International Polar Year 2007–2008 - WMO

Fig. 2.3-7. Dissolved
iron, salinity and
oxygen distributions
in the full-water
column along
the SR3 transect
between Tasmania
and Antarctica.
The position of the
transect is shown
in the insert in the
upper panel.
(Image: Bowie et al.,
unpublished data)
198
IPY 20 07–20 08
suggested that, based on atmospheric observations
and modelling, the sink function has recently been
decreasing due to a southward shift of the westerly
winds associated with changes in the Southern
Annular Mode. This suggestion in turn has been
challenged by several investigators and is the subject
of ongoing research. Although Le Quéré’s conclusions
have been supported by another modelling study
(Lovenduski et al., 2008), it should be noted that
Böning et al., (2008) have questioned this saturation of
the Southern Ocean CO 2 sink, arguing that the effect
of increased eddy formation could compensate for the
extra energy imparted to the ocean by the winds, with
no significant change in the overturning.
While the exploitation of the wealth of carbon data
is still underway, first results are starting to emerge.
The precipitation of CaCO 3. 6H 2 O (ikaite) was observed
for the first time in sea ice, a process likely to have a
significant impact on the carbon cycle in ice covered
areas (Dieckmann et al., 2008). CO 2 oversaturation was
observed under the sea ice in the eastern Weddell
gyre at the end of winter and early spring, with a shift
to undersaturation within a few days as a result of
biological activity thus preventing CO 2 outgassing to
the atmosphere (Bakker et al., 2008). This mechanism
may well be responsible for the annual sink function
of this region. Drifters measuring pCO 2 in the surface
ocean developed in the CARIOCA program (www.
lodyc.jussieu.fr/carioca) indicated the Subantarctic
Zone is a strong sink for atmospheric CO 2 (Boutin et
al., 2008). Decadal trends of anthropogenic CO 2 in the
Weddell Gyre were estimated from repeat sections
along the prime meridian, providing a benchmark for
future investigations (Hauck et al., 2010; Van Heuven et
al., 2010).
A significant achievement of IPY was the first fulldepth
measurements of iron and other trace elements
in the Southern Ocean (e.g. Klunder et al., 2010). For
example, the distribution of dissolved iron along the SR3
section south of Tasmania (Fig. 2.3-7) shows maximum
surface water concentrations between the latitudes of
60° and 65°S. The salinity (Fig. 2.3-7, lower left panel)
and oxygen (Fig. 2.3-7, lower right panel) distributions
along this section indicate that high salinity, lowoxygen,
nutrient-rich circumpolar deep water upwells
within this latitude band. These results, in combination
with much lower dissolved iron concentrations north
of 60°S, support the view that upwelling is more