Past environmental and circulation changes in the South China Sea: Input from the magnetic properties of deep-sea sediments

The South China Sea, located at the transition between the Pacific and the Indian Ocean, receives every year, mainly during the rain season, enormous amounts of river sediments originating from the erosion/weathering of rocks in the catchment basins. At sea, these sediments are carried by different water masses to their deposition site and they constitute a unique archive for past environmental studies in this region. The magnetic fraction of deep-sea sediments, though forming a minority in volume, provides incredibly valuable information for paleoceanographic reconstructions, as long as its provenance and source-to-sink processes are well constrained. After a brief description of the climatic, sedimentological and oceanographic context of the South China Sea (SCS), a review of the information available so far in the literature about the magnetic properties of SCS sediments is presented. It shows a large variety of interpretations/conclusions that finally results in a rather unclear picture. Because in such a context, the characterization of the sediment at the source is critical, the magnetic properties recently obtained from a set of samples from rivers and marine surface sediments are summarized to describe the present day situation. They are then used to interpret paleorecords from a set of seven marine cores distributed from the southern to the northern basins at different water depths and all covering at least the last climatic cycle. The results reported here for the first time suggest that the magnetic mineralogy remains rather stable in time on land and that its time and spatial distribution at sea is an interplay of changes in sea level and deep-sea circulation. During low sea level periods, bottom deep-sea circulation is weak and the deposited sediment originates from the proximal rivers. On the contrary, during high sea level, the circulation is enhanced, transporting more sediment most likely from Taiwan and Luzon, to the northwestern part of the SCS and also, in smaller proportion, to the southern basin where it mixes with the local river-borne sediment. By comparing the two longest records, we observe that this pattern is repeated over the last 900 ka. Superimposed to the 100 kyr cyclicity we also observe a longer-term evolution with a maximum in the bottom current strength around 500 ka coinciding with global changes in the deep ocean circulation and carbon cycle. These new results, based on a wide spectrum of magnetic properties of numerous marine sedimentary cores from the SCS, show that the magnetic fraction yields important insights into past changes of the sedimentary pathways, in particular the dynamic of the deep-sea circulation, depending on the global climatic context.