Distribution and sources of organic matter in size-fractionated nearshore sediments off the Barcelona city (NW Mediterranean)

We have found a wide range of OC contents, from 0.13 to 8.68%, depending on water depth and sediment particle size. The highest OC concentration was always found in the clay fraction (<4 μm), followed by the sand fraction (>63 μm) that contained terrestrial plant debris. Wave activity, discharge of the Besòs River and the “Espigó de Ginebra” outfall were the main mechanisms controlling the sorting of sediments by their grain size and thus the distribution of OC in the inner shelf off Barcelona. In addition, we observed that the organic matter in clay particles was progressively degraded seawards, probably because these particles remain suspended in the water column much more time compared to those that are heavier and, therefore, they are exposed for longer time periods to oxygenated conditions. Both OC/TN ratios and δ13C values found suggest that the organic matter preserved was predominantly land supplied.