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Organic Carbon in Antarctic Snow: Spatial Trends and Possible Sources Runa Antony,*,† K. Mahalinganathan,† Meloth Thamban,† and Shanta Nair‡ † ‡
National Centre for Antarctic and Ocean Research, Headland Sada, Vasco-da-Gama, Goa-403 804, India National Institute of Oceanography, Dona Paula, Goa-403 004, India
bS Supporting Information ABSTRACT: Organic carbon records in Antarctic snow are sparse despite the fact that it is of great significance to global carbon dynamics, snow photochemistry, and airsnow exchange processes. Here, surface snow total organic carbon (TOC) along with sea-salt Na+, dust, and microbial load of two geographically distinct traverses in East Antarctica are presented, viz. Princess Elizabeth Land (PEL, coast to 180 km inland, Indian Ocean sector) and Dronning Maud Land (DML, ∼110300 km inland, Atlantic Ocean sector). TOC ranged from 88 ( 4 to 928 ( 21 μg L1 in PEL and 13 ( 1 to 345 ( 6 μg L1 in DML. TOC exhibited considerable spatial variation with significantly higher values in the coastal samples (p < 0.001), but regional variation was insignificant within the two transects beyond 100 km (p > 0.1). Both distance from the sea and elevation influenced TOC concentrations. TOC also showed a strong positive correlation with seasalt Na+ (p < 0.001). In addition to marine contribution, in situ microorganisms accounted for 365 and 320 ng carbon L1 in PEL and DML, respectively. Correlation with dust suggests that crustal contribution of organic carbon was marginal. Though TOC was predominantly influenced by marine sources associated with sea-spray aerosols, local microbial contributions were significant in distant locations having minimal sea-spray input.
’ INTRODUCTION Snow covers a significant portion of the Earth’s surface and forms an integral part of the global climate system. Snow packs, through the various inorganic and organic compounds present in it and through the physical and photochemical process occurring within it, can have a major impact on atmospheric chemistry.15 Yet, most studies on snow have focused only on inorganic components. Studies on organic carbon are few in number, making it one of the least understood fractions in snow. Calculations reveal that the prokaryotic carbon pool from the polar regions exceeds that of all surface fresh waters by more than 2 orders of magnitude, while dissolved organic carbon levels exceed by about 20-fold.6 These estimates, however, are tentative and need to be refined as more data are made available on organic carbon concentrations in Antarctic environments. Nevertheless, it implies that polar environments, particularly Antarctic ice, contain an organic carbon reservoir that must be seriously considered while addressing issues concerning global carbon dynamics.6,7 Additionally, it has been shown that organic carbon in snow undergoes photochemical reactions, releasing reactive gas-phase species to the overlying atmosphere.810 Despite its importance in airsnow exchange processes and the global carbon cycle, very little is known on the distribution and sources of organic carbon in Antarctic snow. While previous r 2011 American Chemical Society
studies have indicated the importance of marine aerosols in the transport of organic carbon in snow,11 no attempt has been made to systematically measure organic carbon concentrations along a transect from coast to interior to elucidate the marine influence on organic carbon concentrations in snow. Further, most studies on organic carbon in the East Antarctic region are concentrated in the coastal area near the Ross Sea and in the Dome C area,1113 and currently, little information exists on the spatial variation of organic carbon in other regions of Antarctica. These observations highlight the need to quantify and also identify the sources and distribution of organic carbon in snow. One specific challenge in quantifying organic carbon in snow has been the analytical limitations in detecting and measuring them at trace levels. In this work, we carried out high sensitivity measurements of total organic carbon (TOC) along with sea-salt Na+, dust, and microbiological components, systematically in two transects perpendicular to the coast in the East Antarctic region. The sampling sites were fixed close to the coast to about 180 km inland in the Princess Elizabeth Land transect and ∼110 km Received: October 4, 2011 Accepted: October 21, 2011 Revised: October 20, 2011 Published: October 21, 2011 9944
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Figure 1. Maps showing snow sampling locations along the Princess Elizabeth Land and Dronning Maud Land transect. Maps were plotted using the high-resolution Radarsat Antarctic Mapping Project (RAMP version 2) digital elevation model (DEM) data (http://nsidc.org/data/docs/daac/ nsidc0082_ramp_dem_v2.gd.html). Base map features were obtained from the United States Geological Survey (USGS).
away from the coast to ∼300 km inland in the Dronning Maud Land transect, in order to have a better understanding of the organic carbon concentrations and its sources in Antarctic snow. We report the first measurements of organic carbon in snow from the above study regions in East Antarctica and provide information on the spatial distribution of TOC as a function of increasing elevation and distance from the coast.
’ EXPERIMENTAL SECTION Sampling. Surface snow samples were collected along two traverses in East Antarctica representing the Atlantic and Indian Ocean sectors (Figure 1). The first sampling transect extended from the coastal area of Larsemann Hills to about 180 km inland in the Princess Elizabeth Land (Indian Ocean sector). In this transect, sampling was carried out at near sea level to about 2210 m above sea level (m asl). The second transect extended from ∼110 km from the coast (inland to the Indian Antarctic station Maitri) to ∼300 km inland in the polar plateau of Dronning Maud Land (Atlantic Ocean sector). The sampling sites were located at an elevation between 610 and 3015 m asl. Snow
samples (∼10 cm deep) were collected using a precleaned polypropylene scoop and stored in well-sealed and precleaned LDPE bags. In order to avoid contamination due to vehicular movement of the sampling team, sampling was always carried out 50 m upwind from the landing site at each location. Thirty eight samples were collected from the PEL transect and 20 samples from the DML transect. Samples were kept frozen at 20 °C in dark conditions during shipment, storage, and until analysis. Precautions Taken during Sample Handling and Processing. As high-sensitivity analysis was involved, stringent precautions were taken to minimize carbon contamination from the environment. Only containers and vials made of glass were used during standard preparation, sample processing, and analysis to avoid problems of organic carbon contamination known to occur in the case of plasticware. All glassware was soaked in 0.5% ultrapure nitric acid for 48 h, rinsed thoroughly with ultrapure (Milli-Q Element) water, and combusted at 450 °C for 5 h. Only freshly purified ultrapure water was used during every stage of the analysis. Ultrapure water was collected directly into combusted glass bottles, leaving no headspace and immediately sealed tight. In order to determine the magnitude of contamination from the 9945
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Environmental Science & Technology sample bag material, we analyzed the outer layer of snow sample that was in contact with the material of the bag as well as snow from the center of the bag that was at no time in contact with the sample bag material. The average TOC concentration in snow from the outer region was about 108 μg L1 higher than snow from the central region. Thus, in order to avoid contamination, utmost care was taken during subsampling to ensure that only snow from the inner region that was not in contact with the sampling bag material was collected for analysis. Total Organic Carbon Measurements. In the laboratory, snow samples were transferred to acid-cleaned and precombusted (450 °C, 5 h) screw-capped glass bottles under a laminar-flow bench housed in a 15 °C cold room processing facility. Care was taken to ensure that the bottles were filled leaving no head space and were tightly sealed in order to minimize contamination from the atmosphere. Samples were allowed to melt in the dark in a class-100 clean room and analyzed immediately for TOC using a high-sensitivity TOC analyzer (Shimadzu TOC-VCPH). In order to check for possible contamination during the sample handling and melting, blanks comprising ultrapure water were processed in a similar manner as that of the samples. Since prolonged storage of samples can lead to an increase in organic carbon concentrations, only two samples were melted at a time and analyzed within 20 min. Measurements were carried out in triplicate by the nonpurgeable organic carbon (NPOC) method. Samples were automatically drawn into the syringe followed by autoaddition of 2 M ultrapure HCl to lower the pH to ∼2 and hydrocarbon-free high purity N2 sparged at a flow rate of 150 mL min1 to eliminate inorganic carbon. The sample was then introduced into the quartz combustion tube containing the high-sensitivity platinum catalyst (0.5% platinum on quartz wool) and heated to 680 °C. During the high-temperature catalytic oxidation, the remaining carbon (organic carbon) in the sample is converted to CO2 and is swept along with the carrier gas through a dehumidifier and halogen scrubber and finally into a nondispersive infrared (NDIR) detector cell to be quantified. Standards and additional blanks comprising fresh, ultrapure water were run intermittently between samples to check for contamination during sample analysis. During TOC measurements, samples were contained within wellsealed, combusted glass vials from where the instrument automatically withdrew sample by piercing through the vial seal. The samples were therefore never exposed to the atmosphere during the course of analysis, thus avoiding CO2 contamination from the air. Calibration was carried out using potassium hydrogen phthalate as the standard. All standard solutions for generation of calibration curve were analyzed immediately after preparation to avoid concentration changes as a result of storage. Replicate analysis of the standard yielded a precision better than 7%. Relative standard deviation of the measurements was 0.1) from that of the samples collected beyond 100 km in the PEL transect. This comparison shows that while organic carbon exhibits considerable spatial variation from coast to the interior in a given area, no significant regional variation exists in the inland concentrations 9946
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Figure 2. Spatial distribution of TOC, sea-salt Na+, dust, and cell carbon along the Princess Elizabeth Land (ad) and Dronning Maud Land transect (eh) as a function of increasing distance from the coast. Note the break in scale of the X-axis (ad) and the Y-axis (f).
between the two different transects in the East Antarctic region. Earlier reports have shown that organic carbon is present in substantial amounts in snow collected from the East Antarctic region (Pacific Ocean sector) and from the South Pole.1012 The values obtained from the inland region of Princess Elizabeth Land and from Dronning Maud Land are comparable with the TOC concentration (10 km from coast12 but were lower than that of the South Pole snow (mean 400 ( 130 μg L1).10 TOC concentrations in an ice core from Talos Dome (East Antarctica) ranged from 80 to 360 μg L1,13 and these values are also comparable with the present data. Measurement of TOC at remote northern high latitude sites such as Alert in Canada and Greenland ranged from 30 to 700 μg L1.16,10 In general, the TOC profile closely resembled that of Na+ and showed a strong positive correlation (p < 0.001). Since Na+ is considered to be the most conservative ionic proxy for sea spray in coastal Antarctica, 17,18 the strong correlation suggests that sea spray may have contributed to the organic carbon load in PEL. Although Na+ in Antarctic snow is predominantly from marine source, it may also have a continental dust source.19 Therefore, in order to evaluate the sea-spray contribution of Na+, we estimated the sea-salt Na+ (ssNa+) fraction in the samples as in R€othlisberger et al.19 using the following two equations by assuming the mean Ca2+/Na+ ratio for marine aerosols (Rm) to be 0.038 and for the average crust (Rt) to be 1.78.20 nssCa2þ ¼ Ca2þ ðRm ssNaþ Þ ssNaþ ¼ Naþ ðnssCa2þ =Rt Þ
where nssCa2+ is the non-sea-salt Ca2+ and ssNa+ is the sea-salt Na+, while Ca2+ and Na+ are the measured concentrations of these ions in snow. The ssNa+ fraction of the total Na+ was found to be >85% for most sites, indicating that a substantial portion of the Na+ in these snow samples was derived from sea-spray. Parts a and b of Figure 2 show a plot of TOC and ssNa+ concentrations, respectively in the PEL transect versus distance from the coast. The strong correlation obtained between TOC and ssNa+ suggests that TOC probably has a source similar to that of Na+. Previous studies have shown that 77% of the submicrometer-sized fraction of marine aerosols is composed of organic carbon, largely comprising microorganisms, small water insoluble particles, exopolymeric material, and phytoplankton exudates.21,22 Similarly, a study by Calace et al.11 on humic acids in Antarctic snow has shown the importance of marine aerosols in the transport of organic matter in Antarctic snow. The elevated TOC values in the coastal region of the Princess Elizabeth Land transect indicate that proximity of the sea has an important influence on TOC concentrations in snow. TOC in the PEL transect showed a statistically significant inverse relationship both with distance from the sea (r = 0.39, n = 38) and altitude (r = 0.49) (Figure 3). Earlier studies have shown that distance from the coast is one of the most important factors influencing the spatial distribution of sea-salt ions and components originating from marine biogenic activity.18 While TOC concentrations decreased with increasing distance from the coast, with significantly lower values (mean 182 μg L1) in the inland sites compared to the coastal sites (mean 354 μg L1), beyond 20 km inland the values remained fairly constant. This is unlike ssNa+ concentrations, which decreased systematically beyond a distance of 20 km. Similarly, when plotted as a function 9947
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Figure 3. Correlation of total organic carbon with (a) distance from the coast and (b) elevation in the Princess Elizabeth Land transect.
of altitude (Figure 3b), TOC concentrations decreased with increasing elevation from near sea level to about 500 m asl and then remained fairly constant up to 2210 m asl in the PEL transect. Comparing the trends of TOC and ssNa+, it appears that, in the inland sites, the reduced organic carbon input from sea-spray is partially counterbalanced by the presence of alternative sources. It is also possible that organic carbon may be transported to longer distances from the coast than ssNa+, as organic carbon in marine aerosols is mainly concentrated in the submicrometer size fraction and have higher mobility and life span.11,23,24 Jaenicke25 calculated the tropospheric residence time of aerosols as a function of size and showed that aerosol particles that had the longest residence time were those ranging in size from a few tenths of a micrometer to a few micrometers. Aerosol particles in this size range remain airborne for more than 1 week, traveling long distances. In contrast, sea-salt ions such as Na+ are concentrated primarily in the coarser fraction23,24 and therefore get transported shorter distances. In comparison, the snow samples from the Dronning Maud Land transect showed fairly constant TOC values throughout the transect (Figure 2e) and no trends were seen with ssNa+, distance from the coast, and altitude. This may be due to the fact that the sampling sites in the DML transect were located in the interior region at a distance >110 km from the coast and an elevation >610 m asl. As seen in the PEL transect, the influence of sea-spray is diminished at this distance and elevation and no significant relation exists between TOC and these two geographical parameters. Although organic carbon concentrations in snow appear to be dominated by marine influence, especially in the coastal areas, as suggested earlier, the possibility of additional sources of organic carbon cannot be ruled out. Organic carbon concentrations in snow may also be influenced by local biogenic sources, such as the indigenous microorganisms inhabiting snow. Snow harbors diverse kinds of microorganisms, such as bacteria, fungi, algae, diatoms, and viruses.26 Microscopic examination of snow samples revealed the presence of bacteria and a diverse consortium of microalgae, which
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Figure 4. SEM images showing the microbial diversity in snow: (a, b) diatoms, (cg) pico-like single-celled microalgae (note the exopolymer secretion around the cells in g, indicated by the arrow), (h) unidentified microbe, (i) cyano-like filamentous microbe, (j) dividing bacteria, and (k) short rod-shaped bacteria.
included the pico- and nanoplankton (Figure 4). Total cell numbers (bacteria and picoplankton) ranged from 9.43 103 to 9.27 104 cells mL1 in the PEL transect and from 1.29 104 to 6.43 104 cells mL1 in the DML transect. About 55% of the samples in the PEL transect comprised picoplankton with concentrations ranging from 3.93 102 to 5.5 104 cells mL1 and forming up to 71% of the total cells. In comparison, 30% of the samples in DML harbored picoplankton with density ranging from 2.95 102 to 1.32 104 cells mL1 and constituting up to 74% of the total cells. Estimation of the bacterial carbon in Antarctic ice sheets has revealed that bacterial carbon represents a substantial reservoir of organic matter.7 In order to determine the contribution of bacterial and picoplankton cells to the total organic load in snow, cell abundance was converted to bacterial and picoplankton carbon assuming a carbon content of 116 and 20 fg carbon cel1,27 respectively. Both bacteria and picoplankton together accounted for 1041351 ng carbon L1 (mean 365 ng L1) in the PEL samples (Figure 2c), while along the DML transect they accounted for 142707 ng carbon L1 (mean 320 ng L1) (Figure 2g). Thus, despite the differences in regional geography and proximity to the ocean, the contribution of microbial cell carbon to the total carbon was closely similar in snow from the PEL and DML transects. However, the fraction of microbial cell carbon may be an underestimate, as the contribution of nanoplankton-derived carbon to the total carbon was not determined. Although the cellular carbon pool accounted for only a small fraction of the TOC, these values when used to compute microbial cell carbon for the Antarctic ice sheets (3.01 107 km3) equal to about 1.1 1013 g C. This estimated cellular carbon pool is about 1 order higher than that of all surface fresh waters combined28 and is higher than that reported for ice sheets by Priscu et al.6 The higher estimate obtained from this study may be due to the fact that (1) microbial abundance data in this study that was used to compute cellular carbon in the ice sheets were higher than that used by Priscu et al.6 (2) Here, cellular 9948
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Environmental Science & Technology carbon estimates include both bacterial and picoplankton-derived carbon, while only bacterial carbon was accounted for by Priscu et al.6 Nevertheless, these crude estimates reflect the immense pool of cellular carbon held in the Antarctic snow and ice sheets. In addition to the microbial cell carbon, the byproduct of their metabolic activity could also be a significant source of carbon in snow. Previous studies have shown that microorganisms are metabolically active at subzero temperatures.2931 While microbial activity can alter and degrade dissolved organic matter, it can simultaneously release newly synthesized dissolved organic species.32 SEM examination showed that the microalgae in some of the samples produce exopolymeric material (Figure 4g). Exopolymers are primarily composed of polysaccharides, amino acids, amino sugars, glycoproteins, etc. and are important sources of autochthonous organic compounds.33 The photo-autotrophic pico- and nanoplankton can fix atmospheric CO2 into organic matter, thereby adding carbon to glacial systems. Measurements of in situ microbial primary production and community respiration have suggested that glaciers are largely autotrophic systems.34,35 Microbially derived fulvic acids detected in supraglacial samples have been attributed to primary productivity of algae and bacteria on the glacial surface.36,37 Also, molecular level characterization of dissolved organic matter in Antarctic and Greenland glacial ice has revealed that they are predominantly composed of compounds originating from in situ microbial processes.38,39 Thus, the photosynthetic and metabolic activity of these microorganisms together with the release of microbial exudates and exopolymeric substances could add to the total organic carbon content in the snow samples. Organic carbon and microbial cells in Antarctic snow could also have an allochthonous source through wind-borne mineral particles deposited on the snow surface.40,41 These dust particles are mainly of crustal origin coming from areas of locally exposed bed-rock or transported long distance from the continents. In order to elucidate the possibility of dust-borne organic matter deposition in snow, we studied the relation between TOC and dust particles of different size fractions from 1 to 10 μm. Total dust concentrations ranged from 141 to 3500 μg L1 in the PEL transect (Figure 2d) and from 427 to 2018 μg L1 in the DML transect (Figure 2h). The majority (>70%) of particles were concentrated in the 1 μm size fraction. TOC did not show any relation with the total dust concentration or with dust concentration in any of the size fractions studied. The present study thus suggests that the crustal contribution to organic carbon load in the Antarctic snow was not significant. Our study underscores the need for further investigations on the microbial population and activity in snow in addition to studies on snow chemistry, molecular-level characterization of organic carbon, and specific environmental factors in order to better understand the sources, transport, and distribution of organic matter in snow and its role in global carbon dynamics.
’ ASSOCIATED CONTENT
bS
Supporting Information. Method used to determine the instrument blank, and details including the sample locations, elevation of the sampling sites, measured concentrations of TOC, cellular carbon, ssNa+, and dust in the Princess Elizabeth Land and Dronning Maud Land transect are reported in Tables S1 and S2, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Phone: +91 832 2525635; fax: +91 832 2520877; e-mail: runa@ ncaor.org.
’ ACKNOWLEDGMENT The authors thank the Ministry of Earth Science and Director, NCAOR for support. Many thanks to Dr. Rahul Mohan for extending the facility for SEM and to Sahina Gazi for the analysis. Thanks are also due to Sunaina Wadekar for Coulter Counter measurements, Trupti S. Naik for ion chromatography measurements, and Ashlesha Saxena for help with ArcGis. The authors are grateful to Prof. P. Buford Price and Dr. C. T. Achuthankutty for reviewing the manuscript and improving the contents. Abstract photo credit K. Mahalinganathan. This is NCAOR publication no. 34/2011. ’ REFERENCES (1) Dibb, J. E.; Talbot, R. W.; Munger, J. W.; Jacob, D. J.; Fan, S.-M. Airsnow exchange of HNO3 and NOy at Summit, Greenland. J. Geophys. Res. 1998, 103 (D3), 3475–3486. (2) Jones, A. E.; Weller, R.; Anderson, P. S.; Jacobi, H.-W.; Wolff, E. W.; Schrems, O.; Miller, H. Measurements of NOx emissions from the Antarctic snowpack. Geophys. Res. Lett. 2001, 28 (8), 1499–1502. (3) Domine, F.; Shepson, P. B. Airsnow interactions and atmospheric chemistry. Science 2002, 297 (5586), 1506–1510. (4) Jacobi, H.-W.; Frey, M. M.; Hutterli, M. A.; Bales, R. C.; Schrems, O.; Cullen, N. J.; Steffen, K.; Koehler, C. Measurements of hydrogen peroxide and formaldehyde exchange between the atmosphere and surface snow at Summit, Greenland. Atmos. Environ. 2002, 36, 2619–2628. (5) Grannas, A. M; Jones, A. E; Dibb, J.; Ammann, M.; Anastasio, C.; Beine, H.; Bergin, M.; Bottenheim, J.; Boxe, C. S.; Carver, G.; Crawford, J. H.; Domine, F.; Frey, M. M.; Guzman, M. I.; Heard, D.; Helmig, D.; Hoffmann, M. R.; Honrath, R. E.; Huey, L. G.; Jacobi, H.-W.; Klan, P.; McConnell, J.; Sander, R.; Savarino, J.; Shepson, P. B.; Simpson, W. R.; Sodeau, J.; von Glasgow, R.; Weller, R.; Wolff, E.; Zhu, T. An overview of snow photochemistry: Evidence, mechanisms and impacts. Atmos. Chem. Phys. 2007, 7, 4329–4373. (6) Priscu, J. C.; et al. Antarctic subglacial water: Origin, evolution, and ecology. In Polar Lakes and Rivers; Vincent, W. F., Laybourn-Parry, J., Eds.; Oxford University Press: UK, 2008; pp 119. (7) Priscu, J. C.; Christner, B. C. Earth’s icy biosphere. In Microbial Diversity and Bioprospecting; Bull, A. T., Ed.; American Society for Microbiology Press: Washington, DC, 2004; pp 130. (8) Sumner, A. L.; Shepson, P. B. Snowpack production of formaldehyde and its effect on the Arctic troposphere. Nature 1999, 398, 230–233. (9) Guimbaud, C.; Grannas, A. M.; Shepson, P. B.; Fuentes, J. D.; Boudries, H.; Bottenheim, J. W.; Domine, F.; Houdier, S.; Perrier, S.; Biesenthal, T. B.; Splawn, B. G. Snowpack processing of acetaldehyde and acetone in the Arctic atmospheric boundary layer. Atmos. Environ. 2002, 36, 2743–2752. (10) Grannas, A. M.; Shepson, P. B.; Filley, T. R. Photochemistry and nature of organic matter in Arctic and Antarctic snow. Global Biogeochem. Cycles 2004, 18, GB1006. (11) Calace, N.; Cantafora, E.; Mirante, S.; Petronio, B. M.; Pietroletti, M. Transport and modification of humic substances present in Antarctic snow and ancient ice. J. Environ. Monit. 2005, 7, 1320–1325. (12) Lyons, W. B.; Welch, K. A.; Doggett, J. K. Organic carbon in Antarctic snow. Geophys. Res. Lett. 2007, 34, L02501. (13) Federer, U.; Kaufmann, P. R.; Hutterli, M.; Sch€upbach, S.; Stocker, T. F. Continuous flow analysis of total organic carbon in polar ice cores. Environ. Sci. Technol. 2008, 42, 8039–8043. (14) Thamban, M.; Laluraj, C. M.; Mahalinganathan, K.; Redkar, B. L.; Naik, S. S.; Shrivastava, P. K. Glaciochemistry of surface snow from 9949
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