Do stable isotope data from calcrete record late Pleistocene monsoonal climate variation in the Thar Desert of India?

Abstract

Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I-IV of ca. 70,000-60,000, ca. 60,000-55,000, ca. 55,000-43,000, and ca. 43,000- ~25,000 yr, respectively. The youngest eolian unit (V) is < 10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal ¹⁸O-enriched "normal continental" waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000-60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ¹³C is 1.7‰ (0.0-1.7‰), and δ¹³C covaries positively and linearly with δ¹⁸O. δ¹³C values are highest when δ¹⁸O values indicate the most arid climatic conditions. This is best explained by expansion of C₄ grasses at the expense of C₃plants at low latitudes during glacial periods when atmospheric pCO₂ was lowered. C₄ dominance was overridingly influenced by global change in atmospheric pCO₂ despite the lowered summer rainfall.