the topography and morphology of low shields and associated landforms of plains volcanism in the tharsis region of mars abstract the morphology and topography of volcanic landforms provide critical information to the investigation of their tectonic setting and the physical characteristics e g rheology of their eruption products their investigation is also an important prerequisite for studies of comparative planetology e g the comparison between surface features of the earth and other planetary bodies numerous small and low shield volcanoes on mars and associated vents and lava flows have previously been compared to terrestrial plains style volcanism which is defined as being an intermediate style between flood basalts and the hawai ian shields this study investigates the topography and morphology of martian landforms associated with plains volcanism using mola moc themis hrsc and hirise data low shields and other landforms of plains volcanism on mars display similarities with terrestrial basaltic volcanic fields and we do not observe any features that do not have morphologic analogues on earth the most typical landforms of martian plains style volcanism are low shields defined as volcanic shields with diameters of typically less than 50 km heights of a few hundred meters only and extremely shallow flank slopes of less than 0 5 other surface features related to plains style volcanism on mars are craters fissure vents cinder and spatter cones lava flows that are commonly associated with lava channels and tubes lava inflation features and volcanic rift zones our results reconfirm the viking orbiter based conclusion that plains volcanism in the eastern snake river plains is perhaps the best terrestrial morphological analogue for these martian surface features icelandic shields distinct structures in hawai i and other basaltic landforms also show similarities to martian plains volcanism sinuous rilles previously not described in association with plains volcanism on mars are interpreted as evidence for high eruption rates the extremely shallow flank slopes of the low shields suggest the eruption of shield building lavas with low viscosity which might be the result of high eruption temperatures high effusion rates a low si and a high mg content along with a possible high fe content or a combination of these factors the spatial distribution of low shield clusters in tharsis does not show any obvious association with large scale tectonic features plains volcanism might represent a relatively recent type of volcanism on mars which is not related to mantle plumes but to a zone of partial melting in an anomalously warm mantle underneath a thickened crust schumacher and breuer geophys res lett 34 l14202 doi 10 1029 2007gl030083 2007

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

earth and mars water inventories as clues to accretional histories abstract the earth has 2 7 km of water on its surface its mantle contains at least 150 ppm water and probably significantly more depending on the amount of undepleted mantle and subducted crustal water that is present geologic evidence suggests that a few hundred meters of water are close to the martian surface but evidence from snc meteorites indicates that the martian mantle is very dry containing no more than about 35 ppm water part of the difference in water content of the mantles of the two planets is attributed to plate tectonics however the earth s mantle appears to contain at least several times the water content of the martian mantle even accounting for plate tectonics we attribute the difference to two possible causes the first possibility is that melting of the earth s surface during accretion as a result of the development of a steam atmosphere allowed impact devolatized water at the surface to dissolve into the earth s interior in contrast because of mars smaller size and greater distance from the sun the martian surface may not have melted so that the devolatilized water could not dissolve into the surface a second and preferred possibility is that mars like the earth acquired a late volatile rich veneer but it did not get folded into the interior as with the earth but instead remained as a water rich veneer the perception of mars as having a wet surface but a dry interior is consistent with what we know of the geologic history of mars which can be viewed as the progressive intrusion and overplating of a water rich crust by dry mantle derived volcanic rocks

oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

manganese nodules nod a 1 and nod p 1 implications of pre treatment on oxygen isotopes and mineralogy abstract manganese nodules are a type of ferromanganese chemical precipitate containing a plethora of economically important metals increasingly being used in the high and green technology sectors by measuring oxygen isotope ratios in these materials we can increase our understanding of how they form and constrain which metals are likely to be present in them there is a lack of suitable reference materials for use in oxygen isotopic studies of samples dominated by mn oxide minerals i e mn nodules and crusts and therefore a need for their development the samples nod a 1 and nod p 1 united states geological survey mn nodules from the atlantic and pacific oceans respectively provide the best opportunity for such a reference material but their application has been hampered by their incomplete mineralogical characterization and lack of consensus on how to pre treat if at all these type of samples for 18o analysis we have performed a detailed mineralogical study using scanning electron microscopy energy dispersive spectroscopy sem eds powder x ray diffraction pxrd and raman spectroscopy on these reference materials in sample nod a 1 todorokite goethite and or hematite mg calcite phillipsite or chabazite quartz and barite were identified the minerals todorokite birnessite asbolane vernadite goethite pyrochroite quartz phillipsite or chabazite anorthite kaolinite and barite were identified in nod p 1 we conducted experiments to assess the effect on 18o and mineralogy resulting from acidification with hydrochloric acid hcl pre fluorination with bromine pentafluoride brf5 at ambient temperature or both acidification and pre fluorination treatments results from samples given both treatments were deemed most meaningful to the study of mn crusts and nodules and therefore nodule research in general by providing 18o values representative of the relatively pure oxide silicate fraction only we report a 18o value of 11 0 1 0 for the oxide silicate fraction of nod a 1 and a value of 11 1 0 7 for the same fraction in nod p 1 with results showing a 18o shift of up to 10 3 as a function of nodule and the pre treatment used the 18o values of samples given both treatments converge and are substantially 18o depleted relative to that of corresponding untreated material which was found to contain various 18o enriched phases such as carbonate minerals and non stoichiometric water based on our results we recommend that for all future analyses of mn nodules and crusts the samples receive both treatments to ensure accurate and consistent results

oxygen isotope relationships in iron meteorites iron meteorites with oxygen bearing phases can be classified in terms of their oxygen isotopic abundances these iron meteorite classes are isotopically similar to various stony meteorite classes which may indicate a common origin the group iab and iiicd irons may be related to the winonaites group iie irons may be related to h chondrites group iva irons may be related to l or ll chondrites

in situ stress determination from hydraulic injection test data laboratory and field results have shown that the orientation of a hydraulically propagated fracture is not always perpendicular to that of the minimum principal stress but may be influenced by local weakness planes such as preexisting natural fissures the lower the flow rate the stronger the weakness planes influence this observation has led to a new stress determination method based on the measurement of the normal stress supported by fracture planes with various dips and strikes this new method provides means to evaluate the precision of the stress determination and can be applied to situations in which none of the principal stresses is vertical when all the fracture planes are parallel to the borehole axis the method is still applicable if fast flow rate reopening pressures can be measured and if the principal stress directions are assumed to be independent of depth validity of the method is illustrated by analyzing results from two sets of data one of which has been discussed previously in the literature

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

water flow in the barbados accretionary complex we present a simple analytic kinematic model of the barbados accretionary complex assuming a porosity depth law and deformation by pure vertical shear the model is applied to two sections a northern one with a 900 m thick basin sedimentary column near 16 n and a southern one with a 4 5 km thick one to the south this simple model is compatible with growth of the barbados accretionary complex since 50 ma assuming a constant subduction rate and a constant basin sedimentary column equal to the present one this conclusion is not affected by reasonable changes in the porosity depth law it suggests that the totality of the entering sedimentary column is either accreted or subcreted to the wedge the accretion velocity decreases dramatically with age and its variation should be taken into account in any quantitative model the average strain rates show remarkably different distributions between the two sections the thinner wedge having a much higher rate in a narrow area at the toe we then use the model to compute the sediment grains flow the fluid flow and their thermal effects these effects are small and do not exceed 5 of the heat flow in the absence of other internal flows in contrast the thermal effect of the burial of the slab can result in a lowering of the surface heat flow by as much as 40 the temperature at the base of the wedge on its arc side is close to 300 c on the southern section and half that amount on the northern one this high temperature could account for the absence of large earthquakes to the south and their presence to the north the amount of water discharged if collected by faults over the whole toe area is unable to account for some of the manifestations observed over the toe or seaward of the toe in the barbados area we suggest two possible other sources one for the southern section is the orinoco fan area the other for the northern section might be seismic pumping at the limit of the seismogenic zone along the backthrusts between the accretionary wedge and the forearc basin

chemical and sr isotopic variations during diagenesis of miocene siliceous sediments of the monterey formation california the monterey formation of california preserves miocene siliceous sediments at progressive generally with depth diagenetic stages diatomite opal a ct chert opai ct quartz chert for initially clay poor sediments and diatomaceous shale ct porcelanite porous opal ct quartz porcelanite for sediments that were initially clay rich the chemistry of most diatomite is controlled by three primary phases biogenic silica carbonate and detrital clay and several subsidiary phases organic material apatite pyrite and detrital minerals including quartz and plagioclase the elemental abundances of the clay poor diagenetic sequence decrease sharply with advancing diagenetic state for those elements controlled by clay al ti k rb cs fe some group b metals carbonate ca mg sr ba apatite p ca sr ba v and organics v ni while silica shows a congruent increase these first order variations reflect the initial compositions of the siliceous sediments modified by the addition of silica the removal of carbonate and apatite and the migration of petroleum during silica diagenesis anomalous abundances of mn ni cr mo and some other group b metals are associated with trapped refractory organic material and suggest that petroleum catagenesis and migration occurred during silica diagenesis migration may have been facilitated by the release of bound water during silica dehydration reactions second order variations in k rb k cs rb sr 87 sr 86 sr with advancing diagenetic state suggest ion exchange and ion expulsion by reaction of detrital phases with pore fluids diagenetic processes partially equilibrated but did not homogenize depositional 87 rb 86 sr 87 sr 86 sr mixing lines generated by detrital clays and authigenic carbonate and silica in siliceous sediments from the santa maria region however similar mixing lines in diatomites from the santa lucia area are reset to nearly horizontal slopes in associated ct porcelanites the degree of equilibration may be a function of the rate of formation of relatively impermeable quartz chert which isolated the siliceous system from further interaction with pore fluids the rapid formation of quartz chert in the santa maria region is believed to have prevented isotopic homogenization processes from reaching completion initial chemistry of the siliceous systems controlled the rates and pathways of silica diagenesis silica diagenesis in turn altered the chemistry of the siliceous sediment

an internally consistent triple oxygen isotope calibration of standards for silicates carbonates and air relative to vsmow2 and slap2 abstract high precision triple oxygen isotope measurements are becoming a more common analysis in laboratories there is a lack of calibrated standards to use for triple oxygen isotope measurements and this has led to data being presented on different scales rather than to the traditional vsmow2 slap2 scale here we present triple oxygen isotope values of standard carbonates co2 liberated from carbonates silicates and air calibrated to the vsmow2 slap2 scale we analyzed vsmow2 and slap2 to calibrate our reference gas our measured 18o value of slap2 is 55 55 indistinguishable from the accepted value of 55 5 our 17o value of slap2 0 528 is not zero but rather 0 015 corresponding to a 17o value of 29 741 the 17o values of carbonate standards nbs19 iaea603 and nbs18 are 0 102 0 100 and 0 048 respectively 0 010 for co2 of calcite liberated by phosphoric acid digestion at 25 c the acid value at 25 c is 0 5230 0 0003 these results can be used to correct triple oxygen isotope measurements of co2 released by phosphoric acid digestion in other laboratories we present triple oxygen isotope values for uw garnet 2 nbs 28 san carlos olivine nm sco and our in house quartz standard nm q aliquots of nm q and nm sco are available from the center for stable isotopes csi new mexico for interlaboratory comparison our 17o and 18o values for air are 12 178 0 066 and 24 046 0 117 respectively with a corresponding 17o value of 0 441 0 012 with the availability of common standards all laboratories making 17o 18o measurements can calibrate their reference gas relative to the vsmow2 slap2 scale laboratories making triple oxygen isotope measurements on co2 released from carbonates using phosphoric acid digestion can correct to the bulk carbonate value

helium isotopes in ferromanganese crusts from the central pacific ocean abstract helium isotopes have been measured in samples of two ferromanganese crusts va13 2 and cd29 2 from the central pacific ocean with the exception of the deepest part of crust cd29 2 the data can be explained by a mixture of implanted solar and galactic cosmic ray produced gcr he in extraterrestrial grains and radiogenic he in wind borne continental dust grains 4he concentrations are invariant and require retention of less than 12 of the in situ he produced since crust formation loss has occurred by recoil and diffusion high 4he in cd29 2 samples older than 42 ma are correlated with phosphatization and can be explained by retention of up to 12 of the in situ produced 4he 3he 4he of va13 2 samples varies from 18 5 to 1852 ra due almost entirely to variation in the extraterrestrial he contribution the highest 3he 4he is comparable to the highest values measured in interplanetary dust particles idps and micrometeorites mms helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in falling terrestrial and extraterrestrial grains of fe mn crusts the extraterrestrial 3he concentration of the crusts rules out whole undegassed 4 40 m diameter idps as the host instead it requires that the extraterrestrial he inventory is carried by numerous particles with significantly lower he concentrations and occasional high concentration gcr he bearing particles

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

fluid flow in accretionary prisms evidence for focused time variable discharge accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration these latter processes in conjunction with fermentation or thermal maturation of entrained organic matter yield hydrocarbon bearing pore fluids that are expelled from the prism regional fluxes of heat and a number of dissolved chemical species most notably carbon are controlled by the advective expulsion of the pore waters numerical modeling observation and monitoring of flow patterns and rates and recent in situ hydrogeological tests quantify the conditions that control rates of fluid flow dispersed intergranular flow 10 8 to 10 11 m s controlled by the vertical permeability of the prism 10 14 to 10 20 m is limited by low permeability lithologies and seems not to vary much from margin to margin focused flow 10 1 to 10 8 m s above the decollement is controlled by fault zones or sedimentary intrusions diapiric structures at low fluid pressures fault zone permeability may be similar to that of adjacent wall rock but as fluid pressure increases from hydrostatic 0 to near lithostatic levels 1 0 fault zones dilate and fracture permeability increases by 2 4 orders of magnitude 10 10 to 10 16 m similarly mud volcanoes and diapirs provide high permeability fluid conduits to the sediment water interface as a result faults and intrusions become primary flow paths and support surface vents at which syntectonic deposits carbonate and gas hydrates accumulate and chemosynthetic organisms cluster models of thermal and chemical anomalies and epigenetic deposits indicate that flow is temporally variable that conclusion has been quantified by extended 1 10 months seafloor and borehole experiments that measured temperature anomalies associated with flow events on the cascadia prism flow estimated velocity 3 10 5 m s 950 m yr confined to a thrust fault that cuts upward through the prism has brought thermogenic hydrocarbons from depths 1 5 km to the surface within the last 400 years

elysium planitia lava flows crater count chronology and geological implications mars global surveyor mgs and viking images allow analysis of chronological and geological relations among lava flows in southern elysium planitia based on crater populations mgs and viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces there is no evidence of substantial dust mantling on the young flows and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves the youngest flows have some of the lowest crater densities that we have seen on mars some 1 of lunar mare values at crater diameters below about 180 to 500 m we infer that thin scattered volcanic flows some tens of meters thick have been emplaced atop stratigraphic units within the last 100 myr and possibly within the last 10 myr volcanism is thus a continuing process in the recent geologic history of mars and this must constrain geophysical models of the planet

plumbing accretionary prisms effects of permeability variations fault zones focus fluid expulsion in the muddy northern barbados ridge accretionary prism with fault parallel permeabilities about 1000 times greater than intergranular permeabilities in the adjacent sediment in the oregon prism the low bedding perpendicular permeability due to mudstones inhibits intergranular dewatering however intergranular flow is concentrated where submarine erosion breaches high permeability sandy layers even so faults can capture fluid flow from these exposed sandy layers suggesting the faults have a still higher permeability such observations coupled with laboratory measurements permeabilities suggest that faults off oregon may have fault parallel permeabilities at least 10 10000 times greater than the adjacent sediments results from barbados and oregon suggest fluid flow is concentrated along the most active faults at the toe of prisms the fault zones are being progressively loaded by the thickening wedge and are undergoing compaction preliminary experiments show that permeability decreases relative to the surrounding wall rocks along faults within this compactive deformation regime we believe that these faults must undergo dilation perhaps linked to transient increases in pore pressure if they are to be preferential fluid conduits farther upslope erosion exposes rocks that are more consolidated commonly more cemented and generally of lower intergranular permeability than rocks of equivalent burial further seaward because of their lithification and overconsolidation these rocks dilate during faulting locally enhancing fracture permeability in such dilative regimes faults become evermore focused zones of fluid expulsion relative to occluded intergranular pathways

recent and episodic volcanic and glacial activity on mars revealed by the high resolution stereo camera the large area coverage at a resolution of 10 20 metres per pixel in colour and three dimensions with the high resolution stereo camera experiment on the european space agency mars express mission has made it possible to study the time stratigraphic relationships of volcanic and glacial structures in unprecedented detail and give insight into the geological evolution of mars here we show that calderas on five major volcanoes on mars have undergone repeated activation and resurfacing during the last 20 per cent of martian history with phases of activity as young as two million years suggesting that the volcanoes are potentially still active today glacial deposits at the base of the olympus mons escarpment show evidence for repeated phases of activity as recently as about four million years ago morphological evidence is found that snow and ice deposition on the olympus construct at elevations of more than 7 000 metres led to episodes of glacial activity at this height even now water ice protected by an insulating layer of dust may be present at high altitudes on olympus mons

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

implications of carbon flux from the cascadia accretionary prism results from long term in situ measurements at odp site 892b a 403 day in situ field experiment at ocean drilling program site 892b sought to quantify the flux of methane along a fluid active fault and to experimentally determine rates of methane hydrate and authigenic carbonate deposition associated with fluid expulsion from the borehole an instrument package was deployed that osmotically sampled fluid measured borehole pressure and flow rates and contained reaction chambers in which deposition of gas hydrates and carbonates was anticipated and from which microbial communities might be extracted flow is highly variable in the three phase water methane system that exists at site 892b flow rates fluctuate over two orders of magnitude in response to tidally induced pressure variations and gas hydrate formation and dissociation hydrate formation began 45 days into the experiment and reduced the initial flow 2 l day to 20 ml day unexpectedly the hydrate destabilized after about 125 days tidally induced flow reversals are common 25 of time in this setting characterized by overpressured pore waters these reversals pump sulfate rich bottom water into near surface sediments where archaea anaerobically oxidize ch4 and induce carbonate precipitation at the sediment water interface authigenic carbonates are undergoing dissolution methanotrophs dominated the microbial community where fluid is discharged to ambient seawater all expelled methane is apparently oxidized in the water column

fluid flow along a strike slip fault at the toe of the oregon accretionary prism implications for the geometry of frontal accretion sediment offscraping and accretion takes place predominantly by landward vergent thrusting along most of the oregon accretionary prism north of latitude 44 50 n this indicates very low basal shear stress probably due to near lithostatic fluid pressure on the decollement horizon recent multichannel seismic and sidescan sonar studies have demonstrated the existence of an oblique basement involved left lateral strike slip fault termed the wecoma fault it cuts abyssal plain sediments and oceanic basement of the subducting plate where the wecoma fault intersects the wedge thrust front at 45 09 n accretion to the wedge occurs through an anomalous seaward vergent thrust linear fault parallel erosional gullies which trend obliquely across the slope on the seaward flank of the frontal anticline in the accretionary wedge are interpreted as marking the surface traces of splays of the wecoma fault data collected during six alvin dives on and around these gullies demonstrate that fluid expulsion from the deforming sediments is preferentially concentrated along the fault traces evidence includes the presence of chemosynthetic biological communities pervasive veining and fracturing of the rock and extensive carbonate cementation all forming preferentially within the gullies this strike slip fault apparently acts as a high angle fluid escape conduit channeling fluids from deep within the wedge to the surface we propose that this fluid conduit causes the local inversion of thrusting to seaward vergence by allowing the escape of highly overpressured fluids from the incipient decollement horizon the consequent increase in basal effective normal stress favors the development of a seaward vergent thrust the discovery of this strike slip fault acting as an efficient fluid conduit to the surface of the wedge emphasizes the generally unappreciated importance of strike slip and normal faults in dewatering accretionary prisms such dewatering features may significantly influence the development of large scale structures and they have broad applicability to other tectonic settings including collision zones and continental fold and thrust belts

correction to influence of a variable thermal conductivity on the thermochemical evolution of mars in the paper influence of a variable thermal con ductivity on the thermochemical evolution of mars bysandra schumacher and doris breuer journal of geophys ical research 111 e02006 doi 10 1029 2005je002429 2006 the temperature difference between the solidus tem perature and the calculated present day mantle tempera ture was displayed here we describe a correction of thecalculated solidus temperature and therefore a correctionof the presented temperature differences we also discussthe implications of the new results with regard to theplausibility of the models under the aspect of the exis tence of partial melt as several studies hartmann et al 1999 hartmann and berman 2000 neukum et al 2004 have found evidence for recent volcanism onmars there has to be at least locally a zone of partialmelt within lithosphere and mantle this fact is used tovalidate the models

frictional properties of the south china sea oceanic basalt and implications for strength of the manila subduction seismogenic zone abstract we conduct friction experiments on a basalt sample no u1431e41r6w from international oceanic discovery program iodp expedition 349 in the temperature range of 100 600 c and effective normal stress of 50 mpa with 100 mpa pore fluid pressure loading rates are stepped up and down in the range of 0 04 1 0 m s to obtain the velocity dependence of friction based on the rate and state friction law we numerically fit the experimental curves to acquire constitutive parameters that cannot be obtained by direct measurement experimental results show that basalt friction coefficient varies from 0 70 to 0 74 similar to the range of gabbro documented in a previous study he et al 2007 stable velocity strengthening behavior is observed at temperatures of 100 c and 200 c and unstable velocity weakening at temperatures above 300 c with occurrences of periodic stick slips both a and b values inferred by numerical fittings show positive temperature dependence in the temperature range of 100 600 c the stick slip motions above 300 c seem to be due to a dramatic decrease in the characteristic slip distance dc from 11 to 15 m to 0 4 2 5 m based on our results and plastic flow laws of mafic rocks from a previous study fault strength profile and slip stability of manila subduction zone are discussed with calculations in the context of a unified friction flow model the inferred lower boundary of seismogenic zone of manila subduction fault is around 40 km which is in a reasonable range compared with seismicity data

sequential leaching of marine ferromanganese precipitates genetic implications abstract sequential leaching experiments were carried out on twenty one hydrogenetic crust samples from different locations in the central pacific and the results are compared with four crust and nodule samples of different genetic origin fe vernadite mno2 is the most important minor and trace metal bearing mineral phase and shows high concentrations of co ni cd ti ba zn and cu which are present in seawater mainly as hydrated and labile complexed cations elements forming carbonate and hydroxide complexes and oxyanions in seawater like pb mo v and minor proportions of cu and zn are bound in the fe00h fraction the ca carbonate phase and the residual fraction which consists of aluminosilicates and crystalline oxides show only minor heavy metal associations e g cu and zn only a small part of the high ti concentrations in hydrogenetic crusts is of detrital origin ti mainly forms a hydrogenetic phase probably consisting of ti02 2h20 intergrown with the amorphous fe00h phase analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements the most notable change was observed for pb which during phosphatization of crusts is transferred from the mn fe oxide fraction to the apatite fraction a colloid chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed in the first stage mn2 rich water from the oxygen minimum zone is mixed with oxygen rich deep water and oxidized mn iv and other metals like fe ti al and si form oxide and hydroxide colloids these form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species in stage two the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations incorporating the sorbed heavy metals into the mineral phases

kinematic constraints on porosity change in the toe of the cascadia accretionary prism evidence for cementation and brittle deformation in the footwall of the frontal thrust kinematic analyses are performed on a seismic profile across the deformed protothrust zone ptz of the cascadia accretionary prism off the central coast of oregon in order to quantify the deformation in this zone and to explore mechanisms and controls on deformation and dewatering the analyses rely on seismically defined stratigraphy and seismic interval velocities to constrain distortional and volumetric changes within the ptz permitting the calculation of the complete deformation field two solutions are offered the first is based on porosity estimates derived from seismic interval velocities by cochrane et al 1994b which suggest extremely high volume loss in the immediate footwall of the frontal thrust the second solution employs a modified volume distribution in which lateral shear deformation is minimized this latter approach is justified by evidence from seismic profiles and in drill cores for subhorizontal maximum principal stresses in many prisms the two solutions yield total shortening of about 1 46 km 12 with a general landward increase in horizontal shortening and vertical extension this appears to be accompanied by increasing brittle deformation and porosity decrease perhaps enhanced by dewatering along protothrusts the greatest difference between the solutions lies in estimates for tectonically induced volume loss in the footwall of the frontal thrust which is significantly reduced in the second solution relying on kinematic and mechanical arguments that support this result we propose that the high seismic velocities observed in the footwall of the thrust may result from the presence of intergranular cement perhaps preferentially precipitated adjacent to the frontal thrust fault in sediments with relatively high fracture permeability and enhanced fluid flow a high degree of lithification in this zone is consistent with the inferred increase in brittle deformation in the footwall of the thrust

the dole effect and its variations during the last 130 000 years as measured in the vostok ice core we review the current understanding of the dole effect the observed difference between the delta sup 18 o of atmospheric o sub 2 and that of seawater and its causes extended the record of variations in the dole effect back to 130 kyr before present using data on the delta sup 18 o of o sub 2 obtained from studying the vostok ice core and discuss the significance of temporal variations the dole effect reflects oxygen isotope fractionation during photosynthesis respiration and hydrologic processes evaporation precipitation and evapotranspiration our best prediction of the present day dole effect 20 8 per thousand is considerably lower than the observed value 23 5 per thousand and we discuss possible causes of this discrepancy during the past 130 kyr the dole effect has been 0 05 per thousand lower than the present value on average the standard deviation of the dole effect from the mean has been only per thousand and the dole effect is nearly unchanged between glacial maxima and interglacial periods the small variability in the dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant most periodic variability in the dole effect is in the precession band suggesting that changes in this global biogeochemical termmore reflects variations in low latitude land hydrology and productivity or possibly variability in low latitude oceanic productivity 65 refs 4 figs 3 tabs less

ferromanganese crusts as recorders of marine dissolved oxygen abstract the distinct triple oxygen isotope composition of tropospheric o2 relative to seawater is the result of biogeochemical reactions e g primary productivity respiration exchange with the stratosphere and the relative size of different oxygen containing reservoirs namely o2 o3 and co2 this difference in isotopic composition gives tropospheric o2 utility as a record of biogeochemical and atmospheric processes and may also be used for determining where in the rock record isotopic fingerprints of tropospheric oxygen may be preserved the isotopic record of tropospheric oxygen in previous studies is largely limited to analyses of gas trapped in continental glaciers and a patchwork of other proxies most notably the triple oxygen signature of sulfate here we show the uppermost layers of hydrogenetic deep ocean ferromanganese crusts from each of the major ocean basins have a triple oxygen isotope composition consistent with the direct incorporation of dissolved oxygen the range of 18o and 17o in ferromanganese crusts suggests the mn oxide endmember contains a near 50 50 mixture of oxygen from water and dissolved o2 our data indicate this signal also persists into older layers of the crusts potentially preserving near 75 million years of the oxygen isotopic composition of the lower troposphere and subsequent deep ocean respiration our analysis of oxygen isotope values bulk chemistry and estimated local dissolved oxygen for crust top samples reveals that variations in bulk chemistry ultimately exhibit more influence on the oxygen mass balance than changes in dissolved oxygen presenting a challenge for unambiguous determination of local dissolved oxygen although analytical challenges remain these widespread layered deposits of ferromanganese crust may offer a viable path for future interrogation of the history or relative history of the oxygen cycle of the troposphere and deep ocean millions of years into the past

biological oxygen productivity during the last 60 000 years from triple oxygen isotope measurements during isotope exchange between o2 and co2 in the stratosphere the relative rates of biologic o2 production and stratospheric processing determine the relationship between d 17 o and d 18 oo f o2 in the atmosphere variations of this relationship thus allow us to estimate changes in the rate of mass dependent o2 production by photosynthesis versus the rate of o2 co2 exchange in the stratosphere with about equal fractionations of d 17 o and d 18 o in this study we reconstruct total oxygen productivity for the last glacial the last glacial termination and the early holocene from the triple isotope composition of atmospheric oxygen trapped in ice cores with a box model we estimate that total biogenic productivity was only 76 83 of today for the glacial and was probably lower than today during the glacial interglacial transition and the early holocene depending on how reduced the oxygen flux from the land biosphere was during the glacial the oxygen flux from the glacial ocean biosphere was 88 140 of its present value index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4870 oceanography biological and chemical stable isotopes 1615 global change biogeochemical processes 4805 0315 atmospheric composition and structure biosphere atmosphere interactions keywords gisp2 siple ice cores oxygen isotopes past oxygen productivity stratospheric isotope exchange respiration

evidence for recent volcanism on mars from crater counts impact craters help characterize the age of a planetary surface because they accumulate with time they also provide useful constraints on the importance of surface erosion as such processes will preferentially remove the smaller craters earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on mars they disagreed however on the age of the youngest volcanism these earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter here we report an analysis using new images obtained by the mars global surveyor spacecraft of crater populations that extend the size distribution down to about 16 m our results indicate a wide range of surface ages with one region lava flows within the arsia mons caldera that we estimate to be no older than 40 100 million years we suggest that volcanism is a continuing process on mars

interaction between accretionary thrust faulting and slope sedimentation at the frontal makran accretionary prism and its implications for hydrocarbon fluid seepage 1 using high resolution seismic profiles and other geophysical data collected during r v meteor cruise m74 2 we investigate the distribution patterns of shallow sediments their structure and deformation processes and their role in the migration accumulation and seepage of hydrocarbon rich fluids here we show that rapid syn kinematic sedimentation at the frontal makran accretionary prism provides a mechanism by which emerging imbricated thrust packets override the footwall at the seafloor without significant mass wasting and destruction of fault related anticline in the hanging wall these anticlines may rise high above the seafloor to form plate boundary parallel ridges and distinguish from simple thrust blocks seen at convergent margins elsewhere with the fast burial of many thrust faults by the syn kinematic sediments anticlinal growth structures form in these syn kinematic sediments by continuous thrust activity the anticlinal structures preserved within the cores of the ridges or formed from these syn kinematic sediments act as structural traps for migrating hydrocarbon rich fluids above which fluid escape structures are generated leading to seafloor seeps most of the discovered hydrocarbon seeps around sixth ridge are sourced from these traps despite the compressional environment and the rapid syn kinematic sedimentation rates shallow subsurface of the frontal makran is a normally pressured regime in which the buoyancy of hydrocarbons may account for the fluid migration in this important respect the makran accretionary prism differs from many other convergent margins and accretionary prisms where fluid flow is largely driven by tectonically induced overpressure

lead concentrations in the northeast pacific evidence for global anthropogenic perturbations lead concentrations were determined by isotope dilution mass spectrometry in 34 surface and deep water samples collected in the northeast pacific between hawaii and california and off the california coast using a deep water sampler protected against fouling by contamination from the ship and hydrowire measured concentrations lie more than one order of magnitude below previously published open ocean values and they show that in most cases 90 or more of the total lead is in a dissolved form lead concentrations are about 10 fold higher in surface and thermocline waters than in deep waters values drop as low as 1 ng kg 5 pmol kg below 3500 m depth lead profiles thus appear different compared to those of most trace metals which show enrichment in deep waters compared to surface concentrations lead concentrations in surface waters increase from 5 to 15 ng kg 25 to 75 pmol kg along a transect starting from a location 200 km off the california coast and continuing towards the center of the north pacific gyre this increase is congruent with that observed for 210 pb concentrations in the same waters sampled at the same time lead is supplied to the open north pacific largely from the atmosphere at a rate of about 60 ng cm 2 yr which exceeds the prehistoric oceanic output flux of authigenic lead recorded in pelagic sediments about tenfold this excess originates from emissions from smelters and combustion of leaded gasoline overwhelming natural lead inputs that entered the ocean during prehistoric time probably largely through rivers vertical lead concentration profiles below the surface mixed layer are probably not in steady state there concentrations must be increasing in response to the increase of anthropogenic inputs because the estimated lead residence times are more than 20 years in the thermocline and about 80 years in deep waters based on an estimated 10 fold input and concentration increase since the mid eighteenth century in the surface mixed layer in the central northeast pacific it is calculated that the 10 ng kg average concentration between 100 and 900 m is 2 to 5 times larger and the 1 8 ng kg average concentration between 900 and 5000 m depth is about 2 times larger than it was in the mid eighteenth century profiles of lead concentrations in the north atlantic are expected to be shifted generally to larger values by a factor of 2 to 3 compared to those in the north pacific because of the effects of greater industrial lead contamination while lead concentration profiles in the south pacific are expected to be shifted generally to lower values compared to the north pacific by about this same factor because of the effects of lesser industrial lead contamination

surface expression of fluid venting at the toe of the nankai wedge and implications for flow paths abstract cold seeps have been observed with manned submersibles at many sites on the tokai margin most sites are found on the hanging wall of major thrust faults within 1 2 km of their outcrop few sites relate to purely erosional features this relationship between thrusting and venting may be explained either by fluid flow along fault zones by fluid flow along permeable layers outcropping at the fault scarp or by diffuse upward flow of fluids released during footwall compaction we use deep towed side scan sonar and sediment sounder profiles acquired at the toe of the nankai accretionary wedge to identify the context of seepage at a smaller scale modes of fluid expulsion inferred from observations in this zone include flow channelling along strata along thrusts along gravity faults and along joint fractures as well as diffuse flow we then combine visual observation and monitoring of electrochemical potential eh to assess the relative importance of seepage sites we show that bacterial mats are better visual indicators of active seepage than bivalve beds and that the most active seepage sites correlate with outcrops of coarse turbidites cemented by authigenic carbonates these were sampled and have permeability high enough to channel fluid flow 1 4 10 14 m2 we suggest they are the main conduits near the seafloor however seepage sites are also observed directly on the outcrop of major thrusts indicating that they do play a role as fluid conduits we observe negative polarity seismic reflections on these faults below 1 km depth and wonder about their significance

mechanisms to explain the platinum concentration in ferromanganese seamount crusts abstract ferromanganese seamount crusts cover large areas of hard substrates of sea floor edifices in the central pacific they can be distinctly enriched in pt and show pt pd ratios which are several orders of magnitude greater than those orders of magnitude greater than those of seawater thirty four selected samples of the younger and the older crust generation were analyzed for mn fe ni co and pt the bulk pt concentration varies between 0 14 and 1 02 ppm and averages to 0 51 0 24 ppm the older generation is generally somewhat richer in pt 0 63 0 25 ppm than the younger one 0 39 0 15 ppm compared to the earth s upper continental crust the ferromanganese material contains about 100 times more pt in the studied depth range between 1100 and 3800 m the pt concentration is controlled by the water depth in addition to the bulk data the distribution of pt and pd is recorded through a complete ferromanganese crust profile the average pd value amounts to 16 2 8 ppb for the younger and 16 4 8 ppb for the older generation two immediate sources may be responsible for the pt concentration in the ferromanganese crusts seawater and cosmic spherules the presented thermodynamic approaches are based on the presumption that the pt tetrachlorocomplex is the prevailing species under seawater conditions precipitation of pt in seawater may take place if this complex is decomposed by reduction of pt 2 to the elemental state on the other hand the mno 2 precipitation is caused by oxidation of dissolved mn 2 ions which are supplied from the oxygen minimum zone a model reaction by combining the two half reactions is formulated assuming a coprecipitation of pt with mno 2 an alternative mechanism to explain pt enrichment in ferromanganese crusts might be surface adsorption of anionic pt tetrachloro complexes onto positively charged amorphous iron hydroxide particles cosmic spherules were discovered in samples of the older ferromanaganese crust generation the spherules are mostly concentrated in the upper part of this crust layer iron meteorites often have pt concentrations up to 20 ppm the relative quantity recovered in the grain size class 0 1 0 4 mm only amounts to 2 10 4 which is significantly too low to explain the pt content of the crust samples however smaller cosmic particles may exist and also contribute to the pt enrichment suggesting that rough crust surfaces are excellent collectors of denser detrital material the maximum pt values observed in the youngest part of the older crust generation may be explained by physical incorporation of cosmic particles

hydrogeology and mechanics of subduction zone forearcs fluid flow and pore pressure at subduction zones fluid flow pore pressure and tectonic processes are tightly interconnected excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration and it has profound effects on fault and earthquake mechanics through its control on effective stress the egress of these overpressured fluids which is in part governed by the presence of permeable fault zones is a primary mechanism of volatile and solute transport to the oceans recent field measurements new constraints gained from laboratory studies and numerical modeling efforts have led to a greatly improved understanding of these coupled processes here we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs and focus on recent advances that have quantified permeability architecture fluxes the nature and timing of transience and pressure distribution thus providing new insights into the connections between fluid metamorphic mechanical and fault slip proc

triple oxygen isotope analysis of bioapatite as tracer for diagenetic alteration of bones and teeth abstract the detection of diagenetic alteration is critical for palaeoclimate reconstruction that is based on the oxygen isotope composition of fossil bones and teeth so far no direct chemical proxy has been found to track diagenetic modification of the oxygen isotope ratios here a new approach to identify diagenetic changes of 18opo4 values in skeletal apatite of small mammals by means of triple oxygen isotope analysis 16o 17o and18o is presented our method is based on the fact that inhaled air oxygen o2 has an isotope anomaly on its rare isotope 17o inhaled air o2 is a major source of oxygen in small land living mammals a fraction of the anomaly is transferred via body water to skeletal apatite where it can be detected by means of 17o and 18o analyses the approach considering the current analytical uncertainty is restricted to small mammals with body masses 1 kg this is due to the low specific metabolic rates of large mammals resulting in a lower fraction of oxygen inhaled via breathing relative to oxygen from other sources in their body water remnant negative 17o anomalies derived from in vivo inhaled o2 have been detected in enamel bioapatite of eocene to miocene rodent teeth while dentine of the same teeth lacks significant 17o anomalies this suggests preservation of the original phosphate oxygen isotope composition in enamel of these small mammal teeth in contrast 17o anomalies in dentine have been erased due to diagenetic alteration with isotopically normal diagenetic fluids triple oxygen isotope analysis of bioapatite thus seems to be a useful new proxy to directly detect diagenetic alterations of the 18opo4 values of small mammal teeth

tectonically driven fluid flow in the kodiak accretionary complex alaska sandstone and chert boudins that have syntectonic extensional veins are dominant structural elements in melanges of the kodiak accretionary complex alaska detailed analyses of cogenetic methane rich and water rich fluid inclusions in syntectonic quartz veins indicate that fluid pressures dropped by 20 to 45 during crystal growth this drop occurred faster than uplift of the rock and probably represents the local effect of fluid escaping to shallow levels in the subduction complex during melange deformation possibly along an interconnected fracture network an important implication of channelized fracture flow to higher levels in a subduction complex is that fluids may transport heat and solutes thereby generating thermal and chemical anomalies along shallow faults

geochemistry of a sealed deep sea borehole on the cascadia margin the deep sea borehole seal cork was deployed for the first time on a modern accretionary prism during odp leg 146 to the cascadia margin ten months after the deployment the fluid flow and geochemistry of the borehole fluids was investigated during several dives by dsrv alvin the chemical analysis of the borehole fluids revealed methane concentrations of more than 3 5 mm whereas oxygen and dissolved ions as cl no3 or po4 are still close to the ambient seawater composition the exceedingly high methane content measured at the top of the sealed borehole and the observed degassing during the ascent of the submersible indicates that the sampled fluid was initially saturated or close to saturation with respect to ch4 the hydrocarbons are characterized by ratios of 170 200 and 13c values of 59 5 to 62 4 o which indicates a considerable admixture of thermogenic hydrocarbon gases the occurrence of methane of partly thermogenic origin demonstrates that ch4 enters the sealed borehole in the lower perforated section 94 178 mbsf and accumulates at the top of the borehole this suggests the occurrence of free gas within the encapsulated borehole considering the stability field of ch4 hydrates the formation of these ice like structures may take place and potentially results in a clogging of the top of the borehole such precipitates could result in a decoupling of the top of the borehole from the hydraulic and geochemical regime of the accretionary complex an important aspect for future plans of cork deployments

fluid expulsion from the cascadia accretionary prism evidence from porosity distribution direct measurements and gloria imagery fluid expulsion from the cascadia accretionary prism off oregon results from porosity reduction by compaction and by cementation as methane rich pore waters precipitate diagenetic carbonate deposits near the sediment water interface porosity changes suggest that dewatering begins 5 6 km west of the base of the slope in a proto deformation zone gloria imagery of surficial carbonate deposits confirms that fluid is actively expelled from this zone there is no such evidence further west in cascadia basin within the uncertainties of the data porosities do not decrease landward beneath the prism this pattern is consistent with imbricate thrust faulting on the slope which provides the vertical load to induce compactive dewatering and may physically import as much as 50 of the total fluid volume in the section a simple vertical compaction model suggests that significant pore water volumes have been expelled from the lower slope but at flux rates 10 11 10 12 m 3 m 2 s 1 which are orders of magnitude less than those measured at individual vent sites 10 6 m 3 m 2 s 1 faulting clearly controls some fluid expulsion but gloria data suggest that repeated local discharge cementation and abandonment lead to dispersed accumulations of diagenetic carbonate

60 myr records of major elements and pb nd isotopes from hydrogenous ferromanganese crusts reconstruction of seawater paleochemistry we compare the time series of major element geochemical and pb and nd isotopic composition obtained for seven hydrogenous ferromanganese crusts from the atlantic indian and pacific oceans which cover the last 60 myr average crust growth rates and age depth relationships were determined directly for the last about 10 myr using be 10 be 9 profiles in the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial be 10 be 9 ratios due to the lack of additional information co contents have also been used previously to estimate growth rates in co rich pacific and atlantic seamount crusts puteanus and halbach 1988 a comparison of be 10 be 9 and co based dating of three co rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated be 10 be 9 based growth rates back to 60 ma our data show that the flux of co into co poor crusts has been considerably lower the relationship between growth rate and co content for the co poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits manheim 1986 the results suggest that the co content provides detailed information on the growth history of ferromanganese crusts particularly prior to 10 12 ma where the be 10 based method is not applicable the distributions of pb and nd isotopes in the deep oceans over the last 60 myr are expected to be controlled by two main factors a variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and b variability of supply rates or provenance of detrital material delivered to the ocean linked to climate change glaciations or major tectonic uplift the major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust consequently any interpretation of time series of major element concentrations of a single crust in terms of paleoceanograghic variations must be considered with caution this is because local processes appear to have dominated over more basin wide paleoceanographic effects in this study co is the only element which shows a relationship to pb and nd isotopes in pacific crusts a possible link to changes of pacific deep water properties associated with an enhanced northward advection of antarctic bottom water from about 14 ma is consistent with the pb but not with the nd isotopic results the self consistent profiles of the pb and nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts including phosphatization events have been insignificant for particle reactive elements such as pb be and nd isotope time series of pb and nd show no systematic relationships with major element contents of the crusts which supports their use as tracers of paleo seawater isotopic composition

barbados ridge hydrogeologic tests implications for fluid migration along an active decollement hydrogeologic tests were conducted at a sealed borehole penetrating the decollement of the barbados ridge accretionary complex at low excess pore pressures p p p h p l p h 0 0 to 0 36 where p p pore pressure p h hydrostatic pressure and p l lithostatic pressure estimated permeabilities were comparable to those of similar unfractured sediment these tests complement shipboard packer tests completed at higher fluid pressures 0 5 to 1 0 during ocean drilling program odp leg 156 together the test results suggest a 4 to 5 order of magnitude permeability increase as fluid pressure varied from hydrostatic 0 to lithostatic 1 however unlike the results of the shipboard packer tests the test results presented here exhibit no evidence of a relationship between permeability and pore pressure the combined findings from the two sets of hydrogeologic tests indicate that significant permeability increases can occur within the decollement at pore pressures below lithostatic pressure

hydrothermal mn deposits of the franciscan assemblage ii isotope and trace element geochemistry and implications for hydrothermal convection at spreading centers abstract major element trace element and sr nd pb and o isotopic data for a franciscan mn deposit suggest an origin by seafloor hydrothermal circulation based on q mode factor analysis the cherts and mn lenses of the blue jay mine formed from a combination of 4 components representing 1 biogenic 1 hydrothermal and 2 detrital sources rb sr u th pb and o isotopic systematics in the mn lenses were affected by input from the hydrothermal circulation of material leached from the underlying basalts nd isotopic compositions in both cherts and mn lenses are identical and within the range measured for pacific ocean water suggesting the ree were not mobilized by hydrothermal activity correlation of 18 o with sio 2 and mno 2 in the mn lenses implies the lenses formed by simple mixing of hydrothermally derived mn oxides with seawater and biogenic silica 18 o of the cherts is both uniform and depleted relative to dsdp jurassic cherts but similar to microquartz bearing cherts of the monterey formation this suggests that diagenetic activity exerted more control on oxygen isotope compositions then hydrothermal alteration or metamorphism finally a well defined rb sr isochron of 158 5myr was obtained for these cherts and opens the possibility of determining absolute radiometric ages for similar cherts throughout the geologic record

hydrogeological modeling of porous flow in the oregon accretionary prism venting of pore fluid was recently reported near the front of the oregon accretionary prism and was directly measured at one of the venting sites during alvin dives in this study the authors model the flow of pore fluid in the oregon accretionaly prism they use seismic reflection sections to constrain subsurface structures drill hole results from deep sea drilling project leg 18 to constrain sediment properties and regional tectonic information to constrain the timing and the rate of plate convergence and sediment imbrication and thickening the results of two dimensional time element finite element modeling show that high pore pressures are generated beneath the decollement due to the load of imbricated and thickened sediments diffusive flow at a rate of several millimetres per year is predicted within the accretionary prism if fault zone permeability is discounted the presence of a decollement or permeable fault penetrating the accretionary prism as imaged on the seismic reflection section can drastically alter the patterns of fluid flow and produce focused venting of fluid as observed on the other hand fluid venting along faults may not significantly alter the high pore pressures below the decollement

fluid expulsion sites on the cascadia accretionary prism mapping diagenetic deposits with processed gloria imagery point discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near surface 10 m below seafloor mbsf hemipelagic sediment the contrasting clastic and diagenetic lithologies should be apparent in side scan images however sonar also responds to variations in bottom slope so unprocessed images mix topographic and lithologic information we have processed gloria imagery from the oregon continental margin to remove topographic effects a synthetic side scan image was created initially from sea beam bathymetric data and then was subtracted iteratively from the original gloria data until topographic features disappeared the residual image contains high amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge based on submersible mapping ocean drilling program drilling and collected samples diagenetic deposits are concentrated 1 near an out of sequence thrust fault on the second ridge landward of the base of the continental slope 2 along zones characterized by deep seated strikeslip faults that cut transversely across the margin and 3 in undeformed cascadia basin deposits which overlie incipient thrust faults seaward of the toe of the prism there is no evidence of diagenetic deposition associated with the frontal thrust that rises from the decollement if the decollement is an important aquifer apparently the fluids are passed either to the strike slip faults which intersect the decollement or to the incipient faults in cascadia basin for expulsion diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates

fluid generation and distribution in the highest sediment input accretionary margin the makran fluids in subduction zones can influence seismogenic behaviour and prism morphology the eastern makran subduction zone offshore pakistan has a very thick incoming sediment section of up to 7 5 km providing a large potential fluid source to the accretionary prism a hydrate related bottom simulating reflector bsr zones of high amplitude reflectivity seafloor seep sites and reflective thrust faults are present across the accretionary prism indicating the presence of fluids and suggesting active fluid migration high amplitude free gas zones and seep sites are primarily associated with anticlinal hinge traps and fluids here appear to be sourced from shallow biogenic sources and migrate to the seafloor along minor normal faults there are no observed seep sites associated with the surface expression of the wedge thrust faults potentially due to burial of the surface trace by failure of the steep thrust ridge slopes thrust fault reflectivity is restricted to the upper 3 km of sediment and the deeper decollement is non reflective we interpret that fluids and overpressure are not common in the deeper stratigraphic section thermal modelling of sediments at the deformation front suggests that the deeper sediment section is relatively dewatered and not currently contributing to fluid expulsion in the makran accretionary prism

heterogeneous hydrofracture development and accretionary fault dynamics ocean drilling program observations at many accretionary wedges indicate that fluid flow in low permeability sediments is focused along faults rich in clay minerals recent ring shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads it is thus unlikely that observed focused flow occurs through the matrix of muddy faults and we suggest that focused flow can only be explained by the development of open fracture systems the lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot however exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse we propose that the regional hydrofracture networks must propagate heterogeneously leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions

isotopic evidence for channeled fluid flow in low grade metamorphosed jurassic accretionary complex in the northern chichibu belt western shikoku japan the low grade metamorphosed jurassic accretionary complex of the northern chichibu belt hijikawa area western shikoku is divided into two units the hijikawa and kanogawa units that are separated by an out of sequence thrust oost the ozu kawabegawa fault the kanogawa unit south of the ozu kawabegawa fault consists mainly of sandstone shale broken formation of alternating sandstone and shale greenstone chert and pelitic melange while the hijikawa unit is characterized by a stack of subunits separated by small scale thrusts the subunits are mainly made up of basic pelitic and psammitic semischists schistose pelitic melange and chert petrological and mineralogical constraints suggest peak metamorphic conditions of 204 247 c at 1 3 kbar in the kanogawa unit and 228 289 c at 3 5 6 kbar in the hijikawa unit quartz and quartz calcite veins are widely developed in the study area especially in the hijikawa unit regional variations in stable isotopic data show that the 18oquartz and 18ocalcite values in veins tend to increase towards the ozu kawabegawa fault the 18owhole rock values are remarkably high in some subunits close to oosts within the hijikawa unit oxygen isotopic compositions from vein quartz indicate that a higher 18o fluid migrated upward from depth along the ozu kawabegawa fault within the accretionary prism during prehnite pumpellyite facies metamorphism the fluid source is inferred to be pelitic rocks at higher temperatures and greater depths within the accretionary prism

influence of a variable thermal conductivity on the thermochemical evolution of mars 1 the influences of variations in thermal conductivity due to composition temperature and pressure in mantle and crust on the thermochemical evolution of mars have been examined with parameterized convection models we use a temperature and pressure dependent thermal conductivity in the mantle and column averaged values between 1 5 and 4 wm 1k 1 for the crust with 2 wm 1k 1 as the representative value for a basaltic crust the major characteristic of the models is a slower cooling of the upper mantle with a long existing global partial melt zone and a stronger crustal production late in the evolution compared to earlier models which assume a constant thermal conductivity of 4 wm 1k 1 to explain the observed crustal thickness and the early crustal formation on mars the formation of an enriched primordial crust is required recent volcanism can be explained by lateral temperature variations that are caused by thickness variations of the low conducting crust these melt zones which occur underneath thicker than average crust may be the source region for the observed longstanding volcanism in tharsis and elysium instead of plumes rising from the core mantle boundary the evolution of the elastic lithosphere is characterized by a transition at which the elastic strength is first provided by the crust and later also by the upper mantle this finding may explain the estimated elastic lithosphere thickness of less than 16 km in the noachian and more than 100 km in the hesperian and amazonian

on conducting the modified slug test in tight formations the method introduced by bredehoeft and papadopulos 1980 for conducting a modified slug test in tight formations does not assure the condition of approximate equilibrium necessary at the start of the test in addition compressibility in the shut in well can be significantly larger than the compressibility of water which bredehoeft and papadopulos indicate should be used in the calculations errors that may result from using the original test procedure are discussed and a modified procedure and testing arrangement are presented

interrelationship of fluid venting and structural evolution alvin observations from the frontal accretionary prism oregon seismic reflection and sea beam bathymetric data plus submarine geological measurements define a ramp anticline at the deformatoin front of the central oregon subduction zone at its northern termination the ramp anticline is deeply incised by a large 500 m deep submarine canyon and cut by a probable backthrust to the south along the strike of the fold a smaller submarine canyon shallowly erodes the anticline and backthrusting is not apparent in the submersible observations two alvin dives along a transect through the southern canyon show active fluid vents demarked by biological communities at the frontal thrust and at the breached crest of the anticline along a northern transect encompassing the large submarine canyon 10 alvin dives indicated no venting on the formal thrust limited venting in the canyon but numerous biological communities along a scarp interpreted as the surface trace of the backthrust these observations suggest a scenario of vent and structural geomorphic development consisting of 1 frontal thrust faulting and associated venting facilitated by high fluid pressure 2 erosion of the oversteepened seaward flank of the ramp anticline assisted by seepage forces and leading to fluid flow out of stratigraphically controlled conduits in the limbs of the overthrust deposits 3 lockingmore of the frontal thrust due to dewatering or a local decrease in wedge taper associated with development of the large canyon leading to failure along the backthrust and 4 redirection of fluid flow by the backthrust thus within le 0 3 m y deformation of the relatively permeable sediments of the oregon margin results in stratigraphically controlled flow being partially captured by faults less

hydrogeologic properties of a thrust fault within the oregon accretionary prism two sets of hydrogeologic tests conducted at ocean drilling program odp hole 892 on the oregon accretionary prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone the first set of tests consisted of shipboard packer tests conducted during odp leg 146 fall 1992 while the second set of tests were constant drawdown and constant discharge tests conducted in fall 1993 using the submersible alvin pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure relative to hydrostatic dropped below 0 315 to 0 325 mpa 0 53 to 0 54 where pore pressure hydrostatic lithostatic hydrostatic analysis of the packer test data suggested an apparent background pressure of 0 25 mpa 0 42 to 0 50 because the borehole had been open for 12 hours prior to the packer tests formation pore pressures may have exceeded this value prior to drilling of the borehole these overpressures dissipated by the time the second set of tests were conducted one possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone the second set of tests were conducted at pressures 0 019 mpa or 0 03 below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests when fractures were open constraints on fluid flow rate along the fault are provided by observed displacement in a bottom simulating reflector bsr at its intersection with the fault zone the closed fracture transmissivities are insufficient to produce flow rates capable of displacing the bsr therefore open fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed bsr displacement in addition calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor

dynamic model constraints on oxygen 17 depletion in atmospheric o2 after a snowball earth a large perturbation in atmospheric co2 and o2 or bioproductivity will result in a drastic pulse of 17o change in atmospheric o2 as seen in the marinoan oxygen 17 depletion mosd event in the immediate aftermath of a global deglaciation 635 mya the exact nature of the perturbation however is debated here we constructed a coupled four box and quick response biosphere atmosphere model to examine both the steady state and dynamics of the mosd event our model shows that the ultra high co2 concentrations proposed by the snowball earth hypothesis produce a typical mosd duration of less than 106 y and a magnitude of 17o depletion reaching approximately 35 both numbers are in remarkable agreement with geological constraints from south china and svalbard moderate co2 and low o2 concentration e g 3 200 parts per million by volume and 0 01 bar respectively could produce distinct sulfate 17o depletion only if postglacial marine bioproductivity was impossibly low our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean atmosphere exchange only if partial pressure of atmospheric o2 is larger than 0 02 bar during the snowball period and records of weathering derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown in any case a snowball earth is a precondition for the observed mosd event

an alternative mechanism for recent volcanism on mars images of the martian surface indicate that volcanism is a recent process on mars with ages of the youngest lava flows of about 2 ma interestingly these young lava flows are confined to the large old volcanic centers of tharsis and elysium which exhibit a thickened crust compared to surrounding areas the origin of the volcanic centers but also their longstanding volcanic activity is often explained by strong mantle plumes arising from the core mantle boundary however their existence and sustainment during the last few billion years is uncertain under martian conditions alternatively the thermal insulation by the crust could cause the longstanding volcanic activity we show that a locally thickened crust which has a lower thermal conductivity and is enriched in radioactive heat sources in comparison to the mantle leads to significant lateral temperature variations in the upper mantle sufficient to generate partial melt even in the present martian mantle

density and lithospheric thickness of the tharsis province from mex mars and mro gravity data 1 radio science tracking of mars express mars experiment and mars reconnaissance orbiter produced high resolution martian gravity data applying localized spectral analysis on the new gravity data sets we study the surface density and lithospheric elastic thickness in the tharsis province the gravity signal is predicted with geophysical models either including bottom loading top bottom model or taking into account the loading history top top model volcanic shields are mainly composed of high density lava but their construction could have begun with lower density lava with the exception of ascraeus mons which has a top of lower density buoyant bottom loading may have been present in the form of a mantle plume under olympus mons the elastic thickness was much larger at olympus mons than at other volcanoes suggesting large spatial variations of heat flux during the hesperian alternatively small elastic thicknesses could be artifacts reflecting the presence of very localized high density crustal intrusions beneath the volcanoes thaumasia highlands were probably supported by a mantle plume at the time of formation in valles marineris top bottom models predict low densities and large elastic thicknesses in conflict with the basaltic rock composition and hesperian age of the valley dense mafic dikes underlie the western part of the valley the top top model serves to test another scenario in which the trough is formed with sedimentary infilling removed much later by erosion the elastic thickness increasing in between at the large volcanoes the relation between gravity and topography is anisotropic probably because of density variations

transient fluid flow through the toe of the barbados accretionary complex constraints from ocean drilling program leg 110 heat row studies and simple models thirty four sediment and mud line temperatures were collected from six drill holes on ocean drilling program odp leg 110 near the toe of the barbados accretionary complex when combined with thermal conductivity measurements from sediment cores and results from earlier surveys these data delineate the complicated thermal structure on the edge of this convergent margin heat flow values at the seafloor of 92 192 mw m sup 2 are 80 300 higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude heat flow tends to decrease downhole at four sites suggesting the presence of heat sources within the sediments these results are consistent with the flow of warm fluids through the complex along high permeability conduits including thrust faults the major decollement zone and sandy intervals simple calculations suggest that this fluid flow is transient occurring on time scales of tens of thousands of years fluid flow velocities along the decollement zone estimated with a simple thermal model are about 10 sup minus 7 m s 100 times faster than predicted by numerical steady state analyses the estimated maximum sediment permeability within the decollement zone which was basedmore on the above fluid flow velocity is about 10 sup minus 12 m sup 2 also 100 times higher that that calculated numerically high heat flow in the vicinity of 15 degree 30 prime n and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism water discharge zone with dewatering more active here than elsewhere along the accretionary complex less

triple oxygen isotope variations in sedimentary rocks relatively large 0 2 17 o anomalies in the geologic record have been used to recognize atmospheric processes such as photochemical reactions and to trace changes in the partial pressures of o 2 and co 2 in earth s atmosphere through time however recent oxygen isotope measurements of terrestrial rocks minerals and waters also reveal common smaller but statistically significant deviations from a single mass dependent fractionation line these subtle anomalies have been explained through differences in mass dependent isotopic fractionations for various equilibrium and kinetic mechanisms here we present triple oxygen isotope data on sedimentary silica and oxides including archean and phanerozoic cherts and iron formations the distribution of data reflects the mass fractionation laws of low temperature precipitation reactions during growth of authigenic minerals variation in 17 o of the waters from which sedimentary minerals precipitate and equilibrium exchange after initial authigenic formation we use these results to illustrate the potential for small mass dependent variations in 17 o values of sedimentary rocks to provide constraints on the environmental and climatic conditions in which they formed

chemical and stable isotope fractionation in manganese oxide phosphorite mineralization timna valley israel element carbon and oxygen isotope fractionations have been studied in laminated deposits manganese nodules and phosphorite lenses in clastic sediments of the cambrian timna formation southern israel comparisons of element abundances show that only manganese phosphorus and uranium were mobilized and concentrated during diagenesis element fractionations among the diagenetic manganese components particularly resemble those observed in present day suboxic pelagic marine sediments epigenetic processes involving reducing solutions reversed the diagenetic geochemical trends with the redox sensitive elements manganese and uranium showing considerable depletions in nodules and iron and lead showing corresponding enrichments the manganese oxides of nodules have calculated 18 o compositions varying from 1 6 to 3 1 these compositions are significantly lower than a 18 o value of 7 9 obtained for the manganese oxides of an ocean floor nodule and are taken to largely represent temperature differences between the ocean floor and the shallow marine timna conditions slight decreases in 18 o accompany the transformation from diagenetic type a to epigenetic type b manganese nodules corresponding shifts to lower 18 o are also inferred for the structural carbonate of apatites such shifts are compatible with re equilibration accompanying a slight temperature increase and or low temperature meteoric water alteration 13 c values of 7 to 12 obtained for the apatites of laminated deposits and phosphorite lenses are consistent with phosphorite diagenesis in suboxic to anoxic pore waters early diagenetic redox conditions are indicated to be major controls governing the development of the assemblages

permeability of a decollement zone results from a two well experiment in the barbados accretionary complex fault zones are hypothesized to play a major role in focusing fluid expulsion from accretionary complexes a small number of previous investigations have examined the hydrogeologic properties of these fault zones using borehole packer or submersible based slug and constant rate flow tests while these single well experiments yielded the first data on the permeability of active fault zones they could only investigate a small area around the borehole and could not ensure reliable determination of formation storage properties recent data from an unplanned experiment in the decollement zone of the barbados accretionary prism demonstrate the potential for multiple well hydrogeologic tests pressure data from a sealed borehole show a clear signal of drilling activity 45 m away analysis of the pressure response suggests that decollement zone permeability lies between 1 0 and 1 2 x 10 14 m 2 this permeability is 2 4 orders of magnitude greater than overlying and underlying sediment and thus appears sufficient to focus fluid flow along the decollement zone this inadvertent test evaluated fault zone hydrogeologic parameters over a larger radius and longer timescale and at pressures closer to in situ than previous single well tests