Abstract

A study of 56 sediment cores and 121 nephelometer profiles from Nitinat deep-sea fan shows variations in late Holocene accumulation rates and sediment texture which parallel variations in thickness and suspended sediment load of the bottom nepheloid layer. Furthermore, accumulation rates, sediment texture, and nepheloid layer variables all show a substantial degree of correlation with fan topography. In general, the nepheloid layer thickens (>100 m) and suspended sediment loads increase (>100 µg/cm2) above Cascadia Channel (the major channel crossing the fan) as well as above the northern flank of the fan. Over levees and the western portion of the fan, the nepheloid layer thins to <50 m, and suspended sediment loads fall below 100 µg/cm2. Cascadia Channel and the northern flank of the fan have been loci of rapid sedimentation, with accumulation rates ranging from approximately 5 to greater than 12 mg/cm2/yr. In contrast, the apex and western reaches of the fan have significantly lower accumulation rates: 1 to 5 mg/cm2/yr. Detailed size analysis of bottom sediments shows that areas characterized by rapid sedimentation and a thick, heavily loaded nepheloid layer have more medium to fine silt (5 to 7 ϕ) than the clay-rich (>8 ϕ) sediments from interchannel areas with low accumulation rates and a thin, lightly loaded nepheloid layer. The data suggest that turbid water moves continuously down Cascadia Channel and the northern flank. The transport mechanism is size-selective and topographically controlled, concentrating silt-sized detritus in topographic lows. The data also suggest a positive downward flux of sediment particles within the nepheloid layer, at least when averaged over a significant period of time.

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