Molecular and bulk isotopic analyses of organic matter in marls of the Mulhouse basin (Tertiary, Alsace, France)

Abstract

Contents of 13C in kerogens and carbonates in 21 samples from a core of the MAX borehole, Mulhouse Evaporite Basin, range from −27.3 to −23.5 and −3.7 to −1.8 vs PDB, respectively. Organic nitrogen in the same samples is enriched in 15N relative to atmospheric N2 by 12.2 15.7 . Hydrogen indices and values for kerogens vary systematically with facies, averaging 493 mg HC/g Corg and −25.7 in the most saline facies (dominated by inputs from aquatic sources) and 267 mg HC/g Corg and −23.7 in the least saline facies (50/50 aquatic/terrigenous). Values of were measured for individual aliphatic hydrocarbons from three samples representing three different organic facies. For all samples, terrigenous inputs were unusually rich in 13C, the estimated value for bulk terrigenous debris, apparently derived partly from CAM plants, being −22.5 . In the most saline facies, isotopic evidence indicates the mixing of 13C-depleted products of photosynthetic bacteria with 13C-enriched products of halotolerant eukaryotic algae. At lower salinities, a change in the producer community is marked by a decrease in the 13C content of algal lipids. The content of 13C in algal lipids increases in the least saline facies, due either to succession of different organisms or to decreased concentrations of dissolved CO2.