Evidence from authigenic uranium for increased productivity of the glacial Subantarctic Ocean

Authigenic uranium is precipitated in reducing sediments and therefore responds both to changes in particulate organic carbon flux to the sediment and to changes in the oxygen concentration of bottom waters. By examining a large number of cores over a wide latitudinal and depth range in the Atlantic sector of the Southern Ocean, we hope to distinguish between a predominantly productivity-driven authigenic U signal and a circulation or sediment-focusing signal. We find little to no authigenic U in Holocene sediments throughout the South Atlantic Ocean. Glacial sediments north of ∼40°S lack authigenic U, whereas sediments from the Subantarctic Zone show substantial enrichments in authigenic U relative to the Holocene (up to ∼5.3 ppm). The widespread distribution of glacial U enrichment, even in cores with no glacial-interglacial change in mass accumulation rate, implies that U deposition was not caused by an increased supply of organic carbon via sediment focusing. Authigenic U and organic carbon in a shallow core (∼1000 m) from the Subantarctic region show the same glacial increase found in the deep cores. Because this site was well ventilated during the Last Glacial Maximum, its record provides further evidence that increased glacial productivity of the Subantarctic region contributed to the high concentrations of authigenic U found in the glacial sections of deep cores.