Orbital-Scale Variability of Deep-Water Circulation in the South China Sea

Sedimentary records covering approximately the last four glacial-interglacial cycles from two cores from the northern and southeastern South China Sea were used to reconstruct variations in deep-water circulation. Core MD972142 (12°41.33'N, 119°27.90'E) was retrieved at a water depth of 1557 m at the continental slope off Palawan Island during the 1997 IMAGES-III-IPHIS Cruise, and the 1215 cm long piston core GIK17925-3 (119°2.8'E, 19°51.2'N, 2980 m water depth) was taken from the north-eastern South China Sea during the SONNE 95 cruise in 1994. Sedimentary organic carbon (TOC) content shows a strong glacial-interglacial cyclicity with enhanced values during glacial periods, but also higher frequency oscillations corresponding to precession and obliquity controlled insolation variations. Stable carbon isotopes of the organic carbon suggest that increased marine productivity rather than changes in terrestrial input is responsible for the fluctuations in TOC. Proposed explanations for these increased glacial levels of TOC include enhanced primary productivity due to stronger mixing of surface waters by the winter monsoon or nutrient trapping caused by decreased exchange with the open ocean during sea-level low-stands. Manganese/Aluminum and Phosphorus/Aluminum ratios, however, suggest major variations in bottom water oxygenation related to sea level changes with clearly decreased bottom water oxygenation during glacial maxima and well-oxygenated conditions during peak interglacial conditions. We therefore propose that the TOC record is a palimpsest record of short-term productivity signals primarily controlled by insolation variations and longer-term preservation signals governed by variations in deep-water circulation, ultimately controlled ice-volume/sea-level variations.