A search for U.S. Geological Survey (USGS) instruments missing in Monterey Canyon failed to recover the instruments but gathered valuable new data about the canyon bottom.
The missing instruments were part of three deep-sea moorings deployed in the canyon by the USGS from December 2002 to November 2003. Instruments recovered from all three moorings, at 820-, 1,010-, and 1,450-m water depth, have provided direct field measurements, for the first time ever in turbidity-current research, of four turbidity currents. The nearbottom instrument packages on two moorings were not recovered because of a parted wire (on the 820-m mooring) and a malfunctioning acoustic release (on the 1,010-m mooring).
Each package included a sediment trap, a transmissometer, and a Microcat CTD (conductivity-temperature-depth sensor). We have waited for opportunities to recover these two packages, not only for the expensive instruments but also for the data they contain, which would give us more insights into the patterns of near-bed temperatures and suspended-sediment concentrations within turbidity currents. Fortunately, we didn't have to wait long for such an opportunity. Charlie Paull, our Monterey Bay Aquarium Research Institute (MBARI) collaborator, offered two days of ship time, using MBARI's research vessel Point Lobos and remotely operated vehicle (ROV) Ventana to search for the missing instrument packages.
On February 19, a team consisting of Jingping Xu (USGS), Charlie Paull and Rendy Keaton (MBARI), and Marla Stone and Keith Wyckoff (Naval Postgraduate School) departed from Moss Landing, CA, on the Point Lobos for a "search and rescue" mission. The Point Lobos is known for its notorious "rock 'n' roll" in rough weather, and the 14-second, 2- to 3-m swells didn't help. A Dramamine pill a day helped keep the breakfast down, but that was one of the worst boat rides I've ever taken in my 10-plus years of going to sea!
We went first to the 1,010-m mooring site, where the acoustic release on the missing instrument package had been contacted in November 2003 during the first recovery effort. Although the acoustic release indicated that it had let go, it failed to come free, and the package had to be left behind. During that effort in November, an accurate position was triangulated, and we decided during February's search to send the ROV directly down to the determined position. It took the ROV about 45 minutes to dive approximately 1,000 m before it safely landed on the canyon floor. For the next few hours, the ROV searched an area of several tens of square kilometers.
Because visibility at the bottom of the canyon was poor, we often had to rely on the ROV's sonar to look for evidence of the instruments. Some nongeologic targets showed up on the sonar image, but when the pilot maneuvered the ROV to reach these targets for a closer look, they all turned out to be something else: a large log, a clump of bamboo, a tire, and a big mudstone boulder that appeared to have fresh breaks. In one case, we saw a curled, dark, ropelike object on the floor, which very much resembled the black-jacketed wire used on the mooring. As soon as the pilot picked it up, using the ROV's hydraulic arm, it broke into piecesit turned out to be a strand of sea grass. The canyon floor was mostly covered with soft, fine sediment, but we also saw gravel beds in bars and strips.
During all these hours, Marla was ranging on the acoustic release with an Edgetech deckboxsending an acoustic signal to the acoustic release, receiving a response, and using the elapsed time to calculate distance from the ship to the acoustic release. Even though the ranging showed that the ship and ROV were very near to the acoustic release (Marla said the deckset could hear the acoustic release loud and clear), we couldn't see any target on the ROV's sonar or video images. The coded response from the acoustic release indicated that it was released and tilted (that is, in a horizontal position).
Unable to find the target, we suspected that the release and instrument package had been either buried or moved by another turbidity current. At about 1:00 p.m., we decided to pull the ROV onboard and conduct another triangulation to see whether the package had moved since November. We were able to "pinpoint" an area the size of half a football field (60 by 80 m) where the acoustic release appeared to be. This new location was only about 40 m from the site identified in November, and it had been searched during the ROV dive. Nevertheless, we decided to conduct a second ROV dive the next day, to go over the area once more. On the way back to the harbor for the night, we stopped at the 820-m mooring site. Marla attempted to contact the acoustic release at that site, but there was no response.
We returned to the 1,010-m site the next morning for the second ROV dive. The pilot virtually "mowed the lawn" with the ROV, examining overlapping strips of sea floor in the targeted area. There was one exciting moment when we saw a linear target on the sonar image. As the ROV approached the target, we saw a thin, straight object lying on the canyon floor whose length (about 2 m) and size made it look similar to the stainless-steel rod (called the "strongback") of the instrument package. But no sooner had I shouted, "We got it!" than the ROV's hydraulic arm picked it up, and it turned out to be a wood stick.
Several more sonar targets were found and visited, but they all turned out to be something else. We became convinced that the package must have been buried in the sediment. If nongeologic features, such as a wood stick and a large flounder, as well as geologic features, such as the boundary between a rippled bed and a flat bed, could all generate strong sonar signals, the stainless-steel strongback, along with the anchors, the chains, and the release, must have shown even stronger sonar signals if they had been lying on the surface of the canyon floor. With great disappointment, we called off the search. Marla disabled the acoustic release to conserve battery power, in hopes that it will be still alive if we come back in the future, and we moved on to the 820-m mooring site.
The prospect of finding the instrument package at the 820-m site was even smaller than at the 1,010-m site, because the shallower site's acoustic release had never responded to signals from the surface. Therefore, the main mission of this dive was to browse the canyon floor for any interesting geologic features. As the pilot drove the ROV down the canyon, we saw several fresh-cut ledges parallel to the north canyon wall. At the toe of one of these 1.5-m-high ledges, a slab of clay (0.5 by 0.5 m) was lying on the floor. Several cracks were visible on the surface of the clay slab, and the material was soft enough to be penetrated by a leg of the ROV. Charlie conjectured that it was a big slab of soft mudstone which fell from somewhere (for example, the canyon wall) and broke into pieces when it landed. The ROV dive at this site lasted only about 30 minutes.
Although we were unable to find the missing instrument package at either mooring site, the ROV dives were nonetheless scientifically fruitful. Most interesting are the observations of several geologic features, including gravel beds, sand ripples, cobbles and slabs of mudstone, clay balls, and fresh-cut ledges. In the coming months, scientists from all three institutions will be busy digesting the tons of photographs, videotapes, and navigational data to better understand the relations between these geologic features and the turbidity currents observed from the mooring instruments.
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