a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

microbially induced cementation to control sand response to undrained shear current methods to improve the engineering properties of sands are diverse with respect to methodology treatment uniformity cost environmental impact site accessibility requirements etc all of these methods have benefits and drawbacks and there continues to be a need to explore new possibilities of soil improvement particularly as suitable land for development becomes more scarce this paper presents the results of a study in which natural microbial biological processes were used to engineer a cemented soil matrix within initially loose collapsible sand microbially induced calcite precipitation micp was achieved using the microorganism bacillus pasteurii an aerobic bacterium pervasive in natural soil deposits the microbes were introduced to the sand specimens in a liquid growth medium amended with urea and a dissolved calcium source subsequent cementation treatments were passed through the specimen to increase the cementation level of the sand particle matrix the results of both micp and

subduction induced pore fluid venting and the formation of authigenic carbonates along the cascadia continental margin implications for the global ca cycle abstract pore fluid venting associated with subduction induced sediment deformation causes precipitation of calcium carbonate as prominent carbonate chimneys or cement in the accreted sediments across the active continental margin off oregon and washington a depletion of interstitial ca 2 with a maximum decrease of 50 relative to seawater ca 2 over only 1 5m depth and reduction in porosity in the deformed sediments suggest that interstitial ca 2 is removed to form calcium carbonate cement in contrast the pore waters of the undeformed abyssal plain sediments show no depletion in dissolved ca 2 they are either enriched to a maximum of 5 or show no change in dissolved ca 2 here the background level of caco 3 content in the sediment is only 0 1 to 1 calcium carbonate precipitation in the deformed sediments probably occurs as the result of upward migration and oxidation of biogenic methane and of the increase in carbonate saturation due to release of excess pore pressure during fluid venting upward advection of fluids at rates of 1 28 cm y 1 is predicted from diffusion advection reaction models applied to the downcore concentration profiles of dissolved ca 2 and nh 4 in the tectonically deformed sediments the range of predicted flow rates is related to the type of calcium carbonate lithification i e slow rates generate cement and fast rates generate chimneys carbonate mineral precipitation associated with pore fluid venting requires direct transfer of ca 2 from the oceanic basement to the accretionary complex such a mechanism leads us to propose that the accretionary complexes of the global plate subduction zones are a major sink for crustal ca 2 a global flux of crustal ca 2 that is removed by carbonate mineral precipitation may be as muc3 as the hydrothermal ca input this significant ca flux not previously considered in the global geochemical budget implies that pore fluid venting in subduction zones may also act as a global sink or source for other elements

variations in mode of formation and temperature of oceanic deep waters over the past 125 000 years marked changes in the deep water temperature of the different oceanic basins during the past 125 kyr are revealed by a detailed comparison of the oxygen isotopic composition ofbenthicforaminiferafrom sediment cores taken from the norwegian sea and the pacific atlantic and southern oceans the last interglacial deep water temperatures were similar to the modern ones at the 5e 5d transition around 115 kyr bp the deep water temperatures dropped below approximately 0 c in the pacific and southern indian oceans in the atlantic ocean the deep water temperature dropped in 2 successive steps at 115 and 75ky bp the global ocean deep water temperatures were in the 0 to 1 c range for the whole of the last ice age isotope stages 4 2

calibration of the planktonic foraminiferal mg ca paleothermometer sediment trap results from the guaymas basin gulf of california 1 the mg ca ratio has been measured in two surface dwelling species of planktonic foraminifera globigerinoides ruber and globigerina bulloides from biweekly sediment trap samples collected in guaymas basin gulf of california between august 1992 and november 1997 the guaymas basin experiences significant seasonal changes in sea surface temperature sst 16 33 c and thus is an ideal location for testing the temperature dependence of mg incorporation into foraminiferal calcite the planktonic foraminiferal mg ca ratios are directly compared with concurrent temperature measurements from the study site the results from the sediment trap study reveal a strong positive correlation between sst and the mg ca ratio in both g ruber r2 0 86 and g bulloides r2 0 90 the mg ca ratio increases exponentially by 7 per 1 c change in temperature for g ruber and by 6 for g bulloides these results indicate that the mg ca ratio in g ruber and g bulloides accurately records the measured seasonal surface temperature cycle and interannual variability in the guaymas basin within 1 c the greatest deviation between mg ca estimated sst and observed sst occurs in the spring and fall for both species this variability is attributed to rapidly changing hydrographic conditions and possible offset between the time of calcification and the temperature measurements in addition g ruber and g bulloides exhibit very high mg ca ratios during the strong el nino southern oscillation years of 1992 and 1997 verifying their potential to record sst up to 33 c and 28 c respectively the trace metal data are compared with foraminiferal oxygen isotope and alkenone measurements made on the same suite of sediment trap samples to assess the internal consistency of these temperature proxies strong correlations exist between the 18o and mg ca for g ruber r2 0 86 and g bulloides r2 0 79 similarly sea surface temperature estimates from mg ca ratios are in good agreement with alkenone derived temperature estimates r2 0 89 for g ruber and r2 0 81 for g bulloides

helium isotopes as geochemical indicators of a serpentinized fore arc mantle wedge new helium isotope data from hot springs on eastern kyushu island were obtained in order to elucidate the geographic distribution of 3 he 4 he ratios in the fore arc regions of southwest japan systematic regional variations in 3 he 4 he ratios are clearly distinguished across the fore arc from kyushu island 0 093 0 79 r a where r a denotes the atmospheric 3 he 4 he ratio of 1 4 x 10 6 to shikoku island 0 17 3 6 r a and to the kii peninsula 0 24 7 4 r a the distribution of mantle helium relative to total helium reveals a close association with the geophysical evidence of a serpentinized fore arc mantle wedge including the occurrence of deep nonvolcanic low frequency tremors lfts believed to be caused by upwardly mobile aqueous fluids derived from slab dehydration flux of mantle helium carried by aqueous fluids via a non serpentinized fore arc mantle wedge is very low on kyushu island where lfts have not been detected and 3 he 4 he ratios are lower than the atmospheric ratio on the other hand hot springs above the regions where lfts occur in the kii peninsula and northern shikoku island have high 3 he 4 he ratios indicating a significant contribution of mantle helium carried by aqueous fluids via a serpentinized fore arc mantle wedge the helium isotopes in hot and natural spring gases may be a useful geochemical indicator of a serpentinized fore arc mantle wedge especially in areas with sparse seismic station coverage

release of mantle helium from forearc region of the southwest japan arc abstract shikoku japan is an island located on the forearc region of the western sector of the southwest sw japan arc that was formed by subduction of the philippine sea plate beneath the eurasian plate noble gas isotope analyses were carried out for 30 bubbling gas samples and spring water samples collected in shikoku in addition chemical and carbon isotope analyses were conducted for six gas samples observed 3 he 4 he ratios were 0 17 r r a to 3 56 r r a showing that several samples contain mantle helium components although some samples contain helium of crustal origin geographical distributions of 3 he 4 he ratios show that high 3 he 4 he ratios originate mainly from two areas one along the median tectonic line mtl a major active fault in japan the other coinciding with a region in which non volcanic long period tremors occur about 30 km deep obara k 2002 nonvolcanic deep tremor associated with subduction in southwest japan science 296 1679 1681 this fact indicates that fluids liberated from the slab in the forearc region cause deep tremors and fracturing within the crust thereby easing the transfer of fluids to the surface that are mixed with mantle helium to the surface an active fault system can also provide an efficient path for mantle helium transfer

in situ benthic fluxes from an intermittently active mud volcano at the costa rica convergent margin along the erosive convergent margin off costa rica a large number of mound shaped structures exist built by mud diapirism or mud volcanism one of these mound 12 an intermittently active mud volcano currently emits large amounts of aqueous dissolved species and water chemosynthetic vent communities authigenic carbonates and methane plumes in the water column are manifestations of that activity benthic flux measurements were obtained by a video guided benthic chamber lander bcl deployed at a vent site located in the most active part of mound 12 the lander was equipped with 4 independent chambers covering adjacent areas of the seafloor benthic fluxes were recorded by repeated sampling of the enclosed bottom waters while the underlying surface sediments were recovered with the lander after a deployment time of one day one of the chambers was placed directly in the centre of an active vent marked by the occurrence of a bacterial mat while the other chambers were located at the fringe of the same vent system at a lateral distance of only 40 cm a transport reaction model was developed and applied to describe the concentration profiles in the pore water of the recovered surface sediments and the temporal evolution of the enclosed bottom water repeated model runs revealed that the best fit to the pore water and benthic chamber data is obtained with a flow velocity of 10 cm yr 1 at the centre of the vent the flux rates to the bottom water are strongly modified by the benthic turnover benthic filter the methane flux from below at the bacterial mat site is as high as 1032 mol cm 2 yr 1 out of which 588 mol cm 2 yr 1 is oxidised in the surface sediments by microbial consortia using sulphate as terminal electron acceptor and 440 mol cm 2 yr 1 are seeping into the overlaying bottom water sulphide is transported to the surface by ascending fluids 238 mol cm 2 yr 1 and is formed within the surface sediment by the anaerobic oxidation of methane aom 588 mol cm 2 yr 1 however sulphide is not released into the bottom water but completely oxidized by oxygen and nitrate at the sediment water interface the oxygen and nitrate fluxes into the sediment are high 781 and 700 mol cm 2 yr 1 respectively and are mainly driven by the microbial oxidation of sulphide benthic fluxes were much lower in the other chambers placed in the fringe of the vent system thus methane and oxygen fluxes of only 28 and 89 mol cm 2 yr 1 respectively were recorded in one of these chambers our study shows that the aerobic oxidation of methane is much less efficient than the anaerobic oxidation of methane so that methane which is not oxidized within the sediment by aom is almost completely released into the bottom water hence anaerobic rather than aerobic methane oxidation plays the major role in the regulation of benthic methane fluxes moreover we demonstrate that methane and oxygen fluxes at cold vent sites may vary up to 3 orders of magnitude over a lateral distance of only 40 cm indicating an extreme focussing of fluid flow and methane release at the seafloor

a simple evaluation of cleaning procedures on fossil benthic foraminiferal mg ca 1 recent studies suggest that benthic foraminiferal mg ca may be useful in reconstructing deep water temperature changes during the quaternary and on longer timescales rosenthal et al 1997 lear et al 2000 martin et al 2002 because changes in deep water temperature may be small sample preparation could play an important role in reconstructing accurate temperatures from shell mg ca a simple examination of changes in measured shell chemistry of benthic foraminifera from two intervals subjected to increasingly rigorous cleaning reveals that fossil benthic foraminiferal samples cleaned by oxidation yield lower mg ca values than samples only rinsed and weak acid leached reductive cleaning further lowered the shell mg ca of one of the two benthic species examined lower mg ca values following oxidation and reduction appear to reflect removal of contaminant mg associated with remnant organic matter and adsorbed phases

the 89 ma tortugal komatiitic suite costa rica implications for a common geological origin of the caribbean and eastern pacific region from a mantle plume komatiites are reported for the first time in the northern part of the gulf of nicoya costa rica these rocks dated at 89 7 1 4 ma turonian by 40 ar 39 ar methods occur as a large elongated 14 km long 1 5 km wide n60 w striking body in the ophiolitic nicoya complex these lavas have high mgo 26 29 ni and cr have high cao al 2 o 3 0 98 1 08 and moderate al 2 o 3 tio 2 5 55 8 44 ratios and are depleted in al 2 o 3 4 5 5 k 2 o 0 02 0 37 and tio 2 0 59 0 9 although these lavas are cumulates their geochemical composition indicates an origin from a primary komatiitic magma with a melting temperature of 1700 c at a depth of 150 km similarities in the petrology and age 88 90 ma of gorgona curaiao and nicoya tortugal mafic and ultramafic volcanic rocks suggest that these rocks had a common origin these occurrences suggest a single hotspot center over a large area of the caribbean and eastern pacific mesozoic region due to a major thermal anomaly in the mantle such as a hot rising convective plume

indications of a link between seismotectonics and ch4 release from seeps off costa rica measurements of ch4 concentrations in the bottom water during two discrete sampling periods in subsequent years above different cold seeps at the pacific margin off costa rica indicate large scale variations of ch4 release ch4 is emitted from mud extrusions and a slide scar at 1000 2300 m water depth maximum ch4 concentrations were found to be lower above all investigated sites in autumn 2003 than in autumn 2002 although seep sites are up to 300 km apart tidal and current changes were observed but found to apply only to individual seep sites increased seismic activity connected to the moment magnitude m w 6 4 earthquake offshore costa rica in june 2002 could have had an impact on all seep sites and thereby caused an increase in ch4 emission this is supported by the largest variations of ch4 concentration found above mud extrusions located above faults likely more strongly affected by tectonic movements even though our data indicate a relation between seismicity and ch4 seepage the relation is not proven and future work is needed to comprehensively test this hypothesis

he and sr isotopic constraints on subduction contributions to woodlark basin volcanism solomon islands in order to assess the nature and spatial extent of subduction contributions to arc volcanism sr and he isotopic compositions are measured for dredged volcanic rocks from the woodlark basin in the western pacific sup 87 sr sup 86 sr ratios increase geographically from ocean ridge values 7025 7029 at the woodlark spreading center to island arc ratios 7035 7039 in the solomon islands forearc with intermediate values near the triple junction where the woodlark spreading center subducts beneath the solomon islands sup 3 he sup 4 he ratios are also more radiogenic in the forearc 6 9 plus minus 2 r sub a at active kavachi volcano than along the spreading center where values typical of major ocean ridges were found 8 2 9 3 r sub a very low sup 3 he sup 4 he ratios occur in many triple junction rocks 1 to 5 r sub a but consideration of he isotopic differences between crushing and melting analyses suggests that the low ratios were caused by atmospheric 1 r sub a and radiogenic approx 0 2 r sub a helium addition after eruption variations in unaltered magnetic sup 3 he sup 4 he and sup 87 sr sup 86 sr ratios are best explained by subduction related fluid or silicate melt contributions to the magma source region perhaps from ancient pacific lithosphere however more mantle volatiles dominate the generation of woodlark basin rocks despite extensive subduction in the region less

using 3d seismic reflection data to find fluid seeps from the costa rica accretionary prism submersible dives planned using 2d and 3d seismic reflection data off costa rica successfully found numerous sites of fluid expulsion predominantly along surface fault scarps these data were used because more typical data such as deep towed side scan sonar bottom photography and heat flow were unavailable because fluids and fluid pressure distribution profoundly affect the way an accretionary prism deforms detecting fluid pathways and recovering fluids is a key to its deformation processes detailed seismic interpretation of the shallow subsurface identified apparently active faults which are most likely to act as fluid pathways due to a quasi inverse relationship between surface dip and reflection amplitude the amplitude of the seafloor reflection displayed in map view shows the orientation and extent of surface scarps many of which appear related to subsurface faults because the seafloor amplitude response depends on seafloor morphology as well as near surface physical properties removing the effect of the surface dip calculated directly from picked surface structure leaves a residual that may be related to variation in physical properties although the search for fluid seeps by submersible dives over the costa rica prism was sparse and irregular the seeps that have been discovered are all on structural highs of the apron prism boundary

formation of carbonate concretions in surface sediments of two mud mounds offshore costa rica a stable isotope study the surface sediments of two mud mounds mound 11 and mound 12 offshore southwest costa rica contain abundant authigenic carbonate concretions dominated by high mg calcite 14 20 mol mgco3 pore fluid geochemical profiles sulfate sulfide methane alkalinity ca and mg indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane aom at variable depths the current location of the authigenic carbonate concretions is however not related to the present location of the aom zone suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18ocarbonate values ranging between 34 0 and 37 7 vienna standard mean ocean water vsmow and d13ccarbonate values from 52 2 to 14 2 vienna pee dee belemnite vpdb assuming that no temperature changes occurred during mineral formation the authigenic carbonate concretions have been formed at in situ temperature of 4 5 c the d18ocarbonate values suggest mineral formation from seawater derived pore fluid d18oporefluid 0 vsmow for mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid d18oporefluid 5 in mound 11 a positive correlation between d13ccarbonate and d18ocarbonate is observed indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds the carbon of these sources are 1 marine bicarbonate d13cporefluid 0 and 2 bicarbonate which formed during the aom d13cporefluid 70 furthermore the d18oporefluid composition with values up to 4 7 vienna standard mean ocean water vsmow is interpreted to be affected by the presence of emanating freshened and boronenriched fluids earlier it has been shown that the origin of 18o enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature d13cch4 38 a combination of present geochemical data with geophysical observations indicates that mounds 11 and 12 represent a single fluid system interconnected by deep seated fault s

magmatic helium in subduction zone natural gases abstract he isotope ratios provide important constraints on the input of magmatic gases into crustal systems natural gas associated with active continental margins from taiwan r o c new zealand the philippines indonesia thailand and alaska u s a contain a magmatic he component based on measured 3 he 4 he ratios r ranging from 0 5 to 3 8 times the atmospheric value ra the methane helium results can be interpreted as the result of a mixture between a low 3 he 4 he 0 01r a high ch 4 3 he crustal end member and a high 3 he 4 he 0 6 3 9r a low ch 4 3 he 6 magmatic component the presence of mantle he does not require significant amounts of mantle methane a better fit to the data is obtained using a zero methane magmatic end member the worldwide association of 3he and magmatic activity suggests that volcanism plays a role in methane generation by supplying the heat needed to convert organic matter to methane thermogenically only gases from the accretionary prism of barbados west indies appear totally devoid of magmatic he 3 he 4 he ratio 0 02ra the he is completely of crustal origin there is no evidence that faults within the accretionary prism of this fore arc act as conduits for the migration of mantle he many of the gases studied contain significant quantities of co2 which appear to result from two processes 3 he 4 he high co 2 3 he gases 0 01ra 1011 possibly result from decarbonation reactions in the crust while the high 3 he 4 he low co 2 3 he gases 2 4ra

fluid flow methane fluxes carbonate precipitation and biogeochemical turnover in gas hydrate bearing sediments at hydrate ridge cascadia margin numerical modeling and mass balances a numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate bearing surface sediments from a cold vent site situated at hydrate ridge an accretionary structure located in the cascadia margin subduction zone steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below the model results underline the dominance of advective fluid flow that forces a large inflow of methane from below 869 mumol cm 2 a 1 inducing high oxidation rates in the surface layers anaerobic methane oxidation is the major process proceeding at a depth integrated rate of 870 mumol cm 2 a 1 a significant fraction 14 of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite the total rate of carbonate precipitation 120 mumol cm 2 a 1 allows for the build up of a massive carbonate layer with a thickness of i m over a period of 20 000 years aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane charge fluids is high enough greater than or equal to10 cm a 1 to maintain super saturation with respect to this highly soluble carbonate phase it precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments suggesting microbial catalysis the investigated station is characterized by high carbon and oxygen turnover rates approximate to1000 mumol cm 2 a 1 that are well beyond the rates observed at other continental slope sites not affected by fluid venting this underlines the strong impact of fluid venting on the benthic system even though the flow velocity of 10 cm a 1 derived by the model is relative low compared to fluid flow rates found at other cold vent sites non steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow moreover they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities 80 cm a 1 only at flow rates exceeding approximately 100 cm a 1 does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 mumol ch4 cm 2 a 1 into the overlying bottom water

effect of subducting sea floor roughness on fore arc kinematics pacific coast costa rica fault kinematics and uplift in the costa rican fore arc of the middle america convergent margin are controlled to a large extent by roughness on the subducting cocos plate along the northwest flank of the incoming cocos ridge seafloor is characterized by short wavelength roughness related to northeast trending seamount chains onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore arc thrust belt and separate blocks with contrasting uplift rates trunk segments of pacific slope fluvial systems typically follow these margin perpendicular faults regionally developed marine and fluvial terraces are correlated between drainages and across faults along the costa rican pacific coast terrace separations across block bounding faults reveal a pattern of fore arc uplift that coincides roughly with the distribution of incoming seamounts magnitude and distribution of quaternary uplift along the costa rican pacific coast suggests that despite a thin incoming sediment pile the inner fore arc shows an accumulation of mass a characteristic that may be due to underplating of seamounts beneath the fore arc high

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

dynamic fluid flow and chemical fluxes associated with a seafloor gas hydrate deposit on the northern gulf of mexico slope abstract gas hydrates outcrop on the seafloor at the bush hill hydrocarbon seep site in the northern gulf of mexico four newly designed fluid flux meters chemical samplers called the mosquito were deployed for 430 days at bush hill to determine how dynamic subsurface fluid flow influences gas hydrate stability and to quantify the associated methane fluxes into the ocean three of the flux meters were deployed adjacent to an outcropping gas hydrate mound while the fourth monitored background conditions the flux meter measurements reveal that the subsurface hydrology in the vicinity of the mound is complex and variable with frequent changes from downward to upward flow ranging from 161 to 273 cm yr and with temporal variations in the horizontal component of flow the continuous record of fluid chemistry indicates that gas hydrate actively formed in the sediments we propose that long periods of downward flow of seawater adjacent to gas vents up to 4 months are driven by local sub pressure resulting from gas ebullition through faults and fractures due to overpressure at depth high frequency variations in flow rates days to weeks are likely due to temporal changes in sediment permeability and the 3 d fluid flow field as a result of active gas hydrate and authigenic carbonate precipitation as well as the presence of free gas gas hydrate formation occurred as a result of long term emanation of ch 4 at focused gas vents followed by a more diffuse intergranular methane flux the estimated ch 4 flux to the water column from focused gas vents across the bush hill seep is 5 10 6 mol yr this significant flux suggests that bush hill and similar hydrocarbon seeps in the northwestern gulf of mexico may be important natural sources of methane to the ocean and possibly the atmosphere

underthrust sediments fluid migration paths and mud volcanoes associated with the accretionary wedge off costa rica middle america trench styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments we estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the middle america trench near costa rica reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion within 4 km of the deformation front where overburden is 800 m more than half of the total water in the pore spaces may have been expelled the rate of fluid transfer from the underthrust section is highest in the zone 0 4 km arcward of the deformation front the clarity of the reflection associated with the decollement is exceptional and shows a phase reversal relative to the seafloor this high amplitude phase reversed reflection ends about 4 km arcward of the deformation front suggesting a rapid reduction in the density and or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m 1000 m a reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge slope cover boundary and may mark a fluid conduit locally high amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section near the trench where fluid expulsion rates are highest water may exit to the seafloor seaward of the downslope pinch out of the slope section

cementation of sand soil by microbially induced calcite precipitation at various degrees of saturation a newly emerging microbiological soil stabilization method known as microbially induced calcite precipitation micp has been tested for geotechnical engineering applications micp is a promising technique that utilizes the metabolic pathways of bacteria to form calcite precipitation throughout the soil matrix leading to an increase in soil strength and stiffness this paper investigates the geotechnical properties of sand bio cemented under different degrees of saturation a series of laboratory experiments was conducted including sieve analysis permeability unconfined compressive strength consolidated undrained triaxial and durability tests the results indicate that higher soil strength can be obtained at similar caco3 content when the treatment is performed under a low degree of saturation the experimental results are further explained with a mathematical model which shows that the crystallization efficiency i e actual volume of crystals forming at the contact point where they contribute the most to strength can be calculated from the degree of saturation and grain size fine sand samples exhibited higher cohesion but lower friction angle than coarse sand samples with similar caco3 content the results also confirm the potential of micp as a viable alternative technique for soil improvement in many geotechnical engineering applications including liquefiable sand deposits slope stabilization and subgrade reinforcement the freeze thaw and acid rain resistance of micp treated sand has also been tested

estimates of methane output from mud extrusions at the erosive convergent margin off costa rica four mud extrusions were investigated along the erosive subduction zone off costa rica active fluid seepage from these structures is indicated by chemosynthetic communities authigenic carbonates and methane plumes in the water column we estimate the methane output from the individual mud extrusions using two independent approaches the first is based on the amount of ch4 that becomes anaerobically oxidized in the sediment beneath areas covered by chemosynthetic communities which ranges from 104 to 105 mol yr 1 the remaining portion of ch4 which is released into the ocean has been estimated to be 102 104 mol yr 1 per mud extrusion the second approach estimates the amount of ch4 discharging into the water column based on measurements of the near bottom methane distribution and current velocities this approach yields estimates between 104 105 mol yr 1 the discrepancy of the amount of ch4 emitted into the bottom water derived from the two approaches hints to methane seepage that cannot be accounted for by faunal growth e g focused fluid emission through channels in sediments and fractures in carbonates extrapolated over the 48 mud extrusions discovered off costa rica we estimate a ch4 output of 20 106 mol yr 1 from mud extrusions along this 350 km long section of the continental margin these estimates of methane emissions at an erosional continental margin are considerably lower than those reported from mud extrusion at accretionary and passive margins almost half of the continental margins are described as non accretionary assuming that the moderate emission of methane at the mud extrusions off costa rica are typical for this kind of setting then global estimates of methane emissions from submarine mud extrusions which are based on data of mud extrusions located at accretionary and passive continental margins appear to be significantly too high

multifrequency geoacoustic imaging of fluid escape structures offshore costa rica implications for the quantification of seep processes quantification of fluid fluxes from cold seeps depends on accurate estimates of the spatial validity of flux measurements these estimates are strongly influenced by the choice of geoacoustic mapping tools multibeam bathymetry side scan sonar and chirp subbottom profiler data of several mound shaped cold seeps offshore central costa rica show great variety in morphology and structure although the features are only a few kilometers apart mound 11 a 35 m high and 1000 m in diameter structure situated in the se of the study area has an irregular morphology but a smooth surface on side scan sonar data while mound 12 30 m high 600 m across is a cone of more regular outline but with a rough surface and mound grillo 5 m high 500 m across shows the same rough surface as mound 12 but without relief video observations and sediment cores indicate that the structures are formed by the precipitation of authigenic carbonates and indications for extensive mud extrusion are absent except for one possible mudflow at mound 11 different sonar frequencies result in variable estimates of the extent of these mounds with low frequencies suggesting much wider cold seeps consequently overestimating fluid fluxes the absence of mud volcanism compared to accretionary prisms where mud volcanism occurs is related to different tectonic styles strong sediment overpressure and thrust faulting in typical accretionary prisms can generate mud volcanism while subduction erosion and normal faulting extension of the overriding plate at the costa rican margin result in fluid venting driven by only slight fluid overpressures

surficial evidence of fluid expulsion from the costa rica accretionary prism the nature and distribution of authigenic carbonates chemosynthetic bacterial mats and unique macrobenthic chemosynthetic communities of bivalves and tube worms are important for evaluating and reconstructing present and past fluid venting of accretionary complexes this paper describes the authigenic carbonates chemosynthetic fauna and fluid venting observed at the four tectonic regions of the costa rica accretionary wedge in february 1994 during an alvin diving program of 20 submersible dives we found no surficial evidence of highly focused fluid venting at the toe of the prism outermost 3 km as implied by the absence of authigenic carbonates and chemosynthetic fauna the absence of vent communities on the lower 3 km of the prism and the relatively elevated heat flow with respect to the adjacent incoming cocos plate langseth and silver this issue suggests diffuse rather than focused flow through the toe of the prism twelve active and relict vent sites marked by small clusters of live vesicomyid clams are localized at the bases and tops of out of sequence thrusts implicating fracture permeability as the fluid conduit in the lower slope region but upslope from the toe vast authigenic carbonates and seven active and relict vent sites marked by large dense clusters of chemosynthetic organisms predominate the largest mud diapir in the mid slope region fluid expulsion appears to be more restricted on the upper slope with only 2 small but dense vents marked by chemosynthetic fauna observed at one wall of one canyon

noble gas anomalies related to high intensity methane gas seeps in the black sea dissolved noble gases and tritium were analyzed at a series of high intensity methane gas seeps in the black sea to study the transport and gas exchange induced by bubble streams in the water column these processes affect marine methane emissions to the atmosphere and are therefore relevant to climate warming the seep areas investigated are located in the dnepr paleo delta west of crimea and in the sorokin trough mud volcano area south east of crimea noble gas concentration profiles at active seep sites revealed prominent anomalies compared to reference profiles that are unaffected by outgassing supersaturations of the light noble gases helium and neon observed relatively close to the sea floor are interpreted as effects of gas exchange between the water and the rising bubbles depletions of the heavy noble gases argon krypton and xenon that were detected above an active bubble releasing mud volcano appear to be related to the injection of fluids depleted in noble gases that undergo vertical transport in the water column due to small density differences in both cases the noble gas anomalies clearly document seep specific processes which are difficult to detect by other methods helium is generally enriched in the deep water of the black sea due to terrigenic input although exceptionally high helium concentrations observed in one seep area indicate a locally elevated helium flux most of the seeps studied seem to be negligible sources of terrigenic helium noble gas analyses of sediment pore waters from the vicinity of a mud volcano showed large vertical gradients in helium concentrations the helium isotope signature of the pore waters points to a crustal origin for helium whereas the deep water of the black sea also contains a small mantle type component

fluid sources and pathways of the costa rica erosional convergent margin 1 the margins community has only relatively recently begun to examine the tectonics and associated hydrologic systems of erosive convergent margins which are substantially different as compared with accretionary margins in this respect the type example erosive margin is the costa rica system which has been the subject of numerous recent large scale investigations here pore fluids expelled at the wedge toe and at midslope mounds and mud volcanoes have been interpreted to have a common deep source of dehydrating clays analogous to that at accretionary margins however we report unusually high b li molar ratios in pore fluids from a recent mudflow on mound 11 offshore costa rica which together with unusually low b li ratios previously reported at the wedge toe reveal that alternative fluid sources and or processes must be operating at the costa rica margin as serpentine formation is the only subduction zone process that significantly fractionates b and li we propose that the difference in fluid chemical composition is the result of erosion of upper plate serpentinites ongoing serpentinization and serpentine mineral phase transitions in the subduction channel these processes provide both a source of fluids and fluid pathways that lead to the unique geochemical signature observed at this erosional margin this conclusion is compatible with and supported by the current view of the tectonics geology and hydrogeology of the costa rica margin and the similarity of the pore fluid to that of two other convergent margins both with known fluid serpentinite interactions

3he 4he ratios in well gases in the kinki district sw japan surface appearance of slab derived fluids in a non volcanic area in kii peninsula abstract a total of 27 well gas samples collected from sw japan were subjected to precise analysis of he ne and ar isotopic compositions the sampling area covers localities where mantle derived helium emanates in the fore arc of the kii peninsula and is known as the kinki spot e g sano and wakita j geophys res 90 1985 8728 8741 the kinki spot apparently is located within the fore arc with no obvious magmatic activity which contrasts to the occurrence of similarly high 3 he 4 he ratios along the volcanic front of ne japan our high density sampling revealed more clearly that the high 3 he 4 he ratio wells are distributed in the kii peninsula above the area where the young and hot philippine sea plate is subducting with a relatively steeper dip than in the adjacent area this situation favors the near trench side of the subducting slab entering eclogite facies conditions involving slab dehydration without inducing melting of the mantle wedge the aqueous fluids derived from the slab acquire mantle he during their passage through the mantle wedge thus the observed association of mantle he and the fore arc dehydration of the philippine sea plate indicates that fluid derived from the subducting slab is contributing to the occurrence of mantle he in well gases of the kii peninsula the high 3 he 4 he region of kii peninsula also coincides with the occurrence of long period tremors these are caused by the movement of fluids derived from the subducting slab localized stresses cause significant uplift in the area interpreted as indicating that intra crustal fractures provide pathways through the crust for the fluids derived from depth thus magma is not necessarily required as the carrier of mantle derived helium to the surface of the fore arc region of kii peninsula

deformation structures and implications for fluid flow at the costa rica convergent margin odp sites 1040 and 1043 leg 170 abstract during odp leg 170 five sites were drilled and sampled off the costa rica pacific margin two of them site 1040 and 1043 yielded material from a wedge of deformed sediments the main decollement zone and the underthrusting sedimentary sequence of the subducting cocos plate detailed mesoscopic and microscopic analyses of the deformation features characteristic of each domain help to define four different structural hydrologic regimes unexpectedly the wedge of deformed hemipelagic sediments does not represent an accretionary wedge because little or no transfer of material from the subducting plate has occurred the deformed sedimentary wedge records periods of tectonic bulk strain in which fluid pressure intermittently rises and induces fracturing alternating with periods of gravitational bulk strain the last stage of this alternating strain regime is represented by the growth of thick sub horizontal shear zones the sediments caught up in the decollement zone show distinctly different deformation features reflecting different mechanical behaviour and a different response to fluid flow but again transient pulses of increased pore pressure are interpreted as the driving mechanism of dewatering the underthrust pelagic and hemipelagic section is also characterised by dewatering features but only in the uppermost part the basal sediments of this section in contact with underlying gabbro intrusions have a completely different structural history and hydrologic regime structures here are interpreted as related to near ridge processes and the hydrologic system is not linked to the upper sediment dewatering regime but rather seems to have been fed by a seawater source

origin of carbon in fumarolic gas from island arcs abstract we have measured 3 he 4 he ratios 13c values and co 2 3 he ratios of fumaroles and hot spring gases in three volcanic areas in japan the 3 he 4 he ratios 13c values and co 2 3 he ratios vary from 02 05 ratm 9 7 to 2 7 and from 7 74 109 to 1 18 1011 respectively based on these data and those in the literature we present the first quantitative estimates of the upper mantle sediment and limestone including slab components in co2 of volcanic and hydrothermal fluids from subduction zones taking compiled c isotopic data of limestone and organic sediment assuming that the co 2 3 he ratio and 13c value of mid ocean ridge basalt morb are representative of the upper mantle component and that later isotopic and elemental fractionations are minimal it is possible to deconvole each fraction of c on the basis of simple mixing equations up to 20 of the carbon in high temperature volcanic gases in subduction zones is derived from a morb type source while the major fraction of the gases is attributable to co2 produced by decarbonation of subducted marine limestone and slab carbonate

along strike variability in the seismogenic zone below nicoya peninsula costa rica 1 at the subduction zone in northwestern costa rica the seismogenic zone lies directly beneath the nicoya peninsula allowing for near source seismic studies of earthquake activity we located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the nicoya peninsula using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth 10 km in the south to 20 km in the north that occurs where the subducting oceanic crust changes from warmer cocos nazca spreading center cns origin to colder east pacific rise epr origin we argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath nicoya costa rica subducting colder oceanic crust deepens the seismogenic updip limit

helium systematics of cold seep fluids at monterey bay california usa temporal variations and mantle contributions 1 we report helium isotope ratios 3he 4he as well as helium and neon abundance results for submarine cold seep fluids from extrovert cliff in monterey bay california samples were collected in copper tubing attached to submarine flux meters operating in continuous pumping mode following instrumentation recovery the tubing was sectioned to produce for the first time a high resolution time series of dissolved he and ne variations over a time span of several days noble gas concentrations are variable and appear affected by interaction with a hydrocarbon phase within the aquifer however it is still possible to resolve the he signal into components associated with air equilibration excess air entrainment and terrigenic fluxes both crustal and mantle derived the mantle he contribution reaches 25 30 in some samples up to 2 3 ra where ra air 3he 4he our quasi continuous he ne record shows remarkable fluctuations over time scales of only a few hours and reflects the combined effects of gas stripping by hydrocarbons and an episodic input of mantle derived fluids

sediment accretion subduction and dewatering at the base of the trench slope off costa rica a seismic reflection view of the decollement sea beam bathymetric and water gun seismic reflection data off the west coast of costa rica image the base of the trench slope its underlying structure and the behavior of a decollement surface in the presence of relief on the subducting plate a low angle landward dipping reflection marks the master decollement above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope a small horst block at least 9 km long 1 5 km wide and 300 m high is being obliquely subducted beneath the landward trench slope on five seismic lines the decollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level the undulating geometry of the decollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front the dewatering may favor development of bedding parallel zones of low shear strength within the subducting sediments and the master decollement may seek one of these zones

pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical atlantic indicate that the oxygen isotopic composition 18 o of seawater at this site during the last glacial maximum was 0 8 0 1 per mil higher than it is today combined with the 18 o change in benthic foraminifera from this region the elevated ratio indicates that the temperature of deep water in the tropical atlantic ocean was 4 c colder during the last glacial maximum extrapolation from this site to a global average suggests that the ice volume contribution to the change in 18 o of foraminifera is 1 0 per mil which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from barbados corals and terrestrial climate proxies

aqueous fluids derived from a subducting slab observed high 3he emanation and conductive anomaly in a non volcanic region kii peninsula southwest japan abstract it has long been recognized that the kii peninsula in the southwest japan arc is peculiar in a non volcanic region indicated by the presence of high temperature hot springs high terrestrial heat flow and high 3 he content in hot spring gases geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the kii peninsula most of the measured 3 he 4 he ratios are similar or higher than air indicating wide spread incorporation of mantle derived helium into meteoric water a region with rather high 3 he 4 he ratios 4 r a on the west side of the omine mountains coincides with the occurrence of high temperature hot springs a deep crustal resistivity structure across the omine mountains was imaged by wide band magnetotelluric soundings a 2 d inversion with n s strike using both tm and te modes reveals two conductors one in the upper 3 7 km depth and the other in lower crust 25 35 km depth to the west of the omine mountains the distribution of microearthquakes and low frequency tremors and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the philippine sea plate the upper crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust which are presumably derived from the subducting slab considering the occurrence of seismic events in the subducting slab beneath the southern kii peninsula the aqueous fluids generated by dehydration of the slab mantle could plausibly include morb type helium derived from the residual lithospheric mantle therefore the high temperature hot springs and high 3 he emanations in hot spring gases and other geotectonic events in the southern kii peninsula may be due to heat flux and mantle derived helium discharged from aqueous fluid in the upper crust

flow of mantle fluids through the ductile lower crust helium isotope trends heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust minimum 3he 4he ratios in surface fluids from the northern basin and range province western north america increase systematically from low crustal values in the east to high mantle values in the west a regional trend that correlates with the rates of active crustal deformation the highest ratios occur where the extension and shear strain rates are greatest the correspondence of helium isotope ratios and active transtensional deformation indicates a deformation enhanced permeability and that mantle fluids can penetrate the ductile lithosphere even in regions where there is no substantial magmatism superimposed on the regional trend are local high 3he 4he anomalies indicating hidden magmatic activity and or deep fluid production with locally enhanced permeability identifying zones with high resource potential particularly for geothermal energy development

release of gas bubbles from lake sediment traced by noble gas isotopes in the sediment pore water abstract the release of gas bubbles from lacustrine or oceanic sediments into the overlying water ebullition is a major mechanism for the discharge of biogenic or geogenic gases into the water body ebullition of methane or carbon dioxide for instance contributes considerably to the release of these potent greenhouse gases through the sediment water interface depending on the rate of ebullition the pore water will show a depletion in dissolved atmospheric noble gases because the poorly soluble noble gases escape from the pore water into the gas bubbles in this study the depletion of dissolved noble gases in sediment pore water was analyzed for the first time to study bubble formation and ebullition in sediments the noble gases in the pore water of the sediments of soppensee switzerland show a distinct depletion due to ebullition of biologically produced methane this depletion is lowest in the deep sediment and increases towards the sediment surface the noble gas isotope ratios in the pore water indicate that vertical diffusion barely affects the observed noble gas profiles the isotope ratios further show that the methane bubbles remain long enough in the sediment to attain noble gas solubility equilibrium before escaping into the overlying water the volume of gas released from the sediment by ebullition can therefore be reconstructed from the extent of the noble gas depletion in the pore water using a simple gas equilibration model the noble gas profiles in the sediment indicate that ebullition increased in soppensee during the holocene and that ebullition contributed strongly to the release of methane from the sediment our case study thus illustrates that noble gases are sensitive proxies for the release of gas from lacustrine and marine sediments or similar aquatic environments

a new hydrothermal scenario for the 2006 lusi eruption indonesia insights from gas geochemistry abstract the 29th of may 2006 gas and mud eruptions suddenly appeared along the watukosek fault in the north east of java indonesia within a few weeks several villages were submerged by boiling mud the most prominent eruption site was named lusi to date november 2011 lusi is still active and a 7 km2 area is covered by the burst mud breccia the mechanisms responsible for this devastating eruption remain elusive while there is consensus about the origin of the erupted mud the source of water is uncertain the origin of the gas is unknown and the trigger of the eruption is still debated in order to shed light on these unknowns we acquired a wide set of data of molecular and isotopic composition of gas sampled in several lusi vents in the surrounding mud volcanoes in the closest natural gas field wunut and in the hydrothermal vents at the neighbouring volcanic complex in the period 2006 2011 the boiling fluids erupted in the crater zone are apparently co2 dominated while colder ch4 dominated and c2 c3 bearing fluids are identified at several sites around the crater zone gas genetic diagrams maturity plots and gas generation modelling suggest that the hydrocarbons are thermogenic 13c1 up to 35 13c2 up to 20 deriving from marine kerogen with maturity of at least 1 5 ro for instance in the 4400 m deep ngimbang source rocks co2 released from the crater and surrounding seeps is also thermogenic 13c from 15 to 24 related to kerogen decarboxylation or thermal ch4 oxidation in deep rocks although three vents just outside the crater showed an apparent inorganic signature 7 5 4000 m magmatic intrusions and hydrothermal fluids responsible for the enhanced heat that altered source rocks and or gas reservoirs the neighbouring magmatic arjuno complex and its fluid pressure system combined with high seismic activity could have played a key role in the lusi genesis and evolution within this new model framework lusi is better understood as a sediment hosted hydrothermal system rather than a mud volcano

millennial scale variability of deep water temperature and δ18odw indicating deep water source variations in the northeast atlantic 0 34 cal ka bp 1 paired measurements of mg ca and 18occ calcite 18o in benthic foraminifera from a deep sea core recovered on the iberian margin md99 2334k 37 48 n 10 10 w 3 146 m have been performed in parallel with planktonic 18occ analyses and counts of ice rafted debris ird the synchrony of temperature changes recorded in the greenland ice cores and in north atlantic planktonic 18occ allows the proxy records from md99 2334k to be placed confidently on the gisp2 time scale this correlation is further corroborated by ams 14c dates benthic mg ca measurements in md99 2334k permit the reconstruction of past deep water temperature tdw changes since 34 cal ka bp calendar kiloyears before present using these tdw estimates and parallel benthic 18occ measurements a record of deep water 18o 18odw has been calculated results indicate greatly reduced tdw in the deep northeast atlantic during the last glaciation until 15 cal ka bp when tdw warmed abruptly to near modern values in parallel with the onset of the bolling allerod interstadial subsequently tdw reverted to cold glacial values between 13 4 and 11 4 cal ka bp in parallel with the younger dryas cold reversal and the h0 ice rafting event similar millennial scale tdw changes also occurred during the last glaciation indeed throughout the last 34 cal ka millennial 18odw and tdw changes have remained well coupled and are linked with ird pulses coincident with heinrich events 3 2 1 and the younger dryas when transitions to lower tdw and 18odw conditions occurred in general millennial tdw and 18odw variations recorded in md99 2334k describe an alternation between colder low 18odw and warmer high 18odw conditions which suggests the changing local dominance of northern sourced north atlantic deep water nadw versus southern sourced antarctic bottom water aabw the observed similarity of the tdw and gisp2 18oice records would therefore suggest a common component of variability resulting from the coupling of nadw formation and greenland climate a link between greenland stadials and the incursion of cold low 18odw aabw in the deep northeast atlantic is thus implied which contributes to the relationship between greenland climate and the millennial benthic 18occ signal since 34 cal ka bp

a new syringe pump apparatus for the retrieval and temporal analysis of helium in groundwaters and geothermal fluids 1 we present details of a newly designed syringe pump apparatus for the retrieval and temporal analysis of helium spartah the device is composed of a commercially available syringe pump connected to coils of cu tubing which interface the syringe and the groundwater or geothermal wellhead through test deployments at geothermal wells in iceland and california we show that well fluids are drawn smoothly accurately and continuously into the cu tubing and can be time stamped through user determined operating parameters in the laboratory the tubing is sectioned to reveal helium he characteristics of the fluids at times and for durations of interest the device is capable of prolonged deployments up to 6 months or more with minimal maintenance it can be used to produce detailed time series records of he or any other geochemical parameter in groundwaters and geothermal fluids spartah has application in monitoring projects assessing the relationship between external transient events e g earthquakes and geochemical signals in aqueous fluids

measurements of transience and downward fluid flow near episodic methane gas vents hydrate ridge cascadia aqueous flux measurements within cold seep regions on northern hydrate ridge cascadia indicate a high degree of variability with extended periods of downflow and reversals of flow direction over periods of weeks to months local episodic venting of free methane gas was also observed the instruments recorded similar changes in hydrologic flow patterns both on and off clam fields the magnitude of the flow rates decreasing away from the clam field the coincidence of episodic gas venting with nearby highly variable aqueous fluid flow suggests that they may be coupled we propose that these observations are consistent with the action of a gas driven pump that operates somewhat like a geyser the hypothesis of gas driven pumping of seawater through northern hydrate ridge has important ramifications for the mass fluxes through this region

enhancing the axial compression response of pervious concrete ground improvement piles using biogrouting abstractthis paper presents an innovative grouted ground improvement pile alternative biogrouted pervious concrete pile biogrouting is a potential ground improvement technique that utilizes soil bacteria to induce calcium carbonate caco3 precipitation to cement soil particles the most commonly investigated biogrouting method is microbial induced carbonate precipitation micp previous large scale applications of micp have encountered practical difficulties including bioclogging which results in a limited zone of cemented soil around injection points and heterogeneous distribution of caco3 the research presented in this paper focuses on evaluating the feasibility of micp biogrouting of a limited soil zone surrounding permeable piles to improve their responses when subjected to axial compression loading to investigate this two instrumented pervious concrete piles with diameters of 76 mm and lengths of 1 07 m with and without micp biogrouting were subjected to compression loading at the soil struc

carbon release from submarine seeps at the costa rica fore arc implications for the volatile cycle at the central america convergent margin 1 we report total dissolved inorganic carbon dic abundances and isotope ratios as well as helium isotope ratios 3he 4he of cold seep fluids sampled at the costa rica fore arc in order to evaluate the extent of carbon loss from the submarine segment of the central america convergent margin seep fluids were collected over a 12 month period at mound 11 mound 12 and jaco scar using copper tubing attached to submarine flux meters operating in continuous pumping mode the fluids show minimum 3he 4he ratios of 1 3 ra where ra is air 3he 4he consistent with a small but discernable contribution of mantle derived helium at mound 11 13c co2 values between 23 9 and 11 6 indicate that dic is predominantly derived from deep methanogenesis and is carried to the surface by fluids derived from sediments of the subducting slab in contrast at mound 12 most of the ascending dissolved methane is oxidized due to lower flow rates giving extremely low 13c co2 values ranging from 68 2 to 60 3 we estimate that the carbon flux co2 plus methane through submarine fluid venting at the outer fore arc is 8 0 105 g c km 1 yr 1 which is virtually negligible compared to the total sedimentary carbon input to the margin and the output at the volcanic front unless there is a significant but hitherto unidentified carbon flux at the inner fore arc the implication is that most of the carbon being subducted in costa rica must be transferred to the deeper mantle i e beyond the depth of arc magma generation

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

10be imaging of sediment accretion and subduction along the northeast japan and costa rica convergent margins sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity the seismogenic zone and the subduction recycling of volatiles and continentally derived materials to the earth s mantle identifying and quantifying sediment accretion essential for a complete mass balance across the margin can be difficult seismic images do not define the processes by which a prism was built and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy this contribution reports the first use of cosmogenic 10be depth profiles to define the origin and structural evolution of forearc sedimentary prisms biostratigraphy and 10be model ages generally are in good agreement for sediments drilled at deep sea drilling project site 434 in the japan forearc and support an origin by imbricate thrusting for the upper section forearc sediments from ocean drilling program site 1040 in costa rica lack good fossil or paleomagnetic age control above the decollement low and homogeneous 10be concentrations show that the prism sediments are older than 3 4 ma and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments low 10be in costa rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion

the internal structure of oceanic plateaus inferences from obducted cretaceous terranes in western colombia and the caribbean although the structure of mantle plume derived oceanic plateaus has recently been assessed using remote geophysical techniques combined with petrological modelling it is nevertheless desirable to test whether these theoretical rock types dunites gabbros and basalts actually exist and to establish their geochemical nature oceanic plateaus may have initially formed above or near sea level during a short vigorous pulse and thereafter commonly subside to abyssal depths as the lithosphere cools thus making sampling of their deeper levels extremely difficult however the cretaceous age colombian caribbean oceanic plateau was partially accreted against the south american continent so making the imbricated segments available for study during the process of plateau accretion and imbrication it is predominantly the basaltic layers which are obducted but parts of the sequence down to layered and banded gabbros with associated pyroxenites and dunites sometimes foliated can be exposed where the imbricate thrusting brings up deeper levels most of the upper crustal sequence in western colombia is composed of basaltic pillowed and massive flows and sills that are chemically uniform and undepleted relative to normal mid ocean ridge basalts komatiites and more abundant picrites are found at intervals and appear to occur near the base of the sequence in these zones both depleted and moderately enriched basalt and komatiite compositions occur and may result from dynamic partial melting and mixing processes associated with the high temperature part of the plume it is possible to integrate these compositional characteristics into a general model for oceanic plateau structure where the rate of magma supply is in excess of that which can be accommodated by normal spreading processes thus leading to extrusion of flows and the emplacement of sills and high level magma chambers but relatively few dykes the dense and chemically heterogeneous ultramafic magmas intrude the base of the pile and undergo fractionation to form ultramafic cumulates whereas the well mixed basaltic magmas are erupted to form the homogeneous plateau basalts based on these observations a possible structure of the caribbean colombian plateau is proposed which is compatible with geophysical models for other less well exposed oceanic plateaus

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

biocalcification of sand through ureolysis biological processes may provide great and previously unexplored opportunities for cost effective in situ improvement of the engineering properties of soil a laboratory study was conducted to evaluate the changes in geomechanical properties of sand attributable to the formation of calcium precipitates induced through ureolysis catalyzed by sporosarcina pasteurii s pasteurii specifically direct shear and california bearing ratio cbr tests were conducted on sand specimens subjected to treatment by growing resting and dead s pasteurii cells in completely stirred tank reactors and completely mixed biofilm reactors respectively scanning electron microscopy analyses were also conducted to evaluate microbially induced precipitation the results of the study show that the bacterial cells effectively improved the geomechanical properties of the sand growing cells improved the sand properties owing to microbially induced precipitation and related pore volume changes whereas dead and resting cells ge

evidence for serpentinization of the forearc mantle wedge along the nicoya peninsula costa rica 1 characterizing the hydration state of the forearc mantle wedge yields valuable information on frictional stability at the downdip edge of subduction megathrusts simultaneous inversion of p and s wave arrival times collected as part of the costa rica seismogenic zone experiment yields 1d and 3d p and s wave velocity models vp and vs for the nicoya peninsula segment of the middle america trench nicoya peninsula 1d velocity models show similar velocity gradients to country wide 1d velocity models from 5 30 km depth but diverge at expected moho depths due to proximity to the subducting cocos plate 3d vp values range from 7 2 7 6 km sec in the forearc mantle wedge receiver functions computed at global seismic network station jts in northwestern costa rica confirm these vp values yield vp vs of 1 85 and place the continental moho at 36 4 km depth vp and vp vs are consistent with 15 25 serpentinization of the forearc mantle wedge

along strike variations in underthrust sediment dewatering on the nicoya margin costa rica related to the updip limit of seismicity 1 along the costa rican subduction zone offshore the nicoya peninsula an offset in the updip limit of seismicity coincides with a transition between subduction of warm crust generated at the cocos nazca spreading center and cool crust formed at the east pacific rise we evaluate whether the observed difference in thermal state of incoming crust would result in significant differences in sediment dehydration reaction progress along strike thought to be a control on the updip limit of seismicity we combine thermal models with models of dehydration reaction kinetics for opal and smectite to estimate the distribution of diagenetic fluid sources the modeled distribution of diagenetic fluid sources mimics the pattern of the updip limit of seismicity seismicity begins 15 20 km landward of most of the smectite to illite transition this suggests that the location of the updip limit of seismicity may be influenced by the dissipation of fluid overpressures landward of smectite to illite dehydration

mantle volatiles in spring gases in the basin and range province on the west of beijing china constraints from helium and carbon isotopes abstract the mantle degassing observed at the earth surface demonstrates both a provenance of fluids in the mantle and a pathway to the surface quantities of this process are discovered on the plate boundaries where there are plenty of active volcanoes and active faults releasing plenty of mantle volatiles however in intraplate tectonic settings without obvious mantle plume the work for mantle degassing observed in spring gasses seems comparatively limited we selected the basin and range province on the west of beijing an area in the inner part of north china craton to discuss the mantle degassing based on the helium and carbon isotopes of spring gasses and the previous works on seismic tomography and fault slip rate the spring gas helium and carbon co2 isotopes indicate the mixture of crustal and mantle materials the helium ratios reported as rc ra air corrected 3he 4he ratio ra 1 4 10 6 ra is the air ratio vary in the range of 0 33 2 08 the calculated mantle helium contributes 4 26 of helium in spring gasses and the remaining is generated in the crust by radiogenic decay of u th series with tiny air mixture co2 acquires analytical 13cv pdb values in the range from 20 3 to 10 2 affected by carbonate precipitation the unaffected values are calculated to be 8 5 5 1 by temperature dependent isotope fractionation indicating the mixture of mantle and crustal limestone materials the mantle volatiles are possibly generated in the upwelling asthenosphere in that the 3he 4he ratio corresponds well with the negative anomaly of p wave velocity at the depth of 70 km imaged by seismic tomography the 3he 4he ratio also correlates with time averaged fault slip rate suggesting higher slip rate renders more permeable mantle vent these consequences help to construct a conceptual model for intraplate mantle degassing that the mantle volatiles generate in the upwelling asthenosphere and uprise through faults and fractures whose permeabilities are controlled by slip rates

quantifying submarine fluid seep activity along the north anatolian fault zone in the sea of marmara abstract the sea of marmara presents the rare case where active seafloor venting sites are found on the surface trace of a major plate boundary fault the north anatolian fault zone the objective of the 2007 marnaut project was to quantify the level of activity of these venting sites and the source of the fluids emitted with the goal of understanding the processes involved and setting a baseline for long term studies of the relationship between seismic activity and fluid migration expulsion processes sites for flow meter and fluid sampler deployment and coring included basin bounding transtensional faults and strike slip faults cutting through the topographic highs significant fluid flow appears to be primarily an episodic phenomenon at all sites with background rates on the order of mm yr to cm yr except at or very near rare focused vents basin bounding faults expel primarily shallow sourced fluid with a strong influence of brackish pleistocene lake marmara water with the notable exception of the main active fault along the northern edge of the cinarcik basin expulsion sites where the main fault crosses topographic highs are more complex with evidence for deep sourced fluids including thermogenic gas one site on the western high displayed two mound structures that appear to be chemoherms atop a deep seated fluid conduit the fluids being expelled are brines with an exotic fluid chemistry along with thermogenic gas and oil our work shows that submerged continental transform plate boundaries can be hydrologically active and exhibit a diversity of sources and processes

temperature and carbonate ion effects on mg ca and sr ca ratios in benthic foraminifera aragonitic species hoeglundina elegans received 16 march 2005 revised 20 june 2005 accepted 3 november 2005 published 21 february 2006 1 core top samples from atlantic little bahama banks lbb and pacific hawaii and indonesia depth transects have been analyzed in order to assess the influence of bottom water temperature bwt and aragonite saturation levels on mg ca and sr ca ratios in the aragonitic benthic foraminifer hoeglundina elegans both the mg ca and sr ca ratios in h elegans tests show a general decrease with increasing water depth although at each site the decreasing trends are consistent with the in situ temperature profile mg ca and sr ca ratios in lbb are substantially higher than in indonesia and hawaii at comparable water depths with a greater difference observed with increasing water depth because we find no significant difference between results obtained on live and dead specimens we propose that these differences are due to primary effects on the metal uptake during test formation evaluation of the water column properties at each site suggests that in situ co3 ion concentrations play an important role in determining the h elegans mg ca and sr ca ratios the co3 ion effect is limited however only to aragonite saturation levels dco3 aragonite below 15 mmol kg 1 above this level temperature exerts a dominant effect accordingly we propose that mg ca and sr ca in h elegans tests can be used to reconstruct thermocline temperatures only in waters oversaturated with respect to the mineral aragonite using the following relationships mg ca 0 034 0 002 bwt 0 96 0 03 and sr ca 0 060 0 002 bwt 1 53 0 03 for dco3 aragonite 1 5mmol kg 1 the standard error associated with these equations is about 1 1 c reconstruction of deeper water temperatures is complicated because in undersaturated waters changes in mg ca and sr ca ratios reflect a combination of changes in co3 and bwt overall we find that sr ca rather than mg ca in h elegans may be a more accurate proxy for reconstructing paleotemperatures

biogrouting coarse materials using soil lift treatment strategy this paper investigates the feasibility of using a soil lift biogrouting treatment strategy to improve the mechanical properties of coarse sand with the view of applying it to stone columns sand piles and rammed earth columns type of applications a two phase percolation approach was adopted in this study that included percolating a bacterial suspension sporosarcina pasteurii in the first phase and a cementation solution in the second phase this process was repeated every two treatments the study reveals that an increase in the number of soil lifts negatively influenced the mechanical properties of the biocemented coarse sand however the minimum strength and stiffness achieved 2 8 mpa in this study was sufficient to mitigate slumping of a soil column that may occur during installation or excessive radial expansion furthermore it is shown that a single lift treatment can lead to a very high increase in strength and stiffness up to 8 9 mpa and 2 3 gpa respectively however calcite distribution

magnesium and strontium compositions of recent benthic foraminifera from the coral sea australia and prydz bay antarctica analyses of the mg ca and sr ca ratios of the modern benthic foraminifera cibicides wuellerstorfi epifaunal and uvigerina species infaunal from the coral sea and cibicides refulgens epifaunal and trifarina angulosa infaunal from prydz bay antarctica revealed relationships with temperature that have possible applications for reconstructions of bottom water paleotemperatures a positive relationship exists between the mg ca and sr ca ratios of cibicides wuellerstorfi and cibicides refulgens and ambient temperatures at least within the range of 2 and 6 c for the correlation between mg ca compositions and temperature the r2 values range from 0 78 c wuellerstorfi alone to 0 88 c wuellerstorfi and c refulgens together at present the mg ca temperature relationship must be regarded as tentative because of significant overlap of standard error values the relationship between the sr ca compositions of c wuellerstorfi and bottom water temperature yields an r2 value of 0 95 these results indicate that sr ca and possibly mg ca ratios of cibicides wuellerstorfi may provide useful information for the assessment of paleotemperature single species data are presently insufficient to assess the influence of ambient temperature on trace element compositions of uvigerina species trifarina angulosa may have mg ca compositions which are positively related to temperature but sr ca values seem unaffected by temperature this may be due to pore water influences on infaunal tests or to vital effects although more modern data are needed our present results suggest that sr ca ratios and possibly mg ca ratios of some benthic foraminifera have the potential to be useful paleothermometers at least within a temperature range of 2 to 6 c

complex flow patterns through hydrate ridge and their impact on seep biota our recent measurements of long term fluid flux rates and output fluid chemistry at seep sites on hydrate ridge cascadia suggest that the mechanisms driving fluid flow in gas hydrate bearing forearc settings may be significantly more complex than previously thought temporal changes in the polarity and magnitude of flow and in seep fluid chemistry were observed the nature of flow appears to generate a strong response in the chemosynthetic seep community ecology we document here 1 the frequency and range of temporal variability in flow rates 2 the relatively rapid changes in outflow composition 3 strong outflow of seawater like fluids 4 the association of calyptogena sp clams with oscillatory and inflow settings and 5 microbial mat communities associated with strong advection of altered fluids these results suggest that a reappraisal may be required of the nature of the hydrologic system and its effect on related processes such as benthic seep community ecology and biogeochemical cycles

geophysical evidence for dewatering and deformation processes in the odp leg 170 area offshore costa rica abstract we use a combination of borehole data from ocean drilling program odp leg 170 and multichannel seismic reflection mcs data to quantify thickness changes in underthrust sediments away from the boreholes sediments thrust beneath the upper plate at convergent margins may be more rapidly loaded than in any other environment depending on the porosity and permeability of the available fluid pathways these sediments can compact and dewater very rapidly as observed in this area offshore the nicoya peninsula costa rica rapid thinning and dewatering was previously interpreted in this area from mcs data but the lack of velocity data in this deep water environment caused ambiguity in the estimates of thickness change we employ a non linear inversion technique using detailed density data primarily logs and some laboratory measurements and coincident mcs data to create 1d synthetic seismograms and detailed velocity functions at three odp drill sites because only a small part of one hole was logged with a sonic tool and the laboratory measurements significantly underestimate in situ velocities these results provide the most accurate estimate of the velocity profiles we used these velocity functions to depth migrate seven mcs lines in the vicinity of the trench and lower slope spanning a distance of 9 km along strike analysis of the depth migrated images shows that there is significant variation along strike in how the underthrust section compacts which appears to be related to the distribution of normal faults on the cocos plate we interpret that preferentially rapid dewatering in the upper part of the underthrust section may lead to deformation below the original decollement and detachments at deeper stratigraphic levels

compaction and dewatering processes of the oceanic sediments in the costa rica and barbados subduction zones estimates from in situ physical property measurements abstract during ocean drilling program legs 170 and 171a logging while drilling lwd tools were deployed on the costa rica and barbados subduction margins high quality density resistivity and natural ray logs were acquired across the decollement zones on both margins based on a new method of layer by layer correlation of the logs changes in thickness and volume between incoming and subducted or accreted sediments are determined with 15 m resolution and 1 accuracy the change in sediment thickness and volume generally decreases with depth however this change strongly depends on the lithology siliceous layers such as diatomaceous and radiolarian clay tend to be fluid bearing and the stratigraphic position of such zones is a critical factor in the fate of the subducted sediment section on the costa rica margin the sediment section on the cocos plate is underthrust intact beneath the toe of the caribbean plate with no frontal offscraping where a siliceous fluid bearing zone is present only in the upper part of the section on the barbados margin a layer of radiolarian clay exists providing a narrow zone of mechanical weakness and anomalously high dewatering in the middle of the sediment section this layer divides the sediments that are subducted from those that are accreted accreted and subducted sediments show different compaction styles accreted sediments are characterized by rapid compaction with vertical thickening whereas subducted sediments are characterized by slow compaction with vertical flattening the vertical thickening of the accreted sediments is due to horizontal tectonic compaction and contributes to the vertical thickening of the accretionary prism as a whole in the early stages of deformation dewatering flux is calculated by the volume change of the sediment sequences across the trench the dewatering flux computed from the lwd data provides an estimate of the minimum fluid flux in the subduction zone and is significantly greater than the flux estimated from laboratory experiments because of meter scale fluid conduits which influence the downhole logs but not the centimeter scale sample measurements

quantification of methane emission from bacterial mat sites at quepos slide offshore costa rica seafloor methane emission from the quepos slide on the submarine segment of the costa rica fore arc margin was estimated by extrapolating flux measurements from individual seeps to the total area covered by bacterial mats this approach is based on the combination of detailed mapping to determine the abundance of seeps and the application of a numerical model to estimate the amount of benthic methane fluxes model results suggest that the majority of the studied seeps transport rather limited amount of methane on average 177 mol cm 2 a 1 into the water column due to moderate upward advection allowing for intense anaerobic oxidation of methane aom on average 53 of the methane flux is consumed depth integrated aom rates 56 1 538 mol ch4 cm 2 a 1 are comparable with values reported from other active seep sites the overall amount of dissolved methane released into the water column from the entire area covered by bacterial mats on the quepos slide is estimated to be about 0 28 106 mol a 1 this conservative estimate which relies on rather accurate determinations of seafloor methane fluxes emphasizes the potential importance of submarine slides as sites of natural methane seepage however at present the global extent of methane seepage from submarine slides is largely unknown

the mg ca temperature relationship of benthic foraminiferal calcite new core top calibrations in the 4 c temperature range abstract calcite magnesium calcium ratios were determined for four species of benthic foraminifera cibicidoides wuellerstorfi cibicidoides mundulus oridorsalis umbonatus and pyrgo murrhina from core top samples collected from the atlantic indian and pacific oceans where modern bottom water temperatures are less than 4 c mg ca of all four species increase with increasing temperature and the mg ca temperature relationship does not appear to be necessarily exponential mg ca of c mundulus is consistently higher than c wuellerstorfi implying differences in the uptake of mg by different species within this genus the observed mg ca for c wuellerstorfi at temperatures less than 3 c is lower than those predicted from published mg ca temperature relationships derived for cibicidoides spp at higher temperatures previous studies have attributed this change in the mg ca temperature relationship of c wuellerstorfi to the effect of decreased carbonate ion saturation co32 on mg ca at low temperatures a relationship between the co32 and mg ca of 0 0083 0 002 mmol mol mol kg has been established for c wuellerstorfi from this study