INTEGRATED CHEMOSTRATIGRAPHY OF TOARCIAN BLACK SHALES
During the Toarcian widespread deposition of organic-rich mudrocks occurs across Europe. A high degree of organic matter preservation and a covariant negative d13Corg/d13Ccarb isotope excursion in the exaratum Subzone, have been explained by a variety of causes linked to a 3rd order sea-level rise during this period. Models imply flooding of nutrient-enriched shelf areas with silled basin characteristics, expansion of an oxygen minimum zone, recycling of isotopically light CO2 within a stratified basin, or massive dissociation of methane hydrates.
We provide a high resolution (10 to 30 samples per ammonite subzone) multi-proxy chemostratigraphy study of two sections in SW-Germany and NE-England embedded in a stratigraphic and palecological framework. Chemostratigraphy and paleoenvironmental reconstruction is based on Rock Eval and elemental analysis (CNS) of bulk organic matter, d13Corg and d13Ccarb data, biomarker analysis of aliphatic and aromatic hydrocarbons, main and trace element distribution and sediment petrography.
Sea level change exerts the major control on the composition of primary produced organic matter as well as on its preservation, controlled by the prevailing redox-regime. Reconstruction of redox regimes applying organic and inorganic geochemical proxies reveals comparable chemofacies for both study sites. Sea-level variation, which is difficult to determine in black shale settings, is deducted from sterane biomarker composition and sediment petrography. Biomarker composition and carbon isotopic signatures indicate the extent of microbial utilization of organic matter and the degree of recycling of diagenetic CO2 into the photic zone. In addition to dominating bacterial sulphate reduction and CO2-recycling, intensive methanogenesis/methanotrophy occurs within the exaratum subzone. This previously overlooked process but not methane hydrate release seems to enhance the isotope excursion. Changes in primary production or the ratio of terrestrial vs. marine derived organic matter are of significance at the tenuicostatum/falciferum transition. Episodic occurrence of photic zone anoxia through the entire tenuicostatum to bifrons zone is discussed on the basis of a monsoonal climate model with annually changing deep-water mass conditions on the Toarcian shelf.