Solar irradiance and vegetation dynamics at the K/Pg boundary.

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In 1980 Luis Alvarez and colleagues published their ground breaking paper which linked the death of the dinosaurs along with numerous other species at the Cretaceous Palaeogene boundary to the impact of a large meteorite on the Earth's surface. The team proposed that the most likely kill mechanism was the development of a global dust cloud that reduced the amount of sun light reaching the Earth's surface causing global cooling and a reduction in photosynthesis. This would have culminated in a reduction in net primary productivity that would have cascaded up the food chain driving extinction. However, despite being published over 35 years ago this elegant hypothesis has never been directly tested. To test this idea we require techniques that allow us to directly infer this information from the fossil record. The pollen and spore record is one of the most complete temporal and spatial records available to palaeontologists to study. Our proposal sets outs an innovative framework that will use this record to test these ideas. Over the past decade our research programme has demonstrated that the chemical composition of the pollen and spore wall is regulated by the amount of light the parent plant receives. This allows us to use the chemical changes in spores and pollen grains to track sunshine through time. In this application we will use this technique to deliver an understanding of how post impact light environment changed. By collecting high resolution samples across a broad latitudinal range we will be able to constrain the temporal and spatial dynamics of the dust cloud, testing for the first time the major kill mechanism proposed by the Alvarez team. In parallel will generate new records of K/Pg vegetation as represented in the pollen and spore record to generate broad predictions about how vegetation responded to this iconic global extinction event. We will use this new record to test if the level of extinction is linked to the severity of the Impact winter as measured by changes in spore chemistry. This high resolution record will also then be used to infer changes in vegetation dynamics through the extinction event and determine how this changes with latitude. Specifically we will be able to test if key plant traits such as genome size or pollination strategy (wind of insect pollinated) were associated with extinction and/ or survivorship across the K/Pg in North America.