Preprints
https://doi.org/10.5194/gmd-2021-5
https://doi.org/10.5194/gmd-2021-5

Submitted as: model description paper 06 May 2021

Submitted as: model description paper | 06 May 2021

Review status: this preprint is currently under review for the journal GMD.

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

Yusuke Sasaki, Hidetaka Kobayashi, and Akira Oka Yusuke Sasaki et al.
  • Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan

Abstract. Sedimentary 231Pa / 230Th ratios provide clues to estimate the strength of past ocean circulation. For its estimation, understanding the processes controlling the distributions of dissolved 231Pa and 230Th in the ocean is important. However, simulations of dissolved and particulate 231Pa and 230Th in the modern ocean, recently obtained from the GEOTRACES project, remain challenging. Here we show an improved model simulation of 231Pa and 230Th 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 231Pa and 230Th 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 230Th in the Southern Ocean. Our best simulation can well reproduce not only the oceanic distribution of 231Pa and 230Th but also the sedimentary 231Pa / 230Th ratios. Sensitivity analysis reveals that oceanic advection of 231Pa primarily determines sedimentary 231Pa / 230Th ratios. On the other hand, 230Th advection and bottom scavenging have an opposite effect to 231Pa advection on the sedimentary 231Pa / 230Th ratios, reducing their latitudinal contrast. Our model realistically simulates the residence times of 231Pa and 230Th, whereas previous models that reported the similar distribution of sedimentary 231Pa / 230Th ratios significantly overestimate the residence times for both 231Pa and 230Th.

Yusuke Sasaki et al.

Status: open (until 17 Jul 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Yusuke Sasaki et al.

Yusuke Sasaki et al.

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Short summary
For realistically simulating the recently observed distributions of dissolved 230Th and 231Pa in the ocean, we highlight the importance of the removal process of 231Pa and 230Th at the seafloor (bottom scavenging) and the dependence of scavenging efficiency on particle concentration. We show that consideration of these two processes can well reproduce not only the oceanic distribution of 231Pa and 230Th but also the sedimentary 231Pa / 230Th ratios.