A Cenozoic seawater redox record derived from ²³⁸U/²³⁵U in ferromanganese crusts

Abstract

Oceanic oxygen levels are projected to drop in certain areas due to warming climate, but the net effect to the overall ocean redox state is difficult to predict. Here we measured the “stable” uranium isotope composition (²³⁸U/²³⁵U) in globally representative hydrogenous ferromanganese crusts in order to reconstruct the redox evolution of the global ocean throughout the Cenozoic. Samples averaging ∼3 Myr intervals have analytically indistinguishable ²³⁸U/²³⁵U throughout the Cenozoic. Combined with a U isotope mass balance model, we suggest that the overall ocean redox state has remained remarkably stable on million year time scales throughout the Cenozoic, despite large surface temperature fluctuations during this time. This suggests that stabilizing feedbacks (for example, nutrient limitation in low oxygen zones) may have prevented dramatic large-scale shifts in oxygen levels in the ocean. However, the Fe-Mn crust record will be unlikely to reflect rapid perturbations in ocean redox state. To investigate these events, sediment archives with faster accumulation rates and redox proxies with faster response time must be explored.