A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with realistic particle dynamics (ProThorP 0.1)

In this paper, we set forth a 3-D ocean model of the radioactive trace isotopes Th and Pa. The interest arises from the fact that these isotopes are extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles. Our simulations of Th and Pa are based on the NEMO–PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and biogenic silica. Small and big lithogenic particles from dust deposition are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEOTRACES programme, except for boundary nepheloid layers for which, as up to today, there are no non-trivial, prognostic models available on a global scale. Our simulations reproduce Th and Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate Th and Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the Pa/Th ratio observed in the sediments, and supports a moderate affinity of Pa to biogenic silica as suggested by recent observations, relative to Th. Future model development may further improve understanding, especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model, as its source code is readily available.