SCIENCE ADVANCES | RESEARCH ARTICLE GEOLOGY Copyright © 2021 The Authors, some rights reserved; Nonuniform Late Pleistocene glacier fluctuations exclusive licensee American Association in tropical Eastern Africa for the Advancement Alexander R. Groos1*, Naki Akçar2, Serdar Yesilyurt2,3, Georg Miehe4, of Science. No claim to 5 1 original U.S. Government Christof Vockenhuber , Heinz Veit Works. Distributed under a Creative Today’s ice caps and glaciers in Africa are restricted to the highest peaks, but during the Pleistocene, several Commons Attribution mountains on the continent were extensively glaciated. However, little is known about regional differences in the License 4.0 (CC BY). timing and extent of past glaciations and the impact of paleoclimatic changes on the afro-alpine environment and settlement history. Here, we present a glacial chronology for the Ethiopian Highlands in comparison with other East African Mountains. In the Ethiopian Highlands, glaciers reached their maximum 42 to 28 ka thousand years ago before the global Last Glacial Maximum. The local maximum was accompanied by a temperature depression of 4.4° to 6.0°C and a ~700-m downward shift of the afro-alpine vegetation belt, reshaping the human and natural habitats. The chronological comparison reveals that glaciers in Eastern Africa responded in a nonuniform way to past climatic changes, indicating a regionally varying influence of precipitation, temperature, and orography on paleoglacier dynamics. INTRODUCTION ~29 to 20 ka (kiloanni) and hypothesizes that high-latitude warm- The extant glaciers in Africa are restricted to the summit areas of Mount ing initiated the onset of deglaciation in the tropics at ~20 to 19 ka Kenya, Kilimanjaro, and Rwenzori Mountains (1, 2), but during the (11). However, first 36Cl surface exposure ages of erratic boulders cold periods of the Late Pleistocene, several mountain ranges on the from two moraine sequences in the Bale Mountains (Ethiopian continent were extensively glaciated (table S1) (3–5). Well-preserved Highlands) indicate a more complex and nonuniform response of moraine sequences and other glacial landforms testify to multiple tropical glaciers to Late Pleistocene climate changes (7). On the ba- glacier advances in the High Atlas, East African Mountains, and sis of existing data, the glaciers in the northwestern valleys of the Ethiopian Highlands (Fig. 1). Since past glacial fluctuations mainly Bale Mountains reached their local Last Glacial Maximum (LGM) reflect long-term changes in temperature, precipitation, cloudiness, (7) during marine isotope stage (MIS) 3 and, therefore, far earlier and insolation, glacial landforms are an appropriate proxy for re- than in the Rwenzori Mountains during MIS 2 (11). A major glacier constructing regional paleoclimatic variations and paleoecological advance in the Bale Mountains during the hypothesized pantropical changes in alpine environments (4). Studying the climate and gla- LGM (~29 to 20 ka) (11) has not yet been verified (7). The emerging cial history of the mountains in Eastern Africa is of particular inter- discrepancy between the timing of the local LGM in the East African est since the topography on both sides of the East African Rift along Mountains (1°N to 3°S) and more northern Ethiopian Highlands with an amplified regional cooling at high elevations favored the (7° to 13°N) may indicate a regionally varying influence of precipi- formation of numerous ice caps and valley glaciers (3–6). Further- tation, temperature, and orography on past glacial fluctuations. To more, assessing the impacts of past glaciations on the afro-alpine investigate possible regional differences in the timing and extent of environment plays a key role in understanding the circumstances of past glaciations in Eastern Africa and to detect changes in large-scale the early migration of Middle Stone Age foragers into the high ele- tropical atmospheric circulation systems, a representative glacial vations of the glaciated Ethiopian Highlands 47 to 31 cal ka BP chronology of the Ethiopian Highlands including direct moraine (calibrated kiloanni before present) (7). Information on paleocli- ages from valleys and mountains over a wide area is necessary. matic and paleoecological changes in the afro-alpine environment Here, we present an extensive glacial chronology dataset for the during the Pleistocene is also of high relevance for elucidating why Ethiopian Highlands based on 21 previously published (7) and 59 tropical mountains are biodiversity hot spots (8). Latest glacial new 36Cl surface exposure ages of boulders from moraines and peri- chronological and paleoclimatological studies from the East African glacial features from the Bale and nearby Arsi Mountains. By com- Mountains and Ethiopian Highlands indicate distinct climatic and eco- bining the new 36Cl ages from the Ethiopian Highlands with existing 36Cl logical changes at high elevations as they provide evidence for multiple and 10Be glacial chronologies from Mount Kenya, Kilimanjaro, and glacier advances, a pronounced cooling, and depression of altitudinal the Rwenzori Mountains (9–12), this study aims to investigate the vegetation belts during the Late Pleistocene (7, 9–12). response of tropical glaciers in Eastern Africa to Late Pleistocene cli- A recent comparative study focusing on paleoglacier fluctuations mate changes and elaborate on the paleoclimatic and paleoecological in the Rwenzori Mountains (Eastern Africa) and Andes (South America) implications of past glacial fluctuations. Since the extent and dynamic suggests that tropical glaciers reached their last glacial maxima at of past glaciations are determined not only by the prevailing climatic conditions but also by orography and the potential surface area above the former equilibrium line altitude (ELA) (13), this study also in- 1Institute of Geography, University of Bern, Bern, Switzerland. 2Institute of Geological cludes the first terrain analysis of the most extensively glaciated African Sciences, University of Bern, Bern, Switzerland. 3Department of Geography, Ankara 4 mountains (3). Considering the varying hypsography of the mountains University, Ankara, Turkey. Faculty of Geography, Philipps University Marburg, Marburg, Germany. 5Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland. is crucial for comparing and interpreting regional differences of past *Corresponding author. Email: [email protected] glacial fluctuations and drawing conclusions about the paleoclimate. Groos et al., Sci. Adv. 2021; 7 : eabb6826 12 March 2021 1 of 15 SCIENCE ADVANCES | RESEARCH ARTICLE Fig. 1. Quaternary glaciations in Africa. (A) Topographic map showing mountain localities with clear and controversial geomorphological evidence of past glaciations (3, 4). (B) Overview of cosmogenic glacial chronologies from the Ethiopian Highlands and East African Mountains [sites IV and V in (A)] based on 36Cl and 10Be exposure ages (7, 9–12). The blue circles and black arrow indicate the location of the paleoclimate archives (Lake Tana, Chew Bahir, and Tanganyika) considered for Fig. 6 and Discussion. RESULTS The varying hypsography of the mountains has implications for Hypsography of Africa’s glaciated mountains present and past glaciations on the continent. Under present cli- Africa’s high mountains cover less than 0.1% of the entire area of matic conditions, the ELA is located far above the maximum eleva- the continent and are located mainly along the East African Rift tion of most of the African mountains. Glaciers can therefore only (Fig. 1). Exceptions are the High Atlas in northwestern Africa, persist in the summit areas of the three highest peaks of the East Mount Cameroon in western Africa, and the Drakensberg in southern African Mountains. However, once the ELA decreases below 4000 Africa as well as the Tibesti in the Sahara and Jebel Marra in the to 4500 m as during the last glaciation (3), the potential surface area Sahel. The afro-alpine area above 3500 m equals 0.016% (5041 km2) in Africa for the accumulation of snow and formation of ice increas- and that above 4000 m is 0.0028% (880 km2) of the continent’s total es drastically. While the entire area above 5000 m on the continent area of 31.6 million km2. is limited to 22 km2 and the area between 4500 and 5000 m is limited The extent, elevation range, and hypsography of the Ethiopian to 61 km2, 796 km2 is available for snow accumulation between Highlands, East African Mountains, and High Atlas in northern 4000 and 4500 m and an additional 4161 km2 is available between Africa differ widely (Fig. 2). The Ethiopian Highlands (Arsi, Bale, 3500 and 4000 m. and Simien Mountains) reach maximum elevations between 4200 and 4500 m. In contrast, the highest East African Mountains (Kilimanjaro, Glacial history of the Bale Mountains Mount Kenya, and Rwenzori) rise above 5000 m and are defined by The central Sanetti Plateau above 3800 m in the Bale Mountains their topographic prominence and smaller area-elevation ratio. hosts several glacial and periglacial features (Fig. 3 and figs. S1 and Africa’s highest mountain, Kilimanjaro, reaches almost 5900 m and S2). Large boulders (up to 8 m wide and 5 m high) encircle the high- covers an area of 387 km2 for elevations above 3500 m and 190 km2 est peak (Tullu Dimtu, 4377 m) in the central part of the plateau at above 4000 m. Mount Kenya and the Rwenzori Mountains also cover a distance of about 2.5 and 5 km from the peak (fig. S2, A and B). an area of more than 320 km2 above 3500 m and 120 km2 above Because of their rounded shape and circular distribution around the 4000 m. Although the Ethiopian Highlands are lower in elevation, peak, it is likely that a former ice cap transported the boulders and they comprise the largest part in total surface area of Africa’s alpine deposited them along its ice margins. The scatter of the erratic boul- environment above 3000, 3500, and 4000 m due to the broad base of ders on the plateau has previously been defined as the Big Boulder the mountains (Fig. 2). The hypsography of the Bale Mountains is Moraine (14).