Worldwide Precambrian tillites have been recognised for some time. The closest to us is the Port Askaig Tillite in the Dalradian. Many contain striated boulders; the Smalfjord diamictite in northern Norway rests upon a striated pavement. The term diamictite is now generally used for tillites and rocks with a similar lithology. As geological knowledge and geophysical and geochemical applications have developed and been applied to these deposits, it has given rise to the idea that our planet was once completely covered by ice; hence the term ‘snowball Earth’.
As palaeomagnetic data was applied to these regions, and as a picture of the distribution of the continental masses was built up, it was becoming evident that these deposits were mostly formed in tropical regions close to the equator. They are of similar age, in the Neoproterozoic, which is the last period before the start of the Cambrian. A picture emerges of a large continent, Rodinia, straddling the equator and as it broke up by rifting, these glacial deposits are found. There is no dispute that there was a severe Neoproterozoic glaciation, but worldwide? Surely that is impossible?
There are explanations as to how this may come about, and they are to do with CO2 levels and albedo – the reflection of the heat of the sun back into space by the surface of the Earth. It is suggested that if a severe glaciation covered half of the Earth’s surface with snow and ice, the albedo of this surface, (ice and snow reflect 90% of the sun’s heat), would lead to a runaway positive feedback loop, i.e. more snow means more albedo which in turn lower temperatures over a wider area causing more snow. There is also the role of carbon dioxide that as a greenhouse gas raises temperatures. It is removed from the atmosphere during the chemical weathering of silicate minerals. During the Quaternary glaciations the bulk of the continental masses were in the cold and temperate latitudes, and once covered in ice, chemical weathering ceased so leaving more CO2 in the atmosphere to help counter the lowering of temperatures. If, however, the bulk of the continental masses are in tropical latitudes, in a glaciation, this extraction of CO2 by weathering would continue much longer. The Earth could be completely covered by snow and ice and temperatures could drop to -50°C, which is the proposed scenario for snowball Earth.
Other evidence brought forward to support the idea is the existence of Banded Iron Formations at this time. These BIF’s are formed when the sea water is lacking in oxygen and Ferrous Iron (Fe2+) is deposited in thick red beds. They were common in the earlier Precambrian before oxygen producing life was developing, but then ceased as the sea became more oxygenated and Ferric Iron (Fe3+) dominated. But these BIF’s return at the time of snowball Earth. Was the sea becoming deoxygenated again because ice prevented it from reacting with the atmosphere? The BIF’s also contain dropstones, large boulders carried by ice and dropped when it melted. There are also pieces of geochemical evidence quoted, notably about the variations in carbon isotopes and the whole package seems to give strong evidence for the idea of a frozen planet. There are questions, such as how did life survive and how did we come out of it? If the theory is correct life did survive even though photosynthesis must have been much impaired. Coming out of the big freeze is easier to explain; even snowball Earth would not stop plate tectonics and volcanic activity, and a big volcanic phase would pump carbon dioxide into the atmosphere with no chemical weathering to use it. A huge greenhouse effect and melting could cause rising sea levels, and the evidence for this is that on top of nearly all these Proterozoic diamictites are ‘Cap Carbonates’. As CO2 levels rise alongside temperatures precipitation is produced which would deposit limestones and dolostones into the sea, on top of the now flooded diamictites.
It is all very neat, but as with all paradigm-shifting ideas, it will be attacked by the resident scientific community, whether it be Plate Tectonics or Newtonian Physics. In the next blog I will put the case against.
* The illustration shows boulders dropped by icebergs into laminated marine sediment in late Precambrian time, Narachaampspos, Kaokoveld, Namibia. The chief proponent of the Snowball Earth hypothesis, Paul Hoffman, points to the transition to carbonate rocks which indicate the sudden termination of this frigid event. Photograph by courtesy of Prof. Mike Hambrey, Aberystwyth University.