Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation
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(11): 868-873. https://dx.doi.org/10.1038/s41561-021-00843-9
In: Nature Geoscience. Nature Publishing Group: London. ISSN 1752-0894; e-ISSN 1752-0908, more
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| Authors | | Top |
- Pohl, A.
- Lu, Z.
- Lu, W.
- Stockey, R.G.
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- Elrick, M.
- Li, M.
- Desrochers, A.
- Shen, Y.
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- He, R.
- Finnegan, S.
- Ridgwell, A.
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| Abstract |
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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