Determination of Late Cenozoic variations in UV-B radiation using fossil sporopollenin

了解过去入射紫外线辐射（UV-B）的变化是气候科学的一大未知数。由于过去云量的分布未知，因此建模极其困难。然而，估计表明，随着时间的推移，UV-B发生了很大的变化；例如，在过去250年里，传入的UV-B的数量和幅度的变化估计在当前值的- 26%到+32%之间。对UV-B升高（+20%）下生态系统功能各方面的实验室研究表明，过去的变化对生物群落的影响将是显著的，包括氮循环、群落动态、基因突变，甚至可能是物种形成的变化。因此，有一些重要的气候学和生物学研究途径与了解紫外线- b随时间的变化有关，包括：i)了解紫外线- b通量随时间的变化对轨道强迫、臭氧层耗竭和过去云量变化的响应；ii)了解生物（基因、物种、群落、景观）对入射UV-B的这些变化的反应。英国和荷兰的其他研究小组最近的工作表明，在不同UV-B光强度下生长的现代植物中，有可能检测到花粉和孢子细胞壁中吸收UV-B的化合物（孢粉）的变化。迄今为止，这种UV-B信号的差异已经在蚕豆、白柳、桤木、栎树和石松、L. cernua和L. clavatum孢子的现存和亚化石花粉（~ 100年）中被鉴定出来。然而，这项技术还没有系统地应用于更深层的化石记录。这个概念验证应用程序旨在建立在这些非常令人兴奋的结果之上，以了解将该技术应用于晚新生代有机湖泊沉积序列中包含的化石花粉和孢子的能力。目的是通过分析化石石松（lycopodium annotinum）孢子和桤木（Alnus glutinosa）花粉孢粉中吸收UV-B的化合物，建立一种测定UV-B过去变化的方法。化石花粉和孢子将从欧洲三个年代久远的有机湖泊沉积序列中提取，这些沉积序列跨越了过去14000年。这项工作将集中在这两个物种上，因为：i)它们的化学成分是众所周知的，以前的工作已经表明，在不同光强下生长的种群之间，吸收UV-B的化合物存在可量化的差异；Ii)它们是独特的颗粒，可以在化石序列中识别到物种水平；第三，它们有长期连续的化石记录。然后将从三个化石记录中获得的结果与现代估计和过去14000年的UV-B模型估计进行比较，以证明是否有可能使用这种方法通过深时间构建UV-B的代理。

An understanding of past variations in incoming ultra-violet radiation (UV-B) is one of the great unknowns of climate science. It is extremely difficult to model because of the unknown distribution of past cloud cover. Estimates suggest, however, that there have been large changes in UV-B through time; variation in the amount and amplitude of incoming UV-B over the past 250 kyr, for example, is estimated to have ranged between -26 to +32% of present day values. Laboratory studies on various aspects of ecosystem functioning under elevated UV-B (+20%) indicate that the impact of past variations upon biological communities would have been significant including changes to nitrogen cycling, community dynamics, genetic mutations, and even possibly speciation. There are therefore important avenues of climatological and biological research associated with understanding variations in UV-B through time including i) understanding changes in UV-B flux through time in response to orbital forcing, ozone layer depletion and past variations in cloud cover; ii) understanding biological responses (genes, species, communities, landscapes) to these variations in incoming UV-B. Recent work by other groups in the UK and Netherlands indicate that it is possible to detect changes in modern plants grown in different UV-B light intensities in the UV-B absorbing compounds in the cell wall of pollen and spores (sporopollenin). To date, differences in this UV-B signal have been identified in extant and sub-fossil pollen (~ 100 years old) of Vicia fabia, Salix alba, Alnus glutinosa, Quercus robur and Lycopodium annotinum , L. cernua and L. clavatum spores. However, this technique has yet to be systematically applied to the deeper fossil record. This proof-of-concept application aims to build on these extremely exciting results to understand the capabilities of applying this technique to fossil pollen and spores contained in Late Cenozoic organic lake sedimentary sequences. It aims to develop a method of determining past variations in UV-B through analysis of the UV-B absorbing compounds in the sporopollenin of fossil lycopodium (Lycopodium annotinum) spores and alder (Alnus glutinosa) pollen. Fossil pollen and spores will be extracted from three well-date organic lake sedimentary sequences from Europe that span the last 14,000 years. The work will focus on these two species because: i) their chemical composition is well-known and previous work has indicated a quantifiable differences in the UV-B absorbing compounds between populations grown at difference light intensities; ii) they are distinctive grains that can identified to species level in fossil sequences; and iii) they have a long continuous fossil record. Results obtained from the three fossil records will then be compared with modern day estimates and also modelled estimates of UV-B over the past 14,000 years in order to demonstrate whether it is possible to construct a proxy for UV-B through deep time using this method.