An evaluation of benthic foraminiferal U/Ca and U/Mn proxies for deep ocean carbonate chemistry and redox conditions

Abstract

The deep ocean is thought to have played a crucial role in modulating atmospheric CO2 changes, and thus reconstructions of deep ocean conditions can place important constraints on the past global carbon cycle. Some previous studies suggested that foraminiferal U/Ca could be used to infer seawater carbonate chemistry changes, but others showed complications from diagenesis and temperature. A recent downcore study suggested that foraminiferal U/Mn may be used for sedimentary redox-conditions, but no core-top work has been done to investigate factors affecting U/Mn. We investigate controlling factors on U/Ca and U/Mn in two benthic foraminiferal species from 120 global core-tops and three Atlantic sediment cores. Our core-top data reveal no significant correlation between core-top benthic U/Ca and carbonate system parameters. The lack of an influence of deep-water [CO2-3] on U/Ca is further supported by our downcore results. Together, our data highlight complications to use benthic U/Ca for deep-water carbonate chemistry reconstructions. Although no correlation is found between core-top U/Mn and hydrographic data, high-resolution U/Mn and U/Ca in core TNO57-21 show similar patterns to authigenic U (aU) and vary in tandem with atmospheric CO2 on millennial timescales. Changes in U/Mn, U/Ca and aU in TNO57-21 may reflect postdepositional diagenesis linked to sedimentary oxygen, which is controlled by subantarctic surface productivity and ventilation of deep South Atlantic in the past. We suggest that benthic U/Mn and U/Ca may be used as auxiliary indicators for past sedimentary redox-conditions and along with other proxies could reflect deep-water oxygenation.