  In this paper, we set forth a 3-D ocean model of the radioactive trace
isotopes Th-230 and Pa-231. 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-230 and Pa-231 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-230 and Pa-231 dissolved concentrations: they
compare well with recent GEOTRACES observations in many parts of the ocean.
Particulate Th-230 and Pa-231 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-231/Th-230 ratio observed in the sediments, and
supports a moderate affinity of Pa-231 to biogenic silica as suggested by
recent observations, relative to Th-230.
  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.
