Reconstruction of late Quaternary monsoon oscillations based on clay mineral proxies using sediment cores from the western margin of India

In order to reconstruct the variations in the intensity of summer monsoon precipitation during the late Quaternary, two sediment cores from the southwestern continental margin of India were studied for their clay mineral composition and grain size parameters. Kaolinite, illite and gibbsite were dominant in a core off Cochin. Illite with increasing kaolinite content towards the core top was found to dominate in a core from the continental slope off Goa. Major sources of clays at both the core sites were from the hinterland rocks and soils. Careful evaluations of several factors that could complicate the clay distribution in marine environment indicate that the clay mineral parameters can be used as proxies for the intensity of summer monsoon precipitation in the past. The relatively low values of humidity proxies (kaolinite content, K/C and K/I ratios) and higher illite crystallinity with significant variations indicate that the summer monsoons in general were weaker during the late glaciation, with distinct events of intensification at ∼28 000 and 22 000 yr BP. The last deglaciation was characterised by an increased terrigenous input with high values of humidity proxies during 15 700–14 800 cal yr BP, indicating an early strengthening of summer monsoon activity in the region. The most remarkable increase in kaolinite content, K/C and K/I ratios associated with much reduced values of chlorite and illite and illite crystallinity, however, occur between 8800 and 6400 cal yr BP. It appears that the Holocene precipitation maxima occurred after 9000 cal yr BP and lagged behind the precessional forcing. The late Holocene witnessed reduced rainfall activity and resultant decrease in hydrolysis starting at 5600 cal yr BP.