Geology Reference
In-Depth Information
Three-quarters of a billion years ago, Earth entered a period of climate instability the likes
of which have not been seen before or since. It all began with a brutal ice age.
Glaciersleaveanunambiguoussuiteofsedimentaryfeatures.Foremostarethick,irregu-
larlayersofdiagnosticrockscalledtillites,whichpreservechaoticjumblesofsand,gravel,
angularrockfragments,andfinerockflour.Glaciersalsoleavebehindroundedoutcropsof
bedrock that have been scratched and polished by the slowly advancing ice sheets. Erratic
boulders and moundlike moraines add to the evidence, as do finely layered varved sedi-
ments, representing seasonal runoff into glacial lakes.
Field geologists around the world have discovered these glacial features in rocks
between740and580millionyearsoldjustabouteverywherethey'velooked.Indeed,evid-
ence for an abrupt and drastic climate change about 740 million years ago had been accu-
mulatingfordecadeswhengeologistPaulHoffmanandthreecolleaguesatHarvardandthe
University of Maryland published their short, electrifying paper “A Neoproterozoic Snow-
ball Earth” in the August 28, 1998, issue of
Science
. Hoffman and his coworkers made the
extraordinary leap that at least twice during that interval, Earth had not just experienced
an ice age but had completely frozen over, from poles to Equator. They based their claim
in part on meticulous field observations of a sequence of rocks from the Skeleton Coast
of Namibia: thick glacial tillite deposits, side by side with paleomagnetic signals that the
glaciers were close to the Equator, at about 12 degrees latitude. And these weren't alpine
glaciers from high mountains either; the tillites were clearly deposited in shallow coastal
waters, at sea level. The climate must have been correspondingly frigid near the Equator.
By contrast, during Earth's most recent ice age, advancing glaciers never got farther south
thanabout45degreeslatitude,andfossilevidencepointstoarelativelywarmtropicalzone
even during the maximum extent of ice. The Harvard team had unassailable evidence for
Neoproterozoic ice accumulations at sea level, close to the Equator. Hence the snowball
Earth.
For many scientists reading Hoffman's article in 1998, carbon isotopes provided the
smoking-gun evidence for such a sudden and catastrophic change. During the millions of
years prior to the first presumed snowball Earth episode—before about 740 million years
ago—the rapid growth of algal biomass had concentrated isotopically light carbon. Con-
temporaneous limestones deposited in coastal waters around the fragmenting Rodinia su-
percontinent are correspondingly heavy. On the flip side, if microbial productivity slows
or stops, the carbon isotopes in limestone must become on average much lighter. That's
exactly what Hoffman and his colleagues found—a huge decrease of more than 1 percent
in heavy carbon, just before and just after the appearance of glacial deposits about seven
hundred million years ago.
The model that emerged relies on nested positive feedback loops, each of which drove
Earth to a colder and colder state. One feedback depended on continental weathering—a
