The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE;~183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock– Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n- alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ13C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other Toarcian basins with black shale deposition. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated organic compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n-C27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ13C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).
Stephen Ajuaba is currently a research associate and Ph.D. applicant from Cameroon and works on “Enhanced oilsource correlation using biomarkers and compound-specific isotopes". His work covers the application of various geochemical and petrographic techniques for the analysis of the molecular and isotopic composition of oil, natural gas and rock extracts, to enable accurate an efficient characterization and correlation of oils to their source rocks and also to reconstruct their paleo depositional environments. He currently works on the organic- rich Toarcian shales in SW Germany and the Permo-carboniferous rocks in Weiach, Switzerland. Previously, he gained work experience in sedimentary geology during his M.Sc. project in Italy and Spain (2018) and in organic geochemistry during his post-graduate traineeship (2019) at the Montanuniversitaet Leoben.
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