Isotopic variability in the intertidal acorn barnacle Semibalanus balanoides : a potentially novel sea-level proxy indicator

Abstract

Abstract We report variations in the δ 13 C and δ 18 O values of barnacle skeletal carbonate as well as the δ 13 C and δ 15 N 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 δ 18 O values (2.44±0.13‰ [1σ], n=45) and tissue δ 15 N values (8.17±0.42‰, n=15) do not vary systematically with elevation. The δ 13 C 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 δ 13 C 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 δ 13 C 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 δ 18 O values suggests that oxygen isotopes in FBIs can be used to constrain water δ 18 O values if an independent estimate of temperature is available, or temperature if an independent estimate of salinity is available.