Geochemical signals for anthropogenic eutrophication and climate change from the Kallar Kahar Lake, Salt Range, Pakistan

Small, shallow lakes are under severe stress from the combined impact of anthropogenic eutrophication and modern climate change. Therefore, such lacustrine sediments provide key information for understanding the lake’s eutrophication and the ongoing unprecedented climate change. The present work focuses on the saline Kallar Kahar Lake located in the Salt Range of Pakistan, to unravel the extend of anthropogenic impacts and climate change on sedimentation in lakes using a core section of 180 cm. Dark gray marly mud dominates with interlayered greenish-gray sandy/silty layers in the lower part. Geochemical results indicate that the SiO₂ and Al₂O₃ contents drop to around one-third (from 54.6% to 16.4% and from 9.8% to 3.5% respectively) vertically upward in the cored interval, whereas the CaO content displays a three-fold upward increase (from 11% to around 30%). Similarly, the Total Organic Carbon (TOC) and Total Organic Sulphur (TOS) contents show a three-fold vertical upward increase (from 3% to > 15% and < 0.3%  to 0.9%). The paleoredox proxies V/(V + Ni), V/Cr, and especially Th/U indicate an upward decrease in the oxygenation level and establishment of an anoxic setting in the lake. Paleoclimate proxies including Mg/Ca, Rb/Sr, and Sr/V indicate a progressive upward increase in the aridity. Paleosalinity proxies Sr/Ba and Rb/K  suggest an upward increase in the salinity of the lake. The anthropogenic impact proxy elements Mo and Hg indicate enrichment and display strong negative correlations with detrital supply. Similarly, As, Pb, Zn, and P neither correlate with the detrital influx proxies nor with in situ sedimentation proxies thereby pointing to their anthropogenic source. These results indicate that urbanization and anthropogenic activities have blocked the natural drainage of the Kallar Kahar Lake, reducing the detrital influx to around one-third. The increasing aridity of the area due to modern climate change has transformed the lake into a closed-water body where evaporation has increased the salinity forcing a nearly three-fold increase in the in-situ organic carbonate production in the sampled interval. Thus the Kallar Kahar Lake provides an ideal case study site to understand the eutrophication of shallow lakes due to anthropogenic drainage blockage, pollutants inputs and impacts of modern climate change that is observed in many small shallow lakes globally.