Interannual controls on Weddell Sea surface water fCO2 during the autumn–winter transition phase

Abstract

The fugacity of carbon dioxide (fCO 2 ) of the surface waters of the Weddell Sea along the prime meridian has been described for the austral autumn in 1996 and 1998. For individual years, fCO 2 has a strong linear relationship with sea surface temperature, although the relationships cannot be reconciled to provide an interannually consistent algorithm for remotely sensed assessment of fCO 2 . However, from the assumption that Weddell Sea surface water has a single end member (upwelled Warm Deep Water) we have determined the relative contributions of heating, ice-melt, and biological activity on fCO 2 . A breakdown of the controls shows that the measured annual fCO 2 distributions can be recreated for both transects by adjusting solely for thermodynamic forcing, and model adjustments for salinity are small except in regions of significant upwelling during 1998. The incorporation of nitrate utilisation into the model results in a general and significant underestimation of fCO 2 . This runs contrary to the earlier findings of Sabine and Key (Mar. Chem. 60 (1998) 95) in the Southern Ocean although it is consistent with models in the area (Louanchi et al., Deep-Sea Res. I 48 (2001) 1581). A major caveat to these findings is the significant departure of the thermodynamic model and a tightening of the nitrate-adjusted model in 1998 in areas with deeper mixing in the southern Weddell Sea. We propose that there are two reasons for the discrepancies in our model: the source waters are not as homogenous as the model assumes; and there are geographical and seasonal variations of CO 2 exchange with the atmosphere and the input of inorganic carbon and nitrate from below the mixed layer resulting in imbalances in the mixed layer concentration ratios.

A rapid transition from ice covered CO<sub>2</sub>–rich waters to a biologically mediated CO<sub>2</sub> sink in the eastern Weddell Gyre

“…In April 1996 modest supersaturation by 10 µatm and undersaturation by 15 µatm were observed north and south of about 60 • S, respectively (Hoppema et al, 2000). By contrast, in April to May 1998 surface water fCO 2 was supersaturated by 20 to 30 µatm south of 64 • S, while mixed layers were significantly deeper and more saline in autumn 1998 than in April 1996 (Bellerby et al, 2004). This suggested that (more) entrainment of WDW into the mixed layer had occurred in 1998 than in 1996 (Bellerby et al, 2004).…”

“…Such a system has been used before on Polarstern (e.g. Hoppema et al, 2000;Bellerby et al, 2004). Every three hours the measurements were calibrated by two out of three calibration gases, bracketing surface water fCO 2 .…”

Winter-to-summer evolution of <i>p</i>CO<sub>2</sub> in surface water and air–sea CO<sub>2</sub> flux in the seasonal ice zone of the Southern Ocean

“…The distribution of surface-water pCO 2 in the SIZ in the Southern Ocean is characterized by large variability in space and time. In winter, both supersaturation and undersaturation of surface-water pCO 2 with respect to the atmosphere has been reported (Hoppema et al,1995;Bakker et al, 1997;Rubin et al, 1998;Gibson and Trull, 1999;Stoll et al, 1999;Bellerby et al, 2004;Metzl et al, 2006;Takahashi et al, 2009). McNeil et al (2007) demonstrated the seasonal difference of surface-water pCO 2 between winter and summer in the Southern Ocean, and both positive and negative differences were distributed inhomogeneously.…”

“…Recently, McNeil et al (2007) computed a total CO 2 uptake of −0.4 Gt C year −1 by the Southern Ocean on the basis of CO 2 partial pressure (pCO 2 ) in surface water, calculated from carbonate system parameters in seawater that were parameterized separately for the summer and winter seasons as a function of temperature, salinity and nutrient levels. More recently, however, Takahashi et al (2009) estimated a smaller CO 2 uptake of −0.05 Gt C year −1 by incorporating new pCO 2 data from the 2000s and from the seasonal ice zone (SIZ), where relatively high pCO 2 exists in the water under the ice (Bakker et al, 1997(Bakker et al, , 2008Rubin et al, 1998;Bellerby et al, 2004) and where the release of CO 2 to the atmosphere is limited to small areas of open water (e.g., polynyas, leads and cracks) in the ice-covered area.…”

Phytoplankton dynamics in relation to hydrography, nutrients and zooplankton at the onset of sea ice formation in the eastern Weddell Sea (Antarctica)

“…In the area of this investigation both presumptions are important, since it is both a frontal region and in close proximity to the coast and edge of the ice shelf, respectively. Therefore this frontal region may also provide a major contribution in the way of CO 2 drawdown by subsequent sinking of particles (Priddle et al 1992;Bellerby et al 2004). …”