oceanic transverse ridges a flexural response to fracture zone normal extension one of the most puzzling characteristics of sea floor morphology is the occurrence of anomalously shallow fracture zone parallel oceanic transverse ridges a model is proposed for the formation of transverse ridges near lat 21 and 24 n on the mid atlantic ridge in which the differential responses of large offset and small offset fracture zones to recent changes in spreading direction result in the generation of normal faults that coincide with the off axis traces of fracture zones numerical models of the flexural response of the lithosphere to normal faulting suggest that modest amounts of extension 5 km along low angle faults 45 are responsible for the transverse ridges

mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

fracture propagation to the base of the greenland ice sheet during supraglacial lake drainage surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change the process whereby such a pathway is created through thick cold ice has not however been previously observed we describe the rapid 2 hours drainage of a large supraglacial lake down 980 meters through to the bed of the greenland ice sheet initiated by water driven fracture propagation evolving into moulin flow drainage coincided with increased seismicity transient acceleration ice sheet uplift and horizontal displacement subsidence and deceleration occurred over the subsequent 24 hours the short lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially the integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup

greenland meltwater as a significant and potentially bioavailable source of iron to the ocean the micronutrient iron is thought to limit primary production in large regions of the global ocean meltwater measurements suggest that the greenland ice sheet serves as a significant source of potentially bioavailable iron to the surrounding coastal ocean

marine geophysical investigation of the chain fracture zone in the equatorial atlantic from the pi lab experiment the chain fracture zone is a 300 km long transform fault that offsets the mid atlantic ridge we analyzed new multibeam bathymetry backscatter gravity and magnetic data with 100 multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the passive imaging of the lithosphere asthenosphere boundary pi lab experiment analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 myr we model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic magmatic history of the region within the active transform fault zone we observe four distinct positive flower structures with several en echelon fault scarps visible in the backscatter data we find up to 10 mgal residual mantle bouguer anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the atlantic the extensional compressional features observed in the chain transform are less pronounced than those observed further north in the vema st paul and romanche and may be due to local ridge segment adjustments

refined spreading history at the southwest indian ridge for the last 96 ma with the aid of satellite gravity data summary the spreading history of the oceans is modelled mostly by using magnetic anomalies and the fracture zone geometry the high quality satellite derived gravity data that became available in recent years reveal the details of fracture zones which can be used as flow lines to control spreading models we have applied this approach to the southwest indian ridge swir in order to refine its spreading history this is particularly useful for the period of complex spreading between magnetic anomalies 33 and 23 where the magnetic anomalies alone cannot resolve the detailed spreading history we find four main stages in the spreading history of the swir since 96 ma including two that were not noted previously between 96 ma and anomaly 33 76 3 ma and between anomalies 23o 51 7 ma and 18o 40 1 ma o denotes old boundaries of normal magnetization period we also find that the start of the period of complex spreading was at anomaly 33 somewhat earlier than previously proposed we discuss the characteristics of the extension that the old transform faults underwent during the complex spreading phase in response to the counterclockwise rotation of spreading new transform faults appeared at that time considerably widening the transform zones

seasonal evolution of water contributions to discharge from a greenland outlet glacier insight from a new isotope mixing model the greenland ice sheet gris subglacial hydrological system may undergo a seasonal evolution with significant geophysical and biogeochemical implications we present results from a new isotope mixing model to quantify the relative contributions of surface snow glacial ice and delayed flow to the bulk meltwater discharge from a small 5k m 2 land terminating gris outlet glacier during melt onset may and at peak melt july we use radioactive 222 rn and stable isotopes 18 o deuterium to differentiate the water source contributions atmospherically derived 7 be further constrains meltwater transit time from the glacier surface to the ice margin we show that 1 222 rn is a promising tracer for glacial waters stored at the bed and 2 a quantitative chemical mixing model can be constructed by combining 222 rn and the stable water isotopes applying this model to the bulk subglacial outflow from our study area we find a constant delayed flow stored component from melt onset through peak melt this component is diluted first by snowmelt and then by increasing glacial ice melt as the season progresses results from this pilot study are consistent with the hypothesis that subglacial drainage beneath land terminating sections of the gris undergoes a seasonal evolution from a distributed to a channelized system

extreme mantle uplift and exhumation along a transpressive transform fault earth s crust diverges and extends along mid ocean ridges analyses of gravity and seismic data from the equatorial atlantic show that propagation of ridge segments can compress the crust and create sufficient uplift to create small islands

hydro biogeochemical coupling beneath a large polythermal arctic glacier implications for subice sheet biogeochemistry 1 we analyze the interannual chemical and isotopic composition of runoff from a large high arctic valley glacier over a 5 year period during which drainage evolved from a long residence time drainage system feeding an artesian subglacial upwelling sgu at the glacier terminus to a shorter residence time drainage system feeding an ice marginal channel imc increased icemelt inputs to the sgu are thought to have triggered this evolution this sequence of events provides a unique opportunity to identify coupling between subglacial hydrology and biogeochemical processes within drainage systems of differing residence time the biogeochemistry of the sgu is consistent with prolonged contact between meltwaters and subglacial sediments in which silicate dissolution is enhanced anoxic processes e g sulphate reduction prevail and microbially generated co2 and sulphide oxidation drive mineral dissolution solute in the imc was mainly derived from moraine pore waters which are added to the channel via extraglacial streams these pore waters acquire solute predominantly via sulphide oxidation coupled to carbonate silicate dissolution we present the first evidence that microbially mediated processes may contribute a substantial proportion 80 in this case of the total glacial solute flux which includes coupling between microbial co2 generation and silicate carbonate dissolution the latter suggests the presence of biofilms in subglacial ice marginal sediments where local perturbation of the geochemical environment by release of protons organic acids and ligands stimulates mineral dissolution these data enable inferences to be made regarding biogeochemical processes in longer residence time glacial systems with implications for the future exploration of antarctic subglacial lakes and other wet based ice sheet environments

time domain observations of a slow precursor to the 1994 romanche transform earthquake low frequency spectral anomalies have indicated that some large earthquakes are preceded by extended episodes of smooth moment release but the reality of these slow precursors has been debated because they have not been directly observed in the time domain high gain seismograms from the 14 march 1994 romanche transform event moment magnitude m w 7 0 show a precursory ramp with a moment of 7 1018 newton meters beginning about 100 seconds before the arrival of the high frequency p waves this precursor was the initial phase of a slow component of slip that released nearly half of the total moment of the earthquake such behavior may be typical for large earthquakes on the oceanic ridge transform system

extension along the australian pacific transpressional transform plate boundary near macquarie island the australian pacific transform plate boundary fault zone along the macquarie and mcdougall segments of the macquarie ridge complex mrc south of new zealand is characterized by dominantly normal faults and pull apart basins in apparent conflict with the regional transpressional tectonic setting we propose that present day curvature of the transform is inherited from a preexisting divergent plate boundary and that the overall extensional kinematics shown by faults along the main plate boundary trace and exposed on macquarie island result from local stresses related to right lateral right stepping en echelon plate boundary faults and not to the current transpressional setting transpression along the australian pacific transform plate boundary has resulted in uplift along the 1500 km long macquarie ridge complex macquarie island the only subaerial exposure of the complex sits atop a 5 km high 50 km wide submarine ridge of oceanic crust and lies 4 5 km east of the major active plate boundary fault zone thus macquarie island and the surrounding seafloor provide a unique opportunity to study an active oceanic transform fault using complementary marine geophysical and land based geological data mapping of recent faults affecting the topography of macquarie island shows that the island is extensively cut by high angle normal faults forming pull apart basins furthermore evidence for reverse motion is rare using marine geophysical data including swath bathymetry reflectivity and seismic reflection data collected along the australian pacific plate boundary north and south of the island we have defined a 5 15 km wide plate boundary zone a series of right stepping en echelon faults within this zone lies along the main plate boundary trace at the right stepping fault terminations elongate depressions 10 km wide and 1 2 km deep parallel the plate boundary which we interpret as extensional relay zones or pull apart basins we propose that transpression is partitioned into en echelon strike slip faults at the plate boundary and a convergent component that flexes the crust causing the anomalous bathymetric ridge and trough morphology of the mcdougall and macquarie segments of the mrc

can great quakes extend their reach seismologybig earthquakes can trigger other quakes thousands of kilometers away and decades later according to calculations presented on page 1245 1 of this issue of science the researchers simulated how stress travels through deep viscous rock and found that the great earthquakes that struck the far north pacific in the 1950s and 60s could have set off a slow moving stress wave that triggered a pulse of seismic activity in california in the 1980s 1 http www sciencemag org cgi content short 280 5367 1245

characterization of dissolved organic matter dom from glacial environments using total fluorescence spectroscopy and parallel factor analysis aquatic dissolved organic matter dom is a major reservoir of reduced organic carbon and has a significant influence on heterotrophic biological productivity and water quality in marine and freshwater environments although the forms and transformations of dom in temperate aquatic and soil environments have been studied extensively this is not the case for glacial environments in this study fluorescent excitation emission matrices eems parallel factor analysis parafac and cluster analysis were used to characterize the fluorescing components of dom in ice and water samples from supraglacial englacial subglacial and proglacial environments of seven glaciers in the canadian arctic norway and antarctica at least five significant fluorescent dom fractions were identified which accounted for 98 2 of the variance in the dataset these included four protein like components and one humic like component the predominantly proteinaceous character of dom from these glaciers is very different from the more humic character of dom described previously from lacustrine fluvial estuarine and marine environments dom from the sampled glaciers is broadly similar in character despite their geographically distinct locations different thermal regimes and inter and intra site differences in potential organic matter sources glacier ice samples had a relatively low ratio of humic like protein like fluorescence while meltwater samples had a higher ratio

segment scale and intrasegment lithospheric thickness and melt variations near the andrew bain megatransform fault and marion hot spot southwest indian ridge 25 5 e 35 e we analyze bathymetric gravimetric and magnetic data collected on cruise kn145l16 between 25 5 e and 35 e on the ultraslow spreading southwest indian ridge where the 750 km long andrew bain transform domain separates two accretionary segments to the northeast from a single segment to the southwest similar along axis asymmetries in seafloor texture rift valley curvature magnetic anomaly amplitude magnetization intensity and mantle bouguer anomaly mba amplitude within all three segments suggest that a single mechanism may produce variable intrasegment lithospheric thickness and melt delivery however closer analysis reveals that a single mechanism is unlikely in the northeast mba lows shallow axial depths and large abyssal hills indicate that the marion hot spot enhances the melt supply to the segments we argue that along axis asthenospheric flow from the hot spot dammed by major transform faults produces the inferred asymmetries in lithospheric thickness and melt delivery in the southwest strong rift valley curvature and nonvolcanic seafloor near the andrew bain transform fault indicate very thick subaxial lithosphere at the end of the single segment we suggest that cold lithosphere adjacent to the eastern end of the ridge axis cools and thickens the subaxial lithosphere suppresses melt production and focuses melt to the west this limits the amount of melt emplaced at shallow levels near the transform fault our analysis suggests that the andrew bain divides a high melt supply region to the northeast from an intermediate to low melt supply region to the southwest thus this transform fault represents not only a major topographic feature but also a major melt supply boundary on the southwest indian ridge

basal ice microbiology at the margin of the greenland ice sheet basal ice at the margin of the greenland ice sheet was studied with respect to its physical characteristics and microbiological community the basal ice contained high concentrations of dissolved ferrous fe and must therefore be anoxic oxygen consumption experiments indicate that 50 of the oxidation was due to biological activity while the rest could be attributed to chemical processes most likely weathering reactions with ferrous fe at least six different fe containing mineral sources were detected in basal ice together with potential bioavailable fe nanoparticles an active denitrifier population was identified due to formation of 30 n dinitrogen gas after amendment of anoxic sediment slurries with 15 n no3 sulfate reduction could not be detected the solid ice facies contained an abundant 10 8 cells cm 3 and complex microbial community that harbored representatives of at least eight major phyla within the domain bacteria the clone library was dominated by members of the subdivision of proteobacteria of which the largest proportion was affiliated to the genus rhodoferax that comprises facultative aerobic iron reducers the second most abundant phylum was bacteroidetes the solid ice facies had many physical similarities with the overlying debris rich banded ice facies indicating that they formed by similar subglacial processes and harbor similar microbial communities this study extends our knowledge of life in subglacial environments such as beneath ice sheets genbank accession numbers hm439882 hm439950 hq144215 hq144221

subglacial methanogenesis a potential climatic amplifier 1 subglacial environments are a previously neglected component of the earth s global carbon cycle a reflection of the view held until recently that they are dominated by abiotic and oxic conditions here we provide a realistic assessment of the theory that the basal regions of the ice sheets that formed over north america and europe during glaciations were host to significant populations of anaerobic microorganisms including methanogens able to metabolize organic carbon sequestered during interglacials and overridden during quaternary glacials in doing so we review the current evidence for subglacial methane release during deglaciation estimate the size of the subglacial reservoir of organic carbon soc and assess the amount of soc available to subglacial microbes and the likely pathways and rates of carbon turnover we then discuss the fate of subglacial methane and the potential impact of its release on atmospheric methane concentrations we calculate that the soc equates to 418 610 pg c and includes carbon from terrestrial soils vegetation peatlands lake and marine sediments the soc that is potentially available for microbial conversion to methane is smaller than this estimate due to 1 glacial erosion 2 accumulation of recalcitrant organic carbon compounds over time 3 conversion to carbon dioxide by aerobic anaerobic respiration and 4 incomplete conversion of labile organic matter to methane we estimate that the total soc available for conversion to methane is 63 pg c our estimates of methane production potentials span a wide range because of the current uncertainty surrounding subglacial metabolic rates we believe however that there is a strong likelihood that subglacial microbes could convert 63 pg of soc to methane during a glacial cycle if this were the case release of this methane from the ice sheet margins during retreat would need to be episodic in order to significantly impact atmospheric methane concentrations our findings suggest that it may well be important to consider subglacial environments as active components of the earth s carbon cycle conclusive determination of the potential impact of subglacial methane production on atmospheric methane concentrations during deglaciation however awaits more precise determination of the ability of subglacial microbes to degrade organic carbon components and their associated rates of metabolism under in situ conditions

the cryoconite ecosystem on the greenland ice sheet this paper presents an assessment of biological activity associated with ice surface debris cryoconite at the ice sheet scale estimates of the mass distribution of cryoconite over the greenland ice sheet gis and the biological activity associated with it are presented and then coupled with a surface mass balance model to estimate total carbon fluxes due to respiration and photosynthesis we find an average loading of 66 g m 2 at kangerlussuaq southwest greenland which compares well with recent estimates from kronprins christians land 17 440 g m 2 boggild and others 2010 in northeast greenland we also report a significant microbial biomass in cryoconite at both these places 10 3 10 4 cells mg 1 and carbon fluxes of the order of 1 3 m mcg 1 d 1 for both respiration and photosynthesis the modelling indicates that total respiration and photosynthesis fluxes are likely to be 10 1 10 2 gg c a 1 and thus far from trivial however estimation of the net ecosystem impact across the entire ice sheet on atmospheric co2 concentrations is problematic because photosynthesis rates were almost certainly low during our field campaign therefore like its water balance the carbon balance of the gis is now known to be important but its accurate quantification will remain elusive until more data are forthcoming

oceanic broad multifault transform plate boundaries oceanic transform plate boundaries consist of a single narrow a few kilometers wide strike slip seismic zone offsetting two mid ocean ridge segments however we define here a new class of oceanic transform boundaries with broad complex multifault zones of deformation similar to some continental strike slip systems examples are the 750 km long 120 km wide andrew bain transform on the southwest indian ridge and the romanche transform where the mid atlantic ridge is offset by a lens shaped 900 km long 100 km wide sliver of deformed lithosphere bound by two major transform valleys one of the valleys is seismically highly active and constitutes the present day principal transform boundary however strike slip seismic events also occur in the second valley and elsewhere in the deformed zone some of these events may be triggered by earthquakes from the principal boundary numerical modeling predicts the development of wide multiple transform boundaries when the age offset is above a threshold value of 30 m y i e in extra long 500 km slow slip transforms multiple boundaries develop so that strike slip ruptures avoid very thick and strong lithosphere

calculating the balance between atmospheric co2 drawdown and organic carbon oxidation in subglacial hydrochemical systems in order to constrain co2 fluxes from biogeochemical processes in subglacial environments we model the evolution of ph and alkalinity over a range of subglacial weathering conditions we show that subglacial waters reach or exceed atmospheric pco2 levels when atmospheric gases are able to partially access the subglacial environment subsequently closed system oxidation of sulfides is capable of producing pco2 levels well in excess of atmosphere levels without any input from the decay of organic matter we compared this model to published ph and alkalinity measurements from 21 glaciers and ice sheets most subglacial waters are near atmospheric pco2 values the assumption of an initial period of open system weathering requires substantial organic carbon oxidation in only 4 of the 21 analyzed ice bodies if the subglacial environment is assumed to be closed from any input of atmospheric gas large organic carbon inputs are required in nearly all cases these closed system assumptions imply that order of 10 g m 2 y 1 of organic carbon are removed from a typical subglacial environment a rate too high to represent soil carbon built up over previous interglacial periods and far in excess of fluxes of surface deposited organic carbon partial open system input of atmospheric gases is therefore likely in most subglacial environments the decay of organic carbon is still important to subglacial inorganic chemistry where substantial reserves of ancient organic carbon are found in bedrock in glaciers and ice sheets on silicate bedrock substantial long term drawdown of atmospheric co2 occurs

the mw 5 4 reggio emilia 1996 earthquake active compressional tectonics in the po plain italy summary we have analysed the seismic sequence that occurred in october 1996 near the town of reggio emilia on the southern edge of the po plain the onset of the sequence was marked by a 5 4 moment magnitude main shock located at 15 km depth the main shock focal mechanism is a reverse solution with a strike slip component and the scalar moment is 1 46 1017 n m we used broad band digital recordings from a borehole station located at about 70 km from the epicentre for a spectral analysis in order to estimate attenuation and source parameters for the main shock in addition the empirical green s function method has been applied to evaluate the source time function in terms of both moment rate and stress rate we infer an asperity like rupture process for the main shock as suggested by the short duration of the stress release with respect to the overall duration of the moment rate function this analysis also allows us to estimate the average dynamic stress drop of the main shock 600 bar we analysed the digital recordings of the temporary local seismic network deployed after the main shock and of a permanent local network maintained by the italian petroleum agency agip during 15 days of field experiments we recorded more than 800 aftershocks which delineate a 9 km long ne elongated distribution confined between 12 and 15 km depth suggesting that the basement is involved in the deformation processes 102 focal mechanism of aftershocks have been computed from p wave polarities showing mainly pure reverse solutions we calculate the principal stress axes from a selected population of earthquakes providing a constraint on the stress regime of this part of the po plain the focal mechanisms are consistent with a n s subhorizontal 1 all the seismological data we have analysed confirm that this region is undergoing active compressional tectonics as already inferred from recent earthquakes geomorphological data and other stress indicators moreover the elongation of the reggio emilia aftershock sequence is consistent with the regional direction of the thrust fronts cropping out in the area suggesting that they are still active

northern latitude chemical weathering rates clues from the mackenzie river basin canada abstract the main scope of this study is to investigate parameters controlling chemical weathering rates for a large river system submitted to subarctic climate more than 110 river water samples from the mackenzie river system northern canada have been sampled and analyzed for major and trace elements and sr isotopic ratios in the dissolved phase the three main morphological units are reflected in water chemistry rivers from the canadian shield are very dilute dominated by silicate weathering millot et al 2002 whereas the rivers of the rocky and mackenzie mountains as well as the rivers of the sedimentary interior platform are dominated by carbonate weathering and are so 4 rich compared to the rivers of the mackenzie and rocky mountains the rivers of the interior plains are organic silica and na rich and constitute the dominant input term to the mackenzie river mainstream rivers of the canadian shield area do not significantly contribute to the mackenzie river system using elemental ratios and sr isotopic ratios a mathematical inversion procedure is presented that distinguishes between solutes derived from silicate weathering and solutes derived from carbonate weathering carbonate weathering rates are mostly controlled by runoff which is higher in the mountainous part of the mackenzie basin these rates are comparable to the carbonate weathering rates of warmer areas of the world it is possible that part of the carbonate weathering is controlled by sulfide oxidative weathering but its extent remains difficult to assess conversely to what was stated by edmond and huh 1997 overall silicate weathering rates in the mackenzie basin are low ranging from 0 13 to 4 3 tons km 2 yr na k ca mg and confirm the negative action of temperature on silicate weathering rates for river basins in cold climates in contrast to what has been observed in other large river systems such as the amazon and ganges rivers silicate weathering rates appear 3 to 4 times more elevated in the plains than in the mountainous headwaters this contradicts the raymo hypothesis raymo and ruddiman 1992 isotopic characterization of suspended material clearly shows that the higher weathering rates reported for the plains are not due to the weathering of fine sediments produced in the mountains e g by glaciers and deposited in the plains rather the relatively high chemical denudation rates in the plains are attributed to lithology uncompacted shales high mechanical denudation and the abundance of soil organic matter derived from incomplete degradation and promoting crystal lattice degradation by element complexation the three to fourfold factor of chemical weathering enhancement between the plains and mountains is similar to the fourfold factor of enhancement found by moulton et al 2000 between unvegetated and vegetated watershed this study confirms the negative action of temperature on silicate weathering for cold climate but shows that additional factors such as organic matter associated with northern watersheds are able to counteract the effect of temperature this acceleration by a factor of 4 in the plains is equivalent to a 6 c increase in temperature

biogeochemical weathering under ice size matters 1 the basal regions of continental ice sheets are gaps in our current understanding of the earth s biosphere and biogeochemical cycles we draw on existing and new chemical data sets for subglacial meltwaters to provide the first comprehensive assessment of sub ice sheet biogeochemical weathering we show that size of the ice mass is a critical control on the balance of chemical weathering processes and that microbial activity is ubiquitous in driving dissolution carbonate dissolution fueled by sulfide oxidation and microbial co2 dominate beneath small valley glaciers prolonged meltwater residence times and greater isolation characteristic of ice sheets lead to the development of anoxia and enhanced silicate dissolution due to calcite saturation we show that sub ice sheet environments are highly geochemically reactive and should be considered in regional and global solute budgets for example calculated solute fluxes from antarctica 72 130 t yr 1 are the same order of magnitude as those from some of the world s largest rivers and rates of chemical weathering 10 17 t km 2 yr 1 are high for the annual specific discharge 2 3 4 1 10 3 m our model of chemical weathering dynamics provides important information on subglacial biodiversity and global biogeochemical cycles and may be used to design strategies for the first sampling of antarctic subglacial lakes and other sub ice sheet environments for the next decade

accretion mode of oceanic ridges governed by axial mechanical strength oceanic spreading ridges exhibit structural changes as a function of spreading rate mantle temperature and the balance of tectonic and magmatic accretion the role that these or other processes have in governing the overall shape of oceanic ridges is unclear here we use laboratory experiments to simulate ridge spreading in colloidal aqueous dispersions whose rheology evolves from purely viscous to elastic and brittle when placed in contact with a saline water solution we find that ridge shape becomes increasingly linear with spreading rate until reaching a minimum tortuosity this behaviour is predicted by the axial failure parameter f a dimensionless number describing the balance of brittle and plastic failure of axial lithosphere slow spreading fault dominated and fast spreading fluid intrusion dominated ridges on earth and in the laboratory are separated by the same critical f value suggesting that the axial failure mode governs ridge geometry values of f can also be calculated for different mantle temperatures and applied to other planets or the early earth for higher mantle temperatures during the archaean our results preclude the predicted formation of large tectonic plates at high spreading velocity the shape of mid ocean ridges is influenced by lithospheric mechanical strength according to laboratory simulations of diverging plates the results imply that large tectonic plates probably could not have formed on a younger hotter earth

k rich mantle metasomatism control of localization and initiation of lithospheric strike slip faulting a conceptual model is proposed in which bulk transtension or local transtension during bulk simple shear resulting from mantle anisotropy contrasts or lithosphere rheology contrasts of heterogeneously enriched lithospheric mantle triggers localized k rich magmatism which focuses strain and causes nucleation of lithosphere scale transtensional or strike slip shear zones transtension triggered magmatism is most likely to be located at sites of maximum metasomatism of the lithospheric mantle magma generated fractures propagate upwards nucleating zones of lithospheric weakness which focus shear in narrow transcurrent faults or at basin margins in this way magmatism controls fault timing and location although volcanism will be coeval with fault development and volcanoes will appear fault controlled counterintuitively our model suggests that faults are in a sense volcanocontrolled we suggest that this new transtension k rich magmatism transcurrent faulting association represents a hitherto unrecognized genetic relationship as significant as for example the ocean island magma series

the mid rivera transform discordance morphology and tectonic development to better define the morphotectonic elements and tectonic development of the mid rivera transform discordance multibeam bathymetric seafloor backscatter multichannel seismic reflection and total field marine magnetic data were collected along the entire rivera transform west of 107 w during the bart and famex campaigns of the n o l atalante conducted in 2002 these data show that although the transform tectonized zone of the rivera transform west of 107 30 w is a single continuous morphologic basin this basin consists of two distinct morphotectonic domains an eastern domain which morphologically is a deep rhombochasm within which organized seafloor spreading has occurred and a western leaky transform domain these new data in conjunction with the results of previous studies support the idea that the rivera pacific euler pole is migrating southward towards the eastern half of the rivera transform and further indicate a recent 0 14 ma and most likely ongoing clockwise reorganization of the principle transform displacement zones of the rivera transform west of 108 w we propose that the mid rivera transform discordance owes its origin to this eastward progressing clockwise reorganization of the transform segments that is occurring in response to recent changes in rivera pacific relative plate motion

organic carbon export from the greenland ice sheet abstract glacial meltwater exports a unique type of organic carbon to marine systems distinct from non glacially derived riverine export potentially capable of stimulating downstream marine primary productivity here we describe for the first time the bulk level dissolved organic carbon doc and particulate organic carbon poc isotopic composition of glacial meltwater draining the greenland ice sheet gris these data in conjunction with an earlier study that investigated the molecular level composition of gris dissolved organic matter collectively describe the concentration radiocarbon content and lability of organic carbon in subglacial discharge from a land terminating outlet glacier during a melt season by scaling up our measurements across the ice sheet we estimate that the annual doc flux from the gris 0 08 tg y is equivalent to that from a small arctic river discharge q 3 y and that the annual poc flux from the gris 0 9 tg y may be comparable to that of a large arctic river q 200 km 3 y the doc flux is derived primarily from beneath the glacier subglacial 75 in the early season and from surface ice melt up to 100 transmitting through the subglacial environment at the peak of the meltseason the poc flux is primarily derived from the subglacial environment throughout the meltseason the early season low flow glacier discharge contains higher doc concentrations 0 5 4 1 mg l 1 and exports more enriched carbon 14 c doc 250 compared to the peak season high flow discharge when the concentrations are lower 0 1 0 6 mg l 1 and the 14 c is more depleted 14 c doc 400 conversely the poc export 1 4 13 2 mg l 1 14 c poc 250 shows no temporal variation in either concentration or radiocarbon content throughout the meltseason dissolved ion loads in concomitant samples reflected the seasonal evolution of the subglacial drainage system confirming the influence of subglacial hydrology on the composition of the bulk carbon pools based on this work we conclude that 1 different mechanisms control the doc and poc flux from glacial systems 2 chemically distinct doc pools are accessed by seasonally evolving hydrological flow paths and 3 the gris can deliver labile carbon which may also be 14 c depleted to downstream proglacial and marine environments

recent large increases in freshwater fluxes from greenland into the north atlantic 1 freshwater fw fluxes from river runoff and precipitation minus evaporation for the pan arctic seas are relatively well documented and prescribed in ocean gcms fluxes from greenland on the other hand are generally ignored altogether despite their potential impacts on ocean circulation and marine biology here we present a reconstruction of the spatially distributed fw flux from greenland for 1958 2010 we find a modest increase into the arctic ocean during this period fluxes into the irminger basin however have increased by fifty percent 6 3 0 5 km3 yr 2 in less than twenty years this greatly exceeds previous estimates for the ice sheet as a whole the rate of increase since 1992 is 16 9 1 8 km3 yr 2 the cumulative fw anomaly since 1995 is 3200 358 km3 which is about a third of the magnitude of the great salinity anomaly gsa of the 1970s if this trend continues into the future the anomaly will exceed that of the gsa by about 2025

molecular characterization of dissolved organic matter associated with the greenland ice sheet abstract subsurface microbial oxidation of overridden soils and vegetation beneath glaciers and ice sheets may affect global carbon budgets on glacial interglacial timescales the likelihood and magnitude of this process depends on the chemical nature and reactivity of the subglacial organic carbon stores we examined the composition of carbon pools associated with different regions of the greenland ice sheet subglacial supraglacial proglacial in order to elucidate the type of dissolved organic matter dom present in the subglacial discharge over a melt season electrospray ionization esi fourier transform ion cyclotron resonance ft icr mass spectrometry coupled to multivariate statistics permitted unprecedented molecular level characterization of this material and revealed that carbon pools associated with discrete glacial regions are comprised of different compound classes specifically a larger proportion of protein like compounds were observed in the supraglacial samples and in the early melt season spring subglacial discharge in contrast the late melt season summer subglacial discharge contained a greater fraction of lignin like and other material presumably derived from underlying vegetation and soil these results suggest 1 that the majority of supraglacial dom originates from autochthonous microbial processes on the ice sheet surface 2 that the subglacial dom contains allochthonous carbon derived from overridden soils and vegetation as well as autochthonous carbon derived from in situ microbial metabolism and 3 that the relative contribution of allochthonous and autochthonous material in subglacial discharge varies during the melt season these conclusions are consistent with the hypothesis that given sufficient time e g overwinter storage resident subglacial microbial communities may oxidize terrestrial material beneath the greenland ice sheet

effects of increase glacier discharge on phytoplankton bloom dynamics and pelagic geochemistry in a high arctic fjord arctic fjords experience extremely pronounced seasonal variability and spatial heterogeneity associated with changes in ice cover glacial retreat and the intrusion of continental shelf s adjacent water masses global warming intensifies natural environmental variability on these important systems yet the regional and global effects of these processes are still poorly understood in the present study we examine seasonal and spatial variability in kongsfjorden on the western coast of spitsbergen svalbard we report hydrological biological and biogeochemical data collected during spring summer and fall 2012 our results show a strong phytoplankton bloom with the highest chlorophyll a chla levels ever reported in this area peaking 15 5 g l during late may and completely dominated by large diatoms at the inner fjord that may sustain both pelagic and benthic production under weakly stratified conditions at the glacier front a progressively stronger stratification of the water column during summer and fall was shaped by the intrusion of warm atlantic water t 3 c and sal 34 65 into the fjord at around 100 m depth and by turbid freshwater plumes t 1 c and sal 34 65 at the surface due to glacier meltwater input biopolymeric carbon fractions and isotopic signatures of the particulate organic material pom revealed very fresh and labile material produced during the spring bloom 13c enriched with values up to 22 7 at the highest chl a peak and high in carbohydrates and proteins content up to 167 and 148 g l respectively and a clear and strong continental signature of the pom present during late summer and fall 13c depleted with values averaging 26 5 and high in lipid content up to 92 g l when freshwater melting is accentuated our data evidence the importance of combining both physical i e water mass dominance and geochemical i e characteristics of material released by glacier runoff data in order to understand the timing intensity and characteristics of the phytoplankton bloom in kongsfjorden a continuously changing system due to sustained warming in a scenario where glacial retreat is predicted to increase the impacts of meltwater discharge and associated delivery of organic and inorganic material to the surrounding waters special attention is required in order to predict the consequences for arctic fjords and adjacent shelf ecosystems

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

source and biolability of ancient dissolved organic matter in glacier and lake ecosystems on the tibetan plateau the tibetan plateau is the world s largest and highest plateau and holds the largest mass of ice on earth outside the icesheets of greenland and antarctica as well as abundant lakes this study examined the molecular and isotopic signatures of dissolved organic matter dom along with its biolability in glacier ice glacier fed streams and alpine lakes on the tibetan plateau the aim was to assess the sources of dom and the potential of dom to provide a carbon subsidy to downstream ecosystems tibetan glaciers and glacier streams exhibited low dissolved organic carbon doc concentrations 17 7 27 9 lm and ancient doc radiocarbon ages 749 2350 ybp the optical properties stable carbon isotope ratios d 13 cdoc and the molecular composition fourier transform ion cyclotron resonance mass spectrometry of tibetan glacier dom are consistent with data from other glacier systems around the world the geochemical signatures and the ancient apparent ages of tibetan glacier dom suggest a significant fraction is derived from the atmospheric deposition of pre aged possibly fossil fuel derived organics within the tibetan alpine lakes doc was also ancient 525 675 ybp due to either inputs of pre aged organics from glacier runoff direct deposition or due to the aging of organics in situ i e a radiocarbon reservoir effect the glacier ice and glacier stream sites exhibited high biolability of doc and so provide a carbon subsidy to downstream environments that will change as glaciers on the tibetan plateau recede

tectonic interpretation of the andrew bain transform fault southwest indian ocean 1 between 25 e and 35 e a suite of four transform faults du toit andrew bain marion and prince edward offsets the southwest indian ridge swir left laterally 1230 km the andrew bain the largest has a length of 750 km and a maximum transform domain width of 120 km we show that currently the nubia somalia plate boundary intersects the swir east of the prince edward placing the andrew bain on the nubia antarctica plate boundary however the overall trend of its transform domain lies 10 clockwise of the predicted direction of motion for this boundary we use four transform parallel multibeam and magnetic anomaly profiles together with relocated earthquakes and focal mechanism solutions to characterize the morphology and tectonics of the andrew bain starting at the southwestern ridge transform intersection the relocated epicenters follow a 450 km long 20 km wide 6 km deep western valley they cross the transform domain within a series of deep overlapping basins bounded by steep inward dipping arcuate scarps eight strike slip and three dip slip focal mechanism solutions lie within these basins the earthquakes can be traced to the northeastern ridge transform intersection via a straight 100 km long 10 km wide 4 5 km deep eastern valley a striking set of seismically inactive ne sw trending en echelon ridges and valleys lying to the south of the overlapping basins dominates the eastern central section of the transform domain we interpret the deep overlapping basins as two pull apart features connected by a strike slip basin that have created a relay zone similar to those observed on continental transforms this transform relay zone connects three closely spaced overlapping transform faults in the southwest to a single transform fault in the northeast the existence of the transform relay zone accounts for the difference between the observed and predicted trend of the andrew bain transform domain we speculate that between 20 and 3 2 ma an oblique accretionary zone jumping successively northward created the en echelon ridges and valleys in the eastern central portion of the domain the style of accretion changed to that of a transform relay zone during a final northward jump at 3 2 ma

geometrical statistical modelling of systems of fracture zones along oceanic ridges summary a mechanical statistical model is presented that aims to help to understand the history and geometry of the process of formation of fracture zones along oceanic ridges it uses ideas of statistical fracture theory used in engineering namely the weibull fracture model the approximate parallelism of the fracture zones along ridges makes it possible to use a onedimensional point process model with points along the ridge axes which represent the transform faults the ratios of the lengths of the corresponding fracture zones to the ocean width are used to obtain a rough estimate of the weibull modulus which is an important material parameter in fracture theory the theory is refined by introducing a hard core point process model the corresponding positive minimum distance between subsequent fracture zones results from stress relaxation in the vicinity of a given fracture zone

successive shifts of the india africa transform plate boundary during the late cretaceous paleogene interval implications for ophiolite emplacement along transforms abstract the arabian sea in the nw indian ocean is a place where two major transform boundaries are currently active the owen fracture zone between india and arabia and the owen transform between india and somalia these transform systems result from the fragmentation of the india africa transform boundary which initiated about 90 myrs ago when the india seychelles block separated from madagascar to move towards eurasia therefore the geological record of the arabian sea makes it possible to investigate the sensitivity of a transform system to several major geodynamic changes here we focus on the evolution of the india africa transform system during the 47 90 ma interval we identify the late cretaceous 90 65 ma transform plate boundary along chain ridge in the north somali basin from 65 to 42 47 ma the india africa transform is identified at the chain fracture zone which crossed both the owen basin and the north east oman margin finally the transform system jumped to its present day location in the vicinity of the owen ridge these shifts of the india africa boundary with time provide a consistent paleogeographic framework for the emplacement of the masirah ophiolitic belt which constitutes a case of ophiolite emplaced along a transform boundary the successive locations of the india africa boundary further highlight the origin of the owen basin lithosphere incoming into the makran subduction zone

trickle or treat the dynamics of nutrient export from polar glaciers cold based polar glacier watersheds contain well defined supraglacial ice marginal and proglacial elements that differ in their degree of hydrologic connectivity sources of water e g snow ice and or sediment pore water meltwater residence times allochthonous and autochthonous nutrient and sediment loads we investigated 11 distinct hydrological units along the supraglacial ice marginal and proglacial flow paths that drain joyce glacier in the mcmurdo dry valleys of antarctica we found that these units play unique and important roles as sources and or sinks for dissolved inorganic nitrogen and dissolved inorganic phosphorus and for specific fractions of dissolved organic matter dom as waters are routed from the glacier into nutrient poor downstream ecosystems changes in nutrient export from the glacial system as a whole were observed as the routing and residence times of meltwater changed throughout the melt season the concentrations of major ions in the proglacial stream were inversely proportional to discharge such that there was a relatively constant trickle of these solutes into downstream ecosystems in contrast no3 concentrations generally increased with discharge resulting in delivery of episodic pulses of dissolved inorganic nitrogen rich water treats into those same ecosystems during high discharge events dom concentrations or fluorescence did not correlate with discharge rate but high variability in dom concentrations or fluorescence suggests that dom may be exported downstream as episodic treats but with spatial and or temporal patterns that remain poorly understood the strong nutrient specific responses to changes in hydrology suggest that polar glacier drainage systems may export meltwater with nutrient compositions that vary within and between melt seasons and watersheds because nutrient dynamics identified in this study differ between glacier watersheds with broadly similar hydrology climate and geology we emphasize the need to develop conceptual models of nutrient export that thoroughly integrate the biogeochemical and

the iron isotopic composition of subglacial streams draining the greenland ice sheet abstract in this study we present the first measurements of iron fe stable isotopic composition 56 fe of subglacial streams draining the greenland ice sheet gis we measure the 56 fe values 56 fe 56 fe 54 fe sample 56 fe 54 fe standard 1 10 3 of both dissolved and suspended sediment fe in subglacial outflows from five distinct land terminating glaciers suspended sediments have 56 fe values that lie within the crustal array 56 fe 0 in contrast the 56 fe values of dissolved fe in subglacial outflows are consistently less than 0 reaching a minimum of 2 1 in the outflow from the russell glacier the 56 fe values of dissolved fe vary geographically and on daily time scales major element chemistry and mineral saturation state modeling suggest that incongruent silicate weathering and sulfide oxidation are the likely drivers of subglacial stream fe chemistry and that the extent of chemical weathering influences the 56 fe of dissolved fe the largest difference in 56 fe between dissolved and suspended load is 2 1 and occurs in the subglacial system from the russell glacier southwest gis major element chemistry indicates this outflow to be the least chemically weathered while more mature subglacial systems i e that exhibit greater extents of subglacial weathering have dissolved loads with 56 fe that are indistinguishable from suspended sediments 56 fe suspended dissolved 0 ultimately the dissolved fe generated in some subglacial systems from the gis is a previously unrecognized source of isotopically light fe into the hydrosphere the data illustrate that the dissolved fe supplied by subglacial weathering can have variable 56 fe values depending on the degree of chemical weathering thus fe isotopes have potential as a proxy for subglacial chemical weathering intensity or mode finally based on our regional fe concentration measurements from each glacial outflow we estimate a flux weighted continental scale dissolved iron export of 2 1 gg fe yr 1 to the coastal ocean which is within the range of previous estimates

holocene tectonic uplift of the st peter and st paul rocks equatorial atlantic consistent with emplacement by extrusion abstract st peter and st paul rocks spspr are emergent portions of a peridotitic mass within the st paul fracture zone of the equatorial mid atlantic ridge where local seismicity is dominated by strike slip mechanisms we used subtidal red algae in living position death assemblages in intertidal conglomerates and marine terraces as sea level markers the highest deposits correspond to a marine terrace with a shoreline angle at 8 1 m asl that formed ca 6400 6200 cal yr bp after correction for the sea level changes derived by glacioisostatic modelling ams and radiometric 14 c age measurements on the holocene sea level markers indicate emergence during the last 6600 a at a minimum average rate of 1 5 mm a we propose that uplift resulted from peridotite emplacement by extrusion rather than volcanic activity or fault slip with the implication that the ridge has been subject to compression during the holocene

multispectral terrain analysis of europa from galileo images abstract galileo s solid state imaging camera recorded six images at wavelengths from 0 41 to 0 99 m of europa s trailing hemisphere 1 6 km pixel resolution during the g1 orbit 1st orbit target ganymede of the nominal mission we have photometrically corrected these data and extracted spectra representing europa s diverse geologic terrains the goals of the analysis of these spectra are 1 to determine whether europa s geologic units differ spectrally from one another 2 to determine the number of color components necessary to explain europa s dark material spectral behavior and 3 to examine how europan dark materials may change in color with time our data indicate that europan dark spots lineaments and triple band side materials represent a single dark and reddish endmember component we see no evidence for more than one dark endmember however the dark endmember seen in these g1 data is probably not a pure exposure of europan dark material mottled terrains brighter lineaments and aging triple bands can be modeled by a mixture of bright plains materials and the dark component exposures of europan dark materials may thus be brightening with time eventually blending into the surrounding bright plains

deformation and structural evolution of mantle peridotites during exhumation on transform faults a forced transition from ductile to brittle regime abstract the exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile and brittle deformation until the complete emplacement in the oceanic lithosphere the sao pedro and sao paulo archipelago spspa in the equatorial atlantic records the deformational products of ductile brittle and the rocks fluid interaction generating specific structures in each domain the deformational stages are related to the transpressional and transtensional geodynamics of sao paulo transform fault firstly during transpression exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700 and 800 c defined by olivine and orthopyroxene recrystallization the interaction with fluids initially originated from the mantle generates amphibole and oxide rich layers marking the passage to a semi brittle deformation the continuation of peridotite exhumation associated with an nw se shortening and transpressional led to a higher availability of hydrothermal fluids as a consequence four serpentinization episodes are recorded which are associated with semi brittle to brittle transition under temperatures between 300 and 400 c finally the complete exhumation and establishment of brittle mechanisms led to carbonatation phase near the surface with temperatures ranging from 300 to 150 c the active nw se tectonic stress generated e w strike slip faults that were filled by carbonates recording the final exhumation stage

last interglacial mis5e hydrographic shifts linked to meltwater discharges from the east greenland margin abstract proximal evidence of the surface ocean response to size reduction of the greenland ice sheet gis during the last interglacial mis5e and preceding glacial termination t2 remains largely elusive using a new sediment record from the western iceland sea the behavior of the northeastern gis and its relation to the subpolar north atlantic surface hydrography is examined extremely light oxygen isotopic 18o values are found off central east greenland during early mis5e and point to enhanced meltwater release potentially from the northeastern sector of the gis data from downstream the cold east greenland current egc and its eastward branches suggest a far reaching effect of this meltwater not only in the nordic seas but also in the se labrador sea in particular whereas an early mis5e warming at 128 5 126 5 ka in the two regions coincided with the relative reduction of meltwater input into the egc the subsequent cooling noted at 126 5 ka followed a renewed major freshwater event off central east greenland our data further indicate persistent freshwater influence from the east greenland margin over the entire mis5e interval and in addition also reveal a late mis5e meltwater event the latter event occurred just prior to the last glacial inception and emphasizes the importance of greenland meltwater as forcing factor on interglacial climates

rupture processes of the 1999 august 17 izmit and november 12 duzce turkey earthquakes summary we derive the rupture history of the 1999 august 17 izmit mw 7 4 and 1999 november 12 duzce mw 7 1 earthquakes in turkey from teleseismic body waves using broad band data of the global seismograph network aftershock locations and mapped surface breaks the centroid solutions indicate strike slip mechanisms for both events the izmit earthquake was characterized by rupture propagating predominantly eastwards it consisted of a main rupture lasting about 25 s followed within 1 min by two more events of mw 6 9 and mw 7 0 with the teleseismic data we could not resolve the westward extent of rupture into the marmara sea however an upper bound of the seismic moment release west of the epicentre of the izmit event is estimated to be 1 9 1019 n m the duzce earthquake lasted about 14 s and was characterized by a bilateral mode of rupture in excellent agreement with mapped surface breaks and aftershock locations

abyssal hill deflections at pacific antarctic ridge transform intersections nearly complete coverage of shipboard multibeam bathymetry data at the right stepping menard and pitman fracture zones allowed us to map abyssal hill deviations along their traces in this study we distinguish between 1 j shaped curvatures at their origin where modeling is addressing primary volcanism and faulting following a curved zone and 2 straight abyssal hills getting bent in anti j shaped curvatures in response to increased coupling across the transform fault after they were formed we compared the mapped abyssal hill deflections to a detailed plate motion model for the pacific antarctic ridge to test how abyssal hill curvature correlates to changes in plate motion direction which lead to periods of transtension or transpression this test was based on the number and size of the abyssal hill deflections the observations show a high abundance of j shaped abyssal hills during periods of significant clockwise change in plate motion direction which leads to transtension the tip of the ridge axis can deflect up to 60 into the transform fault in response to changes in the stress field at ridge transform intersections this is observed in particular at the pitman fracture zone where there has been a 15 clockwise rotation of the spreading direction azimuth during the last 9 myr in addition we observed anti j shaped curvatures at menard pitman and heirtzler fracture zones during periods of transpression when increased coupling across an oceanic transform fault is partially accommodated by distributed strike slip deformation rather than solely by discontinuous displacement at the transform fault anti j shaped deflections typically develop in seafloor less than 2 myr old when the oceanic lithosphere is thin

earthquake scaling relations for mid ocean ridge transform faults 1 a mid ocean ridge transform fault rtf of length l slip rate v and moment release rate can be characterized by a seismic coupling coefficient ae at where ae v is an effective seismic area and at l3 2v 1 2 is the area above an isotherm tref a global set of 65 rtfs with a combined length of 16 410 km is well described by a linear scaling relation 1 ae at which yields 0 15 0 05 for tref 600 c therefore about 85 of the slip above the 600 c isotherm must be accommodated by subseismic mechanisms and this slip partitioning does not depend systematically on either v or l rtf seismicity can be fit by a truncated gutenberg richter distribution with a slope 2 3 in which the cumulative number of events n0 and the upper cutoff moment mc dcac depend on at data for the largest events are consistent with a self similar slip scaling dc ac1 2 and a square root areal scaling 2 ac at1 2 if relations 1 and 2 apply then moment balance requires that the dimensionless seismic productivity 0 0 atv should scale as 0 at 1 4 which we confirm using small events hence the frequencies of both small and large earthquakes adjust with at to maintain constant coupling rtf scaling relations appear to violate the single mode hypothesis which states that a fault patch is either fully seismic or fully aseismic and thus implies ac ae the heterogeneities in the stress distribution and fault structure responsible for relation 2 may arise from a thermally regulated dynamic balance between the growth and coalescence of fault segments within a rapidly evolving fault zone

oscillatory spreading explanation of anomalously old uplifted crust near oceanic transforms rocks significantly older than their theoretically predicted age have been recovered in the vicinity of some atlantic transform zones rocks of palaeocene 55 58 myr and possibly even of cretaceous age are found near the vema transform on tectonically uplifted crust which should not be older than 30 myr we present here a model which might explain the presence of uplifted anomalously old crust near the vema transform zone tz the proposed model involves axial rift propagation and reorientation and migration of the transform as a result of a change in the regional stress field during transform migration a crustal wedge formerly part of the african plate is transferred to the south american plate the ensuing oscillatory spreading can explain the anomalous age of the uplifted crustal block compression resulting from rift propagation and transform migration provides a mechanism for crustal uplift

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

geochemical weathering at the bed of haut glacier d arolla switzerland a new model waters were sampled from 17 boreholes at haut glacier d arolla during the 1993 and 1994 ablation seasons three types of concentrated subglacial water were identified based on the relative proportions of ca 2c h co 3 and so4 2 to si type a waters are the most solute rich and have the lowest relative proportion of si they are believed to form in hydrologically inefficient areas of a distributed drainage system most solute is obtained from coupled sulphide oxidation and carbonate dissolution so cd it is possible that there is a subglacial source of o2 perhaps from gas bubbles released during regelation because the high so4 2 levels found up to 1200 eq l are greater than could be achieved if sulphides are oxidized by oxygen in saturated water at 0 c c 414 eq l a more likely alternative is that sulphide is oxidized by fe 3c in anoxic environments if this is the case exchange reactions involving fe iii and fe ii from silicates are possible these have the potential to generate relatively high concentrations of hco3 with respect to so4 2 formation of secondary weathering products such as clays may explain the low si concentrations of type a waters type b waters were the most frequently sampled subglacial water they are believed to be representative of waters flowing in more efficient parts of a distributed drainage system residence time and reaction kinetics help determine the solute composition of these waters the initial water rock reactions are carbonate and silicate hydrolysis and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites hydrolysis is followed by so cd the so4 2 concentrations usually are 414 eq l although some range up to 580 eq l which suggests that elements of the distributed drainage system may become anoxic type c waters were the most dilute yet they were very turbid their chemical composition is characterized by low so4 2 h co3 ratios and high ph type c waters were usually artefacts of the borehole chemical weathering environment true type c waters are believed to flow through sulphide poor basal debris particularly in the channel marginal zone the composition of bulk runoff was most similar to diluted type b waters at high discharge and was similar to a mixture of type b and c waters at lower discharge these observations suggest that some supraglacial meltwaters input to the bed are stored temporarily in the channel marginal zone during rising discharge and are released during declining flow little of the subglacial chemical weathering we infer is associated with the sequestration of atmospheric co2 t he progression of reactions is from carbonate and silicate hydrolysis through sulphide oxidation by first oxygen and then fe iii which drives further carbonate and silicate weathering a crude estimate of the ratio of carbonate to silicate weathering following hydrolysis is 4 1 we speculate that microbial oxidation of organic carbon also may occur both sulphide oxidation and microbial oxidation of organic carbon are likely to drive the bed towards suboxic conditions hence we believe that subglacial chemical weathering does not sequester significant quantities of atmospheric co2 and that one of the key controls on the rate and magnitude of solute acquisition is microbial activity which catalyses the reduction of fe iii and the oxidation of fes2 copyright 2002 john wiley sons ltd

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

exogenic and endogenic albedo and color patterns on europa new global and high resolution multispectral mosaics of europa have been produced from the voyager imaging data photometric normalizations are based on multiple image techniques that explicitly account for intrinsic albedo variations through pixel by pixel solutions the exogenic color and albedo pattern on europa is described by a second order function of the cosine of the angular distance from the apex of orbital motion on the basis of this second order function and of color trends that are different on the leading and trailing hemispheres the exogenic pattern is interpreted as being due to equilibrium between two dominant processes 1 impact gardening and 2 magnetospheric interactions including sulfur ion implantation and sputtering redistribution removal of the model exogenic pattern in the mosaics reveals the endogenic variations consisting of only two major units darker redder and bright materials therefore europa s visual spectral reflectivity is simple having one continuous exogenic pattern and two discrete endogenic units

chemical weathering across the western foreland of the greenland ice sheet abstract proglacial streams deliver melt water and chemical weathering products including nutrients and radiogenic isotopes from continental ice sheets to the ocean weathering products are also delivered to the ocean in non glacial streams that form following ice sheet retreat and are disconnected from ice sheet meltwater by hydrologic divides if weathering reactions differ in non glacial and proglacial stream catchments the streams could deliver different types and magnitudes of solutes to the ocean depending on relative discharge volumes unlike proglacial streams however little is known of non glacial stream solute compositions or discharge here we show specific discharges are similar from a proglacial stream draining the greenland ice sheet gris with several streams disconnected from the ice sheet we also evaluate weathering reactions across a 170 km transect in western greenland that contains one proglacial stream draining the gris and two coastal ice distal and three inland ice proximal areas with non glacial streams non glacial streams exhibit solute compositions and offsets between dissolved and bedload sr isotope ratios that indicate weathering increases toward the coast with exposure age and precipitation major element mass balance calculations show weathering reactions shift from predominately carbonic acid weathering of carbonate minerals inland near the ice sheet to predominately sulfuric acid weathering of carbonate minerals near the coast strontium concentrations and isotopic ratios of the proglacial stream reflect mixing of at least two subglacial sources and minor in stream weathering that consumes co2 about 5 times less co2 is consumed per liter in the proglacial than inland non glacial streams however arid conditions inland suggest limited discharge from the ungauged inland streams leads to less total co2 weathering than proglacial stream one coastal area consumes less co2 per liter than the proglacial stream and another coastal area exhibits net co2 production these results indicate estimates for glacial foreland solute fluxes and co2 weathering consumption and production should include estimates from both non glacial and proglacial streams understanding weathering fluxes from these two types of streams will be important for evaluations of past ice sheet retreat and predictions of future solute and co2 fluxes associated with continued ice sheet retreat

widespread bacterial populations at glacier beds and their relationship to rock weathering and carbon cycling bacterial populations found in subglacial meltwaters and basal ice are comparable to those in the active layer of permafrost and orders of magnitude larger than those found in ice cores from large ice sheets populations increase with sediment concentration and 5 24 of the bacteria are dividing or have just divided suggesting that the populations are active these findings 1 support inferences from recent studies of basal ice and meltwater chemistry that microbially mediated redox reactions may be important at glacier beds 2 challenge the view that chemical weathering in glacial environments arises from purely inorganic reactions and 3 raise the possibilities that redox reactions are a major source of protons consumed in subglacial weathering and that these reactions may be the dominant proton source beneath ice sheets where meltwaters are isolated from an atmospheric source of co 2 microbial mediation may increase the rate of sulfide oxidation under subglacial conditions a suggestion supported by the results of simple weathering experiments if subglacial bacterial populations can oxidize and ferment organic carbon it is important to reconsider the fate of soil organic carbon accumulated under interglacial conditions in areas subsequently overridden by pleistocene ice sheets

length scales for continental deformation in convergent divergent and strike slip environments analytical and approximate solutions for a thin viscous sheet model the deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere exact analytical solutions for the case of a newtonian fluid and approximate solutions for the case of fluid with power law rheology show that when the imposed velocity vector is normal to the boundary congressional or extensional regime the deformation field decays away from the boundary with a characteristic length scale 1 3 to 1 10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10 where n is the stress strain exponent in the rheology in contrast when the imposed velocity vector is parallel to the boundary transcurrent regime the length scale of the deformation field is approximately 4 times smaller in each case these length scales decrease approximately as n the difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust such forces would modify the ratio of length scales but not sufficiently to affect this result

nonvolcanic tectonic islands in ancient and modern oceans 1 most oceanic islands are due to excess volcanism caused by thermal and or compositional mantle melting anomalies we call attention here to another class of oceanic islands due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics sunken tectonic islands capped by carbonate platforms have been previously identified along the vema and romanche transforms in the equatorial atlantic we reprocessed seismic reflection lines did new facies analyses and 87sr 86sr dating of carbonate samples from the carbonate platforms a 50 km long narrow paleoisland flanking the vema transform underwent subsidence erosion and truncation at sea level it was then capped by a 500 m thick carbonate platform dated by 87sr 86sr at 11 10 ma three former islands on the crest of the romanche transverse ridge are now at 900 m bsl they show horizontal truncated surfaces of oceanic crust capped by 300 m thick carbonate platforms with 10 6 ma sr isotopic ages these sunken islands formed due to vertical tectonics related to transtension transpression along long offset slow slip transforms another tectonic sunken island is atlantis bank an uplifted gabbroic block along the atlantis ii transform sw indian ridge 700 m bsl a modern tectonic island is st peter and st paul rocks a rising slab of upper mantle located at the st paul transform equatorial atlantic cold tectonic islands contrast with hot volcanic islands related to mantle thermal and or compositional anomalies along accretionary boundaries and within oceanic plates or to supra subduction mantle melting that gives rise to islands arcs

segmentation of the blanco transform fault zone from earthquake analysis complex tectonics of an oceanic transform fault 1 the blanco transform fault zone btfz forms the 350 km long pacific juan de fuca plate boundary between the gorda and juan de fuca ridges nearby broadband seismic networks provide a unique framework for a detailed long term seismotectonic study of an entire oceanic transform fault otf system we use regional waveforms to determine 129 earthquake source parameters combined with 28 harvard moment tensors they represent the largest waveform derived otf source parameter data set joint epicenter determination removes the northeasterly routine location bias projecting seismicity onto the btfz we determine along fault seismic slip rate variations earthquake source parameters and morphology indicate several transform segments separated by extensional step overs the eastern segment from gorda ridge to gorda depression is a pull apart basin the longest transform 150 km following blanco ridge from the gorda to cascadia depression is seismically very active seismically fully coupled has a wider seismic zone 9 km than other btfz transform segments and accommodates the largest mw 6 4 6 5 btfz earthquakes interpretation of cascadia depression as spreading ridge is supported by plate motion parallel normal faulting t axes spreading is currently tectonic 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling a short transform connects to the pull apart surveyor depression widely spread seismicity along the western btfz reflects complex morphology indicating ongoing plate boundary reorganization along short narrow width subparallel faults seismic coupling is low in extensional 15 compared to transform areas 35 100 implying different mechanical properties centroid depth variations are consistent with seismic slip cutoff near 600 c

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

tracking the paleogene india arabia plate boundary the location of the india arabia plate boundary prior to the formation of the sheba ridge in the gulf of aden is a matter of debate a seismic dataset crossing the owen fracture zone the owen basin and the oman margin was acquired to track the past locations of the india arabia plate boundary we highlight the composite age of the owen basin basement made of paleocene oceanic crust drilled on its eastern part and composed of pre maastrichtian continental and oceanic crust overlaid by ophiolites emplaced in early paleocene on its western side a major fossil transform fault system crossing the owen basin juxtaposed these two slivers of lithosphere of different ages and controlled the uplift of marginal ridges along the oman margin this transform system deactivated 40 myrs ago coeval with the onset of ultra slow spreading at the carlsberg ridge the transform boundary then jumped to the edge of the present day owen ridge during the late eocene oligocene period before seafloor spreading began at the sheba ridge this migration of the plate boundary involved the transfer of a part of the indian oceanic lithosphere formed at the carlsberg ridge to arabia this late eocene oligocene tectonic episode at the india arabia plate boundary is synchronous with a global plate reorganization event corresponding to geological events at the zagros and himalaya belts the owen ridge uplifted later in late miocene times and is unrelated to any major migration of the india arabia boundary

transformation of organic matter in a barents sea sediment profile coupled geochemical and microbiological processes process based mechanistic investigations of organic matter transformation and diagenesis directly beneath the sedimentwater interface swi in arctic continental shelves are vital as these regions

brine volcanism and the interior structures of asteroids and icy satellites abstract cryovolcanism is among the foremost processes responsible for modifying the surfaces of icy satellites volcanic brine petrogenesis in ammonia deficient satellites should mainly involve eutectic melting in relevant salt water systems carbonaceous chondrites provide useful insights into the compositions of salts and aqueously altered rock in icy satellites and asteroids c1 chondrites contain about one fifth by mass of salts in various states of hydration many aspects of the petrogenesis and physical volcanology of icy satellite brines should be well described in the system h2o mgso4 na2so4 minor components include sulfates of k ni mn and ca chondrites also contain abundant carbonates but these are probably not very important in brine magmatism due to their low solubilities under expected conditions chlorides are also unimportant under most circumstances because of the low cosmic abundance ratio of cl s soluble salts may have profound effects on the geology and structure of icy satellites and asteroids in some models late episodes of water volcanism are facilitated by high buoyant forces due to the relatively high densities of sulfate rich mantle and crustal layers in other models early hypersaline brine volcanism quickly yields to plutonic magmatism due to low crustal densities europa probably has a layered crust composed of anhydrous mg na sulfates near the base and a frozen or partially molten eutectic mixture of ice and hydrated mg and na sulfates near the surface ganymede may have a crust about 300 km thick composed of a 10 1 ratio of ice mirabilite and a mantle 500 km thick composed of 50 ice phases plus 50 hydrated mg and na sulfates

further evidence for the compound nature of slow earthquakes the prince edward island earthquake of april 28 1997 seismograms of the april 28 1997 prince edward island transform fault earthquake mw 6 8 show clear evidence for the superposition of two types of rupture a normal earthquake and a slow component which primarily radiated energy at low frequencies we combined low frequency spectral data with p waveform data to invert for a source time function that satisfied data from 0 001 to 2 5 hz the results show that the earthquake started with a slow event which began 15 s before the fast mainshock and was part of a slow earthquake which lasted over 30 s the smoothness of the initial event is inferred from the low amplitude of high frequency energy at regional stations during the initial portion of the p wave arrival directivity analysis shows that the two components of the rupture occurred on different faults the slow event occurred on the main transform and the fast event occurred on a parallel fault 40 km to the east a similar compound sequence involving both slow and fast components and rupture on multiple faults occurred on the romanche transform in 1994 these and other results for slow earthquakes suggest that many large events on oceanic transform faults are compound events

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

seasonal evolution of subglacial drainage and acceleration in a greenland outlet glacier an acceleration of ice mass loss has been observed near the margin of the greenland ice sheet partly as a result of faster ice motion observations by gps receivers reveal high seasonal variability in ice motion with summer motion up to 220 higher than winter background levels the greenland ice sheet contains enough water to raise sea levels by 7 m however its present mass balance and future contribution to sea level rise is poorly understood1 accelerated mass loss has been observed near the ice sheet margin partly as a result of faster ice motion2 3 4 surface melt waters can reach the base of the ice sheet and enhance basal ice motion5 6 however the response of ice motion to seasonal variations in meltwater supply is poorly constrained both in space and time here we present ice motion data obtained with global positioning system receivers located along a 35 km transect at the western margin of the greenland ice sheet throughout a summer melt season our measurements reveal substantial increases in ice velocity during summer up to 220 above winter background values these speed up events migrate up the glacier over the course of the summer the relationship between melt and ice motion varies both at each site throughout the melt season and between sites we suggest that these patterns can be explained by the seasonal evolution of the subglacial drainage system similar to hydraulic forcing mechanisms for ice dynamics that have been observed at smaller glaciers

carbon dating reveals a seasonal progression in the source of particulate organic carbon exported from the greenland ice sheet surface melt from the greenland ice sheet gris collects particulate organic carbon poc as it drains into subglacial environments and transports it downstream where it serves as a microbial substrate we hypothesized that older poc is entrained by meltwaters as the subglacial drainage network expands upglacier over the summer to test this poc samples were collected from a meltwater river exiting the gris over an ablation season and 14c dated resulting values were compared with meltwater hydrochemistry and satellite observations of the catchment area we found that poc ages increased from 5000 to 9000 years b p until peak discharge and catchment size afterward significant fluctuations in poc age were observed interpreted to result from periods of high and low subglacial hydrological pressure and sediment supply and subsequent exhaustion these observations suggest a seasonal progression in the source of poc exported from the gris and provide evidence for a seasonally evolving subglacial drainage system

small scale models of oceanic transform zones previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets in order to determine how kinematic and mechanic parameters control the deformation pattern we perform small scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere respectively the three dimensional depth geometry of these layers follows the thermal structure of the ridge transform system the structure of the deformed transform zone is very sensitive to input parameters specially to the offset the obliquity of the deformed zone with the extension direction i e spreading direction in a natural system depends on the offset and on the brittle layer thickness for greater thickness or a shorter offset the transform domain widens and the components of extension become significant for smaller lithosphere thickness or larger offset the transform domain becomes narrow with steeper walls and mainly undergoes strike slip deformation such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones a comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone ptdz and spreading direction a relationship between offset and width is clearly established for mid atlantic ridge mar transforms as on analog models a similar relationship is observed for the east pacific rise epr transforms but with greater variabilty this difference is interpreted as resulting either from greater complexity of transform zones on epr than on mar making it difficult to estimate some parameters from different mechanical behaviors between slow and fast spreading systems or from higher rheological complexity of natural systems than out model

tyre and pwyll galileo orbital remote sensing of mineralogy versus morphology at two selected sites on europa observational data from the galileo orbiter s remote sensing instruments including solid state imaging ssi the near infrared mapping spectrometer nims and the ultraviolet spectrometer uvs are analyzed and interpreted in terms of the history of the tyre and pwyll impact sites and the nature of europa s crustal zonal structure material characterized by asymmetric 1 4 and 2 0 xm bands and visible coloration characterizes each site where morphologic evidence suggests disruption of the topmost crust the material is not h20 ice is endogenic and is common to the linea impact basins and dark trailing side terrain differences between tyre and 11 are interpreted in terms of europa s peculiar energy history while other evidence may suggest flooding of the endogenic material as a liquid the pure ice appearance of most of europa s crust is probably superficial while beneath a thin patina of sputtered h20 molecules the crust is everywhere laced with numerous generations of intrusions and extrusions of an aqueous phase our results together with numerous laboratory experiments and theoretical analysis suggest that the aqueous phase is dominated by the so2 anion nothing is rich but the inexhaustible wealth of nature she shows us only surfaces but she is a million fathoms deep

hydrochemistry of ice stream beds evaporitic or microbial effects recently new data have been acquired on the chemical composition of waters within the glacial till beneath the kamb ice stream kis formerly ice stream c and the bindschadler ice stream bis formerly ice stream d two of the arteries of ice loss from the antarctic ice sheet ais we compare this water chemistry with basally stored subglacial waters from arctic and alpine glaciers and the only other measurements of sub ice sheet chemistry to date that emerging from beneath law dome near casey station sw antarctica during a jokulhlaup we find that the chemistries pose significant questions about the hydrology and biogeochemistry of ice sheet beds copyright 2010 john wiley sons ltd