Chemostratigraphy of the Posidonia Black Shale, SW Germany: I. Influence of sea-level variation on organic facies evolution

Abstract

During the Toarcian, widespread deposition of organic-rich mudrocks (black shales) occurred across Europe, linked to a 3rd-order sea-level rise. We provide a high-resolution, multi-proxy chemostratigraphic study of the Dotternhausen sections in SW Germany embedded in a stratigraphic and palecological framework. Chemostratigraphy and paleoenvironmental reconstruction is based on Rock–Eval and elemental analysis of bulk organic matter and biomarker analysis of aliphatic hydrocarbon fractions. Aliphatic biomarker ratios applied to reconstruct the facies development include pristane/phytane and (pristane+phytane)/( nC 17+ nC 18) ratios, carbon preference index, C 27/C 29 sterane, and hopane/sterane distribution. Results show that sea-level change exerted the major control on the composition of primary produced organic matter and on its redox-controlled preservation. Changes in primary production and in the ratio of terrestrially versus marine-derived organic matter take place drastically at the onset of black shale deposition. The relative proportion of terrestrial versus marine biomass accumulation is independently assessed by highly correlative bulk organic matter properties (HI values) and biomarker ratios (CPI). The terrestrially derived end-member yields HI values of 50–100 mg HC/TOC and CPI values of 1.4–2.0, whereas the marine end-member has HI values of 600–700 mg HC/TOC and CPI values of 0.95–1.05. Correspondingly, the shift in plant- versus bacteria-derived biomass is discriminated by HI values and hopane/sterane ratios. Data were grouped according to their position in the lowstand (LST), lower and upper transgressive (TST), highstand systems tract (HST), and maximum flooding surface (mfz) of the sequence stratigraphic framework. Occurrence of organic matter with a significant bacterial contribution is restricted to the LST and lowermost TST and shows hopane/sterane ratios of 0.25–0.5. Marine phytoplanktonic matter dominates the upper TST, HST, and mfz with organic matter giving hopane/sterane ratios between 0.5 and 1.7. Samples from the HST yield HI values between 450 and 500 mg HC/TOC, which is consistently lower by 150 mg HC/TOC when compared to the TST. The lower quality of the organic matter deposited in the HST is due to its longer settling times through a thicker oxygenated water column. A strong correlation with relative sea-level stand and the C 27/C 29 sterane ratio is observed. LST exhibits C 27/C 29 sterane ratios of 0.7, during transgression values increase to 1.0, reaching a maximum at 1.4 during sea-level highstand, thus reflecting decreasing terrigenous input and shifts in near-shore versus open marine algal communities. Episodic fluctuation of anoxia is discussed on the basis of a monsoonal climate model, which infers significant surface run-off during summer monsoon providing high nutrient supply and associated high primary productivity. In combination with a density-stratified water column, excellent preservation conditions for organic matter were established. Dry-winter conditions with inversion of the surface-water flow regime led to seasonal export of bottom waters and episodic establishment of dysoxic conditions. Variations in the extent of annual flushing of sub-basins on the Toarcian shelf controlled long-term organic matter accumulation rate. Paleoclimate variability and its connection with sea-level stand is thus of paramount importance for different modes of organic matter accumulation and redox system stability.