comparison of arsenic concentrations in simultaneously collected groundwater and aquifer particles from bangladesh india vietnam and nepal one of the reasons the processes resulting in as release to groundwater in southern asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers in an attempt to overcome this difficulty a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in bangladesh recently published results from bangladesh and india relying on the needle sampler are augmented here with new data from 37 intervals of grey aquifer material of likely holocene age in vietnam and nepal a total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for as 1 1000 lg l fe 0 01 40 mg l mn 0 2 4 mg l and s 0 04 14 mg l are compared the p extractable 0 01 36 mg kg and hcl extractable as 0 04 36 mg kg content of the particulate phase was determined in the same suite of samples in addition to fe ii fe ratios 0 2 1 0 in the acid leachable fraction of the particulate phase needle sampler data from bangladesh indicated a relationship between dissolved as in groundwater and p extractable as in the particulate phase that was interpreted as an indication of adsorptive equilibrium under sufficiently reducing conditions across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 ml g the more recent observations from india vietnam and nepal show groundwater as concentrations that are often an order of magnitude lower at a given level of p extractable as compared to bangladesh even if only the subset of particularly reducing intervals characterized by leachable fe ii fe 0 5 and dissolved fe 0 2 mg l are considered without attempting to explain why as appears to be particularly mobile in reducing aquifers of bangladesh compared to the other regions the consequences of increasing the distribution coefficient for as between the particulate and dissolved phase to 40 ml g for the flushing of shallow aquifers of their initial as content are explored

compositional variations in manganese carbonate micronodules from the lower proterozoic nsuta deposit ghana product of authigenic precipitation or post formational diagenesis abstract detailed petrographic and microprobe studies of micronodules identified in the manganese carbonate orebody and adjacent mn rich rocks at the 2 2 ga nsuta deposit ghana indicate distinct lithologic control on nodule structure and chemical composition nodules in the mn rich rocks adjacent to the manganese carbonate orebody conform with the well banded nature of the rock and are distinctly zoned into alternating bands compositionally dominated by mg kutnahorite and mg ca rhodochrosite individual nodules are partially or completely cemented by thin dark silica and carbonates of ca mg mn solid solution nodules in the orebody on the contrary are generally massive homogeneous consist predominantly of rhodochrosite and are only partially cemented by dark silica based on the stratigraphic relationship of the nodule bearing rocks to the manganese carbonate orebody and the variations in texture and composition observed in the nodules it would appear that the mn carbonate nodules precipitated diagenetically in response to changing pore water chemistry and or metal mn and ca supply the overall mineralogical and geochemical features indicate in addition that the nodules may have preserved original carbonate compositions both metastable and stable during burial diagenesis and low grade regional metamorphism

evaluation of the application of rare earth elements to paleoceanography anomalous departures of rare earth elements ree from similarity of chemical behaviour are most evident in a comparison of ce with its neighbors under reducing conditions ce3 acts like other ree but oxidized ce4 is adsorbed on mn fe oxyhydroxides or organic particulates and consequently the remaining solution will tend to exhibit a negative ce anomaly in addition more subtle fractionations may emphasize the light or the heavy or the middle ree a pattern of ree may be recorded by immediate diagenesis on a suitable chemical substrate such as apatite fish debris conodonts carbonate or chert this record was originally thought to reflect the paleoredox conditions of primary deposition however any such primary record may be overprinted by late diagenetic alteration early analyses of ree in conodonts from a wide range of sediments suggested a prevalence of reducing conditions no ce anomaly in pre carboniferous oceans and more common oxidation negative ce anomaly in the late paleozoic however recent research by other investigators indicates that variations in cean are more diverse in origin and in particular many are distorted to an unknown extent by late diagenesis in any case it is now clear that geological age does not show a consistent relationship to the level of the ce anomaly the ce anomaly apparently does not play any regular role in the isotopic events that accompany prominent mass extinctions further research is required to understand and thus make useful the controls on the ce anomaly

aluminous granulites of the archean craton of kasai zaire petrology and p t conditions aluminous granulites of the archean 2 8 ga kasai craton zaire consist of two main mineral assemblages grt opx and sil grt crd rocks the high grade metamorphic conditions as deduced from grt opx and grt opx pl qtz equilibria are 720 c 6 7 kb considering the zoning of the same minerals the slope of the p t path is estimated at 15 b c thermobarometry involving crd is consistent with those p t conditions three cordierite forming reactions have been observed petrographically grt qtz crd opx r1 grt sil crd spl r2 grt sil crd mag r3 these equilibria are continuous reactions end member reactions have slopes less than 15 b c they are decompression reactions occurring after the metamorphic climax using available thermodynamic data r3 fixes the oxygen fugacity to a value below the qfm buffer log 10f o2 17 6 at 720 c 6 7 kb and xh2ofl 0 3 in the graphite stability field the absence of graphite in the rocks shows that the end of the granulite facies metamorphism did not occur under important co 2 streaming the polymetamorphic history of this archean craton is considered 1988 oxford university press

supergene manganese ore records 75 myr long campanian to pleistocene geodynamic evolution and weathering history of the central african great lakes region tectonics drives climate assists abstract the southeastern part of the democratic republic of the congo locally hosts proterozoic manganese deposits the deposits of kisenge kamata are the most significant but manganese ores are also known to occur at kasekelesa former katanga province and mwene ditu former kasai province for the present study cryptomelane rich samples from these two localities were dated using the 40ar 39ar step heating method with a co2 laser probe the ages obtained are within a range of c 77 myr to c 2 myr cryptomelane formation took place at c 76 4 ma c 59 6 ma c 45 ma c 35 ma c 23 8 ma c 15 4 ma and c 13 3 ma at kasekelesa and it occurred at c 35 ma c 22 4 ma c 15 ma c 5 5 7 2 ma c 3 6 ma and c 2 1 2 3 ma at mwene ditu the campanian age c 76 4 ma recorded at kasekelesa is the oldest 40ar 39ar age that has up to now been recorded for mn ores from africa it documents the formation of oxidized ore along a campanian or older erosion surface which could be part of the african erosion surface the complete age record suggests that continent wide tectonics accounts for most of the recognized supergene ore formation episodes controlled by vertical lithospheric movements that are ultimately responsible for alternating stages of landscape stability and erosion tectonics is thus regarded as the first order control for secondary ore formation in central africa over the last 80 myr climate is a second order control because sufficient water supply is needed for supergene enrichment whereby climatic conditions are recognized to have been favourable during some relatively cold late mesozoic and paleogene periods as well as during some humid and warm neogene stages

characterization of space weathering from lunar reconnaissance orbiter camera ultraviolet observations of the moon we investigate the effects of space weathering at ultraviolet wavelengths using a near global seven band 321 689 nm mosaic from the lunar reconnaissance orbiter camera lroc wide angle camera wac we confirm that for moderate to high iron compositions 5 wt feo the steeply positive uv slope at wavelengths 415 nm shallows with increasing exposure to space weathering we measure these differences in lroc wac data as variations in the 321 415 nm ratio which has low values for fresh craters in the mare and moderate iron highlands for low iron highland compositions the break in slope occurs at shorter wavelengths and it is instead the 321 360 nm ratio that increases with exposure to the space weathering environment whereas the 321 415 nm ratio appears to be largely controlled by the degree of shock experienced during the impact the effects of shock may be more important at highland craters because modest shock pressures result in the solid state transformation of plagioclase to its glass equivalent maskelynite and can help distinguish between primary shocked ejecta and locally exposed fresh material in rays while all of the fresh craters we examined have uv spectral properties consistent with substantial alteration due to space weathering the uv spectra of lunar swirls magnetically shielded from the solar wind are consistent with exposure of immature crystalline material together these results suggest that lunar space weathering is dominated by the solar wind and saturates in the uv at is feo values of 40 submature

origin of strong lunar magnetic anomalies further mapping and examinations of lroc imagery in regions antipodal to young large impact basins work at the university of arizona was supported under grant nnx12aj03g from the nasa lunar advanced science and exploration research laser program

pathways of coupled arsenic and iron cycling in high arsenic groundwater of the hetao basin inner mongolia china an iron isotope approach abstract high as groundwater is widely distributed all over the world which has posed a significant health impact on millions of people iron isotopes have recently been used to characterize fe cycling in aqueous environments but there is no information on fe isotope characteristics in the groundwater since groundwater as behavior is closely associated with fe cycling in the aquifers fe isotope signatures may help to characterize geochemical processes controlling as concentrations of shallow groundwaters this study provides the first observation of fe isotope fractionation in high as groundwater and evaluation of fe cycling and as behaviors in shallow aquifers in terms of fe isotope signatures thirty groundwater samples were taken for chemical and isotopic analysis in the hetao basin inner mongolia thirty two sediments were sampled as well from shallow aquifers for fe isotope analysis results showed that groundwater was normally enriched in isotopically light fe with 56fe values between 3 40 and 0 58 and median of 1 14 while heavier 56fe values were observed in the sediments between 1 10 and 0 75 median 0 36 in reducing conditions groundwaters generally had higher 56fe values in comparison with oxic conditions high as groundwaters generally occurring in reducing conditions had high 56fe values while low as groundwaters normally had low 56fe values although sediment 56fe values were generally independent of lithological conditions a large variation in sediment 56fe values was observed in the oxidation reduction transition zone three pathways were identified for fe cycling in shallow groundwater including dissimilatory reduction of fe iii oxides re adsorption of fe ii and precipitation of pyrite and siderite dissimilatory reduction of fe iii oxides resulted in light 56fe values around 1 0 and high as concentration 50 g l in groundwater in anoxic conditions re adsorption of isotopically heavy fe ii produced by microbially mediated reduction of fe iii oxides led to further enrichment of isotopically light fe in groundwater up to 3 4 of 56fe in anoxic suboxic conditions arsenic re adsorption was expected to occur along with fe ii re adsorption decreasing groundwater as concentrations in strongly reducing conditions precipitation of isotopically light fe pyrite and or siderite increased groundwater 56fe values reaching 0 58 56fe with a subsequent decrease in as concentrations via co precipitation the mixed effect of those pathways would regulate as and fe cycling in most groundwaters

latitudinal variation in spectral properties of the lunar maria and implications for space weathering abstract space weathering alters the optical properties of exposed surfaces over time complicating the interpretation of spectroscopic observations of airless bodies like asteroids mercury and the moon solar wind and micrometeoroids are likely the dominant agents of space weathering but their relative contributions are not yet well understood based primarily on clementine mosaics we report a previously unrecognized systematic latitudinal variation in the near infrared spectral properties of the lunar maria and show that the characteristics of this latitudinal trend match those observed at lunar swirls where magnetic fields alter local solar wind flux without affecting the flux of micrometeoroids we show that the observed latitudinal color variations are not artifacts of phase angle effects and cannot be accounted for by compositional variation alone we propose that reduced solar wind flux which should occur both at swirls and toward higher latitudes is the common mechanism behind these color variations this model helps us quantify the distinct effects of solar wind and micrometeoroid weathering and could aid in interpreting the spectra of airless bodies throughout the solar system

nature and origin of arsenic carriers in shallow aquifer sediments of bengal delta india sediments from shallow aquifers in bengal delta india have been found to contain arsenic rivers of ganga brahmaputra system responsible for depositing these sediments in the delta have created a store of arsenic geomorphological domains with different depositional styles regulate the pattern of distribution of zones with widely different content of groundwater arsenic the high arsenic zones occur as narrow sinuous strips confined to channel deposits a few iron bearing clastic minerals and two post depositional secondary products are arsenic carriers secondary siderite concretions have grown on the surface of the clastic carriers in variable intensity the quantity of arsenic in all clastic carriers is in excess of what is generally expected excess arsenic is contributed by the element adsorbed on the concretion grown on the surface of the carriers which adds up to the arsenic in the structure of the minerals variable abundance of concretions is responsible for the variable quantity of arsenic in the carriers and the sediment samples fe2 for the growth of siderite concretions is obtained from the iron bearing clastic carriers the reaction involves reduction of trivalent iron to bivalent and the required electron is obtained by transformation of as3 to as5 it is suggested that oxidation of as3 to as5 is microbially mediated in the safe zone arsenic is retained in the carriers and groundwater arsenic is maintained below 0 05 mg l in the unsafe zone sorbed arsenic is released from the carriers in the water through desorption and dissolution of concretion thereby elevating the groundwater arsenic level to above 0 05 mg l

coticules of the belgian type area stavelot venn massif limy turbidites within the nascent rheic oceanic basin abstract coticule is the name originating in belgium for a fine grained metasedimentary yellowish rock mainly composed of quartz spessartine and mica which is repeatedly interstratified in the hematite rich purple slates of the lower ordovician of the ardenne region it was described for the first time in the nineteenth century in the stavelot venn massif ardenne which is accordingly the type area for this peculiar lithology since then numerous occurrences were described all over the world in metamorphic rocks of various grades and ages the exact nature of the belgian coticule protolith remained hypothetical although intensively discussed for a long time in the literature after an extensive review of all the stratigraphical sedimentological and geochemical results and new field geochemical major and trace elements and nd isotopic investigations we propose a significantly improved and sustained genetic model the thin coticule layers were deposited offshore in a deep oceanic basin as limy mud turbidites during the early floian c 477 ma early ordovician density currents came from the north from the shelf which bordered the emergent brabant massif at that time and flowed down the slope to the deep basin plain the purple shales enclosing the coticule layers represent the normal pelagic sedimentation in the basin these shales are exceptionally rich in fe and mn because of the hydrothermal activity of the nearby and young rheic ocean ridge indeed the rheic ocean opened in the early ordovician c 482 ma during diagenesis the strongly oxidizing depositional environment favoured the mobility of mn 2 as opposed to fe 3 that remained insoluble and immobile this allowed for the migration of divalent manganese from the pelagic shales and for replacing calcium in the turbiditic carbonate fraction to form rhodochrosite later epizonal metamorphism transformed the clay quartz rhodochrosite paragenesis of the protolith into the muscovite quartz spessartine paragenesis of the coticule hence in the type area coticule genesis needed a peculiar environment including a continental shelf with limy mud deposits a continental slope generating periodical turbidites and a nearby oceanic ridge here the nascent rheic ocean delivering hydrothermal iron and manganese

hydrogeochemical zonation and its implication for arsenic mobilization in deep groundwaters near alluvial fans in the hetao basin inner mongolia summary high as groundwater has been found in shallow aquifers of the flat plain of the hetao basin but little is known about as concentration in deep groundwaters around piedmont areas which are the major drinking water resources one hundred and three groundwater samples from wells with depths 50 m and seven samples from one multi level monitoring well 89 m in depth were analyzed for chemical compositions and 18o and d isotopes to examine the geochemical processes controlling as mobilization according to hydrogeological setting chemical and isotopic characteristics of deep groundwater three distinguished hydrogeochemical zones are delineated including recharge oxic zone zone i groundwater flow moderate reducing zone zone ii and groundwater flow reducing zone zone iii zone i is located in proximal fans in the recharge area with oxic conditions where low as groundwater generally occurs in zone ii located in the intermediate between the fans and the flat plain with fe reduction predominated groundwater as is moderate zone iii lies in the flat plain with the occurrence of so 4 2 reduction where high as groundwater is mostly found this indicates that release of as to groundwater is primarily determined by reduction sequences arsenic is immobilized in o 2 no 3 reduction stage in zone i and released in fe reducing conditions of zone ii and displays a significant elevated concentration in so4 reducing stage in zone iii dissolution of carbonate minerals occurs in zone i while ca2 and mg2 are expected to precipitate in zone ii and zone iii in the multi level monitoring well both chemical and isotopic compositions are dependent of sampling depths with the similar trend to the hydrogeochemical zonation along the flow path the apparent increases in d and 18o values in zone iii reveal the possibility of high as shallow groundwater recharge to deep groundwater it indicates that deep groundwaters in proximal fans have low as concentrations and are considered as safe drinking water resources in the hetao basin however high as concentration is frequently observed in deep groundwater in the flat plain which should be routinely monitored in order to avoid chronic as poisoning

biogeochemical cycling of iron in the archean paleoproterozoic earth constraints from iron isotope variations in sedimentary rocks from the kaapvaal and pilbara cratons abstract iron isotope compositions of low metamorphic grade samples of archean paleoproterozoic sedimentary rocks obtained from fresh drill core from the kaapvaal craton in south africa and from the pilbara craton in australia vary by 3 in 56 fe 54 fe ratios reflecting a variety of weathering and diagenetic processes depositional ages for the 120 samples studied range from 3 3 to 2 2 ga and fe c and s contents define several compositional groups including samples rich in fe organic carbon carbonate and sulfide the 56 fe values for low c org low c carb and low s sedimentary rocks are close to 0 the average of igneous rocks this range is essentially the same as that of c org poor late cenozoic loess aerosol river loads and marine sediments and those of c org poor phanerozoic proterozoic shales that these 56 fe values are the same as those of igneous rocks suggests that fe has behaved conservatively in bulk sediments during sedimentary transport diagenesis and lithification since the archean these observations indicate that if atmospheric o 2 contents rose dramatically between 2 4 and 2 2 ga as proposed by many workers such a rise did not produce a significant change in the bulk fe budget of the terrestrial sedimentary system if the archean atmosphere was anoxic and fe was lost from bedrock during soil formation any isotopic fractionation between aqueous ferrous fe fe aq 2 and fe bearing minerals must have been negligible in contrast if the archean atmosphere was oxic fe would have been retained as fe 3 hydroxides during weathering as it is today which would produce minimal net isotopic fractionation in bulk detrital sediments siderite rich samples have 56 fe values of 0 5 0 5 and experimentally determined fe aq 2 siderite fractionation factors suggest that these rocks formed from fe aq 2 that had similar or slightly higher 56 fe values the 56 fe values calculated for fe aq 2 overlaps those of modern submarine hydrothermal fluids but it is also possible that fe aq 2 had 56 fe values higher than those of modern hydrothermal fluids depending upon the fe aq 2 fe carbonate fractionation factor that is used in contrast c org rich samples and magnetite rich samples have strongly negative 56 fe values generally between 2 3 and 1 0 and available fluid mineral fractionation factors suggest that the fe bearing minerals siderite and magnetite in these rocks formed in the presence of fe aq 2 that had very low 56 fe values between 3 and 1 reduction of fe 3 hydroxide by sulfide precipitation of sulfide minerals or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low 56 fe fe aq 2 we interpret microbial dissimilatory fe 3 reduction dir as the best explanation for producing such low 56 fe values for fe aq 2 and our results suggest that dir was a significant form of respiration since at least 2 9 ga

measuring moonlight an overview of the spatial properties lunar coverage selenolocation and related level 1b products of the moon mineralogy mapper 1 the moon mineralogy mapper m3 a high resolution high precision imaging spectrometer flew on board india s chandrayaan 1 mission from october 2008 through august 2009 this paper describes some of the spatial sampling aspects of the instrument the planned mission and the mission as flown we also outline the content and context of the resulting level 1b spatial products that form part of the m3 archive while designed and planned to operate for 2 years in a 100 km lunar orbit m3 was able to meet its lunar coverage requirements despite the shortened mission an increase of the orbit altitude to 200 km and several relevant problems with spacecraft attitude timing and ephemeris the unexpected spacecraft issues required us to invent a novel two step approach for selenolocation leveraging newly available lunar reconnaissance orbiter lunar orbiter laser altimeter lola topography and an improved spacecraft ephemeris we have created a method that permits us to bootstrap spacecraft attitude estimates from the image data themselves this process performs a nonlinear optimization to honor a set of data derived image to image tie points and image to lola control points error analysis of the final results suggests we have converged to a selenolocation result that has image to image root mean square rms errors less than 200 m and image to lola rms errors less than 450 m despite using data derived spacecraft attitude results the level 1b products include the lunar coordinates resulting from this inversion process and 10 relevant observational geometry parameters that fully characterize the ray tracing geometry on a pixel by pixel basis

timing and pt evolution of whiteschist metamorphism in the lufilian arc zambezi belt orogen zambia implications for the assembly of gondwana abstract one of the world s most extensive occurrences of whiteschists talc kyanite schists is located in south central africa with several exposures along a ca 700 km northwest southeast striking zone the metamorphic evolution and age relations of whiteschists and associated rocks from four localities three in the lufilian arc and one in the zambezi belt were investigated in the lufilian arc whiteschists associated garnet amphibolites and biotite kyanite garnet gneisses record peak metamorphic conditions of about documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes

sources and mobility of arsenic in the salt river watershed arizona sources and mobility of arsenic in the upper salt river arizona were examined using a 6 year long mass balance for nine u s geological survey chemical monitoring stations a more spatially intensive low flow synoptic study and laboratory leaching studies an arsenic rich lacustrine deposit the verde formation comprised only 4 of the watershed but contributed a third of the total watershed arsenic loading export from this part of the watershed was 10 20 times higher than export from most other parts of the watershed laboratory experiments confirmed the high as leaching potential for this formation although elevated levels of as were found in a geothermal spring and in several mine seeps these sources of arsenic contributed little to the overall as loading about 15 of the as loading from the upper verde river watershed was retained by a two reservoir system sedimentation and chemical precipitation appear to be likely mechanisms of as retention in the reservoirs

chemical evolution in the high arsenic groundwater of the huhhot basin inner mongolia pr china and its difference from the western bengal basin india elevated as concentrations in groundwater of the huhhot basin hb inner mongolia china and the western bengal basin wbb india have been known for decades however few studies have been performed to comprehend the processes controlling overall groundwater chemistry in the hb in this study the controls on solute chemistry in the hb have been interpreted and compared with the well studied wbb which has a very different climate physiography lithology and aquifer characteristics than the hb in general there are marked differences in solute chemistry between hb and wbb groundwaters stable isotopic signatures indicate meteoric recharge in the hb in a colder climate distant from the source of moisture in comparison to the warm humid wbb the major ion composition of the moderately reducing hb groundwater is dominated by a mixed ion ca na hco3 cl hydrochemical facies with an evolutionary trend along the regional hydraulic gradient molar ratios and thermodynamic calculations show that hb groundwater has not been affected by cation exchange but is dominated by weathering of feldspars allitization and equilibrium with gibbsite and anorthite mineral weathering and mobilization of as could occur as recharging water flows through fractured argillaceous metamorphic or volcanic rocks in the adjoining mountain front areas and deposits solutes near the center of the basin in contrast wbb groundwater is ca hco3 dominated indicative of calcite weathering with some cation exchange and silicate weathering monosiallitization

sedimentary manganese metallogenesis in response to the evolution of the earth system abstract the concentration of manganese in solution and its precipitation in inorganic systems are primarily redox controlled guided by several earth processes most of which were tectonically induced the early archean atmosphere hydrosphere system was extremely o2 deficient thus the very high mantle heat flux producing superplumes severe outgassing and high temperature hydrothermal activity introduced substantial mn2 in anoxic oceans but prevented its precipitation during the late archean centered at ca 2 75 ga the introduction of photosystem ii and decrease of the oxygen sinks led to a limited buildup of surface o2 content locally initiating modest deposition of manganese in shallow basin margin oxygenated niches e g deposits in india and brazil rapid burial of organic matter decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the archean proterozoic transition concurrently a massive drawdown of atmospheric co2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused paleoproterozoic glaciations 2 45 2 22 ga the spectacular sedimentary manganese deposits at ca 2 4 ga of transvaal supergroup south africa were formed by oxidation of hydrothermally derived mn2 transferred from a stratified ocean to the continental shelf by transgression episodes of increased burial rate of organic matter during ca 2 4 and 2 06 ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere black shale hosted mn carbonate deposits in the birimian sequence ca 2 3 2 0 ga west africa its equivalents in south america and those in the francevillian sequence ca 2 2 2 1 ga gabon are correlatable to this period tectonically forced doming up attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric co2 drawdown causing glaciation on the neoproterozoic rodinia supercontinent tectonic rifting and mantle outgassing led to deglaciation dissolved mn2 and fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation during the sturtian glacial interglacial event ca 750 700 ma interstratified mn oxide and bif deposits of damara sequence namibia was formed the varangian marinoan ca 600 ma cryogenic event produced mn oxide and bif deposits at urucum jacadigo group brazil the datangpo interglacial sequence south china liantuo nantuo varangian event contains black shale hosted mn carbonate deposits the early paleozoic witnessed several glacioeustatic sea level changes producing small mn carbonate deposits of tiantaishan early cambrian and taojiang mid ordovician in black shale sequences china and the major mn oxide carbonate deposits of karadzhal type central kazakhstan late devonian the mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans during the early jurassic oae organic rich sediments host many mn carbonate deposits in europe e g urkut hungary in black shale sequences the late jurassic giant mn carbonate deposit at molango mexico was also genetically related to sea level change mn carbonates were always derived from mn oxyhydroxides during early diagenesis large mn oxide deposits of cretaceous age at groote eylandt australia and imini tasdremt morocco were also formed during transgression regression in greenhouse climate the early oligocene giant mn oxide carbonate deposit of chiatura georgia and nikopol ukraine were developed in a similar situation thereafter manganese sedimentation was entirely shifted to the deep seafloor and since ca 15 ma b p was climatically controlled glaciation deglaciation assisted by oxygenated polar bottom currents aabw nadw the changes in climate and the sea level were mainly tectonically forced

the surficial nature of lunar swirls as revealed by the mini rf instrument abstract lunar swirls are optically bright sinuous albedo features found on the moon the mini rf synthetic aperture radar on the lunar reconnaissance orbiter has provided a comprehensive set of x and s band radar images of these enigmatic features including the first radar observations of swirls on the farside of the moon a few general remarks can be made about the nature of the lunar swirls from this data set first the average radar properties of lunar swirls are identical to nearby non swirl regions in both total radar backscatter and circular polarization ratio cpr this implies that average centimeter scale roughness and composition within the high albedo portions of the swirls do not differ appreciably from the surroundings and that the high optical reflectance of the swirls is related to a very thin surface phenomenon less than several decimeters thick not observable with x or s band radar secondly bright swirl material appears to be stratigraphically younger than a newly discovered impact melt flow at gerasimovich d this observation indicates that the swirls are capable of forming over timescales less than the age of the crater the mini rf data set also provides clues to the origin of the lunar swirls in at least one case the presence of an enhanced crustal magnetic field appears to be responsible for the preservation of a high albedo ejecta blanket around an otherwise degraded crater descartes c the degree of degradation of descartes c suggests it should not be optically bright yet it is this implies that the enhanced albedo is related to its location within a magnetic anomaly and hence supports an origin hypothesis that invokes interaction between the solar wind and the magnetic anomaly

decoupling of as and fe release to bangladesh groundwater under reducing conditions part i evidence from sediment profiles this study reexamines the notion that extensive as mobilization in anoxic groundwater of bangladesh is intimately linked to the dissolution of fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater as concentrations were known to span a wide range the sediment properties that were measured include 1 the proportion of fe ii in the fe fraction leached in hot 1 2 n hcl 2 diffuse spectral reflectance and 3 magnetic susceptibility in parallel with local concentrations of dissolved as ranging from 5t o 600g l fe ii fe ratios in shallow gray holocene sands tended to gradually increase with depth from values of 0 3 to 0 5 to up to 0 9 in deeper orange aquifers of presumed pleistocene age that were separated from shallow sands by a clay layer and contained 5 g l dissolved as leachable fe ii fe ratios averaged 0 2 there was no consistent relation between sediment fe ii fe and dissolved fe concentrations in groundwater in nearby wells the reflectance measurements indicate a systematic linear relation r 2 of 0 66 n 151 between the first derivative transform of the reflectance at 520 nm and fe ii fe the magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 x 10 9 m 3 kg si and was linearly related r 2 of 0 75 n 29 to the concentrations of minerals that could be magnetically separated 0 03 to 0 79 dry weight no systematic depth trends in magnetic susceptibility were observed within the shallow sands although the susceptibility of deeper low as aquifers was low up to 200 10 9 m 3 kg si

fe c and o isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in 2 5 ga marine environments abstract combined fe c and o isotope measurements of 2 5 ga banded iron formation bif carbonates from the kuruman iron formation and underlying bif and platform ca mg carbonates of the gamohaan formation south africa constrain the biologic and abiologic formation pathways in these extensive bif deposits vertical intervals of up to 100 m were sampled in three cores that cover a lateral extent of 250 km bif fe carbonates have significant fe isotope variability 56fe 1 to 1 and relatively low 13c down to 12 and 18o values 18o 21 in contrast gamohaan and stratigraphically equivalent campbellrand ca mg carbonates have near zero 13c values and higher 18o values these findings argue against siderite precipitation from seawater as the origin of bif fe rich carbonates instead the c o and fe isotope compositions of bif fe carbonates reflect authigenic pathways of formation in the sedimentary pile prior to lithification where microbial dissimilatory iron reduction dir was the major process that controlled the c o and fe isotope compositions of siderite isotope mass balance reactions indicate that the low 13c and low 18o values of bif siderite relative to those expected for precipitation from seawater reflect inheritance of c and o isotope compositions of precursor organic carbon and ferric hydroxide that were generated in the photic zone and deposited on the seafloor carbon fe isotope relations suggest that bif fe carbonates formed through two end member pathways low 13c low 56fe fe carbonates formed from remobilized low 56fe aqueous fe2 produced by partial dir of iron oxide whereas low 13c high 56fe fe carbonates formed by near complete dir of high 56fe iron oxides that were residual from prior partial dir an important observation is the common occurrence of iron oxide inclusions in the high 56fe siderite supporting a model where such compositions reflect dir in place in the soft sediment in contrast the isotopic composition of low fe carbonates in limestone dolomite may constitute a record of seawater environments although our petrographic studies indicate that the presence of pyrite in most low fe carbonates may influence the fe isotope compositions the combined fe c and o isotope data from kuruman bif carbonates indicate that bif siderites that have negative near zero or positive 56fe values may all record biological fe cycling where the range in 56fe values records differential fe mobilization via dir in the sediment prior to lithification our results demonstrate that the inventory of low 56fe marine sedimentary rocks of neoarchean to paleoproterozoic age although impressive in volume may represent only a minimum of the total inventory of fe that was cycled by bacteria

fractionation of fe isotopes by soil microbes and organic acids small natural variations in fe isotopes have been attributed to biological cycling however without understanding the mechanisms of fractionation it is impossible to interpret such variations here we show that the d 56 fe of fe dissolved from a silicate soil mineral by siderophore producing bacteria is as much as 0 8 lighter than bulk fe in the mineral a smaller isotopic shift is observed for fe released abiotically by two chelates and the magnitude of the shift increases with affinity of the ligand for fe consistent with a kinetic isotope effect during hydrolysis of fe at the mineral surface fe dissolved abiotically without chelates shows no isotopic shift the d 56 fe of the exchange fraction on soil grains is also lighter by 0 6 1 than fe from both hornblende and iron oxyhydroxides the kinetic isotope effect is therefore preserved in open systems such as soils when recorded in the rock record fe isotopic fractionation could document fe transport by organic molecules or by microbes where such entities were present in the geologic past

distribution of groundwater arsenic in xinjiang p r china although potential contamination of groundwater as is expected to occur in xinjiang p r china few data are available for the regional distribution of groundwater as in this study the spatial distribution of groundwater as was investigated in the chepaizi cpz n and shihezi shz n areas of northern xinjiang the balikun yiwu basin by e in eastern xinjiang and the tarim trm s and yanqi yq s basins in southern xinjiang arsenic concentrations greater than 10 g l were found in 12 of analyzed groundwaters all groundwater samples collected in cpz n had as concentrations greater than 10 g l 25 185 g l 30 in shz n 10 g l was found in the eastern and southern trm s and yq s distribution of groundwater as showed a tremendous spatial variability which greatly varied over a short distance horizontally arsenic concentration generally increased with increasing sampling depth the spatial distribution of groundwater as would be regulated by as source and hydrogeochemical processes higher ph and or lower orp values were generally observed in high as groundwater 10 g l in comparison with low as groundwater 10 g l arsenic mobility in shz n and cpz n may result from as desorption under relatively high ph conditions and more tentatively from reductive dissolution of fe iii oxides in by e and trm s however detailed mechanisms of as mobilization in these regions need further investigation

compositional diversity and geologic insights of the aristarchus crater from moon mineralogy mapper data 1 the moon mineralogy mapper m3 acquired high spatial and spectral resolution data of the aristarchus plateau with 140 m pixel in 85 spectral bands from 0 43 to 3 0 m the data were collected as radiance and converted to reflectance using the observational constraints and a solar spectrum scaled to the moon sun distance summary spectral parameters for the area of mafic silicate 1 and 2 m bands were calculated from the m3 data and used to map the distribution of key units that were then analyzed in detail with the spectral data this analysis focuses on five key compositional units in the region 1 the central peaks are shown to be strongly enriched in feldspar and are likely from the upper plagioclase rich crust of the moon 2 the impact melt is compositionally diverse with clear signatures of feldspathic crust olivine and glass 3 the crater walls and ejecta show a high degree of spatial heterogeneity and evidence for massive breccia blocks 4 olivine strongly concentrated on the rim wall and exterior of the southeastern quadrant of the crater is commonly associated the impact melt 5 there are at least two types of glass deposits observed pyroclastic glass and impact glass

lunar swirls examining crustal magnetic anomalies and space weathering trends 1 we have used multispectral images from clementine and data from lunar prospector s magnetometer to conduct a survey of lunar crustal magnetic anomalies prominent lunar swirls and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies we find that all swirls and swirl like albedo patterns are associated with areas of magnetized crust but not all areas of magnetized crust are colocated with swirl like albedo anomalies all observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower feo values compared with their surroundings as determined with a multispectral iron mapping method we discuss these results in relation to the various hypotheses for swirl formation the comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined feo content between swirls and nearby ordinary surfaces the compositional difference could be explained as a consequence of 1 magnetic shielding of the surface from the solar wind which could produce anomalous space weathering little darkening with limited reddening and potentially alter the predictions of the multispectral iron mapping algorithm while the compositional contrast could be enhanced by delivery of lower feo ejecta from outside the swirl and 2 accumulation of fine plagioclase rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly therefore we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation we describe future measurements that could contribute to solution of the puzzle of swirl origin

iron isotope fractionation in subterranean estuaries dissolved fe concentrations in subterranean estuaries like their river seawater counterparts are strongly controlled by non conservative behavior during mixing of groundwater and seawater in coastal aquifers previous studies at a subterranean estuary of waquoit bay on cape cod usa demonstrate extensive precipitation of groundwater borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands waquoit bay is thus an excellent natural laboratory to assess the mechanisms of fe isotope fractionation in redox stratified environments and determine potential fe isotope signatures of groundwater sources to coastal seawater here we report fe isotope compositions of iron coated sands and porewaters beneath the intertidal zone of waquoit bay the distribution of pore water fe shows two distinct sources of fe one residing in the upward rising plume of fe rich groundwater and the second in the salt wedge zone of pore water the groundwater source has high fe ii concentration consistent with anoxic conditions and yield 56fe values between 0 3 and 1 3 in contrast sediment porewaters occurring in the mixing zone of the subterranean estuary have very low 56fe values down to 5 these low 56fe values reflect fe redox cycling and result from the preferential retention of heavy fe isotopes onto newly formed fe oxyhydroxides analysis of fe oxides precipitated onto subsurface sands in two cores from the subterranean estuary revealed strong 56fe and fe concentration gradients over less than 2m yielding an overall range of 56fe values between 2 and 1 5 the relationship between fe concentration and 56fe of fe rich sands can be modeled by the progressive precipitation of fe oxides along fluid flow through the subterranean estuary these results demonstrate that large scale fe isotope fractionation up to 5 can occur in subterranean estuaries which could lead to coastal seawater characterized by very low 56fe values relative to river values

the lunar far uv albedo indicator of hydration and weathering 1 we present an analysis of lunar reconnaissance orbiter lro lyman alpha mapping project lamp measurements of the dayside lunar surface at far ultraviolet wavelengths we use the strong 165 nm h2o absorption edge to look for diurnal variations in hydration we find that diurnal variations in spectral slope are indeed present they are superimposed on latitudinal and spatial variations related to composition and weathering we use two different spectral regions 164 173 nm and 175 190 nm to separate out these effects highlands and mare regions have distinct reflectance spectra with mare regions being spectrally bluer than highlands regions a consequence of the greater abundance of opaque minerals in mare regions bright ray terrains and areas known to be young such as giordano bruno crater are found to be relatively spectrally flat or red in the far uv this is consistent with a lack of space weathering which tends to make the far uv spectrum bluer due to the spectral behavior of nanophase iron large scale latitudinal variations in fuv slope are distinct and are likely due to a gradient in space weathering the diurnal variation in hydration is consistent with a solar wind origin and with loss of h2o at temperatures above 320 k far uv spectroscopy is thus shown to represent a viable method for mapping aqueous alteration even on the dayside of the moon and potentially elsewhere in the solar system

the effect of early diagenesis on the fe isotope compositions of porewaters and authigenic minerals in continental margin sediments abstract iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the california continental margin where isotope compositions range from 56fe 3 0 to 0 4 at one site near monterey canyon off central california organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction dir and bacterial sulfate reduction whereas at our other site in the santa barbara basin dir appears to be comparatively small and production of sulfides fes and pyrite was extensive the largest range in fe isotope compositions is observed for fe ii aq in porewaters which generally have the lowest 56fe values minimum 3 0 near the sediment surface and increase with burial depth 56fe values for fes inferred from hcl extractions vary between 0 4 and 0 4 but pyrite is similar at both stations where an average 56fe value of 0 8 0 2 was measured we interpret variations in dissolved fe isotope compositions to be best explained by open system behavior that involves extensive recycling of feflux this study is the first to examine fe isotope variations in modern marine sediments and the results show that fe isotopes in the various reactive fe pools undergo isotopic fractionation during early diagenesis importantly processes dominated by sulfide formation produce high 56fe values for porewaters whereas the opposite occurs when fe iii oxides are present and dir is a major pathway of organic carbon respiration because shelf pore fluids may carry a negative 56fe signature it is possible that the fe isotope composition of ocean water reflects a significant contribution of shelf derived iron to the open ocean such a signature would be an important means for tracing iron sources to the ocean and water mass circulation

geochemical evidence for iron mediated anaerobic oxidation of methane anaerobic oxidation of methane aom by sulfate has been recognized as a critical process to maintain this greenhouse gas stability by limiting methane flux to the atmosphere we show geochemical evidence for aom in deep lake sediments and demonstrate that aom is likely driven by iron fe reduction pore water profiles from lake kinneret sea of galilee israel show that this sink for methane is located below the 20 cm depth in the sediment which is well below the depths at which nitrate and sulfate are completely exhausted as well as below the zone of methanogenesis iron dependant aom was verified by fe iii amended mesocosm studies using intact sediment cores and native iron oxides were detectable throughout the sediments because anaerobic fe iii respiration is thermodynamically more favorable than both sulfate dependent methanotrophy and methanogenesis its occurrence below the zone of methane production supports the idea that reduction of sedimentary iron oxides is kinetically or biologically limited similar conditions are likely to prevail in other incompletely pyritized aquatic sediments indicating that aom with fe iii is an important global sink for methane

geochemical controls on arsenic and rare earth elements approximately along a groundwater flow path in the shallow aquifer of the hetao basin inner mongolia article i nfo groundwaters and sediments were sampled approximately along a groundwater flow path in the shallow aquifer of the hetao basin inner mongolia along a flow path with a length of about 9 km groundwater evolves from ca hco3 so4 through mg ca hco3 cl to na cl hco3 from the recharge zone to the discharge

geodynamic and climate controls in the formation of mio pliocene world class oxidized cobalt and manganese ores in the katanga province dr congo the katanga province democratic republic of congo hosts world class cobalt deposits accounting for 50 of the world reserves they originated from sediment hosted stratiform copper and cobalt sulfide deposits within neoproterozoic metasedimentary rocks heterogenite the main oxidized cobalt mineral is concentrated as cobalt caps along the top of silicified dolomite inselbergs the supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world class manganese cap deposits in western katanga i e the black earths that are exploited by both industrial and artisanal mining here we provide constraints on the genesis and the timing of these deposits ar ar analyses of oxidized mn ore and in situ u pb sims measurements of heterogenite yield mio pliocene ages the ar ar ages suggest a multi phase process starting in the late miocene 10 5 ma when the metal rich substratum was exposed to the action of meteoric fluids due to major regional uplift further oxidation took place in the pliocene 3 7 2 3 ma and formed most of the observed deposits under humid conditions co and mn caps on metal rich substrata and coeval fe laterites on barren areas these deposits formed prior to the regional shift toward more arid conditions in central africa arid conditions still prevailed during the quaternary and resulted in erosion and valley incision which dismantled the metal bearing caps and led to ore accumulation in valleys and along foot slopes

photometric anomalies of the lunar surface results from clementine data 1 we mapped the photometric characteristics of the lunar surface for several small areas using clementine uvvis camera images the maps of the phase function steepness showed several anomalous sites several small fresh impact craters have anomalous halos in these maps the phase function within the halos is less steep than for the surrounding mare surface we interpret these halos to be due to geologically recent impact caused alteration of the equilibrium millimeter scale regolith structure this equilibrium structure is established through micrometeoritic bombardment at a geologically short timescale an anomaly of the same signature was found at the apollo 15 landing site we interpret it as being a result of the regolith structure alteration with the lander jets a unique photometric anomaly not correlated with albedo was found within the reiner gamma formation we suggest that this anomaly is genetically related to the formation which indicates its young age this favors the impact hypothesis for the nature of the reiner gamma formation our study showed that mapping of photometric characteristics is a new powerful tool in studies of the surfaces of atmosphereless bodies future photometric studies of the moon with existing and new data sets are promising for a search for traces of recent seismic events studies of the recent population of meteoroids in the inner solar system an advance in the understanding of swirls etc

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

iron isotope fractionation in sediments of an oligotrophic freshwater lake abstract in situ iron fe isotope compositions of pore water and solid bound fe phases were measured in sediments of an oligotrophic sulfate poor freshwater lake lake tantare quebec canada previous work has shown that dissimilatory fe iii reduction dir is the main fe reduction pathway in this setting hence the lake sediments provide a well characterized natural environment in which to assess the transferability of dir produced fe isotope fractionations obtained in laboratory model systems iron redox cycling within the sediments produces isotopically light pore water fe ii average 56 fe 2 1 0 6 and sorbed fe ii average 56 fe 1 2 0 2 compared to the fe iii oxyhydroxide average 56 fe 0 6 0 2 and tightly solid bound fe ii average 56 fe 0 3 0 2 sediment pools the apparent isotope fractionation factor between aqueous fe ii and fe iii oxyhydroxides of 2 6 0 5 derived for lake tantare sediments falls within the range reported for experimentally determined isotopic fractionations during dir our results yield the first comprehensive set of fe isotope signatures associated with microbially driven fe redox cycling obtained directly in sediments of an oligotrophic freshwater lake

arsenic incorporation into authigenic pyrite bengal basin sediment bangladesh abstract sediment from two deep boreholes 400 m approximately 90 km apart in southern bangladesh was analyzed by x ray absorption spectroscopy xas total chemical analyses chemical extractions and electron probe microanalysis to establish the importance of authigenic pyrite as a sink for arsenic in the bengal basin authigenic framboidal and massive pyrite median values 1500 and 3200 ppm as respectively is the principal arsenic residence in sediment from both boreholes although pyrite is dominant ferric oxyhydroxides and secondary iron phases contain a large fraction of the sediment bound arsenic between approximately 20 and 100 m which is the depth range of wells containing the greatest amount of dissolved arsenic the lack of pyrite in this interval is attributed to rapid sediment deposition and a low sulfur flux from riverine and atmospheric sources the ability of deeper aquifers 150 m to produce ground water with low dissolved arsenic in southern bangladesh reflects adequate sulfur supplies and sufficient time to redistribute the arsenic into pyrite during diagenesis

imaging of lunar surface maturity the physical evolution of the lunar surface with exposure to the space environment is termed maturation and maturity is the degree to which a particular lunar soil possesses quantitative characteristics consistent with that exposure several quantitative measures or indices of maturity have been proposed and employed including the abundance of solar wind gas abundance of various types of agglutinates various measures of grain size and is feo among the changes attendant with space exposure are striking changes in the optical characteristics of soils mature lunar soils are dark red and exhibit reduced spectral contrast relative to immature soils this paper presents an optical maturity index that quantifies the spectral effects of maturation we show that this optical maturity index correlates with other maturity indices about as well as the accepted maturity indices correlate among themselves and is only weakly coupled to composition the modest correlations among maturity indices suggest important controls on individual maturity indices other than age uncorrelated variations in the rates of accumulation of individual indicators or variations in depths over which indicators are emplaced in addition to mixing these effects conspire to reduce the equivalence of maturity and duration of surface exposure optical maturity illustrates some of these effects showing that ejecta of large and small craters mature at different rates and that the interiors and ejecta of large craters exhibit systematically different optical maturities the same or analogous effects are likely to influence other maturity indices

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

fe isotope exchange between fe ii aq and nanoparticulate mackinawite fesm during nanoparticle growth we detail the results of an experimental study on the kinetics of fe isotope exchange between aqueous fe ii aq and nanoparticulate mackinawite fesm at 25 c and 2 c over a one month period the rate of isotopic exchange decreases synchronously with the growth of fesm nanoparticles 100 isotopic exchange between bulk fesm and the solution is never reached and the extent of isotope exchange asymptotes to a maximum of 75 we demonstrate that particle growth driven by ostwald ripening would produce much faster isotopic exchange than observed and would be limited by the extent of dissolution recrystallisation we show that fe isotope exchange kinetics are consistent with i fesm nanoparticles that have a core shell structure in which fe isotope mobility is restricted to exchange between the surface shell and the solution and ii a nanoparticle growth via an aggregation growth mechanism we argue that because of the structure of fesm nanoparticles the approach to isotopic equilibrium is kinetically restricted at low temperatures fesm is a reactive component in diagenetic pyrite forming systems since fesm dissolves and reacts to form pyrite isotopic mobility and potential equilibration between fesm and fe ii aq thus have direct implications for the ultimate fe isotope signature recorded in sedimentary pyrite

iron isotope fractionation between aqueous ferrous iron and goethite abstract the equilibrium fe isotope fractionation factor between aqueous fe ii and goethite has been experimentally measured to be 1 05 0 08 in 56 fe 54 fe 2 at 22 c using the three isotope method experiments were done using two sizes of goethite 81 11 nm and 590 42 nm and the experimental products were subjected to serial extraction using acid partial dissolution techniques to determine if surface fe iii atoms have different isotopic properties than the bulk goethite these experiments indicate that the interaction of fe ii aq and goethite is dynamic and results in complete or near complete fe isotope exchange over 30 days involving at least four components fe ii aq goethite sorbed fe ii and fe iii surface the equilibrium fractionation factor between fe ii aq and fe ii sorb is the same for both sizes of goethite at 56 fe fe ii aq fe ii sorb 1 24 0 14 this fractionation factor is significantly different than the results of previous studies on fe ii sorption to goethite the proportion of the fe iii surface component is greatest in the experiments that used the smallest goethite and the fe iii surface fe ii aq fractionation is estimated to be at least 2 1 the high fe iii surface fe ii aq fractionation may exert a significant influence on the fe isotope compositions of aqueous fe ii in natural systems that contain nanoparticulate goethite including those involving bacterial iron reduction these results demonstrate that the isotopic properties of nano scale minerals may be distinct from micron scale or larger minerals as is the case for other thermodynamic properties of nanoparticles

on the scalability of hydrogeochemical factors controlling arsenic mobility in three major inland basins of p r china abstract high as groundwater has been widely found in inland basins of p r china which has posed a serious health risk to the local residents although these inland basins experience the same arid to semiarid climate and are filled with quaternary sediments as concentrations show big variations three inlands basins have been investigated to characterize hydraulic conductivities and geochemistry of the groundwater and sediments and evaluate their controls on dissolved as concentrations dissolved as concentrations ranged between

contrasting parageneses in the manganese silicate carbonate rocks from parseoni sausar group india and their interpretation mn silicate carbonate rocks at parseoni occur as conformable lenses within metapelites and calc silicate rocks of the precambrian sausar group india the host rocks are estimated to have been metamorphosed at uppermost p t conditions of 500 550 c and 3 4 kbar the mn rich rocks contain appreciable fe reflected in the occurrence of magnetite 1 mno 1 magnetite 2 mno 15 and magnetite 3 mno 10 two contrasting associations of pyroxmangite with and without tephroite developed in the mn silicate carbonate rocks under isothermal isobaric conditions the former assemblage formed in relatively fe rich bulk compositions and equilibrated with a metamorphic fluid having a low xco2 0 2 and the latter equilibrated with a co2 rich fluid rhodochrosite magnetite 1 quartz protoliths produced the observed mineral assemblages on metamorphism partitioning of major elements between coexisting phases is somewhat variable fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism oxygen fugacity during metamorphism was monitored at or near the qfm buffer in tephroite bearing domains and the fluid composition was buffered by mineral reactions in respective domains as compared to other metamorphosed mn deposits of the sausar group the mn silicate carbonate rocks at parseoni were therefore metamorphosed at much lower fo2 through complex mineral fluid interactions

sedimentogenesis and hydrobiogeochemistry of high arsenic late pleistocene holocene aquifer systems geogenic enrichment of arsenic as in groundwater has been a topic of worldwide concern over the past several decades due to the severe health threat to an estimated over one hundred million people despite the genetic diversity and the spatial variability of as concentrations the most common geogenic as enriched groundwater occurs in flat low lying river floodplains and fluvial lacustrine plains within river systems draining the himalaya tectonic movement sedimentological processes and palaeoclimatic optima after the last glacial maximum have created favorable conditions for the formation of high as aquifer systems mainly within the late pleistocene holocene deposits the co deposition of as bearing fe oxides and reactive organic matter within sediments of deltas floodplains or fluvial lacustrine plains forms the premise for as release under reducing geochemical environments however depending on the reactivity of organic matter and as bearing fe oxides as well as the availability of aqueous sulfate geochemical mechanisms controlling as enrichment in the groundwater differ between the coastal floodplains of south and southeast asia and the fluvial lacustrine plains of north and central china the indigenous geomicrobes thriving to adapt to specific aquifer environments may mediate fe s as biogeochemical cycling responsible for as speciation transformation dissolution desorption and hence mobilization hydrologic processes regulated by the local and regional groundwater flow regimes control as transport as well as its spatial distribution special attention must be paid to anthropogenic perturbation from intensive groundwater pumping and land use changes that affect the aquifer hydrological conditions and the biogeochemical processes thus altering groundwater as levels to decipher the patterns of as variation within the aquifers an integrated approach characterizing the major sedimentological hydrobiogeochemical and anthropogenic processes influencing as release and transport needs to be applied to ensure sustainable water supply for domestic and agricultural needs

the age of supergene manganese deposits in katanga and its implications for the neogene evolution of the african great lakes region abstract supergene manganese deposits commonly contain k rich mn oxides with tunnel structure such as cryptomelane which are suitable for radiometric dating using the 39ar 40ar method in africa mn deposits have been dated by this method for localities in western and southern parts of the continent whereas only some preliminary data are available for central africa here we present new 39ar 40ar ages for mn oxide samples of the kisenge deposit in southwestern katanga democratic republic of the congo the samples represent supergene mn oxide deposits that formed at the expense of primary paleoproterozoic rhodochrosite dominated carbonate ores main phases of mn oxide formation are dated at c 10 5 ma 3 6 ma and 2 6 ma for a core that crosses a mineralized interval the latter shows a decrease in age with increasing depth recording downward penetration of a weathering front surface samples of the kisenge deposits also record a c 19 2 ma phase as well as c 15 7 ma 14 2 ma and 13 6 ma phases the obtained ages correspond to distinct periods of paleosurface development and stability during the mio pliocene in katanga because katanga is a key area bordered to the north by the congo basin and to the east by the east african rift system these ages also provide constraints for the geodynamic evolution of the entire region for the mio pliocene the kisenge deposits record ages that are not systematically found elsewhere in africa although the 10 5 11 ma event corresponds to a roughly simultaneous event in the kalahari manganese field south africa the rest of the katanga paleosurface record differs somewhat from records for other parts of africa for which older eocene ages have been obtained this difference is most probably related to the specific regional geodynamic context uplift of the east african plateau with associated erosion and the opening of the east african rift system at c 25 ma are events whose effects in the study area interfere with those of processes responsible for the development of continent wide paleosurfaces

comparison between datangpo type manganese ores and modern marine ferromanganese oxyhydroxide precipitates based on rare earth elements abstract datangpo type sedimentary manganese deposits which are located in northeastern guizhou province and its adjacent areas are mn carbonate type deposits hosted in black carbonaceous shale that represent a series of medium to large deposits containing a huge tonnage of reserves paas normalized rare earth element distribution patterns of manganese ores record hat shaped rey ree y plots characterized by pronounced middle rare earth element enrichment evident positive ce anomalies weak to strong positive eu anomalies and negligible negative y anomalies these rey geochemical characteristics are different from those of country rocks and record the processes and features of sedimentation and diagenesis manganese was precipitated as mn oxyhydroxide particles in oxidized water columns with the sorption of a certain amount of rare earth elements subsequently transforming from mn oxyhydroxides to rhodochrosite and redistributing rey in reducing alkaline pore water during early diagenesis a number of similarities can be observed through a comparison of datangpo type manganese ores and modern marine ferromanganese oxyhydroxide precipitates based on their rare earth elements the precipitation of datangpo type manganese ores is similar to that of hydrogenetic crusts and nodules based on their positive ce anomalies and relatively higher total rey concentrations however several differences also exist compared to hydrogenetic crusts and nodules datangpo type manganese ores record smaller positive ce anomalies lower total rey concentrations unobvious fractionation between y and ho and weak to strong positive eu anomalies these were caused by quicker sedimentary rates in the oxic water columns of the shallower basin after which pore water became strongly reducing and alkaline due to the degradation of organic matter in the early diagenetic stage in addition compared to typical deposits in the world datangpo type manganese ores are similar to hydrogenetic deposits and different than hydrothermal deposits all of these characteristics of manganese ores indicate that datangpo type manganese ores the principal metallogenic factors of which include oxidation conditions during deposition and reducing conditions during early diagenetic stages represent hydrogenetic deposits

how earth s atmosphere evolved to an oxic state a status report the evolution of the earth s atmosphere is essentially the story of atmospheric oxygen virtually every realm of the earth sciences biology geology geochemistry oceanography and atmospheric science is needed to piece together an understanding of the history of oxygen over the past decade new data from these fields has shown that there were two significant increases in atmospheric o2 levels at around 2 4 2 3 and 0 8 0 6 billion years ago respectively throughout earth history oceanic sulfate concentrations appear to have increased in accord with greater o2 levels while levels of methane a strong greenhouse gas may have inversely mirrored o2 both oxic transitions occurred in eras characterized by bsnowball earthq events and significant disturbances in the carbon cycle perhaps associated with increases in o2 and losses of methane to understand what controlled the oxygenation of the atmosphere it is necessary to determine how o2 is consumed on geologic time scales through reaction with reductants released from the earth s crust and mantle there was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere and a plausible explanation is that excess reductants scavenged photosynthetic o2 from the early atmosphere however a quantitative understanding of how and why o2 became abundant on our reducing planet is still lacking thus the study of the early atmosphere remains a frontier field with much to be discovered d 2005 elsevier b v all rights reserved

mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the huhhot basin inner mongolia observed as concentrations in groundwater from boreholes and wells in the huhhot basin of inner mongolia northern china range between 1 g l 1 and 1480 g l 1 the aquifers are composed of quaternary largely holocene lacustrine and fluvial sediments high concentrations are found in groundwater from both shallow and deep boreholes as well as from some dug wells well depths ranging between 10 m and 400 m populations from the affected areas experience a number of as related health problems the most notable of which are skin lesions keratosis melanosis skin cancer but with internal cancers lung and bladder cancer also having been reported in both the shallow and deep aquifers groundwaters evolve down the flow gradient from oxidising conditions along the basin margins to reducing conditions in the low lying central part of the basin high as concentrations occur in anaerobic groundwaters from this low lying area and are associated with moderately high dissolved fe as well as high mn nh4 dissolved organic c doc hco3 and p concentrations many of the deep groundwaters have particularly enriched doc concentrations up to 30 mg l 1 and are often brown as a result of the high concentrations of organic acid in the reducing groundwaters inorganic as iii constitutes typically more than 60 of the total dissolved as the highest as concentrations tend to be found in groundwater with low so4 concentrations and indicate that as mobilisation occurs under strongly reducing conditions where so4 reduction has been an active process high concentrations of fe mn nh4 hco3 and p are a common feature of reducing high as groundwater provinces e g bangladesh west bengal high concentrations of organic acid humic fulvic acid are not a universal feature of such aquifers but have been found in groundwaters from taiwan and hungary for example the observed range of total as concentrations in sediments is 3 29 mg kg 1 n 12 and the concentrations correlate positively with total fe up to 30 of the as is oxalate extractable and taken to be associated largely with fe oxides the release of as into solution under the reducing conditions is believed to be by desorption coupled with reductive dissolution of the fe oxide minerals the association of dissolved as with constituents such as hco3 doc and p may be a coincidence related to the prevalent reducing conditions and slow groundwater flow but they may also be directly involved because of their competition with as for binding sites on the fe oxides the huhhot groundwaters also have some high concentrations of dissolved u up to 53 g l 1 and f up to 6 8 mg l 1 in contrast to as u occurs predominantly under the more oxidising conditions along the basin margins fluoride occurs dominantly in the shallow groundwaters which have na and hco3 as the dominant ions the combination of slow flow of groundwater and the young age of the aquifer sediments are also considered potentially important causes of the high dissolved as concentrations observed as the sediments are likely to contain newly formed and reactive minerals and have not been well flushed since burial

lunar optical maturity investigations a possible recent impact crater and a magnetic anomaly 1 we have used maps of the omat optical maturity parameter along with other clementine uv vis image products to study two interesting lunar features related to albedo and optical maturity examination of the region of a small crater whose formation has been suggested to be a candidate for the flash photographed in 1953 by stuart demonstrates that the candidate crater is not unusually fresh compared to other small craters in the vicinity therefore it is unlikely that the formation of this impact crater generated the observed flash an area of unusually high albedo in the descartes highlands is also known to be a magnetic anomaly it has been proposed that the presence of the magnetic anomaly has prevented the solar wind from reaching the surface for billions of years and therefore that the area has not undergone normal space weathering and preserved a high albedo the high albedo is caused only by maturity differences with the surroundings not by an exotic composition the fact that the albedo anomaly has not darkened or reddened to the extent expected may be the result of the surface texture as revealed in 3 8 cm radar images we suggest that the fresh appearance is largely caused by overlapping ejecta from two nearby craters and continual exposure of immature material by the erosion of 1 to 10 cm sized fragments within the deposit the magnetic shielding mechanism if operative probably plays only a minor role in producing the descartes albedo anomaly

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

gld100 the near global lunar 100 m raster dtm from lroc wac stereo image data 1 we derived near global lunar topography from stereo image data acquired by the wide angle camera wac of the lunar reconnaissance orbiter camera lroc system from polar orbit tracks the lroc wac provides image data with a mean ground resolution at nadir of 75 m pixel with substantial cross track stereo overlap wac stereo images from the one year nominal mission and the first months of the science mission phase are combined to produce a near global digital terrain model dtm with a pixel spacing of 100 m the global lunar dtm 100 m or gld100 it covers 79 s to 79 n latitudes 98 2 of the entire lunar surface we compare the gld100 with results from previous stereo and altimetry based products particularly with the lunar orbiter laser altimeter lola altimetry which is the current topographic reference for the moon we describe typical characteristics of the gld100 and based upon the comparison to the lola data set assess its vertical and lateral resolution and accuracy we conclude that the introduced first version of the stereo based gld100 is a valuable topographic representation of the lunar surface complementary to the lola altimetry data set further improvements can be expected from continuative investigations

hydrological and sedimentary controls leading to arsenic contamination of groundwater in the hanoi area vietnam the impact of iron arsenic ratios peat river bank deposits and excessive groundwater abstraction groundwater contamination by arsenic in vietnam poses a serious health threat to millions of people in the larger hanoi area elevated arsenic levels are present in both the holocene and pleistocene aquifers family based tubewells predominantly tap the holocene aquifer while the hanoi water works extract more than 600 000 m3 day of groundwater from the pleistocene aquifer detailed groundwater and sediment investigations were conducted at three locations exhibiting distinct geochemical conditions i e i high levels of dissolved arsenic av 121 g l at the river bank ii low levels of dissolved arsenic av 21 g l at the river bank and iii medium levels of dissolved arsenic 60 g l in an area of buried peat and excessive groundwater abstraction seasonal fluctuations in water chemistry were studied over a time span of 14 months sediment bound arsenic 1 3 22 g g is in a natural range arsenic correlates with iron r2 0 8 with variation related to grain size sediment leaching experiments showed that arsenic can readily be mobilized at each of the three locations low levels of arsenic in groundwater 10 g l generally exhibit manganese reducing conditions whereas elevated levels are caused by reductive dissolution under iron and sulphate reducing conditions average arsenic concentrations in groundwater are twofold higher at the river bank than in the peat area the lower levels of arsenic contamination in the peat area are likely controlled by the high abundance of iron present in both the aqueous and sediment phases with median molar fe as ratios of 350 in water and 8700 in the sediments of the peat area reduced iron possibly forms new mineral phases that resorb or sequester previously released arsenic to the sediment despite similar redox conditions resorption is much less significant at the river bank fe as aq 68 fe as s 4700 and hence arsenic concentrations in groundwater reach considerably higher levels drawdown of holocene water to the pleistocene aquifer in the peat area caused by the pumping for the hanoi water works clearly promotes reducing conditions in pleistocene groundwater this demonstrates that excessive abstraction of water from deep wells i e wells tapping water below the arsenic burdened depth can cause a downward shift of iron reducing conditions and concurrently mobilize arsenic along the way vertical migration of reduced groundwater may also impact aquifers under natural hydrological conditions seepage of doc enriched groundwater derived from degradation of organic matter in the clayey sediments at the river bank was observed to enhance and maintain iron reducing conditions in the aquifer where organic matter is scarce once the aquifer becomes reduced arsenic is released from the aquifer solid hosts but additionally derives from the arsenic enriched groundwater seeping from the clay into the aquifer this behaviour is an important mechanism for arsenic contamination in aquifers that might not necessarily contain enough organic matter in their sediments to induce reducing conditions independently

experimental determination of fe isotope fractionation between aqueous fe ii siderite and green rust in abiotic systems abstract iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at 20 c applying a rayleigh distillation model to three experiments in which siderite feco3 was continuously precipitated from solutions over the course of tens of hours the derived fractionation factor is 103ln fe ii siderite 0 48 0 22 2 this experimentally determined fractionation factor is significantly smaller by 1 5 than that calculated from vibrational frequencies and mossbauer shifts geochim cosmochim acta 64 2000 849 865 geochim cosmochim acta 65 2001 2487 2497 and lies on the high end of that inferred from natural mineral assemblages where the fe ii aq siderite fractionation factor was inferred to lie between 1 7 and 0 3 contrib mineral petrol 144 2003 523 547 the measured fe ii aq siderite fractionation from this study suggests that siderite from archean and proterozoic rocks which have moderately negative 56fe values 1 5 to 0 5 may reflect precipitation from fe ii aq whose origin was hydrothermal in contrast natural siderites that have higher 56fe values apparently require non hydrothermal sources of fe and or different pathways of formation or may reflect the effects of cation substitution on the fe ii aq carbonate fractionation factor

a sm 1bnd isotopic study of heterogeneous granulites from the archean kasai lomami gabbro norite and charnockite complex zaire africa abstract sm 1bnd model ages t dm of acid granulites from the kasai complex agree with previous rb 1bsr and u 1bpb ages of 2 8 ga for the early and main stage of granulite facies metamorphism these sm 1bnd results give an upper limit of 3 1 ga for the age of the protoliths of the charno enderbites and metapelitic gneisses which represent the majority of the acid rocks of the complex a sm 1bnd study whole rock and internal isochrons and t dm ages of the associated basic granulites meta gabbro norite massif and metadolerite dykes crosscutting the acid granulites demonstrates that the second stage of granulite metamorphism corresponding to regional deformation of all the rocks is nearly coeval with the emplacement of the basic rocks at 2 4 ga

geochemistry of aquifer sediments and arsenic rich groundwaters from kandal province cambodia abstract elevated as is well known to be present in aquifers utilised for drinking water and irrigation in west bengal and bangladesh this problem has also more recently been discovered in other parts of asia including vietnam cambodia inner mongolia and the middle ganges plain analysis of groundwaters in kandal province of cambodia found waters with comparable geochemistry to the as rich groundwaters of the west bengali delta similarities included high but heterogeneous as distributions predominantly in the form as iii high fe moderate to high hco 3 circumneutral ph low so 4 2 and geochemical components indicative of reducing conditions good positive correlations between as fe hco 3 and nh 4 and dissolved organic c is consistent with as release predominantly via microbially mediated reductive dissolution of as bearing fe iii oxides further evidence for such a process is found from correlations between as fe and organic matter from analysis of aquifer sediments by the presence of goethite in the finer fractions and from the association of as with amorphous poorly crystalline and well crystallised hydrous fe oxides the presence of several high as but low fe wells implies that microbes could have a more direct role in mediating as release via the direct utilisation of fe iii or as v as electron acceptors the presence of elevated as in waters with short aquifer residence times as indicated by their geochemical signature highlights the possible vulnerability of these aquifers to the influx of surface derived waters providing an additional source of labile organic c that could exacerbate as release by stimulating microbial activity

global mapping of lunar crustal magnetic fields by lunar prospector abstract the lunar prospector electron reflectometer has obtained the first global map of lunar crustal magnetic fields revealing that the effects of basin forming impacts dominate the large scale distribution of remanent magnetic fields on the moon the weakest surface magnetic fields 40 nt are diametrically opposite to these same basins this pattern is present though less pronounced for several other post nectarian impact basins larger than 500 km in diameter the reduced strength and clarity of the pattern for older basins may be attributed to 1 demagnetization from many smaller impacts which erases antipodal magnetic signatures over time 2 superposition effects from other large impacts and 3 variation in the strength of the ambient magnetizing field the absence of fringing fields stronger than 1 nt around the perimeter of the imbrium basin or associated with craters within the basin implies that any uniform magnetization of the impact melt must be weaker than 10 6 g cm3 g 1 this limits the strength of any steady ambient magnetic field to no more than 0 1 oe at the lunar surface while the basin cooled for tens of millions of years following the imbrium impact 3 8 billion years ago

influence of ph and dissolved si on fe isotope fractionation during dissimilatory microbial reduction of hematite microbial dissimilatory iron reduction dir has been identified as a mechanism for production of aqueous fe ii that has low 56 fe 54 fe ratios in modern and ancient suboxic environments that contain ferric oxides or hydroxides these studies suggest that dir could have played an important role in producing distinct fe isotope compositions in precambrian banded iron formations or other marine sedimentary rocks however the applicability of experimental studies of fe isotope fractionation produced by dir in geochemically simple systems to ancient marine environments remains unclear here we report fe isotope fractionations produced during dissimilatory microbial reduction of hematite by geobacter sulfurreducens in the presence and absence of dissolved si at neutral and alkaline ph hematite reduction was significantly decreased by si at alkaline but not neutral ph presumably due to si polymerization at the hematite surface the presence of si altered fe isotope fractionation factors between aqueous fe ii or sorbed fe ii and reactive fe iii reflecting changes in bonding environment of the reactive fe iii component at the oxide surface despite these changes in isotopic fractionations our results demonstrate that microbial fe iii oxide reduction produces fe ii with negative d 56 fe values under conditions of variable ph and dissolved si similar to the large inventory of negative d 56 fe in neoarchean and paleoproterozoic age marine sedimentary rocks

coupled fe and s isotope evidence for archean microbial fe iii and sulfate reduction direct fossil evidence for early microbial life on earth is rare microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in earth9s history we present fe isotopic data for sedimentary pyrite from the 2 7 ga belingwe sedimentary basin in zimbabwe isotopically light fe and a remarkable covariation between fe and s isotopes provide strong evidence for coexisting fe and s reduction our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial fe reduction the covariation of fe and s isotopes may provide a useful new tracer of microbial evolution early in earth history

aqueous geochemistry and its influence on the partitioning of arsenic between aquifer sediments and groundwater a case study in the northwest of the hetao basin the consumption of groundwater containing high arsenic concentrations has caused serious public health problems in the hetao basin in inner mongolia however the relationship between as contents of aquifer sediments and in groundwater at the same depth under different redox conditions is currently poorly understood in order to determine this relationship in the hetao basin four multilevel wells were set up to monitor as and other physiochemical parameters in groundwater at different depths in the aquifer sediment samples were also collected at equivalent depths to groundwater sampling intervals during the drilling of boreholes results showed that groundwater as concentrations ranged between 2 26 and 583 g l 1 in suboxic shallow groundwater depths 14 m with relatively lower eh values the as contents of sediments had a negative correlation with the grain size of sediments showing the high affinity of fine grained sediments to as positive relationships were observed between dissolved as concentrations and contents of extractable as including s1 weakly bound as s2 strongly bound as and f3 fe mn oxide bound as in aquifer sediments for a set of depth matched samples in reducing and suboxic environments the relationships showed 263 g l 1 dissolved as per 1 mg kg 1 s2 extracted as in reducing environment and 9 4 g l 1 dissolved as per 1 mg kg 1 s2 extracted as in suboxic environment in the reducing environment as tended to be partitioned into the liquid phase where s2 as showed a better correlation with dissolved as than s1 as in the suboxic environment as tended to be fixed in the solid phase where s1 as of aquifer sediments was closely related to concentrations of dissolved as in groundwater

exsolution lamellae of kirschsteinite in magnesium iron olivine from an angrite meteorite exsolution phenomena of kirschsteinite cafesi04 in olivine have been studied by single crystal x ray diffraction techniques and scanning electron microscopy sem of oriented polished thin sections pts of three single crystals separated from the antarctic angrite lew86010 supplemented by micro area x ray diffraction with the laue method mxl by synchrotron radiation sr for pts of a rock chip of lew8601o the cell dimensions of the host olivine and exsolved kirschsteinite are a 4 79 3 b 10 39 5 and c 6 06 3 a and a 4 87 5 b 11 14 10 and c 6 36 5 a respectively from the precession photos the pts of olivine single crystals oriented parallel to 100 show exsolution lamellae of kirschsteinite up to i 0 m in width the two sets of lamellae are symmetrically related and parallel to 031 and 031 electron microprobe analysis gave si02 33 1 ti02 0 07 al203 0 03 feo 49 4 mno 0 61 mgo 13 4 cao 2 2 cr203 0 02 v203 0 01 nio 0 05 sum 99 4 wt for the host olivine and si02 33 3 ti02 0 03 feo 31 5 mno 0 39 mgo 5 4 cao 28 5 cr203 0 02 nio 0 05 sum 99 2 wt for the exsolved kirschsteinite the results from mxl for the olivine crystals on the rock pts are compatible with the observation on the single crystals that the lamellae are parallel to 031 and 031 the 031 and 031 planes have been known to be twin planes for olivine and the twinning is by reticular pseudomerohedry based on a quadruple lattice although other reported exsolved precipitates in meteoritic olivines exist as inclusions kirschsteinite in lew86010 olivine takes the form of lamellae our explanation is that lew86010 olivine is fe rich and that lamellar precipitates are more easily formed than inclusions because exsolution lamellae along 031 in fe rich olivine maintain lattice coherency

arsenic enrichment in groundwater of west bengal india geochemical evidence for mobilization of as under reducing conditions abstract the mechanism of as release and source s of as has been investigated in a small part of a watershed in the murshidabad district of west bengal analyses include major ion and trace element concentrations as well as o h and s isotope ratios of groundwater surface water and a thermal spring the results indicate that all water samples belong to the ca hco 3 type except for the thermal spring which is of the na hco 3 type shallow and deeper groundwaters have distinct hydrochemical features high as contents were registered only in the deeper groundwater horizon factor analysis and the distribution pattern of major and trace elements indicate that as is present in the aquifer as a scavenged phase by fe iii and to a lesser extent by mn iv phases the release of as into the groundwater occurs gradually in successive stages corresponding to the actual redox state in the aquifer the main stage of as release is related to the bacterial reduction of fe iii to fe ii i e to the simultaneous dissolution of fe oxyhydroxides low redox conditions in highly polluted areas are indicated by low so 4 concentration and high 34 s values during bacterial so 4 reduction residual so 4 in groundwater is depleted in the lighter s isotope 32 s however the cause of the gradual decrease of the redox state in the groundwater is still not well understood

petrology of mn carbonate silicate rocks from the gangpur group india abstract metamorphosed mn carbonate silicate rocks with or without oxides assemblage i and mn silicate oxide rocks with minor mn carbonate assemblage ii occur as conformable lenses within metapelites and metacherts of the precambrian gangpur group india the petrology of the carbonate minerals rhodochrosite kutnahorite and calcite that occur in these two assemblages is reported early stabilisation of spessartine aegirine quartz and carbonates in a wide solid solution range was followed by pyroxmangite tephroite rhodonite through decarbonation reactions subsequently jacobsite hematite braunite hollandite and hausmannite have formed by decarbonation oxidation processes during prograde metamorphism textural characteristics and chemical composition of constituent phases suggest that the mineral assemblages reflect a complex relationship between protolithic composition variation of xco2

iron isotope fractionation in a sulfide bearing subterranean estuary and its potential influence on oceanic fe isotope flux we trace pathways of fe reactions in the indian river lagoon florida usa subterranean estuary using fe isotopes to provide new constraints on fe isotopic fractionation in a sulfide bearing subterranean estuary porewater 56fe values increase from 1 16 at 115cm depth to 0 2 at 7cm depth due to isotope fractionation in three distinct lithostratigraphic zones the deepest zone contains orange sands with elevated fe oxide contents 0 2wt that dissolve through diagenetic fe oxide reduction and elevate fe concentrations in porewaters 100 to 300 m l this reaction causes porewater 56fe values to be 1 lighter than the sediment 56fe values an intermediate zone contains white fe poor sands with fe oxide contents 0 1wt and dissolved fe concentrations 20 m l this zone is a sink for dissolved fe through adsorption of isotopically heavy dissolved fe ii onto mineral surfaces this adsorption results in porewater 56fe values that are as much as 1 8 lighter than sediment 56fe values the uppermost zone contains organic carbon and fe sulfide rich black sediments with low dissolved fe 1 m l and elevated porewater sulfide up to 600 m l concentrations precipitation of isotopically light fe sulfides increases the porewater 56fe values as much as 0 68 more than corresponding sediment 56fe values the near surface fe sulfide precipitation delivers to the lagoon dissolved fe with slightly positive 56fe values averaging about 0 24 via submarine groundwater discharge sgd iron sulfide precipitation in sulfide containing subterranean estuaries thus may result in a previously unidentified source of isotopically heavy fe to the coastal oceans

paleoproterozoic manganese and base metals deposits at kisenge kamata katanga d r congo the kisenge kamata manganese deposit katanga d r congo belongs to a group of paleoproterozoic sedimentary manganese ore occurrences that have been recognized for various parts of africa it is a relatively small deposit with total estimated reserves of similar to 12 mt ore but it is of major importance for understanding key aspects of the geology of the kasai block of the congo craton the available constraints indicate that the deposit formed in the 2 0 1 9 ga time interval in the orosirian period the manganese rich facies of the kisenge kamata deposits range in composition from manganese carbonate rocks to non calcareous deposits dominated by manganese rich garnet associated rocks are mainly grey graphitic shales the kisenge kamata sediments are significantly enriched in cu co ni zn and other accessory metals v mo ga field constraints and geochemical data suggest that the kisenge kamata sediments were probably deposited in a relatively small intracratonic basin graben within the kasai block with an inlet allowing periodical access for ocean water influx carrying mn ree and other metals primary carbonate ore has relatively low delta c 13 3 to 6 parts per thousand and high delta o 18 13 to 20 parts per thousand values confirming their derivation from marine fluids

the distribution and extent of lunar swirls abstract the mysterious high reflectance loops and ribbons known as swirls are not uncommon on the moon but are apparently unique to this body we mapped their distribution and extent using ultraviolet visible images from the lunar reconnaissance orbiter camera we find two main geographic groupings of swirls south pole aitken basin and marginis king and a host of smaller features including swirls near craters abel crozier dewar and dufay x all mapped swirls are associated with magnetic anomalies and swirls have magnetic field strengths shifted to higher values than their background though there is not a 1 1 correspondence between the locations of swirls and magnetic anomalies swirls are also found in regions with iron abundances shifted to higher than background values which could indicate that their formation is inhibited by low iron content the most distinguishing characteristic of swirls is a low 321 415 nm ratio coupled with moderate to high reflectance and swirls generally have high optical maturity omat parameter values stronger 1 m bands and shallower normalized continuum slopes than their surroundings consistent with a surface that has experienced less space weathering however some swirls cannot be discerned in omat or band depth images areas with low 321 415 nm ratios but non distinct visible near infrared properties could be related to the presence of fresh silicates or a glassy component that does not have a substantial abundance of embedded large submicroscopic iron grains i e a difference in the agglutinate fraction of the soil swirl color properties vary with distance from copernican and some eratosthenian craters their association with eratostheninan craters suggests fresh material may be preserved longer in swirls than in non swirl regions

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

optical maturity of ejecta from large rayed lunar craters lucey et al 2000 have developed a methodology for extracting an optical maturity parameter omat from multispectral clementine images the omat parameter characterizes the overall maturity of lunar soils and crater ejecta by changes in reflectance spectra using these omat images we surveyed large craters 20 km diameter on the moon that had previously been mapped as possessing or possibly possessing rayed ejecta we generated average radial profiles of omat values for rays of these large craters from these profiles we classified the craters into three relative age groups 1 older than copernicus inferred age of 810 myr 2 intermediate and 3 as young or younger than tycho inferred age of 109 myr we suspect that there is a bias to our classification scheme such that the omat profiles of smaller craters look like that of larger but older craters nevertheless some large craters such as eudoxus 67 km and aristillus 55 km are now known from this study to have optically mature ejecta and therefore are suspected to be older than copernicus this is consistent with an age of 1 3 gyr suggested for aristillus by ryder et al 1991 such craters were included by mcewen et al 1997 when estimating the density of craters younger than or contemporaneous with copernicus therefore the case for a modest increase in the cratering rate in the past 800 myr versus the previous 2 4 gyr indicated from that work has been weakened grier and mcewen 2001 given current constraints on dating large and recent lunar craters we cannot support or disprove the hypothesis that there has been a significant increase in the rate of large terrestrial impact events in the past 100 400 myr

earthquake impact on iron isotope signatures recorded in mineral spring water we investigated the iron isotope signatures of dissolved fe in the water of the wettinquelle mineral spring bad brambach germany by time series sampling covering seismically active periods related to tectonic activity near the eger rift system in central europe our objective was to test whether fe isotopes trace earthquake induced abiotic and biotic changes in the fluid rock interaction of the deep fissured granitic aquifer we found that the dissolved fe isotope signatures in spring water are distinct from the granitic source signature 56fe 0 09 particularly we discovered that water 56fe values are remarkably stable 0 01 0 11 2sd n 4 before and during a strong seismic swarm period in 2000 local magnitudes ml 3 while o2 and h2 concentrations in water decrease and dissolved fe content increases later recurring events of lower 56fe values down to 0 3 are observed in the period from 2001 to 2003 with intermittent seismic events 1 ml 2 6 the observations indicate a time lag between tectonic forcing and fe isotope response the role of abiotic fluid rock interaction and fe utilizing bacteria identified in the mineral spring water on fe isotope fractionation is discussed we explain recurring changes towards isotopically lighter values by a combination of fe dissolution from deep granite and admixture of isotopically light fe generated by a complex combination of abiotic and biotic processes operating in the aquifer when disturbed by swarm earthquakes events we propose a conceptual model scenario of earthquake triggered changes in biogeochemical processes

manganese deposits in the global lithogenetic system quantitative approach abstract a computerized data base including 330 localities and believed to represent at least 95 of the presently recorded spot accumulations of ore grade mn on land is the basis for a quantitative analysis of terrestrial mn resources this file is reprinted in full appendix the present subaerially exposed global ore mn resources are calculated as 17 9 109 t mn 1 in this figure are included the actually mineable straight mn deposits 8 7 109 t mn potentially mineable land based mn accumulations 9 2 109 t mn and actual or potential mn that could be extracted as a byproduct of mining other metals 0 6 109 t mn this distribution is strongly influenced by giant accumulations where the single exceptional kalahari mn field contains over 50 of the presently economic mn ore reserves or 23 42 of the global land mn resources a set of attributes has been selected to treat the global ore mn population in terms of genesis geotectonic and environmental setting and lithologic associations in terms of genesis precipitation from aqueous solutions was responsible for at least 99 of the contemporaneous and probably also the past mn accumulations now exposed on land weathering of mn orebodies has left its mark on 93 of the mn localities and 24 are now represented entirely by supergene assemblages less than 0 01 of the ore mn resources however are formed by weathering related accumulation over silicate rocks ultramafics in terms of geotectonic environments the bulk of the land based mn deposits 97 formed in intraplate and stable continental margin settings 3 1 formed along pacific type and rift type continental margins and only 0 00045 of the deposits formed in an oceanic setting this is in contrast with the outstanding mn accumulating capacity of the present ocean and is a consequence of the low preservation potential of the oceanic domain in terms of lithologic associations 96 of the mn in land based deposits is present in marine sedimentary associations 70 of mn is in banded iron formations 14 4 is in detrital and 11 1 is in carbonate dominated associations chert and jasper limestone sandstone shale and banded iron formation are statistically the most common immediate hosts to mn ores with recorded hosting frequencies of 79 50 45 37 and 35 respectively in terms of geological history the lower proterozoic accounts for 58 9 of the prreserved ore mn on land followed by oligocene 17 2 jurassic 6 2 and middle proterozoic 4 5 in terms of the intensity of mn accumulation per one million years of geological time oligocene 110 106 t mn ma is two orders of magnitude greater than the nearest time periods jurassic 8 9 106 t mn ma and lower proterozoic 6 5 106 t mn ma the historical distribution pattern of the land based mn deposits seems to indicate that accumulation of the bulk of the present ore grade mn is the result of repeated recycling with a land ocean trend abruptly initiated at the time of early cratonization about 2 5 ga this has been supplemented by a substantially less significant but remarkably steady reverse trend of addition of juvenile mn released from the mantle into the crust mafics and particularly basalts are the most important intermediaries in the cumulative secular increase of liberated and accumulated mn in the crust direct to indirect proven to hypothetical spatial coincidence of basalts and mn ores can be demonstrated on at least 169 localities out of 330 51 evaluated

natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water the example of west bengal and its worldwide implications in order to investigate the mechanism of as release to anoxic ground water in alluvial aquifers the authors sampled ground waters from 3 piezometer nests 79 shallow 80 m wells in an area 750 m by 450 m just north of barasat near kolkata calcutta in southern west bengal high concentrations of as 200 1180 mug l 1 are accompanied by high concentrations of fe 3 13 7 mgl 1 and po4 1 6 5 mg l 1 ground water that is rich in mn 1 5 3 mg l 1 contains 50 mug l 1 of as the composition of shallow ground water varies at the 100 m scale laterally and the metre scale vertically with vertical gradients in as concentration reaching 200 mug l 1 m 1 the as is supplied by reductive dissolution of feooh and release of the sorbed as to solution the process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit in well waters thermotolerant coliforms a proxy for faecal contamination are not present in high numbers 10 cfu 100 ml in 85 of wells showing that faecally derived organic matter does not enter the aquifer does not drive reduction of feooh and so does not release as to ground water arsenic concentrations are high much greater than50 mug l 1 where reduction of feooh is complete and its entire load of sorbed as is released to solution at which point the aquifer sediments become grey in colour as feooh vanishes where reduction is incomplete the sediments are brown in colour and resorption of as to residual feooh keeps as concentrations below 10 mug l 1 in the presence of dissolved fe sorbed as released by reduction of mn oxides does not increase as in ground water because the as resorbs to feooh high concentrations of as are common in alluvial aquifers of the bengal basin arise because himalayan erosion supplies immature sediments with low surface loadings of feooh on mineral grains to a depositional environment that is rich in organic mater so that complete reduction of feooh is common c 2004 published by elsevier ltd

continuous growth of hydrogenetic ferromanganese crusts since 17 myr ago on takuyo daigo seamount nw pacific at water depths of 800 5500 m ferromanganese crusts cover all outcrops on takuyo daigo seamount traversed during remotely operated underwater vehicle rov dives except in places covered by foraminifera sand takuyo daigo is a cretaceous seamount located in the northwest pacific ocean geological and bathymetric mapping provide the framework for this study chemical and mineralogical analyses of the hydrogenetic ferromanganese crusts show temporal and spatial variations typical of those found in previous studies outcrops from 800 to 5500 m water depths are covered with ferromanganese crusts up to 105 mm thick beryllium isotope dating shows that the crusts have apparently been growing continuously at all water depths even through the modern oxygen minimum zone omz contrary to some earlier models for deposition growth rates vary from 2 3 to 3 5 mm myr with fe or mn fluxes of 0 07 0 11 g cm2 myr since the early middle miocene co mn ratios decrease with water depth while fe mn and other metallic elements increase or show no change based on the analysis of the uppermost crust surface this is probably because co is the most abundant redox sensitive element derived from seawater that occurs in crusts

multistage genesis of the late cretaceous manganese karst hosted tasdremt deposit high atlas morocco the eastern part of the souss basin morocco contains several mn deposits in the tasdremt district three mn orebodies occur within the cenomanian turonian dolostones and the senonian coniacian to maastrichtian detrital series the main orebody being located at the boundary between them the mn ores consist of coronadite group minerals mostly coronadite and hollandite in a karstified dolostone new field observations petrographic analyses and geochemical data define the tasdremt deposits as a karst hosted accumulation 11 60 wt mn particularly enriched in ba 1 5 8 2 wt and pb 1 0 5 0 wt with poor contaminations in al fe and p this study shows that the ore forming process is similar to that occurring in the imini c3 level located 100 km to the north east such similarities with the high grade pyrolusite bearing ore suggest that the tasdremt deposit is a lateral equivalent of the imini deposits however the scarcity of pyrolusite in tasdremt results in lower mn grades the tasdremt ores being considered an aborted incomplete system in comparison with the imini deposits 40ar 39ar geochronology of k bearing mn oxides yields late cretaceous ages defining three phases at 91 5 ma 77 5 82 ma and 65 67 ma although the source of metals remains hypothetical mineralizing fluids were carried by o2 free groundwater that mixed with o2 rich shallow meteoric waters at the tasdremt depositional site the dissolution of the host dolostones and the karst environment have provided suitable conditions for the precipitation of mn oxides causing the coeval increase of ph and eh respectively the early atlasic deformation during the late cretaceous is associated with mineralization events and was responsible for creation of low stand reliefs from tasdremt to imini this period enabled karstification and mineralization connecting the tasdremt deposits to other african mn deposits is difficult since the latter consist of laterite resting above paleoproterozoic mn protores and consequently formed under different conditions from karst hosted deposits it is likely that other mn occurrences formed along the atlas belt in similar settings

fe isotope fractionation on fes formation in ambient aqueous solution iron sulfide phases are the ultimate repository of iron and reduced sulfur in sediments the sulfur isotope geochemistry of pyrite has had much to tell us about modern and ancient earth environments and it is likely that fe isotopes will too once fractionations for key processes are known we report the results of an experimental study of fe isotope fractionation on precipitation of fes synthetic mackinawite from excess aqueous fe ii solutions by addition of sodium sulfide solution at 2 40 c the results show a significant kinetic isotope effect in the absence of a redox process no detectable effect of temperature on the fractionation factor was observed the fe isotope fractionation for zero age fes is fe ii fes 0 85 0 30 across the temperature range studied giving a kinetic isotope fractionation factor of fe ii fes 1 0009 0 0003 on ageing the fes in contact with aqueous fe ii becomes progressively isotopically heavier indicating that the initial fractionations are kinetic rather than equilibrium from published reaction mechanisms the opportunity for fe isotope fractionation appears to occur during inner sphere ligand exchange between hexaqua fe ii and aqueous sulfide complexes fe isotope fractionation on mackinawite formation is expected to be most significant under early diagenetic situations where a readily available reactive fe source is available since fes aq is a key reactive component in natural pyrite formation kinetic fe isotope fractionations will contribute to the fe isotope signatures sequestered by pyrite subject to the relative rate of fes2 formation versus fes fe ii aq isotopic equilibration

a statistical analysis of the carbon isotope record from the archean to phanerozoic and implications for the rise of oxygen organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales permanent burial of organic carbon in the crust or mantle oxidizes the surface environment atmosphere ocean and biosphere by removing reduced carbon it has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout earth9s history thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen schidlowski 1988 catling and others 2001 holland 2002 holland 2009 kump and others 2009 rothman 2015 however the opposite conclusion has been drawn from the same carbon isotope record des marais and others 1992 bjerrum and canfield 2004 to test these opposing claims we compiled an updated carbon isotope database and applied both parametric and non parametric statistical models to the data to quantify trends and mean level changes in fractional organic carbon burial with associated uncertainties and confidence levels we first consider a conventional mass balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon for this model statistical analysis implies fractional organic burial has increased over earth history by a factor of 1 5 relative to organic burial at 3 6 ga with the 95 percent confidence interval ranging from factors of 1 2 to 2 0 an increase in organic burial by a factor of 1 2 cannot explain the rise of oxygen whereas an increase by a factor of 2 could conceivably explain the rise of oxygen there is however a highly significant and well constrained increase in organic burial from the proterozoic to the phanerozoic we also analyze changes in the difference between carbonate and organic carbon isotopic ratios over earth history there is a statistically significant increase in this difference from the early to late archean possibly caused by increased biological fractionation due to methanotrophic recycling this transition is consistent with the evolution of oxygenic photosynthesis at 2 8 ga or earlier finally we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes specifically ocean crust carbonatization and authigenic carbonates because the size of these fluxes has a large poorly constrained range our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all however it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time in which case conclusions from the conventional model would be valid

malachite an indicator of major pliocene cu remobilization in a karstic environment katanga democratic republic of congo abstract malachite cu3 oh 2 co3 is a common mineral in the central african copperbelt where it constitutes a high grade 57 cu copper ore and is exploited industrially and by artisanal miners the genesis of this supergene ore in katanga has yet received little attention here we focus on the genetic processes giving rise to this regionally important mineral malachite is secondary to primary copper sulfides which indicates that it ultimately formed by oxidation of the latter in fine grained siliciclastic host rocks it is often partly impregnative commonly with variations in malachite content parallel to the bedding this type may form either by absolute enrichment or by in situ replacement of sulfides within voids of various types including dissolution cavities in carbonate host rocks it precipitates in distinctive forms such as thick laminated crusts and speleothemes up to several tens of centimeters in length ree geochemistry suggests that malachite formed in groundwater environments through in situ recombination of carbonate ions originating from dissolving host rock carbonates and downward percolating cu2 leached from copper sulfides thermodynamic modeling confirms this genetic environment and shows that chrysocolla cu al 2h2si2o5 oh 4 nh2o coexists with malachite when si content increases in the groundwater fluids the importance of karstic features in katanga is also re evaluated karstic features appear to be rather frequent and to constitute ideal site traps for the formation of secondary base metal ores

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

a review of high arsenic groundwater in mainland and taiwan china distribution characteristics and geochemical processes china is a typical high as region where 20 provinces have high as groundwaters among 34 provinces these groundwaters usually occur in both arid semiarid inland basins and river deltas in the inland basins mainly distributed in the northwest of china shallow groundwaters usually have high as concentrations in alluvial lacustrine or lacustrine sediment aquifers while high as groundwater mainly occurs in fluvial marine sedimentary aquifers in the river deltas which have been affected by transgression in both the inland basins and the river deltas high as groundwaters mainly occurring in reducing conditions are characterized by high fe and mn concentrations high ph and hco3 concentration and relatively low no3 and so42 concentrations although as contents are well correlated to fe mn contents in the aquifer sediments groundwater as concentrations are generally independent of sediment as contents redox processes microbe related reduction and desorption processes are the major geochemical processes for as enrichment in groundwaters in reducing conditions both reductive dissolution of fe oxides and reductive desorption of as are believed to result in as mobilization which would be catalyzed by indigenous microbes although decomposition of the low molecular weight organic matter during microbe metabolization would also release the colloid bound as into groundwater the cycling of colloidal as still needs to be further investigated during redox processes besides high ph and high hco3 lead to as desorption from adsorption sites in the aquifer systems however the contribution of competitive desorption to high as concentrations is still unknown and remains to be discovered relative to reductive dissolution of fe oxides especially in the inland basins

effect of indigenous bacteria on geochemical behavior of arsenic in aquifer sediments from the hetao basin inner mongolia evidence from sediment incubations abstract incubation studies were carried out using 5 freshly collected sediments from shallow aquifers of the hetao basin inner mongolia the aquifer sediments covering a range of redox conditions as indicated by their deep grey to yellow color were mixed with degassed artificial as solution or degassed deionized water at a ratio of solid to water of about 1 10 wt wt suspensions which were either amended with glucose or autoclaved were incubated in parallel with unamended suspensions five microcosm cultures of unamended sediments gradually release the equivalent of 0 03 0 30 g g as to the dissolved phase the addition of glucose as a potential electron donor results in a marked stimulation in the mobilization of as 0 71 3 81 g g in the amended incubations for all sediments the quantity of as released accounts for 60 70 of as bound to fe mn oxides in the original sediments the microbially mediated mobilization of as with the organic nutrient as an electron donor is strongly associated with the as bound to fe mn oxides as well as the exchangeable as during the incubations amended with glucose 2 4 of the sediment fe is released the results suggest that the introduction of labile dissolved organic c into the yellowish sediment aquifers with as free groundwater would reduce a significant proportion of the fe iii oxyhydroxides mediated by anaerobic bacteria respiration and increase groundwater as concentrations

oxygen isotope perspective on crustal evolution on early earth a record of precambrian shales with emphasis on paleoproterozoic glaciations and great oxygenation event abstract we present stable isotope and chemical data for 206 precambrian bulk shale and tillite samples that were collected mostly from drillholes on all continents and span the age range from 0 5 to 3 5 ga with a dense coverage for 2 5 2 2 ga time interval when earth experienced four snowball earth glaciations and the irreversible rise in atmospheric o2 we observe significant downward shift of several and a smaller range of 18 o values 7 to 9 in shales that are associated with the paleoproterozoic and potentially neoproterozoic glaciations the paleoproterozoic samples consist of more than 50 mica minerals and have equal or higher chemical index of alteration than overlying and underlying formations and thus underwent equal or greater degrees of chemical weathering their pervasively low 18 o and d down to 85 values provide strong evidence of alteration and diagenesis in contact with ultra low 18 o glacial meltwaters in lacustrine deltaic or periglacial lake sikussak type environments associated with the paleoproterozoic glaciations the d silicate values for the rest of precambrian shales range from 75 to 50 and are comparable to those for phanerozoic and archean shales likewise these samples have similar ranges in 13 c org values 23 to 33 pdb and corg content 0 0 to 10 wt to phanerozoic shales precambrian shales have a large range of 18 o values comparable to that of the phanerozoic shales in each age group and formation suggesting similar variability in the provenance and intensity of chemical weathering except for the earliest 3 3 3 5 ga archean shales which have consistently lower 18 o values moreover paleoproterozoic shales that bracket in age the great oxidation event goe overlap in 18 o values absence of a step wise increase in 18 o and d values suggests that despite the first order change in the composition of the atmosphere weathering cycle was not dramatically affected by the goe at 2 4 2 3 ga shales do not show comparable 18 o rise in the early phanerozoic as is observed in the coeval 18 o trends for cherts and carbonates there is however a sharp increase in the average 18 o value from the early archean to the late archean followed by a progressively decelerating increase into the phanerozoic this decelerating increase with time likely reflects declining contribution of mantle extracted normal 18 o crust and lends support to crustal maturation and increasing 18o sequestration into the crust and recycling of high 18 o and 87sr 86sr sedimentary rocks this secular increase in the 18 o composition of the continental crust could have also had a mild effect on seawater 18 o composition