C11A-0465
Characteristics of basal ice and subglacial water at Dome Fuji, Antarctica ice sheet
(Introduction): The second deep ice coring project at Dome Fuji, Antarctica reached a depth of 3035.22 m
during the austral summer season in 2006/2007. The recovered ice cores contain records of global
environmental changes going back about 720,000 years.
(Estimation of basal ice melt): The borehole measurement was carried out on January 2nd in 2007 when the
temperature disturbance in the borehole calmed down by the rest of drilling for 2 days. Temperature
measurement was performed after 0 C thermometer test was done in the ground. The temperature sensor of
pt100 installed in the skate-like anti-torque was used. We did not have the enough time until the temperature
of thermometer was matched with the temperature of ice sheet. Some error was included in ice temperature
data. The resistance of pt100 sensor was converted to temperature in the borehole measurement machine.
But we used only two electrical lines for pt100 sensor. Rate of heat flow in the ice sheet was calculated using
the vertical temperature gradient of the ice sheet and rate of heat conductivity of ice. The deepest part of
heat flux using temperatures at 3000m and 3030m was about 45mW/m2. We assumed that this value was the
heat flux from the bedrock in the ice sheet. Heat flux to the bedrock surface in the ground was assumed
54.6mW/m2 adopted by ice sheet model (P. Huybrechts, 2006). Then the heat flux for basal ice melt was
about 10mW/m2. This value was equaled to melting of 1.1mm of ice thickness per year. On the other hand,
the annual layer thickness under 2500m was not changed so much and its average was 1.3mm of ice
thickness. So the annual layer thickness and melting rate of basal ice was the same in ordering way. Or ice
equivalent in annual layer is melting every year. The age of the deepest part of ice core is guessed at
720,000 years old and the ice older than basal ice has melted away.
(The state of basal ice): When the ice core drilling depth passed 3031.44m, amount of ice chip more
abundant than the cutting chips has been collected. When the drilling passed 3033.46m, the amount of ice
chip was decreased. But the amount of ice chip collected increase again from 3034.59m and many large ices
have taken the upper part of ice core. The temperature of ice sheet near the bedrock is the pressure
melting point. So the liquid water can exist easy there. The water like groundwater infiltrated into the borehole
and froze in drilling liquid from 3031.44m to 3033.46m. Under 3034.59m, the subglacial water infiltrated into
the borehole and froze in drilling liquid. The existence of water channel in the ice core was found. We think
that the liquid water has been flowing through the boundary of ice crystal.
(Characteristics of chemical constituents): The melted ice was analyzed every 10cm per 50cm from 2400m to
3028m and continuously every 10cm from 3028m to 3034m. The analytical items were water isotopes (d18O
and dD), micro particles (dust) and major ion components. The variations of water isotope and dust in ice
near the bedrock have no conspicuous change. But, the concentrations of Cl- and Na+ ions had interesting
behavior. The concentration of Cl- ion increased and Na+ ion was decreased deeper than 3020m. Further
the concentrations of all ions were decreased suddenly deeper than 3034m. The concentration of ions will be
decrease in turn according to the solubility of the ion.
http://polaris.nipr.ac.jp/~domef/icc-
home/
C11A-0466
U-Th-dated subglacial calcite preserves a record of basal meltwater discharge events of the East Antarctic Ice Sheet spanning the last 340,000 years
Uranium-rich calcite rinds adjacent to the Rennick Glacier, Northern Victoria Land formed during periods of increased glacier extent over the past 340,000 years, representing regional thickening of the East Antarctic Ice Sheet margin. Calcite of up to a few cm in thickness is found on ice-abraded bedrock surfaces and exhibits extremely high U:Th ratios making it suitable for U-series geochronology. Most preserved material grew in the interval 29.5-17.0 ka as expected for precipitation during the last glacial maximum. The remainder is unambiguously dated as belonging to stadials covering the last four glacial cycles, the most significant accumulations immediately preceding known glacial terminations. The calcite preserves two distinct depositional environments: sediment-free spar growth in still, clear subglacial water; and briefer intervals of calcite-cemented sediment including fluvial clasts as large as 15 mm, representing basal discharge events. Oxygen isotopic analysis of the calcite demonstrates significant, rapidly-switching variation in meltwater source, reflecting widespread transport beneath the ice sheet.
C11A-0467
Physical Conditions in Subglacial Lake Ellsworth, West Antarctica
Subglacial lakes are of particular interest for any life they contain, for their record of ice sheet history and for their potential impact on ice dynamics. So far, none has been accessed directly, but Subglacial Lake Ellsworth (SLE) has been identified as the most suitable target in West Antarctica where access should proceed. We have completed a geophysical reconnaissance of Subglacial Lake Ellsworth and present the first results. Seismic, radar and GPS surveys were conducted over SLE between November 2007 and February 2008. The radar surveys mapped the lake outline and the ice thickness in the region, and showed the internal structure within the ice sheet. GPS receivers measured the ice flow over the lake and the surrounding area. The seismic data gave information on the water depth, the ice-water interface and the bed of the lake itself, and we concentrate on these findings. The aims of the seismic surveys were to determine the lake bathymetry and to give some indication of both the physical conditions within the lake, and the nature and structure of any sediments at the lake floor. The lake surface covers an area approximately 22.5 km2. Maximum water depth is more than 150 m, showing that SLE is a substantial body of water. The ice-water interface shows no evidence for freeze-on of basal ice, indicating that the ice is melting at its base over the whole of the lake. The lake bed is composed of high-porosity, low-density sediments with acoustic properties very similar to material found on the deep ocean floor. Seismic reflections indicate a substantial thickness of this soft sedimentary material, accumulated at the lake bed in a low-energy environment. These results have implications for subglacial conditions and ice sheet history, as well as significant practical implications for lake access operations, including access location, preparations for break-through and expectations for the bed sedimentary sequence.
C11A-0468
Mapping Glacier Structure in Three-Dimensions Using an Optical Borehole Televiewer.
Glacier structure provides information about ice flow dynamics, sediment transport and past climatic conditions. We use an optical borehole televiewer for the first time in hot-water boreholes to map the internal structure and sediment distribution of a polythermal valley glacier: Midre Lovénbreen, Svalbard. Surface structure and sediment mapping complement data acquired from boreholes. These data identify: a) primary stratification; b) longitudinal foliation; c) crevasse traces; d) downglacier-dipping shear planes; e) upglacier-dipping shear planes. Sediment transport within the ice mass is observed in associated with the deformation of primary stratification, laminated basal ice and upglacier-dipping shear planes. A three- dimensional map of glacier structure and sediment transport dynamics has allowed us to evaluate existing geophysical data and numerically-derived ice-flow vectors for the glacier. These data also help to further refine conceptual models of glacial sediment transport.
C11A-0469
Subglacial recharge into carbonate bedrock aquifers and generation of microbial methane in adjacent organic-rich shales: southwestern Ontario, Canada
A growing body of literature suggests that regional carbonate aquifer systems, extensive within Paleozoic basins across North America, acted as drains for subglacial meltwater during periods of continental glaciation, with feedbacks to ice sheet dynamics. Recharge of pristine meltwaters displaced and diluted saline brines to depths over 1 km, storing freshwater resources on the continents. In addition, subglacial recharge into organic-rich formations adjacent to these aquifers, enhanced generation of microbial methane, an economic natural gas resource. This study investigates the impacts of Pleistocene glaciation on microbial methane generation in the Upper Devonian Shale (Kettle Point Formation) and recharge to underlying carbonate bedrock aquifers in southwestern Ontario, straddling the Michigan and Appalachian basins. Twenty-one water and dissolved gas samples were collected from domestic wells screened in the Kettle Point and Dundee formations. Twenty- three brine samples were collected from oil wells completed in the underlying Devonian Dundee Formation. Preliminary results show high concentrations of dissolved methane in groundwater wells screened in the Kettle Point Formation; C and H isotopes of CH4, CO2, and H2O will be used to determine the origin of the gas. Low chloride concentrations of Kettle Point and Dundee formation waters show freshwater dilution of saline brines. The δ18O values of groundwaters near the basin margin range from -17.1 to -7.6‰, compared to -11‰ for annual average precipitation for the area; the waters plot along the meteoric water line. The low salinity and relatively negative isotopic value of groundwaters in the Kettle Point and Dundee formations suggest the presence of remnant Pleistocene meltwaters at shallow depths across the basin margin. Carbon-14 concentrations, measured on select groundwaters, will help to further constrain groundwater residence times.
C11A-0470
Glacitectonic deformation around the retreating margin of the last Irish ice sheet
Evidence for ice-marginal glacitectonic shunting and deformation of bedrock slabs is described from three sites around the west coast of Ireland. These sites (Brandon Bay, County Kerry; Pigeon Point, County Mayo; Inishcrone, County Sligo) are all locations where the late Devensian ice margin retreated on land and was confined to within limestone bedrock embayments. At these sites, flat-lying bedrock slabs (< 8 m long) have been dissociated from rockhead and moved seaward (in the direction of ice flow) by glacitectonic shunting. At all of the sites, bedrock slabs have been variously stacked, rotated, deformed into open folds, and brecciated. Separating the bedrock slabs is either a thin layer (< 20 cm) of brecciated and mylonitised cemented bedrock that shows internal folding; or a thicker (< 50 cm) normally-graded diamicton with a fine matrix. Together, the presence of these features suggests oscillation of a polythermal and clean basal ice margin that was strongly associated with basal freeze-on and the presence of proglacial permafrost. Subglacial sediment-laden meltwater was focused from behind the ice margin and through permafrost taliks. It is suggested that hydrofracturing under high hydraulic pressure, and through a frozen-bed ice margin, forced sediment injection into bedrock fractures and bedding planes and away from the ice margin, and that bedrock slabs were moved in part by hydraulic lift as well as thrust-style ice-marginal tectonics. The presence of a mosaic of warm and frozen ice-bed patches, in combination with strong geologic control and meltwater generation from behind the ice margin, can help explain formation of these unusual bedrock slab features.
C11A-0471
Evaluating Subglacial Bedform Internal Composition as a Control on Elongation - a Case Study From the Southern Margin of the Laurentide Ice Sheet
Paleo-ice streams played a crucial role in the behavior and stability of the Laurentide Ice Sheet (LIS). Several studies have identified the signature of fast-moving ice on the landscape once occupied by the LIS. A growing body of literature supports the use of subglacial bedform elongation (length/width) as a relative measure of paleo-ice flow velocity. Internal sediment rheology has been proposed as a control on elongation in addition to the velocity of overriding ice. To test this hypothesis we investigate the internal properties of drumlins and megaflutes in the New York Drumlin Field that display extreme differences in elongation. Exposures created by wave action along the southern shore of Lake Ontario provide access to the internal structure of several drumlins. In addition, roadcuts and quarrying expose drumlins and megaflutes within the interior of the field. Several bedforms are dominated by matrix-supported diamicton with occasional macroscale sand stringers and intraclasts. In contrast, others display sections of massive, well-sorted sand. One exposure (Parmalee) presents a complex stratigraphy with sharp contacts between units of significantly different rheology. Upon comparison, little spatial correlation is found between observed internal composition and bedform elongation. This finding further supports the use of bedform elongation as a proxy measure of ice velocity thereby providing confidence in ice sheet reconstructions based upon geomorphologic evidence.
C11A-0472
30000 years ground surface temperature history of Southern Canada: From temperatures at the base of the Laurentide ice sheet to the Holocene Climate Optimum
Using temperature profiles measured in 7 very deep boreholes (~2000m) located across south central and eastern Canada, we infer ground surface temperature histories from the Last Glacial Maximum (LGM) throughout the subsequent warming of the Holocene until present. Visual inspection of the profiles reveals regional differences between sites, some showing strong signs of post-glacial warming while others indicate small changes in ground surface temperature. These differences are confirmed by the inversions of the temperature profiles. For most sites, the temperatures at the base of the ice sheet are at or slightly below the melting point of ice. One site located at the eastern edge of the ice sheet (Sept-Iles, Quebec) shows basal temperatures a few degrees below the melting point of ice. These results are consistent with field observations and model predictions suggesting high velocity basal flows of the ice sheet above the studied regions. These new data on basal temperatures will provide better quantitative constraints on glacier flow dynamics. The chronology of ice sheet retreat inferred from the inversions is comparable to other proxies. Inverse and direct models also show signs of the Holocene Climate Optimum between 2 and 5ka, with temperatures 1-2K higher than present.
C11A-0473
Subglacial Volcanoes and Their Relationship With Paleo-ice Sheet Thicknesses
Poor control on the thickness of past ice sheets has lead to considerable uncertainty in estimates of the volume of ice stored in Pleistocene ice sheets and glaciers. Moraines and other glacial deposits provide constraints on glacier extent but proper data on ice thickness is often lacking. Subglacially-formed volcanoes in previously glaciated areas are a type of bedform providing clues on ice thickness in volcanic regions. Tuyas in Iceland and northwestern Canada are considered to be built in subglacial eruptions that melted through the overlying ice sheet, forming flat-topped, lava-capped and steep-sided mountains. The lake level of an englacial lake in which a tuya was formed is preserved by the distinct boundary between subaqueously- formed hyaloclastite breccias and subaerially-formed lavas. It has long been recognized that the height of a tuya and the thickness of the ice sheet in which it forms is correlated although good models of this relationship have been lacking. However, useful constraints can be obtained on this relationship by considering the energy used to melt ice in tuya-forming eruptions and the volume of ice melted in such eruptions. It is found that basaltic tuyas should commonly melt about 5 times their own volume while for rhyolitic tuyas this ratio is between 2 and 3. By combining this melted volume with ice rheology, an estimate of the depth of depressions in the ice surface can be obtained. Using representative values of density, rheology and thermal properties for ice and rock, it is found that 200-300 m deep depressions should form around a typical basaltic tuya in Iceland during its buildup in an eruption. This suggests typical ice thicknesses of 500-1000 m in central Iceland, considerably less than expected for e.g. the last glacial maximum (LGM) ice sheet. This may indicate that tuyas did not form during the LGM. Apparently most tuya forming eruptions occurred within a glacier with a volume that was only 10-20% the LGM ice sheet in Iceland.
C11A-0474
Constraint on Basal Conditions in the Onset Region of Jakobshavn Isbrae, Greenland, through Seismic Amplitude Variation with Offset (AVO) Analysis
Jakobshavn Isbrae is responsible for approximately 7% of the ice discharge from the Greenland Ice Sheet, with its flow focused through a subglacial trough that extends tens of kilometers into the ice sheet. Constraints on the basal conditions in the onset region of streaming ice flow are necessary to determine the evolution of this outlet glacier. Here we present the results of a seismic reflection experiment from the onset region of Jakobshavn Isbrae, where we suggest that ice flow occurs over a non-deforming bed. We analyze the amplitude of the reflected seismic energy from the ice bottom reflection to draw this conclusion. Seismic amplitude variation with offset (AVO) analysis in glaciated regions relates the observed reflectivity of the ice-bed interface to the physical properties of the subglacial bed. Here the amplitude and phase of the ice bottom reflection over a range of source-receiver offsets can be shown to fit a unique set of subglacial bed properties. This approach is applied to a set of wide-angle data, in which we observe PP, PS, and SS reflections from the ice-bed interface (where PP is the compressional wave reflection, PS is the converted wave from compressional to shear, and SS is the shear wave reflection). In addition, we analyze data from two seismic reflection profiles in the region, where we observe PP reflections from the ice-bed interface. Our results from the wide-angle data point to a hard bed in the onset region of Jakobshavn Isbrae, with ice flow dominated by basal sliding over a non-deforming bed. Though variations in basal reflectivity are observed over the approximately 16 km of the bed imaged, a hard bed dominates. The implications of this finding for ice sheet modeling will be explored. In particular, a non-deforming bed may lead to less variability in flow speed as the glacier size changes in response to warming (we assume that sediments are more variable subglacially than crystalline basement). However, water input to the bed from surface lakes may be more effective at accelerating the ice because of the lower permeability of basement rock.
C11A-0475
The Moulin Explorer: A Novel Instrument to Study Greenland Ice Sheet Melt-Water Flow.
Recent data shows that the Greenland ice sheet has been melting at an accelerated rate over the past
decade. This melt water flows from the surface of the glacier to the bedrock below by draining into tubular
crevasses known as moulins. Some believe these pathways eventually converge to nearby lakes and
possibly the ocean. The Moulin Explorer Probe has been developed to traverse autonomously through these
moulins. It uses in-situ pressure, temperature, and three-axis accelerometer sensors to log data. At the end
of its journey, the probe will surface and send GPS coordinates using an Iridium satellite tracker so it may be
retrieved via helicopter or boat. The information gathered when retrieved can be used to map the pathways
and water flow rate through the moulins. This work was performed at the Jet Propulsion Laboratory-
California Institute of Technology, under contract to NASA. Support was provided by the NASA Earth
Science, Cryosphere program
http://eis.jpl.nasa.gov/~behar/moulin/index.html
C11A-0476
Evolution of a Temperate Glaciated Shelf: Insights From the Bering Trough Region, Gulf of Alaska
Sequence stratigraphic analysis on low-latitude continental margins has been used to evaluate the dynamic relationship between sea level, tectonics, and climate. The locus of sediment flux along these margins is spatially and temporally controlled by changes in relative sea-level. However, along high-latitude glaciated continental margins, such variation in sediment discharge is primarily controlled by the position of the grounding line. Therefore, the geometry of glacial sequences and their associated systems tracts can be used to identify glacial regime and infer past glacial dynamics, continental shelf evolution, relative sea-level variation, and tectonic processes. Here we present results from a sequence stratigraphic analysis of high- resolution seismic reflection profiles within the Bering Trough region of the Gulf of Alaska, which provide insight into the regional history of the Cordilleran Ice Sheet and the formation of temperate glaciated continental shelves. Marine glacial sequences are identified by the presence of a basal glacial erosional surface formed during glacial advance followed by an upwards progression of ice-contact, ice-proximal, and ice-distal sediments, which are formed during the retreat phase of glaciation. The identification of multiple glacial erosional surfaces indicates that the shelf has been actively glaciated multiple times prior to Mid- and Late-Pleistocene maximum advances. Partial erosion of underlying sequences during subsequent glacial advances results in stacked sequences composed of ice contact and ice-proximal sediments deposited from the grounding line during phases of glacial retreat. These sediments are represented by chaotic seismic facies and variable clinoform seismic facies, respectively. Interpretation of these seismic facies indicates the remnants of grounding line fans and banks within the shelf, which are characteristic of the temperate glacimarine environment due to large meltwater and sediment fluxes. The position of paleo-shelf breaks reveals that the shelf has been locally constructed by a combination of aggradational and progradational deposition, demonstrating a net increase in accommodation space. Variation in systems tract geometry and interpreted surfaces also shows deformation has occurred along the shelf. Such deformation can be attributed to some combination of glaciogenic loading and local subduction-related tectonic processes.
C11A-0477
Exploring the links between transient water inputs and glacier velocity in a small temperate glacier in southeastern Alaska
Glaciers along the Gulf of Alaska are thinning and retreating rapidly and over the last century this loss of ice has contributed measurably to global sea level rise. An important control on the rate at which ice is being lost is basal motion because higher glacier velocities increase the rate at which ice is delivered to ablation zones. Recent research has focused on understanding the effects of sub-glacial water storage on glacier basal motion. In this study, we examined how water inputs from large rainfall events as well as a glacier lake outburst flood affected the velocity of the Lemon Creek Glacier in southeastern Alaska. Lemon Creek Glacier is a moderately sized (~16~km2) temperate glacier at the margin of the Juneau Icefield. An ice- marginal lake forms at the head of the glacier and catastrophically drains once or twice every melt season. We have instrumented the glacier with two meteorological stations: one at the head of the glacier near the ice-marginal lake and another several kilometers below the terminus. These stations measure temperature, relative humidity, precipitation, incoming solar radiation and wind speed and direction. Lake stage in the ice- marginal lake was monitored with a pressure transducer. In addition, Lemon Creek was instrumented with a water quality sonde at the location of a US Geological Survey gaging station approximately 3 km downstream from the glacier terminus. The sonde provides continuous measurements of water temperature, dissolved oxygen, turbidity and conductivity. Finally, two Trimble NetRS dual frequency, differential GPS units were deployed on the glacier at approximately 1/3 and 2/3 down the centerline of the glacier. All of the instruments were run continuously from May-September 2008 and captured the outburst flood associated with the ice-marginal lake drainage as well as several large (>3~cm) rainfall events associated with frontal storms off of the Gulf of Alaska in late summer. Taken together, these data allow us to test the hypothesis that water inputs which overwhelm subglacial drainage networks result in increased rates of basal motion.
C11A-0478
Characterization of Freshwater-Saltwater Interactions Within the Terminal Moraine Separating Vitus Lake and the Gulf of Alaska
Meltwater from the Bering Glacier flows into Vitus Lake, at its terminus, out of the lake via the Seal River, and then into the Gulf of Alaska. Only a small percentage of the estimated glacial meltwater appears as flow in the Seal River based on continuous monitoring, suggesting that lake water is also discharging through the terminal moraine on the lake boundary and discharging into the Gulf of Alaska. Conductivity profiles in the lake measured during summer show the presence of saltwater at about 40-m depth, further supporting the hypothesis that the lake and the gulf are hydraulically connected in the subsurface. Mapping apparent resistivity values of the subsurface allows for a distinction between layers with differing conductive characteristics. Here, we attempted to map an inferred saltwater-freshwater interface believed to exist in the aquifer between the Gulf of Alaska and Lake Vitus at the base of the Bering Glacier. To map the resistivity contrasts in the subsurface, we used the SuperSting R1/IP multielectrode instrument. Using linear 330-m long Schlumberger and Bipole-Dipole arrays, we were able to map sufficiently deep to find the top of the saltwater table. At various sites between the ocean and the lake, the resistivity surveys were conducted in an attempt to find a correlation between the ocean water and the saltwater found at the bottom of Vitus Lake. From the Schlumberger resistivity surveys conducted near the coast, we determined the freshwater- saltwater interface to be at a depth of 35-40-m. A Bipole-Dipole survey at an intermediate site roughly 6-km from the ocean and east of the lake suggested the depth to this interface to be 65-70-m. By comparing the results at several locations, we were able to deduce how the saltwater intrudes into the terminal moraine of the glacier, mixing with groundwater. The saltwater movement in the ground and into Vitus Lake may be due to tidal effects and to the porous nature of the glacial sediments, which provides an easier path than more consolidated sediments. We hope that understanding this intrusion of saltwater will lead to a better understanding of how the glacial melt in Lake Vitus discharges.
C11A-0479
Seismic and Acoustic Observations of Bering Glacier Calving Events
During the summer field season of 2007, four seismograph stations were deployed for ten days near the terminus of the Bering Glacier in southeast Alaska. While the array was sparse and only recorded for a limited time, it was found that events were recorded with sources both near the terminal face of the glacier, and also by grounded and/or floating icebergs within Vitus Lake. In order to further investigate the source mechanisms and locations of these events, an experiment was designed for the following 2008 field season to create a more robust seismic array and to use infrasound detectors to aid in event location and characterization. In June 2008, seven L-22 short period geophones were deployed on islands and along the shores of Vitus Lake near the terminus of the Bering Glacier. For nearly two months, these stations recorded calving events from the terminus and from grounded and/or floating icebergs within Vitus Lake. In early August, three of the existing seismograph stations were implemented with an additional equilateral-triangular array of infrasound detectors. The infrasound and seismic stations operated simultaneously for five days before they were recovered. The combined infrasound and seismic data sets provide the ability to characterize different types of sources, and trends were inferred over the longer term deployment of the seismic data. By studying the spectral characteristics of the ground and air signals, we attempt to improve our understanding of the nature of the sources, which may include submarine, as well as subaerial, calving events. Additional data used in this analysis include boundary constraints such as water depth and ice edge heights at locations as measured during the summer 2007 and 2008 field seasons.
C11A-0480
Seismic Monitoring of Ice Generated Events at the Bering Glacier
The Bering Glacier, located in southeast Alaska, is the largest glacier in North America with a surface area of approximately 5,175 square kilometers. It extends from its source in the Bagley Icefield to its terminus in tidal Vitus Lake, which drains into the Gulf of Alaska. It is known that the glacier progresses downhill through the mechanisms of plastic crystal deformation and basal sliding. However, the basal processes which take place tens to hundreds of meters below the surface are not well understood, except through the study of sub- glacial landforms and passive seismology. Additionally, the sub-glacial processes enabling the surges, which occur approximately every two decades, are poorly understood. Two summer field campaigns in 2007 and 2008 were designed to investigate this process near the terminus of the glacier. During the summer of 2007, a field experiment at the Bering Glacier was conducted using a sparse array of L-22 short period sensors to monitor ice-related events. The array was in place for slightly over a week in August and consisted of five stations centered about the final turn of the glacier west of the Grindle Hills. Many events were observed, but due to the large distance between stations and the highly attenuating surface ice, few events were large enough to be recorded on sufficient stations to be accurately located and described. During August 2008, six stations were deployed for a similar length of time, but with a closer spacing. With this improved array, events were located and described more accurately, leading to additional conclusions about the surface, interior, and sub-glacial ice processes producing seismic signals. While the glacier was not surging during the experiment, this study may provide information on the non-surging, sub-glacial base level activity. It is generally expected that another surge will take place within a few years, and baseline studies such as this may assist in understanding the nature of surges.
C11A-0481 INVITED
Dissolved and Colloidal Trace Elements in Glacial Watersheds of the Yukon River Basin
Dissolved and colloidal trace elements were determined in rivers and streams within the Yukon River Basin in Alaska and Canada. Glacially-affected catchments tend to have low dissolved organic matter (DOC) and high suspended loads (SPM). Thus, for elements where a balance between complexation and adsorption/solubility determines the dissolved/particulate partitioning (e.g., Fe, Cu, light rare earths), dissolved (< 0.02 μm) concentrations tend to be much lower in glacial catchments than in other stream basins. We also observe that the percentage of these and other elements found in the colloidal suspended phase (0.02 - 0.45 μm) is higher in glacial systems than in non-glacial systems. Additionally, the glacial colloidal suspended phase frequently has significant Si and Al, suggesting that some of colloidal metals may be associated with fine alumino-silicates rather than iron oxides. Seasonally in the early spring, some glacial catchments look more like non-glacial systems due to a snowmelt-driven soil-flush input of DOC and low SPM. For the longer term, to examine possible changes resulting from climate change induced glacial retreat, we collected samples from extra-glacial streams in the immediate foreland of the Gulkana Glacier. These streams had similar compositional trends as the sub-glacial streams. This suggests that from a stream chemistry standpoint, conversion of a glacial basin to non-glacial requires further landscape changes such as the development of stabilizing vegetation and generation of DOC. Finally, we conducted an experiment in which enriched 57Fe was added to samples from glacial and non-glacial streams. In the glacial streams, the 57Fe spike partitioned immediately to the particulate phase whereas in non-glacial systems most of the spike remained in the colloidal phase with measurable amounts also in the dissolved phase. This again emphasizes that the high SPM of glacial streams plays an important role in the partitioning of their trace element loads.
C11A-0482
Post-Wisconsinan Chemical Weathering Rates and Trajectories From a 13,400-Year Sediment Core Record of Lead Isotopic Ratios in Maine
Lead isotopic ratios recorded in a 5.3-m 13.4-ka 14C-dated lake sediment core from Sargent Mountain Pond, Maine (USA) are interpreted as an archive of post-glacial chemical weathering. Early weathering yielded highly radiogenic sediment from the preferential release of U and Th decay products (206Pb, 207Pb, and 208Pb) from accessory mineral phases in the catchment's predominantly-granitic till and bedrock relative to non-radiogenic 204Pb from the more abundant primary minerals. Values for 207Pb/206Pb in the sediment increased rapidly from 0.799 to 0.814 in the catchment's first 4,000 years of post-Wisconsinan weathering, and thereafter increased only slightly to just prior to the 19th century. Values for 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb decline over the same time-scale, as a result of decreasing radiogenic Pb being released from catchment weathering. Our results are consistent with: (1) the published interpretation of Pb isotopic variation in ferromanganese ocean crusts as a reflection of continental-scale glacial-interglacial chemical weathering cycles, (2) bench-scale whole-rock weathering experiments, and (3) soil chronosequence Pb isotope dissolution experiments and bridge the gap between short-term, mineral-scale experiments and long-term, ocean sediment records. We establish a time-scale for depletion of accessory minerals, and loss of their Pb isotopic signature at one catchment, and document the concurrent shift to slower primary mineral-controlled chemical weathering after deglaciation.
C11A-0483
Lithium and Magnesium Isotope Behaviour in Arctic Rivers of Western Greenland
Lithium (Li) and magnesium (Mg) isotopes are potentially useful tracers of continental weathering processes because they preserve information on both inorganic and biogenic processes that affect water chemistry in soils and rivers. Li isotope variations principally reflect removal into secondary phases formed during weathering, whereas Mg isotopes are affected both by the formation of secondary phases and by biomass uptake. This study examines the behaviour of these isotopes in a glacial-river, where physical weathering dominates, in order to assess the potential climatic influence on isotope fractionation and the mechanism of delivery of these elements to seawater. River water samples were collected in August 2006 from the Watson river, fed by the Russell Glacier in Western Greenland, site of a major Jökulhlaup in August 2007. Some tributaries receive water directly from the ice sheet ('glacial'), while others have no direct link to the ice sheet ('non glacial'). The glacial rivers have low concentrations of dissolved ions but they are sediment-rich, while the non-glacial rivers have much higher concentrations of dissolved ions but low levels of suspended particulate material. Results of lithium and magnesium isotope analyses show that the glacial rivers have (δ7Li values of 25‰ and δ26Mg values of -1.1‰); that are distinctly different from those of the underlying bedrock (δ7Li ~5‰ and δ26Mg ~ -0.4‰). Given that fractionation of Li and Mg isotopes principally occurs during secondary mineral formation, this result is perhaps surprising as secondary minerals are not supersaturated in these rivers. However, analysis of the suspended load indicates that 6Li is taken up onto iron oxyhydroxides that form during the oxidation of sulphides in the subglacial environment. By contrast, the Mg isotope composition of the glacial waters is best explained by preferential dissolution of trace carbonate phases (with low δ26Mg) in the subglacial meltwaters. Thus, despite low chemical weathering rates and the low concentration of dissolved ions in the glacial waters, weathering in the sub-glacial environment appears to impart an elemental and isotopic signal to the river waters that is distinct from the composition of the bedrock, and in turn has potential to influence seawater chemistry.
C11A-0484
Seasonal Variations Of Chemical Composition Of Soil Porewaters, Streams And Rivers In Basaltic Watersheds Of Central Siberia: The Origin Of River Dissolved Load In The Permafrost Zone
Rivers and streams draining basalts in Central Siberia allow thorough evaluation of the intensity of chemical weathering and its potential change induced by permafrost degradation. In order to get new insights to the origin of dissolved and suspended matter in this important subarctic region, we follow continuously over 4 years the chemical composition of dissolved load in two large rivers (N. Tunguska and Kochechumo), small experimental watershed (Kulingdakan) in the Yenissey River Basin and interstitial soils solutions from different landscape positions and soil horizons. The site (Tura, 64oN, 100oE) offers a unique opportunity for studying the processes occurring at high and low permafrost distribution watersheds. Indeed, at global scale we deal with two equivalent size rivers draining the northern and southern part of the Central Siberia (Kochechumo River and N. Tunguska River , respectively). At the local scale, there are north-facing and south-facing slopes of the watershed that receive equivalent precipitation but exhibit totally different heat input and consequently above-ground biomass and active layer thickness. There is a clear and strongly pronounced seasonal variability in trace and major elements concentration and organic carbon (OC) over the hydrological cycle with most of OC and trace element flux occurring during the spring flood. In summer time, the north-facing slope acts as a primary source of water and dissolved elements to the river as it exhibits the lowest thickness of the active layer. Indeed, the chemical composition of the suprapermafrost flow and that of the river water are very similar. In contrast, the south-facing slopes, although they exhibit twice higher concentrations of OC and TE in porewaters, do not deliver enough fluids to the river as these fluids are fully absorbed by thick active layer, mosses and vascular vegetation. Nevertheless, in early spring, the degradation products of plant litter from the S-facing slopes are abundant in stream waters. In winter time, when all soil fluid migration is stopped and the small rivers are fully frozen, the main source of solutes to the large rivers become groundwaters located in the thawed zone below the river channel. In collaboration with other researches, our results should allow quantitative modeling of the evolution of large boreal continental systems under global warming accompanied by the shift from the permafrost-dominating to permafrost-free environments.
C11A-0485 INVITED
Influence Of Plants On Elements Fluxes Within Central Siberian Ecosystems Dominated By Continuous Permafrost.
The number of studies carried out on boreal forests, located mainly between the latitudes 46°N and 66°N, has recently increased in our community. This relies on the fact that these environments play a key role in regulating climate and global carbon cycle and are probably the most sensitive areas to global warming (IPCC, January 2007). Our efforts focus on weathering processes and elements transport mechanisms between the different chemical reservoirs (soil and litter, plants, atmosphere) using a multidisciplinary approach. The aim of the presentation is to propose a conceptual model of the present-day "biogeochemical" functioning of a watershed located in central Siberia. The role of plants dominated by larches, dwarf shrubs and mosses will be particularly considered. Larches and mosses account for more than 85% of the above-ground biomass. The sampling site is located in the drainage basin of the Nizhniya Tungunska River (eastern tributary of the Yenissey River) on the basaltic plateau of Putorana. Central Siberia spread over more than 4,100,000 km2 at an altitude of about 600m at the east the Yenissey River. Climate is cold with a mean annual air temperature of -9.5°C and presents an annual precipitation of 350mm (Tura Meteorogical Station). 60% of rain is falling during the active period that is the period where the soil temperature is above zero degrees between May and September. At local scale, there are north- facing and south-facing slopes that receive equivalent precipitation but exhibit totally different heat input and consequently above-ground biomass and active layer thickness. We analyzed an important number of samples including soil samples, larch needles, mosses, and dwarf shrubs collected in the various local environments. Plants have been regularly collected from May to September 2007. Also, plant litter degradation experiments have been conducted in situ during the same period. Several main questions arise from this study: Are there differences of elements concentrations in plants as a function of time (active period) and between south-facing and north-facing slopes? Are transfer coefficients of elements between soils and plants different between south-facing and north-facing slopes? Are transfer coefficients of elements different from those calculated in other climatic environments? The present work will answer to these different questions. Finally, we used our data to estimate the importance of the litter fall flux in the riverine flux at the outlet of the catchment.
C11A-0486
Effect of the permafrost thawing on the organic carbon and microbial activity in thermokarst lakes of Western Siberia: important source of carbon dioxide in the atmosphere
Ongoing processes of the permafrost thawing in Western Siberia are likely to increase the surface of water reservoirs via forming so-called thermokarst lakes, mobilizing the organic carbon (OC) from the soil pool to the rivers and, finally, to the ocean, and thus modifying the fluxes of methane and CO2 to the atmosphere. In order to understand the mechanisms of carbon mobilization and biodegradation during permafrost thawing and to establish the link between the OC and microbial activity in forming thermokarstic lakes, we performed a comparative multidisciplinary study on the biogeochemistry of OC and metals in lakes located in the northern part of Western Siberia. About 10 lakes and small ponds of various size and age were sampled for dissolved and colloidal organic carbon and metals and total bacterial cell number. There is a systematic evolution of DOC, pH, trace elements and biological activity during successions of thermokarst lakes encountered in the present study. At the beginning of permafrost thawing at the scale of several meter size ponds, fast lixiviation of unaltered peat yields significant amount of OC, major and trace elements; the pH of these waters is between 3.5 and 4.0 and the conductivity is 20-30 μS. The intermediate stage of lake formation still preserve low pHs, high DOC and conductivity, even in relatively large, up to 1 km diameter but fast growing lakes. At these stages, there is no any productivity as phytoplankton cannot live in these acidic waters and bacterial mineralization intensity is around 0.3 mg C/L/day both in the surface and bottom horizons. Once the lake border is stabilized, there are no new "unaltered peat" sources and the biological processes start to consume the OC and nutrients. At this stage, there is still no production in the water column (< 0.01 mg C/L/day) but the bacterial mineralization intensity remains high, up to 0.3 mg C/L/day. At this final stage, the remaining part of the lake located in the centre of the "khasyrei" exhibits the highest pH (5.5-6.0), presence of green algae and plants, very low conductivity (4-9 μS) and the lowest OC concentrations. However, there is still quite weak primary productivity (¡Ü 0.01 mg/C/L) and significant mineralization rate (0.3 mg C/L/day). The bacterial biomass and the number of colony-forming units for different nutrient media demonstrate clear decrease with the temporal evolution of the lake from dystrophic (7000 CFU/mL) at the begging of permafrost thawing to oligotrophic/mesotrophic at the final "khasyrey" stage (80 CFU/mL). Two main conclusions can be drawn from this work: 1) At all stages of lake evolution, there is always high amount of active OM-mineralizing heterotrophic bacteria, and 2) mineralization processes by these microorganisms clearly dominate any possible CO2 consumption by the phytoplankton. As a result, Western Siberia thermokarstic lakes are likely to act as important source of CO2 to the atmosphere. The CO2 flux associated with this microbial activity (i.e., 400 +/- 200 t/km2/y, assuming average lake depth of 1 m) is at least one order of magnitude higher than the riverine DOC flux, bearing in mind very high proportion of newly formed lakes in the Western Siberia (up to 30 percent).
C11A-0487
Trace element speciation and origin of colloids in surface waters of subarctic zone (NW of Russia and Central Siberia)
Geochemistry of trace elements (TE) in boreal regions attracts large attention of researchers in view of on- going environmental changes that can affect both the fluxes of these elements to the ocean, their speciation and thus their bioavailability. Most of trace elements in waters of boreal zone are transported via organic and organo-mineral colloids. In order to better understand the processes of colloids formation in surface waters draining watersheds of various lithology and permafrost abundance, comparative study of TE speciation in various geographic zones is necessary. In this work we attempted to generalize the typical features of trace element speciation in boreal arctic and subarctic zones assessed via in-situ dialysis and ultrafiltration. Surface waters of three circumpolar regions in Arkhangelsk region, NW Russia and Central Siberia were studied using unique and rigorous procedure via combination of in-situ dialysis and ultrafiltration (1 kDa, 3.5 kDa, 10 kDa, 100 kDa, 0.22 µm, 0.45 µm, 1 μm, 5 µm). In both filtrates and dialysates, all major and trace elements and dissolved organic carbon were analyzed. In all studied regions, three typical features of colloid speciation have been revealed: i) high proportion of large-size colloids (10 kDa – 0.22 μm and 0.22 μm – 5 µm), mostly composed of Fe oxy(hydr)oxides stabilized by organic matter; ii) presence of organic-rich, small size colloids and conventionally "dissolved" substances (< 1 kDa and 1 – 10 kDa), presumably, fulvic acids, and iii) strong association of all trivalent and tetravalent elements with large-size mineral colloids. Results of the present work allow distinguishing between two possible pathways of colloids formation: 1) Groundwater-borne Fe(II) oxidation and TE coprecipitation in the presence of organic matter originated from plant litter and peat layers of surface horizons at the surface redox front between anoxic groundwaters and surficial OM-rich waters of the riparian zone, and 2) Plant litter-borne Fe(II) and TE originated from decomposing plant litter and mosses and coprecipitating together in the very surface horizons above the permafrost layer. The first pathway is typical for non-permafrost bearing zones such as European Russian Arctic where as in Central Siberia, plant litter degradation in surficial horizons is largely responsible for colloids formation. In both cases, dissolved organic matter in the form of essentially fulvic acids helps to stabilize the mineral colloids and prevents the aggregation of Fe oxyhydroxides. As a result, conventional thermodynamic modeling of TE association with dissolved organic matter can be misleading for these systems, where the processes of irreversible coprecipitation, not reversible complexation/adsorption on the surface, control the TE interaction with colloids. It is anticipated that knowledge of speciation of metal pollutants and organic carbon both in surficial fluids and in the permafrost ice and soils will allow the prediction of metals bioavailability change induced by permafrost thawing and human pollution – for example, around the smelters located in the arctic-zone. Elaborated models can be extended to other permafrost- bearing territories of the world which are likely to be highly sensitive to the global warming.
C11A-0488
Geochemistry of organic carbon and trace elements in boreal stratified lakes during different seasons
Our knowledge of chemical fluxes in the system rock-soils-rivers-ocean of boreal and glacial landscapes is limited by the least studied part, i.e., the river water transformation between the lake and the river systems. Dissolved organic carbon (DOC), nutrients, major and trace elements are being leached from soil profile to the river but subjected to chemical transformation in the lakes due to phytoplankton and bacterial activity. As a result, many lakes in boreal regions are quite different in chemical composition compared to surrounding rivers and demonstrate important chemical stratification. The main processes responsible for chemical stratification in lakes are considered to be i) diffusion fluxes from the sediment to the bottom water accompanied by sulfate reduction and methanogenesis in the sediments and ii) dissolution/mineralization of precipitating organic matter (mineral fraction, detritus, plankton pellets) in the bottom layer horizons under anoxic conditions. Up to present time, distinguishing between two processes remains difficult. This paper is aimed at filling this gap via detailed geochemical analysis of DOC and trace elements in the water column profiles of three typical stratified lakes of Arkhangelsk region in Kenozersky National Parc (64° N) in winter (glacial) and in summer period. Concentration of most trace elements (Li, B, Al, Ti, V, Cr, Ni, Co, Zn, As, Rb, Sr, Y, Zr, Mo, Sb, Ba, REEs, Th, U) are not subjected to strong variations along the water column, despite the presence of strong or partial redox stratification. Apparently, these elements are not significantly controlled by production/mineralization processes and redox phenomena in the water column, or the influence of these processes is not pronounced under the control by the allochtonous river water input. In particularly, the stability of titanium and aluminum concentration along the depth profile and their independence of iron behavior suggest the important control by dissolved organic matter. Therefore, organo-ferric colloids controlling petrogenic elements speciation in soil and river waters are being replaced by autochthonous organic colloids in the lake system. The same observation is true for some heavy metals such as nickel, copper and zinc, whereas cobalt, as limiting component, is being strongly removed from the photic zone or it is coprecipitating with manganese hydroxide. Results of the present work allow quantitative evaluation of the role of redox processes in the bottom horizons and organic detritus degradation in the creation of chemical stratification of small lakes with high DOC concentration. Further insights on geochemical migration of trace elements in lakes require : i) study of colloidal speciation using in-situ dialysis; ii) monitoring the annual and seasonal dynamics of redox processes and TE concentration variation along the profile; iii) quantitative assessment of bacterial degradation of suspended OM and Mn and Fe redox reactions along the depth profile; iv) setting the sedimentary traps for evaluation of suspended material fluxes, and, v) thorough study of chemical composition of interstitial pore waters.
C11A-0489
Temporal Variations of Geochemical Fluxes in Boreal Rivers in Permafrost Context: Trace Element and Sr and U Isotope Data (Nizhnaya Tunguska Watershed - Central Siberia).
We propose to characterize the temporal geochemical variability of dissolved chemical fluxes carried by boreal rivers under permafrost context. This is a significant issue as high latitude regions present specific hydrological systems likely to be strongly affected by global warming. For this purpose, a geochemical survey of two rivers draining the South of the basaltic plateau of Putorana in Central Siberia (Kochechumo and Nizhnaya Tunguska) has been performed, with a regular sampling of the rivers at the outlet of their watershed over two years. In the frame of our work, the dissolved loads of the water samples were analysed for major and trace element concentrations as well as for strontium and uranium isotopic compositions. Our results show that chemical variations along the year follow hydrological variations and define three contrasted periods : (1) a very low water period from October to May, during which soluble elements are affected by concentration processes and chemical fractionation processes, (2) a spring flood in May/June characterized by an important water input and also by the mobilization of organic and inorganic colloids together with traditionally insoluble element, (3) an intermediate high water period from June to the end of September. Important strontium and uranium isotopic variations are observed over a hydrological cycle. The data clearly indicate the contribution of several sources to the geochemical flux carried by the stream water over the year. In particular, deep underground reservoirs and suprapermafrost flow would contribute differently to the global geochimical flux carried by these rivers, depending on the time of the year. Such temporal variations in the geochemical fluxes of boreal rivers have now to be taken into account for correct and realistic determination of chemical budgets and weathering rates in permafrost regions.
C11A-0490 INVITED
Direct Evidence of the Feedback Between Climate and Weathering in Glaciated River Catchments
Long-term climate moderation is commonly attributed to chemical weathering; the higher the temperature and precipitation the faster the weathering rate. Weathering releases divalent cations to the ocean via riverine transport where they promote the drawdown of CO2 from the atmosphere by carbonate mineral precipitation. To test this widely-held hypothesis, we performed a field study determining the weathering rates of 8 nearly pristine north-eastern Iceland river catchments with varying glacial cover over 44 years. The mean annual temperature and annual precipitation of these catchments varied by 3.2 to 4.5°C and 80 to 530 percent,respectively during the study period. Statistically significant linear positive correlations were found between mean annual temperature and chemical weathering in all 8 catchments and between mean annual temperature and both mechanical weathering and runoff in 7 of the 8 catchments. For each degree of temperature increase, the runoff, mechanical weathering flux, and chemical weathering fluxes in these catchments are found to increase from 6 to 16 percent, 8 to 30 percent, and 4 to 14 percent respectively, depending on the catchment. In contrast, annual precipitation is less clearly related to the measured fluxes, showing statistically significant positive correlations with runoff, mechanical weathering, and chemical weathering only in 3 of the least glaciated catchments. Mechanical and chemical weathering increased with time in all catchments over the 44 year period. These correlations were statistically significant for only 2 of the 8 catchments due to scatter in corresponding annual runoff and average annual temperature versus time plots. Taken together, these results 1) demonstrate a significant feedback between climate and Earth surface weathering, and 2) suggest that this weathering rate is currently increasing with time due to global warming.
C11A-0491
Iron Isotope Fractionation in Artic Weathering Environments
While our current knowledge of Fe isotope systematics in rivers is still limited, Fe isotopes provide a new approach to important questions concerning Fe sources, Fe speciation and its bioavailability in the hydrological cycles. Here, we investigated several Arctic rivers for elemental and Fe-isotopic composition with specific emphasis on silicate weathering and organic C content in order to provide new insights into the delivery of trace nutrients, such as Fe, that are important for the biological productivity of the Arctic Ocean. Without such characterization of the present state of the system, future changes in the response of these river systems to global change cannot be properly evaluated. Our Fe isotope systematics results in Siberian rivers, include (1) a time-series of water samples from large rivers (Ob and Lena) focusing on the peak flow that has not yet been investigated despite its critical importance for annual budgets; (2) the colloidal fractions (1 kDa to 0.45 μm) of rivers and their estuaries in the White Sea, including the Severnaya Dvina River to assess the influence of Fe-rich colloids flocculation on the Fe isotopic composition of the estuarine waters. Dissolved iron fractions (<0.22 μm) in large rivers have restricted δ56Fe values, with the Ob River varying from -0.29 to 0.01‰, averaging -0.11‰ (n= 20) and the Lena River varying from -0.24 to -0.01‰, averaging -0.11‰ (n=15). These values are slightly lower than bulk silicate earth values (defined at 0.09‰ relative to IRMM-14) and do not display any relationships with ice break-up on the rivers. In contrast, dissolved iron fractions in the Severnaya Dvina River and its tributaries yield more positive δ56Fe values, with values ranging from -0.09 to up to 0.64‰, averaging 0.2‰ (n=19). Small swamp rivers from the South White Sea coast reveal an even higher spread of dissolved δ56Fe values, from -1.1 to +0.8‰ with some clear variations among the various colloidal fractions (1 kDa-10 kDa - 100 kDa). These results, together with the high Fe and DOC concentrations in the water samples, suggest that Fe-redox cycling in soil aquifer, as well as the formation of organic-rich colloids, may impart specific Fe-isotope signatures in dissolved Fe reaching the Artic Ocean. Recent studies (e.g. Fantle and De Paolo, 2004 and Bergquist and Boyle, 2006) have demonstrated that rivers present an isotopically light Fe source to the oceans. Our study further suggests that δ56Fe composition of Artic rivers is not unique and depending on Fe speciation in organo- mineral colloids and their origin, may be characterized by more positive and negative δ56Fe values relative to the crust than previously reported.
C11A-0492
Multi-element Composition of Terricolous Lichens in the Northwest European Russia
Lichens (fungi living with an alga or cyanobacterium) absorb substances, including trace elements through dry and wet deposition, often to very high concentrations, and have been widely used as biomonitors. We studied multi-element composition of terricolous lichens (mostly of genera Cladonia and Cetraria) collected in 2004–2006 in Kola Peninsula, Karelia and Arkhangelsk Region of NW Russia. 31 samples were analyzed. The unwashed lichen samples were air dried and homogenised to a fine powder in an agate crusher, in order to exclude any contamination of trace metals. Samples were treated in a four-step chemical digestion procedure (full dissolution via acid attack). Major element concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES), and trace element concentrations by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500). An enrichment factor (EF) was calculated for each element (X) relative to the composition of earth's crust: EF = ((X/Al) in lichen) / ((X/Al) in the earth's crust). Al was used as a crustal indicator. In most samples contents of Ti, V, Cr, Mn, Fe, Co, rare earth elements (REEs), Th, U are at the background level and their EFs are less than 10. These low EF values indicated that, relative to average values for crustal rocks, there was no enrichment of these elements in the lichen concerned. For some elements (Se, Cd, Zn, Sb, Pb, As, Ni, Cu) consistently higher EF values were obtained. These higher values were interpreted in terms of sources of both anthropogenic and natural sources other than crustal rock and (or) soil. For instance, these elements could be derived by long-range atmospheric transport. Highest concentrations of Cu, Ni, Pb in lichens and EF by these elements were registered in Lovozersky Tundry (Kola Peninsula) and in Paanayarvi area (NW Karelia). Lichens here are polluted by Monchegorsk and Nickel Cu-Ni smelters. In the vicinity of Kostomukshsky Ore-dressing Mill lichens are enriched by Fe. In all lichens collected at the White Sea coast (Onezhsky Peninsula and Kumbysh Island), high Na content and EF values were revealed. In general, elemental composition of terricolous lichens in the Northwest European Russia reflects complex influence of atmospheric deposition of aerosols from both natural and anthropogenic sources. Our studies were supported by the grants of RFBR No. 07-05-00691 and 05-05-65159, project "Nanoparticles". The authors are indebted to Academician A.P. Lisitzin for valuable recommendations.
C11A-0493
Rates of Chemical Weathering in Glacial Moraines of the French Alps
Rates of mineral weathering were evaluated in soil profiles from 9 glacial moraines in the French Alps ranging in age from 360 years to ~140 Ky. The soils with age up to 17.5 Ka are located near Chamonix, whereas the older tills are just east of Grenoble. The mineralogy of the <2 mm size fractions of the soils were determined by quantitative XRD. The mineralogy of the 360 yr moraine indicates the alteration of a small amount of chlorite to kaolinite in the top ~10 cm of the soil profile (~20 mg chlorite per cm2), but no other alteration. The 17.5 Ky till has an incipient A horizon, and most of the chlorite and amphibole in the upper ~30 cm has been altered to kaolinite plus goethite. There was no indication of plagioclase weathering at 17.5 Ka, even in the top 5 cm. The tills close to Grenoble are older than 45 Ka, and all the soils have well developed A and B horizons, with the B horizon depth increasing with the till age from ~65 to ~150 cm. Carbonates in the A and B horizons are severely depleted. The mass of carbonate depletion is approximately linear with soil age, dissolving at a rate of ~105 kg/ha/yr. There appears to be a correspondence between carbonate depletion and silicate weathering, with both carbonate depletion and feldspar weathering ceasing abruptly at the base of the B horizon, below which the till is unaltered. The amount of plagioclase and K-feldspar weathering in the soil horizons increased with age, as did the development of an argillic kaolinite horizon. Weathering rates of plagioclase and K-feldspar in this soil chronosequence were compared with yields from small granitic watersheds and other soil chronosequences. The rate of plagioclase loss per land surface area in this study are in the lower range of those observed from unglaciated granitic watersheds. Plagioclase dissolution rates normalized to mineral surface areas are between 10-19.1 and 10-19.7 mol/cm2/sec, a factor of ~10 slower than soil chronosequences in similar climatic regimes. The slower than expected feldspar dissolution rates suggests that the high surface area of the primary silicates in the clay-size fraction of these tills played a subordinate role in controlling the rate of chemical weathering. Rather, the presence of carbonates in these tills may have suppressed the feldspar dissolution rates below the rates of most non-glaciated watersheds.
C11A-0494
Salt Provenance and Migration in Sediment Core Samples From Dry Valleys, Antarctica
The McMurdo Dry Valleys are characterized by hyperarid conditions and extensive salt accumulation. Soluble ions are present in high concentrations in soil and sediment from multiple potential sources, including weathering of minerals, inheritance from original deposits, and eolian transport of marine salts and atmospheric dust. We are exploring the usefulness of magnesium isotope variations as an additional constraint on ion provenance and migration. We extracted soluble salts from soil and sediment using three sequential water extractions. Magnesium is purified by removing all other soluble ions present in salt extracts using a series of two different ion exchange resins. Finally, magnesium isotope ratios are determined using a Nu Plasma multi-collector inductively coupled plasma mass spectrometer. We analyzed core samples collected in the 1970s during the Dry Valley Drilling Project (DVDP). Soil salt extracts from Taylor Valley, at low altitude near the Ross Sea, possess magnesium isotope values of -0.87‰ to -1.00‰ δ25MgDSM3 and -0.48‰ to -0.54‰ δ26MgDSM3. These values overlap with published values for modern marine water. Ca/(Na+K) and Mg/(Na+K) ratios in these near marine samples are nearly identical to modern marine water. Samples from Wright Valley, further from the Ross Sea, contain proportionally more Ca and Mg than (Na+K), suggesting an alternate provenance. We plan to analyze the salt extracts from the Wright Valley DVDP core for magnesium isotope ratios to assess variability of salt provenance. Additionally, potential magnesium isotope variation with depth in the core may indicate migration of water and salt below the soil surface.