Different types of sediment gravity flows detected in the Var submarine canyon (northwestern Mediterranean Sea)

Abstract

Current velocities and vertical sediment fluxes in the Var submarine canyon were assessed at three stations respectively at 800m, 1200m and 1800m depth, using moorings deployed for 4months during winter 2008–2009. During this period, we observed three major sediment gravity flows, all characterized by sudden increases in current velocity that lasted 2–5h and by downward particle fluxes. Each gravity flow, described using a high frequency current meter and two Acoustic Doppler Current Profiler (75 and 300kHz ADCP) showed distinctive features. The first event, triggered during a flood of the Var River, was determined to be a hyperpycnal current with a large vertical extent (>100m high) and relatively low velocity (40cms−1). The second event, observed after a Var River flood, was more energetic with a maximum horizontal current peak of 60cms−1 but with a low vertical extent (30m high). This event was considered to be a turbidity landslide. The third was the result of a local canyon wall failure. It was characterized by a speed of >85cms−1. These peaks of current speed were associated with large clouds of material that transported sediment along the canyon and reached up to 200gm−2d−1 of sediment (>1gm−2d−1 of organic carbon). Our measurements in the Var canyon show the important role of gravity flows transporting particulate matter to the deep-sea floor. These large inputs of sediment and organic carbon may have a significant impact on deep-sea carbon storage in the Mediterranean Sea.