T15 Understanding the Links between Life, Biogeochemical Cycles, and Environmental Feedbacks throughout the Mesozoic

Sunday, 22 September 2024
8:00 AM - 12:00 PM
207B

DEOXYGENATION PRIOR TO OAE-2

Paper number: T15-11
Program: Technical Sessions
Time: 10:35 AM – 10:50 AM

Authors

Abstract

Understanding and constraining redox conditions surrounding “Oceanic Anoxic Events” (OAEs), as they represent relatively recent intervals of major climate upheaval, are of significant interest given the direct connection to modern deoxygenation and climate change. These ancient events bury a significant portion of organic matter (OM) that is recorded by large carbon isotope excursions (δ13C). The enhanced burial of OM led to the consumption of dissolved oxygen in some portions of the marine water column (and some freshwater environments), global cooling due to the consumption of atmospheric carbon dioxide, and major reserves of fossil fuels.

The term OAE suggests that the reducing conditions during the Mesozoic were restricted to these intervals, defined by δ13C chemostratigraphy. However, there is growing evidence that these OAEs were the culmination of much longer climatically driven events. Applying a multiproxy approach to samples prior to the OAE document local reducing conditions at the continuously organic-rich samples from Demerara Rise. The sedimentary thallium (Tl) isotope proxy, paired with proxies for local redox, provide a global record of changes in ocean oxygen by tracking the burial of manganese oxides, which require the presence of dissolved marine oxygen – one of the first compounds to respond to redox changes. Thus, perturbations in Tl isotopes may provide a record of the earliest changes in marine oxygen. This study provides a unique window into studying an expanded Cretaceous record. Leading up to the Late Cretaceous Cenomanian-Turonian boundary (OAE-2), Tl isotopes document widespread reducing conditions while trace metals suggest limited euxinia on a global scale. Current data suggest that OAEs represent the final event within a longer-term climate scenario, and also that local redox conditions are more variable spatiotemporally. This is likely the result of the expansion and contraction of the oxygen minimum zone (OMZ). Thus, during this time interval that leads up to OAE-2 redox conditions are far more dynamic than generally appreciated. The combination of local and global redox proxies from locations around the world provides a more complete understanding of the entire redox ladder across OAEs, which may be paramount to our predictions of environmental change in a deoxygenating world.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-402938

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