Sedimentary dynamic processes of a contourite drift formation in the South China Sea: from long-term in situ observations to geological records

Abstract:

Contourites are sediments deposited or substantially reworked by thermohaline-induced deepwater bottom currents. The study of contourites with growing interests is widely conducted in seismic stratigraphy, paleoceanography, paleoclimatology, and hydrocarbon exploration. However, the sedimentary dynamic process producing contourites in the deepwater environment is still poorly understood. This research presents an interdisciplinary approach from long-term in situ mooring and tripod observations, multi-beam seabed morphology, seismic stratigraphy, to IMAGES (Marion Dufresne) piston coring and ODP (JOIDES Resolution) drilling studies on the formation of a contourite drift on the lower slope of the northern South China Sea. The contourite drift with ~520 m thick is distributed in water depths ranging from 1650 m to 2500 m and has been accumulated since 1.5 Ma in early Pleistocene. The nowadays contour currents in the northern South China Sea were observed with velocities generally ranging in 0-2 cm/s with a dominant flow direction of ~250º (southwestward/along-slope). However, the relatively stable contour currents were disturbed by several bursts of increased velocities up to 8-11 cm/s, each lasting 2-3 weeks and followed by a direction reversal, which were caused by passing-through of deep-reaching mesoscale eddies. The along-slope sediment transport is induced by both mesoscale eddy and contour currents, and these suspended sediments are mainly derived from Taiwan according to provenance analysis of sediments traps equipped on moorings. Seismic stratigraphy and core sample analysis (oxygen isotope stratigraphy, clay mineralogy, and grain size) reveal a long sedimentary history with strong influence of deepwater currents that have carried the majority of Taiwan-sourced sediments moving westward since early Pleistocene. The glacial-cyclic terrigenous input from various surrounding drainage systems and their transport processes from fluvial source to deep-sea sink are semi-quantitatively reconstructed. The results highlight the significance of contour currents on the long-term evolution of the contourite drift and the sea-level-change controlled glacial-cyclic sedimentary dynamic processes since early Pleistocene in the northern South China Sea.