Seasonal variability in turbidity currents in Lake Ohau, New Zealand, and their influence on sedimentation

Abstract

Layers of sediment that are deposited on the floor of Lake Ohau, New Zealand, offer a means to reconstruct past climate conditions in the Southern Hemisphere at subdecadal and annual resolution. A robust understanding of the modern physical processes that control the influx and dispersal of sediment in the lake is required to reconstruct climate from these sedimentary archives. In this study, water temperature and velocity measurements collected during 2012–13 were analysed to determine the primary physical processes that influence sediment transport in the lake. Sediment input from river inflow occurs throughout the year but exhibits strong seasonal variation. Large inflow events (Q>500m3s–1) that follow strong summer rainstorms trigger high-concentration turbidity currents, which are the main agents for sediment delivery and deposition. During winter, smaller turbidity currents also occur after rain events and contribute to annual sediment accumulation. In addition, large internal waves were observed during the summer and may influence sedimentation. In conclusion, several processes including river inflow, internal waves and convectively driven flows control sediment deposition and accumulation in the Lake Ohau system. We utilise these observations to establish a conceptual model to explain the observed infill stratigraphy in Lake Ohau and guide interpretation of the longer sedimentary record.