Abstract
The preservation of archaeological remains and environmental information in a sediment accumulation can vary in caves and rockshelters, depending on external climatic conditions, and the circumstances within the shelter. Several sediment stratigraphies in the Horn of Africa are characterized by erosion layers, discordances and chronological gaps, that create uncertainties about the impact of climatic and environmental shifts on human settlements. Archaeological sites in Ethiopia that preserve information about human occupation during the Upper Pleistocene and Holocene often deal with major gaps during a period corresponding to MIS 2. In this study we present the first results of sedimentological, geochemical analyses and radiocarbon dating at Sodicho Rockshelter (1930 m above sea level) that provide evidence on high altitude settlement during this mentioned chronostratigraphic gap and subsequent time slices. This new archaeological site in the southwestern Ethiopian Highlands hosts a 2-m-long sediment record. So far, a stratigraphy has been excavated that dates back to ∼27 ka, including several settlement phases of Late Pleistocene and Holocene hunter-gatherers and providing information on environmental changes. A multiproxy approach was chosen to establish a first general stratigraphy of the site and to disentangle the sediment composition as well as site formation processes. The results suggest a variation of allochthonous and autochthonous geogenic deposits, and anthropogenic accumulation processes. With the help of radiocarbon dating, anthropogenic layers were dated covering the arid Last Glacial Maximum (LGM, ∼21 ± 2 ka). The occupation phases were interrupted in cause of environmental changes. The most prominent is the accumulation of reddish, archaeological sterile deposits that can be chronologically associated with the African Humid Period (AHP, ∼15–5 ka BP). Geochemical records point to dry spells within this humid phase, suggesting correlations with regional climate signals of lacustrine sediments. These sediment accumulations of past wet conditions are covered by alternating layers of Holocene volcanic fallout and sediments with preserved cultural material. Our study provides a preliminary impression of still poorly understood time periods of human occupation in the southwestern Ethiopian Highlands. The data obtained from Sodicho Rockshelter could validate the current state of knowledge and partially reduce the chronostratigraphic gap.
Highlights
The development of a sediment stratigraphy within a rockshelter setting is driven by diverse sediment origins and changing conditions of the regional environment, including human agency
The south facing opening of the rock shelter is almost completely protected by big trachytic boulders (2 diam.) that originate from the partial collapse of the originally larger front overhang (Figure 2A)
The interior of Sodicho Rockshelter is characterized by the effects of moisture, which seeps through cracks in the ceiling and the walls, generating dark staining on the rock surface, and creating shallow pools and drip holes with stagnant water during wet seasons (Figure 2)
Summary
The development of a sediment stratigraphy within a rockshelter setting is driven by diverse sediment origins and changing conditions of the regional environment, including human agency. The use of caves and rockshelters by prehistoric humans has been part of geoarchaeological research in several regions, such as southern and northern Africa, Europe or the Levant (Goldberg and BarYosef, 1998; Goldberg et al, 2009; Marean et al, 2010; Miller et al, 2013; Kehl et al, 2014; Klasen et al, 2018; Inglis et al, 2018). In the Horn of Africa, especially in Ethiopia, these gaps in stratification are quite common. In most cases, they correlate with the transition from the Middle Stone Age (MSA) to the Late Stone Age (LSA) during the period of the MIS 2 (Tribolo et al, 2017). Several studies at sites such as Goda Buticha, the Ziway-Shala basin and Mochena Borago identified a stratigraphical and a chronological gap, spanning from ∼38 ka and reaching into the Holocene (Figure 1A) (Tribolo et al, 2017)
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