Evaluation of processes occurring in the bottom nepheloid layer (BNL) of an eastern Mediterranean area using 234Th/238U disequilibria

Particle-reactive radionuclide ²³⁴Th and its ratios with the conservative ²³⁸U were used to trace the marine processes occurring over short timescales in the bottom nepheloid layer (BNL) of seven stations in the Saronikos Gulf and the Elefsis Bay (Greece) during three seasons (summer 2008, autumn 2008 and winter 2009). Summer was considered as a steady season where low physical processes occur and stratification is well established, autumn as a commutative period and winter as period of extensive trawling and physical activities. The obtained ratio profiles showed excess of ²³⁴Th relative to ²³⁸U in the BNL of the sampling area during summer, caused by the dissolved fraction of ²³⁴Th. During autumn, the situation was different with large ²³⁴Th deficit throughout the water column leading to large export fluxes of particles from the water column. Finally, during winter the ratios showed that predominant phenomenon in the area was likely resuspension of bottom sediments. The resuspension signature was additionally evaluated by total suspended matter (TSM) inventories in the BNL. Despite the intense resuspension, small scavenging of dissolved ²³⁴Th was recorded in the BNL resulting in high residence times of dissolved ²³⁴Th. A 1 order of magnitude difference between dissolved and particulate ²³⁴Th residence times was observed indicating that scavenging from dissolved to particulate ²³⁴Th could be highly variable and, as a result, the Saronikos Gulf is a highly dynamic environment, in terms of temporal and spatial particle uptake and removal. Comparing these values to literature ones consistent results were obtained. The possibility of sediment resuspension in the BNL during winter was amplified by the bloom of phytoplankton resulting in even decreased residence times of particulate ²³⁴Th (average values). In contrast, the respective residence times of the dissolved fraction of ²³⁴Th in the BNL were higher showing a maximum in winter at the stations where resuspension concluded. Nevertheless, ²³⁴Th cycling in the area is not controlled by TSM, probably due to the presence of colloids, which could play an essential role in ²³⁴Th scavenging.