In-situ Measurements of Turbidity Currents in the Monterey Submarine Canyon
- Jingping Xu
United States Geological Survey (USGS)
- Marlene Noble
United States Geological Survey (USGS)
End Date: November 30, 2003
Turbidity currents are thought to be the main mechanism to move approximately 500,000 cubic meters of sediment annually from the head of the Monterey Submarine Canyon to the deep-sea fan. Indirect evidence has shown frequent occurrences of such turbidity currents in the canyon, but the dynamic properties of turbidity currents such as maximum speed, duration, and dimensions are still unknown. In this study, we present the first-ever in-situ measurements of velocity profiles of four turbidity currents whose maximum along-canyon velocity reached 190 centimeters per second (cm/s), which is equivalent to 4.25 miles per hour. Two turbidity currents coincided with storms that produced the highest swells and the biggest stream flows during the year-long deployment.
Summary to DateA 2002-2003 study by the USGS and the Naval Postgraduate School measured the velocity and character of four turbidity currents in the Monterey Canyon. Until this study, there had been no detailed documentation of the velocity structure within turbidity currents. During past attempts to measure these events, the instruments were destroyed or were too high off the seafloor to get a detailed profile.
Three moorings were deployed in December 2002 at depths of 820 m, 1020 m, and 1450 m in the Monterey Submarine Canyon (Figure 1). The mooring lines at each site had a variety of oceanographic instrument packages attached to them at different depths to derive profiles of oceanic conditions (Figure 2). The instruments included: downward-looking acoustic Doppler current profilers (ADCPs), CTDs (which measure conductivity, temperature, and depth), transmissometers (which measure the transmission of light as a proxy for turbidity), sediment traps, and single point current meters. In November 2003, the instruments were recovered and the data collected over the year-long deployment was downloaded and analyzed.
Evidence for four turbidity currents was documented by the three moorings deployed during the 2002-2003 study:
* Event 1 - December 17, 2002: This event most likely began in Soquel Canyon because it was not recorded by the R1 mooring and water temperatures indicated that it originated above 500 m. The sediment-laden current lasted at least 5 hours, but turbidity within the canyon remained elevated for several days. Maximum velocities at the R2 mooring were 60 cm/s at 12.5 meters above the bed (MAB); at the R3 mooring, the current moved faster and closer to the sea bed (75 cm/s at 9.3 MAB) (Figure 1 & Figure 3 (A,B,C)).
*Event 2 - December 20, 2002: This turbidity current was the fastest recorded and was documented at all three moorings. The maximum velocities were: 190 cm/s at 12.2 MAB (R1), 160 cm/s at 10.5 MAB (R2), and 180 cm/s at 5.3 MAB (R3) (Figure 1 & Figure 3 (D,E,F)). The actual speed at the front of the turbidity current was calculated to be 6-10 km/hr. This event lasted for 5-8 hours and, despite being faster than Event 1, had less pervasive turbidity than the first event.
*Event 3 - March 14, 2003: This event was similar in profile to the two previous events, but was much weaker. The sensors on the R3 mooring did not record the event (Figure 1 & Figure 4 (A,B,C)).
*Event 4 - November 9, 2003: This event was also similar in profile to the first two events, with a maximum velocity of 155 cm/s recoreded at 10.5 MAB (R2). The current slowed as it moved down-canyon (110 cm/s at 7.3 MAB (R3)) (Figure 1 & Figure 4 (D,E,F)). Also during this event, the R1 mooring cable broke, leaving the lower 70 m buried on the seafloor. The portion of the mooring above 70 m was recovered approximately 8 km away. (For more information on the search for the missing instruments, visit: http://soundwaves.usgs.gov/2004/03/index.html.)
The cause or trigger for these turbidity currents is not fully understood, but can be somewhat constrained. By correlating seismic activity to the timing of the turbidity events, earthquakes can be ruled out as a trigger. River discharge from the Salinas, San Lorenzo and Pajaro Rivers was also not great enough during this time period to create hyperpycnal (high density due to suspended sediment) flows down the canyon.
In the case of Event 3 in March, the disposal of 16,000 cubic meters of dredge spoils from the Moss Landing Harbor at the head of the canyon may have created an overburden that triggered the small turbidity event seen in the mooring data. Events 1 and 2 occurred during major winter storms, when the highest waves were recorded for that season. This intense storm activity most likely triggered Events 1 and 2. The origin for Event 4 is still uncertain.
Reference: Xu J. P., M. A. Noble, L. K. Rosenfeld (2004). In-situ measurements of velocity structure within turbidity currents. Geophys. Res. Lett. v. 31, L09311, doi:10.1029/2004GL019718.
- The turbidity events documented in this year-long study all occurred in winter. Two of these turbidity currents appeared to be related to storms and another was likely human-induced.
- The bodies of the turbidity currents were confined within 50 m of the sea bed, but the resulting sediment plumes reached as high as 170 m above the canyon floor.
- The maximum speed of these events occurred at the head of the turbidity currents between 5 and 12 meters above the bed (MAB). As the turbidity currents moved down canyon, their heads became closer to the canyon floor.
- Geological characterization
- Substrate characterization
Study MethodsScientists from the USGS and the Naval Postgraduate School deployed three moorings in December 2002 at depths of 820 m, 1020 m, and 1450 m in the Monterey Submarine Canyon (Figure 1). The mooring lines at each site had a variety of oceanographic instrument packages attached to them at different depths to derive profiles of oceanic conditions. The instruments included downward looking acoustic Doppler current profilers (ADCPs), CTDs (which measure conductivity, temperature, and depth), transmissometers (which measure the transmission of light as a proxy for turbidity), sediment traps, and single point current meters (Figure 2).
In November 2003, the instruments were recovered and the data collected over the year-long deployment was downloaded and analyzed.
Figures and Images
Figure 1: Map showing the locations of the three moorings (R1,R2,R3) deployed from December 2002 to November 2003 in Monterey Canyon.
Figure 2: Schematic drawing showing the various oceanographic instruments attached to the three moorings. Having instruments at different depths at three mooring locations allowed researchers to make detailed profiles of turbidity currents.
Figure 3: Hourly vertical velocity profiles of Events 1 (A,B,C) and 2 (D,E,F). The profiles are color-coded in the following order (from beginning of the flow in time to the end): black, red, yellow, green, cyan, blue, and purple. The deepest data point in each profile may not be accurate because of acoustic interference from the canyon floor. Velocities are interpolated to a common, hourly time-base, so the maximum velocities depicted are smaller than the measured maxima. Notice the different velocity scales for the events.
Figure 4: Same as Figure 3, but for Events 3 (A,B,C) and 4 (D,E,F).