Geology Reference
In-Depth Information
sametimeasiceisthoughttohavecoveredtheglobe.It'shardtoexplainsuchdeposits,for
the oceans had been stripped of their iron more than a billion years earlier, before the start
of the boring billion. How, then, could the oceans become recharged by iron? One model
suggeststhatthesnowballepisodesealedtheocean,cuttingoffalloxygentotheoceanwa-
ter column. Meanwhile seafloor hydrothermal vents continued to pump fresh iron from the
mantle into the deep ocean. Gradually iron concentrations rose, only to be rapidly depos-
ited as new banded iron formations when the glacial episodes ended.
Snowball versus slushball: such controversies are nothing new in science, and this one
has remained low-key, friendlier than most. Paul Hoffman has retired, and a new genera-
tion has taken up the challenge, for the answers still lie hidden in the rocks.
The Mystery of the Ice
A greater mystery remains. The snowball/slushball Earth episodes were by no means the
first periods of glaciation on Earth, nor would they be the last, but the three major Neo-
proterozoic intervals stand out against the rest. To the best of our knowledge, never before
and never since has such an extreme cold snap occurred on Earth. Why should that be?
How can one brief period of Earth history have been so different from any other?
Two earlier periods of glaciation, both well preserved in the rock record, were evidently
a lot less severe. The earliest known ice advance, a relatively brief event revealed by tillite
deposits on ancient South African cratons, occurred about 2.9 billion years ago, in the
middle of the Archean Eon. That it should have taken so long for Earth's ice caps to ex-
pandfromthepolesisinitselfsomethingofamystery.EarlierinEarthhistorytheSunwas
muchfainter—only70percentofitspresentradianceinthefirstfewhundredmillionyears,
and not more than about 80 percent during the mid-Archean glaciation. With so much less
energy coming from the Sun, other warming mechanisms must have been at play. Many
scientists point to much higher levels of greenhouse gases—carbon dioxide, methane, and
anorangehydrocarbonhaze—aschiefamongthesuspectedmoderatinginfluences.Higher
heatflowsfromEarth'sturbulentdeepinterior,andgreatervolcanicoutputsalsomusthave
played a role in moderating climate.
Ironically, Earth's first glacial episode may have resulted in part from too much green-
housegas.Iftheatmosphere'smethanecontentrose,thenreactionshighinthestratosphere
would have produced more and more of the big hydrocarbon molecules that may have giv-
en early Earth a hazy orange sky. If that haze became too thick, then some of the Sun's
energy would have been blocked and Earth would have cooled.
A second, longer cooling episode, marked by extensive glacial deposits between about
2.4 and 2.2 billion years ago, followed the breakup of the equatorial Kenorland supercon-
tinent. Atmospheric modeling suggests that increased weathering and sediment deposition
