Geochemical characterisation of the Pliensbachian-Toarcian boundary during the onset of the Toarcian Oceanic Anoxic Event, North Yorkshire, UK

Abstract

The lower Whitby Mudstone Formation of the Cleveland Basin in North Yorkshire (UK) is a world renowned location for the Early Toarcian (T-OAE). Detailed climate records of the event have been reported from this location that shed new light on the forcing and timing of climate perturbations and associated development of ocean anoxia. Despite this extensive previous work, few studies have explored the well-preserved sediments below the event that document different phases finally leading to large-scale (global) anoxia, which is the focus of this project. We resampled the underlying Grey Shale Member at cm-scale resolution and conducted a detailed multi-proxy geochemical approach to reconstruct the redox history prior to the Toarcian OAE. The lower Whitby Mudstone Formation, subdivided into the Grey Shale Member overlain by the Jet Rock (T-OAE), is a cyclic transgressive succession that evolved from the relatively shallow water sediments of the Cleveland Ironstone Formation. The Grey Shale Member is characterised by three distinct layers of organic rich shales (~10-60 cm thick), locally named as the ‘sulphur bands’. Directly above and below these conspicuous beds, the sediments represent more normal marine mudstones. Further upwards the sequence sediments become increasingly laminated and organic carbon rich (up to 14 wt %) representing a period of maximum flooding that culminated in the deposition of the Jet Rock (T-OAE). Detailed analyses of the Grey Shale Member, with a focus on the sulphur bands, for TOC and total sulfur concentrations, microscopy, iron speciation (FeHR/FeT, FePyrite/FeHR), trace element concentrations, molecular biomarkers, and bulk carbon and sulphur isotopes confirm highly variable redox conditions prior to the Toarcian OAE, with repeated anoxia/euxinia during periods of sulphur band deposition. Cm-scale geochemical records from the lower sulphur band actually suggest significant, short term variations in redox within the bed, with one full cycle from anoxia/euxinia to oxic conditions and back. We speculate that these cyclic variations in redox during and possibly also between sulphur band formation were driven by orbital forcing, however, better chronological information is necessary to validate this interpretation. The bioturbated mudstones between and below the sulphur bands show less enrichment of TOC, reactive iron and trace elements, but still suggest conditions close to the Fe-proxy threshold characteristic of anoxia (FeHR/FeT = 0.38). Further up the section in the bioturbated mudstones, highly reactive iron and trace elements are significantly depleted, indicating a return to more oxic conditions, which persisted up the top laminated unit of the Grey Shale. This observation challenges the general concept that anoxia/euxinia was limited to the Toarcian OAE, at least in the Cleveland Basin of North Yorkshire. This thesis discusses the detailed dynamics of redox variations and biogeochemical elemental cycling in the run-up to this major event in Earth history. The mechanisms behind the short redox events documented in the sulphur bands may have had some similarities to those proposed for the small hyperthermals post-dating the Paleocene-Eocene thermal maximum (PETM) and other OAE’s. Enhancement of run off from land via enhanced hydrological cycling could have temporarily created conditions in the Jurassic Cleveland III Basin that favoured stratification, high productivity, and development of extreme (euxinic) redox condition with high sulphur and carbon burial. The increasing amplitude of perturbations from the first sulphur band to the T-OAE, and their progressive spatial expansion from proximal to global, may argue for a unifying causal connection, orbital forcing. Further work will have to validate these conclusions and assumption.