Characterizing the sediment dynamics through in-situ measurements in the abyssal Manila Trench, northeast South China Sea

Abyssal sedimentary dynamic processes are crucial to be understanding the transport and distribution of material (sediment, carbon, plastics, etc.) and the formation of deep-sea bedforms. Accurately characterizing such complicated processes requires a multi-faceted approaches (e.g. numercial models, physical experiments, field observation), among which in-situ field data gathering has been the most challenging. In this study, we collected data from four bottom moorings deployed along the Manila Trench in the northeast South China Sea to investigate the sediment transport processes at the trench bottom. The events with high turbidity, including strong and fast gravity flow and weaker turbidity currents with speed similar to tidal currents, transport sediment down-trench. Sediment particles in the northern part of the trench were generally found to move northward (i.e. up the trench) with seasonal variations primarily influenced by the asymmetrical subtidal currents. During the intensification of northbound subtidal current, the benthic nepheloid layers in Gaoping Canyon, which feeds into the trench, might be transported to the north of trench. Furthermore, a weaker turbidity oscillation at S2 tidal frequencies was observed before turbidity current occurred, increasing sediment transport to the deep. It was hypothesized that multiple small turbidity currents may have been triggered by an earthquake, despite not reaching the mooring S2, with the fine sediment being transported downstream by abyssal tidal currents. •First study to quantify the sediment flux in Manila Trench based on the long-term field measurements is presented.•The directions of mean currents and net sediment fluxes in the northern part of Manila Trench are up trench.•The estimated volume of up-trench sediment transport during the nearly one-year non-turbidity-current conditions was of the same order of magnitude as the down-trench volume by two turbidity currents.