A Multi-Proxy Pedosedimentary Insight into Calcium Origin, Sediment Provenance, and Depositional Environments in a NW Botswana Landform

Calcium carbonates are key components of the global terrestrial geochemical carbon cycle and can influence carbon sequestration, climate interactions, and agricultural productivity. In the soils of the Chobe Enclave (northern Botswana), calcium carbonates are widespread, yet their origin and accumulation processes remain poorly understood. This study investigates a pedosedimentary section using multiple proxies, including morphological descriptions, physicochemical properties, particle size inclusive statistic, bulk and clay mineralogy, and geochemical indices such as the Chemical Index of Alteration (CIA), the Weathering Index of Parker (WIP), the Robust Index of Chemical Weathering (RW), the Index of Compositional Variability (ICV), and the calcium-strontium ratio (Ca/Sr). In the upper part of the section, CIA (1.7–6.9) and RW (10.5–23.2) are of low values, while the lower part records higher values (CIA: 40.0–72.2, RW: 26.0–33.3). Conversely, WIP (13.4–40.5) and ICV (9.9–37.7) are higher in the upper part but decrease in the lower part (WIP: 2.4–10.5, ICV: 0.3–0.9). The Mafic-Felsic-RW projection suggests a felsic origin for the entire sediment sequence. The Ca/Sr ratio ranges from 384 to 803 in the carbonate-rich (5–23 wt%) upper section and from 190 to 1011 in the carbonate-poor (< 1 wt%) lower section. These results indicate a shift in depositional environment from shallow lacustrine to alluvial floodplain with temporary flow. Calcium in pedogenic carbonates primarily originates from alluvial sediments, derived from the weathering of felsic rocks. While strontium isotope ratio analysis may provide robust complementary insights, the multiproxy approach used in this study effectively traces calcium origin. Unravelling calcium carbonate origin is essential for improved understanding of soil formation, function and management, especially in drylands.