Mid–late Holocene monsoonal variations from mainland Gujarat, India: A multi-proxy study for evaluating climate culture relationship
A multi-proxy study involving palynology, phytoliths, sedimentology, clay mineralogy, carbon isotopes and magnetic mineralogy was carried out on Wadhwana Lake sediments from sub-humid zone of mainland Gujarat to determine the mid-Holocene climatic fluctuations, and its possible impact on the Harappan culture. The 1.03 m sediment profile of Wadhwana Lake shows five paleoclimatic phases. The study reveals high lake stand during Phase I (~ 7500-5560 cal yr BP). A considerable cool and moist climatic condition during Phase I is inferred due to the presence of pollen belonging to wet evergreen taxa and high phytolith climatic index 'Ic'. Later part of Phase I show gradual replacement of evergreen to deciduous pollen taxa, decrease in Ic value and dominance of smectite over kaolinite, indicating a reduction in wet climatic condition due to decline in the precipitation and prevalence of seasonally dry climate. However, a large variety of Madhuca, Meliaceae, Asteraceae pollen taxa, abundant fresh-water algae Chrozophora and Ceralia pollen in this period provides evidence of intensified arboriculture and agricultural activity. Low lake level and dry climate have been documented during Phase II (5560 to ~ 4255 cal yr BP) and are synchronous with the lake records of western Indian region. High values of phytolith aridity index Iph, high primary minerals, increase in the I´13C values provide evidence for excessive dry climatic conditions at ~ 4255 cal yr BP. Phase III shows a gradual strengthening of SW monsoon after ~ 3500 cal yr BP. Phase IV shows a short pulse of dry climatic conditions (~ 3238 to ~ 2709 cal yr BP) followed by somewhat similar to present day climate for Phase V. The study concludes that onset of dry climate after 5500 cal yr BP is a regionally spread synchronous event that has been documented in several lake records of western India. It is surmised that the urbanization in Harappan civilization in North West India was coincident with the initial phase of declining rainfall of mid-Holocene. The emergence of cultural complexity of Harappan civilization should be seen as an initial adaptation to earliest phase of environmental deterioration and its subsequent decline is probably linked with the changing seasonality pattern and excessive dry climate of later phase (~ 4200-4255 cal yr BP) of mid-Holocene.