Trace element variations in Indian speleothems: Insights into the Holocene climate

Speleothems from the Gupteswar and Kadapa caves were investigated using trace element (TE) ratios (Mg, Ba, Sr, Rb, Mn, Ti, U) over the last 7000 years, supported by ²³⁰Th/²³⁴U dating, to understand the regional and local climate variability induced by the Indian Summer Monsoon. The behavior of TE in calcite is influenced by temperature, seepage pathways, bedrock chemistry, and vegetation cover above the cave. A continuous decrease in the Ba/Sr ratio since 7 ka BP indicates changes in regional precipitation or local moisture availability during this period. However, determining changes in drip water rate and chemical composition is challenging due to the lack of supporting data. Variations in the TE ratios suggest significant climate and depositional changes at ∼4.2 ka BP and ∼2.8 ka BP. A decline in precipitation was observed between 4.2 and 3.8 ka BP and at ∼2.8 ka BP, with an increase in aeolian deposition at the study site since 3.8 ka BP because of preceding dry conditions. The moist and dry climate phases deduced from stable isotope values corroborate the variations in trace element ratios, affirming the paleoclimatic significance of the studied trace elements in tropical cave deposits. A probable explanation for the decline in precipitation at the study location around 4.2 ka BP is the southward migration of the Intertropical Convergence Zone (ITCZ) along with a negative phase of the Indian Ocean Dipole. The prevailing dry conditions in this region could have impacted the northward monsoonal winds, contributing to the decline of the Indus Valley Civilization. Another abrupt change observed at ∼2.8 ka BP can be linked to low solar activity and the southward movement of the ITCZ, coupled with enhanced El Niño-Southern Oscillation activity.