Deep-water renewal by turbidity currents in the Sulu Sea

Abstract

THE ventilation of the intermediate and deep layers of the ocean is governed by convection: dense water formed at the sea surface through air–sea interaction processes (mainly at high latitudes) sinks to great depths. Convection determines the heat, salt and dissolved-gas budgets of the ocean's interior and therefore exerts a significant influence on the Earth's climate. Convective processes can also be initiated by the flow of dense water over submarine sills separating ocean basins with water masses of different buoyancy. Here we report the observation of relatively light deep water in the overflow region near the sill in the northern part of the 5,000-m-deep Sulu Sea. Because of its buoyancy, this water could not have reached the sea floor of its own accord. Turbidity currents, induced by changes in water density that result from sediment influx, occur in this region at intervals of several decades, as indicated by the frequent deposition of graded sequences of silty mud on the abyssal plain. We suggest that such turbidity currents, combined with the effects of classical hydrological plume convection, are capable of bringing light water to large depths. In tropical regions, where normal oceanic convection is relatively weak, this mechanism may contribute significantly to the ventilation of deep basins.