Environments - Geochemistry: An Introduction - page 193

the atmospheric system was severely tipped out of balance with the return of oxygen
deficiency. These layers are overlain by rather poorly dated Neoproterozoic glacial
deposits. These cold spells are known as the Sturtian ( ≈ 730 Ma), Marinoan ( ≈ 640 Ma),

and Gaskiers ( 590 Ma) glaciations and are of worldwide extension. The deposits are

characterized by laminated layers of fine clays and siltstones, known as diamictites, in
which are embedded large clastic fragments; some of them are angular, others rounded
and decorated with striations. These rocks are identified beyond doubt as glacial deposits.
Above the diamictites occur discordant massive layers of dolomite-rich carbonates, known
as the cap-carbonates. Associated with these dolostones, or on top of them, phosphorites
are commonly found, which contain wonderfully preserved multicellular organisms, e.g.
in Doushantuo in China. These are the first incontrovertible Metazoans, also endowed
with body symmetry and motility. With these layers began the Ediacaran radiation and the
advent of multicellular life. Although its members were unequally successful with respect
to their descendance, the Ediacaran fauna was incredibly rich and seemed to bloom very
rapidly in the aftermath of the Gaskiers event.
Although many of these geological observations were made up to 40 years ago, it is
only during the last decade that we have been able to start making sense of them. In the
late 1980s, paleomagnetic data on some of the iron-rich glacial layers showed that glaciers
reached sea level even under tropical and even equatorial latitudes. Such a surprising obser-
vation is unlikely to reflect a tilted terrestrial spin axis, because the equatorial bulge of our
planet, like the bulge of a top, is stabilized near its current position by the attraction of
the Moon. The term of Snowball Earth was coined in 1992 by Joe Kirschvink to reflect
the extensive glaciation of the planet to very low latitudes, possibly sealing off most of
the ocean surface from sunlight. With extensive ice blanketing, increased reflection of the
solar radiation cooled the Earth even more (in jargon, the albedo became very high), and if
it were not for the greenhouse effect, the Earth could have remained frozen for eons. These
glacial bouts probably lasted millions of years, long enough for most previous life forms
to become extinct. It is believed that volcanic emissions of CO 2 saved our planet from the
ice doom: by progressively building up in the atmosphere, CO 2 captured more and more of
the solar radiation, surface temperature increased and the ice quickly melted. Weathering
under high atmospheric P CO 2 acting on a barren crust triggered an alkalinity rush to the
ocean and led to the massive sedimentation of carbonates.
Carbon isotopic compositions of the cap-carbonates were found to be quite informative.
The

≈

13 C values nose-dive by more than 15 per mil over a very narrow sedimentary inter-
val ( Fig. 9.11 ) . One of the early interpretations appealed to the nearly complete cessation
of photosynthetic activity and to the demise of the biomass as a result of the ice blocking
sunlight. Carbon would have the isotopic composition of mantle inputs, i.e.

δ

≈− 7 per mil.

13 C lower than mantle values rather suggests that an oceanic reservoir
of isotopic light carbon was being oxidized. Destabilization of gas hydrates has been sug-
gested to provide enough carbon with very negative

The discovery of δ

13 C values, but such speculation still
remains to be tested. Regardless of which process caused the carbon isotope shift, it left
the terrestrial ocean and the atmosphere much more oxidized than in the pre-glacial times.
It is believed that access to higher P O 2 allowed multicellular organisms to increase their
metabolism and their motility so as to lead the way to the thriving Phanerozoic life.

δ