The objective of this project was to gather new data on past environmental changes in the semi-arid region of Peninsular India in order to understand how this zone, located in the rainshadow of the Western Ghats, has reacted to the quaternary climatic fluctuations (i.e intensification of the south-west indian monsoon). The presence of calcretes in this area provided an alternative to the study of biological indicators rarely found in semi-arid zones.
Calcretes, which are largely present in the arid and semi-arid lands, constitute good indicators of paleo-climatic and paleo-ecological transformations due to their formations in the interfacing of lithosphere, hydrosphere, atmosphere and biosphere (pedological environment). Since the studied calcrete formed on the rocks of the pre-cambrian basement and hence on non-calcareous rocks, the intermediary goals were to determine the origins of calcium (atmospheric input, in situ weathering or lateral transfer at lanscape scale) and to date the different calcrete morphologies.
Materials and Methods:
The two main sites studied were located in the regions of Gunlupet (South Karnataka) and Coimbatore (north-west of Tamil Nadu). These areas were particularly interesting because of several morphological varieties of calcretes developed on silicate rocks formed in different lithological and geomorphological settings. The origin of calcium in this instance is only partially determined.
The understanding of calcium dynamics through the landscapes requires a multidisciplinary approach coupling geomorphology, pedology and geochemistry carried out at different scales. The geomorphic and pedological analysis enables the establishment of a typology of the main morhologies of carbonate concentrations and situating them in their landscape setting. The petrographical analysis (optical microscopy, cathodoluminescence) enables the identification of relations between the diverse forms of calcrete and the causes of their formation. The origins of calcium accumulated in these carbonated indurations is determined by the utilisation of strontium isotopes (87/Sr/86Sr according to the material). The absolute datings which provide an indispensable chronological dimension to the understanding of the paleoenvironmental changes are obtained through the Uranium/Thorium (230Th/234U) method.