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| Compounds made of gas and
water: marine methane hydrate and its amazing properties
Erwin Suess
GEOMAR Research Center
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Natural gas hydrate forms under elevated pressures
and low temperatures in the presence of sufficiently high concentrations
of methane. Trace gases and salinity affect its stability, e.g.
hydrogen sulfide enlarges the stability field whereas carbon dioxide
and higher salinities diminish it. The hydrate structure accomodates
trace gases as well as light hydrocarbons to the extent that under
standard conditions one volume of solid hydrate contains >100-times
its volume in gas. Marine gas hydrate occurs worldwide along active
and passive continental margins. The high primary productivity in
these regions coupled with rapid sedimentation leads to early diagenetic
methane formation mediated by fermentative micro-organisms. Accordingly,
most occurrences of methane hydrate at and below the seafloor are
biogenic, few are thermogenic. In sediments a sequence of hydrate-cemented
strata followed by strata with free methane below has acoustic properties
which affect the velocity and polarity of seismic waves. These properties
result in the characteristic bottom simulating reflector, BSR, which
is used to estimate the size of hydrate reservoirs.
Hydrate deposits found at the seafloor, such as off the eastern
North Pacific coast, in the eastern Mediterranean Sea or in the
Black Sea document highly dynamic formation and dissociation processes.
Fabric analyses show that natural hydrate is less dense than experimentally
formed phases. It coexists with free methane which migrates upwards
from beneath the hydrate stability zone. Several types of hydrate
fabrics, interlayered with carbonate crusts and hemipelagic sediment
clasts, are described. In most cases pure hydrate occurs in layers
millimeters to several decimeters thick. On a macroscopic scale
the fabric varies from highly porous, with pore diameters up to
several cm, to massive with no visible pores. Bulk densities range
from 0.35–0.75 g/cm3 and are inversely correlated with
the pore volume. These data allow estimates for an end-member density
of pure natural methane hydrate of 0.79±0.13 g/cm3 which
is approx. 10% less than the theoretical material density. This
difference is attributed to nano-pores, a fabric observed by field
emission scanning electron microscopy on samples of natural hydrate.
Low densities create a high positive buoyancy force as well as a
low acoustic velocity of hydrated sediments. The low acoustic velocity
affects the subsurface distribution of the bottom simulating reflector
(BSR) as well as the estimated thickness of hydrated formations
and hence the amount of hydrate stored in sedimentary strata. The
strong buoyancy facilitates rapid transfer of solid gas hydrate
from the seafloor to the atmosphere, by hydrate floats, and hence
affects the greenhouse gas budget. Temperature and pressure fluctuations
throughout the Earth’s history have apparently favored periodic
methane release such as by hydrate floats or large-scale eruptions.
Several episodes of anomalously warm climate have been linked to
short-term methane release from hydrate deposits. Such a mobilization
of methane hydrate may also cause submarine slides through increased
pore pressure and water release, which destabilizes continental
margin sediments. As a consequence gas eruptions and tsunamogenic
slumps may have been triggered. Paleo-environmental studies have
documented several sites where chaotic debris deposited from turbidity
currents is linked to a release of methane from gas hydrate. In
all cases the characteristic 12C-enrichment of methane
C is used as a proxy for the methane hydrate source. Consortia of
methane-oxidizing archeae and sulfate-reducing bacteria metabolize
methane from gas hydrates, as shown by the characteristic 12C-enrichment
in biomarkers. This metabolism generates enormous amounts of hydrogen
sulfide, which supports a specialized deep sea macro-faunal ecosystem
represented by bivalves and tube worms. Anaerobic oxidation of methane
(AOM) further results in a continuous precipitation of aragonite
and Mg-calcites. These carbonates as well as the biomass synthesized
from methane-12C may easily be diagnosed by their 12C-isotope
signature in recent and ancient deposits. The total amount of carbon
contained in natural gas hydrate exceeds the fossil fuel based carbon
by far and therefore represents a potentially important energy store.
However, before exploitation can commence, a thorough assessment
of the environmental impact is necessary.
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