Distribution and Composition of Suspended Sediment in the Bottom Waters of the Washington Continental Shelf and Slope

Abstract

ABSTRACT In order to study the dispersal of river-borne suspended sediments through the marine environment, suspensate samples were collected at 26 stations from the lower Columbia River and bottom waters of the adjacent continental shelf and slope during the summer and early fall of 1971. X-ray diffraction analysis has shown the montmorillonite/chlorite (M/C) ratio to be a useful indicator of Columbia River-derived sediments. M/C values vary from high (> 1.0) in the Columbia River and shelf bottom waters, through moderate (0.96-0.65) in the head and upper reaches of three major submarine canyons on the slope, to low (<0.48) in samples from the distal portions of the canyons and the open slope areas between the canyons. These trends appear to develop from physical sedimentary processes, such as differential sedimentation and resuspension, which result in a continually changing clay mineral composition of the Columbia River suspended load as it diffuses into the coastal ocean environment. There is no evidence to indicate that these changes in clay mineralogy are due either to dilution of the Columbia River suspensate by other continental or marine sources, or transformation of the clay minerals due to reactions with seawater. Comparison of the M/C values of the suspended sediment with surficial bottom sediment collected at the same stations indicates that naturally occurring differences between the two can be falsely exaggerated if similar particle size ranges are not used in each analysis. Agreement of the clay mineral trends is best when comparing the M/C ratios of the total inorganic suspended sediment to the M/C ratios of the 0-62µ, rather than the 0-20µ or 0-2µ, fraction of the bottom sediments.