Posted on 02/13/2007 10:38:15 PM PST by SunkenCiv
Several severe glaciations occurred during the Neoproterozoic eon, and especially near its end in the Cryogenian period (630–850 Ma). While the glacial periods themselves were probably related to the continental positions being appropriate for glaciation, the general coldness of the Neoproterozoic and Cryogenian as a whole lacks specific explanation. The Cryogenian was immediately followed by the Ediacaran biota and Cambrian Metazoan, thus understanding the climate-biosphere interactions around the Cryogenian period is central to understanding the development of complex multicellular life in general. Here we present a feedback mechanism between growth of eukaryotic algal phytoplankton and climate which explains how the Earth system gradually entered the Cryogenian icehouse from the warm Mesoproterozoic greenhouse. The more abrupt termination of the Cryogenian is explained by the increase in gaseous carbon release caused by the more complex planktonic and benthic foodwebs and enhanced by a diversification of metazoan zooplankton and benthic animals. The increased ecosystem complexity caused a decrease in organic carbon burial rate, breaking the algal-climatic feedback loop of the earlier Neoproterozoic eon. Prior to the Neoproterozoic eon, eukaryotic evolution took place in a slow timescale regulated by interior cooling of the Earth and solar brightening. Evolution could have proceeded faster had these geophysical processes been faster. Thus, complex life could theoretically also be found around stars that are more massive than the Sun and have main sequence life shorter than 10 Ga. We also suggest that snow and glaciers are, in a statistical sense, important markers for conditions that may possibly promote the development of complex life on extrasolar planets. [emphasis added]
(Excerpt) Read more at plosone.org ...
The Big ChillThe fossil record, say geologists, makes it perfectly clear that for most of the 4.5 billion years that the Earth has orbited the sun even the most northern climes have been warm and balmy places. Then, about 40 million years ago something happened to really turn down the thermostat. Pinning down an exact reason has proved difficult. Some scientists have long suspected that the shift in the positions of the continents may have altered ocean currents enough to change the way their waters carry heat from the equatorial to the polar regions. Another possibility was that changing wind patterns had driven the change. Both ideas, while not entirely ruled out, have been shown to have had a smaller effect than originally estimated... Calcium carbonate contains several forms of the element strontium. The "heavy" variety, known as strontium 87, is associated with the chemical weathering of certain types of rocks. Its lighter chemical twin, strontium 86, originates from deep inside the Earth. A core drilled from the ocean floor will show varying amounts of the two forms of strontium, laid down at different points in the planet's distant past. Strontium 86 will dominate during periods when seafloor spreading was in progress. Strontium 87 will dominate if heavy chemical weathering was taking place. If Raymo's theory was correct, strontium 87 would have been the dominant chemical species when the CO2 levels, hence temperatures, were bottoming out. Unknown to Raymo, some of her colleagues were already making precisely these measurements.
by Jim Wilson
October 1999
Scientists Poke Holes in 'Snowball Earth' HypothesisFrank Corsetti of USC, a co-author on the study, said "this is the first real evidence that substantial photosynthesis occurred in the Earth's oceans during the extreme ice age 700 million years ago, which is a challenge for the snowball theory."
National Science Foundation
Press Release 05-173
September 29, 2005
The evidence does not prove large parts of the ocean remained free of sheet ice during the pre-Cambrian glaciation. Although unlikely, Olcott said it is possible one of the tiny "refugia" under the "Snowball Earth" hypothesis allowed such marine life to exist.
But, she said, "finding the one anomalous spot would be quite unlikely," adding that the samples she studied came from an extensive formation of rocks with similar characteristics.
"At what point does an enormous refugium become open ocean?" she asked.
Popsicle PlanetThe atmospheric concentration of this heat-trapping greenhouse gas dropped so low in the Neoproterozoic that the planet froze over completely for 10 million years, killing off most life, which at the time consisted primarily of microbes and algae. In a dramatic reversal of fortunes, carbon dioxide then grew so abundant in the atmosphere that the ice thawed and Earth roasted, according to the hypothesis. The planet swung between icehouse and greenhouse at least four times, severely stressing the established organisms. Ultimately, this crisis may have triggered the evolution of the first recognizable animals. Seen from space during the Neoproterozoic glacial periods, Earth would have looked like a well-packed snowball, according to the new hypothesis. No patches of blue ocean. No clouds in the sky. Just ice, with perhaps some bare rock. The Snowball Earth hypothesis comes at an opportune time, when many scientists are starting to focus attention on the Precambrian time, especially its turbulent ending.
by Richard Monastersky
August 22, 1998
Albedo 0.80
Sorry, but we're alone, IMHO. :-(
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