Annually resolved δ13C shell chronologies of long-lived bivalve mollusks ( Arctica islandica) reveal oceanic carbon dynamics in the temperate North Atlantic during recent centuries

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 ( δ 13C DIC) data, especially for the temperate North Atlantic, which is the major oceanic sink for anthropogenic CO 2. Here, we report shell carbon isotope values ( δ 13C shell), a potential proxy for δ 13C DIC, of old-grown specimens of the long-lived bivalve mollusk, Arctica islandica. This paper presents the first absolutely dated, annually resolved δ 13C shell 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 δ 13C shell 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 CO 2 (− 25‰). The offset did not appear to vary through the lifetime of individual specimens and among specimens. Therefore, the δ 13C shell data of this species can very likely be used as a measure of δ 13C DIC. Furthermore, shell stable carbon isotope chronologies exhibited habitat-specific differences and a significant inter-annual and decadal variability related to the natural carbon cycle. In addition, a distinct negative δ 13C shell shift was found reflecting the oceanic Suess effect, i.e. the admixture of anthropogenic CO 2. However, this shift only occurred after the early 1920s when a major climate regime shift led to a northward movement of the oceanic Polar Front in the Nordic Seas and a large-scale reorganization of atmospheric and oceanic currents in the North Atlantic. This likely resulted in a reduced admixture of cold Polar water onto the North Icelandic shelf (through the East Iceland Current) and the Gulf of Maine (through the Labrador Current) with an increased volume of warmer, isotopically well-equilibrated Atlantic waters. Our shell-based δ 13C DIC proxy record provides the basis to quantitatively assess natural and anthropogenically induced patterns of carbon uptake in the North Atlantic.