EVIDENCE FOR MODERATELY LOW RATES OF HOLOCENE SEDIMENT ACCUMULATION ON THE MID SHELF ADJACENT TO THE COLUMBIA RIVER LITTORAL CELL, WASHINGTON

Radiocarbon dating of vibracores and gravity cores collected on the mid shelf adjacent to the Columbia River littoral cell (Washington) provides evidence of moderately low rates of late Holocene sediment accumulation (average=30 cm/1000 yrs). Previously published rates of recent/historical sediment accumulation on the mid shelf of 2-4 mm/yr were determined from Pb210 analysis of box cores and extrapolated to 2-4 m/1000 yrs for the late Holocene (Nittrouer, 1978). Seismic data have defined the extent of a post-lowstand deposit that extends NNW from the mouth of the Columbia River, and it is up to 50 m thick. Much of the deposit is thought to be Holocene in age (<10,000 BP), although the basal portion was likely deposited during the early post-glacial rise in sea level. The mid-shelf silt deposit (MSSD), as identified previously by surface samples, cores and seismic data, has been found to thin and fine to the NW in the direction of net bottom current flow.

In March, 1998 and August, 2002, ten cores were collected on the mid shelf, adjacent to and north of the Columbia River mouth, in water depths of 50-106 m. These cores are the first relatively long cores (1-2+ m) to be obtained from the MSSD. Twelve samples of molluscan material from 45-115 cm (below the seabed) in multiple cores have been dated and yield radiocarbon ages ranging from 1000-6700 BP (average age=2950 BP). The average rate of sediment accumulation on the MSSD (30 cm/1000 yrs), calculated from C14 dates, has been substantially slower during the late Holocene than previously suggested by Pb210 results. Lower rates of sediment accumulation also imply that much of the thick shelf deposit was deposited prior to 5000 BP, when sea level was rising and rates of sediment accumulation may have been much higher. The MSSD may be comprised of an upper late Holocene, silty unit and a basal, transgressive-aged unit. We believe that our longer-term rates of mid-shelf sediment accumulation derived from C14 dates, as compared to published short-term rates based on Pb210, may have substantial implications for the Holocene sediment budget for the Washington shelf.