Isotopic variability in the intertidal acorn barnacle Semibalanus balanoides: a potentially novel sea-level proxy indicator
K. F. Craven, M. I. Bird, W. E. N. Austin, J. Wynn, 2008. "Isotopic variability in the intertidal acorn barnacle Semibalanus balanoides: a potentially novel sea-level proxy indicator", Biogeochemical Controls on Palaeoceanographic Environmental Proxies, W. E. N. Austin, R. H. James
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We report variations in the δ13C and δ18O values of barnacle skeletal carbonate as well as the δ13C and δ15N value of tissue from specimens of the acorn barnacle S. balanoides, as a function of elevation within its living range on the Isle of May, Scotland. Individuals were sampled over a 3.50 m range at 0.25 m intervals (1.00–4.50 m above ordnance datum). Carbonate δ18O values (2.44±0.13‰ [1σ], n=45) and tissue δ15N values (8.17±0.42‰, n=15) do not vary systematically with elevation. The δ13C value of shell carbonate increases with elevation by c. 1‰ (total range: −0.77‰ to +0.63‰), but the variability between samples at the same elevation suggests that this relationship will be of limited use in constraining palaeo-elevation. By contrast, tissue δ13C values show systematic variation with elevation, increasing by c. 8‰ (total range: −19.36‰ to −8.77‰) with increasing elevation.
These results suggest that there is potential to use the tissue δ13C values to determine the elevation of a Fixed Biological Indicator (FBI) such as S. balanoides within its living range. If this is also true of the organic matrix of the carbonate skeleton, and if this organic matrix is preserved in Holocene FBIs, then the potential exists to use carbon isotopes to more precisely constrain the palaeo-elevation of FBIs within their living range and hence palaeo sea-level. The small range of carbonate δ18O values suggests that oxygen isotopes in FBIs can be used to constrain water δ18O values if an independent estimate of temperature is available, or temperature if an independent estimate of salinity is available.
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Most of our information about the evolution of Earth’s ocean-climate system comes from the analysis of sediments laid down in the past. For example, the microfossil assemblage reflects the temperature, salinity and nutrient abundance of the water in which the organisms lived, while the chemical and isotopic composition of biogenic carbonates may be used to reconstruct past variations in the operation of the carbon cycle, as well as changes in ocean circulation.
Nevertheless, understanding the link between these sediment variables (or ‘proxies’) and environmental conditions is not straightforward. This volume adopts a novel approach by bringing together palaeontologists, geochemists and palaeoceanographers, who contribute evidence that is required to better constrain these proxies. Topics include: (i) processes of biomineralization, and their effect on the chemical and isotopic composition of different organisms; (ii) proxy validation, including field, laboratory and theoretical studies; (iii) the links between modern and fossil organisms.