Magnitude and Pacing of Early Jurassic Palaeoclimate Change: Chemostratigraphy and Cyclostratigraphy of the British Lower Jurassic (Sinemurian – Pliensbachian)

Abstract

Palaeoclimate research in the Early Jurassic (201.4–174.1 million years ago) has historically focussed on geologically short, large-magnitude climatic events such as the Triassic-Jurassic (T-J) boundary mass extinction and the Toarcian Oceanic Anoxic Event (T-OAE). However, the climate system in the remaining ~18 myr of the Early Jurassic is significantly under studied; particularly given the identification of several smaller magnitude and crucially, less well understood carbon-cycle perturbations. In this thesis, I examine the Sinemurian-Early Pliensbachian from three sites: two archived British Geological Survey (BGS) boreholes (Mochras Borehole Cardigan Bay, Wales and Burton Row Borehole, Somerset, England) as-well-as the Robin Hood’s Bay coastal exposure, Yorkshire, England. I present new, high-resolution multiple-proxy chemostratigraphy (hand-held X-ray fluorescence, stable isotope (δ13Cbulk-org & wood), Rock Eval pyrolysis and Mercury concentration data) through the interval, in order to elucidate the climate and carbon cycle evolution. I demonstrate the reproducibility of two negative carbon cycle excursions of ~4 ‰ δ13Corg associated with enhanced organic burial, marine flooding surfaces and shallow marine dysoxia; the first across the obtusum-oxynotum zone and the second at the Sinemurian-Pliensbachian boundary (raricostatum-jamesoni Zones). Mercury analysis of samples from the Burton Row borehole suggest LIP volcanism did not force climatic change in the Sinemurian and Early Pliensbachian with largely background values of Hg/TOC and no correlation of Mercury enrichment with intervals of isotopically light carbon (or dates of late phase CAMP). Spectral analysis of the high-resolution chemostratigraphy reveals evidence of Milankovitch cyclicity embedded within all studied records and proxies. This finding allows the generation of a robust orbitally tuned age model for the Sinemurian providing an estimate of 7.18 myr for the stage duration. The astronomically tuned age model suggests approximately 0.6-0.8 myr for the obtusum-oxynotum zone CIE and 1.8 myr for the SPBE. The events coincide precisely with the 2.4 myr long eccentricity maxima alluding to an orbital eccentricity control on the carbon cycle and climate. A novel methodology for quantifying the amount of time held within cryptic hiatuses is presented based upon integrating sequence stratigraphy and astrochronology. This method allows the quantification of at least 200-400 kyr of missing time held within several levels in the Robin Hoods bay Sinemurian record. To this end, the results of this thesis provide a compelling insight into an interval of time seldom studied. The analysis of the competing effects of large igneous province volcanism and orbital forcing gives strong evidence for the latter as the primary driver of palaeoclimate change in the Sinemurian and raises intriguing questions about the context of the larger- magnitude Early Jurassic OAEs. Geochemical data from multiple sites provides the means to generate a robust astrochronology for the Sinemurian that has implications for the apportioned time in the Early Jurassic.