Two distinct types of turbidity currents observed in the Manila Trench, South China Sea

Abstract

Sediment gravity flows are the most direct and efficient transport mechanisms for moving terrestrial sediments into deep oceans. Scarcity of firsthand measurements, however, has hindered the quantitative, even qualitative characterization of such flows. Here we present a unique year-long data record from ~4000 m depth in the Manila Trench that captured two very different gravity flows in terms of their hydraulic and sedimentary properties. The first flow was of slow speed (~40 cm s−1) and long duration (~150 h), thus nicknamed ‘Tortoises’, and carried very fine sediment with low concentration (~0.01%). The fast (~150 cm s−1) but short-lived (~40 h) flow, nicknamed ‘Hares’, carried much coarser sediment with higher concentration (~1.2%). Clay mineral compositions suggest that the ‘Tortoises’ originated from upstream canyon wall slumping, whereas the ‘Hares’ was likely submarine canyons southwest of Taiwan Island due to typhoon. Grain size is a key factor in determining evolution of turbidity currents.