Stable isotope abundances in calcretes

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Summary About 300 published and unpublished measurements of carbon and oxygen stable isotope ratios of calcretes are compared in an attempt to define general trends.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.