Distributions of 210Pb and 210Po in surface water surrounding Taiwan: A synoptic observation

Distributions of 210Pb and 210Po in surface water surrounding Taiwan:A synoptic observation

Wei Ching-Linga,n, Pin-Ruei Chen a, Shiao-Yu Lin a, David D. Sheu b, Liang-Saw Wen a,Wen-Chen Chou c

a Institute of Oceanography, National Taiwan University, Taipei, Taiwanb Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwanc Institute of Marine Environmental Chemistry and Ecology, National Taiwan Ocean University, Keelung, Taiwan

a r t i c l e i n f o

Keywords:210Pb210PoParticle scavengingTaiwan

a b s t r a c t

A total of 85 large-volume surface samples were simultaneously collected for the determination ofdissolved and particulate 210Pb (210Pbd and 210Pbp) and 210Po (210Pod and 210Pop) during the JointHydrographic Survey (JHS) on board four research vessels from 31 May to 17 June 2007. Generally, thespatial distributions of the two radionuclides in the surface water surrounding Taiwan are controlled bythe current system and the extent of particle scavenging. Except in the nearshore waters along the coastof China and Taiwan, a higher proportion of 210Po (�25%) is associated with particles than that of 210Pb(2–8%) in the surface water of the study area, demonstrating the different affinities to particles of thetwo radionuclides. The results show large geographic variations in terms of scavenging and removalrates of the two radionuclides. Along the shelf-edge of the marginal seas of the western North Pacific,where Kuroshio Water mixes with shelf water, 210Pb and 210Po results in a linear correlation between thetwo radionuclides. This shelf-edge exchange phenomenon is also found in the region off southeasternTaiwan where the spreading of the mixture of Kuroshio Branch Water and South China Sea SurfaceWater occurs. The residence time of 210Pb increases from 0.2 years in coastal waters to 2.6 years inKuroshio Water, and the residence time of 210Po ranges from 0.2 to 0.3 years. An atmospheric 210Pb fluxof 0.6–2.1 dpm cm�2 a�1 in the seas surrounding Taiwan is estimated.

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1. Introduction

The rate of CO2 sequestration via particle sinking in the surfacelayer of the ocean, the so-called “biological pump,” has been animportant subject for the study of carbon cycling. Hence, con-sidering the important role played by particles in carbon cycling inthe ocean, it is crucial to understand how the removal rate ofparticles varies in different ocean regions. To this end, thenaturally occurring particle-reactive radionuclides, e.g. 234Th,210Pb, and 210Po, have been extensively used as powerful tracersfor quantifying the magnitude of the particulate flux from thesurface layer of the ocean (Murray et al., 2005; Stewart et al., 2011;Verdeny et al., 2009; Wei et al., 2011).

210Pb (t1/2¼22.2 yrs) and 210Po (t1/2¼138.4 days) belong to thenatural 238U decay series. The sources of 210Pb include 226Ra decayand atmospheric deposition, and 210Pb decay is the dominant sourceof 210Po in the surface water of the ocean. Although both

radionuclides are particle-reactive, the geochemical mechanisms thatcontrol their fates in the marine environment are different. 210Pbtends to be adsorbed by inorganic particles, whereas 210Po revealscharacteristics of class B metals or of sulfur analogs, and showsstrong affinity to biological particles in natural environments (Baconet al., 1988; Bacon et al., 1976; Fisher et al., 1983; Masqué et al., 2002;Shannon et al., 1970; Stewart et al., 2007; Stewart and Fisher, 2003;Stewart et al., 2005).

Due to the interaction of Kuroshio water with coastal waters, thegeochemical behavior of particle-reactive elements in marginal seas ofthe western North Pacific tends to be dynamic (Nozaki et al., 1991).Although several profiles (Chung andWu, 2005; Wei et al., 2009; Yangand Lin, 1992) and a small number of surface values (Chen and Chung,1997; Lin and Chung, 1991; Nozaki et al., 1998) have been reported inthe vicinity of Taiwan, there is no extensive 210Pb and 210Po data thatprovides a comprehensive picture of their distribution around theisland. With sponsorship from the Joint Hydrographic Survey (JHS),extensive efforts were undertaken to coordinate four research vesselsto carry out the surveying and sampling tasks in the geographic areaspanning from the Taiwan Strait, the southern East China Sea, and thenorthern South China Sea, all the way to the region off eastern Taiwan,

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n Corresponding author.E-mail address: [email protected] (C.-L. Wei).

Please cite this article as: Wei, C.-L., et al., Distributions of 210Pb and 210Po in surface water surrounding Taiwan: A synoptic observation.Deep-Sea Res. II (2014), http://dx.doi.org/10.1016/j.dsr2.2014.04.010i

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during May and June of 2007. Simultaneous sampling by the fourvessels enabled the drawing of synoptic pictures of various oceano-graphic properties in this dynamic region. The sampling stations weredesigned to produce a detailed picture of oceanographic features inthe region.

Taking advantage of this unique opportunity, we collectedsurface seawater for the determination of dissolved and particu-late 210Pb and 210Po. The purposes of this study were (1) toestablish distribution patterns, (2) to investigate geochemicalbehaviors, and (3) to estimate the removal rates of 210Pb and210Po in the surface water surrounding Taiwan.

2. Materials and methods

Seawater samples from the surface layer (5 m depth) werecollected by CTD/Rosette assembly on board R/V Ocean Researcher I(cruise ORI-833), R/V Ocean Researcher II (cruise ORII-1444), R/VOcean Researcher III (cruise ORIII-1226), and R/V Fishery Research I(cruise FRI-95650) during a joint hydrographic survey betweenMay 31 and June 11, 2007 (cf. http://www.ncor.ntu.edu.tw/odbs/JHS/2007JHS/index.html). Along the cruise routes, continuousmeasurements of temperature, salinity, fluorescence and lighttransmission were made by the underway system (Wetlabs C-Star)installed on board. The intake of the underway system is about5 m (R/Vs Ocean Researcher I and Fishery Research I) or 2 m (R/VsOcean Researcher II and III) below the surface. Locations of sea-water sampling stations are shown in Fig. 1. A total of 85 large-volume (20 L) seawater samples were collected for the determina-tion of dissolved and particulate 210Pb and 210Po concentrations.The seawater was immediately divided into two 10-L subsamples,for separate 210Pb and 210Po determinations, and was pressure-filtered by compressed air through a pre-weighed 142 mm Nucle-pore filter (0.45 mm) mounted in a Plexiglas filter holder.

The filtrate from the 210Po sample was acidified with about 10 mlconcentrated HCl and spiked with 2.2 dpm of 209Po and 30mg of Fecarrier. Given 2 days of isotopic equilibration time, concentratedNH4OH was then added to raise the pH to �8 in order to precipitateFe(OH)3. The Fe(OH)3 precipitate was collected by decanting andcentrifuging and was dissolved in HCl and digested with HNO3.210Po and 209Po were spontaneously plated onto silver plates followingFlynn (1968). The particulate samples collected on the Nuclepore

filters were dried in a desiccator and weighed to estimate theconcentration of total suspended matter. The filter was then decom-posed and digested with HNO3/HCl/HF/HClO4. The same proceduresutilized for dissolved samples were followed in order to plate 210Poand 209Po on silver plates.

The filtrate and filter samples for 210Pb determination werestored for at least two years to let 210Po grow in towards secularequilibrium with 210Pb, after which time the same procedures fordissolved and particulate 210Po were followed. The silver discswere counted by alpha spectrometry (EG&G Ortec 576). Propercorrections for in-growth and decay were applied for all data.

3. Results

The distributions of hydrographic and geochemical parameters arepresented as contour lines on a geographic map. Data of salinity,temperature, concentrations of total suspended matter, chlorophyll-a,dissolved and particulate 210Pb (denoted as 210Pbd and 210Pbp, respec-tively) and 210Po (denoted as 210Podand 210Pop, respectively) is pro-vided in the Appendix A. A summary of various parameters indifferent water masses in the study region is proffered in Table 1.

3.1. Temperature, salinity, fluorescence, and total suspended matter

Geographical distributions of temperature, salinity, fluores-cence, and total suspended matter (TSM) concentrations in thesurface waters of the study area are shown in Fig. 2(A)–(D).Generally, the hydrography in the seas surrounding Taiwan iscontrolled by the interactions of Kuroshio Water, Kuroshio BranchWater (KBW), South China Sea Surface Water (SCSSW), and ChinaCoastal Water (CCW) (Jan et al., 2010). The CCW, which ischaracterized by low temperature and low salinity (To26.5 1C,So33.5), is limited to the northwestern Taiwan Strait. Significantfreshening, shown in salinity distribution in the northwesternTaiwan Strait, is evidently caused by the influence of riverine inputfrom the coast of China. The KBW, with its high temperature(T429.8 1C) and high salinity (S434.4), dominates the south-eastern part of the Taiwan Strait, and reaches as far north as24130'N in the coastal region of western Taiwan. With its inter-mediate temperature and salinity values, the SCSSW occupies thenorthern South China Sea and most parts of the Taiwan Strait.

Fluorescence, an indicator of the abundance of phytoplanktonbiomass, depicts a general distribution that conforms to oceano-graphic characteristics. The highest fluorescence readings werefound in coastal waters immediately off the land mass of Chinaand southwestern Taiwan. The Taiwan Strait reveals an intermedi-ate level of fluorescence, whereas the area to the east of Taiwan,where the Kuroshio Current flows, contains the lowest levels. Highfluorescence levels were also found in the sea off northeasternTaiwan, where a year-long upwelling was reported (Liu et al.,1992). Similar to the results found in the previous year (Wei et al.,2010), the geographic distribution of TSM in the Taiwan Straitreveals the influence of riverine input. Elevated TSM concentra-tions are clearly seen along the coasts of China and Taiwan. As wasthe case with fluorescence, a high TSM level was found in theupwelling region off northeastern Taiwan.

3.2. 210Pb

The distributions of dissolved (210Pbd) and particulate (210Pbp)210Pb concentrations are shown in Fig. 3(A) and (B), respectively.The contour lines of 210Pbd generally follow temperature andsalinity contours (Fig. 2(A) and (B)). Highest 210Pbd levels of420 dpm 100 L�1 were found in the region off northeasternTaiwan. Two tongue-like forms of 210Pbd flank the two sides of

Fig. 1. Locations of sampling stations occupied by R/V Ocean Researcher I (circles),R/V Ocean Researcher II (squares), R/V Ocean Researcher III (triangles), and R/VFishery Researcher I (crosses). Bathymetries of 50 m, 200 m, and 500 m are shownin gray lines. Schematic of the circulation system based on Jan et al. (2010) andLiang et al. (2003) is shown as gray arrows (1: Kuroshio; 2: Kuroshio BranchCurrent; 3: Northern SCS Current).

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southern Taiwan: one bears a relatively low concentration andextends northward in the region off southeastern Taiwan; theother carries a relatively high 210Pbd value and intrudes into theLuzon Strait before extending northward into the Taiwan Strait.The 210Pbd level in the Taiwan Strait is generally lower than that inthe northern South China Sea and in the Kuroshio. The concentra-tion of 210Pbd tends to be lowest in the nearshore water of Chinaand Taiwan. The concentration of 210Pbp is an order of magnitudelower than that of 210Pbd and, unlike 210Pbd, its spatial distributionshows less systematic variability. However, it can be seen that the210Pbp concentration values are evidently higher off the west coast

of Taiwan. High 210Pbp concentrations can also be found in thenorthern South China Sea. 210Pbpvalues are low (o0.5 dpm100 L�1) in the Luzon Strait and the region off easternTaiwan, which is in agreement with data reported by Chen andChung (1997).

3.3. 210Po

The distributions of dissolved (210Pod) and particulate (210Pop)210Po are shown in Fig. 3(C) and (D), respectively. Instead of twotongue-like forms like those shown by the distribution pattern of

Table 1Ranges and average values of salinity, temperature, chlorophyll-a, total suspended matter concentration, dissolved, particulate, and total activities of 210Pb and 210Po in thefour water masses surrounding Taiwan.

Water mass Salinity Temperature Chlorophyll-a μg L�1 TSM msg L�1 210Pbd 210Pbp 210Pod 210Pop 210Pbt 210Pot

CCW 33.4–34.1 24.1–28.1 0.19–0.96 0.27–1.80 1.6–3.3 0.5–1.3 1.2–4.6 0.8–4.6 2.5–4.2 2.6–6.033.870.3 25.671.4 0.5470.31 0.7670.56 2.570.5 0.870.4 3.071.4 1.670.9 3.470.6 4.671.2

SCSSW 33.7–34.4 29.2–30.5 0.06–0.10 0.11–0.24 7.9–14.1 0.5–3.6 2.5–4.0 0.8–4.0 8.5–15.3 3.4–5.434.170.2 29.770.4 0.0870.01 0.1670.04 10.671.9 1.871.3 3.270.5 1.270.4 12.472.2 4.470.7

KBW 33.7–34.6 28.3–30.3 0.06–0.15 0.12–0.59 10.4–20.2 0.3–1.9 2.4–6.9 0.4–6.9 11.0–20.8 3.4–9.034.170.3 29.570.6 0.1070.04 0.2470.16 13.373.5 0.770.4 4.071.7 1.170.6 14.070.35 5.172.1

KW 34.0–34.8 25.2–30.2 0.05–0.94 0.18–0.57 5.0–26.6 0.3–2.0 2.9–11.2 1.0–11.2 5.5–18.2 4.5–13.234.470.2 28.271.3 0.2070.33 0.3470.12 15.576.0 0.770.4 5.572.2 1.770.5 16.27 7.272.4

Fig. 2. Distributions of (A) temperature, (B) salinity, (C) fluorescence, and (D) total suspended matter (TSM) concentration in the surface water surrounding Taiwan.

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210Pbd, only one tongue of high 210Pod concentration extendsnorthwesterly from the northern Luzon Strait. The concentrationof 210Pod in the region off eastern Taiwan is evidently higher thanin other regions. 210Pod levels in the nearshore water adjacent tothe landmass show very low concentrations of o2 dpm 100 L�1.210Pop concentrations are consistently higher than those of 210Pbpand their spatial distributions show a less systematic pattern.Higher 210Pop levels are found in nearshore waters at the Chinacoast, off the Taiwan Bank, and in the upwelling region offnortheastern Taiwan.

4. Discussion

4.1. Spatial distributions of 210Pb and 210Po concentrations

The distributions of 210Pb and 210Po in the surface water on abasin-wide scale have been reported (Nozaki et al., 1998; Nozakiand Tsunogai, 1976; Tsunogai and Nozaki, 1971), and wide rangesfor both radionuclides were found. On a smaller scale, simulta-neous measurements of 210Pb and 210Po in the surface waters ofthe coastal oceans, e.g., the New York Bight (Li et al., 1981), theNorth Sea (Zuo and Eisma, 1993), the Mediterranean (Masqué etal., 2002), and the East China Sea (Nozaki et al., 1991), were alsopresented. Here, we present in greater detail the synoptic picturesof 210Pb and 210Po distributions in the seas surrounding Taiwan

where dynamic interactions of Kuroshio and coastal water occur.The distributions of 210Pbt and 210Pot are shown in Fig. 4(A) and(B), respectively. The average 210Pbt and 210Pot are generally inagreement with that reported by Nozaki et al. (1991) in thenorthern East China Sea and by Nozaki et al. (1998) and Yanget al. (2006) in the South China Sea.

The 210Pb levels in the ocean's surface are controlled by severalfactors, e.g., atmospheric 210Pb deposition flux, particle removalefficiency, horizontal mixing, and mixed layer thickness (Nozaki etal., 1976). The most noticeable feature of 210Pb distributions in thestudy area are the two tongue-like shapes of 210Pbt and 210Pbd(Fig. 3(A)), which flank the two sides of southern Taiwan. The210Pbp level in the Kuroshio and KBW is low (�0.5 dpm 100 L�1,Fig. 3(B)), and it contributes o10% of the 210Pbt in the surfaceseawater of deeper regions. These patterns can only be revealed bysufficient coverage of sampling stations and were previouslymissed by Lin and Chung (1991) and Chen and Chung (1997),who presented surface 210Pb data at very few stations in the regionoff eastern and southern Taiwan, respectively. Such a pronouncedfeature is a result of the horizontal transport of water from theKuroshio through the Luzon Strait. The Kuroshio Branch Currentbrings water with relatively higher 210Pb content into the TaiwanStrait to form a tongue of high 210Pbt and 210Pbd concentrations offthe coast of southwestern Taiwan. This water subsequently entersthe Taiwan Strait along the axis of the Peng-Hu Channel. It isnoteworthy that dissolved 234Th, a particle-reactive radionuclide

Fig. 3. Distributions of (A) dissolved 210Pb (210Pbd), (B) particulate 210Pb (210Pbp), (C) dissolved 210Po (210Pod), and (D) particulate 210Po (210Pop) in the surface watersurrounding Taiwan.






with a much shorter half-life (t1/2¼24.1 d), also revealed a similartongue-like extension along the Peng-Hu Channel (Wei et al.,2010). After mixing with SCSSW, part of the KBW may loopthrough the northern Luzon Strait and spread into the region offsoutheastern Taiwan to reunite with the Kuroshio (Huang et al.,2010; Jan et al., 2010; Liang et al., 2003). The mixing of the KBW inthe Kuroshio results in a tongue-shaped flow of 210Pbt and 210Pbdoff southeastern Taiwan. Following the path of the Kuroshio, theinfluence of the KBW can be traced to 24130'N. In the areacharacterized by low temperature (Fig. 2(A)) and highchlorophyll-a content (Fig. 2(C)), maximum concentrations of210Pbt and 210Pbd were found off northeastern Taiwan. This patchof high 210Pbt and 210Pbd levels is caused by the prevailingupwelling, which is induced by the topographic effect when theKuroshio turns northeastward (Liu et al., 1992). The concentrationsof 210Pbt in this water are consistent with210Pb values, 12–15 dpm100 L�1, in the subsurface water of the Okinawa Trough (Yang andLin, 1992).

The 210Pbt is very low (o5 dpm 100 L�1) along the coast ofChina, suggesting fast removal from seawater by sorption andsubsequent settling of particles in nearshore waters. Although therivers in western Taiwan are small, the sediment yields of thesemountainous rivers are high (Milliman and Meade, 1983). Theserivers contribute a large amount of terrigenous particles withannual sediment flux of 180 Mt into the Taiwan Strait (Dadson etal., 2003). Enhanced scavenging by elevated TSM concentration(Fig. 2(D)) results in higher 210Pbp and particulate 234Th concen-trations (Wei et al., 2010) in the coastal water. A significant

correlation (R2¼0.72, n¼17) between the ratio of particulate tototal 210Pb and TSM concentration was found in the coastal waterof the eastern Taiwan Strait, depicting the influence of particulateinput of rivers to the scavenging of 210Pb. Elevated concentrationsof 210Pbp can also be identified in the region along the shelf breakof the northern South China Sea. Higher levels of 210Pbp in thisregion may be a result of enhanced scavenging due to mixingwhen westward internal waves impinge on the shelf (Nozaki et al.,1991; Wang et al., 2007) or they could be caused by eastwardtransport of 210Pb-enriched particulates originating from the shelfof southern China. Extensive analysis of the extant data has notenabled the present researchers to discern which process is thecause of the phenomenon.

Although radioactive decay of 210Pb is the dominant source of210Po in surface water, the distribution pattern of 210Po is notnecessarily similar to that of its progenitor. In addition to theinfluence of its source material, the concentrations and thedistributions of 210Po are also controlled by scavenging andremoval rates. Except in the nearshore water along the Chinacoast, the 210Pod (Fig. 3(C)) and 210Pot concentrations (Fig. 4(B)) areconsistently lower than those of 210Pbd and 210Pbt ,respectively,due to atmospheric 210Pb deposition into and preferential 210Poremoval from the surface ocean. The distribution pattern of 210Potin the region off eastern Taiwan is thus similar to the distributionsrepresented in the contour map drawn by Lin and Chung (1991).The meandering of 210Pod and 210Pot contours along the path of theKuroshio corroborates the S¼34.2 contour (Fig. 2(B)), reflectingthe interplay of scavenging and mixing in the region. The 210Podand 210Pop concentrations in the Luzon Strait are also very similarto the summer values measured at seven stations by Chen andChung (1997). Like 210Pb (Fig. 3(A) and Fig. 4(A)) and 234Th (Wei etal., 2010), the extension of the 210Po-enriched tongue off south-western Taiwan is manifest in both 210Pod and 210Pot distributions.Elevated 210Popconcentrations in the upwelling region off north-eastern Taiwan are also evident, indicating enhanced 210Po scaven-ging by biological particles (Cherry and Shannon, 1974; Fisher etal., 1983). The distribution pattern of 210Pbp and 210Pop will befurther discussed below, when the partitioning of the radionu-clides is presented.

4.2. 210Pb/226Ra and 210Po/210Pb disequilibria

The atmospheric 210Pb flux, the predominant source of210Pb insurface water, results in an excess of 210Pb relative to 226Ra in thesurface layer of the ocean (Nozaki et al., 1976). Activity of 226Rawas notmeasured in this research; hence, the degree of 210Pb/226Ra disequili-brium cannot be specifically determined in this study. However, 226Raactivity in the northern East China Sea (Nozaki et al., 1991) and theKuroshio (Chen and Chung, 1997) falls in the range of 6–14 dpm100 L�1, and tends to linearly correlate with salinity in the region thatlies away from the Yangtze River plume (S432). Hence, the210Pbt/226Ra can be approximated by the 226Ra activity calculated fromsalinity. Using this method, the distribution is found to be very similarto that of 210Pbt (Fig. 4(A)). The contour of unity generally follows the200 m bathymetry, which reveals 210Pb excess with respect to 226Ra atdeep stations. The highest 210Pbt/226Ra ratio of 43 is discovered in theKuroshio. By contrast, except in the tongue of KBW extending to thePeng-Hu Channel (Fig. 4(A)), all stations in the Taiwan Strait and onthe East China Sea shelf show a deficiency of 210Pb relative to 226Ra,indicating a fast removal rate in coastal water on the 210Pb time scale.

It has been reported that the 210Po/210Pb ratio in seawaterranges from o0.3 to �1 in the surface ocean (Nozaki et al., 1976).Since both 210Po and 210Pb were determined in dissolved andparticulate phases in this study, in addition to the ratio in the totalphase, we may calculate the ratio in the two phases (dissolved andparticulate) and present their spatial distributions in the seas

Fig. 4. Distributions of (A) total 210Pb (210Pbt) and (B) total 210Po (210Pot) in thesurface water surrounding Taiwan.






surrounding Taiwan (Fig. 5). The range of the210Pot/210Pbt ratio islarge, with values ranging from low (o0.2) off eastern Taiwan togreater than unity (41.7) in the coastal waters of China. Theexcess 210Po phenomenon in nearshore waters of China was alsofound in other regions, e.g., in the Yangtze River estuary (Nozaki etal., 1991), the New York Bight (Li et al., 1981), and the North Sea(Zuo and Eisma, 1993). The 210Po excess can be attributed topreferential regeneration of 210Po as compared to 210Pb fromscavenged biogenic particles in sediments that then diffuse intothe surface (Bacon et al., 1988; Kim and Yang, 2004; Li et al., 1981).It is plausible that the regeneration of 210Po is caused by areduction of insoluble Po(IV) to more soluble Po(II) in sediments(Benoit and Hemond, 1990). The 0.5 contour, which is parallel to200 m bathymetry, indicates a moderate deficiency of 210Po, with210Pot/210Pbt ratios of 0.5–1.0 in most regions of the Taiwan Strait.The 210Pod in the surface waters usually shows a depletion, with210Pod/210Pbdo1, whereas 210Pop/210Pbp ratios are usually greaterthan unity except at the nearshore stations. This situation indi-cates a preferential scavenging of 210Po which leads to its removalout of the surface layer via particle settling (Bacon et al., 1976).Field data (Shannon et al., 1970) and laboratory experiments(Stewart et al., 2005) suggest that there is a preferential bioaccu-mulation of 210Po over 210Pb in plankton, which may result in anexcess of 210Po in biological particles. By contrast, 210Pop/210Pbpratios at nearshore stations surrounding Taiwan show a depletionof 210Po relative to 210Pb in particulate matter, which can be causedby riverine input of terrestrial detritus with a low 210Pop/210Pbpratio. At the river mouths of the mountainous rivers of westernTaiwan, a 210Po/210Pb ratio of lower than unity in the suspendedparticles was found (Wei et al., 2012).

4.3. Partitioning between the dissolved and particulate phases

The adsorption or uptake by particulate matter and subsequentremoval by particle settling are the mechanisms of 210Pb and 210Poscavenging in the ocean (Bacon et al., 1976). Hence, the partition-ing between the dissolved and particulate phases may determinethe rate of scavenging-removal of particle-reactive elements fromthe ocean. The percentage of particulate 210Pb and 210Po activitiesin the seawater is shown in Fig. 6. As is shown in Fig. 6, 490% of210Pb and 475% of 210Po exist in dissolved form in deeper regionsof the studied area. Contrary to the circumstances in the deepocean, a significant portion of 210Pb and 210Po on the shelf isassociated with particulate matter on the shelf. It is noted that theregion between China and the Dongsha Atoll contains a higherpercentage of particulate 210Pb and 210Po. This finding is compar-able with the values on the southern shelf of the South China Sea

(Yang et al., 2006). As discussed in the previous section, this maybe a result of enhanced scavenging due to mixing when westwardinternal waves impinge on the shelf (Wang et al., 2007) or ofeastward transport of particulates originating from the shelf ofsouthern China. Along the coastline of China and western Taiwan,a very high percentage of the radionuclides is found in theparticulate phase, similar to findings along the shallow coast of

Fig. 5. Distributions of 210Po/ 210Pb ratio in (A) total, (B) dissolved, and (C) particulate phases of surface water in the surface waters surrounding Taiwan.

Fig. 6. Distributions of percentage of particulate to total phase of (A) 210Pb and(B) 210Po in the surface waters surrounding Taiwan.






the southern North Sea (Zuo and Eisma, 1993), the Yellow Sea(Hong et al., 1999) and the northwestern Mediterranean Sea(Masqué et al., 2002).

The partition coefficients of 210Pb (Kd(Pb)) and 210Po (Kd(Po))have been used as indicators of their affinity to marine particles(Bacon et al., 1976; Baskaran and Santschi, 1993; Santschi et al.,1979; Wei and Murray, 1994). Kd(Pb) and Kd(Po) are calculated by

KdðPbÞ ¼210Pbp

210Pbd � TSM� 106 ð1Þ

and

KdðPoÞ ¼210Pop

210Pod � TSM�106 ð2Þ

Both 210Pb and 210Po show a high affinity for particulate matter,as their log Kd lay mostly between 4.6 and 6.5. These figures aresimilar to the values reported for other shelf seas (Bacon et al.,1988; Masqué et al., 2002; Zuo and Eisma, 1993). The log–logcorrelations of Kd(Pb) and Kd(Po) with TSM are shown in Fig. 7.Within more than two orders of magnitude of TSM concentrations,a decreasing trend for both Kd(Pb) and Kd(Po) was found, which isroughly similar to findings shown previously (Wei and Murray,1994). However, it is noted that the Kd(Pb) in nearshore water offwestern Taiwan and China, which contains TSM concentration41 mg L�1, deviates from the log Kd(Pb)–log TSM relationshipobtained from other regions, indicating the riverine influence onthe partitioning of 210Pb in turbid waters. Although not as evidentas in the log Kd(Pb)–log TSM correlations, the log Kd(Po) alsoshows a lower slope with log TSM in more turbid waters. Honget al.(1999) and Wei et al. (2012) have found that both Kd(Pb) andKd(Po) appear to be less variable with regard to the particulateload in the turbid waters of the Yellow Sea. Baskaran and Santschi(1993) and Baskaran et al.(1997) reported Kd(Pb) values rangingbetween 104.6 and 105.8 ml g�1 in estuarine waters, similar to ournearshore values. Higher Kd(Pb) values are found here when 210Pb-bearing terrestrial detritus is washed from watersheds into near-shore waters. An inverse relationship between Kd and TSM hasbeen widely reported for particle-reactive trace elements andradionuclides and has been extensively discussed and attributedto the presence of colloids in the filter-passing fraction (Baskaranand Santschi, 1993; Honeyman et al., 1988; Honeyman andSantschi, 1989; Santschi et al., 1999). The negative correlation ofKd(Pb)–TSM and Kd(Po)–TSM suggests that surface coordinationand colloid aggregation reactions may also play a critical role in

controlling the partitioning of 210Pb and 210Po between thedissolved and particulate phases.

4.4. Shelf-edge exchange phenomenon

Nozaki et al.(1991) previously reported that the distributions of210Pb and 210Po in surface seawater of the northern East China Seaare strongly affected by mixing processes in the vicinity of theshelf edge. Indicators for removal of 210Pb and 210Po in surfacewaters of the western North Pacific were evident when theactivities of the two radionuclides were plotted against salinity(Nozaki et al., 1991). The concave curves of S-210Pb and S-210Porelationships showed a larger curvature in the high salinity range(S434.5), implying that greater scavenging and removal ratesoccur in the frontal region of the shelf water and the Kuroshio.Along with other data obtained by Nozaki et al. (1991), thecorrelation of 210Pbt and 210Pot is shown in Fig. 8(A). The 1:1relation observable in the figure indicates that there is secularequilibrium between 210Pb and 210Po is achieved, and that there isno fractionation between the two radionuclides. It has been foundthat a linear correlation of 210Pb and 210Po exists (r2¼0.85, n¼38)for those stations shown in Fig. 8(B), which are located in thevicinity of the shelf edge of the marginal seas of the western NorthPacific and in the region off southeastern Taiwan. The 210Pot/210Pbtratio at these stations ranged from 0.17 to 0.50 (Fig. 5(A)),indicating a greater removal of 210Po from surface waters and

Fig. 7. Correlation of Kd(Pb) (solid squares) and Kd(Po) (open squares) with TSMconcentration. Data collected from nearshore waters off China and Taiwan coast arecircled by dotted line.

Fig. 8. (A) Correlation of 210Pbt and 210Pbt in the study area (solid circles) and thenorthern East China Sea by Nozaki et al. (1991) (open circles). (B) Geographiclocations of stations enclosed in the dotted area in (A). Bathymetry of 200 m isshown in gray line.






significant fractionation of 210Po and 210Pb during the mixing ofKuroshio and shelf waters. This analysis opposes the conclusion ofNozaki et al. (1991), which stated that “no fractionation between210Pb and 210Po is taking place during removal in the shelf-edgemixing zone.” It should be noted that the spreading of the mixtureof SCSSW and KBW in the region off southeastern Taiwan, whichoriginates from the northern South China Sea through the LuzonStrait (Huang et al., 2010; Liang et al., 2003), also reveals shelf-edge exchange characteristics. Chou et al. (2007) found that theoutflow of SCS subsurface water, which was enriched withnutrients and dissolved inorganic carbon, results in significantchange in carbonate chemistry in the western Philippine Sea. It isplausible that production and remineralization of biological parti-cles may be enhanced by this outflow, which process may facilitatethe fractionation of 210Pb and 210Po in this region.

4.5. Residence times of 210Pb and 210Po

For the estimation of the residence time of 210Pb and 210Po, theformulation given in the model of Bacon et al. (1976) is adoptedhere. Neglecting atmospheric deposition of 210Po (Turekian et al.,1977), the mass balance for dissolved 210Po in the surface water is

λPo210Pbd ¼ λPo210Podþ JPo ð3Þwhere JPo is the rate of scavenging of dissolved 210Po and λPo is thedecay constant of 210Po. The mass balance for particulate 210Po inthe surface water is

λPo210Pbpþ JPo ¼ λPo210PopþPPo ð4Þwhere PPo is the rate of removal of particulate 210Po.

With respect to the scavenging and particle removal rates, theresidence times of dissolved, particulate, and total 210Po (τPo withsubscript of d, p, and t, respectively) can be calculated by

τPod ¼210PodJPo

ð5Þ

τPop ¼210PopPPo

ð6Þ

τPot ¼210PotPPo

ð7Þ

To estimate the removal rate of 210Pb, it is assumed that 210Pb isremoved from seawater by the same particles for 210Po, hence

PPb ¼ PPo �210Pbp210Pop

ð8Þ

where PPb is the removal rate of 210Pb from the surface water. Theimplicit assumption of Eq. (8) is that the 210Po/210Pb ratio ofsuspended particles is the same as the ratio of the sinking particlesexiting from the mixed layer. Except for unpublished data on theshelf break off northeastern Taiwan, which shows a ratio of1.170.4 (n¼5), there is no information on the 210Po/210Pb ratioin sinking particles from the mixed layer in the study region.However, in the deep layer (640 m) of the Okinawa Trough, Naritaet al. (1990) reported the 210Po/210Pb ratio of 1.05 in settlingparticles collected by sediment traps. In addition, the 210Po/210Pbratio in the sinking particles collected at 30 m in the central SouthChina Sea ranges from 1.3 to 3.5 (Wei et al., 2011). As seen in Fig. 5(C), most of the 210Po/210Pb ratios in suspended particles are in therange between 1 and 3.5, which is similar to the limited data of theratio of sinking particles. Hence, we believe that the PPb estimatedusing Eq. (8) is valid.

Using this PPb, the mass balance for particulate 210Pb is

JPb ¼ λPb210PbpþPPb ð9Þ

where JPb is the rate of scavenging of dissolved 210Pb and λPb is thedecay constant of 210Pb. The mass balance of dissolved 210Pb is

λPb226Raþ IPb

Z¼ λPb

210Pbdþ JPb ð10Þ

where IPb is the atmospheric 210Pb deposition flux(dpm cm�2 a�1) and Z is the average penetration depth of theatmospheric 210Pb deposition. With respect to the scavenging andparticle removal rates, the residence times of dissolved, particu-late, and total 210Pb (τPb with subscript of d, p, and t, respectively)can be calculated by

τPbd¼

210Pbd

JPbð11Þ

τPbp ¼210Pbp

PPbð12Þ

τPbt¼

210Pbt

PPbð13Þ

The residence times of dissolved and particulate 210Pb and210Po with respect to scavenging and particle removal rates indifferent water masses calculated by Eqs. (5)–(7) and (11)–(13) aresummarized in Table 2, in which associated uncertainties esti-mated by error propagation are also given. Since excess activitiesof 210Po (210Pod4210Pbd and 210Pop4210Pbp, Table 1) are found fordissolved and particulate phases in CCW, the residence times of210Po (and hence 210Pb) cannot be estimated by the model due tothe lack of information of other source-terms (i.e., terrestrial inputand sedimentary regeneration). In other water masses, althoughthe errors are generally not insignificant, the τPod and τPbd

are inthe range of 0.2–0.3 years and 0.5–2.5 years, respectively. The τPopand τPbp show similar values of 0.1 year in all water masses, whichresults in the τPot and τPbt fall in the ranges of 0.3–0.4 years and0.6–2.6 years, respectively. Among the three water masses, thelongest τPot of 0.3 years and τPbt of 2.6 years, respectively, werefound in the Kuroshio Water. These values are shorter and longerthan the residences time of 210Po(0.6 years) and 210Pb (1.7 years),respectively, reported by Nozaki et al. (1976) in the surface waterof the Pacific. The long τPbt indicates a low removal rate of 210Pb inoligotrophic Kuroshio water. A large deficiency of 210Po withrespect to 210Pb in the water column of the Okinawa Trough wasfound, which fact was attributed to the low removal of 210Pb andthe accumulation of atmospheric 210Pb input along the flow pathof the Kuroshio (Nozaki et al., 1990). In the SCSSW, the τPot of0.3 years and the τPbt of 0.6 years were estimated, which areshorter than the values reported by Nozaki et al. (1998), whoestimated the residence time of 210Po and 210Pb, 1.4 and 1.1years,respectively, in the northern South China Sea. The difference mustbe caused by the low atmospheric 210Pb flux of 0.4 dpm cm�2 a�1

assumed by Nozaki et al. (1998). The atmospheric 210Pb flux (IPb)into different water masses can be calculated by Eq. (10) andthe results are shown in Table 2. The IPb decreases from2.1 dpm cm�2 a�1 in the SCSSW to 0.6 dpm cm�2 a�1in the KW.Because this study covers a relatively large geographic area in thewestern marginal seas neighboring the Asian continent, the atmo-spheric deposition of 210Pb may vary significantly under theinfluence of prevailing air masses. The IPb estimated from thisstudy is consistent with the limit measurements of atmospheric 210Pb,including 0.75 dpm cm�2 a�1 in the South China Sea (Xu et al., 2010),0.7–1.5 dpm cm�2 a�1 in the south Eastern China Sea (Su et al., 2003),and1–2.8 dpm cm�2 a�1 on the continental shelf of the westernNorth Pacific (Tsunogai et al., 1985). Using the atmospheric 210Pb fluxof 2.1 dpm cm�2 a�1in the SCSSW, a relatively short τPbt of 0.2 year inthe CCW can be estimated by Eqs. (10) and (13). These results areconsistent with Nozaki et al. (1991), who estimated the τPb ranges






from 0.2 years over the shelf to 0.5 years in the slope region of thenorthern East China Sea.

5. Conclusions

Near-simultaneous sampling by four research vessels provided asynoptic picture of oceanographic properties in the seas surroundingTaiwan. Regional particle scavenging processes were investigated bydetermining and analyzing the spatial distributions of dissolved andparticulate 210Pb and 210Po concentrations in the study area. It wasfound that the spatial distributions of the two radionuclides in thesurface waters were controlled by the different hydrographic settings,i.e., by the current systems and by the extent of particle scavenging. Asa result of horizontal transport and mixing of the Kuroshio throughthe Luzon Strait, a tongue-shaped form of high 210Pb concentrationextended into the Taiwan Strait along the axis of the Peng-Hu Channel.Another tongue of low 210Pb flanking southeastern Taiwanwas relatedto the spreading of the KBW via the northern Luzon Strait andsubsequent northward transport of the Kuroshio. Due to atmospheric210Pb deposition into and preferential removal of 210Po from thesurface ocean, 210Po concentration was generally lower than that of210Pb except in the nearshore water, and its spatial distributionshowed a less dynamic pattern than that of 210Pb. A linear relationshipbetween the 210Pb and 210Po concentrations at those stations locatednear the shelf break and at those in the region of the KBW spreadingindicated the two radionuclides were removed proportionally whenthe Kuroshio mixed with coastal waters. Based on the partitioningbetween their dissolved and particulate phases, it was found that490% of 210Pb and 475% of 210Po existed in the dissolved form in thedeeper regions, whereas a significant portion of 210Pb (�20%) and210Po (�30%) was associated with particulate matter on the shelf. The

inverse correlation of the partitioning coefficients of 210Pb and 210Poand TSM concentration indicate that surface coordination and thepresence of colloids in the filter-passing fraction, as well as colloidaggregation reactions, may play an important role in controlling thepartitioning of 210Pb and 210Po between the dissolved and particulatephases. Mass balances for dissolved and particulate 210Pb and 210Powere used to estimate the scavenging and particle removal rates indifferent water masses. With respect to the particle removal rate, theresidence times of 210Pb and 210Po are in the range of 0.2–2.6 and 0.2–0.3 years, respectively. The atmospheric 210Pb flux of 0.6–2.1 dpm cm�2 a�1 estimated by the scavenging model is consistentwith values reported in the literature on the region studied.

Acknowledgments

Assistance provided by the captains and crew of R/Vs OceanResearcher I, II, and III and Fishery Researcher I is appreciated. Onboard the ships, L.-H. Chou, and Y.-R. Hou helped with seawatersample collection and processing. We are thankful for the help ofMs. W.-H. Lee and Mr. H.-S.Wang for processing and plotting CTDdata. This research was supported by the National Science Councilthrough grant NSC 97-2611-M-002-004.

Appendix A

Station, longitude, latitude, temperature, salinity, total sus-pended matter concentration (TSM), chlorophyll-a (Chl-a), dis-solved (210Pbd)and particulate (210Pbp)210Pb, dissolved (210Pod) andparticulate (210Pop)210Po. Uncertainties represent counting errors(71s). See Table A1.

Table 2Residence times of dissolved, particulate, and total 210Pb and 210Po and atmospheric 210Pb flux in the four water masses surrounding Taiwan.

Water mass τPodα τPot τPbd

τPbpτPbt IPb

a

(year) (dpm cm�2 a�1)

CCW – – – – – 0.2b –

SCSSW 0.270.2 0.170.04 0.370.1 0.570.4 0.170.1 0.670.5 2.1KBW 0.270.3 0.170.05 0.370.2 1.471.3 0.170.1 1.471.3 1.0KW 0.370.3 0.170.08 0.470.3 2.572.5 0.170.1 2.672.5 0.6

a Values estimated by assuming the mixing depth of 100 m.b Result calculated by assuming the atmospheric 210Pb flux of SCSSW.






Table A1

Stationa Long. Lat. T S TSM Chl-a 210Pbd210Pbp

210Pod210Pop

(1E) (1N) (1C) PSU (mg L�1) mg L�1 (dpm 100 L�1)

1-1 120.00 22.00 29.21 33.99 0.13 0.28 11.1770.69 0.3970.08 5.9870.53 0.9570.161-3 119.50 21.50 29.56 33.87 0.14 0.11 13.7970.98 0.2570.05 3.3670.33 0.9270.141-6 119.50 20.00 29.36 34.12 0.20 0.12 7.9371.01 0.5670.11 3.5870.44 1.1570.281-9 120.00 21.00 29.97 33.72 0.14 0.10 11.3770.82 0.4870.11 3.1670.37 0.8070.151-11 120.50 22.00 29.42 33.84 0.17 0.14 9.3470.63 0.2570.06 6.5670.53 0.7370.171-17 120.50 20.50 29.69 34.45 0.18 5.4670.53 0.3070.06 3.7370.42 1.0670.201-20 121.00 20.00 29.97 33.82 0.26 0.37 6.1970.54 0.4070.08 8.9970.97 2.0470.471-22 121.00 21.00 29.79 34.37 0.16 0.13 10.4470.72 0.5170.11 6.8770.58 1.7670.311-26 121.75 21.50 29.20 34.52 0.18 15.4271.16 0.4270.08 7.6970.47 1.4270.241-29 122.50 21.50 28.81 34.52 0.34 0.07 17.6771.35 0.5470.09 7.6970.45 1.9370.261-31 123.00 21.50 29.34 34.44 0.32 0.09 15.7670.90 0.4670.09 7.9070.61 1.6670.292-1 122.00 25.50 26.38 34.43 0.34 0.27 5.5970.53 0.6370.05 3.4870.38 0.9770.142-4 121.00 26.00 26.16 33.94 0.21 0.42 5.5170.61 0.7570.06 3.4070.36 0.9470.152-6 120.00 26.00 24.12 33.52 0.27 0.59 3.3470.46 0.4970.04 4.5870.39 0.8570.162-8 119.25 25.00 25.60 33.98 1.80 1.31 2.9170.46 1.3370.08 1.2370.22 3.4070.292–9 118.70 24.50 26.27 33.42 1.22 1.66 1.6370.30 0.8670.08 3.1270.38 1.2470.222-11 118.17 24.00 25.93 33.98 0.39 0.41 2.1570.37 0.4970.06 1.8870.24 1.5470.272-12 117.50 23.50 24.05 34.13 0.70 0.57 2.5870.33 0.8270.06 4.0770.43 1.5370.252-13 117.00 23.00 28.11 33.62 0.34 0.31 2.5470.38 0.5870.06 1.6070.23 0.9870.192-14 117.50 23.00 27.65 34.04 0.43 4.9870.59 0.9070.06 3.0070.29 0.7370.172-15 118.00 23.00 26.82 34.27 0.25 0.34 6.3670.62 1.3270.10 4.1570.42 1.7570.252-18 119.25 23.00 25.49 34.45 0.28 0.48 8.3770.65 0.7670.06 7.1670.43 0.9870.242-20 118.00 23.67 25.43 34.08 0.57 1.10 2.5470.38 1.2770.08 4.4670.46 1.5270.282-27 121.00 25.50 26.45 33.77 0.24 0.40 4.6670.57 1.0370.10 3.1870.37 1.7970.282-29 120.00 24.50 26.52 34.20 0.47 0.38 9.1970.73 1.8170.20 5.7470.46 2.1070.292-30 119.50 24.00 26.97 34.40 0.38 0.29 5.6170.54 0.9270.10 5.3670.36 0.9270.182-31 119.50 23.50 24.81 34.44 0.79 1.09 6.9370.61 2.2370.12 5.4870.49 1.2970.192-32 119.75 23.00 29.20 34.25 0.33 0.15 8.7770.78 0.8270.09 5.7770.52 0.9170.172-35 119.98 23.50 28.62 34.06 1.54 0.93 7.3770.59 3.2070.28 3.5470.35 1.9670.282-36 120.00 24.00 27.86 34.33 1.44 0.60 6.5170.69 2.5970.37 4.7870.46 2.5070.342-D4 121.36 25.18 28.19 32.62 2.84 1.43 1.7570.31 3.6670.29 1.7170.28 1.6270.282-B7 120.92 25.00 28.34 32.97 3.50 1.42 2.3970.33 4.8370.31 1.9370.25 2.8470.342-B10 120.65 24.51 28.18 32.54 4.72 2.31 3.0470.38 6.0370.24 2.0670.29 3.1870.352-B11 120.52 24.34 28.58 33.29 5.01 1.80 4.1570.52 3.1270.32 2.3770.28 2.7270.412-B12 120.44 24.21 28.84 34.00 21.22 1.43 3.0770.34 7.7070.68 2.4770.31 4.4370.452-D14 120.20 23.88 28.95 34.13 3.35 2.34 2.6870.39 1.7270.20 1.8570.25 2.1170.382-B17 120.08 23.34 29.35 33.92 2.95 0.66 1.5870.32 2.3570.33 2.9170.38 2.3870.342-B18 120.06 23.25 29.37 34.02 1.82 2.05 4.4170.49 1.9570.25 2.2170.30 1.3670.252-D19 120.06 23.02 29.00 33.33 2.21 1.15 4.2070.49 3.8070.14 3.5670.42 2.6670.403-3 119.00 21.00 29.51 33.91 0.24 0.09 9.4570.53 0.5470.10 3.2270.22 1.2070.203-5 119.00 19.99 30.14 34.16 0.13 0.18 14.0570.62 1.2970.17 3.3270.33 1.0270.233-6 118.50 20.00 30.46 34.06 0.11 0.14 9.5470.46 0.6470.10 2.4570.27 1.0670.253-8 117.51 20.00 29.47 33.95 0.15 0.09 12.9270.61 1.5470.17 4.0370.32 1.0670.193-10 117.00 20.50 29.45 34.13 0.12 0.11 10.1270.52 0.9870.12 3.2370.31 1.4770.303-14 117.75 21.00 29.21 34.40 0.14 0.13 9.4170.52 3.3470.25 3.2670.25 2.1870.323-16 117.00 21.50 29.20 33.93 0.17 0.14 10.8570.49 3.6170.27 2.8170.22 1.3070.253-18 118.50 21.50 29.62 34.12 0.24 0.15 9.3270.48 3.3370.30 2.5070.18 0.9070.163-22 117.00 22.50 28.83 33.10 0.48 0.40 6.3270.34 2.9470.16 1.8870.23 1.3170.233-23 118.00 22.50 26.56 34.35 0.24 0.45 7.4470.45 0.9170.12 9.3770.52 2.2270.403-24 119.00 22.50 28.91 34.04 0.24 0.13 9.0370.47 6.4270.39 2.7370.20 1.8470.233-25 119.50 22.50 28.95 34.24 0.08 0.12 8.7870.42 0.3370.08 2.5570.23 0.8970.173-26 120.00 22.50 29.67 34.21 0.12 0.16 12.2870.56 1.8670.15 2.6470.24 2.0270.26F-1 122.51 25.00 28.04 34.03 0.36 0.22 10.8870.53 0.7070.08 4.3170.38 1.1970.26F-2 122.50 25.50 25.24 34.45 0.57 1.69 13.7370.63 1.9570.17 3.5370.30 2.9670.37F-3 122.50 26.00 26.25 34.43 0.55 0.26 9.7070.46 1.4270.12 3.9170.29 1.9270.31

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F-4 123.00 26.00 26.88 34.35 0.45 0.45 7.4770.38 0.6170.10 3.3270.31 1.3170.23F-5 122.99 25.75 26.10 34.42 0.52 0.86 10.3970.44 0.5970.10 3.4870.27 1.4070.25F-6 123.00 25.50 27.85 34.19 0.41 0.32 9.2270.40 0.6570.10 2.8770.23 1.6270.33F-7 123.00 25.25 27.72 34.24 0.39 0.18 12.5070.56 0.8970.12 4.3370.37 0.9870.23F-8 123.00 25.01 27.31 34.73 0.31 0.09 18.6470.86 0.7670.07 7.8270.43 1.9270.33F-9 122.50 24.50 29.09 34.23 0.44 0.11 21.8070.99 0.4670.09 7.9670.43 1.8070.30F-10 123.00 24.00 27.99 34.67 0.33 0.09 26.5571.31 0.5870.09 5.7370.33 1.7770.31F-11 122.50 24.01 28.73 34.23 0.33 0.19 26.5071.20 0.6370.09 3.2570.24 1.5070.30F-12 121.99 24.00 30.34 33.79 0.37 0.14 12.9670.59 0.9870.14 3.2270.35 0.8770.22F-13 122.00 23.51 30.05 34.00 0.23 0.10 11.1070.56 0.6170.06 2.9370.25 1.1870.26F-14 122.50 23.50 29.62 33.90 0.29 0.16 26.3670.85 0.4270.06 9.7270.65 1.4570.29F-15 123.00 23.50 28.45 34.76 0.22 0.07 23.6770.98 0.5570.10 11.1570.47 2.0670.30F-16 122.79 23.00 28.44 34.57 0.24 0.10 20.3170.84 0.3670.04 7.6070.47 2.6470.40F-18 122.00 23.00 29.33 34.29 0.25 0.14 13.2270.65 0.5370.07 4.5770.33 1.8870.40F-19 121.25 22.50 29.51 33.95 0.13 0.14 10.5670.50 0.5570.09 2.9070.30 0.6570.19F-20 121.49 22.50 29.71 34.13 0.23 0.20 12.9570.60 0.3470.03 3.6370.30 1.2470.27F-21 122.00 22.50 29.29 34.21 0.22 0.13 16.5970.81 0.4670.09 4.8370.39 1.1170.25F-23 122.99 22.50 28.27 34.65 0.20 0.09 20.2270.85 0.5970.09 6.8970.32 2.1070.39F-24 123.00 22.00 28.93 34.29 0.22 0.15 15.4670.74 0.5270.08 4.8670.37 1.2970.23F-25 122.50 22.00 29.15 34.43 0.21 0.16 17.8370.75 0.3370.05 7.8470.38 1.4370.28F-26 122.00 22.00 29.44 34.42 0.25 0.12 16.5370.62 0.3170.04 5.9170.43 1.5470.24F-27 121.50 22.01 29.79 34.41 0.19 0.15 15.3470.67 0.4970.08 4.8570.34 1.8170.37F-28 121.26 22.01 30.17 34.00 0.18 0.13 13.6270.65 0.3770.06 3.9270.33 1.2970.29F-29 121.01 22.01 27.54 34.25 0.37 1.43 14.7070.64 0.7570.07 3.9270.35 1.6770.31F-30 121.00 22.49 28.81 34.07 0.43 0.25 19.8471.02 0.8570.10 2.4470.20 0.9570.21F-31 121.50 23.00 29.28 34.19 0.59 0.23 13.0170.57 0.4770.06 3.1870.31 0.4370.16F-32 121.58 23.51 29.73 34.31 0.19 0.26 26.0070.79 0.4970.06 4.0770.28 1.6870.23F-33 121.67 24.00 28.58 33.45 0.60 0.46 26.0071.06 1.8670.15 2.1870.20 1.0970.24F-34 122.00 24.50 28.00 33.85 0.35 0.43 25.9271.16 0.5370.07 3.8770.25 1.0970.26F-35 122.09 24.98 26.73 34.13 0.30 0.74 25.7471.19 1.3170.12 3.4270.28 1.6070.21

a Coded by R/V-station number. R/V Ocean Researcher I, II, III, and R/V Fishery Researcher I are abbreviated by 1, 2, 3, and F, respectively.

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Distributions of 210Pb and 210Po in surface water surrounding Taiwan: A synoptic observation