Monsoon Fluctuations in Two 20,000-Yr B.P. Oxygen-Isotope/Pollen Records Off Southwest India

Abstract

Two marine pollen diagrams for sediments off the southwest coast of India provide a 20,000-yr history of the nearby continental vegetation, determined by the monsoon climate, within the framework of the isotopic stratigraphy. Two important phases of the evolution of the monsoon climate are a very arid period about 22,000–18,000 yr B.P. and a very humid period culminating at 11,000 yr B.P. The very arid period corresponds to the lowest pollen representation of mangrove vegetation, which is used here as an indirect indicator of monsoonal runoff. This aridity is due to a very weak southwest airflow, a great reduction of summer monsoonal rainfall, and reduced runoff of the western Ghats rivers. The extension of the monsoon over India after the last glacial maximum has been a gradual process following the northward progression of the Intertropical Convergene Zone. The very humid period, corresponding to the highest pollen representation of mangrove vegetation and the heaviest summer rainfall, is a climatic response to the maximum summer insolation of the Northern Hemisphere at 11,000 yr B.P.