Changes in organic matter composition during the Toarcian Oceanic Anoxic Event (T-OAE) in the Posidonia Shale Formation from Dormettingen (SW-Germany)

Francesca Galasso,Annette Schmid-Röhl,Susanne Feist-Burkhardt,Stefano M Bernasconi,Elke Schneebeli-Hermann

doi:10.1016/j.palaeo.2021.110327

Francesca Galasso, Annette Schmid-Röhl + Show 3 more

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https://doi.org/10.1016/j.palaeo.2021.110327

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Abstract

During the Early Toarcian, deposition of organic carbon-rich-shales occurred throughout the epicontinental sea across Europe. Climate instability and high extinction rates in the marine realm were associated with profound environmental changes. The Toarcian Oceanic Anoxic Event (T-OAE) has been linked to the injection of greenhouse gases (e.g. oceanic methane) into the atmosphere triggered by the emplacement of the Karoo-Ferrar large igneous province (LIP) volcanism. The data presented are obtained from the Posidonia Shale Formation in Dormettingen (southwestern Germany), ~2 km from the well-known Dotternhausen section. Despite the intense palaeontological and geochemical research, studies on the particulate organic matter (POM) across the T-OAE are scarce. Here, we provide a detailed study of POM of the Dormettingen section as a tool to evaluate changes in the depositional environment. Integrated POM (i.e. amorphous organic matter, marine and terrestrial palynomorphs) and geochemical (i.e. carbon isotope δ13C) analyses reveal different episodes of palaeoecological upheavals during the studied time interval. In this study, we will integrate new palynofacies data and combine it with the existing sedimentological and palaeoecological data of Dotternhausen in order to interpret relative sea-level fluctuations and climatic changes at the local palaeogeographic setting.

Highlights

During the Early Jurassic, organic carbon-rich black shales, associated with the Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) were deposited in wide-ranging areas (Fig. 1a)
Particulate organic matter (POM) and bulk organic carbon isotope data are used to re-interpret the depositional environment of the Posidonia Shale Formation at Dormettingen, SW Germany (Late Pliensbachian to Early Toarcian)
The oxic conditions in the earliest Toarcian were interrupted by these anoxic phases, which are marked by changes in the lithology, changes in the microfacies and elevated TOC and by strong variations in particulate organic matter (POM) composition and bulk organic carbon isotope values

Summary

Introduction

During the Early Jurassic, organic carbon-rich black shales, associated with the Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) were deposited in wide-ranging areas (Fig. 1a). The deposition of black shales is documented in two main palaeogeographic regions with characteristic micro-and macrofauna (Stevens, 1971; Bucefalo Palliani and Riding, 2003) These are: (1) the Tethyan continental margins (Tethyan Realm) encompassing Italy and southern Europe (Stevens, 1971; Jenkyns and Clayton, 1997) and (2) the European epicontinental seas (Boreal Realm) comprising Germany, UK and northern Europe. The Central European Basin (CEB) was located between 20◦ and 40◦ N palaeolatitude during the Early Jurassic when the organic-rich black shales were deposited. This W-E trending shallow shelf sea basin was subdivided into several sub-basins (Ziegler et al, 1983; Rohl and Schmid-Rohl, 2005; van Acken et al, 2019). Due to its high organic matter content, the Lower Toarcian deposits of the Posidonia Shale Formation in Germany were the focus of early geochemical studies with the purpose of oil exploration and preliminary

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