by Markus Fischer,
Eberhard Karls University Tübingen, Section Geosciences, AG Mikropalaeoontology
The Ethiopian rift in eastern Africa is known for its diverse landscape, ranging from arid and semi-arid savannahs to high and lush mountainous regions, where anatomically modern humans were present since at least 195 ka BP. Lacustrine sediments and paleo-shorelines near-by modern shallow lakes in the rift indicate that lakes have fluctuated dramatically in the past between deep fresh water lakes, to shallow highly alkaline lakes, down to completely desiccated lakes and thus moisture availability was not constant over time. In order to understand the impact and feedback of different paleoenvironmental changes due to a changing climate (e.g. vegetation, lake sizes) in southern Ethiopia, we here present a comprehensive study that focuses on the time between 15,000 and 5,000 years, a well-known time called the last African Humid Period (AHP). This time period was not only climatically completely different to today, also a big cultural shift occurred during this time. Here we present an interdisciplinary approach that links environmental modelling with lacustrine sediment analysis. This includes a comprehensive hydro-balance modelling approach of multiple rift lakes from the southern Ethiopian Rift (Abaya, Chamo, Chew Bahir) providing insights into paleo-precipitation conditions and lake dynamics. This model is supported by the use of Sr-Isotopes measured on microfossils from drill core sediments to reconstruct water connectivity between the investigated lakes and to backtrack annual precipitation amounts. The model outcome is then used to model paleo-vegetation dynamics using remote sensing based boosted regression trees under different assumptions of paleo-seasonality changes. This model is supported by phytolith analysis from the drill core sediments. The results suggest that an increase in precipitation of 20-30% throughout the whole Southern Ethiopian Rift was necessary to fill quickly the deepest of the investigated lakes, Lake Chew Bahir, to its overflow level, but also small changes in the water budget allowed the lake to disappear within decades. This study highlights a) the sensitivity of the rift lakes to even moderate climate changes, b) that slight seasonal precipitation changes can have a huge impact, and c) that hydrological connection plays an important role.