Abstract: The ability of the ocean to absorb carbon dioxide is likely to be adversely affected by recent climate change. However, relatively little is known about the spatiotemporal variability in the oceanic carbon cycle due to the lack of long-term, high-resolution dissolved inorganic carbon isotope (13CDIC) data, especially for the temperate North Atlantic, which is the major oceanic sink for anthropogenic CO2. Here, we report shell carbon isotope values (13Cshell), a potential proxy for 13CDIC, of old-grown specimens of the long-lived bivalve mollusk, Arctica islandica. This paper presents the first absolutely dated, annually resolved 13Cshell record from surface waters of the North Atlantic (Iceland, Gulf of Maine) covering the time interval between 1753 and 2003. According to our results, the 13Cshell data were unaffected by trends related to ontogenetic age. However, the shell carbonate was precipitated with a constant offset from expected equilibrium by 1.54 to 2.7 ± 0.2 corresponding to a 6.2 to 10.8 ± 0.8% contribution of respiratory CO2 ( 25). The offset did not appear to vary through the lifetime of individual specimens and among specimens. Therefore, the 13Cshell data of this species can very likely be used as a measure of 13CDIC.