seasonality of dissolved rare earth elements in the lower mississippi river 1 dissolved rare earth element ree concentrations were determined in a 27 month time series of the lower mississippi river overall the results agree with limited previous investigations that is the river shows enrichment of heavy rees relative to light rees and also has a significant ce anomaly however the previous investigations relied on only single samples from the river this seasonal investigation reveals significant temporal variations in the river s ree chemistry in particular large approximately fivefold variations in light ree concentrations are observed the light rees follow a seasonality similar to particle reactive trace elements also the ce anomaly shows a corresponding seasonality with greatest fractionation when dissolved concentrations are lowest the time series encompassed a period of extreme flooding in the u s midwest however this event did not appear to affect dissolved ree concentrations the results of this study are compatible with previous observations of dissolved trace elements in the lower mississippi river which suggested the importance of redox processes within the river system in controlling seasonal concentration variability for many of these elements additionally speciation modeling indicates that carbonate complexes likely dominate the solution chemistry of rees in the lower mississippi the observed behavior of the dissolved rees is likewise compatible with this speciation

two cryogenian glacial successions compared aspects of the sturt and elatina sediment records of south australia abstract two cryogenian glacial sequences the sturt and elatina occur within the adelaide rift complex s australia and are part of an 8 km thick succession recording rodinia fragmentation and subsequent passive margin development we compare and contrast these glacial successions presenting new data including detailed logged sections for both glacial sequences facies analysis reveals a total of six facies associations recording a range of glacial and glacially related processes these are a diamictite facies association ice proximal debris flows a sheet sandstone facies association fluvial deposits a flaser and rhythmically bedded facies association tidal flat a hummocky cross stratified sandstone facies association shoreface to offshore transition zone deposits and an underflow facies association recording offshore deposition a sixth facies association a ferruginous lonestone bearing siltstone is restricted to the top of the elatina formation and records rapid environmental amelioration and emergence from glaciation in the central flinders ranges the sturt succession commences with a glacial erosion surface ges which in agreement with previous workers was probably enhanced by rifting this ges is overlain in ascending order by the pualco tillite holowilena ironstone and wilyerpa formation with the warcowie dolomite member at the base of this latter formation a second ges is preserved within this latter unit and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits significant base level falls within the wilyerpa formation are identified by fluvial deposits sitting disconformably upon offshore deposits in contrast within the elatina formation a single ges is proposed which defines the base of the formation the elatina formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation apart from a locally preserved basal diamictite the lower member consists of fluvial deposits whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits the boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others deglaciation left contrasting deposits the sturt deglaciation culminated in deposition of black shale of the tindelpina shale member of the tapley hill formation seemingly as a sheet like deposit over the adelaide rift complex and beyond into the amadeus basin of central australia the elatina deglaciation meanwhile began first with deposition of the sixth facies association ferruginous dropstone bearing siltstone then proceeded with deposition of cap dolostone of the nuccaleena formation

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

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

anthropogenic pb component in hydrothermal ochres from marsili seamount tyrrhenian sea abstract recent hydrothermal fe oxyhydroxide precipitates ochres were sampled during two submersible expeditions to the ne part of the marsili seamount summit se tyrrhenian sea the marsili ochres belong to type 1 primary sulfide poor fe oxyhydroxides according to the classification of hekinian et al hekinian r hoffert m larque p cheminee j l stoffers p bideau d 1993 hydrothermal fe and si oxyhydroxide deposits from south pacific intraplate volcanoes and east pacific rise axial and off axial regions econ geol 88 2099 2121 they are x ray amorphous highly metalliferous al al mn fe 0 4 low in si transition metals co ni cu zn pb and rare earth elements ree and enriched in b v and mo their chemical composition and ree distribution patterns indicate precipitation from low temperature hydrothermal fluid mixed with seawater measured pb isotope compositions in the ochres indicate that the pb is dominantly anthropogenic in origin by contrast the pb isotope ratios in the nearby higher temperature hydrothermal deposits from the panarea arc volcano show that the pb is derived mainly from local volcanic rocks the fact that the pb isotopic signature of the marsili ochres is controlled by recent anthropogenic input suggests that the fe oxyhydroxides precipitated from fluids depleted in transition metals the low 206pb 207pb ratios in the ochres reflect the contribution of pb from australian ores indicating precipitation of the ochres somewhere between present day and the beginning of australian pb import to europe the young age of the ochres and the presence of other hydrothermal deposits along the marsili crest suggest that the magmatic chamber beneath the seamount may still be alive this study represents the first observation of anthropogenic influence on submarine hydrothermal deposits

manganese deposits of the devonian magnitogorsk palaeovolcanic belt southern urals russia abstract recent studies provide an overview on hydrothermal volcaniclastic sedimentary manganese deposits from the magnitigorsk palaeovolcanic belt in southern urals russia 100 deposits this report presents data on the structures of ore bodies chemical composition and mineralogy of manganese ores these deposits are divided into two types with different ore bearing band structures chemical and mineral ore compositions and ore component accumulation conditions manganese ores of the type 1 deposit are associated with jasperites ores from type 2 deposits are localised within thin banded jaspers type 1 deposits are thought to form during hydrothermal activity in areas where hydrothermal fluids leak into the seafloor type 2 deposits are formed within a closed basin as hydrothermal activity decreases away from hydrothermal vents and typically contain a greater volcaniclastic contribution manganese ores comprised more than 70 mineral types and form three groups of mineral assemblages a minerals formed during low grade regional metamorphism t 200 260 c p 2 3 kbar that compose the main part of ore bodies b mineral assemblages in segregatic and metasomatic veins filling late stage tectonic fractures and c an assemblage of subsurface supergene minerals most of these minerals which total approximately 38 minerals are metamorphic silicates the formation of metamorphic mineral assemblages is controlled by the presence of organic material om in primary sediments the conditions under which type 1 deposits formed favour the accumulation of om the metamorphism of these sediments is responsible for the formation of oxide carbonate silicate reduced ores which consist of mn silicates and carbonates such as tephroite rhodonite caryopilite and rhodochrosite andradite quartz hematite and hausmannite insignificant preserved quantities of om were found in type 2 deposits during the metamorphic alteration of these sediments oxide silicate oxidised ores were formed with the dominance of braunite

mineralogical constraints on the paleoenvironments of the ediacaran doushantuo formation assemblages of clay minerals are routinely used as proxies for paleoclimatic change and paleoenvironmental conditions in phanerozoic rocks however this tool is rarely applied in older sedimentary units in this paper the clay mineralogy of the doushantuo formation in south china is documented providing constraints on depositional conditions of the ediacaran yangtze platform that host the earliest animal fossils in the geological record in multiple sections from the yangtze gorges area trioctahedral smectite saponite and its diagenetic products mixed layer chlorite smectite corrensite and chlorite are the dominant clays through the lower 80 m of the formation and constitute up to 30 wt of the bulk rock saponite is interpreted as an in situ early diagenetic phase that formed in alkaline conditions ph 9 the absence of saponite in stratigraphically equivalent basin sections 200 400 km to the south indicates that alkaline conditions were localized in a nonmarine basin near the yangtze gorges region this interpretation is consistent with crustal abundances of redox sensitive trace elements in saponitic mudstones deposited under anoxic conditions as well as a 10 difference in the carbon isotope record between yangtze gorges and basin sections our findings suggest that nonmarine environments may have been hospitable for the fauna preserved in the yangtze gorges which includes the oldest examples of animal embryo fossils and acanthomorphic acritarchs

raman geothermometry of carbonaceous material in the basal ediacaran doushantuo cap dolostone the thermal history of extremely negative δ13c signatures in the aftermath of the terminal cryogenian snowball earth glaciation abstract extremely negative 13 c values as low as 48 of blocky calcite cements filling cavities sheet cracks and fractures in the basal ediacaran cap dolostone of the doushantuo formation in south china have been considered as evidence for methane release possibly associated with gas hydrate destabilization in the aftermath of the terminal cryogenian snowball earth glaciation alternatively these extremely negative 13 c values may have resulted from oxidation of thermogenic methane derived from localized hydrothermal activity long after the deposition of the doushantuo cap dolostone the latter interpretation was based on carbonate clumped isotope thermometry suggesting that the precipitation of extremely 13 c depleted calcite occurred at temperatures up to 476 c mean 378 c n 4 whereas carbonate fabrics with moderately negative 13 c values generally 10 were formed at much lower temperatures mean 112 c n 4 to assess these competing hypotheses we systematically investigated the petrography carbonate carbon and oxygen isotopes and carbonaceous material cm raman spectroscopy of the doushantuo cap carbonate raman geothermometry suggests that cm associated with different microfabrics and 13 c signatures including sedimentary fabrics with moderately negative 13 c values e g peloids stromatolite laminae microbial mat fragments diagenetic fabrics with moderately negative 13 c values e g some void filling or crack filling cements and diagenetic fabrics with extremely negative 13 c values e g void filling or crack filling blocky and bladed calcites experienced temperatures generally 13 c signatures and paleotemperatures whereas the origins of these extremely negative 13 c values remain unresolved the raman geothermometric data challenge the previous interpretation that they were derived from thermogenic methane related to localized hydrothermal activity at temperatures up to almost 500 c that only affected the thermal history of the calcite with extremely negative 13 c values instead the new data suggest that the analyzed microfabrics experienced the same peak temperatures

aragonite stromatolitic buildups from santorini aegean sea greece geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the minoan eruption ca 3600 yr bp this study received financial support from projects cgl2008 00594 and cgl2011 2348 plan nacional ministerio de ciencia e innovacion spain pa additional funding for analyses was provided by the mexican in103211 papiit project cc

formation of fe si mn oxyhydroxides at the pacmanus hydrothermal field eastern manus basin mineralogical and geochemical evidence samples of fe si mn oxyhydroxides were collected from the pacmanus hydrothermal field which lies in a young back arc setting in the eastern manus basin the purpose of the study was to understand the origin and characteristics of fe si mn oxyhydroxides associated with massive sulfides in a back arc basin the pacmanus fe si mn oxyhydroxides are composed of fe oxyhydroxides and mn oxyhydroxides with opal a and nontronite they have very low concentrations of trace elements except for ba mo v and u and rare earth elements and they show ree distribution patterns with positive eu anomalies and slight enrichments of lrees the fe si mn oxyhydroxides appear to be precipitated mainly from hydrothermal fluid with limited seawater contamination and scavenged trace metals are predominantly from the ambient seawater the differences in the ree distribution patterns between the fe oxyhydroxide fraction and mn oxyhydroxide fraction originate from diagenetic processes there are diverse filamentous microtextures resembling unique microbial populations suggesting microbially mediated mineralization during the precipitation of the fe si mn oxyhydroxides

carbonate deposition climate stability and neoproterozoic ice ages the evolutionary success of planktic calcifiers during the phanerozoic stabilized the climate system by introducing a new mechanism that acts to buffer ocean carbonate ion concentration the saturation dependent preservation of carbonate in sea floor sediments before this buffering was primarily accomplished by adjustment of shallow water carbonate deposition to balance oceanic inputs from weathering on land neoproterozoic ice ages of near global extent and multimillion year duration and the formation of distinctive sedimentary cap carbonates can thus be understood in terms of the greater sensitivity of the precambrian carbon cycle to the loss of shallow water environments and co 2 climate feedback on ice sheet growth

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

precambrian permafrost horizons as indicators of palaeoclimate abstract periglacial phenomena such as polygonal wedge structures and microrelief features commonly preserved in fossil permafrost horizons can provide valuable information on palaeoclimate including indications of mean annual air temperature seasonal temperature range relative precipitation and evidence for long term 104 years oscillations of climate such features are known from early and late proterozoic rocks and indicate mean annual air temperatures as low as 12 to 20 c or lower and strongly seasonal periglacial climates these palaeoclimatic data together with information on palaeolatitude and proximity to palaeo sea level may impose strong constraints on palaeoclimate models for proterozoic glaciation

geochemistry of vein and wallrock carbonates from the ediacaran system in south china insights into the origins of depositional and post depositional fluids abstract a combined study of carbon oxygen isotopes and major trace elements was carried out for vein and wallrock carbonates from the lantian formation of the ediacaran system in south china the results provide geochemical constraints on the origins of depositional and post depositional fluids with respect to recognition of the post depositional alteration the lantian formation was subdivided into the upper and lower carbonate units for the upper unit uu petrographic observations indicate that wallrock carbonates only experienced limited post depositional alteration the wallrock exhibits 18 o values of 19 2 to 13 1 and 13 c values of 9 6 to 8 9 both relative to vpdb the vein carbonates are divided into groups a and b based on their 18 o values and ree y patterns group a veins show nearly similar 18 o and 13 c ranges to but different ree y patterns from the wallrock these data indicate that the origin of fluids for group a veins may be similar to that of wallrock which was probably the mixture of continental and marine glacial meltwaters group b veins show both different 18 o values and ree y patterns from but similar 13 c values to the wallrock this indicates that post depositional fluids are different from the depositional fluids and thus of external origin low 18 o values 23 7 to 18 1 for group b veins indicate that the post depositional fluids were derived from continental glacial meltwater there is no correlation between 18 o and 13 c values for these vein and wallrock carbonates suggesting that the wallrock was not extensively altered by the post depositional fluids for the lower unit lu veins exhibit different c o isotope compositions and ree y patterns from wallrock carbonate indicating that post depositional fluids have different geochemical compositions from depositional fluids and thus are of external origin the simultaneous changes in 18 o and 13 c values with different laminations in the wallrock carbonates suggest the preservation of the primary geochemical compositions the 18 o values and ree y patterns for the lu veins indicate that the post depositional fluids were probably the mixtures of continental and marine glacial meltwaters therefore the lantian carbonates record the incorporation of continental glacial meltwater into the marginal sea of ediacaran age nevertheless the geochemical screening can provide constraints on the preservation of primary signatures and thus on the origins of depositional and post depositional fluids in the neoproterozoic marine carbonates

extreme winds and waves in the aftermath of a neoproterozoic glaciation the most severe excursions in the earth s climatic history are thought to be associated with proterozoic glaciations according to the snowball earth hypothesis the marinoan glaciation which ended about 635 million years ago involved global or nearly global ice cover at the termination of this glacial period rapid melting of continental ice sheets must have caused a large rise in sea level here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples these structures occur at homologous stratigraphic levels in australia brazil canada namibia and svalbard our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds significantly longer than those typical for today s oceans the reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch unlimited ocean basins we propose that these extraordinary wind and wave conditions were characteristic of the climatic transit and provide observational targets for atmospheric circulation models

morphology mineralogy and chemistry of hydrothermal deposits from the tag area 26 n mid atlantic ridge abstract metalliferous hydrothermal deposits have been observed and sampled by submersible from the tag hydrothermal field on the mid atlantic ridge at 26 n the deposits include surficial metal rich staining on the surface of carbonate ooze deposits over considerable areas of the sea floor as well as discrete massive layered deposits the latter include hydrothermal precipitates of fe silicate nontronite mn oxide birnessite and fe oxide amorphous the deposits range in size from less than 1 m to 15 m 20 m the composition and morphology of individual deposits varies from thick laminated crystalline birnessite precipitates through fe rich tubular vents to loose earthy birnessite nontronite or amorphous fe oxide interlayered deposits fractionation and crystallinity are presumably controlled by local eh conditions and the hydrodynamics of circulation in the upper crust they are presumed to represent deposition from low temperature hydrothermal solutions which had originally reacted with basaltic oceanic crust at depth at high temperature and had precipitated metal rich sulfides within the crust occasional short duration high temperature black smoker type venting is inferred to have occurred based upon cu fe zn rich layers in the sediments and observations of clam shells in bottom photographs

depositional environments of the lower permian dwyka diamictite and prince albert shale inferred from the geochemistry of early diagenetic concretions southwest karoo basin south africa abstract the upper dwyka and lower ecca groups in the karoo basin of south africa document the climatic and palaeoenvironmental changes associated with the final permo carboniferous deglaciation of the gondwana supercontinent the depositional environments of these groups have until recently been interpreted on the basis of sedimentological and palaeontological evidence here we use the geochemistry of early diagenetic concretions septarian calcite concretions from the upper dwyka group and phosphatic chert concretions and beds from the lower ecca group to infer the depositional environment of these rocks in the southwestern karoo basin 18o values 7 8 to 8 9 smow suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than permian seawater 13c values 15 to 3 pdb indicate that the carbon was derived from a mixture of craton derived calcareous material and organic matter bacterially degraded in the lower sulphate reduction to upper methanogenesis zones during early burial diagenesis the rare earth element ree patterns sr concentrations and 87sr 86sr ratios 0 716 0 737 significantly greater than permian seawater 0 708 together also support the interpretation that calcite and phosphatic concretions formed in glacial fresh water sediments

precious metals in barite silica chimneys from franklin seamount woodlark basin papua new guinea abstract barite silica chimneys with disseminated sulphides and elevated contents of gold and silver occur at two sites on the caldera floor of the basaltic andesite franklin seamount situated near the western tip of the woodlark spreading axis where it propagates into continental crust the mineralogical residences of silver and gold in these chimneys are very different gold occurs in tiny electrum particles dispersed through very late stage sulphide outgrowths from barite into cavities these represent the fourth reported mineralogical occurrence of gold in seafloor hydrothermal deposits silver by contrast was deposited as real or occult sulphosalt inclusions in pyrites apparently formed throughout the local paragenetic sequence in each growth zone of the chimney walls correlations between silver lead and antimony indicate that the main silver habitat is an unrecognized ag dominant pb sb sulphosalt but unresolved inclusions of a pyrargyrite like sulphosalt in colloidal pyrite may also occur electron and proton probe microanalyses have together established a balanced budget for many trace elements in minerals and bulk contents except it is necessary to appeal to variations throughout the chimney in distribution of as cd sb and hg the residence of mo which correlates with gold remains unknown isotopic studies indicate that the local volcanic pile or its parental magma chamber is the source of chalcophile metals in the chimneys but neither the source rocks nor the tectonic setting offer definitive explanations for the enrichment of precious metals at franklin seamount chemical aspects of the probably small scale and short lived hydrothermal system appear the controlling factors including low fluid rock ratios leading to selective leaching of highly extractable constituents and substantial subsurface mixing with seawater leading to pyrite deposition before venting

an estimate of hydrothermal fluid residence times and vent chimney growth rates based on210pb pb ratios and mineralogic studies of sulfides dredged from the juan de fuca ridge abstract the210pbpb ratios across two sulfide samples dredged from the juan de fuca ridge are used to estimate the growth rate of the sulfide material and the residence time of the hydrothermal fluid within the oceanic crust from the onset of basalt alteration 210pb is added to the hydrothermal fluid by two processes 1 high temperature alteration of basalt and 2 if the residence time of the fluid is on the order of the 22 3 year half life of210pb by in situ growth from222rn krishnaswami and turekian 1982 stable lead is derived only from the alteration of basalt the210pb pb ratio across one sample was 0 5 dpm 10 6 g pb and across the other it was 0 4 dpm 10 6 g pb these values are quite close to the238up b ratios of basalts from the area suggesting that the residence time of the hydrothermal fluid from the onset of basalt alteration is appreciably less than the mean life of210pb i e the time required for ingrowth from the radon an apparent growth rate of 1 2 cm yr is derived from the slope of the210pb pb curve for one of the samples this is consistent with its mineralogy and texture which suggest an accretionary pattern of development there is no obvious sequential growth pattern and virtually no gradient in210pb pb across the second sample this is consistent with alteration of the original210pb pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample

discovery of ancient and active hydrothermal systems along the ultra slow spreading southwest indian ridge 10 16 e 1 we report the discovery of active and fossil hydrothermal systems during r v knorr cruise 162 legs vii and ix along a 400 km long segment of the ultra slow southwest indian ridge swir between 10 and 16 e where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date 8 4 mm yr 1 full rate eight of forty one optical temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias four deposits ofsepiolite and silica as well as mn oxide and nontronite cemented breccias the massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep seated normal fault bounding the southern rift valley wall silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low to moderate temperature ultramafic hosted hydrothermal activity the abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates and hence the magmatic heat input along this section of the swir are lower than on any other explored segment of the global mid ocean ridge system this observation suggests that high mantle upwelling and magma supply rates are not required to drive mid ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra slow end of the ridge spreading spectrum the frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation with hydrothermal vent sites being preferentially associated with long lived faults that provide fluid pathways our results suggest the contribution of magma starved ridge segments to the global ocean lithosphere geochemical budget is potentially much larger than previously thought

mineralized microbes from giggenbach submarine volcano 1 the giggenbach submarine volcano which forms part of the kermadec active arc front is located 780 km nne of the north island of new zealand samples collected from chimneys associated with seafloor hydrothermal vents on this volcano at a depth of 160 180 m contain silicified microbes and microbes entombed in reticular fe rich precipitates the mineralized biota includes filamentous rod shaped and rare coccoid microbes in the absence of organic carbon for rdna analysis or preserved cells the taxonomic affinity of these microbes in terms of extant taxa remains questionable because of their architectural simplicity and the paucity of taxonomically significant features the three dimensional preservation of the microbes indicates rapid mineralization with a steady supply of supersaturated fluids to the nucleation sites present on the surfaces of the microbes the mineralization styles evident in the microbes from the giggenbach submarine volcano are similar to those associated with mineralized microbes found in terrestrial hot spring deposits in new zealand iceland yellowstone and kenya these similarities exist even though the microbes are probably different and the fluids become supersaturated with respect to opal a by different mechanisms for ancient rocks it means that interpretations of the depositional settings cannot be based solely on the silicified microbes or their style of silicification

tectonic and volcanic implications of a cratered seamount off nicobar island andaman sea the region of the nicobar earthquake swarm of january 2005 has been explored during a recent cruise using multibeam swath bathymetry seafloor imaging and tv guided sampling to decipher the seafloor morphology nature and tectonic frame work a seamount with well developed crater at the summit was discovered near to the center of the nicobar swarm rock samples collected by tv guided grab from the seamount crater are dacite rhyolite and andesite type with a veneer of ferromanganese oxide coating the aggregates recovered from the slope consisted of manganese globules the geochemistry of the globules suggests 66 97 mn oxide indicative of hydrothermal origin the morphology of the seamount seafloor video footage and geochemistry of the seabed samples suggest that the cratered seamount has erupted in the recent geological past and is dormant at present this is the first documented report of submarine arc volcanism in the andaman sea this finding substantiates the prediction of a submarine volcano east of nicobar island study of the seismicity and the stress pattern in the region suggest that the earthquake swarm in the region occurred due to 2004 sumatra andaman megathrust earthquake the derived stress pattern suggests that the 2004 earthquake greatly reduced the normal stress in the region of west andaman fault from the planes or unclamped them thus unclamping of the region by the 2004 sumatra andaman earthquake led to the initiation of the swarm through predominantly strike slip faulting the newly discovered dormant submarine volcano indicates the volcanic nature of the region we suggest that this submarine volcano is a link between sub aerial volcanoes of barren narcondam islands of the andaman sea and the volcanoes of sumatra

low temperature hydrothermal deposits of franklin seamount woodlark basin papua new guinea abstract hydrothermal deposits of siliceous fe mn oxides from a few cm to 7 m in height are widespread at the summit and caldera floor of franklin seamount a 250 m high edifice of basaltic andesite lava tubes pillows and talus located near the western propagating tip of the woodlark spreading axis some deposits are venting clear fluid at about 30 c the hydrothermal constructions are composed predominantly of x ray amorphous protoferrihydrite mixed with variable amounts of opaline silica and greenish nontronite most deposits are covered by mm thick black crusts of vernadite and feroxyhyte which are mineralised bacterial formations genus leptothrix of hydrogenous origin microbes also influenced formation of the fe rich parts of the deposits as evidenced by filamentous protoferrihydrite pseudomorphs of gallionella blocky inorganic particles of protoferrihydrite and ferrihydrite occur as rare interior phases nontronite formed by precipitation from hydrothermal solutions commonly involving replacement of microbial protoferrihydrite and silica patchy and lenticular birnessite partly altered by biogenic processes to vernadite occurs within siliceous protoferrihydrite matrices in dark cm thick outer margins of some edifices this is considered hydrothermal in origin reflecting percolation of hydrothermal fluids through porous protoferrihydrite opal aggregates to more oxidised conditions near the exterior related to mixing with seawater the different mineralogical associations are reflected by geochemistry trace elements in the manganiferous phases were derived partly from the primary hydrothermal solution and partly from seawater with which this was extensively diluted at the depositional site and also deeper within the volcanic structure of franklin seamount most of the fe mn cu zn and pb was derived from the primary hydrothermal fluid from limited data nontronitic portions of the deposits contain more cu zn and pb than the siliceous protoferrihydrite rich portions migration of redox boundaries and redistribution of elements during flow through processes influence development of the typical zoned structure of the deposits uranium thorium decay series equilibria at both actively venting and inactive sites at franklin seamount yield ages not exceeding 43 years

ediacaran growth of the marine sulfate reservoir abstract the sulfur isotope record as preserved in sedimentary sulfate and sulfide minerals reflects the progressive oxidation of earth s surface environment the latest proterozoic to early paleozoic marine sulfur isotope record is distinguished by unusually high 34 s values in marine sulfates and sulfides geological considerations and sulfur isotopic data indicate that the 34 s enrichment of the marine sulfate reservoir was the result of a rapid increase in the net fractionation between sedimentary sulfate and pyrite 34 s without a compensatory decline in the fractional burial rate of pyrite f pyr a simple one box model in which 34 s is linked non linearly to marine so 4 is used to explore the response in sedimentary sulfur isotope records 34 s sulf and 34 s pyr to an increasing marine sulfate reservoir the model results when compared with a compilation of existing 34 s sulf and 34 s pyr data suggest that the marine sulfate reservoir grew rapidly to roughly phanerozoic levels in the middle ediacaran period before the gaskiers glaciation and prior to the onset of the shuram wonoka negative 13 c anomaly it is further hypothesized that p o 2 levels rose prior to the onset of this extreme perturbation to the global carbon cycle permitting the oxidation of a previously euxinic deep ocean and appearance of the first ediacaran fauna by ca 575 ma despite the enormous demand on oxidants implied by this large magnitude and long duration carbon isotope anomaly

condensation origin for neoproterozoic cap carbonates during deglaciation neoproterozoic deglacial stratigraphy is commonly characterized by a sharp contact separating glacial sediments from laminated capping carbonates this stratigraphic relation is generally assumed to have time significance and to reflect an abrupt shift from icehouse to greenhouse conditions in contrast to this sequence stratigraphic field studies of an ediacaran ca 635 ma glacial to postglacial transition in the amadeus basin of central australia reveal a complex deglacial stratigraphy in which more than 175 m of conglomerate sandstone marl and carbonate at the basin margin and portions of four unconformity bounded sequences pass basinward into no more than 3 m of laminated dolomicrite of typical cap carbonate facies the unconformities which are characterized by as much as several tens of meters of erosional relief oblique sections of incised valleys separate intervals of contrasting sediment provenance and are confidently mapped on the basis of both criteria comparable unconformities are absent in the overlying neoproterozoic succession which is 2 km thick and encompasses many tens of millions of years the amadeus basin cap carbonate was thus deposited during a protracted interval of multiphase cyclical transgression more similar to phanerozoic cyclical sea level rise than to the single catastrophic deglaciation and instantaneous precipitation invoked by popular current models to explain the classic cap carbonate the superposition of carbonate on glacial facies in distal sections evidently records condensation in the absence of siliciclastic sediment rather than abrupt shifts between glacial and tropical conditions facies lithologically similar to cap carbonates may be less obvious in phanerozoic successions because of a secular change in carbonate composition to reefal and deep sea pelagic deposits

mn ba hg mineralization at shallow submarine hydrothermal vents in bahia concepcion baja california sur mexico coastal submarine hydrothermal venting occurs on the west shore of the fault bounded bay of bahia concepcion along a stretch of about 700 m of rocky shoreline diffuse hydrothermal seepage of water and continuous gas bubbling mainly co2 and n2 take place through the sediment covered seafloor at shallow depth 5 to 15 m in addition at about 500 m se a cluster of hot springs occurs within the intertidal zone the temperature and ph of the hydrothermal fluids varies from 72 to 87 c and 5 95 to 6 02 in the submarine diffuse venting area and reaches 62 c and ph of 6 68 in the intertidal hot springs mn ba hg mineralization is presently forming as a result of the hydrothermal venting at the intertidal hot springs moss like crusts of manganese oxides and structureless detrital aggregates cemented by opal a barite and calcite occur around the main discharge conduits these aggregates are overlaid by silica carbonate fine layered stromatolitic aggregates that extend some meters from the hot springs forming crusts and coalescing rims over volcanic pebbles and boulders manganese crusts are made up of barium rich x ray amorphous todorokite like and romanechite phases and represent a rare example of a manganese deposit formed in a shallow submarine hydrothermal environment in the submarine diffuse venting area the hydrothermal precipitates are less conspicuous and consist in millimeter thick iron oxyhydroxide coatings on volcanic cobbles and boulders these coatings are composed of poorly crystallized six line ferrihydrite accompanied by minor cinnabar and fine grained colloform pyrite textures and fabrics of the mineral assemblages suggest microbial mediation for mineral deposition the vent precipitates display a significant enrichment of hg as and sb arsenic is probably adsorbed onto mn and fe oxide substrates the nasc north american shale composite normalized ree patterns of the vent precipitates show a pronounced positive eu anomaly that is probably inherited from the hydrothermal fluid

anatomy and origin of carbonate structures in a miocene cold seep field miocene calcite concretions resembling modern carbonate structures that form at cold seeps are present in fractured opal ct porcelanites that are interbedded with mudstones in coastal cliffs at santa cruz california the morphologies of the carbonate structures differ markedly from conventional concretions and are spatially aligned with orthogonal joints in the porcelanites the structures contain tubular holes that are identical to fluid and gas conduits in modern carbonate seep structures the orientations of these tubes suggest that fluid and gas flow was both vertical and horizontal the latter along extensional joints that formed preferentially in the brittle silica rich layers that had enhanced bedding parallel permeability petrographic and isotopic characteristics of the carbonate structures indicate that calcite precipitation occurred in a shallow subseafloor environment in either the zone of microbial sulfate reduction or of methanogenesis prior to or possibly simultaneously with the silica phase transformation of opal a in diatom shells to opal ct

s and sr isotopic compositions in barite silica chimney from the franklin seamount woodlark basin papua new guinea constraints on genesis and temporal variability of hydrothermal fluid isotopic ratios of strontium and sulfur in six layers across a horizontal section of a hydrothermal barite silica chimney from franklin seamount of western woodlark basin have been investigated sr isotopic ratios in barite samples 87sr 86sr 0 70478 0 70493 are less radiogenic than seawater 87sr 86sr 0 70917 indicating that substantial leaching of sub seafloor magma was involved in the genesis of hydrothermal fluid the so2 of magma likely contributed a considerable amount of lighter s isotope in fluid and responsible for the formation of barite which is isotopically lighter 34s 19 4 20 5 than modern seawater 34s 21 the systematic changes in isotopic compositions across the chimney wall suggest temporal changes in the mode of mineral formation during the growth of the chimney enrichment of heavy s and sr isotopes 34s 20 58 87sr 86sr 0 70493 in the outermost periphery of the chimney indicates that at the initial stage of chimney development there was a significant contribution of seawater sulfate during barite mineralization thereafter thickening of chimney wall occurred due to precipitation of fluid carrying more magmatic components relative to seawater this led to a gradual enrichment of lighter isotopes 34s 20 42 19 48 87sr 86sr 0 70491 0 704787 toward the inner portion of the chimney wall in contrast the innermost layer surrounding the fluid conduit is characterized by heavier and more radiogenic isotopes 34s 20 3 87sr 86sr 0 7049 this suggests there was increasing influence of percolating seawater on the mineral paragenesis at the waning phase of the chimney development

submarine hydrothermal activity and mineralization on the kurile and western aleutian island arcs n w pacific the kurile arc consists of at least 100 submarine volcanoes and 5 submarine caldera located mainly in the rear arc the arc is seismically very active particularly in the south and is characterized by extensive volcanism and hydrothermal activity at least one subaerial volcano on the arc medvezhy located on iturup island has an extremely shallow magma chamber and is intensely active three types of submarine hydrothermal deposit were recovered from this area hydrothermal manganese crusts nontronite and hydrothermal manganese crusts overlain by hydrogenous manganese oxides in addition submarine hydrothermal fe oxyhydroxides enriched in p were sampled from the submerged caldera of kraternaya bight in the central part of the arc these deposits appear to be analagous to deposits from santorini caldera on the aegean arc piip submarine volcano located in the western aleutian arc is characterized by two shallow summit craters both intermediate temperature anhydrite gypsum barite amorphous silica pyrite calcite and aragonite and low temperature nontronite fe oxyhydroxides hisingerite and ferromanganese crusts hydrothermal minerals were recovered from this seamount together with bacterial mats and giant clams the maximum measured temperature of the hydrothermal fluids was 133 c although the actual temperature may have reached 250 c both the kurile and aleutian arcs have the potential to host major submarine hydrothermal systems detailed exploration of these poorly studied areas using modern geophysical geochemical and sampling techniques is therefore strongly recommended

geochemical and mineralogical studies of ca 12 ma hydrothermal manganese rich rocks in the hokuroku district in japan abstract geological and geochemical studies were performed on ca 12 ma mn rich rocks in the hokuroku district in japan to constrain the origin of mn rich hydrothermal fluids and precipitation pathways of mn oxides mn rich rocks occur in hyaloclastites of rhyolite lavas in the studied areas colloform textures of mn oxides were common in those mn rich rocks carbonaceous layers uniquely alternate with mn oxides in the colloform textures the mn oxides layers in the colloform textures were composed of aggregates of nano scale todorokite birnessite and pyrolusite the carbonaceous layers also host apatite nano crystals these findings imply that the carbonaceous layers most likely represent fossilized microbial mats of mn oxidizing bacteria the close association of nano scale mn oxides with organic matter clay and silica minerals suggests that mn oxides initially precipitated near venting areas of low temperature hydrothermal fluids the early mn oxides were trapped in the hyaloclastites and experienced diagenetic modifications combined with hydrothermal alteration seawater penetration and microbial activities in the hyaloclastites such secondary processes further enriched mn in the hyaloclastites forming various mn silicates e g pennantite and shirozulite and veins made of mn ii and iii silicate and oxide silicate e g rhodonite and braunite later hydrothermal fluids also brecciated the massive mn rich rocks rhodochrosite and rhodonite occurred in such brecciated mn rich rocks or sheared hyaloclastites preferentially the carbon isotope compositions of rhodochrosite ranged from 26 7 to 20 8 vpdb the mudstone and early colloform textures in the hyaloclastites contained appreciable amounts of organic matter such organic matter was degraded by microbial processes under the influence of the later hydrothermal activities followed by the formation of rhodochrosite these findings contribute not only to understanding the formation mechanisms of hydrothermal mn rich rocks but also to future exploration of mn deposits

the opening of the woodlark basin subduction of the woodlark spreading system and the evolution of northern melanesia since mid pliocene time abstract magnetic anomaly and seismological data define segments of active seafloor spreading and associated magnetic lineations trending ene in the woodlark basin the total opening rate has been approximately 6 cm yr for the last 1 m y spreading rates diminish by over 10 from east to west along the woodlark spreading system implying a pole of current opening 15 20 to the west commencement of seafloor spreading in the basin has apparently been time transgressive beginning prior to 3 5 m y in the east and at successively later times to the west earthquake focal mechanisms and geological evidence suggest that the land areas bounding the western end of the woodlark basin are undergoing tensional deformation we believe that eventually the woodlark basin plate boundary will propagate westward through the d entrecasteaux islands into the papuan peninsula hitherto unreported shallow seismicity associated with the northern margin of the ne trending section of the woodlark rise probably reflects partial decoupling of the woodlark and solomon basins possibly due to mechanical difficulties in subducting the young woodlark lithosphere analysis of the relative motions between the solomon indo australian and pacific plates shows that the woodlark spreading system has been subducted at high rates 10 cm yr beneath the solomon islands during the opening of the woodlark basin several tectonic and geological features limited to the region of interaction of the woodlark basin with the solomon trench and arc may be symptomatic of ridge subduction these features include high heat flow in the solomon trench which shoals to 4 km low levels of seismicity and only shallow hypocenters and voluminous eruptions of high k 2 o tio 2 olivine basalts and basaltic andesites extremely close to the trench axis this close association in space and time of an unusual volcanic suite with ridge subduction implies a strong dependence of the petrogenesis on the tectonic regime a combination of this study of the woodlark basin and the previous study of the bismarck basin taylor 1979 provides a reconstruction of the positions of the continents ocean basins and island chains in northern melanesia for mid pliocene time in accepting the existence of a solomon plate we can explain the trench like structure off the trobriand margin of new guinea the occurrence of late cenozoic calc alkaline volcanism along the papuan peninsula and the presence of intermediate depth seismicity beneath the north papuan peninsula the rapid changes in relative motions along or across the new ireland solomons chain over the past 3 5 m y may explain the spatial and temporal changes in igneous activity observed on these islands

snowball earth termination by destabilization of equatorial permafrost methane clathrate the recovery from the marinoan snowball ice age of about 635 million years ago is one of the most severe climate change events known to have occurred on earth hypotheses accounting for the abruptness of deglaciation include ice albedo feedback deep ocean out gassing during post glacial oceanic overturn or methane hydrate destabilization martin kennedy and colleagues now report the broadest range of oxygen isotope values yet measured in marine sediments in methane seeps in marinoan deglacial sediments underlying a thin interval of carbonate these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise the range of values is likely the result of mixing between ice sheet derived meteoric waters and clathrate derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater their findings suggest that methane released from low latitude permafrost clathrates acted as a trigger and or strong positive feedback for deglaciation and warming this paper reports the broadest range of oxygen isotope values yet measured in marine sediments in methane seeps in marinoan deglacial sediments underlying a thin interval of carbonate these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise the range of values is likely the result of mixing between ice sheet derived meteoric waters and clathrate derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater the start of the ediacaran period is defined by one of the most severe climate change events recorded in earth history the recovery from the marinoan snowball ice age 635 myr ago ref 1 marinoan glacial marine deposits occur at equatorial palaeolatitudes2 and are sharply overlain by a thin interval of carbonate that preserves marine carbon and sulphur isotopic excursions of about 5 and 15 parts per thousand respectively3 4 5 these deposits are thought to record widespread oceanic carbonate precipitation during postglacial sea level rise1 3 4 this abrupt transition records a climate system in profound disequilibrium3 6 and contrasts sharply with the cyclical stratigraphic signal imparted by the balanced feedbacks modulating phanerozoic deglaciation hypotheses accounting for the abruptness of deglaciation include ice albedo feedback3 deep ocean out gassing during post glacial oceanic overturn7 or methane hydrate destabilization8 9 10 here we report the broadest range of oxygen isotope values yet measured in marine sediments 25 to 12 in methane seeps in marinoan deglacial sediments underlying the cap carbonate this range of values is likely to be the result of mixing between ice sheet derived meteoric waters and clathrate derived fluids during the flushing and destabilization of a clathrate field by glacial meltwater the equatorial palaeolatitude implies a highly volatile shelf permafrost pool that is an order of magnitude larger than that of the present day a pool of this size could have provided a massive biogeochemical feedback capable of triggering deglaciation and accounting for the global postglacial marine carbon and sulphur isotopic excursions abrupt unidirectional warming cap carbonate deposition and a marine oxygen crisis our findings suggest that methane released from low latitude permafrost clathrates therefore acted as a trigger and or strong positive feedback for deglaciation and warming methane hydrate destabilization is increasingly suspected as an important positive feedback to climate change11 12 13 that coincides with critical boundaries in the geological record14 15 and may represent one particularly important mechanism active during conditions of strong climate forcing

dynamic response of oceanic hydrate deposits to ocean temperature change the dynamic response of oceanic hydrate deposits to ocean temperature change matthew t reagan and george j moridis earth sciences division lawrence berkeley national laboratory 1 cyclotron rd berkeley ca 94720 usa

hydrothermal nontronite formation at eolo seamount aeolian volcanic arc tyrrhenian sea abstract a sediment core containing a yellowish green clay bed was recovered from an area of extensive hydrothermal deposition at the se slope of the eolo seamount tyrrhenian sea the clay bed is composed of pure nontronite described for the first time in the tyrrhenian sea which appears to be the most aluminous nontronite ever found among the seafloor hydrothermal deposits the high al content suggests precipitation from al containing hydrothermal solutions the ree distribution of the eolo nontronite has a v shape pattern the heavy ree enrichment is in part due to their preferential partitioning in the nontronite structure this enrichment was possibly further enhanced by the hree preferential sorption on bacterial cell walls the light ree enrichment is the result of scavenging uptake by one of the nontronite precursors i e poorly ordered fe oxyhydroxides from the hydrothermal fluids oxygen isotopic composition of the nontronite yields a formation temperature of 30 c consistent with a low temperature hydrothermal origin the relatively radiogenic nd isotopic signature of the nontronite compared to the present day mediterranean seawater indicates that approximately half of nd and presumably the rest of the lree are derived from local volcanic sources on the other hand 87 sr 86 sr is dominated by present day seawater sr scanning electron microscopy investigation revealed that the nontronite is composed of aggregates of lepispheres and tube like filaments which are indicative of bacteria assisted precipitation bacteria inhabiting this hydrothermal site likely acted as reactive geochemical surfaces on which poorly ordered hydrothermal fe oxyhydroxides and silica precipitated upon aging the interactions of these primary hydrothermal precipitates coating bacterial filaments and cell walls likely led to the formation of nontronite finally the well balanced interlayer and layer charges of the crystal lattice of seafloor hydrothermal nontronite decrease its sorption capacity to zero thus the ubiquitous nontronite precipitation along the active plate boundaries and around the hot spots has no significant impact on oceanic trace element chemistry

rare earth elements in hamersley bif minerals abstract minerals from the hamersley banded iron formation western australia were analyzed for y and rare earth elements yrees by laser ablation icp ms to investigate diagenetic pathways from precursor phases to bif minerals one group of apatites carries the seawater ree signature giving evidence that p and rees thoroughly scavenged from the water column by si ferrihydrite particles were released upon microbial fe 3 reductive dissolution of si ferrihydrite in pore water and finally sequestered mainly in authigenic apatite the absence of fractionation between apatite and seawater suggests that ree were first incorporated into an amorphous calcium phosphate as fully hydrated cations i e as outer sphere complexes the iron oxides and carbonates carry only a small fraction of the whole rock ree budget their ree patterns are distinctly enriched in yb and show some m type tetrad effect consistent with experimental k d ree between solid and saline solution with low carbonate ion concentrations it is deduced that hematite formed at an incipient stage of fe 2 catalyzed dissolution of si ferrihydrite via a dissolution reprecipitation pathway the ree pattern of greenalite found as sub micron particles in quartz in a chert siderite sample is consistent with its authigenic origin by precipitation in pore water after dissolution of a small amount of si ferrihydrite magnetite carries very low yrees ppb level has an homogeneous pattern distinctly enriched in the mid rees compared to hematite and includes a late population depleted in light rees ba and as magnetite forming aggregates and massive laminae is tentatively interpreted as reflecting some fluid aided hematite magnetite re equilibration or transformation at low grade metamorphic temperatures

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

tectonics of rift propagation into a continental margin western woodlark basin papua new guinea oblique convergence along the irregular boundary between the pacific and indo australian plates in the sw pacific has resulted in a transpressional regime the drag of the overriding pacific plate has produced a breakup of the solomon microplate from the indo australian plate and has induced its rotation within the mega shear zone between the two major plates the microplate is being separated from the indo australian plate by passive rifting and seafloor spreading in the woodlark basin which is propagating westward along the continental margin of papua new guinea the present day pole of the microplate rotation with respect to indo australian plate is estimated to be near the owen stanley fault zone which represents the suture after paleocene eocene arc continent collision the average rate of the seafloor spreading propagation is estimated to be 150 mm yr 1 over the last 3 4 ma the transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault the style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks

tectonic and magmatic controls on hydrothermal activity in the woodlark basin the woodlark basin is one of the rare places on earth where the transition from continental breakup to seafloor spreading can be observed the potential juxtaposition of continental rocks a large magmatic heat source crustal scale faulting and hydrothermal circulation has made the woodlark basin a prime target for seafloor mineral exploration however over the past 20 years only two locations of active hydrothermalism had been found in 2009 we surveyed 435 km of the spreading axis for the presence of hydrothermal plumes only one additional plume was found bringing the total number of plumes known over 520 km of ridge axis to only 3 much less than at ridges with similar spreading rates globally particularly the western half of the basin 280 km of axis is apparently devoid of high temperature plumes despite having thick crust and a presumably high magmatic budget this paucity of hydrothermal activity may be related to the peculiar tectonic setting at woodlark where repeated ridge jumps and a re location of the rotation pole both lead to axial magmatism being more widely distributed than at many other more mature and stable mid ocean ridges these factors could inhibit the development of both a stable magmatic heat source and the deeply penetrating faults needed to create long lived hydrothermal systems we conclude that large seafloor massive sulfide deposits potential targets for seafloor mineral exploration will probably not be present along the spreading axis of the woodlark basin especially in its younger western portion

sediments of the active rift zone in the western woodlark basin and the development of hydrothermal and volcanic activity abstract in the western woodlark basin within the neovolcanic zone sediments form ponds in fissures and between individual pillows and tubes a continuous thin sediment cover appears only at the base of the neovolcanic zone its thickness increases with the distance from the zone data on the lithology mineralogy and chemistry of sediment cores from the spreading centre in the western woodlark basin and adjacent areas show that these calcareous sediments avg 44 1 caco 3 contain significant amounts of terrigenous arc volcanic hyaloclastic and hydrothermal material the latter constituting up to 15 of the non carbonate fraction two types of sedimentary sequences occur in the studied area those of the first type were formed by particle by particle sedimentation horizons of redeposited sediments occur in sequences of the second type redeposition of sediments is widespread and is tentatively correlated with cycles of volcanic activity at swells along the spreading centre these alternate with hydrothermal cycles which were determined by the presence of hydrothermal matter in sediments radiocarbon dating of sediments showed that both cycles are of similar duration of the order of 10 000 years they are not synchronous along the spreading axis each swell having a specific evolution based on these determinations we can say that within the studied spreading centre only the franklin seamount swell is prospective for contemporary hydrothermal activity

texture mineralogy and geochemistry of hydrothermally altered submarine volcanics recovered southeast of cheshire seamount western woodlark basin the western woodlark basin lies within a gold rich metallogenic province this area is characterized by detachment faults that record ongoing extension and act as major pathways for the circulation of hydrothermal fluids dredging from the flanks of a submarine ridge southeast of cheshire seamount western woodlark basin retrieved hydrothermally altered monomictic to polymictic crackle mosaic and chaotic breccias with at least 30 clasts n2 mmin diameter the precursor rocks are andesitic to rhyolitic in composition but have been intensely hydrothermally altered with about 90 of the volcanic glass replaced by secondaryminerals the breccias show five generations of quartz growth with the first being related to magmatic processes and the remaining four to alteration stages including silicification chloritization illitization sericitization albitization and sulfidation needle like crystals ofmordenite zeolite withmultiple growth centers growon the fourth generation of quartz notable textural variants in the breccias are vesicles perlitic cracks and zoned alteration halos that mantle the rims of clasts electron microprobe analyses on chlorite from breccia samples have identified clinochlore as the main chlorite type and indicate a formation temperature in the range of 210 304 c this and the elevated au as ag hg zn pb sb contents of a mineralized sample indicate hydrothermal alteration temperatures n200 c suggesting that these breccias may represent the upflow zone of a hydrothermal system and highlight the potential for seafloor massive sulfides in the area the breccias show elevated contents of immobile trace elements and lree as well as a depletion in ta and nb suggesting that the precursor rocks were formed in a rift related suprasubduction environment

the end of the ice age methane gas hydrates may have contributed to the rapid rise in atmospheric ch4 co2 and global temperatures at the end of the last major glaciation about 13 500 years ago given suitable orbital conditions and insolation at high latitude a small triggering event such as the release of one or more arctic gas pools may have initiated massive release of methane from hydrate under ice and in permafrost the consequent greenhouse warming would have provided strong positive feedback amplifying emission this warming driven by ch4 may have induced the release of co2 from the oceans to the biosphere stabilizing the interglacial carbon cycle

toward the snowball earth deglaciation the current state of knowledge suggests that the neoproterozoic snowball earth is far from deglaciation even at 0 2 bars of co2 since understanding the termination of the fully ice covered state is essential to sustain or not the snowball earth theory we used an atmospheric general climate model agcm to explore some key factors which could induce deglaciation after testing the models sensitivity to their parameterizations of clouds co2 and snow we investigated the warming effect caused by a dusty surface associated with ash release during a mega volcanic eruption we found that the snow aging process its dirtiness and the ash deposition on the snow free ice are key factors for deglaciation our modelling study suggests that under a co2 enriched atmosphere a dusty snowball earth could reach the deglaciation threshold

neoproterozoic loess and limits to snowball earth abstract neoproterozoic tillites overlain by limestones and dolostones cap carbonates have been interpreted as evidence of abrupt climate change from glaciers to tropical seas on the assumption that cap carbonates were marine this assumption is here challenged by evidence for loess deposition of the nuccaleena formation of south australia from granulometry angular randomly oriented silt modal diameter 7 and sedimentary structures climbing translatent cross stratification linear dunes in addition a variety of palaeosols in the nuccaleena formation have red clayey horizons replacive micritization expansion cracks and thufur mounds as well as low carbon and oxygen isotope values high strontium isotope ratios and geochemical evidence of leaching and clay formation this cap carbonate has no definitively marine features and is here interpreted as periglacial loess overlying the fluvioglacial intertidal elatina formation so it is not a record of abrupt global warming from snowball earth other cap carbonates around the world differ in various ways and may have formed in different depositional environments but could profitably be reconsidered from the new perspective of the loess depositional model proposed here supplementary material tabular comparison of marine and non marine interpretations of cap carbonates petrographic textures and composition and new isotopic and chemical analyses of nuccaleena formation are available at http www geolsoc org uk sup18443

gas hydrate destabilization and methane release events in the krishna godavari basin bay of bengal abstract methane release events have been linked to global warming alteration of the carbon cycle and influence on biota however unequivocal evidence of paleomethane release events are limited we report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core md161 8 from the krishna godavari k g basin bay of bengal the most negative 13 c spikes are recorded during the marine isotope stages mis 4 and at the transition of mis 5 to 4 occurrence of highly 13 c depleted average 13 c 48 2 4 vpdb authigenic high magnesian calcite are also reported within this time window from the core md161 8 in the present work an unequivocal explanation for the observed 13 c depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical electron microscopy or c o stable isotope ratio analyses due to possible influence of diagenetic alteration we attribute the observed episodic methane expulsion events as inferred from the negative 13 c excursions and earlier reports on the occurrence chemosynthetic bivalves and mo concentration anomaly to the destabilization of the base of gas hydrate stability zone bghsz sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage shale and salt tectonics in the passive continental margins being a globally observed phenomenon its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time additional evidence of methane emission from site md161 15 further supports the link between shale tectonics and methane emission

hydrothermal nontronite formation associated with microbes from low temperature diffuse hydrothermal vents at the south mid atlantic ridge oceanic nontronite deposits have been identified to be closely related to low temperature hydrothermal activities however their formation mechanisms associated with microbes in diffuse hydrothermal vents still remain largely unknown the friable deposits collected from the low temperature diffuse flow at the southern atlantic ridge display a layered structure scanning electron microscope sem and transmission electron microscope tem analyses reveal that abundant filamentous spherical and rod shaped mineralized forms are preserved in the yellowish green layer of the deposits these mineralized forms primarily consist of si and fe selected area electron diffraction saed patterns of the mineralized forms indicate that they are composed of nontronite high intensities of 12c and 12c14n signals derived from cellular structures determined by nano secondary ion mass spectrometry nanosims suggest the intimate relationship between nontronite and microbes the results of 454 pyrosequencing analyses provide insights into the microbial communities involved in the biologically induced mineralization in the yellowish green layer we propose an evolutionary model for establishing paragenetic sequences among nontronite mn oxide and fe oxyhydroxide in the deposits this paragenetic sequence could be widespread in modern and ancient low temperature hydrothermal fields

unraveling sedimentary provenance and tectonothermal history of high temperature metapelites using zircon and monazite chemistry a case study from the eastern ghats belt india abstract the geochemical behavior of detrital zircon and monazite during granulite facies anatexis in metapelites from the eastern ghats belt egb india is explored using u pb geochronology hf isotopes and trace elements in a metapelite from the ongole domain detrital zircon reequilibrated by coupled dissolution reprecipitation and diffusion reaction during ultrahigh temperature metamorphism at 1 63 ga the event completely reset the u pb systems but hf isotopes and trace elements were only partially reequilibrated overgrowths on the altered cores date migmatization at 1 61 ga monazite yields metamorphic ages similar to those of zircon in metapelites from the eastern ghats province egp detrital zircon grains give 2 44 1 40 ga ages and metamorphic ones 1 2 0 5 ga ages metamorphic components include detrital grains reequilibrated by coupled dissolution reprecipitation in the presence of anatectic melt and newly crystallized overgrowths and grains in reequilibrated domains the u pb system wa