Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

Abstract

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 basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated 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).

Molecular fossils of Aptian–Albian blue marls of the Vocontian Basin (France), depositional conditions and connections to the Tethys Ocean

“…The abundance of 2-methylhopanoids at the four studied sections is distinctive of the Goguel Level compared to the other OLs of the Vocontian Basin. The abundance of 2-methylhopanoids has been previously described in different organic-rich deposits of OAE1a [Kuypers et al, 2004b, Dumitrescu and Brassell, 2005, 2006, Karakitsios et al, 2018, Castro et al, 2019, but also in the deposits of the Toarcian OAE [Farrimond et al, 1994, Blumenberg and Wiese, 2012, Ruebsam et al, 2018, Ajuaba et al, 2022, and OAE2 [Farrimond et al, 1990, Kuypers et al, 2004a, Forster et al, 2008. The significance of elevated concentrations of 2-methyl hopanoids is still highly discussed [Kusch and Rush, 2022].…”