meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

carbon isotopes and event stratigraphy near the ordovician silurian boundary yichang south china abstract the organic carbon isotope data through the unweathered ordovician silurian boundary strata at the wangjiawan riverside section which is c 180 m southeast of the wangjiawan north section the gssp for the base of the hirnantian stage upper ordovician show that a positive 13 c org excursion begins just below the base of the hirnantian stage and peaks in the lower part of the n extraordinarius biozone this is followed by an interval of slightly reduced 13 c values and a second peak of 2 above pre hirnantian values which occurs in the lower part of the n persculptus biozone upper hirnantian the peaks in 13 c org values can be correlated well with episodes of glacial expansion described from africa and peri gondwanan europe evidence from sedimentological faunal and geochemical data from south china as well as the evidence of glacial sediments in north africa all suggests a short lived glaciation of 1 ma in the ordovician south pole region consisting of at least two major pulses the first phase of gradual glacial expansion began just before the hirnantian epoch and the second phase ended with rapid melting in the late hirnantian early n persculptus biozone

role of fluids in the earthquake occurrence around aswan reservoir egypt 1 continuing seismicity for about 30 years near a large western embayment of the lake nasser about 50 km from the aswan high dam in egypt has led to a debate about the possibility of its relation with the reservoir impoundment the largest event in the region occurred on 14 november 1981 m 5 3 20 km beneath the wadi kalabsha embayment a westward extension of the lake aswan since then continuous monitoring of seismic activity has given an excellent opportunity to study the spatiotemporal distribution of seismicity in the area most of the immediate aftershocks of the 1981 main shock were located in the gebel marawa area at depths between 15 and 30 km depths of almost all earthquakes away from this zone were shallower than 12 km to quantify the effect of the reservoir impoundment on the seismicity of the aswan area we calculated changes in stress and pore pressure due to the reservoir impoundment using green s function approach the change in coulomb stress s is calculated on the fault planes responsible for majority of the seismicity of the region we found that for all the seismogenic faults s is negative i e stabilizing when we consider the effect of the reservoir load only whereas it is positive i e destabilizing when we include pore pressure for example at the hypocenter of the main earthquake shear stress normal stress and pore pressure due to reservoir operation are estimated as 5 5 13 2 and 13 5 kpa respectively which suggest that s is 3 1 kpa when we do not consider the effect of pore pressure and 5 7 kpa when contribution from pore pressure is considered hence the seismicity in the aswan lake region is driven by the pore pressure due to reservoir impoundment

widespread prolonged late middle to late ordovician upwelling in north america a proxy record of glaciation late middle to late ordovician subtidal ramp carbonates of new mexico texas and oklahoma contain abundant spiculitic chert to 70 chert by volume and phosphate 1 5 wt indicating that these rocks formed in an extensive upwelling zone upwelling began in the late middle ordovician ca 454 ma and persisted until the end of the ordovician late ordovician cherty carbonates also occur along the u s cordilleran margin in board of organic rich graptolitic shale and chert the widespread occurrence of late ordovician cherty and phosphatic carbonates on southern and western laurentia in addition to phosphate rich cool water carbonates over much of the north american midcontinent suggests vigorous thermohaline circulation related to prolonged 10 14 ma gondwana glaciation

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

carbon and sulfur isotope geochemistry of the hirnantian late ordovician wangjiawan riverside section south china global correlation and environmental event interpretation detailed geochemical analyses 13ccarb 13corg 34ssulfide and abundance of sulfide carbonate and organic carbon were performed on samples from the wangjiawan riverside section close to the gssp global stratotype section and point for the hirnantian stage of the ordovician new data show two increases in carbonate content coincident with two glacial pulses that reduced detrital input the new 34ssulfide data show distinct changes in this section with relatively high values in the kuanyinchiao formation a pattern observed throughout the yangtze platform however there is no consensus on the cause of these changes the new 13ccarb data show a sharp rise and peak in the extraordinarius zone below the previously published 13corg peak in the persculptus zone a compilation of the new results with other sections indicates the hirnantian carbon isotope excursion starts near the pacificus extraordinarius boundary and elevated values remain until the end of the excursion in the persculptus zone for both 13corg and 13ccarb the controversy over correlating hirnantian graptolite zones with chitinozoan zones can also be addressed the new 13ccarb data also allow direct comparison with hirnantian 13ccarb data from anticosti island and the baltic region which are zoned by chitinozoan fossils this comparison favors a correlation of the taugourdeaui and scabra chitinozoan zones with the extraordinarius graptolite zone

divergent response of an intertidal swash bar this paper examines the processes responsible for the morphodynamics of an intertidal swash bar at skallingen denmark during seven successive storms one with a large surge of 3 02 m dnn during this period a subtidal bar migrated landward onto the foreshore and continued to migrate across the intertidal zone as a swash bar the onshore migration of the inner subtidal bar resulted from the erosion of sediment from the upper foreshore and dune ramp during the large storm surge that was transported seaward causing the landward displacement of the bar through accretion on the landward slope the magnitude and direction of suspended sediment transport within the intertidal zone and more specifically at and close to the crest of the swash bar varied with the ratio of both the significant hs and average havg wave heights to the water depth hcr at the swash bar crest the local depth minimum the transition between onshore and offshore suspended sediment transport was associated with the average wave of the incident distribution breaking on the swash bar crest havgh 0 33 while the onshore directed transport was largest at infragravity frequencies sediment resuspension was best explained by the skewed accelerations under the surf bores offshore transport was dominated by the cross shore mean currents undertow that developed when the significant wave of the distribution broke on the swash bar crest hsh 0 33 and weakened as the average wave of the distribution started to break at the crest havgh 0 33 and the surf zone approached saturation in contrast to subtidal bars the swash bar at skallingen exhibited a divergent behaviour with respect to the cross shore position of the breaker zone migrating onshore when the average wave broke seaward of the crest and migrating offshore when the average wave broke landward of the crest copyright 2006 john wiley sons ltd

effects of crystal shape and size modality on magma rheology erupting magma often contains crystals over a wide range of sizes and shapes potentially affecting magma viscosity over many orders of magnitude a robust relation between viscosity and the modality of crystal sizes and shapes remains lacking principally because of the dimensional complexity and size of the governing parameter space we have performed a suite of shear viscosity measurements on liquid particle suspensions of dynamical similarity to crystal bearing magma our experiments encompass five suspension types each consisting of unique mixtures of two different particle sizes and shapes the experiments span two orthogonal subspaces of particle concentration as well as particle size and shape for each suspension type thereby providing insight into the topology of parameter space for each suspension type we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses the results were fitted using a herschel bulkley model and augment existing predictive capabilities we demonstrate that our results are consistent with previous work including friction based constitutive laws for granular materials we conclude that predictions for ascent rates of crystal rich magmas must take the shear rate dependence of viscosity into account shear rate dependence depends first and foremost on the volume fraction of crystals relative to the maximum packing fraction which in turn depends on crystal size and shape distribution

high resolution carbon isotope stratigraphy of the basal silurian stratotype dob s linn scotland and its global correlation since its designation as the global stratotype section and point gssp for the base of the silurian system the choice of dob9s linn southern scotland has received criticism due to the difficulties of relating its well constrained graptolite biostratigraphy to shallow water sequences elsewhere kerogen samples from across the ordovician silurian boundary interval at dob9s linn have yielded carbon stable isotope signatures consistent with those recorded elsewhere in particular showing a clear positive 13 c excursion in the terminal ordovician the architecture of the 13 c curve from dob9s linn enables very high resolution stratigraphic subdivision and direct correlation between the deep water dob9s linn section and time equivalent carbonate shelf deposits an integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites conodonts and shelly faunas has been constructed this direct correlation shows that the shallow water successions including the former stratotype candidate at anticosti island are generally incomplete with hiatuses related to the rapid sea level changes during the hirnantian stage this confirms and greatly increases the global utility of dob9s linn as a boundary stratotype

borehole water level response to barometric pressure as an indicator of aquifer vulnerability the response of borehole water levels to barometric pressure is a function of the confining layer and aquifer properties this study aims to use this response as an aid towards quantitative assessment of groundwater vulnerability applying the techniques to the confined semi confined part of the chalk aquifer in east yorkshire uk time series analysis techniques are applied to data collected from twelve monitoring boreholes to characterize and remove components contributing to the borehole water level signal other than barometric pressure such as recharge and earth tides barometric response functions are estimated using the cross spectral deconvolutionaveraging technique performed with up to five overlapping frequency bands a theoretical model was then fitted to the observed barometric response functions in order to obtain estimates of aquifer and confining layer properties derived ranges for pneumatic and hydraulic diffusivities of the confining layer vary over four orders of magnitudes 0 9 to 128 0 m2 day and 10 0 to 5 0 104 m2 day respectively indicating that the aquifer is nowhere purely confined discrepancies between estimates of aquifer transmissivity derived from the barometric response function and pumping tests have been explored using slug tests and results suggest that aquifer model transmissivity are highly sensitive to borehole construction a simple flow model constructed to test the potential impact of confining layer heterogeneity on the barometric response function shows that while high frequencies reflect the immediate vicinity of the borehole low frequencies detect confining layer properties up to some 500 meters distant from the borehole a characteristic time scale is introduced as a function of derived properties of the confining layer and is used as a quantitative measure of the degree of aquifer confinement it is concluded that barometric response functions are sensitive to confining layer properties and thus can provide a useful tool for the assessment of aquifer vulnerability

detecting the vulnerability of groundwater in semi confined aquifers using barometric response functions summary the use of barometric response functions brfs for detecting the presence of fully penetrating highly conductive bodies within aquifer confining layers that present potential pathways for contaminants is explored brfs are determined from borehole water level wl and barometric pressure bp records past studies have shown that confining layer properties can be estimated from brfs providing a potential link between brfs and the concept of groundwater vulnerability existing analytical models that predict the brf from system properties assume homogeneity within the aquifer and its confining layer conditions which are seldom satisfied in nature the impact of partially and fully penetrating high diffusivity heterogeneities within a confining layer representing potential high flow pathways for contaminants on the brf is investigated through a suite of three dimensional transient numerical simulations of the confining layer aquifer system the results are interpreted through comparison with a modified pre existing analytical model for the brf comparison of numerically and analytically calculated brfs reveals that the key effect of a localised fully penetrating high diffusivity heterogeneity within a low diffusivity confining layer is to reduce the brf gain with only minor changes to the phase this impact on the brf decreases with increasing distance from the heterogeneity the importance of heterogeneity size is secondary to distance from the borehole and partially penetrating heterogeneities affect the brf to only a minor extent data from a study of the chalk aquifer e yorkshire england which is semi confined by heterogeneous glacial sediments display variations in brfs which are qualitatively similar to those shown by the numerical results it is suggested that the variation in brfs estimated from borehole records across a semi confined aquifer could be used to assess the degree of spatial continuity of low diffusivity lithologies within the confining layer which protect the aquifer against surface sourced contamination and thus provide a tool to improve groundwater vulnerability assessment

oxygen isotopes from conodont apatite of the midcontinent us implications for late ordovician climate evolution abstract the major glaciation at the end of the ordovician is associated with the 2nd largest mass extinction event of the phanerozoic growth of late ordovician ice sheets requires a dramatic cooling from the greenhouse conditions that prevailed for most of the ordovician but when and how fast this cooling occurred are controversial issues the controversy is due in large part to a lack of good geochemical constraints on the temperature history of the katian 453 445 2 ma to address this uncertainty we measured phosphate 18o values from 3 conodont species collected from sections in the midcontinent region of the united states that span an 5 7 m yr long interval covering most of the katian results reveal a statistically significant offset in 18o values between some taxa and show up to 2 differences among samples however there are no apparent long term trends within or between sections rather values fluctuate around a 18o mean of 19 vsmow our study provides the longest relatively high resolution species specific conodont record generated for this interval and we found no evidence supporting progressive cooling during the katian

determination of aquifer transmissivity from earth tide analysis the water level in an open well tapping an artesian aquifer responds to pressure head disturbance caused by earth tide dilatation of the aquifer because a finite amount of time is needed for water to flow into and out of the well there exists a phase shift or time lag between the tidal dilatation of the aquifer and the water level response in the well we derive an analytical solution that expresses the phase shift as a function of the aquifer transmissivity storage coefficient well radius and the period of the harmonic disturbance this solution is rather insensitive to the storage coefficient thus if the phase shift is known for a harmonic disturbance the transmissivity can be calculated given a rough estimate of the storage coefficient theoretical analysis shows that a significant phase shift may be present even if the disturbance is slowly varying as in the case of earth tides this opens the possibility of estimating aquifer transmissivity from water level records that show earth tide fluctuations a case study using data from a site near parkfield california is presented to illustrate application of the theory phase shifts of the o1 25 82 hour period and m2 12 42 hour period tidal constituents are chosen for analysis because they are free of systematic contamination by fluctuations in barometric pressure a brief error analysis suggests that the computed o1 phase shift is subject to large uncertainty while the computed m2 phase shift is substantially more accurate based on an assumed storage coefficient range of 10 4 to 10 6 the estimated transmissivity range is 8 10 6 to 2 10 5 m2 s while hydraulic tests have not been performed to validate these estimates the range is consistent with the transmissivity value determined by other investigators from analysis of the water level response to an earthquake

interpretation of earth tide response of three deep confined aquifers the response of a confined areally infinite aquifer to external loads imposed by earth tides is examined because the gravitational influence of celestial objects occurs over large areas of the earth the confined aquifer is assumed to respond in an undrained fashion since undrained response is controlled by water compressibility earth tide response can be directly used only to evaluate porous medium compressibility if porosity is known moreover since specific storage ss quantifies a drained behavior of the porous medium one cannot directly estimate ss from earth tide response except for the fact that barometric changes act both on the water surface in the well and on the aquifer as a whole while stress changes associated with earth tides act only in the aquifer the two phenomena influence the confined aquifer in much the same way in other words barometric response contains only as much information on the elastic properties of the aquifer as the earth tide response does factors such as well bore storage aquifer transmissivity and storage coefficient contribute to time lag and damping of the aquifer response as observed in the well analysis shows that the observation of fluid pressure changes alone without concurrent measurement of external stress changes is insufficient to interpret uniquely earth tide response in the present work change in external stress is estimated from dilatation by assuming a reasonable value for bulk modulus earth tide response of geothermal aquifers from marysville montana east mesa california and raft river valley idaho were analyzed and the ratio of ss to porosity was estimated comparison of these estimates with independent pumping tests shows reasonable agreement

hydrological response to earthquakes in the haibara well central japan ii possible mechanism inferred from time varying hydraulic properties summary 28 coseismic groundwater level decreases have been observed at the haibara well shizuoka prefecture central japan from 1981 to 1997 these groundwater level changes cannot be explained as the poroelastic response to coseismic static strain we use the atmospheric pressure and tidal responses of the well rock properties measured on core samples from the same formation and pumping test results to characterize the hydraulic and mechanical properties of the aquifer the responses of the haibara well to the m2 earth tide constituent and to atmospheric pressure have varied over time in particular increasing amplitude and decreasing phase lags were observed after the 1993 pumping test as well as after earthquakes that caused coseismic water level changes the tidal response together with the surface load efficiency derived from the atmospheric pressure response is used to estimate the mechanical properties of the aquifer the largest amplitude of the m2 constituent 2 2 mm is small enough to imply that pore fluid in this system is approximately twice as compressible as water possibly due to the presence of a small amount of exsolved gas diffusion of a coseismic pressure drop near the well could account for the observed time histories of the water level changes the time histories of the water level drops are well matched by the decay of a coseismic pressure drop at least 80 m away from the well removal of a small amount of gas from the formation in that location might in turn explain the coseismic pressure drops

glacioeustasy meteoric diagenesis and the carbon cycle during the middle carboniferous middle carboniferous carbonates in the western u s have undergone pleistocene bahamas style meteoric diagenesis that may be associated with expanding late paleozoic ice sheets fourteen stratigraphic sections from carbonate platforms illustrate the regional distribution and variable intensity of physical and chemical diagenesis just below the middle carboniferous unconformity these sections contain top negative carbon isotope excursions that terminate in regional exposure surfaces that are associated with some combination of karst towers desiccation cracks fabric destructive recrystallization or extensive root systems the timing of the diagenesis is synchronous with similarly scaled top negative carbon isotope excursions observed by others in england kazakhstan and china the mass flux of negative carbon required to generate similar isotopic profiles across the areal extent of middle carboniferous platform carbonates is a significant component of the global carbon cycle we present a simple carbon box model to illustrate that the 13c of dissolved inorganic carbon in the ocean could be elevated by 1 4 as isotopically light carbon from the weathering of terrestrial organic matter reacts with exposed platforms before reaching the ocean and atmosphere these results represent an improvement on global biogeochemical models that have struggled to provide a congruent solution to the high 13c of the late paleozoic icehouse

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

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

sea level rise due to polar ice sheet mass loss during past warm periods background although thermal expansion of seawater and melting of mountain glaciers have dominated global mean sea level gmsl rise over the last century mass loss from the greenland and antarctic ice sheets is expected to exceed other contributions to gmsl rise under future warming to better constrain polarice sheetresponse to warmer temperatures we draw on evidence from in terglacial periods in the geologic record that ex perienced warmer polar temperatures and higher gmsls than present coastal records of sea level from these previous warm periods dem onstrate geographic variability because of the influence of several geophysical processes that operate across a range of magnitudes and time scales inferring gmsl and ice volume changes from these reconstructions is nontrivial and generally requires the use of geophysical models advances interdisciplinary studies of geo logic archives have ushered in a new era of deciphering magnitudes rates and sources of sea level rise advances in our understanding of polar ice sheet response to warmer climates have been made through an increase in the number and geographic distribution of sea level reconstructions better ice sheet constraints and the recognition that several geophysical processes cause spatially complex patterns in sea level in particular accounting for glacial isostatic processes helps to decipher spatial variability in coastal sea level records and has reconciled a number of site specific sea level reconstructions for warm periods that have oc curred within the past several hundred thou sand years this enables us to infer that during recent interglacial periods small increases in

estimating aquifer parameters from analysis of forced fluctuations in well level an example from the nubian formation near aswan egypt 2 poroelastic properties we discuss the in situ estimation of aquifer poroelastic properties from observed water level fluctuations in five wells sampling the 0 30 porosity nubian sandstone near aswan reservoir egypt the study area is close to ideal for the application of poroelastic theory because of the relatively simple stratigraphy and the absence of cultural and climatological noise in this paper we use approximately 2 years data sampled at 0 2 hour intervals to calculate frequency dependent admittances between the well levels and the recognized forcing functions of the earth tides and barometric fluctuations the effect of variations in lake level is treated in a companion paper the time series analysis techniques that we employ are described in detail three of the wells show equilibrium confined response at tidal frequencies which allows various in situ poroelastic parameters of the aquifer to be derived from the tidal and barometric admittance values we make extensions to earlier treatments of poroelastic theory in order to include the effect of horizontal strain resulting from barometric loading the modifications proved of minor importance for our data set because of the high compliance of the aquifer material and the shallow depth to basement in situ shear modulus is determined as 5 4 6 4 109 pa and uniaxial strain loading efficiency as 0 33 each of these parameters depends on the field measurements of well level and air pressure and also depends strongly on a theoretical estimate of the solid earth areal strain tides the principal source of error in lies in these areal strain estimates we measured poisson s ratio in the laboratory and found that it lies in the range 0 13 0 18 this together with the admittance values allows estimates of uniaxial strain modulus 13 19 109 pa with the smaller values at shallower depth specific storage 1 5 1 9 10 6 m 1 with the larger values at shallower depth porosity 0 2 0 3 skempton s constant 0 54 0 58 and biot s constant 0 78 0 85 laboratory measurements of uniaxial strain modulus are about 50 higher than the in situ estimates probably reflecting scale effects despite the high porosity use of a theory of aquifer response in which the solid constituent is considered incompressible produces significant errors in these estimates

katian upper ordovician delta c 13 chemostratigraphy and sequence stratigraphy in the united states and baltoscandia a regional comparison delta c 13 values of numerous limestone samples from katian upper ordovician successions in oklahoma and the upper mississippi valley document the presence of at least three of the four positive excursions that have been recognized in the lower middle katian interval in the cincinnati region in ohio kentucky and indiana a comparison with estonia reveals that five out of the six positive excursions now recorded through the katian succession in the cincinnati region are present in the coeval stratigraphic intervals in estonia which suggests that these represent global perturbations in the carbon cycle a study of the relations between flooding events and delta c 13 excursions shows that only some of the excursions occur in transgressive intervals whereas other excursions are in regressive strata and hence this is in conflict with the idea that positive carbon isotope excursions are forced by rises in sea level part of a previously published attempt to use upper ordovician sequence stratigraphy for trans atlantic correlation is shown to be seriously out of phase with the biostratigraphy and chemostratigraphy several of the sequences recognized in the cincinnati region do not appear to have counterparts in estonia which suggests that they do not reflect eustatic but local conditions c 2010 elsevier b v all rights reserved less

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

carbon isotope stratigraphy and correlation of depositional sequences in the upper ordovician ely springs dolostone eastern great basin usa abstract shallow water carbonate shelf strata of the ely springs dolostone accumulated on the western laurentian margin as relative sea level fluctuated during the late ordovician icehouse the ely springs dolostone is divided into four upper katian depositional sequences and one hirnantian sequence here we present new carbon isotope data from the ely springs in the context of the sequence stratigraphic architecture these data derive from a shelf transect of five stratigraphic sections exposed in the eastern great basin usa and provide records of carbon cycling on the laurentian margin prior to and during the hirnantian glacial maximum the 13 c values range between 2 and 4 at each section and chemostratigraphic tie points occur in all sections of the transect intrabasinal correlations based on the 13 c records are consistent with correlations derived from sequence stratigraphy and conodont biostratigraphy the ely springs dolostone carbon isotope curve can also be correlated with the late katian cincinnatian composite derived from epicontinental deposits of the laurentian midcontinent the globally documented hirnantian positive carbon isotope excursion commonly referred to as hice occurs within the uppermost depositional sequence immediately below a regional unconformity coinciding with peak glacial conditions on gondwana the late katian elkhorn and waynseville 13 c excursions are also present in the ely springs dolostone carbon isotope record the peak value of the hirnantian carbon excursion increases from 2 6 to 3 8 across the shallow shelf and reaches 7 3 in coeval limestones of the hanson creek formation deposited in a deep dysoxic setting this lateral gradient is not the result of changes to the 13 c of weathered carbon supplied by rivers as previously proposed it may be instead related to authigenic carbonate cement precipitation during early diagenesis of the hanson creek strata that these pervasively dolomitized shallow water carbonates retain 13 c signatures useful for regional and global stratigraphic correlation indicates that 13 c chemostratigraphy may be an appropriate correlation tool for dolostones with few biostratigraphically useful fossils

analysis and application of in situ pore pressure measurements in marine sediments recent measurements of in situ subseafloor pore pressure in sediment ponds on the flank of the mid atlantic ridge using a pop up pore pressure instrument have provided important information regarding fluid exchange processes at the seafloor as well as in situ estimates of physical properties of marine sediments the in situ pore pressure recordings of 4 5 days duration contain three distinct components the early part of the record is dominated by the decay of a pressure pulse associated with probe penetration the rate of decay of this pulse is used to estimate in situ permeabilities which range from 3 1 10 16 m2 to 6 5 10 15 m2 the recording also exhibits a low amplitude oscillation that is associated with tidal pressure variation on the seafloor a study based on biot s theory shows that amplitudes and phases of tidally induced pore pressure oscillations are determined by both the frame compressibility and the permeability of the sediments and thus the oscillation components are used to estimate these parameters the frame compressibilities and the permeabilities estimated from tidal oscillations range from 1 2 10 8 pa 1 to 4 1 10 8 pa 1 and from 1 1 10 15 m2 to 4 8 10 15 m2 respectively laboratory measured permeabilities range from 7 6 10 16 m1 to 1 2 10 15 m2 permeabilities determined by these three methods are comparable the third component is the ambient equilibrium pore pressure negative pore pressure gradients were identified on all deployments when combined with the permeabilities of the sediments the estimated average rates of drawdown of water through the sedimentary layers in the ponds are 2 7 mm yr the pore pressure gradient near the seafloor can be used to construct a pore pressure profile through the sediment extrapolated pore pressure at the top of the basement is about 80 kpa 0 8 bar below the hydrostatic pressure

response of well aquifer systems to earth tides problem revisited two recent works cause us to reexamine bredehoeft s 1967 analysis of earthtide response of water wells narasimhan et al 1984 raise several questions regarding bredehoeft s 1967 analysis and suggest that the analysis is internally inconsistent they argue that one cannot directly estimate the specific storage which characterizes the drained behavior of a porous medium from earth tide response which is an undrained phenomenon we resolve the questions raised by narasimhan et al 1984 and show that bredehoeft s analysis is internally consistent in addition we show that it is possible to determine the specific storage from undrained loading while bredehoeft s analysis is somewhat heuristic and neglects grain compressibility van der kamp and gale 1983 present a more rigorous analysis that is based on biot s 1941 constitutive relationships and accounts for grain compressibility however their results reduce to bredehoeft s results when grains are assumed incompressible this suggests that bredehoeft s analysis has incorporated all the essential features of biot s relationships except for grain compressibility upon reexamining bredehoeft s analysis we find that this is indeed the case

earthquake activity in the aswan region egypt the november 14 1981 aswan earthquake m l 5 7 which was related to the impoundment of lake aswan was followed by an extended sequence of earthquakes and is investigated in this study earthquake data from june 1982 to late 1991 collected from the aswan network are classified into two sets on the basis of focal depth i e shallow or deeper than 10 km it is determined that a shallow seismicity is characterized by swarm activity whereas deep seismicity is characterized by a foreshock main shock aftershock sequence b the b value is equal to 0 77 and 0 99 for the shallow and deep sequences respectively and c observations clearly indicate that the temporal variations of shallow seismic activity were associated with a high rate of water level fluctuation in lake aswan a correlation with the deeper earthquake sequence however is not evident these features as well as the tomographic characteristics of the aswan region awad and mizoue this issue imply that the aswan seismic activity must be regarded as consisting of two distinct earthquake groups

mg and ca isotope signatures of authigenic dolomite in siliceous deep sea sediments abstract authigenic carbonates in marine sediments frequently have carbon isotope ratios that reflect local organic carbon processing rather than the c 13 of the global dic dissolved inorganic carbon reservoir but their contributions to ancient sedimentary sections are difficult to assess in this study of authigenic dolomite from the miocene age monterey formation of offshore california mg and ca isotopes are shown to vary with stratigraphic depth as a result of early diagenetic processes the dolomite is a pre compaction authigenic phase that occurs as beds and nodules with c 13 ranging from 16 to 9 light c 13 values were likely acquired from the sedimentary zone of microbial sulfate reduction while heavy c 13 values were acquired from the zone of methanogenesis mg and ca isotopes are roughly anti correlated with intervals of negative c 13 associated with low mg 26 and higher ca 44 40 values the variability is observed over a wide range of length scales from 10 2 meters within individual authigenic beds nodules to 10 2 meters over the entire stratigraphic column and can be understood as the consequence of dolomite precipitation in pore fluids where mg supply is limited by diffusive transport the relationship of mg 26 and ca 44 40 to the more common stable isotope measurements of c 13 and o 18 represents a new diagenetically robust geochemical fingerprint for identifying synsedimentary authigenic carbonates in the geological record

nd isotope records of late ordovician sea level change implications for glaciation frequency and global stratigraphic correlation abstract detailed records of ice sheet advances and retreats are reconstructed for the hirnantian and katian ages of the late ordovician using nd isotopes e nd as a sea level proxy in three study sections from the western margin of laurentia two shallow water platform sections located south and north of the paleo equator and one deep water section located in a continental slope rise setting when sea level was high and paleo shorelines had migrated eastward the e nd value of seawater in the vicinity of each of the study sections shifted toward the e nd value of the eastern panthalassa ocean 4 0 by contrast when sea level was low and paleo shorelines had migrated westward the e nd value of seawater shifted toward the e nd value of the continental weathering flux from laurentia 8 5 0 2 2 mean these stratigraphic patterns of changing e nd values are interpreted to reflect the eustatic sea level fluctuations that previous studies have documented in response to gondwanan ice sheet advances and retreats thus linking the e nd sea level proxy to late ordovician global scale climate changes the e nd profiles for the two platform sections yielded similar proxy sea level curves with five cycles of oscillation recorded during the latest katian and hirnantian three additional cycles of late katian sea level change are recognized in the e nd profile of the deep water continental slope rise section the combination of e nd 13 c and graptolite biostratigraphic data facilitates a precise interregional correlation of the hirnantian age and the paleoclimate changes that took place during this interval the new correlations support previous findings that the hirnantian ice age comprised two major glacial periods separated by a minor interglacial during the early part of the metabolograptus persculptus biozone the younger glacial confined to mid m persculptus biozone time led to more extensive sea surface cooling than did the earlier one and resulted in extensive eustatic sea level drawdown and c cycle changes it records the strata most often recognized as hice the hirnantian carbon isotope excursion in sedimentary successions worldwide such as anticosti island scotland estonia siberia and south china the results of this study support and strengthen the view that glaciation predated the hirnantian age in the late ordovician and that the records of small positive 13 c excursions in katian successions from baltica and eastern north america are themselves proxy indicators of glaciation frequency and eustatic sea level changes

depletion of 13c in seawater σc02 on modern carbonate platforms significance for the carbon isotopic record of carbonates seawater co2 from modern carbonate platforms bahama banks and florida is depleted in 13c by as much as 4 relative to open ocean water depletion in 13c is caused by isotopically light co2 input from respiration of marine and terrestrial organic matter during water mass residence on the bank as such depletion in 13c is related to changes in water chemistry driven by evaporation freshwater discharge and caco3 withdrawal deviation of modern platform seawater 13c values away from those of surface seawater suggests that 13c values of ancient cratonic carbonate must be cautiously interpreted initial depositional environment as well as alteration during diagenesis must be considered in extracting valid secular trends for oceanic 13c variation

techniques for assessing spatial heterogeneity of carbonate δ13c values implications for craton wide isotope gradients the sedimentary record of carbonate carbon isotopes 13ccarb provides one of the best methods for correlating marine strata and understanding the long term evolution of the global carbon cycle this work focuses on the late ordovician guttenberg isotopic carbon excursion a ca 2 5 positive 13ccarb excursion that is found in strata globally substantial variability in the apparent magnitude and stratigraphic morphology of the guttenberg excursion at different localities has hampered high resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle this work investigates the magnitude spatial scale and sources of isotopic variability of the guttenberg excursion in two sections from missouri usa centimetre scale isotope transects revealed variations in 13ccarb and 18ocarb greater than 2 across individual beds linear 13ccarb to 18ocarb mixing lines together with petrographic and elemental abundance data demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components these patterns facilitated objective sample screening to determine the least altered data a 18ocarb filter based on empirical 18ocarb values of well preserved carbonate mudstones allowed further sample discrimination the resulting least altered 13ccarb profile improves the understanding of regional as well as continental scale stratigraphic relations in this interval correlations with other laurentian sections strongly suggest that i small scale variability in guttenberg excursion 13ccarb values may result in part from local diagenetic overprinting ii peak guttenberg excursion 13ccarb values of the midcontinent are not distinct from their taconic equivalents and iii no primary continental scale spatial gradient in 13ccarb for example arising from chemically distinct aquafacies is required during guttenberg excursion time this study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for 13ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry

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

estimating aquifer parameters from analysis of forced fluctuations in well level an example from the nubian formation near aswan egypt 3 diffusivity estimates for saturated and unsaturated zones atmospheric pressure variations provide a broadband signal that may force a sympathetic response in well water levels in this paper time series analysis techniques are used to estimate the response as a frequency dependent admittance function which is then modeled to provide estimates of the fluid transport properties of strata the data derive from five cased piezometer wells sampling aquifers in the nubian formation southwest of aswan egypt three shallow wells 100 140 m deep sample a water table aquifer a fourth w3 400 m deep samples a basal aquifer in the same area that behaves in a confined manner up to a period of several years the fifth well samples another basal aquifer and shows evidence of partial blockage nontidal water level variations in the shallow wells are due almost entirely to barometrically driven flow of air and water using a simple model to fit the observed barometric admittance spectra we obtain estimates of horizontal and vertical permeabilities for water in the saturated zone local horizontal permeability is constrained by modeling the effects of flow induced pressure gradients near the screen for the w3 deep well sampling the basal aquifer the resulting values 0 15 0 3 2 are marginally lower than the large scale 5 km estimates 0 32 0 43 2 derived in a previous paper however the values for the three wells sampling the water table aquifer although consistent among themselves 0 2 0 5 2 are significantly lower than the large scale estimate 1 0 1 5 2 this is contrary to what might be expected given that the wells are preferentially screened in clean sandstones vertical permeability estimated by modeling partial confinement effects is constrained for only one well a low value was obtained because of the presence of claystone beds in the diffusion path between screen and water table at this well the effects of air wave diffusion are clearly manifest in the spectra of one well where the water table lay at a depth of about 40 m the form of the spectra was well fit by ascribing a uniform pneumatic diffusivity of 1 75 10 m 3m2 s to the unsaturated zone however it was also necessary to include an apparent attenuation of the air wave at the capillary fringe of about 0 5 we propose that the effect is due not to attenuation but that it reflects compression of the phreatic surface arising from the presence of trapped air pockets in the underlying saturated zone a 40 m rise in the water table at the site during the decade prior to the measurements might explain the presence of significant quantities of trapped air this rise in water table together with the arid climate might be taken to suggest that the moisture content of the unsaturated zone is negligible except within a meter or so of the water table however calculation of the intrinsic rock permeability from pneumatic diffusivity assuming zero moisture content yields an estimate which is considered to be too low the likely explanation is that the assumption of zero moisture content is in error despite conditions which are as favorable as are ever likely to be realized under field conditions

estimating aquifer parameters from analysis of forced fluctuations in well level an example from the nubian formation near aswan egypt 1 hydrogeological background and large scale permeability estimates this is the first of three papers concerning the extraction of aquifer parameters through the application of time series analysis to water level data from boreholes the analysis is performed on data from six wells located in the nubian desert near the shores of lake nasser egypt these wells are ideally suited to this study since they are remote from most sources of hydrological noise and hence respond primarily to forcing functions that can be measured or modeled for four of the wells which were part of a monitoring system installed prior to impoundment of lake nasser monthly at least water level measurements began in 1964 in 1985 two additional wells were drilled and high precision piezometers that sampled data every 0 2 hour were installed in five of the wells thus data are available which span the period band 23 years to 0 4 hour this paper provides background information and stratigraphy relevant to assessing the significance of the aquifer parameters derived using spectral methods a regional scale three dimensional hydrogeological model for the study area is developed based on the 23 year record of water level changes that occurred at the various well sites in response to the changing level of lake nasser the model that best fits the data and is consistent with recent stratigraphic studies of the nubian formation in the area consists of three horizontal units of total thickness 400 m the lowermost unit overlying the granitic basement is an aquifer of fluvial sandstones and has a large scale several kilometer horizontal permeability of 0 32 0 43 m2 the middle unit is an aquaclude that extends unbroken under the lake and leaks at periods longer than several years thus the lower aquifer is confined up to periods of several years the uppermost unit is the water table aquifer which is composed of 25 30 porosity sandstones with interspersed claystone lenses and has a large scale permeability of 1 0 1 5 m2 of the wells studied two are screened in the basal sandstone and four are screened in the water table aquifer analysis of tidally and atmospherically forced fluctuations in five of these wells is presented in the companion papers

a late cambrian positive carbon isotope excursion in the southern appalachians relation to biostratigraphy sequence stratigraphy environments of deposition and diagenesis abstract a positive carbon isotope excursion is recorded within the upper cambrian sedimentary succession in the southern appalachians that consists of the nolichucky shale the maynardville formation and the copper ridge dolomite the lower part of the succession contains aphelaspis zone fauna early steptoean the extensively dolomitized and poorly fossiliferous nature of the upper part of the succession precludes any detailed biostratigraphic determinations correlation with similar positive carbon isotope excursions in coeval successions elsewhere suggests that this excursion represents a perturbation in the global cycling of carbon comparison of excursions at different localities in north america provides a means for the application of carbon isotope stratigraphy in the southern appalachians the excursion started during deposition of the upper nolichucky shale maximum 13c values 4 to 5o oo pdb are associated with the conformable interval at the maynardville copper ridge dolomite transition which has been interpreted as a correlative conformity in sequence stratigraphic terms the excursion ended during deposition of the lower copper ridge dolomite in western north america the excursion started at the base of the pterocephaliid biomere near the base of the aphelaspis zone this well documented excursion ended prior to the end of the pterocephaliid biomere with the maximum excursion at the sauk ii sauk iii unconformity this supports the correlation between late steptoean dresbachian franconian sea level fall and the sequence boundary at the end of cambrian grand cycle deposition in the southern appalachians the cause of this carbon isotope excursion remains unclear the excursion most likely reflects the enhanced burial of organic carbon promoted by ocean stratification a warm nonglacial climate and a sea level maximum during the early late cambrian the onset of regression may have contributed to the maximum carbon isotope excursion by enhancing sedimentation rates and by increasing organic productivity because of increased nutrient availability the removal of carbon from the ocean surface may have caused a decrease in pco2 of the atmosphere the resulting cooling episode could have triggered an oceanic overturn bringing 12c enriched bottom waters to the surface which in conjunction with oxidation of organic matter during the sea level fall ended the carbon isotope excursion comparison of 13c and 18o values of matrix samples to the associated cement phases provides insights into the relationship between isotope variations and depositional and diagenetic environments 13c values of peritidal dolomicrite define a rather smooth stratigraphic variation curve whereas the values for subtidal micrite have significant scatter resulting from involvement of organic matter in diagenesis fibrous to bladed calcite cement from the subtidal deposits has 13c and 18o values comparable to the associated micrite suggesting precipitation from marine water and similar diagenetic modifications meteoric diagenesis may be responsible for the depletion of 13c and 18o in equant calcite cement relative to the micrite for saddle dolomite cement the depletion of 18o and 13c values similar to those for the peritidal dolomicrite are consistent with formation during burial at elevated temperatures in a rock dominated system this study demonstrates the potential of applying carbon isotope stratigraphy developed in highly fossiliferous successions to stratigraphic intervals with poorly constrained biostratigraphy such studies require evaluation of the effects of depositional environments and diagenesis upon the preservation of marine isotope signatures

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

large perturbations of the carbon and sulfur cycle associated with the late ordovician mass extinction in south china high resolution 13 c data of organic carbon from a continuous section of the late ordovician early silurian reveal two positive 13 c excursions that are associated with the mass extinction in south china the fi rst stratigraphic 34 s measurements on pyrite tied to well established biostratigraphy indicate a large perturbation of the sulfur cycle consistent with major sea level changes related to the glaciation the elevated 34 s values of pyrites and a large short lived negative 34 s excursion of 20 associated with the decay of the glaciation suggest deep water anoxia during the hirnantian stage in contrast to the conventional view that the global oceans were oxygenated we suggest that deep water anoxia may have contributed to the late ordovician mass extinction in south china and possibly elsewhere

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

tightly coupled records of ca and c isotope changes during the hirnantian glaciation event in an epeiric sea setting abstract a 44 40 ca excursion of 0 5 is recorded during the hirnantian glaciation event in carbonate rocks of the monitor range section in nevada the timeframe for the glaciation is 1 03 0 2 ma the 44 40 ca changes are synchronous with lithofacies and biofacies indicators of sea level change and co vary negatively with a large positive 13 c excursion of 7 that was previously interpreted to reflect local c cycling effects in circulation restricted waters of the epeiric martin ridge basin the synchronousness of the ca and c isotope excursions is inconsistent with what we know of ca and c geochemical cycles in the modern ocean because the residence time of ca is longer than that of c a box model is used to illustrate that the negative 44 40 ca excursion would take longer to recover than the positive 13 c excursion if the relative timescales of the hirnantian ca and c cycles were scaled similarly to those in the ocean today one way to speed up the timescale of the ca isotope response is to decrease the size of the ca reservoir this is what is accomplished when seawater circulation is restricted between oceans and epeiric seas approximately 30 of the 0 5 shift in hirnantian 44 40 ca values may be attributed to lower sea surface temperatures in the tropics during glaciation the remainder is attributed to local ca cycling effects in the martin ridge basin which include an increase in isotopically light ca inputs by submarine groundwater discharge sgd and a larger kinetic isotope fractionation effect due to increased carbonate precipitation rate the hypothesized increase in carbonate precipitation rate may have been caused by high photosynthesis rates maintained by high nutrient fluxes from sgd which increased the local carbonate saturation state of the waters additionally the conditions in this setting may have fostered a suite of organisms employing a calcification mechanism that involved microenvironments bacterial or algal surfaces biofilms or sedimentary pore fluids where biological activity could effectively alter the carbonate chemistry in very small c reservoirs a similar scenario was presented by laporte et al 2009 to explain the bulk of the anomalously large 7 positive 13 c excursion in the same section the strong negative coupling between the stratigraphic records of ca and c isotope changes in the martin ridge basin may serve as a tool to identify other epeiric sea settings in the rock record where similar physical and ecological attributes developed during periods of restricted circulation brought on by low sea level

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

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

tidal calibration of plate boundary observatory borehole strainmeters roles of vertical and shear coupling 1 a multicomponent borehole strainmeter directly measures changes in the diameter of its cylindrical housing at several azimuths to transform these measurements to formation strains requires a calibration matrix which must be estimated by analyzing the installed strainmeter s response to known strains typically theoretical calculations of earth tidal strains serve as the known strains this paper carries out such an analysis for 12 plate boundary observatory pbo borehole strainmeters postulating that each of the strainmeters four gauges responds couples to all three horizontal components of the formation strain tensor as well as to vertical strain orientation corrections are also estimated the fourth extensometer in each pbo strainmeter provides redundant information used to reduce the chance that coupling coefficients could be misleadingly fit to inappropriate theoretical tides satisfactory fits between observed and theoretically calculated tides were obtained for three pbo strainmeters in california where the calculated tides are corroborated by other instrumentation as well as for six strainmeters in oregon and washington where no other instruments have ever recorded earth tidal strain several strainmeters have unexpectedly large coupling coefficients for vertical strain which increases the strainmeter s response to atmospheric pressure vertical coupling diminishes or even changes the sign of the apparent response to areal strain caused by earth tides or deep earth processes because near the free surface vertical strains are opposite in sign to areal strain vertical coupling does not impair the shear strain response however pbo borehole strainmeters can provide calibrated shear strain time series of transient strain associated with tectonic or magmatic processes

coseismic fluid pressure response estimated from prediction error filtering of tidal band loading a methodology combining prediction error filters pefs and transfer functions was developed to identify the quasi static fluid pressure response observed in wells due to coseismic strain water levels in confined aquifers respond to long term and seasonal trends recharge events barometric and ocean tide loading tidal strain and tectonic strain low frequency features can be neglected from the quasi static coseismic response estimation transfer functions were constructed to decon volve the fluid pressure response due to measured barometric loading because direct tidal strain and ocean tide loading measurements are rarely available theoretical tidal loading is often calculated from astronomical data however the calculations are subject to many assumptions because tidal driving processes are cyclic pefs are a natural choice for removing the fluid pressure response without assuming a theo retical forcing function in the tidal band the method was applied to hourly fluid pressure data collected over a 3 year period from two wells in the villages of gaocun and tayuan china results of this analysis yielded coseismic fluid pressure heads of 1 6 10 2 and 7 6 10 2 m for the respective wells in response to the datong yanggao earthquake swarm mainshock m s 6 1 18 24 october 1989 epi central distances to the wells were about 200 km the coseismic fluid pressure re sponse for each well was also predicted from dislocation model strain scaled by material dependent volumetric strain sensitivity parameters these parameters were determined from the static confined response to o1 and m 2 earth tide strain constit uents the predicted response was 2 9 10 3 m for the gaocun well and 2 1 10 3 m for the tayuan well although predicted and observed response phases were consistent both predictions underestimated observed response amplitudes as has been true in other reported instances

conodont apatite δ18o values from a platform margin setting oklahoma usa implications for initiation of late ordovician icehouse conditions abstract the uppermost sandbian lower katian stratigraphic record documents a critical time period in earth history lithological evidence and geochemical proxy records suggest that the uppermost sandbian lower katian interval could represent a transitional phase from a long cambrian ordovician greenhouse world to the late ordovician icehouse world to examine this interval of earth s history oxygen isotopes of conodont apatite from the bromide and viola springs formations in southern oklahoma were analyzed in order to provide sea surface temperature estimates at a location that had little influence by terrigenous sedimentation our results are inconsistent with 1 previous suggestions that global cooling occurred across a prominent sequence boundary close to the sandbian katian boundary across north america and 2 the hypothesis that there is a direct connection between volcanic mega eruption s and a significant long term impact on climate during the sandbian katian of laurentia furthermore 18 o values spanning the guttenberg carbon isotope excursion gice an interval that has been interpreted as evidence for an early onset of the late ordovician glaciation are variable a 1 5 18 o shift is observed in the lower gice that is followed by a return to 18 o values that fluctuate around a mean of 19 0 in the upper gice together these observations suggest a short lived cooling event within the lower gice this cooling event may represent enhanced carbon burial and drawdown of atmospheric p co 2 followed by a return to more stable low latitude temperatures in the upper gice

departures from eustasy in pliocene sea level records estimates for sea level three million years ago a period with similar atmospheric co2 levels to today vary from 10 to 40 m above present glacial isostatic adjustment modelling suggests that variations in the height of palaeoshorelines result from the residual adjustment of continental flexure following recent glaciations

upper ordovician sequences of western estonia abstract the upper ordovician uppermost caradoc ashgill section of western estonia consists of a series of seven open shelf carbonate sequences depositional facies grade laterally through a series of shelf to basin facies belts grain supported facies shallow shelf mixed facies middle shelf mud supported facies deep shelf and slope and black shale facies basin locally a stromatactis mud mound occurs in a middle to deep shelf position shallow to deep shelf facies occur widely across the estonian shelf and grade laterally through a transitional slope belt into the basinal deposits of the livonian basin each sequence consists of a shallowing upward prograding facies succession sequences 1 upper nabala stage and 2 vormsi stage record step wise drowning of underlying shelf units lower nabala that culminated in the deposition of the most basinal facies fjacka shale in the livonian basin sequences 3 6 comprise the overlying pirgu stage and record the gradual expansion of shallow and middle shelf facies across the estonian shelf the porkuni stage sequence 7 is bracketed by erosional surfaces and contains the shallowest water facies of the preserved strata the uppermost part of the section normalograptus persculptus biozone is restricted to the livonian basin and includes redeposited carbonate and siliciclastic grains it is the lowstand systems tract of the lowest silurian sequence 8 sequence 7 and the overlying basinal redeposited material i e the lowstand of sequence 8 correspond to the latest ordovician hirnantian glacial interval and the bracketing unconformities are interpreted as the widely recognized early and late hirnantian glacial maximums the sequences appear correlative to upper ordovician sequences in laurentia graptolite biozones indicated that the estonian sequences are equivalent to carbonate ramp sequences in the western united states great basin and mixed carbonate siliciclastic sequences in the eastern united states appalachian basin cincinnati arch region these correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

silurian carbonate shelf and slope evolution in nevada a history of faulting drowning and progradation in the early silurian a broad subtidal to peritidal shallow carbonate ramp was situated in central and northern nevada along both the northern and western margins the shallow ramp merged into a deep water basin the northern shallow ramp margin coincident with the tooele arch had a narrow grainstone shoal facies belt that passed downslope into deep ramp environments the grainstone shoal facies at the western shallow ramp margin formed a wide belt to the north and a narrow belt to the south shallow ramp sediments thinned northward toward the shallow ramp margin indicating slight differential down to south subsidence of the shallow ramp later in the silurian c 4 llandovery the ramp was abruptly drowned simultaneous down to north subsidence along the northern shallow ramp margin possibly led to surface faulting and the formation of an escarpment bounded shelf subsequently the drowned ramp evolved into a rimmed shelf the rim consisting of high energy skeletal sand shoals eastward progradation of peritidal sediments from the shelf rim filled in the shelf during the llandovery and wenlock the rim facies also prograded oceanward over the western slope during the later silurian along the northern shelf margin the rim facies was less well developed and oceanward progradation over the northern slope was slow

changes in marine isotopic composition and the late ordovician glaciation well preserved brachiopod shells and marine cements from limestone coquinas which cap carbonate mudmounds in the siljan area of central sweden have the heaviest stable oxygen and carbon isotope values yet reported for lower palaeozoic marine sediments the coquinas formed during the eustatic regression at the onset of the late ordovician glaciation the isotopic compositions reflect simultaneous shifts in both carbon and oxygen more than 5 13 c and up to 2o 18 o away from more normal lower palaeozoic values in similar carbonates from within the mounds oxygen isotope data are consistent with a change in the isotopic composition of the sea water probably accompanied by a decrease in temperature the changes in carbon values suggest enhanced deposition of organic carbon a process which would have decreased p co 2 in the ocean and the atmosphere and thus contributed to rapid global cooling