Temporal Pattern and Profile of a Coastal-Deep Sea Conveyor at a Marginal Deep Oligotrophic Sea

Sediment trap data set and ²³⁴Th profiles (deep water excesses and deficits) reveal that particulate organic carbon (POC) export at the highly oligotrophic Levantine Sea is dominated by lateral transport from the nearby margin. These intermediate nepheloid layers (INL) operate at multi-depth, with the silt-to-clay size particulate matter (PM) fraction transported at water depths of about 100–500 m, while finer fraction arrives also at deeper depths. The shallow NIL is triggered by winter storms, manipulated by coastal flash floods and shelf resuspension and assisted by cross-shore currents, which allow the arrival of PM at a distance of 50 km within about 10 days. The deeper INL could be related to sediments initially driven to depth by density currents. Our data show that inter-annual differences in sediment trap fluxes were related to changes in both the intensity of coastal floods and current velocity. The frequent observation of deep-water ²³⁴Th excesses during a (relatively) low export winter (2018) is related to lessened cleansing of the water column, that is, reduced removal of fine-grained PM by sinking coarser-grained material. These observations highlight the importance of winter storm intensity in the POC budget of marginal seas like the Levantine Basin (LB) even in areas with limited river discharge. This further suggests that the anticipated increase in extreme weather events due to the on-going climate change should have an impact on this coastal-deep sea conveyor and on POC export in the LB.