MOLECULAR AND BULK ISOTOPIC ANALYSES OF ORGANIC-MATTER IN MARLS OF THE MULHOUSE BASIN (TERTIARY, ALSACE, FRANCE)

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