The stable isotopic composition of dissolved inorganic carbon of seawater (δ¹³C_DIC) can provide a powerful means to investigate atmospheric and oceanic carbon dynamics. Records of past δ¹³C_DIC values of seawater, especially from the extratropical oceans, are needed to better understand how recent climate change and anthropogenic activity (namely fossil fuel emissions) have impacted the global carbon cycle. However, long-term reconstructions of marine δ¹³C_DIC are limited (both spatially and temporally) by a lack of suitable proxy archives, which have undergone a rigorous calibration process. Marine mollusks represent a potential δ¹³C_DIC archive given that the primary source of carbon to their shell material is ambient dissolved inorganic carbon. However, interpretation of this archive is confounded by the additional contribution of respired (or metabolic) carbon to the carbon isotope ratio of shell material (δ¹³C_S).

The research presented here was aimed at the development and application of a new proxy archive of dissolved inorganic carbon. We sought to develop a transfer function relating shell carbonate (CaCO₃) from the marine bivalve mollusk, Arctica islandica, to DIC of ambient seawater through the use of stable carbon isotopes, thereby assessing the suitability of A. islandica as a proxy for paleo-DIC. The marine bivalve species, Arctica islandica, was reared under experimental conditions for 29 weeks in the Gulf of Maine in order to determine the relationship between the carbon isotope composition of shell carbonate (δ¹³C_S) and ambient seawater dissolved inorganic carbon (δ¹³C_DIC), as well as to approximate the metabolic contribution (C_M) to shell material. Both juvenile (2-3 yrs) and adult (19-64 yrs) specimens displayed average percent CM of less than 10% when using three different proxies of respired carbon: digestive gland, adductor muscle and sediment. Given the low percent metabolic contribution to δ¹³C_S and the relative stability of C_M over the ontogenetic ages examined in this study, A. islandica carbonate appeared to be a promising archive of paleo-δ¹³C_DIC.

The experimentally determined relationship between δ13CS and δ¹³C_DIC for A. islandica was applied to existing δ¹³C_S data sets from the Gulf of Maine, the North Sea and Iceland. A comparison between new marine bivalve δ¹³C values and previously published marine and atmospheric δ¹³C records through the last millennium shows evidence of a major δ¹³C excursion, possibly related to an upwelling event in the Pacific Ocean in the early 1600s. Globally distributed specimens of corals, sclerosponges, and bivalves collected from 1 - 146 m depth are included in the record, yet the timing and magnitude of the modern δ¹³C decrease associated with the Suess Effect are closely matched among all records. These findings suggest that in spite of regional controls on the absolute value of δ¹³C_DIC, the modern trend in δ¹³C_DIC is dominated by a response to atmospheric δ¹³C depletion from fossil fuel combustion.