Late Quaternary Continental Carbonates of the Chobe Enclave (Botswana): A Multi‑Proxy Study Using Stable, Clumped, and Radiocarbon Isotopes

Continental carbonates in semi-arid regions represent valuable archives of past hydrological and climatic variability. In northern Botswana, the Chobe Enclave hosts widespread carbonates occurring in two major facies: (1) “islands” carbonates and (2) surface calcretes. The exact origin, formation mechanisms, and paleoenvironmental significance of these carbonates remain debated.

This study investigates the genesis of these two facies using an integrated sedimentological, mineralogical, and geochemical approach. Field observations, petrography, cathodoluminescence, and X-ray diffraction are combined with stable and clumped isotope geochemistry (Δ47) and radiocarbon dating. These data are further contextualized using modern surface and groundwater isotopic compositions.

Carbonate island facies samples that generally occur at similar altitudes throughout the area, give Late Pleistocene ages (~50–10 ka) and recorded precipitation from persistent, moderately evaporated waters at relatively low temperatures (17.1 ± 3.6 °C), consistent with palustrine to lacustrine environments linked to highstands of paleo-Lake Makgadikgadi. In contrast, the younger surface calcretes (<15 ka) have greater isotopic and textural heterogeneity and higher formation temperatures (24.1 ± 3.6 °C), reflecting surface precipitation driven by seasonal precipitation and evaporation under more arid conditions at temperatures broadly comparable to modern mean annual temperatures of this region. In addition, intermediate samples with mixed isotope compositions and fabrics are interpreted as having formed during hydrological/climatic shifts and/or by partial diagenetic reworking of older carbonate islands into calcrete-like deposits.

The results of this study highlight persistent hydrological controls on carbonate formation in the Chobe Enclave throughout the Late Quaternary and demonstrate the potential of multi-proxy carbonate archives for reconstructing paleoenvironmental changes in semi-arid continental settings.