The influence of air‐sea exchange on the isotopic composition of oceanic carbon: Observations and modeling

Although the carbon isotopic composition of ocean waters after they leave the surface ocean is determined by biological cycling, air‐sea exchange affects the carbon isotopic composition of surface waters in two ways. The equilibrium fractionation between oceanic and atmospheric carbon increases with decreasing temperature. In Southern Ocean Surface Waters this isotopic equilibration enriches δ 13 C relative to the δ 13 C expected from uptake and release of carbon by biological processes alone. Similarly, surface waters in the subtropical gyres are depleted in δ 13 C due to extensive air‐sea exchange at warm temperatures. Countering the tendency toward isotopic equilibration with the atmosphere (a relatively slow process), are the effects of the equilibration of CO 2 itself (a much faster process). In regions where there is a net transfer of isotopically light CO 2 from the ocean to the atmosphere (e.g., the equator) surface waters become enriched in 13 C, whereas in regions where isotopically light CO 2 is entering the ocean (e.g., the North Atlantic) surface waters become depleted in 13 C. A compilation of high quality oceanic δ 13 C measurements along with experiments performed using a zonally averaged three‐basin dynamic ocean model are used to explore these processes.