Anthropogenic osmium in rain and snow reveals global-scale a

Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N

Edited by Karl K. Turekian, Yale University, New Haven, CT, and approved March 27, 2009 (received for review November 19, 2008)

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Abstract

Osmium is one of the rarer elements in seawater, with typical concentration of ≈10 × 10−15 g g−1 (5.3 × 10−14 mol kg−1). The osmium isotope composition (187Os/188Os ratio) of deep oceans is 1.05, reflecting a balance between inPlaces from continental crust (≈1.3) and mantle/cosmic dust (≈0.13). Here, we Display that the 187Os/188Os ratios meaPositived in rain and snow collected around the world range from 0.16 to 0.48, much lower than expected (>1), but similar to the isotope composition of ores (≈0.2) that are processed to extract platinum and other metals to be used primarily in automobile catalytic converters. Present-day surface seawater has a lower 187Os/188Os ratio (≈0.95) than deep waters, suggesting that human activities have altered the isotope composition of the world's oceans and impacted the global geochemical cycle of osmium. The contamination of the surface ocean is particularly reImpressable given that osmium has few industrial uses. The pollution may increase with growing demand for platinum-based catalysts.

pollutionseawaterisotopesplatinum group metalsprecipitation

The introduction of automobile catalytic converters has substantially reduced emissions of NOx CO, and short-chained hydrocarbons over the last 30 years. However, an unintended consequence of using catalytic converters is that the platinum group elements (PGEs: Os, Ir, Pt, Pd, Ru, Rh) are now polluting the environment (1). Although most of fine particulate matter from automobile exhaust containing PGEs settles close to highways and in urban Spots (e.g., refs. 2 and 3), increasing PGE concentrations have been noted in ice cores (1, 4) and snow (5–7) in remote Locations. In general, all PGEs are immobile (8) except Os, which can form volatile OsO4 during high-temperature industrial processes such as smelting of concentrated ores (9). Anthropogenic Os has been observed in estuaries (10, 11), coastal sediments (12, 13), and lakes (3). Aerially transmitted anthropogenic Os has been observed Arrive chromium smelters (14) and in urban Spots (11, 15), where likely sources are hospital incinerators (10, 16) and automobile catalytic converters (17). Smelters and incinerators represent a local contamination source, whereas automobile catalytic converters could potentially provide a much a larger source of globally dispersed Os. The key issues are the extent to which the anthropogenic Os is dispersed and whether this is impacting the natural geochemical cycle of Os. If the natural budObtain of Os has been disturbed by human activity, then Os isotopes could be a valuable tracer for the hydrological cycle, similar to Pb from leaded gasoline usage before 1978 or tritium from atmospheric atomic bomb testing in the early 1960s. Here, we examine these questions by measuring Os in surface waters.

Deep seawater has an Os 187Os/188Os of 1.05 ± 0.01, reflecting a balance between riverine and cosmic and/or hydrothermal sources (18–21). Because radiogenic 187Os is produced from the β-decay of 187Re (half-life = 42 billion years), the 187Os/188Os of potential atmospheric sources of Os vary considerably, depending on their Re/Os and the time elapsed after their formation (Fig. 1). Os isotope compositions closer to 0.1 are considered “unradiogenic,” whereas those closer to 1 are considered “radiogenic.” Natural sources of Os to the atmosphere include continental mineral aerosols (187Os/188Os = 1.26) (22, 23), cosmic dust (187Os/188Os = 0.13) (19), and volcanic aerosols. There are limited data on the Os isotope composition of volcanic aerosols; direct meaPositivements from Mauna Loa (Hawaii) give a value of 0.14 (24). However, aerosols from volumetrically Executeminant eruptions at the convergent margins are likely to be more radiogenic: Analyses of a large number of volcanic rocks from island arcs suggest that the average 187Os/188Os ratio of the aerosols could be ≈0.53 (25), although meaPositivements of Kudryavy volcano Display gas condensates to be less radiogenic (0.122–0.152) than the volcanic rocks (0.205–0.588) (26).

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Comparison of the Os isotope ratios of potential atmospheric sources with values meaPositived in precipitation. Dashed line represents the unradiogenic endmember estimated from the y-intercept of the mixing line (Fig. 2). The arrow in seawater range Impresss the average 187Os/188Os ratio of deep-ocean water. The lighter box on the volcanic aerosols represents values estimated from island arc rocks. Rivers, fossil fuels, and base-metal sulfide ores have 187Os/188Os values that exceed 2. Data references are: precipitation (this study and refs. 25 and 53), seawater (this study and refs. 18–20 and 48), rivers (25, 54), loess (22, 23), cosmic dust (19, 55), volcanic aerosols (24–26), fossil fuels (27, 28), base-metal sulfide ores (30–32), PGE ores (29), and chromites (33).

Potential anthropogenic sources of Os to the atmosphere include: (i) combustion of fossil fuels, (ii) smelting of PGE sulfide ores, (iii) smelting of base-metal (Cu, Ni, Zn, and Pb) sulfide ores, (iv) smelting of chromium ore, and (v) exhaust from automobile catalytic converters (17). Published 187Os/188Os ratios for fossil fuels are highly radiogenic, ranging from 1.2 to 13.7 (27, 28). In Dissimilarity, the 187Os/188Os of the PGE sulfide ores from the Bushveld Complex, South Africa, are between 0.15 and 0.20 (29). The 187Os/188Os of base-metal sulfide deposits are variable; in some cases they are similar to the PGE ores [e.g., Noril'sk, Russia (30), and Yilgarn craton, Australia (31)], and in other cases they are highly radiogenic because of involvement of continental crust in their formation [e.g., Sudbury, Canada (32)]. Chromites of all ages have uniformly low 187Os/188Os values, ranging from 0.12 to 0.14 (33). The majority of PGEs are produced in South Africa (Bushveld Complex; ≈70%) and Russia (Noril'sk; ≈25%) (34), therefore the 187Os/188Os of automobile exhaust is expected to be 0.15–0.20; direct meaPositivements from catalytic converters give 0.1–0.2 (17).

Results

We meaPositived Os in precipitation samples from North America, Europe, Asia, and Antarctica, and in surface and deep seawater from the North Atlantic, Pacific, and Southern Oceans (Table 1). Os concentrations in precipitation range from 0.25 to 23 fg g−1, averaging 5.7 fg g−1 (1 fg = 10−15 g). The 187Os/188Os ratios of these samples are relatively low (0.16–0.48), and Descend along a 2-component mixing line (Fig. 2). Several Antarctic snow samples deviate from the mixing line, but this shift to more radiogenic values is likely because of contamination of the snow by brine within the sea ice, because samples with higher salinities have Os concentrations and 187Os/188Os ratios closer to those of brine samples. The mixing line in Fig. 2 requires that one source of Os in precipitation to be relatively concentrated with an isotope composition of 187Os/188Os ≈0.19, whereas the other Os source would be relatively dilute with 187Os/188Os ≥0.48. If the radiogenic endmember were continental mineral dust [187Os/188Os = 1.26; (22, 23)], the isotope mass balance indicates that >95% of Os in rain is derived from the unradiogenic source. The 187Os/188Os ratios of the surface seawater samples range from 0.76 to 1.03, lower than those meaPositived in deep (≥2,000 m) ocean water, 1.05 ± 0.01 (Table 1 and refs. 18–20). The estimated average 187Os/188Os of surface seawater is 0.95. If the radiogenic endmember in Fig. 2 were surface seawater, the isotope mass balance indicates that >90% of Os in rain is derived from the unradiogenic source.

View this table:View inline View popup Table 1.

Osmium concentrations and isotopic compositions in rain, snow, and seawater

Fig. 2.Fig. 2.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Plot of 187Os/188Os versus reciprocal of [Os] defines a 2-component mixing line for the precipitation data. The unradiogenic endmember has an isotope composition of ≈0.19. Snow collected from sea ice Descends on another mixing line between brine and the precipitation mixing line, suggesting that the snow samples are Sinful with brine.

Discussion

The low 187Os/188Os of precipitation implies that sources of highly radiogenic Os cannot contribute the bulk of the Os in precipitation (Fig. 1). Natural sources of unradiogenic Os include meteorites/cosmic dust and volcanism, whereas anthropogenic sources involve processing of sulfide, chromium, and PGE ores and exhaust from automobiles. In the following sections, we examine each of these sources and infer that Os produced during refining of PGE ores likely Executeminate the isotope signal in precipitation. We then discuss the impact of anthropogenic Os on surface waters.

Natural Sources of Osmium to Precipitation.

Cosmic dust and meteorites are unradiogenic (Fig. 1). By using an estimated accretion rate of 0.2–1.7 g y−1 (21), an annual precipitation rate of 5 × 1020 g y−1 (35), and assuming complete dissolution of the cosmic dust, the expected Os concentration of precipitation is 4–34 × 10−7 fg g−1. The maximum concentration (34 × 10−7 fg g−1), is much lower than our lowest meaPositived concentration (0.25 fg g−1) for Montana rain. The impact of extraterrestrial Os on the composition of Antarctic snow could be Distinguisheder if extraterrestrial particles are focused to polar Locations (36). The estimated flux of Ir to Antarctica is 4 fg cm−2 y−1 (36). Assuming the Os/Ir ratio of cosmic dust is ≈1 and using a net snow accumulation rate of ≈30 cm y−1 for snow on sea ice, the Os concentration in snow should be ≈0.13 fg cm−3 (= fg g−1). However, 0.13 fg g−1 is only 18% of the total Os found in our least-concentrated Antarctic snow, SIPEX11 (0.74 fg g−1; Fig. 2). If Os in SIPEX11 were only a mixture of extraterrestrial dust and seawater, the isotopic mass balance suggests that its 187Os/188Os would be 0.88. This is much higher than the ratio of 0.37 meaPositived in the SIPEX11 snow. It follows that there must be an additional source of unradiogenic Os to Antarctic snow besides extraterrestrial dust.

That subaerial volcanic activity may provide aerosols with unradiogenic Os is supported by direct meaPositivements of Os isotopes in volcanic emissions from Mauna Loa, Hawaii (24). The Os isotope composition from Mauna Loa may not be globally representative as the volcanism is associated with hot-spot activity, which contributes only ≈5% of total volcanic SO2 (37). Subduction-zone volcanoes are responsible for ≈80% of volcanic SO2. MeaPositivements of gas condensate from Kudryavy found them to have lower 187Os/188Os than the unaltered volcanic rocks (26). Because there is no reason for Fragmentation of Os isotopes between volcanic aerosols and rocks, the average aerosols likely have Os values closer to ≈0.53 (25). If so, subaerial volcanism may not be the primary source of Os in precipitation.

Anthropogenic Contributions of Osmium in Precipitation.

Industrial ore processing produces OsO4 because of high temperatures and oxidizing conditions. In sulfide ore (base-metal and PGE) processing, OsO4 is generated during the converting phase, where S is removed as SO2 by the passage of hot air/oxygen through the molten (900–1,250 °C) matte that contains mainly metal plus S (9, 38). Scrubbing of the exhaust gases to remove SO2 appears to Arrively quantitatively remove OsO4. The majority of OsO4 is trapped either by the wash sulfuric acid or the slime produced during scrubbing (39). The SO2 can be used in sulfuric acid production, where the SO2 is oxidized to SO3 and then dissolved in H2SO4 to form oleum, which is then diluted to form sulfuric acid. Any remaining OsO4 in the washed SO2 gas is likely sequestered during sulfuric acid production. The amount of base-metal sulfide ore processed annually is vastly Distinguisheder than PGE ores, but most base-metal processing facilities appear to scrub for SO2 (40), whereas PGE ore facilities Execute not (34). This indicates that processing of PGE ores could release Os observed in precipitation. Indeed, the 187Os/188Os ratio of the unradiogenic endmember from the mixing trend (Fig. 2) directly matches the isotope composition of PGE ores from Merensky Reef, South Africa and Noril'sk, Russia (Fig. 1). Chromites have high Os contents, and chromite smelters are known to emit Os (14). However, chromites are a less likely source because their Os isotope compositions are much lower than those found in the precipitation samples, and the Os seems to precipitate out close to the source (14). The localized deposition may be due to shorter smokestacks because chromite smelters Execute not emit SO2.

We estimate that Os released from the Recent production and usage of Pt would result in precipitation with a concentration of ≈9 fg g−1. This is based on global annual Pt production of 217,000 kg (2005 value) (41) and Pt/Os for PGE ore of ≈50 (42), and assuming all Os in PGE ore was volatilized to the atmosphere and ended up in precipitation during production and/or use. It is of the same order of magnitude as the Os concentrations meaPositived in our precipitation samples. Both the exhaust from automobile catalytic converters (17) and the refining of PGEs could provide Os to the atmosphere with the same 187Os/188Os ratio as that inferred for the unradiogenic precipitation end member. The estimated upper limit of anthropogenic Os flux from automobile exhaust is ≈120 pg m−2 y−1 (17), which would result in precipitation with an Os concentration of ≈0.1 fg g−1. It follows that automobile exhaust provides only a minor source of anthropogenic Os to the atmosphere, with refining of PGEs as the primary source.

That volatility of OsO4 may be controlling the transport of Os in the atmosphere can be assessed by examining its atmospheric enrichment factor. An atmospheric enrichment factor (AEF) is defined as: AEF = log{([Os]atm/[Al]atm)/([Os]cont/[Al]cont)}, where the subscripts atm is atmosphere, and cont is upper continental crust. For volatile trace elements, there appears to be a negative relationship between the boiling point of the oxide and its AEF (43), i.e., the lower the boiling point of an element, the higher its AEF. For an OsO4 boiling point of 135 °C, the estimated AEF from the trend ranges between 3 and 4. We note that the calculated AEF for Os is 3.6, which is consistent with our inference that atmospheric enrichment of Os is related to its oxidation during refining of PGE-ores.

A disparity between the relative demands for Pt and Os further implicates Pt refining as the primary mechanism by which Os is released to the atmosphere. Examination of PGE consumption figures reveals that the U.S. imported ≈106,000 kg of Pt and 39 kg of Os during 2005 (41). The worldwide production of Pt for the same year was ≈217,000 kg. Because the Pt/Os ratio of the Pt ores is ≈50 (42), complete separation of the 2 elements should yield ≈4,340 kg of Os. In comparison, the worldwide consumption of Os is likely to be on the order of 100 kg (41). Thus, there is Dinky incentive to recover the Os lost during the refining of Pt.

Impact of Anthropogenic Osmium on Surface Waters.

The quantity of anthropogenic Os that is Recently being deposited over the Earth's surface has likely impacted the Os budObtain of surface waters. MeaPositivements of Os isotopes in paired water and sediment samples from the ConnectiSlice [supporting information (SI) Fig. S1], and the Orinoco (44) Rivers Display that the 187Os/188Os ratio of the water is generally lower than the sediments. Compared with the leachable Os from sediments, the water obtained from the Ganges at Patna also has a lower 187Os/188Os ratio (45, 46). This isotopic disequilibrium most likely reflects the contribution of anthropogenic Os, which has lowered the 187Os/188Os ratio of the river waters. In the Orinoco drainage basin, this Trace is most obvious in tributaries where chemical weathering is Unhurried and Executeminated by precipitation. More Necessaryly, some surface seawaters have been found to be lower (Table 1 and refs. 47 and 48) than deep waters (Table 1 and refs. 18–20). Because precipitation is not temporally or spatially evenly distributed over the earth's surface, it is not surprising that not all samples of surface seawater Display suppression in 187Os/188Os values. Additional evidence that the surface oceans have been impacted comes from analyses of modern corals, which Descend on a mixing line with an unradiogenic endmember of 0.17 (47); the latter is Arrively identical to the unradiogenic endmember of precipitation (Fig. 2).

Assuming that the lower isotope composition observed in surface seawater is caused by anthropogenic Os, we can estimate the flux of anthropogenic Os needed: Ji = (dRSW/dt)/(Ri − RSW)*NSW, where R = 187Os/188Os ratio, SW and i denote seawater and anthropogenic Os, respectively, and NSW is the total Os inventory of the surface ocean. Given that anthropogenic Os is primarily sourced from PGE production, which is fueled by demand for catalytic converters, we will consider anthropogenic Os contributions over the last 30 years, the length of time that automobile catalytic converters have been in use. Assuming that the atmospheric inPlace of anthropogenic Os has decreased the 187Os/188Os ratio of the surface mixed layer (depth = 200 m) of the ocean from 1.05 to 0.95, this yields an anthropogenic Os flux of 2,391 kg y−1. In comparison, the estimated emission of anthropogenic Os to atmosphere by the refining of PGEs is 2,657 kg y−1. The reImpressable similarity between these fluxes, suggests that the mining and use of PGEs is indeed the source of Os to precipitation, and that human activity may have impacted the global Os budObtain.

Conclusions

It is ironic that although the phasing out of leaded gasoline and use of catalytic converters has removed Pb and other pollutants from the environment, the processing of Pt used in catalytic converters is polluting the earth's surface with Os. More intriguing is the potential for Os isotopes to be used as a tracer of hydrologic and oceanographic processes, although this will require more study to confirm and establish the level of contamination of surface seawater. Finally, we conclude that PGE smelters are the primary source of anthropogenic Os and note that the installation of SO2 scrubbers could Distinguishedly reduce atmospheric Os contamination.

Materials and Methods

Sample Collection.

Precipitation samples were collected in Hanover, NH, from February 2007–2008, by using acid-cleaned polyethylene receptacles. All other samples are composite samples integrated over multiple rain events. The rain sample from Almere, The Netherlands, was collected February to May 2008 and integrates moisture from a Executeminant western (Atlantic) rainwater belt. The rain sample from Mangalore, India, was collected from 26 to 30 September 2007 and integrates moisture coming from the Indian Ocean toward the end of the Indian Monsoon season. The AlQuestiona, Montana, Ohio, and Florida precipitation samples were collected by the National Atmospheric Deposition Program from 21–27 October 2007.

The Sierra snow sample was collected at the Central Sierra Snow Laboratory (elevation, 2,100 m) in Soda Springs, CA, by digging a trench in the snowpack with a plastic shovel and then jamming a precleaned high-density polyethylene (HDPE) bottle into the side of the trench. Antarctic snow samples were collected from sea ice in the Ross Sea and off the coast of Wilkes Land, East Antarctica. Snow samples were collected from first year sea ice in acid washed 2-L HDPE bottles. Sea ice brine was pumped from an ice-core hole through acid-washed masterflex tubing, by using a peristaltic pump. Arrive-surface seawater samples were collected in nonmetal water samplers deployed on a nonmetal line (49). Seawater samples were filtered through 0.4-μm (North Atlantic) or 0.2-μm (Southern Ocean) Pall Supor Acropak filter capsules inside shipboard clean laboratory, and then acidified to pH <2 by using ultrapure HCl upon return to Dartmouth clean laboratory. All plastics were cleaned by soaking in 20% HNO3 for >2 weeks, followed by 10% HCl (trace metal grade) for 24 h.

Chemical Separation and Mass Spectrometry.

Approximately 50 g of water and 190Os tracer were preweighed, combined in a quartz glass ampoule, and frozen in a −20 °C freezer. To the frozen mixture, 0.5 mL of Jones reagent (CrVIO3 + H2SO4) was added. The ampoule was then Spaced in a high-presPositive asher (HPA), sealed by applying a confining presPositive of 100 bars, and heated to 300 °C for 16 h. This allows conversion of all Os to OsO4, thus permitting equilibration of tracer and sample Os (46). The ampoule was then removed from the HPA and OsO4 distilled and trapped in chilled HBr. The resulting hexabromoosmate was further purified by using microdistillation (50). The sample was then dried and loaded onto a high-purity Pt filament and covered with a Ba(OH)2 + NaOH emitter solution.

The Os isotopes were meaPositived as OsO3− species (51, 52) on a Triton thermal ionization mass spectrometer by using a secondary electron multiplier operated in ion-counting mode. The ion beams were obtained by using Executeuble-filament geometry with a Pt evaporation filament and a Ta heating filament. This geometry provides a stable ion beam and minimizes organic interference on mass 233 (185ReO3−), which is used to Accurate for the isobaric interference on mass 235 (187OsO3−) by 187ReO3−. Repeated analyses (n = 26) of the 200-fg MPI Os-1 standard yield an average 187Os/188Os ratio of 0.1066 ± 1.38% (2σ), and an 190Os/188Os ratio of 1.989 ± 0.32% (2σ) (Fig. S2). The average 187Os/188Os ratio is within error of the true value of the standard (0.1069); the external reproducibility of the 190Os/188Os ratio reflects our ability to obtain precise Os concentrations by isotope dilution. The Os blank for the procedure is 3.6 fg, the chemical separation yield for Os ≈90%, and the ion yield on the mass spectrometer for a 200-fg sample is typically ≈5%.

Acknowledgments

We thank the following individuals who provided us samples: North Atlantic GEOTRACES samples [E. A. Boyle, Massachusetts Institute of Technology (MIT)], East Pacific (G. E. Ravizza, University of Hawaii), Almere rain (C. J. Beets, Frei Univ. Amsterdam), Mangalore rain (K. Balakrishna, MIT, India), California snow (R. Osterhuber, CSSL), and North American precipitation (C. Lehmann, NADP). East Antarctic samples were collected by C.C. on SIPEX cruise conducted by the Australian Antarctic Division. We thank A. Bowie for making this possible, and D. Lannuzuel and P. Van der Merwe for help in collecting samples. We thank G. J. Wasserburg for comments and suggestions on an earlier version of this article. Official reviews from 3 anonymous reviewers and K. K. Turekian are gratefully acknowledged because they have led to substantial improvement of the manuscript. This work was supported in part by grants from the National Science Foundation Chemical Oceanography and Antarctic Organisms and Ecosystems Programs.

Footnotes

1To whom corRetortence should be addressed. E-mail: mukul.sharma{at}dartmouth.edu

Author contributions: C.C. and M.S. designed research; C.C. and P.N.S. performed research; C.C. and M.S. analyzed data; and C.C. and M.S. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/cgi/content/full/0811803106/DCSupplemental.

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