Neoproterozoic
Carbon Isotopes of the Neoproterozoic Shuram and Wonoka Formations
Le Guerroue, Erwan1, Martin
Kennedy1 (1)
Carbonates δ13C values
provide a useful monitor of changes in the global carbon cycle because they can
record the ratio of organic to carbonate carbon burial. Changes in the carbon
isotopic composition of the carbonate through time can identify times of
anomalous organic carbon burial and related climate events, important for
recognizing source rock intervals. The most pronounced isotope excursions in
the geologic record occur in the Neoproterozoic and have assumed a central role
in the interpretation of biogeochemical events preceding the Ediacaran and
Cambrian radiations. These excursions are less well understood than those in
the Phanerozoic because of their magnitude, duration and the difficulty of
correlation between basins to independently confirm that they do indeed record
global change in the mixed ocean reservoir. The absence of carbonate secreting
organisms and the poorly understood organic carbon cycling also adds to the
uncertainty. Here we described some of the geologic and geochemical constraints
on the Earth's most profound negative carbon isotope excursion which is recorded
in the Shuram Formation of Oman and potentially equivalent Wonoka Formation of
South Australia. Using Phanerozoic carbon cycle models, the -12permil excursion
sustained over many millions of years implies a prolonged interval of complete
oxidation of the DOC carbon reservoir, no organic carbon burial and complete
consumption of atmospheric oxygen. It is apparent from the geological record
that these conditions are not met. Alternatives to this scenario include a
diagenetic origin for these values, or more methanogens mode of carbon cycling
with longer DOC residence times.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California