Influence of particle size and type on 231Pa and 230Th simulation with a global coupled biogeochemical-ocean general circulation model: A first approach

International audienceThe oceanic distributions of 231Pa and 230Th are simulated with the global coupled biogeochemical-ocean general circulation model NEMO-PISCES. These natural nonconservative tracers, which are removed from the water column by reversible scavenging processes onto particles, have been used to study modern and past ocean circulation. Our model includes three different types of particles: particulate organic matter (POM), calcium carbonate (CaCO3), and biogenic silica (BSi). It also considers two particle classes: small particles (POM) that sink slowly (3 m/d) and large particles (POM, CaCO3, BSi) that sink much more rapidly (50 m/d to 200 m/d) in the water column. 231Pa and 230Th are simulated with a reversible scavenging model that uses partition coefficients between dissolved and particulate phases that depend on particle type and size. Model results are then compared with 231Pa and 230Th observations in the water column and modern sediments. A preliminary evaluation of the particle fields simulated by the PISCES model has revealed that particle concentrations are reasonable at the surface but largely underestimated in the deep ocean. Largely to compensate for this, we find it necessary to use partition coefficients that vary as a function of particle size by significantly more that observed to obtain relatively realistic results. In the water column, 231Pa and 230Th fluxes are mainly controlled by the slowly sinking particles and partition coefficients need to be parameterized as a function of particle flux, as suggested by observations. Considering discrepancies between the modeling particle fields and those observed, we were forced to use exaggerated values for partition coefficients in order to get realistic tracer distributions. These 231Pa and 230Th simulations have provided an opportunity to propose some future developments of the PISCES model, in order to make progress in the simulation of trace elements. Assigning calcium carbonate, biogenic silica, and aluminosilicates to the small particle pool represents a credible approach to increase its concentration and subsequently simulate realistic tracer distributions in the water column using reasonable values for the partition coefficients, as well as a realistic fractionation in the sediments at all depths