Structural and chemical variability and provenance of lacustrine organic matter in the Miocene Nördlinger Ries post-impact sediments

Abstract

The Nördlinger Ries in southern Germany contains a sequence of Miocene post-impact lacustrine sediments. To gain knowledge on the chemical composition and provenance of the organic matter (OM) through the up to 250 m thick sediment profiles, pyrolytic, organic geochemical and infrared spectroscopy measurements were performed on both sediments and concentrated OM. Type I kerogen prevails through the examined samples with high atomic H/C ratios (1.6 on average), hydrogen index (HI mostly >600 mg HC/g TOC), and spectral A- and C-factor values (mostly >0.8), but low oxygen index values (OI mostly <25 mg CO2/g TOC) for the concentrated OM. Exceptions occur in the uppermost Miocene sediments at the central site (Nördlingen-1973) and some marginal (SUBO-18) sediments containing hydrogen-lean type III kerogen having low H/C ratios (<1), HI (<300 mg HC/g TOC), spectral A- and C-factor values (<0.6), but high OI values (>40 mg CO2/g TOC). Mineral matrix effects induced by abundant clay minerals and zeolites significantly reduce the HI of bulk sediments as compared to concentrated OM. Organic sulfur-rich kerogen occurs in the basal units in the NR-1973 and occasionally in the SUBO-18 boreholes with high atomic Sorg/Corg ratios >0.04 and pyrolytic thiophenes/toluene ratio > 13. Abundant pyrolytic alkanes/alkenes, high spectral A-factor and CH2/CH3 values suggest prominent algal input to the OM for most of the samples. The aliphatic chain length of the kerogen decreases and the branching degree increases from the older, deeper to the younger, shallower deposits. While oxygen-bearing organic constituents are common, nitrogen-bearing moieties are rare. A general structural model is established for the type I sulfur-rich, Miocene kerogen of the Nördlinger Ries paleo-lake.