FY 1986

Suspended particle movement in and around Quinault submarine canyon

Hickey, B., E.T. Baker, and N. Kachel

Mar. Geol., 71(1-2), 35–83, doi: 10.1016/0025-3227(86)90032-0 (1986)

Time- and space-dependent light transmission, temperature, salinity, and velocity data obtained during two winter experiments in the vicinity of Quinault submarine canyon are used to examine processes controlling subtidal (<1 cpd) fluctuations in suspended particulate concentration in the region; in particular, local resuspension of particles by waves and currents, horizontal advection and vertical advection. Data include time series from four to eight mooring locations (as many as 16 current meters and 10 transmissometers) on the shelf, over the open slope, and in the canyon during each experiment, spatial distributions from five CTD/transmission surveys, and gravity wave information. Particle concentration in the bottom boundary layer at the shelf edge was dominated by resuspension events. The mechanism for resuspension had a large interannual variation; in the first year, wave-induced resuspension dominated, whereas, in the second year, current-induced resuspension dominated. The cut-off depth for storm-related resuspension was generally <250 m. Subtidal advection across isobaths (upwelling/downwelling) accounted for 20% of the variance in concentration fluctuations at the shelf edge in the bottom boundary layer. Horizontal advection of material suspended in the bottom nepheloid layer on the shelf resulted in the formation of shelf break depth intermediate nepheloid layers over the canyon in regions where the flow crosses isobaths. The existence, strength, onshore-offshore extent, and vertical structure of such nepheloid layers at a given time in this asymmetrical canyon appear to be a function of the direction of flow over the outer shelf and over the upper slope in the canyon; whether active resuspension is occurring over the shelf and the maximum resuspension depth and magnitude of the active resuspension event; the frequency, timing, and magnitude of previous resuspension events; spatial and temporal continuity, magnitude and direction of flow over the open slope north and south of the canyon; vertical settling; and upwelling or downwelling related to the along-isobath flow over the upper slope. Fluctuations in particle concentration below the shelf break depth over the canyon slope were controlled by vertical, rather than horizontal, advection. Vertical excursions by the water mass of ±100 m occurred both as a result of isopycnal adjustment to the quasi-geostrophic along-isobath flow and as a result of semidiurnal tidal oscillations. When an overhead nepheloid layer was present, such water mass excursions produced concentration fluctuations at depths below the maximum depth of storm-related local resuspension. In open slope regions, where strong INLs (= intermediate depth nepheloid layers) were absent, the portion of concentration variance that could be explained by advective models with constant spatial gradients was almost negligible. Only the resuspension event was documented in the bottom boundary layer within the deep canyon (bottom depth 1000 m), suggesting that advection must account for the observed variance in particle concentration. However, the large-scale gradients at these depths are sufficiently weak, cf. the smaller-scale gradients or "noise", that advective control could only be demonstrated during the second winter experiment when velocities along the canyon axis were relatively large. Finally, the data demonstrate that subtidal concentration fluctuations within 50 m of the canyon floor are correlated to those at the shelf edge. This correlation reflects a correlation in the velocity field rather than a transfer of particles, as might occur with a turbidity current.