Late Quaternary climate changes in the Horn of Africa

Abstract

The Horn of Africa extends from the Sahara desert eastwards to the Red Sea, the Gulf of Aden and the Indian Ocean. Due to its complex volcano-tectonic evolution over the past 15 million years (Mohr 1971), the region is characterised by considerable changes in elevation within short distances. The Ethiopian highlands, which rise to altitudes exceeding 4000m above sea level, form an extensive uplifted plateau, delimited by pronounced escarpments on both east and west. In Southern Ethiopia the 1000 km-wide uplifted volcanic province is divided asymmetrically into northwestern and southeastern plateaux by the Main Ethiopian Rift which runs SSW-NNE and represents the northern end of the continental East African Rift. Eastward, the continental Afar Rift (Ethiopia and Republic of Djibouti) sits astride the Gulf of Aden—Red Sea sea-floor spreading axis, with several areas below sea level (e.g.,–155mat Lake Asal). These marked altitudinal gradients result in a wide variety of climates and environments, making the region particularly suitable for investigating past environmental change. The region is subject to the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) and is very sensitive to monsoon variability. Several studies have revealed climate variability from millennial to inter-annual timescales during the Late Quaternary (Gasse 2000). This paper provides a detailed overview of this region complementary to those of Hoelzmann et al. (this volume) and Barker et al. (this volume). It reviews Late Quaternary palaeoenvironmental and palaeoclimatic changes, as recorded from various archives (lakes, high altitude peat-bogs, glacial landforms, palaeosols), and proxies (geomorphology, sedimentology, geochemistry, pollen, diatoms). The chronological framework of most records is based on radiocarbon ages (14C yr BP). Radiocarbon ages are converted into calendar estimates (cal. yr) using the CALIB 4.0 program (Stuiver et al. 1998) back to 18,000 14C yr BP, or the polynomial equations established by Bard (1998) which allow calendar estimates back to 36,000 14C yr BP (41,516 cal. yr). Ages are first given in the time-scale provided in the original literature (14C kyr BP or cal. kyr), then the calibrated scale is indicated in brackets.