Simulation of global distribution of 231Pa and 230Th and its ratio in the ocean sediment by using an ocean general circulation model

Akira Oka, Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan, Yusuke Sasaki, Atmosphere and Ocean Research Institute, University of Tokyo, Japan and Hidetaka Kobayashi, Atmosphere and Ocean Research Institute University of Tokyo, Tokyo, Japan
As in Nd isotope ratio, the sediment 231Pa/230Th ratio is an important proxy to estimate the strength of ocean circulation in the past. Previous numerical simulations successfully reproduced the present-day global distribution of sediment 231Pa/230Th ratio, but it is still difficult to realistically simulate the observed distributions of dissolved 230Th and 231Pa in the ocean recently obtained from GEOTRACES project. In this study, we report the results of our ocean general circulation simulation which can realistically reproduce both sediment 231Pa/230Th ratio and the observed distribution of 231Pa and 230Th in the ocean. In addition to a reversible scavenging process commonly considered in previous studies, our simulations incorporated additional processes such as bottom scavenging in nepheloid layer at sea bottom and the dependency of scavenging efficiency on particle concentration. Our model successfully captured the observational features of Atlantic transects of dissolved 231Pa and 230Th presented by the GEOTRACES project better than previous modeling studies. In addition, by performing additional sensitivity simulations, we quantitatively evaluated the processes that control the sediment 231Pa/230Th ratio.