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

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.