Deep-Sea Research II 81–84 (2012) 53–62

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Deep-Sea Research II

0967-06

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Importance of lateral transport processes to 210Pb budget in the easternChukchi Sea during summer 2003

Min Chen a,b,n, Qiang Ma a, Laodong Guo b,c, Yusheng Qiu a,b, Yanping Li a, Weifeng Yang a,b

a College of Ocean and Earth Sciences, Xiamen University, Xiamen 361005, Chinab State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, Chinac School of Freshwater Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53204, USA

a r t i c l e i n f o

Available online 24 March 2012

Keywords:210Pb

Boundary scavenging

Lateral transport

The Chukchi Sea

The Bering Sea

Arctic Ocean

45/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.dsr2.2012.03.011

esponding author.

ail address: mchen@xmu.edu.cn (M. Chen).

a b s t r a c t

Activity concentrations of dissolved and particulate 210Pb and 226Ra in the water column were

measured in the eastern Chukchi Sea during summer 2003. 234Th/238U disequilibria were used to

estimate the scavenging fluxes of 210Pb from the water column to the underlying sediments. Our results

showed that concentrations of 210Pb and its distributions were mainly influenced by mixing processes

of water masses and sediment resuspension. The residence times of 210Pb in the eastern Chukchi Sea

ranged from 5 to 103 d. Short residence times were mostly observed at the shelf stations, indicating a

more effective particle scavenging in the shelf region. A mass balance model was constructed to

evaluate the contribution of lateral transport to 210Pb budget in the water column. The lateral transport

fluxes of 210Pb ranged from 17 to 177 Bq/m2/a, comprising up to 63–94% of the total supply of 210Pb in

the eastern Chukchi Sea. We hypothesize that the accumulative removal of 210Pb in the Pacific inflow

waters during their transport across the Chukchi Sea and the import of 210Pb from sea ice rafted

sediments are the two major lateral transport pathways for the import of 210Pb to the eastern Chukchi

Sea. Our results highlight the importance of lateral transport processes to the geographical distribution

of particle-reactive elements and their biogeochemical cycles in the Arctic Ocean.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The Arctic Ocean is a high latitude area with distinctive physicaland chemical characteristics, including the long-term ice cover,extended periods of darkness during the polar night and the widecontinental shelves covering about 36% of its total surface area(Grebmeier et al., 2006). These factors would tend to favor theremoval of particle reactive elements and contaminants on the highproductive continental shelf compared to the deep basin. Recentstudies have indicated that the shelf-basin interaction in theArctic Ocean seems active, and the signals of the enhanced shelfscavenging could be rapidly transmitted into deep basin (Smith et al.,2003; Cooper et al., 2005; Grebmeier et al., 2006; Chen et al., 2008).

The influence of shelf processes on the deep basin in the Arctic ismainly through the lateral movement of water masses and/or seaice-rafted sediments. Moore and Smith (1986) studied the disequili-bria among 226Ra, 210Pb and 210Po at the Canadian Expedition toStudy the Alpha Ridge (CESAR) ice station (81143.40N, 931250W), anda deficit of 210Pb, compared to its soluble precursor 226Ra, wasobserved at depths corresponding to the nutrient maximum and the

ll rights reserved.

upper halocline, which was attributed to scavenging over thecontinental shelf and the subsequent transported into the interiorof the Canada Basin. During the Arctic Ocean Section (AOS, 1994)and Beaufort Sea (1995) expeditions, low 210Pb concentrations wereobserved in intermediate waters of the Makarov and Canada Basin,which further testified the lateral spreading of shelf waters with low210Pb signals (Smith et al., 2003). As to the role of sea ice raftedsediments in the redistribution of chemicals, several studies havesuggested that sea ice can incorporate shelf sediments and asso-ciated chemical species during its formation in shallow shelfenvironments and can also intercept atmospherically transportedmaterial during transit in the Arctic (Reimnitz et al., 1987; Hebbeln,2000; Eicken et al., 2005; Masque et al., 2007). Release of thematerial from rafted ice would have enhanced the export fluxes ofboth sediments and associated chemical species in the Arctic Ocean(Masque et al., 2007). Some previous studies (Eicken et al., 1997;Pfirman et al., 1995) also suggested that transportation of thesediments associated with the sea ice was controlled by the surfacecurrents in the Arctic Ocean which is clockwise circulation of theBeaufort and the Amerasian Basin and transpolar circulation in theEurasian Basin. Baskaran (2005) measured high activities of excess210Pb in a suite of ice-rafted sediments (IRS) collected from theCanada Basin, and proposed that in addition to atmospheric deposi-tion, IRS may scavenge directly dissolved 210Pb from surface waters.

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–6254

Wheeler et al. (1996) estimated biological production in theArctic Ocean at values higher than previously thought. Higherbiological production and successive higher particle fluxes alsohighlight the potential importance of boundary scavenging in theArctic productive shelf. Recent measurements of 234Th indicatedthat particle export rates over the seasonally ice-free regions ofthe Chukchi and Beaufort Sea shelves were indeed high andcomparable to those measured for shelf regions in low latitudes(Moran et al., 1997; Moran and Smith, 2000; Chen et al., 2002;2003). The lateral gradients in particle flux (i.e., scavengingintensity) would lead to a net transport of particle-reactivesubstances from deep basin to ocean margins where they aresubsequently buried in marginal sediments (Spencer et al., 1981).Nevertheless, the quantitative importance of boundary scaven-ging and its relation to environmental changes in the Arcticremains poorly understood.

Naturally-occurring radionuclides with high particle-reactivity(such as 210Pb, 230Th and 231Pa) have often been used to examineboundary scavenging processes. Because the sources of theseradionuclides in the water column are well known, their exportfluxes to marginal sediments can be compared to their localsupply rates, and used to evaluate the net transport from the openocean (Anderson et al., 1994; Bacon et al., 1994). In this study,210Pb-combined with 234Th-derived particle export fluxes wereused to quantify the boundary scavenging flux of 210Pb in theeastern Chukchi shelf and slope regions. Due to the large differ-ence in their half-lives (210Pb: t1/2¼22.3 a; 234Th: t1/2¼24.1 d), itwould be expected that nearly all of 234Th produced from 238U inthe water column would be scavenged into local sediments, while210Pb would be significantly influenced by boundary scavengingin the Arctic Ocean (Verdeny et al., 2009). Thus, measurements of210Pb, along with 234Th in ocean margins, may provide informa-tion on the intensity of boundary scavenging. The goal of this

Fig. 1. Sampling locations for 210Pb measuremen

work was to provide a more detailed understanding of 210Pbbudget and boundary scavenging in the eastern Chukchi Sea.

2. Methods

2.1. Sample collection

Stations occupied by the R/V Xuelong during the SecondChinese National Arctic Research Expedition in 2003 (July 21–September 15) were illustrated in Fig. 1. Water samples for 210Pbdetermination were collected at stations from the Bering Strait(BS01, BS07 and BS10), the eastern Chukchi shelf (R01, R03, R07,R15, C34, C26, C15, S11 and S32) and slope regions (S25, B11 andB13). Stations in the slope regions usually have a water depth41000 m, but only the upper 100 m water samples were col-lected. For each sample, 5–10 dm3 of seawater was collected by aCTD-Rosette system. Water samples for 210Pb measurementswere filtered on board ship immediately after collection throughnitrocellulose membrane filters to separate particulate fromdissolved (o0.45 mm) phase. Filters containing the particulatematter samples were kept in plastic bags and stored in a freezerbefore analysis. The dissolved fraction of 210Pb was transferredinto a clean polyethylene bottle and acidified with concentratedHCl to a pH�1 for further processing in the laboratory.

Water samples for 234Th measurements were collected from26 stations (Ma et al., 2005). Among the 26 stations, 8 stations(R07, R15, C34, C26, C15, S32, B11 and B13) were sampledconcurrently for both 234Th and 210Pb.

For 226Ra measurements, water samples were collected with asubmersible pump, and then stored in a polyethylene tank. Notethat sampling depths at some stations between submersible pumpand Niskin bottles may not be exactly the same as those for 210Pb

ts in the Chukchi Sea during summer 2003.

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–62 55

(Table 1). For the extraction of dissolved 226Ra, 120–140 dm3 ofseawater were passed through a column packed with 12 g MnO2-fiber. A flow rate of 200–250 cm3/min was maintained for

Table 1Activity concentrations of dissolved, particulate and total 210Pb, and 210Pb/226Ra activi

Station Longitude (1W) Latitude (1N) Depth (m) Temperature (1C) Salinity

BS01 171.508 64.334 0 9.344 31.613

10 9.323 31.607

20 2.020 32.746

30 1.972 32.765

40 1.984 32.784

BS07 168.500 64.340 0 7.564 31.841

10 7.632 31.962

20 7.602 32.011

30 5.539 32.001

BS10 167.010 64.334 0 10.499 28.568

10 10.223 28.635

20 2.487 31.436

R01 169.014 66.991 0 5.725 31.948

10 5.718 31.958

20 3.339 32.567

30 3.333 32.568

40 3.336 32.567

R03 169.000 68.000 0 5.129 32.394

10 5.147 32.401

20 5.147 32.403

30 2.458 32.842

40 1.927 32.916

50 1.413 32.958

R07 168.999 69.996 0 4.624 31.987

10 4.584 31.995

30 �0.153 32.422

R15 168.991 73.998 10 �1.466 29.494

20 �1.526 30.742

40 �1.656 31.451

C15 164.013 71.579 0 �0.874 30.454

20 �1.374 31.344

30 �1.610 32.738

40 �1.603 32.753

C26 162.976 70.495 0 4.959 31.202

10 4.942 31.207

20 5.304 31.720

30 5.200 31.774

C34 167.009 68.919 0 7.940 30.930

10 7.957 30.913

S32 150.376 71.261 0 1.524 29.367

20 3.992 30.437

S25 153.403 72.742 0 �0.648 28.154

25 �0.785 29.188

50 �0.145 31.661

75 �0.909 32.028

100 �1.233 32.267

S11 159.000 72.490 0 �1.024 28.854

20 �1.035 29.147

40 �1.576 31.669

50 �1.647 32.466

B11 156.332 73.995 0 �0.979 27.978

10 �0.999 28.007

20 �1.024 28.751

30 �1.049 29.450

B13 151.883 73.380 0 �1.022 27.559

25 �1.183 27.791

50 �0.650 31.152

75 �0.815 31.788

100 �0.980 32.423

a nd means no data.

quantitative Ra isotope extraction. After the enrichment, MnO2-fiber was enveloped in a plastic bag, and taken back to theonshore laboratory for 226Ra analysis.

ty ratios in the eastern Chukchi Sea during summer 2003.

POC (mmol/m3) DPb (Bq/m3) PPb (Bq/m3) TPb (Bq/m3) TPb/Ra a

10.5 0.8970.08 0.4070.08 1.2970.12 3.2270.43

8.76 0.6370.08 0.6270.08 1.2470.11 4.4470.88

7.68 0.3770.09 0.3670.03 0.7370.09 0.5470.08

6.85 0.4470.08 0.4370.02 0.8770.08 0.5870.07

6.82 0.5470.07 0.4070.02 0.9470.07 0.7670.09

10.2 0.3370.05 0.3970.02 0.7270.05 1.8670.31

10.2 0.3170.05 0.4570.02 0.7670.06 2.1170.33

9.38 0.2470.06 0.2670.02 0.5070.06 0.9870.23

12.9 0.3270.05 0.7270.02 1.0470.05 1.3770.21

8.61 0.2770.05 2.8270.02 3.0870.05 2.5570.15

11.0 0.1170.24 0.5470.01 0.6570.24 0.7570.28

8.19 0.2870.06 0.4870.02 0.7570.06 0.6270.08

17.6 0.1570.10 0.2870.05 0.4370.11 0.5870.15

18.7 0.1870.05 0.1070.03 0.2870.06 0.4170.10

8.81 0.1970.06 0.1670.05 0.3470.08 0.2670.06

9.17 0.2070.06 0.2770.02 0.4770.07 0.3270.05

11.9 0.2370.07 0.3470.02 0.5870.07 0.4770.07

11.9 0.2070.05 0.5670.03 0.7570.06 0.7670.07

8.60 0.1770.05 1.0970.01 1.2670.06 1.5870.12

6.21 0.1570.08 0.1470.02 0.2970.08 0.2770.08

9.85 0.1970.05 0.2670.03 0.4570.06 0.3470.05

8.86 0.0770.07 1.1970.01 1.2670.07 0.7470.22

28.3 0.3770.05 0.8470.02 1.2170.06 nd

6.78 0.1070.07 1.0670.02 1.1670.07 4.1670.79

6.59 0.1270.10 0.1270.09 0.2470.14 0.6870.39

12.7 0.1670.08 0.2770.02 0.4470.08 0.5970.13

8.05 0.1770.05 0.6470.02 0.8170.05 0.8170.08

8.27 0.1770.04 0.3670.02 0.5370.05 0.7070.08

2.65 0.0970.10 0.6670.02 0.7570.10 0.5970.09

11.5 0.1570.04 0.3370.04 0.4870.06 0.7570.11

14.9 0.2470.06 0.2270.03 0.4570.07 0.5370.09

nd 0.2870.05 0.2270.03 0.5170.06 0.3570.04

24.4 0.2570.06 0.4270.02 0.6770.06 0.4070.04

5.44 0.2170.07 0.4070.03 0.6170.07 2.2670.50

3.70 0.2270.06 1.8070.01 2.0170.06 nd

nd 0.1170.10 0.2770.03 0.3770.10 0.8570.25

6.78 0.1370.08 0.3370.03 0.4670.08 0.6770.13

8.87 0.2070.08 0.2670.05 0.4670.09 0.8970.20

8.42 0.1170.12 0.0670.08 0.1770.14 0.4470.36

5.92 0.1470.09 0.8370.04 0.9770.10 1.0870.13

4.39 0.1070.11 0.2570.02 0.3570.11 0.5270.11

2.72 1.5270.02 0.0970.05 1.6270.05 1.4770.11

3.55 1.2770.03 1.2070.02 2.4770.04 nd

5.96 0.8770.02 0.2070.02 1.0770.03 nd

3.96 0.3570.04 0.7370.01 1.0870.04 nd

1.98 0.2970.05 0.2170.03 0.5070.06 nd

9.22 0.5170.02 0.2770.13 0.7870.13 0.9270.16

nd 0.6470.03 0.3170.06 0.9570.07 0.9270.08

7.61 0.1870.08 0.2070.07 0.3870.10 0.2770.07

76.6 0.3170.02 2.6070.03 2.9170.04 nd

4.50 0.6370.02 0.9870.02 1.6170.03 1.9070.14

3.86 0.6270.02 1.0570.02 1.6770.03 1.2670.08

4.05 0.4770.07 1.5070.02 1.9770.07 1.9070.13

nd 0.6470.03 0.8270.02 1.4770.03 1.0970.06

2.26 2.1470.01 0.4070.02 2.5470.03 3.1770.24

4.47 1.8570.02 0.4770.03 2.3270.04 nd

5.27 0.5270.03 1.2970.02 1.8170.04 nd

3.35 0.3670.03 0.2070.05 0.5670.06 nd

2.31 0.2870.04 0.1970.04 0.4770.05 nd

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–6256

2.2. 210Pb analysis

The dissolved and particulate 210Pb samples were storedfor 4720 d before analysis, allowing a secular radioactiveequilibrium to be established between 210Po and 210Pb. 210Pbactivities in the samples were determined through its daughter of210Po (t1/2¼138.4 d).

The analytical procedures for 210Pb in dissolved and particulatefractions have been described in Yang et al. (2006). Briefly, carrierand yield tracers, including 50 mg Fe3þ , 20 mg Pb2þ and accu-rately known amount of 209Po-spike, were added while stirring.After 24 h, the samples were neutralized with NH4OH solution to apH of 9.0. The Fe(OH)3 precipitates were collected through cen-trifugation and re-dissolved with 6 mol/dm3 HCl. After transferredinto a 100 cm3 Teflon beaker, ascorbic acid was added until theyellow color (Fe3þ) disappeared. 1 cm3 of 20% hydroxylaminehydrochloride and 1 cm3 of 25% sodium citrate solution wereadded, and the pH of the solution was adjusted to 1.5. 210Po and209Po were deposited on a silver disk at 90 1C for 4 h. The activitiesof 210Po and 209Po on the disk were determined by an alphaspectrometer (OcteteTM PC, EG&G ORTEC). Errors reported for 210Pbwere based on 1s counting statistics.

The particulate 210Pb samples on filters were digested withmixed acids (i.e., HF, HNO3 and HClO4), followed by proceduresdescribed for the dissolved samples. Reagent blanks, instrumentbackgrounds, and error propagation have been taken into accountfor the calculation of 210Pb activity concentrations.

2.3. Measurements of POC, 234Th and 226Ra

For POC measurements, about 2 dm3 of seawater sampleswere filtered through a precombusted (400 1C for 4 h) WhatmanGF/F membrane (f 25 mm). The filter was subsequently rinsedwith 10 cm3 0.1 mol/dm3 HCl and 30 cm3 Milli-Q water to removeinorganic carbonate. The de-carbonate filters were preserved at�20 1C before analysis. In land laboratory, the filter samples weredried at 60 1C, and wrapped into tin capsules for measurements.Concentrations of POC were measured by an elemental analyzer(Carlo Erba NC2500).

Detailed procedures for 234Th and 226Ra measurements weredescribed in Ma et al. (2005) and Xing et al. (2003), respectively.Briefly, 234Th activity was determined via counting its daughter234mPa on a low-level beta counter (BH1216, Beijing NuclearInstrument Factory). 226Ra activity was measured by 222Rnemanation with a Rn–Th analyzer (FD-125, Beijing NuclearInstrument Factory). Samples were counted two to four timesuntil the standard deviation was less 10%. Both 234Th and 226Radata were used here to construct the mass balance of 210Pb, anddetailed results of 234Th and 226Ra have been published elsewhere(Ma et al., 2005; Li, 2004).

3. Results

3.1. Activity concentrations of dissolved and particulate 210Pb

The activities of dissolved, particulate, and total 210Pb are listedin Table 1. The dissolved (DPb) and particulate 210Pb (PPb) activitiesranged from 0.07 to 2.14 Bq/m3 and from 0.06 to 2.82 Bq/m3,respectively, with the averages of 0.39 and 0.58 Bq/m3. These valuesare higher than those reported previously for the northeasternChukchi Sea with 0.03–0.49 Bq/m3 for DPb and 0.03–0.18 Bq/m3

for PPb in the upper 100 m water column (Lepore et al., 2009).The reason for this activity difference can be attributed to thedifferent sampling time and locations between both studies. Mostof our sampling stations are located in the shallow shelf regions

in the Chukchi Sea, with significant influences from sedimentresuspension and lateral transport processes, which may resultin higher 210Pb in the water column (see discussion below).Nevertheless, activity concentrations of total 210Pb, ranging from0.17 to 3.08 Bq/m3 with a mean of 0.98 Bq/m3, are comparableto those reported for the Chukchi shelf (0.46–0.54 Bq/m3, Smithet al., 2003), the Northeast Water Polynya Greenland (0.1–3.1 Bq/m3,Roberts et al., 1997), the Canadian Ice Island (0.49–0.93 Bq/m3,Smith and Ellis, 1995), the Nanshen Basin (0.55–1.76 Bq/m3,Cochran et al., 1995) and the Bering Sea (1.72–2.37 Bq/m3,Nozaki et al., 1997).

Activity ratios of total 210Pb to 226Ra (TPb/Ra) ranged from 0.26to 4.44. Values of the TPb/Ra in surface waters at most stationswere higher than 1, reflecting the contribution of the atmosphericdeposition for 210Pb in surface water.

3.2. Distributions of 210Pb

There were three transects among our sampling stations:(1) a longitudinal transect in the Bering Strait (BS01, BS07,BS10), (2) a latitudinal transect along 1691W (R01, R03, R07,R15), and (3) a shelf-slope transect extending from the SW to NEin the study area (R03, C34, R07, C26, C15, S11, S25, B13, B11).The distributions of 210Pb from these three transects are depictedin Figs. 2–4.

The hydrological characteristics along the longitudinal sectionin the Bering Strait clearly show the influence of Pacific inflow,with more saline, low temperature Anadyr Water on the westernside and fresher, high temperature Alaska Coastal Water on theeastern side (Fig. 2(a) and (b)). A strong salinity front developedbetween these two water masses. The Anadyr Water to the westwas characterized with high dissolved 210Pb and low particulate210Pb, while the Alaska Coastal Water to the east was character-ized with low dissolved 210Pb and high particulate 210Pb. Dis-tributions of dissolved 210Pb along this transect mirrored thedistributions of POC and particulate 210Pb (Fig. 2(c)–(e)). Theactivity concentrations of total 210Pb were highest in the surfacewaters, with a minimum centered at �20 m (Fig. 2(f)). Ratios ofTPb/Ra showed a decrease with increasing depth, with the highestgradient observed at the BS01 station in the west part of theBering Strait transect (Fig. 2(g)).

As the Pacific waters flowed through the Bering Strait, theymoved northward over the shelf and slope areas of the Chukchiand Beaufort Seas into the Arctic Basin. During their northwardtransport, both temperature and salinity decreased due to the lossof heat and the influence of sea ice melt waters (Fig. 3(a) and(b) and Fig. 4(a), (b)). Distributions of 210Pb at the latitudinaltransect along 1691W showed high concentrations of particulateand total 210Pb in the bottom waters (Fig. 3(e) and (f)), coincidedwith high POC concentrations in these waters (Fig. 3(c)), whichcould be attributed to the effect of sediment resuspension. Thespatial distribution pattern of total 210Pb along this transect wasin parallel with that of particulate 210Pb (Fig. 3(e) and (f)),indicating the dominative role of particulate phase.

Distributions of salinity along the shelf-slope transect showeda front centered at 40–50 m in the outer shelf and slope regions(e.g., Stn. S25, B13, B11, Fig. 4(a) and (b)), indicating the effect ofsea ice melted water on salinity in the upper water column. Highconcentrations of dissolved and particulate 210Pb were observedin the uppermost waters with low salinity. Compared to the sloperegions, concentrations of dissolved 210Pb in the shelf waters werelow (Fig. 4(d)). Similar to the latitudinal transect along 1691W,high POC and particulate 210Pb concentrations were also observedin the bottom waters, and the distribution of total 210Pb resembledthat of particulate 210Pb (Fig. 4(e), (f)).

Fig. 3. Distributions of temperature (1C) (a), salinity (b), POC (mmol/m3) (c), dissolved 210Pb (Bq/m3) (d), particulate 210Pb (Bq/m3) (e), total 210Pb (Bq/m3) (f), and activity

ratio of total 210Pb to 226Ra (g) at a latitudinal transect along 1691W.

Fig. 2. Distributions of temperature (1C) (a), salinity (b), POC (mmol/m3) (c), dissolved 210Pb (Bq/m3) (d), particulate 210Pb (Bq/m3) (e), total 210Pb (Bq/m3) (f), and activity

ratio of total 210Pb to 226Ra (g) at a longitudinal transect in the Bering Strait.

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–62 57

4. Discussion

4.1. 210Pb scavenging

Both Pb and Th are particle-reactive elements and can be readilyadsorbed onto particle surfaces and subsequently removed to thesediments by particle settling. 234Th (t1/2¼24.1 d) is produced from

the radioactive decay of 238U in the water column. Due to its wellconstrained source term and suitable decay time scale, 234Th hasbeen used as an excellent tracer of water column particle dynamicsand export fluxes on seasonal and biologically driven time scales(Buesseler et al., 2006; 2007; Waples et al., 2006). During past years,234Th/238U has been used to estimate the particle export inthe Arctic Ocean during the ice-free seasons (Moran et al., 1997;

Fig. 4. Distributions of temperature (1C) (a), salinity (b), POC (mmol/m3) (c), dissolved 210Pb (Bq/m3) (d), particulate 210Pb (Bq/m3) (e), total 210Pb (Bq/m3) (f), and activity

ratio of total 210Pb to 226Ra (g) at a SW–NE shelf-slope transect.

Table 2Input and removal fluxes of 210Pb and residence times of total 210Pb in the eastern Chukchi Sea during summer 2003.

Station Export

depth (m)

FTh

(Bq/m2/d)

Particulate210Pb/234Th)A.R.

FPb

(Bq/m2/d)

Fscaa

(Bq/m2/a)

FPb decay

(Bq/m2/d)

Fatm

(Bq/m2/a)

FRa growth

(Bq/m2/a)

Fadv

(Bq/m2/a)

tPb (d)

R07 30 21.6 0.037 0.80 80 0.43 10 0.44 70.0 17

R15 40 10.9 0.049 0.53 53 0.61 10 0.91 42.7 37

C34 10 3.5 0.080 0.28 28 0.04 10 0.29 17.8 5

C26 30 12.5 0.043 0.54 54 0.91 10 0.40 44.5 54

C15 40 32.8 0.057 1.87 187 0.62 10 1.32 176.3 11

S32 20 10.1 0.036 0.36 36 0.41 10 0.49 25.9 36

B11 30 9.1 0.067 0.61 61 1.61 10 1.08 51.5 85

B13 100 26.1 0.058 1.51 151 4.82 10 2.49 143.3 103

a Assuming a constant particle export during ice-free season of 100 d.

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–6258

Moran and Smith, 2000; Chen et al., 2003; Trimble and Baskaran,2005; Lalande et al., 2007). Slightly different from 234Th, 210Pb issupplied to the water column by both atmospheric deposition andin situ production from 226Ra, a parent nuclide of 210Pb. Due to itsparticle reactivity, 210Pb has also been widely used to trace particleexport and POC fluxes, but over annual to decadal timescales (Mooreand Dymond, 1988; Heussner et al., 1990; Kim et al., 1997;Radakovitch et al., 1999; Shimmield et al., 1995). While 234Th caneffectively trace scavenging history and water column processes ona time scale of days to weeks, it becomes less sensitive for tracinglonger time scale processes due to its short half-life. On the otherhand, 210Pb (half life of 22.3 years) can be used to trace particle-water and particle–particle interaction processes over a time scale ofmonths to decades. The longer half-life of 210Pb implied that theinput of lateral transport could be an important source for 210Pb,compared to 234Th (Smoak et al., 1996; Moran et al., 1997). Thus,234Th and 210Pb are complementary with respect to their constrainedtimescales. In this study, 234Th/238U disequilibria were used toestimate the scavenging fluxes of 210Pb in the eastern Chukchi Sea.

Scavenging fluxes of 210Pb were estimated at 8 stations(Table 2) where 210Pb and 234Th were synchronously sampled.Similar to the empirical relationship for 234Th and POC export

proposed by Buesseler et al. (1992), the scavenging flux of 210Pbfrom the water column to sediment (FPb) is given by

FPb ¼ FTh �PPb

PTh

� �export

ð1Þ

where FTh is the 234Th export flux from the water column (Bq/m2/d),and ðPPb=PThÞexport represents the activity ratio of particulate 210Pbto particulate 234Th at the export depth. With the calculated FPb, theresidence time of total 210Pb (tPb) with respect to scavenging andparticle removal can be calculated by:

tPb ¼

RTPb

FPbð2Þ

whereR

TPb is the inventory of total 210Pb in the water column(Bq/m2).

The values of FTh, calculated from the 234Th/238U disequilibriausing a 1-D vertical export model (Ma et al., 2005) are listed inTable 2. Activity ratios of ðPPb=PThÞexport in the eastern Chukchi Searanged from 0.036 to 0.080 with an average of 0.053. Although thereare no reported ðPPb=PThÞexport ratios in the study area, these valuesare similar to those reported for the sediment trap samples collectedfrom the Middle Atlantic Bight (0.032–0.054, Santshchi et al., 1999),

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–62 59

but lower than those in suspended particles in the Tampa Bay(a mean of 0.64, Baskaran and Swarzenski, 2007).

The calculated scavenging fluxes of 210Pb (FPb) and the residencetimes of total 210Pb (tPb) in the eastern Chukchi Sea were in theranges of 0.28–1.87 Bq/m2/d and 5–103 d, respectively (Table 2).Short residence times of 210Pb indicated its efficient removal fromthe water column in the eastern Chukchi Sea. As shown in Table 2,the residence times of total 210Pb at shelf stations (5–54 d) weresignificantly shorter than those at slope stations (85–103 d), indicat-ing a more effective particle scavenging in the shelf regions. The lowvalues of tPb in the shelf waters are consistent with the characteristichigh rates of productivity in this region (Wheeler et al., 1996; Chenet al., 2002; Grebmeier et al., 2006). They are also consistent with thehigh export production (Moran et al., 1997; Moran and Smith, 2000;Trimble and Baskaran, 2005; Ma et al., 2005) and sediment commu-nity oxygen consumption (Grebmeier et al., 2006) observed in theshelf regions. Indeed, a significant linear relationship was observedbetween the scavenging fluxes of 210Pb (FPb) and the POC inventoriesin the water column (Fig. 5), further demonstrating the active role ofbiological pump in particle scavenging in the eastern Chukchi Sea.

4.2. 210Pb Budget in the water column

Based on the mass balance of 210Pb in the water column,variation in total 210Pb activity (TPb) with time can be described as:

@TPb

@t¼ FatmþFRa growthþFadv�Fsca�FPb decay ð3Þ

where the positive terms in the right hand side of the equationrepresent the supply rates of 210Pb, and the negative terms are theremoval rates of 210Pb in the water column. Fatm is the atmosphericflux of 210Pb (Bq/m2/a), FRa growth is the production rate of 210Pb byradioactive decay of 226Ra (Bq/m2/a), and Fadv represents the inputflux of 210Pb by lateral advection or diffusion in the water column(Bq/m2/a). Similarly, Fsca and FPb decay represent the rate of 210Pbremoval by particle scavenging and radioactive decay (Bq/m2/a),respectively. Under steady-state conditions, ð@TPb=@tÞ ¼ 0, the lateraltransport flux of 210Pb (Fadv) is thus given by:

Fadv ¼ FscaþFPb decay�Fatm�FRa growth

¼ FscaþlPb

ZTPb�Fatm�lPb

ZRa ð4Þ

where lPb is the decay constant for 210Pb (0.0311 a�1) andR

TPb andRRa are the inventories of total 210Pb and 226Ra in the water column

(Bq/m2), respectively.

Inverntory of POC (mmol/m2)

0 100 200 300 400 500 600

F sca

(Bq/

m2 /

d)

0.0

0.5

1.0

1.5

2.0

2.5

y=0.019+0.0038x(n=8, r2=0.95, p<0.0001)

Fig. 5. Relationship between the scavenging fluxes of total 210Pb and the inventories

of POC in the water column.

Atmospheric deposition rates of 210Pb are low in the Arcticregion owing to reduced rates of radon exhalation from thelimited soil coverage and low rates of precipitation (Dibb, 1990;Hermanson, 1990). The reported Fatm ranged from 6.7 to 45 Bq/m2/a (Baskaran and Naidu, 1995; Cornwell, 1985; Weiss andNaidu, 1986; Nijampurkar and Clausen, 1990; Dibb, 1990; Huhet al., 1997). For the following discussion, a value of 10 Bq/m2/awas used as the atmospheric deposition rate (Huh et al., 1997;Smith et al., 2003; Lepore et al., 2009).

As shown in Table 2, the in situ production rates (FRa growth) of210Pb from 226Ra in the eastern Chukchi Sea (0.44–2.49 Bq/m2/a)were significantly lower compared to the atmospheric input,indicating the relative importance of atmospheric deposition inthe shallow shelf waters of the Chukchi Sea. However, the localsupplies of 210Pb by the atmospheric input (Fatm) and in situproduction (FRa growth) in the eastern Chukchi Sea are insufficientto account for the local removal of 210Pb by particle scavenging(Fsca) and radioactive decay (FPb decay). This 210Pb imbalanceunderlines the significance of the lateral import of 210Pb into thiscontinental margin. Based on the mass balance given by Eq. (4),the lateral transport fluxes of 210Pb (Fadv) in the eastern ChukchiSea ranged from 17 to 177 Bq/m2/a (Table 2), indicating that up to63–94% of the total 210Pb supply in the eastern Chukchi Sea wasderived from the lateral inputs. Even when a maximum atmo-spheric flux of 45 Bq/m2/a was adopted, the calculated value ofFadv for this shelf-slope region of the Chukchi Sea remainedpositive except at C34 and S32 stations.

Our estimates of Fadv can be validated by the previouslyreported 210Pb data in the water column and sediments. Thefocusing factor (f), defined as the ratio of the 210Pb inventory inthe sediment to the deficit of 210Pb in the water column, was usedto quantify the efficiency of 210Pb transport from the water columnto local, underlying sediments. According to the conversionbetween sediment inventory and the flux to the sediments (i.e.,Inventory¼ ðflux=lPbÞ), Fadv can be expressed as a function of f:

Fadv ¼210Pbsed inventory� lPb � 1�

1

f

� �ð5Þ

Based on the reported f and 210Pb sediment inventories at5 stations with depths o150 m in the Chukchi Sea (Smith et al.,2003; Lepore et al., 2009), the calculated lateral fluxes of 210Pb,Fadv, by Eq. (5) were 26–159 Bq/m2/a, which agreed well with ourestimates by Eq. (4).

High Fadv values of 210Pb indicated an active boundary scaven-ging in the eastern Chukchi Sea. The implication is that theeastern Chukchi Sea may act as a sink for many particle reactivechemical species (such as transuranic radionuclides, organiccontaminants and trace metals) that are transported from theadjacent seas through water circulation. Boundary scavenging hasbeen observed throughout the Arctic Ocean for radionuclides,including 210Pb, 230Th and 231Pa (Huh et al., 1997; Smith et al.,2003; Moran et al., 2005). Together with our data, these observa-tions demonstrated consistently that particle export rates overseasonally ice-free regions of the Chukchi and Beaufort Seashelves are indeed high and comparable to those measured inshelf regions in low latitudes (Moran et al., 1997; Moran andSmith, 2000; Chen et al., 2002; Grebmeier et al., 2006).

Major processes for the import of 210Pb to the eastern ChukchiSea may include the accumulative removal of 210Pb in Pacificwaters during their transport across the Chukchi Sea and sea icerafting. Pacific water flows northward through the Bering Straitmainly from two branches: one is Anadyr Water (AW) with moresaline transiting on the western side of the northern Bering Sea andChukchi Sea, and the other is Alaska Coastal Water (ACW) that isfresher and flows on the eastern side of the northern Bering Seaand Chukchi Sea (Weingartner et al., 2005; Grebmeier et al., 2006).

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–6260

The distribution of Fadv values in our study area shows a broadcorrelation to the water circulation pattern, with high Fadv valuesoccurring along the AW-influenced stations and low Fadv alongthe ACW-influenced regions (Fig. 6). According to the distribu-tion of dissolved 210Pb along the BS transect, dissolved 210Pbconcentrations in the AW were higher than those in the ACW(Fig. 2(d)), suggesting the AW has higher potential for transport-ing more 210Pb into the Chukchi Sea. Grebmeier et al. (2006)summarized the integrated Chl-a and sediment communityoxygen consumption in the northern Bering Sea, Chukchi,eastern Siberian and western Beaufort Seas, and showed that,compared to the ACW-influenced regions, the influence ofeutrophic AW to the west gave rise to a chlorophyll-rich watercolumn and organic carbon export to the underlying sediments insummer, consistent with our results on the Fadv distribution, asshown in Fig. 6.

Another potential pathway for the import of 210Pb to the studyareas may be through sea ice sediment entrainment and rafting.Sea ice could acquire 210Pb through direct atmospheric depositiononto ice surface and in association with particles. Additionally,sea ice formed in shallow shelf incorporates suspended sediments(Pfirman et al., 1990). Such sediments are likely to have excess210Pb (Masque et al., 2007). Based on the buoy drift in May/June2002, the ice-transported sediments were conveyed from theentire Beaufort shelf into the western Chukchi Sea (Eicken et al.,2005), and released in an area of high water column and benthic

Fig. 6. Distributions of the lateral transport fluxes of 210Pb (Fadv

production (Naidu et al., 2003). Thus, sediment entrainment andsea ice rafting and the subsequently melting of ice in the ChukchiSea might release excess 210Pb into the water column, and con-tribute to the Fadv observed in the study area.

Our results indicated that particle-reactive chemicals wereredistributed within the Arctic Ocean by lateral transport andpreferentially removed in the marginal areas, such as the Chukchishelf. High rates of primary productivity and sediment resuspen-sion events over the wide continental shelf of the Chukchi Sealikely promoted the removal of particle-reactive chemical species,including trace metals, organic pollutants, and radionuclides such as210Pb. The re-distribution of these materials may play an importantrole in regulating their distributions in the water column and theirbiogeochemical cycling in the Arctic Ocean. Therefore, sedimenta-tion rates based on excess 210Pb in the shelf regions may still berepresentative. However, paleo-environments reconstructed fromsedimentary proxies in these areas may not quantitatively reflectthe local information due to the profound influence of lateraltransport processes.

5. Conclusions

Distributions of dissolved and particulate 210Pb in the easternChukchi Sea were highly influenced by the Pacific inflow andsediment resuspension. The Anadyr Water on the western side of

, Bq/m2/a) in the eastern Chukchi Sea during summer 2003.

M. Chen et al. / Deep-Sea Research II 81–84 (2012) 53–62 61

Bering Strait was characterized with high dissolved 210Pb and lowparticulate 210Pb, while the Alaska Coastal Water on the easternside was characterized with low dissolved 210Pb and high parti-culate 210Pb. High concentrations of particulate 210Pb in the shelfbottom waters were observed and could be ascribed to the impactof sediment resuspension. 234Th/238U disequilibrium was used toestimate the scavenging fluxes of 210Pb in the eastern ChukchiSea. The calculated residence times of total 210Pb with respectto scavenging and particle removal ranged from 5 to 103 d in theeastern Chukchi Sea. The residence times at shelf stations(5–54 d) were significantly shorter than those observed at slopestations (85–103 d), indicating a more effective particle scaven-ging in the shelf region, which is consistent with the high rates ofproductivity in the study region. Based on the mass balance of210Pb in the water column, the contribution of lateral transporton 210Pb budget in the eastern Chukchi Sea was estimated to be63–94% of the total supply of 210Pb in the eastern Chukchi Sea,which highlights the importance of boundary scavenging in there-distribution of particle-reactive trace elements. The majorpathways for the import of 210Pb to the eastern Chukchi Seaincluded the cumulative removal of 210Pb during the transport ofPacific inflow water across the Chukchi Sea and the import of210Pb from sea ice sediment entrainment and rafting.

Acknowledgments

Without the assistance of the officers and crew of the RVXuelong, this work would not have been possible. This work wassupported, in part, by a Chinese IPY Research Program, theNational Natural Science Foundation of China (41125020), aFujian Natural Science Foundation (2009J06026), a special scien-tific research project for public welfare supported by the StateOceanic Administration of China (201105022-4), and a CheungKong Scholarship through Ministry of Education of China.

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