Fluxes of planktic foraminifera in response to monsoonal upwelling on the Somalia Basin margin

Abstract Sediment trap samples collected off Somalia (bi)weekly from early June 1992 through mid February 1993 show large seasonal variations in the shell flux and species composition of planktic foraminifera. These variations mirror the monsoon-driven circulation, which resulted in massive upwelling and offshore eddy transport in summer during the SW Monsoon (June–September) and accounted for nearly 90% of the estimated annual shell flux of planktic foraminifera. G. bulloides dominated the SW Monsoon along with G. glutinata , T. quinqueloba , N. dutertrei and T. iota in a fauna-rich species but with a low diversity and equitability. During the autumn intermonsoon (November–December) the ocean became stratified and the nutrient exhausted, whereas during the NE Monsoon (January–March) a warm surface mixed layer developed with the nutrients entrained by deep wind mixing. It was dominated by G. ruber in association with G. tenella , G. aequilateralis , G. trilobus / G. sacculifer and G. menardii in a fauna poorer in species but with a higher diversity and equitability. Consequently, the ratio in the abundance of G. bulloides and G. ruber follows the monsoonal cycle closely, as both species persist throughout the year. Only the rare G. theyeri seems to be restricted to the SW Monsoon. In October at the end of the SW Monsoon, the trap at 265 m above the bottom intercepted massive amounts of fine-grained carbonate ( μm ) resuspended from the shelf and upper slope. It contained extreme fluxes of bioclastic fragments, benthic foraminifera and benthic ostracods as well as enhanced fluxes of small-sized planktic foraminifera μm , notably G. rubescens . Integrated over the year, the species composition intercepted by the trap shows a very good match with the shell fauna in a core top sediment from the same site. Apparently, the year-estimated monsoonal record of planktic foraminifera, which is strongly dominated by the SW Monsoon production, survives burial intact. Consequently, the species composition of planktic foraminifera and measures such as the bulloides / ruber ratio can be applied as a proxy for past changes in the intensity of the SW Monsoon off Somalia compared to the modern condition.