Stable isotope abundances in calcretes

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Abstract
Summary: About 300 published and unpublished measurements of carbon and oxygen stable isotope ratios of calcretes are compared in an attempt to define general trends. The δ 13 C values of the carbonates range from − 12 to + 4‰ PDB. On a global scale the main factor responsible for the average 13 C content of calcrete in an area appears to be the 13 C content of the dominant plants in the region. No correlation could be found between 13 C contents and radiocarbon age. The carbonate δ 18 O values range from − 9 to +3‰ PDB. On a global scale, the factors discussed as possibly responsible for the 18 O content of a calcrete are the 18 O content of the local rainfall, temperature, and the extent to which the water underwent free surface evaporation before or during calcrete formation. Little or no correlation was found between 13 C or 18 O contents and rainfall, temperature, or evaporation rate on a world basis. Some relation between the 18 O content of calcrete and rain on a regional basis was confirmed. The range of 13 C and 18 O values suggest a diversity of origins and environments of formation.