Pursuing a proxy for carbon cycling in the temperate North Atlantic: Investigation of the utility of Arctica islandicashell carbonate to millennial scale dissolved inorganic carbon reconstructions

The stable isotopic composition of dissolved inorganic carbon of seawater (delta13CDIC) can provide a powerful means to investigate atmospheric and oceanic carbon dynamics. Records of past delta 13CDIC 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 delta13C 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 delta13CDIC 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 (delta13CS).;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 (delta13C S) and ambient seawater dissolved inorganic carbon (delta13 CDIC), as well as to approximate the metabolic contribution (CM) 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 delta 13CS and the relative stability of CM over the ontogenetic ages examined in this study, A. islandica carbonate appeared to be a promising archive of paleo-delta13C DIC.;The experimentally determined relationship between delta13C S and delta13CDIC for A. islandica was applied to existing delta13CS data sets from the Gulf of Maine, the North Sea and Iceland. A comparison between new marine bivalve delta13C values and previously published marine and atmospheric delta13C records through the last millennium shows evidence of a major delta13C 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 delta13C 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 delta13C DIC, the modern trend in delta13CDIC is dominated by a response to atmospheric delta13C depletion from fossil fuel combustion.