Spatial Variation in the Origin of Dissolved Organic Carbon in Snow

Article pubs.acs.org/est

Spatial Variation in the Origin of Dissolved Organic Carbon in Snow on the Juneau Icefield, Southeast Alaska Jason B. Fellman,*,† Eran Hood,† Peter A. Raymond,‡ Aron Stubbins,§ and Robert G.M. Spencer∥ †

Environmental Science Program, University of Alaska Southeast, Juneau, Alaska 99801, United States School of Forestry and Environmental Sciences, Yale University, New Haven, Connecticut 06511, United States § Skidaway Institute of Oceanography, Department of Marine Sciences, University of Georgia, Savannah, Georgia 31411, United States ∥ Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida 32306, United States ‡

Downloaded by UNIV OF SUSSEX on September 9, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.est.5b02685

S Supporting Information *

ABSTRACT: Dissolved organic carbon (DOC) plays a fundamental role in the biogeochemistry of glacier ecosystems. However, the specific sources of glacier DOC remain unresolved. To assess the origin and nature of glacier DOC, we collected snow from 10 locations along a transect across the Juneau Icefield, Alaska extending from the coast toward the interior. The Δ14C-DOC of snow varied from −743 to −420‰ showing progressive depletion across the Icefield as δ18O of water became more depleted (R2 = 0.56). Older DOC corresponded to lower DOC concentrations in snow (R2 = 0.31) and a decrease in percent humic-like fluorescence (R2 = 0.36), indicating an overall decrease in modern DOC across the Icefield. Carbon isotopic signatures (13C and 14C) combined with a three-source mixing model showed that DOC deposited in snow across the Icefield reflects fossil fuel combustion products (43−73%) and to a lesser extent marine (21−41%) and terrestrial sources (1− 26%). Our finding that combustion aerosols are a large source of DOC to the glacier ecosystem during the early spring (April−May) together with the pronounced rates of glacier melting in the region suggests that the delivery of relic DOC to the ocean may be increasing and consequently impacting the biogeochemistry of glacial and proglacial ecosystems in unanticipated ways.

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with typical bulk radiocarbon ages of 750−5000 years BP.2,4,9 Possible sources of glacier-derived DOM include supra and subglacial biological processes,2,12−14 soils and vegetation overrun during glacial advance,9,13 deposition of vascular plant and soil organic matter,3 and aerosols from forest fires,15 and fossil fuel combustion byproducts.4,16 This diversity of sources is illustrated in a recent molecular level analysis of DOM from surface snow collected on the Antarctic Ice Sheet that documented the presence of microbial, vascular plant, and black carbon-like material in the supraglacial DOM pool.11 These studies highlight the need to further resolve the source, particularly for the ancient fraction, of DOM inputs to glacier ecosystems considering current and projected rates of glacier change and the fact that glaciers can impact carbon cycling in downstream ecosystems.3 In this context, resolving the origin and character of glacier-derived DOM is important for understanding how the storage and release of organic carbon from glaciers will change in the future.

INTRODUCTION Glaciers and ice sheets store an estimated 6 Pg of organic carbon and deliver an annual flux of 1.0 ± 0.2 Tg C yr−1 to rivers and coastal oceans.1 Glaciers are among the most sensitive ecosystems to climate warming making this large store of organic carbon vulnerable to export as glaciers melt. In particular, changes in glacier volume that alter runoff and release ice-locked stores of organic matter will influence the magnitude of dissolved organic matter (DOM) delivered to proglacial aquatic ecosystems. This in turn could impact the metabolic balance of downstream ecosystems because glacier DOM is highly bioavailable to aquatic microbes relative to DOM from other catchment sources, such as plant, and soil organic matter.2−4 Globally, the rate of glacier ice loss is accelerating5 with some of the most rapid changes observed along coastal margins in regions like Greenland, Patagonia, and the Gulf of Alaska.6−8 However, there have been few attempts to systematically assess the deposition and accumulation of DOM on and within icefields in these regions. Dissolved organic matter in glacial and proglacial aquatic ecosystems varies considerably in its concentration and quality showing high variability both temporally across the glacial meltwater season,9,10 and spatially across polar ice caps.11 A number of studies have shown that glacier-derived DOM is old © XXXX American Chemical Society

Received: June 1, 2015 Revised: August 27, 2015 Accepted: August 28, 2015

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DOI: 10.1021/acs.est.5b02685 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Map of the 10 study sites and elevation (m) on the Juneau Icefield, southeast Alaska.

depositing heavy rain and snow on the Coast Mountains of western North America. Sample Collection and Analytical Methods. Snow samples were collected in the accumulation zone of the Juneau Icefield on a single day in May, 2013 immediately following one of the last major snowfall events of the winter. The 10 sampling sites were located on a transect extending East/Northeast from the coast (Figure 1). The sample sites ranged from 17 to 64 km from the coast and had an elevation range of 911 to 1847 m above sea level. At each site, a 1 m deep snow pit was dug ∼100 m upwind from the helicopter landing location. One composite snow sample integrating the entire 1 m deep snow pit was collected using a precleaned polyethylene shovel and stored in acid-washed high density polyethylene bottles (wide-mouth 4 L). Samples were allowed to melt at room temperature and immediately filtered through precleaned Whatman Polycap 36TC filters (0.20 μm). Samples for DOC concentration and fluorescence characterization were analyzed immediately following filtration and samples for 13C and 14C were frozen until analysis. Concentrations of DOC were determined by high-temperature nonpurgeable organic carbon analysis on a Shimadzu TOC-V CSH analyzer using a high-sensitivity catalyst to enable the detection of low DOC concentrations.20 Analytical precision for DOC was no more than 0.02 mg C L−1, which was determined from the standard deviation of identical samples reanalyzed during the sample run. Samples for δ18O of water were stored in glass bottles with zero headspace and were analyzed on a Picarro L2120-i analyzer. Values for δ18O were reported in per mil (‰) after normalization to Vienna standard mean ocean water (VSMOW).21 The δ13C of DOC (δ13C-DOC) was analyzed using an O.I. Analytical Model 1010 TOC analyzer interfaced to a PDZ Europa 20−20 IRMS (Sercon Ltd.) at the University of California Davis. Values for δ13C-DOC were reported in per mil (‰) relative to VPDB

We collected snow from 10 locations along a transect extending from the coast east/northeast across the Juneau Icefield in southeast Alaska. Snow samples were analyzed for δ18O and DOM, including 13C, 14C and fluorescence characteristics. Our goal was to assess the origin and nature of DOM in snow that will eventually contribute to glacier ice and test the hypothesis that anthropogenic combustion products are an important source of DOM to the Icefield. This study builds on previous research on the Mendenhall Glacier, a major outflow glacier from the Juneau Icefield;10,16 however, our study design allows us to spatially asses if the deposition of anthropogenic combustion products varies across the Juneau Icefield. Moreover, we hypothesized that snow accumulating on the Juneau Icefield would be more isotopically depleted (δ18O) and have a lower DOM concentration with increasing distance from the coast because of a greater degree of washout of solutes commonly observed as storms move inland over coastal mountains.

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MATERIALS AND METHODS Study Area. The ∼3800 km2 Juneau Icefield blankets the Coast Mountains along the western margin of the mainland in northern southeast Alaska (Figure 1). The city of Juneau, located on the western fringe of the Icefield, has a cool, maritime climate with a mean average temperature of 4.7 °C and a mean annual precipitation of 1400 mm at sea level. However, winter snowfall at high elevations on the Icefield can be more than 10 m water equivalent.17 Since the end of the Little Ice Age (ca. 1870), glacier thinning and retreat from many areas of the Juneau Icefield has been pronounced with low elevation ice thinning rates estimated at 2−8 m yr−1 from several of the major outflow glaciers.18,19 Precipitation on the Icefield generally results from large frontal storms that form in the northern Pacific Ocean and move eastward across the Gulf of Alaska and through Southeast Alaska into Canada typically B


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Environmental Science & Technology scale (Pee Dee Belemnite).22 Bulk Δ14C of DOC (Δ14C-DOC) was measured as described in ref 23. The 14C analysis was performed by accelerator mass spectrometry (AMS) on purified CO2 collected on a vacuum extraction line at Woods Hole Oceanographic Institution and values were corrected for sample δ13C. The fluorescence characteristics of DOM were measured on a Fluoromax-4 (Jobin Yvon Horiba) on water samples warmed to room temperature. Excitation−emission matrices (EEMs) were collected by measuring fluorescence intensity across excitation wavelengths from 240 to 450 at 5 nm increments and emission wavelengths from 300 to 600 at 2 nm increments. Excitation and emission slit widths were 5 nm. Samples were not corrected for inner filter effects because of very low optical densities,24 as determined by measuring absorbance at 254 nm on a Genesys 5 spectrophotometer. All EEMs were corrected for instrument bias and Raman normalized using the area under the water Raman peak at excitation 350 nm.25 Statistical Analyses. We used parallel factor analysis (PARAFAC) to decompose the EEMs into individual fluorescence components using the PLS_toolbox 7.9 (eigenvector Research Inc.) in MATLAB. The data set for the PARAFAC model included 10 EEMs from the Juneau Icefield transect and 10 EEMs from local rainfall and snowfall events collected at sea level. Although our sample size for the model has a limited ability to adequately resolve the fluorescence spectra of DOM, 20 EEMs is generally considered to be the minimum necessary for PARAFAC.26 Rayleigh and Raman scatter were removed from the EEMs prior to modeling. Our PARAFAC model identified a total of four components and was validated using split-half analysis (Figures S1−S2) and core consistency diagnostics. Fluorescence components were reported as a percent relative contribution determined from the maximum fluorescence intensity of each component divided by the total fluorescence of all the modeled PARAFAC components within a sample. The fluorescence characteristics of component 1 (C1) had a maximum excitation (MaxEx) of 240 nm (secondary Ex = 290 nm) and maximum emission (MaxEm) of 354 nm. This uncommonly observed component has spectral characteristics indicative of protein-like fluorescence.27,28 Component 2 (MaxEx = 275 nm, MaxEm = 338 nm) has spectral characteristics similar to the aromatic amino acid tryptophan25,29 and contributes greatly to the fluorescence spectra in glacial ecosystems.10,30−32 It has recently been shown that lignin phenols may contribute to the fluorescence signature of protein-like components33 and that the total population of molecules associated with protein-like fluorescence are actually nitrogen-depleted, but the aromatics associated with its fluorescence are nitrogen-enriched.34 Therefore, we refer to these components simply as C1 and C2 because we have no independent evidence of the specific moieties responsible for their fluorescence. The humic-like PARAFAC components, C3 (MaxEx = 240 nm, SecEx = 300 nm, MaxEm = 430 nm) and C4 (MaxEx = 285/335 nm, MaxEm = 410 nm), have fluorescence characteristics that fall within the traditionally defined humic-like region.29,35 Humic-like C3 was considered to originate mainly from vascular plant sources25,29 and C4 was attributed to marine sources29 but may also be related to microbial abundance in glacial environments.36 The 13C and 14C values were used in a mixing model to determine the relative contribution of three different sources to snow DOC: marine aerosols, terrestrial plant material and

anthropogenic aerosols. These three sources are regarded as the main sources of DOC in precipitation with increasing recognition of the importance of fossil fuel contributions to DOC in both rainwater23,37,38 and snow.4,16,39 Carbon isotope analysis using mixing models has been used to estimate source contributions to rainwater DOC.23,38,40 For the three-source isotopic mixing model, Δ14C values of −50, −1000, 100‰ were used for the marine, fossil and modern terrestrial sources, respectively, while δ13C values of −21, −28, and −26‰ were used.23,40−42 The sensitivity of the mixing model to changes in the endmember values for carbon isotopes was examined by varying the δ13C values ±1‰ (for all three end members) and Δ14C values ±50‰ for the marine end member and −100‰ for the terrestrial end member.41

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RESULTS The δ18O values of snow varied from −16.2‰ to −19.8‰ across the 10 sites and values became significantly more depleted with distance from the coast (R2 = 0.84, P < 0.01, Table 1, Figure S3). This is consistent with the gradual δ18O Table 1. Distance from the Coast for Each Site, DOC Concentrations and Isotopic Values for the 10 Sites of the Juneau Icefield Transect site Site Site Site Site Site Site Site Site Site Site

1 2 3 4 5 6 7 8 9 10

distance km 23 22 17 27 38 47 54 64 59 56

δ18O ‰

DOC mg C L−1

δ13CDOC ‰

Δ14CDOC ‰

Δ14Cage ybp

−16.7 −16.7 −16.2 −17.6 −18.7 −18.7 −18.7 −19.8 −18.4 −18.9

0.3 0.3 0.2 0.2 0.3 0.1 0.2 0.1 0.2 0.1

−25.2 −25.2 −25.4 −25.9 −25.1 −24.9 −24.5 −24.1 −22.9 −22.8

−543 −420 −538 −645 −708 −599 −651 −648 −677 −743

6217 4300 6142 8254 9845 7291 8422 8371 9014 10 824

depletion of snow and rain during atmospheric transport as storms move inland.43 Therefore, we used δ18O as a proxy for distance from the coast because air mass trajectories for individual storms are somewhat variable, which should be reflected in the snow δ18O values. Snow DOC concentrations for all sites were low ( 0.23). Humic-like C3 showed a contrasting spatial pattern to C1, where the percentage of C3 (Figure 2c) was lowest in the sites with the most depleted δ18O values but significantly increased as snow δ18O values became more enriched moving closer to the coast (R2 = 0.64, P < 0.01, Figure S3). Younger DOC was somewhat related to the percentage of humic-like C3 (R2 = 0.35, P = 0.07), which is consistent with the results from the mixing model indicating an overall decrease in modern terrestrial/marine-derived DOC across the Icefield transect (Figure 3). The percentage of the humic-like C4 was also lowest in sites furthest from the coast (Figure 2c) but demonstrated only a mild coherence with δ18O values (R2 = 0.36, P = 0.07, Figure S3) and no relationship with the source

Figure 2. (a) Percentage of fossil, terrestrial and marine sources to DOC in snow based on a three-source, two-isotope mixing model, (b) concentrations of DOC derived from fossil, terrestrial and marine sources, and (c) percentage of the four fluorescence components identified by PARAFAC for the 10 study sites.

contribution of marine-derived DOC (R2 = 0.12, P = 0.33). This result was somewhat surprising given previous studies of DOC fluorescence in rainfall showing this component typically reflects storms with a marine origin.47 Overall, total DOM fluorescence was not related to DOC concentrations (R2 = 0.05, P > 0.50).

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DISCUSSION Carbon isotopic signatures combined with mixing model analysis showed that DOM in snow in the Coast Mountains D


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Figure 3. Changes in the source contribution of marine (blue), terrestrial (brown), and fossil (gray) organic carbon deposited in snow across the Juneau Icefield. Snowflake pies represent the mean of the Icefield transect sites 1−3 (near the coast) and 8−10 (inland sites).

Concentrations of DOC ranged from ∼0.1 to 0.3 mg C L−1 for snow on the Juneau Icefield, which are comparable to snow collected from accumulation areas in glaciers and ice sheets in Antarctica,11,48 Greenland45,49 and the European Alps.50 For major outflow glaciers from the Juneau Icefield, DOC typically ranges from 0.2 to 0.5 mg C L−1,10,51 which is 2−4 times higher than the average DOC concentration for the Icefield transect. Moreover, DOC in the accumulation zone snow was highly 14 C-depleted (−743 to −420‰) relative to glacier outflow DOC (−355 to −100‰) from the Icefield.10 This modernization of DOC from the accumulation zone to glacier outflow suggests additional sources, such as supra and subglacial biological processes3,9,12 may produce substantial contemporary DOC within the glacier ecosystem or organics deposited prior to or earlier in the Anthropocene when fossil fuel inputs were lower, contribute to the DOC being exported today.52 Whatever the source of this modern DOC, its incorporation into glacier runoff would account for the progressive enrichment of Δ14C-DOC values as meltwater transits through the glacier hydrologic system to the outflow. The δ18O values for snow on the Icefield became increasingly depleted moving away from the coast consistent with the natural fractionation or fallout of heavy δ18O that occurs when precipitation cools with increasing elevation (altitude effect) and the “continental or rainout effect” that typically occurs as storms move inland.43 Snow δ18O values (−16.2 to −19.9‰) were somewhat more depleted than what is typically observed in both proglacial and snowmelt-dominated streams (−13.0 to −16.5‰) in the region.51,53 This enrichment in δ18O between the Icefield accumulation zone and its outflow streams reflects the fact that glacier runoff in the Coast Mountains is a mixture of snowmelt and other more enriched source waters including firn, glacier ice, and rainfall. The depletion in the isotopic signature (δ18O) of snow across the Icefield transect was mirrored by changes in the origin and character of DOM. For instance, sites with the most

Figure 4. Regression model describing the relationship between Δ14CDOC values and the percent contribution of fluorescence component 1 (C1) across all sites.

of Alaska reflects a mixture of sources with the largest contribution from fossil carbon. These results support previous research on the Mendenhall Glacier, which is a major outflow glacier of the Juneau Icefield, as well as from glacial ecosystems worldwide showing that anthropogenic aerosols contribute organic matter to glacial ecosystems.11,16,39 This provides further evidence of a considerable anthropogenic footprint in some of the most pristine ecosystems on Earth. Thus, we suggest that the deposition of anthropogenic combustion products in glacial environments maybe impacting the biogeochemistry of glacial and proglacial aquatic ecosystems in many ways that are currently not well understood. E


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The strong coherence between the fluorescence characteristics of C1 and the age and source of fossil DOM (Figure 4) is of particular interest. Although C1 is generally attributed to protein-like fluorescence,27,28 it has characteristics similar to a component identified in aerosols collected from diesel exhaust (component 3 from ref 62)62 suggesting this component may be an indicator of fossil fuel combustion byproducts. This finding supports the idea that DOM fluorescence is useful for distinguishing between anthropogenic pollution (C1) and vascular plant-derived (C3) organic matter in precipitation.47,63 The fact that there was no relationship between total DOM fluorescence and DOC concentration is consistent with other studies showing that snow/ice collected from glacial environments is enriched in non-fluorescent organic matter including aliphatic compounds, amino acids, and lipids and low in the aromatics that dominate the fluorescence characteristics of DOM in non-glacial environments.11,16,32 The Coast Mountains of southeast Alaska are currently experiencing high rates of glacier thinning and recession8,64 and runoff from glaciers averages almost 50% of total freshwater discharge into the Gulf of Alaska.65 Our findings suggest that anthropogenic aerosols contribute abundant DOM to glacier ecosystems in the Alaska Coast Mountains and thus the pronounced rates of glacier melting in the region may be delivering increasing quantities of relic DOM to the Gulf of Alaska. Moreover, fossil fuel combustion products (typically of low molecular weight) that are initially deposited as snow on the glacier surface may eventually be incorporated into proglacial aquatic food webs.66,67 The potential ecological consequences of changes in the flux and character of DOM to coastal ecosystems that receive glacial runoff remain unknown. Our results underscore the need to further quantify and source organic matter inputs to glacier ecosystems in order to both identify the anthropogenic imprint on rapidly changing glacier ecosystems and better constrain the biogeochemical coupling between glaciers and the ecosystems downstream of them.

depleted δ18O values were the most 14C-depleted and contained the greatest contribution of C1 and lowest contribution of humic-like C3 fluorescence. This suggests that contemporary DOM that is likely higher in molecular weight is effectively washed out from the atmosphere with the initial orographic precipitation generated near the coast (Figure 3). Hence, sites closest to the coast demonstrated a higher “humic-like influence” (i.e., derived from terrestrial or marine sources) relative to inland sites with the most depleted δ18O values. Although we found a greater fraction of fossil sources to snow DOC in inland sites, the concentration of DOC attributed to fossil sources showed near constant deposition across the Icefield transect (Figures 2b, 3). Thus, the deposition of combustion aerosols is a widespread phenomenon on the Juneau Icefield and there are not likely to be dramatic shifts in the snow/ice albedo with distance from the coast. Interestingly, the source contribution of marine organic matter to snow DOM was not related to the gradient in δ18O values across sites but was generally greatest in the coastal sites. This suggests there is a small nonreactive (both biological and photochemically) pool of marine organic matter that is capable of transport over long distances before eventual removal, as hypothesized to occur in other coastal regions.54 Our finding that fossil fuel combustion products are the largest contributor to snow DOM, particularly in sites farther from the coast, is not surprising because black carbon in aerosols is small in size, has a low molecular weight55 and can remain airborne for up to three months.56 Moreover, these combustion aerosols likely originate from distant sources, such as Asia. Satellite observations and air monitoring stations have shown that the trans-Pacific transport of combustion aerosols and dust from Asia to western North America occurs throughout the year.57 However, spring (when our snow sampled occurred) is the most active season for trans-Pacific transport because strong winter and spring mid-latitude cyclones can advect Asian dust and pollution into the troposphere where it can be readily transported to western North America by the prevailing westerly winds.58,59 Although our findings show that deposition of combustion aerosols is a large source of carbon to the Juneau Icefield during early spring, they are limited in their temporal extent and possibly only representative of this period of time when trans-Pacific transport of dust and pollution is highest. Therefore, it is necessary to determine if combustion aerosols are still the main source of carbon in snow during the winter months (November through March), which is when much of the snowfall usually occurs on the Juneau Icefield. The deposition of anthropogenic aerosols on the Juneau Icefield is currently not well understood due mainly to a lack of measurements of black carbon deposition in the region. However, several lines of evidence indicate that deposition of fossil fuel combustion products is increasing in the Coast Mountains of Alaska. Analysis of an ice core from nearby Mt. Logan in the Yukon, Canada shows a 10-fold increase in lead concentrations at the turn of the 20th century relative to natural background.60 Records of lake cores in southeast Alaska have similarly shown that mercury deposition rates increased throughout the 20th century.61 These records together with recent documentation of fossil fuel combustion products on the Mendenhall Glacier, southeast Alaska10,16 support our finding of anthropogenically derived carbonaceous material across the accumulation zone of the Juneau Icefield.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02685. S1, Split half analysis of the four component PARAFAC model. S2, Contour plots of the four PARAFAC components. S3, Regression analyses of δ18O in snow versus DOM concentration and character (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Kim Homan for map preparation and Anny Boyette for the preparation of Figure 3. This study was supported by the Department of Interior Alaska Climate Science Center and the National Science Foundation (EAR-0943599, DEB1145885, DEB-1146161, and Alaska EPSCoR OIA-1208927). F


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