Snowball Earth at 2.3 gya

http://pr.caltech.edu/media/Press_Releases/PR12723.html

Evolutionary Accident Probably Caused The Worst Snowball Earth Episode

PASADENA--For several years geologists have been gathering evidence
indicating that Earth has gone into a deep freeze on several occasions,
with ice covering even the equator and with potentially devastating
consequences for life. The theory, known as "Snowball Earth," has been
lacking a good explanation for what triggered the global glaciations.

Now, the California Institute of Technology research group that
originated the Snowball Earth theory has proposed that the culprit
for the earliest and most severe episode may have been lowly bacteria
that, by releasing oxygen, destroyed a key gas keeping the planet warm.

In the current issue of the Proceedings of the National Academy of
Sciences (PNAS), Caltech graduate student Robert Kopp and his supervising
professor, Joe Kirschvink, along with alumnus Isaac Hilburn (now a
graduate student at the Massachusetts Institute of Technology) and
graduate student Cody Nash, argue that cyanobacteria (or blue-green
algae) suddenly evolved the ability to break water and release oxygen
about 2.3 billion years ago. Oxygen destroyed the greenhouse gas
methane that was then abundant in the atmosphere, throwing the global
climate completely out of kilter.
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The abstract of the PNAS article:

Although biomarker, trace element, and isotopic evidence have been
used to claim that oxygenic photosynthesis evolved by 2.8 giga-annum
before present (Ga) and perhaps as early as 3.7 Ga, a skeptical
examination raises considerable doubt about the presence of oxygen
producers at these times. Geological features suggestive of oxygen,
such as red beds, lateritic paleosols, and the return of sedimentary
sulfate deposits after a approximately 900-million year hiatus, occur
shortly before the approximately 2.3-2.2 Ga Makganyene "snowball Earth"
(global glaciation). The massive deposition of Mn, which has a high
redox potential, practically requires the presence of environmental
oxygen after the snowball. New age constraints from the Transvaal
Supergroup of South Africa suggest that all three glaciations in the
Huronian Supergroup of Canada predate the Snowball event. A simple
cyanobacterial growth model incorporating the range of C, Fe, and P
fluxes expected during a partial glaciation in an anoxic world with
high-Fe oceans indicates that oxygenic photosynthesis could have
destroyed a methane greenhouse and triggered a snowball event on
timescales as short as 1 million years. As the geological evidence
requiring oxygen does not appear during the Pongola glaciation at
2.9 Ga or during the Huronian glaciations, we argue that oxygenic
cyanobacteria evolved and radiated shortly before the Makganyene
snowball.
-----------------
My comment:

I tend to agree with the claim that oxygenic photosynthesis did
not arise until 2.3 gya. But I am a little more skeptical of their
claim that it triggered a catastrophic snowball event (1 km thick
oceanic ice-cap at the equator!) that pretty much wiped out the
too clever cyanobacteria and all other life except near vents.

Interesting, though.

There is also an article about this at NASA's "Astrobiology Magazine"
http://www.astrobio.net/news/index.php


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