geochemical modelling of the principal source rocks of the barinas and maracaibo basins western venezuela this study presents a geochemical modelling of the principal source rocks in the western venezuelan basins the area covers more than 100 000 km sup 2 and includes lake maracaibo and barinas basins the geochemical modelling recognizes three source rocks 1 a principal k3 k4 k5 cretaceous sequences represented by la luna capacho and navay formations 2 a secondary corresponding to the t4 oligocene sequence represent by the carbonera formation and 3 an accessory source rock k7 k8 paleocene sequences represented by the carbonaceous shales and coals of the orocue group and marcelina formation three periods of hydrocarbon expulsion were defined for la luna formation early eocene late eocene middle miocene early miocene and early miocene holocene and a principal period of hydrocarbon expulsion for orocue group and carbonera formation plio pleistocene and middle miocene plio pleistocene the 90 of hydrocarbons generated correspond to the principal source rock la luna formation and the 10 to tertiary source rocks carbonera formation and orocue group five petroleum systems were identified lake maracaibo southwest of the lake maracaibo basin the lara nappes the extensive basins of eastern zulia and the barinas subbasin

antarctic ice sheet melting in the southeast pacific the first oceanographic measurements across a deep channel beneath the calving front of pine island glacier reveal a sub ice circulation driven by basal melting of 10 12 m yr 1 a salt box model described here gives a melt rate similar to that of ice balance and numerical models 5 50 times higher than averages for the george vi and ross ice shelves melting is fueled by relatively warm circumpolar deep water that floods the deep floor of the amundsen and bellingshausen sea continental shelves reaching the deep draft of this floating glacier a revised melt rate for ice shelves in the southeast pacific sector raises circumpolar ice shelf melting to 756 gt yr 1 given prior estimates of surface accumulation and iceberg calving this suggests that the antarctic ice sheet is currently losing mass to the ocean

evidence for mixed and biodegraded crude oils in the socororo field eastern venezuela basin this work presents a geochemical study of crude oils from the socororo field socororo major area in the maturin sub basin of the eastern venezuelan basin the v ni total concentrations and v v ni ratios suggest the presence of two oil groups these oils are heavy aromatic asphaltic and according to their sulfur content api gravity and hydrocarbon compositions fall into the same two groups established from the v and ni proxies although the analyzed crude oils show evidence of biodegradation it was possible to determine based on various biomarkers parameters that the oils originated from marine organic matter in a marine carbonate or marine marl source rock deposited under anoxic to suboxic conditions the oils were generated near the peak of the oil window distributions of n alkanes isoprenoids hopanes and steranes combined with a prominent unresolved complex mixture ucm indicate that the oils are biodegraded moreover the presence of pristane and phytane along with alteration of hopanes and steranes and the presence of 25 norhopanes reveals that these oils are mixtures showing different levels of biodegradation sulfur content v and ni concentration and saturate aromatic resin and asphaltene sara composition provide additional evidence for oil mixing such mixed oils originate from several episodes of charging and biodegradation in the reservoirs the first charge was severely biodegraded and many biomarkers susceptible to degradation n alkanes acyclic isoprenoids terpanes methyl hopanes methyl steranes and steranes were extensively altered or completely removed the second oil charge was subjected to a lower degree of biodegradation and only the n alkanes show significant biodegradation both oil charges seem to come from the same source rock which was deposited under variable redox conditions moreover due to their similar maturity the generation and migration of these oils to reservoirs likely occurred close together in time

marine 187os 188os isotope stratigraphy reveals the interaction of volcanism and ocean circulation during oceanic anoxic event 2 high resolution osmium os isotope stratigraphy across the cenomanian turonian boundary interval from 6 sections for four transcontinental settings has produced a record of seawater chemistry that demonstrates regional variability as a function of terrestrial and hydrothermal inputs revealing the impact of palaeoenvironmental processes in every section the 187os 188os profiles show a comparable trend radiogenic values in the lead up to oceanic anoxic event 2 oae 2 an abrupt unradiogenic trend at the onset of oae 2 an unradiogenic interval during the first part of oae 2 and a return to radiogenic values towards the end of the event above the cenomanian turonian boundary the unradiogenic trend in 187os 188os is synchronous in all sections previous work suggests that activity of the caribbean lip large igneous province was the source of unradiogenic os across the oae 2 and possibly an instigator of anoxia in the oceans here we assess this hypothesis and consider the influence of activity from other lips such as the high arctic lip a brief shift to high radiogenic 187os 188os values occurred in the western interior seaway before the onset of oae 2 we evaluate this trend and suggest that a combination of factors collectively played critical roles in the initiation of oae 2 differential input of nutrients from continental and volcanogenic sources coupled with efficient palaeocirculation of the global ocean and epeiric seas enhanced productivity due to higher nutrient availability which permitted penecontemporaneous transport of continental and lip derived nutrients to trans equatorial basins

study of the snow melt freeze cycle using multi sensor data and snow modelling the melt cycle of snow is investigated by combining ground based microwave radiometric measurements with conventional and meteorological data and by using a hydrological snow model measurements at 2000 m a s l in the basin of the cor devole river in the eastern italian alps confirm the high sensitivity of microwave emission at 19 and 37 ghz to the snow melt freeze cycle while the brightness at 6 8 ghz is mostly related to underlying soil simulations of snowpack changes performed by means of hydrological and electromagnetic models driven with meteorological and snow data provide additional insight into these processes and contribute to the interpretation of the experimental data

plio pleistocene diatom biostratigraphy from odp leg 177 atlantic sector of the southern ocean abstract seven sites were drilled during ocean drilling program leg 177 in the atlantic sector of the southern ocean so on a transect over the antarctic circumpolar current from the subantarctic to the antarctic zone at four sites sediments were recovered with a pliocene pleistocene sediment package of up to 580 m allowing the refinement of previous diatom zonation concepts samples were analyzed on stratigraphic distribution and abundance of diatom species a refined diatom biozonation tied to the geomagnetic polarity record is proposed for the middle and late pleistocene two zonations applicable to the northern and southern area of the so were constructed considering different latitudinal distributions of biostratigraphic diatom marker species the southern zonation for the pleistocene relies on the occurrence of species of the genus rouxia r leventerae and r constricta n sp as well as on a revised last occurrence datum lod of actinocyclus ingens 0 38 ma late marine isotope stage mis 11 the use of these new stratigraphic marker species refines the temporal resolution for biostratigraphic age assignment to up to 0 1 myr in particular the lod of r leventerae as an indicator for the mis 6 5 boundary termination ii will improve future dating of carbonate free antarctic sediments these new data were obtained from sediments of sites 1093 and 1094 antarctic zone the northern zonation for the middle and late pleistocene time interval is based on the pleistocene abundance pattern of hemidiscus karstenii which was already proposed by previous investigations e g gersonde and barcena 1998 one new species r constricta and two new combinations fragilariopsis clementia fragilariopsis reinholdii are proposed in this study

spatial and temporal evolution of pine island glacier thinning 1995 2006 we use ers 2 and envisat satellite radar altimetry to examine spatial and temporal changes in the rate of thinning of the pine island glacier west antarctica during the period 1995 to 2006 we show that the pattern of thinning has both accelerated and spread inland to encompass tributaries flowing into the central trunk of the glacier within the 5 400 km 2 central trunk the average rate of volume loss quadrupled from 2 6 0 3 km 3 yr 1 in 1995 to 10 1 0 3 km 3 yr 1 in 2006 the region of lightly grounded ice at the glacier terminus is extending upstream and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow such as the effects of ocean driven melting if the acceleration continues at its present rate the main trunk of pig will be afloat within some 100 years six times sooner than anticipated citation wingham d j d w wallis and a shepherd 2009 spatial and temporal evolution of pine island glacier thinning 1995 2006 geophys res lett 36 l17501 doi 10 1029 2009gl039126

amundsen sea ice production and transport 1 drift and variability of sea ice in the amundsen sea are investigated with ice buoys deployed in march 2000 and a coupled ice ocean model the bremerhaven regional ice ocean simulations brios model results are compared with in situ ocean atmosphere and sea ice measurements satellite observations and 8 19 months of buoy drift data we identify a zone of coastal westward drift and a band of faster eastward drift separated by a broad transition region characterized by variable ice motions the model represents drift events at scales approaching its resolution but is limited at smaller scales and by deficiencies in the national centers for environmental prediction forcing two thirds of the modeled ice production in the southern amundsen moves westward near the coast its transport modulated by meridional wind strength damping sea ice formation in the eastern ross sea half of the ice exported from the ross moves eastward into the northern amundsen sea a net sea ice sink that also receives more than one third of the ice generated to its south a low rate of exchange occurs with the bellingshausen sea which must have a more independent ice regime snow ice formation resulting from high precipitation accounts for one quarter of the ice volume in the amundsen sea aiding the formation of thick ice in a region with generally divergent ice drift freshwater extraction by sea ice formation is roughly balanced by precipitation and ice shelf melting but a positive trend in the surface flux is consistent with an amundsen source for reported freshening in the ross sea

surface melting derived from microwave radiometers a climatic indicator in antarctica this paper aims at presenting a new dataset including the melt events derived from microwave remote sensing occurring in antarctica from summer 1979 80 to 2005 06 the method for detecting melt events and sources of error is presented and then trends in melt duration for every pixel are extracted from the dataset mapped and analyzed the analysis focuses on two particular cases and the main results show that 1 the trends over the period 1980 2006 in the antarctic peninsula match with lengthening of the melt season on the ice shelves and surprisingly shortening of the melt season in the mountainous area of the peninsula and 2 the trends over the period 1996 2006 on the entire continent show a dipolar pattern with the western regions experiencing decreasing melt duration whereas east antarctica and the ross ice shelf experience increasing melt duration this pattern closely mirrors the temperature pattern expected when the southern annular mode is in a decreasing trend as it is over the period 1996 2006 for further analysis and validation the dataset has been made available at http www lgge obs ujf grenoble fr picard melting

mid late pleistocene glacimarine sedimentary processes of a high latitude deep sea sediment drift antarctic peninsula pacific margin the effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the antarctic peninsula pacific margin nineteen sediment cores were visually described logged for magnetic susceptibility and x radiographed about 1000 analyses were performed for grain size clay minerals and biostratigraphy foraminifera nannofossils and diatoms four sediment types associated with distinct sedimentary processes are recognised based on textural compositional analysis 1 hemipelagic mud forms the bulk of the interglacial sediment and accumulated from the pelagic settling of bioclasts and ice rafted wind transported detritus 2 terrigenous mud forms the bulk of the glacial sediment and accumulated from a combination of sedimentary processes including turbidity currents turbid plumes and bottom current reworking of nepheloid layers 3 silty deposits occurring as laminated layers and lenses represent the lateral spillout of low density turbidity currents 4 lastly glacial interglacial gravelly mud layers derive from settling of ice rafted detritus five depositional settings are interpreted within sediment drift 7 each characterised by the dominance interaction of one or several depositional processes the repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages glaciation glacial deglaciation and interglacial each one characterised by a distinctive clay mineral assemblage and bioclastic content variations in clay mineral assemblage within interglacial stage 5 core sed 06 suggest minor colder climatic fluctuations possibly correlatable with substages 5a to 5e

re os geochronology for the nist rm 8505 crude oil the importance of analytical protocol and uncertainty abstract the national institute of science and technology nist research material rm 8505 is a venezuelan crude oil certified for its vanadium concentration it has recently been proposed as a matrix matched standard for interlaboratory comparison of re os isotopic analyses while serving as a benchmark for petroleum based re os method development here the nist rm 8505 oil is characterized using a multi solvent asphaltene precipitation permitting interlaboratory re os data comparison and evaluation of techniques used to derive re os isochron ages we present a detailed discussion of analytical blank sources critical evaluation of analytical uncertainties method improvements and data corrections additionally we provide a comprehensive discussion on the effect of data population size and analytical uncertainty on isochron statistics and the robustness of isochron ages in addition to nist rm 8505 2 43 0 15 ng g re 30 0 0 7 pg g os we provide the first re os data for nist srm 1634c 1 73 0 01 ng g re 32 5 0 8 pg g os and enviromat scp science 39 1 pg g re 1 01 0 02 pg g os two other commercially available hydrocarbon trace metal standards the application of several asphaltene asph precipitations using different n alkane solvents pentane heptane and decane for the nist rm 8505 significantly improved the spread of data in the 187re 188os 187os 188os space multi solvent asphaltene precipitations yield an asphaltene only model 1 isochron 75 24 ma and the maltene crude oil asphaltene triplets yield a more precise model 1 isochron age of 78 5 2 3 ma with a 187os 188osinitial of 0 877 0 021 our age is considerably older compared to a previously reported model 1 age of 62 7 5 7 ma based on c7 asph crude oil malt triplets liu and selby 2017 and considerably younger than the model 1 age of 98 4 9 5 ma based on asph separates liu et al 2019 the analytical reasons leading to these age differences are discussed in this paper the geological context for the nist 8505 oil is unknown therefore we use trace metal data ni v fe mo etc to derive plausible constraints for the origin of this oil the nist 8505 oil shows trace metal data similar to published data for oils from the urdaneta district the la paz mara and lagunillas oil fields and source rocks of the la luna formation in the maracaibo basin venezuela in this geological context our 79 ma age for the nist 8505 oil implies a late cretaceous early hydrocarbon generation event from older upper cretaceous source rocks and overlaps with the youngest age of potential source rocks in the maracaibo basin however the age of large scale oil generation in the maracaibo basin is significantly younger lasting from the oligocene to the pliocene

modelling circumpolar deep water intrusions on the amundsen sea continental shelf antarctica results are presented from an isopycnic coordinate model of ocean circulation in the amundsen sea focusing on the delivery of circumpolar deep water cdw to the inner continental shelf around pine island bay the warmest waters to reach this region are channeled through a submarine trough accessed via bathymetric irregularities along the shelf break temporal variability in the influx of cdw is related to regional wind forcing easterly winds over the shelf edge change to westerlies when the amundsen sea low migrates west and south in winter spring this drives seasonal on shelf flow while inter annual changes in the wind forcing lead to inflow variability on a decadal timescale a modelled period of warming following low cdw influx in the late 1980 s and early 1990 s coincides with a period of observed thinning and acceleration of pine island glacier

re os geochronology and fingerprinting of united kingdom atlantic margin oil temporal implications for regional petroleum systems we demonstrate that the re os system can be used to understand the temporal evolution of a petroleum system as well as aid in the identification of source units traditional geochemical methods high performance liquid chromatography carbon isotope gas chromatography and biomarker analysis indicate that the main source of 18 oils analyzed for re os geochronology from the united kingdom atlantic margin clair schiehallion cuillin and foinaven fields is late jurassic marine shales this is supported by the os isotopic composition 187 os 188 os of the late jurassic source 0 9 2 4 at the timing of oil generation being similar to that of the oil 68 ma 0 92 1 12 indicating that the re os system can potentially be used to fingerprint the source of an oil the re os data for the 18 oils yield an age 68 13 ma that agrees with both the relative basin models and absolute ar ar geochronology timing of oil generation indicating that that re os oil geochronology records oil generation events

surface lithofacies biofacies and diatom diversity patterns as models for delineation of climatic change in the southeast atlantic ocean abstract distribution of diatom species in surface sediments of the southeast atlantic ocean is regulated by present day oceanographic and hydrodynamic processes five assemblages vectors defined by factor vector analysis reflect different environments and conditions assemblage a is a high diversity flora associated with the nutrient rich relatively cold waters south of the polar front in the diatom ooze belt occurrence of this flora on maud rise and vicinity may reflect periodic occurrence of a polynya in that area assemblage b is a reworked assemblage that dominates the southern portion of the study area productivity there is low reflecting sea ice cover during most of the year the area north of the polar front is dominated by assemblage c whose characteristic species reflect the relatively warm subantarctic surface water winnowing and frustule breakage have altered assemblage d found in three isolated samples by removing relatively delicate forms leaving lag deposits of more robust species assemblage e is a low diversity stress flora reflecting unstable unpredictable environments along the polar front antarctic slope and the northern boundary of winter sea ice these sites are characterized by the sinking of cold water downcore analysis of cores lying adjacent to the polar front and the diatom ooze pelagic clay boundary show evidence for past climatic variation the low trophic level occupied by diatoms and the subsequent sensitivity of these organisms to abiotic environmental parameters such as light quality make relative diatom abundance a useful tool for monitoring fluctuations of winter sea ice and the temperature changes responsible for these fluctuations the position of the polar front has migrated at least three times within the last 0 015 m y b p within the last 0 3 m y b p warm maxima have occurred at approximately 0 0 0 015 0 125 and 0 3 m y b p

improved estimation of the mass balance of glaciers draining into the amundsen sea sector of west antarctica from the cecs nasa 2002 campaign in november december 2002 a joint airborne experiment by centro de estudios cientificos and nasa flew over the antarctic ice sheet to collect laser altimetry and radio echo sounding data over glaciers flowing into the amundsen sea a p 3 aircraft on loan from the chilean navy made four flights over pine island thwaites pope smith and kohler glaciers with each flight yielding 1 5 2 hours of data the thickness measurements reveal that these glaciers flow into deep troughs which extend far inland implying a high potential for rapid retreat interferometric synthetic aperture radar data insar and satellite altimetry data from the european remote sensing satellites ers 1 2 show rapid grounding line retreat and ice thinning of these glaciers using the new thickness data we have re evaluated glacier fluxes and the present state of mass balance which was previously estimated using ice thicknesses deduced largely from inversion of elevation data assuming hydrostatic equilibrium the revised total ice discharge of 241 5 km 3 a 1 exceeds snow accumulation by 81 17 km 3 a 1 of ice equivalent to a sea level rise of 0 21 0 04 mm a 1 this magnitude of ice loss is too large to be caused by atmospheric forcing and implies dynamic thinning of the glaciers this is confirmed by ice flow acceleration observed with insar we attribute the flow acceleration and ice thinning to enhanced bottom melting of the ice shelves by a warmer ocean which reduces buttressing of the glaciers and in turn accelerates them out of balance

biostratigraphic characterization and quaternary microfossil palaeoecology in sediment drifts west of the antarctic peninsula implications for cyclic glacial interglacial deposition abstract the sedano project recovered 19 gravity cores on sediment drifts from the pacific continental margin of the antarctic peninsula camerlenghi et al 1997a high resolution terrigenous sedimentary record of the sediment drifts on the antarctic peninsula pacific margin in ricci c a ed the antarctic region museo nazionale dell antartide siena pp 705 710 fifteen cores were sampled with the aim of developing an integrated biostratigraphy and palaeoecology based on calcareous nannofossils diatoms planktonic and benthic foraminifera framed in a depositional process reconstruction barren gray laminated and brown bioturbated hemipelagic sediments characterize glacial and interglacial cycles respectively analyses from both intervals allow comparison between microfossil occurrence in glacial and interglacial cycles the unit boundaries were drawn more accurately by means of the microfossil distribution on the basis of micropalaeontological and sedimentological evidence interglacial unit c is correlated to oxygen isotope stage 5 thus dating the unit boundaries at 127 and 70 ka peaks in diatom abundance correlate well with interglacial units and indicate high productivity and an open ocean environment a calcareous nannofossil cold taxa association is present in most cores examined and its consistent distribution within interglacial unit c indicates key environmental relationships the occurrence of calcareous nannofossils has been related to temperature tolerance sea ice cover reduction nutrient availability and factors limiting primary productivity our results confirm that coccolithophorids occurred at southern high latitudes in the western marginal basins of the antarctic peninsula during short periods of the late quaternary a foraminiferal assemblage made up of sinistral neogloboquadrina pachyderma and few benthics occurs only in interglacial units correlation of microfossil occurrences with climatic cycles adds information on their palaeoecology and palaeoproductivity in the southern high latitudes

vanadium nickel and sulfur in crude oils and source rocks and their relationship with biomarkers implications for the origin of crude oils in venezuelan basins abstract this work presents a study of vanadium nickel and sulfur concentrations and biomarkers in a suite of crude oils and source rocks from three venezuelan basins maracaibo and eastern basins and barinas sub basin crude oils range from unaltered to altered by biodegradation and source rocks are characterized by having different kerogen types type ii iii or iv and maturity levels early mature to post mature vanadium nickel and sulfur concentrations v ni or v v ni and saturate biomarkers were used to classify the oils and source rocks based on inferred paleo redox environment lithology and maturity of the source rock oils are classified into five groups based on v ni ratios they appear to be related to variations in the paleo redox environment from suboxic dysoxic to anoxic suboxic of source rocks with variable lithological composition and organic matter input regardless of their maturity or biodegradation level these five groups are also classified based on biomarkers related to maturity organic matter type paleo redox environment and source rock lithology in source rocks vanadium nickel and sulfur concentrations together with v ni or v v ni ratios provide information about the paleo redox environment and are related to lithology regardless of their maturity these results also indicate variable paleo redox conditions during the sedimentation of venezuelan source rocks all this suggests that while the main source rocks in venezuelan basins are well established there are still uncertainties regarding their lateral and vertical variations in organic and inorganic facies and paleo redox conditions the presence of other source rocks contributing to the accumulation of hydrocarbons in the venezuela basins is also a possibility

plio pleistocene climate evolution trends and transitions in glacial cycle dynamics abstract we describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5 3 myr in particular we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic 18 o when the exponential trend in 18 o variance is removed its spectral power exhibits strong proportional responses to amplitude modulations in orbital forcing over most of the plio pleistocene precession responses correlate with modulations in forcing for the last 5 myr but 41 kyr response is sensitive to obliquity modulation only before 1 4 myr where responses are sensitive to modulations in forcing we demonstrate that glacial cycles are orbitally forced rather than being self sustained or paced by orbital changes the shapes of glacial cycles have several nonlinear properties which may be indicative of glacial interglacial differences in climate sensitivity or response time the saw tooth asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation and the relative duration of interglacial stages decreases at 1 4 myr collectively trends in the shape of glacial cycles and the sensitivity of 18 o to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2 5 and 1 4 myr but show no significant change associated with the appearance of strong 100 kyr cycles during the mid pleistocene transition

subglacial bedforms reveal complex basal regime in a zone of paleo ice stream convergence amundsen sea embayment west antarctica the flow of ice streams which account for most discharge from large ice sheets is controlled by processes operating at the ice stream bed data from modern ice stream beds are difficult to obtain but where ice advanced onto continental shelves during glacial periods extensive areas of the former bed can be imaged using modern swath sonar tools we present new multibeam swath bathymetry data analyzed alongside sparse preexisting data from the amundsen sea embayment the compilation is the most extensive continuous area of multi beam data coverage yet obtained on the inner continental shelf of antarctica the data reveal streamlined subglacial bedforms that define a zone of paleo ice stream convergence but in contrast to previous models do not show a simple downflow progression of bedform types along paleo ice stream troughs we interpret high spatial variability of bedforms as indicating a complex mechanical and hydrodynamic regime at the former ice stream beds consistent with observations from some modern ice streams we conclude that care must be taken when using bedforms to infer paleo ice stream velocities

comprehensive evolution of a petroleum system in absolute time the example of brynhild norwegian north sea abstract petroleum systems are inherently complex comprising multiple components and processes which variably interact in space and time to form petroleum accumulations improved understanding of these complex systems requires a holistic approach of integrating diverse geological geophysical and organic and inorganic geochemical data here we present an applied and comprehensive re os geochronological and geochemical study on the brynhild petroleum system from the norwegian north sea essential components of this system include shale source rocks of the mandal and farsund formations reservoir sandstones of the ula formation and their bitumen extracts and produced brynhild crude oil rock eval trace metal and re os data reveal the excellent oil generation potential of the shaley source rocks high contents of organic matter containing large amounts of type i kerogens and constrain their depositional conditions anoxic bottom waters in a high productivity regime driven by enhanced supply of land derived nutrients seven re os shale ages between 146 and 134 ma are consistent with existing volgian ryazanian age estimates and constrain the initial 187os 188os of the shales to 0 52 in the lowermost farsund formation and 0 75 in the uppermost mandal formation solvent extracted reservoir rocks of the ula formation contain insoluble organic matter with type iii type iv kerogens likely supplied by erosion of older a precise 179 ma re os age with unrealistically low initial 187os 188os coal bearing sediments or organic rich shales the brynhild crude oil has relatively low 187os 188os and a generation age of 44 ma based on an asphaltene only re os isochron maltenes from the brynhild crude oil are isotopically affected by interaction with organic rich source rocks that represent the seal for the brynhild oil field compared with the crude oil bitumen from ula formation sandstones have contrasting re os properties high 187os 188os ratios and young generation ages between 8 and 0 ma based on asphaltene bitumen maltene re os triplets asphaltene only re os regressions and re os model ages for components with extremely high 187re 188os the 44 ma and 8 0 ma re os ages agree well with regional burial models that indicate oil generation from 44 ma to present day a genetic link between crude oil and bitumen is indicated by their similar alkane patterns v and ni contents and v ni and pristane phytane ratios modeling of 187os 188os for individual organic components through time shows that shales of the mandal formation developed 187os 188os ratios that match those of the crude oil and bitumen at 44 ma and 8 ma respectively and therefore confirm the source for both hydrocarbon phases we suggest that bitumen formed during in reservoir mixing of a 44 ma old early mature asphaltene poor oil brynhild crude oil with an 8 ma old asphaltene rich oil

cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode oceanic anoxic events oaes were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen deficient bottom waters oae2 occurring at the cenomanian turonian boundary about 93 5 myr ago is the most widespread and best defined oae of the mid cretaceous although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios the actual trigger mechanism corresponding closely to the onset of these episodes of increased carbon sequestration has not been clearly identified it has been postulated that large scale magmatic activity initially triggered oae2 refs 4 5 but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of oae2 has not yet been found here we report seawater osmium isotope ratios in organic rich sediments from two distant sites we find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of oae2 using a simple two component mixing equation we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin a approximately 30 50 fold increase relative to pre oae conditions furthermore the magmatic osmium isotope signal appears slightly before the oae2 as indicated by carbon isotope ratios suggesting a time lag of up to approximately 23 kyr between magmatism and the onset of significant organic carbon burial which may reflect the reaction time of the global ocean system our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of oae2 which may have triggered the subsequent deposition of large amounts of organic matter

comparison of chemical procedures for re isotopic measurements by n tims abstract precise and accurate measurements of the isotopic composition of rhenium re in geologic samples are of paramount importance for the successful application of the re os geochronometer two methods for chemical isolation of re are used in the geochronology community column chemistry using anion exchange resin and solvent solvent extraction using a sodium hydroxide acetone mixture despite their ubiquitous application a systematic comparison of these two methods is missing here we compare re isotopic data on a variety of samples sulfides hydrocarbons shales and in house standard solutions that were processed by both chemical methods measured 185re 187re ratios for both methods have similar precision and overlap within the propagated long term analytical reproducibility of 1 for samples that yield moderate to strong intensities 50 mv on the weaker re mass in contrast measured 185re 187re ratios for most samples that yield less intense re signals

oceanic heat transport onto the amundsen sea shelf through a submarine glacial trough glaciers which drain the west antarctic ice sheet wais into the amundsen sea are accelerating and thinning rapidly these observations have been attributed to the regional oceanography whereby heat contained within circumpolar deep water cdw drives the basal melting of floating glaciers on the basis of new data we calculate that 2 8 terra watts 1012 of oceanic heat flow onto the continental shelf and toward the glaciers via a submarine glacial trough this is enough to account for most of the basal melting in the entire region suggesting the ocean is supplying an excess of heat toward the antarctic continent

a computer based system simulating snowpack structures as a tool for regional avalanche forecasting this paper describes the current state of a complete automatic system of three numerical models that simulate snow cover stratigraphy and avalanche risks for operational avalanche forecasting the first model safran estimates relevant meteorological parameters affecting snowpack evolution the second crocus is a snow numerical model which simulates the physical processes inside the snowpack and its stratigraphy the last model mepra is an expert system based on an assessment of snowpack stability it deduces natural and accidental avalanche risks to describe the great variability of the snowpack and the associated avalanche risks this automatic system simulates snow cover evolution and its stability for many typical slopes elevations and aspects representative of the different french massifs to achieve this result different kinds of validations have been carried out since winter 1981 they are mainly based on comparisons with different sets of measurements and on the opinion of users although the routinely obtained results do not yet take into account all small scale effects such as wind transport they have been considered as valuable information by avalanche forecasters since 1992 93 and used operationally since then

pleistocene collapse of the west antarctic ice sheet some glacial sediment samples recovered from beneath the west antarctic ice sheet at ice stream b contain quaternary diatoms and up to 108 atoms of beryllium 10 per gram other samples contain no quaternary diatoms and only background levels of beryllium 10 less than 10 6 atoms per gram the occurrence of young diatoms and high concentrations of beryllium 10 beneath grounded ice indicates that the ross embayment was an open marine environment after a late pleistocene collapse of the marine ice sheet

morphometric variability in the diatom fragilariopsis kerguelensis implications for southern ocean paleoceanography abstract diatoms play a central role in the ecosystem of the southern ocean where they represent the main producers and carriers of organic carbon and dissolved silicon towards the deep ocean variability in space and time of the size of the most abundant species fragilariopsis kerguelensis may directly impact the way this ecosystem functions and also affect the nutrient balance of the global ocean as important water masses e g the antarctic intermediate water get their nutrient signature in this area we used a biometric approach and analyzed the size variability of f kerguelensis valves in sixty four surface sediment samples from the southern ocean in sediment trap samples from a mooring site pf3 at the antarctic polar front apf and along a nearby piston core ps1654 2 the average length and valve area of f kerguelensis displayed a distinct increase in the proximity of the apf and lower values to the north and south of it and also changed markedly over seasonal cycles as diatom blooms and nutrient availability waxe and wane the last glacial to interglacial transition also witnesses important changes in the general shape and valve size of f kerguelensis with specimens from glacial intervals being ca 30 larger than their interglacial counterparts 195 versus 150 m 2 valve area a sharp peak in average area and a very specific valve morphology correspond to the first signs of deglaciation and concurring sst sea surface temperature increase and sea ice retreat the latter together with melting icebergs may have contributed dissolved iron to surface waters the highest average sizes during glacial intervals are higher than what found anywhere in the southern ocean today the newly proposed proxy valve area and shape of f kerguelensis may thus prove useful for the reconstruction of the past position and nutrient characteristics of the apf and opal belt a region characterized by high production and export of biogenic silica in a particularly sensitive and dynamic area such as the southern ocean

validation of the energy budget of an alpine snowpack simulated by several snow models snowmip project many snow models have been developed for various applications such as hydrology global atmospheric circulation models and avalanche forecasting the degree of complexity of these models is highly variable ranging from simple index methods to multi layer models that simulate snow cover stratigraphy and texture in the framework of the snow model intercomparison project snowmip 23 models were compared using observed meteorological parameters from two mountainous alpine sites the analysis here focuses on validation of snow energy budget simulations albedo and snow surface tem perature observations allow identification of the more realistic simulations and quantifi cation of errors for two components of the energy budget the net short and longwave radiation in particular the different albedo parameterizations are evaluated for different snowpack states in winter and spring analysis of results during the melting period allows an investigation of the different ways of partitioning the energy fluxes and reveals the complex feedbacks which occur when simulating the snow energy budget particular attention is paid to the impact of model complexity on the energy budget components the model complexity has a major role for the net longwave radiation calculation whereas the albedo parameterization is the most significant factor explaining the accu racy of the net shortwave radiation simulation

interpretation of recent antarctic sea ice variability 1 trends in the satellite derived antarctic sea ice concentrations 1979 2002 show pronounced increase decrease in the central pacific sector bellingshausen western weddell sector by 4 10 per decade confidence levels for these regional trends exceed 95 positive polarities of the antarctic oscillation aao lead to more less ice in the eastern ross amundsen sector bellingshausen northern weddell sector which are qualitatively opposite to the impacts of positive polarities of the el nino southern oscillation enso the mechanisms responsible for the covariability between the ice and the a aao and b enso are demonstrated over the last 24 years a positive aao trend and a slightly negative enso trend produce a spatial pattern of ice changes similar to the regional ice trends however the magnitude of the ice changes associated with the aao and enso is much smaller than the regional ice trends more local or less understood large scale processes should be investigated for the explanations

diatom distribution in southern ocean surface sediments atlantic sector implications for paleoenvironmental reconstructions abstract a study of 230 surface sediment samples collected in the atlantic sector of the southern ocean between the southernmost weddell sea and the subtropical zone documents the modern distribution of diatoms revealing patterns of paleoenvironmental significance estimations of diatom valves per gram dry sediment display numbers of 50 200 106 in the zone of high opal burial located between the mean position of the winter sea ice edge and the polar front and maximum values of greater than 200 106 in the near shore sedimentary basins off the antarctic peninsula lowest diatom concentrations and assemblages strongly affected by dissolution were encountered in the weddell basin despite alteration of the diatom assemblages prior to their incorporation into the sediment record the biogeographic distribution and the abundance pattern of most of the 35 studied diatom species shows a close relationship with the surface hydrography water temperature these relationships can be used to estimate past surface water temperatures based on statistical treatments of the assemblages or on simple relations of species occurrences in the geological record another close link occurs between the distribution of sea ice and sea ice related diatoms

regional geologic and tectonic setting of the maracaibo supergiant basin western venezuela this special issue contains eight topical studies on the structure stratigraphy and petroleum system of the maracaibo basin a supergiant basin in western venezuela most of the work reported in this special issue is the product of thesis related research by master s and doctoral level students at the jackson school of geosciences of the university of texas at austin during a collaborative relationship with the venezuelan national oil company petrleos de venezuela s a that was initiated in the late 1980s this introductory article presents a regional overview of the tectonic setting and geology of the maracaibo basin with a cumulative oil production of more than 30 billion bbl since the first production well was drilled in 1914 and estimated ultimate oil reserves of more than 44 billion bbl the maracaibo basin is the most prolific hydrocarbon basin in the western hemisphere unlike the more extensive gulf of mexico giant hydrocarbon provinces the relatively small size 50 000 km2 19 305 mi2 relative simplicity in its structure and stratigraphy and wealth of surface and subsurface data make the maracaibo basin an attractive target for basinwide synthesis the objective of this article is to present a regional compilation of two dimensional 2 d and three dimensional 3 d seismic data wells and outcrop data at a basinwide scale to reveal the basin s 3 d structure and stratigraphy moreover we show regional tectonic reconstructions regional geologic maps and basin subsidence history to better constrain four major tectonic events that affected the basin and that are critical for understanding the timing and distribution of major unconformities and clastic wedges the distribution of the reservoir rocks the reactivation of older fault trends and the timing of maturation for underlying source rocks many of these topics are discussed in greater detail in the other eight articles in this special issue

an overview of the petroleum system of maracaibo basin the geologically complex maracaibo basin in northwestern venezuela is one of the most prolific hydrocarbon basins in the world having a basinal area of 50 000 km2 19 300 mi2 the basin has produced more than 30 billion bbl of oil with estimated recoverable oil reserves of more than 44 billion bbl the central elements of the petroleum system of the basin include 1 a world class source rock upper cretaceous la luna formation deposited on a shelf to slope environment under anoxic conditions and modified by intermittent oxygenated periods and tectonic events 2 high quality clastic reservoir rocks deposited in eocene and miocene fluviodeltaic settings 3 two main periods of rapid tectonic subsidence responsible for two pulses of voluminous hydrocarbon generation first during paleogene caribbean south american oblique plate collision and second during the neogene uplift of the sierra de perij mrida andes and 4 lateral and vertical migration of oil along strike slip normal and inverted faults as well as a regional unconformity of late eocene oligocene age the maturation migration and trapping of hydrocarbons were closely controlled by the tectonic evolution of the maracaibo basin during the paleogene the development of a foredeep along the northeastern margin of the basin and the strike slip reactivation of the rift related jurassic faults on the maracaibo platform controlled the early structural setting of the source and reservoir rocks hydrocarbons migrated updip from source rocks beneath the north northeastern margin of the basin along north south strike slip faults and into overlying eocene clastic reservoirs in the south central parts of the basin the second period of the maracaibo basin petroleum system developed during subaerial exposure of most of the maracaibo basin during oligocene miocene uplift of the adjacent sierra de perij and mrida andes uplift of mountain ranges surrounding the basin folded and depressed the interior of the basin to form the extensive maracaibo syncline because of the formation of the maracaibo syncline oil generation began in the central and southern parts of the synclinal basin and migrated northward hydrocarbons migrated up the flanks of the maracaibo syncline along reactivated strike slip faults and into miocene rocks adjacent to the uplifted mountain ranges escaping oil has formed numerous surface seeps along the edges of the maracaibo basin

diatom distributions in northern north pacific surface sediments and their relationship to modern environmental variables abstract the quantitative analysis of diatom assemblages preserved in 52 samples from the bellingshausen and the amundsen seas provides the first comprehensive view on the distribution of diatoms in surface sediments of the eastern and central pacific sector of the southern ocean on a latitudinal transect along 120 w diatom valve accumulation rates ar reach maximum values 8 10 108 valves m 2 yr 1 in a zone extending over ca 900 km between the antarctic polar front and the maximum average winter sea ice extent and exceed those ars obtained from an eastern transect along 90 w by one order of magnitude lowest diatom concentrations 1 3 106 valves g 1 were encountered in sediments of the sea ice zone affected by winter and summer sea ice the accumulation rate pattern of the most abundant diatom fragilariopsis kerguelensis 50 abundance in 47 samples mirrors the pattern of the total diatom valve ar and the biogenic silica bsi ar making f kerguelensis the major contributor to the bsi preserved at the sea floor relative abundances of diatom species and species groups were statistically compared with a selection of environmental variables such as the mean summer sea surface temperature and salinity mean annual surface nutrient concentration nitrate phosphate silicon mean annual water column stratification mixed layer depth in summer and mean summer and winter sea ice concentrations polynomial canonical redundancy analysis rda revealed the biogeographic distribution of diatom species had the strongest relationship with summer sea surface temperature ssst out of the nine tested environmental variables this relationship accounted for 69 6 of the total variance of the diatom distribution with 29 7 explained by the first gradient significantly correlated to ssst with r2 0 941 and 15 6 explained by the second gradient correlated to both summer and winter sea ice and silicon concentration azpeitia tabularis hemidiscus cuneiformis and roperia tesselata were associated with warmer water conditions 4 c whereas fragilariopsis curta f separanda f rhombica and thalassiosira gracilis were correlated with cold ssst confirming these environmental relationships is crucial for the development of reference data sets used in quantitative estimations of palaeoclimatic and palaeoceanographic conditions with statistical methods this new data set represents the first modernised treatment of diatom remains from the se pacific ocean and generally supports the use of a circum polar database for the determination of summer sst sea ice and potentially biogenic silica distribution of the southern ocean back into the late quaternary

palaeoproductivity at the antarctic continental margin opal and barium records for the last 400 ka abstract records of biogenic opal and barium were measured in sediment cores at the antarctic continental margin in the area of the weddell lazarev and cosmonaut seas these records provide a qualitative and quantitative tool to estimate changes in palaeoproductivity over the last 400 ka the stratigraphy of the investigated cores is calibrated to a lithostratigraphy adjusted to a stable isotope record from the eastern weddell sea which is supported by a th dating method we present evidence that interglacial productivity along the antarctic continental margin is twice as high compared to subantarctic sites near south orkney a glacial interglacial pattern with high productivity during peak warm stages can be observed back to 400 ka high interglacial productivity is linked to a reduced sea ice coverage which is regulated by the heat flux introduced by north atlantic deep water nadw to the antarctic ocean generally good correlations between the barium and opal records of the sediment cores indicate that dissolution of opal in the water column and the sediment does not obscure the surface productivity signal therefore in this area biogenic opal in combination with other proxies can be used for palaeoproductivity estimates palaeoproductivity is also assessed quantitatively from the barium record using the approaches of dymond et al dymond j suess e lyle m 1992 barium in deep sea sediments a geochemical proxy for paleoproductivity paleoceanography 7 163 181 and francois et al francois r honjo s manganini s j ravizza g e 1995 biogenic barium fluxes to the deep sea implications for paleoproductivity reconstructions glob biochem cycles 9 2 289 303 palaeoproductivity rates obtained by both methods show a good temporal correspondence in peak warm stages higher values are computed with the approach of dymond et al 1992 though some barium may be provided by lateral advection of material as indicated by th data near south orkney both methods provide values which are representative of a high productivity area they are drastically reduced during glacial times the extent and duration of sea ice coverage and the persistence of coastal polynyas is considered to be of primary importance in controlling the flux of biogenic material to sediments of the antarctic continental margin

flow and retreat of the late quaternary pine island thwaites palaeo ice stream west antarctica multibeam swath bathymetry and sub bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo ice stream that carried the combined discharge from the parts of the west antarctic ice sheet now occupied by the pine island and thwaites glacier basins sets of highly elongated bedforms show that at the last glacial maximum the route of the pine island thwaites palaeo ice stream arced north northeast following a prominent cross shelf trough in this area the grounding line advanced to within similar to 68 km of and probably reached the shelf edge minimum ice thickness is estimated at 715 m on the outer shelf and we estimate a minimum ice discharge of similar to 108 km 3 yr 1 assuming velocities similar to today s pine island glacier similar to 2 5 km yr 1 additional bed forms observed in a trough northwest of pine island bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior the style of ice retreat is also evident in five grounding zone wedges which suggest episodic deglaciation characterized by halts in grounding line migration up trough stillstands occurred in association with changes in ice bed gradient and phases of inferred rapid retreat correlate to higher bed slopes supporting theoretical studies that show bed geometry as a control on ice margin recession however estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat our findings show that the ice stream channeled a substantial fraction of west antarctica s discharge in the past just as the pine island and thwaites glaciers do today

last occurrence datums of the diatoms rouxia leventerae and rouxia constricta indicators for marine isotope stages 6 and 8 in southern ocean sediments abstract diatom analysis of sediments recovered during odp leg 177 and rv polarstern expeditions in the atlantic sector of the southern ocean reveals that the abundance pattern and the stratigraphic distribution of the two recently described diatom taxa rouxia leventerae and rouxia constricta represent a new tool for dating sequences that have been deposited under southern ocean cold water conditions in sediments from the present antarctic zone az the last occurrence datums of r leventerae and r constricta can be placed into the upper portion of the marine isotope stage mis 6 0 14 0 13 ma and the lower portion of mis 8 0 3 0 28 ma respectively this allows the identification of isotope stages in pleistocene southern ocean sediments that are often characterized by the lack or discontinuous occurrences of calcareous microfossils occurrences of r leventerae are restricted to glacial sediments deposited in the az highlighting that this taxon is typically restricted to cold waters in contrast r constricta occurs in glacial and interglacial sequences and displays a broader geographical distribution between the northern az and the polar front zone

re os geochronology of the el salvador porphyry cu mo deposit chile tracking analytical improvements in accuracy and precision over the past decade abstract sulfide minerals from the el salvador porphyry cu mo deposit chile were dated by re os geochronology to clarify the timing and duration of mineralization as these data are collected over the past 10 years they chronicle the evolution of re os analytical procedures and improvements in spike sample equilibration mass spectrometry and data reduction included in the data is the first tennantite pyrite re os isochron along with tennantite enargite pyrite os tracing of ore metals based on initial osmium systematics 187os 188osi porphyry associated mineralization follows the traditional early a transitional b and late d stage classification of relative timing of vein formation most cu and mo bearing sulfides were deposited during stages a and b field relationships clearly show the b stage veins are older than the d stage veins samples analyzed with carius tube digestion and mixed double spike indicate b stage molybdenite mineralization between 41 8 and 41 2 ma these ages best represent the timing of main stage mineralization at el salvador a five point 187re 187os isochron for pyrite tennantite from the latest d stage veins yields an age of 42 37 0 45 ma the isochron age while older is within error of early analyses of molybdenite by alkaline fusion digestion using single re and os spikes 42 4 42 0 ma a separate pyrite and tennantite enargite pair containing significant common os yields an initial 187os 188os ratio of 0 134 0 042 compatible with a predominantly mantle origin we appreciate the early analyses for their historical significance while emphasize the geological implications of the 41 8 41 2 ma molybdenite ages temperature estimates by sulfur thermometry and fluid inclusions are 390 510 c for the b stage molybdenites and 230 240 c for the latest d stage pyrite and tennantite analyses based on carius tube digestion and updated spike calibrations suggest a 0 6 m y duration 41 8 to 41 2 ma in line with published deposit geochronology the timing and duration of mineralization from re os dating of ore minerals is more precise than estimates from previously reported 40ar 39ar and k ar ages on alteration minerals the re os results suggest that the mineralization is temporally distinct from pre mineral rhyolite porphyry 42 63 0 28 ma and is immediately prior to or overlapping with post mineral latite dike emplacement 41 16 0 48 ma based on the re os and other geochronologic data the middle eocene intrusive activity in the el salvador district is divided into three pulses 1 44 42 5 ma for weakly mineralized porphyry intrusions 2 41 8 41 2 ma for intensely mineralized porphyry intrusions and 3 41 ma for small latite dike intrusions without major porphyry stocks the orientation of igneous dikes and porphyry stocks changed from nne ssw during the first pulse to wnw ese for the second and third pulses this implies that the wnw ese striking stress changed from 3 minimum principal compressive stress during the first pulse to hmax maximum principal compressional stress in a horizontal plane during the second and third pulses therefore the focus of intense porphyry cu mo mineralization occurred during a transient geodynamic reconfiguration just before extinction of major intrusive activity in the region

pyrolysis kinetics applied to prediction of oil generation in the maracaibo basin venezuela abstract we use chemical kinetic parameters for oil generation derived from modified rock eval and pyromat instruments coupled with thermal history models to predict the timing and extent of oil generation in the maracaibo basin of venezuela the vitrinite reflectance model developed at lawrence livermore national laboratory is used to calibrate thermal history models with measured vitrinite reflectance profiles we examine the way differences in the kinetic parameters affect predictions of maturation in several parts of the basin with different thermal histories maturity indicators such as h c atomic ratio of residual kerogen and api gravity of reservoired oil are compared to the calculated extent of oil generation we use the comparison to check the accuracy of the coupled oil generation and thermal history models

plio pleistocene ice volume antarctic climate and the global δ18o record we propose that from 3 to 1 million years ago ice volume changes occurred in both the northern and southern hemispheres each controlled by local summer insolation because earth9s orbital precession is out of phase between hemispheres 23 000 year changes in ice volume in each hemisphere cancel out in globally integrated proxies such as ocean 18 o or sea level leaving the in phase obliquity 41 000 years component of insolation to dominate those records only a modest ice mass change in antarctica is required to effectively cancel out a much larger northern ice volume signal at the mid pleistocene transition we propose that marine based ice sheet margins replaced terrestrial ice margins around the perimeter of east antarctica resulting in a shift to in phase behavior of northern and southern ice sheets as well as the strengthening of 23 000 year cyclicity in the marine 18 o record

direct radiometric dating of hydrocarbon deposits using rhenium osmium isotopes rhenium osmium re os data from migrated hydrocarbons establish the timing of petroleum emplacement for the giant oil sand deposits of alberta canada at 112 5 3 million years ago this date does not support models that invoke oil generation and migration for these deposits in the late cretaceous most re os data from a variety of deposits within the giant hydrocarbon system show similar characteristics supporting the notion of a single source for these hydrocarbons the re os data disqualify cretaceous rocks as the primary hydrocarbon source but suggest an origin from older source rocks this approach should be applicable to dating oil deposits worldwide

evaluation of bitumen as a 187re 187os geochronometer for hydrocarbon maturation and migration a test case from the polaris mvt deposit canada abstract re os analyses of bitumen from the polaris mississippi valley type zn pb deposit demonstrate the potential utility of the re os system for establishing the absolute timing of hydrocarbon migration and generation the polaris deposit is hosted by carbonate rocks of the upper ordovician thumb mountain formation and sulphide mineralization during late devonian time is constrained by a rb sr sphalerite date of 366 15 ma and a paleomagnetic date bitumen occurs in open space fillings and coatings and is interpreted to have formed during the main and late stages of sulphide mineralization samples of bitumen were taken from a variety of locations from the main deposit at polaris re and os contents are 3 78 ppb and 55 1155 ppt respectively the 187re 188os ratios are very high ranging from 300 to 1800 and the os isotopic composition is moderately to highly radiogenic with 187os 188os ratios ranging from 3 3 to 13 7 calculated 187os 188os ratios at 366 ma are variable but an obvious grouping is defined by 11 of 16 analyses on a re os isochron diagram and these data yield a re os date of 374 9 ma mswd 12 model 3 in agreement with rb sr sphalerite and paleomagnetic dates for mineralization at polaris the combined dates suggest that the migration of fluids responsible for both pb zn mineralization and bitumen formation at polaris was broadly contemporaneous and was coeval with the late devonian ellesmerian orogeny though considerable further research is required to understand the re os isotopic systematics of migrated hydrocarbon systems bitumen re os geochronology may provide the possibility of establishing a more comprehensive understanding of mvt deposits sedimentary basin and petroleum systems

pronounced mid pleistocene southward shift of the polar front in the atlantic sector of the southern ocean abstract ocean drilling program odp site 1094 situated in the atlantic sector of the antarctic region below the polar front pf is dominated by silica rich sediments during the pleistocene but does contain occasional calcium carbonate rich intervals the analysis of the coccolithophore assemblages of one of these intervals in the mid pleistocene allows us to interpret a prominent southward displacement of the pf an age model constructed using paleomagnetic and biostratigraphic information places this event around 1 ma close to the jaramillo subchron in an interval of strong eccentricity and high amplitude in the insolation record we distinguish five intervals based on coccolithophore assemblage and abundances according to the age model used this calcium carbonate rich period should include marine isotope stage 31 the most significant interglacial period recorded at this time however this model cannot decipher if adjacent isotope stages are partially represented interval v the oldest is interpreted as an episode with progressive abundance of coccolithophores with a characteristic assemblage from the polar front zone pfz but with dominance of siliceous micro organisms interval iv contains the maximum abundance of coccolithophores with an assemblage characteristic of the subantarctic zone saz with peaks in abundance of calcidiscus leptoporus coccolithus pelagicus helicosphaera carteri and syracosphaera spp related with the proximity of the subantarctic front saf the southward displacement of the frontal system linked to an increase in sea surface temperature produced increases in diversity and in productivity conversely we consider that the presence of some coccolithophore taxa linked in other environments with anomalies in salinity could reflect a pulse in the melting of west antarctic ice sheet as suggested previously for this interval interval iii is characterized by a pfz assemblage reflecting again a northward shift of the pf interval ii is considered in this context as another southward pulse but less important than interval iv and with the absence of saf and or melting related taxa interval i is considered a transition from a situation similar to the present day pfz to the antarctic zone conditions as consequence of a northward displacement of pf and weddell gyre antarctic circumpolar current boundary

metamorphic signature of snow revealed in ssm i measurements brightness temperatures 19 22 37 85 ghz measured by the special sensor microwave imager ssm i are analyzed using data from the snow monitoring network within the former soviet union during the 1987 1988 winter period it is shown that in the beginning of winter the ssm i measurements display the classical snow scattering signature i e the brightness temperatures decrease with increasing depth and the largest decrease occurs at the highest frequency dramatic deviations from this pattern are observed in the middle of winter where the brightness temperature approaches a minimum and then begins to increase despite the fact that the snow depth remains constant or continues to grow the two stream radiative transfer model is combined with results from dense media theory to help explain the phenomenon model results suggest that the increase in brightness temperature is due to a decrease of the single scattering albedo as the snowpack ages this decrease of the albedo is related to changes in the snow crystalline structure due to metamorphism consequences for the interpretation of satellite measurements and development of algorithms for deriving snow water equivalent are discussed

reassessment of the potential sea level rise from a collapse of the west antarctic ice sheet theory has suggested that the west antarctic ice sheet may be inherently unstable recent observations lend weight to this hypothesis we reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution we obtain a value for the global eustatic sea level rise contribution of about 3 3 meters with important regional variations the maximum increase is concentrated along the pacific and atlantic seaboard of the united states where the value is about 25 greater than the global mean even for the case of a partial collapse

pacific 187 os 188 os isotope chemistry and u pb geochronology synchroneity of global os isotope change across oae 2 studies of oae 2 sections beyond the atlantic ocean western interior seaway wis and european pelagic shelf are limited here we present initial osmium isotope stratigraphy 187os 188os osiosi from two proto pacific sites that span the cenomanian turonian boundary interval ctbi the yezo group yg section hokkaido japan and the great valley sequence gvs california usa to evaluate the 187os 188os seawater chemistry of the proto pacific additionally we combine new 206pb 238u zircon ca id tims geochronology from five volcanic tuff horizons of the yezo group section to test and facilitate inter basinal integration with the wis using radio isotopically constrained age depth models for both sections and quantitatively constrain the absolute timing and duration of events across the ctbi the yg shows an almost identical osiosi profile to that of the wis and very similar to that of other sites of the proto atlantic and european pelagic oceans turgeon and creaser 2008 and du vivier et al 2014 the characteristics of the osiosi profile are radiogenic and heterogeneous 0 55 0 85 prior to the oae 2 and synchronous with the inferred oae 2 onset the osiosi abruptly become unradiogenic and remain relatively homogeneous 0 20 0 30 before showing a gradual return to more radiogenic view the mathml sourceosi 0 70 throughout the middle to late oae 2 a206a206pb 238u zircon age of an interbedded tuff hk017 in the adjacent horizon to the first unradiogenic osiosi value constrains the age of the osiosi inflection at 94 44 0 14 ma94 44 0 14 ma this age including uncertainty agrees with the interpolated age of the same point in the osiosi profile 94 28 0 25 ma94 28 0 25 ma in the only other dated oae 2 section the wis indicating a coeval shift in seawater chemistry associated with volcanism at the oae 2 onset at the levels of temporal resolution ca 0 1 myr further prior to the onset of oae 2 an enhanced radiogenic inflection in the osiosi profile of the yg is correlative within uncertainty with a similar trend in the wis based on the u pb age depth model the interpolated ages 94 78 0 12 ma94 78 0 12 ma and 94 66 0 25 ma94 66 0 25 ma for this osiosi inflection in the yg and wis respectively indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry in contrast the pre oae 2 osiosi profile for the gvs is disparate to that of the yg and those of the proto atlantic and european pelagic shelf locations we interpret the pre oae 2 heterogeneous osiosi values 0 30 0 95 to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic hydrothermal flux and radiogenic continental flux os we conclude that the osiosi profiles from the proto pacific sections record both trends that are consistent globally oae 2 onset syn and post oae 2 but also show regional differences pre oae 2 between oae 2 sections worldwide as such the osiosi profiles coupled with u pb geochronology facilitate the correlation of oae 2 stratigraphy and demonstrate both regional and global ocean dynamics

antarctic records of precession paced insolation driven warming during early pleistocene marine isotope stage 31 precisely dated antarctic continental margin and southern ocean geological records show that the early pleistocene interglacial marine isotope stage 31 mis 31 was characterized by warmer than present surface waters with reduced sea ice and enhanced high latitude marine carbonate production micropaleontologic isotopic and paleomagnetic evidence from drill cores at 77 s cape roberts project 1 and 53 s odp site 1094 indicate circumantarctic changes in sea surface temperature and water mass stratification that are in phase with high southern latitude insolation changes during mis 31 these changes imply a significant though as yet unquantifiable reduction in antarctic ice volume this study supports the hypothesis that the interhemispheric antiphased relationship of the precession cycle attenuates a potentially significant antarctic ice volume signal in the deep sea oxygen isotope record the implications are that antarctic marine ice sheets may be more susceptible to warming and high insolation driven retreat than has been widely recognized

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

further evaluation of the re os systematics of crude oil implications for re os geochronology of petroleum systems abstract here we further demonstrate asphaltene to be the main carrier of re and os in crude oil and additionally show that generally the less soluble fractions of asphaltene contain higher concentrations of re and os with the 187os 188os and 187re 188os values showing different trends between oils these observations are considered as evidence of the existence of different re and os carriers in crude oil the multiple heteroatomic ligands and porphyrins proposed as re and os hosts in crude oil are likely present as free molecules initially absorbed and occluded in the asphaltene aggregates and co precipitate with asphaltene in response to the addition of n alkanes the re os elemental and isotopic systematic behaviour revealed herein are interpreted to be the result of the involvement of re and os carriers in the aggregation and precipitation of asphaltene and the chemical make up of each particular crude oil asphaltene and maltene separated by a series of different alkanes show that asphaltene yields decrease from n pentane to n heptane and then tend to stay stable until n decane the 187os 188os values of the series of asphaltenes are extremely similar for each oil sample inhibiting the possibility of determining any re os dates with the asphaltenes of a single crude oil in contrast the maltene fractions exhibit re os isotopic variation nevertheless only the maltenes of one oil defined a reasonable isochron with an age similar to that of source rock as such this study highlights the need for a greater understanding in order to determine whether and in what cases fractions separated from a single crude oil by different alkanes can provide meaningful re os ages

a marine diatom record from the amundsen sea insights into oceanographic and climatic response to the mid pleistocene transition in the west antarctic sector of the southern ocean abstract the goal of this study is to assess the changes that have occurred during the mid pleistocene transition an important transitional period in cryosphere evolution by examining the siliceous microfossil record of sediments collected proximal to a major ice drainage outlet for the west antarctic ice sheet core ps58 254 was collected from a sediment drift on the upper continental rise in the amundsen sea directly offshore from pine island bay one of the three main discharge areas for the west antarctic ice sheet wais published data on physical properties geochemical composition grain size and clay mineral assemblages are complemented here by a high resolution record sample spacing 10 cm of the siliceous microfossil assemblages diatoms and silicoflagellates between 1200 ka and 621 ka the assemblage is relatively diverse with actinocyclus ingens thalassiothrix antarctica and fragilariopsis kerguelensis dominating the assemblages but diatom abundance is variable from low to barren additionally the occurrence of a ingens thalassiosira elliptipora and thalassiosira fasciculata is used to confirm and further refine the existing age model and extend it back to 1200 ka species composition during the last ca 621 ka is dominated by f kerguelensis which consistently comprises 80 90 of the assemblage a clear relationship between diatom abundance and glacial interglacial variability is apparent after 621 ka which resembles the glacial interglacial variability previously observed in other proxy data a significant change in both sediment composition and diatom assemblages is observed at 621 ka this change concurs with the last abundant occurrence of a ingens and the end of the mid pleistocene transition mpt i e the onset of modern eccentricity precession paced glacial cycles around 650 ka we suggest that during interglacial periods after 621 ka the amundsen sea low pressure system shifted seasonally southwestwards towards the shelf and thereby increased the advection of relatively warm circumpolar deep water cdw onto the amundsen sea shelf which is a major factor for present ice sheet melting in this part of west antarctica

inland thinning of pine island glacier west antarctica the pine island glacier pig transports 69 cubic kilometers of ice each year from 10 of the west antarctic ice sheet wais it is possible that a retreat of the pig may accelerate ice discharge from the wais interior satellite altimetry and interferometry show that the grounded pig thinned by up to 1 6 meters per year between 1992 and 1999 affecting 150 kilometers of the inland glacier the thinning cannot be explained by short term variability in accumulation and must result from glacier dynamics

re os dating of maltenes and asphaltenes within single samples of crude oil abstract re os geochronology of oil may constrain the timing of oil formation and improve oil source and oil oil correlations typically asphaltene asph the heaviest and most re os rich oil fraction from multiple oils within an oil field or a larger petroleum system are analyzed to obtain sufficient spread in re os isotopic ratios a mathematical necessity for precise re os isochrons here we offer a new approach for re os geochronology of oil based on isotopic analyses of different fractions within a single sample of crude oil we studied three oils from the gela oil field southern sicily italy recovered from triassic jurassic stratigraphic intervals streppenosa noto and sciacca formations within the gela 1 well asph insoluble in n alkane and maltene malt soluble in n alkane fractions of oil were separated using n pentane n hexane n heptane and n decane solvents the asph contents of the sciacca and noto oils 26 33 wt are notably higher compared to the streppenosa oil 7 12 wt asph we present an optimized re os procedure with sample digestion in a high pressure asher followed by isotopic measurements using negative thermal ionization mass spectrometry very high metal contents of gela oils allowed acquisition of precise re os data systematic variations between the type of solvent used for asph precipitation and the asph content of the oil also known from the literature and the re os contents of the asph and malt fractions first observed in this study provide important practical applications for re os analyses of oil most re and os 96 98 in the noto oil are hosted in the asph fraction in contrast a significant portion of re and os 33 34 is stored in the malt fraction of the lighter but heavily biodegraded streppenosa oil collectively our new data on alkane distribution hopane and sterane biomarkers major and trace element contents and re os concentrations and isotopic ratios of the oils and their fractions support the presence of two oil families streppenosa oil a heavily biodegraded oil generated at an early stage of thermal maturation from a shaly source rock is clearly distinct from the noto and sciacca oils both generated at peak maturation from a carbonate source rock with no subsequent biodegradation two re os ages were obtained crude oil and asph from noto and sciacca oil yield a model 1 isochron age of 27 5 4 6 ma with an initial 187 os 188 os of 3 89 0 43 mswd 1 6 n 19 we interpret this oligocene age as the time of initial oil generation that was probably related to the onset of regional collisional tectonics in southern sicily asph and crude oil from the streppenosa oil yield scattered data perhaps related to effects of biodegradation streppenosa malt yield a model 1 age of 200 0 5 2 ma and initial 187 os 188 os of 1 39 0 11 mswd 0 52 n 4 this age and initial os isotopic composition are consistent with rapid oil generation shortly after late triassic source rock formation perhaps driven by magmatic related heating here we document a potentially powerful geochronological tool that can be applied to single samples of crude oil within each oil the 187 os 188 os and most of the 187 re 188 os ratios in the malt fractions are lower than in the corresponding asph fractions crude oils being physical mixtures of asph and malt have intermediate re os isotopic ratios hence the needed spread in data points for a re os isochron is obtained by analyzing components of a single crude oil this approach eliminates pitfalls in interpretation of re os data from multiple oils with possibly different generation migration or reservoir modification history

trace metals and organic geochemistry of the machiques member aptian albian and la luna formation cenomanian campanian venezuela abstract two proposed source rock sequences in the lower aptian albian and the upper cenomanian campanian cretaceous sediments of the perija range maracaibo basin were geochemically analyzed in order to find environmentally controlled characteristic fingerprints trace metals and sulfur concentrations were found to be closely related with the physicochemical conditions during deposition of the rocks based on trace metal analyses and sulfur relationships with toc the aptian albian sequence machiques member shows alternate oxic and anoxic episodes close to the bottom water surface where mixed marine and terrestrial organic matter were deposited the cenomanian campanian sequence la luna formation persistently shows anoxic euxinic conditions that allowed the preservation of marine organic matter ratios of trace metals v mo and zn characterize both cretaceous sedimentary columns however some limitations are visualized in using trace metals as correlation tools in further geochemical studies biomarker ratios are related to the organic matter type and lithology some specific ratios and individual compounds allow a clear differentiation between these two sequences and offer in the context of 13 c an excellent tool for further oil source rock correlation studies

a sustained 21 m sea level highstand during mis 11 400 ka direct fossil and sedimentary evidence from bermuda abstract a small protected karstic feature exposed in a limestone quarry in bermuda preserved abundant sedimentary and biogenic materials documenting a transgressive phase still stand and regressive phase of a sea level in excess of 21 3 m above present during marine isotope stage mis 11 400 ka as determined by u th dating and amino acid racemization cobbles and marine sediments deposited during the high energy transgressive phase exhibit rim cements indicating a subsequent phreatic environment this was succeeded stratigraphically by a still stand deposition of fine calcareous lagoonal sediments containing bioclasts of red algae and benthic and planktonic foraminifera that was intensely burrowed by marine invertebrates probably upogebiid shrimp that could not be produced under any condition other than sustained marine submergence overlying this were pure carbonate beach sands of a low energy regressive phase containing abundant remains of terrestrial and marine vertebrates and invertebrates the considerable diversity of this fauna along with taphonomic evidence from seabird remains indicates deposition by high run up waves over a minimum duration of months if not years the maximum duration has yet to be determined but probably did not exceed one or two thousand years the most abundant snails in this fauna are two species indicative of brackish water and high tide line showing that a ghyben herzberg lens must have existed at 20 m the nature of these sediments and fossil accumulation is incompatible with tsunami deposition and given the absence of evidence for tectonic uplift of the bermuda pedestal or platform provide proof that sea level during mis 11 exceeded 20 m a fact that has widespread ramifications for geologists biogeographers and human demographics along the world s coastlines

a practical biodegradation scale for use in reservoir geochemical studies of biodegraded oils existing scales widely used to describe the extent of biodegradation of petroleum have insufficient resolution to usefully characterize many heavy oil and bitumen occurrences including the volumetrically dominant heavily and severely biodegraded oil accumulations in the foreland basins of western canada and venezuela in these and other deposits existing classifications or descriptions of the biodegradation level may vary only slightly yet oil may vary in viscosity by orders of magnitude the manco biodegradation scale proposed here is based on integrating the extent of degradation of various members of compound classes not included in previous biodegradation scales they include alkyl aromatic and alkyl thiophenic compounds that show variable extent of alteration in samples degraded to uniform levels on standard scales but which may show variation in local degradation systematics related to biodegradation mechanisms and extent of oil mixing the manco scale uses a combination of a consideration of the extent of alteration within a compound class together with a consideration of biodegradation across a range of compound classes it can be reliably used as a basis for interpreting geochemical changes in heavily biodegraded oil suites and can also be used to differentiate biodegraded oil samples likely to be amenable to cold production from those requiring production strategies such as steam or chemical flooding as with other biodegradation scales the scale may also provide evidence for the influx of later higher quality oil into a reservoir fluid that had been previously biodegraded

inflow of warm circumpolar deep water in the central amundsen shelf the thinning and acceleration of the west antarctic ice sheet has been attributed to basal melting induced by intrusions of relatively warm salty water across the continental shelf a hydrographic section including lowered acoustic doppler current profiler measurements showing such an inflow in the channel leading to the getz and dotson ice shelves is presented here the flow rate was 0 3 0 4 sv 1 sv 10 6 m 3 s 21 and the subsurface heat loss was estimated to be 1 2 1 6 tw assuming that the inflow persists throughout the year it corresponds to an ice melt of 110 130 km 3 yr 21 which exceeds recent estimates of the net ice glacier ice volume loss in the amundsen sea the results also show a 100 150 m thick intermediate water mass consisting of circumpolar deep water that has been modified cooled and freshened by subsurface melting of ice shelves and or icebergs this water mass has not previously been reported in the region possibly because of the paucity of historical data

hot acidic late permian seas stifle life in record time abstract the end of permian time 252 251 ma hosts the largest mass extinction in earth history yet events heralding this global catastrophe remain intensely disputed we present a chemostratigraphic marker the 187re 188os ratio which soars to unprecedented levels approaching the permo triassic boundary these ratios are tied to profound trace element changes and a precise re os time record at 252 ma preserved in black shales from east greenland and the mid norwegian shelf within a 36 meter shale section an 80 fold increase in re concentrations two fold for os signals seawater conditions that became increasingly inhospitable to life unwavering initial 187os 188os ratios of 0 6 preclude mafic volcanism and meteorite impact as the direct cause of late permian anoxia we argue that extraordinarily high 187re 188os ratios are the hallmark of simultaneously rising ocean temperature and acidity leading to loss of oxygen and the stifling of life in latest permian time

trace elements as palaeoenvironmental markers in strongly mature hydrocarbon source rocks the cretaceous la luna formation of venezuela abstract the late cretaceous la luna formation is the main hydrocarbon source rock formation in venezuela in the lake maracaibo area the formation shows a steep maturation gradient affecting sedimentary organic matter om and clay minerals related to local burial history and thrusting events the influence of the gradient is examined through geochemical organic and inorganic and mineralogical investigations carried out on samples from five cored boreholes in the maracaibo basin the analytical procedure comprises rock eval pyrolysis vitrinite reflectance measurement palynofacies observation major and trace element content determination performed on bulk rock samples or after a sequential leach protocol dop calculation and the examination and analysis of clay mineral assemblages the maturation gradient seems to have had no effect upon the distribution of the trace elements referred to as proxy indicators for palaeodepositional conditions of om rich sediments ba p v mo ni co cu cd cr and mn consequently trace elements appear to be reliable palaeoenvironmental markers they indicate that during the la luna fm deposition the water column must have had variable conditions with high primary production near the surface and the correlative development of euxinic conditions at the bottom this provides further arguments for an upwelling system affecting the northern platform of south america during late cretaceous time the relative scarcity of reactive iron in the depositional environment must have taken an active part in a process involving sulphur and acting on the environmental conditions as well as on om vulcanisation this process favours aliphatic om storage and may be relevant for many other om rich facies or hydrocarbon source rocks

record of a mid pleistocene depositional anomaly in west antarctic continental margin sediments an indicator for ice sheet collapse abstract modern global warming is likely to cause future melting of earth s polar ice sheets that may result in dramatic sea level rise a possible collapse of the west antarctic ice sheet wais alone which is considered highly vulnerable as it is mainly based below sea level may raise global sea level by up to 5 6 m despite the importance of the wais for changes in global sea level its response to the glacial interglacial cycles of the quaternary is poorly constrained moreover the geological evidence for the disintegration of the wais at some time within the last ca 750 kyr possibly during marine isotope stage mis 11 424 374 ka is ambiguous here we present physical properties palaeomagnetic geochemical and clay mineralogical data from a glaciomarine sedimentary sequence that was recovered from the west antarctic continental margin in the amundsen sea and spans more than the last 1 myr within the sedimentary sequence proxies for biological productivity such as biogenic opal and the barium aluminum ratio and the supply of lithogenic detritus from the west antarctic hinterland such as ice rafted debris and clay minerals exhibit cyclic fluctuations in accordance with the glacial interglacial cycles of the quaternary a prominent depositional anomaly spans mis 15 mis 13 621 478 ka the proxies for biological productivity and lithogenic sediment supply indicate that this interval has the characteristics of a single prolonged interglacial period even though no proxy suggests environmental conditions much different from today we conclude that if the wais collapsed during the last 800 kyr then mis 15 mis 13 was the most likely time period apparently the duration rather than the strength of interglacial conditions was the crucial factor for the wais drawdown a comparison with various marine and terrestrial climate archives from around the world corroborates that unusual environmental conditions prevailed throughout mis 15 mis 13 some of these anomalies are observed in the pelagic southern ocean and the south atlantic and might originate in major ice sheet drawdown in antarctica but further research is required to test this hypothesis

fault charged mantle fluid contamination of united kingdom north sea oils insights from re os isotopes 187 os 188 os as expected from a kimmer idge clay formation source however oils from the viking graben with which the brent fi eld is associated and main east shetland basin display unradiogenic 187 os 188 os in this study we seek to demonstrate that these anomalous results are due to interaction with a mantle like fl uid that propagated through the main fault zone of the viking graben and east shetland basin this study enhances our understanding of the re os oil systematics and supports cur rent ideas regarding fault architecture crustal thinning and associated fl uid fl ow and interac tion in the north sea oil system abstract united kingdom north sea oils sourced from the kimmeridge clay formation contain unradiogenic 0 17 to 0 48 and radiogenic 1 04 to 3 34 187 os 188 os values the unra diogenic 187 os 188 os values are spatially associated with the main basin bounding faults of the viking graben and east shetland basin in contrast the radiogenic 187 os 188 os values are associated with north sea basins located farther away from the basin boundary faults we suggest that crustal thinning and strain localization within the viking graben and east shetland basin are suffi cient to have allowed basin bounding faults to propagate to suffi cient depth to act as conduits for mantle derived fl uids to interact with oil this hypothesis is sup ported by previous geochemical data for north sea oil fi elds furthermore we demonstrate the use of os isotopes as an important tracker of crustal scale fl uid dynamics and petroleum migration pathways in extensional basins

geometry and development of glacial continental margin depositional systems in the bellingshausen sea we present multi channel seismic mcs reflection profiles and bathymetry data acquired across a remote and poorly surveyed part of the antarctic continental margin in the bellingshausen and amundsen seas this new information has been combined with published data and used to interpret the style of sedimentation on the continental shelf slope and rise and to describe sedimentation processes which have been active in this region most seismic reflection profiles crossing the continental margin show prograded sequences beneath the outer shelf and upper slope and we infer that the stratal characteristics of these sequences indicate that grounded ice sheets reached the shelf edge during previous glacial times although there are general similarities in stratal geometry on these profiles in detail they reveal significant longitudinal variations in sediment input from the shelf to the upper slope on several profiles we found evidence of mass wasting of the continental slope in the form of slump and debris flow deposits at greater depth turbidity flows bottom currents and coriolis force have controlled the further transportation and deposition of sediment which has resulted in the development of mounds channels and sediment wave fields the distribution and variations in the size and geometry of the mounds reflect sediment input and the relative contribution of these other factors which control sedimentation on the continental rise

the biogeography of major diatom taxa in southern ocean surface sediments 3 tropical subtropical species abstract this paper gives a modern circumscription of tropical subtropical diatoms regarding their relationship with sea surface temperatures sst and sea ice cover diatoms from 228 core top sediment samples collected from the southern ocean were studied to determine the geographic distribution of eight major diatom species taxa preserved in surface sediments generally located north of the subantarctic front the comparison of the relative contribution of diatom species with modern february sst and sea ice cover reveals species specific sedimentary distributions regulated both by water temperatures and sea ice conditions although selective preservation might have played some role their presence in surface and downcore sediments from the southern ocean are reliable indicators of high sst and poleward transport of waters from the tropical subtropical atlantic our work supports the use of diatom remains to reconstruct past variations of these environmental parameters via qualitative and transfer function approaches

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

the permeability of a melting snow cover data from snow lysimeters in california and vermont are used to find the saturated permeability of a melting snow cover in the range of 10 40 10 10 m2 depending on snow density the unsaturated permeability increases as about the third power of liquid saturation the gravity flow theory is shown to be an accurate representation of meltwater drainage from snow covers in two diverse areas even though the snow covers are treated as homogeneous units the variation of saturated permeability with snow density occurs about as predicted by shimizu s formula for dry snow although ice layers decrease the permeability somewhat

stronger ocean circulation and increased melting under pine island glacier ice shelf the ice shelf buttressing antarctica s pine island glacier has been melting rapidly observations taken between 1994 and 2009 show that meltwater production has increased by about 50 since 1994 as a result of a stronger circulation below the ice shelf

kinematic wave modelling of vertical movement of snowmelt water through a snowpack vertical movement of snowmelt water through snowpacks is modelled by applying the kinematic wave theory analytical solutions are obtained for moisture flux particle velocity time history and velocity of meltwater front and total moisture content for a single melt event assuming that the melt rate is constant these solutions are extended to the case involving more than one event

rhenium osmium geochronology and oil source correlation of the duvernay petroleum system western canada sedimentary basin implications for the application of the rhenium osmium geochronometer to petroleum systems the re os geochronometer has been applied to many petroleum systems worldwide however it is debated if the re os systematics in petroleum actually record the timing of oil generation here we investigate the re os isotope systematics of the duvernay petroleum system in the western canada sedimentary basin which has been shown to be a relatively simple petroleum system that is associated with oil generated during the late cretaceous early eocene laramide orogeny from a single source the organic geochemistry of the duvernay oils pristane phytane ratios of 1 5 smooth homohopane profile c29 c27 c28 regular sterane distribution and predominance of diasteranes over regular steranes strongly suggests the oil source to be that of type i ii marine organic matter of the upper devonian duvernay formation the asphaltene fraction re os isotope data of the duvernay oil yield an age of 66 31 ma which is in excellent agreement with the timing of the main stage hydrocarbon generation of the duvernay formation based on basin models further the similarity between the 187os 188os compositions of the duvernay formation source rock 0 46 1 48 and the oil 0 55 1 06 at the time of oil generation supports the hypothesis that the 187os 188os composition of an oil is inherited from the source unit at the time of oil generation and therefore shows no or limited influence from interaction with basin fluids this study supports that the re os isotope systematics of an oil can yield the timing of oil generation and be used to trace its source

reconstruction of the west antarctic ice sheet in pine island bay during the last glacial maximum and its subsequent retreat history abstract marine geological and geophysical studies were conducted to determine the late quaternary glacial history of the west antarctic ice sheet in pine island bay antarctica the data set includes sea floor imagery in the form of multibeam swath bathymetry and deep tow side scan sonar surveys distribution of sedimentary deposits and basal surface conditions are revealed through intermediate resolution seismic data and chirp 3 5 khz sub bottom profiles sedimentary deposits were sampled with piston cores and calcareous material was dated using ams radiocarbon techniques results from the study indicate the ice sheet was grounded on the outer shelf during its maximum extent sometime in the past thick ice flowing slowly and resting on crystalline bedrock was influenced by significant amounts of subglacial meltwater on the inner shelf thinner but more rapidly flowing ice characterized the seaward extent of the ice sheet which was grounded on a sedimentary substrate deglaciation occurred in two phases beginning soon after the last glacial maximum the outer most extension of the ice sheet retreated gradually pausing on a bathymetric high on the middle shelf by 16 000 14c years ago between 16 000 and 12 000 14c years ago ice sheet retreat resumed and may have proceeded rapidly to reach a present day position by 10 000 14c years ago

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

sedimentation on the continental rise west of the antarctic peninsula over the last three glacial cycles abstract the continental rise west of the antarctic peninsula includes a number of large sediment mounds interpreted as contourite drifts cores from six sediment drifts spanning some 650 km of the margin and 4 of latitude have been dated using chemical and isotopic tracers of palaeoproductivity and diatom biostratigraphy interglacial sedimentation rates range from 1 1 to 4 3 cm ka glacial sedimentation rates range from 1 8 to 13 5 cm ka and decrease from proximal to distal sites on each drift late quaternary sedimentation was cyclic with brown biogenic burrowed mud containing ice rafted debris ird in interglacials and grey barren laminated mud in glacials foraminiferal intervals occur in interglacial stages 5 and 7 but not in the holocene processes of terrigenous sediment supply during glacial stages differed meltwater plumes were more important in stages 2 4 turbidity currents and ice rafting in stage 6 the terrigenous component shows compositional changes along the margin more marked in glacials the major oxides al2o3 and k2o are higher in the southwest and cao and tio2 higher in the northeast there is more smectite among the clay minerals in the northeast magnetic susceptibility varies along and between drifts these changes reflect source variations along the margin interglacial sediments show less clear trends and their ird was derived from a wider area downslope processes were dominant in glacials but alongslope processes may have attained equal importance in interglacials the area contrasts with the east antarctic continental slope in the se weddell sea where ice rafting is the dominant process and where interglacial sedimentation rates are much higher than glacial the differences in glacial setting and margin physiography can account for these contrasts

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

re os geochronology and os isotope fingerprinting of petroleum sourced from a type i lacustrine kerogen insights from the natural green river petroleum system in the uinta basin and hydrous pyrolysis experiments abstract rhenium osmium re os geochronology of marine petroleum systems has allowed the determination of the depositional age of source rocks as well as the timing of petroleum generation in addition os isotopes have been applied as a fingerprinting tool to correlate oil to its source unit to date only classic marine petroleum systems have been studied here we present re os geochronology and os isotope fingerprinting of different petroleum phases oils tar sands and gilsonite derived from the lacustrine green river petroleum system in the uinta basin usa in addition we use an experimental approach hydrous pyrolysis experiments to compare to the re os data of naturally generated petroleum in order to further understand the mechanisms of re and os transfer to petroleum the re os geochronology of petroleum from the lacustrine green river petroleum system 19 14 ma all petroleum phases broadly agrees with previous petroleum generation basin models 25 ma suggesting that re os geochronology of variable petroleum phases derived from lacustrine type i kerogen has similar systematics to type ii kerogen e g selby and creaser 2005a selby and creaser 2005b finlay et al 2010 however the large uncertainties over 100 in some cases produced for the petroleum re os geochronology are a result of multiple generation events occurring through a 3000 m thick source unit that creates a mixture of initial os isotope compositions in the produced petroleum phases the 187os 188os values for the petroleum and source rocks at the time of oil generation vary from 1 4 to 1 9 with the mode at 1 6 oil to source correlation using os isotopes is consistent with previous correlation studies in the green river petroleum system and illustrates the potential utility of os isotopes to characterize the spatial variations within a petroleum system hydrous pyrolysis experiments on the green river formation source rocks show that re and os transfer are mimicking the natural system this transfer from source to bitumen to oil does not affect source rock re os systematics or os isotopic compositions this confirms that os isotope compositions are transferred intact from source to petroleum during petroleum generation and can be used as a powerful correlation tool these experiments further confirm that re os systematics in source rocks are not adversely affected by petroleum maturation overall this study illustrates that the re os petroleum geochronometer and os isotope fingerprinting tools can be used on a wide range of petroleum types sourced from variable kerogen types