Decoupling climate and local hydrology in Central Botswana: new insights from carbonate δ13C, δ18O isotopes, and elemental geochemistry

Résumé

Pedogenic carbonates provide valuable archives of past environmental conditions through their elemental and isotopic compositions. This study integrates stable carbon (δ 13 C) and oxygen (δ 18 O) isotopes, obtained from carbonate features alongside manganese (Mn) and iron (Fe) concentrations in carbonate features and soil horizons across four pedosedimentary sections (SC01–SC04), to reconstruct vegetation dynamics, climatic stability, and local hydrological variability. Elemental concentrations of Mn and Fe are reported in molar percent (mol%). Soil profiles exhibit low Mn content (0.04%–0.11%), in contrast to higher Fe concentrations (2.68%–7.47%). Carbonate features are systematically depleted in Fe relative to their host matrices (0.42%–3.13%), while Mn is selectively enriched in several nodules, reaching up to 0.22% (Nd07, SC02). Stable isotope data reveal tightly clustered values within each section. δ 13 C values range from −3.17‰ to −0.24‰ for nodules, which is consistent with a C4-dominated savannah ecosystem. However, deep-seated pseudomycelia exhibit a distinct δ 13 C signature of −6.85‰, interpreted as a biogenic signal from C3 shrub roots in a closed, riparian sub-surface environment rather than a vegetation shift. δ 18 O values display a narrow range of −5.52‰ to −3.39‰ across all sections, indicating limited variability in meteoric water composition and evaporative conditions. High Mn/Fe ratios correlate with depleted δ 18 O values (≤−5‰), marking periods of increased freshwater input and seasonal waterlogging. Crucially, the decoupling of redox signals in some nodules, characterised by a combined high Mn/Fe ratio with δ 18 O enrichment, highlights the occurrence of episodic fluvial pulses and overbank flooding superimposed on a generally semi-arid background. These findings demonstrate that while the regional macro-climate remained stable, local hydrology was highly dynamic, driven by high-frequency fluctuations in river-water influence.

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