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，营养丰富的沼泽，对气候变化极其敏感。降水是沼泽唯一的水源和养分来源，因此优势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部分腐烂形成的，随着时间的推移，这种腐烂会积累成泥炭沉积物。这些泥炭层的放射性碳测年结果显示，泥炭层的形成非常迅速，每十年可以达到10毫米的数量级。泥炭层中包含了过去气候条件的各种指标，也被称为古气候指标，可以用来研究过去的气候变化。例如，大型化石，即植物的遗骸和花粉，反映了最初生长在沼泽表面的植物的丰度。这些植物对气候非常敏感，因此降雨量或温度等因素的变化反映在推断的宏化石记录中。然而，泥炭记录到目前为止还无法提供有关地区温度的信息。鉴于目前关于全球变暖的争论，显然有必要更准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，研究过去的气候就变得困难了。然而，环境科学的进步导致了新的分子工具的发现，这些工具弥合了古气候重建方面的这些差距。对生长在泥炭沼泽表面的现代主要泥炭形成植物，如泥炭苔藓、莎草和石南草的脂类进行了化学分析，发现它们含有明显的化学特征，即脂类生物标志物。例如，脂类生物标记物n-C23烷烃是叶蜡的一种成分，在泥炭藓中发现浓度很高，而在其他泥炭植被中，如石楠，n-C23要么不存在，要么丰度很低。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。脂类的一个显着特征是其疏水性，这意味着它们在泥炭沼泽中不能移动，因此留在了沉淀点。除了脂类生物标记物外，用于古气候重建的其他分子工具也是稳定同位素。在泥炭中的大型化石遗骸中测量的大量稳定同位素值已被用来推断湿度的相对变化，然而，大量同位素技术需要保存完好的大型化石。一种名为化合物特定同位素分析的新分子方法可以测量特定化合物的同位素，如脂类。例如，在布里斯托尔开展的工作表明，利用气相色谱-热转化-同位素比质谱仪(GC-TC-IRMS)新技术测量的泥炭沼泽中的脂肪生物标记物n-C23烷烃的DD值与过去植被的变化相关。此外，n-C23烷烃Dd值与19世纪和20世纪记录的温度变化有很强的相关性，因此表明这种化合物特有的Dd值，也许还有其他类脂生物标志物，在泥炭沼泽中作为新的陆基古温度替代指标(类似于地质温度计)具有潜在的价值。这项拟议工作的总体目标是根据泥炭沼泽植被生物标志物的DD值开发新的陆地古温度替代指标。这将通过测量从横跨欧洲的4个泥炭沼泽中提取的n-C23烷烃在东西纬度断面上的DD值来实现。然后，将这些DD记录与其他泥炭替代物以及过去200年收集的温度数据进行关联，这将使我们能够校准特定生物标志物的DD记录，以便我们能够开发古温度替代物。这项拟议的工作可以为我们提供一个宝贵的工具，在其中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.