recent progress in snow and ice research a review of snow and ice research in 1987 1990 is presented focusing on the effects of layers in seasonal snow covers ice mechanics on fresh water and sea ice and remote sensig of polar ice sheets these topics provide useful examples of general needs in snow and ice research applicable to most areas such as better representation in models of detailed processes controlled laboratory experiments to quantify processes and field studies to provide the appropriate context for interpretation of processes from remote sensing

anomalies in global carbon cycling and extinction at the triassic jurassic boundary evidence from a marine c isotope record this study investigates whether the end triassic biotic crisis was coupled with a perturbation of the marine c isotope budget the marine c isotope signature serves as a proxy of the marine carbon reservoir and ultimately of the global c cycle a continuous shallow water marine limestone succession from the western southern alps bergamasc alps northern italy provides information on the end triassic biotic crisis and on the evolution of the marine carbon reservoir across the triassic jurassic t j boundary the established carbonate c isotope curve is marked by a negative c isotope pulse coinciding with the disappearance of the end triassic benthic faunal assemblage and a widespread rhaetian carbonate platform drowning event the negative spike is followed by a positive c isotope excursion starting at the palynological t j transition the negative c isotope pulse may have resulted from the sudden release of gas hydrates the positive isotope excursion records a global change in organic carbon burial rates probably in response to elevated atmospheric co2 levels at a time of massive volcanic activity in the central atlantic magmatic province high co2 levels were responsible for the end triassic biocalcification crisis carbonate platform collapse and possibly the sudden release of methane from gas hydrate

deep air convection in the firn at a zero accumulation site central antarctica abstract ice cores provide unique archives of past atmospheres and climate but interpretation of trapped gas records and their climatic significance has been hampered by a poor knowledge of the prevalence of air convection in the firn layer on top of polar ice sheets in particular the phasing of greenhouse gases and climate from ice cores has been obscured by a discrepancy between empirical and model based estimates of the age difference between trapped gases and enclosing ice which may be due to air convection here we show that deep air convection 23 m occurs at a windy near zero accumulation rate site in central antarctica known informally as the megadunes site 80 77914 s 124 48796 e deep convection is evident in depth profiles of air withdrawn from the firn layer in the observed pattern of the nitrogen isotope ratio 15 n 14 n the argon isotope ratio 40 ar 36 ar and in the mixing ratios of the anthropogenic halocarbons methyl chloroform ch 3 ccl 3 and hfc 134a ch 2 fcf 3 transport parameters diffusivities were inferred and air was dated using measured carbon dioxide co 2 and methane ch 4 mixing ratios by comparing with the law dome atmospheric record which shows that these are the oldest firn air samples ever recovered co 2 mean age 1863 ad the low accumulation rate and the consequent intense metamorphism of the firn due to prolonged exposure to seasonal temperature cycling likely contribute to deep air convection via large grain size and vertical cracks that act as conduits for vigorous air motion the megadunes site provides a possible modern analog for the glacial conditions in the vostok dome fuji and dome c ice core records and a possible explanation for lower than expected 15 n 14 n ratios in trapped air bubbles at these times a general conclusion is that very low accumulation rate causes deep air convection via its effect on firn structural characteristics

a phase changing dry snowpack model an interac ting continua fra mework is adopted to model a dr y snowpack which is vi ewed as a thr ee co nsti tuent mixtur e composed of an ice matrix who se pore space is occupie d by wat er vapour and dr y air we foc us on the response of a one dimen sion al vertical sno wpack to changes in pressure and tem perature at it s su rf ace the ti me scale of the sur face forcing is assume d to be muc h lo nger than the time scale for thermal transfers and pha se change to take place the constitu ents are ther ef ore in thermal equilib ri um with a common tempe rature t which is governed by a single bulk energy balance in addition each constituent satisf ie s a mass and momentum balance the con stitut ive postulates and external prescriptions necessar y to clo se the system of equa tions are discus sed in detail non dimensional varia bles are then introduced formally to draw out the majo r balances in the equations and construct a red uced system that accur ately model s the dominant features in the snowpack it is sho wn how the effects of phase caa nge ent er the leadi ng o rder balance an ite rative pro ced ure is cons tructed to solve the system illu strations for the case of a sin usoidal ann ual tem perature grad ient imposed at the surf ace are presented

effects of firn ventilation on isotopic exchange a new model of isotopic diffusion in the upper few meters of firn tracks the isotopic composition of both the ice matrix and the pore space vapor through time in two dimensions stable isotopes in the vapor phase move through the firn by diffusion along concentration gradients and by advection wind driven ventilation carries atmospheric water vapor into the firn where it mixes with existing pore space vapor unlike previous models our model allows disequilibrium between pore space vapor and the surrounding snow grains we also calculate the isotopic effects of ventilation driven sublimation and condensation in the firn model predictions of isotopic diffusion in firn compare favorably with existing diffusion models model results quantify what other investigators have suggested isotopic change in the upper few meters is more rapid than can be explained by the whillans and grootes 1985 model isotopic equilibration with atmospheric vapor is an important component of post depositional isotopic change and ventilation enhances isotopic exchange by creating regions of relatively rapid sublimation and condensation in the firn

soil moisture wind speed and depth hoar formation in the arctic snowpack basal depth hoar that forms in arctic snowpacks often has a low thermal conductivity strongly contributing to the snowpack thermal insulance and impacting the permafrost thermal regime at ward hunt island canadian high arctic 83 05 n 74 07 w almost no depth hoar was observed in spring 2016 despite favorable thermal conditions we hypothesize that depth hoar formation was impeded by the combination of two factors 1 strong winds in fall that formed hard dense wind slabs where water vapor transport was slow and 2 low soil moisture that led to rapid ground cooling with no zero curtain period which reduced soil temperature and the temperature gradient in the snowpack comparisons with detailed data from the subsequent winter at ward hunt and from bylot island 73 09 n 80 00 w and with data from barrow and alert indicate that both high wind speeds after snow onset and low soil moisture are necessary to prevent arctic depth hoar formation the role of con vection to form depth hoar is discussed a simple preliminary strategy to parameterize depth hoar thermal conductivity in snow schemes is proposed based on wind speed and soil moisture finally warming induced vegetation growth and soil moisture increase should reduce depth hoar thermal conductivity potentially affecting permafrost temperature

isotope calibrated greenland temperature record over marine isotope stage 3 and its relation to ch4 large temperature variations on millennial time scales in greenland characterised the last ice age abrupt warmings known as dansgaard oeschger do events can be traced in the 18oice record of greenland ice cores however it has been shown that 18oice is not a direct temperature proxy measurements of the isotopic composition of gases trapped in the ice can be used to calibrate the paleothermometer here we present a continuous temperature record based on high resolution 15n measurements and firn model studies it covers a sequence of 9 do events 9 17 during the time period from 38 to 64 kyr bp for which temperature changes of 8 to 15 c were estimated the difference between the modern and the glacial 18oice t relationship can be explained by a combination of source temperature changes and changes in the annual distribution of precipitation a detailed comparison of the temperature evolution with reconstructions of the atmospheric methane ch4 concentration shows that ch4 rises lag temperature increases at the onset of do events by 25 to 70 yr within data resolution the strong correlation between greenland temperature and ch4 on millennial and submillennial time scales suggests that variations on these time scales were probably of hemispheric extent

δ15n of n2 in air trapped in polar ice a tracer of gas transport in the firn and a possible constraint on ice age gas age differences with respect to gas transport the fim near the surface of an ice sheet can be divided into three zones the uppermost is a convective zone located just below the surface of the ice sheet in which the air is rapidly flushed by convective exchange with the overlying atmosphere below the convective zone there is a diffusive air column in which diffusion is rapid but there is no convection between the bottom of the diffusive air column and the bubble close off region there may be a nondiffusive zone in which diffusion is so slow that negligible gas transport occurs the diffusive air column is characterized by progressive enrichment with depth of 15n in n2 and heavy isotopes of gases in general as predicted using the barometric equation in this paper we present data on the 15n of n2 in recently trapped air samples from 12 ice cores along with numerous downcore samples from byrd vostok and dome c bubble close off depths for these cores calculated from a densification model ranged from 51 to 114 meters below the surface mbs we used these data and the barometric equation to calculate the thickness of the diffusive air column and found that it comprised 46 to 93 of the total firn thickness at our study sites paleo close off depths calculated from the densification model for glacial sections of byrd vostok and dome c are 15 25 m deeper than close off depths today diffusive column heights calculated from 15n varied in a more complex manner the diffusive column height at byrd appears to have decreased from 74 m during the last glacial period to 50 m during the holocene at vostok and dome c the diffusive column height calculated from 15n increase from about 65 m during the last glacial period to about 80 m in the holocene we use records of surface temperature and co2 at byrd and vostok along with their respective chronologies to constrain the ice age gas age difference age throughout the section of the vostok ice core corresponding to the last glacial termination in principle age values calculated from these data can be used to discriminate whether gas in the fim mixes to the bubble close off depth or to a depth equivalent to the diffusive column height in practice however uncertainties in the chronology of byrd and vostok are too great to allow us to distinguish between these two possibilities one can only say that at the time of the last termination age for vostok was between 3 and 10 5 kyr previous estimates fall within this range

tomography based characterization of ice air interface dynamics of temperature gradient snow metamorphism under advective conditions snow at or close to the surface commonly undergoes temperature gradient metamorphism under advective flow which alters its microstructure and physical properties a functional understanding of this process is essential for many disciplines from modeling the effects of snow on regional and global climate to assessing avalanche formation time lapse x ray microtomography was applied to investigate the structural dynamics of temperature gradient snow metamorphism exposed to an advective airflow in controlled laboratory conditions experiments specifically analyzed sublimation and deposition of water vapor on the ice structure in addition an analysis of the ice air interface dynamics was carried out using a macroscopic equivalent model of heat and water vapor transport through a snow layer the results indicate that sublimation of the ice matrix dominated for flow rates 10 6 m3 s 1 while during increased mass flow rates the water vapor deposition supplied by the advective flow counteracted sublimation a flow rate dependence of water vapor deposition at the ice interface was observed asymptotically approaching an average estimated maximum deposition rate on the whole sample of 1 05 10 4 kg m 3 s 1 the growth of microsized whisker like crystals on larger ice crystals was detected on microscope photographs leading to an increase of the specific surface area and thus suggest a change of the physical and optical properties of the snow the estimated values of the curvature effect of the ice crystals and the interface kinetic coefficient are in good agreement with previously published values

reconstructing thermal properties of firn at summit greenland from a temperature profile time series we have constrained the value for thermal diffusivity of near surface snow and firn at summit station greenland using a fourier type analysis applied to hourly temperature measurements collected from eight thermistors in a closed off air filled borehole between may 2004 and july 2008 an implicit finite difference method suggests that a bulk diffusivity of 25 3m2 a 1 is the most reasonable for representing macroscale heat transport in the top 30 m of firn and snow this value represents an average diffusivity and in a conduction only model generates temperature series whose phase shifts with depth most closely match those of the summit borehole data rms difference between measurements and model output is 6 days this bulk value derived numerically and corroborated analytically is useful over large tracts of the greenland ice sheet where density and microstructure are unknown

temperature measurements and heat transfer in near surface snow at the south pole to study near surface heat flow on the antarctic ice sheet snow temperatures were measured at south pole station to a depth of 3 m at 15 min intervals during most of 1992 solar heating and water vapor transport were negligible during the 6 month winter as was inter grain net thermal radiation leaving conduction as the dominant heat transport mechanism the rate of temperature change at depth over 15 min intervals was smaller than that at the surface by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth a finite difference model with conduction as the only heat transfer mechanism and measured temperatures as the upper and lower boundary conditions was applied to foursets of three thermistors each the thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor the thermal conductivity obtained at shallow depths above 40 cm was lower than that given by existing parameterizations based on density probably because the snow grains were freshly deposited cold and poorly bonded a model using only vertical conduction explains on average 87 ofthe observed 15 min temperature changes at less than 60 cm depth and 92 below 60 cm the difference between modeled andmeasured temperature changes decreased with depth the discrepancies between model and observation correlated more strongly with the air snow temperature difference than with the product of that difference with the square of the wind speed suggesting that the residual errors are due more to non vertical conduction and to sub grid scale variabilis of the conductivity than to windpumping the residual heating rate not explained by the model of vertical conduction exceeds 0 2 w m 3 only in the top 60 cm of the near surface snow

convective mixing of air in firn at four polar sites air withdrawn from the firn at four polar sites dome fuji h72 and ym85 antarctica and north grip greenland was measured for 15 no f n2 and 18 oo f o2 to test for the presence of convective air mixing in the top part of the firn known as the convective zone understanding the convective zone and its possible relationship to surface conditions is important for constructing accurate ice core greenhouse gas chronologies and their phasing with respect to climate change the thickness of the convective zone was inferred from a regression line with barometric slope of the data in the deep firn it is less than a few meters at h72 and ngrip whereas a substantial convective zone is found at dome fuji 8 6 2 6 m and ym85 14 0 1 8 m by matching the outputs of a diffusion model to the data effective eddy diffusivities required to mix the firn air are found at the surface of dome fuji and ym85 these are found to be several times greater than the molecular diffusivity in free air the crossover from dominance of convection to molecular diffusion takes place at 7 2 11 2 and 0 5 0 5 m at dome fuji ym85 and ngrip respectively these depths can be used as an alternative definition of the convective zone thickness the firn permeability at dome fuji is expected to be high because of intense firn metamorphism due to the low accumulation rate and large seasonal air temperature variation at the site the firn layers in the top several meters are exposed to strong temperature gradients for several decades leading to large firn grains and depth hoar that enhance permeability the thick convective zone at ym85 is unexpected because the temperature accumulation rate and near surface density are comparable to ngrip the strong katabatic wind at ym85 is probably responsible for creating the deep convection the largest convective zone found in this study is still only half of the current inconsistency implied from the deep ice core gas isotopes and firn densification models 2006 elsevier b v all rights reserved

biogeochemical constraints on the triassic jurassic boundary carbon cycle event 1 the end triassic mass extinctions represent one of the five most severe biotic crises in earth history yet remain one of the most enigmatic ongoing debate concerns the environmental effects of the central atlantic magmatic province camp eruptions and their linkage with the mass extinction event across the triassic jurassic boundary there is conflicting paleo evidence for changes in atmospheric pco2 during the extrusion of the camp basalts studies on sediments from european and pacific localities have however identified a substantial negative isotopic anomaly up to 3 5 across the tr j boundary providing an important indicator of changes in the operation of the ancient global carbon cycle we sought to explain the paleo evidence by utilizing a carbon cycle model for the hothouse world of the end triassic that emphasizes the chemical weathering of silicate and carbonate rocks and the ocean carbonate chemistry we find that volcanic co2 outgassing fails to fully account for either a sufficient rise in atmospheric pco2 indicated by the stomata of fossil leaves or the sedimentary isotopic fingerprint instead the scenario that best fits all of the geologic evidence is a positive feedback loop in which warming due to a buildup of volcanically derived co2 triggers destabilization of seafloor methane hydrates and the catastrophic release of ch4 palfy et al 2001 we calculate that this carbon cycle perturbation was huge involving the release of 8000 9000 gt c as co2 during the camp basaltic eruptions and 5000 gt c as ch4 in the model the initial isotopic excursion is assumed to take place over 70 kyr while complete reequilibration of the ocean atmosphere system with respect to co2 is accomplished over 700 1000 kyr our results thus provide a preliminary theoretical explanation for the bioevents estimated pco2 changes and isotopic excursions observed in marine and continental sediments at this time

the 1985 86 western bohemia earthquakes modelling source parameters with synthetic seismograms abstract a major swarm of microearthquakes main shock m l 4 6 occurred in december 1985 and january 1986 in the western bohemia region 50 2 n 12 4 e here we revisit the 1985 86 series by modelling some pronounced features of the observed p waveforms frequently seen in the records of the cll brg and the grf seismograph stations travel times and waveforms can be explained by a simple two layer crustal model consisting of an upper crust with v p 5 9 km s and thickness t 18 km and a lower crust with v p 6 8 km s and t 11 km the crust mantle boundary is at 29 km depth waveforms recorded at distances between about 100 and 120 km consist of a low amplitude first p wave which has travelled through the lower crust this phase is followed about 0 5 to 1 s later by the large amplitude main event about 3 s after the main event another phase stands out with similar amplitude it probably represents the depth phase sp m p which leaves the focus upwards as an s wave is converted at the surface to p and then follows the path of p m p the reflection from the crust mantle boundary to the receiver we used this depth phase to constrain the focal depth of the main shock to 9 km depths of the stronger events m l 3 of the swarm are within the depth range from 7 to 10 km inversion of body wave amplitudes and p wave polarities for the focal mechanism of the main event reveals a strike slip mechanism which agrees well with previously published results

the age of the air in the firn and the ice at summit greenland in the framework of the eurocore drilling project 1989 at summit central greenland air samples were collected from a drill hole in the firn at several depth levels the samples have been analyzed for 85kr co2 ch4 cfc s f 11 and f 12 contents and for the isotopic composition of nitrogen and oxygen the measured data are compared with the results of a diffusion model co2 for example takes 12 years with a standard deviation of 7 5 years to diffuse from the surface to the firn ice transition under the present climatic conditions at summit the difference between the age of the ice and the mean age of the occluded air is approximately 210 years an enrichment of the heavier isotopes due to separation by gravity has been observed molecular diffusion alone suffices to explain the observed concentration trends in the firn column we can however not exclude the existence of a convective zone below the surface we have also measured the density and closed porosity along the firn core that has been retrieved when drilling the hole the porosity data reveal that the air is occluded into bubbles mainly between 65 and 80 m corresponding to densities of 790 and 830 kg m3 respectively

temperatures heating rates and vapour pressures in near surface snow at the south pole a finite volume model is used to simulate 9 years 1995 2003 of snow temperatures at the south pole the upper boundary condition is skin surface temperature derived from routine upwelling longwave radiation measurements while the lower boundary condition is set to the seasonal temperature gradient at 6 5 m depth taken from prior measurements at the south pole we focus on statistics of temperature heat fluxes heating rates and vapour pressures in the top metre of snow but present results from the full depth of the model 6 5 m the monthly mean net heat flux into the snow agrees with results from previous studies performed at the south pole on shorter timescales the heating rates and vapour pressures show large variability the net heat flux into the snow which is between 5wm 2 in the monthly mean can be greater than 20 w m 2 on hourly timescales on sub daily timescales heating rates exceed 40 k d 1 in the top 10 cm of the snow subsurface temperatures and therefore heating rates are more variable during winter april september due to increased synoptic activity and the presence of a strong surface based atmospheric temperature inversion the largest vapour pressures 60 70 pa and vertical gradients of vapour pressure are found in the top metre of snow during the short summer december january in contrast during the long winter the low temperatures result in very small vapour pressures and insignificant vapour pressure gradients the high summertime vapour pressure gradients may be important in altering the isotopic composition of snow and ice on the antarctic plateau

model of wind pumping for layered snow layering affects the air flow through snow caused by surface pressure variations the horizontal and total fluxes are high in hoar layers but the pressure perturbations and vertical components of the flow do not penetrate as deeply as in homogeneous snow that is because the layers pipe the flow horizontally toward the area of low pressure an ice layer at the surface reduces the total flow everywhere the flow decreases as ice layer thickness increases and in general flow changes with permeability however the magnitude of the effect is proportionately weaker when the ice layers are further from the surface the residence time is reduced when hoar layers are present due to shorter flow paths reduced penetration into the deeper snow and higher speeds

ground surface temperature and humidity ground temperature cycles and the ice table depths in university valley mcmurdo dry valleys of antarctica in the upper mcmurdo dry valleys 90 of the measured ice table depths range from 0 to 80 cm however numerical models predict that the ice table is not in equilibrium with current climate conditions and should be deeper than measured this study explored the effects of boundary conditions air vs ground surface temperature and humidity ground temperature cycles and their diminishing amplitude with depth and advective flows darcy flow and wind pumping on water vapor fluxes in soils and ice table depths using the rego vapor diffusion model we conducted a series of numerical experiments that illustrated different hypothetical scenarios and estimated the water vapor flux and ice table depth using the conditions in university valley a small high elevation valley in situ measurements showed that while the mean annual ground surface temperature approximates that in the air the mean annual ground surface relative humidity 85 ice was significantly higher than in the atmosphere 50 ice when ground surface temperature and humidity were used as boundary conditions along with damping diurnal and annual temperature cycles within the sandy soil rego predicted that measured ice table depths in the valley were in equilibrium with contemporary conditions based on model results a dry soil column can become saturated with ice within centuries overall the results from the new soil data and modeling have implications regarding the factors and boundary conditions that affect the stability of ground ice in cold and hyper arid regions where liquid water is rare

floral changes across the triassic jurassic boundary linked to flood basalt volcanism one of the five largest mass extinctions of the past 600 million years occurred at the boundary of the triassic and jurassic periods 201 6 million years ago the loss of marine biodiversity at the time has been linked to extreme greenhouse warming triggered by the release of carbon dioxide from flood basalt volcanism in the central atlantic ocean in contrast the biotic turnover in terrestrial ecosystems is not well understood and cannot be readily reconciled with the effects of massive volcanism here we present pollen spore and geochemical analyses across the triassic jurassic boundary from three drill cores from germany and sweden we show that gymnosperm forests in northwest europe were transiently replaced by fern and fern associated vegetation a pioneer assemblage commonly found in disturbed ecosystems the triassic jurassic boundary is also marked by an enrichment of polycyclic aromatic hydrocarbons which in the absence of charcoal peaks we interpret as an indication of incomplete combustion of organic matter by ascending flood basalt lava we conclude that the terrestrial vegetation shift is so severe and wide ranging that it is unlikely to have been triggered by greenhouse warming alone instead we suggest that the release of pollutants such as sulphur dioxide and toxic compounds such as the polycyclic aromatic hydrocarbons may have contributed to the extinction

40k 40ar dating of the main deccan large igneous province further evidence of ktb age and short duration most mass extinctions coincide in time with outpourings of continental flood basalts cfb some 20 years ago it was shown courtillot v besse j vandamme d montigny r jaeger j j cappetta h 1986 deccan flood basalts at the cretaceous tertiary boundary earth planet sci lett 80 361 374 courtillot v feraud g maluski h vandamme d moreau m g besse j 1988 deccan flood basalts and the cretaceous tertiary boundary nature 333 843 846 duncan r a pyle d g 1988 rapid eruption of the deccan flood basalts at the cretaceous tertiary boundary nature 333 841 843 that the age of the deccan traps was close to the cretaceous tertiary kt boundary and its duration under 1 myr we have undertaken a new geochronological study using the unconventional 40k 40ar cassignol gillot technique which is particularly well suited to the potassium poor deccan lavas the mean of 4 determinations from the topmost ambenali and mahabaleshwar formations is 64 5 0 6 ma they straddle the c29r c29n reversal boundary for which they provide a new constraint the mean age of 3 determinations from the oldest jawhar formation is 64 8 0 6 ma the difference in age between top and bottom of a 3500 m composite section probably comprising 80 of the total deccan volume is statistically insignificant with the overall mean age being 64 7 0 6 ma n 7 our results are consistent with the most recent 40ar 39ar determinations knight k b renne p r halkett a white n 2003 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps earth planet sci lett 208 85 99 knight k b renne p r baker j waight t white n 2005 reply to 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps discussion by a k baksi earth planet sci lett 239 374 382 confirming that there should be no systematic difference between the two methods when they are used in an optimal way an earlier smaller but significant pulse of volcanism between 68 and 67 ma extending over at least 500 km in latitude in the northern part of the deccan cfb has also been identified after 2 to 3 ma of quiescence the second major phase of volcanism occurred near 65 ma expanding over most of the area covered by the first pulse and another 500 km to the south consistent with drift of india by 300 to 450 km at not vert similar 150 mm yr during the quiescence period new paleontological data from the remote rajahmundry section keller g adatte t gardin s bartolini a bajpai s humler e in prep the cretaceous tertiary boundary in deccan traps of the krishna godavari basin of southeastern india epsl to be submitted suggest that this second pulse can itself be divided into two major pulses one starting in c29r and ending at the kt boundary the second starting in the upper part of c29r and ending within c29n

delta15n measurements as a calibration tool for the paleothermometer and gas ice age differences a case study for the 8200 b p event on grip ice isotopic composition measurement on atmospheric nitrogen 15n trapped in ice cores is established as a gravitational enrichment indicator recently it was discovered that thermal diffusion has a significant influence on this parameter during periods of large temperature fluctuations severinghaus et al 1998 here we present results for the 8200 b p event the largest temperature excursion during the holocene which underpin this view and extend it to medium range temperature changes the thermal diffusion component is about one third of the gravitational enrichment for this event the shortness of this cooling event does not significantly affect the gravitational enrichment as documented by rather constant close off depths therefore the 15n residuals can be directly used to estimate temperature changes hence calibrating the paleothermometer since our 15n record can resolve temperature variations as short as a few decades it complements the calibration technique of borehole temperature measurements for which the resolution decreases rapidly with increasing age we obtained a relation 18oice temperature of 0 30 0 11 k which agrees nicely with the temporal dependence for cold periods about 0 30 k and is significantly lower than today s greenland spatial relation 0 67 k this corresponds to a range of the surface temperature shift for the 8200 b p event in central greenland between 5 4 and 11 7 k with a best estimate of 7 4 k by matching the 15n variations with 18oice variations we estimate the mean gas ice age difference to be 209 15 years which is in good agreement with model estimates

4 1 5 c abrupt warming 11 270 yr ago identified from trapped air in greenland ice nitrogen and argon isotopes in air trapped in a greenland ice core gisp2 show two prominent peaks in the interval 11 800 10 800 b p which indicate two large abrupt warming events the first abrupt warming 10 4 c is the widely documented event at the end of the younger dryas here we report on the second abrupt warming 4 1 5 c which occurred at the end of a short lived cooler interval known as the preboreal oscillation 11 270 30 b p a rapid snow accumulation increase suggests that the climatic transition may have occurred within a few years the character of the preboreal oscillation and the subsequent abrupt warming is similar to the dansgaard oeschger d o events in the last glacial period suggestive of a common mechanism but different from another large climate change at 8 200 b p in which cooling was abrupt but subsequent warming was gradual the large abrupt warming at 11 270 b p may be considered to be the final d o event prior to the arrival of the present stable and warm epoch

thermal tracing of retained meltwater in the lower accumulation area of the southwestern greenland ice sheet we present in situ firn temperatures from the extreme 2012 melt season in the southwestern lower accumulation area of the greenland ice sheet the upper 2 5 m of snow and firn was temperate during the melt season when vertical meltwater percolation was inefficient due to a 5 5 m thick ice layer underlying the temperate firn meltwater percolation and refreezing beneath 2 5 m depth only occurred after the melt season deviations from temperatures predicted by pure conductivity suggest that meltwater refroze in discrete bands at depths of 2 0 2 5 5 0 6 0 and 8 0 9 0 m while we find no indication of meltwater percolation below 9 m depth or complete filling of pore volume above firn at 10 and 15 m depth was respectively 4 2 4 5 c and 1 7 c higher than in a conductivity only simulation even though meltwater percolation in 2012 was inefficient firn between 2 and 15 m depth the following winter was on average 4 7 c warmer due to meltwater refreezing our observations also suggest that the 2012 firn conditions were preconditioned by two warm summers and ice layer formation in 2010 and 2011 overall firn temperatures during the years 2009 13 increased by 0 6 c

thermal conductivity and heat transfer through the snow on the ice of the beaufort sea 1 eighty nine point measurements of the thermal conductivity ks of the snow on the sea ice of the beaufort sea were made using a heated needle probe average values ranged from 0 078 w m 1 k 1 for new snow to 0 290 w m 1 k 1 for an ubiquitous wind slab ks increased with increasing density consistent with published equations but could also be reliably estimated from the metamorphic state of the snow using measured values of ks and snow stratigraphy the average bulk value for the full snowpack was 0 14 w m 1 k 1 in contrast ks inferred from ice growth and temperature gradients in the snow was 0 33 w m 1 k 1 the mismatch arises in part because the second estimate is based on measurements from an aggregate scale that includes enhanced heat flow due to two and three dimensional snow and ice geometry a finite element model suggests that the complex geometry produces areas of concentrated heat loss at the snow surface these hot spots however increase the apparent conductivity only by a factor of 1 4 not enough to fully explain the mismatch nonconductive heat transfer mechanisms like natural and forced air convection may also be operating in the snowpack though the ubiquitous presence of low permeability wind slabs potentially limits their effectiveness the relative contributions of effects due to snow and ice geometric and nonconductive processes within the snowpack remain uncertain

snow crystal growth with varying surface temperatures and radiation penetration the temperature field is derived for a sinusoidally varying surface temperature with varying solar radiation penetration the growth rates of snow crystals are calculated to explain the rapidly growing layers of faceted crystals i e depth hoar that form just below the surface at high altitudes and in polar snow the solutions also show that a layer of wet snow can exist just below the surface even on days when the surface temperature remains sub freezing

snow bedforms a review new data and a formation model snow bedforms like sand bedforms consist of various shapes that form under the action of wind on mobile particles throughout a year they can cover up to 11 of the earth surface concentrated toward the poles these forms impact the local surface energy balance and the distribution of precipitation only a few studies have concentrated on their genesis their size ranges from 2 cm ripple marks to 2 5 m tall whaleback dunes we counted a total of seven forms that are widely recognized among them sastrugi an erosional shape is the most widespread from laser scans we compared scaling of snow versus sand barchan morphology we found that both have proportionally the same footprint but snow barchans are flatter the key difference is that snow can sinter immobilizing the bedform and creating an erodible material using a model we investigated the effect of sintering on snow dune dynamics we found that sintering limits their size because it progressively hardens the snow and requires an ever increasing wind speed to maintain snow transport from the literature and results from this model we have reclassified snow bedforms based on two parameters wind speed and snow surface conditions the new data show that snow dune behavior mirrors that of sand dunes with merging calving and collision however isolated snow barchans are rare with most of the snow surfaces encountered in the field consisting of several superimposed bedforms formed sequentially during multiple weather events spatially variable snow properties and geometry can explain qualitatively these widespread compound snow surfaces

firn air δ15n in modern polar sites and glacial interglacial ice a model data mismatch during glacial periods in antarctica the phase lag between atmospheric composition air bubbles and temperature water isotopes can be quantified from ice cores provided that the age difference between entrapped air and the surrounding air can be correctly estimated this difference depends on the lock in depth lid when air no longer mixes with the atmosphere the lid can be estimated from firnification models or from the air isotopic composition 15n and 40ar both methods give consistent results for greenland and one coastal site in antarctica byrd new firn measurements in greenland northgrip and antarctica berkner island bas depot dome c confirm that firnification models correctly reproduce the present lid over a large range of surface conditions however a systematic mismatch is observed for the last glacial maximum lgm in east antarctic sites vostok dome c dome f questioning the model s validity here we use new 15n measurements from two coastal antarctic sites kohnen station and law dome providing depth estimates again distinct from firnification model calculations we show that this discrepancy can be resolved by revising the estimate of past accumulation rates 15n measurements can therefore help to constrain past accumulation rate and improve ice core dating

modeling time series of microwave brightness temperature in antarctica this paper aims to interpret the temporal variations of microwave brightness temperature at 19 and 37 ghz and at vertical and horizontal polarizations in antarctica using a physically based snow dynamic and emission model sdem sdem predicts time series of top of atmosphere brightness temperature from widely available surface meteorological data era 40 re analysis to do so it successively computes the heat flux incoming the snowpack the snow temperature profile the microwaves emitted by the snow and finally the propagation of the microwaves through the atmosphere up to the satellite since the model contains several parameters whose value is variable and uncertain across the continent the parameter values are optimized for every 50 km 50 km pixel simulation results show that the model is inadequate in the melt zone where surface melting occurs on at least a few days a year because the snowpack structure and its temporal variations are too complex in contrast the accuracy is reasonably good in the dry zone and varies between 2 and 4 k depending on the frequency and polarization of observations and on the location at the antarctic scale the error is larger where wind is usually stronger suggesting either that meteorological data are less accurate in windy regions or that some neglected processes e g windpumping surface scouring are important at dome c in calm conditions a detailed analysis shows that most of the error is due to inaccuracy of the era 40 air temperature 2 k finally the paper discusses the values of the optimized parameters and their spatial variations across the antarctic

effect of atmospheric water vapor on modification of stable isotopes in near surface snow on ice sheets south pole are used to simulate the d 18 o of near surface snow as it evolves in time the sensitivity of d 18 o of near surface snow to wind speed surface temperature and accumulation rate is tested through model experiments a steady wind of 5 10 m s 1 results in annual mean d 18 o enrichment of 3 7 enrichment of the heavy isotope occurs predominantly in buried winter snow layers during summer however in many simulations there is a slight depletion of d 18 o summer layers that occurs as atmospheric water vapor is deposited within the snow during winter postdepositional modification of water stable isotopes in near surface snow also depends on the snow accumulation rate high accumulation rates quickly advect snow layers away from the influence of the atmosphere preventing significant modification low accumulation rates e g at vostok and dome c allow significant postdepositional modification because the near surface snow is exposed to forced ventilation for several annual cycles postdepositional modification during the last glacial maximum lgm at summit greenland is estimated to be greater than the present day postdepositional modification therefore the temperature difference for lgm may be larger than that previously inferred from the d 18 o record however because of the compensating effects of lower temperatures and smaller accumulation during the lgm in antarctica the postdepositional enrichment in east antarctica may be approximately the same for lgm as for the modern climate

triassic jurassic organic carbon isotope stratigraphy of key sections in the western tethys realm austria the late triassic period is recognized as one of the five major mass extinctions in the fossil record all these important intervals in earth history are associated with excursions in c isotope records thought to have been caused by perturbations in the global carbon cycle the nature and causes of c isotopic events across the triassic jurassic t j transition however are poorly understood we present several new high resolution organic c isotope records from the eiberg basin austria including the proposed global boundary stratotype section and point gssp for the base of the jurassic the triassic jurassic boundary interval in these records is characterized by the initial and main negative organic carbon isotope excursions cie of up to 8 the initial and main cies are biostratigraphically constrained by first and last occurrences of boundary defining macro and microfossils e g ammonites high resolution c isotope records appear to be an excellent correlation proxy for this period in the eiberg basin pyrolysis analysis demonstrates increased hydrogen index hi values for organic matter coinciding with the initial cie terrestrial organic matter influx and mass occurrences of green algae remains may have influenced the c isotope composition of the sedimentary organic matter this may have contributed to the extreme amplitude of the initial cie in the eiberg basin

non climate influences on stable isotopes at taylor mouth antarctica the late holocene trends in 18 o differ significantly in two ice cores 30 km apart from the area of taylor dome antarctica it is unlikely that the trend in the core from taylor mouth the flank site is due to a standard 18 o surface temperature relationship assuming that the taylor dome near summit core records local climate variations common to both cores we assess two leading possible causes for the observed differences 1 relative to taylor dome taylor mouth may collect snow from more sources with distinct isotopic compositions 2 vapor motion during prolonged near surface exposure may cause post depositional isotope enrichment at taylor mouth where the accumulation rate is low our model of firn pore space vapor and sublimating ice grains suggests that post depositional processes can modify 18 o values by several o isotopic samples from areas with significantly different accumulation rates near taylor mouth could differentiate between possibilities 1 and 2

triassic jurassic weathering and clay mineral dispersal in basement areas and sedimentary basins of southern sweden abstract the basement rocks of southern sweden were deeply weathered in late triassic through cretaceous times and possibly even earlier sub mesozoic saprolites formed presumably due to long term warmth and humidity in conjunction with decomposition of abundant organic matter and subsequent infiltration of acidic water the upper parts of the weathering profiles were subjected to intense leaching and kaolinitisation whereas the less evolved deep weathering front is enriched in smectite thus any erosion prior to full regolith development or extraordinary deep erosion released less developed smectite rich weathering material to the late triassic jurassic receiving basins mesozoic sedimentary successions in southern sweden show high detrital smectite and or kaolinite contents moderate illite contents and low chlorite contents compatible with expected mid latitude warm equable greenhouse conditions norian and older triassic haematite cemented continental arkoses and smectite dominated clays smectite kaolinite were clearly formed in arid settings from the rhaetian and throughout the jurassic humid onshore conditions sustained abundant plant and peat accumulation early diagenetic meteoric flushing and intense chemical weathering kaolinite is therefore a dominating detrital clay mineral in these sedimentary successions kaolinite illite chlorite and smectite i s minor quantitative variations in clay mineralogy within the humid climate rhaetian jurassic sedimentary successions are difficult to interpret in deltaic coal bearing successions the detrital clay mineral composition is conspicuously constant kaolinite illite chlorite possibly due to post depositional clay mineral transformation conversely in shallow marine deposits some quantitative variation is notable kaolinite illite variable smectite chlorite this may reflect that the original detrital composition was better preserved in shallow marine settings smectitic clay minerals associated with gypsum and caliche nodules ambiguously indicate increased aridity towards the end of the jurassic

age scale of the air in the summit ice implication for glacial interglacial temperature change the air occluded in ice sheets and glaciers has in general a younger age defined as the time after its isolation from the atmosphere than the surrounding ice matrix because snow is first transformed into open porous firn in which the air can exchange with the atmosphere only at a certain depth firn ice transition the pores are pinched off and the air is definitely isolated from the atmosphere the firn ice transition depth is at around 70 m under present climatic conditions at summit central greenland the air at this depth is roughly 10 years old due to diffusive mixing whereas the ice is about 220 years old this results in an age difference between the air and the ice of 210 years this difference depends on temperature and accumulation rate and did thus not remain constant during the past we used a dynamic firn densification model to calculate the firn ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition past temperatures and accumulation rates have been deduced from the 18o record using time independent functions we present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the greenland ice core project grip and the greenland ice sheet project two gisp2 ice cores for the last 100 000 years during the holocene the age difference stayed rather stable around 200 years while it reached values up to 1400 years during the last glaciation for the colder scenario the model results are compared with age differences obtained independently by matching corresponding climate events in the methane and 18o records assuming a very small phase lag between variations in the greenland surface temperature and the atmospheric methane the past firn ice transition depths are compared with diffusive column heights obtained from 15n of n2 measurements the results of this study corroborate the large temperature change of 20 to 25 k from the coldest glacial to holocene climate found by evaluating borehole temperature profiles

the layered character of snow covers snow studies have generally ignored the layered nature of snow covers having achieved a good understanding of the properties and processes in homogeneous snow snow scientists should develop more insight into the evolution of the layers and their effects on overall snow response many of the outstanding problems in snow studies can only be solved by dealing with snow as a layered medium the various mechanisms by which layers are generated their effects and some of the outstanding research problems are described here

volatile fluxes during flood basalt eruptions and potential effects on the global environment a deccan perspective abstract we examine the role that flood basalt eruptions may have played during times of mass extinction through the release of volcanic gases continental flood basalt provinces have formed by numerous eruptions over a short period of geologic time characteristically a few million years within this period a short lived climactic phase that lasts about 1 ma typically emplaces a large proportion of the lava volume this phase consists of a series of huge eruptions each yielding 10 3 10 4 km 3 of magma each eruption lasted on the order of a decade or more and built an immense p hoehoe dominated lava flow field by eruptive activity along fissures tens to hundreds of km long high fire fountains emanating from vents along the fissures at times sustained eruption columns that lofted gas and ash into the upper troposphere and lower stratosphere while the lava flows covered huge areas the combination of large eruption magnitudes maintained high effusion rates during eruptions and the repeated nature of the characteristic large scale eruptive activity occurs in earth history only during periods of flood basalt volcanism based on recent analogs and determination of volatile contents of ancient flood basalt lavas we estimate that individual eruptions were capable of releasing 10 000 tg of so 2 resulting in atmospheric loadings of 1000 tg a 1 during a sustained decade long eruptive event we apply this model of flood basalt volcanism to estimate the potential mass of co 2 and so 2 released during formation of the 65 ma deccan province the deccan lava pile contains the record of hundreds of enormous p hoehoe flow fields erupted within a period of about 1 ma consequently atmospheric perturbations associated with so 2 emissions from just one of these long lasting eruptions were likely to have been severe and constantly augmented over a decade or longer by contrast the amounts of co 2 released would have been small compared with the mass already present in the atmosphere and thus much more limited in effect individual eruptions were followed by hiatuses of hundreds to thousands of years during which the gas contributions to the atmosphere would be recycled it is clear that the nature and potential atmospheric impact of a series of huge volume repeated long term degassing events requires further investigation in conjunction with appropriate climate models

effect of accumulation rate on water stable isotopes of near surface snow in inland antarctica 1 postdepositional changes in water stable isotopes in polar firn were investigated at three sites characterized by different accumulation rates along the east antarctic ice divide near dome fuji water stable isotopes major ion concentrations and tritium contents of three 2 4 m deep pits were measured at high resolution 2 cm temporally the snow pits cover the past 50 years with snow accumulation rates in the range of 29 41 kg m 2 a 1 around dome fuji oxygen isotopic profiles in the three pits do not show annual fluctuations but instead exhibit multiyear cycles these multiyear cycles are lower in frequency at dome fuji as compared with the other two sites peaks of water stable isotopes in the multiyear cycles correspond to some ion concentration minima in the pits although such relationships are not observed in coastal regions we propose that the extremely low accumulation environment keeps the snow layer at the near surface which result in postdepositional modifications of isotopic signals by processes such as ventilation and vapor condensation sublimation we estimate that oxygen isotopic ratios could be modified by 10 and that the original seasonal cycle could be completely overprinted under the accumulation conditions at dome fuji moreover stake measurements at dome fuji suggest that the large variability in snow accumulation rate is the cause of the multiyear cycles

a method for precise measurement of argon 40 36 and krypton argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in greenland and at siple dome antarctica we describe a method for measuring the 40ar 36ar ratio and the 84kr 36ar ratio in air from bubbles trapped in ice cores these ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured 15n 14n both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on earth s climate the overall precision of the 40ar 36ar method 1 standard error of the mean is 0 012 for a sample analyzed in duplicate corresponding to 0 6 m in reconstructed firn thickness we use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation about 100 g of ice is used for a duplicate pair of analyses an example of the technique applied to the gisp2 ice core yields an estimate of 11 3k of abrupt warming at the end of the last glacial period 15 000 years ago the krypton argon ratio can provide a diagnostic of argon leakage out of the bubbles which may happen naturally during bubble close off or artifactually if samples are warmed near the freezing point during core retrieval or storage argon leakage may fractionate the remaining 40ar 36ar ratio by 0 007 per change in 84kr 36ar introducing a possible bias in reconstructed firn thickness of about 2 m if thermal diffusion is not accounted for or 6 m if thermal diffusion effects are quantified with measured 15n 14n reproducibility of 84kr 36ar measured in air is about 0 2 1 standard error of the mean but is about 1 for ice core samples ice core samples are systematically enriched in 84kr 36ar relative to atmosphere by 5 probably reflecting preferential size dependent exclusion of the smaller argon atom during bubble entrapment recent results from the siple dome ice core reveal two climate events during the last deglaciation including an 18 m reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr bp and a partial or total removal of the firn during an ablation event at 15 3 kyr bp

stratigraphy and palaeoenvironmental analysis of the triassic jurassic transition in the western southern alps northern italy abstract high resolution stratigraphic studies were carried out on the rhaetian hettangian succession of the bergamasc alps western southern alps italy in order to characterise the stratigraphic and palaeoenvironmental evolution of the lombardian succession across the triassic jurassic t j boundary the t j boundary in the bergamasc alps is sandwiched between the rhaetian carbonate ramp succession of the zu limestone formation and the malanotte formation a new lithostratigraphic unit introduced in this paper deposited during relative sea level rise in the earliest hettangian the zu limestone is organised into subtidal mixed shale carbonate asymmetric cycles documenting the regional evolution from a distal to a proximal depositional environment of an articulated homoclinal carbonate ramp the malanotte formation lacks cyclicity and shows thin bedding and a dominant micritic facies documenting the subtidal evolution of a relatively homogenous outer carbonate ramp environment the microfloral associations record a gradual evolutionary change at the t j boundary whereas the biocalcifying communities appear to be severely affected by the end triassic biotic crisis

water isotope diffusion rates from the northgrip ice core for the last 16 000 years glaciological and paleoclimatic implications abstract a high resolution 0 05 m water isotopic record o 18 is available from the northgrip ice core in this study we look into the water isotope diffusion history as estimated by the spectral characteristics of the o 18 time series covering the last 16 000 years the diffusion of water vapor in the porous medium of the firn pack attenuates the initial isotopic signal predominantly having an impact on the high frequency components of the power spectrum higher temperatures induce higher rates of smoothing and thus the signal can be used as a firn paleothermometer we use a water isotope diffusion model coupled to a steady state densification model in order to infer the temperature signal from the site assuming the accumulation and strain rate history as estimated using the gicc05 layer counted chronology and a dansgaard johnsen ice flow model the temperature reconstruction accurately captures the timing and magnitude of the bolling allerod and younger dryas transitions a holocene climatic optimum is seen between 7 and 9 ky b2k with an apparent cooling trend thereafter our temperature estimate for the holocene climatic optimum points to a necessary adjustment of the ice thinning function indicating that the ice flow model overestimates past accumulation rates by about 10 at 8 ky b2k this result is supported by recent gas isotopic fractionation studies proposing a similar reduction for glacial conditions finally the record presents a climatic variability over the holocene spanning millennial and centennial scales with a profound cooling occurring at approximately 4000 years b2k the new reconstruction technique is able to provide past temperature estimates by overcoming the issues apparent in the use of the classical o 18 slope method it can at the same time resolve temperature signals at low and high frequencies

thermal fractionation of air in polar firn by seasonal temperature gradients 1 air withdrawn from the top 5 15 m of the polar snowpack firn shows anomalous enrichment of heavy gases during summer including inert gases following earlier work we ascribe this to thermal diffusion the tendency of a gas mixture to separate in a temperature gradient with heavier molecules migrating toward colder regions summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion here we explore and quantify this process further in order to 1 correct for bias caused by thermal diffusion in firn air and ice core air isotope records 2 help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion severinghaus et al 1998 and 3 address whether air in polar snow convects during winter and if so whether it creates a rectification of seasonality that could bias the ice core record we sampled air at 2 m depth intervals from the top 15 m of the firn at two antarctic sites siple dome and south pole including a winter sampling at the pole we analyzed 15n 14n 40ar 36ar 40ar 38ar 18o 16o of o2 o2 n2 84kr 36ar and 132xe 36ar the results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30 although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision relative values of different gas pairs may at siple dome 40ar 4 is 66 2 as sensitive to thermal diffusion as 15n in agreement with laboratory calibration 18o 2 is 83 3 and 84kr 48 is 33 3 as sensitive as 15n the corresponding figures for summer south pole are 64 2 81 3 and 34 3 accounting for atmospheric change the figure for o2 n2 4 is 90 3 at siple dome winter south pole shows a strong depletion of heavy gases as expected however the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities except for kr constant at 34 4 suggesting the action of some other process that is not currently understood no evidence for wintertime convection or a rectifier effect is seen

co2 and o2 n2 variations in and just below the bubble clathrate transformation zone of antarctic ice cores co2 measurements on the epica european project for ice coring in antarctica dml ice core in depth levels just below the bubble ice clathrate ice transformation zone 1230 2240 m depth were performed in the youngest part 1230 1600 m they reveal variations of up to 25 ppmv around the mean atmospheric concentration within centimetres corresponding to a snow deposition interval of a few years similar results are found at corresponding depth regions of the dome c and the talos dome ice cores since we can exclude all hitherto known processes altering the concentration of co2 in ice cores we present a hypothesis about spatial fractionation of air components related to episodically increasing clathrate formation followed by diffusion processes from bubbles to clathrates this hypothesis is supported by optical line scan observations and by o2 n2 measurements at the same depth where strong co2 variations are detected below the clathrate formation zone this small scale fractionation process is slowly smoothed out most likely by diffusion regaining the initial mean atmospheric concentration although this process compromises the representativeness of a single co2 measurement on small ice samples in the clathrate formation zone of an ice core it does not affect the mean atmospheric co2 concentration if co2 values are averaged over a sufficiently long depth scale 10 cm in case of the epica dml ice core

thermal effects due to air flow and vapor transport in dry snow the thermal effects of air flow forced through a snow sample are investigated numerically a new method for calculating vapor transport in snow is presented which allows for the determination of the effects of sublimation in this method the snow is not assumed to be saturated with water vapor results of the model show very good agreement with analytical and experimental results

natural convection in the subarctic snow cover the purpose of this study was to determine if air convects in a natural snow cover to detect convection the temperature field in the subarctic snow cover in fairbanks alaska was measured hourly during three winters 1984 1987 using an array of thermistors which were suspended on threads and allowed to be buried by snowfall the results indicate that convection occurred sporadically in 1984 1985 and almost continuously in 1985 1986 and 1986 1987 the evidence was 1 simultaneous warming and cooling at different locations in a horizontal plane in the snow and 2 horizontal temperature gradients of up to 16 c m 1 during the winter warm and cold zones developed in the snow and remained relatively fixed in space we interpret these zones to be the result of a diffuse plumelike convection pattern linked to spatial variations in the temperature of the snow soil interface air flow was inferred to have been primarily horizontal near the base of the snow and vertical elsewhere calculated flow speeds were of the order of 0 2 mm s 1 with a maximum value of 2 mm s 1 the convective circulation was time dependent with perturbations such as high wind or rapid changes in air temperature triggering periods when horizontal temperature gradients were strongest suggesting that these were also periods when the air flow was fastest the coincidence of depth hoar crystals with horizontal c axes and the horizontal flow lines at the base of the snow suggests that convection may have affected crystal growth directions

feasibility of using sand dunes as archives of old air large unaltered samples of the atmosphere covering the past century would complement the history of atmospheric gases obtained from bubbles in ice cores enabling measurement of geochemically important species such as o2 14ch4 and 14co sand dunes are a porous media with interstitial air in diffusive contact with the atmosphere somewhat analogous to the unconsolidated layer of firn atop glaciers recent studies have demonstrated the value of firn as an archive of old air battle et al 1996 bender et al 1994a unlike firn sand dunes are incompressible and so remain permeable to greater depths and may extend the firn record into the past century to evaluate the feasibility of using sand dunes as archives of old air we drilled 60 m deep test holes in the algodones dunes imperial valley california the main objective was to see if the air in a sand dune is as old as predicted by a diffusion model or if the dune is rapidly flushed by advective pumping during windstorms and barometric pressure changes we dated the air with chlorofluorocarbons and krypton 85 anthropogenic tracers whose atmospheric concentrations are known and have been increasing rapidly in the past half century these tracer data match the pure diffusion model well showing that advection in this dune is negligible compared to diffusion as a transport mechanism and that the mean age of the air at 61 m depth is 10 years dunes therefore do contain old air however dunes appear to suffer from two serious drawbacks as archives microbial metabolism is evident in elevated co2 and n2o and depressed ch4 and o2 concentrations in this dune corrupting the signals of interest in this and probably most dunes second isotopic analyses of n2 and o2 from the dune show that fractionation of the gases occurs due to diffusion of water vapor complicating the interpretation of the o2 signal beyond the point of viability for an air archive sand dunes may be useful for relatively inert gases with large atmospheric concentration changes such as chlorofluorocarbons

firn processes and δ15n potential for a gas phase climate proxy abstract in order to quantify the sequence of events between changes in atmospheric composition and climate changes recorded in ice cores we must accurately account for the age difference between ice and gas at a given depth this gas age ice age difference depends on the age of the ice at the bottom of the firn layer where the bubbles are closed off firn densification models are used to calculate how this age difference varied in the past but have an uncertainty on the order of 1000 years for central antarctic sites here we explore the possibility that 15 n of n 2 is a gas phase proxy of climate which can be used to synchronize gas and ice records we present the 15 n record from the epica dome c edc ice core covering the last three glacial terminations and five glacial interglacial cycles between 300 and 800 ka previous studies have shown that gravitational settling enriches 15 n as a function of the diffusive column height in the firn if densification models prediction of deeper firn close off under glacial conditions is correct then we would expect heavier 15 n during glacial periods and a negative correlation with temperature instead edc 15 n is positively correlated with the ice deuterium content a proxy for temperature as previously reported at vostok dome fuji and epica dronning maud land we propose a mechanism that links accumulation rate firn permeability and convective mixing in the top meters of the firn to explain this correlation between 15 n and ice deuterium content the tightest correlation is observed over glacial terminations supporting the idea that 15 n is a property in the gas phase that records changes in surface conditions linked to deglacial warming

hyporheic exchange with heterogeneous streambeds laboratory experiments and modeling 1 hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone in reality streambed sediments have a heterogeneous structure and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns to assess the basic effects of sediment structure on hyporheic exchange we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negative exponential correlated random hydraulic conductivity fields dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high and low permeability regions of the streambed as no existing model could represent these effects we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form induced boundary head distribution and the heterogeneous sediment structure a numerical particle tracking approach was then used to assess the resulting hyporheic exchange the combined flow transport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t 30 hours the heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed the layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth as a result solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed the combination of faster near surface transport and shallower solute penetration produced a shorter mean hyporheic residence time on the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate

physical and climatic parameters which influence the air content in polar ice abstract under present day climatic conditions the air content of polar ice v generally shows a high sensitivity to the atmospheric pressure and hence to the surface elevation of the ice sheet wherethe ice is formed the results presented here are from sixteen different sites fourteen in antartica one in greenland and one in the yukon territories canada and they allow a better understanding of the parameters which influence the air content in ice it is demonstrated the v can be expressed very simply as a function of the atmospheric pressure p i the temperature t i and the porous volume of ice v i at which the air in the firn becomes isolated in terms of pressure from the atmosphere during the process of pore close off our results confirm a v i increase with temperature and show no clear v i dependence on snow accumulation rate the possible non linearity of the v i t i relation we obtain could be due to a wind influence on v i and or to a second order effect of the accumulation rate our v i results are compared to measurements of the volume of closed pores versus depth in the firn to close we discuss the parameterization that we can obtain to interpret the air content variations observed in deep ice cores over periods covering glacial interglacial transitions in terms of paleoclimatic conditions prevailing at the surface of the ice sheet

modelling temperature variations in polar snow using daisy a physics based snow model has been calibrated using data collected at halley bay antarctica during the international geophysical year variations in snow temperature and density are well simulated using values for the model parameters within the range reported from other polar field experiments the effect of uncertainty in the parameter values on the accuracy of the predictions is no greater than the effect of instrumental error in the input data thus this model can be used with parameters determined a priori rather than by optimization the model has been validated using an independent data set from halley bay and then used to estimate 10 m temperatures on the antarctic peninsula plateau over the last half century

the late rhaetian transgression in southern sweden regional and global recognition and relation to the triassic jurassic boundary palynological investigations of rhaetian outcrops and drillcores in combination with subsurface data from wells in scania southern sweden have revealed the presence of marine dinoflagellate cysts in sedimentary strata that were previously considered to have been deposited in a mainly terrestrial environment two distinct dinocyst events are identified one older event where persistent rare to common occurrences of rhaetogonyaulax rhaetica and lunnomidinium scaniense indicates deposition in shallow marine and marginal marine environments this lunnomidinium interval is preceded and succeeded by assemblages with rare to common r rhaetica and can be correlated with the mid to late rhaetian r rhaetica zone it is associated with spore pollen assemblages of the mid rhaetian rhaetipollis limbosporites zone the younger event the r rhaetica maximum interval is characterised by mass occurrence of r rhaetica with less common or absent dapcodinium priscum in association with a dark grey to black mudstone shale the introduction of d priscum in association with the over whelming abundance of r rhaetica allows correlation with the transition between the r rhaetica zone and the succeeding d priscum zone and signals fully marine conditions the r rhaetica maximum interval is associated with spore pollen assemblages of the late rhaetian ricciisporites polypodiisporites zone and is in sweden succeeded both lithostratigraphically and palynostratigraphically by the t j transition the r rhaetica maximum interval corresponds to a late rhaetian maximum flooding event that took place in a shallow gently sloping embayment that covered the danish basin area during the late triassic early jurassic this maximum flooding event can be recognised in late rhaetian strata from different parts of the northern hemisphere it is an important temporal constraint on the t j boundary as it is associated with the disappearance of triassic marine faunas appears to coincide with the onset of the main pulse of camp volcanism immediately precedes the initial carbon isotope excursion and the global sea level drop that characterises the t j transition

extreme snow metamorphism in the allan hills antarctica as an analogue for glacial conditions with implications for stable isotope composition understanding physical processes in near zero accumulation areas can help us to better understand polar ice core records particularly during periods when accumulation rates were lower than today we report measurements from a 5 m firn core from the allan hills antarctica which include physical properties using computer tomography stable isotope ratios d and 18 o and 210 pb activity the core shows a highly metamorphosed firn with homogeneous and stable structure but with discrete layers near the surface the observed firn structure is caused by a combination of unique depositional and post depositional processes the irregular d and 18 o signal does not follow the stratigraphic sequence and implies post depositional modification caused by microscopic pressure gradients in the firn that can result from either forced ventilation over rough surfaces in the presence of wind or alternating temperature gradients between the firn and the atmosphere our results also indicate impact snow deposition under high winds and with a high initial density and air exchange between the atmosphere and the snowpack 210 pb activity below 0 3 m falls below the detection limit implying that most of the core is more than 100 years old we conclude that the allan hills record provides a unique opportunity to investigate important processes that would have affected ice core records from glacial periods

fractionation of gases in polar ice during bubble close off new constraints from firn air ne kr and xe observations gas ratios in air withdrawn from polar firn snowpack show systematic enrichments of ne n2 o 2 n2 and ar n2 in the firn ice transition region where bubbles are closing off air from the bubbles in polar ice is correspondingly depleted in these ratios after accounting for gravitational effects gas in the bubbles becomes fractionated during the process of bubble close off and fractionation may continue as ice cores are stored prior to analysis we present results from firn air studies at south pole and siple dome antarctica which add ne kr and xe measurements to the suite of observations ne o2 and ar appear to be preferentially excluded from the shrinking and occluding bubbles and these gases therefore accumulate in the residual firn air creating a progressive enrichment with time and depth in firn air early sealing of gases by thin horizontal impermeable layers into a nondiffusive zone or lock in zone greatly enhances this enrichment a simple model of the bubble close off fractionation and lock in zone enrichment fits the data adequately the model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles the effect appears to be size dependent because ne o2 and ar have smaller effective molecular diameters than n2 and fractionation increases strongly with decreasing size ne is fractionated 34 2 times more than o2 in south pole firn air and reaches an enrichment of 90 in the deepest sample the large atoms kr and xe do not appear to be fractionated by this process despite the large size difference between the two gases suggesting a threshold atomic diameter of 3 6a above which the probability becomes very small that the gas will escape from the bubble these findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature chronology and past atmospheric composition

high resolution ammonite and carbon isotope stratigraphy across the triassic jurassic boundary at new york canyon nevada the triassic jurassic boundary is generally considered as one of the major extinctions in the history of phanerozoic the high resolution ammonite correlations and carbon isotope marine record in the new york canyon area allow to distinguish two negative carbon excursions across this boundary with different paleoenvironmental meanings the late rhaetian negative excursion is related to the extinction and regressive phase the early hettangian 13corg negative excursion is associated with a major floristic turnover and major ammonite and radiolarian radiation the end triassic extinction early jurassic recovery is fully compatible with a volcanism triggered crisis probably related to the central atlantic magmatic province the main environmental stress might have been generated by repeated release of so2 gas heavy metals emissions darkening and subsequent cooling this phase was followed by a major long term co2 accumulation during the early hettangian with development of nutrient rich marine waters favouring the recovery of productivity and deposition of black shales

attenuation of wind induced pressure perturbations in alpine snow windpumping has been identified as a process that could potentially enhance sublimation of surface snow at high forcing frequency and spawn air movement deeper in firn at lower frequencies we performed an experiment to examine the relationship between high frequency wind and pressure measurements within the top meter of an alpine snowpack and compared experimental results with two theoretical predictions we find that both theoretical predictions underestimate high frequency perturbation pressure attenuation with depth in the near surface snowpack and the discrepancy between theory and measurement increases with perturbation pressure frequency the impact of this result for near surface snow is that potential enhanced sublimation will occur over a shallower snow depth than these two theories predict correspondingly interstitial air mixing at depth in firn will be driven by lower frequencies than these two theories predict while direct measurement of these energy rich lower frequencies is beyond the scope of this paper stationary pressure measurements validate the presence of a pressure field that could drive near surface circulation

carbon cycle perturbation and stabilization in the wake of the triassic jurassic boundary mass extinction event 1 the triassic jurassic boundary mass extinction event t j 199 6 ma is associated with major perturbations in the carbon cycle recorded in stable carbon isotopes two rapid negative isotope excursions in bulk organic carbon 13corg occur within the immediate boundary interval at multiple locations and have been linked to the outgassing of 12c enriched co2 from the central atlantic magmatic province in british columbia a positive 13corg excursion of 5 vienna peedee belemnite v pdb spans part or all of the subsequent hettangian stage here we examine the significance of these carbon isotope excursions as records of global carbon cycle dynamics across the t j boundary and test the link between carbon cycle perturbation stabilization and biotic extinction recovery patterns a combination of 13corg and palynological analyses from the late triassic to early jurassic in the mingolsheim core germany suggests that organic carbon isotope variations are best explained as the result of both compositional changes in terrestrial versus marine input and disturbance and recovery patterns of major terrestrial plant groups across the t j boundary a new high resolution 13ccarb record from the val adrara section in the southern alps italy spanning from the uppermost rhaetian through lower sinemurian does not exhibit a negative excursion at the t j boundary but does record a large positive 13ccarb excursion of 4 v pdb in bulk carbonate that begins at the t j boundary and reaches a local maximum at the early late hettangian boundary values then gradually decrease reaching 0 5 at the hettangian sinemurian boundary and remain relatively constant into the early sinemurian complementary 13ccarb data from 4 more sections that span the hettangian sinemurian boundary support carbon cycle stabilization within the upper hettangian our analyses suggest that isotope changes in organic carbon reservoirs do not necessarily require a shift in the global exogenic carbon reservoir and that the positive excursion in the carbonate carbon isotope record is best explained as the combined result of an increase in atmospheric pco2 leading to accelerated carbon cycling decreased skeletal carbonate production and increased organic carbon burial lasting several hundred thousand years the termination of the positive inorganic carbon isotope excursion coincides with the recovery of marine skeletal carbonate producers and coeval changes in terrestrial vegetation and reflects the gradual reduction in pco2 and the stabilization of the global carbon cycle during the sinemurian

carbon isotope stratigraphy and palynostratigraphy of the triassic jurassic transition in the tiefengraben section northern calcareous alps austria abstract the end triassic is characterized by enhanced rates of biotic turnover in both the marine and terrestrial realms carbon isotope anomalies have been reported worldwide that may indicate coeval global disturbances in the biogeochemical cycles here we present the results of an integrated study of the palynomorph assemblages and the carbon isotope composition of organic matter in the tiefengraben section near st wolfgang austria carbon isotope values from bulk organic matter show significant fluctuations through the triassic jurassic t j boundary interval with two prominent negative 13 c org excursions an initial negative carbon isotope excursion occurs at the base of the kendlbach formation tiefengraben member pre planorbis beds although triassic ammonoids and conodonts have their highest occurrence at the top of the underlying kossen formation the palynomorph and calcareous microfossil assemblages indicate still a rhaetian age for the lower part of the tiefengraben member high abundance of prasinophytes during the interval of the initial carbon isotope excursion may represent a bloom of green algae that flourished as a result of the disturbance of the marine ecosystems increased spore abundance together with the disappearance of distinctive late triassic pollen types e g ovalipollis rhaetipollis indicate vegetation changes on land that are contemporaneous with the marine biotic crisis a second negative carbon isotope excursion occurs in the middle part of the tiefengraben member based on palynological evidence the t j boundary may be drawn at about this level by the lowest occurrence of cerebropollenites thiergartii the established carbon isotope curve as well as palynostratigraphic and other biostratigraphic events can be correlated with st audrie s bay england

argon and nitrogen isotopes of trapped air in the gisp2 ice core during the holocene epoch 0 11 500 b p methodology and implications for gas loss processes argon and nitrogen isotopes of air in polar ice cores provide constraints on past temperature and firn thickness with relevance to past climate we developed a method to simultaneously measure nitrogen and argon isotopes in trapped air from the same sample of polar ice this method reduces the time required for analysis allowing large numbers of measurements we applied this method to the entire holocene sequence of the gisp2 ice core 82 37 1692 22 m with a 10 20 year sampling interval 670 depths d 40 ar and d 15 n show elevated values in the oldest part of the dataset consistent with a thicker firn layer and increased temperature gradient in the firn due to the legacy of the abrupt warming at the end of the younger dryas interval and the gradual warming during the preboreal interval 11 5 10 0 ka the preboreal oscillation and the 8 2k event are clearly recorded the data show remarkable stability after the 8 2k event available data suggests that post coring gas loss involves two distinct types of fractionation first smaller molecules with less than a certain threshold size leak through the ice lattice with little isotopic fractionation second gas composition changes via gas loss through microcracks which induces isotopic fractionation these two gas loss processes can explain most trends in our data and in other ice core records 2008 elsevier ltd all rights reserved