238U/235U in deep-sea corals reflects limited expansion of seafloor anoxia in last ice age

Although much evidence suggests a decrease in deep ocean oxygen levels during the last glacial period, a quantitative global constraint on anoxic versus suboxic seafloor area is still lacking. Establishing such a constraint is challenging, because while changes in the biological pump are thought to drive deep ocean oxygen depletion during glacial time, concurrent changes in sea level and ocean circulation may have variable effects on the prevalence of anoxia on continental shelves. Here we use the uranium isotope redox proxy in cold-water corals to constrain anoxic seafloor area over the last 220 kyr. All samples show modern-like δ²³⁸U values within tight bounds (<0.05 %0), allowing very little change in anoxic seafloor over this interval. This contrasts with coeval carbonate sediments that show large δ²³⁸U variations due to diagenetic alteration. It also contrasts with other redox proxy records, including authigenic uranium enrichments and sedimentary thallium isotopes, that show evidence of glacial oxygen depletion. We conclude that while the glacial ocean experienced an expansion of deep water suboxia, global seafloor anoxia remained roughly constant.

Duke Scholars

Author Michael Kipp Earth and Climate Sciences