Gas Hydrates- Climate and Hydrate Interactions Active

Over the past 15 years, hydrate-climate studies have taken on a central role in the USGS Gas Hydrates Project. Research focuses on the effects of Late Pleistocene to contemporary climate change on the stability of methane hydrate deposits. The goal is to determine how much, if any, methane hydrate is currently dissociating on Earth in response to global warming and to estimate the amount of methane that would directly reach the atmosphere from such degassing.  

Methane is a strong greenhouse gas, and gas hydrates globally trap huge amounts of methane in a frozen form that is stable only a certain pressure and temperature conditions. If large amounts of methane were to reach the atmosphere from degassing gas hydrates, global warming might be exacerbated. In Earth's deep past, rapid warming may have led to widespread breakdown of methane hydrates, but there is as yet no strong evidence that methane release from dissociating gas hydrates triggered past greenhouse warming events. 

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A 2017 review paper coauthored by the USGS provides a modern perspective on the interaction of gas hydrates with the climate system, with a particular focus on warming ocean temperatures and changing sea levels. On contemporary Earth, some of the methane that might be released at the seafloor from gas hydrates dissociating within shallow marine sediments is injected into the deep oceans, where it dissolves and is usually oxidized to carbon dioxide. This carbon dioxide contributes to deep ocean acidification, and some of the carbon dioxide may reach the atmosphere in a few hundred years' time. For seeps at water depths greater than ~100 m, most of the methane likely never reaches the sea-air interface. 

In contrast, a fraction of the methane emitted from shallow (< 100 m water depth) seafloor on continental shelves can reach the atmosphere directly. However, except at high latitudes, gas hydrate is not stable in such shallow seafloor, meaning that methane seeps on most continental shelves are not connected to gas hydrate dynamics. At high latitudes, remnant, now-inundated Pleistocene permafrost that lies beneath some continental shelves is thawing.  Whether methane hydrates associated with such permafrost break down and release methane that reaches the seafloor and overlying ocean-atmosphere system remains the subject of active study.   

The USGS Gas Hydrates Project primarily focuses its climate studies on areas where gas hydrates may actively be degrading due to warming climate.  Onshore, investigations in areas of continuous permafrost on the Alaskan North Slope in 2010-2012 indicated that methane emissions were unlikely to be connected to degrading gas hydrates.   

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Marine study areas include (a) upper continental slopes, where the gas hydrate stability zone thins to zero thickness and is therefore highly susceptible to small perturbations in ocean temperature; and (b) continental shelves in the circum-Arctic Ocean, where subsea permafrost and associated gas hydrate continue to degrade. Upper continental slope studies have been conducted on the U.S. Atlantic, Cascadia, U.S. Beaufort Sea, and Svalbard margins, with additional investigations in the Baltic and North Seas and offshore Greenland. Continental shelf studies in areas with subsea permafrost have been carried out for the U.S. Beaufort Sea margin offshore the Atlantic North Slope, where the USGS has used seismic and borehole data to show that subsea permafrost extends a maximum distance of only a few tens of kilometers offshore.  These findings have contributed to circum-Arctic studies documenting the contemporary state of subsea permafrost. 

In addition to choosing study areas in geographic regions where key questions about climate-hydrate interactions can be addressed, the USGS Gas Hydrates Project also focuses on specific aspects of methane dynamics most critical for assessing the sources and sinks in natural systems. In the marine environment, one emphasis is the sediment-water interface.  At this boundary, researchers study the transfer of methane into ocean waters at cold seeps, along with the underlying dynamics of the gas hydrate systems that may feed the seeps, seafloor chemosynthetic communities and authigenic carbonates around the cold seeps, and methane oxidation and the fate of methane bubbles in the water column.  

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Studies also focus on the shallow part of the ocean’s water column, where researchers map methane concentrations and undertake carbon isotopic analyses to determine the origin of the gas. Key findings are that elevated methane concentrations near the sea surface are generally not spatially coincident with deepwater methane seeps, nor does the methane at shallow depths in the water column have the characteristic signature of seep methane.  In fact, methane hotspots near the sea-air interface are often related to plankton, not seepage of methane into the deep ocean from below the seafloor.