Chlorofluoromethanes in the western Indian sector of the Southern Ocean and their relations with geochemical tracers

Marine ChemistryTS (1991) 151-167

[5 1

Elsevier Science Publ ishers B. V., Amsterdam

Chlorofluoromethanes in the western Indian sector of the Southern Ocean and their relations with geochemical tracers F. Mantisiv' , C. Beauverger", A. Poisson" and N. Metzl " "Laboratoire de Physique et Chimie Marines. Universite Pierre et MarieCurie, T24, Seme etage, 4, place Jussieu, 75252 Paris Cedex 05, France bInsljlul Oceanographique, 195, rue Saint-Jacques, 75005 Paris, France (Received IS November 1990; accepted 27 March 1991 )

ABSTRACT Mantisi, F.. Beauverger, C; Poisson, A. and Metzl , N., [99 1. Chlorofluoromethanes in the western Indian sector of the Southern Ocean and thei r relations with geochem ical tracers. Mal'. Chem., 35: [51-167 . The first verti cal profiles of chlorofluorornethanes (Freons FII and F[2) measured during the austral summer [987 (INDIGO-3 cruise) in the region of Enderby Land (30 0 E ) and the Princess Elizabeth Trough (90 0 E) arc presented in relation to hydrological and geochemical characteristics. In the open ocean , transient tracer penetration reaches [000 m. Off the West Ice Shelf and Enderby Land, a significant decrease in Freons is found below the cold Winter Water and just abo ve the deep oxygen m inimum and temperature max imum of the upper Circumpolar Deep Water (20 0-400 m ). In the region off MacRobertson Land, where the oxygen minimum is deeper (1000 m) , the Freon gradients are less abrupt. In deep open ocean waters, no Freons were detected in the cor e of the Circumpolar Deep Water. However, ncar the continental shelf, we hav e encountered Freon minima associ ated with salinity ma xima, indicating significant mi xing between deep and (recent) ventilated waters. Over the whole water column, a strong zonal contrast emerges in tracer d istr ibutions between stations situated to the east and to the west of MacRobertson Land (65 'E) , wh ich may be associated with the Weddell Gyre extension. Freon max ima associated with oxygen maxima and temperature and salinity minima that characterize Antarctic Bottom Water (AABW) have been found over all the region studied; the tracers indicate three main bottom waters that are related to WeddeJl Sea, Ross Sea and local origins, At two stations located on the edge of the continental shelf, Freon measurements suggest that the AABW formation was recent, and the tracers' continuity reveals a preferential westward flow of bottom waters. Although it is clear that bottom water formation takes place around 60-70 0 E, the information is too sparse to specify the source regions.

INTRODUCTION

Chlorofluoromethanes-Freons 11 (CC1 3F or Fll) and 12 (CChF 2 or F 12)-are inert components of anthropogenic origin in the troposphere and I Present

address: ATOCHEM, CAL, 95 rue Danton, 92300 Levallois-Perret, France.

0304-4203 /91/$03 ,50 © 1991 Elsevier Science Publishers B.V. All rights rese rved.

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CHLOROFLUOROMETHANES IN THE SOUTHERN OCEAN

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seawater (Sanders, 1965; Rowland and Molina, 1975). Their injection into the atmosphere, which began in the 1950s, is transient and constantly increasing (Gormigy, 1987). Futhermore, their physico-chemical inertia in relation to natural environments gives Fl l and F12 a lifetime of 90 and 140 years respectively (Prinn et al., 1983). Thus they lend themselves to study of the mixture and movements of water masses with characteristic lifespans ranging from several years to several decades. Freons have been used successfully to quantify volume transports and mixing rates (Gammon et al., 1982; Bullister, 1984; Wallace and Moore, 1985; Weiss et al., 1985; Fine et al., 1988; Mantisi, 1989; Smethie et al., 1989), and qualitative description of their distributions can show whether a water mass has been recently ventilated or not (Bullister and Weiss, 1983; Wallace and Lazier, 1988; Anderson et al., 1989; Mantisietal.,1989). Many studies have been made on the formation of bottom waters in the regions of the Weddell and Ross Seas, from both the dynamic and the geochemical point of view (Deacon, 1937; Jacobs et al., 1970; Carmack and Foster, 1975; Weiss et al., 1979). In other areas of the Southern Ocean, this process has received little attention and literature on the subject is sparse (Gordon and Tchernia, 1972; Gordon, 1974; Jacobs and Georgi, 1977). In the Amery Basin and off the coasts of MacRobertson Land, Australian investigations between 1981 and 1985 led to a detailed hydrological description of this region. By using hydrographic and XBT data, Smith et al. (1984) came to the conclusion that bottom waters had probably formed in the zone west of the Amery Sea. More recently, with the support of temperature and salinity data, Middleton and Humphries (1989) found also evidence of bottom water formation on the edge of the continental shelf, west of the Amery Basin. We also investigated this domain as part of the INDIGO-3 cruise (3 January-20 February 1987) in the southwest Indian Ocean and the Indian sector of the Antarctic Ocean. In this paper; we present the first Freon vertical profiles in the region of the Amery Basin, MacRobertson Land and Enderby Land (Fig. 1). The stations (79-87) were located near the base of the continental shelf slope at depths of more than 3000 m; only two stations (82 and 83) were on the edge of the continental shelf near the Amery Sea. The Freon profiles are described in relation to other hydrological (temperature and salinity) and geochemical (oxygen and silicate) characteristics, and we focus on deep and bottom water distributions to provide evidence of local bottom water formation. SAMPLING AND ANALYSIS

Samples were collected with a cluster of twelve 12-1 bottles and a Neil Brown CTD (conductivity-temperature-depth) Mark III instrument. The measuring techniques for geochemical parameters (salinity, oxygen and nutrients) have been described by Poisson et al. (1990). Accuracy is ± 0.002 for salin-

154

F. MANTISI ET AL

ity, ±0.002°C for temperature measured by the CTD instrument (Gamberoni et al., 1990) and 0.1% for oxygen using a method derived from Winkler (in Carpenter, 1965). The technique for analysing Freons was derived from that of Bullister ( 1984) and Weiss et al. (1985), and was described in detail by Mantisi ( 1989). Freons were sampled (the first parameter sampled on the Niskin bottles after helium-tritium) in 100 ml glass syringes which were placed immediately into a box continuously flushed with seawater, to avoid ship-air pollution while the measurements were made (which took about 6 h for a three-cast station) . As usual, samples were measured from bottom to surface depths. The measurements were made with a gas chromatograph equipped with a 63 Ni electron capture detector functioning on a pulse current. They have been calibrated with sea air, comparing its composition with the Weiss SIO (Scripps Institution of Oceanography) scale (R.F. Weiss, personal communication, 1987). The SIO calibration scale has an estimated accuracy of 1.3% for Fll and 0.5% for F12 (Bullister and Weiss, 1988). The detection limits of our system were 0.01 pmol kg- J for Fll and F12. The blank values (0 .03 pmol kg-I for Fll and 0.01 pmol kg-I for F12) were based on a statistical study on Freon concentrations measured in the Circumpolar Deep Water and with the help of 14C profiles (Ostlund and Grall, 1988) , also measured during the INDIGO-3 cruise. The standard deviations in a series of eight measurements on the sea air standard and on the seawater were respectively 0.2% and 2% for F 11 and 0.7% and 2% for F 12. RESULTS

We describe the oceanic domain explored offshore and near the Antarctic continental shelf (Fig. 1) following the four main waters masses that characterize the Antarctic Ocean ( Deacon, 1937; Jacobs et al., 1970; Carmack and Foster, 1975; Jacobs and Georgi, 1977; Weiss et al., 1979; Smith et al., 1984; Gordon, 1986) : the Summer Surface Water (SSW) , the Winter Water (WW) , the Circumpolar Deep Water (CDW) and the Antarctic Bottom Water (AABW). Summer Surface Water

The SSW, which has been reheated and made fresher by the melting of the sea ice, is not deeper than 50 m in the area. In the T-S diagram (Fig . 2a), Stations 79 and 84-87 are representative (T= 1°C, S=33 .8) of the austral open-ocean summer conditions in the area (Gordon, 1986) . Stations 80-83 present distinctive hydrological characteristics in surface waters, showing the considerable variability of antarctic surface waters. In the SSW layer, Freon concentrations are fairly homogeneous (Fig. 3a) but from station to station the range is large (1-4 pmol kg- I for F 11 and 1-2 pmol kg- I for F 12). Concentrations are lowest (less than 2 pmol kg-I for Fl1 and less than 1 pmol

155

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kg- 1 for F 12 ) at the southernmost stations (80, 82 and 83 ) and the contrast with open-ocean concentrations is relatively less marked for F12. This implies lower FII/FI2 ratios in the south ( 1.25- 1.75) than in open ocean

156

F. MANTISI ET AL.

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Fig. 3. (a ) Profiles of F II (pmol kg-I ) and F 12 (pmol kg- I) between 0 and 500 m for Stat ions 79-87 (west is on the right ). Wimer water (WW ) layer extension is also shown. (b) Entire profil es of FII (pmol kg-I ) and Fl2 (pmol kg- I) for Stations 79-87 . Positions of oxygen min imum (Om), temperature maximum (TM) and salinity maximum (SM) are specified on F II profiles.

158

F. MANTISI ET AL.

(2-2.5) surface waters. The theoretical (atmospheric equilibrium) ratios for January 1987, as deduced from Warner and Weiss' (1985) solubility coefficients and atmospheric concentrations (222.18 ppt Fll and 367.81 ppt F12) would be 2.46 at Station 79 (T = 1°C, S= 33.75, z= 18 m) and 2.48 at Station 83 (T=O°C, S=33.83, z=2l m). It is likely that seasonal sea ice coverage, which blocks air-sea exchanges (Schlosser et al., 1987) and mixing with old deep waters (which have a lower F 11fF 12 ratio) are responsible for sea surface differences in measured F 11 IF 12 ratios. Winter Water

The SSW layer covers a strip of the residual winter surface water (WW) which presents a very distinct temperature minimum ( < -1.5 DC). At all stations, except station 87 which is further north, salinities in the WW layer are in the 34.2-34.56 range which was explored during FIBEX (summer 19801981 in the 60-90 oE region; Smith et al., 1984). The WW layer is much thicker in the east (150-200 m thick at Stations 80 and 81) of the area studied than in the west (20 m thick at Stations 85-87). In the WW layer Freon concentrations are slightly below or equal to SSW values (Fig. 3a) and they are relatively homogeneous. Typical values are 2 pmol kg- 1 for Fll and 1 pmol kg- 1 for F12, but we measured higher concentrations (3 pmol kg- I F 11 and 1.5 pmol kg- 1 F12) at Station 87, where the WW layer is shallow. The F IlfF 12 ratio ranges between 1.9 and 2.8 but it is low at Station 85 (1.2), where the WW layer is deeper than at Stations 86 and 87 (western sector). Gordon and Huber (1990) quantified significant rates (45 m year- I) of Weddell Sea Deep Water intrusions into the winter mixed layer for the region 5°W-lOoE. Freon profiles (Fig. 3a) clearly reveal that WW layer thickness is connected to mixing rates with deeper waters: for open-ocean stations where the WW layer is thin (Stations 84-87 in the west), the decrease in Freons is much more pronounced than for stations in the east (Stations 79-81). This implies stronger exchanges with deeper waters in the west. This is also true near the continental shelf: the WW layer is much deeper at Station 83 (230 m) than at Station 82 (130 m) where more deep water infiltrations have eroded Freon profiles at a depth of 200-400 m (Fig. 3b). No temperature below -1.5°C has been found at Station 84; the temperature minimum ( - 0.448 °C at 70 m) is much higher than elsewhere in the area studied. However, below the temperature minimum «O°C), the Freon shapes at Station 84 resemble others in the western region (Stations 86 and 87). Circumpolar Deep Water

Beneath this cold water layer is the Circumpolar Deep Water (CDW) which is characterized by temperature and salinity maxima (Figs. 4a and 4b). The relatively warm upper layer of this CDW (UCDW) lies around 200-400 m

159

CHLOROFLUOROMETHANES IN THE SOUTHERN OCEAN

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160

F. M ANTIS I ET AL.

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162

F. MANTISI ET AL.

in the open ocean. It is very close to the deep oxygen minimum except at Station 81, which appears as a boundary in zonal distributions (Fig. 4c). In the UCDW layer, Freon concentrations tend to zero towards the core ofCDW (as represented by the salinity maximum). In this core layer, no Freon was detected, except near the shelf (Stations 82 and 83) where CDW is well defined by Freon minima (Fig. 3b): 0.36 pmol kg- 1 for F 11 and 0.15 pmol kg- 1 for Fl2 at Station 82 at 894 m, and 0.32 pmol kg- 1 for F11 and 0.27 pmo1 kg:" for F12 at Station 83 at 925 m. At Station 82, the temperature maximum is shallower and the Freon minima are thicker; as mentioned above, it is likely that the rates of deep water intrusion are higher at Station 82 between 300 and 1000 m. This would also explain why the WW layer is deeper at Station 83. Below the salinity maximum, the shape of Freon distributions also indicates mixing of Lower Circumpolar Deep Water (LCDW) with Freon-rich bottom waters (1000-1300 m at these stations). Antarctic Bottom Water

In the studied area, three types of bottom water may be distinguished (Fig. 2b): (1) In the west (Stations 85-87) low temperature (-0.67 to -0.63°C) and salinity (34.65-34.67) are representative of Weddell Sea origin (Gordon,1986). (2) In the east (Stations 79-81) higher temperature (-OA2°C) and salinity (34.673) are representative of Ross Sea (Gordon, 1974) and Adelie Coast (Gordon and Tchernia, 1972) sources. (3) Near the shelf (Stations 82-84), as in surface waters, there is a variety of more or less cold and saline bottom waters. All stations present oxygen and Freon maxima in bottom waters (Figs. 4c-f), which indicates recent ventilation. However, maxima concentrations are variable from place to place, indicating different bottom water origin and more or less ventilated areas. The oxygen bottom water concentration represents approximately 70% of that of the surface water (about 340 ,umol kg- I at the surface and 240 ,umo1 kg- 1 at the bottom). As for temperature and salinity, oxygen and nutrient concentrations of bottom water are very similar for each of the eastern (79-81) and western (85-87) station groups (Table 1), and we note that oxygen and silicate are higher in the west (Figs. 4c and d). Freon bottom concentrations range from 004 to 1.6 pmol kg- 1 for F11 and from 0.1 to 0.8 pmol kg- 1 for F 12, and they are higher in the east than in the west, in contrast to oxygen (Table 1), except for Station 84 (see below). This would indicate either a more recent bottom water formation in the east or a stronger mixing between bottom and deep waters in the west. Residence time in the Weddell-

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TABLE I

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Data for bott om sam ples at Stations 79-87 (P0 4 and Si data from Boulahd id and Caisso, in Poisson et al., 1990)

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0

"T1

S

79 80 81 82 83 84 85 86 87

z

T ( potential )

(m)

(OC)

3587 3175 3077 1279 J 192 4259 4994 4654 4545

-0.408 - 0.420 - 0.4 13 - 0.655 -0.387 - 0.791 - 0.652 -0.662 -0.618

Salinit y

ao

O2 (zzmol kg -

34.673 34.685 34.673 34.622 34.641 34.634 34.659 34.651 34.651

27.863 27.873 27.863 27.833 27.836 27.848 27.862 27.857 27.855

235.4 237.7 236.1 257.8 243.0 262.1 243.5 245.3 245.7

NO] 1)

(pmol kg -I)

30.4 30.5 32.6 31.4 31.2 30.5 31.2 31.7

P0 4 (pm ol kg - I )

Si

2.35 2.35 2.3 1 2.27 2.25

120.8 124.0 131.6 98.6 114.4 102.8 143.0 142.5 137.3

2.29

2.34 2.37 2.3 1

"For low Freon concentrations the F II IF12 ratio is not accurat e (e.g. Mantisi , 1989) .

(pmol kg" ")

FI I (pm ol kg -I)

F 12 (p mol kg-I)

FII /FI2

1.63 1.25 0.64 1.52 1.25 1.09 0.41

0.77 0.54 0.23 0.62 0.46 0.45 0. 19 0. 17 0.12

2.12 2.31 2.78 2.45 2.72 2.42 2.16 2.35 4.67"

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F. MANTISI ET AL.

Enderby basin (Edmond et al., 1979) would support the second explanation of the zonal differences. Between east and west open-ocean stations, Station 84 presents very distinctive near-bottom water characteristics: oxygen maximum (262 ,umol kg-I), silicate and phosphate minima (Table 1) and Freon concentrations are much higher than at Stations 81 and 85 (Figs. 4e and f). Therefore, bottom waters at Station 84 must have another origin than from the east (Ross and Adelie sources) and the west (Weddell source). This origin is probably local and may be found on the continental shelf, as indicated by the bottom water characteristics at Station 82 and 83 (Table 1) where we observed high oxygen, low silicate and high Freon concentrations (F 11 concentrations were 1.52 pmol kg- 1 and 1.25 pmol kg- 1 for Station 82 and 83 respectively), about 70-80% of that of the surface waters. These latter characteristics can be taken as representative of the direct ventilated water from the shelf in the following discussion. From both topography (Fig. 1) and bottom water data (Table 1), Stations 81 and 84 are potential candidates to receive the signal of the ventilation. The dynamics of this ventilation are often explained by the existence of cascading down the slope of the continental shelf, much like that which occurs in the Weddell Sea. This should induce a preferential flow of surface water to the bottom, guided by the irregular topographic features. Part of the bottom water of Station 82 should therefore enter that of Station 81. However, hydrological and geochemical parameters rule out this mechanism: bottom water characteristics at Station 81 are similar to those of the eastern group (Fig. 2b and Table 1). Furthermore, there is an incompatibility between the T -8 and Freon dilution factors if we assume the continental shelf region (around 70 to be the unique source of Station 81 bottom water. On the other hand, Station 84 bottom water characteristics are strongly related to those at Station 82 (very low temperature, low salinity, high oxygen and low silicate; see Table 1). Freon concentrations (1. 09 pmol kg- I F 11 and 0.45 pmol kg- I F 12 at 4259 m) are the highest values encountered below 4000 m; they indicate recent ventilation. It thus appears, as already described by Jacobs and Georgi (1977) and Middleton and Humphries (1989), that the general bottom water flow is oriented westward (from Station 80 towards 81 and from Station 82 towards 84). 0E)

CONCLUSION

In the southwestern Indian sector of the Southern Ocean, we found closed relations between vertical distributions in Freon and water mass characteristics: in the temperature minimum layer of Winter Water, Freons are fairly homogeneous; there are large Freon vertical gradients below the deep oxygen minimum and temperature maximum ofUCDW; in the open ocean CDW no

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Freons were detected but a minimum was clearly observed in the CDW on the edge of the continental shelf. In Antarctic Bottom Water Freon maxima were found throughout the region, with higher values in the east. Near the continental shelf the signal was more pronounced, indicating a more intensive or recent ventilation. In particular, at 58°E (Station 84), Freon and oxygen bottom water concentrations suggest a recent ventilation from the southeastern area. We are aware that stations were too sparse to quantify mixing rates; however, we found evidence of regional bottom water formation, and our data indicate that the general flow of these waters is westward. The relationships between Freons and geochemical parameters exist throughout the region, but we found a significant contrast between the eastern and the western areas. This can be explained by a contrast in mixing or deep water intrusions within subsurface and intermediate layers. Such transient tracer distributions would be strong constraints to control the mixing and upwelling rates in physical and biogeochemical models of this region. ACKNOWLEDGEMENTS

The INDIGO-3 cruise took place on board the R/V "Marion Dufresne", thanks to the support of the Terres Australes et Antarctiques Francaises; the authors would like to thank Y. Ballut, B. Ollivier and all the crew for their valuable assistance. F. Mantisi is most grateful to T.D. Foster for a particularly instructive discussion on the formation of bottom waters in the Antarctic Ocean. The experimental aspects of the investigation were made possible by support from the CNRS and from IFREMER, which provided a contract and a doctoral grant.

REFERENCES Anderson, L.G., Jones, E.P., Ko1termann, K.P., Schlosser, P., Swift, J.R and Wallace,D.W.R., 1989. The first oceanographic section across the Nansen Basin in the Arctic Ocean. DeepSea Res., 36 (3): 475-482. Bullister, J.L., 1984. Atmospheric ch1orofluoromethanes as tracers of ocean circulation and mixing: measurement and calibration techniques and studies in the Greenland and Norwegian Seas. Ph.D. Thesis, University of California, San Diego, 172 pp, Bullister, J.L. and Weiss, R.F., 1983. Anthropogenic chlorofluoromethanes in the Greenland and Norwegian Seas. Science, 221: 265-268. Bullister, J.L. and Weiss, R.F., 1988. Determination ofCC1 3F and CCl3F2 in seawater and air. Deep-Sea Res., 35 (5): 839-853. Carmack, E.C. and Foster, T.D., 1975. Circulation and distributions of oceanographic properties near the Filchner Ice Shelf. Deep-Sea Res., 22: 77-90. Carpenter, lH., 1965. The Chesapeake Bay Institute technique for the Winkler dissolved oxygen method. Limnol. Oceanogr., 10: 141-143. Deacon, G.E.R., 1937. The Hydrography of the Southern Ocean. Discovery Rep., 15, 124 pp.

166

F. MANTISl ET AL.

Edmond, J.M., Jacobs, S.S., Gordon, A.L., Mantyla, A.W. and Weiss, R.E, 1979. Water column anomalies in dissolved silica over opaline pelagic sediments and the origin of the deep silica maximum. J. Geophys. Res., 84(C12): 7809-7826. Fine, R.A., Warner, M.J. and Weiss, RF., 1988. Water mass modification at the Agulhas retroflection: chlorofluoromethanes studies. Deep-Sea Res., 35: 311-332. Gamberoni, L., Charriaud, E. and Kartavtseff, A., 1990. Les rapports des campagnes la mer: MD 53/INDIGO 3/SUZAN 2, fasciicule 1. Terres Australes et Antarctiques Francaises, 8704, 141 pp. Gammon, R.H., Cline, J. and Wisegarver, D., 1982. Chlorofluoromethanes in the Northeast Pacific Ocean: measured vertical distribution and application as transient tracers of upper ocean mixing. J. Geophys, Res., 87: 9441-9454. Gordon, A.L., 1974. Varieties and variability of Antarctic Bottom Water. Colloque International du Centre National de la Recherche Scientifique, 215, Processus de formation des eaux oceaniques profondes, pp. 33-47. Gordon, A.L., 1986. Southern Ocean Atlas. National Science Foundation. Washington, DC, Amerind Publishing, New Delhi. Gordon, A.L. and Huber, B.A., 1990. Southern Ocean Winter Mixed Layer. J. Geophys. Res., 95(C7): II 655-11672. Gordon, A.L. and Tchernia, P., 1972. Waters of the continental margin off Adelie Coast, Antarctica. In: D.E. Hayes (Editor), Antarctic Oceanography II: The Australian-New Zealand sector, Antarctic Research Series, Vol. 9. American Geophysical Union, Washington, DC, pp.59-69. Gorrnigy, E.F. (Editor), 1987. Production, sales and calculated release of FII and 12 through 1986. Report of the Fluorocarbon Panel. Chemical Manufacturers Association, Washington, DC. Jacobs, S.S. and Georgi, D., 1977. Observations on the Southwest Indian/Antarctic Ocean. In: M. Angel (Editor), A Voyage of Discovery. Pergamon Press, Oxford, pp. 43-84. Jacobs, S.S., Amos, A.F. and Bruchausen, P.M., 1970. Ross Sea oceanography and Antarctic Bottom Water formation, Deep-Sea Res., 17: 935-962. Mantisi, F., 1989. Perspectives quant it la penetration du CO 2 anthropique, Utilisation des freons II et 12 comme traceurs des masses d'eaux oceaniques dans l'ocean Indien Sud Ouest. These de doctorat en Oceanologie de l'Universite Paris 6, 178 pp. Mantisi, F., Poisson, A. and Bres, B., 1989. Etude qualitative de la ventilation du systerne tropical du sud-ouest de I'Ocean Indien it l'aide du freon 12. Annales du CNFRA, Actes du Colloque de Strasbourg, pp. 497-5 II. Middleton, J.H. and Humphries, S.E., 1989. Thermohaline structure and mixing in the region of Prydz Bay, Antarctica. Deep-Sea Res., 36: 1255-1266. Ostlund, H.G. and Grall, C, 1988. INDIGO 1985-1987 Indian Ocean Radiocarbon. Tritium Lab. Data Rep. 17, Univ, Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL., 40 pp. Poisson, A., Schauer, B. and Brunet, C., 1990. Les rapports des campagnes it la mer: MD/53 INDIGO 3, fascicule 2. Terres Australes et Antarctiques Francaises, 87-02,269 pp. Prinn, R.G., Simmonds, P.G., Rasmussen, R.A., Rosen, R.D., Alyea, EN., Cardelino, CA., Crawford, A.J., Cunnold, D.M., Fraser, P.J. and Lovelock, J.E., 1983. The Atmospheric Lifetime Experiment. J. Geophys. Res., 88: 8353-8414, Rowland, ES. and Molina, M.J., 1975. Chlorofluoromethanes in the environment. Rev. Geophys. Space Phys., 13: I. Sanders, P.A., 1965. Reaction of propellant II with water. Soap Chern. Spec., 41: 117-124. Schlosser, P., Roether, W. and Rohardt, G., 1987. Helium-3 balance of the upper layers of the northwestern Weddell Sea. Deep-Sea Res., 34: 365-377. Smethie, W.M., Jr., Chipman, D.W., Swift, J.H. and Koltermann, K.P., 1989. Chlorofluoro-

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CHLOROFLUOROMETHANES IN THE SOUTHERN OCEAN

167

methanes in the Arctic Mediterranean seas: evidence for formation of bottom water in the Eurasian Basin and deep-water exchange through Fram Strait. Deep-Sea Res., 35(3): 347369. Smith, N.R., Zhaoqian, D., Kerry, K.R. and Wright, S., 1984. Water masses and circulation in the region ofPrydz Bay, Antarctica. Deep-Sea Res., 31: 1121-1147. Wallace, D.W.R. and Lazier, J.R.N., 1988. Anthropogenic chlorofluoromethanes in newlyformed Labrador Sea Water. Nature, 332: 61-63. Wallace, D.W.R. and Moore, R.M., 1985. Vertical profiles ofCCI 3F (F-ll) and CCI2F2 (F-12) in the Central Arctic Ocean Basin. Geophys. Res., 90( CI ): 1155-1166. Warner, M.J. and Weiss, R.E, 1985. Solubilities of chlorofluorocarbons II and 12 in water and seawater. Deep-Sea Res., 32{12): 1485-1497. Weiss, R.F., Ostlund, H.G. and Craig, H., 1979. Geochemical studies of the Weddell Sea. DeepSea Res., 26: 1093-1120. Weiss, R.F., Bullister, J.L., Gammon, R.H and Warner, M.J., 1985. Atmospheric chlorofluoromethanes in the deep equatorial Atlantic. Nature, 314: 608-610.