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

mass balance of east antarctic glaciers and ice shelves from satellite data thevelocityandmassdischarge of ninemajoreast antarcticglaciersnot draining into the ross or filchner ronne ice shelves is investigated using interferometric syntheticapertureradar insar datafromtheeuropeanremote sensingsatellite1and2 ers 1 2 andradarsat 1 theglaciersare david ninnis mertz totten scott denman lambert shiraseandstancomb wills insarisusedtolocatetheirgroundinglinewithpre cision ice velocity is measured with either insar or a speckle tracking technique ice thickness is deduced from prior determined ice shelf elevation assuming hydrostatic equi librium mass fluxes are calculated both at the grounding line and at a flux gate located downstream the grounding line flux is compared to a mass input calculated from snow accumulation to deduce the glacier mass balance the calculation is repeated at the flux gate downstream of the grounding line to estimate the average bottom melt rate of the ice shelf under steady state conditions the main results are 1 grounding lines are found several tens of km upstream of prior identified positions not because of a recent ice sheet retreatbutbecause of the inadequacy of prior determined grounding line positions 2 no gross imbalance between outflow and inflow is detected on the nine glaciersbeing investi gated with anuncertainty of 10 20 prior determined largely positive mass imbalances wereduetoanincorrectlocalizationofthegroundingline 3 highratesofbottommelting 24 7 micea 1 are inferred neargrounding zones where ice reaches the deepestdraft a few glaciers exhibit lower bottom melt rates 4 7 micea 1 bottom melting however appearsto be a major source of mass loss on antarctic ice shelves

a two phase approach to the simulation of the combined isotope salinity signal of marine ice a modified boundary layer box diffusion model is used to simulate the production of the combined oxygen isotope salinity signal of a body of marine ice the final signal is considered to be the result of fractionation and segregation processes occurring first upon the formation of frazil crystals in suspension and second during their accumulation and consolidation due to the advancement of a freezing front the model results are compared to oxygen isotope and salinity values in a 45 m marine ice core from the nansen ice shelf antarctica parameters more suitable to the modeling of marine ice as opposed to sea ice are introduced and conclusions are drawn on the origin of the impurity characteristics of marine ice including the still unresolved question of its anomalously low salinity

seasonal changes in silica deposition in hot spring systems abstract monthly monitoring of water chemistry at octopus spring yellowstone national park reveals no changes in the chemical composition over this eight month study differences in silica deposition are attributed to flow dynamics and seasonal air temperatures colder air temperatures in winter promote visible accumulation of siliceous sinter probably as a result of decreased solubility periodic drying promotes accumulation and subsequent dehydration of the silica this combination of intermittent supply and periodic dehydration may account for the laminated siliceous deposits observed in the geological record the processes described herein are important in the siliceous preservation of microbial fossils but the following observations underscore the difficulty in preserving microbial fabrics in the geological record pools populated by cyanobacteria are bounded by margins built of silicified microbial mats silicification of the pool margins occurs through an evaporative process called wicking silica rich water is transported upward through the microbial mats by capillary action depositing silica at the surface of the mat as water evaporates the upper surface of such wicking deposits is extremely resistant to brecciation but the underlying support is highly friable and would be difficult to preserve intact in winter months temporary isolation of some pools from active flow causes fragile silica deposition around biological and other debris film coated gas bubbles on the surfaces of microbial mats are observed during all seasons and probably are filled with oxygen released during photosynthesis although this has not been confirmed they become coated with silica during winter months presumably because of increased cooling possibly forming thin discontinuous silica sheets as observed in the geological record

filamentous microfossils in a 3 235 million year old volcanogenic massive sulphide deposit the record of archaean microfossils is sparse1 of the few bona fide fossil assemblages most are from shallow water settings and they are typically associated with laminated stromatolitic sedimentary rocks2 3 4 microfossils from deep sea hydrothermal systems have not been reported in precambrian rocks 544 million years old although thermophilic microbes are ubiquitous in modern sea floor hydrothermal settings5 6 and apparently have the most ancient lineages7 8 here i report the discovery of pyritic filaments the probable fossil remains of thread like microorganisms in a 3 235 million year old deep sea volcanogenic massive sulphide deposit from the pilbara craton of australia from their mode of occurrence the micro organisms were probably thermophilic chemotropic prokaryotes which inhabited sub sea floor hydrothermal environments they represent the first fossil evidence for microbial life in a precambrian submarine thermal spring system and extend the known range of submarine hydrothermal biota by more than 2 700 million years9 such environments may have hosted the first living systems on earth consistent with proposals for a thermophilic origin of life10 11 12 13

surface velocity and mass balance of ice streams d and e west antarctica over 75 000 surface velocity measurements are extracted from sequential satellite imagery of ice streams d and e to reveal a complex pattern of flow not apparent from previous measurements horizontal and vertical strain rates calculated from surface velocity indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher ten airborne radar profiles and one surface based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream improved data collection methods increase data density substantially reducing random errors in velocity however systematic errors continue to limit the ability of the flux differencing technique used here to resolve local variations in mass balance nevertheless significant local variations in mass balance are revealed while overall ice streams d and e are in approximate equilibrium an earlier estimate of the net mass balance for ice stream d is improved

reassessment of net surface mass balance in antarctica abstract recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over antarctica a new elevation model of antarctica derived from ers 1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across antarctica the areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines the delineation of drainage basins was achieved using an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive microwave satellite data being employed as a forcing field to control the interpolation of the sparse in situ observations basinwide integrals of net surface mass balance were calculated using t

investigating processes of marine ice formation in a floating ice tongue by a high resolution isotopic study a better knowledge of boundary conditions near a grounding line is critical for understanding the behavior of ice shelves and floating glaciers we show here thatsignificant information can be gained from a high resolution isotopic and textural studyof marine ice accreted at the bottom of a floating glacier near its grounding line two different types of marine ice have been found type 1 is bubble and debris free icewith properties which we believe can be explained by intrusion of brackish water inopen basal fissures closing of the fissures by progression of a freezing front from thesides is precluded and filling by frazil ice is favored type 2 is made of thin clear iceand debris layers which are thought to have formed when a subglacial water filledsediment enters into contact with seawater and is subjected to freezing under a double diffusion process the paper also stresses that in a d 18o diagram the alignment of marine ice samples on a mixing line does not necessarily imply a mixture of continental water and seawater in varying proportions

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

concretionary methane seep carbonates and associated microbial communities in black sea sediments abstract gas seeps in the euxinic northwestern black sea provide an excellent opportunity to study anaerobic methane based ecosystems with minimum interference from oxygen dependent processes an integrated approach using fluorescence and electron microscopy fluorescence in situ hybridization lipid biomarkers stable isotopes 13c and petrography revealed insight into the anatomy of concretionary methane derived carbonates currently forming within the sediment around seeps some of the carbonate concretions have been found to be surrounded by microbial mats the mats harbour colonies of sulphate reducing bacteria dss group and archaea anme 1 putative players in the anaerobic oxidation of methane isotopically depleted lipid biomarkers indicate an uptake of methane carbon into the biomass of the mat biota microbial metabolism sustains the precipitation of concretionary carbonates significantly depleted in 13c the concretions consist of rectangularly orientated rod to dumbbell shaped crystal aggregates made of fibrous high mg calcite the sulphate reducing bacteria exhibit intracellular storage inclusions and magnetosomes with greigite fe3s4 indicating that iron cycling is involved in the metabolism of the microbial population transfer of fe3 into the cells is apparently mediated by abundant extracellular vesicles resembling known bacterial siderophore vesicles marinobactine in size 20 to 100 nm and structure

magnetic anisotropy magnetostatic interactions and identification of magnetofossils 1 single domain magnetite particles produced by magnetotactic bacteria mtb and aligned in chains called magnetosomes are potentially important recorders of paleomagnetic paleoenvironmental and paleolife signals rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes magnetofossils preserved in geological materials however ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post depositional diagenesis in this paper magnetic properties of three sets of samples containing extracted magnetosomes of the culturedmagnetospirillum magneticumstrain amb 1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties intact mtb and well dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction this results in a change of the magnetic signature toward properties typical of interacting single domain particles i e a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization decreasing in the crossing point of the wohlfarth cisowski test and in the delta ratio between losses of field and zero field cooled remanent magnetization across the verwey transition as well as vertical broadening of the first order reversal curve distribution we propose a new diagram that summarizes the verwey transition properties with diagnostic limits for intact and collapsed chains of magnetosomes this diagram can be used in conjunction with other parameters to identify unoxidized magnetofossils in sediments and rocks

a new velocity map for byrd glacier east antarctica from sequential aster satellite imagery this is the publisher s version also available electronically from http www ingentaconnect com

a model of marine ice formation within antarctic ice shelf rifts large rifts that open in antarctic ice shelves are known to be filled by snow accumulations ice shelf fragments and sea ice this work demonstrates how these rifts may also be filled from below through their interaction with ocean water by marine ice the model presented here quantifies both the rate of marine ice accumulation at the top of the rift which results from melt driven convection at its sides and the impact of this process on waters occupying the confined environment of the rift the results show that such a system could fill the rift with tens of meters of marine ice in the process the temperature and salinity of the ambient water in the rift evolve through two distinct phases the first is characterized by rapid change that takes the properties of the ambient water to near equilibrium values followed by a second phase of much slower change the melting rate at the lower part of the walls of the rift emerges as the principal factor influencing many aspects of model behavior testing the model against field observations from an antarctic rift showed it to be robust and successful in reproducing the main observed features including the presence of a thick layer of supercooled ocean water inside the rift

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

metallic ion binding by bacillus subtilis implications for the fossilization of microorganisms the silicification of bacterial cells has been followed in a laboratory simulation by electron microscopy and energy dispersive x ray spectroscopy structural degradation of the bacteria was not influenced by silica availability but the remains of degraded cells did promote the precipitation of silica in contrast cells reacted with ferric iron before aging remained intact and structures conforming to the original morphology of the bacteria were preserved by silicification the binding of metallic ions by microbial cells in particular the retention of iron is therefore considered to be an important contributing factor to the fossilization of microorganisms

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

ice discharge of eastern dome c drainage area antarctica determined from airborne radar survey and satellite image analysis eastern dome c southern talos dome and northern taylor dome are drained by the priestley reeves david mawson and mackay outlet glaciers which flow into the scott coast on the west side of the ross sea antarctica airborne radar surveys were conducted on these glaciers to determine ice thickness and bed morphology along transverse and longitudinal profiles of the grounded and floating segments a new analysis of a i andsat thematic mapper satellite image using a tracking technique was used to measure ice velocity at grounding lines and along ice tongues the integration of radar and satellite data helped to locate grounding lines and to calculate the ice discharge changes in ice fluxes of floating glaciers were used to determine basal melting and freezing rates the ice discharge calculated is less than half that required for a zero net surface mass balance according to the inputs given by the accumulation estimates widely adopted at present the basal melting rates of meteoric ice represent 50 of the net ablation rate

rock magnetic properties of uncultured magnetotactic bacteria abstract understanding the magnetic properties of magnetite crystals fe3o4 produced by magnetotactic bacteria mtbs is of fundamental interest in fields of geosciences biomineralization fine particle magnetism and planetary sciences the database of bulk magnetic measurements on mtbs is however still too sparse to allow for generalizations due to difficulties in obtaining bacteria cells in sufficient quantities from natural environments and the fact that only a few cultivable strains are available here we report the first series of magnetic measurements on two air dried samples containing solely mtbs wild type cocci and magnetobacterium bavaricum which were directly isolated from carbonaceous lake sediments systematic rock magnetic studies show that 1 the magnetosomes in cells are dominated by single domain sd magnetite 2 the samples have delta ratios fc zfc higher than 2 3 the measured low temperature remanence cycling curves as well as the first order reversal curve forc diagrams are significantly different to those measured on synthetic sd magnetite samples and 4 the verwey transition temperature tv 100 k of mtb cells is distinctly lower than that from stoichiometric magnetite tv 120 125 k our results provide new insights on the magnetic properties of bacterial magnetite and advance the use of magnetic proxies for decoding the paleomagnetic signals of sediments containing bacterial magnetite

satellite image derived velocity field of an antarctic ice stream the surface velocity of a rapidly moving ice stream has been determined to high accuracy and spatial density with the use of sequential satellite imagery variations of ice velocity are spatially related to surface undulations and transverse velocity variations of up to 30 percent occur such large variations negate the concept of plug flow and call into question earlier mass balance calculations for this and other ice streams where sparse velocity data were used the coregistration of images with the use of the topographic undulations of the ice stream and the measurement of feature displacement with cross correlation of image windows provide significant improvements in the use of satellite imagery for ice flow determination

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

properties of intracellular magnetite crystals produced by desulfovibrio magneticus strain rs 1 desuffiovibrio magneticus strain rs 1 is an anaerobic sulfate reducing bacterium cells form intracellular nanocrystals of magnetite but are only weakly magnetotactic in order to understand the unusual magnetic response of this strain we studied magnetite crystals within cells grown with fumarate and sulfate many cells grown under either condition did not form magnetic crystals while others contained only i to 18 small similar to 40 nm magnetite containing magnetosomes bulk magnetic measurements of whole cells showed a superparamagnetic like behavior indicating that many of the magnetite crystals are too small to have a permanent magnetic moment at ambient temperature the temperature of the verwey transition is lower similar to 86 k than of magnetite from other magnetotactic strains likely indicating partial oxidation of magnetite into maghemite as a result of the small size and small number of magnetite magnetosomes the magnetic moments of most cells grown anaerobically with fumarate or sulfate are insufficient for magnetotaxis in addition to intracellular magnetite in some cultures another iron oxide hematite formed on the surfaces of cells the hematite grains are embedded in an extracellular polymeric material indicating that the crystals likely resulted from a biologically induced mineralization process although the hematite particles appear to consist of aggregations of many small 5 to 10 nm grains the grains have a consensus orientation and thus the whole particle diffracts as a single crystal the aligned arrangement of nanoparticles within larger clusters may reflect either a templated nucleation of hematite crystallites in an extracellular organic matrix or result from a self assembling process during the crystallization of hematite from ferric gels or ferrihydrite

significant marine ice accumulation in the ablation zone beneath an antarctic ice shelf high resolution crystallographic salinity and isotopic analyses of a 45 m ice core reveal the presence of a thick layer of marine ice near the grounding line of the nansen ice shelf antarctica the anomalous formation of marine ice in a zone assumed to be the site of active basal melting leads us to propose the hypothesis of large basal crevasses as a favorable environment for important marine ice accretion this hitherto unexplored possibility is supported by the overall field configuration and by the discrepancy in some ice properties between this core and the marine ice sections of previous drilling projects these findings could have important implications for the general stability of ice shelves and their disintegration processes the specific properties of this core reveal that marine ice is post genetically deformed

detection of noninteracting single domain particles using first order reversal curve diagrams we present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles unisd in rocks and sediments the method is based on high resolution measurements of first order reversal curves forcs unisd particles have a unique forc signature that can be used to isolate their contribution among other magnetic components this signature has a narrow ridge along the hc axis of the forc diagram called the central ridge which is proportional to the switching field distribution of the particles therefore the central ridge is directly comparable with other magnetic measurements such as remanent magnetization curves with the advantage of being fully selective to sd particles rather than other magnetic components this selectivity is unmatched by other magnetic unmixing methods and offers useful applications ranging from characterization of sd particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments

snow accumulation rates in northern victoria land antarctica by firn core analysis a multiparametric chemical isotopic and physical study on three shallow firn cores sampled in northernvictoria land was carried out to obtain glaciological information and climatic data in this antarctic region sampling areas were accurately prospected to identify sites located at different altitudes and distances from the sea where the snow accumulation was not influenced by katabatic wind redistribution or summer melting stratigraphic isotopic 18 o and chemical h 2 o 2 msa and nssso 4 2 profiles were mutually examined for dating purposes and to determine the mean snow accumulation rates at three different stations annual accumulation rates of 85 420 kg m 2 a 1 were determined in the period 1971 92 an inverse pattern between accumulation rate and altitude was shown by the progression of the mean annual rates of 160 203 and 260 kg m 2 a 1 respectively in the highest medium and lowest stations the mean accumulation value of all northern victoria land data available 170 kg m 2 a 1 represents a decrease of up to 35 with respect to the estimated value most widely used until now our accumulation value is very close to that required for a zero net surface mass balance according to ice discharge a linear relationship with a gradient of 0 81 c has been found between mean 18 o values and mean annual surface temperature for different ice cores drilled in northern victoria land

rapid in situ silicification of microbes at loburu hot springs lake bogoria kenya rift valley microbial mats located along the margins of hot spring pools and outflow channels at lake bogoria kenya are commonly silicified forming friable laminated crusts columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small 2 m silica spheroids rapid silicification of the microbes which may begin while some are still alive can preserve sheaths and in some examples the filaments capsules and cells although this provides evidence of their general morphology the biological features that are required for taxonomic identifications are commonly poorly preserved the silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes almost all the silica at the loburu springs nucleates on microbial substrates this affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

evidence for exclusively inorganic formation of magnetite in martian meteorite alh84001 magnetite crystals produced by terrestrial magnetotactic bacterium mv 1 are elongated on a 111 crystallographic axis in a so called truncated hexa octahedral shape this morphology has been proposed to constitute a biomarker i e formed only in biogenic processes a subpopulation of magnetite crystals associated with carbonate globules in martian meteorite alh84001 is reported to have this morphology and the observation has been taken as evidence for biological activity on mars in this study we present evidence for the exclusively inorganic origin of 111 elongated magnetite crystals in alh84001 we report three dimensional 3 d morphologies for 1000 magnetite crystals extracted from 1 thermal decomposition products of fe rich carbonate produced by inorganic hydrothermal precipitation in laboratory experiments 2 carbonate globules in martian meteorite alh84001 and 3 cells of magnetotactic bacterial strain mv 1 the 3 d morphologies were derived by fitting 3 d shape models to two dimensional bright field transmission electron microscope tem images obtained at a series of viewing angles the view down the 110 axes closest to the 111 elongation axis of magnetite crystals 111 110 0 provides a 2 d projection that uniquely discriminates among the three 111 elongated magnetite morphologies found in these samples 111 elongated truncated hexa octahedron 111 tho 111 elongated cubo octahedron 111 eco and 111 elongated simple octahedron 111 eso all 111 elongated morphologies are present in the three types of sample but in different proportions in the alh84001 martian meteorite and in our inorganic laboratory products the most common 111 elongated magnetite crystal morphology is 111 eco in contrast the most common morphology for magnetotactic bacterial strain mv 1 is 111 tho these results show that 1 the morphology of 111 elongated magnetite crystals associated with the carbonate globules in martian meteorite alh84001 is replicated by an inorganic process and 2 the most common crystal morphology for biogenic mv 1 magnetite is distinctly different from that in both alh84001 and our inorganic laboratory products therefore 111 elongated magnetite crystals in alh84001 do not constitute as previously claimed a robust biosignature and in fact an exclusively inorganic origin for the magnetite is fully consistent with our results furthermore the inorganic synthesis method i e the thermal decomposition of hydrothermally precipitated fe rich carbonate is a process analogue for formation of the magnetite on mars namely precipitation of carbonate globules from carbonate rich hydrothermal solutions followed at some later time by a thermal pulse perhaps in association with meteoritic impact or volcanic processes on the martian surface

influence of thermophilic bacteria on calcite and silica precipitation in hot springs with water temperatures above 90 c evidence from kenya and new zealand hot and boiling springs in kenya and new zealand that are emitting water with temperatures more than 90 c are commonly characterized by a complex array of caco3 and sio2 precipitates that have been formed through abiogenic and biogenic processes thermophilic bacteria are the only microbes that can survive in the boiling water that is discharged into pools around the spring orifice analysis of modern substrates from various springs in the kenya rift valley and the taupo volcanic zone in new zealand shows that they are inhabited by a diverse array of coccoid and filamentous bacteria in some areas these bacteria produce copious amounts of mucus that coat the substrates although the coccoid and filamentous bacteria provide substrates for caco3 and sio2 precipitation the microbes do not seem to have any direct influence on the morphology of the precipitates that are produced conversely the mucus found in these hot spring pools selectively takes up elements such as si mg al and fe but is not calcifi

biogenic mn oxide minerals coating in a subsurface granite environment abstract this paper reports the results of a study of biogenic mn oxide minerals coating in a subsurface granite environment this subterranean environment corresponds to galleries of a 16th century spring water tunnel dug throughout the granite bedrock located in porto city nw portugal several techniques xrd ftir raman icp ms tem eds fesem eds stem eds and stxm nexafs were used to assess the mechanisms involved in the formation of manganese oxides specifically birnessite and todorokite these manganese oxides presented peculiar shapes nano dimensions low degree of crystallinity and high levels of some trace elements such as p ca c al si ba and zn they were associated with large amounts of extracellular polymeric substances exuded by filamentous bacterial communities which serve as nuclei for preferential precipitation of manganese oxides on the extracellular sheaths scientific assessment revealed that biological activity played a major role in the development of these manganese oxides

ice flow physical processes derived from the ers 1 high resolution map of the antarctica and greenland ice sheets accepted 1999 july 7 received 1999 june 7 in original form 1998 june 24 summary the ers 1 satellite launched in 1991 has provided altimetric observations of the greenland ice sheet and 80 per cent of the antarctica ice sheet north of 82 s it was placed in a geodetic 168 day repeat orbit between april 1994 and march 1995 yielding a 1 5 km across track spacing at latitude 70 with a higher along track sampling of 350 m we have analysed the waveform altimetric data from this period to compute maps with a 1 30 grid size data processing consists of correcting for environmental factors and editing and retracking the waveforms a further step consists of reducing the radial orbit error through crossover analysis and correcting the slope error to second order the high resolution topography of both ice sheets reveals numerous details a kilometre scale surface roughness running at 45 from the flow direction is the dominant topographic characteristic of both continents antarctica also exhibits many scars due to local flow anomalies several physical processes can be identified abrupt transitions from deformation to sliding and vice versa and impressive strike slip phenomena inducing en echelon folds

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

motion of an antarctic glacier by repeated tidally modulated earthquakes subglacial seismicity reveals information about glacier behaviour analysis of repeated seismic events beneath an antarctic outlet glacier is consistent with sliding of debris laden ice over a bedrock asperity with an event frequency that is modulated by the ocean tides

ice composition evidence of marine ice transfer along the bottom of a small antarctic ice shelf the existence of marine ice transfer along the underside of the hell s gate ice shelf victoria land is indicated by an isotopic and chemical study of ice cores because of top surface ablation the marine ice formed at the ice shelf ocean interface ultimately appears at shelf surface a succession of congelation platelet and frazil ice is shown to occur the combined study of stable isotope composition and of the sodium content of these different ice types proves to be a valuable tool for specifying the ice shelf ocean interactions in this area two different freezing zones separated by a melting rone exist the parent water for the frazil ice is meltwater from congelation ice which appears in the upstream zone

multiscale characterization of pyritized plant tissues in blueschist facies metamorphic rocks pyritized plant tissues with well preserved morphology were studied in rocks from vanoise western alps france that experienced high pressure low temperature metamorphic conditions in the blueschist facies during the alpine orogeny organic and inorganic phases composing these fossils were characterized down to the nanometer scale by raman microspectroscopy scanning transmission x ray microscopy and transmission electron microscopy the graphitic but disordered organic matter composing these fossils is chemically and structurally homogeneous and mostly contains aromatic functional groups its original chemistry remains undefined likely because it was significantly transformed by diagenetic processes and or thermal degradation during metamorphism various mineral phases are closely associated with this organic matter including sulphides such as pyrite and pyrrhotite carbonates such as ankerite and calcite and iron oxides a tentative time sequence of formation of these diverse mineral phases relative to organic matter decay is proposed the absence of traces of organic matter sulphurization the pervasive pyritization of the vascular tissues and the presence of ankerite suggest that the depositional diagenetic environment of these metasediments was likely rich in reactive iron fe sulphides and ankerite likely precipitated early and might have promoted the preservation of the fossilized biological soft tissues by providing mechanical resistance to compaction during diagenesis and subsequent metamorphism in contrast iron oxides which form rims of 100 nm in thickness at the interface between organic matter and fe sulphides may result from metamorphic processes this study illustrates that it may be possible in some instances to deconvolve metamorphic from diagenetic imprints and opens new avenues to better constrain processes that may allow the preservation of organic fossils during diagenesis and metamorphism

exogenous carbonaceous microstructures in early archaean cherts and bifs from the isua greenstone belt implications for the search for life in ancient rocks abstract the microstructure of hf etched samples of early archaean banded iron formations bifs and cherts from the 3 7 b y old isua greenstone belt southwestern greenland was investigated using high resolution scanning electron microscopy equipped with an electron diffraction system capable of analysing light elements the rocks contain both endogenous of internal origin and exogenous of external origin carbonaceous microstructures the former consist of inclusions of graphite and possibly small amorphous carbonaceous particles both embedded in metacherts however further in situ tem studies are needed to verify the endogeneity of the amorphous particles moreover these rocks also contain endolithic microorganisms i e inhabiting cracks in rocks as well as undifferentiated carbonaceous matter that occur in fractures and cracks between grains the microorganisms include cyanobacteria filamentous microorganisms such as fungal hyphae and possibly bacteria as well as large unidentified cells or spores most of the microorganisms appear to have been fossilised the endoliths are evidently younger than the host rock but must have infiltrated at different periods most likely after the inland ice retreated 8000 years ago the presence of endolithic carbonaceous matter in cracks and microfissures in these rocks will affect any analyses of bulk samples such as carbon isotopes and chemical biomarkers as well as analyses of acid macerated residues thus previous isotope measurements made on bifs and cherts from isua may reflect younger contamination rather than an endogenous original signal likewise some of the previously described isuan microorganisms probably represent recent endolithic contamination

morphological features of elongated anisotropic magnetosome crystals in magnetotactic bacteria of the nitrospirae phylum and the deltaproteobacteria class high resolution transmission electron microscopy was used to study the crystallographic habits of the elongated magnetite crystals variously described as bullet tooth or arrowhead shaped in two recently described uncultured magnetotactic bacteria belonging to the nitrospirae phylum designated candidatus magnetoovum mohavensis strain lo 1 and candidatus thermomagnetovibrio paiutensis strain hsmv 1 and a cultured sulfate reducing magnetotactic bacterium of the deltaproteobacteria class of the proteobacteria phylum designated strain av 1 the elongation axes of the magnetosomes do not coincide with the easy magnetization axis which is 111 but they are parallel to 100 in lo 1 and av 1 and parallel to 110 in hsmv 1 in all three strains magnetosome magnetite crystals appear to elongate at constant width resulting in asymmetric shapes idealized crystal morphologies are proposed neither the control mechanism over crystal growth nor the adaptiveness if any of such unusual crystal habits are known at the moment since similar elongated and asymmetric morphologies are unknown in inorganically formed magnetite crystals these forms of magnetosome magnetite appear to be excellent biomarkers

new velocity map and mass balance estimate of mertz glacier east antarctica derived from landsat sequential imagery automatic feature tracking on two landsat images acquired in january 2000 and december 2001 generates a complete velocity field of mertz glacier east antarctica this velocity field shows two main tributaries to the ice stream between the tributaries a likely obstruction feature in the bedrock results in a slow down of the flow a third landsat image acquired in 1989 and combined with the 2000 image permits the determination of the glacier mean velocity during the 1990s although some parts of the mertz glacier system show evidence of slight speed increase we conclude that the mertz flow speed is constant within our uncertainty 35 m a 1 using this complete velocity field new estimates of the ice discharge flux 17 8 km 3 a 16 4 gt a and of the basal melting of the tongue 11m a of ice are given our results lead to an apparent imbalance of the drainage basin ice discharge 3 5 km 3 a lower than the accumulation considering previous studies in the mertz glacier area we discuss the uncertainty of this imbalance and the problems with accumulation mapping for this region

carbonaceous cherts of the barberton greenstone belt south africa isotopic chemical and structural characteristics of individual microstructures abstract carbonaceous matter occurring in chert deposits of the 3 4 3 2 ga old barberton greenstone belt bgb south africa has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration here a detailed study is presented of in situ analysis of carbonaceous particles by lrs laser raman spectroscopy and sims secondary ion mass spectrometry reporting degree of structural disorder carbon isotope ratio and nitrogen to carbon ratio this combination of in situ analytical tools is used to interpret the 13 c values of only the best preserved carbonaceous remains enabling the rejection of non indigenous unmetamorphosed material as well as the exclusion of strongly hydrothermally altered carbonaceous particles raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub to lower greenschist facies metamorphic event although this identifies these organics as indigenous to the cherts it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re deposition of organics this suggest that morphological interpretation of these carbonaceous particles and in general of putative microfossils or microlaminae in hydrothermally altered early archean cherts should be made with caution a chert in the hooggenoeg formation which is older than and has been hydrothermally altered by a volcanic event 3445 ma ago contains strongly altered carbonaceous particles with a uniform n c ratio of 0 001 and a range of 13 c that is shifted from its original value cherts of the kromberg formation post date this volcanic event and contain carbonaceous particles with a n c ratio between 0 002 and 0 006 both the buck reef chert and the footbridge chert of the kromberg formation have retained fairly well preserved 13 c values with ranges from 34 to 24 and 40 to 32 respectively abiologic reactions associated with hydrothermal serpentinization of ultramafic crust such as fischer tropsch synthesis were an unlikely source for carbonaceous material in these cherts the carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

taphonomy of very ancient microfossils from the 3400ma strelley pool formation and 1900ma gunflint formation new insights using a focused ion beam abstract focused ion beam fib milling permits the accurate extraction of ultrathin c 100 nm cross sectional lamellae from microfossils found in geological thin sections subsequent tem analysis of these lamellae can provide unique insights into the ultrastructure chemistry and taphonomy of precambrian microfossils at the micrometer to nanometer scale combining serial fib milling with sem imaging extends this capability to three dimensional 3d tomographic reconstruction and visualization of precambrian microfossils revealing information not available in light microscopy here we apply these techniques to two iconic silicified microfossil assemblages from the 3400 ma strelley pool formation of western australia and the 1900 ma gunflint formation of canada all the examined microfossils have carbonaceous walls surrounded by pure silica impregnation of microfossil walls by nano grains of silica is common together with variable degrees of wall displacement and replacement by silica all microfossils are rigidly preserved in 3d and show little or no folding or compression however there are also notable differences in taphonomic preservation our examples of the spheroidal gunflint microfossil huroniospora showed the highest fidelity of preservation with a continuous carbonaceous wall fossilized by spheroidal nano silica grains that resemble those found on bacterial surfaces in modern silicifying hot spring environments the nucleation of these silica nano spheres on the microfossil walls has induced an artificial saw tooth like ridged wall texture that may subsequently hinder species level identification the strelley pool microfossils in comparison show a lower fidelity of preservation with small parts of the microfossil walls completely replaced by silica plus extensive recrystallization of spheroidal silica nano grains to angular micro quartz our examples of the sheath like filamentous gunflint microfossil siphonophycus showed the lowest fidelity of preservation with many gaps in the carbonaceous walls and significant redistribution of carbon by recrystallizing silica grains a model is presented to explain these observations criteria for distinguishing highly probable microfossils from non cellular carbonaceous microstructures e g botryoids and grain coatings using fib based imaging are put forward for the first time here using examples drawn from the strelley pool formation and comparisons with younger gunflint material the combined in situ techniques of fib tem and fib sem nano tomography potentially provide a wealth of new nano scale information regarding the biogenicity antiquity and taphonomy of precambrian microfossils however the destructive nature of both techniques makes their application to unique palaeontological specimens problematical

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

an overview of biomineralization processes and the problem of the vital effect biomineralization links soft organic tissues which are compositionally akin to the atmosphere and oceans with the hard materials of the solid earth it provides organisms with skeletons and shells while they are alive and when they die these are deposited as sediment in environments from river plains to the deep ocean floor it is also these hard resistant products of life which are mainly responsible for the earth s fossil record consequently biomineralization involves biologists chemists and geologists in interdisciplinary studies at one of the interfaces between earth and life leadbeater and riding 1986 biomineralization refers to the processes by which organisms form minerals the control exerted by many organisms over mineral formation distinguishes these processes from abiotic mineralization the latter was the primary focus of earth scientists over the last century but the emergence of biogeochemistry and the urgency of understanding the past and future evolution of the earth are moving biological mineralization to the forefront of various fields of science including the earth sciences the growth in biogeochemistry has led to a number of new exciting research areas where the distinctions between the biological chemical and earth sciences disciplines melt away of the wonderful topics that are receiving renewed attention the study of biomineral formation is perhaps the most fascinating truly at the interface of earth and life biomineralization is a discipline that is certain to see major advancements as a new generation of scientists brings cross disciplinary training and new experimental and computational methods to the most daunting problems it is however by no means a new field the first book on biomineralization was published in 1924 in german by w j schmidt schmidt 1924 and the subject has continued to intrigue a dedicated community of scientists for many years until the early 1980s the field was known as

nucleation of calcium carbonate on bacterial nanoglobules nucleation of calcium carbonate on microbial cell material may have been the dominant mode of microbial carbonate formation during most of earth s history current knowledge predicts that nucleation takes place on the cell surface or on extracellular polymeric substances however the initial nucleation steps have not been described in detail and the process remains elusive here we describe the bacterial nucleation of calcium carbonate at the nanometer scale in our precipitation experiment with sulfate reducing bacteria srb the bulk of calcium carbonate precipitates on hundreds of individual globules 60 200 nm in diameter globules originate from the srb cell surface but calcify significantly only when released to the culture medium similar globules have been observed albeit at a much larger scale in other bacterial precipitation experiments and in many natural microbial carbonates suggesting that the process we describe could be an important step in microbial calcification

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

dynamics and mass balance of four large east antarctic outlet glaciers this is the publisher s version also available electronically from http www ingentaconnect com

nanosims insights to biogenicity and syngeneity of archaean carbonaceous structures nanosims is a relatively new technology that is being applied to ancient carbonaceous structures to gain insight into their biogenicity and syngeneity nanosims studies of well preserved organic microfossils from the neoproterozoic similar to 0 8ga bitter springs formation have established elemental distributions in undisputedly biogenic structures results demonstrate that sub micron scale maps of metabolically important elements carbon c nitrogen measured as cn ion and sulfur s can be correlated with kerogenous structures identified by optical microscopy spatial distributions of c cn and s in individual microfossils are nearly identical and variations in concentrations of these elements parallel one another in elemental maps c cn and s appear as globules aligned to form remnant walls or sheaths of fossiliferous structures the aligned character and parallel variation of c and cn are the strongest indicators of biogenicity

the dynamics of ice sheet outlets a comparison of data from aircraft altimetry landsat imagery and radia echo sounding has shown characteristic surface topographies associated with sheet and stream flow the transition between the two is abrupt and occurs at a step in the subglacial topography this marks the onset of basal sliding and high velocities caused by subglacial water it results in crevassed amphitheatre like basins round the head of outlet glaciers it is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase this abrupt transition appears to be topographically controlled since basal temperatures are at the pressure melting point well inland of the change in regime the marie byrd land ice streams exhibit qualitative differences from other ice sheet outlets however the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice the repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion this is concentrated towards the heads of outlet glaciers up stream of the region where significant basal decoupling occurs this may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

in situ silicification of an icelandic hot spring microbial mat implications for microfossil formation transmission electron microscopy and energy dispersive x ray analysis revealed that filamentous phototrophic bacteria resembling chlorojlexus aurantiacus underwent rapid silicification in an icelandic hot spring microbial mat the mineralization associated with the cells occurred both extracellularly within and on the external sheaths of the bacteria and intracellularly within the cytoplasm the exceptional preservation of the bacterial sheaths is due to the presence of distinct mineral nucleation sites this results in the production of silica casts of the bacteria which bear a striking resemblance to microbial remains in ancient microfossil assemblages rcsumc les analyses par microscopie clectronique i transmission et par 1 6nergie dispersive des rayons x ont rcvcl6 que les bactkries filamenteuses phototrophes ressemblant b chlorojlexus aurantiacus ont subi une silicification rapide au sein d un tapis microbien de source therrnale en islande la mineralisation associee aux cellules se presente sous forme extracellulaire b l interieur de et sur la membrane externe des bacteries et sous forrne intracellulaire i l interieur du cytoplasme la preservation exceptionnelle de la membrane des bacteries est due b la presence de sites de nucleation minerale disperses le resultat est la production des moulages siliceux des bactkries affichant une resemblance remarquable avec les vestiges microbiens d anciens assemblages microfossiles traduit par la raaction fossil evidence of the earth s earliest biosphere shows that complex communities of microorganisms were abundant and well distributed throughout the precambrian era among the richest beds of microbial fossils are those found in the gunflint iron formation north of lake superior in canada barghoorn and tyler 1965 the early archaean apex chert of australia schopf and packer 1987 schopf 1993 and the riphean section of the anabar uplift in russia sergeev 1993 all ranging in age from 0 8 to 3 5 ga the distribution and types of microorganisms preserved as cellularly intact microfossils are remarkably similar to those in modern microbial mats in which organic rich layers formed by cyanobacteria and or other types of filamentous bacteria alternate with layers that are comprised of detrital inorganic sediment and authigenic mineral precipitates morse and mackenzie 1990 these benthic microbial communities are considered to be analogous and possibly homologous with those that were preserved billions of years ago as stromato

snow dunes and glazed surfaces in antarctica new field and remote sensing data as part of the international trans antarctic scientific expedition project the italian antarctic programme undertook two traverses from the terra nova station to talos dome and to dome c along the traverses the party carried out several tasks drilling glaciological and geophysical exploration the difference in spectral response between glazed surfaces and snow makes it simple to identify these areas on visible near infrared satellite images integration of field observation and remotely sensed data allows the description of different mega morphologic features wide glazed surfaces sastrugi glazed surface fields transverse dunes and megadunes topography global positioning system ground penetrating radar and detailed snow surface surveys have been carried out providing new information about the formation and evolution of mega morphologic features the extensive presence up to 30 of glazed surface caused by a long hiatus in accumulation with an accumulation rate of nil or slightly negative has a significant impact on the surface mass balance of a wide area of the interior part of east antarctica the aeolian processes creating these features have important implications for the selection of optimum sites for ice coring because orographic variations of even a few metres per kilometre have a significant impact on the snow accumulation process remote sensing surveys of aeolian macro morphology provide a proven high quality method for detailed mapping of the interior of the ice sheet s prevalent wind direction and could provide a relative indication of wind intensity

repeating earthquakes from rupture of an asperity under an antarctic outlet glacier the inner regions of the antarctic continent are generally regarded as nearly aseismic although microseismicity is known to occur beneath some outlet ice streams related to the interaction between the fast flowing ice and the bedrock here we show the occurrence of unusual earthquakes beneath an antarctic outlet glacier that share almost the same magnitude pointing to the repeated rupture of a single asperity these seismic events produce waveforms with very high similarity and uncommon spectrum and are tightly clustered in space but unlike other reported instances of repeating earthquakes on a patch of the san andreas fault they occur in frequent irregular swarms evidence locates these events at the rock ice interface under the glacier and shows the existence of stick slip motion on a smaller scale than the large slow slip events detected by global seismographs seismic behaviour of large glaciers can presumably be connected to surges in ice motion this study determines a little known environment for fracture dynamics studies while also contributing to the understanding of the coupling processes between fast flowing glaciers and bedrock that influence ice stream evolution and stability

early archean fossil bacteria and biofilms in hydrothermally influenced sediments from the barberton greenstone belt south africa abstract sem imaging of hf etched 3 3 3 5 ga cherts from the onverwacht group south africa reveals small spherical 1 m diameter and rod shaped structures 2 3 8 m in length which are interpreted as probable fossil coccoid and bacillar bacteria prokaryotes respectively preserved by mineral replacement other possibly biogenic structures include smaller rod shaped bacteriomorphs 900 m textures in the sediments in which these biogenic structures occur suggest that they were probably deposited in a shallow water environment which was subjected to intermittent subaerial exposure pervasive hydrothermal activity is evidenced by oxygen isotope studies as well as the penecontemporaneous silicification of all rock types by low temperature 220 c hydrothermal solutions

post lgm dynamic deglaciation along the victoria land coast antarctica abstract the post last glacial maximum lgm deglaciation of antarctica holds important clues for understanding past environmental changes and predicting future changes in the antarctic ice sheet cosmogenic nuclide exposure dating of glacial erratics documents the spatial and temporal glacier changes during the most recent deglaciation we collected 55 erratic cobbles from the eight glaciated benches on inexpressible island which is at the terminal area of priestley glacier terra nova bay victoria land to elucidate its post lgm deglaciation pattern analyses of the 10be ages and 26al 10be ratios suggest that priestley glacier underwent 254 m of lowering during the mid holocene between 8 9 and 5 9 ka this lowering rate 0 09 m a 1 is faster than those observed along other victoria land outlet glaciers to the north tucker and aviator but slower than the one to the south mackay the post lgm lowering of these outlet glaciers was triggered by marine ice sheet instability with their asynchronous onsets of deglaciation derived from their diachronous response times to the southwestward migration of the grounding line retreat until their synchronous termination of deglaciation at 6 ka a post lgm deglaciation model with the southwestward migration of the western ross ice shelf grounding line provides the best match to the terrestrial exposure dating results of the thinning patterns of the outlet glaciers along the victoria land coast

biogenic magnetite in stromatolites ii occurrence in ancient sedimentary environments abstract in this paper we report the discovery of fossil bacterial single domain magnetite particles in ancient stromatolites the biogenicity of the crystals was determined by the following criteria 1 distinctive morphology and habit 2 composition and 3 environment of deposition stromatolites ranging in age from the middle archean to pleistocene composed of both carbonate and chert were analyzed for the presence of single domain magnetite using rock magnetic methods the granulometry and composition of the ultra fine grained magnetite crystals extracted were determined by transmission electron microscopy and electron diffraction the oldest magnetofossils were extracted from stromatolitic chert of the gunflint iron formation which is approximately 2000 ma old the implications of these findings and the potential uses of fossil bacterial magnetite in studies of the evolution of biomineralization and prokaryotic metabolic processes paleomagnetism and as an indicator of ancient oxygen levels are discussed bacterial magnetite represents the oldest evidence of biomineralization yet discovered in the fossil record

in situ imaging of organic sulfur in 700 800 my old neoproterozoic microfossils using x ray spectromicroscopy at the s k edge the study of very ancient microfossils has recently raised contentious issues regarding interpretation of the biogenicity of the structures in situ investigation of certain elements such as sulfur within potential microfossils is a powerful complement to other methods of investigation that can provide valuable information on biogenicity we present here a first such study on precambrian microfossils from the 700 800 my old neoproterozoic draken formation svalbard using scanning x ray microscopy sxm in the fluorescence mode and x ray absorption near edge spectroscopy xanes at the sulfur k edge sxm allowed mapping of up to 300 ppm of probably endogenous sulfur within the kerogenous walls of myxococcoides chlorelloidea microfossils xanes showed that the sulfur is most likely contained in heterocyclic organic compounds such as thiophene s

mercury content of equisetum plants around mount st helens one year after the major eruption the mercury content of young equisetum plants collected around mount st helens was higher in the direction of yakima and toppenish washington northeast to east northeast than at any other compass heading and was about 20 times that measured around portland oregon the increase in substratum mercury was not as pronounced as that in plants but was also higher toward the northeast the direction taken by the may 1980 volcanic plume

the link between biomineralization and fossilization of bacteria insights from field and experimental studies abstract fossil biominerals and fossils of microorganisms in ancient rocks contain important biogeochemical signals decoding this record may reveal crucial information about the evolution of life on earth and terrestrial paleoenvironments however the identification of traces of life especially in very old rocks is extremely challenging because the morphological and chemical signatures of biominerals and microfossils are subtle of microscopic size and inevitably altered with aging in this review we stress on the fact that biomineralization is often the first step of fossilization and produces particular chemical structural and morphological features that can be preserved in fossil biominerals or microfossils with a special focus on fe biomineralization the taphonomic processes affecting biominerals and microfossils and altering their morphology and or chemical composition over time are then discussed in light of experimental fossilization simulations and field sample analyses we suggest that taxonomic biases observed in the fossil record may be related to differential abilities of species to trigger biomineralization this calls for studies of the effective biomineralizing activity and fossilization potential of each species present in highly diverse natural microbial communities finally recent analytical developments leave little doubt that very substantial progress in the study of biomineralization processes and ancient biostructures will be achieved in the near future

molecular evidence for life in the 3 5 billion year old warrawoona chert article i nfo the biological origin of organic matter in the oldest siliceous sediments cherts is still debated to address this issue the insoluble organic matter kerogen was isolated from a chert of the warrawoona group the chemical structure of the kerogen was investigated through a combination of analytical techniques including solid state 13 c nuclear magnetic resonance and pyrolysis although dominated by aromatic hydrocarbons the pyrolysate comprises a homologous series of long chain aliphatic hydrocarbons characterized by odd over even carbon number predominance this distribution is only consistent with a biological origin as kerogen must be contemporaneous of the solidification of the chert this observation should be regarded as an evidence for the presence of life on earth 3 5 by ago

crystal size and shape distributions of magnetite from uncultured magnetotactic bacteria as a potential biomarker we studied the sizes and shapes of magnetite nanocrystals produced by several types of uncultured magnetotactic bacteria to understand whether their size distributions can be used for identifying the biogenic origin of magnetite crystals in geological samples the two dimensional projections of the crystals were measured on transmission electron microscope tem images and features of crystal size and shape factor distributions csd and sfd respectively were analyzed in agreement with previous results most magnetite csd curves are asymmetric and negatively skewed however one magnetotactic strain produced particles that have a normal size distribution a statistical analysis of csds and sfds both from this and previous studies reveals similarities among magnetite from magnetotactic strains from various locations in particular crystals in a cultured marine strain mc 2 were indistinguishable from magnetite from a freshwater strain we tested whether csds of distinct magnetosome types can be recovered from the shape and size data of all particles combined in samples that contain several types of magnetosomes such samples can be used as models for rocks that contain magnetite nanocrystals of unknown and presumably various origins if the sfds of the distinct magnetosome types occurring in the same sample differ the csds of individual magnetosome types can be retrieved from bulk data in such cases the characteristic shape of the size distribution can be used for identifying magnetite as originating from magnetotactic bacteria

filamentous microfossils from the 3 500 myr old onverwacht group barberton mountain land south africa the swaziland supergroup barberton mountain land south africa has long been regarded as a promising location for the earth s oldest fossils because it includes some of the most ancient well preserved sedimentary rocks many of which contain carbonaceous matter although there have been numerous reports of microfossils from swaziland group rocks1 7 the biogenicity of most of the structures has been questioned8 10 although some of the organic spheroids are probably biogenic10 11 the best early archaean simple spheroids are generally regarded as possible microfossils 10 because organic spheroids may form abiotically in several ways12 the discovery of less simple biological morphologies is therefore important in establishing the existence of early life forms in the early archaean uniformly sized curving filaments especially tubular ones are difficult to explain as anything other than the fossil remains of filamentous organisms here we report the discovery of numerous filaments from two different stratigraphical positions in the 3 500 myr old onverwacht group of the swaziland supergroup their morphologies and abundance provide convincing evidence for the existence of bacteria or cyanobacteria like organisms on the earth during the early archaean this supports recent reports of similar filamentous microfossils from 3 500 myr old rocks from western australia13

antarctica measuring glacier velocity from satellite images many landsat images of antarctica show distinctive flow and crevasse features in the floating part of ice streams and outlet glaciers immediately below their grounding zones some of the features which move with the glacier or ice stream remain visible over many years and thus allow time lapse measurements of ice velocities measurements taken from landsat images of features on byrd glacier agree well with detailed ground and aerial observations the satellite image technique thus offers a rapid and cost effective method of obtaining average velocities to a first order of accuracy of many ice streams and outlet glaciers near their termini

microbially influenced formation of 2 724 million year old stromatolites nanoscale evidence suggests that the tumbiana formation stromatolites in australia were influenced by microbial activity in the stromatolites clusters of organic globules are closely associated with 2 724 million year old aragonite crystals