Project Database

Abstract:

Fire plays a critical role shaping both the distribution and composition of the Afroalpine and Afromontane communities. Some plant communities are resilient to certain fire regimes, presenting post-fire traits such as resprouting, as is evident in the sub-Alpine Ericaceous zone, while others lacking these strategies are more fire-prone. The fire regime concept includes the biological role in fire occurrence because some of the parameters driving fire regimes – frequency, type or intensity - are often regulated by the vegetation through its structure, flammability and connectivity.

Abstract:

The lakes of the eastern Africa Rift often contain great thicknesses of sediment that may provide continuous records of environmental change over decadal to million-year timescales. However interpretation of these changes is greatly compromised without a reliable chronology. Luminescence dating has not been used extensively in lacustrine settings; instead previous studies have often relied upon radiocarbon dating, using extrapolation beyond the upper limit of that technique, and employing opportunistic sampling of tephra and palaeomagnetic signatures where possible. This study from Lake Tana, Ethiopia, demonstrates that recent advances in luminescence methodology can provide long chronologies for lake sediments that are not dependent on the intermittent presence of dateable material, as is the case for radiocarbon and tephra-based methods. Specifically, this study generates luminescence ages that agree with independent chronology based on radiocarbon dating in the upper part of the core, and extends significantly beyond the range of radiocarbon dating to provide one of the longest independently dated lacustrine sediment records in eastern Africa, thus demonstrating the tremendous potential of luminescence for constructing lacustrine sediment chronologies over 100,000 year timescales.

Abstract:

Garba Guracha is a cirque lake located at 3950 m altitude in the Bale Mountains National Park, Ethiopia, at the boundary between the Afro-alpine and sub-alpine Ericaceous vegetation belts. Jean-Jacques Tiercelin and Mohammed Umer cored its sediments in about 2002, leading to publication of one of the longest, best-dated, and most continuous Late Pleistocene-Holocene palaeoenvironmental records from above 3000 m in Africa. Their 2007 and 2008 papers document the vegetation and deglaciation history of the Bale Mountain plateau, which supports the most extensive area of Afro-alpine vegetation on the planet, and an exceptionally rich endemic biodiversity. However, those publications revealed little about human exploitation of the Bale Mountain environment. We again cored Garba Guracha (‘Black Lake’ in the Oromo language) in February 2017, as part of a major DFG-funded research unit (“The Mountain Exile Hypothesis - How humans benefited from and re-shaped African high altitude ecosystems during Quaternary climate changes”) that aims to test the hypothesis that human settlement of the Bale Mountains dates from at least the late Pleistocene. We also drilled cores from seven other sites in the area in early 2018, in partnership with archaeological, glacial geological and pedological investigations. Here we attempt an early synthesis of the results of ?XRF geochemistry, stable isotope, biomarker, pollen and charcoal analyses of the Garba Guracha sediments with a new age model based on radiocarbon and 210Pb ages, and suggest an initial interpretation of anthropogenic impact on the Bale Mountains.

Abstract:

Low‑altitude lakes in eastern Africa have long been investigated and have provided valuable information about the Late Quaternary paleohydrological evolution, such as the African Humid Period. However, records often suffer from poor age control, resolution, and/or ambiguous proxy interpretation, and only little focus has been put on high‑ altitude regions despite their sensitivity to global, regional, and local climate change phenomena. Here we report on Last Glacial environmental fluctuations at about 4000 m asl on the Sanetti Plateau in the Bale Mountains (SE Ethiopia), based on biogeochemical and palynological analyses of laminated lacustrine sediments. After deglaciation at about 18 cal kyr BP, a steppe‑like herb‑rich grassland with maximum Chenopodiaceae/Amaranthaceae and Plantago existed. Between 16.6 and 15.7 cal kyr BP, conditions were dry with a desiccation layer at ~ 16.3 cal kyr BP, documenting a temporary phase of maximum aridity on the plateau. While that local event lasted for only a few decades, concentra‑ tions of various elements (e.g. Zr, HF, Nb, Nd, and Na) started to increase and reached a maximum at ~ 15.8–15.7 cal kyr BP. We interpret those elements to reflect allochthonous, aeolian dust input via dry northerly winds and increasingly arid conditions in the lowlands. We suggest an abrupt versus delayed response at high and low altitudes, respectively, in response to Northern Hemispheric cooling events (the Heinrich Event 1). The delayed response at low altitudes might be caused by slow negative vegetation and monsoon feedbacks that make the ecosystem somewhat resilient. At ~ 15.7 cal kyr BP, our record shows an abrupt onset of the African Humid Period, almost 1000 years before the onset of the Bølling–Allerød warming in the North‑Atlantic region, and about 300 years earlier than in the Lake Tana region. Erica pollen increased significantly between 14.4 and 13.6 cal kyr BP in agreement with periodically wet and regionally warm conditions. Similarly, intense fire events, documented by increased black carbon, correlate with wet and warm environmental conditions that promote the growth of Erica shrubs. This allows to conclude that biomass and thus fuel availability is one important factor controlling fire events in the Bale Mountains.