²³¹Pa and ²³⁰Th in the ocean model of the Community Earth System Model (CESM1.3)

Abstract. The sediment ²³¹Pa ∕ ²³⁰Th activity ratio is emerging as an important proxy for deep ocean circulation in the past. In order to allow for a direct model–data comparison and to improve our understanding of the sediment ²³¹Pa ∕ ²³⁰Th activity ratio, we implement ²³¹Pa and ²³⁰Th in the ocean component of the Community Earth System Model (CESM). In addition to the fully coupled implementation of the scavenging behavior of ²³¹Pa and ²³⁰Th with the active marine ecosystem module (particle-coupled: hereafter p-coupled), another form of ²³¹Pa and ²³⁰Th have also been implemented with prescribed particle flux fields of the present climate (particle-fixed: hereafter p-fixed). The comparison of the two forms of ²³¹Pa and ²³⁰Th helps to isolate the influence of the particle fluxes from that of ocean circulation. Under present-day climate forcing, our model is able to simulate water column ²³¹Pa and ²³⁰Th activity and the sediment ²³¹Pa ∕ ²³⁰Th activity ratio in good agreement with available observations. In addition, in response to freshwater forcing, the p-coupled and p-fixed sediment ²³¹Pa ∕ ²³⁰Th activity ratios behave similarly over large areas of low productivity on long timescales, but can differ substantially in some regions of high productivity and on short timescales, indicating the importance of biological productivity in addition to ocean transport. Therefore, our model provides a potentially powerful tool to help the interpretation of sediment ²³¹Pa ∕ ²³⁰Th reconstructions and to improve our understanding of past ocean circulation and climate changes.