Diurnal Variation of Carbon Monoxide Concentration in the Equatorial Pacific Region

Abstract

Carbon monoxide (CO) concentrations in seawater were measured in the upwelling region of the equatorial Pacific (0°N, 159°W) during the Hakuho Maru KH-93-4 cruise. The surface water containing CO of 42-173 nl/l, which showed a marked diurnal variation with a maximum and minimum occurring early in the afternoon and morning, respectively, was supersaturated with respect to atmospheric CO mixing ratios. The estimated sea-toair fluxes of CO fell within the range of 0.11-0.12 mg/m 2 /d. The CO concentrations in the water column decreased with depth to approach relatively constant and low levels of around 10 nl/l at depths beneath 70 m. A diurnal variation of CO concentrations was noticed down to depths of about 60 m where the light intensity reached 2.5% of the surface level, although it was most pronounced in the surface water layers. The diurnal variation should be caused by the photochemical CO production followed by in situ biological CO oxidations. A rate constant of the biological oxidation of 0.13 ± 0.01 (h -1 ) was estimated based on the CO decrease observed in the euphotic zone in the evening. The rapid biological CO oxidation would be due to a high bacterial activity depending on a high primary productivity and nutrient contents in the water