Quantitative palynofacies analysis as a new tool to study transfers of fossil organic matter in recent terrestrial environments

Classical palynofacies method, which consists of an organic concentrate microscopic qualitative observation after mineral phase dissolution, is commonly used in order to study sedimentary organic matter. In the present study we develop a new quantitative palynofacies method that allows organic particles mass concentrations to be determined in studied samples. This method was developed to help quantify the input of fossil organic matter (FOM) into modern environments as a result of sedimentary rocks weathering. Studied samples were collected from different pools, like bedrocks, weathering profiles, soils and riverine particles in an experimental watershed “Le Laval”. This watershed overlying Callovo–Oxfordian marls (1 km2 in area) is located near Digne, Alpes-de-Haute-Provence, in France. In addition to palynofacies techniques, Rock-Eval 6 pyrolysis and Al2O3 content measurements (inductively coupled plasma emission spectrometry) were carried out on the samples. Obtained results show that this quantitative palynofacies method is suitable for FOM studies in modern environments, and FOM particles are quantified in the different pools. Results also give evidence that FOM alteration depends on the type of weathering, but also on the kind of organic particles. Soil formation under vegetation, resulting from the (bio)chemical weathering, lead to fossil organic particles concentration losses that do not exceed 30%. Elsewhere, mechanical weathering appears extremely fast and has no qualitative or quantitative influence on the observed FOM particles, which feeds directly into riverine stocks. FOM appears to be very resistant to weathering processes, this highlights its occurrence into supergene pools and then into carbon cycle. Quantitative palynofacies analysis is a new method adapted to such study, but can also be applied to other palynological, paleoenvironmental or archeological studies.

Research Highlights
► Quantitative palynofacies enables taking into account organic matter diversity.
► Quantitative palynofacies enables quantifying organic matter.
► Fossil organic matter can resist to weathering process.
► Fossil contribution should be taken into account into modern carbon pool.