Development of a New Terrestrial Palaeotemperature Proxy Based on dD Values of Lipid Biomarkers of Peat Bog Vegetation

泥炭沼泽，或者更具体地说是营养型沼泽，对气候变化极其敏感。降水是沼泽唯一的水和养分来源，因此主要植被受到当地气候条件的强烈影响。泥炭是由生长在沼泽表面的植物的部分腐烂形成的，随着时间的推移，这些植物积累成泥炭沉积物。对这些泥炭层进行放射性碳定年的结果显示，泥炭的堆积速度相当快，每十年可达10毫米。泥炭层中包含了过去气候条件的各种指标，也被称为古气候代用物，可用于研究过去的气候变化。例如，大化石（即植物的残骸）和花粉反映了原来生长在沼泽表面的植物的丰度。这些植物对气候非常敏感，因此降水或温度等因素的变化都反映在推断的大化石记录中。然而，泥炭记录到目前为止还不能提供有关区域温度的信息。鉴于目前关于全球变暖的争论，显然需要更精确地重建过去的温度。在泥炭记录中无法识别大型化石的地方，对过去气候的研究就变得困难了。然而，环境科学的进步导致了新的分子工具的发现，这些工具弥补了古气候重建中的这些空白。对生长在泥炭沼泽表面的现代主要泥炭形成植物（如泥炭藓、莎草和石南）的脂质进行化学分析，揭示了它们具有独特的化学特征或脂质生物标志物。例如，脂质生物标志物n-C23烷烃是叶蜡的一种成分，在泥炭植物中含量很高，而在其他泥炭植物如石南中，n-C23要么不存在，要么含量很低。进一步的研究表明，n-C23烷烃丰度可以追踪泥炭苔藓对泥炭记录的贡献。脂类的一个显著特征是它们的疏水性，这意味着它们在泥炭沼泽中是不可移动的，因此保持在它们的沉积点上。除脂类生物标志物外，其他用于古气候重建的分子工具是稳定同位素。在泥炭大化石遗骸中测量的体积稳定同位素值已被用于推断湿度的相对变化，然而，体积同位素技术需要保存完好的大化石。一种被称为化合物特异性同位素分析的新分子方法可以测量特定化合物（如脂类）的同位素。例如，在布里斯托尔进行的工作表明，使用气相色谱-热转换-同位素比值质谱（GC-TC-IRMS）的新技术测量泥炭沼泽的脂质生物标志物n-C23烷烃的dD值与过去的植被变化有关。此外，n-C23烷烃dD值与19世纪和20世纪记录的温度变化密切相关，这表明该化合物的特定dD值，以及其他脂质生物标志物，作为泥炭沼泽新的陆源古温度代用物（类似于地质温度计）具有潜在价值。本文的总体目标是基于泥炭沼泽植被生物标志物的dD值，开发一种新的陆相古温标。这将通过测量从横跨欧洲的4个泥炭沼泽中提取的n-C23烷烃的dD值来实现。然后，这些dD记录将与其他泥炭代用品和过去200年收集的温度数据相关联，这将使我们能够校准特定生物标志物的dD记录，从而开发古温度代用品。这项提议的工作可以为我们提供一个宝贵的工具，我们可以研究过去的区域温度变化。

Peat bogs, or more specifically ombrotrophic mires, are extremely sensitive to climate change. Precipitation is the only source of water and nutrients to the bog and so therefore the dominant vegetation is strongly influenced by the local climatic conditions. Peat forms from the partial decay of plants growing on the surface of the bog and over time this accumulates into peat deposits. Results from radiocarbon dating of these peat layers reveal the build up of the peat quite rapid and can be up to the order of 10 mm every ten years. Contained within peat layers are various indicators of past climatic conditions, also known as palaeoclimate proxies, which can be used to study climate changes in the past. For example, macrofossils, i.e. the remains of the plants, and pollen reflect the abundance of the plants originally growing at the bog surface. These plants are very sensitive to climate and so changes such factors as rainfall or temperature are reflected in the macrofossil records inferred. However, peat records have up to now been unable to provide information on regional temperatures. There is obviously a need for more exact reconstruction of past temperatures given the current debates on global warming. Where macrofossils are unidentifiable in the peat record then the study of past climates becomes difficult. However, advances in environmental sciences have lead to the discovery of new molecular tools which bridge these gaps in palaeoclimate reconstruction. Chemical analysis of lipids in modern major peat forming plants growing on the peat bog surface, such as the Sphagnum mosses, sedges and heathers, has revealed they contain distinct chemical characteristics, or lipid biomarkers. For example, the lipid biomarker n-C23 alkane which is a component of leaf wax, is found in high concentrations in Sphagnum species where as in other peat vegetation such as heathers the n-C23 is either absent or present in very low abundance. Further work has revealed that n-C23 alkane abundances can track the contribution of Sphagnum mosses to the peat records. A notable feature of lipids is their hydrophobic nature which means they are immobile in peat bog and so remain at their point of deposition. In addition to lipid biomarkers, other molecular tools which have been used in palaeoclimate reconstruction are stable isotopes. Bulk stable isotope values measured in macrofossil remains in peat have been used to infer relative changes in wetness, however, bulk isotope techniques require well preserved macrofossils. A new molecular approach known as compound-specific isotope analysis can measure isotopes of specific compounds such as lipids. For example, work carried out at Bristol has shown that the dD values of the lipid biomarker n-C23 alkane from peat bogs, measured using the new technique of gas chromatography-thermal conversion-isotope ratio mass spectrometry (GC-TC-IRMS), correlate with vegetation changes in the past. Furthermore n-C23 alkane dD values strongly correlate with recorded temperature variations during the 19th and 20th century so suggests compound-specific dD values of this, and perhaps other lipid biomarkers, have potential value as a new terrestrial-based palaeotemperature proxy (akin to a geological thermometer) in the peat bogs. The overall aim of the proposed work is to develop a new terrestrial palaeotemperature proxy based on dD values of biomarkers of peat bog vegetation. This will be achieved by measuring the dD values of the n-C23 alkane extracted from 4 peat bogs across Europe on a west-east latitudinal transect. These dD records will then be correlated with other peat proxies, and temperature data collected over the past 200 years, which will allow us to calibrate the biomarker-specific dD records so we can then develop the palaeotemperature proxy. This proposed work could provide us with an invaluable tool in which we could study past regional temperature variations.