Evidence of global and regional climate oscillation during the late Pleistocene-Holocene epochs is well documented (Konecky et al., 2011; Timmermann and Friedrich, 2016). In the semi-arid areas of southern Africa, it is marked by increased frequency of extreme climatic events and hydrological responses (Kusangaya et al., 2014). The Kalahari Basin covers a large expanse of lowland area in Botswana and extends into Namibia, South Africa, Angola, Zambia, and Zimbabwe (Fig. 1). Being a huge basin of internal drainage into which aeolian, fluvial and palustrine sediments have accumulated since the late Cretaceous (Grove, 1969), the middle Kalahari has been a hotspot for paleoenvironmental reconstructions mostly through geological, geomorphological, and lately ecological archives (e.g. Burrough et al., 2007; Cordova et al., 2017). Multiple proxy evidence has confirmed a varying Quaternary history for the middle Kalahari, but climate dynamics in this area is still poorly understood even after five decades of paleoenvironmental research (Burrough et al., 2009).
Chobe Enclave in the middle Kalahari Basin of northern Botswana has been influenced by the Pleistocene-Holocene climate variability (Burrough and Thomas, 2009). The present day Chobe Enclave is under the influence of: (i) the Congo Air Boundary (CAB) and Intertropical Convergence Zone (ITCZ), which are major tropical air mass convergence systems; (ii) three main river systems of the Okavango, Kwando-Linyanti, and the Zambezi-Chobe which bring waters from the equatorial Angolan highlands, and (iii) tectonics, given that the local fault system is an extension of the East African Rift Zone (Moore et al., 2012). Therefore, Chobe Enclave owes it geomorphic properties to the complex Quaternary palaeohydrological dynamics influenced by both climate and/or tectonic changes. Most paleoenvironmental reconstruction studies of the middle Kalahari have been focused on the Mababe sub-Basin (Fig. 1). This study is the first to focus on the potential of alluvial paleosols (fossil soils) from Chobe Enclave to contribute to our knowledge of the paleoenvironments of the middle Kalahari.
Climate parameters, the most active of which are precipitation and temperature, exert strong influence on the nature and properties of soils (Jenny, 1994). Sitting at the interface between the atmosphere, lithosphere, hydrosphere and biosphere, soils and paleosols have the potential to archive the prevalent environmental and climatic conditions under which they formed (Beverly et al., 2018). Globally, paleosols have been successfully used for regional and site-specific palaeoenvironmental reconstruction. Proxies that include particle sizes distribution, macromorphology, geochemistry and clay mineralogy of paleosols and sediments are strong indicators of a combination of pedogenesis, provenance, weathering intensity, hydraulic sorting, abrasion, redox condition and diagenesis (Eze and Meadows, 2014a; Eze et al., 2016a; Huntsman-Mapila et al., 2006). This implies that geochemical and pedogenic proxies are responsive to palaeoenvironmental and paleoclimate changes.
It has been shown that Chobe Enclave has more soil diversity than earlier reported (Romanens et al., 2019). A rare exposure of a deep sand quarry in Chobe Enclave reveals a soil stratigraphic (pedostratigraphic) unit consisting of two levels: a thick unit of paleosol carbonate overlying a genetically immature soil profile developed on alluvial sediments. In a geochronological study, Diaz et al. (2019) reported different phases of active deposition and landscape stability spanning MIS6 to MIS1 during which carbonate islands were formed in the Chobe Enclave. Using geochemical indicators of weathering and pedogenesis, we seek to understand the environmental and climate dynamics, and the pedosedimentary processes at a deep pedostratigraphic section in the Chobe Enclave. In consequence, the objectives of this study are: (i) to characterise the paleosol carbonate of the pedostratigraphic unit using their macromorphological, physico-chemical, geochemical, and clay mineralogical properties; and (ii) to assess weathering and pedogenesis in the unit; and (iii) to reconstruct the environmental conditions of the area. The findings will help us to establish the relationship between pedogenic and geomorphic processes during periods of stability and instability in a semi-arid climate. Trace element compositions are particularly useful as these elements generally remain immobile during the processes of weathering, transport and deposition.