CARBON-DIOXIDE IN SURFACE SEAWATERS OF THE WESTERN TROPICAL PACIFIC-OCEAN

The carbon dioxide in seawater (pCO(2)) was measured at the equatorial station (0.0 degrees N, 156.0 degrees E) during the R/V Hakuho Maru C ruise as part of the TOGA COARE program extending from October 31 to D ecember 7, 1992. There is no significant equatorial upwelling during w hole experimental period. The pCO(2) shows the diurnal variation with low values in the daylight hours and high values at nighttime. The mea n value of pCO(2) in the morning (9 h) is smaller by about 4.8 ppm (st andard deviation = 4.6 ppm) than that in the evening (21 h). The maxim um value of downward transport of carbon dioxide (4 x 10(-4) mol m(-2) d(-1)) is obtained when the wind speed is 4.1 m s(-1). The downward tr ansport of carbon dioxide results in the increase in pCO(2) by about 0 .06 ppm, This is negligibly small compared with the real diurnal chang e in pCO(2). The vertical profile of pCO(2) shows that there is a depl etion layer of pCO(2) at around 50 m depth. This depletion layer of pC O(2) is roughly coincident with the layer of high chlorophyll-a concen tration, high dissolved oxygen concentration, and low nitrate concentr ation measured during the Sagittarius Expedition near our station (0.0 degrees S, 150.0 degrees E). These results denote that the biological activity plays an important role in causing the diurnal variation of pCO(2) in seawater. pCO(2) values measured at the equatorial station a re undersaturated with respect to the atmospheric carbon dioxide. This denotes the possibility that the equatorial oceans act as a sink of t he atmospheric carbon dioxide under conditions of high seawater temper ature during cessation periods of upwelling.