Classifying past climate change in the Chew Bahir basin, southern Ethiopia, using recurrence quantification analysis

Abstract

The Chew Bahir Drilling Project (CBDP) aims to test possible linkages between climate and evolution in Africa through the analysis of sediment cores that have recorded environmental changes in the Chew Bahir basin. In this statistical project we consider the Chew Bahir palaeolake to be a dynamical system consisting of interactions between its different components, such as the waterbody, the sediment beneath lake, and the organisms living within and around the lake. Recurrence is a common feature of such dynamical systems, with recurring patterns in the state of the system reflecting typical influences. Identifying and defining these influences contributes significantly to our understanding of the dynamics of the system. Different recurring changes in precipitation, evaporation, and wind speed in the Chew Bahir basin could result in similar (but not identical) conditions in the lake (e.g., depth and area of the lake, alkalinity and salinity of the lake water, species assemblages in the water body, and diagenesis in the sediments). Recurrence plots (RPs) are graphic displays of such recurring states within a system. Measures of complexity were subsequently introduced to complement the visual inspection of recurrence plots, and provide quantitative descriptions for use in recurrence quantification analysis (RQA). We present and discuss herein results from an RQA on the environmental record from six short (<17 m) sediment cores collected during the CBDP, spanning the last 45 kyrs. The different types of variability and transitions in these records were classified to improve our understanding of the response of the biosphere to climate change, and especially the response of humans in the area.

Combining geomorphological–hydrological analyses and the location of settlement and raw material sites – a case study on understanding prehistoric human settlement activity in the southwestern Ethiopian Highlands

“…Accordingly, several studies demonstrated variable climatic conditions at the Horn of Africa during the Late Pleistocene and Holocene. Transitions from humid to arid conditions and vice versa led to rise and shrinkage of lake levels and also affected the connecting drainage networks (Sagri et al, 2008;Carnicelli et al, 2009;Foerster et al, 2012;Junginger and Trauth, 2013;Foerster et al, 2015;Trauth et al, 2019). We propose that the connected drainage systems in our research area, such as the Bisare River, might have reacted to these transitions with changes in their depositional and erosional behaviour.…”

“…The Ethiopian Highlands receive more than 2000 mm of annual rainfall, which is the highest average amount at the Horn of Africa (Griffith, 1972;Viste and Sorteberg, 2013). Annual climatic variations in Ethiopia are related to moisture brought by the summer monsoon that interacts with the dry northeastern "Harmattan" wind system and to changes in the north-south directed pressure gradient (Viste and Sorteberg, E. A. Hensel et al: Prehistoric human settlement activity 205 2013; Nicholson, 2018).…”

“…Annual climatic variations in Ethiopia are related to moisture brought by the summer monsoon that interacts with the dry northeastern "Harmattan" wind system and to changes in the north-south directed pressure gradient (Viste and Sorteberg, E. A. Hensel et al: Prehistoric human settlement activity 205 2013; Nicholson, 2018). A further influence is changes in sea surface temperatures of the adjacent Indian Ocean (Griffith, 1972;Tierney et al, 2008;Segele et al, 2009;Viste et al, 2013). Inter-annual changes in precipitation are linked to the shifting of the tropical rain belt (TRB), low-level convergences, and the role of the accentuated topography (Nicholson, 2018) ( Fig.…”

“…1c). The paleoclimate of southwestern Ethiopia of the last 45 ka was characterized by fluctuations of moister and drier conditions (Foerster et al, 2012;Trauth et al, 2019). According to the paleoclimate record of the Chew Bahir drill cores, arid conditions during the Last Glacial Maximum (LGM, ∼ 24-18 ka) led to a desiccation of the former paleolake (Foerster et al, 2012(Foerster et al, , 2015Trauth et al, 2019) (Fig.…”

“…higher precipitation after arid phases, loose topsoil material, and sparse vegetation due to intense land use (Billi and Dramis, 2003;Fryirs and Brierley, 2013;Mukai, 2017). By studying swamp formation in the upper Bisare catchment, the influence of alternating wet and dry phases on the regional landscape dynamics could exemplarily be investigated for the last several years (for long-term climatic fluctuations, please compare Trauth et al, 2019). These alternations lead to changes in erosion and deposition.…”