The most fundamental problem facing any oceanographer wishing to study the seafloor is all that pesky water in the way. Unlike our terrestrial counterparts, marine geologists cannot see deep seafloor features except in tiny patches illuminated by the lights of expensive and slow submersibles or by the strobes of a towed camera. Another approach is to use sound for imaging. That's just what the TAMU2* digital side-scan sonar does. This device sends a fan-shaped ping, or sound pulse, toward the seafloor beneath and to the sides of the instrument. TAMU2 then listens to the sound scattered back to it and composes an image based on the intensity of the returned signals. At the same time, TAMU2 determines seafloor depths using the signals' round-trip times and their return angles.
For twelve days in June, my colleagues and I used TAMU2 on the R/V Gyre to map three areas of the Louisiana continental slope seafloor where oil seeps are known to occur along faults. We mapped these areas because they contain sites where chemosynthetic organisms have been intensively studied. We wished to determine whether these sites are common or rare. Chemosynthetic organisms such as tube worms and bivalves live off of a food chain based on bacterial degradation of hydrocarbons rather than photosynthesis. Prior studies have focused on a few sites found by chance without surveys to show whether these sites are surrounded by others equally important.
Before starting out, we knew our two largest survey areas each contained approximately six to ten hydrocarbon seeps based on oil slicks they produce at the sea surface. Furthermore, prior geologic studies and bathymetry data show numerous faults and moving salt domes, tectonic elements known to rupture sedimentary layers and release hydrocarbons.
Our TAMU2 data revealed a rich bounty of seafloor features. The records show mud mounds formed by fluid expulsion, carbonate precipitate mounds related to seepage, brine lakes, faults, and sediment flows. In one area we found evidence for massive mudflows emanating from a probable brine-lake atop a mud mound. In the area where most studies of chemosynthetic organisms took place, we confirmed eleven additional mud mounds like the famous Bush Hill chemosynthetic site, which is also within our survey. Our data imply that as many as 50 of these features exist in the area.
Faults are of particular interest and the TAMU2 data showed a network of faults extending from one salt dome to another and ringing the basins between them. Along one fault complex we found numerous places where sediment flows emanate and extend downslope. These are probably sites of significant hydrocarbon venting.
In our deeper survey area, where salt domes are larger and dome slopes steeper, we saw numerous sediment flows on basin floors and surrounding dome flanks. Although some of these may be related to hydrocarbon seepage, many doubtlessly result from destabilization of sediments by salt movement underneath them.
Have you heard the joke about how the operation was a success but the patient died? Our survey ended on a sour note when, after days of flawless operations, the $500,000 instrument package parted its cable and sank while being hauled in at the end of the cruise. The neutrally buoyant package is supposed to float, but this time it sank, reinforcing the adage that an oceano-grapher should never put anything in the water without first saying good-bye. Plans are underway to rebuild the sonar.
[~31K] Map made from a portion of the TAMU2 survey.
