Underwater Pavement: Asphalt deposits cover parts of Gulf of Mexico

During explorations of the seafloor in the southern Gulf of Mexico late last year, researchers discovered peculiar lavalike flows of asphalt that had gushed down the slopes of a steep undersea knoll. The now-solid swaths of hydrocarbon-based material are home to a thriving ecosystem, the scientists have found.

In the first stage of research, the multinational team of scientists used sonar to map a 60-kilometer-by-90-km patch of ocean bottom about 200 km west of the Yucatán. Within that area lie the Campeche Knolls, 22 elongated hills that stand anywhere from 450 meters to 800 m above the surrounding abyssal plain. Slopes of the knolls can measure as much as 12°, says Ian R. MacDonald, an oceanographer at Texas A&M University in Corpus Christi.

The sonar maps revealed that the crests and flanks of nine of the undersea knolls were riddled with fractures and covered with blocks of slumped material.

Satellite images of the region’s ocean surface often show small slicks of oil floating directly over the knolls. When MacDonald and his colleagues used remotely controlled undersea cameras to investigate, the scientists spotted extensive deposits of solidified asphalt emanating from a fault near one knoll’s crest. One 15-m-wide deposit there, consisting of several layers, chronicles a series of eruptions, says MacDonald. The researchers describe their find in the May 14 Science.

Asphalt samples retrieved from the seafloor didn’t contain any oil and held only traces of hydrocarbon gases. A rind on the deposits, as well as a certain type of fracturing throughout the material, suggests the erupting asphalt flowed readily but quickly cooled when it hit the 4°C water of the Gulf’s depths. To have flowed easily, the asphalt either must have erupted at temperatures above 200°C or contained low-viscosity hydrocarbons that have since dispersed, says MacDonald.

Thick mats of microbes blanket some areas of the asphalt deposits. In other spots, ghostly white crabs skitter among clusters of tubeworms and mussels. DNA analyses ought to reveal whether these asphalt-dwelling organisms are genetically distinct from similar creatures found elsewhere in the Gulf’s waters, says MacDonald.

In many regions of the world, hydrocarbons slowly seep from the ocean floor, says biologist Cindy L. Van Dover of the College of William and Mary in Williamsburg, Va. The Campeche Knolls site, however, is the “first site I’ve seen where the hydrocarbons formed a hard surface” that could be colonized by organisms, she notes.

The researchers suggest that microbes at the base of the ecosystem’s food chain are probably feeding either on hydrocarbons in some of the asphalt or on small amounts of dissolved hydrocarbons or sulfides still emerging from the ocean floor.

The species living on the Campeche Knolls asphalt deposits are a “particularly cool twist on chemosynthetic life,” says Charles Paull of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. The frigid temperatures at the site stand in marked contrast to the hellish conditions of the ecosystems thriving near the hydrothermal vents typically associated with midocean ridges.