Currents and Sediment Movement in Monterey Submarine Canyon
Marlene A. Noble, Cynthia H. Pilskaln, Leslie K. Rosenfeld and Frank Schwing
In August, 1993, six moorings were deployed in Monterey Canyon for a period of one year. Two moorings were deployed across a narrow portion of the canyon, axis depth 1450, 3 across a wider portion of the canyon, axis depth 2837 m and 1 at a site where the canyon cuts across the fan, axis depth 3223 m. The narrow, wide and fan moorings were separated by 43 and 32 km along the canyon respectively. Each mooring consisted of one or more current meters, transmissometers and sediment traps. All measurements were made at least 100 m above the bed for 5 of the 6 moorings. Measurements at the deepest site were obtained 10 m above the bed.
The currents were mainly tidal, with the largest tides found closest to the bed at each location. The tidal ellipses were oriented mainly along canyon. Tidal amplitudes in the narrow portion of the canyon were 3 to 4 times larger than in the wider portion. The semi-major axis of the M2 tidal ellipse 100 m above the bed was 11 cm/s. The mean flow in the narrow portion of the canyon was upcanyon near the canyon rim and downcanyon at the deeper measurement site. There was no strong mean flow in the wider portion of the canyon. A 3-day oscillation was commonly found at most measurement sites in the canyon. The oscillation had large spatial scales because it was coherent both across the canyon and between the narrow and wide measurement sites.
To date, we have analyzed the particulate material collected in the two sediment traps deployed at 780 and 1360 m on the two sediment traps deployed at 780 and 1360 m on the north axis canyon mooring. The two traps documented dramatically different sediment flux regimes over the 12 month sampling period. Extremely large particulate sediment fluxes of 22-60 grams per square meter per day were recorded in the lower trap, located 80 m off the bottom at 1360 m, as compared with mass flux values which ranged from 2-4 grams per square meter per day in the upper trap. The 1360 m trap actually overflowed its collection tube after the end of November with the result that only four 30-day flux events were recorded in the trap sample tube.
Two significantly important events were seen in the bottom trap only. Near the end of the first period (8/3-8/31) and into the second month (8/31-9/30), a large amount of black, hydrogen sulfide-rich material formed a very distinctive layer within the trap sample tube. The material most likely originated from the slumping of soft sediments off the adjacent wall. Subsequently, on February 8, a significant turbidity event that lasted 5 to 7 days was recorded. Transmission dropped from values greater than 4 to values near 0 in 4 hours. At this same time, a large change was recorded in the pressure record. No significant events were noted in current speed, though the water at the time became slightly warmer and fresher. Unfortunately, the trap had already overflowed so no sediment was collected in the event.