Environmental modeling and remote sensing linked to lacustrine climate proxies: the rift basin Chew Bahir (Ethiopia) during the late Pleistocene

Abstract

Eastern Africa, a hotspot of hominin evolution, is a diverse and fragile landscape, shaped by the ongoing rifting and the large orographic and climatic gradients between green and lush mountainous regions and dry and hot rift floors, which are partly deserted today, such as the Chew Bahir basin, in southern Ethiopia and north-east of Lake Turkana. Orbital-induced climate change caused the last African Humid Period (AHP) from 15–5 ka (thousand years before present), leading to large rift lakes, such as the paleo-lake Chew Bahir with up to 2500 km² in size. Drilled lacustrine sediment proxies from the Chew Bahir Basin revealed the climate dynamics for the last AHP and the past 620 ka. Because proxies provide qualitative spatially point-wise and temporally continuous data about the past climate and environment, this thesis aims to complement these results using catchment-scaled environmental models to gain quantitative and spatial data about the past hydroclimate and vegetation in the vicinity of Chew Bahir. Therefore, this cumulative thesis provides a collection of model applications in the Chew Bahir vicinity and discusses the consequences for early humans living in this region. First, a Lake Balance Model (LBM) extrapolated the necessary paleo-precipitation of +25–41% compared to today during the AHP and revealed the high importance of paleo-hydro-connectivity between different lake basins in the East African Rift System. Model results showed paleo-lake Chew Bahir may have desiccated within decades and flooded within the same time afterward as soon as humid conditions prevailed. Second, a Predictive Vegetation Model (PVM) linked with the LBM provided spatial estimates of the paleo-vegetation during the Last Glacial Maximum (LGM) time and the AHP. A comparison with the archeological record indicates a human preference for open landscapes in southern Ethiopia. The model yields a precipitation reduction of -17.5% during LGM times. Third, a comprehensive perspective on the lake level evolution within the Turkana Depression indicates that besides paleo-lake Chew Bahir, there was a paleo-lake Chalbi, similar in size and, presumably, even more climate-sensitive than paleo-lake Chew Bahir. Fourth, developing an isotope-enabled quantitative model using the geological gradient between the modern-day catchment and the extended paleo-catchment may be possible for the Chew Bahir lacustrine sediments, as this modern-analog study shows. Fifth, the interdisciplinary discussion about the risk assessment of humans living close-by Chew Bahir and Lake Turkana during the termination of the AHP is shown. Sixth and last, the application of a hydrological model is shown exemplary in a different landscape in Georgia, and the impact of human behavior on the hydrosphere during the Holocene is revealed.