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1 Department of Isotope Geology, Institute of Earth Sciences, Free University Amsterdam, de Boelelaan 1085, 1081 HV Amsterdam, Netherlands Jung{at}geo.vu.nl
2 University of Utrecht, Faculty of Earth Sciences, Budapestlaan 4, 3584 CD Utrecht, Netherlands
We present a multi-proxy study of sediment Core 905 from the Arabian Sea offshore Somalia to assess the validity of a number of proxies for productivity, temperature and wind strength, to reconstruct the monsoon history in the western Arabian Sea. The present-day seasonal variation in productivity in the modern Arabian Sea off Somalia reflects the change from the high-productivity SW monsoon to the low-productivity NE monsoon seasons. Annual productivity is therefore largely controlled by SW monsoon driven upwelling. The geochemical records of Core 905 document millennial-scale variations, for example, in Ba/Al and Corg content. The Younger Dryas and the time equivalent period to Heinrich event 1 show low annual productivity whereas the early Holocene and Bølling-Allerød periods are characterized by high productivity. The upwelling-productivity peaked during Early Holocene time and was followed by a decrease toward the modern values. The total flux of planktic foraminifera and the concentration of the planktic foraminifera G. bulloides are not always controlled by the total productivity. Variations in calcite dissolution, the advection of expatriate fauna or a seasonal decoupling of primary and secondary production appear to hamper straightforward interpretations of those foraminifera records. We conclude that at significantly changed climatic boundary conditions compared with the present day, bulk-sediment-related proxies of productivity more consistently record the local upwelling history than foraminifer-based productivity proxies.
