EarthRef.org Reference Database (ERR) -- Iyer et al. 1995

Iyer, S.S., Babinski, M., Krouse, H.R. and Chemale, F. (1995). Highly C-13-Enriched Carbonate and Organic-Matter in the Neoproterozoic Sediments of the Bambui Group, Brazil. Precambrian Research 73(1-4): 271-282. doi: 10.1016/0301-9268(94)00082-3 .

Paleontological and radiometric dating methods (Rb-Sr, K-Ar and Pb-Pb) suggest Neoproterozoic ages for sedimentary carbonate rocks of the Bambui Group in the southern part of the Sao Francisco Basin of central Brazil. The delta(13)C values for carbonates fall within two ranges, -2 to +2 parts per thousand and +10 to +16 parts per thousand. For both groups coexisting organic carbon is depleted in C-13 by 28 +/- 2 parts per thousand relative to the carbonates. Similar to numerous Neoproterozoic examples worldwide, secular variations in delta(13)C in the Bambui Group show a significant positive excursion beginning with values near 0 parts per thousand in carbonates immediately above a diamictite and rising to values as high as +16 parts per thousand upsection. The Sr isotope data for carbonates from the Bambui Group agree with the age inferred from the secular variation in oceanic delta(13)C values. The striking feature in this study is that both the carbonate and total organic carbon are considerably more enriched in C-13 than previously reported for Neoproterozoic carbonates elsewhere. It is not clear if these unusual delta(13)C values are caused by primary of secondary processes.