A soil stratigraphic unit provides evidence for Late Quaternary climate change in the Chobe Enclave, northern Botswana

Résumé

Abstract A quarry in the Chobe Enclave, northern Botswana, exposes a prominent pedostratigraphic unit of approximately 8 metres consisting of two pedostratigraphic levels (PL), older PL1 and younger PL2. In a region where proxies for palaeoenvironmental reconstruction are terribly lacking, this study assessed the potentials of paleosols (fossil soils) to answer fundamental questions about weathering, pedogenic intensity and environmental change using macromorphology, geochemistry and clay mineralogy. SiO2 is the dominant major oxide (40.6- 98.9 wt. %) followed by CaO (0.02-29.6 wt. %), Fe2O3 (0.48- 2.64 wt. %), MgO (0.14 – 1.81 wt. %) and Al2O3 (0.29 – 0.93 wt. %). The clay-sized minerals present in the paleosols are sepiolite, quartz, calcite and kaolinite. The carbonates had strong positive correlation with Sr (R2 = 0.935), while Fe2O3 had weak positive correlation with TiO2 (R2 = 0.0187). PL1 developed on materials of fluvial origin and is genetically immature with A/C horizons. In comparison with modern soil, PL1 would qualify as Entisols (Fluvent or Fluvaquent in Soil Taxonomy). PL2 is comparable to modern Calsisol and formed from protracted period of post burial dissolution, downward translocation and recrystallization of the palustrine carbonates to form pedogenic carbonates. Calcification, incipient ferralization and leaching of soluble salts are the dominant pedogenic processes in the pedostratigraphic section. Weathering and pedogenesis were generally incipient in the unit. Late Quaternary hydrological dynamics in the Chobe Enclave was the major driver for the formation of the soil stratigraphic unit. This study demonstrates the applicability of paleosols for reconstructing and discerning palaeoenvironmental and climate dynamics and gives the possibility of connecting pedosedimentary processes in the area to regional palaeoenvironmental archives.

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