Gravity Flow Event in Monterey Canyon, 20 December 2001
- Bill Ussler
Monterey Bay Aquarium Research Institute
- Charlie Paull
Monterey Bay Aquarium Research Institute
End Date: March 01, 2002
A sediment gravity flow descended through the axis of Monterey Canyon on 20 December 2001 at 13:35 Pacific Standard Time. The timing of this event is documented by a current meter package which recorded an 11.9-dbar pressure increase in less than 10 min and was found 550 m down-canyon from its deployment site, buried completely within a >70-cm thick gravity flow deposit. This event is believed to have started in less than 290 m of water because an instrument at this location was also lost at the same time.
A 178-cm core collected after the event from the axis of the canyon at 1,297-m water depth contained fresh, greenish chlorophyll-rich organic material at 32-cm sub-bottom depth, suggesting the event extended to this water depth. The only trigger identified for this mass movement event appears to be moderate sea and surf conditions. Thus, gravity flow events of this magnitude do not require an exceptional triggering event.
Summary to DateThe current record from the S4/CTD instrument package deployed in the canyon from 11 September to 17 November 2000 shows that flow in the canyon is tidally modulated and strongest up-canyon. Apparently, common daily flow conditions are unimportant for coarse sediment transport in Monterey Canyon. Our results suggest that a sediment gravity flow occurred within the canyon at 13:35 PST on 20 December 2001, carrying one instrument package 550 m down-canyon in less than 10 min and burying it within a >70-cm-thick sand, pebble, and cobble deposit. Two homers deployed in the canyon at this time were also impacted by this event. One (homer #60) moved 110 m down-canyon and was recovered with sand and pebbles filling voids in its flotation buoy. The other homer (#61) was not relocated, presumable because it was swept away in this event.
Dislodgement of instruments and analysis of sediment cores from the canyon axis indicate that this event impacted the canyon from at least 290 to 1,297 m water depths. The shallow end range is constrained by the loss of homer #61which was deployed in 293 m of water, 10.5 km upstream of where the displaced homer and buried TCM frame were originally deployed. High levels of chlorophyll-a at 32 cm depth, within >178 cm thick gravity flow deposit at 1,297-m water depth, suggest a recent event occurred in this area. We infer that these observations all reflect the same gravity flow event. While the thickness of gravity flow deposits may vary considerably along the axial channel, these observations provide some constraints on the size of this event.
A rough estimate of the volume of material involved in this mass transport event can be made by assuming that a 70-cm-thick layer of sandy sediment was deposited along the 31-km stretch of the canyon floor between 290 and 1,297 m water depths. The average channel width is over 100 m. Thus, ~2,200,000 m3 of material could have been involved in this event. For comparison, the volume of sand estimated to enter the canyon head each year from long-shore sediment transport is ~400,000 m3. This calculation suggests that events of this magnitude would have to occur approximately every five years to maintain the sand budget. Knowledge of the timing of this gravity flow event allows us to assess whether an earthquake, hyperpycnal flow, or storm may have triggered the event.
A search of the Northern California Earthquake Data Center (NCEDC) indicated that no seismic events greater than magnitude 2 occurred within 24 km (the distance to the San Andreas Fault) of the canyon during any of the mooring deployments. Moreover, no appreciable seismic events occurred in California near 13:35 PST on 20 December 2002. Johnson et al. (2001) indicated that hyperpycnal flows occur within Monterey Canyon when the Salinas River discharge exceeds 300 m3 s-1. Although river runoff increased in response to the winter storm on 20 December 2001, maximum flow was only 3 m3 s-1 (USGS gauging station 111525000). Thus, flow conditions suitable for generating a hyperpycnal flow did not exist at that time. Storm conditions existed in Monterey Bay on the afternoon of 20 December 2001 when the gravity flow event occurred. NOAA weather buoy #46042 records sea conditions ~100 km offshore from the head of Monterey Canyon. The gravity flow event occurred near the beginning of a 6-hour interval when significant wave heights exceeded 6 m at this buoy.
We suspect that these sea conditions probably triggered the 20 December 2001 gravity flow event. While these are large swells, they are not unusually big for Monterey Bay. This weather buoy recorded similar or greater significant wave heights on eight other occasions during 2001. Thus, we conclude that sediment gravity flow events in submarine canyons do not require exceptional triggering events.
- Instrument package deployed in the canyon from 11 September to 17 November 2000 shows that flow in the canyon is tidally modulated and strongest up-canyon.
- Dislodgement of instruments and analysis of sediment cores from the canyon axis indicate that this event impacted the canyon from at least 290 to 1,297 m water depths.
- ~2,200,000 m3 of material could have been involved in this gravity flow event.
- Sea conditions probably triggered the 20 December 2001 gravity flow event. Sediment gravity flow events in submarine canyons do not require exceptional triggering events.
- Geological characterization