Abstract
Abstract. Using a combination of seismic oceanographic and physical oceanographic data acquired across the Faroe-Shetland Channel we present evidence of a turbidity layer that transports suspended sediment along the western boundary of the Channel. We focus on reflections observed on seismic data close to the sea-bed on the Faroese side of the Channel below 900 m. Forward modelling based on independent physical oceanographic data show that thermohaline structure does not explain these near sea-bed reflections but they are consistent with optical backscatter data, dry matter concentrations from water samples and from seabed sediment traps. Hence we conclude that an impedance contrast in water column caused by turbidity layers is strong enough to be seen in seismic sections and this provides a new way to visualise this type of current and its lateral structure. By inverting the seismic data we estimate a sediment concentration in the turbidity layers, present at the time of the survey, of 45 ± 25 mg l−1. We believe this is the first direct observation of a turbidity current using Seismic Oceanography.
Highlights
Turbidity layers are some of the largest sediment-laden underflows that occur in ocean basins
In this study we used data from Optical Back-Scatter (OBS) measurements of suspended sediment, using a Seapoint STM sensor, and water samples taken during Conductivity Temperature Depth (CTD) casts during the PROCS programme in 1999, together with measurements from sediment traps attached to moorings deployed on a transect across the Shetland side of the channel (Bonnin et al, 2002; Hosegood et al, 2005) (Fig. 1c)
Using the calibration for medium size particles from Benns and Pilgrim (1994) who suggest that the sediment load for the observed suspended particulate matter (SPM) data could be 75–100 mg l−1 which is of the same order as estimated from the seismic data
Summary
Turbidity layers are some of the largest sediment-laden underflows that occur in ocean basins. These layers play an important role in transporting fluvial, littoral and shelf sediments into deep ocean environments. They may be sourced from sediment-laden river flow cascading down submarine canyons, slope failure, or by the remobilisation of unconsolidated sediment by strong currents. A large number of experimental studies on turbidity layers are available Middleton, 1966; Sumner et al., 2009), natural turbidity layers and other sedimentladen transient currents are hard to observe and study, due to their irregular occurrence and often destructive nature (Hay, 1987). Our knowledge of the turbidity layers is based largely on indirect observations of the modern seafloor from multibeam bathymetry surveys (Kuijpers et al, 2002), highresolution seismic surveys especially those designed for observations of geohazards (Bulat and Long, 2001; Meiburg and Kneller, 2010) and the study of contourites (Masson et al, 2010; Koenitz et al, 2008); together with direct observation of suspended sediment of the neptheloid layer from optical backscatter or transmissometer and sampling either in Niskin bottles or sediment traps (Bonnin et al, 2002; van Raaphorst et al, 2001; Hosegood and van Haren, 2004)
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