implications of till deformation on glacier dynamics the dynamics of glacier motion are governed to a large extent by the properties of the basal interface in this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of black rapids glacier alaska u s a we also investigate whether or not a till layer has a clear surface observable signature in the dynamics of the glacier towards this end we use a finite element ice flow model with a coulomb failure criterion within the basal till layer we find that simple till physics can be used to describe decadal seasonal and short term hours to days velocity variations and possibly uplift events mechanisms for each of these variations involve an increase in the extent of till at failure a transfer of shear stress across the bed and a consequent increase in ice deformation effective shape factors are calculated that permit a simple incorporation of this boundary condition into glacier response models our analyses however have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier

equilibrium line altitude variations with latitude today and during the late wisconsin abstract present day and late wisconsin glacier equilibrium line altitudes ela were compared on a transect from patagonia along the coastal cordillera to alaska and a transect from mexico along the sierra madre rocky mountains to alaska late wisconsin ela s were determined from cirques occupied during the late wisconsin and present day ela from observations of active cirque glaciers the mean change in ela from the late wisconsin to the present was 735 m with a standard deviation of 120 m this indicates that the change in snowline altitude from the present to the late wisconsin was approximately uniform and not latitudinally dependent

a new technique to determine element amounts down to femtograms in dust using femtosecond laser ablation inductively coupled plasma mass spectrometry abstract atmospheric dust has a substantial impact on climate and climate in turn affects the dust flux and origin knowing its composition helps to determine dust sources and climate processes we have therefore determined 46 major and trace elements in dust samples using in situ 200 nm femtosecond laser ablation inductively coupled plasma mass spectrometry la icp ms very small test portions of 4 7 g were enough to enable analyses down to detection limits of femtograms and ng g 1 respectively the data agree within 40 to 15 with the reference values for test portions of 1 to 100 g respectively the femtosecond laser results are compared to those obtained using a 213 nm nanosecond laser which show the advantages of the femtosecond laser such as the ability to use non matrix matched calibration reliable measurements of volatile elements and smaller quantities of material ablation with similar results for the repeatability the technique is especially useful for measurements of small amounts of dust on filters and for 2d distribution maps of selected elements to identify minerals or contamination to demonstrate the applicability five dust samples from niger and chad in the sahara zone were analyzed the samples are characterized by enrichment in light rees and a distinct eu anomaly with surface soils as their source

magnetostratigraphy of late miocene continental deposits in samos greece abstract we present a detailed magnetostratigraphic study of the upper part of the mytilini formation marker tuffs and main bone beds of samos island greece seven successive polarity zones have been identified and a correlation with the geomagnetic polarity time scale has established that the sediments of this section were accumulated between 5 7 and 6 8 ma according to these results the upper mammalian fossiliferous levels are found within the reversed polarity zone lying in the time interval 6 1 6 4 ma

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

saharan wind regimes traced by the sr nd isotopic composition of subtropical atlantic sediments last glacial maximum vs today new nd sr isotopic data on the 30 m lithic particles of surface and last glacial maximum sediments recovered along the african margin between the equator and the gibraltar strait are presented in combination with grain size measurements this 30 m size fraction allows us to eliminate any hemipelagic contribution that could occur in the coarser fractions in the eolian fraction both sr and nd isotopic tracers reveal the same major northwestern origin mauritania mali southern algeria and morocco the archaean formations of the western saharan shield could be the source of the very unradiogenic ratios observed here the more southern regions senegal guinea act only as secondary sources a similar pattern is observed for the lgm lithic particles are mostly transported by both trade and saharan air layer sal winds along an approximate ne sw axis this main feature matches the southern plume characterizing the dust transport observed during winter no significant latitudinal shift of the belt winds is observed between the lgm and today at the lgm however dust fluxes were 2 4 times higher than today leading to a more archaean type imprint in the deposits we do not observe any clear relationship between the latitudinal variability of the upwelling systems identified in this region at the lgm and the location of the major wind systems both enhanced aridity on the continent and increased wind speed probably occurred together over western tropical africa during the last glacial period

rock varnish on petroglyphs from the hima region southwestern saudi arabia chemical composition growth rates and tentative ages we investigated rock varnish formed on sandstone and petroglyphs in the hima area southwestern saudi arabia to characterize the rock varnish we made in situ measurements by portable x ray fluore

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

laurentide ice sheet instability during the last deglaciation the factors leading to the full retreat of ice sheets during deglaciation are debated numerical modelling suggests that the laurentide ice sheet retreated only after a threshold for warming and radiative forcing was passed in the holocene

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

magnetostratigraphy of the middle miocene continental sedimentary sequences of the mae moh basin in northern thailand evidence for counterclockwise block rotation abstract a magnetostratigraphic study has been performed on the mae moh basin well known for its lignite mining in the lampang province paleomagnetic samples were collected from 66 stratigraphic levels rock magnetic investigations indicate the presence of low coercivity minerals specimens subjected to progressive thermal or alternating field demagnetization procedures show that nearly all of them exhibit a low temperature magnetization component and a high temperature characteristic component chrm with either positive or negative virtual geomagnetic pole latitudes and opposite polarity considered as the characteristic miocene magnetization the overall mean paleomagnetic direction incl 22 2 decl 358 3 15 95 4 documents a counterclockwise vertical axis rotation of about 13 1 32 with respect to the expected miocene direction derived from the eurasian polar wander curve incl 42 decl 11 our paleomagnetic results are not consistent with the previously reported paleomagnetic data the rotation observed in the mae moh basin can be the response to local tectonics a section sampled for magnetostratigraphy reveals a polarity sequence of nine magnetozones that can reliably be correlated to the geomagnetic polarity time scale according to biochronological constraints the magnetostratigraphic results from the mammal bearing succession correlate with chron c5abn c5an2n between 13 5 and 12 1 ma according to the mean sedimentation rate of about 17 5 cm ka ages of 12 5 and 12 8 ma are proposed for the fossiliferous levels j5 and k1 k2 lignite zones were the mammal remains were found

grain size control on sr nd isotope provenance studies and impact on paleoclimate reconstructions an example from deep sea sediments offshore nw africa five sediment cores from offshore nw africa were analyzed for strontium and neodymium isotope ratios to reconstruct temporal variations in continental weathering regimes sediments were taken from three time slices with well known and distinctive environmental conditions present day dry and warm similar to 6 ka wet and warm and similar to 12 ka dry and cold terrigenous sediment samples were split into two size fractions to distinguish between the two dominant transport mechanisms offshore nw africa fluvial 0 10 mu m and aeolian 10 40 mu m sr isotope data record evidence of marked grain size control with higher isotopic ratios in the fine fraction in contrast epsilon nd values are largely unaffected by grain size minor variability in nd isotope data at each sampling site indicates near constant sources of terrigenous matter over the last similar to 12 ka variations in sr isotope ratios are interpreted to reflect major changes in the evaporation precipitation balance we suggest that the sr nd isotope data record a latitudinal shift of the northern limit of the african rainbelt and associated wind systems causing changes in the humidity and rate of chemical weathering over nw africa while hyperarid conditions prevailed similar to 12 ka more humid conditions and intensified monsoonal rainfall at similar to 6 ka resulted in greater breakdown of easily weathered k bearing phases and increased sr 87 sr 86 in the detritus in late holocene times the monsoonal circulation diminished resulting in a return to arid conditions our results clearly show that it is of vital importance in paleoenvironmental studies to carry out isotopic analyses on individual sediment fractions that were carried to the studied deposition site by distinct sediment transport mechanisms if isotopic analyses are carried out on bulk sediments the observed variability in isotopic values most likely represents changes in the particle size and mixing proportions of the sediment subpopulations

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

shallow shelf approximation as a sliding law in a thermomechanically coupled ice sheet model 1 the shallow shelf approximation a balance of membrane stresses for ice flow is an effective sliding law for ice sheet modeling our use of it as a sliding law becomes a standard model for ice stream flow when the sliding velocity is large 100 m a 1 and faster following schoof 2006a we describe the basal resistance as plastic till for which the yield stress is given by a mohr coulomb formula pore water pressure is related to basal melt rate the velocity field used in the mass continuity and conservation of energy equations is an average of velocities from the shallow shelf approximation and the nonsliding shallow ice approximation using this scheme our model has realistic time dependent ice streams which exhibit the range of surface velocities seen in actual ice streams we demonstrate the model at high spatial resolution 5 km grid over multiple millenia using its implementation in the parallel ice sheet model numerical experiments show that the entire scheme is stable with respect to many parameter changes some experiments reveal significant ice stream variability in a hypothetical steady climate with characteristic cycles on the order of 1000 years we believe this is the first practical whole ice sheet model with a unified treatment of vertical shear stresses and membrane stresses it is capable of high resolution thermomechanically coupled multimillenia simulations of ice sheets containing ice streams

microanalytical methods for in situ high resolution analysis of rock varnish at the micrometer to nanometer scale abstract a wide range of analytical techniques were used to investigate rock varnish from different locations negev israel knersvlakte south africa death valley and mojave desert california a 200 nm femtosecond laser ablation inductively coupled plasma mass spectrometer la icp ms an electron probe microanalyzer epma focused ion beam fib slicing and scanning transmission x ray microscopy near edge x ray absorption fine structure spectroscopy stxm nexafs this combination enables comprehensive high spatial resolution analysis of rock varnish femtosecond la icp ms and epma were used for quantitative determination of element concentrations in situ measurements were conducted on thick and thin sections with a resolution of 10 40 m and 2 m respectively the results demonstrate that some elements such as mn co pb ni and cu are highly enriched in varnish relative to the upper continental crust up to a factor of 100 the varnish composition is not influenced by the composition of the underlying rock which is witnessed by plots of mno 2 vs sio 2 contents furthermore the mn free end members fall in the range of average dust compositions the varnishes from the various locations show distinct differences in some elemental ratios in particular mn fe 0 3 25 1 mn ba 4 170 ni co 0 03 1 8 and pb ni 0 4 23 the rare earth element ree patterns vary with la n yb n 3 5 12 and different degrees of ce anomalies ce ce 1 5 5 3 to study the internal structure of the varnish 100 200 nm thick fib slices were prepared and mappings of fe mn n co 3 2 ca c and si at the nm scale performed banded internal structures of mn fe and organic c were observed in the israeli and californian samples however no fe rich layers are present in the south african rock varnish samples furthermore cavities were found that are partly filled by c fe and mn rich material internal structures are different for varnish from different locations which might reflect different types of genesis the results of the combined microanalytical techniques give important detailed insights towards unraveling the genesis of rock varnish

stress gradient coupling in glacier flow i longitudinal averaging of the influence of ice thickness and surface slope for a glacier flowing over a bed of longitudinally varying slope the influence of longitudinal stress gradients on the flow is analyzed by means of a longitudinal flow coupling equation derived from the vertically cross sectionally integrated longitudinal stress equilibrium equation by an extension of an approach originally developed by budd 1968 linearization of the flow coupling equation by treating the flow velocity u vertically averaged ice thickness h and surface slope in terms of small deviations u h and a from overall average datum values u 0 h 0 and 0 results in a differential equation that can be solved by green s function methods giving u x as a function of h x and x x being the longitudinal coordinate the result has the form of a longitudinal averaging integral of the influence of local h x and x on the flow u x u x u0 2l l 0 ln n nh n l exp x x l dx where the integration is over the length l of the glacier the operator specified deviations from the datum state and the term on which it operates which is a function of the integration variable x represents the influence of local h x x and channel shape factor x at longitudinal coordinate x on the flow u at coordinate x the influence being weighted by the influence transfer function exp x x l in the integral the quantity l that appears as the scale length in the exponential weighting function is called the longitudinal coupling length it is determined by rheological parameters via the relationship l 2h n n 3n where n is the flowlaw exponent n the effective longitudinal viscosity and n the effective shear viscosity of the ice profile n is an average of the local effective viscosity n over the ice cross section and n 1 is an average of n 1 that gives strongly increased weight to values near the base theoretically the coupling length l is generally in the range one to three times the ice thickness for valley glaciers and four to ten times for ice sheets for a glacier in surge it is even longer l 12h it is distinctly longer for non linear n 3 than for linear rheology so that the flow coupling effects of longitudinal stress gradients are markedly greater for non linear flow the averaging integral indicates that the longitudinal variations in flow that occur under the influence of sinusoidal longitudinal variations in h or with wavelength are attenuated by the factor 1 1 2 l 2 relative to what they would be without longitudinal coupling the short intermediate and long scales of glacier motion raymond 1980 over which the longitudinal flow variations are strongly partially and little attenuated are for 2l 2l 20l and 20l for practical glacier flow calculations the exponential weighting function can be approximated by a symmetrical triangular averaging window of length 4l called the longitudinal averaging length the traditional rectangular window is a poor approximation because of the exponential weighting the local surface slope has an appreciable though muted effect on the local flow which is clearly seen in field examples contrary to what would result from a rectangular averaging window tested with field data for variegated glacier alaska and blue glacier washington the longitudinal averaging theory is able to account semi quantitatively for the observed longitudinal variations in flow of these glaciers and for the representation of flow in terms of effective surface slope values exceptions occur where the flow is augmented by large contributions from basal sliding in the ice fall and terminal zone of blue glacier and in the reach of surge initiation in variegated glacier the averaging length 4l that gives the best agreement between calculated and observed flow pattern is 2 5 km for variegated glacier and 1 8 km for blue glacier corresponding to l h 2 in both cases if l varies with x but not too rapidly the exponential weighting function remains a fairly good approximation to the exact green s function of the differential equation for longitudinal flow coupling in this approximation l in the averaging integral is l x but is not a function of x effects of longitudinal variation of l are probably important near the glacier terminus and head and near ice falls the longitudinal averaging formulation can also be used to express the local basal shear stress in terms of longitudinal variations in the local stope stress with the mediation of longitudinal stress gradients

numerical study of winter water formation in the chukchi sea roles and impacts of coastal polynyas winter water formation is examined in the chukchi sea for the winters of 1992 2006 using a primitive equation ocean model forced by ncep wind and surface salinity flux derived from ssm i thin ice thickness estimates the model is also forced by an external inflow of 0 8 sv through the bering strait the model successfully reproduces the oceanic circulation on the chukchi shelf thus providing numerous insights into behaviors of salt enriched water produced on the shelf the experiments show that under northeasterly winds northward throughflow across barrow canyon is reduced this results in salinity buildup under freezing conditions and ultimately in greater enhancement of salinity of the waters carried into the arctic basin the flow and salinity enhancement of the flow through herald canyon is less extreme but more steady than through barrow canyon together with moored salinity in the bering strait the model results estimate the actual salinity to be 32 9 0 8 psu and 32 7 0 3 psu respectively for waters moving through the barrow and herald canyons both estimates are less than 33 1 psu that is typically observed for the cold halostad layer in the canada basin suggesting the importance of diapycnal mixing with saltier atlantic origin water

sensitivity experiments with a model of the ice age cycle the response to milankovitch forcing abstract we address the question of the extent to which a simple one dimensional time dependent climate model is able to explain the fluctuations of continental ice volume which have occurred during the last quarter of the pleistocene epoch the model includes explicit ice physics based on the glen flow law and explicit glacial isostatic adjustment based on a realistic viscoelastic field theory when the model is forced by the milankovitch variations of insulation it delivers ice volume fluctuations which are very similar to those implied by the specmap 18o record in particular the model appears capable of explaining the observed correlation between eccentricity and ice volume variations the dominance of the 105 year period in the power spectrum of the ice volume time series and the thickness and latitudinal extents of the continental ice sheets which existed at wurm wisconsin maximum 18 000 years ago

deglaciation of a soft bedded laurentide ice sheet we present a series of numerical reconstructions of the laurentide ice sheet during the last deglaciation 18 7 14c ka that evaluates the sensitivity of ice sheet geometry to subglacial sediment deformation these reconstructions assume that the laurentide ice sheet flowed over extensive areas of water saturated deforming sediment soft beds corresponding to the st lawrence lowland the great lakes region the western prairies of the u s and canada and the hudson bay and hudson strait regions sediment rheology is based on a constitutive law that incorporates experimental results from late wisconsin till deposited by the laurentide ice sheet which suggest only mildly nonlinear viscoplastic behavior by varying the effective viscosity of till we produced four reconstructions for the ice sheet during the last glacial maximum 18 14c ka and two reconstructions each of the ice sheet at 14 13 12 11 and 10 14c ka we also produced one reconstruction for 9 8 4 8 and 7 14c ka reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl shaped depression over hudson bay and thin ice 1000 m above modern sea level over the western and southern margins those reconstructions that assume a higher effective viscosity of till in the hudson bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over hudson bay and a more northerly position of the central ice divide these two different geometries may represent ice sheet orographic changes associated with a heinrich event further increases in effective viscosity of till approaching the effective viscosity of ice would result in a high monolithic ice dome centered over hudson bay reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology modeled ice surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds our reconstructions with higher effective till viscosities in hudson bay also agree with the ice 4g reconstructions peltier 1994 which are based on inversion of relative sea level data for the early part of the last deglaciation 18 13 14c ka but then depart significantly from ice 4g beginning at about 12 14c ka due to differing assumptions of the history of deglaciation modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50 55 m by a soft bedded laurentide ice sheet this would increase as the effective viscosity of till increases subsequent ice volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at barbados new guinea and tahiti but suggest that the laurentide ice sheet was not the source of meltwater pulse 1a

a low order dynamical model of global climatic variability over the full pleistocene a previously formulated dynamical model of the late pleistocene ice ages based on the hypothesis that the global co sub 2 system can provide the instability to drive a natural oscillation involving feedbacks between the cryosphere atmosphere and ocean is extended to include 1 additive earth orbital forcing summer insolation changes at 65 degree n and 2 tectonic forcing in the form of a postulated variation in the multiplicative parameters rate constants of the model system the structural e g bifurcation properties of the model are examined in detail to reveal the regions of parameter space wherein the geologically inferred features of the full pleistocene can be simulated including the observed chronology the phase relationships between ice co sub 2 and north atlantic deep water formation and the mid pleistocene transition

bulk composition of northern african dust and its source sediments a compilation abstract this paper presents a review of bulk compositional data of northern african dust and its potential source sediments and includes elemental isotope and mineralogical data northern african dust represents about one half of the total global atmospheric mineral dust burden and its uplift transport and deposition have strong impacts on climate and various terrestrial and marine ecosystems the chemical data set shows that an average northern african dust exhibits comparable si fe and mn contents with respect to the average composition of the upper continental crust is slightly depleted in the alkali metals k and na and enriched in ti and p however the complete data set yields clear evidence that northern african dust and its source sediments are compositionally heterogeneous on a regional scale and that this heterogeneity can be used to differentiate between major potential source areas on the basis of so called source markers an evaluation of these compositional fingerprints shows that the following parameters and especially their combination are effective in the discrimination of the most active source areas in northern africa ratio of ca mg fe wt calcite or carbonate content palygorskite occurrence and abundance illite kaolinite ratio nd 0 value and 87sr 86sr ratio for example the data set corroborates previous ideas which assign carbonate illite and palygorskite rich mineral dusts to north west ern source regions because most of the above listed source markers do not change substantially during transport even far traveled dusts may be assigned to specific potential source areas in northern africa some limitations of the presented data set are also discussed our compilation reveals some substantial gaps in the knowledge of the composition of source sediments and mineral dusts from important potential source areas that should be filled in the future the here compiled data set can be used as a reference frame when incorporating the composition of source sediments e g mineralogy into global or regional dust transport models and can be compared with source analysis by remote sensing or back trajectory analysis however source apportionment studies supported by our data set will not only be useful for actual dust samples but will also be helpful for the understanding of paleo wind directions and hence paleo climatological conditions through the investigation of quaternary eolian sediments deposited in and around northern africa

a thermomechanical model of ice flow in west antarctica this study uses a three dimensional thermo mechanical model to investigate the internal flow dynamics of the west antarctic ice sheet wais the model allows ice thickness flow and temperature to interact freely its domain is prescribed as that of the present day grounded wais realistic present day climatic and topographical boundary conditions are employed the analysis of a series of experiments pays particular attention to the location and dynamics of concentrations of ice flow ice streams underlying topographic troughs are crucial in determining the strength and location of these concentrations of flow the flow pattern generated by subglacial troughs is made more distinct by the inclusion of ice flow temperature coupling the inclusion of sliding leads to the generation of limit cycles in the ice flow they are concentrated around the present day ice streams b and c of the siple coast and have a period of 5 to 10 ky there appears to be competition between several preferred ice flow pathways in this area the two end members of the flow regime are a strong ice stream c with a weakened ice stream a b complex and strong ice streams a and b with a dormant ice stream c ice streams appear to require ice discharges above a certain threshold in order to maintain frictional heat generation and fast flow individual ice streams can therefore interact through changes in catchment area size a reduction in catchment area reduces the volume of ice entering a stream and can cause stagnation as the amount of frictional heating falls

geochemical insights into the relationship of rock varnish and adjacent mineral dust fractions abstract rock varnishes are m thin dark manganese mn rich crusts that accrete in the order of few m ka on weathering resistant lithologies although these crusts can form in all climates they are best known in arid to semi arid settings aeolian dust is understood as a major contributor to the distinct trace metal and ree enrichments in rock varnish however the exact proportions of abiotic and biotic formation mechanisms that may explain the oxidation reactions of mn2 to mn4 present as mn oxyhydroxides in the varnish are still a matter of ongoing debate we present here the first systematic study of trace element enrichment processes between the uppermost layer of the varnish sequence and their adjacent 50 m dust fractions across a selection of 41 major and trace elements this approach is used to investigate samples from three fully arid deserts the an nafud in saudi arabia the negev in israel and the mojave in the usa which are compared to the significantly different environment of the semi arid knersvlakte in south africa a new in situ trace element analysis protocol was developed to perform femto and nanosecond sector field la icp ms microanalyses at high spatial resolution which allows us to measure the most recent varnish layers for their comparison with recent dust in agreement with previous studies all varnishes are enriched in mn pb ce co ba zn ni and the rare earth elements ree here we demonstrate that fine 50 m has low trace element mass fractions and enrichment patterns plotting distinctly away from varnish and fine dust based on these geochemical patterns our results indicate a general enrichment mechanism from fine dust to varnish previous studies suggested dust to play an integral role in providing the trace elements that are incorporated into the varnish by ph eh fluctuations in short term rain and fog events we amalgamate and refine previous growth models by providing direct evidence that leaching of about 10 of the mn and other trace elements from clay minerals in rain or fog droplets at ph 5 and subsequent scavenging on varnish surfaces at ph 8 leads integrated over time to the distinct enrichment patterns of the varnish while initial mn oxyhydroxide formation is suggested to follow pathways of metal oxide mediated photo catalysis for the semi arid occurrence in the knersvlakte we present a distinct growth model as both environment varnish and dust composition differ significantly from the arid settings here thick metallic looking varnish occurs mainly on the rim of quartz pebbles lacks microlamination and likely has upscaled growth processes among other aspects we suggest a more complex interplay between photo catalysis nocturnal condensation events on quartz pebbles with subsequent water trapping at the rock soil atmosphere interface due to a salic top soil layer to mainly account for these differences

eocene krabi basin southern thailand paleontology and magnetostratigraphy we present the magnetostratigraphy of the late eocene krabi section from the krabi basin thailand the investigated section is about 105 m thick and consists of red and yellow siltstones sandstones gray claystones lignites and limestones we collected 86 samples at 43 different stratigraphic levels from the 105 m thick section the samples yielded a paleomagnetic direction and paleomagnetic analyses give favorable demagnetization behaviors two magnetic components can be isolated rock magnetic experiments show that the natural remanent magnetization generally results from the presence of titanomagnetite the first component has roughly the direction of the present day field the second component displays only reversed polarity with a mean direction declination 192 34 inclination 15 5 n 71 95 4 7 k 14 after bedding correction mammalian biostratigraphy indicates a late eocene age correlation to the geomagnetic polarity time scale relies on biostratigraphic data previously proposed for the fauna found in the same section these correlations have been achieved by comparing variations in the sedimentation rate derived from the alternative correlation the proposed correlation puts the krabi section in chron c12r or chron c13r this suggests sedimentation rates of 4 9 and 9 4 cm k y respectively this correlation suggests that the krabi section is older than 31 ma and younger than 34 ma

rapid grounding line migration induced by internal ice stream variability observations indicate that the grounding line position of west antarctica is sensitive to both forced and unforced ice stream variabilities this study endeavors to characterize and understand unforced ice stream variability and associated grounding line migration we use a flowline ice stream model with an undrained plastic bed lateral shear stresses and a stretched grid refined in the grounding zone this model exhibits parameter space structure and hysteresis behavior similar to simpler ice stream models low prescribed temperature at the ice surface or weak geothermal heating produces thermal oscillations between active and stagnant phases as in previous spatially resolved ice flow models thermal activation propagates as an activation wave this model s fine resolution of the grounding zone allows for accurate simulations of transient unforced grounding line migration upstream of the grounding zone horizontal grid spacing of 1 km is required to accurately resolve activation waves activation waves induce the grounding line to migrate over 100 km at a rate that can exceed 1 km yr this is followed during the active phase by retreat which then continues for the duration of the stagnant phase grounding line retreat is the result of a negative mass balance near the grounding line but is not necessarily associated with negative mass balance for the entire ice stream in our simulations of internal variability the novel approach and experiments described in this study show that there can be large excursions in grounding line position in the absence of either external forcing or retrograde slopes

sea ice as the glacial cycles climate switch role of seasonal and orbital forcing a box model of the coupled ocean atmosphere sea ice and land ice climate system is used to study glacial interglacial oscillations under seasonally and orbitally varying solar forcing the dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records and to zeroth order it does not depend on the seasonal and milankovitch forcing the sea ice controls via its albedo and insulating effects the atmospheric moisture fluxes and precipitation that enable the land ice sheet growth this control and the rapid growth and melting of the sea ice allow the sea ice to rapidly switch the climate system from a growing ice sheet phase to a retreating ice sheet phase and to shape the oscillation s sawtooth structure a specific physical mechanism is proposed by which the insolation changes act as a pacemaker setting the phase of the oscillation by directly controlling summer melting of ice sheets this mechanism is shown to induce deglaciations during periods of lower summer insolation superimposed on the 100 kyr are the linear milankovitch forced frequencies of 19 23 and 41 kyr the transition from 41 kyr glacial cycles to 100 kyr cycles one million years ago may be explained as being due to the activation of the sea ice switch at that time this would be the case if sea ice extent was more limited during the warmer climate of the early pleistocene

paleomagnetism of the middle late jurassic to cretaceous red beds from the peninsular thailand implications for collision tectonics abstract jurassic to cretaceous red sandstones were sampled at 33 sites from the khlong min and lam thap formations of the trang syncline 7 6 n 99 6 e the peninsular thailand rock magnetic experiments generally revealed hematite as a carrier of natural remanent magnetization stepwise thermal demagnetization isolates remanent components with unblocking temperatures of 620 690 c an easterly deflected declination d 31 1 i 12 2 95 13 9 n 9 in stratigraphic coordinates is observed as pre folding remanent magnetization from north trang syncline whereas westerly deflected declination d 342 8 i 22 3 95 12 7 n 13 in geographic coordinates appears in the post folding remanent magnetization from west trang syncline these observations suggest an occurrence of two opposite tectonic rotations in the trang area which as a part of thai malay peninsula received clockwise rotation after jurassic together with shan thai and indochina blocks between the late cretaceous and middle miocene this area as a part of southern sundaland block experienced up to 24 5 11 5 counter clockwise rotation with respect to south china block this post cretaceous tectonic rotation in trang area is considered as a part of large scale counter clockwise rotation experienced by the southern sundaland block including the peninsular malaysia borneo and south sulawesi areas as a result of australian plate collision with southeast asia within the framework of sundaland block the northern boundary of counter clockwise rotated zone lies between the trang area and the khorat basin

development of cenozoic basins in thailand abstract there are over 60 cenozoic basins distributed in various parts of thailand both onshore and offshore these basins are mainly n s trending half grabens or grabens and most were initiated in the late oligocene the structural framework of the basins is generally governed by n s trending extensional faults which are spatially related to the movement of the nw se and ne sw trending strike slip faults that have been active since the oligocene the nw se faults are the principal dextral strike slip faults whereas the ne sw faults are the conjugate sinistral strike slip faults the collision of the india craton with southern asia since 40 50 ma caused clockwise rotation of southeast asia and movements on the strike slip faults with the associated development of cenozoic basins in this region four major cenozoic basins mergui pattani malay and phitsanulok contain up to 8000 m of sediments generally the cenozoic fills are continental sediments except in the mergui basin in the andaman sea where the sedimentary fill is mainly marine sediments fluviatile with occasional lacustrine deposits were prominent in the gulf of thailand and onshore basins during the oligocene to early miocene from early to middle miocene widespread perennial lake conditions were well developed in the onshore basins and in the western graben area of the gulf of thailand a change in tectonic and or climatic conditions took place in the middle to late miocene times resulting in the cessation of perennial lake conditions and development of a regional unconformity

sea level as a stabilizing factor for marine ice sheet grounding lines climate change could potentially destabilize marine ice sheets which would affect projections of future sea level rise1 2 3 4 specifically an instability mechanism5 6 7 8 has been predicted for marine ice sheets such as the west antarctic ice sheet that rest on reversed bed slopes whereby ice sheet thinning or rising sea level leads to irreversible retreat of the grounding line however existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea level fall at the margin of a rapidly shrinking ice sheet9 10 11 here we present a suite of predictions of gravitationally self consistent sea level change following grounding line migration our predictions vary the initial ice sheet size and also consider the contribution to sea level change from various subregions of the simulated ice sheet using these results we revisit a canonical analysis of marine ice sheet stability5 and demonstrate that gravity and deformation induced sea level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes we conclude that accurate treatments of sea level change should be incorporated into analyses of past and future marine ice sheet dynamics

new insights into regional tectonics of the indochina peninsula inferred from lower middle jurassic paleomagnetic data of the sibumasu terrane abstract the post jurassic occurrence of differential tectonic rotation between the indochina and south sundaland blocks remains an issue to be properly investigated new paleomagnetic study is used here to find the role of sibumasu terrane in this rotation which is located between a clockwise rotated indochina block and a counter clockwise rotated south sundaland block for this purpose lower to middle jurassic red sandstones of the umphang group in the sibumasu terrane were sampled at 21 sites in the ratchaburi area 13 6 e 99 6 e thailand stepwise thermal demagnetization by 680 c unblocked a pre folding characteristic remanent magnetization a mean direction of this component at 100 unfolding is ds 348 5 is 24 7 95 10 5 k 10 7 n 20 corresponding to an early middle jurassic pole of 78 6 n 10 6 e a95 9 3 comparison of this direction with those reported from other localities of the umphang group kalaw mae sot and north trang syncline localities reveal variable declinations between 348 5 and 44 7 for the sibumasu terrane we ascribe this variation to differential tectonic deformation in the sibumasu terrane as reflected from sinusoidal shaped structural features in the study area the presence of such features in the granitic rocks indicates the occurrences of deformational activities after their intrusion which took place in the period between 130 ma and 51 ma the sibumasu terrane behaved as an independent fragment at a time when indochina was undergoing a clockwise rotation and southward displacement as a result of extrusion tectonics after the gigantic india asia collision taking into consideration a westerly deflected declination d 342 8 from the west trang area in peninsular thailand a counterclockwise rotation of 15 is estimated for the sibumasu terrane as a result of continuous northward indentation of the australian plate into south sundaland block

a numerical investigation of ice lobe permafrost interaction around the southern laurentide ice sheet permafrost existed around and under marginal parts of the southern laurentide ice sheet during the last glacial maximum the presence of permafrost was important in determining the extent form and dynamics of ice lobes and the landforms they produced because of influences on resistance to basal motion and subglacial hydrology we develop a two dimensional time dependent model of permafrost and glacier ice dynamics along a flowline to examine i the extent to which permafrost survives under an advancing ice lobe and how it influences landform development and hydrology and ii the influence of permafrost on ice motion and surface profile the model is applied to the green bay lobe which terminated near madison wisconsin during the last glacial maximum simulations of ice advance over permafrost indicate that the bed upstream of the ice sheet margin was frozen for 60 200 km at the glacial maximum permafrost remained for centuries to a few thousand years under advancing ice and penetrated sufficiently deep tens of meters into the underlying aquifer that drainage of basal meltwater became inefficient likely resulting in water storage beneath the glacier our results highlight the influence of permafrost on subglacial conditions even though uncertainties in boundary conditions such as climate exist

links between ocean temperature and iceberg discharge during heinrich events the last glacial period was punctuated by several periods of massive iceberg discharge from the northern hemisphere ice sheets numerical simulations indicate that these discharge events are linked to an interplay between ice sheet accumulation marine ice shelf stability and periodically oscillating surface ocean temperature

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

coal facies in a cenozoic paralic lignite bed krabi basin southern thailand changing peat forming conditions related to relative sea level controlled watertable variations abstract the cenozoic krabi basin in the southern part of peninsular thailand contains about 112 million tons proven coal reserves at present coal is only produced from the bang mark mine located in the southern part of the basin where the main lignite bed is 7 20 m thick the lignite bed occurs in an overall paralic succession the present paper investigates the depositional conditions of an approximately 8 m thick lignite bed main seam in the bang mark mine using organic petrography including maceral ratios and geochemistry the results are further interpreted in a sequence stratigraphic context the lignite is of low rank and is completely dominated by huminite indicating generally oxygen deficient conditions in the precursor mire very low inertinite contents suggest rare occurrences of wildfires the lower part of the lignite bed represents a topogenous fresh water peat mire with open water areas that in few cases may have experienced influx of saline water the peat mire was subjected to periodic inundations and deposition of siliciclastics tissue preservation was relatively poor the upper part of the lignite bed represents a slightly domed fresh water ombrogenous peat mire with a stable watertable and a balance between peat accumulation and accommodation space creation that favoured preservation of plant tissues in general the mire vegetation changed from less woody in the topogenous mire to more arborescent in the ombrogenous mire where plants with suberinised wood cell walls also were more frequent decompacted the lignite bed corresponds to a minimum 11 m thick peat deposit that records from 22 000 to 55 000 years of peat accumulation watertable rise in the peat mire was controlled overall by relative sea level rise in a sequence stratigraphic context the lignite bed overlies a terrestrialisation surface tes sensu diessel 2007 and the lowermost part records peat formation during a falling watertable and a decreasing accommodation peat accumulation ratio terrestrialisation an accommodation reversal surface ars sensu diessel 2007 indicates a change to paludification style of peat formation characterised by rising watertable and a high accommodation peat accumulation ratio another ars marks a gradual change to a situation with a balanced accommodation peat accumulation ratio the overall watertable rise throughout peat formation but at a gradually slower rate from base to top suggests that the lignite bed could be located in the late transgressive systems tract tst

the 7 13 march 2006 major saharan outbreak multiproxy characterization of mineral dust deposited on the west african margin mineral dust deposits were collected at mbour senegal throughout the spring of 2006 and especially during the well documented march 7 13 large saharan dust outbreak during this 7 day period significant changes in mass flux grain size clay mineralogy and sr and nd isotopic compositions were recorded indicating distinct provenances for the dust transported and deposited during and outside the event all these terrigenous proxies as well as freshwater diatom taxa also showed significant temporal variations during the outbreak implying contributions from at least two different provenance regions tri dimensional back trajectories and satellite imaging enabled us to link those distinct signatures to regions increasingly to the southeast within a large area covering mauritania mali and southern algeria identified by the total ozone mapping spectrometer toms as the main source of the prominent winter spring plume over the tropical atlantic the multiproxy characterization of the march 7 13 dust fall therefore enables us to typify the terrigenous signature of two different regions supplying dust off west africa and provide valuable clues for the interpretation of northeastern tropical atlantic ocean dust sedimentary records in terms of changes in provenance regions and transport systems additionally because dust deposition data are scarce flux and grain size data obtained in this study among other parameters such as clay assemblages provide important constraints for atmospheric transport models and dust deposition budget estimates in this area

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

numerical modeling of late glacial laurentide advance of ice across hudson strait insights into terrestrial and marine geology mass balance and calving flux a time dependent finite element model was used to reconstruct the advance of ice from a late glacial dome on northern quebec labrador across hudson strait to meta incognita peninsula baffin island and subsequently to the 9 9 9 6 ka 14c gold cove position on hall peninsula terrestrial geological and geophysical information from quebec and labrador was used to constrain initial and boundary conditions and the model results are compared with terrestrial geological information from baffin island and considered in the context of the marine event dc 0 and the younger dryas cooling we conclude that advance across hudson strait from ungava bay to baffin island is possible using realistic glacier physics under a variety of reasonable boundary conditions production of ice flux from a dome centered on northeastern quebec and labrador sufficient to deliver geologically inferred ice thickness at gold cove hall peninsula appears to require extensive penetration of sliding south from ungava bay the discharge of ice into the ocean associated with advance and retreat across hudson strait does not peak at a time coincident with the start of the younger dryas and is less than minimum values proposed to influence north atlantic thermohaline circulation nevertheless a significant fraction of freshwater input to the north atlantic may have been provided abruptly and at a critical time by this event

a climate model with cryodynamics and geodynamics we present a highly simplified zero dimensional model of the climatic system the model attempts to incorporate mechanisms important on the time scale of glaciation cycles namely 104 105 years in particular the radiation balance of the ocean atmosphere the plastic flow of continental ice sheets and the viscous flow of the upper mantle are taken into account the stress is on the interaction between the cryosphere ice sheets and the asthenosphere upper mantle which had not been taken into account in previous energy balance or ice sheet models of climate weertman 1976 kallen et al 1979 the model exhibits free self sustained oscillations of an amplitude and period comparable to those found in the paleoclimatic record of glaciations viz o 10 deg k and o 104 years respectively such oscillations had already been found in the simpler model of kallen et al 1978 1979 the fact that nonlinear self sustained oscillations also obtain for the present slightly more complex model is interesting it offers mild support to the idea that unforced oscillations can actually exist in the real climatic system itself these oscillations are characterized by a quarter phase lag between ice extent and global temperature given that the climatic system is a nonlinear oscillator with a period of o 104 years it is expected that astronomical forcing at 19 000 23 000 and 41 000 years will lead to subharmonic oscillations with a period close to 100 000 years the dominant period of glaciation cycles such effects have already been noticed in the work of birchfield and weertman 1978 with an even simpler model including only glacial dynamics the careful study of the interplay between internal mechanisms and external forcing with comparable time scales represents an interesting challenge to the theory of ice ages the results of such theoretical studies can also provide guidance for observational work in particular periodicities of o 104 years in the climate record and phase lags between ice volume and temperature appear to be supported by the most recent paleogeochemical investigations ruddiman and mclntyre 1981

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

numerical modeling of subglacial sediment deformation implications for the behavior of the lake michigan lobe laurentide ice sheet we apply a numerical model of the late wisconsin circa 20 000 years b p lake michigan lobe lml laurentide ice sheet to investigate how fine grained subglacial sediment might influence lobe behavior particularly rapid millennial scale marginal oscillations observed in the geologic record over the canadian shield we assume a rigid bed hard bedded basal boundary condition in areas overlain by fine grained sediment soft bedded the base of the ice is coupled to a deformable sediment layer using a rate dependent stress strain law geotechnical tests of clay rich till deposited by the lml provide control for sediment rheologic parameters simulated cross sectional profiles are consistent with reconstructions from geologic evidence time dependent simulations suggest that a soft bedded lobe could have reached steady state in about 20 000 years or less in contrast to 50 000 to 60 000 years for an otherwise identical hard bedded lobe a soft bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial scale shifts in accumulation or ablation as a nonsliding hard bedded lobe but in both cases the response is slower than that indicated by the geologic record results suggest that while strong millennial scale changes in accumulation and ablation can produce responses in hard bedded or soft bedded ice that are consistent with the geologic record changes in subglacial sediment viscosity even relatively modest changes whether independent or in conjunction with climate change might more readily account for millennial and submillennial scale fluctuations of the lobe margin these observations do not exclude a role for sliding but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior

resolution dependent performance of grounding line motion in a shallow model compared with a full stokes model according to the mismip3d intercomparison making confident statements about the evolution of an ice sheet shelf system with a numerical model requires the capability to reproduce the migration of the grounding line here we show that the shallow ice approximation shallow shelf approximation hybrid type parallel ice sheet model pism with its recent improvements is capable of modeling the grounding line motion in a perturbed ice sheet shelf system the model is set up according to the three dimensional marine ice sheet model intercomparison project mismip3d and simulations are carried out across a broad range of spatial resolutions using 1 a linear interpolation of the grounding line with locally interpolated basal friction and 2 an improved driving stress computation across the grounding line the reversibility of the grounding line i e its retreat after an advance forced by a local perturbation of basal resistance is captured by the model even at medium and low resolutions x 10 km the transient model response is qualitatively similar to that of higher order models but reveals a higher initial sensitivity to perturbations on very short timescales our findings support the application of pism to the antarctic ice sheet from regional up to continental scales and on relatively low spatial resolutions

the basins of sundaland se asia evolution and boundary conditions abstract most of the basins developed in the continental core of se asia sundaland evolved since the late cretaceous in a manner that may be correlated to the conditions of the subduction in the sunda trench by the end of mesozoic times sundaland was an elevated area composed of granite and metamorphic basement on the rims which suffered collapse and incipient extension whereas the central part was stable this promontory was surrounded by a large subduction zone except in the north and was a free boundary in the early cenozoic starting from the palaeogene and following fractures initiated during the india eurasia collision rifting began along large faults mostly n s and nnw sse strike slip which crosscut the whole region the basins remained in a continental fluvio lacustrine or shallow marine environment for a long time and some are marked by extremely stretched crust phu khanh natuna n makassar or even reached the ocean floor spreading stage celebes flores western sundaland was a combination of basin opening and strike slip transpressional deformation the configuration suggests a free boundary particularly to the east trench pull associated with the proto south china sea subduction java sulawesi trench subduction rollback in the early miocene australian blocks reached the sunda subduction zone and imposed local shortening in the south and southeast whereas the western part was free from compression after the indian continent had moved away to the north this suggests an important coupling of the sunda plate with the indo australian plate both to se and nw possibly further west rollback had ceased in the java sumatra subduction zone and compressional stress was being transferred northwards across the plate boundary the internal compression is expressed to the south by shortening which is transmitted as far as the malay basin in the late miocene most of the sunda plate was under compression except the tectonically isolated andaman sea and the damar basins in the pliocene collision north of australia propagated toward the north and west causing subduction reversal and compression in the short lived damar basin docking of the philippine plate confined the eastern side of sundaland and created local compression and uplift such as in nw borneo palawan and taiwan transpressional deformation created extensive folding strike slip faulting and uplift of the central basin and arakan yoma in myanmar minor inversion affected many thailand rift basins all the other basins record subsidence the uplift is responsible for gravity tectonics where thick sediments were accumulated sarawak ne luconia bangladesh wedge

the role of ice stream dynamics in deglaciation since the mid pleistocene transition deglaciation has occurred only after ice sheets have grown large while experiencing several precession and obliquity cycles indicating that large ice sheets are more sensitive to milankovitch forcing than small ice sheets are observations and model simulations suggest that the development of ice streams in the laurentide ice sheet played an as yet unknown role in deglaciations in this study we propose a mechanism by which ice streams may enhance deglaciation and render large ice sheets more sensitive to milankovitch forcing we use an idealized configuration of the parallel ice sheet model that permits the formation of ice streams when the ice sheet is large and ice streams are sufficiently developed an upward shift in equilibrium line altitude commensurate with milankovitch forcing results in rapid deglaciation while the same shift applied to an ice sheet without fully formed ice streams results in continued ice sheet growth or slower deglaciation rapid deglaciation in ice sheets with significant streaming behavior is caused by ice stream acceleration and the attendant enhancement of calving and surface melting at low elevations ice stream acceleration is ultimately the result of steepening of the ice surface and increased driving stresses in ice stream onset zones which come about due to the dependence of surface mass balance on elevation these ice sheet simulations match the broad features of geomorphological observations and add ice stream dynamics that are missing from previous model studies of deglaciation

geochemical and isotopic characterization of the bodele depression dust source and implications for transatlantic dust transport to the amazon basin the bodele depression chad in the central sahara sahel region of northern africa is the most important source of mineral dust to the atmosphere globally the bodele depression is purportedly the largest source of saharan dust reaching the amazon basin by transatlantic transport here we have undertaken a comprehensive study of surface sediments from the bodele depression and dust deposits chad niger in order to characterize geochemically and isotopically sr nd and pb isotopes this dust source and evaluate its importance in present and past african dust records we similarly analyzed sedimentary deposits from the amazonian lowlands in order to assess postulated accumulation of african mineral dust in the amazon basin as well as its possible impact in fertilizing the amazon rainforest our results identify distinct sources of different ages and provenance in the bodele depression versus the amazon basin effectively ruling out an origin for the amazonian deposits such as the belterra clay layer by long term deposition of bodele depression material similarly no evidence for contributions from other potential source areas is provided by existing isotope data sr nd on saharan dusts instead the composition of these amazonian deposits is entirely consistent with derivation from in situ weathering and erosion of the precambrian amazonian craton with little if any andean contribution in the amazon basin the mass accumulation rate of eolian dust is only around one third of the vertical erosion rate in shield areas suggesting that saharan dust is consumed by tropical weathering contributing nutrients and stimulating plant growth but never accumulates as such in the amazon basin the chemical and isotope compositions found in the bodele depression are varied at the local scale and have contrasting signatures in the silica rich dry lake bed sediments and in the calcium rich mixed diatomites and surrounding sand material this unexpected finding implies that the bodele depression material is not pre mixed at the source to provide a homogeneous source of dust rather different isotope signatures can be emitted depending on subtle vagaries of dust producing events our characterization of the bodele depression components indicate that the bodele calcium rich component identified here is most likely released via eolian processes of sand grain saltation and abrasion and may be significant in the overall global budget of dusts carried out by the harmattan low level jet during the winter

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

ice streaming in the laurentide ice sheet a first comparison between data calibrated numerical model output and geological evidence 1 despite the importance of rapidly flowing ice streams to ice sheet mass balance their incorporation into numerical ice sheet models is a major scientific challenge this introduces large uncertainties in model output and inhibits a more complete understanding of the role of ice streams in overall ice sheet stability recent computational advances have enabled more realistic representations of ice streaming but few studies have attempted to compare model output against known locations of ice streams this paper compares predictions of ice streaming derived from a large ensemble analysis of a glacial systems model of the laurentide ice sheet against independent geological evidence compiled from previously published studies although the precise dating of paleo ice stream locations is problematic our analysis includes comparisons at six different time steps 18 to 10 cal ka bp during deglaciation results indicate that the model is successful in predicting all of the major marine terminating ice streams but there is mixed success in simulating terrestrial ice streams in the right place and at the right time which is vital in guiding future model development the model also reveals that whilst some ice streams persist throughout deglaciation the focus of mass loss associated with ice streaming switches through time with dynamic changes in ice stream catchments and tributaries this implies that major changes in ice stream activity are to be expected in a deglaciating ice sheet with important implications for contemporary ice sheet dynamics

results from the ice sheet model intercomparison project heinrich event intercomparison ismip heino results from the heinrich event intercomparison heino topic of the ice sheet model intercomparison project ismip are presented ismip heino was designed to explore internal large scale ice sheet instabilities in different contemporary ice sheet models these instabilities are of interest because they are a possible cause of heinrich events a simplified geometry experiment reproduces the main characteristics of the laurentide ice sheet including the sedimented region over hudson bay and hudson strait the model experiments include a standard run plus seven variations nine dynamic thermodynamic ice sheet models were investigated one of these models contains a combination of the shallow shelf ssa and shallow ice approximation sia while the remaining eight models are of sia type only seven models including the sia ssa model exhibit oscillatory surges with a period of 1000 years for a broad range of parameters while two models remain in a permanent state of streaming for most parameter settings in a number of models the oscillations disappear for high surface temperatures strong snowfall and small sediment sliding parameters in turn low surface temperatures and low snowfall are favourable for the ice surge cycles we conclude that further improvement of ice sheet models is crucial for adequate robust simulations of cyclic large scale instabilities

characterization and differentiation of rock varnish types from different environments by microanalytical techniques abstract we investigated rock varnishes collected from several locations and environments worldwide by a broad range of microanalytical techniques these techniques were selected to address the challenges posed by the chemical and structural complexity within the micrometer to nanometer sized structures in these geological materials femtosecond laser ablation inductively coupled plasma mass spectrometry fs la icp ms scanning transmission x ray microscopy near edge x ray adsorption fine structure spectroscopy stxm nexafs in combination with scanning electron microscopy sem of focused ion beam fib ultra thin 100 200 nm sections conventional and polarization microscopy as well as electron paramagnetic resonance epr measurements were used to obtain information about these rock varnishes rock varnishes from different environments which cannot readily be distinguished based on their macroscopic appearance differ significantly in their constituent elemental mass fractions e g of mn fe ni co ba and pb and their rare earth element ree patterns structural characteristics such as the particle sizes of embedded dust grains internal structures such as layers of mn fe and ca rich material and structures such as cavities varied between varnishes from different environments and regions in the world the epr spectra were consistent with aged biogenic mn oxides in all samples but showed subtle differences between samples of different origin our observations allow us to separate rock varnishes into different types with differences that might be indicators of distinct geneses five different types of rock varnish could be distinguished type i v of which only type i might be used as potential paleoclimate archive each varnish type has specific characteristics in terms of their elemental composition element distribution and structures the combination of element ratios mn ba al ni mn rey mn ce mn pb lan ybn and ce ce total ree contents and structures can be used to separate the different types of rock varnish from each other

magnetic polarity stratigraphy and ages of siwalik group rocks of the potwar plateau pakistan abstract six magnetic polarity sections have been established over the potwar plateau region of pakistan including the major stratotypes of the siwalik group in all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone which is contained within the nagri formation this conspicuous normal polarity zone has been radiometrically dated at 9 5 0 6 m y which identifies it as magnetic chron 9 radiometric dates from the upper siwalik formation have also been used to identify the chron 2 3 boundary in two of the sections the magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale so that the ages of the local stratigraphy are indexed accordingly based exclusively on data from stratotype sections the chinji nagri and dhok pathan formations have nominal age ranges of 10 1 13 1 7 9 10 1 and 5 1 7 9 m y age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype mean sediment accumulation rates during the chinji nagri and dhok pathan interval range from 13 to 52 cm 103 yr essentially linear sediment accumulations are locally maintained over time intervals of several million years the chinji nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the potwar plateau indicating a fundamental sedimentary tectonic change at this time

climate forcing of the pb isotope record of terrigenous input into the equatorial atlantic abstract pb isotopic compositions of bulk sediment cores from the tropical atlantic are used here to infer variations in the provenance of terrigenous input to the tropical atlantic during pleistocene climate cycles the 200 ka high precision 2 100 ppm pb isotope records of the ceara rise western atlantic and sierra leone rise eastern atlantic cores show a clear glacial interglacial cyclicity reflected by alternating unradiogenic pb and radiogenic pb in both cores the glacial interglacial pb isotopic contrast is also observed in pb pb space and can be explained in terms of binary mixing variations along the mixing lines reflecting changes in the relative proportions of the glacial unradiogenic and interglacial radiogenic pb source s the pleistocene pb isotopic variability of the terrigenous input to the ceara rise can be linked to changes in the weathering styles in the amazon basin and as a result in the amazon river discharge these changes are reflected by a greater contribution from the highlands andes pb source during glacial times and strengthening of the lowlands shield pb source during interglacials on the other hand pb isotopic variations in the sierra leone rise core indicate increased pb inputs from the saharan dust plume during glacial times in agreement with the wind patterns over africa furthermore this result bears some implications on the cause of the enhanced glacial terrigenous fluxes to the tropical atlantic which we infer to be due to increase in winter wind transport rather than glacial hyperaridity the cyclicity of the ceara rise and sierra leone rise pb isotopic records together with changes in the proportions of mixing sources throughout the last 200 ka monitor changes in the hydrological cycle over south america and the wind systems over africa respectively both of which are linked to the seasonal latitudinal migration of the intertropical convergence zone the correlation found between pb isotope cycles and other paleoclimate proxies suggests that pb isotopes may be responding to variations in earth s orbital parameters

early rhinocerotids mammalia perissodactyla from south asia and a review of the holarctic paleogene rhinocerotid record the earliest rhinocerotids from south asia are identified on the basis of few dental remains originating from the late eocene of thailand wai lek mine krabi basin and the early oligocene of pakistan paali nala c2 bugti hills once synthesized the holarctic paleogene rhinocerotid record points out a westward diachronism of rhinocerotid first appearance data from north america to europe via asia throughout mid cenozoic times the faunal similarity among mammal localities from the late eocene and early oligocene of peninsular thailand southern china and pakistan suggests the existence of a single south asian paleoprovince during this interval and the persistence of a tropical subtropical climate substantial faunal changes recorded in eastern balochistan reveal a significant climatic deterioration from the middle part of the oligocene neither provinciality nor endemism is noticeable for rhinocerotoid taxa recognized in the oligocene of the indian subcontinent neither the himalayas nor the tibet plateau was a paleogeographic barrier for large mammals during this interval resume les plus anciens rhinocerotides d asie du sud sont identifies gr ce a quelques restes dentaires provenant de l eocene superieur de thailande mine de wai lek bassin de krabi et de l oligocene inferieur du pakistan paali nala c2 collines bugti la synthese du registre fossile paleogene holarctique des rhinocerotides revele un net diachronisme des premieres apparitions de rhinocerotides depuis l est amerique du nord vers l ouest asie puis europe au cours du cenozoique moyen la similarite des communautes de mammiferes entre les localites de l eocene superieur et de l oligocene inferieur de thailande peninsulaire de chine du sud et du pakistan suggere l existence d une seule paleoprovince sud asiatique pendant la periode consideree sous un climat tropical a subtropical les changements fauniques enregistres au balouchistan oriental revelent une deterioration climatique significative a partir du milieu de l oligocene aucun indice de provincialisme ou d endemisme n est releve pour les rhinocerotoides oligocenes du sous continent indien ni les himalayas ni le plateau du tibet ne constituaient a l epoque une barriere paleogeographique pour les grands mammiferes

a conceptual model of the deglaciation of atlantic canada abstract we present a conceptual model for the deglaciation of atlantic canada in which a role is played by ice streams the ice margin lay close to the edge of the continental shelf at the last glacial maximum ice streams occupying shelf troughs comprised 1 a major ice stream in the laurentian channel 2 secondary streams in the bay of fundy gulf of maine trinity trough and notre dame channel and 3 lesser ice streams elsewhere early retreat in the northeast notre dame bay and in the southwest bay of fundy gulf of maine was mainly by calving along embayments by 18 ka radiocarbon years a calving embayment had opened on emerald basin scotian shelf retreat along channels marooned ice on intervening banks a major calving episode beginning just before 14 ka removed ice from the gulf of st lawrence triggering re advances down secondary troughs isolation of a newfoundland ice cap produced radial drainage via fiords and more extensive flow into the st george s bay lowlands on the west coast by 13 ka most ice was on land and was ablating by melting rather than calving shelf ice caps persisted until 11 ka

first magnetostratigraphic study of the pondaung formation implications for the age of the middle eocene anthropoids of myanmar abstract we report the results of a magnetostratigraphic investigation to improve the stratigraphical and chronological resolution of the pondaung formation of central myanmar a total of 98 samples were collected from 45 sites through a 319 m thick section at the fossiliferous locality of yashe kyitchaung or the primate resort yielding primate species bahinia pondaungensis amphipithecus mogaungensis and myanmarpithecus yarshensis near the bahin village thermal and alternating field demagnetization allowed separation of two remanence components the high temperature component is interpreted as the characteristic magnetization rock magnetic experiments show that the remanence magnetization is mainly carried by magnetite this investigation documents normal polarity remanent magnetization with a mean direction documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usep

deformation style of the mesozoic sedimentary rocks in southern thailand abstract mesozoic sedimentary rocks in southern thailand are widespread from nne ssw and n s in chumphon and trang provinces the mesozoic stratigraphic units are the marine triassic sai bon formation and the non marine jurassic cretaceous thung yai group the latter subdivided into khlong min lam thap sam chom and phun phin formations these units overlie permian carbonate rocks with an angular unconformity and are overlain unconformably by cenozoic units and the quaternary sediments the mesozoic rocks have been folded to form two huge first ordered syncline or synclinoria the chumphon and surat thani krabi trang synclinoria these synclinoria are elongated in nne ssw to n s direction and incorporate asymmetric lower order parasitic folds the folds have moderately to steeply dipping eastward limbs and more gently dipping westward limbs these folds were transected by brittle fractures in four major directions these geologic structures indicate wnw ese to e w contraction with top to the east simple shear at some time before the deposition of the cenozoic sedimentary units no major deformation has affected the rocks subsequently apart from the formation of the fault controlled cenozoic basins