Determining rainfall seasonality from high-resolution carbon isotope profiles in evergreen wood and charcoal

Rainfall seasonality is a critical parameter in palaeoclimate reconstructions, both for improving climate models, and understanding past human-environment relationships. One promising approach is to infer rainfall seasonality from sub-annual variations in the stable carbon isotope composition (δ¹³C) of evergreen growth rings. Here, we test this proxy using modern samples of Protea (angiosperm) and Podocarpus (gymnosperm) collected across South Africa’s three major rainfall zones. High-resolution δ¹³C profiles from wood show consistent seasonal patterns that broadly correspond to rainfall seasonality, while comparable – though dampened – signals are retained in charcoal. We further show that isotopic boundaries can substitute for visible ring increments, enabling proxy application even when ring structure is absent, as is often the case in archaeological material. Well-dated charcoal fragments from Boomplaas Cave, an archaeological site in the Southern Cape, indicate the presence of winter-rainfall in the Last Glacial Maximum, aligning with climate models and other proxies. The results support the use of a new proxy for rainfall seasonality in evergreen wood and charcoal, but also highlight the need for local calibration, especially under water stressed conditions. This approach offers new potential for reconstructing seasonal rainfall from archaeological charcoal.