2 Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China;
3 Key Lab of Global Change and Marine-Atmospheric Chemistry, State Oceanic Administration, Xiamen 361005, China;
4 Department of Engineering Physics, Tsinghua University, Beijing 100084, China
About 30% of anthropogenic CO2 is taken up bythe ocean，especially the Arctic and Southern oceans，owing to high biological production in the large oceanmargin areas and low temperatures(Cai et al.，2010).The Chukchi Shelf is one of the most productivecontinental shelves in the Arctic Ocean and isimportant in carbon sequestration(Woodgate et al.，2005; Yu，2010). The magnitude of atmospheric CO2absorbed in these areas has been of concern tooceanographers worldwide(Baskaran et al.，1995;Anderson et al.，1998; Pipco et al.，2002; Bates et al.，2006; Liu et al.，2007; Else et al.，2008).
As an interface for exchange of CO2 between thesurface ocean and interior ocean，the euphotic zoneplays a key role in the growth，removal and cycling ofbiomass. Fluxes within the euphotic zone of carbon，nutrients and other associated elements involved in biogeochemical cycles are very important in the studyof global CO2(Yang et al.，2004; He et al.，2008).Radioactive isotopes，especially natural particlereactiveradionuclides(e.g.，234Th and 210Po)provideanother possible means for quantifying export fl ux ofparticulate organic carbon(POC)from the surfaceocean at various time scales，because of their specifi chalf-lives(Verdeny et al.，2009; Stewart et al.，2011).
Both 210Po and 210Pb are progeny of the 238U decaychain，and are naturally occurring particle-reactiveradionuclides. 210Pb(t1/2 =22.3 a)is produced in situ bydecay of its longer-lived gr and parent 226Ra and fromatmospheric deposition，whereas 210Po is supplied almost exclusively in situ by the decay of itsgr and parent 210Pb，with a minor additional sourcefrom atmospheric deposition(Baskaran，2011).Although 210Pb and 210Po are particle-reactiveradionuclides，the preferential uptake of 210Po byplankton and its transfer in the food chain can be usedto quantify organic carbon fl uxes(Carvalho，2011;Kim et al.，2012). Furthermore，export on a timescaleof several months can be derived by 210Po/ 210Pbdisequilibrium，longer than the several weeks of234Th/ 238U disequilibrium(Friedrich et al.，2002).
In this study，we estimated 210Po-derived exportfl uxes of POC from the euphotic zone at differentstations during the 4th Chinese National ArcticResearch Expedition(CHINARE-4).2 MATERIAL AND METHOD 2.1 Location and sample collection
During CHINARE-4 from July 1 to September 20，2010，a conductivity，temperature and depth(CTD)rosette(SEABIRD 911/17)was used to collect 10-Lseawater samples at various depths in the upper0–100 m water column at four stations in the ArcticOcean，for analysis of 210Po and 210Pb. At stations R03 and R09，sampling was done during July 21–24，whereas that at R12 and R20 was during August24–29. A map of the study area and stations isprovided in Fig. 1.
One liter of each 10-L seawater sample was fi lteredthrough a pre-combusted(450°C，2 h)0.45-μm QMAmembrane immediately after sampling，and thenparticles and the fi lter were sealed inside tin foil and stored frozen for subsequent POC analysis. Theremaining nine liters of seawater from each sample were fi ltered through a 0.45-μm membrane fi lter(120 mm diameter)，to obtain dissolved samples and particulate samples. After fi ltration，dissolved sampleswere acidifi ed to pH 1 with concentrated HCl. Allsamples were stored until laboratory analysis of POC，210Po and 210Pb.2.2 Radionuclide analysis
For dissolved and particulate 210Po and 210Pb，theanalytical methods used followed the modifi cationsof Nozaki(1986) and Masque et al.(2002). Briefl y，fi lter samples were digested using sequential aquaregia，HNO3 and HCl，with HClO4 and HF，and thedissolved material was eventually taken up in1.5 mol/L HCl. During digestion，samples werespiked with 209Po as a yield tracer and stable lead as arecovery tracer. The dissolved 210Po was coprecipitatedwith Fe(OH)3 and redissolved with1 mol/L HCl. Po was plated onto nickel discs usingstirring hot plates and counted on a low backgroundalpha spectrometer(Canberra 7200-08; CT，USA).The plating solution was then replated to remove anyresidual 210Po and 209Po，then re-spiked with 209Po and retained for at least 6 months for ingrowth of 210Po todetermine 210Pb. Pb yields were determined throughmeasurement of stable Pb by inductively coupledplasma mass spectrometry(ICP-MS)，and recoverywas generally between 80% and 90%. The 210Po and 210Pb activities were decay-corrected to the time ofsampling according to Fleer and Bacon(1984). 3 RESULT AND DISCUSSION 3.1 Distribution of particulate and dissolved 210Po and 210Pb
The activity of 210Po and 210Pb in different phases and the activity ratio 210Po/ 210Pb)A.R are given inTable 1. Vertical profi les of 210Po/ 210Pb)A.R in differentwater columns are shown in Fig. 2. The averagedissolved 210Pb of various stations shows an increasingtrend from 1.12 to 1.78 Bq/m3 northward from July toAugust，which may have resulted from atmosphericdeposition of 210Pb during ice melt. The averageparticulate 210Pb(2.22 Bq/m3)was higher than thedissolved 210Pb(1.49 Bq/m3)，indicating intenseparticle scavenging of 210Pb(Bacon et al.，1976;Shimmield et al.，1995).
The vertical profi le of POC in different watercolumns and relationship between POC content and particulate Po are shown in Fig. 3. The average contentof POC decreased northward. However，the highest value for surface water appeared at R09，perhapscaused by ice melt，which resulted in biomass bloomfrom the bottom ice(Yu et al.，2012). Positivecorrelation(R2 =0.027)between POC content and particulate Po confi rms that Po may be used as a tracerof POC export.3.2 Total Po and Pb，residence time of 210Po，watercolumndisequilibrium and flux
The half-life of 210Po constrains the seasonal timescale of disequilibrium of 210Po/ 210Pb for quantifyingthe export fl uxes of POC. Boundary scavenging，especially advective processes，is important for 210Pb，owing to its half-life of 22.3 years(Friedrich et al.，2002; Kim et al.，2012). The spatially comparableactivity of 210Po and 210Pb further constrains the fl uxesbecause of advective processes，which are minorrelative to vertical fluxes.
An irreversible，steady state model of 210Po wasbuilt without advective and diffusive items and atmospheric deposition. To estimate the rates of 210Po and 210Pb scavenged by suspended particles and theirremoval via sinking particles，the following equations were used for the disequilibrium between 210Po and 210Pb in seawater(Bacon et al.，1976):Table 2. This shows that mean residence times ofparticulate and dissolved 210Po in the euphotic zone ofdifferent stations were almost all negative，which maybe caused by regeneration of particles(He et al.，2008). However，the residence time of particulate210Po was higher than that of dissolved 210Po，indicatingrapid biogeochemical cycling in the study area(Rutgers van der Loeff et al.，1995). 3.3 Distribution of particulate organic carbonconcentrations
Concentrations of POC in the water column aregiven in Table 1. POC concentration was 9–601 μgC/L in the study area，with average 106 μg C/L. Thehighest concentration of 601 μg C/L were found atR03 during the fi rst sampling period，this decreasednorthwards and offshore during the second samplingperiod，consistent with the distribution of chlorophyll-a(Yu et al.，2012). POC concentrations in surfacewater were lower than in deeper water，owing tosediment resuspension(Yu et al.，2012).3.4 POC export fl uxes from euphotic zoneThe distribution of POC export fl uxes in the ArcticOcean has signifi cant temporal and regional variability(Yu et al.，2012). Export fl uxes of particulate 210Po and its residence time in the euphotic zone were calculatedin the irreversible scavenging model(Yang，2005).Furthermore，POC export fl uxes were calculated withthe ratio of POC content to activity of 210Po，per theequation of Buesseler et al.(1992) and Yang(2005):Table 2，and the POC fluxes(Method B，marked with BPOC-flux)are given in Table 3. At stationR03，the residence time of 210Po was the shortest，only0.38 a，and the POC export fl ux was the highest，at1 848 mmol C/(m2 ∙a). The latter may have beencaused by the upwelling in the area(Yu，2010).
Another equation to calculate POC export flux atsteady state was developed by Eppley(1979)，which assumes the residence time of 210Po and POC asTable 3.Compared with the export fl uxes derived from methodB，the results calculated by different methods arealmost equal(within 40%; He et al.，2008).Considering errors for the activities of 210Po and content of POC，we assume that the results derived bythe above two methods are consistent.
Average POC export fl uxes 945 mmol C/(m2 ∙a)(Method E) and 1 848 mmol C/(m2 ∙a)(Method B)inthe euphotic zone at R03 were much higher than thoseat R20，126 mmol C/(m2 ∙a)(Method E) and 109 mmolC/(m2 ∙a)(Method B). This shows a decreasing trendnorthward，comparable to prior work in the sameregion(Yu et al.，2012). Compared with other reportedPOC export fl uxes(Table 3)in certain areas of theworld ocean based on various approaches，the fl uxeson the Chukchi Shelf in summer were very high. It isconcluded that there is high export production on theChukchi Shelf in summer. Because available lightlimits marine primary production at high latitude，thisproduction is increased by sea ice melt during summer.Seawater in the Chukchi Sea is derived from theBering Sea，with substantial nutrients to supportprimary production. Consequently，the Chukchi Shelfhas a signifi cant role as CO2 sink during summer.4 CONCLUSION
In the present study，average dissolved 210Pb atdifferent stations had an increasing trend northwardfrom 1.12 to 1.78 Bq/m3 during July and August，and average particulate 210Pb 2.22 Bq/m3 was higher th and issolved 210Pb，at 1.49 Bq/m3 . Residence time ofparticulate 210Po was greater than that of dissolved 210Po，indicating rapid biogeochemical cycling in the studyarea. The highest concentration of POC(601 μg C/L)was at station R03 during the fi rst sampling period，which decreased northward during the second samplingperiod，consistent with the distribution of chlorophyll-a .Average POC export fl uxes in the euphotic zone werederived by two methods with an irreversible scavengingmodel. These fl uxes had a decreasing trend northward and were comparable to prior work in the study area.This suggests an effi cient biological pump in the ArcticOcean and confi rms relatively high new production inthat ocean during summer，indicative of a signifi cantrole as CO2 sink. The results may exp and knowledge ofthe carbon cycle in the western Arctic Ocean.5 ACKNOWLEDGMENT
We thank the captain and crew members of R/VXuelong and members of the oceanographic group foronboard assistance during CHINARE-4.
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