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 800 m, 1200 m and 1800 m depth, using moorings deployed for 4 months during winter 2008-2009. During this period, we observed three major sediment gravity flows, all characterized by sudden increases in current velocity that lasted 2-5 h and by downward particle fluxes. Each gravity flow, described using a high frequency current meter and two Acoustic Doppler Current Profiler (75 and 300 kHz 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 (>100 m high) and relatively low velocity (40 cm s −1). The second event, observed after a Var River flood, was more energetic with a maximum horizontal current peak of 60 cm s −1 but with a low vertical extent (30 m 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 >85 cm s −1. These peaks of current speed were associated with large clouds of material that transported sediment along the canyon and reached up to 200 g m −2 d −1 of sediment (>1 g m −2 d −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. Highlights ► The floods of the Var River explain the trigger of the hyperpycnal currents in the Var submarine canyon. ► Three gravity flows were observed in the Var canyon characterized by increase in current and particles. ► Gravity flows transported lot of material along the entire canyon. ► The large input of sediment has a significant impact on the bottom of the deep Mediterranean Sea.

Key Future Directions For Research On Turbidity Currents and Their Deposits

“…The Santa Monica Basin and Southern California Borderland canyon-fan systems also offer excellent opportunities for extending existing relatively detailed field datasets that include IODP cores Normark et al 2009). The Var Canyon-fan system in the Mediterranean may also provide a suitable location to extend previous unusually detailed monitoring work (Khripounoff et al 2012) and detailed studies of sediment deposits (Mas et al 2010).…”

“…Sediment concentration is the critical parameter controlling such flows, yet it has never been measured directly for flows that reach and build submarine fans. Indeed, studies in only six locations (Xu et al , 2013(Xu et al , 2014Xu 2011;Hughes Clarke et al 2012, 2014Cooper et al 2012;Khripounoff et al 2003Khripounoff et al , 2012Vangriesheim et al 2009;Ayranci et al 2012; Lintern and Hill, personal communication; see Talling et al 2013) have estimated sediment concentrations for turbidity currents in shallow water, typically using backscatter values from acoustic Doppler current profilers (ADCPs). This indirect technique for measuring sediment concentration (backscatter is also affected strongly by additional parameters including grain size) has large uncertainties and cannot penetrate into high sediment concentrations at the base of the flow.…”

How are subaqueous sediment density flows triggered, what is their internal structure and how does it evolve? Direct observations from monitoring of active flows

“…Most of the cable failures occurred during the relatively wet and stormy winter months of November, December and January. , Mediterranean, 2005, Mediterranean, -2010 In addition to the previously described 1979 Nice Airport event triggered by slope failure, monitoring of the Var Canyon has documented much slower and shorter run-out events, which are very dilute (Khripounoff et al, , 2012. A total of six events were observed during a two year period in 2005-2008, with three further events observed during a 4 month period in [2008][2009].…”

“…Several of these flows coincided with floods from the Var River, and these flows were most likely initiated by discharge of hyperpycnal river floodwater (Khripounoff et al, 2012;Fig. 4).…”

“…4). ADCPs at three sites (Khripounoff et al, 2012) provided some of the few velocity profile time series available for submarine turbidity currents (cf., Xu et al, 2010;Xu, 2011;Hughes Clarke et al, 2012).…”