C41B-0502
Emerging Arctic Amplification as Seen in the NCEP/NCAR Reanalysis
Arctic amplification, a near universal feature of climate model simulations, refers to the idea that rises in surface air temperature (SAT) in response to increasing atmospheric greenhouse gas concentrations will be larger in the Arctic compared to the Northern Hemisphere as a whole. Model-projected Arctic amplification is focused over the Arctic Ocean. As the climate warms, the summer melt season lengthens and intensifies, leading to less sea ice at summer's end. Summertime absorption of solar energy in expanding open water areas increases the sensible heat content of the ocean. Ice formation in autumn and winter, important for insulating the warm ocean from the cooling atmosphere is delayed. This promotes enhanced upward heat fluxes, seen as strong warming at the surface and in the lower troposphere. Arctic amplification is not prominent in summer itself, when energy is used to melt remaining sea ice and increase the sensible heat content of the upper ocean, limiting changes in surface and lower troposphere temperatures. Data from the NCEP/NCAR reanalysis show that starting in the mid 1990s and relative to the 1979-2007 time period, Arctic Ocean SAT anomalies turned positive in autumn and have subsequently grown. Consistent with an anomalous surface heating source, development of the autumn warming pattern aligns with the observed reduction in September sea ice extent, and increases from the lower troposphere to the surface. This recent autumn warming is stronger in the Arctic than in lower latitudes. There is no enhanced surface warming in summer. These findings are consistent with the emergence of model-projected Arctic amplification associated with declining sea ice.
C41B-0503
The Combined Role of ENSO-driven Sea Surface Temperature Variation and Arctic Sea Ice Extent in Defining Climate Conditions in the Southwestern United States
Previous research indicates that future reductions in Arctic sea ice could alter storm tracks and precipitation patterns in western North America and negatively impact water resources in the southwestern United States. Other research suggests that the multiple periods of increased precipitation and/or cooler temperatures in the desert Southwest during the Little Ice Age were due to strong El Niño events, and historical records also describe a greater extent of Arctic sea ice at this time. This research explores the relative influences of and interactions between ENSO-related variation in sea surface temperature (SST) and varying Arctic sea ice cover in controlling storm tracks, precipitation patterns, and overall climate conditions in the desert Southwest. We use 15th to 19th century Arctic sea ice extent/concentration records from the ACSYS Historical Ice Chart Archive as a basis for expanding Arctic sea ice from 1870 HadISST data. Then, in a suite of sensitivity studies, we investigate the effects of expanded Arctic ice cover on storm tracks and climate conditions in the northern hemisphere under ENSO-neutral SST conditions and also under strong La Niña and El Niño events. We find a complex interplay between Arctic sea ice cover and tropical Pacific SSTs in which northern hemisphere storm track response to increased Arctic sea ice is strongest when SSTs are ENSO-neutral. A similar, but muted, response is seen under El Niño SST conditions, and we find very little change in northern hemisphere storm tracks when La Niña SSTs are combined with the expanded Arctic sea ice.
C41B-0504
Seasonality of Spectral Albedo and Transmission of Sea Ice in the Transpolar Drift, Arctic Ocean
The physical and optical properties of snow and sea ice in the Polar regions control the amount of solar short-wave radiation, reflected at the surface, scattered and absorbed within snow and ice, and transmitted into the ocean beneath. Albedo and transmissivity of snow and sea ice strongly influence heat fluxes within the coupled atmosphere-ice-ocean system, and by that the evolution of the sea ice. Spectral optical properties are crucial for primary production and evolution of sea ice related microorganisms and various bio-chemical processes. Furthermore, the increasing importance of remote sensors when studying snow and sea ice, raises the need for ground truth data of spectral optical and other physical properties of snow and sea ice. Spectral albedo and transmission were measured continuously within high spectral and temporal resolution during the transpolar drift of the drifting schooner "Tara" through the Arctic Basin between 30 April and 05 September 2007. Simultaneous in-situ measurements of snow and sea ice properties as well as a comprehensive meteorological program complement the dataset and allow common analysis and an integrated dataset. Results show significant seasonal changes and highlight key events during the transitions from spring to summer and summer to autumn; including formation, evolution, and freeze up of melt ponds. We were able to derive absolute values of energy transfer through snow and ice and into the upper ocean. The timing of changes in surface energy balance can be determined and characterized by including results from detailed snow and sea ice studies as temperature and density profiles, snow stratigraphy, and sea ice texture. Among others, they show that the melting period lasted 80 days which was about 20 days longer than on average. Interestingly, the observation period coincides with the time prior to the Arctic sea ice extent minimum in autumn 2007. Consequently, the findings might assist to understand and explain processes linked to the strong retreat of sea ice that year.
C41B-0505
Recent changes in the circulation pattern of Arctic sea ice
We contrast the circulation of the winter and summer sea ice using a 29-year record of ice motion derived from satellite passive microwave observations. While the recent pattern of winter Arctic sea ice circulation and sea-level pressure (SLP) has nearly reverted to conditions typical of the 1980s, the summer has not. Compared to the 1980s, recent summer SLP distributions show lower SLPs (2-3 hPa) over much of the Arctic Ocean. Overall, the recent summer circulation patterns show a strengthening of the Transpolar Drift Stream (TDS) that is associated with the deeper lows. This has contributed to increased area export and to the decline of summer ice extent. Regionally, this creates more open water, reduces the survivability of the ice cover and lowers the ice volume/storage in the Pacific Sector of the Arctic Ocean. This also suggest that because of the inflow of this sea ice into the Atlantic Sector in summer, the summer ice extent in this sector has remained relatively stable compared to that in the Pacific. It is highly likely that these decadal changes in summer circulation could partly explain the recent record minimums in summer ice coverage in the Pacific Sector while the mean ice coverage in the Atlantic Sector has remained unchanged. This also alters our notion of relatively flat SLP fields during the summer months.
C41B-0506
Effects of a Changing Arctic Sea-Ice Cover Upon On-Ice Snow Depths
Snow cover is an important aspect of the heat and mass budget of Arctic sea-ice: it impacts the ice thermal regime as well as transmission and reflection of incoming radiation. Recent changes in Arctic sea-ice extent warrant a re-examination of the relationship between sea-ice and its snow cover in greater detail. Previously examined measurements from Soviet drifting stations have been combined with data from Arctic coastal stations and the International Arctic Buoy Programme and interpolated onto a pan-Arctic Ocean grid. From these snow-depth fields, monthly snow accumulation rates have been calculated over perennial ice. These accumulation rates are combined with freeze-up/melt dates, derived from QuikSCAT and SSM/I satellite data, in a simple model in which the amount of snow accumulation on ice of varying age is determined. Total snow accumulation has been calculated annually at each grid point, based on ice type and melt season duration for the time period 1979-2007. Calculated values have been compared with previous studies, as well as changes in Arctic precipitation and sea-ice patterns over the past three decades, to assess any changes in snow depth distribution over sea-ice. In addition to changes in precipitation patterns driven by sea-ice change, a delay in the onset of fall ice formation and a reduction in multiyear ice have led to a significant reduction in on-ice snow volume.
C41B-0507 TI: Over the past several decades, the minimum Northern Hemisphere summer sea ice extent has decreased substantially. We present an analysis of the influence of declining Arctic Sea Ice cover on the atmosphere, specifically during the autumn/early winter following an extreme summer minimum event. Using simulations from the Weather Research and Forecast model (v 3.0.1), we compare the atmospheric response for the case of the extreme sea ice minimum of 2007 to the corresponding response for the more typical ice conditions of 1984 - the year with median ice extent for the 1979-2008 satellite era. Increases in the area of open water lead to enhanced heat and moisture flux from the Arctic Ocean to the atmosphere during the subsequent autumn. Vertical fluxes of heat and moisture in late autumn and early winter 2007 rival those found in high-profile regions like the Great Lakes and Gulf Stream. We characterize the fluxes both horizontally and vertically and determine the spatial breadth of the influence of these fluxes. In particular, we focus on the influence of these fluxes on storm tracks and the spatial distribution of high and low pressure systems, emphasizing their potential impacts on subpolar latitudes.
C41B-0508
Recent trends in sea ice-associated phytoplankton blooms in the northern Bering and Chukchi Seas
The northern Bering and Chukchi Seas are among the most productive marine ecosystems in the world and act as important carbon sinks, particularly during May and June when seasonal sea ice-associated phytoplankton blooms occur throughout the region. Sea ice melt and breakup during spring strongly drive this production by enhancing light availability in the water column, enabling stratification and stabilization of the water column, and introducing a new source of nutrients to surface waters. Recent variability in sea ice extent in the northern Bering and Chukchi Seas may therefore have profound impacts on spring phytoplankton production. In particular, passive microwave (SMMR and SSM/I) satellite data show strong interannual variability in sea ice area during winter and early spring months over the 1978-2008 time series, but no statistically significant trends. However, for months adjacent to the ice-free season (June–November), significant declines occurred for June (-3222 km2 or -8.5%/decade), July (-902 km2 or - 24.5%/decade), September (-1105 km2 or -23.8%/decade), October (-3879 km2 or - 20.0%/decade), and November (-10521 km2 or -13.0%/decade), suggesting that sea ice is breaking up earlier and reforming later each year. In this study, we focus on utilizing multi-sensor satellite data to investigate key temporal and spatial linkages between sea ice variability and chlorophyll biomass throughout the northern Bering and Chukchi Sea regions. We incorporate AMSR-E passive microwave measurements of sea ice extent and Aqua-MODIS derived concentrations of chlorophyll-a over the years 2002-2008. Remotely sensed measurements are validated with in situ data collected onboard the US Coast Guard Cutter Healy during the spring of 2006, 2007 and 2008. Surface phytoplankton blooms and peaks in chlorophyll-a concentrations are nearly coincident with the onset of sea ice degradation and begin to decline within approximately two weeks of the start of the blooms. Recent trends of earlier sea ice breakup in the northern Bering and Chukchi Seas therefore directly affect the timing and also likely the intensity of phytoplankton blooms. With earlier sea ice breakup, although sea ice melt still enables stratification and provides nutrients to surface waters, sunlight conditions may not yet be ideal for phytoplankton blooms and the intensity of chlorophyll-a peaks may thus be dampened. The potential implications of these shifts in the timing of sea ice breakup might reasonably include lowered overall productivity if seasonal sea ice retreat continues as expected.
C41B-0509
A Comparison of Particle and Organic Carbon Fluxes to the Deep Canada Basin Between 2004-2005 and 2007-2008
Two of the most blatant impacts of climate change in the Arctic are destabilization of permafrost soils, and a reduction in sea-ice cover, altering carbon cycling both on land and in the ocean. In particular, higher terrestrial carbon fluxes to the ocean, and both enhanced and an offshore extension of marine productivity, are anticipated. However, the fate of this carbon (i.e., remineralization versus burial) in the Arctic Ocean, as well as the extent to which the deep basin waters and underlying sediments will sequester carbon in the face of rapidly changing sea-ice, hydrographic and biogeochemical conditions, remain unknown. Answers to these questions are crucial to our understanding of whether the Arctic will serve as a net carbon dioxide source or sink to the atmosphere, and how pelagic ecosystems will respond to these perturbations. In this study, we compare the flux of particulate carbon and associated elements to the deep Arctic Ocean from a bottom-tethered sediment trap deployed at 3000m in the southwest Canada Basin during 2004-2005 and 2007-2008. Particle fluxes and compositions are compared in the context of different seasonal sea-ice and hydrographic conditions between these deployment periods. Overall particle fluxes during the 2004-5 deployment were extremely low, and mass flux variations were temporally decoupled from the cycle of primary productivity in overlying surface waters. Moreover, geochemical analyses showed that the sinking particulate matter was characterized by aged organic carbon and abundant lithogenic material, which together suggest that the majority of the particulate organic carbon entering the deep Canada Basin was supplied from the surrounding margins (Hwang et al. 2008, Geophys. Res. Lett. 35, L11607). These data will be compared with particle fluxes during the 2007-2008 year when summer sea ice was less extensive.
C41B-0510
Summer minimum Arctic sea ice extent and the associated summer atmospheric circulation
Interrelationships between year-to-year variations in September Arctic sea ice extent and summer sea level pressure and surface air temperature at high northern latitudes are examined making use of microwave satellite imagery and atmospheric data for the period 1979-2006. Linear trends and year-to-year variability about the linear trend lines are considered separately: the latter gives a clearer indication of the physical linkages between fields. Years with low September sea ice extent tend to be characterized by anticyclonic circulation anomalies over the Arctic, with easterly wind anomalies over the marginal seas where the year-to- year variability of sea ice concentration is largest. It is hypothesized that the summer circulation anomalies cause sea ice extent principally by way of the Ekman drift in the marginal seas. The associated surface air temperature anomalies also tend to be largest over the marginal seas, with positive anomalies over the regions of reduced sea ice.
C41B-0511
Holocene Arctic Paleoceanography from Ostracode Shell Chemistry and Faunas
Sediment records from the Lomonosov, Mendeleev, and Northwind Ridges, Morris Jesup Rise and Yermak Plateau were used to reconstruct Arctic Ocean paleoceanography and sea ice during glacial, deglacial (Termination 1) and Holocene Arctic records to provide a long-term context for today's climatic changes and rapidly declining sea ice cover. We focus on improving benthic and epipelagic (sea-ice dwelling) ostracode shell chemistry (Mg/Ca, Sr/Ca ratios) and faunal assemblages as proxies for bottom water temperature, the strength of inflowing Atlantic water, sea-ice extent, surface productivity, and ice rafting from continental shelf areas using a 470-sample coretop database and Holocene sediments recovered during expeditions over the last 20 years. Three useful proxies are the epipelagic species Acetabulastoma arcticum (Arctic sea ice), Mg/Ca ratios in the deep benthic species Krithe glacialis (paleothermometery), and the genus Polycope (surface productivity indicator). Results indicate a period of seasonally ice-free conditions and high-surface water productivity in the Amerasian and central Arctic basins beginning at the Younger Dryas- Preboreal transition (11.5 ka) lasting several millennia. The mid-late Holocene is characterized by progressive sea-ice development coincident with large-scale climate changes during the Neoglaciation. We will also present preliminary coretop and Holocene results on Sr/Ca content in ostracodes, which varies by 10-15% in deglacial-Holocene Krithe, and may signify changes in salinity and nutrients.
C41B-0512
New JOPL special issue on late Holocene climate inferred from Arctic lake sediments
–Late Holocene Climate Change Inferred from Arctic Lake Sediments" is a collection of new papers currently
in press as a Special Issue of the Journal of Paleolimnology. It is a contribution from the arctic
paleolimnology community to the 2007-2008 International Polar Year, and is a coordinated response to the
pressing need for additional, high-quality proxy-climate records from high latitudes. The 14 papers in the
volume contribute to the long-term perspective on natural climate variability that is needed to understand
historically unprecedented changes now occurring in the Arctic. This Special Issue includes lakes from across
the North American Arctic and subarctic, as well as northwestern Europe. The studies generally focus on the
last 2000 years, and several extend further back in time. The new paleoclimate records include some of the
longest and highest-resolution records currently available from the Arctic. The studies rely on several proxy
indicators to reconstruct past climate, including: varve thicknesses, chironomid, diatom, and pollen
assemblages, biogenic-silica and organic-matter content, oxygen-isotope ratios in diatoms, and the
frequency of lake-ice-rafted aggregates. These proxies primarily document changes in past summer
temperatures, the main control on physical and biological processes in lakes at high latitudes. Some reflect
other aspects of the climate system, including nutrient availability and storm-track trajectories. The proxy data
for all records are available at the World Data Center for Paleoclimatology. The records will be integrated
into a larger network of paleoclimate sites to investigate the spatial and temporal variability of climate change
and to compare the paleoclimate inferences with the output of general circulation models.
http://www.arcus.org/synthesis2k/
C41B-0513
A Millennial-Scale Sea Surface Temperature Record From the North Atlantic Based on Diatoms
Sea surfaces temperatures (SSTs) are generated from a 1000-year-long sediment core from the eastern flank of Reykjanes Ridge in the subpolar North Atlantic with a time resolution of 2-10 years. 54.3 cm long box core (Rapid 21-12B) and 370 cm long gravity core (RAPID 21-3K) were recovered from deep-sea sediments (2630 m water depth) during the RRS Charles Darwin cruise 159 in 2004. The box core is dated using the 210Pb method and it is continuously subsampled and investigated at 0.5 cm intervals for the last 230 years with a two years average time resolution. The gravity core is dated 14C AMS method and it is investigated continuously at 1.0 cm intervals with a ten years average resolution for the interval representing 230-1000 cal. years BP. August SSTs are reconstructed using marine planktonic diatom species with the Weighted Averages - Partial Least Squares (WA-PLS) method. Results achieved from the box core indicate August SST warming of c. 1 °C from 1773 AD to the present. The interval 1773-1830 represents the cold period at the investigated site. It is followed by warm period between 1830 and 1885. After this the temperature frequency is more stable with short cool events around 1890 and 1930. The last 60 years represent the warm period with a slow warming trend, especially during the past 25 years. However, results do not indicate distinct SST warming since 1870s. The most high-frequency SST variability with amplitude of c. 1 °C appears after 1970 indicating several very warm years, but also coldest years since 1820s.
C41B-0514
Late Holocene Storminess in the Labrador Sea Region: Exotic Pollen and Aeolian Dust as Indicators of Variability?
The spatial distribution of sea ice in the Arctic is strongly affected by wind stress. Assessing any modern changes in sea ice distribution requires them to be set in the context of natural (background) climate variability over longer time periods than those supplied by instrumental and/or historical records. Palaeo- indicators of regional storm activity are, however, rather rare and although ice cores records can provide continuous, high-altitude, proxy archives of air circulation, ice caps may not be representative of conditions at sea level. The Labrador Sea region is noted for being crucially located with respect to the main track of North Atlantic cyclone systems. Analysis of wind-blown sediments retrieved from fjords both in the southwestern (Placentia Bay, Newfoundland) and in the northeastern (Narsaq Sund, Greenland) Labrador Sea may, therefore, highlight any variability in regional storm activity through time. The wind-blown sediment includes the aeolian dust fraction and long distance (exotic) pollen. The aeolian dust fraction is determined by particle size analysis and end-member modelling and mostly indicative of the windier, winter conditions produced by seasonally steep atmospheric gradients. Variability in past air circulation in spring and early summer is inferred by the concentrations of exotic pollen. Information about storminess variability in South Greenland and Newfoundland regions may contribute to our improved understanding of the interaction between ocean circulation and sea ice distribution and large-scale North Atlantic atmospheric circulation patterns.
C41B-0515
Causes of 15th century warming event in the Arctic in a coupled climate model including data assimilation
A group of simulations performed over the last millennium with a three-dimensional climate model of intermediate complexity are forced to follow temperature histories obtained from a recent compilation of well- calibrated surface temperature proxies using a simple data assimilation technique. Those simulations provide a reconstruction of the climate of the Arctic that is compatible with model physics, the forcing applied and the proxy records. Available observational data, proxy-based reconstructions and our model results suggest that the Arctic climate is characterized by important variations of temperature over the last millennium. Even though the last decades are likely to be the warmest of the last millennium, other particular warming episodes also took place previously in the Arctic. For instance, the Arctic experienced an important warming during the 1920-1940 period. Temperatures at this time were similar to the recent ones. Likewise, our model reconstructions show a particularly warm period at the end of the 15th century. This warm event is likely related to the internal variability of the climate system rather than being caused by external forcing. We point out the role of the different mechanisms that could have been responsible for the changes during this period and we advance the coherent hypothesis that changes in atmospheric circulation could be the main cause of the Arctic warming during the late 15th century warming period.
C41B-0516
Possible Evidence for Enhanced Seasonality During the Little Ice Age Indicated by Multiple Isotopes from Kepler Lake, South-Central Alaska
We present multiple-proxy data from two short cores (85 cm and 101 cm) from Kepler Lake, an evaporation- insensitive, groundwater-fed marl lake in South-Central Alaska to reconstruct climate and environmental changes in the recent centuries. The proxies used include calcite C and O isotopes, organic matter (OM) C and N isotopes, and loss on ignition (LOI) analysis. Two cores can be visually correlated based upon LOI results. A 600 year chronology was established based on 3 calibrated AMS 14C dates of terrestrial macrofossils and 210Pb analysis. δ18OVPDB values of inorganic calcite range from - 17.0‰ to -15.7‰, with the highest values between 1470 and 1840 AD during the Little Ice Age (LIA). The relatively high δ18O values during the cold LIA contrast with the conventional temperature interpretation of O isotopes. Therefore, the isotopic shifts around the LIA were likely caused by a shift in atmospheric circulation. A weakening of the wintertime Aleutian Low pressure system residing over the Gulf of Alaska would result in enriched 18O in precipitation as well as a colder winter climate in SC Alaska. During the LIA period CaCO3 contents were elevated by ~15% to >80%. Calcite precipitation in freshwater lakes is primarily a function of summer temperature; we propose that the LIA in SC Alaska represents a period of colder winters and warmer summers. This interpretation is also supported by C isotopes, reflecting aquatic productivity. Both δ13C OM and δ13Ccalcite (relative to VPDB) exhibit relatively high values at 1600 to 1840 AD, after which δ13COM remains constant throughout the rest of the record and δ13Ccalcite declines after the LIA. The delayed increase in δ13C of OM and calcites at 1600 AD may reflect the differential responses of calcite precipitation and aquatic productivity. The negative shift of ~1‰ in δ18O in the 1840s has also been documented in ice cores from Mt. Logan and in marl lake sediments from the southern Yukon, suggesting a broad-scale shift in atmospheric circulation at the end of the LIA. The enhanced seasonality during the LIA may explain some complex moisture and temperature changes as observed from glaciers, lakes and tree-ring records in the region. We speculate that these shifts during and after the LIA may be basin-wide events in the Pacific Ocean.
C41B-0517
Annual and Seasonal Dynamics of Benthic Foraminifera and Their Sensitivity to Environmental Changes in Arctic Fjord
Benthic foraminifera are widely used as climatic and environmental proxy in marine records. Their usefulness as proxy of past environmental conditions, however, depends entirely on how well they are calibrated to their biological, chemical, and physical surroundings. From the Arctic regions there is still limited knowledge on how the benthic foraminifera relate to the present polar conditions. In this study we investigate how the benthic foraminifera relate to the present environment, and in particular how rapidly they respond to the changes in the flow of warm and saline Atlantic water into the Arctic. Kongsfjorden (Svalbard, Europen Arctic) is a glaciated Arctic fjord characterized by the presence of Arctic and Atlantic waters. Tidewater glaciers at the head of the fjord discharge freshwater and suspended sediment into the inner parts of the fjord, whereas predominantly marine conditions prevail at the fjord mouth. In summer, frontal instabilities at the outer and inner parts of Kongsfjorden result in the intrusion of warm and saline Atlantic waters into the fjord. The volume of such intrusions varies interannually, and in the 2006 and 2007 they have been significant. Concurrent with CTD measurements we sampled multicore samples annually (years 2005-2007) in a transect from the mouth of the fjord to the inner part of fjord and analysed the content of living (Rosa Bengal stained) benthic foraminifera. The recent alterations in Kongsfjorden hydrology shifting from 'cold' (2005) to 'warm' (2006-2007) years resulted in changes in the relative composition of the benthic fauna. Two common species Elphidium excavatum f. clavata and Cassidulina reniforme known as co-dominant species in harsh glaciomarine environment decrease in abundance during the warmer years in the entire Kongsfjord area. We have also multicore samples from spring (April 2008) and summer (August 2007) that allows for preliminary investigations of the seasonal faunal changes. Comparing spring (April 2008) and summer (August 2007) shows that some species mainly occur during spring which probably is related to different stages of primary production in the fjord. In the early spring one species Silicosigmoilina groenlandica show much higher abundance than during summer, suggesting it is exploiting the early spring bloom and is then successively replaced by other species during the summer.
C41B-0518
Paleohydrology of the Yukon River Basin during the last 2 millennia; century-scale groundwater flux variations in a closed-basin lake
The Yukon River Basin (YRB) encompasses 330,000 km2 of interior Alaska, U.S.A and Yukon Territory, Canada and recent warming is raising concern about detrimental effects on permafrost, surface and sub- surface hydrology, terrestrial vegetation and consequences for carbon storage, respiration and/or delivery to marine ecosystems. To evaluate recent changes within the context of past natural variations, we are using multi-proxy data from lake sediments within the YRB to extend our understanding of hydroclimatic variability beyond the historic period. Oxygen isotope data from lakes and ice cores in the upper reaches of the YRB in the southwest Yukon Territory have documented regional changes in moisture delivery that had varied by atmospheric circulation around the Aleutian Low (AL). These previous lake studies utilized low (high) lake- water oxygen isotope sensitivity to evaporation caused by their relatively high (low) groundwater fluxes and short (long) lake-water residence times, respectively. Between these two extremes are lakes with oxygen isotope sensitivity to groundwater flux and evaporation that we present here from Seven Mile Lake, located in the central Yukon Territory. Seven Mile Lake sediments from the nearly anoxic hypolimnion at 12m water depth are alternating units of homogenous organic gyttja and fine-scale endogenic carbonate laminations. These visual sediment variations are recorded at 0.5 cm increments by fluctuating inorganic carbon concentrations that are inversely related to carbonate oxygen isotope ratios (high carbonate = low d18O). This suggests higher lake water calcium concentrations caused by increased groundwater fluxes due to a cooler/wetter climate compared to periods with low carbonate and 18O-enrichment caused by more evaporation. The data indicate that century-scale groundwater flux variations were superimposed on a long-term trend towards increased aridity during the last 2000 years. Higher groundwater fluxes occurred during the Little Ice Age (1300-1700 AD) but a number of shorter, decade-long dry intervals also regularly occurred. These data provide new, continuous detail to YRB climate reconstructions that assist estimations of past basin wide water budgets and better inform how recent trends compare with longer term variations.
C41B-0519
High-resolution Paleolimnological Evidence of Climate Variability in the Central Canadian Arctic During the Past 2000 Years
Our intent is to assess whether widely recorded thermal events such as the widespread cooling typical of the Little Ice Age (LIA ~1300 to 1850 AD) and the recent warming of the 20th century are captured in lake records in the central Canadian Arctic. Instrumental climate records from the central Canadian treeline zone display a pattern of variation similar to general Northern Hemisphere temperature trends. To examine whether this general correspondence extends back beyond the instrumental record we obtained a sediment core from Lake S41, a small lake in the Northwest Territories of Canada at 63°43.11' N, 109°19.07' W. The sediments were analyzed for organic-matter content by loss on ignition (LOI), biogenic-silica content (BSi), and midge community composition to reconstruct July air temperature and summer water temperature. The paleolimnological records were compared with records of atmospheric CO2 concentration, solar variability, and hemispheric temperature variations over the past 2000 years. The results of the analyses suggest that widely documented long-term variations in Northern Hemisphere temperature, namely the cooling during the Little Ice Age (LIA), and 20th century warming, are represented in the central Canadian treeline zone. We also recovered a sediment core from Lake V57 (69 39.41' N, 105 21.44 W), a small lake located in southeast Victoria Island, one of the largest islands in the Canadian Arctic archipelago, to asses whether evidence of radiative forcing and hemispheric patterns of temperature change exist north of the central Canadian treeline zone. Midge-based inference models for average July air temperature (AJAT) and summer surface water temperature (SSWT) were applied to the subfossil midge stratigraphy from Lake V57 to develop a reconstruction of past thermal conditions. The results from Lake V57 are compared to other temperature sensitive paleolimnological records from adjacent portions of the central Canadian Arctic to assess the geographic extent of changes in thermal conditions during the 2000 years. The pattern of warming over recent centuries seen in southeast Victoria Island is broadly consistent with temperature- sensitive biogenic silica (BSi) records from a number of sites in the central Canadian Arctic and the treeline zone to the south suggesting a regionally synchronous response in the recent past to climate forcing.
C41B-0520
Late Glacial and Holocene Environments of South-Central Melville Island, Northwest Territories, Canada
A sediment core from Lake BC01, on Melville Island, in the western Canadian Arctic Archipelago, provides multiple-proxy evidence to assess climatic variability over multiple timescales. Analysis of fossil pollen, diatoms, and several sediment parameters indicate significant transitions at 7.3, 5.0, 3.2, 1.0 and 0.07 ka. Fossil pollen is present throughout the record, and is dominated by grass (Poaceae), indicating the presence of a High Arctic vegetation community since Late Glacial times. Prior to 7.3 ka, diatoms were nearly absent, and high magnetic susceptibility and low organic matter content suggest an unstable landscape and perhaps cool conditions. Organic matter accumulation in the sediment increased beginning at 7.3 ka, although there were no diatoms in the sediment until 5.0 ka. Relatively high diatom concentrations suggest warmest conditions in the region between 5 and 3.2 ka. Low diatom production, but high richness, occurred between 3.2 to 1.0 ka; diversity decreased during the Medieval Warm Period, but increased again during the Little Ice Age. An abrupt warming occurred during the past 70 years at the site, as indicated by a rapid increase in diatom and biogenic silica influx. The results presented here suggest that the magnitude of 20th century climate change in the region is comparable to that of the last 2000 years.
C41B-0521
Recent Expansion and Retreat of Independent Glaciers in Greenland
One measure of the cryosphere behavior over the recent past is glacier extent. Here we report on small ice cap and glaciers in eastern Greenland that were at least as restricted 1000 yr ago as they were in 2005. A preliminary survey along Scoresby Sund revealed at least 4 locations where organic remains, mostly arctic willow and mosses, are being uncovered as the glaciers melt. These sites range from the present coast to some 100 km inland at the head of Scoresby Sund and are typically about 500-800 m in elevation. Some are ice caps so very sensitive to small ELA changes. All independent glaciers have distinct Historical moraines. Drift outside these moraines is at least 10,000 yr old based on 10Be dating results. These results indicate that Historical ice extents were the most extensive since early Holocene time. The organic remains are present on the melting ice surface, within debris bands in the glacier, and in growth position. The ages of plants on the coast as as young as 1,040±20 14C yr BP, whereas the remains from sites further inland are slightly younger ~865±25 14C yr BP. Some remains are as old as 2760±30 14C yr BP to as young as 260±30 14C yr BP. Along the edge of a small ice cap in Milne Land, the ice cliffs expose at least three unconformities suggesting that partial melting and regrowth of the ice cap has occurred in the past. A wood sample from the youngest unconformity yields an age of 440±35 14C yr BP which may be considered a maximum bracket on the last growth of the ice cap. Thus the ice caps grew to a extended position in the late Holocene then may have undergone minor decay and regeneration before retreating to their present positions.
C41B-0522
Ice-Tethered Profiler Observations of Dissolved Oxygen in the Arctic Ocean Under Permanent Ice Cover
Two Ice-Tethered Profilers (ITP), deployed in 2006 and 2007, have provided year-round dissolved oxygen (DO) measurements from the surface mixed layer to 760 m depth under permanent ice cover in the central Canada Basin of the Arctic Ocean. A further two ITPs deployed in August 2008 are presently returning DO measurements from the northern Canada Basin and the Makarov Basin. The two ITPs that have returned year-long time series exhibit seasonal variability in DO that primarily reflects primary production and respiration; fractional oxygen saturation varies accordingly. The time evolution of DO is similar for the two ITPs, indicating a predominately temporal variability. Beginning in April, under-ice DO in the central basin increases due to photosynthetic production, reaching a maximum in July. Under-ice DO decreases between August and November and a prominent shallow oxygen maximum is observed immediately below the surface mixed layer. This DO maximum is likely the result of accumulation of oxygen in summer which is then prevented from escaping by the strong stratification. The maximum is destroyed at the onset of winter by convection and deepening of the surface layer. The ongoing DO measurements are key to understanding changes to primary production under thinning ice conditions characterized by higher under-ice light levels, and changing Arctic seasonality.
C41B-0523
Investigation of processes operating at the transition to ice-free conditions: a study with a global climate model
The environmental impacts of a shrinking Arctic sea ice cover are explored with a coupled climate model, CCSM3.0, via simulations in which atmospheric carbon dioxide is increased by 1% per year until stabilization at 8xCO2. The transitions to seasonally ice-free conditions and year-around ice-free conditions are investigated and compared against each other. The reversibility of ice loss is explored with two simulations: a first one for which pre-industrial atmospheric carbon dioxide is instantaneously reestablished, and a second one with a gradual decrease. The role of different feedbacks on the evolution of ice cover and its recovery after complete removal is analyzed. Among those feedbacks, special focus is given to cloud feedbacks, albedo feedbacks and changes in ocean circulation and Arctic ocean stratification.
C41B-0524
Observing Recent Changes in the Large-Scale Arctic Energy Budget
Changes in the large-scale energy budget of the Arctic are examined using a variety of next-generation reanalysis and observational data. An effort is made to construct a best-guess of the current arctic energy budget using a variety of atmospheric data. For the period of 2000-2005, monthly means from the Clouds and the Earth's Radiant Energy System (CERES) data represents the current most-reliable top of atmosphere radiation budget. The remaining components of the energy budget system in the arctic polar cap (defined as 70 degrees North latitude circle), comprising of the vertically-integrated storage and horizontal transports of energy, and net heat transfers between the atmosphere and the subsurface column, are diagnosed using the Japanese 25-year Reanalysis Project (JRA-25) and the NCEP/NCAR Reanalysis (NRA). The as then record-setting minimum sea-ice extent during the 2005 melt season is used as a marker of recent changes occurring in the arctic climate system. However, changes in each reanalysis differs than the satellite observations. In one example, when compared to the 2000-2005 climatology, CERES shows a shift in the peak TOA radiation from July to June in 2005, a change that is absent in the reanalyses and directly attributable to the early and pronounced albedo reduction. An earlier peak in TOA radiation can strongly modulate the flux energy convergence from lower latitudes through circulation changes. Here, the energy budget framework provides a simplified view of the pathway through which changes of key component parings occur.