MANFRED MENSCH and WILLIAM M. SMETHIE, JR., Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964
PETER SCHLOSSER, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 and Department of Earth and Environmental Sciences, Columbia University, New York, New York 10027
As part of the U.S. contribution to the international DOVETAIL (Deep Ocean Ventilation Through Antarctic Intermediate Layers) project, water samples for the measurement of geochemical tracers were collected in the Weddell-Scotia Confluence during cruise NBP 97-5 of R/V Nathaniel B. Palmer . Muench ( Antarctic Journal , in this issue) gives a general introduction to the overall objectives of DOVETAIL and an overview of the work carried out during NBP 97-5. In total, 1,837 chlorofluorocarbon (CFC) samples, 723 helium/neon samples, 840 oxygen isotope samples, and 615 tritium samples were obtained. The analyses for the CFC species CFC 11, CFC 12, and CFC 113 were performed onboard using an electron capture detector (ECD) gas chromatography (Bullister and Weiss 1988; Smethie et al. 1988). The samples for helium, neon, and oxygen isotopes and for tritium were shipped back to Lamont-Doherty Earth Observatory. Their mass-spectrometrical analyses (Bayer et al. 1989; Ludin et al. 1997) will take about 1 year, but first results should be available in mid 1998.
Tracer data increase the dimensionality of the multiparameter space that is available for water-mass analysis. For this application, stable steady-state tracers—acting essentially as dyes—are especially valuable. Transient tracers provide additional information. Because of their time-dependent delivery to the oceanic surface layer, mean residence times and circulation patterns of specific water masses, as well as the relative age structure of the deep ocean, can be obtained.
The steady-state tracer helium-3 (3He) will be used to assess the contribution of deep waters that are advected from the Antarctic Circumpolar Current into the Weddell-Scotia Confluence. The characteristic high isotopic helium-3 (d3He) signal of these waters is caused by the release of mantle helium at the Mid Ocean Ridges, predominantly along the crest of the East Pacific Rise (Craig and Lupton 1981), into the deep waters of the Pacific, from where it penetrates into the Antarctic Circumpolar Current.
The stable isotopes helium-4 (4He) and oxygen-18 (18O) are powerful tools to distinguish between the various near-surface water masses contributing to Weddell Sea waters observed in the Weddell-Scotia Confluence. This potential is due to the fact that shelf waters around Antarctica frequently contain certain amounts of glacial meltwater (Carmack and Foster 1975; Weiss, Östlund, and Craig 1979; Foldvik, Gammelsrød, and Tørresen 1985a,b). Glacial ice is tagged by low d18O values [approximately -40‰ to -50‰ (Morgan 1982; Grootes and Stuiver 1983)] and high 4He concentrations [about 14-fold super-saturation in pure glacial melt (Schlosser 1986)], so that very small fractions (down to 0.5‰) of glacial meltwater are detectable (Schlosser 1986; Schlosser et al. 1990). The d18O signal of the glacial ice is due to progressive depletion of water vapor in H218O with precipitation and, hence, distance from the evaporation site, i.e., the coast (Morgan 1982). As the glacial ice forms from snow through firnification, air bubbles are trapped in the ice matrix. Eventually, they constitute about 10 percent of the entire volume (Gow and Williamson 1975). Together with the low solubility of helium in water (Weiss 1971), these bubbles generate a significant 4He super-saturation when the ice melts at depth. Shelf waters that acquire a glacial meltwater component and then come in contact with the atmosphere for a limited time lose their 4He excess due to gas exchange but retain the d18O signal. Shelf waters that have never been in contact with glacial ice do not carry any of these signals. Their contribution can be identified through their d3He values close to solubility equilibrium with the atmosphere [-1.8 percent (Benson and Krause 1980)].
Tritium [radioactive hydrogen, half-life 12.43 years (Unterweger et al. 1980)] was delivered to the atmosphere in a pulselike fashion as a result of the atmospheric thermonuclear weapon tests in the late 1950s and early 1960s. Its oceanic input function (surface concentration as a function of time) (Mensch, Simon, and Bayer in press) mimics the atmospheric time history. By now, tritium concentrations of oceanic surface waters have returned to the natural background level. Although this means that tritium has lost its transient properties in the near-surface layer, it will continue to be a good tracer. The observation of the further penetration of the bomb peak into the deep waters remote from their formation areas, as well as its subsequent removal from these waters, will provide valuable information on the spreading of, for example, Antarctic Bottom Water throughout the world ocean. Furthermore, tritium with its relatively short half-life will be well suited for time-series investigations of interannual variabilities.
Production and release into the atmosphere of CFC 11 and CFC 12 began in the 1930s and increased quasi-exponentially until the mid-1970s when the rise became linear. Since the early 1990s, the atmospheric CFC 11 and CFC 12 concentrations have leveled off, and CFC 11 started to decrease in 1995. The introduction of CFC 113 into the ocean-atmosphere system started only in the 1960s. Its concentration increased rapidly until the early 1990s. As for tritium, the temporal evolution of the CFC concentrations in oceanic surface waters is controlled by the atmospheric record.
Tracer ratios (CFC 11/CFC 12, CFC 113/CFC 11, CFC 11/tritium) serve as additional transient tracers with distinct input functions. They have the advantage of being less sensitive to the admixture of tracer-depleted waters and, therefore, provide better access to the age of the young component in a water-mass mixture. Although the CFC 11/CFC 12 ratio has been more or less constant since the mid-1970s and, hence, is not suited for dating water masses younger than about 20 years, the CFC 113/CFC 11 and CFC 11/tritium ratios are monotonously increasing functions of time since the early 1980s and the early 1970s, respectively. Together, the three transient tracer ratios available from the NBP 97-5 data set cover the time period from 1950 to present.
The figure shows the distribution of CFC 11 (preliminary shipboard data) along the NBP 97-1 section from the southern Scotia Sea across the South Orkney Plateau into the Jane and Weddell Basins [cf. figure 1 in Muench ( Antarctic Journal , in this issue) for station positions]. Gordon, Visbeck, and Huber ( Antarctic Journal , in this issue) present the hydrographic and current structure along this section. The CFC 11 concentration in the surface layer is close to the solubility equilibrium with the atmosphere (Warner and Weiss 1985) except for stations 23 to 29 in the Jane Basin. These are located in the northern limb of the Weddell Gyre and the CFC 11 undersaturation reflects the advection of waters from the ice-covered regions to the southwest.
At a depth of about 300 meters (m), the CFC 11 concentration drops to less than 1 picomole per kilogram (pmol kg-1). Minimum CFC 11 concentrations approaching 0.1 pmol kg-1 are observed at the southeastern end of the section at about 1,000 m depth. In the Scotia Sea, minimum CFC 11 concentrations slightly above 0.2 pmol kg-1 seem to coincide with the lower 1°C isotherm at stations 1 to 7. In the southern part of the South Orkney Trough (stations 14 to 17), the lowest CFC 11 concentrations are about 0.45 pmol kg-1.
In the Weddell Sea, CFC 11 concentrations increase rapidly as the bottom is approached. In the Jane Basin, the increase seems to start at a potential temperature of about -0.3°C, whereas in the Weddell Basin, the -0.7°C isotherm appears to mark the transition. Maximum observed values in both the Jane and the Weddell Basins are about 2.5 pmol kg-1, providing evidence that a significant contribution to these water masses has been in recent contact with the atmosphere. This finding is further supported by their high oxygen content. In the Scotia Basin north of Pirie Bank (stations 2 to 4), maximum CFC 11 concentrations at the bottom are slightly below 0.8 pmol kg -1. The water-mass properties observed at the bottom of station 15 in the South Orkney Trough (q about -0.6°C, S equals approximately 34.65, O2 equals approximately 5.94 milliliters per liter, CFC 11 equals approximately 1.32 pmol kg-1 ) are observed about 800 m off the seafloor at stations 25 to 27 in the Jane Basin. In the Weddell Basin, no water mass having the properties of the bottom water in the South Orkney Trough was observed. This finding suggests that the bottom waters in the South Orkney Trough are replenished from the Jane Basin through the South Orkney Gap.
We thank Guy and Sally Mathieu, Dee and Lois Breger, and Benjamin Gordon for their contributions to a successful research cruise. Crew and officers of Nathaniel B. Palmer provided excellent support. The personnel of Antarctic Support Associates were very helpful throughout the entire cruise. They were also involved in the logistics of this expedition. The National Science Foundation funded the tracer work through grants OPP 95-28806 and OPP 95-28805 to William M. Smethie, Jr., and Peter Schlosser, respectively. This is Lamont-Doherty Earth Observatory contribution 5768.
Bayer, R., P. Schlosser, G. Bönisch, H. Rupp, F. Zaucker, and G. Zimmek. 1989. Performance and blank components of a mass spectrometric system for routine measurements of helium isotopes and tritium by the 3 He ingrowth method (Sitzungsberichte der Heidelberger Akademie der Wissenschaften, Mathematisch-naturwissenschaftliche Klasse, 5. Abhandlung). Heidelberg: Springer-Verlag.
Benson, B.B., and D. Krause. 1980. Isotopic fractionation of helium during solution: A probe for the liquid state, Journal of Solution Chemistry , 9(12), 895-909.
Bullister, J.L., and R.F. Weiss. 1988. Determination of CCl3F and CCl2F2 in seawater and air. Deep-Sea Research , 35(5), 839-853.
Carmack, E.C., and T.D. Foster. 1975. Circulation and distribution of oceanographic properties near the Filchner Ice Shelf. Deep-Sea Research , 22, 77-90.
Craig, H., and J.E. Lupton. 1981. Helium-3 and mantle volatiles in the oceans and the oceanic crust. In C. Emiliani (Ed.), The sea (Vol. 5). New York: John Wiley and Sons.
Foldvik, A., T. Gammelsrød, and T. Tørresen. 1985a. Circulation and water masses on the southern Weddell Sea shelf. In S.S. Jacobs (Ed.), Oceanology of the antarctic continental shelf (Antarctic Research Series, Vol. 43). Washington, D.C.: American Geophysical Union.
Foldvik, A., T. Gammelsrød, and T. Tørresen. 1985b. Hydrographic observations from the Weddell Sea during the Norwegian Antarctic Research Expedition 1976/77. Polar Research , 3(n.s.), 177-193.
Gordon, A.L., M. Visbeck, and B.A. Huber. 1997. Export of Weddell Sea water along and over the South Scotia Ridge. Antarctic Journal of the U.S. , 32(5).
Gow, A.J., and T. Williamson. 1975. Gas inclusions in the antarctic ice sheet and their glaciological significance. Journal of Geophysical Research , 80, 5101-5108.
Grootes, P.M., and M. Stuiver. 1983. Ross Ice Shelf oxygen isotope profile at J-9. Antarctic Journal of the U.S. , 18(5), 107-109.
Ludin, A., R. Weppernig, G. Bönisch, and P. Schlosser. 1997. Mass spectrometric measurement of helium isotopes and tritium in water samples (Internal Report). Palisades, New York: Lamont-Doherty Earth Observatory of Columbia University.
Mensch, M., A. Simon, and R. Bayer. In press. [Article title to be determined.] Journal of Geophysical Research.
Morgan, V.I. 1982. Antarctic ice sheet surface oxygen isotope values, Journal of Glaciology , 28(99), 315-323.
Muench, R.D. 1997. Deep Ocean Ventilation Through Antarctic Intermediate Layers: The DOVETAIL program. Antarctic Journal of the U.S. , 32(5).
Schlosser, P. 1986. Helium: A new tracer in antarctic oceanography. Nature , 321, 233-235.
Schlosser, P., R. Bayer, A. Foldvik, T. Gammelsrød, G. Rohardt, and K.O. Münnich. 1990. Oxygen 18 and helium as tracers of ice shelf water and water/ice interaction in the Weddell Sea. Journal of Geophysical Research , 95, 3253-3263.
Smethie, W., Jr., D. Chipman, J. Swift, and K. Koltermann. 1988. Chlorofluoromethanes i n the Arctic Mediterranean seas: Evidence for formation of bottom water in the Eurasian Basin and deep-water exchange through Fram Strait. Deep-Sea Research , 35(3), 347-369.
Unterweger, M.P., B.M. Coursey, F.J. Schima, and W.B. Mann. 1980. Preparation and calibration of the 1978 National Bureau of Standards tritiated water standards. International Journal of Applied Radiation and Isotopes , 31, 611-614.
Warner, M.J., and R.F. Weiss. 1985. Solubilities of chlorofluorocarbons 11 and 12 in water and seawater. Deep-Sea Research, 32(12), 1485-1497.
Weiss, R.F. 1971. The solubility of helium and neon in water and seawater. Journal of Chemical and Engineering Data , 16(12), 235-241.
Weiss, R.F., H.G. Östlund, and H. Craig. 1979. Geochemical studies of the Weddell Sea. Deep-Sea Research , 26, 1093-1120.