Astronomical forcing and rapid climate change in the Jurassic

One of the most effective ways we can understand how the Earth works is by studying how it has behaved in the past. In particular, an understanding of how and why climate has changed over Earth history is of critical importance if we are to more fully understand how the Earth may respond to climate change in the future. Importantly, it is the geological record of rocks and fossils that provides us with the tools to understand ancient climate change beyond ~1 million years ago. A number of events in the Jurassic period (from ~200 to ~145 million years ago) have now been recognised that reveal evidence for severe and rapid climate change. Our understanding of these events relies crucially on knowing the timing, duration and rates of environmental change and defining the precise pattern, structure and detail of environmental change. In this fellowship, both these factors will be addressed in order to further understand severe climatic changes that occurred during a short interval of the Early Jurassic (the late Pliensbachian to early Toarcian stages, ~180 million years ago) and at the Middle to Late Jurassic transition (the late Callovian to early Oxfordian stages, ~160 million years ago). In marked contrast to the warm, equable and polar ice-free conditions inferred for most of the Jurassic, the Middle-Late Jurassic transition has revealed geochemical and fossil evidence for a marked deterioration of climate that resulted in a pronounced but short-lived interval of relatively cold temperatures and possible polar ice sheet growth. During the Early Jurassic interval, there is also evidence for a similar abrupt global cooling event. The cooling event of the Early Jurassic was followed by severe warming that has been associated with a marked reduction in global seawater oxygen levels, a large increase in continental weathering rates, mass extinction of marine species, and a major perturbation to the global carbon cycle. A full understanding of all these events can only be gained if their rapidity and timescale is accurately known. One way in which geologists can quantify time in successions of ancient rocks is by looking for evidence of astronomical cycles. Astronomical cycles arise because the Earth's orbit around the sun is not perfectly smooth; for example the angle of Earth's tilt and shape of its elliptical path round the sun changes through time. Importantly, these 'wobbles' in Earth's orbit are regular, and thus cause small, cyclical changes in the Earth-Sun distance with periodicities typically between ~20,000 and 400,000 years. In climatically sensitive sedimentary rocks, these climate cycles will manifest themselves as cyclic changes in rock chemistry, and these cycles can thus be counted and used to construct timescales for ancient sedimentary successions. Such cycles are a common phenomenon in the geological record, and are probably responsible for pacing the timing of the Ice Ages, which have recurred every ~100,000 years or so over the last million years or so. Critically, this also demonstrates how these cycles, although subtle, can actually drive large and severe changes in climate, probably due to feedback effects in the climate system. Nevertheless, the possibility that these cycles drove the major climate change events in the Jurassic detailed above has received little attention. In this research, I will look for evidence of astronomical cycles across the chosen study intervals in order to establish the rapidity and duration of the climate change events, whilst also assessing the potential role astronomical cycles played in driving climate change. In addition to looking for astronomical cycles, the precise causes and consequences of climate change will be assessed by making chemical measurements on rocks and fossils spanning the events. These data will be used to infer changes in seawater temperatures, weathering rates, volcanic activity, and the global carbon cycle.