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Fieldwork

Sampling Methane Seeps and Plumes on the U.S. Atlantic Margin



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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.

Northern U.S. Atlantic margin, showing major canyons that cut through the continental shelf, seeps, and piston cores
Above: Northern U.S. Atlantic margin, showing major canyons that cut through the continental shelf (light blue), seeps identified in 2014 (red circles; see http://dx.doi.org/10.1038/ngeo2232), and piston cores (black triangles) and multicores (stars) acquired on the research vessel (R/V) Hugh R. Sharp in September 2015. Cruise track is shown in orange. Many of the multicore sites coincide with piston-core sites and obscure the piston-core sites on the map. [larger version]

The cruise, which was supported by the U.S. Department of Energy, built on data collected during an April 2015 cruise (see “Imaging Methane Seeps and Plumes on the U.S. Atlantic Margin”) 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 (see “Natural Methane Seepage Is Widespread on the U.S. Atlantic Ocean Margin”).

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 (learn more about subbottom profiling). 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.

Scientists deploying a piston core from the stern of the R/V Sharp
Above: Jenny White and Pete Dal Ferro deploying a piston core from the stern of the R/V Sharp. USGS photograph by Gerry Hatcher. [larger version]

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.

Researchers collect some of the overlying water of a multicore for analysis
Above: (Clockwise from bottom) David Brankovits (Texas A&M University at Galveston), John Pohlman (USGS), and Stefan Krause (GEOMAR Helmholtz Centre for Ocean Research) collect some of the overlying water of a multicore in order to extract methane and carbon dioxide for onboard stable carbon isotope and concentration analysis with a laser-based analytical system under development at the USGS Woods Hole Coastal and Marine Science Center. Photograph by Michael Graw, Oregon State University. [larger version]

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.

The Mini Muc on deck, with major components of the real-time video system labeled
Above: The Mini Muc on deck, with major components of the real-time video system labeled. The footprint of the system is approximately 1 square meter. USGS figure by Gerry Hatcher and Carolyn Ruppel. [larger version]

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.

Researchers monitor the view of the seafloor and control the Mini Muc
Above: Gerry Hatcher (left) monitors the view of the seafloor from the camera mounted on the Mini Muc and communicates with the ship’s bridge while Pete Dal Ferro uses a remote winch control to position the Mini Muc and trigger coring. USGS photograph by Jenny White. [larger version]

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.

Above: Core collection with the Mini Multicorer (Mini Muc) viewed from a wide-angle camera attached to the system’s frame. The weight of the Mini-Muc drives four core barrels (only three are visible) into the sediment. After pausing several seconds to ensure that the core barrels have settled completely into the seabed, the winch operator begins pulling up the unit. The upward pull triggers caps to snap closed on the tops of the core tubes and mechanical arms to swing down and cap off the bottoms. Live video feed is vital to choosing sediment sites suitable for coring and ensuring that the Mini Muc functions properly.
Video: MP4 format | Ogg format | WebM format

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.

Acknowledgments

This cruise was supported by the Methane Hydrates Research Program at the U.S. Department of Energy (DOE). Costs for instrumentation were borne by USGS-DOE interagency agreements DE-FE0002911 and DE-FE0005806. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.


Related Sound Waves Stories
Imaging Methane Seeps and Plumes on the U.S. Atlantic Margin
March–June 2015 Double Issue
Natural Methane Seepage Is Widespread on the U.S. Atlantic Ocean Margin
September / October 2014

Related Websites
Gas Hydrates Project
USGS
Sub-bottom Profiling
NOAA
Methane Hydrates
DOE

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in this issue:

Fieldwork
cover story:
Sampling Atlantic Margin Methane Seeps and Plumes

USGS Completes Second Expedition for Atlantic Submarine-Landslide Studies

Spotlight on Sandy
“Team Delmarva” Completes Seafloor Mapping off Delmarva Peninsula

Research
Pathways to the Abyss

Outreach
USGS Residual Oil Research Presented at Two Public Seminars

USGS Hosts USF Oceanography Camp for Girls

USGS Continues Collaboration for Native Youth in Science

USGS Assists in Another Year of Woods Hole Partnership in Education

Staff
New Production Team for Sound Waves

Publications
Oct. / Nov. Publications

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