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

neoproterozoic glacial rainout intervals observations and implications stratigraphically discrete glacigenic dropstone intervals have been identified within six separate neoproterozoic glaciomarine successions the ca 720 ma chuos formation southwestern congo craton and surprise diamictite southwestern laurentia the ca 600 ma ghaub formation congo craton blasskrans formation northwestern kalahari craton and wildrose diamictite southwestern laurentia and the post 595 ma lower southern highland group northeast laurentia these dropstone intervals are interstratified with hemipelagic dropstone free lithologies that record periods devoid of glacial rainout such episodic deposition requires at a most rudimentary level temporally compatible environmental fluctuations to generate melting of ice the ghaub diamictite is particularly revealing in this regard given that according to the snowball earth hypothesis it had to be deposited either during glacial maxima totally frozen seas or melt back geologically instantaneous owing to its low latitude setting and association with carbonates that record highly depleted 1 3 c values the consistency of facies features shared by all six glaciomarine successions leads us to speculate that the temporally discrete glacial rainout events including diamictites deposited in low latitudes record a dynamic glacial environment and that such conditions were characteristic of neoproterozoic glaciations in general these observations indicate that neoproterozoic seas were not totally frozen and that the hydrologic cycle was functioning

sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

850 790 ma bimodal volcanic and intrusive rocks in northern zhejiang south china a major episode of continental rift magmatism during the breakup of rodinia abstract shrimp u pb zircon ages and geochemical and nd isotopic data are reported for mid neoproterozoic igneous rocks in northern zhejiang province south china the shenwu dolerite dykes the daolinshan granite diabase complex and the shangshu bimodal basalt rhyolite volcanic rocks were dated at 849 7 ma 794 9 ma and 792 5 ma respectively all the studied mafic rocks are tholeiitic in compositions and exhibit overall moderate enrichments in most incompatible trace elements resembling intraplate basaltic rocks in continental rifts the 850 ma shenwu dolerites are the earliest anorogenic basaltic rocks identified so far after the ca 1 1 0 9 ga sibao orogeny but seem to predate the neoproterozoic rifting our results are clearly at variance with the alternative tectonic model that an active continental margin existed along the southern margin of the yangtze block until ca 800 ma the intensive 790 ma bimodal volcanic and plutonic rocks part of a major phase of mid neoproterozoic rifting are concurrent with the widespread mid neoproterozoic intraplate magmatism throughout south china they are most likely related to the proposed mantle superplume that led to the breakup of the supercontinent rodinia

is there a conflict between the neoproterozoic glacial deposits and the snowball earth interpretation an improved understanding with numerical modeling the behavior of the terrestrial glacial regime during the neoproterozoic glaciations is still a matter of debate some papers claim that the glacial sequences cannot be explained with the snowball earth scenario indeed the near shutdown of the hydrological cycle simulated by climatic models once the earth is entirely glaciated has been put in contrast with the need for active wet based continental ice sheets to produce the observed thick glacial deposits a climate ice sheet model is applied to the older extreme neoproterozoic glaciation around 750 ma with a realistic paleogeographic reconstruction of rodinia our climate model shows that a small quantity of precipitation remains once the ocean is completely ice covered thanks to sublimation processes over the sea ice at low latitudes acting as a water vapor source after 10 ka of the ice sheet model the ice volume in the tropics is small and confined as separate ice caps on coastal areas where water vapor condenses however after 180 ka large ice sheets can extend over most of the supercontinent rodinia several areas of basal melting appear while ice sheets reach their ice volume equilibrium state at 400 ka they are located either under the two single domed ice sheets covering the antarctica and the laurentia cratons or near the ice sheet margins where fast flow occurs only the isolated and high latitude cratons stay cold based finally among the simulated ice sheets most have a dynamic behavior in good agreement with the needs inferred by the preserved thick formations of diamictite and share the features of the antarctica present day ice sheet therefore our conclusion is that a global glaciation would not have hindered the formation of the typical glacial structures seen everywhere in the rock record of neoproterozoic times

episodic reactivation of large scale push moraines in front of the advancing taku glacier alaska 1 taku glacier an advancing former tidewater glacier in alaska has been actively pushing its proglacial sediments along part of its terminus over the last 50 years producing so called push moraines the mobilization of these sediments which were locally lifted more than 20 m above sea level by 2004 has happened episodically rather than steadily the last major event of proglacial sediment deformation occurred in 2001 presumably caused by sliding along a basal detachment layer since then most deformation has been localized within a few meters of the terminus including impressive deformational features of the terminal ice where slabs of ice tens of centimeters in thickness have undercut proglacial sediments and vegetation and lifted them up between 2002 and 2004 surface velocities and horizontal displacements were measured across the terminus and in the proglacial push moraine area sediment displacement was highest between the end of march and mid june a decrease in displacement with distance from the terminus revealed that the sediments were deforming internally rather than along a

a field based classification scheme for gravel beaches a tripartite classification of gravel beaches based upon morphodynamic properties is proposed and demonstrated for 42 new zealand beaches the main advantage of this scheme is that it is based on simple visual classification that can be applied globally in the field and is underpinned by morphodynamic differences between the beach types the three types identified from the results are 1 pure gravel beach 2 mixed sand and gravel beach 3 composite gravel beach pure gravel beaches have steep slopes tan 0 08 0 24 and gravels extending from the storm berm to below mean low water spring tide level mixed sand and gravel beaches have moderate slopes tan 0 04 0 13 with sand and gravel entirely mixed both cross shore and at depth composite gravel beaches have a steep gravel berm fronted by a low angle intertidal terrace with overall beach slopes of tan 0 05 0 14 on composite beaches there is distinct hydrodynamic cross shore sorting of the sand and gravel component these broad types are tested using discriminant analysis and the three classes are shown to be highly significant f 16 24 df 8 p 0 00005 the key discriminating variables are iribarren number beach width average grain size and storm berm height

new constraints on equatorial temperatures during a late neoproterozoic snowball earth glaciation intense glaciation during the end of cryogenian time 635 million years ago marks the coldest climate state in earth history a time when glacial deposits accumulated at low tropical paleolatitudes the leading idea to explain these deposits the snowball earth hypothesis predicts globally frozen surface conditions and subfreezing temperatures with global climate models placing surface temperatures in the tropics between 20 c and 60 c however precise paleosurface temperatures based upon geologic constraints have remained elusive and the global severity of the glaciation undetermined here we make new geologic observations of tropical periglacial aeolian and fluvial sedimentary structures formed during the end cryogenian marinoan glaciation in south australia these observations allow us to constrain ancient surface temperatures we find periglacial sand wedges and associated deformation suggest that ground temperatures were sufficiently warm to allow for ductile deformation of a sandy regolith the wide range of deformation structures likely indicate the presence of a paleoactive layer that penetrated 2 4 m below the ground surface these observations paired with a model of ground temperature forced by solar insolation constrain the local mean annual surface temperature to within a few degrees of freezing this temperature constraint matches well with our observations of fluvial deposits which require temperatures sufficiently warm for surface runoff although this estimate coincides with one of the coldest near sea level tropical temperatures in earth history if these structures represent peak marinaon glacial conditions they do not support the persistent deep freeze of the snowball earth hypothesis rather surface temperatures near 0 c allow for regions of seasonal surface melting atmosphere ocean coupling and possible tropical refugia for early metazoans if instead these structures formed during glacial onset or deglaciation then they have implications for the timescale and character for the transition into or out of a snowball state

glacier changes in southeast alaska and northwest british columbia and contribution to sea level rise 1 the digital elevation model dem from the 2000 shuttle radar topography mission srtm was differenced from a composite dem based on air photos dating from 1948 to 1987 to determine glacier volume changes in southeast alaska and adjoining canada srtm accuracy was assessed at 5 m through comparison with airborne laser altimetry and control locations measured with gps glacier surface elevations lowered over 95 of the 14 580 km2 glacier covered area analyzed with some glaciers thinning as much as 640 m a combination of factors have contributed to this wastage including calving retreats of tidewater and lacustrine glaciers and climate change many glaciers in this region are particularly sensitive to climate change as they have large areas at low elevations however several tidewater glaciers that had historically undergone calving retreats are now expanding and appear to be in the advancing stage of the tidewater glacier cycle the net average rate of ice loss is estimated at 16 7 4 4 km3 yr equivalent to a global sea level rise contribution of 0 04 0 01 mm yr

experimental study of the transport of mixed sand and gravel we measured a wide range of transport rates for five different sand gravel mixtures in a laboratory flume each mixture used the same gravel and sand was added to produce mixtures containing 6 15 21 27 and 34 sand control of other variables allows us to isolate the effect of bed sand content on transport as sand content increases gravel transport rates increase by orders of magnitude even though the proportion of gravel in the bed decreases the increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27 the increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content bed surface grain size was measured at the end of each run surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate this casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate

biomarker evidence for photosynthesis during neoproterozoic glaciation laterally extensive black shales were deposited on the sao francisco craton in southeastern brazil during low latitude neoproterozoic glaciation 740 to 700 million years ago these rocks contain up to 3 0 weight organic carbon which we interpret as representing the preserved record of abundant marine primary productivity from glacial times extractable biomarkers reflect a complex and productive microbial ecosystem including both phototrophic bacteria and eukaryotes living in a stratified ocean with thin or absent sea ice oxic surface waters and euxinic conditions within the photic zone such an environment provides important constraints for parts of the snowball earth hypothesis

a simple mechanism for irreversible tidewater glacier retreat 1 tidewater glacier retreat is abrupt and typically irreversible the mechanics of abrupt onset are evaluated by considering the behavior of a sliding law that depends inversely upon effective pressure pice pwater for such a law flux can increase with decreasing thickness if the loss of bed traction exceeds the loss of driving stress characteristic values of increased flux and glacier thinning are shown to propagate upstream if ice flux increases with decreasing thickness below an ice thickness threshold determined by the chosen sliding law upstream propagation leads to instability described in earlier literature in which increased flow leads to thinning loss of bed traction and continued acceleration in simplest terms the instability arises when alterations in glacier geometry act to reduce resistive stresses more than driving stresses the effects of diffusion are also analyzed and it is shown that for the form of sliding law considered here diffusion will not necessarily eliminate upstream propagating instabilities application of this theory shows that a transition from stable to unstable conditions occurred at columbia glacier alaska near the time of onset of its retreat in the early 1980s the theory also successfully predicts stable or rapidly retreating states for 12 other alaska tidewater glaciers and its applicability to greenland outlet glaciers is discussed

ring shear studies of till deformation coulomb plastic behavior and distributed strain in glacier beds a ring shear device was used to study the factors that control the ultimate steady strength of till at high shear strains tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26 3 and 18 6 for tills containing 4 and 30 clay sized particles respectively other tests at steady normal stresses and variable shear strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate this weakening may be due to small increases in till porosity these results provide no evidence of viscous behavior and suggest that a coulomb plastic idealization is reasonable for till deformation however viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till we hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure during a deformation event transient dilation ofdiscrete shear zones should cause a reduction in internal pore water pressure that should strengthen these zones relative to the surrounding till a process called dilatant hardening consequent changes in shear zone position when integrated over time may yield the observed distributed strain

the age of the nantuo formation and nantuo glaciation in south china a u pb sensitive high resolution ion microprobe shrimp age of 654 5 3 8 ma from an ash bed immediately below the nantuo formation in south china provides the lowest age constraint for the nantuo glaciation which has been correlated with the global marinoan glaciation a u pb shrimp age of 636 3 4 9 ma from a fallout tuff within the basal layer of the nantuo formation along with the existing age of 635 2 0 6 ma from its overlying doushantuo cap carbonate suggests that the traditionally defined nantuo formation may have been deposited in a short time period at the end of the nantuo glaciation in combination with available ages globally the data support a relatively short duration and rapid termination of marinoan glaciations

dodging snowballs geochronology of the gaskiers glaciation and the first appearance of the ediacaran biota the snowball earth hypothesis predicts that low latitude glaciation lasted millions of years while co2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate the gaskiers formation of eastern newfoundland canada has been attributed to a snowball glaciation event but the lack of robust paleomagnetic data and precise geochronological constraints has precluded tests of the hypothesis here we present high precision u pb zircon geochronology chemical abrasion isotope dilution thermal ionization mass spectrometry from eight tuffs from multiple distant stratigraphic sections that bracket glacial diamictites and the first appearance of large ediacaran fossils including internal error deposition of the gaskiers diamictite on the avalon peninsula is constrained to have been between 580 90 0 40 and 579 88 0 44 ma and the trinity diamictite on bonavista peninsula was deposited between 579 63 0 15 and 579 24 0 17 ma assuming approximately synchronous deglaciation these results imply a maximum duration for deposition of the trinity diamictite of 340 k y this is inconsistent with the multimillion year duration predicted by the snowball earth hypothesis our geochronologic data also constrain the first appearance datum of ediacaran fossils to 9 5 m y after the gaskiers glaciation thus despite existing paleomagnetic constraints that indicate that marine ice sheets extended to low to middle latitudes it appears that earth narrowly escaped a third neoproterozoic snowball glaciation just prior to the late ediacaran expansion of metazoan ecosystems

a review and assessment of longshore sediment transport equations for coarse grained beaches previous assessments of analytical longshore sediment transport formulae have been heavily biased towards sand sized sediment all have noted the shortage of high quality field data from coarse grained beaches against which to test predictions of longshore transport rates in this paper 12 existing formulae were identified as being potentially applicable for coarse grained sediments and predictions from these formulae are compared using a measured annual transport rate from a shingle beach and a concurrent hindcast wave climate two new empirical equations are also derived one from a numerical model calibrated against the same data set the other derived from field experiments on coarse grained beaches energetics based equations are found to give reasonable predictions of the shingle transport despite being derived for sand beaches in contrast those dimensional analysis type equations which had been validated using laboratory data grossly over predicted the measured transport rates the most accurate predictions were from formulae previously validated at sites similar to that used for this comparison and therefore require further testing against field data from dissimilar sites

application of image cross correlation to the measurement of glacier velocity using satellite image data abstract image to image cross correlation software is applied to pairs of digital satellite images to map the velocity field of moving ice this technique uses small scale glacial surface features such as crevasse scars and snow dunes as markers on the surface of the moving ice displacements of the surface features are mapped by selecting small image areas centered on distinct features or by dividing a large area of densely featured glacial surface into a grid of areas and searching a subsequent image for matching areas using a cross correlation algorithm interpolation of the peak correlation values allows the displacements to be measured to subpixel accuracy resulting in very precise velocity measurements cross correlation is also applied to provide image coregistration in areas devoid of bedrock exposures in such areas subtle large scale topographic undulations in the ice surface related to underlying bedrock structure may be correlated by using large image areas and low pass filtered images both types of applications are demonstrated using ice stream d and ice stream e in west antarctica as test areas a high resolution map of the velocity field of the central portion of ice stream e generated by the displacement measuring technique is presented the use of cross correlation software is a significant improvement over previous manually based photogrammetric methods for velocity measurement and is far more cost effective than in situ methods in remote polar areas

palaeoclimate snowball climate conundrum evidence for a neoproterozoic snowball earth in the sedimentary record has been controversial a weathered horizon preserved in sedimentary rocks from svalbard may provide a rare signature of prolonged global glaciation

sedimentary and structural evolution of a pleistocene small scale push moraine in eastern poland new insight into paleoenvironmental conditions at the margin of an advancing ice lobe abstract recent studies of push moraines have focused on the interplay between the dynamics of ice margins and the environmental variables of the foreland into which they advance these studies showed that the spatial distribution geometry and style of the glaciotectonic deformation of push moraines are controlled by ice induced stresses the strain rate the rheology of the deposits and hydraulic conductivity in this work we provide new insight into this interplay at a small spatio temporal scale specifically the ancient glacial system of the liwiec ice lobe within the younger saalian ice sheet in eastern poland the paleoenvironmental variables that are analysed here refer to the dynamics of the hydrological processes that affected the patterns and sediment deposition rate on the terminoglacial fan and the resulting mechanical stratigraphy and hydraulic conductivity of the foreland we document the progradational sequence of the fan deposits that developed as a result of the ice lobe thickening and the steepening of its stationary front the sedimentary features of the fan the lithology of its basement and the hydraulic conductivity of the foreland strongly influenced the geometry and kinematics of fold growth during the advance of the ice lobe the predominance of flexural slip and the development of fractures including fold accommodation faults were interpreted to be an effect of buckle folding due to horizontal shortening induced by ice advance the partial overriding of the push moraine by the ice lobe and thus the submarginal conditions for deformation were inferred from the significant hinge migration and internal deformation of the strata under undrained conditions in one of the folds the synfolding deposition pattern of the fan growth strata allowed us to suggest that the push moraine was probably formed by a sustained advance rather than surge

late cretaceous cenozoic climatic variations of the northern antarctic peninsula new geochemical evidence and review aptian to pliocene time series of x ray fluorescence data from the antarctic peninsula are used to assess the degree of chemical weathering and maturity of sediments deposited prior and subsequent to the establishment of glaciation in west antarctica a continuous palaeoclimatic signal is inferred from comparing the results with previously published palaeotemperature and palaeobotanical data aptian cenomanian warm cool and santonian early paleocene warm cold cycles were followed by the global late paleocene early eocene climatic optimum subsequent steady decline in temperatures resulted in glacial conditions in the northern antarctic peninsula by early oligocene time under alternating cenozoic frigid cold climates and in sedimentary regimes dominated by physical weathering geochemical signatures were primarily indicative of provenance the development of cretaceous arboreal vegetation in the northern antarctic peninsula can be correlated with the major climatic cycles

satellite derived volume loss rates and glacier speeds for the juneau icefield alaska we provide a high resolution map of elevation change rates dh dt at the juneau icefield jif southeastern alaska in order to quantify its contribution to sea level rise between 2000 and 2009 2013 we also produce the first high resolution map of ice speeds at the jif which we use to constrain flux and look for acceleration we calculate dh dt using stacked digital elevation models dems from the advanced spaceborne thermal emission and reflection radiometer aster instrument and the shuttle radar topography mission srtm taking into account srtm c band penetration via comparison with srtm x band elevations overall the jif is losing mass less rapidly 0 13 0 12mw e a than other alaskan icefields 0 79mw e a we determine glacier speeds using pixel tracking on optical image pairs acquired from 2001 to 2010 by aster from radar image pairs acquired between 2007 and 2011 and from radar interferometry in 1995 we detect seasonal speed variations but no interannual acceleration ruling out dynamics as the cause of the observed thinning thinning must therefore be due to the documented warming in the region flux measurements confirm this for mendenhall glacier showing that calving constitutes only 2 5 5 of mass loss there

dynamic controls on glacier basal motion inferred from surface ice motion 1 current heuristic laws that relate the motion of glaciers due to sliding along the bed to the subglacial water pressure fail to reproduce variations in sliding speed on timescales of specific hydrologic events such as lake drainage rainfall or surging this may be due to the importance of subglacial cavity evolution and shifts in the glacier stress field both of which are not accounted for in typical sliding laws we use multiple time series of surface motion over a 66 day period at brei amerkurjokull iceland to infer changes in bed separation and longitudinal force budget we observe multiple distinct periods of increased surface motion and uplift corresponding to periods of rainfall and or increased temperatures we find consistent hysteresis and lags between motion and variations in both the bed separation and longitudinal stress gradient that we attribute to the redistribution of normal stresses at the bed during cavity growth increases in the longitudinal stress gradient suggest a downglacier stress transfer during increased basal motion that is consistent with increased drainage system efficiency toward the terminus our results suggest that the transient evolution of the subglacial drainage system and shifts in the glacier stress field are important controls on basal motion

strong feedbacks between hydrology and sliding of a small alpine glacier 1 we report the spatial and temporal pattern of sliding on the 7 km long bench glacier alaska using five continuously recording gps antennas following motion of the surface ice distributed at 1 km spacing along the glacier center line we documented surface ice motion over 50 days during summer 2002 surface speeds in two previous winters constrain the motion component associated with ice deformation allowing isolation of the sliding speed history we observed two speedup events bracketing 2 weeks of steady slow sliding the first event was not associated with a meteorological trigger was more subtle than the second and propagated up glacier at a rate of several hundred meters per day the second event coincided with a warm up valley wind which triggered considerable melt of the glacier surface sliding speeds in this event reached 0 3 m d 1 and began almost simultaneously at all sites in the ablation area both the horizontal and vertical displacement time series can be explained by growth and collapse of cavities in the lee of bumps in the bedrock bed cavities grow during rapid sliding and decay by viscous creep we posit that effective pressure averaged over some large area of the bed is inversely proportional to the sliding speed this effective pressure then controls the collapse rate of cavities whose dimensions are estimated from a plausible stepped bed geometry this model explains well the horizontal and vertical surface displacement history through the first event and beginning of the second event the vertical record demands a substantial and abrupt drop in water pressure that departs from the posited sliding effective pressure relationship we argue that this pressure drop reflects establishment of efficient subglacial drainage also manifested in a nearly simultaneous step increase in water discharge in the exit stream the establishment of an efficient conduit system terminates sliding its maintenance inhibits further sliding over the remainder of the summer

rapid erosion of soft sediments by tidewater glacier advance taku glacier alaska usa 2 0 0 1 m a 1 1940 2005 at l 3 km h 350 m and 1 5 0 2 m a 1 1952 2005 at l 1 5 km h 250 m detailed mapping over a 4 km 2 area of the terminus revealed a deeply incised channel in line with a major outlet stream glaciofluvial processes must play the dominant role in the subglacial erosion and removal of these unlithified sediments citation motyka r j m truffer e m kuriger and a k bucki 2006 rapid erosion of soft sediments by tidewater glacier advance taku glacier alaska usa geophys res lett 33 l24504 doi 10 1029 2006gl028467

cyclostratigraphic constraints on the duration of the datangpo formation and the onset age of the nantuo marinoan glaciation in south china abstract constructing an accurate timeline is critical for reconstructing the earth systems through critical transitions in climate geochemistry and life existing dates constrain synchronous initiation ca 717 ma and termination ca 660 ma of the sturtian glaciation from multiple continents the termination of the younger marinoan glaciation is also well dated at ca 635 ma but the onset of this glaciation is only roughly constrained as ca 654 ma south china and ca 639 ma namibia to test if the marinoan glaciation started close to ca 654 ma or ca 639 ma we have conducted a cyclostratigraphic study on the cryogenian non glacial datangpo formation that conformably overlies and underlies sturtian and marinoan glacial diamictites respectively in a deep water basin section in south china a total of 28 765 magnetic susceptibility ms measurements from a drillcore of the 292 m thick muddy siltstone and shale dominated datangpo formation are used for cyclostratigraphic analysis the results reveal significant decameter to meter scale sedimentary cycles of 16 12 m 3 6 3 0 m 1 0 0 8 m and 0 6 0 4 m the ratios of these cycle wavelengths match well with those of the milankovitch cycles calibrated for the cryogenian period the established astrochronologic time scale suggests that the duration of the datangpo formation is about 9 8 million years together with the radiometric age of ca 660 ma for the termination of the sturtian glaciation the cyclostratigraphic data suggest that the nantuo marinoan glaciation in south china initiated at ca 650 ma which is slightly younger than but consistent with the ca 654 ma u pb age from the top of the datangpo formation in shelf sections this age however is significantly older than the ages obtained from marinoan age glacial diamictites in south china ca 636 ma and namibia ca 639 ma given that most of the shelf sections may have suffered from glacial erosion obtaining the onset age of the marinoan glaciation should focus on relatively complete deep water successions

cryogenian of yukon abstract cryogenian strata of the windermere supergroup were deposited in tectonically active basins throughout the north american cordillera from alaska to mexico the windermere supergroup of yukon canada hosts key geochronological constraints on the start of the cryogenian period and the onset of the sturtian snowball earth glaciation at ca 717 ma a 57 million year duration for the sturtian glaciation has been suggested by correlations of the glacigenic rapitan group in yukon with equivalent strata in the mackenzie mountains of northwest territories where it is overlain by organic rich argillaceous carbonate of the ca 660 ma twitya formation however syn depositional faulting and large lateral facies variation complicate these stratigraphic correlations here we describe cryogenian strata of the northern wernecke mountains yukon where correlations with the rapitan and hay creek groups of the mackenzie mountains have been previously established these units are then traced south across cryogenian structures to the southern wernecke mountains and west through the hart river inlier to the coal creek and tatonduk inliers of the ogilvie mountains in order to construct a new regional stratigraphic framework the coal creek inlier preserves volcanic rocks in the mount harper group and the eagle creek formation of the rapitan group for which we provide new u pb ca idtims zircon ages we further formalize the glacigenic cryogenian eagle creek formation together the new correlations and geochronological data allow us to more precisely integrate geological data from northwest canada with the rest of the cordillera and beyond particularly these data refine the onset of the sturtian glaciation in northwestern canada to between 716 9 0 4 and 717 4 0 2 ma and demonstrate that syn sedimentary tectonism resulted in unconformities throughout northwest laurentia between the end of the sturtian glaciation at 660 ma and the end of the marinoan glaciation at 635 ma these unconformities confound correlations throughout the cordillera by commonly preserving only one of the two cryogenian glacial deposits

terminus dynamics at an advancing glacier taku glacier alaska taku glacier alaska usa is currently in the advance stage of the tidewater glacier cycle we investigated the near terminus dynamics by measuring surface velocities surface elevation changes ice thickness and ablation velocities vary on sub daily diurnal seasonal and interannual timescales flowline modeling shows that the modeled surface velocities are sensitive to changes in till yield strength and thus effective basal pressures the glacier bed deepens in the up glacier direction and this imposes a minimum subglacial water pressure necessary for water to drain along the bed in a simple model we impose water pressure gradients based on phreatic surfaces of constant slopes to simulate the winter summer transitions this proves sufficient to explain an observed early season switch from compressional to block flow velocities also vary between years changing basal conditions can result in lower horizontal velocities which decrease the ice supply to the terminus and result in temporary surface lowering but a decrease in ice flux to the terminus must lead to ice storage further upstream and that ice mass will eventually reach the terminus this can explain the observed episodic nature of terminus advance

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

swash zone sediment transport step dynamics and morphological response on a gravel beach abstract state of the art morphodynamic field data were collected from slapton sands uk a fine gravel beach d50 2 10 mm over a 10 day period under a range of wave tide conditions comprising a mixture of southerly swell and easterly wind waves hs 0 5 1 m ts 4 8 s tide range 3 4 m over the field period the upper beach experienced accretion and berm formation during tides with low steepness swell dominated wave conditions h l 0 01 surprisingly the intertidal beach showed an opposite trend with sediment losses under swell conditions and an increase in sediment volume due to wind waves and suggests that upper beach morphological change can be decoupled from the overall sediment budget a submerged accretionary feature the beach step grew and migrated onshore on the rising tide typically reaching a maximum relief of around 0 5 m then gradually decreased in prominence while migrating offshore as the tide fell resulting in little net morphological change the step was better developed during the falling tide than the rising tide and through its control on the swash hydro and sediment dynamics the step had a significant effect on the development of the berm swash flow velocities under the conditions measured were often in excess of 2 m s 1 for both the uprush and the backwash onshore mean flows occurred in the upper swash region 40 inundation was characterised by offshore mean flows swash infiltration losses in the swash zone increased from 40 in the lower swash region to 100 in the upper swash region the most energetic swash flows and the strongest offshore mean flows occurred at the transition between swash zone and surf zone 90 100 inundation

equal mobility of gravel in streams the remains of the day 1 the concept of equal mobility of gravel transport is 2 decades old the weak form of the hypothesis states that in order to transport the coarse half of the mean annual gravel load through a river reach at a rate equal to the fine half the coarser material must be overrepresented on the bed surface giving rise to a mobile bed armor the degree of mobile bed armoring necessary to accomplish this diminishes with increasing bed shear stress the strong form of the hypothesis states that the grain size distribution of the gravel portion of the bed load should be similar to that of the substrate and finer than that of the surface layer here the hypothesis is subjected to a rigorous test in the laboratory under controlled conditions which are nevertheless chosen to be as representative as possible of actual field conditions the grain size distributions used in the experiment included both model gravel and model sand at the end of the experiment the entire surface layer and a significant depth of substrate were removed from an entire reach of the channel and sieved the results of the experiment confirmed the weak form of equal mobility the surface layer was significantly coarsened compared with the bed load even when the sand was excluded from the size distributions the strong form of equal mobility was also supported by the experiment in that the size distribution of the gravel substrate was nearly identical to the size distribution of the gravel portion of the load the gravel substrate was however slightly but measurably coarser than the gravel load a tendency that has been observed in the field in cases for which the strong form is not satisfied

global continental weathering trends across the early permian glacial to postglacial transition correlating high and low paleolatitude sedimentary records time equivalent early permian sedimentary successions from high latitude gondwana basins and from equatorial accumulations in north china covering the glacial to postglacial transition display correlatable trends in continental weathering intensity based on chemical index of alteration cia values the successions display a pattern of cia values that varies with latitude similar to modern estuarine suspended sediments based on the modern day cia temperature correlation the glacial early permian low to high latitude land surface temperature gradient is estimated as 20 c slightly higher than the calculated values for the contemporaneous sea surface temperature gradient

magmatic evolution and crustal recycling for neoproterozoic strongly peraluminous granitoids from southern china hf and o isotopes in zircon zircons can retain a grain scale record of granitoid compositional evolution that is accessible through microanalysis in situ u pb hf and o isotope data yield new insights into the petrogenesis and evolution of the neoproterozoic strongly peraluminous granitoids of the jiangnan orogen jo southern china a negative correlation of th u versus d 18 o is found for most analyses some zircons from eastern jo granitoids show d 18 o variations of 3 6 from core to rim indicating a dramatic shift toward higher oxygen isotope values by voluminous partial melting of supracrustal rocks and signaling a transition from i type like to s type like magmas during the later stage of magmatic evolution this mechanism provides a reasonable explanation why some granitoids have intermediate geochemical features between s type and i type granites hf isotope trends indicate that a larger proportion of mature continental crust was incorporated into the magma sources of the western jo granitoids whereas more juvenile arc crust was incorporated into the eastern jo magmas no significant depleted mantle derived mafic magma was injected into the jo granitoid magmas instead radiogenic hf and nd signatures in jo granitoids reflect incorporated juvenile arc crust and document crustal growth in southern china during the early neoproterozoic ca 900 ma thus our zircon data suggest that strongly peraluminous granitoids which are widely regarded as the products of orogenesis that primarily recycle evolved crust can also record important information about early crustal growth

the survival of benthic macroscopic phototrophs on a neoproterozoic snowball earth the greatest ice ages in earth s history occurred during the 654 635 ma marinoan glaciation when glaciers reached tropical oceans and our planet approached a snowball earth condition paleontological and genomic data suggest that several eukaryotic groups must have survived the marinoan glaciation but their fossil record is scarce and limited to microbes whose ecological and physiological ranges are poorly constrained thus hampering a full understanding of how and where eukaryotic life particularly macroscopic phototrophs survived this snowball earth here we report carbonaceous compression fossils from the marinoan age nantuo formation in south china these fossils are preserved in thin black shales sandwiched between glacial diamictites deposited in inner shelf environments of the mid latitudinal yangtze block some of these fossils are interpreted as benthic macroalgae thus the marinoan glaciation must have been punctuated by episodes of open waters where habitable benthic substrates were available in the photic zone and along the coast of mid latitudinal continents such open waters may have been the refugia where macroscopic phototrophs survived the marinoan glaciation and subsequently diversified in the early ediacaran period

breaker turbulence and sediment suspension in the surf zone field measurements of fluid velocities and suspended sediment concentrations were used to investigate the relative role of coherent vortices related to wave breaking in suspended sediment dynamics the measurements were obtained from a barred intermediate type beach and the instrument rig was exposed to a range of wave types during the experiment including shoaling waves and both spilling and plunging breakers the time scale of the observed breaker vortices was 5 hz and thus in the frequency range of wave orbital motion coherent breaker vortices were separated from wave orbital motions and incoherent higher frequency turbulence using a threshold magnitude for the vertical velocity component plunging breakers displayed the largest root mean square rms and maximum vertical velocities the latter reaching approximately 1 m s 1 these measurements were made at an elevation of 0 15 m above the bed given the observed scale of the breaker vortices and the phase match between suspended sediment concentrations and fluid velocities the turbulence injected into the water column by the plunging breakers played a significant role in suspending bed sediment on the other hand the role of coherent vortex structures in suspending sediment under spilling breakers and shoaling waves is more limited rms vertical velocities under the spilling breakers and shoaling waves in this experiment were respectively a factor 2 and 4 smaller than for plunging breakers different phase relationships were observed between the horizontal fluid velocity and vortex induced sediment suspension for each of the wave types which may contribute to an explanation of net offshore and onshore sediment transport observed during episodes of beach erosion and recovery respectively

reconstructing early permian tropical climates from chemical weathering indices this study was financially supported by the national natural science foundation of china grants 41302083 41572078 the national basic research program of china grant 2011cb808800 and the fundamental research funds for the central universities grant cugl140402 china university of geosciences wuhan p a cawood acknowledges support from nerc grant ne j021822 1

growth and collapse of the distributed subglacial hydrologic system of kennicott glacier alaska usa and its effects on basal motion nearly 100 days of hourly glacier motion hydrology and hydrochemistry measurements on kennicott glacier alaska usa demonstrate the complicated relationship between water and motion at the glacier bed our observations capture the transient glacier response to seasonal and daily melt cycles and to a jokulhlaup that prompts a sixfold increase in glacier speed sliding is promoted whenever the water inputs to the glacier exceed the capacity of the subglacial hydrologic system to transmit the water sensitivity of sliding to daily meltwater inputs varies strongly through the season implying that the state of the hydrologic system governs the sensitivity of basal sliding a numerical model constructed to explore these relationships reveals the roles of the effective pressure the exponent to which this is taken in the sliding law 0 1 0 6 glacier macroporosity 2 and the cavity generating capacity of the glacier bed which encapsulates the sizes and spacing of bed roughness elements temporal changes in the effective pressure associated with evolution of both water inputs and subglacial water transmission capacity can explain the varying strength of diurnal velocity fluctuations of kennicott glacier spatial patterns of glacier macroporosity and of basal roughness govern variation in sensitivity of sliding to water inputs

post orogenic extension in the eastern part of the jiangnan orogen evidence from ca 800 760ma volcanic rocks we present a systematic geochronological and geochemical study on ca 800 760 ma volcanic rocks in the eastern part of the jiangnan orogen the xucun composite dykes are dated at ca 805 ma the mafic components have oib like trace element patterns and positive anomalies in zr and hf the least contaminated sample has relatively depleted nd isotopic features suggesting the xucun mafic dykes may have been generated from the partial melting of oib like asthenosphere with later crustal contamination the xucun felsic dykes have decoupled nd hf isotopes and the hf isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early neoproterozoic juvenile crustal materials with minor incorporation of paleoproterozoic crustal components the ca 800 790 ma shangshu volcanics include two compositional series calc alkaline and tholeiitic the shangshu calc alkaline volcanics in the minjiawu area have low abundances of lile hfse and high na2o contents and sr y ratios similar to adakitic rocks the evident arc like geochemical features and radiogenic nd isotopes nd t values of 3 7 to 4 8 suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by na rich melts released from the subducted slab the tholeiitic mafic rocks from the shangshu bimodal volcanics represent two different magma sources the partial melting of metasomatized lithospheric mantle led to the formation of arc like basalts with low tio2 contents negative anomalies in zr and hf and high values of mg and nd t 6 2 whereas the partial melting of asthenospheric mantle generated volcanic rocks with high tio2 contents and low positive nd t 1 4 to 2 7 without negative anomalies of nb ta zr and hf the shangshu felsic rocks were formed by the reworking of early neoproterozoic juvenile arc crustal materials the ca 760 ma mafic rocks from the puling bimodal volcanics generally have low tio2 contents 0 9 wt nearly flat ree distributions and arc like trace element patterns they may have been generated from the high degree partial melting of metasomatized lithospheric mantle one sample has a high tio2 content 2 41 wt and high nd t 6 2 with overall oib like trace element patterns implying the local partial melting of asthenospheric mantle the occurrence of significant volumes of bimodal volcanics in the eastern part of the jiangnan orogen suggests an extensional setting in the period ca 800 760 ma the evident partial melting of newly metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post orogenic extension shortly after the neoproterozoic orogenesis may be a better explanation for the genesis of the mid neoproterozoic magmatic rocks in the eastern part of the jiangnan orogen post orogenic extension may be diachronous along the whole orogenic belt and probably has no direct relationship with the rodinia rifting event a more detailed model is presented to illustrate the evolution of the eastern part of the jiangnan orogen

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

observations of a reversal in vertical and horizontal strain rate regime during a motion event on unteraargletscher bernese alps switzerland during a motion event on unteraargletscher bernese alps switzerland in spring 1996 surface velocities were measured up to eight times a day at four different locations along the central flowline using global positioning system equipment in addition accumulated vertical strains over the uppermost 50 and 100 m were measured at a location where the total ice thickness is 260 m the motion event was accompanied by high horizontal and vertical strain rates as compared to annual mean values a reversal in strain regime was observed with horizontal strain rates changing to extension while vertical strain rates became compressive this strain rate reversal coincided within the temporal resolution of the data with a maximum in vertical ice displacement at the surface within a day variations in vertical strain from 0 04 a 1 to 0 06 a 1 were observed over the uppermost 100 m vertical stretching is estimated to have contributed to at least 20 of the anomalous vertical ice movement at the surface there were significant differences between measured longitudinal strain averaged over a distance corresponding to a few ice thicknesses and measured vertical strain in spring 1997 a similar but more detailed set of measurements was collected at the same measuring site and vertical strain rates were found to vary non uniformly with depth with the largest values closest to the surface

duration and nature of the end cryogenian marinoan glaciation the end cryogenian glaciation marinoan is portrayed commonly as the archetype of snowball earth yet its duration and character remain uncertain here we report u pb zircon ages for two ash beds from widely separated localities of the marinoan equivalent ghaub formation in namibia 639 29 0 26 ma and 635 21 0 59 ma these findings verify for the first time the key prediction of the snowball earth hypothesis for the marinoan glaciation i e longevity with a duration of 4 m y they also show that the nonglacial interlude of cryogenian time spanned 20 m y or less and that glacigenic erosion and sedimentation and at least intermittent open water conditions occurred 4 m y prior to termination of the marinoan glaciation

silicified glendonites in the ediacaran doushantuo formation south china and their potential paleoclimatic implications the ediacaran period is punctuated by the ca 580 ma gaskiers glaciation in newfoundland however paleoclimatic data are scarce in ediacaran successions in south china where abundant geochemical and paleobiological data are shaping current understanding of ediacaran evolutionary and environmental history here we report the occurrence of silicified glendonites in the ediacaran doushantuo formation deposited in an inner shelf environment on the south china block petrographic evidence suggests that these silicified glendonites are pseudomorphs after syndepositional or early authigenic ikaites formed at near freezing temperatures the glendonite bearing stratigraphic interval is characterized by positive 13c values it predates both the negative 13c excursion en3 widely believed to be an equivalent of the shuram negative excursion and excursion en2 although alternative interpretations may be possible these glendonites may be related to and correlated with the gaskiers glaciation if confirmed this correlation suggests that the shuram event postdates the gaskiers glaciation thus having important implications for ediacaran climate changes carbon cycles and biological evolution

the ciw index a new chemical index of weathering abstract a new chemical index of weathering that can be applied to modern soils and precambrian paleosis is proposed ciw al2o3 the value of this index increases as the degree of weathering increases and the difference between ciw index values of the silicate parent rock and soil or sediment reflects the amount of weathering experienced by the weathered material

behavior of major and trace elements including ree during paleoproterozoic pedogenesis and diagenetic alteration of an archean granite near ville marie quebec canada abstract this paper presents a complete set of chemical data from a pre 2 0 ga weathering profile that formed on the archean granite near ville marie quebec examination of data from two sections of the paleosol indicates that na ca p and sr were lost from the paleosol whereas ti nb th zr hf and ta remained immobile during chemical weathering and all subsequent alteration rare earth mobility during genesis of the ville marie paleosol closely follows the pattern of early stages of chemical weathering in modern profiles average concentration of ree in both profiles is 20 40 lower than what is found in the fresh granite suggesting mobilization during pedogenesis hrees seem to be less mobilized than the lrees la sm n ratios gradually decrease toward the top of the paleosol but the values remain very close to that of the protolith gd yb n values remain unchanged in the weathering profile la yb n values in the weathering profile are quite variable and generally lower than that of the protolith eu eu and ce ce values are essentially the same as those of the protolith but two samples from the uppermost part of the profile have developed positive cerium anomaly of 0 3 relative to the protolith

climatic cycles during a neoproterozoic snowball glacial epoch the profound glaciations of the neoproterozoic cryogenian period ca 850 544 ma represent an extreme climatic mode when it is claimed earth was fully or almost completely covered with ice for millions of years we show that the geochemistry and mineralogy of fine grained neoproterozoic sedimentary rocks in oman are best explained by climatic oscillations that drove variations in the intensity of chemical weathering on contemporary land surfaces the cold climate modes of the cryogenian were therefore cyclical punctuated with well defined warm humid interglacial periods the hydrological cycle and the routing of sediment were active throughout the glacial epoch which requires substantial open ocean water this reconstruction represents a significantly different target for numerical climate models at this critical time in the evolution of earth s biosphere

where glaciers meet water subaqueous melt and its relevance to glaciers in various settings glacier change is ubiquitous but the fastest and largest magnitude changes occur in glaciers that terminate in water this includes the most rapidly retreating glaciers and also several advancing ones often in similar regional climate settings furthermore water terminating glaciers show a large range in morphology particularly when ice flow into ocean water is compared to that into freshwater lakes all water terminating glaciers share the ability to lose significant volume of ice at the front either through mechanical calving or direct melt from the water in contact here we present a review of the subaqueous melt process we discuss the relevant physics and show how different physical settings can lead to different glacial responses we find that subaqueous melt can be an important trigger for glacier change it can explain many of the morphological differences such as the existence or absence of floating tongues subaqueous melting is influenced by glacial runoff which is largely a function of atmospheric conditions this shows a tight connection between atmosphere oceans and lakes and glaciers subaqueous melt rates even if shown to be large should always be discussed in the context of ice supply to the glacier front to assess its overall relevance we find that melt is often relevant to explain seasonal evolution can be instrumental in shifting a glacier into a different dynamical regime and often forms a large part of a glacier s mass loss on the other hand in some cases melt is a small component of mass loss and does not significantly affect glacier response

modeling the evolution of the juneau icefield between 1971 and 2100 using the parallel ice sheet model pism we study the evolution of the juneau icefield one of the largest icefields in north america 3700 km 2 using the parallel ice sheet model pism we test two climate datasets 20 km weather research and forecasting model wrf output and data from the scenarios network for alaska planning snap derived from spatial interpolation of observations good agreement between simulated and observed surface mass balance was achieved only after substantially adjusting wrf precipitation to account for unresolved orographic effects while snap s climate pattern is incompatible with observations of surface mass balance using the wrf data forced with the rcp6 0 emission scenario the model projects a decrease in ice volume by 58 68 and a 57 63 area loss by 2099 compared with 2010 if the modeled 2070 99 climate is held constant beyond 2099 the icefield is eliminated by 2200 with constant 1971 2010 climate the icefield stabilizes at 86 of its present day volume experiments started from an ice free state indicate that steady state volumes are largely independent of the initial ice volume when forced by identical scenarios of climate stabilization despite large projected volume losses the complex high mountain topography makes the juneau icefield less susceptible to climate warming than low lying alaskan icefields

beach morphology and change along the mixed grain size delta of the dammed elwha river washington abstract sediment supply provides a fundamental control on the morphology of river deltas and humans have significantly modified these supplies for centuries here we examine the effects of almost a century of sediment supply reduction from the damming of the elwha river in washington on shoreline position and beach morphology of its wave dominated delta the mean rate of shoreline erosion during 1939 2006 is 0 6 m yr which is equivalent to 24 000 m3 yr of sediment divergence in the littoral cell a rate approximately equal to 25 50 of the littoral grade sediment trapped by the dams semi annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer shoreline change and morphology also differ spatially negligible shoreline change has occurred updrift west of the river mouth where the beach is mixed sand to cobble cuspate and reflective the beach downdrift east of the river mouth has had significant and persistent erosion but this beach differs in that it has a reflective foreshore with a dissipative low tide terrace downdrift beach erosion results from foreshore retreat which broadens the low tide terrace with time and the rate of this kind of erosion has increased significantly from 0 8 m yr during 1939 1990 to 1 4 m yr during 1990 2006 erosion rates for the downdrift beach derived from the 2004 2007 topographic surveys vary between 0 and 13 m yr with an average of 3 8 m yr we note that the low tide terrace is significantly coarser mean grain size 100 mm than the foreshore mean grain size 30 mm a pattern contrary to the typical observation of fining low tide terraces in the region and worldwide because this cobble low tide terrace is created by foreshore erosion has been steady over intervals of at least years is predicted to have negligible longshore transport compared to the foreshore portion of the beach and is inconsistent with oral history of abundant shellfish collections from the low tide beach we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta thus the cobble low tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta which in turn is related to the impacts of the dams on the elwha river to sediment fluxes to the coast

observations of coarse sediment movements on the mixed beach of the elwha delta washington abstract mixed beaches with poorly sorted grains of multiple sizes are a common and globally distributed shoreline type despite this rates and mechanisms of sediment transport on mixed beaches are poorly understood a series of tracer deployments using native clasts implanted with radio frequency identifier rfid tags was used to develop a better understanding of sediment transport directions and magnitudes on the mixed grain size beach of the elwha river delta using tracer samples selected to match the distribution of the coarse fraction on the beach we find that all grain sizes up to large cobbles 128 256 mm were mobile under most measured wave conditions and move in relationship to the direction of the alongshore component of wave energy as estimated by incident breaking wave angles in locations where the breaking wave is normal to the shoreline we find that tracers move in both alongshore directions with approximately equal frequency in locations where breaking waves are oblique to the shoreline we find that alongshore transport is more unidirectional and tracers can approach average velocities of 100 m day under winter wave conditions we use the tracer cloud to estimate the beach active width the mobile layer depth and sediment velocity our results suggest that while sediment velocity increases under increased incident wave angles the active layer depth and width decrease reducing sediment flux at the site with the more oblique breaking waves this result is contrary to what is suggested by traditional wave energy transport models of alongshore sediment transport

quantifying temperature variation between neoproterozoic cryochron nonglacial interlude nanhua basin south china abstract three major late neoproterozoic global cryochrons mark the coldest climate state in earth history with glaciers covering most if not all continents however the terrestrial paleosurface temperature differential between cryochron and nonglacial interlude is unknown time equivalent cryogenian and ediacaran sedimentary successions from south china and oman show multiple negative excursions of the corrected values of their chemical index of alteration ciacorr these excursions reflect climate cooling and correlate with the sturtian marinoan and gaskiers cryochrons based on the na mat transfer function sodium chemical depletion index mean annual temperature we estimate the temperature differential between cryogenian cryochron and nonglacial interlude reached 20 5 4 at around 30 paleolatitude and 12 5 4 temperature difference between the ediacaran cryochron and nonglacial interlude near the equator the fluctuation of ciacorr and mat trends within individual cryochrons likely indicate cold warm climate cycling during the cryogenian cryochron

low latitude glaciation in the neoproterozoic of oman although earth is widely believed to have undergone a series of extreme low latitude snowball glaciations during the neoproterozoic ca 1000 543 ma only one reliable paleomagnetic result from elatina south australia places glacial rocks close to the equator we report new paleomagnetic data from the neoproterozoic huqf supergroup of oman that pass fold and reversal tests and yield a paleopole at 52 3 s 074 4 e n 25 sites 95 7 3 this paleopole places the muscat region of oman at a latitude of 13 in the late neoproterozoic and provides the first direct evidence that both glacial and overlying cap carbonate units were deposited in the tropics the presence of glacial interglacial cyclicity within the huqf supergroup indicates that areas close to the equator may have been largely free of ice at the time of deposition a result that is inconsistent with the classic snowball earth model our result precludes the possibility that contrasting lithologies mark a phase of rapid plate motion and provides the first evidence for low latitude glaciation in arabia a series of magnetic reversals in the fiq tillite and the overlying hadash dolomite in northern and central oman correlates well with a similar sequence in the mirbat formation in southern oman and indicates that recovery from glacial conditions took place over long time scales possibly 105 106 yr

late quaternary deglaciation and sea level history of eastern juan de fuca strait cascadia abstract eastern juan de fuca strait of southwestern british columbia northwestern washington state is near the western and southern terminus of late wisconsinan glaciation in north america seismic reflection profiles bathymetric data sediment core data and 41 new radiocarbon dates provide insight into the history of ice retreat and sea level change in the eastern portion of the strait the retreating ice margin appears to have passed through the area between 14 460 200 and 13 595 145 14c yr glacial marine sediments date between 12 889 50 and 11 110 50 14c yr bp wave eroded unconformities drowned prograded deposits and a partially isolated low stand basin are evidence attributed to low stand sea levels following deglaciation and five new control points on the local eustatic sea level curve are assigned based on radiocarbon dating of these features sea level change was rapid from 75 m immediately after deglaciation to possibly 60 4 m at 9920 60 14c yr bp after this time gradual sea level transgression took place relative sea level reached its approximate present position by 5470 115 14c yr bp since then relative sea level has remained constant suggesting eustatic rise is balanced by crustal uplift

reconstructing cryogenian to ediacaran successions and paleogeography of the south china block abstract neoproterozoic paleogeography of the south china is dominated by the formation of rodinia its break up and the subsequent amalgamation of gondwana two negative excursions of the chemical index of alteration cia and the chemical index of weathering ciw recorded in the cryogenian to ediacaran sedimentary rocks of the cathaysia block south china indicate two cooling events in combination with available age constraints this data suggests correlation with the global marinoan ca 650 635 ma and gaskiers ca 579 ma glaciations u pb ages of detrital zircons from the cryogenian to lower ediacaran strata define two distinctive age peaks at 1056 ma and 998 ma whereas the upper ediacaran strata show only one age peak at 957 ma these data demonstrate neoproterozoic sedimentary rocks in the cathaysia block were derived from a source external to the craton the predominant late mesoproterozoic to early neoproterozoic 1140 870 ma detrital zircons were most likely derived from a source dominated by north india and east antarctica with limited input from western australia furthermore ehf t values of detrital zircons are similar to the coeval detrital zircons from rayner eastern ghats the detrital record of the late cryogenian and ediacaran strata in the cathaysia block suggests that the south china block was connected to the northern margin of india during the dispersal of rodinia and the assembly of east gondwana detrital zircons from cryogenian strata overlying the jiangnan orogen are largely in the range 900 700 ma with scattered archean and paleoproterozoic grains these ages are consistent with derivation from local sources within south china the timing of accumulation of these units after collisional assembly of south china likely accounts for their more restricted provenance with collision generated relief providing both a local source and restricting input from further south within india and or the rest of gondwana

slow net sediment accumulation sets snowball earth apart from all younger glacial episodes during the sturtian and marinoan snowball earth episodes ice cover is thought to have extended from polar to tropical latitudes we test the supposition that such an extreme glacial climate not repeated in the subsequent 635 m y of earth history would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans with 500 sediment accumulation rates to characterize sturtian and marinoan deposits we find median accumulation rates at least four to 15 times slower than expected for phanerozoic glaciomarine deposits as characterized by 10 000 rates our comparison is conservative with respect to time span latitude and distance from the ice margin phanerozoic accumulation rates decrease systematically when averaged over longer time spans comparisons were drawn therefore at 5 and 57 m y time spans to match minimum marinoan and sturtian durations respectively cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin after accounting for time span snowball earth deposits at low latitude are found to be thinner than would be expected either for high latitude cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice rafted debris the rate discrepancy is not readily attributed to overestimates of the total marinoan or sturtian durations if sediment fluxes during warm melt intervals did approach phanerozoic rates these intervals must have occupied a much smaller proportion of snowball earth episodes than in younger glacial climates

time dependent basal stress conditions beneath black rapids glacier alaska usa inferred from measurements of ice deformation and surface motion observations of surface motion and ice deformation from 2002 03 were used to infer mean stress fields in a cross section of black rapids glacier alaska usa over seasonal timescales basal shear stresses in a well defined zone north of the center line orographic left were approximately 7 and 16 lower in spring and summer respectively than in winter correspondingly higher stresses were found near the margins these changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1 2 and 1 5 times larger in spring and summer than in winter these results were inferred with a simple inverse finite element flow model that can successfully reproduce bulk surface velocities and tiltmeter data stress redistribution between the well defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions ice bed decoupling or enhanced till deformation thereby causing large variations in surface velocity and strongly influencing the glacier s net motion during summer

arc magmatism in the delhi fold belt shrimp u pb zircon ages of granitoids and implications for neoproterozoic convergent margin tectonics in nw india abstract the northwestern region of peninsular india preserves important records of precambrian plate tectonics and the role of indian continent within proterozoic supercontinents in this study we report precise shrimp zircon u pb ages from granitoids from the sirohi terrane located along the western fringe of the delhi fold belt in rajasthan nw india the data reveal a range of neoproterozoic ages from plagiogranite of peshua foliated granite of devala and porphyritic granite of sai with zircon crystallization from magmas at 1015 4 4 ma 966 5 3 5 and 808 3 1 respectively the plagiogranite shows high sio2 na2o and extremely low k2o rb ba comparable with typical oceanic plagiogranites these rocks possess low lree and hree concentrations and a relatively flat lree hree slope a well developed negative eu anomaly and conspicuous nb and ti anomalies compared to the plagiogranite the foliated devala granite shows higher sio2 and moderate na2o together with high k2o and comparatively higher rb ba sr and ree with steep ree profiles and a weak positive eu anomaly in contrast to the plagiogranite and foliated granite the porphrytic sai granite has comparatively lower sio2 moderately higher na2o extremely high y zr nb and elevated ree the geochemical features of the granitoids hfse depletion and lile enrichment nb and ta negative anomalies and their plots in the fields of volcanic arc granites and those from active continental margins in tectonic discrimination diagrams suggest widespread neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm our results offer important insights into a long lived active continental margin in nw india during early and mid neoproterozoic consistent with recent similar observations on cryogenian magmatic arcs widely distributed along the margins of the east african orogen and challenge some of the alternate models which link the magmatism to extensional tectonics associated with rodinia supercontinent breakup

a rapid and synchronous initiation of the wide spread cryogenian glaciations abstract the neoproterozoic glaciations represent a remarkable milestone in earth evolution due to their major influence on atmosphere biosphere and hydrosphere the early cryogenian sturtian glaciations evidence for which has been recorded worldwide may have lasted for about 50 million years but the precise timing and synchroneity of these glaciations have been disputed jiangkou glacigenic strata in south china can be attributed to the sturtian glacial episode as they share sedimentological stratigraphic and lithological similarities with sturtian strata in south australia as such dating the onset of the jiangkou glaciation would contribute to testing the synchroneity of the worldwide sturtian glaciation in this paper we demonstrate by means of sims u pb zircon dating that the jiangkou glaciation in south china was initiated at 715 9 2 8 ma the new age is within uncertainty synchronous with dates obtained from the lower parts of glacigenic successions in laurentia and oman thereby indicating thereby indicating a synchroneity and global extent to the sturtian glaciation the synchronous occurrence of the sturtian glaciation in mid to low latitude regions implies rapid ice sheet advancement in response to ice albedo feedback as supported by numerical climatic modeling

variational methods for glacier flow over plastic till we investigate the mechanics of ice streams and glaciers flowing over a bed consisting of coulomb plastic subglacial sediment or more generally of channel flows with coulomb or solid friction laws at the boundary sliding is assumed to occur if shear stress at the glacier bed attains a prescribed locally defined yield stress while no sliding is assumed possible below that yield stress importantly the location of regions of slip and no slip at the bed is not known a priori but forms part of the solution by analogy with friction problems in elasticity we derive a weak formulation as a semi coercive variational inequality which admits a unique solution provided a solvability condition ensuring force balance is satisfied the variational formulation is then exploited to calculate numerical solutions and we investigate the effect of variations in subglacial water pressure ice thickness and surface slope on the discharge of a valley glacier with a plastic bed significant differences are found between the behaviour of wide and narrow as well as steep and shallow angled glaciers and our results further indicate the need to develop models capable of accounting for longitudinal stresses

active seismic studies in valley glacier settings strategies and limitations subglacial tills play an important role in glacier dynamics but are difficult to characterize in situ amplitude variation with angle ava analysis of seismic reflection data can distinguish between stiff tills and deformable tills however ava analysis in mountain glacier environments can be problematic reflections can be obscured by rayleigh wave energy scattered from crevasses and complex basal topography can impede the location of reflection points in 2 d acquisitions we use a forward model to produce challenging synthetic seismic records in order to test the efficacy of ava in crevassed and geometrically complex environments we find that we can distinguish subglacial till types in moderately crevassed environments where moderate depends on crevasse spacing and orientation the forward model serves as a planning tool as it can predict ava success or failure based on characteristics of the study glacier applying lessons from the forward model we perform ava on a seismic dataset collected from taku glacier in southeast alaska in march 2016 taku glacier is a valley glacier thought to overlay thick sediment deposits a near offset polarity reversal confirms that the tills are deformable

measuring sediment transport and bed disturbance with tracers on a mixed beach abstract a new tracer deployment method is applied to the problem of resolving rates and patterns of littoral sediment transport on mixed beaches we applied the method at the morphologically complex meso tidal beach fringing the elwha river delta washington clasts were transformed into tracers using radio frequency identifier rfid tags these tracers were deployed in and on the bed along a cross shore oriented beach transect along and cross shore variations in tracer velocity were used to estimate sediment transport patterns including the depth and cross sectional area of the disturbed bed and bulk alongshore transport rates we found that the peak bed disturbance averaged 22 of the tidal maximum significant wave height and that tracer velocity declined approximately logarithmically with depth in the bed across all deployments the maximum alongshore tracer velocity occurred between 1 0 and 2 0 m above mean lower low water mllw corresponding to elevations 0 3 m below and 0 7 m above mean sea level msl at this location alongshore bulk sediment transport rates estimated from the advection of tracers ranged from 1 1 to 173 1 m3 d under significant wave heights of 0 1 to 2 1 m and these transport rates related linearly to measured wave energy transport both the volumetric sediment transport rates and a cerc type k coefficient derived from the results agreed well with results from other published studies

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

the use of passive integrated transponder pit tags to trace cobble transport in a mixed sand and gravel beach on the high energy oregon coast usa abstract cobble transport is investigated on a mixed sand and gravel beach on the high energy oregon coast using radio frequency identification technology rfid to detect passive integrated transponder pit tags inserted in the gravels pit tags are glass encapsulated transponders that are activated when an antenna passes near them with each tag characterized by its own unique identification number the tags are sealed within the cobbles minimizing any effect on the hydrodynamic character of the particles the tags draw power from the antenna itself and have a potential lifespan of up to 50 years pit tags are simple to use and inexpensive permitting the release and tracking of large numbers of gravel particles as a result pit tags have tremendous potential for examining the complexities of particle movement and the parameters that control their distribution on the beach our experiments have been conducted at cape lookout state park on the northern oregon coast on a composite beach where a seasonally variable sand beach fronts a mixed sand and gravel beach with the latter impacted by waves only during the winter when the sand beach is cut back a total of 400 cobble tracers have been released at five different locations subsequent monitoring of the gravel tracers revealed that they can be detected up to 1 m below the surface of the beach using a 1 0 m diameter antenna and up to 0 5 m below the surface using a smaller 0 3 m diameter antenna while their positions were determined using a trimble 5700 5800 rtk dgps survey system gravel tracer recovery rates were found to vary significantly between the different release points from as high as 90 recovery after 8 months to a low of 18 after 17 months the lowest recovery rates are thought to be due to the extent of aggradation of the sandy beach during the summer partially covering the gravel berm and burying the tracer particles to depths where they could not be detected by the antennas used with our rfid system relocation of the gravel tracers has revealed a predominant northward migration of the cobbles and locally demonstrate a strong cross shore transport both upslope and downslope movements initial attempts to examine the relationship between the transport distances and particle sizes and shapes have thus far revealed that the larger gravels are tending to outrun the smaller particles this is despite the fact that both small and large particles were found to be capable of being transported both short and long distances in contrast our results revealed no relationships between particle shape and distance moved in time it is expected that the pit tag tracers may begin to yield more definitive patterns of sorting the results did highlight that the gravel movement occurs predominantly near the sand gravel beach junction on the profiles a region dominated by a higher incidence of wave breaking and strong swash activity finally our analyses revealed the potential for using pit tags to provide field assessments of the abrasion rates of gravel particles having already found measurable degrees of weight loss to abrasion in our experiments

rapid wastage of alaska glaciers and their contribution to rising sea level we have used airborne laser altimetry to estimate volume changes of 67 glaciers in alaska from the mid 1950s to the mid 1990s the average rate of thickness change of these glaciers was 0 52 m year extrapolation to all glaciers in alaska yields an estimated total annual volume change of 52 15 km 3 year water equivalent equivalent to a rise in sea level sle of 0 14 0 04 mm year repeat measurements of 28 glaciers from the mid 1990s to 2000 2001 suggest an increased average rate of thinning 1 8 m year this leads to an extrapolated annual volume loss from alaska glaciers equal to 96 35 km 3 year or 0 27 0 10 mm year sle during the past decade these recent losses are nearly double the estimated annual loss from the entire greenland ice sheet during the same time period and are much higher than previously published loss estimates for alaska glaciers they form the largest glaciological contribution to rising sea level yet measured

ice thickness measurements of taku glacier alaska u s a and their relevance to its recent behavior using radio echo soundings and seismic reflections we measured cross sections of taku glacier near juneau alaska to resolve inconsistencies in previous measurements and to understand better the glacier s dynamics the maximum thickness is about 1477 m and the minimum bed elevation is about 600 m below sea level which establishes taku glacier as the thickest and deepest temperate glacier yet measured our data indicate that during the 19th century the terminus of taku glacier may have begun its rapid advance at a position where the ice bed was greater than 300 m below sea level and more than 25 km from the inland end of its submarine trough this behavior is uncharacteristic of temperate tide water glaciers the glacier which no longer calves has eroded a sediment layer 100 m thick since 1890 at an average rate of about 3 m a 1 since 1948 this high erosion rate retards advance by entrenching the glacier into the terminal moraine calculations based on ice deformation theory indicate significant basal ice motion near the terminus and high basal shear stress 140 220 kpa along much of its length estimated differences between ice flux and balance flux are consistent with observed thickening and positive net mass balance these data indicate that ice volume is increasing and that further advance is likely

hydraulic and mechanical properties of glacial sediments beneath unteraargletscher switzerland implications for glacier basal motion the force on a ploughmeter and subglacial water pressure have been measured in the same borehole at unteraargletscher switzerland in order to investigate ice sediment coupling and the motion at the base of a soft bedded glacier a strong inverse correlation of the recorded pressure and force fluctuations in conjunction with a significant time lag between the two signals suggests that pore water pressures directly affect the strength of the subglacial sediment the lag is interpreted to reflect the time required for the water pressure wave to propagate through the pores of the sediment to the depth of the ploughmeter analysis of the propagation velocity of this pressure wave yielded an estimate of the hydraulic diffusivity a key parameter necessary to characterize transient pore water flow furthermore the inferred inverse relationship between pore water pressure and sediment strength implies that coulomb plastic deformation is an appropriate rheological model for the sediment underlying unteraargletscher however the sediment strength as derived from the ploughmeter data was found to be one order of magnitude smaller than that calculated for a coulomb frictional material using the water pressure measurements this significant discrepancy might result from pore water pressures in excess of hydrostatic down glacier from the ploughmeter as the ploughmeter is dragged through the sediment sediment is compressed if the rate of this compression is large relative to the rate at which pore water can drain away excess pore water pressures will develop that have the potential to weaken the sediment the same process could lead to highly fluid sediment down glacier from clasts that protrude into the glacier sole and thus would otherwise provide the roughness to couple the glacier to its bed iverson 1999 rapidly sliding glaciers overlying sediments might therefore move predominantly by ploughing which tends to focus basal motion near the glacier sole rather than at depth in the bed

rigorous gps data processing strategies for glaciological applications global positioning system gps data are now routinely used for many glaciological applications in some common cases systematic errors are unmodelled at the data processing stage although they are often presumed insignificant in this paper i investigate these assumptions for three different scenarios 1 measurements on a moving glacier 2 measurements on a floating ice shelf and 3 precise height determination over large elevation ranges such as for aircraft positioning in lidar laser altimeter missions in each case systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the gps processing software under certain conditions if these coordinate biases go undetected short and long term measurements of horizontal ice velocity or rates of ice thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques more rigorous processing strategies are discussed that allow for bias free parameter estimation

fast tidewater glaciers some iceberg calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets no temperate tidewater glaciers are known to have floating termini but many polar and subpolar tidewater glaciers do the fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed which may approach zero and the longitudinal back stress on the terminus the terminus boundary condition the calving relation is imperfectly known yet is vital to the dynamics of these glaciers calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested the calving relations for floating termini are virtually unknown this together with the imperfect understanding of basal sliding inhibits confidence in our understanding of the stability of these glaciers columbia glacier alaska is an instructive example because observations have been made on the major changes in its geometry calving rate and dynamics that have occurred in less than 10 years the calving flux has increased more rapidly than the glacier flux causing thinning and retreat as a result the ice velocity has increased markedly the short term velocity changes relate to changes in back pressure ice recession tidal changes and the flux of water injected to the bed these results have relevance to the mechanisms of basal sliding glacier surges and the stability of marine ice streams

a cryogenian chronology two long lasting synchronous neoproterozoic glaciations the snowball earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons here we present four new re os geochronology age constraints on sturtian 717 660 ma and marinoan 635 ma termination glacial deposits from three different paleocontinents a 752 7 5 5 ma age from the base of the callison lake formation in yukon canada confirms nonglacial sedimentation on the western margin of laurentia between ca 753 and 717 ma coupled with a new 727 3 4 9 ma age directly below the glacigenic deposits of the grand conglomerate on the congo craton africa these data refute the notion of a global ca 740 ma kaigas glaciation a 659 0 4 5 ma age directly above the maikhan uul diamictite in mongolia confirms previous constraints on a long duration for the 717 660 ma sturtian glacial epoch and a relatively short nonglacial interlude in addition we provide the first direct radiometric age constraint for the termination of the marinoan glaciation in laurentia with an age of 632 3 5 9 ma from the basal sheepbed formation of northwest canada which is identical within uncertainty to u pb zircon ages from china australia and namibia together these data unite re os and u pb geochronological constraints and provide a refined temporal framework for cryogenian earth history

taku glacier alaska u s a on the move again active deformation of proglacial sediments taku glacier is one of the few glaciers in alaska u s a that has advanced over the last century 7 km since 1890 this advance slowed substantially during the past decade but in summer 2001 the glacier terminus began to readvance at a rate of 30 cm d 1 the advance produced dramatic proglacial sediment deformation up to 200 m in from of the terminus two to three large bulges and several secondary bulges developed in the proglacial sediments as a result of glacial compression along a 1 km wide portion of the terminus the bulge nearest the terminus was 10 m high and 65 m wide the middle bulge 7 m high advanced at 15 cm d 1 and the distal bulge 3 m high and 50 m wide at 9 cm d 1 crenulations and prominent fractures developed in the overlying vegetation layer the frontal lobes of the bulges were steep and overlaid a shear zonc where sediments were being thrust up and over the ground surface ice proximal push moraines 1 10 m high formed along much of the 9 km wide terminus although deformation was minimal at some locations

ca 830 ma back arc type volcanic rocks in the eastern part of the jiangnan orogen implications for the neoproterozoic tectonic evolution of south china block abstract the volcanic rocks in the eastern part of jiangnan orogen provide an important suite to investigate magma petrogenesis and tectonic evolution of south china block we carried out a combined study of geochronology and geochemistry for the bimodal association of basalts and rhyolitic tuffs from the shuangqiaoshan group in south anhui province zircons from a basalt and two rhyolitic tuff samples yield crystallization ages of 829 12 ma 832 9 ma and 832 10 ma respectively the basalts are characterized by left sloping ree distribution patterns enriched large iron lithosphere elements liles e g rb ba k and depleted high field strength elements hfses e g nb ta relative to those in n morb resembling those of modern central lau back arc basin basalts babb in addition they have positive nd t values 4 6 to 5 7 with initial 87sr 86sr ratios in the range of 0 70706 0 70719 these geochemical and isotopic signatures indicate that the basalts were derived from an n morb like source modified by slab derived fluids the rhyolitic tuffs have high sio2 72 9 78 3 wt contents and high 10 000 ga al ratios mostly 2 6 taking into account their aluminous nature a cnk 1 1 they are affinitive to aluminous a type granites they invariably show a relative enrichment in light ree and strong negative eu anomalies eu eu 0 16 0 31 and negative anomalies in hfse sr and p compared with the neighboring elements in their trace element spidergrams their nd t values range from 2 1 to 0 1 detailed elemental and isotopic data suggest that the rhyolitic tuffs formed by partial melting of mafic lower crust triggered by asthenosphere upwelling the occurrence of bimodal volcanic rocks with the basalts showing geochemical affinity to intra oceanic babb appears to support the development of a neoproterozoic back arc basin in the eastern part of jiangnan orogen at ca 830 ma combined with the published data from central and western part of jiangnan orogen it is inferred that the back arc basin developed in the entire jiangnan orogen during 850 810 ma considering the age of the strata overlying the folded basement sequence we suggest that the final collision of the yangtze and cathaysia blocks happened during 820 810 ma along the jiangnan orogen

gcm simulations of snowball earth conditions during the late proterozoic in order to simulate the snowball earth conditions that may have existed during the late proterozoic we have conducted a series of gcm simulations using a simple 50 meter slab ocean a reduced solar constant of 6 and varied co2 concentrations in this study we vary the co2 concentration from 100 to 3400 ppmv and use rotation rates corresponding to 18 and 24 hour day lengths we also examine the effects of increasing the poleward transport of heat by the oceans our results show that below a critical value of approximately 1700 ppmv of atmospheric co2 sea ice and sub freezing temperatures occur from the poles to the equator a global mean annual two meter air temperature of 221 k is found for boundary conditions of 100 ppmv atmospheric co2 6 reduction in solar forcing and a rotation rate corresponding to an 18 hour day these results confirm those of earlier studies suggesting or implying that low latitude glaciation occurred during the late proterozoic however since the ocean is the critical factor for low latitude glaciation the results should be view cautiously because of the simple slab ocean used in this study