Concordant estimates of oceanic carbon monoxide source and sink processes in the Pacific yield a balanced global "blue-water" CO budget

Studies to characterize sources and sinks of carbon monoxide (CO) in the mixed layer were carried out at sites covering large regions of the north and south Pacific. Apparent quantum yield spectra for the photochemical production of CO from colored dissolve organic matter were measured, as were first-order net microbial CO consumption rate constants. Contrary to initial expectations, neither photoproduction nor biooxidation parameters exhibited strong regional variations, except that in the Southern Ocean CO biooxidation rate coefficients were very low. Global "blue-water" CO flux terms derived from the data (in Tg carbon 'from CO per year, CO-C a^-1) are: photochemical source, 50 (estimated range, 30-70), microbial sink, 32 (estimated range, 10-60) and total CO sink (microbial plus gas exchange), 38 (estimated range, 13-60). Considering uncertainties and extrapolation biases, these independently estimated source and sink terms are thus in, or close to, balance at ∼40 (range of overlap, 30-60) Tg CO-C a^-1. The simplest interpretation of this balance is that no major net sources or sinks (i.e., light-independent production, photoproduction at >450 nm) remain undiscovered, though considerable uncertainty in actual process rates remains. These CO fluxes are, however, very much smaller than some recently estimated values. The origins and implications of these discrepancies are discussed, and the coastal budget term is approximated.