, plus the command line needed to run each one, are given in the readme.txt file in that directory. All other configuration files and boundary conditions are provided as part of the code release.
A manual detailing code installation, basic model configuration, tutorials covering various aspects of model configuration and experimental design, plus results output and processing, are available at https://doi.org/10.5281/zenodo.4615662.
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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 838373 (to A.P.), from NSF EAR-2121445, NSF EAR-1349252, OCE-1232620 and OCE-1736542 (to Z.L.), from the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery Grant) and NSF 17-536 (to A.D.), from the David and Lucile Packard Foundation (to S.F.), from NSF grants 1736771 and EAR-2121165 and the Heising-Simons Foundation (2015-145) (to A.R.) and from the National Natural Science Foundation of China (41520104007, 41721002) (to Y.S. and M.L). Calculations were partly performed using HPC resources from the Centre de Calcul de l’Université de Bourgogne of Direction du Numérique.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Michael Melchin, Daniel Horton and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor(s): James Super.
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(a) Latest Ordovician (445 Ma) reconstruction of Scotese and Wright22. (b) Bathymetry used in the cGENIE simulations, derived from (a). In the absence of a better estimate, the deep ocean is a flat bottom (ca. –4200 m). Names of the main continental masses and the Panthalassa Ocean are indicated on the map. Red dots show the location of the 2 sedimentary sections discussed in the text, where I/Ca data have been collected. km a.s.l.: km above sea level.
Boxes mark the 25th and 75th percentiles of values at each time frame, horizontal lines in the box represent the median, and the whiskers show the maximum and minimum. See Fig. 1 for the age of each box and whisker plot. Two-group Mann-Whitney test indicates that I/Ca values in mid HICE versus late HICE are significantly different (p =2.3E-5 for Anticosti Island; p = 0.014 for Copenhagen Canyon).
Extended Data Fig. 4 Density plot illustrating the distribution of feux values (the fraction of global seafloor with euxinic bottom-waters) compatible with carbonate δ238U data from the Hirnantian-Rhuddanian ocean anoxic event7, using a stochastic, three-sink mass balance model8.
In this study we treat these estimates of euxinia as synonymous with anoxia, both due to limited understanding of uranium cycling in ferruginous environments and the lack of complex iron cycling in the current configuration of cGENIE.
Extended Data Fig. 5 Seafloor oxygen concentration simulated using an atmospheric pO2 of ×0.4 and pCO2 and PO4 inventory values of respectively (a) ×24 CO2 and ×0.4 [PO4], (b) ×8.5 CO2 and ×0.4 [PO4], (c) ×7 CO2 and ×0.2 [PO4] and (d) ×5 CO2 and ×0.6 [PO4].
Shifts from (a) to (b) and from (c) to (d) respectively represent Scenarios #1 and #2 (see Fig. 2). Emerged landmasses are shaded white and contoured with a thick black line. Shallow-water platforms are contoured with a thick black line. Seafloor oxygen concentration here represents the oxygen concentration simulated, for each point of the model grid, in the deepest ocean level. It does not represent the oxygen concentration at any given depth in the model but a draped benthic surface.
Extended Data Fig. 6 Extent of seafloor anoxia (defined as percentage of the total seafloor area characterized by a benthic ocean [O2] ≤ 0 μmol kg−1) simulated as a function of pCO2 (x-axis) and ocean PO4 inventory (y-axis), for pO2 levels of (a) ×1.0, (b) ×0.8, (c) ×0.6, (d) ×0.4 and (e) ×0.2.
Each cell is a cGENIE simulation. Values inside each cell and the colormap both represent the extent of anoxia.
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Pohl, A., Lu, Z., Lu, W. et al. Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation. Nat. Geosci. 14, 868–873 (2021). https://doi.org/10.1038/s41561-021-00843-9
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