Real-Time Mapping of Seawater and Atmospheric Methane Concentrations Offshore of Alaska's North Slope
The continental shelf offshore of the Alaskan North Slope has been flooded by rising seawaters since at least the beginning of the Holocene Epoch, about 11,700 years ago. Like the well-studied East Siberian Arctic Shelf, the U.S. Beaufort Shelf (named for the overlying Beaufort Sea) may be underlain by thawing permafrost and dissociating methane hydrate, an ice-like crystalline solid in which methane molecules are trapped. (For a primer on gas hydrates and their relation to climate warming, see “Gas Hydrates and Climate Warming…,” this issue.)
Methane is a powerful greenhouse gas, and the methane hydrates on circum-Arctic Ocean continental shelves are some of the most climate-susceptible gas hydrates on Earth. Methane released on these shallow-water shelves—whether from dissociating gas hydrates or other sources—is likely to reach the atmosphere without dissolving or being converted to carbon dioxide by microbial processes in seawater.
Geophysical and Geochemical Survey
In August 2011, the U.S. Geological Survey (USGS) Gas Hydrates Project conducted a combined geophysical and geochemical survey on the central part of the U.S. Beaufort Shelf in an area largely underlain by subsea permafrost (see map). USGS research geologist Tim Collett and his collaborators have established the existence of gas hydrates onshore in the eastern part of our study area; the British Petroleum Exploration Alaska (BPXA)-U.S. Department of Energy (DOE)-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well was also drilled in this area. The 2011 offshore research was carried out aboard the research vessel (R/V) Ukpik, a 48.5-foot vessel operated by Southern Cross LLC. The surveys focused on parts of Harrison Bay and on the area landward and seaward of the barrier islands that protect the coast between the Colville River Delta and Prudhoe Bay. USGS analyses of seismic data that were collected by the oil industry 20 to 35 years ago (see http://walrus.wr.usgs.gov/NAMSS/) indicate that both areas should have transitions from nearshore subsea permafrost to an absence of permafrost farther offshore.
During the first phase of the 2011 cruise, we collected simultaneous bathymetric (seafloor depth) data with the SEA SWATHPlus-M sonar system, high-resolution subbottom imagery (down to several meters below the seafloor) with the EdgeTech SB-424 (Chirp) subbottom profiler, and profiles of the sediments down to approximately 150 m below the seafloor with a SIG 2Mille mini-sparker and receiver system. This work built on an August 2010 cruise that was the first non-industry survey to obtain high-resolution geophysical data along this coastline in more than 3 decades. (See “Degradation of Subsea Permafrost and Associated Gas Hydrates Offshore of Alaska in Response to Climate Change,” Sound Waves, October/November 2010.) The geophysical data from the two cruises constrain the distribution of shallow gas and ice scours and provide images of reflectors that correspond to high-velocity layers mapped as permafrost in the legacy seismic data.
The second phase of the August 2011 cruise acquired real-time measurements of seawater and atmospheric methane and carbon dioxide concentrations by using a Picarro G2301-f “cavity ring-down spectrometer” (CRDS; described below) connected to a gas-extraction system that was constructed from a design by Shari Yvon-Lewis of Texas A&M University. Other Texas A&M University scientists were supported by the National Science Foundation to participate in the cruise. The 2011 survey marked the first use of a CRDS by the USGS Coastal and Marine Geology Program for this purpose and provided an outstanding demonstration of the potential of CRDS technology for a variety of greenhouse-gas studies.
Cavity Ring-Down Spectroscopy for Measuring Dissolved Gases
For many years, measurement of dissolved gases has relied on gas-chromatographic analyses of gases extracted from the headspace (the space above the sample in a sealed container) of seawater samples. Although this process can be automated to operate on continuously pumped seawater, the measurements are more routinely carried out on water samples obtained during discrete “casts,” when instruments are lowered from a vessel to measure such seawater properties as temperature and salinity and to collect water samples. Cavity ring-down spectroscopy, in contrast, measures dissolved gases every few seconds from continuously pumped seawater while the vessel is underway.
Cavity ring-down spectroscopy relies on a non-destructive laser to analyze gas concentrations. The laser light bounces between highly reflective mirrors within a pressure- and temperature-regulated cavity until the laser-light energy dissipates to a level below the sensitivity of the cavity’s optic sensors. The instrument measures the duration of this dissipation process, which is known as ring-down time. Because the dissipation is partly caused by the collisions between laser light and gas molecules, the ring-down time correlates to gas concentration, with shorter ring-down times associated with higher gas concentrations. Recording ring-down times instead of changes in absolute intensity of the laser light provides increased accuracy. In comparison with other spectroscopic methods, which transmit laser light along a fixed-path length, the laser light in the CRDS cavity can travel tens of kilometers, providing increased sensitivity. The laser can be “tuned” to different light wavelengths to measure different gases.
During the 2011 Beaufort Sea cruise, the CRDS produced discrete measurements of methane and carbon dioxide concentrations in seawater every few seconds. The intake was periodically switched to sample air, which was dried before measurement, or to measure calibration gases. The ship was operated at 3 to 4 knots during the 2011 cruise, but a key advantage of CRDS-based methane mapping is the ability to acquire measurements in real time regardless of the ship’s speed.
Gas Fluxes Across the Ocean-Air Interface
One of the most important uses of the continuous CRDS measurements in marine settings is the determination of gas fluxes across the ocean-air interface. These calculations provide the most direct evidence that methane released at the seafloor may be contributing to methane concentrations in the atmosphere. Calculating gas fluxes from the measured gas concentrations requires water-quality parameters, such as temperature and salinity, and meteorological data. We continuously recorded seawater salinity and temperature by using a YSI sonde, and we measured wind speed and other parameters by using an Airmar PB200 WeatherStation integrated with the CRDS. The CRDS was linked to real-time kinematic Global Positioning System (GPS) navigation, and information from all systems was recorded simultaneously with Hypack software.
The map shows relative seawater methane concentrations measured in the Beaufort Shelf study area. At most sites, the seawater is supersaturated in methane, meaning that methane is expected to flux from the ocean to the atmosphere. This observation is consistent with the results reported by chemical oceanographer Natalia Shakhova (International Arctic Research Center, University of Alaska, Fairbanks) and coworkers for methane measured in discrete water samples on the East Siberian Arctic Shelf. Our data reveal a possible methane hotspot in eastern Harrison Bay, above an area of seafloor that the geophysical data shows to be charged with shallow gas. Neither the CRDS data nor our geophysical results yield evidence for massive ebullition (bubbling) of methane as the primary means of methane emissions at the seafloor. This observation contrasts with those on the East Siberian Arctic Shelf and in the Mackenzie Delta area of the Canadian Beaufort Shelf. Numerical models predict that the most dramatic dissociation of methane hydrate should be just beyond the present-day seaward extent of subsea permafrost; however, the CRDS data do not detect systematic changes in methane concentrations when crossing from nearshore sediments underlain by seismically detected permafrost to sediments lacking such permafrost.
Future Plans for Cavity Ring-Down Spectroscopy
The successful first deployment of the CRDS and seawater equilibrator system for real-time mapping of greenhouse-gas concentrations and fluxes on the U.S. Beaufort Shelf was an important step for the USGS Gas Hydrates Project. On the basis of lessons from the initial deployment, USGS engineer Emile Bergeron has constructed a smaller, more portable version of some system components, which will simplify operations in remote areas or from small open boats in lakes. Using multiple CRDSes or a switching system among several pumps, we may eventually measure methane at several depths within the water column nearly simultaneously, providing higher quality data than can be obtained from numerous discrete casts that commonly occur over periods of hours or longer. We are also collaborating with USGS marine geochemist Kevin Kroeger to measure lateral fluxes of dissolved greenhouse gases from salt marshes and plan to incorporate a CRDS that simultaneously measures the concentrations and carbon-isotopic compositions of methane and carbon dioxide. This instrument will be deployed in summer 2012 when we conduct a multidisciplinary survey across the Beaufort Shelf with support from the U.S. Department of Energy’s Methane Hydrate R&D Program. The potential applications of CRDS technology reach far beyond the USGS Gas Hydrates Project—collaborations are already developing within the USGS and with outside partners to deploy the instrument to support greenhouse-gas studies in terrestrial settings, wetlands, estuaries, and deepwater marine environments.
in this issue:
Real-Time Mapping of Methane Concentrations