Early animals and plants and their effects on the Earth system: Geochemistry and biogeochemical modelling

Introduction
This project aims to understand and quantify the potential for dramatic climate shifts associated with the rise of land plants and the rapid diversification of animal life in the early Paleozoic era. This will be achieved using a combination of state-of-the-art geochemical analyses on ancient marine rocks and the application of simple mathematical models, with the potential for targeted fieldwork to collect additional samples covering specific periods of interest.

Background
The geological record of the Cambrian period (541-485 million years ago, Ma) documents the explosion of animal life on planet Earth, and the following Ordovician and Silurian periods (together 485-416 Ma) saw the evolution of the first land plants and terrestrial arthropods. Until recently, it has been thought that these newly developed terrestrial and marine ecosystems did not significantly impact the global elemental cycles that regulate climate, but this assumption has now begun to be questioned.

The early Paleozoic is characterised by widespread ocean anoxia, uncertain fluctuations in atmospheric oxygen, and substantial glacial episodes. Current research suggests that these changes in surface conditions may be related to the evolution of both the animal [Boyle et al., 2014] and plant [Lenton et al., 2012] kingdoms over this timeframe. However, these ideas require more rigorous testing and further analyses, for example with respect to the abiotic factors that affect global biogeochemistry and climate – such as the mountain building of the Taconic and Caledonian orogenies, and changing volcanic sources of CO2.

Aims and approach
This project aims to improve the geochemical record of ocean redox conditions, in addition to applying novel geochemical techniques to evaluate changes in the oceanic influx of nutrients and their behaviour, during the early Paleozoic. Via biogeochemical modelling of these analyses, the project will elucidate the mechanisms behind long term global environmental change during this pivotal period of Earth history.

The following key questions will guide the research:
1. Was the oxygen content of the atmosphere in the early Paleozoic significantly different from the present day?

2. What were the dynamics of deep ocean oxygenation throughout the Cambrian, Ordovician and Silurian periods?

3. Did the initial expansion of plant and animal species drive long term climate shifts, or can these be attributed to tectonic or geomorphological factors?

The initial focus of the project will be the sampling of excellently preserved continuous drill core sections held by the British Geological Survey. Measurements of the speciation of iron and phosphorus will provide unprecedented insight into ocean redox conditions and their influence on marine nutrient cycles. These analyses will be combined with bulk element analyses and isotopic ratios of carbon and sulphur, which respond to changes in global tectonic processes, weathering and biogeochemical cycling.

Some of these techniques have been developed by the project team, and the research will be carried out in the modern and well-equipped Cohen Geochemistry Laboratories within the School of Earth and Environment at Leeds. There will be opportunities to visit field sites and collect additional carbonate and black shale samples as the project progresses.

Earth system ‘box’ models link the global cycles of carbon, sulphur, oxygen and phosphorus (among others) to reconstruct long term climate [Berner, 2006; Bergman et al., 2004]. The researcher will work to improve the ongoing modelling efforts of the project team by combining newly recognised mechanisms from studies of more recent geological eras [e.g. Mills et al., 2014] and more detailed ocean modelling [e.g. Wallman, 2003; Clarkson et al., 2015] with new functions representing the evolving global biota.

The models will be ‘driven’ using the geochemical data obtained in the project (e.g. ?13C, ?34S, chemical index of alteration) in order to investigate the potential changes in biogeochemical cycling, and may also be used as a predictive tool whereby the geochemical records are estimated for a given hypothesis and then compared to data (for the redox state of the deep ocean, for example).