Résumé
A marked change in the vegetation of the Western Ghats at ca. 3500 yr B.P. has been observed from a study of two marine cores taken from the inner shelf off Karwar (Karnataka, western India), at the mouth of the Kalinadi River. This change is indicated by a decrease in forest pollen, particularly from evergreen forests and by a corresponding increase in savanna pollen, as well as by a reduction in mangrove pollen. At the same time, the values of the stable isotope δ
13C of sedimentary organic matter increased suddenly due to a higher contribution of organic matter of margine origin. All these modifications were probably induced by a less humid climate (the term “drier” can hardly be used for an area where the annual rainfall still attains 6 m in the hills).
Above this clear limit, the same trends (steady but considerably weaker), also resulting a reduction in the humid conditions, were recorded up to ca. 2200 yr B.P.: reduction in forest vs. increase in savannas; decrease in the mangrove vegetation; higher δ
13C values. From there onwards the pollen assemblages have remained uniform up to the present day. It may be deduced that a climate quite similar to the present conditions was established about 2200 yr ago, and still prevails.
The curves representing the percentages of marine microfossils-dinoflagellates, organic tests of foraminifera, eggs of copepods-show little variation at the 3500 yr B.P. boundary. The marine assemblages were modified drastically only at about 2200 yr B.P. These sharp modifications are probably due to an increase in the marine influence as a result of a reduction in the fresh water supply from the Kalinadi. The stability in the representation of marine assemblages which has persisted from about 2200 yr B.P. to the present confirms the establishment of the present-day climate around that date.
This paleoclimatic reconstruction is in conformity with the evidence for the beginning of drier conditions recorded at about the same time in the low latitudes in many parts of the world: India, northern Arabian Sea, Africa (Sahara, tropical forest, East African lakes), Australia and the Carribbean.