Stable isotope variations in the Neoproterozoic Beck Spring Dolomite and Mesoproterozoic Mescal Limestone paleokarst: Implications for life on land in the Precambrian

Proterozoic karst events towered delta C-13 values by as much as 11 parts p er thousand for the 800 Ma Beck Spring Dolomite, California, and as much as 8.5 parts per thousand for the 1.1 Ga Mescal Limestone, central Arizona, r elative to the originally deposited carbonate. The C-13 changes are attribu ted to input of C-13-depleted organic CO2 derived from photosynthetic organ isms that colonized the ancient land surface. The large isotopic shift and its presence at two separate Localities suggest that Proterozoic karst surf aces were colonized by significant photosynthetic communities with phytomas ses possibly approaching those of today.