Modeling the Nd isotopic composition in the North Atlantic basin using an eddy-permitting model

Boundary Exchange (BE - exchange of elements between continental margins and the open ocean) has been emphasized as a key process in the oceanic cycle of neodymium (Nd) (Lacan and Jeandel, 2005a). Here, we use a regional eddy-permitting resolution Ocean General Circulation Model (1/4A degrees) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental margins. Although the parameterization of the BE is the same in both configurations of the model, the characteristic exchange time in the eddy-permitting configuration is significantly lower than the previous evaluations using a low resolution configuration (6 months to 10 years), but however in agreement with the available seawater Nd isotope data. This results highlights the importance of the model dynamics in simulating the BE process.