Explorations | Expedition to the Deep Slope 2007 | Logs

Mission Summary

Bob Carney
Louisiana State University

The 2007 Expedition to the Deep Slope completed exploration begun during the 2006 Expedition. Spending a month at sea on NOAA’s Research Vessel Ronald Brown, the explorers carried out extensive investigations using the remotely operated vehicle (ROV) Jason to revisit known sites and explore never-before visited locations. While each team of investigators sought answers to many specific questions, the central theme of the expedition was to discover and better understand special deep habitats where geology and biology interact to create atypical seafloor ecologies.

The very high likelihood of finding new seafloor habitats in the Gulf of Mexico is a consequence of an unusual history. The slopes of the North American continent within the United States EEZ contain examples of almost every type of continental margin. This diversity of form and geological process provide ocean scientists with many opportunities to discover and to understand habitats in which biology and geology interact in unanticipated ways. The slope of the Gulf of Mexico is unique within our EEZ. Increasingly industrialized by the offshore oil and gas industry, the Gulf is an ancient basin.

Of particular interest to the explorers of the Deep Slope expedition is the unusually thick layer of sediments in the northern Gulf. Contained within these layers is the history of erosion and river transport from the Appalachians, the Rockies, and smaller mountain ranges formed and worn away on the North American continent. Also within these layers are oil and methane gas arising from the high primary productivity in the ancient Gulf. Rather than simply being trapped, these hydrocarbons and other chemicals normally locked deep within formations actively seep to and through the seafloor along faults caused by movements of a thick layer of mineral salt deposited early in the Gulf’s complex history. At these sites of seepage a unique biota develops in response to the unique geochemistry.

The pace and mode of operations centering around the ROV JASON proved quite different than the 2006 experience with a human occupied vehicle (HOV), the submersible ALVIN . Gone were the morning launch, afternoon recovery, and extensive briefings where each scientist tried to communicate their highest-priority needs to the three people who would actually make the dive. The unmanned JASON is lowered to the seafloor where it can operate for days without resurfacing. Shifts of three Jason crew members attend to all the operational details, while scientists took turns logging activities or carrying out specific tasks one after the other around the clock. This all takes place in a control van strapped to the deck of the R/V Ronald Brown. Illuminated only by the light of instruments and monitors, up to ten people would crowd into the heavily air conditioned room to watch progress over the seafloor.

Beginning June 7, 15 successful descents of Jason were executed with a maximum duration of 42 hours and 41 min. Ten separate areas were explored with the shallowest at 1014m and the deepest at 2750m. The tasks executed on most descents included an initial acoustic mapping of the seafloor to define the area of investigation, sediment sampling, faunal sampling, imaging surveying, and detection of dissolved chemicals with an underwater mass spectrometer. Analysis of these samples will require up to two years in the labs of the explorers. When final results emerge, we will have the most detailed description ever developed of chemosynthetic communities east to west and upper slope to abyss.

While final results will require time to develop, we can already be certain about a few things. Our methods of exploration have proven sound. Especially effective is the targeting of general deployment areas in advance based on the examination of seismic survey data collected by the offshore industry and archived by the Minerals Management Service. All seeps have a distinct seismic signature. Once in a target area, initial bathymetric survey using the acoustic systems of the JASON, we were able to develop high-resolution seafloor maps. These required several hours of bottom time and data analysis, but they provided an invaluable guide to subsequent bottom activities. We were also successful in adjusting our shipboard lab work to the schedule of the JASON through the use of elevators which brought samples to the surface for timely analyses. This was especially important for the microbiologists who had to work for many hours on the freshest of samples.

An added benefit of ROV exploration is that time on bottom is less rushed; allowing scientists time to observe the seafloor between and around the targeted seep communities. This lets us better understand how the combined seep and normal slope systems interact rather than take a myopic view of seeps alone. Using the analytical tool of stable isotope analysis, following laboratory analysis of samples we will better understand who eats whom within the communities. We will also be able to determine if the chemosynthetic production of seeps provides and significant source of food to the more typical deep sea fauna of the surrounding slope. Based on initial observations, we see little suggestion of heavy utilization of seep production, but lab analyses will tell the full story. We were also able to reexamine dense populations of sea urchins found in association with seeps. First noted during explorations at the Blake Ridge seeps in the Atlantic, it is anticipated that observations in two years will help explain the origins and persistence of these animals.

Of special interest among non-chemosynthetic fauna on the continental slope are aggregations of various types of sessile, suspension feeding corals. There existence is somewhat problematic since few suspension feeding animals subsist in the deep sea where food is quite scarce and hard bottoms needed for attachment are rare. Off the southeastern states, explorations have shown deep coral assemblages to be relative common. So far exploration of the northern Gulf of Mexico has found them to be present but less common. Below 1500m they have been rarely encountered even though hard substrates are present.

Hydrocarbons in nature as well as in industrial chemistry come in many forms and formulations. Gases such as methane contain one or just a few carbon atoms. At the other extreme of carbon numbers are thick dark solids commonly referred to as asphalt. It was discovered about fifty years ago that large deposits of asphalt exist as mounds in the deepest part of the Gulf of Mexico, the Sigsbee Abyssal Plain. More recently, it has been determined that chemosynthetic fauna is associated with some of these mounds. Therefore it was especially interesting that lumps of asphalt were found at Garden Bank 647. These were small enough to be picked up by JASON but did not support chemosynthetic fauna. Found at a depth of only 1014m, this discovery seriously challenges speculation that superheated water at much greater depth is needed to create the formations. As with brines, our new results force us to rethink the processes that produce all types of hydrocarbons on the seafloor.

Because of the large size and mobility of the LuAnn salt sheet, it is relatively common to find brine one the seafloor. There are probably several mechanisms which create this water with very high concentrations of salt, but the easiest to envision is the dissolution of the salt layer when in contact with normal seawater. The resulting solution is so dense that it mixes into the overlying ocean water very slowing. We encountered brine in so many different situations during our JASON deployments that we will be forced to reconsider our preconceptions. Especially interesting was an apparent brine-filled crater approximately 150m in diameter at a site designated Alaminos Canyon 601. This distinct feature supported a large mussel community and had the coloration found in other brine pools. After exploring it extensively, we were quite surprised when trying to take a water sample to find far more mud in the crater than brine. Quite possibly, this crater had experienced a recent breech in its wall, allowing the brine to escape. Brine systems, it seems, can be quite transient.

The at-sea component of the Deep Slope Explorations is now complete. The 11 student participants will complete analyses in the coming moths, ponder the results, and possibly devote their careers to wider exploration and greater understanding. The eight more established investigators will fit their experiences and greater understanding into their careers. Some looking back to first experiences in the deep sea nearly a half century ago will have to adjust long-held ideas. Some looking forward to beginning careers that will span another half century will be forming new ideas about the working of the deep.