An investigation into the processes controlling the global distribution of dissolved 231Pa and 230Th in the ocean and the sedimentary 231Pa / 230Th ratios by using an ocean general circulation model COCO ver4.0

• Main Authors: Sasaki, Yusuke, Kobayashi, Hidetaka, Oka, Akira
• Format: Text
• Language: English
• Published: 2021
• Subjects: Southern Ocean
• Online Access: https://doi.org/10.5194/gmd-2021-5; https://gmd.copernicus.org/preprints/gmd-2021-5/

Sedimentary 231 Pa / 230 Th ratios provide clues to estimate the strength of past ocean circulation. For its estimation, understanding the processes controlling the distributions of dissolved 231 Pa and 230 Th in the ocean is important. However, simulations of dissolved and particulate 231 Pa and 230 Th in the modern ocean, recently obtained from the GEOTRACES project, remain challenging. Here we show an improved model simulation of 231 Pa and 230 Th in the global ocean by introducing bottom scavenging and the dependence of scavenging efficiency on particle concentration with water-column reversible scavenging. The incorporation of bottom scavenging improves the simulated distribution of dissolved 231 Pa and 230 Th in the deep ocean, which has been overestimated in models not considering the bottom scavenging. We further demonstrate that introducing the dependence of scavenging efficiency on particle concentration results in a high concentration of dissolved 230 Th in the Southern Ocean. Our best simulation can well reproduce not only the oceanic distribution of 231 Pa and 230 Th but also the sedimentary 231 Pa / 230 Th ratios. Sensitivity analysis reveals that oceanic advection of 231 Pa primarily determines sedimentary 231 Pa / 230 Th ratios. On the other hand, 230 Th advection and bottom scavenging have an opposite effect to 231 Pa advection on the sedimentary 231 Pa / 230 Th ratios, reducing their latitudinal contrast. Our model realistically simulates the residence times of 231 Pa and 230 Th, whereas previous models that reported the similar distribution of sedimentary 231 Pa / 230 Th ratios significantly overestimate the residence times for both 231 Pa and 230 Th.