radial upper mantle attenuation structure of inactive back arc basins from differential shear wave measurements we measure differential attenuation between ss s and sscs scs phase pairs to characterize the variation of attenuation with depth in the upper mantle of five inactive back arc basins the kuril basin sea of japan banda sea the celebes and sulu seas and the shikoku basin a spectral ratio technique is used to measure the differential attenuation operator of the transversely polarized waveforms over a frequency band of 10 to 83 mhz two algorithms are employed to compute the vertically averaged attenuation structure a spectral stacking procedure and a least squares inversion in the spectral stacking method the individual spectra are corrected for the elastic structure at the ss or sscs bounce point and the differential attenuation operator is computed by spectral division the attenuation operators are then normalized and stacked by source depth to obtain more stable spectra and an average t for sources within a restricted depth range is obtained from the slope of the log amplitude spectrum a model for the depth dependent q structure is then calculated from the t measurements assuming q is frequency independent alternatively t measurements for individual phase pairs are made using a similar technique and analyzed by ray tracing and a least squares inversion to obtain the q 1 estimates the q results obtained from the stacking and inversion methods are generally in good agreement the q 1 structures for the various back arc regions are similar to each other within the uncertainties of the derived q models in addition to computing q models for each individual region we partitioned the data into three tectonic provinces based upon bounce point locations the resulting average radial q structure for an inactive basin shows a q of 54 in the uppermost mantle 115 at intermediate depths and 173 in the transition zone we also find a low q zone of 36 beneath active island arcs the q values for inactive back arc basins are lower than the global averages derived from normal modes but are generally consistent with previous body and surface wave studies of other young oceanic regions the most striking feature of this study is the observation of very strong attenuation concentrated at shallow depths 160 km in the upper mantle beneath these basins

organic haze in earth s early atmosphere source of low 13c late archean kerogens high concentrations of greenhouse gases would have been required to offset low solar luminosity early in earth9s history enhanced co 2 levels are probably at least part of the solution but ch 4 may have played a significant role as well particularly during the late archean era 2 5 3 0 ga when methanogenic bacteria were almost certainly present indeed biological ch 4 production should have led to co 2 drawdown by way of a negative feedback loop involving the carbonate silicate geochemical cycle we suggest here that the atmospheric ch 4 co 2 ratio approached the value of 1 needed to trigger formation of titan like organic haze this haze was strongly depleted in 13 c relative to 12 c and was produced at a rate comparable to the modern rate of organic carbon burial in marine sediments therefore it could provide a novel explanation for the presence of extremely low 13 c kerogens in late archean sediments

observations of first order mantle reverberations we have observed first order mantle reverberations specifically sh polarized scsn and sscsn phases reflected at near normal incidence from upper mantle discontinuities as discrete phases on long period digital seismograms of the hglp and sro networks such arrivals correspond to an n or n 1 member dynamic ray family denoted by scsn sd s where n is the multiple number of the parent phase d signifies a reflection from the top of an internal discontinuity at depth d and d signifies a reflection from the bottom the travel times and attenuation of these phases place important constraints on the nature of the transition zone we have employed the phase equalization and stacking algorithm of jordan and sipkin 1977 to obtain the differential attenuation operators of sscsn s650 s sscsn and scsn s650 s scsn phase pairs from deep focus tonga events recorded by the hglp station kip on the island of oahu the apparent q s of the zeroth order and first order reverberations over the frequency band of 10 to 30 mhz have been inverted for the average quality factors of the upper mantle qum and lower mantle qlm and the reflection coefficient of the 650 km discontinuity r 650 at 20 mhz the results are qum 82 18 qlm 231 60 and r 650 0 080 0 004 at normal incidence the qlm estimate is significantly less than recent normal mode solutions suggesting that lower mantle attenuation structure is frequency dependent at very low frequencies we have measured the differential travel times between the first order reverberations sscsn s400 s and sscsn s650 s which yields a direct estimate of the vertical shear wave travel time between the discontinuities along the tonga to kip path the observed two way time 92 2 1 5 sec is less than predicted by some recent oceanic upper mantle models indicating either a smaller separation of the discontinuities and or greater shear velocity in the transition zone measurements of differential travel times between first order reverberations and multiple scs phases suggest that the former is more likely and are consistent with depths of 400 and 650 km for the transition zone discontinuities observations of first order mantle reverberations can potentially provide unique constraints on lateral heterogeneity of the earth s mantle especially within the transition zone

a study of mantle layering beneath the western pacific the reverberative interval of the seismogram defined as the portion following the surface wave train propagating along the minor arc and ending with the first body wave arrivals from the major arc provides an excellent window in which to observe subcritical reflections from mantle discontinuities on an sh polarized seismogram the intervals between scsn and sscsn wave groups zeroth order reverberations are approximately 15 min long and consist almost entirely of waves reflected one or more times from mantle discontinuities first and higher order reverberations we have developed signal enhancement and waveform inversion schemes to progressively extract information pertaining to mantle layering from the waveforms of zeroth and first order reverberations and have applied them to a data set of long period digital seismograms drawn from a number of tectonic regions in and around the western pacific the results place important constraints on the nature of the transition zone discontinuities measurements of the two way vertical shear wave travel time from the free surface to the 400 km and 650 km discontinuities 400 and 650 respectively yield estimates of transition zone travel time tz largely uncontaminated by outside heterogeneity variations in the path averaged estimates of tz are of the order of 5 4 s indicating significant lateral heterogeneity in the transition zone correlations of tz with 400 and lm travel time through the lower mantle are consistent with but do not require the thermal deflection of an exothermic phase transition near 400 km and an endothermic transition at 650 km with 12 km of long wavelength 500 5000 km topography on each the amount of topography estimated for the 650 km discontinuity is significantly less than predicted for a rigorously stratified mantle 100 km in a search for other mantle discontinuities we find evidence for a strong reflector possibly associated with the onset of partial melting in the upper mantle average depth of 86 km due to ambiguity inherent to the method this feature can also be interpreted as an impedance increase in the d region of the lower mantle 70 170 km above the core mantle boundary no evidence is found for a 220 km discontinuity or the d discontinuity proposed by lay and helmberger 1983 suggesting that if such features are present in the western pacific they are either small normal incidence sh reflection coefficient r 1 5 or intermittent or have lateral depth variations well in excess of 50 km

influence of organics and silica on fe ii oxidation rates and cell mineral aggregate formation by the green sulfur fe ii oxidizing bacterium chlorobium ferrooxidans kofox implications for fe ii oxidation in ancient oceans most studies on microbial phototrophic fe ii oxidation photoferrotrophy have focused on purple bacteria but recent evidence points to the importance of green sulfur bacteria gsb their recovery from modern ferruginous environments suggests that these photoferrotrophs can offer insights into how their ancient counterparts grew in archean oceans at the time of banded iron formation bif deposition it is unknown however how fe ii oxidation rates cell mineral aggregate formation and fe mineralogy vary under environmental conditions reminiscent of the geological past to address this we studied the fe ii oxidizer chlorobium ferrooxidans kofox a gsb living in co culture with the heterotrophic geospirillum strain kofum we investigated the mineralogy of fe iii metabolic products at low high light intensity and in the presence of dissolved silica and or fumarate silica and fumarate influenced the crystallinity and particle size of the produced fe iii minerals the presence of silica also enhanced fe ii oxidation rates especially at high light intensities potentially by lowering fe ii toxicity to the cells electron microscopic imaging showed no encrustation of either kofox or kofum cells with fe iii minerals though weak associations were observed suggesting co sedimentation of fe iii with at least some biomass via these aggregates which could support diagenetic fe iii reduction given that gsb are presumably one of the most ancient photosynthetic organisms and pre date cyanobacteria our findings on the one hand strengthen arguments for photoferrotrophic activity as a likely mechanism for bif deposition on a predominantly anoxic early earth but on the other hand also suggest that preservation of remnants of fe ii oxidizing gsb as microfossils in the rock record is unlikely

shallow marine ironstones formed by microaerophilic iron oxidizing bacteria in terminal paleoproterozoic abstract the microbial origin of precambrian iron formations is debated due to the lack of direct fossil evidence in order to reveal the genesis of ironstones under low oxygen levels integrative studies of sedimentology petrography mineralogy and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal paleoproterozoic chuanlinggou formation ca 1 65 1 64 ga of north china using microscopy sem eds icp oes icp ms and mc icp ms techniques mineralogical study shows that the fe rich mineral is predominantly hematite that resulted from dehydration of amorphous fe oxyhydroxide during diagenesis petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus basinward increases of the ironstone abundance eu anomalies from 1 39 to 1 56 and 56fe values of the ironstones from 0 5 to 1 0 suggest that the iron was mainly sourced from seafloor hydrothermal fluids and partially oxidized and precipitated in shallow subtidal to intertidal environments the common existence of fe oxide coated sheaths spiral stalks residual extracellular polymer substances eps and other biogenic fabrics indicates that microaerophilic iron oxidizing bacteria feob may have played an important role in precipitating the chuanlinggou ironstones the extremely low oxygen concentrations implied by the proliferation of microaerophilic feob in the shallow waters the weak positive ce anomalies 0 94 1 12 and low mn concentrations in the ironstones are broadly consistent with the previous result of a cr isotope study thus the establishment of a microaerophilic feob genetic model for the widespread chuanlinggou ironstones in north china provides new insight into the origin of precambrian iron formations and the redox evolution of ocean atmosphere systems during the boring billion

systematic analysis of deep seismicity 200 centroid moment tensor solutions for earthquakes between 1977 and 1980 summary a systematic study of deep seismicity is performed using waveform data from the gdsn between 1977 and 1980 and the centroid moment tensor method of dziewonski et al two hundred new cmt solutions are found for events deeper than 100km their moments obtained using the same algorithm range between 7 x and 1 4 x lo2 dyne cm a factor of 20 000 a statistical analysis of the cmt parameters provides important conclusions among these the shift in origin time for deep events is found to be almost independent of the seismic moment providing an indication of the complexity of deep earthquakes the deviation from the double couple model appears to be correlated to the state of stress of the subducting slabs mechanisms showing down dip compression are generally dominated by the eigenvalue corresponding to the compression axis and down dip tension correlates with the predominance of the tensional eigenvalue comparison with moment tensors obtained by other authors using different waveform data shows good agreement between different methods comparison with published focal plane mechanisms confirms the precision and reliability of the cmt method examples of cmt solutions used to identify patterns of seismicity associated with deep fault planes are shown

thermal structure of jupiter s atmosphere near the edge of a 5 μm hot spot in the north equatorial belt thermal structure of the atmosphere of jupiter was measured from 1029 km above to 133 km below the 1 bar level during entry and descent of the galileo probe the data confirm the hot exosphere observed by voyager 900 k at 1 nanobar the deep atmosphere which reached 429 k at 22 bars was close to dry adiabatic from 6 to 16 bars within an uncertainty 0 1 k km the upper atmosphere was dominated by gravity waves from the tropopause to the exosphere shorter waves were fully absorbed below 300 km while longer wave amplitudes first grew then were damped at the higher altitudes a remarkably deep isothermal layer was found in the stratosphere from 90 to 290 km with t 160 k just above the tropopause at 260 mbar there was a second isothermal region 25 km deep with t 112 k between 10 and 1000 mbar the data substantially agree with voyager radio occultations the voyager 1 equatorial occultation was similar in detail to the present sounding through the tropopause region the voyager iris average thermal structure in the north equatorial belt neb approximates a smoothed fit to the present data between 0 03 and 400 mbar differences are partly a result of large differences in vertical resolution but may also reflect differences between a hot spot and the average neb at 15 4 bars probe descent velocities derived from the data are consistently unsteady suggesting the presence of large scale turbulence or gravity waves however there was no evidence of turbulent temperature fluctuations 0 12 k a conspicuous pause in the rate of decrease of descent velocity between 1 1 and 1 35 bars where a disturbance was also detected by the two radio doppler experiments implies strong vertical flow in the cloud seen by the probe nephelometer at p 0 6 bar measured temperatures were 3 k warmer than the dry adiabat possible evidence of radiative warming this could be associated with a tenuous cloud detected by the probe nephelometer above the 0 51 bar level for an ammonia cloud to form at this level the required abundance is 0 20 solar

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

differential shear wave attenuation and its lateral variation in the north atlantic region because the attenuation of seismic waves is sensitive to variations in temperature and to partial melting mapping seismic q in the upper mantle is an important tool for assessing the processes responsible for lateral heterogeneity utilizing a digital data base of over 150 seismograms and a spectral ratio technique we have measured ss s differential attenuation in the north atlantic region differential attenuation is positively correlated with ss s travel time residual and both differential attenuation and travel time residual decrease with increasing seafloor age in an earlier paper we showed that the age dependence of ss s travel time residual can be explaned entirely by the cooling of the oceanic lithosphere i e contributions from the asthenosphere or from a mantle melt fraction are not required the assumption that plate cooling also dominates the variation of differential attenuation with age permits the derivation of an empirical relation between q 1 and temperature for the oceanic thermal lithosphere i e the depth interval 0 125 km however both the absolute values and the depth distribution of q we obtain under this assumption are at variance with the results of surface wave attenuation studies and the variation of q 1 with temperature that we derive is not as strongly dependent on temperature as indicated by some laboratory measurements we therefore develop models for q in which lateral variations include contributions from the asthenospheric low q zone as well as from lithospheric cooling the q models obtained under this alternative assumption are in good agreement with those obtained from surface wave studies and are therefore preferred over those models with lateral variations confined to the upper 125 km systematic long wavelength 1000 7000 km variations in differential attenuation corrected for seafloor age are evident along the axis of the mid atlantic ridge these variations can be qualitatively correlated with long wavelength variations in ss s differential travel time residuals and are attributed to along axis differences in upper mantle temperature

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

emplacement of a large igneous province as a possible cause of banded iron formation 2 45 billion years ago latest archaean and earliest palaeoproterozoic times from 2 6 to 2 2 billion years ago have generally been viewed as a largely quiescent period of earth history the geological record indicates the very slow deposition of pelagic and chemical sediments1 2 and bears only a limited record of magmatic and tectonic activity3 5 such quiescence is consistent with the contention that the earth s main banded iron formations bifs finely laminated chemical sedimentary rocks rich in iron oxide formed slowly as oxygen abundances in the oceans gradually increased thus reducing the capacity of sea water to retain dissolved iron6 10 here we show that a large igneous province comprising 30 000km3 of dolerite basalt and rhyolite accompanied deposition of a hamersley province bif 2 449 3 million years ago this observation indicates that hamersley bifs formed during a major tectono magmatic event and were deposited very much faster than previously thought at similar rates to or faster than modern pelagic sediments thus the largest palaeoproterozoic bifs rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence are more likely to have formed as a result of an increased supply of suboxic iron and silica rich sea water upwelling onto continental shelves during a pulse or pulses of increased submarine magmatic and hydrothermal activity

a far field hydrothermal plume from loihi seamount an extensive helium plume in the north central pacific emanates from loihi seamount on the flanks of hawaii the maximum helium signal is found at a depth of about 1100 meters the same depth as the near field plume directly above loihi seamount although this helium plume is strongest near hawaii where the 3he 4he ratio at a depth of about 1100 meters reaches values 28 percent above the atmospheric ratio it can be detected quite clearly at latitude 24 n over 400 kilometers to the north excess 3he is also present on the same isopycnal between 15 n and 20 n at 135 w some 2000 kilometers east of the hawaiian islands

deformation induced silica redistribution in banded iron formation hamersley province australia abstract the formation of banded iron formations bif remains controversial despite their potential to provide key information on precambrian atmospheres and hydrospheres it is widely agreed that bif are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases many models address the chemical transformation of such precursor iron oxide phases into bif during compaction and diagenesis however the formation of chert and the influence of physical forces in this process have received less attention microstructural analysis of bif from the hamersley province western australia reveals that significant amounts of silica were redistributed by dissolution precipitation creep during both diagenesis and regional scale deformation this physicochemical process led to silica remobilisation and volume loss by stress induced dissolution of microcrystalline quartz in an aqueous fluid the dissolved solid phase was transported by diffusion and fluid flow along grain boundaries or within available porosity and then reprecipitated in low pressure zones leading to local volume increase these processes were further enhanced by rheological contrasts between different minerals resulting in significant variations of chert band thickness microstructural observations combined with quantitative microfabric analysis reveal domains of crystallographic preferred orientations cpo in quartz grains within chert layers the cpo fabrics record strain regimes e g pure and simple shear extension and shortening that modified quartz aggregates by dissolution precipitation creep providing new insights into the metamorphic and deformation history of bif we document microstructures that indicate that non coaxial deformation was active during diagenesis and subsequent deformation of the hamersley province bif further relatively undeformed chert layers may have been similarly affected by significant amounts of dissolution precipitation creep providing a possible explanation for the destruction of primary features we show that different generations of chert alternate on a micro scale suggesting that the role of deformation has been underestimated in many aspects of bif research the discovery of dissolution precipitation creep in low grade metamorphic bif provides a potential link between the physical and chemical processes that occur within these rocks after deposition in the hamersley province dissolution precipitation creep was active from the compaction or diagenesis stage and probably operated during all major extensional and compressional deformation events we also suggest that chert layers without cpo record the least overprint by diagenesis and as the most pristine form of bif should be targeted for future paleo environmental research

evaluation of siderite and magnetite formation in bifs by pressure temperature experiments of fe iii minerals and microbial biomass abstract anoxygenic phototrophic fe ii oxidizing bacteria potentially contributed to the deposition of archean banded iron formations bifs before the evolution of cyanobacterially generated molecular oxygen o 2 by using sunlight to oxidize aqueous fe ii and precipitate fe iii oxyhydr oxides once deposited at the seafloor diagenetic reduction of the fe iii oxyhydr oxides by heterotrophic bacteria produced secondary fe ii bearing minerals such as siderite feco 3 and magnetite fe 3 o 4 via the oxidation of microbial organic carbon i e cellular biomass during deeper burial at temperatures above the threshold for life thermochemical fe iii reduction has the potential to form bif like minerals however the role of thermochemical fe iii reduction of primary bif minerals during metamorphism and its impact on mineralogy and geochemical signatures in bifs is poorly understood consequently we simulated the metamorphism of the precursor and diagenetic iron rich minerals ferrihydrite goethite hematite at low grade metamorphic conditions 170 c 1 2 kbar for 14 days by using 1 mixtures of abiotically synthesized fe iii minerals and either microbial biomass or glucose as a proxy for biomass and 2 using biogenic minerals formed by phototrophic fe ii oxidizing bacteria mossbauer spectroscopy and xrd showed that thermochemical magnetite formation was limited to samples containing ferrihydrite and glucose or goethite and glucose no magnetite was formed from fe iii minerals when microbial biomass was present as the carbon and electron sources for thermochemical fe iii reduction this could be due to biomass derived organic molecules binding to the mineral surfaces and preventing solid state conversion to magnetite mossbauer spectroscopy revealed siderite contents of up to 17 after only 14 days of incubation at elevated temperature and pressure for all samples with synthetic fe iii minerals and biomass whereas 6 of the initial fe iii was reduced to sideritic fe ii in biogenic fe iii minerals during incubation based on our data magnetite in bif is unlikely to be formed by thermochemical fe iii reduction in sediments of biogenic ferrihydrite hematite or goethite dominated sediments with complex microbial biomass present instead our results suggest that diagenetic magnetite in bif was either formed by microbial fe iii reduction during early diagenesis i e below 120 c or by thermochemical fe iii reduction with simple organic compounds at higher temperatures whereas siderite was formed by both microbial diagenetic fe iii reduction and thermogenic fe iii reduction with complex biomass thermochemical fe iii reduction coupled to biomass oxidation during metamorphism provides another origin for bif siderites and could have led to a significant increase in fe ii content in bif after deposition over geological timescales

iron and carbon isotope evidence for microbial iron respiration throughout the archean abstract banded iron formations bifs are voluminous chemical sediments that are rich in iron oxide carbonate and silica and whose occurrence is unique to the precambrian their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early earth however many details regarding the role of microbial activity in bif deposition and diagenesis are unresolved laboratory studies have shown that reaction between carbon and iron through microbial iron respiration 2fe2o3 nh2o ch2o 7h 4fe2 hco3 2n 4 h2o chemical energy can impart fractionation to the isotopic compositions of these elements here we report iron 56fe vs irmm 014 and carbon isotopic 13c vs v pdb compositions of magnetite and of iron rich and iron poor carbonates in bifs from the late archean 2 5 ga hamersley basin australia and the early archean 3 8 ga isua supracrustal belt isb greenland the range of 56fe values measured in the hamersley basin including light values in magnetite and heavy values in iron rich carbonates up to 1 2 are incompatible with their precipitation in equilibrium with seawater rather the data together with previously reported light 13c values in iron rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration i e dissimilatory iron reduction dir iron and carbon isotope data of iron rich metacarbonates from the isb are similar to those of late archean bifs the isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on earth

seismic structure of the upper mantle beneath the western philippine sea abstract we investigate the seismic structure of the western philippine sea using two sets of seismological observations scs reverberations which provide the layering framework for a regional upper mantle model and observations of frequency dependent phase delays for direct s waves surface reflected phases ss ss sss and surface waves r1 g1 which constrain the velocity and anisotropy structure within the layers the combined data set comprising 17 discontinuity amplitudes and layer travel times from the scs reverberation stack and more than 1000 frequency dependent phase delays was inverted for a path averaged radially anisotropic model mineralogical estimates of the bulk sound velocity and density are incorporated as complementary constraints the final model phb3 is characterized by a 11 5 km thick crust an anisotropic lid bounded by a sharp negative g discontinuity at 89 km an anisotropic low velocity layer extending to 166 km a subjacent high gradient region and transition zone discontinuities at depths of 408 km 520 km and 664 km the lid is slower than in a comparable model for the tonga hawaii corridor pa5 but also significantly thicker requiring a compositional variation between the two regions we explore the hypothesis that the thickness of the oceanic lid is controlled by the melting depth at the spreading centers during crust formation and that the thicker crust and lid in the philippine sea results from deeper melting owing to a higher potential temperature and perhaps a higher water content in the upper mantle

imaging global body wave phases by stacking long period seismograms stacked record sections combining over 5 years of long period global digital seismograph network gdsn seismograms from shallow events show dozens of seismic body wave phases separate images are obtained for the vertical radial and transverse components using an automatic gain control algorithm which normalizes the amplitude of each trace prior to stacking these images provide an indication of the relative signal to noise characteristics of different parts of the gdsn data set core mantle boundary diffracted phases are particularly prominent in these stacks which show pdiff to 150 pkpdiff to 200 ppdiff to 230 sdiff to 160 and ssdiff to 220 high order s surface multiples can be seen extending through the surface wave train up to ranges of 540 two examples of possible upper mantle discontinuity reflected phases are visible one following ppp between about 95 and 125 the other following scs3 between about 210 and 240 an apparent offset between sh and sv arrivals is seen which becomes larger for the higher order s multiples sss ssss etc and appears consistent with models containing transverse isotropy in the upper mantle

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

evidence for a thick mantle transition zone beneath the philippine sea from multiple scs waves recorded by jisnet 1 we deployed the jisnet broadband seismic network in west and central indonesia using data from jisnet and other broadband stations we estimate the thickness of the mantle transition zone beneath east and southeast asia and the philippine sea using the scs reverberation method the thickness is about 240 km beneath east and southeast asia 280 290 km beneath the western edge of the philippine sea 260 280 km beneath the central part of the philippine sea and 260 km beneath the izu bonin arc these results imply that the temperature is about 300 k lower than the global average beneath the western edge of philippine sea 200 k lower beneath the mid part of the philippine sea 130 k lower beneath the izu bonin arc and almost equal to the average beneath east and southeast asia

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

iron in precambrian rocks implications for the global oxygen budget of the ancient earth abstract banded iron formations bif are prominent in sediments older than 2 ga however little is known about the absolute abundance of bif in archean and early proterozoic sediments and the source of the fe is still somewhat uncertain also unknown is the role that fe may have played in the maintenance of low oxygen pressures in the archean and early proterozoic atmosphere an analysis of the chemical composition of precambrian rocks provides some insight into the role of fe in precambrian geochemical cycles the fe content of igneous rocks is well correlated with their ti content plots of fe vs ti in precambrian sandstones and graywackes fall very close to the igneous rock trend plots of fe vs ti in precambrian shales also follow this trend but show a definite scatter toward an excess of fe phanerozoic shales and sandstones lie essentially on the igneous rock trend and show surprisingly little scatter mn ti relations show a stronger indication of precambrian mn loss perhaps due to weathering under a less oxidizing early atmosphere these data show that fe was neither substantially added to nor significantly redistributed in archean and early proterozoic sediments enough hydrothermal fe was added to these sediments to increase the average fe content of shales by at most a factor of 2 this enrichment would probably not have greatly affected the near surface redox cycle or atmospheric oxygen levels continued redistribution of fe and mixing with weathered igneous rocks during the recycling of precambrian sediments account for the excellent correlation of fe with ti in phanerozoic shales and for the similarity between their fe ti ratio and that of igneous rocks

primary and diagenetic controls of isotopic compositions of iron formation carbonates mineralogic chemical and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early proterozoic 2 5 ga dales gorge member of the brockman iron formation hamersley basin western australia samples were obtained both from banded iron formation bif macrobands 9 12 at paraburdoo on the southern margin of the basin and from bif macroband 13 at wittenoom 130 km nw at paraburdoo oxygen isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60 160 degrees c this range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin cf t approximately 300 degrees c at wittenoom becker and clayton 1976 carbon isotopic compositions of carbonates are near 7 vs pdb at paraburdoo and 10 5 at wittenoom but the greater isotopic depletion at wittenoom appears related to primary or diagenetic processes not metamorphism contents of organic carbon are consistently low isotopic depletion is roughly correlated with iron abundance and together with petrographic observations and chemical balances is consistent with the model of bif deposition introduced by beukes et al 1990 primary siderite delta approximately 5 precipitated from an anoxic water column depleted in 13c additional depletion of 13c is associated with coprecipitation of iron oxides and organic carbon oxygen isotopic abundances of microbanded carbonates are similar to those under and overlying massive marine carbonates ranging from 17 6 to 21 0 vs smow 9 6 to 12 9 vs pdb millimeter scale variations in abundances of 13c and 18o are associated with diagenetic replacement of primary siderite by secondary ankerite and or magnetite it is shown that these isotopic variations cannot result from mineral dependent fractionations metamorphism or the influence of large volumes of water in an open system

the geoid small scale convection and differential travel time anomalies of shear waves in the central indian ocean we have measured 37 differential travel times for the phase pairs ss s sss ss scs2 scs scs2 s sscs2 sscs and sscs2 ss each pair included a surface bounce point in the central indian ocean between 4 n and 21 s and 69 and 85 e residuals were calculated relative to both the j b travel time tables and the pem c earth model and were corrected for the elevation of the bounce points the mean j b residual associated with bounce points in the central indian basin is 2 8 0 7 s fast this contrasts with 4 2 s slow reported for the western pacific by sipkin and jordan and is similar to the 4 0 s found for old continental nucleii after correcting for known differences between continental and oceanic crustal structure and for differences in measurement technique we find that travel times in the mantle beneath the central indian basin are midway in character between those beneath pre cambrian shields and those beneath the western pacific ocean basin the fastest differential travel times occur along a line passing close to the center of the large indian ocean geoidal low there is a clear increase in travel time with distance from the center of the geoid low suggesting that the source of the geoid anomaly may also be largely responsible for the difference between the central indian and pacific basins the chagos laccadive ridge seems to coincide with a transition from the fast differential travel times of the central indian basin to slower differential travel times associated with bounce points beneath young seafloor near active spreading centers there are also variations within the central indian basin that are not related to surface geological features or to the age of the seafloor these variations are periodic with a wavelength of about 640 km and with a nne trend which agrees approximately with the trend that would be expected for linear convective rolls in the upper mantle

mapping of the 410 and 660 km discontinuities beneath the japanese islands 1 a family of core reflected shear waves generated by the 28 june 2002 vladivostok deep earthquake including near vertical reflections from the 410 and 660 km discontinuities were recorded by 500 tiltmeters of the hi net and 60 broadband seismometers of the f net in japan the observed upper mantle reflections were cross correlated with synthetics calculated on the basis of the spectral element method for a fully three dimensional earth model using the earth simulator supercomputer to accurately determine the depths of the reflection points the mapped upper mantle discontinuities were compared with a high resolution p wave tomographic image the 660 km discontinuity is depressed at a constant level of 15 km along the bottom of the horizontally extending aseismic slab under southwestern japan the transition from the normal to the depressed level occurs within a lateral distance of less than 200 km observations suggest that the reflections from the 410 km discontinuity interfere with those from slab related structures on top of this discontinuity leading to a spuriously large elevation of the 410 km discontinuity in and near the subducted slab records at stations relatively free from such interference effects however still imply elevation of this discontinuity within the slab

galileo imaging of jupiter s atmosphere the great red spot equatorial region and white ovals during the first six orbits of the galileo spacecraft s prime mission the solid state imaging ssi system acquired multispectral image mosaics of jupiter s great red spot an equatorial belt zone boundary a 5 m hot spot similar to the galileo probe entry site and two of the classic white ovals we present mosaics of each region approximating their appearance at visible wavelengths and showing cloud height and opacity variations the local wind field is derived by tracking cloud motions between multiple observations of each region with time separations of roughly 1 and 10 hr vertical cloud structure is derived in a companion paper by banfieldet al icarus135 230 250 galileo s brief high resolution observations complement earth based and voyager studies and offer local meteorological context for the galileo probe results our results show that the dynamics of the zonal jets and large vortices have changed little since voyager with a few exceptions we detect a cyclonic current within the center of the predominantly anticyclonic great red spot the zonal velocity difference between 0 s and 6 s has increased by 20 m sec 1 we measure a strong northeast flow approaching the hot spot this flow indicates either massive horizontal convergence or the presence of a large anticyclonic vortex southeast of the hot spot the current compact arrangement of two white ovals and a cyclonic structure greatly perturbs the zonal jets in that region

comparison of the structure and dynamics of jupiter s great red spot between the voyager 1 and 2 encounters abstract we have consistently navigated the highest resolution images available of jupiter s great red spot grs from the voyager iss experiment from them we have measured apparent displacements of cloud features around and within the red spot which can be interpreted as wind vectors we have used the wind field information to determine the circulation characteristics of the collar and to map the relative vorticity and horizontal divergence of the central grs region color ratios and ground based data have been used to complement these observations derived quantities from the iris data set taken simultaneously with the voyager images have been obtained in order to determine the thermal structure infrared cloud opacity nh 3 abundance and para h 2 fraction above the great red spot there is evidence of non chaotic activity in the central region of the red spot small eddy structures consisting of coherent cloud features with apparently two dimensional turbulent motions are present in this region along with larger classical cyclonic vortices the surrounding anticyclonic collar circulates with higher velocities and appears to be detached from the core by an abrupt transition region the central region appears to be elevated over the surrounding collar and exhibits evidence for upwelling and some outflow of material differences in the collar structure and velocity profiles between the voyager encounters can be attributed to differences in the ingestion rate of westward moving south equatorial belt vortices caused by a blocking south tropical zone disturbance which moving eastward overtook and passed the red spot between the two encounters this disturbance appears to temporarily increase in brightness during conjunctions with the red spot and has been tracked for 20 years

regional variation of attenuation and travel time in china from analysis of multple scs phases long period multiple scs waveforms from intermediate and deep focus earthquakes surrounding china recorded by the chinese digital seismograph network are used to estimate the multiple scs attenuation operator and differential travel times using both frequency phase equalization and stacking and time domain waveform inversion techniques theoretical considerations augmented by synthetic testing indicate that for continental regions the frequency domain approach yields a more stable estimate of whole mantle attenuation qscs than the time domain approach which is better for whole mantle travel time scs when teamed together the two afford unprecedented success with continental paths the results 16 path averaged estimates of qscs and scs constitute a significant addition to the catalog of multiple scs studies of subcontinental mantle and provide sufficiently dense coverage to invert path estimates of attenuation and travel time for regional quantities of direct tectonic relevance across the frequency band of 10 to 50 mhz we obtain high 215 values of regionalized qscs scs the tarim platform and the fold systems of northeastern china and low 170 values elsewhere on the continent defining a pattern that is well correlated with present day levels of tectonic activity the corresponding two way travel time residuals with respect to the jeffreys bullen j b tables scs scs 935 7 s vary from 1 3 1 4 to 0 8 4 3 s and are typical of other fast continental regions unlike qscs scs varies little from region to region and is correlated poorly with tectonic activity indicating that qscs is much more sensitive to changes in the current tectonic environment offshore scs for the sea of japan is 2 7 3 6 s and for the combined east china and philippine seas region is 9 6 2 7 s the latter very large residual likely reflects the low velocities associated with back arc spreading throughout the study region scs correlates well with the residuals predicted by recent upper mantle tomographic models although the observed variance exceeds the model predictions by roughly a factor of 3

h3 as an ionospheric sounder of jupiter and giant planets an observational perspective thirty years of observations of h3 on jupiter have addressed many complex questions about the physics of the ionospheres of the giant planets spectroscopy imaging and imaging spectroscopy in the

geologic evidence for archean atmospheric and climatic evolution fluctuating levels of co2 ch4 and o2 with an overriding tectonic control the 3 5 3 2 ga sedimentary record shows evidence of surface temperatures of 70 15 c nahcolite nahco 3 as a primary evaporitic mineral and an aggressive weathering regime even in the absence of land vegetation these features are best explained by a mixed ch 4 and co 2 atmospheric greenhouse in which ch 4 co 2 ratios were 1 and pco 2 was at least 100 1000 times the present value and perhaps as high as several bars the formation of large areas of continental crust at 3 2 3 0 ga including the kaapvaal and pilbara cratons resulted in the gradual depletion of atmospheric co 2 through weathering by 2 9 2 7 ga declining pco 2 was associated with climatic cooling and siderite free soils transitory ch 4 co 2 ratios of 1 may have resulted in the sporadic formation of organic haze from atmospheric ch 4 reflected in one or more isotopic excursions involving global deposition of abnormally 1 3 c depleted organic c surface temperatures of 60 c after 2 9 ga may have also increased the distribution and productivity of oxygenic photosynthetic microbes eventual lowering of new continental blocks by erosion reduced loss of atmospheric co 2 due to weathering and continued long term tectonic recycling of co 2 resulted in rising pco 2 and decreasing ch 4 co 2 ratios in the later archean and eventual reestablishment of a mainly co 2 greenhouse similar events may have been repeated in the latest archean and earliest proterozoic but gradually rising production of o 2 effectively kept ch 4 co 2 ratios to 1 at this time

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

seismic structure of the upper mantle in a central pacific corridor the seismic structure of the tonga hawaii corridor has been investigated by combining two data sets revenaugh and jordan s reflectivity profile from scs reverberations which provides travel times to and impedance contrasts across the major mantle discontinuities and 1500 new observations of frequency dependent phase delays for the three component s ss and sss body waves and the r1 and g1 surface waves which constrain the velocity structure within this layered framework the shear waves turning in the upper mantle showed significant splitting of the sh and sv components indicative of shallow polarization anisotropy the data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric radially anisotropic structure the final model pa5 is characterized by a high velocity anisotropic lid bounded at 68 km depth by a large negative g discontinuity a low velocity anisotropic layer below g extending to a small l discontinuity at 166 km an isotropic steep gradient region between 166 km and 415 km and transition zone discontinuities at 415 507 and 651 km the depth of the radial anisotropy in pa5 is shallower than in most previous studies based on surface waves and higher modes the average value of radial shear anisotropy in the lid 3 7 is consistent with the magnitude expected from the spreading controlled models of olivine orientation while anisotropy in the low velocity zone which is required by our data set could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present day plate motions on the basis of recent work by hirth and kohlstedt we suggest that the g discontinuity is caused by a rapid increase in the water content of mantle minerals with depth marking the fossilized lower boundary of the melt separation zone active during crust formation the high gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval the average shear velocity gradient in the transition zone is lower than in most previous seismic models in better agreement with the predictions for a pyrolitic composition

why is the h3 hot spot above jupiter s great red spot so hot recent observations of jupiter s great red spot indicate that the thermosphere above the storm is hotter than its surroundings by more than 700 k possible suggested sources for this heating have t

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

upper mantle shear structure beneath the northwest atlantic ocean we have modeled the sh motion from earthquakes in the northwest atlantic ocean to derive an oceanic upper mantle shear velocity model the signals were recorded on long period wwssn and canadian network stations on the east coast of north america the travel times and waveforms of seismograms in the distance range 11 16 were used to constrain the lid structure these data indicate a fast 4 75 km s lid of about 100 km thickness in the older western atlantic from 16 to 28 the s waves pass through triplications due to velocity jumps near 400 and 660 km the branches from both discontinuities are visible in the s wave data these arrivals were modeled using synthetic seismograms to obtain accurate travel times the two triplications are also apparent in ss data from 30 to 50 the triplication curves derived from the s waves agree with the ss data given the lid structure the waveforms and travel times from the more distant data put tight constraints on the shear velocities at greater depths we find shear velocities from 200 to 400 km to be slow in relation to a shear model of the canadian shield derived by a similar technique indicating continent ocean heterogeneity to about 400 km

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

mantle layering from scs reverberations 1 waveform inversion of zeroth order reverberations this is the first paper in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms here we present path averaged estimates of the two way travel times through the crust mantle scs and crust alone m the whole mantle quality factor qscs and the reflection coefficient at the m discontinuity r0 zm obtained by iterative waveform inversion of scs reverberations we show that qscs estimates derived from direct waveform inversion can be biased significantly if reverberations originating within the crust are ignored especially in continental regions where the crust is thick applying our technique which explicitly models crustal reverberations we substantially expand the geographical coverage of scs and qscs measurements available from previous studies the scs data correlate well with whole mantle travel times predicted by the harvard tomographic models both the scs data and the qscs data support the hypothesis that variations in velocity and attenuation structure between old continents and oceans persist to depths of several hundred kilometers in particular the value of qscs derived for paths crossing stable continent is very high 280 30 at 30 mhz compared with that for mature ocean basin 189 16 consistent with the thick plate model of cratons requiring that differences in thermal structure extend to several hundred kilometers the crustal thicknesses implied by the observations of path averaged m are consistent with those inferred from short range refraction data gravity measurements and surface topography the path averaged reflectivity of the m discontinuity is also tectonically correlated becoming brighter as the percentage of continental crust encountered along the path increases this result implies that on average the impedance at the base of the continental crust is significantly lower than at the base of the oceanic crust

making magnetite late again evidence for widespread magnetite growth by thermal decomposition of siderite in hamersley banded iron formations abstract banded iron formations bifs are marine chemical sedimentary rocks that were widely deposited before the start of the great oxidation event 2 45 2 32 ga they represent an important archive into the chemistry of the precambrian oceans and atmosphere and the evolution of the marine biosphere it is believed that bifs were derived from ferric oxides hydroxides that were deposited by iron oxidizing bacteria or by reaction with oxygen released by cyanobacteria hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide hydroxide precipitate whereas magnetite is thought to have formed from ferric oxides hydroxides via bacterial iron reduction and oxidation of microbial biomass however studies are equivocal about when and how magnetite formed in bifs we present the results of a detailed petrographic study of magnetite in bifs of the hamersley group western australia we find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction instead most of the magnetite formed by replacement of siderite and other ferrous rich minerals after burial our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern pilbara craton 200 350 c the thermal decomposition of siderite could account for much of the magnetite in the hamersley bifs 3feco3 h2o fe3o4 3co2 h2 the relationship between magnetite and the primary precipitates which are preserved as greenalite nanoparticles and microgranules in chert bands suggests that much of the magnetite formed in siderite rich iron silicate bands that developed by compaction of non silicified iron silicate muds these bands acted as reactive pathways for fluid flow and the formation of magnetite if most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron our results are consistent with models for the deposition of bifs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial

new observational results concerning jupiter s great red spot the changing size and aspect ratio of the great red spot from 1880 to 2000 are reviewed indicating that the length of the system has decreased significantly over the last 100 years and continues to decrease at present at a rate of 0 19 degrees per year voyager iris maps of para hydrogen fraction and potential temperature over the system are presented showing the internal structure and surrounding areas at pressures from 100 to 500 mbars in these maps the grs appears as a cold temperature anomaly and a low para fraction anomaly vertical structure analyses and wind velocity measurements are presented from data acquired by the galileo spacecraft in 1996 and 2000 respectively the vertical structure data suggest a very thick white cloud deck with a top near 800 100 mbars and an extended blue absorbing optically thick haze up to about 200 mbars with a thin stratospheric haze above it the wind field shows a substantial increase in the maximum tangential velocity from 150 to 190 m s 1 over the galileo time period the vertical structure analyses from galileo data and the wind and temperature fields from voyager data all show evidence of a tilt to the great red spot at the cloud deck from north to south and more subtly from east to west the data support the hypothesis of the grs as a tilted pancake with temporal variability to the tilt

decoupled sources of the 2 3 2 2ga yuanjiacun banded iron formation implications for the nd cycle in earth s early oceans abstract the recognized worldwide gap in bif deposition between 2 4 and 2 0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time however the recently dated 2 3 2 2 ga yuanjiacun banded iron formation bif in the north china craton ncc offers a possibility to redress these uncertainties shale normalized rare earth element yttrium ree y patterns of the bif and interlayered meta chert samples show features characteristic of other archean and paleoproterozoic bifs with hree enrichment relative to lree positive la and eu anomalies and superchondritic y ho ratios comparable to modern seawater very low al 2 o 3 e nd t 3 5 derived from interaction with a depleted mantle source and associated with high fe fluxes the second is ambient surface seawater e nd t 2 4 which obtained its signature through weathering of the nearby landmasses and associated with high si fluxes our findings suggest that the ree budget of the oceans prior to 2 3 ga was generally dominated by hydrothermal circulation of seawater through depleted mantle derived source rocks however where evolved local continental crust and or an enriched mantle source were present this positive mantle nd signal became discernible in the bifs by comparing the nd isotopic features of similar aged bifs and marine carbonates it is concluded that similar to modern oceans the early precambrian ocean was not well mixed with respect to its nd isotopic composition

the joffre banded iron formation hamersley group western australia assessing the palaeoenvironment through detailed petrology and chemostratigraphy abstract the joffre member of the brockman iron formation is by volume the largest single known banded iron formation bif in the world here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this 2 45 billion year old unit oxide bif and silicate carbonate oxide bif dominate the lithology with minor amounts of interbedded stilpnomelane mudrock stilpnomelane rich tuffaceous mudrock and calcareous mudrock besides chert and magnetite the prominent mineralogy is riebeckite ankerite hematite stilpnomelane and crocidolite the bif is characterized by an average of 50 wt sio 2 and 44 5 wt fe 2 o 3 and an overall low abundance of al 2 o 3 2 sn of 0 24 the rey patterns also show a positive la sn anomaly no ce sn anomaly and a weakly developed positive y sn anomaly iron isotopes 56 fe with positive 56 fe values of 0 04 to 1 21 suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides no epiclastic grains have been found rather submarine hydrothermal fluids and fine grained volcanogenic detritus controlled bif chemistry the former source is reflected through a constant positive eu sn anomaly throughout the core average eu sn anomaly of 1 6 with a peak of 2 1 between 100 and 155 m depth while the latter source is best reflected through the stilpnomelane rich tuffaceous mudrock consisting of volcanic ash fall tuff with relict shards set in a stilpnomelane matrix the mudrock is overlain by well preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re worked volcanic ash formed either on the seafloor or in the water column prior to deposition an enriched hree to lree pattern a high iron content 30 wt and a 56 fe value of 0 59 collectively imply that the mudrock facies interacted with the fe rich seawater prior to deposition the tio 2 zr ratio of the bif and the associated mudrocks suggest a felsic only source related to the same style of volcanics as the slightly younger woongarra rhyolites given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics we speculate that the fine grained pelagic ash particles may have sourced bio available nutrients to the surface water this would have facilitated enhanced biological productivity including bacterial fe ii oxidation which is now recorded as the positively fractionated 56 fe iron oxide minerals in the joffre bif alongside submarine hydrothermal input to the basin the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways thereby making the joffre bif poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

thermal structure and composition of jupiter s great red spot from high resolution thermal imaging abstract thermal ir imaging from space borne and ground based observatories was used to investigate the temperature composition and aerosol structure of jupiter s great red spot grs and its temporal variability between 1995 and 2008 an elliptical warm core extending over 8 of longitude and 3 of latitude was observed within the cold anticyclonic vortex at 21 s the warm airmass is co located with the deepest red coloration of the grs interior the maximum contrast between the core and the coldest regions of the grs was 3 0 3 5 k in the north south direction at 400 mbar atmospheric pressure although the warmer temperatures are present throughout the 150 500 mbar range the resulting thermal gradients cause counter rotating flow in the grs center to decay with altitude into the lower stratosphere the elliptical warm airmass was too small to be observed in irtf imaging prior to 2006 but was present throughout the 2006 2008 period in vlt subaru and gemini imaging spatially resolved maps of mid ir tropospheric aerosol opacity revealed a well defined lane of depleted aerosols around the grs periphery and a correlation with visibly dark jovian clouds and bright 4 8 m emission ammonia showed a similar but broader ring of depletion encircling the grs this narrow lane of subsidence keeps red aerosols physically separate from white aerosols external to the grs the visibility of the 4 8 m bright periphery varies with the mid ir aerosol opacity of the upper troposphere compositional maps of ammonia phosphine and para h 2 within the grs interior all exhibit north south asymmetries with evidence for higher concentrations north of the warm central core and the strongest depletions in a symmetric arc near the southern periphery small scale enhancements in temperature nh 3 and aerosol opacity associated with localized convection are observed within the generally warm and aerosol free south equatorial belt seb northwest of the grs the extent of 4 8 m emission from the seb varied as a part of the 2007 global upheaval though changes during this period were restricted to pressures greater than 500 mbar finally a region of enhanced temperatures extended southwest of the grs during the survey restricted to the 100 400 mbar range and with no counterpart in visible imaging or compositional mapping the warm airmass was perturbed by frequent encounters with the cold airmass of oval ba but no internal thermal or compositional effects were noted in either vortex during the close encounters

investigation of upper mantle discontinuities near northwestern pacific subduction zones using precursors to ssh teleseismic long period word wide standard seismograph network wwssn tangential component recordings of deep and intermediate depth earthquakes are analyzed for the presence of ss precursors denoted sxs signifying underside reflections from discontinuities at depth x below the ss reflection point above the source these ss precursors can be used to place constraints on upper mantle discontinuities in the vidnity of subducfion zones the clearest precursor is usually the sins phase the underside reflection from the moho which is observed for a number of events in the northwestem pacific for paths reflecting under the sea of okhotsk and north korea the amplitude and timing of s s relative to ss are modeled using synthetic seismograms to determine the shear wave impedance contrast at the moho and the crustal thickness in these regions the results are compared with previous work on pp precursors reflected from the moho for similar paths with consistent crustal thickness being found s s is strong for continental reflection points under north korea which has laterally varying 31 to 36 km crustal thickness and 31 2 to 18 shear wave impedance contrast at the moho in the northem sea of okhotsk region the crustal thickness is around 21 to 29 km with 26 to 28 shear wave impedance contrast at the moho stacked sins waveforms for the sea of okhotsk vary from strong isolated arrivals in the north to weak poorly defined precursors toward the south reflecting a transition from continental to oceanic crust s s is not clearly separated from ss for oceanic paths under the izu and japan regions and our procedure cannot resolve oceanic moho properties unless broadband data are used other precursors sxs are much weaker than sins and are difficult to identify in individual waveforms we use slant stacking to enhance the signal to noise and search for precursors with various slownesses using recordings from 13 deep events four sii wave reflectors may exist below the moho in our study area the shallowest is the 80 km discontinuity which varies in depth laterally being near 80 to 85 km below the sea of okhotsk near 66 km beneath izu japan and near 90 km under north korea a reflector near 200 to 210 km depth is indicated by data from japan but there is no evidence for any shear wave impedance contrast around this depth under north korea or the sea of okhotsk weak evidence is found for a 330 km discontinuity in these regions with the depth of a few percent impedance contrast varying from 325 to 335 km relatively strong and consistent arrivals indicate the presence of a 400 km discontinuity with the depth varying from 380 to 400 kin perhaps indicating slight elevation of the olivine phase transformation near the slabs

from bif to red beds sedimentology and sequence stratigraphy of the paleoproterozoic koegas subgroup south africa abstract the 2 4 ga old koegas subgroup of the transvaal supergroup contains siliciclastics and iron formations it occupies a transitional stratigraphic position between the major iron formations on the kaapvaal craton and strata with evidence for oxygenation of the environment koegas siliciclastics reflect a new phase of uplift and erosion on the craton after a period of chemical deposition paleocurrent data indicate transport to the west and northwest away from the craton however no precise source area can as yet be defined rocks were deposited in foreshore to offshore environments of a prograding delta or submarine fan system immature arkosic wackes and subarkoses occur in proximal parts and pass to terrigenous mudstones and chemical iron formations in offshore environments with low detrital input lateral facies transitions and the hierarchical stratigraphic stacking pattern of the koegas subgroup allow a sequence stratigraphic interpretation in which regressive coarse siliciclastics prograded repeatedly over transgressive units of chemical iron formations metals in the transgressive units have an intrabasinal hydrothermal source similar to underlying iron formations localized iron stained detrital units resemble red beds they reflect additional diagenetic fe remobilization from deeper water iron formations or possibly a continental fe source the second possibility would imply the presence of some free oxygen in the environment

the antiquity of oxygenic photosynthesis evidence from stromatolites in sulphate deficient archaean lakes the tumbiana formation about 2700 million years old was largely deposited in ephemeral saline lakes as judged by the unusual evaporite paragenesis of carbonate and halite with no sulfate stromatolites of diverse morphology occur in the lacustrine sediments some with palimpsest fabrics after erect filaments these stromatolites were probably accreted by phototropic microbes that from their habitat in shallow isolated basins with negligible sulfate concentrations almost certainly metabolized by ozygenic photosynthesis

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

jupiter s ionosphere results from the first galileo radio occultation experiment the galileo spacecraft passed behind jupiter on december 8 1995 allowing the first radio occultation measurements of its ionospheric structure in 16 years at ingress 24 s 68 w the principal peak of electron density is located at an altitude of 900 km above the 1 bar pressure level with a peak density of 105 cm 3 and a thickness of 200 km at egress 43 s 28 w the main peak is centered near 2000 km altitude with a peak density of 2 104 cm 3 and a thickness of 1000 km two thin layers possibly forced by upwardly propagating gravity waves appear at lower altitudes in the ingress profile this is the first in a two year series of observations that should help to resolve long standing questions about jupiter s ionosphere

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

deposition of banded iron formations by anoxygenic phototrophic fe ii oxidizing bacteria the mechanism of banded iron formation bif deposition is controversial but classically has been interpreted to reflect ferrous iron fe ii oxidation by molecular oxygen after cyanobacteria evolved on earth anoxygenic photoautotrophic bacteria can also catalyze fe ii oxidation under anoxic conditions calculations based on experimentally determined fe ii oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that even in the presence of cyanobacteria anoxygenic phototrophs living beneath a wind mixed surface layer provide the most likely explanation for bif deposition in a stratified ancient ocean and the absence of fe in precambrian surface waters

thickness of the mantle transition zone beneath the south pacific as inferred from analyses of scs reverberated and ps converted waves abstract we study the thickness of the mantle transition zone beneath the south pacific in order to determine temperature anomalies associated with presumed hot plumes in the south pacific superswell the scs reverberation method is used to determine the average thickness between an event station pair and the velocity spectrum stacking vss method is used to determine the thickness near each station the thickness obtained by the scs reverberation method shows that the average thickness beneath the superswell is less than the globally averaged thickness by 6 km while the average thickness beneath the tonga fiji subduction is greater than the global average by 10 km the vss method shows nearly normal thickness beneath stations used in the superswell region we conclude that substantial thinning of the transition zone associated with presumed hot plumes occurs locally beneath the society hot spot as revealed by our previous ss precursor study and is not widespread beneath the whole area of the superswell

quasi love phases between tonga and hawaii observations simulations and explanations seismograms of some shallow tonga earthquakes observed at hawaii contain sv polarized phases in the love wave time window most prominently on the vertical component given the geometry of the observations 40 45 such phases may be explained either as body waves or as mode converted surface waves detailed synthetic seismogram modeling of representative events reveals several instances where the body wave explanation is inadequate even when plausible uncertainties in the source mechanism are taken into account the observed sv polarized phase can instead be generated through love rayleigh scattering which requires laterally varying seismic anisotropy along the tonga hawaii path trial and error forward modeling with simple structures based on the transversely isotropic mid pacific velocity model pa5 of gaherty et al 1996 obtains velocity structure that yields synthetic seismograms matching the observations this model while non unique suggests first order constraints on the lateral variation in anisotropic properties and associated mantle flow along the tonga hawaii path by examining trade offs in model parameters we conclude that robust features of the model are 1 a transition from radial to mixed radial and azimuthal anisotropy 3 5 from hawaii 2 the nw se alignment of the axis of azimuthal anisotropy 3 higher degree of p anisotropy relative to s anisotropy and 4 the presence of azimuthal anisotropy within upper 200 250 km of the mantle taken together these features imply a disruption of mantle fabric by the processes forming hawaii emperor volcanic system a model with anisotropic gradients in both the lithospheric lid and shallow asthenosphere is the simplest extension of our starting model however an equivalent data fit can be obtained if the azimuthal anisotropy gradients are restricted to line beneath the high velocity lid of model pa5 so that mantle hot spot flow need not penetrate the lithospheric lid

alternating si and fe deposition caused by temperature fluctuations in precambrian oceans precambrian banded iron formations contain thin alternating layers of iron and silica rich minerals microbial culture experiments show that these alternations could be caused by changes in the relative dominance of microbial iron oxidation and abiotic silica precipitation driven by fluctuating ocean temperature

teleseismic travel time residuals in north america and anelasticity of the asthenosphere abstract the slope a of the relation t s a t p b for the teleseismic s and p travel time residuals t s and t p is practically the only robust observational value that is equally sensitive to the lateral s and p velocity variations in the upper mantle published estimates of a for north america are about twice higher than those at ultrasonic frequencies in olivine at comparable pressures and temperatures in our study lateral variations of the s residuals in north america are found not from the s wave readings which can be easily biased but from accurately determined delays relative to p of the ps phases converted from 410 km and 660 km discontinuities nevertheless the estimates of a thus derived are close to the values obtained with the standard technique the discrepancy between the ultrasonic and seismic estimates of a can be explained by physical velocity dispersion on the assumption of a single absorption band our value of a is consistent with the published values of q in the seismic frequency band and velocities in the ultrasonic range if the high frequency cutoff of the absorption band in the upper mantle beneath western north america is on the order of several hundred hertz however in the literature there are indications of a cutoff at around 1 hz then our data imply the presence of another absorption band at high frequencies outside the seismic frequency band this band may correspond to the viscous dissipation in the walsh walsh j b 1969 a new analysis of attenuation in partially melted rock j geophys res 74 4333 4337 model of penny shaped inclusions of melt

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

topography of the 410 km and 660 km discontinuities beneath the japan sea and adjacent regions by analysis of multiple scs waves the northwest pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities due to the sparse distribution of seismic stations in the sea previous studies mostly focus on mantle transition zone mtz structures beneath continents or island arcs leaving the vast area of the japan sea and okhotsk sea untouched in this study we analyzed multiple scs reverberation waves and a common reflection point stacking technique was applied to enhance consistent signals beneath reflection points a topographic image of the 410 km and 660 km discontinuities is obtained beneath the japan sea and adjacent regions one dimensional and 3 d velocity models are adapted to obtain the apparent and true depth we observe a systematic pattern of depression 10 20 km and elevation 5 10 km of the 660 with the topography being roughly consistent with the shift of the olivine phase transition boundary caused by the subducting pacific plate the behavior of the 410 is more complex it is generally 5 15 km shallower at the location where the slab penetrates and deepened by 5 10 km oceanward of the slab where a low velocity anomaly is observed in tomography images moreover we observe a wide distribution of depressed 410 beneath the southern okhotsk sea and western japan sea the hydrous wadsleyite boundary caused by the high water content at the top of the mtz could explain the depression the long history trench rollback motion of pacific slab might be responsible for the widely distributed depression of the 410 ranging upward and landward from the slab

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

reactions between basalt and co2 rich seawater at 250 and 350 c 500bars implications for the co2 sequestration into the modern oceanic crust and the composition of hydrothermal vent fluid in the co2 rich early ocean abstract this study aims to understand how basaltic rocks absorb co 2 in high temperature alteration zones in the subseafloor and to reconstruct hydrothermal alteration processes such as carbonatization of archean greenstones to this end we conducted two laboratory experiments simulating hydrothermal reactions between basalt synthesized under quartz fayalite magnetite oxygen fugacity and co 2 rich nacl fluid ph 6 5 at 25 c at high temperature and pressure as the water rock reactions progressed at 250 c and 350 c 500 bars total carbonic acid concentration co 2 reduced from its initial 400 mmol kg to near 0 and 100 mmol kg respectively meanwhile calcite was formed in the basalt as an alteration mineral this indicates that calcite destabilizes as temperature increases in the h 2 o co 2 basalt system and that crustal basalts can absorb almost all co 2 in the fluid as calcite at least at temperatures and initial co 2 concentrations below 250 c and 400 mmol kg respectively although the second aim was realized in the experiments minerals such as sericite dolomite ankerite and siderite present in archean greenstones were not identified in the alteration products possibly because k mg and fe were lacking in the initial solutions steady state concentrations of sio 2 mg and k in the fluids during water rock reactions were similar to those of high temperature fluids 250 c in modern basalt hosted hydrothermal systems however the final experimental ph in situ was 6 6 and 7 2 at 250 c and 350 c respectively higher than that in modern hydrothermal fluids approximately 5 and higher than the neutral ph 5 5 5 6 at 250 350 c 500 bars the results suggest that the presence of abundant co 2 in the initial fluid induced carbonatization of basalt consequently ph was buffered by precipitation and dissolution of calcite because ph in situ was elevated the dissolved fe and mn concentrations in the fluid were two to three orders of magnitude lower than those of modern hydrothermal fluids in modern oceans high temperature hydrothermal vent fluids are the second largest iron source after riverine input however because alkaline metal poor hydrothermal fluids are generated in co 2 rich systems co 2 rich seafloor hydrothermal systems may have behaved as iron sinks in early oceans

tectonic sedimentology sediment systems deciphering global to local tectonics tectonic sedimentology has broad appeal because it can be applied at many time and length scales such as sedimentary consequences of changing rates of plate creation at mid ocean ridges continent continent collisions plateaux uplift active plate margins extensional rift tectonics and the birth growth and death of individual folds and faults tectonic sedimentology deals with the direct influence of neotectonics on sedimentation and basin development and with the inverse problem of reconstructing the order and development of palaeotectonic structures this review attempts to show how the scope and importance of the subject has developed over the last decade and is divided into five themes i at the largest tectonic scale constraints on mid ocean ridge activity and continent continent collisions come from a precise chemical analyses of fluid inclusions in marine halite deposits dating back to ca 550 ma b sr isotopic ratios of marine phanerozoic calcitic fossils and c chemical composition of archaean and proterozoic banded iron formations ii at regional scales constraints on the timing and extent of continental plateau uplift incision and cratonic tilting are given by a palaeoaltimetry estimates from oxygen isotope depletion in carbonate palaeosols b radiometric dating of palaeowater table speleothems and c preferential river migration iii at basin scales catchment processes are emphasized as key controls on sediment flux from tectonic uplands to sedimentary basins the inadequacy of sediment transport algorithms based on tractive stress and flow power to describe erosion incision and sediment transport during the most formative catchment hydrological events is emphasized iv rates of basin scale fault and fold growth displacement death and basement uplift are provided by application of facies analysis and sequence stratigraphy to accurately dated sediments in sedimentary basins v the effects of changing climate and varying basin base level in modifying tectonic influences on erosion and sedimentation are examined in such cases experimental and numerical modelling using multi variate parameters of subsidence rate sediment supply and sea level change can shed light on the problem future developments will undoubtedly improve such models hopefully establishing full linkage and feedback between sediment water supply and changing climate within an overall tectonic framework that may itself be changing

multiple scs attenuation and travel times beneath western north america variations in attenuation and travel time of multiple scs whole mantle reverberations indicate very strong lateral gradients in upper mantle properties beneath the tectonically active province of western north america whole mantle attenuation estimates qscs obtained by spectral stacking of phases sampling discrete subregions span the entire range previously determined using a similar procedure for a global set of continental and oceanic qscs measurements the relationship between travel time and attenuation anomalies which has been used to argue for predominantly thermal control on the variations in oceanic environments is poorly defined within the continental regime however a correlation between surface heat flow and qscs variations in the united states indicates that thermal variations do control attenuation heterogeneity for paths beneath mexico stacking 13 pairs of scs phases from the 24 october 1980 huajuapan mexico and the 19 june 1982 el salvador events yields qscs 142 10 in the frequency band 7 8 to 46 9 mhz time domain measurements for the same phases yield a similar estimate of qs s 148 9 the average scsn l scs n jb jeffreys bullen travel time residual htscs is 0 9 sec slower than jb all of the differential travel time estimates are corrected for dispersion effects in the passband of the observations and are appropriate for a reference period of about 30 sec multiple scs arrivals from the 29 april 1965 puget sound earthquake recorded by wwssn csn lrsm and pasadena stations provide the first determination of whole mantle attenuation beneath the westernmost united states pasadena recordings from three instruments provide scs paths traversing the cascade and sierra nevada ranges stacking 9 scs pairs yields qscs 208 39 7 8 to 54 7 mhz and the same phases give htscs 2 5 sec six pairs of scs reverberations under the pacific northwest give qs s 344 88 15 6 to 54 7 mhz and atscs 5 5 sec a combined estimate for the relatively low heat flow region of the pacific borderlands gives qscs 257 20 7 8 to 46 9 mhz comparable to the high qscs estimates previously determined for south america on the other hand 12 scsn pairs sampling the northern basin and range which has high heat flow yield a very low qs s 95 4 7 8 to 31 3 mhz with htscs 5 1 sec the latter qscs value is even lower than estimates for young oceanic regions suggesting extensive partial melting in the low velocity zone under the northern basin and range

benthic perspective on earth s oldest evidence for oxygenic photosynthesis the great oxidation event goe is currently viewed as a protracted process during which atmospheric oxygen increased above 10 5 times the present atmospheric level pal this threshold represents an estimated upper limit for sulfur isotope mass independent fractionation s mif an archean signature of atmospheric anoxia that begins to disappear from the rock record at 2 45 ga however an increasing number of papers have suggested that the timing for oxidative continental weathering and by conventional thinking the onset of atmospheric oxygenation was hundreds of million years earlier than previously thought despite the presence of s mif we suggest that this apparent discrepancy can be resolved by the earliest oxidative weathering reactions occurring in benthic and soil environments at profound redox disequilibrium with the atmosphere such as biological soil crusts and freshwater microbial mats covering riverbed lacustrine and estuarine sediments we calculate that oxygenic photosynthesis in these millimeter thick ecosystems provides sufficient oxidizing equivalents to mobilize sulfate and redox sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant s mif signature as continental freeboard increased significantly between 3 0 and 2 5 ga the chemical and isotopic signatures of benthic oxidative weathering would have become more globally significant from a mass balance perspective these observations help reconcile evidence for pre goe oxidative weathering with the history of atmospheric chemistry and support the plausible antiquity of a terrestrial biosphere populated by cyanobacteria well before the goe

loihi seamount hawaii a mid plate volcano with a distinctive hydrothermal system the initial discovery of deep sea hydrothermal vents at the galapagos rift1 2 has resulted in a decade of exploration and experimentation subsequent discoveries in the pacific3 8 and atlantic9 ocean basins have established the importance of these features as sources of mantle derived gases and solutes10 2 and as loci for heat dissipation13 14 polymetallic sulphide mineral deposition15 16 and geochemical exchange17 19 the presence of dense communities of bacteria and specialized macrofauna20 22 has called into question the absolute role of sunlight as the principal energy source for marine organisms until recently these deep sea vents were known only at or near plate boundaries but recent dredging23 24 near bottom water sampling25 27 and sea floor camera survey28 programmes have indicated that hydrothermal vents are present at the summit of loihi seamount a mid plate hotspot submarine volcano at the southern end of the hawaiian island chain29 here we report results obtained from sampling two active vent fields the fluid chemistry especially high co2 and fe presence of alkalic lavas and iron depositing bacteria and absence of macrofauna suggest that mid plate vents may differ from the hydrothermal vents found at mid ocean ridges

mantle layering from scs reverberations 4 the lower mantle and core mantle boundary this is the final installment in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms in this paper long period sh polarized multiple scs phases reflected once from a mantle discontinuity first order reverberations are used to search for abrupt shear wave impedance contrasts in the lower mantle beneath the geographic regions sampled the depth interval of 1000 2600 km bullen s region d appears free of any distinct radial layering in agreement with the majority of recent seismic models and the notion of near adiabatic compression of a compositionally homogeneous lower mantle to pass undetected discontinuities in d must be either small sh polarized reflection coefficient r0 1 0 highly intermittent or subject to lateral depth variations well in excess of 50 km at greater depths corresponding to the d region of the lowermost mantle we find evidence for a reflector of long period seismic energy situated an average of 325 km above the core mantle boundary cmb similar to the discontinuity proposed by t lay and coworkers our results indicate a 4 4 increase in shear wave impedance and a 1 7 increase in density if the 2 75 increase in shear velocity proposed by lay and helmberger 1983 holds true for our study area the discontinuity is observed beneath less than one third of the 18 seismic corridors examined and does not appear to be a ubiquitous feature of the lower mantle this plus an observed correlation of discontinuity depth and lower mantle velocity heterogeneity and the lack of experimental evidence for deep mantle phase transformations strongly favor a compositional origin mantle side layering nearer the cmb perhaps associated with a thin chemical boundary layer would mimic the effects of the surface crust producing a complex wave train of first and higher order reverberations superposed upon all other reverberative interval arrivals if present and unaccounted for cmb structure would drive estimates of crustal structure and qscs obtained by multiple scs waveform inversion away from their true values compared to independent estimates our results admit little bias monte carlo tests are used to assess the sensitivity of this technique to cmb layering which decreases with decreasing boundary layer thickness nonetheless the results limit cmb transition zones in excess of 20 km to impedance contrasts less than 12 8 for a 40 km transition beneath much of the western pacific australia melanesia and indonesia