hydrogen isotope composition of early proterozoic seawater the hydrogen isotope composition of early proterozoic seawater has been indirectly determined on the basis of d values measured in mafic ultramafic samples from a hydrothermally altered layered flow from the chukotat group of the lower proterozoic 2 0 1 9 ga cape smith fold belt northern quebec this flow and other mafic ultramafic units of the fold belt represent some of the oldest relict oceanic crust identified on earth measured d values of whole rock samples range from 53 to 40 matching closely values found in modern metavolcanic rocks and phanerozoic ophiolites a model of water rock interaction for conditions of metamorphism deduced from mineral parageneses and sr and o isotope data t 200 250 c water rock ratio close to 1 0 indicates that the d values of chukotat flow samples reflect interaction with seawater that had a d 0 20 this result implies that the hydrogen isotope composition of early proterozoic seawater was certainly similar to the modern value and that subduction and ridge processes that control the global water cycle have not changed significantly since the early proterozoic

genesis of palimpsest cool water carbonate sediment on the continental margin of southern australia abstract the continental margin of southern australia is a site of extensive quaternary cool water carbonate sedimentation seafloor sediments are a mixture of holocene biofragments and late pleistocene relict and stranded particles relict sediments produced during marine isotope stages 3 and 4 are generally preserved as iron stained intraclasts such grains in the great australian bight are photozoan in character they include symbiont bearing benthic foraminifers and coralline algal particles and indicate that the wide shelves were bathed in shallow relatively warm oligotrophic waters and floored by marine grasses similar to many modern shallow inboard embayments in the region today in contrast east of the great australian bight relict sediments are more heterozoan in character with coralline algal and molluscan facies on the inner shelf and bryozoan sediments on the outer shelf stranded sediments marooned during the sea level rise associated with marine isotope stage 2 are gray to buff in color and composed of both biofragments and intraclasts these particles are mainly heterozoan in character throughout the region with subtropical photozoan marine grassbank facies localized to portions of the inner great australian bight stranded sediments are interpreted to have accumulated in an areally restricted photic zone and upon an overall narrow shelf covered in cool nutrient rich waters swell driven seafloor sediment disturbance to depths of 100 m along the southern australian margin in conjunction with reduced rates of sedimentation that characterize the cool water carbonate realm have resulted in mixing of sediment currently being produced with exhumed relict and stranded sediments generated during previous sea level stands the composition of these deposits and the nature of their pre burial alteration are a function of sea level history late pleistocene relict sediments originally accumulated in low energy inboard environments but were modified in the surf zone during subsequent sea level fluctuations this resulted in the formation of intraclasts abraded infilled and cemented biofragments of local origin the widespread distribution of intraclasts and of composite grain assemblages derived from multiple sedimentary environments that display highly variable degrees of seafloor alteration may be signatures of modern cool water carbonate assemblages worldwide while such palimpsest sediments may complicate paleoenvironmental interpretation their differentiation in the rock record can be utilized to discern subtleties in paleoceanography and sea level history

the relative timing of serpentinisation and mantle exhumation at the ocean continent transition iberia constraints from oxygen isotopes legs 173 and 149 of the ocean drilling program profiled a zone of exhumed mantle peridotite at the ocean continent transition oct beneath the iberia abyssal plain the zone of exhumed peridotite appears to be tens of kilometers wide and is situated betw

δ2h of water from fluid inclusions in proterozoic halite evidence for a deuterium depleted hydrosphere abstract 2h were measured in water present in tiny primary fluid inclusions trapped in mesoproterozoic 1 4 ga and neoproterozoic 0 8 ga halite crystals while the hydrogen concentrations range from 300 to 1500 ppm for the whole sample collection 2h values range from 74 to 54 vsmow for the mesoproterozoic halite sampled from the sibley group ontario canada and from 89 to 38 for the neoproterozoic halite from the browne formation officer basin australia the amount of evaporation required to precipitate halite is accompanied by a deuterium enrichment of 30 to 90 it means that the original aqueous solutions had 2h values comprised between 140 and 90 considering the estimated tropical paleolatitudes for both depositional environments surface waters were most likely significantly deuterium depleted relative to modern ones if such interpretation is partly at variance with the geological record it could be reconciliated with some modelling outputs of the long term water cycle indeed several mechanisms produce a progressive deuterium enrichment of the oceans those identified so far are the addition of deuterium rich extraterrestrial water cometary dust the loss of h during water photodissociation by ultraviolet radiation in the upper atmosphere and the isotopic fractionation that takes place during the hydroxylation of nominally anhydrous mantle rock forming minerals we also propose that the major process able to shift through time the 2 of the hydrosphere is the reduction of h2o into h2 during the serpentinization of mantle rocks at mid ocean ridges

stable isotope compositions of serpentinite seamounts in the mariana forearc serpentinization processes fluid sources and sulfur metasomatism the mariana and izu bonin arcs in the western pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge we present stable isotope o h s and c data for serpentinites from conical seamount in the mariana forearc and s isotope data for torishima seamount in the izu bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge and to investigate the transport of sulfur from the slab to the mantle wedge six serpentine mineral separates have a restricted range of 18o 6 5 8 5 antigorite separates have d values of 29 5 to 45 5 that reflect serpentinization within the mantle wedge whereas chrysotile has low d values 51 8 to 84 0 as the result of re equilibration with fluids at low temperatures fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300 375 c two late cross fiber chrysotile veins have higher 18o values of 8 9 to 10 8 and formed at lower temperatures as low as 100 c aqueous fluids in equilibrium with serpentine at 300 375 c had 18o 6 5 9 and d 4 to 26 consistent with sediment dehydration reactions at temperatures 200 c in the subducting slab rather than a basaltic slab source three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have 18o 16 7 24 5 consistent with the serpentinizing fluids at temperatures 250 c 13c values of 0 1 2 5 suggest a source in subducting carbonate sediments the 34s values of sulfide in serpentinites on conical seamount 6 7 to 9 8 result from metasomatism through variable reduction of aqueous sulfate 34s 14 derived from slab sediments despite sulfur metasomatism serpentinites have low sulfur contents generally 164 ppm that reflect the highly depleted nature of the mantle wedge the serpentinites are mostly enriched in 34s median 34ssulfide 4 5 consistent with a 34s enriched mantle wedge as inferred from arc lavas

a vestige of earth s oldest ophiolite a sheeted dike complex within the 3 8 billion year old isua supracrustal belt isb in southwest greenland provides the oldest evidence of oceanic crustal accretion by spreading the geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid ocean ridge like setting and their oxygen isotopes suggest a hydrothermal ocean floor type metamorphism the pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association the paleoarchean isua ophiolite complex these sheeted dikes offer evidence for remnants of oceanic crust formed by sea floor spreading of the earliest intact rocks on earth

halogen chemistry and hydrogen isotopes of apatite from the 3 7 ga isua supracrustal belt sw greenland abstract the origin and evolution of volatiles particularly water in the abiotic early earth environment have been intensively studied and this is a topic of high relevance when considering the timing and conditions of life s emergence investigation of hydrogen isotopes in the oldest crystals of minerals from the apatite group ca10 po4 6 f cl oh 2 should bring new insight to this topic as the d h ratio of apatite has proved useful for establishing the evolution of volatiles in other solar system bodies apatite crystals from metasedimentary and metavolcanic rocks collected from a low strain domain of the eoarchean isua supracrustal belt have been investigated for their d h signatures using secondary ion mass spectrometry and for major and trace element abundances using epma and laser ablation icp ms apatite crystals from isua have a broad range in volatile compositions both within samples from the same lithology e g banded iron formation bif and between different lithologies bif metacarbonate mafic dyke most of the crystals are chlorine rich which is in contrast to typical modern sedimentary apatite that is predominantly carbonate fluorapatite we have used the ree distribution patterns from isua apatite to distinguish those crystals having sedimentary metasomatic and magmatic origins the d h ratios for all samples were determined to be very low with dvsmow values falling within the range from 93 to 56 d values of isua apatite do not correlate with ree signatures of crystals nor could we relate d to the host lithologies we conclude that this geochemical system has been reset during long and multi phase geologic history making it difficult if not impossible to use hydrogen isotopes for assessment of the primary signatures of the archean hydrosphere

high archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3 5 ga swaziland supergroup south africa new and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3 5 3 2 ga swaziland supergroup barberton greenstone belt south africa the low 1 8 o values of these cherts indicate extremely high ocean temperatures of 55 85 c previously the large depletion in 1 8 o shown by all barberton cherts relative to their phanerozoic counterparts has been attributed to low 1 8 o in archean oceans chert formation during late diagenesis wholesale loss of 1 8 o during alteration and or regional silicification of sediments around hot springs these alternative explanations are not compatible with the new results cherts in the onverwacht group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying fig tree and moodies groups demonstrating that the chert 8 0 o values were fixed prior to archean uplift and erosion which started at 3 26 ga the maximum 1 8 o value in barberton cherts 22 is lower than the minimum values 23 in phanerozoic bedded cherts precluding late diagenesis as the explanation of the overall low 1 8 o values regional metamorphic hydrothermal or long term resetting of original 1 8 o values is also precluded by preservation of 1 8 o values across different metamorphic grades and by systematic 1 8 o differences among interbedded chert types stratigraphic units and conglomerate clasts the 7 1 8 o variation of these archean cherts is similar to that of phanerozoic deep sea cherts formed when opal converted to microquartz during burial but the actual archean values are 10 lower marine opal was apparently converted to microquartz during burial to depths of 1 km cherts with 1 8 o 15 reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and or hydrothermal activity cherts with higher 1 8 o values formed during early diagenesis and indicate an extremely hot archean ocean and surface environment

evolution of holocene coastal environments near robe southeastern south australia abstract coastal robe range and adjacent woakwine range situated 10 15 km inland are the most southwesterly and youngest of a series of parallel low altitude pleistocene beach dune barriers in southeastern south australia in the early holocene the post glacial marine transgression flooded the robe woakwine corridor thus forming a shallow back barrier lagoon open to the southern ocean near the present towns of robe and beachport marine to lagoonal bioclastic carbonate quartz sedimentation prevailed within the corridor from ca 7500 2000 yr bp along the seaward side of robe range transgressive sands were deposited initially as ephemeral sand flats but were mostly redistributed as coastal dunes which today remain active and blanket the pleistocene core of the range sand was also transported along the coast and deposited within the protected embayments of guichen and rivoli bays in the form of prograded beach ridge plains eventually isolating the back barrier lagoon from the open ocean sediment aggradation within the lagoon cessation of marine influence and regional uplift of 0 7 m during the holocene combined to transform the corridor to a lacustrine landscape fossil molluscs foraminifera and ostracods are diagnostic of successive palaeo environments and thus signify stages in the rapid geomorphic evolution of the coastal robe woakwine corridor

successive foraminiferal faunas and inferred palaeoenvironments associated with the postglacial holocene marine transgression gulf st vincent south australia gulf st vincent is one of a pair of elongate triangular shallow water embayments into the southern coast of continental australia from the southern floor of this embayment within a shallow basi

oxygen isotopes and sea level from the time that detailed oxygen isotope records derived from foraminifera living in the constant temperature environment of the abyssal ocean became available there has been a discrepancy between the ice volume record that these records imply and that derived from the altitude of dated coral terraces around the world here we re examine the data and conclude that the temperature of the abyssal ocean has been an actively varying component of the climate system

control of soil acidification by fluvial sedimentation in an estuarine floodplain eastern australia abstract a shallow stratigraphic sequence with associated pyrite induced soil acidification was investigated along a transect from the levee to the backswamp in an estuarine floodplain of eastern australia three sedimentary layers were identified and interpreted to correspond with three depositional stages firstly a layer of humic pyrite rich silty mud was deposited under a saline mangrove inhabited intertidal environment during the present high sea level episode this pyritic layer is buried by the second sedimentary layer of grey brown mud with limited pyrite content that was deposited in a brackish lagoonal environment this material now represents much of the contemporary backswamp surface the third sedimentary layer is a sandy mud without pyrite that has been deposited by freshwater overbank floods it is concluded that fluvial sedimentation has been increasingly important in the development of the stratigraphic sequence controlling the pyrite content thickness and occurrence depth of the pyritic layer the present drainage conditions have allowed oxidation of pyrite in the soils of the backswamp and the resulting acidification has caused elevated concentrations of toxic aluminium that threaten the local environment however in the levee the pyritic layer is covered by thick non pyritic freshwater sediments and low pyritic lagoonal sediments and the soil profiles are unlikely to contribute to any acidification hazard

oxygen isotope study of mid ocean ridge hydrothermal fluids implication for the oxygen 18 budget of the oceans new data are reported for the 18o composition of hot hydrothermal fluids from various sites with different tectonic settings the measured 18o in the range 0 53 2 3 confirm earlier data suggesting that mid ocean ridge hydrothermal vents are a major source of 18o to the ocean in light of this new data we attempt to re evaluate the marine 18o budget from direct quantification of the main sources and sinks of oceanic oxygen 18 within the uncertainty of the flux estimates 18o inputs and outputs are found to balance each other showing that oceanic 18o is presently at a steady state using these present fluxes we examine the past evolution of the 18o composition of the ocean and speculate that the initial 18o oceanic content may have evolved rapidly towards its present value this suggests as already pointed out in previous studies that ocean water may have had a rather constant 18o composition over most of the earth s history and that 18o measurements in ancient cherts and carbonates may primarily record the evolution of the ocean temperature

influence of continental growth on mid ocean ridge depth the interconnectedness of life water and plate tectonics is strikingly apparent along mid ocean ridges mor where communities of organisms flourish off the disequilibrium of chemical potentials created by circulation of hydrothermal fluids driven by earth s heat nisbet and sleep 2001 staudigel et al 2004 moreover submarine hydrothermal environments may be critical for the emergence of life on earth nisbet and sleep 2001 oceans were likely present in the hadean valley et al 2002 harrison 2009 but questions remain about early earth s global tectonics van hunen and moyen 2012 including when seafloor spreading began and whether mid oceanic ridges were deep enough for maximum hydrothermal activities kasting et al 2006 for example plate tectonics influences global sea level by driving secular variations in the volume of ocean basins due to continental growth flament et al 2008 similarly variations in the distribution of seafloor age and associated subsidence flament et al 2008 due to assembly and dispersal of supercontinents coltice et al 2012 explains the largest sea level variation over the past 140 myr muller et al 2008 using synthetic plate configurations derived from numerical models of mantle convection coltice et al 2012 2014 appropriate for early earth we show that mor has remained submerged and its depths potentially constant over geologic time thus conditions in the early earth existed for hydrothermal vents at similar depths as today providing environments conducive for the development of life and allowing for processes such as hydrothermal alteration of oceanic crust to influence the mantle s geochemical evolution this article is protected by copyright all rights reserved

an integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems chemical and isotopic changes accompanying seawater basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs incorporating provision for addition and subtraction of a wide range of reactant and product minerals as well as cation and oxygen and hydrogen isotopic exchange equilibria the models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100 to 350 c with an overall water rock ratio of about 0 5 being constrained by an assumed 18 o h 2o at 350 c of 2 0 per mil h craig personal communication 1984 this is a realistic model because 1 the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust 2 the water rock ratio is high 50 at low temperatures and low 0 5 at high temperatures and 3 it allows for back reaction of earlier formed minerals during the course of reaction progress the results closely match the major element chemistry von damm et al 1985 and isotopic compositions craig et al 1980 of the hydrothermal solutions presently emanating from vents at 21 n on the east pacific rise the calculated solution chemistry for example correctly predicts complete loss of mg and so 4 and substantial increases in si and fe however discrepancies exist in the predicted ph 5 5 versus 3 5 measured and state of saturation of the solution with respect to greenschist facies minerals the calculated d h 2o is 2 6 per mil in excellent agreement with analytical determinations the calculated chemical mineralogic and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges humphris and thompson 1978 stakes and o neil 1982 as well as with data from ophiolites gregory and taylor 1981 predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite epidote chlorite tremolite greenschist assemblage at 350 c the models demand that the major portion of the water rock interaction occur at temperatures of 300 350 c interaction at temperatures below approximately 250 c results in negative 18 o h 2o shifts contrary to the observed positive 18 o values of the fluids exiting at midocean ridge vents hydrogen isotope fractionation curves by suzuoki and epstein 1976 lambert and epstein 1980 and liu and epstein 1984 among others are compatible with the model and require d h 2o to increase at all temperatures as a result of seawater basalt interaction

triple oxygen and hydrogen isotopic study of hydrothermally altered rocks from the 2 43 2 41 ga vetreny belt russia an insight into the early paleoproterozoic seawater abstract the early paleoproterozoic represents a period of rapid changes in earth systems that could have affected the stable isotopic composition of seawater the well preserved pillow structures hyaloclastites and komatiitic basalts of the 2 43 2 41 ga vetreny belt baltic shield provide a record of high temperature water rock interaction induced by contemporaneous seawater here we present results of mineralogical fluid inclusion hydrogen and triple oxygen isotopic analysis of hydrothermal alteration products emphasis is given to vein filling quartz and epidote as they likely formed at high water rock ratios ten minerals pairs quartz epidote and quartz calcite returned temperatures of isotopic equilibrium between 286 and 387 c which compares well to the homogenization temperatures measured for saline fluid inclusions hosted in vein quartz the computed d and 18o values of equilibrium fluids range between 31 and 12 and 1 36 and 3 20 respectively which overlap with the isotopic composition of ice free world seawater and fluids generated at submarine hydrothermal systems this is the earliest piece of evidence suggesting that early paleoproterozoic seawater had a d value close to 0 we also present triple oxygen isotopic composition of quartz and epidote that formed in similar facies of hydrothermal alteration from the relatively young 6 7 ma oceanic crust as sampled by the odp hole 504b in the eastern pacific ocean these data show similarity to the triple oxygen isotope analyses of the vetreny belt rocks indicating that the 2 43 2 41 ga seawater had the 17o value close to that of modern day seawater due to small fractionation at 300 390 c epidote water 1 epidotes present a strong evidence that equilibrium fluids had 17o values close to 0 based on the previously published quartz water calibration the computed 17o values of equilibrium fluids range between 0 11 and 0 03 significantly lower than that of seawater or inferred seawater derived fluids at low water rock ratios this can be explained by multiple factors including phase separation of fluids or and presence of low temperature quartz overgrowths both are reflected in the fluid inclusion data and in situ 18o measurements by ion microprobe sims presented here overall our study suggests that the 18o 17o and d values of the 2 43 2 41 ga seawater were 1 7 1 1 0 001 0 011 and 0 20 respectively similar to the modern values which reflects the dominant role of submarine hydrothermal alteration in the stable isotopic budget of seawater throughout earth s history

early archean serpentine mud volcanoes at isua greenland as a niche for early life the isua supracrustal belt greenland of early archean age 3 81 3 70 ga represents the oldest crustal segment on earth its complex lithology comprises an ophiolite like unit and volcanic rocks reminiscent of boninites which tie isua supracrustals to an island arc environment we here present zinc zn isotope compositions measured on serpentinites and other rocks from the isua supracrustal sequence and on serpentinites from modern ophiolites midocean ridges and the mariana forearc in stark contrast to modern midocean ridge and ophiolite serpentinites zn in isua and mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average based on recent results of zn isotope fractionation between coexisting species in solution the isua serpentinites were permeated by carbonate rich high ph hydrothermal solutions at medium temperature 100 300 c zinc isotopes therefore stand out as a ph meter for fossil hydrothermal solutions the geochemical features of the isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the mariana forearc the reduced character and the high ph inferred for these fluids make archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids

stable isotope δ18o δd δ37cl evidence for multiple fluid histories in mid atlantic abyssal peridotites odp leg 209 abstract serpentinized abyssal peridotites sampled by the ocean drilling program leg 209 along the mid atlantic ridge near the 15 20 n fracture zone have been analyzed for oxygen hydrogen and chlorine isotope compositions in order to determine isotopic behavior under a wide range of serpentinization conditions and place constraints on fluid history oxygen and hydrogen thermometry suggests peak serpentinization temperatures of 300 500 c serpentine separates have low d values possibly due to a magmatic fluid component or low temperature exchange during seafloor weathering chlorine geochemistry focused on three holes 1274a and 1272a serpentinized peridotites and 1268a serpentinite locally altered to talc concentrations of both water soluble chloride wsc and structurally bound chloride sbc are significantly lower at hole 1268a compared to holes 1274a and 1272a the 37cl values for wsc and sbc of serpentinites in holes 1274a and 1272a are slightly positive avg wsc 0 20 n 22 and avg sbc 0 35 n 22 representing typical seawater hydration conditions commonly determined for abyssal peridotite the sbc of serpentinites from hole 1268a are also positive avg 0 63 whereas the sbc in talc dominated samples is negative avg 1 22 the wsc of both talc and serpentine dominated samples are also negative avg 0 15 we interpret the chlorine isotope data to preserve a record of multiple fluid events as seawater hydrated the peridotite 37cl was preferentially incorporated into the forming serpentine and water soluble salts yielding similar 37cl values on a regional scale as sampled by holes 1268a 1274a and 1272a the resultant pore fluid was left depleted in 37cl locally hole 1268a this evolved fluid was remobilized possibly due to the initiation of hydrothermal circulation in response to emplacement of a mafic magma body the low 37cl pore fluids attained elevated sio2 and sulfur concentrations due to interaction with the gabbroic intrusion and when ascending through the surrounding serpentinite caused formation of isotopically negative talc this secondary fluid also flushed the preserved serpentinite of its previously formed salts resulting in negative 37clwsc values the 37clsbc values of the serpentinite samples remained unmodified by reaction with the secondary fluid

a synvolcanic origin for magnetite rich orebodies hosted by bif in the weld range district western australia abstract iron ore deposits hosted by archean banded iron formation bif in the weld range greenstone belt are representative of most of the documented iron ore deposits in the yilgarn craton they include near surface supergene goethite hematite orebodies that overlie and partly modify deeper occurrences of hypogene magnetite and specular hematite ores the cenozoic goethite hematite rich orebodies in these deposits are unequivocally the product of meteoric fluid alteration affecting bif in the near surface supergene environment however the deeper and likely older magnetite and specular hematite rich orebodies have a more contentious origin this study is the first to present a fluid alteration model for hypogene iron mineralization in the yilgarn craton that uses fluid inclusion and mineral chemistry data to constrain the physical chemical characteristics and source of hypogene fluids responsible for mineralization high grade 57 wt fe magnetite rich iron ore at the beebyn deposit defines a discontinuous series of 440 c and co2 rich paired o and c stable isotope data for stage 1 ferroan dolomite suggest that these fluids had a magmatic source while stage 1 magnetite chemistry e g enrichments in mg mn ca and na indicates chemical exchange took place between the fluids and mafic igneous rocks prior to crystallization of magnetite the presence of monophase carbonic fluid inclusions in stage 1 ferroan dolomite suggest that phase separation of a bicarbonate rich aqueous fluid took place in deeper parts of the hydrothermal system which led to the separation of the resultant volatile rich and brine phases during transport of the stage 1 fluid to shallower crustal levels cooling of the hydrothermal system during the stage 2 fluid event involved i an early brine 275 327 c 36 40 equiv wt nacl with cl br and c and o isotopes values that overlap the ranges for magmatic fluids with minor involvement of archean seawater and extensive chemical exchange with country rocks followed by ii pulses of moderate and lower temperature stage 2 brines 125 260 c 2 24 equiv wt cacl2 with cl br o and c isotope na br and ca ca na signatures that suggest cooling of magmatic derived fluids that mixed with archean seawater and reacted with mafic igneous country rocks in areas more distal to fluid pathways the last stage of formation of magnetite rich ore at the beebyn deposit involved the flow of stage 3 fluids through the existing fault network that controlled earlier fluids stage 3 fluids are lower temperature 98 175 c low to high salinity brines with cl br values that overlap reported ranges for 3 2 ga vent fluids and seawater thus they are likely the product of heated archean seawater that was chemically modified through interaction with mafic country rocks magnetite talc veins at the madoonga deposit have fluid halogen ratios for fluid inclusions hosted by magnetite that are compatible with a range of possible sources including low grade metamorphic fluids geothermal brines or oil field formation waters although their common spatial association with semi massive sulfides suggests their likely precipitation from brines derived from heated archean seawater more locally developed specular hematite quartz veins that cut folded magnetite rich ores at the beebyn and madoonga deposits are the product of iron redistribution in bif by heated meteoric fluids or seawater with precipitation of specular hematite as a consequence of oxidation and cooling of the fluid

climatic variability in the eastern united states over the past millennium from chesapeake bay sediments salinity oscillations caused by multidecadal climatic variability had major impacts on the chesapeake bay estuarine ecosystem during the past 1000 yr microfossils from sediments dated by radiometry 14 c 137 cs 210 pb and pollen stratigraphy indicate that salinity in mesohaline regions oscillated 10 15 ppt during periods of extreme drought low fresh water discharge and wet climate high discharge during the past 500 yr 14 wet dry cycles occurred including sixteenth and early seventeenth century megadroughts that exceeded twentieth century droughts in their severity these droughts correspond to extremely dry climate also recorded in north american tree ring records and by early colonists wet periods occurred every 60 70 yr began abruptly lasted 20 yr and had mean annual rainfall 25 30 and fresh water discharge 40 50 greater than during droughts a shift toward wetter regional climate occurred in the early nineteenth century lowering salinity and compounding the effects of agricultural land clearance on bay ecosystems

origin of waters responsible for serpentinization of the izu ogasawara mariana forearc seamounts in view of hydrogen and oxygen isotope ratios many serpentinite seamounts occur over a region 20 120 km west of the trench axis in the izu ogasawara mariana forearc regions the hydrogen and oxygen isotopic compositions of serpentine from these regions indicate that there are at least two kinds of waters responsible for serpentinization seawater and water derived from dehydration of the descending slab serpentine from two mariana and two torishima samples with microscopically ductile and sheared texture sheared type have lower d 63to 52 and slightly higher 18o values 6 1 to 8 2 than that of other nine ogasawara samples with mesh texture mesh type d 43to 49 and 18o 5 8to 6 7 this suggests that the sheared type serpentine with lower d and slightly higher 18o values was formed within the wedge mantle by interaction with water derived from a descending slab the sheared texture is likely to have been produced during diapiric uplift the unaltered portion of the ultramafic bodies later interacted with seawater after emplacement at or near the seafloor resulting in formation of the mesh type serpentine with higher d values

isotope composition and volume of earth s early oceans oxygen and hydrogen isotope compositions of earth s seawater are controlled by volatile fluxes among mantle lithospheric oceanic and continental crust and atmospheric reservoirs throughout geologic time the oxygen mass budget was likely conserved within these earth system reservoirs but hydrogen s was not as it can escape to space isotopic properties of serpentine from the approximately 3 8 ga isua supracrustal belt in west greenland are used to characterize hydrogen and oxygen isotope compositions of ancient seawater archaean oceans were depleted in deuterium expressed as d relative to vienna standard mean ocean water vsmow by at most 25 5 but oxygen isotope ratios were comparable to modern oceans mass balance of the global hydrogen budget constrains the contribution of continental growth and planetary hydrogen loss to the secular evolution of hydrogen isotope ratios in earth s oceans our calculations predict that the oceans of early earth were up to 26 more voluminous and atmospheric ch4 and co2 concentrations determined from limits on hydrogen escape to space are consistent with clement conditions on archaean earth

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

what caused the rise of atmospheric o2 abstract oxygenic photosynthesis appears to have evolved well before o 2 levels increased in the atmosphere at around 2 4 ga this has led to numerous suggestions as to what may have kept o 2 suppressed and then eventually allowed it to rise these suggestions include changes in the recycling of carbon and sulfur relative to water or hydrogen a switch from dominantly submarine to dominantly subaerial volcanism gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases a decline in deposition of banded iron formations a decline in nickel availability and various proposals to increase the efficiency of photosynthesis several of these different mechanisms could have contributed to the rise of o 2 although not all of them are equally effective to be considered successful any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates which shows relatively little change with time apart from transient but occasionally spectacular excursions the reasons for this constancy are explored here but are not fully resolved in the process of making these comparisons a self consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on earth like exoplanets

late pleistocene and holocene sea level change along the australian coast abstract tectonically the australian continent is relatively stable and as such it provides a good platform for studying late pleistocene and holocene sea level change the observations indicate that present sea level was reached at about 6000 years ago and that since then level has remained constant to within a few metres considerable spatial variability in the amplitude of the emergence and in the time at which sea levels first reached their present values have been observed spatial variability has also been observed in the sea level during the last glacial maximum and during the late phase of the holocene transgression much of this variaiblity is the result of the earth s adjustment to the glacial melting in the late pleistocene early holocene time and the concomitant distribution of this meltwater into the oceans conclusions that are drawn from matching observations with this glacio hydro isostatic model include the following 1 the mantle viscosity increases with depth the average upper mantle viscosity is about 2 3 10 20 pa s and the average lower mantle defined as below 670 km depth viscosity is about 10 22 pa s the average lithospheric thickness for the coastal and offshore region is about 70 80 km lateral variations in these mantle parameters are indicated 2 the addition of melwater into the oceans did not cease 6000 years ago but continued at a reduced rate up to more recent times adding an amount of water equivalent to a 3 m sea level rise in this interval 3 the amplitude of the holocene emergence is a function inter alia of coastal geometry and significant variations occur over distances of a few hundred kilometres and more holocene highstands of 1 2 m are generally well developed at the continental margins but are less well developed or do not developed at all at offshore islands and reefs 4 variations in the time of maximum emergence also occur being earlier at upstream tidal river sites than at coastal sites at offshore reefs or islands the sea level curve lags the coastal curves by 1000 years or more 5 considerable spatial variability occurs in the sea levels at the time of the last glacial maximum with variations of up to 30 m occurring across the continental shelves 6 there is generally no need to invoke tectonic movements to explain the sea level data along the australian coast but possible exceptions occur for the perth region and for tasmania

free energy distribution and hydrothermal mineral precipitation in hadean submarine alkaline vent systems importance of iron redox reactions under anoxic conditions abstract thermodynamic calculations of mixing between hypothetical seawater and hydrothermal fluid in the hadean deep ocean were carried out to predict saturation states of mineral precipitates and redox reactions that could occur in hadean submarine alkaline hydrothermal systems associated with the serpentinization of ultramafic rocks in the calculations the seawater was assumed to be weakly acidic ph 5 5 and to include carbon dioxide ferrous iron and silica with or without nitrate while the hadean hydrothermal fluid was assumed to be highly alkaline ph 11 and to contain abundant molecular hydrogen methane and bisulfide based on the archean geologic record the modern low temperature alkaline hydrothermal vent fluid lost city field and experimental and theoretical considerations the modeling indicates that potential mineral precipitates in the mixing zone hydrothermal chimney structures could consist mainly of iron sulfides but also of ferrous serpentine and brucite siderite and ferric iron bearing minerals such as goethite hematite and or magnetite as minor phases the precipitation of ferric iron bearing minerals suggests that chemical iron oxidation would be made possible by ph shift even under anoxic condition in the mixing zone comprising an inorganic barrier precipitated at the interface of the two contrasting solutions various redox reactions release free energy with the potential to drive endergonic reactions assuming the involvement of coupling inorganic protoenzymes hydrogenotrophic methanogenesis and acetogenesis long considered the most ancient forms of biological energy metabolisms are able to achieve higher maximum energy yield 0 5 kj kg hydrothermal fluid than those in the modern serpentinization associated seafloor hydrothermal systems e g kairei field furthermore the recently proposed methanotrophic acetogenesis pathway was also thermodynamically investigated it is known that methanotrophic acetogenesis would require additional exergonic reactions to compensate its most endergonic methane to methanol conversion reaction at the oxidative entry to the metabolic pathway our calculations support the view that this thermodynamic barrier could be overcome by the reduction of nitrate in seawater at low temperature as previously suggested however the thermodynamic calculations also revealed that the reduction of ferric iron bearing minerals would occur at the outer margin and within the hydrothermal chimney wall the maximum available energy of iron reducing methanotrophic acetogenesis was calculated to be 0 25 0 35 kj kg hydrothermal fluid although this value is lower than theoretically available through nitrate reduction which approaches 0 45 1 25 kj kg hydrothermal fluid on the outer cool margins of a putative hadean alkaline chimney it is higher than that of sulfate reducing anaerobic oxidation of methane in the lost city field these results suggest that iron reduction had the potential to drive methanotrophy and that the hadean hydrothermal mixing zone was energetically more favorable to methanotrophy than previously thought we conclude that iron oxidation and reduction in oxyhydroxides probably played important roles in the early evolution of energy metabolisms in the hadean alkaline hydrothermal system

serpentinization of oceanic peridotites temperature dependence of mineralogy and boron content serpentine mineralogy 18o 16o ratios and boron content were determined in equatorial atlantic serpentinized ultramafic rocks of mantle derivation the oxygen isotope data were used to estimate the temperatures of serpentinization a relationship appears to exist between temperature of serpentinization mineralogy of the serpentinites and their boron content orthochrysotile is the dominant serpentine phase in the 200 120 c range of serpentinization temperatures lizardite and polygonal serpentine are dominant in the 120 80 c range and clinochrysotile and lizardite in the 80 30 c temperature range these results are interpreted in terms of p t stability of serpentine phases and of changes in chemistry of circulating seawater during ascent of the ultramafic bodies in the oceanic crust an inverse relationship was found between temperature of serpentinization and boron content of the serpentinites suggesting that boron is acquired by the serpentinites from seawater at low temperature

further amino acid racemisation evidence for glacial age multiple lowstand deposition on the new south wales outer continental shelf southeastern australia abstract amino acid racemisation aar analyses total acid hydrolysate on the fossil bivalve molluscs pecten fumatus and glycymeris radians from 10 vibracores from the new south wales continental shelf in present water depths ranging between 123 and 152 m below present sea level and extending some 300 km of latitude provide further evidence for multiple lowstand deposition at times of glacial maxima the extent of racemisation as determined for the amino acids alanine ala aspartic acid asp leucine leu and valine val indicate that the fossil molluscs plot into three principal aminogroups of contrasting age and in general accord with results previously obtained from three vibracores for the molluscs p fumatus and placamen placidium comparable extents of racemisation are evident in the fossil molluscs from the different cores providing a framework for a shelf wide regional aminostratigraphy fossil molluscs of the past three glacial maxima marine oxygen isotope stages ois 2 6 and 8 are identified within the vibracores based on amino acid racemisation evidence calibrated by radiocarbon dating of the same shell associations and correlation with independently dated deposits in southeastern australia the results extend the known geographical range of glacial age lowstand deposits on the mid and outer continental shelf of new south wales southeastern australia and help constrain the magnitude of sea level fall for the past three glaciations to at least 120 m below present sea level the extent of racemisation in specimens of p fumatus of marine ois 6 age with strongly indurated sediment encrustations adhering to the shell surfaces is significantly higher than for shells of equivalent age but without adhering sediment the higher extent of racemisation is consistent with a higher diagenetic temperature history for these specimens by implication these individual shells are likely to have remained within a relatively shallow water environment during the last glacial maximum resulting in the high degree of sediment induration and the higher extent of racemisation

a warm or a cold early earth new insights from a 3 d climate carbon model abstract oxygen isotopes in marine cherts have been used to infer hot oceans during the archean with temperatures between 60 c 333 k and 80 c 353 k such climates are challenging for the early earth warmed by the faint young sun the interpretation of the data has therefore been controversial 1d climate modeling inferred that such hot climates would require very high levels of co2 2 6 bars previous carbon cycle modeling concluded that such stable hot climates were impossible and that the carbon cycle should lead to cold climates during the hadean and the archean here we revisit the climate and carbon cycle of the early earth at 3 8 ga using a 3d climate carbon model we find that co2 partial pressures of around 1 bar could have produced hot climates given a low land fraction and cloud feedback effects however such high co2 partial pressures should not have been stable because of the weathering of terrestrial and oceanic basalts producing an efficient stabilizing feedback moreover the weathering of impact ejecta during the late heavy bombardment lhb would have strongly reduced the co2 partial pressure leading to cold climates and potentially snowball earth events after large impacts our results therefore favor cold or temperate climates with global mean temperatures between around 8 c 281 k and 30 c 303 k and with 0 1 0 36 bar of co2 for the late hadean and early archean finally our model suggests that the carbon cycle was efficient for preserving clement conditions on the early earth without necessarily requiring any other greenhouse gas or warming process

2 7 ga metamorphic peridotites from southeast greenland record the oxygen isotope composition of archean seawater abstract the 18 o value of seawater is presently buffered by high temperature hydrothermal alteration and low temperature weathering of lithospheric rocks it is much debated whether the 18 o of seawater has been buffered to this steady state value throughout earth s history or alternatively whether it gradually increased towards the present value since the archean a third possibility is that the 18 o of seawater was buffered at a higher value before continents emerged and has been buffered at its current value 18 o 1 since the emergence of continents in this contribution we reconstruct the 18 o of archean seawater from triple oxygen isotope 17 o 18 o variations in 2 7 ga ultramafic rocks that reacted with fluids that in turn had been derived from seawater in the archean the samples that were studied are peridotites from the ivnartivaq complex an ultramafic lens in the archean kuummiut terrane rae craton southeast greenland that have unusually low 18 o values for peridotites in olivine 1 7 18 o 4 6 relative to vsmow bulk rock trace element concentrations mineral compositions and u pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites that formed by the dehydration of serpentinites 2 7 ga ago the serpentinite protoliths in turn had formed by alteration of ultramafic cumulate rocks at high temperatures 250 450 c by fluids that were derived from seawater triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower 18 o value than that of seawater in the phanerozoic but could have been derived from seawater with a value of 18 o 1 i e the modern ice free seawater composition we conclude that the 18 o of seawater had reached its current steady state value by 2 7 ga ago

sea level at the last glacial maximum evidence from northwestern australia to constrain ice volumes for oxygen isotope stage 2 new sea level information from the bonaparte gulf in northwestern australia is used to constrain the magnitude and rates of change of ice volumes during the last glacial maximum lgm the region is tectonically stable and far from the former ice covered regions the glacio hydro isostatic adjustment of the coast is therefore relatively small and the corrections for this evect are not sensitive to details of the rebound model microfossil analysis and ams radiocarbon dating of 11 gravity cores taken across the shelf and bonaparte gulf demonstrate that 1 the lgm sea levels were locally at 125 4 m 2 the lgm terminated abruptly at 19 000 cal yr bp with a rapid rise in sea level of about 15 m over the next 500 years and 3 the onset of the minimum sea levels occurred before 22 000 cal yr bp when corrected for the glacio hydro isostatic evects the increase of lgm ice volumes over the present day ice volume is 52 5 106 km3 the termination of the lgm is marked by a rapid ice discharge of 5 2 106 km3 2001 elsevier science b v all rights reserved

holocene foraminifera as indicators of relative estuarine lagoonal and oceanic influences in estuarine sediments of the river murray south australia abstract in southeastern south australia the river murray debouches through a coastal barrier separating euryhaline estuarine lagoonal waters from the southern ocean depending upon the relative freshwater outflow of the river and ingress of the ocean water salinity varies greatly within the lower estuary ammonia beccarii and elphidium articulatum are euryhaline species of foraminifera that characterize the estuary and back barrier coorong lagoon the inner shelf marine environment hosts an assemblage in which discorbis dimidiatus e crispum e macelliforme and various cibicidid species predominate in cored sediments recovered from the shallow lower estuary the relative abundance of a beccarii e articulatum was compared with that of d dimidiatus e crispum e macelliforme other species these data and ams radiocarbon ages determined for foraminifera and ostracods provide evidence of a change from maximum oceanic influence 5255 60 yr b p to maximum estuarine influence 3605 70 yr b p over this same time interval sea level fell relatively by about 2 m however the event was also contemporaneous with falling water levels in several victorian lakes and it is thus attributed to onset of climatic aridity reduced precipitation in the river murray catchment and reduced freshwater outflow enhanced development of the flood tide delta and constriction of the mouth

the neoarchaean storo supracrustal belt nuuk region southern west greenland an arc related basin with continent derived sedimentation we present new major and trace element data as well as sm nd isotope compositions for archaean supracrustal rocks and a gabbro anorthosite complex from the island of storo in godthabsfjord southern west greenland we also provide new u pb isotope data for zircon extracted from these rocks the storo rocks have experienced amphibolite facies metamorphism and the entire sequence forms an east dipping frontal thrust ramp in tectonic contact with mesoarchaean nuk gneisses to the west and eoarchaean itsaq gneisses to the east these orthogneisses record a complex regional accretion history as established by previous work the present study aims at explaining the geochemical features of the storo supracrustal belt ssb which comprises metavolcanic and metasedimentary rocks that are situated within this collisional zone the ssb has a maximum age of ca 2800 ma constrained by the youngest detrital zircon population found in a metasedimentary unit the minimum age of the ssb is 2707 8 ma defined by previously published re os isotope data the metavolcanic rocks have a tholeiitic basaltic composition with relatively flat primitive mantle normalised trace element patterns and generally negative nb anomalies nb nb 0 30 0 90 they plot above the mantle array in th yb nb yb space consistent with a subduction zone affinity as also proposed by previous studies a thin fault contact separates the ssb from a gabbro anorthosite complex formally defined here as the storo anorthosite complex which has an age of ca 3050 ma the sm nd isotope data of the ssb suggest that these metavolcanic rocks experienced contamination by a crustal source that was isotopically similar to the storo anorthosite complex sac this in turn suggests that the sac could have formed the basement for the younger volcanic sequence of the ssb or alternatively that the mantle source of the ssb was contaminated by melts derived from sac aged crust or sediments the metasedimentary rocks of ssb show a mixed mafic felsic component with variable degrees of maturity highly mature metasediments of the ssb contain several age populations of regionally well known magmatic events and thus support a significant local crustal provenance furthermore the youngest documented detrital zircon is found in a thin metasedimentary unit within the metavolcanic rocks and thereby shows that the ssb formed in close proximity to subaerially exposed continental crust i e a back arc environment and not a distal island arc setting as previously proposed finally this suggests that relatively cool lower continental crust which was capable of supporting subaerial mountains existed at least locally during the meso neoarchaean

oxygen and hydrogen isotope evidence for a temperate climate 3 42 billion years ago the widely held view that the archaean climate around 3 5 billion years ago was remarkably warm with ocean temperatures perhaps as high as 80 c has been questioned on the grounds that the established method of estimating ancient ocean temperature from the oxygen isotope ratios of sedimentary deposits is subject to significant uncertainty michael hren et al have adopted a different approach to estimating ocean temperature based on an analysis of both oxygen and hydrogen isotopes of 3 4 billion year old buck reef chert sediments from south africa the isotopes sampled are consistent with formation in waters no warmer than about 40 c suggesting that earth s early oceans may have been far cooler than previously thought the study of stable oxygen isotope ratios 18o of precambrian cherts suggests that ocean temperatures during the archaean era about 3 5 billion years ago were between 55 c and 85 c but uncertainty about the 18o of the primitive ocean has led to considerable debate regarding this conclusion here a combined analysis of oxygen and hydrogen istopes sampled from 3 42 billion year old buck reef chert rocks in south africa indicates that the ancient ocean was much cooler than previously thought stable oxygen isotope ratios 18o of precambrian cherts have been used to establish much of our understanding of the early climate history of earth1 2 3 and suggest that ocean temperatures during the archaean era 3 5 billion years ago were between 55 c and 85 c ref 2 but because of uncertainty in the 18o of the primitive ocean there is considerable debate regarding this conclusion examination of modern and ancient cherts indicates that another approach using a combined analysis of 18o and hydrogen isotopes d rather than 18o alone can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters4 5 here we show that 18o and d sampled from 3 42 billion year old buck reef chert rocks in south africa are consistent with formation from waters at varied low temperatures the most 18o enriched buck reef chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below 40 c geochemical and sedimentary evidence suggests that the buck reef chert was formed in shallow to deep marine conditions so our results indicate that the palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler 40 c than previously thought2

oxygen isotopes in authigenic quartz from massive salt deposits abstract we describe a new geochemical tool that could provide temperatures of ancient basins in which massive evaporites were deposited the oxygen isotope composition of fine crystalline quartz found in large halite bodies such quartz is well preserved from post depositional alterations and it can be relatively easily separated for the purpose of this study five halite samples were selected from four various evaporite basins spanning in age from the early cambrian to the late jurassic the obtained isotope temperatures refer to locations where evaporites were deposited i e in subtropical zones of the earth as it may be estimated from continental distribution during phanerozoic times reasonable temperatures are obtained with an assumption for the 18o of brines ranging from 1 to 0 during halite deposition the assumption of higher 18o values leads to unrealistic temperatures higher than the homogenization temperatures of fluid inclusions in halite

precambrian paleosols and atmospheric co2 levels abstract precambrian atmospheric pco2 levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols in addition to theoretical flaws with that model recalculation of pco2 levels using more recent thermodynamic data gives significantly lower values a new model based on the mass balance of weathering in paleosols gives consistent results from three separate 2 2 ga old paleosols the calculated pco2 value of 23 3 3 times present atmospheric levels is insufficient to overcome the faint young sun paradox suggesting that another greenhouse gas such as ch4 was present at elevated levels relative to the present atmosphere to account for the discrepancy applying this new approach to other precambrian paleosols a proterozoic pco2 curve is generated which indicates consistently high pco2 from 2 5 to 1 8 ga ago and a substantial drop in atmospheric pco2 at some time between 1 8 and 1 1 ga ago

clumped isotope thermometry of magnesium carbonates in ultramafic rocks abstract magnesium carbonate minerals produced by reaction of h 2 o co 2 with ultramafic rocks occur in a wide range of paragenetic and tectonic settings and can thus provide insights into a variety of geologic processes including 1 deposition of ore grade massive vein cryptocrystalline magnesite 2 formation of hydrous magnesium carbonates in weathering environments and 3 metamorphic carbonate alteration of ultramafic rocks however the application of traditional geochemical and isotopic methods to infer temperatures of mineralization the nature of mineralizing fluids and the mechanisms controlling the transformation of dissolved co 2 into magnesium carbonates in these settings is difficult because the fluids are usually not preserved clumped isotope compositions of magnesium carbonates provide a means to determine primary mineralization or re equilibration temperature which permits the reconstruction of geologic processes that govern magnesium carbonate formation we first provide an evaluation of the acid fractionation correction for magnesium carbonates using synthetic magnesite and hydromagnesite along with natural metamorphic magnesite and low temperature hydromagnesite precipitated within a mine adit we show that the acid fractionation correction for magnesium carbonates is virtually indistinguishable from other carbonate acid fractionation corrections given current mass spectrometer resolution and error in addition we employ carbonate clumped isotope thermometry on natural magnesium carbonates from various geologic environments and tectonic settings cryptocrystalline magnesite vein deposits from california red mountain magnesite mine austria kraubath locality turkey tutluca mine eskisehir district and iran derakht senjed deposit exhibit broadly uniform 47 compositions that yield apparent clumped isotope temperatures that average 23 7 5 0 c based on oxygen isotope thermometry these clumped isotope temperatures suggest mineralization at shallow crustal depths in the presence of meteoric water hydrous magnesium carbonates from a 400 km latitudinal transect along the serpentinized peridotite bodies of the california coast ranges record clumped isotope temperatures between 14 2 and 22 7 2 8 c in agreement with historical maximum temperatures during the rainy season for california talc carbonate alteration of ultramafic rocks in greenland isua supracrustal belt and vermont ludlow yields clumped isotope alteration temperatures of magnesite and dolomite between 326 and 490 c broadly consistent with paragenesis and thermodynamic analysis for co 2 metasomatism of ultramafic rocks these metamorphic carbonates extend the applicability of clumped isotope thermometry to high temperature magnesium carbonate systems and indicate equilibrium blocking temperatures for magnesite of 490 c our study demonstrates the applicability of the clumped isotope approach to provide information on the formation of magnesium carbonates as ore resources surface records of climate and metamorphic assemblages

on eustatic sea level history last glacial maximum to holocene abstract this paper addresses the question of the magnitude and time dependence of the globally averaged eustatic rise of sea level that occurred subsequent to the time of last glacial maximum lgm at approximately 21 000 calendar years before present through the analysis of relative sea level rsl histories predicted by a realistic mass conserving and gravitationally self consistent theory of postglacial sea level change it is demonstrated that there are preferred oceanic locations at which this eustatic function is well approximated by local sea level history one such location is the island of barbados in the caribbean sea a site from which a coral based record exists that extends from mid holocene to lgm because of the sea level ambiguity that is inherent to coral based records however it is important that the global ice equivalent eustatic sea level curve inferred on the basis of the barbados data be tested against observations at other locations from which similarly extensive records are also available but which are derived on the basis of sea level indicators which are not subject to the ambiguities inherent to corals it is shown that when the eustatic function employed in the global theoretical model is tuned so as to enable the model to fit the barbados observations where the maximum relative sea level rsl depression is assumed to be near 120 m then the theory misfits the record from the sunda shelf in the indonesian archipelago as well as the record from j bonaparte gulf in northern australia both of these recently published records appear to constrain the lgm low stand of rsl to a value above 120 m the implications of these results for interpretation of the long coral derived records from the huon peninsula of papua new guinea and the island of tahiti are also discussed

oxygen isotope analysis of fossil organic matter by secondary ion mass spectrometry we have developed an analytical procedure for the measurement of oxygen isotope composition of fossil organic matter by secondary ion mass spectrometry sims at the sub per mill level with a spatial resolution of 20 30 m the oxygen isotope composition of coal and kerogen samples determined by sims are on average consistent with the bulk oxygen isotope compositions determined by temperature conversion elemental analysis isotope ratio mass spectrometry tc ea irms but display large spreads of 18o of 5 10 attributed to mixing of remnants of organic compounds with distinct 18o signatures most of the 18o values obtained on two kerogen residues extracted from the eocene clarno and early devonian rhynie continental chert samples and on two immature coal samples range between 10 and 25 based on the average 18o values of these samples and on the o isotope composition of water processed by plants that now constitute the eocene clarno kerogen we estimated 18owater values ranging between around 11 and 1 which overall correspond well within the range of o isotope compositions for present day continental waters sims analyses of cyanobacteria derived organic matter from the silurian zdanow chert sample yielded 18o values in the range 12 20 based on the o isotope composition measured on recent cyanobacteria from the hypersaline lake natron tanzania and on the o isotope composition of the lake waters in which they lived we propose that 18o values of cyanobacteria remnants are enriched by about 18 2 to 22 2 relative to coeval waters this relationship suggests that deep ocean waters in which the zdanow cyanobacteria lived during early silurian times were characterised by 18o values of around 5 4 this study establishing the feasibility of micro analysis of phanerozoic fossil organic matter samples by sims opens the way for future investigations of kerogens preserved in archean cherts and of the o isotopic composition of ocean water at that period in time

the ph of enceladus ocean abstract saturn s moon enceladus is a geologically active waterworld the prevailing paradigm is that there is a subsurface ocean that erupts to the surface which leads to the formation of a plume of vapor and ice above the south polar region the chemistry of the ocean is just beginning to be understood but is of profound geochemical and astrobiological interest here we determine the ph of the ocean using a thermodynamic model of carbonate speciation observational data from the cassini spacecraft are used to make a chemical model of ocean water on enceladus the model suggests that enceladus ocean is a na cl co3 solution with an alkaline ph of 11 12 the dominance of aqueous nacl is a feature that enceladus ocean shares with terrestrial seawater but the ubiquity of dissolved na2co3 suggests that soda lakes are more analogous to the enceladus ocean the high ph implies that the hydroxide ion should be relatively abundant while divalent metals should be present at low concentrations owing to buffering by carbonates and phyllosilicates on the ocean floor carboxyl groups in dissolved organic species would be negatively charged while amino groups would exist predominately in the neutral form knowledge of the ph improves our understanding of geochemical processes in enceladus ocean the high ph is interpreted to be a key consequence of serpentinization of chondritic rock as predicted by prior geochemical reaction path models although degassing of co2 from the ocean may also play a role depending on the efficiency of mixing processes in the ocean serpentinization leads to the generation of h2 a geochemical fuel that can support both abiotic and biological synthesis of organic molecules such as those that have been detected in enceladus plume serpentinization and h2 generation should have occurred on enceladus like on the parent bodies of aqueously altered meteorites but it is unknown whether these critical processes are still taking place or if enceladus rocky core has been completely altered by past hydrothermal activity the presence of native h2 in the plume would provide strong evidence for contemporary aqueous alteration that replenishes this source of energy for possible life the high ph also suggests that the delivery of strong oxidants from the surface to the ocean has not been significant otherwise sulfuric acid would be produced which would be consistent with geophysical models of episodic resurfacing activity on enceladus this paper represents an expansion of chemical oceanography to an ocean planet beyond earth

iron deposits hosted by banded iron formations in the yilgarn craton products of sequential iron enrichment by magmatic marine and meteoric fluids abstract mesoarchean algoma type bif in the yilgarn craton are locally enriched by secondary processes to iron concentrations approaching 72 wt fe while their near surface goethite hematite ores are unequivocally the product of supergene fluid alteration their deeper magnetite and crystalline hematite martite ores have a contentious origin this study shows that magnetite ores formed after diagenesis and peak greenschist facies metamorphism of bif coinciding with regional deformation events the ores formed by a multi stage process involving initial replacement of primary quartz bands in bif by hypogene carbonate minerals followed by the dissolution of the carbonate and concentration of magnetite at the beebyn deposit the earliest fluids were hot 440 c co2 vapor rich and were likely derived from felsic magmatic fluids in contrast the fluids responsible for carbonate alteration at the matthew ridge koolyanobbing and windarling deposits were cooler 366 to 125 c and aqueous rich with salinities from 0 3 to 43 9 wt nacl equiv their cl br and c o isotopic ratios suggest they are the product of mixing between magmatic fluids and heated seawater in all deposits later cooling 178 to 98 c of these fluids led to dissolution of carbonate minerals and concentration of magnetite dated monazite and xenotime from magnetite ores demonstrates that fluid alteration of bif began in the mesoarchean and continued episodically through to the paleoproterozoic later specular hematite rich veins that cut and modify magnetite ores crystallized from aqueous rich fluids that are more oxidized with salinities from 0 to 28 7 wt nacl equiv and crystallization temperatures of 243 to 85 c their cl br and c o isotopic fluid ratios indicate that they formed from mixing between heated seawater and meteoric waters the bif hosted iron deposits in the yilgarn craton show remarkable similarities with the well studied iron deposits in the carajas mineral province and hamersley basin they all show the same transition through time from magnetite to hematite rich ores subsequent episodic tectonically driven fluid alteration events led to further iron enrichment in these deposits the transition in ore types most likely corresponds to a common guiding process namely the uplift of crustal rocks containing bif resulting in the progressive shallowing of fluid alteration pathways and the greater influence of cooler more oxidized hydrothermal fluids unlike iron deposits hosted by algoma type bif in the yilgarn and carajas iron deposits hosted by superior type bif in the hamersley basin display a greater influence of oxidized marine and meteoric waters without involvement of magmatic fluids

limited archaean continental emergence reflected in an early archaean 18 o enriched ocean the origin and evolution of earth s biosphere were shaped by the physical and chemical histories of the oceans marine chemical sediments and altered oceanic crust preserve a geochemical record of these histories marine chemical sediments for example exhibit an increase in their 18o 16o ratio through time the implications of this signal are ambiguous but are typically cast in terms of two endmember but not mutually exclusive scenarios the oceans may have been much warmer in the deep past if they had an oxygen isotope composition similar to that of today alternatively the nature of fluid rock interactions including the weathering processes associated with continental emergence may have been different in the past leading to an evolving oceanic oxygen isotope composition here we examine approximately 3 24 billion year old hydrothermally altered oceanic crust from the panorama district in the pilbara craton of western australia as an alternative oxygen isotope archive to marine chemical sediments we find that at that time seawater at panorama had an oxygen isotope composition enriched in 18o relative to the modern ocean with a 18o of 3 3 0 1 vsmow we suggest that seawater 18o may have decreased through time in contrast to the large increases seen in marine chemical sediments to explain this possibility we construct an oxygen isotope exchange model of the geologic water cycle which suggests that the initiation of continental weathering in the late archaean between 3 and 2 5 billion years ago would have drawn down an 18o enriched early archaean ocean to 18o values similar to those of modern seawater we conclude that earth s water cycle may have gone through two separate phases of steady state behaviour before and after the emergence of the continents the water cycle was in two different steady states before and after continental emergence as recorded in the decreasing oxygen isotope values of seawater since the archaean according to an inverse geochemical model of the oceanic crustal record

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

sea level history 45 000 to 30 000 yr b p inferred from benthic foraminifera gulf st vincent south australia surficial sediments of gulf st vincent south australia are predominantly bioclastic cool temperate carbonates benthic foraminifera are abundant and distribution of species is closely related to water depth for example massilina milletti is most common at depths ca 40 m while discorbis dimidiatus is characteristics of shallow subtidal environments elphidium crispum a shallow water species and e macelliforme favoring deeper water provide a useful numerical ratio their logarithmic relative abundance in the sediment size fraction 0 50 0 25 mm correlates strongly with water depth vibrocores sv 4 and sv 5 recovered undisturbed sections of quaternary strata from the deepest part ca 40 m of gulf st vincent amino acid racemization and radiocarbon age determinations show that late pleistocene sections of the cores were deposited over the time ca 45 000 to 30 000 yr b p species of fossil foraminifera recovered from these sections are mostly extant in modern gulf st vincent thus allowing paleoecological inferences of late pleistocene sea levels these inferred sea level maxima can be correlated with those determined from study of huon peninsula coral reef terraces initial estimates of tectonically corrected sea levels for transgressions in gulf st vincent at 40 000 and 31 000 yr b p are 22 5 m and 22 m respectively the intervening regression lowered sea level to 28 m

lizardite versus antigorite serpentinite magnetite hydrogen and life the serpentinization of peridotite operates according to one or the other or a combination of two end member mechanisms in low temperature environments 50 300 c where lizardite is the predominant serpentine mineral olivine is consumed by reaction with h2o but its composition mg remains unchanged mg rich lizardite magnetite and dihydrogen gas brucite are products of the reaction at higher temperatures 400 600 c rates of mgfe diffusion in olivine are orders of magnitude faster with the result that the growth of mg rich antigorite can be accommodated by a compositional adjustment of olivine eliminating the need to precipitate magnetite and evolve hydrogen this latter end member mechanism probably best reflects the situation in the forearc mantle wedge

spatial and temporal variability in the holocene sea level record of the south australian coastline abstract studies of past sea levels based on radiocarbon dated field samples suffer variously from compilation of information from disparate locations and the imprecise nature of the dated indicators geographic segregation and systematic elimination of poor samples from time depth data sets leads to improved interpretation of holocene sea level history an example is presented from southern australia where this is achieved through selection of higher resolution palaeoenvironmental indicators separation of transgressive from regressive populations and geographic regionalisation of data in southern australia fossil sea level indicators preserved in prograding coastal plain settings include seagrass sandflat mangrove samphire and chenier ridge organo sedimentary remains these provide sea level time depth data points with a variety of elevational and dating errors preserved organic remains at the transition from posidonia seagrass to intertidal sandflat environment and from sandflat to avicennia mangrove environment provide the most precise local data for tracing sea level change time depth plots of 233 dated sea level indicators from south australia generate a broad sea level envelope tracing the southern australian transgression from 10 000 to ca 6000 radiocarbon years bp followed by a more or less consistent level to the present finer details of sea level behaviour are only apparent after systematic selection separation and regionalisation of the data the data indicate a very rapid sea level rise in the early holocene at about 16 mm year reaching present levels at 6400 years bp this was followed by regionally variable regression and emergence of the land of 1 3 m a process that continues to the present the systematic increase from 1 to 3 m in the elevation of the 6400 year bp highstand with distance away from the shelf margin is consistent with a hydro isostatic origin for the emergence as predicted by geophysical models

diagenesis and compositional partitioning of quaternary cool water carbonate aeolianites southeastern australia abstract middle to late pleistocene stranded carbonate sea beaches adjacent aeolian dunes and pleistocene aeolianites together with holocene marginal marine calcareous sediments across southeastern australia contain the longest record of such deposition in the modern world stretching back to at least 900 ky mis 23 the grains are were derived from a suite of cool water heterozoan carbonate sediments produced on the adjacent offshore shelf most modern shelf carbonate sediments in the region today are aragonite rich but the aragonite is presently slowly being dissolved on or just below the seafloor before the deposits enter the rock record beach dune sediments are derived mainly from the shallow 30 meters deep adjacent seafloor they are mollusk rich but with most bryozoan particles having been destroyed in the surf zone the mollusks are dominantly aragonite these nearshore sediments are swept onshore before any significant seafloor diagenesis can take place and so are compositionally different from the eventual aragonite poor open shelf sediments aragonite skeletal fragments in the dunes progressively dissolve with time under a semi arid climate and produce diagenetic low magnesium calcite meteoric cements a well known process common in tropical aragonitic pleistocene aeolianites the loss of metastable minerals with time is matched by a progressive and well understood change in stable isotopes such alteration in this environment however seems to take longer than in tropical carbonates with lithification herein not beginning until 200 ka and complete cementation being achieved in 450 ky thus neither aeolianites nor offshore marine carbonates record the composition of the original sediment aeolianites lack the important bryozoan constituents whereas neritic deposits lack the prolific mollusks this partitioning revealed for the first time answers the conundrum as to why some cool water neritic shelf and coastal sediments in the rock record are compositionally different

contrasting serpentinization processes in the eastern central alps stable isotope compositions have been determined for serpentinites from between davos arosa platta nappe switzerland and the valmalenco italy d and 18o values 120 to 60 and 6 10 respectively in the arosa platta nappe indicate that serpentinization took place on the continent at relatively low temperatures in the presence of limited amounts of metamorphic fluids that contained a component of meteoric water one sample of chrysotile has a 18o value of 13 providing evidence of high w r ratios and low formation temperature of lizardite chrysotile in this area in contrast relatively high d values 42 to 34 and low 18o values 4 4 7 4 for serpentine in the eastern part of the valmalenco suggest a serpentinization process that took place at moderate temperatures in fluids that were dominated by ocean water the antigorite in the valmalenco is the first reported example of continental antigorite with an ocean water signature an amphibole sample from a metasomatically overprinted contact zone to metasediments d 36 indicates that the metasomatic event also took place in the presence of ocean water lower d values 93 to 60 of serpentines in the western part of the valmalenco suggest a different alteration history possibly influenced by fluids associated with contact metamorphism low water rock ratios during regional metamorphism and metasomatism have to be assumed for both regions

benthic foraminiferal response to late glacial and holocene sea level rise and rainfall variability off east africa analogous to west and north africa east africa experienced more humid conditions between approximately 12 and 5 kyr bp relative to today while timing and extension of wet phases in the north and west are well constrained this is not the case for the east african humid period here we present a record of benthic foraminiferal assemblages and sediment elemental compositions of a sediment core from the east african continental slope in order to provide insight into the regional shallow indian ocean paleoceanography and east african climate history of the last 40 kyr during glacial times the dominance of a benthic foraminiferal assemblage characterized by bulimina aculeata suggests enhanced surface productivity and sustained flux of organic carbon to the sea floor during heinrich stadial 1 h1 the nuttallides rugosus assemblage indicates oligotrophic bottom water conditions and therefore implies a stronger flow of southern sourced aaiw to the study site during the east african humid period the saidovina karreriana assemblage in combination with sedimentary c n and fe ca ratios suggest higher river runoff to the indian ocean and hence more humid conditions in east africa between 8 5 and 8 1 kyr contemporaneous to the globally documented 8 2 kyr event a severe reduction in river deposits implies more arid conditions on the continent comparison of our marine data with terrestrial studies suggests that additional moisture from the atlantic ocean delivered by an eastward migration of the congo air boundary during that time period could have contributed to east african rainfall since approximately 9 kyr the gaining influence of the millettiana millettii assemblage indicates a redevelopment of the east african fringe reefs

sulfur and oxygen isotope constraints on the sedimentary history of apparent conglomerates from the nuvvuagittuq greenstone belt nunavik quebec abstract mafic igneous rocks of the nuvvuagittuq greenstone belt ngb crystallized before 3 8 ga and possibly as early as 4 3 ga potentially making the belt the oldest known supracrustal sequence on earth however detrital zircons from a rare quartz biotite schist in the ngb yield significantly younger ages of 3 77 ga or less these appear to be inconsistent with the ages of the mafic igneous rocks as the quartz biotite schist has been interpreted as a metaconglomerate formed by the dismantling of preexisting lithologies in order to assess this genetic interpretation we performed a sulfur and oxygen isotope study of the quartz biotite schist sulfide grains found in quartz clasts and the matrix show significant mass independent fractionation of sulfur isotopes 0 2 33s 1 0 mean 33s 0 5 0 1 secondary sulfides from crosscutting veins do not show mass independent fractionation of sulfur isotopes 0 1 33s 0 3 mean 33s 0 1 0 1 oxygen isotope compositions of quartz from clasts matrix and a fine grained lens are highly enriched in 18o 16 9 18o 26 7 non zero 33s values indicate a surficial origin for sulfur probably the eoarchean atmosphere while high 18o values suggest a low temperature 65 18 c origin for the quartz likely as chemical precipitation of a chert precursor from si saturated seawater therefore the coupled s and o isotope measurements show that primary isotopic signatures characteristic of surficial environments survived the protracted metamorphic history of the ngb and suggest that the quartz biotite schist contains material that originated as chemical metasediments the near mono mineralic compositions of the clasts quartz and their shared 18o enrichment suggest that they had a common protolith which was deposited prior to the formation of the schist and subsequently reworked whether the quartz biotite schist represents a metaconglomerate or a structural melange it preserves remnants of some of the oldest chemical sediments on earth

o and h stable isotope compositions of oceanic crust and upper mantle rocks exposed in the hess deep near the galapagos triple junction abstract a large variety of rocks consisting of basalts dolerites gabbros and ultramafics are exposed on the intra rift ridge in the hess deep representing a section of the oceanic crust formed at the east pacific ridge 1 we conducted a stable isotope study of o and h to assess the nature of the interaction between seawater and the oceanic crust in this region fresh basalt and dolerite samples have magmatic morb values 18o 6 0 105 d 70 and low h2o content 57 and h2o contents 1 3 most gabbros have 18o 5 6 essentially identical to their initial magmatic values their d values are generally higher than 57 suggesting that they have reacted with some seawater derived fluids as is also suggested by their mineralogy in gabbros displaying little or no visual evidence of alteration of their plagioclases and pyroxenes 18oplagioclase pyroxene is small 0 3 and consistent with magmatic values the calcic an 50 plagioclases have magmatic 18o 5 8 albitized prehnitized plagioclases have variable 18o values 4 8 it also appears that the 18o modified gabbros are those in which plagioclases are transformed into albite and or prehnite which are characteristic of greenschist facies conditions the lack of calcic an 50 plagioclases depleted in 18o and hornblende suggests that most of the gabbros did not react extensively under amphibolite facies conditions with seawater derived fluids t 400 c or that they were subsequently altered to greenschist facies gabbros the serpentinized peridotites were depleted in 18o 3 3 4 9 in one sample the 18oserpentine magnetite 5 7 corresponds to a temperature of about 325 50 c these values are compatible with serpentinization by seawater derived fluids at temperatures between 200 and 350 c another sample has a higher 18o value of 10 corresponding to a serpentinization temperature of less than 50 c the 18o mass balance between seawater and the oceanic crust seems to have attained steady state where enrichment in 18o of metabasalts is balanced by the depletion in 18o of metadolerites the contribution of the gabbros is minor the depletion in 18o of the serpentinized peridotites is not taken into account since we believe that the high temperature serpentinization is characteristic of the hess deep and cannot be extrapolated to the normal oceanic crust

the holocene non tropical coastal and shelf carbonate province of southern australia abstract carbonate dominant sediments are currently forming and accumulating over the extensive marine shelf of the passive margin of southern australia a dearth of continental detritus results from both a very low relief and a predominantly arid climate the wide continental shelf is bathed by cold upwelling ocean waters that support luxuriant growths of bryozoans and coralline algae together with sponges molluscs asteroids benthic and some planktonic foraminifera the open ocean coast is battered by a persistent southwest swell resulting in erosion of calcrete encrusted pleistocene eolianites much sediment is reworked and overall shelf sedimentation rates are low high energy microtidal beach dune systems occur between headlands and along the very long ocean beach in the coorong region the northern more arid coastal areas also contain saline lakes that precipitate gypsum from infiltrated sea water and display marginal facies of aragonite boxwork to fenestral carbonate crusts with stromatolites and tepee structures in contrast the southern seasonally humid coorong region has a predominantly continental groundwater regime where sulphate is rare and the high summer evaporation precipitates dolomite magnesite and aragonite muds fenestral crusts breccias tepees and some stromatolites are also present st vincent and spencer gulfs both afford some protection from ocean swell but tidal amplitude and currents increase and a depth and inundation related zonation of plants and animals is established muddy carbonate sand accumulates on the sea floor below 30 m where filter feeding bryozoans bivalves and sponges dominate in shallower regions seagrass meadows contain a rich fauna that results in rapid accumulation of an unsorted muddy bioclastic sand mangrove woodlands backed by saline marsh with cyanobacterial mats are common and accumulate mud rich and gastropod bearing sediment as tidal amplitude and desiccation increase northward into both gulfs a supratidal zone bare of vegetation sabkha becomes the site for deposition of gypsum rich and fenestral calcitic mud

hydrothermal metasomatic and tectono metamorphic processes in the isua supracrustal belt west greenland a multi isotopic investigation of their effects on the earth s oldest oceanic crustal sequence abstract despite superimposed metamorphic overprinting and metasomatic alterations primary volcanic features remain preserved in low strain domains of mafic volcanic sequences in the western isua supracrustal belt isb west greenland these basaltic successions represent the hitherto oldest known fragments of oceanic crust on earth early archean metasomatic fluids rich in light rare earth elements lree th u pb ba and alkalies invaded the supracrustal package and distinctively altered the basaltic sequences field relationships source characteristics traced by pb isotopes and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the isb between 3 81 and 3 74 ga ago subsequent early archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks allanite occurs as the most characteristic and critical secondary metasomatic metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration even in domains of low strain because of its high concentration of lree th and u this secondary mineral accounts for much of the disturbances recorded by the sm nd and th u pb isotope systematics of the pillowed metabasalts the supracrustal sequences were tectono metamorphically affected to varying degrees during a late archean 2 6 to 2 8 ga old event also recognized in the adjacent gneiss terranes of the isuakasia area the degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite hosted parent daughter elements to the overall whole rock mass budget of the respective isotope systems although low strained initially only weakly metasomatized pillow basalts remained more or less closed with respect to the u pb and rb sr systems since 3 74 ga the sm nd system appears to have been partially opened on a whole rock scale during the late archean event this diversified behavior of the whole rock isotope systems with respect to late archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late archean recrystallization processes with associated renewed fluid flow in reactivated zones of high strain where primary metasomatic alteration is most prominently developed late archean partial resetting also of the u pb isotope system on a whole rock scale occurred this is consistent with an apparent late archean age of kyanite which initially crystallized during the early archean metamorphism its age is controlled by the u pb systematics of allanite inclusions which have exchanged their isotopic properties during the tectono metamorphic event that overprinted the oceanic crustal sequence at isua more than 1000 myr later these results underline the need for care in the interpretation of whole rock geochemical data from polymetamorphic rocks in general and from the isua oceanic crustal sequences in particular to constrain isotopic models of early earth s evolution likewise this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from isua to infer models for geotectonic settings relevant for their formation

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

subduction and atmospheric escape of earth s seawater constrained by hydrogen isotopes abstract the hydrogen isotopic d h ratio reflects the global cycling and evolution of water on earth as it fractionates through planetary processes we model the water cycle taking seafloor hydrothermal alteration chemical alteration of continental crust slab subduction hydrogen escape from the early earth and degassing at mid ocean ridges hot spots and arcs into account the differences in d h ratios between present day oceans oceanic and continental crust and mantle are thought to reflect isotopic fractionation through seafloor alteration chemical alteration and slab dehydration however if the speed of plate tectonics has been nearly constant throughout earth s history the degassing and regassing rates are too small to reach the present day d h ratios we show that a hydrogen escape from reduced early atmosphere b secular net regassing or c faster plate tectonics on early earth is needed to reproduce the present day d h ratios of the water reservoirs the low d h ratio of archean seawater at 3 8 ga has previously been interpreted as a signature of a hydrogen escape but we find it can also be explained either by b secular net degassing or by c faster plate tectonics on early earth the rates of hydrogen escape from early earth and secular regassing on present day earth are constrained to be lower than 2 1 10 11 kg yr and 3 9 10 11 kg yr consequently the volume of water in the present day mantle could result entirely from the regassing through earth s history in that case the volume of initial oceans could be 2 to 3 times larger than that of current earth we suggest that in addition to the d h ratio of archean seawater identifying the d h ratios of both seawater and mantle throughout earth s history would allow to distinguish these evolutionary scenarios

amino acid racemization and radiocarbon dating of interstadial marine strata oxygen isotope stage 3 gulf st vincent south australia abstract concordant results for the extent of racemization of the amino acids aspartic acid leucine and valine and isoleucine epimerization in several mollusc genera support radiocarbon data that indicate an interstadial age oxygen isotope stage 3 45 000 to 30 000 yr b p for a sequence of mollusc rich carbonate muds in gulf st vincent south australia a numeric age of 46 500 9100 yr b p was obtained on marine molluscs from these sediments using a model based on apparent parabolic racemization kinetics in a global context the sequence represents one of the few well documented occurrences of marine strata of this age from a tectonically quiescent setting a sea level as high as 22 m is indicated for this interval amino acid racemization provides an important method for validating conventional finite radiocarbon ages close to the practical limits of radiocarbon dating and in studies of reworking

aspartic acid racemisation dating of mid holocene to recent estuarine sedimentation in new south wales australia a pilot study abstract the degree of aspartic acid racemisation measured in radiocarbon dated specimens of fossil estuarine molluscs collected from lake illawarra a holocene barrier estuary in new south wales australia is evaluated in the context of results of laboratory induced racemisation established in heating simulated ageing experiments the general kinetic trend of aspartic acid racemisation in both heating experiments and fossil molluscs anadara trapezia and notospisula trigonella conforms to a model of apparent parabolic kinetics using the apparent parabolic kinetic model numeric ages based on the degree of aspartic acid racemisation in the fossil molluscs have been calculated an aminostratigraphy of the lake illawarra holocene basin fill has been established based on the extent of aspartic acid racemisation measured in 29 specimens of fossil molluscs aspartic acid d l ratios in a trapezia and n trigonella range from 0 079 0 005 to 0 436 0 004 representing an age range from

from evaporated seawater to uranium mineralizing brines isotopic and trace element study of quartz dolomite veins in the athabasca system abstract stable isotope o h c radiogenic isotope sr nd and trace element analyses have been applied to quartz dolomite veins and their uranium u bearing fluid inclusions associated with proterozoic unconformity related uo 2 uraninite ores in the athabasca basin canada in order to trace the evolution of pristine evaporated seawater towards u mineralizing brines during their migration through sediments and basement rocks fluid inclusion data show that quartz and dolomite have precipitated from brines of comparable chemistry excepted for relatively small amounts of co 2 found in dolomite hosted fluid inclusions however 18 o values of quartz veins 18 o 11 to 18 and dolomite veins 18 o 13 to 24 clearly indicate isotopic disequilibrium between quartz and dolomite hence it is inferred that this isotopic disequilibrium primarily reflects a decrease in temperature between the quartz stage 180 c and the dolomite stage 120 c the 13 c values of co 2 dissolved in dolomite hosted fluid inclusions 13 c 30 to 4 and the 13 c values of dolomite 13 c 23 5 to 3 5 indicate that the co 2 dissolved in the mineralizing brines originated from brine graphite interactions in the basement the resulting slight increase in the fluid partial pressure of co 2 p co 2 may have triggered dolomite precipitation instead of quartz 18 o values of quartz veins and previously published 18 o values of the main alteration minerals around the u ores illite chlorite and tourmaline show that quartz and alteration minerals were isotopically equilibrated with the same fluid at 180 c the ree concentrations in dolomite produce paas normalized patterns that show some similarities with that of uo 2 and are clearly distinct from that of the other main ree bearing minerals in these environments monazite zircon and aluminum phosphate sulfate aps minerals the radiogenic isotope compositions of dolomite 87 sr 86 sr i 0 7053 to 0 7161 and e nd t 8 8 to 20 3 differ from one deposit to another reflecting both heterogeneity in the basement geology and variable preservation of the original composition of brines the previously published 87 sr 86 sr i and e nd t values of uo 2 compare with the most evolved dolomites i e dolomites precipitated from brines that exchanged the most with the basement this reinforces a close genetic link between dolomites and uo 2 deposition and implies that uo 2 deposition occurred in a cooling system during the transition from quartz to dolomite formation the 18 o and d values of the mineralizing brines 18 o 1 to 8 and d 150 to 50 are considerably shifted from that of their theoretical original values acquired during evaporation of seawater 18 o 3 and d 40 the positive 18 o shift is explained by protracted fluid rock interaction within the basin and basement rocks the negative d shift is attributed to incomplete mixing between the u mineralizing brines and low d water this low d water was likely produced during the abiogenic synthesis of bitumen by fisher tropsch like reactions involving co 2 derived from brine graphite interaction in the basement and radiolytic h 2 the resulting low d brines have been equilibrated with alteration minerals this may explain why some alteration minerals yield anomalously low d values whose significance has long been debated

the stratigraphy of coastal carbonate banks and holocene sea levels of northern spencer gulf south australia abstract stratigraphic and chronologic studies of the coastal sediments of northern spencer gulf in the vicinity of redcliff are described two pleistocene marine units are recognised beneath an extensive holocene peritidal sequence the holocene sequence is dominated by sediments of the posidonia australis seagrass facies which form a distinctive carbonate bank fringing the coastline peritidal sedimentation commenced prior to 6600 radiocarbon yrs b p and a 2 5 m higher relative sea level existed from about 6000 until around 1700 yrs b p the subsequent relative fall to present level probably resulted from local tectonic uplift after the regression mangrove and samphire colonization and beach ridge progradation occurred over the former seagrass bank

foraminiferal biofacies eco succession and holocene sealevels port pirie south australia abstract at port pirie on the eastern coast of northern spencer gulf south australia 10 cores taken along a 3 km transect recovered a succession of holocene marine sediments facies representing shallow subtidal posidonia seagrass and intertidal sandflat mangrove and back storm ridge coastal lagoon environments are recognized on the basis of lithological characteristics and their preserved foraminifera an assemblage dominated by nubecularia lucifuga peneroplis planatus and discorbis dimidiatus signifies sediments of shallow subtidal posidonia seagrass meadows subtle changes in the numerical distribution of these species upcore are used to infer the change from subtidal to intertidal sandflat facies elphidium crispum and elphidium macelliforme become more numerous across this transition the mangrove facies is characterized by trochammina inflata this species is present significantly in only one seaward core where modern mangrove woodland continues to grow today and no equivalent biofacies are recognized in other cores the lagoonal sediments preserve a rich assemblage of species of euryhaline foraminifera together with those of the mangrove woodland they exhibit an ecological succession which can be related to decreasing intervals of tidal inundation and increasing salinity helenina anderseni with subordinate ammonia beccarii and elphidium cf articulatum are the pioneer species in the euryhaline setting giving way to trichohyalus tropicus and miliolinella schauinslandi late stages of hypersaline sedimentation are characterized by triloculina inflata triloculina oblonga in the mangroves h anderseni a beccarii and elphidium cf articulatum are replaced by trochammina inflata as the dominant species in turn as further sediment aggradation leads to ever shorter intervals of tidal inundation at the landward side of the mangrove woodland trochammina inflata is overtaken by ammobaculites barwonensis quantitative foraminiferal biofacies analysis confirms and refines the sedimentological interpretation of intertidal sediment facies from macro observations of the core materials it provides independent estimates of the elevation of key facies boundaries in cores and confirmation of a general relative fall in sealevel in northern spencer gulf over the past 7000 yr

reconciliation of late quaternary sea levels derived from coral terraces at huon peninsula with deep sea oxygen isotope records a major discrepancy between the late quaternary sea level changes derived from raised coral reef terraces at the huon peninsula in papua new guinea and from oxygen isotopes in deep sea cores is resolved the two methods agree closely from 120 ka to 80 ka and from 20 ka to 0 ka ka 1000 yr before present but between 70 and 30 ka the isotopic sea levels are 20 40 m lower than the huon peninsula sea levels derived in earlier studies new high precision u series age measurements and revised stratigraphic data for huon peninsula terraces aged between 30 and 70 ka now give similar sea levels to those based on deep sea oxygen isotope data planktonic and benthic 18o data using the sea level and deep sea isotopic data oxygen isotope ratios are calculated for the northern continental ice sheets through the last glacial cycle and are consistent with results from greenland ice cores the record of ice volume changes through the last glacial cycle now appears to be reasonably complete