River signature over coastal area (Eastern Mediterranean): Grain size and geochemical analyses of sediments

Abstract

Rivers and coastal areas are two ecosystems in permanent interaction. Thus, to understand the process occurring at the land–sea continuum, the determination of the origin and fate of sediment and its associated organic matter is critical.Moreover, tracing the source of particulate matter from terrestrial origin has not been adequately carried out in the eastern Mediterranean basin, especially in Lebanon. In order to differentiate between the terrestrial and marine sources of particulate matter and characterize the depositional environments, grain size composition, organic carbon (OC), total nitrogen (TN), Isotopic Ratio (δ13C), the terrestrial and labile fractions of organic matter, as well as, photosynthetic pigments were analyzed for surface sediment samples collected from the coastal marine area facing the mouth of Ibrahim river and from the river watershed.This study shows that the combined use of the grain size composition of sediments and carbon isotopic signatures (δ13C) of its associated organic matter is an efficient tool to track the source and fate of organic matter in highly dynamic estuarine and coastal ecosystems.The obtained results illustrate the occurrence of two grain size compositional types associated with two different depositional environments. Shallow stations (≤30 m) are characterized by a hydrological sorting and strong hydrodynamic conditions leading to the winnowing of the finer fraction to deep stations (≥60 m) found to be impacted by river inputs.Organic carbon and total nitrogen values are strongly correlated with the mud fraction controlling their distribution, and therefore leading to their accumulation in the sediment of deep stations (≥60 m). On the other hand, δ13C values and terrestrial fraction percentage suggest the occurrence of riverine materials at the stations (≥60 m), while at the shallow stations, addition of local autochthonous production materials occurs associated with high values of chlorophyll and labile organic matter fraction.