Carbonate system in the water masses of the Southeast Atlantic sector of the Southern Ocean during February and March 2008

Abstract

Abstract. 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) from February to March 2008. Eddies detached from the retroflection of the Agulhas Current increased the gradients observed along the fronts. Minima in the fugacity of CO 2 , f CO 2 , and maxima in pH on either side of the frontal zone were observed, noting that within the frontal zone f CO 2 reached maximum values and pH was at a minimum.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) offered pH T,25 values of 7.56 and 7.61, respectively. The UCDW also had higher concentrations of CFC-12 (>0.2 pmol kg −1 ) as compared to deeper waters, revealing that UCDW was mixed with recently ventilated waters. Calcite and aragonite saturation states ( ) were also affected by the presence of these two water masses with high carbonate concentrations. The aragonite saturation horizon was observed at 1000 m in the subtropical area and north of the Subantarctic Front. At the position of the Polar Front, and under the influence of UCDW and LCDW, the aragonite saturation horizon deepened from 800 m to 1500 m at 50.37 • S, and reached 700 m south of 57.5 • S. High latitudes proved to be the most sensitive areas to predicted anthropogenic carbon increase. Buffer coefficients related to changes in [CO 2 ], [H + ] and with changes in dissolved inorganic carbon (C T )Correspondence to: M. González-Dávila (mgonzalez@dqui.ulpgc.es) and total alkalinity (A T ) offered minima values in the Antarctic Intermediate Water and UCDW layers. These coefficients suggest that a small increase in C T will sharply decrease the status of pH and carbonate saturation. Here we present data that suggest that south of 55 • S, surface water will be under-saturated with respect to aragonite within the next few decades.

Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean

“…The highest rates of acidification were found in the surface waters, where we also observe the greatest rates of C ant increase, with SACW showing a rate of pH decrease of 0.0016 yr −1 . The latter value is in line with that calculated for the same water mass on the eastern side of the North Atlantic Ocean at the ESTOC site (0.0017 yr −1 ) for the period 1995(Santana-Casiano et al, 2007González-Dávila et al, 2011). The SAMW demonstrates the next greatest rate of decline of 0.0014 yr −1 , followed by AAIW and uCDW both showing acidification rates of 0.001 yr −1 , which are comparable with values from other recently ventilated water masses in the North Atlantic: acidification rates of 0.0019 and 0.0012 yr −1 have been reported for SAIW and SPMW, respectively (Vázquez-Rodríguez et al, 2012).…”

“…These variations have been attributed to a combination of climatic and biological effects. The greater sensitivity of some water masses to acidification has been well documented by González-Dávila et al (2011) through the application of the buffering factors described by Egleston et al (2010). González-Dávila et al (2011) highlighted waters originating at high latitudes as particularly sensitive to increases in the concentration of dissolved CO 2 ([CO 2 (aq)]), in particular Antarctic Intermediate Water (AAIW) and upper Circumpolar Deep Water (uCDW) due to low ratios of total alkalinity (A T ) to DIC.…”

Global variability and changes in ocean total alkalinity from Aquarius satellite data

“…This is about 50% more than the ± 8.1 µmol/kg uncertainty of Lee et al empirical relationship for TA. In general, we expect that the errors will be large in regions of high eddy kinetic energy, where higher resolution is needed to lower the signal to noise ratio [ Gonzalez‐Davila et al ., ]. We summarize errors contributing to satellite TA as follows: satellite SSS bias ±11 µmol/kg, Lee et al empirical relationship ± 8.1 µmol/kg, and averaging errors due to eddies ~ ±10 µmol/kg (after Gonzalez‐Davilla et al).…”

Estimating carbonate parameters from hydrographic data for the intermediate and deep waters of the Southern Hemisphere oceans

“…The IDW is characterised by a salinity maximum, intermediate alkalinity and DIC. These IDWs have been influenced by mixing with NADW, especially in the deep western boundary current (Ganachaud et al, 2000; Fig. 1).…”