Impact of storm-induced remobilization on grain size distribution and organic carbon content in sediments from the Blanes Canyon area, NW Mediterranean Sea

Interactions between organic carbon and mineral particles are known to play a key role for the preservation of organic carbon in marine sediments. In this study we combine meteorological, hydrological and oceanographic information with grain size and geochemical analyses (organic carbon, OC, its stable isotope δ13C, and the nitrogen to OC ratio) in different grain size fractions of surface sediments to determine the impact of a major storm on organic matter redistribution in a deep continental margin of the NW Mediterranean Sea. Sediment cores were obtained from the continental shelf, along the axis of the Blanes submarine canyon (BC), and in the adjacent southern open slope before (November 2008) and after (February and May 2009) the occurrence of one of the most severe coastal storms recorded in the last century in the study area.The storm occurred the 26 of December 2008 and dramatically impacted the North Catalan coast and its offshore area. The high shear stress produced by this event on the shelf floor led to sediment erosion and, subsequently, to its transport and deposition into BC. While sand and gravel-sized particles up to 2500μm in diameter were deposited at 300m of water depth in the canyon head, the finer particles (<4μm) were transported farther offshore towards the deep margin reaching at least 1500m depth.The results obtained show that the hydrodynamic processes that controlled the sorting and differential deposition of particles according to their grain size also controlled the redistribution of particles with different loads of OC. Thus, whereas coarse particles transported to the canyon head were poorly loaded with OC, fine particles reaching greater depths were relatively rich in OC, a significant proportion of which was of marine origin (i.e. derived from marine primary production). This adds to evidence that the physical action of waves and currents during storms plays a key role in determining the amount of OC stored in surface sediments not only in the shelf but also in the deep margin and possibly deeper.