Surprising undersea discovery chronicled by MSU geologist


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etween 2011 and 2014, a team of scientists discovered 570 methane plumes escaping the sea floor between North Carolina and Cape Cod.
Mississippi State geosciences assistant professor Adam Skarke, then a physical scientist on the NOAA OER’s scientist-sailor team aboard the Okeanos Explorer, played a primary role in gathering and disseminating the data gathered on the ship. He was the lead author of “Widespread methane leakage from the sea floor on the U.S. Atlantic margin,” a recently published article in Nature Geoscience, the premier peer-reviewed research journal for earth scientists. The article suggests that the discovery of so many methane seeps is raising new questions about geology, oceanography and sea floor ecosystems. Along with Skarke, researchers for this project included Brown University NOAA Hollings Scholar Mali’o Kodis, USGS Gas Hydrates Project Chief Carolyn Ruppel, USGS research physicist Daniel Brothers and Earth Resources Technology physical scientist Elizabeth Lobecker.

  • Methane seeps escape from the Atlantic coastal margin sea floor near Virginia at depths lower than 1,400 meters, or 4,593 feet. Scientists had thought that cold seeps occur near tectonically active plates or petroleum basins, but neither are present near this bed of cup and bubblegum corals. “The seepage was not necessarily expected there because the tectonically passive area lacks an underlying petroleum basin,” Skarke says. The methane plumes, visible here as bubbles, support microorganisms that transform methane into energy. “Many of these newly discovered seeps may be related to the breakdown of a special kind of ‘methane ice,’ or gas hydrate,” Skarke says. Though the chemical process makes the ocean waters more acidic, chemosynthetic organisms, including some types of mussels, thrive in these kinds of environments. Other life-forms at this seep site include quill worms and anemones.

  • Mapping Team Lead Adam Skarke conducts an Expendable Bathythermograph (XBT) cast to measure temperatures through the water column in the Gulf of Mexico, which features numerous methane plumes. The XBT scanner’s handheld launcher deploys a probe into the water that transmits temperature data back to the ship. Okeanos Explorer also has a deepwater multibeam echosounder that can locate bubbles of gas rising through the water, which made the sonar tool an essential instrument in identifying so many methane seeps along the Atlantic continental margin seafloor. “A cornerstone of the NOAA OER program is the collection of data that can lead to new discoveries for the scientific community,” Skarke says. “One unique aspect of the program that made it so enjoyable to work there was the fact that we collected many types of data about U.S. oceans and made the data immediately available to the scientific community for studies that could not otherwise have been completed.”

  • When Okeanos Explorer’s imaging equipment first photographed this area in 2012 with multibeam sonar technology, scientists could see that they needed to investigate further. They hypothesized that methane could be escaping, so the team returned in May 2013 to record further images and take samples. Jason took the deepest dive the ROV had ever yet taken at that time and was submerged 1,600 meters, or 5,249 feet, and yes, these bubbles did indicate the team had found yet another methane seep site.

  • The Okeanos Explorer’s control room allows the scientist-sailors to review the data as ROVs and other sensory equipment gathers it. The multibeam sonar imaging system aboard the Explorer, the Kongsberg Simrad EM 302, creates a band of signals using multiple sensors pointed at different angles on either side of the ship. The time interval between the signal transmission and its returning echo suggests the depth over the band. The resulting data helps scientists map habitats and develop three-dimensional models.

  • This lithodid crab scuttles across this patch of chemosynthetic mussels at a depth of 1,600 meters, or 5,249 feet. Scientists aboard the Okeanos Explorer took samples of the animals around the seeps, microbial mats and other specimens to study how the methane provides food for animals near the methane plume. This particular mussel bed is located about 35 miles off the Virginia coastline.

  • A rockling rests among the mussels. Usually, rocklings swim close to shore around rocks, pools and sandbars, but this fish rests among a chemosynthetic mussel bed near a methane plume. While some species are commonly found near seep sites—some fish and certain crabs—do appear in Atlantic coastal margin seep communities, others—tube worms and galatheoid crabs—were not found.

  • This gas hydrate formed above methane escaping from the ocean floor, but it remains stable and solid at sediments below 500 meters, or 1,640 feet. While oftentimes, methane ice indicates a potential natural gas source, these gas hydrates are so deep that ocean temperatures would have to warm 6.5 to 11 degrees Celsius, or 43 to 52 degrees Fahrenheit, for the methane ice become unstable.

  • Gas hydrate shown here appears white. It formed under a rock overhang, but bubbles being emitted from the seafloor are shown in the crack below the rock. These bubbles do not directly enter the atmosphere because they dissolve in the water and microbes generally convert it to carbon dioxide. The laser scale, marked by the red laser dots, denotes 10 centimeters, or about 4 inches.

  • Living and dead mussels, along with white bacterial mats, were discovered at this seep site among the brown rocks, also known as carbonates. The large size of the carbonates suggest this particular methane seep has been leaking methane for a long time, and while chemosynthetic mussels were not as prevalent at this particular site, the bacterial mats were well established. Scientists must learn more before they can draw definite conclusions.

  • This massive chemosynthetic mussel community appears to be thriving; the white coating on some of the mussel shells is from bacteria. “Two years ago, no human had ever seen these seafloor communities that have now been found,” Skarke says. “These newly discovered seeps have expanded the number of locations that deep sea ecologists can study.” Wide distribution of mussels is common near many of the methane plumes and may indicate methane seeps over a widespread area. Thanks to the discovery of so many of these mussel beds adjacent to methane plumes, researchers are now questioning how the methane seeps are colonized, what other kinds of structures are present and how they relate to other forms of life in the sea floor ecosystem. The scientific team discovered this bed in 2013 on the Northeast U.S. Canyons Expedition, the research mission that confirmed several methane seepage and gas hydrate sites along the sea floor of the Atlantic coastal margin.

  • Octopus Graneledone verrucosa explores the mussel bed adjacent to this methane seep site. The white patches are microbial mats of bacteria that feed on the methane, sulfides and shells of dead mussels. “Their metabolism may depend on methane or hydrogen sulfide, a common seep gas toxic to many life-forms,” Skarke says. While scientists have already gathered various data about mussels living at these sites, questions remain about them and other life-forms. For example, scientists continue to investigate how the octopus, as well as other ocean fauna and microbes, respond and adapt to the more acidic ocean environments near the plumes. In these ecosystems, methane does not likely indicate natural gas reservoirs. However, with samples of the methane, geochemists can determine the chemical composition of the seeps, as well as whether shallow or deep gas sources are the underlying source.

  • After scientists identified bubbles escaping from the sea floor during a routine survey in 2012, the team returned in 2013 with Jason, a precision multi-sensory imaging and sampling remotely operated vehicle (ROV), to learn more. Not only can Jason dive to a maximum depth of 6,500 meters, or 21,385 feet, the ROV has six thrusters providing about 600 pounds of thrust, which makes it very navigable on the ocean floor as it surveys and collects samples from the environment. In this extensive, densely packed mussel bed, Jason collects sediment to investigate the microbial mat, as well as any other animals in the mud. The ROV also uses its manipulator arm to collect a sea urchin and a few mussels. Researchers are using Jason’s collection and seafloor surveys to determine whether the escaping methane indirectly provides food for animals living around the seep.

 

The Next Steps

For decades, upper ocean temperatures have been increasing all over the world, Skarke says. While some of the methane seeps are similar to those found on the Arctic Ocean margins, data suggests that some seeps have been active for more than a century.

“With small changes in ocean temperature, gas hydrate can release its methane into the sediments, and the gas may escape at the sea floor to form plumes in the water column (so) a key question is how the long-term seepage and short-term warming of the ocean are related to methane escape,” he says.

The research completed thus far does not provide conclusive evidence regarding the presence of a relationship between global climate change and the seeps along the Atlantic continental margin sea floor. Most of the methane plumes discovered are located so deep that the gas never reaches the atmosphere directly.

Skarke appreciates all the support he received from Mississippi State administrators, especially from the Department of Geosciences, as he worked with the team of NOAA researchers to compile the data and submit it to the top-tier journal. Likewise, administrators agree that Skarke’s expertise is offering both the university and its geosciences students the opportunity to engage in enhanced research collaborations at Mississippi State and other institutions.

“By hiring Adam, we bring research-based learning to both undergraduate and graduate students in areas of inquiry that are outside the purview of our traditional geoscience research and learning activities,” says geosciences department head Bill Cooke. “Adam’s research demonstrates that MSU geosciences are successfully engaging in very high-level research in the field of geology and beyond.”

By Leah Barbour
Images courtesy of Deepwater Canyons 2013 – Pathways to the Abyss, NOAA-OER/BOEM/USGS and NOAA Okeanos Explorer Program, 2013 Northeast U.S. Canyons Expedition.