Plant wax n-alkanes track C3-C4 vegetation transitions and hydrological responses along elevation gradients in Lesotho, southern Africa

Understanding human adaptations to high altitude contexts, and the impact of climatic change and associated vegetation change on human societies is a key research theme in archaeological, palaeoecological, and palaeoanthropological research. In many highland regions, variations in temperature related to elevation and aspect are associated with the distribution of plants following the C₃ and C₄ photosynthetic pathways. In Lesotho, δ¹³C of soil organic matter and faunal material is commonly used in archaeological research to infer palaeoenvironmental change and its connection to changes in human behavioural and settlement patterns. However, there is limited modern reference information for δ¹³C in the region with which to interpret past dynamics.

We present the results of bulk δ¹³C and compound-specific isotope analysis (CSIA; δ¹³C and δ²H) of long-chain n-alkanes from modern soil samples collected from four elevation transects covering 1,800 to 3,300 m a.s.l. in eastern Lesotho, from both north- and south-facing slopes. Consistent with modern ecological observations, elevation, and, in turn, temperature, are major controls on the distribution of C₃ and C₄ vegetation along the transects. Bulk sediment and n-alkane (C₂₉, C₃₁ and C₃₃) δ¹³C results from north-facing slopes track the transition from C₄ dominated to C₃ dominated vegetation with increasing elevation, although this transition point varies by n-alkane chain-length. On south-facing slopes, which receive less solar radiation, CSIA δ¹³C values largely reflect C₃ dominated vegetation, including at lower elevations than on the north-facing slopes. Differences observed between the bulk sediment and CSIA δ¹³C results are likely related to the impact of microbial activity in the bulk sediment samples, and highlight the advantages of compound specific analysis in identifying and exploring these discrepancies.

Variability in δ²H along the transects is less distinct, but also observed, particularly between north- and south-facing slopes. Mean δ²H values between north- and south-facing slopes are offset by up to 30 ‰. The results of this study provide a reference baseline for studying past ecological change in this region, where the application of compound specific isotope analysis of sediments from archaeological sites is increasingly used to improve our understanding of past human-environment interactions in highland areas.