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Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes.

ABSTRACT: The Paleocene Eocene Thermal Maximum (PETM) represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global extent and temporal progression of de-oxygenation is poorly constrained. Here we present carbonate associated uranium isotope data for the PETM. A lack of resolvable perturbation to the U-cycle during the event suggests a limited expansion of seafloor anoxia on a global scale. We use this result, in conjunction with a biogeochemical model, to set an upper limit on the extent of global seafloor de-oxygenation. The model suggests that the new U isotope data, whilst also being consistent with plausible carbon emission scenarios and observations of carbon cycle recovery, permit a maximum ~10-fold expansion of anoxia, covering <2% of seafloor area.

SUBMITTER: Clarkson MO

PROVIDER: S-EPMC7810695 | biostudies-literature | 2021 Jan

REPOSITORIES: biostudies-literature

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Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes.

Clarkson Matthew O MO Lenton Timothy M TM Andersen Morten B MB Bagard Marie-Laure ML Dickson Alexander J AJ Vance Derek D

Nature communications 20210115 1

The Paleocene Eocene Thermal Maximum (PETM) represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global extent and temporal progression of de-oxygenation is poorly constrained. Here we present carbonate associated uranium isotope data for the PETM. A lack o ...[more]

PMID: 33452243

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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.

Project description:Abstract The Paleocene‐Eocene Thermal Maximum (PETM) is recognized globally by a negative excursion in stable carbon isotope ratios (δ13C) in sedimentary records, termed the carbon isotope excursion (CIE). Based on the CIE, the cause, duration, and mechanisms of recovery of the event have been assessed. Here, we focus on the role of increased organic carbon burial on continental margins as a key driver of CO2 drawdown and global exogenic δ13C during the recovery phase. Using new and previously published sediment proxy data, we show evidence for widespread enhanced primary production, low oxygen waters, and high organic carbon (Corg) burial in marginal and restricted environments throughout the δ13C excursion. With a new biogeochemical box model for deep and marginal environments, we show that increased phosphorus availability and water column stratification on continental margins can explain the increased Corg burial during the PETM. Deoxygenation and recycling of phosphorus relative to Corg were relatively mild, compared to modern day anoxic marine systems. Our model reproduces the conditions reconstructed by field data, resulting in a burial of 6,000 Pg across the PETM, in excess of late Paleocene burial, and ∼3,300 Pg C for the critical first 40 kyr of the recovery, primarily located on continental margins. This value is consistent with prior data and model estimates (∼2,000–3,000 Pg C). To reproduce global exogenic δ13C patterns, this Corg burial implies an injection of 5,000–10,000 Pg C during the first ∼100–150 kyr of the PETM, depending on the source's δ13C (−11‰ to −55‰). Key Points Data and modeling show increased productivity, deoxygenation, and organic C burial in marginal and restricted environments during the PETM Excess burial of ∼3,300 Pg organic C burial at the early stages of the PETM recovery Excess organic C burial during recovery requires several thousands of Pg of 13C‐depleted carbon input to match global exogenic δ13C trends

Dataset Information

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes.

Publications

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes.

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