Calcium Isotopes in Stalagmites as a Quantitative Precipitation Proxy for Comparison to Instrumental Records

Abstract:

Stable oxygen and carbon isotopes from stalagmites in caves have been frequently used to create qualitative maps of precipitation over the course of history. In order to determine if calcium isotopes (δ^44/40Ca) in speleothems can be used to provide accurate, quantitative precipitation reconstructions, we measured δ^44/40 Ca in a modern stalagmite from Mawmluh Cave in Northeast India. The part of the stalagmite studied grew from 1969-1996 AD, and it was sampled for δ^44/40 Ca at annual resolution. Drip water, host rock, glass plate, and modern precipitation data from Mawmluh Cave were used to calibrate a Rayleigh Fractionation Model. This determined the fraction of carbonate remaining in the solution after prior carbonate precipitation (PCP) occurred. This information was then compared with the glass plate calcite and modern rainfall to link the amount of PCP occurring in the cave system to rainfall amount. We compared calculated precipitation amounts based on stalagmite δ^44/40 Ca to corresponding instrumental annual rainfall for Northeast India from 1969-1996. Although the magnitude of rainfall changes calculated from the speleothem record did not match the observed rainfall amounts, the stalagmite-based rainfall record displayed some of the same trends, such as increasing between 1983 to 1985. A previous study using δ^44/40 Ca in a modern stalagmite from Heshang Cave, China, produced more accurate rainfall reconstructions. Although both caves are located in monsoon regions, Mawmluh Cave receives more annual rainfall. It is possible that δ^44/40 Ca as a proxy is most suitable for picking up long periods of aridity that are not present in our modern record from this site.