Accumulation of modern sediment in Quinault submarine canyon

Abstract

Twenty box cores have been studied from Quinault submarine canyon, a major pathway between the Washington continental shelf and the deep sea for silt and clay originating from the Columbia River. Sediment accumulation rates (on a 100-yr time scale) are calculated from profiles of excess 210Pb and of 137Cs. Accumulation rates decrease with increasing distance downcanyon: from 14 × 10 −2 g cm −2 yr −1 in the upper canyon (lower head) to 3 × 10 −2 g cm −2 yr −1 in the lower canyon. In the upper canyon (lower head), channel-floor areas have a higher mean accumulation rate ( 12 × 10 −2 g cm −2 yr −1 ) than channel-wall areas ( 5 × 10 −2 g cm −2 yr −1 ). The difference in accumulation rate between channel-floor and channel-wall areas indicates preferential accumulation (i.e., focusing) in topographic lows. The decrease in accumulation rate (from high rates in the channel floors of the upper canyon to low rates in the lower canyon) may be caused by progressive depletion of a nepheloid-layer source and/or may be an artifact of sediment focusing in the upper canyon. The silty clay and clayey silt accumulating in the canyon show no significant trend in grain size or clay mineralogy with depth in cores or with distance downcanyon. The lack of sorting probably results from the transport of sediment in aggregate form, with a nearly random distribution of size and mineralogy within aggregates. Quinault Canyon is accumulating approximately 5.5 × 10 5 MT of sediment yr −1. The lithogenic component (64%) accounts for 1–4% of the Columbia River discharge. Similarity of sediment-trap deposition rates (including storm events) and 210Pb accumulation rates indicates that the vertical flux is a major input of sediment to Quinault Canyon and that storms are important in the accumulation of sediment on a 100-yr time scale.