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The Kothiyakhad sedimentary sequence of Mahi estuary in mainland Gujarat, India, contains valuable information on late- to mid-Holocene climatic conditions as inferred by phytolith, palynofacies, magnetic susceptibility and clay mineralogical studies. Three distinct climatic regimes, ie, Phase I, II and III, were established. Phase I (3660—~3400 yr BP) shows a gradual weakening of SW monsoonal activity, though overlapped by enhanced western disturbances which led to the development of cool climatic conditions. The coupled effect of SW monsoon and enhanced winter precipitation produced improved hydrological conditions, which supported the agrarian society of the Indus Valley civilization until the beginning of Phase II (~3400—~3000 yr BP). During Phase II the SW monsoon was in a state of severe recession, leading to severe drought-like conditions, other than for a brief but intensely warm and humid pulse recorded at ~3320 yr BP, associated with SW monsoonal activity. In Phase III (~3000—2850 yr BP), SW precipitation fluctuated greatly with a considerable increase in warm summer conditions, similar to present-day conditions. The weak SW monsoonal activity ~3500 yr BP also coincided with a global cool and arid phase and this probably explains the timing as well as the cause of why the population of the Indus civilization migrated to more humid areas to sustain their livelihoods.
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