A Detailed View on the Seasonality of Stable Carbon Isotopes Across the North Atlantic

The North Atlantic Ocean plays a major role in climate change not the least due to its importance in CO2 uptake and thus natural carbon sequestration. The CO2 concentration in its surface waters, which determines the ocean's CO2 sink/source function, varies on seasonal and interannual time scales and is mainly driven by air‐sea gas exchange, temperature variability, and biological production/respiration. The variability in stable carbon isotope signatures can provide further insight and help to improve the understanding of the controls of the surface ocean carbon system. In this work, a cavity ringdown spectrometer was coupled to a classical, equilibrator‐based pCO2 system on a voluntary observing ship line that regularly sails across the subpolar North Atlantic between North America and Europe. From 2012 to 2014, a 3‐year time series of underway surface δ13C(CO2) data was obtained along with continuous measurements of temperature, salinity, and fCO2. We perform a decomposition of thermal and nonthermal drivers of fCO2 and δ13C(CO2). The direct measurement of the surface ocean δ13C(CO2) allows us to estimate the mass flux and also the stable carbon isotope fractionation during air‐sea gas exchange. While the CO2 mass flow was in the range of 1–2 mol CO2·m−2·year−1 on the shelves and 2.5–3.5 mol CO2·m−2·year−1 in the open ocean, the isotope signature of this CO2 flux with respect to the sea surface ranged from −2.6 ± 1.4‰ on the shelves to −6.6 ± 0.9‰ in the western and −4.5 ± 0.9‰ in the eastern part of the open ocean section.