dating young zircon a case study from southeast asian megacrysts abstract u pb geochronology of quaternary zircon crystals is challenging due to their low levels of radiogenic pb the importance of accurate common pb correction and the potential need for secular disequilibrium correction u th he dating offers an independent benchmark for the quality of u pb data and their reduction including the various corrections as the closure temperature of the he system in zircon is 900 c hence u pb ages must predate u th he ages or at least be equal to them in the case of rapid cooling analyses of southeast asian mantle derived zircon megacrysts from tram poung tr tok phrom to bang kacha ba and pailin pa provide new information about their pre eruptive and eruptive history sims u pb zircon ages of 6 7 3 4 ma tr 2 45 0 23 ma to 1 44 0 13 ma ba and 0 97 0 41 ma pa correspond to u th he ages of 5 30 0 52 ma tr 2 42 0 10 ma to 1 05 0 05 ma ba and 0 91 0 02 ma pa respectively the youngest of these u pb ages pa is corroborated by la icpms mapping of a grain that yields an age of 1 017 0 067 ma the grain mapping reveals no evidence for pb diffusion and the calculated pb diffusion temperatures for these megacrysts of 1110 1250 c are significantly greater than ti in zircon apparent temperatures of 659 c these observations may imply that the u pb ages reflect magmatic crystallization and hence the close similarity of the u pb and u th he dates observed in all samples documents high zircon megacryst growth rates of 1 10 12 to to 2 10 14 cm s tr

biostratigraphic and palaeoenvironmental controls on the trilobite associations from the lower ordovician fezouata shale of the central anti atlas morocco abstract trilobites are a major component of both biomineralized and soft bodied assemblages of the fezouata biota central anti atlas morocco trilobite taxa of the fezouata shale tremadocian floian in the zagora area are typical of lower ordovician communities from shallow shelf environments and show strong affinities with early ordovician assemblages from the montagne noire southern france dramatic variations in both taxonomic composition and taphonomic attributes together with the sedimentological facies changes allow us to discriminate the palaeoenvironmental versus stratigraphic control on trilobite assemblages three communities are described along an onshore offshore profile namely the pilekiid bavarillid agerina and raphiophorid biofacies additionally three stratigraphic assemblages are identified assemblage 1 is confined to the late tremadocian assemblage 2 ranges from late tremadocian to middle floian and assemblage 3 is restricted to the floian the higher diversity of some levels is explained by storm induced inputs of debris from taxa living in adjacent environments additionally control of relative seal level variation on some peculiarities of the trilobite fauna specific endemicity lack of some expected taxa is evidenced in the fezouata shale finally temporary dysoxic events are highlighted by preliminary geochemical analyses probably allowing episodic exceptional preservation

survival of ancient landforms in a collisional setting as revealed by combined fission track and u th he thermochronometry a case study from corsica france abstractthe age of high elevation planation surfaces in corsica is constrained using new apatite u th he data field observations and published work zircon fission track apatite fission track aft data and landform stratigraphical analysis thermal modeling results based on aft and u th he data and the eocene sediments uncomformably overlapping the variscan crystalline basement indicate that present day elevated planation surfaces in corsica are the remnants of an erosion surface formed on the basement between 120 and 60 ma during the alpine collision in the paleocene eocene the variscan crystalline basement was buried beneath a westward thinning wedge of flysch and the eastern portion was overridden by the alpine nappes resetting of the apatite fission track thermochronometer suggests an overburden thickness of 4 km covering variscan corsica protected by soft sediment the planation surface was preserved in the latest oligocene to miocene times the surface was re exposed and offset by

the bivalve fauna from the fezouata formation lower ordovician of morocco and its significance for palaeobiogeography palaeoecology and early diversification of bivalves early ordovician bivalves are rare worldwide thus discovery of any new bivalve fauna of this age presents an exceptional event a new abundant bivalve fauna from the fezouata formation of morocco includes nine genera alococoncha babinka cardiolaria coxiconchia ekaterodonta glyptarca cienagomya praenucula and redonia modern evaluation of the recently collected bivalves as an important infaunal element substantially supplements our knowledge of the fossil assemblage of the fezouta formation bivalves from the fezouata formation show close affinities with assemblages of comparable age known from the montagne noire as well as with middle ordovician bivalve faunas from perunica iberia avalonia and australia some genera are in common with early ordovician bivalves from south american west gondwana argentina and bolivia during the early ordovician the migration of the bivalves was limited only to gondwana and peri gondwanan terrains the montagne noire and morocco played an important role in the distribution of important groups of bivalves to other regions of european peri gondwana armorica perunica iberia and probably similarly during the middle ordovician for the expansion of bivalves through avalonia to other continents like baltica and laurentia quantitatively the early ordovician bivalve fauna of morocco is dominated by heteroconchs associated with protobranchs and pteriomorphs the dominance of heteroconchs and poor evidence of pteriomorphs are conspicuous here and both of these features characterise a high latitude bivalve fauna exclusively infaunal burrowers filter feeders and deposit feeders are present in siliciclastic facies of the fezouata formation the genus redonia completely dominates the early ordovician bivalve fauna of morocco functional morphology of muscle scars in early ordovician and cambrian bivalves is compared two conspicuous complex muscular systems decisive for an efficient burrowing appear already in the early ordovician myophoric buttress in early ordovician protobranchs and heteroconchs seems to be an important character for serving as the attachment point for muscles which helped to hold the visceral sac during burrowing

ordovician faunas of burgess shale type the renowned soft bodied faunas of the cambrian period which include the burgess shale disappear from the fossil record in the late middle cambrian after which the palaeozoic fauna dominates the disappearance of faunas of burgess shale type curtails the stratigraphic record of a number of iconic cambrian taxa one possible explanation for this loss is a major extinction but more probably it reflects the absence of preservation of similar soft bodied faunas in later periods here we report the discovery of numerous diverse soft bodied assemblages in the lower and upper fezouata formations lower ordovician of morocco which include a range of remarkable stem group morphologies normally considered characteristic of the cambrian it is clear that biotas of burgess shale type persisted after the cambrian and are preserved where suitable facies occur the fezouata biota provides a link between the burgess shale communities and the early stages of the great ordovician biodiversification event

rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

post collisional tertiary quaternary mafic alkalic magmatism in the carpathian pannonian region a review mafic alkalic volcanism was widespread in the carpathian pannonian region cpr between 11 and 0 2 ma it followed the miocene continental collision of the alcapa and tisia blocks with the european plate as subduction related calc alkaline magmatism was waning several groups of mafic alkalic rocks from different regions within the cpr have been distinguished on the basis of ages and or trace element compositions their trace element and sr nd pb isotope systematics are consistent with derivation from complex mantle source regions which included both depleted asthenosphere and metasomatized lithosphere the mixing of dmm himu emii mantle components within asthenosphere derived magmas indicates variable contamination of the shallow asthenosphere and or thermal boundary layer of the lithosphere by a himu like component prior to and following the introduction of subduction components various mantle sources have been identified lower lithospheric mantle modified by several ancient asthenospheric enrichments source a young asthenospheric plumes with oib like trace element signatures that are either isotopically enriched source b or variably depleted source c old upper asthenosphere heterogeneously contaminated by dm himuemii emi components and slightly influenced by miocene subduction related enrichment source d old upper asthenosphere heterogeneously contaminated by dm himu emii components and significantly influenced by miocene subduction related enrichment source e melt generation was initiated either by i finger like young asthenospheric plumes rising to and heating up the base of the lithosphere below the alcapa block or ii decompressional melting of old asthenosphere upwelling to replace any lower lithosphere or heating and melting former subducted slabs the tisia block d 2004 published by elsevier b v

a 56 million year rhythm in north american sedimentation during the phanerozoic abstract long term 10 myr fluctuations in climate sea level and sedimentation have been documented in the stratigraphic record but the lack of well constrained data series has made it difficult to rigorously evaluate cyclic periodic or quasi periodic changes at this scale here we utilize a new compilation of the coverage area of sedimentary rocks in north america to investigate the dominant modes orders of stratigraphic variability and to evaluate potential long period cyclic changes in sedimentation on the continent during the phanerozoic our analysis resolves two principal temporal modes of variability 1 a strongly sinusoidal mode with a periodicity of 56 myr 3 myr and 2 a longer term phanerozoic mode the m curve linked to the wilson cycle which is indistinguishable from a stochastic autoregressive process the newly identified 56 myr cycle in sedimentation delineates most of the cratonic sequences that have previously been identified qualitatively in north america but here we propose a quantitative redefinition that includes nine distinct units and two mega sequences the timing of the 56 myr beat in sedimentation is consistent with an orogenic oscillator source or an oscillatory dynamic in mantle convection and its tempo is statistically similar to a known rhythm in number of marine animal genera in the global fossil record thus the identification of a significant periodic signal in the sedimentary record of north america provides new evidence for an important tectonic and or mantle scale cyclic process that links both large scale biological evolution and physical environmental change

welsh gold a new exceptionally preserved pyritized ordovician biota few konservat lagerstatten are known from the ordovician and most preserve atypical marginal marine communities thus we have little idea of how animals with a low preservation potential evolved during the great ordovician biodiversification event here we report the newly discovered llanfawr mudstones lagerstatte from the basal sandbian late ordovician of central wales uk this biota which has been studied through x ray radiography and microtomography is dominated by sponges it also includes cnidarians the oldest known solitary hydroid arthropods priapulids various worm like forms entoproct like organisms and a variety of enigmatic fossils the fauna includes taxa that are rarely preserved even in exceptional fossil biotas and offers the potential for a new perspective on ordovician ecosystems the dominantly filter feeding assemblage resembles modern abyssal sponge dominated communities although it was formed in much shallower water the unusual llanfawr mudstones fauna shows that ordovician ecological development was considerably more advanced in offshore environments than the mineralized fossil record implies

palynomorphs of the fezouata shale lower ordovician morocco age and environmental constraints of the fezouata biota abstract the present study documents new palynological investigations of the fezouata shale from the anti atlas morocco palynomorphs were extracted from samples collected from both outcrops and drill cuttings outcrop samples were taken near zagora and include some that were collected during excavation of stratigraphic horizons where exceptionally well preserved fossils of the fezouata lagerstatte occur subsurface samples were taken from the az 1 adrar zouggar mountain borehole which was extracted some 300 km to the southwest of zagora the palynological samples yielded acritarchs chitinozoans scolecodonts conodonts and fragments of graptolites the abundance and quality of preservation of palynomorphs varies greatly but rich and diverse assemblages were recovered from several samples the diversity and composition of the assemblages points to an open shelf environment generally mixed preservation states suggest resedimentation or differential transport histories the acritarchs can be assigned to the diagnostic messaoudensis trifidum acritarch assemblage which is typical of the tremadocian floian boundary interval of the gondwanan margin in high southern palaeolatitudes the acritarch taxa present in some of the lower parts of the fezouata shale including levels of exceptional preservation can be attributed to sub assemblages 1 2 of the messaoudensis trifidum assemblage and thus point to a late tremadocian age of the fezouata lagerstatte confirming biostratigraphic data provided by graptolites of the araneograptus murrayi graptolite biozone chitinozoans from the fezouata shale are from the e symmetrica and e brevis biozones and include various species that are well known from several localities on the gondwanan margin and from other palaeocontinents baltica laurentia and south china demarcating broad links between those regions the coincidence between index fossils of these three groups hitherto not all found in the same level suggests that the age assignments of chitinozoan biozones may be in need of revision

fe and h2o2 distributions in the upper water column in the indian sector of the southern ocean total dissolvable iron tdfe particulate iron pfe and hydrogen peroxide h2o2 measurements were performed along a n s transect in the upper 250 m in the southern ocean 62 00e 66 42s 49 00s antares ii cruise february 1994 tdfe was organically extracted apdc dddc chloroform and analysed by graphite furnace atomic absorption spectrometry gfaas pfe was analysed by gfaas following a strong mixed acid leach and h2o2 was analysed on board by fluorometry the respective detection limits are equal to 0 13 nmol kg 0 02 nmol kg and 3 0 nmol kg tdfe concentrations vary from 0 4 to 6 2 nmol kg and profiles are not completely depleted in the surface pfe concentrations vary from 0 02 to 0 2 nmol kg iron carbon fe c uptake ratios for phytoplankton were calculated either from seawater or particle measurements they are variable along the transect but are consistent when they could be compared all the observed ratios are within the range of values proposed for the fe c uptake ratios by phytoplankton using our uptake ratio calculated in the permanent open ocean zone 4 10 6 mol mol we estimate that the primary production which can be supported by the iron input flux into the surface waters is two times higher than the measured primary production in the same area in the surface waters h2o2 concentrations vary from 5 0 to 19 7 nmol kg such low concentrations are due to strong vertical mixing low dissolved organic matter concentrations and the latitude of the site

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

high pressure aragonite phenocrysts in carbonatite and carbonated syenite xenoliths within an alkali basalt we describe the first observation of primary magmatic aragonite in carbonatite and carbonated syenite occurring as xenoliths in a pliocene basaltic diatreme located near the hungary slovakia border the aragonite hosting matrix consists of disordered p rich calcite occasionally associated with trachyte glass we interpret the aragonite growth as evidence of supra lithostatic overpressure in the magmatic plumbing system that connected the crustal basaltic reservoir with the partial melting zone of the lithospheric mantle and the disordered calcite trachyte as quenched residual immiscible melts generated close to the solidus of the carbonated alkali basalt differentiated in the crustal reservoir the quenching event was a phreato magmatic eruption within the stability field of the low pressure calcite this was triggered by advective overpressure caused by expanding gas bubbles in a quasi incompressible silicate melt system the high pressure pre eruption origin of aragonite is indicated by enrichment in 13c compared to the associated calcite interpreted as a record of co2 degassing at t 500 c the oxygen 18o ranges of 22 1 24 5 v smow in aragonite 21 6 22 7 in calcite and carbon 13c ranges of 4 4 to 5 9 v pdb in aragonite 11 9 to 12 7 in calcite isotope signatures are consistent with a degassed carbonatite melt primarily derived from a subduction zone

solving the production diffusion equation for finite diffusion domains of various shapes part 2 application to cases with alpha ejection and non homogeneous distribution of the source abstract in this paper numerical methods derived in part i chem geol 2002 are expanded to accommodate the effects of emission and zonation of u and th as relevant for u th he thermochronology expressions for the required coefficients are derived for spheres finite and infinite cylinders and rectangular blocks for parent nuclide distributions which are either uniform or have stepwise zoning we show that the effects of emission and zonation on the he retention in a nonspherical crystal of modest aspect ratio can be approximated by a sphere of identical surface to volume ratio s v if the zonation is transposed on to the sphere the currently usual correction method is to perform first a diffusive calculation without considering emission and zonation and to multiply afterward with a correction factor that has been derived originally for nondiffusive calculations our results demonstrate that more accurate calculations yield different results and that the differences are often too large to be ignored in practice especially for thermal histories that include a protracted residence in the partial retention zone the difference between the usual correction and our correction can be as high as 20 for homogeneous sources for zoned crystals the difference can be even larger

the fezouata fossils of morocco an extraordinary record of marine life in the early ordovician the discovery of the fezouata biota in the latest tremadocian of southeastern morocco has significantly changed our understanding of the early phanerozoic radiation the shelly fossil record shows a well recognized pattern of macroevolutionary stasis between the cambrian explosion and the great ordovician biodiversification event but the rich soft bodied fezouata biota paints a different evolutionary picture the fezouata assemblage includes a considerable component of cambrian holdovers alongside a surprising number of crown group taxa previously unknown to have evolved by the early ordovician study of the fezouata biota is in its early stages and future discoveries will continue to enrich our view of the dynamics of the early phanerozoic radiation and of the nature of the fossil record supplementary material a complete faunal list is available at http www geolsoc org uk sup18843

acigol rhyolite field central anatolia part 1 high resolution dating of eruption episodes and zircon growth rates protracted pre eruptive zircon residence is frequently detected in continental rhyolites and can conflict with thermal models indicating briefer magma cooling durations if scaled to erupted volumes here we present combined u th and u th he zircon ages from the acigol rhyolite field central anatolia turkey which is part of a quaternary bimodal volcanic complex unlike other geochronometers this approach dates crystallization and eruption on the same crystals allowing for internal consistency testing despite the overall longevity of acigol rhyolite volcanism and systematic trends of progressive depletion in compatible trace elements and decreasing zircon saturation temperatures we find that zircon crystallized in two brief pulses corresponding to eruptions in the eastern and western part of the field during middle and late pleistocene times respectively for late pleistocene zircon resolvable differences exist between interior average 30 7 0 9 ka 1 error and rim 21 9 1 3 ka crystallization ages these translate into radial crystal growth rates of 10 13 to 10 14 cm s broadly consistent with those constrained by diffusion experiments rim crystallization and u th he eruption ages 24 2 0 4 ka overlap within uncertainty evidence for brief zircon residence at acigol contrasts with many other rhyolite fields suggesting that protracted zircon crystallization in or recycling from long lived crystal mushes is not ubiquitous in continental silicic magma systems instead the span of pre eruptive zircon ages is consistent with autochthonous crystallization in individual small volume magma batches that originated from basaltic precursors

pyritization of soft bodied fossils beecher s trilobite bed upper ordovician new york state although pyrite is ubiquitous in fine grained organic carbon bearing marine sediments it is only rarely involved in the preservation of soft bodied organisms beecher9s trilobite bed in upper ordovician strata of new york state is an exception it is a classic locality for trilobites having appendages and other soft tissues preserved in pyrite the relative timing and duration of the formation of pyrite associated with the fossils and their host sediments were determined by use of sulfur isotope ratios the exoskeleton and appendages of the trilobites show relatively light sulfur isotope values in contrast to the enclosing sediment which is characterized by a substantial excursion to heavy isotope values preservation of soft parts requires rapid burial of carcasses in sediments otherwise low in metabolizable organic matter in these circumstances pyrite formation within the sediments is suppressed thus concentrations of sulfate and reactive iron are initially high enough to promote early rapid and extensive pyritization of nonmineralized tissue

exsolution of thortveitite yttrialite and xenotime during low temperature recrystallization of zircon from new caledonia and their significance for trace element incorporation in zircon recrystallized zircon grains in a phengite epidote chlorite schist from north eastern new caledonia contain as inclusions a mineral assemblage consisting of celadonite kaolinite quartz fe oxy hydroxide smectite chlorite xenotime y thortveitite yttrialite and allanite ce this assemblage formed during low temperature 100 c seafloor alteration of a plagioclase rich mafic rock and represents the first documented evidence of this alteration event in the high p belt of northeastern new caledonia the survival of this low temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions thortveitite sc2si2o7 and yttrialite y2si2o7 inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized the altered zircon cores generally have lower trace element contents than the pristine igneous zircon cores and we suggest that thortveitite yttrialite and xenotime y formed as a result of trace element expulsion from zircon during low temperature recrystallization trace element concentrations in the zircon cores indicate that trivalent cations ree y sc in zircon cannot be charge balanced by xenotime substitution alone pentavalent cation concentrations nb as are also insufficient for charge balance the presence of thortveitite sc2si2o7 and yttrialite y2si2o7 suggests that trivalent cations in zircon might be charge balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as h occurring interstitially in the zircon lattice

tertiary subduction and slab break off model of the carpathian pannonian region abstract this paper illustrates the application of multidisciplinary data analysis to the carpathian pannonian region and presents a new tertiary development model that incorporates all multidisciplinary parameters during the late cretaceous eocene the future carpathian orogen was part of the larger alpine carpathian orogen formed by the southeastward subduction 135 55 ma of the penninic ocean and the ultimate collision 55 40 ma of the european and adriatic continents the subduction in the alpine region ended by the slab break off of the subducting oceanic lithosphere from the continental one and by the related magmatism 43 24 ma while the eocene alps were under continental collision the eocene remnant carpathian flysch basin rcfb was still subducting the carpathians and eastern parts of the alps started to occupy the region of the present carpathians during the early miocene the oceanic plate underlying the rcfb subducted southwestwards later westwards while the northeastward and eastward migration velocity of the accreting plate boundary decreased through time the slower subduction rate caused a steeper dip of the subducting slab the beginning of the eggenburgian 22 19 ma and following duration of extension in the orogenic hinterland accompanied by the asthenosphere upwelling the onset of the crustally derived volcanism above the risen asthenosphere and the younger mantle derived volcanism progressively less contaminated by the crustal material all contemporaneous with a shortening in the carpathian accretionary wedge the subduction of the rcfb finally led at the end of the early miocene to a collision with the continental margin along the westernmost part of the present carpathian arc the subduction roll back of the subducting slab of the rcfb under the advancing inner carpathians the oblique closure of the basin the progressive change of the subduction to collision from the west to the east along the carpathian arc drove a scissors like break off of the subducting oceanic slab along the west carpathians the break off in the east carpathians was driven by the weight of a subducting oceanic slab and buoyancy of the attached continental slab the break off started in the west of the carpathian arc in the early miocene ran along the arc to its present position in the bend area between the east and south carpathians and separated the oceanic slab that subducted passively under its own weight from the european continental margin the lateral propagation rate of the slab tear decreased with time the break off related volcanism was synchronous with the final stages of the collision

a new lagerstatte from the middle ordovician st peter formation in northeast iowa usa a new fossil fauna has been discovered from a recently recognized shale unit within the middle ordovician st peter formation in northeast iowa it contains a variety of invertebrates and vertebrates including soft body tissues impressions and 3 dimensionalpreservations the exceptional preservation reveals a new konservat lagerstatte the winneshiek lagerstatte and opens a unique window into the community that inhabited the margins of the laurentian cratonic seaway during middle ordovician transgression among the fossils several conodont assemblages including the apparatus of enigmatic coleodontids are preserved some conodont assemblages associated with soft body tissues are particularly noteworthy

a new hypothesis for organic preservation of burgess shale taxa in the middle cambrian wheeler formation house range utah cambrian konservat lagerstatten are the most significant fossil deposits for our understanding of the initiation of phanerozoic life although many modes of preservation may occur these deposits most frequently contain nonmineralized fossils preserved in the form of kerogenized carbon films a rare yet important taphonomic pathway that has not previously been explained for any unit by a comprehensive model the middle cambrian wheeler formation of utah one of these lagerstatten contains abundant kerogenized preservation of nonmineralized tissues which occurs within a distinctive taphofacies that accumulated under the following conditions 1 domination of the siliciclastic fraction by clay sized particles 2 close proximity to a carbonate platform which resulted in mixed carbonate clay sediments 3 a well developed oxygen minimum precluding benthic colonization and burrowing and 4 relative proximity to oxic bottom waters facilitating transport of organisms from a habitable environment to one that favored their preservation we propose that preservation of nonmineralized tissues in the wheeler formation may have resulted from a combination of influences that reduced permeability and thus lowered oxidant flux which in turn may have restricted microbial decomposition of some nonmineralized tissues those influences include near bottom anoxia preventing sediment irrigation by restriction of bioturbation reducing conditions near the sediment water interface that may have acted to deflocculate aggregations of clay minerals resulting in low permeability face to face contacts early diagenetic pore occluding carbonate cements and an absence of coarse grains such as silt skeletonized microfossils fecal pellets or bioclasts this model may be applicable to kerogenized preservation of macrofossils in other fossil lagerstatten

re anchoring the late pleistocene tephrochronology of new zealand based on concordant radiocarbon ages and combined 238u 230th disequilibrium and u th he zircon ages abstract the caldera forming rotoiti eruption from okataina volcano was one of the largest rhyolite events of the last 100 ka in the taupo volcanic zone new zealand its associated widespread tephra layer rotoehu tephra is a significant time marker in terrestrial marine paleoclimate correlation studies in the sw pacific however the accurate and precise age of this eruption and of the subsequent smaller volume earthquake flat eqf eruption has been the subject of controversy despite numerous studies we have applied combined 238u 230th disequilibrium and u th he dating of zircon from rotoiti and eqf deposits and obtained overlapping ages of 45 1 7 3 3 ka and 45 1 6 2 9 ka respectively these results are supported by new and published high precision radiocarbon data bracketing the age of the rotoiti eruption between 44 8 0 3 and 47 5 2 1 ka cal bp these age data are also in good agreement with a range of previously published estimates based on paleoclimate palynology luminescence dating of enclosing sediment and sedimentation rates in terrestrial and marine settings however these results are at variance with a commonly quoted age of 60 ka largely constrained by a single 40ar 39ar age of an overlying glassy lava flow at a distal tephra site this study demonstrates the potential for combined 238u 230th disequilibrium and u th he dating of zircon in tephra and other volcanic deposits to provide age control in the 40 100 ka time interval a period difficult to constrain using more traditional radiocarbon k ar and 40ar 39ar methods

the great ordovician biodiversification event gobe the palaeoecological dimension abstract the great ordovician biodiversification event gobe saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so called cambrian explosion the diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them the present paper reviews the palaeoecological dimension of the gobe it involved major increases in s and biodiversity largely associated with the rise of the paleozoic evolutionary fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the ecological evolutionary units p1 and p2 in the benthos these include more complex food webs than those of the cambrian greater tiering especially above the sediment water interface and the development of guild structures indicating increased competition between taxa for particular resources the ordovician is characterized by a profound change in reef composition with a switch from microbial dominated reefs in the early and middle ordovician to metazoan dominated reefs in the late ordovician increases in complexity of deep water trace fossil assemblages began in the early ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities within the water column the gobe involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton including planktotrophic larvae from a range of invertebrate clades this revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates molluscs and arthropods and promoted the rise of suspension feeders in the benthos an escalation amongst predators and thus community evolution may also have been a major driver of biodiversification

combined u pb and u th he geochronometry of basalt maars in western carpathians implications for age of intraplate volcanism and origin of zircon metasomatism the age of intraplate volcanism in northern pannonian basin of carpathians is revisited using a combination of zircon u pb zircon u th he and apatite u th he dating techniques complemented by electron microprobe emp characterisation of dated minerals a total of six maar structures and diatremes in the south slovakian volcanic field ssvf were dated and the obtained new ages yielded the following key findings two isolated maars in se part indirectly dated by geomorphologic constraints to late pleistocene are actually of pliocene 2 8 0 2 ma and late miocene 5 5 0 6 ma ages in contrast two maars in nw part of the study area are of late pliocene age 4 1 0 4 and 5 2 5 4 ma younger than the late miocene age 6 5 ma inferred previously from k ar data on the proximal basaltic lava flows these maars therefore belong to the second volcanic phase that was previously identified only in se part of the ssvf in the light of the new geochronologic data it seems likely that the pliocene phreatomagmatic eruptions may have occurred along extension related nw and ne trending orthogonal faults emp analyses and imaging revealed an extensive syn and post growth metasomatic replacement by dissolution reprecipitation in the majority of zircons abundant silicate melt inclusions in porous metasomatised parts of the zircons are diagnostic of magmatic rather than hydrothermal metasomatism consistent ages of the metasomatised and non metasomatised zones do not indicate disturbance of the u pb system during the metasomatism enrichment in u and th loss in the metasomatised zircons are diagnostic of an increasing oxygen fugacity triggered by degassing of the volatile residual melt during the final stages of alkali basalt fractionation rare zircon to baddeleyite transformation was probably connected with lowered silica activity in carbonated basaltic magmas in south eastern part of the study area

systematics of xenocrystic contamination preservation of discrete feldspar populations at mccullough pass caldera revealed by 40ar 39ar dating single crystal 40ar 39ar dating of k feldspars from silicic volcanic rocks containing xenocrysts often yields a spectrum of ages slightly older than those of juvenile sanidine phenocrysts in contrast feldspars from thin low volume units of the tertiary 14 ma mccullough pass tuff define discrete age populations at 14 ma 15 ma and 1 3 ga reflecting the time of eruption xenocrysts from an older ignimbrite exposed in the caldera wall and proterozoic basement k feldspars respectively conductive cooling and diffusion modelling suggests preservation of such discrete populations is likely only when xenocrystic material is incorporated into the magma very near or at the surface or is engulfed in thin rapidly cooled pyroclastic flows during emplacement incorporation of xenocrysts into the subvolcanic magma chamber into thick rhyolite domes or lava flows or into large welded ignimbrite sheets will result in partial or total resetting of the k ar isotopic system similarly petrographic evidence such as exsolution lamellae may be homogenized under these conditions but not in thin ignimbrites extremely low diffusion rates for disordering of the al si tetrahedral siting of basement feldspars suggests that they will retain their ordered structural state given rhyolitic magma temperatures thus even when petrographic and k ar isotopic evidence for xenocrystic contamination is obscured it may be preserved in the form of al si ordering

a unified model for bedform development and equilibrium under unidirectional oscillatory and combined flows the development of bedforms under unidirectional oscillatory and combined flows results from temporal changes in sediment transport flow and morphological response in such flows the bedform characteristics for example height wavelength and shape change over time from their initiation to equilibrium with the imposed conditions even if the flow conditions remain unchanged these variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record hence understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record this article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional purely oscillatory and combined flow conditions which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples the present results yield a unified model for bedform development and equilibrium under unidirectional oscillatory and combined flows the experimental results show that the processes of bedform genesis and growth are common to all types of flows and can be characterized into four stages i incipient bedforms ii growing bedforms iii stabilizing bedforms and iv fully developed bedforms furthermore the development path of bedform growth exhibits the same general trend for different flow types for example unidirectional oscillatory and combined flows bedform size for example small versus large ripples bedform shape for example symmetrical or rounded bedform planform geometry for example two dimensional versus three dimensional flow velocities and sediment grain sizes the equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition

a review of wave dominated clastic shoreline deposits abstract barrier islands beaches wave dominated deltas and chenier plains probably form more than one fifth of present coasts and constitute an even larger proportion of the subsiding lowland coasts likely to be preserved in the stratigraphic record the geometry vertical sequences and composition of their deposits are the result of 1 shoreline behaviour whether transgressive regressive or stable 2 physical processes particularly the relative intensities of waves and tides 3 climate 4 global tectonic setting and 5 topography of underlying deposits published interpretations indicate that equivalent deposits are abundant in the geological record sandstones are mostly quartzose cemented by carbonates and quartz and have sheet or linear geometries ancient deposits can provide evidence of palaeogeography and some are economically significant particularly as hydrocarbon reservoir rocks four end member sedimentation models are proposed for the interpretation of ancient deposits 1 transgressive sheet sands associated with deltas 2 non deltaic transgressive sheet sands 3 regressive sheet sands and 4 linear sand bodies probable depositional settings geometries characteristics of deposits and palaeocurrent patterns are discussed

a tale of two eras pliocene pleistocene unroofing of cenozoic and late archean zircons from active metamorphic core complexes solomon sea papua new guinea u pb ion microprobe analyses of zircons from gneisses and granodiorites exposed in the d9entrecasteaux islands and from conglomerate sections of the goodenough no 1 well in the adjacent trobriand basin provide constraints on the age of magmatism peak metamorphism and nature of rocks unroofed during initial stages of metamorphic core complex formation in the solomon sea the youngest populations of zircons from felsic gneisses and granodiorites indicate late pliocene 206 pb 238 u ages no inherited zircons were identified in the granodiorites and the 206 pb 238 u ages 1 65 0 18 ma 1 98 0 08 ma 2 are interpreted as crystallization ages these synkinematically emplaced granodiorites intruded into actively extending continental crust are some of the youngest known granitoids currently exposed at the earth9 surface zircon ages from felsic gneisses 2 63 0 16 ma 2 72 0 28 ma 2 are interpreted to date zircon growth subsequent to eclogite facies metamorphism felsic gneiss samples also contained zircon xenocrysts from cretaceous miocene protoliths in striking contrast zircons from igneous and metamorphic clasts from the goodenough no 1 well indicate a single population with a 207 pb 206 pb age of 2781 9 ma 2 we speculate that they are derived from basement rocks unroofed during initial stages of development of the d entrecasteaux metamorphic core complexes these results provide the first direct evidence for the existence of archean protoliths in the basement rocks of southeastern papua new guinea

storm generated sedimentary structures in subtidal marine facies with examples from the middle and upper ordovician of southwestern virginia abstract the role of storms is of major importance in forming stratification in open shelf facies though storm processes on modern shelves are not fully understood enough is known to see how sediments are influenced storms transport sand away from coastal areas and suspend and transport sand and mud on the open shelf when wave base is lowered storms generate winnowed lag deposits of shells or gravel but coarser sediment and shells of megafauna are little transported except in very near shore environments the martinsburg formation middle and upper ordovician of southwestern virginia was deposited on a storm influenced open marine platform in contrast to the classic turbidite flysch basin of the martinsburg in the great valley of virginia to the northeast deposition of most martinsburg sediments was due to storms sediments deposited during single storm events occur in characteristic fining upward sequences 1 to 80 cm thick in both carbonate and terrigenous clastic dominated areas of the platform individual storm deposits consist of a sharp erosional base overlain by a couplet of whole fossil packstone and laminated calcarenite siltite or quartzose siltstone to very fine sandstone which is gradational to shale packstones are typically intraclastic have shells aligned parallel o bedding and convex up often with adhering or contained mud and have abundant infiltration textures such as cement filled shelter voids packstones grade abruptly to a laminated bed with horizontal plane lamination large and small scale in phase undulatory lamination and wave ripple cross lamination commonly occurring in ordered vertical sequences laminated beds contain escape burrows are weekly graded and have irregularly mounded upper bed surfaces laminated beds grade upward to shale containing horizontal branching burrows and marine fossils the faunal content of the fining upward sequences matches that of the interbedded shale storm generated beds vary greatly in thickness laterally and frequently pinch out completely fining upward sequences formed by scouring winnowing and suspension of the sand and mud during the height of the storm to form the erosional base and autochthonous lag of whole fossils storms preferentially preserve whole fossils rather than fragmenting them infiltration of finer sediment into the lag deposit and deposition of the laminated bed occurred as the storm waned laminated beds are molded into a distinctive suite of sedimentary structures by wave generated currents the shale represents deposition of finest storm suspended material and continued hemipelagic sedimentation similar sequences are widely distributed in the geologic record variability of storm generated sequences can be related to changing water depth thus storm deposits may be useful in basin analysis to estimate relative water depths and trace transgressive and regressive events

gastropoda tergomya and paragastropoda mollusca from the lower ordovician fezouata formation morocco abstract gastropoda tergomya and paragastropoda gtp are a small but recognizable part of the collective fezouata biota from the lower ordovician tremadocian floian fezouata formation in morocco gtp range through the sequence but become more abundant and diverse in the stratigraphically higher and shallower marine parts of the succession about 200 rock samples in existing collections have gtp but usually each slab contains several specimens so the number of individual is many times higher a total of seven species are recognized of which four were known earlier gastropods are represented by the planispiral bellerophontoid sinuites sp recognized for the first time in the tremadocian part of the succession and the anisostrophic nearly planispiral lesueurilla prima barrande in perner tergomyans are most abundant dominated by the genus carcassonnella with carcassonnella courtessolei horn and peel carcassonnella vizcainoi horn and peel and carcassonnella sp the latter encompass several specimens from different localities and stratigraphical levels and may represent one of the named species or new varieties carcassonnella is for the first time recorded in the tremadocian part of the succession a second tergomyan is thoralispira laevis thoral while paragastropoda are represented by pelecyogyra fezouataensis ebbestad and lefebvre in the peri gondwana area carcassonnella thoralispira and lesueurilla are considered signature taxa and the fezouata gtp compare closely with those of montagne noire france both in composition and distribution the bohemian fauna is slightly younger floian darriwilian with different species except for lesueurilla prima the latter may have a wider distribution being tentatively recognized in the lower ordovician of argentina and spain

immiscible separation of metalliferous fe ti oxide melts from fractionating alkali basalt p t f o2 conditions and two liquid elemental partitioning globules of iron dominated 59 69 wt feotot and titanium dominated 43 5 wt tio2 oxide melts have been detected in igneous xenoliths from pliocene to pleistocene alkali basalts of the western carpathians fluid inclusion and mineral composition data indicate immiscible separation of the high iron oxide melt him at magmatic temperatures the him separation occurred during clinopyroxene augite accumulation in an alkali trachybasalt and continued during crystallization of amphibole kaersutite and k feldspar anorthoclase the latter coexisting with trachyte and alkalic rhyolite residual melts some him was also expelled from sub alkalic rhyolite 70 77 sio2 coexisting with an27 45 plagioclase and quartz in granitic tonalite trondhjemite xenoliths oxygen fugacities during him separation range from 1 4 to 0 6 log units around the qfm buffer a close genetic relationship between him hosted xenoliths and mantle derived basaltic magma is documented by mineral 18o values ranging from 4 9 to 5 9 v smow d values of gabbroic kaersutite between 61 and 86 v smow are in agreement with a presumed primary magmatic water source most trace elements except li rb and cs have preferentially partitioned into the him the him si melt partition coefficients for transition elements sc v cr co ni and base metals zn cu mo are between 2 160 resulting in extreme enrichment in the him la and ce also concentrate in the silicic melt whereas tb tm in the him hence the immiscible separation causes ree fractionation and produces residual silicic melt enriched in lree and depleted in hree the weak fractionation among tb tm and yb lu can be attributed to recurrent extraction of the him from the magmatic system while flat hree chondrite normalized patterns are interpreted to indicate no or little loss of the him

biostratigraphy and palaeoecology of lower ordovician graptolites from the fezouata shale moroccan anti atlas abstract the fezouata shale fezouata formation has produced the most complete lower ordovician graptolite succession known from the african continent which was studied from 32 sections located along the western central and eastern moroccan anti atlas a composite biostratigraphic and chronostratigraphic scheme is presented for the entire formation that in ascending order comprises the anisograptus matanensis rhabdinopora flabelliformis anglica zones uppermost lower tremadocian adelograptus tenellus and aorograptus victoriae zones middle tremadocian araneograptus murrayi and hunnegraptus copiosus zones upper tremadocian cymatograptus protobalticus zone lower floian baltograptus jacksoni zone middle floian and the baltograptus minutus zone plus an azygograptus interval upper floian most of these zones or biozones are recorded for the first time in africa together with important graptolite species such as ancoragraptus bulmani spjeldnaes cym protobalticus monsen b jacksoni rushton clonograptus multiplex nicholson b minutus tornquist and azygograptus eivionicus elles a correlation with the graptolite sucession from the algerian sahara is also suggested reinforced by the common record of the minute anisograptid choristograptus louhai legrand providing its second world occurrence in the tremadocian of morocco the prior late tremadocian dating of the fezouata lagerstatte is here extended to the middle floian as revealed by the graptolite biochronology the repeated record along the lower and middle parts of the fezouata formation of deep water mesopelagic species of the genera araneograptus with mass occurrences of conical colonies together with large horizontal rhabdosomes of paratemnograptus paradelograptus clonograptus tetragraptus and holograptus schizograptus do not fit the sedimentological interpretation of deposition of these strata in much shallower inshore to mid shelf environments this inconsistency has implications when inferring the living conditions of the fezouata soft bodied assemblages

high resolution 5 μm u th pb isotope dating of monazite with excimer laser ablation ela icpms abstract monazite lree po 4 a common accessory mineral in magmatic and metamorphic rocks is complementary to zircon in u th pb geochronology because the mineral can record successive growth phases it is useful for unravelling complex geological histories a high spatial resolution is required to identify contrasted age domains that may occur at the crystal scale bulk mineral techniques such as id tims applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages laser ablation la icpms has been demonstrated to be a useful technique for u th pb dating of zircons and this study tests its analytical capabilities for dating monazite a sector field high resolution icpms coupled with a 193 nm arf excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements the u th pb systematic was applied to monazite grains from several samples a lower palaeozoic lens from high grade terrains in southern madagascar neogene hydrothermal crystals from the western alps a palaeoproterozoic very high temperature granulite from central madagascar and a variscan leucogranite from spain directly on a polished thin section the major aim was to compare and or reproduce tims and emp ages of monazite from a variety of settings and ages the three independent 206 pb 238 u 207 pb 235 u and 208 pb 232 th ratios and ages were calculated isotope fractionation effects mass bias laser induced fractionation were corrected using a chemically homogeneous and u pb concordant monazite as external standard this study demonstrates that excimer laser ablation ela icpms allows u th pb dating of monazite with a high level of repeatability accuracy and precision as well as rapidity of analysis a spatial resolution almost comparable to that of emp in terms of crater width 5 m produced precise 208 pb 232 th 206 pb 238 u and 207 pb 235 u ratios for dating palaeozoic to precambrian monazites the advantages of ela icpms isotope dating are precision accuracy and the ability to detect discordance in the case of late miocene hydrothermal monazites from the alps a larger spot size of 25 m diameter is required and precise and accurate ages were obtained only for 208 pb 232 th systematics results from the variscan granite show that in situ u th pb dating of monazites with ela icpms is possible using a 5 m spot directly on thin sections so that age data can be placed in a textural context

origin and provenance of 2 ma 2 ga zircons ejected by phreatomagmatic eruptions of pliocene basalts in southern slovakia u pb ages of zircons recovered from pliocene pyroclastic deposits in northern part of the cenozoic intra carpathian back arc basin pannonian basin span the interval from pliocene 2 2 ma to paleoproterozoic orosirian rhyacian 1850 2115 ma the scattered u pb ages reflect eruption ages of the host basaltic volcanic centres two episodes of post eocene magmatic crustal growth and the possible tectonic affiliation provenance and age of the subjacent basement or the sedimentary basin detritus sampled by the basaltic magma the youngest zircons define the maximum ages of phreatomagmatic eruptions during the late miocene pliocene extension these zircons are distinguished from older zircons by zr hf 40 90 and th u ratios 0 5 4 5 as well as super chondritic ehf t values ranging from 7 to 14 indicating mantle derived parental magmas the locally increased th u ratios up to 8 accompanied by zr hf 60 are diagnostic of evolved phonolite parental melt hence the youngest zircons can be interpreted as antecrysts originating from evolved melts cogenetic with the host alkali basalts in contrast older zircons represent xenocrysts scavenged by the uprising basalt from surrounding rocks subordinate eocene early oligocene 29 38 ma sub group of zircon xenocrysts is coincidental with the magmatism and volcanism along the periadriatic lineament and the middle hungarian zone the early miocene 18 ma cluster is coeval with the deposition of the bukk mountains felsic ignimbrite correlated with the onset of the back arc extension that triggered miocene sedimentation within the pannonian basin the eocene early oligocene zircons have been likely scavenged from pyroclastic and ash fall deposits of the palaeogene retroarc basin subjacent to the miocene basin infilling sub chondritic ehf t values between 2 5 and 8 in the eocene early miocene zircons attest their crystallization from subduction related felsic to intermediate melts containing large amounts of recycled crustal material palaeozoic proterozoic zircons create a heterogeneous population with variable trace element abundances and ehf t values the determined age clusters are reminiscent of some basement units cropping out recently in central western carpathians zircon hf isotope data indicate recycling of up to 3 4 ga old mafic crust and also the presence of 2 ga old juvenile mafic crust these units had either underlain the northern part of the pannonian basin during pliocene or had been exposed during the deposition of miocene clastic sediments the absence of mesoproterozoic grenvillian zircons 0 9 1 8 ga in the pre cenozoic population of zircon xenocrysts is provisionally interpreted as indicating the evolution of the zircon source area within the west african craton

hydrographic section aeross the kuroshio near 35 n 143 e a closely spaced conductivity temperature depth hydrographic section was conducted off the east coast of japan in july 1992 the southeastward section crossed the japan trench and the kuroshio in the vicinity of the kashima 1 seamount vertical sections of temperature salinity density oxygen and nutrients are discussed in conjunction with the movement and interleaving of water masses complicated vertical and horizontal mixings of water masses are inferred from the temperature and salinity relationships mixing processes are patchy and not continuous beneath the front warm salty water found beneath the kuroshio may result from upward mixing of water from intermediate depths the main axis of the kuroshio indicated by the 14 c isotherm at 200 m is at 35 7 n 142 6 e about 20 km from the north wall surface thermal front geostrophic speeds exceed 170 cm s 1 at the surface volume transport through the section is 81 106 m3 s 1

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

x ray microtomography applied to fossils preserved in compression palaeoscolescid worms from the lower ordovician fezouata shale abstract computed microtomography xr ct was used to study the three dimensional morphology of palaeoscolex cf tenensis a palaeoscolecid worm from the lower ordovician fezouata lagerstatte central anti atlas morocco this method revealed important aspects of its internal anatomy such as a pharynx bearing longitudinal rows of tiny teeth tubular structures are interpreted as the fragments of a cylindrical gut tract running from mouth to anus energy dispersive x ray analysis edx confirm the primary phosphatic composition of the microplates associated with the cuticle of the worm and suggests that the preservation of soft tissues took place via early bacteria mediated pyritization followed by strong weathering of fe rich aluminosilicates during diagenesis

in situ benthos and paleo oxygenation in the middle cambrian burgess shale british columbia canada the middle cambrian burgess shale was deposited under variable levels of oxygenation during periods of oxygenation low diversity shell beds were formed and the muds were colonized by infauna under these circumstances pyrite was restricted to anoxic microenvironments and formed small 1 mm discrete aggregated masses soft bodied organisms were preserved under low oxygen conditions typified by evenly dispersed pyrite and an absence of trace fossils and shell beds

the lower ordovician fezouata konservat lagerstatte from morocco age environment and evolutionary perspectives abstract the lower ordovician fezouata konservat lagerstatte from southern morocco has been one of the major palaeontological discoveries of the last decade it provides a unique insight into one of the most critical periods in the evolution of marine life the cambrian ordovician transition however its potential for deciphering key trends in animal diversification was hitherto largely limited by major uncertainties concerning its stratigraphic position age and environmental setting based on extensive fieldwork fossil evidence and facies recognition our study provides clarification on these three crucial issues exceptional preservation is limited to two intervals within the fezouata shale graptolites indicate a late tremadocian age for the fezouata konservat lagerstatte as a whole which is supported by biostratigraphical evidence provided by acritarchs sedimentological features and reconstructed patterns of relative sea level changes indicate relatively shallow water environmental conditions under distal storm influence in an offshore to lower shoreface siliciclastic ramp setting the fezouata biota represents a unique and exceptional window into the palaeobiodiversity in open marine conditions thus contrasting with the other ordovician konservat lagerstatten presently known in our analyses of this new set of data we pave the way for accurate temporal faunal and environmental comparisons with other lower palaeozoic konservat lagerstatten and unlock the full potential of the fezouata biota to better understand the processes and scenarios of early animal radiations

stratigraphic controls of soft bodied fossil occurrences in the cambrian chengjiang biota lagerstatte maotianshan shale yunnan province china abstract the early cambrian maotianshan shale exposed near kunming china is one of the oldest and best preserved konservat lagerstatten containing the chengjiang biota although the fossils have been well studied the depositional processes and stratigraphic relationships amongst the fossil bearing localities are poorly understood largely due to limited continuous exposures and structural complexity in this area in this study we use a detailed facies and stacking pattern analysis to delineate the depositional history and stratigraphic relationship of the maotianshan sedimentologic observations from five prolific fossil localities of the chengjiang biota have yielded six distinct facies within the maotianshan shale the three coarse grained facies composed of siltstones and very fine sandstones f1 f3 contain sedimentary structures including hummocky cross stratification and combined flow ripples that are consistent with deposition within storm wave base the fine grained facies composed of interbedded silty mudstones and clay rich mudstones f4 f6 were deposited below the wave base and represent episodes of lower storm frequency and pelagic sedimentation these six facies have specific stacking patterns indicating the presence of five distinct depositional units each of those depositional units is bound on its upper and lower contact by a flooding surface fining upward successions near the base of each stratigraphic unit are interpreted as short lived marine transgressions these are overlain by coarsening upward stacked facies interpreted as a period of shoreline progradation the fossils of the chengjiang fauna are exclusively found in the coarsening upward part of the depositional units the oldest fossil bearing intervals are associated with the most westerns sections in the study area in contrast the youngest fossil bearing intervals are found in the most eastern sections this time transgressive relationship is interpreted to reflect a genetic relation between distinct sedimentary processes and environments and the burial and preservation of the chengjiang biota

mechanism for burgess shale type preservation exceptionally preserved fossil biotas of the burgess shale and a handful of other similar cambrian deposits provide rare but critical insights into the early diversification of animals the extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft bodied fossil assemblages has remained enigmatic since walcott s initial discovery in 1909 here we demonstrate the mechanism of burgess shale type preservation using sedimentologic and geochemical data from the chengjiang burgess shale and five other principal burgess shale type deposits sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation low sulfate concentrations in the global ocean and low oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability subsequently rapid entombment of fossils in fine grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments a permeability barrier provided by bed capping cements that were emplaced at the seafloor is a feature that is shared among burgess shale type deposits and resulted from the unusually high alkalinity of cambrian oceans thus burgess shale type preservation of soft bodied fossil assemblages worldwide was promoted by unique aspects of early paleozoic seawater chemistry that strongly impacted sediment diagenesis providing a fundamentally unique record of the immediate aftermath of the cambrian explosion

exceptionally preserved late ordovician biotas from manitoba canada there are few body fossil biotas known from early paleozoic accretionary shorelines and very few examples of ordovician soft bodied assemblages this study documents two recently discovered biotas from separate sedimentary basins in manitoba canada that provide unique information about tropical shoreline communities shortly before the late ordovician extinction event each site represents a distinct depositional environment but they share biotic elements including eurypterids xiphosurids and large problematic tubes the william lake biota representing more restricted conditions includes jellyfish that are among the best hydromedusan body fossils known rocks at the airport cove site deposited under more open circulation contain scolecodonts and noncalcified algae these biotas have some parallels with the recently described middle ordovician winneshiek lagerstatte but are also similar to some late silurian assemblages considered together early paleozoic marginal marine deposits are a rich but as yet poorly known source of paleobiodiversity data

estimating palaeo water depth from the physical rock record abstract palaeo water depth and its change with time is a key factor in environmental analysis and in sedimentology in general the literature published on this topic and on related subjects however is substantial albeit scattered within and between different communities geologists oceanographers engineers physicists etc here i focus on proxies for palaeo bathymetry as obtained from the physical rock record floral faunal and chemical evidence for palaeo water depth being equally important must be dealt with elsewhere an assessment of the main depth related parameters in actualistic settings is presented and their applicability to the fossil sedimentary record discussed the outcome reveals the full complexity of this topic there is for example no such thing as an average wave base depth applicable to a wide range of fossil case settings this because wave and current climates in general and particularly so in neritic and coastal settings are intriguingly complex furthermore observations from modern oceans are not a priori applicable to fossil ones acknowledging these problems and limitations critically evaluated quantitative proposals for depth indicative facies and features in the sedimentary record are presented and error bars discussed

distribution of hydrogen peroxide in the northwest pacific ocean 1 hydrogen peroxide h2o2 is a reactive oxygen intermediate involved in the cycling of metals and dissolved organic matter because little is known of its distribution in the north pacific ocean we determined h2o2 in surface waters continuously and obtained vertical profiles at nine stations during a cruise from japan to hawaii surface water h2o2 varied from less than 10 to more than 250 nmol dm 3 a diel cycle in surface water h2o2 25 nmol dm 3 was observed only on one day during the monthlong cruise this is contrary to expectations based on the usual assumption of photo production as the dominant input of h2o2 experiments were also conducted during the cruise to examine both photo production and dark decay the net rate of photo production at a station near hawaii was determined to be 8 nmol dm 3 h 1 similar to rates reported for the central atlantic ocean and antarctic however this maximum estimate of photo production is also similar to probable rates of h2o2 input by other mechanisms biological production and rain the pseudo first order rate constant for dark decay varied from 0 1 to 0 2 d 1 which is toward the low end of previous reports of h2o2 decay rates and was observed to increase proportionately to the dissolved organic carbon concentration taken together these results suggest that photo production of h2o2 in open ocean waters may be less important than previously thought and therefore h2o2 is likely less of an indicator of the photo chemical reactivity of surface waters than hoped for furthermore we observed that the h2o2 inventory for the upper 200 m of the water column has a maximum at midlatitudes we suggest that this results from diminished inputs at high latitude as well as increased decay rates at low latitudes

a genetic link between konzentrat and konservat lagerstatten in the fezouata shale lower ordovician morocco abstract in the central anti atlas morocco zagora region the fezouata shale has yielded an extraordinary fossil lagerstatte documenting the initial stages of the great ordovician biodiversification event this lagerstatte contains abundant and exceptionally well preserved fossils epf exhibiting soft bodied preservation as well as hard mineralized parts of extinct organisms while soft bodied preservation in the fezouata shale is confined to a few stratigraphic intervals other fossiliferous intervals contain only shelly fossils in sandstone lenses sl placed in the context of a previously established depositional model epf are interpreted as autochthonous assemblages buried by storm deposits close to storm wave base there the current component of storms is reduced compared to proximal settings and weak oscillation is dominant resulting in the record of oscillation ripples in contrast bioclastic accumulations were generated upslope in shallower more proximal lower shoreface environments there the current component of storms was significant as suggested by the disarticulation of skeletal remains bioclastic materials are localized in sl which are lenses a few tens of centimeters in thickness and a few meters in width and length these lenses commonly display erosive bases and convex tops resulting in a lobate or a channel lobe morphology a large amount of sediment was thus rapidly deposited resulting in the rapid burial of allochthonous material across a strongly localized surface of the sea floor integrating both sedimentological and paleontological evidence intervals yielding epf are considered as konservat lagerstatten ksl in contrast with bioclastic accumulations defined as konzentrat lagerstatten kzl thus ksl assemblages are interpreted to approximate biocenoses in which organisms experienced rapid burial by distal tempestites while kzl assemblages more likely correspond to thanatocenoses locally deposited by storm back currents here a genetic link between ksl and kzl is suggested within the fezouata shale both types of lagerstatten were related to storm activity but at different depths

storm influenced inner shelf sand lobes in the caradoc ordovician of shropshire england abstract a sequence through the middle caradoc ordovician of shropshire shows the horderley sandstone with thick beds of hummocky cross stratified sandstone succeeded by the cheney longville flags consisting of interbedded mudstones siltstones and sandstones some of which also exhibit hummocky bedding in the upper formation the mudstones are commonly bioturbated but also have many wave rippled silty horizons the interbedded thick siltstones and sandstones all lack current ripples and appear to have been rapidly deposited as a result of storm episodes the siltstone and sandstone beds can be separated into two types type 1 beds are massive or faintly planar laminated and have good lateral continuity type 2 beds are lenticular coarser well laminated and have hummocky bedding the f rmer are believed to have been deposited from suspension and the latter from powerful bottom traction currents flowing offshore during storm surges two varieties of hummocky bedding have been recognized within the sequence the more usual type has laminae mantling a hummocky erosional topography a second variety confined to the cheney longville flags has laminae that accreted upwards from a planar base to produce undulating bedforms wavelength up to 3 m wave height up to 15 cm that have symmetrical internal laminae that are form concordant the bedforms resemble wave ripples in cross section but are about an order of magnitude larger than usual wave ripples in the cheney longville flags the argillaceous sediments and their fauna show virtually no change through 80 m of the succession the type 1 beds however increase in proportion upwards whereas the type 2 beds show two strong maxima because the overall environment apparently remained nearly constant and there is no evidence for progradation we interpret the two developments of type 2 beds as reflecting two localized lobes of sand carried offshore into an inner shelf environment by strong laterally restricted offshore currents analogous to but on a larger scale and more powerful than rip currents the independent upward increase in type 1 beds might reflect regional changes in paleogeography which caused a progressive exposure of the shoreface to an oceanic wave climate

palaeoecological aspects of the diversification of echinoderms in the lower ordovician of central anti atlas morocco abstract echinoderms are one of the major components of benthic faunas in the lower ordovician sequence near zagora central anti atlas morocco the fezouata shale tremadocian late floian has yielded numerous exquisitely preserved echinoderm assemblages ranging through several stratigraphic levels and palaeoenvironmental conditions these associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high latitude siliciclastic dominated western gondwana sediments where rapid in situ burials facilitated excellent faunal census conditions lower shoreface deposits of the fezouata shale provide the most complete record of successive echinoderm faunas in late tremadocian times these relatively shallow shelf deposits show the progressive replacement of low diversity opportunistic cambrian like dwarfed communities dominated by cornute stylophorans in unhospitable dysoxic environmental conditions by higher diversity benthic assemblages more typical of the great ordovician biodiversification event and dominated by blastozoans on well oxygenated sea floors the turnover of the ordovician radiation was apparently slightly delayed in more proximal settings eocrinoid meadows persisted in shallower environmental conditions up to the middle floian in the late floian they were replaced by diploporite dominated communities typical of later ordovician high latitude peri gondwanan faunas from a palaeobiogeographic point of view low diversity assemblages display relatively strong affinities with cosmopolitan late cambrian echinoderm faunas whereas high diversity communities are dominated by peri gondwanan taxa

u th pb geochronology of zircon and monazite from syenite and pincinite xenoliths in pliocene alkali basalts of the intra carpathian back arc basin abstract phreatomagmatic eruptions in the lucenec basin in slovakia ejected fragments of igneous rocks of unknown age and ranging in composition from gabbro to syenite and orthopyroxene granodiorite pincinite two kinds of syenites with different fe3 contents composed of anorthoclase and interstitial metaluminous trachyte to alkali rhyolite glass represent the final products of alkali basalt differentiation at depths of 10 and 15 18 km pincinite composed of plagioclase quartz orthopyroxene ilmenite and interstitial peraluminous dacite glass is a quenched supersolidus anatectic melt formed at the depth of 17 20 km by high temperature dehydration melting of biotite bearing gneissic protolith zircon from pincinite and syenite and monazite from syenite were dated using excimer laser ablation icp ms the obtained ages cluster at 5 8 0 1 and 5 4 0 1 ma thus corroborating the genetic link between the two rock types and constraining the residence time of at least 300 100 ka of the parental basalt in the deep crustal magma storage zones the inferred ages overlap the upper miocene pliocene boundary and are considerably younger than spatially associated basaltic lava flows dated using the k ar method consequently the fluviolacustrine poltar formation overlying the maar structure must have sedimented during the pliocene and not during the upper miocene pontian as previously assumed

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

u pb geochronology of zircons from fossiliferous sediments of the hajnacka i maar slovakia type locality of the mn 16a biostratigraphic subzone lacustrine sediments of the hajnacka i maar of southern slovakia contain teeth and skeletal remains of a mammal fauna including index early villanyian arvicoline rodents u pb dating of magmatic zircons extracted from the redeposited fossiliferous maar sediments revealed a total of six volcanic events the oldest age of 3 43 ma was interpreted as that corresponding to the initial phreato magmatic eruption that created the maar most zircons grouped around 3 06 0 03 ma the age attributed to the catastrophic eruption that killed the mammal assemblage thriving within and around the maar studied this age coincides with the 3 1 ma boundary between the mn 16a and 16b subzones of the european mammal neogene chronostratigraphic scale estimated from the succession of early villanyian palaeontological localities according to the hypsometry of rodent teeth younger zircon ages overlapped those of the neighbouring basaltic lava flows necks and dykes thus recording superimposed effusions the youngest 1 58 ma old zircon defines the age limit for the final redeposition of the fossiliferous sediments in the maar drainage lake regime