Whispers from the Depths: tracing a century of environmental shifts in the Central and Kashmir Himalayas

Environmental changes in the Indian Himalayas due to human activities remain poorly understood. To investigate this, we analyzed lake sediment cores from the Central and Kashmir Himalayas to reconstruct pollution, eutrophication, and fire history over the past-century. We measured concentrations, compositional variations, and temporal trends of alkylated-polycyclic aromatic hydrocarbons (PAHs), monosaccharide anhydrides, fecal biomarkers, and aliphatic hydrocarbons, including n-alkanes, acyclic and cyclic isoprenoids. We hypothesize that pyrogenic processes, particularly low-intensity forest-fires, play a significant role in lake eutrophication by delivering nutrient-rich ash and altering catchment biogeochemistry, thus accelerating ecological changes in these sensitive alpine ecosystems.

Our findings show a steady rise in total PAHs since the 1970s, with compositional shifts reflecting intensified socio-economic activities and pollution. Diagnostic ratios of multiple methylated and non-methylated PAHs suggest that pyrogenic sources are the main contributors to sedimentary PAHs. Botryococcenes (C_31-34) and n-alkanes concentrations also displayed an increasing trend, peaking in the 1980s. This peak coincided with changes in the composition of long (nC_26-36), mid (nC_21-25), and short-chain (<nC₂₁) n-alkanes, reflecting environmental shifts likely driven by both pollution and fire-related inputs. The detection of monosaccharide anhydrides supports the occurrence of regional low-intensity fires, likely contributing to nutrient influx through ash deposition and post-fire runoff, further altering lake trophic states.

This study proposes a fire-induced nutrient enrichment mechanism 'pyroeutrophication', acting in tandem with anthropogenic pollution. As the first of its kind from the Indian subcontinent, our findings offer valuable molecular insights into historical environmental changes and shed light on mitigation strategies for fragile Himalayan landscapes.