late stage water eruptions from ascraeus mons volcano mars implications for its structure and history abstract ascraeus mons was one of the first of the martian volcanoes to be imaged by the high resolution stereo camera hrsc experiment onboard the esa mars express spacecraft these images show much of the volcano at a higher resolution than previously and details of its lava flows sinuous rilles flank vents and tectonic features indicate an unexpected origin for some of these features we establish the time stratigraphic sequence for these features and use a numerical model on hrsc stereo dtms of the sinuous rilles and conclude that they were formed by water erosion terrestrial analogues for such features are found at reunion island and other volcanoes we then examine the overall structure of the volcano which is dissimilar to that of large terrestrial volcanoes in important respects and perform laboratory analogue experiments of its deformation concluding that the tectonic features were formed by sinking of the volcano into a substratum that was much weaker than the volcanic edifice an ice rich substratum melted by a combination of pressure melting and magmatic heating seems the most likely mechanism analogous water escape structures in a similar volcanic situation have been identified at mt haddington in the antarctic the possible role of a hydrological cycle and a hydrothermal system within the volcano are discussed based on field evidence we propose that much of the broad aprons of lobate flows issuing from the ne and ssw foot of ascraeus mons are composed of mudflows rather than lava flows these different approaches are linked into a coherent history of this volcano the similarity of ascraeus mons to pavonis mons and arsia mons though ascraeus is younger suggests that some of our conclusions may apply to these volcanoes too

novel intact glycolipids in sediments from an antarctic lake ace lake abstract two novel glycolipids docosanyl 3 o methyl rhamnopyranoside and docosanyl 3 o methylxylopyranoside were identified as the most abundant gc amenable components of extracts of sediments of ace lake an antarctic meromictic lake docosanyl 3 o methyl rhamnopyranoside was identified by isolation and high field two dimensional nmr techniques docosanyl 3 o methylxylopyranoside was tentatively identified by characterisation of its sugar moiety by methanolysis and demethylation both glycolipids also have a minor homologue containing a c24 instead of a c22 n alkyl chain stable carbon isotope measurements indicate that the sugar moieties of these glycolipids are ca 8 9 enriched relative to the alkyl chains concentrations increased markedly with depth reaching 2 mg g sediment at 25 cm depth structurally related glycolipids occur in specialised cells of nitrogen fixing cyanobacteria so a cyanobacterial origin is suggested for these sedimentary glycolipids

airborne gravity reveals interior of antarctic volcano abstract understanding antarctic volcanoes is important as they provide a window on magmatic and tectonic processes of the antarctic plate and contain datable records of ice sheet changes we present the results from the first detailed airborne radar and gravity surveys across james ross island northern antarctic peninsula which is dominated by mt haddington an ice covered miocene recent alkaline stratovolcano the surveys provide new insights into the subsurface structure of the volcano and hence its development which are unavailable from the surface geology alone we show that mt haddington is associated with a significant negative bouguer gravity anomaly 26 mgal which suggests that there has not been significant pooling and solidification of a dense shallow level mafic magma chamber during the growth of the volcano over at least the past 6 m y which is consistent with independent geochemical evidence simple flexural isostatic models cannot explain the localised negative bouguer anomaly 3d modelling techniques show that the negative anomaly is best explained by a shallow low density intra crustal body with its top close to or at the surface although comparable gravity anomalies are commonly associated with large 20 km ash filled calderas as seen at yellowstone or toba there is no geological evidence on james ross island for a similar structure we therefore propose that the james ross island volcanic edifice subsided into the thick underlying pile of relatively soft jurassic and cretaceous sediments which were displaced by low density hyaloclastite breccia the type of deformation envisaged is similar to that associated with concepciou or iwaki volcanoes in south america although mt haddington is much larger

silicate weathering rates decoupled from the 87sr 86sr ratio of the dissolved load during himalayan erosion this study establishes that carbonate weathering dominates the supply of sr with elevated 87sr 86sr ratios in the bhote kosi sun kosi river system central nepal a major himalayan tributary to the ganges the dissolved load of the bhote kosi sun kosi displays a rapid increase in the 87sr 86sr ratio in its lower section downstream of the main central thrust a feature common to other himalayan tributaries of the ganges mass balance analysis of the contributing sources to the dissolved load identifies the weathering of palaeoproterozoic impure carbonates comprised largely of dolomite and mica from the lesser himalaya as the most significant lithological control upon elevated 87sr 86sr ratios in situ laser ablation analysis of the dolomite component yields 87sr 86sr ratios of 0 93 to 1 11 indistinguishable from bulk rock this confirms the presence of a highly soluble source of radiogenic strontium and suggests that diffusive exchange during a protracted metamorphic history has homogenised sr isotopes between silicate and carbonate phases within the assemblage weathering of the carbonate component of such rocks has yielded unusually high 87sr 86sr at significant sr fluxes in the dissolved load approximately 60 of the impact of the bhote kosi sun kosi on the marine 87sr 86sr ratio can be traced to weathering of this carbonate since similar lithologies outcrop within the catchments of most himalayan tributaries that feed the ganges and brahmaputra rivers the presence of such radiogenic carbonates undermines the inference that the impact of these rivers on sr isotopes in seawater is indicative of high silicate weathering rates that result from the uplift of tibet and the himalaya

sedimentological and stable isotope evolution of lakes in the vestfold hills antarctica the sedimentological chemical and isotopic characteristics of sediment cores from three slightly saline to hypersaline lakes highway ace and organic lakes and two marine inlets ellis fjord and taynaya bay in the vestfold hills antarctica have been examined sections of the cores deposited in marine environments are characterized by uniform regularly laminated fine grained organic rich sediments with uniform organic 13c values 18 0 to 19 4 vs pdb and sulfur contents in contrast sediments deposited in lacustrine environments are extremely heterogeneous varying from finely laminated mat like sequences to poorly sorted clastic rich sediments authigenic monohydrocalcite and aragonite occur in some lake sediments the 13c values of organic matter in the lacustrine sediments exhibit an extremely wide range 10 5 to 25 3 that can be related to variations in physico chemical conditions in the lake waters strongly negative organic 13c values coupled with high sulfur contents are indicative of an anoxic zone in the overlying lake waters whereas less negative organic 13c values coupled with low sulfur contents are indicative of well mixed oxic conditions particularly high organic 13c values result during high levels of microbial activity in the lakes due to high rates of photosynthetic co2 fixation the large shifts in organic 13c are not necessarily accompanied by any change in macroscopic sedimentological characteristics illustrating the utility of isotopic investigations in these environments the 13c composition of authigenic carbonate in hypersaline organic lake sediments provides a record of changes in palaeoproductivity while the 18o of the carbonate provides information on rates of meltwater input and evaporation in the lake 14c dating suggests that highway lake was isolated from the sea by isostatic uplift at least 4600 yr before present b p whereas organic lake was isolated at approximately 2700 yr b p apparent emergence rates calculated from the 14c ages range from 1 0 to 2 1 mm yr 1 the reservoir effect in the lacustrine and marine environments is variable but probably does not exceed 1000 yr in any of the lakes examined

fossil rotifers and the early colonization of an antarctic lake abstract early holocene sediments from a continental antarctic lake ace lake vestfold hills east antarctica contained abundant fossil rotifers of the genus notholca the fossil is similar to specimens of notholca sp present in modern day ace lake and other fresh and brackish lakes of the vestfold hills cyanobacteria and protists chrysophyte cysts dinoflagellate cysts and rhizopod tests were also recovered from the core samples these sediments were deposited early in the freshwater phase of ace lake soon after deglaciation of the area the occurrence of this trophically diverse assemblage of organisms at an early stage in the evolution of the lake suggests either that they were part of an endemic antarctic flora and fauna which pre dated the last glacial maximum and survived in glacial refugia or that efficient intercontinental dispersal had occurred

sampling sediment under warm deep water abstract the following devices and their operation are described a modifications to the mackereth corer allowing 12 m long cores to be taken routinely b a development of the frozen finger which recovers two plates of mud each 120 cm 12 cm at each operation together with equipment for facilitating its sub sampling and c a new design of hammer driven piston corer

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

chemical weathering of a marine terrace chronosequence santa cruz california i interpreting rates and controls based on soil concentration depth profiles abstract the spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal california are interpreted in terms of chemical weathering rates and processes in regoliths up to 15 m deep and 226 kyrs old quartz normalized mass transfer coefficients indicate non stoichiometric preferential release of sr ca na from plagioclase along with lesser amounts of k rb and ba derived from k feldspar smectite weathering results in the loss of mg and concurrent incorporation of al and fe into secondary kaolinite and fe oxides in shallow argillic horizons elemental losses from weathering of the santa cruz terraces fall within the range of those for other marine terraces along the pacific coast of north america residual amounts of plagioclase and k feldspar decrease with terrace depth and increasing age the gradient of the weathering profile bs is defined by the ratio of the weathering rate r to the velocity at which the profile penetrates into the protolith a spreadsheet calculator further refines profile geometries demonstrating that the non linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface to volume ratios and at high residual feldspar concentrations at greater depth by the approach to thermodynamic saturation these parameters are of secondary importance to the fluid flux qh which in thermodynamically saturated pore water controls the weathering velocity and mineral losses from the profiles long term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute cl balances qh 0 025 0 17 m yr 1 during saturation controlled and solute limited weathering the greater loss of plagioclase relative to k feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics the steady state weathering rate under such conditions is defined as r q h m sol m total 1 s v b s the product of qh and the ratio of solubilized to solid state feldspar msat mtotal define the weathering velocity the weathering gradient bs reflects the kinetic rate of reaction where sv is the volumetric surface area of the residual feldspar both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates r 5 14 10 16 mol m 2 s 1 which agree with those reported for other environments of comparable climate and age weathering dependent concentration profiles are commonly described in literature the present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved

calcium and magnesium isotope systematics in rivers draining the himalaya tibetan plateau region lithological or fractionation control abstract in rivers draining the himalaya tibetan plateau region the 26mg 24mg ratio has a range of 2 and the 44ca 42ca ratio has a range of 0 6 the average 26 mg values of tributaries from each of the main lithotectonic units tethyan sedimentary series tss high himalayan crystalline series hhcs and lesser himalayan series lhs are within 2 standard deviation analytical uncertainty 0 14 the consistency of average riverine 26 mg values is in contrast to the main rock types limestone dolostone and silicate which range in their average 26 mg values by more than 2 tributaries draining the dolostones of the lhs differ in their 44 42 ca values compared to tributaries from the tss and hhcs the chemistry of these river waters is strongly influenced by dolostone solute mg ca close to unity and both 26 mg 1 31 and 44 42 ca 0 64 values are within analytical uncertainty of the lhs dolostone these are the most elevated 44 42 ca values in rivers and rock reported so far demonstrating that both riverine and bedrock 44 42 ca values may show greater variability than previously thought although rivers draining tss limestone have the lowest 26 mg and 44 42 ca values at 1 41 and 0 42 respectively both are offset to higher values compared to bedrock tss limestone the average 26 mg value of rivers draining mainly silicate rock of the hhcs is 1 25 lower by 0 63 than the average silicate rock these differences are consistent with a fractionation of 26 mg values during silicate weathering given that the proportion of mg exported from the himalaya as solute mg is small the difference in 26mg 24mg ratios between silicate rock and solute mg reflects the 26mg 24mg isotopic fractionation factor silicate dissolved mg between silicate and dissolved mg during incongruent silicate weathering the value of silicate dissolved mg of 0 99937 implies that in the tss solute mg is primarily derived from silicate weathering whereas the source of ca is overwhelmingly derived from carbonate weathering the average 44 42 ca value in hhcs rivers is within uncertainty of silicate rock at 0 39 the widespread hot springs of the high himalaya have an average 26 mg value of 0 46 and an average 44 42 ca value of 0 5 distinct from riverine values for 26 mg but similar to riverine 44 42 ca values although rivers draining each major rock type have 44 42 ca and 26 mg values in part inherited from bedrock there is no correlation with proxies for carbonate or silicate lithology such as na ca ratios suggesting that ca and mg are in part recycled however in spite of the vast contrast in vegetation density between the arid tibetan plateau and the tropical lesser himalaya the isotopic fractionation factor for ca and mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of ca and mg in these catchments the discrepancy between solute and solid ca and mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of ca and mg bound as minerals in rock and released as cations in rivers the fractionation of ca and mg isotope ratios may prove useful for tracing mechanisms of chemical alteration ca isotope ratios of solute riverine ca show a greater variability than previously acknowledged the variability of ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global ca cycling if past variations in oceanic ca isotope ratios are to be of use in constraining the past carbon cycle

a diatom based palaeosalinity history of ace lake vestfold hills antarctica a comprehensive diatom stratigraphy is used to calculate a palaeosalinity history for an antarctic lake via an established diatom salinity transfer function for the vestfold hills antarctica a sediment core taken from ace lake in 1995 shows three distinct changes in diatom assemblage constituents initial benthic hyposaline freshwater taxa are replaced by marine planktonic and sea ice taxa with these taxa in turn replaced by the benthic hypersaline taxa dominant in the lake today these changes in assemblage composition enable the lakewater salinity of each stage to be determined and the holocene evolution of the lake to be refined deglaciation of the vestfold hills at the beginning of the holocene exposed ace lake basin following this fresh lacustrine diatoms were deposited from 11 380 to 8110 corrected 14c yr bp relative sea level rise after this time led to the progressive marine inundation of the lake and the deposition of marine diatom taxa marine taxa were dominant in the sediment for mo

the strontium isotopic budget of himalayan rivers in nepal and bangladesh abstract himalayan rivers have very unusual sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic we present sr o and c isotopic data from river and rain water bedload and bedrock samples for the western and central nepal himalaya and bangladesh including the monsoon season central himalayan rivers receive sr from several sources carbonate and clastic tethyan sediments high himalayan crystalline hhc gneisses and granitoids with minor marbles carbonates and metasediments of the lesser himalaya lh and miocene recent foreland basin sediment from the siwaliks group and the modern flood plain in the tethyan himalaya rivers have dissolved sr 6 mol l and 87sr 86sr 0 717 with a large contribution from moderately radiogenic carbonate rivers draining hhc gneisses are very dilute with sr 0 2 mol l and 87sr 86sr 0 74 lesser himalayan streams also have low sr 0 4 mol l and are highly radiogenic 87sr 86sr 0 78 highly radiogenic carbonates of the lh do not contribute significantly to the sr budget because they are sparse and have very low sr in large rivers exiting the himalaya sr systematics can be modeled as a mixture between tethyan rivers where slightly radiogenic carbonates mean 87sr 86sr 0 715 are the main source of sr and lesser himalaya waters where extremely radiogenic silicates 0 8 are the main source of sr hhc waters are less important because of their low sr rivers draining the siwaliks foreland basin sediments have sr 4 mol l and 87sr 86sr 0 725 weathering of silicates in the siwaliks and the flood plain results in a probably significant radiogenic 0 72 0 74 input to the ganges and brahmaputra g b but quantification of this flux is limited by uncertainties in the hydrologic budget the g b in bangladesh show strong seasonal variability with low sr and high 87sr 86sr during the monsoon sr in the brahmaputra ranges from 0 9 mol l and 0 722 in march to 0 3 mol l and 0 741 in august we estimate the seasonally weighted flux from the g b to be 6 5 108 mol yr with 87sr 86sr 0 7295

major ion chemistry of the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal the ganga brahmaputra one of the world s largest river systems is first in terms of sediment transport and fourth in terms of water discharge a detailed and systematic study of the major ion chemistry of these rivers and their tributaries as well as the clay mineral composition of the bed sediments has been conducted the chemistry of the highland rivers upper reaches of the ganga the yamuna the brahmaputra the gandak and the ghaghra are all dominated by carbonate weathering ca mg and hco3 account for about 80 of the cations and anions in the lowland rivers the chambal the betwa and the ken hco3 excess over ca mg and a relatively high contribution of na k to the total cations indicate that silicate weathering and or contributions from alkaline saline soils and groundwaters could be important sources of major ions to these waters the chemistry of the ganga and the yamuna in the lower reaches is by and large dictated by the chemistry of their tributaries and their mixing proportions illite is the dominant clay mineral about 80 in the bedload sediments of the highland rivers kaolinite and chlorite together constitute the remaining 20 of the clays in the chambal betwa and ken smectite accounts for about 80 of the clays this difference in the clay mineral composition of the bed sediments is a reflection of the differences in the geology of their drainage basins the highland rivers weather acidic rocks whereas the others flow initially through basic effusives the ganga brahmaputra river system transports about 130 million tons of dissolved salts to the bay of bengal which is nearly 3 of the global river flux to the oceans the chemical denudation rates for the ganga and the brahmaputra basins are about 72 and 105 tons km yr 1 respectively which are factors of 2 to 3 higher than the global average the high denudation rate particularly in the brahmaputra is attributable to high relief and heavy rainfall

partitioning of strontium between calcite and fluid the sr ca ratio of biogenic carbonate is widely used as a proxy for paleotemperature this application is supported by empirical calibrations of sr ca as a function of temperature but it is also known that sr uptake in calcite gauged by kdsr is affected by other variables including bulk precipitation rate kdsr increases with increasing precipitation rate there are no data from controlled experiments specifically addressing the effect of radial growth rate of individual crystals on kdsr for this reason we conducted two series of experiments to explore sr partitioning at varying growth rates 1 growth from a cacl2 nh4cl srcl2 solution by diffusion of co2 from an ammonium carbonate source drift experiments and 2 drip precipitation of calcite on a substrate using a steady flow of cacl2 srcl2 and na2co3 solutions mixed just before passage through a tube and dripped onto a glass slide precoated with calcite cave type experiments the growth rates of individual crystals were determined by periodic monitoring of crystal size through time or roughly by comparison of the final size with the duration of the experiment electron microprobe analyses across sectioned crystals grown in the drift experiments show that the concentration of sr is high in the center where radial growth rates are highest and decreases systematically toward the edge the center to edge drop in sr concentration is a consequence of the slowing radial growth rate as individual crystals become larger in general high crystal growth rate v enhances sr uptake in calcite due to a type of kinetic disequilibrium we refer to as growth entrapment the apparent kdsr ranges from 0 12 to 0 35 as v increases from 0 01 nm s to 1 m s at 25 c

chemical and strontium oxygen and carbon isotopic compositions of carbonates from the lesser himalaya implications to the strontium isotope composition of the source waters of the ganga ghaghara and the indus rivers samples of precambrian carbonate mostly dolomite outcrops collected across the lesser himalaya have been analysed for their mineralogy chemical composition and isotope ratios of sr o and c to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the ganga ghaghara and the indus their sr concentrations range from 20 to 363 ppm 18opdb 1 4 to 12 8 and mn 11 2036 ppm the petrography of the samples their low sr concentrations and wide range of 18o values are suggestive of their postdepositional alteration the 87sr 86sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0 7064 to 0 8935 and are generally more radiogenic than that of contemporaneous seawater comparison of the 87sr 86sr and sr ca ratios among the carbonates and silicates from the lesser himalaya and the source waters of the ganga ghaghara and the indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates an upper limit of carbonate sr in the various source waters is calculated to be between 6 and 43 assuming that all the ca in the rivers is of carbonate origin the results show that on the average weathering of the precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters however they can be a dominant supplier of radiogenic sr to some rivers on a regional scale the silicate sr component in some of the source waters of the ganga bhagirathi bhilangna alaknanda and ganga ghaghara kali and sarju and the indus sutlej was calculated from the ca na sr na ratios and strontium isotope compositions of these rivers and the silicate endmember these calculations suggest that 33 89 of sr in the bhagirathi bhilangna alaknanda ganga and sarju rivers is of silicate origin whereas in the kali and the sutlej it is much lower only 8 the remaining sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites this study underscores the importance of weathering of silicates carbonates and evaporites in contributing to the sr mass balance and 87sr 86sr of the source waters of the ganga ghaghara and the indus the present day silicate and carbonate sr contributions to the sr budget of the rivers vary considerably but among the major source waters of the ganga silicate sr exerts a more dominant control on their sr abundance and 87sr 86sr

the holocene evolution and palaeosalinity history of beall lake windmill islands east antarctica using an expanded diatom based weighted averaging model abstract a mid holocene climate optimum is inferred from a palaeosalinity reconstruction of a closed saline lake beall lake from the windmill islands east antarctica using an expanded diatom salinity weighted averaging wa regression and calibration model the addition of 14 lakes and ponds from the windmill islands east antarctica to an existing weighted averaging regression and calibration palaeosalinity model of 33 lakes from the vestfold hills east antarctica expands the number of taxa and lakes and the range of salinity in the existing model and improves the model s predictive ability this improved model was used to infer holocene changes in lake water salinity in beall lake windmill islands six changes in diatom inferred salinity in beall lake are put into broad chronological context based on three radiocarbon dates as the east antarctic ice sheet eais retreated from the windmill islands during the early holocene 9000 8130 corr yr bp beall lake formed as a melt water fed freshwater lake which gradually became more saline as marine influence increased from 8000 corr yr bp between 8000 and 4800 corr yr bp the diatom assemblage included planktonic marine taxa such as chaetoceros spp and cryophilic taxa such as fragilariopsis cylindrus which indicate favourable summer growth conditions a mid holocene warm period produced a climate that was warmer and more humid with increased precipitation and snow accumulation this is reflected in the beall lake core as a reduction in the salinity of the lake diatom assemblage from 4800 4600 corr yr bp holocene isostatic uplift rates in the windmill islands vary from 5 6 m 1000 yr by applying this uplift rate it is calculated that the bedrock would have risen above sea level by 4000 yr bp the beall lake core diatom assemblage from 4600 2900 corr yr bp includes both marine cryophilic and planktonic taxa together with freshwater benthic and planktonic lacustrine taxa this mix of species indicates the emergence of the lake from the sea around 4600 corr yr bp from 2800 corr yr bp retreat of the ice margin led to increasing melt water inputs and associated freshening of the lake basin until 1900 corr yr bp the lake basin had no oceanic influence by this time allowing a terrestrial freshwater flora to establish and thrive for the next 1000 yr at 1850 corr yr bp a sudden and rapid salinity change is evident in beall lake a late holocene warm period between 2000 and 1000 yr bp has been observed in ice core records from law dome an ice cap abutting the windmill islands to the east and north it is therefore inferred that at 1850 corr yr bp summer temperatures within the beall lake catchment area were much higher than present summer temperatures the climate optimum identified in the beall lake core 4800 yr bp confirms mid holocene warming of the windmill islands and suggests a synchronous mid holocene climate optimum occurred across coastal east antarctica in addition the abrupt climate change inferred at 1850 yr bp suggests that higher resolution sampling of sediment cores from coastal east antarctic limnological oases will provide more evidence of rapid climate change events over coastal east antarctica in future

spatial and temporal variation of sublimation on antarctica results of a high resolution general circulation model in this paper we use output of a high resolution general circulation model echam 3 t106 resolution 1 1 x 1 1 to study the spatial and temporal variation of sublimation on antarctica first we compare model results with available observations of sublimation rates the yearly cycle with small latent heat fluxes during the winter is well reproduced and the agreement with sparsely available spot observations is fair the model results suggest that a significant 10 15 of the annual precipitation over antarctica is lost through sublimation and that sublimation plays an important role in the formation of blue ice areas a preliminary analysis of the atmospheric boundary layer moisture budget shows that the spatial variation of sublimation in the coastal zone of east antarctica can be explained by variations of horizontal advection of dry air dry air advection and thus surface sublimation is enhanced in areas where katabatic winds are strong and have a large downslope component and where the antarctic topography drops suddenly from the plateau to the coastal zone in areas where horizontal advection is small like the plateau and the large ice shelves special conditions must be met to make significant sublimation at the surface possible

combined sr nd pb and li isotope geochemistry of alkaline lavas from northern james ross island antarctic peninsula and implications for back arc magma formation abstract we present a comprehensive geochemical data set for a suite of back arc alkaline volcanic rocks from james ross island volcanic group jrivg antarctic peninsula the elemental and isotopic sr nd pb and li composition of these cenozoic basalts emplaced east of the antarctic peninsula is different from the compositions of the fore arc alkaline volcanic rocks in southern shetlands and nearby bransfield strait the variability in elemental and isotopic composition is not consistent with the jrivg derivation from a single mantle source but rather it suggests that the magma was mainly derived from a depleted mantle with subordinate oib like enriched mantle component em ii the isotopic data are consistent with mantle melting during extension and possible roll back of the subducted lithosphere of the antarctic plate magma contamination by triassic early tertiary clastic sediments deposited in the back arc basin was only localized and affected li isotopic composition in two of the samples while most of the basalts show very little variation in 7 li values as anticipated for mantle driven li isotopic composition these variations are difficult to resolve with radiogenic isotope systematics but li isotopes may prove sensitive in tracking complex geochemical processes acting through the oceanic crust pile including hydrothermal leaching and seawater equilibration

strontium isotopes and rubidium in the ganga brahmaputra river system weathering in the himalaya fluxes to the bay of bengal and contributions to the evolution of oceanic87sr 86sr the concentrations of rb and sr and87sr 86sr isotopic ratios have been measured in the dissolved load of the ganga brahmaputra g b river system the ganga was sampled extensively from its source at gangotri in the higher himalaya to patna on the alluvial plains the brahmaputra was sampled in its stretch in assam in india the average sr concentration in the ganga at patna is 1 2 mol l and that in the brahmaputra at goalpara is 0 73 mol l the mean87sr 86sr ratios are 0 7239 and 0 7192 respectively the87sr 86sr in the ganga source waters the alaknanda the bhagirathi and their tributaries range between 0 7300 and 0 7986 considerably higher than the global average runoff value of 0 7119 the high87sr 86sr in the ganga source waters result from the intense weathering of precambrian granites and gneisses enriched in radiogenic sr the sr isotope systematics of the ganga waters is dominated by silicate weathering whereas carbonate weathering plays a significant role in their major ion chemistry the g b system transports about 910 million moles of dissolved sr annually to the bay of bengal with an average87sr 86sr of 0 7213 the sr transported by the g b system is about 2 7 of the global dissolved sr flux to the oceans via rivers model calculations reveal that the g b system has contributed significantly to the sr isotope evolution of seawater during the past 20 ma the flux of rb through the g b system is 24 million moles per year this is about 5 of the global river supply of rb to the oceans nearly twice the contribution of water via the g b system to the oceans our study suggests that the marine geochemistries of sr and rb and possibly u may have been influenced considerably by the himalayan orogeny

new aerogeophysical view of the antarctic peninsula more pieces less puzzle new airborne geophysical data reveal subglacial imprints of crustal growth of the antarctic peninsula by mesozoic arc magmatism and terrane accretion along the paleo pacific margin of gondwana potential field signatures indicate that the antarctic peninsula batholith is a composite magmatic arc terrane comprising two distinct arcs separated by a 1500 km long suture zone similar to the peninsular ranges batholith in southern and baja california aeromagnetic aerogravity and geological data suggest that a mafic early cretaceous western arc was juxtaposed against a more felsic eastern arc which in mid cretaceous times was intruded by highly magnetic tonalitic granodioritic plutons of island arc affinity suturing of the two arcs against the gondwana margin caused the mid cretaceous palmer land orogenic event convergence and suturing may have been driven by two subduction zones or alternatively by a decrease in slab dip leading to an inboard migration of the arc as in california

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

controls over the strontium isotope composition of river water abstract strontium concentrations and isotope ratios have been measured in river and ground waters from the ganges orinoco and amazon river basins when compared with major element concentrations the data set has allowed a detailed examination of the controls over the strontium isotope systematics of riverine input to the oceans in the following environments 1 1 typical drainage basins containing limestones evaporites shales and alumino silicate metamorphic and igneous rocks 2 2 shield terrains containing no chemical or biogenic sediments and 3 3 the floodplains that constitute the largest areas of many large rivers the strontium concentration and isotope composition of river waters are largely defined by mixing of strontium derived from limestones and evaporites with strontium derived from silicate rocks the strontium isotope composition of the limestone endmember generally lies within the phanerozoic seawater range which buffers the 87 sr 86 sr ratios of major rivers a major exception is provided by the rivers draining the himalayas where widescale regional metamorphism appears to have led to an enrichment in limestones of radiogenic strontium derived from coexisting silicate rocks the strontium isotope systematics of rivers draining shield areas are controlled by the intense transportlimited nature of the weathering reactions and thereby limits variations in the strontium flux from these terrains floodplains are only a minor source of dissolved strontium to river waters and precipitation of soil salts in some floodplains can reduce the riverine flux of dissolved strontium to the oceans the most effective mechanisms for altering the isotope ratio and flux of riverine strontium to the oceans are increased glaciation and large scale regional metamorphism of the type produced during continental collision both mechanisms provide a means for increasing the 87 sr 86 sr ratio of the global riverine flux

volcanic and tectonic evolution of the northern antarctic peninsula late cenozoic to recent abstract field investigation together with a number of geochemical petrographical analyses as well as absolute k ar age determinations and geophysical data allow the recognition of an evolutionary sequence of geodynamic events which have affected the northern region of antarctic peninsula and the adjacent islands a significant volcanic calc alkaline belt which developed on the northwestern margin of the antarctic peninsula during the cretaceous to middle tertiary is indicative of active subduction of the antarctic plate in that area this activity decreases during the lower miocene giving way to an expansive phase represented by the bransfield rift these extensional processes are dominant during the pliocene creating a rift system in southeastern bransfield towards larsen both the bransfield and larsen systems comprise one fan like rift system associated with the prince gustav rift and the scotia arc micro plate ejection of abundant pyroclastic material generated a large plateau of palagonite hyaloclastites of basaltic alkaline composition during the pleistocene recent the extensional activity continued as evidenced by the active volcanic fractures represented in bransfield by the deception penguin and bridgeman volcanic centres in the prince gustav rift by paulet islands and others and in larsen by the coley seal nunatak and argo volcanic centres the latter is characterized by basaltic olivine alkaline effusions these rifts and the continental blocks are affected by a series of fractures with a n60 70 w strike which could be directly associated with the hero fracture zone extending northwest of the south shetland islands trench

relative contributions of silicate and carbonate rocks to riverine sr fluxes in the headwaters of the ganges abstract exhumation of the himalayan tibetan orogen is implicated in the marked rise in seawater 87sr 86sr ratios since 40 ma however both silicate and carbonate rocks in the himalaya have elevated 87sr 86sr ratios and there is disagreement as to how much of the 87sr flux is derived from silicate weathering most previous studies have used element ratios from bedrock to constrain the proportions of silicate and carbonate derived sr in river waters here we use arrays of water compositions sampled from the head waters of the ganges in the indian and nepalese himalaya to constrain the end member element ratios the compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two component mixing of silicate and carbonate derived components and lie on a plane in multicomponent composition space key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes na 0 for carbonate and constant ca na for silicate the fractions of sr derived from silicate and carbonate sources are then calculated by mass balance in sr ca mg na composition space comparison of end member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low mg trace calcite in silicate rocks may explain discrepancies in sr ca na mg covariation alternatively composition dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation ratio diagrams however the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained silicates are calculated to provide 50 of the dissolved sr flux from the head waters of the ganges assuming that discrepancies between ca mg na covariation and the silicate rock compositions arise from addition of trace calcite if the ca mg na mixing plane is rotated by composition dependent secondary calcite deposition this estimate would be increased moreover when 87sr 86sr ratios of the sr inputs are considered silicate sr is responsible for 70 16 1 of the 87sr flux forcing changes in seawater sr isotopic composition since earlier studies predict that silicate weathering generates as little as 20 of the total sr flux in himalayan river systems this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated

himalayan tectonics weathering processes and the strontium isotope record in marine limestones the time evolution of the isotopic composition of seawater strontium the ratio of strontium 87 to strontium 86 over the last 500 million years has the form of an asymmetric trough the values are highest in the cambrian and recent 0 7091 and lowest in the jurassic 0 7067 superimposed on this trend are a number of smaller oscillations consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only himalayan style continental collisions can influence the isotope ratio on the scale observed the contemporary himalayan orogeny is by far the largest since the late precambrian pan african event that produced the high in the cambrian

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

long term average mineral weathering rates from watershed geochemical mass balance methods using mineral modal abundances to solve more equations in more unknowns abstract the number of phases for which weathering rates can be determined by watershed geochemical mass balance is limited by the number of equations that can be constructed from elemental flux losses from the watershed and mineral stoichiometries mass balance studies of watershed weathering rates routinely use the flux losses of the six major cations sio 2 al na k mg and ca analyses of these species in water are common but following matrix algebraic methods limits the number of weathering rates that can be calculated to six for the brubaker run watershed located in the northern piedmont physiographic province of pennsylvania usa long term 10 3 10 6 year watershed chemical flux losses have been determined using 10 be derived total denudation rates and zirconium normalized total chemical concentrations from bedrock and soils chemical flux losses calculated from solid phase data have three advantages they 1 permit generation of a relatively large number of equations because both major and trace analyses are included 2 eliminate the need for many years of regular e g weekly sampling and chemical analyses of stream water and atmospheric precipitation and measurement of hydrologic parameters i e precipitation stream discharge etc and 3 long term weathering rate calculations need not address biomass for brubaker run eight minerals are involved in weathering the five primary minerals are ree rich epidote ankerite almandine spessartine garnet muscovite chlorite and the three secondary products are weathered muscovite kaolinite and gibbsite the long term average weathering rates of these minerals were calculated using the six major cations and two trace elements selected from rb sr ba la pr nd sm gd and dy despite having the eight equations needed geochemically reasonable weathering rates e g positive primary mineral rates that reflect destruction could not be achieved regardless of the two trace elements used in the mass balance calculations for brubaker run this is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral grains with trace element stoichiometries in some minerals varying by as much as an order of magnitude because the trace elements are hosted by a relatively small number of minerals the computed weathering rates of other minerals become very sensitive to small variations in trace cation stoichiometry ree rich epidote garnet and ankerite within the brubaker run watershed together host nearly all of the ca in the bedrock and completely dissolve at or near the weathering front consequently approximately all of the ca in bedrock is lost from the regolith in bedrock the mole percentages of ca hosted by ree rich epidote garnet and ankerite are 49 mol 4 mol and 43 mol respectively and are determined by the modal abundance of the mineral in the bedrock and its ca stoichiometry the weathering rates of ree rich epidote garnet and ankerite can be determined by distributing to each mineral that fraction of the total watershed ca flux loss for which it is responsible based on its mole percent ca in bedrock by using a base cation that is completely lost from the regolith and knowing the mole percentage of that element in the mineral s undergoing weathering additional equations may be added to the mass balance matrix we term this technique the flux distribution method the flux distribution method eliminates the need for additional equations established using trace elements based on the mineral weathering rates for the brubaker run watershed determined using the flux distribution method the rates at which the weathering front penetrated the bedrock the saprolitization rate are 4 5 m myr 1 and 6 5 m myr 1 for chlorite and muscovite respectively these measured long term average saprolitization rates compare very favorably with published theoretical values for the nearby northern maryland piedmont which range from 2 2 to 5 3 m myr 1

geochemistry and tectonic setting of alkaline volcanic rocks in the antarctic peninsula a review abstract the numerous miocene recent alkaline volcanic outcrops in the antarctic peninsula form a substantial volcanic province the least well known part of a major belt of alkaline volcanism that extends between south america and new zealand the outcrops consists mainly of aa and pahoehoe lavas and hyaloclastites which locally contain accidental nodules of spinel lherzolite and other mantle derived lithologies the province is predominantly basaltic with two major differentiation lineages 1 a sodic series of olivine and alkali basalt hawaiite mugearite trachy phonolite and trachyte and 2 a relatively potassic highly undersaturated series of basanite tephrite and phono tephrite all the lavas show varying effects of fractionation by crystallization of olivine and clinopyroxene joined by plagioclase in the hawaiites to trachytes fractional crystallization can probably explain most of the chemical variation observed within each outcrop but variable partial melting is necessary to account for the differences in incompatible element enrichment between the two series and between the individual outcrops the degree of partial melting may not have exceeded 3 as is the case for many other alkaline magmas the volcanism is an intraplate phenomenon but there is no correlation in timing between the cessation of subduction and the inception of alkaline volcanism the activity cannot be related to the passage of the coupled pacific antarctic plate over a stationary mantle hot spot although the precise causal relationship with tectonic setting is unknown regional extension was a prerequisite for giving the magmas rapid access to the surface

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

faunal microfossils indicators of holocene ecological change in a saline antarctic lake abstract the sediment record of the fauna of ace lake a saline meromictic lake in the vestfold hills antarctica consists of copepod eggs spermatophores and exoskeletal fragments rotifer and tintinnid loricae and foraminiferal and folliculinid tests the relative abundance of these remains along with other characteristics of the core allows the development of a coherent picture of the progress of ace lake from a species poor freshwater lake early in the holocene to a biodiverse marine basin following a marine transgression subsequent sea level fall reformed ace lake as a saline lake and productivity initially increased after isolation after a major event possibly associated with overturn of the meromictic lake biodiversity and productivity decreased and have continued to do so until the present

geologic control of sr and major element chemistry in himalayan rivers nepal our study of the seti river in far western nepal shows that the solute chemistry of the river and its tributaries is strongly controlled by geology the seti flows through four distinct terranes starting with the tethyan sedimentary series tss and greater himalayan series ghs tss ghs waters display 87 sr 86 sr ratios of 0 73 and high sr and ca consistent with the composition of limestone and marble common in these terranes the 87 sr 86 sr ratio and mg increase markedly as the river passes into the lesser himalayan series lhs where tributaries have 87 sr 86 sr ratios from 0 75 to 1 02 and high sr ca and mg the high mg in lhs waters correlate with high 87 sr 86 sr ratios which we attribute to weathering of highly radiogenic 0 71 0 82 dolostones tributaries to the seti river draining the largely carbonate free dadeldhura thrust sheet dts have ratios near 0 74 but low sr ca and mg and therefore have little impact on seti mainstem chemistry mass balance calculations and camg weathering indices show that carbonate weathering accounts for 70 of total dissolved solids to the seti river sr ca ratios of river waters provide a minimum estimate of the carbonate weathering contribution to sr due to partitioning of sr and ca during incongruent dissolution and reprecipitation of calcite overall we attribute high 87 sr 86 sr ratios in the seti river and its tributaries to the weathering of metacarbonates especially dolostones in the upper nawakhot group which have exchanged sr with silicates during metamorphism our modeling of sr fluxes in the seti river indicates that the tss ghs accounts for 36 39 of the sr the lhs for 40 53 and 8 23 for the dts prior to exposure of lhs rocks at 12 ma tss and ghs carbonates with low 87 sr 86 sr ratios dominated himalayan rivers we attribute the elevated 87 sr 86 sr ratios of himalayan paleorivers during the late miocene and pliocene to exposure and weathering of lhs metacarbonates copyright 2000 elsevier science ltd

late holocene east antarctic climate trends from ice core and lake sediment proxies a high resolution record of evaporation for the last 650 years was derived from the diatom salinity signal preserved in a sediment core taken from ace lake vestfold hills 68 28 s 78 11 e antarctica the seasonal oxygen isotope signal preserved in an ice core from law dome 66 44 s 112 50 e antarctica revealed a high resolution summer temperature record for the same time period the two proxies show highly correlated behaviour despite having significantly different climatic response mechanisms and a large geographic separation the correlation observed between proxies based on such differing processes and analytical method ologies provides not only a climate record for the past 650 years that is both robust and regionally representative of coastal east antarctica but also confirmation of the utility of reconstructions using these methods

glacial removal of late cenozoic subglacially emplaced volcanic edifices by the west antarctic ice sheet local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the ross sea northwestern ross ice shelf and the west antarctic ice sheet reveal a linear magnetic rift fabric and numerous subcircular high amplitude anomalies most of these anomalies have sources that probably resulted from late cenozoic volcanism some of these volcanic structures penetrate the neogene sediments beneath the deglaciated continental shelf and are present at the base of the present grounded ice sheet and beneath the ice shelf geophysical data indicate two or three youthful volcanic edifices at widely separated areas beneath the sea and ice cover in the west antarctic rift system in contrast we suggest glacial removal of edifices of volcanic sources of many more anomalies magnetic models controlled by marine seismic reflection and radar ice sounding data allow us to infer that glacial removal of the associated late cenozoic volcanic edifices probably debris comprising pillow breccias and hyaloclastites has occurred essentially concomitantly with their subglacial eruption removal of unconsolidated volcanic debris erupted beneath the ice is probably a more appropriate term than erosion given its fragmented ice contact origin the exposed volcanoes may have been protected from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet glacial removal may be the general case exposed late cenozoic volcanic peaks and outcrops consisting primarily of flows which erupted during antarctic glacial conditions since approx 30 ma may be the exceptions the volume of the exposed volcanoes is small in contrast to the much greater volume 106 km3 of late cenozoic magmatic rock remaining at volcanic centers beneath the continental shelf ross ice shelf and west antarctic ice sheet

bathymetry depth to magnetic basement and sediment thickness estimates from aerogeophysical data over the western weddell basin we estimated the bathymetry and sediment thickness of a remote and difficult to access portion of the antarctic continental margin using aerogeophysical surveying techniques the u s argentina chile aerogeophysical survey collected magnetic and gravity data over the basins surrounding the antarctic peninsula thirty seven of these flight lines were used to estimate bathymetry and depth to magnetic basement for the western weddell basin a wavenumber technique was applied to individual magnetic anomaly profiles in an automated fashion to obtain estimates of the depth to magnetic basement the bathymetric estimates were obtained by admittance inversions of the gravity field the results were then gridded at a 40 km interval for the region 64 w 44 w 73 s and 62 s bathymetric estimates and depth to magnetic basement estimates were differenced at each grid point to obtain a regional estimate of the thickness of nonmagnetic overburden assumed to be sediment subsequent spot measurements of topography in the estimated region of the continental margin generally agree to about 52 m the estimated magnetic basement deepens from the antarctic peninsula margin eastward to a maximum of 10 12 km near 54 w we also postulate the existence of two moderately large basins flanking the eastward continuation of the jason peninsula farther east the basement steps upward with a correspondent thinning of the sedimentary layer along the east coast of the peninsula results agree well with seismic studies on james ross island and magnetotelluric studies on marambio island and the larsen nunatak as well as the british antarctic survey basement estimates from aeromagnetic data this study further demonstrates the utility of combined application of airborne and satellite geophysical techniques in the study of structure and tectonic evolution of continental margins and marine basins

combined dna and lipid analyses of sediments reveal changes in holocene haptophyte and diatom populations in an antarctic lake preserved ribosomal dna of planktonic phototrophic algae was recovered from holocene anoxic sediments of ace lake antarctica and the ancient community members were identified based on comparative sequence analysis the similar concentration profiles of dna of haptophytes and their traditional lipid biomarkers alkenones and alkenoates revealed that fossil rdna also served as quantitative biomarkers in this environment the dna data clearly revealed the presence of six novel phylotypes related to known alkenone and alkenoate biosynthesizing haptophytes with isochrysis galbana uio 102 as their closest relative the relative abundance of these phylotypes changed as the lake chemistry particularly salinity evolved over time changes in the alkenone distributions reflect these population changes rather than a physiological response to salinity by a single haptophyte using this novel paleo ecological approach of combining data from lipid biomarkers and preserved dna we showed that the post glacial development of ace lake from freshwater basin to marine inlet and the present day lacustrine saline system caused major qualitative and quantitative changes in the biodiversity of the planktonic populations over time

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

diatom based transfer functions for inferring past hydrochemical characteristics of african lakes abstract a new dataset of 282 modern diatom samples and associated environmental information has been created by merging existing regional datasets from north and east africa and niger the relationships between diatom species distributions and hydrochemistry are examined using canonical correspondence analysis cca and partial cca variables reflecting water conductivity ph and cation and anion composition account for significant and independent components of the total variation in the diatom data predictive models transfer functions are developed using the method of weighted averaging for conductivity r 2 0 87 ph r 2 0 77 and ratios between alkali and alkaline earth metals r 2 0 81 and carbonate bicarbonate and sulphate chloride ions r 2 0 82 prediction errors are estimated using the computer intensive method of jackknifing these transfer functions enlarge the potential domain for reconstruction of past hydrochemistry from fossil diatoms preserved in lake sediments

holocene sea level change and ice sheet history in the vestfold hills east antarctica abstract a new holocene sea level record from the vestfold hills antarctica has been obtained by dating the lacustrine marine and marine lacustrine transitions that occur in sediment cores from lakes which were formerly connected to the sea from an elevation of 7 5 m 8000 yr ago relative sea level rose to a maximum 9 m above present sea level 6200 yr ago since then sea level has fallen monotonically until the present the precision of the new record makes it suitable for constraining the recent history of the ice sheet in that region using numerical models of glacio hydro isostasy simplified regional models suggest that the ice sheet margin has retreated 30 40 km since the last glacial maximum with 600 700 m of thinning occurring at the present coastline

coastal oceanographic conditions in the prydz bay region east antarctica during the holocene recorded in an isolation basin information on east antarctic coastal environments during the holocene is relatively sparse this is surprising as sedimentary records from the interface between land and sea can provide chronologies of climatic change isostatic uplift relative sea level and the colonization of newly formed biomes here we examine a sediment core from pup lagoon a coastal lake in the larsemann hills east antarctica sediment stratigraphy fossil pigments and diatoms were used to infer the sequence of holocene environmental and climatic change results show that between 5800 and 5500 cal yr bp the marine coast of prydz bay was characterized by stratified open water conditions during spring and summer and seasonally warm conditions from 5500 to 2750 cal yr bp sea ice duration in prydz bay increased with the coast being ice free for 2 3 months each year conditions which are similar to the present day a return to stratified open water conditions and a reduction in winter sea ice extent between 2750 and 2200 cal yr bp is signalled by enhanced biogenic production and more open water diatom taxa this is consistent with evidence for the mid holocene hypsithermal detected in other records in east antarctica isostatic isolation of the pup lagoon basin from the sea between 2200 and 2000 cal yr bp slightly precedes the emergence of lakes with comparable sill heights from the nearby vestfold hills the colonization of pup lagoon after its isolation as a freshwater lake was initiated by a siliceous flora dominated by stomatocysts with microbial mat development being prevented by mechanical or physical stress a brief period of marine incursion following the mid holocene hypsithermal may be related to local events such as iceberg calving or to minor sea level change weighted averaging regression used to infer salinity in the lacustrine zone shows that from 1500 cal yr bp pup lagoon is a freshwater lake where the flora is dominated by stratified cyanobacterial mats with green algae and diatoms as co dominants comparable to modem pup lagoon and other lakes in the larsemann hills

microlevelling procedures applied to regional aeromagnetic data an example from the transantarctic mountains antarctica the extensive application of digital enhancement and filtering as a powerful tool for aeromagnetic interpretation not only of high resolution but also of regional data requires an improved levelling two microlevelling techniques were thus compared in order to find an effective but relatively simple procedure to remove or at least to reduce residual magnetic errors remaining after standard levelling processes this study was carried out on regional aeromagnetic data recently acquired at high magnetic latitudes along the transantarctic mountains in antarctica where it is particularly critical to remove time dependent magnetic variations two dimensional fft filters applied to the gridded data namely the butterworth and a directional cosine filter proved to be more effective than previously proposed one dimensional space domain filters in the reduction of the residual corrugation not removed by statistical levelling tectonic interpretation of trends detected in the total field magnetic anomaly map and in the 3d analytic signal improved after application of frequency domain microlevelling however we also show that when interpreting microlevelled data two factors must be considered i the possible presence of real geological trends aligned along the flight lines ii modifications in the results yielded by depth estimates of magnetic sources due to the fft filters applied during the microlevelling procedure such changes were seen both in the well established 2d fft method based on the slope of the energy spectrum and in the more recent 3d euler deconvolution technique overall our results indicate that microlevelling could profitably be applied to older gridded aeromagnetic data sets in antarctica thus improving the accuracy and geological significance of future regional magnetic compilations as already seen in other continents

animated tectonic reconstruction of the southern pacific and alkaline volcanism at its convergent margins since eocene times abstract an animated reconstruction shows south pacific plate kinematics in the reference frame of west antarctica between 55 ma and the present day the ocean floor in the region formed due to seafloor spreading between the antarctic pacific phoenix and nazca plates a plate formed by fragmentation of the farallon plate early in oligocene times the pacific antarctic ridge remained fairly stable throughout this time migrating relatively northwestwards by various mechanisms behind the rapidly moving pacific plate the nazca and phoenix plates also moved quickly but relatively towards the east or southeast and were subducted in these directions beneath the south american and antarctic plates segments of spreading centres forming at the trailing edges of the nazca and phoenix plates periodically collided with these subduction zones resulting in the total destruction of the nazca phoenix spreading centre and the partial destruction of the nazca antarctica spreading centre the chile ridge and antarctic phoenix ridge which ceased to operate shortly before its northeasternmost three segments could collide with the antarctic margin following collision of segments of the chile ridge parts of the antarctic plate underwent subduction at the chile trench after these collisions slab windows should have formed beneath both the south american and antarctic convergent margins and the animation shows occurrences of alkaline volcanism that have been or can newly be related to them further occurrences of alkali basalts at the margins of the powell basin and more speculatively james ross island can be related to the formation of a slab window beneath them following the collision of segments of the south america antarctica spreading centre in the northwest weddell sea

seasonality in late holocene climate from ice core records high resolution ice core 18o data from a site with well preserved seasonal cycles are used to extract seasonal temperature trends over the last 700 years with an effective resolution of a few mont

on discrimination between carbonate and silicate inputs to himalayan rivers we review new and published analyses of river waters bedloads and their constituent minerals from the dhauli ganga and alaknanda headwaters of the ganges in garhwal and the marsyandi in nepal and their tributaries these data are used to discriminate between the inputs of major cations and sr from silicate and carbonate sources methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in sr ca mg na k 87sr 86sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component this results in factor of two overestimates of the fractions of silicate derived ca mg and sr to correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of sr ca mg na k 87sr 86sr space the compositions of the carbonate and silicate end members in the bedload are determined by sequential leaching the results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron microprobe analyses of the minerals in the bedload in the upper marsyandi catchment which drains low grade tethyan sedimentary series formations a set of mainstem samples collected over a two year period define tight correlations in sr ca mg na k 87sr 86sr space modeling of the magnitude of secondary carbonate precipitation and fractions of silicate derived ca mg and sr in sr ca mg 87sr 86sr space gives self consistent results that are compatible with both the calculations of mineral modes and published mg isotopic compositions if the ratio of chlorite to biotite weathering is high or if there is another silicate source of mg these calculations imply that between 12 and 31 percent of the sr and 44 and 72 percent of the mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate derived to carbonate derived cations modeling in sr ca na and or k space is inconsistent with the sr isotopic and mg isotopic constraints and we conclude that in this catchment dissolution of na and k are incongruent relative to sr ca mg potassium is preferentially retained in micas whereas the controls on na are unclear modeling of the catchments underlain by high himalayan crystalline and lesser himalayan series in garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature up to 60 percent of the sr in the catchment on high himalayan crystalline series and 20 to 30 percent of sr in the catchments on lesser himalayan series are estimated to be derived from silicates however it should be noted that the element budgets are not all self consistent and the use of bedrock element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties

palaeohydrological modelling of ace lake vestfold hills antarctica the major influences on the salinity and water level of an antarctic lake are parameterized and a palaeohydrological model linking the palaeosalinity of an antarctic lake to the palaeohydrology of the lake is developed climatic change in this lake is recorded in the evaporative loss trend reconstructed from water level and lakewater salinity estimates a decrease in salinity between 700 and 200 years bp corresponds with c 5 m increase in water level over the same time period both water level and lakewater salinity then stabilize in the last 200 years bp the palaeohydrological model derived from the changes inferred in salinity and water level shows that there is no significant change in evaporation for the last 700 years but that a lower evaporation period is evident at 150 200 years bp suggestive of a mild little ice age event in the vestfold hills

analysis of james ross island volcanic complex and sedimentary basin based on high resolution aeromagnetic data abstract high resolution aeromagnetic surveys provide a geophysical tool to help image the subsurface structure of volcanoes and their tectonic framework here we interpret high resolution aeromagnetic data and models for james ross archipelago and surrounding regions to provide a new perspective on neogene magmatism emplaced along the eastern margin of the antarctic peninsula based on the analysis and modelling of magnetic anomalies we were able to image the subglacial extent of miocene to recent alkaline rocks of the james ross island volcanic group and map tectonic structures that appear to have exerted important controls on neogene magmatism high amplitude linear anomalies detected over mount haddington stratovolcano were modelled as caused by subvertical feeder bodies extending to a depth of at least 3 km these feeder bodies may have been emplaced along a n s oriented volcano tectonic rift zone we also identified several effusive subglacial centres and imaged two concentric magnetic arcs which we related to neogene volcanic and subvolcanic lineaments likely controlled by mid cretaceous strike slip fault belts and associated deformation zones the regional magnetic quiet zone that encompasses james ross island is caused by the thick sedimentary infill of the larsen basin and a low amplitude magnetic high within the basin is inferred to reflect a basement push up structure associated with strike slip faulting along the eastern edge of the antarctic peninsula in the offshore regions magnetic anomalies southwest of tabarin peninsula and west of vega island may reflect recent volcanic structures that have yet to emerge from the sea floor

reconciling the elemental and sr isotope composition of himalayan weathering fluxes insights from the carbonate geochemistry of stream waters determining the relative proportions of silicate vs carbonate weathering in the himalaya is important for understanding atmospheric co2 consumption rates and the temporal evolution of seawater sr however recent studies have shown that major element mass balance equations attribute less co2 consumption to silicate weathering than methods utilizing ca sr and 87sr 86sr mixing equations to investigate this problem we compiled literature data providing elemental and 87sr 86sr analyses for stream waters and bedrock from tributary watersheds throughout the himalaya mountains in addition carbonate system parameters pco2 mineral saturation states were evaluated for a selected suite of stream waters the apparent discrepancy between the dominant weathering source of dissolved major elements vs sr can be reconciled in terms of carbonate mineral equilibria himalayan streams are predominantly ca2 mg2 hco3 waters derived from calcite and dolomite dissolution and mass balance calculations demonstrate that carbonate weathering contributes 87 and 76 of the dissolved ca2 and sr2 respectively however calculated ca sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and pco2 gradients in the himalaya the state of calcite and dolomite saturation was evaluated across these gradients and the data show that upon descending through the himalaya 50 of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas co2 stream water ca mg and ca sr ratios decrease as the degree of supersaturation with respect to calcite increases and mg2 ca2 and hco3 mass balances support interpretations of preferential ca2 removal by calcite precipitation on the basis of patterns of saturation state and pco2 changes calcite precipitation was estimated to remove up to 70 of the ca2 originally derived from carbonate weathering accounting for the nonconservative behavior of ca2 during riverine transport brings the ca sr and 87sr 86sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock thereby permitting consistency between elemental and sr isotope approaches to partitioning stream water solute sources these results resolve the dissolved sr2 budget and suggest that the conventional application of two component ca sr and 87sr 86sr mixing equations has overestimated silicate derived sr2 and hco3 fluxes from the himalaya in addition these findings demonstrate that integrating stream water carbonate mineral equilibria divalent cation compositional trends and sr isotope inventories provides a powerful approach for examining weathering fluxes