control of pre rift lithospheric structure on the architecture and evolution of continental rifts insights from the main ethiopian rift east africa we investigate the along axis variations in architecture segmentation and evolution of the main ethiopian rift mer east africa and relate these characteristics to the regional geology lithospheric structure and surface processes we first illustrate significant along axis variations in basin architecture through analysis of simplified geological cross sections in different rift sectors we then integrate this information with a new analysis of ethiopian topography and hydrography to illustrate how rift architecture basin symmetry asymmetry is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics flexural uplift and surface processes fluvial erosion unloading this analysis shows that 70 of the 500 km long mer is asymmetric with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin we finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere we provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous strong somalian plateau and the weaker more heterogeneous ethiopian plateau differences originating from the presence of pre rift zones of weakness on the ethiopian plateau and likely amplified by surface processes the data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic magmatic activity without transitioning into a symmetric rifting stage these observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide

hypocentre determination offshore of eastern taiwan using the maximum intersection method summary the maximum intersection maxi method which derives from the master station method msm determines within a 3 d velocity model the absolute hypocentral location based on observed arrival times first the spatial node that better satisfies the arrival time differences computed at all station pairs plus or minus an error tolerance value in seconds is defined as the preliminary hypocentral solution pred second because pred depends neither on the estimate of origin time nor on the residual root mean square rms residual outliers are objectively detected and cleaned out from the original data set without any iterative process or weighting third a statistical minimization residual rms is conducted in a small domain around the pred node which results in a unique final solution the maxi method is applied to the determination of earthquake hypocentres with the proper station correction terms in the southernmost extremity of the ryukyu subduction zone where several dense seismic clusters occur near the seismogenic plate interface the location of earthquakes recorded at both the taiwanese and japanese networks is obtained for about a thousand events between 1992 and 1997 the process uses a detailed 3 d velocity model based on multiple geophysical data sources obtained in the junction area between subduction and collision east of taiwan the earthquake clustering and the significant drop in residual statistics 1 20 0 80 and 0 35 s for taiwanese catalogue msm and maxim solutions respectively indicate the accuracy of the method which can be used to routinely determine absolute hypocentre location based on observed arrival times

the permo triassic transition in spitsbergen δ13corg chemostratigraphy fe and s geochemistry facies fauna and trace fossils new 13 c org analyses of two boundary sections between the late permian kapp starostin formation and the early triassic vardebukta formation of western spitsbergen confirm field evidence that their contact is a conformable one thus contrary to previous reports some spitsbergen sections contain a complete record of the environmental and faunal changes during the crisis interval of the end permian mass extinction no environmental deterioration is recorded in the late permian until near the end of the terminal changxingian stage whereupon the abundant siliceous sponge fauna of the kapp starostin formation disappears along with the deep burrowing fauna responsible for the zoophycus trace fossil a low diversity dysaerobic trace fossil assemblage is briefly developed before a transition to finely laminated pyritic facies immediately beneath the permo triassic boundary analysis of the s c ratios from the laminated strata suggests that free h 2 s was present in the water column euxinic conditions even in relatively nearshore settings subject to storm sandstone deposition the mass extinction crisis in spitsbergen is therefore coincident with the extensive development of oxygen poor conditions in the water column and compares closely both in timing and nature with the crisis seen in lower latitude tethyan settings however the subsequent aftermath and recovery in the boreal sections of spitsbergen was more rapid than in tethys thus a shoreface sandstone body within the dienerian stage contains an appreciable diversity of fauna by the standards of the early triassic including bryozoans calcareous algae and deep infaunal bivalves that suggests the marine ecosystem recovery began earliest in higher palaeolatitudes

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

microbial deposits in the aftermath of the end permian mass extinction a diverging case from the mineral mountains utah usa the lower triassic mineral mountains area utah usa preserves diversified smithian and spathian reefs and bioaccumulations that contain fenestral microbialites and various benthic and pelagic organisms ecological and environmental changes during the early triassic are commonly assumed to be associated with numerous perturbations productivity changes acidifica tion redox changes hypercapnia eustatism and temperature changes post dating the permian triassic mass extinction new data acquired in the mineral mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures these data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the smithian early spathian the studied section records a large scale depositional sequence during the dienerian spathian interval during the transgression depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral microbial limestones shallow subtidal marine sponge microbial reefs to deep subtidal mud dominated limestones storm induced deposits microbialite sponge reefs and shallow subtidal deposits indicate the regression three microbialite associations occur in ascending order i a red beds microbialite association deposited in low energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure ii a smithian microbialite association formed in moderate to high energy tidal conditions where microbialites include stromatolites and associated carbonate grains oncoids ooids and peloids and iii a spathian microbialite association developed in low energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation hydrodynamics bathymetry and grain supply this study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions due to the presence of numerous metazoans associated with microbialites throughout the smithian increase in accommodation and spathian decrease in accommodation the commonly assumed anachronistic character of the early triassic microbialites and the traditional view of prolonged deleterious conditions during the early triassic time interval is questioned

contrasting styles of post caldera volcanism along the main ethiopian rift implications for contemporary volcanic hazards abstract the main ethiopian rift mer 7 9 n is the type example of a magma assisted continental rift the rift axis is populated with regularly spaced silicic caldera complexes and central stratovolcanoes interspersed with large fields of small mafic scoria cones the recent latest pleistocene to holocene history of volcanism in the mer is poorly known and no eruptions have occurred in the living memory of the local population assessment of contemporary volcanic hazards and associated risk is primarily based on the study of the most recent eruptive products typically those emplaced within the last 10 20 ky we integrate new and published field observations and geochemical data on tephra deposits from the main late quaternary volcanic centres in the central mer to assess contemporary volcanic hazards most central volcanoes in the mer host large mid pleistocene calderas with typical diameters of 5 15 km and associated ignimbrites of trachyte and peralkaline rhyolite composition in contrast post caldera activity at most centres comprises eruptions of peralkaline rhyolitic magmas as obsidian flows domes and pumice cones the frequency and magnitude of events varies between individual volcanoes some volcanoes have predominantly erupted obsidian lava flows in their most recent post caldera stage fentale whereas other have had up to 3 moderate scale vei 3 4 explosive eruptions per millennium aluto at some volcanoes we find evidence for multiple large explosive eruptions corbetti bora baricha boset bericha which have deposited several centimetres to metres of pumice and ash in currently densely populated regions this new overview has important implications when assessing the present day volcanic hazard in this rapidly developing region

carbon isotope stratigraphy across the permian triassic boundary a review abstract the palaeozoic mesozoic transition is marked by distinct perturbations in the global carbon cycle resulting in a prominent negative carbon isotope excursion at the permian triassic p t boundary well known from a plethora of marine and continental sediments potential causes for this negative 13 c trend and their links to the latest permian mass extinction have been intensively debated in the literature in order to draw conclusions regarding causation a general 13 c curve was defined after consideration of all available datasets and with due reference to the biostratigraphic background the most important features of the p t carbon isotope trend are the following the 4 7 13 c decline lasting 500 000 years is gradual and began in the changhsingian at the stratigraphic level of the c bachmanni zone the decreasing trend is interrupted by a short term positive event that starts at about the latest permian low latitude marine main extinction event horizon eh indicating that the extinction itself cannot have caused the negative carbon isotope excursion after this short term positive excursion the 13 c decline continues to a first minimum at about the p t boundary a subsequent slight increase is followed by a second occasionally two peaked minimum in the lower and middle i isarcica zone the negative carbon isotope excursion was most likely a consequence of a combination of different causes that may include 1 direct and indirect effects of the siberian trap and contemporaneous volcanism and 2 anoxic deep waters occasionally reaching very shallow sea levels a sudden release of isotopically light methane from oceanic sediment piles or permafrost soils as a source for the negative carbon isotope trend is questionable at least for the time span a little below the eh and somewhat above the p t boundary

progressive environmental deterioration in northwestern pangea leading to the latest permian extinction stratigraphic records from northwestern pangea provide unique insight into global processes that occurred during the latest permian extinction lpe we examined a detailed geochemical record of the festningen section spitsbergen a stepwise extinction is noted as starting with 1 loss of carbonate shelly macrofauna followed by 2 loss of siliceous sponges in conjunction with an abrupt change in ichnofabrics as well as dramatic change in the terrestrial environment and 3 final loss of all trace fossils we interpret loss of carbonate producers as related to shoaling of the lysocline in higher latitudes in relationship to building atmospheric co 2 the loss of siliceous sponges is coincident with the global lpe event and is related to onset of high loading rates of toxic metals hg as co that we suggest are derived from siberian trap eruptions the final extinction stage is coincident with redox sensitive trace metal and other proxy data that suggest onset of anoxia after the other extinction events these results show a remarkable record of progressive environmental deterioration in northwestern pangea during the extinction crises

local seismicity near the actively deforming corbetti volcano in the main ethiopian rift abstract corbetti is currently one of the fastest uplifting volcanoes globally with strong evidence from geodetic and gravity data for a subsurface inflating magma body a dense network of 18 stations has been deployed around corbetti and hawassa calderas between february 2016 and october 2017 to place seismic constraints on the magmatic hydrothermal and tectonic processes in the region we locate 122 events of magnitudes between 0 4 and 4 2 using a new local velocity model the seismicity is focused in two areas directly beneath corbetti caldera and beneath the city of hawassa the shallower 0 5 km depth below sea level b s l earthquakes beneath corbetti are mainly focused in ew to ns elongated clusters at urji and chabbi volcanic centres this distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids away from a cross rift pre existing fault source mechanisms are predominantly strike slip and different to the normal faulting away from the volcano suggesting a local rotation of the stress field these observations along with a low vp vs ratio are consistent with the inflation of a gas rich sill likely of silicic composition beneath corbetti in contrast the seismicity beneath hawassa extends to greater depth 16 km b s l these earthquakes are focused on 8 10 km long segmented faults which are active in seismic swarms one of these swarms in august 2016 is focused between 5 and 16 km depth b s l along a steep normal fault beneath the city of hawassa highlighting the earthquake hazard for the local population

early triassic productivity crises delayed recovery from world s worst mass extinction the recovery of life after the latest permian extinction was protracted over early triassic time detailed geochemistry of marine sections along northwest pangea indicates that upwelling ceased at the extinction event nitrogen stable isotope data suggest that this was associated with progressive increase in nutrient stress throughout the early triassic coincident with a significant decrease in organic carbon content despite pervasive anoxic to euxinic conditions we argue that the early triassic hothouse both reduced marine productivity and deepened the nutricline reducing the overall rate of nutrient delivery to the photic zone creating an early triassic nutrient gap when oceans finally cooled by middle triassic time renewed nutrient upwelling and onset of organic rich shale deposition occurred across northwest pangea marking the final return of global marine productivity

carbon isotope record of the p t boundary and the lower triassic in the southern alps evidence for rapid changes in storage of organic carbon abstract the carbonate carbon isotope record of lower triassic shelf sediments was investigated in the pufels and l om picol uomo sections of the western central dolomites to derive a complete record from the permian triassic transition to the uppermost lower triassic from the western tethys the carbon isotope record shows the well studied decrease at the permian triassic boundary ptb followed by an increase to about 1 5 at the dienerian smithian boundary the 13c values increase to a pronounced 6 maximum a steady rapid drop until the upper smithian below 2 follows this 13c peak at the smithian spathian transition a steep rise to positive values is documented the positive carbon isotope peak is interpreted to reflect an interval of enhanced burial of organic carbon whereas the isotopic minima provide evidence of periods of oxidation of organic matter and or release of isotopically light carbon this may be ascribed to quickly changing environmental conditions which have already been postulated by paleontologists due to the delayed biotic recovery from the permian triassic extinction event because of the short time interval these positive isotope excursions represent their generation by burial of organic carbon in periods of high productivity and or anoxia appears plausible concurrent carbon isotopic excursions are reported from iran pakistan and oman as well as from southern china which indicate an at least tethys wide event if the excursion proves to be of global significance dark shale deposited in the pelagic panthalassa ocean may have acted as sink for organic carbon supporting the hypothesis of a periodically stratified ocean in the lower triassic

formation processes of framboidal pyrite abstract pyrite framboid formation may be the result of four consecutive processes 1 nucleation and growth of initial iron monosulfide microcrystals 2 reaction of the microcrystals to greigite fe 3 s 4 3 aggregation of uniformly sized greigite microcrystals i e framboid growth and 4 replacement of greigite framboids by pyrite the uniform morphology uniform size range and ordering of the microcrystals in individual framboids as well as the range of observed framboid structures from irregular aggregates to densely packed spherical aggregates and polyframboids are consequences of these processes using dlvo theory derjaguin landau verwey and overbeek we have evaluated the stability of colloidal iron monosulfide suspensions with ionic strengths typical of marine and lacustrine waters in addition to van der waals attractive and double layer repulsive forces a term is included to account for the ferrimagnetic properties of greigite numerical models predict that magnetically saturated greigite particles 0 1 m in diameter will rapidly aggregate in either marine or fresh water the aggregation model is in agreement with the sequence of greigite formation followed by pyrite framboid formation established in a previous experimental study sweeney and kaplan 1973 and is consistent with the occurrence of framboids composed of other magnetic minerals e g greigite magnetite and magnesioferrite based on the temperature dependent magnetic properties of greigite and aging experiments in hydrothermal solutions this mechanism for framboid formation via precursor greigite could operate to temperatures of 200 c consistent with the occasional occurrence of pyrite framboids in the paragenesis of metalliferous ore deposits

anomalously diverse early triassic ichnofossil assemblages in northwest pangea a case for a shallow marine habitable zone early triassic trace fossil assemblages from the northwest margin of pangea record a diverse suite of postextinction infauna these ichnofossil assemblages occurred within well oxygenated shallow marine refuges in a panthalassa ocean otherwise characterized by widespread anoxia we propose an environmentally controlled model for their distribution in which wave aeration enhanced by frequent storms gave rise to an optimal zone for benthic colonization within this habitable zone extinction pressures were ameliorated and postextinction recovery duration was minimized

the eruptive history and magmatic evolution of aluto volcano new insights into silicic peralkaline volcanism in the ethiopian rift the silicic peralkaline volcanoes of the east african rift are some of the least studied volcanoes on earth here we bring together new constraints from fieldwork remote sensing geochronology and geochemistry to present the first detailed account of the eruptive history of aluto a restless silicic volcano located in a densely populated section of the main ethiopian rift prior to the growth of the aluto volcanic complex before 500 ka the region was characterized by a significant period of fault development and mafic fissure eruptions the earliest volcanism at aluto built up a trachytic complex over 8 km in diameter aluto then underwent large volume ignimbrite eruptions at 316 19 ka and 306 12 ka developing a 42 km2 collapse structure after a hiatus of 250 ka a phase of post caldera volcanism initiated at 55 19 ka and the most recent eruption of aluto has a radiocarbon age of 0 40 0 05 cal ka bp during this post caldera phase highly evolved peralkaline rhyolite lavas ignimbrites and pumice fall deposits have erupted from vents across the complex geochemical modelling is consistent with rhyolite genesis from protracted fractionation 80 of basalt that is compositionally similar to rift related basalts found east of the complex based on the style and volume of recent eruptions we suggest that silicic eruptions occur at an average rate of 1 per 1000 years and that future eruptions of aluto will involve explosive emplacement of localised pumice cones and effusive obsidian coulees of volumes in the range 1 100 106 m3

molybdenum isotopic evidence for oxic marine conditions during the latest permian extinction the latest permian extinction lpe ca 252 ma represents the most severe extinction event in earth s history the cause is still debated but widespread marine anoxic to euxinic h2s rich conditions from deep to shallow water environments are commonly suggested as a proxy for marine oxygen levels we analyzed 98 95mo of two lpe sections that represent a gradient in water depth on the northwest margin of pangea results from deep water slope environments show a large shift in 98 95mo values from 2 02 to 2 23 at the extinction horizon consistent with onset of euxinic conditions in contrast sub storm wave base shelf environments show little change in the molybdenum isotopic composition 1 34 to 0 05 indicating ongoing oxic conditions across the lpe these results indicate that areas of the continental shelf of northwest pangea underwent mass extinction under oxic conditions throughout the lpe event and that shallow water anoxia was therefore not a global phenomenon

quaternary oblique extensional tectonics in the ethiopian rift horn of africa abstract the ethiopian rift extends in a northeasterly direction from southern ethiopia to the afar region it shows a complex fault pattern characterised by the interplay of a n30 e n40 e trending border fault system with the quaternary wonji fault belt which is constituted by right stepping en echelon n s to n20 e trending faults the wonji fault belt affects mainly the rift floor but it also overlaps with some segments of the margins its en echelon arrangement indicates a left lateral component of displacement along the rift trend the general fault pattern of the ethiopian rift as well as mesoscopic fault analyses and structural features of some key areas indicate the occurrence of a roughly e w extension which is compatible with the sinistral shear component of motion along the rift this paper proposes that oblique rifting related to the e w direction of extension has been active since the beginning of the quaternary and that it came after an earlier phase of roughly pure extension orthogonal to the rift trend

calcium carbonate seafloor precipitates from the outer shelf to slope facies of the lower triassic smithian spathian union wash formation california usa sedimentology and palaeobiologic significance abstract formerly aragonite seafloor fans from the lower triassic smithian spathian union wash formation of east central california were examined to determine the processes active during their growth the seafloor fans were deposited in an outer shelf to slope environment along the western margin of pangaea and are contained within two stratigraphic intervals the lower precipitate bearing interval is 130 m thick comprised of micritic limestone and contains precipitates that fall into 2 end member morphologies 1 laminated precipitate masses that contain distinct hemispheres or fans and 2 precipitate masses that are composed of a complex interlocking mosaic of hemispheres or fans precipitates found in the bedded lower half of the interval are typically 2 3 cm tall and 10 20 cm wide and are commonly undercut disturbed and in the case of the interlocking mosaics of hemispheres show evidence of entrainment and transport by density currents precipitates are larger in the upper massive portion of the lower precipitate bearing interval and occur only as interlocking hemisphere fan mosaics fans and hemispheres attain heights as tall as 30 cm and precipitate bodies made of interlocking mosaics of hemispheres and fans attain thicknesses of up to 50 cm and widths of 1 2 m chaotic bedding broken precipitates and brecciation of precipitate bodies imply an unstable and mobile substrate the upper precipitate bearing interval is 30 m thick comprised of bivalve ammonoid and microgastropod bearing wackestone and is separated from lower precipitate bearing strata by 23 m of interbedded shale and calcareous siltstone much of the upper precipitate bearing unit was deposited by subaqueous debris flows as suggested by multiple breccias contained within the interval seafloor cements are found in undisturbed units and rarely within breccia clasts suggesting that precipitates formed locally as well as higher on the shelf precipitate growth was likely the result of an increase in the saturation state of calcium carbonate in early triassic oceans related to widespread oceanic anoxia the presence of unusual carbonates during the period and their correlation with depauperate faunas in the equivalent shallow facies of the virgin limestone suggest that the remarkable oceanographic conditions associated with precipitate growth likely dampened the biotic recovery in the region until those conditions disappeared in the middle triassic

intrabasin variability of the carbon isotope record across the permian triassic transition sverdrup basin arctic canada abstract a major late paleozoic depocentre the sverdrup basin canadian high arctic has been largely left out of the latest permian extinction debate as early workers presumed middle to late permian strata were absent basin scale sequence stratigraphic and chemostratigraphic correlations indicate late permian strata are only missing on the basin margins where they were removed by sub triassic erosion whereas continuous deposition is recorded in the basin centre the varying degree of sub triassic erosion has significant impact on the carbon isotope record across the latest permian extinction event where both the apparent rate and magnitude of carbon isotope shift vary as a function of basin position the intrabasin variability in apparent 13corg shift across the event is equivalent to that observed globally in contrast to the abrupt isotope shifts recorded on the basin margin similar to many records reported globally the basin centre section shows a systematic shift associated with the latest permian extinction the earth likely underwent a prolonged period of increasing environmental stress leading up to the event

trace fossil evidence for restoration of marine ecosystems following the end permian mass extinction in the lower yangtze region south china abstract unlike the high abundance low diversity macrofaunas that characterize many early triassic benthic palaeocommunities ichnofossils were relatively common in the aftermath of the end permian mass extinction worldwide ichnofossils therefore are a good proxy for ecosystem recovery after the end permian biotic crisis this paper documents 14 ichnogenera and one problematic form from lower triassic successions exposed in the lower yangtze region south china post extinction ichnodiversity remained rather low throughout the griesbachian early smithian period and abruptly increased in the late smithian however several lines of evidence including extent of bioturbation burrow size trace fossil complexity and tiering levels indicate that diversification of ichnotaxa in the late smithian did not signal full marine ecosystem recovery from the permian triassic p tr mass extinction marine ichnocoenoses did not recover until the late spathian in south china the marginal sea provided hospitable habitats for tracemakers to proliferate in the aftermath of the end permian mass extinction

textured organic surfaces associated with the ediacara biota in south australia abstract the ediacaran period takes its name from the fossils of the ediacara biota which represent the first appearance of large and diverse assemblages of organisms in the fossil record although the global record of these distinctive body fossils is now well known a previously unrecognized megascopic organic record of textured organic surfaces tos occurs in the ediacara biota however tos is also a feature over a wider range of paleoenvironmental settings where body fossils are unknown in ediacaran siliciclastic successions that have been studied in australia namibia and western north america paleoecological analysis of successive bedding planes of strata from the late ediacaran rawnsley quartzite in the flinders ranges of south australia reveals that tos represent the most common organic features in bedding surface assemblages of the ediacara biota the tos consist of preserved patterned assemblages of textured organic mats fibers and simple tubular body fossils complex ediacara body fossils while striking for their distinctive body plans and dominating some of the beds are relatively minor components of combined overall surface area many elements of tos have previously been miss diagnosed as trace fossils which are in practice limited to two or at most three morphotypes that indicate the presence of bilateria although tos represent a simpler grade of organismic construction than discrete and more complex ediacara body fossils they were preserved in a similar manner marked variability in all components of the biota between successive surfaces suggests that ediacara ecologies fluctuated at short intervals despite an apparently consistent sedimentary regime

ultra shallow marine anoxia in an early triassic shallow marine clastic ramp spitsbergen and the suppression of benthic radiation lower triassic marine strata in spitsbergen accumulated on a mid to high latitude ramp in which high energy foreshore and shoreface facies passed offshore into sheet sandstones of probable hyperpycnite origin more distal facies include siltstones shales and dolomitic limestones carbon isotope chemostratigraphy comparison allows improved age dating of the boreal sections and shows a significant hiatus in the upper spathian two major deepening events in earliest griesbachian and late smithian time are separated by shallowing upwards trends that culminated in the dienerian and spathian substages the redox record revealed by changes in bioturbation palaeoecology pyrite framboid content and trace metal concentrations shows anoxic phases alternating with intervals of better ventilation only dienerian early smithian time witnessed persistent oxygenation that was sufficient to support a diverse benthic community the most intensely anoxic usually euxinic conditions are best developed in offshore settings but at times euxinia also developed in upper offshore settings where it is even recorded in hyperpycnite and storm origin sandstone beds an extraordinary facet of spitsbergen s record the euxinic phases do not track relative water depth changes for example the continuous shallowing upwards from the griesbachian to lower dienerian was witness to several euxinic phases separated by intervals of more oxic bioturbated sediments it is likely that the euxinia was controlled by climatic oscillations rather than intra basinal factors it remains to be seen if all the anoxic phases found in spitsbergen are seen elsewhere although the wide spread of anoxic facies in the smithian spathian boundary interval is clearly a global event

onset development and cessation of basal early triassic microbialites betm in the nanpanjiang pull apart basin south china block abstract new investigations in the nanpanjiang basin indicate that the onset of the iconic microbialites associated with the paleozoic mesozoic boundary was early triassic in age bathymetry water agitation oxygenation light penetration and clastic load are shown to have exerted a direct control on the growth of microbialites carbonate supersaturation is also required for the deposition of the microbialites bathymetric control is further corroborated by the inheritance of the topography of a latest permian pull apart basin into early triassic times with a distribution of basal early triassic microbialites betm restricted to uplifted blocks and the accumulation of carbonaceous black shales in adjacent troughs the geographically most extensive nanpanjiang betm bloomed on a large nw se trending uplifted block exceeding 12 000 km 2 luolou platform centered on northwestern guangxi post triassic displacements along the youjiang fault obscure the paleogeographic relation of betm exposed west of this fault triassic foraminifers occur in the basal most betm episode which is locally bracketed by high energy grainstones made of reworked permian foraminifers therefore the permian triassic boundary ptb is within the unconformity that separates the late permian heshan fm from the basal most betm where accommodation space was sufficient up to five event surfaces are associated with the unconformity microfacies analysis supports chemical dissolution but did not reveal evidence for subaerial erosion although intercalated grainstone made of permian foraminifers indicate reworking chemical dissolution and mechanical erosion both conceivably contributed to the genesis of the unconformity the upward shift from tabulated to domical microbial build ups is accompanied by accumulation of coquinoid lenses between domes which indicates deepening of the luolou platform betm the main drowning resulting from both regional tectonic subsidence and a global sea level rise led to the cessation of the betm that were buried under predominant fine siliciclastics any concomitant change in sea water chemistry appears unlikely

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

glacial interglacial modulation of the marine nitrogen cycle by high latitude o2 supply to the global thermocline 1 an analysis of sedimentary nitrogen isotope records compiled from widely distributed marine environments emphasizes the global synchrony of denitrification changes and provides evidence for a strong temporal coupling of these variations to changes in nitrogen fixation as previously inferred we explain the global coherence of these records by a simple physical control on the flux of dissolved oxygen to suboxic zones and the coupling to fixation via the supply of phosphorus to diazotrophs in suitable environments according to our hypothesis lower glacial stage sea surface temperature increased oxygen solubility while stronger winds in high latitude regions enhanced the rate of thermocline ventilation the resultant colder rapidly flushed thermocline lessened the spatial extent of denitrification and consequently n fixation during warm periods sluggish circulation of warmer less oxygen rich thermocline waters caused expansion of denitrification zones and a concomitant increase in n fixation local fluctuations in export productivity would have modulated this global signal

the physical environment of the peruvian upwelling system abstract knowledge of the characteristics of the physical environment of the peru coastal upwelling zone has increased greatly during the past decade observations made during 1976 and 1977 many made as part of the coastal upwelling ecosystem analysis joint ii expedition provide the basis for an interpretative description of the physical processes and phenomena occurring in peru coastal waters four important phenomena are discussed wind driven upwelling the poleward undercurrent surface mixed layer deepening and low frequency coastal trapped waves the coastal winds were invariably favorable for coastal upwelling even during the 1976 el nino the agreement between the offshore transport in the relatively shallow 30 m surface layer and the ekman transport deduced from the local wind was good for both the mean and variable state the agreement with the deeper onshore transport was less good consistent with the marked three dimensionally and spatial variability of the upwelling the poleward undercurrent was apparently continuous from 5 s to at least 15 s flowing just below the surface layer over the continental shelf and slope and supplying the upwelling water the variability in the alongshore velocity field was dominated by baroclinic free coastal trapped waves with periods of 5 20 days although there is appreciable temporal and spatial variability in the peru coastal upwelling system the upwelling the undercurrent and the low frequency coastal trapped waves were ubiquitous

obliquity along plate boundaries most of the plate boundaries are activated obliquely with respect to the direction of far field stresses as roughly only 8 of the plate boundaries total length shows a very low obliquity ranging from 0 to 10 sub orthogonal to the plate displacement the obliquity along plate boundaries is controlled by i lateral rheological variations within the lithosphere and ii consistency with the global plate circuit indeed plate tectonics and magmatism drive rheological changes within the lithosphere and consequently influence strain localization geodynamical evolution controls large scale mantle convection and plate formation consumption and re organization thus triggering plate kinematics variations and the adjustment and re orientation of far field stresses these geological processes may thus result in plate boundaries that are not perpendicular but oblique to the direction of far field stresses this paper reviews the global patterns of obliquity along plate boundaries using gplate we provide a statistical analysis of present day obliquity along plate boundaries within this framework by comparing natural examples and geological models we discuss deformation patterns and kinematics recorded along oblique plate boundaries

the unusual sedimentary rock record of the early triassic a case study from the southwestern united states abstract sedimentary systems as well as the earth s biota changed dramatically following the end permian mass extinction this global sedimentological transformation is reflected in the widespread occurrence of flat pebble conglomerates subtidal wrinkle structures microbialites and carbonate seafloor fans in various lower triassic sections as a case study a facies analysis of the lower triassic virgin limestone member of the moenkopi formation and the middle and upper members of the union wash formation southwestern united states was conducted and all unusual facies were documented within a palaeoenvironmental framework the occurrence of the flat pebble conglomerates and wrinkle structures is significant because these reflect a long term reduction in vertical bioturbation in the case of the subtidal microbialites and carbonate seafloor fans their formation is linked to an increase in alkalinity the presence of these facies in the uppermost lower triassic strata of the southwestern united states indicates that reduced levels of infaunal bioturbation as well as and perhaps a consequence of unusual ocean chemistry extended for 5 6 million years after the end permian mass extinction suggesting that depositional environments as well as the biota were significantly affected by the extinction event

wrinkle structures a critical review abstract in this paper a variety of so called wrinkle structures is reviewed in an attempt to help distinguish between crinkly decorations arising from physical processes that acted on siliciclastic bedding surfaces and true microbially induced wrinkle structures two types of small scale microbially induced sedimentary structures are prominent due to their distinct geometry and mode of occurrence 1 elephant skin textures characterized by reticulate patterns of sharp crested ridges forming mm to cm scale polygons occurring on argillite or argillaceous veneers above fine grained sandstone and likely reflecting growth structures of microbial mats 2 kinneyia structures characterized by mm scale flat topped winding ridges and intervening troughs and pits sometimes resembling small scale interference ripples kinneyia structures usually occur on upper surfaces of siltstone sandstone beds themselves frequently event deposits and are thought to have formed beneath microbial mats additionally more linear variations of mat growth structures partly resembling small scale petees may be developed finally some wrinkly structures resulting from tractional mat deformation or mat slumping are occasionally preserved these may appear as arcuate belts of non penetrative small scale folds or as wrinkled bulges on otherwise flat surfaces wrinkle structures as indicators for the former presence of mats gain in importance if other mat related structures are additionally observed in the same clastic succession e g sand chips sandy intraclasts or spindle shaped or sinuously curved to circular sand cracks frequently combined in networks furthermore appropriate lithologies and facies are required for instance if compared with the distribution of modern cohesive microbial mats laminated siltstone argillite with intercalated siltstone sandstone beds representing event deposits in tidal flat successions would be compatible with microbial mat development within a variety of physically induced small scale wrinkly structures miniature load structures may above all be misinterpreted as microbially induced wrinkle structures due to their similar size and appearance and their comparatively frequent occurrence

microbially induced sedimentary structures a new category within the classification of primary sedimentary structures abstract cyanobacterial films and mats syndepositonally influence erosion deposition and deformation of sediments the biomass levels surface morphologies and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing growing microbial mats dredge grains from their substrate upwards whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle trap and bind suspended particles these and similar biotic physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures we distinguish structures on bedding planes leveled bedding surfaces wrinkle structures microbial mat chips erosional remnants and pockets multidirectional ripple marks and mat curls and internal bedding structures sponge pore fabrics gas domes fenestrae structures sinoidal laminae oriented grains benthic ooids biolaminites mat layer bound grain sizes we propose to place this group of microbially mediated structures as a fifth category bedding modified by microbial mats and biofilms in pettijohn and potter s 1964 existing classification of primary sedimentary structures

denitrification under glacial and interglacial conditions a physical approach 1 recent analysis of sedimentary 15n records over the last 200 000 years has shown an expansion of water column denitrification zones during warmer periods and a shrinkage of these oxygen poor regions during glacial periods two different mechanisms could be responsible for driving the changes in the denitrification records variations in local productivity leading to a change in remineralization rates and or changes in the ocean circulation and ocean temperature resulting in variations of the physical supply of dissolved oxygen here we focus on the supply mechanism by using a fully coupled atmosphere ocean sea ice land surface scheme model the university of victoria earth system climate model to examine oxygen supply under varying physical conditions while maintaining an invariant biological oxygen utilization scheme we show that circulation changes can be the cause for the observed changes in dissolved oxygen in the glacial ocean lower temperatures and enhanced formation of antarctic intermediate water and north pacific intermediate water during glacial periods increase the physical supply of oxygen and therefore decrease water column denitrification in our last glacial maximum simulations the change in water characteristics in the eastern tropical pacific is important enough to reduce denitrification by 46 to 65 compared to present day conditions depending on the wind fields used as boundary conditions the consequences of our findings could be substantial for the near future with a warming climate denitrification zones could expand leading to changes in the biological pump and the flux of n2o into the atmosphere

control of rift obliquity on the evolution and segmentation of the main ethiopian rift the main ethiopian rift has evolved in two stages with successive activation of differently oriented fault systems analogue modelling of the earth s lithosphere demonstrates that such a rift evolution requires neither magma weakening nor a change in plate kinematics the main ethiopian rift is an active rift in the break up stage and it marks the incipient boundary between the nubia and somalia plates1 rifting started with the activation of large boundary faults and diffuse volcanism followed by focused magmatism and faulting in the rift floor along step like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading2 this concentration of volcano tectonic activity has been thought to be either magma assisted2 3 or controlled by a change in rift kinematics with a late oblique rifting phase that would have caused the development of the step like fault segments that focused magma upwelling4 5 geodetic6 7 seismic8 and stress field9 data confirm current oblique rifting kinematics but plate kinematics models do not predict a change in nubia somalia motion in the past 11 million years10 here i use lithospheric scale analogue models of oblique rifting to analyse the development of the main ethiopian rift i find that neither magma weakening nor a change in plate kinematics are required to simulate a two phase evolution with successive activation of differently oriented fault systems i conclude that rift evolution and segmentation are controlled by rift obliquity independent of magmatic processes

slip re orientation in oblique rifts oblique extension is expected to result in a combination of dip slip and strike slip displacement along faults with strike orthogonal and oblique to the extension direction respectively this general concept is in disagreement with observations from natural oblique rifts where faults show dip slip kinematics indicating pure extension irrespective of the fault strike with respect to the regional extension direction consequently along oblique structures slip is re oriented and oblique to the applied extension direction besides at fault scale slip is re oriented along strike such that it is dip slip at the fault center and becomes highly oblique slip toward the fault tips here we use analogue experiments to show that this discrepancy can be resolved when a preexisting weak zone wz is present in the crust at the onset of oblique extension the wz is implemented within the lower crust and strikes oblique to the extension direction our experimental results show that an inherited wz within the ductile crust favors the re orientation of slip such that oblique extension results in pure dip slip displacement on faults that strike oblique with respect to the extension direction furthermore we show that slip is re oriented along strike of major faults such that the fault center shows dip slip kinematics whereas its tips display strike slip kinematics these findings call into question the use of paleostress reconstructions to constrain plate kinematics in oblique extensional tectonic settings

fault architecture in the main ethiopian rift and comparison with experimental models implications for rift evolution and nubia somalia kinematics abstract the main ethiopian rift mer offers a complete record of the time space evolution of a continental rift we have characterized the brittle deformation in different rift sectors through the statistical analysis of a new database of faults obtained from the integration between satellite images and digital elevation models and implemented with field controls this analysis has been compared with the results of lithospheric scale analogue models reproducing the kinematical conditions of orthogonal and oblique rifting integration of these approaches suggests substantial differences in fault architecture in the different rift sectors that in turn reflect an along axis variation of the rift development and southward decrease in rift evolution the northernmost mer sector is in a mature stage of incipient continental rupture with deformation localised within the rift floor along discrete tectono magmatic segments and almost inactive boundary faults the central mer sector records a transitional stage in which migration of deformation from boundary faults to faults internal to the rift valley is in an incipient phase the southernmost mer sector is instead in an early continental stage with the largest part of deformation being accommodated by boundary faults and almost absent internal faults the mer thus records along its axis the typical evolution of continental rifting from fault dominated rift morphology in the early stages of extension toward magma dominated extension during break up the extrapolation of modelling results suggests that a variable rift obliquity contributes to the observed along axis variations in rift architecture and evolutionary stage being oblique rifting conditions controlling the mer evolution since its birth in the late miocene in relation to a constant post ca 11 ma n100 e nubia somalia motion

morphological comparison of distributed volcanic fields in the main ethiopian rift using high resolution digital elevation models abstract distributed volcanic fields of scoria cones maars tuff cones and lava flows represent a crucial expression of the interplay between magmatism tectonics and surface processes in continental rifts we compare the cone morphology of two fields butajira and east ziway in the main ethiopian rift mer using high resolution digital elevation models dems to investigate volumes surface environment and age of emplacement volumes of individual cones are similar in both fields mean 4 106 m3 and lower than in other distributed volcanic fields located in extensional environments volcanism at butajira has created linear clusters of superimposed cones suggesting a small number of larger 107 108 m3 fissure eruptions compared to the isolated events at east ziway the presence of maars and deep craters within cones at butajira indicates phreatomagmatic activity not seen at east ziway where cones may have been emplaced during a dry climate phase shape parameters including curvature of cone rims and ratios of cone height width and crater depth suggest younger eruption ages at butajira compared to east ziway and the possibility of ongoing activity in this area a broader survey of cone fields across the mer using the aster global digital elevation model fails to identify systematic variation in height width and volume from cone morphology limited by low resolution high resolution topography provides a useful insight into rift environments at the time of emplacement and further surveys across the main ethiopian rift may advance our understanding of distributed volcanism in extensional settings and the hazards they present

bromine counts from xrf scanning as an estimate of the marine organic carbon content of sediment cores 1 xrf sediment core scanning technology is increasingly used to quantify sediment composition the overall good correlation between biophilic halogen bromine br and sedimentary total organic carbon toc potentially allows the fast estimation of down core toc profiles by xrf scanning in order to test this approach we present data from the arabian sea and a mediterranean brine basin comparing xrf core scanning br data with discrete sample toc analyses overall br counts and toc show a clear correlation except when stable carbon isotope and c n data indicate intervals characterized by enhanced input of terrestrial organic matter hence solid phase br is exclusively associated with marine organic matter moc and can be used as a direct estimate of this parameter after a calibration is established high pore water br in the brine core steepens the br toc correlation but after salt correction shows a nearly identical gradient to that of the arabian sea core

environmental analysis of paleoceanographic systems based on molybdenum uranium covariation abstract modern low oxygen marine systems exhibit patterns of molybdenum uranium covariation that can be linked to specific attributes and processes of the depositional system including 1 variation in benthic redox conditions 2 the operation of particulate shuttles within the water column and 3 the evolution of watermass chemistry the importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum mo auth and uranium u auth and from the mo u auth ratio of the sediment relative to the seawater mo u molar ratio of 7 5 7 9 in open ocean systems with suboxic bottomwaters u auth enrichment tends to exceed that of mo auth owing to onset of u auth accumulation at the fe ii fe iii redox boundary resulting in sediment mo u auth ratios less than that of seawater as bottomwaters become increasingly reducing and at least occasionally sulfidic the rate of accumulation of mo auth increases relative to that of u auth and sediment mo u auth ratios equal or exceed that of seawater in restricted marine systems with permanently sulfidic deepwaters the relative enrichment of mo auth and u auth depends on additional factors in the cariaco basin which has an aqueous mo u ratio similar to that of seawater the operation of a particulate mn fe oxyhydroxide shuttle serves to accelerate the transfer of mo to the seafloor leading to strong enrichments in mo auth relative to u auth in the black sea the chemistry of the deep watermass has evolved to the point where its aqueous mo u ratio is only 0 04 that of seawater as a consequence of which sediments deposited under deepwater influence are depleted in mo auth relative to u auth these mo u covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low oxygen marine systems analysis of anoxic facies from two north american paleomarine systems the late pennsylvanian midcontinent sea lpms and the late devonian seaway lds reveals authigenic mo u relationships similar to those of the modern marine environments above implying similar redox and hydrographic controls the observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the lpms and with markedly restricted deepwater circulation in silled basins of the lds patterns of authigenic mo u covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics

trace fossils and the aftermath of the permo triassic mass extinction evidence from northern italy abstract the duration of early triassic anoxia in the carbonate ramp settings of the werfen formation dolomites northern italy has been constrained using ichnofabric and sedimentological data this includes fine lamination and abundant pyrite framboids commonly reworked into lags by storm events the anoxic event spanned the entire griesbachian stage and extended from deep waters into extraordinarily shallow water settings evidence for oxygen deficiency can be found in mid and inner ramp facies the extent and duration of this event may at least partially explain the magnitude of the permo triassic mass extinction normal benthic conditions returned rapidly in the dienerian stage and pervasively bioturbated distal ramp facies testify to the ventilation of the deeper water areas of the region in post griesbachian times a post mass extinction radiation is not particularly well displayed in either trace fossil or body fossil diversity compilations from the dolomites moderate low levels of trace fossil diversity are maintained throughout the post griesbachian werfen formation whilst the body fossil data shows a moderate increase from the mid smithian stage there is thus a curious lag period spanning the dienerian to mid smithian when environmental conditions were apparently normal but the marine fauna failed to show any recovery

secular and tidal strain across the main ethiopian rift using a combination of laser ranging and gps data acquired between 1969 and 1997 we derive a separation velocity for the somali and nubian plates in ethiopia 4 5 1 mm yr at n108 10e this vector is orthogonal to the nne trending neotectonic axis wonji fault belt of the ethiopian rift axis current rifting is concentrated within a 33 km wide zone that includes a 7 km wide belt of late quaternary faulting where maximum surface strain rates are comparable to those at active plate boundaries 0 1 strain yr the strain field suggests that thin 5 km elastic crust separates thick continental lithosphere a geometry quite different from oceanic rifting and a mechanical configuration that favors the amplification of regional strain semidiurnal strain tides however as measured by kinematic gps methods are not amplified along or across the rift indicating that the rift zone s low rigidity applies only at periods of years

microbial mats in the terrestrial lower triassic of north china and implications for the permian triassic mass extinction abstract evidence for microbial mats has been reported repeatedly from marine lower triassic rocks but scarcely mentioned in post mass extinction terrestrial facies here we report from the terrestrial lower triassic liujiagou formation in north china the presence of five kinds of microbially induced sedimentary structures miss or sedimentary surface textures including old elephant skin textures wrinkle structures palimpsest ripples manchuriophycus structures and sand cracks terrestrial microbial communities that produced these miss adapted not only to periodically desiccated conditions but also to the storm dominated palaeoenvironments in the liujiagou formation the permian triassic mass extinction ptme in north china is marked by the die off of plants disappearance of coal beds extinction of pareiasaurs among tetrapods decreased bioturbation levels and a dramatic change of sedimentary systems through the sunjiagou formation the sunjiagou formation recorded the turnover from an ever wet to a progressively drier and hotter climate and it spans the ptme in north china following this mass extinction miss became much more common and widespread suggesting that the mass extinction provided favourable biological and environmental conditions for the development of the miss in terrestrial ecosystems especially the decreased bioturbation intensity and grazing pressure associated with increased temperature and climatic drying in the upper part of the liujiagou formation and overlying heshanggou formation the disappearance of miss coupled with increased bioturbation might indicate an improvement of terrestrial ecosystems and the beginning of the triassic biotic recovery however as the investigation of miss in space and time through the geological record is in its early stages further geobiologic and geochemical studies as well as high precision isotopic dating from permian triassic terrestrial successions are needed to fully reveal the timing and pattern of the early triassic terrestrial ecosystem reconstruction

three dimensional seismic imaging of a protoridge axis in the main ethiopian rift models of continental breakup remain uncertain because of a lack of knowledge of strain accommodation immediately before breakup our new three dimensional seismic velocity model from the main ethiopian rift clearly images mid crustal intrusions in this active transitional rift setting supporting breakup models based on dike intrusion and magma supply the most striking features of our velocity model are anomalously fast elongate bodies velocity v p 6 5 6 8 km s extending along the rift axis interpreted as cooled mafic intrusions these 20 km wide and 50 km long bodies are separated and laterally offset from one another in a right stepping en echelon pattern approximately mimicking surface segmentation of quaternary volcanic centers our crustal velocity model combined with results from geologic studies indicates that below a depth of 7 km extension is controlled by magmatic intrusion in a ductile middle to lower crust whereas normal faulting and dike intrusion in a narrow zone in the center of the rift valley control extension in the brittle upper crust this zone is inferred to be the protoridge axis for future seafloor spreading

the deja vu effect recurrent patterns in exploitation of ecospace establishment of the mixed layer and distribution of matgrounds although the notion of recurrent trace fossil assemblages through geologic time as a result of similar sets of environmental conditions has a long history in ichnology the idea that recurrent ichnofaunas may have occurred in connection with macroevolutionary events has not yet been put forward we refer to this phenomenon as the deja vu effect and suggest that understanding its signifi cance may shed light on how organisms colonize underutilized or empty ecospace the establishment of the mixed layer and the temporal and spatial distribution of matgrounds these recurrent trace fossil assemblages record the activity of the epifauna and a very shallow tier infauna in the absence of mid to deep tier trace fossils and mottled textures they typically consist of grazing trails very shallow feeding burrows and arthropod trackways shallow tier burrows and trails with well developed scratch marks may be present these ichnofaunas suggest that the initial exploitation of underutilized or empty ecospace is linked to a set of recurrent behavioral strategies to obtain food from matgrounds and fi rmgrounds in the absence of a well developed mixed layer preservation of these behaviors in the form of trace fossils is mediated by a set of taphonomic conditions these taphonomic windows occur during the initial colonization of empty or underutilized ecospace such as the ediacaran cambrian transition or the subsequent colonization of the land but also after the end permian mass extinction which may have had a negative impact on the mixed layer in marine environments producing a return to the ecologic conditions of the early paleozoic

proliferation of early triassic wrinkle structures implications for environmental stress following the end permian mass extinction lower triassic strata in the western united states and northern italy have yielded the first reported occurrence of microbially mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the cambrian we hypothesize that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end permian mass extinction these wrinkle structures occur in siliciclastic sedimentary rocks in association with hummocky cross stratification and in some cases trace fossils such as asteriacites rhizocorallium planolites and gyrochorte that indicate a subtidal paleoenvironment wrinkle structures were found in the campil member in the werfen formation of northern italy and in the virgin limestone member of the moenkopi formation and the upper member of the thaynes formation exposed in the western united states the presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm dominated siliciclastic shelves wrinkle structures commonly formed in a variety of subtidal paleoenvironments during the proterozoic cambrian but thereafter became restricted to intertidal supratidal and deep sea environments this restriction has been attributed to the increase in infaunalization of metazoans following the cambrian radiation the proliferation of wrinkle structures in subtidal settings during the early triassic suggests that infaunal bioturbation was reduced after the end permian mass extinction and that this reduction lasted for millions of years

distribution of quaternary deformation in the central main ethiopian rift east africa 1 the main ethiopian rift mer is a narrow continental rift characterized by an along axis variation in rift evolution with early stages in the south evolving to incipient breakup in the north although distribution and style of quaternary volcanotectonic deformation is well known in the northern rift sector knowledge of these characteristics is comparatively less constrained southward in this paper we present the results of a field structural study carried out to better constrain the time space distribution of faulting in the central sector of the mer central mer the new field structural data coupled with new 14c radiometric dating of faulted rocks suggest a localization of faulting at both rift margins of the central mer where radiometric dating of faulted material has allowed establishing a late pleistocene holocene activity of border faults conversely in rift faulting wonji fault belt wfb is subordinate highlighting a major difference with the northern sector of the mer where deformation is essentially accommodated in the axial zone this is consistent with an along axis variation in rift evolution showing the central mer less evolved than the northern rift sector inversion of cumulative fault slip data reveals a variation in the extension direction between the rift margins n105 110 e and the rift floor n90 95 e which accords well with the current nubia somalia plate kinematics the variation in extension direction across the rift could manifest a slip partitioning between the boundary faults and in rift wfb faults

the end permian mass extinction and its aftermath on an equatorial carbonate platform insights from ichnology terra nova 22 195 202 2010 abstract a semi quantitative analysis of the endobenthic activity across the permo triassic boundary ptb was carried out on a 445 m thick well core interval from the persian gulf by logging the degree of bioturbation trace fossil distribution and ichnodiversity id this analysis enables for the first time the evaluation of recovery of endobenthic organisms after the end permian mass extinction by direct comparison of post extinction lower triassic with pre extinction upper permian units the results indicate that the studied shallow marine carbonate platform within an equatorial setting acted as oxygenated refugee for the biotic recovery traces of suspension and deposit feeders reappear simultaneously shortly after the extinction event but are reduced in size macroborings so far unknown from the lower triassic occur where suitable firm or hard substrate was available recovery took place in two phases early induan 50 pre extinction id and 20 30 bioturbation and late induan 75 id and almost 90 bioturbation re established

shear wave splitting in ethiopia precambrian mantle anisotropy locally modified by neogene rifting 1 twenty six broadband seismic stations in an areal array spanning 500 500 km across ethiopia were used for shear wave splitting studies our results show small to moderate delay times 0 5 1 7s with fast polarization azimuths sub parallel to the orientation of the east african rift nne ssw and also to the proterozoic tectonic fabric across the entire studied area our results imply ethiopian upper mantle anisotropy is controlled largely by the proterozoic accretion of the mozambique belt with possible minor effects within the rift due to aligned cracks or melt pockets parallel to the rift axis our observations are not consistent with anisotropy created by asthenospheric flow parallel either to the cenozoic extension direction nw se or to the modern absolute plate motion direction nnw sse or to asthenospheric radial flow from the afar plume

thermal models of dyke intrusion during development of continent ocean transition a consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching dyke intrusion also plays an important role magma intrusion can accommodate extension at lower yield stresses than are required to extend thick strong unmodified continental lithosphere mechanically thereby aiding the breakup process dyke intrusion is also expected to heat and thereby weaken the plate but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood to address this issue a numerical solution to the heat flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting the thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in ethiopia finite difference models demonstrate that magmatic extension rate exerts a first order control on the crustal thermal structure once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly less than 1 ma in the main ethiopian rift mer the majority of present day extension is focused on 20 km wide quaternary recent axial magmatic segments that are mostly seismogenic to mid crustal depths and show p wave seismic velocities characteristic of heavily intruded continental crust when reviewed in light of our models these observations require that no more than half of the mer s extension since 2 ma has been achieved by dyke intrusion magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm yr extension rate in the older faster extending 16 mm yr red sea rift rsr in afar dyke intrusion is expected to have had a more dramatic impact on crustal rheology accordingly effective elastic plate thickness and moho depth in the danakil region of northernmost afar are markedly reduced and seismicity is shallower than in the mer thermally driven variations in crustal rheology over time in response to dyke intrusion thus play an important role in the development of continent ocean transition

new trace fossil evidence for an early recovery signal in the aftermath of the end permian mass extinction abstract new ichnological data from the lower triassic werfen formation dolomites italy revealed an unexpectedly diverse and complex ichnofauna in mixed siliclastic carbonate shelf sediments of the western tropical tethys shortly after the end permian mass extinction event common elements are thalassinoides palaeophycus and planolites other associated ichnogenera include spongeliomorpha rhizocorallium lockeia catenichnus helminthopsis and taenidium one structure is tentatively determined as curvolithus biostratigraphic data conodonts and bivalves of the trace fossil interval lower seis siusi member suggest a late griesbachian age less than 1 ma after the end permian mass extinction event similarly diverse ichnofossil communities from this time interval have recently been described from the boreal realm but this is the first record of a diverse griesbachian ichnoassemblage from the tropics indicating that the early recovery of trace fossil producers was not latitudinally restricted as previously proposed accordingly relatively advanced recovery stages were reached on a global scale much earlier than the spathian as is commonly acknowledged the early and interregional peak in ichnofaunal recovery implies subsequent ecological setbacks in post griesbachian times that explain the overall delay of benthic recovery until the spathian rather than persistent catastrophic conditions during the griesbachian a global and synchronous succession of crises and relaxation phases after the griesbachian is proposed as an explanatory model for the recovery pattern of benthic ecosystems

early diagenesis of redox sensitive trace metals in the peru upwelling area response to enso related oxygen fluctuations in the water column pore water and solid phase data for redox sensitive metals mn fe v mo and u were collected on a transect across the peru upwelling area 11 s at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from 0 to 93 m by comparing authigenic mass accumulation rates and diffusive benthic fluxes we evaluate the respective mechanisms of trace metal accumulation retention and remobilization across the oxygen minimum zone omz and with respect to oxygen fluctuations in the water column related to the el nino southern oscillation enso sediments within the permanent omz are characterized by diffusive uptake and authigenic fixation of u v and mo as well as diffusive loss of mn and fe across the benthic boundary some of the dissolved mn and fe in the water column re precipitate at the oxycline and shuttle particle reactive trace metals to the sediment surface at the lower and upper boundary of the omz at the lower boundary pore waters are not sufficiently sulfidic as to enable an efficient authigenic v and mo fixation as a consequence sediments below the omz are preferentially enriched in u which is delivered via both in situ pre cipitation and lateral supply of u rich phosphorites from further upslope trace metal cycling on the peruvian shelf is strongly affected by enso related oxygen fluctuations in bottom water during periods of shelf oxygenation surface sediments receive particulate v and mo with metal oxyhydr oxides that derive from both terrigenous sources and precipitation at the retreating oxycline after the recurrence of anoxic conditions metal oxyhydr oxides are reductively dissolved and the hereby liberated v and mo are authigenically removed this alternation between supply of particle reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of v and mo compared to u on the peruvian shelf the decoupling of v mo and u accumulation is further accentuated by the varying susceptibility to re oxidation of the different authigenic metal phases while authigenic u and v are readily re oxidized and recycled during periods of shelf oxygenation the sequestration of mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions our findings reveal that redox sensitive trace metals respond in specific manner to short term oxygen fluctuations in the water column the relative enrichment patterns identified might be useful for the reconstruction of past omz extension and large scale redox oscillations in the geological record

unusual intraclastic limestones in lower triassic carbonates and their bearing on the aftermath of the end permian mass extinction flat pebble conglomerates were a common carbonate facies in cambrian to early ordovician open marine settings but they become extremely rare in these environments after this time however the early triassic witnessed an anachronistic reappearance of flat pebbles together with other intraclast types in a range of carbonate depositional settings in south china flat pebble conglomerates are encountered in storm dominated platform carbonates to deep basinal settings while prefossilized bivalve intraclasts and flat pebbles are common in mid ramp facies of northern italy the emplacement mechanisms of the intraclast bearing beds appear to have been diverse and to have included basinal turbidity flows and storm generated hyperconcentrated flows true storm beds deposited under combined flow conditions are rare the cause of the widespread early lithification implied by the early triassic intraclasts appears to have been twofold suppression of bioturbation allowing the preservation of thin beds and rapid submarine lithification both features appear to be a response to the widespread development of benthic dysoxia anoxia during and following the end permian mass extinction this event appears to have temporarily recreated the conditions that pertained in cambro ordovician shelf seas flat pebble conglomerates may therefore constitute a proxy indicator of stressed environmental conditions associated with global anoxic dysoxic events

coalescence microseismic mapping earthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations if mis picks are made these will contribute to the location therefore causing potential bias for data recorded by a dense seismic array direct imaging methods can be applied instead we describe the coalescence microseismic mapping method which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array by continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio temporal coordinates of the origins of seismic events noisy data are migrated away from the correct origin so do not contribute to errors in location the method is rooted in a bayesian formulation of event location traveltime inversion allows imaging of source locations and has the capacity to handle errors in modelled traveltimes it has the advantage of working with any 3 d velocity model which therefore may include anisotropy it also automatically incorporates both p and s wave data a multiresolution grid search leads to an efficient implementation with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality we discuss the theory and implementation of this method and illustrate it with real data from microseismic events in iceland caused by melt intrusion in the crust

glacial interglacial variability in the eastern tropical north pacific oxygen minimum zone recorded by redox sensitive trace metals 1 changes in the intensity of the oxygen minimum zone omz in the eastern tropical north pacific over the past 140 kyr are recorded as enrichments and depletions of redox sensitive metals in sediments of two piston cores one within and one below the modern omz from the continental margin off mazatlan mexico 22 41 n 106 28 w concentrations of al 7 7 0 6 ti 0 37 0 03 fe 3 1 0 25 mn 320 31 ppm and ba 560 82 ppm in core nh15p within the omz at 420 m water depth were relatively constant over the last 110 kyr in contrast concentrations of cd 4 8 2 ppm cu 29 8 9 1 ppm u 9 1 3 7 ppm mo 12 1 5 0 ppm v 138 2 51 9 ppm and re 45 8 25 5 ppb were all at least 30 higher in interglacial stages compared to glacial stages concentrations of al 7 4 0 3 ti 0 35 0 03 fe 3 5 0 4 and mn 385 77 ppm in core nh22p below the omz at 2025 m water depth were comparable to those in core nh15p while concentrations of ba 1662 292 ppm were about a factor of three higher in contrast concentrations of cd 0 9 0 3 ppm u 6 6 0 9 ppm mo 3 2 1 2 ppm v 81 8 10 1 ppm and re 25 4 12 ppb were lower in sediments of comparable age than the concentrations of these same metals in the omz core and the differences in their concentrations over glacial interglacial cycles were less pronounced than those in shallower core comparison of the nonlithogenic fraction of metals in the sediments with their estimated contribution from plankton suggests that organic matter is probably the major source of cu ba and perhaps cd to the sediment whereas the indirect effects of organic carbon and low bottom water oxygen concentrations on sediment redox state appear to be more important controls on the distributions u re mo and v changes in the depth at which re and mo precipitated in the sediments and in the re mo ratio suggest that the redox state of the surface sediment and overlying water at both core depths varied over time re and mo removal depths were shallower and re mo ratios were lower at the omz site than in deeper water suggesting that a more reducing environment prevailed over time at the shallower site although the redox state of the waters and sediment at both sites varied over glacial interglacial cycles the omz likely was never anoxic in the last 140 kyr this variability in redox state could be attributed to changes in regional export productivity changes in ocean circulation or a combination of both processes a paleoproductivity reconstruction from biogenic ba data suggests that glacial productivity was considerable lower than it was during interglacial stages model results suggest that the oxygen penetration depth an indicator of sediment redox state changed less than 1 cm as a result of the change in productivity changes in oxygen penetration depth estimated from re and u removal depths are significantly larger suggesting that changes in ventilation are a more important control on sediment redox state overall trace metal results confirm the tight coupling between ocean circulation marine productivity and global change

the impact of ocean deoxygenation on iron release from continental margin sediments in the oceans high nitrate low chlorophyll regions such as the peru humboldt current system and the adjacent eastern equatorial pacific1 primary productivity is limited by the micronutrient iron within the peruvian upwelling area bioavailable iron is released from the reducing continental margin sediments2 the magnitude of this seafloor source could change with fluctuations in the extension or intensity of the oxygen minimum zones3 4 here we show that measurements of molybdenum uranium and iron concentrations can be used as a proxy for sedimentary iron release and use this proxy to assess iron release from the sea floor beneath the peru upwelling system during the past 140 000 years we observe a coupling between levels of denitrification as indicated by nitrogen isotopes trace metal proxies for oxygenation and sedimentary iron concentrations specifically periods with poor upper ocean oxygenation are characterized by more efficient iron retention in the sediment and a diminished iron supply to the water column we attribute efficient iron retention under more reducing conditions to widespread sulphidic conditions in the surface sediment and concomitant precipitation of iron sulphides we argue that iron release from continental margin sediments is most effective in a narrow redox window where neither oxygen nor sulphide is present we therefore suggest that future deoxygenation in the peru upwelling area would be unlikely to result in increased iron availability whereas in weaker oxygen minimum zones partial deoxygenation may enhance the iron supply

the history of coastal upwelling off peru 11 s odp leg 112 site 680b over the past 650 000 years the coarse sediment fraction 63 m and benthic foraminiferal composition of quaternary sediment cores from odp leg 112 hole 680b 252 5 m water depth were sampled from the upwelling area off peru high resolution oxygen isotope stratigraphy based on analyses of the benthic foraminifera bolivina seminuda allows a detailed reconstruction of the upwelling history since oxygen isotope stage 16 variations in the relative abundances of the benthic foraminiferal species nonionella auris bolivina costata and bolivina seminuda document changes in fertility which were found to be enhanced during interglacial stages 1 3 5 7 9 and 11 generally bioturbated sediments are dominated by the genera cassidulina epistominella gyroidina trifarina and valvulineria these benthic foraminifera are members of the undercurrent fauna which is commonly associated with sediments that have a high terrigenous detrital component and a low organic carbon content 6 ml l o2 the entire core interval investigated in hole 680b appears to have been affected by the peruvian undercurrent generally undercurrent dominated sections were found to correspond with glacial oxygen isotope stages

topography river network and recent fault activity at the margins of the central main ethiopian rift east africa abstract along its length the main ethiopian rift mer in east africa records a transition from early fault dominated morphology in the south to axial magma assisted rifting typical of continental break up in the north it is one of the few locations on earth offering a complete picture of the evolution of continental rifting and thus provides a unique opportunity to directly analyze how the drainage network reorganizes under extensional tectonic forcing in this paper we present a new analysis of the river network and relative landforms complemented with a summary of recent geological data at both rift margins of the central mer a key sector of the rift capturing the phase of drainage reorganization between incipient and mature rifting this analysis shows that hydrography is strongly influenced by recent tectonics rectangular drainage patterns windgaps and lacustrine swampy areas formed by structural dams document that the rivers are in continuous competition with fault activity the irregular longitudinal profiles with knickpoints knickzones in correspondence with faults also suggest that rivers are in a transient state of disequilibrium related to recent tectonic activity at rift margins in agreement with previous geological and seismological data a more regional analysis extended to the adjoining northern and southern mer indicates that rifting evolves from initial stages characterized by margins poorly incised by rivers with gentle channel gradients except in correspondence with faults to mature phases in which rift margins are highly incised by a well organized fluvial network composed by concave and steep rivers our regional analysis also indicates a stronger and or more recent tectonic activity at the rift margins proceeding to the south in line with previous models of rift development

muddy prodeltaic hyperpycnites in the lower genesee group of central new york usa implications for mud transport in epicontinental seas abstract recent studies of marine continental shelves show that hyperpycnal flows are responsible for offshore transport of large volumes of sediment detailed facies analysis and petrography of the lower genesee group in the northern appalachian basin nab shows a wealth of sedimentary textures and fabrics that indicate mud deposition by lateral transport across and along the shelf under energetic conditions intervals of silt rich mudstones and muddy siltstones with internal scours diffuse stratification soft sediment deformation normal and inverse lamina set grading and a reduced intensity and diversity of bioturbation occur in multiple facies types and interrupt what appears to be the overall background sedimentation these intervals and their sedimentary features are interpreted as products of high density fluvial discharge events which generated turbulent flows that carried fine grained clastics several tens of kilometers offshore from the paleoshoreline recognizing these sediments as products of river flood and storm wave generated offshore directed underflows challenges previous depositional models for organic rich mudstones in the lower genesee succession which call for clastic starvation and suspension settling of clay and silt in a deep stratified basin rapid deposition of fine grained intervals from hyperpycnal plumes in a setting favoring preservation of organic rich mudstones calls for a reappraisal of the depositional setting of not only the genesee group but also of comparable mudstone successions in the appalachian basin and elsewhere

seismicity of the bora tullu moye volcanic field 2016 2017 the bora tullu moye tm volcanic field is a geothermal energy prospect in the central main ethiopian rift but little is known about the seismicity of the region here we document seismic activity between february 2016 and october 2017 locating more than a 1 000 seismic events of local magnitude 0 to 2 7 this provides new insights into fluid movement and deformation beneath what we only now realize is a complicated volcanic system a discrete cluster of events lies beneath tm but surprisingly most of the seismicity lies in two clusters that are beneath neither the bora or tm edifices in these regions we use earthquake cluster orientations fault plane solutions and fast seismic shear wave orientations to show that seismicity is triggered by hydrothermal circulation of fluids along preexisting fractures the fractures trend in multiple directions and are in general not parallel to rifting related structures instead the fractures are parallel to structures created during previous caldera forming eruptions at both bora and tm highly fractured regions such as this could be attractive targets for geothermal power generation we estimate a minimum depth for a magmatic body beneath tm to be 6 5 km using the mapped brittle ductile transition frequency analysis of the earthquake waveforms reveal the bulk of the events to be volcano tectonic but some low frequency seismicity is present at a depth of 5 km beneath the tm edifice triggered by high pore fluid pressures

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

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

orientation of late precambrian sutures in the arabian nubian shield recent tectonic models have resulted in conflicting descriptions of how the late precambrian sutures of the arabian nubian shield extend into northeast africa the hamisana shear zone in northeastern sudan is critical to this discussion because it truncates and disrupts two sutures the allagi heiani and the onib sol harried analysis of field structural data thematic mapper imagery and rb sr and u pb geochronology suggests that the allagi heiani suture is the western extension of the onib sol harried suture and that both make up the exposed parts of a far traveled polydeformed ophiolitic nappe complex subsequent deformation localized in the hamisana shear zone disrupted this nappe and displaced the suture between 660 and 550 ma during regional deformation associated with the najd fault system these results indicate that at least one suture extends westward into the interior of northern africa

recurrent early triassic ocean anoxia the early triassic record from the smithian stratotype shows that the organic carbon isotope record from northwest pangea closely corresponds to major fluctuations in the inorganic carbon records from the tethys indicating truly global perturbations of the carbon cycle occurred during this time geochemical proxies for anoxia are strongly correlated with carbon isotopes whereby negative shifts in 13 c org are associated with shifts to more anoxic to euxinic conditions and positive shifts are related to return to more oxic conditions rather than by a delayed or prolonged recovery the early triassic is better characterized by a series of aborted biotic recoveries related to shifts back to ocean anoxia potentially driven by recurrent volcanism

two pulses of extinction during the permian triassic crisis over 90 of marine species were lost during the end permian extinction fossil data show that the crisis in china was marked by two distinct phases of marine extinction separated by a 180 000 year recovery period

re orientation of the extension direction and pure extensional faulting at oblique rift margins comparison between the main ethiopian rift and laboratory experiments in this study we draw on a unique combination of well resolved fault slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending main ethiopian rift east africa these data show that both boundary and internal faults oblique and orthogonal to the plate divergence pd respectively exhibit almost pure dip slip motion and indicate significant local deflection in orientation of the extension direction at rift margins scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep seated pre existing weakness oblique to the direction of pd that is able to cause a local rotation in the orientation of the extension direction at rift margins minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems as supported by existing and new palaeomagnetic data from the rift

off rift volcanism in rift zones determined by crustal unloading when continents are stretched over a long period of time deep elongated rift valleys form at earth s surface and zones of ponded magma centred beneath the rift form at the crust mantle boundary1 2 ascending magma sometimes erupts within the rift valley3 4 or counterintuitively at volcanic fields away from the rift valley that are offset by tens of kilometres from the source of magma at depth5 6 7 8 the controls on the distribution of this off rift volcanism are unclear here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley whereas others are confined to the valley we find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure caused by crustal thinning and faulting along the rift borders when gravitational unloading dominates over tectonic stretching forces dykes ascending from the ponded magma are steered towards the rift sides eventually causing off rift eruptions our model also predicts the formation of stacked magma sills in the lower crust above the magma ponding zone as well as the along rift propagation of shallow dykes during rifting events consistent with observations of magmatism and volcanism in rift zones globally we conclude that rift topography induced stress changes provide a fundamental control on the transfer of magma from depth to the surface

high temperature and low oxygen perturbations drive contrasting benthic recovery dynamics following the end permian mass extinction the end permian mass extinction event was the greatest loss of biodiversity ever experienced on the planet the event is thought to have been triggered by the initiation of the volcanic eruptions of the siberian traps the five million year recovery interval that followed the extinction event was strongly influenced by the environmental effects of sustained volcanic eruptions including extreme temperature events and persistent global and regional oxygen minimum zones the effects of these environmental perturbations on the paleoecological recovery of the benthic marine fauna were studied in two depositional units from the southwestern united states representing two substages during the early triassic recovery the smithian sinbad limestone was influenced by high sea surface temperatures and contains a relatively high diversity fauna that exhibits extremely small body size gastropods that lived in this environment were size limited a possible result of metabolic stress due to extreme temperatures these microgastropods were able to become dominate components of the benthic fauna by occupying niche space vacated by other taxa that were excluded by high temperatures the spathian virgin limestone shows evidence for low oxygen conditions the resulting low diversity benthic fauna had a more ecologically complex community structure than the smithian sinbad limestone including the occupation of epifaunal tiering space by crinoids as the prevalence of aerobic facies increased through time diversity body size and the complexity of faunal interactions also increased suggesting that low oxygen conditions were the limiting factor for the benthic recovery in that region the differences in diversity and community structure between the two units highlight the importance of environmental and temporal differences in driving the recovery patterns of the benthic fauna following the extinction event high temperature events and low oxygen conditions restricted the benthic fauna in different ways but both contributed to the delay in recovery from the end permian mass extinction

continental rifting parallel to ancient collisional belts an effect of the mechanical anisotropy of the lithospheric mantle analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process rift propagation is not random but tends to follow the trend of the orogenic fabric of the plates systematically reactivating ancient lithospheric structures continental rifts often display a clear component of strike slip deformation in particular in the early rifting stage moreover although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts these observations suggest that the deformation of the lithosphere especially during rifting is controlled by its preexisting structure on the other hand 1 the plasticity anisotropy of olivine single crystal and aggregates 2 the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs and 3 seismic anisotropy data which require a tectonic fabric in the upper mantle coherent over large areas suggest that preservation within the lithospheric mantle of a lattice preferred orientation lpo of olivine crystals may induce a large scale mechanical anisotropy of the lithospheric mantle we use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process we assess the deformation of an anisotropic continental lithosphere in response to an axi symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head model results show that a lpo induced mechanical anisotropy of the lithospheric mantle may result in directional softening leading to heterogeneous deformation during continental rifting this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique 30 55 to the preexisting mantle fabric this directional softening associated with olivine lpo frozen in the lithospheric mantle may also guide the propagation of the initial instability that will follow the preexisting structural trend the preexisting mantle fabric also controls the deformation regime imposing a strong strike slip shear component a lpoinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting structural inheritance the plume rift paradox and the onset of transtension within continental rifts fl 2001 elsevier science b v all rights reserved

loss of the sedimentary mixed layer as a result of the end permian extinction abstract the end permian mass extinction resulted in the most dramatic degradation of marine bottom communities during the phanerozoic one result of this extinction was the long recognized extreme reduction in bioturbation of the early triassic seafloor several lines of evidence i e preferential preservation of epifaunal and very shallow tier infaunal trace fossils paucity of mid and deep tier trace fossils absence of mottled bioturbation textures dominance of cohesive substrates widespread occurrence of microbially induced sedimentary structures in open marine environments ecological composition of early triassic communities show that the reduction in bioturbation was so extreme that the sedimentary mixed layer was eradicated at an interregional scale for the only time since it was established in the early palaeozoic the consequences of this for ecosystem function and geochemical cycling must have been profound and yet they have received little consideration biogenic mixing of sediments is fundamental to geochemical cycling in extant marine ecosystems and it also governs ecologically limiting factors such as nutrient fluxes benthic primary production and availability of ecospace the collapse of biogenic sediment mixing during the early triassic must have affected geochemical properties of sediments and the seawater as reflected in the geologic record of the sulphur cycle additionally many of the proxies traditionally used to infer early triassic seawater anoxia may rather reflect poor sediment oxygenation arising from the extinction of bioturbators because of its impact on seawater and sediment chemistry the loss of the mixed layer may have been an important but hitherto little considered constraint on the recovery from the end permian mass extinction

the paleoenvironmental distribution of phanerozoic wrinkle structures abstract wrinkle structures are microbially induced sedimentary structures that are known to have formed in ancient marine siliciclastic environments this study reviews the sedimentary context and paleoenvironmental distribution of these sedimentary structures throughout three distinct intervals of the phanerozoic the cambrian the post cambrian paleozoic and the lower triassic during these times wrinkle structures are found primarily within two sedimentary environments storm dominated subtidal environments and the intertidal zone subtidal occurrences of wrinkle structures during the post ordovician phanerozoic only appear to occur during intervals of environmental stress while earlier examples from the cambrian and lowermost ordovician likely formed prior to significant increases in extent and depth of bioturbation that hallmark most of the phanerozoic intertidal examples occur regardless of severe environmental stress and may have formed under low levels of bioturbation due to inherent vagaries of a marginal marine setting these environmental preferences appear to be conservative throughout the phanerozoic and may very well extend throughout earth s history

rapid biotic rebound during the late griesbachian indicates heterogeneous recovery patterns after the permian triassic mass extinction new fossil data from two early triassic griesbachian to dienerian sections from south china show unusually high levels of both benthic and nektonic taxonomic richness occurring in the late griesbachian in total 68 species including 26 newly originated species representing mollusks brachiopods foraminifers conodonts ostracods and echinoderms occur in the late griesbachian indicating well established and relatively complex marine communities furthermore the nekton shows higher origination rates than the benthos analyses of the sedimentary facies and size distribution of pyrite framboids show that this high diversity interval is associated with well oxygenated environments in contrast to the previously suggested scenario which inferred that persistently harsh environmental conditions impeded the biotic recovery during the early triassic our new findings combined with recent work indicate a fitful regional recovery pattern after the permian triassic crisis resulting in three main diversity highs late griesbachian early dienerian early middle smithian and spathian the transient rebound episodes were therefore influenced by both extrinsic local e g redox condition temperature and intrinsic e g biological tolerances origination rates parameters

extensive microbial mats and their influences on the erosional and depositional dynamics of a siliciclastic cold water environment lower arenigian montagne noire france abstract lower arenigian ordovician rocks of the montagne noire france represent a shallow marine environment of a high latitude position along the northern margin of gondwana within the weakly metamorphosed siliciclastics six depositional units were recognized 1 outer shelf 2 foreshore zone below wave base 3 foreshore zone above wave base 4 sand bars of shoreface zone 5 intertidal deposits and 6 lagoonal zone wrinkle structures and various other phenomena were mediated by coherent cyano bacterial mats the structures can be related to the different facies zones of the paleoenvironment whereas the muddy outer shelf and the high energetic sand bars were not overgrown by any mat constructing microbial populations fine sands of the foreshore zone the tidal flats and lagoonal areas were widely colonized restriction of mats due to competition of space by endobenthic macroorganisms or by grazers was low the microbially induced structures are composed of organic material pyrite clay minerals illite chamsonite chlorite and chert the minerals precipitated in situ during degradation of the organic layers by the activity of heterotrophic bacteria at low temperatures because the microorganisms formed a dense organic carpet covering extensive areas of the ancient sea bottom they influenced significantly the erosional and depositional dynamics of the sedimentary system of the local arenigian biostabilization counteracted erosion and baffling trapping and binding enriched mineral particles additionally the in situ formed minerals contributed to the total amount of sediment the biotic influence lead to increased accumulation of sediment within the depositional area the study shows that microbial mats of great extension occur within phanerozoic siliciclastics of cold paleoclimate zones and that preservation of the mat fabrics was possible the significant mats influenced the local sedimentary system an aspect scarcely taken into account for such paleoenvironments

controls on the formation of microbially induced sedimentary structures and biotic recovery in the lower triassic of arctic canada microbially induced sedimentary structures miss are reportedly widespread in the early triassic and their occurrence is attributed to either the extinction of marine grazers allowing mat preservation during the permo triassic mass extinction or the suppression of grazing due to harsh oxygen poor conditions in its aftermath here we report on the abundant occurrence of miss in the lower triassic blind fiord formation of the sverdrup basin arctic canada sedimentological analysis shows that mid shelf settings were dominated by deposition from cohesive sand mud flows that produced heterolithic rippled sandstone facies that pass down dip into laminated siltstones and ultimately basinal mudrocks the absence of storm beds and any other event beds points to an unusual climatic regime of humid quiet conditions characterized by near continuous run off geochemical proxies for oxygenation mo al th u and pyrite framboid analysis indicate that lower dysoxic conditions prevailed in the basin for much of the early triassic the resultant lack of bioturbation allowed the development and preservation of miss including wrinkle structures and bubble textures the microbial mats responsible for these structures are envisaged to have thrived on sandy substrates within the photic zone in oxygen poor conditions the dysoxic history was punctuated by better oxygenated phases which coincide with the loss of miss thus permo triassic boundary and griesbachian mudrocks from the deepest water settings have common benthos and a well developed tiered burrow profile dominated by phycosiphon the presence of the intense burrowing in the earliest triassic contradicts the notion that bioturbation was severely suppressed at this time due to extinction losses at the end of the permian the notion that early triassic miss preservation was caused by the extinction of mat grazers is not tenable