Geology Reference
In-Depth Information
Cycles of Change
Back on Neoproterozoic planet Earth, at the tail end of the first snowball episode seven
hundred million years ago, the tipping point of climate change had been reached. The in-
evitable rise in carbon dioxide played a big role; the sudden release of methane from clath-
rates may have contributed as well. In a geological blink of an eye—perhaps much less
than a thousand years—the climate lurched. The snowball Earth transformed to the hot-
house Earth as temperatures soared to record levels.
Foralongtime, perhaps thirty million years, awarm climate prevailed, butthehothouse
ensureditsowndemise. Theelevated atmospheric concentrations ofcarbondioxide gradu-
ally fell from their extremes. Some of the greenhouse gas was removed by reactions with
rocks. The denuded land, exposed to rainfall laced with corrosive carbonic acid (a con-
sequence of high atmospheric CO
2
), weathered rapidly. The influx of mineral nutrients,
coupled with the resurgence of sunshine, led to explosive algae blooms that devoured the
greenhouse gas. All of these events are duly preserved in the carbon isotope record.
And so for the next 150 million years, Earth cycled between these extremes. Not once,
nottwice,butatleastthreetimestheicegatheredandretreated,theglobalclimateswinging
drunkenlyfromarctictotropicalandbackagain.Thefirstepisode,calledtheSturtianglaci-
ation, reached a maximum about 720 million years ago. The Marinoan glaciation followed
at 650 million years, and the less severe Gaskiers glaciation occurred at 580 million years.
Thick accumulations of rock in a dozen countries reveal details of this dramatic cycle. As
iceretreated,theglaciersleftbehindhugepilesofplucked-upbouldersandground-uprock,
lumpy tillites and polished rounded bedrock. Soon thereafter thick crystalline deposits of
carbonate minerals covered the tillite layers—another telltale sign of warming oceans. The
carbonates formed so rapidly in the CO
2
-supersaturated seas that giant crystals several feet
longblanketedtheshallowoceanfloor.ThesehastydroppingsspeakofatimewhenEarth's
tortured surface had lost its chemical equilibrium—forever abandoning its boring billion
stasis.
For a time following Paul Hoffman's 1998 publication about snowball Earth, geologists
embraced the frozen-planet scenario, but the bloom is now falling off the rose. Climate
modelers have found it difficult to encase all of Earth in ice, for their calculations suggest
thatevenattimesofsignificantcooling,theEquatorshouldremaintemperate.Fieldgeolo-
gists now find evidence of moving ice, surface waves, and ocean currents during the max-
imum freeze—signs of at least some open water. For most geologists, the hard snowball
has been replaced by a more benign “slushball” scenario, the new model to beat. Hoffman
counters that the slush could represent conditions just before or just after the glacial max-
imum.
How might we tell the difference? One intriguing line of evidence supporting a hard
snowball is a striking, short-lived pulse of banded iron formations laid down about the
