Downward fluxes of settling particles in the deep Cretan Sea (NE Mediterranean)

Abstract

During the CINCS project (Pelagic–benthic Coupling IN the oligotrophic Cretan Sea—NE Mediterranean), a single mooring with two sediment traps (at 200 and 1515m water depth) and two current meters was deployed in the southern Cretan Sea margin at a depth of 1550 m. A second mooring deployed at the 500 m station was lost, as a result of fishing activities. The duration of the study was 12 months (November 1994 to November 1995) with sampling intervals of 15 or 16 days. The traps were retrieved, serviced and the sedimented material was collected every 6 months. In total, 48 samples were collected (24 from each trap) throughout the study period and fluxes of total particulate mass, opal, organic matter, carbonates, and lithogenic component were measured. Natural radionuclides ( 210Po and 210Pb) were determined for all trap samples. Total mass flux and the fluxes of four major constituents increased with depth, the total mass flux reaching values of nearly 550 mg m −2 d −1 at 1515 m and 187 mg m −2 d −1 at 200 m depth, following the same patterns observed in other experiments (ECOMARGE, SEEP-I, SEEP-II). The mean annual mass fluxes were 209 and 49.8 mg m −2 d −1 at the near bottom and near surface trap respectively. This suggests that lateral transport of particulate matter is of importance in the area. Total mass fluxes at the two depths were characterized by different seasonal fluctuations, although a general decreasing trend was observed from the I (winter) to the II (summer) deployment at both depths. This was mainly a result of reductions in aluminosilicate inputs during the summer dry period. At 200 m depth carbonates were more important during winter, because of a large carbonate input consisting mainly of coccoliths of Emiliania huxleyi, while during the summer decreased fluxes of carbonates and aluminosilicates resulted in a reduction of the mass flux. In contrast, at 1515 m depth the lithogenic component was the dominant component during the winter deployment, indicating a terrigenous input. During the summer period the decrease in mass flux was strongly effected by the decrease in aluminosilicates. There was a diminution in the organic carbon content with a concomitant increase in total mass flux, which, together with the almost negligible increase in the annual 210Pb activity with depth and the increase of 210Po activity with depth could be interpreted as indicating a contribution of resuspended material to the input at 1515 m. The complex mesoscale circulation of the Cretan Sea, consisting of a cyclone (east)–anticyclone (west) system, controls particle transfer in the area. This hydrodynamic system seems to move water masses towards the southern Cretan Sea margin, and consequently carry materials from the open sea to the upper slope and shelf.