Sediment sorting in the deposits of turbidity currents created by experimental modeling of explosive subaqueous eruptions

Deposits of turbidity currents induced by subaqueous volcanic eruptions are increasingly recognized in settings ranging from lakes to the deep sea. These "eruption-fed density currents" have high current temperatures and low-density particles, which affect current dynamics and deposit features. Field data indicate that deposit thickness and grain size decrease downflow at a changing rate. This trend is similar for deposits of noneruptive turbidity currents, but with volcanic activity, the deposits often exhibit a break in slope of the profile of deposit thickness and grain size distribution. Here we present laboratory results on density currents that address the effects of variations in grain density and water temperature on flow properties and depositional processes. Parallel runs were performed under identical flow conditions using quartz sand and pyroclastic sediment. Each grain type was used in one series of runs in which a hot density current enters a cold environment and one series in which a cold current enters a similarly cold environment. Experiments in which the inflow contained hot water and low-density sediment tended to emplace the deposit center of mass proximally; sedimentation also reduced bulk current density below that of the ambient water, giving rise to a buoyant plume from which suspension sedimentation occurred. By contrast, experiments in which the inflow contained cold water and high-density sediment tended to deposit mass distally, with no accompanying plume. Results of the study have special relevance to submarine volcaniclastic deposits but also illuminate fundamental aspects of how grain properties and water temperature affect the driving force of density currents.