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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, G03007, doi:10.1029/2009JG001077, 2010

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Contemporary geochemical composition and flux of aeolian dust to the San Juan Mountains, Colorado, United States Corey R. Lawrence,1,2 T. H. Painter,3,4 C. C. Landry,5 and J. C. Neff1,6 Received 12 June 2009; revised 1 February 2010; accepted 2 March 2010; published 16 July 2010.

[1] Dust deposition in the Rocky Mountains may be an important biogeochemical flux from upwind ecosystems. Seasonal (winter/spring) dust mass fluxes to the San Juan Mountains during the period from 2004 to 2008 ranged from 5 to 10 g m−2, with individual deposition events reaching as high as 2 g m−2. Dust deposited in the San Juan Mountains was primarily composed of silt‐ and clay‐sized particles, indicating a regional source area. The concentrations of most major and minor elements in this dust were similar to or less than average upper continental crustal concentrations, whereas trace element concentrations were often enriched. In particular, dust collected from the San Juan Mountain snowpack was characterized by enrichments of heavy metals including As, Cu, Cd, Mo, Pb, and Zn. The mineral composition of dust partially explained dust geochemistry; however, based on results of a sequential leaching procedure it appeared that trace element enrichments were associated with the organic‐, and not the mineral‐, fraction of dust. Our observations show that the dust‐derived fluxes of several nutrients and trace metals are substantial and, because many elements are deposited in a mobile form, could be important controls of vegetation, soil, or surface water chemistry. The flux measurements reported here are useful benchmarks for the characterization of ecosystem biogeochemical cycling in the Rocky Mountains. Citation: Lawrence, C. R., T. H. Painter, C. C. Landry, and J. C. Neff (2010), Contemporary geochemical composition and flux of aeolian dust to the San Juan Mountains, Colorado, United States, J. Geophys. Res., 115, G03007, doi:10.1029/2009JG001077.

1. Introduction [2] The deposition of aeolian or wind‐blown dust is an important and widespread flux to a variety of ecosystems [Derry and Chadwick, 2007; Lawrence and Neff, 2009]. Dust can provide a source of biologically essential nutrients required for productivity in terrestrial [Chadwick et al., 1999; Neff et al., 2006; Soderberg and Compton, 2007; Swap et al., 1992] and aquatic ecosystems [Jickells, 1995; Psenner, 1999]. In addition, dust deposition may influence ecosystem biogeochemistry through several other processes including buffering soil and surface water against acidification [Hedin and Likens, 1996; Loye‐Pilot and Morelli, 1988; Psenner, 1999; Schwikowski et al., 1995], altering the physical and chemical nature of soils and sediments [Birkeland et al., 2003; Litaor, 1987; Simonson, 1995],

1 Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado, USA. 2 Now at U.S. Geological Survey, Menlo Park, California, USA. 3 National Snow and Ice Data Center, University of Colorado at Boulder, Boulder, Colorado, USA. 4 Now at Jet Propulsion Laboratory, Pasadena, California, USA. 5 Center for Snow and Avalanche Studies, Silverton, Colorado, USA. 6 Environmental Studies Program, University of Colorado at Boulder, Boulder, Colorado, USA.

Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2009JG001077

influencing the timing and rate of snowmelt [Conway et al., 1996; Painter et al., 2007], and providing a carbon substrate for microbial productivity [Ley et al., 2004]. [3] There is suggestion that dust fluxes have increased during the 20th century and growing evidence that land use and climate change are altering regional and global scale dust processes. Past dust deposition rates are recorded in Antarctic ice, and this record suggests that dust inputs have doubled during the 20th relative to the 19th century. These increases in Antarctic dust have been attributed to land use and climate changes in South America [McConnell et al., 2007]. Similarly, a study of alpine lakes in the southern Rocky Mountains found that rates of lake sedimentation have increased by as much as fivefold during the past two centuries, likely as a result of human activity [Neff et al., 2008]. These and other records imply that contemporary climate variability and land use change in dust source regions may influence the future flux of aeolian material to downwind ecosystems; however, model estimates vary with regard to the magnitude and direction of changes [Mahowald and Luo, 2003; Tegen et al., 2002b]. Despite these uncertainties, it is clear that contemporary dust deposition is a widespread and important ecological process [Lawrence and Neff, 2009]. To understand the local and regional importance of dynamic aeolian processes, it is vital to establish a benchmark of modern dust fluxes. [4] The goal of this research is to quantify the contemporary flux and geochemical composition of aeolian dust to

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LAWRENCE ET AL.: DUST FLUXES TO THE SAN JUAN MOUNTAINS

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Figure 1. The San Juan Mountains are located downwind from the arid and semi‐arid dust producing regions of the Colorado Plateua and Mojave Desert. The expanded region shows the location and topographic detail of the dust collection site at the Senator Beck Basin Study Area (SBBSA) and the Interagency Monitoring Program for Protected Visual Environments (IMPROVE) suspended particulate sampling site (WEMI1). The topographic hillshade was generated from U.S. Geological Survey 7.5 min digital elevation models. ecosystems to the southern Rocky Mountains, United States. The widespread and long‐term accumulation of dust in high‐elevation environments of the Rocky Mountains is suggested by soils, where several researchers have noted enrichments of silt‐sized particles [Bockheim and Koerner, 1997; Dahms, 1992; Lawrence and Neff, 2009; Litaor, 1987; Muhs and Benedict, 2006]. Although there is some debate as to the source of this enrichment, aeolian inputs are often considered the most likely. For instance, Muhs and Benedict [2006] compared trace element ratios of soil particles in the Indian Peaks Wilderness of the central Rocky Mountains with potential local and regional parent material sources and found that soil silt‐sized particles were derived from aeolian, and not local sources. If dust inputs are responsible for the observed enrichments of soil silt‐sized particles, it is plausible that dust inputs also play an important role in the biogeochemical cycling of elements in the terrestrial ecosystems of the Rocky Mountains. [5] The biogeochemical relevance of aeolian dust deposition depends on several factors including the timing of deposition, the chemical availability of elements contained within dust minerals and organic matter, and the fate of aeolian material once it has been deposited. We hypothesize that through time, inputs of dust to high‐elevation ecosystems in the Rocky Mountains have shaped the geochemical landscape of these environments and represent an important flux of elements to both terrestrial and aquatic ecosystems. This work represents the first step toward testing this hypothesis. Specifically, we address three relevant questions. (1) What is the average mass of dust deposited in the San Juan Mountains each winter/spring? (2) What is the geochemical composition of this dust and how does it compare with average crustal material? (3) What is the form and availability of biologically sensitive elements associated

with dust inputs? Answering these questions is critical to furthering our understanding of the geochemical linkages between upwind dust source regions and the biogeochemical cycling of elements in high‐elevation terrestrial and aquatic ecosystems of the Rocky Mountains. Furthermore, this work provides a baseline for evaluating future changes of dust processes in the southwestern United States.

2. Site Description [6] The San Juan Mountains (SJM) are located in the southwestern corner of Colorado and are a southern subrange of the Rocky Mountains Belt. The SJM provide an excellent location for the study of aeolian dust deposition because they form a substantial geographic barrier to storms blowing across the arid and semi‐arid regions of the Mojave Desert and Colorado Plateau (Figure 1). Spring storms in these desert regions can mobilize large quantities of soil and transport particle‐rich air masses toward the SJM. As these dust‐laden air masses are pushed up and over the SJM, gravitational settling and orographic precipitation deposit particulates on terrestrial and aquatic surfaces. Dust deposited during these spring weather events is often visible in the high‐elevation snowpack of the SJM and other areas of Rocky Mountains, where it gives snow a distinctly reddish tint. It is estimated that the dry‐lands of the Mojave Desert and Colorado Plateau are the source of approximately 0.77 Tg of dust annually [Simonson, 1995] and much of that material may end up in downwind mountain ecosystems. Once deposited, the fate and function of this dust in high‐ elevation terrestrial ecosystems remains unclear. [7] The dust samples described in this study were collected from the Senator Beck Basin Study Area (SBBSA) on Red Mountain Pass (elevation ∼3500 m) north of Silverton,

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Colorado. The SBBSA sampling plot is situated in a large clearing below tree line. The average temperature at Red Mountain Pass is −9°C during the winter months and 11°C during the summer months [Natural Resources Conservation Service, 2009]. Annual precipitation inputs for this region range from 40 to 90 cm and the 19‐year average is 77 cm [National Atmospheric Deposition Program, 2009]. Approximately half of the annual precipitation is deposited as snow. Monsoon precipitation patterns dominate from July to October while Pacific cyclones bring winter snows from November to June. High‐elevation regions of the SJM are typically snow covered from November to late June or early July.

3. Methods 3.1. Collection and Characterization of Dust Deposition Events [8] Occurrences of dust‐on‐snow deposition events at the SBBSA were recorded by the Center for Snow and Avalanche Studies (CSAS) from 2004 to the present. The Interagency Monitoring of Protected Visual Environments (IMPROVE) maintains a regular record of suspended atmospheric particulates at a nearby site. The IMPROVE Weminuche Wilderness site (WEMI1) is situated < 30 km from the SBBSA at an elevation of 2750 m. Situated at a lower elevation, the total deposition flux to the WEMI1 site may differ from the fluxes measured at SBBSA; however, because deposition events are a regional process, we expect the timing of dust‐on‐snow deposition events at the SBBSA should be reflected in suspended particulate record at the WEMI1 site. If true, the IMPROVE record may also provide an indication of particulate flux to SJM ecosystems during the summer/fall seasons when dust‐on‐snow collections are not possible. We compared the concentration of atmospherically suspended particles < 10 mm in diameter (PM10) from the WEMI1 site with the timing of discrete observation of dust‐on‐snow deposition events over the period from 2004 to 2007 (data from IMPROVE [2009] ). Atmospheric PM10 concentrations from the WEMI1 site are sampled bi‐weekly by IMPROVE, as a result, the temporal comparisons of bi‐weekly IMPROVE measurements are qualitative at best. [9] From 2004 to 2008, we collected samples of dust‐ events from the snowpack of the SBBSA. Following each dust deposition event, we collected a layer of fresh snow containing the most recent dust inputs from a 0.5 m2 sampling area. We typically collected the samples within 24 to 48 h after the deposition event and transported them in clean plastic bags to the CSAS facilities in Silverton, CO where they were thawed at room temperature. We then transferred the thawed dust‐containing snow into large acid‐washed carboys. Within two days of the field collection, carboys were shipped to the Environmental Biogeochemistry Laboratory (EBL) of the University of Colorado, Boulder. Upon arrival at the EBL, samples were evaporated using a multistep process. First, each melted snow sample containing dust was transferred from the carboy to a large acid‐washed basin and warmed to ∼60°C. We kept these evaporation basins in a clean fume hood to facilitate evaporation and to prevent contamination from dust fall within the laboratory. When the volume of liquid in the samples reached ∼500 ml,

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we transferred the sample solution to a clean acid‐washed 1000 ml polyethylene bottle. We then stored the condensed samples frozen (up to 4 months) until drying could be completed using an industrial freeze‐dryer (Virtis K‐series Benchtop, SP Industries, Warminster, PA). After freeze‐ drying, we weighed the dry‐mass collected to determine the dust deposition flux (g m−2). During sample collection, transport, and evaporation we made a concerted effort to avoid sources of contamination by using only clean and acid‐washed sampling equipment and containers. Although we did not collect field blanks, we are confident that background contamination was minimal. 3.2. Texture and Mineralogy [10] After freeze‐drying, dust samples were weighed and sieved into three size‐fractions including < 250 mm, 250 to 850 mm, and > 850 mm. We then split the size‐separates for physical and geochemical analyses; however, because the masses of the > 850 mm size‐class samples were very small and usually dominated by large organic material (discussed below), this fraction was not analyzed. For some deposition events, the sampling of dust from the snowpack did not yield enough sample mass for all analyses to be completed. In such cases, analysis of the bulk elemental chemistry was assigned highest priority. When a sufficient mass of dust was collected, we also analyzed dust particle size distribution (PSD) and mineralogy. [11] The PSDs of six dust events were measured by laser light‐scattering particle‐size analysis (Matsizer 2000, Malvern Instruments, Worcestershire, England) at the Earth Surface Processes Laboratory of the United States Geological Survey, Lakewood, CO. We made these particle‐size analyses after removing organics in 30% peroxide and carbonates in 15% hydrochloric acid. The mineral compositions of 4 dust events were measured by X‐ray diffraction (XRD, Siemens D5000 X‐Ray Diffractometer, New York, NY) at the Institute of Arctic and Alpine Research, Soil Mineralogy Laboratory, Boulder, CO. Mineral abundance was quantified from the XRD data using the RockJock software package following the methods of Ebrel [2003]. 3.3. Bulk‐Dust Geochemical Analyses [12] We measured the total C and N concentrations of dust at the EBL by combustion analysis (Costech Elemental Combustion System, Valencia, CA). Inorganic carbon content was calculated by difference between total carbon measured in a bulk sample and organic carbon measured in a sub‐sample treated with 10% HCL. After the complete digestion of bulk samples in a mixture of concentrated HCL, HNO3, and HF, we measured the major, minor, and trace element composition of bulk aeolian dust samples by inductively coupled ‐atomic emission spectrometry and ‐ mass spectrometry (ICP‐AES, ICP‐MS) at the United States Geological Survey, Lakewood, CO. 3.4. Dust Sequential Leaching [13] To assess the distribution of elements among unique chemical fractions, we subjected three dust samples to a multistep leaching process prior to geochemical analyses. The goal of this leaching process was to isolate exchangeable, carbonate bound, organic, and residual element fractions; however, because of difficulty leaching exchangeable

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ions from dust samples without simultaneously dissolving soluble salts, we combined the measurements of exchangeable and carbonate fractions into a single pool, which we refer to as the extractable fraction. [14] For each sample that was sequentially leached, approximately 2 g of dust was weighed into a clean 30 ml Teflon beaker. To this we added 20 ml of 1M NH4Ac (adjusted to a pH of 7) and stirred the sample slurry vigorously for approximately 1 min. Following stirring we allowed samples to equilibrate for at least 24 h. After the equilibration period, we again stirred the samples for 1 min and then centrifuged the slurry for 7 min at 5000 rpm in an acid‐cleaned glass centrifuge tube. Then, we transferred 8 ml of the supernatant to a sterile 15 ml polypropylene sample tube for element analysis. Finally, we re‐suspended the soil or dust sample residue in ultra‐pure deionized H2O, transferred it back to the original Teflon beaker and allowed it to dry overnight at 60°C. [15] After drying overnight, we removed the sample beaker from the drying oven and carefully broke apart any physical crust. Next, 20 ml of distilled 1M acetic acid was added to the beaker and stirred vigorously for 1 min. The beaker was then covered and allowed to equilibrate for 24 h. After the equilibration period, the sample slurries were centrifuged, and the supernatant was collected for element analysis. Again, we re‐suspended the solid residue in ultra‐ pure deionized H2O, transferred it back to the original Teflon beaker, and dried it overnight at 60°C. [16] The following day, using a procedure identical to that described above, we leached the dry samples with 1M HNO3 for 24 h, centrifuged the slurry, and collected the supernatant for element analysis. The remaining solid was dried overnight at 60°C. After drying, we transferred the samples to aluminum weigh boats and ashed them in a muffle furnace at 500°C for 8 h. We then leached the ashed samples with ultra‐pure deionized H2O for 24 h. The samples were then centrifuged, the supernatant was collected for element chemistry, and the remaining solids were again dried overnight at 60°C. [17] After the final oven‐drying step, we ground the remaining sample mass with an agate mortar and pestle and transferred the ground samples to clean Teflon beakers. We then digested the samples using a combination of concentrated HCL, HNO3, HF, and perchloric acid. After the digestion, we evaporated the samples to dryness and re‐dissolved them in 20 ml of 1M HNO3. Finally, we measured the element concentrations of each sample generated from the sequential leaching protocol by ICP‐AES and ‐MS at the Laboratory for Environmental Geochemistry (LEG) of the University of Colorado, Boulder, CO. In addition, we also measured three reagent blanks for each of the reagents used in the sequential leaching procedure described above. In the event that the concentration of any element was detectable in the blanks, the average value of the three replicates was subtracted from the sample value. For all leaching steps, we calculated concentrations as the mass of each element measured relative to the original starting mass of dust (mg g−1). As mentioned above, the measurements of the first two leaching steps were combined. Similarly, the 1M HNO3 and the deionized H2O leaching steps were combined and are hereafter referred to as the organic fraction.

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3.5. Snowpack Sampling and Analysis [18] On several occasions during the study period we collected depth‐profile samples from snow pits in the SBBSA in order to assess the influence of dust deposition on dissolved ion concentrations in the snowpack and to begin to evaluate the mobility of dust derived elements in a field setting. We collected these samples after the period of maximum snow accumulation and thus samples were likely influenced to some degree by horizontal and vertical melt flow through the snowpack. The depth increments we sampled varied corresponding to snowpack stratigraphy, which was identified based on crystal size and structure, temperature, and density. In total, we sampled 135 different stratigraphic layers from 17 snow pits. We collected and weighed each sample using a clean plastic coring‐tube then transferred them to a clean plastic bag. For each sample, we made note of the presence or absence of visible dust layers. After collection, samples were transported frozen to the EBL in Boulder, CO where they were thawed at room temperature and filtered through 0.7 mm pore size glass‐ fiber filters (Whatman GF/F 25mm filters, Maidstone, England). The filtrate solution was then stored frozen until analysis for major cation and anion concentrations by ion chromatography (DX‐500, Dionex Corporation, Sunnyvale, CA). In addition, several analytical control samples were filtered and analyzed by the same methods.

4. Results 4.1. Mass Flux and Texture [19] Over five seasons of study, a total of 30 dust deposition events were observed on the snowpack of the SBBSA. Deposition events occurred as early as December and often as late as May, but overall, April tended to be the dustiest month (Table S1, available as auxiliary material).1 Most deposition events were associated with strong southwesterly winds and often coincided with new snowfall; however, from 2004 to 2008, approximately 30% of the deposition events were thought to be dry‐events. From our data set, it is difficult to determine if dry‐events characteristically deposit more or less material than wet‐events. Many of the dry deposition events were small enough that we were not able to collect enough sample‐mass to measure, whereas one of the largest single deposition events recorded during our study was a dry‐event. The total winter/spring seasonal deposition flux of dust ranged from 5 to 10 g m−2 (Table 1). [20] Only 20% of dust deposition events observed in the SBBSA corresponded to periods of elevated (≥ mean + 1 standard deviation) atmospheric PM10 concentrations at the WEM1 IMPROVE site. Additionally, almost 40% of dust deposition observations occurred during periods of PM10 concentrations that were below the long‐term mean (Figure S1). Further analysis of these data shows that there are several winter/spring increases in atmospheric PM10 at WEMI1 each year, which are not associated with observed dust deposition events. Similarly, the WEMI1 record indicates periods of elevated atmospheric particulate concentrations during the summer and fall seasons, when we have no adequate record of dust deposition inputs to SBBSA. Overall, 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2009JG001077.

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Table 1. A Summary of Seasonal Deposition at the Senator Beck Study Area, Including the Number of Dust‐on‐Snow Deposition Events, the Seasonal Average Event Deposition Flux, and the Total Seasonal Flux

Season

Deposition Events (n)

Average Event Flux (g m−2)

Total Seasonal Flux (g m−2 yr−1)

2003/2004 2004/2005 2005/2006 2006/2007 2007/2008 Average

3 4 8 8 7 6

NA 2.41 0.82 0.72 NA 1.31

NA 9.62 4.90 5.07 NA 6.53

this comparison indicates that IMPROVE PM10 concentrations measured at the WEM1 site are not a good predictor of major dust deposition events at the SBBSA site, which is located almost 30 km away. [21] Dust isolated from the SJM snowpack consisted mainly of what appeared to be soil‐derived mineral and organic matter but several samples also contained small but visible quantities of non‐soil organic matter. For example, the > 850 mm fraction accounted for less than 2% of the total event based deposition mass and was typically dominated by spruce‐fir needles, bark, and fur or hair. We suspect that the material making up the > 850 mm fraction is locally derived. Correspondingly, we have excluded these non soil‐ derived materials (>850 mm) from our estimates of total dust mass flux. The remaining mass flux of individual deposition

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events (250 to 850 mm) ranged from 0.12 to 4.55 g m−2 and averaged 1.31 g m−2 (n = 17). The total seasonal flux from dust‐on‐snow deposition events ranged from 5 to 10 g m−2 yr−1 and averaged 6.53 g m−2 yr−1 (Table 1). [22] On average, the < 250 mm size‐class accounted for 78% (n = 20) of the total dust mass deposited during each event. The remaining portion of the sample mass (22%) appeared to be comprised of particles sized from 250 to 850 mm; however, visual inspection of this portion of the samples revealed it was actually comprised of clay and silt aggregates and small particles trapped in fibrous organic material. The physical structure of this portion of the dust samples prevented separation of the smaller particles during the sieving process. As a result, we have likely underestimated the mass fraction of < 250 mm size particles with this approach. This is corroborated by the lack of significant difference in the concentration of most mineral derived element concentrations of these two size‐fractions (data not shown) and the greater abundance of organic C in the larger size‐fraction (Table S2). If the 250 to 850 mm size‐fraction were dominated by larger sand‐sized particles, the concentration of many trace elements would likely be diluted by the larger abundance of quartz and feldspars. Based on these observations, we conclude dust deposited in the SJM is dominated by particles ranging in size from small sands to clays and is classified as silt to clayey‐silt (Figure 2). From high resolution particle size analysis of the < 250 mm fraction, the mean particle size in phi units is 6.08 ± 0.09 (n = 5) and the mean degree of sorting is 2.27 ± 0.08 (n = 5). On average, 70% of the < 250 mm particles collected from the

Figure 2. The average particle‐size distribution is plotted for the < 250 mm size‐split of 4 dust‐events. Data are shown as the mean ± standard error of the percent of total volume contributed by dust particles divided into descrite particle‐size bins. The inset shows the textural classification of the same four dust‐ evetns (solid triangles) as well as the mean textural classification of those samples (open triangles). Particle‐size data were not meaured for the > 250 mm dust‐size split. Bulk measurements of sample mass after sieving suggest that particles < 250 mm make up the dominant particle size‐mode (see discussion in text). 5 of 15

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Table 2. The Abundance of Dust Minerals as a Percent of the Total Mineral Massa Minerals

2005 E3 (wt %)

2005 E5 (wt %)

2007 E2 (wt %)

2007 E3 (wt %)

Mean (wt %)

SE (wt %)

Quartz K‐feldspars Plagioclase feldspars Calcite Dolomite Amphibole Goethite Maghemite Volcanic glass Kaolinite Illites + smectites Muscovite Biotite Chlorites

35.4 12.7 10.5 3.9 1.2 1.1 0.8 1.4 2.6 3.9 13.9 4.7 4.7 2.8

36.5 9.2 11.1 4.5 1.1 0.4 0.3 3.8 6.9 0.9 17.2 3.4 1.8 2.5

29.8 9.5 6.3 9.1 1.2 0.4 0.1 3.2 6.4 0.2 25.9 1.3 1.4 4.1

24.0 12.2 12.1 0.7 0.1 0.8 0.6 3.8 8.7 1.7 24.3 4.5 1.6 4.0

31.4 10.9 10.0 4.6 0.9 0.7 0.4 3.0 6.1 1.7 20.3 3.4 2.4 3.3

3.3 1.0 1.5 2.0 0.3 0.2 0.2 0.7 1.5 0.9 3.3 0.9 0.9 0.5

Measurements were made on the < 250 mm size‐fraction only.

a

snowpack have diameters between 105 and 9.3 mm, while 27.6% of particles have diameters less than 9.3 mm and 14.1% have diameters less than 2.8 mm. The PSD exhibits a steep decline in particles from 53 to 250 mm (Figure 2). 4.2. Mineralogy [23] The mineral composition of four aeolian dust samples (20%) in the organic fraction including Cd, Ce, Cu, La, Pb, Y, and Zn. Other elements including Al, Fe, K, Na, Ti, As, Cs, Li, Nb, Rb, V, and Zr were found predominantly in the residual fraction (Figure 3 and Table 3). 4.5. Element Enrichments [27] We compared the element concentrations measured in dust with concentrations of the average upper continental crust (UCC) [Wedepohl, 1995]. The enrichment or depletion of elements in dust relative to the UCC provides some evidence for the origin of dust particles [Schutz and Rahn, 1982; Schutz and Sebert, 1987]. Schutz and Rahn [1982] showed that because the geochemical composition of aerosol sized particles from desert soils are, in general, quite similar in element concentration to the UCC, the enrichment or depletion of dust relative to the UCC is a good point for comparison when the specific source soil is unknown. Figure 4 shows the enrichment or depletion of major and minor element concentrations of bulk and residual dust relative to the average. In general, most major and minor elements (Si, Al, Fe, K, Na, Mg and Ti) are depleted relative to average UCC concentrations but a few elements are enriched including Ca, P, and to a lesser extent Mn. After the sequential leaching, residual concentrations of all major

and minor elements of dust are depleted including those elements that were enriched in bulk dust. In comparison, most trace element concentrations were similar to (Ba, La and Y) or enriched (As, Cr, Cu, Li, Mo, Ni, Pb, V and Zn) relative to the average UCC values. Most notably, several heavy metals including As, Cd, Cu, Mo, Pb and Zn were enriched 1‐ to 10‐fold, compared with average UCC concentrations (Figure 5). When considering only the residual chemical fraction, all major, minor, and trace elements (with the exception of As) are depleted relative to UCC concentrations (Figures 4 and 5). Comparisons of UCC normalized element concentrations between bulk‐ and residual‐dust show that the leaching process removed a substantial proportion of several elements including Ca, P, As, Cd, Cu, Mo, Pb, and Zn. 4.6. Snowpack Dissolved Ions [28] The concentration of dissolved ions in snowpack of the SBBSA was greater in samples containing dust compared with those that did not (Figures S2 and S3). Specifically, the mean (n = 23) concentrations of cations in snow layers containing dust were 27.41 ± 5.54, 7.32 ± 1.17, 5.54 ± 1.18, 63.27 ± 15.82, and 9.4 ± 1.03 meq L−1 for Na+, Mg2+, K+, Ca2+, and NH+4 , respectively. In comparison, the mean (n = 113) concentrations of these cations in snow samples devoid of visible amounts of dust were 10.50 ± 1.15, 3.01 ± 0.49, 5.05 ± 1.33, 18.08 ± 2.17, and 8.39 ± 0.51 meq L−1. The mean (n = 23) anion concentrations of snowpack samples

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Figure 4. To examine the enrichment/depletions of elements in dust, we normalized the major element concentrations of dust to the corresponding average concentration of upper continental crustal material [from Wedepohl, 1995]. Mean values are shown for the bulk composition of thirteen dust deposition events (black bars) and for the residual chemical‐fraction (gray bars) of 3 dust events that were subjected to a squential chemical leaching procedure. containing dust were 7.29 ± 1.47, 10.72 ± 2.29, and 11.57 ± 2.91 meq L−1 for Cl−, NO−3 , and SO2− 4 , respectively. Comparatively, in snowpack layers without the visible presence of dust, the mean concentrations of these same anions were 4.35 ± 0.61, 7.47 ± 0.50, and 5.49 ± 0.45 meq L−1. Differences between snowpack layers with and without dust were statistically significant for Na+, Ca2+, Mg+, and SO2− 4 (p ≤ 0.05, Student’s t‐test, two‐tailed). For many of the dissolved ions measured, there were several snowpack layers without dust, which were outliers exhibiting higher concentrations of dissolved ions (Figures S2 and S3). These data points tended to minimize the significance of difference between the observed mean concentrations and likely were likely the result of downward translocation of dissolved ions from dust layers during snowpack melting.

5. Discussion 5.1. Contemporary San Juan Dust Deposition [29] Our estimates of dust deposition rates in the SJM are similar to, or higher than, estimates from other parts of the Rocky Mountains. For example, we estimate the total winter/spring deposition flux to the SJM snowpack to range from 5 to 10 g m−2. In comparison, dust deposition rates of ∼5 g m−2 yr−1 [Ley et al., 2004] and ∼4 g m−2 yr−1 [Dahms and Rawlins, 1996] have been measured in the Front Range of Colorado and the Wind River Range of central Wyoming, respectively. Although similar in magnitude, it must be

noted that our measurements represent conservative estimates of annual dust flux to the SJM since they do not include exogenous dust inputs during snow‐free periods. [30] There is some indirect evidence that exogenous dust deposition occurs all yearlong in the SJM. For instance, bi‐weekly IMPROVE network sampling at a nearby site, approximately 30 km from our study area, suggests seasonal mean PM10 concentrations during summer/fall that are similar to or higher than winter/spring concentrations (Figure S1). This is consistent with trends observed at other sites in the western United States, where the highest concentrations of dust are observed during the summer but several large dust events also occur during the spring [Wells et al., 2007]. While seasonally averaged PM10 concentrations are suggestive of dust inputs during the summer and fall, these data do not provide a proxy for dust deposition. Specifically, the lack of correspondence between the IMPROVE PM10 record and the dust‐on‐snow events identified at SBBSA reveal limitations of using the atmospheric PM10 concentration as a proxy for dust deposition. Furthermore, it is difficult to determine whether above average PM10 concentrations observed in the IMPROVE record during summer and fall periods are related to exogenous dust or to elevated concentrations of local dust, pollen, or forest fire aerosols. Although local inputs complicate measurements of exogenous dust fluxes during the summer/fall season, it is unlikely that these or other sources of aerosols are prevalent during the winter/spring deposition

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Figure 5. To examine the enrichment/depletions of elements in dust, we normalized the trace element concentrations of dust to the corresponding average concentration of upper continental crustal material [from Wedepohl, 1995]. Mean values are shown for the bulk composition of thirteen dust deposition events (black bars) and for the residual chemical‐fraction (gray bars) of 3 dust events that were subjected to a squential chemical leaching procedure.

events measured on the snowpack. During this time, local sources of dust are snow covered and the forest fire and pollen aerosols are at seasonal minimums. More detailed year‐round sampling of particulate deposition inputs are required to better constrain the total yearly flux of exogenous dust as well as local particulate fluxes; however, our estimates of winter/spring dust fluxes provide a lower bound estimate of dust fluxes to the SJM. [31] The contemporary flux of dust to the SJM reported here must be considered in the context of changing regional climate and land use. During the period of our observations, the western United States was in a severe drought [Cook et al., 2007]. The western United States has experienced droughts of a similar or greater intensity at least twice in the past century [Cook et al., 2007] and several times during the past 2000 years [Hughes and Diaz, 2008]. Although the most‐recent drought is not unprecedented, its influence on contemporary dust processes may be substantial. We cannot say how drought conditions influenced the dust deposition rates we measured in the SJM, but there is evidence that, in general, drought conditions do enhance dust emissions [Okin and Reheis, 2002; Prospero and Lamb, 2003]. In addition, changes in human land use throughout the western United States also affects dust transport and deposition [Belnap and Gillette, 1998; Neff et al., 2008]. Predictions are varied as to how interactions between climate and land use change will alter the emission, transport, and deposition of dust on the global scale [Mahowald and Luo, 2003; Tegen et al., 2002b]. In general, climate models predict

more intensive and extensive drought conditions during the next 50 years [Hughes and Diaz, 2008], which is conducive to increases in dust. In contrast, fertilization of plant growth due to increasing atmospheric CO2 and expanded pavement of dust producing surfaces in the southwestern United States could lead to decreased future dust fluxes. Continued monitoring of dust transport throughout the western United States is required to better understand the relationship between climate, land use, and dust. 5.2. Confidence in Dust Flux Estimates and Scaling Up [32] Our estimates of dust fluxes to the SJM were based on observations from a single location with a small spatial sampling area (0.5 m2), which may complicates efforts to scale up deposition fluxes to regional estimates. There is clear evidence that landscape position can influences deposition rates [Goossens, 2000]. Vegetation can also influence dust deposition [Hoffmann et al., 2008; Tegen et al., 2002a] and interception of dust by sub‐alpine forests may increase dust accumulation rates below tree line. Despite these factors, which are known to contribute to the heterogeneity of deposition fluxes, remote sensing observations suggest dust is deposited homogeneously on the SJM alpine snowpack (T. H. Painter, unpublished data, 2010). This provides some confidence for extending our measurements to larger spatial scales. [33] Distinct from the spatial distribution of the dust deposition process, the redistribution and accumulation of dust on the landscape is likely to be influenced by wind and

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avalanching. As such it is possible that areas of heavy snow accumulation could also be characterized by the greatest accumulation of dust. While we have not explored how these mechanisms influence the spatial distribution of dust deposition rates in the SJM, our event based sampling protocol minimizes the effects of local dust redistribution on our flux estimates. Specifically, samples were collected from the snowpack immediately following the deposition events so the redistribution of dust and snow is limited only to what occurs during the event. [34] An added benefit of sampling dust deposition from the snowpack is that such measurements are not subject to the same sampling biases prone to dust collecting apparatuses. For example, dust deposition rates measured from passive collectors may not represent true rates because of sampler biases, which vary depending on the type of sampler used [Goossens and Offer, 1994; Goossens and Rajot, 2008]. Furthermore, calculating deposition rates from active dust samplers requires detailed characterization of surface properties, wind speed, as well as particulate concentrations [Wesely and Hicks, 2000]. Active collection also often excludes particles greater than 10 mm. Snowpack dust collection captures natural deposition rates and the full range of particle sizes that have been deposited. 5.3. Dust Provenance [35] Two studies of dust deposition in the SJM have suggested the provenance of winter/spring dust is non‐local and likely centered in the arid and semi‐arid regions of the southwestern United States [Neff et al., 2008; Painter et al., 2007]. Specifically, the Sr and Nd isotope compositions of dust deposited in the SJM preclude local San Juan volcanic rocks as a dust sources but are consistent with a derivation from basement rocks of southwestern United States sources; however, these isotope data do not rule out the contribution of far traveled dust from Asian sources [Neff et al., 2008]. Observations of large Asian dust storms tracking across the western United States [Chin et al., 2007; Heald et al., 2006; Wells et al., 2007] imply that distant sources may contribute to dust deposition fluxes in the SJM and other areas of the Rocky Mountains. Although Asian dust sources may sometimes contribute to the flux of dust to the SJM, these distant source regions are unlikely to result in the deposition rates and particle size distributions observed on the SJM snowpack. In particular, the PSD of dust samples from the SJM snowpack is dominated by silt‐sized particles between 30 and 53 mm in diameter (Figure 2), which is not consistent with far‐traveled dust deposition that is typically dominated by comparatively finer textured material [e.g., Prospero et al., 1989]. In general, the rate and PSD of dust deposited in the SJM are characteristic of a regional source [Lawrence and Neff, 2009]. Furthermore, geostationary remote sensing data and ensemble back‐trajectory data show that large masses of dust are emitted from the Colorado Plateau, Great Basin, and Mojave Desert at the same time as deposition is observed in the SJM [Painter et al., 2007]. Overall, we cannot completely eliminate arid regions in Asia as a source of some to the SJM, but the evidence reported here strongly suggests that regional sources dominate winter/spring dust inputs.

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5.4. Geochemistry of Dust Deposited in the San Juan Mountains [36] Several minerals dominate the composition of dust deposited in the SJM including quartz, k‐spar, plagioclase, illite, smectite, and occasionally calcite (Table 2). The prevalence of these minerals is generally consistent with the composition of dust measured in other regions [Leinen et al., 1994; Moreno et al., 2006; Schutz and Sebert, 1987]. Despite these general similarities, detailed dust mineralogy can be indicative of source‐region soils [Claquin et al., 1999; Jeong, 2008; Maher et al., 2009; Merrill et al., 1994; Shao et al., 2008] as well as the distance that the particles have traveled through the atmosphere before deposition [Rea and Hovan, 1995]. Specifically, with increasing distance from the source region, the texture of dust becomes finer as the larger particles are removed via gravitational settling. As a result of these systematic changes in the PSD of dust, the mineral composition is also likely to vary with the distance from the source region [Leinen et al., 1994]. For instance, dust from local source areas typically contains greater amounts of quartz and feldspars associated with the larger abundance of sand‐ and silt‐sized particles, while dust from distant source areas is comparatively enriched in phyllosilicate minerals associated with the greater abundance clay‐sized particles. The dust deposited in the SJM primarily originates from regional source areas and as a result it is composed primarily of silt‐ and clay‐sized particles. Correspondingly, the mineral composition of dust deposited in the SJM exhibits substantial abundances of quartz (31.4 ± 3.3%), feldspars (20.9 ± 1.8%), and phyllosilicate clay minerals (31.2 ± 3.7%). [37] With the exception of Ca and P, concentrations of all major and minor elements in SJM dust are depleted relative to average UCC (Figure 4). Although it is possible that the original dust source soils are naturally depleted, we feel it is more likely that these depletions in dust result from excess organic matter and soluble salts. This conclusion is supported by the observed enrichment of Ca and P and the depletion of the immobile element, Ti. The sequential leaching results (described below) suggest Ca enrichment results from the inclusion of soluble carbonates, whereas P enrichment results from the inclusion of organic matter. The inclusion of calcium carbonates and organic matter in dust leads to enrichments of Ca and P but also leads to the depletion or other rock derived elements, through the dilution of silicate mineral abundance. For instance, Ti is an immobile element and is therefore typically enriched, relative to the average UCC, in fine‐textured soils (or dust) that have undergone some degree of chemical weathering [Schutz and Rahn, 1982]. The observed depletion of Ti in dust compared to the UCC is consistent with the addition on more labile element pools. [38] In contrast to most major and minor elements, the bulk concentrations of many trace elements including As, Cd, Cu, Mo, Pb, and Zn are enriched in dust relative to the UCC (Figure 5). Enrichments of these heavy metals are not uncommon for aeolian dust [Arimoto et al., 1990; Erel et al., 2006; Holmes and Miller, 2004; Reheis and Kihl, 1995]; however, the enrichments of metals in SJM dust are some of the highest observed in passively collected samples [Lawrence and Neff, 2009]. High concentrations of trace

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elements in dust are sometimes attributed to particle‐size fractionation during the dust emission process [Schutz and Rahn, 1982]; however, Castillo et al. [2008] make the case that the enrichments of heavy metals in dust are often large enough that they cannot be explained by physical fractionation alone. We use the results of the sequential leaching protocol for further our interpretation of element concentrations in the aeolian dust of the SJM. 5.5. Form and Availability of Elements in Dust [39] The most easily available or mobile portion of SJM dust, the extractable‐fraction, is dominated by the presence of soluble minerals including carbonates. On average carbonates account for ∼5% of the mineral mass of dust deposited in the SJM (Table 2) and appears to be the predominant source of Ca, Sr, and Mg. These elements occur in greatest abundance in the extractable fraction (Figure 3). Cadmium is also abundant in the extractable leach fraction of dust but is negatively correlated with Ca indicating that it is not directly associated with carbonate minerals. Instead, Cd is highly correlated with macronutrients including C, N and to a lesser extent P indicating an association with organic matter. [40] Organic matter also appeared to be a very important component of dust deposited in the SJM. For instance, dust particles contained a substantial abundance of organic C and total N compared with surface soils from potential southwestern source regions [Fernandez et al., 2006, 2008]. The high concentrations of organic C and N relative to surface soils from potential southwestern source regions may reflect an enrichment of organic material in dust during the emission process [e.g., Li et al., 2008], suggest a different and more organic rich source region, or indicate the mixing of an additional source of organics at some point prior to collection. In addition, organic P appears to be the dominant form in dust as the organic leach accounts for 65% of the total abundance of P (Figure 3). [41] The extractable‐ and organic‐fractions are the most likely sources of trace metal enrichments measured in bulk dust relative to the average UCC. Combined, these fractions account for between 40 and 85% of the enriched trace elements (Table 3). In addition, the bulk concentration of organic C and total N are positively correlated with concentrations of the enriched trace metals (Table S3). When considering only the concentrations of the dust residual mineral fraction (extractable and organic fractions removed) the large enrichments are no longer observable for Cd, Cu, Pb, or Zn and instead, these elements appear slightly depleted (Figure 5). Thus, consistent with the results of a sequential extraction study of dust collected in the eastern Mediterranean [Erel et al., 2006], we conclude excess heavy metal concentrations in dust relative to average crustal materials are attributable to exchangeable ions or surface coatings on clays or organic particles, and not as part of the internal structure of silicate minerals. However, it remains unclear how and when these extractable and/or organic associated trace metals become associated with dust. 5.6. Anthropogenic Influence on San Juan Mountain Dust Geochemistry [42] Interactions of dust with volcanic gasses [Hinkley et al., 1999] or anthropogenic emissions [Castillo et al.,

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2008; Erel et al., 2006; Reheis et al., 2009] have been proposed as a source of excess trace element enrichments. There is abundant evidence of the modification of mineral particle surface chemistry during atmospheric transport [Dentener et al., 1996; Falkovich et al., 2004; Grassian, 2002; Wurzler et al., 2000]. In particular, dust particles may scavenge pollutants from the atmosphere through mineral surface reactions with compounds including sulfur oxides [Usher et al., 2002], nitrogen oxides [Goodman et al., 2000; Laskin et al., 2005; Underwood et al., 2001], O3 [Michel et al., 2002; Mogili et al., 2006; Usher et al., 2003], chlorine [Sullivan et al., 2007b], and volatile organic carbons [Falkovich et al., 2004; Li et al., 2001; Russell et al., 2002]. Furthermore, mineral particles that have interacted with anthropogenic emissions during transport often exhibit organic surface coatings [Falkovich et al., 2004; Kandler et al., 2007; Perry et al., 2004]. Although the reactivity of dust surfaces during atmospheric transport is dependent on the mineral composition [Krueger et al., 2005; Sullivan et al., 2007a], overall these studies show that dust particles are reactive during atmospheric transport and may act to scavenge pollutants and subsequently deposit them in terrestrial or aquatic ecosystems. [43] It is unclear if these observed association of trace element enrichment with organic mater in dust deposited in the SJM results from natural processes or from reaction with anthropogenic aerosols. A recent review of the global cycle of several heavy metals show that anthropogenic driven fluxes are now on the same order of magnitude as natural fluxes [Rauch and Pacyna, 2009]. In particular, industrial coal emissions remain a large source of As, Cd, Cu, Mo, Ni, Pb, and Zn [Davison et al., 1974; Finkelman, 1999; Rauch and Pacyna, 2009]. There are several large coal‐fired power plants upwind of the SJM that could contribute to elevated metal concentrations associated with soil‐dust deposition. Aerosol emission from anthropogenic (and other) sources may bind to the large reactive surface area of silt‐ and clay‐ sized particles of soil‐dust as they mix in the atmosphere [Erel et al., 2006]. Whether this is the source of metal deposition to the SJM remains uncertain and is an important area for future research. 5.7. Potential Role of Dust in Alpine and Sub‐alpine Ecosystems [44] The winter/spring deposition of dust represents a substantial element flux to ecosystems of the SJM (Table 4). These estimates suggest dust deposition may be an important flux of several biologically relevant elements. For instance, the flux of Ca from dust deposition during the winter/spring seasons at SBBSA is an order of magnitude greater than the average annual atmospheric flux of Ca to the Hubbard Brook ecosystem in New England, United States [Likens et al., 1998]. In contrast, the flux of N estimated from dust deposition to the SJM is small (∼4.0%) compared with the inorganic N flux observed in the Front Range of Colorado [Burns, 2004]. In comparison to the estimated nutrient use of spruce‐fir forests of the SJM (S. C. Castle, unpublished data, 2010), the winter/spring flux from dust could supply 7, 10, 41, 14, and 90% of the annual uptake of N, P, K, Ca, and Mg, respectively; however, this is a very rough estimate and the actual amount of plant nutrient demand that is met by aeolian inputs is much more difficult

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Table 4. Estimates of Element Flux Based on the Average Concentration of Each Element Observed in San Juan Dust and the Minimum, Maximum, and Mean Observed Rates of Dust Deposition Element

Minimum (mg m−2 yr−1)

Maximum (mg m−2 yr−1)

Mean (mg m−2 yr−1)

OC C N Si Al Fe Mg Ca K Na Ti Mn P As Ba Be Cd Ce Co Cr Cu La Li Mo Ni Pb Sr V Y Zn

206.00 266.50 22.00 1321.94 297.23 116.86 57.30 159.09 100.59 48.61 14.21 2.65 4.54 0.04 3.17 0.01 0.01 0.30 0.05 0.20 0.56 0.15 0.16 0.02 0.10 0.16 1.16 0.30 0.10 0.66

412.00 533.00 44.00 2643.89 594.45 233.73 114.60 318.18 201.18 97.22 28.42 5.30 9.07 0.07 6.34 0.02 0.01 0.59 0.10 0.39 1.12 0.31 0.32 0.03 0.21 0.33 2.31 0.61 0.21 1.32

267.80 346.45 28.60 1718.53 386.40 151.92 74.49 206.82 130.77 63.19 18.47 3.45 5.90 0.05 4.12 0.01 0.01 0.38 0.07 0.25 0.73 0.20 0.21 0.02 0.14 0.21 1.50 0.39 0.14 0.86

to determine. The biological utilization of dust‐derived nutrients in terrestrial (and aquatic) ecosystems in the SJM will depend on the availability of those elements. Results from our sequential leaching of dust samples suggest that a large proportion of several biologically cycled elements including Ca, Mg, and P are present in readily available forms. Consequently, these elements may also be mobile and easily transferred from terrestrial to aquatic systems. In addition, the sequential leaching results also suggest that dust provides a source of bioavailable and mobile trace elements, including several heavy metals. [45] From our measurements of snowpack dissolved ion chemistry, we know that a portion of dust deposited on the SJM snowpack is soluble and influences the concentration of dissolved base cations during snowmelt. For example, our results show that snowpack layers containing dust deposition events have significantly higher concentrations of Na+, Mg2+, Ca2+, and SO2− 4 than layers that do not contain dust (Figures S2 and S3). Differences between snowpack layers with and without dust would likely be more pronounced if the snow samples used for these measurements had been collected immediately following deposition and before any snowpack melting occurred. Melting presumably results in horizontal and vertical transport of solutes through the snowpack, masking chemical differences between layers containing dust and those that do not. [46] Comparisons of snowpack cation concentrations measured in southwestern Colorado with other measure-

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ments from the Rocky Mountain region further support a regional maximum dust input in the SJM. For instance, a transect of snowpack chemistry measurements across the Rocky Mountains found the highest concentrations of Ca2+ in the southern Rocky Mountains [Turk et al., 2001]. The dissolved ion concentrations that we have measured in the SJM snowpack layers containing dust are higher than those reported from other areas of the Rocky Mountains [Clow et al., 2002; Turk et al., 2001], especially Ca2+. However, when we consider only snowpack layers that do not contain dust, the mean concentration of dissolved ions that we measured in the SJM are similar to values reported for other areas of the Rocky Mountain region. [47] Overall, our observations support the notion that the rapid weathering of the labile components of aeolian dust may influence aqueous chemistry in the SJM. For example, using Sr isotopes as a proxy, Clow et al. [1997] showed that ∼26% of Ca in streams draining a granitic watershed of the central Rocky Mountains was derived from aeolian carbonates. Similarly, inputs of dissolved ions to soil water from the weathering of the labile fraction of aeolian dust may be an important source of essential elements for alpine and sub‐ alpine vegetation during the early growing season [Castle and Neff, 2009]. In addition, the dissolution of carbonates and other soluble salts may also influence the acid neutralizing capacity of soils and surface waters [Delmas et al., 1996; Hedin and Likens, 1996; Psenner, 1999]. For instance, based on our estimated range of winter/spring dust deposition (5 to 10 g m−2 yr−1), the abundance of carbonate minerals in dust (calcite + dolomite = 5.5% mineral mass), and the average snow water equivalence at the SBBSA each season (38.5 cm yr−1), we estimate that carbonate dissolution from dust deposition could account for 14 to 28 meq L−1 of snowmelt acid neutralizing capacity (1 mg L−1 CaCO3 = 20 meq L−1 ANC). This is comparable to the total acid neutralizing capacity measured in streams (∼32 meq L−1) of the nearby Weminuche Wilderness Area [Mast et al., 2000]. Thus, in poorly buffered alpine basins, dust deposition may be an important control of surface water ANC. [48] The deposition of dust to the SJM is likely to have an influence on the physical and geochemical properties of local soils and sediments. For example, The mean particle size and degree of particle sorting observed for dust samples collected in this study are consistent with silt‐enriched surface soils of the central Rocky Mountains and with Mojave Desert dust samples [Muhs and Benedict, 2006]. The dominance of silt‐sized dust particles provides further support for the aeolian explanation of silt‐enrichment in Rocky Mountain soils [Litaor, 1987; Muhs and Benedict, 2006]. Accumulations of aeolian sediments are likely to influence soil geochemistry if the composition of dust is substantially different than that of the local geologic substrates. For instance, variations in trace element concentration and isotopic composition in soils of Mount Cameroon, located on the western coast of Africa near the Gulf of Guinea, have been attributed to the accretion of Saharan dust [Dia et al., 2006]. If concentrations of elements in dust are greater than those of the local bedrock, then dust may enrich soils. Conversely, dust accumulation could dilute the concentration of some elements in soils and sediments, if the concentration of those elements in dust is less than that of local bedrock. More detailed estimates of the effect of dust

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deposition on soils in the SJM must be made through comparisons of the composition of dust with that of local bedrock and soils.

6. Summary and Conclusions [49] The SJM in southwestern Colorado receive 5 to 10 g m−2 of aeolian dust deposition from December to June each year. This dust is deposited on the snowpack and is composed of organic (∼8%) and mineral matter (∼92%). The particle‐size of the mineral portion of dust is predominately silt and clays. The source of this dust is non‐local and likely centered in arid and semi‐arid regions of the southwestern United States; however, long‐range transport of Asian dust to the western United States may be an additional, albeit smaller source. The geochemical fluxes from dust inputs are likely an important factor for high‐elevation ecosystems in the southern Rocky Mountains. [50] The geochemistry of dust deposited in the SJM is generally indicative of fine‐textured soil particles; however, both soluble salts and organic matter have a large influence on the chemical composition of dust. For example, the variable presence of carbonate minerals appears to be a major control of Ca, Mg, and Sr concentration in dust. The organic fraction of dust appears relatively large when compared with potential source soils of the Colorado Plateau, possibly indicating a potential bias toward the emission of organic rich soil particle classes during dust emission or possibly the development of an organic coating on dust particles as they are transported through the atmosphere. The later possibility is supported by the observation that the organic fraction of dust appears to be linked to the high concentration of many heavy metals. Although we cannot say for certain what is the source of this metal enrichment relative to the average UCC, it is possible that anthropogenic emissions such as those from coal combustion could influence the chemical composition of aeolian dust deposited in Rocky Mountain ecosystems. [51] Based on the measurements of dust flux and geochemistry presented here, we cannot reject the hypothesis that dust accumulation is an important factor for the geochemical composition and biogeochemical cycling in Rocky Mountain ecosystems. When estimates of the dust fluxes are considered in light of the widespread observations of silt‐ enrichment in alpine soils of the Rocky Mountains [Dahms, 1993; Litaor, 1987; Muhs and Benedict, 2006], the potential importance of dust for soil properties including mineralogy, texture, and biogeochemistry are apparent. For example, the geochemical fluxes resulting from aeolian dust deposition represent a substantial exogenous source of many ecologically relevant elements as well as heavy metal contaminants. Several elements essential for biological productivity, such as Ca and Mg, are supplied by dust inputs in forms that are relevant to plant uptake, soil and surface water acid neutralizing capacity, and aqueous exports. In addition, several metal contaminants deposited with dust including Cd, Cu, and Pb occur in mobile forms suggesting they may also be available for biological uptake or aqueous export. These data further indicate the potential importance of dust inputs for biogeochemical cycling in Rocky Mountain ecosystems and should be considered when evaluating element budgets for both terrestrial and aquatic ecosystems.

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[52] Acknowledgments. This project was partially supported by an A. W. Mellon Foundation grant to J.C.N., two U.S. National Science Foundation grants to T.H.P. (ATM‐0432327 and ATM‐0431955), and a U.S. National Science Foundation Doctoral Dissertation Improvement Grant to C.R.L. (DEB‐0808535). Additional research support was provided to C. R.L. by the Geological Society of America and the Colorado Mountain Club. The University of Colorado Laboratory of Environmental and Geological Studies and the U.S. Geological Survey Earth Surface Dynamics Program provided assistance with ICP and particle size analyses, respectively. We would like to acknowledge the constructive feedback of two anonymous reviewers, whose suggestions have greatly improved the quality of this work. We also thank S. Castle, E. Costello, G. L. Farmer, R. Reynolds, and E. Verplanck for field, laboratory, and/or technical assistance during the course of this project.

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