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Volume 11, issue 9 | Copyright

Special issue: Nucleus for European Modelling of the Ocean - NEMO

Geosci. Model Dev., 11, 3537-3556, 2018
https://doi.org/10.5194/gmd-11-3537-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model description paper 31 Aug 2018

Model description paper | 31 Aug 2018

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

Marco van Hulten1,2, Jean-Claude Dutay1, and Matthieu Roy-Barman1 Marco van Hulten et al.
  • 1Laboratoire des Sciences du Climat et de l'Environnement, IPSL, CEA–Orme des Merisiers, 91191 Gif-sur-Yvette, France
  • 2Geophysical Institute, University of Bergen, Bergen, Norway

Abstract. In this paper we set forth a 3-D ocean model of the radioactive trace isotopes 230Th and 231Pa. 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 230Th and 231Pa 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 of today, there are no non-trivial prognostic models available on a global scale. Our simulations reproduce 230Th and 231Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230Th and 231Pa 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 231Pa∕230Th ratio observed in the sediments and supports a moderate affinity of 231Pa to biogenic silica as suggested by recent observations relative to 230Th.

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.

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We present an ocean model of the natural radioactive isotopes thorium-230 and protactinium-231. These isotopes are often used to investigate past ocean circulation and particle transport. They are removed by particles produced by plankton and from uplifted desert dust that is deposited into the ocean. We approach observed dissolved and adsorbed Th-230 and Pa-231 activities. The Pa-231 / Th-230 sedimentation ratio is reproduced as well; this quantity can be used as a proxy for ocean circulation.
We present an ocean model of the natural radioactive isotopes thorium-230 and protactinium-231....
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