Using sporopollenin chemistry to reconstruct changes in UV-B radiation in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand. EMBARGOED until 2 September 2027.

Abstract

In this work the first Southern Hemisphere surface ultraviolet-B (UV-B) reconstructions based on UV absorbing compounds (UACs) in sporopollenin recovered from New Zealand lacustrine sediments are presented. UACs were measured utilising Fourier Transform Infrared microspectroscopy. The first records are in subdecadal resolution and were derived from (sub)fossil Pinus and Prumnopitys taxifolia pollen recovered from Lake Ohau sediment cores. Here, an interspecies comparison between Pinus and Prumnopitys taxifolia pollen is conducted covering the period between 1954 to 2016 CE. Prumnopitys taxifolia sporopollenin chemistry was also utilised to reconstruct surface UV-B during the Maunder Minimum (1645 – 1715 CE), a period of lower solar activity. These results show that UACs do not necessarily track 11-year solar cycles, but rather show similarities with modelled surface UV-B and erythemally weighted UV-B, and have a negative relationship with total column ozone. This suggests UACs are modulated by local factors rather than short-term changes in total solar irradiance (TSI) linked to the 11-year solar cycle. Observed differences between the Pinus and Prumnopitys UAC records may also be explained by these reasons, as they are likely sourced from different locations, and thus have been exposed to distinct local conditions. These observations stay consistent with the Maunder Minimum record. The second UAC records are based on Lophozonia menziesii, Halocarpus and Poaceae pollen from Lake Bright. These records are on an approximately centennial scale, and extend back approximately 4000 years in time. There is high coupling between Lophozonia menziesii and Poaceae UACs, while Halocarpus only reveals an intermediate relationship with the other taxa. Yet, all taxa show a trend that appears to track longer-term changes in spring insulation which is influenced by Milankovitch cycle components. This suggests that long-term Milankovitch cycles do, at least, partially govern UAC regulation in sporopollenin. Last, a modern survey was conducted to see if plants respond similarly in UAC modulation when grown under different UV-blocking films, and if short-term responses in UAC production can be observed by translocating plants to a different UV environment during pollen production. The results here are negative. It is unclear if this is because plants have a baseline in UAC production which was never crossed, or if other local factors, such as insect exposure, influenced plants to produce a similar level of UACs even when in different UV environments.