FY 1986

Contemporary sedimentation processes in and around an active west coast submarine canyon

Baker, E.T., and B.M. Hickey

Mar. Geol., 71, 15–34, doi: 10.1016/0025-3227(86)90031-9 (1986)

Contemporary processes controlling the off-shelf transport of suspended particles in the vicinity of Quinault submarine canyon on the Washington continental shelf were identified using CTD/transmissometer surveys and a three-month deployment of current meter/transmissometer/sequentially-sampling sediment trap arrays. Resuspension events on the shelf create a shelf-break-depth nepheloid layer that transports sediment off the shelf. Fluctuations of the particle concentration in this layer were positively and significantly correlated with the short-term (9.5 days) temporal variation of trap deposition rates in the canyon and on the adjacent open slope. Contributions to the deposition rate from downslope transport along the canyon floor, from local resuspension of canyon sediments, and from particles in the ambient bottom nepheloid layer were negligible. These observations indicate that deposition on the slope is controlled by fast-sinking (100-200 m day) amorphous aggregates that scavenge fine-grained particles from the shelf-break-depth nepheloid layer. Areal variations in the deposition rate result from interaction between the regional flow pattern and the canyon geometry. Rates were high ( 60 g m day) in the canyon upper head because it indents the shelf and thus underlies the shelf-break-depth nepheloid layer created by across-isobath advective transport of resuspended mid-shelf silty sediments. Rates were low (3-6 g m day) elsewhere in the canyon and on the open slope because the transport is much less rapid and direct, occurring primarily by horizontal diffusion rather than across-isobath advection of particles, and because the availability of erodable fine-grained particles is reduced in the relatively coarser outer-shelf sediments. Trap deposition rate, extrapolated to an annual rate based on a historical seasonal pattern of shelf storms, was consistent with published radiometric bottom sediment accumulation rates throughout the study area.