太平洋西部熱帯海域における表面海水中の二酸化炭素濃度

Abstract

The carbon dioxide in seawater (pCO2) was measured at the equatorial station (0.0°N, 156.0°E) during the R/V Hakuho Maru Cruise as part of the TOGA COARE program extending from October 31 to December 7, 1992. There is no significant equatorial upwelling during whole experimental period. The pCO2 shows the diurnal variation with low values in the daylight hours and high values at nighttime. The mean value of pCO2 in the morning (9h) is smaller by about 4.8ppm (standard deviation=4.6ppm) than that in the evening (21h). The maximum value of downward transport of carbon dioxide (4×10-4 mol m-2d-1) is obtained when the wind speed is 4.1ms-1. The downward transport of carbon dioxide results in the increase in pCO2 by about 0.06ppm. This is negligibly small compared with the real diurnal change in pCO2. The vertical profile of pCO2 shows that there is a depletion layer of pCO2 at around 50m depth. This depletion layer of pCO2 is roughly coincident with the layer of high chlorophyll-a concentration, high dissolved oxygen concentration, and low nitrate concentration measured during the Sagittarius Expedition near our station (0.0°S, 150.0°E). These results denote that the biological activity plays an important rôle in causing the diurnal variation of pCO2 in seawater. pCO2 values measured at the equatorial station are undersaturated with respect to the atmospheric carbon dioxide. This denotes the possibility that the equatorial oceans act as a sink of the atmospheric carbon dioxide under conditions of high seawater temperature during cessation periods of upwelling.