Late Quaternary climate variability in the source region of Homo sapiens: Dry-wet cycles in Chew Bahir, southern Ethiopia
Climate change, as a key topic in our society, has far reaching implications for many aspects of our lives, in the past present and evermore in the future. Climatic variability and a rapidly changing environment are considered to have had a significant influence on human evolution, migration and cultural and technological innovation. However, to evaluate the impact that climatic shifts on different timescales might have had on the living conditions of prehistoric humans, an understanding and continuous reconstruction of these climatic fluctuations and their underlying driving mechanisms are essential.
This work presents results from such a high resolution (up to 3 years) lake-sediment record from the palaeolake Chew Bahir, a newly invested climate archive in a tectonic-bound basin in southern Ethiopia. The record was obtained from six 9–18.8 long cores along a 17 km NW-SE transect across the basin, today an extensive saline mudflat. The objective of this work is to understand and reconstruct the sensitive patterns and expressions of East Africa‘s highly variable climate, as the climatic context for important cultural transitions in the source region of Homo sapiens.
Now, the multi-proxy analyses and interpretation being the heart piece of this thesis, provide the climatic history of the past ~60 ka cal BP and show that Chew Bahir responded sensitively with pronounced shifts in moisture availability towards climatic fluctuations on millennial to centennial timescales, and to the precessional cycle.
The first part of this work concentrates on a) the reconstruction of the velocity and character of these late-Quaternary wet-dry transitions on different time scales (orbital, millennial–centennial and decadal) and b) a basic proxy concept for Chew Bahir for the last two insolation controlled wet-dry cycles. This concept comprises be- sides the deciphering of major intra-basin dynamics and mechanisms controlling the way from source to sink, an initial understanding of site-specific proxies: especially potassium as a sensitive indicator for aridity and chlorine as a humidity proxy. The records are based on a set of geochemical, physical and biological indicators as well as a suite of AMS radiocarbon dates. The Chew Bahir cores document a highly non-linear response to the last insolation controlled dry-wet cycle, the so-called African Humid Period (~15–5 ka BP) with a pronounced abrupt onset of humid conditions within <500 yrs and a disproportionally gradual decline of moisture availability, as compared to the decrease in insolation. Feedback mechanisms and a complex interrelationship with the monsoon circulation and the diverse topography of the East African Rift have been suggested as possible key factors. The AHP frames a sharply defined arid phase, corresponding to the Younger Dryas chronozone (~12.8–11.6 ka BP). During the overall arid phase of MIS 3, several oscillations to wetter conditions have been recorded, that resemble the high latitude Dansgaard-Oeschger cycles. Heinrich-events are suggested to be expressed in several episodes of extreme aridity. The full humid conditions of the Holocene wet period [AHP], are punctuated by several abrupt droughts on a centennial to millennial time-scale, the termination of the AHP is though gradual, a textbook example for climatic instability during a transition. A series of 20–80 yr long droughts modulate the 1,500 yr long shift from full wet to arid conditions.
In a broader spatio-temporal context this Mid Holocene wet-dry transition in Chew Bahir is evaluated together with two other examples of a change from stable to unstable environmental conditions: the MIS 5–4 transition in the Naivasha basin (central Kenya rift) and thirdly, the Mid Pleistocene Transition in the Olorgesaille basin (Southern Kenya Rift). The concept of hominin speciation, dispersal and cultural innovation being possibly influenced by this transition from stable to unstable environmental conditions is tested on the three different timescales provided by the three records.
As a contribution towards a better understanding of human-climate interaction, we compared the last 20 ka of the paleo-climate record from Chew Bahir with the settlement history of adjacent possible refugia in the Ethiopian highlands and around lake margins. Shifts in and out of favourable living conditions are deducted from the climatic history, which shows besides orbitally driven longterm transitions several short abrupt climate events. These are expressed as shifts to pronounced aridity, suggesting phases of climatic stress. Comparing the frequency of archaeological findings as a parameter for human occupation in refugia to this close-by climate record, allows us to outline how complex the interplay between humans and environment during the last 20 ka really was.
The results comprised in this work represent an important prerequisite for the ICDP “Hominid Sites and Paleolakes Drilling Project” and for the CRC-806 programme “Our Way to Europe”, which aim to determine climatic and environmental context of human evolution and dispersal. The potential of this deep terrestrial climate archive has been evaluated herein and proved that the sediment deposits are suitable to provide a longer climate history, to be precise to cover with a 400 m core the climatic history of >500,000 yrs in the source region of modern humans.
2010 – 2013: Ph.D. Student at Institute of Geography, University of Cologne.
2009: State Examination (1st) in Geography and English.
Thesis title: “Pollen sedimentation since the High Middle Ages in Laguna Potrok Aike, Southern Patagonia”.
2004 – 2009: Studies in Geography and English, University of Cologne, Additional degree course BLUE (Bilingual Teaching).