Norwegian Seas and the Nansen Basin of the Arctic Ocean

Prog. Oceanog.Vol. 35, pp. 1-28, 1995 Pergamon 0079-6611(95)00003-8

Copyright© 1995Elsevier Science Ltd Printed in GreatBritain. All rightsreserved 0079 - 6611/95 $29.00

Mid-1980s distribution of tritium, 3He, I4C and 39Ar in the Greenland/Norwegian Seas and the Nansen Basin of the Arctic Ocean PETER SCHLOSSER1'2,GERHARDBONISCH1, BERNDKROMER3, H. Huoo LOOSLI4, RENEDIKTBOHLER4, REINHOLDBAYER3, GEORGESBONANI5 and KLAUSPETERKOLTERMANN6

~Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA 2Department of Geological Sciences, Columbia University, New York, NY 10027, USA 3Institutj~r Umweltphysik der Universitiit Heidelberg, Im Neuenheimer FeM 366, D-69120 Heidelberg, Germany 4Physikalisches Institut der Universitdt Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland ~TnstitutJ~r Mittelenergiephysik, ETH Ziirich, Hdnggerberg, CH-8093 Zfirich, Switzerland 6Bundesamtfiir Seeschiffahrt und Hydrographie, Bernhard-Nocht-Str. 78, D-20359 Hamburg 36, Germany A b s t r a c t - The distributions oftritium/3He, 14Cand 39Arobserved in the period between 1985 and 1987 in the Greenland/Norwegian Seas and the Nansen Basin of the Arctic Ocean are presented. The data are used to outline aspects of the large-scale circulation and the exchange of deep water between the Greenland/Norwegian Seas and the Nansen Basin. Additionally, semi-quantitative estimates of mean ages of the main water masses found in these regions are obtained. Apparent tritiumPHe ages of the upper waters (depth <500m) vary from close to zero in the Norwegian Current to about 15 years at the lower boundary of the Arctic halocline. The deep waters (>l,500m depth) of the Greenland/ Norwegian Seas show apparent tritium/3He ages between about 17 years in the Greenland Sea and 30 years in the Norwegian Sea. 39Arbased estimates of the Nansen Basin intermediate, deep and bottom +26 161 .~o and 277.31 water ages are 91-23, +33year s for Arctic Intermediate Water (AIW), Eurasian Basin Deep Water (EBDW) and Eurasian Basin Bottom Water (EBBW), respectively. Within the errors, age estimates ofEBDW andEBBW based on 14C/tritium correlations are consistent withthose derived from 39Ar (163 to 287 years for EBDW and 244 to 368 years for EBBW). A quantitative evaluation of the data in terms of deep water formation and exchange rates based on box model calculations is presented in an accompanying paper.

CONTENTS 1. 2. 3.

4.

Introduction Sample collection and measurement Water masses 3.1 Surface waters 3.2 Intermediate waters 3.3 Deep waters Results 4.1 Greenland/Norwegian Seas 4.1.1 North/south section 4.1.2 East/west section 4.2 Arctic Ocean

2 2 5 5 5 5 6 6 6 15 15

2

P. SCHLOSSERet al.

5.

6. 7. 8.

Discussion 5.1 Upperwaters 5.1.1 Greenland/NorwegianSeas 5.1.2 ArcticOcean 5.2 Deepwaters 5.2.1 Greenland/NorwegianSeas 5.2.2 ArcticOcean Conclusions Acknowledgements References

18 18 18 20 22 22 23 24 26 26

1. INTRODUCTION

The average formation rate of North Atlantic Deep Water (NADW) has been estimated to be about 15 to 20Sv (BROECKERand PENG, 1982; WPdGHTand STOCKER, 1992; MAIER-REIMER, MIKOLAJEWlCZand HASSELMANN,1993). The regions north of the Scotland/Iceland/Greenland ridges contribute a substantial fraction of the source waters of NADW (about 5.6Sv; DICKSONand BROWN, 1994). These overflow waters are mainly intermediate waters from the Iceland Sea. They are formed as part of a fairly complex circulation and water mass transformation scheme in the closely coupled system of the Norwegian/Greenland/Icelandseas and the Arctic Ocean. Understanding the sensitivity of the deep water formation processes and rates in this coupled system requires knowledge of its hydrographyand circulation including the mean residence times of its main water masses. Measurement of transient and steady=state tracers have been used in the past to study specific aspects of the water mass structure and circulation in this region including deep water formation rates in the Greenland Sea and exchange of deep waters between the Greenland/Norwegian Seas and the Arctic Ocean, as well as the mean residence times of the halocline waters (e.g. PETERSON and ROOTH, 1976; BULLISTERand WEISS, 1983; SMETHIE, OSTLUNDand LOOSLI,1986, SMETHIE,CHIPMAN,SWIFTand KOLTERMANN,1988; KRYSELLand WALLACE, 1988; HEINZE, SCHLOSSER, KOLTERMANN and MEINCKE, 1990; RR~IN, 1991; SCHLOSSER, BONISCH, KROMER, MONNICH and KOLTERMANN, 1990; SCHLOSSER, BONISCH, RHEIN and BAYER, 1991; SCHLOSSER, BAUCH, FAIRBANKSand BONISCH, 1994; WALLACE, SCHLOSSER, KRYSELLand BONISCH, 1992). Most of these studies have been focused on certain

regions or on specific tracers. The purpose of this contribution is to present a multi-tracer data set collected in 1985 and 1987 covering both the Greenland/Norwegian Seas and the Nansen Basin and to discuss its relevance for studies of large scale circulation patterns and estimations of mean ages of the main water masses. The data provide the basis for model-based evaluations of the deep water formation and exchange rates in the Greenland/Norwegian Seas and the Eurasian Basin of the Arctic Ocean which are discussed in an accompanyingpaper (BONISCHand SCHLOSSER,1995). 2. SAMPLECOLLECTIONAND MEASUREMENT

The tritium/3He, 14C,and 39Ardata sets discussed below were collected during cruise no.71 of the RIV Meteor to the Greenland/Norwegian Seas (June to August 1985; tritium/3He samples only) and on leg 3 o f Polarstern cruise ARK IV to the Arctic Ocean (July to September 1987, for

geographical position oftbe stations see Fig. 1). Tritium samples were taken from standard Niskin samplers (volume of 5 or 10 litres) mounted on a CTD/rosette system equipped with a Neil Brown CTD. Tritium samples of the 1985 Meteor cruise were collected in 1L glass bottles and measured radiometricallyin the Heidelberg low-level counting laboratory. Precision of these measurements is ±5% or +0.08TU (WEISS,ROETHERand

Distributionof tritium,3He,14Cand 39Arin the Nordic Seas

3

BADER, 1986), where one TU (tritium unit) means a tritium to hydrogen ratio of 10-18. The tritium samples collected during the 1987 Polarstern cruise (about 40g)were degassed in the Heidelberg helium isotope laboratory using a vacuum extraction unit and stored for several months in glass bulbs for 3He ingrowth. After this time, the 3He produced by tritium decay was measured in a dedicated helium isotope mass spectrometer (for details, see BAYER,SCHLOSSER,BONISCH,RUPP, ZAUCKERand ZIMMEK,1989). Precision of these measurements is about ± 1to 2% and the detection limit was about 0.03TU. 3He/4He ratios were measured with a precision of about ±0.2% and are reported in the 5 notation where 83Hemeans the percent deviationofthe 3He/4Heratio of a sample from that ofan air standard (3He/4He ratio: 1.384 106; CLARKE,JENKINSand TOP, 1976).

FIG.1. Geographicpositionsof the Meteor 71 and ARK IV~3 stations.

4

P. SCHLOSSERet al.

The tritium/3He age, Xae.3, is calculated according to Tta

In ~ ) +[3HeJ - -

ln2

[3H]

"tHe.3 --

where T 1/2is the half life of tritium (12.43 years), [3HeJ is the tritiogenic 3He concentration (in TU) and [3H] is the tritium concentration at the time of sampling (in TU). Samples for 14C measured by the low-level counting technique were collected using stainless steel Gerard-Ewing samplers (volume of270L). Total inorganic carbon and dissolved gases were extracted on board using two vacuum extraction systems. After acidification of the water with hydrochloric acid to a pH of 2, it was sprayed into the vacuum chamber of one of the extraction systems at a flow rate of 6 to 8L min-l. The pressure of the vacuum chamber was held at water vapor pressure by removing the released gases. The extracted gases were dried and bubbled through purified sodiumhydroxide to absorb the CO 2 for 14C measurement. At several stations the gases extracted from 5 Gerard samplers (wire length between the samplers: 15m) were collected in stainless steel cylinders for 39Armeasurement. Extraction efficiency was about 80to 90%. Samples for AMS 14C were collected from the rosette water sampler (10L Niskin bottles) using pretreated, evacuated 1L glass bulbs with O-ring sealed valves. The bulbs contained HgC12to prevent changes in the CO 2 content of the water by biological activity. Large volume 14C samples were measured at Heidelberg by gas counting of the CO 2 set free by addition of acid to the NaOH solution (SCHOCH, BRUNS, M1]NNICHand MI]NNICH, 1980; SCHOCHand MI3NNICH,1981). The 14Cblank of the NaOH was negligible (---0.2%0 in A14C). The data are reported as age corrected A14C, following STUIVERand POLACH(1977). The overall error of the A14C values is +7%0. The data refer to the 1983 recalibration of the Heidelberg sodium carbonate substandard to NBS oxalic acid ( K R O M E R , 1984). The small volume t4C samples were measured at the Ziirich AMS facility after CO2 extraction and targetpreparation at Heidelberg. The precision of the AMS 14C data is about +5 to 6%0. Details of the procedure are described in KROMER, PFLEIDERER,SCHLOSSER,LEVIN,MONN1CH,BONANI, SUTERand WOLFLI(1987). 39A1" measurements were performed at Bern using low-level counting techniques in an underground laboratory. Ar and Kr fractions were separated by gas chromatography followed by purification. The extraction and separation yield was checked by comparison of the amount of the extracted gases with that expected from the amount of water and the argon solubility in water. Additionally, 85Kr was measured on each 3 9 A t sample as an indicator of contamination with air. The 39At data are reported in 'units' of'% modem'. This notation means the activity of the sample expressed as percent fraction of a modem tropospheric air standard. The 3 9 A t data are decay corrected to the date of sampling. The precision of the data is about +4 to 5%. Details of the procedures are described by LOOSLI(1983). The 39Ar age, "CAr.39,is calculated as follows:

Ar-39 -- "

In2

~ [39A.rmod], J

where TI/2 is the half life of 39At (269 years), [39Arobs]is the measured 39At concentration in % modern, and [39Armor] is the 39At concentration of water in solubility equilibrium with the atmosphere (100% modem).

Distribution of tritium, 3He, 14Cand 39Arin the Nordic Seas

5

3. WATERMASSES In this chapter the main water masses in the Greenland/Norwegian Seas and the Nansen Basin of the Arctic Ocean are briefly described to provide the framework for the discussion of the tracer distributions in the following chapter. 3.1 Surface waters

The surface waters of the Greenland and Norwegian seas consist mainly of two water masses: warm and saline Atlantic Water (AW; S>35; O.>3 °C; SWIFTand AAGAARD,1981), and fresh, cold, Polar Water (PW; S<34.4; O.<0°C except in summer when solar radiation can heat the surface to temperatures of about 4 to 5°C; SWIFTand AAGAARD, 1981). AW is transported north in the Norwegian Current. One branch of AW continues to flow north through Fram Strait into the Arctic Ocean where it subducts beneath the Arctic halocline. A second branch ofAW reeirculates in Fram Strait and constitutes a significantheat and salt source of the Greenland Sea. Finally, a third branch of AW enters the Arctic Ocean through the Barents Sea. Modification of AW by cooling and salt addition during sea-ice formationpotentiallyprovides a source of Arctic Ocean intermediate and deep water. PW is formed in the Arctic Ocean by mixtures of shelf waters, Bering Strait inflow water, and AW. It exits the Arctic Ocean through Fram Strait as part of the East Greenland Current (EGC). Northward flowing AW and southward flowing PW are separated by the Arctic domain which is defined as the region between the Polar Front in the west (i.e. the front between the Arctic domain and the EGC) and the Arctic Front in the east (i.e. the front between the Arctic domain and AW or the Atlantic domain) (SWIFTand AAGAARD, 1981). The main feature of the Arctic domain is the weakly stratified Greenland gyre. The upper waters of the Nansen Basin consist of a cold surface mixed layer (SML) with temperatures close to the freezingpoint and salinities ranging from about 33.5 in the northern part of the Nansen Basin to about 34.5 in its southern part. The thickness of the SML is typically about 10 to 25m. Below the SML we find Lower Halocline Water (LHW) defined by JONES and ANDERSON(1986) as water with a salinity of about 34.2 and a minimum in NO. Upper Halocline Water (UHW) ori~nating in the Canadian Basin of the Arctic Ocean and characterized by salinities of about 33.1 and a distinct nutrient maximum acquired in the Chukchi/East Siberian seas (JONES and ANDERSON, 1986) was practically absent in the Nansen Basin during our 1987 section. 3.2 Intermediate waters

In the Greenland/Norwegian Seas, SWIFTand AAGAARD(1981) definedtwo intermediate water masses. Upper Arctic Intermediate Water (uAIW; 34.734.9; 0°C
Greenland Sea Deep Water (GSDW; S=34.889 to 34.892; O~- 1.26 to - 1.29°C in 1981; SWIFT, TAKAHASHIand LIVINGSTON 1983) is the coldest and freshest among the deep waters in the Greenland/Norwegian Seas and the Eurasian Basin of the Arctic Ocean. Norwegian Sea Deep

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Distribution of tritium, 3He, 14C and 39Ar in the Nordic Seas

7

pot. Temperature [C] 79

0__~

109

114

140

148

150

151

-100. -200

-

-300 -400-500 -1000 -

-1500-2~0°

-2500 -3000-3500-4000- I 67

-2 69

71

(a)

73

75

Latitude [N]

Tritium [TU] ^

67 (b)

"]~

48

69

10~

114

71

l~IN

73

l~

I~,/~

I~|

75

Latitude [N]

FIG.2. Distribution of(a) potential temperature, (b) tritium, (c) 53He, and (d) tritium/3He age along a N/S section across the Greenland/Norwegian Seas (for geographical position of the stations, see Fig. 1).

I

i

. . . .,. .

,. . . . , . . .,.

. ,. . .

....

..... ,......

............

, .... ,

depth[m] . . . . .~. . . ., . . .,. . ., ,

,

a~

elp

,,,,)

o

o

o

o

o

o

o

o

o

depth[m] o

o

o

o

i l

~= e'D

{,/J

Distribution of tritium, 3He, ~4C and 39AFin the Nordic Seas

9

pot.Temperature [C] 144b

144a

144

142

140

137

, 135

133

131

-2 -16

-12

-8

-4

0

(a)

4

8

12

[ W ] L o n g i t u d e [El

Tritium [TU] ,

"

J

m

M

l

10

P. SCHLOSSERet al.

~3He [% ] 144b

-.3~UU

144a

135

142

140

-12

-8

0 4 -4 [W] Longitude [E]

144

142

144

1~7

131

133

-2

|

-16

(c)

12

TISHe age[ys] t~ 1Orb

144a

140

135

137

131

133

40

-1~ -2£ 30

-3( -4( -5{

20 ~

-1~

~" -15{ 10

-3111 -3~

1 -16

(m

-12

-8

-4

0

[W] Longitude IE]

4

12

Distribution of tritium, 3He, '4C and 39At in the Nordic Seas

11

pot. Temp. [°C] 285

2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 81.5

82.0

82.5

83.0

(a)

83.5

84.o

84.5

85.0

85.5

86.0

latitude [N]

Salinity [psu] 285

35.0

34.5

34.0

33.5

81.5

(b)

82.0

82.5

83.0

83.5

84.0

84.5

85.0

85.5

' I ' 86.0

~

latitude [N]

FIG.4. Distribution of(a) potential temperature, (b) salinity, (c) A14C, (d) tritium, (e) 83He and (f) tritium/3He age along a section across the Nansen Basin (for geographical position of the stations, see Fig. 1).

33.0

12

P. SCHLOSSERet al.

A14C [ %o1 285

100 80 60 40 20 0 -20 -40 -60 -80 -100 81.5

82.0

82.5

83.0

83.5

84.0

84.5

85.0

85.5

86.0

latitude IN]

(c)

Tritium [TU] 285 m

10

I

i ¸

m -300q, -4~| -500q |

81.5 (d)

82.0

82.5

83.o

83.5 84.o latitude [N]

84.5

85.0

85.5

86.0

Distribution o f tritium, 3He, J4C and 39Ax in the Nordic Seas

13

~He[%] 285

30

25

20

15

lO

81.5

82.0

82.5

83.0

83.5

84.0

84.5

85.0

85.5

86.0

latitude IN]

(e)

TI3He ageLvs] 285

10

20

30

40

50

60 81.5 (f)

82.0

82.5

83.0

83.5 84.0 latitude IN]

84.5

85.0

85.5

86.0

Distribution of tritium,3He, J4Cand 39Arin the Nordic Seas

15

Apparent tritium/3He ages reach values of up to about 30 years in NSDW and of about 15 to 17 years in GSDW (Fig.2d). The upper water apparent tritium/3He ages are close to zero in the near-surface waters of the Norwegian Sea south of a strong transition at about 70.5 °N, while they tend to be significantly higher in the Greenland Sea (about 5 to 10 years between about 100 and 500m depth). The vertical apparent tritium/3He age gradient is more monotonous in the upper 1,500m of the Greenland Sea compared to the Norwegian Sea where a strong gradient is observed around 1,000m depth (Fig.2d). 4.1.2. East/west section. The E/W section is located at about 73.5°N, i.e. slightly south of the centre of the Greenland gyre at about 74 to 75°N. The Norwegian Current is reflected in the relatively high surface water temperatures (about 8°C) east of about 5°E (Fig.3a). In the western part of the section (west of station 137) is a thin temperature minimum layer at about 50m depth, i.e. below the seasonally warmed surface layer. The recirculation of Atlantic water produces a potential temperature maximum at about 100 to 300m depth which is most pronounced between stations 144 and 144b. The lowest potential temperatures with values below - 1.2°C are observed below about 2,400m depth between about 5°E and 14°W. The influence of the Greenland gyre is reflected in the doming of the isotherms in the central Greenland Sea. The surface layer (0 to about 500m depth) has fairly homogeneous tritium concentrations of about 3.5TU east of station 137 in the EAV section (Fig.3b). This regime is separated by a strong lateral tritium gradient from waters with lower tritium concentrations found in the western part of the section. Below this layer, tritium concentrations in the eastern part of the section drop monotonically over the next 1,000m depth to values of 0.75TU or less in the deep water. West of station 137, the tritium concentrations in the surface waters (< 100m depth) show a strong east/west tritium gradient with values reaching from about 3.1TU at station 137 to more than 6TU at station 144b. Below this layer tritium concentrations in the Greenland Sea drop to values between 2 and 3TU between 100 and 500m depth. Deep water tritium concentrations in the Greenland Sea are fairly homogeneous with values close to 1TU. 83He values are fairly homogeneous in the deep Greenland Sea with values slightly above 4% (Fig.3c). There is a weak maximum centred around 800m depth in the eastern part of the section (63He values close to 6%) below which the 63He values drop below 4%. The surface fi3Hevalues are close to solubilityequilibrium with the atmosphere in the upper 300m between stations 131 and 135. There is a strong lateral gradient at about 4°E west of which fi3He values are about 3 to 4% between about 100 and 500m depth. A distinct 3He maximum is found around 50 to 100m depth between stations 142 and 144b. At the western boundary of the section (station 144b) we observe a 3He minimum at about 200m depth. The apparent tritium/3He ages in the deep water of the E/W section are about 15 to 20 years in the Greenland Sea (Fig.3d). There is a significant increase in the apparent tritium/3He age towards the east with maximum tritium/3He ages between 20 and 25 years in the bottom waters of stations 131 and 135. Surface waters in the Norwegian Current have low apparent tritium/3He ages (below 1 year in the upper 200m between stations 131 and 135). In the surface waters of the Greenland Sea, apparent tritium/3He ages reach a maximum of more than 10 years around 50m depth at stations 144a and 144b. There is a distinct minimum in the tritium/3He age distribution between about 100 and 300m depth west of station 144 with values below 5 years (minimum value: 1 year at station 144b). 4.2 Arctic Ocean

Stations 269 to 371 span a section across the Nansen Basin of the Arctic Ocean reaching from the Barents shelf in the south (about 81 °N) to the Gakkel Ridge in the north (about 86°N; Fig. 1).

16

P. ScnLoss~ret al.

The most pronounced feature of the potential temperature section (Fig.4a; ANDERSON, KOLTERMANN,SCHLOSSER,SWIFTand WALLACE,1989) is the temperature maximum with values above 1.7°C centred at about 250m depth and potential temperatures close to the freezing point in the surface layer (50 to 100m depth) north of station 310. In the intermediate and deep waters, potential temperatures decrease monotonically from about 1°C at about 500m depth to values slightly above - 1°C near the bottom. The strongest salinity gradient is found near the surface (down to about 200m depth; Fig.4b; ANDERSONet al, 1989) in the well pronounced halocline. Below the halocline, there is a salinity maximum (salinities >34.94) associated with the temperature maximum at about 200 to 400m depth. A weak salinity minimum with values below 34.92 separates this feature from the deep salinity maximum which reaches values above 34.94 near the bottom. The tritium distribution (Fig.4d) is fairly homogeneous with values between 3.5 and 4TU in the surface waters (<400m depth) of the southern part of the section (stas 269 to 310). North of station 310 surface water tritium concentrations increase to values of more than 8TU at station 371. The high tritium waters are confined to the upper 100 to 200m. Below this depth, tritium concentrations decrease monotonically to values of about 1 TU at about 1,500m depth. Tritium concentrations in the deep and bottom waters (>1,500m) decrease to minimum values of about 0.03TU near the bottom in the centre of the Nansen Basin. Below 1,500m depth tritium concentrations increase significantly from the centre of the basin towards the continental slope of the Barents Sea and towards the Gakkel Ridge where the highest tritium concentrations are observed at station 364, i.e. at the southern slope of the ridge. The most remarkable features of the 3He distribution are the relatively high 83He values of the surface waters and the distinct 3He maximum observed at about 150m depth north of station 340 (Fig.4e). South of station 340, both features are much less pronounced and disappear towards the Barents shelf. At about 250 to 300m depth, a 3He minimum extends from the continental slope northward to station 362. Below this 3He minimum, a high 3He layer centred at about 600m depth (about 400 to 1,000m) extends from the northern end of the section southward to station 310. The deep waters have relatively low 83He values (typically between 2.5 and 3%). There are boundary effects with relatively low ~3He values (<0%) near the continental slope of the Barents shelf. This feature extends from the surface down to about 500m depth. Below this depth the 83He values observed close to the continental slope are slightly higher than those observed in the central basin. There is a slight 3He maximum in the vicinity of the Gakkel Ridge (depths below 1,500m) which extends into the central Nansen Basin at a depth of about 3,000m. The tritium/3He age distribution shows relatively low apparent ages (<2 years) close to the continental slope of the Barents Shelfbetween the surface and about 500m depth (Fig.4f). Towards the north, surface water tfitium/3He ages initially increase to slightly above 5 years between stations 358 and 364, but then further north they decrease again to values below 5 years. A tritium/3He age maximum centered around 150m depth extends from the northern end of the section to station 310. Below this maximum, relatively low tritium/3He ages are observed between the continental slope and the northern end of the section. Below about 300 to 500m depth tritiurrd3He ages increase more or less monotonically to values close to 50 years near the bottom of the central Nansen Basin. Towards the northern and southern boundaries, apparent tritium/3He ages of the deep and bottom waters decrease significantly compared to those observed in the central basin. The A14C distribution (Fig.4c) is somewhat similar to the tritium distribution (Fig.4d). There is apronounced northward gradient in AI4C in the upper 500m of the water column with maximum A I4C values of> 100% in the surface waters north of station 362. The -50%o isoline is found between 1,000 and 2,000m depth. Deep and bottom water AI4Cvalues range from -50 to about-85%o with

Distribution of tritium, 3He, ~4Cand 39Atin the Nordic Seas

17

the tendency for higher Al4C values to occur near the boundaries of the Nansen Basin. The 39At measurements obtained from the 1987 Polarstern cruise are summarized as a composite depth profile (Fig.5). 39Ar concentrations decrease monotonically from the upper water column (92% modern at about 30m depth at station 269) to about 46% modern in the bottom waters of station 358. Apparent ages calculated from the observed 39At concentrations are indicated in Fig.5 (slanted numbers).

92+-6%mod.

32

5(

I0(

15(

20q

E "rl-- 25( (2. IJJ Q 50(

351

40,

45

5O( 0

(kin) FIO.5.39Ardistribution in the main water masses of the Nansen Basin. The symbols display the 39At concentration in O'~ modem. Full symbols would mean 100% modem, open circles would indicate 0% modem.

18

P. SCHLOSSERet al.

5. DISCUSSION The observed tracer fields are discussed in terms of the water mass structure in the Greenland/ Norwegian Seas and the Nansen Basin and the relative 'ages' of the individual water masses. We use the term age for the time that has elapsed since a water parcel left the surface after acquiring its initial concentration by exchange with the atmosphere. At this point gases typicallyhave reached solubility equilibrium with the atmosphere while t4C and tritium are set to a certain value that can be reconstructed as a function of time. The term 'age' has to be distinguished from the term mean residence time which stands for the average time a water parcel spends in a certain reservoir such as a deep-sea basin. The ages derived from measured tritium/3He ratios are apparent ages which may differ significantlyfrom true ages as a result of non-linear effects during mixing of waters with different tritium/3 He ratios. It has been shown by WALLACEet al (1992) on the basis of a simple box model calculation that apparent ages observed in 1987 with values below about 10 to 13 years closely reflect the real mean residence time of a water mass even in the worst case scenario of a well-mixed water reservoir (box). Apparent tritium/3He ages with values higher than 15 years are more likely to be affected by mixing and so can not be interpreted in a straightforward manner. The measured Alqc values are converted into mean ages, after subtraction of the bomb component by means of a tritium/14C correlation, that reflect the mean time that has elapsed since equilibration with the atmosphere. Apparent 39Arages are calculated by straightforward application of the radioactive decay equation to the measured concentration differences between equilibrated surface water (100% modem) and the individual water masses for which we obtained 39Armeasurements. While this approach is useful to obtain a first order estimation of the relative age structure of the waters found in the Greenland/Norwegian Seas and the Nansen Basin, it does not allow realistic estimations of deep water formation and exchange rates. To obtain this information, we have to apply models which correct for mixing. Such a studywas done by BONISCHand SCHLOSSER(1995) in an accompanying paper. 5.1 Upper waters 5.1.1. Greenland~Norwegian Seas. The low 63He values along with tritiurn/3He ages of less than one year in the upper 250m of the Norwegian Sea indicate that these waters are close to solubility equilibrium with the atmosphere and the tzitiogenic 3He is removed from the water by gas exchange. The sharp lateral tritium gradient at about 7 I°N is related to the Jan Mayen current which transports water with high tritium concentrations from the East Greenland Current towards the centre of the Greenland gyre. The original source of the high tritium concentrations is Arctic river-runoff(e.g. 0 STLUND,1982). This observation adds evidence to the hypothesisthat the Arctic river-runoff might directly impact the surface salinities in the centres of the convective gyres forming dense deep and bottom waters (AAGAARDand CARMACK, 1989). The relatively low tritium concentrations observed between about 600m depth and the surface at about 75°N mark the centre of the Greenland gyre where deep convectionleads to vertical exchange between surface and deep waters. The relativelyhigh apparent tdtium/3He ages in the upper layers of the Greenland gyre indicate that these waters are not as well equilibrated with the atmosphere ('less ventilated') as those found at similar depths in the Norwegian Sea. The EAV section displays the same basic features as the N/S section. As the section is located south of the centre of the Greenland gyre, the high surface tritium concentrations related to the Jan Mayen Current extend from the East Greenland Current to about 2 °W (Fig.3b). The tritium versus salinity plot of stations located in the Greenland Sea (Fig.6) shows a linear correlation between

Distribution of tritium, 3He, ~C and 39Arin the Nordic Seas

19

tritium and salinity for most of the stations. Extrapolation of the tritium/salinity correlation to a salinity of zero yields tritium values of about 60TU. Such high tritium concentrations can only be explained by addition of Arctic river-runoff. The sharp lateral gradients in tritium, 3He and tritium/ 3He age at about 4°E (Figs 3b-d) mark the Arctic front (SWIFT, 1986), i.e. the transition between the Arctic domain and the Norwegian Current (Atlantic domain). The 3He maximum at about 75m depth extending from the East Greenland Current to the centre of the Greenland gyre (Fig.3c) seems to be a remnant of the pronounced 3He maximum which marks the lower boundary of the Arctic halocline (see Fig.4e). Apparent tritium/3He ages in this water layer reach values of more than 10 years. This value is almost as high as the apparent tritium/3He ages observed in the lower boundary of the Arctic halocline. The 3He andtritium/3He age minima, centred around 200m depth at station 144b, mark relatively young (<2 years) Atlantic water recirculated in Fram Strait. The mean ages of the waters in the upper 500m of the Arctic domain (excluding the top 50 to 100m) is approximately 5 to 10 years, while the waters found in the upper 500m of the Norwegian Current have a mean age of less than 1 to 2 years. This means that the upper waters of the Norwegian Sea are much closer to equilibrium with the atmosphere than those in the Greenland Sea (they are better 'ventilated'). I0

f-

2

A

I.,J

o

--IO I--

i

i

I

I

i

I

I

I

I

|

I

I

I

I

i

n.I- 8

8

0

32

I

33

34

SALINITY

i 35

[ psul

FIG.6. Tritium versus salinity plot of the upper waters of stations locatedin the Greenland Sea. The tritium concentrationat a salinity of 0 is 60TU, i.e. a clear indicationof a significantfiver-runoff component.

20

P. SCHLOSSERet al.

5.1.2. Arctic Ocean. The upper waters of the 1987 Polarstern section consist primarily of a surface mixed layer and lower halocline waters as defined by JONESand ANDERSON(1986). Surface tritium and 3He concentrations are fairly homogeneous south of about 83°N (southern boundary of the Transpolar Drift) and increase north of this latitude as a result of the increasing amount of river-runoff transported across the Arctic Ocean in the Transpolar Drift (e.g. SCHLOSSERet al, 1994). The contribution of Arctic river-runoffto the surface layer tritium concentrations is reflected in a tritium versus salinity plot (Fig. 7). In this plot stations with significant river-runoffcontributions (stations 358 to 371, Fig.7b) show a larger tritium/salinity slope compared to stations influenced by sea-ice meltwater (stations 269 to 340, Fig.7a). Extrapolation to a salinity of 0 yields a tritium concentration of the freshwater component of 194TU. This feature is consistent with 180 data from the same section which have been used for quantitative separation of the river-runoffand seaice meltwater components (SCHLOSSER, e t al, 1994; BAUCH, SCHLOSSERand FAIRBANKS, 1995).

I0,

I

I

I 269 D 276 I~ 2 8 0 • 285 • 287

6

b 296 • 310 + 340

'~ 2 I'-" L.--.I

+

++

+

A

o L$ V V

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00 o

4

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2-

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C 33

I

I

I

33.5

34

34.5

35

S A L I N I T Y I~psu] FIo.7. Tritium versus salinity plot of the upper waters of stations located in the southern (A) and northern (B) part of the Nansen Basin. Extrapolation of the correlation in Fig.7B to a salinity of O yields a tritium concentration of 194TU reflecting the high fraction of fiver-runoffin the surface waters observed in the northern part of the section. This feature is not seen in the southern part of the section (Fig.7A) where the freshwater consists predominantly of sea-ice meltwater.

Distributionof tritium, 3He,14Cand 39Atin the Nordic Seas

21

Apparent tritium/3He ages of the surface waters in the Transpolar Drift are fairly high (close to 5 years) since the perennial sea-ice cover practically prevents loss of tritiogenic 3He to the atmosphere (Fig.Sb). South of the Transpolar Drift, tritiurrd3He ages of the surface waters are significantly lower reaching values as low as one year or less on the Barents Shelf(Fig.Sa). The strong 3He maximum observed at the lower boundary of the halocline is centred around salinities of about 34.5 and marks relatively old waters with apparent tritium/3He ages of about l0 to 15 years (Fig.Sb). This maximum marks a 1:1 mixture of Atlantic Water and lower halocline water (see potential temperature versus salinity plot in Fig.9). The reason for this statement is the fact that the waters above and below the tritium/3He age maximum are advectively dominated with different flow directions. The tritium/3He maximum therefore marks a zone of high shear and most likely low velocity which is dominated by mixing of LHW (overlying the tritium/3He age maximum) and Atlantic derived water (underlyingthe tritium/3He age maximum). The underlying water layer (about 200 to 400m depth) is primarily of Atlantic origin and has strong lateral 3He and tritium/3He age gradients while it is almost homogeneous in tritium. The variation of the tritium/3He age in the Atlantic layer results from the southern part of the section consisting of relatively young waters having entered the Arctic Ocean via the West Spitzbergen Current relatively recently, while the northern part of the section is dominated by recirculated Atlantic water which has already spent a significant time in the Arctic Ocean (COACHMANand BARNES, 1963). More detailed interpretations of the tritium/3He age distribution of the upper water column can be found in WALLACEet al (1992) and SCHLOSSERet al (1994). 20

i

i

T

12 + +

++

+

+

?o ILl 2 0 C9

, ,o t AO

O~ ~V V

12

Or.I [] r.].

eo 8 A

V °°o

o

4

b q .

t

~5.5

I 34

I :54.5

35

SALINITY [psu] FIG.8. Tritium/3Heageversussalinityplots of the upper waters (surface to Atlantic Water) ofstations

in the southern(a) and northern(b) part of the NansenBasin.

22

P. SCHLOSSERet al.

++ • • 296 I

,3,o I +3.o I

r-1

•11 +V O

o

P

I a IJ.i

-

2

1 I

i

,

LLI

0

o

-I

" 33

,,o I

,

¢ 33.5

SALINITY

34

34.5

35

Cpsu]

FIG.9. Potentialtemperatureversus salinityplot of the upper haloeline waters in the southern (a) and northern (b) pan of the Nansen Basin. 5.2 Deep waters 5.2.1. Greenland~Norwegian Seas The deep waters of the Norwegian Sea are a mixture of Arctic Ocean deep waters and GSDW which is entrained into the Arctic outflow (AAGAARD,SWIFT and CARMACK, 1985; SWIFTand KOLTERMANN, 1988). The mixture (so called 'new NSDW'; SMETHIE et al, 1986) enters the Norwegian Sea through the Jan Mayen Fracture Zone. The relatively low tritium concentrations found in NSDW (<0.5TU) compared to GSDW (about 0.75 to I TU, Fig.2b), is the result of relatively low tritium concentrations of the Arctic Ocean component contributing to NSDW and the fact that new NSDW mixes into a large reservoir of old NSDW with very low tritium concentrations (e.g. HEINZEet al, 1990). The highest tritium concentrations in GSDW are observed at stations 140 and 148 at about 74°N. Although GSDW has lower apparent tdtium/3He ages than NSDW (Fig.2d) indicating faster renewal from the surface, the 83He values of GSDW are higher than those ofNSDW (Fig.2e) because of the higher tritium concentration ofGSDW. The low tritium concentrations observed in the deep waters of the eastem part of the E/W section (stations 131 to 133, Fig.3b) reflect NSDW flowing northward toward the Arctic Ocean. The northward flowingNSDW can also be recognized in its relatively low 83He values (Fig.3c) and its relatively high apparent tritium/3He age (Fig.3d).

Distribution of tritium, 3He,14Cand 39Arin the Nordic Seas

23

The tritiurrg3He ages of NSDW and GSDW are apparent ages affected by mixing and can not be interpreted directly as mean residence times. To obtain mean residence times we have to use models which correct for the non-linear effects of mixing (e.g. BONISCHand SCHLOSSER,1995). The only straightforward information we can derive from the observed apparent tdtium/3He ages of GSDW and NSDW is the fact that GSDW is renewed more rapidly from the surface than NSDW. 5.2.2. Arctic Ocean. Tritium and ~3Hevalues are relativelylow in the deep waters of the Arctic Ocean (generally less than 1TU and 3% respectively, below 1,500m depth). Therefore, apparent tritium/3 He ages are most likely heavily affectedby mixing and non-tritiogenichelium sources (e.g. SCHLOSSERet al, 1990), i.e. they are not an appropriate tool for determination of the ages of the deep waters. However, they provide valuable constraints for model-based estimates of the deep water renewal times (e.g. SCHLOSSERet al, 1991). We used the data for such a model-based study in an accompanying study (BONISCHand SCHLOSSER,1995). The increase of tritium, 3Heand 14Ctowards the southem and northern boundaries of the deep Nansen Basin (depth >l,500m) is consistent with hydrographic and CFC data (ANDERSONe t al, 1989; KRYSELLand WALLACE,1988) and CFC observations from a 1985 Polarstern cruise to the southern tip of the Eurasian Basin (SMETHIEet al, 1988). This observation is difficult to explain on the basis of the sparse observations available from this region. The lower salinity of the deep and bottom waters towards the boundary, together with the higher transient tracer concentrations, suggest inflow of water from the Norwegian (and maybe Greenland) Sea. Northward flow of Norwegian Sea Deep Water is also required by box model calculations (e.g. BONISCH and SCHLOSSER,1995). However, to maintain the higher salinities in the centre of the basin a source of high-salinity water is required which in the present setting can only be supplied from the Arctic shelves (most likely Barents and Kara seas, e.g. BAUCHet al, 1995). Such a source would most likely contain high transient tracer concentrations because of its short mean residence time on the shelf and the related intense exchange with the atmosphere. The transient tracer concentrations of the bottom waters of the central Nansen Basin are very close to zero (Fig.3d, see also JONES, ANDERSON and WALLACE,1991). This leaves us with the following somewhat speculative conclusions: (1) The higher tracer concentrations observed in the deep boundary currents of the Nansen Basin reflect inflow of water with relatively low salinities and high tracer concentrations from south of Fram Strait. These waters, together with the deep water formed within the Arctic Ocean, make up the deep boundary currents. (2) The formation of EBBW is variable in time and the high-salinity, low-tracer waters of the central Nansen Basin are the remnant of EBBW formed decades or centuries ago under different hydrographic conditions (see also JONESet al, 1991 ). (3) The waters contained in the boundary current mix slowly into the interior of the basin (SMETHIE et al, 1988). The spreading from the boundaries into the interior of the basin is supported by a weak but significant 3He maximum extending at about 3,000m depth from the Gakkel Ridge to the centre of the Nansen Basin (Fig.4e). In cases where transient tracer concentrations are low, natural radioactive tracers with relatively long half lives such as 39Ar or 14C can be used to derive reliable mean residence times of specific water masses. 14C has been used for this purpose before by OSTLUND,TOP and LEE (1982) and SCHLOSSERet al (1990). In the Nansen Basin, ~4C can be applied more or less straightforward in the bottom waters because the contribution of bomb 14C to the total 14C content is negligible, as indicated by the very low tritium concentrations (Fig.4d). In the depth range between about 1,500m and about 2,600m we have to subtract the bomb component from the measured AI4C values if we want to use ~4Cas an age indicator. One way to accomplish this is to correlate 14C with tritium and to extrapolate the correlation to a tritium value of zero. The intercept of the ~4C axis can then be used as a first order approximation of the pre-bomb (zero-tritium) 14C concentration. The

24

P. SCHLOSSSRet al.

difference between this value and the pre-bomb surface water A14C value can then be converted into a mean age. Because the tritium to 14C ratio o f surface water is not constant in time, a non-linear tritium versus 14C correlation has to be expected for the deep waters of the Nansen Basin. In a tritium versus 14C plot containing all our Nansen Basin data we find such a non-linear correlation (Fig. 10a). If we plot tritium versus 14C for individual water masses such as Eurasian Basin Deep Water (EBDW, Fig. 10b), or Eurasian Basin Bottom Water (EBBW, Fig. 10c), we find within the scatter o f the data more or less linear correlations. Extrapolation of these correlations to zero tritium yield pre-bomb Am4C values of-77+6%0 and -86+6%0 for EBDW and EBBW, respectively. If we convert these values into mean residence times using OSTLtYND'Sestimate of the pre-bomb surface A~4C value of-51.5+3.5%o (-48 to -55%0, OSTLtmD,POSSNERTand SWIFT, 1987), we obtain values o f 163-287 and 244-368 years for EBDW and EBBW respectively (Fig. 11). The upper and lower limits are calculated taking into account the errors of both the pre-bomb A~4Cvalue (+3.5%o) and the extrapolation o f the tritium/A~4C correlation to a tritium concentration of zero (+6%0). Our EBDW age estimate (--200 to 250 years) is significantly higher than the value derived by OSTLUND et al (1982) from Fram I I I data (<80 years at 2,500m depth). The reason for this difference is that F r a m II1was located at the southern tip of the Eurasian Basin, which is influenced by water originating in the Greenland/Norwegian Seas and more rapidly ventilated boundary currents. 39At does not have the problem of a bomb component that has to be subtracted from the observed concentrations. It is therefore the better tracer for straightforward estimates of the mean age times o f the intermediate and deep waters. On the basis of observed 39mrconcentrations we obtain mean ages o f AIW, EBDW and EBBW of 68-117, 117-211 and 246-310 years, respectively. The mean ages derived from lac after correction for the bomb component are consistent with these values (Table 1). TABLE 1: Mean ages of A/W, EBDW and EBBW based on correlations

measurements and 14C/tritium

AIW

EBDW

EBBW

79 + 5% 68 - 117

-77 5:6 163- 287 66 + 8% 117 - 211

-86 5:6 244- 368 49 + 4% 246 - 310

AI4C '4Ca~ 39Atcone. 39Atage

39At

6. CONCLUSIONS The tracer data sets from the Greenland/Norwegian Seas and the Nansen Basin o f the Arctic Ocean discussed in this contribution outline the potential o f multi-tracer studies in terms o f estimating mean ages o f the waters north of the Scotland/Iceland/Greenland sills. Straightforward, semi-quantitative interpretation o f the data provides valuable firstrough estimates of the time scales involved in the circulation o f these waters. Additionally, gradients in measured tracer distributions provide insight into the pathways o f the individual water masses. In order to derive quantitative information from the observed tracer distributions, models have to be applied. A study extending our interpretation to this level o f quantification is presented by BONISCH and SCHLOSSER(1995).

Distribution of tritium, 3He, ~4Cand 39Arin the Nordic Seas

t!iiil

-

al-

o ~,r""

- I00 1.4

-50

J

0

I

50

I

I

i

I00 i /

1.2

I

L-J

0.8 02

Ixl; l

p"

0.6 0.4

~

0.2 0

1.4

,

/

I

-80 I

+ I

b i

-~ I

I

I

I -40 I

l -20 I

1.2

I 0.8 0.6 0.4 0.2 0

-80

-60 -40 14C [%01

-20

FIG.10. AI4C versBs tritiumplots for the Nanse~ Basin. (a) all datapoints; Co)EBDW (1,500-2,600m); (c) EBBW (2,600 to bottom).

25

P. SCHLOSSERet al.

26

0

r-I

I

I

I

I

I

I

I

AIW

I000

E

t_J

"lt-" Q. LtJ 1:3

2000

.'EBDW

3SAr i

I

I

,,c

9Ar

3000

EBBW 400%

I

'

I~)0

I

i,tC ' 20O ' AGE ry ]

II

I

300

I

FIG.11. Estimated 39Ar and t4C ages ofAIW (39Ar only), EBDW and EBBW. For explanation see text.

7. ACKNOWLEDGEMENTS J6m Thiede, chiefscientist on ARK IV/3, provided time for an extensive tracer program. L. Anderson, P. Jones, Jose Rodriguez, J. Swift and D. Wallace contributed to various aspects o fthe data collection. The Alfred-Wegener-Institute for Polar and Marine Research assisted in many logistical aspects. The officers and crews ofR/V Polarstern and R/V Meteor provided excellent technical support. Patty Catanzaro drafted the figures. Financial support by the Deutsche Forschungsgemeinschaft, the National Science Foundation under grant no. DPP 90-22890, the Office of NavalResearch under grant no. N00014-90-J-1362, and the Swiss National Science Foundation is gratefully acknowledged. LDEO contribution no. 5297.

8. REFERENCES

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Distribution of tritium, 3He, 14Cand 39Arin the Nordic Seas

27

BONISCH,(3. and P. SCHLOSSER(1995) Deep water formation and exchange rates in the Greenland/Norwegian Seas and the Eurasian Basin of the Arctic Ocean derived from tracer balances. Progress in Oceanography, 35, 29-52. BROECKER,W.S. and T.H. PENO(1982) Tracers in the Sea, Eldigio Press, Palisades, New York, 692pp. BULLISTER,J.L. and ILF. WEISS(1983) Anthropogenic chlorofluoromethanes in the Greenland and Norwegian Seas, Science, 221,265-268. CLARKE,W.B., W.J. JENKINSand Z. TOP (1976) Determination of tritium by mass spectrometric measurement of 3He. International Journal of Applied Radiation and Isotopes, 27, 515-522. COACHMAN,L.K. and C.A. BARNES(1963) The movement of Atlantic Water in the Arctic Ocean. Arctic, 16, 8-16. DICKSON,R.R. and J. BROWN(1994) The production of North Atlantic Deep Water: sources, rates, and pathways. Journal of Geophysical Research, 99, 12,319-12,341. HEINZE,C., P. SCHLOSSER,K.P. KOLTERMANNand J. MEINCKE(1990) A tracer study of the deep water renewal in the European Polar Seas. Deep-Sea Research, 37, 1425-1453. JONES,E.P. and L.G. ANDERSON(1986) On the origin of the chemical properties of the Arctic Ocean halocline. Journal of Geophysical Research, 91, 10,759-10,767. JONES, E.P., L.G. ANDERSONand D.W.R. WALLACE(1991) Tracers of near-surface, halocline and deep waters in the Arctic Ocean: implications for circulation. Journal of Marine Systems, 2,241-255. KROMER,B. (1984) Recalibration of Heidelberg 14C laboratory data. Radiocarbon, 26, 148. KROMER,B., C. PFLEIDERER,P. SCHLOSSER,I. L EVIN,K.O. MUNNICH,G. BONANI,M. SUTERand W. Wt)LFLI(1987) AMS t4C measurement of small volume oceanic water samples: Experimental procedure and comparison with low-level counting technique. Nuclear Instruments and Methods, B29, 302-305. KRYSELL,M. and D.W.R. WALLACE(1988) Arctic Ocean ventilation studied using carbon tetrachloride and other anthropogenic halocarbon tracers. Science, 242, 746-749. LOOSLI, H.H. (1983) A dating method with 39Ar.Earth and Planetary Science Letters, 63, 51-62. MAIER-REIMER,E., U. MIKOLAJEWICZand K. HASSELMANN(1993) Mean circulation of the Hamburg LSG OGCM and its sensitivity to the thermohaline surface forcing. Journal of Physical Oceanography, 23, 731-757. (~)STLUND,H.G. (1982) The residence time of the freshwater component in th Arctic Ocean. Journal of Geophysical Research, 87, 2,035-2,043. OSTLUND,H.G., G. POSSNERTand J.H. SWIFF(1987)Ventilation rate of the deep Arctic Ocean from Carbon 14 data. Journal of Geophysical Research, 92, 3769-3777. OSTLUND,H.G., Z. TOP and V.E. LEE (1982) Isotope dating of waters at Fram Ill. GeophysicalResearch Letters, 9, 1117-1119. PETERSON, W.H. and C.G. ROOTH (1976) Formation and exchange of the deep water in the Greenland and Norwegian Seas. Deep-Sea Research, 23,273-283. PdqEIN, M. (1991) Ventilation rates of the Greenland and Norwegian Seas derived from distributions of the chlorofluoromethanes F 11 and F 12. Deep-Sea Research, 38, 485-503. SCHLOSSER,P., D. BAUCH,R. FAIRBANKSand G. BONISCH(1994) Arctic river-runoff: mean residence time on the shelves and in the halocline. Deep-Sea Research, 41, 1053-1068. SCHLOSSER,P., G. BONISCH,M. RHEINand R. BAYER( 1991) Reduction of deepwater formation in the Greenland Sea during the 1980s: evidence from tracer data. Science, 251, 1054-1056. SCHLOSSER,P., G. BONISCH,B. KROMER,K.O. MONNICHand K.P. KOLTERMANN(1990) Ventilation rates of the waters in the Nansen Basin of the Arctic Ocean derived from a multi-tracer approach. Journal of Geophysical Research, 95, 3,265-3,272. SCHOCH,H. and K.O. MONNICH(1981) Routine performance of a new multi-counter system for high-precision 14C dating. In: Methods oflow-levelcountingandspectrometry, Vienna, International Atomic Energy Agency. SCHOCH,H., M. BRUNS,K.O. MONNICHand M. MONNICH(1980) A multi-counter system for high-precision carbon14 measurements. Radiocarbon, 22, 442-447. SMETHIE,W.M.JR, H.G. ~)STLUNDand H.H. LOOSLI(1986) Ventilation of the deep Greenland and Norwegian Seas: evidence from krypton-85, tritium, carbon-14 and argon-39. Deep-Sea Research, 33,675-703. SMETH1E,W.M.JR, D.W. CHIPMAN,J.H. SWIFTand K.P. KOLTERMANN(1988) Chlorofluoromethanes in 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, 347-369. STUIVER,M. and H.A. POLACH(1977) Reporting of 14C data. Radiocarbon, 19, 355-363. SWIFT,J.H. (1986) The Arctic waters. In: The Nordic Seas, B.G. HURDLE,editor, Springer-Verlag, 129-153.

28

P. SCHLOSSERet al.

SWIFT,J.H. and K. AAGAARD(1981) Seasonal transition and water mass formation in the Iceland and Greenland Seas. Deep-Sea Research, 28, 1107-1129. SWIFT, J.H. and K.P. KOLTERMANN(1988) The origin of Norwegian Sea Deep Water. Journal of Geophysical Research, 93, 3563-3569. SWIFT,J.H., T. TAKAHASHIand H.D. LIVINGSTON(1983) The contribution of the Greenland and Barents Seas to the deep water of the Arctic Ocean. Journal of Geophysical Research, 88, 6981-6986. WALLACE,D.W.R., P. SCHLOSSER,M. KRYSELLand G. BC3NISCH(1992) Halocarbon and tritium/3He dating of water masses in the Nansen Basin, Arctic Ocean. Deep-Sea Research,39, $435-$458. WEISS, W., W. ROETHERand G. BADER (1976) Determination of blanks in low-level tritium measurement. International Journal of Applied Radiation and Isotopes, 27, 217-225. WRIGI-IT,D.G. and T.F. STOCKER(1992) Sensitivities o fa zonally averaged global ocean circulation model. Journal of Geophysical Research, 97, 12,707-12,730.