Proterozoic karst events lowered δ13C values by as much as 11‰ for the 800 Ma Beck Spring Dolomite, California, and as much as 8.5‰ for the 1.1 Ga Mescal Limestone, central Arizona, relative to the originally deposited carbonate. The 13C changes are attributed to input of 13C-depleted organic CO2 derived from photosynthetic organisms that colonized the ancient land surface. The large isotopic shift and its presence at two separate localities suggest that Proterozoic karst surfaces were colonized by significant photosynthetic communities with phytomasses possibly approaching those of today.

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