late ordovician jaw bearing polychaetes from anticosti island eastern canada and their biogeographic significance the upper ordovician to lower silurian shallow marine succession of anticosti island eastern canada provides one of the most complete records across the hirnantian in the world this study reports a diverse assemblage of scolecodonts polychaete jaws from the upper katian and hirnantian vaureal ellis bay and basal becscie formations of western anticosti the collection of 10 samples includes ca 30 species representing 10 families the fauna is dominated by polychaetaspids mochtyellids paulinitids and polychaeturids the family xanioprionidae and genera pistoprion tetraprion and rakvereprion are documented for the first time from the ordovician of laurentia the anticosti polychaete fauna shows great similarity to the contemporaneous faunas of baltoscandia this is evidenced by a high relative abundance of mochtyellids and polychaeturids and a number of common species thus suggesting that the closing iapetus ocean at that time did not constitute a barrier for the dispersal of jaw bearing polych

systematic occurrences of malformed teratological acritarchs in the run up of early palaeozoic δ13c isotope excursions abstract the late ordovician and silurian are characterised by several strong global positive 13 c carb excursions some of them exceed 5 and thus belong to the strongest perturbations of the carbon cycle in the phanerozoic the onset of the excursions is characterised by extinction and or turnover events of several groups of marine invertebrates the causal mechanisms of the carbon cycle perturbations however are still unknown and currently a matter of vigorous scientific debate our own investigations in the hirnantian latest ordovician have shown that the onset of the major 13 c excursion hice is characterised by very high abundances of acritarchs showing abnormal teratological forms a critical review of published reports of abnormal acritarchs from the late ordovician to early devonian and a correlation of their occurrences with the global stable carbon isotope curve show that high abundances of teratological forms of acritarchs are often coeval to the run up of 13 c excursions high abundances of teratological forms in modern marine protists are commonly observed in environments with a high degree of environmental stress in the fossil record the challenge is to attribute abnormal forms of organisms to specific environmental circumstances our study implies that they are somehow related to the global carbon cycle i e to carbon isotopic composition of the ambient sea water and that they share a common extrinsic cause with the contemporaneous extinction and or turnover events in other fossil groups

fold axis parallel extension in an arcuate fold and thrust belt the case of the helvetic nappes abstract the helvetic nappes of western switzerland are discussed as an example of an arcuate foreland fold and thrust belt in which active fold axis parallel stretching occurred fold axis parallel extension is recorded by 1 1 incremental strain data from pressure shadow fibres the significance of pressure shadow fibres for the determination of the deformation history of a region is discussed pressure shadows are used to quantify the amount of and to describe the distribution of fold axis parallel extension occurring in the helvetic nappes 2 2 the extension directions of conjugate systems of en echelon veins it is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern the relative age of the conjugate en echelon vein systems in the helvetic deformation history can be assessed the geometry of the conjugate sets relative to the local anisotropy plane is described and the significance of the preferred orientation of their extension directions is discussed 3 3 fold axis parallel sections a comparison of the regional distribution of the fold axis parallel strain with the shape of the helvetic nappes in fold axis parallel sections shows that the fold axis parallel strain cannot be related to the footwall topography of the nappes it is concluded that the fold axis parallel extension in the helvetic nappes was induced by a change of direction of overthrust shear this change occurred late in the deformation history and was superposed on the already formed nappes the changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting alpine nappe pile relative to the european plate a rotation which lead to the arcuate shape of the western alps

biodiversity patterns of ordovician marine microphytoplankton from baltica comparison with other fossil groups and sea level changes abstract based on the extensive literature on ordovician acritarchs biodiversity curves of marine microphytoplankton of the palaeocontinent baltica have been compiled the dataset is derived from more than 100 publications and includes over 600 species whose ranges can be used in diversity analysis stratigraphically well constrained data from the rapla and mannamaa boreholes northern estonia are analysed separately in order to provide additional information on the middle to late ordovician microphytoplankton evolution on shallow shelf settings the total species diversity and normalised diversity curves based on range through data show a slight decrease from the tremadocian to floian which may partly be attributed to limited data available a continuous increase in diversity from the base of the dapingian to late darriwilian early katian can be observed with highest total diversity approaching 250 species in the keila regional stage the highest appearance rates are recorded in the dapingian and darriwilian diversity progressively diminished after the early katian with a more pronounced decrease in the upper ordovician porkuni regional stage corresponding to the hirnantian a significant faunal turnover took place in the uppermost ordovician pirgu and porkuni stages with high extinction rates as well as the appearance of many acritarch taxa that are typical of the silurian floras the phytoplankton diversity curves match rather well with those of several other fossil groups in baltica notably brachiopods and ostracods comparison with other palynomorphs reveals both similarities and differences the chitinozoans show the highest diversities in the upper darriwilian but their later decline predates that of acritarchs a rapid diversification of scolecodonts jawed polychaetes can also be observed in the upper darriwilian however unlike acritarchs or chitinozoans their diversity continues to increase until the uppermost katian the acritarch diversity changes are analysed and discussed in the context of palaeogeographic northwards drifting of baltica and palaeoclimatologic changes rising sea levels up to the middle part of the upper ordovician the increasing diversity of the phytoplankton not only roughly correlates with the baltic and global sea level changes but also with the diversification of marine invertebrate groups

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

a far field record of the end ordovician glaciation the ellis bay formation anticosti island eastern canada abstract the end ordovician was a critical period in earth history it was a time of global climatic change with large scale continental glaciations a wealth of recent work has documented the stratigraphical records from the near field areas i e from various gondwanan continental relics where glaciation was more severely felt there is an urgent need to re examine the stratigraphic record from the far field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing duration and character at the glaciation centred around the ordovician silurian o s boundary one of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on anticosti island in eastern canada the east west trending 200 km long ellis bay hirnantian outcrop belt with superb coastal exposures at both ends of anticosti island is slightly oblique to the paleoshoreline with near shore siliciclastic dominated facies restricted to the eastern part of the island and more offshore carbonate dominated facies present along the central and western parts these mixed carbonate siliciclastic facies accumulated along a storm dominated beach to offshore profile they are organized into a stacked set of shallowing upward offshore to shoreline facies successions typically coarsening upward separated by deepening upward facies successions typically fining upward formed during intervening transgressions high frequency glacio eustatic fluctuations were the dominant control on the development of tr sequences observed in the ellis bay succession of anticosti island although tectonic subsidence had a substantial secondary effect on thickness five tr sequences are recognized allowing for precise correlation along the ellis bay outcrop belt in the eastern sections relatively thin tr sequences are locally bounded by unconformities a typical sequence begins with thin transgressive limestone overlain by a maximum flooding calcareous shale grading up into regressive storm influenced proximal to transitional zone fine sandstone grainstone and shale which are sharply overlain by shoreface to foreshore sandstone grainstone in the central and western sections the thicker tr sequences have more symmetrical transgressive regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply two discrete glacial periods are recognized during the deposition of the hirnantian ellis bay succession with the younger one recording large sea level changes 50 m in magnitude caused by the development of very large ice sheets over western gondwana abundant ice conditions on the globe during the end ordovician and the resulting high amplitude eustasy appear to have left distinctive stratigraphical imprints on mixed paleotropical siliciclastic carbonate ramps

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

acid resistant microfossils used in the determination of palaeozoic palaeoenvironments in libya abstract a study of silurian acid resistant microfossils from the ghadames basin in tripolitania libya using palynological techniques revealed that the following may be used as palaeoenvironmental indicators spores acritarchs chitinozoa tasmanitids scolecodonts plant tissue eurypterid cuticle and graptolite siculae the palynofacies were defined by quantitative and qualitative diversification of these assemblages sedimentological features were used to confirm these proposed facies six distinct types of palynofacies were identified and related to models of distance from the shoreline it is tentatively suggested that they may relate to lagoonal intermediate and open marine environments

global climatic forcing of deep sea benthic foraminiferal test size during the past 120 m y stratigraphic distribution of the maximum diameter of the largest deep water 1500 m water depths calcareous trochospiral benthic foraminiferal tests in samples representing the past 120 m y shows that average values of the maximum diameter have doubled since early cretaceous time fluctuations in test size show six minima and six maxima it is significant that the fluctuations correspond to changes in global climate and in deep water dissolved oxygen conditions this latter synchronism indicates that deep sea foraminiferal test size was likely controlled by fluctuations in the level of dissolved oxygen or related factors in the deep ocean caused by global climatic changes

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

the wathaman batholith an early proterozoic continental arc in the trans hudson orogenic belt canada the wathaman batholith a major element of the trans hudson orogenic belt in northern saskatchewan and manitoba has an exposed strike length of 900 km this immense body separates dominantly archean rae hearne provinces to the northwest from early proterozoic arc related terranes of the orogen to the southeast major and trace element data for 42 samples from the saskatchewan part of the batholith are similar to those of phanerozoic arc batboliths for example sierra nevada and andean batholiths data plotted on trace element discrimination diagrams show that the batholith has continental arc affinities u pb zircon geochronology of five main phase granite samples suggests that most of the batholith formed between 1865 and 1850 ma and is coeval with smaller plutons of the la rouge arc the main deformation of the batholith is limited by the 1830 ma age of a late granite sheet that crosscuts folds and foliation of main phase granites the 15 to 25 m y crystallization span of the batholith is similar to the histories of many more recent continental magmatic arcs

plutonism deformation and metamorphism in the proterozoic flin flon greenstone belt canada limits on timing provided by the single zircon pb evaporation technique the authors have used the single zircon pb evaporation technique to determine the age of granitoids in the western flin flon domain of the trans hudson orogen in canada and to constrain the timing of molasse sedimentation deformation metamorphism and mesothermal gold mineralization the plutons were intruded between 1 860 and 1 834 ma about 30 m y after volcanism and provide further evidence of a major period of intrusive activity throughout the trans hudson orogen at that time the older plutons 1 860 to 1 848 ma are synchronous with p2 deformation and the early stages of peak metamorphism whereas the younger plutons are synchronous with or postdate the p3 deformation event molasse sedimentary units are deformed and metamorphosed thus deposition must have begun at about 1 860 ma mesothermal gold mineralization postdates all plutons and may be related to high grade peak metamorphism in adjacent terranes at about 1 815 ma this study also provides further support for the use of the pb evaporation technique as a suitable method for dating single zircons and indicates that the sup 208 pb sup 206 pb ratio of zircons may be a useful tool in detecting the presence and source of xenocrystic zircons

structural development of the snow lake allochthon and its role in the evolution of the southeastern trans hudson orogen in manitoba central canada the snow lake allochthon is a zone of tectonic interleaving of sedimentary rocks of an inverted marginal basin kisseynew domain with island arc and oceanic rocks it is located in the southeaster

post hunnebergian ordovician carbon isotope trend in baltoscandia its environmental implications and some similarities with that of nevada abstract carbon isotope changes during post hunnebergian ordovician time were investigated as the corresponding sequence of rocks is stratigraphically nearly complete in baltoscandia an attempt was made to use it to elaborate the general pattern of carbon isotope changes in most of the ordovician complications caused by several local hiatuses condensed sections and facies changes were mitigated by the study of overlapping sections a total of 326 new whole rock analyses and published data from 18 drill cores and 4 outcrops in estonia latvia and sweden were considered the following positive carbon isotope events were observed 1 the mid darriwilian excursion peak 13c value 1 9 in the aseri stage 2 the mid caradoc excursion 2 2 in the uppermost part of the keila stage 3 the first late caradoc excursion 2 3 in the lower part of the rakvere stage 4 the second late caradoc excursion 2 4 in the upper part of the nabala stage 5 the early ashgill excursion 2 5 in the lowermost part of the pirgu stage 6 the mid ashgill excursion 2 0 in the upper part of the pirgu stage 7 the widely known large hirnantian excursion in estonia the peak value reaches 6 7 in the porkuni stage the study interval comprises a long 19 ma period characterized by low magnitude carbon isotope changes and a following much briefer 12 ma interval with often medium scale shifts and a major event at the end the latter interval reflects onset in the mid caradoc of a period of variable climate alternation of arid and humid episodes which ended with a serious cooling and glaciation the baltoscandian and north american nevada 13c trends are in general lines very similar providing good prospects for global chemostratigraphical correlation and understanding of role of oceanic processes in carbon cycling

late ordovician sand wave complexes on anticosti island quebec a marine tidal embayment laterally discontinuous mixed carbonate siliciclastic sandstones in the upper vaureal and lower ellis bay formations of anticosti island were deposited on an equatorial carbonate ramp with a slope of less than 1 the 10 18 m thick sandstones are interpreted as subaqueous sand wave complexes analogous to detached parts of modern shoreface connected sand ridges these record storm enhanced tidal modification of a northerly derived shoal retreat massif that may have formed in response to recovery from global sea level lowstands in the late ordovician ashgill late rawtheyan hirnantian the sand wave complexes formed within a tidal embayment that was confined by the precambrian shield to the north and northwest by rising tectonic highlands of the humber zone in newfoundland to the east and by active tectonic highlands in the quebec appalachians gaspesie to the south paleocurrent distributions parallel to the western margins of the strait of belle isle suggest that the north end of the embayment w

acritarch marine microphytoplankton diversity in an early ordovician deep water setting the skiddaw group northern england implications for the relationship between sea level change and phytoplankton diversity early ordovician acritarch microphytoplankton assemblages from an outer shelf to slope facies in northern england show changes in diversity that parallel changes in sea level with high diversity occurring during lowstands and low diversity during highstands the pattern is the reverse of that recorded in the literature in which high acritarch diversity in continental platform successions has been related to periods of transgression and highstand and low diversity to periods of regression and lowstand the apparent contradiction can be explained if high diversity assemblages move further onto continental platforms during transgression and highstand but towards basins during regression and lowstand the hypothesis that high diversity acritarch assemblages track onshore and offshore during episodes of sea level change implies that there is a direct causal link between sea level and microphytoplankton diversity at least in so far as changes in sea level influence the location of high diversity the direct physical influence of sea level change on phytoplankton diversity might have been restricted to relatively small scales however its most obvious effect being perhaps to shift the locus of maximum diversity rather than affect the overall level of diversity at larger temporal and spatial scales the direct influence of sea level change on acritarch diversity is less certain parallel large scale changes in sea level and phytoplankton diversity have been documented for the phanerozoic as a whole but possibly reflect either independent responses to other factors such as the effects of tectonic cycles and changes in palaeogeography ocean circulation and climate or else indicate an indirect influence in the sense that sea level rise created conditions under which phytoplankton diversity could increase in contrast to the results discussed in this paper other analyses of ordovician acritarch diversity at larger spatial scales than those considered here have concluded that there is no evidence for any causal link between sea level change and diversity if however the controls on diversity exerted directly by changes in sea level are scale dependent and only apparent at relatively small scales analyses at larger spatial scales that compile measures of diversity across a range of depositional environments i e that sample both onshore and offshore environments and combine the results might not evince a correlation between changes in sea level and diversity in addition to producing shifts in the location of high diversity assemblages sea level changes also appear to be accompanied by at least local turnover in species composition during the early ordovician in northern england the high diversity assemblages associated with lowstands are very different in composition elsewhere in cratonic settings maximum flooding intervals within successive early palaeozoic stratigraphic sequences are characterised by markedly different acritarch assemblages hence changes in sea level appear to be associated with local extinctions and originations as well as changes in the locus of high diversity possibly as a result of ecological stresses arising from changes in the location of acritarch assemblages

lipid biomarkers record fundamental changes in the microbial community structure of tropical seas during the late ordovician hirnantian glaciation the late ordovician mass extinction was linked to climate cooling and glaciation of gondwana during the terminal ordovician hirnantian age 444 7 443 4 ma extinction patterns have been well described for many marine taxa but much less is known about marine microbial communities through this interval to elucidate the structure of microbial communities in tropical marine basins through the late ordovician we analyzed lipid biomarkers in thermally well preserved strata from the taconic foreland anticosti island canada the cincinnati arch midwestern united states and the western continental margin vinini formation nevada united states despite clear oceanographic differences lipid biomarker profiles show similarities between these three localities major shifts in biomarker distributions of anticosti island and the vinini formation mainly hopane sterane ratios record changes in the balance of bacterial versus algal primary production bacterial contributions to sedimentary organic matter were highest during warm intervals both before and after hirnantian cooling in particular 3 methylhopanes likely sourced from aerobic methanotrophic bacteria occur in high relative abundance many times the phanerozoic average across laurentia throughout most of the interval studied 3 methylhopane abundances also reveal an overall positive relationship with paleotemperature proxies implying increased methane cycling during warm intervals these results suggest that enhanced methane cycling could have provided an important positive feedback on climate during extended intervals of early paleozoic time

a high resolution continuous δ13c record spanning the ordovician silurian boundary on anticosti island eastern canada1 one of the best exposed and most complete stratigraphic records from paleotropical areas spanning the ordovician silurian o s boundary is located on anticosti island eastern canada our study is

first record of the hirnantian upper ordovician δ13c excursion in the north american midcontinent and its regional implications the most prominent of the two major global delta c 13 excursions in the ordovician the hirnantian delta c 13 excursion hice which is previously recorded from the uppermost ordovician in a few sections in nevada quebec arctic canada baltoscandia scotland and china is documented for the first time from the north american midcontinent samples through the girardeau limestone and leemon formation in missouri and illinois show elevated delta c 13 values of 4 parts per thousand to 5 parts per thousand although not determined precisely the beginning of the hice is likely to be in the upper part of the orchard creek shale and it ends in the upper leemon formation being extraordinarily useful chronostratigraphically the presence of the hice makes it possible to provide a firm dating of the study interval whose age has long been controversial comparison between the study sections and coeval hice sequences in north america and europe show striking similarities especially in sea level history indicating that major local lowstands reflect eustatic sea level changes a comparison with hirnantian diamictite successions in north and south africa and argentina suggests that these lowstands correspond to two major gondwanan glacial episodes less

structural setting and fluid characteristics of the proterozoic tartan lake gold deposit trans hudson orogen northern manitoba the late kinematic structural architecture of the flin flon domain is dominated by a network of anastomosing shear zones formed by compressional deformation during the proterozoic trans hudson orogeny ca 1900 1800 ma a number of structurally controlled mesothermal gold deposits occur throughout the region associated with this shear zone network the tartan lake gold deposit occurs in the tartan lake shear zone at the contact between a layered gabbroic intrusive complex and metavolcanic rocks of the amisk group ca 1875 ma five phases of deformation have been recognized in the flin flon region p1 p5 midgreenschist facies peak metamorphic conditions phase 3 at tartan lake are recorded by a mineral assemblage of biotite actinolite albite muscovite epidote and carbonate in volcani clastic rocks along with a well developed phase 3 schistosity early movement of the tartan lake shear zone was initiated with phase 3 deformation during which oblique slip occurred during phase 4 this shear zone was overprinted by a major regional fold and the shear zone was reactivated by strike slip movement regional phase 3 schistosity is refolded in a series of steeply plunging mesoscopic phase 4 fold closures that were synchronous with the shear zones a network of conjugate strike slip shear zones that vary in width from 5 to 35 m comprise the 0 3 km wide structure at tartan lake deformation within the shear zones is marked by 1 development of s and c schistosities in which c is dominant 2 weakly developed sub horizontal mineral lineations and 3 folding and kink banding of these schistosities within shear zones sericite chlorite quartz carbonate albite tourmaline and pyrite are the dominant minerals and biotite and actinolite are retrograded to chlorite such that this deformation and au mineralization are postpeak metamorphism five stages of veining occurred within the shear zones and these are characterized by distinctive orientations and mineral parageneses gold mineralization occurred with two of these stages of veining along with pyrite chalcopyrite tellurides monazite scheelite and rutfie the timing of this mineralizing event has been constrained by the 40 ar 39 ar method at 1791 or 4 ma this is 20 to 40 m y postpeak metamorphism fluids were of h 2 o co 2 nacl composition throughout the ore forming vein events with modal salinity 6 9 wt percent equiv co 2 density 0 64 g cm 3 and homogenization temperatures ranging from 250 degrees to 390 degrees c with a median of 295 degrees c stable isotope compositions of quartz delta 18 o 10 8 13 8ppm chlorite delta 18 o 3 0 4 8ppm muscovite delta 18 o 7 5 9 0ppm scheelite delta 18 o 4 0 5 0ppm albite delta 18 o 9 0 9 6ppm tourmaline delta 18 o 7 9 8 8ppm pyrite delta 34 s 1 7 3 1ppm and chalcopyrite delta 34 s 1 4 1 7ppm establish the temperature of gold mineralization between 315 degrees and 472 degrees c with an average at 372 degrees or 41 degrees c 1sigma ore forming fluids had delta 18 o 5 2 to 6 5 per mil and delta d 50 to 86 per mil and fe dolomites from mineralized veins exhibit restricted ranges of delta 13 c 7 0 to 5 2ppm and delta 18 o 9 3 to 10 9ppm suggesting a uniform fluid composition these veins formed at a pressure of 1 2 to 2 4 kbars corresponding to 4 3 to 8 6 km if lithostatic fluid pressure is assumed constraints on the depth at which these veins precipitated and the character of the fluids suggest that this gold deposit formed from advecting metamorphic fluids a postmineralization stage of brittle fracturing is represented by barren veins that exhibit significant depletion in delta 34 s 29 to 30ppm this may be the result of an increase in fluid f sub o 2 in the final stages of crustal deformation during the trans hudson orogeny

sponge dominated offshore benthic ecosystems across south china in the aftermath of the end ordovician mass extinction abstract offshore benthic communities were sparse after the end ordovician mass extinction the recent discovery of diverse abundant sponges in south eastern china anhui and zhejiang provinces that flourished during the hirnantian late ordovician post extinction interval raises questions over the extent and ecological significance of the sponge community at that time this paper dramatically expands the scale of the hirnantian sponge fauna within south china with seven new occurrences across 2000 km these occurrences cross the ordovician silurian boundary interval in the marginal and submerged platform areas of the yangtze sea in jiangsu hubei hunan chongqing and sichuan provinces in condensed sections platform margin facies the occurrences are normally represented by extensive spiculite layers but several sites yield entire exceptionally preserved body fossils at ganggangshan near nanjing jiangsu province such preservation is abundant the new occurrences are combined with palaeogeographic and tectonic reconstructions to produce a model for the distribution and preservation of the sponge fauna we argue that the exceptional preservation was due largely to nepheloid layers that were generated during rapid post glacial transgression that flooded the weathered land surface of the uplifted cathaysia block nepheloid layer collapse led to rapid deposition of suspended sediment within the deeper parts of the intra plate restricted basin and on adjacent platform margins and slopes the suspended sediment influx also introduced abundant nutrients to the water column as indicated by a significant 13c excursion during the sponge bearing mudstone interval at ganggangshan the combined sponge assemblage was dominant over large areas of the hirnantian offshore sea floor across south china we predict that similar occurrences with exceptional preservation will also be found in other continental blocks abundant articulated sponges may be encountered particularly in restricted basin successions containing rapidly deposited graptolitic mudstones near the ordovician silurian boundary

silurian tectonic sedimentary setting and basin evolution in the sichuan area southwest china implications for palaeogeographic reconstructions abstract silurian black shales are among the most significant hydrocarbon source rocks globally and are widespread across the southern and eastern sichuan basin and adjacent areas their tremendous production potential has been demonstrated by the discovery of self sourced shale gas reservoirs in the jiaoshiba field in the fuling area of the sichuan basin in this study we reconstruct the tectonic and palaeogeographic framework of the silurian sichuan basin analyze the distribution of sedimentary facies and subdivide the sedimentary succession into a number of stages understanding the formation and evolution of the silurian basin is necessary to predict its potential for future hydrocarbon exploration and development and may also help to better understand global palaeoceanographic conditions during this interval the sichuan basin in the silurian period was located along the northwestern margin of the yangtze plate which was itself located on the northern margin of gondwana at a low near equatorial latitude the basin was initially composed of an intracratonic depression surrounded by three structural uplifts forming a semi enclosed shallow sea however the basin experienced intraplate tectonic deformation at an early stage followed by continued flexural subsidence and intracratonic deformation preserved silurian strata comprising the longmaxi shiniulan and hanjiadian formations reflect several depositional stages including 1 slow deposition of the basal black shale of the longmaxi formation in a deep water environment due to a transgressive lag following glacial melting in gondwana the resulting sea level rise and intraplate tectonic activity 2 rapid sedimentation during the later stage of longmaxi deposition 3 deposition of the shiniulan formation on a neritic carbonate platform in the southern of the basin 4 deposition of the siliciclastic hanjiadian formation in a deltaic environment

deformation within foreland thrust sheets by populations of minor faults the dominant deformation mechanism within foreland thrust sheets is the displacement of blocks of weakly deformed rock on discrete mineral coated minor faults in highly deformed layers at the bases of three southern appalachian foreland sheets such minor faults are members of families or populations of consistently oriented structures the deformation style is similar in all three sheets rock nearest each thrust was shortened in the direction of sheet transport and thickened by a family of faults oriented at low angles to the thrusts rock further above each thrust experienced only the first of these two deformation stages statistical analyses of fault populations from these three sheets demonstrate that their deformation is quantitatively as well as qualitatively similar deformation due to movement on minor faults is statistically homogeneous at the scale of an outcrop in one of these sheets strain due to minor faults can be measured in that sheet a strain profile across this sheet is similar to strain profiles across continuously deformed sheets from internal portions of mountain belts suggesting that their responses to emplacement are similar

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

co2 forced climate thresholds during the phanerozoic abstract the correspondence between atmospheric co2 concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity here i compare 490 published proxy records of co2 spanning the ordovician to neogene with records of global cool events to evaluate the strength of co2 temperature coupling over the phanerozoic last 542 my for periods with sufficient co2 coverage all cool events are associated with co2 levels below 1000 ppm a co2 threshold of below 500 ppm is suggested for the initiation of widespread continental glaciations although this threshold was likely higher during the paleozoic due to a lower solar luminosity at that time also based on data from the jurassic and cretaceous a co2 threshold of below 1000 ppm is proposed for the initiation of cool non glacial conditions a pervasive tight correlation between co2 and temperature is found both at coarse 10 my timescales and fine resolutions up to the temporal limits of the data set million year timescales indicating that co2 operating in combination with many other factors such as solar luminosity and paleogeography has imparted strong control over global temperatures for much of the phanerozoic

the ordovician silurian boundary at the eastern end of anticosti island a hirnantia fauna is recorded for the first time from anticosti island situated 6 m below the beds interpreted here as representing the maximum regression at the eastern end of the island the latter coinciding with the local top of the revised ellis bay formation the ordovician silurian boundary on the island is discussed

a late ordovician age for the whirlpool and power glen formations new york a restudy of the palynology of the whirlpool formation and power glen formation in new york usa yielded a diverse fossil assemblage with cryptospores glomalean fungi acritarchs chitinozoans scolecodonts and small carbonaceous fossils these new data and particularly the presence of the chitinozoan index fossil hercochitina crickmayi combined with emerging stable carbon isotope data suggest a late ordovician katian or hirnantian age for these formations which is older than their previously suggested silurian rhuddanian age

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

palaeoenvironmental interpretation of microphytoplankton diversity trends in the cambrian ordovician of the northern sahara platform abstract analysis of vertical fluctuations in assemblage diversity and relative abundances of microphytoplankton morphological classes permits palaeoenvironmental reconstructions of the cambrian ordovician stratal sequences in the hassi r mel area north central algeria and northern rhadames basin southern tunisia abundance fluctuations of acritarch morphological classes and assemblage diversity appear to be related to changes in depositional facies the present palaeoenvironmental interpretation is primarily based on a comparison with microphytoplankton distributional trends in modern depositional environments where possible the observations are tested against palaeoenvironmental information derived independently from previous lithostratigraphical macropalaeontological and petrographical studies of the formations investigated the present data support the following palaeoenvironmental scenario during late cambrian times relatively shallow water neritic conditions prevailed throughout the study area with evidence for a more proximal environment in southern tunisia in the early tremadoc open marine offshore conditions characterized the central algerian region but a shallower water setting is indicated for southern tunisia during the late arenig llanvirn interval an open marine offshore setting is envisaged for the southern tunisian region with more proximal conditions prevailing in the algerian hassi r mel area during the entire ashgill an essentially restricted marine environment was established throughout the study area some stratigraphically important acritarch species e g acanthodiacrodium angustum appear to be facies sensitive the recognition of palaeoecological control on microphytoplankton distribution has important consequences on the biostratigraphic application of these microfossils

the end ordovician glaciation and the hirnantian stage a global review and questions about late ordovician event stratigraphy abstract this paper proposes a global review of hirnantian event stratigraphy the hirnantian gssp in south china is tentatively correlated with latest ordovician strata from the peri gondwanan glacial regions problems of biostratigraphical correlation are highlighted at a worldwide scale the major biostratigraphically useful fossil groups graptolites chitinozoans brachiopods conodonts acritarchs are analysed and their limits for global correlation of the uppermost ordovician are discussed palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective late ordovician global biostratigraphical scheme as an alternative correlative tool the hice hirnantian isotopic curve excursion event is often put forward in the literature however carbon isotope chemostratigraphy shows like biostratigraphy some limits to the present state of knowledge no good independent biostratigraphical control of the hice exists in both shallow carbonate deposits and deeper shaly ones recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic 13corg and inorganic 13ccarb carbon species further complicating the use of the hice as an isochronous benchmark all of these difficulties for hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future precise late ordovician and early silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the big five extinctions

geodynamic evolution and thermal history of the central flin flon domain trans hudson orogen constraints from structural development 40ar 39ar and stable isotope geothermometry shear zones in the flin flon area record five episodes of development these have been studied using structural analysis and 40ar 39ar thermochronology deformation occurred as a result of collision between the archean rae hearne and archean superior plates during the trans hudson orogeny 1900 to 1700 ma the geodynamic evolution encompasses a transition from island arc supracrustal development 1900 ma to tectonic mountain building under brittleductile conditions 1860 ma two episodes of ductile lineation schistosity l s shear zones on the prograde metamorphic path 1860 1810 ma and two episodes of retrograde displacement between 1790 and 1690 ma during final suturing of the archean cratons with paleoproterozoic assemblages metamorphism evolved in three stages contact aureoles to granitoid bodies formed at 1860 1840 ma hornblende homfels facies 40ar 39ar ages indicate cooling through 500 c at 1840 ma this was followed by regional heating burial which produced regional peak thermal recrystallization at 1820 1790 ma under lower greenschist to lower amphibolite conditions lastly retrograde metamorphism took place during unroofing between 1790 and 1690 ma with a regional cooling rate of 2 c m y

reefs during the multiple crises towards the ordovician silurian boundary anticosti island eastern canada and worldwide multiple latest ordovician rawtheyan hirnantian glaciations in central africa with concomitant global sea level lowstands and cooler restricted equatorial carbonate shelves and ramps interrupted by warmer interstadial highstands had a dramatic global impact on the tropical shallow water reef ecosystem and carbonate production with the ordovician silurian boundary strata on anticosti island as a global standard for a carbonate shelf ramp setting the latest ordovician and earliest silurian reveal three reef phases ended by three extinctions the first extinction towards the end of the rawtheyan affected the last richmondian type reefs vaureal formation mill bay member the second extinction was less pronounced ending with reefs at the base of the prinsta member ellis bay formation interpreted as the top of the normalograptus extraordinarius graptolite subzone the third and most severe extinction phase capped the laframboise patch reef complex ellis bay formation at the top of the no

biotic turnover driven by eutrophication before the sturtian low latitude glaciation a dramatic oceanic biotic shift from eukaryotic phytoplankton to bacteria occurred about 740 million years ago microfossil and geochemical data from the chuar group in the southwestern united states link this biotic turnover to widespread eutrophication of surface waters reconstructions of the diversity of precambrian microorganisms suggest a pronounced biotic turnover coinciding with the onset of neoproterozoic low latitude glaciation in which diverse assemblages of organic walled microfossils known as acritarchs were replaced by assemblages of simple smooth walled forms called leiosphaerids and the remnants of bacterial blooms1 2 3 4 this turnover has been interpreted as the mass extinction of eukaryotic phytoplankton1 2 3 4 and the subsequent proliferation of bacteria5 however the causes of this mass extinction and its exact temporal relationship to the glaciations remain unclear1 2 3 4 here we present palaeontological data from the 742 6 myr old chuar group from arizona which indicate that the biotic turnover occurred before the first low latitude sturtian glaciation constrained to be between 726 and 660 myr in age6 in our record the turnover is associated with the appearance of abundant and diverse protozoan fossils and a shift to rising total organic carbon suggestive of increased primary productivity spurred by the influx of nutrients this is followed by an increase in the ratio of highly reactive iron to total iron which we interpret as persistent water column anoxia we therefore conclude that the biotic turnover recorded in the chuar group was driven by widespread eutrophication of surface waters rather than low latitude glaciation

improved efficiency of the biological pump as a trigger for the late ordovician glaciation the first of the big five phanerozoic mass extinctions occurred in tandem with an episode of glaciation during the hirnantian age of the late ordovician the mechanism or change in the carbon cycle that promoted this glaciation thereby resulting in the extinction is still debated here we report new coupled nitrogen isotope analyses of bulk sediments and chlorophyll degradation products porphyrins from the vinini creek section vinini formation nevada usa to show that eukaryotes increasingly dominated marine export production in the lead up to the hirnantian extinction we then use these findings to evaluate changes in the carbon cycle by incorporating them into a biogeochemical model in which production is increased in response to an elevated phosphorus inventory potentially caused by enhanced continental weathering in response to the activity of land plants and or an episode of volcanism the results suggest that expanded eukaryotic algal production may have increased the community average cell size leading to higher export efficiency during the late katian the coincidence of this community shift with a large scale marine transgression increased organic carbon burial drawing down co2 and triggering the hirnantian glaciation this episode may mark an early palaeozoic strengthening of the biological pump which for a short while may have made eukaryotic algae indirect killers enhanced algal productivity during the late ordovician may have led to carbon drawdown and the inception of the hirnantian glaciation according to sediment geochemistry and carbon cycle modelling

time calibration of a pt path from the western tauern window eastern alps the problem of closure temperatures new hornblende k ar and 39ar 40ar and mica rb sr and k ar ages are used to place specific timemarks on a well constrained pressure temperature path for the late alpine metamorphism in the western tauern window after identification of excess 40ar the closure behavior of ar in hornblende is compared with that of sr and ar in phengite and biotite samples were collected in three locations whose maximum temperatures were 570 c zemmgrund 550 c pfitscher joch and 500 540 c landshuter hutte

transient dwarfism of soil fauna during the paleocene eocene thermal maximum soil organisms as recorded by trace fossils in paleosols of the willwood formation wyoming show significant body size reductions and increased abundances during the paleocene eocene thermal maximum petm paleobotanical paleopedologic and oxygen isotope studies indicate high temperatures during the petm and sharp declines in precipitation compared with late paleocene estimates insect and oligochaete burrows increase in abundance during the petm suggesting longer periods of soil development and improved drainage conditions crayfish burrows and molluscan body fossils abundant below and above the petm interval are significantly less abundant during the petm likely because of drier floodplain conditions and lower water tables burrow diameters of the most abundant ichnofossils are 30 46 smaller within the petm interval as burrow size is a proxy for body size significant reductions in burrow diameter suggest that their tracemakers were smaller bodied smaller body sizes may have resulted from higher subsurface temperatures lower soil moisture conditions or nutritionally deficient vegetation in the high co2 atmosphere inferred for the petm smaller soil fauna co occur with dwarf mammal taxa during the petm thus a common forcing mechanism may have selected for small size in both above and below ground terrestrial communities we predict that soil fauna have already shown reductions in size over the last 150 years of increased atmospheric co2 and surface temperatures or that they will exhibit this pattern over the next century we retrodict also that soil fauna across the permian triassic and triassic jurassic boundary events show significant size decreases because of similar forcing mechanisms driven by rapid global warming

thermochronology of the needle falls shear zone a post collisional high strain zone of the trans hudson orogen the needle falls shear zone is a major paleoproterozoic discontinuity separating an archean craton from accreted paleoproterozoic terranes in the western trans hudson orogen it is a 1 km wide subvertical mylonite zone forming the southwestern sector of a northeast trending shear system extending for more than 400 km kinematic data indicate oblique east side up dextral strike slip displacement throughout its evolution from ductile through late brittle deformation for most of its exposed length it marks the northwestern margin of the wathaman batholith the oldest pb evaporation ages of zircons from mylonite average 1857 ma consistent with published zircon ages of the wathaman batholith confirming the batholith is protolith for much of the shear zone the synkinematic metamorphic assemblage of amphibole biotite quartz feldspar signifies deformation under amphibolite facies conditions 500 630 c oxygen isotope fractionations between quartz hornblende and quartz biotite pairs in textural equilibrium

sterane biomarkers as indicators of palaeozoic algal evolution and extinction events abstract algae are among the oldest organisms known to inhabit the earth s biosphere with a record dating back some 2 7 ga only few marine algae produce hard parts that can be preserved over geologic time and consequently their fossil record is incomplete molecular fossils or biomarkers add complementary information to the fossil palynomorph record steroids are important constituents of eukaryotic cell membranes and are preserved in sediments as steranes c28 and c29 steranes are indicators for the presence of green and c27 steranes for the presence of red algae respectively the relative abundance of steranes allows the investigation of the fossil record for palaeozoic algal diversification and evolution in this study 500 palaeozoic rock samples representing a broad facies variety from the upper ordovician to the p t boundary were investigated for sterane compositions in addition three profiles covering the hirnantian the f f and the hangenberg event were analyzed for possible shifts in the sterane distribution associated with extinction events a sharp increase of the c28 c29 sterane ratio from 0 70 at the devonian carboniferous boundary implies a fundamental change in the green algae assemblage from more primitive mainly c29 sterane producing algae to modern c28 sterane producing algae for all investigated extinction events a pronounced but short lived rise in the c28 sterane content occurs that is attributed to an episodic increase in prasinophytes the gradual radiation of algae may have been triggered by frequent mass extinctions in the upper devonian culminating with the massive decline of acritarchs at the d c boundary the coeval rise in the c28 c29 sterane ratio indicates the presence of an non encysting algal group and coincides with the global augmentation of numerous filamentous codiacea siphonales and the rise of euspondyle and metaspondyle dasycladales

high resolution stable isotope stratigraphy of upper ordovician sequences constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation the two phases of the late ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the gondwanan glaciation in this paper we argue that the distinctive late ordovician carbon isotope profile provides a chronostratigraphic ruler against which a sequence of environmental and biotic events may be located the ruler also allows regional and global high resolution correlation of successions representing very different environments cores from the upper ordovician succession of estonia and latvia record a large 1 3 c c a r b o n a t e excursion up to 6 with a similar profile shape the consistent relationship between the chemostratigraphy and biostratigraphy in the baltic region suggests that the isotope profile has a regional chronostratigraphic value the presence of similar profiles in nevada united states suggests that the excursion is a global chronostratigraphic signal this interpretation enables a detailed correlation to be made between upper ordovician shallow marine and basinal sequences that have wholly different faunas successions in the baltic area and in canada that do not display the model profile are interpreted as incomplete reinter pretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction key levels of environmental change have been located against the carbon isotope profile new oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events cooling and sea level fall started at the same stratigraphic level as the start of the carbon isotope excursion the later rise in sea level and fall in oxygen isotope values record the end of the glaciation these restrict the duration of the main glaciation to only 1 5 graptolite zones we propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling sea level fall and temperature change documented here the chronostratigraphic ruler provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high resolution correlation of biotic events at different locations this approach identifies regional similarities and differences in the patterns of extinction the main phase of graptolite extinction in the monitor range nevada for example is synchronous with the chitinozoan extinction in the baltic region but chitinozoan taxa survive to higher levels in nevada the benthic faunas in the baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the hirnantian the level at which the marine environment started to change rapidly but that there were further extinctions of species within the early hirnantian the cold adapted hirnantia fauna did not appear immediately after the extinction in this area the relationship between the second phase of extinction and the carbon isotope excursion is less clear but the available data suggest that the extinction coincides with a time of rapid environmental change but not at the inception of environmental change as happened in the first phase

some remarks on high temperature low pressure metamorphism in convergent orogens many high temperature low pressure high t low p metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating cooling cycle such terranes are not readily explained as the conductive response to crustal thickening since the resulting moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric scale strains must greatly outlast the deformation consequently high t low p metamorphism may be generated during crustal thickening only when significant heat is advected within the crust as for example may occur during the segregation of granitic melts we show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially for example by removing the mantle part of the lithosphere a simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust c and the lithosphere 1 respectively allows the potential energy of the evolving orogen to be readily evaluated using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high t low p metamorphism during crustal thickening with c1 is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations fc f1 for the same initial lithospheric configuration

oxygen and carbon isotopes and event stratigraphy near the ordovician silurian boundary anticosti island quebec the growth and destruction of continental ice sheets in north africa and adjacent parts of gondwana during the hirnantian stage had profound influence on the sedimentary style of mixed carbonate siliciclastic sequences on anticosti island quebec canada these strata deposited in a tropical mid shelf setting show direct evidence of local sea level changes of up to 40 m after correction for tectonic and load induced subsidence this indicates maximum global glacio eustatic changes of 32 to 36 m this implies maximum ice volume of about 12 to 14 106 km3 and related changes in the oxygen isotopic composition of global sea water amounting to as little as 0 5 to 0 6 given that the oxygen isotope ratios in syn glacial samples from the anticosti sequence range from 8 9 to 2 8 18o it is not possible to clearly distinguish the effects of ice cap development from temperature and diagenetic controlled variation the strong negative shift in 13c values at the top of the ellis bay formation may indicate a pronounced increase in atmospheric co2 coincident with destruction of the late ordovician ice caps

accretion history of the trans hudson orogen in manitoba and saskatchewan from paleomagnetism lithoprobe s sponsorship has led to the acquisition of paleomagnetic data from 20 units throughout the paleoproterozoic trans hudson orogen tho of saskatchewan and manitoba essentially the first such data for the region discussed summarily in this paper they provide an initial framework for the tho s tectonic evolution they show that the archean hearne and superior cratons were at subtropical and subpolar paleolatitudes respectively at 1875 ma with the lynn lake laronge arc midway between them in the manikewan ocean by 1855 ma this ocean was still 5500 700 km wide and its seafloor was subducting northwestward under the hearne craton and pericratonic peter lake domain margin with the coeval development of an andean type continental magmatic arc the wathaman chipewyan batholith between 1855 and 1810 ma coalescing apparent polar wander path apwp segments record closing of the ocean at a rate of 12 cm a trapping and accreting the several separate intervening terranes flin flon hanson lake lynn lake laronge rottenstone and presumably also other tho terranes from 1815 to 1775 ma the assembled terranes drifted as a coherent craton yielding a stillstand and hairpin in the apwp

stratigraphy of the upper ordovician upper vaureal and ellis bay formations eastern anticosti island quebec marked facies changes occur in late ordovician strata assigned to the uppermost vaureal and ellis bay formations ashgill rawtheyan hirnantian on anticosti island quebec western anticosti features shales and carbonates whereas outcrops along the eastern coast contain prominent discontinuous mixed siliciclastic carbonate units detailed section measurement along the northeast coast allows for the first time accurate definition of seven new members within this uninterrupted sequence sands present in the upper vaureal and lower ellis bay formations in the east appear to have deterred the growth of muddy bottom brachiopod communities comparable to those in the western and central regions of anticosti sand units within the upper vaureal formation contain 1 m diameter colonies of paleofavosites coeval small coral patch reefs are found in the central part of the island where sands are absent the uppermost ellis bay formation of northeast anticosti is marked by a shallow subtidal coral algal onco

dynamic sulfur and carbon cycling through the end ordovician extinction revealed by paired sulfate pyrite δ34s geochemical records of the end ordovician hirnantian stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur 34spyr organic carbon 13corg and carbonate carbon 13ccarb these isotope excursions coincide with the end ordovician glaciation and mass extinction a relative increase in pyrite burial fpyr attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna an increase in fpyr would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate 34sso4 34sso4 here we present new high resolution paired sulfur isotope data from carbonate associated sulfate 34scas and pyrite from the hirnantian stage of western anticosti island quebec canada these data document a positive 20 enrichment in 34spyr comparable in magnitude to previous reports but no parallel excursion in 34scas this pattern provides new constraints on the origin of the 34spyr excursion and the nature of carbon sulfur coupling through hirnantian time specifically these observations preclude enhanced pyrite burial as the cause of the hirnantian 34spyr excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated rather the global 34spyr excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling epyr the epyr record shows a strong inverse correlation with 13c suggesting a mechanistic link between carbon cycling and processes controlling epyr during the hirnantian changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation e g through syndepositional sediment reworking and or chemocline migration and resultant restricted exchange of porewater sulfate the magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates which are sensitive to the abundance type and lability of metabolically relevant substrates environmental change associated with the end ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments leading to increased microbial sulfate reduction rates and reduced epyr as such the hirnantian 34spyr excursion may be viewed as a dynamic biological response to global climate change highlighting the connections between the carbon and sulfur biogeochemical cycles

tempestite frequency curves a key to late ordovician and early silurian subsidence sea level change and orbital forcing in the anticosti foreland basin quebec canada following partial closure of the northern iapetus ocean along the newfoundland segment of the st lawrence promontory subsidence along the anticosti platform was influenced by residual thermal subsidence renewed tectonic loading by thrust sheets to the south and sediment loading basement subsidence calculated by removing the effects of sediment loading was between 2 and 5 cm ka in the caradocian increasing to 8 6 cm ka in the pusgillian and cautleyan and reaching a maximum of 17 7 cm ka in the rawtheyan during deposition of the princeton lake and vaureal formations a marked decline in subsidence beginning in the hirnantian and continuing into the early silurian may reflect decoupling of thrust loads to the south although a further stage of thrust loading may have occurred in the aeronian during deposition of the lower part of the jupiter formation storm frequency curves produced using tempestite abundance thickness and grain size through more than a kilometre of carbonate strata on antico

assessing mechanisms of environmental change palynological signals across the late ludlow silurian positive isotope excursion δ13c δ18o on gotland sweden abstract the silurian is characterised by strong environmental changes as indicated by several pronounced positive 13c and 18o excursions the mechanisms responsible for these isotopic shifts are a matter of much debate the purpose of the present study is a quantitative high resolution evaluation of the palynomorph distribution across the late ludlow ludfordian isotope excursion on gotland marine and terrestrial palynomorphs have been chosen as palaeoenvironmental indicators because they are widely distributed and exceptionally well preserved moreover the combined analysis of marine and terrestrial palynomorphs allows the reconstruction of contemporaneous palaeoenvironmental change in the marine and terrestrial realms to correlate our results with the carbon isotope stratigraphy we have sampled only localities with published isotope data from diagenetically unaltered brachiopod shells our results show that fluctuations in the composition of the acritarch assemblages are closely correlated with the stable isotope development low abundances of acritarchs occur in times of high stable isotope values and vice versa indicating that an increase in marine productivity cannot have been the reason for the positive 13c excursion the results are in good agreement with climatic models for the silurian assuming alternating humid and arid climatic conditions in low latitudes times of high stable isotope values correspond to arid climatic conditions in low latitudes with low input of terrestrial nutrients resulting in impoverished acritarch and conodont communities whereas times of low stable isotope values correlate with humid climate high nutrient input and abundant and diverse acritarch communities the major change in acritarch communities took place during the increase of the stable isotope values after hemipelagic planktonic or nektonic organisms graptolites conodonts have been affected this indicates that environmental changes connected with the stable isotope excursion first affected deeper water settings and later the photic zone the absolute abundance of terrestrial spores closely mirrors the marine 13c and 18o development showing that both the marine and the terrestrial realm were synchronously affected by the climatic changes the high abundance of spores during the isotope excursion might be explained by increased aeolian input

the onset of the ordovician plankton revolution in the late cambrian abstract the great ordovician biodiversification event comprises the rapid diversification of marine organisms during the ordovician period it is now clear that this adaptive radiation started for some organisms already in the cambrian and continued for others beyond the end of the ordovician making the great ordovician biodiversification event part of a long term late proterozoic and early palaeozoic radiation that in part is expressed by the fossil record as the cambrian explosion a significant diversification of different groups of the plankton is observed in the late cambrian early ordovician interval leading to the subsequent ordovician plankton revolution the possible causes of this plankton revolution are currently debated they include changes in palaeoclimate palaeogeography or tectonic and volcanic activity as well as a modified nutrient supply in this context the steptoean positive carbon isotope excursion 13 c carb spice event in the late cambrian paibian stage furongian series has been related to a major increase in atmospheric o 2 from 10 18 to some 20 29 and to increased oceanic nutrient availability here we analyze the diversification of the planktonic groups during the late cambrian and early ordovician in particular in relation to the spice event our analyses include the changing diversities of the phytoplankton acritarchs diverse groups of zooplankton e g radiolarians graptolites chitinozoans and the switch to a planktonic mode of life of fossil groups e g arthropods molluscs that were part of the cambrian benthos and that later occupied pelagic niches in addition we focus also on data indicating evidence for a late cambrian to ordovician origin of planktotrophy in invertebrate larvae it can be concluded that none of the diversifications of the different planktonic organisms can be related directly to the spice event however a long term 10 20 million years oxygenation pulse related to the spice event might have fuelled the explosion of phytoplankton diversity observed in the latest cambrian early ordovician that led to completely modified trophic structures permitting an increase in diversity and abundance of plankton feeding groups during the ordovician

a hirnantian latest ordovician reefal bryozoan fauna from anticosti island eastern canada taxonomy and chemostratigraphy the natiscotec outcrop on anticosti island canada ellis bay formation laframboise member hirnantian late ordovician exposes a patch reef some 20 30 m in diameter 2 3 m thick with abundant rugose corals as well as favositids and heliolitids reef capping and flanking sediments include typical hirnantian brachiopods such as hirnantia hindella and eospirigerina within the reef peloidal microbialites encrusting bryozoan colonies are common the bryozoan fauna includes three cystoporates seven trepostomes and three phylloporines two genera and two species are new the cystoporate natiscotecella tenuis n gen and n sp and the phylloporine dilaminocladia natiscotecensis n gen and n sp three more species are also new the cystoporates ceramopora clara n sp and acanthoceramoporella spinigera n sp and the trepostome revalotrypa honguedensis n sp furthermore we identify the three trepostomes atactoporella aff ortoni nicholson 1874 hallopora elegantula hall 1852 and monotrypell

late ordovician mass extinction a new perspective from stratigraphic sections in central nevada integrated sequence stratigraphic biostratigraphic and chemostratigraphic analyses of three stratigraphic sections in central nevada indicate that late ordovician glaciation induced sea level fall produced diachronous stepwise faunal turnover in graptolites conodonts chitinozoans and radiolarians and also triggered a strong but transient positive 13c excursion this pattern is very different from that described for most mass extinction events

phytoplankton dynamics across the ordovician silurian boundary at low palaeolatitudes correlations with carbon isotopic and glacial events the late ordovician culminated in a major glacial period that has been related to one of the strongest mass extinctions recorded during the phanerozoic during this interval anticosti island quebec eastern canada was located at low to intermediate palaeolatitudes 15 30 s on the eastern margin of laurentia it displays a relatively complete section across the ordovician silurian o s boundary upper ordovician to lower silurian strata of anticosti studied here comprise the vaureal katian ellis bay hirnantian and becscie latest hirnantian rhuddanian formations phytoplankton dynamics inferred from the acritarch content of these strata are locally compared with new geochemical environmental proxies 13c with recent palynological data from baltica and also with phytoplankton dynamics as described from near polar high latitude localities of the gondwana region two positive excursions of the carbon isotope record have been identified in the lower and upper ellis bay formation overall phytoplankton trends are as follows 1 as with chitinozoans conodonts brachiopods and corals acritarchs from anticosti display a major turn over during the early hirnantian uppermost vaureal formation lower ellis bay formation with the appearance of taxa that exhibit silurian affinities e g tylotopalla sp ammonidium sp oppilatala sp evittia sp dilatisphaera spp high polymorphism for some common taxa is observed in the interval corresponding to the first 13c positive excursion using sequence stratigraphy this period may be related to a first glacially driven sea level drawdown corresponding to terrestrial ice sheet growth on gondwana such as recorded in morocco 2 subsequently during the late hirnantian of anticosti uppermost ellis bay formation phytoplanktic communities suffered a crisis exemplified by the disappearance of several typical large late ordovician taxa i e orthosphaeridium spp peteinosphaeridium spp large baltisphaeridum spp sacculidium sp the lower becscie strata corresponding to the time of deglaciation at the o s boundary interval only contain a low diversity assemblage dominated by large acritarchs of the genus hoegklintia and other morphologically related forms and a few tolerant and long ranging taxa e g evittia remota micrhystridium spp leiospheres despite a possible taphonomic bias in the uppermost ellis bay formation the major crisis in the late hirnantian coincides with the second 13c excursion and a second stronger glacially driven sea level fall corresponding to the time of the maximum extend of the ice sheets on gondwana 3 such phytoplankton events have also been detected in gondwanan ice center area but with typical ordovician taxa crises occurring apparently later when plotted against positive carbon isotope excursions as also observed in baltica this time lag may be linked to a basin configuration of the studied areas where environmental stress consequently differed or linked to the presence of omission surfaces coupled with preservation bias of acritarch assemblages in the uppermost ordovician strata of anticosti in addition to their biostratigraphic and palaeobiogeographic utility acritarchs turn out to be more specific in highlighting palaeoenvironmental conditions

applications of single grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites abstract 207 pb 206 pb ages determined by evaporation of single zircon grains from rocks of the new england appalachians in the usa permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age whose u pb upper intercept zircon ages obtained through isotope dilution methods show profound post crystallization discordance demonstrate that the evaporation method is successful in recovering protolith ages uncertainties are 10 15 my for rocks of paleozoic age or older preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics 207 pb 206 pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the new england appalachians reveal the distinct provenances of these rocks the missisquoi formation in southeastern vermont usa is comprised in part of detritus derived from the 1 0 1 2 ga grenvillian province of laurentia whereas this component is conspicuously absent from the plainfield formation in easternmost connecticut the detrital zircon age data suggest that the missisquoi formation accumulated peripheral to the laurentian craton during the early paleozoic whereas the plainfield formation detritus was supplied by non laurentian ossibly west african sources during the late proterozoic or early paleozoic in southern vermont the evaporation method successfully distinguishes between two groups of augen gneisses with ages of 960 ma and 1120 ma which were first recognized through isotope dilution methods collectively these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks

parallel stretching lineations and fold axes oblique to a shear displacement direction a model and observations abstract spatial variations in shear strain rate are expected in ductile shear zones where the variation is a change in shear strain perpendicular to the displacement direction the effect is to rotate the shear slip planes this is a mechanism for giving a rotation of fold axes into parallelism with the slip and extension direction in a rock if such a variation in shear strain affects rocks with a strong planar anisotropy it is possible to produce a fabric with an apparent stretching lineation parallel to fold axes but both significantly oblique to the slip direction a possible example of this is seen in strongly deformed quartz mica schists from syros greece where a stretching lineation is seen parallel to fold hinges over a range of fold axes orientations of at least 40

environmental controls on marine ecosystem recovery following mass extinctions with an example from the early triassic abstract the recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity the pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors here we present an analysis of changes in marine conditions following the end permian mass extinction with the objective of evaluating the role of environmental factors in the protracted 5 myr long recovery of marine ecosystems during the early triassic specifically our study examines changes in weathering productivity and redox proxies in three sections in south china chaohu daxiakou and zuodeng and one in northern india mud our results reveal 1 recurrent environmental perturbations during the early triassic 2 a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early griesbachian late griesbachian mid smithian and more weakly the mid spathian 3 increases in marine productivity during the aforementioned intervals except for the early griesbachian and 4 stronger and more temporally discrete intervals of environmental change in deepwater sections chaohu and daxiakou relative to shallow and intermediate sections zuodeng and mud our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity within community alpha diversity infaunal burrowing and ecosystem tiering we infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the early triassic these perturbations were associated with elevated weathering and productivity fluxes implying that nutrient and energy flows were key influences on recovery more regular secular variation in deepwater relative to shallow water environmental conditions implies that perturbations originated at depth i e within the oceanic thermocline and influenced the ocean surface layer irregularly finally we compared patterns of environmental disturbance and ecosystem recovery following the other four big five phanerozoic mass extinctions to evaluate whether commonalities exist in general the pace of ecosystem recovery depends on the degree of stability of the post crisis marine environment

mesothermal gold mineralization in a proterozoic greenstone belt western flin flon domain saskatchewan canada the flin flon domain trans hudson orogen canada is an example of a proterozoic greenstone belt that hosts a large number of small mesothermal gold occurrences the greenstone belt consists of tholeiitic to calc alkaline volcanic rocks amisk group approximately 1900 1880 ma unconformably overlain by molasse type sedimentary rocks missi group 1850 1840 ma the supracrustal rocks are intruded by gabbroic to granitic rocks ranging in age from synvolcanic to late tectonic 1890 1835 ma metamorphic grade varies from prehnite pumpellyite facies to amphibolite facies and peak thermal conditions generally were attained locally during granitoid intrusion many of the shear zones and the dominant regional foliation developed during ductile deformation coeval with peak metamorphism these shear zones were reactivated and mineralized under brittle ductile conditions during postpeak metamorphic uplift mesothermal gold mineralization is hosted in quartz veins that developed at jogs or zones of competency contrasts along brittle ductile shear zones in all lithologies in the region alteration envelopes which consist of quartz carbonate chlorite albite muscovite pyrite are usually less than a few meters wide and overprint regional metamorphic assemblages the veins consist of milky colored quartz and may also contain tourmaline ankerite chlorite and muscovite pyrite and arsenopyrite are the dominant sulfides and gold grade is proportional to the modal abundance of sulfides the dominant fluids associated with gold mineralization were h 2 o co 2 nacl 0 6 14 7 wt nacl equiv in composition with generally uniform h 2 o co 2 phase ratios in a given plane or zone but variable phase ratios between zones oxygen isotope mineral pairs indicate temperature of mineralization of between 360 degrees and 420 degrees c which when combined with fluid inclusion density estimates indicate that most of the gold mineralization occurred at a pressure of about 2 kbars rapid pressure release in dilatant zones with related phase separation and change in fluid composition and properties and local fluid wall rock interaction are the likely gold precipitation mechanisms the o h s c and sr isotope compositions of hydrothermal minerals in the shear zones indicate that the mineralizing fluids interacted extensively with proterozoic metamorphic and igneous rocks similar in composition to those presently exposed at the surface at high temperatures and at low water rock ratios the o and h isotope compositions are compatible with formation of the fluids from devolatilization reactions during prograde metamorphism at depth the o isotope composition of barren quartz veins varies with the host rocks indicating that these fluids were either locally derived or interacted extensively with the immediate host rocks an rb sr isochron age of 1760 or 9 ma on tourmaline and muscovite from the rio deposit is similar to 40 ar 39 ar plateau ages for muscovites from the premetamorphic laurel lake deposit in the flin flon domain this suggests that fluid advection through the rio deposit was contemporaneous with the thermal event that affected laurel lake muscovites these ages along with an age from the tartan lake mesothermal deposit 1791 or 4 ma in the flin flon domain indicate that fluid flow related to hydrothermal activity along the shear zones in the western flin flon domain occurred periodically over a period of about 30 ma these proterozoic gold occurrences are similar to archean mesothermal gold deposits in terms of geologic structural and tectonic setting alteration and vein mineralogy and fluid composition pressure and temperature however the limited extent of alteration the low gold content the isotopic systematics and the lack of either s type granites or mantle derived lamprophyres suggest that the shear zones hosting the western flin flon domain occurrences sampled limited quantities of fluids and thus were not transcrustal features capable of hosting giant gold deposits

polar front shift and atmospheric co2 during the glacial maximum of the early paleozoic icehouse our new data address the paradox of late ordovician glaciation under supposedly high pco2 8 to 22 pal preindustrial atmospheric level the paleobiogeographical distribution of chitinozoan mixed layer marine zooplankton biotopes for the hirnantian glacial maximum 440 ma are reconstructed and compared to those from the sandbian 460 ma they demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the polar front through time from 55 70 s to 40 s these changes are comparable to those during pleistocene interglacial glacial cycles in comparison with the pleistocene we hypothesize a significant decline in mean global temperature from the sandbian to hirnantian proportional with a fall in pco2 from a modeled sandbian level of 8 pal to 5 pal during the hirnantian our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end ordovician mass extinction

thermal evolution of the kisseynew sedimentary gneiss belt manitoba metamorphism at an early proterozoic accretionary margin

the sanfandila earthquake sequence of 1998 queretaro mexico activation of an undocumented fault in the northern edge of central trans mexican volcanic belt abstract a sequence of small earthquakes occurred in central mexico at the northern edge of the trans mexican volcanic belt tmvb in the state of queretaro during the first 3 months of 1998 medium to large events in the continental regime of central mexico are not common but the seismic history of the region demonstrates that faults there are capable of generating destructive events the sequence was analyzed using data from a temporary network with the goals of identifying the causative fault and its relation to regional tectonics employing a waveform inversion scheme adapted from a method used for regional studies we found that the source mechanisms conform to the style of faulting i e extension in the e w direction representative of the taxco san miguel allende fault system this system has been proposed as the southernmost extension of the basin and range br province the spatial distribution of hypocenters and source mechanisms indicate that the seismogenic segment was a fault with an azimuth of approximately 334 with almost pure dip slip since events which occurred just south from this region show features which are consistent with tmvb tectonics i e extension in an n s direction the sequence may mark the boundary between the tmvb and br stress domains

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

terminal ordovician carbon isotope stratigraphy and glacioeustatic sea level change across anticosti island quebec canada globally documented carbon isotope excursions provide time varying signals that can be used for high resolution stratigraphic correlation we report detailed inorganic and organic carbon isotope curves from carbonate rocks of the ellis bay and becscie formations spanning the ordovician silurian boundary on anticosti island quebec canada strata of the anticosti basin record the development of a storm dominated tropical carbonate ramp these strata host the well known hirnantian positive carbon isotope excursion which attains maximum values of 4 5 in carbonate carbon of the laframboise member or the fox point member of the becscie formation the excursion also occurs in organic carbon and 13 c carb and 13 c org values covary such that no reproducible 13 c 13 c carb 13 c org excursion is observed the most complete stratigraphic section at laframboise point in the west shows the characteristic shape of the hirnantian stage excursion at the global stratotype section and point gssp for the hirnantian stage in china and the silurian system in scotland we therefore suggest that the entire hirnantian stage on anticosti island is confined to the laframboise and lower fox point members by documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin we are able to reconstruct glacioeustatic sea level fluctuations corresponding to maximum glacial conditions associated with the end ordovician ice age the combined litho and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and becscie limestones the former transgresses from west to east and the latter progrades from east to west the sea level curve consistent with our sequence stratigraphic model indicates that glacioeustatic sea level changes and the positive carbon isotope excursion were not perfectly coupled although the start of the isotope excursion and the initial sea level drawdown were coincident the peak of the isotope excursion did not occur until after sea level had begun to rise carbon isotope values did not return to baseline until well after the anticosti ramp was reflooded the sea level 13ccarb relationship proposed here is consistent with the weathering hypothesis for the origin of the hirnantian 13 c carb excursion

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

biostratigraphy taxonomic diversity and patterns of morphological evolution of ordovician acritarchs organic walled microphytoplankton from the northern gondwana margin in relation to palaeoclimatic and palaeogeographic changes abstract acritarchs the fossilizable resting cysts of phytoplanktonic algal protists are the dominant component of marine organic walled microfossils in the palaeozoic the majority of acritarchs show strong similarities with dinoflagellate cysts in morphological and biogeochemical features as well as distributional patterns in the sediments the production of these organic walled microfossils and their distribution and survivorship in the sediments were controlled by differences in ecological tolerances and life cycle autecology of the planktonic parent organisms calculation of evolutionary rates and development of a detailed diversity curve at specific level form the basis for discussing the influence of global palaeoenvironmental perturbations on the evolution of organic walled microphytoplankton in northern gondwana during latest cambrian through ordovician times the potential of acritarchs for biostratigraphic correlation at the regional scale northern gondwana domain is much improved by our detailed revision of distributional patterns of 245 acritarch taxa the most important cambro ordovician acritarch bio events are short periods of diversification which also correspond to introduction of morphological innovations observed in latest cambrian and earliest tremadoc late tremadoc early arenig basal llanvirn and latest ashgill and an important extinction phase in the early caradoc overall acritarch diversity increased from the basal ordovician up to the middle llanvirn then declined in the early and middle caradoc during ashgill times the assemblages are poorly diversified at the generic level as a result of a combined effect of sea level drawdown and onset of glacial conditions but no major extinction event is observed in connection with the end ordovician biotic crisis the peak in acritarch diversity during middle ordovician times appears to be correlated to maximum spread of palaeogeographical assembly acritarch dynamics appear largely uncorrelated to second order sea level oscillations the primary abiotic controls on acritarch evolution were palaeogeographical and the associated palaeoceanographic changes especially during middle ordovician and the end ordovician palaeoclimatic shift the acritarch fossil record provides important information on the evolution of oceanic primary producers however the relationships between acritarch diversity oceanic productivity and evolution of invertebrate animals are proving much more complex than previously thought in particular the hypothesis of a causal relationship between changes in acritarch diversity and metazoan evolution in the palaeozoic is not supported by our data

ordovician and silurian nautiloid cephalopods from anticosti island traject across the ordovician silurian o s mass extinction boundary well preserved shells of ashgill katian hirnantian and llandovery nautiloids from anticosti island include the type species of 10 genera from a diversity high of some 40 described nektic and nek

refining 2 km of ordovician chronostratigraphy beneath anticosti island utilizing integrated chemostratigraphy new high resolution chemostratigraphy in combination with updated biostratigraphy refines the chronostratigraphic resolution of the nearly 2 km thick ordovician section below anticosti island a total of 1414 horizons spaced at 1 0 1 5 m intervals were sampled from the new associated consolidated paper nacp drill core and analyzed for major and trace elemental composition by portable x ray fluorescence analyzer pxrf select micrite and calcareous shale powders were then analyzed for 13ccarb number of samples n 364 and 87sr 86sr n 25 our results indicate a floian to early darriwilian f3 dw1 age for the romaine formation a middle darriwilian to sandbian dw2 sa1 age for the mingan formation a lower to mid katian ka1 ka2 age for the macasty formation an upper katian ka3 age for the lower vaureal formation an uppermost katian ka4 age for the upper vaureal and a hirnantian h1 age for the ellis bay formation this integrated chemostratigraphic and biostratigraphic synthesis es

marine biodiversification in response to evolving phytoplankton stoichiometry abstract diversification of the marine biosphere is intimately linked to the evolution of the biogeochemical cycles of carbon nutrients and primary productivity a meta analysis of the ratio of carbon to phosphorus buried in sedimentary rocks during the past 3 billion years indicates that both food quantity and critically food quality increased through time as a result of the evolving stoichiometry nutrient content of eukaryotic phytoplankton evolving food quantity and quality was primarily a function of broad tectonic cycles that influenced not just carbon burial but also nutrient availability and primary productivity increasing nutrient availability during the middle to late proterozoic culminated in the production of food phytoplankton biomass and fresh dead organic matter with c p redfield ratios sufficient to finally promote geologically rapid biodiversification during the proterozoic phanerozoic transition this resulted in further massive nutrient sequestration into biomass that triggered positive feedback via nutrient recycling bioturbation mesozooplankton grazing on phytoplankton productivity increasing rates and depths of bioturbation through the phanerozoic suggest that nutrient recycling continued to increase increasing bioturbation and nutrient cycling appear to have been necessary to sustain the primary productivity and energetics biomass metabolic rates and physical activity such as predation of the marine biosphere because of the geologically slow input of macronutrients like phosphorus from land and the continued sequestration of nutrients into marine and terrestrial biomass

chitinozoan biostratigraphy of a new upper ordovician stratigraphic framework for anticosti island canada thickness and facies variations in the upper ordovician succession of anticosti island have made correlations between the western and eastern ends of the island difficult this chitinozoan study reveals that most strata of the ellis bay formation at the eastern end of the island correlate with the vaureal formation at the western end in the eastern end chitinozoans typical of the ellis bay formation occur only in the upper part of the lousy cove member whereas chitinozoans diagnostic of the mid upper vaureal occur in the rest of the formation moreover the eastern schmitt creek and mill bay members of the vaureal formation are older than the uppermost vaureal eponym members in the western end new east west correlations supported by geochemical sedimentological and sequence stratigraphic data are proposed comparison with coeval upper ordovician successions suggests a hirnantian age for the gamachiana zone that occurs in the lower members of the ellis bay formation it also indicates that the hirnantian isotopic carbon excursion hice on anticosti begins in the d anceps zone just below the base of the hirnantian stage reaches a first peak in the lower n extraordinarius zone a second modest peak high in the n extraordinarius zone and a third more important one in the lower part of the n persculptus zone these results are in agreement with the biostratigraphic and chemostratigraphic data at the global boundary stratotype section and points gssps for the base of the hirnantian and silurian in south central china and at dob9s linn in scotland

nature s diffusion experiment the cooling rate cooling age correlation correlation between the age of orogeny and the cooling rate of metamorphic rocks through 500 180 c as determined through various radioisotopic dating methods suggests that ancient orogens cooled at rates as much as two orders of magnitude slower than those of active orogens i propose that the correlation has arisen because ancient orogens have undergone long postorogenic periods as long as hundreds of millions of years during which the rocks remained effectively isothermal laboratory experiments suggest that argon diffusion in biotite may be sufficiently rapid at temperatures of 220 c to produce the correlation by slow loss of argon during extended isothermal periods one interesting realization is that nature has provided a long term argon diffusion experiment the results of which have been inverted in this paper to give an independent estimate of the diffusivity of argon in biotite under natural conditions this estimate compares favorably with a projection of the results of laboratory hydrothermal experiments down 10 orders of magnitude the analysis confirms the theory that there is a critical thermal window below accepted closure temperatures in which long residence will result in significant partial resetting of thermochronometers

palynology and palynofacies analysis of a silurian llandovery wenlock marine succession from the precordillera of western argentina palaeobiogeographical and palaeoenvironmental significance abstract a comprehensive survey of the marine and terrestrial palynomorphs from the la chilca formation exposed in the central precordillera of san juan argentina is presented twenty four samples from the quebrada ancha section were analysed marine phytoplankton predominated in the entire stratigraphic unit while the contribution of terrestrial palynomorphs was much less acritarchs and chlorophytes represent a highly diversified assemblage including stratigraphically relevant taxa such as domasia trispinosa tylotopalla caelamenicutis tylotopalla digitifera crassiangulina variacornuta stellinium rabians and percultisphaera cf stiphrospinata terrestrial palynomorphs are exclusively represented by six cryptospore species namely gneudnaspora divellomedia pseudodyadospora laevigata rugosphaera cerebra tetrahedraletes medinensis cf imperfectotriletes vavdovae and velatitetras retimembrana comparisons with coeval phytoplankton assemblages from gondwana and other palaeoplates such as laurentia baltica and avalonia show strong similarities suggesting a cosmopolitan distribution pattern during the llandovery and wenlock the first palynofacies analysis for the early silurian of argentina was performed depositional environments based on palynofacies match with those previously suggested based on sedimentological data two main palynofacies associations are recognised association b occurs in the lower part of the la chilca formation and is mostly composed of amorphous organic matter aom representative of low energy inner shelf environments association a is recognised in the upper part of the formation where sandstone beds become more frequent and is characterised by a greater amount of possible palynomorph remains ppr indicating a proximal platform to shoreface environment three excursions of total organic carbon toc were identified in the section probably as a result of sea level rise episodes that produced anoxic conditions and preservation of organic matter

the stratigraphic and geochemical imprints of late ordovician glaciation on far field neritic carbonates anticosti island eastern canada abstract the storm dominated open marine neritic carbonate facies continuously exposed in coastal cliffs and wide rocky tidal flat exposures at the western end of anticosti island form a 315 m thick expanded section spanning the ordovician silurian boundary these carbonate facies display deepening shallowing trends of different orders resulting in a hierarchical stacking of sedimentary cycles the carbonate facies within these multi order cycles represent sediments that were deposited from shoreline to deeper outer carbonate ramp 80 m in water depth and exhibit sea level changes of several tens of metres in magnitude field petrographic and geochemical evidence support a well preserved primary 18o signal in close equilibrium with the original paleoenvironmental conditions the 18o intracycle variations up to 1 0 in magnitude reveal repeated decreasing and increasing values during the transgressive and regressive portions of decametre tr cycles respectively although temperature salinity and ice volume influenced the recorded primary 18o signal orbital scale glacioeustasy ultimately controlled facies and cycle development and 18o signal during the katian immediately before the latest ordovician hirnantian glaciation the 3 5 myr long 18o signal recorded by our entire succession supports a protracted period of cooling leading into the main phase of latest ordovician hirnantian glaciation spectral analysis of the 18o record also reveals decametre variations which are associated with decametre scale transgressive regressive tr cycles possibly reflecting eccentricity orbital signals our study shows the importance of a thick expanded stratigraphic succession sampled at the highest resolution possible to decipher potential linkages between facies changes multi order stratigraphic cycles 18o signal and the earth s orbital parameters