Across Margin Export of Organic Matter by Cascading Events Traced by Stable Isotopes, Northwestern Mediterranean Sea

We present a 1-yr (2005–2006) record of organic carbon (OC) and total nitrogen (TN) contents and their isotopic ratios (δ¹³Coc and δ¹⁵N) in sinking particles from the western Gulf of Lions. Maximum OC and TN fluxes recorded in January 2006 and March–April 2006 were associated with negative near-bottom temperature anomalies. This reveals large inputs of resuspended organic matter from the shelf basinwards to depths > 1500 m by dense shelf-water cascading. The first pulse of organic matter had low $\delta ^{13} C_{OC} $ (–25.5‰) and N:C (0.08) and high δ¹⁵N (6.6‰) values, indicative of the arrival of organic matter of terrestrial origin. In contrast, the second pulse had high $\delta ^{13} C_{OC} $ (–21.9‰) and N:C (0.15) and extremely low δ¹⁵N (0.0‰) values, indicative of the transfer of organic matter with higher marine contribution. Though downslope export of organic matter from the adjacent shelf predominated, particles escaping from surface waters remained an important source of organic matter during low-energy and low-flux periods and especially during summer conditions, with significant contribution of N₂ fixation to organic matter export. Overall, our results suggest the export of organic matter from different sources to the deep Mediterranean basin, which in turn might alter within a couple of months the quality of the sedimentary organic matter deposited on the sea floor and thus the dynamics of the deep ecosystems.