ORIGINS OF CALCRETE DEVELOPED ON PRECAMBRIAN SILICATE ROCKS USING SR ISOTOPIC TRACING AND MICROMORPHOLOGICAL ANALYSIS: IMPLICATIONS FOR CLIMATE CHANGE IN THE SEMI-ARID INTERIOR OF SOUTH INDIA

This study associates micromorphological analysis and isotopic tracing of the various calcrete occurrences on basement rock outcrops in South India, in order to understand their formation and evolution, and assess their value as indicators of paleoclimatic change. The two main settings are upland Karnataka (ca. 900 m) and lowland Tamil Nadu (ca. 400 m), respectively.

Toposequences and vertical profiles were described and sampled for measuring the isotopic composition of specific phases of carbonates, including susceptible bedrock sources of calcium. ⁸⁷Sr/⁸⁶Sr ratios are specific to different marine and terrestrial reservoirs and materials and are not modified by dissolution and precipitation processes. Sr concentrations and isotopic compositions were determined by thermal ionization mass spectrometry (TIMS). Results on carefully targeted fabric sub-units identified in thin section have given Sr isotopic ratios of 0.709218 and 0.709383 for laminar calcrete capping elevated parts of the lowland landscape in Tamil Nadu. These values are close to ratios known for Quaternary sea water (0.7092), implying advection of Ca-rich dust from exundated carbonate platforms during low sea stands. For sites in upland Karnataka (calcic soil horizon and colluvial lag, respectively) Sr ratios of 0.711237 were obtained. Such a value is closer to those observed for silicate bedrock, and suggests near-field Ca transfer by water through slope wash processes. We therefore reveal three categories of calcrete deposit: (1) where no local commanding relief exists (Palghat Gap), outcropping groundwater paleocalcrete is shown to also incorporate far-field aeolian Ca. (2) Near-field Ca provenance dominates in the upland sites that are more remote from India's shorelines and where a commanding bedrock landform has supplied solutes with no detectable aeolian input. (3) In situ Ca accumulation by primary pedogenesis is variable and difficult to quantify at any given site exhibiting pedogenic carbonates. Based on the diversity of (1) formative hydrological settings, (2) provenances of calcium, and (3) fabric transformations, most calcrete occurrences thus appear to be hybrid, polygenetic and partly relict, but indicative of fairly stable semi-arid climatic conditions in the rainshadow of the Western Ghats at the 100 ky time scale.