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About 300 published and unpublished measurements of carbon and oxygen stable isotope ratios of calcretes are compared in an attempt to define general trends.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.
Summary
Authigenic calcareous accumulations within regoliths can be simply classified for geotechnical purposes as calcareous soils, calcified soils, powder calcretes, nodular calcretes, honeycomb calcretes, hardpan calcretes, and calcrete boulders and cobbles. Each of these categories represents a particular stage in the growth or weathering of a calcrete horizon and possesses a significantly different range of geotechnical properties. A similar classification can be applied to other pedocretes.
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