Abstract
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.
Highlights
Oceanic circulation and the carbon cycle play a major role in the regulation of the past and present climate
Our model is part of a global general circulation model, but instead of solving the Navier–Stokes equations, our tracers are advected by the circulation fields of a previous simulation of the dynamical ocean model
The overturning stream function (OSF) is defined as the zonally and vertically integrated meridional component of the current velocity. We use this as a measure for the Atlantic Meridional Overturning Circulation (AMOC)
Summary
Oceanic circulation and the carbon cycle play a major role in the regulation of the past and present climate. Heat and carbon dioxide in the atmosphere tend to equilibrate with the ocean surface and are transported down into the deep ocean through the Meridional Overturning Circulation (MOC). Biogeochemical cycling generates organic carbon that transfers into the deep ocean through particle sinking. The strength of the MOC and particle removal participate actively in the regulation of the climate on the Earth. Trace elements are affected by these mechanisms and represent useful tools to provide constraints on these processes. It is difficult to measure the MOC strength, and for past climate there are no measurements available at all. Isotopes and trace elements from sediment cores are used as proxies to infer past ocean circulation. Several examples include carbon isotopes (Broecker et al, 1988), the cadmium/calcium ratio (Rosenthal et al, 1997), the ratio between protactinium-231 and thorium-230 (231Pa/230Th) (Böhm et al, 2015), and the neodymium iso-
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