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

inversion of rg waveforms recorded in southern spain abstract group velocity dispersion measurements of r g waves generated either by blasts or by local earthquakes are used to investigate the shallow crustal structure of almeria southern spain in principle the usable frequency range of 250 to 2000 mhz allows determination of structures to depths of about 4 km for this purpose the main operations are a detailed dispersion analysis of high frequency rayleigh waves propagating along very short paths and the inversion of r g wave group velocities a total of 21 seismic events were studied these events had small magnitudes 2 0 to 2 5 approximately and very shallow focal depths about 100 m and were taken from a set of 214 events that occurred in 1991 during a spanish italian seismic experiment the events were recorded at seven single component stations belonging to the regional seismic network of andalucia at approximate distances of between 15 and 57 km from the source these events were grouped into six seismic sources according to specific criteria we used digital filtering techniques providing a significant improvement in signal to noise ratio to determine ray path group velocities and we inverted dispersion data via generalized inversion in order to obtain refined dispersion data we have carried out a further regionalization of group velocities and thus six small subregions have been resolved in group velocities the highest group velocity values from 1 93 to 2 25 km sec 1 correspond to the filabres mountain range which is an area containing materials of the nevado filabride complex of paleozoic and triassic age on the other hand low velocity values between 1 39 and 1 56 km sec 1 correspond to the alhamilla mountain range which belongs to the alpujarride complex and contains conglomerates of the cambrian and tortonian periods the velocities obtained for the neogene quaternary basin of the andarax river with materials of the tortonian and pliocene periods are also very low between 1 35 and 1 68 km sec 1 we inverted the regionalized group velocities in order to obtain the shear velocity structure of the region for depths down to 4 km according to the regional earth models that we obtained we find clear variations in velocity both laterally and vertically for several zones with different composition the filabres mountain range shows high shear velocity values 2 14 to 2 83 km sec 1 in the opposite end we have the andarax basin that presents the lowest shear velocity values consistent with its sedimentary structure 1 56 to 2 55 km sec 1 intermediate shear wave velocities characterize the remaining regions the tabernas sorbas basin the gador mountain range and the volcanic region of nijar cape of gata although the relationship between lateral changes in r g dispersion and geologic structure may not be straightforward in this study we have observed a correlation between those changes and the sharply contrasting geology between adjacent geological formations

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

inversion of rayleigh wave phase and group velocities by simulated annealing abstract a joint inversion of regionalized phase and group velocities of fundamental mode rayleigh waves is performed by means of a simulated annealing scheme which makes use of some thermodynamic analogies based on the gibbs distribution the correlations and the uncertainties on shear wave velocities defining the elastic model are estimated by using a procedure based on the same thermodynamic analogies as a relevant feature the simulated annealing algorithm permits reducing the usual dependence of the solution on the sometimes arbitrary starting model a shortcoming linked to the use of this procedure for solving an inversion problem is the necessity of a previous empirical determination of some parameters controlling the annealing process with this aim a simulation of a joint inversion of synthetic surface wave dispersion data has been performed after that path averaged velocity dispersion curves corresponding to a hercynian area of the iberian peninsula have been inverted to derive two possible crust and upper mantle elastic structures whereas the first structure has been deduced by considering as unknown variables the shear velocities of the stratified earth model the second has been obtained also including the respective layer thicknesses both structures are compared with shear velocity and q models derived by the stochastic inversion procedure in previous studies and from the same rayleigh waves dataset finally advantages and shortcomings of the inversion of phase and group velocity data by means of simulated annealing are discussed

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

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

tomographic images of the iberian subcrustal lithosphere and asthenosphere the s wave velocity structure of the lithopheric mantle and asthenosphere beneath iberia is displayed by means of tomographic images obtained from dispersion data of rayleigh waves propagating across the iberian region we have used long period data recorded at the broadband stations of the network of autonomously recording seismographs nars installed in the iberian peninsula on the occasion of the iberian lithosphere heterogeneity and anisotropy iliha project a total of 64 teleseismic events provided by the iliha array and 143 seismic paths have been studied surface wave dispersion analysis is carried out using various methods from methods for a correct acquisition of data and subsequent two station surface wave velocity measurements to inversion methods for velocity structure and methods for verifying the reliability of the inversion results the phase and group velocity dispersion curves of the fundamental mode rayleigh waves are the basic information from which we have obtained several earth models represented by shear wave velocity distributions with depth using these inversion results we display the most conspicuous features of the velocity stnicture of the iberian lithosphere asthenosphere system and propose a new regional model the iberian lithosphere asthenosphere ila model for the deep structure of the iberian peninsula down to a depth of 200 km we use a representation technique based on an iterative laplacian interpolation method for obtaining tomographic images of the subcrustal lithosphere and asthenosphere of iberia we find significant lateral velocity variations in the peninsula though these differences vary with depth interval a low velocity channel in the lithosphere 41 51 km appears with nonuniform velocity structure in contrast at the greatest lithospheric depths 51 81 km almost the entire peninsula shows a rather uniform velocity structure the asthenosphere 81 181 km is clearly a nonhomogeneous gross layer both laterally and with depth the relatively higher velocities span the shallower depths within the asthenosphere whereas the lower ones span the deeper part

a deep seismic sounding investigation of lithospheric heterogeneity and anisotropy beneath the iberian peninsula abstract the dimensions of the iberian peninsula the facility of firing large charges in the surrounding waters the well known and relatively uniform geology and prior knowledge of the crustal structure resulted in it being chosen as a study area for the investigation of the seismic structure of the lower lithosphere via refraction wide angle reflection seismic profiling the iberian lithosphere heterogeneity and anisotropy experiment iliha with a star shaped arrangement of six long range dss profiles was carried out in october 1989 the models derived from a first interpretation of the recorded data are presented three crustal profiles cover the same western and central part of the hercynian massif as the mantle profiles the resulting interpretations all include a middle as well as a lower crustal layer above the mantle the velocities of the layers in all three models are similar however the layer depths vary beneath the profiles interpretation of the mantle derived data suggests a layered lower lithosphere one reversed line and an intersecting unreversed line indicate the layering penetrates to at least 90 km depth the homogeneity of these layers contrasts strongly with the heterogeneous hercynian surface geology velocities derived from reflected data from the deep layers suggest the constituent materials are either anisotropic or that the layers suffer a slight regional dip

geometrical spreading and attenuation of lg waves a comparison between western anatolia turkey and southern spain abstract we compared the attenuation and geometrical spreading coefficient of lg waves for crustal paths in western anatolia and southern spain at source receiver distances between 80 and 400 km we used the coda normalization method which is based on the elimination of the source and site effects from observed spectra of lg waves q for lg waves in the crust was determined as a function of frequency in the range 1 5 10 0 hz for western anatolia and in the range 1 5 12 hz for southern spain we found that qlg shows a strong frequency dependence in both regions in western anatolia the frequency dependence of qlg is qlg f 82 f1 0 and the geometrical spreading coefficient ranges from 0 6 to 0 9 we found qlg 83 f0 88 and a geometrical spreading coefficient close to 0 5 in southern spain comparing our results with coda q values obtained in both regions qlg qc are similar qlg is close to qi in all frequency bands we interpret the geometrical spreading coefficient higher than 0 5 for western anatolia as due to a strong layered and heterogeneous crustal structure

shallow structure of part of northwestern iberia from short period rayleigh wave observations propagation of 0 2 2 5 s short period rayleigh waves across a part of northwestern iberia is investigated analysis of a seismic profile line with recorded seismograms up to distances of 70 km from the shot point provides group velocities and attenuation coefficients of the fundamental mode first higher mode group velocity has been obtained for only one of the studied cases the studied region has been divided into two subrogions the northwestern and the southeastern side from the shot point inversion procedures applied to the velocity and attenuation data yield the shear velocity and the q 1 models up to a depth of about 1 5 km for the nw and 2 4 for the se side from the shear wave obtained models corresponding to the nw and to the se sides it is possible to infer the existence of some degree of lateral variation of s velocity the nw side presents velocities varying between 1 8 km s 1 at the surface and 3 3 km s 1 at 1 2 km depth the velocities in the se model for the same depths are 2 2 and 3 4 km s 1 respectively using only the fist six traces of the se side it is possible to infer a low velocity region located between 0 4 km and 0 7 km depth it seems that the velocities are higher in the complex geological region of the se side than in the nw one which is in agreement with p velocity models found by others from the tentative anelastic attenuation coefficients q 1 models have been inferred for both sides of the seismic refraction profile although the obtained q 1 models are somehow different in shape the maximum q 1 value is about 0 05 in both cases

lenticular shale fabrics resulting from intermittent erosion of water rich muds interpreting the rock record in the light of recent flume experiments lenticular lamination is a fabric that is known from shales of all ages but its origin and paleoenvironmental significance is poorly understood we have successfully reproduced this fabric in flume experiments beds of water rich mud were eroded in a flume and yielded sub millimeter to centimeter size fragments that can be transported in bedload for distances of ten kilometers or more upon redeposition and compaction these deposits have the same textural qualities as lenticular laminated shales from the rock record although accumulation of fecal pellets or abundant burrow tubes in a shale may produce comparable fabrics upon compaction these can be distinguished from erosion produced lenticular lamination via petrographic criteria lenticular lamination in shales that is due to deposition of water rich mud fragments most likely records intermittent erosion and transport of surficial muds by currents

integrated lower silurian chitinozoan and graptolite biostratigraphy of buttington brick pit wales the chitinozoans recovered from six graptolitic horizons in the buttington brick pit wales are described a correlation between the upper llandovery and lower wenlock chitinozoan and graptolite biostratigraphical schemes is presented these data are compared with the recently refined chitinozoan biostratigraphical scheme in the nearby banwy river section wales chitinozoans from definite turriculatus graptolite biozone strata are described for the first time from the uk chitinozoans indicative of the eisenackitina dolioliformis biozone occur in the turriculatus biozone although the base of the former biozone probably occurs at a lower stratigraphical level the angochitina longicollis chitinozoan biozone has been identified in middle or probably upper spiralis graptolite biozone strata questionably identified a longicollis occurs lower in strata assigned to the middle spiralis biozone where it occurs with the short ranging conochitina mathrafalensis and belonechitina cavei this may indicate that 1 the base of the longicollis biozone occurs slightly lower in the telychian in buttington brick pit than in the more basinal banwy river section 2 the specimen assigned to a longicollis represents an undescribed species or 3 c mathrafalensis and b cavei occur higher in buttington brick pit than in the banwy river section the absence of a longicollis in the turriculatus and crispus graptolite biozones in buttington highlights the need to re examine the specimens assigned to a longicollis from these levels in estonia sweden and norway chitinozoans indicative of the margachitina margaritana chitinozoan biozone occur in murchisoni biozone strata the base of the former biozone has not been observed at buttington and the occurrence of conochitina flamma indicates that the highest graptolite bed examined correlates with a level high in the murchisoni graptolite biozone

whole rock and discrete pyrite geochemistry as complementary tracers of ancient ocean chemistry an example from the neoproterozoic doushantuo formation china the trace element content of pyrite is a recently developed proxy for metal abundance in paleo oceans previous studies have shown that the results broadly match those of whole rock studies through geologic time however no detailed study has evaluated the more traditional proxies for ocean chemistry for comparison to pyrite trace element data from the same samples in this study we compare pyrite trace element data from 14 samples from the wuhe section of the ediacaran age doushantuo formation south china measured by laser ablation inductively coupled plasma mass spectrometry with new and existing whole rock trace element concentrations total organic carbon fe mineral speciation s isotope ratios and pyrite textural relationships this approach allows for comparison of data for individual trace elements within the broader environmental context defined by the other chemical parameters the results for discrete pyrite analyses show that several chalcophile and siderophile elements ag sb se pb cd te bi mo ni and au vary among the samples with patterns that mirror those of the independent whole rock data a comparison with existing databases for sedimentary and hydrothermal pyrite allows us to discriminate between signatures of changing ocean conditions and those of known hydrothermal sources in the case of the wuhe samples the observed patterns for trace element variation point to primary marine controls rather than higher temperature processes specifically our new data are consistent with previous arguments for pulses of redox sensitive trace elements interpreted to be due to marine oxygenation against a backdrop of mostly o2 poor conditions in the ediacaran ocean with important implications for the availability of bioessential elements the agreement between the pyrite and whole rock data supports the use of trace element content of pyrite as a tracer of ocean chemistry in ways that complement existing approaches while also opening additional windows of opportunity for example unlike the potential vulnerability of whole rock data to secondary alteration the pyrite record may survive greenschist facies metamorphism furthermore early formed pyrite can be identified through textural relationships as a proxy of primary marine chemistry even in the presence of hydrothermal overprints on whole rock chemistry via secondary fluids finally pyrite analyses may allow for the possibility of more quantitative interpretations of the ancient ocean once the elemental partitioning between the mineral and host fluids are better constrained collectively these advances can greatly increase the number of basins that may be investigated for early ocean chemistry especially those of precambrian age

shear velocity structure beneath the iberian massif from broadband rayleigh wave data abstract up to a few years ago dispersion analyses of surface waves across the iberian peninsula and adjacent zones were based on analog data recorded at the long period iberian stations the installation of the nars array on iberian territory for a period of one year has provided a greater station density than was previously available with the very few permanent long period seismological stations installed on the peninsula the nars array also provided quality digital records and increased the path coverage for two station surface wave velocity measurements fundamental mode rayleigh waves recorded at broadband stations belonging to this array have been analyzed to produce phase and group velocity dispersion curves for the period range 10 90 s with the dataset now available the elastic structure beneath the iberian massif has been investigated in terms of the shear velocity distribution as a function of depth time variable filtering is employed to remove higher mode interference efficiently and to improve isolation of the fundamental mode rayleigh wave from the seismograms multiple filtering is then used to compute group velocities at each station the interstation rayleigh wave group velocity can thus be easily calculated frequency domain wiener deconvolution is used to determine the interstation phase velocity we perform inversion of velocity dispersion data containing both rayleigh wave phase velocities and group velocities according to the generalized inversion theory by means of the stochastic inverse operator the theoretical models for the lower crust and uppermost mantle beneath the iberian massif obtained by joint inversion show a continental lithosphere with a thickness of 81 km the crustal and subcrustal velocities are greater than in other areas of the iberian peninsula the asthenosphere appears as a layer 100 km thick defined by very low velocities when compared with the rest of the peninsular area both the lithosphere and the asthenosphere exhibit a low velocity channel in the lithosphere a subcrustal low velocity channel has velocity constrained between 4 33 and 4 62 km s 1 in the asthenosphere the low velocity channel is constrained between 4 13 and 4 36 km s 1

lateral variation of the crust in the iberian peninsula new evidence from the betic cordillera abstract new results from a seismic refraction wide angle reflection survey carried out in the betic cordillera in autumn 1989 contribute to a better picture of its deep structure one nw se profile cuts across the iberian massif the external and internal betics the structure of the crust in the iberian massif shows characteristics similar to those found in previous experiments the lower crust is found as a distinct layer 12 km thick with an average velocity of 6 8 km s 1 the moho is found at about 35 km depth this structure extends southeastward until a 3 4 km upwelling of the moho about 30 km north of the present day surface boundary between the external and internal betic units further southeast the moho deepens to 38 km and the lower crust is no longer seismically detected the absence of differentiated lower crust beneath part of the external betics may be related with the mesozoic rifting of the south iberian passive margin instead this absence under the internal betics may be caused by rifting in conjunction with the collisional evolution of the orogen a wnw ese profile lying in the internal betics shows the presence of a prominent reflector at 10 12 km depth this seems to be a widespread feature in the internal betics and may be interpreted as a detachment surface the moho is found at 38 km depth rising strongly in the easternmost betics seismic data suggest a thin crust in the offshore area southwest of malaga probably containing a massive zone of high velocity rocks which is also supported by available geophysical and geological data

crustal shear wave velocity models retrieved from rayleigh wave dispersion data in northeastern north america on 23 june 2010 a moderate earthquake with mw 5 2 occurred near the town of val des bois quebec canada 60 km northeast of ottawa ontario the earthquake generated excellent crustal rayleigh wave records we divided the 54 seismic stations that recorded clear rayleigh wave trains into 14 groups by station azimuth in each group we measured the rayleigh wave dispersion data station by station and formed one dispersion data file for the inversion in this way we ob tained 14 crustal velocity models around the epicenter we compared all 14 models and found that there are low velocity layers in the top 10 km on the north side of the ottawa bonnechere graben based on model similarity we formed one model for the north side by averaging the north side models and another model for the south side by averaging the south side models the separation of the north side and south side mod els appears to follow the ottawa bonnechere graben in the top 10 km the velocities in the south model are obviously slower than those in the north model

the crustal structure in iberia inferred from p wave coda abstract the crustal velocity structure at four locations in the iberian peninsula is inferred from p wave coda recorded by broad band stations the crustal responses of the stations are derived from the p wave coda of deep teleseismic events and the most pronounced features of the p and s velocity structure are obtained by waveform modeling of the vertical and radial component of these responses the method is applied to data of nars stations in toledo alicante and granada in spain and manteigas in portugal the optimum models of the waveform inversions are carefully evaluated and compared to results of deep seismic sounding experiments in general the depths of large velocity gradients are well resolved but the actual values of the seismic velocities are less constrained

teleseismic p residual study in the iberian peninsula abstract teleseismic p wave residuals cleared of the effects of various source regions and deep mantle paths reflect the composition and structure of the lithosphere and asthenosphere beneath seismological stations we use a method with which we compute two components of the relative residuals for each station namely a directionally independent average and azimuth incidence angle dependent terms and also the ach 3 d inversion method the representative averages show consistent variations over the iberian peninsula with a relatively steep gradient in some areas the iberian massif and the betic cordillera these changes attain values around 1 s over a distance of about 200 km the lateral variations of these residual averages are transformed into a tentative model of the lithosphere thickness a thick lithosphere is observed beneath the iberian massif the pyrenees and beneath part of the betic cordillera waves from events at different teleseismic distances and various azimuths provide consistent residual patterns and a systematic directional dependence of the relative residuals especially at stations in the betic cordillera

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

processing and inversion of long period surface wave data collected in the iberian peninsula summary several filtering techniques have been used to remove the effects of multipathing and modal contamination and to isolate the fundamental mode from rayleigh wavetrains group velocity data are obtained by means of the multiple filter technique a time variable filter has allowed the influence of noise as well as the interference produced by higher modes to be removed multiple filtering is then used again to compute group velocities at each station the interstation group velocity for the fundamental mode rayleigh wave is estimated according to the velocities at two stations frequency domain wiener deconvolution is used to compute the phase velocity between two stations the well known three station method is applied to correct the distances travelled by the waves across the array and therefore to determine interstation phase and group velocities in a more accurate manner on the other hand lateral refraction at the atlantic continental edge of the peninsula is also studied phase velocities are corrected for the anelastic effect inversion of the interstation rayleigh wave phase velocities is then made in accordance with generalized inversion theory to obtain theoretical 2 d layered earth models in this paper these methods are applied to rayleigh waves generated by teleseismic events propagating across the iberian peninsula and recorded at wwssn stations as a consequence new and principal features for the iberian lithosphere asthenosphere system are obtained a very interesting feature of the the iberian lithosphere was found a low velocity layer directly under the moho between 39 and 64 km depth with shear velocities ranging from 4 12 to 4 37 km s 1 the iberian asthenosphere which lies between 100 and 180km depth is not an homogeneous layer and shows a negative velocity gradient from top to bottom together with a sudden increase in shear velocity beneath the low velocity zone

a laminated hemipelagic facies from the wenlock and ludlow of the welsh basin a hemipelagic fades from the silurian succession in wales consists of finely laminated dominantly silt grade sediment it can be interbedded with sand or mud grade turbidites or can occur as an independent facies its occurrence and character was controlled mainly by low bottom water oxicity the pelagic fauna of the laminated hemipelagite supports this view many of the silt grade particles have an aggregate structure and probably represent the faecal pellets of oceanic zooplankton the fine lamination is thought to have formed by alternations of planktonic blooms and increased discharges of silt into the basin by comparison with modern examples this alternation was likely to have been annual or seasonal

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

numerical model for local scour under offshore pipelines a numerical model based on potential flow theory is proposed for simulating the equilibrium scour hole formed by unidirectional flow underneath offshore pipelines the model employs a finite difference method to solve the laplace equation in terms of velocity potential in a curvilinear coordinate system a boundary adjustment technique based on the newton raphson method is used to calculate the free boundary formed by the eroded seabed by means of the equilibrium of all forces acting on a sediment particle on a sloping bed because the solution of flow field and adjustment of the seabed topography are carried out in an iterative manner the model takes into account the interactions between the flow pipe and the seabed the comparison of the present model with empirical formulas on the prediction of the maximum scour depth indicates that the present model is useful for approximate estimation of scour depth at a pipeline on the seabed for the case of clear water scour

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

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

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

elastic anelastic regional structures for the iberian peninsula obtained from a rayleigh wave tomography and a causal uncoupled inversion the elastic and anelastic structure of the lithosphere and asthenosphere of the iberian peninsula is derived by means of tomographic techniques applied to local phase and group velocities and local attenuation coefficients of rayleigh wave fundamental mode the database consists of surface wavetrains recorded at the broadband stations located in the iberian peninsula on the occasion of the iliha project path averaged phase and group velocities and attenuation coefficients were previously obtained by standard filtering techniques of surface wavetrains and subsequently local dispersion curves were computed according to the yanovskaya ditmar formulation first a principal component analysis pca and the average linkage al clustering algorithm are applied to these local values in order to classify the iberian peninsula in several rather homogeneous domains from the viewpoint of the similarity of the corresponding local dispersion curves without previous seismotectonic constraints second averaged phase and group velocities and attenuation coefficients representing each homogeneous region are used to derive the respective elastic and anelastic models of the lithosphere and asthenosphere this purpose is achieved by using the uncoupled causal inversion of phase and group velocities and attenuation coefficients the main features of the homogeneous regions are discussed by taking as reference the hercynic alpine and neogene domains of the iberian peninsula and two questions affecting the reliability of the elastic anelastic models are revised first the coherence of the shear velocity and q 1 models obtained by causal uncoupled inversion for each region is analysed second the influence of the causal phase and group velocities on the shear velocity models is evaluated by comparing elastic and anelastic models derived from causal uncoupled inversion with those deduced from non causal inversion

shear wave velocity tomography of the lithosphere asthenosphere system beneath the iranian plateau we conducted a tomographic inversion of rayleigh wave dispersion to obtain 2d phase and group velocity tomographic images in the 10 100 s period range and shear wave velocity structures for the iranian plateau for this purpose the fun damental mode of rayleigh waves recorded along 1586 paths by 29 broadband sta tions was identified by applying the frequency time analysis ftan to each epicenter station path which simultaneously satisfies the two station method condi tions the fundamental modes identified by ftan have been used to determine the path average interstation phase and group velocities at selected periods with this pro cedure 243 group and phase velocity dispersion curves were processed to obtain to mographic maps by applying the yanovskaya ditmar formulation for periods in the 10 100 s range averaged dispersion curves of phase and group velocities which represent six rather homogeneous regions are computed finally we used a fully non linear inversion procedure to derive tomographic images of the elastic structure of the lithosphere and asthenosphere of the six main structural and seismotectonic features of the iranian plateau the dense path coverage in the iranian plateau permits us to pro duce images that have substantially higher lateral resolution compared to images cur rently available from global and regional group velocity studies tomographic maps at high frequencies are well correlated with the upper crust structure especially with sediment layers thickness estimated shear wave velocity structures are well corre lated with the shield like lithosphere structure in zagros a low velocity zone lvz is not detected in alborz or the south caspian basin which can imply thrusting of the oceanic crust of the southern caspian sea under the alborz to the south lvzs are derived for the region east of iran central iran and kopeh dagh

crust and upper mantle structure of the central iberian meseta spain summary quarry blasts recorded along three lines on the central iberian meseta are used in an attempt to interpret the crustal structure the results of the interpretation of the data together with published surface wave and earthquake data suggest a layered structure of the crust having the following features the basement in some areas covered by up to 4 km of sediments has a p velocity of 6 1 km s 1 a low velocity layer between 7 and 11 km depth seems to exist on the basis of both p and s interpretation of seismic data a thick middle crust of 12 km has a p velocity of 6 4 km s 1 and overlies a lower crust with a mean p velocity of 6 9 km s 1 and a possible slight negative gradient the mean vp vs ratio for the crust is about 1 75 the moho is reached at about 31 km depth and consists of a transition zone at least 1 5 km thick the p velocity of the upper mantle is close to 8 1 km s 1 and the s velocity about 4 5 km s 1 which gives a vp vs ratio of 1 8 for the uppermost mantle a tentative petrological interpretation of the velocities and composition of the layers is given

deep structure of the iberian peninsula determined by rayleigh wave velocity inversion summary a rigorous study of velocity dispersion of surface waves generated by teleseismic events propagating across the iberian peninsula and traversing main geological units has been carried out from a set of selected analogue data as digital records have only become available recently dispersed seismic signals have been obtained over a period of 16 years between 1967 and 1982 at the five iberian stations having long period instruments in our study we have considered many earthquakes thus obtaining a fairly good path coverage of most of the peninsula for two station rayleigh wave velocity measurements in all cases the approach azimuths of the wavefronts were carefully checked several digital filtering techniques have been employed to remove the effects of multipathing and modal contamination and to isolate the fundamental mode from rayleigh wavetrains thus we have obtained good estimates for both phase and group velocities a time variable filter has reduced the influence of noise and removed higher mode interference multiple filtering is then used to compute group velocity frequency domain wiener deconvolution is used to compute the interstation phase velocity the determined average rayleigh wave velocities reveal differences in the propagation conditions of the seismic energy across the peninsula a mapping of velocities for various periods of reference together with a mapping of errors in velocity are the basis for obtaining the rayleigh wave velocity distribution in the peninsula theoretical 2 d layered earth models are obtained by joint inversion of phase and group velocity dispersion curves using the stochastic inverse operator in our inversion scheme we use velocities corrected for anelastic effects finally a 3 d mapping of s velocity is performed this study shows important regional features of the deep structure of iberia we see small lateral inhomogeneities and also two low velocity layers one with shear velocities usually ranging from 4 23 to 4 31 km s 1 directly under the moho and another the asthenosphere with a negative velocity gradient for depths between 81 and 181 km terminated at the bottom by a sharp discontinuity

first two station rayleigh wave velocity measurements for the northern iberian region abstract up to now dispersion analysis of surface waves across the iberian peninsula and adjacent zones has been based on analog data recorded at the long period iberian stations also the northern region of the peninsula has never been investigated due to the lack of seismological stations with the iliha data set now available it is possible to investigate the northern part of iberia from quality digital records to efficiently remove higher mode interference and to improve isolation of the fundamental mode rayleigh wave from the seismograms time variable filtering is employed once the signal is filtered multiple filtering is then used to compute group velocities at each station the interstation rayleigh wave group velocity can thus be easily calculated frequency domain wiener deconvolution is used to determine the interstation phase velocity we carried out inversion of velocity dispersion data containing both rayleigh wave phase velocities and group velocities according to the generalized inversion theory by means of the stochastic inverse operator the theoretical 2 d earth models determined by joint inversion allow us to obtain for the first time the distribution of the shear wave velocity both laterally and with depth for the northern iberian region and to emphasize the main features of the crust mantle structure of this area

graptolite biozonation of the wenlock series silurian of the builth wells district central wales the graptolite biozonation of the wenlock series in the builth wells district is reassessed the graptolite fauna is dominated by relatively few but variable species of monograptus s s pristiograptus and monoclimacis these species are mostly long ranging and are of limited biostratigraphical use biozonation is based largely on the relatively rare cyrtograptids the lower wenlock series of the district comprises the widely recognized centrifugus murchisoni and riccartonensis biozones the middle to upper wenlock biostratigraphy differs from that previously recognized in the uk above the riccartonensis biozone dubius rigidus and lundgreni biozones are distinguished separate flexilis and ellesae biozones are not recognized monograptus flexilis appears earlier than then largely co exists with cyrtograptus rigidus s l while cyrtograptus ellesae appears later than cyrtograptus lundgreni the uppermost wenlock nassa ludensis biozone is sparsely fossiliferous its subdivision has not proved possible this revised biostratigraphical scheme for the wenlock series of the builth wells district suggests that the sheinwoodian homerian boundary as defined in the welsh borderland might occur within rather than at the base of the lundgreni biozone however difficulties in correlation with other wenlock sequences particularly overseas suggest significant biofacies control with the wenlock seas of the builth wells district appearing to have been periodically inhospitable to graptolites

graptolite and conodont biostratigraphy of the upper telychian lower sheinwoodian llandovery wenlock strata jabalon river section corral de calatrava central spain a graptolite biostratigraphy is erected for the upper telychian upper crenulata biozone to lower sheinwoodian riccartonensis or dubius biozone strata of the jabalon river section spain two unconformities are recognized in the section one between the lapworthi and murchisoni biozones the other between the murchisoni and riccartonensis biozones these unconformities coincide with intervals of lowered eustatic sea level graptolite assemblages include both cosmopolitan taxa and some which have been recorded previously from morocco and or other spanish sections at some stratigraphical levels pristiograptus or euroclimacis species are abundant monoclimacis streptograptus and mediograptus species are generally uncommon conodonts were examined from the upper spiralis through to lower murchisoni biozone the occurrences of pterospathodus amorphognathoides are consistent with the species known range elsewhere four new graptolite species are described euroclimacis jabalonensis e hamata monoclimacis flexa and stimulograptus pradoi

pyritization of trace metals in anoxic marine sediments the pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as h2s reactive fe pyrite content and salinity in controlling this process the areas studied included anoxic sulfidic sediments baffin bay a hypersaline coastal lagoon from texas and green canyon a hemipelagic oil seepage area anoxic nonsulfidic sediments with high sedimentation rates atchafalaya bay mississippi delta system hemipelagic anoxic nonsulfidic sediments with low sedimentation rates gulf of mexico shelf and slope and orca basin an euxinic hypersaline basin and organic rich marsh sediments atchafalaya bay results indicate that the degree of trace metal pyritization dtmp of all trace metals except cd increased with increasing degree of pyritization dop irrespective of the type of sedimentary environment involved however for as hg and mo the dtmpdop values were generally above the 1 1 ratio line whereas the transition metals co cu mn and ni displayed a close to linear increase in dtmp with dop and moderate incorporation into pyrite chromium and the class b metals pb and zn were also gradually incorporated into the pyrite phase but without reaching the dtmp levels exhibited by the transition metals cadmium was not incorporated to a significant extent these results are consistent with the chemical attributes of these different classes of trace elements availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of co cr cu and ni into pyrite in anoxicsulfidic fe poor and h2s rich environments

high resolution early paleozoic ordovician silurian time scales for much of the geologic time scale resolving power appears to be limited by the duration of biostratigraphic zones and subzones yet these zones exploit the appearance and disappearance events of only a small fraction of the species they span computer algorithms can sequence an order of magnitude more events and make explicit the uncertainties that arise when calibrating the resulting time line an illustrative case history builds an ordovician and silurian time scale from the geologic record of the entire graptolite clade over 1900 species from more than 400 localities and sections worldwide the same approach can be applied to any stratigraphic interval with detailed biostratigraphic observations it provides the foundation for time scales and paleobiologic time lines using logic similar to graphic correlation optimizing algorithms search for a composite sequence of species first and last appearance events that minimizes the implied shortcomings of all the field observations the algorithms minimize the number of species coexistences that are implied but never observed and the net adjustments needed to bring local range charts into agreement with a single composite sequence of events after total section thicknesses have been rescaled to help normalize for variation in depositional rate mean stratigraphic thicknesses are used to scale the intervals between adjacent events in the composite sequence the resulting scaled composite sequence is converted to a relative geologic time scale by identifying stage and zone boundaries within the sequence of graptolite events this relative scale is in turn calibrated by dated volcanic ash beds that were incorporated in the search for the optimal sequence these dated events are also used to test for linearity of the scaled composite the final time scale has a potential resolving power of 0 02 0 1 m y more than ten times better than can be achieved by traditional zones graptolite zones vary widely in duration from as short as 0 1 m y to nearly 5 0 m y the mean duration of zones or zonal groupings calibrated here is 1 44 m y in the ordovician and 0 91 m y in the silurian the average uncertainty in locating zone boundaries in a single composite sequence is about one fourth of the mean zone duration the variance resulting from differences in time scales developed from differing numbers of field observations and in response to changes in the optimization criteria gives a very conservative measure of the overall robustness of the method these differences indicate an average uncertainty in the age of graptolite zone boundaries that is more nearly equal to the mean zone duration for zone duration the mean uncertainty amounts to about one half of the length of an average zone

graptoloid evolutionary rates track ordovician silurian global climate change graptoloid evolutionary dynamics show a marked contrast from the ordovician to the silurian subdued extinction and origination rates during the ordovician give way during the late katian to rates that were highly volatile and of higher mean value through the silurian reflecting the significantly shorter lifespan of silurian species these patterns are revealed in high resolution rate curves derived from the conop constrained optimization scaled and calibrated global composite sequence of 2094 graptoloid species the end ordovician mass depletion was driven primarily by an elevated extinction rate which lasted for c 1 2 ma with two main spikes during the hirnantian the early silurian recovery although initiated by a peak in origination rate was maintained by a complex interplay of origination and extinction rates with both rates rising and falling sharply the global 13 c curve echoes the graptoloid evolutionary rates pattern the prominent and well known positive isotope excursions during the late ordovician and silurian lie on or close to times of sharp decline in graptoloid species richness commonly associated with extinction rate spikes the graptoloid and isotope data point to a relatively steady marine environment in the ordovician with mainly background extinction rates changing during the katian to a more volatile climatic regime that prevailed through the silurian with several sharp extinction episodes triggered by environmental crises the correlation of graptoloid species diversity with isotopic ratios was positive in the ordovician and negative in the silurian suggesting different causal linkages throughout the history of the graptoloid clade all major depletions in species richness except for one were caused by elevated extinction rate rather than decreased origination rate

a test of the great circle approximation in the analysis of surface waves a new way of reconstructing wave fronts is de veloped for testing the validity of the great circle approxima tion in dispersion analysis of surface waves phase and time delays between pairs of stations of an array are used to re construct the wave fronts or surfaces of equal phase this techque is applied to fundamental mode rayleigh waves recorded at the nars stations dudlag the iliha iberian lithosphere heterogeneity and anisotropy project the re sulting wave fronts are compared with the theoretical ones i e calculated assuming that the waves have traveled along the great circle path with a constant phase velocity and the angle between both fronts is calculated waves traveling mosfiy through a laterally homogeneous oceanic path before arriving to the iberian peninsula are analyzed for the fre quency range of 10 to 60 mhz these show angles up to 8 degrees between the theoretical wave fronts and the wave fronts reconstructed using the delay data this implies that we make a relative error less than 1 when calculating path averaged phase velocities ignoring the deviation of the ar rival of the wave from the great circle direction

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

lateral variation and frequency dependence of coda q in the southern part of iberia summary a method based on the coda attenuation law q qo f fo v leads to the determination of the lateral variation of coda q in the southern part of the iberian peninsula using seismograms belonging to the seismological network of the cartuja observatory located in granada the lateral variation of qo q value corresponding to a reference frequency fo of 1 hz and its frequency dependence for the 1 to 5hz frequency range are in general in agreement with coda q values for frequencies less than about 1 hz previously determined in the region under study to determine the coda q values analytical functions have been used to fit the magnification curves of the vertical component short period seismographs belonging to the cartuja network the problem is solved by using least squares techniques and non linear inversion the determined coda q values and its frequency dependence correlate well with several known geophysical parameters in the southern part of the iberian peninsula

the ichnogenus teichichnus seilacher 1955 the ichnotaxonomy of the phanerozoic trace fossil teichichnus seilacher 1955 is revised and its ethology producers and environmental affiliation are re evaluated newly studied material reveals that teichichnus is a sub vertical spreite burrow with an arcuate shape containing stacked laminae and a passively filled terminal causative burrow funnel like extension of the terminal causative burrow and wall ornamentation may occur high variation of burrow morphology is common and has resulted in the erection of 18 ichnospecies of which only four are herein regarded as valid teichichnus rectus t zigzag t patens and t duplex these ichnospecies are characterised by their overall burrow shape spreite morphology and more subordinate occurrence of branching and sculpture as recommended ichnotaxobases combined modes of feeding become evident including deposit and suspension feeding suggesting that teichichnus is a dwelling trace rather than a feeding trace aside from the classical interpretation of polychaetes as producers many features accord with an interpretation of dwelling echiurans and holothurians this interpretation may apply for many post palaeozoic t rectus and t zigzag whereas t duplex and partly t patens are likely produced by arthropods i e trilobites and crustaceans bivalves are also considered for producing teichichnus like traces teichichnus is a marine trace fossil with predominance in shallow marine environments low diversity and high density occurrences of teichichnus characterise marginal marine paralic environments with reduced salinity i e brackish water and oxygen i e dysoxia whereas in shoreface and offshore settings teichichnus occurs in association with many other ichnotaxa and in low density deep marine occurrences of teichichnus are comparably sparse and isolated records may indicate dysoxic bottom conditions

the middle rhuddanian lower silurian hot shale of north africa and arabia an atypical hydrocarbon source rock an important hydrocarbon source rock a hot shale has been identified from the middle rhuddanian llandovery lower silurian of a number of localities in north africa and arabia it is unusual in having been deposited during a eustatic fall in sea level evidence for this sea level fall in libya and jordan is manifested in a wide variety of palynological proxy data including reduced acritarch and chitinozoan total abundance reduced acritarch diversity and changes in the relative abundances of various environmentally sensitive acritarch morphogroups and from the presence of a minor positive 13corg excursion at the same stratigraphical level as the hot shale the environment of deposition was anoxic before during and after deposition of the hot shale except for some very brief incursions of more oxygenated water that enabled the development of a very limited burrowing benthos and graptolite preservation as three dimensional pyrite internal moulds these incursions occurred just before and during deposition of the hot shale and are interpreted as resulting from the influx of sediment laden oxygenated currents from shallower water environments enhanced organic matter preservation within the hot shale is attributed to greater productivity resulting from increased nutrient input particularly from wind blown dust more rapid descent through the water column of organic matter in post bloom marine snow macroaggregates development of geochemically sealing microbial mats at the sea floor and more rapid burial of organic material than had occurred earlier in the rhuddanian

anelastic structure of the iberian peninsula obtained from an automated regionalization algorithm and stochastic inversion abstract an automated regionalization of a reduced domain of the earth s surface without a priori seismotectonic information and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis a clustering procedure and the stochastic inversion algorithm to anelastic attenuation coefficients of rayleigh waves our database is formed by path averaged attenuation coefficients derived from fundamental mode rayleigh waves crossing the iberian peninsula the wavetrains were recorded at the broad band stations installed some years ago in the iberian peninsula for the iliha project before the sequential application of the algorithms the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10 120 s period range all these maps were obtained by applying yanovskaya s formulation for laterally heterogeneous media to the set of path averaged attenuation coefficients mentioned above after that we divided the iberian peninsula into six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis and the clustering algorithm we then obtained by stochastic inversion the respective anelastic structure down to a depth of 225 km for the six homogeneous regions the coefficients vary from 1 0 to 2 0 10 3 km 1 and the q 1 structures ranging from 110 10 3 to 40 10 3 suggest for five of the regions an asthenosphere with its upper and lower boundaries close to 80 and 180 km depth respectively a shallow beginning of the asthenosphere is detected for the sixth homogeneous region this range is very similar to the depths deduced from previous tomographic studies of the iberian peninsula based on rayleigh wave phase and group velocities finally correlations obtained between the regions and some seismotectonic characteristics of the peninsula are discussed it is noteworthy that we cannot associate each of the six homogeneous regions with a single hercynian alpine or neogene domain

chitinozoan biostratigraphy of the silurian wenlock ludlow boundary succession of the long mountain powys wales systematic collecting through the upper wenlock upper homerian and lower ludlow gorstian and lowermost ludfordian silurian rock succession of the long mountain powys wales identifies some 48 chitinozoan species that distinguish four biozones two subzones and an interregnum consideration of the chitinozoan biozones together with those of the graptolites enables a local three fold subdivision of the late homerian lundgreni graptolite biozone and the distinction of lower and upper intervals for the gorstian incipiens graptolite biozone the base of the ludlow series in the long mountain more or less equates to the base of the cingulochitina acme chitinozoan biozone although no key chitinozoan first or last appearance datums are associated with the series boundary itself the new graptolite chitinozoan biozonation allows enhanced correlation between upper wenlock and lower ludlow sedimentary deposits of the lower palaeozoic welsh depositional basin and those of the palaeo shelf in the stratotype wenlock and ludlow areas of shropshire chitinozoans seem affected by the phenomena that caused the late wenlock mulde extinction in graptolites but with the final disappearance of 9 species and re appearance of 11 species following an interval of overall low diversity they seem to have suffered less severely than their macro zooplanktonic contemporaries

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

u pb zircon age constraints on the timing and duration of wenlock silurian paleocommunity collapse and recovery during the big crisis high precision isotope dilution u pb zircon dating was conducted on three volcanic ash fall bentonite samples from the swedish island of gotland and on a fourth bentonite from the west midlands england zircons from the ireviken grotlingbo djupvik gotland and wren s nest hill 15 west midlands bentonites yielded weighted mean pb 206 u 238 ages of 431 83 0 23 0 67 ma 428 45 035 0 73 ma 428 06 0 2110 66 ma and 427 86 032 0 71 ma respectively analytical total uncertainties these biostratigraphically well controlled age dates effectively bracket the wenlock epoch of the silurian period and provide control for the duration of one of the major paleozoic biotic events and associated perturbations to the global carbon cycle the big crisis or lundgreni event graptolites the niulde event conodonts the mulde excursion carbon isotopes these new data suggest an older and shorter duration for the recalibration of the wenlock series and demonstrate that the cascade of biological and chemical events that took place during the big crisis happened on time scales of tens to hundreds of thousands of years less

improvements in the knowledge of the anelastic structure of the iberian peninsula from seismic data of the iliha project summary attenuation coefficients of rayleigh waves propagating across the iberian peninsula and quality factor models were obtained for different two station great circle paths the data used were long period wave trains contained in the seismograms provided by the iliha array installed in the peninsula in the 1980s more than 300 seismograms were analysed to obtain 14 sets of similar paths consisting of either paths totally included in one of the four pure tectonic regions or mixed paths crossing several regions we used the two station method to determine attenuation parameters r and then an inversion method to derive q 1 models the attenuation coefficients of seismic energy in the 10 120 s range are in the range 2 8 10 3 0 1 10 3 km 1 and correlate well with both previous results obtained from analogue wwssn and most seismogeotectonic characteristics of iberia the q models obtained also agree with a few models for iberia determined earlier and they show significant variations from one region to another as a general feature the values are higher in the active seismotectonic regions typical maximum of 2 6 10 3 km 1 than in the more stable ones typical maximum of 1 4 10 3 moreover the maximum of q 1 in the active regions 145 10 3 120 10 3 is higher than in the stable ones 40 10 3 100 10 3 in general both the regionalized attenuation coefficients of rayleigh waves and the quality factor models exhibit a good agreement with previously published lg wave attenuation and coda q distribution patterns in iberia

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

anelastic rayleigh wave attenuation in the iberian peninsula summary fundamental mode attenuation coefficients of the vertical components of rayleigh waves have been obtained for three different tectonic regions of the iberian peninsula the western part of iberia which is tectonically most stable has the lowest attenuation and the largest quality factor q average q at depths from 0 to 80 km is 126 84 in the tectonically stable part of western iberia and 57 22 in the active part of eastern iberia

paleobiogeography high resolution stratigraphy and the future of paleozoic biostratigraphy fine scale diachroneity of the wenlock silurian conodont kockelella walliseri the wenlock epoch of the silurian period has become one of the chronostratigraphically best constrained intervals of the paleozoic the integration of multiple chronostratigraphic tools such as conodont and graptolite biostratigraphy sequence stratigraphy and delta c 13 carb chemostratigraphy has greatly improved global chronostratigraphic correlation and portions of the wenlock can now be correlated with precision better than 100 kyr additionally such detailed and integrated chronostratigraphy provides an opportunity to evaluate the fidelity of individual chronostratigraphic tools here we use conodont biostratigraphy sequence stratigraphy and carbon isotope delta c 13 carb chemostratigraphy to demonstrate that the conodont kockelella walliseri an important guide fossil for middle and upper sheinwoodian strata lower stage of the wenlock series first appears at least one full stratigraphic sequence lower in laurentia than in baltica rather than serving as a demonstration of the unreliability of conodont biostratigraphy this example serves to demonstrate the promise of high resolution paleozoic stratigraphy the temporal difference between the two first occurrences was likely less than 1 million years and although it is conceptually understood that speciation and colonization must have been non instantaneous events paleozoic paleobiogeographic variability on such short timescales tens to hundreds of kyr traditionally has been ignored or considered to be of little practical importance the expansion of high resolution paleozoic stratigraphy in the future will require robust biostratigraphic zonations that embrace the integration of multiple chronostratigraphic tools as well as the paleobiogeographic variability in ranges that they will inevitably demonstrate in addition a better understanding of the paleobiogeographic migration histories of marine organisms will provide a unique tool for future paleozoic paleoceanography and paleobiology research c 2010 elsevier b v all rights reserved less

a global perturbation to the sulfur cycle during the toarcian oceanic anoxic event abstract the mesozoic era was punctuated by intervals of widespread anoxia within the ocean termed oceanic anoxic events or oaes the chemostratigraphy of these intervals also contains evidence of transient perturbations to many biogeochemically important elemental cycles here we present high resolution sulfur isotope data from three stratigraphic sections spanning the toarcian oceanic anoxic event t oae of the early jurassic all sections show a similar increase in the sulfur isotope ratio of sulfate parallel to an overall positive excursion in carbon isotopes during the oae interval based on forward box modeling the sulfate s isotope excursion can be generated by transiently increasing the burial rate of pyrite in marine sediments likely deposited under euxinic i e anoxic and sulfidic conditions in the water column in addition modeling shows that prolonged recovery of the 34 s of seawater sulfate at least 8 ma after the initial rise associated with the oae was due to the relatively long residence time of sulfate in the jurassic ocean estimates from our modeling put the toarcian marine sulfate concentrations at 4 to 8 mm the similarity of the sulfur isotope records from the north european epicontinental or epeiric sea and tethyan continental margin suggests that local modification of the marine sulfur isotope signal was minimal a point explored with isotope mixing models importantly our results indicate that the sulfur isotope excursion reflects a globally significant perturbation in the sulfur cycle and that pyrite burial in the north european epeiric seaway alone cannot account for the excursion this study along with recent work from other phanerozoic intervals of widespread marine oxygen deficiency confirms that the sulfur cycle can be perturbed significantly by enhanced pyrite burial during periods of prolonged oceanic anoxia euxinia

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

a record of late ordovician to silurian oceanographic events on the margin of baltica based on new carbon isotope data elemental geochemistry and biostratigraphy from two boreholes in central poland abstract new stable isotope data from organic carbon 13corg and inorganic elemental geochemistry data have been generated from upper ordovician to silurian strata in two boreholes in the lublin basin of poland grabowiec 6 and zwierzyniec 1 they have been integrated here with biostratigraphical data from graptolites acritarchs and chitinozoans faunal assemblages from grabowiec 6 indicate that it spans from the katian clingani graptolite biozone to the gorstian scanicus graptolite biozone 13corg values from this section record the sheinwoodian ireviken excursion the homerian mulde excursion and a minor positive shift associated with the lower ludfordian leintwardinensis biozone the second section zwierzyniec 1 spans the sandbian through gorstian nilssoni biozone 13corg values record the hirnantian carbon isotope excursion hice and the ireviken excursion as well elemental geochemistry data is used to recognize subtle changes in provenance and lithology significant increases in the abundance of v and mo are recognized in strata deposited above the ireviken excursion the enrichment of these redox sensitive elements suggests that persistent regional anoxia and euxinia may be associated with the aftermath of these oceanographic disturbances some of these same trace elements along with fe2o3 and pb are also abundant in strata coeval with or just below the ireviken and hice excursions this may have a causal link with malformed palynomorphs observed at these intervals by some workers which are thought to reflect toxic levels of dissolved heavy metals in the world s oceans

crustal shear wave velocity models retrieved from rayleigh wave dispersion data in northern canada baffin island is one of the several seismically active regions in the far north of canada in 1933 a strong earthquake with m w 7 3 occurred in the region on 7 july 2009 a relatively strong earthquake with m w 6 0 occurred in the same area this earthquake was very well recorded by many modern seismic stations we systematically organized the rayleigh wave displacement records measured rayleigh wave dispersion data at 28 stations surrounding the epicenter and retrieved the s wave velocity models we used a previous model for the western quebec seismic zone as the initial model in our analyses and found that the velocities of all models at the shallow depths 15 km were obviously slower than those of the initial model in the middle crust the velocities in most models were close to those of the initial model in the directions of azimuths 171 218 and 241 the velocities in the middle crust were faster than those of the initial model in the directions of 263 279 the velocities in the middle crust were slower than those of the initial model the slowest s wave velocities in the top layers occurred in the directions of azimuths 90 and 218 these findings indicate differences in the existing crustal structures

graptolites in british stratigraphy 697 taxa of planktonic graptolites are recorded and their stratigraphical ranges are given through 60 biozones and subzones in the ordovician and silurian strata of england wales and scotland in the first such stratigraphical compilation for great britain since the synthesis of elles wood 1901 1918