B51C-0597
Winter CO2 and CH4 fluxes along a trans-Alaska pipeline during three winters
This research was carried out to estimate the winter fluxes of CO2 and CH4 using the static chamber and the concentration profile methods along a trans-Alaska pipeline during three winters of 2004/5, 2005/6, and 2006/7. The 3-year averaged winter fluxes of CO2 and CH4 in taiga and tundra were 0.60±0.03 (SE; standard error) and 0.39±0.04 gCO2-C/m2/d, and 2.62±0.31 and 2.60±0.27 mgCH4-C/m2/d, respectively. This suggests that CH4 was emitted through the snowpack to the atmosphere along a latitudinal transect during the winter. Winter CH4 emission may be occurred through vascular plants such as Sedges on the tussock. The annually averaged snow depth and SWE (snow water equivalent) as well as winter emissions of CO2 and CH4 were remarkably decreased in taiga temporally. On the other hand, although the snow depth and SWE were decreased in tundra, winter emissions of CO2 and CH4 were increased with yearly. The 3-year averaged wintertime emissions of CO2 and CH4 were 115±6.4 gCO2-C/m2/season and 0.59±0.06 gCH4- C/m2/season along a trans-Alaska pipeline during three winters. This implies that winter emissions of CO2 and CH4 are an important part of the annual carbon budget in seasonally snow-covered terrain of typical boreal forest and tundra soils.
B51C-0598
The Relationship of Carbon Dioxide Flux, Methane Flux, Hyperspectral Reflectance Properties, Leaf Area Index (LAI), and Albedo Over Multiple Land Cover Types in Beringia
The Beringia project is based on a novel rapid assessment technique to quantify decadal scale change in radiative forcing potential in the Beringia region. During the 2005-2007 summer seasons, 16 sites throughout the Beringia region were visited. Sites locations included Chukotka, Wrangel Island, the North Slope of Alaska, and the Seward Peninsula. Study plots of different land cover types were chosen for the purpose of determining their radiative forcing potential based on trace gas exchange (CO2, CH4) potential and albedo. This report examines the relationship of trace gas exchange (CO2 and CH4), hyperspectral reflectance, soil moisture, species cover, leaf area index (LAI), active layer depth, and above-ground plant biomass for different land cover types. The data presented are analyses from three years of data collection.
B51C-0599
The Effects of Ice Shelf Break-up on Changes in Particulate Carbon Distribution and Composition, Examples from the Larsen System, Antarctica
A conceptual model of particulate carbon transport beneath ice shelves greatly expands our understanding of benthic ecosystems and the sedimentologic history of ice shelf systems. Such research is a major step to deciphering the carbon cycle across 1.5 million km2 of mostly unexplored seafloor and water column. We can most effectively study the problem by sampling beneath the former extent of recently collapsed shelves, such as the Larsen system. It is important to evaluate how biogeochemical processes such as those that govern particulate carbon transport in southern polar regions are affected by the loss of permanent ice cover. Thus, we investigate how open marine and ice-covered benthic environments vary in terms of the amount, source, and isotopic composition of total organic carbon (TOC) at the surface and at depth in the Larsen System. We evaluate stable isotopic carbon and the content of TOC from 30 surface samples and one Kasten core collected in water depths of 446-1181m from sites formally located beneath the Larsen A & B Ice Shelves in the NW Weddell Sea. We evaluated the content of TOC within the surface samples with C/N ratios and δ13C values. Sources of particulate TOC may be related to advection from outside the ice shelf system, hemipelagic settling from glacier grounding lines or undermelt of basal debris, or in situ production via chemoautotrophy. δ13C variations among surface and downcore samples (formerly beneath the Larsen Ice Shelf) show remarkable uniformity. The mean value for δ13C is -23.86‰ with a standard deviation of 0.27‰ and a range of 1.41‰ across an area of 7*103 km2. Such uniformity suggests that a similar provenance of particulate carbon transport may exist over time and space, despite differences in sediment source and accumulation beneath the ice shelf. δ13C variations among surface samples located in open marine environments show a similar mean δ13C value (-23.53‰), but the standard deviation (0.76‰) and the range (2.32‰) show that the values are not as uniform as beneath the ice shelf. Mean TOC within all surface sediment samples is 0.41% with a standard deviation of 0.14%; the average C/N ratio of the samples is within the bounds of the Redfield Ratio with a standard deviation of 3.21. Our preliminary δ13C and TOC data suggest dynamic changes in particulate carbon transport due to the removal of permanent ice cover via ice shelf break-up along the Antarctic Peninsula.
B51C-0600
Mercury biomagnification in polar bears ( Ursus maritimus)
Mercury biomagnification occurs in a variety of ecosystems resulting in greater potential for toxicological effects in higher-level trophic feeders. However, Hg transport pathways through different foodweb channels are not well known, particularly in high-latitude systems affected by atmospheric Hg deposition associated with snow and ice. Stable carbon and nitrogen isotope ratios and Hg concentrations determined for 26 late 19th and early 20th century polar bear hair specimens collected from cataloged museum collections elucidate relationships between high latitude marine foodweb structure and Hg transport pathways. Nitrogen and carbon isotopic compositions suggest that polar bears derive nutrition from both open water (pelagic) and ice associated (sympagic) foodweb channels. Correlation between Hg concentrations and nitrogen isotope compositions indicate mercury biomagnification occurred in most of the polar bears investigated. Interpretation of stable isotope based foodweb structure in concert with Hg concentrations further suggests that Hg biomagnification occurred to a greater degree in polar bears participating in pelagic foodweb channels.
B51C-0601
Carbon Biogeochemistry in the Western Arctic Ocean and Inferences on an Ice Free Arctic Ocean
The Chukchi shelf is the site of some of the highest rates of primary production in the global ocean. Each spring, seasonal ice cover retreats and exposes nutrient-rich waters to near continuous sunlight stimulating an intense biological bloom. In 2002, rates of net community production over the shelf were as high as 2,000 mg C m-2 d-1 near Barrow Canyon, and averaged 800 mg C m-2 d-1 across the entire shelf. During this time, concentrations of DOC and POC increased from in situ production and terrestrial fluxes. Using conservative tracers to construct a carbon mass balance, we found that 10 % of the DIC consumed during net community production was converted to DOC and 15 % was converted to suspended POC. The remaining 75 % was exported from the mixed layer as sinking organic particles. At the termination of the bloom, nitrate concentrations had been reduced to near zero and most of the organic matter had been exported from the mixed layer, leaving surface waters undersaturated with respect to atmospheric CO2. Presently, these shelf surface waters are transported offshore beneath the permanent ice cover before any significant re-equilibration with atmospheric CO2 can occur. As such, the biological pump on the Chukchi shelf plays an important role in conditioning the highly oligotrophic waters of the Canada Basin. If the forecasted reduction in Arctic sea ice occurs then this basin will initially act as strong sink for atmospheric CO2 by allowing the re-equilibration. However, the depletion of nutrients from shelf processes will continue to limit any significant biological activity prohibiting the deep central Arctic Ocean from being a long term sink of that CO2.
B51C-0602
Changes in Plant Communities in Northern Alaska Under Scenarios of Climate Change 2003 to 2100: Implications for Climate Feedbacks
Assessing potential future changes in Arctic and boreal plant species productivity, ecosystem composition, and canopy complexity is essential for understanding environmental responses under expected altered forcing. We examined potential changes in the dominant plant functional types of the sedge tundra, shrub tundra, and boreal forest ecosystems in ecotonal northern Alaska for the years 2003 - 2100. We compared the energy feedbacks associated with increases in biomass to the energy feedbacks from changes in the duration of the snow free season. We based our simulations on nine input climate scenarios from the IPCC-SRES storylines, and a new version of a biogeochemistry model, the Terrestrial Ecosystem Model (TEM), that incorporates vegetation dynamics for multiple plant functional types (e.g., trees, shrubs, grasses, sedges, mosses), multiple vegetation pools, and soil thermal regimes. We found mean increases in net primary productivity (NPP) in all plant functional types, but with the Betula spp. in the shrub tundra showing increases that were at least three times larger than that seen by any other plant functional type. Increases in NPP were positively related to increases in growing season length for all of the plant functional types in the sedge tundra, but the response of NPP to growing season length was different for the plant functional types in the shrub tundra and boreal forest. While NPP increased, heterotrophic respiration (Rh) also increased, resulting in decreases or no change in net ecosystem carbon uptake. Greater aboveground biomass from increased NPP in all the vegetation types produced a decrease in summer albedo, greater regional heat absorption (0.34 ± 0.23 W m2 decade-1), and a positive climate feedback. However, the decrease in albedo due to a shorter snow season (-5.1 ± 1.6 days decade-1) resulted in much greater regional heat absorption (3.3 ± 1.24 W m2 decade-1) than that associated with increases in vegetation. Through quantifying feedbacks associated with changes in vegetation and those associated with changes in the snow season, we can reach a more integrated understanding of the potential impact of climate change on the dominant Arctic and boreal ecosystems.
B51C-0603
DMS Sulphate and MSA as tracers of marine biogenic productivity in an Arctic snowpack
Sources, seasonal patterns, and spatial variations of snowpack sulphate of the Prince of Wales Icefield, Ellesmere Island were assessed using δ34S values along with major ion and MSA concentrations. Snowpack sulphate concentrations and δ34S values diminished with elevation and inland distance suggesting that the proximal North Water Polynya was the major sulphate source with additional sources affecting specific regions. Snowpack δ34S values in depth profile reflected seasonal cycles of marine biogenic, anthropogenic, and sea-salt aerosols with appreciable contributions of marine biogenic sulphate to virtually every depth horizon. A proximal April/May marine biogenic source was indicated, with DMS oxidation occurring near the marine boundary layer, and the possibility of additional oxidation at higher atmospheric levels. Depositional patterns of DMS sulphate and MSA indicate they were oxidized from DMS at different elevations in the atmosphere and that their deposition routes differed.
B51C-0604
Linking photochemical transformation of an Antarctica Fulvic Acid to diminished bioavailability and oxidation of organic electron shuttles
Photolysis of fulvic acid isolated from Pony Lake, Antarctica, a hypereutrophic coastal pond located on Ross Island, was evaluated for transformation kinetics and photo-bleaching mechanisms by spectroscopy, as well as changes in bioavailability. The fulvic acid fraction of Pony Lake was isolated by sorption to non-ionic XAD-8 resin, and represents the fraction of the dissolved organic matter considered to be the most photo-reactive fraction. Spectroscopic and electrochemical analysis during Pony Lake fulvic acid photolysis reveals three fundamental alterations to the natural organic matter isolate: decreased molar absorptivity, decreased fluorescence, and a loss of reduced organic functional groups (potential electron shuttles). Surprisingly we observed no carbon loss to mineralization. Evaluation of the light absorbance decay kinetics in the presence/absence of oxygen indicate that approximately 70% of photo-bleaching occurs via direct pathways and 30% is due to reaction with photochemically generated reactive oxygen species (ROS). Of the ROS mediated pathways approximately 70% of the reactivity is attributable to hydroxyl-radical oxidation. Molecular level changes in fulvic acid showed a loss of electron-rich (reduced) components during photolysis, specifically redox active N/S functional groups. Reduced forms of organic nitrogen (amines) decrease in concentration, while sulfur moieties (thiols) are essentially eliminated during photolysis. Furthermore, as the suite of reduced fulvic acid components are photochemically oxidized, we observe a concomitant production of hydrogen peroxide, presumably due to the photo-reduction of dissolved oxygen coupled to organic matter oxidation. Decay kinetics of fluorescent components identified in the fulvic acid isolate were evaluated using parallel factor component analysis (PARAFAC) of excitation-emission matrices (EEMs), and further illustrate a loss of overall fluorescence and a decrease in the redox-active (electron- shuttling) components of the fulvic acid. These molecular-scale changes in photo-chemically altered fulvic acid illustrate a general trend towards broad-scale oxidation of chromophores, fluorophores, and reduced organic N/S functionalities. The loss of electron-donating capacity was correlated to decreases in relative concentrations of redox-active moieties in microbially-derived fulvic acid and suggests that photolysis attacks the electron shuttling properties of the fulvic acid. Photochemical oxidation of electron-rich functional groups and chemical degradation of organic electron shuttles may be related to the decreased bioavailability of photolyzed fulvic acid.
B51C-0605
Experimental Increases in Snow Alter Physical, Chemical and Feedback Processes in the High Arctic.
Winter climate conditions are changing throughout the Arctic. In Greenland, there are observed increases in snowfall across portions of the island while the margins of the Greenland Ice Sheet are thinning. However, these changes and the consequences of altered meteorological surface dynamics on High Arctic terrestrial ecosystems and their potential feedbacks are unclear. Increases in winter snow cover may cause warmer soils in winter, greater rates of winter C losses, increases in winter N mineralization, shorter growing seasons and reduced net C gain in summer due to either reduced gross photosynthesis or increases in ecosystem respiration. In our study, we have constructed replicated snow fences in prostrate dwarf shrub tundra (polar desert and semi- desert) ecosystems in NW Greenland. Our measurements were taken at the deep (1.0 m snow depth) and intermediate (0.35 m snow depth) points along the drift to address these questions: a) how do increases in snow depth alter the surface and subsurface physical and chemical processes of these ecosystems?, and b) to what extent do increases in snow depth alter net CO2 exchange, gross ecosystem photosynthesis and ecosystem respiration? After three years of treatment we have found that in winter, deep snow results in warmer soil temperatures and in the subsequent summer, areas with deep winter snow have colder soil temperatures. This effect is most pronounced immediately following snowmelt and temperatures slowly return to ambient conditions near the end of summer. Deeper snow results in higher soil water contents in early summer, but by mid-July soil water contents have returned to ambient levels. Net ecosystem CO2 exchange rates are consistently negative (CO2 source to the atmosphere) through most of the growing season and vary in their magnitude by snow depth and ecosystem type. Areas with the deepest snow during winter consistently have the largest rates of CO2 loss to the atmosphere. The middle snow depth treatment showed lower rates of respiration than the deep treatment in both ecosystem types and greater photosynthetic gains at the semi-desert site. Our study indicates that surface processes in the High Arctic are sensitive to winter snow depth and that the resultant changes in physical, chemical and biological processes alter the magnitude and patterns of feedbacks between High Arctic landscapes and the arctic atmosphere.
B51C-0606
Hydrologic regulation of methyl chloride and methyl bromide fluxes from Alaskan Arctic tundra
Arctic tundra may be a significant regional sink for CH3Cl and CH3Br, although the magnitude of this sink is poorly constrained because of the limited extent of field measurements. We sought to close this gap in knowledge by comparing gross and net fluxes of CH3Cl and CH3Br in the northern coastal plain and the continental interior. Net and gross fluxes were deconvoluted using stable isotope tracers. Net flux measurements indicated that both regions were net atmospheric sinks for CH3Cl and CH3Br, averaging -590 ± 107 nmol CH3Cl m-2 d-1 and -11.3 ± 2.6 nmol CH3Br m-2 d- 1. Gross uptake rates averaged -793 ± 128 nmol CH3Cl m-2 d-1 and -20.3 ± 2.9 nmol CH3Br m-2 d-1. Hydrology strongly influenced CH3Cl and CH3Br uptake, which varied as a function of hydrologic regime, soil volumetric water content and water table depth. The overall trend showed increasing CH3Cl and CH3Br uptake with decreasing soil moisture. Laboratory incubations suggested that this inverse relationship was the result of mass transfer limitation in wetter soils, rather than because of reduced microbial consumption under anaerobic conditions. Water table depth was one of the best predictors of net and gross uptake, with uptake rates increasing proportionately with water table depth. This finding has potential regional significance as changes in water table depth may alter the Arctic tundra sink for CH3Cl and CH3Br. We also observed CH3Cl and CH3Br uptake under anaerobic conditions suggesting that freshwater anaerobes may be a previously unidentified sink for methyl halides.
B51C-0607
Differential Photosynthetic Responses of two Deciduous Shrub Species to Short and Long Term Snow Accumulation in the Arctic Tundra of Northern Alaska.
Shrub abundance is increasing in many ecosystems in the arctic. This change is likely to have large scale implications on carbon, water and energy balances both at local and global scales. A proposed mechanism underlying increased shrub abundance is a positive feedback where shrubs increase snow accumulation which insulates soils during the winter and increases nitrogen mineralization in these nutrient limited ecosystems. The deciduous shrub Betula nana has been implicated as the primary driver of changes in snow accumulation. We examined the leaf level photosynthetic rates of B. nana and another deciduous shrub species Salix pulchra after short (1 winter) and long-term (12 winters) snow accumulation treatments. Our results indicate differential photosynthetic responses of the two shrub species with B. nana showing a greater increase in photosynthesis in response to long term snow increases but not following 1 year of snow addition. Salix did not exhibit significant increases in leaf level photosynthesis in response to short or long term increases in snow depth. Higher leaf- level carbon fixation by Betula may partially explain it's gradual dominance under deeper snow conditions in winter and may be associated with greater N availability and higher leaf N.
B51C-0608
Effects of Deeper Snow on Ecosystem CO2 Fluxes in Tussock Tundra in Northern Alaska
Climate warming in northern latitudes is projected to lead to increased precipitation and, therefore, increased snow depth in arctic ecosystems. Deeper snow in winter can affect these ecosystems by leading to warmer soils, shorter growing seasons, and increased water availability to plants after snow melt, with subsequent implications to ecosystem biogeochemical cycling. Increases in snow depth in the arctic have been reported to lead to changes in plant community composition and, particularly, increases in shrub abundance in tundra ecosystems. Because of the confounded effects of greater snow depth on arctic ecosystems, it is unclear what the mechanisms underlying observed shrub cover changes are. In this study, we investigated the differential effects of greater snow depth on ecosystem CO2 exchange in tussock tundra near Toolik Lake, Alaska. We measured CO2 uptake and ecosystem respiration throughout the growing season after the first winter of snow depth manipulation and compared these results to CO2 exchange in plots under long-term (14 years) snow depth increase. Our initial findings indicate that long-term increases in snow result in greater relative increases in respiration as opposed to ecosystem photosynthesis, leading to greater net CO2 efflux. However, in the first year of snow additions, these differences were not observed, nor were there any differential effects of winter warming only, or winter warming with added water on CO2 exchange.
B51C-0609
Methane emission rates from the Arctic coastal tundra at Barrow: temporal and spatial variability and response to an experimental carbon addition
We characterized spatial and temporal variability in methane emission rates from the Arctic coastal tundra within the Barrow Environmental Observatory during July 2007. Our chamber-based survey complements ongoing tower-based measurements on the site, and provides baseline data for future upscaling efforts. By taking a Los Gatos methane analyzer into the field, we were able to: 1) conduct an exceptionally large number of chamber measures over a short period of time, 2) gather high precision temporal data in chamber methane concentrations, and 3) follow-up on unusual field observations. Our survey revealed that methane emission rates were most strongly correlated with soil moisture levels, such that dry sites were only weakly emitting while sites with standing water emitted an average 60 mg CH4-C m-2 d-1. On these wettest sites, methane emission rates were most strongly correlated with Carex aquatilis density, such that changes in density from 0 to 20% coverage led to increase in emission rates from 50 to 75 mg CH4-C m-2 d-1. To evaluate the importance of carbon limitation for methane emission, we conducted an acetate addition experiment to inundated (water table > soil surface) and wet (water table = soil surface) soils. Surprisingly, injections of acetate, equivalent to 1 day of methane flux, did not lead to any significant changes in methane emission rates over 72 hours, indicating a lack of C limitation. We found that sites differed little in day-to-day methane emission rates over the course of two weeks, with average 16% CV. However, one point on dry tundra emitted >1000 mg CH4-C m-2 d-1, with 2 orders of magnitude variation over 10 days, and 3 orders of magnitude range of flux rates within a single square meter.
B51C-0610
Sensitivity of the Polar Terrestrial Carbon Cycle to Climate Change
The polar and sub-polar regions represent a major hotspot in the terrestrial carbon (C) cycle, besides the Tropics. The net C fluxes are predominantly controlled by heterotrophic respiration, which is dependent both on temperature and soil moisture. We introduce new simulations by the dynamical vegetation model LPJ and the ocean C model HAMOCC coupled to the atmosphere-ocean general circulation model BCM, developed at the Bjerknes Centre for Climate Research. We focus in these global simulations on the sensitivity and biogeochemical and biogeophysical feedbacks between the land-surface and the atmosphere at high latitudes. The first set of experiments analyse the sensitivity of the terrestrial C cycle to precipitation, temperature and atmospheric CO2 concentrations in offline simulations. Besides the CO2 fertilisation effect on the net primary productivity, we quantify increase of heterotrophic respiration with higher precipitation and temperature. Furthermore, the pronounced temperature increase (5 K) introduces a threshold in vegetation dynamics: higher temperature impedes the regeneration of the boreal plant functional types (PFT), while it is too cold to allow the establishment of temperate PFT. As a consequence, the vegetation biomass decreases in the Arctic and sub- Artic. These conclusions appear to depend strongly on the atmospheric and land-surface model used. We compare the results of the BCM-driven with ECHAM-driven LPJ simulations. We also assess the role the dynamical vegetation by comparing these simulations to a BCM/ECHAM-driven carbon allocation scheme with prescribed vegetation maps (SLAVE). The second set of experiments assesses the biogeochemical and biogeophysical feedbacks between the boreal land-surface and atmosphere. The biogeochemical feedback is analysed by comparing simulations with prescribed atmospheric CO2 concentrations to prognostic CO2 concentrations from oceanic, terrestrial and anthropogenic CO2 emissions. The biogeophysical feedback is analysed through the changes of the surface energy and water budgets, induced by the vegetation dynamics and related changes in leaf area index, albedo and roughness length.
B51C-0611
Modeling transport, fate, and lifetime of riverine DOC in the Arctic Ocean
We use a numerical model to examine the fate of riverine fluxes of dissolved organic carbon (DOC) in the Arctic basin. The model is based on the Arctic sector of an eddy-permitting ocean model (MITgcm), where the spherical domain is projected onto a cube to avoid polar a singularity in the Arctic region. The physical model is forced by time-varying NCEP re-analysis products and an explicit representation of fresh water run-off in the Arctic region. Passive tracers and idealized "DOC" tracers are explicitly represented, transported by the prognostic circulation fields. The tracers have sources near the mouths of major Arctic river systems, distributed in space and time according to observational estimates of regional DOC discharge. The DOC-like tracers have a finite, tunable lifetime in the water column. Comparison of a suite of model sensitivity studies with observed tracer relationships from the region suggest that the model captures the main circulation features and source distributions appropriately. Tracer relationships in the model match those observed most closely when the lifetime of riverine source DOC is about 1 year. We will discuss the role of the riverine DOC sources in regulating regional air-sea carbon fluxes in the context of a more complete biogeochemical model.
B51C-0612
Novel Penta-Unsaturated Alkenones From Lake Fryxell, Antarctica
Novel methyl octatriaconta-pentaen-one (C38:5m), methyl and ethyl nonatriaconta-pentaen-one (C39:5m and C39:5e, respectively) and methyl tetradec-pentaen-one (C40:5m) are identified from chromatographic and mass spectrometric properties of a bottom sediment sample from perennially ice-covered Lake Fryxell in Antarctica. The pentaunsaturated alkenones comprise 15%, 28% and 12% of the total C38, C39 and C40 homologous series. These alkenone biomarkers also consist of tetra , tri-, and diunsaturated methyl and ethyl ketones from C37 to C40. Low salinity and extremely cold conditions year round may have strongly influenced the number of unsaturations in the biomarkers and chain length of the alkenones. As also suggested by accompanying cholesterol and alkene biomarkers, these alkenones are key biomarkers of haptophycean algae of the Class Prymnesiophyceae, but the specific species that biosynthesizes these alkenones in the lake are still unknown.