Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island in the western Arctic Ocean

Pergarnon

Deep&a

0%7-0645@3oOo49-6

Research II. Vol. 42. No. 6, 1449-1470. 1995 Copyright 0 1995 pp. Elsevicr Science Ltd

Printed in Great Britain. All rights reserved 09674645195 S9.50 + 0.00

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island in the western Arctic Ocean J. N. SMITH*

and K. M. ELLIS*

(Received 31 May 1994; in revisedform 21 December 1994; accepted 3 April 1995)

Abstract-Water column profiles of naturally-occurring (‘%b, ?o, U6Ra) and fallout (‘37Cs, 23g*2@Pu) radionuclides measured at the Canadian Ice Island in 1985.1986 and 1989 over the Arctic Ocean continental shelf were compared to profiles measured at the CESAR ice station in the central Arctic Ocean in 1983. Cesium-137 concentrations at CESAR decreased from a mean value of 4.2 Bq/ m3 in the surface mixed layer to less than 1.OBq/m3 in the Atlantic layer and deep waters. Cesium-137 concentrations were similar to CESAR values in the surface mixed layer at the Ice Island, but in halocline water they were lower than values measured at comparable water depths at CESAR, indicating that the arrival of fallout 13’Csin the Ice Island halocline had been delayed compared to its arrival in the CESAR halocline. These data are consistent with a combination of mixing box and lateral advection models with mixing occurring over the Russian continental shelves followed by lateral advection through the interior halocline waters of the Canada Basin. Dissolved 2’sPb and “‘?o concentrations were higher in nutrient maximum water at the Ice Island compared to CESAR. Low values at CESAR are caused by scavenging of particle-reactive radionuclides during modification of Bering Sea water in continental shelf environments. The elevated ‘lcPb and 21cPo concentrations measured in nutrient maximum water at the Ice Island are assumed to result from ingrowth of 21cPb from 226Ra during lateral advection of upper halocline water between the CESAR and Ice Island locations, a process that is estimated to take approximately 11 years. Plutonium-239,240 is also depleted, relative to ‘37Cs in the surface mixed layer and upper ha&line at the Ice Island owing to its scavenging during modification of Pacific water in shelf environments. The 239*2%/137Cs ratio increases to values approaching fallout levels in lower halocline and Atlantic water owing to both reduced 23g*2~u removal in source waters and vertical transport of 239*2~u with sinking particles.

INTRODUCTION Many of the chemical and hydrographic features of the Arctic Ocean halocline are governed by processes occurring over the broad continental shelves. The halocline is maintained partly by the production of brines over the shelf, as a consequence of salt rejection during sea-ice formation, followed by the mixing of this dense water along isopycnal surfaces with interior waters (Aagaard et al., 1981; Aagaard et al., 1985; Melling and Lewis, 1982). Shelf regions also appear to be zones of recycling for nutrients and other biologically utilized trace elements (Moore et al., 1983; Jones and Anderson, 1986), in addition to being sinks for particle-reactive species (Moore and Smith, 1986; Bacon et al., 1989; Macdonald and Thomas, 1991). The extensive shelf regions underlying the Chukchi, Laptev, Kara and Barents seas which occupy approximately 30% of the area of the Arctic Ocean, have most *Marine Chemistry Division, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada B2Y 4A2. 1449

Department

of Fisheries and Oceans,

1450

J. N. Smith and K. M. Ellis

frequently been cited as the principal locations for processes governing dense water formation and nutrient recycling in the arctic (Aagaard et al., 1981; Jones and Anderson, 1986). However, these processes also affect water column properties over the narrower shelf regions of the Canadian Archipelago. As a consequence of the Canadian Ice Island program, it became possible to undertake measurements in the water column overlying the Canadian continental shelf and test various hypotheses related to the maintenance of the Arctic Ocean halocline. One method for studying the dynamics of shelf interactions with halocline water masses is through the use of chemical and radioactive tracers. Measurements of the fallout radionuclide 137Cs permit the determination of shelf-halocline exchange rates, while measurements of the members of the naturally-occurring 238U decay series (210Pb, 2’oPo and 226Ra) and the fallout radionuclide 239*240Pucan provide information on the mechanisms governing sediment-water interactions of particle-reactive species. In the present paper we report measurements of these radionuclides in water column samples collected at the Canadian Ice Island, north of Ellsemere Island in 1985, 1986 and 1989. The results are compared to similar measurements made in 1983 at the CESAR ice station in the central Arctic Ocean. The implications of these results are discussed with regard to water circulation and particle transport in surface and halocline waters of the Arctic Ocean. METHODS Large-volume water samples were collected at the CESAR ice station (Fig. l), located over the Alpha Ridge, during April and May 1983, using a 100 1 GoFlo bottle. Approximately 50 1 of seawater were passed through two KCFC packed resin columns arranged in series in order to extract 137Csfrom solution (Smith et al., 1990). The KCFC columns were subsequently transported to the laboratory at the Bedford Institute of Oceanography where 137Cs was measured using a hyperpure Ge detector having an efficiency of 25%. Some samples were analyzed using a NaI well detector. Hydrographic and nutrient results from CESAR were reported in Jones and Anderson (1986). Water samples were collected at the Canadian Ice Island in May-June of 1985, 1986 and 1989 (Fig. 1). The Canadian Ice Island was approximately 15 km2 in area with the ice layer being 45 m thick. A permanent ice camp was established on this island, which permitted the sampling of Arctic marine regimes as the island drifted across the continental shelf. In 1985, the Ice Island was held in land-fast ice at 81”434’N 93”25’W, 25 km north of Ellsemere Island (Fig. 2). By June 1986 it had drifted south-westwardly to 80”57.O’N, 97”36’W. At both locations the water depth was approximately 300 m. Finally, by 1989 the Ice Island had drifted into the Peary Channel in water 500 m deep (Fig. 2). Water samples from depths less than 40 m were collected by pumping through small holes near the edge of the Ice Island. Deeper samples were collected through the main sampling hole, 1 m in diameter, which was melted near the centre of the Ice Island. These samples were collected prior to the collection of sediment samples, because the latter operation tended to contaminate the sampling hole with particulate matter. Water samples were collected using 5 1 and 100 1 Niskin bottles on a Kevlar hydrowire with water depths determined using a meter wheel. Temperatures were determined using reversing thermometers. Analyses were performed for salinity, alkalinity, dissolved oxygen and nutrients immediately after sample collection (Jones and Anderson, 1990). A 5 1 bottle was positioned immediately above the 100 1 bottle. Salinity and nutrient

Radionuclide tracer profiles at the CEDAR Ice Station and Canadian Ice Island

1451

Fig. 1. Locations of the CESAR ice station over the Alpha Ridge in 1983 and the Canadian Ice Island in 1985,1986 and 1989. Also illustrated are the LOREX and AIWEX Ice Stations occupied in 1983 and 1985, respectively.

results for samples collected from both bottles were compared to previously determined profiles in order to confirm the reliability of sampling depths. In some cases a 20 1 aliquot was removed from the 100 1 bottle and passed through MnOz-coated acrylic fibres which quantitatively adsorb Ra isotopes. The MnOz was subsequently dissolved in weak HCl solution in the laboratory and 226Ra concentrations were determined by alpha particle counting using a radon gas emanation method (Moore and Smith, 1986). The remaining 80 1 were filtered through a 0.4 pm Nucleopore filter which was subsequently analyzed for particulate 2’oPo and “‘%b using methods previously reported (Moore and Smith, 1986). Twenty litre samples were acidified using HCl and retained for “‘PO and ‘i”Pb analyses. A 208Po tracer and 1 mg of Pb carrier were added to the sample and were co-precipitated using a cobalt APDC precipitation. The precipitate was filtered through a 0.45 pm Millipore filter and returned to the laboratory for analysis using alpha particle spectrometry with the Pb recoveries being determined using flame AAS. The remaining seawater sample (60-100 1) was then passed through a KCFC resin column, which was subsequently analyzed in the laboratory for i3’Cs using a Ge gamma detector. A second KCFC placed in series during the collection of several samples having i3’Cs activities in excess of 5 Bq/m3 showed no evidence of inuomplete removal of i3’Cs by the first column. For a i3’Cs detection limit of 0.12 Bq/ m3, for a 100 1sample, the lower limit for the extraction efficiency is estimated to be 96%. A ferric hydroxide precipitation was used to extract 239V2?u (with 242Pu tracer) from acidified, unfiltered 60 1 water samples. Plutonium-239,240 was concentrated by radiochemical separation using ion-exchange techniques and electro-deposition onto

1452

J. N. Smith and K. M. Ellis

Fig. 2.

Track of the Ice Island during transit along the Canadian continental shelf between 1985 and 1989.

stainless-steel discs and it was subsequently measured using alpha particle spectrometry (Smith et al., 1987). CESIUM-137

RESULTS

Hydrographic properties

The hydrographic features of the water column over the Canadian continental shelf in 1989 (Fig. 3) were similar to those measured during previous studies at the Ice Island in 1985 and 1986 (Jones and Anderson, 1990) and at the CESAR (Jones and Anderson, 1986; Fig. 4), LOREX (Moore et al., 1983) and AIWEX (Casso et al., 1988) ice stations in the central and western Arctic Ocean. The water column in the Canada Basin can be divided into five components, a surface mixed layer, upper halocline, lower halocline, Atlantic layer and deep water (Kinney et al., 1970; Moore et al., 1983). The halocline and thermocline extend down from the surface mixed layer to the Atlantic water layer, which is characterized by a temperature maximum of 0.5”C near the 450 m level at the CESAR ice station (Fig. 4). The upper component of the Atlantic water layer is also evident at the Ice Island locations (water depths of 300-500 m) although the core is not distinguishable. The relatively fresh (32.2 psu), cold (near freezing) surface mixed layer was approximately 50 m thick at the Ice Island and consisted mainly of a mixture of Bering Sea water, sea-ice meltwater and runoff from Siberian rivers. The source water for the upper halocline in the Canada Basin is Pacific water that enters the Arctic Ocean through Bering Strait and is modified by processes occurring over the Chukchi and East Siberian Sea continental

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island

1453

Temperalum (“c) -2

0

2

salhtlv

5. .

??*

. . .

.

I

.

. I

6ooi::.:.:.:.:...:.:.:., * * * . * ..:.:. . .. .. .. .. ...‘ . ......‘ . ......‘ . ..‘,.:‘ . ...:.:.:.:.: . .. . . . ..i.i...i....:..... Fig. 3.

. . . . :::.:...:...............:. .. ..... .... ... ::::::i::.:.:.:.:.:.:.::::

Profiles of salinity, temperature, silicate and phosphate measured at the Ice Island in 1989 were similar to those measured at CESAR (Fig. 4) in 1983.

shelves. The formation of sea-ice over these shelves results in the production of brines or dense water that descend to levels of neutral buoyancy and mix along isopycnal surfaces with interior water to supply and maintain the upper halocline (Aagaard et al., 1981; Melling and Lewis, 1982). Upper halocline water is characterized by a pronounced maximum in dissolved silicate at a salinity of 33.1, located at approximately 100 m over the Canadian continental shelf (Fig. 3) and at 120 m at CESAR (Fig. 4). The silicate maximum is associated with inputs of nutrient rich water from the Bering Sea, possibly augmented by nutrient regeneration and release from shelf sediments during advection into the central regions of the Arctic Ocean (Moore et al., 1983). Lower halocline or thermocline water is generated from Atlantic water that enters the Arctic Ocean in the vicinity of the Barents and Kara Seas and is subsequently modified to form water having a salinity close to 34.2 and a minimum in NO (Jones and Anderson, 1986). The ha&line at the Ice Island was similar, but not identical to that observed at CESAR. The Ice Island halocline measured in 1989 was slightly warmer (Fig. 3), especially in the steepest part of the halo&e, near 34.0, where temperatures at the Ice Island were several tenths of a degree higher than at CESAR. Jones and Anderson (1990) have interpreted these results to mean that halocline water has mixed with overlying surface water and underlying Atlantic layer water to a greater degree over the shelf compared to the central arctic location and suggested that the ha&line water at the Ice Island was “older” compared to central arctic halocline water. The present set of radionuclide tracer data permits an evaluation of their conclusions. Cesium- 137 profiles

Cesium-137 and silicate profiles for water samples collected at the CESAR ice station (1983) and the Ice Island (1985, 1986 and 1987), respectively, are illustrated in Fig. 4 and Fig. 5 and listed in Tables 1 and 2. The mean 137Csactivity (5.3 Bq/m3) in the mixed layer (O-

1454

J. N. Smith and K. M. Ellis 13’Cs(Bq/ma)

13’Cs (Bq/m3)

OOW

I.:, , , ,

0

20

40

Si WI

1800

31

32

33

34

35

36

SALINITY -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 I I I1 I I I I I1 I, TEMPERATURE (“C)

Fig. 4. Cesium-137, salinity and temperature water depth profiles measured at the CESAR ice station in 1983. Data uncertainties are represented by 1 sigma counting uncertainties with all data decay corrected to sampling date. Inset: Elevated 13’Csconcentrations extend well below the nutrient maximum defined by a peak in silicate concentration (solid circles).

60 m) at CESAR lies in the range of surface 13’Cs concentrations (4.4-6.2 Bq/m3) measured at the Ice Island. However, higher surface 13’Cs activities (7.57 Bq/m3; Casso et al., 1988) measured in 1985 at the AIWEX ice station (Fig. 1) indicate that significant lateral gradients may have existed in surface mixed layer r3’Cs concentrations in the Canada Basin during the 1980s. At CESAR, the i3’Cs activity decreases through the surface mixed layer and halocline to values of less than 0.5 Bq/m3 below 500 m. Cesium- 137 activities in all samples collected at water depths below 1000 m were less than 0.3 Bq/m3. Cesium-137 activities similarly decreased through the halocline at the Ice Island locations, although there were several anomalously high values (e.g. 6.3 Bq/m3 at 90 m in 1986) which may represent elevated i3’Cs activities associated specifically with the water layer at the peak of the nutrient maximum. A slightly higher 13’Cs activity (4.4 Bq/m3) was measured in the upper halocline or nutrient maximum water (100-120 m) at CESAR compared to the mean 13’Cs activity (4.0 Bq/m3) in

1455

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island

0 I

10 I

Si (NW

20 I

30 I

40 I

Si (pM)

Si WV 500

I,

13’Cs (Bq /m3)

10 I

20 I

‘%s

30 I

40 I

500

10 I,

I,

(Bq Ima)

20

30 I,

40

50

1

13’Cs (Bq Ima)

500

Fig. 5. Cesium-137 water depth profiles (circles connected by lines) at the Ice Island in 1985, 1986 and 1989 exhibit slightly reduced values through the nutrient maximum (solid circles) and a sharp decline in lower halocline water compared to the CESAR 13’Csprofile.

Table 1. Water depth, salinity (S) and ‘37Cs concentrations (one sigma counting uncertainties) for CESAR Ice Station samples

Water Depth (m)

s

3 25 50 75 100 150 150 200 225 250 300 350 400 500 600 750 875 1000

31.884 31.890 31.900 32.236 33.156 33.575 33.825 34.416 34.420 34.567 34.729 34.802 34.839 34.872 34.879 34.879 34.897 34.909 34.915 34.924 34.924 34.920 34.937 34.942

1100 1200 1300 1330 1500 1740

i3’Cs (Bq/m3) 5.50 5.35 5.18 4.50 4.42 3.57 3.67 3.57 3.20 3.07

* f f k k + + f f +

0.13 0.27 0.25 0.27 0.23 0.30 0.16 0.30 0.13 0.20 1.43 f 0.22

1.05 + 0.18 0.62 + 0.10 0.63 + 0.15 0.28 * 0.12 0.33 f 0.15 < 0.33 < 0.18 0.13 f 0.10 < 0.27 <0.18 <0.22 < 0.17 0.13 * 0.13

105 110 120 130 140 150 175 200 250 270 275 290 300 400 475

Snow 3 5 20 50 60 70 80 90 95 100

Depth

* f f k f

1.35 1.37 0.83 0.23 0.33

34.200 34.320 34.510 34.673 34.712

0.30 0.17 0.28 0.27 0.28

0.30 0.25 0.42 0.37

3.80 + 0.30

33.104

f k & k

3.90 f 0.30 3.82 k 0.30

32.440 32.790

4.17 2.82 2.78 2.57

5.57 f 0.32 5.43 f 0.43

32.156 32.241

33.268 33.513 33.763 33.851

6.23 f 0.73

@s/m’)

“‘Cs

32.266

s

1985

0.62 k 0.13 1.07 + 0.25

0.17 0.28 0.22 0.30 0.18 0.27 0.13 0.18 0.22

34.669 34.701

+ f f + + + k + *

+ 0.35 + 0.40 + 0.20 + 0.30 f 0.35 f 0.22 + 0.27 +_ 0.12 + 0.27

3.78 3.62 2.68 2.65 2.30 2.53 1.83 1.67 0.55

4.97 5.50 4.87 4.88 4.57 4.58 6.30 4.38 3.45

52.1 + 0.5

(Bslm’)

“‘Cs

33.345 33.456 33.641 33.756 33.834 33.886 34.103 34.277 34.605

32.222 32.239 32.381 32.493 32.617 32.769 32.934 33.077 33.191

S

1986

34.759 34.829 34.843

0.73 + 0.18 0.63 + 0.20 0.80 f 0.25

0.40 0.37 0.30 0.28

2.40 1.50 0.77 0.75

33.897 34.204 34.438 34.667

f + f +

4.35 + 0.45

33.298

0.437 + 0.048 0.462 _+ 0.052

0.380 + 0.065

0.505 a 0.048 0.442 rf: 0.045

0.242 + 0.048 0.332 + 0.033

0.413 * 0.057

0.395 * 0.042

0.343 + 0.045

0.593 + 0.050 0.488 + 0.053

0.428 k 0.052 0.420 + 0.063

3.97 f 0.43 3.63 f 0.43

32.915 32.807

0.703 f 0.067

32.424

0.975 + 0.085

4.43 + 0.20 4.23 + 0.40

(mBs/m’)

2’%

0.925 + 0.072

WWm’)

2’opO

1.020 + 0.085

@s/m’)

“‘Cs

31.849

s

1989

3.0 13.5 f 2.3 13.4 + 3.3

13.3 f

15.7 f 2.5

13.2 If: 2.4

9.7 + 2.4

8.9 + 1.2

9.9 $- 1.9

10.4 + 3.0

WWm’)

239.240~”

Table 2. Radionuclhie and salinity resultsfor samples collected at the Ice Island in 1985, 1986 and 1989. Polonium-2 10 and ”'Pb represent dissolved phase activities while 239,240Purefers to an unfiltered (total Pu) sample. Two samples were collected at a water depth of 150 m in 1989, while the single sample collected at 475 m was analyzed in duplicate for 239,240Pu

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island

1457

nutrient maximum water (90-l 10 m) measured (1985-1989) at the Ice Island. However, the most significant difference in 13’Cs profiles at the CESAR and Ice Island locations occurred below the nutrient maximum, where i3’Cs concentrations at the Ice Island rapidly decreased with increasing water depth to values of less than 1.0 Bq/m3 below the 250 m level. In contrast, i3’Cs concentrations decreased much less rapidly through the lower halocline at CESAR, as evidenced by relatively high levels of 13’Cs (3.1 Bq/m3) measured at the 250 m water depth (Fig. 4). This distinction is illustrated in Fig. 6 in which the Ice Island and CESAR 13’Cs data are plotted versus salinity. Cesium-137 concentrations at CESAR are clearly elevated at lower halocline salinities in excess of 33.5 compared to those at the Ice Island. These data can be viewed as representing mixing between Atlantic water (having a salinity of 34.8), water (salinity of 34.2) modified in the Kara and Barents Sea and water (salinity of 33.1) originating in the Chukchi and East Siberian Seas. Differences in the lower halocline, i3’Cs concentrations at CESAR and the Ice Island probably reflect the timedependent 13’Cs input function for the Russian continental shelf, in addition to differences in lower halocline ventilation rates at the two locations. Also illustrated in Fig. 6 are i3’Cs data (Casso et al., 1988; Livingston, 1988) for halocline water at the CESAR and AIWEX ice stations analyzed at the Woods Hole Oceanographic Institution (WHOI). The combined lower halocline 13’Cs data from CESAR indicate the presence of a i3’Cs maximum at a salinity of 34.116, close to the salinity of 34.2 associated with Barents and Kara Sea water. The lower halocline 13’Cs data at AIWEX reflect those of the Ice Island, indicating that the reduced lower halocline i3’Cs concentrations measured at the Ice Island may be typical of the interior of the Canada Basin.

?? -0 0

O0 0

0

O0 +oA A

AIWEX (WHOI) 1985 CESAR (WHOI) 1983 CESAR @IO) 1983

32.0

33.0

34.0

35.0

SALINITY

Fig. 6. Cesium-137 activity versus salinity for CESAR and the combined Ice Island data set. Additional 13’Csdata (Casso et al., 1988) measured at WHOI are from the CESAR and AIWEX ice stations. CESAR 13’Csactivities are elevated in lower halocline (S > 33) compared to both AIWEX and the Canadian Ice Island.

1458

J. N. Smith and K. M. Ellis

Cesium- 137 input function

Rates of diapycnal heat transfer in the arctic are low compared to those for heat loss during ice formation on the continental shelves. The consequent formation of dense water in shelf regions drives the lateral circulation in the halocline. Ventilation rates for the halocline have been estimated in the western Arctic using transient chemical tracers (e.g. tritium: Ostlund, 1982; chlorofluorocarbons: Wallace and Moore, 1985) and ventilation models that stimulate the isopycnal transport occurring between shelf and interior waters. Since 13’Cs circulates through the Arctic Ocean as a mainly conservative water mass tracer, its distribution through the halocline should be similarly consistent with the predictions of lateral ventilation models. However, the applicability of r3’Cs as a tracer is constrained by our knowledge of the history of 13’Cs concentrations in source waters of the arctic continental shelves. Despite recent reports of Russian dumping of radioactive wastes in the Barents and Kara Seas (Yablokov et al., 1993), the two principal sources of 13’Cs in central arctic waters are considered to be fallout from atmospheric nuclear weapons tests and effluent releases from Sellafield and other European nuclear reprocessing plants (Aarkrog, 1993). The earliest radionuclide measurements in the Arctic Ocean were those of “Sr (Bowen and Sugihara, 1964) on surface water collected under the arctic ice in the Canada Basin and in the Chukchi Sea between 1957 and 1962. The 137Csconcentrations in these samples would have ranged from 1 to 1.5 times the “Sr value depending upon the extent to which 13’Cs had been preferentially scavenged in runoff or during transport through shelf regions (Livingston et al., 1984). The paucity of radionuclide data from the Arctic Ocean since that time requires an indirect method to estimate 137Csshelf water concentrations. East Greenland Current. Most Arctic Ocean surface water (AOSW) exits via the East Greenland Current (EGC) on the west side of Fram Strait. Radionuclide measurements conducted in EGC waters during the past 30 years by Aarkrog et al. (1992) can be used as a proxy time series for 13’Cs in AOSW and shelf water. The EGC 13’Cs signal reflects that of AOSW, modified by (i) mixing and radioactive decay of 13’Cs during the 10 year residence time of AOSW in the Arctic (Dorsey and Peterson, 1976) and (ii) the addition of Atlantic water having its own characteristic tracer signal to the extent that water from the northward flowing West Spitzbergen Current recirculates and is entrained into the EGC (Perkin and Lewis, 1984; Quadfasel et al., 1987). From their measurements of Sellafield tracers in EGC waters in 1983, Aarkrog et al. (1983) estimated that the contribution of Atlantic water to the EGC was approximately 50% between 61”N and 66”N. However, the more northerly component of the EGC within Fram Strait itself would have a much reduced component of Atlantic water (Bourke et al., 1987) and be representative of AOSW. Strontium-90 and 13’Cs data (salinities greater than 25 psu) from measurements conducted in EGC water along the East Greenland coast are given as a function of collection date in Fig. 7 (Aarkrog et al., 1992). It is assumed that the 137Cs/g0Srratio in AOSW has been maintained approximately equal to 1.0, compared to the atmospheric fallout value of 1.5, owing to selective scavenging of 13’Cs by suspended particulate material or sediments in coastal regions. Under these conditions, the r3’Cs activities in EGC water for years prior to 1973 can be assumed to be equal to the “Sr activities for those years given in Fig. 7. The 13’Cs shelf source function for upper halocline water, indicated by the dashed

1459

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island

DENS. P.C. WNl~

0 ;

CHUKCM SEA cAN.SAs. BARENTSSEA (bottom water)

A .

: lil

+ I

I

‘nCs SHE-----? INPUT

-r*

P955

FUNCTION

r.’ 1990

1965

1970

1975

1960

1985

1990

YEAR

Fig. 7. Strontium-90 and i3’Cs concentrations measured in Arctic Ocean surface waters and the East Greenland Current since 1955. Sampling locations on the East Greenland coast are Denmarkshavn (77”N; 19”W), Angmagssalik (66”N; 39”W) and Prins Christians Sund (60”N; 41”W). Dashed line corresponds to the 13’Cs shelf input function used in the shelf-interior, mixing box model for the upper halocline. Dotted line corresponds to estimated 13’Cs concentration in Barents Sea bottom water, enhanced by inputs from Sellafield, used as shelf input function for lower halocline water in the mixing box model.

line in Fig. 7, has been estimated from the annual mean 137Csvalues, extrapolating between years for which there are no data. Sellafield inputs. Cesium-137 concentrations in Atlantic water entering the Arctic Ocean by the West Spitzbergen Current were 2-3 times greater than 137Csconcentrations in the EGC, measured simultaneously in the early 1980s as a result of discharges from Sellafield (Casso and Livingston, 1984; Aarkrog et al., 1983; Smith et al., 1990). Cesium-137 concentrations in surface waters of the Barents Sea (Kershaw and Baxter, 1993) were even greater during this period, while Livingston (1988) has reported 137Csvalues in excess of 12 Bq/m3 in bottom water of the Barents Sea in 1985. Because the Barents and Kara Seas are source regions for the formation of lower halocline water (Midttun, 1985; Jones and Anderson, 1986; Anderson and Jones, 1992), the 13’Cs shelf input function should reflect Sellafield 13’Cs transport through this region. The mean 137Csactivity and range of values reported by Livingston (1988) and Casso et al. (1988) for deep water samples (salinities in the range of 34.5-35.0) collected in 1985 from the northeast Barents Sea, proximal to the Santa Anna Trough (Fig. l), are illustrated in Fig. 7. The post-1978 Sellafield contribution to the 13’Cs input function for lower halocline water has been estimated by assuming a constant annual increase in the i3’Cs activity to this mean value of 15.2 Bq/m3. It is assumed that by the CESAR sampling date of 1983, Sellafield i3’Cs had not yet reached the Chukchi-East Siberian Sea source region for upper halocline water and therefore the lower 13’Cs shelf input curve in Fig. 7 can be used to estimate shelf 13’Cs exchange with the upper halocline.

1460

J. N. Smith and K. M. Ellis

Chernobyl inputs. Radioactive contaminants from the Chernobyl nuclear reactor accident in the USSR in late April 1986 were transported to the Canadian Arctic by a route that apparently passed over Greenland (Fudykiewicz, 1988). Water and aerosol samples were collected at the Ice Island between 20 May and 30 May, 1986, approximately 1month after the Chernobyl accident. Analyses of air filters, through which volumes of the order of 500 1 of air had been pumped on the Ice Island, revealed mean ‘37Cs and ‘%s concentrations (unit volume of air) on aerosols of 0.458 mBq/m3 and 0.259 mBq/m3, respectively, while 137Csand 134Csconcentrations in surface layers of fresh snow were 52 Bq/m3 and 24 Bq/m’, respectively. However, levels of 134Csin surface seawater were low, < 0.3 Bq/m3, indicating that the 137Cs water column profile at the Ice Island probably had not been significantly modified by the relatively low inputs of Chernobyl 137Csinto the western Arctic Ocean.

Lateral transport models

Wallace and Moore (1985) and Wallace et al. (1987) employed two lateral transport models, the “freight train” and “mixing box” models, to estimate halocline ventilation rates from chlorofluorcarbon (CFC) distributions at CESAR. The “freight train” model assumes that off-shelf transport is entirely advective, with tracer concentrations in halocline water directly reflecting the shelf tracer concentration during the last year of contact with shelf water. However, 137Cs concentrations in the EGC in the 1960s and 1970s (Fig. 7) were greater (even after correcting for radioactive decay) than 137Cs concentrations in the CESAR and Ice Island haloclines (Fig. 4 and 5), indicating that some reduction of the shelf 137Cssignal by mixing must have occurred prior to, or during, its entry into the halocline. In the “mixing box” model, the halocline is maintained by mixing between shelf and interior water along isopycnals, with each isopycnal cell representing a laterally well mixed, homogeneous water mass. Exchange between interior boxes having a concentration, Ci, with shelf water having a tracer concentration, C,, as a function of time, t, is defined by ddT - (cs ; ci) _ ACi

(1)

where 3,is the radioactive decay constant (0.023 year-‘) for 137Cs.Numerical integration of equation (1) (using a time step of 1 year and an upper integration limit corresponding to the year of sampling) for given values of the shelf exchange time, T, and with C, given by the proxy 137Csrecord (Fig. 7), permits the determination of the interior 137Csconcentration, Ci, as a function of T (Wallace and Moore, 1985). Comparison of model ‘37Cs concentrations with measured water column values at CESAR and the Ice Island then permits the assignment of shelf exchange times, T, to the different water mass layers (Fig. 8). For chemical tracers, such as CFCs, 3He and CCL+,which exchange with the atmosphere, T represents a ventilation age, while for non-volatile tracers, such as 137Cs, T is a fluid residence time whose reciprocal represents the fractional volume of a given interior isopycnal that undergoes mixing with shelf water each year. In Fig. 8, the shelf exchange times, TcS for CESAR are compared to those of the Ice Island, with the latter represented by an envelope of data for the years 1985-1989. The value of T,, (22 years) in nutrient maximum water (100-130 m depth) at CESAR is lower than values (20-40 years) measured in nutrient maximum water (100 m depth) at the Ice Island. TcS increases to values unrealistically high in lower halocline water at the Ice Island, far in excess of the Atlantic layer water residence time of the order of 30 years, while values of TcS

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island

,

1461

CESAR

200 ADVECTION

MODEL)

A+ SELLAFIELD /c::o_, 3001

10

60

20

SHELF &HA&

70

TIME~~NS)

Fig. 8. Cesium-137 shelf exchange times, Tc,, for halocline water at CESAR increase from values of the order of 10 years in the surface mixed layer to values greater than 30 years in the lower halocline. Including Sellafield inputs in the source function for the lower halocline results in higher shelf exchange times, similar to those reported for CFC (F-l 1 and F-12) ventilation ages at CESAR (Wallace and Moore, 1985). Values of Tc, for the Ice Island (1985, 1986 and 1989 data) estimated using the mixing box model alone lie in shaded envelope (labelled Mixing Box) and are greater at the Ice Island compared to comparable water depths at CESAR. However, when advective transport (11 years) to CESAR and the Ice Island is included in the model the envelope of Tc, (Mixing Box + Advection) is shifted to lower shelf exchange times, in better agreement with the CESAR results.

in lower halocline water at CESAR increase to values of 25-40 years. These results suggest that the rate of exchange of shelf and halocline water is lower at the Ice Island compared to CESAR. However, an alternative interpretation which invokes advective lateral transport from shelf to CESAR and Ice Island locations is considered in the following section. Also illustrated in Fig. 8 are ventilation ages, TcFC, determined from applications of the mixing box model to vertical profiles of CClsF (F-11) and CC12F2 (F-12) measured at CESAR by Wallace and Moore (1985), where T cFc is the mean of the F-l 1 and F-12 ventilation ages. The Tcs/TcFc ratio is illustrated as a function of depth through the halochne in Fig. 9. In the upper halocline Tcs exceeds T,-FC,but between 150 and 200 m the two exchange times become equal. Both 13’Cs and CFCs are assumed to be conservative tracers for the production of dense water and subsequent mixing with interior halocline waters, but CFCs exchange across the ocean-atmosphere interface and can, in principle, undergo direct exchange with interior water depending on the degree of saturation and the depth of brine plume formation. Because of their different mechanisms for injection into the water column, variations in the CFC input function may be registered more rapidly in interior waters compared to changes in the i3’Cs input function. Under these conditions, the

1462

J. N.

Smithand K. M. Ellis

-1

!

+o+

1 ioo-

2

1

-

L

I

SELLAFIELO

Fig. 9. The ratio, TQ/TCFC of the ‘37Cs shelfexchangetime fo the CFC ventilationage has been estimated from the mixing box model for CESAR. Dotted points represent shift in ratio caused by Sellafield modification to the ‘37Cs lower ha&line input function.

mixing box model could give lower CFC ventilation ages compared to 137Csshelf exchange times (Doney and Jenkins, 1988) as observed at CESAR. The magnitude of the Tcs/TcFc ratio would depend on the efficiency of CFC exchange with the atmosphere under different oceanographic and climatological constraints. However, if mixing between shelf and interior halocline water occurs in the absence of gas exchange with the atmosphere, CFCs and ‘37Cs would behave similarly and the ventilation age ratio, TCJTCFC would approach unity (Doney and Jenkins, 1988). The decrease in Tcs /T CFC,to values of one or less in lower halocline waters is consistent with the modification of this water on the comparatively deep, ice-covered shelf regions of the Barents and Kara Seas (Midttun, 1985; Moore and Wallace, 1988; Anderson and Jones, 1992) under conditions in which CFC exchange with the atmosphere is significantly reduced. Values of Tcs calculated for lower halocline water, allowing for 137Cs transport from Sellafield through bottom waters of the Barents Sea, are illustrated in Fig. 8. The effect of including Sellafield inputs in the source function for lower halocline water is to produce higher values of Tcs, in closer agreement to the CFC ventilation age. Recent inputs of Sellafield 137Csto lower halocline waters would be expected to produce an increase in the ‘37Cs/90Sr ratio, in excess of the fallout value of 1.5 (Livingston, 1988). However, measurements of the 137Cs/90Srratio on two lower halocline water samples from CESAR (Casso et al., 1988) gave ratios of 1.50 A 0.04 and 1.61 + 0.13, a result that does not prove the presence of Sellafield 137Csin the halocline at CESAR.

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island

LEAD-210,

1463

210Po AND 226Ra RESULTS

Radium-226 water-depth profile

Radium-226 distributions in the water column were similar at the three Ice Island locations in 1985, 1986 and 1989 with the latter results illustrated in Fig. 10. The 226Ra concentration of 1.2 Bq/m3 measured in the surface mixed layer (O-60 m) in 1989 was lower than the value of 1.8 Bq/m3 measured at the CESAR ice station (Moore and Smith, 1986) and similar to values typical of Greenland Sea surface water (Broecker et al., 1976). The 226Ra concentration increased to a maximum value of 2.2 Bq/m3 in the nutrient maximum, a value only slightly lower than values of 2.3 Bq/m3 measured in the nutrient maximum region of the CESAR ice station (Moore and Smith, 1986). Silica values at the nutrient maximum at the Ice Island in 1989 were also only slightly lower than those of 42 PM measured at CESAR, while both sets of data followed the 226Ra:silica relationship observed by Broecker et al. (1976). Lower values of 226Ra, averaging 1.9 Bq/m3, were measured in the nutrient maximum in 1985 and 1986 at the Ice Island, and these were consistent with similarly reduced values (35 PM) of silica (Jones and Anderson, 1990). Below the nutrient maximum, the 226Ra concentration decreased to a mean value of approximately 1.3 Bq/m3, in agreement with results at CESAR (Moore and Smith, 1986). Lead-2 10, 210Po profiles

Dissolved phase 2’oPo and 210Pb profiles for the Ice Island (1989) are compared to the upper water column results from CESAR (Moore and Smith, 1986) in Fig. IO. Lead-2 10 and “‘PO concentrations (0.9 Bq/m3) in the surface mixed layer at the Ice Island in 1989 were significantly greater than levels (0.6 Bq/m3) measured at CESAR (Fig. 10). Atmospheric *l”Pb (Bq I m31

210Po

Fig. 10. Radium-226 water depth profiles at the Ice Island are similar to those measured at CESAR, featuring a distinctive peak in the nutrient maximum. The higher dissolved 21aPb and 2”‘Po levels in the nutrient maximum at the Ice Island compared to CESAR can be explained by ingrowth of *“Pb from 226Raduring an 11year longer transit time to the Ice Island.

1464

J. N. Smith and K. M. Ellis

deposition rates of 21?b are low in the Arctic owing to reduced rates of radon exhalation from the limited soil coverage and low rates of precipitation (Hermanson, 1990; Dibb, 1992). However, 2’?b d eposition rates may be slightly enhanced in Arctic shelf regions compared to the central Arctic, an effect that could account for elevated surface water 21?b concentrations at the Ice Island compared to the CESAR location. For example, the atmospheric flux of 2’%‘b was estimated to be 4.2 Bq/m’ year at CESAR, while higher “%‘b fluxes, 5-30 Bq/m2 year, have been generally measured in soils and snow fields on arctic land masses (Windom, 1969; Weiss and Naidu, 1986; Nijampurkar and Clausen, 1990). The most significant difference between 210Pb and 21?o profiles at the Ice Island and CESAR is in the region of the nutrient maximum and upper halocline. The pronounced minimum in 210Pb and 21@Pomeasured in upper halocline water at CESAR is absent from the Ice Island data where 210Pb and 21%o decrease to relatively constant values of approximately 0.5 Bq/m3 in the nutrient maximum, values that are generally maintained throughout the lower regions of the water column. The 21%b minimum was interpreted by Moore and Smith (1986) as having been caused by accelerated transfer of dissolved 210Pb to the particulate phase during modification of Pacific Ocean water over the arctic continental shelves to form nutrient maximum water. Enhanced scavenging of dissolved 21%‘b in nutrient maximum water has been ascribed to higher concentrations of particles in the shallow shelf waters or to the influence of Mn oxides produced by the oxidation of sedimentsupplied reduced Mn. Particulate “‘Pb and “‘PO activities were extremely low at the Ice Island, and most values were below the detection limit of 0.02 Bq/m3. The absence of particulate 21’%b and 210Podata eliminates the option of calculating vertical particle transport rates. However, the disequilibria between the dissolved 210Pbactivity, A pb, with respect to the activity, Aa,, of its 226Ra precursor can be used to derive an approximate residence time for “%‘b, relative to its transfer to particles. The dependence of the dissolved *l?b activity, Apb, in a given volume element of water on time, t, can be expressed by d&b -==hARa-~Apb-dt

Apb ‘5P

(2)

where the first term on the right hand side of equation (2) represents the rate of production of 210Pb by radioactive decay from 226Ra, the second and third terms represent the rate of 210Pb removal by radioactive decay and net uptake onto particles, respectively, ;1 is the radioactive decay constant (0.0311 year-‘) for ‘l”Pb, and zp is the residence time of dissolved 210Pb with regard to uptake onto particles. The net particle removal flux of ‘l”Pb would include contributions from desorption and re-adsorption of 210Pbassociated with the downward flux of particles from the overlying waters. Under steady state conditions, equation (2) can be set equal to zero and solved for zp R

” =h(l

-R)

where R is the &,/ARa activity ratio. As noted above, the CESAR results were interpreted as an advective feature with the deficiency of “‘Pb, with respect to 226Ra, resulting from scavenging of the former during transport across shelf regions. However, subsequent ingrowth of 210Pb from 226Ra during transport of nutrient maximum water to the Ice Island locations could have resulted in the elevated levels of dissolved 21?b measured in the Ice Island nutrient maximum. Let us

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island

1465

assume that a “freight train” model applies to transport between shelf regions and the CESAR and Ice Island haloclines, and that the water column maintains its integrity with no lateral infusions of water. The transit time difference, at, for the two locations can then be calculated by integrating equation (2) and solving for t for both CESAR and the Ice Island (assuming the same integration constant in each case), and determining the difference (i.e. 6t =

~Ioe-Island

-

kESARh

give%

1 1 - &es(l + 1lArp) Jr = (A + l,rp)ln 1 - RI& + l/Arp)

(4)

where RI, and Rces represent the APIJAR~ activity ratios at the Ice Island and CESAR ice station, respectively. The calculation of 6t requires a value for zp that cannot be determined simply from values of ARa and Apb in the nutrient maximum layer, owing to the absence of a steady state in ‘i”Pb production and removal. However, the similar values of ARK and Apb in lower halocline water at the Ice Island and CESAR locations suggest that a steady-state in the 210Pb activity has been established at these water depths, and can be characterized by approximately the same value of rp (= 16 years) at each location. If it is further assumed that this value of z,, is equally applicable to the net uptake of 210Pb onto particles in the nutrient maximum layer following the off-shelf transport of this water mass, then equation (3) can be solved to give a value of 6t = 11 years. These assumptions are probably valid if the particle assemblages responsible for the scavenging of 210Pb in regions of relatively high productivity over the shelves have been depleted prior to arrival of nutrient maximum water at the CESAR location. Although the concentration of suspended material in nutrient maximum water was higher in 1985 at the Ice Island (22 @g/l;Yeats and Westerlund, 1991) compared to CESAR (5.3 pg/l; Bacon et al., 1989) in 1983, concentrations at both locations were extremely low and would be expected to result in very inefficient scavenging of particlereactive radionuclides. Circulation model The Canadian Ice Island may have traversed a boundary region between branching currents of upper halocline water, one flowing toward Fram Strait and the other flowing toward the west and through the Canadian Archipelago (Jones and Anderson, 1990) closely following the anticyclonic gyre in surface water circulation indicated by ice drift (Colony and Thorndike, 1984). Jones and Anderson (1990) interpreted their observations in 1986 of slightly warmer halocline temperatures and a broader nutrient maximum at the Ice Island compared to CESAR, to imply that greater mixing of halocline and Atlantic layer water had occurred by the time of arrival of this water at the Ice Island. Using a vertical diffusion model, they estimated that the temperature profile at the CESAR ice station would change to match that of the Ice Island in approximately 10 years and suggested that this represented the transit time for halocline water between the CESAR and Ice Island locations. Yeats and Westerlund (1991) also concluded that this quantity of mixing would explain the decrease in trace metal (Cd, Zn, Ni) concentrations in nutrient maximum water observed between CESAR and the Ice Island. Mixing between nutrient maximum and Atlantic layer water also would have diminished the disequilibria in 226Ra and “‘Pb concentrations observed at CESAR by the time this water arrived at the Ice Island. However, this effect would have contributed less than 2 years to the estimated transit time,

1466

J. N. Smith and K. M. Ellis

&, of 11 years between CESAR and the Ice Island, even allowing for a 15% change in each radionuclide concentration. The longer transit time to the Ice Island, compared to CESAR, also explains discrepancies between 13’Cs shelf exchange times determined from the mixing box model which were greater for halocline water at the Ice Island compared to CESAR. A more realistic, two-phase model invokes mixing between shelf and interior water over the Russian continental shelf followed by advective transport (assuming negligible diapycnal mixing) through interior waters of the Canada Basin. Under these conditions, the halocline 13’Cs concentrations at the Ice Island would be equivalent to halocline 13’Cs concentrations 11 years earlier at CESAR, diminished only by radioactive decay. If the Ice Island 13’Cs profiles are decay-corrected by a radioactive decay time of 11 years to make them comparable to those measured at CESAR, then the box mixing model can be applied to these profiles using an upper limit of integration of equation (1) (with the i3’Cs shelf input function in Fig. 7) as the year of sampling minus 11 years (i.e. 1978 for the 1989 data set). The new values of the shelf exchange time, T, calculated for the 1985, 1986 and 1989 Ice Island profiles generally lie within the envelope illustrated in Fig. 8 and, in the upper halocline, are in reasonable agreement with values estimated for CESAR. Lower halocline values of Tare still much greater at the Ice Island compared to CESAR, despite factoring in an advective transport time between the two locations of 11 years. However, these differences could reflect still greater differences in transport times for the advection of lower halocline water from the Barents and Kara Sea shelf regions to the Ice Island compared to the CESAR location. Plutonium-239,240 profile

The 239,240Puwater depth profile at the Ice Island in 1989 was relatively uniform in the upper halocline, with the 239,24aPuactivity being equal to 0.010 Bq/m3 in the nutrient maximum (Fig. 11). Plutonium-239,240 increased to a mean value of 0.013 Bq/m3 in the lower halocline and upper Atlantic layer water. The 239*240Puconcentrations at the Ice Island are in agreement (within analytical and sampling uncertainties) with those (Fig. 11) measured at the LOREX Ice Station in 1979 over the Lomonosov Ridge in the central Arctic Ocean (Livingston et al., 1984). Plutonium-239,240 is a fallout tracer derived from atmospheric nuclear weapons tests and is efficiently scavenged by particles in surface water, shelf environments or other regions of high productivity. The effect of scavenging on 239*240Puprofiles is best evaluated by considering deviations in the 239V240Pu/‘37Cs ratio from the global fallout average of 0.021 (decay corrected to 1989). The low value of this ratio of 0.002 measured in surface water at the Ice Island (Fig. 1 l), is caused by scavenging of 239*240Pu relative to 13’Cs and is consistent with values measured in surface water collected north of the Barents Sea in 1980 at approximately the same latitude (79”-82”N) as the Ice Island (Holm et al., 1986). The increase in the 239*24?u/‘37Cs ratio to a mean value of 0.017 with increasing water depth is a feature typical of measurements conducted during the 1980s in the North Atlantic (Cochran et al., 1987) and North Pacific (Nagaya and Nakamura, 1987) water columns. This increase in the 239,240Pu/‘37Csratio is generally associated with the uptake of 239,240Puonto particles in surface waters and subsequent release from sinking particles during their descent. This mechanism would be expected to be of reduced importance in the Arctic Ocean where productivity and particle concentrations are extremely low. The subsurface increase in the

Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island m.zaPu

(Bq /

1467

m3)

; FALLOUT

-ob ICE ISLAND1089 L LOREX 1972

-

i

. .:.:.:... ****.:::I.: .......i.........

Fig. 11. Plutonium-239,240 concentrations and the 23g*2%/‘37Cs ratio measured at the Ice Island in 1989 are in reasonable agreement with measurements conducted at the Lorex Ice Station in 1979. An increase in 23g*2% concentrations and, particularly the 23g*2%/137Cs ratio, with increasing water depth reflects both the downward transport of 23g*% on particles and enhanced 23g,2@Pu removal from surface mixed layer and upper halodine water duringmodificationin continentalshelf regimes. The 23g,2%/‘37Cs ratio in fallout, decay corrected to 1989 is indicated by dotted line.

239,240Pu/137Csratio could also be an advective feature, having been previously established in Atlantic layer water prior to, or during, its transport through the Arctic Ocean. In 1972, 239,240Pu/137Csratios in surface water in the Greenland Sea were 0.009 (Livingston et al., 1984). This value is equivalent to a ratio of 0.013 (decay corrected to 1989), which is well within analytical uncertainties of the Ice Island, lower halocline results. However, lower halocline 239,240 Pu/137Cs ratios at the Ice Island had not yet been affected by more recent additions of Atlantic water to the Arctic Ocean, as the 239,2‘?u/137Cs ratios in Atlantic water in the Nansen Basin had decreased to levels of 0.002-0.003 by 1987, owing to elevated ratios in inputs of ‘37Cs from Sellafield (Livingston et al., 1993). Although 239*240Pu/137Cs lower halocline and Atlantic layer source water at the Ice Island are consistent with historical values measured in Atlantic source water, they still may have been increased to some extent by 239,240Purelease from sinking particles. CONCLUSIONS 1. Cesium- 137 profiles measured on the Canadian continental shelf from the Canadian Ice Island in 1985, 1986 and 1989 all exhibit similar surface mixed layer concentrations (4.4-6.2 Bq/m3), decreasing to values of less than 0.5 Bq/m3 in lower halocline water below 200 m. A 137Csprofile measured at the CESAR ice station in 1983 has similar values (5.3 Bq/m3) in the surface mixed layer, but significantly elevated concentrations (3.2 Bq/m3), compared to the Ice Island, in lower halocline water below 200 m. 2. A two component, mixing box model was applied to estimate exchange times between

1468

J. N. Smith and K. M. Ellis

shelf and interior halocline waters. The historical record of seSr and 13’Cs concentrations in AOSW leaving the Arctic Ocean in the East Greenland Current was used as a proxy for the shelf 13’Cs input function. Shelf exchange times, Tcs, were significantly greater in halocline water at the Ice Island compared to CESAR. However, when a transport time difference of 11 years for advection to the CESAR and Ice Island locations is factored into the mixing box model results, upper halocline shelf exchange times estimated for the Ice Island were similar to those at CESAR. At CESAR, Tcs increased with increasing water depth to values in excess of 30 years in the lower halocline while the ratio of Tcs to the mean CFC ventilation age ( Tcs/ TCFc) at each water depth decreased from 4 in the upper halocline to less than unity in the lower halocline. 3. Radium-226 activities exhibit maxima, consistent with silica values, in the nutrient maximum at both CESAR and the Ice Island. Dissolved 210Pband 21ePo levels are extremely low in the nutrient maximum at CESAR owing to scavenging by particles during modification of this water in shelf environments, but they are significantly elevated in nutrient maximum water at the Ice Island. By invoking ingrowth from 226Ra as the principle cause of elevated 21sPb/226Ra ratios at the Ice Island, a transit time difference of 11 years was estimated for the lateral advection of upper halocline water to CESAR and the Ice Island. 4. Plutonium-239,240 has been depleted in upper halocline water (and surface mixed layer water) at the Ice Island, relative to 13’Cs, owing to preferential uptake onto particles in shelf regimes. Ratios of 23g*2‘?u/137Cs increase in lower halocline and Atlantic layer water to approach values typical of fallout ratios as a consequence of both downward transport of 23g,24ePuon particles from surface water and to reduced scavenging of 23gV240Pu from lower halocline source water during its interaction with continental shelf regimes. Acknowledgements-The authors thank Richard Nelson (DFO) who assisted in a great deal of the field sampling work at the Canadian Ice Island and Grazyna Folwarczina and Jim Abriel at BIO who performed many of the laboratory analyses.

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Bacon M. P., C. Huh and R. M. Moore (1989) Vertical profiles of some natural radionuclides over the Alpha Ridge, Arctic Ocean. Earth and Planetary Science Letters, 95, 15-22. Bourke R. H., J. L. Newton, R. G. Paquette and M. D. Tunnicliffe (1987) Circulation and water masses of the East Greenland Shelf. Journal of Geophysical Research, 92(C7), 6729-6740. Bowen V. T. and T. T. Sugihara (1964) Fission product concentrations in the Chukchi Sea. Arctic, 17, 198-203. Broecker W. S., J. Goddard and J. L. Sarmiento (1976) The distribution of Ra-226 in the Atlantic Ocean. Earfh and Planetary Science Letters, 32, 220-235.

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Casso S. A. and H. D. Livingston (1984) Radiocesium and Other Nuclides in the Norwegian-Greenland Seas, 1981-1982. Tech. Rep. WHOI-84-40, Woods Hole Oceanographic Inst., Woods Hole, MA, USA, 101 pp. Casso S. A., W. R. Clarke, H. D. Livingston, J. M. Pahnieri and L. D. Surprenant (1988) Cesium and strontium isotopes in the northwestern North Atlantic and Arctic Ocean, 1981-1985. Tech Rep. WHOI-88-8, Woods Hole Oceanographic Inst., Woods Hole, MA, USA, 47 pp. Cochran J. K., H. D. Livingston, D. J. Hirschberg and L. Surprenant (1987) Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean. Earth and Planetary Science Letters, 84, 135 152.

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Dibb J. E. (1992) The accumulation of *lePb at Summit, Greenland since 1855. Tellus, 44B, 72-79. Doney S. C. and W. J. Jenkins (1988) The effect of boundary conditions on tracer estimates of thermocline ventilation rates. Journal of Marine Research, 46, 947-965. Dorsey H. G. and W. H. Peterson (1976) Tritium in the Arctic Ocean and East Greenland Current. Earth and Planetary Science Letters, 32, 342-350. Hermanson M. H. (1990) *i’Pb and 13’Cs chronology of sediments from small, shallow Arctic lakes. Geochimica et Cosmochimica Acta, 54, 1443-1451.

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Livingston H. D., K. 0. Buesseler and J. K. Cochran (1993) Radionuclides in the water of the Arctic Shelf and Interior: 1979-1987. In: Environmental radioactivity in the Arctic and Antarctic, P. Strand and E. Holm, editors, Proceedings of the International Conference on Environmental Radioactivity in the Arctic and Antarctic, Kirkenes, Norway, pp. 257-260. Macdonald R. W. and D. J. Thomas (1991) Chemical interactions and sediments of the western Canadian Arctic Shelf. Continental Shelf Research, ll(&lO), 843-863. Melling H. and E. L. Lewis (1982) Shelf drainage flows in the Beaufort Sea and their effect on the Arctic Ocean pyonocline. Deep-Sea Research, 32, 1233-1241. Midttun L. (1985) Formation of dense water in the Barents Sea. Deep-Sea Research, 32, 1233-1241. Moore R. M. and J. N. Smith (1986) Disequilibria between U6Ra, “‘?b and *?o in the Arctic Ocean and the implications for chemical modification of the Pacific water inflow. Earth and Planetary Science Letters, 77, 285-292.

Moore R. M., M. G. Lowings and F. C. Tan (1983) Geochemical profiles in the central Arctic Ocean: Their relation to freezing and shallow circulation. Journal of Geophysical Research, 88(C4), 2667-2674. Moore R. M. and D. W. R. Wallace (1988) A relationship between heat transfer to sea-ice and temperaturesalinity properties of Arctic Ocean waters. Journal of Geophysical Research, 93&l), 565-571. Nagaya Y. and K. Nakamura (1987) Artificial radionuclides in the western northwest Pacific (II): 13’Cs and 239.2%i inventories in water and sediment columns observed from 1980 to 1986. Journal of the Oceanographic Society of Japan, 43, 345-355.

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