The Toarcian oceanic anoxic event : organic and inorganic geochemical anomalies in organic-carbon-rich mudrocks from the North Yorkshire coast, UK and Dotternhausen Quarry, SW Germany

Harding, Stephen M. (2004). The Toarcian oceanic anoxic event : organic and inorganic geochemical anomalies in organic-carbon-rich mudrocks from the North Yorkshire coast, UK and Dotternhausen Quarry, SW Germany. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f9d5

Abstract

High-resolution organic and inorganic chemostratigraphic profiles have been determined for Toarcian (Lower Jurassic) marine successions of organic-carbon-rich mudrocks from the Yorkshire coast (UK) and Dotternhausen Quarry (SW Germany). This data has been used to understand the nature of the environmental changes and the most plausible mechanism(s) during the Toarcian oceanic anoxic event (OAE, exaratum ammonite Subzone). Possible mechanisms include gas-hydrate release, oxygen minimum zone expansion and oceanic overturn.

Total organic-carbon, total sulfur, total nitrogen and other organic parameters (Rock-Eval pyrolysis) infer that denitrifying/euxinic palaeoredox conditions existed in the sediments and bottom waters during the OAE with conditions becoming less anoxic/euxinic after the end of the exaratum Subzone. The 6‰ negative δ ¹³C_org excursion is confirmed in Yorkshire and Germany and its relationship to the long-term change established. Correlation between Yorkshire and Germany indicates that several carbonate horizons are equivalent and that diagenetic conditions were similar.

Inorganic element abundances have been determined using a combination of X-ray fluorescence and refined Laser ablation ICP-MS techniques. Inorganic elements can be divided into those that have a detrital affiliation (e.g. Si, Al and Ti) and those that are seawater derived or have a biological association (e.g. Cu, V and Mo). Some detrital elements such as Zr and Nb show evidence for hydrodynamic sorting related to a relative sea-level rise that produces a decrease in Zr and Nb during deposition of the bifrons Zone. The Mo/Cu ratio changes from low in the exaratum Subzone to high in the overlying falciferum Subzone. It is suggested that the relatively low levels of Mo/Al₂O₃, Ni/Al₂O₃ and Re/Al₂O₃, in the sedimentary deposits formed during deposition of exaratumSubzone is due to a high sedimentation rate.

The relatively fast sedimentation rates inferred during deposition of the exaratum Subzone are most likely a consequence of increased of continental weathering rates and primary productivity. This supports the hypothesis that the trigger for the Toarcian OAE is massive gas-hydrate dissociation associated with a rise in mean global temperature and an increase in chemical weathering.