isotope and trace element evidence for late stage intra crustal melting in the high andes abstract purico chascon is an acid igneous complex less than 1 5 ma old rising to 5800 m in the north chilean andes and consisting of andesite dacite cones and dacite domes on an ignimbrite shield the rocks are subdivided into two groups those from chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths whereas among those at purico no xenoliths have been found 87sr 86sr 0 7095 0 7081 at purico 0 7079 0 7069 at chascon and 0 7061 0 7057 in the xenoliths from the chascon lavas 143nd 144nd 0 51222 0 51236 overall the purico lavas are characterised by higher sio2 rb sr 87sr 86sr and ree abundances and lower sr nd sr ba and143nd 144nd than most andean igneous suites there is no indication ofselective crustal contamination of sr or any systematic change in isotope ratios during differentiation nonetheless the trend of for example high sr nd and sr contents in rocks with low87sr 86sr 0 704 ecuador to low sr nd and sr and high sio2 in rocks with87sr 86sr 0 7081 0 7095 at purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity the formation of the least evolved purico lavas 60 sio2 is discussed in terms of bulk assimilation of crustal material mixing between crustal and mantle derived magmas and partial melting of pre existing crust although such models are still extremely primitive the simplest explanation of the observed chemical variations is that the purico rocks evolved from parental magmas derived by crustal anatexies thermal considerations suggest that such late stage crustal anatexis is a predictable response to crustal thickening which in the andes is thought to have taken place during the cenozoic

87sr86sr in late proterozoic carbonates evidence for a mantle event at 900 ma ago abstract strontium isotopic measurements were made on late proterozoic carbonates from west african craton comparison of samples with acceptable trace element patterns with coeval data from southern africa and with the published australian results suggests that the 87 sr 86 sr ratio of the late proterozoic sea water evolved in the following manner about 0 7075 at 1000 50 ma 0 7056 to 0 7074 at 900 50 ma 0 7068 to 0 7091 0 7106 at 800 50 ma 0 7074 to 0 7077 at 700 50 ma and 0 7076 to 0 7089 0 7096 at 600 50 ma ago the variations are comparable in magnitude and frequency to those described previously for the phanerozoic strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of sr into late proterozoic oceans was of comparable isotopic composition to its present day counterpart 0 711 consequently the non radiogenic 87 sr 86 sr value at 900 50 ma ago signifies a large flux of mantle strontium into the ocean at this time because the present time resolution is only about 75 25ma additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed late proterozoic sea water curve

an adaptive finite volume solver for ice sheets and glaciers 1 we have developed a new adaptive finite volume scheme for numerical simulations of ice sheets and glaciers the mathematical basis of the adaptive solver is represented by the finite volume method and irregular voronoi cell grids evaluation of spatial derivatives are obtained at cell edges from approximating the ice surface elevation locally by second order polynomials following a least squares approach the proposed scheme includes a simple and accurate method for handling discontinuities in ice thickness gradients along ice margins the flexibility of the voronoi cell structure allows for local mesh refinement in selected areas we show how increased mesh density along a steep ice margin may considerably increase overall numerical accuracy of an ice sheet simulation although the numerical scheme is not restricted to ice sheet representations using the shallow ice approximation sia we here use analytical sia solutions to test the accuracy of the method

depositional environment and mechanism of preservation of microfossils upper proterozoic bitter springs formation australia microfossils preserved in black chert nodules from the upper proterozoic bitter springs formation of australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds groundwaters of halite salinity pickled the microorganisms and thus inhibited their early bacterial degradation these same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions nodules and bipyramidal quartz crystals where the chert nodules formed in areas of buried cyanobacterial mats further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved

erosional feedbacks and the oscillation of ice masses 1 an analytical solution to the development of a glacier or ice sheet in a region where the underlying rock rises steadily shows that the ice mass oscillates in size independefntly of changes in the climate in response to a feedback in which ice thickness is linked to topography by ice induced erosion the rate of vertical movement of the underlying rock uplift and the parameters governing ice erosion ablation and precipitation determine the period of oscillation two dimensional numerical simulations show that the tectonic uplift rate is the most important control on the period of oscillation and that the feedback period is likely to be between 10 and 100 kyr an examination of sediment data from the deep sea drilling project shows evidence of ice mass oscillations in the chugach st elias mountains and in the himalayas with periods consistent with those produced by the proposed mechanism localized changes in glacial extent may not therefore always be the consequence of climate change and care must be taken in interpreting evidence of glacial advance and retreat

precise age of bangiomorpha pubescens dates the origin of eukaryotic photosynthesis although the geological record indicates that eukaryotes evolved by 1 9 1 4 ga their early evolution is poorly resolved taxonomically and chronologically the fossil red alga bangiomorpha pubescens is the only recognized crown group eukaryote older than ca 0 8 ga and marks the earliest known expression of extant forms of multicellularity and eukaryotic photosynthesis because it postdates the divergence between the red and green algae and the prior endosymbiotic event that gave rise to the chloroplast b pubescens is uniquely important for calibrating eukaryotic evolution however molecular clock estimates for the divergence between the red and green algae are highly variable and some analyses estimate this split to be younger than the widely inferred but poorly constrained first appearance age of 1 2 ga for b pubescens as a result many molecular clock studies reject this fossil ex post facto here we present new re os isotopic ages from sedimentary rocks that stratigraphically bracket the occurrence of b pubescens in the bylot supergroup of baffin island and revise its first appearance to 1 047 0 013 0 017 ga this date is 150 m y younger than commonly held interpretations and permits more precise estimates of early eukaryotic evolution using cross calibrated molecular clock analyses with the new fossil age we calculate that photosynthesis within the eukarya emerged ca 1 25 ga this date for primary plastid endosymbiosis serves as a benchmark for interpreting the fossil record of early eukaryotes and evaluating their role in the proterozoic biosphere

relative roles of source composition fractional crystallization and crustal contamination in the petrogenesis of andean volcanic rocks there are well established differences in the chemical and isotopic characteristics of the calc alkaline basalt andesite dacite rhyolite association of the northern n v z central c v z and southern volcanic zones s v z of the south american andes volcanic rocks of the alkaline basalt trachyte association occur within and to the east of these active volcanic zones the chemical and isotopic characteristics of the n v z basaltic andesites and andesites and the s v z basalts basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere conversely the alkaline lavas are derived from basaltic parent magmas formed from mantle of within plate character recent basaltic andesites from the cerro galan volcanic centre to the se of the c v z are derived from mantle containing both subduction zone and within plate components and have experienced assimilation and fractional crystallization a f c during uprise through the continental crust the c v z basaltic andesites are derived from mantle containing subduction zone components probably accompanied by a f c within the continental crust some c v z lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing the petrogenesis of the c v z lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation fractional crystallization and magma mixing during uprise through the continental crust

microfossils in 2000 ma old cherty stromatolites of the franceville group gabon abstract a well preserved microbial assemblage has been found in paleoproterozoic cherty stromatolites from the 2000 ma old franceville group gabon this assemblage contains coccoids and star like forms filaments and budding tubular forms these various forms are mostly of cyanobacterial affinity and among them nostocalean akinetes aerchaeoellipsoides are reported for the first time from rocks older than the mesoproterozoic the most noteworthy discovery in the franceville biota is the gunflint type association huroniospora gunflintia eoastrion archaeorestis known also without archaeorestis in other paleoproterozoic assemblaes throughout the world the present assemblage comprises 13 taxa consisting of 8 species belonging to 10 genera and 2 unnamed forms it is dominated by gunflintia filaments regarded as a mat bulding community and by huroniospora coccoids probably mat dwellers and or plankters the rare and large forms such as akinetes of archaeoellipsoides were allochthonous and presumably planktonic

landscape evolution and glaciation of the rwenzori mountains uganda insights from numerical modeling abstract the rwenzori mountains are a high alpine mountain chain about 40 80 km in size just north of the equator in the western branch of the east african rift system in africa the central part of the mountain chain is located in uganda and the highest peak the margherita peak with 5119 m lies on the border to the democratic republic of congo topography is very pronounced with steeply incised valleys and clear glacial landforms in the upper part of the mountain chain the rwenzori mountains are an unusually high mountain chain located in the extensional setting of the east african rift system and the large elevation poses a challenging problem for geodynamists to explain we have used the landscape evolution model ultima thule which combines hillslope diffusion fluvial erosion and glacial abrasion and is driven by a climate driver simulating the variations in temperature precipitation and relief over several glacial cycles with a simulation time of 800 ka we test the hypothesis of climate tectonic interactions on the uplift of the rwenzori mountains our results show that a moderate cooling of around 6 causes widespread glaciation of the high mountain regions as observed during the peak glacial phases and that morphological processes degrading the landscape allow for a tectonic uplift rate of around 0 5 mm a 1

petroleum generation and migration in the putumayo basin colombia insights from an organic geochemistry and basin modeling study in the foothills this paper discusses the results of 1d modeling and geochemical analyses of rock oil and gas samples performed in the foothills of the putumayo basin colombia a multivariate statistical assessment of biomarker data points to the existence of three oil groups two of the them similar to oil families reported in previous studies and related to the villeta formation source rocks and a novel one tentatively associated with the caballos formation source rocks geochemical data of oil and gas samples points to a complex reservoir filling history in the foothills with at least two migration pulses with distinct maturity levels vitrinite reflectance data together with modeling results shows that the source rock is practically immature in the putumayo foothills even in the footwall of large thrust faults the thermal and burial reconstruction of a pseudo well located in the present cordillera area suggests that the source rocks have started petroleum generation and expulsion during the late oligocene miocene the oil and gas expelled during such a period have migrated over 80 km reaching the present foreland area subsequent tectonic burial and or higher heat flows related to the late miocene pliocene andean paroxysm is thought to account for a second pulse of migration of thermally evolved hydrocarbons limited to the foothills area

microbial metazoan reefs of the terminal proterozoic nama group c 550 543 ma namibia thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal proterozoic nama group c 550 543 ma of central and southern namibia the reefs form integral parts of several carbonate platforms within the nama group including the kuibis platform of the northern nama basin zaris subbasin and huns platform witputs subbasin of the southern nama basin the reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes isolated patch reefs and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width in the majority of cases the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the kuibis and huns platforms this suggests that a regime of increasing accommodation was required to form well developed reefs though reefs also occur sporadically in highstand systems tract settings within a given transgressive systems tract a regime of increasing accommodation through time favours the transition from sheet like biostromal geometries to more isolated patch and pinnacle biohermal geometries similarly increasing accommodation in space such as a transect down depositional dip shows a similar transition from more sheet like geometries in updip positions to more isolated geometries in downdip positions reefal facies consist of thrombolitic domes columns and mounds with well developed internal clotted textures in addition to stromatolitic domes columns and mounds with crudely to moderately well developed internal lamination stromatolites are better developed in conditions of relatively low accommodation and updip locations under conditions of higher current velocities and greater sediment influx thrombolites are better developed in conditions of relatively high accommodation and low sediment influx both types of microbialites are intimately associated with the first calcifying metazoan organisms which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats the appearance of thrombolitic textures during terminal proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans which would have been an important process in generating clotted textures fabrics in the nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate interpreted as former aragonite this was followed by emplacement of geopetal micrite fills and precipitation of dolomite as an isopachous rim cement followed by occlusion of remaining porosity by blocky calcite spar

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

garzon massif basement tectonics structural control on evolution of petroleum systems in upper magdalena and putumayo basins colombia abstract the garzon massif is an active laramide style basement uplift flanked by the upper magdalena valley umv and the putumayo basin in this paper we use new gravity magnetic well and seismic data for the first geophysical interpretation of the garzon massif the garzon algeciras fault has been previously interpreted as a right lateral strike slip fault the new seismic well and gravity data demonstrates that the garzon fault is also a low angle 12 17 andean age fault thrusting precambrian basement 10 17 km northwestward over miocene sediments of the umv in a prospective footwall anticline the new geophysical data as well as previous field mapping were used to produce the first gravity and magnetic maps and retrodeformable structural cross section of the northern garzon massif the new model distinguishes for the first time distinct episodes of thin skinned and thick skinned deformation in the garzon massif the model indicates approximately 43 km of early to middle miocene shortening by thin skinned imbricate thrusting contemporaneous with the uplift of the nearby southern central cordillera 9 16 ma and the main hydrocarbon expulsion event for the umv and putumayo basin this was followed by at least 22 km of late miocene 3 6 ma thick skinned andean shortening and 7 km of uplift on the symmetrical garzon thrust and a se verging basement thrust fault zone the andean uplift interrupted and exposed the hydrocarbon migration pathways to the putumayo basin 3 d volume fracture analysis was used for the first time in this paper together with the first seismic and well data published for the topoyaco and miraflor structures to test closure models for the topoyaco foothills intense fracturing is observed in the topoyaco basement monocline from the near surface to depths of over 3 5 km the high level of fracturing permitted freshwater flushing and oil biodegradation and hydrocarbon escape in contrast the miraflor 1 well located just southwest of the topoyaco block tested light gravity oil and is sealed from groundwater flushing and biodegradation by a backthrust

a new surface processes model combining glacial and fluvial erosion we have developed a new surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion ice thickness and velocity are calculated under a shallow ice approximation simulation experiments in fast growing orogens comparing the efficiencies of fluvial and glacial erosion where the two are operating simultaneously over several glacial cycles show that glacial landscapes can support greater ice masses than fluvial landscapes glacial valley and lake shapes create a disequilibrium between landform and land forming process that leads to pulses of high erosion at the end of glacial periods glacial erosion rates can reach a constant value in a uniformly growing orogen and glacial erosion is capable of eroding drainage divides when the ice is moderately thick

relationship between tertiary volcanic rocks and hydrocarbons in the liaohe basin people s republic of china the liaohe basin is a tertiary rift developed on a paleohigh characterized by widespread volcanic rocks and active fault subsidence source rocks consist of mudstone of paleogene age and reservoir rocks consist of volcanic rocks sandstones of tertiary age and buried hills of mesozoic paleozoic age traps are a combination of structural and stratigraphic with drape features being dominant cap or seal rocks consist of volcanic rocks and mudstones of paleogene age source maturation and migration timing are closely linked to the volcanic episodes the higher geothermal gradient provided by magmatic activity resulted in early maturation of source rocks these hydrocarbons are locally reservoired in volcanic rocks these favorable hydrocarbon play parameters make the liaohe basin an attractive hydrocarbon province

using formation micro imaging wireline logs and onshore analogues to distinguish volcanic lithofacies in boreholes examples from palaeogene successions in the faroe shetland basin ne atlantic

climate and tectonic controls on glaciated critical taper orogens abstract in this work we present the results of a new analytical model that examines the coupling between glacial erosion and orogen development surface processes are assumed to be glacially dominated and tectonic activity is controlled by critical wedge mechanics in these circumstances we find that orogen width is strongly dependent on both the rate of accretion and on the rate of precipitation the orogen size is linked to tectonic and climate changes via proportionality constants the orogen width scales with the rate of accretion to between the 2 3rd and 2nd powers and with the rate of precipitation to the 1 3rd and 5 4th powers the value of the proportionality constants varies with the taper angle and the relative rates of ice deformation and sliding in all cases the sensitivities are higher than those calculated for fluvially eroding critical wedge orogens analytical solutions are supported by the results of a numerical flow line model the flow line model further predicts that uplift rates will be highest at and just below the equilibrium line altitude if glacial ablation is largely a calving process rock uplift in the wedge will be strongly focused towards the toe the predicted response time of glaciated orogens to changes in climate and tectonic forcing is dependent upon the constants of erosion the rheology and the rate of precipitation these analyses predict that actual orogens have variable e folding response times for example approximately 1 5 myr for the southern alps of new zealand and the olympic mountains of washington state and approximately 5 5 myr for the european alps

enigmatic fossils from the upper silurian of bolivia evidence for marine productivity in high latitude gondwana specimens collected recently from the las carmelitas facies of the kirusillas formation exposed in the cochabamba area bolivia and dated as ludlow late silurian by graptolites show bedding surfaces almost completely covered by coalified compressions and impressions the majority comprise mixtures of fragmented amorphous unidentifiable material debris but some horizons are dominated by well defined morphological entities with a particular type or combination of types confined to a particular layer in all five morphotypes have been circumscribed but their affinities based on a comprehensive review of the possibilities remain conjectural algal affinities are suggested tentatively for abundant unbranched narrow strap shaped fossils lacking any further diagnostic features and the numerous coalified discs or three dimensionally preserved spheres leiosphaerid acritarchs occurring isolated in the matrix or less often in clusters following a discussion on the recognition of faecal pellets in the fossil record it is postulated that elliptical spiral or simply segmented structures may be the excreta of planktonic or benthic metazoans although the latter seems less likely in view of the proposed anoxic depositional environment of the shales that lack any bioturbation such coalified remains whether from primary producers or grazers indicate high productivity at high palaeolatitudes in the late silurian continental seas in this region of peri gondwana

coupling glacial erosion and tectonics at active orogens a numerical modeling study 1 climate change indirectly alters the distribution of tectonic uplift at active orogens by modifying the action of surface processes which in turn alters mountain topography the impact of alpine glaciation on tectonic activity is explored here the predictions of previous analytical critical wedge models are compared with the output of a numerical model that explicitly couples rock uplift produced by convergence with surface erosion glacial and fluvial and erosive processes are calculated over a two dimensional grid which uses an ice evolution model to calculate the rate of glacial erosion due to ice sliding tectonic uplift is determined by a two dimensional finite element convergence model the simulations performed support the predictions of previous one dimensional analytical modeling of fluvially dominated orogens increasing precipitation decreases orogen width by the one quarter power the simulations show that glacially dominated landscapes have similar dependencies the total glacial erosion yield is determined to be proportional to the glacial coverage sublinearly dependent on the precipitation rate and linearly dependent on the glacial erosion constant that relates erosion with sliding speed climate also determines the distribution of tectonic uplift in the model if the equilibrium line altitude ela is high and glacial processes are limited to peak regions the center of the mountain range experiences the highest rate of rock uplift extensive glaciation increases rock uplift rates on the flanks of the range the style of glacial erosion is also important a simple glacial buzzsaw does not create the same tectonic response as models that physically simulate glacial erosion the simulations support the idea that a climate cooling of the magnitude recorded in the late cenozoic has the potential to more than double the rate of rock uplift in appropriate orogens the implications of the model are discussed for observations from the olympic mountains of washington state the southern alps of new zealand the alaskan coastal range and the southern andes

uav based detection and spatial analyses of periglacial landforms on demay point king george island south shetland islands antarctica abstract high resolution aerial images allow detailed analyses of periglacial landforms which is of particular importance in light of climate change and resulting changes in active layer thickness the aim of this study is to show possibilities of using uav based photography to perform spatial analysis of periglacial landforms on the demay point peninsula king george island and hence to supplement previous geomorphological studies of the south shetland islands photogrammetric flights were performed using a pw zoom fixed winged unmanned aircraft vehicle digital elevation models dem and maps of slope and contour lines were prepared in esri arcgis 10 3 with the spatial analyst extension and three dimensional visualizations in esri arcscene 10 3 software careful interpretation of orthophoto and dem allowed us to vectorize polygons of landforms such as i solifluction landforms solifluction sheets tongues and lobes ii scarps taluses and a protalus rampart iii patterned ground hummocks sorted circles stripes nets and labyrinths and nonsorted nets and stripes iv coastal landforms cliffs and beaches v landslides and mud flows and vi stone fields and bedrock outcrops we conclude that geomorphological studies based on commonly accessible aerial and satellite images can underestimate the spatial extent of periglacial landforms and result in incomplete inventories the pw zoom uav is well suited to gather detailed geomorphological data and can be used in spatial analysis of periglacial landforms in the western antarctic peninsula region

contact metamorphism and thermogenic gas generation in the voring and more basins offshore norway during the paleocene eocene thermal maximum voluminous volcanic intrusive activity took place in the voring and more basins at the paleocene eocene boundary at about 56 ma this event caused thermal maturation of cretaceous sedimentary rocks in the basins we have estimated the resulting thermogenic gas generation potential from contact metamorphism using numerical simulations calibrated using borehole data the borehole 6607 5 2 from the utgard sill complex in the voring basin contains two c 100 m thick sills and is used as a case study we present both new and compiled data showing that 1 the bulk organic content is reduced towards the sill intrusions 2 a c 1 km thick stratigraphic interval is thermally affected based on vitrinite reflectance data 3 relative emplacement timing can affect the gas yield by up to 25 and 4 some of the thermogenic methane is still present in the aureoles the numerical model is calibrated using data from 11 wells we estimate that the total gas generation potential for the two utgard sills equals that of the troll field c 10 gt ch 4 the largest producing gas field offshore norway we show that in the voring and more basins the total gas generation potential is up to 1500 gt ch 4 c 1100 gt c even from the relatively organic poor cretaceous source rocks with c 1 wt organic carbon with implications for the carbon cycle at the paleocene eocene boundary

andean cenozoic volcanic centers reflect basement isotopic domains isotopic data for quaternary volcanic centers from lat 17 5 to 22 s in northern chile provide evidence for crustal contributions to arc magma genesis in the central andes and delineate basement domains crustal thickness distance from trench height of volcanic edifices above the seismically active subduction zone and sediment supply to the trench are all constant along this segment of the arc the only significant variable is crustal age paleozoic in the south proterozoic in the north pb isotopic compositions of quaternary lavas in the northern sector are lower 206 pb 204 pb 17 89 18 28 and sr and o isotope ratios are less variable 87 sr 86 sr 0 7058 0 7077 18 o 6 5 8 4 than in the southern sector 206 pb 204 pb 18 59 18 79 87 sr 86 sr 0 7053 0 7083 18 o 6 4 9 3 for rocks similar in bulk composition from 53 to 74 wt sio 2 together these features are interpreted to reflect interaction of magmas with crustal contaminants of different isotopic character rather than changes in the mantle source of central andean are magmas resulting from either subduction erosion or involvement of old enriched lithosphere

development of radar glacier zones on the king george island ice cap antarctica during austral summer 1996 97 as observed in ers 2 sar data

tectonic segmentation of the north andean margin impact of the carnegie ridge collision the north andean convergent margin is a region of intense crustal deformation with six great subduction earthquakes mw 7 8 this century the regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the andes segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction this segmentation is related to heterogeneity on the subducting nazca plate in particular the influence of the carnegie ridge collision is investigated four distinct seismotectonic regions can be distinguished region 1 from 6on to 2 5on with steep ese dipping subduction and a narrow volcanic arc region 2 from 2 5on to 1os showing an intermediate depth seismic gap and a broad volcanic arc region 3 from 1os to 2os with steep ne dipping subduction and a narrow volcanic arc region 4 south of 2os with flat subduction and no modern volcanic arc the carnegie ridge has been colliding with the margin since at least 2 ma based on examination of the basement uplift signal along trench parallel transects the subducted prolongation of carnegie ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed broad volcanic arc these findings conflict with previous tectonic models suggesting that the carnegie ridge entered the trench at 1 ma furthermore the anomalous geochemical adakitic signature of the volcanoes in the broad ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant shallowly subducting carnegie ridge from segments of steep subduction in regions 1 and 3 it is further suggested that carnegie ridge supports a local flat slab segment similar to that observed in peru the impact of the carnegie ridge collision on the upper plate causes transpressional deformation extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the san andreas fault system the pattern of instrumental and historical seismicity indicates 1 great earthquakes on the northern and southern flanks of the colliding ridge 2 a slight reduction in observed seismicity at the trench ridge intersection 3 increased stress far into the continent and 4 a nne displacement of the n andes block to be further effects of the collision 1999 elsevier science b v all rights reserved

isotopic composition of late mesozoic basic and ultrabasic rocks from the andes 23 32 s implications for the andean mantle mesozoic mantle derived magmatic rocks from the jurassic early cretaceous active continental margin of n chile near 24 s and from the cretaceous rift system of nw argentina near 26 s central rift and 32 s southern rift are used to constrain the isotopic composition of mantle reservoirs before the onset of the cenozoic andean orogeny they are largely uncontaminated by assimilation of the local palaeozoic basement as seen in the major and trace element patterns and sr and nd isotopic composition of most samples but small amounts of crustal contamination are detected in the pb isotope composition two different types of mantle sources have been identified 1 for rocks from the jurassic arc now at the continental margin at 24 s and for rocks from the central rift at 26 s a depleted mantle source with 143nd 144nd t 0 5127 0 5128 87sr 86sr t ca 0 7033 may be inferred pb isotopic ratios of most samples from the magmatic arc are overall less radiogenic than those from the rift 206pb 204pb t ca 18 0 18 4 vs 18 1 19 1 207pb 204pb t 15 55 15 59 vs 15 58 15 64 and 208pb 204pb t 37 8 38 6 vs 38 4 38 9 2 in the southern rift at ca 30 32 s sierras los condores region lc and sierras chajan region ch an enriched mantle source has been identified which is similar to the composition of the mantle lithosphere beneath the brazilian shield samples from lc and ch show some isotopic differences because of varying degrees of crustal contamination in the lc samples the lc samples have a higher th u ratio than the ch samples and different pb isotopic ratios 206pb 204pb t 18 0 18 6 lc vs 17 7 and 17 8 ch 207pb 204pb t 15 50 15 66 lc vs 15 50 ch and 208pb 204pb t 38 6 39 2 lc vs ca 37 5 ch the nd isotopic ratios of lc and ch samples are similar 143nd 144nd t 0 5123 0 5122 lc vs 0 5123 ch whereas their sr isotopic ratios differ 87sr 86sr t 0 7054 0 7079 lc vs ca 0 704 ch electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00410 001 0344 3

neogene magmatism and its possible causal relationship with hydrocarbon generation in sw colombia the cretaceous oil bearing source and reservoir sedimentary succession in the putumayo basin sw colombia was intruded by gabbroic dykes and sills the petrological and geochemical character of the magmatic rocks shows calc alkaline tendency pointing to a subduction related magmatic event k ar dating of amphibole indicates a late miocene to pliocene age 6 1 0 7 ma for the igneous episode in the basin therefore we assume the intrusions to be part of the andean magmatism of the northern volcanic zone nvz the age of the intrusions has significant tectonic and economic implications because it coincides with two regional events 1 the late miocene pliocene andean orogenic uplift of most of the sub andean regions in peru ecuador and colombia and 2 a pulse of hydrocarbon generation and expulsion that has reached the gas window high la yb k nb and la nb ratios and the obtained sr nd pb isotopic compositions suggest the involvement of subducted sediments and or the assimilation of oceanic crust of the subducting slab we discuss the possibility that magma chamber s west of the basin below the cordillera did increase the heat flow in the basin causing generation and expulsion of hydrocarbons and co2

effect of anigneous intrusion on oil shale at rundle australia abstract elemental analyses x ray diffraction thermal and isotopic analyses gas chromatography mass spectrometry electron spin resonance nuclear magnetic resonance spectroscopy and demineralizing techniques have been used to evaluate the effect of a small igneous intrusion on rundle oil shale each set of results contributes to an overall appreciation of how in situ pyrolysis of a highly aliphatic kerogen occurs as in previous studies application of a range of methods gives a better understanding than any of the techniques on its own significant conclusions are as follows 1 the shale samples closest to the intrusion contain the least organic matter 2 a micro reservoir for expelled volatiles was identified 1 m below the intrusion 3 the hydrogen deficient carbonaceous residue remaining close to the intrusion produces no oil on further heating and 4 the major effect of the intrusive event was the in situ pyrolysis of the oil shale the severity of alteration decreasing with distance from the contact

exceptional preservation of fossils in an upper proterozoic shale late proterozoic organisms must have been diverse and widely distributed but in general their fossil record is both taxonomically and environmentally limited exceptional preservation of proterozoic fossils is not unknown but it is usually associated with silicified carbonates from restricted peritidal or playa lake environments1 2 we report here an exceptionally well preserved and distinctive assemblage of late proterozoic fossils from subtidal marine shales in addition to the sphaeromorphic acritarchs and cyanobacterial sheaths routinely preserved in proterozoic rocks this assemblage includes multicellular algae seaweeds a diverse assortment of morphologically complex protistan vesicles and probable heterotrophic bacteria thus it provides one of the clearest and most taxonomically varied views of proterozoic life yet reported

numerically simulating alpine landscapes the geomorphologic consequences of incorporating glacial erosion in surface process models abstract a numerical model that simulates the evolution of glaciated mountain landscapes is presented by employing a popular sliding based glacial erosion model many common glacial landforms are created the numerical model builds on earlier work as it is fully two dimensional and employs the first order forcing on mountain evolution these forcings include tectonic uplift isostasy hillslope processes fluvial processes and glacial processes a climate dependent model of ice dynamics is employed to determine ice coverage and ice flux two simulations are presented one with generic model parameters and a second with parameters that correspond to conditions in the southern alps of new zealand landforms that are produced by the model include climate climate dependent elevation lowering similar to what might be expected by a glacial buzz saw valley overdeepening terminal moraines and valley retreat the model also predicts that current rates of sedimentation are higher than the long term average and that several tens of thousands of years are required for the landscape to adjust to a change in the dominant erosional forcing therefore glaciated orogens are unlikely to achieve topographic steady state over milankovitch timescales

climate sensitivity of glaciers in southern norway application of an energy balance model to nigardsbreen hellstugubreen and alfotbreen three glaciers in southern norway with very different massbalance characteristics are studied with an energy balance model of the ice snow surface the model simulates the observed mass balance profiles in a satisfactory way and can thus be used with some confidence in a study of climate sensitivity calculated changes in equilibrium line altitude for a 1 k temperature increase are 110 108 and 135 in for nigardsbreen hellstugubreen and alfotbreen respectively the corresponding changes in mass balance averaged over the entire glacier area are 0 88 0 715 and 1 11 in year water equivalent runs with an ice flow model for nigardsbreen to which calculated mass balance profiles are imposed predict that the front will advance by 3 km for a 1 k cooling and will retreat by as much as 6 5 km for a 1 k warming the response to a 10 increase in precipitation would be a 2 km advance of the snout whereas a 4 km retreat is predicted for a 10 decrease this large sensitivity as compared to many other glaciers is to a large extent due to the geometry of nigardsbreen

effect of igneous intrusion on the organic geochemistry of a siltstone and an oil shale horizon in the midland valley of scotland abstract the effect of rapid thermal stress on the organic geochemistry of a siltstone intruded by a quartz dolerite dyke and an oil shale intruded by an alkali dolerite sill has been studied using optical petrography rock eval pyrolysis gas chromatrography and gas chromatography mass spectrometry maturity dependent ratios derived from both the aliphatic and aromatic hydrocarbon fractions corroborate the observation of raymond and murchison 1988b that the alkali dolerite sills which were intruded soon after sedimentation had a lesser effect than the later quartz dolerite intrusions the maturity parameters based on the alkylnaphthalene and alkylphenanthrene isomer ratios are more suitable for studying maturity variations in heat affected samples than those based on the commonly used aliphatic biological markers these aromatic compounds survive to higher ranks and the isomer ratios continue changing

thermal alteration of cretaceous black shale by diabase intrusions in the eastern atlantic ii effects on bitumen and kerogen abstract the thermal effects of three one major and two minor miocene diabase intrusions on cretaceous black shales from dsdp site 41 368 have been analyzed a concentration gradient was observed especially for the hydrocarbons decreasing towards the major intrusion and between the three sills the thermally altered samples in the proximity of and between the sills contained elemental sulfur and an excess of thermally derived pristane over phytane whereas the unaltered sediments contained no elemental sulfur and more phytane than pristane a maximum yield of the extractable hydrocarbons was observed at a depth of 7 m below the major sill two classes of molecular markers were present in this bitumen suite the first was sesqui di and triterpenoids and steranes which could be correlated with both terrigenous and autochthonous sources they were geologically mature and showed no significant changes due to the thermal stress the second class was found in the altered samples which contained only polynuclear aromatic hydrocarbons with low alkyl substitution and sulfur and oxygen heterocyclic aromatic compounds these compounds were derived from pyrolytic reactions during the thermal event kerogen was isolated from all of these samples but only traces of humic substances were present the h c n c 13 c 34 s and d all exhibit the expected effects of thermal stress the kerogen becomes more aromatized and richer in 13c 34s and d in the proximity of and between the sills maturation trends were also measured by the vitrinite reflectance and electron spin resonance where the thermal stress could be correlated with an elevated country rock temperature and an increased degree of aromaticity the effects of in situ thermal stress on the organic rich shales resulted in the generation and expulsion of petroliferous material from the vicinity of the sills

latitudinal variation in glacial erosion rates from patagonia and the antarctic peninsula 46 s 65 s we use extensive sedimentary and marine geophysical data to derive sediment volume based millennial time scale glacial erosion rates from glacially influenced fjords and bays across a broad latitudinal transect from central patagonia 46 s to the antarctic peninsula 65 s and to determine how glacial erosion rates change with increasing latitude and decreasing atmospheric temperatures we also calculate million year time scale erosion rates for the western antarctic peninsula cordillera and inner continental shelf from seismic stratigraphic analysis of the continental margin these results are complemented by erosion rates derived from existing thermochronology data sets apatite fission track and apatite u th he for both patagonia and the antarctic peninsula regions despite considerable regional variability our results show a clear trend of decreasing with increasing latitude millennial values span two orders of magnitude from 0 02 mm yr for illiad glacier on anvers island antarctica 64 5 s to 0 83 mm yr for san rafael glacier in northern patagonia 46 5 s regional averages are three times higher for the patagonian areas than the antarctic peninsula areas this trend is interpreted to result from a general decrease in temperature and water availability at the ice bedrock interface for the antarctic peninsula study sites erosion rates are highly clustered around 0 1 mm yr with the exception of maxwell bay for which the value is 0 36 mm yr in patagonia erosion rates are more variable than in the antarctic peninsula with ranging between 0 14 mm yr europa glacier area and 0 83 mm yr san rafael glacier area this regional variability in is interpreted as due to differences in hypsometry and bedrock resistance to erosion million year time scale values derived from thermochronology ages also decrease with latitude with maximum values decreasing from 0 9 1 1 mm yr north of 46 s to 0 1 0 2 mm yr south of 48 s in patagonia and reaching 0 2 0 3 mm yr in the antarctic peninsula the sediment based million year time scale estimates for the western antarctic peninsula cordillera indicate that glacial erosion rates increased by 25 30 after 5 3 ma from 0 09 mm yr 5 3 9 5 ma to 0 11 0 12 mm yr 5 3 ma for patagonia the decrease in long term erosion rates south of 46 s is interpreted to result from relatively long periods of slow glacial erosion associated with the ice masses having been colder subpolar on the southern patagonian cordillera and having eroded at rates comparable to those we obtained for the antarctic peninsula these long term erosion rates are 1 2 orders of magnitude lower than estimates based on recent sediment yields highlighting the transient nature of high sediment flux events however our sediment volume derived millennial time scale closely approximates the maximum values of tectonic time scale values derived from thermochronology ages our combined millennial and million year time scale glacial erosion data quantify the significant decrease in rates of glacially driven denudation at geological tectonic and millennial time scales with increasing latitude from patagonia to the antarctic peninsula highlighting the influence of climate on mountain denudation

timing of the most recent neoglacial advance and retreat in the south shetland islands antarctic peninsula insights from raised beaches and holocene uplift rates abstract the timing of the most recent neoglacial advance in the antarctic peninsula is important for establishing global climate teleconnections and providing important post glacial rebound corrections to gravity based satellite measurements of ice loss however obtaining accurate ages from terrestrial geomorphic and sedimentary indicators of the most recent neoglacial advance in antarctica has been hampered by the lack of historical records and the difficulty of dating materials in antarctica here we use a new approach to dating flights of raised beaches in the south shetland islands of the northern antarctic peninsula to bracket the age of a neoglacial advance that occurred between 1500 and 1700 ad broadly synchronous with compilations for the timing of the little ice age in the northern hemisphere our approach is based on optically stimulated luminescence of the underside of buried cobbles to obtain the age of beaches previously shown to have been deposited immediately inside and outside the moraines of the most recent neoglacial advance in addition these beaches mark the timing of an apparent change in the rate of isostatic rebound thought to be in response to the same glacial advance within the south shetland islands we use a maxwell viscoelastic model of glacial isostatic adjustment gia to determine whether the rates of uplift calculated from the raised beaches are realistic given the limited constraints on the ice advance during this most recent neoglacial advance our rebound model suggests that the subsequent melting of an additional 16 22 increase in the volume of ice within the south shetland islands would result in a subsequent uplift rate of 12 5 mm yr that lasted until 1840 ad resulting in a cumulative uplift of 2 5 m this uplift rate and magnitude are in close agreement with observed rates and magnitudes calculated from the raised beaches since the most recent neoglacial advance along the south shetland islands and falls within the range of uplift rates from similar settings such as alaska

prokaryotic and eukaryotic microfossils from a proterozoic phanerozoic transition in china cherts of the doushantuo formation in the yangtze gorges south china contain a superbly preserved heterogeneous assemblage of bacteria cyanobacteria planktonic algae submillimetre sized burrows and problematica the microbiota has a bearing on defining the precambrian cambrian boundary baltisphaeridium and associated spinose microfossils in the chert suggest an early cambrian age whereas other acritarchs vendotaenids and pretrilobite small shelly fossils from the conformably overlying the dengying formation indicate a late proterozoic age for the doushantuo this mixed benthic planktonic assemblage preserved in silicified laminated microbial mat like fabrics provides a unique sample of diverse microbiotic elements existing at the dawn of the phanerozoic and records the earliest evidence of meiofauna

sequence stratigraphy in proterozoic successions sedimentological logging and facies mapping have been used to identify depositional sequences bounded by subtle but regionally persistent unconformities in rocks of proterozoic age in the western united states south australia and northwestern canada we conclude from these studies that the sequence stratigraphic approach is of considerable importance for intrabasinal time correlation in the proterozoic and for facies interpretation and basin analysis in proterozoic rocks

a survey of paleozoic microbial fossils in chert abstract non acritarch microbial fossils from paleozoic marine cherts exhibit generally poor preservation with rare exceptional preservation microbial microfossils were discovered in 11 out of 17 stromatolitic and non stromatolitic paleozoic units containing replacement cherts from the continental united states many microfossils likely have a cyanobacterial affinity having both coccoid and filamentous morphotypes examined units range in age from middle cambrian to late permian and were deposited in peritidal open shelf and basinal settings all of which have preserved microbial assemblages cherts from the examined units generally formed during early diagenesis and the quality of microbial preservation does not seem to have changed during the paleozoic which is a period that experienced significant secular changes in the mode of marine silica formation

early neoproterozoic sequence stratigraphy and acritarch biostratigraphy eastern officer basin south australia abstract the early neoproterozoic torrensian latest tonian 800 750 ma succession in the central and eastern officer basin was deposited in an intra cratonic rift basin the outcropping pindyin sandstone consists of basal lowstand conglomerate and upward fining transgressive sandstone and is conformably overlain by marine highstand shale of the alinya formation in the giles 1 well the pindyin sandstone contains aeolian to fluvial deposits and is overlain by peritidal and sabkha sediments of the alinya formation seismic interpretation suggests that the succession is thin on the murnaroo platform and thickens toward the munyarai trough the two sedimentary provinces are separated by the ungoolya hinge which probably marks the southeastern margin of the fraser musgrave fold belt highstand organic rich shale in the alinya formation is probably an important petroleum source rock acritarchs occur in both chert and shale in the alinya formation and their distribution was probably controlled by sedimentary environments similar assemblages from outcrop and subsurface provide a reliable evidence for regional correlation the assemblages in giles 1 are dominated by benthic coccoids and contain many distinct spinose acritarchs which are particularly abundant in the subtidal grey shale whereas mainly phytoplankton is present in the chert the assemblage represented by trachyhystrichosphaera vidalii t aimica cymatiosphaeroides kullingii can be used for global correlation in the early neoproterozoic the abundant microfossils from the alinya formation and equivalent beds around the world probably indicate a microorganism explosion at 750 800 ma prior to the neoproterozoic glaciations thirty seven acritarchs spheroids and seven cyanobacterial filaments are briefly described in this study including 5 new species comasphaeridium tonium eoentophysalis gilesis goniosphaeridium alinyum gorgonisphaeridium pindyium and gorgonisphaeridium torrensium and a newly combined species archaeoellipsoides karatavicus comb nov

regional provenance from southwestern colombia fore arc and intra arc basins implications for middle to late miocene orogeny in the northern andes andean orogenic processes controlled the spatial and temporal distribution of the magmatic and sedimentary record this contribution integrates new u pb zircon ages heavy mineral analyses and biostratigraphic constraints from the neogene sedimentary record of the fore arc and intra arc basins and volcano plutonic rocks of southwestern colombia to reconstruct these orogenic processes the results reveal continuous arc magmatism since the late oligocene with a major post middle miocene magmatic peak and exhumation when integrated with other geological constraints the tectonic evolution of the margin includes eocene oligocene oblique convergence with limited magmatic activity followed by the initiation of a late oligocene early miocene arc that migrated to the east in the middle miocene when it experienced a major increase in magmatic activity crustal deformation exhumation and thickening this orogenic evolution is related to the shallowing of the slab dip due to the subduction of the neogene nazca plate

calving of fuerza aerea glacier greenwich island antarctica observed with terrestrial laser scanning and continuous video monitoring a short term series of quantitative observations of calving activity of fuerza aerea glacier greenwich island the south shetland islands antarctica was conducted in order to test new methods of monitoring calving the volume of single calving events was quantified by combining terrestrial laser scanning tls surveys with continuous video recording of the ice front an empirical formula for area volume scaling of the calved ice block was proposed based on the tls measured calved ice volume and the calved ice front area obtained by manual delineation on the images acquired with the video camera this combination of methods proves to be a valuable tool for glacier monitoring providing both high temporal resolution and precise quantitative measurements of the calving volume the size distribution of calving events is best approximated by a power law and within the short period of observations 14 d calving was found to be an intrinsic process not dependent on environmental forcings over the period of 21 january 04 february 2013 the ice flow velocity at the terminus of fuerza aerea glacier was 0 26 0 07 m d 1 and the calving rate was 0 41 0 07 m d 1

problematic bedding plane markings from the middle proterozoic manganese subgroup bangemall basin western australia abstract enigmatic bedding plane markings impressions resembling isolated strings of beads occur in sandy and argillaceous strata of the stag arrow formation in the middle proterozoic manganese subgroup bangemall group of the bangemall basin east of newman western australia markings are abundant at two localities and traces are found at a third structures similar in both morphology and dimension have previously been reported from the appekunny formation in the middle proterozoic belt supergroup of u s a the manganese subgroup structures provide further details of morphology enabling comparison with a variety of biogenic and non biogenic structures a source for the markings still cannot be determined with certainty but a biogenic origin is favoured and affinities with the green or brown algae suggested such an origin is consistent with records of megascopic algae from rocks of similar age

hydrocarbon reservoirs in a trachyte porphyry intrusion in the eastern depression of the liaohe basin northeast china hydrocarbon reservoirs in the oulituozi area of the liaohe basin northeastern china occur within a subvolcanic trachyte porphyry intrusion this intrusion was emplaced at a depth of less than 1 km during the paleogene the prevalent reservoir porosity is in explosive breccias fracture networks steeply dipping cracks and dilational microfractures in the apical zone of the intrusion these voids were formed by underground explosion during magma solidification and by postsolidification volume contraction minor oil and gas volumes are trapped also in fractures in the overlying brittle beds the massive rocks in the lower part of the porphyry body serve as hydrocarbon seals for reservoir beds lateral to the intrusion we propose a model for reservoir formation related to a subvolcanic intrusion that includes three types of reservoirs 1 reservoirs in the apical zone of the intrusion 2 reservoirs in the upper wall of the intrusion and 3 reservoirs lateral to the intrusion attention should be paid to all the three reservoir types during exploration around a subvolcanic intrusion

patterns of phytoplankton radiation across the precambrian cambrian boundary late proterozoic and early phanerozoic phytoplankton comprise a wide array of microfossils of possibly bacterial and algal affinity their radiation patterns around the proterozoic cambrian boundary interval are becoming increasingly well known for numerous sequences in the east european and siberian platforms china baltoscandia and north america transgressive sequences in the east european platform and northern baltoscandia comprise alternating sandstones mudstones and claystones their sedimentology is well documented and microfossils acritarchs chitinous fossils sabelliditids shelly fossils and trace fossils are well represented rocks immediately overlying the transitional beds have yielded lower cambrian trilobite faunas that are utilized for regional correlations continuous successions of planktic microorganisms consisting of prokaryotic photoautotrophic cyanobacteria and algal protists are known from rocks comprising the transitional vendian cambrian sequence in the east european platform the rock sequence attributed to the lower cambrian platysolenites antiquissimus zone has yielded acritarchs of 9cambrian type9 abruptly replacing a morphologically simple proterozoic type microbiota recorded in beds of the upper proterozoic sabellidites vendotaenia zone the radiation of novel forms of phytoplankton appears to have been synchronous over a wide part of the late proterozoicarly cambrian shelf and is parallelled by the nearly contemporaneous emergence of cambrian shelly fossils and trace fossils along with new acritarch taxa the diagnostic acritarch assemblage of the platysolenites antiquissimus zone consists of widely distributed species occurring also in the lowermost cambrian deposits of the siberian platform baltoscandia greenland north america svalbard and scotland it is suggested that along with shelly faunas trace fossils and geochemical data the general radiation patterns of phytoplankton shed light on the events shaping the important biotic events around the precambrian cambrian boundary it is also believed that a better understanding of the regulatory mechanisms affecting the base of trophic chains may in turn be of significant importance in defining the precambrian cambrian boundary

latest proterozoic plankton from the amadeus basin in central australia the plankton of the proterozoic is preserved mainly as microfossils known as acritarchs most of which are considered to be cysts of eukaryotic algae in recent studies of the diversity of proterozoic and early palaeozoic acritarchs a gradual increase in diversity from the middle into the late proterozoic has been shown to be followed by a sharp decrease in diversity in the latest proterozoic ediacarian or vendian and then a rise again in the early cambrian1 2 these observations have been interpreted in terms of the evolutionary diversification of eukaryotic algae and the ecological effects of the pre ediacarian glaciation occasional discoveries of complex acritarchs in ediacarian rocks3 6 have raised some doubts about these interpretations but more recent work seems to have supported this view7 here we report the discovery of a diverse assemblage of large and morphologically complex acritarchs from the ediacarian age upper pertatataka formation in the amadeus basin of central australia this with a recent report of similar fossils from the upper sinian of china8 provides a significant new perspective on the history of the plankton it is now necessary to suggest an earlier radiation at least in off shore environments or to question the reality of the postulated decrease in diversity in addition there may well have been an extinction event late in the ediacarian neither of these phenomena has been recognized previously

surface velocity and ice discharge of the ice cap on king george island antarctica glaciers on king george island antarctica have shown retreat and surface lowering in recent decades concurrent with increasing air temperatures a large portion of the glacier perimeter is ocean terminating suggesting possible large mass losses due to calving and submarine melting here we estimate the ice discharge into the ocean for the king george island ice cap l band synthetic aperture radar images covering the time span january 2008 to january 2011 over king george island are processed using an intensity tracking algorithm to obtain surface velocity measurements pixel offsets from 40 pairs of radar images are analysed and inverted to estimate a weighted average surface velocity field ice thicknesses are derived from simple principles of ice flow mechanics using the computed surface velocity fields and in situ thickness data the maximum ice surface speeds reach 225 m a 1 and the total ice discharge for the analysed flux gates of king george island is estimated to be 0 720 0 428 gt a 1 corresponding to a specific mass loss of 0 64 0 38 m w e a 1 over the area of the entire ice cap 1127 km 2

eruptive environment of volcanism on brabant island evidence for thin wet based ice in northern antarctic peninsula during the late quaternary terrestrial volcanism occurred extensively on brabant island northern antarctic peninsula during the late quaternary 200 ka probably entirely late pleistocene two compositionally distinct volcanic sequences formed three large shield volcanoes the volcanoes were constructed in association with a 150 m thick non ice stream glacial cover although it was likely to be thicker few hundred metres where it extended onto the continental shelf the glacial cover on the volcanoes was probably mainly permeable snow firn and fractured crevassed ice it was wet based erosive and sediment forming indicating either a temperate or perhaps more likely sub polar thermal regime and would have been more extensive than that present today such a thin ice sheet would have had a low surface gradient during periods of extension across the continental shelf this is the first direct evidence for the thickness and thermal regime of the terrestrial ice sheet in the antarctic peninsula during any quaternary period prior to the last glacial maximum

climate sensitivity of the ice cap of king george island south shetland islands antarctica a two dimensiona l vertically integrated ice flow model has been used to simulate the current state of the ice cap of king george island south shetland islands antarctica as well as the sensitivity of this state to climate change the model was forced by an energy balance model that generates the specific mass balance from climatological input data of two research stations it proved difficult to simulate satisfactorily the entire geometry of the present day ice cap nevertheless it was possible to simulate a steady state ice cap whose volume and areal extent approximate the estimated current situation several experiments have indicated that this state is highly sensitive to climate change the model predicts that cooling by l k will increase the ice volume by 10 and warming by l k will decrease it by 36 a 10 change in precipitation will alter the ice volume by less than 8 application of the ipcc 90 business as usual scenario leads to a 55 reduction in the ice volume by the year ad 2100 compared to the present day situation the response of the ice cap to warming is therefore totally different from the response of the main antarctic ice sheet which is believed to gain mass by increasing temperatures

paleogene igneous intrusion and its effect on thermal maturity of organic rich mudstones in the beibuwan basin south china sea abstract the seismic drilling and logging data reveal that a large scale igneous intrusion with a width of 14 km and a maximum thickness of 170 m intruded within the paleogene liushagang formation in the fushan depression beibuwan basin south china sea scs in this study we report the geochemistry and sr nd pb hf isotopic compositions of the igneous rocks and evaluate the thermal effect induced by this large scale igneous intrusion on the host rocks the analyzed igneous samples exhibit strong enrichment in light rare earth elements lree and large ion lithophile elements lile having characteristics similar to intra plate oceanic island basalts oib the sr nd pb hf isotopic data display narrow ranges e g 87sr 86sri 0 7042 0 7044 143nd 144ndi 0 5128 0 5129 206pb 204pbi 18 90 18 94 ehf t 7 56 9 60 geochemical and isotopic compositions suggest a mixed mantle source between depleted mid ocean ridge basalt morb mantle dmm like mantle and enriched mantle type ii emii possibly the hainan mantle plume vitrinite reflectance values major mineralogical changes together with a maturity related biomarker ts ts tm all reveal significant thermal effect induced by the igneous intrusion vitrinite reflectance values of the host rock are up to 2 5 in the intrusion region whereas lower reflectance values 0 62 0 73 occur in the unaffected area of the same strata moreover our results suggest that the host rocks above the igneous intrusion are characterised by higher maturity than below which should be attributed to the different behavior of hydrothermal fluids these observations suggest that the thermal effect of large scale thick igneous intrusions is much more intense than that of thin igneous intrusions and the behavior of hydrothermal fluids induced by magmatic intrusive events should be a critical impact factor during heat transfer process

ice thickness areal and volumetric changes of davies dome and whisky glacier james ross island antarctic peninsula in 1979 2006 this study calculates area volume and elevation changes of two glaciers on james ross island antarctica during the period 1979 2006 davies dome is a small ice cap whisky glacier is a valley glacier ground penetrating radar surveys indicate ice thickness which was used for calculations of the bed topography and volume of both glaciers maximum measured ice thicknesses of davies dome and whisky glacier are 83 2 and 157 2 m respectively between 1979 and 2006 the area of the ice cap decreased from 6 23 0 05 km2 to 4 94 0 01 km2 20 7 while the area of the valley glacier reduced from 2 69 0 02 km 2 to 2 40 0 01 km2 10 6 over the same period the volume of the ice cap and valley glacier reduced from 0 23 0 03 km3 to 0 16 0 02 km3 30 4 and from 0 27 0 02 km3 to 0 24 0 01 km3 10 6 respectively the mean surface elevation decreased by 8 5 2 8 and 10 1 2 8 m the average areal 0 048 0 011 km2 a 1 and volumetric 0 003 0 001 km3 a 1 changes are higher than the majority of other estimates from antarctic peninsula glaciers