Diel carbon monoxide cycling in the upper Sargasso Sea near Bermuda at the onset of spring and in midsummer

Rapid bio-oxidation of carbon monoxide (CO), a photoproduct of dissolved organic matter, results in diel cycles reflecting photochemical-biogeochemical- physical interactions. These cycles were characterized by timeseries studies of hydrography, meteorology, insolation, optics, and CO concentration ([CO]). Diel patterns of near-surface [CO] generally varied little seasonally, despite different forcings. In summer 30% of CO was below the mixed layer (ML); ML depths and optics indicate that two-layer photochemistry is common elsewhere. [CO] decayed at similar rates in both isolated layers. In spring CO extended to 150 m, reflecting nocturnal mixing. Cruise-average profiles indicated a shallower ML defined by CO than that defined by density, illustrating short-lived CO's sensitivity to recent mixing. Below the ML, [CO] dropped exponentially with depth on scales controlled by photoproduction in summer but mixing in spring. Mass-balance modeling of diurnal variations in CO column burden (CB) gave similar production rates in summer and spring (∼50 μmol m^-2 d^-1 CO), but different biooxidation rates, 1.5 ± 0.2 d^-1 (summer, 0-50 m) and 0.52-0.66 d^-1 (spring, 0-200 m). Outgassing averaged 4% of bio-oxidation in summer and 14% in spring. Similar production and differing sinks resulted in different CBs: 39.3 μmol m^-2 (summer) versus 94.9 μmol m^-2 (spring). Insolation-normalized CO productions were 1.8 times those in the oligotrophic equatorial Pacific. Improved, well-blanked analyses found [CO] at 200 m always <0.1 nmol L^-1 in summer, and usually in spring. Prior reports of [CO] systematically ≥0.2 nmol L^-1 at depth in well-stratified, oxic blue waters may be high because of method artifacts.