silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

the silicon isotope record of early silica diagenesis the heavy isotopes of silicon are strongly enriched in some of the youngest early diagenetically formed porcellanite layers from the southwest indian ridge pleistocene and the maud rise pliocene these porcellanite layers are composed of opal ct and were formed by the conversion of amorphous silica opal a from siliceous sediment via dissolution reprecipitation their bulk si30 values range between 1 7 and 2 3 detritus poor siliceous sediment surrounding these layers is significantly lower at 0 3 to 1 5 sequential chemical extractions of bulk siliceous sediment show i preferential dissolution of diatoms featuring higher si30 than radiolaria and al si components the detailed investigation of porcellanite layers by micro scale si isotope and al si analyses using uv femtosecond laser ablation icp mass spectrometry show that ii precipitation of authigenic aluminum silicates enriched in light si isotopes drives pore waters to even higher si30 we suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert we infer that past environmental conditions can be reconstructed with high fidelity from the stable si isotope composition of chert when initial seawater si concentrations were high such as in the precambrian exchange of si between layers during phase transformation from opal a to opal ct and from opal ct to quartz is impeded when variable amounts of detrital minerals are present because they control rates of silica phase transformation and hence the timing of dissolution reprecipitation during burial

germanium silicon fractionation in the weathering environment abstract we present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the ge si system as a tracer of silicate weathering processes in both modern and ancient environments intensely weathered soils developed on hawaiian basalts have bulk soil ge si ratios 2 to 10 times higher than fresh basalt e g 10 to 25 mol mol vs 2 5 mol mol soil ge concentrations increase with si and decrease with fe suggesting that ge sequestration is related to accumulation of secondary soil silicates rather than retention in soil fe oxy hydroxides sequential extractions of these soils suggest that ge si fractionation takes place by ge sequestration during the initial precipitation of secondary soil aluminosilicates principally allophane further si loss and changes in mineralogy as these soils age result in little additional ge si fractionation ge si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals kaolinite has a ge si ratio 5 9 mol mol higher than the plagioclase from which it forms 3 1 mol mol whereas accumulation of primary quartz ge si 0 5 mol mol prevents granitic soils from attaining high ge si ratios laboratory synthesis of allophane confirms that ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution

ice sheet sources of sea level rise and freshwater discharge during the last deglaciation we review and synthesize the geologic record that constrains the sources of sea level rise and freshwater discharge to the global oceans associated with retreat of ice sheets during the last deglaciation the last glacial maximum 26 19 ka was terminated by a rapid 5 10 m sea level rise at 19 0 19 5 ka sourced largely from northern hemisphere ice sheet retreat in response to high northern latitude insolation forcing sea level rise of 8 20 m from 19 to 14 5 ka can be attributed to continued retreat of the laurentide and eurasian ice sheets with an additional freshwater forcing of uncertain amount delivered by heinrich event 1 the source of the abrupt acceleration in sea level rise at 14 6 ka meltwater pulse 1a 14 15 m includes contributions of 6 5 10 m from northern hemisphere ice sheets of which 2 7 m represents an excess contribution above that derived from ongoing ice sheet retreat widespread retreat of antarctic ice sheets began at 14 0 15 0 ka which together with geophysical modeling of far field sea level records suggests an antarctic contribution to this meltwater pulse as well the cause of the subsequent younger dryas cold event can be attributed to eastward freshwater runoff from the lake agassiz basin to the st lawrence estuary that agrees with existing lake agassiz outlet radiocarbon dates much of the early holocene sea level rise can be explained by laurentide and scandinavian ice sheet retreat with collapse of laurentide ice over hudson bay and drainage of lake agassiz basin runoff at 8 4 8 2 ka to the labrador sea causing the 8 2 ka event

absolute accumulation and isotope fractionation of si and fe during dolomite weathering and terra rossa formation abstract the behavior of silicon si and iron fe and their isotope fractionation during carbonate weathering and terra rossa formation are unclear here we focus on a 4 05 m thick terra rossa profile developed on dolomite in the yunnan guizhou plateau china we measured the mineral and geochemical composition magnetic susceptibility and si and fe isotopic composition of the terra rossa ferromanganese concretions fmcs and the dolomite and its insoluble residues the results indicate that there is absolute accumulation of si and fe during dolomite weathering and terra rossa formation due to the extraneous input of dissolved si and fe this means that the insoluble residues of dolomite are not the only source for terra rossa formation in addition we suggest that the high frequency dependent magnetic susceptibility fd and rubification of the terra rossa are the result of the oxidation of extraneous dissolved fe and the neoformation of finely dispersed fe oxides overall the terra rossa is characterized by the enrichment of light si and fe isotopes compared with insoluble dolomite residues and bulk dolomite the extraneous input of dissolved fe is characterized by a light isotopic signature whereas the si isotopic signature of the extraneous source is unclear due to modification by late desilicification si isotope fractionation in the terra rossa is mainly controlled by chemical weathering and associated mobilization and redistribution of si the 30 si values of terra rossa decrease with increasing weathering intensity while the re precipitation of mobilized si leads to the significant enrichment of isotopically heavy si in the basal terra rossa the 30 si values of terra rossa are not significantly affected by the adsorption of the fe and al oxyhydr oxides by contrast the major process that fractionates fe isotopes in the dolomite terra rossa weathering system is unclear in the terra rossa profile the 56 fe values of the fmcs are significantly lower than those of the surrounding terra rossa matrix these low 56 fe signatures of the fmcs reflect the preferential translocation of the light fe isotope while the fmcs may be able to record the low 56 fe characteristics of pore water

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

glacial interglacial ice stream stability in the weddell sea embayment antarctica the resilience of the antarctic ice sheet and its effect on global sea level depends on the dynamics of ice streams antarctic ice streams are known to be responsive to changes at the ocean interface and as expected most have thinned in response to ocean warming and sea level rise since the last glacial maximum lgm here we provide direct and unexpected evidence that points toward the glacial interglacial stability of the slessor and recovery glaciers ice streams of the east antarctic ice sheet eais which merge with the west antarctic ice sheet wais to form the filchner ice shelf in the weddell sea embayment cosmogenic nuclide measurements in the shackleton range suggest that the slessor and recovery ice streams were not significantly thicker than today during the lgm we hypothesise that the glaciers did not thicken because the grounding line was not able to migrate seaward beyond the deep thiel crary trough beneath the filchner ice shelf immediately offshore this discovery reveals how a topographic threshold can affect the dynamics of ice streams it also reduces uncertainties on the thickness extent and volume of the antarctic ice sheet in a large but unknown sector of the antarctic ice sheet it constrains the potential sea level rise from antarctica it helps explain observed anomalies in glacio isostatic adjustment above all it suggests that the behaviour of the atlantic facing weddell sea sector of the wais contrasts with that of the pacific facing ross and amundsen sea sectors

tracing mechanisms controlling the release of dissolved silicon in forest soil solutions using si isotopes and ge si ratios the terrestrial biogenic si bsi pool in the soil plant system is ubiquitous and substantial likely impacting the land ocean transfer of dissolved si dsi here we consider the mechanisms controlling dsi in forest soil in a temperate granitic ecosystem that would differ from previous works mostly focused on tropical environments this study aims at tracing the source of dsi in forest floor leachates and in soil solutions under various tree species at homogeneous soil and climate conditions using stable si isotopes and ge si ratios relative to granitic bedrock clays minerals were enriched in si 28 and had high ge si ratios while bsi from phytoliths was also enriched in si 28 but had a low ge si ratio such a contrast is useful to infer the relative contribution of silicate weathering and bsi dissolution in the shallow soil on the release of dsi in forest floor leachate solutions the delta si 30 values in forest floor leachates 1 38 parts per thousand to 2 05 parts per thousand are the lightest ever found in natural waters and ge si ratios are higher in forest floor leachates relative to soil solution these results suggest dissolution of si 28 and ge enriched secondary clay minerals incorporated by bioturbation in organic rich horizons in combination with an isotopic fractionation releasing preferentially light si isotopes during this dissolution process ge si ratios in soil solutions are governed by incongruent weathering of primary minerals and neoformation of secondary clays minerals tree species influence si isotopic compositions and ge si ratios in forest floor leachates through differing incorporation of minerals in organic horizons by bioturbation and to a lesser extent through differing si recycling c 2010 elsevier ltd all rights reserved

late cenozoic deposits at reedy glacier transantarctic mountains implications for former thickness of the west antarctic ice sheet abstract deposits corresponding to multiple periods of glaciation are preserved in ice free areas adjacent to reedy glacier southern transantarctic mountains glacial geologic mapping supported by 10be surface exposure dating shows that reedy glacier was significantly thicker than today multiple times during the mid to late cenozoic longitudinal surface profiles reconstructed from the upper limits of deposits indicate greater thickening at the glacier mouth than at the head during these episodes indicating that reedy glacier responded primarily to changes in the thickness of the west antarctic ice sheet surface exposure ages suggest this relationship has been in place since at least 5 ma the last period of thickening of reedy glacier occurred during marine isotope stage 2 at which time the glacier surface near its confluence with the west antarctic ice sheet was at least 500 m higher than today

selective glacial erosion and weathering zones in the coastal mountains of marie byrd land antarctica in the coastal mountains of marie byrd land antarctica there is a juxtaposition of ice moulded landforms at lower altitudes and upstanding summits with weathered surfaces bearing tors this paper uses geomorphological mapping and exposure dating to test two hypotheses commonly used to explain such a landscape contrast either the pattern reflects contrasts in glacial erosion related to the basal thermal regime beneath a former ice sheet or it represents a periglacial trimline marking the upper limit of ice cover during the last glacial maximum lgm cosmogenic nuclide measurements on erratics show that the mountains were covered by ice on several occasions most recently during the lgm similar measurements on bedrock show that fragile landforms on weathered surfaces survived many glaciations nuclide concentrations in bedrock indicate both that subglacial erosion is insignificant and that ice free periods of periglacial weathering are longer at higher elevations we conclude that the pattern of landscape modification reflects both topographically controlled contrasts in the basal thermal regime of overriding ice and the longer exposure to periglacial conditions at higher elevations in addition the combination of nuclide data and the wider pattern of striations shows that the lgm ice cover was thinner than predicted by ice sheet models assuming equilibrium conditions that ice sheet thinning has occurred from 10 400 years ago to the present and that regional flow by overriding ice has been replaced by radial ice flow from local glaciers centred on individual mountain massifs

positive mass balance of the ross ice streams west antarctica we have used ice flow velocity measurements from synthetic aperture radar to reassess the mass balance of the ross ice streams west antarctica we find strong evidence for ice sheet growth 26 8 gigatons per year in contrast to earlier estimates indicating a mass deficit 20 9 gigatons per year average thickening is equal to 25 of the accumulation rate with most of this growth occurring on ice stream c whillans ice stream which was thought to have a significantly negative mass balance is close to balance reflecting its continuing slowdown the overall positive mass balance may signal an end to the holocene retreat of these ice streams

past and future grounding line retreat of the west antarctic ice sheet the history of deglaciation of the west antarctic ice sheet wais gives clues about its future southward grounding line migration was dated past three locations in the ross sea embayment results indicate that most recession occurred during the middle to late holocene in the absence of substantial sea level or climate forcing current grounding line retreat may reflect ongoing ice recession that has been under way since the early holocene if so the wais could continue to retreat even in the absence of further external forcing

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

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

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

the imprint of time on canadian soil landscapes abstract the imprint of time on canadian soil landscapes reflects the strong influence of repeated quaternary glaciations with most soils having formed on late pleistocene glacial sediments and to a lesser extent holocene non glacial sediments soil chronosequences in these youthful landscapes seldom span the full holocene and have been documented principally in areas of coastal isostatic uplift and shoreline evolution as well as on recessional moraine sequences in the cordillera these chronosequences have placed quantitative constraints on rates of podzolization in environments ranging from coastal temperate rainforest to subarctic longer sequences involving soils and relict paleosols on pleistocene morainal and glaciofluvial surfaces in the central yukon territory record more complex legacies of pedogenesis and landscape evolution but with much coarser time control and many potentially confounding influences of climatic change and disturbance processes older weathered bedrock and regolith can make significant contributions to soil parent materials in several settings 1 in favoured locations where local conditions protected these materials from glacial erosion in southern canada 2 beyond the maximum limits of glaciation principally in northwestern canada and 3 in the arctic where cold based non erosive ice allowed preservation of these materials closer collaborations between pedologists and other quaternary scientists will be needed to ensure a more complete understanding of these complex pedogenic legacies in canadian landscapes

propagation and hindered settling of laboratory ash flows the fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects we carried out experiments on flows of fluidized volcanic ash at 170 c which is hot enough to reduce cohesive effects of moisture the flows were generated in a 3 m long horizontal lock exchange flume the ash was fluidized and expanded uniformly in the flume reservoir by up to 43 above loose packing and was then released each flow defluidized progressively down the flume until motion ceased initial expansion e and initial height h0 were varied independently of one another the flows traveled in a laminar manner flow fronts exhibited three main phases of transport 1 a brief initial phase of gravitational slumping 2 a dominant approximately constant velocity phase and 3 a brief stopping phase phase 2 frontal velocities scaled with equation image like other types of dam break flow deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with e despite rates of shear up to 80 s 1 aggradation rates were identical to those determined independently at the same value of e in quasi static collapse tests sedimentation caused the flows to thin progressively during transit until they ran out of volume the dynamics were governed to a first order by two dimensionless parameters 1 the initial aspect ratio h0 x0 in the lock reservoir and 2 the ratio tsett tgrav of two timescales a particle settling time tsett and a gravitational acceleration time tgrav

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

can fe isotope fractionations trace the pedogenetic mechanisms involved in podzolization abstract stable fe isotopes have shown the potential for tracing pedogenetic processes large isotopic fractionations were especially observed in podzols nevertheless a clear link between isotopic fractionation and elementary processes still needs to be established to spatially distinguish the mechanisms successively involved in pedogenesis we studied a podzolic chronosequence from vancouver island british columbia canada we analyzed depth variations in soil properties ph particle size fractions organic carbon si al ratio zr ti fe concentrations in different fe pools and fe isotopic compositions the si al ratio zr and ti demonstrated that the cox bay podzols developed from the same parental material satisfying the requirements of a chronosequence we showed that acidification to a ph of 4 7 was a prerequisite for the start of podzolization this first phase took place after 270 years furthermore we observed that once the required ph was reached 4 7 podzolization occurred rapidly over 50 years we found that fe isotope fractionation in the a e horizon was linked to mineral dissolution while in the podzolic b horizons this fractionation was clearly associated with the accumulation of fe organic complexes and poorly crystalline fe oxyhydroxides

silicon isotope variations accompanying basalt weathering in iceland abstract this study presents the si isotope compositions of the dissolved phase of twenty rivers from across iceland the high resolution nu plasma 1700 mc icp ms was used to provide interference free 30 si 28 si ratios with an average limiting precision of 0 04 30 si the si isotope composition ranges from 0 08 to 1 46 30 si averaging 0 63 0 38 1 sd this is only slightly lighter than the recently reported mean of 0 84 0 19 for high precision measurements of swiss rivers but is significantly lighter than compositions reported for some other rivers the si isotope composition varies slightly with the amount of glacial cover which provides evidence that the continental si input to the oceans may be climatically controlled and thus likely to vary over glacial interglacial cycles after correction for the dissolution of trace amounts of calcite in one region a correlation is found between 30 si and ca si this is consistent with the measured basaltic composition and indicates isotopic fractionation during si removal from the dissolved phase by calculating the fraction of si that is removed from the dissolved phase into secondary phases it is possible to use the riverine si flux si denudation rate to calculate the initial dissolution rate of si within these catchments from this study we estimate that the total amount of si being dissolved by primary weathering of iceland is 3 million tons per yr however only 50 of this si is released into the rivers and contributes to denudation using the si isotope composition and the si fraction removed it is possible to model the si isotope fractionation as either a rayleigh type fractionation or a steady state system both yield similar fractionation factors of about 1 5 30 si implying only one underlying fractionation process that is controlled by external parameters such as runoff furthermore rivers that follow a steady state fractionation model are associated with higher si dissolution rates and lower 30 si values indicating an inverse relationship between weathering rates and dissolved riverine si isotope composition

a new holocene relative sea level curve for terra nova bay victoria land antarctica more than 100 radiocarbon dates of penguin guano and remains shells and seal skin afford ages for raised beaches adjacent to terra nova bay antarctica these dates permit construction of a new relative sea level curve that bears on the timing of deglaciation recession of the ross sea ice sheet grounding line from terra nova bay occurred no earlier than 7200 14c yr 8000 cal yr bp retreat along the victoria land coast may have been rapid possibly contributing to eustatic sea level rise centred at ca 7600 cal yr bp the presence of a significant amount of ice remaining in the ross sea embayment in holocene time lessens the chance that antarctica contributed significantly to meltwater pulse 1a several thousand years earlier copyright 2004 john wiley sons ltd

effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of geysir iceland detectible 30si variations in present day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents interpretations of 30si signatures remain problematic in view of potential post depositional changes of which magnitudes and underlying mechanisms are largely unknown a critical issue in the interpretation of isotope data from cherts concerns the extent to which early diagenetic processes modify original 30si signatures here we report 30si variations in opal a opal a ct and opal ct from fossil sinter deposits in an active discharge apron in the geysir geothermal field iceland opal a samples show an average 30si of 0 7 0 2 while opal ct samples are isotopically lighter with an average 30si of 2 0 0 4 implying a sizable shift of approximately 1 3 between the different phases this shift can be explained by repetitive dissolution re precipitation processes diffusive transport or temperature differences during phase transitions on average the fossil opal a tends to be less negative in 30si than amorphous silica that recently precipitated from the hydrothermal water the difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface or to a post depositional release of surface 28si at the onset of diagenetic formation our results corroborate the perception that original silicon isotope signatures of silica acquired during chemical precipitation from a saturated solution may not be preserved in the geological record and that post depositional changes must be taken into account when interpreting data from ancient chert deposits

natural variations of δ30si ratios during progressive basalt weathering hawaiian islands abstract silicon stable isotopes can be used to trace the biogeochemical pathways of si as it moves from its continental sources to its sink in ocean sediments along the way si is incorporated into clay minerals taken up by plants where it forms plant opal and leached into rivers the major land to ocean conduit compared to igneous rocks the waters that drain continents are enriched in heavy si isotopes but the mechanisms that control fractionation have not been elucidated we studied si isotope fractionation along a 4 million yr basaltic soil chronosequence on the hawaiian islands using the natural context of these samples in combination with laboratory experiments we demonstrate that the isotopic composition of dissolved si in weathering systems is determined by the combined effects of rock disintegration clay mineral neosynthesis and si biocycling weathering preferentially releases 28si into solution whereas secondary mineral formation preferentially removes 28si from solution in humid environments leached soils have lost large amounts of this soluble si thus creating a net loss of 30si from the entire soil system as soils develop and greater fractions of si reside in neoformed clay minerals 30sibulk soil values change progressively toward more negative values basalt 30si values are about 0 5 but older soils have 30si values up to 2 5 the difference between the solid and solution 30si values remains more or less constant with progressive weathering and therefore soil water from older soils has a more negative 30si composition in the upper horizons of the hawaiian soils this weathering driven 30si shift is modified by the addition of unweathered primary minerals via dust carrying 30si values of about 0 5 and by biocycling of si via plants producing negative 30si values in phytoliths and positive 30si values in soil solutions derived from upper horizons due to the high concentrations of dissolved si in these near surface layers rivers have more positive 30si values than predicted based on the weathering status of the lower horizons when combined with published 30si values from large rivers worldwide we find that the results from hawaii point to weathering control of si isotopes delivered to the oceans and thus to an important continent ocean linkage that warrants further investigation

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

precambrian si isotope mass balance weathering and the significance of the authigenic clay silica sink abstract the geological cycles of carbon and silicon are linked via the silicate weathering feedback but growing understanding of reverse weathering the formation of authigenic clay minerals via back reaction of silica with cations indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed recent carbon cycle modeling has suggested that enhanced reverse weathering during precambrian time could have played an important role in regulating climate the topology of the precambrian silica cycle including the size of the authigenic clay sink and its influence on the carbon cycle is challenging to reconstruct due to inherent preservation biases in the rock record here we used an alternative approach using si isotope values to invert for the magnitudes of precambrian silica fluxes by assuming that the silica cycle must be in mass balance with the bulk silicate earth over geological time scales with respect to si isotopes to estimate uncertainties associated with these flux reconstructions we used monte carlo simulations based on the natural variation exhibited in si isotope ratio data from the precambrian sedimentary record n 2118 spanning 3 8 ga to 525 ma and modern authigenic clays n 123 the perspective of si isotope mass balance shows that the previously observed increase in chert 30si values across precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature results yielded a secular increase in the relative contribution of the authigenic clay sink across most of precambrian time this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater this result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust this approach also highlights key areas where improving si isotope records could further enhance our constraints on the precambrian silica cycle and its relationship with the carbon cycle and climate stability

impact of low denudation rates on soil chemical weathering intensity a multiproxy approach abstract quantifying the influence of denudation i e physical erosion and chemical weathering on soil weathering intensity is an important component for a comprehensive understanding of element biogeochemical cycles the relation between the weathering intensity and the denudation rate is not clear and requires further investigation in a variety of climatic and erosional settings here in the betic cordillera southern spain we assess the soil chemical weathering intensity with a multiproxy approach combining different indicators of chemical weathering of the soil the total reserve in bases trb the content in fe oxides the quartz and clay content the soil cation exchange capacity cec and the silicon si isotope composition of the clay sized fraction our multiproxy approach demonstrates that in this semi arid environment at low denudation rates an increasing denudation rate decreases the soil weathering intensity whereas si mobility remains limited our results converge with previous conclusions based on chemical mass balance methods in the same geological setting mass balance methods and particularly chemical depletion fractions cdf are based on the immobility of a refractory element commonly zirconium zr relative to major cations in soils interestingly our study suggests that a weathering index such as the trb may provide a useful complement to assess soil chemical weathering intensity in eroding landscapes where the application of chemical mass balances may be hampered by potential zr mobility in the soil or by heterogeneity of zr concentrations in the bedrock

cosmic ray produced 10be and 26al in antarctic rocks exposure and erosion history we have measured cosmic ray produced t1 2 1 5 million years 10be and t1 2 0 705 million years 26al in purified quartz fractions of selected rock samples from antarctic mountains from these data we calculate 1 mean erosion rates for the limiting case of steady state surface exposure to cosmic rays and 2 minimum exposure ages for the limiting case of no erosion calculated mean erosion rates are very low on the order of a few times 10 5 cm yr we believe the sampling to be sufficient to generalize this result to exposed bedrock in antarctica in favorable cases it is possible to distinguish between the limiting cases steady state erosion seems a better description in such cases most samples including some taken a few meters above the present ice level seem to have been exposed for millions of years without major episodes of burial or abrasion by ice

a 2 million year glacial chronology of the hatherton glacier antarctica and implications for the size of the east antarctic ice sheet at the last glacial maximum abstract a series of distinct glacial deposits flanking the margins of the upper hatherton glacier an outlet glacier in the central transantarctic mountains are used to constrain the behaviour of the antarctic ice sheets cosmogenic exposure ages of 18 erratics from four glacial drifts covering the ice free dubris and bibra valleys range in age from 5 to 1997 ka our results document four glacial advance and retreat events superimposed on an overall long term ice thickness reduction of about 500 m since the mid pleistocene the lack of field evidence and absence of lgm exposure ages in the glacial deposits of the hatherton glacier supports our conclusion that at the lgm the east antarctic ice sheet was of similar size or may have been slightly smaller than present minimum exposure ages from the oldest two glacial events represented by the isca and danum drifts are 1 2 ma and 0 5 ma respectively the britannia ii drift previously assumed to mark the maximum extent of the last glacial maximum advance has a mean 10 be age of 126 3 2 ka n 5 ages from the younger britannia i drift suggest that since the mid holocene 6 5 1 2 ka n 5 approximately 200 m of additional ice has been lost

geomorphological evidence of cold based glacier activity in south victoria land antarctica cold based glaciers have long been recognized as capable of covering and protecting landscapes however recent studies of modern cold based glaciers in antarctica show that in some situations erosion deformation and deposition can occur recognizing the dual ability of cold based glaciers to protect and preserve surfaces on the one hand and erode and modify on the other is important for correctly interpreting the often subtle imprint of cold based glaciers on landscapes a range of geomorphological features related to cold based glacier activity has now been documented along with an improved understanding of cold based glacier structure processes and interaction with various substrates collectively this provides an enhanced ability to understand the impact of cold based glaciers on landscapes and reappraise the geomorphological record such insight allows recognition of previously unknown glacial events and better interpretations of the landscape exposure record this is particularly important at the margins of the antarctic ice sheets where past fluctuations in ice sheet volume and its contributions to post glacial sea level rise are poorly constrained this paper reviews the known geomorphological evidence associated with cold based glaciers in the south victoria land region of the transantarctic mountains it aims to provide progress towards a set of criteria for recognizing cold based glacier activity in other regions and to highlight the implications of cold based glacial activity for surface exposure studies and interpreting glacial history cold based glaciers are below pressure melting point throughout and are therefore frozen to the substrate and move primarily by internal deformation drewry 1986 this led to the long standing view that cold based glaciers cannot slide and are thus incapable of erosion and deposition and therefore protect the underlying surface while numerous studies have shown this to be true in many situations recent investigations of modern cold based glaciers have revealed that in some circumstances complex processes operate at subfreezing temperatures in the basal zone and that cold based glaciers can generate a variety of geomorphological features these studies have forced a reappraisal of the geomorphic effectiveness of cold based glaciers and have confirmed that they can both protect and modify landscapes e g fitzsimons 1996 atkins et al 2002 fitzsimons 2003 waller et al 2012 thus the debate has moved on and the challenge now is to understand the full range of processes and products associated with cold based glaciers and the implications for landscape development hambrey fitzsimons 2010 waller et al 2012 recognizing the often subtle features indicative of cold based glacial activity offers the potential to delineate and date previously unknown or poorly constrained glacial events this is particularly relevant for interpreting glacial history and modelling ice sheet reconstructions for example numerous surface exposure dating studies have been carried out on ice marginal landforms with the aim of constraining past variations in the ice sheet volume however many of these studies have produced inconsistent contradictory results including evidence of complex exposure histories suggesting episodes of burial and reworking by cold based ice e g strasky et al 2008 di nicola et al 2009 compared with warm based glaciers there are still relatively few studies of cold based glaciers and significant gaps in our understanding of the processes and products remains in addition lingering misconceptions regarding the geomorphic effectiveness of cold based glaciers and historic use of ambiguous terminology to describe cold based sediments and landforms mean that the evidence and implications of past cold based glacial events have yet to be fully appreciated by the wider geomorphological community this review provides a summary of known geomorphological features associated with cold based glaciers in the south victoria land region of antarctica it aims to provide a basis for recognizing similar features elsewhere and attempts to draw attention to the implications of cold based glacial geomorphology for the correct application of surface exposure dating studies and interpretation of cold based glacial history

biogenic and pedogenic controls on si distributions and cycling in grasslands of the santa cruz soil chronosequence california abstract biogenic and pedogenic processes control silica cycling in grasslands growing on a soil chronosequence and dominated by strong seasonal variabilities of a mediterranean climate shallow pore water si in spite of significant annual uptake and release by plant growth and dieback exhibits only moderate seasonal fluctuations reflecting strong buffering from labile biogenic si dominated by phytoliths and by secondary pedogenic silicates long phytolith residence times 340 900 yrs reflect the seasonally dry climate and high solute si concentrations water extractable si is closely associated with al indicating seasonal precipitation and dissolution of a highly labile 1 1 hydroxyaluminosilicate has probably allophane which transforms in deeper soil into fine grained poorly crystalline kaolinite shallow plant roots extract greater proportions of biogenic si and deeper plant roots larger amounts pedogenic si high pore water ge si in late winter and spring reflects the reinforcing effects of plant fractionation and concurrent dissolution of ge enriched has the same processes produce pore waters with depleted 30 si 28 si in the summer and fall ge si declines and 30 si 28 si increases reflecting the cessation of plant uptake continued dissolution of soil phytoliths and re precipitation of less soluble has si inputs from weathering 2 90 mmol m 2 yr 1 and losses from pore water discharge 18 68 mm m 2 yr 1 are comparable for individual soils decline with soil age and are significantly less than amounts of si annual cycled through the vegetation 42 171 mm m 2 yr 1 mobile si is generally balanced in the soils with upward bio pumping by the shallow rooted grasses efficiently competing against downward leaching and pore water discharge small net annual increases in si in the present day soils could not have been maintained over the time scale represented by the chronosequence 65 225 yrs implying past changes in environmental conditions

subglacial thermal balance permits ongoing grounding line retreat along the siple coast of west antarctica changes in the discharge of west antarctic ice streams are of potential concern with respect to global sea level the six relatively thin fast flowing ross ice streams are of interest as low slope end members among antarctic ice streams extensive research has demonstrated that these rivers of ice have a history of relatively high frequency asynchronous discharge variations with evolving lateral boundaries amidst this variability a 1300 km grounding line retreat has occurred since the last glacial maximum numerical studies of ice stream d parizek and others 2002 indicate that a proposed thermal regulation mechanism clarke and marshall 1998 hulbe and macayeal 1999 tulaczyk and others 2000a b which could buffer the west antarctic ice sheet against complete collapse may be over ridden by latent heat transport within melt water from beneath inland ice extending these studies to ice stream a whillans ice stream and ice stream c suggests that further grounding line retreat contributing to sea level rise is possible thermodynamically however the efficiency of basal water distribution may be a constraint on the system because local thermal deficits promote basal freeze on especially on topographic highs observed short term variability is likely to persist

dynamics and mass balance of taylor glacier antarctica 1 geometry and surface velocities 1 taylor glacier antarctica exemplifies a little studied type of outlet glacier one that flows slowly through a region of rugged topography and dry climate this glacier in addition connects the east antarctic ice sheet with the mcmurdo dry valleys a region much studied for geomorphology paleoclimate and ecology here we report extensive new measurements of surface velocities ice thicknesses and surface elevations acquired with insar gps and gpr the latter two were used to construct elevation models of the glacier s surface and bed ice velocities in 2002 2004 closely matched those in 2000 and the mid 1970s indicating negligible interannual variations of flow comparing velocities with bed elevations shows that along much of the glacier flow concentrates in a narrow axis of relatively fast flowing ice that overlies a bedrock trough the flow of the glacier over major undulations in its bed can be regarded as a cascade it speeds up over bedrock highs and through valley narrows and slows down over deep basins and in wide spots this pattern is an expected consequence of mass conservation for a glacier near steady state neither theory nor data from this taylor glacier study support the alternative view recently proposed that an outlet glacier of this type trickles slowly over bedrock highs and flows fastest over deep basins

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

rare earth elements dynamics along pedogenesis in a chronosequence of podzolic soils rare earth elements ree total concentration and signature in soils are known to be impacted by successive soil forming processes so it can be used as probe of soil processes however few studies focus on their behavior in podzols podzols result from the combination of two main pedogenic processes 1 the strong weathering in the surface eluvial horizon 2 the downward transfer of dissolved organic matter om and mobile al and fe and their accumulation in the illuvial horizon beneath iron oxides and om are known to have strong affinities with ree and to play an important role in transfer and immobilization of ree in order to decipher the relative importance of fe oxide and om in ree fate during podzolization and to investigate whether ree can trace podzol formation we study here the evolution of ree signatures along five pedons aged from 120 to 530 years in a cambisol podzol chronosequence located in the cox bay of vancouver island our results show that the ree content is strongly correlated to the general loss of elements and mineral weathering furthermore the accumulation of secondary om al and fe bearing phases does not impact the ree signature of the bulk soil both our results and the ones available in the literature indicate that the release of ree induced by weathering and subsequent leaching in percolating water are the main pathways determining the ree fate in podzols furthermore we show that ree can be released and mobilized in very short periods of time during podzolization 330 years

examination of surface exposure ages of antarctic moraines using in situ produced 10be and 26al concentrations of 10be t12 1 5 106 y and 26a1 t12 0 72 106 y have been determined by accelerator mass spectrometry ams in a suite of quartz samples taken from sandstone boulders in several moraines in arena valley a dry valley adjacent to the taylor glacier in the quatermain mountains southern victoria land east antarctica these isotopes are produced in surficial quartz by cosmic ray spallation of o and si the concentrations in these samples ranged from 6 1 105 to 3 0 107 at g 1 for 10be and from 9 4 106 to 1 2 108 at g 1 for 26a1 depending upon the extent of exposure at the surface analyses of 10be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons the value of 142 96 41 g cm 2 is consistent with other published values production rates of 17 6 4 at g 1 y 1 for 10be and 113 54 16 at g 1 y 1 for 26a1 at 1300 m and 87 s and a 26al 10be production ratio of 6 5 1 3 1 3 were calculated from the data these values correspond to sea level production rates at high geomagnetic latitude of 6 4 at g 1 y 1 and 41 7 at g 1 y 1 for 10be and 26a1 respectively consistent with determinations based on 11 ky glacially polished surfaces in the sierra nevada in california these production rates imply exposure ages for the various moraines ranging from 50 ky to 2 5 my in accordance with other geological evidence the 10be and 26al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3he examination of the 3he concentrations in conjunction with the present data set yields an estimate of 3he production of 230 8540 at g 1 y 1 corresponding to 85 31 15 at g 1 y 1 at sea level this is lower than previously reported values of 240 60 at g 1 y 1 and 190 at g 1 y 1 scaled to sea level and high geomagnetic latitude based respectively on volcanic flows less than 2000 years old at low latitude and samples collected from 14 400 year old volcanic flows this discrepancy is likely due to diffusive losses which may become significant up to 50 for surface exposure ages on the order of 2 0 my nevertheless the general correlations among the three isotopes suggest that 3he does not suffer gross losses through diffusion

stable silicon isotopes of groundwater feldspars and clay coatings in the navajo sandstone aquifer black mesa arizona usa abstract we present some of the first analyses of the stable isotopic composition of dissolved silicon si in groundwater the groundwater samples were from the navajo sandstone aquifer at black mesa arizona usa and the si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed silicon isotope compositions were measured using high resolution multi collector inductively coupled mass spectrometry hr mc icp ms nu1700 nuplasma hr the quartz dominated bulk rock and feldspar separates have similar 30 si of 0 09 0 04 and 0 15 0 04 95 sem respectively and clay separates are isotopically lighter by up to 0 4 compared to the feldspars from isotopic mass balance considerations co existing aqueous fluids should have 30 si values heavier than the primary silicates positive 30 si values were only found in the shallow aquifer where si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals however 30 si decreases along the flow path from 0 56 to 1 42 representing the most negative dissolved si isotope composition so far found for natural waters we speculate that the enrichment in 28 si is due to dissolution of partly secondary clay minerals and low temperature silcretes in the navajo sandstone the discovery of the large range and systematic shifts of 30 si values along a groundwater flow path illustrates the potential utility of stable si isotopes for deciphering the si cycling in sedimentary basins tracing fluid flow and evaluating global si cycle

asynchronous behavior of the antarctic ice sheet and local glaciers during and since termination 1 salmon valley antarctica abstract the stability of the antarctic ice sheet under future warming remains an open question with broad implications for sea level prediction and adaptation in particular knowledge of whether the ice sheet has the capacity for rapid drawdown or collapse or whether it can remain stable during periods of warming is essential for predicting its future behavior here we use 55 radiocarbon dates coupled with geomorphologic mapping to reconstruct the timing of changes in ice extent and elevation during the last ice age termination in salmon valley adjacent to mcmurdo sound in the western ross sea embayment results indicate that a grounded ice sheet in the ross sea embayment achieved its maximum elevation and extent along the headlands of salmon valley at 18 000 yr bp during a period of increasing temperatures and accumulation over the antarctic continent this ice remained at or near its maximum on the headlands near the valley mouth until after 14 000 yr bp removal of grounded ross sea ice from salmon valley was complete shortly after 7900 yr bp indicating that the grounding line had retreated through southern mcmurdo sound by that time we suggest the primary driver of ross sea ice removal from mcmurdo sound was marine based either through basal melting or calving due to sea level rise when combined with regional data the salmon valley record suggests that this sector of the antarctic ice sheet did not contribute in a significant way to deglacial meltwater pulses such as meltwater pulse 1a in contrast to the ross sea ice our work also shows that local independent alpine glaciers in salmon valley have advanced through the holocene land terminating glaciers such as these elsewhere in the region show a similar pattern and may reflect the continued influence of increased accumulation following the termination of the last ice age

ice dynamics and thermal regime of taylor glacier south victoria land antarctica les donnees glaciologiques recueillies sur le taylor glacier ont servi a reconstituer les principaux traits de la dynamique et du regime thermique du glacier les etudes du bilan de masse montrent que le glacier est proche de l equilibre les conditions thermiques de la glace basale sur une grande partie de la zone d ablation ont ete calculees a partir d estimation du flux de chaleur geothermique et de mesures des temperatures de la glace pres de la surface des vitesses et de l epaisseur de la glace on a trouve que jusqu a 50 de la zone d ablation la glace basale pouvait etre fondante

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

desilication in archean weathering processes traced by silicon isotopes and ge si ratios abstract the influence of continental weathering on oceanic seawater compositions in the precambrian is poorly understood here we combine si isotopes and ge si ratios of a mesoarchean paleosol 2 95 ga and overlying shales as proxies for weathering processes and si mass transfer at the early earth s surface results show that as with modern soils neoformation of secondary clay minerals in the paleosol was associated with the fractionation of si isotopes and ge si ratios in response to chemical weathering and soil desilication furthermore the loss of fe bearing minerals most likely fe bearing smectites produced additional controls on si and ge mobility through the coupled dissolution and re precipitation of clay minerals under reductive conditions opposite fractionation behaviors are observed neoformed secondary clay minerals acted as a 28 si and ge sink whereas the leaching of fe bearing minerals released 28 si and ge into soil solutions an attempt to estimate 30 si and ge si signatures of released dissolved elements into soil solution provided 30 si and ge si signatures of about 0 1 and 1 4 mol mol respectively furthermore shales deposited shortly after paleosol formation display 30 si and ge si compositions that may be explained as mixtures of the recognized paleosol components the shale record suggests that weathering induced desilication and leaching of fe bearing minerals to a lesser extent might have been widely effective during the mesoarchean for the transfer of si from the continents to the hydrosphere comparison of si isotopes recorded in detrital sediments and chemical precipitates over time might thus provide a useful supplementary tool to decipher the impact of putative increases in continental weathering and desilication relative to changes in hydrothermal and continent derived solute inputs to ocean and ocean cooling

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

reconciling marine and terrestrial evidence for post lgm ice sheet retreat in southern mcmurdo sound antarctica abstract retreat of the antarctic ice sheets since the last glacial maximum lgm contributed to sea level rise but the location amount and timing of ice mass loss has been controversial this paper presents new 10 be exposure ages from glacially transported erratics which record post lgm retreat of grounded ice in the western ross sea ice elevation in southern mcmurdo sound was 520 m above present day sea level on the eastern side of mount discovery during the lgm and the onset of major deglaciation in the region was after 14 ka the ice surface lowered from 520 to 234 m above present day sea level between 14 0 ka and 10 3 ka and from 234 m to 30 m between 10 3 ka and 7 4 ka this late glacial and holocene deglaciation chronology from southern mcmurdo sound is consistent with other records on the margins of the ross embayment and implies that the western margins of the ross sea ice sheet rsis experienced most mass loss during the early to middle holocene these 10 be exposure ages coupled with sediment provenance define a two stage ice flow scenario for mcmurdo sound subdividing differing reconstructions into an early and late phase prior to termination i an expanded koettlitz glacier flowed north and northeast between brown peninsula and mount discovery and coalesced with northward flowing ice fed from the skelton and mulock glaciers thinning and retreat of the koettlitz glacier and perhaps other outlet glaciers flowing through the royal society range allowed ice grounded in the ross sea to flow westward and northward north of brown peninsula grounding line recession in the ross sea during the late glacial and holocene was likely driven by southern ocean warming and sea level rise from the retreat of the northern hemisphere ice sheets and the outer margins of the antarctic ice sheets

reconstructing the antarctic ice sheet at the last glacial maximum abstract a reconstruction of the antarctic ice sheet at the last glacial maximum lgm constrained by geologic data shows expansion across continental shelves accompanied by little inland change this added peripheral ice volume contributed 14 m to the lowering of eustatic sea level at the lgm of the total expansion one third took place in east antarctica and two thirds in west antarctica

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

extensive retreat and re advance of the west antarctic ice sheet during the holocene to predict the future contributions of the antarctic ice sheets to sea level rise numerical models use reconstructions of past ice sheet retreat after the last glacial maximum to tune model parameters 1 reconstructions of the west antarctic ice sheet have assumed that it retreated progressively throughout the holocene epoch the past 11 500 years or so 2 4 here we show however that over this period the grounding line of the west antarctic ice sheet which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf retreated several hundred kilometres inland of today s grounding line before isostatic rebound caused it to re advance to its present position our evidence includes first radiocarbon dating of sediment cores recovered from beneath the ice streams of the ross sea sector indicating widespread holocene marine exposure and second ice penetrating radar observations of englacial structure in the weddell sea sector indicating ice shelf grounding we explore the implications of these findings with an ice sheet model modelled re advance of the grounding line in the holocene requires ice shelf grounding caused by isostatic rebound our findings overturn the assumption of progressive retreat of the grounding line during the holocene in west antarctica and corroborate previous suggestions of ice sheet re advance 5 rebound driven stabilizing processes were apparently able to halt and reverse climate initiated ice loss whether these processes can reverse present day ice loss 6 on millennial timescales will depend on bedrock topography and mantle viscosity parameters that are difficult to measure and to incorporate into ice sheet models

iron and silicon isotope behaviour accompanying weathering in icelandic soils and the implications for iron export from peatlands abstract incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions which has implications for fe export from soils this study uses fe isotopes to assess the processes controlling fe export in a range of icelandic soils including peat soils derived from the same parent basalt where fe isotope variations principally reflect differences in weathering and drainage in poorly weathered well drained soils non peat soils the limited fe isotope fractionation in soil solutions relative to the bulk soil 57fesolution soil 0 11 0 12 is attributed to proton promoted mineral dissolution in the more weathered poorly drained soils peat soils the soil solutions are usually lighter than the bulk soil 57fesolution soil 0 41 0 32 which indicates that fe has been mobilised by reductive mineral dissolution and or ligand controlled dissolution the results highlight the presence of fe organic complexes in solution in anoxic conditions an additional constraint on soil weathering is provided by si isotopes the si isotope composition of the soil solutions relative to the soil 30sisolution soil 0 92 0 26 generally reflects the incorporation of light si isotopes in secondary aluminosilicates under anoxic conditions in peat soils the largest si isotope fractionation in soil solutions relative to the bulk soil is observed 30sisolution soil 1 63 0 40 and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation si supersaturation in solution with respect to amorphous silica is reached upon freezing when al availability to form aluminosilicates is limited by the affinity of al for metal organic complexes therefore the precipitation of amorphous silica in peat soils indirectly supports the formation of metal organic complexes in poorly drained soils these observations highlight that in a scenario of decreasing soil drainage with warming high latitude peatlands fe export from soils as fe organic complexes will increase which in turn has implications for fe transport in rivers and ultimately the delivery of fe to the oceans

silicon isotopes record dissolution and re precipitation of pedogenic clay minerals in a podzolic soil chronosequence by providing the largest part of the reactive surface area of soils secondary minerals play a major role in terrestrial biogeochemical processes the understanding of the mechanisms governing neo trans formation of pedogenic clay minerals in soils is therefore of the utmost importance to learn how soils evolve and impact the chemistry of elements in terrestrial environments soil forming processes governing the evolution of secondary aluminosilicates in podzols are however still not fully understood the evolution of silicon si isotope signature in the clay fraction of a podzolic soil chronosequence can provide new insight into these processes enabling to trace the source of si in secondary aluminosilicates during podzol forming processes characterized by the mobilization transport and precipitation of carbon metals and si the si isotope compositions in the clay fraction comprised of primary and secondary minerals document an increasing light 28 si enrichment and depletion with soil age respectively in illuvial b horizons and eluvial e horizon the mass balance approach demonstrates that secondary minerals in the topsoil eluvial e horizons are isotopically heavier with 30 si values increasing from 0 39 to 0 64 in c a 200years while secondary minerals in the illuvial bhs horizon are isotopically lighter 30 si 2 31 compared to the original unweathered secondary minerals in bc horizon 30 si 1 40 the evolution of si isotope signatures is explained by the dissolution of pedogenic clay minerals in the topsoil which is a source of light 28 si for the re precipitation of new clay minerals in the subsoil this provides consistent evidence that in strong weathering environment such as encountered in podzols si released from secondary minerals is partially used to form tertiary clay minerals over very short time scales ca 300years our dataset demonstrates the usefulness to measure si isotope signatures in the clay fraction to discern clay mineral changes e g neoformation versus solid state transformation during soil evolution this offers new opportunity to better understand clay mineral genesis under environmental changes and the short term impact of the dissolution and re precipitation of pedogenic clay minerals on soil fertility soil carbon budget and elemental cycles in soil plant systems 2014 elsevier b v

late quaternary ice surface fluctuations of hatherton glacier transantarctic mountains abstract former longitudinal profiles of hatherton glacier an outlet through the transantarctic mountains constrain nearby polar plateau elevations and ice shelf grounding in the southwestern ross embayment four gravel drift sheets of late quaternary age beside hatherton glacier are from youngest to oldest hatherton britannia i britannia ii and danum the hatherton drift limit is uniformly 20 to 70 m above the present ice surface the britannia ii drift limit is within 100 m of the present surface of uppermost hatherton glacier but is 450 m above middle hatherton glacier extrapolation of this profile downglacier indicates a surface elevation 1100 m above the present ross ice shelf the britannia i drift limit is parallel to but 50 100 m below britannia ii drift the danum drift limit is parallel to but 50 100 m above the britannia ii profile from correlation with drifts near mcmurdo sound and from local 14 c dates we assign an early holocene age to hatherton drift a late wisconsin age to britannia drifts and an age of marine isotope stage 6 to danum drift by our age model the upper reaches of hatherton glacier and presumably the adjacent polar plateau have not exceeded their current elevations by more than 100 150 m during the last two complete global glacial interglacial cycles whereas the middle and lower reaches of hatherton glacier have thickened considerably during the last two global glaciations late wisconsin and marine isotope stage 6 the effect of ice shelf grounding probably was the major control of these changes of hatherton glacier holocene ice surface lowering probably represents the last pulse of grounding line recession in the southwestern ross embayment

fractionation of silicon isotopes during biogenic silica dissolution abstract silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization primarily by diatoms in surface waters of the ocean interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean as driven by biological fractionation during the production of biogenic silica bsio2 or opal by diatoms however other factors which may influence the silicon isotope composition of diatom opal particularly post formational aging and maturation processes remain largely uninvestigated here we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bsio2 suspended in seawater under closed conditions this fractionation acts counter to that occurring during bsio2 production and at about half its absolute magnitude with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor edsi bsi of 0 55 corresponding to a fractionation factor 30 28 of 0 99945 the enrichment factor did not vary with source material indicating the lack of a significant species effect or with temperature from 3 to 20 c thus the dissolution of bsio2 produces dissolved silicon with a 30si value that is 0 55 more negative than its parent bsio2 an effect that must be accounted for when interpreting oceanic 30si distributions the 30si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed implying a measurable 0 1 change in the relative 30si of opal samples whenever the difference in preservation efficiency between them is 20 this effect could account for 10 30 of the difference in diatom 30si values observed between glacial and interglacial conditions it is unlikely however that the inferred maximum possible change in b30sio2 of 0 55 would be manifested in situ as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage

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

modeling the surface mass balance response of the laurentide ice sheet to bolling warming and its contribution to meltwater pulse 1a meltwater pulse mwp 1a occurred 14 5 14 ka and is the largest abrupt rise in sea level 10 20 m of sea level rise of the last deglaciation the timing of mwp 1a is coincident with or shortly follows the abrupt warming of the north atlantic region into the bolling warm period which could have triggered a large laurentide ice sheet lis contribution to mwp 1a given that outside of the arctic lis iceberg discharge probably did not increase during the bolling much of the lis mwp 1a contribution likely occurred through surface ablation here we test the response of lis surface mass balance to bolling warming by forcing a lis energy mass balance model with climate from an atmosphere ocean general circulation model our modeling approach neglects changes in lis mass from dynamics and iceberg calving allowing us to isolate the surface mass balance response model results suggest that lis surface ablation can explain much of the sea level rise just prior to mwp 1a lis surface mass balance becomes more negative in response to the bolling warming contributing an additional 2 9 1 0 m of sea level rise in 500 yr in addition to the background contribution of 4 0 0 8 m the modeled lis mwp 1a contribution is less than previous assumptions but agrees with geochemical runoff and lis area volume estimates the fraction of mwp 1a attributable to other ice sheets particularly antarctica depends on the total sea level rise that occurred during this mwp

the marine geochemistry of germanium ekasilicon the geochemical behavior of dissolved germanium in rivers estuaries and the oceans is similar to that of silicon it is chemically weathered from continental rocks and transported by rivers to the oceans where it is taken up by siliceous organisms and deposited in biogenic opal the ratio of germanium to silicon buried in siliceous oozes must reflect the oceanic input ratios from rivers crustal weathering and sea floor vents mid ocean ridge hydrothermal weathering the opal germanium silicon ratio may thus monitor relative variations in these weathering processes and provide information about the geochemical history of seawater

the silicon isotope composition of ethmodiscus rex laminated diatom mats from the tropical west pacific implications for silicate cycling during the last glacial maximum the cause of massive blooms of ethmodiscus rex laminated diatom mats ldms in the eastern philippine sea eps during the last glacial maximum lgm remains uncertain in order to better understand the mechanism of formation of e rex ldms from the perspective of dissolved silicon dsi utilization we determined the silicon isotopic composition of single e rex diatom frustules 30sie rex from two sediment cores in the parece vela basin of the eps in the study cores 30sie rex varies from 1 23 to 0 83 average 1 04 a range that is atypical of marine diatom 30si and that corresponds to the lower limit of reported diatom 30si values of any age a binary mixing model upwelled silicon versus eolian silicon accounting for silicon isotopic fractionation during dsi uptake by diatoms was constructed the binary mixing model demonstrates that e rex dominantly utilized dsi from eolian sources i e asian dust with only minor contributions from upwelled seawater sources i e advected from subantarctic mode water antarctic intermediate water or north pacific intermediate water e rex utilized only 24 of available dsi indicating that surface waters of the eps were eutrophic with respect to silicon during the lgm our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool thus revising previously proposed models for the formation of e rex ldms

nickel isotope fractionation during tropical weathering of ultramafic rocks abstract although ni isotopes have been shown to be significantly fractionated in terrestrial samples their use in continental environmental studies has not yet been evaluated the present study focuses on an ultramafic um massif barro alto goias brazil because such areas are naturally rich in ni we present developed lateritic weathering profiles the goal of the study is to evaluate the potential of using ni isotopes in environmental continental studies by combining its isotopic signature with mineralogy in order to better understand the geochemical cycling of ni in um settings during weathering as such ni isotope values were measured in samples from the barro alto um complex in the main stages of the lateritic weathering profile of um rocks including bedrock ores saprolitic and lateritic samples and soil the mineralogical composition of the samples with a focus on the different ni bearing minerals was also determined to decipher the potential links between isotopic fractionation and weathering dynamics isotopic signatures 60 ni from the natural ni geochemical cycle include bedrock samples 60 ni 0 28 0 08 ore samples saprolitic and lateritic 60 ni from 0 60 to 0 30 and soil samples 60 ni from 0 19 to 0 02 an overall trend of heavier isotope depletion was observed in the solid phase during weathering 60 ni soil bedrock 0 47 the mineralogical results were consistent with the literature and showed that the mineralogy of the lateritic part and soil was dominated by fe oxides whereas clay minerals were the primary ni phase scavengers in the saprolitic part of the profile thus the formation of ni bearing clay minerals and fe oxides appeared to lead to depletion in heavier isotopes which indicates preferential export of heavy isotopes in the dissolved phase this result is consistent with isotopic signatures measured in the exchangeable pool of the solid phase 60 ni exch total up to 0 47 and ni isotopes appear to be a promising tracer to better understand the biogeochemical ni cycling on the earth s surface

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

sea level changes at the lgm from ice dynamic reconstructions of the greenland and antarctic ice sheets during the glacial cycles abstract new experiments were performed with three dimensional thermomechanical models of the greenland and antarctic ice sheets to simulate their behaviour during the glacial cycles to reconstruct their thickness and extent at the last glacial maximum lgm and to estimate their glacio eustatic contribution to the global sea level stand the calculations used improved ice dynamic and isostatic treatments updated datasets on higher grid resolutions and refined climatic treatments based on newly calibrated transfer functions between ice core records and climatic perturbations results are discussed from a reference run with standard parameters that is compared with available glacial geological observations and from a series of sensitivity experiments focusing on isostatic adjustment thermomechanical coupling climatic forcing mass balance changes and basal melting rates and viscosity changes of antarctic ice shelves for the antarctic ice sheet we find that volume changes are closely linked with grounding line changes of the west antarctic ice sheet at the lgm the grounding line extended close to the continental shelf break almost everywhere ice over central east antarctica was generally thinner than today and varied mainly in accordance with accumulation fluctuations for the greenland ice sheet melting is important only during interglacial periods and the most sensitive period concerns the size of the ice sheet during the eemian at the lgm the greenland ice sheet extended beyond the present coastline to cover at least the inner continental shelf and thinned by up to several hundred meters in central areas for a plausible range of parameters the experiments indicate that at the time of maximum sea level depression 21 kyr bp the antarctic ice sheet contributed 14 18 m to the sea level lowering and the greenland ice sheet 2 3 m significantly less than the older climap reconstructions whereas both ice sheets were at 21 kyr bp close to their maximum extent the experiments also indicate that their maximum volumes were reached only by 16 5 kyr bp greenland and 10 kyr bp antarctica equal to an additional sea level lowering of respectively 0 4 and 3 7 m holocene retreat was essentially complete by 5 kyr bp in greenland but is found to still continue today in west antarctica before reversing to growth during the next millenium the models were found to reproduce gross features of the ice sheet s history since the lgm in reasonably good agreement with available glacial geological data although observational control on ice thickness changes remains very limited

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

variations of δ30si and ge si with weathering and biogenic input in tropical basaltic ash soils under monoculture in soils silicon released by mineral weathering can be retrieved from soil solution through clay formation si adsorption onto secondary oxides and plant uptake thereby impacting the si isotopic signature and ge si ratio of dissolved si dsi exported to rivers here we use these proxies to study the contribution of biogenic si bsi in a soil plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping delta si 30 and ge si ratios were determined in bulk soils 2 mm sand 50 2000 mu m silt 2 50 mu m amorphous si asi 2 50 mu m and clay 2 mu m fractions delta si 30 by mc icp ms nu plasma in medium resolution operating in dry plasma with mg doping delta si 30 vs nbs28 0 12 parts per thousand 2 sigma sd ge si computed after determination of ge and si concentrations by hr icp ms and icp aes respectively components of the asi fraction were quantified by microscopic counting phytoliths diatoms ashes compared to fresh ash delta si 30 0 38 parts per thousand ge si 2 21 mu mol mol 1 soil clay fractions 2 mu m were enriched in light si isotopes and ge with increasing weathering degree delta si 30 decreased from 1 19 to 2 37 parts per thousand and ge si increased from 4 10 to 5 25 mu mol mol 1 sand and silt fractions displayed delta si 30 values close to fresh ash 0 33 parts per thousand or higher due to saharian dust quartz deposition whose contribution was evaluated by isotopic mass balance calculation si isotopic signatures of bulk soils 2 mm were strongly governed by the relative proportions of primary and secondary minerals the bulk soil si isotopic budget could be closed indicating that all the phases involved were identified microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation delta si 30 and ge si values of clay fractions in poorly developed volcanic soils isotopically heavier and ge depleted in surface horizons support the occurrence of a dsi source from banana phytolith dissolution available for si sequestration in clay sized secondary minerals clay minerals formation and si adsorption onto fe oxide in the soil plant system delta si 30 and ge si are thus highly relevant to trace weathering and input of dsi from phytoliths in secondary minerals although not quantifying the net input of bsi to dsi c 2009 elsevier ltd all rights reserved

holocene ice recession at polygon spur reedy glacier antarctica the holocene history of outlet glaciers affords information on the behavior and mechanisms controlling the extent of the east antarctic ice sheet here we present both new radiocarbon and recalcul

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

silicon isotopes and the tracing of desilication in volcanic soil weathering sequences guadeloupe abstract silicon si stable isotopes have the potential to become a useful weathering proxy given that light si isotopes are preferentially incorporated into secondary clay minerals here we investigate how si depletion in soils and associated clay mineralogy influence the si isotope fractionation associated with clay mineral formation we report 30 si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from guadeloupe that were formed under contrasting climatic conditions strongly desilicated soils containing kaolinite that formed in wet areas high precipitation are compared with less desilicated soils containing smectite formed in drier conditions low precipitation clay fractions are isotopically lighter than the parent andesite 30 si 0 23 and increasingly lighter with si depletion in soils which supports the view that the si isotope composition in secondary clay fractions is controlled by the degree of soil desilication it is shown that the si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for si rich secondary clay minerals such as smectite and larger for si poor secondary clay minerals such as kaolinite this study provides new insights to better define si isotopes as a proxy for environmental conditions for clay neoformation

soil carbon storage controlled by interactions between geochemistry and climate rising temperature can increase soil organic matter decomposition and co2 emissions in a 4 000 km north south transect in chile and antarctica soil geochemistry which can be modified by climate is the dominant direct control of carbon storage soils are an important site of carbon storage1 climate is generally regarded as one of the primary controls over soil organic carbon1 2 but there is still uncertainty about the direction and magnitude of carbon responses to climate change here we show that geochemistry too is an important controlling factor for soil carbon storage we measured a range of soil and climate variables at 24 sites along a 4 000 km long north south transect of natural grassland and shrubland in chile and the antarctic peninsula which spans a broad range of climatic and geochemical conditions we find that soils with high carbon content are characterized by substantial adsorption of carbon compounds onto mineral soil and low rates of respiration per unit of soil carbon and vice versa for soils with low carbon content precipitation and temperature were only secondary predictors for carbon storage respiration residence time and stabilization mechanisms correlations between climatic variables and carbon variables decreased significantly after removing relationships with geochemical predictors we conclude that the interactions of climatic and geochemical factors control soil organic carbon storage and turnover and must be considered for robust prediction of current and future soil carbon storage

the silicon isotopic composition of fine grained river sediments and its relation to climate and lithology abstract the 30si stable isotopic composition of silicon in soils and fine grained sediments can provide insights into weathering processes on continents with important implications on the si budget of modern and past oceans to further constrain the factors controlling the distribution of si isotopes in sediments we have analysed a large number n 50 of separate size fractions of sediments and suspended particulate materials collected near the mouth of rivers worldwide this includes some of the world s largest rivers e g amazon congo mackenzie mississippi murray darling nile yangtze and rivers from the case study areas of the congo river basin and northern ireland silt size fractions exhibit a mean si isotopic composition 30si 0 21 0 19 2 s d similar to that previously inferred for the upper continental crust in contrast clay size fractions display a much larger range of 30si values from 0 11 to 2 16 which yield a global 30siclay of 0 57 0 60 2 s d representative of the mean composition of the average weathered continental crust overall these new data show that the si isotopic signature transported by river clays is controlled by the degree of chemical weathering as inferred from strong relationships with al si ratios at a global scale the clay bound si isotopic composition of the world s largest river systems demonstrates a link with climate defining a general correlation with mean annual temperature mat in corresponding drainage basins while the distribution of si isotopes in river sediments also appears to be influenced by the tectonic setting lithological effects and sediment recycling from former sedimentary cycles our results pave the way for their use as paleo weathering and paleo climate proxies in the sedimentary record

late quaternary glacial history constrains glacio isostatic rebound in enderby land east antarctica measurements of the loss or gain of ice mass from large ice sheets are presently achieved through satellite based techniques such as grace gravity recovery and climate experiment the accuracy of these satellite based measurements to changes in modern ice sheet mass depends on our knowledge of present day glacio isostatic crustal uplift rates caused by past ice sheet changes to improve models of glacio isostatic rebound in east antarctica we investigated ice histories along rayner glacier enderby land and a little explored sector of the ice sheet where grace data had suggested significant mass gain during the last decade observations from a recent glacial geomorphic reconnaissance coupled with cosmogenic nuclide dating indicate that in the lower part of the rayner glacier enderby land ice heights lowered by at least 300 m and the calving margin retreated by at least 10 km in the early holocene 6 to 9 ka b p the magnitude and timing of deglaciation are consistent with ice histories used to model the postglacial rebound corrections for present day grace mass trends these observations strengthen the body of evidence that suggests ice mass gain in enderby land is presently partly offsetting mass loss in other parts of antarctica

late quaternary ice surface fluctuations of beardmore glacier transantarctic mountains abstract former longitudinal profiles of beardmore glacier an outlet through the transantarctic mountains constrain polar plateau elevations near the center of antarctica and ice shelf grouding in the southern ross embayment three gravel drift sheets of late quaternary age occur alongside beardmore glacier plunket drift the youngest is parallel to and 7 30 m above the present ice surface the upper limit of beardmore drift intermediate in age is within 35 40 m of the present ice surface near the polar plateau but about 1100 m above the present ice surface near the glacier mouth the upper limit of meyer drift the oldest is parallel to and 30 50 m above beardmore drift from correlation with numerically dated drifts farther north we assign an early holocene age to plunket drift a late wisconsin age to beardmore drift and an age of marine isotope stage 6 to meyer drift by our age model beardmore glacier was close to current elevations in its upper reaches and thickened considerably in its middle and lower reaches during the last two global glaciations represented by beardmore and meyer drifts most likely grounded ice in the southern ross embayment caused such thickening of beardmore glacier almost to the polar plateau a concomitant decline in precipitation is implied by ice cap retreat on the nearby dominion range and is consistent with little change of upper beardmore glacier ice shelf grounding most likely resulted from lowered sea level and or basal melting lower than present precipitation was probably caused by colder air temperatures and more distant open water the plunket profile records holocene ice surface lowering from increased surface ablation decreased ice flow or grounding line recession

atmospheric scaling of cosmogenic nuclide production climate effect 1 absorption of cosmic rays by atmospheric mass varies temporally due to a redistribution of atmospheric pressure by ice sheets during glaciations the compression and expansion of the atmosphere due to cooling and warming and changes in katabatic winds near large ice masses these atmospheric processes can result in changes in production rates of cosmogenic nuclides which when integrated over long exposure durations may result in 0 to 5 adjustments in site production rates depending on location combining a ccm3 model with imbedded ice sheets for 20 ka we show that production rates changes relative to today are greatest at high elevations 6 7 at 5 km altitude due to atmospheric compression from decreased temperature production rates at specific times for sites near ice sheet margins can be reduced more than 10 due to a combination of katabatic winds draining off the ice sheet margins and atmospheric cooling nunatak settings may be significantly affected by the climate effect due to persistent glacial atmospheric conditions atmospheric variability may explain some of the disparities among cosmogenic nuclide production rate calibrations

a 150 000 year climatic record from antarctic ice during much of the quaternary the earth s climate has undergone drastic changes most notably successive glacial and interglacial episodes the past 150 kyr includes such a climatic cycle the last interglacial the last glacial and the present holocene interglacial a new climatic time series for this period has been obtained using 18 o data from an antarctic ice core

the silicon isotope composition of granites because weathering results in preferential incorporation of the lighter si isotopes into clay minerals the si in magmas derived from meta sedimentary lithologies enriched in such phases should also be isotopically light relative to melts derived from igneous sources with the advent of new high precision techniques it should therefore be possible to resolve such differences hence si isotopes have to potential to distinguish between and characterise different magmatic source regions to explore this here we report the si isotopic compositions of a suite of phanerozoic i s and a type granites and mineral separates as well as samples of associated country rock the whole rock data fall between 30si 0 40 and 0 11 0 04 95 s e this range is more limited than that defined by previous low precision measurements of granites but is significantly broader than that displayed by more recent high precision measurements of high si extrusive igneous rocks as predicted s type granites are on average isotopically lighter than i and a type samples and negative correlations with alumina saturation index asi values and initial 87sr 86sr provide evidence that si isotope variations in granites can be explained by variable incorporation of an enriched sedimentary component however the relatively small isotopic perturbations and apparent degree of overlap between granite affinities suggest that si isotopes are not as sensitive to sedimentary input relative to more established proxies such as for example o isotopes the average si isotope value of all granites analysed in this study is 30si 0 23 0 15 2 s d which is entirely consistent with a recently published estimate for the continental crust based on an andesitic bulk composition and the well defined relationship between si isotopes and magmatic fractionation

estimated storage of amorphous silica in soils of the circum arctic tundra region we investigated the vertical distribution storage landscape partitioning and spatial variability of soil amorphous silica asi at four different sites underlain by continuous permafrost and rep

contrasting environmental memories in relict soils on different parent rocks in the south western italian alps abstract soils on the alps are usually weakly developed both because of the extensive pleistocene glaciations and because of slope steepness and climate enhancing erosion however on some stable relict surfaces particularly in the outermost sections of the alpine range some highly developed soils are apparently in contrast with holocene soil forming conditions in this work we assess the extent of pedogenesis in some of these soils located under montane level vegetation in the ligurian alps sw piemonte italy and relate it to the effects of climate and parent material the considered well developed profiles showed signs of extremely different pedogenetic processes on the different lithotypes in particular podzols with extremely thick e horizons up to more than 2 m thick and very hard thick ortstein or placic horizons were formed on quartzite reddish terra rossa luvisols were formed on limestone red extremely acidic alisols were formed on shales the chemical properties the micromorphology and the clay mineralogy demonstrated high intensity pedogenic trends and were characteristic of processes usually occurring under different climates they may therefore represent excellent pedo signatures of different specific past climatic environmental conditions as a response of different lithologies to specific soil forming environments which range from warm and humid climates typical of red alisols and luvisols to cool and humid wet climates leading to the formation of podzols with ortstein placic horizons

history of the grounded ice sheet in the ross sea sector of antarctica during the last glacial maximum and the last termination knowledge of variations in the extent and thickness of the antarctic ice sheet is key for understanding the behaviour of southern hemisphere glaciers during the last ice age and for addressing issues involving global sea level ocean circulation and climate change insight into past ice sheet behaviour also will aid predictions of future ice sheet stability here we review ter restrial evidence for changes in ice geometry that occurred in the ross sea sector of antarctica at the last glacial maximum lgm and during subsequent deglaciation during the lgm a thick grounded ice sheet extended close to the continental shelf edge in the ross embayment this ice reached surface elevations of more than 1000 m along the coast of the central and southern trans antarctic mountains and marie byrd land the local lgm occurred by 18 ka on the coast but as late as 7 10 ka inland the first significant thinning took place at roughly 13 ka with most ice loss happening in the holocene this history makes it unlikely that the ross sea sector was a major contributor to meltwater pulse 1a mwp 1a resolution of a possible antarctic origin for mwp 1a awaits detailed reconstructions from all sectors of the ice sheet the antarctic ice sheet contains the world s larg est reservoir of fresh water and is a major control on southern hemisphere climate a history of changes in ice sheet extent and thickness is vital for answering questions involving the initiation of southern hemisphere glaciation and deglaciation