relation between depositional environment and the elemental composition of early diagenetic siderite early diagenetic siderites from marine and fresh water depositional environments are characterized by distinctive compositional trends siderite from fresh water environments is often relatively pure i e greater than 90 mol feco sub 3 and commonly attains end member composition siderite from marine environments however is always extremely impure and has extensive substitution of mg up to 41 mol and to a lesser extent ca up to 15 mol for fe in the siderite lattice in addition marine siderite generally contains less mn and has a higher mg ca ratio than fresh water siderite this compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters inasmuch as early marine pore waters generally have a higher mg sup 2 ca sup 2 ratio and contain less mn sup 2 and fe sup 2 and more ca sup 2 and mg sup 2 than meteoric waters

chemical weathering indices applied to weathering profiles developed on heterogeneous felsic metamorphic parent rocks abstract chemical weathering indices are commonly used for characterizing weathering profiles by incorporating bulk major element oxide chemistry into a single metric for each sample generally on homogeneous parent rocks weathering indices change systematically with depth however the weathering of heterogeneous metamorphic rocks confounds the relationship between weathering index and depth in this paper we evaluate previously defined chemical weathering indices for their suitability in characterizing weathering profiles developed on felsic heterogeneous metasedimentary bedrock from coweeta hydrologic laboratory north carolina usa among the weathering indices evaluated here the weathering index of parker wip is the most appropriate for application to weathering profiles on heterogeneous and homogeneous parent rock because the wip includes only the highly mobile alkali and alkaline earth elements in its formulation it yields values that differ greatly from those of the parent rock in addition the wip allows for aluminum mobility unlike other weathering indices these characteristics combine to make the weathering index of parker the most applicable index for studying the weathering of heterogeneous metasedimentary rocks however the wip should be applied judiciously as alkali and alkaline earth metals may be readily depleted during weathering in addition to reflecting weathering the chemical index of alteration cia chemical index of weathering ciw plagioclase index of alteration pia and vogt s residual index v may be sensitive to subtle geochemical changes such as hydrothermal alteration along a fault and or alteration at the water table weathering indices that include iron in their calculations were not included in this study because they do not distinguish between ferric and ferrous iron the oxidation state and distribution of iron in a weathering profile is influenced by the proportion of oxidation states in the pre weathered metamorphic rock and interaction with groundwater during weathering

source investigation of a small event using empirical green s functions and simulated annealing we propose a two step inversion of three component seismograms that 1 recovers the far field source time function at each station and 2 estimates the distribution of co seismic slip on the fault plane for small earthquakes magnitude 3 to 4 the empirical green s function egf method consists of finding a small earthquake located near the one we wish to study and then performing a deconvolution to remove the path site and instrumental effects from the main event signal the deconvolution between the two earthquakes is an unstable procedure we have therefore developed a simulated annealing technique to recover a stable and positive source time function stf in the time domain at each station with an estimation of uncertainties given a good azimuthal coverage we can obtain information on the directivity effect as well as on the rupture process we propose an inversion method by simulated annealing using the stf to recover the distribution of slip on the fault plane with a constant rupture velocity model this method permits estimation of physical quantities on the fault plane as well as possible identification of the real fault plane we apply this two step procedure for an event of magnitude 3 recorded in the gulf of corinth in august 1991 a nearby event of magnitude 2 provides us with empirical green s functions for each station we estimate an active fault area of 0 02 to 0 15 km2 and deduce a stress drop value of 1 to 30 bar and an average slip of 0 1 to 1 6 cm the selected fault of the main event is in good agreement with the existence of a detachment surface inferred from the tectonics of this half graben

nannofacies analysis as a tool to reconstruct paleoenvironmental changes during the early toarcian anoxic event the black shales of lower toarcian schistes carton in france jet rock in england posidonia shale in germany are sediments formed by fine alternations of millimetric to sub millimetric clear and dark laminae a study of polished core surfaces dotternhausen core sw germany was carried out in sem in order to characterize the micro fabrics of different laminae and their content in nannofossils two main intervals were recognized where fabrics and nannofossil content are significantly different in the clay rich interval characterized by high wt toc and a large negative carbon isotope excursion both in carbonates and organic matter the dominant micro fabrics are well foliated or lumpy giving evidence of the absence or reduction of benthic life nannofossils are scarce and only concentrated in discrete laminae separated by large abiotic intervals assemblages are indicative of a deep nutricline but sporadic nutrient inputs occurred as demonstrated by fluctuation in abundance of specimens of the biscutaceae nutrients were probably delivered to the german basin via riverine influx different sized framboids of pyrite were recognized in this interval the smallest ones 2 m being probably precipitated in an anoxic and sulphidic water column the clay rich interval was likely deposited during low sea level in a restricted basin these conditions were followed by a sea level rise and better water mass circulation within the german basin sedimentation records an increase in carbonate deposition organic matter content decreased and carbon isotopes came back to pre excursion values micro fabrics became less markedly laminated pyrite framboids nearly disappeared and rich and diversified nannofossil assemblages are recorded sea water conditions were probably highly fluctuating as indicated by alternation of intervals dominated in turn by schizosphaerella spp flourishing under generally oligotrophic conditions where nutrients were temporarily available in surface waters due to storms and by crepidolithus spp a deep dweller the results of this work fit well with a silled basin model

geochemistry of shales from the archean 3 0 ga buhwa greenstone belt zimbabwe implications for provenance and source area weathering abstract phyllites from the archean 3 0 ga buhwa greenstone belt zimbabwe were deposited on a stable cratonic platform analyses of the phyllites generally define a single geochemical group based on major and trace element abundances the phyllites are strongly depleted in cao na2o and sr with respect to average archean upper crust by contrast k2o ba and rb are enriched several times relative to average archean upper crust reflecting basin scale k metasomatism transition metals are somewhat depleted whereas high field strength elements are typically enriched relative to average archean upper crust samples generally have fractionated lree patterns average cen smn 2 4 with small negative eu anomalies average eu eu 0 78 and generally flat hrees average gdn ybn 1 3 such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite mixing calculations which massbalance the rees and th sc suggest 70 tonalite 15 mafic volcanic rocks and 15 granite as possible proportions for the source of the phyllites a nearby early archean 3 5 ga continental nucleus preserves similar lithologies and is the probable source intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95 100 nearly complete depletion of cao and na2o and high al2o3 na2o ratios fine grained sediments of comparable age and presumed tectonic setting elsewhere in southern africa show similar geochemical characteristics implying that source area compositions and weathering intensities were similar these mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at 3 0 ga across southern africa

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

positive europium anomaly at the permo triassic boundary spiti india a limonitic layer at the base of the otoceras ophiceras zone in the lalung section in spiti valley shows a significant positive europium anomaly with maximum eu eu 1 9 this layer defines the contact between the earliest griesbachian and the permian productus shale sequence the layer shows a small cerium anomaly having the maximum value of ce la n 1 4 the iridium concentration in this layer at 73 pg g is only marginally above the levels found in the adjacent shales although higher than that found in the overlying limestones another horizon having 114 pg g of ir occurs 70 cm below this limonitic layer in the shale sequence these observations have important implications both for the terrestrial mainly volcanic and extra terrestrial impact hypotheses for the permo triassic transition which are discussed

ground motion simulations of the 2004 mw 6 4 les saintes guadeloupe earthquake using ten smaller events the validity and the stability of a ground motion simulation method based on the recordings of a single small event as an empirical green s function egf is tested on a seismic crisis that occurred 25 km offshore of the guadeloupe islands caribbean arc we aim to determine if 1 the method enables us to reproduce the observed ground motion 2 the choice of the small event taken as an egf is crucial for the simulations and 3 the method provides valuable results compared with ground motion prediction equations gmpes we have successively used the recordings of 10 small earthquakes m w 4 2 5 1 to simulate the ground motions generated by the mainshock m w 6 4 at 12 accelerometric stations we first determined the moment and focal mechanisms of the 10 events chosen as an egf as well as the stress drop ratio c between each of these events and the mainshock then we simulated 500 accelerograms for each egf and each station a good reproduction of the mainshock response spectra the peak ground acceleration and the duration of the signal was obtained using 9 out of 10 egfs for stations with site effects the results obtained are much closer to the real data than values given by the gmpes on sediment sites in the case of blind predictive simulation we propose to calibrate the stress drop ratio c through a comparison between the simulated response spectra on rock site stations and the values predicted by gmpes

chemical evolution of rare earth elements fractionation between colloidal and solution phases of filtered river water abstract within the dissolved fraction of connecticut river water there is large scale fractionation of rare earth elements between colloidal and solution phases colloidal particles carry a large part of the dissolved ree pool filtration through filters with progressively smaller pore size 0 45 0 22 and 0 025 m results in filtrates with lower absolute concentrations of ree and more fractionated compositions with decreasing pore size the filtrates have shale normalized abundances which become systematically more depleted in the lree relative to the hree hence both the solution and colloidal phases are significantly fractionated with respect to the dissolved fraction these conclusions are consistent with previous studies of ree in rivers estuaries and soils and with surface solution speciation models colloids play an important role in the aquatic geochemistry of rare earth elements they are responsible for the fractionation of ree during the estuarine removal of ree the removal of suspended and colloidal particles from river water leaves the solution phase of river water with compositions of ree as evolved as those of surface seawater hence the evolution of the composition of ree in river water and seawater is significantly influenced by colloid formation during chemical weathering on the continents

anatomy of a small intraplate earthquake a dissection of its rupture characteristics using regional data summary the magnitude mblg 5 0 mont laurier earthquake of 1990 october 19 in quebec canada was one of the largest to have occurred in eastern north america during the past decade high frequency ground motions recorded on regional network instruments exceeded values anticipated for an event of its size by a factor of 3 a commonly favoured explanation for the discrepancy is that the source was a rare high stress event in this paper detailed fault slip models are derived to fit waveform and spectral characteristics of the regional data the results establish that the effective rupture stress was normal about 100 bars that the fault rupture developed asymmetrically and that the average slip time for points inside the rupture area approx 0 1 s was significantly less than that associated with the standard brune 1970 source spectral model the rupture area developed in at least four distinct episodes each extending the previously ruptured area taken together with similar results for the mblg 6 5 saguenay earthquake of 1988 november the results indicate that a widely used assumption in hazard analyses that earthquake spectra are adequately represented by the standard brune spectral model is unreliable for the interpretation and prediction of strong ground motion

sequencing events across the permian triassic boundary guryul ravine kashmir india abstract the permian triassic boundary ptb section at guryul ravine kashmir india comprises a 100 m thick apparently conformable succession of mixed siliciclastic carbonate sediments deposited in a deep shelf or ramp setting this section although long important in debates concerning placement of the ptb has not previously been the focus of an integrated chemostratigraphic study in the present study samples from a 20 m thick interval straddling the ptb were analyzed for major and trace element concentrations toc tic rees and organic 13c 15n to investigate contemporaneous environmental changes the guryul ravine section exhibits a sequence of discrete events that provide potentially important information about the character and cause s of the ptb two transient negative c isotopic excursions within the upper changxingensis zone predate the late permian event horizon lpeh i e the global mass extinction event by 200 400 kyr and are associated with the onset of a major eustatic rise and with secondary extinction peaks these excursions may record minor environmental disturbances prior to the main end permian crisis supporting an intrinsic mechanism such as volcanism and climate change rather than an extrinsic mechanism such as a bolide impact the onset of a large 4 0 to 4 5 sustained negative c isotope shift marks the lpeh which is located at the zewan khunamuh formation contact 260 cm below the biostratigraphically defined ptb the guryul ravine section exhibits a stepwise extinction pattern with the largest peak between the lpeh and ptb and a secondary peak just above the ptb a pattern similar to that observed in the meishan d gssp

the ml 5 3 epagny french alps earthquake of 1996 july 15 a long awaited event on the vuache fault the ml 5 3 epagny earthquake that occurred on 1996 july 15 in the vicinity of annecy french alps was the strongest event to shake southeastern france in the last 34 years moderate to serious damage in the annecy area is consistent with msk intensities of vii viii this earthquake occurred on the vuache fault a geologically well known morphologically clear nw se trending strike slip fault that links the southern jura mountains with the northern subalpine chains the hypocentre was located in mesozoic limestones at shallow depths 1 3 km the focal mechanism indicates left lateral strike slip motion on a n136 e striking plane dipping 70 to the ne abundant field evidence was gathered in the days following the main shock several hundred aftershocks were recorded thanks to the rapid installation of a 16 station seismic network all aftershocks occurred along the southernmost segment of the vuache fault defining a 5 km long 3 5 km deep n130 e striking rupture zone dipping 73 to the ne the fault plane solutions of 60 aftershocks were found to be consistent with left lateral slip on nw se striking planes at the se tip of the aftershock zone we found ground cracks parallel to the fault close to the annecy meythet airport runway at the nw tip near bromines we observed left lateral displacement of concrete walls in a building we also noticed flow changes in two springs close to that locality geodetic levelling across the fault revealed about 1 cm of uplift for the region north of the fault the recording of aftershocks with a six station accelerometric network showed that lacustrine deposits locally amplified the ground motion up to eight times which explains how this moderate magnitude shock could cause such heavy damage historical records draw attention to the central segment of the vuache fault which has been locked for at least 200 years situated nw of the 1996 aftershock zone between the mandallaz and vuache mountains this segment forms a 12 km long potential seismic gap where other m5 events or one single m6 event might occur

the trace metal content of recent organic carbon rich sediments implications for cretaceous black shale formation organic carbon rich sediments are enriched in several redox sensitive and stable sulfide forming trace metals tm and form an important sink in global tm cycles in today s ocean such sediments are deposited in coastal upwelling areas like those in the gulf of california off peru or off the namibian coast and euxinic basins like the black sea in an attempt to distinguish both environments by their specific tm patterns some important conclusions may be drawn 1 the tm enrichment seen in both environments broadly is rather similar 2 upwelling sediments are often enriched in cd and p and depleted in co and mn 3 sapropels have a tendency towards higher enrichments in ba and mo s re as cu ni sb and fe reflecting the strong sulfidation in an anoxic water column 4 upwelling systems cover a broad range of environmental settings where the steepness of the slope the proximity of h2s to the sediment seawater interface and the intensity of bio accumulation and regeneration play an important role for tm accumulation 5 the tm content of anoxic basins is mainly controlled by tm availability in the water column and sedimentation rate 6 enhanced bio productivity is the main switch turning an oxygenated into an anoxic environment cretaceous black shales oae 2 exhibit tm signatures of both coastal upwelling areas and sapropels the strong enrichments in cd mo tl v cu ni bi and sb demonstrate that bio accumulation and presence of h2s in the water column and associated sulfidation processes persisted during this time interval in large parts of the proto atlantic the usefulness of ba as a paleoproductivity proxy is obscured by diagenesis the extreme accumulation in ag and zn requires either an additional tm source for instance hydrothermal input or a different tm seawater composition during the cretaceous the geochemical data suggest that the major driving force for the widespread occurrence of c t black shales seems to be the increase in volcanic activity and associated co2 input throughout the cretaceous the ocean atmosphere system operated in a mode different from today during this time interval after conversion of the nutrient co2 into organic matter the system switched back into the normal operational mode

analysis of the mw 4 3 lorient earthquake sequence a multidisciplinary approach to the geodynamics of the armorican massif westernmost france summary a mw 4 3 earthquake occurred on 2002 september 30 in the armorican massif nw france since it was one of the largest events ever recorded in this region this was the opportunity to improve our seismotectonic knowledge of the armorican massif we performed a post seismic survey sisbreizh which allowed us to locate accurately 62 aftershocks within 14 days an analysis of the main shock using broadband records provided a normal fault mechanism with a dextral strike slip component located at 12 km depth the aftershock sequence exhibits 1 a combination of almost pure right lateral strike slip and dominant normal faulting similar to the main shock 2 magnitudes ranging from 0 4 to 1 9 and 3 depths ranging from 11 5 to 13 5 km that is close to the main shock hypocenter the distribution of the aftershocks defines a rupture plane dipping 60 to the south with a fault length of 2 km consistent with the source parameters of the main shock beside the sisbreizh survey a morpho structural analysis has been conducted we found fault plane solutions with southward dipping n120 150 normal fault planes the stress tensor computed after the aftershock focal mechanisms is a strike slip regime with a ne sw extensional direction the lorient earthquake appears to reactivate late hercynian structures and the whole sequence is reflecting the regional scale tectonic stress field expressed by a combination of strike slip and normal faulting

rupture characteristics of the three m 4 7 1992 1994 parkfield earthquakes slip on the san andreas fault was determined for three m 4 7 earthquakes using a tomographic inverse system beroza and spudich 1988 to invert seismic source time functions stfs from s waves stfs were obtained by deconvolving mainshock accelerograms by those from collocated smaller earthquakes accelerograms were from the u s geological survey parkfield small aperture array upsar and from a distributed array of digital accelerometer stations at parkfield eight or nine stfs are used in each of the three inversions stfs are typically symmetrical pulses with a duration of about 0 3 0 5 s in the inversion mainshock rise time was set to 0 05 s and we allowed the rupture time to vary slightly from a constant rupture velocity of approximately 0 85 rupture for all three events which are located in or close to the middle mountain preparation zone or box mmb quickly reaches a local maximum in slip and then propagates outward to peaks ridges or plateaus in the slip distribution slip for the october 20 1992 event located just inside the southern edge of the mmb propagates from an initial spike north and updip along a curving ridge for about 2 km the initial spike continued to grow in the november 14 1993 event located north of the october 20 1992 event just beneath the hypocenter of the 1966 parkfield earthquake which shows little directivity although there is a smaller patch of slip updip and to the south in contrast rupture for the december 20 1994 event located just south of the october 20 1992 event propagated north and slightly updip creating a rough plateau in slip a few kilometers wide on a side directivity for this event also is to the north directivity for all three events points in the approximate direction of the 1966 hypocenter small pulses which comprise a coda are found on the stfs for several seconds after the initial impulsive event several tests based on the assumption that the average of all stfs from upsar for each event is an estimate of the true slip at the source suggest that the codas in the stfs are s waves from a long duration source rather than uncorrected site response an initiation phase is found on the array average for the november 14 1993 and december 20 1994 events these precursory phases are the result of a spike in slip at the hypocenter a value of 2 4 4 mm is obtained for d c the slip weakening distance by interpreting the initial spike as a critical patch the few aftershocks for the october 20 1992 event are distributed to the north and updip of the mainshock but the november 14 1993 event had a strong burst of aftershock activity that propagated to the north of its hypocenter at roughly the same depth aftershocks of the december 20 1994 event are mostly updip the november 14 1993 event had the simplest slip distribution appeared to be the most impulsive and had the most active aftershock sequence and the greatest depth if the eventual parkfield earthquake initiates near the 1966 hypocenter then the directivity of the three events studied here will have pointed to it however it is certainly possible that both the initiation of characteristic parkfield shocks and the directivity of smaller events are controlled by fault properties on a larger scale such as by fault bends or jogs

stability and facies association of early diagenetic mineral assemblages an example from a jurassic ironstone mudstone succession u k abstract sedimentological and petrographic data from two lower liassic ooidal ironstones the frodingham and pecten ironstones and associated mudstones from eastern england have been collected within these ironstones early diagenetic berthierine siderite and minor glauconite have a limited lateral extent associated with an intrabasinal high the market weighton swell this mineral assemblage passes laterally and vertically into mudstones and silty mudstones within which pyrite is the dominant early diagenetic iron mineral the ironstones are stratigraphically condensed having experienced low net sediment accumulation rates and shallow high energy conditions thermodynamic considerations indicate that berthierine and siderite should be stable in anoxic environments where sulfide activity is negligible pyrite is rare within the ironstones suggesting that sulfate reduction was a negligible process during ironstone formation where pyrite is present it postdates berthierine and siderite indicating that sulfate reduction may have been a later process in some beds we propose that berthierine and siderite were precipitated during suboxic diagenesis where sulfate reduction was inhibited and iron reduction was the dominant process of organic matter oxidation suboxic diagenesis resulted from the extended periods of physical and biological sediment reworking during ironstone deposition which favored the oxidation of iron minerals and aerobic egradation and loss of organic matter berthierine formed during early suboxic diagenesis at the expense of detrital clays and iron oxides during later suboxic diagenesis pore water bicarbonate activities increased leading to siderite precipitation early glauconite also formed in places and it seems likely that it formed close to the oxic suboxic interface the predominance of pyrite in the mudstones indicates that sulfate reduction was the major process of organic matter oxidation during early diagenesis

variations of zonal currents in the central tropical pacific during 1970 to 1987 sea level and dynamic height measurements the annual and interannual variations of the major zonal currents of the central tropical pacific for the period 1970 1987 are described by analyzing two data sets sea level data from a few selected island stations and ship of opportunity expendable bathythermograph xbt observations the annual cycles of geostrophic surface current indices dynamic height or sea level difference between the ridge and the trough defining the current in the north equatorial current nec and north equatorial countercurrent necc are found to be in phase maxima in november to be strong and to explain about 50 of the total variance the annual cycle of volume transport relative to 400 dbar in the necc is also strong but nec volume transport has a weak annual cycle the discrepancy between the nec surface current and the transport is due to the variation of the surface current being larger than that of the deep currents the annual cycle of the transport of the south equatorial current sec is 6 months out of phase with that of the nec and necc phases of the annual cycles of the sea level and dynamic height indices of flow agree well in general but there are significant differences between the annual amplitudes of the two types of measurements these differences arise from a variety of sources dynamic height reference level assumptions longitudinal spacing of sea level gauges meridional resolution of sea level array and different signal to noise ratios for the two sets of indices the necc shows marked flow maxima during the peaks of the strong ninos of 1972 and 1982 which were followed by near disappearance of the current the following year these fluctuations are clearly present in both the surface current indices and the volume transport in 1976 and 1986 1987 moderate ninos the same pattern is not marked the el nino signal in nec transport is weak there was a strong rise of equatorial sea level during the early phases of the 1972 and 1982 ninos followed by a fall the next year this resulted in a decrease of the transport of the sec in the first year followed by an increase to a maximum during the second year the maximum coincides with a minimum in necc transport

a new asymptotic method for the modeling of near field accelerograms we study high frequency radiation from a dislocation model of rupture propagation at the earthquake source we demonstrate that in this case all the radiation emanates from the rupture front and by a change of variables that at any instant of time the high frequency waves reaching an observer come from a line on the fault plane that we call isochrone an asymptotic approximation to near source velocity and acceleration is obtained that involves a simple integration along the isochrones for every time step it is shown that wave front discontinuities critical or stopping phases are radiated every time an isochrone becomes tangent to a barrier this leads to what we call the critical ray approximation which is given in a closed form the previous results are compared with discrete wavenumber synthetics obtained by bouchon 1982 for the gilroy 6 recording of the coyote lake earthquake of 1980 the fit between the asymptotic and full numerical method is extremely good the critical ray approximation permits the identification of different phases in bouchon s synthetics and the prediction of the behavior of the signal in the vicinity of their arrival time

permian triassic extinction organic δ13c evidence from british columbia canada the permian triassic p t extinction is documented geochemically in a marine sequence deposited in a basinal setting at williston lake northeastern british columbia by using elemental and isotopic organic geochemical data from well preserved sedimentary rocks the 13 c values of kerogens in the rocks exhibit a sudden shift at the p t boundary from latest permian values of 29 1 pdb to a minimum of 32 6 2 m above the p t boundary and then back to the permian value 4 m above the p t boundary after considering various factors we conclude that reduced surface water primary productivity following the p t mass extinction is largely responsible for the observed 13 c shift the abruptness of the 13 c shift in a sequence of continuous deposition argues that the strong pulse of extinction at the p t boundary was sudden rather than gradual marine primary productivity did not recover until at least 50 to 100 ka after the time of the p t boundary so a higher atmospheric p co 2 in the earliest triassic may have resulted from buildup of dissolved co 2 owing to reduced photosynthetic carbon demand in the surface water

trace elements as source indicators in cratonic sediments a case study from the early proterozoic libby creek group southeastern wyoming to understand how grain size and sorting effect trace element distributions in sediments we compare the compositions of closely associated quartzites pelites and glacial diamictites from the 2 ga libby creek group in se wyoming we found similar ree distributions lree enriched negative eu anomalies and that most compositional differences can be explained by variable quartz dilution trace element distributions from mineral separates in a fuchsite quartzite show that fuchsite contains most of the th u ree sc co and ni and controls the whole rock ree distributions zircon contains most of the zr and hf but 5 of the hree in the pelites and 20 in the quartzites other than zr and hf most trace elements in libby creek sediments are chiefly in micas and were probably in clays in the original sediments increased eu anomalies in upper libby creek sediments signal a sudden increase in eu depleted granitoids in sediment sources which reflects late tectonic uplift mixing models favor granite ba

sedimentary chemofacies characterization by means of multivariate analysis abstract multivariate statistical analysis is applied to geochemical data from three sections forming part of the stratigraphic record of the cerro pelado formation oligocene miocene in the central region of the falcon basin northwestern venezuela our main goal is introducing and testing a statistical protocol for the identification of chemofacies in the studied sections the first step involves data preparation and cleaning selection of relevant components convenient replacement of values below the detection limit and determination of outliers second a biplot analysis allows us to infer geochemical processes that can be interpreted from a paleoenvironmental point of view detrital association redox organic matter association and carbonatic association considering such geochemical associations a constrained cluster analysis is then carried out to determine the chemofacies for each section according to the compositional nature of geochemical data all statistical analysis is conducted within a log ratio analysis framework in addition robust statistical methods are considered for outlier detection and biplot representation in order to smooth the influence of potential outliers on the estimates

anoxia and fluctuating climate recorded from the devonian carboniferous black shales tethys himalaya india a multi proxy approach due to inaccessibility and tough conditions high altitude low oxygen region very less information is available about the indian tethys himalayan sequences particularly geochemical data are scarce an attempt is made in the present study to evaluate the sediment geochemistry and paleo redox conditions of devonian carboniferous black shales from the spiti region tethys himalaya using multi proxy approach besides black shales this paper also provides the first in depth study of the geochemistry of phosphatic nodules hosted by the black shales that will allow primary and secondary processes to be understood geochemical investigations particularly redox sensitive trace element ratios for e g v v ni 0 5 ni co 4 3 7 2 v cr 2 and positive ce anomalies suggest deposition of these black shales under sub oxic to anoxic marine conditions thus recording an ocean anoxic event in the region these ocean anoxic events of devonian carboniferous spiti black shales which possess distinctive geochemical signatures can very well be correlated with global events such as hangenberg events and the lower alum shale event the span of the chemical index of alteration cia values 54 78 variation of 4si parameter 82 3 and c values 0 1 0 6 recorded in the black shales strongly indicate varying degrees of weathering conditions in the source area which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in devonian carboniferous period in the spiti region the distinct geochemical characteristics such as y sr and rare earth element ree enrichments particularly middle ree enrichments with a hat shaped pattern with high p2o5 contents up to 14 wt distinguish the nodules from the host rocks black shales sem scanning electron microscope results supplemented by xrd x ray diffraction studies of the nodules show the dominance of calcium silica and phosphatic phases the diversity of clay minerals significant variations in mineralogical composition and lack of any continuous vertical trend in the nodules suggest a relatively low impact on the primary environmental signal by burial diagenesis

intraplate seismicity new seismotectonic data in western europe abstract fault plane solutions for 128 earthquakes which occurred in france and neighbouring areas have been determined stress field information given by these data are discussed in relation to previous tectonic knowledge the western alps are characterized by a compressional regime with a fan shaped distribution of p axes which might be related to the interaction mechanism between europe and the adriatic plate the pyrenean region is mainly submitted to a south north compression tectonic activity in this area seems to be determined by the presence of two subduction zones with reverse dips in the south bay of biscay and in the central pyrenees the transition zone between these two active belts is characterized by significant seismicity with a transpression regime

reservoir quality modeling of tight gas sands in wamsutter field integration of diagenesis petroleum systems and production data the late cretaceous almond formation is a lithic rich tight gas sandstone reservoir in wamsutter field wyoming the almond has measured porosities of less than 12 and permeabilities that are generally less than 0 1 md this is the result of significant mechanical and chemical compaction precipitation of carbonate cements and authigenic clays and deep burial cementation by quartz despite the advanced diagenesis and poor reservoir quality of this tight gas sandstone touchstone diagenetic modeling was used successfully to simulate rock properties during burial basin modeling and fluid inclusion data were integrated with touchstone simulations to constrain the timing of petroleum charge into the reservoir model results indicate that the almond was initially charged by liquid petroleum when the reservoir retained high porosity and permeability subsequent gas charging flushed most of the liquids from the system as the reservoir continued to experience porosity and permeability reduction from continued quartz cementation the almond touchstone model was applied to 15 well locations throughout the field ranging from areas of low thermal stress to high thermal stress a comparison of touchstone predicted permeability and standard thermal stress yielded a correlation that was used to build a pseudopermeability reservoir quality risk map across the wamsutter area a comparison of this map with historical production data demonstrates that the map successfully predicts areas of enhanced well performance the method used to build this risk map may be applied elsewhere as a quick means of high grading areas of risk during field development

source pulse enhancement by deconvolution of an empirical green s function observations of the earthquake source time function are enhanced if path recording site and instrument complexities can be removed from seismograms assuming that a small earthquake has a simple source its seismogram can be treated as an empirical green s function and deconvolved from the seismogram of a larger and or more complex earthquake by spectral division when the deconvolution is well posed the quotient spectrum represents the apparent source time function of the larger event this study shows that with high quality locally recorded earthquake data it is feasible to fourier transform the quotient and obtain a useful result in the time domain in practice the deconvolution can be stabilized by one of several simple techniques in this paper the method is implemented and tested on high quality digital recordings of aftershocks of the jan 9 1982 miramichi new brunswick earthquake in particular seismograms from a jan 17 aftershock 017 13 33 gmt local mag 3 5 exhibit path or site effects which complicate the determination of source parameters after deconvolution the apparent far field source of this event is a simple pulse in displacement with duration 0 07 second for both p and s

observational constraints on earthquake source scaling understanding the limits in resolution abstract i examine the resolution of the type of stress drop estimates that have been used to place observational constraints on the scaling of earthquake source processes i first show that apparent stress and brune stress drop are equivalent to within a constant given any source spectral decay between 1 5 and 3 i e any plausible value and so consistent scaling is expected for the two estimates i then discuss the resolution and scaling of brune stress drop estimates in the context of empirical green s function results from recent earthquake sequences including the 1992 joshua tree california mainshock and its aftershocks i show that no definitive scaling of stress drop with moment is revealed over the moment range 1019 1025 within this sequence however there is a tendency for moderate sized m 4 5 events to be characterized by high stress drops however well resolved results for recent m 6 events are inconsistent with any extrapolated stress increase with moment for the aftershocks focusing on corner frequency estimates for smaller m

quantification of loss of calcite pyrite and organic matter due to weathering of toarcian black shales and effects on kerogen and bitumen characteristics abstract comparison of geochemical data on posidonia shale early toarcian from a shallow unweathered core and from adjacent weathered exposures of the same fades was performed results revealed that quantitatively the most affected petrographic constituent is pyrite and that organic matter and carbonate were also altered by weathering due to this process of weathering the bulk composition of organic matter especially the molecular composition of soluble organic matter and the fluorescence colour of organic particles are changed estimates of weathering rates reveal that the annual release of sulphur and organic carbon from black shales may significantly add to anthropogenic pollution

chondritic meteorite fragments associated with the permian triassic boundary in antarctica multiple chondritic meteorite fragments have been found in two sedimentary rock samples from an end permian bed at graphite peak in antarctica the ni fe co ni and p fe ratios in metal grains the fe mg and mn fe ratios in olivine and pyroxene and the chemistry of fe ni p and s bearing oxide in the meteorite fragments are typical of cm type chondritic meteorites in one sample the meteoritic fragments are accompanied by more abundant discrete metal grains which are also found in an end permian bed at meishan southern china we discuss the implications of this finding for a suggested global impact event at the permian triassic boundary

frequent euxinia in southern neo tethys ocean prior to the end permian biocrisis evidence from the spiti region india abstract in this study we reconstruct water column oxygen availability during the deposition of late permian shales wuchiapingian to approximately early changhsingian using the geochemistry and morphology of sedimentary pyrite fes2 deposition of the shales occurred prior to the end permian mass extinction within the southern neo tethys ocean spiti region himachal pradesh india we found that the variability of pyritic sulfur to organic carbon spyr corg and the pyritic sulfur isotopic composition 34spyr did not record changes in oxygen availability at spiti sections this was due to the removal of original pyritic sulfur by modern oxidative weathering and diagenetic pyrite formation which overwhelmed the original 34spyr values however pyrite framboid size distributions was a reliable recorder of paleoredox conditions the degree of oxygen limitation varied with multiple transient euxinic events interrupting a predominantly oxic dysoxic trend near the top of the gungri formation upper member approximately late wuchiapingian to early changhsingian these transient euxinic intervals provide important insight and regional evidence of environmental stress and instability in the southern neo tethys well prior to the end permian mass extinction

geochemistry of shales of the upper cretaceous hayang group se korea implications for provenance and source weathering at an active continental margin abstract shales of the upper cretaceous hayang group in the yeongyang subbasin southeastern korea were deposited in fluvio lacustrine environments as the paleo pacific izanagi plate obliquely subducted beneath the asian continent major and trace element compositions derived from shales analyzed from the hayang group reveal geochemical signatures that are different from post archean average australian shale paas but are typical of igneous rocks found in the continental margin island arc settings for example most major element concentrations are depleted relative to paas among them mgo and mno depletions are most pronounced in all shales cao enrichment is notable in some formations due to the presence of secondary calcite as dispersed micro calcite nodules sc y and th concentrations are also slightly depleted relative to paas while chondrite normalized ree data for shales from the hayang group show moderate to large uniform lree enrichment but variable hree depletions in addition they are more fractionated than paas and have significant negative eu anomalies moreover the chemical index of alteration and a cn k relations comply that the source area was dominated by non steady state weathering regimes indicative of active uplift along an active continental margin these regions were punctuated by two periods of steady state weathering or alternatively by enhanced recycling of sediments the latter interpretation is in accord with the timing of exposure of a cherty sequence in the source area mixing calculations based on ree data suggest that the average shale in the hayang group was derived from sources composed of precambrian granitic gneiss arc basaltic rocks triassic yeongdeok granite in the subbasin and the mino accretionary complex of sw japan with ratios around 43 7 31 8 73 2 26 4 15 8 36 7 15 4 2 8 33 9 14 5 0 0 33 8 however these proportions change up sequence

the whole rock cerium anomaly a potential indicator of eustatic sea level changes in shales of the anoxic facies abstract the whole rock cerium anomaly tested for outer shelf upper slope stratigraphic sections from the middle ordovician through the lower silurian of scotland is proposed as an empirical technique to develop a eustatic 3rd order or finer scale sea level curve this interval was chosen as it straddles the well documented late ordovician glaciation and can be defined by graptolite zones the anomaly is calculated from neutron activation analysis of low carbonate phosphate free fossil free field identified shales of the graptolite facies by comparison of the normalized cerium content with the linearized trend of the normalized composition of other rare earth elements in order of atomic number for sections originally deposited in the main pycnocline below the surface mixed layer values of the anomaly for a given sample would indicate its position on the redox curves developed for the early paleozoic by wilde 1987 changes in the anomaly that are positive with time would indicate a lowering of sea level as the apparent depth on the redox curve would reflect more oxic conditions relative changes negative with time would indicate a rise in sea level as the apparent depth reflects more anoxic conditions depending on the vertical sample spacing and the time interval sampled resolution of the order of 1 m y might be achieved thus the vail et al 1977 curves of the 3rd order 1 to 10 m y or of finer scale could be obtained by this technique with the proper choice of section accordingly for the early through middle paleozoic when the main pycnocline was anoxic this geochemical technique could be used to develop eustatic sea level curves and additionally offer an independent calibration for seismic stratigraphy as well as an indicator of glacial interglacial climatic sequences or eustatic changes due to fluctuations in global ridge crest volumes

inorganic geochemistry of the type caradoc series sandbian to middle katian upper ordovician onny valley shropshire uk the geochemistry and petrology of the type section of the caradoc series in the onny valley indicate that it was deposited on a marginal basin continental shelf similar to the western side of the present sea of japan the lower beds form a transgressive regressive sequence in which the rocks become less mature upwards all the coarser sediments above the basal quartzites and conglomerates are greywackes in which the apparent muddy and ferrous matrix is due to the breakdown of unstable minerals and particles higher values of na 2 o and na k ratios are found in the coarser shallow water sandstones of the horderley sandstone formation and decrease markedly in the succeeding beds accompanied by an increase in k 2 o higher values of carbonate corrected and hence related other major and minor elements like sio 2 cao p 2 o 5 mno and most trace elements correlate with the transgressive systems and maximum flooding surfaces of the three sequences recognized where they are related to condensation at those horizons chemical indices of alteration cia suggest that the horderley sandstone formation underwent greater predepositional physical weathering than lower and higher beds which is compatible with the petrography and were deposited during a cool phase within overall warm sandbian katian times trace element ratios suggest an oxic to suboxic depositional environment

how does an asperity break new elements from the waveform inversion of accelerograms for the 2319 ut october 15 1979 imperial valley aftershock the 16 accelerograms obtained at less than 30 km during the 2319 ut october 15 1979 imperial valley aftershock were analyzed after having been corrected for the crack induced anisotropy in the shallow sediments both p and s waveforms are used to study the earthquake source process and to infer an estimate of attenuation and site amplification effects on the waveforms at frequencies 0 6 1 hz and 2 4 hz the comparison between the 0 6 1 hz p and s wave seismic moment measures and their dependence on distance has provided an estimate for this parameter 1 9 40 1023 dyn cm and the bound limits for attenuation qs 0 6 1hz 20 25 and near site amplification of shear waves at low frequency the inversions of the 2 4 hz s velocity waveforms for different values of rupture velocities and attenuation parameters have been preliminarily performed to investigate the variability of static slip and static stress drop estimates a circular growth of the rupture at a constant velocity and a final elliptical shape are assumed the bipolar character of the velocity waveform excludes the asperity models consisting of a small highly stressed area inbedded in a large stress free crack therefore only the slip distribution of a quasidynamic crack with healing phases has been considered the dynamic stress release has been independently measured from the amplitude level of the first acceleration pulses for various rupture velocities and attenuation the constraint that the static stress drop does not significantly exceed the dynamic stress drop suggests relatively high qs values 200 at 2 4 hz and small rupture velocities ur 0 5 0 7 is the shear wave velocity the elliptical source is about 1 2 km long and 0 7 km wide 0 2 km elongated in the nw upward direction the nucleation point location indicates a dominant propagation toward the nw top fault direction for the inferred values of attenuation parameter and rupture velocity the minimum values of the dynamic and static stress drop are 300 700 bars and are associated with about l m of average slip on the fault the nucleation of the rupture is explained as an effect of the high loading stress on a larger size barrier left unbroken by the mainshock in which the aftershock fault area is embedded the hetereogeneous stress and rock strength distribution on the barrier region would therefore have controlled the rupture development and arrest an estimate of the s amplification in the considered frequency bands has been obtained through the seismic moment computed from the p wave records which are less sensitive to site amplification and attenuation effects for the sites under study at frequencies below 5 hz the results show a differential amplification of s waves 3 at 1 hz and 2 at 2 4 hz which is attributed to the propagation in the very shallow low velocity sediments of the imperial valley

resolving original signatures from a sea of overprint the geochemistry of the gungri shale upper permian spiti valley india abstract the geochemistry of black shales can faithfully record paleoenvironmental conditions however the original geochemical signatures are obscured by post depositional events such as diagenesis and weathering resolving the relative influence of these processes on the geochemistry of shales requires a combination of geochemical and chemometric chemical statistics approaches to data analysis that identify the sources of variance in major and trace element geochemistry the permian gungri shale kuling group at attargoo spiti valley himachal pradesh india is a gray to black shale capped unconformably by an iron rich pebble sand layer ferruginous layer marking ostensibly the permian triassic boundary high resolution analysis of the gungri at attargoo 3 cm intervals reveals a geochemical record complicated by low grade metamorphism diagenesis and modern weathering using multivariate chemometric techniques e g principal component analysis we identified variance in the geochemical record associated with the original detrital and depositional conditions we also identified 4 beds within the gungri that mark depositional events which may correlate to those identified in other pt sections e g guryul ravine kashmir identification of sources of variance allowed us to identify geochemical signatures that can be used for paleooceanographic reconstruction the attargoo section records a sequence of discrete events including transient iron enrichments attributed to euxinic depositional conditions and late diagenetic formation of siderite and regression condensation surfaces associated with pulsed transgression regression towards the uppermost permian our results show that chemometric analysis of geochemical data enables the resolution of paleoenvironmental conditions in shales impacted by post depositional alteration

water depth and diagenetic constraints on the use of barium as a palaeoproductivity indicator sediments underneath the high productivity areas of the coastal upwclling and equatorial divergence zones in the pacific and indian oceans are enriched in barium and on this basis the barium content of the sediments has long been suggested as a potential palaeoproductivity indicator analyses of sediments from the peru margin corroborate the pattern of enhanced barium accumulation in areas of high primary productivity in the deep ocean but also show that this pattern cannot be extended to shallow water deposits sediments from the peru shelf lack any barium enrichment whereas this element is significantly enriched in slope and basinal deposits in water columns deeper than 2000 m this depth effect is further illustrated at site 682 located in the rapidly subsiding lima basin the quaternary sequence deposited at water depths 3000 m is enriched in organic carbon opal and biogenic barium the miocene sediments although deposited under highly productive waters associated with a coastal upwclling centre do not show an enrichment in the barium record corresponding to the very high organic carbon and opal levels a pattern that is consistent with deposition in shallow basins in addition to the effect of water depth on the barium distribution overprinting of primary signals can be observed in sediments from upwclling areas undergoing strong anoxic diagenesis microcrystalline barite is partly dissolved in intervals depleted in interstitial sulphate as shown for sediments from the peru margin dissolution of barite in sulphate depleted intervals of deep water sections leads to high barium concentrations near the termination of the sulphate reducing zone where downward sulphate and upward barium diffusion foster local barite precipitation in diagenetic fronts the barium distribution in sedimentary oxic and suboxic environments at deep water depositional sites has a high potential as a palaeoproductivity indicator however barium accumulation as a proxy for ocean fertility should be used with caution in areas of varying water depths and in anoxic diagenetic environments where sulphate depletion undcrsatur ates the interstitial waters with respect to barite

formation of syngenetic and early diagenetic iron minerals in the late archean mt mcrae shale hamersley basin australia new insights on the patterns controls and paleoenvironmental implications of authigenic mineral formation a section through the late archean mt mcrae shale comprising in ascending order a lower shale interval lsi a banded iron formation bif an upper shale usi and a carbonate c1 has been analyzed for total fe and al contents and authigenic fe present as carbonate oxide sulfide and silicate phases the authigenic mineralogy is controlled by the episodic addition of fe from hydrothermal activity and removal of fe by sulfide relative to rates of clastic sedimentation the lsi and bif have mean fet al values of 2 and 5 respectively that record iron enrichment from hydrothermal sources iron was precipitated primarily as siderite accompanied by fe rich chlorite from anoxic bottom waters rich in dissolved fe pyrite formation was probably limited by the availability of sulfate which was present at low concentrations and became rapidly depleted the usi has generally lower fet al values 0 6 1 3 similar to those found in paleozoic shales with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle this interval contains authigenic fe predominantly as pyrite where high values for dop 0 8 indicate the existence of a water column that became rich in dissolved sulfide euxinic when sulfate concentrations increased due to a transient or secular increase in ocean atmosphere oxygenation high concentrations of dissolved sulfide maintained low concentrations of dissolved fe which allowed only minor amounts of fe to be precipitated as carbonates and silicates the usi also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin the carbonate c1 represents a basinal chemistry where sulfide has been removed and fet al values are 1 indicating that hydrothermal activity again produced dissolved fe rich bottom waters detailed iron speciation of the mt mcrae shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late archean hamersley basin just prior to the paleoproterozoic rise in atmospheric oxygenation and our refined methods have relevance to all fe rich deposits

the negative eu anomaly in archean sedimentary rocks implications for decomposition age and importance of their granitic sources abstract a negative correlation between eu anomaly eu eu and chemical index of weathering ciw is found for archean pelites in both greenstone belts and high grade terrains from southern africa western greenland north america western australia and china a similar negative correlation betweeneu eu and chemical index of alteration cia is also found for worldwide young deep sea sediments from various tectonic settings the increase in the indices of weathering and alteration coincide with both shifts in mineral composition and mineral decomposition from plagioclase k feldspar to mica kaolinite and from amphibole to chlorite smectite juvenile crustal materials from local sources are only slightly decomposed due to little or no transport they are characterized by low ciw cia and negligible eu anomalies in contrast crustal materials from large provenance areas have usually suffered long transport and recycling which results in decomposition of high temperature minerals and large ciw cia values that are negatively correlated with significant negative eu anomalies thus the observed trends are interpreted as mainly reflecting increasing contributions of recycled and decomposed materials from large provenances relative to juvenile crustal additions through arc magmatism at local sources in this context there is no obvious distinction between archean greenstone and high grade metasediments and young deep sea sediments the large provenance areas have contained a significant proportion of granitic and rhyolitic materials which in part have not resisted decomposition and erosion due to their occurrence in shallow crustal layers granites received their distinctive negative eu anomaly from equilibration with a feldspar rich residuum the negative eu anomaly of archean metaclastic sediments apparently increased with decreasing age trends of minimum and averageeu eu values of metaclastic sediments evolving with time are calculated for the archean and lead us to infer that the negative eu anormaly eu eu

catastrophic environmental transition at the permian triassic neo tethyan margin of gondwanaland geochemical isotopic and sedimentological evidence in the spiti valley india abstract interpreting global consequences of the permian triassic p tr extinction requires examination across paleogeographic realms of pangaea the spiti valley in india remnant of the peri gondwanan shelf preserved trails of this environmental catastrophe in the neo tethys ocean we document new sedimentological observations and high resolution trace element concentrations and carbon oxygen lead isotope data across the p tr boundary in spiti framboidal pyrites fossils and laminated lithology of the late permian shales indicate deeper anoxic depositional environment while 13 c org excursions of 2 4 and 3 1 in atargu and guling outcrops respectively identify the p tr transition across a clayey partly gypsiferous ferruginous layer sedimentological similarities of this layer to other neo tethyan sections from transcaucasia and iran indicate subaqueous oxidation of shallow marine sediments on a regional scale light rare earth element enriched late permian shales with conspicuous ce eu anomalies reflect their source from the adjacent panjal trap basalts ca 289 ma of kashmir continental crustal nb ta and zr hf anomalies appear at the p tr boundary sediments and prevail through the overlying early triassic carbonates original pb isotope ratios along with an increasing pb abundance closer to the p tr boundary distinguish the volcanic source of the late permian shales from the continental crustal siliciclastic signature of the early triassic carbonates our 13 c org trace element and pb isotope record from spiti indicate catastrophic changes in sediment sources and facies with effects on carbon cycle and are consistent with an abrupt episode of marine regression and erosional forcing also observed elsewhere along northern gondwanaland simultaneous eruption of siberian volcanics and bolide impacts in parana basin of brazil and elsewhere implicating impact triggered volcanism left catastrophic regional global imprints on sea level climate marine anoxia and tectonic stability that connected the p tr crisis across terrestrial and marine realms worlwide

effect of diagenesis on trace element partitioning in shales abstract in order to investigate the partitioning of trace elements in mineral phases of black shales and diagenetic remobilization of trace elements within these phases we isolated major black shale components sulfide carbonate silicate and organic and analyzed the isolated fractions as well as corresponding whole rock for their trace element composition by drc icp ms this study focused on black shales of different grades of thermal maturity from the ordovician utica shale magnafacies of quebec usm immature tmax 20 50 c ro 140 c ro 1 analysis of the isolated rock fractions revealed that the carbonate fraction preserves a sea water like rare earth elements ree abundance pattern with enrichment of the heavy rare earth elements hree dy lu relative to light rare earth lree la sm the sulfide and organic fractions show a slight enrichment in the lree mass balance calculations showed that the organic fraction contains up to about 20 of total trace element content of the whole rock with the sulfide and carbonate fractions each containing less than 10 the partitioning and fractionation of trace elements between different phases may have important implications for paleoenvironmental reconstructions and provenance studies our data suggest that the whole rock signature may be less representative of source area or depositional environment but rather reflects the diagenetic history this is especially the case in organic rich sediments such as black shales where the organic fraction hosts up to 20 of the trace element content of the whole rock

properties of the seismic nucleation phase abstract near source observations show that earthquakes begin abruptly at the p wave arrival but that this beginning is weak with a low moment rate relative to the rest of the main shock we term this initial phase of low moment rate the seismic nucleation phase we have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1 1 8 1 the size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake the seismic nucleation phase is characteristically followed by quadratic growth in the moment rate consistent with self similar rupture at constant stress drop in this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it

permo triassic boundary and lower to middle triassic in south tibet abstract the abundance of conodonts increases sharply across the permian triassic boundary in south tibet and as a consequence griesbachian assemblages are much better defined than changxingian ones the otoceras latilobatum bed representing the base of the triassic at selong is a condensed biocalcirudite including abundant macrofossils crinoids corals bryozoans brachiopods of permian aspect associated with a varied conodont assemblage including hindeodus parvus of early griesbachian age condensation and reworking at the p t boundary is interpreted as related to starvation and subaqueous erosion redeposition during rapid transgression at the base of the triassic rather than to subaerial exposure during a first order sea level drop all of the lower triassic is strongly condensed particularly in the west gyirong and selong where total thickness is 6 5 m at most to 8 5 m thickness is more comparable with other himalayan localities at tulong where the dienerian to spathian section reaches 20 m as in central nepal carbonate intervals dated as early griesbachian to dienerian early to mid smithian and latest smithian to earliest aegean are separated by two mudrock intervals locally reduced to millimetric interbeds dated as late dienerian or earliest smithian and late smithian condensed wackestone packstone intervals were deposited during transgressive stages on the outermost shelf to continental slope during highstand stages mudrock intervals slowly accumulated on the outer shelf whereas only a thin veneer of mud was deposited close to the shelf break oceanic currents intruding onto the outermost shelf during peak transgressions are inferred to be the main causes of winnowing and resuspension of mud during the spathian when oxidized packstones in ammonitico rosso facies were deposited at tulong grey limestones locally with bacterial mats or containing agglutinated foraminifers and intercalated black mudrocks indicate instead dysoxic conditions which were widespread from griesbachian to smithian times the middle triassic is mostly represented by marls and marly wackestones with crinoids pelagic bivalves and benthic foraminifers a notable increase in silty to sandy quartzo feldspathic detritus is recorded at mid aegean times and very fine grained subarkoses characterize the mid anisian section at tulong terrigenous supply was renewed probably in the early carnian but accumulation rates sharply increased only in the latest carnian and norian

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

mineralogic controls on ree mobility during black shale diagenesis abstract the llandeilo caradoc black shales of southwest wales u k record a paragenetic sequence that requires redistribution of rare earth elements ree on a mineralogic scale during diagenesis the main evidence for this redistribution is formation and replacement of ree rich apatite and monazite and variations in whole rock ree patterns the processes that produce these mineralogic changes create a diagenetic overprint on the llandeilo caradoc black shales that obscures geochemical information related to both provenance and paleoenvironmental conditions of deposition periods of open system conditions existed during the diagenetic alteration of the llandeilo caradoc black shales which allow an evaluation of the extent and timing of the ree redistribution inferred from petrography the observed variations in la sm ratios in the whole rock ree patterns are strong evidence that ree were redistributed on a scale greater than an individual hand sample this change in the whole rock ree pattern altered the cerium anomaly by as much as 20 and produced a range in the sm nd ratio from 0 14 to 0 20 this change in the whole rock sm nd ratio indicates a disturbance in the sm nd isotopic system at about 460 ma that can be directly linked to the formation and dissolution of diagenetic apatite and monazite during diagenesis one effect of this disturbance is to alter the range in nd model age for the llandeilo caradoc black shales from 1 6 to 1 8 ga to about 1 4 to 2 25 ga thus complicating any interpretation of provenance the ability to link petrographic observations from a black shale to measurable whole rock geochemical changes provides us with one tool to evaluate the presence of a diagenetic overprint on trace elements commonly used in paleoenvironmental reconstruction

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

a unique geochemical record at the permian triassic boundary a 330 metre core drilled through the marine permian triassic boundary in the carnic alps of austria allows closely correlated studies of geochemistry petrography and palaeontology across the boundary the isotope shifts and metal concentrations are extended multiple and complex and do not resemble those seen at the cretaceous tertiary boundary both the carbon isotope shifts and the chemical events including an indium anomaly may have causes related to a major regression of the sea

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

himalayan ironstones superplumes and the breakup of gondwana oolitic ironstones or glauco phosphorites were deposited all along the northern india continental margin during major tectonically enhanced transgressions in frasnian permian norian callovian early cretaceous and paleocene time these regional events of starvation and drowning were commonly associated with alkalic to tholeiitic magmatism and invariably coincided in time with multiple rifting events that punctuated the progressive disintegration of gondwana deposition of major condensed horizons enriched in iron silica and phosphorus was fostered by global factors such as eustatic rise warm greenhouse climates sluggish circulation increased fertility and poor ventilation in the oceans such anomalous sedimentary conditions recurred synchronously during magnetic quiet zones pointing to successive episodes of global fast spreading and accelerated terrestrial heat loss multiple pulsations of earth are inferred to have triggered supercontinent breakup and widespread volcanism thus altering the atmosphere and hydrosphere and dramatically affecting sedimentary and biological systems

long term isovolumetric leaching of aluminum from rocks during weathering implications for the genesis of saprolite abstract isovolumetric geochemical analyses of 26 weathering profiles saprolites on crystalline rocks in brazil and south carolina indicate that on average si and al are leached from rocks in a long terms molar ratio of 8 3 hewever with the exception of the rio negro river the molar ratio of si to al in rivers draining these regions is about 100 if saprolites are still forming the large discrepancy in al mobilization between the saprolite record and modern river chemistry may be due to degassing of dissolved co2 from soil water entering streams with attendant increase in ph and al precipitation if saprolites are relic the rio negro basin may be a model for the environmental conditions under which they form these conditions are 1 low relief 2 abundant rainfall and intense leaching and 3 organic rich soils which produce large concentrations of co2 and dissolved organic acids in the soil abundant rainfall maintains dilute soil water large co2 pressure lowers ph dissolved organic matter complexes al and fe and thus enhances the solubility of secondary clays all of these effects hinder the precipitation of secondary weathering products and thus promote rock dissolution that is more nearly congruent the problem with the rio negro model is to reconcile the low relief of the basin which inhibits rapid groundwater movement with the short contact time indicated by the diluteness of its chemistry

depositional environments provenance and paleoclimatic implications of ordovician siliciclastic rocks of the thango formation spiti valley tethys himalaya northern india abstract recently published findings indicate that the ordovician period has been much more dynamic than previously anticipated thus making this period significant in geological time the ordovician of india can best be studied in the spiti region because the spiti basin records the complete uninterrupted history of excellent marine sedimentary rocks starting from cambrian to paleogene which were deposited along the northern margin of india due to these reasons the geochemical data on the ordovician rocks from the spiti region is uncommon the present geochemical study on the ordovician thango formation sanugba group is mainly aimed to understand the provenance and the paleoclimatic conditions the sandstones are the dominant lithology of the thango formation with intercalations of minor amount of shales detailed petrographic and sedimentological analysis of these rocks suggest that three major depositional environments viz fluvial transitional and marine prevailed in the basin representing transgressive and regressive phases the major and trace element ratios such as sio2 al2o3 k2o na2o and la th sc discrimination diagram suggest that these rocks were deposited in passive margin tectonic settings various geochemical discriminants and elemental ratios such as k2o na2o al2o3 tio2 la sc th sc cr th zr sc gd yb n and pronounced negative eu anomalies indicate the rocks to be the product of weathering of post archean granites the striking similarities of the multi elemental spider diagrams of the studied sediments and the himalayan granitoids indicate that sediments are sourced from the proterozoic orogenic belts of the himalayan region chemical index of alteration cia values of the studied sediments 55 72 suggest that the source rocks underwent low to moderate degree of chemical weathering the span of the cia values 55 72 recorded in the sediments from the spiti region may have resulted from varying degrees of weathering conditions in the source area this in turn reflects variable climatic conditions prevailing during the ordovician period which is consistent with the global studies the high cia values observed by numerous studies on siliciclastic sediments may imply warm paleoclimate attributed to an increase in atmospheric carbon dioxide and or methane levels inducing an acidic weathering environment and enhanced silicate weathering while the low cia values indicates prevalence of arid glacial conditions where negligible chemical alteration is possible this study documents the noteworthy events like fluctuating rise and fall atmospheric co2 and sea levels during the ordovician period a conclusion which is consistent with the other such studies on the phanerozoic glaciation or mass extinction events on gondwana supercontinent

pyrite framboid study of marine permian triassic boundary sections a complex anoxic event and its relationship to contemporaneous mass extinction size analysis of pyrite framboids has been undertaken on epicontinental permian triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia mid paleolatitude sections from the margins of the boreal spitsbergen greenland and neotethyan oceans western australia reveal intense anoxia throughout the permian triassic boundary interval with euxinic conditions frequently developing and dysoxia encountered even in relatively shallow water settings above storm wave base at equatorial paleolatitudes weakly oxygenated dysoxic conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition although euxinia was rare western and eastern tethyan locations re veal a complex and unstable redox history anoxia in the hindeodus praeparvus zone was replaced by oxygenated facies in the permian triassic boundary interval h changxingensis to h parvus zones oxygen poor deposition returned during the succeeding isarcicella isarcica zone the more persistent and intense development of oxygen restriction in cooler water mid paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of permian triassic bound ary anoxia in higher paleolatitudes the ben thic invertebrate extinctions occurred during a prolonged phase of oxygen poor deposition while in equatorial tethyan locations benthic losses occurred at the end of the fi rst anoxic phase in the late h praeparvus zone

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

large scale climatic fluctuations in the latest ordovician on the yangtze block south china likely under warm and humid conditions low cia values however indicate the near absence of chemical weathering thereby refl ecting cool and or arid conditions in this paper the paleo climatic changes during the ordovician silurian transition were reconstructed in the light of cia values derived from the chemical composition of fi ne grained siliciclastic rocks in the mid upper yangtze river region china the infl u ences of paleoclimatic changes on biota are also discussed abstract the ordovician silurian transition was a critical interval in earth s history marked by dramatic climatic oceanic and biological turnovers here we present the chemical index of alteration cia as a proxy of changes in intensity of chemical weathering and its varia tions across the ordovician silurian boundaries wufeng through guanyinqiao to longmaxi formations from wangjiawan and nanbazi on the yangtze block south china in order to explore the climatic changes our data show that the cia values of sediments commonly range from 75 to 90 in the wufeng and longmaxi formations indicating a high degree of chemical weathering and thus a hot and humid climate during deposition in contrast a sharp drop in cia values most 60 70 in the guanyinqiao formation or hirnantian suggests an overall cold and arid climate interrupted by several intervals of warm climate when deposited the temporal coincidence of two phases of massive biotic extinctions with the beginning and end of the cold climate epoch respectively suggests that the large climatic changes could be one of the main controls on the mass extinctions although other factors may also have played a role

rupture complexity of a moderate intraplate earthquake in the alps the 1996 m5 epagny annecy earthquake summary the magnitude 5 epagny annecy earthquake of 1996 july 15 is the largest seismic event to have occurred in the alps since the introduction of modern digital instrumentation this strike slip event was located on the vuache fault near the town of annecy in the northern french alps the aim of our work was to retrieve the main parameters of the rupture process of this earthquake from seismograms recorded at local and regional distances 20 300 km to eliminate path and site evects from the seismograms we compared the main shock recordings at each station with those of the largest aftershocks nearby we used a combination of techniques including pulse width measurements and cross correlation of velocity traces comparison of p wave displacement pulses and empirical green s function deconvolution to retrieve the apparent duration of the rupture process as seen at each station our results demonstrate that in the absence of on scale data p wave pulse width measurements on clipped signals can be misleading if the rupture process is complex in the case of the annecy earthquake comparisons of on scale p wave displacement seismograms and the empirical green s function deconvolutions show that the rupture process consisted of at least two subevents separated by 0 2 0 3 s and with a total duration of about 0 5 s the systematic azimuthal dependence of both the shape and duration of the apparent source time function is consistent with a nearly unilateral propagation of the main rupture phase in a southeast direction along the fault plane and parallel to the direction of slip an isochron analysis reveals that the first subevent occurred slightly to the northwest of the nucleation point but that the second subevent was located further to the southeast thus confirming the overall rupture directivity towards the southeast an interpretation of our results in light of the previously documented aftershock distribution and of observations of ground cracks in the epicentral area suggests that the main shock occurred on the vuache fault and that rupture in a northwest direction was inhibited by a right lateral stepover in the fault accordingly the vast majority of the subsequent aftershocks which include several magnitude 3 4 events occurred on a fault segment that is slightly ovset from the inferred surface trace of the vuache fault and that was activated by the main shock

tracking euxinia in the ancient ocean a multiproxy perspective and proterozoic case study the evolution and extinction of life are tied intimately to the oxygen state of the ocean and particularly to the presence of anoxic and h2s containing euxinic water on a global scale anoxia and euxinia were more common in the past relative to today s 0 5 euxinic seafloor we are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies with particular emphasis on fe s mo analysis of fine grained siliciclastic rocks for the latter establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well dated well preserved materials that span shallow and deep environments across time lines despite these difficulties the multiple approaches synthesized in our case study point to global oxygen deficiency in the deep ocean and perhaps euxinia during most if not all of the proterozoic and likely extending into the early paleozoic

statistical empirical index of chemical weathering in igneous rocks a new tool for evaluating the degree of weathering abstract chemical weathering indices are useful tools in characterizing weathering profiles and determining the extent of weathering however the predictive performance of the conventional indices is critically dependent on the composition of the unweathered parent rock to overcome this limitation the present paper introduces an alternative statistical empirical index of chemical weathering that is extracted by the principal component analysis pca of a large dataset derived from unweathered igneous rocks and their weathering profiles the pca analysis yields two principal components pc1 and pc2 which capture 39 23 and 35 17 of total variability respectively the extent of weathering is reflected by variation along pc1 primarily due to the loss of na 2 o and cao during weathering in contrast pc2 is the direction along which the projections of unweathered felsic intermediate and mafic igneous rocks appear to be best discriminated therefore pc1 and pc2 represent independent latent variables that correspond to the extent of weathering and the chemistry of the unweathered parent rock subsequently pc1 and pc2 were then mapped onto a ternary diagram mfw diagram the m and f vertices characterize mafic and felsic rock source respectively while the w vertex identifies the degree of weathering of these sources independent of the chemistry of the unweathered parent rock the w index has a number of significant properties that are not found in conventional weathering indices first the w index is sensitive to chemical changes that occur during weathering because it is based on eight major oxides whereas most conventional indices are defined by between two and four oxides second the w index provides robust results even for highly weathered sesquioxide rich samples third the w index is applicable to a wide range of felsic intermediate and mafic igneous rock types finally the mfw diagram is expected to facilitate provenance analysis of sedimentary rocks by identifying their weathering trends and thereby enabling a backward estimate of the composition of the unweathered source rock

spatial variation in sediment fluxes redox conditions and productivity in the permian triassic panthalassic ocean abstract two permian triassic boundary sections in central japan provide a rare window into environmental conditions within the panthalassic ocean which encompassed more than half the earth s surface at 252 ma integration of petrographic geochemical and time series data provides new insights regarding the fluxes of major and trace components to the sediment as well as environmental conditions in both the deep and intermediate water masses at each study site the ubara section was located in a high productivity peri equatorial location whereas the gujo hachiman section was located in a moderate productivity location at some distance from the paleoequator an upward transition from gray organic poor cherts to black siliceous mudstones at both sites occurred in conjunction with increased primary productivity intensified euxinia within the oxygen minimum zone omz and decimation of the radiolarian zooplankton community euxinia in the omz of the equatorial panthalassic ocean developed episodically for a 200 250 kyr interval during the late permian followed by an abrupt intensification and lateral expansion of the omz around the permian triassic boundary throughout the study interval bottom waters at both sites remained mostly suboxic a finding that counters hypotheses of development of a superanoxic permo triassic deep ocean as a consequence of stagnation of oceanic overturning circulation

a tethys wide mid carnian upper triassic carbonate productivity crisis evidence for the alpine reingraben event from spiti indian himalaya abstract a significant mid carnian lithological change from carbonatic to siliciclastic sedimentation is described from the spiti himalaya n india the recorded sudden increase of terrigenous influx occurs at the boundary from the chomule to the rama formation it is dated within the late lower carnian uppermost aonoides zone and isochronous all over the tethys himalaya bio and chemostratigraphic data point to a synchrony with changes in carbonate productivity known from on area of wide extent along the nw tethys the reingraben event of the northern calcareous alps austria defines a regional decline in carbonate productivity related to both a platform demise and enhanced siliciclastic input laminated shales found in a distal part of the spiti basin document the onset of anoxic conditions within the deeper shelf synchronous to the reingraben event in austria high resolution biostratigraphy and identical 13ccarb curves are therefore in support of a direct link between the change in spiti and the reingraben event our data combined with a comprehensive review of previously postulated scenarios regarding the reingraben event suggest linking of climatic change with siliciclastic input and a carbonate productivity crisis initiated by onset of a relatively humid megamonsoonal late lower carnian atmospheric system that affected both the southern and northwestern tethys and point to a supra regional trigger of the alpine reingraben event

source study and tectonic implications of the 1995 ventimiglia border of italy and france earthquake ml 4 7 we analyse the source process and the aftershock distribution of the april 21 1995 ventimiglia ml 4 7 earthquake using the records of permanent high dynamic broad band seismic stations and a temporary network deployed on land and at sea few hours after the earthquake this event occurred on the western mediterranean coast near the border between italy and france at a depth of 9 km at a point where alpine tectonic units and late oligocene extensional structure overlap and are currently undergoing compressional stress the focal solutions of the mainshock and three aftershocks depict a dominant reverse faulting with an important strike slip component which underlines two nodal planes a nw se dipping north fault and a ne sw dipping south fault we operate a careful re location of the aftershocks using a master event technique and data from the temporal network and obtain a predominant nw se alignment then we analyse the rupture process using an empirical green function approach we find that the mainshock broke a 0 5 to 1 km fault length and that the rupture propagated during 0 1 0 2 s probably in a se direction those two arguments together with the recent fault trace that exists close to the epicentre leads us to propose that this event expresses the reactivation of an old transverse nw se structure with a dextral movement this study thus emphasizes the role of inherited deep rooted transcurrent features in the tectonic reactivation of this passive margin it also underlines the importance of combining short period and broad band seismology to better resolve and understand regional tectonic processes in areas of moderate seismic activity and complex geology

the geochemical behavior and isotopic composition of hg in a mid pleistocene western mediterranean sapropel abstract the concentrations of mercury hg and other trace metals ni cu zn mo ba re u and the hg isotopic composition were examined across a dramatic redox and productivity transition in a mid pleistocene mediterranean sea sapropel sequence characteristic trace metal enrichment in organic rich layers was observed with organic rich sapropel layers ranging in hg concentration from 314 to 488 ng g avg 385 with an average enrichment in hg by a factor of 5 9 compared to organic poor background sediments which range from 39 to 94 ng g hg avg 66 comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers hg to the seafloor near complete scavenging of hg from the water column renders the sapropel hg isotopic composition representative of mid pleistocene mediterranean seawater sapropels have an average 202hg value of 0 91 0 15 n 5 1 sd and 199hg value of 0 11 0 03 n 5 1 sd background sediments have an average 202hg of 0 76 0 16 n 5 1 sd and 199hg of 0 05 0 01 n 5 1 sd which is indistinguishable from the sapropel values we suggest that the sapropel isotopic composition is most representative of the mid pleistocene tyrrhenian sea

determination of rupture duration and stress drop for earthquakes in southern california a simple technique is developed for determining the rupture duration and stress drop of earthquakes between magnitudes 3 5 and 4 0 using the time between the p wave onset and the first zero crossing 1 2 on seismograms from local seismic networks this method is applied to 10 main shocks in southern california to investigate regional variations in stress drop the initial pulse widths of 65 foreshocks or aftershocks of these events were measured values of 1 2 for small earthquakes below about magnitude 2 2 are generally observed to remain constant with decreasing magnitude in four sequences studied the relative pulse width of a particular main shock m 3 5 at a given station is found to be correlated with the relative pulse width of its aftershocks recorded at that station these observations are interpreted to signify that the waveforms of these small events m 2 2 are essentially the impulse response of the path between the source and receiver values of 1 2 determined from small foreshocks and aftershocks are therefore subtracted in effect deconvolved from those of each main shock to obtain an estimate of the rupture duration of the main shock which is corrected for path effects significant variations in rupture duration and stress drop are observed for the main shocks studied aftershock locations and azimuthal variations in 1 2 both indicate that the rupture zone of one earthquake expanded unilaterally a factor of 10 variation in stress drop is calculated for two adjacent events of similar seismic moments occurring 1 hr apart on the san jacinto fault system the first event in this pair had the highest stress drop of the events studied 860 bars and was followed within 8 months by a magnitude 5 5 earthquake 2 km away

high resolution geochemistry and sequence stratigraphy of the hushpuckney shale swope formation eastern kansas implications for climato environmental dynamics of the late pennsylvanian midcontinent seaway abstract the hushpuckney shale member of the swope formation missourian stage eastern kansas is the core shale of a kansas type cyclothem formed during the late transgressive to early regressive phases of a late pennsylvanian glacio eustatic cycle coeval high frequency climato environmental dynamics of the midcontinent seaway are preserved in the kgs orville edmonds no 1a study core as centimeter scale variation in major components organic carbon authigenic sulfides and phosphate detrital siliciclastics organic macerals trace element redox proxies mo u v zn and ichnofabric features benthic o 2 levels declined sharply from the base of the black shale 0 cm went sulfidic at 4 cm and reached a redox minimum at 21 cm above this level redox potential gradually rose fluctuating between sulfidic and nonsulfidic conditions from 35 cm to the black shale gray shale contact at 52 cm onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft bodied organisms comprised of deep tiered tracemakers tolerant of low o 2 conditions trichichnus and shallow tiered tracemakers favoring soupground helminthopsis or firmground substrates the zoophycos phycosiphon schaubcylindrichnus planolites association most geochemical records exhibit a low order cycle spanning the full 52 cm thickness of the black shale submember reflecting a dominant glacio eustatic control the base and top of the black shale submember record lateral migration of the pycnocline across the midcontinent shelf during the transgressive and regressive phases respectively the maximum flooding surface mfs is at 17 23 cm an interval containing the euxinic peak and characterized by high concentrations of illite representing a cratonic siliciclastic flux and terrestrial organic macerals the product of transient increases in humidity weathering rates and the export of coal swamp vegetation associated with the interglacial highstand of the swope cyclothem although a transgressive surface of maximum starvation sms cannot be confidently identified a phosphate rich regressive condensation surface at 28 34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth a correlative shift in dominance from terrestrial to marine derived organic macerals reflects increased upwelling of nutrient rich deepwaters and enhanced primary productivity penecontemporaneous paleogeographic and climatic factors e g semirestricted circulation monsoonal precipitation predisposed the midcontinent seaway toward sensitivity to high frequency climato environmental fluctuations which are preserved as 2 to 7 cm thick cycles in the study core the 52 cm thick black shale submember contains 12 such high order cycles which have an estimated average duration of 2 to 9 kyr implying a sub milankovitch band i e millennial scale climatic control the results of this study are relevant to the sequence stratigraphic interpretation of kansas type cyclothems the dynamics of gondwanan ice age glacio eustatic cycles and the development of boundary conditions for late pennsylvanian paleoclimate models

rupture directivity and slip distribution of the m 4 3 foreshock to the 1992 joshua tree earthquake southern california details of the m 4 3 foreshock to the joshua tree earthquake were studied using p waves recorded on the southern california seismic network and the anza network deconvolution using an m 2 4 event as an empirical green s function corrected for complicated path and site effects in the seismograms and produced simple far field displacement pulses that were inverted for a slip distribution both possible fault planes north south and east west for the focal mechanism were tested by a least squares inversion procedure with a range of rupture velocities the results showed that the foreshock ruptured the north south plane similar to the mainshock the foreshock initiated a few hundred meters south of the mainshock and ruptured to the north toward the mainshock hypocenter the mainshock m 6 1 initiated near the northern edge of the foreshock rupture 2 hr later the foreshock had a high stress drop 320 to 800 bars and broke a small portion of the fault adjacent to the mainshock but was not able to immediately initiate the mainshock rupture

palaeoenvironments of the permian triassic transition sections in kashmir india abstract detailed measurements of two permian triassic boundary sections at guryul ravine and pahlgam in kashmir indicate continuous sedimentation across the boundary though with a marked change from shallow storm influenced shelf to deeper shelf below wave base successive shell beds through the 2 5 m thick transitional latest permian section at guryul formed as in situ accumulations with little sign of either erosional breaks or condensation faunal change takes place gradually in the late permian transition beds as elsewhere in the tethyan sections there is no major event layer at the permian triassic boundary and little evidence for dysaerobic conditions both the kashmir and east greenland permian triassic sections have sedimentation rates that are orders of magnitude greater than at the condensed meishan reference section they show that the end permian extinction was associated with transgression but not with immediate onset of anoxic conditions macrofaunal extinction seems associated with reduction in primary production which would also explain the dwarfism of fossils found in all permian triassic sections studied so far

chemical transport to the seafloor of the equatorial pacific ocean across a latitudinal transect at 135 w tracking sedimentary major trace and rare earth element fluxes at the equator and the intertropical convergence zone we have analyzed the major trace and rare earth element composition of surface sediments collected from a transect across the equator at 135 w longitude in the pacific ocean comparing the behavior of this suite of elements to the caco3 opal and corg fluxes which record sharp maxima at the equator previously documented at the same sampling stations enables us to assess the relative significance of the various pathways by which trace elements are transported to the equatorial pacific seafloor the 1 1 high biogenic source at the equator associated with equatorial divergence of surface water and upwelling of nutrient rich water and 2 2 high aluminosilicate flux at 4 n associated with increased terrigenous input from elevated rainfall at the intertropical convergence zone itcz of the tradewinds are the two most important fluxes with which elemental transport is affiliated the biogenic flux at the equator transports ca and sr structurally bound to carbonate tests and mn primarily as an adsorbed component trace elements such as cr as pb and the rees are also influenced by the biogenic flux at the equator although this affiliation is not regionally dominant normative calculations suggest that extremely large fluxes of ba and p at the equator are carried by only small proportions of barite and apatite phases the high terrigenous flux at the itcz has a profound effect on chemical transport to the seafloor with elemental fluxes increasing tremendously and in parallel with ti normative calculations however indicate that these fluxes are far in excess of what can be supplied by lattice bound terrigenous phases the accumulation of ba is greater than is affiliated with biogenic transport at the equator while the p flux at the itcz is only 10 less than at the equator this challenges the common view that ba and p are essentially exclusively associated with biogenic fluxes many other elements including mn pb as and rees also record greater accumulation beneath the itcz than at the equator thus adsorptive scavenging by terrigenous paniculate matter or phases intimately associated with them appears to be an extremely important process regulating elemental transport to the equatorial pacific seafloor these findings emphasize the role of vertical transport to the sediment and provide additional constraints on the paleochemical use of trace elements to track biogenic and terrigenous fluxes