magnetic remanence in the murchison meteorite the murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules various inclusions and matrix with occasional porphyroblasts of olivine and or pyroxene it also contains amino acids that may have served as the necessary components for the origin of life magnetic analyses of murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence the rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids the level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition we support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination when using this approach 1 2 of the natural remanence left in terrestrial rocks with trm and or crm determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral with the exception of hematite the same method is applied to the murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200 2000 nt during and or after the formation of the parent body

a bistable organic rich atmosphere on the neoarchaean earth it has been hypothesized that before widespread oxygenation about 2 45 billion years ago the earth s atmosphere contained an organic haze similar to that on titan however these theoretical predictions have not been substantiated by geological evidence here we use multiproxy geochemical analyses of sediments from the 2 65 2 5 billion year old ghaap group in south africa to reconstruct ocean and atmospheric chemistry during this time we find evidence for oxygen production in microbial mats and localized oxygenation of surface waters carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane consistent with the prediction of a hydrocarbon haze we use a photochemical model to corroborate our geochemical data our simulations predict transitions between two stable atmospheric states one with organic haze and the other haze free the transitions are presumably governed by variations in the amount of biological methane production during the archaean eon we find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the archaean before the rise of oxygen the atmosphere of the early earth may have consisted of an organic haze geochemical data and modelling suggest that from 2 65 to 2 5 gyr ago several transitions between hazy and haze free atmospheric conditions occurred potentially linked to variations in biogenic methane production

an ocean floor carbonaceous sedimentary sequence in the 3 2 ga dixon island formation coastal pilbara terrane western australia abstract the dixon island formation of the pilbara terrane western australia extends from cleaverville beach to the dixon island coast and is the only example worldwide of a coastal outcrop of a 3 2 3 1 ga low grade greenstone belt the dixon island formation was situated in an immature island arc setting and comprises siliceous carbonaceous deep water sediments that contain evidence for hydrothermal and microbial activity the extensive outcrop along the coastline makes it possible to examine in detail the characteristics of mesoarchean sedimentation in a hydrothermal environment this study focuses on a continuously exposed carbonaceous black chert succession in the central part of the northern coastline outcrop on dixon island at this site a 20 m thick carbonaceous black chert sequence conformably overlies basement rocks of highly altered komatiite rhyolite tuffs the black chert sequence formed well bedded black chert with carbonaceous peloidal matter and fragmented grains and the sequence is homogeneous and finely laminated in this sequence evidence of low temperature hydrothermal fluid sediments and alteration stractures are well preserved in the lowermost section indicating that high levels of hydrothermal activity occurred on the ocean floor during deposition in particular swarms of black chert veins provide evidence of post volcanic hydrothermal activity that released organic matter and silica to the ocean the carbonaceous peloidal textures and 13 c org values of sediments located just above the basement which hosts the vein swarms suggest that the veins were the conduits for hydrothermal fluid which contained organic rich silica material and that flowed onto the seafloor to form the homogeneous carbonaceous cherts along with hydrothermal related sediment the 13 c org values of organic matter in the black cherts range from 42 to 22 average 31 9 n 313 lighter 13 c values 35 to 42 characterize carbonaceous laminated black chert located 5 m above the basement where biogenic structures e g biomat bed microfossil structures are found the lighter 13 c values might be related to methanotrophic micro organic activity within the sediments during hydrothermal activity in summary we reconstructed the sedimentary environment upon a mesoarchean hydrothermal ocean floor that was a site of microbial activity and local methanogenesis

a giant late archean lake system the meentheena member tumbiana formation fortescue group western australia abstract the 2720 ma old meentheena member of the tumbiana formation fortescue group is a 30 50 m thick succession of alternating siliciclastic and limestone beds that crop out intermittently over a distance of 680 km across the pilbara craton western australia limestones of the member consist of flat pebble intraformational conglomerate oolite grainstone calcisiltite and calcilutite siliciclastics consist of shale siltstone and sandstone and many of these are tuffaceous sedimentary structures include abundant symmetrical ripples ripple cross stratification desiccation cracks teepee structures and planar to wavy lamination the member is superficially similar in appearance wherever it crops out however the correlation of individual beds and units over distances greater than 15 km is problematic there are no regional marker beds radiometric ages from tuffaceous units young to the west by a few million years four lithofacies are defined a flat pebble conglomerate b ripple cross laminated oolite grainstone c planar to wavy laminated calcilutite calcisiltite and d shale siltstone stromatolites are found in all three of the limestone facies evaporites are rare and include halite pseudomorphs the four lithofacies are generally found in a vertical succession that characterizes a lithofacies association and reflects a deepening upward succession associations may not have all four lithofacies but do reflect the same deepening upward pattern the uppermost portions of shale siltstone facies commonly contain symmetrical ripples and desiccation cracks indicating subaerial exposure measured sections contain as many as 25 of these parasequences indicating fluctuating water levels limestone lithology and facies commonly change abruptly laterally at the meter and greater scale and vertically centimeter to meter scale the sedimentary structures differences and abrupt changes in lithofacies patterns the inability to correlate confidently between regions and the differences in age from east to west suggest a complex diachronous depositional system of fluctuating water levels in separate bodies of water the limestone is dominated by sedimentary structures indicative of shallow water within wave base however it is devoid of evidence of any tidal influence possibilities for depositional environments include a tideless sea or a lacustrine system 87sr 86sr values from limestone differ from known archean marine carbonates ree patterns are decidedly non marine as well a lacustrine system is favored for the meentheena member based on sedimentary structures the abruptness of lithologic and facies changes the facies association lateral gradation into fluvial deposits the inability to correlate many units and beds for more than a few to several kilometers geochemical data stromatolites and comparisons with younger lacustrine basins as well as marine successions there is no single line of evidence that unequivocally establishes a lacustrine origin however multiple lines of evidence are collectively consistent with a lacustrine origin the meentheena member would therefore represent the oldest known carbonate rich lacustrine system which we call the meentheena lakes system

anatomy of a pressure induced ferromagnetic to paramagnetic transition in pyrrhotite implications for the formation pressure of diamonds 1 meteorites and diamonds encounter high pressures during their formation or subsequent evolution these materials commonly contain magnetic inclusions of pyrrhotite because magnetic properties are sensitive to strain pyrrhotite can potentially record the shock or formation pressures of its host moreover pyrrhotite undergoes a pressure induced phase transition between 1 6 and 6 2 gpa but the magnetic signature of this transition is poorly known here we report room temperature magnetic measurements on multidomain and single domain pyrrhotite under nonhydrostatic pressure magnetic remanence in single domain pyrrhotite is largely insensitive to pressure until 2 gpa whereas the remanence of multidomain pyrrhotite increases 50 over that of initial conditions by 2 gpa and then decreases until only 33 of the original remanence remains by 4 5 gpa in contrast magnetic coercivity increases with increasing pressure to 4 5 gpa below 1 5 gpa multidomain pyrrhotite obeys neel theory with a positive correlation between coercivity and remanence above 1 5 gpa it behaves single domain like yet distinctly different from uncompressed single domain pyrrhotite the ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure which can potentially be used as a geobarometer owing to the greater thermal expansion of pyrrhotite with respect to diamond pyrrhotite inclusions in diamonds experience a confining pressure at earth s surface applying our experimentally derived magnetic geobarometer to pyrrhotite bearing diamonds from botswana and the central african republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1 3 to 2 1 gpa these overpressures constrain the mantle source pressures from 5 4 to 9 5 gpa depending on which bulk modulus and thermal expansion coefficients of the two phases are used

whole rock 26 al 26 mg systematics of amoeboid olivine aggregates from the oxidized cv3 carbonaceous chondrite allende we report on mineralogy petrography and whole rock 26 al 26 mg systematics of eight amoeboid olivine aggregates aoas from the oxidized cv chondrite allende the aoas consist of forsteritic olivine opaque nodules and variable amounts of ca al rich inclusions cais of different types and show evidence for alteration to varying degrees melilite and anorthite are replaced by nepheline sodalite and grossular spinel is enriched in feo opaque nodules are replaced by fe ni sulfides ferroan olivine and ca fe rich pyroxenes forsteritic olivine is enriched in feo and often overgrown by ferroan olivine the aoas are surrounded by fine grained matrix like rims composed mainly of ferroan olivine and by a discontinuous layer of ca fe rich silicates these observations indicate that aoas experienced in situ elemental open system iron alkali halogen metasomatic alteration during which fe na cl and si were introduced whereas ca was removed from aoas and used to form the ca fe rich silicate rims around aoas the whole rock 26 al 26 mg systematics of the allende aoas plot above the isochron of the whole rock allende cais with a slope of 5 23 0 13 10 5 reported by jacobsen et al 2008 in contrast whole rock 26 al 26 mg isotope systematics of cais and aoas from the reduced cv chondrite efremovka define a single isochron with a slope of 5 25 0 01 10 5 larsen et al 2011 we infer that the excesses in 26 mg present in allende aoas are due to their late stage open system metasomatic alteration thus the 26 al 26 mg isotope systematics of allende cais and aoas are disturbed by parent body alteration processes and may not be suitable for high precision chronology of the early solar system events and processes

uv induced mass independent sulfur isotope fractionation in stratospheric volcanic sulfate 1 sulfuric acid aerosols produced in the stratosphere following massive volcanic eruptions possess a mass independent sulfur isotopic signature acquired when volcanic so2 experiences uv photooxidation the volcanic data are consistent with laboratory so2 photooxidation experiments using uv light at 248 nm maximum absorption of ozone whereas sulfur isotopic anomalies previously observed in archean samples are consistent with photodissociation at 190 220 nm a mechanism of so2 photooxidation occurring in the early stage of a stratospheric volcanic plume in the range of 220 320 nm weak band absorption of so2 is also proposed since mass independent sulfur isotope anomalies in stratospheric volcanic sulfate appear to depend on the exposure of so2 to uv radiation their measurements might therefore offer the possibility to determine the degree of uv penetration in the ozone absorption window for the present and past atmospheres they can also be used to determine the stratospheric or tropospheric nature of volcanic eruptions preserved in glaciological records offering the possibility to reassess the climatic impact of past volcanic eruptions

anomalous fractionation of sulfur isotopes during heterogeneous reactions abstract applying square adsorption well and morse potential models for the adsorption of a sulfur specie on a solid surface we have evaluated the fractionation of four sulfur isotopes 32s 33s 34s and 36s during chemisorption reactions heterogeneous reactions between solid and aqueous or gaseous species are found to produce anomalous fractionations of sulfur isotopes when the adsorption energy is small ab initio calculations for so2 adsorption on a kerogen surface indicate the possibility of creating anomalous isotope signatures for the adsorbed species such as 33s 34s 1 08 36s 34s 0 84 33s 7 0 to 13 6 and 36s 13 0 to 25 2 in a temperature range of 0 to 200 c the magnitude of 33s and 36s increase with increasing temperature these data together with various geochemical data e g organic c and sulfide s contents hydrothermal alteration effects on archean sedimentary rocks with anomalous sulfur isotope fractionations suggest that the anomalous sulfur isotope signatures in such rocks may have been created by heterogeneous reactions between organic matter minerals and sulfur bearing aqueous solutions under hydrothermal conditions rather than by uv photolysis of volcanic so2 in an o2 poor atmosphere

redox control of arsenic mobilization in bangladesh groundwater abstract detailed hydrochemical measurements 34sso4 and 3h analyses were performed on 37 groundwater samples collected during february 1999 january and march 2000 from 6 locations in eastern and southeastern bangladesh to examine redox processes that lead to as mobilization in groundwater the study sites were chosen based on available nation wide as surveys to span the entire spectrum of as concentrations in bangladesh groundwater and to represent 3 of 5 major geological units of the ganges brahmaputra delta uplifted pleistocene terrace fluvial flood plain and delta plain arsenic was found to be mobilized under fe reducing conditions in shallow aquifers 50 g l or 0 7 m were always accompanied by high dissolved hco3 4 mm and were close to saturation with respect to calcite groundwater enriched in as 200 800 g l or 2 7 10 7 m and phosphate 30 100 m but relatively low in dissolved fe 5 40 m probably resulted from re oxidation of reducing as and fe enriched water this history was deduced from isotopic signatures of 34sso4 and 3h2o 3h to delineate the nature of redox changes for some of the reducing groundwaters in contrast as is not mobilized in presumed pleistocene aquifers both shallow 30 60 m and deep 150 270 m because conditions were not reducing enough due to lack of sufficient o2 demand

preservation and detectability of shock induced magnetization preservation and detectability of shock induced magnetization abstract an understanding of the effects of hypervelocity impacts on the magnetization of natural samples is required for interpreting paleomagnetic records of meteorites lunar rocks and cratered planetary surfaces rocks containing ferromagnetic minerals have been shown to acquire shock remanent magnetization srm due to the passage of a shock wave in the presence of an ambient magnetic field in this study we conducted pressure remanent magnetization prm acquisition experiments on a variety of natural samples as an analog for srm acquisition at pressures ranging up to 1 8 gpa comparison of the alternating field af and thermal demagnetization behavior of prm confirms that af demagnetization is a more efficient method for removing srm overprints than thermal demagnetization because srm may persist to unblocking temperatures approaching the curie temperatures of magnetic minerals the blocking of srm to high temperatures suggests that srm could persist without being eradicated by viscous relaxation over geologic timescales however srm has been rarely observed in natural samples likely because of two factors 1 other forms of impact related remanence e g thermal remanent magnetization from impact related heating or chemical remanent magnetization from post impact hydrothermal activity are often acquired by target rocks that overprint srm and 2 low srm acquisition efficiencies may prevent srm from being distinguished from the underlying primary remanence or other overprints due to its low magnetization intensity sonia m tikoo 1 2 jerome gattacceca 3 4 nicholas l swanson hysell 1 benjamin p weiss 1 4 clement suavet 4 and cecile cournede 3 department of earth and planetary science university of california berkeley ca 94720 usa berkeley geochronology center 2455 ridge road berkeley ca 94709 usa cnrs aix marseille university cerege um34 aix en provence france department of earth atmospheric and planetary sciences massachusetts institute of technology mit 77 massachusetts avenue cambridge ma 02139 usa final version for publication in journal of geophysical research planets accepted 07 24 2015 doi 10 1002 2015je004840

the chemical conditions of the late archean hamersley basin inferred from whole rock and pyrite geochemistry with δ33s and δ34s isotope analyses the well preserved late archean sedimentary rocks of the fortescue and hamersley basins in western australia offer fascinating insights into early earth ocean chemistry prior to the great oxidation event goe in this study we use a combination of whole rock geochemistry la icpms trace element analysis of sedimentary pyrite and pyrrhotite and shrimp si sulfur isotope analyses to elucidate the chemical changes in these sedimentary rocks these proxies are used to examine chemical conditions of the ocean during the late archean two to three periods of oxygen enrichment prior to the deposition of banded iron formations bif can be identified one minor stage of general increase in whole rock enrichment factors and trace element content of pyrite is observed up stratigraphy in the jeerinah formation fortescue basin and a more substantial stage is present in the paraburdoo and bee gorge members of the wittenoom formation hamersley basin some of the trace element enrichments indicate organic matter burial flux ni cr zn co and cu which suggests an increase in biological productivity if the increased biological activity reflects an increase in cyanobacteria activity then an associated increase in oxygen is likely to have occurred during the deposition of the bee gorge member an increase in atmospheric oxygen would result in continental weathering of sulfide and other minerals increasing the trace element content of the water column via erosion and avoiding excessive depletion of trace elements due to drawdown in seawater since some of these trace elements may also be limiting nutrients such as mo and se for the cyanobacteria the degree of biological productivity may have further increased due to the increasing amount of trace elements introduced by oxygenation in a positive feedback loop these periods of increased productivity and oxygen rise stopped prior to the onset of bif deposition in the hamersley basin this may be due to the ocean reaching an oxidation threshold enabling the precipitation of hematite and magnetite bif the bif deposition caused depletion of ocean nutrients such as phosphate severely limiting the growth of cyanobacteria and thus limiting further oxygen production

middle archean volcano hydrothermal sequence bacterial microfossil bearing 3 2 ga dixon island formation coastal pilbara terrane australia the 3 2 ga dixon island formation in the cleaverville group of the coastal pilbara terrane australia is one of the most complete and best preserved examples of middle archean oceanic stratigraphy and contains possible microbial material field observations and geochemical evidence suggest that this formation contains a low temperature hydrothermal vent system with a biogenic microbial colony from the archean ocean the dixon island formation is 350 m thick and consists of the rhyolite tuff black chert and varicolored chert members in ascending order the rhyolite tuff member contains many vein swarms such as quartz and black chert veins and highly altered rhyolite tuff layers which are identified as an underground bypass zone for circulating hydrothermal fluid many black chert vein swarms in the rhyolite tuff member imply intensive low temperature hydrothermal activity during deposition of the black chert member which is 10 15 m thick the black chert member is composed of massive black chert laminated black chert dark greenish siliceous shale and tuffaceous laminated chert which are mainly composed of very fine quartz abundant pseudomorphs of silica after aragonite barite and gypsum and a distinctly continuous stromatolite like biomat layer 10 20 cm thick are preserved within the laminated black chert bed the stromatolite like biomat bed is formed of fine iron or iron coated quartz pisolite within fine grained silica the absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at 500 2000 m paleodepth and may have been on the slope of an immature island arc microbial material has been preserved well in the lower part of black chert member the massive black chert has carbonaceous peloids 0 3 2 0 mm in diameter which are similar to those in the black chert veins the massive black chert contains spiral rod and dendrite shaped bacterial material the total organic carbon toc value of massive black chert in the lower part of the black chert member is higher toc 0 15 0 45 than that of the overlying laminated chert section toc 0 02 0 15 and the black chert vein toc 0 1 0 13 and the carbon isotope 13 c values of this lithology 33 to 27 are also lighter than for the black chert veins 29 to 26 and the laminated black chert in the upper part of the black chert member and the varicolored chert member 27 to 13 this evidence suggests that the carbonaceous grains and bacteria shaped material in the lower part of the black chert member are of bio genic origin and were formed close to a low temperature hydrothermal vent system the microbial colony may have been rapidly fossilized by silicification related to hydrothermal activity laminated black chert in the upper part of the black chert and the varicolored chert members may have formed by cyanobacterial sedimentation from the ocean surface

arsenic toxicity of groundwater in parts of the bengal basin in india and bangladesh the role of quaternary stratigraphy and holocene sea level fluctuation arsenic toxicity in groundwater in the ganges delta and some low lying areas in the bengal basin is confined to middle holocene sediments dissected terraces and highlands of pleistocene and early holocene deposits are free of such problems arsenic rich pyrite or other arsenic minerals are rare or absent in the affected sediments arsenic appears to occur adsorbed on iron hydroxide coated sand grains and clay minerals and is transported in soluble form and co precipitated with or is scavenged by fe iii and mn iv in the sediments it became preferentially entrapped in fine grained and organic rich sediments during mid holocene sea level rises in deltaic and some low lying areas of the bengal basin it was liberated subsequently under reducing conditions and mediated further by microbial action intensive extraction of groundwater for irrigation and application of phosphate fertilizer possibly triggered the recent release of arsenic to groundwater this practice has induced groundwater flow mobilizing phosphate derived from fertilizer as well as from decayed organic matter which has promoted the growth of sediment biota and aided the further release of arsenic however the environment is not sufficiently reducing to mobilize iron and arsenic in groundwater in the ganges floodplains upstream of rajmahal thus arsenic toxicity in the groundwater of the bengal basin is caused by its natural setting but also appears to be triggered by recent anthropogenic activities

pyrite paragenesis and multiple sulfur isotope distribution in late archean and early paleoproterozoic hamersley basin sediments abstract the sulfur isotope record in late archean and early paleoproterozoic rocks is of considerable importance because it provides evidence for changes in early earth atmospheric oxygen levels and potentially constrains the origin and relative impact of various microbial metabolisms during the transition from an anoxic to oxic atmosphere mass independently fractionated mif sulfur isotopes reveal late archean and early paleoproterozoic sulfur sources in different pyrite morphologies in western australia s hamersley basin multiple sulfur isotope values in late archean pyrite vary according to morphology fine grained pyrite has positive sulfur mif indicating a reduced elemental sulfur source whereas pyrite nodules have negative sulfur mif potentially derived from soluble sulfate via microbial sulfate reduction the hamersley basin 34 s 33 s record suggests that the extent of oxygenation of the surface ocean fluctuated through the late archean from at least 2 6 ga more than 150 million yr before the great oxidation event in the early paleoproterozoic there is less distinction between pyrite morphologies with respect to sulfur isotope fractionation and pyrite from the brockman iron formation trends toward modern sulfur isotope values an important exception to this is the strong negative mif recorded in layer parallel pyrite in paleoproterozoic carbonate facies iron formation this may suggest that deeper water hydrothermal environments remained anoxic while shallower water environments became more oxidised by the early paleoproterozoic the results of the current study indicate that sulfide paragenesis is highly significant when investigating archean and early paleoproterozoic multiple sulfur isotope compositions and sulfur sources

two coexisting sulfur metabolisms in a ca 3400 ma sandstone a sandstone from the ca 3400 ma strelley pool formation of western australia contains pristine micron sized pyrite intimately associated with organic material coating framework quartz grains a synsedimentary to early diagenetic origin for this pyrite is indicated by its occurrence in black bedded sandstone at the base of the formation and in reworked black clasts higher up in the formation high resolution multiple sulfur isotope analysis 32s 33s 34s using secondary ion mass spectrometry nanosims and large radius ion microprobe reveals 34svcdt vienna canyon diablo troilite values between 12 and 6 and 33s values between 1 65 and 1 43 from pyrite grains within a single thin section a large spread of 34s values over only 5 10 m together with the spatial association of pyrite with carbon and nitrogen indicates biological processing of sulfur the presence of both 33s and 33s signals overprinted by significant mass dependent 34s fractionation in this pyrite population indicates for the first time that both microbial sulfate reduction of aqueous sulfate 33s and microbial disproportionation of elemental sulfur 33s were co occurring in an open marine sedimentary hosted ecosystem in the paleoarchean

a multiple sulfur and organic carbon isotope record from non conglomeratic sedimentary rocks of the mesoarchean witwatersrand supergroup south africa abstract multiple sulfur isotope ratios 36 s 34 s 33 s 32 s and organic carbon isotope ratios 13 c 12 c were measured from 198 non conglomeratic sedimentary samples that were collected from five deep diamond drill cores that cover the majority of the ca 2 96 2 82 ga witwatersrand supergroup 13 c org 34 s py and 33 s py values of the sample set range from 44 3 to 21 9 3 7 to 16 5 and 4 0 to 1 9 respectively these geochemical data vary relative to depositional facies proximal marine distal marine and fluvial dominated and tectonic setting trailing margin and foreland basin in the trailing margin setting of the 2 96 ga hospital hill subgroup the proximal marine depofacies is characterized by relatively high organic carbon contents up to 0 9 wt and 13 c org values around 28 in contrast to the distal marine depofacies that yields low organic carbon contents 0 01 wt and high 13 c org values up to 22 both depofacies yield low sulfur contents 0 02 wt a narrow range of 34 s values 3 2 and positive 33 s values up to 1 9 this data is consistent with photoautotrophic carbon fixation in shallow marine environments and limited organic carbon production preservation in deeper water settings due to longer transport distances and effective biological degradation e g fe reducing microbes positive 33 s values imply that sulfur was largely derived from a photochemical elemental sulfur reservoir in the foreland basin tectonic setting of the 2 94 ga government and 2 92 ga jeppestown subgroups shelf deposits associated with fluvial braidplain depofacies are characterized by 13 c depleted organic carbon 44 to 38 relatively high sulfur contents 0 2 1 3 wt variable 34 s values 3 7 to 16 5 and small negative 33 s values 0 4 these data suggest that the microbial community in the fluvial dominated depofacies may have consisted of photoautotrophs methanogens anaerobic methanotrophs and sulfate reducers sulfate was derived from a variety of sources that include photochemical crustal and marine sulfur reservoirs the occurrence of three regionally persistent diamictite deposits in the government subgroup coincides with the onset of compressional tectonics and development of pyritic shales with highly 13 c depleted organic carbon suggesting that an increase in continental sulfur flux and methane oxidation may have triggered the mesoarchean glaciations drawdown of h 2 and ch 4 however the link between large 33 s anomalies 4 0 to 1 2 and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures

the fall recovery orbit and composition of the tagish lake meteorite a new type of carbonaceous chondrite the preatmospheric mass of the tagish lake meteoroid was about 200 000 kilograms its calculated orbit indicates affinity to the apollo asteroids with a semimajor axis in the middle of the asteroid belt consistent with a linkage to low albedo c d and p type asteroids the mineralogy oxygen isotope and bulk chemical composition of recovered samples of the tagish lake meteorite are intermediate between cm and ci meteorites these data suggest that the tagish lake meteorite may be one of the most primitive solar system materials yet studied

in situ quadruple sulfur isotopic compositions of pyrites in the ca 3 2 2 72 ga metasedimentary rocks from the pilbara craton western australia abstract the sulfur mass independent fractionation s mif of mesoarchean is distinct from the other archean periods by the dampened magnitudes of 33s and a steeper slope of 36s 33s compared with the archean reference array ara according to previous studies however samples available of this interval are limited owing to poor or no preservation additionally the majority of the data were obtained by analysis of bulk rocks thus are essentially the sulfur isotopic compositions of mixed sulfur components due to multiple stages of post diagenesis overprints and modifications this study systematically measured the multiple sulfur isotopic compositions of pyrites nodular disseminated in ca 3 2 2 72 ga metasedimentary rocks from the pilbara craton western australia where one of the best preserved archean records occur using the upgraded shrimp si after detailed pyrite generation and genesis investigation by bse imaging and naocl etching the results show that 33s values of the diagenetic pyrites in these metasedimentary rocks range from 2 4 to 0 8 such a range is inconsistent with the other archean intervals and any experiment producing s mif so far where the magnitude of the maximum positive 33s value is considerably larger than the corresponding minimum negative 33s value the dampened positive 33s is explained in several ways from the perspective of production and preservation including original photochemical products with larger magnitudes of negative 33s than positive 33s in a different atmospheric composition non zero 33s of initial so2 different mechanism producing s mif chain formation and destruction of positive 33s by subduction the downward 36s 33s deviations from ara in the diagenetic pyrites of 2 72 ga tumbiana formation can be attributed to modifications induced by microbial activities this is based on the negative 0 9 33s 36s values similar to biological products and the large range of negative 34s values the downward 36s 33s offsets in the hydrothermally altered diagenetic pyrites of 2 78 ga mt roe basalt are most likely mixing of primary sulfur on ara with later sulfur disobeying ara the upward 36s 33s deviations in the diagenetic pyrites of 2 93 ga mosquito creek formation likely reflect different atmospheric composition

facies architecture and sequence stratigraphic features of the tumbiana formation in the pilbara craton northwestern australia implications for depositional environments of oxygenic stromatolites during the late archean abstract the tumbiana formation ca 2 7 ga is a succession developed in response to late archean crustal extension in the pilbara craton northwestern australia the formation is characterized by intercalations of stromatolitic carbonates and provides a perspective of the evolution of photosynthetic organisms which are interpreted to have been responsible for the oxygenation of earth surface environments in early earth here we investigated the depositional setting of the stromatolitic carbonates for the better understand of the environments which may have controlled the massive development of oxygenic photosynthesis we studied lithofacies and sequence stratigraphic features of the tumbiana formation in the redmont area a southern margin of the northern pilbara craton on the basis of three dimensional analyses of onshore outcrops the tumbiana formation as much as 170 m thick unconformably overlies the early to middle archean basement rocks and is unconformably overlain by the maddina formation that consists of mafic tuffaceous sediments and basalt lavas the lower part of the tumbiana formation is characterized by planar and trough cross stratified conglomerates which are interpreted to be an alluvial fan deposit shed from the northern hinterlands and developed mainly in local depressions of the basement rocks during late lowstand and early transgressive stages the base of the middle part is defined by a transgressive surface of erosion overlain by conglomerates and or breccias that pass upward to mafic tuffaceous mudstones and sandstones intercalated with accretionary lapillis breccias and basalt pillow lavas mudstones are commonly laminated and sandstones contain current and wave ripple lamination parallel lamination hummocky cross stratification and trough cross stratification in general the middle part exhibits an overall fining and coarsening upward pattern and is interpreted to indicate transgressive and regressive shelf to coastal lithofacies successions in particular the uppermost horizon of the middle part is characterized by sandstone beds with herringbone structures and desiccation cracks and is interpreted to indicate an intertidal deposit developed during late highstand and or following falling stages the middle part of the formation in general thickens to the northern proximal area and eventually flattened out the topographic irregularity of the basement rocks the upper part is characterized by stromatolitic carbonates associated with minor mudstones and wave and current rippled sandstones and does not exhibit distinct lateral variation in thickness and vertical lithofacies organization the base of the upper part is characterized by a transgressive lag deposit the ensuing transgression over the intertidal deposits of the uppermost middle part is interpreted to have developed a new accommodation space for the vertical stacking of stromatolites with different external shapes from elongated small domal and columnar types associated with desiccation cracks to stratiform and large domal types these changes may have responded to the increase in paleowater depth possibly from intertidal to subtidal environments in conclusion stromatolites in the upper tumbiana formation in the redmont area are interpreted to have developed in coastal and shallow marine environments in response to the rise in relative sea level rather than in ephemeral saline lakes as proposed by previous studies however the outcome of the present research does not necessarily deny the previously proposed interpretation but suggests spatial variations in depositional environments of the tumbiana formation from the north to south

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

the mesoarchean emergence of modern style subduction abstract well preserved volcanic sequences span the paleoarchean to neoarchean evolution of the pilbara craton in northwestern australia this region provides the best physical evidence bearing on the stage of earth s history when modern style tectonic processes began paleoarchean assemblages in the eastern nucleus of the craton the 3 51 3 24 ga pilbara supergroup show few features that can reasonably be interpreted as evidence for modern style subduction processes incompatible trace element enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust but this evidence on its own can equally well be interpreted in terms of either a subduction enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the pilbara supergroup viewed in context within the thick autochthonous and consistently upward younging pilbara supergroup these felsic units are most likely related to the same plume dominated processes that formed the basalts that dominate the supergroup it is very unlikely that modern style plate tectonic processes played any role in the paleoarchean evolution of the pilbara craton although some form of non uniformitarian e g flat subduction process may have operated in stark contrast the mesoarchean units of the west pilbara terrane and the late tectonic basins that cover that boundary between the west and east pilbara terranes show clear evidence for modern style convergent margin processes igneous rocks in this belt which flanks the old eastern cratonic nuclei have enriched geochemical signatures that cannot be accounted for by crustal contamination this region is also characterised by a linear magmatic and structural fabric by the presence of lithologically and geochronologically exotic belts and by the presence of a broad belt of isotopically more juvenile crust the collective strength of these arguments provides compelling evidence that a modern style oceanic arc fringed the east pilbara terrane at 3 12 ga and accreted to that terrane by 2 97 ga these assemblages mark the minimum age for the birth of modern style plate subduction process

major element chemistry of the changjiang yangtze river the chemistry of major elements ca mg na k hco3 so4 cl and si in the river water of the changjiang yangtze river was studied based on continuously monitored data at 191 stations in the drainage basin for the period 1958 1990 the results show that the total dissolved solid tds concentration of the changjiang varies over an order of magnitude throughout the basin 49 7 518 1 mg l with a medium tds concentration of 205 9 mg l about three times the global average in contrast the tds at a given main channel station varies only slightly in different seasons with a variation factor less than 2 0 despite a substantial water dilution in the summer flood season the major element chemistry of the changjiang is mainly controlled by rock weathering with the anion hco3 and the cation ca dominating the major ion composition due to the abundance of carbonate rocks in the basin a persistently increasing trend has been observed in the concentrations of so4 and to a lesser extent cl in the changjiang a signature of considerable anthropogenic impacts e g acid deposition flux calculations at datong the most downstream main channel station without tidal influence indicate that the changjiang transports ca 154 106 tons year of tds to the sea second only to the amazon in the world

earliest rock fabric formed in the solar system preserved in a chondrule rim rock fabrics record the formation compaction and deformation history of that rock high resolution mapping of tiny grains in the allende cv meteorite reveals preservation of a spherical fabric that may have formed in the solar nebula and could be the oldest rock fabric to have formed in the solar system

a new framework for modeling sediment fining during transport with fragmentation and abrasion 1 a new framework for modeling sediment fining during transport is proposed the model is based on a physical description of both abrasion and fragmentation processes it attempts to describe and explain the changes in the grain size distribution of sediments during their transport in a water flow abrasion and fragmentation are modeled through the use of breakage frequency functions and daughter size distributions these functions are determined experimentally for the specific case study which is presented comparison between the predictions from the model and experimental data from the case study show that 1 both abrasion and fragmentation are involved in the degradation 2 fragmentation efficiency decreases during the experiment and 3 the shape of the pebbles provides feedback between abrasion and fragmentation this type of model is therefore appropriate for explaining degradation mechanisms which is promising for a future application to rivers

progressive alteration in cv3 chondrites more evidence for asteroidal alteration abstract the oxidized cv3 chondrites can be divided into two major subgroups or lithologies bali like cv3oxb and allende like cv3oxa in which chondrules calcium aluminum rich inclusions cais and matrices show characteristic alteration features weisberg et al 1997 krot et al 1997d kimura and ikeda 1997 the cv3oxb lithology is present in bali kaba parts of the mokoia breccia and possibly in grosnaja and allan hills alh 85006 it is characterized by the presence of the secondary low ca phyllosilicates saponite and sodium phlogopite magnetite ni rich sulfides fayalite fa 90 ca fe rich pyroxenes fs10 50wo45 50 and andradite phyllosilicates replace primary ca rich minerals in chondrules and cais which suggests mobilization of ca during aqueous alteration magnetite nodules are replaced to various degrees by fayalite ca fe rich pyroxenes and minor andradite fayalite veins crosscut fine grained rims around chondrules and extend into the matrix thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures 300 c in the presence of aqueous solutions oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large 18ofayalite magnetite fractionation 20 we infer that phyllosilicates magnetite fayalite ca fe rich pyroxenes and andradite formed at relatively low temperatures 300 c by fluid rock interaction in an asteroidal environment secondary fayalite and phyllosilicates are virtually absent in chondrules and cais in the cv3oxa lithology which is present in allende and its dark inclusions axtell alha81258 alh 84028 lewis cliff lew 86006 and parts of the mokoia and vigarano breccias instead secondary nepheline sodalite and fayalitic olivine are common fayalitic olivine in chondrules replaces low ca pyroxenes and rims and veins forsterite grains it also forms coarse lath shaped grains in matrix secondary ca fe rich pyroxenes are abundant we infer that the cv3oxa lithology experienced alteration at higher temperatures than the cv3oxb lithology the presence of the reduced and cv3oxa lithologies in the vigarano breccia and cv3oxa and cv3oxb lithologies in the mokoia breccia indicates that all cv3 chondrites came from one heterogeneously altered asteroid the metamorphosed clasts in mokoia krot and hutcheon 1997 may be rare samples of the hotter interior of the cv asteroid we conclude that the alteration features observed in the oxidized cv3 chondrites resulted from the fluid rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced cv3 chondrites

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

the porosity and permeability of chondritic meteorites and interplanetary dust particles abstract we have investigated the porosity of a large number of chondritic interplanetary dust particles idps and meteorites by three techniques standard liquid gas flow techniques a new noninvasive ultrasonic technique and image processing of backscattered images the latter technique is obviously best suited to sub kilogram sized samples we have also measured the gas and liquid permeabilities of some chondrites by two techniques standard liquid gas flow techniques and a new nondestructive pressure release technique we find that chondritic idps have a somewhat bimodal porosity distribution peaks are present at 0 and 4 porosity a tail then extends to 53 type 1 3 chondrite matrix porosities range up to 30 with a peak at 2 the bulk porosities for type 1 3 chondrites have the same approximate range as exhibited by the matrix which indicates that other components of the bulk meteorites including chondrules and aggregates have the same average porosity as the matrix these results reveal that the porosities of primitive materials at scales ranging from nanogram to kilogram are similar which implies that similar accretion dynamics operated through 12 orders of size magnitude permeabilities of the investigated chondrites vary by several orders of magnitude and there appears to be no simple dependence of permeability with degree of aqueous alteration chondrite type or porosity

fabric analysis of allende matrix using ebsd fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution electron backscatter diffraction ebsd has allowed fabrics in these fine grained materials to be visualized in detail for the first time our data reveal that allende a cv3 chondrite possesses a uniform planar short axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction interference between this matrix fabric and the larger more rigid components such as dark inclusions dis and calcium aluminium rich inclusions cais has lead to the development of locally oriented and intensified matrix fabrics in addition dis possess fabrics that are conformable with the broader matrix fabric these results suggest that dis were in situ prior to the deformational shortening event responsible for these fabrics thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid kojima and tomeoka 1996 fruland et al 1978 moreover both dis and allende matrix are highly porous 25 corrigan et al 1997 mobilizing a highly porous di during impact induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable we favor a model that involves allende dis cais and matrix accreting together and experiencing the same deformation events

magnetization of extraterrestrial allende material may relate to terrestrial descend abstract the origin of magnetization in allende may have significant implications for our understanding of core formation differentiation dynamo processes in chondrite parent bodies the magnetic allende data may contain information that could constrain the magnetic history of allende the measurements on allende chondrules reveal an existence of magnetization component that was likely acquired during the meteorite transit to terrestrial conditions both the pyrrhotite carrying magnetic remanence intensity and direction of the chondrules change erratically when subjecting the allende meteorite s chondrules to temperatures near 77 k and back to room temperature chondrules with more intense original magnetization are denser and contain larger inverse thermoremanent magnetization itrm temperature dependent monitoring of itrm revealed that magnetization was acquired at temperature near 270 k such temperature is consistent with the condition when in addition to temperature increase the atmospheric uniaxial pressure applied during the meteorite entry on the porous material was responsible for meteorite break up in the atmosphere during this process collapse of the pore space in the matrix and some chondrules would generate crystalline anisotropy energy accumulation within pyrrhotite grains in form of parasitic magnetic transition

fluffy type a ca al rich inclusions in the allende meteorite abstract allende fluffy type a s fta s are a distinct sub group of ca al rich inclusions whose primary mineral assemblage consists of al rich melilite ak 0 33 spinel that is commonly very v rich perovskite and frequently hibonite some contain relatively coarse grained melilite up to 1 5 mm that is intensely kink banded and commonly reversely zoned hibonite and v rich spinel others contain much finer grained and strain free melilite 50 m and have not been found to contain hibonite or v rich spinel some fta s contain both coarser and finer grained melilite and textural relationships indicate that the latter is replacing the former fta s are characterized by extremely irregular shapes and 60 75 volume per cent of fine grained secondary alteration products many are aggregates of innumerable nodules each of which is surrounded by a wark lovering type rim sequence these nodules are frequently separated from one another by matrix like clastic rim material other fta s do not have nodular structure structural and mineralogical characteristics of their wark lovering rims suggest that fta s did not achieve their shapes by deformation of a liquid or a hot plastic solid in contrast to those in type b inclusions formation of reverse zoning in the coarser grained melilite crystals in fta s cannot be understood in terms of crystallization from a liquid but are readily explainable by condensation from a solar nebular gas during a period of falling pressure further evidence against a liquid origin is the wide range of spinel compositions within individual coarser grained fta s the fact that the reversely zoned melilite crystals cannot have been produced in any kind of sublimation or distillation process precludes formation of these inclusions as volatilization residues fta s are aggregates in some of which are preserved vapor solid condensate grains that formed at high temperature in the solar nebula

overview of the coordinated ground based observations of titan during the huygens mission coordinated ground based observations of titan were performed around or during the huygens atmospheric probe mission at titan on 14 january 2005 connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground based programs these observations consisted of three different categories 1 radio telescope tracking of the huygens signal at 2040 mhz 2 observations of the atmosphere and surface of titan and 3 attempts to observe radiation emitted during the huygens probe entry into titan s atmosphere the probe radio signal was successfully acquired by a network of terrestrial telescopes recovering a vertical profile of wind speed in titan s atmosphere from 140 km altitude down to the surface ground based observations brought new information on atmosphere and surface properties of the largest saturnian moon no positive detection of phenomena associated with the probe entry was reported this paper reviews all these measurements and highlights the achieved results the ground based observations both radio and optical are of fundamental importance for the interpretation of results from the huygens mission

multiple sulfur and carbon isotope composition of sediments from the belingwe greenstone belt zimbabwe a biogenic methane regulation on mass independent fractionation of sulfur during the neoarchean abstract to explore the linkage between mass independent sulfur isotope fractionation mif s and 13corg excursions during the neoarchean as well as the contemporary redox state and biogeochemical cycling of carbon and sulfur we report the results of a detailed carbon and multiple sulfur 34s 33s 36s isotopic study of the 2 7 ga manjeri and 2 65 ga cheshire formations of the ngezi group belingwe greenstone belt zimbabwe multiple sulfur isotope data show non zero 33s and 36s values for sediments older than 2 4 ga i e prior to the great oxidation event goe indicating mif s thought to be associated with low atmospheric oxygen concentration however in several 2 7 2 5 ga neoarchean localities small scale variations in mif s signal magnitude seem to correlate with negative excursion in 13corg possibly reflecting a global connection between the relative reaction rate of different mif s source reaction and sulfur exit channels and the biogenic flux of methane into the atmosphere during periods of localized microbiologically mediated shallow surface water oxygenation the manjeri formation black shales studied here display a wide range of 13corg between 35 4 and 16 2 average of 30 3 6 0 1 while the cheshire formation shales have 13corg between 47 7 and 35 1 average 41 3 3 1 the 34s values of sedimentary sulfides from manjeri formation vary between 15 15 and 2 37 average 1 71 4 76 1 showing very small and mostly negative 33s values varying from 0 58 to 0 87 average 0 02 0 43 1 cheshire formation black shale sulfide samples measured in this study have 34s values ranging from 2 11 to 2 39 average 0 25 1 08 1 and near zero and solely positive 33s anomalies between 0 14 and 1 17 average 0 56 0 29 1 moreover 36s 33s in the two formations are comparable with a slope of 1 38 manjeri formation and 1 67 cheshire formation respectively this differs from the archean reference line i e 0 9 the sulfur and carbon isotopic signatures recorded here likely reflect different environmental conditions and or ecosystems within the sampled cheshire and manjeri formations paleofacies the differences in carbon isotopes in different sedimentological facies are interpreted as recording different metabolic pathways including photosynthesis methanogenesis and methanotrophy low 13corg values in the cheshire formation black shales suggest a strong influence of methanotrophy likely preceded by bacterial methanogenesis while the large range of manjeri formation 34s may record bacterial sulfate reduction the c and multiple s isotopic variations recorded here may imply different patterns of c and s fractionation perhaps dependent on variations in the biogenic egress of methane to the atmosphere

hydrogeochemical characteristics of central jianghan plain china the central jianghan plain is the semi closed basin in the middle reaches of yangtze river a total of 78 water samples targeting groundwater were collected from 75 sites in this study site the area between yangtze river and han river including rivers and lakes for temperature ph electrical conductivity ec total dissolved solids tds and ion composition measurements correlation matrix was used to assess the geochemical and anthropogenic processes the most confined groundwater was grouped into hco3 ca mg while phreatic groundwater and surface water had a more diversified hydrochemistry the spatial variation in overall water quality as well as comparison with who world health organization standards for drinking water is illustrated mn as and no3 concentrations were found to exceed the allowable limits for drinking water of who guidelines and they also show remarkable spatial variations abnormally high nitrate concentration up to 150 190 mg l was found only in phreatic groundwater which suggested that the nitrate pollution might be caused by agricultural activities the present study may be helpful in further studies concerning water quality issues in this area where groundwater is a vital source for drinking and other activities

aspects of the validation of magnetic remanence in meteorites meteorite magnetic records constitute physical evidence of processes acting during early solar system evolution consequently the validation of these records is important in meteorite research the first step in the validation process should be the rem value the rem value is the ratio of natural remanence nrm to saturation remanent magnetization imparted by a 1 t magnetic field sirm the rem values range over 3 to 4 orders of magnitude for stony meteorites and for chondrules from allende c3v s1 bjurbole l4 s1 and chainpur ll3 s1 meteorites the rem values computed from published nrm and sirm data identify many orders of magnitude range in the rem values including rem values 100 10 3 these data suggest a dependence for the nrm intensity on the curatorial location from which the sample was obtained any earth rock acquiring thermoremanent magnetization trm in the geomagnetic field has a restricted range in rem mostly between 5 and 50 10 3 the exception being the mineral hematite in the multidomain size range the only terrestrial samples with rem much greater than 100 10 3 are those struck by lightning the rem value provides a physical basis for recognition between valid records and those that might be contaminated the isothermal remanence acquisition ra curve is presented as a contamination curve that allows an indication of the level of magnetic field contamination required to give the computed rem rm sirm value in the case of the bjurbole and chainpur chondrules with rem values 100 10 3 the ra curve indicates that unrealistically large contamination magnetic fields would be required to give rem values greater than 100 10 3 this would suggest contamination other than by a hand magnet that is normally available to an experimenter this would require an explanation that would involve large magnetic fields during chondrule formation or some extraordinary remanence acquisition mechanism that remains to be described magnetic contamination experiments using 80 and 40 mt magnets demonstrate that the rem values and extent of modification of the magnetic vector record are mineralogy dependent and this is mostly related to the amount and characteristics of the mineral tetrataenite the complexity of the meteorite records suggest validation of the record as a first step the rem value is the first physical statement that can be made in this validation

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

pathways for neoarchean pyrite formation constrained by mass independent sulfur isotopes it is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the neoarchean it is puzzling therefore that the sulfur isotope record of the neoarchean is characterized by a signal of atmospheric mass independent chemistry rather than a strong overprint by sulfate reducers here we present a study of the four sulfur isotopes obtained using secondary ion ms that seeks to reconcile a number of features seen in the neoarchean sulfur isotope record we suggest that neoarchean ocean basins had two coexisting significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record one of these pools is suggested to be a soluble sulfate pool and the other pool atmospherically derived elemental sulfur is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide we suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite forming environments we also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record

review secular tectonic evolution of archean continental crust interplay between horizontal and vertical processes in the formation of the pilbara craton australia the archean pilbara craton contains five geologically distinct terranes the east pilbara karratha sholl regal and kurrana terranes all of which are unconformably overlain by the 3 02 to 2 93 ga de grey superbasin the 3 53 3 17 ga east pilbara terrane ep represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3 53 3 43 3 35 3 29 and 3 27 3 24 ga each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3 72 ga and melting of crust to generate first tonalite trondhjemite granodiorite ttg and then progressively more evolved granitic magmas in each case plume magmatism was accompanied by uplift and crustal extension the combination of conductive heating from below thermal blanketing from above and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust these mantle melting events caused severe depletion of the subcontinental lithospheric mantle making the ep a stable buoyant unsubductable continent by c 3 2 ga extension accompanying the latest event led to rifting of the protocontinent margins at between 3 2 and 3 17 ga after 3 2 ga horizontal tectonic forces dominated over vertical forces as revealed by the geology of the three terranes karratha sholl and regal of the west pilbara superterrane the c 3 12 ga whundo group of the sholl terrane is a fault bounded 10 km thick volcanic succession with geochemical characteristics of modern oceanic arcs including boninites and evidence for flux melting that indicate steep archean subduction at 3 07 ga the 3 12 ga sholl terrane 3 27 ga karratha terrane and c 3 2 ga regal terrane accreted together and onto the ep during the prinsep orogeny this was followed by development of the de grey superbasin an intracontinental sag basin and widespread plutonism 2 99 2 93 ga as a result of orogenic relaxation and slab break off craton wide compressional deformation at 2 95 2 93 ga culminated with 2 91 ga accretion of the 3 18 ga kurrana terrane with the ep this compression caused amplification of the dome and keel structure in the ep final cratonization was effected by emplacement of 2 89 2 83 ga post tectonic granites

unique thermoremanent magnetization of multidomain sized hematite implications for magnetic anomalies intense magnetic remanence 100 1000 a m associated with md hematite and or titanohematite and associated with high koenigsberger ratios 40 1000 indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant titanohematite behaves similarly to hematite and thus the grain size dependence of trm acquisition in hematite is considered as a generalization the transition between truly md behavior and tendency towards sd behavior in hematite has been established to be between grain sizes of 0 1 and 0 05 mm in contrast to magnetite and titanomagnetite hematite exhibits inverse grain size dependence with md hematite acquiring a relatively intense trm in the geomagnetic field comparable to sub micrometer sized magnetite and only an order of magnitude less than sd magnetite consequently md hematite and by analogy titanohematite remanence may be of significance as a source of magnetic anomalies at all scales md hematite exhibits trm weak field acquisition behavior that is different from all other magnetic minerals being the only magnetic mineral having an rem trm sirm value e0 1 for trm acquisition in the geomagnetic field the very different trm behavior of md hematite in contrast to magnetite is due to two factors the first is the lesser influence of demagnetizing energy with respect to wall pinning energy at temperatures almost up to the curie temperature for hematite the second is the greater importance of the magnetostatic energy in the applied field which for hematite dominates the total energy at high temperatures fl 2000 published by elsevier science b v all rights reserved

evidence for impact induced pressure gradients on the allende cv3 parent body consequences for fluid and volatile transport carbonaceous chondrites such as those associated with the vigarano cv parent body exhibit a diverse range of oxidative reduced alteration mineralogy mcsween 1977 although fluids are often cited as the medium by which this occurs rubin 2012 a mechanism to explain how this fluid migrates and why some meteorite subtypes from the same planetary body are more oxidized than others remains elusive in our study we examined a slab of the well known allende cv3oxa meteorite using several petrological techniques e g fry s and flinn and computerized tomography ct we discover it exhibits a strong penetrative planar fabric resulting from strain partitioning among its major components calcium aluminum rich inclusions cais 64 5 ct matrix 21 5 fry chondrules 17 6 ct in addition to the planar fabric we found a strong lineation defined by the alignment of the maximum elongation of flattened particles interpreted to have developed by an impact event the existence of a lineation could either be non coaxial deformation or the result of a mechanically heterogeneous target material in the later case it could have formed due to discontinuous patches of sub surface ice and or fabrics developed through prior impact compaction macpherson and krot 2014 which would have encouraged preferential flow within the target material immediately following the impact compacting pore spaces we suggest that structurally controlled movement of alteration fluids in the asteroid parent body along pressure gradients contributed to the formation of secondary minerals which may have ultimately lead to the different oxidized subtypes

quadruple sulfur isotope analysis of ca 3 5 ga dresser formation new evidence for microbial sulfate reduction in the early archean abstract multiple sulfur isotope system is a powerful new tracer for atmospheric volcanic and biological influences on sulfur cycles in the anoxic early earth here we report high precision quadruple sulfur isotope analyses 32s 33s 34s 36s of barite pyrite in barite and sulfides in related hydrothermal and igneous rocks occurring in the ca 3 5 ga dresser formation western australia our results indicate that observed isotopic variations are mainly controlled by mixing of mass dependently md and non mass dependently fractionated non md sulfur reservoirs based on the quadruple sulfur isotope systematics 34s 33s 36s for these minerals four end member sulfur reservoirs have been recognized 1 non md sulfate 34s 5 2 33s 3 1 2 md sulfate 34s 10 3 3 non md sulfur 34s 6 33s 4 and 4 igneous md sulfur 34s 33s 0 the first and third components show a clear non md signatures thus probably represent sulfate and sulfur aerosol inputs the md sulfate component 2 is enriched in 34s 10 3 and may have originated from microbial and or abiotic disproportionation of volcanic s or so2 our results reconfirm that the dresser barites contain small amounts of pyrite depleted in 34s by 15 22 relative to the host barite these barite pyrite pairs exhibit a mass dependent relationship of 33s 34s with slope less than 0 512 which is consistent with that expected for microbial sulfate reduction and is significantly different from that of equilibrium fractionation 0 515 the barite pyrite pairs also show up to 1 difference in 36s values and steep 36s 33s slopes which deviate from the main archean array 36s 33s 0 9 and are comparable to isotope effects exhibited by sulfate reducing microbes 36s 33s 5 to 11 these new lines of evidence support the existence of sulfate reducers at ca 3 5 ga whereas microbial sulfur disproportionation may have been more limited than recently suggested

hydrochemical characteristic and interaction process of surface and groundwater in mid lower reach of hanjiang river china hanjiang river the largest tributary of yangtze faces environmental pressures from south to north water transfer project and anthropogenic influences in order to reveal the chemical characteristics of river and contribution of groundwater to the river along mid lower reaches of hanjiang river field works have been conducted in wet and dry seasons the major ion compositions are characterized by the dominance of ca2 and hco3 in surface water and groundwater as a result calcite weathering is a dominant process for surface water and groundwater additionally dolomite weathering also contributes to hydrochemical evolution in the groundwater of the basin stable isotopes 18o d show that both surface and groundwater most probably originate from present day precipitation isotopic of both surface water and groundwater can provide greater confidence in identifying the weathering and extent of interaction processes it was found that river water was mainly recharged by tributaries in wet season while recharged by groundwater in dry season dissolved solute flux from tributaries and groundwater to the river was estimated by water balance budget model and environmental tracer mass balance approach tributaries and groundwater contribute to mid lower reach of hanjiang river in wet season and dry season respectively from section h1 to h9 there were 455 4 kg s of total dissolved solute tds transferred from tributaries to the main channel and 99 8 kg s of tds fluxed from groundwater to river water in dry season

magnetic properties of martian meteorites implications for an ancient martian magnetic field the magnetic properties of samples of seven martian meteorites eet 79001 zagami nakhla lafayette governador valadares chassigny and alh 84001 have been investigated all possess a weak very stable primary natural remanent magnetization nrm and some have less stable secondary components in some cases the latter are associated with magnetic contamination of the samples imparted since their recovery and with viscous magnetization acquired during exposure of the meteorites to the geomagnetic field since they fell the magnetic properties are carried by a small content 1 of titanomagnetite and in alh 84001 possibly by magnetite as well the most likely source of the primary nrm is a thermoremanent magnetization acquired when the meteorite material last cooled from a high temperature in the presence of a magnetic field current evidence is that this was 1 3 ga ago for the nakhlites and chassigny and 180 ma for shergottites the time of the last relevant cooling of alh 84001 is not presently known preliminary estimates of the strength of the magnetizing field are in the range 0 5 5 ut which is at least an order of magnitude greater than the present field it is tentatively concluded that the magnetic field was generated by a dynamo process in a martian core with appropriate structure and properties

fundamental relations of mineral specific magnetic carriers for paleointensity determination abstract a fundamental linear relationship exists between the efficiency of thermoremanent magnetization measured at room temperature and the magnitude of the ambient field at the time of acquisition the magnetic efficiency the ratio of thermoremanent magnetization to saturation remanent magnetization multiplied by the saturation magnetization is proportional to the magnetizing field where the proportionality constant is independent of magnetic mineralogy and domain state the empirical constant for this equation was determined using existing experimental data of single domain through pseudosingle domain to multidomain states of iron meteoritic iron magnetite maghemite pyrrhotite and hematite we show that the acquired magnetic efficiency is closely related to two types of demagnetizing fields that act as barriers against domain wall pinning during magnetic acquisition the first relates to the saturation magnetization that is derived from the distribution of bohr magnetons within the crystal lattice and the second originates from grain shape theoretical considerations imply a factor of two difference in the magnetic efficiency acquired during laboratory and geologic timescales this equation reveals that troilite may be a potentially important magnetic carrier for extraterrestrial magnetism using magnetic scanning techniques our relationship allows for estimating the paleointensity from samples that contain more than one magnetic species

methanotrophs regulated atmospheric sulfur isotope anomalies during the mesoarchean tumbiana formation western australia abstract the 2 73 billion year old tumbiana formation of the fortescue group in the pilbara craton western australia preserves one of the best records of the fortescue negative carbon isotope excursion with organic 13 c values as low as 60 the prevailing environmental conditions at this biological excursion are not well understood here we present an integrated carbon 12 c 13 c and multiple sulfur 32 s 33 s 34 s 36 s isotopes study of a pristine diamond drill core through the tumbiana formation collected at meentheena by the pilbara drilling project our results show centimeter to meter scale heterogeneities in sulfur and carbon contents and isotope compositions organic 13 c values vary at the meter scale and display a bimodal distribution with peaks at about 44 and 34 this bimodal distribution is interpreted to reflect variations in the contribution of methanotrophic metabolism to the biomass the 34 s v cdt values of sedimentary pyrite vary from 5 73 to 2 75 sulfur isotopes sulfur contents and s c ratios are variable suggesting a complex local biotic sulfur cycling sedimentary pyrite preserves mass independent fractionation of sulfur mif s with 33 s v cdt ranging between 0 25 and 1 64 average 0 47 corresponding to the characteristic dampened mesoarchean anomalies positive correlation between sulfide mif s and a 13 c depletion of organic matter argues for a causal link between methanotrophs activity and the preservation of mif s anomalies in the rock record

evaluating the role of microbial sulfate reduction in the early archean using quadruple isotope systematics abstract microscopic pyrites with low 34 s 32 s ratios in 3 47 gyr old sedimentary barites from north pole australia have been interpreted as evidence for microbial sulfate reduction and or sulfur disproportionation in the early archean we show that these microscopic sulfides have similar to slightly less negative 33 s and slightly more negative 36 s values compared to the enclosing sulfate crystals this finding is consistent with a primary mass independent signature overprinted by biological sulfate reduction as calibrated by previous experimental laboratory culture studies however it is inconsistent with an overprint by abiological sulfate reduction or sulfur disproportionation as predicted by isotope exchange theory and laboratory culture studies thus our multiple sulfur isotope measurements support the contention that sulfate reducing microbes had evolved by 3 47 billion years ago

oxygen isotopic zoning of reversely zoned melilite crystals in a fluffy type a ca al rich inclusions from the vigarano meteorite the oxygen isotopic microdistributions within melilite measured using in situ secondary ion mass spectrometry correspond to the chemical zoning profiles in single melilite crystals of a fluffy type a ca al rich inclusions cais of reduced cv3 vigarano meteorite the melilite crystals show chemical reverse zoning within an individual single crystal from the akermanite rich core to the akermanite poor rim the composition changes continuously with the crystal growth the zoning structures suggest that the melilite grew in a hot nebular gas by condensation with decreasing pressure the oxygen isotopic composition of melilite also changes continuously from 16o poor to 16o rich with the crystal growth these observations suggest that the melilite condensation proceeded with change consistent with an astrophysical setting around the inner edge of a protoplanetary disk where both 16o rich solar coronal gas and 16o poor dense protoplanetary disk gas could coexist fluffy type a cais could have been formed around the inner edge of the protoplanetary disk surrounding the early sun

arsenic in groundwater testing pollution mechanisms for sedimentary aquifers in bangladesh in the deltaic plain of the ganges meghna brahmaputra rivers arsenic concentrations in groundwater commonly exceed regulatory limits 50 m gl 21 because feooh is microbially reduced and releases its sorbed load of arsenic to groundwater neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution the most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat concentrations of ammonium up to 23 mg l 21 come from microbial fermentation of buried peat and organic waste in latrines concentrations of phosphorus of up to 5 mg l 21 come from the release of sorbed phosphorus when feooh is reductively dissolved and from degradation of peat and organic waste from latrines calcium and barium in groundwater come from dissolution of detrital and possibly pedogenic carbonate while magnesium is supplied by both carbonate dissolution and weathering of mica the 87 sr 86 sr values of dissolved strontium define a two component mixing trend between monsoonal rainfall 0 711 6 0 001 and detrital carbonate 0 735

lithofacies control on multiple sulfur isotope records and neoarchean sulfur cycles abstract triple sulfur isotope ratios 32 s 33 s 34 s were measured for 141 bulk rock samples from two agouron scientific drill cores gkp01 and gkf01 that recovered neoarchean successions of the transvaal supergroup south africa these two deep time cores are correlated to each other with 14 tie lines using volcanic and impact spherule layers in a sequence stratigraphic framework allowing us to evaluate both lithofacies and temporal controls over multiple sulfur isotope systematics the ca 2 5 ga giga annum before present basinal klein naute formation and the ca 2 6 ga peritidal boomplaas and vryburg formations yield an array of data characterized by 33 s 1 4 34 s these linear trends are found in both shallow water and deepwater facies but are characteristic to rocks with high iron content suggesting these may reflect isotopic compositions of aqueous sulfide elemental sulfur reservoirs in the neoarchean oceans data that deviate from this linear array are interpreted as resulting from additional inputs of sulfide from microbial sulfate reduction with or without contribution from sulfur disproportionation sulfate derived sulfur evolved to be either enriched or depleted in 34 s depending on the local depositional environment for example the reivilo formation in core gkf01 is characterized by abundant microbialite textures and shows an isotopic signature of closed system sulfate reduction rapid cementation of these carbonate fabrics may have attenuated the supply of sulfate to pore waters resulting in the progressive 34 s enrichments during bacterial sulfate reduction below the sediment water interface in contrast signatures of open system sulfate reduction are associated with slope facies dominated by granular dolostones preserved in the upper nauga formations in gkf01 and gkp01 the two different sulfur isotope patterns interpreted to reflect closed versus open system sulfate reduction are both found in the lower reivilo formation in both cores this lateral variation of isotope signals documents that observed 34 s shifts can be controlled locally and may not have temporal significance this study demonstrates critical importance of recovering sulfur isotope data from stratigraphically correlated drill cores to evaluate both geographical and temporal shifts of sulfur cycles and their links to the great oxidation event at the end of the archean eon

evidence for an impact induced magnetic fabric in allende and exogenous alternatives to the core dynamo theory for allende magnetization we conducted a paleomagnetic study of the matrix of allende cv3 chondritic meteorite isolating the matrix s primary remanent magnetization measuring its magnetic fabric and estimating the ancient magnetic field intensity a strong planar magnetic fabric was identified the remanent magnetization of the matrix was aligned within this plane suggesting a mechanism relating the magnetic fabric and remanence the intensity of the matrix s remanent magnetization was found to be consistent and low 6 t the primary magnetic mineral was found to be pyrrhotite given the thermal history of allende we conclude that the remanent magnetization was formed during or after an impact event recent mesoscale impact modeling where chondrules and matrix are resolved has shown that low velocity collisions can generate significant matrix temperatures as pore space compaction attenuates shock energy and dramatically increases the amount of heating nonporous chondrules are unaffected and act as heat sinks so matrix temperature excursions are brief we extend this work to model allende and show that a 1 km s planar impact generates bulk porosity matrix porosity and fabric in our target that match the observed values bimodal mixtures of a highly porous matrix and nominally zero porosity chondrules make chondrites uniquely capable of recording transient or unstable fields targets that have uniform porosity e g terrestrial impact craters will not record transient or unstable fields rather than a core dynamo it is therefore possible that the origin of the magnetic field in allende was the impact itself or a nebula field recorded during transient impact heating

atmospheric sulfur rearrangement 2 7 billion years ago evidence for oxygenic photosynthesis mass independently fractionated sulfur isotopes mif s provide strong evidence for an anoxic atmosphere during the archean moreover the temporal evolution of mif s shows increasing magnitudes between 2 7 and 2 5 ga until the start of the great oxidation event g o e at around 2 4 ga the conclusion of a completely anoxic atmosphere up to the g o e is in contrast to recent studies on redox sensitive elements which suggest slightly oxidizing conditions during continental weathering already several hundred million years prior to the g o e in order to investigate this apparent inconsistency we present multiple sulfur isotopes for 2 71 ga pyritic black shales derived from the kidd creek area ontario canada these samples display high positive 33s values up to 3 8 and the typical late archean slope in 36s 33s of 0 9 in contrast the time period before 3 2 2 73 ga is characterized by greatly attenuated mif s magnitudes and a slope in 36s 33s of 1 5 we attribute the increase in 33s magnitude as well as the contemporaneous change in the slope of 36s 33s to changes in the relative reaction rate of different mif s source reactions and changes in atmospheric sulfur exit channels both of these are dependent on atmospheric ch4 co2 and o2 mixing ratios we propose a distinct change in atmospheric composition at 2 7 ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed neoarchean mif s record our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2 7 billion years ago

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

geologic geomorphic and hydrologic framework and evolution of the bengal basin india and bangladesh abstract the bengal basin the largest fluvio deltaic sedimentary system on earth is located in bangladesh and three eastern states of india sediment accumulates in the basin from the ganges brahmaputra and meghna gbm river systems and is dispersed into the bay of bengal forming the largest submarine fan in the world the basin is located in the himalayan foreland at the junction of the indian eurasian and burmese plates the basin is bounded by the indian craton on the west and the indo burmese fold belts on the east it can be broadly divided into a stable shelf and a foredeep separated by a deep seismic hinge zone basin sediments overlie gondwanan basement and vary in thickness from a few kilometers on the stable shelf to more than 16 km in the foredeep the basin was initiated at the breakup of gondwanaland in the late mesozoic and evolved through the formation of the proto gbm delta to the present delta starting around 10 5 ma the stratigraphy of the different parts of the basin differs considerably because of contrast in depositional history within the several sub basins that were produced by intra plate tectonic activities associated with ongoing himalayan orogeny the present day geomorphology is dominated by the extensive holocene gbm floodplain and delta the vertical succession of the deltaic plain can be classified into five units on the basis of differences in grain size which reflect differing depositional environments the initiation of the modern gbm delta at the onset of the pleistocene glacial maximum and its evolution to the present configuration are intricately related to holocene fluvio dynamic processes eustatic sea level changes and tectonic movements the sedimentology and mineralogy of the different parts of the basin reflect differences in sediment provenance the mineralogy is dominated by detrital quartz some feldspar and minor amounts of carbonates illite and kaolinite are the main clay minerals the basin has profuse groundwater resources but the architecture of the aquifers is not yet well resolved different classification schemes have been proposed on the basis of lithology regional groundwater flow follows a low hydraulic gradient from north to south but pumping for irrigation during the past several decades appears to have severely distorted regional flow recharge occurs primarily during the monsoon season groundwater chemistry is dominated by anoxic ca hco 3 type water with relatively high concentrations of fe mn and as carbonate dissolution silicate weathering feooh reduction and mixing with saline water are the primary processes controlling hydrochemistry

inverse thermoremanent magnetization 1 inverse thermoremanent magnetization itrm is reversed to the thermoremanent magnetization trm process itrm results from warming from low temperature t in a magnetic field while trm results from field cooling from high t the development of itrm was studied in magnetites of grain sizes from submicron to 135 m in pyrrhotites and in hematite crystals all three minerals acquired itrm after warming through their magnetic transitions 35 k for pyrrhotite 120 and 130 k for magnetite 250 k for hematite however when an impacting meteorite s cold interior warms to ambient t in the geomagnetic field magnetite is the most likely candidate for acquiring itrm the magnetite itrm blocking temperature distribution was determined from 12 neighboring partial itrms in nested field on warming plus field off cooling cycles 300 20 k the largest partial itrms are produced in t intervals around magnetite s verwey transition tv 110 120 k and isotropic point tk 130 k both transitions involve large changes in crystalline anisotropy and renucleation of magnetic domains itrm is blocked when initially broad domain walls narrow and are pinned by dislocations itrm has contrasting properties to trm which is mainly due to blocked single domain moments itrm is strongest for 3 to 20 m grains whereas trm peaks for submicron magnetites only 10 20 of itrm survives low temperature demagnetization ltd at 77 k or af demagnetization to 10 15 mt compared to 30 90 for trm itrm decreases quasi linearly with t in thermal demagnetization the median unblocking temperature tub is 300 c and 20 25 survives at 550 c the low tub part of itrm could mimic extraterrestrial nrm of low tub cited as evidence of negligible heating of meteorites in their transfer to earth the high tub itrm would contaminate paleointensity determinations up to the highest t steps the best cure for itrm contamination is af or ltd pretreatment

trm in low magnetic fields a minimum field that can be recorded by large multidomain grains abstract thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling rocks that cool slowly commonly contain magnetic grains of millimeter scale this study investigated 1 mm sized magnetic minerals of iron iron nickel magnetite and hematite and concluded that the thermoremanent magnetization trm acquired by these grains did not accurately record the ambient magnetic fields less than 1 t instead the trm of these grains fluctuated around a constant value consequently the magnetic grain ability to record the ambient field accurately is reduced above the critical field trm acquisition is governed by an empirical law and is proportional to saturation magnetization m s the efficiency of trm is inversely proportional to the mineral s saturation magnetization m s and is related to the number of domains in the magnetic grains the absolute field for which we have an onset of trm sensitivity is inversely proportional to the size of the magnetic grain these results have implications for previous reports of random directions in meteorites during alternating field demagnetization or thermal demagnetization of trm extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude extraterrestrial rocks commonly contain large iron based magnetic minerals as a common part of their composition and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates

structural evolution of the middle archean coastal pilbara terrane western australia 1 the middle archean coastal pilbara terrane is composed of two lithotectonic units the karratha and cleaverville roebourne supercomplexes the karratha supercomplex lies tectonically beneath the cleaverville roebourne supercomplex and consists of 3270 3250 ma granitic batholiths and metamorphosed terrigenous sediments it is identified as a juvenile continental crust with tonalite trondhjemite granodiorite type granites the cleaverville roebourne supercomplex is interpreted as a dismembered immature island arc which has a restored crustal thickness of 20 km this supercomplex consists of 3195 ma bimodal volcanic chemical sedimentary sequences metabasite low k granitic gneiss and peridotite regionally distributed granite porphyries circa 3020 3000 ma have intruded all of the coastal pilbara terrane detailed structural analyses and zircon u pb dating suggest that the coastal pilbara terrane records two phases of deformation the first phase is an immature island arc and micro continental crust collision and is represented by top to the northwest thrusting d1 circa 3100 3020 ma this phase of deformation an arc continent collision appears to represent an important transition in the formation of continental crust the 3020 ma granite porphyry event may be related to syn post collision igneous activity and may reflect a second crustal thickening associated with magmatic underplating the second phase of deformation is characterized by regional scale left lateral strike slip faulting d2 circa 3020 2925 ma and by more localized right lateral strike slip faulting d3 circa 2925 2770 ma this phase of craton scale strike slip faulting may represent stabilization of the newly formed continental crust

mass dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles abstract sulfur isotope studies of post archean terrestrial materials have focused on the ratio 34s 32s because additional isotopes 33s and 36s were thought to carry little information beyond the well known mass dependent relationship among multiple isotope ratios we report high precision analyses of 33s and 36s values defined as deviations of 33s and 36s from ideal mass dependent relationships for international reference materials and sedimentary sulfides of phanerozoic age by using a fluorination technique with a dual inlet isotope ratio mass spectrometer measured variations in 33s and 36s are explained as resulting from processes involve branching reactions two or more reservoirs formed or mixing irreversible processes in closed systems rayleigh distillation amplify the isotope effect we outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation the isotope systematics discussed here cannot explain the much larger variation of 33s and 36s observed in archean rock records furthermore phanerozoic samples we have studied show a characteristic 33s and 36s relationship that differs from those measured in archean rocks and laboratory photolysis experiments thus high precision analysis of 33s and 36s can be used to distinguish small non zero 33s and 36s produced by mass dependent processes from those produced by mass independent processes in archean rocks and extraterrestrial materials

new insights into archean sulfur cycle from mass independent sulfur isotope records from the hamersley basin australia we have measured multiple sulfur isotope ratios 34 s 33 s 32 s for sulfide sulfur in shale and carbonate lithofacies from the hamersley basin western australia the v 33 s values v 33 swn 33 s30 515un 34 s shift from 31 9 to 6 9x over a 22 m core section of the lower mount mcrae shale v2 5 ga likewise sulfide sulfur analyses of the jeerinah formation v2 7 ga yield v 33 s values of 30 1 to 8 1x over a 50 m section of core despite wide variations in v 33 s and n 34 s these two shale units yield a similar positive correlation between v 33 s and n 34 s in contrast pyrite sulfur analyses of the carawine dolomite v2 6 ga yield a broad range in n 34 s 3 2 to 16 2x but a relatively small variation and negative values in v 33 s 32 5 to 31 1x the stratigraphic distribution of n 33 s n 34 s and v 33 s in western australia allows us to speculate on the sulfur isotopic composition of archean sulfur reservoirs and to trace pathways in the archean sulfur cycle our data are explained by a combination of massindependent fractionation mif in the atmosphere and biological mass dependent fractionation in the ocean in the archean volcanic sulfur bearing gas species were photolysed by solar ultraviolet uv radiation in an oxygen free atmosphere resulting in mif of sulfur isotopes aerosols of s8 with v 33 ss 0 and sulfuric acid with v 33 s6 0 formed from the products of uv photolysis and carried mass independently fractionated sulfur into the hydrosphere the signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass dependent fractionation on mass independent atmospheric effects multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur metabolizing organisms a 2003 elsevier science b v all rights reserved

major ion chemistry in the headwaters of the yamuna river system chemical weathering its temperature dependence and co 2 consumption in the himalaya abstract the yamuna river and its tributaries in the himalaya constitute the yamuna river system yrs the yrs basin has a drainage area and discharge comparable in magnitude to those of the bhagirathi and the alaknanda rivers which merge to form the ganga at the foothills of the himalaya a detailed geochemical study of the yrs was carried out to determine i the relative significance of silicate carbonate and evaporite weathering in contributing to its major ion composition ii co2 consumption via silicate weathering and iii the factors regulating chemical weathering of silicates in the basin the results show that the yrs waters are mildly alkaline with a wide range of tds 32 to 620 mg l 1 in these waters the abundances of ca mg and alkalinity which account for most of tds are derived mainly from carbonates many of the tributaries in the lower reaches of the yamuna basin are supersaturated with calcite in addition to carbonic acid sulphuric acid generated by oxidation of pyrites also seems to be supplying protons for chemical weathering silicate weathering in yrs basin contributes on average 25 molar basis of total cations on a basin wide scale silicate weathering however does not seem to be intense in the basin as evident from low si na k in the waters 1 2 and low values of chemical index of alteration cia in bed sediments 60 co2 drawdown resulting from silicate weathering in the yrs basin in the himalaya during monsoon ranges between 4 to 7 105 moles km 2 y 1 this is higher than that estimated for the ganga at rishikesh for the same season the co2 consumption rates in the yamuna and the ganga basins in the himalaya are higher than the global average value suggesting enhanced co2 drawdown in the southern slopes of the himalaya the impact of this enhanced drawdown on the global co2 budget may not be pronounced as the drainage area of the yrs and the ganga in the himalaya is small the co2 drawdown by silicates in the yrs basin is marginally higher than the reported values of co2 release from oxidation of organic rich sediments estimated using re as a proxy this comparison shows the need to constrain co2 sources and sinks better to balance its budget in a regional scale the results also show that silicate weathering rate in the yrs basin is 10 mm ky 1 and on the ganga basin it is 5 mm ky 1 which are several times lower than the carbonate weathering rates the significantly higher silicate weathering rate observed in the yrs basin seems to be governed by rapid physical erosion in this region the apparent activation energy for overall silicate weathering in the yrs basin derived from na and si concentrations and water temperature ranges from 50 to 80 kj mol 1 these values are comparable to those reported for granitoid weathering in natural watersheds and feldspar weathering in laboratory experiments this study brings to light the sources contributing to major ions enhanced chemical weathering rates in the yamuna river basin and interdependence of silicate weathering on physical erosion and temperature