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. Sedimentary 231Pa/230Th ratios provide clues to estimatethe strength of past ocean circulation. For its estimation, understandingthe processes controlling the distributions of 231Pa and 230Th inthe ocean is important. However, simulations of dissolved and particulate231Pa and 230Th in the modern ocean, recently obtained from theGEOTRACES project, remain challenging. Here we report a model simulation of231Pa and 230Th in the global ocean with COCO ver4.0. Startingfrom the basic water-column reversible scavenging model, we also introducedthe bottom scavenging and the dependence of scavenging efficiency onparticle concentration. As demonstrated in a previous study, theincorporation of bottom scavenging improves the simulated distribution ofdissolved 231Pa and 230Th in the deep ocean, which has beenoverestimated in models not considering the bottom scavenging. We furtherdemonstrate that introducing the dependence of scavenging efficiency onparticle concentration results in a high concentration of dissolved230Th in the Southern Ocean as observed in the GEOTRACES data. Our bestsimulation can well reproduce not only the oceanic distribution of231Pa and 230Th but also the sedimentary 231Pa/230Thratios. Sensitivity analysis reveals that oceanic advection of 231Paprimarily determines sedimentary 231Pa/230Th ratios. On the otherhand, 230Th advection and bottom scavenging have an opposite effect to231Pa advection on the sedimentary 231Pa/230Th ratios,reducing their latitudinal contrast. Our best simulation shows the realisticresidence times of 231Pa and 230Th, but simulation without bottomscavenging and dependence of scavenging efficiency on particle concentrationsignificantly overestimates the residence times for both 231Pa and230Th in spite of similar distribution of sedimentary231Pa/230Th ratios to our best simulation.

geoscientific model development

Global simulation of dissolved &amp;lt;sup&amp;gt;231&amp;lt;/sup&amp;gt;Pa and &amp;lt;sup&amp;gt;230&amp;lt;/sup&amp;gt;Th in the ocean and the sedimentary &amp;lt;sup&amp;gt;231&amp;lt;/sup&amp;gt;Pa∕&amp;lt;sup&amp;gt;230&amp;lt;/sup&amp;gt;Th ratios with the ocean general circulation model COCO ver4.0