Anthropogenic Impacts on Sedimentation in Shallow Urban Lakes and Soils: The Kallar Kahar Lake, Salt Range, Pakistan and Soils from Vienna, Austria

Lacustrine sediments and soils (anthrosols) provide key archives for tracing anthropogenic signals in the Earth’s system and have remained central to Anthropocene research.Small, shallow lakes are under severe stress from the combined impact of anthropogenic eutrophication and modern climate change. Therefore, lacustrine sediments provide key information for understanding the lake’s eutrophication and the ongoing unprecedented climate change. However, the onset and geographical extent of anthropogenic impacts in such lakes remain poorly constrained outside Europe and North America (the Global North). This study, in the framework of UNESCO IGCP 732 (LANGUAGE of the Anthropocene), investigates anthropogenic impacts on sedimentation using the Kallar Kahar Lake, a saline lake in Salt Range, Pakistan, as a representative of the Global South. A 180 cm sediment core of the lake was analyzed using X-ray diffraction (XRD) and geochemical methods. The lower part consists of dark gray marly mud with greenish-gray sandy/silty layers. XRD indicates 70% detrital siliciclastic minerals (quartz, feldspars, muscovite, clay) and <30% calcite. In the upper core, siliciclastics drop to < 40% while calcite exceeds 50%. Geochemically, SiO2 and Al2O3 decline upward (54.6% to 16.4%, 9.8% to 3.5%, respectively), as do Fe2O3 (4.5% to 1.7%), K2O (1.6% to 0.6%), and Na2O (1.5% to 0.3%). CaO increases from 11% to ~30%. Total Organic Carbon and Sulphur rise from <3% and <0.3% to >15% and ~0.9%, respectively. Trace element proxies Zr/Rb and (Zr + Rb)/Sr indicate decreased detrital influx, while paleoredox proxies (V/(V + Ni), V/Cr, 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 aridity. Paleosalinity proxies (Sr/Ba and Rb/K) suggest an upward increase in lake salinity. The anthropogenic impact proxies (Mo and Hg) indicate enrichment and display strong negative correlations with detrital supply. Similarly, As, Pb, Zn, and P correlate neither with detrital influx proxies nor with in situ sedimentation proxies, thereby indicating an anthropogenic source. These results indicate that urbanization and anthropogenic activities have blocked the natural drainage of the lake, reducing the detrital influx to around one-third. Increasing aridity of the area due to modern climate change has transformed the lake into a closed-water body where evaporation has increased salinity, forcing a nearly threefold increase in the in-situ organic carbonate production in the sampled interval. Thus, the Kallar Kahar Lake provides an ideal case study site from the Global South to understand the eutrophication of shallow lakes due to anthropogenic drainage blockage, pollutant inputs and impacts of modern climate change that are observed in many small shallow lakes globally. In a further project (Austrian Science Fund FWF PIN4378323), these results from the Global South will be compared with urban sediments and soil profiles (sections Brunn am Gebirge, Liesingbach) of Vienna, Austria.