DE-NT0006558

Integrating Natural Gas Hydrates in the Global Carbon Cycle

DE-NT0006558

Last Reviewed 5/29/2012

Project Goal

The goal of this project is to develop a two-dimensional, basin-scale model for the deep sediment biosphere with methane dynamics to provide a better picture of the distribution of hydrates on the sea floor and their vulnerability to warming of the deep ocean, thus integrating methane hydrates into the global carbon cycle.

Performers

University of Chicago – Chicago, IL 60637
University of California Berkeley – Berkeley, CA

Background

Large but poorly quantified amounts of methane are trapped in the sediments beneath the sea floor, frozen into a form of water ice called clathrate or hydrate. The hydrates could be vulnerable to melting with a temperature increase of a few degrees Celsius (°C) [Buffett and Archer, 2004], an increase which is achievable given the available inventories of fossil fuel carbon for combustion. The hydrate carbon reservoir has probably grown in size over millions of years within the context of gradual ocean cooling, but a release of carbon from the hydrate pool due to melting could take place on a time scale of millennia.

The melting temperature of hydrate increases with pressure, and temperature in the ocean decreases with pressure (depth), so hydrate becomes increasingly stable with increasing ocean depth in the presence of methane gas [Kvenvolden, 1993]. Absence of methane gas in the open ocean, however, means that most of the hydrates are found in the sediment.

Within the sediment column, the temperature increases with depth, so that at a depth of typically a few hundred meters below the sea floor, the temperature exceeds the melting threshold. Therefore, the term hydrate stability zone generally refers to the sediment column from the sea floor down to the melting depth a few hundred meters below the sea floor. Climate warming primarily affects hydrate stability near the base of the stability zone, where temperatures approach the melting point. The sediment column provides a thermal buffer that slows the response of the hydrates to climate warming by many centuries. A change in sea level might also affect the stability of hydrates by altering the pressure. Sea level rise in the future would tend to stabilize the hydrates in the coming centuries, whereas warming would de-stabilize them.

The impact of melting oceanic hydrates on climate depends on whether the carbon reaches the atmosphere in the form of methane. If methane is released on a time-scale which is long relative to its atmospheric lifetime (decade), the result would be an increase in the steady-state concentration of methane in the atmosphere. The oxidation product of methane is CO2, another greenhouse gas although a weaker one. In contrast to methane (a transient chemical species) CO2 accumulates in the atmosphere, ultimately taking hundreds of thousands of years to be consumed by weathering reactions with igneous rocks. Methane that dissolves in the deep ocean would be oxidized to CO2 within a few years [Valentine et al., 2001], in which form it would ultimately equilibrate with the atmosphere, releasing some 15%-25% of the carbon to the air.

No mechanism has been proposed by which more than a Gton or so of methane could be released to the atmosphere within a few years, generating a significant transient spike of atmospheric methane concentration. The more likely impact of a melting hydrate reservoir is, therefore, a long-term, chronic methane source, elevating atmospheric methane and contributing to the total CO2 load on the atmosphere.

The bottom-line question which this project aims to address is whether the methane released from melting hydrates in the sediment column is likely to escape into the ocean or the atmosphere, or remain in place below the sea floor.

Impact of this Research

The project will serve to provide an improved understanding of the mechanisms responsible for methane cycling within the deep sediment column, thus providing constraints on the potential for hydrate response to climate change and the role of hydrates in the global carbon cycle.

Accomplishments

  • A two-dimensional geological time- and basin-scale model of the sedimentary margin in passive and active settings, for the simulation of the deep sedimentary methane cycle including hydrate formation was developed. The model measures the response of various continental margin archetype scenarios to changing oceanographic and geologic forcing over 140 million years. The following four archetypical model formulations were configured for simulations: Passive margin model, Active margin model, Gulf of Mexico model, and an Arctic margin model. The following are results from the numerical model simulations:
    • Hydrate inventory is extremely sensitive to the ability of methane bubbles to rise within the sediment column.
    • The active and passive model configurations are more sensitive to organic carbon deposition, respiration kinetics, and vertical bubble transport, than to ocean temperature.
    • The active margin simulations show a sensitivity to plate subduction velocity.
    • In general, the model inventories of methane hydrate are still sensitive enough to uncertain parameters that the models provide no real strong new constraint on methane inventories of the real ocean, but rather the models can hope to diagnose what the sensitivities of the real ocean hydrate reservoir might be.
  • An Arctic archetype scenario was developed to model the formation of a passive margin in the Beaufort Sea. A model for a subducting margin has also been developed and is being tuned to simulate the Juan de Fuca plate.
  • Simulations were conducted to assess the sensitivity of the model to organic carbon concentrations, respiration kinetics, flow anisotropy, and 50 million-year cycles in ocean temperature. Each run was subjected to 100,000 years of elevated temperature at the end of the cycle to determine the impact of sudden warming. Preliminary results indicate that the 2-D model seems to be less sensitive to temperature cycles than the 1-D model. This is perhaps related to the difference in governing dynamics in the two models.
  • Researchers have tuned the simulation to replicate or be comparable to the eastern margin of the United States, matching as much as possible the sedimentation rates and organic carbon concentrations observed in this region. The model produces methane hydrate in the observed depth range, and at the base of the methane hydrate stability zone as observed.
  • High-resolution simulation scenarios were developed to predict the impact of time-dependent warming on methane release to the open ocean. Researchers can now simulate the development of a passive margin and methane hydrate deposits for 100-200 million years.
  • A number of improvements were made to the model. A radioactive iodine tracer to infer the age of the pore waters in the sediments was included. Anisotropy was added to the permeability of the sediments to allow greater flow rates of both pore fluids and gas bubbles in the horizontal direction. A new option was also added to the code to allow modeling to determine the effect of a 10-100 kyr time period of 5° C global warming of the oceans. Significant changes were made to the advection schemes for the pore fluid and the solid sediment. For the pore fluid the model was enhanced to cope with high-permeability sediments without limiting the time step. For the solid sediment the model was updated to increase the resolution in the critical hydrate stability region and to calculate the total fluid velocity relative to the sea floor, including the effects of pore water burial by the sediment advection.

Current Status (May 2012)
The project has been completed. The final report is available below under "Additional Information".

Project Start: October 1, 2008
Project End: December 31, 2011

Project Cost Information:
Phase 1 - DOE Contribution: $171,126, Performer Contribution: $43,669
Phase 2 - DOE Contribution: $231,224, Performer Contribution: $58,895
Phase 3 - DOE Contribution: $237,924, Performer Contribution: $60,629
Planned Total Funding - DOE Contribution: $640,274, Performer Contribution: $163,193

Contact Information:
NETL – Skip Pratt (Skip.Pratt@netl.doe.gov or 304-285-4396)
University of Chicago – David Archer (d-archer@uchicago.edu or 773-702-0823)
University of California Berkeley – Bruce Buffett (bbuffett@berkeley.edu or 510-559-8167)
If you are unable to reach the above personnel, please contact the content manager.

Additional Information
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 Project Report [PDF-16.8MB] April, 2012

Quarterly Progress Report July - September, 2011 [PDF-187KB] - November, 2011

Quarterly Progress Report April - June, 2011 [PDF-232KB] - August, 2011

Quarterly Progress Report April - June, 2010 [PDF-51KB] - August, 2010

Quarterly Progress Report January - March, 2010 [PDF-61KB] - May, 2010

Quarterly Progress Report October - December, 2009 [PDF-46KB] - January, 2010

Quarterly Progress Report July - September, 2009 [PDF-19KB]

Quarterly Progress Report April - June, 2009 [PDF-30KB]

Quarterly Progress Report January - March, 2009 [PDF-26KB]

Quarterly Progress Report October - December, 2008 [PDF-31KB]

Technology Status Assessment [PDF-31KB] 

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