A close-up mussel aggregation with Chirodota heheva sea cucumbers. Click image for larger view and image credit.
Lake ‘Eerie’ seems like a pretty apt name for this location. The seafloor was dotted with tiny tubeworms and polychaete worms, the small black specs in the image. Click image for larger view and image credit.
The Red Crater
July 3, 2007
Ian MacDonald
Texas A&M - Corpus Christi
26°23.42 N
94°30.58 W
See how scientists use the "Bushmaster" seafloor community characterization sampling tools. (Quicktime, 2.2 Mb.)
See how a typical mussel pot collection helps scientist understand deepwater mussel aggregations. (Quicktime, 1.8 Mb.)
On July 2nd and 3rd, we had been working at a brine pool at 2200 m depths. The pool measures about 140 by 180 m and is filled with concentrated brine, dark blue in color and full of flocculent fragments of grayish white barite. The ‘shore’ of the pool is coated with barite and looks for all the world like a white sandy beach—especially when Jason approaches and pushes ahead of it a bow ‘wave’ of the dense brine, which washes in and breaks over the beach, inundating the sea urchins and amphipods that live just beyond the rim of barite.
Then our work at this pool was largely done and we had some time before the next dive—our last dive of this expedition. Eric Cordes had looked at the geophysical survey charts of the area and noticed a very large, and distinct bull’s-eye about 2 or 3 km south of our position. With a bit of time to spare, why not go explore? After discussing our exploration plan, Jason went into tow mode and we made our way south at, for Jason, the warp speed of 1.5 knots.
Jason arrived at the feature in the afternoon just as a pyrotechnics demonstration was taking place on the fantail. Perhaps in anticipation of July 4th, the Brown’s crew brought out boxes of rockets and smoke bombs and showed us how we could fire these gadgets off in the unlikely event that we should find ourselves bobbing in a life raft and desire to signal our position. Against the excitement of all the bangs and whooshes and clouds of smelly smoke, the news that Jason had found “a really big brine pool” did not at first arouse much interest. But then, one by one, the scientists began disappearing into the control van until the small, dark space was packed with over a dozen spectators, craning to see the monitors. This was really cool!
Jason was following a slightly curving shoreline, deep blue on one side and white on the other. There were white flocs floating in the brine. It was like the shoreline of the previous brine pool, except much bigger. And Jason went on and on. Then the shoreline began to break apart into a series of channels and pools. The pool edge became much less distinct and we began to see patches of mussels. At first there were little clusters of ten or twenty animals, some on ‘tussocks’ of high ground, others seemingly submerged in faintly shimmering brine. Then the clusters began to merge together into mats, carpets, tapestries, bowling alleys, football fields! Jason flew along completely surrounded in every direction. The mussels were stacked on end and layered completely covering the bottom. You could see their siphons pumping amid diaphanous veils of translucent mantle tissue. There were millions and millions of mussels! And scurrying among the mussels were shrimp, snails, amphipods, pink sea cucumbers, fish... Everyone in the van was shouting and pointing and shaking their heads.
After taking a closer look at the AUV data, scientists spotted another geological target of potential interest. The late amended to the dive plan payed off when footage of this mussel ‘carpet’ came into view. They discovered what is probably one of the largest-known mussel beds in the deep Gulf of Mexico. Click image for larger view and image credit.
The Jason group fabricated this niskin bottle that could be lowered into the brine pool and triggered to collect a water sample without disturbing the delicate interface. Click image for larger view and image credit.
Dinner came and went. Jason continued to circumnavigate the crater. The inner crater, which was all we had time to explore, was 280 m across. In the survey data, the circling rim was quite distinct. On the bottom, steering by video that shows only vague shapes and shadows a few meters ahead, it was very difficult to follow the edge. We left the Mussel City behind. It seemed to curve off along what might have been a broad series of outflow channels curving away to the west. We drove out of it to the south, but never saw its limits to the west.
When we eventually finished the circuit and other collections, Jason ventured out into the center of the crater. We thought that like the smaller brine pool to the west, this would be full of fluid across it surface. Instead, soon after leaving the shore with mussels, the brine ‘dried up’ and the bottom became extremely rugged, with folds and ridges in chaotic array. The bottoms of the folds were lined with dark, blue-black sediment. The ridges were light gray with snail trails and occasional fish. Eventually, we crossed the ridge zone and reached the central 50 m or so of the crater.
Now the seafloor changed abruptly, becoming much more level and even. Instead of blue-black and gray, the sediments were chalky white. Then we began to see streaks of a creamy red material. Soon the red matter coalesced and the white material became the streaks. The bottom was perfectly level, appearing solid and with teaming swarms of amphipod-like animals skimming its surface. It was unlike any seafloor anyone of us had ever seen. We kept straining for comparisons and coming up empty—like Mars, like a desert, like the Moon. Fact was, it was new and different enough so that there were no fitting metaphors.
Our task was to collect sediment cores of the briny sediment. Matt the pilot gingerly brought Jason to the bottom, being extremely careful not to stir up a silt cloud. We selected a bright red patch to core and -mat eased the core into the sediment. The core eased into the red layer with no resistance and went down 30 cm until it seem to hit a hard bottom. When Matt pulled it out there was a big cloud of mud and mud coating the tube so it was impossible to tell if we had a sample. I thought the mud had slipped out. We tried several more times. Eventually we decided to try coring with a niskin bottle—this is a device really intended to collect water samples. It too is a tube, but on that is sealed on both ends when two stoppers are pulled together by rubber tubing running inside the cylinder. We eased the niskin into the red muck and hit the trigger. The stoppers snapped shut and we had a sample of red stuff.
