Quantitative estimates of past UV-B irradiance from fossil pollen

Project Overview In the 20th century, emissions of ozone-depleting compounds caused a global environmental crisis through their ability to cause ozone depletion in the atmosphere. Ozone plays a vital role for life on Earth through providing a screening mechanism for dangerous solar UV-B radiation, which is known to have negative effects on plants and animals (including skin cancer in humans). Global environmental policy has been successful in reversing the trend in the emission of ozone-depleting atmospheric chemicals worldwide, so that exposure to dangerous UV-B radiation is likely to be less of a threat to ecosystems and humans in the 21st century. However, large variations in ozone and surface UV-B radiation have also thought to have occurred in the geological past with potentially major implications for climate and ecosystems. For example, swings in the Earth’s geomagnetic field may have caused changes in ozone concentrations, and the increases in solar UV-B radiation at the Earth’s surface that resulted have even contributed to the extinction of the Neanderthals. But although UV-B radiation is an important variable for understand process influencing life on Earth, at present it remains challenging to reconstruct changes in UV-B radiation at the Earth’s surface beyond the instrumental measurements since the 20th century. QUEST-UV will attempt to solve this challenge through the chemical analysis of fossil-pollen grains. Since the 2000s researchers have suggested that chemical sunscreens produced by plants, and which are also found in the walls of pollen grains and then buried in lakes and bogs over thousands of years, may be used to reconstruct UV-B radiation received at the Earth’s surface. QUEST-UV will use experiments to provide the experimental support for this proxy, and then use this understanding to provide the first quantitative reconstruction of UV-B radiation based on sediments representing up to the last 10,000 years. Progress updates Specific achievements of the project in so far: - Completion of preparation of fossil samples related to the Kassjön Lake Sediment record. These are now undergoing analysis using GC-MS. - Successful collection and analysis of vibrational spectroscopy data and GC-MS for greenhouse and fieldwork experiments, with four masters projects associated with this. - Successful establishment of a greenhouse experimental set up to evaluate UV-B responses in pollen using Pinus branches/ cultivars - Development of a method for ‘fossilisation’ of modern pollen grains from experiments with an associate masters project with this. We are currently writing up this paper. - Presentation of results at multiple international symposia by members of the research term in 2023/ 2024. We are now currently analysing results from the field season in 2024 to test the hypothesis that pollen chemical variations can be linked to UV-B exposure in pine pollen grains. There have been some delays because the postdoctoral research fellow successfully obtained a job in 2024 and so left the project. A researcher position will employed in 2025 to complete the analytical and chemometric work in relation to this project. We will conduct one more experimental season in 2025 using Pinus mugo cultivars.

Variations in UV-B at the Earth’s surface are likely to have had major consequences for ecological patterns and biosphere dynamics in the geological past. They are also an important indicator for understanding patterns of global and regional climate change. However, there is no systematic method to quantitatively reconstruct UV-B beyond the instrumental record. This is severely hindering our ability to infer the extent of UV-B changes and their associated climate dynamics in the past, and their impacts on biosphere dynamics and ecological change. A promising new approach proposes to use the chemical compounds of fossil-pollen grains to reconstruct UV-B. However, it remains impossible to quantitatively estimate the magnitude of terrestrially received UV-B radiation based on current understanding. This is because palaeo-UV-B studies have mainly used correlative methods to infer the pollen-chemical response to UV-B radiation. A new approach is required which integrates understanding of the biochemical response at the plant level, in order to provide quantitative reconstructions which are underpinned by process-based understanding. QUEST-UV tackles this problem by developing the first quantitative reconstruction model for UV-B based on fossil pollen. New empirical understanding about the biological response at the plant scale will be combined with modelled changes in solar radiation to predict expected changes in pollen chemistry for two different periods in the Holocene. We will integrate expertise from across a range of chemical techniques to develop a set of new analytical workflows which will also improve understanding biochemical pathways underlying the response to UV-B radiation. This will, for the first time, allow researchers to develop process-based reconstructions of UV-B, resulting in large improvements in the accuracies of, and confidence in, UV-B reconstructions with far-reaching implications for geosciences and plant science communities.