DE-NT0006558

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

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

Background
Large but poorly known 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 warming of a few °C [Buffett and Archer, 2004], a warming which is obtainable given the available inventories of fossil fuel carbon for combustion. The hydrate carbon reservoir has probably accumulated over millions of years, within the context of the gradual cooling of the ocean over geologic time, 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 depth in the ocean 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 hydrates.

The climate impact of melting hydrates in the ocean 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, taking ultimately 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 C or so of methane could be released to the atmosphere within a few years, to generate 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 to the ocean or the atmosphere, or to remain in place below the sea floor.

Potential Impacts
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
Project was initiated October 1, 2008. There are no technical accomplishments under the project to date.

Current Status
The project is to be carried out as a 3 year / 3 Phase effort. Planned activity within Phase 1 is to include:

• Develop parallel version of model code for a two-dimensional, basin scale model for the deep sediment biosphere with methane dynamics
• Addition of specific new capabilities within the model (geo-chemical tracer capability, permafrost simulation capability, multiphase flow capability, subsurface gas migration capability
• Initiation of model configuration such that is capable of performing simulations for the planned regional archetype scenarios
• Initiate development of archetype scenarios to be used for model simulation runs (Arechetype scenarios to include: Atlantic passive margin, accretionary wedge system, Arctic shelf and slope system, and the Gulf of Mexico)

Project Start: October 1, 2008
Project End: September 30, 2011

Project Cost Information:
Phase 1 - DOE Contribution: $171,127, Performer Contribution: $43,669
Phase 2 - DOE Contribution: $164,807, Performer Contribution: $41,898
Phase 3 - DOE Contribution: $169,520, Performer Contribution: $43,732
Planned Total Funding (if project continues through all project phases):
DOE Contribution: $505,454, Performer Contribution: $129,299

Contact Information:
NETL – Sandra McSurdy (Sandra.McSurdy@netl.doe.gov or 412-386-4533)
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)

Additional Information
Technical Status Assessment [PDF-31KB]