A Biostratigraphic and Geochemical Investigation of Early Jurassic Hyperthermal Events

Abstract

The Early Jurassic was an interval characterised by frequent environmental and climatic disruptions of varying severity, with research predominantly focussed on the most severe event of this interval, the early Toarcian Oceanic Anoxic Event (T-OAE). Despite the wealth of literature on the T-OAE, the mechanisms driving its characteristic negative carbon isotope profile are yet to be sufficiently resolved. Similarly, the expression of the T-OAE in high latitudes, and in the terrestrial realm is poorly constrained, with research historically focussed on the lower latitude marine sediments of present-day western Europe. In this thesis, I present new high-resolution organic carbon isotope and Hg concentration data from the Toarcian GSSP site of Peniche, Portugal, to investigate Large Igneous Province (LIP) volcanism as a driver of the T-OAE. I also present new organic carbon isotope and palynological data from a high-palaeolatitude borehole in order to reconstruct the marine and terrestrial vegetation response to the T-OAE. Here I show that there is a close relationship between perturbations of the carbon cycle (as illustrated by negative δ13C steps) and enhanced Hg emissions into the ocean–atmosphere system (evidenced by normalised sedimentary mercury concentrations), and therefore that periodic LIP magmatism overprinted any astronomically forced climate change at the onset of the negative CIE associated with the T-OAE. The terrestrial vegetation record of the high-palaeolatitude borehole indicates that the T-OAE of the Viking Corridor was characterised by significant warming, with taxa such as Classopollis classoides increasing in abundance, followed by a shift towards a more temperate, humid climate during the recovery phase of the event. The Liasidium Event of the Sinemurian is another interval of climatic disruption in the Early Jurassic, and is a hyperthermal event less severe than the T-OAE with a comparatively brief history of study. Here I present high-resolution organic carbon isotope and elemental abundance data form the newly drilled Prees 2 borehole, and show two hitherto unrecorded negative ‘spikes’ in organic carbon-isotope values, in addition to the two previously described in literature from Robin Hood’s Bay. Using elemental proxies, I also show that parasequence stacking patterns indicate that the Liasidium Event occurred immediately above a flooding surface, which is correlated to a maximum flooding surface apparent in Robin Hood’s Bay, confirming sea level rise of at least regional extent at the onset of this event. The work performed here advances the understanding of the T-OAE, both helping to address the debate around forcing mechanisms at the onset of the event, and in describing the vegetation effects at high latitudes. Similarly, the analysis of the Prees 2 core has yielded the most complete and expansive record of the Liasidium Event to-date, and may serve as an effective reference section for all future works on this interval.