Reconstructing ocean oxygenation changes from U/Ca and U/Mn in foraminiferal coatings: Proxy validation and constraints on glacial oxygenation changes

Abstract

Deep-sea oxygen concentrations reflect combined effects of air-sea exchange in high-latitude surface waters, ventilation through ocean circulation and the organic carbon remineralization at depth. Reconstruction of past bottom water oxygen (BWO) concentrations has been challenging due to limitations of each existing BWO proxy whose fidelity may be complicated by diagenetic or depositional factors. Therefore, evaluations on BWO changes with multi-proxy approach are always preferred. In this study, we exploit the authigenic uranium content on mixed planktonic foraminiferal coatings as a BWO proxy by presenting new foraminiferal U/Ca and U/Mn ratios of the Holocene and last glacial maximum (LGM) sediments from 54 sites throughout the Pacific Ocean, covering a range of modern BWO from 8 to 210 μmol/kg. Distinct correlation between Holocene foraminiferal U/Ca and U/Mn with BWO is observed in the modern Pacific Ocean, which has not been obscured by other complicating factors such as sedimentation rate and organic carbon flux. Based on the comparison of our foraminiferal U/Ca and U/Mn ratios between the Holocene and LGM and existing redox proxy data, we provide new constraints on Equatorial and South Pacific oxygenation changes during the LGM. First, the boundary between better oxygenated upper ocean and less oxygenated deeper ocean in the Eastern Equatorial Pacific was limited to a narrower water depth range between ∼0.6 and 0.7 km surrounding the Panama Basin. Second, our data imply better oxygenation in the upper and bottom waters of the Pacific Ocean and mid-depth deoxygenation, which contrasts with findings in the deep Atlantic and Indian Oceans. After excluding influences from other factors such as sedimentation rates and productivity, our study demonstrates foraminiferal U/Ca and U/Mn provide a useful proxy for BWO reconstruction in the Pacific, thus helping to constrain the glacial-interglacial oceanic carbon cycle.