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.

Marine anoxia linked to abrupt global warming during Earth’s penultimate icehouse

“…2 ). Recent studies indicate that the δ 238 U values of shallow-water carbonates are consistently, by an average of ∼0.24 to 0.27‰, more positive than the contemporaneous seawater ( 47 , 48 ), whereas pelagic carbonate δ 238 U values approximate the seawater signature, suggesting a diagenetic gradient may exist among depositional sites of different paleobathymetry ( 16 , 47 , 49 , 50 ). The isotopic offset results from both fractionation of U isotopes during early diagenetic carbonate dissolution and recrystallization, and authigenic enrichment of isotopically heavy U(IV) in carbonates ( 47 , 48 ).…”

“…For greenhouse OAEs with available carbonate δ 238 U data [e.g., the Permian–Triassic boundary ( 37 , 41 , 64 ), Triassic–Jurassic boundary ( 68 ), OAE2 ( 16 ), as well as the PETM ( 49 )], we ran the same linked LOSCAR and U models in two scenarios of low and high C injection as reported in the literature ( SI Appendix , Fig. S8 and Table S2 ).…”

Isotopic filtering reveals high sensitivity of planktic calcifiers to Paleocene–Eocene thermal maximum warming and acidification

“…A warming-induced decrease in oxygen solubility and concomitant expansion of oxygen minimum zones ( 71 ) would have had dire consequences for planktic foraminifers with deeper depth ecologies such as the thermocline-dwelling subbotinids. Though paleoredox records indicate that oceanic bottom waters remained relatively well ventilated at Site 865 during the PETM ( 72 ), it does not preclude the possibility that an expanded oxygen minimum zone within the overlying water column contributed to the demise of local subbotinid populations by impinging upon their thermocline habitat.…”

A Depth‐Transect of Ocean Deoxygenation During the Paleocene‐Eocene Thermal Maximum: Magnetofossils in Sediment Cores From the Southeast Atlantic

“…However, in some cases, problems in interpretations result from the pervasive deep-sea carbonate dissolution during the PETM (McInerney & Wing, 2011;Penman et al, 2016;Zachos et al, 2004Zachos et al, , 2005. Lack of carbonate in the sediment records means that many records utilizing carbonate-based proxies (e.g., Clarkson et al, 2021;Zhou et al, 2014Zhou et al, , 2016 may not resolve the maximum deoxygenation during the PETM. Moreover, most proxies do not show clearly how dissolved oxygen levels varied over a vertical water mass depth transect (e.g., Pälike et al, 2014;Zhou et al, 2016).…”