A buried Arctic carbon store may be less secure than assumed
Scientists studying north-western Greenland say they have identified evidence of a previously underappreciated route for methane release: glacial meltwater destabilizing methane hydrates that were thought to be safely locked in sediment. The work, focused on Melville Bay, raises concern that continued ice-sheet melting could reactivate a process that likely occurred after the last glacial maximum.
Methane hydrates are an unusual form of frozen gas in which methane molecules are trapped inside a lattice of water. They form under cold, high-pressure conditions beneath the ocean, under permafrost, or beneath glaciers. Their significance is enormous because some estimates suggest hydrate deposits contain more carbon than all conventional fossil fuels combined.
That does not mean all of that methane is poised to escape. But it does mean that understanding the stability of these deposits is a major climate question. The new Greenland findings suggest that one assumed line of stability may be weaker than expected.
What researchers found in Melville Bay
The team, led by Mads Huuse at the University of Manchester, examined an area where methane hydrates are known to occur in sediments at the bottom of Melville Bay. In seismic surveys originally carried out by oil and gas companies in 2011 and 2013, the researchers identified 50 large pockmarks in the seafloor, each reaching depths of up to 37 meters.
The pockmarks cluster near a grounding zone wedge, a long berm of earth marking where the floating tongue of the Greenland ice sheet met the sea floor during the last glacial maximum. At first, the features were thought to have been scoured by overturning icebergs. But sediment cores later changed that interpretation.
Those cores showed that the upper sediment layers were largely free of methane even though local temperature and pressure conditions should have been suitable for methane hydrate stability. That discrepancy led the researchers toward a different explanation: the methane had once been present and then had been flushed out.
A new mechanism for release
The proposed trigger is glacial meltwater. As the ice sheet retreated after the last glacial maximum, meltwater appears to have moved through the subsurface and disturbed the methane hydrates held in sediment. Huuse described it as a newly recognized release pathway for methane that scientists had effectively assumed was “in the bank” and stable.
The significance of that phrase is hard to miss. Climate science often distinguishes between active emissions sources and carbon stores assumed to be relatively secure over relevant timescales. If glacial meltwater can destabilize hydrate deposits, then the retreat of major ice masses may do more than raise sea levels and reshape landscapes. It may also open a route for additional greenhouse gas release.
The seafloor pockmarks provide the geological footprint of that past disturbance. They are not merely holes in the sediment. In this interpretation, they are evidence that methane moved upward and altered the sea floor as environmental conditions changed.
Why methane matters so much
Methane is a potent greenhouse gas, and even relatively modest releases can matter for warming. That is why methane hydrates attract so much attention. They represent a large carbon reservoir, but one whose behavior under rapid climate change remains incompletely understood.
The Greenland study does not show that a massive modern methane pulse is inevitable. It does, however, expand the menu of mechanisms by which hydrates can be disrupted. Researchers had already considered warming oceans, thawing permafrost, and pressure changes. Meltwater-driven flushing adds another process to watch, especially in regions where glaciers and marine sediments interact.
That makes the work relevant beyond Greenland. Similar combinations of retreating ice, sediment basins, and hydrate-bearing zones may exist elsewhere in the Arctic. If the proposed mechanism proves general, the climate implications could be broader than one bay.
Past warning, present uncertainty
One of the most striking features of the research is that it reads the past as a warning for the future. The post-glacial world already ran this experiment once. Large ice masses retreated, meltwater pathways changed, and methane appears to have been mobilized. The concern is that modern warming could recreate enough of those conditions to do it again.
That is not the same as forecasting an immediate crisis. Geological systems can operate over long timescales, and release rates matter as much as total volumes. Still, the study sharpens the risk picture. Instead of asking only whether warming oceans will destabilize hydrates from above, scientists may now have to ask whether meltwater can unsettle them from within or below.
For climate research, that is a meaningful shift. It suggests some cryosphere changes may interact more directly with hidden carbon stores than previously recognized.
- Researchers identified 50 large pockmarks in Melville Bay, some up to 37 meters deep.
- Sediment cores suggested methane hydrates had been removed despite favorable stability conditions.
- The team proposes glacial meltwater flushed methane hydrates after the last glacial maximum.
- The study raises concern that continued ice-sheet melting could reactivate a similar process.
This article is based on reporting by New Scientist. Read the original article.
Originally published on newscientist.com
