By Dr. Carolyn Ruppel, Research Geophysicist, U.S. Geological Survey
Dr. Diva Amon, Research Fellow, Natural History Museum, London, United Kingdom
December 20, 2017
NOAA’s Deep Discoverer (D2) remotely operated vehicle (ROV) encountered gas hydrate mounds, ice worms, chemosynthetic communities, and active gas seeps during Dive 17 of the Okeanos Explorer Gulf of Mexico 2017 expedition. Guided by seafloor anomalies delineated by the Bureau of Ocean Energy Management and by Okeanos sonar data that had detected plumes of gas bubbles in the water column, Dive 17 explored the seafloor at ~1,050 meters water depth (3,445 feet) on the northwestern edge of a salt-cored Horne Dome, ~87 kilometers (54 miles) from the Mouth of the Mississippi, Louisiana.
During the dive, D2 discovered low relief seafloor mounds that mantled gas hydrate deposits interspersed with carbonates. Gas hydrate forms when low molecular weight gas (usually methane) combines with water and freezes to form “methane ice” under low temperature and moderate pressure conditions. In the deepwater Gulf of Mexico, the ample supply of gas, coupled with appropriate pressure and temperature conditions, cause gas hydrate to form on or just beneath the seafloor, a phenomenon first described ~25 years ago.
During Dive 17, scientists observed dense orange-stained gas hydrate exposed in several mounds. In some places, the same outcrop had both massive gas hydrate that had probably formed below the seafloor and clear, porous gas hydrate made up of hydrate-encased bubbles. Video recording of methane emissions adjacent to one of the massive hydrate deposits provided insights into seafloor gas dynamics. For example, rapid gas emissions produced bubbles, with some bubbles emerging from clear tubes formed from gas hydrate. Slow gas emissions led to the formation of hydrate-coated gas droplets that initially blocked the gas conduit until the pressure inside the droplet increased, causing the droplet to detach. Similar phenomena were filmed on bare seafloor in the Gulf of Mexico during a 2014 Okeanos Explorer expedition.
During the 2017 Gulf of Mexico expedition, the only macrofauna spotted living directly on methane hydrate outcrops were ice worms, Hesiocaeca methanicola. These pink hesionid polychaetes are 2-4 centimeters (~0.8 to 1.6 inches) long and were first discovered two decades ago living on seafloor hydrate mounds at 800 meters (~2,625 feet) water depth in the Gulf of Mexico. The worms can live in communities as dense as 3,000 individuals per square meter (one square meter is ~10.8 square feet), although far fewer individuals were seen on exposed hydrate during Dive 17.
Imagery collected during the dive revealed that the ice worms had burrowed into the hydrate, creating small depressions. The worms are known to colonize not only exposed hydrate, but also hydrate buried beneath 10 centimeters (~3.9 inches) or more of sediment cover. When first discovered, ice worms were thought to host endosymbiotic chemosynthetic bacteria that used the abundant hydrocarbon-rich fluids to create food. However, it is now known that the worms graze on bacteria growing on the hydrate.