Page 125 - Introduction to Paleobiology and The Fossil Record

112 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 4.10 Snowball Earth

Strong evidence suggests that a number of Late Neoproterozoic ice ages were of global extent
(Hoffman et al. 1998). The occurrence of tillites in close association with carbonates in near-equato-
rial positions has suggested to Paul Hoffman and his colleagues that during these intervals the Earth

was virtually covered by ice. These data supported a model first developed by Brian Harland in the
1960s, subsequently christened “snowball Earth” by Joe Kirschvink in the 1980s. But paleomagnetic
data for low-latitude ice is not the only line of evidence for a global snowball. The majority of these
glacial deposits are overlain by so-called cap carbonates. These rocks suggest deposition in extreme
greenhouse conditions, under an atmosphere of high concentrations of carbon dioxide and seawater
supersaturated with calcium carbonate. Such conditions were promoted by the high temperatures
required to kick the Earth out of its “snowball” state (Fig. 4.27). The incredible buildup of the
greenhouse gas, carbon dioxide, in the atmosphere was a direct consequence of a lack of liquid water
and the cessation of weathering processes; this buildup essentially saved the surface of the planet
from an eternal frozen state. The glacial deposits and the cap carbonates, however, are also strongly
13
depleted in the C isotope; this suggests very little biological productivity was in progress that could
12
13
have removed the lighter C isotope, causing preferential enrichment of the heavier C stable
isotope. And, finally, banded ironstone formations (BIFs) are a feature of the snowball interval sug-

gesting the existence of an anoxic, stratified ocean system. Some BIFs are even associated with ice-
rafted dropstones. Not everyone, of course, agrees with this hypothesis; some have suggested a milder
“slushball Earth” and some even deny the possibility of global ice sheets altogether. But, surely these

“freeze–fry” episodes had an important influence on the mode of organic evolution. Biological evolu-
tion would certainly have continued, not least associated with active volcanic vents deep under the
ice and in other extreme environments. However evidence for metazoan life seems to appear directly
after snowball Earth.

(a) (b)

(c) (d)
Figure 4.27 Snowball Earth scenario. (a) Continents are near the equator, increasing precipitation
removes CO 2 from the atmosphere, and with falling temperatures ice begins to spread from the
poles. (b) Ice continues to spread with temperatures further reduced by the albedo (refl ection of
solar energy) effect. (c) Atmospheric CO 2 increases due to volcanic activity, prompting a reversal
in temperatures. (d) Greenhouse conditions return and the ice sheets recede. (Courtesy of Jørgen
Christiansen and Svend Stouge.)