5d. pCO2 and Total CO2 Concentration in Surface Waters:
The meridional distribution of eight properties in surface mixed-layer waters are shown in Figs. 16, 17, and 18 along the 150.5°W (P-16), 135°W (P-17), and 88°W (P-19) sections. The polar waters, which are located south of about 60°S, are characterized by sub-zero temperatures, lower salinity and high concentrations of total CO2, oxygen and nutrients. The concentrations of nitrate, phosphate and silicate are as high as 30 µmol/kg, 2.1 µmol/kg, and 65 µmol/kg, respectively. The concentration of silicate decreases rapidly northward to nearly zero between 55°S and 65°S, and it remains nearly zero to the north. In contrast to the behavior of silicate, the concentrations of total CO2, nitrate and phosphate tend to decrease gradually but remain to high to about 35°S. Since these quantities decrease together with a proportion similar to the Redfield P/N/C ratio of 1/15/106, the observed decreases are attributed to biological utilization. From the polar front to about 35°S, the temperature increases gradually northward signifying the sub-Antarctic zone. The surface water pCO2 values remain close to the atmospheric value of about 350 atm through out the polar and sub-polar waters. This may be due to the fact that the effect of warming to the north is compensated by the effect of the northward decrease in the total CO2 concentration. Further north in the vicinity of 35°S the concentrations of nitrate and phosphate are decreased to nearly zero and the temperature increases rapidly. These features signify the sub-tropical convergence, where the warm subtropical waters depleted of nutrients meet with the colder sub-Antarctic waters rich in nitrate and phosphate (but depleted in silicates). The surface water pCO2 increases north of the convergence in response to the increase in temperature.
An interesting feature is observed in the equatorial area along the 88°W meridian (P-19 section)(see Fig. 18). A pronounced minimum in salinity (as low as 31 PSU) and total CO2 concentration (as low as 1780 µmol/kg) is observed between 2°N and 8°N in the Panama Basin area. On the other hand, all other properties change none or very little. Since this area receives over 100 cm/yr of rain fall, the low salinity and low CO2 concentration may be attributed to the effect of dilution by rain water. Since both the total CO2 and the alkalinity are proportionally reduced by dilution, the pCO2 should be affected only by the effect of salinity changes on the dissociation constants and solubility of CO2 in seawater. For a reduction of salinity from 35 to 31, the pCO2 in seawater should be reduced by 14% or about 50 µatm. This is consistent with the observations made in the Panama Basin area (Fig. 18).
Using the data obtained during this investigation and others obtained since 1973 (e.g., Murphy et al., 1991; Rubin et al., 1996), distribution maps for the pCO2 and total CO2 concentration data in surface waters of the South Pacific and the Pacific sector of the Southern Ocean have been prepared. In Fig. 19, the surface water pCO2 values are expressed as sea-air pCO2 differences, and represent mean austral summer values between October and April during the twenty-year period, 1973 through 1993. Positive values indicate that the ocean water is a source for atmospheric CO2, whereas negative values indicate that the ocean is a CO2 sink. Fig. 19 shows that the equatorial belt (5°N-10°S) is a strong source for atmospheric CO2 as has been documented before by many others, and that the high latitude Southern Ocean areas south of 50°S are strong CO2 sinks. Over the temperate and subtropical gyres between 10°S and 50°S, the areas east of about 130°W are CO2 sources as first pointed out by Murphy et al. (1991), whereas those west of this longitude are CO2 sinks. Since these two areas are similar in the area and have similar magnitude but opposite signs of sea-air pCO2 values, the temperate-subtropical South Pacific Ocean as a whole is a neutral reservoir for atmospheric CO2 during the austral summer months.
Fig. 20 represents the mean distribution of the total CO2 concentration in surface waters during the austral summer months, mid-October through April. The highest values (greater than 2150 µmol/kg associated with 30 µmol/kg nitrate and 34.0 salinity) are found in the Antarctic and sub-Antarctic waters south of about 60°S. This is attributed to the upward mixing of deep waters rich in CO2. On the other hand, surface waters located just north of the subtropical convergence have a total CO2 concentration of about 2000 µmol/kg with nearly zero nitrate and a salinity of about 35.0. Normalizing the total CO2 value for the polar water to a constant salinity of 35.0 and taking the difference from the subtropical water value, we obtain that the total CO2 concentration in the polar water was reduced by about 213 µmol/kg as 30 µmol/kg nitrate was totally utilized. The total CO2/nitrate ratio of 7.1 (= 213/30) thus estimated for the changes observed in the polar and subtropical waters is consistent with the classic Redfield ratio of 7.1 (=106/15).
Consistent with the greater pCO2 values in the temperate and sub-tropical gyre areas, the total CO2 concentrations in the eastern half of the areas are greater than those in the western half by about 50 µmol/kg. This may be attributed to the fact that the western areas are supplied by the southward flow of the western equatorial waters depleted of nutrients and low in CO2, whereas the eastern areas are supplied by the northward flow of the sub-Antarctic waters rich in nutrients and CO2.