Matthew T. Reagan and George J. Moridis
Earth Sciences Division, Lawrence Berkeley National Laboratory
Scott M. Elliott, Mathew Maltrud, Philip W. Jones
COSIM (Climate Ocean Sea Ice Modeling), Los Alamos National Laboratory
Vast quantities of methane are trapped in oceanic hydrate deposits, with estimates of 3,000, 10,000, or even 74,000 gigatons of methane carbon trapped as hydrate in ocean sediments. An increase in temperatures at the seafloor, driven by climate change, could dissociate some of these hydrates, leading to methane release into the ocean and perhaps eventually into the atmosphere. Because methane is a powerful greenhouse gas, there is concern that such a release could have adverse consequences. This positive feedback has been proposed as a driver of past rapid climate change (the “Clathrate Gun” hypothesis). While this hypothesis is controversial, the role of methane in climate cycles is currently an active area of research, and hydrates are considered a potential source. Interest has increased since a team lead by the University of Birmingham discovered plumes of methane gas bubbles erupting from the seabed off the island of West Spitsbergen in a region where ocean warming has been documented. Are these releases driven or regulated by dissociating gas hydrates? And if so, what could be the climate and ecosystem consequences? A team of researchers from Lawrence Berkeley National Laboratory (including George Moridis and Matt Reagan from the Earth Sciences Division) and Los Alamos National Laboratory has been working over the past year to answer these questions.
Gas saturation, SG, and hydrate saturation, SH, within the 2-D Spitsbergen margin system at t = 50, 100, 130, and 200 yr.
The first step was to quantify local methane release under warming conditions. Using the TOUGH+HYDRATE numerical modeling software and a 1-D representation of heat and fluid flow, hydrate phase behavior, and methane transport in the sub-seafloor, Moridis and Reagan established that the majority of deep-ocean hydrates would be stable under expected warming conditions, but that shallow hydrates, especially those in the shallow Arctic Ocean, may be susceptible to a mere 1oC–3oC of warming. Regions like the Barents Sea are where the first effects of warming should be apparent, and hydrates at 300 m to 400 m depth may already have been affected.
The second step was to consider the possibility that such methane releases may already be occurring, and to test the hypothesis that the Spitsbergen system is hydrate-driven. Moridis and Reagan conducted large-scale, 2-D simulations in conditions representative of the Arctic continental shelf along the western Svalbard margin, using the massively parallel version of the TOUGH+HYDRATE code. These simulations show that such a system should create plumes of methane localized along the receding edge of the hydrate stability, as has been observed. The potential methane release into the ocean from such a system could contribute 0.004 Tg/yr of CH4 to the ocean, small in comparison to the global flux of methane, but potentially significant to the local ocean chemistry and biology. Roughly 700,000 km2 of the Arctic Ocean falls within 300 m–500 m depth, and this release represents a mere 0.02% of that area. If hydrate-driven plume systems exist in other areas with similar depth and temperature conditions, the cumulative effect could be huge in absolute terms.
The third step begins to address the consequences of such a release, and to determine the fate of the methane. Working with the methane fluxes from Step 1, the LBNL-LANL team is conducting simulations of methane release into the oceans using an ocean general circulation model known as POP, the Parallel Ocean Program, with added methane biochemistry. The POP simulations predict depletion of several key seawater components by bacteria as they attempt to deal with the methane plumes—oxygen, nitrogen and trace metals may all be removed from the water column and thus be unavailable to support existing organisms and ecosystems. This implies that these releases would be strong perturbations to the geochemistry of Arctic seawater. Potentially stressed locations include the enclosed and poorly ventilated Okhotsk Sea and Bering Sea, as well as central basin waters directly beneath the pole. The methane-eating bacteria would also become inactive when resources run low, permitting the methane to spread further.
Integrating the local methane releases from the 1-D model across simplified seafloor bathymetry provides the first basin-scale estimates. The first case examined, the Sea of Okhotsk, reveals that a few degrees of ocean warming may release nearly 90 Tg of methane into the basin during the first 30 years after release begins. The coupled POP simulations, coincidentally, indicate that about 95 Tg of methane over 30 years would be sufficient to create widespread hypoxia and nutrient depletion in the basin. This large release occurs despite less that 1% of the estimate Okhotsk hydrate reservoir dissociating.
Hypoxia in the Sea of Okhotsk, after 30 years of methane release into the basin.
Once integrated, these models will form the basis for a new source term to global climate models and allow the first quantitative assessment of the relationship between dissociating hydrates and global climate.
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