Distribution and Transport of Suspended Particulate Matter in Submarine Canyons Off Southern California

Abstract

Studies of the distribution of suspended particulate matter in submarine canyons off southern California are reported. Simultaneously collected light beam transmission and salinity/ temperature data demonstrate an association of particle maxima with the relatively steep segments of the vertical, water-density gradient over shelf, slope, and canyon environments. Within the submarine canyons that cut the mainland shelf, mid-water particle maxima contain high percentages of terrigenous detritus supplied by seacard flew from canyon nepheloid layers. The concentrations of particulate matter in the canyon nepheloid layers rarely, if ever, exceed 10 mg/l even in areas close to major rivers; typical peak values in the canyon heads are 3 to 6 mg/l. It is concluded that turbid-layer, density-excess underflows are not possible at these low concentrations. Further, the suggestion of an earlier report that the slow, net downcanyon transport in Pacific coast canyons could be driven by the suspended particles is not supported by our most recent results from Santa Cruz Canyon. Bottom currents in this canyon are vigorous and show a net downcanyon flow that is far too strong to be explained by turbid-layer flow. While submarine canyons off southern California are an important pathway for fine sediment leaving the shelf, it is likely that no more than 10–15% of the annual terrigenous supply moves to deeper water by way of the canyons.