Carbonate system buffering in the water masses of the Southwest Atlantic sector of the Southern Ocean during February–March 2008

摘要

Carbonate system variables were measured in the South Atlantic sector of the Southern Ocean along a transect from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC) in February–March 2008. Eddies detach from retroflection of the Agulhas Current located north of the Subantarctic Front (SAF). The eddies increase the gradients observed at the fronts so that minima in fCO₂ and maxima in pH in situ on either side of the frontal zone are observed, while within the frontal zone fCO₂ reached maximum values and pH in situ was a minimum. Mixing at the frontal zones, in particular where cyclonic rings were located, brought up CO₂-rich water (low pH and high nutrient) that spread out the fronts where recent biological production favored by the nutrient input increases the pH in situ and decreases the fCO₂ levels. Vertical distributions of water masses were described by their carbonate system properties and their relationship to CFC concentrations. Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) had pH_T,25 values of 7.56 and 7.61, respectively. UCDW also had higher concentrations of CFC-12 (>0.2 pmol kg?1) as compared to deeper waters, revealing the mixing with recently ventilated waters. Calcite and aragonite saturation states (Ω) were also affected by the presence of these two water masses with high carbonate concentration. Ω_arag = 1 was observed at 1000 m in the subtropical area and north of the SAF. At the position of the Polar front and under the influence of UCDW and LCDW Ω_arag = 1 deepen from 600 m to 1500 m at 50.37° S, and it reaches to 700 m south of 57.5° S. High latitudes are the most sensitive areas under future anthropogenic carbon increase. Buffer coefficients related to changes in [CO₂], [H+] and Ω with changes in C_T and A_T showed the minimum values are found in the Antarctic Intermediate Water (AAIW), and UCDW layers. These coefficients suggest that a small increase in C_T will sharply decrease the pH and the carbonate saturation states. Here we present data that are used to suggest that south of 55° S by the year 2045 surface water will be undersaturated in aragonite.