10Be/9Be Ratios of Cauvery River Delta Sediments, Southern India:Implications for Palaeo-Denudation Rates in the Catchment and Variation in Summer Monsoon Rainfall During Late Quaternary

Present and past denudation rates (D) of the Cauvery river catchment determined using meteoric ¹⁰Be/⁹Be on drill-core sediment samples from Uttarangudi and Valangaiman sites in the delta vary from 18.7 ± 1.6 to 48.1 ± 8.3 t/km²/a. The present day denudation rates of 37.7 ± 4.9 t/km²/a and 36.9 ± 5.2 t/km²/a estimated for these sites are higher by a factor of two than that based on solute and suspended load of the Cauvery river. Denudation rates estimated using ¹⁰Be/⁹Be (reactive) is more accurate as it is not affected by damming of rivers. Based on ⁹Be fraction (reactive + dissolved) and previous studies, we infer that sediments for Valangaiman site were mainly sourced from Western Ghats and Mysore plateau, whereas highlands bordering southwestern margin of the delta mostly supplied sediments to the Uttarangudi site. The Western Ghats and the delta received rainfall mainly during summer monsoon and NE monsoon respectively. Comparison of palaeo-denudation rates with various proxies of the Indian summer monsoon shows inverse relationship between them. Lower denudation rates are estimated for Early to Mid-Holocene period which is characterized by intense rainfall. Drier conditions after 4.5 ka BP caused increase in denudation rates after 2.5 ka BP for the Valangaiman core, consistent with a response time of 2 ka required to change ¹⁰Be/⁹Be in sediments of the Cauvery basin.