Snowball earth

Earth might have been a huge snowball when the earth’s surface covered with ice and snow 950 million years ago. This theory is called ‘Snowball Earth Hypothesis In 1960, scientists researched and found out that when the earth’s surface covered with snow and ice. It could have covered hole earth.The temperature would go down and then the hole earth will be covered with ice and snow then the earth is like a huge snowball. But there are some scientists who against with the snowball earth theory because of the problem of the cap carbonates. Cap carbonates are layers of distinctively textured carbonate rocks which typically from the uppermost layer of sedimentary sequences reflecting major glaciations in the geological record. The scientists accept the fact that the earth was covered with ice and snow but another problem is the cap carbonates. They are interpreted as warm-water rocks because dolomite requires hot water to precipitate from solution.

Joe Kirschvink had found the escape route from a "snowball" Earth, but was it a route ever taken. Kirschvink pointed to the results from paleomagnetism, a technique that Harland had employed in attempting to estimate the latitudes of Neoproterozoic glaciation. The latitude at which certain rocks formed can be estimated from the inclination, corrected for subsequent disturbances, of their natural remnant magnetization (NRM), which varies systematically from vertical at the magnetic poles to horizontal at the magnetic equator. Today, the magnetic and geographic poles do not coincide, nor have they at most times in the past.

Joe Kirschvink was unaware of two emerging lines of evidence that would strongly support his "snowball" Earth hypothesis. Ironically, both were first highlighted by the intrepid George Williams. All across Australia, Williams reported, from the Kimberleys to the Flinders, Neoproterozoic glacial intervals are blanketed by peculiar "cap" dolostones (equimolar calcium-magnesium carbonate). The transition from glacial deposits to "cap" dolostone is abrupt and lacks evidence of significant hiatus. Williams argued that the "cap" dolostones are primary, or nearly so, and imply high surface temperatures based on laboratory experiments and modern occurrences. He concluded that Neoproterozoic glacial epochs closed with "abrupt climatic warmings". It was soon apparent that Neoproterozoic "cap" dolostones are a world-wide phenomenon, particularly striking in regions where carbonate rocks are otherwise absent. The "paradoxical" association of glacial and warm-water deposits was widely acknowledged.

A potential explanation for the rare occurrence of "snowball" events in Earth history is an unusual continental configuration. Paleomagnetic evidence suggests that there were few if any continents at high latitudes 600-700 million years ago. When most continents are close to the Equator, the Earth is deprived of a mechanism that keeps the amount of carbon dioxide in the atmosphere above a critical level. If carbon dioxide in the atmosphere were to slowly drop over millions of years due to a slow reduction in volcanic activity, global temperatures would drop and glaciers would cover the high-latitude continents, just as ice sheets cover Antarctica and Greenland today.

Cambrian period
The Cambrian is a major division of the geologic timescale The Cambrian is the earliest period in whose rocks are found numerous large, distinctly fossilizable multicellular organisms that are more complex than sponges or medusoids Understanding of Cambrian fossils, and their evolutionary significance, has been hugely enhanced by the study of fossils from three famous lagerstätten.

The Cambrian period saw most continents located in the southern hemisphere at low paleolatitudes (near the equator). The Ediacaran supercontinent of Pannotia continued to assemble in some regions but fragmented into Gondwana Laurentia, Baltica, and various mostly submerged Asian blocks. Laurentia stretched across the Cambrian equator, partly submerged by the Iapetus ocean, with a mostly mostly submerged Baltica and Siberia approaching from the South-East.

Cap carbonates
Cap carbonates are layers of distinctively textured carbonate rocks which typically form the uppermost layer of sedimentary sequences reflecting major glaciations in the geological record. They are typically 3-30 m thick and occur on platforms, shelves and slopes world-wide, even in regions otherwise lacking carbonate strata.

The cap limestone contains a strongly negative carbon-isotope signature, which is believed to be primary and is similar to many cap carbonate beds deposited globally. Deposition of cap carbonate is a response to a sudden increase in shallow-seawater alkalinity.