FROZEN HEAT | Volume 1

3.2 THE ROLE OF GAS HYDRATE IN PAST CLIMATE CHANGE

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.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 53

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