Occurrence of fossil organic matter in modern environments: optical, geochemical and isotopic evidence

Y. Graz (1) , Christian Di Giovanni (1) , Yoann Copard (2) , M. Elie (3) , P. Faure (3) , Fatima Laggoun-Défarge (1) , J. Lévêque (4) , R. Michels (3) , J.E. Olivier (5)

1 ISTO - Institut des Sciences de la Terre d'Orléans
2 M2C - Morphodynamique Continentale et Côtière
3 G2R - Géologie et gestion des ressources minérales et énergétiques
4 GéoSol - Microbiologie des sols-GéoSol
5 UR ETGR (ETNA) - Érosion torrentielle, neige et avalanches

This study relates to the input and fate of fossil organic matter (FOM) in the modern environment, and focuses on two experimental watersheds overlying Jurassic marls: Le Laval and Le Brusquet (1km2 in area), located near Digne, Alpes-de-Haute-Provence, France. Considering that FOM delivery is mainly a result of different processes affecting sedimentary rocks [(bio)chemical and mechanical weathering], samples from different pools were collected: bedrocks, weathering profiles, soils and riverine particles. The samples were examined using complementary techniques: optical (palynofacies methods), geochemical (Rock-Eval 6 pyrolysis, C/N ratio), molecular (gas chromatography/mass spectrometry) and isotopic (stable C isotopic composition). As a result, FOM markers were identified and tracked through the different pools. The results highlight the contribution of FOM, which can be found in all the studied pools. Transfer of FOM between bedrocks/weathering profiles (governed mainly by chemical weathering) and bedrocks/riverine particles (governed mainly by mechanical weathering) reveals any qualitative change. Weathering profiles/soils transition [governed by (bio)chemical weathering] is characterized by alteration of the FOM, which is difficult to describe because fossil material is mixed with recent organic matter (ROM). Despite this latter point, the study provides evidence for a relative resistance of FOM to weathering processes and points to its contribution to the modern geosystem and the C cycle.

Keywords: storage; variability; haute-provence; marine-sediments; sedimentary-rocks; atmospheric carbon; global carbon balance; rock-eval pyrolysis

Keywords: erosion; modern soils

Domains: Geochemistry; Global Changes

Y. Graz, Christian Di Giovanni, Yoann Copard, M. Elie, P. Faure, et al.. Occurrence of fossil organic matter in modern environments: optical, geochemical and isotopic evidence. Applied Geochemistry, 2011, 26 (8), pp.1302-1314. ⟨10.1016/j.apgeochem.2011.05.004⟩. ⟨insu-00592510⟩