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