T227 Do Carbon Sources or Sinks Drive the Phanerozoic Carbon Cycle?: Understanding Geobiologic Forcings and Feedbacks

Wednesday, 12 October 2022
8:00 AM - 12:00 PM
506

URANIUM-ISOTOPE RECORDS OF GLOBAL OCEAN DEOXYGENATION DURING THE EARLY APTIAN OCEANIC ANOXIC EVENT (OAE 1A)

Paper number: T227-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

The Cretaceous Period was marked by a number of intense environmental perturbations, including a series of episodes of widespread seawater oxygen depletion, or Oceanic Anoxic Events (OAEs). One of the most severe Cretaceous OAEs occurred during the Early Aptian (OAE 1a, ~120 Ma), which is thought to have been triggered by major carbon emissions related to the volcanic formation of the Greater Ontong-Java Plateau. Stratigraphic records of OAE 1a around the world are characterized by a series of carbon-isotope (δ13C)excursions, beginning with a sharp negative shift that is followed by a pair of positive excursions. At several sites, the OAE strata are also marked by the preservation of organic-rich laminated shales, indicative of oxygen-depleted conditions in the water column and at the sediment-water interface. However, there is a relative paucity of open-ocean sedimentary records from the Early Aptian. Thus, the degree to which anoxic conditions spread throughout the global marine realm at that time remains poorly constrained.

Here, we present the first uranium-isotope (δ238U) datasets from four stratigraphic records of OAE 1a. Under reducing conditions, soluble U6+is converted to insoluble U4+, which is sequestered in organic-rich sediments. This conversion is associated with a pronounced isotopic fractionation in favor of 238U in U4+ions. Consequently, the removal of this reduced ion via organic-matter burial (itself enhanced in a reducing environment), causes depletion of 238U in the water column and a shift to an isotopically lighter δ238U composition of seawater. These changes in seawater δ238U are then recorded by carbonates precipitated in the marine realm. Previous studies of the Late Devonian extinction, end-Permian extinction, and Cenomanian–Turonian OAE have all highlighted pronounced negative excursions in seawater δ238U, thought to reflect a major increase in the proportion of the global ocean that became oxygen depleted at these times. By stratigraphically correlating the δ238U results withδ13Crecords of OAE 1a, we show how the geographic extent of marine anoxia changed at that time. Further comparisons with other geochemical data allow discussion of the temporal relationship between the spread of anoxic conditions and onset of submarine volcanic activity thought to have triggered them.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380019

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