Plant-wax biomarkers and their isotopes reveal complex relationships between climate, vegetation and fire during collapse of Indus Valley Civilization

Climate variability, especially monsoonal rainfall, has significantly shaped habitable areas for human populations in South Asia in the past just as it does today. Instances of climate-driven social disruptions and population movements are evident worldwide, as evidenced for example in the Classic Maya and the Indus Valley Civilization (IVC). However, climate change can manifest in very different ways in terms of vegetation and fire regimes, with important implications for regional environmental histories as well as socio-political patterns. As such, it is essential to develop a comprehensive understanding of the intricate interplay between climate, vegetation, fire, and archaeological evidence relating to changes in settlement patterns and continuities. Insights derived from such studies offer a foundation to explore and comprehend present and future human-environment interactions.

Here we present multi-proxy time-series datasets derived from a 2.25-meter geological trench known as ‘Jankipura,’ located within the semi-arid Thar Desert. Jankipura, located near Didwana Lake, holds prehistoric importance, being surrounded by major archaeological sites in the Thar Desert. It is also a part of the Didwana Palaeolithic Complex, surrounded by the IVC, Jodhpura-Ganeshwar, and Ahar-Banas cultural regions. The chronology of the Jankipura trench is constructed based on four ¹⁴C AMS dates ranging from 183 to 4656 cal yr. BP, aligning with the Mature phase of the IVC – a period characterized by population migration and a severe reduction in settlement density. Our analysis encompasses measurements of sediment total organic and bulk carbon isotope (d¹³C_bulk) composition, alongside examinations of plant-wax molecular distributions (n-alkanes and fatty acids). Additionally, we analyzed the δ¹³C and δ²H values of long-chain n-alkanes (C₂₇, C₂₉, C₃₁, and C₃₃) and fatty acids (C₂₆, C₂₈, C₃₀, and C₃₂) extracted from the sediment samples.

Our study also involved the assessment of macro-charcoal concentrations (>125 µm, differentiating grass from wood) to reconstruct the climate-vegetation-fire relationships during and after a major period of disruption of the IVC. The findings highlight an dry phase between 4656 and 2932 cal yr. BP, characterized by a mixed C₃-C₄ vegetational landscape with limited fire episodes. A significant fire episode took place during the period from 2932 to 1960 cal yr. BP, suggesting dry conditions supported by abundant C₄ vegetation. Between 1960 and 183 cal yr. BP, three minor fire events occurred amid fluctuating rainfall conditions and a landscape dominated by mixed C₃-C₄ vegetation. The identified macro-charcoal predominantly comprised woody fragments over grass fragments. Notably, an increasing trend in isotope values, reaching its peak in macro-charcoal, is observed between 183 cal yr. BP and the present, signifying increased aridity compared to the mature phase of the IVC.

Although the study is based on a single trench, our observation of a weak relationship between vegetation and fire suggests that the reconstructed fire events may have originated from anthropogenic activities. This sheds light on the significance of vegetation, especially the utilization of wood, during the Mature phase of the IVC. We recommend generating more records from this region to better comprehend the spatio-temporal interaction of the IVC population with the environment.