Reconstructing Fire, Vegetation, and Climate Variability over the Last ~1800 Years from a High-Resolution Speleothem Record in the Central Balkans

Understanding long-term interactions between fire activity, vegetation dynamics, and climate variability is essential for contextualizing recent environmental change in the Mediterranean/Balkan region. Speleothems represent a promising yet still underutilized archive for paleofire reconstructions, offering robust chronologies and the integration of multiple environmental proxies within a single, continuous terrestrial record.

Here we present biomarker results from a 220 mm long speleothem from Golubarnica Cave (North Macedonia), spanning approximately the last 1800 years. The record is continuous and constrained by U-Th dating, and combines polycyclic aromatic hydrocarbons (PAHs) as indicators of fire activity with n-alkanes reflecting vegetation composition and terrigenous organic matter inputs. Individual sampled layers integrate on average ~30 years, with both integration windows and temporal spacing ranging from sub-annual to multi-centennial scales, allowing the identification of long-term trends and abrupt shifts in fire-related molecular assemblages. This speleothem forms part of the PROMETHEUS project, which investigates fire-climate-ecosystem interactions using speleothem-based multi-proxy approaches.

The PAH record reveals multi-centennial phases of fire activity broadly corresponding to major late-Holocene climatic intervals. Low and relatively stable PAH concentrations characterize the early part of the record (approximately 2nd-6th centuries CE), indicative of a background fire regime. Fire activity increases during the Medieval Climate Anomaly, peaks in the 12th-13th centuries CE, and declines abruptly toward the end of the 13th century, marking the onset of a prolonged phase of reduced fire activity broadly consistent with cooler

conditions during the Little Ice Age. Fire-related signals increase again from the late 16th century onward toward the present. While primarily interpreted in terms of hydroclimatic variability, potential contributions from medieval socio-environmental changes and land-use practices cannot be excluded.

Throughout the record, PAH variability closely mirrors speleothem δ¹⁸O, indicating a persistent hydroclimatic control on regional fire regimes. The main fire maximum is chemically distinct, dominated by an extreme increase in retene (up to two orders of magnitude above background levels) and accompanied by pronounced increases in higher-molecular-weight PAHs, suggesting a major shift in fuel type and/or fire intensity involving resin-rich woody biomass rather than a simple increase in fire frequency. Low-resolution n-alkane data show a synchronous response during this event, including a temporary increase in total n-alkanes, a minimum in average chain length, and a subsequent increase in carbon preference index, pointing to short-lived changes in vegetation-derived organic matter inputs and/or preservation.

Overall, this study highlights the potential of high-resolution speleothem hydrocarbon records to capture multi-decadal to centennial variability in fire regimes and associated environmental processes, identifying hydroclimate as a primary driver of fire activity in the central Balkans during the late Holocene.