in Ice: What Are Gas Hydrates?
In 1810, Sir Humphrey Davy discovered a new class of materials, known
as gas hydrates or clathrates. These ice-like structures form when high
pressure and low temperature trap light natural gases within rigid cages
of water molecules. The trapped gas can be methane (hence methane
hydrate), butane, propane, ethane or a number of other light natural
At relatively low temperatures and high pressures, some light natural gases
can combine with water to create crystalline substances resembling ice. These
solid compounds are called clathrate hydrates of gas, or more conveniently,
Since the late 1960s, researchers have found methane-rich gas hydrates in sediments of the deep ocean and beneath permafrost regions. Such gas hydrates store a tremendous amount of methane, which if liberated, could supply bountiful energy or perturb global climate.
In the Gulf of Mexico, methane rises rapidly from below the seafloor and creates gas hydrate mounds. Courtesy of Ian R. MacDonald, Texas A&M University-Corpus Christi.
Estimates show that oceanic gas hydrates currently hold somewhere between 1,000 and 22,000 gigatons of carbon as methane, with most studies suggesting about 10,000 gigatons. Considering that our atmosphere contains about 700 gigatons of carbon, even the low mass estimates make gas hydrate a major component of the global carbon cycle.
This carbon pool, however, is sensitive to relatively small changes in deep-ocean temperature and sea level. Thus, in the past, gas hydrates may have destabilized, releasing methane into the atmosphere through gas bubbles rising rapidly through the water column or gas hydrates floating to the surface. Because methane is about 10 times more powerful a greenhouse gas than carbon dioxide, its release could have resulted in a potentially abrupt climate change.
This idea has captivated the geoscience community because the effects would have been widespread and significant. Many variables, however, need further examination to demonstrate that such degassing did in fact happen in the geologic past.
Deep-sea sediments host the vast majority of natural gas hydrates, which exist within a pressure- and temperature-limited volume often called the gas hydrate stability zone (GHSZ). Across a typical continental margin, the GHSZ makes a lens beneath the seafloor. Today, this lens begins at 250 to 500 meters water depth because, depending on local conditions, this is where the pressure is high enough and the water temperature low enough to create gas hydrates. From this depth down, the seafloor marks the top of the lens because gas hydrates, like ice, float and cannot accumulate in water. The bottom of the lens lies within the sediment, where temperatures are too warm for gas hydrates.
Although pressures and temperatures conducive for gas hydrates exist throughout the deep ocean, most deposits have been found along continental margins where high burial rates of organic matter drive considerable production of hydrocarbon gases, particularly methane. Gas hydrates form at or near the seafloor in a few locations where conduits such as faults bring gas-charged fluids to very shallow sediment, including the Gulf of Mexico and the Oregon margin. Most gas hydrates, however, occur in the GHSZ well beneath the seafloor as part of a dynamic system.
Photosynthetic organisms produce complex organic molecules, which eventually sink to the seafloor. Once in the sediment, bacteria utilize dissolved oxygen, nitrate and sulfate to convert organic matter to new compounds, including carbon dioxide and acetate. From these simple compounds, other microbes generate carbon-13-depleted methane. The dissolved gas migrates vertically and horizontally via diffusion and fluid flow. Eventually, at sufficient gas concentrations and appropriate pressure and temperature conditions, gas hydrates can precipitate in pore space. Sediment burial over time slowly brings the solid hydrates to higher temperatures. At the base of the GHSZ, gas hydrates are no longer stable and dissociate to water and free methane bubbles. Much of this methane can then migrate upward through the sediments to recycle.
Gas hydrates do not continually accumulate, however, because methane also escapes from sediment. In most places, methane moving up from depth encounters sulfate diffusing down from the seafloor. Microbes step in, using the methane and sulfate as food in an anaerobic process that typically occurs over a thin horizon within the upper 40 meters of sediment and produces bicarbonate ion and hydrogen sulfide. Seafloor vents also discharge methane into deep water at a few locations, notably where conduits bring gas-charged fluids up from below the GHSZ. At present day, aerobic oxidation by bacteria consumes most of this methane in the water column before it reaches the atmosphere.
In certain regards, gas hydrates and underlying free gas represent a major yet overlooked component of the global carbon cycle. Burial and degradation of organic carbon slowly contributes carbon to gas hydrate systems, while anaerobic microbial oxidation and seafloor venting slowly return carbon to the ocean.
Gas hydrates may serve as a capacitor, however, with relatively steady carbon inputs but highly variable carbon outputs, depending on temperature and pressure throughout time. Consider, for example, a rise in seafloor temperatures along continental margins from 0 degrees Celsius to 5 degrees Celsius. This temperature increase would significantly shrink the GHSZ, destabilizing large amounts of gas hydrate into free-gas bubbles. Buildup of free gas within sediment might then cause local pressures to exceed those of overlying sediment thus releasing methane from the seafloor through venting or sediment failure.
The capacitor concept brings some essential elements to discussions of gas hydrates and climate change. Perhaps most important to note is that widely accepted models for the global carbon cycle invariably omit gas hydrates and seafloor methane fluxes. These models remain accurate portrayals of carbon cycling when a small carbon input to gas hydrates roughly balances a small carbon output, which probably describes the present-day situation, but not necessarily the conditions of past time periods. Additionally, sedimentary strata suggest that organic carbon has accumulated in relatively cold deep waters (less than 15 degrees Celsius) throughout the geologic record. Thus, methane production and gas hydrates have likely been ubiquitous phenomena over time. Lastly, sea level has dropped and bottom-water temperature has warmed in the past, sometimes abruptly. Large amounts of carbon-13-depleted methane might escape the seafloor during these intervals, potentially leading to a warming in the atmosphere.
Substantial oxidation of methane in the ocean, however, would also affect the environment, principally by removing dissolved oxygen from seawater and dissolving carbonate on the seafloor. Thus, irrespective of whether methane burst into the atmosphere or ocean, the methane would ultimately convert to carbon dioxide, which would propagate throughout the ocean, atmosphere and terrestrial biomass. A massive release of carbon-13-depleted methane would, therefore, decrease the ratio of carbon-13 to carbon-12 across Earths surface a ratio geologists can measure for different time periods in the past.
Pronounced drops in the carbon-13 to carbon-12 ratio of carbonate and organic matter mark several ancient events of extreme global environmental change. During the Phanerozoic, these times include the Permian/Triassic boundary, 250 million years ago; multiple episodes of the Mesozoic, particularly 183 and 120 million years ago; and the Paleocene/Eocene Thermal Maximum (PETM), 55 million years ago. For each time period, researchers suggest that a massive release of methane from marine gas hydrates is an important ingredient of geologic change. Several researchers have also speculated that marine gas hydrates have influenced Quaternary climate.
Evidence for tremendous methane outgassing from gas hydrates is most compelling for the PETM, a brief interval that happens to coincide with a prominent deep marine extinction, extreme global warming and extraordinary mammal diversification. At least 50 different stable isotope records, constructed using carbonate and organic matter from both marine and terrestrial environments, show a prominent decrease in the ratio of carbon-13 to carbon-12 across the PETM. This truly global isotope excursion begins as an abrupt drop over about 20,000 years, followed by a more gradual return over about 200,000 years. The drop marks a rapid and massive addition of carbon depleted in carbon-13, while the return indicates its subsequent sequestering into the rock cycle.
The best explanation for this carbon input is a massive release of methane into the ocean or atmosphere, given the signatures abruptness and magnitude. Equally important, oxygen isotope records from fossilized sea life suggest a sudden rise in deep-ocean temperatures, perhaps by 6 degrees Celsius. This temperature change would have affected the distribution of gas hydrate dramatically. Deep-marine sequences also indicate a substantial drop in dissolved oxygen and pronounced dissolution of carbonate, consistent with release and oxidation of methane from dissociation of hydrates.
Even for the PETM, however, at least three major problems face the notion of massive release of methane from gas hydrates. First, deep-ocean waters averaged 10 degrees Celsius before the Paleocene/Eocene boundary. This temperature means that the GHSZ on continental margins was much smaller than it is today. To cause the observed isotope excursion, gas hydrates must have been more abundant within the GHSZ during the Paleocene than at present-day levels.
Second, widespread methane release from the seafloor should have left physical traces, such as vent structures or sediment slumps. Although seismic profiles have documented numerous mud volcanoes, apparently formed during the PETM in the North Atlantic, these features vented in relatively shallow water depths, so they cannot signify methane escape from gas hydrate systems.
Lastly, methane release from gas hydrates during the PETM requires that bottom-water warming preceded, at least in part, carbon input. But, evidence for this remains elusive because of intrinsic difficulties in determining the relative timing of rapid environmental changes in ancient strata.
Over the last 90 million years, pressure and temperature conditions affecting gas hydrate stability were most perturbed during the PETM. This event also has the hallmark geologic signatures expected for a massive methane release from the seafloor. Until new evidence emerges, however, gas-hydrate-driven climate change during the PETM or other time intervals remains a fascinating but unproven idea.
Conceivably, we live in a world with an enormous amount of gas hydrate and free gas that affects climate and global systems over time. Most current models for global carbon cycling and climate change, however, have continued to omit the large and dynamic seafloor methane cycle. We may be sitting on the brink of a major shift in thinking about the carbon cycle and climate change, one that would permeate throughout the broad geoscience community. Hopefully, over the next few years, an appropriate understanding will come through new drilling of gas-hydrate-bearing sequences, new carbon cycle models incorporating gas hydrates and free gas, and new records to pinpoint past seafloor methane release.