Astronomical forcing and chronology of the early Toarcian (Early Jurassic) oceanic anoxic event in Yorkshire, UK

Abstract

During the early Toarcian (∼183 Ma ago), a high rate of organic carbon burial globally over a brief interval of time has led to the recognition of a major oceanic anoxic event (OAE). A pronounced negative excursion in the carbon-isotope composition of marine organic matter, marine carbonate and terrestrial plant material is a key feature of this event but the precise timescale and cause(s) of this isotopic anomaly are debated. Associated with the negative carbon-isotope excursion is evidence for a coeval rise in seawater palaeotemperature, an increase in continental weathering rates, and the mass extinction of marine invertebrate species. The early Toarcian OAE provides evidence for the Earth's response during rapid climate change, and critical to our understanding of the event is a high-resolution timescale that allows us to quantify the rates, duration and lead/lag times of environmental processes. In this study, we present 2743 new high-resolution organic carbon, sulphur and carbonate concentration data from samples of well-preserved organic-rich mudrocks spanning the early Toarcian OAE in Yorkshire, UK. We have used these data to document the geochemical changes and significantly extend and refine the astronomical timescale across this event. Our detailed analysis of the relationship between astronomical forcing and carbon isotope changes in both Yorkshire and a section from Peniche, Portugal, indicates that astronomical forcing paced the timing of major shifts in δ 13 C and hence climate in both sections. Our analyses also demonstrate that there was a marked increase in the relative strength of astronomical forcing recorded at the onset of the OAE, and that the recorded nature of astronomical forcing changed during the event. Both the Yorkshire and Peniche cyclostratigraphies suggest that one astronomical forcing parameter paced environmental change through the δ 13 C event, and that this parameter was obliquity or precession. © 2011 by the American Geophysical Union.