numerical calibration of the devonian carboniferous boundary two new u pb isotope dilution thermal ionization mass spectrometry single zircon ages from hasselbachtal sauerland germany the hasselbachtal section sauerland germany is an auxiliary global stratotype of the devonian carboniferous boundary and one of the most important reference sections for the evolution of the latest famennian to earliest tournaisian pelagic fauna biostratigraphically well controlled altered volcanic ash layers metabentonites intercalated in the section afford a perfect opportunity for a numerical fixing of this important paleozoic period boundary we have performed u pb isotope dilution thermal ionization mass spectrometry id tims analyses on sub microgram sized single zircons and zircon fragments extracted from two metabentonites beds 79 and 70 in the lowermost tournaisian part of the section bed 79 metabentonite is positioned directly above the devonian carboniferous boundary within the siphonodella sulcata conodont zone five concordant analyses form a cluster with a 2 0 6 pb 3 3 8 u concordia age of 360 5 0 8 ma zircons of the next younger metabentonite bed 70 in the lower siphonodella duplicata conodont zone yielded a tightly grouped cluster of 10 concordant analyses with a 2 0 6 pb 2 3 8 u concordia age of 360 2 0 7 ma on the basis of these two new single zircon ages and previously published late famennian u pb id tims ages the devonian carboniferous boundary is reinterpolated herein to 360 7 0 7 ma

evidence for large scale eddy driven gyres in the north atlantic analysis of a recent climatological database for the north atlantic has detailed an extensive large scale recirculation system in the intermediate waters of the north atlantic basin the pressure fields that define this recirculation coupled with the potential vorticity fields associated with the recirculating flow provide observational evidence for basin scale eddy driven flow in the global ocean the recirculations are intimately tied to the waters carried southward by the deep western boundary current and are therefore likely to affect the distribution of climatic anomalies in the north atlantic

laurentide cordilleran and montane glaciation in the western peace river grande prairie region alberta and british columbia canada abstract geological investigations over the last decade in the western peace river grande prairie district of northwestern alberta and northeastern british columbia have provided important chronostratigraphic information regarding laurentide cordilleran and montane quaternary glacial events numerous 14 c age determinations as well as stratigraphic and sedimentological analyses of numerous exposures indicate that this area was first glaciated by cordilleran ice sometime before the late wisconsinan during the late wisconsinan ca 22 000 14 000 bp laurentide glacier ice covered much of the area between approximately 15 000 and 10 000 bp montane ice affected only the western part of the peace region there is no evidence for synchroneity of the ice advances and therefore no evidence for ice coalescence in this region the resultant ice free corridor was most likely a laterally fluctuating zone which existed during a several thousand year period in the late pleistocene climatic conditions along the corridor during this period were probably unfavorable for human migration but adequate for some plants and small animals numerous isolated areas along the western foothills of the northern rocky mountains west of the laurentide glaciation limit were never glaciated during the late wisconsinan

decadal response of the kuroshio extension jet to rossby waves observation and thin jet theory abstractthis study examines interannual to decadal variability of the kuroshio extension ke jet using satellite altimeter observations from 1993 to 2010 the leading empirical orthogonal function eof mode of sea level variability in the ke region represents the meridional shift of the ke jet followed by its strength changes with a few month lag this shift of the ke jet lags atmospheric fluctuations over the eastern north pacific by about three years broad sea level anomalies slas emerge in the eastern north pacific 3 4 years before the upstream ke jet shift and propagate westward along the ke jet axis in the course of the propagation the meridional scale of the slas gradually narrows and their amplitude increases this westward propagation of slas with a speed of about 5 cm s 1 is attributed to the westward propagation of the meridional shift of the jet consistent with the thin jet theory whose importance has been suggested by previous numerical studies in addition the westward propagatin

magnetostratigraphy susceptibility of the přidolian lochkovian silurian devonian gssp klonk czech republic and a coeval sequence in anti atlas morocco abstract the magnetosusceptibility event and cyclostratigraphy msec record for the p idolian lochkovian silurian devonian global boundary stratotype section and point gssp klonk prague basin czech republic is described and used to establish a magnetostratigraphy susceptibility profile for the gssp gssp msec data are summarized into three magnetozones msz the tma msz late p idolian with 13 msszs mssz the klonk msz latest p idolian and earliest lochkovian with 17 msszs and the voskop msz early lochkovian with 7 msszs the base of klonk mssz 2 is coincident with the base of lochkovian and the devonian the proposed magnetostratotype for the p idolian lochkovian boundary is supported by ms data from a nearby core through the gssp sequence and into the upper ludlow three additional mszs are recognized for the portion of the core sequence older than the gssp boundary sequence the extension of the mszs and msszs away from the proposed magnetostratotype is tested by comparison with a lochkovian sequence in the western sahara of southeastern morocco mszs are the result of global sea level fluctuations that alter base level and the pattern of erosion and influx of detrital paramagnetic mineral components into the marine environment large scale transgressive and regressive patterns interpreted from the msec data establish that the silurian devonian boundary in the barrandian region falls between a short lived transgressive pulse in the latest p idolian and an equally short lived regressive pulse in the earliest lochkovian msszs identified in the gssp and core sequences are the result of variation in climate cycles and the resulting periodicity can explained by either the 38 9 ka silurian and devonian equivalent of the modern 54 ka obliquity cycle or the eccentricity cycle of 106 ka periodicity use of the obliquity periodicity gives a duration for the barrandian lochkovian of 4 46 ma which agrees with the most recent geochronometric estimate of 4 5 ma for the lochkovian use of the eccentricity periodicity gives a duration for the barrandian lochkovian of 12 16 ma which agrees with the previous geochronometric estimates of 12 2 and 12 5 ma for the lochkovian in any case the cyclic nature of the msec data established by the msszs can be explained by variations in the rate of supply of weathered terrigenous paramagnetic grains to the marine system these variations in supply occurred due to climate changes resulting from the either the obliquity or eccentricity of the earth s orbit knowledge of the periodicity of lochkovian cycles in the barrandian area allows estimation of the rates of sediment accumulation the duration of fossil ranges and the rates of evolution depending on the choice of obliquity or eccentricity to explain the cyclicity in the barrandian lochkovian the estimated rate of sediment accumulation in the gssp outcrop averaged either 51 or 140 ka m the range of the graptoloid monograptus uniformis angustidens wholly contained within the lochkovian sequence at the gssp is also estimated at either 51 or 140 ka

re os depositional ages and seawater os estimates for the frasnian famennian boundary implications for weathering rates land plant evolution and extinction mechanisms four toc rich shale intervals spanning the frasnian famennian f f boundary were recovered in a drillcore west valley nx 1 from western new york usa and radiometrically dated using re os two of the black shale intervals wvc785 from 2 9 m below and wvc754 from 6 4 m above the f f boundary respectively yielded statistically overlapping ages with uncertainties of 1 1 an interpolated age and associated graphically determined uncertainty of 372 4 3 8 ma provides new absolute age constraints on the f f boundary this date is 4 1 ma younger than the latest proposed f f boundary age of 376 1 ma obtained by interpolation of u pb dates from volcanic zircon kaufmann b 2006 calibrating the devonian time scale a synthesis of u pb id tims ages and conodont stratigraphy earth science reviews 76 175 190 and within uncertainty of the international commission on stratigraphy accepted date of 374 5 2 6 ma a third date from sample wvc802 8 2 m beneath the f f boundary yielded an imprecise age of 357 23 ma owing in part to a limited re os range the initial 187os 188os 0 45 to 0 47 reflecting contemporaneous seawater os values are low but similar to the value of 0 42 reported for the exshaw fm canada at the devonian mississippian boundary ca 361 ma selby d creaser r a 2005 direct radiometric dating of the devonian mississippian time scale boundary using the re os black shale geochronometer geology 33 545 548 this may suggest fairly constant and low global continental weathering rates during the late devonian although in view of the short residence time of os in seawater 1 4 104 yr further measurements are needed to assess potential short term variation in seawater os ratios owing to low os and re abundances at the f f boundary our data are inconsistent with long term volcanism and bolide impact as potential late devonian mass extinction mechanisms in addition the frasnian famennian ocean appears to have been depleted with respect to re possibly indicating an exhaustion of the re seawater reservoir owing to high burial rates of redox sensitive elements under dysoxic anoxic conditions leading up to the f f boundary

the formation and failure of natural dams of the numerous kinds of dams that form by natural processes dams formed from landslides glacial ice and late neoglacial moraines present the greatest threat to people and property landslide dams form in a wide range of physiographic settings the most common types of mass movements that form landslide dams are rock and debris avalanches rock and soil slumps and slides and mud debris and earth flows the most common initiation mechanisms for dam forming landslides are excessive rainfall and snowmelt and earthquakes landslide dams can be classified into six categories based on their relation with the valley floor type i dams 11 of 184 landslide dams from around the world that we were able to classify do not reach from one valley side to the other type ii dams 44 span the entire valley floor in some cases depositing material high on opposite valley sides type iii dams 41 move considerable distances both upstream and downstream from the landslide failure type iv dams 1 are rare and involve the contemporaneous failure of material from both sides of a valley type v dams 1 also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river type vi dams 3 involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side many landslide dams fail shortly after formation in our sample of 73 documented landslide dam failures 27 of the landslide dams failed less than 1 day after formation and about 50 failed within 10 days over topping is by far the most common cause of failure the timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry material characteristics of the blockage rate of inflow to the impoundment size and depth of the impoundment bedrock control of flow and engineering controls such as artificial spill ways diversions tunnels and planned breaching by blasting or conventional excavation glacial ice dams can produce at least nine kinds of ice dammed lakes the most dangerous are lakes formed in main valleys dammed by tributary glaciers failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam cold polar ice dams generally drain supraglacially or marginally by downmelting of an outlet channel warmer temperate ice dams tend to fail by sudden englacial or subglacial breaching and drainage late neoglacial moraine dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries these late neoglacial dams pose hazards because 1 they are sufficiently young that vegetation has not stabilized their slopes 2 many dam faces are steeper than the angle of repose 3 these dams and lakes are immediately downslope from steep crevassed glaciers and near vertical rock slopes and 4 downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks the most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls rockfalls or snow or rock avalanches melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam although data are few for the same potential energy at the dam site ownstream flood peaks from the failure of glacier ice dams are smaller than those from landslide moraine and structed earth fill and rock fill dam failures moraine dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011 1012 joules differences in flood peaks natural dam failures appear to be controlled by dam characteristics and failure mechanisms

ocean gyres driven by surface buoyancy forcing midlatitude gyres in the ocean are large scale horizontal circulations that are intensified on the western boundary of the ocean giving rise to currents such as the gulf stream the physical mechanism underlying gyres is widely recognized to involve the curl of the wind stress which injects potential vorticity into the upper ocean however model results have highlighted the role of surface buoyancy fluxes principally heating and cooling of the ocean surface in driving circulation and enhancing gyre variability here we present two numerical simulations one in the fully turbulent regime and the second an eddy permitting ocean model which show that gyre like circulation can be driven by surface buoyancy fluxes alone we explore this phenomenon through a combination of modeling and linear theory to highlight that the transport of ocean gyres depends upon surface buoyancy fluxes as well as wind stress

late frasnian sedimentation cycles in the appalachian basin possible evidence for high frequency eustatic sea level changes abstract during the late frasnian 11 fourth order progradational retrogradational marine sedimentation cycles were deposited in the appalachian foreland basin mapping based primarily on subsurface data demonstrates the continuity of these cycles over a distance of 700 km cyclicity in distal facies occurs as alternations of organic rich and organic poor shales two of the organic shales can be correlated with the transgressive kellwasser beds of europe in more proximal facies recurring lobes of siltstone and sandstone were deposited based on lithologic indices the temporal pattern shows significant variation in the strength of relative facies change during deposition in particular two times of particularly pronounced progradation correspond to previously recognized eustatic sea level falls the correlation of portions of appalachian basin depositional cyclicity with global sea level events suggests that the entire sequence of 11 cycles with estimated average duration of around 100 150 ka were the result of high frequency eustatic sea level changes this in turn would be consistent with a brief period of late frasnian glaciation as others have previously suggested

the eureka river landslide and dam peace river lowlands alberta the eureka river landslide of june 1990 at 50 mm3 is one of the largest historical landslides on the interior plains of canada it is one of seven large translational landslides to have occurred in the peace river lowlands within the last 65 years each landslide occurred in quaternary sediments deposited within a preglacial valley each landslide formed a dam the rupture surface of the eureka river landslide in preglacial lacustrine sediment 125 m below the peace river lowlands plains extended beneath the river channel causing the channel to be elevated the resulting landslide dam was over 20 m high forming a lake exceeding 8 km in length the river cut a new channel around the toe of the landslide abandoning the prelandslide channel as the new channel is free of armour incision has been rapid after 10 years the dam now stands approximately 5 m high key words landslide landslide dam peace river alberta preglacial valley geomorphology

greenhouse to icehouse a biostratigraphic review of latest devonian mississippian glaciations and their global effects the latest devonian mississippian interval records the long term transition from devonian greenhouse conditions into the late palaeozoic ice age lpia this transition was punctuated by three short glaciation events in the latest famennian mid tournaisian and visean stages respectively primary evidence for glaciation is based on diamictite deposits and striated pavements in south america appalachia and africa the aim of this review is to assess the primary biostratigraphic and sedimentological data constraining diamictite deposits through this transition these data are then compared to the wider record of eustasy mass extinction and isotope stratigraphy in the lower palaeolatitudes precise age determinations are vital to integrate high and low palaeolatitude datasets and to understand the glacial control on wider global changes palynological techniques currently provide the best biostratigraphic tool to date these glacial deposits and to correlate the effects of glaciation globally this review highlights a high degree of uncertainty in the known history of early lpia glaciation as much of the primary stratigraphic data are limited and or unpublished future high resolution stratigraphic studies are needed to constrain the history of glaciation both spatially and temporally through the latest devonian and mississippian

calibrating the devonian time scale a synthesis of u pb id tims ages and conodont stratigraphy the recalibrated devonian time scale represents an important improvement of devonian chronology regarding two aspects firstly a data set of 13 biostratigraphically well bracketed u pb id tims zircon and monazite ages constitutes the framework of the scale secondly approximately time linear biostratigraphic scales have been used for the interpolation between the isotopic ages the new construction thus represents a biochronometric time scale which allows the assignment of numeric ages not only to stage boundaries but also to each biozone boundary the method of interpolation applied also enables the projection of the geochronological error onto the time scale according to the newly calibrated scale the devonian lasted 57 4 5 7 ma from 418 1 3 0 to 360 7 2 7 ma this is compared to previously published time scales the longest time span ever calculated for this period the age interpolations for the eight stage boundaries are famennian tournaisian devonian carboniferous 360 7 2 7 ma frasnian famennian 376 1 3 6 ma givetian frasnian 383 7 3 1 ma eifelian givetian 388 1 2 6 ma emsian eifelian 391 9 3 4 ma pragian emsian 409 1 3 8 ma lochkovian pragian 412 3 3 5 ma pridolian lochkovian silurian devonian 418 1 3 0 ma full size table table options view in workspace download as csv the duration of the middle devonian is calibrated here as quite short with 8 2 ma and the emsian and the famennian are the longest stages with interpolated durations of 17 2 and 15 4 ma respectively together all devonian stages contain 57 conodont zones including subzones according to the standard conodont zonation giving a mean duration of about 1 ma in the highly resolved part of the time scale mid eifelian to the devonian carboniferous boundary zonal resolution averages 0 6 ma in contrast the lowest resolution is shown in the early emsian to mid eifelian interval with zonal durations of up to 5 5 ma serotinus zone

u pb zircon dating of paleozoic volcanic rocks from the rheno hercynian zone new age constraints for the steinkopf formation lahn dill area germany small and volumetrically subordinate outcrops of alkali rhyolitic to trachytic metavolcanic rocks are widespread in the lahn dill area of the rheinisches schiefergebirge germany these occurrences are either related to late early devonian or to middle devonian episodes of volcanic activity an unambiguous age assignment for the time of eruption could not be established so far most volcanic rocks of the study area are not suitable for application of u pb geochronology a notable exception is the steinkopf rock suite in the sw lahn dill area but a straightforward interpretation of the la icp ms u pb data is hampered by complex zircon populations recording various processes including crystallization from newly formed melt inheritance assimilation of country rocks variable degrees of pb loss and fluid assisted zr mobility a plausible evaluation of u pb zircon data of two metatrachytes and a metavolcaniclastic rock document a coherent age group at ca 390 384 ma and a probable eruption age at ca 385 ma suggesting a relationship to middle devonian givetian igneous processes zircon bearing veinlets in a metatrachyte indicate post eruptive infiltration of external hydrothermal fluids and or internal zr redistribution in late devonian to early carboniferous time 364 10 ma but this process is of rather limited importance

early devonian conodont graptolite collation and correlations with brachiopod and coral zones central nevada a range chart of some early devonian conodont and graptolite taxa from central nevada is presented it is based on the graphic correlation method of shaw and shows correlations with the established zones for brachiopods and corals as well as conodonts and graptolites it indicates that the hesperius through pesavis conodont zones established for western north america are approximately equivalent to the lochkovian of the czechoslovakian sequence and the pragian begins approximately with the sulcatus zone the chart also indicates that the zones based on different groups do not coincide are of very unequal duration and that intervals of rapid morphologic change correspond to intervals of higher diversity and greater morphologic variation species have significantly diff rent longevities generally those originating during times of greatest morphologic variation being the shorter lived the approximate duration of the species studied is 2 to 10 m y use of the graphic correlation method permits better correlation of shelf and basin faunas and enhances the possibility for constructing accurate shelf margin histories and for correlating provincial shelf faunas with those in other parts of the world

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

the interaction of recirculation gyres and a deep boundary current abstractmotivated by the proximity of the northern recirculation gyre and the deep western boundary current in the north atlantic an idealized model is used to investigate how recirculation gyres

gigantic low gradient landslides in the northern periphery of the crimean mountains ukraine abstract large scale low gradient ancient landslides estimated at 5 4 18 9 km 2 in area and 0 2 1 2 km 3 in volume have been studied in the northern hilly periphery of the crimean mountains ukraine they originated on slopes along wide water gaps of rivers belbek kacha alma and biyuk karasu crossing the cuestas of the northern foothills the slopes generally consist of slightly northward tilting miocene mainly sarmatian limestones overlying weak clay rich lower neogene palaeogene substratum with a significant content of smectite although the region is characterised by the least active contemporary morphodynamics within the crimean mountains the landslides which were studied are of the same size or even larger than various types of landslides occupying active geomorphic domains of the highest mountain range in the southernmost part of the peninsula the landslides are generally a spreading type but the sliding mechanics were probably very complex involving toppling rotational slides gravitational folding and translational block slides all the landslides which were studied are located in the vicinity of regional faults and three of them have headscarps aligned along faults a common feature is also a location close within several km to the mesozoic suture zone which is the most important tectonic feature in the northern periphery of the crimean orogene this suture was formerly classified as aseismic however evidence of strong low frequency palaeoearthquakes was collected during the last decade within both the mesozoic suture and the low lying northern part of the crimean peninsula radiocarbon dating of deposits associated with the landslides has revealed at least two phases of increased landslide activity during the late glacial chronozone and holocene epoch the main landslide phase presumably took place at some time between the late glacial and atlantic chronozones minor reactivation of landslide toes occurred during the subatlantic chronozone and some of them have been active up to recent times the first major landslide phase was possibly triggered by an earthquake whereas late holocene activity can be attributed both to seismic and hydroclimatic factors

direct radiometric dating of the devonian mississippian time scale boundary using the re os black shale geochronometer many phanerozoic time scale boundaries are characterized by oceanic anoxia and mass extinction events with the deposition of black shale the re os isotope system in black shale can be used to provide depositional ages for these rocks thus yielding direct radiometric ages for time scale boundaries we demonstrate that the re os black shale geochronometer can yield precise ages useful for time scale research through study of the devonian mississippian boundary within the exshaw formation canada the re os date determined places the devonian mississippian boundary at 361 3 2 4 ma 2 including uncertainty model 1 mean square of weighted deviates 1 2 in accord with recent calibration interpolated from u pb zircon dates 360 7 0 7 ma for the hasselbachtal section germany

the numerical age of the devonian carboniferous boundary the shrimp ion microprobe has been used to date zircons in a 1 cm thick bentonite located in the hasselbachtal auxiliary global stratotype section through the devonian carboniferous boundary in germany and a tuff located at a similar biostratigraphic level in australia multiple replicate analyses have yielded indistinguishable ages and indicate 353 2 4 0 ma 2 as the age of the boundary obtaining an age for the boundary in the german reference section depended on the ability of shrimp to date a small sample and to target small areas of structurally complex zircons future refinements of the age of this boundary may depend on re analysis of this uniquely placed bentonite and this highlights the value of defining biostratigraphic stratotypes in the vicinity of known dateable horizons

the great whirl observations of its seasonal development and interannual variability 1 in situ measurements are used together with sea surface height data to study the development and variability of the great whirl gw a large quasi stationary anticyclone that appears off the coast of somalia during the southwest monsoon season we find that anticyclonic circulation indicative of the gw appears on average in april almost two months before the onset of the southwest monsoon winds this early initiation is coincident with the arrival of annual rossby waves at the western boundary with the onset of the monsoon winds in early june the gw proper intensifies quickly remaining at its peak throughout july august and september and dissipating about one month after the winds have died the gw is present for an average 166 30 days per year and the position of its northern flank close to 9 n coincides with the latitude of zero wind stress curl the intraseasonal variability of the gw is intense as a result of mutual advection with one to three flanking cyclones which accompany the gw 70 of the time and tend to circulate clockwise around it there is no consistent seasonal pattern for the development or dissipation of the gw although movement to the southwest is common toward the end of the season the gw of 1995 deepened from 200 m in june to over 1000 m in september and strengthened from a swirl transport of 10 to 60 sv cool waters in its core resulted from advection via the somali current and some local vertical mixing

momentum balance of the wind driven and meridional overturning circulation abstractwhen a force is applied to the ocean fluid parcels are accelerated both locally by the applied force and nonlocally by the pressure gradient forces established to maintain continuity and satisfy the kinematic boundary condition the net acceleration can be represented through a rotational force in the rotational component of the momentum equation this approach elucidates the correspondence between momentum and vorticity descriptions of the large scale ocean circulation if two terms balance pointwise in the rotational momentum equation then the equivalent two terms balance pointwise in the vorticity equation the utility of the approach is illustrated for three classical problems barotropic rossby waves wind driven circulation in a homogeneous basin and the meridional overturning circulation in an interhemispheric basin in the hydrostatic limit it is shown that the rotational forces further decompose into depth integrated forces that drive the wind driven gyres and overturning forces

insights on why graphic correlation shaw s method works in 1964 a b shaw presented a method of correlating fossiliferous sedimentary rocks based on interpretation of graphic plots of first and last occurrences of taxa because there is no way to determine the true total ranges of fossil taxa it is instructive to test the accuracy of the method using hypothetical datasets the dataset used here consists of 16 taxa in six sections with differing known rates of rock accumulation in all graphs a single straight line correlation was a reasonable interpretation the resulting ranges after the first and third rounds of compositing reproduce the true ranges but with small errors slight errors in the positioning of individual correlation lines are more likely to lengthen ranges artificially than to shorten them shaw s method works well because whereas actually sampled ranges will be shorter than true ranges errors in correlation will be likely to extend some ranges this or any exercise using simulated data is useful only if the hypothetical situation resem

evolutionary dynamics of pragian dacryoconarida lower devonian tentaculitoidea evidence from palaeontological data and 13c of marine carbonates from czech republic abstract a detailed palaeontological study was performed on 38 successive assemblages of dacryoconarids tentaculitoidea sampled along the cerna rokle section praha basin czech republic this section represents the whole pragian stage lower devonian and roughly corresponds to a 6 ma time span dacryoconarid assemblages change in abundance and diversity oxygen and carbon isotope compositions of the dacryoconarid bearing carbonates were measured to evaluate the possible influence and control of the environmental conditions on the evolutionary dynamics of the dacryoconarid faunas large and rapid 13c excursions occur in the marine carbonate sequence of the cerna rokle section some of which apparently correspond with faunal variations however no clear and simple relationship appears between carbon isotope ratio fluctuations and dacryoconarid evolutionary dynamics occurrence and extinction of the dacryoconarid species appear to be mainly related to biotic factors environmental variations cannot be regarded as having preponderantly influenced the dacryoconarid evolution which more probably resulted from intrinsic biotic factors

two new emsian early devonian u pb zircon ages from volcanic rocks of the rhenish massif germany implications for the devonian time scale so far only few biostratigraphically well constrained isotopic ages constitute the geochronological framework of the devonian period to expand this sparse dataset two new u pb isotope dilution thermal ionization mass spectrometry id tims zircon ages were obtained from volcanic rocks of the rhenish massif germany these rocks are interbedded in emsian lower devonian marine sedimentary successions that are well documented by index fossils given that the u pb zircon data represent primary crystallization and eruption ages they provide an important contribution to the devonian time scale the older sample a stratiform volcaniclastic layer hans platte is collected from bundenbach hunsruck the type locality of the lower emsian hunsruck slate a cluster of five coherent single zircon analyses obtained from this layer yielded a weighted mean 206 pb 238 u age of 407 7 0 7 ma which is regarded as the time of eruption of the pyroclastic rock biostratigraphically the layer can be assigned to the upper part of the polygnathus excavatus conodont zone the younger u pb age was obtained from the global stratotype section and stratigraphic point section at wetteldorf eifel where the k bentonite hercules i is intercalated 13 m below the lower middle devonian boundary in the uppermost part of the polygnathus costatus patulus conodont zone some of the 19 single zircon analyses obtained from this bentonite appear to document significant influences of inheritance the 13 youngest points are however concordant and form an elongated cluster along concordia with 206 pb 238 u ages between 392 2 and 407 7 ma given the risky assumption that this age scattering is exclusively caused by varying amounts of inheritance the youngest point of this cluster represents the analysis with the least or no inheritance in this case its 206 pb 238 u age of 392 2 1 5 ma should approach the eruptional age of the bentonite the ages presented here in combination with u pb id tims zircon and monazite ages from other workers allow a recalibration of the early emsian to early eifelian interval of the devonian time scale the result is a duration of the emsian stage excluding the basal emsian p kitabicus conodont zone of more than 17 2 ma and an emsian eifelian early middle devonian boundary age of 391 8 0 4 ma

the durations of silurian graptolite zones recently published radiometric data show the duration of the silurian period to be approximately 32 ma significantly longer than previous estimates radiometric and graphic correlation studies demonstrate considerable variation in the durations of the four silurian epochs approximately 13 ma for the llandovery 4 ma for the wenlock 5 ma for the ludlow and 10 ma for the p idoli using this new geochronological data figures for average graptolite zone duration are presented for each epoch ranging from as little as 0 44 ma in the wenlock to 1 43 ma in the p idoli the figures provide a refined subdivision of the silurian timescale of value to studies which use silurian graptolite zones to calibrate geological processes

rb sr and 40ar 39ar ages in blueschists from the ile de groix armorican massif france implications for closure mechanisms in isotopic systems rb sr and 40ar 39ar data mainly ranging from 345 5 to 361 5 ma and from 348 4 1 4 to 364 6 1 4 ma respectively have been obtained on phengite from the hp lt rocks from the ile de groix armorican massif france which represents one of the few occurrences of variscan blueschists petrological data show two blueschist facies units recording distinct peak p t conditions about 16 20 kbar 500 c and 12 16 kbar 450 c in the upper and lower units respectively followed by a nearly isothermal decompression in the greenschist facies rb sr and 40ar 39ar results defining two groups of ages mainly around 345 353 ma and 358 366 ma are consistent in well preserved blueschist facies rocks as well as in strongly overprinted samples from both units one sample excepted there is no evidence of inherited or excess argon and resetting of the isotopic systems oldest rb sr and 40ar 39ar ages are found in rocks with well preserved blueschist facies parageneses while younger ages are obtained on samples with a strong greenschist facies overprint this suggests that the closure of the k ar system is diachronous in samples which record the same p t evolution a closure of the k ar clock at around 450 500 c in phengite from rocks left nearly undeformed after the blueschist facies metamorphism is in agreement with petrological geochemical and structural data thus in these samples the oldest rb sr and 40ar 39ar ages around 360 370 ma represent the age of the hp metamorphism in the overprinted samples later closure of the k ar and rb sr clocks is the result of retrogression and continuing ductile deformation the small age difference between the oldest and the youngest rb sr and 40ar 39ar ages around 10 20 ma argues in favour of a relatively fast cooling

decadal variability of the kuroshio extension observations and an eddy resolving model hindcast abstract low frequency variability of the kuroshio extension ke is studied using observations and a multidecadal 1950 2003 hindcast by a high resolution 0 1 eddy resolving global ocean general circulation model for the earth simulator ofes in both the ofes hindcast and satellite altimeter observations low frequency sea surface height ssh variability in the north pacific is high near the ke front an empirical orthogonal function eof analysis indicates that much of the ssh variability in the western north pacific east of japan is explained by two modes with meridional structures tightly trapped along the ke front the first mode represents a southward shift and to a lesser degree an acceleration of the ke jet associated with the 1976 77 shift in basin scale winds the second mode reflects quasi decadal variations in the intensity of the ke jet both the spatial structure and time series of these modes derived from the hindcast are in close agreement with observations a linear rossby wave

millennial scale paleoclimate cycles recorded in widespread palaeozoic deeper water rhythmites of north america rhythmically interbedded limestone and shale or limestone and chert rhythmites are a common feature of many deep water phanerozoic carbonate marine deposits seventeen different palaeozoic rhythmite successions from across north america 8 studied in detail 9 studied in reconnaissance are described and summarized individual rhythmite couplets 4 20 cm thick are composed of fine grained laminated to massive detrital limestone alternating with shale or marl or laminated spiculitic chert stratigraphic and primary depositional features within rhythmites and associated facies indicates that the carbonate rich and carbonate poor interbedding is the result of repetitive changes in sediment input primary rather than due to diagenetic redistribution of calcareous material secondary the average duration of individual rhythmite couplets is calculated using spectral analysis to determine couplet thickness combined with biostratigraphically calibrated undecompacted average sediment accumulation rates the duration of rhythmite couplets from the eight well studied successions ranges between 150 and 4900 yrs with the majority lying between 1000 and 3000 yrs i e millennial scale the paucity and low diversity of skeletal and trace fossils preserved suspension laminae lack of sediment reworking and facies associations indicate rhythmite accumulation was favored by 1 deposition below storm wave base which limited reworking by currents 2 dysaerobic bottom waters which restricted bioturbation and intermixing of interbedded lithologies and 3 proximity to nearshore carbonates supplying abundant fine grained detrital carbonate these combined conditions were best met along flooded subtropical continental shelves or epeiric seas during my scale 3rd order sealevel rises the rhythmic alternation between carbonate rich and carbonate poor layers is interpreted to represent millennial scale paleoclimatic changes related to 1 wet dry climate cycles which influenced the amount of continent derived eolian and or fluvial sediment input 2 variations in offshore transport via storm generated or density currents of nearshore derived terrigenous or carbonate sediments and or 3 changes in wind driven upwelling and availability of recycled biogenic silica the various rhythmite successions accumulated under dramatically different paleoenvironment and paleogeographic conditions including active to passive tectonic settings equatorial to subtropical latitudes long term icehouse through greenhouse climatic conditions calcite versus aragonitic seas variable atmospheric co2 concentrations before and after land plant and animal evolution and across widely varying ocean basin configurations if our short term paleoclimatic interpretations for the rhythmites are correct then it is apparent that millennial scale climate changes occurred over a very wide spectrum of paleoceanographic paleogeographic paleoclimatic tectonic and biologic conditions and over time periods from the cambrian to the quaternary given this it is difficult to invoke models of internally driven thermohaline oceanic oscillations or continental ice sheet instabilities to explain their origin instead we suggest that

the lower permian wasp head formation sydney basin high latitude shallow marine sedimentation following the late asselian to early sakmarian glacial event in eastern australia the lower permian wasp head formation early to middle sakmarian is a 95 m thick unit that was depos ited during the transition to a non glacial period following the late asselian to early sakmarian glacial event in eastern australia this shallow marine sandstone dominated unit can be subdivided into six facies associa tions i the marine sediment gravity flow facies association consists of breccias and conglomerates deposited in upper shoreface water depths ii upper shoreface deposits consist of cross stratified conglomeratic sand stones with an impoverished expression of the skolithos ichnofacies iii middle shoreface deposits consist of hummocky cross stratified sandstones with a trace fossil assemblage that represents the skolithos ichnofacies iv lower shoreface deposits are similar to middle shoreface deposits but contain more pervasive bioturba tion and a distal expression of the skolithos ichnofacies to a proximal expression of the cruziana ichnofacies v delta influenced lower shoreface offshore transition deposits are distinguished by sparsely bioturbated carbo naceous mudstone drapes within a variety of shoreface and offshore deposits trace fossil assemblages repre sent distal expressions of the skolithos ichnofacies to stressed proximal expressions of the cruziana ichnofacies impoverished trace fossil assemblages record variable and episodic environmental stresses possibly caused by fluctuations in sedimentation rates substrate consistencies salinity oxygen levels turbidity and other physio chemical stresses characteristic of deltaic conditions vi the offshore transition offshore facies association con sists of mudstone and admixed sandstone and mudstone with pervasive bioturbation and an archetypal to dis tal expression of the cruziana ichnofacies the lowermost 50 m of the formation consists of a single deepening upward cycle formed as the basin transitioned from glacioisostatic rebound following the asselian to early sak marian glacial to a regime dominated by regional extensional subsidence without significant glacial influence the upper 45 m of the formation can be subdivided into three shallowing upward cycles parasequences that formed in the aftermath of rapid possibly glacioeustatic rises in relative sea level or due to autocyclic progra dation patterns the shift to a parasequence dominated architecture and progressive decrease in ice rafted de bris upwards through the succession records the release from glacioisostatic rebound and amelioration of cli mate that accompanied the transition to broadly non glacial conditions

the formation and impact of landslide dams state of the art abstract the blocking of river courses by mass movements is very common in mountainous areas with deep and narrow valleys landslide dams may pose serious threats to people and their livelihoods downstream in the case of abrupt dam failure since the publication of benchmark reviews of costa and schuster 1988 and korup 2002 there is a growing number of studies focusing on the formation stability and short term impacts of landslide dams this review combines the insights of all these studies builds on current concepts of landslide dams and suggests ways to unify terminologies and classifications we furthermore present a new worldwide database compiled from literature data it contains 410 landslide dams 1 million m3 in volume that were formed since 1900 since these have the most complete data entries these data show that dam longevity is among other factors correlated with the type of landslide forming the dam those formed by rock debris avalanches and rockslides have longest lifespans however the influence of landslide type or material on dam longevity decreases with time after dam formation to ensure consistency in the next database generation we suggest guidelines for data collection to provide a solid basis for evaluating dam stability and governing factors a preliminary classification matrix for landslide dam stability that combines topographic setting and the internal structure of the dam body is another outcome of our review furthermore an evaluation of the various geomorphic stability indices proposed in the literature regarding their suitability and limitations in assessing dam formation and stability shows that they predict the probability of dam formation reasonably well but that their application to longevity estimates requires further assessment the geomorphic impacts of landslide dams in the short medium and long term are summarized and illustrated with key examples finally for a better understanding of the factors controlling dam stability we recommend to 1 include dam composition and sedimentary structures in future case studies 2 maintain and update the worldwide database for sound statistical analyses 3 refine landslide dam stability indices and test them for different landslide types and 4 study hazard cascades related to multiple dams in one watershed for long term landscape evolution studies we suggest to 5 quantify terrestrial sediment flux related to landslide dams 6 detect ancient landslide dams in river profiles and 7 further exploit the sediment archives in former impoundment areas

a model of the inertial recirculation driven by potential vorticity anomalies abstract some essential features of a recirculating inertial gyre the recirculation can be analyzed with a very simple analytically tractable model in wind driven eddy resolving general circulation models the recirculation appears as a strong sub basin scale inertial flow with homogeneous potential vorticity the constant value of potential vorticity decreases with increasing forcing dissipation ratio while the size and the strength of the recirculating gyre increases in the subtropical gyre the recirculating gyre might be driven by anomalous values of low potential vorticity carried northward by the western boundary current we have modeled this process using a barotropic model and prescribing the values of potential vorticity at the edge of the gyre our model gyre is contained in a rectangular box in an attempt to simplify the geometry as much as possible and to isolate the processes occurring in the recirculating region with weak diffusion the prescribed boundary forcing induces a flow with co

the 1939 montagneuse river landslide alberta the 1939 landslide on the west wall of the montagneuse river valley 1300 m long 1400 m wide and 80 m thick with av olume of 76 10 6 m 3 is the largest historic rapid landslide on the interior plains of canada it lasted about a minute dammed the river and formed a reservoir 1 5 km long the reservoir was filled before 1952 by lake deposits which were eroded by the river by 1988 the landslide contained two major blocks the downslope block 950 m long kept intact without obvious tilting indicating that the block moved almost horizontally on a comparatively deep rupture surface the displaced material included part of the preslide river channel so the montagneuse river was pushed eastward the toe of the block moved up and dammed the river the upslope block with a length of 300 350 m slid with little rotation but broke up into at least five smaller blocks most trees on the upslope block postdate the slide two sag ponds on the landslide visible in 1952 had enlarged by 1988 probably a result of water running off from the recently cleared farmland behind the scarp the main scarp has been substantially modified by earth flows and slides landslides in the montagneuse river valley coincide with the shaftesbury buried preglacial channel of the peace river the surfaces of rupture of five historic landslides in the peace river lowland follow clays deposited in preglacial channels confirming that these deposits may be hazardous when eroded

a conodont based silurian chronostratigraphy ranges of 79 conodont species are used to effect a graphic correlation of upper llandoverian pridolian silurian rocks in 30 stratigraphic sections in europe and north america a silurian composite standard section css developed by this method provides the total stratigraphic ranges for all species for which there is information in the system these ranges are stated in terms of the sequence of silurian rocks at cellon austria which is the standard reference section srs the llandoverian wenlockian and ludlovian pridolian boundaries are recognized based on the position of the golden spike in the boundary stratotypes the leasows hughley brook and pozary sections which form parts of the css range data for graptolite species in ten of the stratigraphic sections are used to approximate positions of all other boundaries between silurian series and stages that have been designated in boundary stratotypes that are not parts of the composite standard section the pridolian is shown to have had a longer duration than either the ludlovian or wenlockian and the llandoverian possibly approximated the pridolian in duration the css is divisible into 48 standard time units stu9s of conceptually equal length hence the chrono stratigraphy based on it resolves at a level two to four times higher than any previously proposed for the silurian the css is also divided into a succession of conodont chronozones that are based on zones proposed by walliser 1964 aldridge 1972 and barrick and klapper 1976 conodont chronozones can be recognized with confidence only in sections that are parts of the css framework

movement triggers and mechanisms of two earth slides in the thompson river valley british columbia canada since the construction of the canadian pacific railway and canadian national railway main rail lines in western canada in 1885 and 1905 respectively both companies have had to contend with 11 large translational retrogressive earth slides in the thompson river valley south of ashcroft british columbia the initiation of these slides is associated with the down cutting by the thompson river through the quaternary sediments in its valley the slides move on two subhorizontal weak layers in a glaciolacustrine clay silt unit within this quaternary sediment sequence transient seepage and stability analyses were conducted for two sample slides and the results were in agreement with inclinometer and piezometric data it is concluded that the thompson river triggers the movements in a drawdown mechanism and or erosion mechanism the thompson river affects the stability of these slides in three ways i by changing the pore pressure on the rupture surface ii by changing the supporting force on the toe

on the recirculation of the subtropical gyre summary a hydrographic section through the gulf stream and its recirculation to the south is interpreted in terms of an analytical model based on the baroclinic fofonoff gyre in which layers shielded from forcing are homogenized to a uniform value of potential vorticity given this simple paradigm of the circulation the process of anticyclogenesis is studied in which seasonal changes in the volume of low potential vorticity waters accompany a steepening of the main thermocline and an intensification of the recirculating gyre the consequences of the gyre intensification for the structure of the bowl the circulation within it and the deep abyssal flow are investigated it is shown that as the anticyclogenesis proceeds there is a general southward shift of the current systems the bowl of the circulation strikes the ocean floor further south and there is an intensification of the depth independent recirculation tight in to the axis of the stream these changes in the structure of the recirculation are broadly in accord with observed seasonal variability associated with wintertime mode water production

global correlation of the vazante group sao francisco basin brazil re os and u pb radiometric age constraints abstract the vazante group consists of precambrian carbonate dominated platform deposits that extend along more than 300 km in the external zone of the brasilia fold belt of the sao francisco basin in east central brazil the sequence is about 4 8 km thick and contains a preserved glaciomarine diamictite unit containing dropstone at the top and a lower diamictite unit at the bottom previous c and sr isotope profiles suggested the correlation of the upper diamictite unit with the sturtian glacial event ca 750 643 ma however new re os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 46 and 1100 77 ma u pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 15 and 1000 25 ma respectively based on the re os and u pb ages the best age estimate of the vazante group is constrained to be 1000 1100 ma and thus the two diamictite units are not correlative with the sturtian glaciation s but most likely are records of glacial events that occurred during the late mesoproterozoic

the atmospheric circulation over the north atlantic as induced by the sst field spectral analyses of the sea surface temperature sst in the simple ocean data analysis soda reanalysis for the past half century identify prominent and statistically significant interannual oscillations in two regions along the gulf stream front over the north atlantic a model of the atmospheric marine boundary layer coupled to a baroclinic quasigeostrophic model of the free atmosphere is then forced with thessthistoryfromthesodareanalysis twoextremestatesarefoundintheatmosphericsimulations 1 an eastward extensionofthe westerly jetassociatedwith thefront which occurs mainly during boreal winter and 2 a quiescent state of very weak flow found predominantly in the summer this vacillation of the oceanicfront induced jet in the model is found to exhibit periodicities similar to those identified in the observed gulf stream sstfront itself in addition a close correspondence is found between interannualspectralpeaksin the observed north atlantic oscillation nao index and the soda induced oscillations in the atmospheric model in particular significant oscillatory modes with periods of 8 5 4 2 and 2 8 yr are found in both observedandsimulatedindicesandareshowntobehighlysynchronizedandofsimilarenergyinbothtimeseries theseoscillatorymodesinthesimulationsareshowntobesuppressedwheneither i thegulfstream frontor ii its interannual oscillations are omitted from the sst field moreover these modes also disappear when iii the sst front is spatially smoothed thus confirming that they are indeed induced by the oceanic front

stress induced response from both natural and construction related processes in the deepening of the peace river valley b c the peace river rises in the rocky mountain trench and flows easterly through the rocky mountains before flowing onto the plains in northeastern british columbia near the alberta border stress measurements conducted at two of three damsites near the boundary of the foothills and the plains reveal high remnant lateral stresses and a k0 of about 2 in proceeding 90 km downriver from the w a c bennett dam past the peace canyon dam to the proposed site c dam near fort st john the stratigraphic section changes from competent massive sandstones of lower cretaceous age to soft weak shales at the top of the lower cretaceous section valley downcutting by the peace river through these highly stressed rocks has led to valley rebound features that have both similarities and differences where observed at each of the three damsites differences in rock strength at each site contribute to the variations in features noted these valley rebound features include vertical relief joints horizontal bedding plane join

on the time averaged flow of quasi geostrophic wind driven gyres the time averaged flow of a three layer eddy resolving quasi geostrophic ocean model is considered in the light of two analytical models of wind driven gyres in which the vertical structure of the gyre is set by assuming that potential vorticity is uniform beneath layers exposed to forcing the first due to young and rhines 1982 supposes that the depth integrated meridional transport is set by the imposed wind stress curl in contrast in the baroclinic fofonoff gyres of marshall and nurser 1986 1988 inertial aspects of the flow are emphasized without imposing a sverdrup constraint the mean fields from the model are seen to lie between the two extremes represented by these analytical solutions in the interior of the gyre away from inertial boundary currents and jets the sverdrup constraint is obeyed and the mean flows resemble the young and rhines solution however the essential character of the overspun recirculation including the weakly depth dependent nature of the currents in its return flow seem to be well captured in the inertial limit considered by marshall and nurser

on the origin and significance of pyrite spheres in devonian black shales of north america abstract devonian black shales deposited on the north american craton contain abundant tasmanites cysts that are typically preserved as flattened circular discs on bedding planes work by the present authors shows that cysts can be preserved as pyrite infill casts that are expressed as sand size whole and geopetal half spheres of pyrite at the bases of thin black shale layers these occur in situ at many stratigraphic levels in the distal prodelta facies of the catskill delta complex of new york as well as in laterally equivalent black shales in tennessee and kentucky reworked pyrite casts usually dominated by whole spheres form lenticular lag accumulations and hydraulic placers together with plant debris and phosphatic particles bone debris conodonts an earlier model for the formation of pyrite spheres in gas bubbles is rejected in favor of formation within uncompressed tasmanites cysts direct observation of cyst cuticle in association with pyrite spheres suggests that localized bacterial sulfate reduction in tasmanites interior voids led to formation of localized pyrite deposition in a manner similar to that described from certain ammonoid chamber settings cyst fill commenced with formation of framboidal pyrite followed by later diagenetic pyrite cementation between framboids these fills show geopetal features and appear to have formed within the redox zone below the sediment water interface although described here from the upper devonian comparable pyrite textures are also known from proterozoic cambrian ordovician and silurian sediments they probably occur throughout the sedimentary record and in mudstone successions they may prove to be an important source of sand size grains in areas far removed from the basin margins as such they may be important for detection of erosive events and strong bottom currents and provide valuable information about the depositional history of mudstone successions

upgradient and downgradient potential vorticity fluxes produced by forced rossby waves part i basic experiments abstractthe mean flow and potential vorticity pv flux produced by rossby waves are examined particularly by focusing on the effects of stratification and nonlinearity on upgradient and downgradi

chronometric calibration of mid ordovician to tournaisian conodont zones a compilation from recent graphic correlation and isotope studies three available graphic correlation analyses are used to calibrate mid palaeozoic conodont zonations sweet s scheme for the mid to upper ordovician kleffner s for the mid to upper silurian and murphy berry s for the lower and middle lower devonian the scheme of sweet is scaled by applying the high precision u pb zircon date of tucker and others for the rocklandian and linked with that of kleffner by scaling the graptolite sequence of the ordovician silurian global stratotype section to fit two similarly derived dates from this sequence the top of kleffner s scheme all of murphy berry s as well as standard zones to the frasnian are calibrated by using tie points of the latest cambridge bp time scale gts 89 however the recent microbeam zircon date by claoue long and others for the hasselbachtal devonian carboniferous auxiliary stratotype is used to calibrate the standard famennian zones also the similarly derived but preliminary determination reported by roberts and others from the isismurra formation of new south wales is tentatively taken as the top of the tournaisian and so used to calibrate tournaisian zones despite the considerable extrapolation required to compile these schemes and their inherent errors the resultant time scale closely agrees with other dates of tucker and others from the llanvirn as well as the gts 89 homerian gorstian tie point this suggests that stratigraphic methods can be usefully applied to geochronometry the llandovery appears to have lasted longer 16 m y than usually envisaged and the ordovician silurian boundary may need to be lowered to approximately 443 5 ma certainly chrons varied widely in duration and further stratigraphic studies to estimate their relative durations as well as high resolution dating for their calibration will be crucial to more accurate biochronometries

eddy mean flow interactions in the along stream development of a western boundary current jet an idealized model study a theoretical study on the role of eddy mean flow interactions in the time mean dynamics of a zonally evolving unstable strongly inertial jet in a configuration and parameter regime that is relevant to oceanic western boundary current wbc jets is described progress is made by diagnosing the eddy effect on the time mean circulation examining the mechanism that permits the eddies to drive the time mean recirculation gyres and exploring the dependence of the eddy effect on system parameters it is found that the nature of the eddy mean flow interactions in this idealized configuration is critically dependent on along stream position in particular relative to the along stream evolving stability properties of the time mean jet just after separation from the western boundary eddies act to stabilize the jet through downgradient fluxes of potential vorticity pv downstream of where the time mean jet has through the effect of the eddies been stabilized eddies act to drive the time mean recirculations through the mechanism of an upgradient pv flux this upgradient flux is permitted by an eddy enstrophy convergence downstream of jet stabilization which results from the generation of eddies in the upstream region where the jet is unstable the advection of that eddy activity along stream by the jet and the dissipation of the eddies in the region downstream of jet stabilization it is in this region of eddy decay that eddies drive the time mean recirculations through the mechanism of nonlinear eddy rectification resulting from the radiation of waves from a localized region it is found that similar mechanisms operate in both barotropic and baroclinic configurations although differences in the background pv gradient on which the eddies act implies that the recirculation driving mechanism is more effective in the baroclinic case this study highlights the important roles that eddies play in the idealized wbc jet dynamics considered here of stabilizing the jet and driving the flanking recirculations in the absence of eddy terms the magnitude of the upper ocean jet transport would be significantly less and the abyssal ocean recirculations and their significant enhancement to the jet transport would be missing altogether

global time scale and regional stratigraphic reference scales of central and west europe east europe tethys south china and north america as used in the devonian carboniferous permian correlation chart 2003 dcp 2003 the boundaries of the devonian carboniferous and permian stages of the global stratigraphic reference scale abbreviated to global stratigraphic scale gss are described in relation to the biostratigraphic and or lithostratigraphic units of the regional stratigraphic reference scales abbreviated to regional stratigraphic scales rss of central and west europe east europe tethys south china eastern tethys and north america in their type regions the boundaries of gss units rarely coincide with those of homonymous rss units moreover the definitions of some rss units have changed several times over the last decades and subsequent misunderstanding of the stratigraphical significance of these changes has often introduced errors into proposed global correlation charts the stratigraphic framework proposed in our global devonian carboniferous permian correlation chart 2003 dcp 2003 devonian carboniferous permian correlation chart 2003 menning m schneider j w alekseev a s amon e o becker g von bitter p h boardman d r bogoslovskaya m braun a brocke r chernykh v chuvashov b i clayton g dusar m davydov v i dybova jachowicz s forke h c gibling m gilmour e h goretzki j grunt t a hance l heckel p h izokh n g jansen u jin y g jones p kading k ch kerp h kiersnowski h klets a klug ch korn d kossovaya o kotlyar g v kozur h w laveine j p martens th nemyrovska t l nigmadganov a l paech h j peryt t m rohn r roscher m rubidge b schiappa t a schindler e skompski s ueno k utting j vdovenko m v villa e voigt s wahlman g p wardlaw b r warrington g weddige k werneburg r weyer d wilde v winkler prins c f work d m 2004 abschlusskolloquium dfg schwerpunktprogramm 1054 evolution des systems erde wahrend des jungeren palaozoikums im spiegel der sedimentgeochemie abstracts univ erlangen germany 2004 p 43 herein abbreviated to dcp 2003 and cited as dcp 2003 in references is an attempt to reduce these errors the dcp 2003 is the stratigraphic base for project 1054 of the deutsche forschungsgemeinschaft dfg the evolution of the late palaeozoic in the light of sedimentary geochemistry this composite time scale has been carefully balanced as far as data allows to remove unnecessary artificial compression and expansion of time intervals biozonations and depositional events the ages selected in dcp 2003 are markedly different to those in the geologic time scale 1989 gts 1989 harland w b armstrong r l cox a v craig l e smith a g smith d g 1990 a geologic time scale 1989 cambridge univ press cambridge harland w b armstrong r l cox a v craig l e smith a g smith d g 1990 a geologic time scale 1989 cambridge univ press cambridge pp 1 263 and in gradstein and ogg gradstein f m ogg j 1996 a phanerozoic time scale episodes 19 1 2 3 4 insert whereas they are closer to those of the geologic time scale 2004 gts 2004 gradstein f m ogg j g smith a g 2004 a geologic time scale 2004 cambridge univ press cambridge pp 1 589 mostly the ages are rounded to the nearest 0 5 ma in order to avoid estimates of questionable accuracy whereas ages of 0 1 ma in the gts 2004 and their error bars of 0 4 ma to 2 8 ma for the devonian to permian stage boundaries suggest an improved accuracy in contrast in the dcp 2003 questionable ages and positions of stratigraphic boundaries are marked by arrows c 2006 elsevier b v all rights reserved

assessment of the 187re decay constant by cross calibration of re os molybdenite and u pb zircon chronometers in magmatic ore systems the past decade has seen renewed interest in 187re 187os geochronology using a variety of matrices including sulfide minerals shales and meteorites the most widely used value of the 187re decay constant 187re is 1 666 0 005 10 11 a 1 0 31 which is based on cross calibration of re os and pb pb chronometers for certain meteorites smoliar m i walker r j and morgan j w 1996 re os isotope constraints on the age of group iia iiia iva and ivb iron meteorites science 271 1099 1102 however other recent studies have yielded alternate values of 187re based upon either direct counting experiments or analysis of meteorites here we provide an independent assessment of 187re using methodology sample materials and preparation of os standard solutions different from those of smoliar et al 1996 combining re os age data for molybdenite formed in magmatic ore deposits with the u pb zircon age of the magmatic rocks a refined 187re value is determined by averaging 11 individual cross calibration experiments spanning ca 2700 ma of earth history using the u decay constants of jaffey jaffey a h flynn k f glendenin l e bentley w c and essling a m 1971 precision measurement of half lives and specific activities of 235u and 238u phys rev 4 1889 1906 a value for 187re of 1 6668 0 0034 10 11 a 1 is determined using the 238u value of jaffey et al 1971 and 235u value of schoene schoene b crowley j l condon d j schmitz m d and bowring s a 2006 reassessing the uranium decay constants for geochronology using id tims u pb data geochim cosmochim acta 70 426 445 a value for 187re of 1 6689 0 0031 10 11 a 1 is determined these values are nominally higher ca 0 1 and ca 0 2 than the value determined by smoliar et al smoliar m i walker r j and morgan j w 1996 re os isotope constraints on the age of group iia iiia iva and ivb iron meteorites science 271 1099 1102 but within calculated uncertainty further refinement of 187re by cross calibrating the molybdenite and u pb zircon chronometers should be possible by utilizing high precision single grain chemical abrasion zircon u pb analyses

two different aperiodic phases of wind driven ocean circulation in a double gyre two layer shallow water model abstract a two layer shallow water model is used to investigate the transition of wind driven double gyre circulation from laminar flow to turbulence as the reynolds number re is systematically increased two distinctly different phases of turbulent double gyre patterns and energy trajectories are exhibited before and after at re 95 deterministic and fully developed turbulent circulations in the former phase the inertial subgyres vary between an asymmetric solution and an antisymmetric solution and the double gyre circulations reach the aperiodic solution mainly due to their barotropic instability an integrated kinetic energy in the lower layer is slight and the generated mesoscale eddies are confined in the upper layer the power spectrum of energies integrated over the whole domain at re 70 has peaks at the interannual periods 4 7 yr and the interdecadal period 10 20 yr the loops of the attractors take on one cycle at those periods and display the blue sky catastrophe at re 95 the dou

basin scale wind forced dynamics of the seasonal southern south china sea gyre 1 using observed wind forcing the seasonal development and disappearance of the near surface anticyclonic gyre in the southern south china sea is modeled along with associated dynamic features forced by monsoon winds the ocean circulation in the south china sea alternates between a single cyclonic gyre during the winter northeast monsoon and a double gyre cyclonic anticyclonic in the northern southern south china sea during the summer southwest monsoon reversing the western boundary current along vietnam with each shift of the monsoon regime during the summer southwest monsoon the poleward western boundary current converges with the equatorward western boundary current of the northern gyre turning offshore to create an eastward jet and flanking dipole recirculation near 13 n the development and timing of the eastward jet and recirculations are consistent with large scale basin dynamics the subsequent transition to the winter northeast monsoon regime destroys the recirculation cells and eastward jet employing high resolution 1 5 layer and 2 0 layer numerical models we demonstrate the applicability of large scale dynamics to the deeper central basin of the south china sea specifically showing that the gyre scale circulation governs the development of the eastward jet and dipole recirculations the destruction of the recirculations by the monsoon winds each autumn is also modeled this effort highlights the dominance of monsoon wind forcing in the south china sea s basin scale dynamics

re os and mo isotope systematics of black shales from the middle proterozoic velkerri and wollogorang formations mcarthur basin northern australia joint application of the mo isotope paleoredox proxy and re os deposition age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time here we report new re os and mo isotope data for the mesoproterozoic velkerri formation roper group and paleoproterozoic wollogorang formation tawallah group mcarthur basin northern australia new re os ages of 1361 21 ma 2 n 14 mean square of weighted deviates mswd 1 3 model 1 and 1417 29 ma 2 n 12 mswd 1 3 model 1 constrain the depositional age of the velkerri formation and its contained biomarkers as well as acritarchs and microfossils from the roper group black shales from the upper velkerri formation have high mo abundances 105 119 ppm and degree of pyritization dop values 0 90 0 92 implying quantitative conversion of molybdate moo42 to thiomolybdate mos42 in overlying bottom waters the average 97 95mo 0 72 0 10 2 n 6 of these shales is consistent with previous data but represents a significantly more precise determination for global seawater 97 95mo at 1 4 ga this value is lighter than present day seawater by 0 85 and reflects expanded strongly euxinic deep ocean conditions h2s aq 11 m relative to oxic suboxic and weakly intermittently euxinic h2s aq 11 m marine deposition in the 1 4 ga oceans mass balance modelling suggests mo removal into strongly euxinic and oxic sediments may have comprised 30 70 and less than 15 respectively of the oceanic mo sink at 1 4 ga as opposed to 5 and 35 today respectively the re os radioisotope system in organic rich shales serves as a test for post depositional alteration that could affect the usefulness of paleoredox tracers such as mo stable isotopes re os isotope data for the wollogorang formation black shales are scattered and yield a highly imprecise date of 1359 150 ma 2 n 21 mswd 85 model 3 this age is younger than u pb zircon ages from interbedded tuffs that constrain the age of deposition at ca 1730 ma in conjunction with previous petrological geochemical and paleomagnetic data the re os isotope data suggest hydrothermal fluid flow through the wollogorang formation possibly associated with formation of the ca 1640 ma mcarthur river pb zn ag sedimentary exhalative deposit resulted in post depositional mobilization of re and os based on the degree of deviation of the re os data from a 1730 ma reference line open system behavior of re and os was greatest near the base of the black shale unit wollogorang formation black shales are enriched in mo 41 58 ppm but are characterized by variable 97 95mo 0 3 0 8 and dop 0 57 0 92 the lightest 97 95mo values occur near the base of the black shale unit based on the re os systematics hydrothermal fluids have probably overprinted the authigenic 97 95mo signature in those shales however the heaviest 97 95mo values in the wollogorang formation come from stratigraphically higher shales and are similar to those observed for the velkerri formation and thus may reflect seawater 97 95mo at 1 73 ga

strontium isotope systematics of conodonts middle devonian eifel mountains germany abstract the eifel mountains lithological profile the stratotype for the middle devonian was densely sampled for conodonts to delineate the sr isotopic evolution of coeval sea water the 87sr 86sr curve obtained shows oscillations around a mean of about 0 7078 that have a frequency in the 105 106 yr range and an amplitude in the 10 5 10 4 range these oscillations mimic those obtained on coeval brachiopods diener et al 1996 but are shifted into more radiogenic domains by up to 10 4 model calculations suggest that the shift is due to partial 30 40 early diagenetic equilibration of conodonts with their enclosing bulk rock matrix the reality of partial recrystallisation is supported by visible neoformed blocky apatite crystals on the surface of the conodonts despite their otherwise apparently excellent preservation with cai values of 1 5 2 5 we therefore suggest that studies anticipating use of conodonts as a recorder of paleoceanographic events should be confined to specimens embedded in a relatively clean matrix such as pure limestone even these results should be cross checked by comparison with data on coeval low mg shells such as brachiopods or belemnites

influence of landslides on biophysical diversity a perspective from british columbia landslides have long been overlooked or underestimated as important natural disturbance agents in particular the ecological role of landslides in maintaining biological diversity has been largely ignored here we provide a western canadian british columbian perspective on the influences of landslides on biophysical diversity which is related in several ways to biological diversity we recognize several types of biophysical ecological diversity site diversity soil diversity and the derivative habitat or ecosystem including aquatic ecosystems diversity there are also a variety of landslide types depending on materials and on the rate and style of movement we discuss the roles of different landslide types on various aspects of terrestrial diversity landslides are simultaneously depositional and erosional processes that influence sites by redistributing materials and changing surface expression usually creating a complex microtopography that can include very dry ridges and hummocks and sometimes depressions with standing water landslide impacts to site also influence soil and soil development portions of landslides with exposed parent material are set back to the initial stages of soil development and ecological succession landslides can also change soil density structure porosity surface texture chemistry and microclimate by changing site and soil landslides also influence habitat landslides influence habitat diversity by engendering a mosaic of seral stages often both primary and secondary and in overwhelmingly forested landscapes often create nodes or hotspots of non forested habitat and biota in some areas like the boreal forest there is an important interplay between landslides and fire while on the coast of british columbia debris and snow avalanches can be the dominant disturbance agent low gradient and deep seated landslides are often opportunistically colonized by beaver and other water and shrub loving fauna sag ponds and impounded streams provide aquatic habitat often with standing dead trees landslide rubble and scarps provide denning nesting habitat escape terrain and cliff habitat for vertebrates

mechanisms of decadal variability of the wind driven ocean circulation eddy resolving quasigeostrophic simulations of wind driven circulation in a large ocean basin are presented the results show that strong modes of low frequency variability arise in many parameter regimes and that the strength of these modes depends upon the presence of inertial recirculations in the flow field the inertial recirculations arise through advection of anomalous potential vorticity by the western boundary current and are barotropized by the effect of baroclinic eddies in the flow the mechanism of low frequency oscillations is explored with reference to previous studies and it is found that the observed mode can be linked to the gyre mode but is strongly modified by the effect of eddies

the landslide dam on the saddle river near rycroft alberta alberta s largest historic landslide dammed the saddle river near rycroft alberta and thrust the bed of the river upward against the opposite valley wall forming a type 6 landslide dam the rycroft landslide occurred within a day or two of 17 june 1990 on the north wall of the saddle river 12 km northeast of rycroft alberta the triggers for the landslide are considered to be the lateral and vertical erosion of the river as a result of a severe rainstorm and the ensuing flood the slide scarp arcs for 1 km through a cultivated field the loss of farmed land was 13 h 32 acres a further 10 h 25 acres may be lost by retrogression of the scarp the toe heaved the riverbed upward as much as 20 m and thrust it laterally 60 m to be compressed against the opposite valley wall about 40 mm3 of ground moved the rupture surface dipped steeply through postglacial lake clay and till curved abruptly to pass on a subhorizontal surface through preglacial clay then curved upward under the riverbed this pregla

residual strength of clays in landslides folded strata and the laboratory the post peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages first at relatively small displacements the strength decreases to the fully softened or critical state value owing to an increase in water content dilatancy second after much larger displacements the strength falls to the residual value owing to reorientation of platy clay minerals parallel to the direction of shearing if the clay fraction is less than about 25 the second stage scarcely comes into operation the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20 degrees conversely when the clay fraction is about 50 residual strength is controlled almost entirely by sliding friction of the clay minerals and further increase in clay fraction has little effect the angles of residual shearing resistance of the three most commonly occurring clay minerals are approximately 15 degrees for kaolinite 10 degrees for illite or clay mica and 5 degrees for montmorillonite when the clay fraction lies between 25 and 50 there is a transitional type of behaviour residual strength being dependent on the percentage of clay particles as well as on their nature the post peak drop in strength of a normally consolidated clay is due only to particle reorientation measurements of strength on natural shear surfaces agree within practical limits of variation with values derived from back analysis of reactivated landslides this field residual strength can be recovered by multiple reversal shear box tests on cut plane samples but in high clay fraction materials it is typically somewhat higher than the strength measured in ring shear tests residual strength is little affected by variation in the slow rates of displacement encountered in reactivated landslides and in the usual laboratory tests but at rates faster than about 100 mm min qualitative changes take place in the pattern of behaviour a substantial gain in strength is followed with increasing displacement by a fall to a minimum value in clays and low clay fraction silts this minimum is not less than the slow or static residual but in clayey silts with clay fractions around 15 25 according to tests currently in progress the minimum can be as low as one half of the static value author trrl

entrainment and homogenization of a passive tracer in a numerical model gyre the mechanism by which tracer is entrained into a spatially decaying gyre from an external source is examined as well as the homogenization which subsequently occurs a simple advective diffusive numerical model is used whose streamlines consist of an elongated gyre situated beside a distinct boundary current which inputs tracer into the domain this is meant to represent the deep cyclonic recirculation of the gulf stream and adjacent deep western boundary current a shear flow analysis shows that two parameters dictate the manner in which tracer penetrates across streamlines of the gyre a peclet number and a parameter which measures the strength of the cross stream shear the large values of these parameters cause a plume of tracer to spiral inward toward the center of the gyre at steady state the tracer which has accumulated in the gyre becomes homogenized the size of this uniform area is related to the extent to which the spiral penetrated the gyre and decreases with increasing diffusivity as several examples illustrate