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Figure 2.1. Variation in carbon isotope composition of shallow marine carbonates with d 13 C
measured in % (i.e., parts per 1,000; Purdy, 2003).
formation of one-celled life to multi-celled life that occurred soon after the snow-
ball Earth was coincidental or a byproduct of snowball Earth. As Walker pointed
out, when life is flourishing, ocean water and the carbonates that it produces are
enriched in 13 C/ 12 C compared with periods when life is less extensive. Kerr (2000)
provided a commentary on the theory of snowball Earth. As Hoffman and Schrag
(2002) have explained, carbon supplied to the oceans and atmosphere from out-
gassing of carbon dioxide by volcanoes contains about 1% 13 C and 99% 12 C.
Carbon is removed by the burial of calcium carbonate in the oceans, in addition
to terrestrial removal by silicate weathering. If removal by burial of calcium car-
bonate in the oceans were the only process in effect, calcium carbonate would
have the same 13 C/ 12 C ratio as the volcanic output, but carbon is also removed
from the ocean in the form of organic matter, and organic carbon is depleted in
13 C (2.5% less than in calcium carbonate; Purdy, 2003). In a snowball Earth
scenario, snowball events should drastically decrease the levels of biological
productivity. This drop in biological productivity should trigger decreased levels
of 13 C in sediments. Figure 2.1 shows several periods of extremely low 13 C in the
era near 600 mybp .
Schrag et al. (2002) discussed the possible role of methane in the pre-glacial
buildup of 13 C during the snowball era.
The website http://www.snowballearth.org provides a good summary of data
and theory regarding putative snowball states of the Earth. Macdonald et al.
(2010) found new evidence that sea ice extended to the Equator 716.5 million
years ago, supporting the theory that a snowball Earth event occurred at that
time.
Another possible explanation for snowball Earth is that the Earth may have
undergone a larger tilt for a short time, thus cooling the equator while warming
polar areas. However, no credible mechanism has been offered for how the Earth's
tilt could change so remarkably in a short time.
 
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