URANIUM ISOTOPE EVIDENCE FOR REDOX COUPLING OF THE CARBON CYCLE

Canonical models of the exogenic carbon cycle suggest that the globally-averaged proportional burial of organic matter in sediments broadly controls the carbon isotopic composition of seawater and marine carbonates worldwide. If correct, the redox state of the world oceans, in addition to the extent of primary productivity and sediment accumulation rates, should play a significant role in controlling the extent to which organic carbon is sequestered in sediments vs. remineralized prior to long-term burial. Thus, periods of widespread anoxia, with the development of euxinic conditions along continental margins and in deep basins, should promote organic carbon export and burial, as well as the ¹³C enrichment of carbonates and newly-formed organic matter. To test this relationship, we surveyed time-series trends in carbonate uranium isotopes—a powerful proxy for global ocean redox—across a series of major positive carbon isotope excursions through Earth history. In carbonates deposited from the Ediacaran through Cretaceous periods, a systematic relationship is often observed between negative δ²³⁸U and positive δ¹³C excursions, with the onset of the uranium isotope decline immediately preceding the carbon isotope rise. Insofar as uranium isotope compositions are controlled by the areal extent of euxinic environments in the oceans, this relationship between δ¹³C and δ²³⁸U provides direct evidence for a coupling between the carbon cycle and the redox state of the global oceans. The link between these two isotope systems is strengthened by the tendency for euxinia—to which U isotopes respond—to be concentrated along continental margins where most organic carbon is sequestered. Furthermore, the temporal offset of peak excursion values is consistent with the development of anoxia before the organic carbon burial events. This synthesis supports the standard model for global changes in the δ¹³C compositions of marine carbonates, and highlights the importance of environmental perturbations on the trajectory of animal life.