Are clathrates a credible alternative to farming as a methane source?

One particular sentence caught my eye in the Schmidt 2004 paper that I mentioned in the previous post:

"Most importantly, clathrates can be explosively unstable if the temperature increases or the pressure decreases - which can happen as a function of climate change, tectonic uplift or undersea landslides."

Early Holocene archaeologists get excited by the idea of undersea landslides because a major event dated around 8100 +/-100 BP, the Storegga Slide, is thought to be the cause of the inundation of Doggerland, the area between the British Isles, Denmark and northern Germany now known as the North Sea (Weninger et al, 2008, pp.16-17). The date of the that slide is exciting because it is around the time Ruddiman's carbon dioxide anomaly starts to develop and when there is a methane bounce-back from the 8200 BP Holocene cold event (Ruddiman, 2003, pp.263-4; p.266). Methane has a relatively short atmospheric life before it decays to carbon dioxide and other gases (Archer, 2007, p.523); this last point helped resolve my need to find mechanisms that act both on methane and carbon dioxide.



Location of the Storegga Slide (Nature, 2013)

Beget and Addison (2007) hypothesised that the Storegga Slide released "huge amounts" of methane which caused the sudden and significant increase in atmospheric methane, measured in GRIP ice cores, starting around 8100 BP. This is an example of the "clathrate gun hypothesis" which suggests that undersea landslides periodically release climate-affecting quantities of methane (Kennett et al., 2003).

Sea-bed sulfates are a proxy for the concentration of methane; the depth of the sulfate-methane interface (SMI) is a function of the period during which sulfates have been deposited. Paull et al. (2007) measured SMIs along the Storegga Slide and at its margins. They assumed that the sea-bed exposed by the slide would have contained no sulfate. The depths of the measured SMIs were inconsistent with estimates of large quantities of methane being in the material which was displaced in the slide and so they argue that there was not an associated major release of methane.

Dawson et al. (2011, p.1170), who actually set out to date the slide more accurately, compared Beget and Anderson with Paull et al. and concluded:

"the Storegga Slide represents a case where one of the world’s largest slides does not appear to have released significant quantities of methane gas-hydrates into the atmosphere".

which begs two questions:

1. Why did this huge slide not release large quantities of methane?
2. Where did the atmospheric methane come from?

Question 1 is outside the scope of this blog but it is illustrative of the uncertainties that pervade the research of the complex climate-affecting events of the past.

Question 2, frustratingly, takes us back to the beginning.