Lithium Isotope Reveals Tight Links Between Chemical Weathering Intensity and Indian Summer Monsoon in South Asia

Abstract

The relationship between catchment silicate weathering and short-term (<10(4) yrs) variations in the Indian summer monsoon (ISM) remains a topic of debate. To investigate this link, we present a comprehensive analysis of lithium isotopic composition (delta Li-7), neodymium isotopic composition (epsilon Nd), and chemical index of alteration (CIA) in the silicate phase of clay fractions from a sediment core retrieved from the western Bay of Bengal. The values of epsilon Nd, delta Li-7 and CIA range from -14.7 to -10.5, -1.7-0.1 parts per thousand and 68 to 81, respectively. A source-corrected delta Li-7(clay-source) is used as a proxy for chemical weathering intensity. Our weathering records show strong correlations with the ISM on both orbital and millennial timescales. At the orbital scale, increased weathering coincides with periods of global warming and a strengthened ISM. At the millennial scale, variations in our proxies closely track global (Heinrich stadial 1) and regional (8-6 ka) events, highlighting the sensitivity of silicate weathering to climatic variability. The prevailing warm and humid climatic conditions since the last deglaciation have promoted new soil formation, thereby enhancing the chemical weathering intensity captured by the clay fraction. These findings provide compelling evidence for a tight connection between catchment-scale silicate weathering and short-term ISM variations, offering valuable insights into the complex interplay between earth's surface processes and regional climate dynamics.