|Home||Archived February 20, 2019||(i)|
Although the amount of gas hydrate in the natural environment is enormous, little is known about its distribution in sea-floor sediment or even exactly how it forms. Exploring these and other questions was among the goals of the recently completed coring cruise conducted jointly by the Institut Polaire Franćais, Paul-Émile Victor (IPEV), and the U.S. Geological Survey (USGS) aboard the 120-m-long French research vessel Marion Dufresne. The cruise, partly funded by the U.S. Department of Energy, originated in Cancún, Mexico, on July 1 and ended in Tampa, FL, on July 18.
The Gulf of Mexico is unique in the world for containing significant amounts of both biogenic gas hydrate (hydrate formed in shallow sediment by microbial production of methane) and thermogenic hydrate (hydrate formed by deep natural gas leaking into the shallow subsurface sediment). The possible presence of gas hydrate in the northern Gulf of Mexico has been inferred from geophysical data, but before this cruise, samples have been recovered from only the uppermost few meters of sediment by shallow coring or submersible vessels, typically on sea-floor mounds. Much of this cruise focused on finding evidence for the existence of gas hydrate away from obvious sea-floor gas-hydrate mounds and at depth in the sediment.
The generation of gas caused by hydrate dissociation was spectacularly demonstrated when the uppermost several meters of one core blew vertically out the end of the core barrel, flew at least 10 m into the air, and landed in the gulf waters next to the ship. The gas hydrate remained on the surface of the water because of its low density and floated away as it dissociated. Other gas hydrate, recovered during the cruise, was present either as particles distributed throughout the sediment or as massive chunks that filled the entire 10-cm diameter of the core liner. The hydrate samples were preserved in liquid nitrogen for future shore-based laboratory testing.
Considerable at-sea help was provided by an international group of about 40 scientists under the IMAGES (International Marine Past Global Changes Study) and PAGE (Paleoceanography of the Atlantic and Geochemistry) programs (Laurent Labeyrie and Viviane Bout, cochiefs). The IMAGES program is an international effort to understand the mechanisms and consequences of climatic changes by using the oceanic sedimentary record.
Also deeply appreciated was onshore assistance from Dave Mason (USGS, Woods Hole, MA), Jesse Hunt (Minerals Management Service, New Orleans, LA), Bill Gwilliam (Department of Energy, Morgantown, WV), and Manika Prasad (Stanford University).
Other shipboard studies included the collection of 9-m-long box cores from Pigmy and Orca Basins (see map) for measuring contaminant input to the northern Gulf of Mexico from the Mississippi River, part of a project led by Pete Swarzenski (USGS, St. Petersburg, FL). Pigmy and Orca Basins are nearly adjacent; however, Orca Basin has been covered by a thick brine layer for most of Holocene time, producing anoxic conditions at the basin floor. Oxic conditions exist in Pigmy Basin. In addition to studying contaminant-input history, USGS scientists will compare the effects of anoxic and oxic conditions on pollutant sequestration in the two basins.
The USGS is also working with Catherine Kissel (IPEV) on paleomagnetic studies and with Sabrina Nardozza (Laboratoire des Sciences du Climat et de l'Environnement) on physical-property profiles. The USGS cores collected in the gulf will be archived at Texas A&M University under the guidance of Bill Bryant.
Before docking in the Port of Tampa, FL, the Marion Dufresne obtained two cores in Tampa Bay for Terry Edgar (USGS, St. Petersburg, FL) and Deb Willard and Tom Cronin (USGS, Reston, VA), as part of the Tampa Bay project (see first related story, below). A.B. Wade (USGS, Reston, VA) and Hannah Hamilton (USGS, Gainesville, FL) visited the Marion Dufresne while it was in Tampa Bay and orchestrated a series of interviews and videotaping sessions for both national and local news coverage.
in this issue:
|Home||Archived February 20, 2019|