Comparison of stable isotope values of Quaternary calcretes from Adana and Mersin provinces: implications on controlling factors

Abstract

Calcretes are widespread in Adana and Mersin provinces and form under different morphologies. Most calcrete profiles comprise a hard laminated crust/hardpan at the top, gradually intergrading into the nodular and/or tubular/columnar horizon with depth. This study compares the delta O-18 and delta C-13 values of calcretes from both provinces and discusses the controlling factors and environmental conditions. The delta O-18 and delta C-13 values are characteristic for the pedogenic calcretes. The Adana calcrete mean delta O-18 values of the hardpan, nodules and tubes, and fractures-infills are 0.69 parts per thousand, 0.77 parts per thousand, and 1.04 parts per thousand PDB heavier than those of the Mersin calcretes, respectively. The overall difference between the two groups is 0.78 parts per thousand PDB. The differences are related to the high evaporation rate in Adana province in respect to Mersin province under similar climatic conditions, except for the evaporation rate. The high evaporation rate in Adana province is due to higher ventilation. The delta C-13 values of both provinces are ahnost the same, reflecting calcrete formation in soil with abundant C-3 vegetation similar to contemporary vegetation. In addition, the mean delta C-18 and delta(1)3C values of the hardpan calcretes slightly differ from those of the columnar horizon, showing a depletion in heavy isotopes. The depletion in the mean delta C-18 values of hard laminated crust in respect to the columnar horizon is 0.09 parts per thousand PDB for Adana calcretes and 0.12 parts per thousand PDB for Mersin calcretes. This is related to the relatively thick water film from which the calcretes formed by precipitation and displacive replacement processes. The difference in the mean delta C-13 values is 0.32 parts per thousand PDB for the Adana calcretes and 0.11 parts per thousand PDB for the Mersin calcretes, and the depletion in delta C-13 values of the hard laminated crust reflects proximity of the bioactive horizon in the soil.