Multi-proxy constraints on the origin, dynamics, and geochemical evolution of soil inorganic carbon in semi-arid landscapes of Botswana

Abstract

Soil inorganic carbon (SIC) constitutes a major component of the terrestrial carbon pool, particularly in dryland environments where pedogenic carbonates dominate long-term carbon sequestration. Despite its diverse environmental and ecological importance, SIC remains largely underrepresented in global carbon budgets, a deficit compounded by a lack of research into its formation dynamics and role as a proxy for paleoenvironmental and climatic evolution. This thesis addresses the prominent theoretical and methodological gaps through a progressive, multi-scale investigation of SIC in the semi-arid landscapes of Botswana. Using pedological and sedimentological approaches, the study characterised SIC formation pathways across major landscape units by integrating morphological examinations, mineralogical analyses, major and trace element geochemistry, and stable isotopes (δ13C, δ 18O). The findings highlight a regional dominance of pedogenic carbonate accumulation, driven by the mobilisation of calcium ions within distinct hydrological and geological settings. In the Kalahari Basin, this process is driven by felsic rock weathering and local fluviolacustrine reworking of lithogenic materials. Two mechanisms operate simultaneously. Mobilised calcium may originate from the in-situ dissolution of pre-existing carbonates or from the upward movement of calciumenriched soil solutions through evapotranspiration. While on the Eastern Hardveld, calcium is primarily supplied by the weathering of silicate-rich basaltic rocks. The results highlight an insitu mobilisation mechanism linked to the weathering of basaltic rock fragments reworked into the soils, in addition to upward movement. Stable isotope of carbon (δ13C) and oxygen (δ18O) signatures in carbonates indicate persistent semi-arid conditions and C3-C4 mixed vegetation. Furthermore, REE fractionation patterns and negative cerium anomalies provide robust, independent constraints for distinguishing pedogenic from lithogenic signatures, overcoming limitations of major-element-based interpretations. Collectively, these results advance the geochemical understanding of SIC formation in underrepresented regions and provide a transferable methodological framework for investigating soil carbon dynamics. This work reduces critical uncertainties in the global carbon budget and establishes a transferable framework for tracing SIC formation across semi-arid environments.