Do REEs in mudstones record bottom-water redox?: The Pliensbachian–Toarcian record (Lower Jurassic) and T-OAE in the Cleveland Basin, England

Abstract

The Pliensbachian–Toarcian of the Cleveland Basin provides a global reference for the interval incorporating the Toarcian Oceanic Anoxic Event (T-OAE ~ 183 Ma). Palaeoredox proxies show a progressive shift from oxic bottom waters in the late Pliensbachian through dysoxic–anoxic conditions in the earliest Toarcian to euxinia during the T-OAE. Anoxia–dysoxia persisted into the middle Toarcian. Laminated black shales with TOC contents > 2.5 % characterise the anoxic–euxinic intervals. The rare-earth element (REE) geochemistry of the succession sampled in the Dove's Nest core is described and compared to data from nearby Yorkshire coastal outcrops. Interpretation is based on a review of REE behaviour in modern marine water columns, pore waters and sediments. Mud(stone) REE patterns are insensitive to bottom-water redox conditions. The REEs are principally located in the siliciclastic clay fraction of modern marine muds and ancient mudstones. Bulk mud(stone)s generally exhibit relatively flat REE patterns when normalised to average shale. Cerium anomalies are largely absent. Stratigraphical trends in the Yorkshire succession are related principally to sediment grain size. Authigenic and biogenic phosphates, principally carbonate fluorapatite, when present, dominate the whole-rock REE inventory leading to convex-upward patterns and large positive middle REE (MREE/MREE*) anomalies. These occur sporadically throughout the oxic–euxinic intervals, showing no correlation to bottom-water redox. The REE geochemistry of marine mudstones presents a combination of primary mineralogical and grain-size controls related to sediment provenance combined with the impact of authigenic mineral formation. Bulk mudstone REE patterns do not provide a viable bottom-water redox proxy.