Collapse of the continental margin and methane release

The vast reserves of peculiar methane-water ice deposits (gas hydrate or clathrate) in sea-floor sediments are the most likely source of methane releases that could generate sudden warming events, such as that at the end of the Palaeocene, and left traces in polar ice cores during the last few glacial-interglacial episodes.  Methane probably leaks from the sea floor all the time, but is soon oxidised to the lesser “greenhouse” gas CO2 in the atmosphere, so muting its potential effects to a low background level.  For methane to have a sizeable effect on global warming, lots of it has to blurt out suddenly.  Possibly the only mechanism that can trigger such explosive releases are failures of sea-floor sediments, either by those beneath a steep surface slope collapsing under gravity, or as a result of seismicity.  Geoscientists from University College London and the British Geological Survey have tried to correlate known peaks in atmospheric methane from the recent past (shown by ice cores) with episodes of mass flow on the seabed (Maslin, M. et al. 2004.  Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis.  Geology, v. 32, p. 53-56).  They found that the periods of greatest disturbance of continental-slope sediments over the last 45 ka took place at the tail-end of the last glaciation, between 13 and 15 ka and 8 to 11 ka.  Each correlates with methane highs in the Greenlandic ice cores and with bouts of rapidly rising sea level (the Bølling-Ållerød and Preboreal warming periods).  So they conclude that there is support for a “clathrate gun” model for sudden warming associated with glacial to interglacial transitions.  However, seafloor collapses also correlate with Heinrich events (ice-sheet surges that launched iceberg “armadas” to low latitudes) that punctuated glacial times.  These marked brief periods, repeating every 1000 years or so, which mark cooling when sea-levels were low.  None are associated with upsurges in atmospheric methane., although the following interstadial warmings are.  This lack of correlation rules out a “clathrate gun” influence on millennial-scale climate fluctuations during glaciations.

Super-eruptions and climate

The biggest known, young volcanic crater is that of Toba on Sumatra, which is a caldera complex measuring 30 x 100 km.  Around 74 ka Toba emitted an eruption that dwarfed any in more recent times, and spread a dust cloud around the world – it is present in ice cores from Greenland, and has been linked with a cooling step during the onset of the last glaciation.  It happened around the time that fully modern humans had begun to spread across Asia after migrating from NE Africa – an Acheulean hand-axe has been found in the Toba Tuff – and may have deeply affected those pioneering bands.  There are older ash levels that can also be attributed to Toba eruptions, one found 2500 km away in the sediments of the South China Sea (Lee, M-Y. et al. 2004.  First Toba supereruption revival.  Geology, v. 32, p. 61-64) and at other sites up to 3000 km from Toba.  This gives an age around 800 ka.  Lee and colleagues from Academica Sinica (Taiwan), the National Taiwan University and the University of Rhode Island estimate that almost 1000 km3 of ash was expelled by the eruption.  Unlike the 74 ka ash, this layer falls in the transition from a glaciation to an interglacial period; instead of a possible cooling influence through dust blocking solar heating, there is a warming trend.  Although not quite as big as the 74 ka eruption of Toba, that of 800 ka is still vastly bigger than any other explosive volcanism during the Pleistocene.  So, it suggests that super-eruptions are not significant climate triggers after all.


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