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Monday, February 17, 2014

Snowball Earth, the mysterious mega ice age

Little rain falls in the Karoo. What little rain that does fall seems to barely get the dirt wet before the clouds break and the unrelenting sun returns to bake the red soils back to a dry, fine dust. The Orange river bisects this bleak land in the western regions of South Africa but its waters originate far in the east and the outflow of muddy water that it carries seems more a tease to the vast plains of sparse grass and thorny shrubs than anything else.

Long the roads seem to stretch here for there are no trees to block views. Yet this region of Africa brings in geologists and paleontologists from all over the world because the Karoo is one of the oldest contiguous chunks of continental granitic rocks on the planet. These large uniform bits of continents, called cratons, are the holy grail from which those scientists can piece together the happenings of ancient Earth. Here in the Karoo lay the largest fossil beds of synapsids; the strange mammal like reptiles who dominated land prior to the massive Permian extinction event wiping them out 250 million years ago. That event, known to paleontologists as The Great Dying, was the closest complex life ever came to being wiped off the planet with 99% of everything both on land and in the oceans dying in just a few thousand years. And the Karoo provided many of the key answers to understanding how it happened.

But our interest is in rocks older than the Permian ones. These rocks are from 700 million years ago and tell a story of an event that was even more devastating to the planet than the Permian extinction. And many orders more bizarre. An event that left Earth looking like a strange and alien planet for almost 60 million years. Something that those that study it can only describe as Snowball Earth.

The Cryogenian Period got its name for a reason. This stretch of time 850 - 650 million years ago had long been a sort of enigma. All over the world the rocks that dated from this period showed unmistakable signs of glaciers and large ice sheets. And that just shouldn't be. Even in the worst ice ages glaciers were never everywhere. During the last great ice age that we currently find ourselves on the tail end of the most extensive ice cover never covered all the continents. In North America the ice stopped right near where St Louis now sits. In Europe the ice sheet never went south of the Alps. Yet the Cryogenian rocks, no matter where they were, seemed to show ice.

This graphic shows the distribution of glacial deposits. (snowballearth.org)

The issue was so puzzling to early geologists that they more or less ignored it, focusing on other time periods when things weren't so darned weird. And so the Cryogenian became a kind of black hole of research for some time where few wanted to waste their time trying to unravel something so seemingly inexplicable. It was reasoned that eventually new breakthroughs in geological understanding might allow for research to proceed.

Strangely the first breakthrough almost led geologists astray of the true scale of the mystery. This paradigm shift was the advent of continental drift theory and its successor Plate Tectonics. Part of what always confounded earlier geologists was their adherence to the idea of Uniformitarianism which was the concept pioneered by Charles Lyell which held that the processes that we see now are the processes that shaped the past. Which was immensely sensible but had one glaring flaw: it didn't account for the land itself moving slowly and constantly.

So the early geologists found fossilized deposits of glaciers and ice shelves in places like Australia or the aforementioned Karoo and it just didn't make sense. But when the idea that the continents moved came along suddenly it didn't seem so weird. "Aha" they reasoned "this just means these places were in the polar regions 700 million years ago!"

And that, it seemed, was that. The Cryogenian was certainly weird with all the major landmasses in polar regions but so long as they were in polar regions all the evidence of ice didn't seem the confounding enigma it once was.

But were the landmasses in the polar regions? One man who didn't think so was Paul Hoffman the professor of Geology at Harvard. He had one huge problem with the idea that the glacial deposits could be explained by the continents being in polar regions during the Cryogenian: cap carbonates.

What is a cap carbonate? Well a carbonate rock is formed when acidic rain dissolves massive amounts of limestone. The runoff carries the dissolved limestone into the oceans where they would lay as thick deposits of calcium rich sedimentary rocks. Carbonate rocks required massive amounts of precipitation and temperate to tropical temperatures. So it seemed weird to Hoffman that a huge thick strata of exactly these kinds of rocks lay directly over the Cryogenian glacial rocks.

How could rocks that require warm temperatures form directly over rocks that clearly showed massive glaciers? For Hoffman this showed the existing explanation, that the continents were in polar regions, was either wrong or a very incomplete because clearly the sedimentary evidence showed that the continents of the Cryogenian had first been very cold and then very warm.

So were the continents in polar regions? Thus explaining the glaciers and ice sheets with a succeeding perplexing period of rapid warmth? Or were the continents not near the polar region with a period of mysterious glaciers? That was what Hoffman wanted to find out. And that led him to Joseph Kirschvink another geologist from Caltech.

Along those long roads in the Karoo of South Africa you might have encountered Kirschvink drilling out cores from rocks alongside roads in deep excavated cuts or near rivers. His drilling, while seemingly bizarre, had a purpose, he would use geomagnetism to get to the bottom of the problem that had vexxed Hoffman.

When rocks form they lock in themselves the geomagnetic properties of the Earth at that time. This allows someone with very exacting equipment such as what Kirschvink had in his lab back in Pasadena, California to figure out from what latitude a rock formed at. And Kirschvink had found something kind of neat: the rocks in the Karoo region from the Cryogenian had formed in the tropics!

This meant that the earlier explanation for the massive glaciers no longer applied. The continents were not in polar regions, they were near the equator.

But....how had this happened? How were there glaciers on all the continents when they were near the equator?

The answer to that came thanks to the recent ending of the cold war. In the 1960's Mikhail Budyko had been performing climate research using advanced mathematical models for the Voeikov Main Geophysical Observatory in the USSR. The results of his research had been rather disturbing. He discovered that Earth had an albedo tipping point.

This is best explained by noting the highly reflective nature of snow and ice. What Budyko found was that if sea ice extended past a certain point so much incoming solar radiation would be reflected back into space that a feedback loop would take over as the increase in reflection would lead to colder temperatures creating more sea ice creating more reflective surfaces until the sea ice went all the way to the equator. Once at the equator a new equilibrium would be established as the atmosphere rapidly depleted itself of moisture leaving Earth as a vast, ice bound world with no clouds in the sky and almost unimaginable cold everywhere.

In fact what Budkyo had found was that having the continents so tightly packed around the equator was the just the recipe needed for such a climatological disaster. And as Hoffman applied Kirschvink's magnetic method to new Cryogenian deposits from all over the world it was looking more and more like that period of time matched what Budkyo had found as the ideal conditions to create this ice locked world.

But this brought up a new question. If this horrible feedback loop would so rapidly lock Earth in ice...what could possibly ever reverse it? It was just this question that had led Mikhail Budkyo to abandon this research. It was interesting but it clearly had never happened because there would be no way to reverse it and since Earth today clearly was not a giant ice ball it was intriguing but not something that had ever happened.

But it seemed it had happened. All the evidence was pointing to it having happened. So something broke Earth out of this horrible ice bound funk. What was it?

Kirschvink had an idea. It was a take off from what a French scientists named Jean Baptiste Joesph Fourier had hypothesized in the early 19th century. Fourier was an educated Frenchman who had found himself almost beheaded by the Terror of the late period of the French Revolution. Seeking to leave France he had enlisted with Napoleon to travel with him to Egypt. Along the way he kept journals regarding his observations and thoughts. Always interested in heat and heat flow he eventually calculated just how hot Earth should be given its size and distance from the sun.

What Fourier found was that Earth was too warm. This led him to postulate an atmospheric greenhouse effect that kept Earth warmer than it should be.

Now Kirschvink realized that while the ice bound conditions of Earth during the Cryogenian shutdown all conventional climate greenhouse gases would still proliferate from volcanic processes. Eventually they would build up enough to break the cycle, melt the ice and bring about an Earth more like what we normally see.

In addition this method of rapid melting via large concentrations of Carbon Dioxide would explain the cap carbonates.

With that realization the 700 million year old mystery was solved. Earth had once been a giant snowball. And it was a rapid event of global warming not unlike what we are doing to ourselves now that brought it to an end. Not that this stopped Kirschvink from drilling more cores from the middle of the desolate Karoo.

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