B42A-01 INVITED
The Modern Polar Antarctic Zone: Insight From the Stable Isotopes of Nitrate
We report nitrate N and O isotope measurements from the seasonally sea ice covered Polar Antarctic Zone (AZ). We estimate equivalent N and O isotope effects of 5 ‰ for nitrate assimilation in the upper water column. The amplitude of the N isotope effect is less than previously observed in the Subantarctic Zone (SAZ), > 7 ‰. The difference in isotope effects suggests the potential use of the expressed isotope effect of nitrate assimilation as an indicator of the physiological state of algae growing in the Southern Ocean. We previously hypothesized that the high SAZ isotope effect results from unbalanced growth due to light-limitation (DiFiore et al., 2006). The lower isotope effect in the AZ, which has shallower summer time mixed layer depths, is consistent with that hypothesis. The O isotopes of nitrate can be decoupled from the N isotopes during nitrification and therefore provide a tool for quantifying this additional source of mixed layer nitrate. Wankel et al. (2007) used this approach to identify a significant fraction (up to 30%) of nitrate that may result from mixed layer nitrification off Monterey Bay, California. Our data from the AZ indicates little decoupling of the dual isotopes of nitrate suggesting that nitrification is not a major source of nitrate in the AZ mixed layer, establishing a baseline to which other regions may be compared.
B42A-02 INVITED
Linking Nitrate Uptake and Water Storage in an Antarctic Stream
A nitrate enrichment experiment was performed in Huey Creek, a glacial meltwater stream in the McMurdo Dry Valleys of Antarctica, to determine processes responsible for nitrate loss in a polar desert stream with no visible vegetation. Streamflow in Huey follows a diel cycle, resulting in temporal and spatial variability in two separate storage areas – a near-stream and far-lateral hyporheic zone. Near-stream hyporheic exchange occurred in only one of four monitored stream reaches, with a mean uptake rate of 0.042 umol N/m2/hr. Uptake rates could not be balanced by nitrite, ammonium, and nitrous oxide production, suggesting the importance of biomass as a source and sink of nitrogen. During high flows, nitrate loss is accompanied by a pulse of ammonium that accounts for an average of 42% of the total nitrate loss. Ammonium production is 4.4 times greater than nitrate loss during one hour of the flood, providing further evidence that nitrogen has been stored in the subsurface biomass, and is mineralized as a result of the fresh water penetrating the hyporheic zone. Properties of the far-lateral hyporheic zone were also flow-dependent. Exchange from stream to subsurface occurred during floods, when anabranches moved water laterally across the channel banks. Exchange back to the stream occurred with the recession of flood stage. Pulses of nitrate species downstream of this storage zone suggest significant denitrification in this far-lateral hyporheic zone. The first flood recession plume consisted mainly of nitrite, while the second was dominated by ammonium, suggesting a greater amount of denitrification in the second pulse. Both pulses were accompanied by high quantities of DOC (121 and 287% of mean background mass, respectively) – an unexpected result in this carbon-limited system. These results highlight the linkage between water and desert ecosystems, and challenge researchers to understand both spatial and temporal variability in potential ecological hotspots and stream flow in order to correctly interpret biogeochemically important solute data.
B42A-03
Carbon Cycling in Soils of the McMurdo Dry Valleys, Antarctica
The Antarctic Dry Valleys are among the most extreme soil environments on earth. In 1903 R.F. Scott wrote of an apparent lack of life in the dry valleys, yet recent evidence reveals microbial and invertebrate abundances comparable to hot deserts. While Scott's impression has persisted far into modern times, an emerging view of dry valley biogeochemistry suggests a greater potential for biological activity and influence over element cycling than recognized in the early years of Antarctic exploration and research. For example, due to the lack of vascular plants in the dry valleys, soil organic matter was historically thought to be derived from allochthonous inputs of carbon, but recent evidence shows that soil organic carbon and nitrogen pools have multiple origins, including lacustrine, marine, endolithic and in situ sources derived from both ancient and contemporary processes. Here I present a synthesis of data describing dry valley soil organic matter pools and processes; including a model for soil carbon turnover based upon in situ mineralization rates and kinetic fractionation techniques. Rates of in situ carbon mineralization are among the lowest reported for terrestrial ecosystems, yet estimates of residence times for carbon are shorter than might be expected in an ecosystem so strongly influenced by landscape legacies carried over from the Last Glacial Maximum. Rate kinetics for carbon mineralization indicate that organic matter may cycle over multiple time scales consistent with both ancient and contemporary sources. This model suggests that a slow turnover pool of soil carbon is stable over century to millennial time scales, while a smaller carbon pool cycles more rapidly and responds to seasonal and inter- annual climate variability. This model demonstrates how multiple pool models of soil carbon can account for the apparent paradox of rapid carbon turnover in an ecosystem influenced by biogeochemical legacies.
B42A-04
The Presence and Export of Labile Dissolved Organic Matter from Glacier Systems
Glaciers are the source for many important rivers globally. Increased rates of glacier melt in response to climate warming result in an increased flux of glacially-derived organic material to downstream aquatic ecosystems. This is particularly true in polar regions where glaciers are most abundant and climate warming is most significant. Dissolved OM (DOM) is a significant factor in aquatic biogeochemical processes and its characteristics may exert a fundamental influence of downstream aquatic ecological processes. There have been few studies of DOM export from glacier systems and questions remain regarding the characteristics of glacially derived OM and its biogeochemical behavior in downstream aquatic ecosystems. We studied the characteristics of DOM in three glacier systems using fluorescence spectroscopy (in both synchronous and emission-excitation modes) and analyzed the spectra using principal components (PCA) and parallel factor analyses (PARAFAC). These systems have potentially different OM sources due to their locations. Outre Glacier is an alpine glacier in the Coast Mountains of British Columbia, Canada, John Evans Glacier is a valley glacier in arctic Canada, and Canada Glacier is a valley glacier in the McMurdo Dry Valleys, Antarctica. We collected supraglacial snow, glacier ice and proglacial meltwater and its resident DOM was characterized. All of the supraglacial snow and glacier ice samples exhibit DOM with a common and dominant fluorophore, which we tentatively attribute to a "tyrosine-like" component. The presence of this component does not persist in proglacial streams and is replaced by fluorophores which are indicative of possible "tryptophan-like" and humic components in the DOM. The export of the "tyrosine-like" component from glaciers and its disappearance in proglacial streams may be indicative of biogeochemical processing in proglacial streams and therefore may function as an important initial substrate for microbial metabolism in glacier-fed streams.
B42A-05
Stoichiometry of Carbon, Nitrogen, and Phosphorus Regeneration Interactions in the Hyporheic Zones of Arctic Streams Draining Areas of Continuous Permafrost
We used conservative tracer (Rhodamine WT) additions to examine flow paths in two arctic tundra streams with contrasting physical characteristics (high and low gradient, cobble and peat substrate). We installed mini- piezometers in the same streams to examine nutrient patterns longitudinally and with depth. The combination of the flow and nutrient data allowed us to estimate nutrient regeneration rates. In a separate study, we used whole- stream metabolism methods to estimate whole-system photosynthesis and respiration. Comparison to chamber-based metabolism methods showed that most of the whole-system respiration could be attributed to heterotrophic activity in the hyporheic zone. We found that regeneration of C in the hyporheic zone (respiration) was in reasonable stoichiometric agreement with the regeneration of N and P. Increasing temperature and discharge had relatively modest impacts on ecosystem respiration and photosynthesis. We concluded that a substantial portion of the N and P required to support ecosystem photosynthesis in these permafrost-dominated streams can be obtained from hyporheic regeneration. Second, a substantial portion of the excess C (supersaturated CO2) in these streams may be due to hyporheic respiration rather than terrestrial runoff of CO2-laden groundwater. Third, the expected changes in future climate in the arctic foothills may have only a limited effect on the instantaneous rates of C, N, and P processing. The larger effect is likely to be on annual processing rates, due to the longer flowing water season. http://www.mines.edu/~mgooseff/arctic_proj.html
B42A-06
Biogeochemistry and nitrogen cycling in an Arctic, volcanic ecosystem
As part of a study on Mars Analogue environments, the biogeochemistry of Sverrefjellet Volcano, Bocfjorden, Svalbard, was conducted and compared to surrounding glacial, thermal spring, and sedimentary environments. An understanding of how nitrogen might be distributed in a landscape that had extinct or very cold adapted, slow- growing extant organisms should be useful for detecting unknown life forms. From high elevations (900 m) to the base of the volcano (sea level), soil and rock ammonium concentrations were uniformly low, typically less than 1- 3 micrograms per gm of rock or soil. In weathered volcanic soils, reduced nitrogen concentrations were higher, and oxidized nitrogen concentrations lower. The opposite was found in a weathered Devonian sedimentary soil. Plants and lichens growing on volcanic soils have an unusually wide range in N isotopic compositions from –5 to +12‰, a range rarely measured in temperate ecosystems. Nitrogen contents and isotopic compositions of volcanic soils and rocks were strongly influenced by the presence or absence of terrestrial herbivores or marine avifauna with higher concentrations of N and elevated N isotopic compositions occurring as patches in areas immediately influenced by reindeer, Arctic fox ( Alopex lagopus), and marine birds. Because of the extreme conditions in this area, ephemeral deposition of herbivore feces results in a direct and immediate N pulses into the ecosystem. The lateral extent and distribution of marine- derived nitrogen was measured on a landscape scale surrounding an active fox den. Nitrogen was tracked from the bones of marine birds to soil to vegetation. Because of extreme cold, slow biological rates and nitrogen cycling, a mosaic of N patterns develops on the landscape scale.
B42A-07
Sensitivity of the Carbon Cycle in the Arctic to Climate Change
The recent warming in high latitudes is affecting a broad spectrum of physical, ecological, and human/cultural systems in this region. Some of these changes may be irreversible on century time scales, and have the potential to cause rapid changes in the earth system. The response of the carbon cycle in northern high latitude regions is a major concern. The release of large stores of carbon in land and ocean systems of the Arctic has the potential to substantially increase the concentration of carbon dioxide and methane, which would act as a positive feedback to climate change with worldwide consequences. To address this concern we have conducted the Arctic Carbon Cycle Assessment, an activity sponsored by the Arctic Monitoring and Assessment Program (AMAP), the Climate and Cryosphere (CliC) Project, and the International Arctic Science Committee (IASC). Our overall goal in this assessment is to clarify key uncertainties and vulnerabilities in the response of the carbon cycle in northern high latitude regions to projected climate change. In this assessment we (1) provide a contemporary picture of the stocks and fluxes of the carbon cycle in northern high latitudes, (2) identify the role of northern high latitudes in the contemporary global carbon cycle, and (3) present our current understanding of the sensitivities of the carbon cycle in this region to climate change and the implications of responses for the global carbon cycle. The assessment has identified that there is between 1000 and 2000 Pg (1E+15 g) C stored in northern high latitudes that is potentially vulnerable to climate change over the next century. Currently, it appears that northern high latitude regions are a sink for atmospheric carbon dioxide of approximately 0.5 Pg C per year and a source of atmospheric methane of approximately 50 Tg (1E+12 g) C per year. While these estimates have substantial uncertainty, they do indicate that northern high latitudes are important in the global carbon cycle. A substantial release of volatile carbon stored in and below permafrost of terrestrial ecosystems of northern high latitudes has the potential to impact global climate change significantly. The rate of carbon release will depend on both the rate of warming and whether the landscape becomes wetter or drier. The greatest potential for marine systems in the Arctic to influence the global carbon cycle is through substantial methane release to the ocean and atmosphere from the warming of marine methane hydrates and seabed permafrost. http://www.amap.no/