Principles

[Fig. with schematic of water mixing and map of ice flow track] Addition of glacial meltwater imprints unique isotopic signals on the waters interacting with the floating ice shelves. d18O values are decreased and 4He concentrations are increased (Weiss et al., 1979; Schlosser, 1986; Schlosser et al., 1990, Weppernig et al., 1995). These signals can be used to identify the contributions of the different Shelf water masses to the formation of Weddell Sea Bottom Water (WSBW). Preconditioning of Shelf Water by interaction with glacial ice (Ice Shelf Water/ISW; Foldvik et al., 1985) results in WSBW with a high 4He and a low d18O signal, whereas the bottom water formation following the "Foster/Carmack" process (mixing of Western Shelf Water with Modified Warm Deep Water; Foster and Carmack, 1976) forms WSBW with low 4He and slightly higher d18O values. Here we present and discuss their relevance for understanding the formation of WSBW. 4He and d18O data are from samples collected during the drift of the Ice Station Weddell.

Station Map

4He profiles

At least part of the scatter in the 4He concentration in the surface layer is probably caused by partial melting of icebergs following the drift track of the ice floe. Below the surface layer 4He concentrations generally increase slightly with depth and we observe a maximum at about 100 meters above the bottom, most pronounced in the southern part of the drift track. This 4He peak is a clear indication of bottom water formation involving ISW which contains glacial meltwater. The lower 4He concentrations at the bottom can be explained by formation of WSBW by the Foster/Carmack process, which involves shelf waters with little 4He excess.

4He profiles

d18O section

d18O values in Winter Water (WW) increase from south to north. The highest values can be found close to the temperature maximum at about 500m depth. Below this depth, d18O values decrease towards the bottom where they reach values below those observed in the surface waters. The low d18O values in the bottom layer indicate a high meteoric water component (glacial meltwater, precipitation or snow blown off the ice shelfes).

d18O section

Mixing of WDW with WSW and WW resp. ISW

Mixing of WDW with Western Shelf Water (WSW) and Winter Water (WW) forms the coldest WSBW found along the drift track. The low 4He concentration together with low d18O values are caused by accumulation of glacial meltwater in WSW and at the same time partial re-equilibration of the WSW with the atmosphere prior to detrainment off the shelf resulting in partial loss of the excess 4He. Foldvik et al. (1985) observed that ISW leaving the Filchner Depression flows down the slope in a well confined plume and therefore mixes with rather cold WDW at greater depth to form WSBW with relatively high 4He concentrations. The water which was observed flowing over the sill of the Filchner Trench (OF) is already a mixture of ISW found behind the sill and WSW (Schlosser et al., 1990).

4He vs. Thta, stations AF4-7

4He vs. Thta, stations 9-26

4He vs. Thta, stations 35-63

4He vs. Thta, stations 68-70

Fraction of WSW and ISW layers

[WSW and ISW fraction vs N-S] Most of the excess 4He in the water column is added by ISW, whereas the d18O signal reflects glacial meltwater input from both ISW and WSW. By integrating the total amount of 4He and 18O below the depth corresponding to the position of the 0 oC isotherm, we obtain a rough estimate of the fraction of ISW and WSW in this part of the water column. Generally, the fraction of ISW in WSBW decreases from south to north. Two maxima at about 68 and 70.5 oS are superimposed on this trend. They might be related to channels of ISW flowing down the continental slope in the vicinity of the Filchner/Ronne and Larsen ice shelves, respectively. The general trend of the WSW fraction is almost a mirror picture of that observed for ISW.

WSW and ISW fraction vs N-S

WDW and Shelf Water fraction vs N-S

Conclusions

Examination of the distribution of 4He and d18O, together with hydrographic data, allows us to distinguish between the relative contributions of ISW and WSW/WW to WSBW. Multi-parameter water mass analysis indicates a general S/N decrease of the fraction of ISW inWSBW along the drift track of Ice Station Weddell, while there is an opposite trend in the fraction of WSW There are indications of increased contributions of ISW to WSBW in the vicinity of the Filchner/Ronne and possibly the Larsen Ice Shelf.

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