Determining soil sources by organic matter EPR fingerprints in two modern speleothems

Abstract

Organic matter (OM) sources and transfers are critical questions at the crossroad of ecology, hydrology and paleoenvironmental studies in karst environments. Recently chemical or isotopic characterisations of these organic matters have demonstrated their diversity in karst water and speleothems. However, knowledge of their origin and transfer from soil needs to be improved especially in karst environments where a mosaic of soils occurs. Here we investigate the applicability of the Electron Paramagnetic Resonance (EPR) signature of OM to seek different soil fingerprints in speleothems. The positions and the shapes of semiquinone-type radical’s EPR lines are considered as a robust signature of the organic matter through the different compartment of karst ecosystems. We demonstrate that the combination of EPR lines simulation constitutes a fingerprint that discriminates folic Leptosol developed on limestone outcrops from eutric Cambisols located in the dolines and topographical depressions. We also report the conservation of the thinner OM EPR lines after a water extraction of the organic matter from soils. Finally, OM EPR signatures were detected in two different speleothems sampled near each other: a stalagmite and a subaqueous flowstone. An unexpected two-orders-of-magnitude discrepancy between the free radical concentrations of the subaqueous flowstone and the soils on one hand, and the low free radical concentration of the stalagmite samples on the other, was ascribed to the kinetics of conformational changes in OM occurring during their transfer through karst either in fast or in slow water pathways. A folic Leptosol fingerprint was found in stalagmite and conversely, an eutric Cambisol fingerprint was found in the flowstone sample. This unexpected difference between the EPR signatures of the flowstone sample and stalagmite sample was ascribed to the masking of the Leptosol EPR signature (due to the low concentrations of free radicals) in the case of the flowstone sample and to the fact that the EPR signature of the stalagmite sample clearly shows the Leptosol is the sole source of the OM. Finally, the folic Leptosol fingerprint remains steady over the last four centuries covered by the stalagmite demonstrating that the source of the OM is stable over the growth period strengthening the interest of OM entrapped in speleothem for paleoenvironment studies.