Variability in geomorphic response to millennial-scale hydroclimatic shifts and fire-induced disturbance: a multi-proxy analysis of a peatland archive from South Africa's Mediterranean biodiversity hotspot

Abstract

Anthropogenically-driven shifts in hydroclimatic and wildfire regimes typify fire-prone regions globally, with profound implications for sediment production and transport processes. However, the nature and extent to which past climatic shifts and fire activity have influenced fluvial sediment dynamics over long timescales remains poorly understood, especially in the Mediterranean-climate region of South Africa. This study presents a ca. 2858-year record of fluvial sedimentological change and palaeoecological reconstructions of vegetation (fossil pollen), fire (charcoal) and herbivory (dung spores) from a peatland in the Cape Floristic Region to advance understanding of the linkages between climate, vegetation, fire and geomorphic processes. Analyses of grain-size distribution revealed alternating phases of organic accumulation, punctuated by clastic sediment input. The initiation of clastic sediment pulses supersedes peaks in fire activity and encompasses the period of climatic reorganisation in the region. This highlights the apparent temporal asynchronicity between fire activity and geomorphic responses, where fire may act as a precursor disturbance, with sediment transfer occurring only in response to hydroclimatic forcing. The recovery of pre-disturbance organic sedimentation patterns, especially over the past 200 cal. yr BP, following compounded disturbances, indicates the potential resilience of this wetland. This local-scale resilience likely depends on the capacity of eco-hydro-geomorphic feedbacks to re-establish sediment (dis) connectivity states post-disturbance. By integrating geomorphological and palaeoecological evidence, this study provides novel insights into historical variability in sediment connectivity dynamics in Mediterranean-type fluvial systems. This information is timely, given intensifying human influences, and has implications for catchment management strategies and interpreting wetland evolution trajectories.