Organic matter and clay interaction in a meromictic lake: Implications for source rock OM preservation (Lac Pavin, Puy-de-Dôme, France)

Adoum Mahamat Ahmat (1, 2) , Mohammed Boussafir (1, 2) , Claude Le Milbeau (1, 2) , Régis Guégan (1, 2) , Tiago de Oliveira (1, 2) , Lydie Le Forestier (1, 2)

1 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
2 Biogéosystèmes Continentaux - UMR7327

Investigation of organic matter (OM) preservation mechanisms is crucial for understanding organic accumulation in sedimentary environments. We focus here on the contribution of organo-clay interaction in such preservation. Most studies addressing this issue deal with organo-mineral complexes which have settled, omitting consideration of early aggregation within the water column. We therefore investigated the sorption of OM on montmorillonite (Na MMt) and kaolinite (Kaol) immersed in a permanently stratified water column (Lake Pavin, France). The two types of clay samples were kept immersed for different lengths of time and characterized via analytical methods suited for the organic phase [pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), GC-MS, Fourier transform infrared spectroscopy (FT-IR) and dissolved organic carbon (DOC) analysis, as well as for the mineral phase [X ray diffraction (XRD) and microprobe elementary cartography]. Results suggest that strong organo-clay interaction occurred within the water column to a varying extent depending on water physicochemical conditions and the mineral phase typology. A higher concentration of DOC was observed in the proximal environment of the clay traps, suggesting an attractive effect induced by the clay minerals. Lipid compounds sorbed during the immersion were essentially fatty acids and aromatic compounds. Their distributions were closer to a phytoplankton signature than allochtonous pedogenetic OM. The sorption was particularly efficient in the O2 transition zone, with a greater amount of OM on Na-MMt than Kaol. This was principally due to the high specific surface area and cationic exchange capacity (CEC) of the 2/1 clay mineral. Besides, XRD characterization revealed an increase in Na MMt interlayer spacing, which did not necessarily correspond to organic intercalation. The study confirms that the sorption of dissolved OM cannot alone explain the preservation of hydrocarbon-rich OM in such a lacustrine environment. Indeed, the sorbed OC was very low compared with the amount proposed by several authors as an expression of the monolayer equivalent principle.

Mots clés: Organic Matter; Preservation; Clays; Sorption

Domaines: Géochimie; Interfaces continentales, environnement

Adoum Mahamat Ahmat, Mohammed Boussafir, Claude Le Milbeau, Régis Guégan, Tiago de Oliveira, et al.. Organic matter and clay interaction in a meromictic lake: Implications for source rock OM preservation (Lac Pavin, Puy-de-Dôme, France). Organic Geochemistry, 2017, 109, pp.47-57. ⟨10.1016/j.orggeochem.2017.03.014⟩. ⟨insu-01505079⟩