Preserved stable isotopic signature of subaerial diagenesis in the 1.2-b.y. Mescal Limestone, central Arizona: implications for the timing and development of a terrestrial plant cover.

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. delta ¹⁸ O values of the Mescal carbonates show two distinct data sets: 1) dolomites from an unaltered zone which were unaffected by subaerial diagenesis have delta ¹⁸ O values ranging from +19.9 to +25.6per mille Standard Mean Ocean Water (SMOW) with cherts averaging +30per mille, and 2) dolomites from a dissolution zone subaerially exposed in the Precambrian with delta ¹⁸ 0 values ranging from +13.9 to +22.4per mille and cherts averaging +25per mille. delta ¹³ C of dolomite ranges from +3.7per mille Pee Dee Belemnite (PDB) in the unaltered zone to 0per mille 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 delta ¹⁸ O of the unaltered dolomite was fixed during early meteroic-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 vegetatively covered land surface. Alternative explanations for producing the observed delta ¹³ C variations in the absence of vegetation do not seem feasible. We therfore suggest that the subaerial environment 1.2 b.y. ago was a biologically active zone. -Authors