NEOPROTEROZOIC SNOWBALL EARTH

Neoproterozoic Snowball earth

Neoproterozoic Snowball earth

Introduction

The sudden excursions of the carbon isotopic which occurred periodically throughout the geological history can be found in the carbonate rocks of the sea. This provides great deal of evidence regarding the dissolved inorganic carbon in the ocean.

The severity of the unusual Neoproterozoic glaciations-sheets of ice which extended over continents at sea level to within some degree of the equator- is held up by the severe isotopic reduction of these cap carbonates (Sohl et al., 1999).

The extraordinary event of the formation of cap carbonates across the whole world requires an extraordinary description for explanation. Such an explanation can be provided by The 'Snowball Earth' hypothesis, according to which, the oceans of the world were entirely covered with ice for a time stretching over millions of years within the period of the Cryogenian. Such an event if the world was covered with ice would have actually shut down the water cycle of the earth, paving ways for volcanic carbon dioxide to build up in the atmosphere. Furthermore, the hypothesis addresses that as the atmospheric carbon was transferred back into the oceans through acidic rain, it resulted in the huge increment in the rates of mineral erosion, which caused the cap carbonates to form.

According to the snowball model suggested by Hoffman et al. (1998), it is caused when the surface of the ocean is covered by ice which forms a partition between the ocean and reservoirs of atmospheric CO2. A severe albedo feedback and the silicate weathering allow the atmospheric CO2 to accumulate at high levels before causing the process of rapid deglaciation and melting of the sea ice. CO2 is then delivered back to ocean with excess of alkalinity, leading to the formation of cap carbonate.

Location of the Rocks, age, and correlation

There is an unusual distribution of the Neoproterozoic glacial deposits around the world due to the strange carbonate units which cover them in almost every progression, and the records of the carbon and sulfur isotopic at pre and post glacial occurrences. Majority of the deposits are found to be as glacial by origin through the existence of diamictite and covered argillaceous layers with dropstones. Some of them have faceted and striated clasts indicating glacial processes. Most of the diamictites are rich in carbonate clasts, suggesting if the underlying carbonate deposits were directly eroded by glaciers.

Groupings of silicified microfossil within glacial deposits of Neoproterozoic have been found in the Kingston Peak Formation located in the Death Valley, eastern California. There is no doubt over the Neoproterozoic age of the Kingston Peak Formation. However, it is not obvious which Neoproterozoic glaciation correlates to the unit under discussion. Corsetti et al., (2003) agreed on the older Neoproterozoic glaciation, established on the comparison of lithologic with the present units anywhere in the western North America.

The oncolite bed is found to be deposited in a complex marine environment. In contrast to the diamictites, a shallow marine setting can be represented by the carbonate unit ...