Geochemistry of Calcretes (Calcic Palaeosols and Hardpan), Coimbatore, Southern India: Formation and Paleoenvironment

This paper deals with the late Quaternary calcic nodules formed within vertisols in the foot-hill regions, and hardpan calcretes (greater than 200 ka) formed over the Precambrian substrate that occur as duricrust horizons in the plains around Coimbatore region, Tamil Nadu, India. The bulk chemistry of calcic nodules and the hardpan calcretes show very little variability in CaO, SiO2, Al2O3 and Fe2O3 content. The trend of trace metal content in calcic nodules and in the hardpan calcrete is Mn>Zn>Ni>Cr>Pb>Co>Cu and Zn>Mn>Ni>Co>Pb>Cu>Cr respectively. PAAS- normalised REE data of the calcretes (calcic nodules and the hardpan) demonstrate a positive Eu anomaly. This could be attributed to feldspar, apatite mineral alteration and soil digenetic processes, differential weathering leaving behind plagioclase phenocrysts and apatite grains which are enriched in Eu. The stable isotope values of the calcic nodules do not exhibit a wide range (δ18O -3.39 to -5.84 per mille and δ13C -3.01 to -6.64 per mille), compared to the hardpan calcretes (δ18O -2.91 to -12.98 per mille ) and δ13C (-0.05 to -7.4 per mille). The palaeoenvironment during the formation of the calcretes nodules was dominated by the C4 plants with sparse vegetation cover and that the parent material/bedrock supported a thin veneer of soil column as present day. Calcic nodule accumulations in the soils has resulted from differential weathering and chemical histories within the soil profiles while the hardpan calcretes formed by complex pedogenic and chemical - groundwater processes in arid - semi arid conditions over a long geological period. During the late MIS3 to the LGM period the south west monsoon was weak with mean annual rainfall (MAR) between 300- 500 mm/yr., the sea level was low and the rainfall at the time of carbonate formation would have remained damp enough to allow silicate weathering of the soil sediments. (author)