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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
53
An extreme global warming event in the geological record be-
gan 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
º
C over a 1 to 10 thousand year period
(Dunkley-Jones
et al.
2010). Potential global-scale triggers for a
temperature rise include a change in ocean circulation patterns
(Lunt
et al.
2010), or a change in snow, ice and vegetation cover-
age 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 large-
scale magma eruptions (Storey
et al.
2007; Cohen
et al.
2007).
Triggering mechanisms themselves may not be capable of
generating the full global-scale temperature increase, how-
ever, so many researchers invoke a process proposed by Dick-
ens
et al.
(1995) in which the warming trigger destabilizes a
significant volume of gas hydrate. This link to gas hydrate dis-
sociation is suggested by the numerous stable isotope records
across Earth indicating the PETM warming coincided with at
least 2 000 Gt of isotopically light (
13
C-depleted) carbon to the
ocean and atmosphere (Zeebe
et al.
2009; Cui
et al.
2011), as
well as oxygen depletion in the oceans and widespread carbon-
ate dissolution on the sea floor (Dickens
et al.
1997). Isotopi-
cally light carbon can be an indication of biogenic methane
that has been released from dissociating hydrate. Moreover, as
discussed in Volume 1, Chapter 2, methane released into the
ocean can be oxidized to CO
2
, a process that consumes oxygen
and can also cause carbonate dissolution by making the water
more acidic (see Volume 1 Chapter 2, Text Box 2.1).
Irrespective of the fate of methane, atmospheric carbon
concentrations would increase over relatively short-time
scales, and contribute to the dramatic PETM warming
(e.g. Dickens
et al.
1997; Zeebe
et al.
2009). Gas hydrate’s
role during the PETM continues to be debated, however,
because there are several possible sources for massive and
rapid 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 (Sven-
sen
et al.
2004), or carbon dioxide and methane release
from degrading permafrost (DeConto
et al.
2010).
For the PETM and other past warming events, a few exam-
ples being the Permian-Triassic boundary (Krull and Retal-
lack, 2000), in the early Toarcian (Hesselbo
et al.
2000;
Cohen
et al.
2007), in the Cretaceous (Jenkyns and Wil-
son, 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 is
that a large fraction of the methane released from the sea
floor may be oxidized in the water column, such that a pri-
mary consequence of hydrate dissociation is ocean acidifi-
cation and loss of dissolved oxygen (Dickens 2003). These
past-climate studies help guide our expectations for what
role gas hydrates might play in the future, given current
climate trends.
3.2
THE ROLE OF GAS HYDRATE
IN PAST CLIMATE CHANGE