Marlene Noble, Ph.D.
United States Geological Survey
Wednesday, April 12, 2000
The mechanisms that govern the transport of sediment and water through submarine canyons are not well understood because of difficulties in monitoring these processes. In August 1993, a one-year field program was begun to investigate these mechanisms within and above Monterey Canyon. Six moorings that measure current, temperature, salinity and water clarity were deployed in the mid- and -outer canyon. Four CTD/transmissometer cruises were conducted to measure the spatial variations of water properties.
Current measurements suggest that tidal flows dominate at all sites, but currents above the canyon rim and within the canyon belong to two distinct dynamic systems. Tidal ellipses were oriented mainly along the canyon. Tidal amplitudes in the narrow portion of the canyon were 3 to 4 times larger than in the wider portion. A 3-day oscillation with very large spatial scales was commonly found at all measurement sites within, but not above, the canyon.
Bottom intensification of currents was the strongest at the site in the narrowest portion of the canyon, where the near-bottom shear stress was often high enough that bottom sediments were re-suspended. The bottom nepheloid layer was up to 500 m thick at the narrow site of the canyon where water depth is about 1,500 m. An unusually strong turbidity event occurred 100 m above the bed on February 8 when the dense suspended particulate matter totally blocked the transmissometer for about 2 hours. The water column gradually cleared over the next 7 to 10 days.