The finding suggests that much more fire ice is vulnerable to climate-induced melt than scientists realized, and that it could be a major source of planet-warming gas in the future. “It’s a very, very, very large carbon source,” says Davies. “What we’re showing is that there are pathways through which that carbon is released that we hadn’t appreciated.”
These special smallpox originated at a depth of 330 meters. But before Davies’ team dug into the data, no one was looking for melting fire ice at this location, because it is inland from where hydrate is stable in the current climate, and therefore not an area of interest. At these relatively shallow depths, methane hydrate formation in the sediment stops, where temperatures are too high and pressures too low.
“Everyone has been looking at a particular zone – about 450 to 750 meters below water depth – where hydrates are particularly vulnerable to melting,” says Davies. Hydrate is considered stable below 750 meters, where it is unlikely to release methane into the ocean during climate warming.
But things don’t always go exactly as expected. Temperatures may rise even deeper in the ocean, closer to the heat of the Earth itself. “Every 100 meters it gets a little warmer,” says Davies. “Although the pressure increases, the temperature also increases. They cross each other. And at that point, hydrate goes from stable to unstable.”
Davies thinks that as the oceans warmed over the past million years, fire ice that was very deep, perhaps several hundred meters below the seafloor, at a water depth of about 1 to 2 kilometers, also warmed and destabilized – then released gas that began to migrating slope. As the methane traveled from deeper areas beneath the seabed, it began to leak around 330 meters. “The ‘Eureka!’ The moment was finding these gigantic craters. “Due to interglacials – warm periods over the last million years – the gas moved up great distances and escaped every time it melted,” says Davies. “I thought, wow, [pockmarks are] formation due to hydrate dissociation in the deep water.”
Depth is an extremely important consideration when it comes to methane gas and climate, because it helps limit some of the damage. In the deepest parts of the ocean, fire ice can dissociate and belch methane, but microbes will destroy the gas before it can reach the surface. Methane also dissolves easily in seawater, which will lead to acidification, but at least it will not enter the atmosphere. (Because of the same mechanics, higher carbon dioxide concentrations in the atmosphere acidify the ocean.)