Deliberately Released Tracers

Another "new" tracer in the region during this period is sulphur hexafluoride (SF6). This non-reactive gas is introduced to the ocean via air-sea exchange and it has a linearly and rapidly increasing atmospheric history due to its anthropogenic sources and it has therefore been used as a transient tracer in a few studies (Law and Watson 2001; Watanabe et al. 2003; Tanhua et al. 2004; Bullister et al. 2006). In 1996 a total of 320 kg SF6 was deliberately released into the intermediate layer of the central Greenland Sea gyre (Watson et al. 1999) within the project ESOP-2. Since the primary goal of the experiment was to study convection processes of the Greenland Sea, the tracer was injected at a larger potential density anomaly (28.049 kg m-3) than normally found on the Denmark Strait sill. However, the spreading of tracer-tagged water did provide a unique possibility to follow and map the pathways of Greenland Sea Arctic Intermediate Water (GSAIW) to other basins (Messias et al. 2008). Also the relative magnitude of the contribution of GSAIW to the Denmark Strait Overflow (Olsson et al. 2005b; Tanhua et al. 2005a) and the Iceland-Scotland Overflow (Olsson et al. 2005a) could be estimated, as discussed below. Figure 20.4 shows the strong tracer signal within the East Greenland Current north of the sill (station 44) in 1999, a clear signal at the sill (station 56), but a lack of "excess" SF6

GINS 1999 SF6 [fmol kg-1]

GINS 1999 SF6 [fmol kg-1]

Fig. 20.4 SF6 concentrations in the East Greenland Current and in DSOW sampled in 1999. The Greenland Sea Arctic Intermediate Water (GSAIW) is seen on station 44 as a mid-depth SF6 maximum, and on station 56 at the sill. There is, however, no trace of GSAIW in the DSOW on station 16, just south of the sill

Fig. 20.4 SF6 concentrations in the East Greenland Current and in DSOW sampled in 1999. The Greenland Sea Arctic Intermediate Water (GSAIW) is seen on station 44 as a mid-depth SF6 maximum, and on station 56 at the sill. There is, however, no trace of GSAIW in the DSOW on station 16, just south of the sill just south of the sill at station 16 (Olsson et al. 2005b). The tracer observations has also provided an upper limit for the transit time from the interior of the Greenland Sea to the Denmark Strait sill of 3 years (Olsson et al. 2005b), that can be compared with numerical modelling estimates of 2.5 years (Eldevik et al. 2005), and to the Labrador Sea of 7 years (Tanhua et al. 2005a).

Even though the addition of deliberately released SF6 (excess SF6) distorts the transient SF6 signal (i.e., the anthropogenic SF6 with its source in the atmosphere), the relation between the released and transient parts of the signal can be estimated with aid from other transient tracers such as chlorofluorocarbons (CFCs). The high SF6 concentration in the DSOW plume on the Greenland slope is clear in a section sampled in 2003 during a cruise to the Irminger Sea, Fig. 20.5 (Tanhua et al. 2005a). The section is the northern most of the VEINS/ASOF standard sections and is very close to the TTO-NAS section. The excess SF6 in the DSOW has been estimated to be roughly 0.12 fmol kg-1, while the bulk of the tracer signal (about 1.2 fmol kg-1) is of atmospheric origin (Tanhua et al. 2005a).

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