Modelling the oceanic Nd isotopic composition on a global scale

Since the end of the eighties, it is recognized that the Nd isotopic composition (expressed as εNd) varies from ca -13 in the North Atlantic to ca -5 in the Pacific. It has been clearly established that far from any source of lithogenic material, εNd is a conservative tracer of water mass mixing. Since the range of εNd values of the deep waters are imprinted in the metalliferous sediments, they are used by paleoceanographers to trace the past variations of the thermohaline circulation. However, many studies conducted in the present day ocean highlight that the Nd budgets based only on dust/riverine inputs were not able to reconcile the Nd content and εNd variations between the 3 oceanic basins (the 'Nd paradox'). The 'missing source' was often suspected to be the oceanic margins but was quantified only recently. This talk proposes modelling results of the Nd oceanic distribution (concentration and isotopic composition) on a global scale. For doing that, we first established an extensive compilation of the εNd signatures of the world continental margins (deduced from more than 50 references, figure below). Then, we applied the rate of exchange of Nd between the continental margins and the oceans, deduced from Lacan and Jeandel (in rev.). This allowed to simulate lithogenic Nd inputs into a large scale circulation model (ORCA/OPA). Simulated vertical profiles are compared to those measured in the different oceanic basins