hydrocarbon gas in the costa rica subduction zone primary composition and post genetic alteration abstract at the active continental margin off costa rica substantial amounts of hydrocarbon gases are encountered in sediments the molecular composition c1 c3 of free hydrocarbon gas as well as the isotopic composition 13c of methane and ethane and d of methane was analysed on core samples ranging between 50 and 380 m depth collected at sites 1040 1043 which was drilled during odp leg 170 in addition the molecular composition of the c1 c3 hydrocarbons and the 13c composition of c1 and c2 hydrocarbons was determined on adsorbed gas from selected depth intervals at site 1041 50 380 mbsf the molecular composition and stable carbon and hydrogen isotope signature of low molecular weight hydrocarbons from core sediments and gas pockets indicate that most of the gas was generated by microbial co2 reduction beside 13c values of about 80 for methane which is typical for microbially generated methane extremely light 13c values of 55 were measured for ethane the carbon isotope composition of methane and ethane as well as the c1 c2 c3 ratio display distinct trends with increasing depth gas mixing calculations indicate that the percentage of thermally generated ethane increases from 10 at about 75 mbsf to almost 80 at 380 mbsf the fraction of thermogenic methane in this depth interval is calculated to range from 0 03 to 1 8 of the total methane the small contribution of thermogenic methane would increase the 13c value by

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

the effect of carbonate cementation of underthrust sediments on deformation styles during underplating fluids expelled during deformation at convergent margins lead to a variety of diagenetic reactions including carbonate cementation which alter the rheology of the deforming sediment the volume of carbonate cement in shallow sediments at modern convergent margins is small but sandstones in an ancient accretionary complex in alaska contain significant amounts of carbonate cement that formed before underplating occurred other than vein calcite carbonates are early calcite replacing framework grains or filling pore spaces followed by ankerite that occurs mainly as pore filling cement amounts range from 2 16 weight percent in typical samples averaging about 9 weight percent mean porosity filled by carbonate in kodiak sandstones corresponds to porosities expected at depths of 4 5 km in normal compacting sands underthrust at modem convergent margins the temperatures of formation are difficult to determine precisely but were probably between 100 c and 200 c the bulk of cementation occurred before the formation of thrust faults folds and slaty cleavage related to underplating lithification by cementation changed the rheology of sandstones by 1 filling pore space and increasing peak strength and 2 increasing cohesion so that paniculate flow was no longer a viable deformation mechanism lithification before significant deformation contributed to underplating of the kodiak formation as relatively coherent duplexes

glendonites and methane derived mg calcites in the sea of okhotsk eastern siberia implications of a venting related ikaite glendonite formation mg calcite cemented bioturbation traces glendonite aggregates of idiomorphic bi pyramidal crystals and porous amber colored carbonate concretions were recovered from a methane dominated cold vent area in 380 m water depth at the northern sakhalin slope sea of okhotsk bioturbation traces consist of mg calcite cemented sediment with 13c values between 37 and 46 peedee belemnite pdb which implicate methane as the carbon source glendonite a calcite pseudomorphosis after ikaite caco3 6h2o and amber colored concretions are both composed of varying amounts of calcite and mg calcite with 13c values between 19 and 34 pdb isotope analyses of an ikaite crystal recovered in the vicinity reveals 13c values between 20 and 22 pdb indicating organic matter as the carbon source microscopic investigations of glendonites and amber colored concretions show a porous fabric of a primary calcite phase that is overgrown by a secondary mg calcite cement as ikaite pseudomorphs to porous calcite and as carbon isotope values are the same for the ikaite and the high end member values of glendonite samples the primary calcite phase is suggested to be a former ikaite phase because they share the equal color fabric mineral and isotopic composition with glendonites the amber colored concretions are also suggested to represent calcite that transformed from ikaite a mixture of pseudomorph calcite 13c 20 pdb which originally formed as ikaite from degraded organic matter and mg calcite 13c 43 pdb which crystallized due to the anaerobic oxidation of methane can explain the varying carbon isotope data of the glendonites and the amber colored concretions the growth of ikaite the transformation of ikaite to calcite and the crystallization of mg calcite indicate changing geochemical conditions within a cold vent environment at different times ideal conditions for the ikaite formation are given during the establishment of a cold vent site when upward migrating methane rich fluids enhance the anaerobe decomposition of organic matter which again increases the phosphate and alkalinity concentrations near the sediment surface lower rates of organic matter decomposition during on going venting decrease these high phosphate but also sulphate concentrations and allows other carbonate phases as mg calcite to form this additional carbonate precipitation and the ikaite formation itself lower the high alkalinity and destabilize ikaite which pseudomorphs to porous calcite triggered by the further upward shifting so4 h2s boundary and increasing methane oxidation rates the typical cold vent methane derived carbonate genesis takes place which cements the sediment pore space and induces the secondary mg calcite crystallization within the glendonite fabric taking this scenario into account ikaite formation should be a common process in the beginning of methane dominated vent activity at cold bottom water temperatures glendonite pseudomorphs can be assumed to represent a typical manifestation at fossil and recent cold vents at high latitudes

eiscat svalbard radar observations of ionospheric plasma dynamics in relation to dayside auroral transients the first eiscat svalbard radar esr and ground optical campaign was carried out in january 1998 to cover a variety of dayside auroral activities in the 1000 1400 magnetic local time sector the current paper focuses on 1 hour of combined radar meridian scanning photometer and ground magnetometer observations near the cusp convection throat on january 23 1998 the radar was pointing north at a fixed elevation of 45 in the scan plane of the photometer several plasma dynamical features were revealed by this simplest possible convection mode of the esr such as spontaneous movement of the cusp inflow region and cusp related ionization blobs pushing through the ambient incompressible plasma multiple morning discrete arcs were found to be aligned with convection streamlines equatorward of the flow reversal while plasma in the polar cap inflow region streamed perpendicularly across the optical cusp boundary as earlier anticipated

a proposed production model of rapid subauroral ion drifts and their relationship to substorm evolution multisatellite data are used to examine the temporal relationship between subauroral ion drifts said and the phases of an auroral substorm utilizing images of auroral luminosities taken by the dynamics explorer 1 de 1 spacecraft and observations of particle injection at geosynchronous orbit we identify the time of expansive phase onset and estimate the time at which recovery begins noting the times at which said are observed simultaneously by the de 2 spacecraft we find that said typically occur well after substorm onset more than 30 min during the substorm recovery phase substantial westward ion drifts and field aligned currents are observed well equatorward of the auroral oval during the expansion phase of a substorm but the drifts lack the narrow spike signature associated with said prior to substorm onset and after substorm recovery field aligned currents are absent equatorward of the auroral oval and the ionosphere is very nearly corotating a phenomenological model of said production is proposed that qualitatively agrees with the observed ionospheric signatures and substorm temporal relationship in this model substorm generated subauroral field aligned currents close via pedersen currents with the outward flowing region 1 currents at higher latitudes these pedersen currents flow in the region of low conductivity equatorward of the auroral oval and are associated with relatively large poleward directed electric fields the frictional heating of the ions caused by collisions with the corotating neutral atmosphere substantially increases the rate of ion atom interchange between o and n2 subsequent fast recombination of no with electrons further reduces the subauroral f region conductivities with a corresponding increase in the electric field and the frictional heating this heating leads to thermal expansion substantial field aligned plasma flow and very large depletions in the f peak concentration thus additionally reducing the height integrated pedersen conductivity

new geochemical support for mixing zone dolomitization at golden grove barbados dolomite occurs in the golden grove reef terrace of late pleistocene age in southeastern barbados where fore reef lithologies have been partially to completely dolomitized interpretation of the possible modes of formation of this dolomite has been a subject of disagreement for over a decade among the points of contention are the timing of deposition dolomitization and dolomite cement precipitation and the relationship of replacement dolomite with dolomite cement data from this study indicate that dolomitization occurred during a single relative sea level fall following reef development at marine oxygen isotope stage 7 3 at approximately 216 ka replacement dolomitization affected both micritic matrix and skeletal grains during or after replacement dolomitization dolomite and calcite cements filled primary and secondary pores the cement paragenesis comprises early dolomite cement complexly banded cements of alternating calcite and dolomite late dolomite cement of near stoichiometric composition and a final phase of blocky calcite cement replacement dolomitization and cement precipitation represent a continuum of processes resulting from progressive modification of diagenetic fluids stable carbon and oxygen isotope and strontium elemental compositions of replacive dolomite dolomite cements and calcite cements indicate that dolomitization occurred in a mixing zone between marine and meteoric groundwaters occupying the reef terrace shortly after deposition dolomite oxygen isotope values range from 0 18 to 3 85 pdb and become progressively more depleted throughout the paragenesis indicating a greater influence of meteoric water through time remarkably depleted carbon isotope compositions for both replacement dolomite and dolomite cement 9 53 to 23 04 pdb originated from oxidation of thermogenic methane advected from the underlying accretionary complex methane was oxidized in marine waters that subsequently mixed with meteoric waters as evidenced by a progressive enrichment in carbon isotope composition throughout the paragenesis dolomite strontium contents are substantially elevated above those expected for marine dolomitization ranging from 640 to 1650 ppm and increasing throughout the paragenesis meteoric water provided the excess strontium as aragonite to calcite transformation in the exposed reef terrace increased the fluid sr ca ratio the continuum of geochemical compositions between the replacement dolomite and dolomite cement argues for their genetic association these new data provide additional support to the mixing zone interpretation for golden grove and that the model of mixing zone dolomitization remains viable

syntectonic carbonate veins from the barbados accretionary prism odp leg 110 record of palaeohydrology syntectonic carbonate veins were found along the decollement zone between the atlantic and caribbean plates and in two sites 12 and 17 km landward of the deformation front of the barbados accretionary prism odp leg 110 both calcite and lesser amounts of rhodochrosite mnco3 occur in microcrystalline fibrous and blocky mosaic forms in veins deposited along scaly shear zones descriptions of the occurrences textures mineralogy and stable isotope geochemistry of these veins presented herein help trace the hydrological evolution of the barbados accretionary prism which has seen the impact of more deeply derived fluids on elevated sediment temperatures and altered geochemistry of interstitial fluids at one site for example vein forming fluids have been as much as 10 c warmer and 1 more enriched in 18o than expected carbon isotope ratios of veins indicate a mixture of carbon derived from seawater and from oxidized organic matter during sulphate reduction although dissolution of calcareous skeletons may be an additional component the presence of methane is recorded in one vein sample in the accretionary prism indicating that either the plumbing of the prism has changed with time allowing more fluid to leak into the prism in the past or that materials currently 1 km above the decollement zone were once deformed in the decollement using oxygen isotope analyses of rhodochrosite samples from two sites in conjunction with measurements of fluid temperature and 18o values of interstitial fluids it is proposed that the fractionation of oxygen isotopes between rhodochrosite and water 1000 1n 38 at 25 c 10 greater than the fractionation between calcite and water however it appears that substitution of 10 20 mol ca into rhodochrosite reduces the fractionation factor by 2 9 so impure diagenetic rhodochrosite samples from the deep ocean will rarely be this enriched in 18o

mineral chemical study of u bearing minerals from the dominion reefs south africa the neo archean dominion reefs 3 06 ga are thin meta conglomerate layers with concentrations of u and th bearing heavy minerals higher than in the overlying witwatersrand reefs ore samples from uranium one africa s rietkuil and dominion exploration areas near klerksdorp south africa were investigated for their mineral paragenesis texture and mineral chemical composition the ore and heavy mineral assemblages consist of uraninite other uraniferous minerals fe sulphides ni co sulfarsenides garnet pyrite pyrrhotite monazite zircon chromite magnetite and minor gold sub rounded uraninite grains occur associated with the primary detrital heavy mineral paragenesis u ti u th minerals pitchblende colloform uraninite and coffinite are of secondary re mobilised origin as evidenced by crystal shape and texture most of the uranium mineralisation is represented by detrital uraninite with up to 70 2 wt uo2 and up to 9 3 wt tho2 re crystallised phases such as secondary pitchblende without th coffinite u ti and u th phases are related to hydrothermal overprint during low grade metamorphism and are of minor abundance

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

tubular structures of northern wairarapa new zealand as possible examples of ancient fluid expulsion in an accretionary prism evidence from field and petrographical observations

early carbonate diagenesis in slope sequences and at their boundaries cenozoic offshore new jersey odp leg 150 this paper reports the genetic links among the depth distribution mineralogy and stable isotopic composition of diagenetic carbonates with sedimentation rates and types and preservation of organic matter in the terrigenous and biogenic sediments of oligocene and miocene age on the new jersey slope calcites formed close to the sediment surface at sequence boundaries and maximum flooding surfaces when the profile of early diagenetic reactions was stabilized in the sediment column for extended periods dolomites precipitated in the sulfate reduction zone when diagenetic profiles stabilized during truncation sequence boundary formation and the deposition of lowstand sediments that overlie the sequence boundaries most dolomites occur in distal slope sediments that were deposited before the shelf had prograded into the study area siderites formed during a later stage of burial in the methanogenic zone they are not directly genetically related to the sequence stratigraphy of the new jersey slope the diagene tic dolomites and siderites occur in widely separated depth intervals below the present sea floor the distribution of the diagenetic carbonates and their preferential occurrence in separated depth intervals resulted from different combinations of sedimentation rates and organic matter types and preservation

direct observations of the evolution of polar cap ionization patches patches of ionization are common in the polar ionosphere where their motion and associated density gradients give variable disturbances to high frequency hf radio communications over the horizon radar location errors and disruption and errors to satellite navigation and communication their formation and evolution are poorly understood particularly under disturbed space weather conditions we report direct observations of the full evolution of patches during a geomagnetic storm including formation polar cap entry transpolar evolution polar cap exit and sunward return flow our observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a tongue of ionization toi

earth s ion upflow associated with polar cap patches global and in situ observations we report simultaneous global monitoring of a patch of ionization and in situ observation of ion upflow at the center of the polar cap region during a geomagnetic storm our observations indicate strong fluxes of upwelling o ions originating from frictional heating produced by rapid antisunward flow of the plasma patch the statistical results from the crossings of the central polar cap region by defense meteorological satellite program f16 f18 from 2010 to 2013 confirm that the field aligned flow can turn upward when rapid antisunward flows appear with consequent significant frictional heating of the ions which overcomes the gravity effect we suggest that such rapidly moving patches can provide an important source of upwelling ions in a region where downward flows are usually expected these observations give new insight into the processes of ionosphere magnetosphere coupling

deep scientific dives in the japan and kuril trenches abstract in the summer of 1985 during the french japanese kaiko program ten dives to depths of 6000 m in the japan and kuril trenches were made in the newly launched submersible nautile the sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving seabeam mapping geomagnetic and gravimetric measurements and single channel seismic profiling the results of the dives provide new constraints on the geodynamics of these subduction zones in the japan and kuril trenches huge slump scars were observed on the landward slopes of the trenches slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material the slump scars allowed us to observe the internal structure of the margin the monoclinal structure on the northern japan trench margin deduced from the seismic profiles and dsdp drilling was confirmed several dives on kashima seamount confirmed that this volcano has recently been split into two parts by a normal fault system comparisons of lithology and paleontology on the two separated parts of the seamount were made deep sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches it can be concluded that the whole margin is venting fluids from depths of 2 3 km which is consistent with the indications of overpressure observed in drill sites on the japan trench margin the fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor

deep biological communities in the subduction zone of japan from bottom photographs taken during nautile dives in the kaiko project abstract twenty seven dives of the submersible nautile in the subduction zone around japan conducted in the french japanese project kaiko proved that fairly luxuriant benthic communities dominated by deep sea giant clams of the genus calyptogena family vesicomyidae were consistently present on the accretionary prism at abyssal depths benthic communities characterized by three hitherto undescribed bivalves of the genus calyptogena were found between depths of about 3800 and 4020 m at the mouth of tenryu canyon and at the top of basement swell of the zenisu ridge both situated in the eastern nankai subduction zone sporadic but discrete patches of organisms characterized by one more undescribed bivalve belonging to the genus calyptogena were observed and collected between depths of 5130 and 5960 m on the landward wall of the japan and kouriles trenches photographic inventories were prepared semiquantitatively using each series of bottom photographs taken in these areas with bow cameras of the submersible nautile observations on the sporadic but dense distribution of the clams and other characteristic associated organisms match well with the scheme that communities sustained by chemosynthetic energy sources can be present at connate water seepages in subduction zones these are to date the deepest record of benthic communities supposedly associated with chemosynthetic processes

space time variability of polar cap patches direct evidence for internal plasma structuring 1 coordinated observations of ionospheric variability near the geomagnetic pole using the resolute bay incoherent scatter radar risr n super dual auroral radar network superdarn high frequency hf radars and all sky imagers have clarified the relative contribution of structuring mechanisms operating on polar plasma patches from the multipoint risr n observations a three dimensional image can be constructed of the plasma parameters the colocated coherent echoes from the superdarn radars provide information on field aligned irregularities and from all sky imagers located in resolute bay canada and qaanaaq greenland information is obtained on the emission brightness at different wavelengths a good correlation is found between the location of the coherent incoherent and optical signals of patches from the superdarn radar data it is evident that plasma irregularities seem to be present throughout the region of enhanced electron density the patches are observed to be formed in the cusp region due to bursty flux transfer events and are then transported across the polar cap during the time period of about 10 minutes when a patch drifted through the risr n field of view the patch seemed to undergo significant deformation in all three spatial dimensions with density fluctuations of about 10 and spatial variations leading to stretching and tilting of the patch the findings show that plasma structuring can likely occur within polar cap patches which support previous suggestions that a patch is highly variable as it drifts across the polar cap with a faster spread of irregularities throughout the patch as a result

stratigraphic control of chemistry and mineralogy in metamorphosed witwatersrand quartzites within metamorphosed quartzites of the witwatersrand supergroup bulk chemistry is the major control on the distribution of metamorphic mineral phases and on their chemistry bulk chemistry is in turn controlled by stratigraphic position pyrophyllite distribution varies directly with whereas feldspar distribution varies inversely with bulk al chloritoid distribution is more complex and varies with both bulk al and bulk fe mg despite the overprint of low grade metamorphism and possibly of other alteration processes both detrital and metamorphic phyllosilicates can be recognized and characterized al in metamorphic white mica and metamorphic chlorite varies with bulk al whereas the composition of detrital white mica is unrelated to bulk chemistry similarly mg in metamorphic chlorite varies with bulk mg bulk chemical variation particularly in al mg and na and thus metamorphic mineral chemistry is controlled by stratigraphic position both within individual stratigraphie units and in the strat

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

methane derived carbonate cementation of marine sediments from the kattegat denmark geochemical and geological evidence abstract methane derived carbonate cementation of holocene marine sediment occurs at several locations in the kattegat denmark generally the carbonate cemented sandstones occur at the seafloor as individual slabs or widely distributed thinly lithified pavements in some areas the lithified sandstones take the form of large and spectacular vertical pillar and mushroom like bodies standing up from the seafloor the carbonate cement consists of aragonite high mg calcite and dolomite the high mg calcite is the most widespread carbonate solid phase found aragonite is primarily associated with shell accumulations in the sediments so far dolomite has only been recorded as cement in the large vertical lithified sandstone bodies the carbonates are remarkably depleted in 13c i e generally in the range of 55 to 45 pdb and are therefore interpreted as being the result of methane oxidation by sulphate reducing bacteria in the anaerobic diagenetic zone of the sediments however the oxygen isotopic composition and the concentrations of minor elements indicate that carbonate precipitation took place under relatively constant temperature and salinity conditions in normal marine pore fluids the habit and mineralogy of the carbonate cemented sandstone observed most likely are controlled by the effect of bacterial activity and upward gas migration on the interstitial geochemical microenvironment the lithified sediments were subsequently exposed as spectacular sandstone formations on the seafloor by submarine erosion data from onshore exploration wells reveal that methane is encountered in eemian early weichselian late quaternary marine deposits the chemical composition and the isotopic signatures of the methane suggest a gas of microbial origin and it is thought that the gas is generated within the host marine quaternary sediment the locations of submarine gas seeps along with those of the carbonate cemented sandstone formations and the onshore gas seeps show a linear distribution which largely coincides with the extension of the fennoscandian border zone thus the seeps may be controlled by the structural framework in the northeastern part of the danish subbasin

numerical modeling study of mineralization induced by methane cold seep at the sea bottom abstract in order to investigate the response of authigenic minerals to gas hydrate geo systems the biogeochemical processes and its induced mineralization were predicted by employing the comprehensive reactive transport modeling approach based on the available data extracted from the northern continental slope area of the south china sea a 1 d vertical column model was developed three cases with different upward methane flux rates and three cases with different mineral compositions i e a total of six cases were designed to investigate the effects of variations in the depth of sulfate methane transition zone smtz and in the mineral composition on the formation of authigenic minerals the simulation results indicate that the smtz depth influenced by both the upward methane flux rate and the initial composition played an important role in the formation of authigenic minerals the aom reaction is intensive at the interface and the precipitation amount of calcite is large which is mainly controlled by aom when the methane leakage rate is 20 times higher than the base case aragonite starts to precipitate during the simulation oligoclase k feldspar smectite na smectite ca chlorite dissolved our study specific to this area as a starting point may provide a quantitative approach for investigating carbonate and pyrite formation in hydrate bearing sediments accounting for methane oxidation and sulfate reduction the method presented here and the model built in this study can be used for other sites with similar conditions in addition this study may serve as an indication for the potential natural gas hydrate reservoir in depth and is also significant for marine carbon and sulfur cycle

fluid flow and mass flux determinations at vent sites on the cascadia margin accretionary prism fluid venting from the toe of the accretionary prism off oregon was measured in situ during a series of dives with dsrv alvin in 1987 and 1988 a benthic chamber was placed over active vent sites to sequentially collect samples of venting fluids and to make direct measurements of discharge rates calibrated flow meter measurements and flow rates determined from dissolved methane transfer indicate that discharge from two vent sites alvin 1428 and alvin 1900 ranges roughly between 100 and 500 l m 2 d 1 with the most reliable estimates falling in the range of 125 150 l m 2d 1 these rates imply subsurface advective flow on the order of 100 m yr 1 comparison of observed discharge rates with rates calculated for steady state expulsion supported by accretion related compaction indicates that the observed flow is greater than predicted flow by several orders of magnitude the disparity dictates that fluids are not derived locally but are transported laterally within the prism or that flow is not steady state and that individual vents are short lived features in the ongoing accretion process

deep sea submersible survey in the suruga sagami and japan trenches preliminary results of the 1985 kaiko cruise leg 2 abstract nine submersible dives were made in three trenches off central japan between 2990 and 5900 m of water depth our observations confirm the interpretation that daiichi kashima seamount is a cretaceous guyot formed on the pacific plate that has traveled into the japan trench we also confirmed the previous interpretation of a large normal fault that splits the seamount in two halves the lower one being now subducting beneath the japan margin compressional deformation was identified within the lower part of the inner slope in front of the seamount the pattern of deformation that affects quaternary sediments is in agreement with the present kinematics of the convergence between the pacific plate and japan deep water 5700 m clam colonies are associated with advection of fluids driven by the subduction related overpressures in the northern slope of the boso canyon along the sagami trough system philippine sea plate japan boundary the deformation affecting a thick upper miocene to lower pliocene sequence indicates two directions of shortening a n175 e direction which is consistent with the present relative motion along the sagami trough n285 n300 e and a n30 e direction which could be related to a more northerly direction of convergence that occured during the early quaternary and earlier

reconstructing the history of fluid flow at cold seep sites from ba ca ratios in vesicomyid clam shells hydrogen sulfide discharge at cold seep sites is recorded as enrichment in the barium to calcium ba ca ratio in shells of vesicomyid clams collected live from cold seeps in monterey canyon and the cascadia margin a direct relationship between increased ba fluxes from cold seeps and ba incorporation into shells was established for the cascadia margin site for the monterey canyon site a 2 yr episode of high fluid flow centered on 1992 was inferred from coherent changes in the ba ca profiles of threecalyptogena kilmerishells comparison with precipitation and d 18 o data indicates that this high flow period may have been driven by an increase in rainfall after the 1988 1990 california drought high resolution records preserved in clam shells are shown to be useful in elucidating characteristics history and possible mechanisms driving fluid discharge at continental margin seeps thus establishing their potential use as paleotracers of fluid seepage events an extensive set of geophysical and geochemical measurements is currently available to document the flow and expulsion of fluids at transform and convergent margins it is now recognized that such fluid discharge has far reaching implications that include hydrocarbon prospecting benthic ecosystem structure and function and groundwater research moore 1999 at present we are unable to incorporate geochemical data from cold seeps to models quantifying their effects on marine geochemical budgets because we lack a clear understanding of the spatial and temporal variability of these systems and of the mechanisms responsible for the advective transport however vent macrofauna may afford such an opportunity by recording in their shells the immediate environment in which they grow and thereby allowing us to reconstruct the variability in fluid discharge rates vesicomyid clams live in association with sites of sulfide 1

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

on the symmetry of ionospheric polar cap patch exits around magnetic midnight in this paper we examine how polar cap patches which have been frozen into the antisolar flow over the polar cap are transported into the nighttime auroral oval first we present a detailed case study from 12 january 2002 with continuous observations of polar cap patches exiting into the nighttime auroral oval in the scandinavian sector satellite images of the auroral oval and all sky camera observations of 630 0 nm airglow patches are superimposed onto super dual auroral radar network convection maps these composite plots reveal that polar cap patches exit on both the dusk and on the dawn convection cells then we present statistics based on 8 years of data from the meridian scanning photometer at ny aalesund svalbard to investigate the possible interplanetary magnetic field imf by influence on the distribution of patch exits around magnetic midnight the magnetic local time distribution of patch exits is almost symmetric around magnetic midnight independent of imf by polarity synthesizing these observations with previous results we propose a three step mechanism for why patch material exits symmetrically around midnight first intake of patch material occurs on both convection cells for both imf by polarities second plasma intake by transient magnetopause reconnection stretches the newly cut polar cap patches into dawn dusk elongated forms during their transport into the polar cap and finally at exit dawn dusk elongated patches are split and diverted toward both the dawn and dusk flanks when grabbed by transient tail reconnection

controls on calcium isotope fractionation in sedimentary porewaters abstract the calcium isotopic composition of porewaters and authigenic carbonates in the anoxic sediments of a convergent continental margin drilled during ocean drilling program odp provides first insight into the different processes that control ca geochemistry in clastic marine organic rich sedimentary environments in 4 sites drilled during leg 204 at hydrate ridge cascadia margin offshore oregon usa sulfate is consumed during anaerobic oxidation of methane and of organic matter via sulfate reduction within the upper meters of the sedimentary section these reactions promote the precipitation of authigenic carbonates through the generation of bicarbonate which is reflected in a pronounced decrease in calcium concentration although ca isotope fractionation is observed during carbonate precipitation ca concentration in the pore fluids from odp leg 204 is decoupled from ca isotopy which seems to be mainly controlled by the release of light ca isotopes that completely overprint the carbonate formation effect different processes such as the release of organically bound ca ion exchange and ion pair formation may be responsible for the released light ca deeper within the sedimentary section additional processes such as ash alteration influence the ca isotopic composition of the porewater two sites drilled into the deeper core of the accretionary prism reveal the nature of fluids which have reacted with the oceanic basement these deep fluids are characterized by relatively high ca concentrations and low 44 40ca ratios

multiradar observations of the polar tongue of ionization 1 we present a global view of large scale ionospheric disturbances during the main phase of a major geomagnetic storm we find that the low latitude auroral and polar latitude regions are coupled by processes that redistribute thermal plasma throughout the system for the large geomagnetic storm on 20 november 2003 we examine data from the high latitude incoherent scatter radars at millstone hill sondrestrom and eiscat tromso with superdarn hf radar observations of the high latitude convection pattern and dmsp observations of in situ plasma parameters in the topside ionosphere we combine these with north polar maps of stormtime plumes of enhanced total electron content tec derived from a network of gps receivers the polar tongue of ionization toi is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern the dayside source of the toi is the plume of storm enhanced density sed transported from low latitudes in the postnoon sector by the subauroral disturbance electric field convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral f region is significantly enhanced by the sed material the three incoherent scatter radars provided full altitude profiles of plasma density temperatures and vertical velocity as the toi plume crossed their different positions under the cusp in the center of the polar cap and at the midnight oval polar cap boundary greatly elevated f peak density 1 5e12 m 3 and low electron and ion temperatures 2500 k at the f peak altitude characterize the sed toi plasma observed at all points along its high latitude trajectory for this event sed toi f region tec 150 1000 km was 50 tecu both in the cusp and in the center of the polar cap large upward directed fluxes of o 1 e14 m 2 s 1 were observed in the topside ionosphere

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

impacts of a mesoscale eddy pair on internal solitary waves in the northern south china sea revealed by mooring array observations abstractboth internal solitary waves isws and mesoscale eddies are ubiquitous in the northern south china sea scs in this study the authors examine the impacts of mesoscale eddies on the isws transiting the northern scs deep basin that evolve from the steepening internal tide generated in the luzon strait using in situ data collected from a specifically designed mooring array from november 2013 to january 2014 an energetic mesoscale eddy pair consisting of one anticyclonic eddy ae and one cyclonic eddy ce propagated across the mooring array observations revealed that the amplitude propagation direction and speed of the transbasin isws were significantly modulated by the eddy pair when the moorings were covered by the southern portion of the ae the isw amplitudes decreased by as much as 67 because of the thermocline deepening along the wave direction and the energy divergence along the wave front when the moorings were covered by the northern portions of both eddies the amplitude of is

chemical evidence for advection of hydrothermal solutions in the sediments of the galapagos mounds hydrothermal field pore water profiles of ca mg f po4 3 and mn in the galapagos mounds hydrothermal field are believed to reflect in part upwelling of hydrothermal solutions through the sediments concentration depth profiles in a low heat flow area just north of the mounds field display diagenetic changes typical of those found in pore waters underlying highly productive surface waters consistent with the inference of no water flow or very slow downwelling w 5 cm yr of bottom water through these sediments rates of upward advection calculated from mounds field pore water profiles of ca mg and f profiles agree well with each other averaging about 1 cm yr in the pelagic sediments near the mounds and 15 30 cm yr within the hydrothermal mounds themselves the upward advection also modifies the shape of po4 3 and mn profiles advection rates inferred from the pore water data are generally in reasonable agreement with those made from heat flow data the higher ca and lower mg f po4 3 and mn concentrations in mounds field pore waters compared with those of the low heat flow area suggest chemical exchange between the solution and basalt prior to upwelling li k rb sr and so4 concentrations are indistinguishable from bottom water this suggests very high effective water rock ratios during the reactions which produced the upwelling solutions perhaps due to extensive prior alteration of basalt adjacent to the flow path of water through the crust inferred reaction temperatures are between 70 150 c

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

calcium carbonate hexahydrate from organic rich sediments of the antarctic shelf precursors of glendonites large euhedral crystals of calcium carbonate hexahydrate were recovered from a shelf basin of the bransfield strait antarctic peninsula at a water depth of 1950 meters and sub zero bottom water temperatures the chemistry mineralogy and stable isotope composition of this hydrated calcium carbonate phase its environment of formation and its mode of precipitation confirm the properties variously attributed to hypothetical precursors of the glendonites and thereby greatly expand their use in paleoceanographic interpretation

lithium sources in pore fluids of peru slope sediments and implications for oceanic fluxes high li concentrations up to a maximum of 1155 m are observed in the pore fluids of the peru convergent margin slope sediments at ocean drilling program sites 683 and 685 9 s the li concentration depth gradients are twice as steep as at site 682 and 688 11 s within the sediments the most important li sources are from aluminosilicate minerals biogenic opal a contains little li and thus dilutes the li concentration of the bulk sediments the sediment compositions and the thermal regimes are similar at 9 and 11 s suggesting there is an additional non sedimentary source for the observed high li concentrations in the northern pore fluids at 9 s the 87sr 86sr ratios reach a maximum value of 0 709958 the observed radiogenic 87sr 86sr values in the pore fluids support the suggestion that the additional li may derive from exchange reactions with underlying continental crust the high concentrations of li at 11 s may derive from basalt alteration at moderate to high temperatures as suggested by the non radiogenic 87sr 86sr ratios in these pore fluids which reach a minimum value of 0 707218 based on 1 li concentrations in the pore fluids in slope sediments from peru and several other margins and 2 an approximate estimate of fluid flux from continental margins into the ocean continental margins provide an estimated 1 to 3 1010 moles li yr to the ocean this source of oceanic li which has not been considered previously is of the same order of magnitude as some estimates of hydrothermal and river li fluxes and may have important consequences for the oceanic li isotope budget the sink is unknown for this newly discovered and possibly large li source but it may be more pervasive low temperature alteration of oceanic basement than previously estimated or burial of mineral phases such as authigenic clay minerals or metal oxyhydroxides which may be li rich

relative importance of horizontal and vertical transports to the formation of ionospheric storm enhanced density and polar tongue of ionization there are still uncertainties regarding the formation mechanisms for storm enhanced density sed in the high and subauroral latitude ionosphere in this work we deploy the thermosphere ionosphere electrodynamic general circulation model tiegcm and gps total electron content tec observations to identify the principle mechanisms for sed and the tongue of ionization toi through term by term analysis of the ion continuity equation and also identify the advantages and deficiencies of the tiegcm in capturing high latitude and subauroral latitude ionospheric fine structures for the two geomagnetic storm events occurring on 17 march 2013 and 2015 our results show that in the topside ionosphere upward e b ion drifts are most important in sed formation and are offset by antisunward neutral winds and downward ambipolar diffusion effects in the bottomside f region ionosphere neutral winds play a major role in generating seds sed signature in tec is mainly caused by upward e b ion drifts that lift the ionosphere to higher altitudes where chemical recombination is slower horizontal e b ion drifts play an essential role in transporting plasma from the dayside convection throat region to the polar cap to form tois inconsistencies between model results and gps tec data were found 1 gps relative tec difference between storm time and quiet time has holes in the dayside ion convection entrance region which do not appear in the model results 2 the model tends to overestimate electron density enhancements in the polar region possible causes for these inconsistencies are discussed in this article

nankai trough and zenisu ridge a deep sea submersible survey abstract eight submersible dives between 3000 and 4200 m water depth were made off southern japan in the eastern nankai subduction zone benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone at the toe of the accretionary prism a benthic community was also discovered along a zone of active compression at the foot of zenisu ridge 30 km south of nankai trough temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there studies of water samples indicate advection of methane and light hydrocarbons specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption thus the benthic communities are related to overpressure driven fluid advection along tectonic zones with active surface deformation a 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280 direction which is close to the 305 average direction of subduction in this area the dives establish further that compressive deformation is presently occurring at the foot of zenisu ridge the previous interpretation of the zenisu ridge as a zone of recent north south intraplate shortening 40 km south of the nankai trench is confirmed we conclude that tectonic evolution might well lead to future detachment of the zenisu ridge and overthrusting of this large piece of oceanic crust over the continental margin such a process might be an efficient one to emplace ophiolites over continents

high resolution observations of the small scale flow pattern associated with a poleward moving auroral form in the cusp 1 in this paper we present high resolution observations by the eiscat svalbard radar of the localized ionospheric flow response due to one single flux transfer event and the relative location of a poleward moving auroral form in a fast scan mode the radar was tracking a 50 60 km wide channel of westward flow in the cusp region this flow channel was surrounded by flow running in the opposite direction at the poleward edge of the narrow flow channel a poleward moving auroral form was situated consistent with the auroral form being the signature of an upward birkeland current filament our observations can be interpreted in terms of the southwood 1985 1987 flux transfer model and the fte twin cell flow pattern appears to be a ripple onto the larger scale background convection

on the importance of interplanetary magnetic field by on polar cap patch formation 1 a number of poleward moving events were observed between 1130 and 1300 ut on 11 february 2004 during periods of southward interplanetary magnetic field imf while the steerable antenna of the european incoherent scatter eiscat svalbard radar esr and the tromso vhf radar pointed nearly northward at low elevation in this interval simultaneous superdarn cutlass finland radar measurements showed poleward moving radar aurora forms pmrafs which appeared very similar to the density enhancements observed by the esr northward pointing antenna these events appeared quasiperiodically with a period of about 10 min comparing the observations from the above three radars it is inferred that there is an almost one to one correspondence between the poleward moving plasma concentration enhancements pmpces observed by the esr and the vhf radar and the pmrafs measured by the cutlass finland radar these observations are consistent with the interpretation that the polar cap patch material was generated by photoionization at subauroral latitudes and that the plasma was structured by bursts of magnetopause reconnection giving access to the polar cap there is clear evidence that plasma structuring into patches was dependent on the variability in imf by the duration of these events implies that the average evolution time of the newly opened flux tubes from the subauroral region to the polar cap was about 33 min

interhemispheric study of polar cap patch occurrence based on swarm in situ data the swarm satellites offer an unprecedented opportunity for improving our knowledge about polar cap patches which are regarded as the main space weather issue in the polar caps we present a new robust algorithm that automatically detects polar cap patches using in situ plasma density data from swarm for both hemispheres we compute the spatial and seasonal distributions of the patches identified separately by swarm a and swarm b between december 2013 and august 2016 we show a clear seasonal dependency of patch occurrence in the northern hemisphere nh patches are essentially a winter phenomenon as their occurrence rate is enhanced during local winter and very low during local summer although not as pronounced as in the nh the same pattern is seen for the southern hemisphere sh furthermore the rate of polar cap patch detection is generally higher in the sh than in the nh especially on the dayside at about 77 magnetic latitude additionally we show that in the nh the number of patches is higher in the postnoon and prenoon sectors for interplanetary magnetic field imf by 0 respectively and that this trend is mirrored in the sh consistent with the ionospheric flow convection overall our results confirm previous studies in the nh shed more light regarding the sh and provide further insight into polar cap patch climatology along with this algorithm we provide a large data set of patches automatically detected with in situ measurements which opens new horizons in studies of polar cap phenomena

nankai trough japan trench and kuril trench geochemistry of fluids sampled by submersible nautile abstract deep water samples collected during the kaiko project are often associated with biological communities located on geological structures favorable to fluid venting the evidence of fluid venting are the temperature anomalies the decrease in sulfate concentrations the content in methane and the lowc 1 c 2 c 3 ratio of light hydrocarbons because of large dilution by ambiant seawater during sampling it is difficult to compute the composition of the advected end member pore fluid part of this fluid should originate in the petroleum window i e at temperature about 60 c modeling the upward flow of water taking into account the anomalies of temperature measured on the seafloor and the geochemical anomalies leads to non steady state advection of the pore fluid the occurrence of a deep component in the fluid has implications for the geological and tectonic models of the subduction zones off japan

sulfate reduction using methane in sediments beneath a bathyal cold seep giant clam community off hatsushima island sagami bay japan two sandy sediment cores cores d227 120 and d380 were collected from inside a deep sea giant clam calyptogena soyoae community off hatsushima island western sagami bay central japan 35 59 9 n 139 13 6 e 1160 m deep and a muddy sediment core core d227 202 was obtained from outside the community by the submersibleshinkai 2000 the chloride concentration of the pore waters is constant vertically and sulfate reduction using sedimentary organic matter occurs in core d227 202 21 cm long the chloride concentrations are lower by 7 at the 7 5 9 cm depth in core d227 120 9 cm long and by 3 at the 11 12 cm depth in core d380 16 cm long than those of the overlying bottom waters in the cores from inside of the community sulfate concentration decreases remarkably and dissolved inorganic carbon alkalinity ammonium n and hydrogen sulfide concentrations increase significantly with increasing depth in core d380 34s values of sulfate ions increase from 20 5 to 35 3 and 13c values of dissolved inorganic carbon decrease drastically from 7 0 to 45 with increasing depth from the top to the bottom of the core although the 13c values of the organic carbon of the sediments are 23 7 0 9 in core d380 these results indicate that sulfate reduction using methane is active within the sediments just beneath the living clams and that the hydrogen sulfide produced can be used by endosymbiotic sulfur oxidizing bacteria living in the gills ofc soyoae in the community

high latitude plasma outflow as measured by the dmsp spacecraft 1 we have examined the vertical ion flux in the topside high latitude ionosphere from measurements of the vertical ion drift and ion number density made by the dmsp f13 satellite we found that the average vertical flux over the entire high latitude region near 800 km altitude is quite small this suggests that most of the vertical ion flux is associated with a large scale breathing of the upper ionosphere in the polar cap the vertical ion flux is uniformly downward at all locations however in the auroral zone the ion flux is highly structured and a net upward flux is produced only by spatially and temporally confined events containing upward fluxes in excess of 1010 cm 2 s 1 that have no downward counterparts

bromine and iodine in peru margin sediments and pore fluids implications for fluid origins at the peruvian convergent margin two distinct pore fluid regimes are recognized from differences in their cl concentrations the slope pore fluids are characterized by low cl concentrations but elevated br and i concentrations due to biogenic production the shelf pore fluids exhibit elevated cl and br concentrations due to diffusive mixing with an evaporitic brine in the slope pore fluids the br i and nh4 concentrations are elevated following bacterial decomposition of organic matter but the i concentrations are in excess of those expected based on mass balance calculations using nh4 and br concentrations the slope sediment organic matter which is enriched in iodine from oxidationreduction processes at the oxygenated sediment water interface is responsible for this enrichment the increases in dissolved i and the i enrichments relative to nh4 and br correlate well with sedimentation rates because of differential trapping following regeneration the pore fluid i br ratios suggest that membrane ion fiitration is not a major cause of the decreases in cl concentrations other possible sources for low cl water including meteoric water clathrate dissociation and or mineral dehydration reactions imply that the diluting component of the slope low cl fluids has flowed at least 1 km through the sediment the low bottom water oxygenation in the shelf is responsible for the low if any enrichment of iodine in the shelf sediments fluctuations in bottom water oxygen concentrations in the past however may be responsible for the observed variations in the sediment ibr ratios comparison of na cl and br cl molar ratios in the pore fluids shows that the shelf high cl fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution this dense brine located below the sediment sections drilled appears to have flowed a distance 500 km through the sediment

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

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

seafloor fluid seeps on kimki ridge offshore southern california links to active strike slip faulting abstract the kimki ridge fluid seeps are located in western catalina basin about 60 km southwest of the southern california mainland and at a water depth of approximately 1100 m multichannel seismic reflection profiles collected by the u s geological survey usgs in 2014 show acoustic transparency within the kimki ridge suggesting the possibility of fluid seeps and possible sub seafloor fluid pathways subsequent multibeam bathymetric and backscatter intensity data collected during a cooperative university of washington usgs cruise in early 2016 show subtle seafloor buildups with high acoustic backscatter reflectivity in three places along kimki ridge supporting the existence of fluid seepage a remotely operated vehicle rov dive conducted as part of the nautilus exploration program took place in august 2016 to confirm the presence of these previously unknown seeps and document their characteristics as well as those of any associated biological communities two of the three seeps were explored by rov and showed abundant evidence of fluid seepage including characteristic algal mats chemosynthetic clams and authigenic carbonate formation the seeps are comprised of carbonate buildups 1 3 m thick and 300 500 m across within these areas we interpret broad crater like depressions 30 50 m across and 1 2 m deep to be individual seep vents the seep areas appear to be broad zones of diffuse seepage that support chemosynthetic biologic communities however active venting was not observed geochemical analyses of rock samples collected from the seeps indicate microbially driven anaerobic oxidation of methane at or near the sediment water interface seismic reflection profiles show chimney like fluid pathways along the limbs and in the axis of the fold forming kimki ridge and evidence of methane in shallow sediments can be traced into the adjacent catalina basin a system of closely spaced faults located at the axis of the kimki ridge anticline may serve as pathways to allow fluid flow to the seafloor our data are consistent with other studies that suggest that transpression is an important component in the formation and localization of fluid seeps in a strike slip setting implying that seep formation may be a common occurrence at fault stepovers or transpressional bends in strike slip systems

average characteristics and activity dependence of the subauroral polarization stream 1 data from the millstone hill incoherent scatter radar taken over two solar cycles 1979 2000 are examined to determine the average characteristics of the disturbance convection electric field in the midlatitude ionosphere radar azimuth scans provide a regular database of ionospheric plasma convection observations spanning auroral and subauroral latitudes and these scans have been examined for all local times and activity conditions we examine the occurrence and characteristics of a persistent secondary westward convection peak which lies equatorward of the auroral two cell convection individual scans and average patterns of plasma flow identify and characterize this latitudinally broad and persistent subauroral polarization stream saps which spans the nightside from dusk to the early morning sector for all kp greater than 4 premidnight the saps westward convection lies equatorward of l 4 60 invariant latitude spans 3 5 of latitude and has an average peak amplitude of 900 m s in the predawn sector saps is seen as a region of antisunward convection equatorward of l 3 55 spanning 3 of latitude with an average peak amplitude of 400 m s

global plasmasphere evolution 22 23 april 2001 1 the evolution of the plasmasphere during the moderate 22 23 april 2001 storm is studied by comparing observed plasmasphere behavior to the predictions of a simulation the plasmasphere observations include global images from image euv and density and flow measurements from lanl mpa the subjective uncertainty in the euv plasmapause was found to be 0 2l inward plasmapause motion was correlated with southward interplanetary magnetic field imf with a 27 min delay the electric e field at the plasmapause was approximately 13 of the solar wind e field the observations support the idea that plasmaspheric erosion begins with a partial plasmapause indentation which then gradually widens in magnetic local time mlt to encompass the entire nightside in situ measurements confirm a dayside plume of sunward flowing plasma and images show that the plume subsequently experienced phases of mlt narrowing and rotation wrapping to simulate the event we employed a test particle representation of the plasmapause using a parametric e field model that includes corotation and convection from two sources dayside magnetopause reconnection dmr and the subauroral polarization stream saps the model captures the phases of plume evolution and it reproduces the observed plasmapause with maximum mean error 0 7l 0 24l the model fails to reproduce important subglobal features e g a predawn plasmaspheric shoulder and grossly misplaces the inner edge of the rotating and wrapping plume saps contributes significantly to the duskside flows but the simulated flows are about 50 smaller than those seen by lanl mpa the simulated plasmapause e field is about 70 of the euv inferred value

stratification of east west plasma flow channels observed in the ionospheric cusp in response to imf by polarity changes 1 during a period of predominantly north westward flow for imf bz negative and imf by positive a sequence of three distinct negative excursions of imf by resulted in a train of three eastward directed flow channels interleaved by westward flow enhancements propagating into the polar cap the high resolution of the eiscat svalbard radar data enables us to track formation and movement of the flow channels which are interpreted as a sequence of intermittent reconnection alternating between different reconnection sites our observations are consistent with the view that a new region of reconnected flux manifests as development of a distinct flow channel near the polar cap boundary and that successive events stay separated while pushing each other into the polar cap each flow channel will remain separated from neighboring channels mapping to different reconnection sites as long as the magnetic tension force with its associated field aligned current systems is maintained

using the 87sr 86sr of modern and paleoseep carbonates from northern cascadia to link modern fluid flow to the past abstract most authigenic carbonates previously recovered from the cascadia slope have 87 sr 86 sr signatures that reflect shallow precipitation in equilibrium with coeval seawater there is also evidence for carbonate formation supported by fluids that have been modified by reactions with the incoming juan de fuca plate 87 sr 86 sr 0 7071 teichert et al 2005 or with terrigenous turbidites 87 sr 86 sr 0 70975 to 0 71279 sample et al 1993 we report on the strontium isotopic composition of carbonates and fluids from iodp site u1329 and nearby barkley canyon offshore vancouver island which have strontium isotope ratios as low as 0 70539 whereas the strontium and oxygen isotopic compositions of carbonates from paleoseeps in the uplifted coast range forearc indicate formation in ambient bottom seawater several samples from the pysht sooke fm show a 87 sr depleted signal 87 sr 86 sr 0 70494 and 0 70511 similar to that of the anomalous site u1329 and barkley canyon carbonates our data when analyzed in the context of published elemental and isotopic composition of these carbonates joseph et al 2012 point to two formation mechanisms 1 shallow precipitation driven by the anaerobic oxidation of methane aom with 13 c values as low as 50 and contemporaneous 87 sr 86 sr seawater ratios and 2 carbonate precipitation driven by fluids that have circulated through the oceanic crust which are depleted in 87 sr carbonates formed from the second mechanism precipitate both at depth and at sites of deep sourced fluid seepage on the seafloor the 87 sr depleted carbonates and pore fluids found at barkley canyon represent migration of a deep exotic fluid similar to that found in high permeability conglomerate layers at 188 mbsf of site u1329 and which may have fed paleoseeps in the pysht sooke fm these exotic fluids likely reflect interaction with the 52 57 ma igneous crescent terrane which supplies fluids with high calcium manganese and strontium enriched in the non radiogenic nucleide tectonic compression and dehydration reactions then force these fluids updip where they pick up the thermogenic hydrocarbons and 13 c enriched dissolved inorganic carbon that are manifested in fluids and carbonates sampled at barkley canyon and at site u1329 the crescent terrane may have sourced cold seeps in this margin since at least the late oligocene

pore water geochemistry of eastern mediterranean mud volcanoes implications for fluid transport and fluid origin abstract the pore water chemistry of mud volcanoes from the olimpi mud volcano field and the anaximander mountains in the eastern mediterranean sea have been studied for three major purposes 1 modes and velocities of fluid transport were derived to assess the role of upward advection and bioirrigation for benthic fluxes 2 differences in the fluid chemistry at sites of milano mud volcano olimpi area were compiled in a map to illustrate the spatial heterogeneity reflecting differences in fluid origin and transport in discrete conduits in near proximity 3 formation water temperatures of seeping fluids were calculated from theoretical geothermometers to predict the depth of fluid origin and geochemical reactions in the deeper subsurface no indications for downward advection as required for convection cells have been found instead measured pore water profiles have been simulated successfully by accounting for upward advection and bioirrigation advective flow velocities are found to be generally moderate 3 50 cm y 1 compared to other cold seep areas depth integrated rates of bioirrigation are 1 2 orders of magnitude higher than advective flow velocities documenting the importance of bioirrigation for flux considerations in surface sediments calculated formation water temperatures from the anaximander mountains are in the range of 80 to 145 c suggesting a fluid origin from a depth zone associated with the seismic decollement it is proposed that at that depth clay mineral dehydration leads to the formation and advection of fluids reduced in salinity relative to sea water this explains the ubiquitous pore water freshening observed in surface sediments of the anaximander mountain area multiple fluid sources and formation water temperatures of 55 to 80 c were derived for expelled fluids of the olimpi area

trace element characteristics of different pyrite types in mesoarchaean to palaeoproterozoic placer deposits a comparative in situ la icp ms trace element study on pyrite from three different variably auriferous archaean to palaeoproterozoic palaeoplacer deposits in the ouro fino syncline quadrilatero ferrifero brazil the elliot lake area north of lake huron se canada and several deposits within the witwatersrand basin south africa revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits especially the ni and au concentrations as well as co ni and mo ni ratios were found to be systematically different in detrital compact detrital porous and post sedimentary hydrothermal pyrite grains from different source areas high co ni ratios and low au concentrations are typical of post sedimentary pyrite which is hydrothermal in origin in contrast relatively low co ni ratios and high au contents characterise detrital porous banded and concentric pyrite grains au 1 ppm which are syn sedimentary in origin in the elliot lake area and the witwatersrand basin detrital compact rounded pyrite is characterised by high co ni ratios which is in agreement with derivation from a hydrothermal source low au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks in contrast derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the ouro fino syncline because of low to intermediate co ni ratios high au contents may indicate a genetic relationship between pyrite and gold there systematic differences exist between the three areas with respect to au ni co mo and cu distributions in detrital pyrite which reflects differences in the provenance a predominantly mafic ultramafic source is indicated for the ouro fino a felsic source for the elliot lake and a mixed felsic mafic provenance for the witwatersrand pyrite populations independently of pyrite type the higher au endowment of the studied witwatersrand and ouro fino conglomerates are also reflected by an overall higher au concentration in the respective pyrite grains compared to the relatively au poor samples from elliot lake in general a strong positive correlation between au and pb levels in the various pyrite grains is noted analogous to pb which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete pbs phases au is considered to be present mainly in the form of discrete au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice

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

implications of measured thermal gradients for water movement through the northeast japan accretionary prism thermal gradients of 24 36 c km have been found in the slope cover sediments of the accretionary prism off northeast japan deep sea drilling project dsdp sites 438a 439 and 440 these gradients lead to the prediction of temperatures in excess of 400 c at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity such temperatures conflict with the predictions of temperatures of 150 c at even greater depths from conductive thermal models and petrologic studies of low temperature blueschists at similar ancient margins one dimensional steady state conduction and advection via darcian water flow of heat up through the prism was simulated numerically the measured thermal gradients can be reconciled with temperatures less than 125 c at the prism base by water flowing upward at rates of the order of 10 10 m s such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast japan prism would flow up through a 10 km width of the prism perpendicular to the trench and would require the dsdp sites to have been located fortuitously over the localized zone s of water flow smaller velocities will reproduce the measured gradients and thus greater widths through which water flow occurs are predicted if the temperature at the prism base is higher and or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate where water flow is significant as likely wherever abundant water rich sediments are subducted thermal gradients below a few kilometers in accretionary prisms may be much lower than near surface values this has profound implications for the thermal structure of the prism off northeast japan and elsewhere

chemical and isotopic evidence for sources of fluids in a mud volcano field seaward of the barbados accretionary wedge chemical isotopic and heat budgets are controlled by fluid flow and venting at convergent margins fluids vent from accretionary complex sediments via numerous pathways including mud volcanoes which are common at the barbados convergent margin chemical and isotopic compositions of fluids collected from four mud volcanoes located east of the barbados deformation front at 13 50 n indicate venting is sufficiently rapid to prevent diffusion from seawater in sediments surrounding the mud volcanoes the rate of nonfocused fluid flow is 0 26 cm yr although this rate is 3 orders of magnitude slower than the flow from the mud volcanoes significantly more fluid vents from the sediments surrounding the mud volcanoes than from the restricted areas of rapid flow from the volcanoes chloride concentrations are depleted to a minimum of 211 mm 45 of seawater value primarily by mixing diagenetically altered pore fluids with an 18o enriched fresh water released from the dissociation of methane hydrate this reaction is accompanied by the release of sufficient methane to form a free gas phase and initiate eruption of the mud volcanoes the geochemical compositions of the diagenetically altered pore fluids reflect the interplay between five major reactions 1 clay mineral dehydration which releases h2o and influences a range of chemical and isotopic compositions 2 organic matter regeneration which increases alkalinity nh4 and br and decreases so42 concentrations 3 cation exchange with clay minerals which increases na and decreases nh4 concentrations 4 carbonate recrystallization and precipitation which decrease ca2 but increase sr2 concentrations and 5 conversion of volcanic ash to clay minerals which utilizes h2o and decreases mg2 concentrations the geochemistry of the pore fluids suggests that these reactions occurred at temperatures ranging from 75 to 115 c thus at the regional geothermal gradients of 27 to 40 c km these temperatures indicate that the source regions are at 2 to 4 5 km depth

sulfur sources of sedimentary buckshot pyrite in the auriferous conglomerates of the mesoarchean witwatersrand and ventersdorp supergroups kaapvaal craton south africa large rounded pyrite grains 1 mm commonly referred to as buckshot pyrite grains are a characteristic feature of the auriferous conglomerates reefs in the witwatersrand and ventersdorp supergroups kaapvaal craton south africa detailed petrographic analyses of the reefs indicated that the vast majority of the buckshot pyrite grains are of reworked sedimentary origin i e that the pyrite grains originally formed in the sedimentary environment during sedimentation and diagenesis forty one of these reworked sedimentary pyrite grains from the main vaal basal kalkoenkrans beatrix and ventersdorp contact reefs were analyzed for their multiple sulfur isotope compositions 34s 33s and 36s to determine the source of the pyrite sulfur in addition five epigenetic pyrite samples pyrite formed after sedimentation and lithification from the middelvlei and the ventersdorp contact reefs were measured for comparison the 34s 33s and 36s values of all 41 reworked sedimentary pyrite grains indicate clear signatures of mass dependent and mass independent fractionation and range from 6 8 to 13 8 1 7 to 1 7 and 3 9 to 0 9 respectively in contrast the five epigenetic pyrite samples display a very limited range of 34s 33s and 36s values 0 7 to 4 0 0 3 to 0 0 and 0 3 to 0 1 respectively despite the clear signatures of mass independent sulfur isotope fractionation very few data points plot along the primary archean photochemical array suggesting a weak photolytic control over the data set instead other factors command a greater degree of influence such as pyrite paragenesis the prevailing depositional environment and non photolytic sulfur sources in relation to pyrite paragenesis reworked syngenetic sedimentary pyrite grains pyrite originally precipitated along the sediment water interface are characterized by negative 34s and 33s values suggesting open system conditions with respect to sulfate supply and the presence of microbial sulfate reducers on the contrary most reworked diagenetic sedimentary pyrite grains pyrite originally precipitated below the sediment water interface show positive 34s and negative 33s values suggesting closed system conditions negligible 33s anomalies from epigenetic pyrite suggest that the sulfur was sourced from a mass dependent or isotopically homogenous metamorphic hydrothermal fluid contrasting sulfur isotope compositions were also observed from different depositional environments namely fluvial conglomerates and marine modified fluvial conglomerates the bulk of the pyrite grains from fluvial conglomerates are characterized by a wide range of 34s values 6 2 to 4 8 and small 33s values 0 3 this signature likely represents a crustal sulfate reservoir derived from either volcanic degassing or from weathering of sulfide minerals in the hinterland reworked sedimentary pyrite grains from marine modified fluvial conglomerates share similar isotope compositions but also produce a positive 33s 34s array that overlaps with the composition of archean barite suggesting the introduction of marine sulfur these results demonstrate the presence of multiple sources of sulfur which include atmospheric crustal and marine reservoirs the prevalence of the mass dependent crustal sulfur isotope signature in fluvial conglomerates suggests that sulfate concentrations were probably much higher in terrestrial settings in comparison to marine environments which were sulfate deficient however the optimum conditions for forming terrestrial sedimentary pyrite were probably not during fluvial progradation but rather during the early phases of flooding of low angle unconformities i e during retrogradational fluvial deposition coupled in some cases with marine transgressions immediately following inflection points of maximum rate of relative sea level fall

decay of polar cap patch 1 we report an event in which a polar cap patch was detected with an all sky imager asi at resolute bay canada 74 73 n 265 07 e aacgm latitude 82 9 on the nightside the patch stopped its antisunward motion associated with a northward turning of interplanetary magnetic field and stayed within the field of view of the asi for more than 1 h when the patch stagnated its luminosity decreased gradually which allows us to investigate how the patch plasma decayed in a quantitative manner the decay of the patch can be quantitatively explained by the loss through recombinations of o with ambient n2 and o2 molecules if we assume the altitude of the optical patch to be around 295 km the derived altitude of the patch around 295 km is much higher than the nominal value at 235 km obtained from the msis e90 and iri 2007 models indicating that climatological models such as iri are not suitable for describing the actual density profile of patches this is probably because the loss process was much faster in the lower altitude part of the patch thus the peak altitude of the patch increased as it traveled across the polar cap because of rapid recombination at the bottomside of the f region this suggests that we should employ higher emission altitude when we investigate optical patches transported deep into the nightside polar cap such information is important when we compare the optical data with other instruments such as coherent radars and gps scintillation measurements by mapping the all sky image on the geographic coordinate system with an assumption of the patch emission altitude

chemical and isotopic evidence of gas influenced flow at a transform plate boundary monterey bay california chemical and isotopic compositions of pore fluids document upward flow through communities of vesicomyid clams in monterey bay california within the clam communities the sulfate reduction zone is only 10 cm thick and ca and mg concentrations decrease to values as low as 2 2 mm and 34 5 mm respectively at depths less than 30 cm below the sediment water interface less than 5 m outside the communities the base of the sulfate reduction zone is deeper than the greatest penetration of the cores 30 cm and ca and mg exhibit only minor changes from seawater values the sediment exhibits no significant variation in grain size mineralogy organic carbon nitrogen or carbonate content throughout the region the composition of pore fluid within clam communities results from upward flow of altered fluid rather than different diagenetic reactions within and outside the communities isotopically light dissolved inorganic carbon dic with 13c values ranging from 3 2 to 54 1 could reflect carbon sources from either oxidized thermogenic methane and or a mixture of oxidized microbial methane and solid organic carbon the c1 c2 c3 ratios ranging from 34 to 1142 and the hydrogen and carbon isotopic compositions of methane d values of 109 to 156 13c values of 30 6 to 86 6 suggest that methane is primarily microbial but that a minor component could be thermally generated any thermogenic methane would have migrated from great depths possibly 2 km the presence of methane is likely to contribute to fluid flow by reducing the density of the fluids past fluid migration and venting are reflected by widespread carbonate mineralization at the sediment water interface this mineralization and the geographic distribution and proportions of microbial and thermogenic methane suggest that vent sites migrate when permeability is reduced during carbonate cementation these results demonstrate that along with convergent and divergent plate boundaries transform plate boundaries are characterized by fluid flow and that the flow may be widespread occurring at sites away from fault zones

stromatolitic fabric of authigenic carbonate crusts result of anaerobic methane oxidation at cold seeps in 4 850 m water depth methane seepage leads to mg calcite and aragonite precipitation at a depth of 4 850 m on the aleutian accretionary margin stromatolitic and oncoid growth structures imply encrustation of microorganisms microbial mats in the host sediment with a unique growth direction downward into the sediment forming crust shaped lithologies biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol which contain extremely low 13c values this indicates the presence of methane consuming archaea and 13c values of 42 to 51 pdb indicate that methane is the carbon source for the carbonate crusts thus it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate reducing bacteria and that carbonate precipitation proceeds downward into the sediment where ascending cold fluids provide a methane source strontium and oxygen isotope analyses as well as 14c ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3 000 years ago

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

signatures of storms oceanic floods and forearc tectonism in marine shelf strata of the quinault formation pliocene washington usa marine shelf strata of the quinault formation reflect the influences of storm flood processes and convergent margin tectonism on sedimentation and palaeocommunity distributions in an active forearc basin of early pliocene age western washington usa the sedimentologic ichnologic and invertebrate megafaunal character of coastal sea cliff exposures in the pratt cliff duck creek area quinault indian nation reveal five different sedimentary facies scoured rosselia bioturbated mixed and acharax these facies document the shifting interplay and intensities among storms waves and river flood plumes during transgression in inner to mid shelf settings storm sedimentation on the inner shelf is recorded north of pratt cliff by amalgamated proximal tempestites of the scoured facies which grade up section to thick deposits of hummocky cross stratified sandstone indicative of strong wave influences these hummocky beds alternate in metre scale packages with banded mudstone and siltstone that have distinctive sedimentologic and ichnofaunal characteristics rosselia facies in particular the mudstone and siltstone occur as 1 15 cm thick rhythmic parallel beds that are laterally continuous internally homogeneous to faintly laminated and thus similar in nature to fine grained oceanic flood deposits reported from shelf settings offshore the modern eel river northern california the quinault flood deposits are dominated by the ubiquitous trace fossil rosselia socialis comprising vertical mud packed flaring burrows with a sand filled central shaft which has been inferred as the feeding dwelling structure of a vermiform invertebrate adapted to high sedimentation rates in inner shelf settings fairweather conditions in between the higher energy periods of storms waves and floods are recorded north of pratt cliff by the mixed facies which is interpreted as representing the sand and mud zone of the inner to mid shelf transition quieter deeper mid shelf fairweather settings are typified by the bioturbated facies south of pratt cliff where lower sedimentation rates and lower physical energies produced extensively bioturbated deposits of sandy siltstone punctuated in places by isolated sandy beds of distal tempestites quinault strata also chronicle stratigraphic signatures of subduction of the juan de fuca plate beneath western washington during the pliocene for example the imprint of geochemically unusual authigenic carbonates and a chemosynthetic palaeocommunity acharax facies have been interpreted as a methane seep on the quinault seafloor furthermore a mobile rockground epifauna of pholadid bivalves became established on abundant dark mudstone cobbles and pebbles sourced from the hoh assemblage a miocene accretionary prism that was actively deforming as well as interacting with quinault forearc sediments during the pliocene hoh mudstone clasts were supplied to the quinault shelf via seafloor piercing diapirs and eroding melange shear zones exposures of which today occur in fault contact with quinault strata along the coast from taholah to the raft river

storm time plasma transport at middle and high latitudes associated with the large scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions solar produced f region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector as a result of the offset between the geomagnetic and geographic poles the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 ut and 24 ut a snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge producing a latitudinally narrow region of storm enhanced plasma density sed and increased total electron content which is advected toward higher latitudes in the noon sector the millstone hill incoherent scatter radar regularly observes sed as a spatially continuous large scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid or low latitude source region for local times away from noon the latitude of most probable sed occurrence moves equatorward by 6 for an increase of 2 in the kp index during strong disturbances the topside sed is observed to be convecting sunward at 750 m s 1 with a flux of 1014 m 2 s 1 this feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms called the dusk effect and constitutes a source for the enhanced f region plasma observed in the polar cap during disturbed conditions

the daytime f layer trough and its relation to ionospheric magnetospheric convection the daytime f layer trough is studied by means of an extensive network of ground based ionospheric sounders in the northern hemisphere under conditions of solar maximum near winter solstice the trough is observed to be a continuous band having an instantaneous extent of thousands of kilometers consisting of depletions in the daytime electron density often by an order of magnitude it lies in regions of sunward ionospheric magnetospheric convection an afternoon sector corresponding to the dusk cell a morning sector corresponding to the dawn cell and morphology and activity dependence consistent with convection as detected in the diurnal distributions of f0f2 the trough is a persistent feature at high latitudes appearing on each day of a 31 day period of continuous observation and although highly variable from day to day is apparent in the monthly medians the afternoon trough which is detected independently by at least five and as many as 17 stations on each day is generally continuous and stationary for a duration of many hours in magnetic latitude magnetic local time coordinates the trough contracts during quiet conditions so as to lie above 70 magnetic latitude but expands during disturbed conditions so as to extend from 75 to 52 magnetic latitude the trough has a pronounced dependence on longitude appearing principally in the afternoon in eastern magnetic longitudes but in the morning in western magnetic longitudes an effect so prevalent that it produces large east west local time asymmetries in the diurnal distributions of median daytime f layer electron densities throughout a wide range of latitudes the longitudinal dependence is found to result from the relation between the two principal coordinate systems of the ionosphere magnetosphere interaction solar geomagnetic coordinates in which the convection pattern and the resultant daytime trough reside and solar terrestrial coordinates in which solar ion production and the undisturbed daytime f layer in general reside as a consequence of the fact that these coordinate systems vary with respect to one another with longitude the trough varies within the daytime f layer with longitude

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

large plasma density enhancements occurring in the northern polar region during the 6 april 2000 superstorm we focus on the ionospheric response of northern high latitude region to the 6 april 2000 superstorm and aim to investigate how the storm enhanced density sed plume plasma became distributed in the regions of auroral zone and polar cap plus to study the resultant ionospheric features and their development multi instrument observational results combined with model generated two cell convection maps permitted identifying the high density plasma s origin and the underlying plasma transportation processes results show the plasma density feature of polar cap enhancement pce 600 103 i cm3 appearing for 7 h during the main phase and characterized by increases reaching up to 6 times of the quiet time values meanwhile strong westward convections 17 500 m s created low plasma densities in a wider region of the dusk cell oppositely small 750 m s but rigorous westward drifts drove the sed plume plasma through the auroral zone wherein plasma densities doubled as the sed plume plasma traveled along the convection streamlines and entered the polar cap a continuous enhancement of the tongue of ionization toi developed under steady convection conditions however convection changes caused slow convections and flow stagnations and thus segmented the toi feature by locally depleting the plasma in the affected regions of the auroral zone and polar cap from the strong correspondence of polar cap potential drop and subauroral polarization stream saps we conclude that the saps e field strength remained strong and under its prolonged influence the sed plume provided a continuous supply of downward flowing high density plasma for the development and maintenance of pces

large scale imaging of high latitude convection with super dual auroral radar network hf radar observations the hf radars of the super dual auroral radar network superdarn provide measurements of the e b drift of ionospheric plasma over extended regions of the high latitude ionosphere with the recent augmentation of the northern hemisphere component to six radars a sizable fraction of the entire convection zone approximately one third can be imaged nearly instantaneously 2 min to date the two dimensional convection velocity has been mapped by combining line of sight velocity measurements obtained from pairs of radars within common volume areas we describe a new method of deriving large scale convection maps based on all the available velocity data the measurements are used to determine a solution for the distribution of electrostatic potential expressed as a series expansion in spherical harmonics the addition of data from a statistical model constrains the solution in regions of no data coverage for low order expansions the results provide a gross characterization of the global convection we discuss the processing of the radar velocity data the factors that condition the fitting and the reliability of the results we present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field imf in the case of a sudden polarity change from northward to southward imf the convection is seen to reconfigure globally on very short 6 min timescales

formation of methane related authigenic carbonates in a highly dynamic biogeochemical system in the krishna godavari basin bay of bengal abstract we report the abundant occurrence of authigenic fe rich carbonate high mg calcite hmc and low mg calcite from 11 cores recovered from the krishna godavari basin k g basin bay of bengal the cores were collected as part of the indian gas hydrate exploration program on board r v marion dufresne md 161 may 2007 in different environments including mounds mud diapirs mass flows and hemipelagic sediments over a range of water depths from 647 to 2079 m authigenic carbonates range in size from 1 mm to 12 cm and display various morphologies like roundish or platy micro nodules and tube like forms from the cores 173 carbonate samples have been investigated for their depth distribution mineralogy geochemical and stable isotopic composition the stable carbon isotopic composition of 46 out of 88 measured carbonate samples are around 50 which allows the differentiation into methane related carbonates hmc especially at sites 8 and 15 but also in low abundance at sites 1 5 9 and 12 results indicate that the carbonates at site 8 and 15 represent paleo methane seepage locations the fe rich carbonates occur abundantly at many sites in the k g basin their varying carbon isotopic composition indicates that probably not only sulfate reduction through organic matter degradation but also methanogenesis are the responsible processes for their formation

sedimentological and mineralogical aspects of the newly discovered witwatersrand placer deposit that reflect proterozoic weathering welkom gold field south africa the eldorado paleosurface marks the last major period of erosion in the witwatersrand succession in the welkom gold field fluvial and debris flow sediments on this surface contain placer concentrates that were ultimately derived by the repeated erosion of subcropping placers which occur within a sequence of onlapping formations separated by unconformities near the southern margin of the basin autoradiographs of core recovered by drilling through the eldorado paleosurface indicate that uranium mineralization is associated with detrital pyrite and gold accumulations this mineralization occurs as fluvial bed load concentrates that were deposited in shallow paleochannels and with conglomeratic diamictites comparison of heavy mineral assemblages in crushed concentrates of the subcropping placers with those in the placer alluvium on the eldorado pediment indicates that the suite of heavy minerals is the same all three varieties of pyrite show detrital rounding including secondary pyrite which originally formed in situ in the older placers kerogen particles appear to be rounded allogenic grains of broken columnar kerogen derived from erosion of older placers uraninite is very sparse and inclusions found in the rounded kerogen grains have been largely altered to brannerite the dominant uranium bearing minerals are uraniferous leucoxene and brannerite and various stages of alteration from uraninite to brannerite are evident the rarity of uraninite indicates that most of the grains of that mineral were altered during reworking and prolonged exposure in a slightly oxidizing atmosphere

tectonic significance of the cold seepage zones in the eastern nankai accretionary wedge an outcome of the 15 years kaiko projects abstract the tectonic significance of the cold fluid venting in the eastern nankai accretionary wedge has been elucidated based upon results of precise mapping and submersible observation executed since 1984 under the french japanese cooperative projects kaiko i ii kaiko nankai and kaiko tokai in conjunction with studies of the japan trench sagami bay and other regions the very sharp boundary of benthic communities associated with temperature anomalies indicates the occurrence of focused upflow of nutrient rich fluids through cracks in the carbonate crust existing on a near surface horizon of permeable sediment the linear alignment of venting zones is a surface expression of the shallow detachment faults methane rich fluids expelled from the conduit along a fault migrate in the overlying permeable sediment and react with circulating sea water containing sulfate to produce hydrogen sulfide and carbonate precipitate en echelon or zigzag patterns of the cracks are developed in the carbonate crust under the influence of tectonic shear stress it has been concluded that bivalve communities grow on a thin veneer of sediment above the cracks of carbonate through which fluid containing hydrogen sulfide diluted with sea water flows out on the other hand bacterial mats seem to flourish either on thin sediment overlying the fault conduit or over a thick cover of sediment without carbonate crust in the former case bivalves cannot grow because venting fluids are too dense with methane and hydrogen sulfide to maintain the oxygen rich condition of the bottom water in the western nankai trough fine grained less permeable sediment cover seems to prevent circulation of fluids whereas methane appears to be widespread in the deeper sedimentary layer there major distinctions in modes of seepage among sites are presumed to be caused by 1 the tectonic regime causing channels of upward fluid venting 2 rates and composition of venting fluids affected by mixing rates of sea water and 3 the permeability and thickness of the near bottom sediment

controls on authigenic carbonate precipitation at cold seeps along the convergent margin off costa rica five sediment cores from cold seeps at the forearc off costa rica were used to explore the relationship between fluid advection dissolved ca concentrations in upward migrating fluids and authigenic caco3 precipitation a numerical transport reaction model was used to determine rates of anaerobic oxidation of methane aom caco3 precipitation and benthic fluxes of solutes production of carbonate alkalinity and formation of authigenic carbonates is most intense at intermediate flow rates 3 40 cm a 1 and reduced under low and high flux conditions 0 1 and 200 cm a 1 dissolved ca concentrations observed in the vent fluids off costa rica cover a wide range between 4 and 31 mm clearly exceeding seawater concentrations at two locations systematic model runs showed that high ca concentrations in ascending fluids enhance the rate of authigenic carbonate production at moderate flow rates leading to an almost quantitative fixation of deeply derived ca in authigenic carbonates hence caco3 precipitation is not only controlled by ca diffusing into the sediment from bottom water but also by the ca concentration in ascending fluids thus ca enriched fluids offer a reason for enhanced subsurface caco3 precipitation and the occurrence of carbonate caps on dewatering structures in the central american fore arc based on average precipitation rates deduced from the systematic model runs it is possible to give a rough estimate of the global ca fixation at cold seeps 2 1010 mol ca a 1 which suggests that cold seeps are most likely not of key importance with respect to ca cycling in the ocean

simulations of internal solitary wave interactions with mesoscale eddies in the northeastern south china sea abstractwith the combined analysis of synthetic aperture radar image and satellite altimeter data collected in the northeastern south china sea scs this study found one type of distorted phenomenon of internal solitary wave isw with the long front caused by the oceanic mesoscale eddy motivated by these satellite observations the authors carried out numerical experiments using the fully nonhydrostatic and nonlinear mitgcm to investigate the perturbation of isws by an isolated cyclonic or anticyclonic eddy the results show that the isw front is distorted by these oceanic eddies due to the retardation and acceleration effects at their two sides the isw energy along the front is focused onto scattered from the wave fragment where a concave convex pattern is formed and the previously accumulated energy in the focusing region is gradually released after the isw propagates away from the eddies the isw amplitude is modulated greatly by the eddies due to the energy redistribution along the front se

geological evidence for mud diapirism on the mediterranean ridge accretionary complex abstract a mud breccia has been repeatedly cored from four mud diapir fields located on the crest of the mediterranean ridge this breccia is composed of subrounded clasts supported by a clay rich matrix and ranges in age from cretaceous to early miocene sedimentological analysis allows identification of a primitive intrusive and a reworked mud breccia the latter being formed by mud extrusion and gravitational reworking of the former homogeneous mud sorted by grain size which occurs at the top of the diapirs has been interpreted as a lake deposit based on composition and carbon isotopic ratios gases present within the diapiric material suggest a partial thermogenic origin and gas escape structures have been identified in the mud breccia and host sediments carbonate crusts occurring in the host sediments are not related to bacterial oxidation of methane and consequent authigenic calcite precipitation their carbon and oxygen isotopic ratios reveal that they are characteristic of early deep sea lithification in the mediterranean sea the presence of a marker bed composed of mn nodules and bacterial colonies the mud lake deposits showing lower carbonate content than primitive mud breccia and the gas escape structures suggest the presence of active fluid vents in the vicinity of the diapiric structures evidence of active mud diapirism can be found from about 300 000 yrs b p to the present

major cation gradients in dsdp interstitial waters the role of diffusive exchange between seawater and upper oceanic crust abstract interstitial water analyses from dsdp legs 1 53 were surveyed to identify sites at which the dominant process supporting gradients of major seawater cations is alteration of the underlying oceanic crust sites with thin sediment cover are generally without gradients sites with appreciable sediment accumulation are generally with gradients the transition is probably the result of cessation of convective circulation within the sediments a model is developed for the diffusive flux through an accumulating unreactive sedimentary layer supported by continued reaction within the crust the model is consistent with the observations provided that convection within the crust extends to depths 1 km and that convective flow within the sediments ends once 100 150 m of sediment accumulates the model suggests a steady state gradient inversely proportional to the sedimentation rate this dependence is apparent in the data however some variation in the reaction rate is also required fluxes of mg and na are systematically related to the flux of ca which because of its mobility serves as a tracer of hydrolysis of the basalt when the ca flux is low the mg flux serves as the principal sink for alkalinity produced during basalt hydrolysis when the ca flux is high the mg flux becomes diffusion limited and nabearing alteration phases must form the fluxes resulting from these alteration reactions exert little influence on either the chemistry of seawater or that of the upper crust

interstitial water chemistry of sediments of the costa rica accretionary complex off the nicoya peninsula interstitial water analyses from numerous piston gravity and alvin push cores show that fluid flow at the costa rica accretionary prism is spatially limited and can only be detected by visually directed cores in zones of biogenic activity most of the sites cored show evidence for normal diagenetic processes in rapidly deposited organic carbon rich sediments with little evidence for fluid advection however in visually directed alvin push cores obtained from black sulfidic sediments characterized by the presence of calyptogena clams or tubeworms evidence for fluids advected upward from greater depth horizons is shown these zones are associated with a large mudvolcano on the prism and with the zoes of out of sequence thrusts as changes from sea water concentrations are still relatively small substantial mixing with sea water must have occurred during this upward fluid movement

empirical high latitude electric field models electric field measurements from the dynamics explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn dusk ogo 6 measurements j p heppner 1977 the analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones paralleling the previous analysis the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the y component of the interplanetary magnetic field the objective which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns is met by deriving one pattern model bc for the northern hemisphere with a y interplanetary magnetic field imf and southern hemisphere with a y imf and two patterns models a and de for the northern hemisphere with a y imf and southern hemisphere with a y imf the most significant large scale revisions of the ogo 6 models are 1 on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the imf y component 2 on the nightside where a westward flow region poleward from the harang discontinuity appears under model bc conditions and 3 magnetic local time shifts in the positions of the convection cell foci the modeling above was followed by a detailed examination of cases where the imf z component was clearly positive northward neglecting the seasonally dependent cases where irregularities obscure pattern recognition the observations range from reasonable agreement with the new bc and de models to cases where different characteristics appeared primarily at dayside high latitudes the analysis of these differences for strong bz conditions using passes through different mlt zones led to finding that the deviations from both the bc and de models could be modeled by extending the evening cell foci toward and beyond noon with a deformation that roughly resembles a rotational twist of the extended evening cell the morning cell accommodation to the model bc deformation produces a narrow tongue of eastward convection near 75 invariant latitude on the dayside the model de deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 mlt sector in both cases the deformations of the two cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern thus the nightside dilemmas that plague three and four cell models designed to explain sunward convection in polar regions under bz conditions do not appear

events of enhanced convection and related dayside auroral activity in this paper we study the high latitude plasma flow variations associated with a periodic 8 min sequence of auroral forms moving along the polar cap boundary which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field satellite data on auroral particle precipitation and ionospheric plasma drifts from dmsp f10 and f11 are combined with ground based optical and ion flow measurements for january 7 1992 ionospheric flow measurements of 10 s resolution over the range of invariant latitudes from 71 to 76 were obtained by operating both the european incoherent scatter eiscat uhf and vhf radars simultaneously the optical site ny alesund svalbard and the eiscat radar field of view were located in the postnoon sector during the actual observations the west greenland magnetometers provided information about temporal variations of high latitude convection in the prenoon sector satellite observations of polar cap convection in the northern and southern hemispheres show a standard two cell pattern consistent with a prevailing negative by component of the interplanetary magnetic field the 630 0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the 1440 1540 mlt sector were observed to coincide with magnetosheath like particle precipitation and a secondary population of higher energy ions and they propagated eastward tailward at speeds comparable with the convection velocity it is shown that these optical events were accompanied by bursts of sunward return flow at lower latitudes in both the morning and the afternoon sectors consistent with a modulation of dungey cell convection the background level of convection was low in this case kp 2 the variability of the high latitude convection may be explained as resulting from time varying reconnection at the magnetopause in that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection