Geochemistry, Mineralogy, Volcanology News

Asphalt Volcanoes Erupt in Slow Motion

Natural asphalt seeps on the ocean floor provide a stable home for diverse marine life that sequesters greenhouse gases.

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Underwater volcanoes erupt throughout the world, but in the southern Gulf of Mexico, they churn out something unusual: cold asphalt instead of hot lava. First discovered in 2003, these natural oil seeps at the bottom of the ocean provide a home and fuel for marine life.

Diagram showing how asphalt volcanoes form.
Asphalt volcanoes form above natural oil reservoirs deep below the seabed. Microorganisms degrade the oil, leaving asphalt and methane to seep out of the ocean floor. This diagram shows how these structures form. Credit: Jack Cook, Woods Hole Oceanographic Institution

The highly diverse ecosystems that spring up around asphalt volcanoes do something else: sequester carbon. Federal laws protect deep-sea ecosystems on the U.S. side of the Gulf of Mexico, but on the Mexico side, no such protections exist. Because these sites occupy an area that is open to energy exploration and development, a multinational team of researchers has suggested that it is time to consider the best model to conserve them.

Pavement Under the Sea

Natural asphalt is a sticky, viscous form of oil. When microorganisms degrade oil from reservoirs below the seabed, they leave asphalt behind as a waste product.

A team of German, U.S., and Mexican researchers discovered asphalt volcanoes at the Campeche Knolls in the southern Gulf of Mexico during an expedition in 2003. The researchers named the original site, covering more than a square kilometer in area, Chapopote, the Aztec word for tar.

They found that as the asphalt seeped out of the seabed, it hardened and solidified in the cold water. Few processes add hard surfaces to the deep ocean, according to Ian MacDonald, a biological oceanographer at Florida State University in Gainesville and one of the researchers who discovered Chapopote.

Most organisms that survive in the depths do so by burrowing under layers of the ocean bottom’s sediments, MacDonald said, but asphalts provide a hard surface on which species such as ice worms and some types of mussels can grow. In addition, the seeps provide the starting materials for chemosynthesis—the process by which organisms use energy from inorganic chemical reactions to make their food.

The German government funded return trips in 2006 and again in 2015 to further explore the asphalts and characterize the diverse fauna that inhabit them. MacDonald presented the results of the 2015 expedition at the 2016 Ocean Sciences Meeting in New Orleans.

Slow Ooze, Harboring Hydrates

When the team first discovered the asphalt volcanoes, they found that the asphalt looked strikingly similar to lava flows on land—asphalt flows change size, they get wrinkly, they fold over each other, MacDonald said. They speculated that the asphalt was released quickly in bursts, but when they returned in 2015, a closer look at the asphalt eliminated that possibility.

By creating a photo mosaic of the main asphalt flow and examining its shape and how the asphalt had weathered over time, they realized that the asphalt oozed slowly out of the seabed, rather than erupting in a quick spurt.

“The asphalts come out very slowly—tectonically slowly,” MacDonald said.

At deep asphalt volcano sites, gas hydrate outcrops, like this one seen in a panoramic view, form almost instantly.
At deep asphalt volcano sites, gas hydrate outcrops, like this one seen in a panoramic view, form almost instantly. Credit: Ian MacDonald and Marum Center for Marine Environmental Sciences

Unexpectedly, the research team found large mounds of gas hydrates—clusters of ice with methane trapped inside—on or near the volcanoes. They also found massive aggregations of chemosynthetic tube worms tens of meters long colonizing the hydrate mounds. Some of the tube worms may be hundreds of years old, they noted.

Gas hydrates would normally dissolve quickly in seawater because the concentration of methane in the sea is so low, but the researchers suspect the tube worms help to stabilize the hydrate mounds.

“The tube worms are creating a blanket that essentially sequesters the gas hydrate and stops it from dissolving into the seawater,” MacDonald explained.

Interocean Connectivity

Chemosynthetic communities proliferate around hydrocarbon seeps in many areas along the equator, from the Gulf of Mexico to West Africa’s Atlantic coast and even in the eastern Pacific. Before the Isthmus of Panama closed off the Atlantic from the Pacific, these waters were joined.

“We think that, at some point, all of these communities were connected, and we still see a genetic relationship in some of the crustaceans between these sites,” said Elva Escobar, an aquatic ecologist at the National Autonomous University of Mexico in Mexico City and a member of the research team. The group is now studying how the organisms’ larval stages go from one place to the other and is gathering Pacific specimens to compare with those from the other side of the isthmus.

The asphalt ecosystem is an incredibly diverse community that is still understudied, according to Escobar. “We know very little about how these communities grow, how they are structured, how they change in time, and how they interact with the gas hydrates,” she said.

Chapopote could provide a model for studying and conserving other chemosynthetic communities in the deep sea, she said. It “provides a natural laboratory that allows us to see the diversity of the processes occurring below the asphalt, within the asphalt, and at the asphalt-water interface,” she added.

Hazards for Oil and Gas Extraction

Mexico recently announced the first energy production lease sales in their ultradeep offshore waters, but the extensive asphalt pavements and gas hydrate mounds at those depths pose hazards for drilling operations, according to the researchers.

If oil and gas companies encounter asphalts while drilling, it’s important to know whether the asphalt they hit is above or below the seafloor, MacDonald noted.

“That will tell you whether the asphalt that you’re seeing is likely to be expansive and extensive, so you hit it in many places with your drill bit, or it’s still in the subbottom, where it might be a potential resource,” he said.

Conservation Prospects

Escobar, who serves on the Legal and Technical Commission of the International Seabed Authority, said that to conserve both the seeps and the species they host, researchers need to better understand how far the asphalts extend and how they change in space and time.

The Mexican government is increasing the extent of marine protected areas off its coasts, Escobar said, and should also ensure that these unique ecosystems are included within those zones. According to MacDonald, informing both the Mexican public and the international community about the importance of these ecosystems might help protect them.

“I think the international public should be engaged to recognize yet again an example of the diversity, beauty, and complexity of deep sea environments,” he said.

—Lauren Lipuma, Contributing Writer

Citation: Lipuma, L. (2016), Asphalt volcanoes erupt in slow motion, Eos, 97, doi:10.1029/2016EO048095. Published on 15 March 2016.

© 2016. The authors. CC BY-NC 3.0