biotic geochemical and environmental changes through the early sheinwoodian wenlock silurian carbon isotope excursion escie buttington quarry wales abstract palaeontological sedimentological and geochemical data particularly from la icpms analyses of pyrite from a 12 4 m section through the lower sheinwoodian trewern brook mudstone formation of buttington quarry wales are combined to reveal the environmental changes taking place during the early wenlock epoch of the silurian during the early part of the escie early sheinwoodian carbon isotope excursion the uppermost cyrtograptus murchisoni through to lower monograptus riccartonensis biozones show increased nutrient levels and peaks in palynomorph and graptolite abundance at this time graptolite diversity globally was declining dramatically possibly associated with metal particularly zinc concentrations attaining toxic levels and probably with the base of the euphotic zone being higher in the water column resulting from blooms of phytoplankton causing those graptolites that specialized in feeding on phytoplankton that were outcompeted in this diminished volume of illuminated surface waters to decline in numbers and eventually become extinct correlation of the graptolite and conodont biozonations enables the levels of the main stratigraphically higher ireviken event datum points to be proposed for the buttington section and this in turn allows events recognised elsewhere intervals of malformed palynomorphs and re organization of polychaete faunas together with an oceanic anoxic event to be correlated with the lower part of the studied buttington section the commencement of the oae identified on gotland at ireviken event datum point 4 appears to correlate rather well with a rapid transition marking the top of the butterley mudstone member at buttington from heavily bioturbated mudstones to laminated graptolitic mudstones surprisingly the most rapid change in 13ccarb values within the buttington section from 0 6 at 8 19 8 25 m to 2 09 at 8 50 8 55 m is not associated with any major faunal palynological abundance or geochemical changes these are all coincident with the initial rise in 13ccarb values up to about the 5 m level in the buttington section another unexplained feature of the graptolite record is a sudden change from monograptus dominated to pristiograptus dominated assemblages about half way through the studied section

records of carbon and sulfur cycling during the silurian ireviken event in gotland sweden abstract early silurian 431 ma carbonate rocks record a ca 4 5 positive excursion in the stable isotopic composition of carbonate carbon 13ccarb associated with this isotopic shift is a macroevolutionary turnover pulse known as the ireviken event the onset of this carbon isotope excursion is commonly associated with a shallowing upward facies transition that may have been accompanied by climatic change as indicated by a parallel positive shift 0 6 in the stable isotopic composition of carbonate oxygen 18ocarb however the relationships among carbon cycle perturbations faunal turnover and environmental changes remain enigmatic here we present a suite of new isotopic data across the ireviken event from multiple sections in gotland sweden these samples preserve no systematic change in 18ocarb but show positive excursions of equal magnitude in both carbonate 13ccarb and organic 13corg carbon in addition the data reveal a synchronous perturbation in sulfur isotope ratios manifest as a ca 7 positive excursion in carbonate associated sulfate 34scas and a ca 30 positive excursion in pyrite 34spyr the increase in 34spyr values is accompanied by a substantial concomitant increase in stratigraphic variability of 34spyr the relatively constant offset between the 13ccarb and 13corg excursions throughout the ireviken event could be attributed to increased organic carbon burial or possibly a change in the isotopic composition of co2 sources from weathering however a positive correlation between carbonate abundance and 13ccarb suggests that local to regional changes in dissolved inorganic carbon dic during the shallowing upward sequence may have been at least partly responsible for the observed excursion the positive excursion recorded in 34scas suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir an inverse correlation between cas abundance and 34scas supports the notion of decreased sulfate concentrations at least locally consistent with a concomitant increase in pyrite burial a decrease in the offset between 34scas and 34spyr values during the ireviken event suggests a substantial reduction in the isotopic fractionations epyr expressed during microbial sulfur cycling and pyrite precipitation through this interval decreased epyr and the concomitant increase in stratigraphic variation in 34spyr are typical of isotope systematics observed in modern shallow water environments associated with increased closed system behavior and or oxidative sedimentary reworking during early sediment diagenesis while the isotopic trends associated with the ireviken event have been observed in multiple locations around the globe many sections display different magnitudes of isotopic change and moreover are typically associated with local facies changes due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the ireviken event and their relationship to changes in local depositional environment we surmise that these patterns more closely reflect biogeochemical processes related to deposition and lithification of sediment than global changes in carbon and sulfur burial fluxes

palaeoclimate perturbations before the sheinwoodian glaciation a trigger for extinctiond during the ireviken event telychian to sheinwoodian conodonts pterospathodus eopennatus ssp n 1 zone through the ozarkodina sagitta rhenana superzone were studied in detail for their oxygen isotope composition the upper kockelella ranuliformis and ozarkodina sagitta rhenana zones represent the peak interval of the sheinwoodian positive 18oapatite excursion reflecting the coolest conditions with marine sea water temperatures decreased by more than 6 c in the subtropics the 18oapatite data cover pre through post ireviken event strata the study includes also material from younger levels up to the late wenlock ctenognathodus murchisoni zone however this record is incomplete with one major hiatus reflecting the regression associated with the middle silurian mulde event the stratigraphic levels of the analyzed conodont samples from estonia can be correlated with high precision to the datum points of the ireviken event strata in the lower and upper visby formations on gotland sweden inferred short term climatic changes in the upper telychian part of the studied interval may have been caused by phases of major volcanic activity along the caledonian front across the ireviken event we observe variations in oxygen isotope ratios which allow a direct comparison to the stepwise extinctions and faunal reorganisations in different groups our data suggest that faunal extinctions are connected to time intervals of warming before the establishment of more stable and cooler conditions during the main sheinwoodian glacial the most severe extinctions and faunal turnovers at datum points 2 and 4 occurred during the early warmer interval of the event after the shift into the sheinwoodian icehouse in the lower kockelella ranuliformis zone reflected by rapidly decreasing sea water temperatures faunas restabilized and reefal communities started to flourish sedimentary evidence for the glaciation is discussed for different palaeolatitudes less

the sacta limestone member early wenlock cool water temperate carbonate deposition at the distal foreland of gondwana s active margin bolivia abstract the sacta limestone member is located towards the base of the silurian kirusillas formation in central bolivia and forms part of the sedimentary record of the peru bolivia palaeozoic basin the early wenlock sheinwoodian age of this thin carbonate unit has been established by conodont and foraminifer biostratigraphy during the silurian a thick siliciclastic wedge was deposited in the foredeep of the basin related to tectonism along the active margin of gondwana strong subsidence and downslope resedimentation slides slumps debris flows turbidites dominated towards the west of the basin during the llandovery and early wenlock in contrast a much thinner record was deposited at the distal margin of the foreland basin pinching out towards the northeast and onlapping the stable precambrian basement the sacta member represents cool water temperate carbonate deposition at mid to high latitude on the distal stable margin of this foreland basin its sedimentation took place towards the end of a phase of important tectonic instability and the carbonate bed was itself also partially resedimented late ashgill llandovery glaciation of nearby reliefs along the active margin of gondwana was followed by brief and local cool water temperate carbonate deposition in the early wenlock and subsequently by the development of a storm and wave dominated shallow clastic shelf during the rest of the silurian the sacta member records carbonate deposition in western gondwana during a brief and singular event with important palaeogeographic and palaeoclimatic implications

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

palaeoecology of transported brachiopod assemblages embedded in black shale cape phillips formation silurian arctic canada abstract brachiopod communities have been widely used for palaeoecological studies and reconstruction of sea level changes in earth history previous studies of brachiopod communities emphasised sampling benthic shelly fossils preserved in situ to avoid preservation bias silurian wenlock brachiopod assemblages are well preserved as debris turbidity flows including calcareous concretions embedded in thick sequences of black shale of the cape phillips formation with a mixture of deep water brachiopods and transported shallow water forms in this study the first attempt is made to use the transported assemblages to reconstruct silurian brachiopod communities that once lived along a palaeogradient from the shallow water arctic platform to the deep water franklinian basin seven transported brachiopod assemblages were recognised using multivariate analyses cluster analysis and principal components analysis with varying ratios of shallow versus deep water shells segregated according to their known palaeobathymetric ranges in other regions the wenlock early ludlow sea level fluctuations in the study area reconstructed using the percentage values of shallow and deep water brachiopod specimens as a proxy have a high degree of agreement with the arctic silurian sea level curve and moderate agreements with global sea level histories of other regions such as the british isles and gotland interpreted from graptolite abundance and diversity changes and sequence and event stratigraphical methods respectively

an oceanic model for lithological and faunal changes tested on the silurian record a model for oceanic cycles is presented the cycles are expressed as changes between more humid low latitude and cooler high latitude climates p episodes and dryer low latitude and warmer high latitude climates s episodes the cyclicity may be self regulating through changes in the co 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate the model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance diversity and extinction rate of planktic and benthic faunas sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production the advance and retreat of carbonate deposition on the shelf edge the timing of oolite reef and black shale formation and changes between oxic and anoxic deep water faunal effects include both iterative evolution and intermittently widespread taxa lazarus taxa these predictions agree well with changes observed in well known sequence of wenlock and ludlow age as well as changes seen in selected older and younger sequences

early silurian wenlockian δ13c profiles from the cape phillips formation arctic canada and their relation to biotic events geochemical data from the cape phillips formation arctic canada are examined in association with three silurian biotic crises in the graptolite community the early wenlockian ireviken mid wenlockian cyrtograptus lundgreni and end wenlockian colonograptus ludensis extinction events positive 13corg excursions are associated with the ireviken and c lundgreni events but not the co ludensis event the ireviken and c lundgreni excursions are recognized worldwide and are herein interpreted to be the result of carbonate weathering in response to eustatic sea level drop the c lundgreni excursion is of greater magnitude in the more proximal basin margin section at abbott river cornwallis island and is explained by the amplification of a more strongly positive 13c signature in shallower parts of an epeiric basin during increased exposure and weathering of the carbonate shelf excursion c5 within the c praedeubeli c deubeli zone is also of regional significance as it occurs in both the abbott r

ultrahigh temperature metamorphism followed by two stage decompression of garnet orthopyroxene sillimanite granulites from ganguvarpatti madurai block southern india the mg al granulites from ganguvarpatti consist of orthopyroxene sillimanite garnet quartz as peak assemblage with a few porphyroblasts of cordierite and sapphirine these assemblages were strongly overprinted by late symplectites and coronas orthopyroxene inclusions in garnet and porphyroblast cores have the highest xmg 0 80 and al2o3 content 10 7 wt the estimated near peak metamorphic conditions 1 000 50 c and 11 kbar using garnet orthopyroxene geothermobarometry are consistent with those determined using a petrogenetic grid the proposed multi stage evolution process implies an initial decompression deduced from multi phase symplectites followed by cooling during biotite formation further late decompression is explained from the orthopyroxene rims on biotite this proposed p t path thus suggests a unique and complex evolution history for the uht granulites of southern india present results are comparable with similar adjacent terranes in the gondwana supercontinent but the lack of structural and geochronological data makes a link with any major metamorphic event uncertain

integrated silurian conodont and carbonate carbon isotope stratigraphy of the east central appalachian basin abstract the eastern and southeastern margins of the appalachian basin contain extensive exposures of telychian through pridoli upper silurian strata litho and biostratigraphic analyses of these strata over the last century provided a general means for their chronostratigraphic correlation to global series and stage boundaries however a rarity of zonally diagnostic fauna limited precise correlation both within and beyond the appalachian basin recent conodont biostratigraphic studies substantially revised the chronostratigraphic position of these strata by more than a series in some cases much of the interval however remained biostratigraphically unzoned and the precise position of series and stage boundaries remained tentative to improve the chronostratigraphic correlation of these strata to global series and stage boundaries we provide new integrated conodont and high resolution 13ccarb chemostratigraphic analyses from two outcrops located in west virginia and virginia spanning the upper rose hill through tonoloway formations upper telychian through pridoli these new data show that the silurian carbon isotope record for this portion of the appalachian basin is shifted to more negative 13ccarb values than typical global data however the overall isotopic records appear correlative at least within the appalachian basin thereby providing a means for future correlation within the basin for intervals in which biostratigraphy is traditionally lacking these coupled data demonstrate that the ireviken and mulde excursions are contained within our sections the high amplitude lau excursion was however not observed and the present biostratigraphic data suggest that a majority of the ludlow series was either not recorded or was later removed by erosion these coupled analyses therefore indicate that the ludlow pridoli boundary resides within the wills creek formation and we propose that the overlying isotopic record presented here records the late ludlow carbon isotope excursion llcie and silale low previously identified only within the baltic basin

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

al2o3 tio2 ratio of the clay fraction of late ordovician silurian carbonate rocks as an indicator of paleoclimate of the fennoscandian shield abstract in the fennoscandian shield the fluctuations in arid humid climate conditions are inferred from the al2o3 tio2 ratio of the clay fraction of estonian carbonate rocks of the baltoscandian sedimentary basin the late ordovician climate of the fennoscandian shield was mainly semi humid with several humid pulses in the sandbian mid katian and hirnantian the post hirnantian climate was fluctuating between humid and arid until stable humid conditions were established in the middle and early late llandovery starting from the latest llandovery the climate turned semi arid and arid the ordovician silurian long term trend toward aridity was caused by the equator ward drift of the baltica plate shorter term fluctuations of al2o3 tio2 reflect climate changes of the global extent

early telychian silurian marine siliciclastic red beds in the eastern yangtze platform south china distribution pattern and controlling factors the distribution pattern of early telychian turriculatus crispus graptolite biozone red beds in the eastern yangtze platform of south china is reconstructed based on regional geologic data the red beds are developed in three areas which are separated by regions without red deposition the distribution pattern indicates that the cathaysian oldland was the provenance of sediment rich in ferric oxides which are essential for the formation of red beds silurian marine siliciclastic red beds both in china and worldwide tended to develop during times of relatively low sea level coeval hematitic oolites that formed far from the coast may record a change from reducing to oxidizing conditions in the ocean furthermore it is likely that a fall in global sea level a transition from reducing to oxidizing conditions in the ocean and a cooling climate all of which were closely related to the early telychian valgu event promoted the global development of marine red beds during this period

how does sea level correlate with sea water chemistry a progress report from the ordovician and silurian 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

ultra high resolution multivariate record and multiscale causal analysis of pridoli late silurian implications for global stratigraphy turnover events and climate biota interactions abstract the upper silurian and especially pridoli epoch is a critical interval for the understanding the evolution of earth s biota since it witnessed series of powerful extinction events global reorganizations of paleocommunities and expansion of new clades which assumed dominance in the subsequent devonian period the stratigraphic record of the eastern part of the silurian baltic basin represents an excellent candidate for the fine scale analysis of paleoecological and paleoceanographic dynamics therefore we presenting here stratigraphic record of the milaiciai 103 core south western lithuania of conodonts brachiopods 13ccarb 18ocarb and a series of various geochemical and geophysical proxies most of which were sampled at a very high resolution 11 ka spanning uppermost ludlow pridoli and the lowermost devonian the analysis revealed that the pridoli was characterized by four distinct long term dynamic states here named i normal pridoli i ii silal negative carbon isotopic excursion and extinction event iii normal pridoli ii and iv klonk positive stable carbon isotopic event and excursion the most significant change in most variables is detected approximately in the middle of the pridoli at the beginning of the delotaxis detorta zone at the recovery after silal event of conodonts and at the onset of collapse of brachiopod communities the analysis of dynamic complexity revealed the strong evidence for the presence of 405 ka metronome milankovitch cycles in the record of 13ccarb which constrains the duration of the pridoli epoch to 4 3 0 2 ma the recurrence plot analyses revealed deterministic coordination between 13ccarband conodont paleocommunity dynamics at all scales finally lead lag analysis using conditional joint recurrence revealed that conodont dynamics as a rule were driven by perturbations in the carbon cycle although silal event interval shown opposite pattern thus confirming the unusual nature of this previously unrecognized coordinated climatic and biotic event

extreme thermal metamorphism associated with gondwana assembly evidence from sapphirine bearing granulites of rajapalayam southern india

palynology organic geochemistry and carbon isotope analysis of a latest ordovician through silurian clastic succession from borehole tt1 ghadamis basin southern tunisia north africa palaeoenvironmental interpretation abstract palynological and palynofacies analyses combined with organic carbon isotope measurements have been performed on terminal ordovician through silurian clastic sediments from the north african margin of gondwana southern tunisia a first carbon isotopic signal 13corg from gondwanan silurian sedimentary sequences is presented showing interesting correlation with existing coeval isotopic curves from other areas changes in lithology palynofacies characteristics palynomorph diversity carbon isotope developments and organic geochemistry parameters appear to be all causally linked and to reflect changes in palaeoceanographic conditions in particular the detailed chronostratigraphic correlation and the observed similarities in patterns of carbon isotopic developments and palyno and litho facies changes through the study section has permitted to identify the supposedly global earliest wenlock ireviken event and late ludlow lau event isotopic excursions for the first time in high latitude gondwana this confirms that these excursions were linked to global changes in the oceanic system the present results suggest that from rhuddanian to early wenlock times an extended period of black shale deposition occurred over the north african gondwanan margin progressively transgressing from basin palaeodepressions to basin palaeohighs palynofacies and organic geochemistry support a coastal upwelling promoted productivity increase during this interval associated to a decrease in diversity of the marine microplanktonic communities the earliest wenlock strong positive isotopic shift appears associated with this protracted period of massive black shale deposition and thus of organic carbon burial on continental platforms located in high latitude settings this could well explain the apparent paradox between excessive carbonate deposition simultaneous to a carbon isotopic shift towards lighter values observed in low palaeolatitude localities laurentia and baltica the strong distinct isotope excursion occurring in late ludlow times is possibly linked to the well know lau event evidence for extensive organic carbon burial is lacking to explain this strong excursion some significant changes in the marine palynomorph communities are recorded in connection to the increase in stable isotope values 13corg additionally a strong correlation between abundance of terrestrially derived palynomorphs miospores and 13corg development is recorded which seems to support increased clastic input from a terrestrial source

early silurian sea level changes global sea level fluctuated markedly during the early silurian probably as a result of the waxing and waning of ice sheets in the south american portion of gondwana the highest sea levels of the silurian are recorded by the telychian upper crispus lower griestoniensis and spiralis lower lapworthi biozones other highstands occurred in the early aeronian during the convolutus zone mid aeronian guerichi zone and late turriculatus zone early telychian and early sheinwoodian low sea levels characterized much of the argenteus and sedgwickii zones aeronian the utilis subzone late guerichi early turriculatus zones early telychian the late telychian commencing in the mid lapworthi zone and after a period of apparently only small amplitude sea level fluctuations in the late sheinwoodian and earliest homerian the mid late homerian in particular the early nassa zone facies and faunal changes in the lower silurian do not support the p and s model of jeppsson and others but are consistent with the sea level changes proposed herein mid telychian marine red beds appear to have been deposited during a minor sea level fall immediately after a period of very high sea levels rather than during a transgressive episode as previously suggested comparison of the sea level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early silurian deep water graptolitic and shallow water shelly sequences improving the precision of this correlation should be a priority for future research

beyond black shales the sedimentary and stable isotope records of oceanic anoxic events in a dominantly oxic basin silurian appalachian basin usa abstract episodes of instability in the global climato oceanic system have become the hallmark of the silurian period these events are marked by six large positive carbon isotope excursions associated with major extinctions the widespread environmental consequences of these perturbations of the global carbon cycle remain poorly documented a high resolution chronostratigraphic reconstruction of the appalachian foreland basin afb provides a framework to evaluate environmental signatures of two of these carbon isotope excursions valgu and ireviken events telychian sheinwoodian at a regional scale integration of published biostratigraphic range data mapping of sequence stratigraphic surfaces and facies and generation of high resolution carbon isotope profiles for several transects provides precise correlation of the basin stratigraphy yielding a high resolution temporal framework this chronostratigraphic reconstruction of the afb highlights the importance of time specific facies tsf during these intervals for instance packages of red green and gray marine rocks separately occupied discrete time intervals cutting across local facies belts indicative of shallow to deep marine environments similarly widespread iron mineralization occurred during discreet time slices as bathtub rings rimming the margins of the basin changes from one time specific facies to the next are coincident with changes in carbon isotope values indicating a genetic link between fractionation of the global carbon reservoir and redox changes from generally oxic to anoxic oceanic conditions in parts of the afb the basal part of the study interval was deposited during the valgu event this first event begins with an unconformity marking global lowstand this fall in sea level was coincident with rising carbon isotope values as recorded in adjacent basins a slight sea level rise and flooding of the subaerial unconformity occurred near the peak in carbon isotope values oxidized iron minerals were deposited in a zone that extended from the lower shoreface into the basin center pinching out into shallower parts of the basin thin pyrite phosphorite glauconite layers associated with black shales in the basin center were deposited over these ferruginous strata as the event ended and carbon isotope values shifted negatively to a more stable baseline temperatures warmed sea level reached a silurian high and oxic conditions associated with very low rates of organic carbon oc burial were established these conditions were relatively stable for nearly four million years this long period of stasis was punctuated by the ireviken event which displays a strikingly similar progression of tsf patterns yet more intense and longer lived than the preceding valgu event with the first positive shift in carbon isotopes at the onset of the ireviken event the basin redox system changed from largely oxidizing to reducing coincident with a rapid drop in sea level and an abrupt reappearance of iron mineralization on the basin margins the sea level fall associated with the shift to peak values of the carbon isotope excursion is marked by the progradation of a significant body of sand well out into the basin during this sea level drop conditions in the basin center became highly reducing and periodically sulfidic widespread ironstones and ankeritic carbonates were deposited isochronously rimming the shallow basin margins where they alternated with light gray pyritic shales indicating fluctuating redox conditions a large negative carbon isotope excursion of 3 is coincident with a shift from regression to transgression suggesting glacial melting via methane release continued deposition of ironstones through this time interval suggests a primary link to shallow marine redox processes rather than large scale transgression or regression of the shoreline the ironstones are overlain by gray to black pyritic shales deposited across all environments revealing a final stage of redox progression into sustained sulfidic conditions as peak values of the carbon isotope excursion returned to a high stable baseline near 4 the similar expression of tsfs within the afb of these two global events demonstrates that fractionation of the global atmospheric and oceanic carbon reservoirs had a predictable cascading impact on shallow sea floor environments the associated redox processes reveal the dynamic regional interactions of a shallow oxic epicontinental water mass with a broader anoxic oceanic water mass providing valuable insight into the origin of some of the largest extinctions in the silurian period

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

palaeoecology of the graptoloidea abstract graptoloids class graptolithina were colonial pterobranch hemichordates and formed a major component of the early paleozoic macrozooplankton by analogy with modern pterobranchs they filtered food particles from the water mass in which they lived by means of ciliated lophophores they are inferred to have been able to migrate vertically through the water column for feeding efficiency and predator avoidance as by modern euphausiids but their means of locomotion is still highly conjectural the rhabdosomes which housed the colonies grew by distal accretion and large examples are inferred to have taken up to several years to have reached full size the few studies of survivorship suggest a relatively constant mortality of individuals within the community tubular and blister like outgrowths of cortical tissue are seen rarely and are inferred to be a response to parasitism and rare faecal packages with crumpled rhabdosomes suggest ingestion by predators the function of the monospecific aggradations of conjoined colonies synrhabdosomes seen on rare bedding planes is uncertain but is unlikely to have been part of their normal life history and ecology monospecific assemblages of unconnected colonies on single bedding planes are not rare however and suggest localised populations of single species in the overlying water column in life other bedding planes have a range of species resulting from mass mortality from volcanic eruptions and suggest a diversity of species in the water column the onshore offshore and depth facies preference of some 200 ordovician species has led to an ecological model in which the graptoloid habitat is partitioned vertically and horizontally in waters of the neritic and pelagic realms one group of species group 1 is largely confined to a deep water biotope mesopelagic zone characterised by dysaerobic water where they are thought to have exploited high bioproductivity denitrification zones a second group group 2 occupies an epipelagic biotope and is confined to waters of the epipelagic zone aerobic water a third and less well defined group group 3 occupies the inner shelf waters of the epipelagic zone only the median duration of species in the deep water biotope is significantly shorter than that of species in the epipelagic biotope suggesting greater environmental stress and instability in the dysaerobic zone over geological time consistent with models of disruption of the density structure and circulation patterns of the oceans from climatic instability the diversity and abundance of species were enhanced in both the epipelagic and deep water biotopes in regions of marginal upwelling where nutrients were brought from the deep to the shallow water zones it was in these regions that the classical graptolitic shale developed

ultra high temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the palni hill ranges southern india sapphirine granulites from a new locality in the palni hill ranges southern india occur in a small enclave of migmatitic highly magnesian metapelites mg 85 72 within massive enderbitic orthogneiss they show a variety of multiphase reaction textures that partially overprint a coarse grained high pressure assemblage of bt opx ky grt pl qtz the sequence of reactions as deduced from the corona and symplectite assemblages together with petrogenetic grid considerations records a clockwise p t evolution with four distinct stages 1 equilibration of the initial high p assemblage in deep overthickened crust 12 kbar 800 900 c was followed by a stage of near isobaric heating presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives during heating kyanite was converted to sillimanite and biotite was involved in a series of vapour phase absent melting reactions which resulted in the ultra high temperature assemblage opx crd kfs spr sil grt qtz bt coexisting with melt equilibration at c 950 1000 c 11 10 kbar 2 subsequently as a result of decompression of the order of 4 kbar at ultra high temperature a sequence of symplectite assemblages opx sil spr spr crdopx spr crdopx crdopx crd spl crd spl developed at the expense of garnet orthopyroxene and sillimanite this stage of near isothermal decompression implies rapid ascent of the granulites into mid crustal levels possibly due to extensional collapse and erosion of the overthickened crust 3 development of late biotite through back reaction of melt with residual garnet indicates a stage of near isobaric cooling to c 875 c at 7 8 kbar i e relaxation of the rapidly ascended crust to the stable geotherm 4 a second period of near isothermal exhumation up to c 6 5 kbar 850 c is indicated by the partial breakdown of late biotite through volatile phase absent melting reactions available isotope data suggest that the early part of the evolutionary history stages 1 3 is presumably coeval with the early proterozoic metamorphism in the extended granulite terrane of the nilgiri biligirirangan and shevaroy hills to the north while the exhumation of the granulites from mid crustal levels stage 4 occurred only during the pan african thermotectonic event which led to the accretion of the kerala khondalite belt to the south

integrated biostratigraphy of the lower silurian of the aizpute 41 core latvia integrated graptolite conodont and chitinozoan biostratigraphical data is presented from the rhuddanian through to lower sheinwoodian of the aizpute 41 core latvia correlation of the biozonation schemes based upon the three groups is achieved from the cyphus through to lowermost riccartonensis graptolite biozones except for the upper aeronian and lower telychian which lack both chitinozoans and graptolites and upper lapworthi through to approximately base murchisoni graptolite biozone where there is interpreted to be an unconformity datum 2 of the ireviken event is correlated with a level at the base of or within the murchisoni biozone it is possible that the changes in conodont assemblages at datum 2 on gotland are the result of an unconformity here streptograptus kaljoi sp nov from the lower spiralis graptolite biozone is described

early silurian δ13corg excursions in the foreland basin of baltica both familiar and surprising the sommerodde 1 core from bornholm denmark provides a nearly continuous sedimentary archive from the upper ordovician through to the wenlock series lower silurian as constrained by graptolite biostratigraphy the cored mudstones represent a deep marine depositional setting in the foreland basin fringing baltica and we present high resolution data on the isotopic composition of the section s organic carbon 13 c org this chemostratigraphical record is correlated with previously recognized 13 c excursions in the upper ordovician lower silurian including the hirnantian positive isotope carbon excursion hice the early aeronian positive carbon isotope excursion eacie and the early sheinwoodian positive carbon isotope excursion escie a new positive excursion of high magnitude 4 is discovered in the telychian oktavites spiralis biozone lower silurian and we name it the sommerodde carbon isotope excursion socie the socie appears discernible in 13 c carb data from latvian and estonian cores but it is not yet widely recognized however the magnitude of the excursion within the deep marine depositional setting represented by the sommerodde 1 core suggests that the socie reflects a significant event in addition the chemostratigraphical record of the sommerodde 1 core reveals the negative excursion at the transition from the aeronian to telychian stages the rumba low and suggests that the commencement of the eacie at the base of the demirastrites triangulatus biozone potentially is a useful chemostratigraphical marker for the base of the aeronian stage

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

testing the limits of paleozoic chronostratigraphic correlation via high resolution 500 k y integrated conodont graptolite and carbon isotope δ13ccarb biochemostratigraphy across the llandovery wenlock silurian boundary is a unified phanerozoic time scale achievable the resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration by virtue of deep ocean cores and astrochronology the cenozoic and mesozoic time scales carry error bars of a few thousand years k y to a few hundred k y in contrast most of the paleozoic time scale carries error bars of plus or minus a few million years m y and chronostratigraphic control better than 1 m y is considered high resolution the general lack of paleozoic abyssal sediments and paucity of orbitally tuned paleozoic data series combined with the relative incompleteness of the paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the paleozoic time scale to the level achieved during the mesozoic and cenozoic was viewed as impractical impossible or both here we utilize integrated graptolite conodont and carbonate carbon isotope delta c 13 carb data from three paleocontinents baltica avalonia and laurentia to demonstrate chronostratigraphic control for upper llandovery through middle wenlock telychiansheinwoodian similar to 436 426 ma strata with a resolution of a few hundred k y the interval surrounding the base of the wenlock series can now be correlated globally with precision approaching 100 k y but some intervals e g uppermost telychian and upper shein woodian are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and or extinction or carbon isotopic features to delineate such small time slices although producing such resolution during the paleozoic presents an array of challenges unique to the era we have begun to demonstrate that erecting a paleozoic time scale comparable to that of younger eras is achievable less

a late silurian extinction event and anachronistic period resurgence of microbialites and anachronistic facies in the aftermath of phanerozoic extinction events has hitherto only been documented among the major five based on the classical silurian carbonate platform strata of gotland sweden a minor biotic event is herein for the first time related to a substantial increase in anachronistic facies strata deposited during and shortly after the lau event in the baltic paleobasin eke and burgsvik formations include evidence for reduced grazing and infaunal activity in contemporaneous shallow low latitude seas e g flat pebble conglomerates normal marine stromatolites mass occurrence of oncoids and subtidal wrinkle structures e g kinneyia and microripples the latter structures have recently been suggested to be restricted to proterozoic cambrian and early triassic time and therefore are regarded as anomalous for the phanerozoic the co occurrence of these facies and global extinctions among pentamerid brachiopods and conodonts suggests that minor extinction events also resulted in anachronistic periods less devastating but fully comparable to those in the aftermath of the end ordovician and end permian mass extinctions a global positive stable carbon isotope excursion and the last appearance datum of the conodont polygnathoides siluricus may facilitate identification of the late silurian anachronistic period elsewhere less

first report of sapphirine bearing rocks from the palghat cauvery shear zone system southern india abstract silica deficient mg al granulites from paramati within the palghat cauvery shear system contain sapphirine in association with corundum spinel and sillimanite gedrite which occurs commonly in this locality coexists with cordierite corundum and sillimanite mineral assemblages and reaction textures indicate peak metamorphism at ultrahigh temperature conditions this new locality provides evidence for extreme crustal metamorphism along the archean proterozoic collision boundary in southern india

quantifying the community turnover of the uppermost wenlock and ludlow silurian conodonts in the baltic basin abstract the homerian to ludfordian interval of the mid to late silurian period was a time of significant changes in conodont communities global climate oceanographic patterns and biogeochemical cycles the mulde and the lau events are preeminent examples of globally recognized conodont extinction episodes from this interval in earth s history the silurian baltic basin is the most suitable locality globally for studying these perturbations to the ocean atmosphere system since there is an extensive record of conodont taxa occurrences their communities and their environments spanning across shore face to open ocean settings in this study we present new conodont and 13c data from the upper homerian to ludlow interval from two core sections g luva 99 and g luva 118 representing shelfal environments and the numerical conodont data from the vidukl 61 section from deep water settings and compare them with patterns of conodont diversity change as revealed in the data rich milaiciai 103 core section for this purpose we explored the stratigraphically tied time series of conodont diversity changes employing recurrence and cross recurrence plots the binary similarity matrices that are used for deciphering complex spatial and temporal dynamic patterns the cross recurrence plots were used as a means of synchronizing the geological sections by applying dynamic time warping and the newly described moving window median recurrence point search algorithms the results revealed that the conodont community compositional data are sufficiently temporally and spatially coherent to be reasonably used for synchronizing geological records moreover the sudden state transitions detected in the cross recurrence plots suggest that the lau event was of great importance for conodont community evolution in the studied time slice

present status of the geochronology of the precambrian rocks of rajasthan abstract age determinations mostly by rb sr analyses of the precambrian rocks of rajasthan by us and by others are summarized and discussed broad periods of acid magmatism at 1 3000 2900 m y 2 2600 2500 m y 3 2000 1900 m y 4 1700 1500 m y and 5 850 750 m y were identified the oldest rocks in the area are the yet undated banded gneisses bgc east of udaipur intruded by the untala granite dated at 2950 m y and hence of mid archean age the basal status hitherto attributed to the berach granite dated at 2600 m y is no longer tenable the radiometric control on the beginning and duration of the overlying aravalli supergroup is not yet satisfactory though a lower limit at 2000 m y is indicated heron s original delhi rocks have recorded two magmatic events widely separate in space and time while the earliest granitic activity at 1600 m y is recorded only in the alwar basin in the northeast the younger activity between 850 750 m y is widespread as shown by the nearly concordant ages of erinpura type granites along the aravalli mountain range and the malani rhyolites in the western plains of the aravalli range

carbonate platform evolution and conodont stratigraphy during the middle silurian mulde event gotland sweden evidence from sedimentology and conodont biostratigraphy is used to reinterpret the mid homerian late wenlock succession on gotland sweden a new conodont zonation includes from below the ozarkodina bohemica longa zone including five subzones the kockelella ortus absidata zone and the ctenognathodus murchisoni zone two taxa are named ozarkodina bohemica longa and pseudooneotodus linguicornis these new zones are integrated with facies in order to correlate strata and infer the major depositional environments and the controls on deposition during the mid homerian mulde event reef associated and skeletal carbonate deposition predominated before and after the event i e during the uppermost o s sagitta zone and again in the c murchisoni zone these periods are characterized by the expansion of reefs and shoal facies across marls in the topmost slite group on eastern gotland and in the lower parts of the klinteberg formation on western gotland respectively the intervening o b longa and k o absidata zones are initially characterized by rapid facies changes including siliciclastic deposition and later stabilisation of a carbonate depositional system the composition of sediments and depositional rates are closely related to the creation and destruction of accommodation space and reflects a classical case of depositional bias of the carbonate and siliciclastic depositional systems based on coastline migration stratal boundaries and the stratigraphic position of major reef belts several facies associations can be fitted into a sequence stratigraphic model for platform evolution a highstand systems tract hst situation prevailed prior to and during the early part of the event the upper slite group including the lower frjel formation this hst was characterized by prolific skeletal production and regional reef development except for during the latest stage when carbonate production declined at the onset of the mulde event platform growth was inhibited during a following regressive systems tract rst when regional siliciclastic deposition predominated the gannarve member the subsequent lowstand resulted in regional emersion and karstification i e a complete termination of the platform the post extinction transgressive systems tract tst is exclusively composed of non skeletal carbonates the bara member of the halla formation re occurrence of reefs and a prolific skeletal production marks platform recovery during a second hst the remaining halla and the lower klinteberg formations integration of high resolution biostratigraphy and sequence stratigraphy reveals that the major physical control on platform evolution was a 5th order eustatic sea level change during an early part of the mulde event and that the bulk of the strata accumulated when the platform aggraded and prograded during the highstand systems tracts thus silurian oceanic events and associated sea level changes had profound impact on the neritic carbonate system the gotland based middle and late homerian sea level curve shows two rapid regressions both leading to truncation of highstand systems tracts the first lowstand occurred at the very end of the c lundgreni chron and the second at the end of the co ludensis chron the intervening interval was characterized by stillstand or possibly slow transgression

high resolution correlations between gotland and estonia near the base of the wenlock the strata of the ireviken event can be correlated between gotland sweden and the viki core on saaremaa estonia with a high degree of precision most of the ten datum planes are found with a precision of better than 0 1 m in the lower and upper visby beds on gotland and high in the mustjala member of the jaani formation on saaremaa five of the latest llandovery early wenlock units defined by these datum planes are between c 0 12 and 1 77 m thick on gotland and 0 12 and 0 67 m in estonia the duration of each was between a few thousand and c 31 000 years the stratigraphic resolution achieved in the interval of the ireviken event is 10 000 years ultimate range ends are lacking within a preceding slightly longer interval but the pattern of intermittent presence of some taxa agrees well with an unchanged relative rate of sedimentation their coeval presence in different places indicates ecological regional or more widespread changes

oxygen isotope variations in phosphate of biogenic apatites i fish bone apatite rechecking the rules of the game abstract the major advantage of the oxygen in phosphate isotope paleothermometry is that it is a system which records temperatures with great sensitivity while bone and teeth building organisms are alive and the record is nearly perfectly preserved after death fish from seven water bodies of different temperatures 3 23 c and different 18 o values 16 to 3 of the water were analysed the 18 o values of the analysed po 4 vary from 6 to 25 the system passed the following tests a the temperatures deduced from isotopic analyses of the sequence of fish from lake baikal are in good agreement with the temperatures measured in the thermally stratified lake b the isotopic composition of fish bone phosphate is not influenced by the isotopic composition of the phosphate which is fed to the fish but only by temperature and water composition isotopic analysis of fossil fish in combination with analysis of mammal bones should be a useful tool in deciphering continental paleoclimates

carbon isotope event markers through the wenlock pridoli sequence at ohesaare estonia and priekule latvia abstract the ohesaare borehole in the northern part of the silurian baltic gulf penetrates shallow and deep shelf marlstones limestones and dolostones the priekule borehole in the deep shelf belt of the basin was drilled through marlstones and graptolitic mudstones most of the successions except the upper part are characterized by a high sr ca ratio close to marine calcites suggesting good preservation of carbonate material conodont alteration indices are low 1 2 carbon isotopes were measured using whole rock samples in the ohesaare core section two distinctive 13 c positive excursions were established one in the early wenlock riccartonensis and low rigidus ellesae standard graptolite zones 4 2 and the second in the late wenlock nassa zone 4 6 before and after the latter there are also intervals with low 13 c values the first just above the lundgreni zone 1 8 and another in the ludensis perhaps reaching into nilssoni zone 3 8 in the priekule section the early wenlock 13 c peak is well represented 3 1 but in the late wenlock early ludlow 13 c values show little variation from 2 0 to 1 in this section a high peak of 13 c values 5 9 was determined in the nova beds correlated with the topmost gorstian and lowermost ludfordian ludlow the carbon isotope positive excursions show good correlation with lows of the global sea level curve and with most of the biotic and supposed climatic events glaciation ocean state

middle palaeozoic extinction events faunal and isotopic data abstract at least 9 and possibly as many as 12 extinction events of global or near global impact can be discriminated in the mid palaeozoic earliest silurian to early carboniferous on the basis of brachiopod coral conodont and ammonoid data and the history of carbonate build ups isotopic data from whole rock samples are presented for three of these events based on australian carbonate sequences constrained by conodont data these data represent the initial phase of a more extensive investigation using c o s sr and nd isotopic signatures derived from conodonts articulate and inarticulate brachiopod shell and fish remains from numerous australian and european sequences the aim of the project is to identify isotopic responses to extinction events and address causes for these changes in limestone sections analysed so far variations in carbon isotope compositions on a whole rock scale are most marked at horizons that can be correlated with times of significant reduction in biomass and diversity this is despite the fact that the whole rocks are in fact multicomponent systems with respect to carbon in part arising from diagenetic rearrangements of carbon distribution within the scale of the whole rock thus the carbon isotope data do provide evidence on a whole rock scale for fundamental changes in the global carbon cycle that are correlative with extinction events it is however unlikely that the magnitude of the isotopic shift will be precisely documented from such whole rock analyses the oxygen isotope results are less obviously related to the times of carbon isotope excursion this contrast in apparent resilience to post depositional modification of pristine isotope compositions exemplifies the extreme caution needed in evaluating isotopic data from carbonates

trilobite systematics biostratigraphy and palaeoecology of the lower ordovician komstad limestone and huk formations southern scandinavia

evaluation of the salinic i tectonic cancaniri glacial and ireviken biotic events biochemostratigraphy of the lower siluriansuccession in the niagara gorge area canada and u s a abstract well preserved brachiopods from the niagara gorge area anticosti island britain gotland and estonia were utilised to delineate a complex isotopic evolution for llandovery wenlock seawater the sr isotope record reflects the salinic i tectophase of the late llandovery in the continuous increase in 87sr 86sr values from 0 708070 to 0 708346 the salinic ii tectophase is marked by relative constancy of sr isotope values until the late wenlock when it rises from 0 708345 to 0 708430 the second tectonic phase was therefore likely only of a regional nature the carbon isotopes during the llandovery fall within a band of about 1 to 3 a range comparable to modern low latitude brachiopods a large positive 13c excursion of about 3 identifies the ireviken event excursion characterizes the early wenlock the biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the cancaniri glaciation although a direct causative scenario is as yet unknown the oxygen isotopic trends of well preserved brachiopods clearly reflect a warm climate interval during the latest llandovery associated with the silurian sea level highstand subsequently in the early wenlock the sea level fell with the onset of the cancaniri glaciation in the southern hemisphere this is reflected in a significant positive 18o excursion particularly in brachiopods from the niagara gorge area brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative 18o values indicating a lesser cooling gradient

the silurian nautiloid bearing strata of the cellon section carnic alps austria color variation related to events abstract silurian nautiloid bearing strata llandovery p idoli from the cellon section of the carnic alps austria are compared using the innovative perspective of the color of the enclosing sediment of these fossiliferous beds with coeval regional occurrences along the north gondwana margin prague basin sardinia morocco particular attention was placed on distinguishing and comparing the colors of these levels and their general nautiloid faunal composition in order to identify the prevailing features of their occurrence at precise stratigraphic intervals red brown strata usually reflect time rich or slowly accumulated deposits from a diverse in situ fauna while black gray strata are related to single event beds due to episodic deposition probably influenced by currents other single event beds of distinctive pink gorstian in age and purple strata ludfordian in age may prove useful as marker horizons the overall data set available in relation to these diversely colored nautiloid bearing horizons at the cellon section at precise time slices when compared with global eustatic conditions and biotic events has highlighted that these deposits reflect both local phenomena within the depositional basin and regional depositional episodes these include apparent mass mortality events and the cephalopod limestone biofacies that may be considered as time specific facies

carbon isotope stratigraphy of the upper telychian and lower sheinwoodian llandovery wenlock silurian of the banwy river section wales 13 corg and toc data are presented from the upper spiralis biozone telychian llandovery silurian through to the upper sheinwoodian wenlock silurian of the banwy river section wales in laminated hemipelagites from the telychian 13 corg values rise through the upper lapworthi biozone to a maximum in the lower insectus biozone after which they decline slightly the most conspicuous feature of the 13 corg curve is the prolonged positive excursion in the sheinwoodian commencing in the upper murchisoni biozone and ending in strata yielding monograptus flexilis this sheinwoodian positive 13 c excursion in the banwy river section correlates precisely with that recognized in the east baltic the interval with the highest 13 corg values also records the highest toc values suggesting that for the sheinwoodian at least burial of carbon may have contributed to the positive 13 c excursion bioturbated strata yield very low toc values whether the 13 corg values from these beds reflect a primary signal or the result of biostratinomic or diagenetic modification is uncertain

early silurian glaciations in brazil abstract the opening of the brazilian paleozoic basins during the late ordovician was followed by a tectonic stabilization in the silurian at the time of the early silurian glaciations deposition of continental sandstones prevailed in the basins offshore environments were present in the amazonas basin and to a smaller extent in the parana basin the ice sheets advanced over fluviatile and littoral sediments except for the south margin of the amazonas basin where the glacial diamictites immediately overlie marine facies the ice produced much meltwater even when advancing and this was probably climatically controlled the ice movements were from the northwest at the northern margin of the amazonas basin and towards the northeast at the southern margin in the parnaiba basin and its outliers the jatoba basin and cariri valley the ice came from southeast and probably also from east further to the south in the parana basin it came from the north and northeast four glaciations have been dated to the early silurian the oldest is dated as latest ashgill and or earliest llandovery in the parana basin as evidenced from tillites in the upper rio ivai and lower vila maria formations including the former iapo formation it is contemporaneous with a glaciation in the cape basin south africa the next glaciation is dated as early middle llandovery gregarius graptolite zone and is present in the amazonas basin and the andean belt and probably also in the parnaiba basin and its outliers the two youngest tillites were widespread in the northern brazilian basins amazonas parnaiba and jatoba basins and cariri valley as well as in the andean belt and are dated as early late llandovery possibly starting already in the late middle llandovery and latest llandovery to earliest wenlock the latter tillite is the most widespread one assuming these glaciations are related to polar positions the apparent migration of the south pole ice cap must have been quite rapid from central north africa in late ashgill towards central brazil in the middle late llandovery the glaciation in the parana basin took place up to 45 from the late ashgillian south pole the cool climate continued during the silurian but no new glaciations are known before latest devonian

did intense volcanism trigger the first late ordovician icehouse oxygen isotopes measured on late ordovician conodonts from minnesota and kentucky united states were studied to reconstruct the paleotemperature history during late sandbian to katian mohawkian cincinnatian time this time interval was characterized by intense volcanism as shown by the prominent deicke millbrig and other k bentonite beds a prominent carbon isotope excursion guttenberg 13 c excursion gice postdates the millbrig volcanic eruptions and has been interpreted to reflect a drawdown of atmospheric carbon dioxide and climatic cooling the oxygen isotope record in conodont apatite contradicts this earlier interpretation an increase in 18 o of 1 5 vienna standard mean ocean water just above the deicke k bentonite suggests an abrupt and short lived cooling that possibly initiated a first short term glacial episode well before the major hirnantian glaciation the decrease in 18 o immediately after the mega eruptions indicates warming before the gice and no cooling is shown in the gice interval the coincidence of the deicke mega eruption with a cooling event suggests that this major volcanic event had a profound effect on late ordovician late mohawkian climate

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

palaeokarst evidence for widespread regression and subaerial exposure in the middle katian upper ordovician of baltoscandia significance for global climate we report on widespread and contemporary palaeokarst in the middle katian upper ordovician of sweden estonia and latvia the first major palaeokarst horizon to be reported from the ordovician of baltoscandia the solution features occurwithin a fewmetres of thick zone of limestone immediately belowthe widespread fjacka shale and are related to an unconformity with preserved palaeorelief in the slandrom limestone in sweden and the time equivalent saunja formation in estonia and latvia facies evidence for a karst origin comes fromseveral outcrops and core sections and includes 1 frequent karren likemorphologies interpreted as swiss cheese karst sensu baceta et al 2001 2 local occurrences of solution collapse breccia 3 presence of bladed pseudospar crystals in solution cavities and 4 carbon isotope values indicating meteoric influence to the succession these findings are herein put in context with previously reported large scale erosional channels that may cut down several tens of metres below the fjacka shale in the subsurface baltic sea area and with regional anomalous thickness variations in the slandrom limestone and saunja formation all together forming strong support for regional exposure of the baltoscandian continent in the middle katian high resolution stable isotopic data show that the regression and lowstand of sea level overlap with thewaynesville carbon isotope excursion it resulted in basin wide cessation of carbonate production near the amorphognathus superbus and a ordovicicus conodont zonal boundary the contemporary development of palaeokarst in different confacies belts of the basin suggests that this was an extraordinary sea level lowstand herein interpreted as reflecting a middle katian glaciation less

lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya the stratigraphic record of the eastern murzuq basin has been importantly influenced by deformation resulting in angular and or deeply erosional unconformities though the overall context is intracratonic major transgressive events and the ordovician glaciation are nevertheless documented allowing the delineation of tectonic eustasy or climate driven unconformities lower palaeozoic key events and related unconformities that characterize the north gondwana platform have therefore a signature in the eastern murzuq basin the basement cover unconformity also known as the infra tassilian surface truncates all the deformed and metamorphosed lower cambrian and older rocks above is a middle cambrian to lower ordovician megasequence murizidie and hasawnah fms which is in turn truncated by an intra ordovician angular unconformity this megasequence is unconformably overlain by a middle ordovician hawaz fm to silurian tanzzuft and akakus fms megasequence which includes the upper ordovician glaciogenic unit mamuniyat fm bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations tillites and glaciotectonic structures the middle ordovician to silurian megasequence is finally truncated by a base devonian angular unconformity overlain by fluvial sandstones regarding the possibility that those fluvial deposits may be as younger as late devonian in the eastern murzuq basin based on palaeoflora the so called caledonian unconformity might be here a much younger mid eifelian surface and the occurrence of the lower devonian tadrart fm is questioned the upper ordovician glacial erosion surface which is sometimes referred to as the taconic unconformity usually truncates middle ordovician strata in the murzuq basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra ordovician deformation event in the murizidie southeastern murzuq basin the infra tassilian surface the intra ordovician unconformity and the upper ordovician glacial erosion surface amalgamate together here an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision the main part of which relate to the intra ordovician tectonic event the upper ordovician climate related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the lower palaeozoic although it presents most of the physical attributes of tectonic driven unconformities

paleo environmental cyclicity in the early silurian yangtze sea south china tectonic or glacio eustatic control abstract the yangtze sea of the south china craton experienced strong environmental perturbations during the first 5 6 myr of the silurian the lower silurian lungmachi formation in the jiaoye 1 drillcore chongqing china records three sea level cycles stratigraphic sequences characterized by rapid deepening at the base of each cycle followed by slow shallowing each deepening event was associated with shifts toward higher marine productivity more intensely reducing bottomwater conditions reduced watermass restriction lower detrital input and enhanced siliciclastic weathering intensity we infer control by glacio eustasy in which deglacial sea level rises and concurrent climatic warming triggered the observed environmental changes shallowing was associated with renewed glaciation and characterized by the opposite set of conditions the results of this study thus provide evidence in support of continued continental glaciation during the early silurian i e following the termination of the major end ordovician hirnantian glaciation the study core also exhibits a long term shallowing trend through the entire lower silurian the origin of which may have been tectono epeirogenic uplift of the south china craton during the kwangsian orogeny

depositional and diagenetic constraints on the abundance and spatial variability of carbonate associated sulfate abstract marine carbonate rocks are composed in varying abundance of cement micrite abiotic grains and fossils which can provide information about the physical and chemical environments in which they formed geochemical analyses of these carbonates are not always interpreted alongside the wealth of geologic including petrographic information available resulting in potentially faulty reconstructions of biogeochemical and environmental conditions these concerns have prompted closer scrutiny of the effect of depositional lithofacies and diagenesis on carbonate proxies here we have combined x ray absorption near edge structure xanes spectroscopy and x ray fluorescence xrf imaging to map the speciation and abundance of sulfur in carbonate petrographic thin sections in upper ordovician carbonates from anticosti island canada and early silurian carbonates from gotland sweden across multiple depositional facies lithofacies and fossil communities between anticosti island and gotland are similar which allows for comparison of changes in the dominant s species and their abundance in separate basins associated with variations in glacio eustatic sea level sulfide abundance is greatest in mudstone wackestone and packstone facies where interstitial micrite hosts abundant pyrite sulfate abundance as carbonate associated sulfate cas varies within individual fossil fragments as well as within the same fossil phylum and is particularly high in unaltered brachiopods in contrast sulfate abundance is generally very low in micrite near the detection limit and generally arises in situ from sulfide that has been oxidized as opposed to true cas in different cement fabrics sulfate abundance is greatest in drusy pore filling cements organic sulfur compounds are also detected and although low in abundance are mostly found within micrite the detection and characterization of both inorganic sulfur and organic sulfur compounds provides a platform to understand early processes of biomineralization this approach will broaden our understanding of the source of inorganically bound sulfate in ancient carbonates as well as the effect of depositional setting and diagenesis on cas incorporation re mobilization and ultimate abundance in sedimentary carbonates additionally this work has implications for the cas isotopic value of individual carbonate components that may affect interpretations of stratigraphic variability of numerous cas sections throughout earth history

evidence for extensive methane venting on the southeastern u s atlantic margin we present the first evidence for widespread seabed methane venting along the southeastern united states atlantic margin beyond the well known blake ridge diapir seep recent ship and autonomous underwater vehicle auv collected data resolve multiple water column anomalies 1000 m height and extensive new chemosynthetic seep communities at the blake ridge and cape fear diapirs these results indicate that multiple highly localized fluid conduits punctuate the areally extensive blake ridge gas hydrate province and enable the delivery of significant amounts of methane to the water column thus there appears to be an abundance of seabed fluid flux not previously ascribed to the atlantic margin of the united states

correlation of silurian bentonites from gotland and the eastern baltic using sanidine phenocryst composition and biostratigraphical consequences silurian bentonites from sweden and the eastern baltic are correlated using xrd analysis of sanidine composition identifying the same bentonite in different sections has constrained correlations between the conodont graptolite and chitinozoan zonations three bentonites correlate the cyrtograptus murchisoni zone with the upper part of the pterospathodus amorphognathoides zone and both the lower and the upper pseudooneotodus bicornis conodont biozones at their type locality thus the base of the llandovery wenlock boundary stratotype can be correlated with a level in the upper part of the cyrtograptus murchisoni graptolite biozone identifying the y bentonite gotland sweden in eastern baltic drill cores shows that the base of the eisenackitina lagena chitinozoan biozone is close to the base of the k patula conodont biozone which is coeval with an upper part of the c rigidus graptolite zone sanidine data from the grotlingbo bentonite fit well with previous correlations some bentonites from gotland with distinct sanidine compositions and bulk geochemistries have not yet been identified elsewhere

pan african granulite facies assemblages in kodaikanal anaimalai ranges tamil nadu and mt abu balaram areas of gujarat india madagascar india connection in eastern gondwana assembly

emergence biodiversification and extinction of the chitinozoan group chitinozoans are considered as reproductive bodies of marine invertebrates called chitinozoophorans these chitinozoophorans were most likely to have been small pelagic or necto pelagic soft bodied probably wormlike animals and judging from the size of chitinozoans they probably measured from a few millimetres to a few centimetres in length the chitinozoophorans most likely survived by grazing on phytoplankton there is no evidence of a large colonization of the pelagic niche in the cambrian but from the early ordovician onward this niche was exploited chiefly by graptolites and chitinozoophorans both groups inhabited nearshore and offshore habitats but in contrast to the graptolites the chitinozoans displayed their highest diversity at high latitude in less distal that is upper and lower offshore environments the chitinozoan group evolved rapidly during the ordovician and reached its maximum ordovician diversity in the late darriwilian from the first occurrence of chitinozoans in early tremadocian times to the biodiversity crisis in latest ordovician times nearly 80 of the morphological innovations took place until their extinction in the latest devonian chitinozoans survived through several biodiversity crises in the early late ordovician late hirnantian late wenlock earliest emsian and in the latest frasnian kellwasser event during the melting of the hirnantian ice sheet most ordovician genera and species became extinct but some genera extended beyond the boundary e g spinachitina belonechitina cyathochitina ancyrochitina the hirnantian glaciation was not directly responsible for the dramatic extinction of organic walled microfossils but it certainly accelerated the extinction of lineages that had already been weakened since the early to mid katian the late wenlock and earliest emsian graptolite crises affected the chitinozoophorans to a lesser degree and the latest frasnian kellwasser event did not greatly affect chitinozoophorans the disappearance of the chitinozoan group at the end of the famennian resulted from a combination of factors for example the chitinozoophorans probably no longer had the genetic potential for successful adaptations to successive drastic environmental changes only one species is known from the latest famennian their usual niche was invaded by more efficient groups and their usual food supply disappeared or was no longer sufficient the latter factor is supported by the contemporaneous decline in phytoplankton

carbon and oxygen isotopic composition of silurian brachiopods gotland sweden palaeoceanographic implications abstract one hundred and seventy two non luminescent brachiopod shells from the silurian sequence of gotland were investigated for their carbon and oxygen isotopic composition diagenetic alteration of the shell calcite was excluded by means of petrographic trace element and cathodoluminescence analysis oxygen and carbon isotopic signals vary considerably within the uppermost llandovery to the ludlow average 18o and 13c values of well defined stratigraphic units range from 2 4 to 5 9 and 1 to 8 respectively we observed parallel trends in the carbon and oxygen isotope curves with the most prominent shifts occurring in the lower wenlock upper wenlock and middle ludlow average 18o values 2 4 to 2 9 recorded in the middle ludlow are among the highest 18o values reported for the early palaeozoic the recorded 18o variations cannot solely be explained by changes in sea surface temperature and salinity the variations in the isotopic signals correlate negatively with changes in sea level heavy carbon and oxygen signals coincide with sea level lowstands whereas light signals are recorded during sea level highstands sea level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves inducing oxygen deficiency in the basins alumshale facies the low 18o values are attributed to 18o depletion in surface waters owing to the storage of 18o enriched saline waters in the basins low 13c values are explained by a low surface water productivity since nutrients were not supplied to the photic zone but rather stored in the deeper ocean during sea level lowstands colder waters that formed in higher latitudes may have represented the main intermediate water source a better ventilation of the basins is documented by the deposition of well aerated sediments whereas the high 18o values are explained by the change in oceanic circulation the high 13c values are attributed to an enhanced primary productivity and possibly to an enhanced burial of corg the model discussed here may provide an attractive scenario for accounting for the recorded short term variations in the isotopic signals without the necessity of introducing an isotopic correction factor for silurian sea water

depositional history source rock quality and thermal maturity of upper ordovician lower silurian organic rich sedimentary rocks in the central part of the baltic basin lithuania abstract this study provides a comprehensive geochemical and mineralogical characterisation of the fine grained organic matter rich sedimentary rocks of the lower palaeozoic in the central part of the baltic basin western lithuania the primary objective was to deduce environmental conditions for the lower palaeozoic succession which is very well preserved in the baltic basin while the secondary goal was to obtain information on maturation and thermal history of the rocks the sample set covers different stratigraphic units of the ordovician lower silurian succession and was analysed with a wide range of different methods including elemental analysis organic petrography rock eval pyrolysis molecular geochemical analysis of saturated and aromatic hydrocarbon fractions inorganic geochemistry mineralogy xrf xrd analysis and magnetic susceptibility ms of rock samples their integration allowed a determination of the redox conditions paleoproductivity and clastic input during sediment deposition the fine grained sediments are dominated by clay minerals and quartz and to a lesser extent carbonate pyrite and feldspar microscopic analysis revealed abundant solid bitumen and zooclasts biomarker data indicate a marine origin for the precursor organic matter maturity parameters revealed the presence of a thermal maturity anomaly in the southwestern part of lithuania where the late phase of oil generation and possibly beginning of condensate wet gas generation with corresponding maximum paleotemperatures of 140 180 c has been reached bottom water conditions varied from oxic to anoxic euxinic during deposition driven by interaction of climate states water mass circulation and stratification water geochemistry and sea bottom bathymetry four shaly horizons at the upper ordovician fjacka and mossen fms silurian dobele fm and near llandovery wenlock boundary levels were recognized as being formed in oxygen depleted benthic settings as indicated by high toc and ts contents sedimentation features high metal enrichment or degree of piritization the deposition of highly organic matter rich silurian dobele fm might have coincided with oceanic anoxia and late aeronian bioevent accompanied by an upwelling regime with increased paleoproductivity the interpreted coarsening upward trend at the early silurian adavere regional stage terminates with fall in relative sea level at the llandovery wenlock boundary the geochemical proxies near llandovery wenlock boundary may reflect the perturbations in oceanic water geochemistry which in turn may have eventually led to ireviken mass extinction event

initial plant diversification and dispersal event in upper silurian of the prague basin abstract a relatively rich association of six species of embryophyte plants is known from the upper silurian of the prague basin bohemian massif czech republic all stratigraphically controlled specimens come from the neocolonograptus parultimus neocolonograptus ultimus zone of the p idoli a new genus and species tichavekia grandis psenicka sakala et kraft is established the new combination aberlemnia bohemica schweitzer sakala psenicka et kraft comb nov illustrates its lycophytina affinity the distribution and taphonomy of these plants in the prague basin indicates the proximity of exposed land interpreted to be islands of volcanic origin two local associations show primary differences in vegetated areas of the islands and apparent ecological responses of the primitive vascular plants to their habitats suitable environments in the coastal zones of volcanic islands in the prague basin which was situated at the outer periphery of a broad gondwanan shelf likely represent significant transfer points for an initial dispersion and a first expansion of land plants repeated vegetation of the islands is inferred from the discrete distribution of land plants at different stratigraphic levels these changes are explained partly in context of local changes of the island relief and partly as responses to global climatic and sea level fluctuations and fit environmental dynamics of islands of volcanic origin in general this initial plant diversification and dispersal event ipdde is of considerable significance in the evolution of plants documented from the bohemian massif

oxygen and carbon isotopic composition of silurian brachiopods implications for coeval seawater and glaciations we collected 236 calcitic brachiopod shells covering the entire silurian period 30 m y at high temporal resolution from stratotype sections from anticosti island canada wales united kingdom the oslo region norway gotland sweden and podolia ukraine estonia latvia and lithuania data from petrography scanning electron microscopy cathodoluminescence isotopes and trace elements all confirmed that there was excellent preservation in most shells thus arguing for retention of primary isotope signals exceptions were samples from the oslo region the d18o and d13c values for the well preserved samples range from 2 to 6 5 and from 1 to 7 5 peedee belemnite respectively in terms of temporal trends oxygen and carbon isotopes vary in parallel with a slight decrease with declining age of 1 through silurian time with superimposed short term oscillations that are negatively correlated with sea level changes three successive positive d18o shifts in early aeronian latest aeronian and early wenlock time correlate with sea level lowstands and with glacial diamictite deposits in the amazon basin and in africa the high d18o and d13c values are attributed to cold episodes with low sea levels and low values to warm episodes with high sea levels during warm silurian episodes the d18o value of seawater is suggested to have been about 3 5 standard mean ocean water smow and the global tropical temperatures of about 20 30 c similar to the present day values in summer during glacial episodes seawater is proposed to have had a d18o value of 2 5 smow and temperature of about 14 23 c comparable to tropical temperatures proposed for the last glacial episode 14 to 28 c three positive d13c peaks in early and late wenlock and late ludlow time likely of regional to global significance appear to coincide with sea level lowstands but we are as yet unable to propose a convincing causative geologic scenario that would explain their origin

reconstructing the environmental conditions around the silurian ireviken event using the carbon isotope composition of bulk and palynomorph organic matter the carbon isotope composition 13c of bulk organic matter and two palynomorph groups scolecodonts and chitinozoans from the llandovery wenlock strata of gotland e sweden are compared to gain knowledge about carbon cycling in the silurian sub tropical shelf environment the 13c values of the palynomorphs are mostly lower than the 13c values of the bulk organic matter and the 13c values of the benthic scolecodonts are lower than those of the planktonic chitinozoans while the difference between bulk and palynomorph 13c may be in part a function of trophic state the lower values of the scolecodonts relative to those of chitinozoans which are assumed to live in the well mixed water column might imply an infaunal mode of life for the polychaetes that carried the scolecodonts lower 13c for the scolecodonts in the middle of the section may represent variations in primary marine productivity supported by acritarch abundance data oxidation of organic matter in the bottom waters or genera effects in general however trends between the three data sets are parallel indicating similarities in the low frequency environmentally forced controls the 13c data show a decreasing trend from the base of the section up to a horizon well below the base of the upper visby formation at this level and therefore probably several 10 kyr before the 13c increase in the carbonates the 13c organic values increase by 1 this perhaps is an expression of a changed composition of the bulk organic matter associated with the extinction events prior to the llandovery wenlock boundary

lower wenlock black shales in the northern holy cross mountains poland sedimentary and geochemical controls on the ireviken event in a deep marine setting the stratigraphic variability and geochemistry of llandovery wenlock l w series boundary sediments in poland reveals that hemipelagic sedimentation under an anoxic euxinic water column was interrupted by low density bottom currents or detached diluted turbid layers that resulted in intermittent seafloor oxygenation total organic carbon values and inorganic proxies throughout the wilkow 1 borehole section suggest variable redox conditions u mo ratios 1 throughout much of the aeronian and telychian stages together with an absence of pyrite framboids suggest oxygenated conditions prevailed however elevated total organic carbon near the aeronian telychian boundary together with increased u th and v v ni ratios and populations of small pyrite framboids are consistent with the development of dysoxic anoxic conditions at that time u th v cr and v v ni ratios as well as u authig and mo concentrations suggest that during the ireviken black shale deposition bottom water conditions deteriorated from oxic during telychian time to mostly suboxic anoxic immediately prior to the l w boundary before a brief reoxygenation at the end of the ireviken black shale sedimentation in the sheinwoodian stage rapid fluctuations in u mo during the ireviken event are characteristic of fluctuating redox conditions that culminated in an anoxic euxinic seafloor in sheinwoodian time following ireviken black shale deposition conditions once again became oxygen deficient with the development of a euxinic zone in the water column the aeronian to sheinwoodian deep water redox history was unstable and rapid fluctuations of the chemocline across the l w series boundary probably contributed to the ireviken event extinctions which affected mainly pelagic and hemipelagic fauna

silurian records of carbon and sulfur cycling from estonia the importance of depositional environment on isotopic trends abstract the llandovery wenlock carbonate marl succession of the baltoscandian basin has been analyzed in the viki drill core western estonia for carbonate carbon 13 c carb organic carbon 13 c org carbonate associated sulfate 34 s cas and pyrite 34 s pyr isotopes along with trace element concentrations following the end ordovician glaciation and the hirnantian carbon isotope excursion 13 c carb values rise to an early llandovery carbon isotope maximum 2 subsequently 13 c carb and coeval 13 c org record a large 4 5 excursion in the early wenlock associated with a regression and biotic turnover known as the ireviken bioevent ibe overall 34 s cas values generally fall within a range 25 to 35 similar to that reported in other sections of comparable age two distinct trends are observed low and stable 34 s cas values are associated with deep water facies while 34 s cas becomes higher and more variable in the shallow water facies similarly 34 s pyr values are low and invariant 15 in deep water facies but become higher and more variable ranging between 35 and 16 in shallow water deposits despite abundant recrystallization no obvious evidence for large scale alteration of proxies is apparent in concentrations of fe sr mn and mg or in isotopic correlations although increased variability and lower values of 34 s cas are observed where cas abundance is low a large well resolved positive excursion in 34 s cas observed during the ibe from a time correlative section on gotland sweden is absent in the data presented here this difference demonstrates that counter to general understanding local environmental variations can give rise to divergent 34 s cas profiles in coeval sections the s isotope fluctuations in cas and pyrite in the viki drill core can be explained by local processes involving facies changes early diagenetic processes and associated open vs closed system behavior that can overprint and potentially mask basin or global signals

the silurian climatic transition recorded in the epicontinental baltica sea abstract the transition from the middle to late ordovician cold climate to the silurian greenhouse phase was marked by major climatic and oceanographic changes the vast epereic sea that during the early paleozoic extended over the baltic plate for hundreds of kilometres recorded the dynamic processes that characterized this important climatic evolution this shallow epicontinental sea experienced phases of hypoxic conditions for millions of years during the silurian favouring the sedimentation of organic rich deposits a continuous core drilled in the silurian fine grained succession of the baltic basin was studied integrating sedimentological palynological and geochemical data the succession deposited in a restricted environment in low energy settings recorded three long term variations in bottom water redox conditions our data highlight a direct control of long term climatic changes on oceanic redox cycles phases of cool and dry climate with reduced runoff alternated with periods of warmer and humid conditions with higher runoff and fluvial discharge during warmer and more humid climatic phases the presence of a stable pycnocline hampered deep water renewal efficiency and together with increased nutrients led by enhanced weathering and runoff favoured the onset of anoxic bottom water conditions cooler and drier intervals favoured instead a weaker and less stable pycnocline that coupled with more oligotrophic conditions drove the formation of oxic bottom waters the variation through time in water salinity and temperature influenced both the type of organic matter in the surface waters as well as its preservation along the water column in fact the extended residence time at the stable pycnocline during the anoxic phases led to the degradation of the organic matter and limited its accumulation at the sediment water interface the waning of the latest ordovician cooling to the silurian greenhouse climate was characterized by the progressive alternation of cool phases and warmer periods the effect of this dynamic climatic instability on the redox state of the baltica epicontinental sea protracted over millions of years up to the middle silurian

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

oxygen isotope variability in conodonts implications for reconstructing palaeozoic palaeoclimates and palaeoceanography conodonts have the potential to elucidate the intricacies of palaeozoic climates especially if 18 o values of single apatitic tooth like elements can be used to map evolving sea surface temperatures and differentiate oceanic water masses their ecological distribution as pelagic and nektobenthic organisms high resolution biostratigraphy and abundance in cambrian triassic rocks qualifies them as potentially robust climate archives previous ion microprobe conodont 18 o studies have proceeded directly to palaeotemperature interpretation without appreciation of inter and intra element variability or post mortem artefacts here ion microprobe analyses of ordovician and silurian conodonts establishes that intra element crown tissue 18 o typically varies by 1 53 of conodonts analysed is normally 2 92 of analyses and rarely varies by 2 4 18 o can vary across elements suggesting a microstructural and or diagenetic control 18 o can vary between species representatives by c 3 18 o of pelagic and nektobenthic taxa can be offset by 2 3 elements processed with formic acid have highly variable 18 o and thermal alteration does affect 18 o conodont ion microprobe 18 o values are comparable with those of bulk methods but utilization of material with no consideration of geological context or processing history may introduce significant artefacts a protocol for future conodont oxygen isotope ion microprobe studies is proposed supplementary material full results of oxygen isotope analyses reported in this paper are available at www geolsoc org uk sup18516

reaction textures in a suite of spinel granulites from the eastern ghats belt india evidence for polymetamorphism a partial petrogenetic grid in the system kfmash and the roles of zno and fe2o3 spinel granulites with or without sapphirine occur as lenses in garnetiferous quartzofeldspathic gneisses leptynites near gokavaram in the eastern ghats belt india spinel granulites are mineralogically heterogeneous and six mineral associations occur in closely spaced domains these are i spinel quartz cordierite ii spinel quartz cordierite garnet orthopyroxene sillimanite iii spinel cordierite orthopyroxene sillimanite iv spinel quartz sapphirine sillimanite garnet v spinel quartz sapphirine garnet and iv rhombohedral fe ti oxide cordierite orthopyroxene sillimanite common to all the associations are a porphyroblastic garnet containing an internal schistosify defined by biotite sillimanite and quartz perthite and plagioclase spinel contains variable amounts of exsolved magnetite and is distinctly zn rich in the sapphirine absent associations x mg in the coexisting phases decreases in the order cordierite biotite sapphirine orthopyroxene spinel garnet fe ti oxides textural criteria and compositional characteristics of the phases document several retrograde mineral reactions which occurred subsequent to prograde dehydration melting reactions involving biotite sillimanite quartz plagioclase and spinel the following retrograde mineral reactions are deduced 1 spinel quartz cordierite 2 spinel quartz garnet sillimanite 3 garnet quartz cordierite orthopyroxene 4 garnet quartz sillimanite cordierite 5 spinel cordierite orthopyroxene sillimanite 6 spinel sillimanite quartz sapphirine 7 spinel sapphirine quartz garnet sillimanite and 8 spinel quartz sapphirine garnet a partial petrogenetic grid for the system feo mgo al 2 o 3 sio 2 k 2 o h 2 o at high fo 2 has been constructed and the effects of zno and fe 2 o 3 on this grid have been explored combining available experimental and natural occurrence data the high fo 2 invariant points in the partial grid have been located in p t space geothermobarometric data and consideration of the deduced mineral reactions in the petrogenetic grid show that the spinel granulites evolved through an anticlockwise p t trajectory reaching peak metamorphic conditions 9 kbar and 950 c followed by near isobaric cooling dt dp 150 c kbar this was superimposed by an event of near isothermal decompression dt dp 15 c kbar the studied spinel granulites therefore preserve relic prograde mineral associations and reaction textures despite being metamorphosed at very high temperatures and bear evidence of polymetamorphism

modeling the early paleozoic long term climatic trend the early paleozoic climate has been described as warm and equable however recent data based on conodont oxygen isotopic composition reveal a large long cooling trend through the ordovician followed by an abrupt cooling during the late ordovician glaciation this long term climate change is associated with a major radiation in the earth life history nonetheless the driving mechanisms for this cooling trend remain unknown carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change however the impact of the plate motion context has not been explored yet although it might have a major impact on atmospheric co 2 levels simulations with a climate model coupled to a biogeochemical model geoclim show that the atmospheric co 2 decreased from more than 20 pal 5600 ppmv in the furongian down to approximately 10 pal 2800 ppmv in the llandovery before rising again in the early devonian we suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large co 2 drawdown that led to the onset of cooler to glacial conditions from the middle ordovician to the llandovery the weathering of fresh volcanic rocks is itself responsible for 33 of the late ordovician atmospheric co 2 decrease the rest being related to the continent motion through the intertropical convergence zone itcz mean annual continental temperature falls by 3 c in the early ordovician reaching 13 5 c during the glacial interval and rises to 16 c in the early devonian

low latitude ordovician to triassic brachiopod habitat temperatures bhts determined from δ18o brachiopod calcite a cold hard look at ice house tropical oceans abstract some researchers have suggested that phanerozoic land based ice sheets extended occasionally into the tropical realm if true the tropical ocean at those times must have been distinctly colder than at the last glacial maximum lgm when northern hemisphere ice sheets did not extend below 38 latitude low latitude ocean temperatures derived from oxygen isotopes of brachiopod shells test this hypothesis by comparing the temperature regime for paleozoic and early mesozoic low latitude settings to the tropical temperature regime of the modern interstadial ocean and to mean temperatures estimated for the tropical ocean at lgm a running mean of ph adjusted brachiopod habitat temperatures bhts shows that paleozoic low latitude oceans were on average cool to cold relative to the modern interstadial tropical ocean at times during pennsylvanian serpukhovian tournaisian and ordovician silurian glaciations these tropical seas were indeed significantly colder on average than at the lgm ice sheets within tropical latitudes can be reasonably predicted at those times abundant and diverse paleozoic brachiopod communities reflect these cool tropical oceans consistent with modern brachiopod ecological preference for colder waters amplified paleozoic temperature oscillations suggest recurring global warming events which episodically drove these cold tropical oceans to temperatures significantly higher than the warmest modern tropical ocean

sapphirine bearing granulites from abu balaram area gujarat state implications for india madagascar connection p t conditions of sapphirine bearing granulites from a new locality from the north eastern part of gujarat range from 8 2 8 8 kbars at t of 950 c the discovery of this uht assemblage in the north western part of the indian shield bordering the aravalli delhi fold belt provides additional evidences for india madagascar connection linking the aravalli malani sequences to similar lithologies in northern madagascar

silurian δ13c stratigraphy a view from north america three positive shifts in 13 c are recognized in well dated carbonate successions in north america and can be confidently correlated with shifts in the baltic region by using conodont and graptolite zonations the 13 c excursions reach distinct peaks of 3 to 4 at the ends of the amorphognathoides and siluricus conodont zones and within the ludensis graptolite zone the three events are associated with the most significant extinction horizons in the silurian and occur near clastic carbonate transitions that mark eustatic or oceanographic changes the magnitudes of the two oldest 13 c peaks compare well with the baltic region however the end siluricus excursion reaches far greater values of as much as 11 in sweden likely reflecting local modification of the global seawater signature

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events

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

silurian conodont based correlations between gotland sweden and saaremaa estonia correlations between the silurian successions of gotland and the east baltic are based on the narrowest stratigraphic units that can be distinguished in both areas in order further to increase precision true ends of ranges in measured sections have been used instead of pooled formational faunas where outcrops and collections permit some previous correlations are confirmed in other cases there are considerable changes both in levels of correlation and in precision the major differences are found mostly when one or more of the major gaps have remained unrecognized and stratigraphic units therefore have been extended and translated upwards or downwards at least some of these gaps decrease in magnitude southwards in estonia gaps at these levels are also noted on gotland although they are small and probably do not extend across the island the patterns through time of deposition during secundo episodes and gaps during primo episodes agree very well with what can be predicted from a model of oceanic and climatic cyclicity