effects of sample mass and macrofossil type on radiocarbon dating of arctic and boreal lake sediments dating lake sediments by accelerator mass spectrometry ams 14 c analysis of terrestrial plant macrofossils overcomes one of the main problems associated with dating bulk sediment samples i e the presence of old organic matter even so many ams dates from arctic and boreal sites appear to misrepresent the age of the sediment to understand the nature of these apparent dating anomalies better we conducted a series of 14 c dating experiments using samples from alaskan and siberian lake sediment cores first to test whether our analytical procedures introduced a sample mass bias we obtained 14 c dates for different sized pieces of single woody macrofossils in these sample mass experiments statistically equivalent ages were found for samples as small as 0 05 mg c secondly to assess whether macrofossil type influenced dating results we conducted sample type experiments in which 14 c dates were obtained for different macrofossil types sieved from the same depth in the sediment we dated materials from multiple levels in sediment cores from upper capsule lake north slope northern alaska and grizzly lake copper river basin southern alaska and from single depths in other records from northern alaska in several of the experiments there were significant discrepancies between dates for different plant tissues and in most cases wood and charcoal were older than other macrofossil types usually by several hundred years this pattern suggests that 14 c dates for woody macrofossils may misrepresent the age of the sediment by centuries perhaps because of their longer terrestrial residence time and the potential in built age of long lived plants this study identifies why some 14 c dates appear to be inconsistent with the overall age depth trend of a lake sediment record and it may guide the selection of 14 c samples in future studies

modeling an exogenic origin for the equatorial ridge on iapetus abstract iapetus has a ridge along the equator that extends continuously for more than 110 in longitude parts of the ridge rise as much as 20 km above the surrounding terrains most models for the formation of this enigmatic ridge are endogenic generally requiring the formation of a fast spinning iapetus with an oblate shape due to the rotation speed many of these require specific scenarios and have constraining parameters in order to generate a ridge comparable to what is seen today an exogenic formation mechanism has also been proposed that the ridge represents the remains of an early ring system around iapetus that collapsed onto the surface thus far none of the models have conclusively identified the origin of the ridge in this study an exogenic origin for the ridge is assumed which is derived from a collapsing disk of debris around iapetus without invoking any specific model for the generation of the debris disk here we evaluate whether it is possible to generate a ridge of the size and shape as observed by simulating the impact of the collapsing debris using the cth hydrocode pi scaling calculations suggest that extremely oblique impact angles 1 10 are needed to add to ridge topography these extreme impact angles severely reduce the cratering efficiency compared to a vertical impact adding material rather than eroding it during crater formation furthermore material is likely to be excavated at low angles enhancing downrange accumulation multiple impacts from debris pieces will heighten this effect because infalling debris is predicted to impact at extremely low angles both of these effects might have contributed to ridge formation on iapetus the extreme grazing angles of the impacts modeled here decouple much of the projectile energy from the target and impact heating of the surface is not significant these models suggest that a collapsing disk of debris should have been able to build topography to create a ridge around iapetus

cassini imaging science initial results on phoebe and iapetus the cassini imaging science subsystem acquired high resolution imaging data on the outer saturnian moon phoebe during cassini s close flyby on 11 june 2004 and on iapetus during a flyby on 31 december 2004 phoebe has a heavily cratered and ancient surface shows evidence of ice near the surface has distinct layering of different materials and has a mean density that is indicative of an ice rock mixture iapetus s dark leading side cassini regio is ancient heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief local albedo variations within and bordering cassini regio suggest mass wasting of ballistically deposited material the origin of which remains unknown

coupling of thermal evolution and despinning of early iapetus abstract the cassini mission revealed two spectacular characteristics of iapetus 1 a geologically old and high equatorial ridge which is unique in the solar system and 2 a large flattening of 35 km consistent with the equilibrium figure for a hydrostatic body rotating with a period of 16 h whereas the current spin period is 79 33 days this study describes three dimensional simulations of solid state convection within an undifferentiated iapetus it investigates the implications for the evolution of the interior thermal structure and its spin rate and global shape using radially layered viscoelastic models the role of the concentration in the short lived radiogenic element 26 al just after accretion is completed is specifically addressed the first result is to show that whatever the 26 al value convection occurs as suggested by castillo rogez et al castillo rogez j matson d sotin c johnson t lunine j thomas p 2007 icarus 190 179 202 convection reduces the warming of the interior compared to the conductive evolution and therefore limits the conditions for despinning in our calculations two conceptual linear viscoelastic models are used when considering a maxwell rheology the interior temperature viscosity never reaches a value high low enough to induce despinning in order to promote dissipation at low temperature a burgers rheology which includes an additional dissipation peak is introduced for favorable parameter values this latter rheology leads to despinning however only models associated with large amounts of short lived radiogenic elements 26 al 25 ppb lead to the observed flattening this suggests that the accretion process needs to be completed shortly after the formation of cais calcium aluminum rich inclusions 4 myr for 26 al varying between 72 and 46 ppb the observed flattening is obtained only for a limited range of initial spin period between 9 5 and 10 2 h for 26 al ranging between 30 and 15 ppb initial spin rates smaller than 8 5 h are required for smaller values of 26 al the body is too cold and viscous to acquire a significant flattening even if a rotation period close to the body disruption limit is considered even with a thin lithosphere during the early stage our simulations show that iapetus never reaches the equilibrium figure for a hydrostatic body due to the non zero rigidity of the lithosphere the 35 km value of the flattening is the result of the partial relaxation of an ancient larger flattening ranging between 45 and 80 km depending on the evolution of the lithosphere thickness mainly controlled by the radiogenic content a thin lithosphere is consistent with an early building of the equatorial ridge the lithosphere thickening due to interior cooling can explain the preservation of the ridge throughout the remaining evolution of iapetus

climate and vegetation history from a 14 000 year peatland record kenai peninsula alaska abstract analysis of pollen spores macrofossils and lithology of an ams 14c dated core from a subarctic fen on the kenai peninsula alaska reveals changes in vegetation and climate beginning 14 200 cal yr bp betula expansion and contraction of herb tundra vegetation characterize the younger dryas on the kenai suggesting increased winter snowfall concurrent with cool sunny summers remarkable polypodiaceae fern abundance between 11 500 and 8500 cal yr bp implies a significant change in climate enhanced peat preservation and the occurrence of wet meadow species suggest high moisture from 11 500 to 10 700 cal yr bp in contrast to drier conditions in southeastern alaska this pattern may indicate an intensification and repositioning of the aleutian low al drier conditions on the kenai peninsula from 10 700 to 8500 cal yr bp may signify a weaker al but elevated fern abundance may have been sustained by high seasonality with substantial snowfall and enhanced glacial melt decreased insolation induced seasonality resulted in climatic cooling after 8500 cal yr bp with increased humidity from 8000 to 5000 cal yr bp a dry interval punctuated by volcanic activity occurred between 5000 and 3500 cal yr bp followed by cool moist climate coincident with neoglaciation tsuga mertensiana expanded after 1500 cal yr bp in response to the shift to cooler conditions

delayed formation of the equatorial ridge on iapetus from a subsatellite created in a giant impact 1 the great equatorial ridge on saturn s moon iapetus is arguably the most perplexing landform in the solar system the ridge is a mountain range up to 20 km tall and sitting on the equator of iapetus and explaining its creation is an unresolved challenge models of its formation must satisfy three critical observations why the ridge 1 sits exactly on the equator 2 is found only on the equator and 3 is thus far found only on iapetus we argue that all previously proposed models fail to satisfy these observations and we expand upon our previous proposal that the ridge ultimately formed from an ancient giant impact that produced a subsatellite around iapetus the orbit of this subsatellite would then decay once iapetus itself had despun due to tides raised by saturn until tidal forces from iapetus tore the subsatellite apart the resultant debris formed a transient ring around iapetus the material of which rained down on the surface to build the ridge by sequestering the material in a subsatellite with a tidally evolving orbit formation of the ridge is delayed which increases the likelihood of preservation against the high impact flux early in the solar system s history and allows the ridge to form on thick stiff lithosphere heat flow likely 1 mw m 2 required to support this massive load without apparent flexure this mechanism thus explains the three critical observations

dependence of ice core relative trace element concentration on acidification to assess the role of methodological differences on measured trace element concentrations in ice cores we developed an experiment to test the effects of acidification strength and time on dust dissolution using snow samples collected in west antarctica and alaska we leached antarctic samples for 3 months at room temperature using nitric acid at concentrations of 0 1 1 0 and 10 0 v v at selected intervals 20 min 24 hours 5 days 14 days 28 days 56 days 91 days we analyzed 23 trace elements using inductively coupled plasma mass spectrometry concentrations of lithogenic elements scaled with acid strength and increased by 100 1380 in 3 months incongruent elemental dissolution caused significant variability in calculated crustal enrichment factors through time factor of 1 3 pb to 8 0 cs using snow samples collected in alaska and acidified at 1 v v for 383 days we found that the increase in lithogenic element concentration with time depends strongly on initial concentration and varies by element e g fe linear regression slope 1 66 r 0 98 our results demonstrate that relative trace element concentrations measured in ice cores depend on the acidification method used

massive ice avalanches on iapetus mobilized by friction reduction during flash heating the great distance travelled by long runout landslides observed previously on the earth and mars requires a mechanism of friction reduction identification and analysis of long runout landslides on saturn s moon iapetus suggests that the iapetian landslides are enabled by flash heating of the icy sliding surface

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

east west faults due to planetary contraction abstract contraction expansion and despinning have been common in the past evolution of solar system bodies these processes deform the lithosphere until it breaks along faults their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface while the search for despinning tectonics has not been conclusive there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion though the pattern is seldom isotropic as predicted the cause of the non random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction expansion with another deformation despinning tidal deformation reorientation another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation using thin elastic shells with variable thickness i show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction expansion into a pattern of faults striking east west preferably formed in the equatorial region by contrast lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike slip faults and polar normal faults if contraction is added to despinning the despinning pattern first shifts to thrust faults striking north south and then to thrust faults striking east west if the lithosphere is thinner at the poles the tectonic pattern caused by contraction expansion consists of faults striking north south i start by predicting the main characteristics of the stress pattern with symmetry arguments i further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two making it easy to determine fault orientation for combined contraction and despinning i give two methods for solving the equations of elasticity one numerical and the other semi analytical the latter method yields explicit formulas for stresses as expansions in legendre polynomials about the solution for constant shell thickness though i only discuss the cases of a lithosphere thinner at the equator or at the poles the method is applicable for any latitudinal variation of the lithospheric thickness on iapetus contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge on mercury the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation north south or east west which may be relevant to the orientation of lobate scarps

postglacial vegetation history of the mackenzie river basin abstract three radiocarbon dated cores from small lakes provide pollen records of postglacial vegetation change in the central mackenzie river basin these data are combined with previously published records to reconstruct vegetation development from central alberta to the mackenzie delta the paleoecological data demonstrate that a continuous corridor of herb and shrub dominated vegetation extended between beringia and the great plains of north america from deglaciaiton until about 10 000 yr b p the initial vegetation was replaced by spruce forests between 10 000 and 8500 yr b p significant vegetation change continued until approximately 500 yr b p when the muskeg dominated landscape typical of the present subarctic was established the initial herb and shrub vegetation was likely a reprise of conditions which existed during earlier pleistocene deglaciations these conditions may have provided a favorable environment for animals adapted to nonforested vegetation and promoted migration southward from beringia and northward from the great plains during the last deglaciation and earlier pleistocene deglaciations the sequence of postglacial vegetation change in the mackenzie basin likely reflects a combination of climatic change edaphic change and plant migration patterns and interactions

use of scanning electron microscopy to investigate records of soil weathering preserved in lake sediment reconstruction of soil weathering is one of the more difficult problems for palaeoecologists interested in landscape scale environmental changes here we use scanning electron microscopy and energy dispersive x ray analysis sem eds to study the pitting of feldspars in soils and lake sediments at two sites in the great lakes region usa a semi quantitative feldspar weathering index fwi allows description of the weathering status for large numbers of individual grains and reveals differences among soil horizons and strata of lake sediment as well as between the two sites feldspars in upper soil horizons generally have greater fwi than those in lower horizons but analyses also suggest considerable weathering in the soil c horizon at the site with more rapid water infiltration at one site the fwi of upper soil horizons and surface lake sediment are similar implying that lake sediments have the potential to record changes in soil over time however differences between upper soil horizons and surface sediment at the other site highlight the need to examine more sites and more profiles within sites to better relate catchment and sediment characteristics we interpret increases in the fwi over time within lake sediments as evidence of progressive weathering of soils and conclude that the sites had different histories of soil development however the complexities of sediment deposition in lakes and the paucity of information relating fwi to soil characteristics prevent straightforward reconstruction of soil development variability within each record appears tied to initial grain chemistry depositions environment changes in lake level and progressive degradation of grains as more information relating micromorphology to soil characteristics becomes available this technique should become a more useful complement to chemical studies of lake sediment

a reconstruction of the north atlantic oscillation using tree ring chronologies from north america and europe tree ring records six from eastern north america and four from northwestern europe are used to develop the first reconstruction of the winter north atlantic oscillation nao the reconstructed series spans the interval ad 1701 1980 and explains 41 of the variance in the nao over the ad 1874 1980 calibration period the reconstruction also captures the spectral properties of this index suggesting that the oscillatory character of the nao is a long term feature of the north atlantic climate system

a contraction model for the flattening and equatorial ridge of iapetus others have explained the excess flattening of iapetus by a model in which the moon formed at a high spin rate achieved isostatic equilibrium by very rapid interior heating caused by short lived radioactive isotopes slri and subsequently cooled locking in the excess flattening with respect to an equilibrium shape at its present spin rate here we propose an alternate model that does not require an unusually high initial spin rate or the slri the initial formation of iapetus results in a slightly oblate spheroid with porosity 10 radioactive heating by long lived isotopes warms the interior to about 200 k at which point it becomes ductile and the interior compacts by 10 while the 120 km thick exterior shell remains strong the shell must deform to match the reduced volume of the ductile interior and we propose that this deformation occurs along the equator perhaps focused by a thinner equatorial shell the final shape of the collapsed sphere matches the observed shape of iapetus today described as an oblate ellipse except along the equator where strain concentration forms a broad ridge to maintain this non equilibrium shape the thickness of the shell must exceed 120 km testing the equatorial focusing hypothesis will require a model that includes non linear processes to account for the finite yield strength of the thick lithosphere nevertheless we show that the stress in the lithosphere generated by the contraction of the interior is about 3 times greater than the stress needed to deform the lithosphere so some type of lithospheric deformation is expected

a high resolution atmospheric dust record for 1810 2004 a d derived from an ice core in eastern tien shan central asia centennial scale high resolution records of atmospheric dust conditions are rare in the arid and semiarid regions of central asia limiting our understanding of the regional climate and environmental changes and their potential driving forces in this paper we present an annually resolved atmospheric dust record covering the period of 1810 2004 a d reconstructed from an ice core retrieved at 4512 m above sea level from the miaoergou glacier in the eastern tien shan the time series of dust flux for the past 195 years shows three periods of relatively low values i e 1810 1829 a d 1863 1940 a d and 1979 2004 a d and two periods of relatively high values i e 1830 1862 a d and 1941 1978 a d spatial correlation analysis suggests possible regional factors controlling the dust flux including antecedent summer precipitation spring soil moisture and near surface wind speed in addition the miaoergou dust flux is closely associated with the winter index of the north atlantic oscillation nao over the past two centuries with high low dust periods coinciding with the negative positive phases of the nao the persistent relationship suggests that the nao may have been a key driver on dust flux change over the arid regions between the tien shan and kunlun mountains

boreal ecosystem development in the northwestern alaska range since 11 000 yr b p abstract analyses of pollen plant macrofossils macroscopic charcoal mollusks magnetic susceptibility and geochemical content of a sediment core from farewell lake yield a 11 000 yr record of terrestrial and aquatic ecosystem changes in the northwestern foothills of the alaska range between 11 000 and 8500 yr b p the regional landscape was dominated by abetulashrub tundra in whichpopulus salixcommunities were common abundant charcoal in sediments indicates that fires were common in the lake catchment during this period and high mineral accumulation rates allogenic elemental content and magnetic susceptibility suggest intense soil erosion in addition mollusks pollen and macrofossils of aquatic macrophytes and biogenic silica provide evidence that the lake was substantially shallower and more productive 11 000 8500 yr b p than later low lake level and high aquatic productivity might have been caused by warm and dry summers associated with early postglacial insolation maximum in northern high latitudes about 8000 yr b p picea glaucaarrived within the lake catchment forming a forest tundra association until ca 6000 yr b p alnusshrub thickets established in the region ca 6500 yr b p andbetula papyriferaarrived ca 6000 yr b p closedp glaucaforests developed ca 6000 yr b p picea marianabecame important subsequently and replacedp glaucaas the dominant tree species in the region ca 4000 yr b p an increase in authigenic fe mn ratios suggests that the development of waterlogged soils accompanied this vegetation change fires increased in importance at this time and might have accelerated soil erosion the establishment ofp marianaforests probably reflected complex responses of forest ecosystems to the onset of cooler and wetter climate conditions during the late holocene

lake level reconstruction and paleohydrology of birch lake central alaska based on seismic reflection profiles and core transects lake level history for birch lake alaska was reconstructed using seismic profiles and multiproxy sedimentary analyses including sedimentology geochemistry magnetic susceptibility and palynology twenty two seismic profiles 18 km total and eight sediment cores taken from the lake margin to its depocenter at 13 5 m provide evidence for low lake stands during the late pleistocene and holocene thirty one ams radiocarbon dates of macrofossils and pollen provide a century scale chronology prior to 12 700 14c yr b p the lake which now overflows was either seasonally dry or desiccated for prolonged periods indicating a severe period of aridity lake level rose more than 18 m between 12 700 and 12 200 14c yr b p before falling to 17 m below the level of overflow between 11 600 and 10 600 14c yr b p the water remained between 14 and 17 m below the overflow level onlap sedimentary sequences were formed during a transgression phase between 10 600 and 10 000 14c yr b p between 10 000 and about 8800 14c yr b p the lake was between 6 and 9 m below the overflow level lake level again rose approaching the overflow level between 8800 and 8000 14c yr b p seismic and core evidence of minor erosional events suggest lowstands of 2 6 m until 4800 14c yr b p there have been no prolonged periods of lake level depression since that time the major restructuring of the climate system during deglaciation evidently generated a complex set of fluctuations in effective moisture in interior alaska which likely affected eolian processes and vegetation development as well as lake levels

climatological significance of δ18 o in precipitation and ice cores a case study at the head of the urumqi river tien shan china stable oxygen isotope ratios 18 o of precipitation and ice core samples collected from the headwaters of the urumqi river tien shan china were used to test the relationship between 18 o and contemporaneous surface air temperature t a a strong temporal relationship is found between 18 o in precipitation and t a particularly for the monthly averages which remove synoptic scale influences such as changes in condensation level condensation temperature and moisture sources yao and others 1996 linear fits as high as 0 95 c for precipitation events and 1 23 c for monthly averages are found although the 18 o amplitude in ice cores drilled at the nearby urumqi glacier no 1 2 km from the precipitation sampling site decreased dramatically compared to the precipitation samples the ice core records of annually averaged 18 o are still positively correlated with contemporaneous air temperature especially summer air temperature at the nearby daxigou meteorological station nevertheless the relationship between the ice core 18 o records and contemporaneous air temperature is less significant than that for the precipitation samples due to depositional and post depositional modification processes which are highlighted by the successive snow pit 18 o profiles from the urumqi glacier no 1 our results might extend the application of high altitude and subtropical ice core 18 o t a records for palcoclimate reconstruction

dust storm activity over the tibetan plateau recorded by a shallow ice core from the north slope of mt qomolangma everest tibet himal region 1 based on a 39 8 m ice core drilled at the east rongbuk glacier 27 59 n 86 55 e 6518 m a s l north slope of mt qomolangma everest central himalaya the changes of dust storm over the tibetan plateau tp recorded by insoluble particle and its possible connection with the winter jfm north atlantic oscillation nao are investigated the results show that the level of dust storm activity exhibits a significant decreasing trend during the past 50 years this may be attributed at least partly to a decrease in wind speed over the tp and its surrounding regions the normalized time series of the winter nao index shows a significant inverse correlation with the mass concentration of insoluble particle implying a possible connection between the winter nao and the dust storm activity over the tp

a review of central asian glaciochemical data the glaciers of central asia provide suitable locations from which to recover continuous high resolution glaciochemical records on a continental scale although the glaciochemical investigations undertaken to date in central asia are few in number and limited in terms of spatial coverage and length of record some preliminary observations can be made concerning regional and seasonal trends in snow chemistry in this region the sodium chloride ratio for most snow samples collected in central asia approaches the ratio found in sea water 0 86 in leq kg i reflecting a marine source for these constituents sodium and chloride concentrations are on average 3 10 times higher in the himalayas than in the karakoram demonstrating the greater influence of monsoonal sources of moisture in the himalayas very high sodium concentrations from khel khod glacier probably reflect a local crustal source from surrounding ice free areas low nitrate concentrations were found in snow collected from the southern margin of the himalayas and high concentrations in snow deposited on the north margin of the himalayas this strong regional trend in the spatial distribution of nitrate suggests the influx of continental aerosols rich in nitrate originating from the arid regions of central asia high calcium concentrations measured in snow from mount everest and the north west corner of china are also indicative of dust derived from the arid regions of central asia very high sulfate concentrations found in snow from the tien shan and the bogda shan most likely reflect local anthropogenic sources the altitude effect on isotopic composition is not apparent from snow samples collected in central asia understanding the processes which control the chemical content of snow the local to regiona l scale complexities

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

ridge formation and de spinning of iapetus via an impact generated satellite abstract we present a scenario for building the equatorial ridge and de spinning iapetus through an impact generated disk and satellite this impact puts debris into orbit forming a ring inside the roche limit and a satellite outside this satellite rapidly pushes the ring material down to the surface of iapetus and then itself tidally evolves outward thereby helping to de spin iapetus this scenario can de spin iapetus an order of magnitude faster than when tides due to saturn act alone almost independently of its interior geophysical evolution eventually the satellite is stripped from its orbit by saturn the range of satellite and impactor masses required is compatible with the estimated impact history of iapetus

a 130 0 year history of vegetation and environmental change at jan lake east central alaska pollen stomates and macrofossils were analysed from a 3 63 m sediment core at jan lake eastcentral alaska in order to improve spatial resolution of patterns of vegetation history in this region the chronology was based on 19 ams 14c dates on plant macrofossils and concentrated pollen zone jan i 12400 11 600 yr bp records a unique herb tundra pollen assemblage particularly high in chenopodia ceae adding supporting evidence for a spatial mosaic of lateglacial vegetation in east central alaska unexpected pollen of picea betula and alnus in lateglacial sediments appears to arise from reworking of older pollen bearing deposits according to a radiocarbon date of 22180yr bp on a pollen sample shrub betula tundra dominates zone jan il 11 600 10000 yr bp with indications of climatic amelioration tree species populus picea glauca betula papyrifera began to arrive in zone jan illa 9200 6600 yr bp and the modern boreal forest including alnus and picea mariana became established during zone j

anthropogenic sulfate and asian dust signals in snow from tien shan northwest china snow samples were collected from a 0 5 m snowpack at glacier no i and near bogda feng eastern tien shan northwest china samples that were melted in the field were analyzed for chloride nitrate sulfate sodium potassium magnesium calcium and microparticles eight samples were returned frozen and were analyzed for the above ions plus ammonium acetate formate methylsulfonate and hydrogen peroxide there was no significant difference in measured major ion concentrations between the melted and frozen samples measured cations in both sets of samples were two to three times greater than measured anions calcium and sodium are the dominant cations while sulfate is the dominant measured anion high ion burdens are associated with dusty layers in the snowpack indicating that dust from the vast arid regions of central asia is the dominant source for major ions in tien shan snow the significant increase in sulfate and decrease in the cation anion ratio in bogda eng snow relative to glacier no i snow suggest that anthropogenic emissions from urtimqi are an important source of sulfate to precipitation downwind from the city

littlefield lake michigan carbonate budget of holocene sedimentation in a temperate region lacustrine system littlefield lake is in a late stage of marl lake evolution characterized by reduced rates of carbonate precipitation long and short term carbonate budgets show that the volume of calcite in the lake basin is 3 7 times that expected from annual calcium depletion in the epilimnetic water this holocene decrease in carbonate production is also recorded as a gradual increase in the amount of organic and carbonate material in cores from the deep lake basin late stage reduction in carbonate production is evidently a natural consequence of lakeward progradation of littoral marl benches encroachment of terrestrial vegetation and reduced colonization by carbonate producing lake macrophytes

time dependent flexure of the lithospheres on the icy satellites of jupiter and saturn abstract previous analyses into flexural deformation on the icy satellites of jupiter and saturn have assumed static elastic lithospheres viscous creep within the lithosphere however can cause evolution over time here we apply a finite element model that employs a time dependent elastic viscous plastic rheology in order to investigate flexure on icy satellites factors that affect this time dependent response are those that control creep rates surface temperature heat flow and grain size our results show that surface temperature is by far the dominant factor at higher surface temperatures 100 130 k the evolution of the deformation is such that the thickness of a modeled elastic lithosphere could vary by up to an order of magnitude depending on the time scale over which the deformation occurred because the flexure observed on icy satellites generally indicates transient high heat flow events our results indicate that the duration of the heat pulse is an important factor for the icy worlds of jupiter and saturn static models of lithospheric flexure should be used with caution

recent increases in atmospheric concentrations of bi u cs s and ca from a 350 year mount everest ice core record 1 high resolution major and trace elements sr cs ba la ce pr nd sm eu tb dy ho er tm yb lu bi u tl al s ca ti v cr mn fe and co quantified in a mount everest ice core 6518 m above sea level spanning the period 1650 2002 ad provides the first asian record of trace element concentrations from the pre industrial era and the first continuous high resolution asian record from which natural baseline concentrations and subsequent changes due to anthropogenic activities can be examined modern concentrations of most elements remain within the pre industrial range however bi u and cs concentrations and their enrichment factors ef have increased since the 1950s and s and ca concentrations and their efs have increased since the late 1980s a comparison of the bi u cs s and ca data with other ice core records and production data indicates that the increase in atmospheric concentrations of trace elements is widespread but that enrichment varies regionally likely sources for the recent enrichment of these elements include mining metal smelting oil and coal combustion and end uses for bi and mining and refinement for u and cs the source of the synchronous enrichment of ca and s is less certain but may be related to land use and environmental change

recent changes of atmospheric heavy metals in a high elevation ice core from muztagh ata east pamirs initial results al mn rb sr ba cs bi and sb were measured at various depth intervals of a 41 6 m firn ice core drilled at an elevation of 7010 m near the top of muztagh ata glacier east pamirs 38817 0 n 75806 0 e central asia these data spanning the mid 1950s to 2000 were obtained by analyzing 101 sections using a sector field double focusing inductively coupled plasma mass spectrometer icp ms instrument this study provides the first time series for these metals from central asia concentrations are 11 7 329 ng ml 1 for al 0 33 42 7 ng ml 1 for mn 0 42 17 8 ng ml 1 for sr 0 04 1 4 ng ml 1 for rb 0 18 10 4 ng ml 1 for ba 2 167 pg ml 1 for cs 2 51 pg ml 1 for sb and 1 31 pg ml 1 for bi large variations in metal concentrations were found during the study period pronounced increases in concentrations were observed for sb and bi from the mid 1960s to the beginning of the 1990s suggesting increased anthropogenic sources of sb and bi in central asia during the same period however the decrease of sb and bi concentrations during the mid to late 1990s reflects a reduction in anthropogenic activities in central asia

testing models for the formation of the equatorial ridge on saturn s moon iapetus via crater counting abstract iapetus s equatorial ridge visible in global views of the moon is unique in the solar system the formation of this feature is likely attributed to a key event in the evolution of iapetus and various models have been proposed as the source of the ridge by surveying imagery from the cassini and voyager missions this study aims to compile a database of the impact crater population on and around iapetus s equatorial ridge assess the relative age of the ridge from differences in cratering between on ridge and off ridge and test the various models of ridge formation this work presents a database that contains 7748 craters ranging from 0 83 km to 591 km in diameter the database includes the study area in which the crater is located the latitude and longitude of the crater the major and minor axis lengths and the azimuthal angle of orientation of the major axis analysis of crater orientation over the entire study area reveals that there is no preference for long axis orientation particularly in the area with the highest resolution comparison of the crater size frequency distributions show that the crater distribution on the ridge appears to be depleted in craters larger than 16 km with an abruptly enhanced crater population less than 16 km in diameter up to saturation one possible interpretation is that the ridge is a relatively younger surface with an enhanced small impactor population finally the compiled results are used to examine each ridge formation hypothesis based on these results a model of ridge formation via a tidally disrupted sub satellite appears most consistent with our interpretation of a younger ridge with an enhanced small impactor population

the tidal history of iapetus spin dynamics in the light of a refined dissipation model 1 we study the tidal history of an icy moon basing our approach on a dissipation model which combines viscoelasticity with anelasticity and takes into account the microphysics of attenuation we apply this approach to iapetus the most remote large icy moon in the saturnian system different authors provide very different estimates for iapetus s despinning timescale by several orders of magnitude one reason for these differences is the choice of the dissipation model used for computing the spin evolution as laboratory data on viscoelastic properties of planetary ices are sparse many studies relied on dissipation models that turned out to be inconsistent with experiment a pure water ice composition generally assumed in the previous studies of the kind yields despinning times of the order of 3 7 gyr for most initial conditions we demonstrate that through accounting for the complexity of the material like second phase impurities one arrives at despinning times as short as 0 9 gyr a more exact estimate will remain unavailable until we learn more about the influence of impurities on ice dissipation by including the triaxial shape caused torque we encounter a chaotic behavior at the final stage of despinning with the possibility of entrapments in the intermediate resonances the duration of these entrapments turns out to be sensitive to the dissipation model no long entrapments have been found for iapetus described with our laboratory based dissipation model

formation of iapetus extreme albedo dichotomy by exogenically triggered thermal ice migration the striking appearance of saturn s moon iapetus half black and half white has puzzled astronomers for over three centuries now spencer and denk p 432 1 published online 10 december present an explanation for this asymmetry a thermally controlled runaway migration of water ice triggered by exogenic deposition of dark material on the moon s leading darker side which faces the direction of motion of the moon in its orbit around saturn this mechanism is unique to iapetus because it rotates slowly enough to allow large temperature variations to arise it is small enough to allow long range migration of water and there is a source of dust to trigger the process in a related paper denk et al p 435 2 published online 10 december present data derived from the cassini imaging science subsystem that reveal that both dark and bright materials on the leading side of iapetus are redder than their trailing side counterparts this asymmetry results from the deposition of dust and debris from other moons in the saturnian system the very same process that initiates the thermal segregation proposed above 1 lookup doi 10 1126 science 1177132 2 lookup doi 10 1126 science 1177088

210 pb dating of the miaoergou ice core from the eastern tien shan china in 2005 two ice cores with lengths of 58 7 and 57 6 m respectively to bedrock were recovered from the miaoergou flat topped glacier 43 degrees 03 19 n 94 degrees 19 21 e 4512 m a s l eastern tien shan pb 210 dating of one of the ice cores 57 6 m was performed and an age of ad 1851 6 at a depth of 35 2 m w e was determined for the period ad 1851 2005 a mean annual net accumulation of 229 7 mm w e a 1 was calculated at the nearby oasis city of hami similar to 80 km from the miaoergou flat topped glacier the annual precipitation rate is 38 mm w e a 1 hence glacial meltwater is a major water supply for local residents the surface activity concentration of pb 210 ex was found to be similar to 400 mbq kg 1 which is higher than observed at other continental sites such as belukha russia and tsambagarav mongolia which have surface activity concentrations of 280 mbq kg 1 the pb 210 dating agrees well with the chronological sequence deduced from the annual layer counting resulting from the seasonalities of delta o 18 and trace metals for the period ad 1953 2005 and beta activity horizons resulting from atmospheric nuclear testing during the period ad 1962 63 we conclude that pb 210 analysis is a suitable method for obtaining a continuous dating of the miaoergou ice core for similar to 160 years which can also be applied to other ice cores recovered from the mountains of western china

tectonics on iapetus despinning respinning or something completely different abstract saturn s moon iapetus is unique in that it has apparently despun while retaining a substantial equatorial bulge stresses arising from such a non hydrostatic shape should in principle cause surface deformation tectonics as part of a search for such a tectonic signature lineaments linear surface features on iapetus were mapped on both its bright and dark hemispheres lineament orientations were then compared to model stress patterns predicted for spin down from a rotation period of 16 5 h or less to its present synchronous period and for a range of lithospheric thicknesses many lineaments are straight segments of crater rimwalls which may be faults or joints reactivated during complex crater collapse most striking are several large troughs on the bright trailing hemisphere these troughs appear to be extensional and are distinctive on that hemisphere because the interior floors and walls of the troughs contain dark material globally no specific evidence of strike slip or thrust offsets are seen but this could be due to the age and degraded nature of any such features we find that observed lineament orientations do not correlate with predicted patterns due to despinning on either hemisphere the equatorial ridge was specifically excluded from this analysis and is considered separately modest evidence for preferred orientations 40 from north could be construed as consistent with respinning which is not necessarily far fetched assuming the rigidity of unfractured ice predicted maximum lithospheric differential stresses from despinning range from 1 mpa to 160 mpa for the elastic spheroid and thin lithosphere limits respectively although it is only for thicker elastic lithospheres that we expect a nonhydrostatic state to be maintained over geologic time against lithospheric failure the tectonic signature of despinning may have been obscured over time because the surface of iapetus is very ancient iapetus thick lithosphere may have inhibited the full tectonic expression of despinning or both several prominent lineaments strike e w and are thus parallel to the equatorial ridge though not physically close to it but a tectonic or volcanic origin for the ridge is highly problematic

on a ring origin of the equatorial ridge of iapetus 1 most recent cassini observations by the imaging science subsystem iss showed that the third largest saturnian satellite iapetus has a curious ridge system exactly aligned with its equator porco et al 2005 because iapetus has a large hill sphere for the trapping of circum satellitary material a ring system might have been present during its formation a scenario is proposed to describe how the equatorial ridge system could have been produced by the collisional accretion of a ring remnant subsequent to the formation of the proto iapetus

holocene development of boreal forests and fire regimes on the kenai lowlands of alaska several studies have noted a relationship between vegetation type and fire frequency yet despite the importance of ecosystem processes such as fire the long term relationships between disturbance climate and vegetation type are incompletely understood we analysed pollen plant macrofossils and sedimentary charcoal from three lakes within the kenai lowlands to determine postglacial relationships between disturbance climate and vegetation for the boreal forest of southwest alaska an herb tundra was established in the lowlands following deglaciation by 13 000 cal bp salix alnus and probably betula kenaica expanded in the area after 10 700 cal bp followed by picea glauca by 8500 cal bp picea mariana became established by 4600 cal bp the early holocene was probably the driest time during the postglacial as determined by aquatic plant macrofossils and climate models lake levels reached near modern conditions by at least 8000 cal bp mean fire intervals mfi were longest during the shrubherb tundra phase 1389 65 yr decreased after expansion of b kenaica salix and populus 779 49 yr and picea glauca 819 41 yr and increased again with the arrival of p mariana 1309 66 yr unlike previous studies our data demonstrate the highest fire frequencies during the early to mid holocene and less frequent fire during the late holocene when p mariana forests dominated the lowlands early holocene forests of p glauca and b kenaica existed in summers that were longer and drier than today while the increasingly wetter and cooler climates of the late holocene probably hindered forest fire around paradox lake perhaps because of less frequent summer drought

a framework for interpreting paleoclimatic variations in eastern beringia abstract paleoclimatic variations in a particular region can be viewed as the outcome of the superimposition of the effects of a number of large scale controls a framework for understanding paleoclimatic variations in a region can be established by considering the long term history of those controls jointly with climatic modeling results that illustrate how regional climates respond to changes of individual controls the relevant controls of late quaternary paleoclimatic variations in eastern beringia include 1 the size of the ice sheet which influences both atmospheric circulation and temperature 2 insolation which influences mainly temperature and seasonality 3 carbon dioxide concentration in the atmosphere which influences temperature and 4 feedbacks involving sea ice sea surface temperatures and snow cover which reinforce the insolation effects we review the results of a number of paleoclimatic experiments with general circulation models to illustrate the potential response of eastern beringia to the variations of these controls we then present a framework for understanding paleoclimatic variations in eastern beringia by describing the likely responses of the climate of this region to the known variations in the controls in addition we discuss key times during the quaternary when particular combinations of the controls give rise to natural experiments which lend themselves to the testing of particular hypotheses our understanding of the nature of climatic variations at high latitudes should increase through examination of paleoclimatic evidence that documents the response of the region at those key times

iapetus geophysics rotation rate shape and equatorial ridge iapetus has preserved evidence that constrains the modeling of its geophysical history from the time of its accretion until now the evidence is a its present 79 33 day rotation or spin rate b its shape that corresponds to the equilibrium figure for a hydrostatic body rotating with a period of 16 h and c its high equatorial ridge which is unique in the solar system this paper reports the results of an investigation into the coupling between iapetus thermal and orbital evolution for a wide range of conditions including the spatial distributions with time of composition porosity short lived radioactive isotopes slri and temperature the thermal model uses conductive heat transfer with temperature dependent conductivity only models with a thick lithosphere and an interior viscosity in the range of about the water ice melting point can explain the observed shape short lived radioactive isotopes provide the heat needed to decrease porosity in iapetus early history this increases thermal conductivity and allows the development of the strong lithosphere that is required to preserve the 16 h rotational shape and the high vertical relief of the topography long lived radioactive isotopes and slri raise internal temperatures high enough that significant tidal dissipation can start and despin iapetus to synchronous rotation this occurred several hundred million years after iapetus formed the models also constrain the time when iapetus formed because the successful models are critically dependent upon having just the right amount of heat added by slri decay in this early period the amount of heat available from short lived radioactivity is not a free parameter but is fixed by the time when iapetus accreted by the canonical concentration of 26al and to a lesser extent by the concentration of 60fe the needed amount of heat is available only if iapetus accreted between 2 5 and 5 0 myr after the formation of the calcium aluminum inclusions as found in meteorites models with these features allow us to explain iapetus present synchronous rotation its fossil 16 h shape and the context within which the equatorial ridge arose

the topography of iapetus leading side abstract we have used cassini stereo images to study the topography of iapetus leading side a terrain model derived at resolutions of 4 8 km reveals that iapetus has substantial topography with heights in the range of 10 km to 13 km much more than observed on the other middle sized satellites of saturn so far most of the topography is older than 4 ga neukum g wagner r denk t porco c c 2005 lunar planet sci xxxvi abstract 2034 which implies that iapetus must have had a thick lithosphere early in its history to support this topography models of lithospheric deflection by topographic loads provide an estimate of the required elastic thickness in the range of 50 100 km iapetus prominent equatorial ridge porco c c and 34 colleagues 2005 science 307 1237 1242 reaches widths of 70 km and heights of up to 13 km from their base within the modeled area the morphology of the ridge suggests an endogenous origin rather than a formation by collisional accretion of a ring remnant ip w h 2006 geophys res lett 33 doi 10 1029 2005gl025386 l16203 the transition from simple to complex central peak craters on iapetus occurs at diameters of 11 3 km the central peaks have pronounced conical shapes with flanking slopes of typically 11 and heights that can rise above the surrounding plains crater depths seem to be systematically lower on iapetus than on similarly sized rhea which if true may be related to more pronounced crater wall slumping which widens the craters on iapetus than on rhea there are seven large impact basins with complex morphologies including central peak massifs and terraced walls the largest one reaches 800 km in diameter and has rim topography of up to 10 km generally no rings are observed with the basins consistent with a thick lithosphere but still thin enough to allow for viscous relaxation of the basin floors which is inferred from crater depth to diameter measurements in particular a 400 km basin shows up domed floor topography which is suggestive of viscous relaxation a model of complex crater formation with a viscoplastic bingham rheology melosh h j 1989 impact cratering oxford univ press new york of the impact shocked icy material provides an estimate of the effective cohesion viscosity at 0 04 0 01 mpa 0 6 0 2 gpa s the local distribution of bright and dark material on the surface of iapetus is largely controlled by topography and consistent with the dark material being a sublimation lag deposit originating from a bright icy substrate mixed with the dark components but frost deposits are possible as well

dated wood from alaska and the yukon implications for forest refugia in beringia abstract postulations on the existence of forest refugia in parts of beringia during the last glacial have been in large part based on ambiguous evidence existing data on radiocarbon dated and identified fossil wood and macrofossils from alaska and northwest canada are synthesized here and are augmented by results of palynological studies in an effort to show the persistence of some and total extinction of other tree and large shrub species possible dispersal routes taken by species that reinvaded beringia in postglacial times are also reconstructed from the fossil record macrofossil and pollen evidence when combined with climatic factors makes cottonwood a good candidate for survival during the last glacial larch and aspen are also candidates though the evidence for them is less positive pollen and macrofossils of alder are very scarce in deposits of the last glacial age and if it survived at all it was probably in very isolated vegetatively reproducing clones shrub birch may have been present in beringia but tree birch probably was reintroduced during the holocene spruce also appears to have been absent in alaska from about 30 000 to 11 500 yr ago and probably reinvaded beringia from a refugium south of the laurentide ice sheet

topographic constraints on the origin of the equatorial ridge on iapetus abstract saturn s moon iapetus has an equatorial ridge system which may be as high as 20 km that may have formed by endogenic forces such as tectonic and convective forces or exogenic processes such as debris infall we use high resolution topographic data to conduct a topographic analysis of the ridge which suggests a predominantly triangular morphology with some ridge face slopes reaching 40 allowing for an exogenic formation mechanism

long term retention of impact crater topography on ganymede previous analyses into viscous relaxation of impact craters on ganymede have predicted short characteristic relaxation times of the order of 100 myr or less for larger craters in disagreement with the apparent great ages of ganymede s terrains by applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model we calculate upper limit zero heat flow relaxation times in excess of the age of the solar system eliminating this discrepancy these very long relaxation times are not due to elastic effects but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses

ostracod shell chemistry as proxy for paleoenvironmental change abstract the application of ostracod shell chemistry as a paleoenvironmental tool has grown within the past decades most studies have investigated mg and sr in ostracod shells as proxies for temperature and salinity and the use of a wide range of trace elements as prospective paleoenvironmental indicators has yet to be developed for lacustrine systems only a few preliminary studies have used trace metals in paleolimnological studies such as cd ba and zn as paleonutrient indicators or mn fe and u as redox and oxygenation indicators this paper reviews the state of the art of geochemical analyses in microfossils such as ostracods foraminifera and corals and provides insights in new trace element proxies with the goal to promote the use of trace elements in ostracod shells as paleoenvironmental proxies in paleoceanography foraminifera and corals are most prominently used to reconstruct past climate conditions well established proxies such as 18o mg ca and sr ca provide information about changes in sea surface temperatures in addition a great number of new proxies have been developed recently such as radiogenic isotopes and redox sensitive trace elements in paleolimnology ostracod shell chemistry is widely used to assess paleohydrological changes reconstruction of temperature and salinity changes in lake environments is often achieved by oxygen isotopes as well as mg ca and sr ca ratios but depending on the hydrological and geological settings of the lake system local calibrations are needed to assess which proxy is suited to reflect which processes new proxies need to be tested by novel techniques that recently have become available compared to conventional instrumentation used in ostracod shell chemistry methods such as laser ablation icp ms and nanosims allow single shell analysis and provide high resolution data the potential of ostracods in paleolimnology is not yet fully assessed but can be developed by learning from paleoceanographic studies

despinning and shape evolution of saturn s moon iapetus triggered by a giant impact abstract iapetus possesses two spectacular characteristics i a high equatorial ridge which is unique in the solar system and ii a large flattening a c 34 km inconsistent with its current spin rate these two main characteristics have probably been acquired in iapetus early past as a consequence of coupled interior rotation evolution previous models have suggested that rapid despinning may result either from enhanced internal dissipation due to short lived radioactive elements or from interactions with a sub satellite resulting from a giant impact for the ridge formation different exogenic and endogenic hypotheses have also been proposed but most of the proposed scenarios have not been tested numerically in order to model simultaneously internal heat transfer tidal despinning and shape evolution we have developed a two dimensional axisymmetric thermal convection code with a deformable surface boundary coupled with a viscoelastic code for tidal dissipation the model includes centrifugal and buoyancy forces a composite non linear viscous rheology as well as an andrade rheology for the dissipative part by considering realistic rheological properties and by exploring various grain size values we show that in the absence of additional external interactions despinning of a fast rotating iapetus is impossible even for warm initial conditions t 250 k alternatively the impact of a single body with a radius of 250 350 km at a velocity of 2 km s may be sufficient to slow down the rotation from a period of 6 10 h to more than 30 h by combining despinning due to internal dissipation and an abrupt change of rotation due to a giant impact we determined the parameters leading to a complete despinning and we computed the corresponding shape evolution we show that stresses arising from shape change affect the viscosity structure by enhancing dislocation creep and can lead to the formation of a large scale ridge at the equator as a result of rapid rotation change for initial rotation periods of 6 h

folding of europa s icy lithosphere an analysis of viscous plastic buckling and subsequent topographic relaxation abstract potential contractional folds on jupiter s icy moon europa have been identified the best example is at the extensional band astypalaea linea where a series of subtle topographic undulations 25 km in wavelength possess parasitic tectonic features that support a folding origin a scenario has been qualitatively proposed whereby folds form via unstable contraction of the icy lithosphere compensate for extension elsewhere on europa and then subsequently relax here we quantitatively address this scenario applying a model for viscous plastic buckling of planetary lithospheres and finite element simulations of topographic relaxation our results suggest that the lithosphere of europa could indeed be unstable but the low required surface temperatures limit fold formation to higher latitudes and the high required driving stresses 9 10 mpa are difficult to achieve on the satellite the depth to the brittle ductile transition is well constrained and high thermal gradients are indicated implying heat flows near 100 mw m 2 in addition topographic relaxation progresses so slowly even at these heat flows that it is not a viable mechanism to eliminate such features over the age of europa s surface given the paucity of identified folds we conclude that the necessary conditions for their formation are rare and that lithospheric folding is a minor mechanism for compensating the large amounts of extension seen elsewhere on europa

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

holocene glaciation of alaska and adjoining yukon territory canada abstract holocene glacial fluctuations between arctic central interior and southern maritime alaska are broadly synchronous this synchrony is evident from a review of work in 11 study areas with varying numbers of glaciers 3 100 glacier types subpolar cirque temperate valley tidewater surging and dating methodologies the pattern is temporally and spatially consistent with vegetation data and generally with climatic deteriorations culminating in the little ice age early holocene glacial advances are confirmed for 7 6 5 8 lichen ka bp at 4 of 97 glaciers in the brooks range advances also occurred by 5 85 and 5 7 ka bp at separate glacier tongues in the northern alaska range at least by 6 5 ka bp in lituya bay and possibly also in the malaspina and aleutian islands areas a sharp increase in glacier activity began by 4 4 ka bp in the arctic and glacier bay areas and by about 3 ka bp major glacier advances probably were initiated or well underway throughout alaska during holocene glacial maxima ice filled glacier and lituya bays as well as fjords of prince william sound and massive moraines buried forests in the northeastern st elias mountains area minor recession is recorded directly or implied in most of alaska about 2 ka bp or locally through the following millennium subsequently distinct but locally weak advances occurred by about 1 2 ka bp in most areas again followed by intervals of glacier recession most pervasive and ubiquitous were the little ice age advances initiated about 700 bp these generally culminated in two major advances between about 400 and 50 bp in the arctic those of a d 1600 dominated those of the late 1800s in the gulf of alaska those of the middle 1700s or late 1800s dominated large scale asynchronous advances and retreats of calving tidewater glaciers have been recorded during the twentieth century but most such movements are only indirectly related to climate

climate change and arctic ecosystems 1 vegetation changes north of 55 n between the last glacial maximum mid holocene and present a unified scheme to assign pollen samples to vegetation types was used to reconstruct vegetation patterns north of 55 n at the last glacial maximum lgm and mid holocene 6000 years b p the pollen data set assembled for this purpose represents a comprehensive compilation based on the work of many projects and research groups five tundra types cushion forb tundra graminoid and forb tundra prostrate dwarf shrub tundra erect dwarf shrub tundra and low and high shrub tundra were distinguished and mapped on the basis of modern pollen surface samples the tundra forest boundary and the distributions of boreal and temperate forest types today were realistically reconstructed during the mid holocene the tundra forest boundary was north of its present position in some regions but the pattern of this shift was strongly asymmetrical around the pole with the largest northward shift in central siberia 200 km little change in beringia and a southward shift in keewatin and labrador 200 km low and high shrub tundra extended farther north than today at the lgm forests were absent from high latitudes graminoid and forb tundra abutted on temperate steppe in northwestern eurasia while prostrate dwarf shrub erect dwarf shrub and graminoid and forb tundra formed a mosaic in beringia graminoid and forb tundra is restricted today and does not form a large continuous biome but the pollen data show that it was far more extensive at the lgm while low and high shrub tundra were greatly reduced illustrating the potential for climate change to dramatically alter the relative areas occupied by different vegetation types

evidence from north west canada for an early holocene milankovitch thermal maximum according to the milankovitch theory of global climatic change maximum summer solar radiation at high latitudes of the northern hemisphere occurred at 10 000 yr bp refs 1 2 in particular it predicts summer solstice radiation greater by 9 10 preliminary climate simulation experiments with these increased values of radiation confirm that high latitude land surfaces received maximal insolation at 10 000 yr refs 3 4 paradoxically however the large volume of fossil pollen and other evidence from north america indicates a maximum of holocene warmth at 7 000 6 000 yr ref 5 and a recent review of the evidence from new england suggests that the warming began at 9 000 and ended at 5 000 yr but also stresses the difficulties of interpretation in terms of climate change6 we summarize here data from sites in the north west corner of mainland canada fig 1 that directly support the milankovitch hypothesis

high resolution atmospheric cadmium record for ad 1776 2004 in a high altitude ice core from the eastern tien shan central asia two ice cores drilled to the bottom were recovered from miaoergou flat topped glacier 43 03 19 n 94 19 21 e 4512 ma s l eastern tien shan central asia in 2005 a high resolution record of cadmium was established by applying inductively coupled plasma mass spectrometry to one of the ice cores 57 6 m covering a 228 year period from ad 1776 to 2004 the results showed long term variations of atmospheric transport and deposition of cadmium at high altitudes trend analysis based on the sequential mann kendall test and the analysis of crustal enrichment factors of the cadmium shows that natural contribution mainly from rock and mineral dust dominated the atmospheric cycles of cadmium during the period ad 1776 1957 which was confirmed by the significant correlation between the winter north atlantic oscillation nao index and annual cadmium concentration the concentration of cadmium increased sharply from ad 1957 to 2004 suggesting increasing influence from human activities such as metals production the ice core record indicated increasing atmospheric cadmium pollution in response to rapid economic growth after ad 1957 in the region

a 47 year high resolution chemistry record of atmospheric environment change from the laohugou glacier no 12 north slope of qilian mountains china abstract based on a 20 12 m shallow ice core drilled at the laohugou glacier no 12 39 25 7 n 96 33 4 e 5040 m a s l north slope of qilian mountains in the northeastern qinghai tibetan plateau in june 2006 the changes of atmospheric environment over the qilian mountains and northwestern china recorded by ice chemistry and its possible connection with atmospheric circulation and the winter north atlantic oscillation nao are investigated a total of 606 samples were analyzed for major ions ca2 na k mg2 nh 4 cl so 4 2 and no 3 and stable oxygen isotope ratios 18o the ion chemistry of the ice core covering the past 47 years is mainly characterized by mineral dust species ca2 na mg2 so 4 2 cl and k and anthropogenic species so 4 2 nh 4 and no 3 ca2 is the dominant cation in the ice core with a medium value of 2815 5 eq l 1 accounting for 65 8 of the total cations and so 4 2 is the predominant anion with a medium value of 1500 4 eq l 1 accounting for 45 8 of the total anions compared with ice core records from altai and himalayas at the northern and southern margin of western china respectively dust related species concentrations in the laohugou ice core are much higher due to pronounced regional crustal dust aerosol inputs from surrounding arid and semiarid deserts the results show that dust species concentrations decreased during 1960 2006 a d while non dust species so 4 2 nh 4 and no 3 anthropogenic species concentrations increased which indicating that the ice core record of dust storm activity over qilian mountains exhibits a significant decreasing trend during the past 47 years this change of dust species may be caused by decrease in wind speed and increase in precipitation over the laohugou surrounding areas in qilian mountains and northwestern china and wind speed decrease is probably caused by spatial differences of inter decadal air temperature change of the 600 hpa in central asia the normalized time series of the winter nao index shows a significant inverse correlation with the annual flux of ca2 in dust implying a possible connection between the winter nao and the dust storm activity over the qilian mountains

the reconstruction of boreal forest fire history from lake sediments a comparison of charcoal pollen sedimentological and geochemical indices abstract fossil charcoal fossil pollen sedimentological and geochemical analyses of lake sediments have been used previously to reconstruct a history of local fires and resulting vegetation change the rationale behind these approaches is described and the usefulness of each technique for reconstructing fire history in the boreal forest is assessed empirically historical and dendrochronological records provide regional and local fire histories for a site in wood buffalo national park alberta canada the local and regional history of fires is compared with the microscopic charcoal content macroscopic charcoal content elemental carbon content fossil pollen content sedimentology and geochemistry of annually laminated sediments from a small lake there is no significant correlation between the abundance of microscopic charcoal macroscopic charcoal and total elemental carbon content of the sediments automated measures of microscopic charcoal abundance made with an image analysis system are correlated significantly with optical counts of microscopic charcoal none of the charcoal measures provide unequivocal records of local fire activity and the abundance of microscopic charcoal appears to be influenced by variations in regional fire activity however the highest depositional rate of macrofossil charcoal occurred during the time of a fire that burned within the drainage basin variations in sedimentological measures and geochemistry do not correlate with local fire activity fossil pollen percentages and accumulation rates display a pattern of variation that is consistent with observed vegetation responses to fire in the boreal forest what is likely apparent in the pollen record are the results of a series of burns of sufficient extent and intensity to kill most of the above ground biomass of the vegetation in an area at least as great as the drainage basin the inability of evidence from lake sediments to provide detailed histories of past fire activity is not surprising given the wide range of variation in the spatial extent proximity intensity and impact of individual fires

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

impact basin relaxation at iapetus abstract we investigate impact basin relaxation on iapetus by combining a 3d thermal evolution model robuchon g choblet g tobie g cadek o sotin c grasset o 2010 icarus 207 959 971 with a spherical axisymmetric viscoelastic relaxation code zhong s paulson a wahr j 2003 geophys j int 155 679 695 due to the progressive cooling of iapetus younger basins relax less than older basins for an ice reference viscosity of 10 14 pa s an 800 km diameter basin relaxes by 30 if it formed in the first 50 myr but by 10 if it formed at 1 2 gyr bigger basins relax more rapidly than smaller ones because the inferred thickness of the ice shell exceeds the diameter of all but the largest basins considered stereo topography shows that all basins 600 km in diameter or smaller are relaxed by 25 or less our model can match the relaxation of all the basins considered within error by assuming a single basin formation age 4 36 ga for our nominal viscosity this result is consistent with crater counts which show no detectable age variation between the basins examined

postglacial vegetation and climate change in the northern bristol bay region southwestern alaska pollen analysis of sediment cores from grandfather and ongivinuk lakes reveals a record of postglacial vegetation and climate change in the northern bristol bay region the chronology is based on six conventional 14c dates of bulk organic matter from the grandfather core a mesic herb tundra dominated the landscape 13 000 9800 yr b p betula shrubs probably first appeared in the region 11 300 yr b p but were restricted to favorable microhabitats until 9800 yr b p the later establishment of betula shrubs and relatively low betula pollen abundance in these records compared to other areas of eastern beringia suggest that postglacial warming in southwestern alaska was dampened by regional climatic controls possibly low sea surface temperatures of the north pacific ocean between 10 800 and 9800 yr b p diminished betula shrub cover along with decreased aquatic productivity as recorded by pediastrum cell nets and biogenic silica suggest a brief reversion to colder and drier climatic conditions possibly associated with the younger dryas event around 9800 yr b p betula shrub tundra and meadow communities expanded probably in response to increased temperature and precipitation alnus arrived and formed extensive thickets within the region ca 7400 yr b p the establishment of the modern boreal forest tundra ecotone is marked by the arrival of picea glauca at grandfather lake ca 4000 yr b p and the subsequent increase to present population densities ca 2000 yr b p the unique features of these pollen records emphasize the spatial complexity of late quaternary vegetation and climate history in eastern beringia

late quaternary vegetational history of the black river region in northeastern alaska two pollen diagrams describe the previously unknown vegetational history of the black river and little black river drain ages of northeastern alaska tundra grew throughout much of the region between ca 19 800 and 15 000 years bp prior to 18 000 years bp the vegetation was dominated perhaps by xeric cyperaceae communities but apparently between ca 18 000 and 15 000 years bp a more diverse herb tundra characterized the vegetation herbaceous species continued to be important in the vegetation until ca 9700 years bp although betula nana glandulosa and salix species became more common after 15 000 years bp at ca 9700 years bp a major change in the vegetation occurred with the spread of populus probably p balsamifera b nana glandulosa and salix possibly resulting in a mosaic of shrub tundra and populus gallery forest picea glauca migrated into the black river region ca 7500 years bp followed by alnus ca 7200 years bp and picea mariana ca 6000 years bp herb zone records from the black rive

preliminary results from measurements of selected trace metals in the snow firn pack on urumqi glacier no 1 eastern tien shan china we present preliminary results on the occurrence of pb cd zn al and fe in dated samples collected from snow firn packs at an altitude of 4130 m on urumqi glacier no 1 eastern tien shan extreme precautions for avoiding contamination were taken throughout the sampling processing transportation and analysis procedures the concentrations of trace metals were determined by a double focusing inductively coupled plasma mass spectrometer in an ultra clean room the average concentrations for these metals in surface snow are in ng g 1 pb 2 4 cd 0 05 zn 10 0 al 100 0 and fe 130 0 these are higher concentrations especially for pb and zn than those in the polar and or low latitude remote areas the data show that the input of trace metals to the snow has a clear seasonal change lower concentrations in surface snow can be found in july through september and higher concentrations from october to march with an exception for january the mean concentrations of the elements in the snow firn pack also indicate seasonal variations and show a marked inverse relationship with temperature possibly a result of meltwater percolation in the snow firn pack

particle motion and the theory of charcoal analysis source area transport deposition and sampling principles from particle motion physics were applied to recurring problems of the interpretation of stratigraphic charcoal data 1 fires within catchments of lakes often produce no record in fossil charcoal curves and 2 periods characterized by no local fire e g 20th century fire suppression often display as much charcoal as times when local fire was frequent quantitative theory on source area transport deposition and sampling of charcoal shows the relationship between particle sizes counted by alternative methods of charcoal analysis pollen slides for particles 5 80 m in diameter petrographic thin sections for particles 50 10 000 m diameter and charcoal diagrams the relationship between diameter and critical and deposition velocities results in fundamental aerodynamic differences between the sizes of particles quantified by the two methods charcoal recorded on pollen slides is of a size that is difficult to lift but once entrained it remains in suspension thin section charcoal is lifted at relatively low wind velocities but it is not suspended at normal surface wind speeds thermal buoyancy during fire lofts charcoal above the forest canopy depending on particle size and wind speed pollen slide charcoal sizes are underrepresented near the fire because they remain in suspension and are preferentially exported from the burn area thin section charcoal is convected to lower heights on average and is deposited nearby following fire thin section charcoal is redistributed locally by wind and thus may enter lakes because of cohesive forces and aerodynamics more pollen slide charcoal remains on the ground and less enters lakes source areas for pollen slide charcoal are subcontinental to global and diagrams of pollen slide charcoal are biased toward nonlocal charcoal they can be used to interpret importance of fire for broad spatial and temporal scales thin section charcoal represents the catchment fire regime simulation models that generate charcoal during fire mix sediments and then sample at specified intervals indicate that 1 in the absence of sediment mixing the pollen slide method should consistently resolve individual fires that occur with an expectation of 30 50 yr 2 unless samples are continuous neither method will produce useful estimates of fire frequency and 3 even a modest amount of sediment mixing will obscure the signal