Calibrating Geochemical Proxies in an Aragonite Stalagmite from the West Coast of India

The Indian Monsoon is a major component of the global climatic system. Reconstruction of its past behaviour from stalagmites could be a powerful approach to understand its operation in varied climate states. Such records from the west coast would be valuable for the information they might impart about rainfall intensity and composition as air-masses first move onto the continent from the Arabian Sea, providing important constraints for other sites further inland. Carbonate rocks are sparse on the west coast, however, limiting availability of potential stalagmite sites. Here we report δ18O, δ13C and trace element analyses on the only stalagmite so far reported from the west coast. The sample grew from 1666 to 1997 AD in north Karnataka and is formed of aragonite, rather than calcite which is more commonly used for speleothem paleoclimate reconstruction. We aim to assess the use of aragonite geochemistry for paleoclimate reconstruction in general, and the rainfall at this particular site. Findings include: 1) Carbonate forming in the cave today appears to be in oxygen-isotope equilibrium, but the stalagmite has a kinetic overprint which is influenced by the slope of the growing surface of the sample. This local control on δ18O makes its use for reconstruction challenging. 2) Trace element measurements may provide more robust indicators of local paleoclimate. 3) High resolution sampling of 11 annual growth layers capture a strong seasonal record controlled by prior aragonite precipitation (PAP). PAP, similar to PCP, is a promising proxy for seasonal dryness or seasonal change in the partial pressure of carbon dioxide in this cave.