Pleistocene-Holocene imprint of soil pyrogenic carbon and reversed ages in soil and cave flowstone

Abstract

Wildfire can significantly alter karst surface environments, yet its impact on cave systems and speleothem archives remains poorly understood. This study investigates the relationship between surface fire events and their imprint on inverted ages recorded in soil and flowstone in the Postojna (Slovenia) karst area. We present new radiocarbon ( 14 C) ages from charcoal preserved in a well-developed Luvisol and from a carbon-rich black layer enclosed within cave flowstone, complemented by U/Th dating of flowstone and micromineralogical analyses of soil horizons. Charcoal ages range from modern (110 years BP) to the Late Pleistocene (∼31,000 years BP). Reversed age sequences are observed in the deepest Bt3 soil horizon and in the cave flowstone and are linked to downslope movement triggered by a major palaeofire around 92,000 years BP. This process transported older (∼31,000 years BP) charcoal downslope, hindered soil respiration, and altered meteoric-water infiltration, resulting in anomalously high dead carbon fractions (DCF ≈ 40–90%). δ 13 C signatures support shifts between open Late Pleistocene steppe-tundra and denser Holocene forest vegetation, while highly depleted δ 13 C values in the flowstone black layer reflect soot inputs from high-temperature combustion. Evidence of magnetite, maghemite, and mullite confined to the Bt3 horizon confirms high-temperature wildfire. Integrating soil, cave, and mineralogical data demonstrates that major wildfire events can generate synchronous surface–subsurface signals, affect sediment redistribution on karst slopes, and produce identifiable fire horizons within speleothems. The study highlights the potential of combined soil charcoal and speleothem archives to reconstruct palaeofire activity and landscape evolution in karst environments.