Exploring the causes of mass extinctions

The geological fossil record shows considerable variation in the amount of biodiversity on the Earth over the past 545 million years. This record is punctuated by numerous so-called mass extinctions, which resulted in the annihilation of up to 90% of all species in a relatively short period. A famous example is the end-Cretaceous extinction 65 million years ago which wiped out the dinosaurs. Numerous suggestions have been put forward to explain one or more of these mass extinction events, including volcanism, changes in sea levels and ocean circulation (resulting in very low levels of oxygen in the ocean and the collapse of the marine food chain), meteorite impacts, nearby supernovae or the internal dynamics of the ecosystem. This is a controversial field: Many studies have presented evidence for or against the various scenarios, yet no clear consensus has emerged on the cause of the majority of mass extinction events (or even what constitutes a "mass" extinction). More radical is the suggestion that there are periodic variations in the extinction record. In the 1980s evidence was advanced for a 26 Myr period in the extinction record, and since then there have been claims for other periods, in particular a 62 Myr cycle. Some authors have pointed to an astronomical cause to one of more of these, noting, for example, that 62 Myr is approximately the period of oscillation of the Sun about the Galactic plane in its orbit around the Galaxy. It has even been claimed that the chronology of impact craters on the Earth follows one of these periods. However, many of these studies have been criticised and some have not stood up to rigorous investigation. Some have used poor analysis methods or applied lax statistical standard to identify "significant" periods or correlations. Others have not properly accounted for the fossil record sampling or for noise in the data. Moreover, most authors have focused on just one mechanism or model: no-one has yet undertaken a systematic, robust comparison of a comprehensive set of models.

The goal of this project is to test and compare the different models proposed to explain variation in the geological records (fossil and cratering). This will range from straight forward "phenomenological" models designed to explain the data "as they are" - such as a model for periodic variation - through to more complex, stochastic models of biological population dynamics. As we are particularly interested in whether there are astronomical causes of extinctions, a special focus will be on relevant astronomical models. These include a quasi-periodical dynamical model of the Sun's passage through the Galactic plane and spiral arms, as it has been suggested, for example, that the enhanced cosmic ray flux from supernovae could change climate and thereby affect life on Earth. Another idea is that gravitational perturbations of the cometary Oort cloud could shower the Earth periodically (and certainly there is evidence for numerous large impacts on the Earth). While it is unlikely that we will unambiguously identify a single cause, we should be able to quantify, via probabilities, the plausibility of each model.

This is an interdisciplinary project involving astrophysical and biological population modelling, time series analysis and (Bayesian) statistics. While some of the models we will test are already well known, others will require us to make a proper mathematical formulation. A central tenet of this work is a robust statistical analysis, plus the realization that not all mass extinctions, and certainly not all biodiversity variations, a priori have a single cause.

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