Sampling Methane Seeps and Plumes on the U.S. Atlantic Margin
USGS scientists and partners completed a research cruise on the research vessel (R/V) Hugh R. Sharp to acquire sediment and water samples, heat-flow data, and geophysical imagery to better understand methane dynamics on the northern U.S. Atlantic margin.
Scientists from the U.S. Geological Survey (USGS) Gas Hydrates Project, Oregon State University, and GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, recently completed a research cruise on the research vessel (R/V) Hugh R. Sharp to acquire sediment and water samples, heat-flow data, and geophysical imagery to better understand methane dynamics on the northern U.S. Atlantic margin.
The cruise, which was supported by the U.S. Department of Energy, built on data collected during an April 2015 cruise that focused on acquiring geophysical constraints on methane dynamics on the same part of the margin. In 2014, Carolyn Ruppel, chief of the USGS Gas Hydrates Project based at the Woods Hole Coastal and Marine Science Center (WHCMSC), and Daniel Brothers, a geophysicist based at the Pacific Coastal and Marine Science Center (PCMSC), coauthored a study that described 570 previously unknown methane seeps on the upper continental slope in this area.
The September 2015 R/V Sharp cruise featured unique uses of data and instrumentation that were relatively new to the USGS. Because sampling operations are safer in daylight but geophysical data can be collected day or night, the researchers used the night-time hours to collect high-resolution subbottom imagery and water-column imagery of methane plumes. The following day, they returned to locations identified during the previous night to collect piston cores, multicores, heat-flow measurements, and water-column samples for dissolved methane. Subbottom imagery (cross-sectional views of sediment layers and other features beneath the seafloor) was acquired with the WHCMSC Edgetech SB-512 Chirp instrument, which emits an acoustic pulse and receives the return signal in a single “fish” towed at approximately 10 meters below the sea surface. A Simrad EK60 echo sounder equipped with a 38 kHz split-beam transducer that was mounted in the R/V Sharp’s retractable keel was operated nearly continuously to locate methane plumes in the water column. Repeated EK60 surveys were conducted over certain seep fields to measure how long the methane plumes persisted over hours and days. Bill Danforth and Eric Moore, both of WHCMSC, were responsible for acquisition and processing of both echo sounder and Chirp data.
A major focus of the cruise was piston coring carried out under the direction of Pete Dal Ferro and Jenny White (PCMSC). Twenty-one piston cores were collected in seep and pockmark fields (pockmarks are shallow seafloor depressions thought to be related to past methane expulsion events), on upper continental slope transects, and in Hudson Canyon (see map, above). These cores recovered a total of nearly 100 meters of sediment at water depths of 80 to 1,150 meters. Brian Buczkowski (WHCMSC) tracked subsampling of the cores, a process in which core material was extracted for detailed study to meet physical, geotechnical, microbiological, and geochemical cruise objectives. A biogeochemistry team led by John Pohlman (WHCMSC) and including Michael Casso and Lee-Gray Boze from WHCMSC and David Brankovits from Texas A&M University at Galveston segmented recovered piston cores to extract pore waters, study methane distributions, and provide subsamples to microbiologists Frederick Colwell and Michael Graw (Oregon State University) and Stefan Krause (GEOMAR). On many piston core deployments, sediment temperatures were measured using self-logging thermistors (temperature sensors) attached to the piston core barrels. The thermistors were loaned by Andy Fisher (University of California–Santa Cruz), who maintains the instrumentation as part of a National Science Foundation-sponsored community facility.
The cruise marked the first USGS deployment of a mini-multicorer, the Mini Muc (pronounced “mini muck”), a 4-core system built by K.U.M. Umwelt and Meerestechnik Kiel GmbH and loaned to the USGS by Tina Treude, a geomicrobiologist at University of California, Los Angeles, and a project collaborator. Multicorers are often used to retrieve high-quality, barely disturbed samples of near-seafloor sediment for microbiological, biogeochemical, and paleoceanographic studies. The Mini Muc uses 60-centimeter-long and 5-millimeter-thick Plexiglas tubes with an inner diameter of 90 millimeters. Compared with other multicore systems used in the U.S. research fleet, the Mini Muc requires substantially less deck space and is a lighter, more maneuverable instrument package.
An important component of multicoring is seafloor visualization to ascertain the proximity of cores to methane seeps, bacterial mats, and other features. For the Mini Muc deployments, Gerry Hatcher (PCMSC) designed a real-time high-definition (HD) camera system that was mounted on the Mini Muc frame. The camera system consisted of two SeaLite Sphere LED Lights, an HD Multi SeaCam wide-angle camera from Deepsea Power & Light, and a “control can” custom-built for the cruise. The “control can” is an underwater pressure housing containing voltage regulators and other electronics needed to power the lights and transmit the live HD-video signal over the ship’s fiber-optic sea cable to the surface. Aboard the ship, the video was simultaneously viewed and recorded, with the time stamped for correlation with ship’s positional data.
To collect Mini Muc samples, the vessel was first precisely maneuvered over a seafloor position. The Mini Muc, with attached camera system, was then deployed and lowered to within 1.5 meters of the seafloor using laboratory-based joystick controls. While the Mini Muc hovered above the seafloor, scientists used the live video to determine where to core. During sampling, the Mini Muc gently penetrated the sediments before core caps automatically closed over the top and bottom of the core tubes.
The live video capability developed at the USGS for this cruise was essential to collecting high-quality cores, diagnosing equipment problems, and avoiding hard seafloor that would have damaged the equipment. The video also provided scientists with a visual record of each sample site similar to that available from a remotely operated vehicle or manned submersible, at a fraction of the cost.
Watch two videos of the Mini Muc as it is maneuvered into place before being driven into the "mucky" bottom of the sea. In the first video, a school of little fish float nearby, and even a squid can be seen swimming about. Anemones and crabs are on the seafloor. The second video shows the silty seafloor, numerous mussel shells, and the mechanical arms as they close.
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