FROZEN HEATA GLOBAL OUTLOOK ON METHANE GAS HYDRATESVOLUME ONE

Beaudoin, Y. C., Waite, W., Boswell, R. and Dallimore, S. R. (eds), 2014.
Frozen Heat: A UNEP Global Outlook on Methane Gas Hydrates. Volume 1.
United Nations Environment Programme, GRID-Arendal.
© United Nations Environment Programme, 2014
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FROZEN HEATA GLOBAL OUTLOOK ON METHANE GAS HYDRATESVOLUME ONE
GUEST EDITORS
William Waite, Ray Boswell, Scott Dallimore

Canadian Polar Commission

Commission canadienne des affaires polaires

CONTENTS

11
12
14
16
21

Chapter 1
What are Gas Hydrates?
1.1 Introduction
1.2 What are Gas Hydrates?
1.3 Gas Hydrate Formation, Stability, and Occurrence
1.4 What Forms Do Gas Hydrates Take in Nature?

31

Chapter 2
Methane Gas Hydrates and the Natural Carbon Cycle

33
34
40
42

2.1 Introduction
2.2 Methane Generation and Consumption
2.3 A Gas Hydrate Capacitor in the Global Carbon Cycle?
2.4 Life at Marine Methane Seeps

51
52
53
54
55
57
68
71

Chapter 3
Assessment of the Sensitivity and Response of Methane Gas Hydrate to Global Climate Change
3.1 Introduction
3.2 The Role of Gas Hydrate in Past Climate Change
3.3 Key Issues for Linking Gas Hydrate with Climate Change
3.4 Global Climate Change Projections
3.5 Response of Gas Hydrates to Climate Change
3.6 Review of Sensitivity of Global Gas Hydrate Inventory to Climate Change
3.7 Conclusions

FOREWORD

Growing energy demands, uncertainty about supplies, andthe urgent need to reduce emissions of greenhouse gases
mean that the world faces an uncertain energy future. Many
countries have begun to explore alternative energy sources,
including so-called unconventional fossil fuels such as
natural gas hydrates.
Gas hydrates generally occur in relatively inaccessible polar
and marine environments, which is why they have not been
extensively studied until recently. Research about naturally
occurring gas hydrates has increased markedly over the
past two decades, however, and understanding about where
hydrates occur and how they might be exploited is growing
rapidly. Japan has recently tested offshore production ofnatural gas from a hydrate reservoir located more than 1,300
metres below the sea’s surface and other countries are also
actively exploring production potentials.
Continuing a tradition of identifying emerging issues, theGlobal Outlook on Methane Gas Hydrates is the result ofa rigorous assessment process designed to ensure theavailability of scientifically credible and policy-relevant
information. This assessment format brings together
diverse strands of knowledge and is a key mechanism
through which science informs decision-making.
This report provides a basis for understanding how gas
hydrates occur and the emerging science and knowledge
as to their potential environmental, economic, and social
consequences of their use. The intention of this publication
is to enable sound policy discourse and choices that take into
account a number of important perspectives.

Achim Steiner
UN Under-Secretary Generaland Executive Director of UNEP

PREFACE
Methane gas hydrates are solid, ice-like combinations ofmethane and water (Fig. I.1) that are stable under conditionsof relatively high pressure and low temperature. Gas hydrates
contain most of the world’s methane and account for roughly
a third of the world’s mobile organic carbon. Because gas hydrates tend to occur in relatively inaccessible and harsh polar
and marine environments, they were not studied extensively
until recently. For more than a century after their first creation in the lab by scientists in the early 1800s, gas hydrates
were considered an academic curiosity, with no meaningful
occurrence in nature. In the 1930s, they were recognized as
an industrial hazard forming blockages in oil and gas pipelines. In the late 1960s, scientists in Russia inferred theiroccurrence in nature. However, it wasn’t until after a seriesof deep-ocean scientific drilling expeditions in the late 1970s
and early 1980s that the abundance of gas hydrates in thenatural environment was widely recognized.

Growing energy demands and climate concerns have brought
increased attention to the potentially immense quantity ofmethane held in natural gas hydrates. The result has beena significant acceleration of the investigation of gas hydrates
over the past two decades (Fig. I.2), and the pace of scientificdiscovery about naturally occurring gas hydrates continues
to increase.
Although industry remains focused primarily on mitigating
unwanted gas-hydrate formation in production and transport
infrastructure, it is beginning to invest in understanding thehazards that naturally occurring gas hydrates pose to deepwater and Arctic energy development. Academia, supported
by national programs, is making significant progress in understanding the basic physics and chemistry of gas hydrates,
as well as their impact on the physical properties of sediments. This research furthers our understanding of the roleof gas hydrates in global environmental processes, including
natural geohazards, long-term carbon cycling and – giventhat methane is a potent greenhouse gas – global climatechange. However, the primary driver for much of the current
interest is the prospect of utilizing gas hydrates as an energy
resource. For a world in which energy demands are increasing steadily and future energy supplies are uncertain, thewidespread occurrence of potentially immense gas resources
is motivating intensive investigations in many countries.
Gas hydrate research is shifting from the level of individual
scientists to coordinated national research programs. As aresult, policy makers, business leaders, and private citizens
are now engaged in a discussion about the most appropriate
directions for gas hydrate research, as well as about management and funding issues. The large quantities of naturally
occurring gas hydrates distributed around the globe give riseto numerous societal and scientific concerns.

Figure i.1: Gas hydrate nodules. Nodules (white) recovered while
coring in the East Sea (Sea of Japan) (Courtesy Korea Institute of
Geoscience and Mineral Resources)
6 FROZEN HEAT

To facilitate decisions that must often rely on highly technicaland multidisciplinary information, this comprehensive summary of current issues in global gas hydrate research and de-Gas hydrates landmark findings
Japan discovers
first rich marine GH

Shell conducts GH
geohazard program
offshore Malaysia

Unigue GH-dependantbiota discoveredin the Gulf of Mexico

Japan-Canada complete
extended test of GH
production
US, Japan releaseassessments indicating
significant resource
potential

Ripmeesters’ Structure H
recovered in nature
Makogon predicts substantial
occurrence of GH in nature.
Villiard does firstwork on Methane
Hydrates

1800

Geophysical predictionof rich GH in the Gulf
of Mexico confirmedby drilling

Powell describes “clathrates” - thechemical nature of gas hydrates
is now known

Michael Faraday makes
first measurementof hydrate composition
Sir Humphry Davy
makes
Cl-hydrate in his lab

Studies links CH4, GH,and present climatechangeVan de Waals and Platteeeuw develop
thermodynamic model of GH properties.

Villiard, de Fourchard, others
show hydrates have complex
pressure-temperaturedependencies

1850

Ignik Sikumi test, first
field trial of CO2-CH4
exchange

Hammerschmidt
documents methane
hydrate formation ingas pipelines.

1900

Nankai field site, first
offshore production
test occurs in Japan

1950

2000

Industry discovers and test
GH reservoirs in arctic US-Canada

Unique 150m-thickGH occurrence
discovered off India

Shipley links widespreadgeophysical feature (BSR) to GH
Glomar Challenger recovers GH
in series of expeditions
McIver postulates GH role insubmarine landslides
ODP Leg 146
targets GH

Sowers shows minimal
GH link to Ice Ageclimate changes
Test well in Canada
proves ability to recovergas from GH

Extensive GH occurrence
mapped at “Blake Ridge” - US
East Coast
Dickens suggests the role of GH
during past carbon injection events

An Academic Curiosity
An Industrial Hazard
Energy and Environment

Figure i.2: Timeline of major milestones in gas hydrate (GH) research.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

7

velopment has been compiled: Frozen Heat: A global outlook
on methane-gas hydrates. Frozen Heat is a two-part review
that covers the role of gas hydrates in natural systems (Volume 1) and the potential impact of gas hydrates as a possible
new and global energy resource (Volume 2).

Volume 1 Summary

As a basis for understanding how gas hydrates occur andevolve in nature, Chapter 1 describes the crystal structures ofgas hydrates, their stability requirements, and the environmental settings in which gas hydrates commonly occur. It
also gives estimates of the global quantity and distribution ofgas hydrates. These gas hydrate basics provide a context forthe central message in Chapter 2: gas hydrates are a key partof the global carbon cycle, storing and releasing vast quantities of methane in response to changing environmental
conditions. Chapter 2 summarizes how methane is generated, moved into and out of gas hydrates, and gets consumed.
Chapter 2 also discusses the link between gas hydrates anddeep marine ecosystems. For example, much of the methane
released by gas hydrates into these ecosystems is consumed
by microbes in the upper sediment layers and water column
and never reaches the atmosphere.
Understanding the behaviour of gas hydrates over long time
periods is an important step in understanding how Earthworks. As discussed in Chapter 3, the breakdown of gas hydrates due to natural events, such as long-term increases inbottom-water temperature, could release large volumes of gas
from marine sediments, potentially transferring significant
amounts of methane into the oceans and, to a lesser degree,
into the atmosphere. Chapter 3 considers models of past climate change and future climate conditions and how thosemodels might be affected by potential feedbacks from gas hydrates. It is currently thought that methane from gas hydrates
likely contributed to, but did not trigger, past global warm-

Figure i.3: Left: methane from hydrate flared from the Mallik 5L38 Arctic gas hydrate research well in Canada (Courtesy of the
Mallik 2002 Gas Hydrate Production Testing Program). Right:
well-logging gas-hydrate-bearing sediment in the Gulf of Mexico(Courtesy R. Boswell, DOE)
8 FROZEN HEAT

ing events. Chapter 3 notes that, in the near term, the directcontribution of methane from gas hydrates to Earth’s climate
warming will likely be of minor significance. Despite thetremendous quantity of methane contained in gas hydrates
globally, only a small fraction occurs in environments that will
warm sufficiently over the next century to release methane
capable of reaching the atmosphere. A more significant nearterm result of methane release, particularly in the ocean, may
be the oxygen depletion and acidification of the deep oceanthat occurs when methane is broken down by microbes. Baseline monitoring studies will be important for understanding
the extent of these environmental degradation issues.

Volume 2 Summary

The central message in Volume 2 is that gas hydrates may
represent both an enormous potential energy resource andsource of greenhouse gas for a world with ever-increasingenergy demands and rising carbon emissions. Even if nomore than a small subset of the global resource is accessible
through existing technologies, that portion still represents
a very large quantity of gas. The accessible subset could include highly concentrated gas hydrate accumulations in locations where conventional hydrocarbon production is alreadyplanned or underway, and more diffuse deposits in areas
with strong societal motivations for developing domestic energy resources. To date, a few short-term, pilot-scale methane
production tests have been conducted in research wells. The
results suggest that larger-scale exploitation may be feasible,
but no commercial gas hydrate production has yet occurred.
Several nations, however, are currently researching the energy potential of gas hydrates (Fig. I.3). Recent detailed assessments of the energy potential of methane-gas hydrates

concluded that there are no anticipated technical roadblocks
to producing gas from hydrate deposits (Expert Panel on Gas
Hydrates 2008; Committee on Assessment of the Department of Energy’s Methane Hydrate Research and Development Program 2010).
Ultimately, a combination of technological advances and favourable global/regional market conditions could make gas
hydrate production economically viable. Therefore, Volume
2 provides a summary of gas-hydrate-based, energy-related
information useful in evaluating future energy resource options. Topics addressed in Volume 2 include a review of likely
future trends in energy supply, a characterization of prospective gas hydrate resources, technologies for exploration anddevelopment, and the potential environmental, economic,
and social implications of gas hydrate production.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

9

REFERENCES
Committee on Assessment of the Department of Energy’s Methane
Hydrate Research and Development Program (2010). Realizing theenergy potential of methane hydrate for the United States. 204 p.
National Research Council of the National Academies. The National

10 FROZEN HEAT

Academies Press, Washington, D.C.
Expert Panel on Gas Hydrates (2008). Energy from gas hydrates:
Assessing the opportunities & challenges for Canada. 222 p. Councilof Canadian Academies, Ottawa, Canada

CHAPTER 1

What areGas Hydrates?

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

11

1.1 INTRODUCTION
The English chemistry pioneer Sir Humphry Davy first combined gas and water to produce a solid substance in his lab
in 1810. For more than a century after that landmark moment, a small number of scientists catalogued various solid“hydrates” formed by combining water with an assortmentof gases and liquids. Sloan and Koh (2007) review this early
research, which was aimed at discerning the chemical structures of gas hydrates (Fig. 1.1), as well as the pressures andtemperatures at which they are stable. Because no practicalapplications were found for these synthetic gas hydrates, theyremained an academic curiosity.
That perspective changed in 1934. Natural gas was beginning to be used widely as a fuel and was often transported via
pipelines. Some pipelines were becoming plugged by what
appeared to be ice. E.G. Hammerschmidt (1934) discoveredAB

12 Å 12 Å

A A

12 Å 12 Å

B B
Figure 1.1: Crystal structures of ice and methane gas hydrate. For(A) ordinary hexagonal ice (ice Ih) and (B) structure I methane gas
hydrate (sI methane hydrate). Each sphere represents an atom:
white for hydrogen, red for oxygen, green for carbon. In hexagonal
ice, water molecules (H2O) are arranged in a hexagonal lattice. In
sI methane hydrate, water molecules form a lattice of cages, eachcage potentially holding a methane molecule (CH4). On the rightside of (B) and again toward the middle of (B), the cages line up,
appearing to hold a line of methane molecules (Figures courtesy B.
Anderson, WVU/NETL).

12 FROZEN HEAT

the plugs were not ice, but gas hydrates. This initiated a waveof engineering research – now known widely as flow assurance – dedicated to predicting and preventing the formationof hydrate blockages in industrial equipment. Since then,as hydrocarbon exploration moved into deeper water where
hydrates form more readily, the oil and gas industry has invested heavily in flow assurance research.
Much of the early gas hydrate research was empirical in nature,
as knowledge of the chemical structures of gas hydrates was still
limited. Determining the precise chemical formulation for gas
hydrates was challenging. A breakthrough came in the early
1950s, when a relatively new technology, X-ray diffraction, revealed that gas hydrates were in fact clathrates, a term coined a
few years earlier to describe solids with no fixed chemical composition in which small guest molecules are trapped within ahost lattice. For many years, the combination of the predictive
power of a thermodynamic model for clathrate behaviour (vander Waals and Platteeuw 1959) and crystal structure information from X-ray diffraction provided the cornerstone of efforts
to predict gas hydrate properties based on their crystal structure(Von Stackelberg and Muller 1951; Davidson 1973).
In the mid-1960s, following research into the pressures andtemperatures at which gas hydrates are stable (Pieroen 1955;
van der Waals and Platteeuw 1959), Y. Makogon and colleagues
in Russia recognized the natural association of methane andwater, and that the physical conditions (low temperatures andhigh pressures) necessary to form gas hydrates should occur
naturally on Earth (Makogon 1965). In high-latitude permafrost regions, they predicted, gas hydrates should be foundstarting hundreds of metres below the ground surface. In
marine environments, they should be found in shallow seafloor sediments beneath cold polar bottom waters where water
depths exceed approximately 300 metres, or in the sediments
beneath warmer, lower-latitude bottom waters where water
depths exceed 450-500 metres. Industry drilling in Arctic
permafrost confirmed the existence of naturally-occurringgas hydrates in the early 1970s. It was not until a series ofBox 1.1 Gas Hydrate and the Deep Sea Drilling Project
The Deep Sea Drilling Project (DSDP) (1968-1983, Legs 1-96)
introduced R/V Glomar Challenger, the first international
drilling platform for global studies of gas hydrates in themarine environment (Figure TB1.1). Over the course of several
DSDP legs, scientists obtained the first tangible proof that gas
hydrates exist in a variety of geologic settings, evidence thatgas hydrates could be nearly ubiquitous in continental-marginand slope sediment around the world.
An objective of DSDP Leg 11 in 1970 was to investigate thenature of the anomalous acoustic reflections (called Bottom
Simulating Reflectors or BSRs) that parallel the sea floor.
They had been observed on seismic profiles of the passive
margin along the Blake Outer Ridge in the Atlantic Ocean.
The expedition recovered sediment cores with methane
concentrations so high that, in many cases, gas expansion
was sufficient to extrude sediment from core liners. Although
no obvious gas hydrates were recovered on Leg 11, the highgas concentrations and presence of the BSR were suggestive
enough for the R/V Glomar Challenger to return in 1980
(DSDP Leg 76) with an objective of recovering gas hydrates.
This objective was met with the recovery and testing of a gas
hydrate specimen with a high concentration of methane.
A year earlier, in 1979, gas hydrates were recovered in theactive margin setting along the landward wall of the Middle
America Trench during DSDP Leg 66 off Mexico and Leg 67 off
Guatemala. The primary gas from hydrate specimens at bothsites was methane, which was confirmed by a massive gashydrate specimen recovered in 1983 during Leg 84 near the Leg
66 sites. Although a BSR was present at the Leg 66 hydrate-marine discoveries made in the early 1980s during scientificexpeditions by the Deep Sea Drilling Program’s R/V Glomar
Challenger (see Text Box 1.1), however, that gas hydrates wererecognized as a significant part of the natural environment. It

recovery sites, the Leg 67 hydrate recoveries were in vitric, or
glass-like, sands with no associated BSR.
Yet another hydrate-bearing geologic setting was discoveredin 1983, when DSDP Leg 96 recovered gas-hydrate nodules
and crystals in Gulf of Mexico mud. Taken together, thesesites provided a particularly significant result of the Deep Sea
Drilling Project by showing how gas hydrates were present
in sediments from a wide range of geologic environments.
Extrapolation of these results suggests that gas hydrates
are ubiquitous in continental-margin and slope sedimentaround the world, and this assumption has been confirmed bysubsequent investigations.

Figure TB-1.1: The R/V Glomar Challenger (Courtesy of the U.S.
Geological Survey).

was soon realized that such a large, and previously unappreciated, storehouse of organic carbon and its inherent energy
potential could have profound implications for society and ourunderstanding of Earth (Kvenvolden 1988a, b; 2000).
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

13

1.2 WHAT ARE GAS HYDRATES?
In nature, most substances have a fixed composition of building blocks. For example, in the case of methane (CH4), there
is always one carbon (C) atom for every four hydrogen (H)atoms, and these atoms are locked together in a fixed geometric structure by chemical bonds. It was initially assumed
each gas-hydrate structure had a fixed ratio of gas molecules
to water molecules, but this was later discovered to be incorrect (de Forcrand 1902).

Gas hydrates are classified as clathrates. In a clathrate, thesolid lattice of host molecules is physically stabilized by enclosing a sufficient, but not fixed, number of appropriatelysized guest molecules. The guest molecules reside withincages, which are open cavities within the lattice, and the stability of the structure depends on the co-existence of both
hosts and guests (Fig. 1.1). This combination occurs without
any direct chemical bonding. Furthermore, it is stable evenFigure 1.2: Gas hydrate outcrop on the sea floor of the northern Gulf of Mexico. The hydrate has an orange hue due to the presence of small
volumes of oil. This hydrate outcrop hosts pink “methane ice worms.” These worms, discovered in 1997, are generally 2–4 cm in length,
and graze upon bacteria living on the hydrate (Fisher et al., 2000). Additional ice worm descriptions are in Volume 1, Chapter 2 (Photo
courtesy I. MacDonald, FSU).
14 FROZEN HEAT

if some cages are empty. For methane hydrate to be stable,
only 70 per cent of the available cages need to contain methane (Holder and Hand 1982), although typically more than95 per cent of the cages are filled (Circone et al. 2005). The
occupancy rate can vary, depending on the pressure, temperature, and the gases present. As a result, clathrates are
non-stoichiometric compounds, or compounds without any
fixed chemical composition. Composition measurements
over a wide range of pressure and temperature conditions,however, show methane hydrate has an average compositionof CH4•5.99(+/–0.07)H2O (Circone et al. 2005).
Water is the exclusive lattice-building molecule in natural clathrates (hence the popular term, hydrate). Suitable guest molecules include methane (CH4), carbon dioxide (CO2), nitrogen(N2), ethane (C2H6), propane (C3H8), and other low-molecularweight gases and liquids. Methane has, so far, been the mostcommon clathrate guest molecule observed in nature. Therefore, the term methane hydrate is also common and will be used
occasionally in this report and associated web pages.
Naturally occurring clathrates can fit a variety of gases in theirstructures and create different water lattice shapes or cages
to accommodate the different sizes of available gas molecules(Sloan and Koh 2007). The most common clathrate structure forms in the presence of methane and a few other small
guest atoms or molecules with diameters between 4.2 and 6
Angstroms (Å). An Angstrom is 1/10 000th of a micron or
10-10 metres. This particular clathrate structure is known as
Structure I (Fig. 1.1). A unit cell, the smallest repeatable element of the Structure I hydrate lattice, consists of 46 watermolecules enclosing 2 smaller cavities and 6 larger cavities.
When larger gas molecules (6 to 7 Å), such as ethane andpropane, are present in sufficient quantities, a second clathrate structure (Structure II) forms. The unit cell of Structure
II hydrate consists of 136 water molecules creating 16 small
cavities and 8 large cavities. A third structure, known as
Structure H, has also been found in nature and can accommodate larger molecules (7 to 9 Å) when small molecules arealso present. To date, field studies suggest Structure I hydrate
occurs most often, Structure II is much less common, andStructure H is extremely rare.
Although people do not ordinarily see methane hydrate intheir daily lives, the methane and water molecules that makeup methane hydrate are quite ordinary. In fact, approximately
85 per cent of the molecules in gas hydrates are water molecules, and the chemical similarities between methane hydrate
and common water ice lead to many similarities in physicalproperties. For example, the density of both substances (~0.9
grams per cubic centimetre) is less than that of liquid water(~1 gram per cubic centimetre), so both ice and gas hydrates
will float in water. Visually, large nodules of methane hydrate
tend to look like white, opaque ice, although in nature, small
impurities can result in hydrate that ranges in colour fromorange (Fig. 1.2) to blue.
Ice and methane hydrate are, however, very different in termsof the conditions at which they are stable. In general, freshwater-ice stability on Earth is only a function of temperature,with the water-ice to liquid-water transition occurring at 0ºC(32ºF). As discussed in section 1.3 however, gas hydrate formation requires a suitable combination of temperature, pressure, water chemistry, guest-molecule composition and guest
molecule abundance (Thakore and Holder 1987).
Where gas hydrates do exist, they store gas very effectively.
Methane hydrate stores so much gas that when exposed toan open flame in controlled conditions, the dissociation, or
hydrate breakdown, can free enough flammable methane tocreate what looks like burning ice, surrounded by a growing
pool of water (see front cover of this volume). Dissociating
one unit volume of methane hydrate will release approximately 0.8 unit volumes of pure water and, once the gas isbrought to atmospheric pressure, 164 to 172 unit volumes ofmethane, depending on cage occupancy (Kvenvolden 1993;
Xu and Germanovich 2006). This is true regardless of how
deeply the methane hydrate was initially buried.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

15

1.3 GAS HYDRATE FORMATION,STABILITY, AND OCCURRENCE
Given adequate supplies of gas and water, the fundamental
controls on gas-hydrate formation and stability are pressure
and temperature. In general, a combination of low tempera-ture and high pressure is needed to form methane hydrate(Fig. 1.3). Because of Earth’s geothermal gradient – the natural
increase of temperature with depth below the ground surface

Stability conditions for gas hydrates
Depth (metres)

Depth (metres)

Marine setting00

Sea surface

200

200

400

400

Permafrost setting

Ground surface

Ice freezing temperature

Stability zone
Stability zone

600

600

800

800

Sea floor

1000

1000

1200

1200

1400

1400

1600

0

10

20
Temperature ºC

Base of permafrost

30

1600
-30

-20

-10

0

10

20

30

Temperature ºC

Figure 1.3: Stability conditions for gas hydrates. Idealized phase diagrams illustrating where methane hydrate is stable in marine andpermafrost settings. Hydrate can exist at depths where the temperature (blue curve) is less than the maximum stability temperaturefor gas hydrate (orange curve). Pressure and temperature both increase with depth in the Earth. Although hydrates can exist at warmer
temperatures when the pressure is high (orange curve), the temperature at depth (blue curve) gets too hot for hydrate to be stable, limiting
hydrate stability to the upper ~1km or less of sediment. The presence of salt, a gas hydrate inhibitor, shifts the gas hydrate stability curve(orange) to lower temperatures, decreasing the depth range of the gas hydrate stability zone. For seawater, this decrease is approximately
1.1°C (Dickens and Quinby-Hunt, 1994) (Figure modified from Kvenvolden (1988a)).
16 FROZEN HEAT

Selected gas-hydrates study areas
Prudhoe
Bay area

Svalbard

Mallik test site

Messoyahka

QilianMountains

Cascadia Margin
Northern Gulf
of Mexico

Blake Ridge

ShenhuBasin

MexicoCosta Rica

Ulleung Basin

Indian
Ocean

TaiwanJapan Sea
EasternNankaiTroughGumusutKakap

Peru

New ZealandFigure 1.4: Selected gas hydrate study areas. The yellow squares indicate a few of the historically-significant gas hydrate research sites, alongwith locations where gas hydrates have been recovered from depths greater than 50 meters beneath the sediment surface. Remote sensing
studies have inferred the presence of gas hydrate in numerous other locations. Though widespread, methane gas hydrates are restricted tolocations where adequate supplies of methane are available, which is generally on or near continents (Figure modified from Ruppel et al. 2011).

– gas hydrates are stable only in locations where high pressures can be attained in shallower, cooler sediments. The vertical extent over which these conditions occur at any location is
known as the gas hydrate stability zone (GHSZ). In this report,
unless otherwise stated, the GHSZ is for Structure I methane
hydrate, the most common gas hydrate on Earth.
The GHSZ exists in Arctic regions where cold average airtemperatures create thick zones of permanently frozen soils(permafrost). In these regions, the top of the GHSZ typicallyoccurs about 200 to 300 metres below the land surface, often
within an interval of permafrost. The GHSZ can extend 500
metres or more below the base of the permafrost (Fig. 1.3).
The GHSZ also exists in oceans or deep inland lakes wherehigh pressures are generated by relatively deep water – typically 300 to 500 metres or more, depending on the bottom-

water temperature. The top of the GHSZ occurs within thewater column, with the base of the GHSZ some distance below the sea floor (Fig. 1.3). The thickness of the GHSZ generally increases with increasing water depth. In areas of deep
water and low geothermal gradients, the GHSZ can extend 1
000 metres or so below the sea floor (Milkov 2004), with themost deeply buried deposits being as warm as 20°C or more(see Collett et al. 2009). Even this maximum depth for gas
hydrates is shallow compared to many conventional hydrocarbons, which are now being sought nearly 10 000 metres
below the sediment surface (Lewis et al. 2007; Mason 2009).
Just because a given location satisfies the pressure and temperature requirements for gas-hydrate stability, there is no
guarantee gas hydrates are present. If pressure and temperature were the only determinants, gas hydrates would
be virtually ubiquitous throughout oceanic sediment. In adA GLOBAL OUTLOOK ON METHANE GAS HYDRATES

17

dition to appropriate pressure and temperature conditions,gas hydrate formation requires adequate supplies of waterand hydrate-forming guest molecules (Fig. 1.4). The interval in which gas hydrates actually occur within the GHSZ isdesignated as the gas hydrate occurrence zone or GHOZ. As
discussed in Volume 1 Chapter 2, the methane incorporated
into gas hydrates comes from organic carbon. In shallow

sediments, the organic carbon is broken down by microbes,
with methane being one of the by-products. At significant
depths, it is broken down by thermal processes in whichheat cracks the organic matter into smaller molecules, suchas methane (Fig. 1.5). Organic carbon itself is not uniformly
distributed, nor has it always been distributed in the same
locations. In modern times, for example, approximately 90

Fate of buried organic matter
Organic material
settling to the sea floor
Approximate depth,
metresSea floor hydrate
outcrop above active
methane seep

10
Methane depleted zone.
Hydrate dissolves without active replenishment
100
Biologic processes

Organic materialburiedby sedimentation

Biogenic methane
generated by microbes
from organic matter

Excess methane forms:
Gas hydrate

BGHS

Excess methane forms:
Gas bubbles

along faults or otherpermeable paths

1 000
Geologic processes

10 000
BurialOil and otherhydrocarbons

Thermogenic break-downof organic matter

Increasing
temperatureand pressure

Figure 1.5: Fate of buried organic matter. Buried organic material is degraded by microbes, thermogenically altered by heat and pressure, or buried
more deeply and lost to the surface carbon cycle. Methane produced during microbial (also called “biogenic”) and thermogenic decomposition
can slowly migrate through overlying sediment with fluids or rise rapidly along faults or other permeable paths. As methane-saturated fluids
rise and cool, excess methane forms gas bubbles below the base of gas hydrate stability, BGHS. Above the BGHS, excess methane generally
forms methane hydrate, but can also form bubbles (Suess et al., 1999, Liu and Flemings 2006) (Figure modified from Pohlman et al. 2009).
18 FROZEN HEATper cent of the organic carbon buried in ocean sediment isfound beneath relatively shallow water near the continents(Hedges and Keil 1995; Buffett and Archer 2004). In periods of much lower sea levels, however, organic carbon was
deposited farther from the continents’ current edges, on
what is now the continental slope (Muller and Suess 1979;
Jasper and Gagosian 1990).
Gas hydrate volume estimates rely on two basic parameters: the amount of pore space, or porosity, available for gas
hydrates in the stability zone (Kvenvolden, 1988a; Collett,
1995; Dickens 2001; Klauda and Sandler 2005), and the percentage of that space occupied by gas hydrates, called the gas
hydrate saturation. The gas hydrate saturation is related tothe amount of methane that can be formed from available
organic matter and transported into the GHSZ (Harvey andHuang 1995; Archer et al. 2009). Gas hydrates tend to be
distributed quite unevenly because the porosity, the permeable paths for liquid and gas flow, and the conditions controlling the conversion of organic material into methane gas
can all vary dramatically over short distances (Expert Panelon Gas Hydrates 2008; Frye 2008; Solomon et al. 2008).
The Earth’s heterogeneous gas-hydrate distribution anduncertainties in porosity and gas hydrate saturation have
led to widely varying global estimates of the methane contained in hydrates (Fig. 1.6). Even the lowest estimates,
however, are so large they are given in terms of gigatonnesof carbon (GtC). A gigatonne equals 109 tonnes, equivalent
to 1 petagram or 1015 g. A petagram of water, for example,
takes up 1 cubic kilometre. For a sense of scale, it is estimated that approximately 1.8 Gt of methane carbon was
consumed globally as natural gas in 2011 (U.S. Energy Information Administration 2010).
The earliest global estimates of methane content in gas hydrates were made prior to the first recovery of gas hydrates
from marine sediment (green region in Fig. 1.6). These estimates assumed gas hydrates existed wherever pressure andtemperature conditions for gas hydrate stability were satisfied. This was equivalent to assuming gas hydrates were present in sediments beneath about 93 per cent of the world’s
oceans (Milkov 2004).

Estimates of the methane heldin hydrates worldwide
Gigatonnes carbon
107
106

Methane in marine hydrates (based
only on the pressure and temperature
requirements for hydrate stability)

105

First observation of marine hydrate

104

Methane in marine
gas hydrates

103
10

2

Methane in permafrost
gas hydrates

10
1
1970

1975

1980

1985

1990

1995

Global natural
gas reserves(conventional)

2000

2005

2010

Figure 1.6: Estimates of the methane held in hydrates worldwide. Early
estimates for marine hydrates (encompassed by the green region),made before hydrate had been recovered in the marine environment,
are high because they assume gas hydrates exist in essentially all
the world’s oceanic sediments. Subsequent estimates are lower, but
remain widely scattered (encompassed by the blue region) because ofcontinued uncertainty in the non-uniform, heterogeneous distributionof organic carbon from which the methane in hydrate is generated,
as well as uncertainties in the efficiency with which that methane isproduced and then captured in gas hydrate. Nonetheless, marine
hydrates are expected to contain one to two orders of magnitude moremethane than exists in natural gas reserves worldwide (brown square)(U.S. Energy Information Administration 2010). Continental hydratemass estimates (encompassed by the pink region) tend to be about 1
per cent of the marine estimates (Figure modified from Boswell andCollett (2011)). Estimates are given in Gigatonnes of carbon (GtC) forcomparison with other organic hydrocarbon reservoirs (see Figure1.7). At standard temperature and pressure, 1 GtC (Gigatonnes ofcarbon) represents 1.9 Tcm (trillion cubic meters) of methane which
has an energy equivalent of approximately 74 EJ (exajoules).
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

19

Carbon mass in gas-hydrate-boundmethane compared to othersources of organic carbon
Gigatonnes of Carbon
10 000
6 000
5 000
4 000
3 000
2 000
1 000
0

Gas hydrates

Oil
Natural gas
Coal

Energy resources Other major pools

Detrital organic matter
Peat
Vegetation

Dissolved in water
Frozen soils
Non-frozen soils

Figure 1.7: Carbon mass in gas-hydrate-bound methane comparedto other sources of organic carbon. A 2008 workshop estimatedthe global methane content in gas hydrates to range from 1 000
to 10 000 gigatonnes of carbon (GtC) (Krey et al. 2009). Taking a
midrange value of 5 000 GtC as an example, gas hydrates would
account for ~33 per cent of Earth’s organic carbon (excluding
dispersed carbon such as bitumen and kerogen). Other majorcarbon pools and their mass estimates in GtC are presented
here in top-to-bottom order as they are displayed in the figure.
Values for vegetation and non-frozen soil are taken from Sabine
et al. (2004), frozen soils from Tarnocai et al. (2009), peat from
Limpens et al. (2008b, a), detrital matter from Matthews (1997),
and atmospheric values from Blasing (2013). All other values arefrom Sundquist and Visser (2003).

20 FROZEN HEAT

Estimates of the global methane content in gas hydrates
fell as researchers began linking gas hydrate occurrence tothe supply of organic material from which methane could
be generated. Since the early 1980s, global estimates havevaried widely (blue region in Fig. 1.6), reflecting continued
uncertainties regarding the amount of methane delivered to,and subsequently stored in, the hydrate stability zone (Buffett and Archer 2004; Wood and Jung 2008).
Significant reduction of the uncertainty associated with
global estimates will require additional mapping and coring
to define local and regional patterns of gas hydrate distribution (Archer 2007) and to improve our basis for estimating
porosity and gas hydrate saturation in unexplored regions.
Such assessments are now underway, resulting in more
rigorously constrained estimates for some of the world’s
promising production regions (see Volume 2 Chapter 2), as
well as for regions that are sensitive to climate change (seeVolume 1 Chapter 3).
As shown in Figure 1.7, even a median estimate of 5 000
Gt of methane carbon in methane hydrate represents a significant fraction of the world’s organic carbon, and is ofsimilar magnitude to the combined estimates of carbon in
traditional global fossil fuel resources, such as oil, coal, andnatural gas. Not only is the gas hydrate estimate uncertain,
however, but not all gas hydrates are equally accessible asan energy resource (see Volume 2 Chapter 2) (Boswell andCollett 2011). Nonetheless, with annual global consumption estimated at 1.8 Gt of methane carbon in 2011 and2.15 Gt in 2020 (U.S. Energy Information Administration
2010), recovering even a small fraction of the methane ingas hydrates could significantly affect the global energymix (see Volume 2).

1.4 WHAT FORMS DO
GAS HYDRATES TAKE IN NATURE?
The most visible gas hydrates in nature are massive moundsof solid hydrate, often many metres in diameter, exposed on
the sea floor and frequently covered with thin drapes of sediment (Fig. 1.8, bottom row). These mounds mark locations
where active fluid vents, or seeps, supply methane directly
to the sea floor. Seeps provide the methane for gas-hydrate
mounds to form and grow, but this growth must compete
not only with temperature changes that can destabilize gas
hydrate, but with erosion from the sea water itself, which isundersaturated in methane and will therefore dissolve exposed gas hydrate (Lapham et al. 2010; Zhang et al. 2011).
Gas hydrate mounds have been observed to decay, withchunks of hydrate breaking away from mounds and floating away (MacDonald et al., 1994), but this is not a regularoccurrence (MacDonald et al., 2005). Monitoring studies ofgas hydrate mounds in the Gulf of Mexico (MacDonald et al.,2005) and offshore of Vancouver Island at the Barkley Canyon site (Lapham et al. 2010) demonstrate that gas hydrate
mounds can persist for several years at least, in spite of being
continually dissolved by seawater and exposed to short-term
increases in bottom-water temperature.
The vast majority of gas hydrates, however, lay buried in
sediment. The sediment itself is 30 – 70 per cent pore space(Santamarina et al. 2001), and as shown in Figs. 1.8-1.10,
the manner in which gas hydrates fill or alter that space canbe quite different depending on the abundance of available
methane and whether the sediment is sandy or more finegrained (Fig. 1.9).

Hydrate in sands

The relatively high permeability of sands facilitates the flowof water and methane needed for hydrate formation, and gashydrates have been found filling more than 60 per cent of theavailable pore space with saturations as high as 90 per centin some Arctic sands (Collett et al. 2009) (Fig. 1.10, class F),

as high as 80 – 90 per cent in Gulf of Mexico sand bodies(Boswell et al. 2012) (Fig. 1.10 class C) and as high as 70 per
cent in the sandy sections of interbedded sands and muds
off Japan’s southeastern coast, on the margin of the Nankai
Trough (Tsuji et al. 2004, 2009) (Fig. 1.10 class C). Though
only approximately 10 per cent of the world’s gas hydrates
likely occur in sands (Collett et al. 2009), the high gas hydrate concentrations that can be found in sands have madethem research and development targets for potential gas hydrate exploration (see Volume 2).

Hydrate in fine-grained sediment

Marine drilling conducted initially on the Blake Ridge (offshore eastern United States) in 1995 (Paull et al. 1998)
found gas hydrates occurring as microscopic pore-fillinggrains in fine-grained sediments (clays and muds) (Fig.
1.10 Class E). These accumulations can cover large areas
and extend through thick vertical sequences. It is generally
believed the majority of Earth’s gas hydrates exist in thisdispersed form (Boswell 2009), even though the concentrations are typically low, ranging from 1 or 2 per cent to ashigh as 12 per cent of the pore volume. These low saturations are probably due to the very small pore size and low
permeability of clay-rich sediments, which greatly hinder
the mobility of both gas and water. Gas hydrates likely form
more readily in zones within these fine-grained environments where porous microfossils or slightly coarser grains
provide a small increase in both porosity and permeability(Kraemer et al. 2000; Bahk et al. 2011).
In areas where methane flux is particularly strong, it is possible for gas hydrates to accumulate to greater concentrations
within clay-rich sediments. In 2006, drilling off the coast ofeastern India revealed an approximately 150-metre-thick section of fractured clay sediments with gas hydrate saturationsof 20 to 30 per cent or more (Collett et al. 2008). An expediA GLOBAL OUTLOOK ON METHANE GAS HYDRATES

21

Pore-Filling Gas Hydrates

Conglomerate: Arctic Canada

Sand: Offshore Japan

Clay: Offshore China

Grain Displacing Gas Hydrates
Conglomerate: Arctic Canada

Sand: Offshore Japan

Clay: Offshore China

Grain Displacing Gas Hydrates
Conglomerate: Arctic Canada

Sand: Offshore Japan

Clay: Offshore China

Grain Displacing Gas Hydrates

Massive Layers: Offshore India

Veins: Offshore Korea

Nodules: Offshore India

Veins: Offshore Korea

Nodules: Offshore India

Veins: Offshore Korea

Nodules: Offshore India

Western North Atlantic

Offshore Vancouver Island

Sea-Floor Mounds
Massive Layers: Offshore India

Sea-Floor Mounds

Massive Layers: Offshore India

Sea-Floor Mounds

Gulf of Mexico
22 FROZEN HEATtion to the East Sea of Korea in 2007 found a similar occurrence (Park 2008). X-ray scans conducted on cores from both
India and Korea, which were acquired and analyzed at in situ
pressures (see Text Box 1.2), showed gas hydrates existing as
sporadic lenses of solid hydrate within a pervasive networkof thin, nearly vertical fractures (Holland et al. 2008; Rees etal. 2011) (Fig. 1.8 middle row). Although the mechanisms bywhich such accumulations form are not clear, it may be thatcomparatively vigorous gas migration within gas “chimneys”
can disrupt the sediment enough to create the local permeability needed for enhanced gas-hydrate formation (Fig. 1.10
class A, B). It is not known how many such occurrences exist,
but they could be quite abundant. In 2005, a well-logging expedition in the Gulf of Mexico found a roughly 30-metre interval in which gas hydrates were observed to occupy numerous near-vertical fissures within clay-rich sediments (Ruppel
et al. 2008). In 2009, a similar gas-hydrate occurrence, about
150 metres thick and widespread, was logged elsewhere in
the Gulf of Mexico (Boswell et al. 2012). In both settings, thefissures occurred within distinct sedimentary layers and appeared to be controlled by subtle changes in sediment properties. The interpreted gas hydrate saturations were generally
low, ranging from 5 per cent to perhaps 10 per cent of thepore volume.

The large size and lateral continuity of typical gas hydrate occurrences in mud-rich sediments are conducive to the generation of anomalous and conspicuous features, seen in seismic
data and called bottom-simulating reflectors (BSRs). Initially,
BSRs were used widely to assess the distribution of gas hydrates (Shipley et al. 1979). However, recent drilling resultsfrom Japan (Tsuji et al. 2009) and the Gulf of Mexico (Shedd etal. 2012) have demonstrated that BSRs can appear in many different forms, and gas hydrates can even occur without a BSR(Paull et al. 1996). As discussed in Volume 2, Chapter 2, BSRs
are therefore not considered reliable indicators of the nature
or concentration of gas hydrates, and more sophisticated geological and geophysical exploration approaches are now being
used (Tsuji et al 2009; Boswell and Saeki 2010).
As Chapters 2 and 3 in this volume illustrate, the varied geologic
settings in which gas hydrates are found must be consideredwhen evaluating the role of gas hydrates in natural systems suchas the natural carbon cycle (Chapter 2), the link to chemosynthetic sea-floor communities (Chapter 2), and past and futureclimate change (Chapter 3). In Volume 2, the implications offinding high hydrate saturations in sand layers or as veins in
fine-grained sediment are discussed in terms of the reservoir’s
accessibility and value as a potential energy resource.

Figure 1.8: A selection of gas hydrate forms observed in nature. Unless otherwise noted, gas hydrate is white, and sediment is dark. Top row:
Pore-filling. Left, Mallik site, Canada (courtesy JOGMEC-NRCan-USGS), and centre, Nankai Trough, offshore Japan (courtesy JOGMEC),
show high saturation in sandy sediments. Right (South China Sea, courtesy GMGS-01 Science Party) shows low to moderate saturation in
fine-grained sediments. Middle row: Grain displacing in fine-grained sediments. Left: massive near-horizontal layers from offshore India(Courtesy NGHP-Expedition-01). Centre: array of thin, near-vertical veins from East Sea (Courtesy UBGH-01). Right: large nodule from
Bay of Bengal (Courtesy NGHP-Expedition-01). Bottom row: Gas hydrates exposed at the sea floor. Left: massive sea-floor mound stainedorange with oil in the Gulf of Mexico (Courtesy I. MacDonald). Centre: massive hydrate mass built from methane gas bubbles under a thinsediment layer at Blake Ridge, offshore USA (Courtesy Woods Hole Oceanographic Institution). Right: massive sea-floor mound offshore
Vancouver Island, Canada (Chapman et al. 2004).
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

23

Silt and Sand-rich Host Sediments
Thinly interbedded(Nankai Trough; Gulf of Mexico GC955)

Without Gas Hydrate
Porosity: 30-45%
Permeabilitiy 500-2000 md
Mechanical Strength: Low
With Gas Hydrate
Porosity: 10-15%
Permeability; 0.1 - 0.5 md
Gas Hydrate Saturation: 50-90%

Massively-bedded(Gulf of Mexico WR313; Mallik)

100 microns

Clay-rich Host Sediments
Grain-displacing in disrupted, deformed sediments(KG Basin, Ulleung Basin)

Pore-filling in undisrupted sediments(Blake Ridge)
Without Gas Hydrate
Porosity: 50-70%
Permeability: Diminishes with Depth
to very low values (0.0001 md)
Mechanical Strength: Very Low

100 microns

With Gas Hydrate
Porosity: 45-60%
Permeability: Nil (0.0001 md)
Gas Hydrate Saturation: 1-10%

With Gas Hydrate
Porosity: 45-60%
Permeability: Nil (0.0001 md)
Gas Hydrate Saturation: 5-40%

100 microns

Massive Occurrences (no host sediment)

Consolidated host sediments (rock)(Gulf of Mexico, Cascadia, others)(Messoyahki, Barrow (AK), Qilian Mtns (Tibet))

Variety of Lithologies
Porosity: Reduced due to graincompaction, cementation
Permeabilitiy: Reduced 500-2000 md
Mechanical Strength: Very highFigure 1.9: Summary of how the host sediment controls the form of gas hydrate occurrence. Gas hydrates are primarily found inunconsolidated sands (upper row) or clays (centre row). Hydrates also commonly occur in thin, hydrate-bearing sand layers separated byfine-grained sediment (upper right), and can even form in sands that have themselves been consolidated, or packed more tightly together,to form a rock (lower right). In clays, gas hydrates can exist in low concentrations in the small pores between grains (centre left). Gas
hydrates in higher concentrations tend to displace grains to form veins, lenses, and nodules (centre right), and those veins, lenses or
nodules can occasionally grow large enough to appear as a massive hydrate occurrences (lower left) (adapted from Boswell et al. 2011).
24 FROZEN HEATBox 1.2 Identifying gas hydrate in specimens of natural sedimentGas hydrates are now considered one of the largest
storehouses of potentially mobile organic carbon on theplanet. However, their very existence on Earth was notconfirmed until the first samples were observed during
scientific drilling programs in the early 1980s (see TEXT
BOX 1.1). One reason gas hydrates eluded detection
for so long is that the unique combination of highpressure/low-temperature conditions required for theirstability is restricted to some of the more remote places
on Earth, including in and beneath permafrost in Arctic
regions and within the marine sediments of continental
margins. Like water ice, when a gas hydrate is removed
from the environment in which it is stable, it melts
into a liquid water phase. Gas hydrate also releases its
trapped methane gas in the process. Since gas hydrates
achieve this phase change rather quickly, much of thegas hydrate present in specimens collected at or below
the sea floor in conventional marine studies will havedisappeared (dissociated) by the time the specimens
arrive on deck for inspection. Only the largest solid
masses persist long enough to be physically observed.
Initially, scientists developed special means to infer
the presence of gas hydrates from the impact theirdissociation has on the chemistry of the surrounding
sediment: that is, the stronger the shift of pore-watersalinity to fresher values as compared to the local
background condition, the greater the gas-hydrate
volume that had recently been present. In addition,
infrared scanners are used to detect cold spots inrecovered cores. These spots indicate where gas
hydrates have been and where their melting has cooled
the surrounding sediment. The ability to conduct direct
measurements in situ using geophysical well-logging
tools has advanced significantly (Tsuji et al. 2009),
and currently much can be determined with great
confidence using such tools, particularly when gashydrate concentrations are high. Predicting gas-hydrate
occurrence using remote sensing (such as seismic orelectromagnetic surveys conducted from the surface) is
possible, and this ability becomes more accurate witheach detailed field study.

To fully assess gas-hydrate-bearing sediments, scientists havedevised pressure-coring technologies that allow samples tobe collected and retrieved without ever exiting gas-hydratestability conditions. This technology continues to advance,with increasingly complex measurements being made on
acquired samples. X-ray images taken of such samples havedemonstrated the wide variety of forms gas hydrates can takein the subsurface, ranging from broadly disseminated porefilling grains to complex arrays of delicate tabular veins andfracture-filling forms (see Fig. TB-1.2) (Holland et al. 2008;
Rees et al. 2011). Such measurements and images provide
critical ground-truth data to confirm the impact of gas-hydrate
occurrence on the physical properties of the sediment.

A

B

C

Figure TB-1.2: X-Ray-computed tomography images for gashydrate-bearing clays from the Krishna-Godavari Basin offshore
eastern India. Gas hydrates are shown in white, clay is shownin grey, and blue represents ice. (A) Gas hydrates are generally
observed as near-vertical veins in this 90-centimetre-long core.
The diameter is 5.7 centimetres (Holland et al. 2008). (B) In
this micro-computed tomography scan (Rees et al. 2011), a
23-centimetre-long sample, also 5.7 centimetres in diameter,
illustrates how the large gas hydrate veins observed in the fullcore scan are themselves made up of small, interconnected veins.
Ice has formed in this specimen during sample transfer andhandling, and it is not representative of the in situ environment,
which is well above the freezing temperature of water. (C) A
natural-light image of gas-hydrate-bearing clay from the region.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

25

General schematic showing typical modes of gas hydrate
occurrence relative to the geologic environment

Relict Permafrost on
recently inundated
shelves (most climate sensitive?)

Permafrost
F

Pore-filling in sands athigh concentration
both within and below
permafrost (some pre-existing
free gas accumulations)~350 to 600 m water depth (depending on bottom-water temperature)one

Sea Floor
MoundsD

ted z
e depleethan

M

BIOGENIC GAS GENERATION
A, B

C

d”
oun dsa-b d mutar
“St cturefra

200 to 1000 meters

Minimal Gas Hydrate(reduced gas supply)
far offshoreor
Sea Flo
Disseminated,pore-filling

Hydrate at upper edge of stability(climate sensitive)

E

Hydrate-bearing
Marine Clays

Hydrate-bearing
sand

Hydrate-bearing
Deformed Clay

Water-bearing
sand
Free gas

Deeply-buried, pore filling in sands(resource targets, climate buffered)Gas Hydrate
Stability Boundary“Chimney Structures”:
Dense fracture fills in muds

R. Boswell - 2011

THERMOGENIC GAS GENERATION AND MIGRATION

A
A

B
B

C
C

D
D

E
E

F
F

Figure 1.10: General schematic showing typical modes of gas hydrate occurrence relative to the geologic environment. Thin (A) and thickly
veined (B) sediment-displacing gas hydrates (white) in fine-grained sediment (grey); (C) pore-filling gas hydrates in sand; (D) gas hydrate
mounds on the sea floor (hydrate has an orange coating from oil and is draped with grey sediment); (E) disseminated gas hydrates (whitespecks) in fine-grained sediment (grey); (F) gas hydrates (white) in coarse sands (grey) (adapted from Boswell 2011).

26 FROZEN HEATA GLOBAL OUTLOOK ON METHANE GAS HYDRATES

27

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Res. Lett., 38. doi: 10.1029/2011gl047141

CHAPTER 2

Methane Gas Hydrates
and the Natural
Carbon Cycle

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

31

Atmosphere

780

120

92

6.3

Land use change

59

2.2

90

550

Rivers

-

- -

-

-

-

-

--

-

-

-

-

0.8

58

Vegetation
-

-

-

60
-

-

-

-

-

-

Soil and vegetative
1 500
litter
-

- -

-

- -

-

-

-

-

-

3

- ?
- - ~0.004- - - - - - - - - - - - - - - - Gas- - - - hydrates- - - - - 5 000- -fuels- (Combined)- - Fossil- 5-000 - 10
- 000
- - - - - - Coal
- Oil and- - gas- fields
fields? - - - - - - -- -- -- - Storage:
1 020- Gigatonnes of C- - - -- Fluxes: Gigatonnesof8- C per year
-

-

-

-

-

32 FROZEN HEAT

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Carbon cycle

Methane is a significant component of the near-surface global carbon pool, and it can exist as a free gas, dissolved in water, or held in gas hydrate. Prior to the Industrial Revolution,
methane accounted for 0.05 Gigatonnes of carbon (GtC) in
the oceans, and 1.5 GtC in the atmosphere (Sundquist andVisser, 2003). Modern estimates suggest methane now accounts for 3.7 GtC in the atmosphere (IPCC, 2007). By comparison, midrange estimates discussed in Volume 1, Chapter
1 suggest global methane hydrates sequester ~5000 GtC, representing about a third of the ~15 000 Gt of organic carbon
near the Earth’s surface. Methane in hydrates also accounts
for approximately 10 per cent of the total near-surface carbon
pool (organic and inorganic), which is on the order of 41 000
GtC, with another 5 000 to 10 000 GtC held in coal, gas andoil (Houghton, 2007).

Ocean surface
725

48

33
37

Marine biota

3
11

42

This chapter focuses on the marine environments, where ~99
per cent of the Earth’s methane hydrate exists (See Volume 1,
Chapter 1). A summary of how and where methane is formed
and consumed is presented, along with a discussion of how
gas hydrates, and also sea floor chemosynthetic communities,
fit into methane’s contribution to carbon cycling between theatmosphere, hydrosphere, geosphere, and biosphere.

Deep ocean

38 000

0.2
-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

--

--

-

-

--

-

-

-

--

-

2.1 INTRODUCTION

Sediments
-

-

-

-

-

Figure 2.1: Global carbon cycle. Carbon moves through theatmosphere, biosphere, geosphere, and hydrosphere. Gas hydrates(orange) are shown in marine sediments, but are also buried beneath
permafrost sediment in Arctic regions. The 5 000 GtC cited for gas
hydrates is a midrange estimate from recent global assessments,
and the ~.004 GtC/year carbon flux from hydrates is taken fromthe Intergovernmental Panel on Climate Change (IPCC, 2007). Allother values are compiled from Houghton (2004). Although gas
hydrates are a significant global carbon pool, the precise amountof carbon, the amount of carbon released from gas hydrates to theatmosphere, and the extent to which that release could increase as
the global climate changes are all under active debate. Improving
upon the values for gas hydrates used in this figure will require
extensive mapping and research efforts around the world.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

33

2.2 METHANE GENERATIONAND CONSUMPTION
Methane is the third-most abundant greenhouse gas inEarth’s atmosphere, after water vapour and carbon dioxide. Although the concentration of carbon dioxide in theatmosphere is more than 200 times that of methane (Blasing 2013), the impact of methane is magnified because it
is about 23 times more potent than carbon dioxide as agreenhouse gas. This potency is related to methane’s radiative forcing capacity, which refers to the ability of a gas toabsorb and trap heat radiating off Earth’s surface (Lacis etal. 1981; Hansen et al. 1988). Methane has a relatively short
lifetime in the atmosphere (Boucher et al. 2009), because
within about a decade, a combination of sunlight and chemical processes cause methane molecules in the atmosphereto break down to water and carbon dioxide, the two mostabundant greenhouse gases.
As reviewed by the IPCC (2007), the total flux of methane carbon to the atmosphere from all sources is currently
around 0.45 GtC/year, or 450 TgC/year (Tg = 1012 grammes)(See Fig. 2.2). This flux is more than double the pre-industrial flux, and about 70 per cent of the emissions are due
to human activity (Reeburgh 2007; Colwell and Ussler III
2010). Gas hydrates are estimated to account for only about
1 per cent of the annual methane emissions to the atmosphere (Forster et al. 2007), but as discussed below, hydrates’
true methane contribution is not precisely known.
Since most methane hydrates occur in marine sediments atwater depths greater than ~500 metres, a key factor affecting how much methane released from dissociating gas hydrates reaches the atmosphere is the efficiency of transferring methane through the water column. For most bubbles
released from the sea floor at water depths greater than 100
meters, the methane will be replaced by other gases during
bubble ascent, and the methane will dissolve in the surrounding waters (McGinnis et al. 2006) and can be consumed by
microbes (see Section 2.2.2). Direct transfer of methane to
34 FROZEN HEAT

the atmosphere via bubbles is most relevant in shallow lakes,
estuaries, and river deltas, and on continental shelves. Onlyin the case of Arctic Ocean continental shelves (Shakhova et
al. 2010; Biastoch et al. 2011) could these methane releaseprocesses at shallow water depths be related to gas hydrates.
In special cases, gas hydrate could play a role in enhancing
transport of methane to the ocean-atmosphere interface.
For example, bubbles released from the sea floor within thegas hydrate stability zone (greater than 500 metres) could
form gas hydrate shells in the water column. With such armouring, the bubbles may retain methane to shallower water depths during bubble ascent. Another enhanced transfer
mechanism involves chunks of gas hydrate, which can occasionally break off from sea floor gas hydrate mounds (MacDonald et al. 2005). Because gas hydrate is buoyant, thesechunks may reach the sea surface relatively intact before
releasing their methane. Sea floor gas hydrate mounds are
not widespread, and this process is not an important factor
in transfer of methane to the atmosphere. (For additional
discussion of bubble transfer, see Volume 1 Chapter 3).

2.2.1 Marine methane sources: How ismethane produced?
Most methane that reaches the Earth’s surface is producedby microbial activity in sediments, where a special group ofarchaea called methanogens produce methane via anaerobic (without oxygen) decomposition of organic material.
Intense heating of organic carbon also produces hydrocarbon liquids (e.g., petroleum) and gases, including methane.
These formation mechanisms are summarized here.

Microbial methane production

Methanogens generate methane in organic-rich sediments
in many settings (e.g., marshes, rice paddies, estuaries,
landfills, river deltas, and continental margins). We focus

Methane mass released per year
Teragrammes

Primary sources of methane release

231

Maximum

Ruminant
Biomass emissions
burning

Minimum

Wetlands

100
90
80

Landfills
Oil and Gas(Burned as fuel)70
Rice
Paddies

Coal(Burned as fuel)

60
50
40

TermiteemissionsWildanimalemissions

Oceans30
Geologicalsources20

WildfiresGashydrates

100

Source: IPCC (2007)

Figure 2.2: Primary methane emissions to the atmosphere per year. Methane emissions due to human-related activities, shown to the rightof the volcano, account for approximately 70 per cent of the total emissions (Reeburgh 2007; Colwell and Ussler III 2010). Gas Hydrates
are currently estimated to contribute about 5 Tg (Tg = 1012 grammes) of methane (3.5 Tg carbon) per year to the atmosphere (IPCC, 2007).
Emission rates for gas hydrates are highly speculative, however, and may overestimate the atmospheric methane contribution by not
fully accounting for methane consumed by microbes in the sediment and water column (Fig. 2.3). Additional research will be required toconstrain the methane flux from gas hydrates that reaches the atmosphere.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

35

here on continental margin settings, which combine theappropriate pressure and temperature conditions for methane hydrate formation with regions of high sedimentation
rates and elevated “primary productivity,” which is therate at which organic carbon is produced in surface waters(Reeburgh 2007). As discussed in Volume 1, Chapter 1, therapid burial of organic carbon that “rains” to the sea floor
can promote microbial breakdown of that organic material, with methane as a key by-product. At the appropriatepressure and temperature conditions, this methane can beincorporated into methane hydrates within the sediments,
usually at water depths greater than ~500 m. Because the
combination of high primary productivity and high organiccarbon burial rate is mostly confined to continental margins(Hedges and Keil 1995; Buffett and Archer 2004), continental margins host most of the world’s gas hydrate while sediments of deep ocean basins are relatively free of methane
hydrate, even though the deep-ocean pressure and temperature conditions are suitable for gas-hydrate formation (SeeVolume 1 Chapter 1).

MethaneMethaneconsumption
consumptionin
inthe
theenvironment
environment

Zone
Zone44

Atmosphere
Atmosphere

Aerobic
Aerobicmethane
methaneoxidationoxidation
MethaneMethanechemicallychemicallyreacts
reactstotoform
formmany
many
compounds,
compounds,including
includingcarbon
carbondioxide
dioxideAerobic
Aerobicmethane
methaneoxidationoxidation
MethaneMethaneandandoxygenoxygenarearebiologicallybiologicallyand
andchemicallychemicallyconverted
convertedtotocarbon
carbondioxide
dioxide

Where
Wheremethane
methanerises
risesfromfromthetheseaseafloor
floorininplumes
plumesofof
bubbles,bubbles,muchmuchofofthe
themethanemethanedissolves
dissolvesbefore
beforereaching
reachingthe
thesurface
surface

Zone
Zone33Water
Watercolumn
columnZone
Zone22Oxygenated
Oxygenatedsediments
sediments

Aerobic
Aerobicmethane
methaneoxidationoxidation

MethaneMethaneandandoxygenoxygenarearebiologicallybiologicallyand
andchemicallychemicallyconvertedconvertedtotocarbon
carbondioxide
dioxidenear
nearthetheseaseafloor
floor

Zone
Zone11

AnoxicAnoxicsediments
sediments

Anaerobic
Anaerobicoxidationoxidationofofmethane
methaneMethaneMethanedissolved
dissolvedininpore
porewaterwaterandandsulphatesulphatearearebiologicallybiologicallyconvertedconvertedtotobicarbonate,bicarbonate,hydrogenhydrogensulphidesulphideand
andwaterwater

Gaseous
Gaseousmethane
methanecan
canbypass
bypassthethesedimentsedimentbased
basedbiofilter
biofilterbybymigrating
migratingalong
alongpermeable
permeable
paths,
paths,suchsuchasasfaults
faultsFigure 2.3: Methane consumption in the environment. Near sea-floor methane hydrate is being continuously broken down, releasing
methane dissolved in pore water. As methane moves through sediment into the water column and atmosphere, it is consumed in a varietyof chemically and microbially controlled reactions. As listed on the left, dissolved-phase methane can then be consumed by microbes
as part of an extended chemosynthetic food chain (see also Fig. 2.7) or consumed chemically. As shown on the right, gaseous methane
can bypass the microbially controlled reactions in the sediment because microorganisms can access only dissolved methane (Treude etal. 2005b; Treude and Ziebis 2010). Methane in bubbles entering the water column tends to dissolve into the water, where it can thenbe consumed by aerobic microbes. The methane “biofilter” removes much of the methane that would otherwise be transported into theatmosphere. Figure is not drawn to scale. For hydrates in the marine environment, the water depth (Zone 3) would generally be 300-500
metres or more, Zone 2 would be on the order of 1 centimetre thick, and Zone 1 would be on the order of 10 metres thick.
36 FROZEN HEAT

Thermal methane production

Organic material must be buried beneath a few thousand
metres of sediment to reach the temperatures necessary to
produce methane at significant rates. A portion of the hydrocarbons formed at depth can migrate up toward the sea
floor via faults, fractures, and high permeability sediments.
Along the way, the gases can become trapped in subsurface
structures, be incorporated into gas hydrates, or be released
via seeps at the surface. Thermogenic methane, and theassociated methane hydrates, are most common in active
petroleum areas, such as the Gulf of Mexico (Sassen et al.
2001; Boswell et al. 2012).

2.2.2 Marine methane sinks: Theconversion of methane to otherforms of carbon
Methane can be removed from the global inventory throughbiological, chemical, and physical sinks (summarized inFig. 2.3) (Reeburgh 2003). For example, in the atmosphere,
methane oxidizes to carbon dioxide in about ten years due toa photolytic process. For methane in the marine realm, theprimary methane sinks are anaerobic (without oxygen) oxidation of methane (AOM) and aerobic (with oxygen) oxidationof methane. On present-day Earth, AOM probably dominates
on a global basis (Dickens 2003; Reeburgh 2007).

Anaerobic oxidation of methane (AOM):
Microbes that consume methane without
needing oxygen

Microorganisms consume an estimated 80 to 90 per cent of
the methane that reaches shallow sub-sea floor sediments (Reeburgh 1996; Dickens 2003; Reeburgh 2007). The primary sink
for this methane is AOM (Zone 1 in Fig. 2.3), a reaction that isaccomplished by a consortium of two types of microorganisms:
methanotrophic archaea (called ANME from anaerobic methanotrophs) and sulphate-reducing bacteria (Knittel and Boetius
2009). Sulphate, which is abundant in seawater, penetrates thesediments and is consumed in the methane oxidation process.
The thickness of Zone 1 in Fig. 2.3 is related to the rate of AOM
and the upward flux of methane. This zone can be thin (< 10metres) where upward methane flux is high and thicker in areas of low methane flux (Borowski et al. 1999; Davie and Buffett 2003; Treude et al. 2003; Kastner et al. 2008).

Some methane can still escape the sediment AOM sink.
Where methane flux is very high, such as in fault zones orat mud volcanoes, sulphate cannot penetrate the sediment(Niemann et al. 2006; Joye et al. 2009). In these locations,
AOM is not an efficient benthic filter, and methane vents
directly into the water column (MacDonald et al. 2002; Liuand Flemings 2006; Solomon et al. 2008).

Aerobic oxidation of methane: Microbes that
consume methane but also need oxygen

A second sink for methane is aerobic oxidation. This process
occurs in near-sea-floor sediments that contain both oxygenand methane (Zone 2 in Fig. 2.3), consuming some of themethane that remains following AOM (Sommer et al. 2006;
Ding and Valentine 2008). Aerobic oxidation of methane isalso a dominant methane sink in the water column (Zone 3
in Fig. 2.3) (e.g. Mau et al. 2007), but the accompanying processes remain poorly understood outside a few areas where
sensitive radiotracer techniques have been applied.
Aerobic methane oxidation is believed to be carried out by
methanotrophic bacteria that use methane as their solesource of energy and as a primary source of structural carbon (Hanson and Hanson 1996). A fraction of the oxidized
methane is converted to bacterial biomass, while the remainder is released as dissolved inorganic carbon. In contrast to AOM, which has bicarbonate as its main inorganiccarbon product, aerobic oxidation of methane yields primarily carbon dioxide, which increases ocean acidity (See Text
Box 2.1). In the water column, aerobic methane oxidation
requires time and space for microbes to effectively consume
methane. As reviewed by Hu et al. (2012), aerobic oxidationis quite efficient when methane is diffusing through water
deep enough to stabilize gas hydrates (300-500 metres).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

37

Box 2.1 Could microbial methane oxidation boost acidification and oxygen
depletion in the ocean?
Methane consumption by methane-eating microbes in sediments
and in the water column is an important global mechanism thatprevents methane from reaching the atmosphere. There are twomain processes by which methane is consumed: aerobic andanaerobic methane oxidation. But do these processes generate
ecological issues of their own?
Aerobic oxidation of methane consumes both oxygen and methane
to produce carbon dioxide. Excessive oxygen consumption,
particularly in the deep ocean where it is not easily replenished,
can be detrimental to oxygen-breathing life forms. Carbon dioxide
dissolves in water to form carbonic acid, acidifying the water. In
theory, if methane vents rapidly into the water column, aerobic
oxidation of methane could cause significant local decreases inoxygen levels and increased acidity (lower pH values, see Fig. TB2.1
for an example of potential acidification effects).
There are indications in the geologic record that massive methane
releases from gas hydrates might have driven ocean acidificationin the past (Zachos et al. 2005; Pelejero et al. 2010). Model
predictions for the future (Biastoch et al. 2011) suggest thatmethane consumption could lead to pH values dropping by up to
0.25 units within the next century in some deep areas of the Arctic
Ocean. In addition, microbial consumption of methane could
decrease local bottom-water oxygen concentrations by up to 25
per cent. Regional methane-induced sea-water acidification fromthe sea floor would occur, in addition to ocean-wide acidification
caused by the uptake of anthropogenic carbon dioxide from theatmosphere. The combined effect of the two processes would
accelerate acidification in parts of the Arctic Ocean, including indeeper waters. Research has so far been based on the premise ofa projected pH decrease due to the anthropogenic carbon dioxide
uptake of about 0.3 units by the end of this century. Methaneinduced acidification could nearly double the pH decrease in partsof the Arctic Ocean (Biastoch et al. 2011).
The effects of anaerobic oxidation of methane (AOM) in sediments
are not as easily predicted. AOM consumes no oxygen and produces
bicarbonate instead of carbon dioxide (Barnes and Goldberg 1976).

38 FROZEN HEAT

However, sulphide, another end-product of AOM, might be reoxidized with oxygen by chemoautotrophic organisms (Jørgensen
and Nelson 2004) or simply through abiotic chemical reactions.
So although the microbial process itself does not directly consume
oxygen, consumption occurs during re-oxidation of sulphide at the
sediment-water interface.

Figure TB-2.1: Potential effects of ocean acidification on marineorganisms. The planktonic coccolithophore Calcidiscus leptoporus
cultured under present-day carbon dioxide conditions (pCO2 ~380
µatm, left panel) and under conditions projected for the end of thiscentury, assuming business-as-usual carbon dioxide emissions(pCO2 ~780 µatm, right panel). With increasing carbon-dioxideinduced ocean acidification, the energetic costs of calcification go
up. While some organisms are able to compensate for this, others
find it increasingly difficult to produce their carbonate shells andskeletons (courtesy Ulf Riebesell, GEOMAR, Kiel)).

Field studies show dissolved oxygen is strongly depleted in somecoastal areas and marginal seas, such as the Black Sea and Baltic
Sea, as well as at poorly ventilated and highly productive continental
margins. Observational data show the volume of these depleted
water masses has expanded significantly in recent decades(Stramma et al. 2008). Models predict the ongoing oxygen loss
at continental margins will be amplified over the coming century
in response to anthropogenic carbon dioxide emissions (Oschlies
et al. 2008). Most fauna living at the sea floor depend on oxygen

and will vanish when dissolved oxygen concentrations in ambient
bottom waters fall below a critical threshold value. The spread of
low-oxygen conditions is, therefore, a serious threat to fauna living
at the sea floor of the affected continental margin areas. Although
field studies have not yet demonstrated significant pH decreases
or oxygen depletions in the vicinity of methane seeps, it is possible
these effects may take a long time to become apparent. Baseline
studies might be needed in order to understand these slow
environmental degradation issues.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

39

2.3 A GAS HYDRATE CAPACITOR IN
THE GLOBAL CARBON CYCLE?
Methane is a dynamic component not just of the sub-sea-floor
environment, but the global environment as a whole. The roleof gas hydrate in the movement of methane through the global
carbon cycle can be visualized by characterizing the gas hydrates as a methane “capacitor” (Dickens 2001; Dickens 2003;
Dickens 2011). Like a capacitor in an electrical network, gas
hydrates can become charged with methane over time and also
discharge, releasing a significant quantity of methane.

Under steady-state conditions, methane slowly enters thegas hydrate stability zone through organic carbon degradation, methane production, and methane migration. Methane slowly leaves this volume through gas hydrate dissociation, gas hydrate dissolution, AOM, venting, and burial. If
methane inputs to gas hydrate exceed methane outputs,
gas hydrate volumes grow as long as pore water is available;
otherwise, they shrink. Gas hydrates, therefore, can act as aBox 2.2 Can gas hydrate breakdown trigger submarine slides?
The presence of gas hydrates generally strengthens the hostsediment. When gas hydrates break down into water andfree methane gas, however, what are the consequences for
sediment stability?
When gas hydrate dissociates, the released gas and water occupy
a greater volume than they do in the solid hydrate structure.
This expansion means gas hydrate dissociation in sediment canincrease pressure in the pore space (McIver 1982; Kayen and
Lee 1991; Xu and Germanovich 2006), weakening the sedimentby pushing sediment grains apart. It has been suggested that
dissociated gas hydrate could form a fluid- and gas-rich glide
plane, upon which the overlying sediment might be able to slide(see Fig. TB-2.2) (McIver 1982). The slide-triggering, gas-hydratedissociation might itself be brought on by a pressure decrease
due to an earthquake (Bugge et al. 1987), a drop in sea level(Maslin et al. 2004), or a temperature increase due to risingbottom-water temperatures (Dickens et al. 1995).
Gas hydrates have been tied to submarine slides the world
over, including the colossal Storegga slide offshore Norway(Bugge et al. 1987), along the western Atlantic Margin (Boothet al. 1993; Lee 2009), offshore Brunei (Gee et al. 2007), andon many other continental slopes around the world. While gas

40 FROZEN HEAT

hydrates may have played a role in some isolated slides (Lopez etal. 2010), definitive proof of gas hydrate dissociation substantiallycontributing to major submarine slides remains elusive, even forthe heavily-studied Storegga slide (Mienert, 2008). There are two
drawbacks to the gas-hydrate triggering mechanism theory:
1. Because sediments are generally permeable (meaning fluid canflow through them to some extent), gas hydrate dissociation
may simply push fluid and gas away from the dissociation site
without generating any significant pressure increase. Bouriak
et al. (2000) suggest that, for the Storegga slide, gas hydratedissociation would only have increased the pore pressure by 0.2
per cent, not enough to trigger a slide.
2. The distribution of gas hydrates seldom coincides with the initialslide failure location or the glide plane along which the sedimentsubsequently slides. The Storegga slide, for example, began at thetoe of the slide (Kvalstad et al. 2005). Gas hydrates were likely
to be dissociating in much shallower water, landward toward theslide headwall (Mienert et al. 2005). Moreover, the non-uniform
distribution of gas hydrates does not coincide with the slide
surface, so gas hydrate dissociation did not provide a glide plane
for the Storegga slide (Bryn et al. 2005; Kvalstad et al. 2005).
An alternative gas hydrate breakdown mechanism, in which the
topmost gas hydrates dissolve in response to sea-floor warming, hassink, sequestering methane as the hydrate forms, or act as asource, releasing methane as the hydrate breaks down.
Currently, the global methane hydrate capacitor is thought tobe relatively stable – with balanced methane inputs and emissions from natural gas-hydrate reservoirs. Local variations are
possible, with gas hydrates in some areas of the globe acting
as methane sinks, while others in different environments

may be acting as methane sources (one potential source being methane released from hydrate in submarine slides, as
discussed in Text Box 2.2). The relatively simple gas hydrate
capacitor concept is not intended to capture the complete,interconnected system of controls on gas hydrate formation,
but it does provide a useful analogy for discussing how gas
hydrate volumes change over time in response to natural environmental conditions (see Volume 1, Chapter 3).

been proposed by Sultan et al. (2004) as a method of weakening thesediment to the point where a slide, such as Storegga, would initially
fail near its toe. However, other recognized geologic phenomena areoften sufficient to explain large slides, including earthquakes in areas
where rapid sediment deposition creates vulnerable slopes (Brynet al. 2005) and over-steepening of slopes due to localized tectonic

uplift (Hornbach et al. 2007). Although gas hydrate dissociation
may not trigger major submarine slides, it might contribute to
the slide’s shape. In the Storegga slide scar, for example, the slide
headwall coincides with the location at which gas hydrates arethought to have been susceptible to environmental change about
8 200 years ago, when the slide occurred, (Mienert et al. 2005).

Schematic of a submarine slide triggeredby gas hydrate dissociation
Gas plume

Slide

Hydrate-bearing
sediment

Layer of dissociatedgas hydrate

Figure TB-2.2: Schematic of a submarine slide triggered bygas hydrate dissociation. In theory, gas pressure generated bymethane released during gas hydrate dissociation weakens thesediment and provides a glide plane for sediment failure. In
practice, gas hydrates are rarely located at sites where slides
initially fail, nor are gas hydrates generally distributed uniformly
over large enough spatial scales or in the proper orientation
to provide effective glide planes for submarine slides. (Figure
modified from McIver (1982)).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

41

2.4 LIFE AT MARINEMETHANE SEEPS
Sea floor cold seeps emit methane and sometimes other gases
into the overlying water column. Some cold seeps are associatedwith gas hydrates, while others occur at water depths too shallow
for gas hydrate to be stable. At these seeps, methane and otherfluids are transported to the sea floor through conduits createdby over pressurization, leakage of deep gas reservoirs, salt dome
accommodation, mud volcano emplacement, and tectonic processes (Judd et al. 2002; Suess 2010). Methane seeps are often
characterized by specialized life forms whose metabolism isbased on chemosynthesis (Levin 2005; Suess 2010) (see Text
Box 2.3), and these cold-seep environments are distinct fromthose associated with hydrothermal vents at mid-ocean ridges.
The presence of near-surface hydrates at a methane seep tends
to spread the methane release over a larger sea floor area,
while also increasing the amount of methane dissolved in thepore water. This dissolved methane is more easily consumed
by the chemosynthetic community than is the gaseous methane that can bypass chemosynthetic communities by venting
through focused gas channels outside the hydrate stability

zone (Treude and Ziebis 2010). Near-surface gas hydrates may
also enhance the formation of carbonate pavements in thesediment, produced by anaerobic oxidation of methane (AOM)(Bohrmann et al. 1998). These carbonates, after erosion andexposure, become secondary habitats for deep-sea organisms(e.g. Paull et al. 1984). In this chapter, we will not discriminate
between methane-seep life forms found in the presence or absence of near-surface gas hydrates, because their adaptations
and survival strategies are almost identical.
Because methane seeps associated with gas hydrates werediscovered 30 years ago (Paull et al. 1984), their investigation is still in its infancy. Our knowledge of these systems– especially those located on continental margins – is slowly
increasing with the advance of deep-sea technologies. Nevertheless, we know these ecosystems can be relatively common
features along certain continental margins and in tectonically active areas of the sea floor. Investigations of terrestrial
seep fossils (i.e., authigenic carbonates that are now exposedon land and believed to have formed along the sea floor at

Figure 2.4: Bacterial mats associated with gas hydrates. White and orange mats of sulphur-oxidizing bacteria cover sediments with near-surface
gas hydrates at Hydrate Ridge, Northeast Pacific Ocean (Courtesy of Lisa Levin, Scripps Institution of Oceanography).
42 FROZEN HEATBox 2.3 What is chemosynthesis?
The term chemosynthesis refers to metabolisms that obtainenergy to create biomass by using chemical energy throughreduction-oxidation reactions. If the carbon source used toform the biomass is inorganic (carbon dioxide), the organisms
are called chemoautotrophs. Chemoautotrophs are different
from photoautotrophs, organisms that use light energy
through photosynthesis to obtain energy for growth fromcarbon dioxide. Chemoautotrophs are bacteria, exclusively.
Animals that live symbiotically with chemoautotrophs are calledchemosymbiotic organisms. Animals (including humans) thatfeed on algae, plants, or other animals are called heterotrophs.
Famous examples of chemosymbiotic animals are the giant
tubeworms, clams, and mussels thriving on chemical energy
in the permanent darkness of hydrothermal vent systems
and methane seeps (Corliss et al. 1979; Felbeck 1981; Levin2005). These ecosystems are often described as functioning
completely independent of sunlight, especially when located in
the deep sea. At first glance, this statement appears correct,
because primary producers at the basis of these ecosystems
gain energy and carbon from inorganic compounds, even in
the absence of light. However, the chemicals needed to oxidize
energy-rich molecules such as hydrogen sulphide, are oxygenand nitrate. Both were rare in ancient oceans. Only throughphotosynthesis did oxygen accumulate and react with reduced
nitrogen compounds to form nitrate.
Hence, modern complex chemosynthetic ecosystems, especiallythose with higher organisms, are not completely independentof sunlight. They depend on compounds produced directly or
indirectly through photosynthesis. This dependence is evenmethane seeps) imply that such sea-floor ecosystems havebeen occurring for millions of years (Goedert and Benham
2003; Peckmann and Thiel 2004).

2.4.1 Overview of chemosynthetic
communities at methane seeps
Chemosynthetic communities found at methane seeps include both microorganisms and animals that depend on

stronger at seeps, where methane, and sometimes petroleum,
form the basis for chemosynthetic food chains. Both are often
fossil-transformation products of photosynthetically produced
organic matter.

Figure TB-2.3: Near the sea floor above active methane seeps,
gas hydrate can form mounds such as that pictured above in
the Gulf of Mexico. The gas hydrate mound is tinted orange
by small amounts of oil, and is partially covered by a thinsediment drape (grey material). (Courtesy of Ian MacDonald).

hydrogen sulphide, which is a by-product of AOM (Paull etal. 1984; Sibuet and Olu 1998; Levin 2005), as well as animals that directly consume methane (Childress et al. 1986;
Schmaljohann and Flugel 1987). Free-living chemosynthetic
bacteria include sulphur bacteria such as Beggiatoa, Thioploca, or Thiomargarita (Jørgensen and Nelson 2004), as wellas aerobic methane-oxidizing bacteria. Filamentous sulphur
bacteria can sometimes form extensive white or orange matson the sediment surface (Fig. 2.4).
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

43

B

A

C

D

E

F

G

Figure 2.5: Cold-seep organisms. Examples from the methane seep ecosystem. A, C, E and F are chemosymbiotic animals whose energy sourceis hydrogen sulfide produced by methane-degrading microorganisms in the sediment. A: vestimentiferan tubeworm – Lamellibrachia barhami, B:
lithodid crab embracing tube cores placed in a field of vesicomyid clams and bacterial mat C: solemyid clam – Acharax sp. D: Snail – Neptunea
amianta and their egg towers attached to rock. E: Yeti crabs – Kiwa puravida, the ‘fur’ on their claws is filamentous symbiotic bacteria which theynourish by waving in sulphide-rich fluids, and then consume F: Thyasiridae, Quepos Seep (400 m water depth), Costa Rica margin G: Alvinocarid
shrimp, Mound 12, Costa Rica margin (1 000 m water depth) (Photos courtesy of Greg Rouse and Lisa Levin, Scripps Institution of Oceanography).
44 FROZEN HEAT

Chemosymbiotic animals at methane seeps can be large orsmall, form bushes, dense beds, reefs, or live alone, and they
can grow very quickly or exceptionally slowly. Animal communities at methane seeps include single-celled organisms (protozoans) and multi-celled animals (metazoans). Most of themetazoans are invertebrates. Many are sustained, one way or
another, by microbial activity linked to methane. Common examples include vestimentiferan tubeworms (Fig. 2.5, A), crabs(Fig. 2.5 B, E), and a diversity of clams (Fig. 2.5, C, F).
All of these taxa are relatively large compared to non-seep,
deep-sea fauna. Many seep-endemic organisms have reducedor absent digestive systems. Instead, they provide homes tosymbiotic chemoautotrophic bacteria that provide the hostwith nutrition through aerobic sulphide and/or methane oxidation (Fig. 2.6).
The seeps and seep organisms support a wealth of grazing,
predatory, and deposit-feeding taxa by providing substrate forattachment, access to reduced compounds, entrainment oforganic-rich particles, and access to microbial protozoan ormetazoan prey (Carney 1994; Cordes et al. 2010). Additionally,
the carbonates (limestone is a type of carbonate) precipitatedby microbial AOM consortia form crusts, rocks, boulders,and even vast landscapes at seeps (Teichert et al. 2005). These
seeps can support high densities of mussels, tubeworms, andgrazing gastropods (Olu-Le Roy et al. 1996; Levin et al. 2010).
Because the chemosynthetic life forms described here require different chemical balances and concentrations ofmethane and sulphide (Sibuet and Olu-Le Roy 1998; 2003;
Levin 2005), distinct habitat patches form in response to thefluid chemistry and fluid flow rate (flux). Generally, sediments covered with mats of sulphur-oxidizing bacteria are associated with the strongest fluid and methane fluxes or nearsurface gas hydrates. Mussel and vesicomyid clam beds areassociated with high to moderate fluxes. Solemyid clam beds,
as well as vestimentiferan frenulate tubeworm fields, are associated with lower oscillating fluxes or deeper gas hydrates(Fig. 2.7) (Sahling et al. 2002; Sibuet and Olu-Le Roy 2003;
Levin 2005; Sommer et al. 2006). Such connections havebeen documented in several methane-seep environments(e.g. Van Dover et al. 2003; Olu-Le Roy et al. 2007; 2009).
The combination of microbial mats, the beds, bushes, andMorphology of a tube worm
hosting sulphide-oxidizing symbionts
O2
HSCO2

Plume

Vestimentum

Heart likestructure

Trophosome

DorsalvesselVentralvesselCoelomic
CavityO2HSCO2Bacteria

Figure 2.6: Symbiotic relationships for obtaining energy fromsulphide. Morphology of a tube worm (top) and photo of a clam
hosting sulphide-oxidizing symbionts (bottom, photo courtesy ofGreg Rouse, Scripps Institution of Oceanography). Tube worms
host their symbionts in the trophosome, a specialized organ.
Oxygen (O2), sulphide (HS–), and carbon dioxide (CO2) are taken
up from the surrounding water through the animal’s plume anddelivered via the blood stream to the symbionts. Clams harbour
their symbionts in their gills. Oxygen and carbon dioxide areavailable from the surrounding water, and sulphide is taken up
from the sediment through the clam’s foot.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

45

fields formed by the engineering/foundation species and themicrobially-precipitated carbonates, creates a heterogeneous,
highly patchy habitat structure that contributes significantly
to the overall biodiversity of seep ecosystems and continental
margins (Cordes et al. 2010; Vanreusel et al. 2010).
The animals present at cold seeps are rarely in direct contact
with gas hydrates. Only a single large taxon, the ice worm
Hesiocaeca methanicola (See Chapter 1, Fig. 1.2) (Desbruyeres
and Toulmond 1998; Fisher et al. 2000), has been documented to live directly in or on methane hydrates. This species
attains relatively large size (2–4 centimetres) and occurs at
high densities (2 500 to 3 000 individuals per square metre)

Calyptogena
Beggiatoa

H 2S

AOM
H2S

CH4

Acharax

AOM
CH4

Figure 2.7: Chemosynthetic habitats. Chemosynthetic habitats
generated by different fluid flow rates, including transport ofmethane, as well as the sulphide resulting from anaerobic oxidationof methane (AOM), are colonized by different fauna. Left: free-living,sulphur-oxidizing bacteria mats (e.g., Beggiatoa spp.) in sedimentswith highest fluxes. Centre: vesicomyid clams (e.g., Calyptogenaspp.) in sediments with high-to-moderate fluxes. Right: solemyid
clams (e.g., Acharax spp.) in sediments with low flux.

46 FROZEN HEAT

Aside from the Gulf of Mexico, there has been limited direct sampling of massive methane hydrates to assess metazoan associations. Exposed methane hydrate at Hydrate
Ridge does not appear to be directly colonized by metazoans (Boetius and Suess 2004), although the presence of gas
hydrates supports dense, colourful bacterial mats that canlead to high densities of infauna (animals living inside thesediment) in the near vicinity (Sahling et al. 2002; Levin etal. 2010; Vanreusel et al. 2010). The gas hydrates just below
bacterial mats at Hydrate Ridge may actually act as a barrier,blocking some of the digging clams, tubeworms, and otherspecies (Sahling et al. 2002).

2.4.2 Sensitivities of methane-seepcommunities to climate change andgeological variations

H2S

Gas hydrate

on gas hydrates in the Gulf of Mexico. Studies suggest theice worm consumes free-living microbes associated with thehydrate and that the worm’s activities, which involve forming
depressions and creating small-scale water currents at thehydrate surface, may promote microbial growth and speed
hydrate decomposition. The association of the ice worm withgas hydrates occurs both at the sediment-water interface andat least 10 centimetres below the surface.

There are indications in the geological record that warming/
cooling trends and oscillations in eustatic sea level could influence methane hydrate stability, authigenic carbonate formation, slope stability, and, in turn, the abundance of seep
habitats (Jiang et al. 2006; Archer 2007; Kiel 2009). Undersea
earthquakes, such as the Grand Banks earthquake, can also
produce methane seeps and chemosynthetic habitats (Mayer etal. 1988). It is, so far, unknown how the gas-hydrate response
to ongoing climate change (Discussed in Volume 1, Chapter3) will affect chemosynthetic communities. Dissociation could
create completely new habitats by increasing methane seepage,or rapid gas hydrate dissociation and disappearance might decrease the horizontal extent of existing seep habitats.

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CHAPTER 3

Assessment of theSensitivity and Responseof Methane Gas Hydrate
to Global Climate Change

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51

3.1 INTRODUCTION
As discussed earlier in this report, methane in the atmosphere is a potent greenhouse gas. Methane’s radiative forcing
value, a measure of how it changes Earth’s balance between
incoming and lost solar energy, is about 0.5 watts per square
metre, second only to the 1.66 watts per square metre of carbon dioxide (IPCC 2007).
The atmosphere contains about 3.7 Gt of methane carbon(IPCC 2007). This value is reasonably constant because of
a delicate balance between atmospheric methane input andremoval rates, both in the range of 0.45 Gt of methane carbon per year (IPCC 2007). Taking the midrange estimateof 5 000 Gt of carbon held as methane sequestered in gas
hydrates (see Volume 1 Chapter 1), an instantaneous releaseof just a tenth of a per cent of Earth’s gas hydrates to the

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atmosphere would more than double the IPCC’s estimatedatmospheric methane concentration.
Methane’s greenhouse potency, combined with the amountof methane stored in gas hydrates, has led researchers to suggest methane released from dissociating gas hydrates playeda significant role in past climate changes, and could be an
important factor in future climate change. This chapter presents an example of how gas hydrates have been connected topast climate change and discusses key factors in establishing
the role of gas hydrates in future climate change. Future climate change scenarios and the possible response of marine
and permafrost gas hydrate deposits are then considered.
The chapter concludes with a summarized assessment of thedeposits that are most susceptible to change.

3.2 THE ROLE OF GAS HYDRATE
IN PAST CLIMATE CHANGE
An extreme global warming event in the geological record began at the Paleocene-Eocene Boundary, about 56 million years
ago (Dunkley-Jones et al. 2010; McInerney and Wing 2011).
During this event, now called the Palaeocene-Eocene thermal
maximum (PETM), global surface temperatures, including in
the deep-sea, rose by 5 to 6 ºC over a 1 to 10 thousand year period(Dunkley-Jones et al. 2010). Potential global-scale triggers for atemperature rise include a change in ocean circulation patterns(Lunt et al. 2010), or a change in snow, ice and vegetation coverage that altered the amount of sunlight absorbed by the Earth(Adams et al. 1999). Triggers for warming at a local or regional
scale might include cometary impact (Kent et al. 2003) or largescale magma eruptions (Storey et al. 2007; Cohen et al. 2007).

Irrespective of the fate of methane, atmospheric carbon
concentrations would increase over relatively short-timescales, and contribute to the dramatic PETM warming(e.g. Dickens et al. 1997; Zeebe et al. 2009). Gas hydrate’srole during the PETM continues to be debated, however,
because there are several possible sources for massive andrapid carbon input to the ocean and atmosphere unrelated
to gas hydrates. Other suggested carbon sources during the
PETM include: oxidation or burning of peat (Kurtz et al.
2003), impact of a carbonaceous comet (Kent et al. 2003),
intrusion of volcanic sills into organic-rich sediment (Svensen et al. 2004), or carbon dioxide and methane releasefrom degrading permafrost (DeConto et al. 2010).

Triggering mechanisms themselves may not be capable ofgenerating the full global-scale temperature increase, however, so many researchers invoke a process proposed by Dickens et al. (1995) in which the warming trigger destabilizes asignificant volume of gas hydrate. This link to gas hydrate dissociation is suggested by the numerous stable isotope records
across Earth indicating the PETM warming coincided with atleast 2 000 Gt of isotopically light (13C-depleted) carbon to theocean and atmosphere (Zeebe et al. 2009; Cui et al. 2011), as
well as oxygen depletion in the oceans and widespread carbonate dissolution on the sea floor (Dickens et al. 1997). Isotopically light carbon can be an indication of biogenic methanethat has been released from dissociating hydrate. Moreover, as
discussed in Volume 1, Chapter 2, methane released into theocean can be oxidized to CO2, a process that consumes oxygenand can also cause carbonate dissolution by making the water
more acidic (see Volume 1 Chapter 2, Text Box 2.1).

For the PETM and other past warming events, a few examples being the Permian-Triassic boundary (Krull and Retallack, 2000), in the early Toarcian (Hesselbo et al. 2000;
Cohen et al. 2007), in the Cretaceous (Jenkyns and Wilson, 1999), and in the Quaternary (Hill et al. 2006), one
general conclusion is that if methane hydrate dissociation
was important, it exacerbated, but did not initially trigger,
rapid global warming (Dickens et al. 1995; Dickens 2003;
Zachos et al. 2005; Sluijs et al. 2007; Dunkley-Jones et al.
2010; Maslin et al. 2010). Another important conclusion isthat a large fraction of the methane released from the sea
floor may be oxidized in the water column, such that a primary consequence of hydrate dissociation is ocean acidification and loss of dissolved oxygen (Dickens 2003). These
past-climate studies help guide our expectations for whatrole gas hydrates might play in the future, given current
climate trends.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

53

3.3 KEY ISSUES FOR LINKINGGAS HYDRATE WITH CLIMATE CHANGE
Establishing the importance of methane from gas hydrates
in ongoing and future climate change remains a challenge.
Part of the challenge is the considerable uncertainty about
the total amount of methane involved, as well as the timing
and nature of gas hydrates’ response to future change (Kerr
2010). To establish quantifiable connections between gas hydrates, ongoing climate change, and future climate change,
four main questions must be answered:
How much warming will there be, and where? Surface temperature changes must be linked to global processes, such as
fluctuations in ocean circulation, sea level, and glacier massbalance, in order to provide information about temperaturechanges at the sea floor or in coastal permafrost regions. Predictive models of heat transfer through sediment can then be
applied to forecast the temperature evolution for the buried
gas hydrates themselves.
How much gas hydrate breakdown will result from thatwarming? Answering this question requires knowing the

54 FROZEN HEAT

spatial distribution and concentration of gas hydrates in relation to regions of enhanced warming, as well as verifying thedepth of hydrate occurrence in these locations to determine
whether a given surface temperature change will cause anyor all of the buried gas hydrates to break down.
How long will the warming take to destabilize those gas hydrates? Heat transfer down through sediment can be slow.
The process is further slowed by the heat absorbed in thawing overlying permafrost or dissociating gas hydrates themselves. There can be delays of 3 000 years or more between
ground-surface or sea-floor warming and the ensuing gas
hydrate dissociation at depth (Ruppel 2011).
How much of the methane released from the destabilized gas
hydrates will be transferred to the atmosphere? To increase atmospheric methane concentrations, methane liberated fromgas hydrates must migrate through the sediment and/or watercolumn without being consumed or dissolved. Consumption
rates are not well-constrained and vary by location.

3.4 GLOBAL CLIMATE CHANGE
PROJECTIONS
Future trends in global climate change have been subjected
to intense scientific investigation, with major efforts being
mounted by the Intergovernmental Panel on Climate Change(IPCC). Their fourth assessment report, published in 2007,

documents the growing impact of human activities on global
climate and reviews the challenges of undertaking forward
predictions (IPCC 2007).

Annual temperature increases for 2001-2005
relative to 1951-1980

Average surface temperature anomaly (ºC)-0.8 -0.4 -0.2 0.2 0.4 0.8 1.2 1.6 2.1

Source : Hansen, J., et al.
Global Temperature Changes,
Prot. Natl. Acad. Sci. 103, 2006.

Figure 3.1: Sea-surface and land-surface temperature changes. Change in ocean sea-surface temperature and temperature over land from2001 to 2005, relative to the 1951-1980 mean (Hansen et al. 2006). The Arctic is experiencing some of the Earth’s most significant warming.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

55

Predicted increase in global mean surface-air
temperatures
Global surface warming, ºC
4

Temperature change, ºC
10

3

8

2

6

20th century
A2
A1B
B1
Constant
composition

1
0
-1
-2
1900

Arctic surface air-temperature change

2000

2100

2200

4
2
0

2300
Year

Source: IPCC 2007

-2
1900

1950

2000

2050

2100
Year

Figure 3.2: Predicted increase in global mean surface-air
temperatures. Increases are relative to 1980–1999 for different
emission scenarios (IPCC 2007). The partial pressure of carbon
dioxide in the atmosphere (pCO2) is assumed to attain a value of
800 to 1 000 parts per million (ppm) at the end of this century
for scenario A2. It increases to about 700 ppm in scenario A1B
and reaches approximately 500 ppm in scenario B1. The current
pCO2 value of about 390 ppm is maintained until the end of the
century in the constant composition scenario. (See IPCC (2007)
for further information.)

Figure 3.3: Arctic surface air-temperature change. Change ismeasured relative to measurements from 1901 to 1950 (black curve).
Orange region is the 2001-2100 prediction given the A1B scenario(pCO2 increase to 700 ppm by 2100). Bars to the right indicate
the predictions for 2091–2100 for the scenarios B1 (blue bar, pCO2
increases to 500 ppm by 2100), A1B (orange bar, pCO2 increases to
700 ppm by 2100), and A2 (red bar, pCO2 increases to 800–1 000
ppm by 2100) (IPCC 2007).

According to the IPCC assessments, surface temperaturesrose significantly over the last century, with the strongest
warming signal in the Arctic (Fig. 3.1).

air temperatures may increase by up to 8 °C by the end of
this century.

Future trends in climate change have been estimated by the
IPCC for different greenhouse gas emission scenarios (Fig.
3.2). Predictions of global surface-air-temperature increases
over the next century range from 1.5 to >3 °C. As observedin the historical data, the most substantial warming is anticipated at high northern latitudes (Fig. 3.3) where surface
56 FROZEN HEAT

Methane release from dissociating gas hydrates is not included in the IPCC climate predictions, in part because themagnitude and timing of the induced emissions are poorly
constrained and therefore difficult to forecast. Gas hydratedissociation might nevertheless amplify future warming,
ocean acidification, and oxygen loss, as discussed in Volume 1, Chapter 2.

3.5 RESPONSE OF GAS HYDRATESTO CLIMATE CHANGE
3.5.1 Oceanic response to climatechangeMarine gas-hydrate deposits occur in sediments under 300-500
metres or more of water and at a significant depth beneath thesea floor. As a result, the most important climate change consideration for hydrate dissociation is the possible warming ofbottom waters. Heat conduction is the primary heat transfer
process from the atmosphere into the ground in terrestrial settings, but a number of processes can transport heat from the sea
surface into the ocean’s interior. These include vertical mixing,
convection of water masses and changes in ocean circulation.
First-order predicted trends in bottom-water temperatures overthe next 100 years are shown in Figure 3.4. Bottom-water temperatures could increase by up to 2 °C in shallow water alongcontinental margins by the end of this century, but significantly
smaller temperature changes are predicted for deep-sea settings. However, new result show that even during cold stadials,
persistent intermediate water warming existed (Ezat et al., 2014)
making future scenarios more difficult to predict. Gas hydrates
occurring at shallow burial depths or as outcrops around thecontinental margins could experience significant warming overthe coming decades and centuries. The largest bottom-water
warming is predicted for the Arctic Ocean, where large areasof sea floor are affected by changes in the relatively warm Atlantic waters flowing into the European Nordic seas and the Arctic
Ocean (Biastoch et al. 2011). In some Arctic locations, shallow
bottom waters may warm by up to 5 °C by 2100 (Fig. 3.5).
The increase in bottom-water temperatures is slowed by the highheat capacity of seawater and by slow communication between
surface waters and the deep ocean. Atmospheric temperature increases will however, over the coming centuries and millennia,
raise bottom-water temperatures. The long-term effect of global
warming on sea-floor temperatures has been evaluated by Fyke

and Weaver (2006). According to their model, the bottom-watertemperature at continental margins will eventually increase by
about 4 °C, and as reported by Ruppel (2011), approximately 3.5
per cent of world’s gas hydrate could be dissociated over the next
century due to bottom-water warming (see Section 3.6).
In addition to changes in ocean temperature, the global sea
level will rise in response to global warming. Sea level rise in-

FutureFuture changechange inin bottom-waterbottom-watertemperaturesatthetemperatures at the sea
sea floor
floor

Temperature change °C
Temperature change °C
-2
-2

-1
-1

0
0

1
1

2
2

Source: adapted from Park et al. 2009
Source: adapted from Park et al. 2009

Figure 3.4: Future change in bottom-water temperatures at thesea floor. Changes are given in °C per 100 years as predictedby the Kiel Climate Model (KCM) (Park et al. 2009), for a pCO2
increase scenario (1 per cent increase until current-day values
are doubled). Values are an ensemble average of eight individualmodel realizations starting at different initial states.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

57

Arctic bottom water temperature

Trend in Arctic bottom water temperature

LaptevSea

Laptev
Sea

Arctic Ocean

Arctic Ocean

European
Nordic Sea

European
Nordic Sea

Mean bottom water temperature (1985-2004), °C
-2

-1

0

1

2

400 metres isobath

Source: redrawn from Biastoch, A., et al., Rising Arctic Ocean
temperatures cause gas hydrate destabilization and ocean acidification

Mean trend per 100 years, °C
-2

-1

0

1

2

3

4

5

400 metres isobath

Source: redrawn from Biastoch, A., et al., Rising Arctic Ocean

Figure 3.5: Current values and future changes in Arctic bottom-water temperatures. Left: Map of the modern bottom-water temperatures inan ocean model at 1/2° resolution (1985-2004). Right: Trend in bottom-water warming under elevated pCO2 as predicted by the Kiel Climate
Model (KCM) (in °C per 100 years). The contour line depicts the 400-metre water-depth contour (From Biastoch et al. (2011)).

duces a pressure increase at the sea floor and may help to stabilize marine gas hydrates. However, IPCC projections of eustatic sea-level rise are generally less than two metres by 2100
and not expected to significantly enhance the stability of gas
hydrates, which are more sensitive to temperature than pressure (Ruppel 2000, 2011; Reagan and Moridis 2008). For
example, modelling by Tishchenko et al. (2005) shows how
the complete breakdown of the Greenland ice sheet, and the58 FROZEN HEAT

seven-metre sea level rise it would cause, would only protect
gas hydrates from a ~0.2 °C temperature increase. In fact, as
discussed in Section 3.5.4, sea level rise might actually haveaccelerated gas hydrate dissociation along Arctic shelves bysubmerging and warming the sediment. Other changes in
the ocean regime, such as sea-ice cover in the Arctic, wave
and current regime, or hydrology, are also not expected tohave a great influence on gas hydrate stability.

3.5.2 Response of marine gas
hydrates to sea-floor warming
In deep waters, the upper sediment layers remain within thegas hydrate stability zone upon bottom water warming (Fig.
3.6, left panel). Heat penetrating into the sediment from above
may induce gas hydrate dissociation at the base of the GHSZ,
where gas hydrates are most sensitive to change. However,heat is transferred slowly into marine sediments via conduction through the sediment and pore-water matrix, because
bottom waters cannot penetrate into the sea floor. As such,
hydrate dissociation along much of the middle to lower conti-

nental slope can only occur after a prolonged warming periodof several thousand years (Xu et al. 2001). Moreover, the liberated methane would have a tendency to migrate upwards toshallow sediment, where colder temperatures should induce
gas hydrate formation. Depending on the magnitude of warming and time, considerable amounts of methane would remaintrapped within sediments with a rise in bottom water temperature (Dickens, 2001). Thus, as reported by a number ofauthors, fluxes of methane from gas hydrate regions in deepwater settings are likely quite negligible to warming of bottom
waters over the coming centuries (Reagan and Moridis 2007;
Garg et al. 2008; Reagan and Moridis 2008; Ruppel 2011).

Deep-water system: penetration of heat into
marine sediments beneath 1000 metres of water

Shallow-water system: penetration of heat into
marine sediments beneath 320 metres of water

Sediment depth, metres
0
3
0

6

9

Temperature, ºC
12
15

Phase
boundary

50

Sediment depth, metres
0
0

20

150

30

300

Temperature, ºC
3Phaseboundary

40

200
250

2

Hydratedissociation

10

100

1

t = 0 yrt = 1000 yr
t = 3000 yr
t = 5000 yr

50

Hydratedissociation60

t = 0 yrt = 20 yr
t = 50 yr
t = 100 yr

Figure 3.6: Penetration of heat into marine sediments. The left panel shows the increase in temperature with depth for sediments,
located at 1 000 metres water depth, that are exposed to linear bottom-water warming of 1 °C per 1 000 years. Only the deepest gas
hydrates dissociate, and only after a significant time delay. Methane released in this fashion is likely to migrate to slightly shallower
depths and reform gas hydrate. The right panel depicts the response of permafrost-free Arctic upper-slope sediments at 320 metreswater depth to a linear increase in bottom-water temperature of 3 °C per 100 years. In contrast to the deep marine setting, gas hydrates
in shallow settings can be destabilized more rapidly, and dissociation occurs at the upper hydrate surface, facilitating the methanetransport away from the hydrate. Temperature profiles were calculated by applying a thermal conductivity of 1.2 W m–1 K–1 and a
volumetric thermal capacity of 5.1 J cm–3 K–1. The initial geothermal gradient was implemented as 40 °C km–1. The phase boundary(dotted line) is calculated for methane-gas hydrates in sulphate-depleted pore water with salt content of 35 g per kg of pore water(Tishchenko et al. 2005). Methane-gas hydrate is only stable when ambient sediment temperatures are lower than the temperaturedefined by the phase boundary. Gas hydrates dissociate when ambient temperatures exceed the phase boundary value. Heat absorbed
during gas hydrate dissociation is not considered here.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES59

A very different scenario is possible in the shallower settings
typical of the upper continental slope. Here, even modest
warming can destabilize gas hydrates (Fig. 3.6, right panel).
Hydrate dissociation would start at the top of the deposit, andthe entire gas hydrate inventory in this setting could theoretically be transformed into water and methane. If the methane release rates were high enough, methane could escapethe sediment’s methane biofilter and be released into thewater column (See also Volume 1, Chapter 2). Gas hydrate
outcroppings at the upper-slope sea floor might react instantaneously to sea-floor warming, while gas hydrates situatedat greater water and sediment depth would dissociate only
after a prolonged heating period of one hundred to several
hundred years (Reagan and Moridis 2007; Garg et al. 2008;
Reagan and Moridis 2008; Ruppel 2011).
The effect on gas hydrate stability of the predicted warmingof the Arctic sea floor was estimated by Biastoch et al. (2011).
According to their model, the GHSZ thickness will be significantly reduced at several continental-slope areas due to global
warming (Figs. 3.5 and 3.7). The authors proposed that about1014 Gt of methane carbon might be released from dissociating
gas hydrates deposited in Arctic slopes at greater than 60 °N
over the next 100 years, considering the slow penetration ofheat into the sediments (Fig. 3.6, right panel). Their gas hydrate concentration estimates are based on the work of Klauda
and Sandler (2005), which are at the high end of the publishedestimates. If released completely to the atmosphere, even thisupper-estimate methane release would be too small to significantly enhance global warming in a 100-year time span. Nonetheless, this quantity of methane has the ability to enhanceocean acidification and oxygen depletion along the continental
slope (see Volume 1 Chapter 2, Text Box 2.1). It should also
be noted that the estimated amount of methane likely to bereleased remains uncertain, since the methane release rate depends on the largely unconstrained distribution and inventoryof methane gas hydrates in shallow Arctic slope sediments.

3.5.3 Field evidence for ongoing
marine gas-hydrate dissociation
Methane release as free-gas venting at the sediment-waterinterface is observed in many deep-water environments
around the world. Some of these active gas seeps are from
60 FROZEN HEAT

Changes in thickness of the GHSZ
caused by temperature increase

Laptev
Sea
Arctic Ocean

European
Nordic Sea

Metres
-30

-20

-10

0

10

20

30

400 metres isobath
Source: redrawnfrom Biastoch, A., et al., Rising Arctic Ocean
temperatures cause gas hydrate destabilization and ocean acidification

environments where pressure and temperature settings are
conducive to gas hydrate formation (Ginsburg et al. 1993;
MacDonald et al. 1994; Suess et al. 1999; Van Dover et al.
2003; Tomaru et al. 2007). In many cases, it appears thisphenomenon is not related to gas hydrate dissociation, but
is the result of complex porous-media processes that allow
some free gas to pass through the gas hydrate stability zone
without forming gas hydrates (Liu and Flemings 2006). Potential links between climate change and sea floor methane
release due to dissociating marine gas hydrates have beenfound along the shallow-water-limit of hydrate stability along
the upper continental slope, however (Westbrook et al. 2009;
Mienert et al. 2010; Berndt et al. 2014; see also Text Box 3.1).

Effect of arctic bottom-water warming on gas-hydrate stability
Volume change in the GHSZ
Thousands cubic kilometres-0

Percentage ofsteady-state solution
0

-4

-10%

300
400

-8

-20%

-12
-30%
-16

500
600

Gas

700
-40%

-20
-24

Depth, metres
200

0

100

200

300

400

-50%
500

Years

800
900

Hydrate
0

1

2

3

4

5
6
7
Temperature, ºC

8

9

10

Figure 3.7: Effect of arctic bottom-water warming on gas hydrate stability. Left: Changes in the thickness of the GHSZ caused by thebottom-water temperature increase depicted in Figure 3.5. Above left: Volumetric GHSZ thickness changes north of 60°N as a function oftime, given in absolute numbers (left axis) and as percentage of the steady-state solution, neglecting the transient heat intrusion into thesediment (right axis). Above right: Phase diagram of methane-gas hydrates as a function of pressure and temperature (constant salinity ofS = 35 p.s.u.). Orange symbols mark the current bottom-water temperatures along the European Nordic Sea (circles) and Russian slope
along the Laptev and East Siberian Seas (squares), black symbols mark the predicted bottom-water temperatures in 100 years. Vertical bars
indicate the vertical resolution of the ocean model (From Biastoch et al. (2011)).

3.5.4 Response of permafrost gas
hydrate to climate warming
In the Arctic, where thick occurrences of permafrost are foundat depth, temperature and pressure conditions in the subsurface
create a significant interval where gas hydrates can be stabl­e inand beneath the permafrost (Dallimore and Collett 1995). Permafrost gas hydrates have been described in terrestrial areas
where permafrost is more than 250 metres thick in Siberia,
Arctic Canada, and northern Alaska. Permafrost gas hydrates
are also likely to exist in shallow-shelf settings, associated withrelict permafrost that formed while these areas were exposed as
dry land by low sea levels during Pleistocene ice ages.

As in permafrost-free sediments (Fig 3.6), heat must first
diffuse down into the sediment before gas hydrates canbe warmed and destabilized. The presence of permafrost
slows this heat transfer (Fig. 3.8). Heat from the sedimentsurface is consumed over millennial time scales to warmand eventually thaw the permafrost (Lachenbruch and Marshall 1986; Taylor et al. 1996a,b; Majorowicz et al. 2004;
Taylor et al. 2006; Ruppel 2011). Forward modelling, withthe effects of possible warming over the next century taken
into consideration, shows only negligible changes in terrestrial Arctic gas hydrate stability conditions (Taylor et al.
2006; Ruppel 2011).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

61

Box 3.1 Methane bubble plumes from the sea floor off West Svalbard
Over 200 methane gas plumes have been observed comingfrom the seabed offshore West Svalbard (Fig. TB-3.1.1). There
are plumes on the continental shelf in relatively shallow water
near the shelf edge, on the continental slope in water shallower
than the ~400 m water depth required for methane hydrates tobe stable, and a few plumes have even been observed in deeper
water (Westbrook et al. 2009; Sarkar et al. 2012; Rajan et al. 2012).
From a climate-change perspective, it is of interest to know if 1)
methane in the plumes reaches the atmosphere to directly increase
atmospheric greenhouse gas levels, and 2) if recent climate change
is responsible for creating the methane plumes. It does not appear
methane in these plumes is reaching the atmosphere (Fisher et
al. 2011), but there is ongoing debate about whether the plumes
are caused by gas hydrates that are dissociating in response tobottom-water warming over the past ~30 years (Thatcher et al.
2013; Sarkar et al. 2012; Rajan et al. 2012; Berndt et al. 2014).

Much of the methane feeding into this region appears to be
migrating toward the sea floor from below the gas hydrate stability
zone, further down the continental slope than where most of themethane flares are found (Hustoft et al. 2009, Thatcher et al. 2013;
Sarkar et al. 2012; Rajan et al. 2012). As methane migrates upward
through sediment, it can occasionally find flow conduits throughwhich it can escape vertically through the gas hydrate stability zone
(GHSZ) (Liu and Flemings 2006), but in this region that type offlare is quite rare (Sarkar et al. 2012).
More generally, methane either becomes incorporated into the gas
hydrate or migrates up the continental slope through permeable
sediment layers and conduits, some of which reach all the way tothe shelf (Thatcher et al. 2013; Sarkar et al. 2012; Rajan et al. 2012)(Fig. TB-3.1.2). Of particular interest, however, are the plumes locatedjust upslope from the current limit of gas hydrate stability (Fig. TB-

Figure TB-3.1.1: Methane-richplumes in the water column on theWest Svalbard continental margin.
A: Location of survey area west of
Svalbard; bathymetry (Jakobsson etal. 2008). B: Positions of acousticallyimaged plumes are depictedby “pins” superimposed on aperspective view of the bathymetry ofpart of the area of plume occurrence.
The 396-metre isobath contour is the
expected landward limit of the GHSZ.
C: Part of the record from an acoustic
survey showing examples of observed
plumes. All plumes show a deflection
towards the north caused by the West
Svalbard Current. The sea floor, ataround 240-metre depth, is shown by
the strong (red) response (adaptedfrom Westbrook et al. 2009).
62 FROZEN HEAT

Sedimentary layers and gas migration pathways
for the continental margin and slope offshore Svalbard
Depth (m)0100

Old top of GHSZ

200

GHSZ

Bubble plumes
Base of former GHSZ

Old baseof GHSZ

300

Hemipelagic interbeds

Fractures

400
500
600

Gas released bydissociating
hydrateBase of

700
800

GHSZ

Gas

Predominantly
glacigenic sediment

Predominantly hemipelagicand other marine sediment

Gas

Gas pocket

0

1

2

3

4

5 Km

900
1000

WEST

EAST

Figure TB-3.1.2: Sedimentary layers and gas migration pathways for the continental margin and slope offshore Svalbard. In this conceptualmodel, gas cannot easily reach the sediment surface of the continental slope without being transformed to gas hydrates or diverted
upslope by impermeable hydrate-bearing sediment or glacial debris flows. Instead, gas migrates up through faulted sediment and upslope
through permeable layers before reaching the sediment surface in the gas-flare region near the top of the continental slope (adapted fromThatcher et al. 2013).
3.1.2, inset). It has been postulated that hydrate had been stable inshallower waters, but a 1°C bottom water temperature increase overthe past ~30 years caused that hydrate to begin dissociating andemitting methane from the sea floor (Thatcher and Westbrook 2011;
Sarkar et al. 2012). Marin-Moreno et al. (2013) have extended thisidea to predict the regional methane release over the next 300 years.
They use two different climate models to estimate the distribution of
hydrates in the region, and assuming hydrate dissociation is driven
by long-term temperature increases, they estimate anywhere from~1 – 25 TgC (0.001 – 0.025 GtC) could be released per year from thesection of sea floor between 400 – 550 metres water depth along theEurasian Margin over the next 300 years. Recent observations from
the MASOX autonomous observatory, however, suggest the methane
plumes may be thousands of years old, having already begun hosting

biologic communities that have formed carbonate deposits. Rather
than resulting from modern warming trends, the plumes may instead
come from methane hydrates that form and dissociate in response
to seasonal temperature changes of the bottom water (Berndt et al.
2014). In spite of the many observed plumes, the methane released
from the sea floor contribute a negligible amount of methane to theatmosphere (Fisher et al. 2011), but will instead likely contribute toacidification and oxygen depletion in the ocean.
This region remains an active study area as researchers continue
to investigate the origins and fate of methane in this location. Our
understanding of this system will evolve rapidly over the next few
years as results are released from ongoing studies, as well as fromseveral new research cruises.
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

63

The situation is more complex on Arctic shelves, which werefrozen as permafrost until being flooded during the sea-level
rise that started 7 000 to 15 000 years ago (Shakhova et al.
2010b). Flooding warmed the ground surface to above freezing and began thawing the permafrost. An example of the geothermal response of permafrost and the gas hydrate stability
zone is illustrated in Figure 3.8 for sites on the Beaufort Shelf.

Penetration of heat into
permafrost-bearing sediment
Depth from seabed, metres
0

Permafrost thawing by salt diffusion
Gas hydrate dissociation

8 kaBpMethane hydrate
stablity curve

13.5 kaBp
500

Present time
Permafrost thawing

1000
Gas hydrate dissociation
1500
-15

-10

-5

0

5

10

15

20

Temperature (°C)
Source: Figure courtesy of A. Taylor, Geological Survey of Canada

Figure 3.8: Penetration of heat into permafrost-bearing sedimentthat has been flooded by sea water. Thawing permafrost acts asa thermal buffer, slowing the diffusion of heat into sediment.
Once dissociated, however, gas released at the top of the hydratestability zone can migrate through the sediment without reentering the gas hydrate stability zone. This case is similar to theshallow marine case illustrated in Figure 3.6. Gas liberated from
dissociation at the base of gas hydrate stability will likely reformas gas hydrate as it migrates up through the gas hydrate stability
zone (Figure courtesy of A. Taylor, Geological Survey of Canada).
64 FROZEN HEATThe shift in mean annual sediment-surface temperaturesfrom around –15 °C to near 0 °C, induced by the marine transgression, is far more significant than current air-temperature
increases, and the shelf warming has been going on for approximately 13 500 years. This marine transgression has an
on-shore analogy: the emplacement of a thermokarst lake ona terrestrial landscape. As with the shelf transgression, a lake
can more quickly transport heat to the depths of gas hydratestability than could occur in terrestrial systems subjected only
to atmospheric warming at the ground surface.
Although sub-sea permafrost destabilizes on time scales of 5000 to 10 000 years (Shakhova et al. 2010a), the roughly 4 °C
warming predicted for the Arctic (Fig. 3.5) has the potential
to perturb or accelerate processes that have been going on
for millennia. Without a permafrost cap, underlying methane – either from gas hydrates or other sources – can moreeasily escape through degrading permafrost to the sedimentsurface (Shakhova et al. 2010a; Brothers et al. 2012, Portnov
et al. 2013). Moreover, as illustrated in Figure 3.8, gas hydratedissociation in these flooded permafrost environments canoccur at the top of the GHSZ, as in the upper-continentalslope case (Section 3.5.2). Methane released at the top the
GHSZ will not reform as gas hydrates while migrating to
the sediment surface, increasing the likelihood of methane
reaching the ocean/atmosphere system and contributing toclimate warming.

3.5.5 Field evidence for ongoing
dissociation of permafrost gas
hydrateDirect evidence for the release of methane from dissociating gas hydrates associated with relict subsea permafrost or
terrestrial permafrost is lacking, but Paull et al. (2007, 2011)
have suggested that some features associated with gas releaseon the Beaufort shelf may be related to gas hydrate dissociation initiated by marine transgression. Pingo-like features(PLFs) are one example. Based on shallow geologic studies,geothermal modelling, and the geochemistry of sedimentpore waters/gases, it has been proposed that PLFs on the
Canadian Beaufort Shelf may be formed by sediment, water,and gas movement from depth, resulting from permafrost
gas hydrate dissociation, as shown in Figure 3.9.

On the U.S. Beaufort shelf/slope area, recent measurementssuggest there is no measurable difference in the surface water’s methane content across the zone where methane hydrate may currently be dissociating, meaning that while thesurface waters are methane rich, widespread gas bubbling in
the water column that can be attributed to hydrate dissociation is not observed (Pohlman et al. 2012).

The Laptev Sea and the surrounding Siberian shelf areas arealso quite rich in methane (Shakhova et al. 2010b) and bubble plumes have been observed, but there are many methane
sources in that system and it is not yet known the extent towhich dissociating hydrates are contributing to the observed
methane concentrations (Text Box 3.2).

Evolution of a pingo-like feature (PLF)
Vent gas
Pingo-like
FeatureMoat

Gas hydrate
bearing sediments

Gas hydrate
bearing sediments

Figure 3.9: Evolution of a pingo-like feature (PLF). As the subsurface warms, the top of the gas hydrate stability zone moves downward(yellow arrows in the left panel). Warming results in gas hydrate dissociation in a gradually thickening zone (brown), releasing gaseous
methane into the sediments (yellow bubbles). Bubble formation associated with this phase change creates overpressured conditions. The
right-hand panel shows how material may flow (red arrows) both laterally and vertically in response to overpressure. Displaced sediments
rise upward to form the PLF and allow the gas to vent. As the pressure is dissipated through both the transfer of solids and degassing,subsidence in the area immediately surrounding the PLF (black arrows) creates the moat.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

65

Box 3.2 Methane release along the East Siberian Arctic Shelf
In the East Siberian Arctic Shelf (ESAS, Fig. TB-3.2.1), methane
concentrations in the surface waters exceed typical seawater
values and far exceed the atmosphere’s equilibrium methane
concentrations (Shakhova et al. 2010a). Shakhova et al. (2010b)
suggest methane is released into the atmosphere at an annual
rate of about 8 Tg of carbon (.008 GtC) for the ESAS alone,comparable to the rate expected for the rest of Earth’s oceans
combined. The present-day ESAS methane release rate is nearly 2
per cent of the nearly 450 Tg of methane carbon (.45 GtC) annually
released to the atmosphere from all sources globally (IPCC, 2007). A
current research challenge is to predict whether methane release rates
will increase significantly in response to ongoing climate change.

East Siberian Arctic Shelf

East Siberian Arctic Shelf
NORTH POLEM e n d

so

R.

v

Ye

rm

ak

G
P.

ak

ke

Rid

l

Ri

dg

eleev

Ridge

ChukchiSeagee

EAST SIBERIANARCTIC SHELF

Arctic Ocean

Depth, metres
0100

East Siberian
Sea

Barents Sea
Kara Sea

1000
2000
3000
5000

Source: adapted from NOAA

Figure TB-3.2.1: East Siberian Arctic Shelf (ESAS). The ESAS makes up a quarter of the Arctic shelf area (Shakhova et al. 2010a), with anaverage depth of only 58 metres (Jakobsson 2002) and significant riverine input.

66 FROZEN HEAT

Two key questions are:• What are the methane sources?
• How effective are the methane sinks that consume methane
before it reaches the atmosphere?
Knowing the sources can reveal whether the system has beensteadily releasing methane at these rates in response to long-termclimate change and/or whether the methane release rates are likely
to accelerate as the system responds to short-term warming.
One possible source is methane brought in by the six largest
Eurasian rivers, although Shakhova et al. (2010a) suggest mostof the riverine methane is oxidized in the rivers prior to reaching
the ESAS. Given the geologic history of the ESAS, it is more likelythat methane is coming out of the ESAS sediment (Fig. TB-3.2.2).
The sediment drape on the ESAS is organic-rich (Vetrov andRomankevich 2004; Shakhova et al. 2010a). The upper layers werefrozen as permafrost until increasing sea levels, starting 7 000
to 15 000 years ago, flooded the region (Shakhova et al. 2010b)
and raised the ground-surface temperature above freezing. The
permafrost has been thawing ever since as heat and salt fromoverlying sea water penetrate deeper into the sediment. Shakhova
et al. (2010a) summarize four methane sources in this thawing,organic-rich system:
1. Methane can be produced via microbial breakdown of organic
material in the shallow, modern ESAS sediment, which wasnever frozen.
2. As permafrost thaws, the newly unfrozen, older organic material
also becomes available for microbes to consume, producing
methane as a by-product of that consumption.
3. Gas hydrates, thought to exist across a significant portion of the
ESAS (Soloviev 2002; Shakhova et al. 2010a), may be dissociating
and releasing methane in response to heat transferred downfrom the sea floor.
4. Methane may be leaking up through the thinning or thawed
permafrost from a deeper petroleum system.

The present-day methane release from ESAS sediments isthought to be occurring in response to long-term sedimentwarming resulting from seawater flooding the ESAS region(Shakhova et al. 2010a), rather than to recent atmosphericwarming trends. However, it is not yet certain which sources
contribute to the observed seawater-methane concentrations.
Methane consumption efficiency, the combined removal ofmethane due to dissolution and to microbial processes in thesoil and water column (see Volume 1 Chapter 2), is also not wellconstrained in the ESAS region. Quantifying methane sources
and sinks remains a requirement for establishing the long-termclimatic impact of methane released to the atmosphere.

Figure TB-3.2.2: Methane plumes in the East Siberian Arctic
Shelf (ESAS). The extremely shallow ESAS environment allows
gas-bubble plumes to reach the water surface, facilitating thetransfer of methane from the sediment to the atmosphere (Image
courtesy of I. Semiletov, unpublished data from cruise-2011).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

67

3.6 REVIEW OF SENSITIVITY OF
GLOBAL GAS HYDRATE INVENTORY
TO CLIMATE CHANGE
How long a warming event takes to affect gas hydrate stability and whether methane released from gas hydrates istransported to the atmosphere depend, to a large extent, on
where gas hydrates are located. Based on published distribution estimates, Ruppel (2011) summarizes five generalcategories or “zones” of gas hydrate occurrence and theextent to which predicted climate change would result inthe transport of methane from gas hydrates to the atmosphere (Figure 3.10). Following the work of Ruppel (2011),
estimates of gas hydrate sensitivity to warming are given
as percentages of the global gas hydrate store, assuming
99 per cent of the world’s gas hydrates are located in deepwater marine environments (zones 3-5 in Figure 3.10), and1 per cent are associated with permafrost, either on land or
submerged in shallow Arctic shelf regions (zones 1 and 2 inFigure 3.10) (McIver 1981). The percentages given by Ruppel (2011) depend on whether future studies uphold the assumed balance between marine and permafrost-associatedgas hydrate volumes. For a sense of scale, even 1 per centof the estimated global supply of methane in gas hydrates(5 000 GtC) is equivalent to 25 times the estimated global
consumption of methane in 2020 (2.15 GtC), based on consumption estimates from the (U.S. Energy Information Administration, 2010).

1: Terrestrial Arctic environments

Less than 1 per cent of the world’s gas hydrates are likely toexist in this environment (Zone 1 in Fig. 3.10). Because thepresence of permafrost dramatically slows the transfer ofheat to the depths at which gas hydrates exist, time scales inexcess of 1 000 years are necessary for atmospheric warming to begin dissociating gas hydrates at the top of the gas
hydrate stability zone (Ruppel 2011). On an extremely localized scale, thermokarst lakes may provide a conduit for more
rapid delivery of heat into the subsurface to dissociate gas
68 FROZEN HEAT

hydrates. Gas-venting pockmark features beneath delta lakes
and channels at the edge of the Mackenzie Delta have been
attributed to gas hydrate dissociation (Bowen et al. 2008). As
noted in Ruppel (2011), however, methane seeps in terrestrial
Arctic environments may be carrying methane from deeperhydrocarbon reservoirs, rather than from gas hydrates breaking down due to warming. Identifying the methane source inthis sector is an important research focus.

2: Flooded permafrost environments (<100metres water depth)

Given the assumption that 1 per cent of the world’s gas hydrates exist in polar regions, and much of that 1 per centexists below terrestrial permafrost, Ruppel (2011) estimatesless than 0.25 per cent of the global gas hydrate volume isfound in flooded permafrost regions (Zone 2 in Fig. 3.10).
Gas hydrates in Zone 2 are also buried beneath about 200
metres of sediment, and it is not likely that human-activity-related warming trends are affecting them significantly.
However, this sector has experienced significant warming,
because coastal flooding that occurred about 13 500 years ago
generated up to 17 °C of warming (Shakhova et al. 2010b) at
the sediment surface. This warming continues to thaw anddegrade both permafrost and underlying gas hydrates (Semiletov et al. 2004). In these shallow environments, methanegas released from the sea floor can pass through the watercolumn and enter the atmosphere (McGinnis et al. 2006).
This sector is a likely location for gas hydrates to impact theatmospheric methane concentration over the next few hundred years. However, identifying how much of the methane
release is caused by anthropogenic warming of gas hydrates
requires first distinguishing between methane produced bygas hydrate dissociation and methane from other sources,
such as organic matter decay or migration from deeper
methane sources.

1- Thick onshore
permafrost
Thermokarst
lake

2- Shallow
Arctic shelf

3- Upper edgeof stability

ACTIVE LAYER:
aerobic oxidation

4- Deepwater

5- Sea floor
mound/seep

ATMOSPHERE:
Photolytic CH4 oxidationin around 10 years

Permafrost
Gas hydrate stability zone

Upper edgeof stability

500m

OCEAN: aerobic (microbial)
CH4 oxidation

Methane population sources:

NEAR SEA FLOORSEDIMENTS:
Anaerobic
CH4 oxidation

Young microbial gas
Old microbial gas
Thermogenic gas

Gas hydrate Seepstability zone

Released from gas hydrates (microbial or thermogenic)

Figure 3.10: Pathways for methane release from sediment. This schematic cross-section of a high-latitude ocean margin, broken into five
distinct zones, contains four potential methane sources. Methane is released from sediment along much of the cross-section, but over the
next few hundred years, methane released from gas hydrates, in particular, is likely to reach the atmosphere only from Zone 2, the shallow
Arctic shelf (From Ruppel (2011)).

3: Upper continental slope environments(about 300-500 metres water depth)

Ruppel (2011) calculates that about 3.5 per cent of the world’s
gas hydrates exist in this environment (Zone 3 in Fig. 3.10).
At this feather-edge of stability, bottom-water warming could
destabilize the entire thickness of gas hydrates in the shallow
subsurface. Reagan and Moridis (2007) estimate gas hydrates
in these shallow systems extending nearly 50 metres into thesediment could dissociate within 100 years. If gas hydrates
have not already begun dissociating along the Arctic slope,
the process could begin within the next century (Biastoch et al.

2011, Marin-Moreno et al. 2013) and progress rapidly to lower
latitudes (Reagan et al. 2011). However, as noted by Ruppel(2011), methane gas released from sediments at these water
depths would likely be oxidized prior to reaching the atmosphere (see also Text Box 3.1 for a discussion of evidence ofclimate-related hydrate dissociation in this zone).

4: Deepwater marine environments (greaterthan 500 metres water depth)

These gas hydrate deposits (Zone 4 of Fig. 3.10) probably account for about 95 per cent of all gas hydrates on Earth. In spite
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

69

of their abundance, they would contribute little to no methane to the ocean, even over a 3 000-year period after imposing a 1.25°C bottom-water temperature increase over present
conditions (Ruppel 2011). A heat pulse entering the sediment
would require millennia to reach the vulnerable gas hydrates
at the base of the gas hydrate stability zone. In addition, themethane would likely remain trapped below the GHSZ or beconverted back into gas hydrate as it migrated up through thesediment. An exception to this recycling model could occur
if over pressuring associated with methane gas release generated highly permeable pathways that facilitated the transit of
the gas through the overlying gas hydrate stability zone. Oncereleased from the sea floor at these depths, methane would
likely be consumed in the water column prior to reaching theatmosphere (McGinnis et al. 2006). However, as discussed in
Chapter 2, bubbles released at these depths could form hydrate
shells that would limit the rate at which methane in the bubble
dissolved and allow methane to reach shallower depths.

70 FROZEN HEAT

5. Gas hydrate mounds on the sea floor

In the presence of seeps, gas hydrate mounds can occur inZones 2 to 5 (see Fig. 3.10). Whether methane from mounds
and seeps is being transferred to the atmosphere is a current topic of debate (Solomon et al. 2009; Hu et al. 2012).
The direct exposure of gas hydrate mounds to sea watermeans they are constantly dissolving, and their breakdown
increases with increasing temperature. As with the uppercontinental-slope gas hydrates, methane released frommounds will be subject to dissolution and oxidation in thewater column. If gas hydrate mounds break apart or dislodge from the sediment surface, however, the gas hydrate
can rise through the water column and allow methane toreach the mixed layer near the sea surface and enter theatmosphere (Brewer et al. 2002; Paull et al. 2003). As notedby Ruppel (2011), however, mounds represent only a trace
fraction of the global gas hydrate reservoir.

3.7 CONCLUSIONS
Even in response to present-day warming rates, the vast majority of the world’s gas hydrates will not contribute methane
to the atmosphere over the next century (Archer, 2007). The
roughly 3.5 per cent of the world’s gas hydrates existing at thefeather-edge of stability on the upper continental slope could
break down over the next 100 years, but the released methane
would likely be consumed in the sediment or water column
before entering the atmosphere. Only the approximately 0.25
per cent of the world’s gas hydrates located in flooded permafrost environments (Ruppel, 2011), which have been subjected
to warming over the past 7 000 to 15 000 years, are likely to release, or are releasing, methane that can reach the atmosphere.

drates could transfer directly to the atmosphere. Since
water-column methane oxidation consumes oxygen as wellas methane, quantifying methane oxidation rates is also
necessary for establishing the biologic repercussions of reduced oxygen levels in marine systems; and,3. Identify or fingerprint methane entering the atmospherefrom gas hydrates in order to distinguish gas hydrates
from other active methane sources. The release rates andthe volume scales of methane from the decomposition oforganic material or from deeper hydrocarbon reservoirs
can be quite different from the methane-release patterns
associated with gas hydrates.

Because a significant increase in atmospheric methane concentrations can result from transferring even a very small
fraction of the methane in gas hydrates to the atmosphere,
quantifying the climatic impact of gas hydrate breakdown
will require reducing substantial uncertainties in estimatesof methane transfer to the atmosphere. Three key researchgoals are:
1. Constrain the gas hydrate volumes that currently exist in
the most sensitive environments through in situ sampling,
remote sensing and modelling;
2. Establish water-column methane-consumption rates toconstrain estimates of how much methane from gas hy-

So far, contemporary anthropogenic climate change does
not appear to have triggered significant gas hydrate dissociation. However, the potential climate and environmental
impact of even a limited dissociation of the world’s gas hydrates continues to fuel multidisciplinary research in thisarea (see Text Boxes 3.1 and 3.2). In addition to the climatic
impact of methane release to the atmosphere, methane release can affect other aspects of the environment. Methane
oxidation within the ocean contributes to ocean acidification
and will also affect the budget of dissolved oxygen, carbon
dioxide, and other compounds in the ocean (as discussed inVolume 1, Chapter 2).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

71

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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES

75

EDITORIAL TEAM AND AUTHORSEDITOR
Yannick Beaudoin
GUEST EDITORS
William F. Waite, U.S. Geological Survey
Ray Boswell, U.S. Department of Energy
Scott Dallimore, Geological Survey of Canada – Natural Resources Canada
TECHNICAL EDITORS
Claire Eamer
Patrick Daley
REVIEWER
Carolyn Ruppel, U.S. Geological Survey
AUTHORS
Chapter 1
Ray Boswell, U.S. Department of Energy
William F. Waite, U.S. Geological Survey
Keith Kvenvolden, U.S. Geological Survey
Carolyn A. Koh, Colorado School of Mines
Jeffery B. Klauda, University of Maryland
Bruce A. Buffett, University of California, Berkeley
Matthew Frye, U.S. Bureau of Ocean Energy Management
Mark Maslin, University College London						Chapter 2
Tina Treude, GEOMAR
Kelly Rose, U.S. Department of Energy
Samantha B. Joye, University of Georgia
Lisa A. Levin, Scripps Institution of Oceanography
Gerald Dickens, Rice University
Craig Shipp, Shell
Espen S. Andersen, Statoil
George Claypool, Independent Research Geochemist
Miriam Kastner, Scripps Institution of Oceanography
Craig Smith, University of Hawaii
Andrew Thurber, Oregon State University
Chapter 3
Klaus Wallmann, GEOMAR
Scott Dallimore, Geological Survey of Canada – Natural Resources Canada
Arne Biastoch, GEOMAR
Graham Westbrook, University of Birmingham
Natalia Shakova, University of Alaska-Fairbanks
Jeffrey Severinghaus, Scripps Institution of Oceanography
Gerald Dickens, Rice University
Jürgen Mienert, University of Tromsø

STEERING COMMITTEE
Espen Sletten Andersen, Statoil
Yannick Beaudoin, GRID-Arendal
Jean-Marie Beaulieu, Canadian Polar Commission
Ray Boswell, U.S. Department of Energy
Tim Collett, U.S. Geological Survey
Scott Dallimore, Geological Survey of Canada – Natural Resources Canada
Rolando di Primio, GFZ-Potsdam
Nader C. Dutta, Schlumberger
James Goodman, Forum for the Future
Haflidi Haflidason, University of Bergen
Berit Oline Blihovde Hjelstuen, University of Bergen
Jarle Husebø, Statoil
MV Lall, Directorate General for Hydrocarbon, India
76 FROZEN HEAT

Joo Yong Lee, Korea Institute of Geoscience and Mineral Resources
Yoshihiro Nakatsuka, Japan Oil, Gas and Metals National Corporation (JOGMEC)
Kelly Rose, U.S. Department of Energy
Tatsuo Saeki, Japan Oil, Gas and Metals National Corporation (JOGMEC)Paul Siegel, retired, World Wide Fund for Nature
Anne Solgaard, GRID-Arendal
Jan Stenløkk, Norwegian Petroleum Directorate
Bruce Alastair Tocher, Statoil
Tina Treude, Helmholtz Centre for Ocean Research Kiel (GEOMAR)
William F. Waite, U.S. Geological Survey
Klaus Wallmann, Helmholtz Centre for Ocean Research Kiel (GEOMAR)
Koji Yamamoto, Japan Oil, Gas and Metals National Corporation (JOGMEC)CONTRIBUTORS
We would also like to acknowledge content support provided by:
Chapter 1
Katherine V. Ackerman, U.S. Geological Survey
Timothy S. Collett, U.S. Geological Survey
Tomas D. Lorenson, U.S. Geological Survey
Katherine Y. McMullen, U.S. Geological Survey
John Pohlman, U.S. Geological Survey
John Ripmeester, National Research Council of Canada		
Pieter Tans, National Oceanic and Atmospheric Administration
William J. Winters, U.S. Geological Survey
Fred Wright, Geological Survey of Canada				Chapter 2
Shana K. Goffredi, California Institute of Technology
John Pohlman, U.S. Geological Survey
David Valentine, Univeristy of California, Santa Barbara
Chapter 3
Inez Fung, University of California, Berkeley
Matthew Reagan, Lawrence Berkeley National Laboratory

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