A High Resolution Speleothem Record from NE India: Paleoseismic and Modern Climate Insights through U-Th and δ18O Analysis

Abstract

The state of Meghalaya in NE India is a vibrant landscape where people’s livelihoods are intimately tied to the regional geology and seasonal climate. The climate of Meghalaya is defined by the Indian Summer Monsoon (ISM), which brings immense rainfall during the summer months. Uplift of the Shillong Massif, which dominates the topography of Meghalaya and contributes to orographic ISM precipitation in this region, is associated with historic mega earthquakes. However the long-term seismic history of the region is not well understood. Caves on the southern edge of the Massif, including Mawmluh Cave, provide unique opportunities to both improve understanding of the seismic history of NE India through uranium-thorium (U-Th) dating of tectonically broken speleothems and to study relationships between the ISM and other global climate indices through δ18O analysis of annually laminated speleothems. U-Th dating indicates that new speleothem growth on top of large broken speleothems in Mawmluh Cave initiated within 7 ± 2.2 years following the magnitude 8.7 1950 Assam-Tibet earthquake. These results suggest that dating of older breakdown structures could provide a stronger record of the seismic history of this region. Furthermore, a high-resolution modern speleothem δ18O record from Mawmluh Cave reveals ISM strength and frequency variations correlated with known changes in behavior of the El Niño Southern Oscillation over the past ~60 years, providing a new understanding of the influences on precipitation in NE India and a valuable calibration dataset for long-term speleothem paleoclimate records from this region.