Gas Hydrate and the Environment

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Gas hydrate is an ice-like substance formed when methane or some other gases combine with water at appropriate pressure and temperature conditions. Nearly the entire global inventory of gas hydrates is confined to sediments in a zone tens to hundreds of meters thick close to the seafloor at water depths greater than 350–600 m. On U.S. continental marine margins alone, BOEM estimates that gas hydrate sequesters over 50,000 trillion cubic feet of methane (mean value) or roughly 1800 times U.S. natural gas consumption in 2014. 

Schematic showing the general setting of seeps on the US Atlantic margin and related processes, such as gas hydrate degradation,

Schematic showing the general setting of seeps on the U.S. Atlantic margin and related processes, such as gas hydrate degradation, groundwater seepage, leakage through fractured rocks, or emissions from the seafloor overlying salt diapirs.

(Credit: Carolyn Ruppel, Woods Hole Coastal and Marine Science Center. Public domain.)

A key debate about gas hydrates is whether these shallowly buried deposits will rapidly break down with continued global warming and, if so, on what timescales. If gas hydrates do release methane, a potent greenhouse gas, in response to warming temperatures, questions remain about whether this methane could reach the atmosphere and in turn exacerbate warming. Even if methane released at the seafloor does not cross into the atmosphere, water column processes that transform the methane to carbon dioxide can lead to enhanced deoxygenation/acidification

Gas hydrate at the seafloor on the U.S. Atlantic margin.

Gas hydrate at the seafloor on the U.S. Atlantic margin. The icy deposit formed as gas bubbles emitted from the seafloor transformed into methane hydrate beneath the overhanging rock.

(Public domain.)

The CMHRP has been a leader in defining the scientific priorities for U.S. research on the interaction of gas hydrate and the environment and has led activities to review the state-of-the-art for this research. Oceanographic research involves imaging gas hydrate distribution in the seafloor, detecting methane plumes in the water column, and measuring methane emissions from the ocean to the atmosphere. In permafrost gas hydrate areas, researchers analyze gases, track methane flux, and reconstruct methane emissions histories. The CMHRP collaborates with the U.S. Department of Energy, NOAA, and other agencies to advance this research.

In the U.S. Arctic Ocean, the CMHRP has collected thousands of kilometers of continuous sea-air methane flux data over areas where gas hydrates are present in the sediments. USGS scientists have also studied how fast methane released at the seafloor is consumed by water column bacteria in the Arctic Ocean and evaluated the distribution of gas hydrate associated with remaining subsea permafrost close to the Beaufort Sea coastline. On the Arctic Svalbard margin, USGS Gas Hydrates Project scientists have shown that the net cooling associated with absorption of carbon dioxide by near-surface waters above seafloor methane seeps more than offsets warming that could be attributed to methane escape to the atmosphere from these seeps.

John Pohlman samples seep gas through ice in Lake Qalluuraq

USGS research chemist John Pohlman samples seep gas through ice in Lake Qalluuraq, located in continuous permafrost approximately 97 kilometers (60 miles) south of Barrow, Alaska.

(Credit: Robert Vagnetti, Dept. of Energy. Public domain.)

The latest frontier for CMHRP studies that assess the potential synergies between deepwater marine gas hydrates and the changing environment is in the temperate latitudes of the U.S. Atlantic margin. In 2014, CMHRP scientists and colleagues in academia used data acquired by the NOAA Office of Ocean Exploration and Research (OER) to discover more than 570 previously unknown sites where methane is leaking from the seafloor between Cape Hatteras and outer Georges Bank. Many of the seeps offshore Virginia, Maryland, and Delaware and in Hudson Canyon lie just shallower than the water depths (~550 m) at which pressures become too low and temperatures too high for gas hydrate to remain stable on the upper continental slope. Warming of northwest Atlantic Ocean waters over several decades may be contributing to the breakdown of gas hydrate and the present-day release of methane at these seeps. 

Using state-of-the-art laser-based instrumentation, the CMHRP has directly measured sea-air methane flux over northern U.S. Atlantic margin seeps in real time during ship surveys. The data show that very little of the gas emitted at the seafloor reaches the ocean surface, which implies that even widespread release of methane from degrading gas hydrates on upper continental slopes is unlikely to dramatically increase atmospheric methane concentrations. The CMHRP has also extensively mapped water column methane plumes on the margin, focusing particularly on effusive seeps that lie at water depths where methane should be trapped in gas hydrate, not emitted as bubble streams. CMHRP scientists collaborate with USGS benthic ecologists and academic scientists to conduct multidisciplinary research at the seep sites, sometimes using remotely operated vehicles to explore the seafloor.

Future CMHRP investigations of the interaction of hydrates and the environment will focus on mapping gas hydrate deposits where they are susceptible to warming ocean temperatures, primarily on upper continental slopes on the U.S. Pacific and Atlantic margins. This research may also focus on Arctic Ocean continental shelves, where gas hydrate that once coexisted with permafrost may remain in the warming sediments. Geophysical techniques will refine estimates of the amount of methane trapped in these deposits, while geochemical analyses will constrain the origin of methane seeping from the seafloor and the rates of methane transfer to the atmosphere across the sea-air interface.

Methane seeping on the Virginia margin just shallower than the limit for gas hydrate stability.

Methane seeping on the Virginia margin just shallower than the limit for gas hydrate stability. 

(Public domain.)