Asfawossen Asrat

Temperature variability over Africa during the last 2000 years

A growing number of proxy, historical and instrumental data sets are now available from continental Africa through which past variations in temperature can be assessed. This paper, co-authored by members of the PAGES Africa2k Working Group, synthesises published material to produce a record of temperature variability for Africa as a whole spanning the last 2000 years. The paper focuses on temperature variability during the ‘Medieval Climate Anomaly’ (MCA), ‘Little Ice Age’ (LIA) and late 19th–early 21st centuries. Warmer conditions during the MCA are evident in records from Lake Tanganyika in central Africa, the Ethiopian Highlands in northeastern Africa, and Cango Cave, the Kuiseb River and Wonderkrater in southern Africa. Other records covering the MCA give ambiguous signals. Warming appears to have been greater during the early MCA (c. ad 1000) in parts of southern Africa and during the later MCA (from ad 1100) in Namibia, Ethiopia and at Lake Tanganyika. LIA cooling is evident in Ethiopian and southern African pollen records and in organic biomarker data from Lake Malawi in southeastern tropical Africa, while at Lake Tanganyika the temperature depression appears to have been less consistent. A warming trend in mean annual temperatures is clearly evident from historical and instrumental data covering the late 19th to early 21st centuries. General warming has occurred over Africa since the 1880s punctuated only by a period of cooling in the mid 20th century. The rate of temperature increase appears to have accelerated towards the end of the 20th century. The few long high-resolution proxy records that extend into the late 20th century indicate that average annual temperatures were 1–2°C higher in the last few decades than during the MCA.

Decadal-scale rainfall variability in Ethiopia recorded in an annually laminated, Holocene-age, stalagmite

An annually laminated, uranium-series dated, Holocene stalagmite from southeast Ethiopia has been analysed for growth rate and δ13C and δ18O variations at annual to biennial resolution, in order to provide the first long duration proxy record of decadal-scale rainfall variability in this climatically sensitive region. Our study site (10°N) is climatically influenced by both summer (June—August) and spring (March—May) rainfall caused by the annual movement of the Inter-Tropical Convergence Zone (ITCZ) and modulated by large-scale anomalies in the atmospheric circulation and in ocean temperatures. Here we show that stalagmite growth, episodic throughout the last 7800 years, demonstrates decadal-scale (8—25 yr) variability in both growth rate and δ 18O. A hydrological model was employed and indicates that this decadal variability is due to variations in the relative amounts of rainfall in the two rain seasons. Our record, unique in its combination of length (a total of ~1000 years), annual chronology and high resolution δ18O, shows for the first time that such decadal-scale variability in rainfall in this region has occurred through the Holocene, which implies persistent decadal-scale variability for the large-scale atmospheric and oceanic driving factors.

Continental-Scale Temperature Variability during the Past Two Millennia: Supplementary Information

Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.

Magma emplacement and mafic–felsic magma hybridization: structural evidence from the Pan-African Negash pluton, Northern Ethiopia

Abstract The Negash pluton (50 km 2 ) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics–metasediments. This almost circular massif consists of monzogranites, granodiorites, diorites–gabbrodiorites, and hybrid diorites. The anisotropy of magnetic susceptibility (AMS) method was used to determine internal structures of the pluton. The foliation trajectories are concentric and inward dipping. The lineation pattern displays an external zone characterised by horizontal concentrically oriented lineations and an internal zone with NW–SE oriented lineations. These petro-structural data clearly locate the feeder zone at the north-western tip of the pluton and indicate the subsequent expansion of the magmas towards the SE. The pluton is a result of in-situ assembly of four magma batches, which were forcefully injected into pre-existing foliated country rocks in relation to transpressional tectonic regime. Two types of mafic–felsic magma interactions are recognised: homogeneous and heterogeneous hybrid diorites at the north-western part, and mingled interfaces at the diorite–granodiorite contact zones mainly visible in the eastern and south-eastern parts. The in-situ mingling along diorite–granodiorite contacts was achieved at the level of emplacement during the injection of dioritic magma into the felsic magmas while the hybrid diorites are assumed to result from two-way conduit mixing and mingling during simultaneous rising of mafic and felsic magmas.

Environmental Monitoring in the Mechara caves, Southeastern Ethiopia: Implications for Speleothem Palaeoclimate Studies

Abstract The interpretation of palaeoclimate records in speleothems depends on the understanding of the modern climate of the region, the geology, the hydrology above the caves, and the within-cave climate. Monitoring within-cave climate variability, geochemistry of speleothem-forming drip waters, and associated surface and groundwater, provides a modern baseline for interpretation of speleothem palaeohydrological and palaeoclimate records. Here, we present results of such monitoring of the Mechara caves in southeastern Ethiopia, conducted between 2004-2007. Results show nearly constant within cave climate (temperature and humidity) in all caves, which generally reflects the surface climate; groundwater and surface water geochemistry is similar across the region (except slight modification by local lithological variations); and modern drip water isotope data fall close to regional Meteoric Water Line, but speleothems further from equilibrium. The Holocene and Modern speleothems from these caves give high-resolution climate records, implying that the Mechara caves provide a suitable setting for the deposition of annually laminated speleothems that could record surface climate variability in a region where rainfall is sensitive to both the strength of the intertropical convergence zone as well as Indian Monsoon variability.

The Geomorphological Map of Mt. Amba Aradam Southern Slope (Tigray, Ethiopia)

Abstract Please click here to download the map associated with this article. The geomorphological map described below covers an area of about 100 km, located on the southeastern slope of Mt. Amba Aradam (northern Ethiopia). It has been produced within the Ethio-Italian Cooperation Programme, in order to understand the recent evolution and present-day trends of the area as a basic tool for land reclamation/rehabilitation projects. The survey was carried out in three successive work campaigns (February 1995, February-March 1996 and December 2002) following the Italian Environmental Agency guidelines. The main geomorphological processes responsible for present-day landscape modelling in the investigation area, are gravity-driven mass movements and slope erosion due to running water. They strongly affect human activities, especially in terms of agriculture and infrastructure management. This map may therefore represent a useful document for land management as well as the initial step for the assessment of geomorphological hazard and risk.

Geoheritage conservation in Ethiopia: the case of the Simien Mountains

Geoheritage conservation in Ethiopia: the case of the Simien Mountains Ethiopia constitutes one of the most significant environmental and cultural reserves on Earth. Ethiopias natural and cultural tourist attractions are mostly associated with geological features: the active Ethiopian and Afar rifts as well as the Simien and Bale massifs are few examples. Ethiopias cultural history, religious manifestations and civilization, like the rock-hewn churches of Lalibela and the stelae of Axum, are also imprinted in rock. Geomorphological and geological features, notably the isolation of the north-western highlands from the external world by the harsh Afar depression close to the sea, determined the route of Ethiopian history. Though tourism has been identified as a major sustainable development sector, systematic geoheritage evaluation and conservation strategies are lacking in the country. I this paper the Simien Mountains are presented as major geoheritages which should be prioritized for geoconservation in order to develop sustainable tourism (geotourism) in the area.

Glass compositions and tempo of post-17 ka eruptions from the Afar Triangle recorded in sediments from lakes Ashenge and Hayk, Ethiopia

Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3 1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5 1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region.

Contribution of a Geomorphosite Inventory to the Geoheritage Knowledge in Developing Countries: the Case of the Simien Mountains National Park, Ethiopia

The central objective of this paper is to show how a geomorphosite inventory can improve the knowledge of geomorphological and geological heritage in developing countries. We propose an inclusive and systematic approach of protection and outreach of the geomorphological heritage based on a geomorphosite inventory. Twenty-one geomorphosites were identified and fully documented in the Simien Mountains National Park (SMNP) in Ethiopia. The results show that the geomorphological heritage of the SMNP is in a good conservation state. It is of considerable scientific, educational, touristic and ecological interest. A management program consisting of eight strategic objectives with the aim of increasing awareness and sustainable use of geomorphological heritage of the SMNP (here called geotourism development) was subsequently developed. It includes capacity building of park staff, expansion and diversification of the trekking experience and the introduction of an accreditation system for geo-guided tours. The proposals for the geomorphosite management are indicative of a high potential for international geotourism development in the Simien Mountains. A Geopark could be proposed in the future, for which further research and pioneering work is required.

Potential Geoheritage Sites in Ethiopia: Challenges of Their Promotion and Conservation

Abstract This chapter outlines a summary of potential geoheritage sites in Ethiopia and discusses some promotion and conservation challenges. Ethiopia can be considered as one of the major geoheritage hotspots in the world. It hosts unique and diverse geological features including active volcanic and tectonic processes leading to the birth of a new oceanic crust. It also hosts most of the famous hominid-bearing sedimentary successions where the key to the understanding of human origins lies. Its civilization and cultural history is also strongly recorded in numerous monuments of rock including ancient stelae and rock-hewn churches. However, Ethiopia has yet to capitalize on its immense geoheritage resources and pursue an active geoconservation strategy if its economy is to benefit from a sustainable use of these resources.