charcoal records from thermokarst deposits in central yakutia eastern siberia implications for forest fire history and thermokarst development abstract macroscopic charcoal records from a thermokarst lake deposit in central yakutia eastern siberia were used to reconstruct the history of forest fires and investigate its relationship to thermokarst initiation high accumulation rates of charcoal and pollen were coincident in the basal deposits of the thermokarst lake which suggests that both were initially deposited on the forest floor and subsequently reworked and accumulated in the thermokarst depression high charcoal and pollen accumulation rates in the basal deposits dating to 11 000 9000 cal yr bp also indicate that the thermokarst topography developed during the early holocene a lower charcoal accumulation rate after ca 9000 cal yr bp suggests that thermokarst development has been inhibited since this time it also indicates that a surface fire regime has been predominant at least since ca 9000 cal yr bp in central yakutia

diatom responses to 20th century climate warming in lakes from the northern urals russia changes in diatom assemblages and spheroidal carbonaceous particle scp profiles during the last 200 years in 210pb dated sediment cores from five remote arctic and sub arctic lakes in the northern urals were analysed the study area covers a large territory from arctic tundra in the north to boreal forest on the western slopes of the ural mountains in the south ph was reconstructed using a diatom based model the degrees of compositional turn over and rates of change were estimated numerically the 20th century diatom floristic shifts the rise in diatom accumulation rates and the rates of diatom compositional change in the northern ural lakes correlate well with june temperature in the region and with the overall circum arctic temperature increase from the 1970s the main driving force behind diatom compositional shifts in the study lakes are the changes in the duration of ice free season timing of water turn over and stratification periods and habitat availability changes in spheroidal carbonaceous particles show no pronounced effect on diatom assemblages pollution is restricted to regional sources originating mainly from the vorkuta coal industry changes in diatom plankton are more pronounced than changes in diatom benthos there is no clear north south gradient in degree of compositional changes with greatest changes occurring in lake vankavad situated in northern boreal forest the degree of the 20th century diatom changes in lake vankavad is greater than in most circum arctic and sub arctic lakes from northern europe and canada

a general cooling trend on the central tibetan plateau throughout the holocene recorded by the lake zigetang pollen spectra abstract a 741 cm long laminated sediment core covering the last 10 800 years was collected from lake zigetang central tibetan plateau 90 9 e 32 0 n 4560 m a s l and analysed palynologically at 69 horizons biome reconstruction suggests a dominance of temperate steppe vegetation mainly artemisia and poaceae on the central tibetan plateau during the first half of the holocene 10 8 4 4 cal ka bp while alpine steppes with desert elements mainly cyperaceae poaceae chenopodiaceae and characteristic high alpine herb families tend to dominate the second half 4 4 0 cal ka bp the artemisia cyperaceae ratio a semi quantitative measure for summer temperature indicates a general cooling trend throughout the holocene dense temperate steppe vegetation and maximum desert plant withdrawal however indicate that a suitable balance of wet and warm conditions for optimum vegetation growth likely occurred during the middle holocene 7 3 4 4 cal ka bp severe early holocene cold events have been reconstructed for 8 7 8 3 and 7 4 cal ka bp

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

time reversal imaging of seismic sources and application to the great sumatra earthquake the increasing power of computers and numerical methods like spectral element methods allows continuously improving modelization of the propagation of seismic waves in heterogeneous media and the development of new applications in particular time reversal in the three dimensional earth the concept of time reversal hereafter referred to as tr was previously successfully applied for acoustic waves in many fields like medical imaging underwater acoustics and non destructive testing we present here the first application at the global scale of tr with associated reverse movies of seismic waves propagation by sending back long period time reversed seismograms we show that seismic wave energy is refocused at the right location and the right time of the earthquake when tr is applied to the sumatra andaman earthquake 26 dec 2004 the migration of the rupture from the south towards the north is retrieved therefore tr is potentially interesting for constraining the spatio temporal history of complex earthquakes

late pliocene sedimentation in lake baikal implications for climatic and tectonic change in se siberia abstract within the framework of the baikal drilling project bdp a 192 m long sediment core bdp 96 1 was recovered from the academician ridge a submerged topographic high between the north and central basins of lake baikal sedimentological clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth corresponding to ca 2 4 3 4 ma the aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of northern hemisphere glaciation in late pliocene time a major climate change occurred in the lake baikal area at about 2 65 ma enhanced physical weathering in the catchment mirrored in the illite to smectite ratio and temporarily reduced bioproduction in the lake reflected by the diatom abundance evidence a change towards a colder and more arid climate probably associated with an intensification of the siberian high in addition the coincident onset of distinct fluctuations in these parameters and in the zr al ratio suggests the beginning of the late cenozoic high amplitude climate cycles at about 2 65 ma fluctuations in the zr al ratio are traced back to changes in the aeolian input with high values in warmer more humid phases due to a weaker siberian high assuming that the sand content in the sediment reflects tectonic pulses the lake baikal area was tectonically active during the entire investigated period but in particular around 2 65 ma tectonic movements have likely led to a gradual catchment change since about 3 15 ma from the western towards the eastern lake surroundings as indicated in the geochemistry and clay mineralogy of the sediments the strong coincidence between tectonic and climatic changes in the baikal area hints at the himalayan uplift being one of the triggers for the northern hemisphere glaciation

environmental variability in northeastern siberia during the last 13 300 yr inferred from lake diatoms and sediment geochemical parameters abstract in northeastern siberia russia a 1 2 m sediment core was retrieved and radiocarbon dated from a small and shallow lake located at the western side of the lower lena river n 69 24 e 123 50 81 m a s l the objective of this paper is to reconstruct the palaeoenvironmental variability and to infer major palaeoclimate trends that have occurred since 13 3 cal kyr bp we analysed the diatom assemblages sedimentology grain size total organic carbon toc total nitrogen tn and the elemental and mineralogical composition using x ray fluorescence xrf and x ray diffractometry xrd of the sediment core our results show parallel changes in the diatom species composition and sediment characteristics enhanced minerogenic sediment input and the occurrence of pyrite is indicative of a cold period between 12 7 and 11 6 cal kyr bp the diatom data enable a qualitative inference about the local ecological conditions to be made and reveal an oligotrophic lake system with alkaline and cold conditions during the earliest holocene moderately warmer climates are inferred for the period from 9 1 to 5 7 cal kyr bp the major shift in the diatom assemblage from dominance of small benthic fragilarioid taxa to a more complex diatom flora with an influx of several achnanthoid and naviculoid diatom species occurred after a transitional period of about 1400 yr 7 1 to 5 7 cal kyr bp at 5 7 cal kyr bp indicating a circumneutral and warmer hydrological regime during the holocene thermal maximum htm diatom valve concentrations declined starting 2 8 cal kyr bp but have been rising again since 600 cal yr bp this has occurred in parallel to the increased presence of acidophilous diatom taxa e g eunotia spp and decreased presence of small benthic fragilarioid species in the most recent sediments which is interpreted as the result of neoglacial cooling and subsequent recent climate warming our findings are compared to other lake inferred climate reconstructions along the lena river we conclude that the timing and spatial variability of the htm in the lower lena river area reveal a temporal delay from north to south

marginal ice zone rheology comparison of results from continuum plastic models and discrete particle simulations computer simulations of ice behaviour in the marginal ice zone miz are described two cases were studied a constant velocity wind blowing at certain angle with respect to the coast in what can be considered a simplified miz problem and a vortex wind force field the viscous plastic approach of hibler has been used to model the ice rheology we study the effects of adopting two different rheologies cavitating fluid and mohr coulomb the introduction of shear strength effects through a mohr coulomb yield criterion plays an important role in determining ice drift velocities in the miz the effects that ice floe collisions can have in determining the ice rheological behavior have been tested by using discrete particle simulations in order to do so simulations for the miz were performed using different mean floe sizes as well as different miz widths and wind conditions the predictions of the discrete particle simulations are in general agreement with the results obtained by using the mohr coulomb rheology in the viscous plastic model the continuum models were solved by using a fully lagrangian method known as smoothed particle hydrodynamics sph the method has the advantage of the lagrangian techniques in avoiding problems of numerical diffusion an additional advantage of sph in comparison with classical lagrangian approaches is that sph does not need grids and this makes it well suited to handle problems of large deformations

arctic lake ontogeny across multiple interglaciations abstract few lakes in the arctic preserve sediments older than holocene age because of pervasive glacial scour during the last ice age here we present sediment diatom and geochemical records from a lake on east central baffin island cf8 nunavut canada that captures three successive interglacial periods within the last 200 000 years a portion of marine isotope stage mis 7 the last interglacial mis 5e and the holocene mis 1 an additional unit of diatom rich organic sediment occurs between the latter two intervals and is ascribed to mis 5a interstadial conditions the lake cf8 paleolimnological record reveals similar ontogenetic trends within each interglacial early postglacial environments in both the holocene and mis 5e were characterized by a dominance of colonial benthic fragilarioid diatoms that thrived in relatively alkaline waters these species shifts do not coincide with major changes in base cation delivery to the sediments suggesting that shifts in diatom assemblages are controlled mainly by climate driven ph dynamics diatom assemblages then transitioned into dominance by tychoplanktonic aulacoseira species likely in response to climate driven ph dynamics the highest sustained abundances of aulacoseira occur in sediments ascribed to mis 5e and this is the only interval in which thalassiosiroid centric taxa also occur e g discostella and puncticulata spp given that these planktonic taxa typically reach high abundances during extended periods of open water conditions we surmise that mis 5e was the warmest interval recorded the overall similarity of lake ontogenetic trajectories recorded within each interglacial period suggests that climatic and edaphic factors drive a complex succession of environmental changes through indirect effects on lake ice cover habitat availability and lake water ph in recent decades past 50 yrs an unprecedented increase in periphytic eunotia taxa is evident these diatoms were only present intermittently during previous glacial interglacial cycles the expansion of eunotia is attributed to increased habitat availability associated with declining ice cover during recent 20th century warming by integrating data from several distinct interglacials the cf8 diatom record considerably extends our understanding of arctic lake ontogeny and provides an unparalleled natural archive with which to compare recent changes associated with anthropogenic climate warming

late glacial and holocene landscapes of central beringia abstract new palynological and sedimentological data from st lawrence island present a rare view into late glacial and holocene environments of the central bering land bridge the late glaciation was a time of dynamic landscape changes in south central beringia with active thermokarst processes including the formation and drainage of thaw lakes the presence of such a wet unstable substrate if widespread probably would have had an adverse impact on food sources and mobility for many of the large mammal populations the establishment of betula shrub tundra on the island suggests late glacial summers that were warmer than present consistent with regional paleoclimatic interpretations however the increasing proximity to the bering sea as postglacial sea levels rose modified the intensity of warming and prevented the establishment of deciduous forest as found in other areas of beringia at this time the mid to late holocene is marked by more stable land surfaces and development of sphagnum and cyperaceae peat deposits the accumulation of organic deposits decline of shrub betula and decrease in thermokarst disturbance suggest that conditions were cooler than the previous a recent decline in peat accumulation at the study sites may relate to local geomorphology but similar decreases have been noted for other arctic regions

influence of the water content on x ray fluorescence core scanning measurements in soft marine sediments the x ray fluorescence xrf core scanner provides bulk sediment chemistry data measured nondestructively at the split core sediment surface although this method is widely accepted there is little known about the effects of physical properties such as density and water content on xrf core scanner data comparison of xrf scanner measurements from the sediment surface and dry powder samples of sediment core geob7920 indicates strongly reduced element intensities for the lighter elements al and si we relate the lower element intensities of the measurements taken at the sediment surface to the amount of water in the sample volume analyzed by the xrf core scanner the heavier elements k ca ti and fe remain relatively unaffected by the variation of any physical property within sediment core geob7920 additionally we successfully use the elemental intensity of cl as a proxy for the seawater content in the sample volume analyzed by the xrf core scanner this enables the establishment of a correction function for the elements al and si that corrects for the radiation absorption of the water content in sediment core geob7920 off cape blanc nw africa

asynchronous evolution of the indian and east asian summer monsoon indicated by holocene moisture patterns in monsoonal central asia abstract the numerical meta analysis of 92 proxy records 72 sites of moisture and or temperature change confirms earlier findings that the dominant trends of climatic evolution in monsoonal central asia since the last glacial roughly parallel changes in northern hemisphere summer insolation i e the period following the last glacial maximum was characterized by dry and cold conditions until 15 cal kyr bp followed by a warm wet period coincident with the bolling allerod warm period and terminated by a cold dry reversal during the younger dryas period after an abrupt increase at the start of the holocene warm and wet conditions prevailed until ca 4 cal kyr bp when moisture levels and temperatures started to decrease ordination of moisture records reveals strong spatial heterogeneity in moisture evolution during the last 10 cal kyr the indian summer monsoon ism areas northern india tibetan plateau and southwest china exhibit maximum wet conditions during the early holocene while many records from the area of the east asian summer monsoon indicate relatively dry conditions especially in north central china where the maximum moisture levels occurred during the mid holocene we assign such phenomena to strengthened hadley circulation centered over the tibetan plateau during the early holocene which resulted in subsidence in the east asian monsoonal regions leading to relatively dry conditions our observations of the asynchronous nature of the two asian monsoon subsystems on millennial time scales have also been observed on annual time scales as well as implied through the spatial analysis of vertical air motion patterns after strong ascending airflows over the tibetan plateau area that were calculated from ncep ncar reanalysis data for the last 30 years analogous with the early holocene the enhancement of the ism in a future warming world will result in an increase in the asynchronous nature of the monsoon subsystems this trend is already observed in the meteorological data from the last 15 years

patterns of early lake evolution in boreal landscapes a comparison of stratigraphic inferences with a modern chronosequence in glacier bay alaska the chronosequence approach which infers temporal patterns of environmental change from a spatial array of modem sites has been a major tool for studying successional processes a model of early lake ontogeny in boreal landscapes developed from a chronosequence of lakes in alaska suggests that long term soil development and related hydrological change produce a loss of alkalinity and base cations a decrease in ph an increase in doc and a transient increase followed by a decrease in lakewater nitrogen concentrations over time we compare this model of lake ontogeny with patterns of change reconstructed from diatom assemblages in 10 sediment cores from the same region lakewater ph declined in the majority of lakes but the pattern rate and magnitude of decline varied considerably among sites apparently related to local differences in hydrologic setting inferred trends in nitrogen concentration over time are not spatially uniform and appear to vary because of local differences in vegetation history

diatom shifts as evidence for recent subarctic warming in a remote tundra lake nwt canada abstract changes in diatom assemblage composition were examined from the sediments of slipper lake an isolated tundra lake located 50 km north of current treeline in canada s northwest territories diatom shifts over the last ca 5600 years were related to the response of this ecosystem to climatic and environmental change during the late holocene with particular emphasis on the last few hundred years to date a detailed diatom analysis for this recent time frame has not been undertaken for the central canadian subarctic the first ca 5400 years of the sediment record was marked by modest shifts between benthic alkaliphilous diatom taxa e g fragilaria achnanthes and navicula species and heavily silicified tychoplanktonic aulacoseira species in the 19th century abrupt changes were delineated by a marked shift to a diatom assemblage characteristic of more planktonic habitats consisting of the cyclotella stelligera complex c stelligera c pseudostelligera which was absent in earlier sediments several possible mechanisms for these recent changes are examined including atmospheric deposition of acidifying compounds and anthropogenically derived nutrient enrichment however we conclude that climatically induced limnological changes associated with shorter duration of ice cover and a longer growing season can best explain the direction and magnitude of changes in our diatom record

younger dryas and early holocene peats from northern far east russia abstract macrofossil remains from dated peat sections help clarify paleoclimate patterns inferred from subtle changes in the palynological data from the northern russian far east these data show latitudinal range extensions for tree betula larix gmelinii and picea sp as well as the occurrence of tree sized shrubs of duschekia fruticosa and salix sp in areas that today are low shrub tundra ages of extralimital taxa suggest a postglacial thermal maximum between 11 000 and 8100 14c bp including a region wide warm younger dryas this warm interval contrasts to other areas of siberia where the thermal optimum is 9000 4000 14c bp additionally macrofossils suggest that holocene changes in northern treeline were not synchronous in the far east treeline advance and retreat occurred earlier by 1000 14c y and 4000 14c y respectively as compared to other areas of russia basal and minimum ages of peat deposits indicate that the early holocene was a particularly important time for peat initiation and accumulation across northern siberia

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

diatom inferred climatic and environmental changes over the last 9000 years from a low arctic nunavut canada tundra lake abstract diatom assemblage changes over the holocene were examined from a 14c dated sediment core retrieved from lake tk 2 a small low arctic lake located ca 200 km north of the forest tundra ecotone in mainland nunavut canada notable changes in the diatom assemblages were recorded throughout the core suggesting that the holocene epoch in this region has been environmentally and climatically dynamic the earliest diatom assemblages ca 9000 cal yr bp were dominated by taxa that are atypical of post glacial assemblages commonly recorded throughout the arctic and may suggest that early holocene conditions at lake tk 2 were relatively warmer a shift to dominance by small benthic opportunistic fragilaria taxa followed after ca 8550 cal yr bp more typical of arctic assemblages during initial lake ontogeny suggesting the onset of cooler more alkaline conditions an abrupt and short lived marked decrease in diatoms between ca 8550 and 8500 cal yr bp with corresponding changes in physical and chemical indicators e g sedimentation rate siliciclastic content organic matter content provides potential evidence for the 8 2 k cooling anomaly an event rarely recognized from other paleolimnological studies in the canadian north following ca 7000 cal yr bp a substantial shift occurred to a more complex and diverse diatom assemblage that now included more acidophilic taxa this compositional change is likely indicative of a natural long term loss of alkalinity in the lake and marks the onset of the holocene thermal maximum consistent with the timing of this warm period for this region the relatively stable diatom assemblage composition during the neoglacial period was punctuated by fluctuations in key species potentially correlative with the so called medieval warm period and little ice age in the most recent sediments the post industrial expansion of the small planktonic cyclotella stelligera complex and a concurrent decline in the heavily silicified aulacoseira lirata complex are similar to shifts that are increasingly being recognized as geographically widespread diatom responses to recent climate warming the lake tk 2 diatom record provides important insights into the holocene environmental history of this understudied region of the canadian arctic furthermore it is one of the few arctic lakes in which the 8 2 k cold event is possibly expressed

geochemical response on redox fluctuations in holocene lake sediments lake steisslingen southern germany abstract holocene sediments of lake steisslingen record sedimentation under different redox conditions this is proven by the changing depth of the chemocline and the resulting varying extension of laminated sediments from the deepest part of the lake to shallower water depths redox fluctuations are additionally reflected by colour changes between red and green and the quality of the nonglacial varves major and trace elements were analysed on bulk sediment samples period between 8200 6200 and 2650 2000 cal a bp to test their response to the observed changes in past depositional environments mn mo u fe and s were found to be redox sensitive elements in lake steisslingen fe and s are almost entirely fixed in pyrite because pyrite formation is limited by fe availability a parallel distribution of mo u fe and s between 8200 7200 cal a bp is interpreted as a signal for syngenetic pyrite formation from anoxic and sulfidic hypolimnetic waters since bioproductivity was low this must have been caused by conditions favouring a longterm stable stratification of the lake i e high water level high summer temperatures ice cover wind protection and or low storm activity between 6000 2000 cal a bp frequently changing redox conditions from anoxic to dysoxic are assumed mo and u curves are mirror images of the fe s and mn variations this is interpreted as showing diagenetic pyrite formation from pore waters which is enhanced during times of high fe availability under rather oxygenated conditions i e when high input of fe oxy hydroxides is possible but when u and mo cannot necessarily be fixed in the sediments changes from slightly disturbed lamination dysoxic restricted mixis to homogenous sediments oxic conditions holomixis cannot be traced within the geochemical record the observed redox fluctuations between 6000 and 2000 cal a bp are not correlated to changes in bioproductivity detrital input or cultural influences and must therefore be of a natural probably climatic origin the north atlantic oscillation is discussed as a possible cause

the lateglacial and postglacial vegetation history of the northwestern limits of beringia based on pollen stomate and tree stump evidence abstract in order to reconstruct the late and postglacial vegetation history of the northwestern edge of beringia a sediment core was collected from a lake north of the present treeline along the lower lena river of northeastern siberia and analysed for fossil pollen and stomates in addition fossil tree stumps were collected in the vicinity of the lake eight radiocarbon dates indicate that the lake sediment record spans at least the past 12 300 yr bp the early vegetation at this site was dominated by herb and shrub tundra possible evidence of younger dryas cooling consisting of a decrease in shrub birch and increases in grass and herbaceous plants occurs between 11 000 and 10 000 yr bp forests dominated by larix dahurica and including picea obovata extended northward to the site between 8500 and 3500 yr bp there is an agreement between the pollen stomate and tree stump evidence for this advance the modern vegetation of shrub tundra was established after 3500 yr bp