RECENT MONTEREY CANYON FLUSHING EVENTS: HOW FAR DID THEY GO?

Over the last several years, high-resolution multibeam bathymetry, ROV-guided observations and sampling, and data from canyon monitoring activities (measurements of near-seafloor currents, turbidity, pressure, temperature, and salinity) have been collected throughout the Monterey canyon-fan channel system. These data show that the upper canyon has been quite active in recent time; gravel and coarse sand, with a composition similar to the sand found on local beaches, accumulate near the canyon head, and regularly move down into the canyon as episodic gravity flow events. Measurements of DDT and its naturally-occurring breakdown products (DDT_r) show that fine-grained DDT_r-bearing sediments have been transported down through the canyon-fan system since the beginning of agricultural use of DDT ~60 years ago. In contrast, fine-grained sediments on the flanks of the canyon do not have detectable quantities of DDT_r. The question that remains is whether recent sand-carrying events, observed in the upper canyon, are representative of the flows responsible for transporting the DDT_r-bearing fine-grained sediments throughout the canyon-fan channel system. In 2004, we obtained transects of ROV-collected vibracores across the Monterey fan channel (axial depths of 3400-3800 m) in order to determine the stratigraphic record of the most recent sand carrying events that have moved onto Monterey Fan. These vibracores show recent (likely historic) coarse sand and gravel deposits in the fan channel thalweg. DDT_r concentration measurements in these cores will verify if the youngest sediments were deposited within the last ~60 years. A longer geologic record of these and other events is provided by additional ROV-collected vibracores and previously collected piston and gravity cores from the adjacent levees. Available radiocarbon ages from the channel overbank deposits reveal multiple Holocene episodes of large flows that probably represent large canyon flushing events.