Flow conditions of the Quaternary Deep-water Current reconstructed by sediment waves in the northeastern South China Sea

Abstract

The South China Sea (SCS) plays a key role in maintaining the circulation in the Pacific and Indian oceans. After entering the northeast South China Sea from the Luzon Strait, the Pacific Deep Water transforms into the Deep-water Current (DWC) and flows westward. The upwelling of the DWC in the SCS could outflow into both the Indian and Pacific oceans. However, when and how this modern circulation was established in the SCS remains unclear. By using seismic reflection data tied to the Ocean Drilling Program wells in the northeastern SCS margin, we have discovered fields of previously unreported sediment waves, of which the onset dates back to ∼2.6 Ma. The sediment wave heights increased from 2.0 to 7.5 m, in association with spatial extent from 630 km2 to 800 km2 between ∼2.6 Ma and ∼ 0.7 Ma. After that, the wave heights and spatial extent reduced to ∼5.5 m and 700 km2, respectively. Considering the location, morphological features and water depth, we propose that these sediment waves were formed by the DWC. The morphological changes of the sediment waves are linked to energy increase and decrease of the DWC within ∼2.6–0.7 Ma and ∼ 0.7–0 Ma, respectively. We interpret the intensification as caused by the narrowing and uplifting of the Luzon Strait that is the sole deep-water gateway of the SCS, and speculate that the post-0.7 Ma weakening was probably related to the reduced Kuroshio Current intrusion due to the middle Pleistocene climate transition. This study proposes a novel model for the evolution of the Quaternary DWC hydrodynamics, fostering our understanding of the paleo-oceanographic links between the SCS and the Pacific Ocean.