Uranium-isotope records of global ocean deoxygenation during the Early Aptian Oceanic Anoxic Event (OAE 1a)

The Phanerozoic Aeon was marked by several variations in global oxygenation levels, occuring both gradually over multi-million year timescales and also more abruptly as transient perturbations lasting typically a million years or less. Following an overall trend of rising atmospheric oxygen levels in the mid–late Palaeozoic, marine redox conditions are thought to have been close to modern by the time of the Cretaceous Period (145–66 Ma). However, the Cretaceous Period also featured multiple episodes of geologically abrupt depletions in seawater oxygen levels, known as Oceanic Anoxic Events (OAEs), one of the most severe of which occurred during the Early Aptian (OAE 1a, ~120 Ma). This environmental crisis is thought to have been triggered by major carbon emissions related to the volcanic formation of the Greater Ontong-Java Plateau. Several OAE 1a sites are 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, the relative paucity of Early Aptian open-ocean sedimentary records means that the degree to which anoxic conditions spread throughout the global marine realm during OAE 1a remains poorly constrained.

Here, we aim to verify the nature of seawater oxygen levels prior to, during, and after OAE 1a, using uranium-isotope (δ²³⁸U) records of that event. Under iron-reducing conditions, soluble U⁶⁺ transforms to insoluble U⁴⁺, which is associated with a pronounced isotopic fractionation in favour of ²³⁸U in U⁴⁺ ions. Thus, the subsequent sequestration of U⁴⁺ in organic-rich sediments causes depletion of ²³⁸U in the water column and a shift to an isotopically lighter δ²³⁸U composition of seawater. This change in marine δ²³⁸U is recorded by carbonates precipitated in seawater. Thus, δ²³⁸U trends across various records of OAE 1a enable the hypothesis that background Cretaceous ocean redox conditions were comparable to today to be tested, and the change in geographic extent of anoxic water masses during the environmental change quantified. By further comparing the δ²³⁸U data with other geochemical proxies (e.g., carbon-isotope evidence of organic-matter burial; osmium-isotope evidence of volcanism), we further explore the causes and environmental consequences of transient ocean redox fluctuations in the Early Cretaceous oceans.