(Palaeo-)ecological significance, transport and preservation of organic-walled dinoflagellate cysts in the Somali Basin, NW Arabian Sea

To date, relatively little information is available about factors influenci ng organic-walled cyst production of tropical dinoflagellates and processes influencing the final burial of cysts in bottom sediments, such as transpo rt and preservation. To extend this information, cyst fluxes were documente d for three sediment traps from June 1992 to February 1993 at two sites in the Somali Basin (northwestern Arabian Sea) as well as the cyst association of underlying sediments. By comparing cyst associations of contemporaneous ly collected trap samples at different depths at one site, information abou t transport and processes of decay in the water column was obtained. Neithe r transport nor decay appears to have any detectable influence on cyst asso ciation during cyst settlement through the water column. Comparing the trap associations with the underlying sediments indicates that downslope transp ort appears to have influenced the cyst association on a local scale only. Species-selective decay, probably related to the presence of oxygen bottom sediments, has influenced the cyst association most pronouncedly at the mos t offshore site. Relating variations in the trap associations with environm ental conditions of the overlying surface waters indicates that highest pro duction of both filled and empty cysts occurs during the SW Monsoon upwelli ng. Based on this correlation three groups of species can be distinguished: Species with highest fluxes during (1) the first-half of the SW Monsoon (J une-August); Bitectatodinium spongium, Echinidinium granulatum, Echindinium transparantum, Echinidnium spp., cysts of Protoperidinium compressum and c ysts of Protoperidinium subinerme, (2) the transition between the SW-Monsoo n and inter-Monsoon; Spiniferites mirabilis and Spiniferites spp., (3) no p articular season; all other species. Cyst associations of all trap samples are dominated by cyst of Protoperidinium species. Cysts with highest fluxes during the SW-Monsoon form about a third of the associations. (C) 2000 Els evier Science Ltd. All rights reserved.