Compound-specific radiocarbon dating - approaching the problems of dating Antarctic sediments
Final Report Abstract
Objective of the project was to use compound-specific radiocarbon analysis (CRSA) to improve radiocarbon-based sediment chronologies and to establish CSRA as a broadly applicable method to obtain high-quality age models. Within the project we could obtain compound-specific 14C ages from very different sediment types, with respect to carbon content, organic carbon sources and age. We successfully isolated fatty acids from sediments from a Subantarctic fjord that are highly-clastic and contain high proportions of pertogenic organic carbon. Compound-specific 14 C ages correspond with carbonate ages from the same samples, showing that CSRA has the potential to obtain 14C-based sediment ages in otherwise un-datable sediments. This could be relevant for many records from the Antarctic shelf, were the lack of carbonate fossils and the large and variable proportions of petrogenic carbon hamper good age control. CSRA of multiple biomarkers from the same samples (n-alkanes, fatty acids and n-alcohols) were used to trace reservoir effects by comparing 14C values of lipids of mainly marine or mainly terrestrial origin. In the absence of independent sediment ages for either of the investigated cores, the reservoir effect correction remains problematic even at the compound-specific level. Variable age offsets between biomarkers of terrestrial and marine origin point to variable aquatic reservoir ages, which likely results from variable glacier run-off and water column stratification and has to be considered when 14C values are interpreted in terms of sedimentation ages. Another outcome of the study was that terrestrial biomarkers were generally more depleted in 14C than marine compounds. This shows that the terrestrial organic matter in South Georgia consists of both a “fresh” pool in equilibrium with the atmosphere during deposition and a “pre-aged” pool entrained in soils and/or peats. As in other polar and sub-polar regions changing climatic conditions likely have an effect on carbon cycling processes, which should be further investigated for this Subantarctic environment. In general, CSRA of phytoplankton biomarkers such as C16:0 fatty acids can be useful to refine the age models of sedimentary records in marine records from Antarctica and the Subantarctic, especially when terrestrial input (i.e., of land plant and soil organic carbon) and petrogenic input is high.
Publications
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(2016) Testing models of ice cap extent, South Georgia, sub-Antarctic. Quaternary Science Reviews 154, 157-168
Barlow, N.L.M., Bentley, M.J., Spada, G., Evans, D.J.A., Hansom, J.D., Brader, M.D., White, D.A., Zander, A., Berg, S.
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(2018) Holocene glacier fluctuations and environmental changes in sub-Antarctic South Georgia inferred from a sediment record from a coastal inlet. Quaternary Research 91, 132-148
Berg, S., White, D., Jivcov, S., Melles, M., Leng, M., Rethemeyer, J., Allen, C., Perren, B., Bennike, O., Viehberg, F.
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(2018) Was South Georgia ice free during the Last Glacial Maximum?. Geological Society of London Special Publications 461, 49-60
White, D., Bennike, O., Melles, M., Berg, S.
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(2019) Current sample preparation and analytical capabilities of the radiocarbon laboratory at CologneAMS. Radiocarbon
Rethemeyer, J., Gierga, M., Heinze, S., Stolz, A., Wotte, A., Wischhöfer, P., Berg, S., Melchert, J., Dewald, A.
