Investigating Ocean Deoxygenation During the PETM Through the Cr Isotopic Signature of Foraminifera

Abstract

Over the past several decades, oxygen minimum zones have rapidly expanded due to rising temperatures raising concerns about the impacts of future climate change. One way to better understand the drivers behind this expansion is to evaluate the links between climate and seawater deoxygenation in the past especially in times of geologically abrupt climate change such as the Palaeocene-Eocene Thermal Maximum (PETM), a well-characterized period of rapid warming~56 Ma. We have developed and applied the novel redox proxies of foraminiferal Cr isotopes (δ 53 Cr) and Ce anomalies (Ce/Ce*) to assess changes in paleoredox conditions arising from changes in oxygen availability. Both δ 53 Cr and Cr concentrations decrease notably over the PETM at intermediate to upper abyssal water depths, indicative of widespread reductions in dissolved oxygen concentrations. An apparent correlation between the sizes of δ 53 Cr and benthic δ 18 O excursions during the PETM suggests temperature is one of the main controlling factors of deoxygenation in the open ocean. Ocean Drilling Program Sites 1210 in the Pacific and 1263 in the Southeast Atlantic suggest that deoxygenation is associated with warming and circulation changes, as supported by Ce/Ce* data. Our geochemical data are supported by simulations from an intermediate complexity climate model (cGENIE), which show that during the PETM anoxia was mostly restricted to the Tethys Sea, while hypoxia was more widespread as a result of increasing atmospheric CO 2 (from 1 to 6 times preindustrial values).

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

“…In pelagic sediments, elevated U/Ca in foraminifera is thought to semi-quantitatively track overall authigenic U enrichment 55 which is also likely to be reflected in the bulk carbonate data presented here. Together with the higher δ 238 U CAU values, these trends indicate more reducing conditions and are consistent with other proxy evidence in these sediments, including benthic species turnover patterns 21 , 56 , barium accumulation rates (BARs) 10 , cerium anomaly data, and chromium isotopes 20 (Supplementary Figs. 2 and 3 ; Supplementary Note 1 ).…”

“…Unfortunately, for Site 401, we cannot precisely constrain the timing or severity of local redox changes during the initial phase of the CIE (including the acidification interval) using U/Ca and δ 238 U CAU . But, U enrichment factors, measured on bulk sediment digests that were sampled at higher resolution 13 , and foraminiferal chromium isotope data 20 suggest a brief interval of relatively more reducing conditions at the PETM onset compared to the rest of the measured interval (Supplementary Fig. 3 ).…”

“…Despite extensive study, the overall global extent and severity of anoxia during the PETM is poorly quantified, making it difficult to meaningfully compare it to other periods of oceanic anoxia. This limitation is largely due to reliance on local redox proxies, which act as targets for spatially resolved Earth system models 15 , 20 , but yield patchy geographical information and conflicting interpretations 7 . Global-scale geochemical insights, from molybdenum (Mo) isotopes, suggest that euxinia was more prevalent during the PETM than the modern 11 but less extensive than during Mesozoic OAEs 26 .…”

“…Samples are poorly lithified carbonate-rich sediments with a mixture of benthic and pelagic foraminifera, or carbonate nannofossil ooze, and show variable contributions from detrital clays 35 – 37 . The intermediate-depth equatorial Pacific is thought to have been mostly oxygenated before and during the PETM 14 , 20 , and δ 238 U CAU is likely to reliably record δ 238 U sw . By contrast, the other two sites experienced varying degrees of de-oxygenation with observable effects on local U systematics 13 , 20 .…”

“…The intermediate-depth equatorial Pacific is thought to have been mostly oxygenated before and during the PETM 14 , 20 , and δ 238 U CAU is likely to reliably record δ 238 U sw . By contrast, the other two sites experienced varying degrees of de-oxygenation with observable effects on local U systematics 13 , 20 . The δ 238 U CAU from Site 865 and published δ 13 C data 1 provide constraints for an established carbon–phosphorus–uranium (C–P–U) biogeochemical box model 32 , based on the more comprehensive COPSE model 38 , to quantify the extent of seafloor anoxia during the PETM.…”