Diatom flux and species composition in the Greenland Sea and the Norwegian Sea in 1991–1992

Abstract

Vertical diatom fluxes were investigated in sediment trap material from June 1991 to July 1992 from the Greenland Sea at 72 °N, 007 °W and the Norwegian Sea at 70 °N, 000 °E in order to determine seasonal species compositions and alterations that affect settling assemblages. Different hydrographical conditions, including seasonal ice coverage, are reflected in seasonal variations of total diatom flux and species composition. The purpose of this study is to achieve a better understanding of pelagic and sedimentary processes. The quantification of the annual diatom fluxes with respect to accumulation rates of underlying sediments is a suitable approach in reconstructing palaeoceanographic changes and their effects on the productivity of the oceans. The influx of laterally advected material and the alteration of sedimenting assemblages due to silica dissolution are also considered in this approach. A strong pulse-like sedimentation pattern was observed in the Greenland Sea. Highest values of up to 7.7 × 10 6 valves m −2 d −1 were recorded during the summer (second half of July 1991). Allochthonous diatoms, such as benthic and subtropical-boreal, as well as enriched autochthonous species found in the deep water trap indicate the sporadic occurrence of lateral advection and resuspension processes in the Greenland Sea. In the Norwegian Sea, low diatom flux was observed and three distinct sedimentation events could be differentiated at 500 m water depth. At this site, Zooplankton grazing pressure during spring is known to have more influence on the phytoplankton stock than in the Greenland Sea, which causes a delay in diatom accumulation. This is evident from maximum values of 0.3 × 10 6 valves m −2 d −1 during November/December 1991. Significant enrichment of Arctic diatom species suggests a transport from the Greenland Sea into the Norwegian Sea via deep currents. The assemblages of diatom species that are preserved in underlying surface sediments reflect the local oceanographic conditions and are quite similar to the assemblages found in deep water traps from both regions. However, they do not necessarily reflect the composition of the main sedimentation event, but rather the typical post-bloom species composition with stronger silicified diatoms. Despite the absence of weakly silicified species due to silica dissolution, diatom assemblages were useful in reconstructing modern and ancient oceanographic conditions.