determination of accurate and precise chromium isotope ratios in seawater samples by mc icp ms illustrated by analysis of safe station in the north pacific ocean abstract although the cycling of chromium cr in the ocean may have direct implications on the cr isotope system s application as a paleo redox proxy little seawater cr isotope data has been published this may be partly due to the analytical challenges associated with the preconcentration and the purification of the nano molar trace metal cr from the seawater matrix here we present a reliable method for the determination of accurate and precise 53cr 52cr isotope ratios of total chromium cr iii cr vi in seawater which could be modified to be redox species specific seawater acidification induces the slow conversion of cr vi to cr iii total cr of the sample and an equilibrated 50cr 54cr double spike is preconcentrated as cr iii using mg oh 2 coprecipitation cr captured by the mg oh 2 pellet is oxidized to cr vi using nh4 2s2o8 under carefully controlled ph temperature and time settings in order to avoid h2o2 generation and to control sulfur speciation taking advantage of the differing charges of cr iii and cr vi three ag 1 x8 columns separate cr from the salt matrix and isobaric interferences v ti and fe column 1 sulfur polyatomic interferences column 2 and fe traces column 3 analysis is conducted on a mc icp ms isoprobe featuring a hexapole collision cell in low resolution mode when pure solutions of srm 979 are processed via this low blank method 0 017 nmol of cr 53cr w r t srm 979 0 02 0 06 2 sd is obtained n 10 using this technique the first full water column profile of cr isotope ratios at a station in the pacific ocean safe station 30 n 140 w was generated and demonstrates high oceanographic consistency this article is part of a special issue entitled cycles of trace elements and isotopes in the ocean geotraces and beyond edited by tim m conway tristan horner yves plancherel and aridane g gonzalez

the hafnium isotope composition of pacific ocean water the first hf isotope data for seawater are reported for a series of stations in the northwestern pacific and define a range from epsilon hf 3 5 1 4 to 8 6 1 6 most samples have values within error of the average of epsilon hf 5 9 but significant variations are found in intermediate waters at a depth of 600 m as well as in deep waters the nd and hf isotope compositions of the deep waters fall within the range of values found for surfaces of hydrogenetic ferromanganese crusts in the region confirming that hf in the fe mn crusts has been derived from the overlying water column which thus provide an archive of past seawater compositions although the seawater samples are generally close to the global epsilon nd epsilon hf correlation obtained from ferromanganese crusts there are significant deviations from this correlation indicating that there is some additional decoupling between nd and hf isotope signals most likely caused by local water mass mixing and differences in residence times this is not resolved in the crust samples which integrate seawater signals over 10 4 years the combined use of these two isotope systems in seawater therefore provides an additional dimension for tracing water masses in the oceans studies of the distribution of oceanic hf isotope compositions that have been confined to deep water and boundary waters as recorded in seafloor ferromanganese crusts can now be extended and aimed at characterising the entire present day water column average hf concentrations measured in this study are somewhat lower than previously reported suggesting a shorter residence time for hf in the global oceans although the uncertainty in the extent of hf removal from the water column during estuarine mixing as well as a lack of data on hydrothermal and dust inputs remains a limit on how well the residence time can be defined c 2008 elsevier ltd all rights reserved

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

molybdenum isotope fractionation in pelagic euxinia evidence from the modern black and baltic seas here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the black sea and two deeps of the baltic sea dissolved molybdenum in all water samples is depleted in salinity normalized concentration and enriched in the heavy isotope 98mo values up to 2 9 compared to previously published isotope data of sedimentary molybdenum from the same range of water depths furthermore 98mo values of all water samples from the black sea and anoxic deeps of the baltic sea are heavier than open ocean water the observed isotope fractionation between sediments and the anoxic water column of the black sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions an extrapolation to a theoretical pure mos42 solution indicates a fractionation constant between mos42 and authigenic solid mo of 0 5 0 3 measured waters with all thiomolybdates coexisting in various proportions show larger but non linear fractionation the best explanation for our field observations is mo scavenging by the thiomolybdates dominantly but not exclusively present in the form of mos42 the mo isotopic compositions of samples from the sediments and anoxic water column of the baltic sea are in overall agreement with those of the black sea at intermediate depth and corresponding sulphide concentrations the more dynamic changes of redox conditions in the baltic deeps complicate the black sea derived relationship between thiomolybdates and mo isotopic composition in particular the occasional flushing mixing of the deep waters affects the corresponding water column and sedimentary data 98mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations the results can be explained by non conservative behaviour of mo under suboxic to anoxic conditions in the shallow bottom parts of the basin most pronounced on the nw shelf of the black sea

molybdenum in the northeast pacific ocean1 the concentration of dissolved molybdenum has been determined in several profiles from the northeast pacific ocean the salinity normalized concentration is essentially constant in all open ocean samples 107 nm 1 2 5 nm the dissolved distribution is not significantly affected by biological cycling

molybdenum isotopes in hydrothermal manganese crust from the ryukyu arc system implications for the source of molybdenum abstract a high concentration of molybdenum mo is a striking feature of modern marine hydrothermal manganese mn crusts from both low and high temperature hydrothermal systems however the origin of that mo is poorly constrained in this study we analyzed mo isotopic composition 98 95mo in a mn crust collected from the ryukyu arc system and assessed the possible use of 98 95mo to constrain the origin of mo in hydrothermal mn crusts along with mo isotope analysis we also measured the abundance of major and trace elements and the re os isotopic composition the mn crust sample had a high mn content 42 47 and was low in fe 0 2 0 5 among the manganophile elements co ni cu zn and mo only mo showed high concentrations 550 ppm we also found low contents of os 35 ppt with an isotopic composition of 187os 188os 1 00 these results are consistent with previously reported chemical compositions of hydrothermal mn crusts 98 95mo values in the sample ranged from 0 56 to 0 66 which are 2 7 lighter than the present day seawater 98 95mo but similar to those in modern hydrogenous fe mn crusts and mn nodules although current data do not preclude the possible contribution of hydrothermally derived mo the light 98 95mo values can be explained by isotope fractionation associated with a change in coordination number during the adsorption of mo from seawater onto mn oxides we suggest that the 98 95mo data are useful for constraining the source of mo in hydrothermal mn crusts

causality between interplate north atlantic and intraplate fennoscandia seismicities abstract annual seismic energy release patterns have been calculated for the time period 1917 1987 for fennoscandia and the north atlantic ridge nar between 40 n and 80 n a brief visual comparison shows the great similarity between the energy release patterns in fennoscandia and individual segments along the nar to express the observed resemblance quantitatively and to provide an objective test of the phenomenon systematic cross correlations between seismicities along various nar segments and in fennoscandia were carried out the cross correlations obtained are statistically significant and show maximum values for time lags mostly between 0 and 3 years negative time lags mean that the energy release in the nar precedes that in fennoscandia the closest similarity is found between fennoscandia and the ridge segment between iceland and jan mayen 66 71 n periodicities of 8 13 and 38 years are seen in the energy release from both regions the significant cross correlations and consistent periodicities in the temporal patterns of seismic energy release along the nar and in fennoscandia serve as an indication of a tectonic connection between the interplate nar and intraplate fennoscandia seismicities

barium isotopic fractionation in seawater mediated by barite cycling and oceanic circulation the marine biogeochemical cycle of ba is thought to be controlled by particulate baso4 barite precipitation associated with the microbial oxidation of organic carbon and its subsequent dissolution in the baso4 undersaturated water column despite many of these processes being largely unique to ba cycling concentrations of ba and si in seawater exhibit a strong linear correlation the reasons for this correlation are ambiguous as are the depth ranges corresponding to the most active baso4 cycling and the intermediate sources of ba to particulate baso4 stable isotopic analyses of dissolved ba in seawater should help address these issues as ba isotopic compositions are predicted to be sensitive to the physical and biogeochemical process that cycle ba we report a new methodology for the determination of dissolved ba isotopic compositions in seawater and results from a 4500 m depth profile in the south atlantic at 39 99 s 0 92 e that exhibit oceanographically consistent variation with depth these data reveal that water masses obtain their ba and ba isotopic signatures when at or near the surface which relates to the cycling of marine baso4 the shallow origin of these signatures requires that the substantial ba isotopic variations in the bathypelagic zone were inherited from when those deep waters were last ventilated indeed the water column below 600 m is well explained by conservative mixing of water masses with distinct ba and ba isotopic compositions this leads us to conclude that large scale oceanic circulation is important for sustaining the similar oceanographic distributions of ba and si in the south atlantic and possibly elsewhere these data demonstrate that the processes of organic carbon oxidation baso4 cycling and ba isotopic fractionation in seawater are closely coupled such that ba isotopic analyses harbor great potential as a tracer of the carbon cycle in the modern and paleo oceans

tungsten stable isotope compositions of terrestrial samples and meteorites determined by double spike mc icpms abstract tungsten stable isotopes hold great potential to examine a variety of physical and chemical processes operating during the accretion and differentiation of asteroids and terrestrial planets such as core formation and mantle crust differentiation to assess the magnitude and origin of w isotope fractionations we determined the w stable isotopic compositions of six usgs geological reference materials a nist steel and 24 iron meteorites and chondrites the w isotope data were obtained using multi collector inductively coupled plasma mass spectrometry mc icpms and using a 180 w 183 w double spike chondrites and iron meteorites exhibit a very narrow range in w stable isotope compositions resulting in a mean 184 183 w 0 027 0 007 95 conf relative to the nist 3163 w standard this value represents a good estimate for the w stable isotope composition of bulk planetary bodies from the inner solar system the 184 183 w of some iron meteorites slightly deviates from this value most likely due to w isotope fractionations induced during crystallization of the metal cores of iron meteorite parent bodies the investigated terrestrial silicate rocks exhibit a narrow range in 184 183 w which for most samples is indistinguishable from the mean value of chondrites and iron meteorites however felsic samples tend to be isotopically lighter than mafic samples indicating that magmatic processes on earth induced w isotope fractionations these fractionations are possibly related to the fluid mobility of w in subduction zones but more data are needed to test this hypothesis given that most terrestrial igneous rocks are isotopically indistinguishable from chondrites and iron meteorites core formation on earth does not seem to have induced a measurable isotopic fractionation for w however more data are needed to firmly arrive at this conclusion

mass dependent cadmium isotopic variations in nature with emphasis on the marine environment abstract we report a survey of natural mass dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double spike technique ds tims over sixty samples of natural terrestrial cd from diverse environments including morb oib continental loess hydrogenic and hydrothermal ferromanganese deposits and sphalerites both oceanic and from major continental ore deposits were analysed our results are expressed in terms of e 112 110 cd which are deviations in 112 cd 110 cd from our in house jmc cd standard in parts per 10 4 the total e 112 110 cd variation is relatively small with a range of only 5 e units and is one to two orders of magnitude smaller than that previously found in meteorites the morb oib and loess e 112 110 cd values are similar and provide a good estimate for the bulk silicate earth bse value which is 0 95 0 12 relative to our cd standard e 112 110 cd 0 16 relative to munster jmc cd taken together these data suggest little cd isotope fractionation takes place during crust mantle segregation cd isotopic compositions of continental sphalerite zns deposits worldwide and high temperature oceanic hydrothermal sulphides show remarkably similar e 112 110 cd values consistent with our estimate for the bse in contrast mid temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 e units variation along with the most negative values these variations are most probably caused by precipitation redissolution of sulphide phases en route within the hydrothermal system the e 112 110 cd variability found in worldwide marine fe mn deposits reflects the seawater cd isotope signal upon precipitation from ambient seawater a decrease in e 112 110 cd is observed in passing from shallow water fe mn deposits to those from deeper waters 2000 m depth this shift is explained by biological fractionation related to the uptake of dissolved seawater cd by phytoplankton in the upper water column the relatively uniform e 112 110 cd values close to zero at great depths are consistent with regeneration and remineralization of cd at depth our data suggest that cd isotopes much like the cd ca ratio in foraminifera could potentially serve as a proxy for past changes in biological productivity the temporal cd isotope record in a fe mn crust archive at 2000 m depth from the ne atlantic suggests no gross long term changes in cd cycling took place over the past 8 ma

sulfidity controls molybdenum isotope fractionation into euxinic sediments evidence from the modern black sea molybdenum mo isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry it is therefore fundamental to understand the processes controlling mo partitioning into modern marine environments this study identifies the availability of dissolved sulfide as the dominant control on overall mo removal from the water column in euxinic systems mo isotopic composition of surface sediments from different localities of the black sea demonstrates complete fixation of mo only below 400 m water depth above a critical concentration of 11 mol l 1 aqueous hydrogen sulfide in the bottom water the mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2 3 in contrast significant mo isotope fractionation into less euxinic sediments is evident at shallower depths in the black sea as well as in temporarily euxinic deeps of the baltic sea consistent with the observed lower maximum sulfide concentrations in the respective water columns therefore mo isotope signatures in the modern black sea constrain the processes responsible for global mo removal from the ocean by euxinic sediments furthermore models of past ocean anoxia reconstruction have to consider that the seawater mo isotopic composition is not per se archived in euxinic sediments

tungsten enriched in submarine hydrothermal fluids abstract here we report the first data for w in hydrothermal vent fluids in the deep oceans vented hydrothermal fluids were collected from the kairei field a mid ocean ridge hydrothermal field at the rodriguez triple junction central indian ridge and from arc backarc hydrothermal systems at the suiyo seamount in the izu bonin arc north pacific ocean and at the hatoma and yonaguni knolls in the okinawa trough east china sea while the dissolved w concentration in hydrothermal fluids linearly increased with a decrease in the mg concentration for each system the w concentration in endmember fluids was very different it was 0 21 nmol kg at the kairei field 15 nmol kg at the suiyo seamount and 123 nmol kg at the hatoma knoll which was 4 orders of magnitude above the ambient level in seawater the w concentration was not a simple function of cl alkalinity b and nh 4 the hydrothermal fields are efficiently enriched with w through reaction with fractionated calc alkaline dacite and with terrigenous sediments although mo is a chemical analogue of w in oxic seawater the mo concentration decreased in the hydrothermal fluids to 2 7 nmol kg probably due to precipitation of mo sulfide

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

sedimentary molybdenum cycling in the aftermath of seawater inflow to the intermittently euxinic gotland deep central baltic sea abstract molybdenum mo concentrations and isotope compositions in sediments and shales are commonly used as proxies for anoxic and sulfidic i e euxinic conditions in the water column of paleo marine systems a basic assumption underlying this practice is that the proxy signal extracted from the geological record is controlled by long term order of decades to millennia mo scavenging in the euxinic water column rather than mo deposition during brief episodes or events order of weeks to months to test whether this assumption is viable we studied the biogeochemical cycling of mo and its isotopes in sediments of the intermittently euxinic gotland deep in the central baltic sea here multiannual to decadal periods of euxinia are occasionally interrupted by inflow events during which well oxygenated water from the north sea penetrates into the basin during these events manganese mn oxyhydr oxide minerals are precipitated in the water column which are known to scavenge mo we present sediment and pore water mo and mo isotope data for sediment cores which were taken before and after a series of inflow events between 2014 and 2016 after seawater inflow pore water mo concentrations in anoxic surface sediments exceed the salinity normalized concentration by more than two orders of magnitude and coincide with transient peaks of dissolved mn a fraction of the mo liberated into the pore water is transported by diffusion in a downward direction and sequestered by organic matter within the sulfidic zone of the sediment diffusive flux calculations as well as a mass balance that is based on the sedimentary mo isotope composition suggest that about equal proportions of the mo accumulating in the basin are delivered by mn oxyhydr oxide minerals during inflow events and mo scavenging with hydrogen sulfide during euxinic periods since the anoxic surface sediment where mo is released from mn oxyhydr oxides are separated by several centimeters from the deeper sulfidic layers where mo is removed the solid phase record of mo concentration and isotope composition would be misinterpreted if steady state mo accumulation was assumed based on our observations in the gotland deep we argue that short term redox fluctuations need to be considered when interpreting mo based paleo records

determination of the tungsten isotope composition in seawater the first vertical profile from the western north pacific ocean abstract the stable isotope ratio of w is a new tracer in oceanographic studies and a new proxy in paleoceanographic studies however precise data for modern seawater have not been reported to date because the concentration of w in seawater is as low as 49 pmol kg 1 an 3000 fold preconcentration is necessary prior to measurement by multicollector inductivity coupled plasma mass spectrometry mc icp ms for the preconcentration we investigated solid phase extraction using chelating resins namely nobias chelate pa1 with ethylenediaminetriacetic acid groups and tsk 8hq with 8 hydroxyquinolie groups we report that tsk 8hq is useful because the effects of the seawater matrix are minor thermodynamically and kinetically we present a novel method for analysis of the concentrations and isotope ratios of w and mo in seawater consisting of solid phase extraction chromatographic separation using anion exchange resin ag1 x8 and measurement by mc icp ms both w and mo are quantitatively recovered by this method which was applied to seawater samples collected from the north pacific ocean the measured concentration of w and the concentration and isotope ratio of mo are consistent with those in the literature the isotope ratio of w is found to be uniform throughout the water column in the western north pacific ocean 186 184w is 0 55 0 12 ave 2sd n 7 using nist srm 3163 as a reference for w on the basis of this data we determined that the isotopic difference in 186 184w is 0 49 between seawater and oxic sediments in the modern ocean this value accords with the reported experimental data for the isotope fractionation of w during adsorption on manganese and iron oxyhydr oxides suggesting the validity of our data

the isotope composition of inorganic germanium in seawater and deep sea sponges abstract although dissolved concentrations of germanium ge and silicon si in modern seawater are tightly correlated uncertainties still exist in the modern marine ge cycle germanium stable isotope systematics in marine systems should provide additional constraints on marine ge sources and sinks however the low concentration of ge in seawater presents an analytical challenge for isotopic measurement here we present a new method of pre concentration of inorganic ge from seawater which was applied to measure three ge isotope profiles in the southern ocean and deep seawater from the atlantic and pacific oceans germanium isotopic measurements were performed on ge amounts as low as 2 6 ng using a double spike approach and a hydride generation system coupled to a mc icp ms germanium was co precipitated with iron hydroxide and then purified through anion exchange chromatography results for the deep i e 1000 m depth pacific ocean off hawaii nearby loihi seamount and the deep atlantic off bermuda bats station showed nearly identical 74 70ge values at 3 19 0 31 2sd n 9 and 2 93 0 10 2sd n 2 respectively vertical distributions of ge concentration and isotope composition in the deep southern ocean for water depth 1300 m yielded an average 74 70ge 3 13 0 25 2sd n 14 and ge si 0 80 0 09 mol mol 2sd n 12 significant variations in 74 70ge from 2 62 to 3 71 were measured in the first 1000 m in one station of the southern ocean near sars seamount in the drake passage with the heaviest values measured in surface waters isotope fractionation by diatoms during opal biomineralization may explain the enrichment in heavy isotopes for both ge and si in surface seawater however examination of both oceanographic parameters and 74 70ge values suggest also that water mass mixing and potential contribution of shelf derived ge also could contribute to the variations combining these results with new ge isotope data for deep sea sponges sampled nearby allowed us to determine a ge isotope fractionation factor of 0 87 0 37 2sd n 12 during ge uptake by sponges although ge has long been considered as a geochemical twin of si this work underpins fundamental differences in their isotopic behaviors both during biomineralization processes and in their oceanic distributions this suggests that combined with si isotopes ge isotopes hold significant promise as a complementary proxy for delineating biological versus source effects in the evolution of the marine silicon cycle through time

contrasting behavior of tungsten and molybdenum in the okinawa trough the east china sea and the yellow sea by using catalytic current polarography and high resolution inductively coupled plasma mass spectrometry w and mo in seawater were determined in the okinawa trough a backarc rift and in the east china sea and the yellow sea mo was distributed conservatively throughout the study area and its salinity normalized concentration was 104 6 nm n 105 w was also uniformly distributed south of the kuroshio current 56 7 pm n 51 anomalous high concentrations of w maximum 254 pm were found in the iheya graben in the middle okinawa trough gt 1 000 m depth which were probably supplied by hydrothermal activity the concentrations of mo and w in the east china sea and the yellow sea showed linear correlation with salinity 26 lt s lt 35 the mo data can be explained by mixing of seawater and river water of the changjiang mo 10 nm qu et al 1993 however the values of w extrapolated to s 0 were largely different between two cruises 1 200 pm in may june 1987 and 540 pm in june 1994 and much higher than the reported concentrations of 160 pm for world rivers by turekian 1969 and 30 pm for unpolluted japanesemore rivers by sohrin et al 1989 moreover significantly high w was observed in the bottom water at stations near the changjiang river estuary and the western yellow sea while these data may suggest that w is released from the anoxic sediments of the continental shelf more data are needed to elucidate the mechanism controlling the distribution of w less

frictional properties of simulated shale coal fault gouges implications for induced seismicity in source rocks below europe s largest gas field abstract we report 21 frictional sliding experiments performed on simulated fault gouges prepared from shale coal mixtures our aim was to investigate the effects of local coal seam smearing on the frictional properties and induced seismogenic potential of faults cutting the upper carboniferous source rocks underlying the groningen gas reservoir netherlands we used shale siltstone core recovered from beneath the groningen reservoir plus polish bituminous coal of similar age and origin to coals locally present in the groningen source rocks we performed friction experiments in velocity stepping constant velocity slide hold slide shs and slide unload slide sus modes under near in situ conditions of 100 c and 40 mpa effective normal stress employing sliding velocities of 0 1 100 m s and a variety of pore fluids samples with 0 50 vol coal showed friction coefficients 0 45 with minor slip weakening samples with 50 vol coal showed marked slip weakening from peak friction values of 0 47 to 0 3 regardless of experimental conditions presumably reflecting strain localization in weak coal rich shear bands possibly accompanied by changes in coal molecular structure however re sliding experiments sus showed that slip weakening is limited to small initial displacements 2 3 mm and does not occur during slip reactivation at near steady state almost all experiments performed at in situ stress pore water pressure 15 mpa and temperature conditions exhibited stable velocity strengthening behaviour regardless of coal content by contrast under dry and gas saturated ch4 argon conditions or using water at 1 atm 50 50 vol shale coal mixtures showed velocity weakening and even stick slip our results imply that faults in the groningen carboniferous shale siltstone sequence are not prone to induced earthquake nucleation at in situ conditions even when coal bearing or coal enriched by smearing however the mechanisms controlling coal friction remain unclear at the sliding velocities studied and the evolution of coal friction at seismic slip velocities remains unknown

neodymium isotopic composition of the oceans a compilation of seawater data a global compilation of the neodymium isotopic composition of seawater is presented with 880 data points it confirms the gradual end increase for intermediate deep water masses from the northwest north atlantic via the austral and indian oceans to the pacific this confirms the usefulness of this tracer for studying large scale oceanic circulation the compilation stresses the need for documenting the oceans south of 30 s from which less than 4 of the data are derived the associated neodymium concentrations display heterogeneous vertical gradients among major oceanic basins in addition to particle remineralization along the global thermohaline circulation the database suggests that basin size differences also contribute to that heterogeneity

tungsten molybdenum fractionation in estuarine environments abstract dissolved tungsten w and molybdenum mo concentrations were measured in surface waters and sediment pore waters of terrebonne bay a shallow estuary in the mississippi river delta to investigate the biogeochemical processes that fractionate these group 6 elements relative to one another during transit from weathering to sedimentary environments although many of the chemical properties of w and mo are similar the two elements behave autonomously and the fractionation mechanisms are only partly understood in sulfidic pore waters dissolved mo is depleted relative to river water seawater mixtures whereas dissolved w is 10 fold enriched reductive dissolution of poorly crystalline phases like ferrihydrite which is a preferential host of w relative to mo in grain coatings on river borne particles can explain the dissolved w enrichment dissolved w becomes increasingly enriched as h 2 s aq rises above about 60 m due to transformation of wo 4 2 to thiotungstates as well as to additional reductive dissolution of phases that host w in contrast as rising sulfide transforms moo 4 2 to thiomolybdates in pore waters dissolved mo is suppressed probably owing to equilibration with an fe mo s phase this putative phase appears to control the aqueous ion product q fe 2 mos 4 2 0 6 h 2 s 0 0 4 h 0 8 at a value of 10 7 78 concentrations of dissolved w and mo in pore waters bear no relation to concentrations in surface waters of the same salinity in surface waters dissolved mo is nearly conserved in the estuarine mixing zone dissolved w appears also to be conserved except for several cases where w may have been enhanced by exchange with underlying w rich pore waters with increasing salinity the molar mo w ratio rises from about 10 to about 1000 in surface waters whereas it is mostly vi is more susceptible to reduction than w vi but the latter is more prone to adopting octahedral coordination we propose that the first difference facilitates mo but not w precipitation in sulfidic sediments whereas the second explains tungsten s proportionately greater sequestration on river borne particles and its subsequent release to sulfidic pore waters after the particles are deposited in the delta and become subject to reductive dissolution during diagenesis