Dissolved platinum in major rivers of East Asia: Implications for the oceanic budget


  • Tseren-Ochir Soyol-Erdene,

    1. School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, South Korea
    2. Now at Korea Polar Research Institute, Songdo Techno Park, 7-50, Songdo-dong, Yeonsu-gu, Incheon 406-840, South Korea
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  • Youngsook Huh

    1. School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, South Korea
    2. Research Institute of Oceanography, Seoul National University, Seoul 151-747, South Korea
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[1] Dissolved platinum concentrations of eleven large pristine river systems in East Asia (∼200 samples) were determined to better constrain the oceanic platinum budget. Most samples had concentrations less than 1.4 pM; relatively high concentrations up to 5.8 pM were measured in only approximately 6% of the samples. The median Pt concentrations of the individual river systems had only a small range, from 0.18 pM (Duman) to 0.63 pM (Huang He), and the difference in Pt yield mainly resulted from the difference in runoff. The rivers draining the eastern Tibetan Plateau – the Salween, Mekong, Chang Jiang (Yangtze), Hong (Red), and Huang He (Yellow) – had higher Pt yield than the rivers of the Russian Far East – the Amur, Lena, Yana, Indigirka, and Kolyma. If the discharge-weighted mean Pt concentration of our samples (0.36 pM) is extrapolated globally, the estimated riverine flux of dissolved Pt to the ocean is 13 × 103 mol y−1. Based on this riverine flux, the estimated oceanic residence time of Pt is 24 ± 10 kyrs. A 50% release and 50% uptake of Pt in estuaries would modify this to 16 kyrs and 45 kyrs, respectively.

1. Introduction

[2] The geochemical cycle of platinum (Pt) is estimated to be one of the most anthropogenically dominated of the element cycles [Baioumy et al., 2011; Klee and Graedel, 2004; Rauch et al., 2010]. Its average concentration in the upper continental crust (UCC) is only 0.510 ng g−1 [Peucker-Ehrenbrink and Jahn, 2001], but the production of Pt has increased approximately threefold during past three decades to support the use in automobile catalytic converters in addition to traditional anthropogenic applications (catalyst, medicine, and jewelry) [Rauch and Morrison, 2008; Johnson Matthey, http://www.platinum.matthey.com/publications/pgm-market-reviews/, 2009]. The investigation of the distribution and transport of Pt in the atmosphere and hydrosphere has not kept pace with this increase, and the influence of the current anthropogenic pollution to the oceanic budget is unclear [Soyol-Erdene et al., 2011a, 2011b; Wiseman and Zereini, 2009]. The problem is associated with the lack of reliable Pt data for rivers, groundwaters, and estuaries owing to their extremely low concentrations (pM level).

[3] The seawater profiles of Pt concentration published by three research groups differ: conservative in the Atlantic and western Pacific [Colodner et al., 1993], surface depletion in the eastern Pacific [Hodge et al., 1985], and surface enrichment in the Indian Ocean [Jacinto and van den Berg, 1989; Ravizza, 2001]. This could be due to real variability among the ocean basins or be an artifact of the three different analytical methods used: isotope-dilution inductively coupled plasma mass spectrometry (ID-ICPMS) after anion exchange column extraction [Colodner et al., 1993], graphite furnace atomic absorption spectrometry after anion exchange column extraction [Hodge et al., 1985], and cathodic stripping voltammetry (CSV) after UV irradiation [Jacinto and van den Berg, 1989]. Recently, Obata et al. [2006]showed for estuarine water samples that ID-ICPMS and CSV gave consistent results. Deep-water Pt concentrations seem to fall between 0.3 and 1 pM, but there is uncertainty as to inter-basin differences [Ravizza, 2001].

[4] There is limited data in the literature on riverine Pt. The SLRS4 is a river water certified reference material (CRM) which is also a real filtered river water collected from the Ottawa River, Ontario, Canada. The literature compilation of its dissolved Pt concentrations is rather high at 6.4 ± 1 pM (1.2 ± 0.2 pg mL−1; pico = 10−12) [Rodushkin et al., 2005; Soyol-Erdene et al., 2011b, 2011c], and this may be related to the proximity of Ni-Cu-PGE ores [Coker et al., 1991]. Stream samples from near the Tulameen ultramafic complex, British Columbia, Canada had 3–5 pM Pt (n = 7, median concentration: 4.2 pM) [Cook and Fletcher, 1993]. The Ara and Tama rivers which flow through Tokyo displayed high Pt concentrations (10.4 ± 0.4 pM and 31.4 ± 0.4 pM, respectively), which can be attributed to anthropogenic emissions [Obata et al., 2006]. There are also reports of unidentified rivers in Germany with concentrations 1.1–3.3 pM [Ravindra et al., 2004] and an urban river in Sweden with <10 pM [Hidalgo et al., 1996].

[5] Estuarine behavior of Pt has only been studied for the Ara and Tama rivers draining into the Tokyo Bay [Obata et al., 2006]. There is a hint of mid-estuarine enrichment in dissolved Pt relative to the simple dilution of the riverine end-member, but the number of samples was insufficient to unambiguously establish the fact.Turner [2007] and Cobelo-Garcia et al. [2008] carried out laboratory experiments using mixtures of river and estuarine waters spiked with Pt(IV). Either sediment slurry was added to filtered waters [Turner, 2007], or unfiltered waters were used [Cobelo-Garcia et al., 2008]. They found that the particulate-aqueous distribution coefficient decreased with increasing salinity, suggesting release of Pt in estuaries. This is consistent with what was observed byObata et al. [2006].

[6] The true speciation of Pt in sea and river waters is still speculative, but PtOH+ [Azaroual et al., 2001], Pt(OH)2 [Colombo et al., 2008; Wood et al., 1992], or [Pt(OH)5(H2O)] in river water and [PtCl5(OH)]2− in seawater [Gammons, 1996] have been proposed to be the dominant inorganic species. Strong affinity was indicated for organic sulfur [Wood et al., 1992], and UV-irradiation released additional Pt into the dissolved load which suggests organic-complexation [Obata et al., 2006]. However, specific information regarding interaction with natural aqueous ligands like humic acids are lacking, and organic complexation is predicted to be kinetically hindered for Pt [Cosden et al., 2003].

[7] In this paper we report Pt concentrations in major rivers of East Asia and estimate the riverine flux to the ocean in order to improve our understanding of its natural oceanic mass balance – the fluxes into and out of the ocean and the residence time. As the rivers we studied are in remote areas distant from high population centers and as most were sampled before the introduction of catalytic converters into the region, this will provide information on the baseline levels of dissolved Pt in rivers away from point sources.

2. Materials and Methods

2.1. Laboratory Clean Conditions

[8] Ultrapure water was obtained using a three-stage purification process: reverse osmosis (Innovation, Human Power 1+, Human Corporation), deionization (Puris Esse, UP Basic, Mirae Co.), and trace metal filtration (Protego, Mykrolis Corporation, MA, USA). Commercial ultrapure grade HNO3(Ecoresearch Co. Ltd., Korea) was used in acidification of samples and standards for sector field (SF)-ICPMS.

[9] The cleaning procedure for experimental tools, including high-density polyethylene (HDPE) bottles, was carried out in a laminar flow clean hood located in a HEPA-filtered clean booth. New HDPE bottles were rinsed with deionized ultrapure water, leached with 2–3 M HCl at 60°C for at least 48 h, rinsed again with ultrapure water several times, and leached with ultrapure water at 60°C for 48 h. After rinsing with ultrapure water, they were dried and packed in clean polyethylene bags until use. Polypropylene (PP) pipette tips were leached in 1:1 high-purity HNO3 for a week and repeatedly rinsed with ultrapure water just before use.

2.2. Sample Description

[10] Approximately 200 river water samples collected from major East Asian rivers were used in this study (Figure 1 and Figures S1–S5 and Table S1 in the auxiliary material). The Salween, Mekong, Chang Jiang (Yangtze), Hong (Red), and Huang He (Yellow) drain the eastern Tibetan Plateau. The furthest downstream samples for these rivers were collected significantly upstream from the mouths. The Amur, Lena, Yana, Indigirka, and Kolyma drain the Russian Far East, and the latter four flow into the Arctic Ocean. The Duman River originates from the border between China and North Korea and flow into the East Sea/Sea of Japan. Spring waters of Mt. Baekdu were also analyzed. The total area drained by the rivers that we sampled is 5577 × 103 km2 (∼4% of global land area) and the water discharge is 1541 km3 y−1 (∼4% of the global water discharge to the ocean).

Figure 1.

Location map of the major rivers of East Asia. The black dots indicate sampling sites.

[11] The drainage basins of the Upper Salween, Mekong, and Chang Jiang, and the Hong contain siliciclastic (<10% carbonate), mixed (∼40% carbonate), and carbonate (∼80% carbonate) consolidated sedimentary rock with some outcrops of metamorphic rock [Dürr et al., 2005]. The Upper Huang He drains mixed semi- to un-consolidated sedimentary rock and minor loess [Wu et al., 2005]. The Duman basin is dominantly volcanic rock with exposures of granitic basement in the lower reaches [Han and Huh, 2009]. Unconsolidated sedimentary rock, metamorphic rock, and Quaternary alluvial deposits are found in the Amur basin [Moon et al., 2009]. The left bank tributaries of the Lena drain the Cambrian-Ordovician carbonate platform with Jurassic-Cretaceous evaporites, and the right bank tributaries drain the Proterozoic basement of the Trans-Baikal Highlands [Huh and Edmond, 1999; Huh et al., 1998a]. The right bank tributaries of the Lower Lena, Yana, Indigirka, and Kolyma drain a Mesozoic continental collisional/accretionary zone of complex geology [Huh et al., 1998b].

[12] Sampling expeditions were carried out in July to August of 1992–1998 for the Russian Far East and in May to September of 1999–2004 and in December to January of 2002–2003 for China and Vietnam (Table S1). The Duman River and the springs of Mt. Baekdu were sampled in August of 2005 and 2006. All samples were filtered in the field within 24 h through 0.4 μm polycarbonate filters that had been pre-cleaned with ultrapure HNO3 and water. The filtered samples were acidified to pH ∼2 with ultrapure HCl (1992–1995) or HNO3(1996–2006) and stored in pre-cleaned HDPE bottles.

2.3. Instrumental Analysis

[13] Analysis of dissolved Pt was performed directly using a SF-ICPMS (Element2, Thermo Finnigan MAT, Germany) at the National Center for Inter-University Research Facilities of Seoul National University. An Aridus II high-sensitivity desolvation nebulizer system (Cetac Technogies, USA), which includes a heated Teflon cyclonic chamber, a membrane desolvator, and a microflow PFA nebulizer (100 μl min−1) with additional N2gas, was incorporated to transport sample to the SF-ICPMS. The desolvation nebulizer reduced the oxide production rate (BaO/Ba < 0.02%) and increased the instrumental sensitivity by approximately 10 times relative to the standard instrument (∼0.9 ⋅ 106 cps for 100 pg mL−1indium solution). To obtain the lowest blank level, ultrapure grade (99.999%) argon and nitrogen gases were used, and the SF-ICPMS was located in a class-1000 clean laboratory. The river water samples were introduced to the desolvation system in a class-10 clean bench. Details of the instrumental operating conditions and data acquisition parameters are illustrated inTable 1.

Table 1. Instrumental Conditions and Measurement Parameters for the Sector-Field ICP-MS (Element 2) and the Desolvation System (Aridus II)
  • a

    Daily optimized to obtain maximized intensity.

Gas Flow Rates 
Cool/L min−116.00
Auxiliary/L min−10.80–0.90a
Sample/L min−10.950–1.060a
Sweep gas/L min−12.8
Nitrogen flow rate/mL min−13–4a
Membrane temperature/°C160
Spray chamber temperature/°C110
Washing time/min5
Take up time/min1
Selected isotopes195Pt, 179Hf, 169Tm
ResolutionLow (m·Δm−1 = 300)
Dwell time per acquisition point/ms10
No. of acquisition points per mass segment (sample per peak)200
Total acquisition time (per mass segment)/s0.4
Acquisition window (%)20
Search window (%)0
Integration window (%)20

[14] An external calibration method was applied for the quantification. Reference standard solutions were prepared by sequential dilution from a 10 ppm PGE (platinum group element) multielement standard solution (CMS-2, Inorganic Ventures, Lakewood, New Jersey, USA) using ultrapure 1% (w/w) HNO3. The Pt concentrations of the working standard solutions used for the calibration curve were 0, 50, 100, 200, 500, 1000, and 2000 fg mL−1 (femto = 10−15). For these concentration ranges, the correlation coefficients (r2) of the calibration curves were higher than 0.999.

[15] Less than 1 mL (∼0.5 mL) of sample was used for the analysis. The 0.5 ng mL−1 Tm internal standard was used to monitor and correct for the drift in instrumental sensitivity. The isotope (195Pt) with the highest natural abundance and lowest interference was used for quantification. The most significant potential interference for 195Pt is 179Hf16O. Interference was checked by measuring the analyte (195Pt) in 0–100 pg mL−1single-element standard solutions of interferent (Hf) [Soyol-Erdene et al., 2011b, 2011c], but the desolvation nebulization system rendered such interference negligible up to 100 pg mL−1 Hf. In this study Hf in river water samples was monitored simultaneously with Pt, and the Hf concentrations were always lower than 30 pg mL−1. Thus, mathematical correction was not required.

[16] The instrumental detection limit, three times the standard deviation of the blank solution (1% ultrapure HNO3), was 20 fg mL−1 (0.1 pM). The field blanks, ultrapure water carried through identical filtration and acidification procedure as the samples during the sampling expeditions, were close to the instrumental blank. Thus, the samples were corrected only for the instrumental blank.

[17] The accuracy was confirmed using SLRS4. We diluted SLRS4 ten times to bring it to a concentration range similar to that of our samples and also measured it without dilution. There is no certified value for Pt, but our measurements (1.48 ± 0.12 pg mL−1 Pt, n = 67) were in good agreement with those reported in the literature – 1.3 ± 0.1 pg mL−1 [Rodushkin et al., 2005] and 1.23 ± 0.18 pg mL−1 [Soyol-Erdene et al., 2011b, 2011c]. Analytical reproducibility was checked after every five samples (or every 40–50 min): 0.1–23% for the standard solution (n = 68, 58 ± 6 fg mL−1or 0.30 ± 0.03 pM), 1.9–18% for the 10-times-diluted SLRS4 (n = 11, 130 ± 17 fg mL−1or 0.67 ± 0.09 pM), and 0.1–9% for non-diluted SLRS4 (n = 56, 1500 ± 100 fg mL−1 or 7.7 ± 0.5 pM). To establish reliability of the analysis, we made additional intermediate dilutions of the SLRS4, with dilution factors 2, 5, 10, and 20. The measured concentrations were in good agreement with expected concentrations (Figure S6).

[18] Inter-element ratios among the PGEs often help distinguish the different sources (e.g., anthropogenic, crustal, volcanic, and ore deposit). However, only Pt was analyzed in this study due to analytical difficulties. Osmium requires high temperature oxidation conditions [Chen and Sharma, 2009]. Iridium concentrations were lower than the instrumental detection limit (10 fg mL−1). If the concentration ratio of Ir and Pt in our samples is similar to SLRS4 (Ir: 8 ± 4 fg mL−1, Pt: 1300 ± 100 fg mL−1) [Rodushkin et al., 2005; Soyol-Erdene et al., 2011c], Ir concentrations are expected to be 0.1–2 fg mL−1. This would require non-boiling preconcentration of 10–100 mL samples. Direct measurements of Ru, Rh, and Pd concentrations were hindered by strong interferences from Sr, Rb, Cu, Zn, and Pb and ultra-low levels (fg mL−1 or pM) in river water. Matrix separation by column extraction may require 0.5–1 L of samples [Obata et al., 2006].

[19] Platinum is liable to adsorb onto the walls of containers, which effect HDPE seems to minimize [Azaroual et al., 2001]. Coker et al. [1991] reported that 0.1 M HCl ensures stability of Pt for at least several months and that the loss of Pt to the walls of the containers is not significant within 24 h. The SLRS4 CRM (National Research Council, Canada) distributed in HDPE bottles has shown no change in Pt concentration during several years of storage at similar conditions as our samples [Rodushkin et al., 2005; Soyol-Erdene et al., 2011b, 2011c]. We carried out additional testing for containers using SLRS4 (acidified with ultrapure HNO3to pH ∼1.6) for one week. HDPE, Teflon, and PP containers showed conservative behavior within uncertainty, while some loss was observed for glass, and significant blank was detected for low density polyethylene (LDPE) even though the containers were all pre-cleaned (Figure S7). No difference was observed for storage temperatures (room temperature and refrigerated) within the time frame tested.

2.4. GIS-Based Parameters and Statistical Calculations

[20] Drainage basin area and runoff calculated using ArcGIS software were obtained from literature [Han and Huh, 2009; Moon et al., 2009, 2007; Noh et al., 2009; Wu and Huh, 2007; Wu et al., 2005]. Basin areas were modified from the USGS Global GIS database for South Asia [Hearn et al., 2000] and North Eurasia [Hearn et al., 2001], which were based on the 30 arc-second digital elevation model of the world (GTOPO30). Runoff data were from the UNEP/GRDC Composite Runoff Fields v1.0, which is the interpolated runoff from values measured at hydrological stations [Fekete et al., 2002].

[21] Principal component analysis (PCA) was performed using SPSS v.18. In order to avoid the effect of dilution with varying runoff, Na-normalized concentrations were used. Data for dissolved Na, K, Mg, Ca, HCO3, Cl, SO42−, Si, Sr, and 87Sr/86Sr were obtained from literature [Han and Huh, 2009; Huh and Edmond, 1999; Huh et al., 1998a, 1998b; Moon et al., 2009, 2007; Noh et al., 2009; Wu et al., 2005].

3. Results and Discussion

3.1. Controls on the Level of Dissolved Pt

[22] Concentrations of dissolved Pt ranged from below detection limit (0.1 pM) to 5.8 pM (Table S1). Ninety-four percent of our samples (n = 186) were distinctly low (<0.1–1.4 pM), and the outliers (1.5–5.8 pM) represent approximately 6% (n = 12) of the samples (Figure 2). Nine outliers (RD125, RD124, CJ239, CJ238, RD132, RD130, RD213, RD120, and CJ245) were extracted from the eastern Tibetan Plateau rivers (n = 138) and three (AU108, UL713, and IG106) from the rivers of the Russian Far East (n = 60). The concentrations of Pt in the six samples attributed to the springs of Mt. Baekdu (CB101–106) and in a spring in the Lena basin (UL406) ranged between below detection limit and 5.2 pM and are treated separately from the riverine data. Our measurements are on the lower side of the few reported measurements for rivers (see Introduction).

Figure 2.

Box plot of dissolved riverine Pt concentrations in (a) China and Vietnam – the Salween, Mekong, Chang Jiang (Yangtze), Hong (Red), Huang He (Yellow), and Duman and hot springs of Mt. Baekdu and (b) the Russian Far East (the Amur, Lena, Yana, Indigirka, and Kolyma). n: number of samples analyzed. Stars indicate outliers (n = 12, 6% of the total number of the samples) and are labeled with the sample ID. UL406 is a spring sample.

[23] We carried out PCA on dissolved Pt and other major elements as a first attempt at drawing out the source characteristics of Pt in river waters. The source lithologies indicated by PCA are not expected to correlate with surface exposure areas within the sampled sub-basins due to incongruent weathering. For example, even a small exposure of carbonate can disproportionately affect the river dissolved load composition. Also, different elements in various minerals display diverse dissolution and adsorption/desorption kinetics. The results of PCA would represent the amalgamated effect of mineral type, dissolution kinetics, and exposure. PCA was performed separately for (i) the main group (n = 182, <0.1–1.4 pM Pt) and (ii) the outliers (n = 12, 1.5–5.8 pM Pt). For each calculation, two factors were extracted that explained 67% and 88%, respectively, of the variance in the two data sets (Table 2).

Table 2. Rotated Component Matrix for River Dissolved Pt and Major Elementsa
 Main (<1.4 pM Pt)Outlier (1.5–5.8 pM Pt)
Factor 1Factor 2Factor 1Factor 2
  • a

    The 6 spring samples were excluded. Rotation Method: Varimax with Kaiser Normalization. Blank fields: R < 0.4, considered not important.

Pt/Na.630 .503.788
K/Na .840.408.840
Mg/Na.922 .891.414
Cl/Na −.599.807 
SO42−/Na.674 .940 
Si/Na .950 .918
Sr/Na.696 .913 
87Sr/86Sr.453.662 .829
% of Variance38294642
Cumulative %38674688

[24] For the main group samples, the Pt/Na was best grouped with Mg/Na, HNO3/Na, Ca/Na, Sr/Na, SO42−/Na, and moderately with 87Sr/86Sr in the first factor, which can be interpreted as carbonate weathering (Table 2). The second factor, not associated with Pt, had high positive loadings for Si/Na and K/Na, a moderate negative loading for Cl/Na, and moderate positive loadings for 87Sr/86Sr, Ca/Na, and HNO3/Na. The second factor may be due to biological activities such as phytolith growth and minor weathering of silicates. Negative loading for Cl/Na suggests dilution by halite dissolution. The variability in Pt concentrations for the main group samples is best associated with carbonate weathering. We do not put significant emphasis on this association, as it represents the average composition of the rivers dominated by carbonates and the low Pt concentrations due to low levels in the UCC and to low solubility in the absence of strong organic ligands.

[25] For the outlier samples the first extracted factor had high positive loadings for SO42−/Na, Sr/Na, Mg/Na, and Cl/Na and moderate loadings for HCO3/Na, Ca/Na, Pt/Na, and K/Na. This factor can be related to evaporite and carbonate dissolution (Table 2). The second factor had high positive loadings for Si/Na, K/Na, 87Sr/86Sr, Pt/Na, and Ca/Na and moderate loadings for HCO3/Na and Mg/Na (Table 2). This factor can be interpreted as silicate weathering. The samples with high Pt had 87Sr/86Sr ratios similar to the global average river water (0.7111) [Peucker-Ehrenbrink et al., 2010] and between Phanerozoic marine carbonate (0.7067–0.7091) [Denison et al., 1998] and UCC (0.716) [Goldstein and Jacobsen, 1988] (Figure 3). This indicates that the outlier samples are not related to extensive involvement of old basement at this regional scale. Weathering of young volcanics or silicates with UCC composition can explain much of the variance in Pt concentrations observed in the outliers.

Figure 3.

Dissolved Pt concentration versus 87Sr/86Sr ratio. Solid line, dashed line, and gray shaded box indicate the values for the upper continental crust (UCC) [Goldstein and Jacobsen, 1988], global riverine average [Peucker-Ehrenbrink et al., 2010], and Phanerozoic marine carbonate [Denison et al., 1998], respectively.

[26] The reported Pt content in potential source lithologies are: UCC (0.510 ng g−1) [Peucker-Ehrenbrink and Jahn, 2001], MORB (0.2–0.5 ng g−1), reducing sediments on continental margins (0.6–3.5 ng g−1), and metalliferous sediments (1.6–35.5 ng g−1) [Ravizza and Pyle, 1997]. Unweathered organic-rich shales can be up to 9 ng g−1 [Peucker-Ehrenbrink and Hannigan, 2000]. Very high values are observed for organic- and sulfide- rich shale (up to 150 ng g−1) [Orth et al., 1988] and Cu-Ni-PGE or chromatite deposits [Maier, 2005; Mungall and Naldrett, 2008; Naldrett et al., 2008]. However, in the absence of strong ligands (e.g., thiosulfate or organics) [Anthony and Williams, 1993; Bowles et al., 1995; Wood, 1996; Wood et al., 1992] solubility in aqueous media is limited. This is also consistent with Coker et al.'s [1991] observation of rapidly decreasing Pt from 14 pM in waters related to the gossanous zones to below their detection limit of 1 pM within a few meters. In future studies, the role of organic ligands should be explored further in natural systems.

[27] We examined the relationship between Pt concentration and total dissolved cations (TZ+). Most of the data cluster in a narrow area with Pt concentrations less than 1 pM and TZ+ less than 5 mEq (Figure S8a). The high-Pt samples have low-to-moderate TZ+ values, except CJ239, CJ238 and CJ245 draining arid regions of the Tibetan Plateau, CB101 and CB106 Mt. Baekdu spring samples, and UL713 draining the evaporites of the Russian Far East. These rivers have low or undetermined discharge and therefore their effect on the riverine Pt yield is not significant (Figure S8b). The Si/TZ+ ratio is used as a silicate weathering intensity index [e.g., Huh and Edmond, 1999]. There is no correlation between Pt concentration or yield and Si/TZ+ (Figures S8c and S8d). This suggests that the release and transport of Pt is not a simple function of weathering rates or the intensity of silicate weathering.

[28] Anthropogenic Pt contamination appears to be global, as demand of Pt has increased in the past several decades and its increased deposition has been observed in urban lake and peat bog sediments [Rauch et al., 2004a, 2004b] as well as in remote Antarctic snow samples [Soyol-Erdene et al., 2011a]. Lacking data for other trace elements with potential anthropogenic sources (either from mining or from atmospheric emission) we cannot attempt to resolve the anthropogenic component of the Pt found dissolved in rivers. There may have been a rise in the background level due to aerosol transport, and the possibility exists that the riverine flux includes a significant “contamination” component above the natural level. At least, we do not observe a gross Pt contamination of surface waters in East Asia, perhaps because automobile catalysts have been introduced relatively late into this region and much of the area sampled lacks significant traffic.

3.2. Riverine Pt Flux to the Ocean

[29] To calculate the dissolved Pt flux of each of the eleven river systems, the median concentrations of each river system were multiplied by their respective mean annual discharges at the mouths [Fei, 1999; Gaillardet et al., 1999; Huh et al., 1998b]. Values for the mouth rather than the furthest downstream sample were used, because for the eastern Tibetan Plateau rivers our furthest downstream samples were significantly upstream of the mouth. The relatively homogeneous median values, barring the 12 outliers none of which are main channel samples, justified the choice of median values. Estimated dissolved fluxes of Pt were between 1 mol y−1 (Duman) and 325 mol y−1 (Chang Jiang), the variability arising mainly from that of water discharge (Table 3). The discharge-weighted mean concentration, calculated based on the median Pt concentrations for the eleven rivers and their discharges at mouth, was 0.36 pM (Table 3). This is similar to the seawater concentration of Pt (0.3–1 pM) [Colodner et al., 1993; Hodge et al., 1985; Jacinto and van den Berg, 1989].

Table 3. Dissolved Pt Fluxes of the Major Rivers of East Asia
River SystemNo. of Samples AnalyzedAreaaDischargeaRunoffPt (median)bPt FluxPt Yield
103 km2km3 y−1mm y−1pMmol y−110−3 mol km−2 y−1
Chang Jiang3718089285130.35325180
Huang He1375241550.632634
Total19894382873  1023 
Discharge-weighted mean    0.36 150

[30] The dissolved Pt yield, the flux per unit area, was approximately three times higher in the eastern Tibetan Plateau rivers (Salween, Mekong, Chang Jiang, Hong, and Huang He) than in the rivers of the Russian Far East (Amur, Lena, Yana, Indigirka, and Kolyma) (Table 3). The caveat is that the median Pt concentrations of some rivers are based on only a few samples.

[31] Because dissolved Pt concentrations did not show appreciable variability in our ∼200 samples covering various lithologies and climatic zones of East Asia, we assumed the discharge-weighted mean (0.36 pM) to represent the global riverine influx to the ocean. When multiplied by the global river runoff of 37,400 km3 y−1 [Palmer and Edmond, 1989], the riverine inflow of dissolved Pt is 13 × 103 mol yr−1.

[32] Total riverine flux for other PGEs are 1.6 × 103 mol Os yr−1 [Levasseur et al., 1999] and 0.4 × 103 mol Ir yr−1 [Anbar et al., 1996]. Based on the riverine fluxes of Pt (this study), Os, and Ir, the molar Pt/Os ratio for the dissolved riverine flux is 8.1 and is about half that of the UCC (Pt/Os = 16) [Peucker-Ehrenbrink and Jahn, 2001] and higher than that in surface seawater (5.2) [Ravizza, 2001]. The Pt/Ir ratio for the dissolved riverine flux is 30, considerably higher than its UCC ratio of 24 [Peucker-Ehrenbrink and Jahn, 2001] but one order of magnitude lower than that in surface seawater (520) [Ravizza, 2001]. The differences in Pt/Os and Pt/Ir ratios between the river and surface seawater are likely related to the different mobility in aqueous environments, precipitation mechanisms in the estuaries and/or in the oceans [Goldberg et al., 1986; Sharma and Wasserburg, 1997], and contributions of possible unknown sources of these elements to the ocean [Peucker-Ehrenbrink, 2002].

3.3. The Oceanic Mass Balance

[33] The oceanic residence time of Pt (τPtsw) can be estimated from the rate of addition of Pt to seawater as follows (Figure 4):

display math

where CPtsw is the estimated concentration of Pt in seawater (0.26 pM) [Ravizza, 2001], Mo is the mass of the oceans (1.34 × 1021 kg), the F terms are the Pt fluxes into the ocean, friver is the fraction that survives the estuary and reaches the deep ocean, and faeolian and fmeteorite are the fractions that dissolve. The submarine groundwater discharge is becoming recognized as important for many elemental cycles, but data for Pt is as yet unavailable.

Figure 4.

The oceanic budget of platinum. The fluxes are in  mol y−1. See text for references.

[34] The main continental input is probably through rivers but there is a lack of direct data from the estuaries making it difficult to estimate friver. Considering the results of laboratory particulate-aqueous partitioning experiments [Cobelo-Garcia et al., 2008; Turner, 2007] and the 3-point estuarine transects [Obata et al., 2006], a 50% release (friver = 1.5) yields 20000 mol y−1. However, frivercan be much higher if the river carries high levels of suspended sediment, if the particulate-aqueous distribution coefficient is high, and/or if the riverine Pt concentration is high. On the other hand, Pt could also be taken up in the estuaries through interaction with organic matter. This would be favored in systems with longer residence time in estuaries where the slow kinetics for Pt coordination with organic ligands [Cosden et al., 2003] can be overcome. A 50% uptake of dissolved Pt in the estuaries (friver = 0.5) yields 7000 mol y−1.

[35] The delivery of Pt to the ocean by aeolian dust was calculated by taking the dust deposition rate to the ocean of 450 Tg y−1 [Jickells et al., 2005], assuming the dust Pt content to be that of the UCC (0.510 ng g−1) [Peucker-Ehrenbrink and Jahn, 2001], yielding 1200 mol y−1. For faeolian, we assumed that 3% of the aerosol-borne Pt dissolves in the water column by analogy with gold [Falkner and Edmond, 1990]. The result is an aeolian dust contribution to dissolved seawater Pt of 36 mol y−1.

[36] The cosmic dust flux was calculated by taking the meteorite flux of 30 ± 15 × 109 g y−1 [Peucker-Ehrenbrink and Ravizza, 2000] and average chondritic Pt of 990 ng g−1 [Anders and Grevesse, 1989], which yields 150 mol y−1. For fmeteorite, we assumed that 20% dissolves in the water column using estimates from Os isotopic composition of ferromanganese leachates [Esser and Turekian, 1988]. The result is 30 mol y−1.

[37] The mantle input of Pt to the ocean through high temperature hydrothermal plumes is difficult to estimate. McKibben et al. [1990] found up to 2.5 nM Pt in the Salton Sea geothermal brines, and theoretical and experimental studies predict equilibrium concentrations over 500 nM in conditions typical of submarine hydrothermal solutions [Gammons et al., 1992; Wood, 1990]. However, as is the case with iron, most Pt probably forms sulfides around the orifice of hydrothermal vents and fine grained oxyhydroxides close to the ridge axes. Therefore, we have considered hydrothermal fluid source flux to be equivalent to the hydrothermal sedimentation sink flux, 1000 mol y−1 (see below).

[38] Altogether, by far the most important source of dissolved Pt to the ocean is the input through rivers, and if estuarine effect is ignored (friver = 1), the residence time is 24 kyrs. We evaluated the uncertainty in this estimate by assigning relative uncertainties for the major terms: CPtsw (20%), mean annual runoff (12%) [Schlosser and Houser, 2007], and CPtrw (30%) which yielded a 40% uncertainty on the residence time: 24 ± 10 kyrs. However, if we consider the poorly known friver, the residence time could be from 12 kyrs (friver = 2) to 45 kyrs (friver = 0.5) With currently available information, 16 kyrs (friver = 1.5) would be the best estimate.

[39] The oceanic residence time of Pt can also be estimated from the rate of removal of Pt from seawater (Figure 4). The primary sink for Pt is believed to be burial in hydrogenous sediments. Goldberg et al. [1986] estimated the average sedimentation rate of Pt based on the Pt content of pelagic sediments (2 ng g−1), density (2 g cm−3), and sedimentation rate (1 mm ky−1). We updated their calculation using new data for Pt in oceanic sediments that has become available in the intervening years. We separated the sedimentation flux into three terms: red clay, carbonate, and hydrothermal.

display math

where Ao is the area of the oceans (3.6 × 1018 cm2), F terms are the burial fluxes per unit area and ared clay and acarbonateare the fractions of the deep-seafloor covered by red clays (0.381) and carbonates (0.471) [Berger, 1976]. For the red clay component, we retained the concentration and sedimentation rate estimate of Goldberg et al. [1986], which resulted in 2800 mol y−1. For the carbonate component, we used the 0.926 ng Pt cm−2 kyr−1 determined by Cave et al. [2003] for the NE Atlantic and assumed that 85% was hydrogenous and not lithogenic. The Pt sedimentation rate in carbonate was higher at 6800 mol y−1. The hydrothermal sedimentation flux is estimated as 1000 mol y−1based on sediments accumulating under the Rainbow hydrothermal plume on the Mid-Atlantic Ridge [Cave et al., 2003]. These add up to 10600 mol y−1. The oceanic residence time calculated using the sink terms is 33 kyrs.

[40] Evaluating the uncertainty on the residence time based on sink terms is even more difficult than for that based on source terms. Admittedly the assessment was based on extrapolation of data from limited localities. The Pt concentrations in marine sediments have large ranges, and the sedimentation rates also vary widely: 3–60 mm ky−1 for calcareous ooze and 0.3–15 mm ky−1 for red clay [Schulz and Zabel, 2006]. Considering these uncertainties, the residence time based on sink terms can only provide an order-of-magnitude estimate.

[41] Our estimated oceanic residence time (16 ± 6 kyrs) is lower than the earlier assessment of 106 years using the sink flux [Goldberg et al., 1986]. The residence time of Ir was estimated from a small pristine river in Sweden, the Baltic Sea estuarine environment, seawater concentration based on the Pacific and the North Sea, and burial in sediments, which yielded 2–20 kyrs [Anbar et al., 1996]. Applying the Os/Ir ratio of river water, Sharma [2011]re-evaluated the residence time of Ir as 9–14 kyrs. The oceanic residence time of Os was evaluated using the oceanic Os isotope budget as 24–54 kyrs [Levasseur et al., 1999].

4. Conclusion

[42] We report for the first time the dissolved platinum concentration in large pristine rivers of East Asia with a low detection limit of 0.1 pM. The measured Pt concentration in river water samples ranged from <0.1 to 5.8 pM, but most (186 samples, 94%) were less than 1.4 pM. Calculated discharge-weighted mean concentration for the rivers of East Asia was 0.36 pM, comparable to seawater (0.26 pM) [Ravizza, 2001]. Global riverine dissolved Pt flux was 13 × 103 mol yr−1 and is approximately one order of magnitude higher than the estimates for other platinum group elements i.e., Ir and Os in the literature. The oceanic residence time of Pt was estimated to be on the order of 104 years.


[43] Financial support was provided by the NRF grant (2010–0027586) from the Korean government (MEST) to Y.H. The authors would like to thank Y. Han for useful discussion and the SNU IRF for access to SF-ICPMS. We gratefully acknowledge Editor L. Derry and B. Peucker-Ehrenbrink, M. Sharma, and an anonymous reviewer for constructive criticism on this and an earlier version of the manuscript.