Correlation of tephra deposits frequently relies on the analysis of glass shards separated from their host. Small shards from distal deposits (marine-, ice- or lake-cores, peat bogs) are difficult to analyse. Here current methods for glass shard analysis from (particularly) marine tephra deposits are reviewed. These methods apply equally to other distal deposits, where linking repositories of climatic information is central to many research programmes. Electron probe microanalysis is used widely to determine the major element compositions of volcanic glass. However, electron beam irradiation causes permanent damage to glass, especially hydrated or silica- and alkali-rich compositions. Recent developments have shown that reliable data is obtained with beam diameters >3 µm on basaltic or moderately hydrated rhyolitic glass, provided low electron beam current densities are used. For robust correlation of tephra deposits from different environments, it is recommended that analytical data are normalized to an anhydrous basis. Glass trace element analysis is commonly performed by laser ablation inductively coupled plasma mass spectrometry, which determines approx. 30 elements at <1 ppm from ablation craters 10 µm diameter, and although element fractionation occurs, it can be corrected. Recent developments in both methods should facilitate analysis of smaller, more distal material, expanding the geographical range over which reliable tephra correlations can be achieved.
Corbetti caldera is the southernmost large volcanic system in Ethiopia, and has been categorized at the highest level of uncertainty in terms of hazard and risk. Until now, the number and frequency of past explosive eruptions at Corbetti has been unknown, due to limited studies of frequently incomplete and patchy outcrop sequences. Here we use volcanic ash layers preserved in sediments from three Main Ethiopian Rift lakes to provide the first detailed record of volcanism for the Corbetti caldera. We show that lake sediments yield more comprehensive, stratigraphically resolved dossiers of long-term volcanism than often available in outcrop. Our eruptive history for Corbetti spans the past 10 k.y. and reveals eruptions at an average return period of ~900 yr. The threat posed by Corbetti has until now been underestimated. Future explosive eruptions similar to those of the past 10 k.y. would blanket nearby Awassa and Shashamene, currently home to ~260,000 people, with pumice-fall deposits, and would have significant societal impacts. A lake sediment tephrostratigraphic approach shows significant potential for application throughout the East African Rift system, and will be essential to better understanding volcanic hazards in this rapidly developing region.
Previous paleolimnological studies demonstrated that the sediments of Garba Guracha, situated at 3950 m asl in the afro-alpine zone of the Bale Mountains of Ethiopia, provide a complete Late Glacial and Holocene paleoclimate and environmental archive. We revisited Garba Guracha in order to retrieve new sediment cores and to apply new environmental proxies, e.g. charcoal, diatoms, biomarkers, and stable isotopes. Our chronology is established using 210 Pb dating and radiocarbon dating of bulk sedimentary organic matter, bulk n-alkanes, and charcoal. Although bedrock was not reached during coring, basal ages confirm that sedimentation started at the earliest * 16 cal kyr BP. The absence of a systematic age offset for the n-alkanes suggests that ''pre-aging'' is not a prominent issue in this lake, which is characterised by a very small afro-alpine catchment. X-ray fluorescence scans and total organic carbon contents show a prominent transition from minerogenic to organic-rich sediments around 11 cal kyr BP coinciding with the Holocene onset. While an unambiguous terrestrial versus aquatic source identification seems challenging, the n-alkane-based P aq proxy, TOC/N ratios, d 13 C values, and the sugar Electronic supplementary material The online version of this article (
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