kinematic model for the main central thrust in nepal we present a kinematic model for the himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late miocene pliocene geochronologic and thermochronologic ages from rocks in the main central thrust zone in central nepal at its current exposure level the main central thrust juxtaposes a hanging wall flat in greater himalayan rocks with a footwall flat in lesser himalayan rocks of the ramgarh thrust sheet which is the roof thrust of a large lesser himalayan duplex sequential emplacement of the main central early miocene and ramgarh middle miocene thrust sheets was followed by insertion of thrust sheets within the lesser himalayan duplex and folding of the main central and ramgarh thrusts during late miocene pliocene time thorium lead th pb ages of monazite inclusions in garnets from central nepal record the timing of coeval progressive metamorphism of lesser himalayan rocks in the footwall of the main central thrust although this model does not rule out minor late stage reactivation of the main central thrust major late miocene reactivation is not required

change in climate ocean and ice sheet conditions in the ross embayment antarctica at 6 ka evidence from the ross embayment antarctica suggests an abrupt cooling and a concomitant increase in sea ice cover at about 6000 sp 6 ka stable isotope delta d concentrations in the taylor dome ice core at the western edge of the ross embayment decline rapidly after 6 ka and continue to decline through the late holocene methanesulfonic acid concentrations at taylor dome show opposite trends to delta d sediment cores from the western ross sea show a percentage minimum for the sea ice diatom fragilariopsis curta between 9 and 6 ka when taylor dome delta d values are highest followed by an increase through the late holocene radiocarbon dates from raised beach deposits indicate that the retreat of ice shelves in the ross embayment ceased atabout 6 ka coincident with the environmental changes inferred from the sediment and ice core records the similarity in timing suggests an important role for climate in controlling the evolution of ice shelf margins following the end of the last 9 glaciation

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

inverted metamorphism in the intracontinental ductile shear zone during himalayan collision tectonics collision tectonics in the himalaya ca 50 ma remobilized the proterozoic basement and its cover of the indian plate into the higher himalayan crystalline belt this belt displays an inverted metamorphism progressing from a garnet to a sillimanite k feldspar zone toward higher topographic and structural levels because previously hypothesized mechanisms are not sufficient to explain inverted metamorphism we propose that ductile shear displacements along ubiquitous closely spaced s c shear planes may have caused metamorphic inversion across a 15 20 km thick southwest vergent higher himalayan shear zone

abandoned penguin rookeries as holocene paleoclimatic indicators in antarctica penguins are sensitive indicators of the antarctic climate and of the environmental parameters that limit their presence and distribution paleoenvironmental data obtained from the study of abandoned penguin rookeries pygoscelis adeliae along the victoria land coast in antarctica indicate 14c date of 11 13 ka for the oldest abandoned rookery and supply new information about the timing of glacier retreat in southern victoria land after the last glacial maximum the continuous presence of the adelie penguins is documented from 7 ka according to our data the limiting factors that control the presence of penguins along the coast of victoria land changed during the holocene whereas several colonies were occupied for very long periods other sites were used for more or less extended periods and then abandoned the greatest diffusion of rookeries occurred between 3 and 4 ka a period of particularly favorable environmental conditions that has never been repeated it was followed by a sudden decrease in the number of penguin rookeries shortly after 3 ka this event has been attributed to an increase of the sea ice extension and may have been correlated to a worldwide phase of climate change near the subboreal subatlantic boundary a minor phase of penguin reoccupation occurred locally in the eighth to fourteenth centuries a d because the presence and number of penguins reflect the state of health of the antarctic marine ecosystem it is important to evaluate the variations in their distribution in the past in the absence of human induced changes

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

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

holocene depositional environments and surface level changes at lake fryxell antarctica we report on holocene surface level variations of lake fryxell antarctica as determined from multiproxy analyses of 18 sediment cores during this time accumulating sediments were predominantly aeolian sand with algal and carbonate laminae based on stratigraphy mineralogy and diatom assemblages we suggest some carbonate laminae were deposited when lake level dropped leading to concentration and subsequent precipitation of salts although lake level appears to have remained relatively stable throughout the holocene minor 4 5 m below present lowstands occurred at approximately 6400 4700 3800 and 1600 cal yr bp the stability of lake fryxell during the holocene contrasts with large scale variability at other dry valleys lakes eg lake vanda and with suggestions from chemical diffusion models of a near desiccation at 1200 cal yr bp the reason for the comparative stability of lake fryxell is uncertain but may be the result of basin morphology and the number aspect and proximity of meltwater sources

lateral variation in geometry of thrust planes and its significance as studied in the shumar allochthon lesser himalayas eastern bhutan abstract the features mapped indicate a sse striking displacement vector for the shumar thrust of eastern bhutan sections parallel to this direction show that the frontal flat segment of the thrust plane along with the subthrust and the hanging wall rocks has been folded into a synform a longitudinal section however shows a cuspate lobate shape broad concave upward curves meeting along sharp crests of the thrust plane which laterally cuts up through the subthrust rocks the broad salients and sharp re entrants defining the multiple lobate pattern of the thrust trace is due to the cuspate lobate lateral curvature which is a primary feature of the thrust plane each cusp combined with the forward or backward inclination of the thrust plane imparts a scoop like shape several of which are joined by transverse ridges parallel to the displacement vector the s surfaces transposed parallel to the limbs and axial surfaces of the isoclinal early folds in the allochthon have been progressively reoriented parallel to the curviplanar thrust surface later post thrusting folds superposed on these initially curved surfaces have curved hinges which trend at high angles to the displacement direction as a result the bent hinges are congruent to the laterally curved thrust surface below the evolution of the shumar thrust can be explained by lateral propagation along concave upward trajectories thrust fronts propagating from multiple nuclei subsequently coalesced to form a single thrust surface the ridges along which the thrusts coalesce have the form of lateral ramps

insights into the holocene environmental setting of terra nova bay region ross sea antarctica from oxygen isotope geochemistry of adelie penguin eggshells an oxygen isotope record of adelie penguin pygoscelis adeliae eggshells from terra nova bay victoria land antarctica has been reconstructed in order to provide insights into the palaeoenviroment of this region during the last 8000 years the 18o values measured in eggshell carbonate show a great variability over the course of the studied period with maximum values consistently around 27 5 adopting a metabolic enrichment relative to water source of 28 5 eggshell 18o values can be interpreted in terms of marine water uptake by the penguins the spread in 18o of eggshell carbonate at any given time toward more negative values reflects the existence of a low 18o water end member most plausibly snow or snowmelt samples younger than 2000 years are characterized by a general lack of very negative 18o values as also attested by other proxies this shift toward more positive isotopic values is likely to be related both to a decreasing contribution of snow or meltwater in the seawaters and con

late pleistocene holocene marine conditions in the ross sea antarctica evidence from the diatom record statistical analyses of diatom assemblages from radiocarbon dated sediment cores were used to reconstruct changing palaeoceanographic conditions in the western and west central ross sea antarctica from c 12 14c kyr bp to the present data from three kasten cores support a north to south time transgressive glacial interglacial transition assemblages at the base of each core suggest that glacial processes affecting frustule preservation were significant during the late pleistocene increasing biogenic silic and decreasing per centages of reworked species suggest that upper water column productivity instead of preservational processes increasingly dominates diatom assemblages during grounding line recession c 12to c 6 14c kyr bp a warm ing during the middle to late holocene c 6to c 3 14c kyr bp may have resulted in a spring sea ice melting prior to the annual inception of the ross sea polynya in the west central ross sea and b increased plateletice delivery due to increased melting beneath t

exhumation during crustal folding in the namche barwa syntaxis geological observation in the eastern end of the himalaya shows that the asia india suture is folded metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating establish cooling exhumation of rocks from c 30 km depth within the last 4 myr we argue that exhumation is caused by 10 mm yr 1 erosion coeval with crustal scale folding

a review of the australian new zealand sector of the southern ocean over the last 30ka aus intimate project the australia new zealand region of the southern ocean is influenced by several of the major global water masses of the oceans and is the prime entry point for cold deep waters into the pacific basin during the last glacial there was increased sea ice extent around antarctica as far north as 55 s as well as increased iceberg presence inferred from ice rafted debris evidence from microfossil assemblages suggests that sea surface temperatures sst were up to 7 c cooler consistent with recent estimates of cooling for new zealand derived from glacier modelling and other terrestrial proxies the subtropical front stf subantarctic front saf and polar front pf had migrated north except where the position of the fronts were controlled bathymetrically despite the potential for iron fertilisation by increased dust input into the ocean during the glacial there is limited evidence for higher total biological productivity in the pacific sector of the southern ocean the altered oceanic circulation during the glacial also decreased nutrients in the surface waters and affected the outgassing of co 2 this contributed to an increased storage of co 2 in the deep waters and lowering of the carbonate lysocline during the deglaciation sea ice retreat and sst increased rapidly at 18 ka roughly synchronous with the reinvigoration of deep water circulation in the southern ocean and the release of co 2 stored in the deep waters the gradient in carbon isotopes 13 o benthic between antarctic intermediate water aaiw and lower circumpolar deep water lcdw was greatest at the start of the deglaciation suggesting that the aaiw ventilation preceded lcdw ventilation or there was a significant change in air sea fractionation of 13 o benthic there was a slight enrichment in 18 o planktic decrease in ssts and a reduction in intermediate and deep water circulation between 14 and 12 5 ka bp during the antarctic cold reversal acr coincident with glacier advances in the new zealand southern alps and other terrestrial records of cooling following the acr there was a second more minor release of deep water co 2 most likely related to reinvigoration of north atlantic deep water nadw from 10 ka onwards the modern intermediate and deep water circulation was established air temperatures and ssts reached a maximum in the early holocene with the stf located at its most southerly position and there was a widespread retreat of antarctic ice sheets to their modern position a decline to modern sst and air temperatures in the mid to late holocene followed while millennial cycles overprinted on the sst and 18 o planktic records may be the result of subtle changes in the position and strength of the westerly winds during the holocene

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

paleoceanographic changes in the northeast indian ocean during middle miocene inferred from carbon and oxygen isotopes of foraminiferal fossil shells abstract the middle miocene transition witnessed rapid fluctuations in oceanographic and climatic systems however very limited work has been attempted in miocene sediments of the northern indian ocean to identify these changes here we present the first detailed 13c and 18o records of surface and deep dwelling foraminifera from odp site 758 in the northeast indian ocean located atop the ninetyeast ridge carbon and oxygen isotopic data on globigerinoides quadrilobatus planktonic and cibicidoides wuellerstorfi benthic foraminifera indicated various global and local successions of oceanographic and climatic importance the climatic events recorded at odp 758 are a mid miocene climatic optimum 17 15 ma b monterey excursion 17 14 ma c east antarctica ice sheet formation 13 8 ma d initiation of indian summer monsoon with waning of antarctica ice sheet 12 3 10 4 ma and e evidence of a cooling event 10 2 9 6 ma our benthic foraminifera 18o record showed almost identical deep water evolution to that of the indo pacific for 13 9 11 5 ma interval a significant decrease in 13cben at 10 2 ma was indicative of a complete re adjustment of the carbon cycle after the monterey excursion and export of the north atlantic deep water nadw towards the southern ocean to give rise to the circumpolar deep water cdw with a 13c value of 0 4 the formation of the nadw had given rise to modern deep water like conditions at 10 ma ago a comparison of our record with previously published isotopic records from other regions suggested a good correlation between the south pacific indo pacific and northeast indian ocean during the studied time interval it appears that a strong westerly indian ocean current miojet may have linked these oceans from 14 to 9 ma

piston core based biostratigraphic constraints on pleistocene oscillations of the west antarctic ice sheet in western ross sea between north basin and and 1b drill site abstract new diatom biostratigraphic age data derived from piston core were used to constrain the chronology for a seismic based interpretation of west antarctic ice sheet wais advance and retreat from the north basin outer shelf in western ross sea the data demonstrates that grounded ice occupied the outer shelf on at least ten occasions during the last 2 0 ma of the pleistocene although the biostratigraphic resolution is low the constraints permit correlations to coeval section recently drilled in south mcmurdo sound at the and 1b drill site our interpretation of regional scale correlations suggests that the pleistocene dynamics of the wais involved grounding line translations in excess of 600 km however the data also suggests that only two major advances of grounded ice occurred since 0 71 ma in other words several of the late pleistocene sediment cycles at and 1b must represent only minor advance and retreat of grounded ice to and from south mcmurdo sound the results prove false the long standing tacit assumption that the wais experienced major advances in association with peak glacials and eustatic lowstands during the last 700 ka of the pleistocene these results show that piston cores can be used to constrain the timing of pre holocene grounding events these data also highlight the need for regional scale correlations to distinguish between major versus minor grounding line translations of grounded ice in the marine setting

interdecadal variation in an antarctic sponge and its predators from oceanographic climate shifts during the 1960s there was extensive formation of anchor ice to depths of 30 meters at mcmurdo station antarctica during this period the sponge homaxinella balfourensis was rare as were its predators in that depth zone most of the existing sponges were killed by anchor ice during the 1970s anchor ice formation was reduced and there was a massive recruitment of homaxinella which covered as much as 80 percent of the substrata in that zone many predators appeared but did not control the sponge population and it continued to grow through that decade the early 1980s were characterized by ice formation and almost all of the homaxinella were eliminated leaving an order of magnitude more predators in that zone the interdecadal increases in anchor ice probably result from local upwelling of extremely cold deep water possibly in response to shifts in the strengths of regional currents

the sedimentary legacy of a palaeo ice stream on the shelf of the southern bellingshausen sea clues to west antarctic glacial history during the late quaternary a major trough belgica trough eroded by a palaeo ice stream crosses the continental shelf of the southern bellingshausen sea west antarctica and is associated with a trough mouth fan belgica tmf on the adjacent continental slope previous marine geophysical and geological studies investigated the bathymetry and geomorphology of belgica trough and belgica tmf erosional and depositional processes associated with bedform formation and the temporal and spatial changes in clay mineral provenance of subglacial and glaciomarine sediments here we present multi proxy data from sediment cores recovered from the shelf and uppermost slope in the southern bellingshausen sea and reconstruct the ice sheet history since the last glacial maximum lgm in this poorly studied area of west antarctica we combined new data physical properties sedimentary structures geochemical and grain size data with published data shear strength clay mineral assemblages to refine a previous facies classification for the sediments the multi proxy approach allowed us to distinguish four main facies types and to assign them to the following depositional settings 1 subglacial 2 proximal grounding line 3 distal sub ice shelf sub sea ice and 4 seasonal open marine in the seasonal open marine fades we found evidence for episodic current induced winnowing of near seabed sediments on the middle to outer shelf and at the uppermost slope during the late holocene in addition we obtained data on excess pb 210 activity at three core sites and 44 ams c 14 dates from the acid insoluble fraction of organic matter aio and calcareous micro fossils respectively at 12 sites these chronological data enabled us to reconstruct for the first time the timing of the last advance and retreat of the west antarctic ice sheet wais and the antarctic peninsula ice sheet apis in the southern bellingshausen sea we used the down core variability in sediment provenance inferred from clay mineral changes to identify the most reliable aio c 14 ages for ice sheet retreat the palaeo ice stream advanced through belgica trough after similar to 36 0 corrected c 14 ka before present b p it retreated from the outer shelf at similar to 25 5 ka b p the middle shelf at similar to 19 8 ka b p the inner shelf in eltanin bay at similar to 12 3 ka b p and the inner shelf in ronne entrance at similar to 6 3 ka b p the retreat of the wais and apis occurred slowly and stepwise and may still be in progress this dynamical ice sheet behaviour has to be taken into account for the interpretation of recent and the prediction of future mass balance changes in the study area the glacial history of the southern bellingshausen sea is unique when compared to other regions in west antarctica but some open questions regarding its chronology need to be addressed by future work c 2010 elsevier ltd all rights reserved

metamorphic evolution of the lesser himalayan crystalline sequence siyom valley ne himalaya india abstract the paper reports classical barrovian inverted metamorphism in the lesser himalayan crystalline sequence siyom valley ne himalaya the sequence is thrust bounded and constitutes a part of the western limb of the siang antiform six metamorphic zones are delineated with increasing grade from biotite zone through garnet staurolite kyanite sillimanite to sillimanite k feldspar zone with increasing structural level granulite facies pelitic assemblages marked by the occurrence of garnet cordierite occur in the sillimanite k feldspar zone three phases of folding f1 f2 and f3 and two generations of planar structures s1 and s2 are recognised peak metamorphic mineral growths are dominantly syn f2 metamorphic isograds and boundary thrusts of the sequence are co folded around the siang antiform by f3 deformation garnet shows prograde growth zoning in lower grade rocks and retrograde diffusion zoning in higher grade rocks pelites of the sillimanite k feldspar zone show textural evidence of decompression such as corona of cordierite around kyanite breakdown reactions of muscovite and biotite in this zone are attributed to decompression geothermobarometric calculations show gradual increase of temperature from garnet zone to sillimanite k feldspar zone whereas pressure increases sharply from garnet zone to staurolite zone and remains nearly constant thereafter peak metamorphism reached temperature of 750 c and pressure of 10 kbar in the sillimanite k feldspar zone p t estimates decompression reactions and pseudosection topologies suggest a clockwise path with steep decompression for the evolution of the sillimanite k feldspar zone in the siyom valley

himalayan inverted metamorphism constrained by oxygen isotope thermometry inverted metamorphic field gradients are preserved in two amphibolite facies metapelitic sequences forming the crystalline core zone of the himalayan orogen in the sutlej valley nw india in the high himalayan crystalline sequence hhcs metamorphic conditions increase upwards from the staurolite zone at the base through the kyanite in and sillimanite in isograds finally to reach partial melting conditions at the top the structurally lower lesser himalayan crystalline sequence lhcs shows a gradual superposition of garnet in staurolite in and kyanite sillimanite in isograds although phase equilibria constraints imply inverted temperature field gradients in both units garnet biotite garb rim thermometry indicates final equilibration at a nearly uniform temperature around t 600 c across these sequences the p t path and garnet zoning data show that this apparent lack of thermal field gradient is mainly the consequence of a resetting of the garb equilibria during cooling in order to constrain peak temperature conditions 20 samples along the studied section have been analysed for oxygen isotope thermometry the isotopic fractionations recorded by quartz garnet and quartz aluminosilicate mineral pairs indicate temperatures consistent with phase equilibria and p t path constraints for metamorphic peak conditions together with barometry results based on net transfer continuous reactions the oxygen isotope thermometry indicates peak conditions characterized by 1 a temperature increase from t 570 to 750 c at a nearly constant pressure around p 800 mpa from the base to the top of the hhcs unit 2 a temperature increase from t 610 to 700 c and a pressure decrease from p 900 to 700 mpa from the base to the top of the lhcs metapelites oxygen isotope thermometry thus provides the first quantitative data demonstrating that the himalayan inverted metamorphism can be associated with a complete inversion of the thermal field gradient across the crystalline core zone of this orogen

anchor ice seabed freezing and sediment dynamics in shallow arctic seas diving investigations confirm previous circumstantial evidence of seafloor freezing and anchor ice accretion during freeze up storms in the alaskan beaufort sea these related bottom types were found to be continuous from shore to 2 m depth and spotty to 4 5 m depth spotty anchor ice occurred as pillow shaped crystal aggregates on buried slabs of frozen sand surrounded by unfrozen sand considerations of required conditions for ice bonding and anchor ice growth allows regional extrapolation and suggests the possibility of anchor ice growth out to 20 m depth the estimated maximum depth of supercooling during fall storms anchor ice and seabed freezing apparently do not develop during a calm freeze up because of the abrupt growth of anchor ice during a freezing storm and its release soon after formation of a surface ice cover this ice type has not been documented before the concretelike nature of frozen bottom where present should prohibit sediment transport by any conceivable wave or current regime during the freezing storm but elsewhere particularly where the bonded crust is broken by grounded ice anchor ice lifts coarse material off the bottom and incorporates it into the ice canopy thereby leading to significant ice rafting of shallow shelf sediment and likely sediment loss to the deep sea

recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer the garnet al silicate plagioclase gasp geobarometer has been recalibrated using four recent garnet activity models four analogous garnet biotite temperature models and two recent plagioclase activity models a typical sillimanite bearing sample that formed at about 5 25 kbar 575 c shows a possible p range of 0 7 kbar due to t error 1 kbar due to range of garnet activity model 0 9 kbar due to range of plagioclase activity model and 5 4 kbar due to range of experimental end member reversals extended by one sigma calibrations were further constrained with the kyanite sillimanite k s phase boundary such that the best fit of 76 pelitic schist samples from 11 localities provides an individual end member calibration for each of the eight possible combinations of garnet and plagioclase activity models with the appropriate geothermometer samples with low grossular or anorthite component were rejected the end member calibrations are constrained to pass through the the best determined portion of the gasp experimental reversals at 1230 c 26 6 kbar these individual end member calibrations provide self consistent models that tend to compensate for error in the garnet and plagioclase activity expressions the models were also tested on a set of 59 samples from the alps the recommended calibration is the average garnet activity model and average garnet biotite t model of holdaway 2000 the fuhrman and lindsley 1988 plagioclase activity model and h grs 6628521 s grs 258 76 to combine with the remaining phases in the berman database to produce the optimum end member gasp curve these thermodynamic data are for the gasp geobarometer only error is about 0 8 kbar absolute and about 0 6 kbar relative geological error is the largest component of error in many of these samples care should be taken to be sure that analyzed plagioclase and biotite are near analyzed garnet that the peak t portions of garnet and plagioclase are selected that the peak t al silicate is determined and that the t calculated is the most accurate possible these calibrations represent an improvement over previous published gasp calibrations these eight models are available for distribution as three programs t p p t intersection for the dos based personal computer

the last glaciation of bear peninsula central amundsen sea embayment of antarctica constraints on timing and duration revealed by in situ cosmogenic 14c and 10be dating abstract ice streams in the pine island thwaites region of west antarctica currently dominate contributions to sea level rise from the antarctic ice sheet predictions of future ice mass loss from this area rely on physical models that are validated with geological constraints on past extent thickness and timing of ice cover however terrestrial records of ice sheet history from the region remain sparse resulting in significant model uncertainties we report glacial geological evidence for the duration and timing of the last glaciation of hunt bluff in the central amundsen sea embayment a multi nuclide approach was used measuring cosmogenic 10be and in situ 14c in bedrock surfaces and a perched erratic cobble bedrock 10be ages 118 144 ka reflect multiple periods of exposure and ice cover not continuous exposure since the last interglacial as had previously been hypothesized in situ 14c dating suggests that the last glaciation of hunt bluff did not start until 21 1 5 8 ka probably during the last glacial maximum and finished by 9 6 0 9 ka at the same time as ice sheet retreat from the continental shelf was complete thickening of ice at hunt bluff most likely post dated the maximum extent of grounded ice on the outer continental shelf flow re organisation provides a possible explanation for this with the date for onset of ice cover at hunt bluff providing a minimum age for the timing of convergence of the dotson and getz tributaries to form a single palaeo ice stream this is the first time that timing of onset of ice cover has been constrained in the amundsen sea embayment

diatom evidence for late holocene climatic events in granite harbor antarctica downcore diatom data from a deep basin in granite harbor antarctica provide new high resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years the influence of global scale climatic change during the time of the medieval warm period and possibly the little ice age is revealed by long term changes in floral assemblages as recorded by changes in the relative concentration of nitzschia cylindrus the diatom data suggest that granite harbor experienced less annual ice and pack ice cover during both these periods possibly because of higher temperatures during the medieval warm period and stronger offshore winds during the little ice age the sedimentary record reveals several distinct short lived events in which nearly monospecific blooms of corethron criophilum and chaetoceros spp occurred possibly the result of strong stabilization of the upper water column and were most likely a response to regional rather than global conditions although short lived these events may play a significant role in the delivery of organic carbon and biogenic silica to the seafloor

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

the influence of deformation on the formation of axial planar leucosomes and the segregation of small melt bodies within the migmatitic napperby gneiss central australia in the napperby gneiss central australia a distinctive group of elongate leucosomes occurs along the axial planes of mesoscopic crenulations that have harmonic two dimensional kink band like geometries the leucosomes truncate a fabric defined by biotite quartz k feldspar plagioclase sillimanite and are chiefly composed of k feldspar and quartz with or without plagioclase they are characterized by accessory garnet or aggregates of intergrown ilmenite and magnetite produced during the melting reactions bi plag q sill g ksp melt and bi plag q ru ilm mt ksp melt the absence of garnet and intergrown ilmenitemagnetite aggregates in the gneiss surrounding the crenulation zones indicates that these melting reactions preferentially occurred in the deformation zones in spite of a homogeneous distribution of reactant phases in the gneiss this means that strain partitioning and partial melting are coupled processes in which the garnet and ilmenite magnetite producing melt reactions initially occurred in high strain zones where the melting reaction activation energy was first overstepped this was a consequence of an additional strain induced matrix free energy component on top of the temperature effect once reaction product nucleation had occurred the deformation zones became the sites for continued melting which was energetically favoured over nucleating garnet and ilmenite magnetite elsewhere the melt reactions producing garnet and ilmenite magnetite resulted in the formation of grain supported leucosomes with a higher melt component than the surrounding gneiss and the leucosomes were therefore relatively incompetent in a differential stress field diffusional processes will cause elongation of the leucosome in a plane normal to 1 parallel to the axial planes of folds if sufficient melting has occurred an interconnected leucosome network will result creating a potentially important buoyancy driven melt drainage system in the napperby gneiss interconnected leucosome bearing crenulation zones are mostly vertical which is an ideal orientation for buoyancy driven diffusive melt migration however melt segregations remained small due to a melt impermeable lithological boundary overlying the gneiss

post glacial regional climate variability along the east antarctic coastal margin evidence from shallow marine and coastal terrestrial records we review the post glacial climate variability along the east antarctic coastline using terrestrial and shallow marine geological records and compare these reconstructions with data from elsewhere nearly all east antarctic records show a near synchronous early holocene climate optimum 11 5 9 ka bp coinciding with the deglaciation of currently ice free regions and the optimum recorded in antarctic ice and marine sediment cores shallow marine and coastal terrestrial climate anomalies appear to be out of phase after the early holocene warm period and show complex regional patterns but an overall trend of cooling in the terrestrial records a mid to late holocene warm period is present in many east antarctic lake and shallow coastal marine records although there are some differences in the regional timing of this warm period it typically occurs somewhere between 4 7 and 1 ka bp which overlaps with a similar optimum found in antarctic peninsula terrestrial records the differences in the timing of these sometimes abrupt warm events in different records and regions points to a number of mechanisms that we have yet to identify nearly all records show a neoglacial cooling from 2 ka bp onwards there is no evidence along the east antarctic coastline for an equivalent to the northern hemisphere medieval warm period and there is only weak circumstantial evidence in a few places for a cool event crudely equivalent in time to the northern hemisphere s little ice age there is a need for well dated high resolution climate records in coastal east antarctica and particularly in terre adelie dronning maud land and enderby land to fully understand the regional climate anomalies the disparity between marine and terrestrial records and to determine the significance of the heterogeneous temperature trends being measured in the antarctic today 2010 elsevier b v

penguin colonization following the last glacial interglacial transition in the vestfold hills east antarctica abstract the population history of the adelie penguin pygoscelis adeliae and its response to climatic and environmental changes have been widely studied in maritime antarctica and several glacial refugia of adelie penguins were dated back to the last glacial maximum however the process of colony expansion during the subsequent glacial interglacial transition was less well documented in this paper we investigate an ornithogenic core from vestfold hills east antarctica to improve knowledge of postglacial penguin colonization local deglaciation occurred around 15 6 kyr bp based on a coarse grained detrital layer a date that is earlier than most of those reported in other studies from the same region geochemical analysis of the core suggests penguins started colonizing the northern vestfold hills around 14 6 kyr bp the oldest geological record in east antarctica on penguin occupation and their population exhibited a broadly increasing trend thereafter the population expanded at approximately 6740 yr bp in the mid holocene which corresponds to a local climatic optimum although this inference is based on population changes of local sub colonies it corresponds well to reconstructions of regional climate change and changes in the penguin population based on genetic studies from a consideration of the results of previous studies of the occupation history of sea birds following deglaciation in the arctic as well as other parts of antarctica we propose that sea birds colonized the emerging ice free areas shortly after local glacier retreat over geological time scales the availability of breeding habitat is a key factor controlling the population size and activity range of antarctic adelie penguins

the eastern himalayan syntaxis major tectonic domains ophiolitic melanges and geologic evolution abstract geologic mapping in the eastern himalayan syntaxis confirmed the three regional tectonic elements outlined by previous geologic workers our studies however show that the indus yarlung tsangpo suture iys is a continuous melange zone that forms an inverted u in map view around the namche barwa antiform the namche barwa and nyainqentanglha crystalline complexes lie below and above the iys suture respectively and both were parts of the northern indian plate basement rocks with petrologically and geochronologically correlative protoliths both units were deformed metamorphosed and intruded at the end of the proterozoic the zhibai formation the lower part of the namche barwa group extends along the northwest slope of the himalaya and is mainly composed of highly deformed aluminous felsic gneiss containing sporadic boudins of high pressure granulite in the namche barwa antiform the upper part of the namche barwa group includes a calcareous rock assemblage characteristized by marble and diopside bearing calcsilicate rocks interlayed with felsic gneiss petrochemical studies show that the iys contains lenses of oceanic crustal rocks originated from fore arc trench island arc and back arc basin environments which implies they were derived from a ssz type supra subduction zone ophiolite our field mapping identifies the jiali parlung tsangpo remnant suture jps that lies north of the namche barwa antiform as a possible branch of the neo tethyan oceanic realm subduction of the mesozoic neo tethyan oceanic plate resulted in both mesozoic and cenozoic granite intrusions in the northwest trending gangdise magmatic belt along the southern edge of asia uplift and exhumation have been the most recent dominant tectonic processes in the late cenozoic for the high himalayan crystalline rocks namche barwa group in the core of the namche barwa antiform

oceanographic mechanisms and penguin population increases during the little ice age in the southern ross sea antarctica abstract the adelie penguin is a well known indicator for climate and environmental changes exploring how large scale climate variability affects penguin ecology in the past is essential for understanding the responses of southern ocean ecosystems to future global change using ornithogenic sediments at cape bird ross island antarctica we inferred relative population changes of adelie penguins in the southern ross sea over the past 500 yr and observed an increase in penguin populations during the little ice age lia 1500 1850 ad we used cadmium content in ancient penguin guano as a proxy of ocean upwelling and identified a close linkage between penguin dynamics and atmospheric circulation and oceanic conditions during the cold period of 1600 1825 ad a deepened amundsen sea low asl led to stronger winds intensified ocean upwelling enlarged ross sea and mcmurdo sound polynyas and thus higher food abundance and penguin populations we propose a mechanism linking antarctic marine ecology and atmospheric oceanic dynamics which can help explain and predict responses of antarctic high latitudes ecosystems to climate change

antarctic ice volume and contribution to sea level fall at 20 000 yr bp from raised beaches the contribution of the antarctic ice sheets to global sea level fall at the last glacial maximum lgm depends largely on how the extent and thickness of peripheral ice changed model studies1 3 suggest that there was widespread thickening from 500 m to more than 1 000 m of the ice sheet margins sufficient to induce a drop in sea level of at least 25 m geological evidence4 5 on the other hand indicates only limited ice expansion and a sea level fall of just 8 m here we use recent data on the altitudes and ages of raised beaches from the ross embayment and east antarctica to investigate the timing and extent of antarctic deglaciation these indicate that the ice margin during the lgm was thinner and less extensive than has been formerly thought and that its contribution to the drop in sea level was only 0 5 2 5 m deglaciation was well advanced by 10 kyr bp and was complete by 6 kyr bp these findings imply either that sea level during the lgm fell less than present estimates suggest or that an ice volume considerably greater than currently accepted must have been present in the northern hemisphere

geothermobarometry and development of inverted metamorphism in the darjeeling sikkim region of the eastern himalayan abstract the darjeeling sikkim region provides a classic example of inverted himalayan metamorphism the different parageneses of pelitic rocks containing chlorite biotite garnet staurolite kyanite sillimanite plagioclase and k feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones microprobe data of coexisting minerals show that xmg varies in the order garnet staurolite biotite chlorite white mica is a solid solution between muscovite and phengite garnet is mostly almandine rich and shows normal growth zoning in the lower part of the main central thrust mct zone and reverse zoning in the upper part of the zone chemographical relations and inferred reactions for different zones are portrayed in afm space in the low grade zones oriented chlorites and micas and rolled garnets grew syntectonically and were succeeded by cross cutting chlorites and micas and garnet rims in the upper zones sillimanite kyanite and staurolite crystallized during a static inter kinematic phase p t contitions of metamorphism estimated through different models of geothermobarometry are estimated to have been 580 c for the garnet zone to a maximum of 770 c for the sillimanite zone the preferred values of pressure range from 5 0 kbar to 7 7 kbar models to explain the inverted metamorphism include overthrusting of a hot high himalayan slab along a c 5 km wide ductile mct zone and the syn or post metamorphic folding of isograds

inverted oligo miocene metamorphism in the lesser himalaya sequence arunachal pradesh india age and grade relationships the lesser himalaya sequence in the siyom valley eastern arunachal pradesh exposes a telescoped and inverted greenschist upper amphibolite facies sequence over 20 km tectonically overlain by an upright grade decreasing upward granulite to lower amphibolite facies sequence over 30 km such grade relationships would normally attribute the boundary to a main central thrust mct structure and predict a change from underlying lesser himalaya sequence lhs to greater himalaya sequence rocks across the boundary however all pelitic and psammitic samples have similar detrital zircon age spectra involving c 2500 1750 1500 1200 and 1000 ma gondwanan populations correlated with the lhs isograds are broadly parallel to a penetrative nw dipping s2 foliation developed contemporaneously with the inversion garnet growth in garnet staurolite and kyanite zone schists beneath the thrust commenced at 8 kbar and 550 c before syn to post s2 heating of staurolite and kyanite zone rocks to 640 c at 8 5 kbar most probably at c 18 5 ma kyanite rutile garnet migmatite immediately above the thrust records peak conditions of 10 kbar and 750 c and c 21 5 ma monazite ages slip on a se propagating thrust was likely contemporaneous with early miocene metamorphism based on the distribution of structure metamorphic textures and overlap of age relationships it is inferred to have initially controlled the uplift of granulite to mid crustal levels between 22 and 19 ma thermal relaxation within a disrupted lhs metamorphic profile inducing a post s2 thermal peak in lower grade footwall rocks

fe mg mixing in cordierite constraints from natural data and implications for cordierite garnet geothermometry in granulites from chemicaldata on 108 pairs of naturally occurring garnet and cordierite the interchange energy of fe mg mixing in cordierite wo and the free energy change of the exchange equilibrium feralrsiro t t zmgraiosiro y mgralrsiro yzferalosiro in the temperature range of 700 850 c have been extracted using standard methods of linear regression analysis the input thermochemical parameters for multicomponent camg fe mn garnet solid solutions are based on the updated and revised values after ganguly and saxena 1984 e g wr 2500 cal w w 3000 cal calculated on the basis of one third formula unit on the basis of the derived values at 710 760 800 and 850 c fe mg mixing in cordierite has been approximated to a symmetric regular solution the fe mg substitution shows positive deviation from ideality in cordierite with tzi 1 9 34 4 54kj mol of divalent cation the values derived for the free energy of mixing at xi d xi ds 0 5 are 3 26 kj at 700 c and 4 39 kl at 900 c the ag value in joules for the above equilibrium can be expressed as 15079 8 552 incorporating these parameters the geothermometric expression for the exchange equilibrium is

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

dynamics of penguin population size and food availability at prydz bay east antarctica during the last millennium a solar control abstract the paleoecology of the adelie penguin pygoscelis adeliae is controlled by multiple factors in this study we analyzed two millennial scale ornithogenic sediment cores rnl and rl from the vestfold hills east antarctica using the al normalized phosphorus content of the bulk sediment and carbon nitrogen fractions in penguin remains we reconstructed relative changes in the penguin population size of each sub colony and relative krill abundance in the summer prydz bay polynya both records are correlated to solar irradiance at the centennial scale the penguin population recorded in core rnl decreased from 1120 860 yr bp reached a peak from 860 630 yr bp remained at a low level from 630 320 yr bp and then increased with large fluctuations during the past 400 years these changes are all in phase with the trend of solar irradiance at the decadal scale penguin population minima correspond to solar minima from 490 400 yr bp sporer minimum 290 220 yr bp maunder minimum and from 160 120 yr bp dalton minimum whereas population maxima correspond to solar maxima from 1030 980 yr bp 350 290 yr bp 210 160 yr bp 120 70 yr bp the population recorded in core rl exhibited the same changes as in rnl during the last 480 years the reconstructed krill abundance also corresponds to these trends when data are available this correspondence demonstrates a food chain mechanism that is related to solar activity and light availability at the ocean surface which influence the intensity of photosynthesis and phytoplankton productivity and thus the abundance of krill and apex predators such as penguins our findings highlight the fact that despite the various climatic impacts on penguin populations their effects on the base of the food chain are usually the direct drive

contributions and unrealized potential contributions of cosmogenic nuclide exposure dating to glacier chronology 1990 2010 abstract this paper reviews the application of cosmogenic nuclide exposure dating to glacier chronology exposure dating of glacial landforms has made an outsize impact on this field because the technique filled an obvious need that had already been recognized by glacial geologists by now hundreds of studies have used cosmogenic nuclide exposure dating to date glacial deposits and in fact it is rare to find a study of glacial geology or glacier chronology or any paleoclimate synthesis that makes use of such studies that does not involve exposure dating these developments have resulted in major contributions to glacier chronology and paleoclimate in particular i reconstructing antarctic ice sheet change ii establishing the chronology of late pleistocene and holocene glacier change in mountain regions where it was previously unknown iii establishing the broad chronological outlines of mountain glaciations prior to the last glacial maximum and iv gaining insight into subglacial erosional processes through the observation that many glaciated surfaces preserve cosmogenic nuclide inventories from long past ice free periods as well as the present one an important potential future contribution will be the application of the large data set of exposure dated glacier chronologies to better understand global and regional climate dynamics during lateglacial and holocene millennial scale climate changes however this contribution cannot be realized without significant progress in two areas i understanding and accounting for geologic processes that cause apparent exposure ages on glacial landforms to differ from the true age of the landform and ii minimizing systematic uncertainties in exposure ages that stem from cosmogenic nuclide production rate estimates and scaling schemes at present there exists an enormous data set of exposure ages on glacial deposits but these data cannot be used to their full potential in paleoclimate syntheses due to an inadequate understanding of geologic scatter and production rate uncertainties the intent of this paper is to highlight this situation and suggest some strategies for realizing this potential

pressure temperature time path discontinuity in the main central thrust zone central nepal metapelites collected in central nepal reveal a discontinuity in metamorphic pressuretemperature time p t t paths near the base of the main central thrust zone despite an absence of obvious structural breaks garnets in the structurally lowest rocks grew with increasing t and p loading whereas garnets 1 3 km upsection grew with increasing t but decreasing p exhumation monazite grains in structurally lower rocks yield ionmicroprobe th pb ages of 8 9 ma structurally higher monazite grains range from 10 to 22 ma the p t t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca 8 ma but also reflect thermal relaxation following older 20 ma or older thrust movement the main central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate

marine geological constraints for the grounding line position of the antarctic ice sheet on the southern weddell sea shelf at the last glacial maximum abstract the history of grounded ice sheet extent on the southern weddell sea shelf during the last glacial maximum lgm and the timing of post lgm ice sheet retreat are poorly constrained several glaciological models reconstructed widespread grounding and major thickening of the antarctic ice sheet in the weddell sea sector at the lgm in contrast recently published onshore data and modelling results concluded only very limited lgm thickening of glaciers and ice streams feeding into the modern filchner and ronne ice shelves these studies concluded that during the lgm ice shelves rather than grounded ice covered the filchner and ronne troughs two deep palaeo ice stream troughs eroded into the southern weddell sea shelf here we review previously published and unpublished marine geophysical and geological data from the southern weddell sea shelf the stratigraphy and geometry of reflectors in acoustic sub bottom profiles are similar to those from other west antarctic palaeo ice stream troughs where grounded ice had advanced to the shelf break at the lgm numerous cores from the southern weddell sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments these data sets give evidence that grounded ice had advanced across the shelf during the past thereby grounding in even the deepest parts of the filchner and ronne troughs radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited but indicate that the ice grounding occurred at the lgm and that ice retreat started before 15 1 corrected 14c kyrs before present bp on the outer shelf and before 7 7 corrected 14c kyrs bp on the inner shelf which is broadly synchronous with ice retreat in other antarctic sectors the apparent mismatch between the ice sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles similar to those of ice plains that had advanced through filchner trough and ronne trough at the lgm considering the global sea level low stand of 130 m below present a low surface slope of the expanded lgm ice sheet in the southern weddell sea can reconcile grounding line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland this scenario implies that ice sheet growth in the weddell sea sector during the lgm and ice sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea level fluctuations during these times

unglaciated areas in east antarctica during the last glacial marine isotope stage 3 new evidence from rauer group abstract limited information on the east antarctic ice sheet eais geometry during marine isotope stage 3 mis 3 60 25 ka restricts our understanding of its behaviour during periods of climate and sea level change ice sheet models forced by global parameters suggest an expanded eais compared to the holocene during mis 3 but field evidence from east antarctic coastal areas contradicts such modelling and suggests that the ice sheet margins were no more advanced than at present here we present a new lake sediment record and cosmogenic exposure results from bedrock which confirm that rauer group eastern prydz bay was ice free for much of mis 3 we also refine the likely duration of the last glacial maximum lgm glaciation in the region lacustrine and marine sediments from rauer group indicate the penultimate period of ice retreat predates 50 ka the lacustrine record indicates a change from warmer wetter conditions to cooler drier conditions after ca 35 ka substantive ice sheet re advance however may not have occurred until much closer to 20 ka contemporary coastal areas were still connected to the sea during mis 3 restricting the possible extent of grounded ice in prydz bay on the continental shelf in contrast relative sea levels rsl deduced from field evidence indicate an extra ice load averaging several hundred metres thicker ice across the bay between 45 and 32 ka thus ice must either have been thicker immediately inland with a steeper ice profile or there were additional ice domes on the shallow banks of the outer continental shelf further work is required to reconcile the differences between empirical evidence of past ice sheet histories and the history predicted by ice sheet models from far field temperature and sea level records

thermal model for the zanskar himalaya abstract crustal thickening along the northern margin of the indian plate following the 50 ma collision along the indus suture zone in ladakh caused widespread high temperature medium pressure barrovian facies series metamorphism and anatexis in the zanskar himalaya metamorphic isograds are inverted and structurally telescoped along the main central thrust mct zone at the base of the high himalayan slab along the zanskar valley at the top of the slab isograds are the right way up and are also telescoped along northeast dipping normal faults of the zanskar shear zone zsz which are related to culmination collapse behind the miocene himalayan thrust front between the mct and the zsz a metamorphic anatectic core within sillimanite grade rocks contains abundant leucogranite granite crustal melts of probable himalayan age a thermal model based on a crustal scale cross section across the zanskar himalaya suggests that m1 isograds developed during early himalayan barrovian metamorphism were overprinted during high grade mct related anatexis and folded around a large scale recumbent fold developed in the hanging wall of the mct

anchor ice formation and ice rafting in southwestern lake michigan u s a abstract anchor ice i e ice attached to the bed in a lake stream or ocean is common in the nearshore zone of southwestern lake michigan during winter lacustrine anchor ice has at least four distinct morphologies and was observed on sand pebble and boulder substrates in water depths to 4 m the limit of diving traverses the maximum depth of anchor ice formation may be much greater anchor ice is released from the lake bed on mornings following formation events released floating anchor ice carries sediment to the water surface this sediment is ice rafted along shore and offshore under the influence of prevailing winds we estimate that 0 85 m3 of sand per meter of beach is being removed from the nearshore zone of southwestern lake michigan by anchor ice annually melting ice drops this sand in deep water far from shore this is a significant loss of sand from the sediment starved nearshore zone of southwestern lake michigan

a review of definitions of the himalayan main central thrust most workers regard the main central thrust mct as one of the key high strain zones in the himalaya because it accommodated at least 90 km of shortening because that shortening exhumed and buried hanging wall and footwall rocks and due to geometric and kinematic connections between the main central thrust and the structurally overlying south tibet detachment geologists currently employ three unrelated definitions of the mct metamorphic rheological age of motion structural or protolith boundary structural these disparate definitions generate map and cross section mct positions that vary by up to 5 km of structural distance the lack of consensus and consequent shifting locations impede advances in our understanding of the tectonic development of the orogen here i review pros and cons of the three mct definitions in current use none of these definitions is flawless the metamorphic rheological and age of motion structural definitions routinely fail throughout the orogen whereas the protolith boundary structural definition may fail only in rare cases all limited to sectors of the eastern himalaya accordingly a definition based on high strain zone geometry and kinematics combined with identification of a protolith boundary is the best working definition of the mct

thermobarometry in granulites pitfalls and new approaches abstract computer models of cooling granulite facies rocks with the assemblage garnet biotite sillimanite k feldspar plagioclase quartz are presented that incorporate simultaneous reaction net transfer and exchange and multicomponent diffusion in garnet in models where cooling is accompanied by exchange reactions plus diffusion only the calculated garnet core matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate the size of the garnet and the volumetric ratio of garnet to biotite maximum calculated core temperatures of approximately 700 750 c are to be expected from typical granulite facies garnet biotite assemblages and calculated core temperatures can easily be as low as 600 c in models where both net transfer and exchange reactions are possible calculated garnet core matrix biotite temperatures are typically higher than the metamorphic peak temperatures by more than 150 c for large garnets r 2 mm but may be lower than the metamorphic peak for small garnets r the composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence calculated p t trajectories are very sensitive to the petrologic interpretation of this recrystallization history for models in which all the modal plagioclase reacts the core and rim pressures may be faithfully recovered provided that the temperature estimate for the core is reliable for models in which only a small percentage of the modal plagioclase reacts very small modal amounts of plagioclase may be in equilibrium with garnet rim which if incorrectly interpreted may lead to grossly erroneous p t path interpretations a method to test an inferred p t path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion an example from western new hampshire usa is presented that closely matches the measured profile

a framework for estimating anchor ice extent at potential formation sites in mcmurdo sound antarctica a distinctive feature of polar regions is the formation of ice clusters attached to the seabed known as anchor ice anchor ice plays an important role in mobilizing bed sediments and serves ecological roles providing habitats or as an agent of disturbance creating potentially fatal environments to benthic fauna the sublittoral zone associated with the landward margin represents the most likely environment for anchor ice formation where conditions conducive to the advection of supercooled water from sub ice shelf cavities are favourable we develop a framework to estimate the areal extent of anchor ice formation assuming a northerly flow of 75m deep supercooled water plumes from the ross and mcmurdo ice shelf cavities antarctica in mcmurdo sound our results indicate that regions beneath the mcmurdo ice shelf extending along brown peninsula and white and black islands are likely conducive to anchor ice formation anchor ice may also form along the hut point peninsula and around ross island and in pockets along the southern victoria land coast the limitations of our approach include an imposed northerly flow of ice shelf water poorly constrained sub ice shelf bathymetry and temporal variability in supercooled water depth production particularly in the

variation in the characteristics and development of soils at edmonson point due to abiotic and biotic factors northern victoria land antarctica abstract the lack of baseline surveys of soil environments in many areas of antarctica provides an impediment to understanding their suitability for supporting biotic communities and limits abilities to monitor and predict impacts of environmental changes a soil survey was conducted at edmonson point victoria land within representative local environments to identify their variability and drivers of soil processes and geochemistry the soils were coarse textured and lacked cohesion and structural development the parent material was homogenous and consisted of weathered and unconsolidated basaltic lavas and scoria despite these similarities the soils varied significantly with the variation driven by local environmental and biotic factors penguins had the greatest influence deposited guano altered soil processes leading to profound changes in soil characteristics the ornithogenic soils were rich in penguin bio elements with low c n high ec and large variation in ph contents of n and c declined at comparable rates as c n values did not change with time ornithogenic p was not readily lost resulting in its high concentrations in relict soils abundant bacteria played a key role in these processes but after cessation of guano inputs bacterial numbers and activity declined considerably alternatively rich autotrophic microflora had a little influence on ornithogenic soil geochemistry characteristics and development of mineral soils were driven by hydrology and associated biological processes dry fellfield soils were the most immature as indicated by more coarse grained material neutral ph low ec and c n values scarce microflora and elemental content corresponding to typical background values alternatively soils from moss communities and wetlands had higher amounts of fine grained material c n and c n values and lower ph that resulted from accumulation of organic matter elemental concentrations in soils from moss communities were similar to those in fellfields but were elevated in wetland soils while c and n contents seemed to be controlled by biotic communities geochemistry of other elements was driven by external inputs although some inputs may originate from substratum weathering in the surrounding areas marine derived elements redistributed with penguin guano seemed to be the major source of the enrichment analyses of the origin and distribution of the soil elements may provide valuable records for reconstruction of geochemical and environmental processes in antarctic terrestrial ecosystems

early stall of west antarctic ice sheet advance on the eastern ross sea middle shelf followed by retreat at 27 50014cyrbp abstract marine geological and geophysical data suggest that the west antarctic ice sheet wais deposited three thick seismically resolvable units in the eastern ross sea since the last glacial maximum lgm but the chronology of these grounding events is debated existing ice retreat chronologies for the eastern ross sea are considered suspect because nearly all have been developed using radiocarbon dating of acid insoluble organics aio from bulk sediment we used a new strategy to isolate in situ forams from diamict sediment deposited at the middle shelf grounding zone wedge gzw a subaqueous ice marginal landform previously interpreted to represent deposition during the third grounding event since the lgm advance to the shelf edge here we present a synthesis of our new radiocarbon dates of foram tests and aio with previously published radiocarbon dates we propose that the wais s lgm advance in the eastern ross sea sector reached the middle shelf circa 27 500 14c yr bp and that open marine sedimentation began shortly thereafter there is as of yet no clear explanation for why wais retreat in this sector of antarctica apparently began prior to the peak of the last glacial cold period a precipitation deficit localized warm water intrusion an ice dynamic response in the absence of climate forcing or another as of yet unknown forcing mechanism could be responsible

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

domal structures and high grade metamorphism in the higher himalayan crystalline zanskar region north west himalaya india abstract in the main himalayan range in the ladakh zanskar area domal structures have been observed at structurally deeper levels in the tectonic unit of the higher himalayan crystalline their formation occurred during a second temperature dominated phase m2 of high grade regional metamorphism characterized by the semipelitic paragenesis of sillimanite k feldspar and incipient anatexis the doming event reveals a local system of synmetamorphic uplift superimposed on a regional system of northeast southwest trending compression in the main himalayan range the development of the dominant s2 foliation is related to deformation during the doming phase which started early in the m2 event the deformation propagated continuously north east and south west with time in the north east on the northern slopes of the main himalayan range this deformation is expressed by extensional shear movements of the upper tectonic levels finally leading to the late to postmetamorphic normal fault system of the zanskar shear zone towards the south west deformation is expressed by compressional movements e g at the main central thrust mct in the kishtwar window area the observed compression and extension is inferred to relate to an increased uplift of the domal bulges of the tectonic kishtwar window and of the whole main himalayan range

observations of platelet ice growth and oceanographic conditions during the winter of 2003 in mcmurdo sound antarctica 1 platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice water interface related to the proximity of ice shelves it is known to form in supercooled seawater this study details the conditions for its growth during the austral winter of 2003 in mcmurdo sound antarctica a key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column monitored using the strength of the backscattered signal from an acoustic doppler current profiler as a proxy generally these crystals appeared from mid may as water near the interface became supercooled this near surface supercooling appears to be related to a simultaneous abrupt change in the structure of the upper 250 m of the water column from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature near surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density over the course of winter of the surface mixed layer created by salt rejection during ice growth these processes allowed cold water trapped by buoyancy in a band at the base of the mixed layer in early winter to gain access to the ice water interface by midwinter this band of cold water probably had its origin beneath the mcmurdo ice shelf

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

geology and tectonic history of the lohit valley eastern arunachal pradesh india abstract the lohit river section of eastern arunachal pradesh comprises four tectonic units from sw to ne these are the lesser himalayan rocks the mishmi crystallines the tidding suture zone and the lohit plutonic complex the mishmi thrust underlies the basal lesser himalayan unit while the mishmi crystallines are thrust over the lesser himalayan unit along the main central thrust the grade of metamorphism in the mishmi crystallines increases up the structural section from chlorite to staurolite kyanite zones exhibiting inverted metamorphism the relationship between deformation and metamorphism shows that the metamorphic peak was syn to post tectonic in relation to the main ductile shearing event continued deformation after the metamorphic peak was accommodated along millimetre scale shear zones developed throughout the sequence parallel to the regional schistosity movement along these shear zones has resulted in inversion of the metamorphic zones the rocks of the tidding suture represent an ophiolitic melange thrust over the mishmi crystallines which in turn are overthrust by the lohit plutonic complex along the lohit thrust the lohit plutonic complex is subdivided into western and eastern belts separated by the walong thrust the western belt consists of deformed quartz diorite diorite gabbro and trondhjemite intruded by basic and acid dykes the eastern belt comprises garnet sillimanite gneiss intercalated with crystalline marble bands followed by a complex zone of leucogranites aplites and pegmatites which intrude the early foliated quartz diorite soda rich granite and microdiorite the rocks of the eastern belt are the northward continuation of the mogok gneissic belt of central burma the occurrence of intrusive rocks in the eastern belt suggests that the magmatism related to subduction extended to the east far from the subduction zone the peraluminous leucogranites aplites and pegmatites are the products of crustal melting induced by crustal thickening related to the intracontinental walong thrust subsequent to metamorphism and shearing the whole sequence was folded into an antiform forming the eastern syntaxis and this deformation steeply tilted the earlier low angle thrusts and foliations later compression partitioned into right lateral strike slip motion producing a superimposed sub horizontal lineation observed mostly in the lohit plutonic complex

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

distinguishing subglacial till and glacial marine diamictons in the western ross sea antarctica implications for a last glacial maximum grounding line analyses of lithology stratigraphy and tephra from marine sediment cores collected from the western ross sea during cruises eltanin 32 and 52 and deep freeze 80 and 87 indicate that subglacial till does not extend to the continental shelf edge subglacial till occurs as the lowest unit in most cores landward south of approximately 74 s while seaward of approximately 74 s the lowest diamicton units are glacial marine diamictons glacial marine diamictons are distinguished from subglacial tills by the presence of higher and more variable total organic carbon content downcore distinct tephra layers stratification higher diatom and foraminifera abundances higher sand content and radiocarbon dates in chronological order downcore sand sized tephra layers from two cores on the outer continental shelf are interpreted as single eruptive events one likely to have been derived from the mount melbourne volcano and the other from the pleiades volcano radiocarbon dates from sediment above and below the tephra layer in one of these cores df87 32 show that deposition indicative of open water conditions occurred between 22 and 26 ka in the western ross sea

environmental implication of nitrogen isotopic composition in ornithogenic sediments from the ross sea region east antarctica δ15n as a new proxy for avian influence abstract we analyzed 15n in both acid treated and untreated sediment profiles from mcmurdo sound of the ross sea region east antarctica that were influenced by penguin guano the difference between treated and untreated 15n 15n was significant in three profiles which were heavily impacted by guano and minor in two profiles with less guano influence we determined that the total nitrogen in the sediments is primarily derived from penguin guano and algae and used an n species test to explain the variation of 15n in two profiles it was found that post depositional decomposition and ammonia volatilization which have important roles in the cold and arid environment of east antarctica would render an elevated 15n through kinetic isotopic fractionation in the inorganic nitrogen from guano n species analysis revealed that the percentage of inorganic nitrogen in total nitrogen indicative of the degree of guano influence is the key factor controlling 15n in the sediments this hypothesis successfully explained the nitrogen isotopic composition in the remaining three sediment profiles we conclude that the parameter 15n rather than traditionally used untreated 15n can be taken as an effective proxy for the strength of avian influence on ornithogenic sediments in east antarctica

interhemispheric ice sheet synchronicity during the last glacial maximum the timing of the last maximum extent of the antarctic ice sheets relative to those in the northern hemisphere remains poorly understood we develop a chronology for the weddell sea sector of the east antarctic ice sheet that combined with ages from other antarctic ice sheet sectors indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of northern hemisphere ice sheets surface climate forcing of antarctic mass balance would probably cause an opposite response whereby a warming climate would increase accumulation but not surface melting our new data support teleconnections involving sea level forcing from northern hemisphere ice sheets and changes in north atlantic deep water formation and attendant heat flux to antarctic grounding lines to synchronize the hemispheric ice sheets

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

the metamorphism in the central himalaya all along the himalayan chain an axis of crystalline rocks has been preserved made of the higher himalaya crystalline and the crystalline nappes of the lesser himalaya the salient points of the metamorphism as deduced from data collected in central himalaya central nepal and kumaun are 1 the higher himalaya crystalline also called the tibetan slab displays a polymetamorphic history with a first stage of barrovian type overprinted by a lower pressure and or higher temperature type metamorphism the metamorphism is due to quick and quasi adiabatic uplift of the tibetan slab by transport along an mct ramp accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover the resulting metamorphic pattern is an apparent diachronic inverse zonation with the sillimanite zone above the kyanite zone 2 conversely the famous inverted zonation of the lesser himalaya is basically a primary pattern acquired during a one stage prograde metamorphism its origin must be related to the thrusting along the mct with heat supplied from the overlying hot tibetan slab as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust 3 thermal equilibrium is reached between units above and below the mct far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the tibetan slab during the second metamorphic stage but closer to the mct front the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the tibetan slab

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

holocene climate variability in antarctica based on 11 ice core isotopic records a comparison is made of the holocene records obtained from water isotope measurements along 11 ice cores from coastal and central sites in east antarctica vostok dome b plateau remote komsomolskaia dome c taylor dome dominion range d47 km105 and law dome and west antarctica byrd with temporal resolution from 20 to 50 yr the long term trends possibly reflect local ice sheet elevation fluctuations superimposed on common climatic fluctuations all the records confirm the widespread antarctic early holocene optimum between 11 500 and 9000 yr in the ross sea sector a secondary optimum is identified between 7000 and 5000 yr whereas all eastern antarctic sites show a late optimum between 6000 and 3000 yr superimposed on the long time trend all the records exhibit 9 aperiodic millennial scale oscillations climatic optima show a reduced pacing between warm events typically 800 yr whereas cooler periods are associated with less frequent warm events pacing 1200 yr

entrainment and export of laptev sea ice sediments siberian arctic sea ice sediments sis sediments rafted by ice and shelf surface deposits from the siberian laptev sea were analyzed to study turbulent entrainment processes this study focused on the silt and clay fractions on average representing 63 4 and 30 4 total 93 7 of the bulk sis respectively generally the sis contains 50 more silt than the shelf deposits while the clay percentages in both sample groups are similar sand sized particles are less abundant in sis than in bottom sediment although sis is clearly enriched in silt no evidence was found for preferential ice entrainment of any silt subfraction coarse medium fine statistical tests of sis and shelf silt parameters and the similar clay mineral compositions of both sample groups provide evidence for entrainment of laptev bottom deposits into newly forming ice the entrainment is related to the mechanism known as suspension freezing accordingly coarser clasts sand and plant debris may be lifted upward by buoyant anchor ice while coarse silt particles are supposed to be transported to the surface by rising frazil aggregates additionally turbulent interaction of convergent langmuir circulation and near surface frazil ice streaks is proposed to support ice entrainment of the finest particles 6 4 phi 12 m entrainment and export of sis from the laptev sea were assessed for ten individual flaw leads extended narrow polynyas separating fast and drifting ice using particle concentrations in ice samples and new ice formation rates the results show that roughly 20 mt of sis can be annually exported from the laptev sea by lead ice only thus formation and export of sediment laden lead ice may play a major geological role in siberian arctic shelf erosion and coastal retreat

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

origin of the inverted metamorphism of the lower himalayas central nepal abstract the southern slope of the central nepal himalayas is divided geologically by the main central thrust zone into the underlying midland metasediment zone lower himalayas in the south and the overlying himalayan gneiss zone higher himalayas in the north the former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium and the latter zone consists of high grade crystalline rocks with a north dipping homoclinal structure and is further overlain by the tethys himalayas composed of non or slightly metamorphosed tethyan sediments the main central thrust zone shows a characteristic lithology such as calcareous schist quartzite phyllitic schist and augen gneiss and is intensely mylonitized the mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the midland metasediment to the main central thrust zones that is the metamorphic grade of the midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies and the greenschist amphibolite transitional zone is attained in the thrust zone the himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone on the basis of the compositional zoning of garnets and muscovite compositions however it is concluded that the himalayan gneisses suffered a polymetamorphism the younger metamorphism resulted in a retrograde metamorphism of the himalayan gneisses together with the above mentioned inverted metamorphism and an anatectic melting in depth this was caused by shear heating due to the thrust movement along the main central thrust zone during the alpine tectonism the older metamorphism which seems to be precambrian in age was much higher in metamorphic grade than the younger one and formed the pyrope rich cores of garnets in the himalayan gneisses the maximum metamorphic temperature of 550 c estimated from the mineral paragenesis in the thrust zone is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5 10 cm yr maximum stress of 300 bars and thrusting distance of 250 km bird 1978

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

cenozoic structural and metamorphic evolution of the eastern himalayan syntaxis namche barwa combined geological and geochronological investigations of the eastern himalayan syntaxis in the namche barwa region of tibet reveal the first order elements of its cenozoic tectonic evolution the syntaxis is characterized by a northeast plunging antiform and is bounded by two northeast striking strike slip shear zones a left slip shear zone on the western side and a right slip shear zone on the eastern side these strike slip shear zones are linked by east westtrending thrusts and served either as 1 a roof thrust to a large duplex system or 2 transfer faults to a south directed thrust system that accommodated northward indentation of a folded indian plate an east west trending pop up structure in the core of the antiform juxtaposes a granulite bearing complex over sillimanite bearing gneisses of gangdese affinity to the north and of indian affinity to the south previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at v800 c the first at high pressures 14 15 kbar followed by a second event at 8 10 kbar zircons from mafic granulites yield four populations of concordant u pb ion microprobe ages two groups are at v65 ma and v160 ma and likely crystallized during andean type gangdese magmatism prior to the indo asian collision a third cluster at v40 ma exhibits very low th u ratios and is interpreted to have crystallized in the presence of fluids associated with a high pressure granulite facies metamorphic event during the early stages of the indo asian collision subsequent to high pressure metamorphism in the western himalaya syntaxis between v50 and 43 ma a fourth cluster of zircons yields ages between 11 and 25 ma and th u ratios that decrease systematically with decreasing age we interpret the youngest zircon age v11 ma to represent the timing of moderatepressure high grade metamorphism with the older ages and higher th u ratios being a result of mixing with a restitic igneous component this interpretation coupled with a v 8m a 40 ar 39 ar age on hornblende from a metadiorite within the core of the antiform suggests that the namche barwa syntaxis has been characterized by rapid cooling and exhumation since at least late miocene time despite its contrasting structural setting the miocene and younger metamorphism and cooling history in the namche barwa syntaxis are strikingly similar to those of the nanga parbat syntaxis of the western himalaya fl 2001 elsevier science b v all rights reserved

a 45 000 yr record of adelie penguins and climate change in the ross sea antarctica well preserved remains of bone tissue and eggshell of adelie penguin pygoscelis adeliae have been recovered from numerous abandoned colonies in the ross sea region antarctica radiocarbon dates on these remains provide an occupation history for this species ranging from hundreds to tens of thousands of years ago we completed 62 new radiocarbon dates on these remains which now indicate that an open water marine environment existed in the ross sea from ca 45 000 to 27 000 14 c yr before present b p and provide constraints for the timing of the last advance of the ross ice sheet penguins did not recolonize the ross sea until ca 8000 calendar years cal yr b p after the early holocene retreat of the ross ice sheet two subsequent periods of abandonment at 5000 4000 and 2000 1100 cal yr b p correlate with cooling episodes that caused unfavorable marine conditions for breeding penguins most modern colonies were established only within the past 2000 yr

reconciliation of late quaternary sea levels derived from coral terraces at huon peninsula with deep sea oxygen isotope records a major discrepancy between the late quaternary sea level changes derived from raised coral reef terraces at the huon peninsula in papua new guinea and from oxygen isotopes in deep sea cores is resolved the two methods agree closely from 120 ka to 80 ka and from 20 ka to 0 ka ka 1000 yr before present but between 70 and 30 ka the isotopic sea levels are 20 40 m lower than the huon peninsula sea levels derived in earlier studies new high precision u series age measurements and revised stratigraphic data for huon peninsula terraces aged between 30 and 70 ka now give similar sea levels to those based on deep sea oxygen isotope data planktonic and benthic 18o data using the sea level and deep sea isotopic data oxygen isotope ratios are calculated for the northern continental ice sheets through the last glacial cycle and are consistent with results from greenland ice cores the record of ice volume changes through the last glacial cycle now appears to be reasonably complete