In a paper published today in the journal Science, titled "Carbonate Deposition, Climate, Stability, and Neoproterozoic Ice Ages," post-graduate researcher Andy Ridgwell and Martin Kennedy, an associate professor of sedimentary geology and geochemistry, both at UC Riverside - together with Ken Caldeira at the Lawrence Livermore National Laboratory - suggest that emerging life played a role in moderating ice ages that have resulted in today's moderate climate.
"The catastrophic ice ages that characterized Earth's early record and challenged animal evolution were seemingly brought under control by the animals themselves" said Kennedy. "Ultimately we owe our comparatively moderate climate to the production of calcium carbonate skeletons and shells."
The paper seems to address one major persistent question facing scientists. Why were ancient ice ages so much more extreme than anything that has occurred since? For instance, during the last ice age some 20,000 years ago the great ice sheets reached northern Pennsylvania but progressed no further towards the equator. What kept a 'snowball' glaciation from occurring? The key lies in the presence of calcium carbonate, or its relative scarcity, in ancient marine life.
Ridgwell, Kennedy and Caldeira used a computer model to assess the differences in how carbon is cycled between the atmosphere, ocean, and sedimentary rocks between modern and Precambrian times. The results, published in the Science paper, suggest that the microscopic marine plants and animals that make protective shells and skeletons out of the mineral calcium carbonate - the material that forms chalk and limestone rocks - are critical.
Their computer-model results demonstrate that the presence of carbonate secreting organisms in the oceans help regulate the aquatic chemistry, which ultimately controls the amount of carbon dioxide in the atmosphere. Carbon dioxide warms the planet through the process known as the "green house" effect.
Although the production of useful calcium carbonate structures like shells and skeletons would have originally been driven by a need for protection and to provide body support for the animals to grow larger, this evolutionary innovation also had a profound effect on moderating climate. The researchers concluded that the consequential improvement in the regulation of the amount of carbon dioxide in the atmosphere is why the Earth has not 'snowballed' at any point in the last 600 million years. Such organisms, however, are almost unknown until after the end of the geological ages known as the Precambrian, which lasted until 544 million years ago.
Ancient rocks from the end of the Precambrian age indicate that glaciers were eroding the landscape and depositing layers of crushed rock debris thousands of meters thick. Although the Earth has experienced numerous cold periods since then, with glaciers leaving their tell-tale marks each time, these Precambrian rocks are special because some of them appear to have been deposited in tropical latitudes. If the land was frozen near to the equator then most of the surface of the planet must have been covered in ice. The Earth would have resembled a giant snowball and conditions would have been extremely inhospitable to evolving life.
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