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

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.