PP21B-1374 0800h
Seismic Investigation of El'gygytgyn Lake, Chukotka (NE Siberia)
Lake El'gygytgyn is a 3.6 Mio years old crater lake located in Central Chukotka, NE Russia, with a water depth of 170 m and a diameter of 12 km. Not having been glaciated ever since, it would reveal a paleoclimatic record unique in the Arctic realm. During the last years it has become a major focus of multi-disciplinary international research as a target for deep drilling in the near future. During expeditions in 2000 and 2003, reflection and refraction seismic combined with high resolution 3.5 kHz echosound profiling was carried out. Raytracing of the sonobuoy refraction data reveals a four-layer model of the lake that is interpreted as follows: (a) upper sedimentary unit, consisting of lacustrine muds with velocities of around 1500 m/s and a thickness of about 170 m, (b) lower sedimentary unit, consisting of lacustrine muds with velocities of around 1650 m/s and a thickness between 80 and 200 m, (c) fallback breccia with velocities of about 3000 m/s and a thickness between 50 and 300 m and (d) brecciated bedrock with velocities of > 3600 m/s. The brecciated bedrock forms a central uplift structure which is almost levelled by the overlying fallback breccia. The lower sedimentary unit drapes the smooth topography of the fallback breccia, whereas the upper sedimentary unit is almost flat. Small faults are associated with the central uplift structure and have been active until recently. Reflection seismic data indicate that the upper sedimentary unit is characterized by well stratified sediments, whereas the lower sedimentary unit is more massive. The upper sedimentary unit is locally intercalated with debris flows to a depth of at least 160 m subbottom. Debris flows are more common in the western part of the lake and along the slopes. The 3.5 kHz profiling allows a detailed mapping of the debris flow distribution. At the proposed drillsite near the centre of the lake, the sediments appear to be well stratified and largely unaffected by debris flows and promises a almost undisturbed paleoclimatic record reaching back one million years prior to the first major glaciation of the Northern Hemisphere. A second possible drillsite is located more proximal to the western shoreline and is, therefore, more intercalated with debris flows.
PP21B-1375 0800h
Biomarker investigations of a 250ka old Lake El'gygytgyn sedimentary record - preliminary results
Lake El'gygytgyn is a 3.6 Mio year old impact crater lake located in central Chukotka, NE Siberia. In recent times the lake is frozen from about October till June with only a few months of open water during the summer. The sedimentary record of the lake has become a major focus of multi-disciplinary multi-national paleoclimatic research. In 1998 and 2003, two 13m (PG1351) and 16m (LZ1024) long sediment cores, respectively, were recovered from the central part of the lake which comprise the last approx. 250ka BP. Based on previous lithological and biogeochemical studies the sediments of PG1351 can be sub-divided into four different facies units showing cyclic alternations. These units were associated with warm, peak warm, cold and dry, and cold and more moist climates, respectively. We have carried out biomarker analysis on such sediment units of core LZ1024. Our chronology is based on matching a magnetic susceptibility record of this core with that of the previously dated core PG1351. Biomarkers such as n-alkanes, dinosterol, brassicasterol, cholesterol, isoarborinol, campesterol, and sitosterol have been identified and quantified by gas chromatography and gas chromatography/mass spectrometry. High TOC values during cold phases with perennial lake ice coverage indicate that organic matter was less affected by degradation as a result of water body stagnation compared to warm phases with summer melt and complete mixing. Our biomarker results exhibit ratios of long- to short-chain alkanes being high during warm and peak warm phases. This suggests that terrigenous input (including long-chain alkanes derived from land plants) was enhanced and/or autochthonous organisms (synthesizing short-chain alkanes) were less abundant as a result of a mixing water column during the summer period. Consistently, the records of dinosterol and brassicasterol derived from aquatic organisms (dinoflagellates and diatoms) exhibit distinctively higher (3 to 10 times) concentrations during cold phases as compared to warm phases. This suggests a relatively high aquatic productivity and preservation during perennial lake-ice coverage in comparison to phases with water column mixing during arctic summer periods. Our biomarker results suggest that the sediments of Lake El'gygytgyn reflect past aquatic organism assemblages and the composition and sources of terrigenous organic matter input. Both appears to be mostly controlled by seasonal ice cover. Thus, biomarkers can be used to interpret the evolution of Lake El'gygytgyn as a reflection of changes in the arctic climate.
PP21B-1376 0800h
Slope Evolution At The Margins Of Lake El'gygytgyn Impact Crater In Late Quaternary Time
Periglacial environmental changes are thought to trigger sediment export into Lake El'gygytgyn and we studied the permafrost surrounding and its properties to amend paleoclimate reconstructions using the lake sediment record. We use the following two distinct periglacial formations as complementary paleoenvironmental archives. First, after collecting GPR data for localizing undisturbed sediment layers a five-meter permafrost core was recovered, which penetrated a discrete terrace formation. In contrast to sediment studies in mid and low latitude areas we made use of the chemical composition of the texture ice, which allows defining individual phases of terrace sedimentation derived from solifluctional downhill movement of sediment. The ice-rich cryostructure results from repeated cycles of thawing and freezing within the soil moisture of the active layers. Stable oxygen isotope signatures and composition of light soluble anions and cations within supernatant texture ice shows prominent relative maxima and minima. They are associated with more humid and more arid phases during the terrace formation. Heavy mineral composition shows a constant uphill source area, the predominance of a silt-sized sediment matrix and single grain surface textures reveal tracks of cryogenic weathering. Second, an ice wedge formation contained in the terrace deposits has been sampled. It allows distinguishing a wide-meshed polygonal growth in the bottom part from a more close-meshed polygonal growth to the top. The upper generation of close-meshed polygons is associated with more arid conditions of a modern cold continental climate, a fact resembling the results from frozen sediment studies. Stable oxygen and deuterium isotope composition documents a Holocene age for the ice wedge growth. It suggests an epigenetic origin of the wedge ice within older sediments and it confirms that the wedge ice inhibits a stable isotope signal for winter precipitation, whereas the surrounding texture ice within the frozen sediments contains a mixture of winter and summer precipitations. Finally, a conclusive sedimentation model is given. It illustrates solifluctional terrace formation during interstadial time at a continental permafrost site.
PP21B-1377 0800h
Luminescence Dating: a new Chronometer for Arctic Ocean Sediments
Late Cenozoic sediments from the Arctic Ocean contain a record of variable ice cover and of related paleoclimatic conditions. However, a common chronostratigraphy has been elusive, especially in the range 30-780 ka. Attempts at stratigraphic correlations within this age range depend on assumptions about sedimentation rate, and about correlations with more southern ocean sediments. Luminescence sediment dating potentially can provide direct burial ages of siliclastic mineral grains within such sediment cores. Forty-six "known-age", fine-silt samples from 20 cores along a transect from the northern slope of Alaska to near Fram Strait were analyzed by thermally-stimulated (TL) and infrared-stimulated (IRSL) luminescence sediment-dating procedures. Unsupported ("excess") Th-230 and Pa-231were measured by thick-source alpha-particle counting. The cores represent water depths from 400 m to 4200 m. Compared to C-14 ages from core-top material, IRSL ages are too old by variable amounts, ranging from 4 kyr (insignificant when dating samples $ > $ 50 ka) at the Northwind Ridge (western Arctic Ocean) to ca. 10 kyr over some ridge tops, and to 190 kyr from the deepest sample (near the Gakkel Ridge). The geographically variable age overestimates indicate the spatial irregularity of "inherited" luminescence, attributable to the diverse effects of ice-rafting and other processes, such as turbidity and bottom-current resuspension. Samples likely as old as ca. MIS 5 were analyzed at the Northwind Ridge site. An IRSL age of 85$\pm$5 ka was obtained just above the penultimate foram (N. pachyderma) abundance peak, while TL and IRSL ages of 99$\pm$9 ka were obtained for the younger part of the next deepest abundance peak. These results indicate the great potential for various versions of luminescence sediment dating in some regions of the Arctic Ocean. The TL method has the potential to reach back beyond 500 ka. Interglacial-stage (foram-'rich') sediments from ridge tops are probably most suitable for luminescence dating.
PP21B-1378 0800h
Holocene Vegetation Changes in Eastern Kamchatka Based on Pollen and Macrofossil Records
Little is known about the Quaternary climate and vegetation history of Kamchatka. Only a few previous studies have provided paleoenvironmental information for this area, but these studies have poor age control and are inconsistent To reconstruct paleoclimate and both regional and local vegetation history we are analyzing continuous, high-resolution pollen and macrofossil records from peats on Kamchatka. Thin, well-dated ash layers in these peats provide excellent age control; sections sampled thus far range in age back to 12,000 years. Herein we report results from one example, the Uka peat section (57.8$^{o}$N 162.2$^{o}$E; about 10 m a.s.l.). This section is located on a morainal terrace close to a small lake. The basal section is lacustrine clay with a few spores pointing to scarce vegetation under cold conditions, probably latest Pleistocene. This clay is replaced upward by limnic peat, probably early Holocene (pollen zone 1). This zone is characterized by dominance of shrub alder and birch, herbs, and ferns. The highest value (in the whole section) of sage and the absence of tree pollen suggest a treeless landscape and thin vegetation cover under still cold conditions, while increase in local aquatic pollen indicates lake filling and shrinking. In Zone 2 (ca. 8000-4000 BP), mire vegetation shows successive development of eutrophic fen including three pulses of sphagnum followed by sedge peaks. Pollen concentration is very low, probably indicating high deposition rates. A warming trend is suggested by the appearance ca. 5600 BP of tree birch, increasing to the end of zone 2, while alder strongly decreases. The most pronounced changes in both regional and local vegetation are found ca. 3800 BP, when tree birch pollen reaches its highest value and a few long-transported spruce pollen grains appear. The dominant (eutrophic) sedge is suddenly replaced by a more oligotrophic one. Such local components as grasses, shrub birch and willow increase, and total pollen concentration dramatically increases. All these features suggest drier conditions in zone 3 (3800-1200 BP), however it should be supported by further studies. Zone 4 (1200 BP to present) shows development of an oligotrophic peat bog indicated by high values of heath; the establishment of shrub pine is signaled by a sharp increase in its pollen.
PP21B-1379 0800h
Quantitative Reconstructions of Beringian Climates from Pollen Assemblages
We reconstructed mean July temperature for the past 30 000 years at Zagoskin Lake (Ager. 2003. Quat Res 60:19) using the Modern Analogue Technique (MAT). Calibration was done using a dataset of 4549 modern pollen samples from across North America, with a pollen sum of 106 pollen taxa. Reconstructed temperatures show a cold full-glacial, warming in the late-glacial and early Holocene and a warm Holocene. Experimentation showed the results are robust with respect to various decisions that can be made in application of the method. Although many of the samples in the full-glacial could be considered non-analogue, the temperatures estimated are reasonable, and comparable to those estimated at samples that are considered analogue. Further, these samples are not greatly deviant when ordinated with modern samples from northern Canada and Alaska.
PP21B-1380 0800h
Elemental and Isotopic Constraints on the Late Glacial-Holocene Transgression and Paleoceanography of the Chukchi Sea
Results obtained from analyses of three sediment cores collected from the Chukchi Shelf at 55m, 80m, and 107m water depths show changes in the composition and quantity of sedimentary organic matter delivered to the core locations from the Late-glacial up to near modern times. Two of the cores (80m, and 107m) show an abrupt and substantial increase in the total organic content and a shift in stable carbon and nitrogen isotopic composition of the organic matter at 8000-9000 years BP. We interpret this shift to be indicative of increased marine primary productivity that subsequently led to the onset of denitrification, as observed today in modern Chukchi Shelf sediments. The shift coincides with the probable re-advance of sea ice coverage at 8500 14C yrs BP, as well as a shift in the Trans Polar Drift. Together, these occurrences suggest a major reorganization of Arctic paleoceanography at 8-9ka that has more or less persisted through the Holocene. A small increase in productivity is also observed at ~11ka in the 80m core that could coincide with the first onset of Bering Strait through-flow following the LGM. Additional studies are underway to help constrain sea ice conditions at various times during the Holocene in the Bering and Chukchi Seas in order to elucidate the relationship between former ice coverage regimes and primary production.
http://www.geo.umass.edu/beringia/index.html
PP21B-1381 0800h
Regional Response of Central Beringian Climate to Circumarctic Ice Sheets and Orbital Parameters Since the Last Interglaciation: Model-Data Comparisons
The quaternary history of Beringia is marked by rapid glacial advances during periods of high eustatic sea level versus relatively limited ice extent when circumarctic ice sheets are growing at or near maximum. It is widely understood that changes in insolation are a primary forcing behind the growth and decay of continental ice sheets, but Beringian glaciations appear to be responding, on a regional scale, differently than the major ice sheets of the Northern hemisphere. Building on the work of field researchers in Siberia and Alaska, model-data comparisons are conducted to examine the mechanisms driving Beringian glaciations since the last interglacial. Boundary conditions are based on field data to set initial conditions of Northern hemisphere ice-extent, atmospheric and sea surface temperatures, sea level, and insolation. Simulations are run at 120,000 ka, 70,000ka, and 30,000ka with boundary conditions manipulated to examine the influence of large ice sheets on atmospheric circulation and moisture availability to the Beringian region. Other factors investigated include the effect of changes in sea surface temperature on Beringian climate, as well as alternate flooding and exposure of the Bering land bridge. The model simulations attempt to reproduce conditions evidenced by the data and test hypotheses for the underlying mechanisms driving Beringian glaciations. Ultimately, it is hoped that this research may shed light on the interplay between insolation, temperature, and moisture availability contributing to the growth of circumarctic ice sheets.
PP21B-1382 0800h
Deglaciation to Holocene Sea-ice History Over Navarin and Pervenets Canyons of the Northern Bering Sea Using Diatoms and Alkenones
The presence or absence of sea-ice over the Bering Sea influences the productivity of its waters as well as the regional climate of Beringia. Not only is the extent and distribution of sea-ice an important climate system feedback because of changes in albedo, it also has the potential to impact oceanographic and atmospheric circulation patterns by altering ocean stratification and impeding gas exchange between the atmosphere and ocean (Smith, et al, 2003). Understanding the history of regional climate change and atmosphere-ice-ocean systems across the western Arctic and North Pacific requires high resolution reconstructions of sea ice extent throughout the Bering Sea. The variability in the position of the ice edge since the last glacial maximum (LGM) has not been mapped or analyzed in detail in the Bering Sea. While it has been hypothesized that sea ice persisted in the eastern Bering Sea for at least 9 months per year during the LGM (Sancetta, 1983), sea-ice today covers only the Bering continental shelf areas 2 to 5 months per year (de Vernal and Hillaire-Marcel, 2000). What is not known are conditions during deglaciation and the Holocene when sea ice margin productivity in the Bering could have influenced the quality of nutrient rich waters flowing to the Arctic with submergence of the Bering Strait. This study presents preliminary results from a 1441 cm-long core extracted between Navarin and Pervenets Canyons. Future work will expand the spatial coverage to include several sites across the Bering Sea and will result in a compilation of maps showing the changing extent and duration of sea ice since the LGM. Down-core changes (at 500 year intervals) in diatom assemblages are documented since the LGM in order to detect the presence or absence of sea ice. Diatom species such as {\it Fragilariopsis grunowii}, {\it F. cylindrus},and {\it Thalassiosira gravida} are used as indicators of sea ice. These data are compared with the U$^{K}$$_{37}$ index derived from alkenones and a $\delta$$^{18}$O record from foraminifera in order to document changes in SST, nutrient cycling and productivity since the LGM.
PP21B-1383 0800h
Evidence From Diatom-Bound d15N for Bering Sea Stratification During the Last Ice Age and a Causal Link to North Pacific Denitrification Changes
Using a recently developed method for the isotopic analysis of diatom microfossil-bound N (Robinson et al. 2004), we have adapted the diatom cleaning protocol to be effective even in high-clay sediments of the Bering Sea. Newly generated profiles of diatom-bound d15N and opal content from the Bering Sea appear to reflect glacial-interglacial variations in local nutrient utilization. Interglacial conditions are characterized by low diatom-bound d15N (4-5 permil) and high opal content, indicative of relatively low levels of nutrient consumption and high diatom productivity, whereas glacial periods are predominantly characterized by high diatom-bound d15N ($\sim$8 permil) and low opal content. Taken together, these results suggest stratification of the Bering Sea upper water column during glacial times, with nutrient drawdown resulting from continued iron supply through atmospheric deposition. A steady increase in diatom-bound d15N and steady decrease in opal content throughout stages 5, 3, and 2 suggest a progressive increase in nutrient utilization. Two peaks in diatom-bound d15N are also observed upon deglaciation, before diatom-bound N relaxes to low Holocene d15N, and the presence of laminations during these periods suggests that local denitrification within the Bering Sea may have worked to raise the d15N of the nitrate feeding surface waters at these times. One interpretation of these data is that more complete consumption of nutrients in the Subarctic Pacific surface during the last glacial period lowered the nutrient content of the intermediate water formed in the North Pacific. This water then flowed equatorward to ventilate the low latitude thermocline. The decrease in the preformed nutrient concentration of thermocline water reduced productivity, subsurface O2 consumption, and denitrification in the low latitudes. These changes in denitrification, in turn, modulated the d15N of nitrate in the Bering Sea subsurface.
PP21B-1384 0800h
Penultimate Glaciation Succeeded by Huge Meltwater Pulse in the Northwind Ridge Area, Western Arctic Ocean
Piston core 92 PC 6 from the Northwind Ridge extends our understanding of the glacial history of the western Arctic Ocean back to the late stages of the penultimate glaciation, providing an over 100 cm long record of a full glacial - interglacial cycle. The most interesting aspect is a massive glacial meltwater event at the end of stage 6. The second to last ice age ended with a massive pulse of iceberg calving, indicated by massive amounts of IRD (ice rafted debris) with cm-size stones. The IRD composition points to an origin from the Northwest Territories, Arctic Canada, just like the end of the last glaciation. This event is followed by a quiet period with little IRD. Thereafter follows a massive peak of IRD with almost the same quantities as before, but this time the grains are very well sorted and have an upper size limit of about 2 mm. We attribute this peak to the combined action of glacial meltwaters, sorting the grains, and sea ice rafting to their place of deposition. Supporting this conclusion of a sea ice-rafting event instead of a glacial iceberg event is the abundance of detrital Fe mineral grains of fine sand-size from the Laptev Sea at about the same time. These Fe grains could only be transported by sea ice. Also, there is a spike in Fe grains from the Mackenzie River during the end of the Stage 6 meltwater (IRD) event and coarse IRD typical of glacial icebergs calving from this river have never been found in any Arctic sediments. This evidence of sorted IRD and Fe grains from unglaciated areas at the termination of glacial stages does not preclude that massive amounts of icebergs also calved from ice streams adjacent to Banks Island and also from the Queen Elizabeth Island (Canada) area prior to the meltwater events.
PP21B-1385 0800h
Investigating the Influence of Atmospheric Changes on the Variability of the North Pacific Using a Fully Coupled GCM
In this study we attempt to investigate whether changes in atmospheric concentrations in ozone and greenhouse gases (GHGs), including CO$_{2}$, N$_{2}$O, and methane, have an influence on decadal-scaled oceanic and atmospheric dynamics in the Northeast Pacific. Using a coupled ocean-atmosphere GCM (FOAM1.5) we simulate climatic conditions for the pre-industry and the present day while focusing on the North Pacific. We explore how the ozone hole over the Southern Hemisphere and increased concentrations in GHGs observed in the present day influence the Pacific (Inter)Decadal Oscillation (PDO) and the North Pacific High (NPH), two dominant modes of variability in the North Pacific. In each model we examine the spatial and temporal patterns of the NPH, sea-surface temperatures and salinities (SSTs, SSSs) as well as wind and ocean currents on the order of interannual to interdecadal time scales. We find that within these simulations the influence of the prescribed atmospheric perturbations is small. We observe that the present-day SST and SSS fields of the Northeast Pacific are similar in both model runs as well as sea level pressure. In both simulations the PDO dominates the patterns of variability in the North Pacific and does not appear to change either in character or expression as a result of the atmospheric perturbations. The atmospheric change caused by the rise in GHG concentrations and the decline in ozone is not prominent as a primary influence on the decadal scale variability within the Northeast Pacific.
PP21B-1386 0800h
Fluvial fingerprints in northeast Pacific sediments: Unravelling terrestrial-ocean climate linkages
As the earth's climate history becomes better understood, it becomes clear that the terrestrial and oceanic systems interact in complex ways. This is seen in core sites offshore the Pacific Northwest (PNW) of North America. A correlation can be seen in oceanic biostratigraphic assemblages and down-core changes in terrestrial pollen types. However, it is difficult to determine whether this relationship is the result of a coupled migration of terrestrial vegetation and oceanic fauna on millennial timescales or the result of changes in ocean circulation patterns that create more complex pollen pathways to the core sites. This research begins to unravel the answers to this problem by examining down-core changes in sediment provenance on millennial timescales. Preliminary data characterize sediment of 24 rivers from ten geologic provinces between latitudes 36° N - 47° N. Through clay mineralogy, major and trace element geochemistry and Ar-Ar "province" ages, ten of the 24 rivers can be uniquely identified, while six of the ten geologic provinces can be uniquely constrained geochemically. With further Nd, Sr and Pb isotopic analyses, we hope to constrain the non-unique sediment sources. We will also be presenting initial down-core geochemical results from cores EW 9504-17PC and EW9504-13PC, offshore southern Oregon and central California, respectively.
PP21B-1387 0800h
The Significance of Late Quaternary Mass Movement Events for the Paleoenvironmental Interpretation of Sediment Records from Lake El'gygytgyn, NE Siberia
The paleoclimatic and paleoenvironmental significance of sediment records from Lake El'gygytgyn, NE Siberia, may depend on the impact of mass movement deposits. In order to investigate their genesis, composition, and distribution, a subrecent debris flow at the western lake slope was identified by 3.5 kHz echo sounding and penetrated by two sediment cores. The composition of these cores was compared to that of a 16.7 m long core from the central lake part, where both shallow seismic data and the core composition exclude the existence debris flows. The lower part of the debris flow is stratified, probably reflecting an initial stage with sediment sliding and limited sediment mixture. Massive sediments above and in front of the stratified deposits indicate a second stage with turbulent transport. The debris flow led to basal erosion of ca. 1 m thick unconsolidated sediments along parts of his flow path. It produced a suspension cloud in the lake water, whose deposition caused the formation of a graded subrecent turbidite in the entire lake, i.e. not only in front but also on top of the debris flow. Hence, the turbidite was not a product of a density-driven turbidity current, but of 'pelagic rain' following Stokes' Law. In consequence, it was not erosive. According to the results from the long core, these conclusions are valid for the majority if not for all of the mass movement events that have taken place in Lake El`gygytgyn during the past 300 kyr. Given that one debris flow has produced one turbidite all over the lake, the total number of turbidites in this core (24) reflects the number of debris flows during the last 300 kyr. Altogether 19 turbidites were found in sediments deposited during warm periods. Only five turbidites were settled under cold climatic conditions. The composite thickness of all turbidites within the long core is 2.1 m (13 % of the sequence).