Stable isotopic compositions and fractionations of carbon monoxide at coastal and open ocean stations in the Pacific

Included in this report are the results of the first isotopic studies of seawater‐dissolved carbon monoxide (CO). Large variations in both carbon and oxygen isotopic compositions (δ 13 C: −44.7 ∼ −21.1‰PDB, and δ 18 O: +8.7 ∼ +32.9‰SMOW) were observed in seawater‐dissolved CO due to the combination of isotopically light CO production in seawater and the enrichment of heavy isotopes during the microbial consumption of CO. Compared with the total δ 13 C value of dissolved organic matter (DOM), the isotopic compositions of the photochemically produced CO in seawater are depleted in 13 C by more than 20‰, probably due to large fractionation processes during the photochemical production of CO from DOM. In addition, the isotopic fractionation factors resulting from the microbial oxidation of CO in seawater were estimated to be 1.005 ± 0.002 and 1.006 ± 0.002 for carbon (α c ) and oxygen (α o ) based on the incubation experiments. By calculating the weighted mean of the isotopic composition of CO according to the CO concentration observed in surface seawater, the mean δ 13 C and δ 18 O for CO emitted from the oceans were estimated to be −40‰ PDB and +15‰ SMOW, respectively. Oceanic CO is a unique source of 13 C and 18 O‐depleted CO relative to that of fossil fuel combustion and biomass burning, which will help us create a better isotopic mass balance for both global and local CO budget.