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,
“gas hydrates.”
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.
Current conditions
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.
Carbon cycling
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 Earth’s surface — a ratio geologists
can measure for different time periods in the past.
Abrupt change
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 signature’s 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.
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