Long-term stability of δ13C with respect to biological age in the aragonite shell of mature specimens of the bivalve mollusk Arctica islandica

The stable carbon isotope ratio in bivalve shells (δ13CS) is an enigmatic geochemical archive whose interpretation is often frustrated by the intrusion of variable and unpredictable vital effects which can influence the mix of metabolic and dissolved inorganic carbon (DIC) in the shell material. The impacts of vital effects and rapid changes in calcification rates on the variability and value of δ13CS have been described in a number of studies and in many bivalve species with typical lifespans between a few years and a few decades, δ13CS has been observed to change (usually decreasing) with biological age. Very long-lived animals, by contrast, spend most of their lives in the mature, slow-growing phase, and it might be expected that in these instances the effect of changes in calcification rates on δ13CS would be less marked, or even absent. Analysis of δ13CS in mature (> 40 years old) Arctica islandica, reported here, indicates that this is the case. δ13CS in shell samples with biological ages between 42 and 391 years from four distinct sites in the North Atlantic shelf seas (Gulf of Maine, north Icelandic shelf, Irish Sea and North Sea) shows no age-related trend. This suggests that metabolic vital effects in mature A. islandica may be reasonably stable at the population level. If the drivers of isotopic disequilibrium between shell and ambient environment can be identified and quantified, it may be feasible to adjust for them and use δ13CS in mature A. islandica to investigate long-term changes in nutrient sources and as a robust proxy for δ13C of environmental DIC.