Determine the potential impacts of gas hydrate instability in terms of the release of methane into seafloor sediments, the ocean and the atmosphere.

University of California, San Diego (Scripps Institution of Oceanography) – manage geochemical, hydrological and sedimentological investigations
Texas A&M University – manage field monitoring program

Location

La Jolla, California 92093

This project will monitor, characterize, and quantify the rates of formation and dissociation of methane gas hydrates at and near the seafloor in the northern Gulf of Mexico, and determine linkages between formation/dissociation and physical/chemical parameters of the deposits over the course of a year. The stability and response of shallow gas hydrates to temperature and chemical perturbations will be monitored in situ, and localized seafloor and water column environmental impacts of hydrate formation and dissociation characterized. The following will be determined: 1) The equilibrium/steady state conditions for structure II methane gas hydrates at the field site,2) whether the system is in dynamic equilibrium and the local hydrology is characterized by steady state episodic fluid flow, and 3) how fluid fluxes and fluid composition work together to dynamically influence gas hydrate stability.

This project is designed to increase understanding of the effects of environmental changes on the stability of naturally occurring gas hydrates on the seafloor. This work will help determine the potential environmental impacts of gas hydrate instability which can release methane into seafloor sediments, the ocean, and the atmosphere. The chemistry and structure of the hydrates, the composition of the overlying seawater, and the chemistry, mineralogy and hydrology of nearby sediments and pore waters are being characterized in detail. This project will also provide knowledge that is necessary for the future success of hydrate energy development, planning safe deep-water drilling operations, and preventing sediment slides in areas where gas and/or oil pipelines exist or where drilling is in progress.

Results

• Conducted instrumentation deployment cruise July 2002,
• Collected core and water samples at Green Canyon 185,
• Deployed osmotic membrane samplers and flow meters to collect a time series of pore fluids from sediments and monitor the associated flow rate,
• Deployed time-lapse camera and high-resolution thermistors,
• Recovered deployed instrumentation August 2003,and
• Performed laboratory analysis of collected samples.

Previous observations at Gulf of Mexico field sites suggest that gas hydrates on the seafloor form and dissociate in cycles over periods of several weeks to several months. Many factors, including biochemistry, subsurface hydrology, and temperature play a role in these cycles. Unfortunately, data on the kinetics of natural gas hydrate formation and dissociation in the marine environment are lacking. This project is designed to collect and analyze such data.

This project (informally called the Gas Hydrate Observation, Sampling, and Tracer Study or GHOSTS) is in the final year of a three-year cooperative agreement. The first of two research cruises took place June 6-14, 2002 aboard the R/V Seward Johnson. Two sites, Green Canyon 185 (Bush Hill) and Green Canyon 234, were selected for study based on their well-mapped and well-characterized shallow gas hydrate outcrops.

Using the Johnson Sea Link submersible, core and water samples were collected, time-lapse video and temperature recorders were installed, and geochemical and hydrological monitoring equipment was deployed to collect data during the following year. These instruments provided a record of pore fluid and bottom-water chemical and isotopic compositions, seafloor temperatures, and sediment fluid flow rates. They also recorded a visual record of hydrate responses and changes on the seafloor that occur as a result of environmental variations. A second cruise to retrieve the equipment took place in August 2003.

A time-lapse digital camera assembly installed on the seafloor recorded hourly images of the growth or dissolution of an exposed hydrate deposit. The assembly was equipped with a synoptic temperature recorder.

The fluids and samples retrieved during the 2003 cruise currently reside at the shore-based laboratory at Scripps Institution of Oceanography. An extensive array of geochemical, isotopic, mineralogical, and structural analyses of the gas hydrate, vent water, pore fluid, and sediment samples have been completed . Analyses of petroleum and other high carbon number organic compounds were conducted at Texas A&M. The collective geochemical data from this cruise will provide essential information on the coupling of the ocean and sub-seafloor thermal regime, on heat transfer between the ocean and the sub-seafloor, and on the amount of methane and other low-carbon number organic species that are required to form additional gas hydrates or stabilize existing deposits. The following conclusions can be drawn from the research activities and the subsequent analyses:

• The Bush Hill pore fluid chemistry varies significantly over short distances and gas venting is primarily focused, as reflected by sharp interfaces between distinct benthic biological communities.
• Pore fluid chemistry indicates regional sulfate reduction (and carbonate formation) that is particularly intense in and near active vents. The intensity is manifested by the spatial distribution of chemosynthetic communities and seafloor gas hydrate.
• Active gas hydrate formation in the tubeworm and mussel shell fields during the monitoring period (440 days) was documented by the time-series fluid chemical data.
• In addition to widespread vertical fluid advection, the existence of lateral advection was also documented. This may explain why some benthic biological communities survive away from the active vents.
• The subsurface hydrology is complex with both up-flow and down-flow occurring within each of the sub-environments; up-flow ranged from 0.5 to 214 cm/yr and down-flow from 2-162 cm/yr.
• Fluid flow polarity oscillates at periods of 14 days to 4 months and is coupled between the mound sites and background site.
• The gas hydrate preferentially fractionates ethane, propane, and H2S into its structure, thus, the residual vent gas composition differs from that of the original gas.
• At bush Hill the seafloor gas hydrate mound is presently stable despite bottom water temperature fluctuations. It has been observed in its current position for >12 years and benthic models suggest that it has been accumulating for 10,000 years.
• Based on d13C-DIC (Dissolved Inorganic Carbon) anaerobic oil oxidation at Bush Hill instead of anaerobic methane oxidation is the dominant sulfate reducing reaction in the sub-seafloor, therefore much of the methane is transported across the seafloor into the water column; most of it is venting through faults.
• Methane concentrations in surface waters above plumes are highly supersaturated 200-500 times the equilibrium value.
• Aerobic oxidation of methane (methanotrophy) consumes about 80% to maximum 90% of the methane.
• Water column DIC is depleted in 13C. Minimum bottom water d13C-DIC value observed is -3.63‰.
• Above and near the bubble plumes d13C-DIC values in surface waters are as well slightly negative (0.06 to -0.16‰, PDB).
• Based on methane supersaturation, air-sea exchange, wind speed, and mixed-layer average depth, methane flux into the atmosphere at three vent sites was modeled; the fluxes obtained range from 100-2000 µmol/m2 per day. This result allows further calculations of basin-wide methane flux into the atmosphere, using satellite data for the number of methane plumes, and assuming that these data are regionally representative.
• Leifer and MacDonald used the rate and size of bubbles at seeps and the number of seeps observed via satellites to estimate the methane flux to the atmosphere in the Gulf of Mexico. They concluded that ~0.5 Tg/year methane escapes to the atmosphere in the Gulf of Mexico.
• It is suggested that although supersaturation of methane in surface waters is a persistent feature of most ocean waters, it is considerably enhanced in continental margins, in particular in regions of oil and gas seeps, and gas hydrates, as in the Gulf of Mexico.
• The present day relationships between regional tectonics and hydrology at margins, and sea-air methane exchange, thus the global magnitude of methane flux from surface water continental margins to the atmosphere, are as yet undetermined. Data acquired during this project at the Gulf of Mexico provide an important step forward to answering this question.

All work on this project has been completed. The project final report is available below under "Additional Information".

$348,041

$93,076

NETL – Rick Baker (Richard.Baker@netl.doe.gov or 304-285-4714)
Scripps – Miriam Kastner (mkastner@ucsd.edu or 858-534-2065)
Texas A&M University – Ian R. MacDonald (imacdonald@falcon.tamucc.edu or 316-825-2234)

In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].

Final Report  [PDF-1.2MB] - June, 2006

Publications:
Kastner, M., D. Bartlett, I. MacDonald, and E. Solomon, 2005, CH4 fluxes across the seafloor at three distinct gas hydrate fields: impacts on ocean and atmosphere chemistry, Trondheim, Norway, Proceedings of the Fifth International Conference on Gas Hydrates, Volume 3, p. 709-714.

Solomon, E., M. Kastner, H. Jannasch, Y. Weinstein, and G. Robertson, 2005, Insights into the dynamics of in situ gas hydrate formation and dissociation at Bush Hill gas hydrate field, Gulf of Mexico, Trondheim, Norway, Proceedings of the Fifth International Conference of Gas Hydrates, Volume 3, p. 947-953.

Presentations
Kastner, M., D. Bartlett, I. MacDonald, and E. Solomon, 2005, CH4 fluxes across the seafloor at three distinct gas hydrate fields: impacts on ocean and atmosphere chemistry, Trondheim, Norway, Fifth International Conference on Gas Hydrates, June 13-16.

Kastner, M., C. Solem, D. Bartlett, I. MacDonald, and D. Valentine, 2003, The extent of CH4 emission and oxidation in thermogenic and biogenic gas hydrate environments, American Geophysical Union Fall Meeting, December 8-12.

Solomon, E., M. Kastner, H. Jannasch, Y. Weinstein, G. Robertson, and A. Aubrey, 2004, Long-term continuous monitoring of fluid chemistry and flux at the Bush Hill gas hydrate field, Gulf of Mexico, using a new flowmeter, the MOSQUITO, American Geophysical Union, Fall Meeting, December 13-17.

Solomon, E., M. Kastner, H. Jannasch, Y. Weinstein, and G. Robertson, 2005, Insights into the dynamics of in situ gas hydrate formation and dissociation at Bush Hill gas hydrate field, Gulf of Mexico, Trondheim, Norway, Fifth International Conference of Gas Hydrates, June 13-16.

Weinstein, Y., M. Kastner, and H. Jannasch, 2003, The MOSQUITO: a new sampler for monitoring fluid and solute fluxes between the sediment and the ocean, EGS-AGU-EUG Joint Assembly Meeting, Nice, France, April 6-11.