Paleorainfall during the past two millennia in the Western Ghats, south-western India: Evidence from a multi-proxy lacustrine sedimentary record

The Indian monsoon, a critical component of the global climate system, plays a vital role in shaping the environmental and socio-economic landscape of the Indian subcontinent. A decreasing trend of southwest monsoon rainfall is documented in the southern part of the Western Ghats during the 20^th century. The current study attempts to reconstruct the short- and long-term changes in monsoon during the past 1600 years based on multi-proxy studies on lacustrine sediments from Cheppandikere Lake (CK), situated in the Western Ghats in southwestern India. Three AMS ¹⁴C dates constrain the age of sediment core collected from the lake and span the past ∼1600 years. The environmental magnetic (χ_ARM, χ_fd %, χ_lf, IRM, SIRM, S-ratio, and HIRM) data indicates that the magnetic mineral concentration, magnetic grain size, and mineralogy have varied significantly during the past in response to changing rainfall conditions. The diffuse reflectance spectra (DRS) denote the distinctive peaks of sediment components like hematite, goethite, clay minerals, and organic matter. The temporal variation of the different sediment components is implied by the component scores obtained by Principal Component Analysis (PCA). The aquatic productivity and input of terrestrial organic matter are indicated by organic carbon, C/N, δ¹³C and δ¹⁵N values. The multi-proxy data suggests the presence of two distinct climatic phases in the region during the past two millennia. Phase I (∼300–1150 AD) is characterized by lower magnetite, higher hematite, coarser magnetic grain size, finer particle size, decreased terrestrial organic matter, a higher proportion of clay minerals, enhanced aquatic productivity, and a lower proportion (52 %) of C₃ vegetation (higher C₄; 48 %) pointing towards weak monsoonal conditions. However, higher magnetite, lower hematite, finer magnetic grain size, increased terrestrial organic matter, a higher proportion of clay minerals, reduced aquatic productivity, and a higher proportion (92 %) of C₃ vegetation (lower C₄; 8 %) indicate a stronger monsoon and increasing trend of rainfall during Phase II (∼1150 AD to Present). The study shows that the monsoon in the Western Ghats has strengthened overall in the region and responded to global climatic episodes like the Little Ice Age (weak), Medieval Warm Period (strong) and Dark Age Cold Period (weak). A similar pattern is also seen in other lacustrine records in the Western Ghats, albeit with differences in their short-term variability.