Abrupt weakening of the summer monsoon in northwest India 4100 yr ago

Climate change has been suggested as a possible cause for the decline of urban centers of the Indus Civilization ~4000 yr ago, but extant paleoclimatic evidence has been derived from locations well outside the distribution of Indus settlements. Here we report an oxygen iso- tope record of gastropod aragonite (δ 18 O a ) from Holocene sediments of paleolake Kotla Dahar (Haryana, India), which is adjacent to Indus settlements and documents Indian summer mon- soon (ISM) variability for the past 6.5 k.y. A 4‰ increase in δ 18 O a occurred at ca. 4.1 ka mark- ing a peak in the evaporation/precipitation ratio in the lake catchment related to weakening of the ISM. Although dating uncertainty exists in both climate and archaeological records, the drought event 4.1 ka on the northwestern Indian plains is within the radiocarbon age range for the beginning of Indus de-urbanization, suggesting that climate may have played a role in the Indus cultural transformation.