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

Abstract

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 time‐series 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.