No evidence bacteria ate methane at end of Snowball Earth

Same story as yesterday, but now 635 million years earlier. This time again no evidence can be found that it were microbes that ate climate-disrupting amounts of methane, at the end of the Marinoan ice age, better known as ‘Snowball Earth’.

This time though the new insights do not suggest the methane stayed in the oceans or in the atmosphere, but perhaps that it wasn’t there at all.

The Marinoan ice age fell in the Cryogenian, the coldest geological period in the history of our planet, for as far as scientific evidence and theory can tell. During the two main ice ages (Sturtian and Marinoan) almost all of the oceans were frozen.

That did not bother much, because it was still the Precambrian Eon, albeit closing in -’just’ tens of millions of years away- on the onset of the Cambrian, that brought the wonderful explosion of higher life forms on Earth, both in flora and fauna – that left there prints as fossils, visible for the human eye.

Methane theory

There were already microbes around though, during the Cryogenian. And that let paleoclimatologists to a theory, based on a carbon-rich layer in Chinese dolomite, which may have been a product of large-scale methane breakdown by bacteria.

If so, there may have been enormous amounts of methane, quite suddenly in the Earth’s atmosphere. And just as in the (much better documented) catastrophic warming event some 580 millions years later, the Paleocene Eocene Thermal Maximum or PETM, these ‘must have somehow been’ disturbed methane hydrate deposits – indeed potent enough to force our climate to a many degrees warmer state, so providing a convenient and rather convincing hypothesis for how [not so much why] the Marinoan ice age ended.

Wrong time, wrong source

Until last Wednesday’s publication in Nature. Scientists from the California Institute of Technology who have researched the Chinese Precambrian rocks have proved the (main evidence supporting the) theory wrong.

The carbon rich layer on top of the dolomite was found to be 1.6 million years older – and to have been deposited at much higher temperatures. This means the carbon is likely derived from a hydrothermal source and that the creation of the layer does not coincide with the end of the glaciation period.

It also means creative minds can set themselves to thinking of new theories on why the Earth decided to turn up its thermostat, 635 million years ago. [The methane hypothesis may still hold true, as it is also supported by oxygen isotope anomalies in other marine sediments, we can read in another Nature publication, dating from 2008.]

Paleo limitations

If ever we are to find out, that does not mean there would be much to learn that would apply as a parallel for today’s climate. The Precambrian Earth was very, very different from ours.

Besides, our current warming is very unique too. For our current Geologic time period, the Quaternary, has been more or less binary so far – switching from cold (the ‘modern’ ice ages) to agreeable (interglacials, like the Eemian, or our own Holocene). Never before in these 2.6 million years has there been a third state, the hot one, or ‘Anthropocene,’ the one we will be entering during the 21st century. And although there may have been mass extinctions associated with the Earth’s previous climatic changes, nothing ever happened this fast.

© Rolf Schuttenhelm | www.bitsofscience.org

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