In SE Siberia, carbonate formations with δ^sup 13^C^sub carb^ values ranging between -12[per thousand] and -7[per thousand] (V-PDB) and Sr concentrations of up to 2.5% occupy an area of 40 000 km^sup 2^. Several successions exceed 1000 m in thickness and represent the world's largest known exposures of sedimentary carbonates exhibiting extreme depletion in ^sup 13^C. The carbonates were deposited on a carbonate platform evolving from a mixed carbonate-siliciclastic ramp to a carbonate ramp, and then from a peritidal rimmed shelf to a deep-water open shelf. All sequences reveal a facies-independent, upward rise in marine δ^sup 13^C^sub carb^ from -12[per thousand] to -7[per thousand]. The trend and magnitude of the values mimic those that are characteristic of the 600-550 Ma Shuram-Wonoka isotope event. A coincident stratigraphic rise in ^sup 87^Sr/^sup 86^Sr from 0.70802 to 0.70862 in several sections of limestones, containing 4400 µg g^sup -1^ Sr on average, is considered to be by far the best available constraint on a temporal variation of seawater isotopic composition through the Late Ediacaran. If the greatest temporal rate of change in seawater ^sup 87^Sr/^sup 86^Sr observed in the Cenozoic is applied to the Siberian sections, the calculated minimum duration for the Suram-Wonoka event is 10 Ma.
In 1993, carbonate formations characterized by extreme depletion in ^sup 13^C and yet not related to glaciations were reported simultaneously from Australia (Wonoka Formation, Pell et al. 1993), Oman (Shuram Formation, Burns & Matter 1993) and Siberia (several formations in the Aldan Shield, Pokrovsky & Gertsev 1993). The significance of this discovery presented a major puzzle and it took over a decade for the first realization (Melezhik et al. 2005) and wide international acceptance (Halverson et al. 2005; Le Guerroué et al. 2006) of the fact that the unprecedented negative δ^sup 13^C^sub carb^ excursion, known now as the Shuram-Wonoka event of Ediacaran time, represents a major perturbation of the global carbon cycle. It is, at present, unique in Earth history in terms of the amplitude of depletion in ^sup 13^C (Fike et al. 2006). Halverson et al. (2005) provided a high-resolution composite carbon-isotope record for the Early Neoproterozoic and Ediacaran. This was supplemented by data from Oman, which included a detailed record of the Shuram-Wonoka event (Le Guerroué et al. 2006). Available radiometric dates constrain the termination of the Shuram-Wonoka event to around 550 Ma, whereas there is no consensus on the timing of its onset. Various workers have suggested different constraints ranging from 600 to 560Ma (Condon et al. 2005; Fike et al. 2006; Le Guerroué et al. 2006; Bowring et al. 2007).
The existence of the Shuram-Wonoka carbonates represents a challenging problem for our current understanding of global carbon cycle geodynamics. Changes in the ratio of reduced/ oxidized carbon sequestered in sediments, a methane hydrate release, or 'zero' biological productivity, if applied separately, cannot explain the carbon isotope characteristics and formation of these carbonates. It has been suggested that the event was possibly driven by multiple causes (Melezhik et al. 2005; Le Guerroué et al. 2006; Pokrovsky et al. 2006b). Fike et al. (2006) proposed that the excursion was caused by the oxidation of a large reservoir of organic carbon suspended in the deep ocean by the mechanism suggested by Rothman et al. (2003), who adopted a dynamic systems approach to interpret the unusual behaviour of the carbon cycle during the Neoproterozoic. They showed that a model wherein the ocean's inorganic and organic carbon reservoirs are treated as being interactive but having separate residence times, and with the latter reservoir 2-3 orders of magnitude larger than at the present, can plausibly explain the observed large excursions in sedimentary carbonate δ^sup 13^C (range about -5 to +10[per …