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USGS Instruments Record Turbidity Flows in Monterey Canyon, California
Monterey Canyon is known for its intermittent and dramatic downcanyon turbidity flows, or underwater avalanches of sediment and water. In 1994, tantalizing evidence of a turbidity flow was recorded by a U.S. Geological Survey (USGS) mooring in the canyon axis in 1,450 m of water. That event was violent enough to damage a pressure sensor and totally occlude an optical water-clarity sensor 100 m above the bed. Ever since, USGS researchers have been eager to get a better look at one of these elusive events. Last November, they got their wish when they recovered three moorings that had recorded flow-velocity profiles and collected sediment from four turbidity flows that tumbled down the canyon during the past year. The moorings had been deployed in December 2002 as part of a joint effort between the USGS' Woods Hole Science Center, the Western Coastal and Marine Geology Team, and the Naval Postgraduate School in Monterey, CA. At 820-, 1,010-, and 1,450-m water depth along the canyon axis, the moorings were timed to be in the water simultaneously with instrumented bottom frames deployed by the Monterey Bay Aquarium Research Institute (MBARI) in shallower waters upcanyon. Each USGS mooring was designed by Marinna Martini to suspend instrument packages above the bed at three or four heights specified by Marlene Noble, the project's chief scientist. Each package included an Anderson-type sediment trap, which collected sediment moving downcanyon and falling from the canyon walls, and various sensors to measure temperature, salinity, current speed and direction, and suspended-sediment concentration.
To keep the traps within 10 degrees of vertical and to avoid drag from the anticipated high-speed currents at the bottom, the moorings' design located all the flotation at the top. This design was risky: if too many of the glass flotation spheres imploded, or the mooring wire failed, only a specialized, deep-water remotely operated vehicle (ROV) might be able to find and recover the collapsed mooring. Although they feared that the bottom of the moorings might be buried or battered by debris driven by turbidity flows, researchers were eager to get data and sediment close to the bottom. So, the bottom of each mooring was designed to be sacrificial: one acoustic release (which parts the wire when it receives an acoustic signal from the surface) was placed at the bottom of the mooring, below the lowermost instrument package, and another acoustic release about 60 m above the bottom. If the first acoustic release was buried and became inoperable, the second acoustic release could be activated, and all the instrumentation above the second acoustic release successfully retrieved. Thus, only the lowermost instrument package would be lost in the event of a large, damaging turbidity flow. If the lowermost instrument package could be retrieved intact, however, scientists would get data within 20 m of the seabed, where the greatest and most interesting sediment movement was believed to occur. Also attached to each mooring was a downward-looking acoustic Doppler current profiler (ADCP), which uses sound waves and the Doppler effect to measure flow velocity at numerous points between the instrument and the sea floor. Each ADCP was attached just above the second acoustic release, about 60 m above the bottom, and measured flow-velocity profiles down to the seabed. During the year that the moorings were in the canyon, reports came back from MBARI scientist Charlie Paull that MBARI's bottom frames were being pushed downcanyon by turbidity flows and battered by large, refrigerator-size rocks. The USGS team crossed their fingers and hoped that they would not see messages from the satellite beacons indicating that the moorings had parted and surfaced prematurely.
After 11 months and 20 days of the 12-month deployment, Steve Etchemendy of MBARI reported that a string of floats had been sighted on the surface which looked suspiciously like the USGS' floats. So, 10 days before the planned recovery cruise, a gutsy team led by Marla Stone of the Naval Postgraduate School hopped aboard the research vessel Shana Rae, a 52-ft-long converted fishing boat, and spent 10 seasick hours in the dead of night recovering the 820-m-long mooring. Many hooks were found embedded in the mooring wire and instrumentation, suggesting that a fishing boat may have hooked into the mooring and parted the wire. However, the instrumentation had recorded the onset of a turbidity flow at the same time the wire broke, and so debris in the turbidity flow may have been the cause. The wire broke near the bottom, thus everything was recovered except the sacrificial bottom instruments. A week later, on November 25, 2003, a team from the USGS in Menlo Park, CA (Marlene Noble, Jingping Xu, Joanne Ferreira, Hal Williams, Kevin Orzech), the USGS office in Woods Hole, MA (Marinna Martini, Jonathan Borden, Rick Rendigs, Steve Ruane), and the Naval Postgraduate School (Marla Stone) recovered the rest of the moorings aboard the research vessel Point Sur. The team experienced only one more mishap, losing the sacrificial bottom part of a second mooring, at 1,010-m water depth, to an acoustic-release failure. Given the debris noted by MBARI, the release may have already been damaged, or its mechanical workings clogged. The third mooring, at 1,450-m water depth, was retrieved intact. The data from all recovered instruments on the three moorings are currently being processed. Preliminary examinations showed some spectacular results. On all three moorings, the 1-m-long sediment traps at 64 and 158 m above the canyon bed were full of sediment—several traps were overflowing with it. The sediment layers in the traps (fine sediment overlain by coarse, then again by fine) clearly indicate an event-driven hydrodynamic environment deep within the canyon. USGS scientists Bill Normark, Brian Edwards, Homa Lee, and Kevin Orzech and MBARI scientist Charlie Paull will help Jingping Xu decipher the information now buried in those cores, including analyzing them for contents of DDT and other pesticides, determining their stratigraphy, studying foraminifers, analyzing grain size, and measuring C14 isotopes and chlorophyll concentrations. The downward-looking-ADCP profiles showed four distinct, violent turbidity flows during the year-long deployment: two in December 2002, a third in March 2003, and a fourth in November 2003. The December 2002 and March 2003 events had measured current speeds of more than 1.5 knots near the bed; the highest current speed recorded during the second event was nearly 4 knots. That second event (on Dec. 20, 2002) moved the shallow USGS mooring at 820-m water depth almost a third of a mile downcanyon and may have moved and covered the MBARI moorings deployed in shallower waters in the canyon axis. The USGS moorings at 1,010- and 1,450-m water depth stayed in place during these strong events. A preliminary look suggests that the USGS moorings will provide detailed turbidity-flow data never before recovered from a submarine canyon. In addition to shedding light on the flows themselves, the data set will allow examination of possible triggering mechanisms, which include not only earthquakes, surface waves, and density flows but also the large, near-bottom internal tides that Marlene Noble and Leslie Rosenfeld (Naval Postgraduate School) already noticed in the current-meter records. The entire Monterey Canyon team is waiting breathlessly to see the final processed data from all the recovered instruments.
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in this issue:
Mapping Hawaiian National Parks
Special AGU Sessions on Gas-Hydrate Systems South Louisiana Sea-Level Rise
USGS Book Wins Outstanding Publication Award "Local Heroes" of Western Snowy Plover Protection
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
