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Scavenging of Thorium Isotopes in theArctic Regions: Implications for theFate of Particle-reactive PollutantsM. BASKARAN*Department of Geology, Wayne State University, 0224 Old Main Building, Detroit, MI 48202, USA
The sources of inorganic pollutants to the Arctic areas arereviewed using previously published results. The removalof particle-reactive pollutants is discussed using thoriumscavenging as an analog. The scavenging of 234Th from theupper water column (�100 m) and sediment inventory of230Th from the deep Arctic waters is compared to di�erentocean basins in the subarctic areas. Such a comparisonshows that 234Th is in equilibrium with its parent, 238U, incertain regions of the Canada Basin of the Arctic Ocean,while it is de®cient in other regions of the arctic as well asin sub-polar ocean basins. This implies that the particle-reactive pollutants in the deep Arctic of the Canada Basinare less likely to be removed from the deep waters and willeventually be transported out of this area. We have uti-lized the 230Th inventory in sediments from the Arctic areato determine the removal rates of particle-reactive nuc-lides. The 230Th inventory in the deep Arctic Ocean of theCanada Basin is much lower than the Norwegian Sea andthe Fram Strait of the Arctic as well as all other sub-polarworld oceans. These observations suggest that any pollu-tants into the deep Arctic areas of the Canada Basin areless likely to be removed locally and may be transportedout of this area. In those areas, the colloidal materialcould potentially play a major role in the removal ofparticle-reactive contaminants. Ó 2001 Elsevier ScienceLtd. All rights reserved.
Introduction
The organic and inorganic pollutants in the Arctic areprimarily introduced through discharges into Arcticrivers (from sewage and industrial e�uents) and/orthrough runo�, atmospheric fallout and from the watermass transport from the North Atlantic Ocean. Of thesepollutants, the particle-reactive ones are removed pri-marily by the removal of particles (sedimentation) whilethose that have low particle reactivity, follow the watermasses and eventually exported out of the Arctic to
other oceanic regimes. Biological processes and abio-logical particles primarily remove the particle-reactivepollutants by scavenging.
Most of the water ¯ow into the arctic is derivedfrom the North Atlantic that comes from the directionof o�shore America and Western Europe. The in¯owsand out¯ows to the Arctic Ocean are given in Table 1.The northward ¯ow through the Fram Strait is esti-mated to be �12:7� 106 m3 sÿ1�� 12:7 Sv�. The in-¯ow from the Paci®c through the Bering Strait isabout 9% of the water that ¯ows through the FramStrait (Tolmazin, 1985). It is probable that millions oftons of pollutants of agrochemical and industrial ori-gin from America and Western Europe reach theArctic through the Fram Strait. The riverine ¯owfrom Arctic rivers is estimated to be 0:15 m3sÿ1 whilethe amount of precipitation directly into the Arcticwaters is estimated to be � 0:114 m3sÿ1 (Tolmazin,1985). The ®ve major rivers, the Yenisey, Ob, Koly-ma, Lena and Mackenzie rivers drain a continentalarea of more than 107 km2 (Rey, 1982). The six Arcticseas that receive e�uents from Russia are: White sea,Barents Sea, Kara Sea, East Siberian Sea, Laptev Seaand Chukchi Sea. During the annual maximum ofsea-ice cover, the pack ice reaches an area of 107 km2,which is 30% of the world's sea ice.
In the Arctic Ocean, the shallow seas (4:1� 106 km2)are seasonally ice-covered and the deep basins(8:2� 106 km2) are usually covered by 3±5 m of icethroughout the year (Wheeler et al., 1997). Ice-coverprevents the atmospherically delivered pollutants fromdirectly reaching the water-column. The melting of theice and the displacement of the ice-cover, either by windor moving ice along with ice-rafting of bottom sedi-ments in shallow seas (as well as lakes), eventually leadsto the redistribution of sediments as well as sediment-sorbed particle-reactive pollutants (Nichols, 1967; Her-manson, 1990). In addition, ice rafting has beenreported to be the dominant mechanism responsible forthe transport of ®ne-grained contaminant-laden sedi-ments from coastal zones to the deep Arctic Basin (e.g.,Barnes et al., 1982; Kempema et al., 1989; Reimnitz
Marine Pollution Bulletin Vol. 42, No. 1, pp. 16±22, 2001
Ó 2001 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0025-326X/01 $ - see front matterPII: S0025-326X(00)00194-6
*Tel.: +1-313-577-3262.E-mail address: [email protected] (M. Baskaran).
16
et al., 1993; P®rman et al., 1995; Cooper et al., 1998;Landa et al., 1998).
The primary productivity is maximum during the ice-free, three summer months and the productivity is nearzero during the remaining nine months. A sediment trapdeployed in the Eastern Chukchi Sea (66°370 N) indi-cated that the average mass ¯ux during the ice-covermonths (measured in February) was about 10% of themagnitude of the mass ¯ux during the ice-free months.The dust particles that are delivered to the sea surfaceare retained on the sea ice-cover; thus the suspendedparticle concentration in the ice-cover months is muchlower than in most temperate regions. The particleconcentration in the deep (>3000 m water depth) Arctic(Canada Basin) is �5 lg lÿ1, (Bacon et al., 1989b)compared with 102±103 lg lÿ1 in other places. Since theparticles play a major role in the removal of particle-reactive pollutants, lower particle concentration plays amajor role in decreased removal rates of pollutants.
Particle-reactive Radionuclide Scavenging
Quantitative information on the intensity of scav-enging can be estimated from the disequilibrium be-tween the pairs 234Th and 238U, as well as 230Th and 234U.It has been shown that in the upper 100 m of subarcticwaters, the activity of 234Th is not in equilibrium with itsparent 238U, which is attributed to the removal of 234Thby particles (both abiotic and biotic) on the time scale ofdays to months (Bhat et al., 1969; Coale, 1985, 1987;Baskaran et al. (1995) and the references therein). Thedisequilibrium between 234Th and 238U in the arcticwaters will yield information on the rates of removal ofparticle-reactive pollutants from the Arctic waters. Theresidence time of 230Th in the open ocean is <100 yr(Anderson et al., 1983a,b), which is much shorter thanits half-life (75 200 yr). After its removal from the watercolumn by sorption onto particles, 230Th reaches thesediment. Thus, if the 230Th sorbed particles reach thesediment, where 230Th was removed, then its de®ciencyin the water column (234U±230Th) should be found in thesediment. The ratio of 230Th inventory in sediments to
the production of 230Th is a measure of vertical scav-enging of 230Th in that particular site. In areas wherelateral advection is high and/or the scavenging is veryslow, it is likely that most of in situ-produced 230Th islaterally transported to other oceanic regimes.
In this paper, a review of the sources of pollutants tothe Arctic is attempted. In addition, the disequilibriumbetween 234Th and 238U, as well as the residence times ofdissolved and particulate 234Th in the upper 100 m watercolumn of the major ocean basins is compared to thedeep Arctic basin. The sediment inventory of 230Th inthe world-ocean is compared to the corresponding val-ues in the Canada and Nansen Basins of the Arctic aswell. The goals and objectives of this paper are todelineate the di�erences in scavenging intensity of par-ticle-reactive nuclides between the subarctic and Arcticwaters as well as the di�erences between di�erent basinswithin the Arctic region.
Sources and Fate of Pollutants to the Arctic
Atmospheric input of pollutants in to the Arctic watersThe Arctic atmosphere is expected to be relatively
pollution-free due to its remoteness and distance frommajor population and industrial centers in the Arctic.However, atmospheric transport of contaminants maybe an unde®ned source of contaminants as has beenreported. The Arctic atmosphere is very hazy duringwinter and early spring. This haze appears to be a carrierof acidity, trace metals, organics, etc., derived fromlong-range transport of midlatitude pollution products(Rahn, 1981; Barrie, 1986; Talbot et al., 1992). The ®l-ters that were used to collect aerosols in the Arcticduring winter were found to be gray in color as opposedto the summer aerosols, which are colorless. This graycolor was attributed to the sooty carbon derived fromthe lower latitudes (Rahn, 1981; Barrie, 1986; Talbotet al., 1992). In marine environment, the atmosphericaerosols that are primarily removed by wet precipitationand dry fallout, deposit into the pack ice and are sub-sequently released into the water column. Due to thesmaller size of the Arctic aerosols (because these aero-sols are well-aged and are derived from midlatitudes),the dry fallout should be of less importance; on the otherhand, low amount of precipitation will make the wetdeposition less important. Rahn (1982) concluded thatdry and wet deposition is likely to have approximatelythe same relation in the Arctic as in midlatitudes.
The concentrations of vanadium in the aerosols havepreviously been used as a pollutant index. The twomajor sources of vanadium to the aerosols includesmoke as a result of combustion of fossil fuels and thesoil dust. Higher concentrations of vanadium and sul-fate (SO2 derived from the mid-latitudes and are sub-sequent conversion to sulphate) in the Arctic aerosolsduring winter were observed. It has been reported that90±95% of the vanadium in the Arctic aerosols duringwinter is pollution-derived (Rahn, 1982). In addition,
TABLE 1
Water mass balance for the Arctic Ocean (Tolmazin, 1985).
In¯uxes Transport (Sv) Percent of total
In¯ux from Atlantic Ocean 12.68 89.4In¯ux from Paci®c Ocean 1.14 8.8Precipitation 0.114 0.8River runo� 0.149 1.0
Total 14.1 100.0
Out¯owsDischarge into Atlantic Ocean13.8
98.2
Transport of ice 0.19 1.4Evaporation 0.034 0.4
Total 14.0 100.0
17
Volume 42/Number 1/January 2001
210Pb in Arctic aerosols during the winter months atBarrow, Alaska are highest compared to values fromother places (Rahn and McCa�rey, 1980). The 222Rnexhalation rate from the ocean surface is about twoorders of magnitude slower than that of the continents(Turekian et al., 1977; Baskaran and Naidu, 1995 andthe references therein). Thus, if these aerosols were de-rived from the marine air, the 210Pb concentration wouldbe much lower. The Rn exhalation rate from the per-mafrost-laden soils is considerably lower than the valuesreported for other continental settings (Baskaran andNaidu, 1995). From these observations, it is likely thatmost of the 210Pb is derived from the horizontal advec-tion from middle latitudes within the lower layers of thetroposphere.
Another evidence for horizontal advection is sug-gested by the concentrations of 7Be in aerosols. The 7Bedata for Arctic sites show strong seasonal variationswith peak concentrations in the late winter or earlyspring (Feely et al., 1989). The peak concentration of7Be in air samples coincides with arrival of Arctic hazethat is derived from mid latitude regions of Asia, Europeand North America (Rahn, 1981). It is believed that 7Be(and likely 210Pb) enters the lower troposphere of theArctic as a result of horizontal advection from middlelatitudes within the lower layers of the troposphere(Feely et al., 1989). Thus, it is likely that 210Pb producedfrom degassed 222Rn in the Arctic is not only from the222Rn that emanated from the soil in the Arctic (which islikely to be a minor component), but also from thehorizontal advection from middle latitudes within thelower layers of the troposphere. More recent study byBaskaran and Shaw (2000) indicates that the contami-nant-laden aerosols in the Alaskan Arctic region have arelatively longer residence time during the wintertime.
Riverine input of pollutants to the Arctic watersMost of the riverine input into the Arctic Ocean
comes from the former USSR territories. The total an-nual riverine ¯ow into the Arctic Ocean is estimated tobe 3:75� 1012 m3, which is about 2% of the oceanic¯ow into the Arctic Ocean from the North Atlantic andabout three times greater than the annual precipitationinto the Arctic ocean (Dorsey and Peterson, 1976; Rahn,1982). The relative amounts of pollution through at-mosphere, rivers and oceanic input can be evaluatedfrom a consideration of the inputs of Pb, given in Rahn(1982), into the Arctic. The annual input of Pb from theworld ocean, mainly through the North Atlantic, is es-timated to be 4:1� 103 metric tons (MT) and is about athird of the riverine input (11� 103 MT). The averageannual atmospheric input is also signi®cantly lower thanthe world ocean input (0:88� 103 MT).
Role of sea ice in the transport of contaminants in theArctic Ocean
Ice rafting is a dominant mechanism responsible forthe transport of ®ne-grain-laden contaminants from
coastal zones to the deep Arctic Basin (Barnes et al.,1982; Kempema et al., 1989; Landa et al., 1998). Forexample, ®ne ice crystals that form in suspension insupercooled, turbulent water (known as Frazil ice) hasbeen shown to scavenge ®ne-grained sediments as wellas particle-reactive contaminants from the water col-umn. Transport of continental shelf sediments in sea icefrom shelf areas to the deeper basins could play a sig-ni®cant role in biogeochemical cycling of radionuclidesand other contaminants.
Scavenging Rates of Particle-reactive Pollutants
The scavenging of particle-reactive pollutants in theArctic Ocean critically depends on suspended particleconcentrations in the water column. In areas, where theparticle concentrations are high, the particle-reactivepollutants are rapidly transferred to the sediments. Ithas been shown that the removal behavior of 228Th fromthe water column by the rapid attachment to suspendedparticles and subsequent removal to the sediments isanalogues to Fe, Cr(III) and Po from the water column(Santschi et al., 1984). The production, transport andsettling of particles in polar oceans involves intriguingproblems. The seasonal variations in primary produc-tivity are very large due to solar insolation (heat enteringthe atmosphere, ocean and land from the sun) patternsin the Polar Regions. It has been shown that the prin-cipal components of polar ocean ¯uxes are controlled bybiological interactions in the surface layers, even thoughsedimentation of coarse ice-rafted rock detritus involvesonly physical processes (Honjo, 1990).
The intensity of scavenging can be investigated usingthe disequilibrium between 234Th and its parent, 238U. Inmost world oceans, in the upper 100 m of the watercolumn, there is a de®ciency of 234Th with respect to238U. This de®ciency can be used to investigate the ex-tent of disequilibrium and to determine scavenging rateconstants. The published data on 234Th de®ciency fromthe world ocean, including one station from the Arctic,are presented in Table 2. The average residence time ofdissolved (ss) and particulate Th (sp) in the upper 100 mis calculated using the equations
ss � 234Thd= �238U�
±234Thd�kTh
� �1�and
sp � 234Thp=f �238U� ÿ 234Thd ÿ 234Thp
�kThg; �2�
where kTh is the 234Th decay constant (0.029 dÿ1), �238Ud�and �234Thd� are 238U and 234Th dissolved activity con-centrations (dpm lÿ1), respectively, and 234Thp are theparticulate 234Th concentrations.
The calculated average residence times of dissolvedand particulate 234Th for the upper 100 m of the worldocean are also presented in Table 2. The deep CanadianBasin Arctic is the only place where 234Th is in radio-active equilibrium with 238U. However, recent studies
18
Marine Pollution Bulletin
TABLE
2
Inventories
ofparticles(l
gcmÿ2),dissolved
andparticulate
234Th(dpm
cmÿ2)and
238U�d
pm
cmÿ2)andaverageresidence
times
ofdissolved
andparticulate
234Thin
theupper
100m
ofthemajorocean
basins.
Area
Particle
inventory
(lg
cmÿ2)
Dissolved
234Th
inventory
(dpm
cmÿ2)
Particle
234Th
inventory
(dpm
cmÿ2)
238U
inventory
(dpm
cmÿ2)
234Thde®ciency
(dpm
cmÿ2)
Dissolved
234Th
residence
time(d)
Particulate
234Th
residence
time(d)
Reference
Paci®c±VERTEX
II�1
8°N
109°W
NA
14.99�1.02
3.73�0.22
24.31
5.59�1.04
56�11
23�4
Coale
andBruland
(1987)
Paci®c±VERTEX
III
�16°N
107
°WNA
14.51�
0.61
3:3
9�
0:1
224.28
6:3
8�
0:6
252�
51
8�
2Coale
andBruland
(1987)
Paci®c±VERTEX
IV�2
8°N
155
°WNA
20:2
0�
0:5
12:7
6�
0:1
824.87
1:9
1�
0:5
4150�
23
50�
15
Coale
andBruland
(1987)
EasternTropicalPaci®c
11°310 N
87°16W
(628m)
NA
17.26
)23.59
6.33
95
)BaconandRutgers
vander
Loe�
(1989a)
GulfofMexico
470(summer)
19:4
6�
1:1
51:3
9�
0:0
625.67
4:8
2�
1:1
5109�
33
10�
2Baskaranet
al.(1996)
(26°410 N
;94°590 W
(1500m)
700(spring)
16.90�
0.86
0:9
1�
0:0
525.32
7:5
1�
0:8
670�
12
4�
1Baskaranet
al.(1996)
NorthAtlantic
47°05.40 N
;19°55.1W0
20:1�
2:6
3:5
0�
0:4
224.7
1:1
0�
2:6
3152�
88
111�
266
Buesseleret
al.(1992)
47°05.40 N
;19°55.1W0
16:5�
2:0
5:1
8�
0:5
024.7
3:0
2�
2:0
670�
19
60�
41
47°05.40 N
;19°55.1W0
15:5�
1:3
3:4
1�
0:3
524.7
5:7
9�
1:3
559�
10
20�
547°05.40 N
;19°55.1W0
13:6�
0:8
3:4
6�
0:3
224.7
7:6
4�
0:8
643�
46�
2CESAR
IceCamp,Arctic
Ocean
85°5
00 N
108
°500 W
54.3
22.4
0.33
22.7
)0.03
2597
)Baconet
al.(1989b)
19
Volume 42/Number 1/January 2001
have indicated that there is active scavenging in theChukchi plateau due to higher suspended particle con-centrations and particle ¯uxes (Moran et al., 1997; Ed-monds et al., 1998). In areas where suspended particleconcentrations are very low (as well as low sedimenta-tion rates, as reported by Ku and Broecker (1967);Finkel et al. (1977); and Bacon et al. (1989b)), it is likelythe colloidal material will play a signi®cant role in metalscavenging. Wheeler et al. (1996) have reported thedissolved organic carbon concentrations (which is usedas a proxy for marine colloids, as most of the colloidalmaterial is organic in composition) comparable to mostother ocean basins. The dissolved residence time, cal-culated using Eq. (1), is the longest for the CESAR IceCamp station in the Canada Basin. This observation isin contrast with the results of Cochran et al. (1995), whofound that Th residence times in the Nansen Basinsurface waters were comparable to the values obtainedfor open North Atlantic Ocean waters (Broecker et al.,1973; Li et al., 1980). This di�erence between the Nan-sen Basin surface water and Canada Basin water is at-tributed to the di�erences in the particles ¯uxes betweenthe two basins and/or the di�erences in the ventilationtime scales between the two basins (Cochran et al.,1995). The suspended particle inventory in the watercolumn at this station is approximately an order ofmagnitude higher than the values reported for the Gulfof Mexico (Baskaran et al., 1996).
The slow removal of these particle-reactive nuclideswill result in the building up of these nuclides in thewater column and is likely to result in a low inventory ofparticle-reactive radionuclides in sediments. For exam-ple, 230Th, which is a decay product from the dissolved234U, is produced at a known rate given by the relation:
N � 280Z�atoms cmÿ2 minÿ1�;
where N is the standing crop production of 230Thexcess
and Z is the water depth in km. In situ scavenging bysuspended particles and subsequent settling of theseparticles requires that sediments contain all the 230Th. Inareas, where there is a slow removal of particle-reactiveradionuclides, the measured inventory should be lessthan the theoretically predicted inventory.
Several researchers have reported that unusual phys-ical characteristics, such as seasonal or permanent icecover, very limited primary productivity (and low sus-pended particle concentration), wide continental shelvesand di�erences in the characteristics of glacio-marinesuspended matter, result in relatively slow removal ratesfor biochemically active and particle-reactive trace ra-dionuclides in high-latitude oceans, such as the ArcticOcean (Ku and Broecker, 1967; Finkel et al., 1977;Moore and Smith, 1986; Bacon et al., 1989b; Darbyet al., 1989). It has been reported that the sediment in-ventory of 230Th from Canada Basin were only about30% of what was expected from the overhead produc-tion from the water column (Ku and Broecker, 1967).Finkel et al. (1977), based on 10Be measurements on twosediment cores from Canada Basin of the Arctic (80°N),showed that the deposition rates of 10Be in the Arctic areabout 3±5 times lower than those observed for otheroceanic regimes. This was partly attributed to the de®-ciency of particulate scavengers. Bacon et al. (1989b)reported that the surface water (85°500N, 108°500W) wascharacterized by an exceptionally low rate of Th scav-enging, even in comparison with other oligotrophicoceans. This slow scavenging of Th results in higher230Th concentrations in the water column and Baconet al. (1989b) reported substantially higher 230Th con-centrations compared to other ocean basins at compa-rable depths.
TABLE 3
Measured and expected inventories of 230Th and scavenging e�ciency in sediment cores from the world ocean.
Area Sedimentary 230Thinventory (dpm cmÿ2)
Production of 230Thinventory (dpm cmÿ2)
Scavenginge�ciencya (%)
References
Atlantic and Indian Ocean(compiled data)2000±3000 m 770� 110 800 96:3� 13:83400±4600 m 835� 200 1220 68:4� 16:4 Kadko (1980)4600±5800 m 1110� 135 1540 72:1� 8:7
Norwegian Sea 68°29.70N 0°49.10E(2802 m)
664� 15 785 84:6� 1:9 Scholten et al. (1994a)
Norwegian±Greenland Sea 72°1.80N9°15.90E (2518 m)
866� 13 705 123� 2 Scholten et al. (1994a)
Greenland Sea 76°31.20N 3°57.50E(2485 m)
441� 8 696 63:4� 1:2 Scholten et al. (1994a)
Arctic Ocean 77°380N 173°430W(1856 m)
120 520 23.1 Finkel et al. (1977)
Arctic Ocean 88°350N 149°270W(2871 m)
200 804 24.9 Finkel et al. (1977)
a Scavenging e�ciency � (sediment 230Th inventory/production of 230Th inventory)� 100.
20
Marine Pollution Bulletin
The sediment inventory of 230Th for the world oceanis presented in Table 3. The scavenging e�ciency[� (sediment inventory of 230Th/production of 230Thinventory) �100] in the subpolar oceans varies between68% and 96% (Kadko, 1980). The scavenging e�ciencyis notably low in the Canada Basin. However, it is in-teresting to note that the scavenging e�ciency of 230Thin the Nansen Basin is comparable to subpolar regions.This di�erence in the scavenging e�ciency between theNansen Basin and Canada Basin of the Arctic could beeither due to di�erences in the particle concentrationsand/or ventilation rates. One can infer some observa-tions based on the sedimentation rates from these areas,even though the extensive data base on the particleconcentrations from Arctic Basins do not exist. Thesedimentation rates in the Arctic Ocean, obtained usingexcess 230Th, extrapolated ages from 14C dates andstratigraphic chronologies using d18O isotopes are pre-sented in Table 4. The average sedimentation rate in theCanada Basin is between 0.09±0.24 cm/103 yr (0.09±0.24cm kyÿ1, Ku and Broecker, 1967; Finkel et al., 1977)while the values in high-latitude areas in the Norwe-gian±Greenland Sea and Nansen Basin are approxi-mately an order of magnitude higher (1.3±4.4 cm kyÿ1,Scholten et al., 1990, 1994a,b). Since the sedimentationrates in the Nansen Basin are about an order of mag-nitude higher than the Canada Basin, it is likely that thelow inventory of 230Th in the Canada Basin is likely dueto the low particle ¯ux in that area.
The mean residence times of the Arctic waters spanover three orders of magnitude, ranging from less than ayear to several hundred years (Schlosser et al., 1993).From the observations of Wallace et al. (1987, 1992) ithas been shown that the freon inventories in the watercolumn of the central Nansen Basin are relatively highcompared to the central Canada Basin, indicating rela-tively rapid ventilation in the Nansen Basin comparedwith the Canada Basin (Cochran et al., 1995). Thus, it isnot clear which of the two factors (particle concentra-tions and ventilation rates) that control the removal of
Th and other particle-reactive pollutants in these twoArctic Basins are dominant. More simultaneous studiesdealing with the particle concentrations and their ¯uxesand Th isotope measurements in the Arctic water col-umn and determination of ventilation rates in theseplaces are needed to address this question.
Conclusions
This study clearly shows that there is a distinct dif-ference in the scavenging of Th isotopes, and by impli-cation, other particle-reactive pollutants, between theCanada Basin and the Nansen Basin of the Arctic. Thescavenging e�ciency of 230Th in both regions, for ap-proximately the same latitude, di�ers considerably. Thisdi�erence is attributed to the low particle ¯ux inferredfrom the di�erences in sedimentation rates and/or dif-ferences in the ventilation rates of the two basins. Fromthe published data on the sedimentation rates in thesetwo basins, it appears that di�erences in particle ¯ux isprobably the primary reason for the slow scavenging ofparticle-reactive nuclides in the Canada Basin.
This work was in part supported by the National Science Foundation(grant number: NSF-OPP-9996337).
Anderson, R. F., Bacon, M. P. and Brewer, P. G. (1983a) Removal ofTh-230 and Pa-231 from the open ocean. Earth and PlanetaryScience Letters 62, 7±23.
Anderson, R. F., Bacon, M. P. and Brewer, P. G. (1983b) Removal ofTh-230 and Pa-231 at ocean margins. Earth and Planetary ScienceLetters 66, 73±90.
Bacon, M. P. and Rutgers van der Loe�, M. R. (1989a) Removal ofthorium-234 by scavenging in the bottom nepheloid layer of theocean. Earth and Planetary Science Letters 92, 157±164.
Bacon, M. P., Huh, C.-A. and Moore, R. M. (1989b) Vertical pro®lesof some natural radionuclides over the Alpha Ridge, Arctic Ocean.Earth and Planetary Science Letters 95, 15±22.
Barrie, L. A. (1986) Arctic air pollution: an overview of currentknowledge. Atmospheric Environment 20, 643±663.
Barnes, P. W., Reimnitz, E. and Fox, D. (1982) Ice rafting of ®ne-grained sediment, a sorting and transport mechanism, Beaufort Sea,Alaska. Journal of Sedimentary Petrology 52, 493±502.
TABLE 4
Sedimentation rates in the deep Arctic Basins obtained using 230Thexcess,14C and stratigraphic chronology using d18O isotopes.
Area Depth (m) Sedimentation rates (cm kyÿ1) References
Norwegian Sea 68°29.70N; 0°49.10E 2802 2.0 Scholten et al. (1994a)
Norwegian Sea 2281 1.59 Scholten et al. (1994a)70°18.30N; 3°7.40W
Norwegian±Greenland Sea 72°1.80N 9°15.90E 2518 4.37 Scholten et al. (1994a)
Greenland Sea 76°31.20N 3°57.50E 2485 1.25 Scholten et al. (1994a)
Fram Strait±Nansen Basin 78°300N; 02°E �3000 2±3 Scholten et al. (1990)
Canada Basin±Arctic Ocean 77°380N 173°430W 1856 0.24a Finkel et al. (1977)
Canada Basin±Arctic Ocean 82°560N 155° 540W 3437 0.20 Ku and Broecker (1967)
Canada Basin±Arctic Basin 88°350N 149°70W 2871 0.09a Finkel et al. (1977)
a Linear sedimentation rates were calculated from the porosity and mass accumulation rates.
21
Volume 42/Number 1/January 2001
Baskaran, M. and Naidu, A. S. (1995) 210Pb-derived chronology andthe ¯uxes of 210Pb and 137 Cs isotopes into continental shelfsediments, East Chukchi Sea, Alaskan Arctic. Geochimica Cosmoc-hima Acta. 59, 4435±4438.
Baskaran, M. and G. E. Shaw (2000) Residence time of Arctic hazeaerosols using the concentrations and activity ratios of 210 Po, 210 Pband 7Be. Journal of Aerosol Science (in press).
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