xenon geochronology of schwarzwald pitchblendes the uranium fission xenon systems in eleven samples selected from three occurrences in the schwarz wald were investigated menzenschwand wittichen mullenbach relationships between mineralogical properties xe release profiles and xe xe age spectra provide criteria for sample suitability for u xe and xe xe dating and interpretation of the xe xe age spectra u leaching from pitchblendes was demonstrated for three of the wittichen samples indicating also disturbed u pb relations the results show that the u xe isotopic system is a valuable geochronological tool for studying uranium minerals in samples of two deposits fission xe was well preserved and allowed us to determine xe xe plateau ages 301 11 304 9 and 279 10 ma for menzenschwand and 244 9 ma for wittichen pitchblendes these ages show the closure times of the u xe isotope systems and most probably correspond to the formation of these pitchblendes they are in accord with published u pb data four other samples of these two deposits had very irregular spectra an attempt was made to interpret integrated xe xe ages u xe ages in these samples in terms of alteration events little fission xe was found in material from mullenbach there is a large time gap between the formation of the hosting granites and the u ore depositions about 30 and 90 ma respectively in menzenschwand and wittichen

lack of evidence for a substantial sea level fluctuation within the last interglacial during the last interglacial global mean sea level reached approximately 6 to 9 m above the present level this period of high sea level may have been punctuated by a fall of more than 4 m but a cause for such a widespread sea level fall has been elusive reconstructions of global mean sea level account for solid earth processes and so the rapid growth and decay of ice sheets is the most obvious explanation for the sea level fluctuation here we synthesize published geomorphological and stratigraphic indicators from the last interglacial and find no evidence for ice sheet regrowth within the warm interglacial climate we also identify uncertainties in the interpretation of local relative sea level data that underpin the reconstructions of global mean sea level given this uncertainty and taking into account our inability to identify any plausible processes that would cause global sea level to fall by 4 m during warm climate conditions we question the occurrence of a rapid sea level fluctuation within the last interglacial we therefore recommend caution in interpreting the high rates of global mean sea level rise in excess of 3 to 7 m per 1 000 years that have been proposed for the period following the last interglacial sea level lowstand

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 1 geophysical results in the 1994 1995 and 1998 austral field seasons geophysical research efforts in the ross sea focused on acquiring a high resolution database designed to permit 1 reconstruction of the maximum extent and configuration of the ice sheet during the last glacial maximum i e oxygen isotope stage 2 2 reconstruction of conditions at the base of the ice sheet and 3 assessment of the relative retreat history of the ice sheet following the last glacial maximum five seismic facies are distinguished on the basis of external geometry relationships among features bounding surface amplitude intensity of internal acoustic signature and geometry of internal reflectors seismic facies 1 is a transparent draping unit correlated to diatomaceous mud interpreted as being deposited under open marine conditions seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations these units are interpreted to be grounding zone proximal deposits overridden by the expanded ice sheet seismic facies 2 may be a deforming till unit seismic facies 4a is characterized by an internally massive to chaotic signature an erosional often flat lower surface and a hummocky upper surface displaying glacial lineations this unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion 4a and with an earlier pre last glacial maximum expansion 4b seismic facies 5 is an acoustically laminated ponded and draping deposit interpreted to contain proglacial and sub ice shelf materials deposited continuously since the last interglacial period the associations of these units provide context for the interpretation of the ice edge maximum position conditions at the base of the ice sheet and the relative retreat history of the region there is compelling evidence for a much expanded ice sheet in the ross sea during the last glacial maximum in the western ross sea the maximum grounding position is marked by an isolated grounding zone wedge and placed in the vicinity of coulman island approximately 150 km from the continental shelf edge in the central ross sea the maximum grounding position is close to the continental shelf break based primarily on the presence of an extensive 60 m thick sheet like deposit with a fluted upper surface streaming ice may have occupied bathymetric lows on the continental shelf as suggested by 1 the configuration of bathymetry 2 the presence of glacially eroded troughs 3 the concentration of sediment interpreted to be deforming till within the middle to outer shelf reaches of the troughs and 4 the fluted nature of the upper surface absolute rates of streaming ice flow relative to inter ice stream areas are not implied the western ross sea continental shelf deposits record the retreat history of ice derived predominantly from the east antarctic ice sheet and glaciers of the transantarctic mountains ice flow on the continental shelf is interpreted to have remained fixed in position restricted by the walls of the troughs on the inner shelf of the western ross sea the ice flowed over and eroded lithified sedimentary strata slower moving ice occupied flat topped banks a single grounding zone wedge occurs on the western ross sea central shelf no substantial deposits are observed on the inner continental shelf this lack of grounding zone features reflects a restricted sediment supply and a relatively steady rapid retreat of the ice sheet during retreat ice flow acted independently in each major trough and on the bank tops ice retreated from victoria land basin drygalski trough before it retreated from joides basin ice remained on the bank tops shedding material into the abandoned troughs grounded ice in the central ross sea was derived predominantly from an expanded west antarctic ice sheet the ice sheet remained grounded on the continental shelf edge after retreat of ice from the western ross sea expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf ice stream drainage shifted laterally as recorded by rounded laterally accreting ridges that separate bathymetric troughs the central ross sea is the repository for larger volumes of sediment derived from ice flow across basins of relatively thick unlithified sedimentary deposits two till sheets mark grounding zone positions in the central ross sea reflecting the higher sediment supply and a stepped deglaciation ice remained grounded on the pennell bank to the west a series of moraines marks the retreat of ice across the bank mega scale glacial lineations and other streamlined subglacial geomorphic features occur across the continental shelf primarily within the troughs the lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet this deforming substrate may have contributed to the onset of deglaciation features associated with meltwater are rare or absent suggesting that basal meltwater played a minor role in retreat of the ice sheet

ross sea paleo ice sheet drainage and deglacial history during and since the lgm onshore and offshore studies show that an expanded grounded ice sheet occupied the ross sea embayment during the last glacial maximum lgm results from studies of till provenance and the orienta

west antarctic ice sheet elevations in the ohio range geologic constraints and ice sheet modeling prior to the last highstand abstract knowledge of the timing and extent of past west antarctic ice sheet wais elevation changes provides insight to the wais response to sea level and climate forcing as well as constraints for ice sheet models trimlines and erratics indicate that maximum ice elevation at the ohio range located near the wais divide was 125 m above the present ice surface and was reached at 10 ka however 3 he and 10 be exposure ages of most granitic erratics and supra glacial boulders in adjacent blue ice areas are older than the last highstand and provide constraints on earlier interior wais elevation changes although diffusive loss of 3 he occurs in quartz an excellent correlation between 3 he and 10 be concentrations in samples up to 400 ka is observed in the ohio range demonstrating the utility of 3 he measurements in antarctic quartz the correlation is used to calculate equivalent 10 be exposure ages from the larger 3 he data set the exposure ages occur in distinct clusters 10 20 ka 70 80 ka 130 140 ka and 180 190 ka that we relate to pulses in the emergence of englacial debris in localized blue ice ablation areas a combined ice sheet ice shelf model predicts that wais elevations near the ohio range have varied only 125 m over the last 200 kyr with highstands at 10 ka and 130 ka in good agreement with observations of trimlines and peaks in the exposure age distribution at the ohio range in the model interior wais elevations are controlled by the interaction of accumulation rates and ice dynamics related to ice sheet ocean interactions at the grounding line wais highstands occur early in interglacial periods during intervals of warmer temperatures and high accumulation rates prior to the arrival of a wave of thinning we hypothesize that the correspondence of rising regional wais elevation with debris pulses relates to increased ice debris flux and ablation in the local blue ice areas resulting from the warmer temperatures the occurrence of clasts with exposure ages up to 200 ka on the wais surface indicates elevation changes near the ohio range were limited in extent during stage 5e the geologic observations combined with model results place constraints on wais geometry during stage 5e and limit the wais contribution to the higher sea levels observed during the last interglacial to 3 m

reactivated basement structures affecting the sedimentary facies in a tectonically quiescent epicontinental basin an example from nw switzerland abstract the jurassic deposits in the southern part of central europe accumulated in a shallow epicontinental sea and their deposition has usually been believed to have corresponded to a phase of tectonic quiescence neither on conventional seismic records nor in outcrops obvious indications of synsedimentary tectonics were found nonetheless subtle variations in lithofacies and thickness occur above faults in the crystalline basement in fact preexisting structures became reactivated in the jurassic during major extensional phases when the tethys and the north atlantic opened this reactivation led to differential subsidence and or to rotation of fault bounded blocks but the sediments were deformed mainly flexurally as vertical movements in the basement were dissipated by triassic salt thus the depositional area was morphologically differentiated into swells and depressions in siliciclastic muddy environments swells were characterized by hardbottoms and hiatus beds depressions were filled by distal tempestites and gravity deposits in carbonate settings reefs nucleated on swells marls and muds were deposited in depressions many of them gravity deposits reactivation of faults occurred only during short episodes and was not synchronous throughout the study area and reactivation of individual faults was episodic and probably controlled by the palaeostress field and the faults orientation reactivation of deep rooted faults is also documented by hydrothermal activity which led to vein mineralization and alteration of minerals today exposed in nearby basement units the chronostratigraphic ages of the hydrothermal processes coincide with phases of enhanced subsidence during the sinemurian aalenian bajocian bathonian and oxfordian in turn facies changes of the sedimentary cover should be useful to predict basement structures when no seismic records are available

cosmogenic nuclide exposure ages and glacial history of late quaternary ross sea drift in mcmurdo sound antarctica 3he 10be and26al surface exposure ages were determined for rock samples from late quaternary drift sheets deposited on the coast of mcmurdo sound antarctica by the ross sea ice sheet the youngest drift previously believed to be late wisconsin in age has exposure ages of 8 106 kyr 19 samples these results suggest that the apparent late wisconsin ice margin may represent a number of ice sheet advances during the last glacial period 3he exposure ages from analogous stratigraphically older drift range from 104 to 567 kyr seven samples and clearly distinguish the youngest drift from older deposits comparison of10be and3he concentrations in ross sea drift quartz samples indicates significant3he loss in only one sample suggesting that cosmogenic3he is generally retained in young antarctic quartz samples exposure age 100 kyr of appropriate grain size in two of the five quartz samples analyzed significant excesses of26al relative to10be preclude meaningful26al exposure age calculations these excesses may be due to nucleogenic26al produced as a byproduct of uranium and thorium decay

neotectonics geodesy and seismic hazard in the northern walker lane of western north america thirty kilometers of crustal shear and no strike slip article i nfo roughly 30 km of cumulative right lateral crustal displacement and 5 6 mm yr of the ongoing relative right lateral motion between the pacific and north american plates are observed in the northern walker lane the right lateral shear has been accommodated in large part by the development of a set of discontinuous en echelon normal fault bounded basins and perhaps significant vertical axis rotations of the intervening crust the observations provide an illustrative example of how large amounts of crustal shear may be accom modated in the absence of strike slip faults and point to difficulties attendant to melding geologic and geo detic observations in the analysis of seismic hazard in this particular case the assumption that all geodetically observed shear across the area will be recorded by earthquake displacements may be flawed

mapping blue ice areas in antarctica using etm and modis data blue ice areas bias and their geographical distribution in antarctica were mapped using landsat 7 etm images with 15 m spatial resolution obtained during the 1999 2003 austral summers and covering the area north of 82 58 s and a snow grain size image of the modis based mosaic of antarctica moa dataset with 125 m grid spacing acquired during the 2003 04 austral summer from 82 58 s to the south pole a map of bias was created with algorithms of thresholds based on band ratio and reflectance for etm data and thresholds based on snow grain size for the moa dataset the underlying principle is that blue ice can be separated from snow or rock by their spectral discrepancies and by different grain sizes of snow and ice we estimate the total area of bias in antarctica during the data acquisition period is 234 549 km 2 or 1 67 of the area of the continent blue ice is scattered widely over the continent but is generally located in coastal or mountainous regions the bia dataset presented in this study is the first map covering the entire antarctic continent sourced solely from etm and modis data this dataset can potentially benefit other studies in glaciology meteorology climatology and paleoclimate meteorite collection and airstrip site selection

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

new kinematic models for pacific north america motion from 3 ma to present i evidence for steady motion and biases in the nuvel 1a model we use velocities derived from 2 4 5 years of continuous gps observations at 21 sites on the pacific and north american plates along with a subset of the nuvel 1a data to examine the steadiness of pacific north america motion since 3 16 ma the transfer of baja california to the pacific plate and the magnitude of biases in the nuvel 1a estimate of pacific north america motion we find that pacific north america motion has remained steady since 3 16 ma but at rates significantly faster than predicted by nuvel 1a in the vicinity of baja california our gps derived model and recent seafloor spreading rates in the southern gulf of california both indicate that the nuvel 1a model underestimates pacific north america rates by 4 2 mm yr 1 steady pacific north america motion since 3 16 myr and increasing seafloor spreading rates since 3 58 myr in the gulf of california imply that pacific north america motion was partitioned between seafloor spreading in the gulf of california and decelerating slip along faults in or offshore from the baja peninsula

hydrothermal and unexpected diagenetic alteration in permian shales of the lodeve epigenetic u deposit of southern france traced by k ar illite and k feldspar dating abstract mineralogical and chemical analyses combined with k ar dating of illite and k feldspars were carried out on bulk samples and different fractions from permian shales around the lodeve u deposit to identify diagenetic and hydrothermal alteration episodes and their timing authigenic 2m1 and 1m illite with k ar ages between 270 and 245 ma are the predominant mineral types in the shales they are of early diagenetic origin relative to deposition and formed apparently directly from pyroclastic materials in a lagoonal to deltaic environment under semi arid conditions albite and a first generation of k feldspar developed by a replacement of analcime the k feldspar growing only in the former lagoonal environment that favored also early diagenetic dolomitization at the expense of calcite the hydrothermally altered shales next to the u deposit contain authigenic quartz fe chlorite neoformed illite and a second generation k feldspar with k ar ages beginning at about 220 ma triassic although jurassic and cretaceous ages were also obtained this younger age spectrum jurassic cretaceous is close to previous u pb isotope data from the u deposit probably reflecting several episodes of u mobilization therefore the hydrothermal activity extended episodically from triassic to cretaceous time within the u deposit the hydrothermal temperatures attained locally 220 c as the k ar system of illite from early permian diagenesis is partially reset triassic hot fluids ascended along faults into the northern area of the lodeve basin during an initial rifting of the nearby western tethys ocean further fluid pulses were recorded at lodeve during the opening of the north atlantic and separation of europe from africa jurassic cretaceous which also caused the peak hydrothermal activities in central and western europe as the lodeve area is located between two independently developing oceans it could have been supplied with hot fluids for a long period of time

mesozoic hydrothermal impact on rotliegende and bunter immature sandstones of the high rhine trough and its adjacent eastern area southern black forest germany abstract the mesozoic hydrothermal and geodynamic evolution of a complex region of the european crust is studied here on immature and highly reactive rotliegende and bunter sandstones of the intramontane high rhine trough southern black forest and its adjacent eastern area bounded by the upper rhine graben and the alpine foreland microscopic observations outline varied degrees of illitization of detrital feldspar grains muscovite flakes and illite particles xrd analysis shows that the clay fractions are dominated by illite with minor illite smectite mixed layered minerals of r3 and r1 ordering the chemical composition of these minerals is controlled by precursor minerals since the illitic material inherited its chemical components from detrital muscovite and illite towards the centre of the trough the sandstones were more strongly affected by the hydrothermal impact showing extensive replacement of feldspar grains by al rich illite k ar dating of the authigenic illite type material points to multiple hydrothermal alteration episodes a late jurassic event at 147 ma seems to have affected extensively the study area especially towards the centre of the trough at the periphery of the trough relics of an early jurassic activity are likely preserved an additional early cretaceous hydrothermal event seems to have occurred locally near schopfheim and next to the rhine graben boundary fault long before its main rifting phase the 147 ma event was obtained also nearby in the previously investigated breisgau schramberg offenburg teinach and baden baden troughs and is characteristic for the black forest and some other areas outside this region it concludes obviously a period of regional hydrothermal activities between 160 and 145 ma whereas the early jurassic and cretaceous signatures are also known throughout central and western europe suggesting repeated fluid pulses the hydrothermal flows were controlled by reactivated basement faults due to periods of extensive reorientation of the tectonic stress during the break up of pangaea illitization was triggered by hot and low saline solutions of about 180 c and eq whereas a later fluid pulse of about 80 c and 20 wt nacl eq caused quartz overgrowth fluid flows delivered by the long lasting orogeny of the nearby alps were probably limited towards the black forest becoming part of the european margin of the tethys in the mesozoic the pre alp area evidently developed a separate fluid flow system

ree systematics in hydrothermal fluorite abstract ree patterns of fluorite from 63 post variscan hydrothermal veins of the schwarzwald area germany can be divided into two major groups type a fluorite shows roof shaped paas normalized patterns with enriched mree the turning point of the curvature is at ho or er position all patterns of type a show a distinct positive eu anomaly type b fluorite shows roof shaped patterns with enriched mree and strongly depleted hree and a characteristic steep slope between nd and sm the turning point of the curvature is at gd to dy all patterns of type b show negative or no eu anomalies laser ablation icp ms traverses through single grains combined with cathodoluminescence imagery revealed either i identical ree patterns and total ree concentrations from core to rim ii identical ree patterns but concentration gradients or iii variable ree patterns and concentration gradients from core to rim these features are explained by different degrees of supersaturation or different fluid fluorite ratios during crystallization and by complexly controlled precipitation processes under closed system conditions petrographically late fluorites from several deposits show the characteristics of new fluid batches rather than the expected remobilization features lree enrichment close to the crystal faces in some grains may result from the copreciptiation of a submicrometer sized lree enriched phase or from lree clustering in late cool fluids by comparison with literature data on natural waters from gneissic and granitic aquifers and of leachates of granites and gneisses we show that the hydrothermal vein fluorite preserves the characteristic source related ree feature of the hydrothermal fluid that migrated through either gneissic type a or granitic type b host rocks the immediate host rocks of the veins however are not the source rocks of the ree which indicates large scale fluid convection in the basement gneiss related type a fluorite shows tb ca tb la trends of primary hydrothermal crystallization whereas granite related type b fluorite indicates remobilization of possibly pegmatitic or variscan fluorite

response of the east antarctica ice sheet to sea level rise a nonsteady ice flow model that explicitly includes longitudinal stresses is used to study the effects of marginal changes on the interior portions of an ice sheet sea level rise is found to cause a wave of thinning to propagate upglacier and this wave is slowed diffused and damped at the ice divide at dome c in central east antarctica post wisconsinan sea level rise is modeled as having caused 110 m thinning with response now 70 complete if accumulation rates increased by 10 at the same time then calculated net thinning has been only 75 m ice sheet response to changes in sea level and accumulation rate is relatively rapid response to temperature changes is an order of magnitude slower response times are shorter if the marginal position of an ice sheet is controlled

controls on interior west antarctic ice sheet elevations inferences from geologic constraints and ice sheet modeling abstract knowledge of the west antarctic ice sheet wais response to past sea level and climate forcing is necessary to predict its response to warmer temperatures in the future the timing and extent of past interior wais elevation changes provides insight to wais behavior and constraints for ice sheet models constraints prior to the last glacial maximum lgm however are rare surface exposure ages of glacial erratics near the wais divide at mt waesche in marie byrd land and at the ohio range in the transantarctic mountains range from 10 ka to 500 ka without a dependence on elevation the probability distribution functions pdf of the exposure ages at both locations are remarkably similar during the last glaciation maximum interior ice elevations as recorded by moraines and erratics were reached between 10 ka and 12 ka however most exposure ages are older than the lgm and cluster around 40 ka and 80 ka the peak in the exposure age distributions at 40 ka includes ages of alpine moraine boulders at mercer ridge in the ohio range comparison of the pdf of exposures ages from the ohio range and mt waesche with the temperature record from the fuji dome ice core indicates that the youngest peak in the exposure age distributions corresponds to the abrupt warming during the last glacial termination a prominent peak in the ohio range pdf corresponds to the penultimate termination stage 5e during the intervening glacial period there is not a consistent relationship between the peaks in the pdf at each location and temperature a combined ice sheet ice shelf model with forcing scaled to marine 18 o predicts that interior wais elevations near the ice divide have varied 300 m over the last glacial cycle peaks in the pdf correspond to model highstands over the last 200 ka in the simulated elevation history maximum ice elevations at ohio range 100 m and mt waesche 60 m occur at 10 ka in agreement with observations from these sites during collapse of the marine portion of the wais ice elevations at ohio range and mt waesche are drawn down at least 200 m below the present ice elevation the good correspondence between the model results and observations at both the ohio range and mt waesche supports the conclusion that interior wais highstands do not occur during glacial maximums rather the highstands are controlled primarily by increased accumulation during temperature maximums that occur early in the interglacials interior down draw events follow highstands resulting from the arrival of a wave of thinning triggered by retreat of the wais grounding line coupled with decreasing accumulation rates

range bounding normal fault of smith valley nevada limits on age of last surface rupture earthquake and late pleistocene rate of displacement abstract smith valley is bounded on its western edge by an active normal fault and the pine nut mountains displacement on the fault is responsible for development of one of the westernmost basins of the basin and range province of the western united states interpretation of an exposure afforded by excavation of a trench across the range bounding fault places the most recent surface rupture earthquake after 5176 130 cal yr b p and suggests it was close in time to 3530 82 cal yr b p utilization of tephrochonologic and cosmogenic analyses and mapping of fault scarps across the artesia road fan are the basis for putting forth an initial estimate of the late pleistocene vertical slip rate of the fault at between 0 125 mm yr and 0 33 mm yr

k ar dating of episodic mesozoic fluid migrations along the fault system of gernsbach between the moldanubian and saxothuringian northern black forest germany along the sw ne trending fault zone of gernsbach northern black forest over a distance of about 45 km between baden baden and pforzheim basal series of the sedimentary cover are extensively altered by hydrothermal solutions the fluids were conveyed in the fault zone and because of favourable permeabilities they spread out along the unconformity between the basement and sedimentary cover k ar dates on authigenic white micas illites and hydrothermally altered detrital micas point to the occurrence of two major episodes of fluid migration the extensive alteration along the entire fault system occurred about 150 ma ago during the jurassic in relation to these hydrothermal activities the epigenetic strata bound uranium deposit of mullenbach originated in upper carboniferous sediments near the fault and the fluorite quartz vein type mineralization of kafersteige was formed along the fault about 100 ma ago during the cretaceous the results indicate various geological processes of the fault system of gernsbach which is located between the saxothuringian and moldanubian zone the mobilization of the fluids and the formation of the ore deposits were probably caused by and connected with tectonic activities in mid europe

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

the antarctic ice sheet during the last glacial maximum and its subsequent retreat history a review abstract an emerging body of evidence from studies of the last glacial interglacial cycle suggests that the east and west antarctic ice sheets have not advanced and retreated in concert the west antarctic ice sheet advanced to the outer shelf in most regions during the last glacial maximum lgm the retreat from the shelf commenced shortly after the lgm and continued into the late holocene the west antarctic ice sheet in ross and amundsen seas slid across a deforming bed at least during the final phases of extended glaciation this implies that at this time the ice sheet had a low profile differences in the number and locations of grounding zone wedges and smaller grounding zone features from trough to trough imply that individual west antarctic ice sheet ice streams retreated episodically details concerning the expansion retreat and behavior of the east antarctic ice sheet eais are more sparse the picture emerging is that the eais did not expand to the continental shelf edge during the lgm rather it achieved a maximum extent of a mid shelf position in some locations while in other areas the ice terminus was situated near its present location the timing of retreat along sectors within the eais appears diachronous and in places occurred prior to the lgm the antarctic peninsula shelf contained considerably more ice during the lgm than previously proposed the results presented in this paper support more recent published ice sheet models that call for greater contributions of melting ice from west antarctica including the antarctic peninsula to the post glacial rise in sea level

late quaternary evolution of reedy glacier antarctica abstract fresh deposits above the margins of reedy glacier show that maximum ice levels during the last glaciation were several hundred meters above present near the glacier mouth and converged to less than 60 m above the present day surface at the head of the glacier exposure ages of samples from five sites along its margin show that reedy glacier and its tributaries thickened asynchronously between 17 and 7 kyr bp at the quartz hills located midway along the glacier maximum ice levels were reached during the period 17 14 kyr bp farther up glacier the ice surface reached its maximum elevation more recently 14 7 10 2 kyr bp at the caloplaca hills 9 1 7 7 kyr bp at mims spur and around 7 kyr bp at hatcher bluffs we attribute this time transgressive behavior to two different processes at the glacier mouth growth of grounded ice and subsequent deglaciation in the ross sea embayment caused a wave of thickening and then thinning to propagate up glacier during the lateglacial and holocene increased snow accumulation on the east antarctic ice sheet caused transient thickening at the head of the glacier an important result of this work is that moraines deposited along reedy glacier during the last ice age cannot be correlated to reconstruct a single glacial maximum longitudinal profile the profile steepened during deglaciation of the ross sea thinning at the quartz hills after 13 kyr bp while thickening upstream near its confluence with mercer ice stream rapid thinning beginning prior to 7 8 kyr bp reduced the level of reedy glacier to close to its present level thinning over the past 1000 years has lowered the glacier by less than 20 m

holocene deglaciation of marie byrd land west antarctica surface exposure ages of glacial deposits in the ford ranges of western marie byrd land indicate continuous thinning of the west antarctic ice sheet by more than 700 meters near the coast throughout the past 10 000 years deglaciation lagged the disappearance of ice sheets in the northern hemisphere by thousands of years and may still be under way these results provide further evidence that parts of the west antarctic ice sheet are on a long term trajectory of decline west antarctic melting contributed water to the oceans in the late holocene and may continue to do so in the future

decoding the dipstick thickness of siple dome west antarctica at the last glacial maximum ice thickness in west antarctica at the last glacial maximum lgm is poorly known yet is key information for understanding ice streams and interpreting ice cores although trim lines moraine limits and exposure age dating provide geologic constraints on ice thickness near the transantarctic mountains and in marie byrd land lack of exposed bedrock hampers traditional geologic methods in a central 2 3 10 6 km 2 region here we infer ice sheet thickness changes in the central ross sea embayment by using a tran sient ice flow model to find combinations of accumulation rate and ice sheet thickness histories that match the depth age relationship and the measured layer thickness pattern in the siple dome ice core after we reject unreasonable accumulation rate histories the remaining history pairs indicate thinning of 200 400 m since the lgm our estimate is lower than previous reconstructions that were constrained by geologic evidence from the transantarctic mountains and by marine data from the ross sea floor which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period low surface elevations in the central ross sea embayment during the lgm do not preclude thicker ice along its boundaries however if this grounded ice sheet came over 1000 km from interior west antarctica as is usually assumed then it had very low surface slope requiring a very slippery bed alternatively the grounded ross sea ice might have flowed from the transantarctic mountains and western marie byrd land

highest pluvial lake shorelines and pleistocene climate of the western great basin abstract shoreline altitudes of several pluvial lakes in the western great basin of north america record successively smaller lakes from the early to the late pleistocene this decrease in lake size indicates a long term drying trend in the regional climate that is not seen in global marine oxygen isotope records at 70 m above its late pleistocene shoreline lake lahontan in the early middle pleistocene submerged some basins previously thought to have been isolated other basins known to contain records of older pluvial lakes that exceeded late pleistocene levels include columbus fish lake lake columbus rennie kobeh diamond lakes jonathan and diamond newark long lake hubbs and clover very high stands of some of these lakes probably triggered overflows of previously internally drained basins adding to the size of lake lahontan simple calculations based on differences in lake area suggest that the highest levels of these pluvial lakes required a regional increase in effective moisture by a factor of 1 2 to 3 relative to late pleistocene pluvial amounts assuming that effective moisture is directly proportional to the hydrologic index or lake area tributary basin area these previously unknown lake levels reflect significant changes in climate tectonics and or drainage basin configurations and could have facilitated migration of aquatic species in the great basin

measurements of past ice sheet elevations in interior west antarctica a lateral moraine band on mount waesche a volcanic nunatak in marie byrd land provides estimates of past ice sheet surface elevations in west antarctica helium 3 and chlorine 36 surface exposure ages indicate that the proximal part of the moraine up to 45 meters above the present ice surface was deposited about 10 000 years ago substantially later than the maximum ice extent in the ross embayment the upper distal part of the moraine may record multiple earlier ice sheet high stands a nonequilibrium ice sheet model predicts a delay of several thousand years in maximum ice levels at mount waesche relative to the maximum ice extent in the ross sea the glacial geologic evidence coupled with the ice sheet model indicates that the contribution of the ross sea sector of the west antarctic ice sheet to holocene sea level rise was only about 3 meters these results eliminate west antarctic ice as the principle source of the large meltwater pulse during the early holocene

rapid early holocene deglaciation in the ross sea antarctica deglaciation of the ross sea following the last ice age provides an important opportunity to examine the stability of marine ice sheets and their susceptibility to changing environmental conditions insufficient chronology for ross sea deglaciation has helped sustain i the theory that this region contributed significantly to meltwater pulse 1a mwp 1a and ii the idea that ross sea grounding line retreat occurred in a swinging gate pattern hinged north of roosevelt island we present deglaciation records from southern transantarctic mountain glaciers which delivered ice to the central ross sea abrupt thinning of these glaciers 9 8 kyr bp coincided with deglaciation of the scott coast 800 km to the north and ended with the ross sea grounding line near shackleton glacier this deglaciation removed grounded ice from most of the central and western ross sea in less than 2 kyr the ross sea sector neither contributed nor responded significantly to mwp 1a

beryllium 10 exposure ages of erratic boulders in southern norway and implications for the history of the fennoscandian ice sheet the margins of the fennoscandian ice sheet fis during the last glacial maximum lgm are fairly well constrained yet the thickness of the fis and its temporal evolution are poorly known here we use beryllium 10 be 10 exposure ages of glacially transported boulders along three vertical transects in southern norway to determine past ice sheet elevations the ice surface elevation during the lgm at our westernmost site skala was likely at approximately 1440 in coinciding with a previously mapped trimline at our central site blaho the ice surface elevation at the lgm was at least at similar to 1620m likely closer to 1800m at elgahogna our easternmost site lgm ice was above the summit elevation of 1460m rapid deglaciation began at skala and blaho at approximately 13 5 14 5 10 beka and at approximately 12 be 10 ka at elgdhogna by approximately 9 be 10 ka the fis had thinned to below our lowest sample at each location periods of rapid thinning correlate with an exposure dated ice margin history as well as the glaciation curve for western scandinavia our data when combined with previous bedrock and blockfield highly weathered bedrock on summits exposure ages resolve a long standing debate over the age and geomorphic interpretation of blockfields and weathering zones in southern norway frozen bed low erosive ice clearly covered the summit blockfields at blaho and especially at elgdhogna where we find a large and progressive divergence between boulder and bedrock exposure ages with elevation we conclude that the lower limit of blockfields in these areas should be interpreted as an englacial thermal boundary rather than the upper limit of the lgm fis and that blockfields have survived multiple glacial cycles we also compare our deglaciation record with several models of the fis models that predict how the fis changed during deglaciation either through inversion of crustal rebound or through climate dependent forward modeling generally match our exposure ages reasonably well during deglaciation but predict larger lgm surface elevations than our data indicate on the other hand the climap maximum model which provides lgm elevations only agrees with our results fairly well c 2007 elsevier ltd all rights reserved

evidence for late pleistocene thinning of siple dome west antarctica 1 we use a two dimensional full stress thermomechanical flow band model to investigate late pleistocene and holocene histories of ice thickness and ice flow at siple dome in west antarctica measurements obtained from an ice core and borehole near the ice divide and measurements of radar detected internal stratigraphy are used as targets for modeling models that account for the climate history accumulation rate and temperature alone yield a poor match to observations histories of ice dynamics ice thickness ice divide location and ice flow enhancement also need to be included our favored model includes 350 m of thinning at siple dome between 15 000 and 14 000 years ago onset of divide flow 3000 years ago ongoing divide migration starting 2500 years ago and evolution of ice fabric over the past 30 000 years the timing of thinning is earlier than that inferred from geologic evidence from the margins but is coincident with a 20 m rise in global sea level about 14 500 years ago meltwater pulse 1a this opens the possibility that thinning was a result of increased activity of the ice streams in the late pleistocene forced by meltwater pulse 1a the contribution to meltwater pulse 1a as a result of this thinning was likely small

northern hemisphere forcing of climatic cycles in antarctica over the past 360 000 years the milankovitch theory of climate change proposes that glacial interglacial cycles are driven by changes in summer insolation at high northern latitudes the timing of climate change in the southern hemisphere at glacial interglacial transitions which are known as terminations relative to variations in summer insolation in the northern hemisphere is an important test of this hypothesis so far it has only been possible to apply this test to the most recent termination because the dating uncertainty associated with older terminations is too large to allow phase relationships to be determined here we present a new chronology of antarctic climate change over the past 360 000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the dome fuji and vostok ice cores this ratio is a proxy for local summer insolation5 and thus allows the chronology to be constructed by orbital tuning without the need to assume a lag between a climate record and an orbital parameter the accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores and changes in insolation our results indicate that orbital scale antarctic climate change lags northern hemisphere insolation by a few millennia and that the increases in antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of northern hemisphere summer insolation these results support the milankovitch theory that northern hemisphere summer insolation triggered the last four deglaciations

u th he evidence of jurassic continuous hydrothermal activity in the schwarzwald basement germany abstract u th he model and isochron ages of 21 botryoidal hematites collected from the hohberg base metal vein system in the triberg granite complex mittelschwarzwald germany cover most of the jurassic period the possibility of quantitative 4 he loss triggered by successive hydrothermal events although conceivable for individual hematites based on a 4 he retentivity study and calculated opening temperatures of hematite for 4 he of 90 180 c can largely be ruled out thus the range of model and isochron ages indicates that the majority of the hematites are not cogenetic this is supported by a broad range of pb total concentrations in the hohberg botryoidal hematites 150 2700 ppm by comparison with virtually identical pb total concentrations found in cogenetic hematites from four different vein systems in the schwarzwald the th u ratios of the hematites vary by a factor of 700 0 01 7 25 mainly as a result of the range of th concentrations 0 1 74 ppm the incorporation of substantial amounts of th in hematite is thought to result from increased th transport mobility possibly caused by association with colloids and or complexing in the ore solution 18 o analyses of a 160 ma old cogenetic hematite quartz pair indicate that precipitation occurred from a fluid of magmatic origin at a temperature of 180 c the regional temperature peak in the basement at the present erosional surface during the jurassic 130 c provides a lower temperature limit of hematite formation this implies that hematite emplacement occurred at temperatures roughly ranging between 130 and 180 c the jurassic continuous hydrothermal activity suggests that the pre rifting european craton is characterized by local thermal fluxes during a long time span

chronology of late wisconsin ice retreat from the western ross sea antarctica lithologic data from marine sedimentary cores and accelerator mass spectrometer ams radiocarbon dates indicate that grounded ice did not advance to the western ross sea continental shelf edge during the last glacial maximum lgm a chronology of the timing of ice retreat was provided by 26 ams dates obtained from 12 cores dates ranging from 20 to 29 ka suggest that the outer continental shelf beyond ca lat 74 s was not covered by grounded glacial ice prior to and during the early stages of the lgm 14c dates just above transitions from subglacial diamictons to marine muds indicate that the area around the drygalski ice tongue was deglaciated by at least 11 5 ka the ross ice shelf reached its present day position near ross island by about 7 ka a hiatus of about 15 ka is apparent in radiocarbon dates in cores from the outer continental shelf the hiatus is interpreted to represent an absence of sedimentation caused by the presence of an ice shelf

ice elevation near the west antarctic ice sheet divide during the last glaciation 1 interior ice elevations of the west antarctic ice sheet wais during the last glaciation which can serve as benchmarks for ice sheet models are largely unconstrained here we report past ice elevation data from the ohio range located near the wais divide and the onset region of the mercer ice stream cosmogenic exposure ages of glacial erratics that record a wais highstand 125 m above the present surface date to 11 5 ka the deglacial chronology prohibits an interior wais contribution to meltwater pulse 1a our observational data of ice elevation changes compare well with predictions of a thermomechanical icesheet model that incorporates very low basal shear stress downstream of the present day grounding line we conclude that ice streams in the ross sea embayment had thin lowslope profiles during the last glaciation and interior wais ice elevations during this period were several hundred meters lower than previous reconstructions citation ackert r p jr s mukhopadhyay b r parizek and h w borns 2007 ice elevation near the west antarctic ice sheet divide during the last glaciation geophys res lett 34 l21506 doi 10 1029 2007gl031412

deglacial history of the west antarctic ice sheet in the weddell sea embayment constraints on past ice volume change the retreat history of the west antarctic ice sheet wais since the last glacial maximum is important for understanding the process of rapid deglaciation constraining models that seek to predict the future trajectory of the ice sheet and for estimating rates of sea level change here we report new glacial geologic data from the southwestern weddell sea embayment that demonstrate that this part of the wais was thinner than previously suggested and that there was progressive thinning of the ice sheet by 230 480 m since ca 15 ka we use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the weddell sea at the last glacial maximum the volume of this ice would have added between 1 4 and 2 0 m to postglacial sea level rise and would not have been sufficient to contribute significantly to meltwater pulse 1a a rapid rise in sea level 14 200 yr ago

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

chronology of taylor glacier advances in arena valley antarctica using in situ cosmogenic 3he and 10be abstract in situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits in situ3he and 10be in quartz sandstone boulders from arena valley southern victoria land antarctica provide chronological constraints for a sequence of moraines taylor ii ivb moraines related to expansions of taylor glacier and the east antarctic ice sheet mean 3he ages are 113 000 45 000 yr 208 000 67 000 yr 335 000 187 000 yr and 1 2 0 2 myr for taylor ii iii iva and ivb moraines respectively mean 1 corresponding mean 10be ages for taylor ii and ivb moraines are 117 000 51 000 yr and 2 1 0 1 myr for the older deposits the 3he ages are probably lower limits due to diffusive loss although the exposure ages appear consistent with the few previous age estimates particularly with an isotope stage 5 age for taylor ii each moraine exhibits a broad age distribution the distribution probably results from a variety of factors which may include prior exposure to cosmic rays 3he loss erosion postdepositional boulder movement and radiogenic production of 3he nonetheless the exposure ages provide direct chronological constraints for the moraine sequence and suggest a maximum thickening of taylor glacier relative to the present ice surface of 500 m since the late pliocene early pleistocene

climatic oscillations 10 000 155 000 yr b p at owens lake california reflected in glacial rock flour abundance and lake salinity in core ol 92 earlier wetter cycles the lake overflowed to fill a series chemical analyses of the acid soluble and clay size fractions of of downstream basins including china lake basin searles sediment samples 1500 yr resolution reveal oscillations of lake valley panamint valley and ultimately death valley salinity and of glacial advances in core ol 92 back to 155 000 yr smith and street perrott 1983 in 1992 the u s geological b p relatively saline conditions are indicated by the abundance survey drilled a 323 m deep core ol 92 into owens lake of carbonate and smectite both pedogenic and authigenic re sediments near the depocenter of the basin to obtain a continflected by ca sr and mg in the acid soluble suite and by cs2o uous record of silty clay sediment spanning the last 800 000 excess mgo and loi loss on ignition in the clay size fraction yr a multiparameter reconnaissance study of the entire core rock flour produced during glacial advances is represented by ca 7000 yr resolution reported in a 13 chapter summary the abundance of detrital plagioclase and biotite in the clay size volume smith and bischoff 1997 revealed cycles characfraction the ratio of which remains essentially constant over the terized by closed and overflowing lake conditions entire time span these phases are quantitatively represented by na2o tio2 ba and mn in the clay fraction the rock flour record we report here the results of a second part of an intermediindicates two major ice advances during the penultimate glacial ate resolution study aimed at resolving shorter term events cycle corresponding to marine isotope stage mis 6 no major over just the past 155 000 yr using sediment samples of ca advances during the last interglaciation entire mis 5 and three 1500 yr resolution taken over the uppermost 90 m of olmajor advances during the last glacial cycle mis 2 3 and 4 92 the companion study menking et al 1997 reports on the ages of the latter three correspond rather well to 36 cl dates grain size tic total inorganic carbon content and stable reported for sierra nevada moraines the onset of the last intergla isotopic measurements proxies which tend to reflect variaciation is shown by abrupt increases in authigenic caco3 and an tions in water flow and which are most representative of abrupt decrease in rock flour at about 118 000 yr b p according variations during closed lake conditions the present study to our time scale in contrast the boundary appears to be gradual was concerned primarily with seeking a proxy for glacial in the d 18 o record in which the change from light to heavy values advances we found that the mineralogy and chemistry of begins at about 140 000 yrs b p the exact position of the termination therefore may be proxy dependent conditions of high car the clay fraction provide indices of glacial rock flour we bonate and low rock flour prevailed during the entire period from also report on the composition of the acid soluble carbonate 118 000 yr b p until the glacial advance at 53 000 yr b p signaled fraction which gives further detail on closed lake conditions

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 2 sedimentologic and stratigraphic signature sedimentologic geotechnical geochemical and accelerator mass spectrometer ams radiocarbon data from two marine geologic cruises in the ross sea have allowed us to constrain facies relationships and temporal changes in the west antarctic ice sheet the selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality chirp subbottom reflection system a complex but consistent succession of facies documents a number of environments from subglacial to open marine massive mud rich diamictons have low water contents contain in places a calcareous microfossil assemblage show minimal textural variation and contain low and uniform total organic carbon values this reflects a subglacial setting this unit passes upward into a stratified thin granulated facies consisting of pelletized sandy muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble sized clasts this facies reflects the lift off zone or thin water film between the basal debris and sea floor overlying this unit are silty clays that contain a well sorted very fine grained sand component there are no coarse grains within this unit this facies is in part laminated and reflects deposition beneath an ice shelf near and away from the grounding line zone the ice shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open marine setting a sandy volcaniclastic rich subfacies marks the transition from ice shelf to open marine environments found at the calving line the 86 ams radiocarbon dates on organic matter provide an accurate chronology for 19 cores ice shelf conditions were established in the outer drygalski trough by 11 0 25 ka and perhaps earlier this transition took place in the joides basin by 10 8 ka the calving front of the ross ice shelf passed over the drygalski trough at 74 s by 9 5 0 25 ka the timing of deglaciation in the ross sea calls into question current models for the contribution of antarctic glacial ice to holocene sea level rise and suggests that recession was relatively gradual and more closely aligned with northern hemisphere deglaciation and its associated eustatic pulse

the fossil geothermal system of the baden baden trough northern black forest f r germany abstract limnic sediments of the upper carboniferous lower permian and triassic from the baden baden trough were hydrothermally altered during the jurassic concentric alteration halos each having a characteristic mineral composition were formed from center to margin zones of sericitization albitization and weak alteration covering an area of 100 km2 are evident these zones indicate decreasing hydrothermal influence whereby the solutions must have risen in the center the chemistry of mica changing systematically from a muscovitic toward an illitic composition also reflects this trend the boron content increases from 75 to 205 ppm the estimated maximum alteration temperature ranges between 240 and 290 c these remarkable changes are obviously early manifestations of geothermal events related to the development of a continental rift rhine graben

exposure ages from mountain dipsticks in mac robertson land east antarctica indicate little change in ice sheet thickness since the last glacial maximum past changes in east antarctic ice sheet eais volume are poorly known and diffi cult to measure yet are critical for predicting the response of the ice sheet to modern climate change in particular it is important to identify the sources of sea level rise since the last glacial maximum lgm and ascertain the present day stability of the world s largest ice sheet we present altitudinal transects of 10 be and 26 al exposure ages across the framnes mountains in mac robertson land that allow the magnitude and timing of eais retreat to be quantifi ed our data show that the coastal eais thinned by at most 350 m in this region during the past 13 k y this reduction in ice sheet volume occurred over a 5 k y period and the present icesheet profi le was attained ca 7 ka in contrast to the west antarctic ice sheet which continues to retreat today combined with regional offshore and terrestrial geologic evidence our data suggest that the reduction in eais volume since the lgm was smaller than that indicated by contemporary ice sheet models and added little meltwater to the global oceans stability of the ice margin since the middle holocene provides support for the hypothesis that eais volume changes are controlled by growth and decay of northern hemisphere ice sheets and associated global sea level changes

surface exposure chronology using in situ cosmogenic 3he in antarctic quartz sandstone boulders abstract in situ produced cosmogenic helium 3 he c provides a new tool for constraining histories of quaternary geomorphic surfaces before general application of the technique however the systematics and production rates of 3 he c must be well understood in a companion study 3 he and 10 be data from sandstone and granite boulders in the dry valleys region of antarctica have been used to constrain the ages of an important moraine sequence formed by the taylor glacier data from these deposits also provide information about the systematics of 3 he in quartz that has important implications for geochronology based on 3 hec in contrast to previous results from olivine and clinopyroxene crushing quartz in vacuo releases helium with high 3 he 4 he ratios up to 148 r a where r a is the atmospheric 3 he 4 he ratio indicating that crushing cannot be used to determine the isotopic composition of trapped i e noncosmogenic helium in quartz analysis of 3 he in different size fractions of the same samples indicates significant 3 he loss not predicted by existing 3 he diffusion data for quartz the origin of the discrepancy is not clear but loss from these samples is not as significant as suggested by the limited data of previous studies

deglaciation of pope glacier implies widespread early holocene ice sheet thinning in the amundsen sea sector of antarctica abstract the amundsen sea sector of the antarctic ice sheet presently dominates the contribution from antarctica to sea level rise several large ice streams that currently drain the sector have experienced rapid flow acceleration grounding line retreat and thinning during the past few decades however little is known of their longer term millennial scale retreat history despite the reliance of several ice sheet and glacial isostatic adjustment models on such data for improving sea level prediction from this critical region this study investigates the timing and extent of surface lowering of one of those ice streams pope glacier since the last glacial maximum lgm using glacial geological evidence for former ice cover we present a new deglacial chronology for the glacier derived from surface exposure dating of glacially deposited cobbles and ice scoured bedrock from mount murphy and its surrounding peaks cosmogenic 10be exposure ages from 44 erratic cobbles and 5 bedrock samples and in situ 14c exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5 5 16 ka although 10be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples none of the ages pre date the lgm from these results we infer that the surface of pope glacier lowered by 560 m during the early to mid holocene 9 6 ka at an average rate of 0 13 0 09 0 04 m yr 1 the lowering coincided with a period of enhanced upwelling of warm circumpolar deep water onto the continental shelf in the region a reduction in buttressing facilitated by such upwelling by an ice shelf that is thought to have spanned the embayment until 10 6 cal kyr bp could have triggered simultaneous early holocene thinning of pope glacier and glaciers elsewhere in the amundsen sea embayment

crustal deformation across the sierra nevada northern walker lane basin and range transition western united states measured with gps 2000 2004 1 global positioning system gps data collected in campaigns in 2000 and 2004 were processed and interpreted with other gps data in the western basin and range province to provide new constraints on the rate style and pattern of deformation of the central and northern walker lane wl which lies near the western boundary of the basin and range across the central wl near 38n latitude the velocities with respect to north america increase westward by 10 mm yr inducing dextral shear farther north between 40 and 41n latitude a western zone of 7 mm yr relative motion undergoes dextral shear and an eastern zone of 3 mm yr relative motion undergoes extension and shear these data show that the northern wl is essentially a dextral shear zone experiencing minor net dilatation ed 2 6 0 8 nstrain yr near most holocene normal faults dilatation inferred from the velocity field is not greater than the uncertainties however near the central nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts ed 23 0 3 9 nstrain yr some of which is attributable to transient postseismic deformation following large historic earthquakes a block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the honey lake warm springs east pyramid fault system and mohawk valley faults is 7 mm yr the wl is a zone whose width and dilatation rate increase northwestward consistent with counterclockwise rotation of the sierra nevada microplate and transfer of deformation into the pacific northwest

reassessment of the potential sea level rise from a collapse of the west antarctic ice sheet theory has suggested that the west antarctic ice sheet may be inherently unstable recent observations lend weight to this hypothesis we reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution we obtain a value for the global eustatic sea level rise contribution of about 3 3 meters with important regional variations the maximum increase is concentrated along the pacific and atlantic seaboard of the united states where the value is about 25 greater than the global mean even for the case of a partial collapse