5c. Total CO2 Concentration and pCO2 along 53°S

An east-west section along about 53°S between 74°W and 135°W is shown in Fig. 14. The eastern extreme (on the right hand side) of this section is located on the continental slope of South America. A CO2 maximum (as high as 2300 µmol/kg) centered at a depth of approximately 2600 meters is seen east of about 128°W. This water contains low concentrations of oxygen and can be traced across the Drake Passage (Chipman et al., 1992). It shoals southward across the Passage and is found near the southern end of the Drake Passage at depths less than 200 meters deep, where it is ventilated with the atmosphere. The high CO2 and low oxygen water may be formed locally by the oxidation of biogenic debris falling through the continental slope waters or accumulated as sedimented on the slope. Alternatively, it may represent a narrow strip of the North Pacific water flowing southward as a boundary current. The origin of this water is being investigated.

The distribution along 53°S of pCO2 in seawater at 4.0°C is shown in Fig. 15. In contrast to the distribution of TCO2 (Fig. 14), the pCO2 maximum layer is located at about 1700 meters, and continues across the section at this depth. Although the high pCO2 zone becomes broader and deeper (down to about 3000 meters) toward the continental slope and overlaps with the high TCO2 zone east of about 90°W, the TCO2 maximum lies about 1000 meters deeper than the pCO2 maximum. This may be explained by the dissolution of calcium carbonate, which starts at a depth of about 2000 meters. This would reduce the pCO2 (due to increased alkalinity) below this depth while it adds extra CO2 to that produced by the oxidation of organic debris. Thus, the total CO2 maximum centered around 2600 meters may be attributed to the combined effects of the oxidation of organic debris and dissolution of calcium carbonate.

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