A 35,000 year record of changes in the eastern Indian Ocean offshore Sumatra
ARTICLE I NFO We have examined the upper 276 cm of deep-sea core BAR9403 taken at a water depth of 2034 m offshore the southern portion of Sumatra in the eastern Indian Ocean using several micropalaeontological proxies. Faunal counts and stable isotopes of oxygen and carbon of planktic and benthic foraminifers, as well as floral counts of dinoflagellates were obtained to reconstruct conditions in the oceans over the last 35,000 years. At times, we found that when benthic foraminifers indicate high organic content values at the bottom of the ocean this is not paralleled by high productivity signals at the sea surface, but instead must relate to changes in bottom-water circulation as a result of slower water circulation. The marine isotopic stages (MIS) 3-1 are clearly differentiated by benthic and foraminiferal assemblages as well as dinoflagellates and their cysts. MIS 3 is characterised by a much sharper (than today) thermocline that was closer to the sea surface and by the absence of a low-salinity 'barrier layer' which today results from high monsoonal rains. The absence of the latter persisted during the last glacial period (MIS 2) when bottom circulation must have been reduced and high percentages of organic matter occurred on the sea floor combined with low dissolved-oxygen levels. The deglaciation is marked by a change in salinity at the sea surface as seen by the dinoflagellates and planktic foraminifers and progressive alteration of the thermocline was detected by foraminifers suggesting a less productive deep chlorophyll maximum in contrast with MIS 3 and 2. Monsoonal activity commenced around 15,000 cal years ago and was well established 2000 years later. The Holocene is marked by a significant increase in river discharge to the ocean, pulsed by the delivery of organic matter to the sea floor, despite overall oligotrophic conditions at and near the sea surface induced by a permanent low-salinity cap. We did not identify obvious and persistent upwelling conditions offshore Sumatra for the last 35,000 years.