Precipitation variability over Northwest Himalaya from ∼4.0 to 1.9 ka BP with likely impact on civilization in the foreland areas

We present a stalagmite based high resolution climatic record between similar to 4.0 and 1.9 ka BP from Dharamjali Cave in Pithoragarh (Kumaun Himalaya), an area which is influenced primarily by the Indian Summer Monsoon (ISM) and supplemented by the Indian Winter Monsoon (IWM). The chronology of the 41.5 cm long DH-1 stalagmite was constructed using a StalAge model on six Th-230/U dates. However, some samples have higher Th-232 concentrations, leading to an increase in the final age uncertainties, hence the chronology supports only the possible climatic boundaries. However, the significance of this work lies in its being one of the rare studies of speleothems to reconstruct high resolution climatic changes in the NW India for the Upper Holocene. The delta O-18 values, ranging between - 5.3 parts per thousand and -10 parts per thousand, show a large variation, compared to the areas dominated by a single monsoon, and this can be ascribed to the two sources of moisture (e.g., ISM and IWM) in the study area during the Upper Holocene. The sample consists of aragonite except for two sections of calcite growth from 0-7.3 cm and 8.5-13.5 cm from the top. The climatic reconstruction, within the age uncertainties, indicates strengthened precipitation from 4.0 ka BP with a sharp drop ( > - 2 parts per thousand) in delta O-18 values, peaking at similar to 3.7 ka BP. A gradual decline in precipitation is observed from similar to 3.7 to 3.0 ka BP with possible droughts, centered at similar to 3.4, similar to 3.2 and similar to 3.0 ka BP. Subsequently, climatic amelioration took place between similar to 3.0 and 2.9 ka BP, showing fluctuating trend in delta O-18 values with comparatively more rainfall, possibly generated by the IWM in the form of thunderstorms and hailstorms from similar to 2.9 to 2.7 ka BP. Precipitation declined from similar to 2.7 to 2.4 ka BP with a decadal scale major drought event, strongest in the present data set, at similar to 2.5-2.4 ka BP, whereas, an abrupt drop in stalagmite delta O-18 values from similar to 2.4 to 2.3 ka BP points to increased precipitation intensity. Thereafter, the precipitation gradually decreased until ca. 2.1 ka BP with one of the driest events at similar to 2.1 ka BP. A century scale increasing trend in the precipitation intensity is observed from similar to 2.1 to 2.0 ka BP, following which the precipitation again declined. Accordingly, five sudden drought events are documented, centering at similar to 3.4, similar to 3.2, similar to 3.0, similar to 2.5-2.4 and similar to 2.1 ka BP. A gradual reduction in precipitation from similar to 3.7 to 3.0 ka BP coincides with reduction and deurbanisation and step -wise disintegration of the Harappan civilization along the Indus-Ghaggar-Sarasvati valleys in the foreland areas of northwestern India.