Diatom fluxes in surface sediments of the Goban Spur continental margin, NE Atlantic Ocean

Continental slopes are presumed key areas for deposition of organic carbon exported from the shelf. Analysis of across-slope differences in diatom and silicoflagellate fluxes recorded in bottom sediments of the Goban Spur mar gin, a typical North Atlantic slope environment, was carried out to test if they can provide information on the magnitude of advection of material fro m the shelf into deeper waters. Total diatom and silicoflagellate accumulat ion rates showed strong across-slope differences. Minimum values are record ed at the shelf break where maximum surface productivity conditions occur w hile the deeper sampling stations record fluxes as high as 183x10(6) valves cm(-2) ka(-1). While high diatom fluxes show a clear correspondence with t he activity of a permanent bottom nepheloid layer operating in the region, they do not correlate with productivity patterns observed in the water colu mn. Diatom assemblages are mainly composed of Chaetoceros resting spores an d Thalassionema nitzschioides (Grunow) Grunow ex Hustedt, typical indicator s of spring bloom conditions in the area. The absence of clear across-slope trends in the diatom assemblages is interpreted as the effect of random mi xing driven by the strong hydrodynamic regime provoked by the activity of t he bottom nepheloid layer. The dominance of Chaetoceros resting spores acro ss the slope is related to important exportation of shelf-derived productio n. However, due to the broad ecological tolerances of the main taxa composi ng the diatom assemblages, they do not allow precise estimations on the mag nitude of the primary vertical flux vs, the secondary lateral flux in this slope environment. Use of the tychoplanktonic and benthic diatoms, which ar e restricted to the neritic realm, allows only the estimation of the minimu m amount of shelf-derived diatoms reaching the slope sediments (at least 13 % of the total diatom assemblage for the upper slope area of the Goban Spur ). This study shows that major limitations exist for the use of diatoms pre served in surface sediments of this area as tracers of shelf-derived produc tion transported to the continental slope.