Sediments, soils, and the expansion of farmers into a forager's world: A geoarchaeological study of the mid-to-late Holocene in Hwange National Park, Zimbabwe

Abstract

The transition from foraging to food production in interior southern Africa is still not well understood, despite being central to hypothesized migration and diffusion routes connecting the earliest agropastoralist sites in central Africa with the southern African subcontinent. Hwange National Park (14,650 sq. km), Zimbabwe is located along these proposed routes and has both Late Stone Age and Early Iron Age sites that can address this transition.To better understand this change in lifeways and any environmental and/or topographic factors that influenced it, geoarchaeological investigations characterized the soils, sediments, and landforms in six different areas of the park. Forty-eight stratigraphic sections varying from two to four m thick were described, six of which are detailed here. These stratigraphic sections include dated material that allows correlation between basins of different substrates and comparisons of carbon isotopic signatures to reveal whether conditions were locally favorable to grasses (hot/dry) or woodlands (cool/wet) through time.Examined river cuts and augered samples show that some river basins have sediment textures, soil types, and topographic relief that should have been attractive to farmers. However, radiocarbon ages reveal variable potential for finding remains of any early farmers from basin-to-basin, with some bedrock-constrained valleys having no preserved deposits of appropriate age. The repeated scouring of these river basins testifies to sometimes volatile conditions that would have challenged sustained food production.In fact, in the study area landscape instability and variable environmental conditions were commonplace during the past three millennia. Drought between ca. 2.8 and 2.6 ka may have lessened the risk of disease vectors for the earliest agropastoralists that arrived during more mesic conditions between 1.9 and 1.65 ka. At this time basins began to accumulate fill and soil development suggests landscape stability even in headland basins. Changes in soil carbon isotope values imply relatively hot/dry conditions prevailed around 0.73 ka and 0.41 ka and cool/wet conditions around 0.53 ka, but much information has been lost due to the numerous erosional events that wiped away swaths of sediments during intervening droughts. The landscape in Hwange National Park continues to change rapidly as reflected by several meters of river downcutting in response to recent man-made disturbance and/or drought. This downcutting both reveals and endangers the few remaining archaeological sites that are critical to understanding the first food producers in the region.