The first tritium/He data from the center of the Greenland gyre were collected during GEOSECS in 1972 (Östlund et al. [1987]). These data, together with subsequent surveys in 1979 (Me 52, Heinze et al. [1990]), 1981 (TTO/NAS, WHOI He Isotope Lab. Data Release 3.0, Smethie et al. [1986]), 1982 (Me 61, Heinze et al. [1990]; Hudson 82, Östlund and Grall [1993]), 1985 (Me 71, Schlosser et al. [1995]), and 1988 (M 8, Bönisch and Schlosser [1995]), and the JOHAN HJORT cruises form the longest transient tracer time series for the central Greenland Sea. Tritium concentrations in surface waters decrease from nearly 5 TU in 1972 to 1.9 TU in 1994 (Fig. 2d). In the depth range between 750m and 1500m, tritium concentrations changed little over this time period. Tritium concentrations below 1500m remained nearly constant during the 1970s. The GEOSECS tritium measurements show a relatively large scatter compared to the more recent data. Tritium values in the deep water (> 2000m) ranged from 1.1 TU to 1.7 TU, without a significant depth gradient. The Me 52 data in 1979 and the surveys in the early 1980s (TTO/NAS 1981; Me 61 and HUDSON 82, 1982) also show relatively high tritium concentrations (> 1.0 TU) in the deep water, but with less scatter. The data obtained in the mid 1980s (Me 71, 1985) and in the late 1980s (M 8, 1988) show lower tritium concentrations. This trend continued during the JOHAN HJORT cruises in the early 1990s; in 1994, tritium concentrations of the deep water reached values below 0.6 TU.
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Fig. 2: Time series of (d) tritium in the central Greenland Sea. The time series are presented as sections with time as the horizontal and depth as the vertical axes. The color bars are chosen to reflect changes in the deep water. Each year marker indicates the first of January of the particular year. |
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With the exception of the HUDSON 82 cruise, He isotope data were obtained simultaneously with the tritium data (Fig. 2e). The tritium/He-3 age time series (Fig. 2f) is established by combining the tritium and tritiogenic He-3 data according to equation (1). Because of gas exchange with the atmosphere, surface values are close to solubility equilibrium with the atmosphere (-1.8%, Benson and Krause [1980]) and the tritium/He-3 age is close to zero. An intermediate He-3 maximum found in most regions of the oceans in the northern hemisphere indicating the penetration depth of tritium (e.g., Schlosser et al. [1995]) is completely missing in the central Greenland Sea. Instead, we observe monotonically increasing values between the surface and the deep water. Both He-3 values and tritium/ ages show the lowest values below 3000m depth during the 1970s. During the 1980s and early 1990s, He-3 values increase from about 3 % (1981) to 6 % (1994) and the tritium/He-3 ages increase from about 12 years in 1981 to about 25 years in 1994. Over 13 calendar years, the tritium/ age of this part of the deep water increased by 13 years, suggesting that this water body was isolated from exchange with young waters between 1981 and 1994. A more detailed examination in Section 4 however shows that this was not the case.
