Depositional mechanisms for upper Miocene sediments in the South China Sea central basin: Evidence from calcareous nannofossils

In 2014, the International Ocean Discovery Program (IODP) Expedition (Exp.) 349 discovered carbonate layers at sites located in the South China Sea (SCS) central basin. Preliminary results from shipboard investigation showed that most of these carbonate layers are calcareous nannofossil ooze and interpreted them as turbidites. However, more detailed studies are required to determine their depositional mechanisms and paleoenvironmental implications. In this study, we have analyzed CaCO3 content, calcareous nannofossil abundance, assemblage composition and preservation in carbonate and clay samples from the upper Miocene sections of Site U1433 in the SCS central basin. We confirm that these carbonate layers are of turbidity current genesis but are not typical nannofossil ooze, while the clays represent regular in situ deep sea sediments. The study interval spanned approximately from 10 to 5 million years before present (Ma). Calcareous nannofossil abundance and assemblage are used to reconstruct the evolution of the upper seawater structure in the SCS central basin. Our results indicate that the seawater shifted from strongly stratified to well-mixed due to enhanced monsoon. This shift was accompanied by an increase in pelagic productivity at ~8 Ma. This environmental change is critical for component variations of the carbonate turbidites since we suggest that these turbidites mainly sourced from periplatform ooze in the southern SCS composed by of mixed neritic and pelagic particles. The neritic and pelagic CaCO3 production have inversed responses to the change of seawater environment in the SCS and thus cause the component variations in the carbonate turbidites of Site U1433. •Upper Miocene carbonate layers in the South China Sea central basin comprise varying amounts of calcareous nannofossils.•The carbonate layers and pelagic clay layers of Site U1433 represent turbidity-dominated and in situ productivity-dominated depositions respectively.•A prominent calcareous nannofossil change at about 8 Ma was related to the enhanced East Asian Monsoon and the increas in the Mekong River discharge.