Modern environmental conditions recorded in surface sediment samples off W and SW Indonesia: Planktonic foraminifera and biogenic compounds analyses Mahyar Mohtadi a,b, ⁎ , Lars Max b , Dierk Hebbeln a,b , Anne Baumgart c , Nils Krück c , Tim Jennerjahn c a Center for Marine Environmental Sciences (MARUM), University of Bremen, 28359 Bremen, Germany b Geosciences Department, University of Bremen, 28359 Bremen, Germany c Center for Tropical Marine Ecology (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany Abstract A total of 69 surface sediment samples from several fore-arc basins located west and southwest of the Indonesian Archipelago was analyzed with respect to the faunal composition of planktonic foraminifera, the stable oxygen and carbon isotopic signal of a surface- dwelling (Globigerinoides ruber) and a thermocline-dwelling (Neogloboquadrina dutertrei) species, and the opal and CaCO 3 contents in bulk sediment. Our results show that the distribution pattern of opal in surface sediments corresponds well to the upwelling-induced chlorophyll concentration in the upper water column and thus, represents a reliable proxy for marine productivity in the coastal upwelling area off S and SW Indonesia. Present-day oceanography and marine productivity are also reflected in the tropical to subtropical and upwelling assemblages of planktonic foraminifera in the surface sediments, which in part differ from previous studies in this region probably due to different coring methods and dissolution effects. The average stable oxygen isotopic values (δ 18 O) of G. ruber in surface sediments vary between 2.9‰ and 3.2‰ from basin to basin and correspond to the oceanographic settings during the SE monsoon (July–October) off west Sumatra, whereas off southern Indonesia, they reflect the NW monsoon (December–March) or annual average conditions. The δ 18 O values of N. dutertrei show a stronger interbasinal variation between 1.6‰ and 2.2‰ and correspond to the upper thermocline hydrology in July–October. In addition, the difference between the shell carbon isotopic values (δ 13 C) of G. ruber and N. dutertrei (Δδ 13 C) appears to be an appropriate productivity recorder only in the non- upwelling areas off west Sumatra. Consequently, joint interpretation of the isotopic values of these species is distinctive for different fore-arc basins W and SW of Indonesia and should be considered in paleoceanographic studies. Keywords: Planktonic foraminifera; Oxygen isotopes; Carbon isotopes; Surface sediment; Indonesia 1. Introduction The Indonesian region, also termed as the “Maritime Continent”, is an area of major climatic importance (e.g. Schneider, 1998). As a key area along the return branch of the global conveyor belt and the only low-latitude ⁎ Corresponding author. Center for Marine Environmental Sciences (MARUM), University of Bremen, 28359 Bremen, Germany. Tel.: +49 421 21865660; fax: +49 421 21865505. E-mail address: mohtadi@uni-bremen.de (M. Mohtadi).