Isotope data from Ice Station Weddell: Implications for deep water formation in the Weddell Sea

Helium isotope data (³He/⁴He ratios and ⁴He concentrations) and H₂¹⁸O/H₂¹⁶O ratios obtained from stations occupied during the drift of Ice Station Weddell (February to June 1992) are used, together with hydrographic data, to study formation of deep and bottom water in the western Weddell Sea. The data indicate deep and bottom water formation along the entire track of the ice station (71.4 to 65.8°S, ≈53°W). Ice Shelf Water (ISW) seems to contribute significantly to the formation of Weddell Sea Deep Water (WSDW) and Weddell Sea Bottom Water (WSBW) in the southern part of the drift track. Toward the north, the fraction of ISW contained in WSDW/WSBW decreases. This trend is overlaid by high ISW fractions in the deep and bottom waters found in the vicinity of the Larsen Ice Shelf. The fraction of Western Shelf Water (WSW) in WSBW shows the opposite trend, increasing from south to north. The combined fraction of ISW and WSW in waters with potential temperatures below 0°C is about 20%, corresponding to a roughly 200 m thick layer. Overall, WSW seems to contribute approximately 2 to 3 times more water than ISW to the water column below the 0°C isotherm. Using the estimated flow of ISW over the sill north of the Filchner Depression of 1 Sv [Foldvik et al., 1985a] together with the ratio of WSW plus Winter Water (WW) to ISW, we calculate a value of about 5 Sv for the formation rate of WSBW with a potential temperature of -0.7°C. About one third of this flux represents near-surface waters (WSW/WW) which have recently been equilibrated with the atmosphere, whereas pure ISW contributes about 10%.