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Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2.

ABSTRACT: Oceanic Anoxic Event 2 (OAE 2), occurring ?94 million years ago, was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. It was typified by a rapid rise in atmospheric CO2, global warming, and marine anoxia, leading to the widespread devastation of marine ecosystems. However, the precise timing and extent to which oceanic anoxic conditions expanded during OAE 2 remains unresolved. We present a record of global ocean redox changes during OAE 2 using a combined geochemical and carbon cycle modeling approach. We utilize a continuous, high-resolution record of uranium isotopes in pelagic and platform carbonate sediments to quantify the global extent of seafloor anoxia during OAE 2. This dataset is then compared with a dynamic model of the coupled global carbon, phosphorus, and uranium cycles to test hypotheses for OAE 2 initiation. This unique approach highlights an intra-OAE complexity that has previously been underconstrained, characterized by two expansions of anoxia separated by an episode of globally significant reoxygenation coincident with the "Plenus Cold Event." Each anoxic expansion event was likely driven by rapid atmospheric CO2 injections from multiphase Large Igneous Province activity.

SUBMITTER: Clarkson MO

PROVIDER: S-EPMC5866551 | biostudies-literature | 2018 Mar

REPOSITORIES: biostudies-literature

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Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2.

Clarkson Matthew O MO Stirling Claudine H CH Jenkyns Hugh C HC Dickson Alexander J AJ Porcelli Don D Moy Christopher M CM Pogge von Strandmann Philip A E PAE Cooke Ilsa R IR Lenton Timothy M TM

Proceedings of the National Academy of Sciences of the United States of America 20180305 12

Oceanic Anoxic Event 2 (OAE 2), occurring ∼94 million years ago, was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. It was typified by a rapid rise in atmospheric CO<sub>2</sub>, global warming, and marine anoxia, leading to the widespread devastation of marine ecosystems. However, the precise timing and extent to which oceanic anoxic conditions expanded during OAE 2 remains unresolved. We present a record of global ocean redox changes during OAE 2 using ...[more]

PMID: 29507196

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Project description:Anoxic marine zones were common in early Paleozoic oceans (542-400 Ma), and present a potential link to atmospheric pO2 via feedbacks linking global marine phosphorous recycling, primary production and organic carbon burial. Uranium (U) isotopes in carbonate rocks track the extent of ocean anoxia, whereas carbon (C) and sulfur (S) isotopes track the burial of organic carbon and pyrite sulfur (primary long-term sources of atmospheric oxygen). In combination, these proxies therefore reveal the comparative dynamics of ocean anoxia and oxygen liberation to the atmosphere over million-year time scales. Here we report high-precision uranium isotopic data in marine carbonates deposited during the Late Cambrian 'SPICE' event, at ca. 499 Ma, documenting a well-defined -0.18‰ negative ?238U excursion that occurs at the onset of the SPICE event's positive ?13C and ?34S excursions, but peaks (and tails off) before them. Dynamic modelling shows that the different response of the U reservoir cannot be attributed solely to differences in residence times or reservoir sizes - suggesting that two chemically distinct ocean states occurred within the SPICE event. The first ocean stage involved a global expansion of euxinic waters, triggering the spike in U burial, and peaking in conjunction with a well-known trilobite extinction event. During the second stage widespread euxinia waned, causing U removal to tail off, but enhanced organic carbon and pyrite burial continued, coinciding with evidence for severe sulfate depletion in the oceans (Gill et al., 2011). We discuss scenarios for how an interval of elevated pyrite and organic carbon burial could have been sustained without widespread euxinia in the water column (both non-sulfidic anoxia and/or a more oxygenated ocean state are possibilities). Either way, the SPICE event encompasses two different stages of elevated organic carbon and pyrite burial maintained by high nutrient fluxes to the ocean, and potentially sustained by internal marine geochemical feedbacks.

Dataset Information

Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2.

Publications

Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2.

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