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

Abstract

We present a stalagmite based high resolution climatic record between ∼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 230Th/U dates. However, some samples have higher 232Th 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 δ18O values, ranging between −5.3‰ and −10‰, 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‰) in δ18O values, peaking at ∼3.7 ka BP. A gradual decline in precipitation is observed from ∼3.7 to 3.0 ka BP with possible droughts, centered at ∼3.4, ∼3.2 and ∼3.0 ka BP. Subsequently, climatic amelioration took place between ∼3.0 and 2.9 ka BP, showing fluctuating trend in δ18O values with comparatively more rainfall, possibly generated by the IWM in the form of thunderstorms and hailstorms from ∼2.9 to 2.7 ka BP. Precipitation declined from ∼2.7 to 2.4 ka BP with a decadal scale major drought event, strongest in the present data set, at ∼2.5–2.4 ka BP, whereas, an abrupt drop in stalagmite δ18O values from ∼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 ∼2.1 ka BP. A century scale increasing trend in the precipitation intensity is observed from ∼2.1 to 2.0 ka BP, following which the precipitation again declined. Accordingly, five sudden drought events are documented, centering at ∼3.4, ∼3.2, ∼3.0, ∼2.5–2.4 and ∼2.1 ka BP. A gradual reduction in precipitation from ∼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.