Variability of benthic foraminifera north and south of the Denmark Strait
The Denmark Strait is a key area for the exchange of water masses and functions as a deep-water gateway between the Nordic Seas and the North Atlantic. Thus it constitutes an important part of the global thermohaline circulation (THC). The gateway aperture is influenced by eustatically and isostatically controlled changes of the Denmark Strait sill and by alteration in density contrasts between the water masses in North and South. In order to determine the faunal exchange through the changing aperture, benthic foraminiferal assemblages were analysed. Eight surface sediment samples were taken along the East Greenland continental margin during the RV "Polarstern" cruise ARK XVIII/1 in July 2002 by means of a multicorer and a box corer respectively. Faunal assemblages of living (stained) and dead benthic foraminifera north and south of the Denmark Strait (980-2564 m water depth) were compared with each other. Northern and southern stations exhibit clear differences in abundances, diversities, number of species, and species composition. Due to strong bottom currents and low carbonate dissolution rates, high abundances with predominating calcareous species of dead and evidently allochthonous foraminifera were recorded south of the sill. Rather low abundances of dead autochthonous foraminiferal assemblages, containing mainly agglutinated species, were found north of the sill because of weak bottom currents and high carbonate dissolution rates. Southern stations showed a large portion of epibenthic species living as suspension feeders. In contrast endobenthic species living as substrate feeders dominated northern stations, pointing to lateral advection of food particles in South and sedimentation of food particles in North. Morphomertic measurements were performed on the high abundant foraminifer Cibicidoides wuellerstorfi. In order to supplement the data of the surface sediment samples and obtain morphometric data of glacial specimens, two long sediment cores were investigated as well. These cores were taken during the RV "Polarstern" cruise ARK XVIII/1 north of the Denmark Strait and the RV "Poseidon" cruise POS 210/2 in August 1995 south of the sill. Northern glacial samples contained no C. wuellerstorfi. The morphometric investigations included counting of chambers within the last whorl, determination of the maximum test diameter, identification of the coiling direction, measurement of the proloculus (initial chamber) diameter, and weighing of individual tests. It was statistically significant that chamber numbers decreased form glacial over northern to southern stations. Maximum diameters, percentages of coiling direction, proloculus diameters, and weight of single tests showed no differences. Evidences of a faunal exchange over the sill lead to the conclusion that C. wuellerstorfi from both sides of the sill response to diverse environmental conditions and develop two different ecophenotyes rather than genotypes. For C. wuellerstorfi the Denmark Strait is more likely a migration passage than a barrier.