Revisiting the fingerprint of organic matters in speleothem by Electron Paramagnetic Resonance

Abstract

The evolution of soil organic carbon (SOC) stocks is critical for both food production and climate change mitigation. This study uses advanced electron paramagnetic resonance (EPR) spectroscopy to investigate the spatial localisation and characterisation of organic carbon in speleothems, with a particular focus on methodological advances in recent decades. A speleothem sample from the Choranche cave in France was analysed using UV laser-induced fluorescence (LIF) and continuous wave EPR spectroscopy. The LIF analysis identified three main types of organic compounds − aromatic amino acids, aliphatic aromatics and larger aromatic compounds − distributed throughout the sample. EPR spectroscopy revealed the presence of Fe³⁺ and Mn²⁺ ions, along the entire sample for Fe³⁺ and more localised for Mn²⁺. When radical organic matter (ROM) is detected, first and second harmonic EPR imaging shows its collocation with Fe³⁺ and Mn²⁺, suggesting specific embedding conditions or source events. The study highlights a significant discrepancy between fluorescent organic matter (FOM) and ROM, challenging previous assumptions about their co-transfer from soil to speleothems. The results suggest that ROM is likely to be associated with specific soil redox conditions or high-energy events, whereas FOM represents a continuous background transfer. This distinction is crucial for accurate interpretations of soil organic carbon loss and its environmental implications. Future research should integrate detailed spectroscopic and isotopic analyses to better quantify organic carbon dynamics and their environmental proxies. Our results highlight the importance of distinguishing between different types of organic matter in speleothems to improve our understanding of soil organic carbon fluxes in relation to climate and human land use.