bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modelling the impacts of a nitrogen pollution event on the biogeochemistry of an Arctic glacier
Authors: Roberts Tjarda J. , Hodson Andy , Evans Chris D. , Holmén Kim 
Published: June 2011

AbstractA highly polluted rain event deposited ammonium and nitrate on Midtre Lovénbreen, Svalbard, European High Arctic, during the melt season in June 1999. Quasi-daily sampling of glacial runoff showed elevated ion concentrations of both ammonium (NH4+) and nitrate (NO3−), collectively dissolved inorganic nitrogen (DIN) in the two supraglacial meltwater flows, but only elevated NO3− in the subglacial outburst. Time-series analysis and flow-chemistry modelling showed that supra- and subglacial assimilation of NH4+ were major impacts of this deposition event. Supraglacial assimilation likely occurred while the pollution-event DIN resided within a/the supraglacial slush layer (estimated DIN half-life 40–50 hours, with the lifetime of NO3− exceeding that of NH4+ by 30%). Potentially, such processes could affect preservation of DIN in melt-influenced ice cores. Subglacial routing of event DIN and its multi-day storage beneath the glacier also enabled significant assimilation of NH4+ to occur here (60% of input), which may have been either released as particulate N later during the melt season, or stored until the following year. Our results complement existing mass-balance approaches to the study of glacial biogeochemistry, show how modelling can enable time-resolved interpretation of process dynamics within the biologically active melt season, and highlight the importance of episodic polluted precipitation events as DIN inputs to Arctic glacial ecosystems.
Organic matter content and quality in supraglacial debris across the ablation zone of the Greenland ice sheet
Authors: Stibal Marek , Lawson Emily C. , Lis Grzegorz P. , Mak Ka Man , Wadham Jemma L. , Anesio Alexandre M. 
Published: June 2011

AbstractQuantifying the biogeochemical cycling of carbon in glacial ecosystems is of great significance for regional, and potentially global, carbon flow estimations. The concentration and quality of organic carbon (OC) is an important indicator of biogeochemical and physical processes that prevail in an ice-sheet ecosystem. Here we determine the content and quality of OC in debris from the surface of the Greenland ice sheet (GrIS) using microscopic, chromatographic, spectrophotometric and high-temperature combustion techniques. The total OC content in the debris increased with distance from the edge of the ice sheet, from virtually zero to >6% dry weight at 50 km inland, and there was a peak in the carbohydrate proportion and the microbial abundance at ∼6km inland. The highest (galactose + mannose)/(arabinose + xylose) ratios, indicating maximum autochthonous microbial production, were found at >10km inland. We propose that three key processes influence the carbon cycling on the GrIS: aeolian input of microbial inoculum and nutrients, in situ biological C transformation and the wash-away of supraglacial debris by meltwaters. We show that all these processes have significant spatial variability. While the total OC content of the debris on the ice sheet is probably controlled by the physical processes of wind transport and wash-away by meltwater, the microbial abundance and the quantity of the labile cell-contained OC within the debris is likely to be driven by the balance between the wash-away and the microbial productivity.
Detecting biogeochemical activity in basal ice using fluorescence spectroscopy
Authors: Barker J.D. , Klassen J.L. , Sharp M.J. , Fitzsimons S.J. , Turner R.J. 
Published: June 2011

AbstractDissolved organic matter (DOM) is an important component of aquatic carbon and nutrient budgets and is a metabolic substrate for organisms at the base of aquatic food chains. Active microbial communities in glaciers affect the abundance and characteristics of organic matter (OM) that is exported to downstream ecosystems. However, how OM is biogeochemically altered in glaciers remains unknown and studies documenting active microbial activity by detecting in situ biogeochemical modifications of OM are lacking due to difficulties characterizing OM and the low concentrations of DOM typical of glacier environments. To address this issue, we measure the abundance and fluorescence characteristics of DOM in basal ice at Victoria Upper Glacier (VUG), McMurdo Dry Valleys, Antarctica. We compare these observations with the results of microbial incubations from the same basal ice samples to determine whether the occurrence of fluorophores indicative of recent microbial activity is linked to the presence of culturable microbial communities containing organisms that could have produced them. Psychrotolerant bacteria were isolated from basal ice samples and were associated with marine humic-like fluorescence. This is interpreted as being indicative of in situ microbial degradation of OM within basal ice at VUG. Marine humic-like material is a recalcitrant form of OM, and its biogeochemical transformation from a relatively labile form of OM in glacier ice may function as a carbon sink.
Basal ice microbiology at the margin of the Greenland ice sheet
Authors: Yde Jacob C. , Finster Kai W. , Raiswell Rob , Steffensen Jørgen P. , Heinemeier Jan , Olsen Jesper , Gunnlaugsson Haraldur P. , Nielsen Ole B. 
Published: June 2011

AbstractBasal ice at the margin of the Greenland ice sheet was studied with respect to its physical characteristics and microbiological community. The basal ice contained high concentrations of dissolved ferrous Fe and must therefore be anoxic. Oxygen consumption experiments indicate that 50% of the oxidation was due to biological activity while the rest could be attributed to chemical processes, most likely weathering reactions with ferrous Fe. At least six different Fe-containing mineral sources were detected in basal ice together with potential bioavailable Fe nanoparticles. An active denitrifier population was identified due to formation of 30N-dinitrogen gas after amendment of anoxic sediment slurries with 15N-NO3−. Sulfate reduction could not be detected. The solid ice facies contained an abundant (∼108 cells cm−3) and complex microbial community that harbored representatives of at least eight major phyla within the domain Bacteria. The clone library was dominated by members of the β-subdivision of proteobacteria of which the largest proportion was affiliated to the genus Rhodoferax that comprises facultative aerobic iron reducers. The second most abundant phylum was Bacteroidetes. The solid ice facies had many physical similarities with the overlying debris-rich banded ice facies, indicating that they formed by similar subglacial processes and harbor similar microbial communities. This study extends our knowledge of life in subglacial environments such as beneath ice sheets. GenBank accession numbers: HM439882-HM439950; HQ144215-HQ144221.
The cryoconite ecosystem on the Greenland ice sheet
Authors: Hodson Andy , Bøggild Carl , Hanna Edward , Huybrechts Phillipe , Langford Harry , Cameron Karen , Houldsworth Alexandra 
Published: June 2011

AbstractThis paper presents an assessment of biological activity associated with ice surface debris (cryoconite) at the ice-sheet scale. Estimates of the mass distribution of cryoconite over the Greenland ice sheet (GIS) and the biological activity associated with it are presented and then coupled with a surface mass-balance model to estimate total carbon fluxes due to respiration and photosynthesis. We find an average loading of 66gm−2 at Kangerlussuaq, southwest Greenland, which compares well with recent estimates from Kronprins Christians Land (17–440 gm−2: Bøggild and others, 2010) in northeast Greenland. We also report a significant microbial biomass in cryoconite at both these places (103–104 cells mg−1) and carbon fluxes of the order of 1–3 μM C g−1d−1 for both respiration and photosynthesis. The modelling indicates that total respiration and photosynthesis fluxes are likely to be ∼101–102 GgCa−1 and thus far from trivial. However, estimation of the net ecosystem impact across the entire ice sheet on atmospheric CO2 concentrations is problematic because photosynthesis rates were almost certainly low during our field campaign. Therefore, like its water balance, the carbon balance of the GIS is now known to be important, but its accurate quantification will remain elusive until more data are forthcoming.
Laser-induced fluorescence emission (LIFE) from Lake Fryxell (Antarctica) cryoconites
Authors: Sattler Birgit , Storrie-Lombardi Michael C. , Foreman Christine M. , Tilg Markus , Psenner Roland 
Published: June 2011

AbstractLaser-induced fluorescence emission (LIFE) images were obtained in situ from a 27 cm long ice core at Lake Fryxell, Antarctica. The excitation was accomplished with a simple 532 nm green laser pen light, and the fluorescence images were captured with a small compact digital camera. The targets for the experiment were mm-scale cryoconite assemblages found in the ice covers of this perennially frozen Antarctic lake. The fluorescence response originates from photo-pigments in cyanobacteria-dominated cryoconite assemblages with phycoerythrin (PE) exhibiting the optimal target cross section. This inexpensive, low-mass, low-energy method avoids manipulation of the in situ habitat and individual target organisms and does not disturb the microbial community or the surrounding ice matrix. We establish the correlation between fluorescence intensity and PE concentration. We show that cryoconite fluorescence response does not appear to decrease with depth in the ice cover, in agreement with similar findings at Lake Untersee, a perennially ice-covered lake in Dronning Maud Land, Antarctica. Optical reflection and refraction events at the air/ice interface can complicate quantitative estimates of total pigment concentrations. Laser targeting of a single mm-scale cryoconite can result in multiple neighboring excitation events secondary to reflection and refraction phenomena in the multiple air/ice interface of the bubbles surrounding the primary target.
Structure and formation process of cryoconite granules on Ürümqi glacier No. 1 , Tien Shan, China
Authors: Takeuchi Nozomu , Nishiyama Hiromu , Li Zhongqin 
Published: June 2011

AbstractCryoconite granules are dark-colored spherical aggregates of organic and inorganic material on glacier ice, and are commonly observed on glaciers the world over. The structure of cryoconite granules on Ürümqi glacier No. 1, Tien Shan, China, was analyzed. Granules were distributed over the entire ice surface of the ablation area, and ranged in size from 0.26 to 3.5 mm (mean 1.1 mm). The granule surface was densely covered with filamentous cyanobacteria. Microscopy of a thin section revealed various inner structures. Most granules had concentric layers of dense organic matter, which are probably derived from annual growth of the granules by the activity of cyanobacteria. The number of layers averaged 3.5 and ranged up to 7, which is likely to indicate their mean and maximum growth ages, respectively. Some granules contained two or more subgranules, showing that small granules had combined and enlarged. Such structures suggest that granule formation was mainly due to the activity of filamentous cyanobacteria, and that the granules repeatedly grew and disintegrated over a cycle of several years on the glacier.
The mass–area relationship within cryoconite holes and its implications for primary production
Authors: Cook J. , Hodson A. , Telling J. , Anesio A. , Irvine-Fynn T. , Bellas C. 
Published: June 2011

AbstractLinear relationships between the mass of sediment present in a cryoconite hole and the hole area are described for a range of glacier and ice-sheet surfaces. The strong relationships found indicate that some mechanism regulates the thickness of the layer of sediment occupying the ‘floor’ of the hole. We find that this regulation process responds immediately to the addition of new debris to a hole and infer that it is caused by lateral thermal conduction from the debris to the hole wall. This causes hole widening by melt, and a redistribution of the debris within then takes place, usually resulting in 0.04–0.20 gcm−2 of debris in a layer of single cryoconite grains. The thinning of the debris layer during hole widening also reduces self-shading and thus maximizes the exposure of cryoconite to solar radiation. We explore the implications of the hole widening for biological production. Net photosynthesis (CO2 fixation) is shown to be favoured by thin debris layers, whilst net heterotrophy (CO2 respiration) occurs when debris layers are in excess of 2–4 mm. We conclude that the carbon balance of cryoconite holes is sensitive to the thickness of the debris and that the thermodynamic equilibration of the debris thickness helps the ecosystem to maximize primary production during the summer.
Measuring rates of gross photosynthesis and net community production in cryoconite holes: a comparison of field methods
Authors: Telling J. , Anesio A.M. , Hawkings J. , Tranter M. , Wadham J.L. , Hodson A.J. , Irvine-Fynn T. , Yallop M.L. 
Published: June 2011

AbstractPhotosynthesis by microbes on the surfaces of glaciers and ice sheets has the potential to fix carbon, alter the albedo of ice surfaces via the production of organic matter and so enhance ice melt. It could also be important for supplying labile organic matter and nutrients to in situ and downstream ecosystems. This study compares in situ 24 hour incubation methods for measuring rates of gross photosynthesis, respiration and net community production (NCP) in cryoconite holes on three Svalbard valley glaciers. Rates of gross photosynthesis and respiration measured by the ΔCO2 method were closely balanced, resulting in rates of NCP close to the detection limit (mean of –1.3 μg C g−1 d–1) consistent with previous measurements in Arctic cryoconite holes. This suggests that organic matter within cryoconite holes may be derived largely from allochthonous sources. The molar ratio of ΔO2 to ΔCO2 in incubations gave mean respiratory and photosynthetic quotients of 0.80 ± 0.17 (1 × SD) and 1.24 ± 0.20 (1 × SD), respectively. The 14C method typically underestimated rates of gross photosynthesis (ΔCO2 method) by more than one order of magnitude and measured a rate closer to NCP.
In-stream uptake and retention of C, N and P in a supraglacial stream
Authors: Scott Durelle , Hood Eran , Nassry Michael 
Published: June 2011

AbstractSupraglacial streams form annually during the melt season, transporting dissolved solutes from the melting ice and snowpack to subglacial flow paths and the glacier terminus. Although nutrient and carbon processing has been documented in other supraglacial environments (cryoconite holes, snowpack), little work has examined the potential for in-stream nutrient retention in supraglacial streams. Here we carried out a solute nutrient injection experiment to quantify NH3+, PO43− and labile dissolved organic carbon (DOC) retention in a supraglacial stream. The experiment was performed on a 100 m stream reach on Mendenhall Glacier, an outlet glacier on the Juneau Icefield, southeastern Alaska, USA. The study stream contained two distinct reaches of equal length. The first reach had a lower velocity (0.04 ms−1) and contained abundant gravel sediment lining the ice–water interface, while the second reach was devoid of bedload sediment and had an order-of-magnitude higher velocity. At the end of the second reach, the stream emptied into a moulin, which is typical of supraglacial streams on this and other temperate glaciers. We found that N and P were transported largely conservatively, although NO3− increased along the reach, suggestive of nitrification. Labile DOC was retained slightly within the stream, although rates were low relative to the travel times observed within the supraglacial stream. Although our findings show that these streams have low processing rates, measurable in-stream nitrification and dissolved organic matter uptake within this biologically unfavorable environment suggests that supraglacial streams with longer residence times and abundant fine substrate have the potential to modify and retain nutrients during transport to the glacier terminus.
Recent hydrologic change in a Colorado alpine basin: an indicator of permafrost thaw?
Authors: Caine Nel 
Published: June 2011

AbstractHydrologic and hydrochemical studies have been conducted in Green Lakes Valley, Colorado Front Range, USA, above 3550 m since 1982. They show a classic seasonal hydrograph dominated by snowmelt and an earlier date for the start of spring flow and for peak flow over the period of record. This is consistent with patterns found at lower elevations in Colorado and throughout western North America. They also show an increasing trend in flows in September and October of 2.6 ±0.7 mm a−1 which is not found elsewhere and cannot be accounted for by increased autumn precipitation and the melting of surface ice. Because this late-season increase is not found at the highest elevations or in basins in which there is no evidence of permafrost, it seems best explained by the thawing of alpine permafrost at intermediate elevations. This is corroborated by an increase in the concentration of base cations and silica, and particularly in Ca2+ and SO4–, in the stream discharge starting in 2000. As with the physical hydrology, the geochemical signals have not been detected at the higher elevations in the basin, though they have previously been associated with streamflow from a small rock glacier in the valley. The combined evidence suggests the degradation of ice-rich permafrost on the north-facing slopes of the valley below 3700 m, where it has been detected at 3 m depth by geophysical surveys.
The microstructure and biogeochemistry of Arctic cryoconite granules
Authors: Langford Harry , Hodson Andy , Banwart Steve , Bøggild Carl 
Published: June 2011

AbstractA cryoconite granule is a biologically active aggregation of microorganisms, mineral particles and organic matter found on glacier surfaces, often within shallow pools or cryoconite holes. Observations of the microstructure of a range of cryoconite granules from locations in Svalbard and Greenland reveal their structure and composition. Whereas bulk analyses show that the mineralogy and geochemistry of these granules are broadly similar, analyses of their microstructure, using optical, epifluorescence and confocal microscopy, indicate differences in the location and quantity of photosynthetic microorganisms, heterotrophic bacteria and organic matter. Using these findings, a hypothesis on the aggregation of cryoconite is presented, centred upon multilevel aggregation by bioflocculation and filamentous binding.
Carbon fluxes through bacterial communities on glacier surfaces
Authors: Anesio Alexandre M. , Sattler Birgit , Foreman Christine , Telling Jon , Hodson Andy , Tranter Martyn , Psenner Roland 
Published: June 2011

AbstractThere is very little information about the activity of microbial communities on the surface of glaciers, though there is an increasing body of evidence to show that they strongly influence the biogeochemistry of these habitats. We measured bacterial abundance and production in cryoconite holes on Arctic, Antarctic and Alpine glaciers in order to estimate the role of heterotrophic bacteria within the carbon budget of glacial ecosystems. Our results demonstrate an active bacterial community on the surface of glaciers with doubling times that vary from a few hours to hundreds of days depending on the glacier and position (water or sediments) within the cryoconite hole. However, bacterial production is only ∼2–3% of the published literature values of community respiration from similar habitats, indicating that other types of microbes (e.g. eukaryotic organisms) may also play a role in the C cycle of glaciers. We estimate that only up to 7% of the organic C in cryoconite sediments is utilized by the heterotrophic bacterial community annually, suggesting that the surface of glaciers can accumulate organic carbon, and that this C may be important for biogeochemical activity downstream to adjacent ecosystems.
Prediction of Countershock Success in Patients Using the Autoregressive Spectral Estimation
Authors: Neurauter A. , Wieser L. , Wenzel V. , Abella B. , Myklebust H. , Steen P. A. , Strohmenger H.-U. , Nowak C. N. 
Published: June 2011

SummaryObjectives: Ventricular fibrillation (VF) is a life-threatening cardiac arrhythmia and within of minutes of its occurrence, optimal timing of countershock therapy is highly warranted to improve the chance of survival. This study was designed to investigate whether the autoregressive (AR) estimation technique was capable to reliably predict countershock success in VF cardiac arrest patients.Methods: ECG data of 1077 countershocks applied to 197 cardiac arrest patients with out-of-hospital and in-hospital cardiac arrest between March 2002 and July 2004 were retrospectively analyzed. The ECG from the 2.5 s interval of the precountershock VF ECG was used for computing the AR based features Spectral Pole Power (SPP) and Spectral Pole Power with Dominant Frequency weighing (SPPDF) and Centroid Frequency (CF) and Amplitude Spectrum Area (AMSA) based on Fast Fourier Transformation (FFT).Results: With ROC AUC values up to 84.1 % and diagnostic odds ratio up to 19.12 AR based features SPP and SPPDF have better prediction power than the FFT based features CF (80.5 %; 6.56) and AMSA (82.1 %; 8.79).Conclusions: AR estimation based features are promising alternatives to FFT based features for countershock outcome when analyzing human data.
Early Outcomes and the Virological Effect of Delayed Treatment Switching to Second-Line Therapy in An Antiretroviral Roll-Out Programme in South Africa
Authors: Levison Julie H , Orrell Catherine , Losina Elena , Lu Zhigang , Freedberg Kenneth A , Wood Robin 
Published: June 2011

Background More patients in resource-limited settings are starting second-line antiretroviral treatment (ART) following first-line ART failure. We aimed to describe predictors of lack of virological suppression in HIV-infected patients on second-line ART in a roll-out programme in South Africa. Methods A retrospective analysis was performed on an adult HIV treatment cohort who started second-line ART (lopinavir/ritonavir, didanosine and zidovudine) after virological failure of first-line ART (two consecutive HIV RNA>1,000 copies/ml). Predictors of week 24 lack of suppression (HIV RNA>400 copies/ml) on second-line ART were determined by bivariate analysis where missing equals failure. A multivariable model that adjusted for gender, age and time to ART switch was used. We tested these findings in sensitivity analyses defining lack of suppression at week 24 as HIV RNA>1,000 and >5,000 copies/ml. Results Of 6,339 patients on ART, 202 started second-line ART. At week 24, an estimated 41% (95% CI 34–47) did not achieve virological suppression. Female sex (adjusted OR 2.25, 95% CI 1.03–4.88) and time to ART switch (adjusted OR 1.07, 95% CI 1.01–1.14 for each additional month) increased the risk of lack of virological suppression. Age, CD4+ T-cell count and HIV RNA at second-line ART initiation did not predict this outcome. In multivariate models, these findings were insensitive to the definition of lack of virological suppression. Conclusions A substantial number of HIV-infected patients do not achieve virological suppression by week 24 of second-line ART. Women and patients with delayed start of second-line ART after first-line ART failure were at an increased risk of lack of virological suppression.