June Editorial      

What caused the end of 'Snowball Earth'?

There are still many things which we do not understand about this very cold period of the Earth's history and one of those things is what caused this ice age to come to an abrupt end some 600 million years ago.

A number of theories have been formed to describe what caused the changes leading to the massive meltdown. In 1998 Paul Hoffman and his colleagues suggested a sudden climate change as the result of an extreme greenhouse effect. They considered that there might be a direct link with the rapid precipitation of calcium carbonate in warm surface waters. These produced thick layers of rock referred to as 'cap carbonate rocks' (ref 1) - those sedimentary deposits which today directly overlay the glacial deposits.

These cap carbonates have an unusual chemical composition and strange sedimentary structures that are often interpreted as large ripples. The cap carbonates are predominantly continuous layers of limestone (CaCO₃) and/or dolostone (see below) which are typically 3-30 m thick. They occur on platforms, shelves and slopes world-wide, even in regions which otherwise lack carbonate strata.

Dolostone is a sedimentary carbonate rock which is composed predominately of dolomite. Most dolostone is formed by when some of the calcium in limestone is replaced by magnesium prior to lithification. (Lithification is the process of material hardening into a solid rock).

The formation of sedimentary rocks like cap carbonates could be caused by a large influx of positively charged ions. Such ions would be produced by rapid weathering during the extreme greenhouse conditions following a Snowball Earth event.

Another interesting signature of the cap carbonates is the low level of carbon-13 in their structure in comparison to the levels in other carbonates.

There are two stable carbon isotopes - 12C and 13C. The dominant isotope is ¹²C while ¹³C is measured as a trace element. The quantities of the different isotopes can be measured by mass spectrometry - an analytical technique that measures the mass-to-charge ratio of charged particles and compares this to a standard. The result of mass spectrometry analysis of ¹³C is the delta of the ¹³C = δC-13 which is expressed as parts per thousand (‰).

Hoffman and his colleagues analysed 'cap carbonate' rocks in samples and found that when compared to other sedimentary rocks, cap carbonates are depleted of 13C with a δC-13 of about -5‰.

One of the more interesting theories proposed to explain the formation of cap carbonates is that the rapid weathering responsible for the climatic changes at the end of snowball earth dramatically increased the level of the CO₂ in the atmosphere. As the ice melted, the CO₂ dissolved in the water and formed carbonic acid which fell as acid rain. This in turn weathered (destroyed) the exposed silicates and carbonate rocks releasing a large amount of calcium which then washed into the ocean. Here it formed the distinctively textured layers of carbonate sedimentary rock seen today.

There are problems with this theory, not least because a high carbon dioxide concentration in the atmosphere would also cause the oceans to become acidic, causing seawater to dissolve carbonates instead of creating them. Also, not all glacial deposits are capped by carbonates.

In 2008 a new study of samples of cap dolostone from south China showed even more dramatic depletion of ¹³C; to the value of -48‰. (ref 2) The authors of this study analysed samples from the Doushantuo Formation. This is known to be one of the oldest fossil beds to contain highly preserved fossils. The researchers estimated that the cap carbonates they analysed were approximately 635 million years old, which would put them at exactly the time that the glacial period ended.

The authors proposed that at this time there was a rapid and widespread release of methane from methane clathrate following clathrate destabilization. The methane may have contributed to the unusual sedimentary and isotope features of cap carbonates. The theory suggested that as the methane was released, it was oxidised by existing microbes and its carbon wastes were incorporated into the dolostones. Since methane is low in Carbon-13, if the methane was indeed incorporated into the cap carbonates this would explain the low level of ¹³C in those rocks. Furthermore, this hypothesis would not only explain the the extremely low level of Carbon-13 in the cap carbonates but also explain the dramatic changes which caused the end of the 'Snowball Earth'. Methane is a strong greenhouse gas and large amounts of methane bubbling up through ocean sediments would heat up the atmosphere.

However, new research recently published in the journal Nature (ref 3) disputes the methane-microbe theory. A team of scientists led by researchers from the California Institute of Technology (Caltech) analysed the same dolostones from South China.

Using carbonate clumped isotope thermometry, ⁸⁷Sr/⁸⁶Sr isotope ratios, trace element content and clay mineral evidence they showed that carbonates bearing the ¹³C-depleted signatures (-48‰) crystallized more than 160 million years after the deposition of the cap dolostone. The researchers used carbonate-clumped isotope thermometry. This is a technique developed in Caltech that looks at the way in which rare isotopes (such as the carbon-13 in dolostone) group or ‘clump’ in crystalline structures such as bone or rock.

The researchers were able to show that this clumping is highly dependent upon the temperature of the immediate environment in which the crystals form. High temperatures mean less clumping; low temperatures mean more. The results show that to obtain the very low level of ¹³C in the samples, temperatures would have to be extremely high, indeed, too high for any microbes to survive.

What the team's thermometer made very clear, says Eiler (a co-author of the latest Nature paper), is that 'the carbon source was not oxidized and turned into carbonate at Earth's surface. This was happening in a very hot hydrothermal environment, underground.'

So it seems that the Doushantuo dolostones, although interesting, tell us nothing about the events which took place at the end of Snowball Earth. The field is once again wide open for new theories. And although it is still unknown whether methane clathrate destabilization had a role in the exit from the ‘snowball’ state, it is now clear that the process would not have left the extreme carbon isotope signature found in cap dolostones.

Reference

1. Paul F. Hoffman, Alan J. Kaufman, Galen P. Halverson, Daniel P. Schrag. A Neoproterozoic Snowball Earth (1998) Science, 281, pp 1342-46.

2. Wang, Jiasheng; Jiang, Ganqing; Xiao, Shuhai; Li, Qing; Wei, Qing . Carbon isotope evidence for widespread methane seeps in the ca. 635 Ma Doushantuo cap carbonate in south China. (2008) Geology, 36, pp 347-350.

3. Thomas F. Bristow, Magali Bonifacie, Arkadiusz Derkowski, John M. Eiler, John P. Grotzinger. A hydrothermal origin for isotopically anomalous cap dolostone cements from south China. (2011) Nature, 474, pp 68-71