A Precambrian exposure surface in the 1.2-b.y. Mescal Limestone has been isotopically examined for indications of a carbon isotopic signature that might indicate the presence of a subaerial vegetative cover in the middle Proterozoic.
δ18O values of the Mescal carbonates show two distinct data sets: (1) dolomites from an unaltered zone which were unaffected by sub-aerial diagenesis having δ18O values ranging from +19.9 to +25.6‰ Standard Mean Ocean Water (SMOW) with cherts averaging +30‰, and (2) dolomites from a dissolution zone subaerially exposed in the Precambrian with δ18O values ranging from +13.9 to +22.4‰ and cherts averaging +25‰. δ13C of dolomite ranges from +3.7‰ Pee Dee Belemnite (PDB) in the unaltered zone to 0‰ in the dissolution zone. The dissolution zone consists of a karst breccia of recemented dolomite and chert fragments with numerous clastic solution dikes.
Isotopic and field data indicate that the δ18O of the unaltered dolomite was fixed during early meteoric-water diagenesis, including dissolution-silicification of evaporites and dolomitization. During a later subaerial exposure event, a large flux of meteoric water flushed through the dissolution zone and produced the same isotopic patterns that Allan and Matthews documented for younger examples as indicative of a vegetatively covered land surface. Alternative explanations for producing the observed δ13C variations in the absence of vegetation do not seem feasible. We therefore suggest that the subaerial environment 1.2 b.y. ago was a biologically active zone.