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The uranium-isotopic composition of Saharan dust collected over the central Atlantic Ocean
Aeolian Research 17 (2015) 61–66
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The uranium-isotopic composition of Saharan dust collected over the central Atlantic Ocean Sarah M. Aciego a,⇑, Sarah M. Aarons a, Kenneth W.W. Sims b a b
Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, United States Department of Geology and Geophysics, University of Wyoming, Laramie, WY, United States
a r t i c l e
i n f o
Article history: Received 10 May 2014 Revised 1 November 2014 Accepted 5 January 2015 Available online 26 February 2015 Keywords: Sahara Dust Soils Uranium isotopes
a b s t r a c t Uranium isotopic compositions, 234 U=238 U activ ity , are utilized by earth surface disciplines as chronometers and source tracers, including in soil science where aeolian dust is a significant source to the total nutrient pool. However, the 234 U=238 U activ ity composition of dust is under characterized due to material and analytical constraints. Here we present new uranium isotope data measured by high precision MCICP-MS on ten airborne dust samples collected on the M55 trans-Atlantic cruise in 2002. Two pairs of samples are presented with different size fractions, coarse (1–30 lm) and fine (<1 lm), and all samples were processed to separate the water soluble component in order to assess the controls on the 234 U=238 U activ ity of mineral aerosols transported from the Sahara across the Atlantic. Our results indicate 234 U=238 U activ ity above one for both the water soluble (1.13–1.17) and the residual solid (1.06–1.18) fractions of the dust; no significant correlation is found between isotopic composition and travel distance. Residual solids indicate a slight dependance of 234 U=238 U activ ity on particle size. Future modeling work that incorporates dust isotopic compositions into mixing or isotopic fractionation models will need to account for the wide variability in dust 234 U=238 U activ ity . Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The deposition of aeolian dust to the continental biosphere can be a significant source of nutrients to soils and ecosystems (Chadwick et al., 1999; Reynolds et al., 2001), especially in nutrient limited locations such as the tropics, with high weathering (leaching) rates (Chadwick et al., 1999; Wardle et al., 2004; Muhs et al., 2007; Pett-Ridge et al., 2009), or soils derived from nutrient-poor bedrock (Richardson et al., 2004; Hahm et al., 2014). Assessing the relative contributions of bedrock and aeolian dust to the nutrient cycles in these locations can be done using physical and chemical tracers, including radiogenic isotopes (Borg and Banner, 1996; Reid et al., 2003; Prospero et al., 2010). Similarly, quantitative weathering studies constrain the formation of soils related to biology, ecology, long term climate change, and erosion using elemental concentrations, radiogenic isotope tracers, and radioactive isotope chronometers (Miller et al., 1993; Bullen et al., 1997; Huggett, 1998; Wardle et al., 2004; Dosseto et al., 2012; Poggevon Strandmann et al., 2012; Chabaux et al., 2013). Recently, work in these areas has focused on a combination of radiogenic (e.g. strontium, Sr) and uranium-series (U-series) isotopes to ⇑ Corresponding author. http://dx.doi.org/10.1016/j.aeolia.2015.01.003 1875-9637/Ó 2015 Elsevier B.V. All rights reserved.
elucidate the relationship between dust deposition, soil formation and nutrient cycling (Pelt et al., 2013). The U-series decay chains have distinct advantages over other systems for determining timescales of processes due to the variable decay rates of daughters within the chain. The 238 U decay chain includes the daughter isotopes 234 U (t1=2 = 248 kyr). In undisturbed rocks and minerals older than 1 Ma, activities of the daughters should be equal to those of the parents: 238 U k238 ¼ 234 U k234 (where brackets indicate atom concentrations and k refers to the decay constant). Therefore, the activity ratio 234 U=238 U activ ity will be equal to one and in ‘‘equilibrium’’. Departure from equilibrium can occur during water–rock interaction via the preferential release of daughter products from the solid by recoil associated with the high-energy alpha decay of the parent nuclide (see Semkow, 1991; DePaolo et al., 2006; Aciego et al., 2011 for model descriptions). As a result, primary solids contain depleted daughter radionuclides to the parent ones whereas liquid phase have enriched daughter radionuclides: 234 U=238 U activ ity ratios of 0.5–0.9 are commonly measured in fine-grained solids (Skwarzec et al., 2002; Maher et al., 2004, 2006; Dosseto et al., 2008) and activity ratios of 1.05–11 have been measured in natural waters and ice (Chabaux et al., 2003; Porcelli and Swarzenski,
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2003; Goldstein et al., 2004; Maher et al., 2004, 2006; Dosseto et al., 2008; Aciego et al., 2011). Dust emission source materials are not principally derived from ‘‘primary’’ minerals; instead most materials that form dust are secondary weathering products such as clays and silts (Biscaye et al., 1997) that precipitate out of solutions likely enriched in 234 U com pared to 238 U (Oster et al., 2012). The ultimate 234 U=238 U activ ity ratio will depend on the (1) composition of the fluid from which the solid precipitated and (2) time the grain has been that size and composition. For instance, dust source materials in the Sahara were weathered under humid conditions 6000 years ago (Schuster et al., 2006) and precipitates from liquids (and weathering) would lead to 234 U=238 U activ ity ratios greater than 1. Since that time, these Saharan mineral particles will have lost some 234 U relative to 238 U due to recoil. The actual 234 U=238 U activ ity ratio will reflect the balance between initial enrichment and decay back to an equilibrium value. Previous research has shown that the radiogenic composition of aerosol dust is size dependent (Biscaye et al., 1997). As DePaolo et al. (2006) and Maher et al. (2004, 2006) have shown, recoil loss of 234 U should be greater in minerals with large surface to volume ratios (e.g. small grains), but no 234 U=238 U activ ity ratios of different size fractions of aerosols have been published. Because coarse particles will be deposited faster than fine particles due to gravitational settling, the isotopic composition of the dust can fractionate with distance from source. One final aspect of aerosol deposition that is often neglected is the isotopic difference between the soluble and insoluble masses, only recently addressed for Sr isotopes (Aarons et al., 2013), but no measurements have been published for 234 U=238 U activ ity . U-series chronometers make assumptions about the initial 234 U=238 U activ ity of the source material of interest and the conservative behavior of dust from source to sink (e.g. dust in soils, Pelt et al., 2013). Therefore the uranium isotopic composition of aeolian dust is an important constraint on earth surface processes. In the equatorial Atlantic and Caribbean, the Sahara is wellestablished as the dominant source of aeolian dust material (McDowell et al., 1990; Muhs et al., 1990; Borg and Banner, 1996; Colarco et al., 2003; Pett-Ridge et al., 2007, 2009; Prospero et al., 2010). However, the literature is sparse on the uranium isotopic composition of aeolian dust and only one study is published on the uranium isotopic composition of Saharan dust from aerosol samples collected in Barbadoes and analyzed approximately 40 years ago (Rydell and Prospero, 1972). In the intervening period the technology for measuring U-series isotopes has changed considerably (Goldstein and Stirling, 2003; Ball et al., 2008) and errors in measurements have gone from 5% to 10% (Rydell and Prospero, 1972) to better than 0.5% today (Oster et al., 2012; Keech et al., 2013; Pelt et al., 2013; Arendt et al., 2014). Recent work has started to provide more comprehensive spatial variability in global dust uranium compositions (Oster et al., 2012), but soil formation studies in the equatorial Atlantic and Carribean still rely on the work of Rydell and Prospero (1972). Given the large errors in the measurements by alpha counting, it is difficult to determine if the large range in uranium isotopic compositions ( 234 U=238 U activ ity = 1.00– 1.22, one standard deviation = 0.06) in the Rydell and Prospero (1972) study can be attributed to a true variability in the composition of the dust or a gaussian distribution of the error; typical 1sigma errors for the alpha-counting technique employed were 0.05, similar to 1 S.D. of all the measurements of 0.06. Data are moderately skewed, moderate skewness = 0.79, by the value of 1.22, but still have a 1 S.D. of 0.05 if the outlier is removed, low skewness = 0.31. Dust deposition and soil formation models that rely on this one study may be under-estimating the variability in the uranium composition of Saharan dust.
The objectives of this study are threefold: (1) assess the vari ability 234 U=238 U activ ity of Saharan dust, (2) determine if the isotopic composition is size-dependent, and (3) examine the controls on the 234 U=238 U activ ity of the soluble and insoluble components of aerosols transported from the Sahara across the Atlantic. In order to address these objectives we examine the 234 U=238 U activ ity of aeolian dust sourced from a Saharan dust storm and deposited over several thousand kilometers across the equatorial Atlantic Ocean. 2. Materials and methods Airborne dust samples were collected during Meteor Cruise 55 (M55), a 6000 km west to east transect of the tropical Atlantic Ocean in October/November 2002; the latitude and longitude of sample collection locations along with sample numbers are noted in Table 1 and mapped in Fig. 1. The cruise transect intersected a dust storm originating in the Sahara, as indicated by back-trajectory analysis (Wallace and Bange, 2004). Aerosol collection was monitored for anthropogenic source corruption (e.g. ship exhaust), and collection suspended if there was a possibility of contamination (Baker and Jickells, 2006). Enough aerosol material was collected to measure the radiogenic Sr, Nd, Hf (Aarons et al., 2013; Rickli et al., 2010) and radioactive U isotopic composition (presented here) of the dust. Samples of 1–10 mg of airborne dust were collected on cellulose Whatman 41 filters in coarse (1–30 lm) and fine (<1 lm) fractions; a full description of sampling techniques is described in Baker (2004), Baker and Jickells (2006). Dust samples were removed from filters using ultrasonication and centrifugation under Class 100 clean lab conditions using ultrapure reagents (>18 MX water and Seastar acids) as described in Aarons et al. (2013). Filter membranes on which the aerosol Ò samples were collected were placed on precleaned Teflon baskets Ò Ò in 90 mL Savillex Teflon beakers. 15 mL of water was added to the beakers and samples were ultrasonicated for 1 h, the liquid decantÒ Ò ed into precleaned Savillex Teflon centrifuge tubes, centrifuged Ò Ò for 10 min, the water was pipetted into 30 mL Savillex Teflon beakers; these vessels contained the water soluble fraction. 7 mL of water was then added to the original 90 mL beakers (and samples), ultrasonicated for 1 h, decanted into centrifuge tubes, centrifuged again for 10 min, then the water was pipetted into Ò Ò Savillex Teflon beakers. The centrifuge tubes containing the insoluble dust were treated with 10 drops of concentrated HNO3, shaken, and poured into 7 mL beakers; these vessels contained the insoluble fraction. An additional 1 mL of 7.5 M HNO3 was added to centrifuge tubes, shaken, and poured into the insoluble fraction beakers. After removal and separation of the water soluble and residual dust solids, the two fractions were digested and dissolved in acids. The water soluble fraction was dried and redissolved in 9 M HCl, while the insoluble portion was digested in aqua regia followed by a mixed HF–HNO3–HClO4 digestion step. After digestion, samples were chemically separated using the techniques described in Aciego et al. (2009), which separated the Sr, Nd, Hf and U into elemental fractions. Analysis of the Sr, Nd, and Hf isotopic compositions are described in Aarons et al. (2013). The U fractions from the chemical separation were retained and the uranium isotopic compositions measured at the University of Wyoming. Uranium isotopic compositions were measured at the Wyoming High Precision Isotope Laboratory at the University of Wyoming on a Neptune PLUS ultra high-precision, high-sensitivity multi collector inductively coupled mass spectrometer using established techniques (Ball et al., 2008). Each sample was loaded into a sample solution of 0.01M HCl and diluted to approximately 10 ppb
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Table 1 Uranium isotopic composition of Saharan dust, sample names ending with ‘F’ are the fine fraction and sample names ending with ‘C’ are the coarse fraction. 2 S.D. errors in the activity are all 0.001 based on the long-term reproducibility of external standard NBS960 (Arendt et al., 2014). Sample name
Latitude
Longitude
3F 4C 7F 7C 9F 11F 11C 20F 24F 25F
11.0 N 10.6 N 10.0 N 10.0 N 10.0 N 10.0 N 10.0 N 8.4 N 10.9 N 11.0 N
58.7 53.7 44.8 44.8 39.0 33.3 33.3 24.6 17.1 19.0
a
W W W W W W W W W W
Dust size (lm)
Water soluble 234 U=238 U act
Residual solid 234 U=238 U act
Water soluble 87 Sr/86Sra
Residual solid 87 Sr/86Sra
1–2 2–30 1–2 2–30 1–2 1–2 2–30 1–2 1–2 1–2
1.123 1.150 1.133 1.149 1.152 1.152 1.152 1.158 1.151 1.170
1.134 1.089 1.060 1.066 1.092 1.093 1.144 1.179 1.125 1.115
0.710693 n.d. 0.710534 0.709842 0.710698 0.712097 0.710713 0.710975 n.d. 0.711483
0.716669 n.d. 0.717743 0.713584 n.d. 0.719167 0.714815 0.716028 0.718134 0.716606
Indicates previously published data from Aarons et al. (2013), where external 2 S.D. errors in 87Sr/86Sr are 0.000030 based on 10 mg splits of USGS reference material BCR-2.
Fig. 1. Sample collection sites: numbers shown are the sample names; latitude and longitude are compiled in Table 1. Map modified from Aarons et al. (2013).
uranium concentration. The sample intake system was rinsed with ultrapure 18 MX water and dilute HCl between analyses. Sample measurements were bracketed with measurements of New Brunswick Laboratory international reference uranium standard U010 to correct for mass bias and ion counting efficiency, using the U=234 U ratio of 18,354 (Richter and Goldberg, 2003). NIST international reference uranium standard NBS960 was run periodically to test the precision and accuracy of the Neptune PLUS over the entire sample run and the activity ratio compared to the accepted value of Cheng et al. (2000), 0.963. The long term average measured from August 2011 to March 2014 of standard NBS960 on the University of Wyoming Neptune PLUS is 0.963, with a 2 S.D. of 0.001 (Arendt et al., 2014).
238
3. Results and discussion The uranium isotopic compositions of the coarse and fine fractions of the M55 cruise dust samples are presented in Table 1. In seawater and crustal materials concentrations of Sr are higher than that of neodymium (Nd), hafnium (Hf) and U, which results in a higher measurement success rate of Sr isotopic compositions compared to the remaining elements given the limited sample size. Nevertheless, robust 234 U=238 U activ ity ratios were obtained on ten samples out of a total of sixteen, including two paired coarseand-fine fraction samples (samples 7 and 11). The 234 U=238 U activ ity compositions of the dust samples were in the range 1.06–1.18, narrower than that found by Rydell and
Prospero (1972) (1.00–1.22) but the analytical errors of this measurement are much smaller than that of previous works. This find 234 U=238 U activ ity variability is ing indicates that the compositionally driven and not an artifact of high analytical errors. The relationship between the particles sizes and 234 U=238 U activ ity does follow the hypothesized relationship outlined in the Introduction: fine grained materials are more depleted in 234 U than the corresponding coarse ones (Fig. 2). The difference in 234 U=238 U activ ity between the grain sizes is inconsistent; the fine-grained and coarse-grained composition of sample 7 are almost identical (1.060 and 1.066), while the fine-grained and coarse-grained composition of sample 11 are significantly different (1.093 and 1.144). Although these samples support the hypothesized relationship, the small number of sample pairs (two) is not enough for a definitive conclusion and additional studies will be required to verify a broad relationship. For the purposes of this study we use the distance measurements from Aarons et al. (2013) to assess the relationship between fractionation of isotopic composition and travel distance in dust (e.g. due to gravitational settling). The starting point chosen was the coast of west Africa (Dakar), as Laurent et al. (2008) and Tanaka and Chiba (2006) have shown, dust emissions are sourced from across the Sahara encompassing both interior sites (e.g. Mauritania) and coastal sites (e.g. Senegal), but the last possible Saharan source on a westward trajectory is the Senegal coast. It is possible and likely that some dust was sourced from the interior of west Africa, which would make the distances provided here
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uble 87Sr/86Sr and the seawater 87Sr/86Sr suggested that 70% of the Sr in the water soluble fraction was sourced from seasalt; the similar correlation found here in 234 U=238 U activ ity suggests seasalt is the dominant uranium contributor to the water soluble fraction. Fig. 4 illustrates the similar behavior of Sr and U in the water soluble fraction; previously measured 87Sr/86Sr (Aarons et al., 2013) and 234 U=238 U activ ity measured in this study show that the water soluble fraction of both Sr and U pools are clustered close to seawater. The actual source of the soluble fraction is likely a combination of seaspray and water-soluble minerals or mineral coatings on the insoluble material, which provides an explanation for excursions away from the seawater isotopic composition. Mineral coatings on African dust have been invoked to explain isotopic differences in hafnium space (Bayon et al., 2009), and their presence would provide an explanation for the 234 U=238 U activ ity behavior observed
238
Fig. 2. 234 U= U for the water soluble (blue circles) and solid residuals activ ity (brown squares for coarse fraction and brown diamonds for fine fraction) of Saharan dust as a function of distance from dust source. Also plotted is the isotopic composition of modern seawater (blue line). Note that samples 11F and 11C have identical sampling locations and water soluble 234 U=238 U activ ity compositions, so the data symbols plot in the same location.
minimum travel distances. With the estimated dust source as the coast of west Africa, there is a slight negative correlation between the 234 U=238 U activ ity of the insoluble dust and distance from the
dust source (Fig. 2), but regression analysis provides an r2 value of 0.16 and a p-value of 0.249 indicating that the correlation is insignificant. If the travel distance was longer due to a more interior source or tortuous trajectories, additional distance between points would decrease the negative trend and reinforce the current observation of little correlation between distance and composition. Based on the data presented, 234 U=238 U activ ity isotopic fractionation is not transport-distance dependent. With the exception of two samples, the water-soluble samples had higher 234 U=238 U activ ity than their corresponding insoluble counterparts, and are uniformly closer to the known modern 234 U=238 U activ ity seawater ratio of 1.149 ± 0.002 (Stirling et al.,
here. Mineral coatings or evaporitic minerals are precipitated out of fluids interacting with the surface environment and can be differently aged than insoluble mineral grains. The variability in 234 U=238 U activ ity ratios in earth surface fluids (see Introduction) means that soluble minerals and coatings can also have highly variable 234 U=238 U activ ity ratios, which when mixed with seaspray will lead to excursions away from 1.149. While this observation supports their existance, the Sr and U isotopic compositions are not positive proof and future work should address this additional ‘‘source’’ component. In contrast, there is no observable correlation between the 87 Sr/86Sr and 234 U=238 U activ ity ratios in the water-insoluble dust fraction (Fig. 4). This lack of correlation suggests that there are different processes controlling the isotopic compositions in the water-insoluble dust. The 87Sr/86Sr composition depends generally on the age of the continental crust from which the dust is derived and mineralogy of that dust (see Grousset and Biscaye, 2005 and references within). The long, 48.8 109 year, half-life of 87Rb, which is the parent nuclide of 87Sr, means that the 87Sr/86Sr in mineral dust will not depend on it’s age assuming that the age of the mineral dust is less than several million years old. In contrast, the 234 U=238 U activ ity composition does not depend on the composi-
1998; Robinson et al., 2004) as shown in Fig. 3. In the previous work of Aarons et al. (2013), the correlation between the water sol-
Fig. 3. Comparison of the 234 U=238 U of the water soluble fraction to the activ ity residual fraction; symbology is the same as in Fig. 2. Note that the compositions of samples 9F and 11F are almost identical so the data symbols plot in the same location. In all cases, the error in the measurement is smaller than the size of the symbol.
87 86 Fig. 4. Plot of radiogenic strontium isotopic composition ( Sr/ Sr) versus 234 U=238 U for the water soluble (blue circles) and solid residuals (brown activ ity squares for coarse fraction and brown diamonds for fine fraction) of Saharan dust. Also plotted is the isotopic composition of modern seawater (blue circle). Strontium compositions were measured on the same sample digestions as the uranium compositions and taken directly from Aarons et al. (2013); values of isotopic compositions are provided in Table 1. In all cases, the error in the measurement is smaller than the size of the symbol. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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tion or age of the continental crust, but instead on the geometry of the dust grains, the interaction with waters with high 234 U=238 U activ ity and the age of the dust. The relatively short half-life of 234 U and fine particles containing uranium means that the dust composition can change on timescales between 10 ka and 1 Ma (DePaolo et al., 2006; Aciego et al., 2011), leading to high ly variable 234 U=238 U activ ity ratios in dynamic dust source environments such as the Sahara, a conclusion that is supported by our results. The 234 U=238 U activ ity ratios measured in this study have immediate implications for studies utilizing U-series isotopes where dust deposition is a significant process in the environment. Models that incorporate a soluble dust fraction (e.g. Oster et al. (2012), Andersen et al. (2013)) into their soil development can use an end-member 234 U=238 U activ ity ratio centered around the seawater value with a limited variability. If the soluble portion was the only component supplying uranium to the system, a simple linear mixing model – that depends on limited variability in end-member compositions (Parnell et al., 2010) – would be appropriate for deconvolving the source contributions of uranium present in a soil or water system. However, because silicate dust weathers and releases material into the environment after deposition, simple mixing models are inappropriate. The water-insoluble 234 U=238 U activ ity ratios, whose variability is more than 10%, are
not correlated with the corresponding 87Sr/86Sr ratios, a factor not accounted for in linear mixing models (Parnell et al., 2010). In order to incorporate source isotopic composition variability into mixing models, more complex approaches will be required such as Bayesian techniques employed in stable isotope mixing models (Parnell et al., 2010; Cable et al., 2011; Arendt et al., submitted for publication). 4. Conclusions Our study confirms the wider variability of the
234
U=238 U
activ ity
ratios in the Saharan airborne dust observed by Rydell and Prospero (1972) and indicates that the range is based on source composition variability rather than analytical errors. The 234 U=238 U activ ity ratios reveal that uranium in the water soluble fraction of the dust is dominantly derived from sea salt, which is in agreement with the previous work on the 87Sr/86Sr composition of the water soluble and residual solids (Aarons et al., 2013). How ever, the water soluble 234 U=238 U activ ity variations away from the seasalt value are not well correlated with the solid composition, indicating that there may be a mineral coating derived from the dust source region with a different isotopic composition. Analysis of different size fractions of the same sample indicates a that the fine fractions are less enriched in 234 U than their coarse fraction counterparts, but more work will need to be done to fully understand particle-size dependency. The lack of correlation between the 234 U=238 U activ ity and 87Sr/86Sr compositions of the residual solids indicates that there is large source composition variability with respect to 234 U=238 U activ ity , which renders simple linear mixing models inappropriate for dust deposition modeling of 234 U=238 U activ ity in soil systems. Acknowledgements The samples for this research were kindly provided by A.R. Baker. Funding for this project was provided by grants from the Rackham Graduate School and the Turner Award from the Department of Earth and Environmental Sciences at the University of Michigan
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Porcelli, D., Swarzenski, P.W., 2003. The behavior of U- and Th-series nuclides in groundwater. In: Uranium-Series Geochemistry, vol. 52, pp. 317–361. Prospero, J., Landing, W., Schulz, M., 2010. African dust deposition to Florida: temporal and spatial variability and comparisons to models. J. Geophys. Res. 115 (D13), D13304. Reid, E., Reid, J., Meier, M., Dunlap, M., Cliff, S., Broumas, A., Perry, K., Maring, H., 2003. Characterization of African dust transported to Puerto Rico by individual particle and size segregated bulk analysis. J. Geophys. Res. 108 (D19), 8591. Reynolds, R., Belnap, J., Reheis, M., Lamothe, P., Luiszer, F., 2001. Aeolian dust in Colorado Plateau soils: nutrient inputs and recent change in source. Proc. Natl. Acad. Sci. USA 98 (13), 7123–7127. Richardson, S., Peltzer, D., Allen, R., McGone, M., Parfitt, R., 2004. Rapid development of phosphorous limitation in temperate rainforest along the Franz Josef soil chronosequence. Oecologia 139, 267–276. Richter, S., Goldberg, S.A., 2003. Improved techniques for high accuracy isotope ratio measurements of nuclear materials using thermal ionization mass spectrometry. Int. J. Mass Spectrom. 229 (3), 181–197. Rickli, J., Frank, M., Baker, A., Aciego, S., deSouza, G., Georg, R., Halliday, A., 2010. Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: implications for sources and inputs of trace metals to the ocean. Geochim. Cosmochim. Acta 74, 540–557. Robinson, L.F., Belshaw, N.S., Henderson, G.M., 2004. U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas. Geochim. Cosmochim. Acta 68, 1777–1789. Rydell, H., Prospero, J., 1972. Uranium and thorium concentrations in wind-borne Saharan dust over the Western Equatorial North Atlantic Ocean. Earth Planet. Sci. Lett. 14 (3), 397–402. Schuster, M., Duringer, P., Ghienne, J.-F., Vignaud, P., Mackaye, H., Likius, A., Brunet, M., 2006. The age of the Sahara desert. Science 311 (5762), 821. Semkow, T., 1991. Fractal model of radon emanation from solids. Phys. Rev. Lett. 66 (23), 3012–3015. Skwarzec, B., Borylo, A., Struminska, D., 2002. U-234 and U-238 isotopes in water and sediments of the southern Baltic. J. Environ. Radioact. 61 (3), 345–363. Stirling, C.H., Esat, T., Lambeck, K., McCulloch, M.T., 1998. Timing and duration of the last interglacial: evidence for a restricted interval of widespread coral reef growth. Earth Planet. Sci. Lett. 160, 745–762. Tanaka, T., Chiba, M., 2006. A numerical study of the contributions of dust source regions to the global dust budget. Global Planet. Change 52, 88–104. Wallace, D., Bange, H., 2004. Introduction to special section: results of the meteor 55: tropical solas expedition. Geophys. Res. Lett. 31, L23S01. Wardle, D., Walker, L., Bardgett, R., 2004. Ecosystem properties and forest decline in contrasting long-term chronosequence. Science 305 (5683), 509–513.
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Aeolian Research journal homepage: www.elsevier.com/locate/aeolia
The uranium-isotopic composition of Saharan dust collected over the central Atlantic Ocean Sarah M. Aciego a,⇑, Sarah M. Aarons a, Kenneth W.W. Sims b a b
Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, United States Department of Geology and Geophysics, University of Wyoming, Laramie, WY, United States
a r t i c l e
i n f o
Article history: Received 10 May 2014 Revised 1 November 2014 Accepted 5 January 2015 Available online 26 February 2015 Keywords: Sahara Dust Soils Uranium isotopes
a b s t r a c t Uranium isotopic compositions, 234 U=238 U activ ity , are utilized by earth surface disciplines as chronometers and source tracers, including in soil science where aeolian dust is a significant source to the total nutrient pool. However, the 234 U=238 U activ ity composition of dust is under characterized due to material and analytical constraints. Here we present new uranium isotope data measured by high precision MCICP-MS on ten airborne dust samples collected on the M55 trans-Atlantic cruise in 2002. Two pairs of samples are presented with different size fractions, coarse (1–30 lm) and fine (<1 lm), and all samples were processed to separate the water soluble component in order to assess the controls on the 234 U=238 U activ ity of mineral aerosols transported from the Sahara across the Atlantic. Our results indicate 234 U=238 U activ ity above one for both the water soluble (1.13–1.17) and the residual solid (1.06–1.18) fractions of the dust; no significant correlation is found between isotopic composition and travel distance. Residual solids indicate a slight dependance of 234 U=238 U activ ity on particle size. Future modeling work that incorporates dust isotopic compositions into mixing or isotopic fractionation models will need to account for the wide variability in dust 234 U=238 U activ ity . Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The deposition of aeolian dust to the continental biosphere can be a significant source of nutrients to soils and ecosystems (Chadwick et al., 1999; Reynolds et al., 2001), especially in nutrient limited locations such as the tropics, with high weathering (leaching) rates (Chadwick et al., 1999; Wardle et al., 2004; Muhs et al., 2007; Pett-Ridge et al., 2009), or soils derived from nutrient-poor bedrock (Richardson et al., 2004; Hahm et al., 2014). Assessing the relative contributions of bedrock and aeolian dust to the nutrient cycles in these locations can be done using physical and chemical tracers, including radiogenic isotopes (Borg and Banner, 1996; Reid et al., 2003; Prospero et al., 2010). Similarly, quantitative weathering studies constrain the formation of soils related to biology, ecology, long term climate change, and erosion using elemental concentrations, radiogenic isotope tracers, and radioactive isotope chronometers (Miller et al., 1993; Bullen et al., 1997; Huggett, 1998; Wardle et al., 2004; Dosseto et al., 2012; Poggevon Strandmann et al., 2012; Chabaux et al., 2013). Recently, work in these areas has focused on a combination of radiogenic (e.g. strontium, Sr) and uranium-series (U-series) isotopes to ⇑ Corresponding author. http://dx.doi.org/10.1016/j.aeolia.2015.01.003 1875-9637/Ó 2015 Elsevier B.V. All rights reserved.
elucidate the relationship between dust deposition, soil formation and nutrient cycling (Pelt et al., 2013). The U-series decay chains have distinct advantages over other systems for determining timescales of processes due to the variable decay rates of daughters within the chain. The 238 U decay chain includes the daughter isotopes 234 U (t1=2 = 248 kyr). In undisturbed rocks and minerals older than 1 Ma, activities of the daughters should be equal to those of the parents: 238 U k238 ¼ 234 U k234 (where brackets indicate atom concentrations and k refers to the decay constant). Therefore, the activity ratio 234 U=238 U activ ity will be equal to one and in ‘‘equilibrium’’. Departure from equilibrium can occur during water–rock interaction via the preferential release of daughter products from the solid by recoil associated with the high-energy alpha decay of the parent nuclide (see Semkow, 1991; DePaolo et al., 2006; Aciego et al., 2011 for model descriptions). As a result, primary solids contain depleted daughter radionuclides to the parent ones whereas liquid phase have enriched daughter radionuclides: 234 U=238 U activ ity ratios of 0.5–0.9 are commonly measured in fine-grained solids (Skwarzec et al., 2002; Maher et al., 2004, 2006; Dosseto et al., 2008) and activity ratios of 1.05–11 have been measured in natural waters and ice (Chabaux et al., 2003; Porcelli and Swarzenski,
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2003; Goldstein et al., 2004; Maher et al., 2004, 2006; Dosseto et al., 2008; Aciego et al., 2011). Dust emission source materials are not principally derived from ‘‘primary’’ minerals; instead most materials that form dust are secondary weathering products such as clays and silts (Biscaye et al., 1997) that precipitate out of solutions likely enriched in 234 U com pared to 238 U (Oster et al., 2012). The ultimate 234 U=238 U activ ity ratio will depend on the (1) composition of the fluid from which the solid precipitated and (2) time the grain has been that size and composition. For instance, dust source materials in the Sahara were weathered under humid conditions 6000 years ago (Schuster et al., 2006) and precipitates from liquids (and weathering) would lead to 234 U=238 U activ ity ratios greater than 1. Since that time, these Saharan mineral particles will have lost some 234 U relative to 238 U due to recoil. The actual 234 U=238 U activ ity ratio will reflect the balance between initial enrichment and decay back to an equilibrium value. Previous research has shown that the radiogenic composition of aerosol dust is size dependent (Biscaye et al., 1997). As DePaolo et al. (2006) and Maher et al. (2004, 2006) have shown, recoil loss of 234 U should be greater in minerals with large surface to volume ratios (e.g. small grains), but no 234 U=238 U activ ity ratios of different size fractions of aerosols have been published. Because coarse particles will be deposited faster than fine particles due to gravitational settling, the isotopic composition of the dust can fractionate with distance from source. One final aspect of aerosol deposition that is often neglected is the isotopic difference between the soluble and insoluble masses, only recently addressed for Sr isotopes (Aarons et al., 2013), but no measurements have been published for 234 U=238 U activ ity . U-series chronometers make assumptions about the initial 234 U=238 U activ ity of the source material of interest and the conservative behavior of dust from source to sink (e.g. dust in soils, Pelt et al., 2013). Therefore the uranium isotopic composition of aeolian dust is an important constraint on earth surface processes. In the equatorial Atlantic and Caribbean, the Sahara is wellestablished as the dominant source of aeolian dust material (McDowell et al., 1990; Muhs et al., 1990; Borg and Banner, 1996; Colarco et al., 2003; Pett-Ridge et al., 2007, 2009; Prospero et al., 2010). However, the literature is sparse on the uranium isotopic composition of aeolian dust and only one study is published on the uranium isotopic composition of Saharan dust from aerosol samples collected in Barbadoes and analyzed approximately 40 years ago (Rydell and Prospero, 1972). In the intervening period the technology for measuring U-series isotopes has changed considerably (Goldstein and Stirling, 2003; Ball et al., 2008) and errors in measurements have gone from 5% to 10% (Rydell and Prospero, 1972) to better than 0.5% today (Oster et al., 2012; Keech et al., 2013; Pelt et al., 2013; Arendt et al., 2014). Recent work has started to provide more comprehensive spatial variability in global dust uranium compositions (Oster et al., 2012), but soil formation studies in the equatorial Atlantic and Carribean still rely on the work of Rydell and Prospero (1972). Given the large errors in the measurements by alpha counting, it is difficult to determine if the large range in uranium isotopic compositions ( 234 U=238 U activ ity = 1.00– 1.22, one standard deviation = 0.06) in the Rydell and Prospero (1972) study can be attributed to a true variability in the composition of the dust or a gaussian distribution of the error; typical 1sigma errors for the alpha-counting technique employed were 0.05, similar to 1 S.D. of all the measurements of 0.06. Data are moderately skewed, moderate skewness = 0.79, by the value of 1.22, but still have a 1 S.D. of 0.05 if the outlier is removed, low skewness = 0.31. Dust deposition and soil formation models that rely on this one study may be under-estimating the variability in the uranium composition of Saharan dust.
The objectives of this study are threefold: (1) assess the vari ability 234 U=238 U activ ity of Saharan dust, (2) determine if the isotopic composition is size-dependent, and (3) examine the controls on the 234 U=238 U activ ity of the soluble and insoluble components of aerosols transported from the Sahara across the Atlantic. In order to address these objectives we examine the 234 U=238 U activ ity of aeolian dust sourced from a Saharan dust storm and deposited over several thousand kilometers across the equatorial Atlantic Ocean. 2. Materials and methods Airborne dust samples were collected during Meteor Cruise 55 (M55), a 6000 km west to east transect of the tropical Atlantic Ocean in October/November 2002; the latitude and longitude of sample collection locations along with sample numbers are noted in Table 1 and mapped in Fig. 1. The cruise transect intersected a dust storm originating in the Sahara, as indicated by back-trajectory analysis (Wallace and Bange, 2004). Aerosol collection was monitored for anthropogenic source corruption (e.g. ship exhaust), and collection suspended if there was a possibility of contamination (Baker and Jickells, 2006). Enough aerosol material was collected to measure the radiogenic Sr, Nd, Hf (Aarons et al., 2013; Rickli et al., 2010) and radioactive U isotopic composition (presented here) of the dust. Samples of 1–10 mg of airborne dust were collected on cellulose Whatman 41 filters in coarse (1–30 lm) and fine (<1 lm) fractions; a full description of sampling techniques is described in Baker (2004), Baker and Jickells (2006). Dust samples were removed from filters using ultrasonication and centrifugation under Class 100 clean lab conditions using ultrapure reagents (>18 MX water and Seastar acids) as described in Aarons et al. (2013). Filter membranes on which the aerosol Ò samples were collected were placed on precleaned Teflon baskets Ò Ò in 90 mL Savillex Teflon beakers. 15 mL of water was added to the beakers and samples were ultrasonicated for 1 h, the liquid decantÒ Ò ed into precleaned Savillex Teflon centrifuge tubes, centrifuged Ò Ò for 10 min, the water was pipetted into 30 mL Savillex Teflon beakers; these vessels contained the water soluble fraction. 7 mL of water was then added to the original 90 mL beakers (and samples), ultrasonicated for 1 h, decanted into centrifuge tubes, centrifuged again for 10 min, then the water was pipetted into Ò Ò Savillex Teflon beakers. The centrifuge tubes containing the insoluble dust were treated with 10 drops of concentrated HNO3, shaken, and poured into 7 mL beakers; these vessels contained the insoluble fraction. An additional 1 mL of 7.5 M HNO3 was added to centrifuge tubes, shaken, and poured into the insoluble fraction beakers. After removal and separation of the water soluble and residual dust solids, the two fractions were digested and dissolved in acids. The water soluble fraction was dried and redissolved in 9 M HCl, while the insoluble portion was digested in aqua regia followed by a mixed HF–HNO3–HClO4 digestion step. After digestion, samples were chemically separated using the techniques described in Aciego et al. (2009), which separated the Sr, Nd, Hf and U into elemental fractions. Analysis of the Sr, Nd, and Hf isotopic compositions are described in Aarons et al. (2013). The U fractions from the chemical separation were retained and the uranium isotopic compositions measured at the University of Wyoming. Uranium isotopic compositions were measured at the Wyoming High Precision Isotope Laboratory at the University of Wyoming on a Neptune PLUS ultra high-precision, high-sensitivity multi collector inductively coupled mass spectrometer using established techniques (Ball et al., 2008). Each sample was loaded into a sample solution of 0.01M HCl and diluted to approximately 10 ppb
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Table 1 Uranium isotopic composition of Saharan dust, sample names ending with ‘F’ are the fine fraction and sample names ending with ‘C’ are the coarse fraction. 2 S.D. errors in the activity are all 0.001 based on the long-term reproducibility of external standard NBS960 (Arendt et al., 2014). Sample name
Latitude
Longitude
3F 4C 7F 7C 9F 11F 11C 20F 24F 25F
11.0 N 10.6 N 10.0 N 10.0 N 10.0 N 10.0 N 10.0 N 8.4 N 10.9 N 11.0 N
58.7 53.7 44.8 44.8 39.0 33.3 33.3 24.6 17.1 19.0
a
W W W W W W W W W W
Dust size (lm)
Water soluble 234 U=238 U act
Residual solid 234 U=238 U act
Water soluble 87 Sr/86Sra
Residual solid 87 Sr/86Sra
1–2 2–30 1–2 2–30 1–2 1–2 2–30 1–2 1–2 1–2
1.123 1.150 1.133 1.149 1.152 1.152 1.152 1.158 1.151 1.170
1.134 1.089 1.060 1.066 1.092 1.093 1.144 1.179 1.125 1.115
0.710693 n.d. 0.710534 0.709842 0.710698 0.712097 0.710713 0.710975 n.d. 0.711483
0.716669 n.d. 0.717743 0.713584 n.d. 0.719167 0.714815 0.716028 0.718134 0.716606
Indicates previously published data from Aarons et al. (2013), where external 2 S.D. errors in 87Sr/86Sr are 0.000030 based on 10 mg splits of USGS reference material BCR-2.
Fig. 1. Sample collection sites: numbers shown are the sample names; latitude and longitude are compiled in Table 1. Map modified from Aarons et al. (2013).
uranium concentration. The sample intake system was rinsed with ultrapure 18 MX water and dilute HCl between analyses. Sample measurements were bracketed with measurements of New Brunswick Laboratory international reference uranium standard U010 to correct for mass bias and ion counting efficiency, using the U=234 U ratio of 18,354 (Richter and Goldberg, 2003). NIST international reference uranium standard NBS960 was run periodically to test the precision and accuracy of the Neptune PLUS over the entire sample run and the activity ratio compared to the accepted value of Cheng et al. (2000), 0.963. The long term average measured from August 2011 to March 2014 of standard NBS960 on the University of Wyoming Neptune PLUS is 0.963, with a 2 S.D. of 0.001 (Arendt et al., 2014).
238
3. Results and discussion The uranium isotopic compositions of the coarse and fine fractions of the M55 cruise dust samples are presented in Table 1. In seawater and crustal materials concentrations of Sr are higher than that of neodymium (Nd), hafnium (Hf) and U, which results in a higher measurement success rate of Sr isotopic compositions compared to the remaining elements given the limited sample size. Nevertheless, robust 234 U=238 U activ ity ratios were obtained on ten samples out of a total of sixteen, including two paired coarseand-fine fraction samples (samples 7 and 11). The 234 U=238 U activ ity compositions of the dust samples were in the range 1.06–1.18, narrower than that found by Rydell and
Prospero (1972) (1.00–1.22) but the analytical errors of this measurement are much smaller than that of previous works. This find 234 U=238 U activ ity variability is ing indicates that the compositionally driven and not an artifact of high analytical errors. The relationship between the particles sizes and 234 U=238 U activ ity does follow the hypothesized relationship outlined in the Introduction: fine grained materials are more depleted in 234 U than the corresponding coarse ones (Fig. 2). The difference in 234 U=238 U activ ity between the grain sizes is inconsistent; the fine-grained and coarse-grained composition of sample 7 are almost identical (1.060 and 1.066), while the fine-grained and coarse-grained composition of sample 11 are significantly different (1.093 and 1.144). Although these samples support the hypothesized relationship, the small number of sample pairs (two) is not enough for a definitive conclusion and additional studies will be required to verify a broad relationship. For the purposes of this study we use the distance measurements from Aarons et al. (2013) to assess the relationship between fractionation of isotopic composition and travel distance in dust (e.g. due to gravitational settling). The starting point chosen was the coast of west Africa (Dakar), as Laurent et al. (2008) and Tanaka and Chiba (2006) have shown, dust emissions are sourced from across the Sahara encompassing both interior sites (e.g. Mauritania) and coastal sites (e.g. Senegal), but the last possible Saharan source on a westward trajectory is the Senegal coast. It is possible and likely that some dust was sourced from the interior of west Africa, which would make the distances provided here
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uble 87Sr/86Sr and the seawater 87Sr/86Sr suggested that 70% of the Sr in the water soluble fraction was sourced from seasalt; the similar correlation found here in 234 U=238 U activ ity suggests seasalt is the dominant uranium contributor to the water soluble fraction. Fig. 4 illustrates the similar behavior of Sr and U in the water soluble fraction; previously measured 87Sr/86Sr (Aarons et al., 2013) and 234 U=238 U activ ity measured in this study show that the water soluble fraction of both Sr and U pools are clustered close to seawater. The actual source of the soluble fraction is likely a combination of seaspray and water-soluble minerals or mineral coatings on the insoluble material, which provides an explanation for excursions away from the seawater isotopic composition. Mineral coatings on African dust have been invoked to explain isotopic differences in hafnium space (Bayon et al., 2009), and their presence would provide an explanation for the 234 U=238 U activ ity behavior observed
238
Fig. 2. 234 U= U for the water soluble (blue circles) and solid residuals activ ity (brown squares for coarse fraction and brown diamonds for fine fraction) of Saharan dust as a function of distance from dust source. Also plotted is the isotopic composition of modern seawater (blue line). Note that samples 11F and 11C have identical sampling locations and water soluble 234 U=238 U activ ity compositions, so the data symbols plot in the same location.
minimum travel distances. With the estimated dust source as the coast of west Africa, there is a slight negative correlation between the 234 U=238 U activ ity of the insoluble dust and distance from the
dust source (Fig. 2), but regression analysis provides an r2 value of 0.16 and a p-value of 0.249 indicating that the correlation is insignificant. If the travel distance was longer due to a more interior source or tortuous trajectories, additional distance between points would decrease the negative trend and reinforce the current observation of little correlation between distance and composition. Based on the data presented, 234 U=238 U activ ity isotopic fractionation is not transport-distance dependent. With the exception of two samples, the water-soluble samples had higher 234 U=238 U activ ity than their corresponding insoluble counterparts, and are uniformly closer to the known modern 234 U=238 U activ ity seawater ratio of 1.149 ± 0.002 (Stirling et al.,
here. Mineral coatings or evaporitic minerals are precipitated out of fluids interacting with the surface environment and can be differently aged than insoluble mineral grains. The variability in 234 U=238 U activ ity ratios in earth surface fluids (see Introduction) means that soluble minerals and coatings can also have highly variable 234 U=238 U activ ity ratios, which when mixed with seaspray will lead to excursions away from 1.149. While this observation supports their existance, the Sr and U isotopic compositions are not positive proof and future work should address this additional ‘‘source’’ component. In contrast, there is no observable correlation between the 87 Sr/86Sr and 234 U=238 U activ ity ratios in the water-insoluble dust fraction (Fig. 4). This lack of correlation suggests that there are different processes controlling the isotopic compositions in the water-insoluble dust. The 87Sr/86Sr composition depends generally on the age of the continental crust from which the dust is derived and mineralogy of that dust (see Grousset and Biscaye, 2005 and references within). The long, 48.8 109 year, half-life of 87Rb, which is the parent nuclide of 87Sr, means that the 87Sr/86Sr in mineral dust will not depend on it’s age assuming that the age of the mineral dust is less than several million years old. In contrast, the 234 U=238 U activ ity composition does not depend on the composi-
1998; Robinson et al., 2004) as shown in Fig. 3. In the previous work of Aarons et al. (2013), the correlation between the water sol-
Fig. 3. Comparison of the 234 U=238 U of the water soluble fraction to the activ ity residual fraction; symbology is the same as in Fig. 2. Note that the compositions of samples 9F and 11F are almost identical so the data symbols plot in the same location. In all cases, the error in the measurement is smaller than the size of the symbol.
87 86 Fig. 4. Plot of radiogenic strontium isotopic composition ( Sr/ Sr) versus 234 U=238 U for the water soluble (blue circles) and solid residuals (brown activ ity squares for coarse fraction and brown diamonds for fine fraction) of Saharan dust. Also plotted is the isotopic composition of modern seawater (blue circle). Strontium compositions were measured on the same sample digestions as the uranium compositions and taken directly from Aarons et al. (2013); values of isotopic compositions are provided in Table 1. In all cases, the error in the measurement is smaller than the size of the symbol. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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tion or age of the continental crust, but instead on the geometry of the dust grains, the interaction with waters with high 234 U=238 U activ ity and the age of the dust. The relatively short half-life of 234 U and fine particles containing uranium means that the dust composition can change on timescales between 10 ka and 1 Ma (DePaolo et al., 2006; Aciego et al., 2011), leading to high ly variable 234 U=238 U activ ity ratios in dynamic dust source environments such as the Sahara, a conclusion that is supported by our results. The 234 U=238 U activ ity ratios measured in this study have immediate implications for studies utilizing U-series isotopes where dust deposition is a significant process in the environment. Models that incorporate a soluble dust fraction (e.g. Oster et al. (2012), Andersen et al. (2013)) into their soil development can use an end-member 234 U=238 U activ ity ratio centered around the seawater value with a limited variability. If the soluble portion was the only component supplying uranium to the system, a simple linear mixing model – that depends on limited variability in end-member compositions (Parnell et al., 2010) – would be appropriate for deconvolving the source contributions of uranium present in a soil or water system. However, because silicate dust weathers and releases material into the environment after deposition, simple mixing models are inappropriate. The water-insoluble 234 U=238 U activ ity ratios, whose variability is more than 10%, are
not correlated with the corresponding 87Sr/86Sr ratios, a factor not accounted for in linear mixing models (Parnell et al., 2010). In order to incorporate source isotopic composition variability into mixing models, more complex approaches will be required such as Bayesian techniques employed in stable isotope mixing models (Parnell et al., 2010; Cable et al., 2011; Arendt et al., submitted for publication). 4. Conclusions Our study confirms the wider variability of the
234
U=238 U
activ ity
ratios in the Saharan airborne dust observed by Rydell and Prospero (1972) and indicates that the range is based on source composition variability rather than analytical errors. The 234 U=238 U activ ity ratios reveal that uranium in the water soluble fraction of the dust is dominantly derived from sea salt, which is in agreement with the previous work on the 87Sr/86Sr composition of the water soluble and residual solids (Aarons et al., 2013). How ever, the water soluble 234 U=238 U activ ity variations away from the seasalt value are not well correlated with the solid composition, indicating that there may be a mineral coating derived from the dust source region with a different isotopic composition. Analysis of different size fractions of the same sample indicates a that the fine fractions are less enriched in 234 U than their coarse fraction counterparts, but more work will need to be done to fully understand particle-size dependency. The lack of correlation between the 234 U=238 U activ ity and 87Sr/86Sr compositions of the residual solids indicates that there is large source composition variability with respect to 234 U=238 U activ ity , which renders simple linear mixing models inappropriate for dust deposition modeling of 234 U=238 U activ ity in soil systems. Acknowledgements The samples for this research were kindly provided by A.R. Baker. Funding for this project was provided by grants from the Rackham Graduate School and the Turner Award from the Department of Earth and Environmental Sciences at the University of Michigan
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