Molecular fossils as a tool for investigating Quaternary sea-level change in sediments

Climate fluctuations affect environment and sea levels, understanding the drivers of past sea levels is key for predicting future scenarios. Studies of sea-level change are carried out conventionally by microfossil analyses, but these can occur in low numbers. In this work, Quaternary transgressions and palaeoenvironmental changes were studied using molecular fossils. The widely used glycerol dialkyl glycerol tetraether (GDGTs) biomarkers can occur in sediments in trace amounts. To enhance their detection limits, lipids were derivatised with fluorescent groups. High yields were recorded for derivatisation of lipid standards but, surprisingly, derivatisation of GDGT samples from sediments indicated lower MS response than for the native lipids. Thus, GDGTs were analysed as native compounds. Interpretation of the molecular fossil distributions from transgressions in a well-constrained Holocene core (Loch of Stenness, Orkney) and in five Middle Pleistocene cores (East Anglia) enabled corroboration of earlier results and recognition of characteristics that were not evident from microfossils. In the Loch of Stenness, these included the earlier detection of the onset and cessation of the transgression, the redox conditions of the water column, and the shift in vegetation from forest to grassland. Six different type distributions were recognised among the molecular fossils of four cores from the Nar Valley (Norfolk). The timing of the transgression was refined, marine influence was proposed for the first time in one core, and vegetation of the hinterland was reconstructed. The overall similarity in the types of molecular fossil distributions in the East Anglian cores may indicate that they were deposited under similar environmental conditions. Novel glycerol monoalkanediol diether (GMD) lipids with straight alkyl chains and methyl branches were identified from aquatic, marine and terrestrial sediments discussed here. Molecular fossils have been shown to provide useful additional data on Quaternary sea-level change and it is recommended that they become an integral part of sea-level studies.