cambrian grand cycles a northern appalachian perspective large scale sedimentary cycles or grand cycles are the most obvious features of the port au port group in southwestern newfoundland each cycle is interpreted to represent deposition in outer platform muddy tidal flats lower shaly half cycle and ooid shoal complexes upper carbonate half cycle unpredictable metre scale assemblages of oolite and carbonate laminite constituting the carbonate half cycles are attributed to frequent migration of the shoal complexes in response to hydrographic factors such as tidal fluctuations and storms predictable metre scale shallowing upward cycles of parted limestone and shale in the shaly half cycles were probably controlled mainly by variable rates of carbonate sedimentation a eustatic mechanism however cannot be discounted comparison with other north american grand cycles suggests a eustatic mechanism for grand cycle formation possibly involving variations in the rate of sea level rise the interpreted sequence of formation established on the basis of the port au port group involves 1 a rapid sea level rise which flooded the platform with shoreward derived siliciclastic muds and resulted in the development of muddy tidal flats and 2 the gradual development of ooid shoal complexes which encroached onto the tidal flats with decreasing rates of sea level rise in contrast to western north american examples newfoundland grand cycles are composed almost entirely of peritidal sediments they are also thinner and have a smaller areal extent than do those in the west these differences may be due to such factors as narrower shelves slower rates of sedimentation and lower amplitudes of sea level rise on the present northeastern margin of the north american craton than on the western margin

test ban treaty verification with regional data a review this paper summarizes the use of regional 30 seismic data in a test ban context for detecting locating identifying and determining the yield of underground nuclear explosions in many areas of the world eastern north america africa eastern ussr lg is the largest amplitude wave recorded on standard seismograph systems and thus is the most appropriate phase for monitoring small magnitude events excellent location capability for near regional events has been demonstrated at the norwegian small aperture array noress using lg and p waves lg and other regional phases may contain information on source depth but such information has not been exploited to date fifteen classes of regional discriminants have been identified including 1 1 first motion 2 2 ms mb 3 3 p2 p1 ratio 4 4 excitation of short period sh waves 5 5 lg rg amplitude ratios 6 6 pn lg pg lg and pmax lg amplitude ratios 7 7 lg group velocity and energy ratios in lg 8 8 excitation of sn 9 9 third moment of frequency 10 10 generation of higher mode surface waves 11 11 peak amplitudes of love and rayleigh waves and long period surface wave energy density 12 12 prevailing period of long period love waves 13 13 spectral ratio long period s waves to rayleigh waves 14 14 spectral ratio long period love waves to rayleigh waves 15 15 frequency of the peak spectral amplitudes and spectral ratios in pn pg s and lg each of these proposed discriminants has with differing degrees of success separated some explosions from some earthquakes however most have been tested only on limited data usually from one geographic region and only one or two recording stations no systematic analyses have been done to determine the best individual discriminant or combination of them preliminary evaluation of the use of lg for yield determination suggests that regional waves hold promise in this application theoretical studies have contributed significantly to the understanding of propagation characteristics of regional waves but further studies are required emphasizing modeling for realistic anisotropic sources the major conclusion of this study is that a systematic and comparative evaluation of all the proposed regional discriminants is now required utilizing a common data base derived from all present day test sites this evaluation would suggest the optimal discrimination procedure using regional waves and would also define areas of needed research without such an integrated evaluation it is still possible to speculate using existing results on the most promising regional discriminants

guided wave propagation in laterally varying media ii lg waves in north western europe summary lg wave observations at norsar from a sequence of explosions straddling the central graben in the north sea show that the phase is almost extinguished by passing through this structure for models of the graben structure based on seismic refraction experiments with crustal thinning beneath a zone of thickened sediments numerical modelling of lg propagation shows very poor transmission the inclusion of attenuation simulating the effects of scattering from faults and other complex structures further diminishes the transmittivity the theoretical effect of a crustal pinch is to redistribute s energy over a wide range of groups velocities a pattern that fits well with the observations three components recordings at norsar of explosive charges fired in water covering a wide range of azimuths show significant transverse components to the seismograms the transverse energy builds up with travel time and for lg is of the same order as the vertical component these results are consistent with the theoretical prediction of progressive net transfer of energy from rayleigh to love modes as the wavetrain propagates through a three dimensionally heterogeneous medium

response of the indian ocean to the tibetan plateau thermal forcing in late spring abstractthe thermal effect of the tibetan plateau tp is known to exert substantial impacts on the atmospheric general circulation suggesting that it may also influence the wind driven circulatio

theoretical study of the excitation spectral characteristics and geometrical attenuation of regional seismic phases abstract we present a theoretical study of the generation and geometrical attenuation of regional crustal phases we do this through the computation of seismograms in the epicentral distance range from 60 to 500 km the geometrical attenuation of lg waves with epicentral distance is of the form r 0 83 pg wave amplitudes display a much stronger decay of the form r 1 5 the spectral density of the crustal transfer function for pg waves is relatively flat for frequencies between 0 1 and 5 hz while lg wave spectra strongly fall off beyond 2 to 3 hz the excitation of pg wave is insensitive to the depth of the source within the crust while the lg amplitude is about 50 per cent higher for a source in the upper and middle crust than in the lower crust the amplitudes of these two phases drastically decrease when the source is below the moho these results illustrate the important role of wave guide played by the crust for the propagation of lg and pg we find that the geometrical attenuation of pg and lg waves is independent of source mechanisms in the case of an explosion the excitation of pg is insensitive to the source depth the lg wave amplitude is small in comparison to pg and rayleigh waves and depends on the closeness of the source to an interface or to the free surface

the complete synthesis of seismic crustal phases at regional distances using a new method to evaluate green s functions for elastic layered media we compute complete synthetic seismograms at regional distances for an earthquake in the earth crust we choose as source parameters the ones inferred for an earthquake in central france and compare the computed seismograms to the ones recorded at several short period stations of the french seismic network the major features of the records including complexity and duration are well reproduced by the synthetics lg waves which are the prominant phase are shown to be guided waves made up of sh and sv waves incident on the moho at angles of incidence more grazing than the critical angle and multiply reflected within the crust the length of the computed lg wave coda extends between group velocities of 3 5 and 2 8 km s and is in good agreement with the data

late cambrian steptoean sedimentation and responses to sea level change along the northeastern laurentian margin insights from carbon isotope stratigraphy carbon isotopes are applied as tools for stratigraphic correlation of poorly fossiliferous upper cambrian carbonate strata in the northern u s appalachians upper cambrian steptoean marine carbonate rocks record a significant global positive carbon isotope excursion 13c 4 5 vienna peedee belemnite vpdb the timing of which is well documented in fossiliferous sections elsewhere the steptoean excursion peaks at a sea level lowstand that produced the sauk ii sauk iii sequence boundary on the north american craton in this study this excursion is documented for the first time in the northern u s appalachians in poorly exposed limestone debris flow and olistolith deposits interbedded within 20 m of continental slope shales of the schodack formation these deposits contain the only reported pre elvinia zone steptoean fauna in new york and record 13c values of up to 3 the slope carbonate sediment was mainly derived from the shelf margin and is mixed with common coarse grained silici clastic material these deposits reflect a seaward migration of the siliciclastic source area exposed craton suppressed carbonate platform sedimentation and shelf bypassing during the sauk ii sauk iii sea level fall nonfossiliferous dolostones and dolomitic marbles of the proposed carbonate platform correlatives the pine plains formation in southeastern new york and the stockbridge formation from western massachusetts also contain common coarse siliciclastics however sampled sections do not record elevated 13c values indicating that these strata are probably not of steptoean age this suggests that steptoean time is represented in the carbonate platform to slope succession of the northeastern present day laurentian margin by an extremely condensed stratigraphic interval or even a hiatus

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

comparative analysis of methods used to define eustatic variations in outcrop late cambrian interbasinal sequence development interbasinal correlation of late cambrian cyclic carbonates from the appalachian and cordilleran passive margins the texas craton and the southern oklahoma aulacogen defines six major third order depositional sequences graphic correlation of biostratigraphically constrained strata was used to establish equivalency of stratigraphic sequences between the individual sections relatively isochronous biomere boundaries were used as time datums for lithostratigraphic correlation although the individual sections are composed of different types of meter scale cycles and component lithofacies that reflect the various environmental settings of the localities the overall upward shallowing character of individual sequences is evident the sequences are late cedaria mid crepicephalus late crepicephalus aphelaspis to earliest elvinia elvinia to early saukia and saukia to the cambrian ordovician boundary interbasinal correlation of stratigraphic sequences permits an evaluation of quantitative techniques for determining accommodation history correlation of fischer plots of cyclic successions from separate basins supports a eustatic control of late cambrian sequence development r2 r3 curves derived from subsidence analysis of the late cambrian sections provide good resolution of the second and third order scales of accommodation change and interbasinal correlations of r2 r3 curves also support eustatic control on sequence development comparing the accomodation curves and subsidence analysis with paleobathymetric trends of late cambrian cyclic strata suggests that themore curves may approximate the form of the eustatic sealevel signal a composite eustatic sealevel curve for late cambrian time in north america was created by qualitatively combining the accommodation curves defined by the different techniques for each of the four localities 129 refs 16 figs 3 tabs less

average qlg qsn and observation of lg blockage in the continental margin of nova scotia the term lg blockage refers to the sudden disappearance of the lg phase along a particular propagation path which is commonly seen at continental oceanic transition zones in this paper we present observational evidence of lg blockage across the continental margin of nova scotia in eastern canada regional lg and sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2 5 and 4 7 are inverted simultaneously for the source spectrum site amplification and average attenuation the vertical displacement spectra were estimated between 0 9 and 10 75 hz the assumptions include a fixed frequency independent geometric spreading rate for lg and a frequency dependent spreading model for the sn estimates for the apparent regional attenuations are qlg f 615 25 f0 35 0 04 and qsn f 404 23 f0 45 0 03 results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region based on the observation of a strong trade off between sn and lg amplitudes which have different attenuation characteristics we conclude any attenuation study based on measuring amplitude of a package of several different phases without taking into consideration the propagation characteristics of individual waveforms at the region of study may bias the estimation of average regional q

calculation of synthetic seismograms in a laterally varying medium by the boundary element discrete wavenumber method abstract we investigate the propagation of lg waves in laterally varying crustal structures by numerical simulation the method of calculation is formulated in terms of boundary integral equations where the green9s functions are evaluated by wavenumber summation the approach is well suited to study the propagation of seismic waves in a layered medium where the interfaces are flat in some regions and irregular in others we investigate the effect of a crustal fault with vertical offset and study the case of a lateral change in crustal thickness the results show that the lg wave amplitude is only slightly affected by the presence of these heterogeneities they confirm the robustness of lg wave propagation in presence of lateral heterogeneities observed in other numerical simulations they show that large scale geometric features of the crust cannot account alone for the strong attenuation of lg waves observed in many regions the results also suggest a possible relation between the level of lg wave coda and the degree of roughness of the moho they further indicate the importance of backscattering and suggest a possible use of the backscattered wave field to map strong crustal heterogeneities

domain reduction method for three dimensional earthquake modeling in localized regions part i theory this article reports on the development of a modular two step finite element methodology for modeling earthquake ground motion in highly heteroge neous localized regions with large contrasts in wavelengths we target complex geo logical structures such as sedimentary basins and ridges that are some distance away from the earthquake source we overcome the problem of multiple physical scales by subdividing the original problem into two simpler ones the first is an auxiliary problem that simulates the earthquake source and propagation path effects with a model that encompasses the source and a background structure from which the lo calized feature has been removed the second problem models local site effects its input is a set of equivalent localized forces derived from the first step these forces act only within a single layer of elements adjacent to the interface between the exterior region and the geological feature of interest this enables us to reduce the domain size in the second step if the background subsurface structure is simple one can replace the finite element method in the first step with an alternative efficient method the methodology is illustrated in a companion paper yoshimura et al 2003 for several 3d problems of increasing physical and computational complexity we consider first a flat layered stratigraphic system for this simple case the first step can be carried out by means of 3d green s function evaluations the extension to more general problems is illustrated by two examples a basin and a hill with the same background stratigraphy to verify the two step procedure with a problem for which the finite element method is used throughout we model ground motion in a small region of the los angeles basin using both the two step domain reduction method and the traditional approach in which the computational domain contains both the source and the geological region of interest

the middle late cambrian reef transition and related geological events a review and new view abstract the reefs of the middle late cambrian cambrian epoch 3 furongian have long been simply considered as microbialites that flourished in the aftermath of the archaeocyath extinction thorough review of the reefs in these periods shows however that the cambrian epoch 3 and the furongian actually yield different types of reefs the cambrian epoch 3 reefs are dominated by thrombolites and dendrolites largely constructed by the calcified microbes epiphyton and renalcis on the other hand the furongian reefs consist mainly of maze like maceriate reefs and columnar stromatolites the maceriate reefs most likely formed by siliceous sponges and calcified microbes including girvanella and tarthinia whereas the columnar stromatolites were mainly constructed by girvanella tarthinia and minor siliceous sponges other microbial reefs e g non columnar stromatolites persisted during the cambrian epoch 3 and the furongian mainly as small patch reefs or reefal crusts lithistid sponge microbial reefs initially formed in the cambrian epoch 3 and occurred throughout the furongian but occupied only a minor portion during these periods several geological events occurred across the boundary between the cambrian epoch 3 and the furongian including positive carbon and sulfur isotope excursions a eustatic sea level drop major faunal turnover of trilobites and diversification of new organisms the coincidence of these events and the transition in reefal fabrics suggest that reef ecosystems were significantly influenced by at least some of these events among the events it is likely that sea level change would have affected reefs although further studies are required in order to test whether and how the other events influenced the reef transition epiphyton and renalcis and thrombolites and dendrolites formed by them declined greatly most likely due to sea level fall and thus decreased in shallow marine habitat at the end of cambrian epoch 3 the cambrian epoch 3 thrombolites and dendrolites were gradually replaced by a sponge microbial association of the maze like maceriate reefs and the columnar stromatolites in the furongian coincident with a sea level rise

on the nature of regional seismic phases ii on the influence of structural barriers summary the blockage of the lg wave by crustal barriers such as continental margins and graben structures has long been recognized as providing a very useful tool for mapping large scale lateral crustal variations along the propagation path numerical simulation of lg wave propagation in complex anelastic media using the pseudospectral method provides insight into the nature of the propagation process using both snapshots of the wavefield and synthetic seismograms a variety of 2 d structures have been investigated including the influence of sediments crustal thickness and attenuation thick sedimentary basins covering a graben structure can have a major influence since they remove lg energy by generating p conversion and scattering the principal mechanisms for strong lg attenuation across a graben the reduction of the lg energy is reinforced by anelastic attenuation in the sediments as well as the influence of the gradually thinning crustal waveguide associated with an elevated moho the extinction of lg in a sequence of explosions fired across the central graben of the north sea can be simulated by numerical calculations for the structure derived from refraction experiments

attenuation of multiphase surface waves in the basin and range province part i lg and lg coda summary broad band data recorded by the lawrence livermore laboratory network are used to investigate whether or not lg q and lg coda q have similar values in the basin and range province of north america in order to retrieve spectral estimates of the broad band lg and lg coda signals a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies a stochastic approach is used to model the lg signal and ambient noise and to analyse results from two station determinations of lg q in the basin and range the effects of ambient noise frequency dependent station site response and focusing defocusing on lg q inversion are analysed it is shown that after averaging spectral ratios determined from repeated observations over four two station paths a modest site effect will not cause significant errors in the resulting lg q relation using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0 2 and 2 5 hz we obtain the relation qlg f 267 f 56 f0 37 0 06 for the basin and range province further constraints on lg q were obtained from lg coda which due to the natural averaging effect of scattering is insensitive to effects of focusing defocusing to investigate the validity of the single isotropic scattering model for lg coda in the basin and range province we calculated ischrons for scatterers corresponding to later lg coda signals as required by the single scattering model it is found that when significant portions of these isochrons are oceanic lg coda amplitude decay becomes irregular and abnormally slow suggesting a strong scattering reflection effect caused by the rapidly changing crustal structure along the coast applying the single isotropic scattering model to lg coda records with isochrons mostly confined to the continental crust results in q f 275 f 26 f 0 36 0 03 which is nearly identical to our qlg relation this indicates that even in a complicated geological province a joint interpretation of the dynamic properties of lg and lg coda can be successful and fruitful inconsistent lg q and coda q relations found in earlier studies in the basin and range province may be attributed to different settings of parameter trade offs different seismic phases or different dynamic properties being used

neoproterozoic stratigraphy of the centralian superbasin australia abstract the basement of central australia includes the arunta block and the 1200 1100 ma musgrave block succeeded by 1075 1000 ma bimodal volcanic rifts and dolerite dykes a regional hiatus at 1000 800 ma interpreted as reflecting a central post volcanic underplated upland was followed at 800 ma by a second swarm of dykes in the musgrave block gawler craton and stuart shelf associated with the crustal sagging that initiated the centralian superbasin the centralian superbasin is defined here to encompass the neoproterozoic fill of the amadeus georgina ngalia officer and savory basins the intra cratonic superbasin was disrupted internally 540 600 ma ago petermann ranges orogeny by a central uplift and associated thrusts and by mid carboniferous 320 ma tectonism alice springs orogeny to form the structural basins listed above the stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions new results from acritarch biostratigraphy and isotope chemostratigraphy in conjunction with conventional lithostratigraphy and sequence analysis allow better stratigraphic resolution than previously possible field observations in the recently discovered savory basin in western australia provide a basis for correlations with the adjacent basins

evaluating the relationship between the carbon and sulfur cycles in the later cambrian ocean an example from the port au port group western newfoundland canada abstract we present a high resolution 34 s sulfate and pyrite and 13 c carbonate record from the middle upper cambrian port au port group a mixed carbonate siliciclastic succession exposed in western newfoundland canada the results illustrate systematic 34 s sulfate shifts of 15 over relatively short stratigraphic intervals 10 m likely 34 s sulfate pyrite ca 23 and a generally positive coupling between changes in 13 c carbonate and 34 s sulfate together these results indicate that middle to late cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to change than it was in either terminal neoproterozoic or cenozoic oceans however a simple carbon c and sulfur s isotope box model of the late cambrian ocean illustrates that low sulfate concentrations alone fail to account for the 15 34 s sulfate shifts recognized in port au port strata such large shifts can be generated only if fluctuating oceanic redox is invoked marine anoxia forces reduced c s burial and elevated 34 s driving larger 34 s changes per mole of organic carbon buried the conclusion that later cambrian oceans featured both low sulfate levels and widespread subsurface anoxia supports hypotheses that link fluctuating marine redox conditions in the delayed recovery of skeletal animals and metazoan reefs from late early cambrian extinction

upper cambrian carbonate sequences of the argentine precordillera and the steptoean c isotope positive excursion spice abstract carbon and sr isotope profiles in upper cambrian platformal carbonate formations in the precordillera western argentina zonda la flecha and la silla formations were constructed for three representative sections a quebrada de la flecha eastern precordillera b cerro la silla central precordillera and c quebrada de la angostura northern part of the central precordillera at quebrada de la angostura upper part of the la flecha formation 13ccarb varies continuously up section from 2 0 to 5 6 pdb and records the spice anomaly 5 reported for the first time in south america the peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea level change the steptoean zonda formation dolomites at the quebrada de la flecha exhibit a total 13c range from 2 7 to 0 6 with discrete positive anomaly about 200 m from the transition to the overlying sunwaptan la flecha formation pronounced c isotope anomaly 5 6 is observed in the la flecha formation at about 300 m below the transition to the la silla formation at the cerro la silla section the zonda formation exhibit 13c values of 1 increasing slightly at the transition to the la flecha formation 1 to 0 the transition of the la flecha to the la silla formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive c isotope excursion probably corresponding to the spice at the quebrada de la flecha 87sr 86sr for the zonda formation varies from 0 70924 to 0 70955 and for the la flecha formation from 0 70908 to 0 70942 at cerro la silla this ratio varies from 0 70914 to 0 70923 for the la flecha formation and from 0 70898 to 0 70980 for the la silla formation at the quebrada de la angostura ratios for the la flecha carbonates range from 0 70918 to 0 70993 the overall variation of 87sr 86sr is consistent with globally reported upper cambrian seawater values at ca 500 ma the unambiguous record of spice in the la flecha formation at the quebrada de la angostura supports a steptoean age for its deposition and allows precise local regional and global stratigraphic correlation the pronounced negative c isotope excursion recorded in the la flecha formation carbonates at the quebrada de la flecha is likely equivalent to that registered in sunwaptan carbonates of north america and australia and might be tied to a global event as a valuable tool in stratigraphic correlation snice acronym for sunwaptan negative isotope carbon excursion

attenuation of regional phases in western europe abstract the purpose of this study is to evaluate the attenuation of local seismic phases in france we use the 25 short period vertical stations of the french seismic network and select a set of six earthquakes which occurred in france or in its vicinity and within a range of epicentral distances from 1 to 10 we have determined the attenuation versus distance for pn sn pg and lg phases and analyzed the influence of frequency attenuation coefficients for pn and sn waves are of the order of 2 they are slightly higher for pg 2 3 and lg 2 5 waves but the amplitude of these waves is largely dependent on the azimuth of propagation lg pg ratios present a large scatter between 1 and 10 with a mean trend around 3 to 4 we found that the lg anelastic attenuation coefficient is of the order of 0 2 deg 1 at 1 hz a result to be compared with the values of 0 07 deg 1 eastern united states 0 15 deg 1 northern ussr and 0 35 deg 1 south of the caspian sea obtained by nuttli 1973 1981 the quality factor computed for each one of these local phases shows a clear dependence on frequency increasing from around 100 at 1 hz to around 1000 at 16 hz

lg wave modelling for the north sea summary theoretical seismograms for inhomogencous 2 d media are computed via summation of surface wave modes and use of transmission coefficient matrices in an attempt to explain the observed lg wave barrier within thc continental structure of the north sea where other scientists have been able to trcat lg wave propagation in the north sea for one frequency many surface wave modes and a complicated structural model of the north sea grabens our method takes into account many frequencies and is consequentiy limited to a few significant modes and a relatively simple structural model

a hybrid method for the estimation of ground motion in sedimentary basins quantitative modeling for mexico city to estimate the ground motion in two dimensional 2d laterally heterogeneous anelastic media a hybrid technique has been developed that combines modal summation and the finite difference method in the calculation of the local wave field owing to a seismic event both for small and large epicentral distances it is possible to take into account the source path and local soil effects as a practical application we have simulated the ground motion in mexico city caused by the michoacan earthquake of september 19 1985 by studying the one dimensional 1d response of the two sedimentary layers present in mexico city it is possible to explain the difference in amplitudes observed between records for receivers inside and outside the lake bed zone these simple models show that the sedimentary cover produces the concentration of high frequency waves 0 2 to 0 5 hz on the horizontal components of motion the large amplitude coda of ground motion observed inside the lake bed zone and the spectral ratios between signals observed inside and outside the lake bed zone can only be explained by 2d models of the sedimentary basin in such models the ground motion is mainly controlled by the response of the uppermost clay layer the synthetic signals explain the major characteristics relative amplitudes spectral ratios and frequency content of the observed ground motion the large amplitude coda of the ground motion observed in the lake bed zone can be explained as resonance effects and the excitation of local surface waves in the laterally heterogeneous clay layer also for the 1985 michoacan event the energy contributions of the three subevents are important to explain the observed durations

facies distribution and coral microbialite reef development on a low energy carbonate ramp chay peninsula kimmeridgian western france abstract the chay peninsula western france succession corresponds to a kimmeridgian shallow and low energy ramp system that developed in the northeast of the aquitaine basin the sedimentation shows a transition from a carbonate dominated to a mixed carbonate siliciclastic depositional environment that takes place at the base of a major third order sea level transgression facies partitioning and coral microbialite reef levels i e reef windows suggest higher frequency fourth order sea level oscillations reef growth phases i e high frequency reef windows are interpreted to have been controlled by fifth order sea level oscillations in the mixed siliciclastic carbonate system successive prograding units of shoreface sediments pass into thick marl limestone alternations over a distance of tens of metres such remarkable short distance lateral facies variations suggest a complex sedimentary system made of juxtaposed deposits that rapidly shift through time terrigenous fluxes in modifying the light intensity and the nutrient level in the water column strongly controlled the facies distribution and notably the reef composition and growth identified coral genera suggest mixed photo heterotrophic mode of nutrition and low mesotrophic conditions in marine waters for each reef growth phase nutrient increase favoured large amounts of microbialites to develop forming a crust that entirely capped the coral reef surface thus high frequency fluctuations in nutrient input in tune with fifth or higher order sea level fluctuations controlled the reef development at a millennial scale the chay peninsula reef distribution and development provide insight into understanding the response of reef communities under climatically induced environmental changes

primary aragonite and high mg calcite in the late cambrian furongian potential evidence from marine carbonates in oman acknowledgements fieldwork and sampling was funded by petroleum development oman during s al marjibis s ph d their help is gratefully acknowledged we also thank colleagues at the university of aberdeen julie dougans suerc for assisting with stable isotope analysis and dr richard hinton eimf for assistance with ion microprobe analysis profs kiessling tucker bosence coleman dr dickson and an anonymous reviewer are thanked for their helpful and encouraging comments

some recent lg phase displacement spectral densities and their implications with respect to the prediction of ground motions in eastern north america abstract lg phase displacement spectral density corner periods of recent earthquakes in central new brunswick and central arkansas are used to point out a potential source of dispersion in the relative scaling of eastern north american strong motion data the corner periods of one of the central new brunswick and several of the larger central arkansas earthquakes are atypical of other events of similar magnitude and seismic moment in the region the maximum far field ground motion of an earthquake is a function of its corner period therefore strong motion data of earthquakes that exhibit atypical characteristics such as a displaced corner period are a source of dispersion in the scaling of that strong motion data if plotted as a function of magnitude or seismic moment

correlation of lg amplitude ratios from peaceful nuclear explosions to crustal structure in northern eurasia because the lg phase is strongly affected by the variations of crustal structure in some areas understanding these factors is important for nuclear test monitoring this study examines the effects of regional tectonic structure on lg propagation by using the peaceful nuclear explosion pne profiles in russia the logarithms of lg sn and lg pcoda pne amplitude ratios within the 0 5 to 3 hz frequency band are measured and correlated with the regional crustal structures both ratios are found to decrease within the areas with thick low velocity sedimentary cover and across tectonic boundaries with abrupt variations of crustal thickness for the offset derivatives of both the logarithmic ratios a linear relationship to the slopes of the moho and the vertical travel times within the sedimentary cover is determined a strong negative slope is observed along both crustal thinning and crustal thickening segments and a positive slope follows the negative slope across a crustal thickening segment therefore crustal thinning appears to affect the lg propagation stronger than crustal thickening which could be due to focusing lg energy within the crustal wave guide the log amplitude regression provided in this article could be useful for predicting lg behavior in areas where amplitude measurements are not available

impact of mongolian plateau versus tibetan plateau on the westerly jet over north pacific ocean mountains have long been considered to play an important role in the formation of modern climate particularly in the asia pacific sector the existence of tibetan plateau tp is believed to maintain the stationary planetary wave patterns and to intensify the westerly jet over north pacific however the potential role of mongolian plateau mp has often been neglected in these studies with an atmospheric general circulation model we show that the mp despite its smaller size exerts a great influence on the planetary scale circulation and the subtropical westerly jet the mp amplifies the meridional thermal gradient at the mid upper troposphere and thus strengthens the jet which is primarily facilitated by its high latitude location by blocking westerly winds the mp forces their northern branch further northward which allows the southward penetration of cold air at the lee side and intensifies the east asian trough in contrast the impact of the tp itself is not as large as expected hence one should not simply ascribe all the mountain induced climate change to the tp uplift because other smaller topography might be also very important

questioning a widespread euxinia for the furongian late cambrian spice event indications from δ13c δ18o δ34s and biostratigraphic constraints results from a high resolution study of 13ccarb 18ocarb 34scas 34scrs and elemental concentrations ca fe mg mn and sr in the furongian kyrshabakty section southern kazakhstan are reported here the investigated interval covers the drumian to jiangshanian stages of the cambrian period respectively the regional ptychagnostus atavus to ivshinagnostus ivshini irvingella major trilobite zones 13ccarb data include the steptoean positive carbon isotope excursion spice with a local peak value of 5 the onset of spice corresponds to the kormagnostus simplex glyptagnostus stolidotus zones and pre dates the base of the paibian stage furongian series 34scas data already increase during the lejopyge armata biozone culminating in three positive excursions prior and after the spice maximum differences in onset peak values and shape of the 13ccarb and or 34scas excursions at kyrshabakty but also in almost all sections characterized by the spice are pointing towards furongian seawater that was low in sulphate concentration and heterogeneous in its carbonate carbon and sulphate sulphur isotopic composition the occurrence of benthic faunal elements in almost all spice related sections strongly supports oxygenated conditions at the seafloor therefore excluding widespread anoxia or euxinia regional anoxic conditions are most probable a positive 18ocarb excursion parallel to the spice could probably be explained by a decline in seawater ph associated with a sea level rise again no euxinic conditions would be mandatory for explaining the spice event

monsoon source shifts during the drying mid holocene biomarker isotope based evidence from the core monsoon zone cmz of india abstract a better understanding of past variations of the indian summer monsoon ism that plays a vital role for the still largely agro based economy in india can lead to a better assessment of its potential impact under global climate change scenarios however our knowledge of spatiotemporal patterns of ism strength is limited due to the lack of high resolution continental paleohydrological records here we reconstruct centennial scale hydrological variability during the holocene associated to changes in the intensity of the ism based on a record of lipid biomarker abundances and compound specific stable isotopic composition of a 10 m long sediment core from saline alkaline lonar lake situated in the core monsoon zone of central india we identified three main periods of distinct hydrology during the holocene in central india the period between 10 1 and 6 cal ka bp was likely the wettest during the holocene lower average chain length acl index values 29 4 28 6 and negative 13 c wax values 34 8 to 27 8 of leaf wax n alkanes indicate the dominance of woody c 3 vegetation in the catchment and negative d wax values concentration weighted average 171 to 147 suggest a wet period due to an intensified monsoon after 6 cal ka bp a gradual shift to less negative 13 c wax values particularly for the grass derived n c 31 and appearance of the triterpene lipid tetrahymanol generally considered as a marker for salinity and water column stratification mark the onset of drier conditions at 5 1 cal ka bp an increasing flux of leaf wax n alkanes along with the highest flux of tetrahymanol indicate a major lowering of the lake level between 4 8 and 4 cal ka bp we find evidence for a transition to arid conditions indicated by high and strongly variable tetrahymanol flux in addition a pronounced shift to less negative 13 c wax values in particular for n c 31 25 2 to 22 8 during this period indicates a change of dominant vegetation to c 4 grasses in agreement with other proxy data such as deposition of evaporite minerals we interpret this period to reflect the driest conditions in the region during the last 10 1 ka this transition led to protracted late holocene arid conditions after 4 ka with the presence of a permanent saline lake supported by the sustained presence of tetrahymanol and more positive average d wax values 122 to 141 a late holocene peak of cyanobacterial biomarker input at 1 3 cal ka bp might represent an event of lake eutrophication possibly due to human impact and the onset of cattle livestock farming in the catchment a unique feature of our record is the presence of a distinct transitional period between 4 8 and 4 cal ka bp which was characterized by some of the most negative d wax values during the holocene up to 180 when all other proxy data indicate the driest conditions during the holocene these negative d wax values can as such most reasonably be explained by a shift in moisture source area and or pathways or rainfall seasonality during this transitional period we hypothesize that orbital induced weakening of the summer solar insolation and associated reorganization of the general atmospheric circulation as a possible southward displacement of the tropical rainbelt led to an unstable hydroclimate in central india between 4 8 and 4 ka our findings shed light onto the sequence of changes during mean state changes of the monsoonal system once an insolation driven threshold has been passed and show that small changes in solar insolation can be associated with major hydroclimate changes on the continents a scenario that may be relevant with respect to future changes in the ism system

the north sea lg blockage puzzle summary the north sea lg blockage for wave paths across crustal graben structures is a well established observational fact analysis of such observations implies that lg blockage takes place in graben areas associated with sedimentary basin formation and crustal thinning these intriguing observations have triggered many theoretical studies aimed at highlighting specific lg loss mechanisms albeit so far with only moderate success our approach to this problem is to simulate seismic wavefield propagation through the crustal waveguide using 2 d finite difference techniques the graben structures are known in detail from oil exploration works in the north sea which has enabled us to use realistic crustal models in our lg synthetics in the most extreme model tested the crystalline crust thickness beneath the graben amounted to only 5 km while the overlying sedimentary pile is nearly 10 km thick at the base of the crust in the graben area the moho is elevated nearly 10 km this model has similarities to the oceanic crustal waveguide where total lg blockage is claimed for path lengths exceeding 100 km the synthetic wavefields are displayed in terms of snapshots semblance velocity analysis and time space rms amplitudes the dominant structural lg loss mechanisms are the delay of the lg waves in the thick sediments lg to rg conversions scattering by lateral heterogeneities in the sediments and s wave leakage out of the crustal wave guide and into the upper mantle a fraction of these upper mantle s waves return to the crust and appear as sn coda observationally strong sn phases of long duration are often associated with weak lg phases and vice versa our synthetics produced lg amplitude decay amounting at most to 6 10 db while observational data imply blockage amounting to 15 20 db the latter is equivalent to a pn lg magnitude difference of nearly one magnitude unit the main outcome of this study is therefore that lg wave propagation is very robust and that a dominant blockage effect associated with intrinsic attenuation that is q values of the order of 100 at 2 hz for a path length of minimum 100 km is necessary to conform to observations

physical description of lg waves in inhomogeneous continental crust summary the view of lg waves as a superposition of seismic surface waves is very useful for explaining the amplitudes of the late parts of seismograms from regional earthquakes we have recently made computations for rayleigh waves interpreting the special attenuation of lg waves in the north sea as being caused by 1 the distribution of energy over longer time intervals combined with 2 the upwards or downwards pumping of wave energy in the diverent crustal structures combined with 3 anelastic absorption no special discussion is necessary of the claim that anelastic absorption in a particular area gives a reduction in wave amplitude whilst a physical description of the geometrical evect is warranted a physical description of the wave propagation phenomena involved is given in the present paper by classifying the mode conversion that is involved in the propagation from one crustal structure into another the physics is described via relations between the phase velocities group velocities and energy dependences on depths of the various surface wave modes that exist in the diverent crustal structures we introduce the concept of energy depth similarity a parameter that is almost as easily computed as the ratios of phase velocities or group velocities but is a far better measure of the transition character the crustal structures in question are representative of 1 the north sea and its surroundings in general and 2 the viking and central grabens in the middle of the north sea wave propagation is fundamentally diverent for diverent wave frequencies it is coherent for long period waves and incoherent for short period waves the propagation may generally be termed well behaved for wave periods longer than several seconds that is energy cross coupling between incoming and outgoing modes follows some regular pattern this regularity is highly dependent on the similarity of the two modes on either side of a discontinuity and also on the similarity of their neighbouring modes a characteristic of the regularity is that there is no mode conversion for very longperiod modes in contrast a pattern cannot be recognized for waves of shorter period and one may use the term ill behaved the lg waves in the continental crust are in the short period range because their wavelengths are short compared to thickness changes as well as to the thicknesses of crustal structures this leads to the view that lg waves are superpositions of many modes changing into even more modes at any discontinuity since rayleigh and love waves are coupled at non perpendicular incidence and the earth has many discontinuities the superposition of modes is constantly changing in relative proportions rayleigh and love modes being intermingled

regional seismic phases across the ligurian sea lg blockage and oceanic propagation abstract the ligurian sea west mediterranean is an example of a narrow oceanic basin which stops lg wave propagation locating the exact point of lg extinction was one of the goals of the sisbalig ii seismological experiment which deployed both inland stations in provence and corsica and ocean bottom seismometers obs in the ligurian sea a precise analysis of all available regional seismological data using energy diagrams reveals the role of the provence margin in the lg phase extinction for seismic events occurring in the southwestern alps the extinction is not a progressive phenomenon along oceanic travel path but occurs in a narrow zone of about 20 km width in the vicinity of the provence margin the extinction is accompanied by the generation of two types of diffracted waves the mantle phase sn v s 3 5 km s 1 and very low velocity s waves appearing both near the provence margin and in the middle of the ligurian basin we performed numerical simulation of sh wave propagation in two dimensional 2d media to investigate the mechanisms of lg extinction the simulations confirm that simple crustal thinning at the continent ocean boundary alone cannot cause the observed extinction of lg waves although the change in moho depth does induce the diffraction of a part of the incident lg wavetrain into mantle waves giving rise to the sn phase on the other side of the basin the existence of soft oceanic sediments is a very important factor in explaining the extinction as a large part of the lg energy is transferred into slow s waves trapped in the sediments moreover comparison of synthetic seismograms and observed data shows that the observed two packets of low velocity waves are an indication of the asymmetry of the ligurian basin with the existence of two shallow basins with very low s velocity 1 0 km s 1 near the provence margin and in the middle of the ligurian sea however the attenuation of lg waves in the vicinity of the provence margin is more abrupt in the data than in the simulations this discrepancy could result from the complex geometry of the continent ocean transition zone and its possibly very diffractive nature two factors that could add to lg extinction at the margin and are impossible to take into account in models

chemostratigraphy and palaeo environmental characterisation of the cambrian stratigraphy in the amadeus basin australia abstract the sedimentary succession of the central australian amadeus basin consists of neoproterozoic to carboniferous sedimentary rocks and contains shallow marine subtidal carbonates of middle to late series 2 to furongian cambrian age a combination of sequence stratigraphy geochemistry and mineralogy shows a transgressive 2nd order cyclicity deposited between 511 490 ma and a change from arid low energy to humid high energy depositional environments this is reflected in an initially evaporitic sequence with upward decreasing halite and anhydrite abundance and transition from oxygenated to anoxic conditions reflected by the fe mineral species change from hematite to pyrite during transgression sequence boundaries of several 3rd order cycles consisting of hst carbonate rocks and lst siltstones correlate with globally recognised sequence boundaries linked to the inferred eustatic sea level record for the upper two series of the cambrian system the carbon isotope record for this 1400 m thick succession in combination with biostratigraphic age correlation allowed the identification of the globally recognised steptoean positive carbon isotope excursion spice drumian carbon isotope excursion dice and redlichiid olenellid extinction carbon isotope excursion roece

elucidating the relationship between the later cambrian end marjuman extinctions and spice event abstract the late cambrian early ordovician transition contains several discrete marine extinction events the first of these extinctions the end marjuman occurs in two phases and is thought to coincide with the beginning of the steptoean positive carbon isotope excursion or spice a large excursion in the marine carbon isotope record that represents a large perturbation to the carbon cycle during this time additionally the carbon isotope record from the deadwood formation in the black hills of south dakota usa displays a small negative 13c excursion near the end marjuman extinctions here we examine the carbon isotope stratigraphy of the upper cambrian portion of the conasauga group of the southern appalachians usa to determine the relative timing between the extinction events and changes in the carbon cycle represented by excursions within the carbon isotope record previous high resolution biostratigraphic studies have identified a thick stratigraphic record of end marjuman extinctions within the conasauga group making it an excellent target for a high resolution chemostratigraphic study in the conasauga group there is no change in carbon isotope stratigraphy across the first phase of the end marjuman extinctions suggesting no major change occurred in the carbon cycle during this time further a negative 13c excursion is absent in the conasauga group across the interval that contains the end marjuman extinctions this suggests that the excursion in the deadwood formation is either a local oceanographic signal or a diagenetic feature finally the onset of the spice occurs at the same stratigraphic point as the second phase of the end marjuman extinctions and at the appearance of a low diversity potentially low oxygen tolerant trilobite fauna the stratigraphic positions of these biological and geochemical events suggest a role for marine anoxia in the second phase of the end marjuman extinctions

seismic event discrimination abstract discrimination of small events begins with detection association and location recent advances in detection include use of updated measures of the noise variance to control the false alarm rate and updating the noise amplitude spectrum n for application of the optimum detection filter s n 2 where s is the signal spectrum postdetection processes of interest include determination of signal azimuth and emergence angle from linear three component processing association programs have recently been improved by implementation of techniques taking advantage of information provided by arrays location advances include modification of the a priori travel time residual covariance matrix to account for the fact that travel time residuals are correlated at stations near to each other simultaneous determination of location and phase velocity when using regional phases and direct use of phases other than p e g pg lg s pp and pkpbc according to most source theories small earthquakes and explosions have the same flat displacement spectrum for p waves at teleseismic frequencies aside from location discrimination therefore rests on focal mechanism differences and generation of shear waves by earthquakes the m s m b discriminant rests on the shear wave generation but there is overlap of explosions and earthquakes at low magnitudes for dip slip earthquakes low frequency 0 3 hz p waves are weak from explosions due to the surface pp reflections p pp amplitude ratios for earthquakes vary over the focal sphere in a way impossible for a pair of explosions regional discrimination shows promise in the ratio of maximum amplitudes before and after sn and in the observed fact that shear phases from explosions detonated near high impedance contrasts are of lower frequency than those from earthquakes in the same region possibly due to the generation of the explosion shear phases by p to s scattering recent experimental analyses show that there is almost no decoupling at high frequencies in agreement with earlier theories these frequencies may be observable at regional distances

role of the tibetan plateau thermal forcing in the summer climate patterns over subtropical asia the mechanism of the tibetan plateau tp thermal forcing in influencing the summer climate patterns over subtropical asia is investigated by means of ncep ncar reanalysis diagnosis results show that since the tp is a huge elevated heating source with the strongest heating in the surface layers in summer the thermal adaptation results in a shallow cyclonic circulation near the surface and a deep anticyclonic circulation above it according to the steady barotropic vorticity equation for large scales airflow must converge in the lower layers and diverge in the higher layers over the eastern side of the tp however the western side of the tp is characterized by a reversed structure i e divergence in lower layers but convergence in higher layers hence pumping and sucking processes bring in upward and downward movement over the east and west sides of the tp respectively such a circulation is embedded in the large scale circulation that is forced by the eurasian continental heating because the tp together with iran plateau are located at the central and eastern parts of the continent and because the orography induced circulation is in phase with the continental scale circulation the role of the tp thermal forcing is to intensify the east asian monsoon to its east and the dry and hot desert climate in mid asia to its west the summertime thermal forcing of the rockies and andes can generate similar circulations along the two subtopics as the tp does since they are located near the western coasts but the lower troposphere poleward flow that is induced by orographic thermal forcing does not coincide with the poleward flows over the eastern coastal region that is induced by continental heating and the monsoon rainfall in north and south america is not as strong as in east asia however the equatorward flow and the associated subsidence induced by the two mountain ranges along the western coasts of both north and south america are in phase with those induced by continental heating these contribute to the formation of the stable low stratus clouds and strong long wave radiative cooling over the eastern subtropical pacific regions just off the western coast of the continent

high resolution stable isotope stratigraphy of the upper cambrian and ordovician in the argentine precordillera carbon isotope excursions and correlations abstract we report the occurrence of important carbon isotope excursions in early paleozoic formations of the eastern and central argentine precordillera the steptoean positive isotope carbon excursion spice is known from north america kazakhstan south china australia and south america and the negative isotope carbon excursion snice has been described for the first time in south america we report here the record of the spice and snice in a single section in the eastern precordillera in the central precordillera a minor middle darriwilian positive carbon isotope excursion mdice and a late sandbian positive isotope carbon excursion the gice 3 c bicornis biozone are reported from two sections one pre gice positive carbon isotope excursion sandbian sa1 n gracilis biozone in the central precordillera with a 13c peak of 2 is perhaps equivalent to the positive spechts ferry 13c excursion of north america a positive 13c excursion 6 n persculptus biozone recorded at the base of the late hirnantian la chilca formation probably corresponds to hice these carbon isotope excursions resulted from more than one factor where paleoceanographic events probably played a major role a sea level fall and vigorous fluctuations in the steptoean spice b sea level rise in the sunwaptan snice c important transgression in the sandbian pre gice and gice and d sea level fall in the late hirnantian hice in the darriwilian and sandbian stages organic burial has led to a large 12c sequestration in the deep anoxic ocean with saline bottom water recorded by the graptoliferous black shales of the gualcamayo and los azules formations in central precordillera helped the building of the mdice and gice anomalies 18o values for the upper cambrian carbonates are likely near primary isotope signals that point to progressive cooling from the spice to the snice whereas for sandbian carbonates they suggest strong temperature fluctuations the 13c peak of the gice coincides with a cooler period with temperatures warming up towards the late hirnantian to rhuddanian the upper cambrian to the middle ordovician of the precordillera recorded a decrease of seawater 87sr 86sr ratios in accordance to the global picture this decrease probably reflects the influence of widespread volcanic activity from arc terranes in low latitude settings along eastern margins of laurentia and in the argentine precordillera in the early to middle ordovician five carbonate samples from san juan gualcamayo and los azules formations yielded end values that along seven already published values seem to plot along the nd isotopic evolution trend of the iapetus ocean together the record of global upper cambrian and ordovician carbon isotope excursions in the argentine precordillera is a valuable proxy in refining early paleozoic stratigraphy establishing of regional global high resolution correlations and sea level change history in south america

ray theoretical analysis of lg ray theory is used to investigate the effect that irregular crustal interfaces have on the different phases that comprise lg we concentrate on the effects of a variable moho in this initial study while keeping the surface flat the moho models considered are a cosinusoid a model of the moho below the german part of the european geo traverse egt and a model representing an ocean continent transition it is found that even slight undulations of the moho give rise to extensive multipathing this affects the amplitudes of the supercritically reflected crustal multiples comprising lg in a dramatic way two different types of ray behavior are distinguished the first type corresponding to regular or weakly chaotic ray behavior occurs at smaller values of the ray parameter and gives rise to large amplitudes at distinct epicentral distances due to focusing accompanied by gaps containing very little energy due to defocusing in between the second type corresponding to strongly chaotic ray behavior occurs at larger values of the ray parameter small changes in the take off angle in this chaotic regime produce large fluctuations in the epicentral distance of arrivals the moho below germany for example produces 140 supercritically reflected arrivals at an epicentral distance of 400 km since the amplitudes of arrivals in the chaotic region are smeared out over a large epicentral distance range they are much less prominent than the arrivals from the regular region even though they are supercritically reflected travel time fluctuations are less sensitive to the undulations in the moho than amplitude variations they are more affected by the magnitude than by the wavelength of the undulations amplitude undulations of more than 3 km produce multiple arrivals that have differences in travel time of up to 10 sec

uranium isotopes distinguish two geochemically distinct stages during the later cambrian spice event a b s t r a c t anoxic marine zones were common in early paleozoic oceans 542 400 ma and present a potential link to atmospheric po2 via feedbacks linking global marine phosphorous recycling primary production and organic carbon burial uranium u isotopes in carbonate rocks track the extent of ocean anoxia whereas carbon c and sulfur s isotopes track the burial of organic carbon and pyrite sulfur primary long term sources of atmospheric oxygen in combination these proxies therefore reveal the comparative dynamics of ocean anoxia and oxygen liberation to the atmosphere over million year time scales here we report high precision uranium isotopic data in marine carbonates deposited during the late cambrian spice event at ca 499 ma documenting a well defined 0 18 negative 238 u excursion that occurs at the onset of the spice event s positive 13

on the attenuation of lg waves in western and central asia and their use as a discriminant between earthquakes and explosions data from earthquakes and explosions in the western and central parts of the soviet union are used to determine the values of the coefficient of anelastic attenuation of 1 sec period lg waves from the russian uplands region and for the region from the caspian sea to the southern perimeter of the soviet union for the former region the value is approximately 0 15 deg 1 and for the latter 0 35 deg 1 using the empirical veith and clawson 1972 curve for the amplitude of p waves at regional distances and theoretical curves for the attenuation of higher mode surface waves for lg curves are constructed which show the variation of the amplitude ratio of lg to p with epicentral distance with the coefficient of anelastic attenuation as a parameter when data from explosions and earthquakes in the soviet union are compared with the calculated curves the amplitude ratios of lg and p on the average are larger for earthquakes than explosions however there is much overlap so that this ratio by itself does not appear to be a useful discriminant between earthquakes and explosions

the link between tibetan plateau monsoon and indian summer precipitation a linear diagnostic perspective the thermal forcing of the tibetan plateau tp is analyzed to investigate the formation and variability of tibetan plateau summer monsoon tpsm which affects the climates of the surrounding regions in particular the indian summer monsoon precipitation dynamic composites and statistical analyses indicate that the indian summer monsoon precipitation is less greater than normal during the strong weak tpsm strong weak tpsm is associated with an anomalous near surface cyclone anticyclone over the western part of the tibetan plateau enhancing reducing the westerly flow along its southern flank suppressing favoring the meridional flow of warm and moist air from the indian ocean and thus cutting providing moisture supply for the northern part of india and its monsoonal rainfall these results are complemented by a dynamic and thermodynamic analysis i a linear thermal vorticity forcing primarily describes the influence of the asymmetric heating of tp generating an anomalous stationary wave flux composite analysis of anomalous stationary wave flux activity after plumb in j atmos sci 42 217 229 1985 strongly indicate that non orographic effects diabatic heating and or interaction with transient eddies of the tibetan plateau contribute to the generation of an anomalous cyclone anti cyclone over the western tp ii anomalous tpsm generation shows that strong tpsm years are related to the positive surface sensible heating anomalies over the eastern tp favoring the strong diabatic heating in summer while negative tpsm years are associated with the atmospheric circulation anomalies during the preceding spring enhancing northerly dry cold air intrusions into tp which may weaken the condensational heat release in the middle and upper troposphere leading to a weaker than normal summer monsoon over the tp in summer

phosphorus sources for phosphatic cambrian carbonates the fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating but time bounded window into ediacaran cambrian animal evolution for reasons that remain unknown phosphatic fossil preservation declined significantly through cambrian series 2 here we investigate the phosphorus p sources for phosphatic cambrian carbonates presenting sedimentological petrographic and geochemical data from the cambrian series 2 3 thorntonia limestone australia some of the youngest cambrian strata to display exceptional phosphatic preservation of small shelly fossils we find that within thorntonia sediments phosphate was remobilized by organic decay and bacterial iron reduction with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils we discuss the merits of bioclastic derived organic matter bound or iron bound p as potential sources to these strata petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the p for fossil preservation in contrast high organic carbon contents imply significant organic fluxes of p to thorntonia sediments sedimentology and iron speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic ferruginous conditions in subsurface water masses suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second significant p source to thorntonia sediments simple stoichiometric models suggest that by themselves neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment thus we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the thorntonia limestone

lateral variation of lg wave propagation in southern mexico 1 in this study we investigated lateral variation of lg wave propagation in southern mexico from recordings of 92 crustal earthquakes along 591 travel paths the efficiency of lg propagation was measured in terms of lgto pn spectral ratio it was found that lgpropagation is inefficient for travel paths through the gulf of mexico coastal plains and the gulf of tehuantepec areas with thick layers of sediments an average lg quality factor qlg as a function of frequency for southern mexico was estimated for the efficient lg travel paths the relation obtained for qlg in the frequency range 1 6 8 hz is qlg f 204 f0 85 the lateral variation of qlg 1was solved as a mixed determined inverse tomography problem separately for each frequency in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions the spatial resolution obtained was about 200 km it was found that the trans mexican volcanic belt the gulf of mexico coastal plains and the area east of 94 w are characterized by lower than average qlgvalues i e higher attenuation high qlg values were obtained for the mixteco oaxaca terranes while for the guerrero terrane values similar to the average were obtained the results show a correlation between qlg and crustal structure and provide valuable information on lateral variation of qlg which is needed for reliable prediction of ground motion during future earthquakes

lg and rg waves on the california regional networks from the december 23 1985 nahanni earthquake we investigate lg and rg propagation in california using the central and southern california regional networks approximately 550 stations constitute these two short period networks providing a dense coverage of almost the entire state the waveforms recorded from the december 23 1985 nahanni canada earthquake are used to construct three profiles along the propagation path almost n s and three perpendicular to the propagation path almost e w to look at the nature of propagation of these two types of surface waves groups of records from stations in various geological and tectonic provinces in california are also examined in order to establish regional characteristics of the surface waves we find that the propagation characteristics of lg differ from those of rg across california lg waves are apparently more sensitive to crustal heterogeneities the most striking observations are the similarity of coda for both the lg and the rg waves within geologic provinces and the marked difference in coda between regions these differences are seen in the amplitudes coda duration shape of the energy envelope frequency content and sharpness of the phase initiation in general a decrease in the moho depth near the pacific coast is correlated with a decrease in the surface wave amplitude especially at higher frequencies 0 15 0 2 hz most interesting is the association of the san andreas fault with abrupt changes in the wave train amplitudes the surface waves are amplified in the vicinity of the fault zone and then decrease in amplitude after the zone is crossed in the coast ranges amplitudes are low and waveform coherence is poor the rg phase dominates the record in the sierra nevada and both surface waves are amplified by the thick sedimentary sequence of the great valley

the effect of crustal structure on the regional phases pg and lg at the nevada test site the effect of local structure on the characteristics of the regional phases pg and lg was investigated by analyzing events at epicentral distances less than 10 from the nevada test site the stations used are on markedly different geological structures three on granitic intrusions two on pahute mesa a feature overlaying a buried volcanic caldera and finally four on yucca flats a deep alluvial valley the crustal effect on amplitudes is quite significant for both lg and pg yucca flats gives amplitudes as much as a full order of magnitude higher than the stations on granite while pahute mesa stations are 0 7 0 8 magnitude units higher in amplitude than the granite sites besides gross amplitude effects the local crust also induces prolonged ringing at yucca flats resulting in the lengthening of the signal wave trains the observed site amplification effects show no clear dependence on the azimuths of arrivals indicating that the phenomenon is due to local structures and is not caused by lateral refraction of surface wave modes the observed amplification is similar in magnitude for the vertical component of both pg and lg thus not affecting the pg lg amplitude ratio a possible discriminant between earthquakes and explosions at regional distances finite difference theoretical simulations using two dimensional models of the yucca basin reproduce in a qualitative sense the ringing effect observed at the yucca flats any attempt at matching characteristics of pg and lg with synthetic seismograms computed for laterally homogeneous layered structures is apt to lead to false conclusions about the source and path properties unless the recording sites used are carefully chosen for small crustal effects

carbon isotope stratigraphy of upper cambrian steptoean stage sequences of the eastern great basin record of a global oceanographic event a large global positive excursion in 13 c from 0 5 to 4 5 during the late cambrian pterocephaliid biomere steptoean stage aphelaspis elvinia zones is documented at high stratigraphic resolution in three sections in the eastern great basin the excursion which we refer to as the steptoean positive carbon isotope excursion or spice excursion began coincident with a world wide extinction event the 13 c data from the great basin reflect global seawater values in a wide range of lithologies including oolitic grainstones wackestones thrombolitic boundstones and flat pebble conglomerates we use a section at shingle pass in the southern egan range to divide the pterocephaliid biomere into eight isotope steps that represent equal divisions of the 13 c rise and fall 4 this provides a basis for recognition of a revised chronostratigraphic framework for the pterocephaliid biomere steptoean stage strata deposited during the beginning of the spice excursion record a major change in the pattern of sedimentation in the eastern great basin this is reflected in a siliciclastic carbonate transition at shingle pass nevada and a carbonate siliciclastic transition at the house range and lawson cove sections in utah a regional siliciclastic influx recognized throughout the great basin occurs near the peak of the spice excursion carbon isotope analyses from cratonal sections in wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the spice excursion the correlated changes in 13 c relative sea level and the marine biota during the spice excursion provide remarkably detailed records of a major paleoceanographic event we speculate that changes in sea level climate or tectonics may have triggered the spice excursion and coeval extinction event subsequent burial of organic carbon caused the increase in 13 c and may have led to an interval of global cooling the results of this study lend confidence to carbon isotopic studies of pre mesozoic rocks

sensitivity of east asian monsoon climate to the uplift of the tibetan plateau abstract using the cola center for ocean land atmosphere studies general circulation model a series of numerical experiments were conducted to examine the impact of uplift of the tibetan plateau on the evolution of the monsoon climate in east asia an attempt was made to isolate the effect of the plateau uplift from the changes in other forcing mechanisms we examined the spatial and temporal variation patterns of a variety of climatic variables and indices including monsoon intensity sea level pressure surface and upper air winds temperature at different levels precipitation air humidity and soil moisture our results suggest that the evolution of the east asia monsoon may be more sensitive to the uplift of the tibetan plateau than that of the south asia monsoon moreover the effect of the plateau uplift on the east asia winter monsoon is more significant than that on the summer monsoon in the northern part of east asia north china the formation of the monsoon climate is marked mainly by the establishment of the northerly winds in winter or the winter monsoon system corresponding to the uplift of the plateau when the height reaches approximately 50 of its current elevation on the other hand the establishment of the monsoon climate in the southern part of east asia the yangtze river valley and the area south of it may be much earlier than that in the northern part

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

a numerical model study on the behaviour of asian summer monsoon and amoc due to orographic forcing of tibetan plateau simulations using the echam5 mpi om coupled atmosphere ocean model both with and without the tibetan plateau are performed in order to study the large scale effects of orographic forcing on the behaviour of the asian summer monsoon system our analysis emphasises the significant impact of plateau forcing on the atmosphere ocean interactions it is argued that in addition to the orographic forcing of the tibetan plateau on the climate of asia such as sensible heat pumping and thermal insulation other significant direct processes exist which link the asian summer monsoon to the sea surface temperatures in the north atlantic ocean the removal of the tibetan plateau modifies the wind driven ocean circulations over the north atlantic which leads to a decrease in the surface heat advection over the north atlantic ocean and a decrease in the atlantic meridional overturning circulation this in turn affects via teleconnections both the monsoon rainfall and the position of the intertropical convergence zone

the late cambrian spice δ13c event and the sauk ii sauk iii regression new evidence from laurentian basins in utah iowa and newfoundland carbon isotope data from upper cambrian sections in three laurentian basins in northern utah central iowa and western newfoundland record a large positive d 13 c excursion spice event of up to 1 5 peak d 13 c ratios are well dated by trilobite collections to the middle of the steptoean stage dunderbergia zone and occur during maximum regression associated with formation of the sauk ii sauk iii subsequence boundary on the north american craton max imum regression was marked by an influx of quartz sand into carbon ate platform settings in all three widely separated basins in northern utah this quartz sand formed a thick sequence known as the worm creek quartzite which marks a conspicuous interruption of carbonate deposition during the middle to late cambrian in the region in west ern newfoundland the thickness of the quartz sand unit is much re duced but still marks a brief shutdown of the carbonate factory that is unique to the cambrian shelf succession of the area in the central iowa area of the cratonic interior an upward shallowing carbonate succession culminates in cross stratified trilobite grainstones at the peak of the spice in dunderbergia zone time and the lowest point on the relative sea level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface although it is difficult to determine precisely the departure from baseline d 13 c that marks the beginning of the spice excursion in the stratigraphic successions analyzed our results are consistent with a rise and subsequent fall in d 13 c tracking a major regressive transgressive event recorded across northern laurentia the correlation of a major d 13 c excursion with regression is similar to that described for the late ordovician for which the pattern has been attributed to either in creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic matter containing silicates at high latitudes or high productivity and organic carbon burial driven by oceanic overturn the lack of known steptoean age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic carbon burial was the likely cause of the spice event we suggest that increased weathering and erosion rates during relative sea level fall sauk ii iii increased the burial fraction of organic carbon in an expanded region of fine grained siliciclastic deposits in shelf and upper slope environments during the steptoean

oxygen isotope trends and seawater temperature changes across the late cambrian steptoean positive carbon isotope excursion spice event the globally recognized late cambrian steptoean positive c isotope excursion spice is characterized by a 3 5 positive 13 c shift spanning org and pyrite we analyzed 18 o values of apatitic inarticulate brachiopods from three upper cambrian successions across laurentia to evaluate paleotemperatures during the spice 18 o values range from 12 5 to 16 5 estimated seawater temperatures associated with the spice are unreasonably warm suggesting that the brachiopod 18 o values were altered during early diagenesis despite this all three localities show similar trends with respect to the spice 13 c curve suggesting that the brachiopod apatite preserves a record of relative 18 o and temperature changes the trends include relatively high 18 o values at the onset of the spice decreasing and lowest values during the main event and an increase in values at the end of the event the higher 18 o values during the global extinction at the onset of the spice suggests seawater cooling and supports earlier hypotheses of upwelling of cool waters onto the shallow shelf decreasing and low 18 o values coincident with the rising limb of the spice support the hypothesis that seawater warming and associated reduced thermohaline circulation rates contributed to decreased dissolved o 2 concentrations which enhanced the preservation burial of c org causing the positive 13 c shift

regional relationships among earthquake magnitude scales various magnitude scales commonly used and the interrelationships among them are reviewed it is shown that problems exist with all of the magnitude scales being used in the united states when using regional catalogs for example it is often necessary to determine how the reported magnitudes were determined often such information is not available although the potential errors are quite large both the ms and the mb scales were designed to be universal scales however both ms and mb magnitudes are often determined beyond the applicable range of the equations used to define the two scales furthermore the ms magnitudes are not generally available for moderate to small earthquakes in most earthquake catalogs the mb magnitudes are more generally available than ms values however there is also much greater variation in the way mb is determined in particular a significant change in the mb scale occurred in the early 1960 s when the world wide standard seismograph network wwssn was established this change in instrumentation used to determine mb values had a significant effect on estimated magnitudes post 1960 values are lower and the saturation level of the mb scale the older longer period instruments recorded larger mb magnitudes than can be recorded with the wwssn instruments in addition great care must be taken when selecting the mb magnitudes of western u s earthquakes because the values are often in considerable error owing to the fact that they were determined at distances less than 25 and were not properly corrected for attenuation in the upper mantle or asthenosphere the seismic body wave magnitude mb of an earthquake is strongly affected by regional variations in the q structure composition and physical state within the earth therefore because of differences in attenuation of p waves between the western and eastern united states a problem arises when comparing mb values for the two regions a regional mb magnitude bias exists which depending on where the earthquake occurs and where the p waves are recorded can lead to magnitude errors as large as unit there is also a significant difference between mb and ml values for earthquakes in the western united states an empirical link between the mb of an eastern u s earthquake and the ml of an equivalent western earthquake is given by ml 0 57 0 92 mb east this result is important when comparing ground motion between the two regions and for choosing a set of real western u s earthquake records to represent eastern earthquakes

impacts of extremely asymmetrical polar ice sheets on the east asian summer monsoon during the mis 13 interglacial abstract marine isotope stage mis 13 500 ka ago was a relatively weak interglacial according to the benthic 18o and antarctic ice core records however proxy records from the northern hemisphere indicate that mis 13 was at least as warm as or even warmer than more recent interglacials with an extremely strong summer monsoon and possible melting of the greenland ice sheet in this study assuming an asymmetric hemispheric climate we investigate the response of the east asian summer monsoon to various greenland and antarctic ice sheet sizes using a set of sensitivity experiments with the hadcm3 model through factor separation analysis results show that with no greenland ice sheet and a large antarctic ice sheet there is an enhancement of summer monsoon precipitation in the east asian monsoon region of up to 7 in agreement with the direction of change from proxy reconstructions for china this enhancement is associated with a stronger land sea thermal contrast more water vapor transport to east asia and a northward shift of the intertropical convergence zone the response to the greenland ice sheet removal is related to thermodynamic and topographical effects that influence the east asian monsoon through an orographically induced wave train the response to the larger antarctic ice sheet involves stronger upwelling of circumpolar deep water in the southern ocean melts the sea ice leading to a warmer sst this warming propagates to the north affecting the east asian summer monsoon

numerical modelling of sh lg waves in and near continental margins summary the effect of transition regions between continental and oceanic structures on the propagation of l waves from continental sources is examined in particular the attenuation due to variations in layer thickness in such transition regions is calculated and explained for a suite of simple models the measured attenuation due tq the geometry of the transition regions between the oceanic and continental structures within a partially oceanic path with source and receiver in a continental structure is at most a factor of four for frequencies from 0 01 to 1 hz this is inadequate to explain the observed extinction of l along such paths this extinction has previously been attributed to the effects of the transition region geometry the method used to calculate the results presented in this study is developed and its validity and accuracy are demonstrated propagator matrix seismograms are coupled into a finite element calculation to produce hybrid teleseismic sh mode sum seismograms these hybrid synthetics can be determined for paths including any regional transition zone or other heterogeneity that exists as part of a longer mostly plane layered path numerical results presented for a suite of transition models show distinct trends in each of the regions through which the wavefield passes the wavefield passes through a continent ocean transition region then a region of oceanic structure and finally through an ocean continent transition region when an l wavefront passes through a continent ocean transition the amplitude and coda duration of the l wave at the surface both increase at the same time much of the modal l energy previously trapped in the continental crust is able to escape from the lower crust into the subcrustal layers as body waves the magnitude of both these effects increases as the length of the transition region increases when the wavefront passes through the region of oceanic structure further energy escapes from the crustal layer and produces a decrease in l amplitude at the surface the rate of amplitude decrease is maximum near the transition region and decreases with distance from it when the wavefield passes through the oceancontinent transition region a rapid decrease in the l amplitude at the surface of the crust results the energy previously trapped in the oceanic crustal layer spreads throughout the thickening crustal layer some of the body wave phases produced when the wavefield passes through the continent ocean transition region are incident on the continental crust in the ocean continent transition region these waves are predominantly transmitted back into the crust the other body wave phases reach depths below the depth of the base of the continental crust before reaching the ocean continent transition and thus escape from the system

a basin wide record of the late cambrian steptoean positive carbon isotope excursion spice in the amadeus basin australia abstract the cambrian paibian sedimentary succession of the central australian amadeus basin contains a sequence of supratidal to subtidal shallow marine siliciclastic and oolitic stromatolitic limestones and dolostones basin wide sequence stratigraphy in combination with biostratigraphy revealed the g stolidotus zone within a 3rd order transgressive systems tract tst the westward transgression caused changes from a fluvial dominated depositional environment towards a shallow marine oolitic carbonate shoal environment the eastern succession is dominated by stromatolitic oolitic carbonate rocks with 2 to 5 m 5th order shoaling upward cycles with several 4th order cycles the change from tst to hst highstand systems tract is marked by a maximum flooding surface within the goyder formation which coincides with the peak of the steptoean positive carbon isotope excursion spice the spice shows a facies independent synchronous positive 13c excursion of 5 in a 130 m interval in 8 sections across a 460 km transect the spice peak is lowest in the nearshore successions 0 4 13c and highest in the platform succession 4 9 13c and is interpreted to be related to the chemical gradient of seawater and mixing of the dic with atmospheric co2 derived i e terrestrial bicarbonate the recovery from spice is recorded by 4th order shoaling upward cycles that compose the 3rd order hst this is the first time that sequence stratigraphy and biostratigraphy has been combined with carbon isotope chemostratigraphy across the nearshore to subtidal cambrian basin succession in australia

finite difference modelling of lg blockage summary a p sv finite difference method is used to calculate seismic wave propagation in the crust and upper mantle the anelastic effect is excluded from the calculation and therefore any wavefield changes can be attributed to either geometrical or structural effects snapshots of displacement distrihutions within and synthetic seismograms at the free surface of different models revealed that the inclusion of a water column over an appropriate section of the crust is necessary to explain the severe lg attenuation widely observed in many parts of the world in particular for an oceanic propagation path they clearly show that the effect of a water column over the crust is an important factor in blocking lg piupagation a propagation path of more than 100 km over an oceanic crust can completely block the transmission of lg waves originating in a neighbouring continental crust the energy of the lg wavetrain is redistributed at continental margins most of the incoming energy is either converted to waves which propagate into the mantle or is backscattered to form waves which propagatc in the continental crust in the opposite direction very little energy is presented at the expected arrival times at the other side of the oceanic crust which is consistent with the lg blocking phenomena simplified 2 d models of the north sea central graben are used to simulate lg propagation in a laterally varying continental path the inclusion of a water column over the crust enhances the attenuation of lg the deeper the water column the greater the attenuation the undulation of sea bottom can further enhance the attenuation of lg

revised wonoka isotopic anomaly in south australia and late ediacaran mass extinction the global late ediacaran shuram wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean assuming that all ediacaran carbonate is marine disregarding carbonate in south australia shown here to be palaeosol or palaeokarst the synchronous marine organic carbonate excursion is only 8 for 13 c organic and 6 for 13 c carbonate and lasted less than a million years this revised magnitude and duration is comparable with perturbations across the permian triassic boundary and correlative with a global late ediacaran acritarch mass extinction like permian triassic isotopic excursions the revised organic carbonate wonoka excursion may also have been a greenhouse palaeoclimatic warm spike which terminated valley incision and glacioeustatic drawdown during the mid ediacaran fauquier glaciation and preceded chill of the late ediacaran billy springs glaciation supplementary material measured sections and tables of mineral and grain size proportions major element and stable isotope analyses are available at www geolsoc org uk sup18756

biostratigraphy and carbon and nitrogen geochemistry of the spice event in cambrian low grade metamorphic black shale southern norway abstract we present new chemo and biostratigraphical data from 90 m of alum shale formation stratigraphy from the middle cambrian series 3 to the lowermost ordovician tremadocian at krekling southern norway the alum shale was deposited during a transgression covering parts of the baltic craton and contains up to 13 5 wt total organic carbon toc increasing up section through the furongian pyrolysis confirms that the alum shale is affected by low grade metamorphism either from a nearby pluton or during deep burial where the remaining organic matter has lost most of the hydrocarbon productivity s2 0 0 9 mghc g rock compared to equivalent formations in sweden toc 8 22 wt s2 20 80 still the carbon isotopic trend 13 c org shows that the positive carbon isotope excursion known as the spice event furongian is preserved and that the carbon isotopes did not fractionate significantly while undergoing metamorphism the 13 c org increases steadily from the base of the section from 30 8 to a maximum of 27 3 likely due to increasing atmospheric oxygen concentrations and cyanobacteria productivity the heaviest values are recorded in an interval with abundant pyrrhotite nodules suggesting that the organic productivity led to euxinic conditions as shown from several other spice sections worldwide the evolution of bulk rock nitrogen isotopes 15 n shows positive correlations with 13 c org and toc with an increase from 2 to 1 5 before the onset of the spice negative 15 n values indicate that atmospheric nitrogen was fixed in the shallow ocean during the transition from low to high toc shale at about 18 m in the section both the nitrogen content and the 15 n increase but the 15 n is not high enough to imply water mass nitrogen reduction denitrification as a dominant process instead we suggest that the krekling section was affected by sediment denitrification during spice a process resulting in a ca 1 5 post depositional increase in 15 n at the same time in the upper parts of the water column atmospheric nitrogen was still fixed by cyanobacteria we conclude that the cambrian ocean transgressed the baltic craton and inhibited deep water upwelling leaving geochemical signals characteristic of other greenhouse climates spanning the phanerozoic

evidence of spice related anoxia on the laurentian passive margin paired δ13c and trace element chemostratigraphy of the upper conasauga group central appalachian basin abstract this study reports data from the upper cambrian conasauga group and overlying copper ridge formation of the central appalachian region eastern u s a geochemical isotopic and petrographic analysis of core material was carried out to constrain the extent of marine anoxia and to characterize sediment fluxes on the carbonate platform and continental shelf of laurentia contemporaneous with the steptoean positive carbon isotope excursion spice a late cambrian global marine anoxic event carbonate rocks primarily dolostone record a positive 13ccarb excursion starting in the middle nolichucky formation reaching its peak 4 3 at the boundary between the maynardville and copper ridge formations strontium isotope ratios in the dolostone units are only slightly offset from the expected cambrian seawater values suggesting minimal post diagenetic disturbance of isotopic and trace element systematics selective leaching of carbonate units reveal upward decreasing trends in dissolved redox sensitive trace metals e g u ni v indicative of drawdown from regional and global marine anoxia during the peak of the late cambrian spice event

the neoproterozoic cambrian record in australia a stable isotope study abstract shallow intracratonic basins developed across a large area of the central part of the australian craton during the neoproterozoic and continued to accumulate marine sedimentary successions until well into the early palaeozoic a half billion year sedimentary record carbonate successions ranging in age from neoproterozoic cryogenian to ostensibly early ordovician have been sampled at eleven sites across four of the northernmost of these basins amadeus georgina wiso and daly basins a total of 996 samples were then analyzed for stable carbon 13 c carb and oxygen 18 o carb isotopes to develop a regional secular stable carbon isotopic curve the curve complements earlier work and provides an important framework for global correlation at a critical time in earth history during which the biosphere and atmosphere were evolving rapidly at one site in the amadeus basin diagenesis proved to have significantly modified the isotopic composition of the carbonates and the data were eliminated analysis of the remaining samples suggests that they reflect the global secular stable carbon isotope curve and tie well with established palaeontological timescales in particular key isotopic excursions can be identified in the earliest part of the neoproterozoic succession notably the negative excursion following the sturtian glaciation in the early palaeozoic record a major excursion during ordian early templetonian 509 506 ma time can be identified across the region as can the so called spice excursion which has been associated globally with the steptoean stage of laurentia 497 5 495 ma a smaller late templetonian floran 506 503 ma positive excursion has also been recognized the stable carbon isotope record provides a strong regional correlation in both fossiliferous and unfossiliferous neoproterozoic and early palaeozoic successions and allows a global correlation to be established

euxinia linked to the cambrian drumian carbon isotope excursion dice in australia geochemical and chemostratigraphic evidence abstract the cambrian drumian carbon isotope excursion dice event is a negative carbon isotope 13 c excursion it is associated with the first appearance datum fad of the agnostoid arthropod ptychagnostus atavus whose first appearance in the global boundary stratotype section and point gssp defines the base of the cambrian series 3 drumian stage although this isotopic event has been clearly identified in the great basin usa it has not been consistently reported in other areas of the world here we report a study integrating isotopic and geochemical data from middle cambrian series 3 sections from the northeastern georgina basin australia to investigate the occurrence of the dice event in australia and the paleoenvironmental conditions associated with this event the similarity between the isotopic excursions observed in the georgina basin 2 to 3 2 13 c excursion and that observed in the great basin 2 to 3 6 13 c excursion highlighted the presence of the dice in australia and emphasized the potential use of this event for refining global isotopic correlations the multi proxy correlation used in this study also showed a pulse of euxinia in association with the dice suggesting strongly euxinic conditions near the base of the drumian stage

regional phases in continental and oceanic environments summary the very different crustal thicknesses between continental and oceanic crust lead to the establishment of distinct styles of regional seismic wavefield observations and numerical simulations of events near subduction zones in japan allow a direct comparison of seismic wave propagation in oceanic and continental structures the thick zone of lower seismic velocities in the continental crust acts to trap s wave energy in particular and generates crustally guided phases such as pg lg in the oceanic environment there is no efficient s wave trapping and crustal energy progressively leaks into the mantle reverberations in the crust and the water above help to sustain the amplitude of the mantle arrivals pn sn to large distances

magnitude the relation of ml to mblg abstract the m blg magnitude developed by nuttli 1973 for the central united states is adapted to other regions of the world by incorporating a specific term for the spatial coefficient of anelastic attenuation using region specific values m blg values are determined for earthquakes occurring near the wwssn stations bks gsc and dug a comparison of the m blg values with published m l magnitudes shows that these two magnitude scales are essentially equivalent between magnitudes 3 to 5 because of this observation we now have a direct way of comparing eastern and western united states earthquake sizes for the purpose of adapting western strong motion data bases to the east and also for understanding teleseismic magnitude biases

rapid northward shift of the indian monsoon on the tibetan plateau at the end of the little ice age variations in the indian monsoon im and westerlies ws significantly affect the climate on the tibetan plateau tp and have widespread ecological and socioeconomic impacts on the whole of asian society so far however the rate and magnitude of changes in the im have still remained unclear here we report for the first time that the im rapidly shifted northward at the end of the little ice age lia we used sediment proxies for humidity and moisture sources from the taro co lake which is located in the transition zone between the ws and im our comprehensive survey of climate records for the tp and its peripheral mountain ranges revealed that the northern boundary of the im i e the southern boundary of the ws lay along the southern slope of the gandise range 29 5 n in the late lia in contrast it passed quickly over the gandise range by at least 1 5 in latitude at the end of the lia our results suggest that this rapid climatic shift was potentially triggered by the counteracting effects of blocking by the tp and its marginal orography which hindered the northward movement of the im and the pulling thermal gradient of the tp

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

isotopic fingerprints of microbial respiration in aragonite from bahamian stromatolites authigenic aragonite preserves a carbon isotopic record of heterotrophic microbial influences on dissolved inorganic carbon dic in microenvironments within shallow subtidal stromatolites from highborne cay bahamas a greater amount of aragonite precipitates when and where respiration rather than photosynthesis influences local dic which is consistent with sulfate reduction promoting carbonate precipitation and calcium release during decay of exopolymeric substances thus heterotrophs play a more direct role than phototrophs in stromatolite lithification cyanobacteria are spatially associated with aragonite containing heterotrophic isotopic signatures hence the absence of an autotrophic isotopic signature in the rock record does not imply the absence of photosynthetic organisms

carbon isotope stratigraphy using carbonate cements in the triassic sherwood sandstone group corrib field west of ireland abstract carbon stable isotopes from carbonate minerals mainly dolomite from six wells from the lower triassic sherwood sandstones of the corrib gas field slyne basin west of ireland allow stratigraphic correlation the results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks the triassic reservoir rocks have been buried to 4000 m and heated to 165 c and now contain methane rich gas although the oxygen isotopic signal has been at least partially reset during burial and heating a primary carbon isotopic signal appears to have survived diagenesis the carbon isotope ratio varies from 3 2 to 2 1 all six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon 13 c increases from about 3 at the base of the sherwood to about 2 170 m above the base 13 c then decreases to about 2 for the next 70 m and remains steady for the following 50 m the top 20 m of the sherwood contains carbonate with a 13 c values decreasing to about 3 the occurrence of a stratigraphically correlatable carbon isotope pattern implies that the primary evolution signal has been preserved the change in 13 c correlates with indicators of aridity and biological stress such that the highest 13 c values are in sedimentary rocks deposited in a playa lake arid times these rocks contain the greatest quantity of dolomite cement conversely the lowest 13 c values correspond to sedimentary rocks deposited from well developed rivers relatively humid times from the lowest quantity of dolomite cement the same carbon isotope evolution has been found in another well in the slyne basin and in belgium suggesting that the palaeoenvironmental isotope signal in the triassic sedimentary rocks of the corrib field may have a regional significance

lg wave propagation and crustal structure differences near denmark and the north sea summary recordings of regional earthquakes near denmark and the north sea have been collected in an attempt to find the lg wave propagation properties in the area propagation paths of lengths between 300 and 1100km are investigated the amplitudes of the lg waves and the other seismic phases in the seismograms are compared the propagation paths are classified according to whether the lg wave propagation compared to p and s wave propagation is good intermediate or poor it is noted that there is a distinct regional pattern in the lg wave attenuation the lg wave propagation is poor across the north sea the north sea is surrounded by areas of good continental lg wave propagation except towards the south east in the netherlands and western germany an area in skagerrak is characterized by good but slow propagation of a surface wave similar to lg since this is exactly the area of a cluster of earthquakes in skagerrak it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area just off the coast of north western denmark the lg wave propagation is as good in the northern part of denmark as it is in the fennoscandian shield the extent to which the propagation is poor across the north sea is dependent on the frequency of the lg waves observed with different seismographs the lg waves are more attenuated relative to the other seismic phases the higher the frequency is between 0 25 and 2 hz as the structural differences between the fennoscandian bedrock area and the norwegian danish basin with 8 10 km of sediments has no effect on the lg wave propagation it is suggested that the structural anomalies causing the poor lg wave propagation across the north sea are deep crustal anomalies possibly significant variations in crustal thickness it is likely that such deep crustal differences are located along the most prominent upper crustal differences in the sedimentary grabens in the middle of the north sea the poor propagation across the north sea is similar to that observed across a sedimentary basin in eastern greenland which also developed mainly before seafloor spreading separated greenland from europe the frequency dependent propagation of the lg waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

the nonlinear response of the atmosphere to large scale mechanical and thermal forcing abstract the subject of large scale mountain waves is reviewed briefly existing mountain wave theory based on a linear system is shown to give an inadequate description of the balance of angular momentum the response of the atmosphere to mechanical forcing in a nonlinear framework is then discussed using a two level quasi geostrophic long wave spectral model based on spherical coordinates including diabatic heating surface friction and mountains the nonlinear theory shows that there exists a critical mountain height hc which is a function of the frictional coefficient as well as the phase difference between the mountain and the surface pressure field if and only if the mountain height is less than this critical value can the deflection effect of the mountain be neglected and the response regarded as approximately linear this critical mountain height is only about 1 km thus most of the atmospheric response to large scale mountains must be nonlinear in the nonlinear case as the mountain heigh

a global carbon isotope excursion spice during the late cambrian relation to trilobite extinctions organic matter burial and sea level abstract the steptoean positive carbon isotope excursion spice marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue water trilobites the spice excursion is documented from sections in laurentia kazakhstan china and australia where it begins with the mass extinction at the base of the pterocephaliid biomere steptoean stage in laurentia and at coeval extinction horizons in gondwana and perigondwana terranes the peak of spice 5 coincided with a time of maximum regression in laurentia spice is similar in this regard to excursions that coincide with glacio eustatic falls such as in the late ordovician a plausible scenario involves the transformation of ocean circulation between two states which led to enhanced coastal upwelling and benthic extinctions the lack of evidence for glaciation indicates that the coeval sea level fall sauk ii sauk iii event resulted from tectonic or hydrologic changes that remain poorly understood at this time