Authigenic uranium deposition in the glacial North Atlantic: Implications for changes in oxygenation, carbon storage, and deep water-mass geometry

Oxygen in the ocean has essential ecological and climatic functions, and can be an important indicator of deep-ocean ventilation and carbon storage. Previous studies are divided on whether the subsurface North Atlantic, which today is well-oxygenated, had higher or lower oxygen levels during the Last Glacial Maximum (LGM). Crucially, the limited number of previous reconstructions precludes any conclusions regarding basin-wide patterns in past changes in oxygenation. Authigenic uranium in deep-sea sediments is a sensitive redox tracer that can shed light on bottom water oxygen. Here, we leverage published and new U- and Th-series isotope measurements from North Atlantic sediments to calculate the mass accumulation rate of authigenic uranium (aU MAR) during the Holocene and the LGM. We find that greater aU burial, reflecting lower-than-Holocene oxygen levels and correspondingly greater respired carbon storage, were persistent features of the LGM in the deep North Atlantic. The eastern basin was substantially less well-oxygenated than the western basin. This zonal contrast is possibly related to the farther advance and greater infilling to the east of the mid-Atlantic Ridge of deep waters originating from the Southern Ocean. An alternative explanation is the different residence time in the two basins of deep waters originating from the subpolar North Atlantic. Previous compilations of two nutrient tracers, δ13C and CdW, are consistent with the varying-deep-circulation interpretation of our aU MAR dataset. The observed threshold behavior of aU or the pattern of export productivity, especially at high latitudes, may also have enhanced this west-east difference.