87sr86sr in late proterozoic carbonates evidence for a mantle event at 900 ma ago abstract strontium isotopic measurements were made on late proterozoic carbonates from west african craton comparison of samples with acceptable trace element patterns with coeval data from southern africa and with the published australian results suggests that the 87 sr 86 sr ratio of the late proterozoic sea water evolved in the following manner about 0 7075 at 1000 50 ma 0 7056 to 0 7074 at 900 50 ma 0 7068 to 0 7091 0 7106 at 800 50 ma 0 7074 to 0 7077 at 700 50 ma and 0 7076 to 0 7089 0 7096 at 600 50 ma ago the variations are comparable in magnitude and frequency to those described previously for the phanerozoic strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of sr into late proterozoic oceans was of comparable isotopic composition to its present day counterpart 0 711 consequently the non radiogenic 87 sr 86 sr value at 900 50 ma ago signifies a large flux of mantle strontium into the ocean at this time because the present time resolution is only about 75 25ma additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed late proterozoic sea water curve

facies and sequence controls on the appearance of the cambrian biota in southwestern mongolia implications for the precambrian cambrian boundary neoproterozoic cambrian rocks of the zavkhan basin govi altay western mongolia com prise large scale alternations of siliciclastic and carbonate dominated units cf grand cycles analysis of such dcpositional sequences near the base of the cambrian confirms that the distribution of trace fossils small shelly fossils and calcimicrobial structures was strongly controlled by ecology and taphonomy corre sponding to specific points in a sea level cycle evolution of the cambrian biota is thus viewed through a series of narrow time windows once only for each dcpositional cycle correlation of the precambrian cambrian boundary level on the basis of the first appearance of the phycodes pedum assem blage is also fraught with difficulty since stratigraphic resolution may be limited to a single sea level cycle c 1 5 ma it is suggested that in many cases basin analysis will need to be undertaken before this bound ary can be drawn

geophysical evidence for the locations shapes and sizes and internal structures of magma chambers beneath regions of quaternary volcanism this paper is a review of seismic gravity magnetic and electromagnetic techniques to detect and delineate magma chambers of a few cubic kilometres to several thousand cubic kilometres volume a dramatic decrease in density and seismic velocity and an increase in seismic attenuation and electrical conductivity occurs at the onset of partial melting in rocks the geophysical techniques are based on detecting these differences in physical properties between solid and partially molten rock although seismic refraction techniques with sophisticated instrumentation and analytical procedures are routinely used for detailed studies of crustal structure in volcanic regions their application for magma detection has been quite limited in one study in yellowstone national park u s a fan shooting and time term techniques have been used to detect an upper crustal magma chamber attenuation and velocity changes in seismic waves from explosions and earthquakes diffracted around magma chambers are observed near some volcanoes in kamchatka strong attenuation of shear waves from regional earthquakes interpreted as a diffraction effect has been used to model magma chambers in alaska kamchatka iceland and new zealand one of the most powerful techniques in modern seismology the seismic reflection technique with vibrators was used to confirm the existence of a strong reflector in the crust near socorro new mexico in the rio grande rift this reflector discovered earlier from data from local earthquakes is interpreted as a sill like magma body in the kilauea volcano hawaii mapping seismicity patterns in the upper crust has enabled the modelling of the complex magma conduits in the crust and upper mantle on the other hand in the usu volcano japan the magma conduits are delineated by zones of seismic quiescence three dimensional modelling of laterally varying structures using teleseismic residuals is proving to be a very promising technique for detecting and delineating magma chambers with minimum horizontal and vertical dimensions of about 6 km this technique has been used successfully to detect low velocity anomalies interpreted as magma bodies in the volume range 10 3 10 6 km 3 in several volcanic centres in the u s a and in mt etna sicily velocity models developed using teleseismic residuals of the cascades volcanoes of oregon and california and kilauea volcano hawaii do not show appreciable storage of magma in the crust however regional models imply that large volumes of parental magma may be present in the upper mantle of these regions in some volcanic centres teleseismic delays are accompanied by p wave attenuation and linear inversion of spectral data have enabled computation of three dimensional q models for these areas the use of gravity data for magma chamber studies is illustrated by a study in the geysers clear lake volcanic field in california where a strong gravity low has been modelled as a low density body in the upper crust this body is approximately in the same location as the low velocity body delineated with teleseismic delays and is interpreted as a magma body in yellowstone national park magnetic field data have been used to map the depth to the curie isotherm and the results show that high temperatures may be present at shallow depths beneath the yellowstone caldera the main application of electrical techniques in magma related studies has been to understand the deep structure of continental rifts electromagnetic studies in several rift zones of the world provide constraints on the thermal structure and magma storage beneath these regions

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

proterozoic supercontinent its latest precambrian rifting breakup dispersal into smaller continents and subsidence of their margins evidence from asia several large cratons and numerous smaller blocks all with precambrian sialic crust are seen clearly in the present day structure of asia the outer parts of larger precambrian terranes and entire smaller ones are covered with vendian early paleozoic shallow water carbonate sedimentary rocks coupled with the so called old late vendian early cambrian phosphorites in siberia southern china india kazakhstan and mongolia these sedimentary rocks are interpreted to represent a global belt of near equatorial sedimentation on passive continental margins of numerous vendian early paleozoic continents with free access to the world ocean later precambrian rifting is recorded clearly in many precambrian terranes of asia rifting was repeated during cenozoic time with the same pattern and in the same places it was preceded and accompanied by various magmatic activity from 750 to 600 ma tillites filling rifts are about 650 ma and are believed to be a feature of a single proterozoic super continent rift to drift transition as attested by the beginning of thick and laterally continuous carbonate sedimentation on passive margins seems to be diachronous it started in early vendian time or even earlier in some places but not earlier than cambrian in others

methane hydrate in the global organic carbon cycle the global occurrence of methane hydrate in outer continental margins and in polar regions and the magnitude of the amount of methane sequestered in methane hydrate suggest that methane hydrate is an important component in the global organic carbon cycle various versions of this cycle have emphasized the importance of methane hydrate and in the latest version the role of methane hydrate is considered to be analogous to the workings of an electrical circuit in this circuit the methane hydrate is a condenser and the consequences of methane hydrate dissociation are depicted as a resistor and inductor reflecting temperature change and changes in earth surface history these consequences may have implications for global change including global climate change

late precambrian pennatulids sea pens from the eastern yangtze gorge china paracharnia gen nov abstract pennatulids preserved as membranous carbonised compressions from the shibantan member dengying formation sinian system in the eastern yangtze gorge hubei province represent the first genuine discovery of the late precambrian ediacara type metazoan remains in china the specimens were initially referred to charnia ford 1958 and described as c dengyingensis by ding and chen in 1981 a re examination of the type material and comparing it with other late precambrian frond like forms particularly those from the ediacara assemblage of south australia and from the charnwood assemblage of england indicate that this chinese species represents a distinct genus paracharnia gen nov its characteristic polyp leaves individually free standing on distinct leaf stalks along the lateral sides of the median stem resemble those of many living sea pens represented by pennatula and contribute a significant feature to the precambrian charniidae glaessner 1979 its close association with abundant macroscopic algal remains of vendotaenia sp and its proximity to the overlying basal cambrian small shelly fossil assemblages in the same succession emphasise its palaeontological and biostratigraphic significance

diverse lavas from closely spaced volcanoes drawing from a common parent emmons lake volcanic center eastern aleutian arc abstract emmons lake volcanic center elvc on the lower alaskan peninsula is one of the largest and most diverse volcanic centers in the aleutian arc since the middle pleistocene eruption of 350 km3 of basalt through rhyolite has produced a 30 km arc front chain of nested calderas and overlapping stratovolcanoes elvc has experienced as many as five major caldera forming eruptions the most recent at 27 ka produced 50 km3 of rhyolitic ignimbrite and ash fall these violent silicic events were interspersed with less energetic but prodigious outpourings of basalt through dacite holocene eruptions are mostly basaltic andesite to andesite and historically recorded activity includes over 40 eruptions within the last 200 yr all from pavlof volcano the most active site in the aleutian arc geochemical and geophysical observations suggest that although all elvc eruptions derive from a common clinopyroxene spinel plagioclase fractionating high aluminum basalt parent in the lower crust magma follows one of two closely spaced but distinct paths to the surface under the eastern end of the chain magma moves rapidly and cleanly through a relatively young 28 ka hydraulically connected dike plexus steady supply short magma residence times and limited interaction with crustal rocks preserve the geochemistry of deep crustal processes below the western part of the chain magma moves haltingly through a long lived 500 ka and complex intrusive column in which many generations of basaltic to andesitic melts have mingled and fractionated buoyant silicic melts periodically separate from the lower parts of the column to feed voluminous eruptions of dacite and rhyolite mafic lavas record a complicated passage through cumulate zones and hydrous silicic residues as manifested by disequilibrium phenocryst textures incompatible element enrichments and decoupling of rees and hfses ratios such features are absent in mafic lavas from the younger part of the chain highlighting the importance of plumbing architecture and longevity in creating petrologic diversity supplemental data include 156 major element xrf and 128 trace element icp ms whole rock analyses 23 new 40ar 39ar ages a generalized geologic map with associated unit descriptions and field photographs and photomicrographs of key petrographic features

new c isotope stratigraphy from southwest china implications for the placement of the precambrian cambrian boundary on the yangtze platform and global correlations a high resolution c isotope profile from a fossiliferous section spanning the precambrian cambrian pc c boundary on the yangtze platform southwest china is used to constrain c isotope chemostratigraphy and its global correlation the discovery of a strong negative c isotopic anomaly just below the china marker a indicates that this negative anomaly can be correlated globally and demonstrates that the pc c boundary of the chinese successions should be close to marker a on the basis of c isotope chemostratigraphic constraints and biostratigraphic data we find that c isotopic peak s2 also first reported here from the chinese successions can be correlated with c isotopic peak i in northern siberia and c isotope feature d in southwest mongolia by integrating the data from archaeocyaeothan fauna the c isotopic peak s3 in the middle dahai carbonates may be correlative with feature g in the mongolian section

first ediacaran fauna found in western africa and evidence for an early cambrian glaciation ediacara type body fossils are reported here for the first time from the west african craton in algeria this assemblage comprises medusoids comparable to medusinites sp and nimbia a trace fossil and a new genus of body fossil with uncertain affinity the fossils are in sandstones of the neoproterozoic cover separated by a major unconformity from the overlying tillite considered until now to be late neoproterozoic most ediacaran fauna radiated after the varanger ice age and tillites associated in ahaggar with molasse of the pan african belt are overlain by carbonates that were deposited during deglaciation and that locally contain early cambrian shelly fossils therefore we interpret the glaciogenic diamictite as early cambrian in age

structure and evolution of the hengill grensdalur volcanic complex iceland geology geophysics and seismic tomography recent geological and geophysical research indicates the presence of three volcanic systems in the hengill grensdalur area and progressive westerly migration of the accretionary plate boundary the hengill system comprises the active hengill central volcano and a nne trending fissure swarm that is the present locus of crustal accretion in the area extinct volcanic systems are identified with the mountain hromundartindur and the grensdalur area and associated nne trending zones the grensdalur system was formerly the locus of accretion in the area a widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems a tomographic study of the upper 5 km of crust using local earthquakes imaged three bodies with velocities up to 15 higher than the average background velocities and volumes of several tens of cubic kilometers one of these underlies the grensdalur volcano and a second underlies the olkelduhals area within the hromundartindur system these are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them the third high velocity body underlies the extinct basalt shield husmuli which is not associated with geothermal resources that body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site a low velocity body with a volume of a few cubic kilometers was imaged in the depth range 2 4 km beneath the northern part of the presently active hengill central volcano this volume may contain partial melt and represent the heat source or part of the heat source fueling the hengill field heat balance calculations show that subsurface magmatism continued in the extinct grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the hengill system

preliminary hydrogeological assessment of late cretaceous tertiary ardley coals in part of the alberta basin alberta canada abstract a preliminary hydrogeological evaluation was undertaken on the gas potential of shallow coals in the pembina warburg exploration area in the alberta basin alberta canada regional data for the late cretaceous tertiary ardley coal zone acz were compiled and supplemented with site specific data collected from a key test well drilled as part of a regional exploration program limited regional pressure data suggest hydraulic communication between the uppermost ardley with the overlying paskapoo formation a comparison between hydraulic head and topography suggests that flow at least in part of the ardley paskapoo is gravity driven however a decoupling of the hydraulic regime appears evident from pressure test data in beds stratigraphically below the uppermost scollard ardley and above the base of scollard at least in the eastern part of the study area where the test well was drilled the decoupling is evident in regional pressure data but the precise stratigraphic position may vary regionally formation waters typically are na hco 3 type with salinities as tds of less than 2000 mg l anomalously high bicarbonate dissolved inorganic carbon or dic concentrations exceeding 1500 mg l with 13 c dic 22 50 and dissolved methane identified in formation waters collected directly from ardley coal in test well 103 point to the presence of secondary biogenic gas the 13 c isotopes for dic coupled with 18 o and 2 h isotopes for associated groundwater and regional hydraulic data suggest that the uppermost ardley coal zone in the eastern part of the study area is part of a regional topographically driven dynamic flow system in which methanogenic processes are modifying groundwater chemistry and gas charging parts of the area whether or not biogenic gas charging in the ardley is pervasive is uncertain the relatively small coal data set requires that further exploration in the study area should consider the presence of microbial gas and the potential for hydrogeological controls on its distribution however further detailed testing will be necessary to develop a consistent and useful database for exploration and development

the behavior of rare earth elements in seawater precise determination of variations in the north pacific water column abstract we present new highly precise and accurate data for ten rare earth elements rees in two water column profiles and in bottom waters from three additional localities in the north pacific our results presented here and in an earlier publication indicate that the water column in the western north pacific is stratified with respect to the isotopic composition of nd as well as nd la sm nd and er la both the nd isotopic and the ree data show that ree transport is strongly influenced by horizontal advection even in the sluggish circulation of the pacific in general rees increase in concentration with depth at mid latitude they reach a maximum concentration at about 3000 m and then decrease to the bottom at high latitude heavy rees reach a maximum concentration near 3000 m whereas light rees continue to increase in concentration with depth the deep water maximum and bottom water depletion is interpreted as an advective feature resulting from the flow of a lower concentration of rees in antarctic bottom water under a higher concentration of rees in the north pacific concentrations of rees do not show a linear relationship with those of si indicating that the ree cycle is not directly coupled to that of the si cycle ree element ratios i e nd la correlate with hydrographie properties which suggests that major features of the ree patterns may be conserved during lateral transport comparison of our ree data to those in previously published reports on pacific seawater shows that the identification of ree distributions affected by advection is only made possible by the high precision analysis we developed for this study the ce concentration we observe is relatively constant over 200 4500 m depth at both sites where we have complete profiles this unique constancy of ce is in contrast to the contiguous rees la and nd which show substantial concentration variations with depth

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

seismicity patterns 1963 1983 as stress indicators in the shumagin seismic gap alaska earthquakes 1963 to 1983 of magnitude 5 0 to 6 5 form a rim of high seismic activity around a central region of relative seismic quiescence that coincides with the shumagin seismic gap this pattern which can be inferred both from local network and teleseismic earthquake locations shows high activity near the eastern and western ends of the seismic gap and along the down dip end of the main thrust zone the rim of seismicity surrounding an area of seismic quiescence may reflect strong couplign along the elastic brittle part of the plate boundary where the great earthquakes occur the temporal behavior of the microseismicity recorded by the shumagin network is characterized by a burst of activity in 1978 and 1979 and a low or average level of activity from 1980 to january 1983 the increased microearthquake activity during 1978 1979 is located mainly near the down dip end of the main thrust zone and along the benioff zone down to 100 to 200 km depth composite fault plane solutions of earthquakes occurring in 1978 1979 show down dip tension between 50 to 120 km depth in the upper plane of the benioff zone composite fault plane solutions of 1981 earthquakes however indicate in plate compression in the same region hence the rate of occurrence and focal mechanisms of microearthquakes located in the benioff zone below the main thrust both show coincident temporal and spatial variations that may reflect fluctuations in local stresses

extinction of cloudina and namacalathus at the precambrian cambrian boundary in oman biostratigraphic carbon isotope and u pb zircon geochronological data from the ara group of oman indicate an abrupt last appearance of cloudina and namacalathus coincident with a large magnitude but short lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the precambrian cambrian boundary u pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542 0 0 3 ma consistent with previous age constraints from siberia and namibia combined with the global biostratigraphic record these new data strengthen hypotheses invoking mass extinction within terminal proterozoic ecosystems at or near the precambrian cambrian boundary

ediacara type fossils in cambrian sediments fossil assemblages that preserve soft bodied organisms are essential for our understanding of the composition and diversity of past life the worldwide terminal proterozoic ediacara type fossils from 600 544 myr bp are unique in consisting of soft bodied animals which are typically preserved as impressions in coarse grained sediments1 2 3 4 these lagerstatten are also special because they pre date the major burst of skeletonization which occurred near the start of the cambrian period3 most ediacara type fossils are interpreted to be cnidarians but higher metazoans such as annelids and molluscs may also be represented1 2 3 4 however the unique style of preservation and difficulties in finding convincing morphological homologies with definite animals have led some specialists to prefer non metazoan interpretations such as vendobionta5 in addition the rarity of ediacara type fossils in younger sediments has led to suggestions of a terminal proterozoic mass extinction6 here we report typical ediarcara type frond shaped fossils that occur together with an assemblage of cambrian type trace fossils in unequivocally cambrian aged sediments of the uratanna formation south australia this occurrence bridges the apparent divide between the terminal proterozoic and cambrian fossil assemblages and also suggests that closure of a taphonomic window an interval of time with unique preservational conditions was as important as extinction in the disappearance of ediacara type organisms

multidisciplinary observations of the 2011 explosive eruption of telica volcano nicaragua implications for the dynamics of low explosivity ash eruptions we present multidisciplinary observations of the march june 2011 vei 2 eruptive episode of the basalticandesite telica volcano nicaragua which allow for a comprehensive study of the eruption mechanics of low explosivity eruptions at persistently active volcanoes the observations are from a dense network of seismic and gps instrumentation augmented by visual observations of the eruptive episode geochemical and petrologic analysis of eruptive products plume so2 measurements and temperature measurements of fumaroles inside and outside the active vent the 2011 eruptive episode was telica s most explosive since 1999 and consisted of numerous vulcanian explosions with maximum column heights of 1 5 2 km above the crater rim depositing a low volume of dominantly hydrothermally altered ash based on observed variations in seismicity temperature and so2 flux the lack of deformation of the edifice the non juvenile origin of and predominance of accretionary lapilli in the ash we propose that temporary sealing of the hydrothermal system between similar to 0 5 and 2 km depth allowed pressure to build up prior to vulcanian explosions making this a phreatic eruptive episode

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

repeating coupled earthquakes at shishaldin volcano alaska since it last erupted in 1999 shishaldin volcano aleutian islands alaska has produced hundreds to thousands of longperiod 1 2 hz lp earthquakes every day with no other sign of volcanic unrest in 2002 the earthquakes also exhibited a short period 4 7 hz sp signal occurring between 3 and 15 s before the lp phase although the sp phase contains higher frequencies than the lp phase its spectral content is still well below that expected of brittle failure events the sp phase was never observed without the lp phase although lp events continued to occur in the absence of the precursory signal the twophased events are termed bcoupled eventsq reflecting a triggered relationship between two discrete event types both phases are highly repetitive in time series suggestive of stable non destructive sources waveform cross correlation and spectral coherence are used to extract waveforms from the continuous record and determine precise p wave arrivals for the sp phase although depths are poorly constrained the sp phase is believed to lie at shallow b4 km depths just west of shishaldin s summit the variable timing between the sp and lp arrivals indicates that the trigger mechanism between the phases itself moves at variable speeds a model is proposed in which the sp phase results from fluid moving within the conduit possibly around an obstruction and the lp phase results from the coalescence of a shallow gas bubble the variable timing is attributed to changes in gas content within the conduit the destruction of the conduit obstacle on november 21 2002 resulted in the abrupt disappearance of the sp phase published by elsevier b v

application of wave theoretical seismoacoustic models to the interpretation of explosion and eruption tremor signals radiated by pavlof volcano alaska tremor and explosion signals recorded on september 29 during the fall 1996 pavlof eruption are interpreted using video images field observations and seismic data waveform analysis of tremor and explosions provided estimates of the melt s volcano acoustic parameters and the magma conduit dimensions initial mass fractions of 0 25 water and 0 025 carbon dioxide in the melt can explain the resonance characteristics of the tremor and explosion pulses inferred from seismic data the magma conduit is modeled as a three section rectangular crack we infer that the tremor radiating region consists of the lowermost two sections both with cross sectional areas of 10 m2 the deeper section is 43 m long with magma sound speed of 230 m s density of 2600 kg m3 and viscosity of 1 0 106 pa s the section above it defined by the water nucleation depth is 64 m long with magma sound speed of 91 m s density of 2000 kg m3 and viscosity of 1 4 l06 pa s an average magma flow velocity of 1 2 m s with superposed random oscillations acts as the tremor source explosions are postulated to occur in the uppermost part of the magma conduit after water comes out of solution the explosion source region consists of a 15 m long section with cross sectional area of 20 m2 sound speed of 51 m s density of 1000 kg m3 and viscosity of 1 5 103 pa s a burst pressure of 220 mpa at 14 m depth would generate an acoustic pulse whose amplitude and character match the observed signal waveform analysis of the explosion pulses shows that the explosive event may be preceded by a long period fluid transient which may trigger the metastable magma gas mixture the modeling procedure illustrates the synergy of fluid dynamic seismic and acoustic models and data with geological and visual observations

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

a deep low velocity body under the yellowstone caldera wyoming delineation using teleseismic p wave residuals and tectonic interpretation in previously published papers iyer 1975 iyer and stewart 1977 iyer 1979 evidence was presented for the presence of a large body of anomalous material with compressional velocities considerably lower than normal from the surrounding rock in the crust and upper mantle under yellowstone national park wyoming in this paper we give a detailed analysis of p residual data from the 26 element telemetered seismic array and three groups of portable stations operated by the u s geological survey in the yellowstone region using a variety of analytical techniques we delineate the shape of the low velocity body determine the velocity structure inside the body and interpret the nature of the material that constitutes the body we will also outline some tectonic implications of our model the spectacular surface geothermal phenomena for which yellowstone is famous result from deep seated volcanism the volcanic evolution of yellowstone is discussed by christiansen and blank 1972 and summarized by eaton and others 1975 briefly the geothermal manifestations in yellowstone are results of three cycles of quaternary volcanism each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows the oldest cycle ended about 1 9 m y ago forming a caldera from island park to perhaps the central part of the present yellowstone caldera the second cycle climaxed about 1 2 m y ago and was confined to the island park caldera the third cycle began soon after the second and is responsible for the 75 by 45 km yellowstone caldera as it is outlined today this caldera is the result of a catastrophic eruption approximately 600 000 yr ago the most recent episode of volcanism probably began about 150 000 yr ago in the western half of the caldera near old faithful geyser and the youngest flows in yellowstone about 70 000 yr old are probably associated with this episode

understanding carbonate ramp dynamics using δ13c profiles examples from the neoproterozoic buah formation of oman the late neoproterozoic buah formation nafun group oman is a carbonate unit outcropping in the jabal akhdar and huqf areas it is composed mostly of shallow water carbonates deposited on a distally steepened carbonate ramp correlation of two 13c isotope shifts shows that in the jabal akhdar ramp differentiation into fast and slow subsiding areas was followed by lateral progradation in the huqf area however a uniform scenario of upward shallowing of the facies and lateral progradation is demonstrated by chemostratigraphic timelines cross cutting the facies belts the chemostratigraphic profiles show that the buah formation was deposited during sea level highstand conditions and that ramp differentiation was due to synsedimentary tectonics high resolution correlation of 13c profiles from the same lithostratigraphic unit whether precambrian or phanerozoic in age lacking biostratigraphic data can shed light on carbonate systems dynamics tectonic vs eustatic controls on depositional sequences and basin subsidence

structure of the benioff zone beneath the shumagin islands alaska relocation of local earthquakes using three dimensional ray tracing seismic rays are traced through a prescribed three dimensional inhomogeneity that simulates the subducted slab below the shumagin islands region to calculate local station delays for a given hypocenter and a slab model hypocenters determined by the shumagin seismic network are then relocated using the station delays a flat layered velocity structure and a standard earthquake location computer program station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data a set of path corrections that is calculated for each grid point station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km application of this method to data from 1982 for the digitally recording shumagin seismic network shows the following results 1 a previously observed apparent increase in dip of the subducted slab at depths of 100 km disappears 2 the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45 toward north northwest at depths between 80 and 250 300 km and has a 7 higher velocity than the surrounding mantle 3 hypocenters determined from shumagin network data are located only 10 20 km south of high quality hypocenters determined from teleseismic data alone 4 qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high frequency coda possible converted phases and low amplitudes of first p arrivals for rays that travel mostly along the slab conversely for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first p arrivals that are followed by an insignificant coda and low amplitude s waves

mass extinctions and sea level changes abstract review of sea level changes during the big five mass extinctions and several lesser extinction events reveals that the majority coincide with large eustatic inflexions the degree of certainty with which these eustatic oscillations are known varies considerably thus the late ordovician and end cretaceous extinctions are associated with unequivocal major regressions demonstrated from numerous widespread regions in contrast the multiple high frequency sea level changes reported for the frasnian famennian crisis based on the supposed depth preferences of conodont taxa have little support from sequence stratigraphic analyses which reveals the interval to be one of highstand the end permian mass extinction has long been related to a severe first order lowstand of sea level newell n d 1967 revolutions in the history of life geol soc am spec pap 89 63 91 based primarily on the widespread absence of latest permian ammonoid markers but field evidence reveals that the interval coincides with a major transgression newell s hypothesis that marine extinctions are related to shelf habitat loss during severe regression remains tenable for the end guadalupian and end triassic extinction events but not for other crises rapid high amplitude regressive transgressive couplets are the most frequently observed eustatic changes at times of mass extinction with the majority of extinctions occurring during the transgressive pulse when anoxic bottom waters often became extensive the ultimate cause of the sea level changes is generally unclear a glacioeustatic driving mechanism can only be convincingly demonstrated for the end ordovician and end devonian events at other times it is speculated that they may relate to the widespread regional doming and subsequent collapse caused by the impingement of superplumes and ultimate eruption on the base of the lithosphere

the problems and potential of using animal fossils and trace fossils in terminal proterozoic biostratigraphy abstract despite the discovery of soft bodied megascopic fossils of late neoproterozoic age at numerous localities world wide there has been slow acceptance of their potential for intercontinental correlation the stratigraphic thickness of sediments separating individual occurrences of such fossils from the base of the cambrian is very varied basic questions have also been posed as to the classification of the organisms represented and hence their likely time significance recent geochronological studies support notions of a narrow time distribution well studied neoproterozoic sequences in eurasia eastern and western north america and southern australia show strikingly similar patterns of increase in the numbers and diversity of trace fossils and soft bodied megascopic remains up section though strata immediately preceding the cambrian are relatively barren other than for a sparse palynoflora and compressions of primitive plants trace fossils from australian sections relating to an early part of the ediacaran apparently overlap a time of abundance of large process bearing acritarchs and big leiosphaerids this overlap of early animal traces and distinctive acritarchs may prove to be of significant biostratigraphic importance rare discoveries of forms resembling rangea gurich in the flinders ranges of south australia precede the classical ediacara assemblage and may provide a possible correlation with occurrences of this genus in namibia the distinctive remains and trace fossils at this level constituting the rangeid hiemalora assemblage zone resemble a biota from the podolia region of the ukraine and show a similarity to finds in parts of the windermere supergroup of the northwest territories canada similarly the offshore frondose assemblage of newfoundland is comparable with the biota of charnwood forest england the type ediacara assemblage of the flinders ranges characterizing the dickinsonia costata assemblage zone is a climax biota with diverse taxa and large individual species as well as some 20 ichnogenera it closely correlates with a late redkinian or early kotlinian assemblage in the upper part of the section on the zimny gory coast of the white sea northern russia the trace fossils of these levels show that significant diversification began before the cambrian

seismo acoustic signals associated with degassing explosions recorded at shishaldin volcano alaska 2003 2004 in summer 2003 a chaparral model 2 microphone was deployed at shishaldin volcano aleutian islands alaska the pressure sensor was co located with a short period seismometer on the volcano s north flank at a distance of 6 62 km from the active summit vent the seismo acoustic data exhibit a correlation between impulsive acoustic signals 1 2 pa and long period lp 1 2 hz earthquakes since it last erupted in 1999 shishaldin has been characterized by sustained seismicity consisting of many hundreds to two thousand lp events per day the activity is accompanied by up to 200 m high discrete gas puffs exiting the small summit vent but no significant eruptive activity has been confirmed the acoustic waveforms possess similarity throughout the data set july 2003 november 2004 indicating a repetitive source mechanism the simplicity of the acoustic waveforms the impulsive onsets with relatively short 10 20 s gradually decaying codas and the waveform similarities suggest that the acoustic pulses are generated at the fluid air interface within an open vent system so2 measurements have revealed a low so2 flux suggesting a hydrothermal system with magmatic gases leaking through this hypothesis is supported by the steady state nature of shishaldin s volcanic system since 1999 time delays between the seismic lp and infrasound onsets were acquired from a representative day of seismo acoustic data a simple model was used to estimate source depths the short seismo acoustic delay times have revealed that the seismic and acoustic sources are co located at a depth of 240 200 m below the crater rim this shallow depth is confirmed by resonance of the upper portion of the open conduit which produces standing waves with f 0 3 hz in the acoustic waveform codas the infrasound data has allowed us to relate shishaldin s lp earthquakes to degassing explosions created by gas volume ruptures from a fluid air interface

crystal chemical controls on rare earth element concentrations in fossil biogenic apatites and implications for paleoenvironmental reconstructions abstract fossil biogenic apatites display rare earth element ree patterns which have been proposed to reflect environmental and biological controls on past seawater composition these patterns can be separated in two groups 1 patterns that are similar to those of open ocean and epicontinental waters and 2 patterns that exhibit a strong enrichment in intermediate ree bell shaped patterns because ree trapping in biogenic apatites occurs mostly post mortem it is essential to understand the crystal chemical inorganic factors that can influence the fractionation of ree between apatite and water a model is proposed by which partition coefficients of ree between apatite and water for a substitution mechanism can be extrapolated from mineral melt partition data to seawater or sedimentary basin conditions these are compared with available experimental partition coefficients for ree adsorption mechanism and used to discuss ree patterns in fossil biogenic apatites calculations of ree patterns for apatites at equilibrium with water show that the bell shaped patterns in fossil apatites can be explained by fractionation with seawater or continental fluids at low temperature under crystal chemical control involving a substitution mechanism and in the context of extensive or late diagenesis in that case the equilibrium composition of the fluid cannot be unequivocally determined from observed ree concentrations in fossil apatites this type of sample is inappropriate to discuss the past environmental or biological controls on ree concentrations in the ocean other sample compositions suggest little to moderate alteration of the original oceanic to freshwater pattern through the adsorption mechanism in the context of weak or early diagenesis and may be used with caution for that purpose

neoproterozoic sulfur isotope variation in australia and global implications abstract seven sulfur isotope stages are apparent in australian neoproterozoic sections 1 from 840 ma to the sturtian glaciation 700 ma 34ssulfate varied little 19 to 17 5 and 34ssulfide ranged from 20 to 23 2 the sturtian glaciation was followed by 3 a rise in 34ssulfide to an average of 30 seen also in china namibia and canada and in 34ssulfate at least to 26 and possibly to 45 the sharp rise in 34s of all sulfur fractions sulfate sulfide and organic leads us to speculate that sulfide depleted in 34s was deposited on the abyssal plain and that residual sulfate enriched in 34s remained beneath a stagnant ice covered ocean during the sturtian glaciation and was brought to the shallow continental margin during the post glacial transgression 4 34ssulfide and 34ssulfate fall from 640 to 607 ma 34ssulfate down to 17 probably from oxygen excess through massive organic carbon burial which causes weathering of sulfide depleted in 34s continental sulfide and previously deposited sulfide on the abyssal plain 5 the oxic environment of the marinoan 605 595 ma glaciation minimises sulfate reduction unlike the sturtian so that 34ssulfate is unvarying 6 the post glacial environment has sulfide depleted in 34s average 34ssulfide

palynology of uppermost proterozoic and lowermost cambrian formations central mackenzie mountains northwestern canada the acritarch assemblages of strata from the base of the upper proterozoic sheepbed formation to the base of the lower cambrian atdabanian sekwi formation are described the sections sampled are in southwestern internal structural units where erosion beneath the sub cambrian unconformity is least evident problems of lithostratigraphic correlation of post sheepbed pre backbone ranges formations remain acritarchs indicate the age of the sheepbed formation and the blue flower formation above it is latest proterozoic vendian whereas that of the vampire formation is early cambrian atdabanian the backbone ranges formation has not yielded datable acritarchs but it is for the most part cambrian in age based on other fossil evidence comparisons are made with the russian platform and southern canadian rocky mountains successions the total number of acritarch genera increases markedly across the precambrian cambrian transition

isotope stratigraphy of the ediacarian neoproterozoic iii of the adelaide rift complex australia and the overprint of water column stratification abstract this study establishes a high resolution 13c chemostratigraphic framework for the type ediacarian terminal proterozoic section in the adelaide rift complex a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the earth s surface environment only 15 m of the 3 5 km thick type section consists of shallow water carbonates of the kind that have formed the basis of most previous studies of neoproterozoic chemostratigraphy however the organic 13corg record reveals a surprising richness of information and shows excellent utility in correlation between sections in the adelaide rift complex lateral variations in 13corg profiles correspond to expected diachronous patterns of basin fill with the addition of 87sr 86sr data correlation can also be made to the officer and amadeus basins consistent with known lithostratigraphic and sequence stratigraphic constraints sublittoral micritic carbonates of the upper nuccaleena formation cap dolostone and lower and middle wonoka formation show strong basinwide 13c depletion 13ccarb 3 to 10 despite compelling evidence for preservation of depositional 13ccarb such as good textural preservation evidence for sea floor lithification and in the wonoka formation pristine 87sr 86sr and low mn sr associated with the depleted carbonates are rare subaqueous evaporite relatively heavy 13corg and anomalously 13c depleted organic carbon of benthic microbial mat origin and a common cause salinity stratification of the water column is postulated the indicated palaeogeographic setting an evaporative mediterranean like basin tenuously connected to the world ocean is entirely consistent with previous suggestions of one or more messinian type desiccation events to explain very deep 1 km incised valleys or canyons in the adelaide rift complex and officer basin at horizons within the wonoka formation and correlates positive excursions in 13corg during basin stratification may in part be due to 12c depletion of surface waters by photosynthetic carbon fixation despite the proposed semi isolation of australian ediacarian basins from the world ocean major 13c excursions which can to some extent be differentiated by the monotonic rise in ediacarian marine 87sr 86sr appear to be correlatable to the upper windermere supergroup northwest canada

stable and unstable phases of elevated seismic activity at the persistently restless telica volcano nicaragua abstract telica volcano nicaragua is a persistently restless volcano with daily seismicity rates that can vary by orders of magnitude without apparent connection to eruptive activity low frequency lf events are dominant and peaks in seismicity rate show little correlation with eruptive episodes presenting a challenge for seismic monitoring and eruption forecasting a short period seismic station teln has been operated on telica s summit since 1993 and in 2010 the installation of a six station broadband seismic and eleven station continuous gps network the tesand network was completed to document in detail the seismic characteristics of a persistently restless volcano between our study period of november 2009 and may 2013 over 400 000 events were detected at the tesand summit station tbtn with daily event rates ranging from 5 to 1400 we present spectral analyses and classifications of 200 000 events recorded by the tesand network between april 2010 and march 2013 and earthquake locations for a sub set of events between july 2010 and february 2012 in 2011 telica erupted in a series of phreatic vulcanian explosions six months before the 2011 eruption we observe a sudden decrease in lf events concurrent with a swarm of high frequency hf events followed by a decline in overall event rates which reached a minimum at the eruption onset we observe repeated periods of high and low seismicity rates and suggest these changes in seismicity represent repeated transitions between open system and closed system degassing we suggest that these short and long term transitions between open to closed system degassing form part of a long term pattern of stable vs unstable phases at telica stable phases are characterised by steady high rate seismicity and represent stable open system degassing whereas unstable phases are characterised by highly variable seismicity rates and represent repeated transitions from open to closed system degassing where the system is unable to sustain steady open system degassing these observations have implications for seismic monitoring at persistently restless volcanoes as the recognition of unstable seismic phases may indicate the open system degassing process cannot be sustained and explosions are likely

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

a composite reference section for terminal proterozoic strata of southern namibia integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal proterozoic to cambrian witvlei and nama groups of namibia coupled with precise u pb zircon age constraints these correlations make it possible to construct a composite reference section for use in calibrating terminal proterozoic chronostratigraphy the namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the proterozoic cambrian boundary the two glacial horizons may represent each of two distinct varanger age glaciations better known from the north atlantic region from the higher of the two glacial horizons up the composite stratigraphy preserves one of the thickest and most complete available records of carbon isotope variability in post varanger terminal proterozoic seawater four carbon isotope chemostratigraphic intervals are recognized 1 a postglacial negative 13 c excursion npg interval 2 a rising interval pr interval of increasing positive delta 13 c values 3 a falling interval pf interval characterized by decreasing positive 13 c and culminating in near zero or negative values and 4 an interval of moderately positive relatively invariant 13 c values i interval that extends up to the unconformity that contains the proterozoic cambrian boundary each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world by comparing sediment accumulation rate in the radiometrically calibrated namibian stratigraphy with sediment accumulation rates in correlative sections in arctic canada and oman a maximum age of 564 ma is estimated for the end of the younger varanger glaciation 25 m y younger than previous estimates

eruption characteristics and cycles at pavlof volcano alaska and their relation to regional earthquake activity abstract pavlof volcano 55 25 n 161 54 w exhibits two eruption styles magmatic eruptions of one to two days duration and phreatic phreatomagmatic activity lasting several days to two months thirty four eruptions have occurred in historic times of these the largest are volcano explosivity index 3 nine magmatic and 13 phreatomagmatic eruptions occurred between 1973 1983 all the magmatic eruptions occurred in the fall between sept 9 nov 20 four magmatic eruptions occurred during november 11 15 but in four different years a 3 year long period of eruptive activity between 1973 1976 bears striking resemblance to a period of activity between 1980 1983 no locatable shallow earthquakes 100 km are clustered beneath pavlof and several other volcanoes their t axes show downdip tension within the slab deep teleseisms 160 km mostly occurred between 1977 1979 when the volcano was not erupting catalogued volcanic activity throughout the alaska aleutian arc shows a weak tendency to increase around the time of great m 7 8 earthquakes

neoproterozoic chemostratigraphy and correlation of the port askaig glaciation dalradian supergroup of scotland the 13 c of preglacial carbonates beneath the glaciogenic port askaig formation falls from 5 to negative values while least altered 87 sr 86 sr values are close to 0 7067 postglacial cap carbonates of the bonahaven formation begin with negative 13 c carb values jumping sharply to 11 7 near the top which we correlate with excursions above the elbobreen wilsonbreen glaciations of svalbard and putative sturtian not varanger glaciations elsewhere dated at c 720 ma much of the succeeding terminal neoproterozoic is missing in ne svalbard and east greenland raising doubts about their use for global sr and c isotope curves in the neoproterozoic

integrated chronostratigraphy of the vendian system of siberia implications for a global stratigraphy integrated 13 c chemostratigraphy and sequence stratigraphy are developed for a succession of vendian strata nokhtuisk section on the southern margin of the siberian platform the carbonates exhibit secular variations in 13 c and at least three major depositional sequences that encompass the last c 20 million years

a sea level test for inertial interchange true polar wander events summary the possibility of inertial interchange true polar wander iitpw events in which the rotation pole moves 90 with respect to the solid earth in a matter of 10 myr has been discussed in the geophysical literature for more than three decades recent evidence for an iitpw event in early cambrian time has renewed interest in the issue however the veracity of supporting palaeomagnetic evidence remains a matter of significant debate we propose that sea level variations driven by polar wander provide an important independent test for the occurrence of iitpw events our numerical simulations of the response of a viscoelastic planet to an iitpw induced forcing predict sea level changes of up to 200 m depending on the details of the earth model the location of the site relative to the rotation path and the elapsed time for the reorientation of the pole a preliminary comparison of our predictions to early middle cambrian sea level records for australia laurentia and baltica shows qualitative agreement this comparison suggests that a definitive test for the cambrian iitpw hypothesis is possible given a suyciently accurate and globally distributed database of sea level histories

the vendian cambrian δ13c record north iran evidence for overturning of the ocean before the cambrian explosion continuous fossilliferous successions across the precambrian cambrian pc c boundary in the elburz mountains of northern iran show a remarkable negative 13c excursion just below the pc c boundary high concentrations of manganese phosphorus barium and high abundances of fossil phytoplankton and black shale coincide with the excursion worldwide stratigraphic correlation shows that the isotopic anomaly is a global event the initial metazoan diversification coupled with13c enrichment occurs stratigraphically just above the excursion we propose the following scenario for oceanic environmental changes before the cambrian faunal explosion based on new data from iran a global warm climate following the last precambrian glaciation resulted in a generally stagnant oceanic condition so that surface water was oxic deep water was dysoxic depleted in13c and enriched in nutrients massive upwelling of deep water vertical advection of nutrients and13c depleted co2 caused enhanced phytoplankton productivity and a sharp drop in 13c in shallow water carbonate and organic carbon we conclude that latest cryptozoic overturning of ocean stratification preceded the cambrian explosion

latest proterozoic microfossils from the nama group namibia south west africa abstract gray to black shales from the kuibis and schwarzrand subgroups of the nama group south west africa namibia contain a distinctive if depauperate assemblage of organic walled microfossils the assemblage is dominated by leiosphaerid acritarchs and fragments of the ribbon like macrofossil vendotaenia small unicells and non septate filaments interpreted as the sheaths of oscillatorian cyanobacteria occur with variable frequency throughout the subgroups while chuaria circularis bavlinella faveolata and a comasphaeridium like form known from a single specimen are found as rarer components of the biota assemblage composition shows no strong stratigraphic trends within the subgroups the nama assemblage compares closely with biotas previously described from uppermost proterozoic valdaian sequences of the baltic region scandinavia and australia and confirms a latest proterozoic age for the kuibis and schwarzrand subgroups and their contained metazoan fossils although its significance is unclear nama microfossils tend to be strongly corroded this diagenetic feature is widespread among valdaian assemblages but appears to be much less characteristic of older or younger biotas

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

ground coupled airwaves at pavlof volcano alaska and their potential for eruption monitoring an abnormally high number of explosion quakes were noted during the monitoring effort for the 2007 eruption of pavlof volcano on the alaska peninsula in this study we manually cataloged the explosion quakes from their characteristic ground coupled airwaves this study investigates how the ground coupled airwaves might be used in a monitoring or analysis effort by estimating energy release and gas mass release over 3 104 quakes were recorded the energy release from the explosions is approximated to be 3 1011 j and the total gas mass assuming 100 water released was 450 t the tracking of explosion quakes has the potential to estimate relative eruption intensity as a function of time and is thus a useful component of a seismic monitoring program

prokaryotic and eukaryotic microfossils from a proterozoic phanerozoic transition in china cherts of the doushantuo formation in the yangtze gorges south china contain a superbly preserved heterogeneous assemblage of bacteria cyanobacteria planktonic algae submillimetre sized burrows and problematica the microbiota has a bearing on defining the precambrian cambrian boundary baltisphaeridium and associated spinose microfossils in the chert suggest an early cambrian age whereas other acritarchs vendotaenids and pretrilobite small shelly fossils from the conformably overlying the dengying formation indicate a late proterozoic age for the doushantuo this mixed benthic planktonic assemblage preserved in silicified laminated microbial mat like fabrics provides a unique sample of diverse microbiotic elements existing at the dawn of the phanerozoic and records the earliest evidence of meiofauna

palaeoenvironment and carbon isotope stratigraphy of upper proterozoic carbonates of the yangtze platform the 13c values for phanerozoic marine carbonates display secular trends mainly in the range 1 to 2 1 these have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1 2 carbon isotope trends are not as well established for the precambrian but there are indications that upper proterozoic sequences contain carbonates with unusually high 13c values3 9 here we present the results of the first carbon isotopic study of the upper proterozoic carbonate strata from the type locality of the sinian system in the yangtze gorges sections near yichang south china and discuss their implications for accumulation of organic matter we also compare the results for these chinese strata with 13c trends in other upper proterozoic cambrian sequences and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences

determination of large scale velocity structure of the crust and upper mantle in the vicinity of pavlof volcano alaska pavlof volcano is a 2715 m high stratovolcano located at latitude 55 4 n and longitude 161 9 w near the western end of the alaska peninsula in this study the large scale lateral perturbations to a vertical reference velocity structure in the vicinity of pavlof volcano are determined using modified versions of standard three dimensional inversion techniques we use as the primary data set p wave arrival times and residuals from shallow 40 km deep regional earthquakes located by the shumagin islands seismic network relative residuals mainly from refracted e g pn arrivals allow us to constrain models for a laterally varying crustal velocity structure several strong ray path dependent patterns are seen in the residuals the most pronounced is that rays passing through the crust beneath the volcano are delayed positive residuals by up to 1 0 s while those rays traveling subparallel to and behind the volcanic axis on the retroarc bering sea side arrive as much as 0 5 s early negative residuals the corresponding results of a formal velocity inversion show the presence of a lowvelocity body beneath the volcanic arc that coincides geographically with emmons caldera 10 km sw of pavlof computed velocity values are about 12 14 lower than in the surrounding crust and are comparable to values found at other volcanic areas the exact shape and position of the low velocity body and its suitability for geothermal power development have not yet been sufficiently determined for such an assessment a much denser station distribution is required

sequence stratigraphy and the interpretation of neoproterozoic earth history abstract the application of sequence stratigraphy to neoproterozoic successions is important for improving the resolution of time correlation within individual sedimentary basins and potentially at a global scale the methodology is illustrated in this paper by reference to two contrasting examples from the flinders ranges adelaide geosyncline of south australia where the younger part of the neoproterozoic to earliest cambrian succession 770 ma to 540 ma has been divided into thirteen unconformity bounded depositional sequences one of the most prominent sequence boundaries at or near the base of the wonoka formation is characterized by a series of buried canyons as much as 1 km deep high resolution sequence stratigraphic studies at umberatana syncline continue to support the view that the canyons were cut subaerially and filled by fluvial and shallow water sediments in contrast to the wonoka canyons sequence boundaries interpreted at the base of the nuccaleena formation seacliff sandstone and near the top of the abc range quartzite are relatively subtle with only limited evidence for erosion and valley incision four sequence boundaries at the level of the sturtian and marinoan varanger glacial deposits and in the vicinity of the precambrian cambrian boundary are thought to correlate with surfaces in the amadeus basin of central australia other prominent sequence boundaries including the wonoka canyons and surfaces within the upper part of the wonoka formation and at the base of the ediacara member of the rawnsley quartzite correspond with a relatively condensed section in the amadeus basin and their lateral persistence beyond the adelaide geosyncline is therefore difficult to evaluate given the lack of precision in biostratigraphy and isotope geochemistry in neoproterozoic rocks and in a marked departure from phanerozoic practice we recommend placement of a terminal proterozoic gssp at a sequence boundary a prime candidate in australia is the sequence boundary at the base of the nuccaleena formation seacliff sandstone immediately above the marinoan glacial rocks in the adelaide geosyncline and its likely correlative at or near the base of the gaylad sandstone in the amadeus basin

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

geochemical investigation of a neoproterozoic glacial unit the mineral fork formation in the wasatch range utah the mineral fork formation is an isolated unit of neoproterozoic glaciogenic rocks in the wasatch range in northwestern utah geochemical analyses of diamictite and mudstone samples were carried out to document its major and trace element geochemistry comparison of the lower and upper parts of the formation which have been ascribed to different tectonic and sedimentological settings revealed no striking differences but the analyzed samples show a gradual upward increase in cao mgo fe 2 o 3 mno and p 2 o 5 and a decrease in sio 2 tio 2 al 2 o 3 ratios that suggest a provenance throughout the formation with an average composition slightly more mafic than average upper crust the mudstones are more k rich than the diamictites calculation of a chemical index of alteration cia is complicated by the presence of carbonate minerals after making corrections for carbonate content cia values range from 65 to 70 these values are higher than those from paleoproterozoic glaciogenic deposits some mudstones in the upper part of the formation are fe rich 15 fe 2 o 3 approaching the composition of iron formations the ferruginous nature supports previous correlation with fe rich glaciogenic rocks of the rapitan group in the northern canadian cordillera trace elements including rare earth elements suggest that the materials in the mineral fork formation were largely derived from a post archean source most of the widespread glaciogenic rocks of the neoproterozoic were derived from older sedimentary materials whereas the deposits of the earlier great glacial episode in the paleoproterozoic contain a much higher proportion of crystalline materials

observations of deep long period dlp seismic events beneath aleutian arc volcanoes 1989 2002 abstract between october 12 1989 and december 31 2002 the alaska volcano observatory avo located 162 deep long period dlp events beneath 11 volcanic centers in the aleutian arc these events generally occur at mid to lower crustal depths 10 45 km and are characterized by emergent phases extended codas and a strong spectral peak between 1 0 and 3 0 hz observed wave velocities and particle motions indicate that the dominant phases are p and s waves dlp epicenters often extend over broad areas 5 20 km surrounding the active volcanoes the average reduced displacement of aleutian dlps is 26 5 cm2 and the largest event has a reduced displacement of 589 cm2 or ml 2 5 aleutian dlp events occur both as solitary events and as sequences of events with several occurring over a period of 1 30 min within the sequences individual dlps are often separated by lower amplitude volcanic tremor with a similar spectral character occasionally volcano tectonic earthquakes that locate at similar depths are contained within the dlp sequences at most aleutian volcanoes dlps appear to loosely surround the main volcanic vent and occur as part of background seismicity a likely explanation is that they reflect a relatively steady state process of magma ascent over broad areas in the lower and middle portions of the crust at mount spurr dlp seismicity was initiated by the 1992 eruptions and then slowly declined until 1997 at shishaldin volcano a short lived increase in dlp seismicity occurred about 10 months prior to the april 19 1999 eruption these observations suggest a link between eruptive activity and magma flux in the mid to lower crust and uppermost mantle

integrated stratigraphy of the upper neoproterozoic succession in yunnan province of south china re evaluation of global correlation and carbon cycle abstract this study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub cambrian marine sedimentary succession in yunnan province of south china our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late neoproterozoic the succession has a basal glacial diamictite nantuo formation and comprises two large scale depositional cycles towards the precambrian cambrian pc c boundary each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal in the absence of biostratigraphic control attribution of the tillite has remained equivocal between the 710 ma sturtian and 635 ma marinoan glaciations a compilation of known paleomagnetic and other geochronologic constraints suggests that the nantuo glaciation in south china was at least diachronous in which the nantuo tillite in yunnan belongs to the older glaciation stratigraphic patterns of the diagenetically screened 13c variation and depositional sequences during the carbonate dominated interval show a striking similarity with those of the upper cryogenian and entire terminal proterozoic carbonates at widely separated other localities rather than the terminal proterozoic alone supporting the view that deposition of the nantuo tillite in yunnan reflects the sturtian rather than marinoan glaciation further considerations of the 13c results provide three important implications for the late neoproterozoic marine carbon cycle first comparison of different 13c profiles suggests considerable variability in the absolute values of carbonate on both intra and inter basinal scales which differs profoundly from their relative homogeneity in marine phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late neoproterozoic second an empirical correlation between the 13c and relative sea level changes was applied to evaluate hypothesized explanations for 13c temporal variation in surface sea yielding short term inverse and long term positive relationships the short term trend marks periods of the negative 13c excursions and is compatible to most of the existing arguments for the 13c events on the other hand mechanisms causing the long term trend remain enigmatic in the absence of comparable data from other localities our observation however suggests that at least two different mechanisms operated marine carbon cycle during the late neoproterozoic third the negative 13c anomaly characterizing the period following the marinoan glaciation worldwide was not coincident between kerogen and carbonate but occurs as a lagged drop in yunnan where a 7 depletion in kerogen preceded an 3 depletion in carbonate their lagged trends likely reflect different sources as kerogen and carbonate signals are considered to monitor surface and deeper waters respectively the negative 13c event would therefore have occurred in surface water followed by a smaller 13c depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site this mechanism supports the proposed hypothesis that injection of the 13c depleted atmospheric co2 into the surface sea deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post glacial warming contributed to the negative 13c anomaly

origins of carbonate in neoproterozoic stromatolites and the identification of modern analogues abstract the viewpoint in this paper is that the distinction of detrital from in situ precipitated carbonate in stromatolites is vital so that the appropriate modern analogues can be sought reviewing literature on modern stromatolites it is clear that marine stromatolites are dominated by detrital sediment although diagenetic processes can obscure the detrital nature of carbonate in mats in low salinity conditions carbonate is dominantly of precipitate origin being triggered by photosynthesis in mats in saline conditions petrographically similar precipitates occur apparently triggered variously by photosynthesis evaporation and organic matter degradation in different cases light carbon isotopes in the carbonate are a good pointer to organic degradation as a carbon source a review of criteria proposed for identifying the origin of carbonate in precambrian stromatolites shows that in different cases reliable evidence both for sediment incorporation and carbonate precipitation has been obtained although many arguments are equivocal the utility of luminescence petrography for positive identification of detritus and carbon isotope data for identification of carbon sources is outlined new petrographic and carbon isotope data are presented from four units canyon formation of east greenland draken and wilsonbreen formations of spitsbergen and atar group mauritania although environmentally diverse each displays evidence for both sediment incorporation and localized carbonate precipitation although in widely varying proportions it is recommended that luminescence studies be carried out for stromatolite microstructural work carbon isotope data indicate no distinction between early inorganic cements stromatolitic and non stromatolitic dolo micrites and indicate a common origin related to water column and microbial mat photosynthesis variably accompanied by evaporation

problematic fossils in the late neoproterozoic wonoka formation south australia abstract complex biogenic structures are described from the upper wonoka formation late neoproterozoic of the adelaide geosyncline south australia the structures are preserved within silty limestones and calcareous siltstones as dark bedding plane markings defining branching arrays of parallel elongate curved elements the fossils bear at least a superficial resemblance to the putative trace fossil palaeopascichnus normally preserved in siliciclastic sediments and reported from a number of late neoproterozoic sections worldwide the material from the wonoka formation is not a trace fossil and seems most likely to represent a surface encrusting alga the nature of palaeopascichnus needs to be carefully reassessed in the light of the possibility that the wonoka fossils represent a different preservational window into the same phenomenon the thin ca 2 m but widespread nature of the fossil horizon makes it a useful stratigraphic marker within the wonoka formation with the potential to aid in regional correlation within the adelaide geosyncline

sustained long period seismicity at shishaldin volcano alaska abstract from september 1999 through april 2004 shishaldin volcano aleutian islands alaska exhibited a continuous and extremely high level of background seismicity this activity consisted of many hundreds to thousands of long period lp 1 2 hz earthquakes per day recorded by a 6 station monitoring network around shishaldin the lp events originate beneath the summit at shallow depths 0 3 km volcano tectonic events and tremor have rarely been observed in the summit region such a high rate of lp events with no eruption suggests that a steady state process has been occurring ever since shishaldin last erupted in april may 1999 following the eruption the only other signs of volcanic unrest have been occasional weak thermal anomalies and an omnipresent puffing volcanic plume the lp waveforms are nearly identical for time spans of days to months but vary over longer time scales the observations imply that the spatially close source processes are repeating stable and non destructive event sizes vary but the rate of occurrence remains roughly constant the events range from magnitude 0 1 to 1 8 with most events having magnitudes

eruptions of pavlof volcano and their possible modulation by ocean load and tectonic stresses all nine magmatic eruptions at pavlof volcano in six of the years from 1973 to 1984 have occurred between september 9 and november 20 volumes of erupted material range from 0 1 to 15 9 106 m3 dense rock equivalent at an average rate of 3 106 m3 yr 1 the volumes are estimated from eyewitness reports for two eruptions the others are estimated from a relationship that we derive between eruption volume and harmonic tremor duration and amplitude the volume of lava erupted is approximately time predictable a significant correlation exists between the eruptions and yearly nontidal variations in sea level and may result from ocean loading calculated volume changes beneath the volcano due to ocean loading are from 0 02 to 2 times eruption volumes and we postulate that the volcano acts as a long period several month volume strain meter with lava being preferentially erupted when strain beneath the volcano is compressive the volcano did not erupt during the period 1978 1980 when tilt seismic data and sea level data indicate that deep aseismic slip may have occurred models of this event predict a volume increase beneath the volcano that might have compensated strain from magma injection these observations indicate that pavlof volcano may be responsive to small slow changes in ambient stresses or strains and that these changes may modify or trigger eruptions

seismic detection of the summit magma complex of kilauea volcano hawaii application of simultaneous inversion of seismic p wave arrival time data to the investigation of the crust beneath kilauea volcano yields a detailed picture of the volcano s heterogeneous structure zones of anomalously high seismic velocity are found associated with the volcano s rift zones a low velocity zone at shallow depth directly beneath the caldera coincides with an aseismic region interpreted as being the locus of kilauea s summit magma complex