hummocky cross stratification and megaripples a geological double standard abstract hummocky cross stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits the geometry of the cross strata sets implies a low relief oval bedform of a few meters spacing orientation is poorly defined or random hummocky cross strata sets have been attributed to the action of strong storm wave surges megaripples of 2 5 m spacing are typical responses of the inner atlantic shelf of north america to storm flows side scan sonar records have shown that megaripples on the inner shelf are commonly hummocky oval to circular in plan view and lack asymmetry box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure the megaripples am produced by combined flow storm currents these are alongcoast geostrophic flows with a mean flow component of 20 60 cm sec 1 a high frequency wave orbital current of similar or weaker intensity is present but its role in megaripple formation is subordinate to that of the mean flow the high velocity coastal margin of the flow is often jetlike and downwelling in nature and is characterize by a down current decrease in sand flux and rapid deposition of sand the bottom boundary layer of such flows is characterized by dense sediment suspensions 40 60 mgl 1 and is highly turbulent due to the interaction of the mean flow and wave orbital current components eddy diffusivity up to 200 cm2 sect at 40 cm above the bed sand transport in this coastal boundary zone is obliquely offshore but neither the density of the sediment suspension nor storm surge ebb appear to be important driving forces it is difficult to account for either modern hummocky megaripples or ancient hummocky cross stratification as responses to purely oscillatory flows combined flow currents are the typical response of mostmodern shelves and epicontinental seas to storms and presumably were so in the past hummocky cross stratified beds are generally too thick 20 80 cm to be accounted for by in situ resuspension they are more likely to have been despoited by combined flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm s duration evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime measurements on the atlantic shelf likewi e suggest a transitionlike regime whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes but more intense turbulence and a higher suspended sand concentration the lack of normal cross bedding in bothmodern and ancient hummocky bedforms indicates that these features did not migrate this characteristic may be a consequence of the complex trajectories followed by near bottom water particles in response to combined flow currents

burrowing mechanics burrow extension by crack propagation a worm minimizes its energy expenditure as it forges a path through mud sediment until now the analysis of burrowing mechanics has neglected the mechanical properties of impeding muddy cohesive sediments which behave like elastic solids1 here we show that burrowers can progress through such sediments by using a mechanically efficient previously unsuspected mechanism crack propagation1 2 in which an alternating anchor system of burrowing serves as a wedge to extend the crack shaped burrow the force required to propagate cracks through sediment1 2 in this way is relatively small we find that the force exerted by the annelid worm nereis virens in making and moving into such a burrow amounts to less than one tenth of the force it needs to use against rigid aquarium walls3

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

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

orbital scale climate change and glacioeustasy during the early late ordovician pre hirnantian determined from δ18o values in marine apatite this study focuses on the 10 m y before the latest ordovician hirnantian glaciation we test whether orbital scale climatic fluctuations controlled the growth and melting of continental glaciers resulting in glacioeustatic sea level changes and the development of widespread marine sedimentary cycles 18o values of conodont apatite from 14 late ordovician katian cycles range from 17 to 21 isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow water facies supporting the hypothesis that glacioeustasy was the dominant control on water depth changes measured intracycle 18o changes of 0 7 2 5 were controlled by changes in ice volume 60 m sea level changes sea surface temperatures 5 c and potentially local increases in seawater evaporation during drier and or windier glacial stages these interpreted orbital scale climate changes and resultant large glacial ice volume changes support recent interpretations of a dynamic and prolonged ordovician greenhouse to icehouse transition

calibrating the late ordovician glaciation and mass extinction by the eccentricity cycles of earth s orbit a process based sedimentological analysis of upper ordovician glacial marine rocks in africa suggests that full glaciation of the continental shelf started in the late extraordinarius zone of the hirnantian two cycles of ice sheet growth are represented during full glaciation initial terrestrial ice sheet growth in the early extraordinarius zone influenced the first event of the late ordovician mass extinction retreat of the ice sheet from the shelf ended by the persculptus zone when the second event of the late ordovician mass extinction began eccentricity controlled ice sheet growth is assumed periodicity 0 1 m y hence two cycles of full glaciation lasted 0 2 m y the duration of the extraordinarius zone is estimated as 0 5 m y therefore the minimum duration of the first extinction event was 0 3 m y

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

wrinkle structures microbially mediated sedimentary structures common in subtidal siliciclastic settings at the proterozoic phanerozoic transition sedimentary structures produced by microbial activity are well known from ancient carbonate facies but little is known about equivalent microbial structures in ancient siliciclastic facies wrinkle marks and kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics and they may represent microbial activity to evaluate this possibility these structures were studied in vendian cambrian strata of north america and compared to structures formed by modern microbial mat communities in redfish bay texas striking similarities in sedimentologic petrographic and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes the relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the ordovician microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea floor environments

a high resolution continuous δ13c record spanning the ordovician silurian boundary on anticosti island eastern canada1 one of the best exposed and most complete stratigraphic records from paleotropical areas spanning the ordovician silurian o s boundary is located on anticosti island eastern canada our study is

late neoproterozoic to permian tectonic evolution of the quebec appalachians canada abstract due to the unique location of the quebec appalachians in a re entrant of the paleo north american margin and to the fact that the area largely escaped late paleozoic deformation this segment of the orogen preserves evidence of the early orogenic stages that are partly obscured elsewhere despite diachronicity both along and across strike of the orogen five tectonic events of regional significance are recognized 1 the earliest event is mainly recorded by neoproterozoic to early ordovician normal faults associated with extension of the laurentian craton and the establishment of a passive continental margin 2 the second event the ordovician taconian orogeny is marked by foreland directed deformation and corresponds to the principal deformational and metamorphic events recorded by the laurentian margin 3 the third event early salinic is early silurian it is restricted to the southern part of the quebec appalachians and consists of hinterland se directed thrusts folds and retrograde metamorphism 4 the fourth event late salinic is recorded by regional scale late silurian to early devonian normal or transtensional faults and is associated with the formation of the connecticut valley gaspe trough and 5 the last event the devonian acadian orogeny affects with variable intensity all the pre carboniferous lithologies this tectonic synthesis implies that the subdivision of the peri laurentian oceanic tracts proposed in other segments of the appalachians should be applied with caution in quebec and that the structural style the number of oceanic domains and possibly the vergence of subduction change along the strike of the orogen in particular the similarities of the thetford mines and boil mountain ophiolitic complexes and sedimentary covers and the lack of evidence for a major intervening suture are consistent with the maximum allochtony hypothesis i e the assumption that the southern quebec ophiolites are rooted east of the chain lake massif a geodynamic model involving the taconian obduction of a large ophiolitic nappe s the docking of a peri gondwanan gander medial new england terrane in early salinic the formation of large sedimentary basins connecticut valley gaspe and merrimack troughs due to trench rollback and or delamination during the late salinic event and collision at the outer margin of composite laurentia during the acadian orogeny can account for the protracted structural and metamorphic history of the quebec appalachians

grain fabric of hummocky and swaley cross stratification abstract bed form and grain fabric of two types of hummocky and swaley cross stratification from the pleistocene formations in the kanto plain cental japan display clearly distinctive flow conditions of storm typhoon generated combined flow the grain alignment in the vertical profile of a domal structure of hummocky cross set together with variable strike directions of a lamina suggest that the cross stratification was produced under the waning stage of oscillatory dominant combined flow elsewhere the preferred grain orientation along the maximum dip direction of a large swaley structure shows a small angle of upcurrent imbrication both on the stoss and lee sides of the laminae the stratification within the swale is asymmetric but the strike directions of a lamina are persistently oriented in a preferred direction the fabric and the structure of the swale indicate that the cross stratification was formed as a very low angle mega ripple under unidirectional dominant combined flow the grain fabric of both cross sets clearly shows that in spite of a superficial resemblance to antidunes neither of the two structures was formed in flow conditions for antidunes

episodic sedimentation on an early silurian storm dominated carbonate ramp becscie and merrimack formations anticosti island canada the 150 160 m thick lowermost silurian rhuddanian becscie and merrimack formations of anticosti island canada represent continuous deposition on a shallow open marine carbonate ramp several rock types are identified a laminated and homogenous mudstone b laminated and homogenous packstone c argillaceous mudstone and packstone d calcareous shale e laminated calcisiltite f medium to fine grained grainstone and g bio intraclastic rudstone these rock types are arranged into five distinct lithofacies lf1 calcareous mudstone shale lf2 laminated homogenous mudstone lf3 calcareous grainstone shale lf4 laminated mudstone grainstone and lf5 laminated calcisiltite grainstone the sequence reflects deposition on a low energy muddy carbonate to argillaceous ramp subject to short lived episodic high energy storms these events produced fining upwards storm units 5 80 cm thick or tempestites consisting of a sharp scoured base overlain by intra bioclastic rudstone grading upwards into medium grained grainstone finely laminated calcisiltite and mudstone or shale these are interbedded with low energy fairweather mudstones and calcareous shales deposition progressed from a carbonate mud dominated ramp in the becscie formation to an argillaceous mud dominated ramp in the merrimack formation lateral tempestite proximality trends and lithofacies distribution indicate that the anticosti basin deepened to the south east into the iapetus ocean and shallowed towards a sw ne orientated shoreline to the north west vertical tempestite proximality trends and lithofacies changes identify third order eustatic sea level changes after an initial deepening at the base of the formation a shallowing deepening event dominated the sequence several higher order fluctuations defined by lithofacies and tempestite proximality trends are superimposed on these changes the fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods

bioturbation in the abyssal arabian sea influence of fauna and food supply abstract in order to evaluate bioturbation in abyssal arabian sea sediments of the indus fan profiles of 210 pb half life 22 3 yr and 234 th half life 24 1 d were measured in cores collected during september and october 1995 and april 1997 respectively the density and composition of epibenthic megafauna and lebensspuren were determined in vertical seafloor photographs during april 1997 mean eddy diffusive mixing coefficients according to the distribution of excess 210 pb 210 pb db were 0 072 0 028 0 068 0 055 0 373 0 119 0 037 0 009 and 0 079 0 119 cm2 yr 1 in the northern western central eastern and southern abyssal arabian sea respectively mean eddy diffusive mixing coefficients according to the distribution of excess 234 th 234 th db were 0 53 1 64 and 0 47 cm2 yr 1 in the northern western and central abyssal arabian sea respectively mobile epibenthic megafauna at the western northern central and southern study sites were dominated by ophiuroids holothurians ophiuroids and natant decapods the respective densities were 100 82 29 and 6 individuals 1000 m 2 the northern study site was characterized by a high abundance of spoke traces and fecal casts the central site showed spoke traces and many tracks the southern site displayed the highest abundance of spoke traces whereas at the western site hardly any lebensspuren were observed there is evidence for at least two functional endmember communities in the arabian sea in the northwestern arabian sea wast vertical particle displacement seems to be dominated by macrofauna and primarily eddy diffusive in the southern arabian sea sast non local and incidental mixing due to spoke trace producers might become more important and superimpose reduced eddy diffusive mixing with respect to biological data cast is an intermediate location given the biological data average 210 pb db is higher and decimeter scale variability of 210 pb db smaller at cast than expected these findings indicate that in a mixture of both endmember communities the organisms may interact in way that increases values of biodiffusivity as reflected by 210 pb db and reduces decimeter scale 210 pb db heterogeneity in comparison to the simple sum of the isolated effects of the endmembers for time scales 210 pb db and 234 th db bioturbation intensity should be controlled primarily by the composition of the benthic fauna its specific adaptation to the environmental setting and the abundance of each species of the benthic community food supply can have only an indirect influence on bioturbation intensity in certain parts of the ocean the a priori overall positive relationship between poc flux and biodiffusivity might include restricted intervals displaying no or even negative relations

natural remanent magnetization acquisition in bioturbated sediment general theory and implications for relative paleointensity reconstructions we present a general theory for the acquisition of natural remanent magnetizations nrm in sediment under the influence of a magnetic torques b randomizing torques and c torques resulting from interaction forces dynamic equilibrium between a and b in the water column and at the sediment water interface generates a detrital remanent magnetization drm while much stronger randomizing torques may be provided by bioturbation inside the mixed layer these generate a so called mixed remanent magnetization mrm which is stabilized by mechanical interaction forces during the time required to cross the surface mixed layer drm is lost and mrm is acquired at a rate that depends on bioturbation intensity both processes are governed by a mrm lock in function the final nrm intensity is controlled mainly by a single parameter that is defined as the product of rotational diffusion and mixed layer thickness divided by sedimentation rate this parameter defines three regimes 1 slow mixing 10 with mrm acquisition and full drm randomization and 3 intermediate mixing because the acquisition efficiency of drm is larger than that of mrm nrm intensity is particularly sensitive to in case of mixed regimes generating variable nrm acquisition efficiencies this model explains 1 lock in delays that can be matched with empirical reconstructions from paleomagnetic records 2 the existence of small lock in depths that lead to drm preservation 3 specific nrm acquisition efficiencies of magnetofossil rich sediments and 4 some relative paleointensity artifacts

a generalized stochastic approach to particle dispersal in soils and sediments abstract we explore a generalized stochastic description for the dispersal of particles in soils and sediments the model is based on the continuous time random walk ctrw which enables a very general description of particle mixing due to biological activity a community of burrowing organisms is characterized by a bioturbation fingerprint that consists of two probability distributions the jump length distribution describes the distance a particle travels within a given bioturbation event the waiting time distribution describes the time interval a particle waits at a given location in between two bioturbation events our stochastic analysis provides insight on the applicability of the widely used biodiffusion model which forms the asymptotic limit of the ctrw model this asymptotic behaviour of diffusion is always reached independent of the shape of bioturbation fingerprint provided that the waiting time distribution has a finite mean and the jump length distribution has a finite variance the biodiffusion coefficient can be decomposed as the variance of the jump length distribution over the mean of the waiting time distribution since the variance enables a squared weighing of the jump distance long range displacements will contribute more to the mixing intensity d b than short range displacements therefore large macrofauna will contribute disproportionately to the mixing intensity relative to their abundance as they displace particles over longer distances than smaller organisms finally we review the potential for bioturbation of so called anomalous transport models where either the mean of the waiting time distribution or the variance of the jump length distribution becomes infinite

particle mixing rates in sediments of the northeast tropical atlantic evidence from 210pbxs 137cs 228thxs and 234thxs downcore distributions particle mixing rates were estimated from distribution of 137cs and excess activities of 210pb 228th and 234th measured in nine sediment cores taken from the northeast tropical atlantic i e at the three sites of the french eumeli eutrophic mesotrophic and oligotrophic programme at the oligotrophic site activities of 137cs and excess 210pb decrease smoothly with depth on the other hand at the mesotrophic and eutrophic sites 210pbxs and 137cs profiles suggest a mixing process with three different regimes with subsurface sediments being mixed more rapidly than those at the surface bioturbation rates were found to be slowest at the oligotrophic site intermediate at the mesotrophic site and fastest at the eutrophic site these mixing intensities appear closely linked to benthic fauna abundance organic carbon flux to the sediments and primary production in overlying surface waters the use of a multitracer approach for studying sediment particle reworking has allowed bioturbation rates to be better constrained and an assessment of whether radionuclide distributions in surface sediments are in steady state for only one core from the mesotrophic site were surface sediments not in steady state perhaps because of the occurrence of a benthic animal i e a xenophyophore at the surface of this core

periodicity in the early palaeozoic marine realm evidence from the british isles suggests that during 3 intervals in the early palaeozoic conditions approximating to those of a non uniformitarian greenhouse g state earth were realized middle early upper cambrian st davids merioneth ages 30 ma mid ordovician llandeilo caradoc ages 32 ma and early silurian llandovery early wenlock ages 12 ma during these times sea levels were high and ocean waters below the zone of surface overturning poorly aerated and at times euxinic pelagic biotas graptoloids were diverse and in aerated portions of the highly productive shelf seas benthic biotas brachiopods flourished opposing conditions marking a well oxygenated o state earth occurred to varying degrees in the intervening intervals early cambrian comley age 20 ma late cambrian mid ordovician tremadoc late llandeilo ages 46 ma late ordovician ashgill age 4 ma and mid to late silurian post early wenlock age 15 ma within the constraints of the present data perturbations in the early palaeozoic marine realm do not appear to follow the same regular periodicity as in the mesozoic and caenozoic among a number of likely causes for this the greater tendency for the early palaeozoic seas to become anoxic seems the most important

late ordovician obliquity forced glacio eustasy recorded in the yangtze block south china abstract a high precision time framework is critical for improving our understanding of three major events the late ordovician mass extinction the hirnantian glaciation and the hirnantian isotopic carbon excursion hice in the present study we conducted detailed cyclostratigraphic analysis of a high resolution 1 2 cm intervals anhysteretic remanent magnetization series obtained from the wanhe section in south china covering the lower katian through hirnantian power spectra and derived average spectral misfit results show that the section records the ensemble of milankovitch cycles with 405 kyr long eccentricity 93 125 kyr short eccentricity 33 8 kyr main obliquity and 17 22 kyr precession periods the obliquity period of 33 8 0 46 kyr indicates a precession constant k of 57 19 0 53 arcsec yr a length of day of 22 37 0 12 h and an earth moon distance of 375 330 722 km vs 384 000 km today during the late ordovician 445 ma the ratios of obliquity band power to total power reveal periodicities of 1 32 myr for the s4 s3 term and 173 4 kyr for the s3 s6 term aided by the highly fluctuating magnetic susceptibility ms we recognized a series of 1 32 myr obliquity forced glacio eustatic cycles with three intervals of sea level rise and climate warming trend characterized by high ms values and two episodes of sea level fall and climate cooling trend before the hirnantian the 405 kyr and subsequent 33 8 kyr calibrations produce an 7 36 myr floating astronomical time scale fats for the studied succession indicating that the estimated durations for the hirnantian stage the worldwide hirnantian glaciation and the first and second phases of the late ordovician mass extinction were 1225 kyr 830 kyr 440 kyr and 540 kyr respectively this work provides a reliable high resolution time framework for further investigation of the major geological events occurred in the late ordovician and improves our knowledge of solar system behaviour and earth moon history during that period

tempestite frequency curves a key to late ordovician and early silurian subsidence sea level change and orbital forcing in the anticosti foreland basin quebec canada following partial closure of the northern iapetus ocean along the newfoundland segment of the st lawrence promontory subsidence along the anticosti platform was influenced by residual thermal subsidence renewed tectonic loading by thrust sheets to the south and sediment loading basement subsidence calculated by removing the effects of sediment loading was between 2 and 5 cm ka in the caradocian increasing to 8 6 cm ka in the pusgillian and cautleyan and reaching a maximum of 17 7 cm ka in the rawtheyan during deposition of the princeton lake and vaureal formations a marked decline in subsidence beginning in the hirnantian and continuing into the early silurian may reflect decoupling of thrust loads to the south although a further stage of thrust loading may have occurred in the aeronian during deposition of the lower part of the jupiter formation storm frequency curves produced using tempestite abundance thickness and grain size through more than a kilometre of carbonate strata on antico

flexural modeling of paleozoic foreland basin subsidence offshore western newfoundland evidence for substantial post taconian thrust transport multichannel seismic reflection data gathered by the petroleum industry offshore of western newfoundland constrain the absolute thicknesses of the middle ordovician and post middle ordovician portions of the anticosti foreland basin immediately adjacent to the appalachian structural front these seismic lines which include 1970s vintage data in the public domain and recently acquired proprietary data indicate that the middle ordovician taconian portion of the foreland basin is more than an order of magnitude thinner than the preserved post middle ordovician salinian acadian portion the mechanical lithospheric coupling of foreland basins and their adjacent overthrust belts is a well understood phenomenon a thin taconian foreland succession and a thick salinian acadian succession imply that the overthrust loads responsible for flexural depression of the anticosti basin were much more distant and or much smaller during the middle ordovician taconian orogeny than they are today we quantify thi

benthic island community on the back of a snail silurian anticosti island canada a single specimen of the large silurian gastropod phanerotrema sp from the goeland member on anticosti island preserves an outstanding microcosm of its surrounding community including multiple trophic groups strategies for niche partitioning and innovative adaptations for survival in a mud dominated marine ecosystem at least seven associations are recorded in this fossil specimen including the gastropod itself healed breakage from an apparently failed attempt at durophagous predation domichnial bioerosion encrustation by a stromatoporoid nestling behaviour and commensal embedment by a lingulid brachiopod and epifaunal attachment by two cornulitids as a benthic island the gastropod conch provided a favourable isolated habitat for hard substrate colonizers this unique specimen sheds light on the broader paleozoic ecosystem including the potential importance of benthic islands in the evolution of niche partitioning strategies in space limited communities

lower paleozoic configuration of the quebec reentrant based on improved along strike paleogeography an improved paleogeography of the quebec reentrant has been achieved by examining the nature and distribution of major sediment buildups within this reentrant the quebec reentrant has one side produced by rift faults and one side by a major transform fault the oak hill group was deposited near a triple rift zone clastic sediments of this group formed a large delta that was fed by a major river flowing in the rift valley of the ottawa graben the delta must have prograded on a gentle slope platform no other thick deltaic deposits are known towards the northeast instead facies analyses favor the existence of a promontory near quebec still farther to the northeast the saguenay graben was not the product of a triple rift junction however it served to funnel sediments to a deep sea fan and the adjacent platform was narrow with a steep slope the lateral distribution of sediments from the ordovician taconic orogeny can be explained by this improved paleogeographic reconstruction the configuration of

wrinkle marks an intertidal sedimentary structure due to aseismic soft sediment loading abstract the structures have a wide distribution intertidally in the macrotidal severn estuary and inner bristol channel occurring on the active sedimentary surface chiefly in association with a millimetre scale sand over mud doublet the sand surface is marked by depressions and ridges of a wide variety of shapes and orientations relative to slope that are matched antithetically by ridges and depressions on the mud sand interface below the sand layer consequently takes the form of a series of touching to detached pillow shaped masses whose centres lie 2 4 sand layer thicknesses apart wrinkle marks originate soon after the emergence of the sedimentary surface into the atmosphere and are considered to be soft sediment deformation structures on account of their 1 occasional displacement and or severance of early emergence surface organic trails and feeding marks 2 similarity in shape and internal structure to pseudo nodules recorded from the field and produced experimentally and 3 association with gravitationally unstable sediment layers in settings that favour the vigorous seepage of pore water through water logged sediments owing to their origin at the sedimentary surface wrinkle marks appear to be preserved mainly as surface markings on eroded and discordantly reburied mud sand interfaces

chitinozoan biostratigraphy of a new upper ordovician stratigraphic framework for anticosti island canada thickness and facies variations in the upper ordovician succession of anticosti island have made correlations between the western and eastern ends of the island difficult this chitinozoan study reveals that most strata of the ellis bay formation at the eastern end of the island correlate with the vaureal formation at the western end in the eastern end chitinozoans typical of the ellis bay formation occur only in the upper part of the lousy cove member whereas chitinozoans diagnostic of the mid upper vaureal occur in the rest of the formation moreover the eastern schmitt creek and mill bay members of the vaureal formation are older than the uppermost vaureal eponym members in the western end new east west correlations supported by geochemical sedimentological and sequence stratigraphic data are proposed comparison with coeval upper ordovician successions suggests a hirnantian age for the gamachiana zone that occurs in the lower members of the ellis bay formation it also indicates that the hirnantian isotopic carbon excursion hice on anticosti begins in the d anceps zone just below the base of the hirnantian stage reaches a first peak in the lower n extraordinarius zone a second modest peak high in the n extraordinarius zone and a third more important one in the lower part of the n persculptus zone these results are in agreement with the biostratigraphic and chemostratigraphic data at the global boundary stratotype section and points gssps for the base of the hirnantian and silurian in south central china and at dob9s linn in scotland

late ordovician massive bedded thalassinoides ichnofacies along the palaeoequator of laurentia abstract massive bedded thalassinoides ichnofacies occurs pervasively in widely separated areas on the vast upper ordovician carbonate platforms along the palaeoequator of laurentia from the borglum river formation in north greenland through the red river formation and bighorn dolomite of the williston basin manitoba canada to wyoming usa to the ely springs dolomite and equivalent strata of the eastern great basin utah and nevada usa the late ordovician thalassinoides is characterised by complex three dimensional anastomosing or multi level galleries that are consistent in size pattern and density with preserved depth of 1 m or greater in addition to the similar thalassinoides burrows the ichnofacies across the palaeocontinent also contains typical taxa of the red river biota such as the soccer ball sized receptaculitid fisherites large shelled diverse and abundant nautiloids the distinctively planispiral gastropod maclurina as well as diverse tabulate and rugose corals the consistent characteristics of the red river biofacies and thalassinoides ichnofacies suggest a palaeogeographically homogeneous and temporally stable depositional environment along the palaeoequatorial belt of laurentia during the pre hirnatian late ordovician the thalassinoides ichnofacies marked the first phase of mega scale invasion of burrowers into relatively deep water platform and shelf environments by successively creating a well oxygenated deep burrow system for protection and deposit feeding such substrate tiering expansion constituted an important aspect of the great ordovician biodiversification event

a laboratory investigation of assimilation at the top of a basaltic magma chamber the fluid dynamic processes affecting the mechanism and amount of assimilation of the roof of a magma chamber have been studied in a series of analogue laboratory experiments an aqueous solution of na 2 co 3 was first cooled and crystallized at the top of a laboratory tank to form a solid roof layer of prescribed composition the tank was then drained and refilled with warmer solution either na 2 co 3 of the same or different composition or in two experiments kno 3 the metal top in all cases was held below the eutectic temperature for the system na 2 co 3 h 2 o the experiments were continued until substantial crystallization and in some cases complete solidification were achieved starting with a dense solid roof layer of na 2 co 3 10h 2 o and ice on the na 2 co 3 side of the eutectic and a warmer eutectic na 2 co 3 solution below there was rapid convective cooling of the liquid layer accompanied by melting of the roof producing a compositional

late ordovician sand wave complexes on anticosti island quebec a marine tidal embayment laterally discontinuous mixed carbonate siliciclastic sandstones in the upper vaureal and lower ellis bay formations of anticosti island were deposited on an equatorial carbonate ramp with a slope of less than 1 the 10 18 m thick sandstones are interpreted as subaqueous sand wave complexes analogous to detached parts of modern shoreface connected sand ridges these record storm enhanced tidal modification of a northerly derived shoal retreat massif that may have formed in response to recovery from global sea level lowstands in the late ordovician ashgill late rawtheyan hirnantian the sand wave complexes formed within a tidal embayment that was confined by the precambrian shield to the north and northwest by rising tectonic highlands of the humber zone in newfoundland to the east and by active tectonic highlands in the quebec appalachians gaspesie to the south paleocurrent distributions parallel to the western margins of the strait of belle isle suggest that the north end of the embayment w

the unusual sedimentary rock record of the early triassic a case study from the southwestern united states abstract sedimentary systems as well as the earth s biota changed dramatically following the end permian mass extinction this global sedimentological transformation is reflected in the widespread occurrence of flat pebble conglomerates subtidal wrinkle structures microbialites and carbonate seafloor fans in various lower triassic sections as a case study a facies analysis of the lower triassic virgin limestone member of the moenkopi formation and the middle and upper members of the union wash formation southwestern united states was conducted and all unusual facies were documented within a palaeoenvironmental framework the occurrence of the flat pebble conglomerates and wrinkle structures is significant because these reflect a long term reduction in vertical bioturbation in the case of the subtidal microbialites and carbonate seafloor fans their formation is linked to an increase in alkalinity the presence of these facies in the uppermost lower triassic strata of the southwestern united states indicates that reduced levels of infaunal bioturbation as well as and perhaps a consequence of unusual ocean chemistry extended for 5 6 million years after the end permian mass extinction suggesting that depositional environments as well as the biota were significantly affected by the extinction event

sequence stratigraphy of the red river formation williston basin usa stratigraphic signature of the ordovician katian greenhouse to icehouse transition abstract the upper katian upper ordovician red river formation is a carbonate evaporite supersequence that formed in the epicratonic williston basin of western north america this study focuses on the upper part of the formation in the southern us part of the basin the studied succession is made up of three transgressive regressive sequences rr1 rr2 and rr3 each 0 5 1 m y in duration and composed of relatively few meter scale high frequency cycles parasequences sequence boundaries are characterized by intraclastic breccia laminated dolomite successions that have been truncated and capped by evaporite deposits subaqueous anhydrites within each sequence represent evaporite lowstand systems tracts lsts that formed during relative sea level lows that lead to the precipitation of gypsum later recrystallized into anhydrite in a quiet salina type environment transgressive tst and early highstand systems tracts hst are composed of subtidal facies of predominantly skeletal mudstone to wackestone packstone with abundant burrow mottling late hsts are predominantly composed of peritidal porous laminated dolomite the study illustrates how the sequence stratigraphic framework of carbonate evaporite deposits from the williston basin relates to the late ordovician glacio eustasy transitional climates and low basin subsidence in an arid setting

plutonium and210pb distributions in northeast atlantic sediments subsurface anomalies caused by non local mixing abstract the depth distributions of 210 pb and 239 240 pu measured in a suite of box cores collected from water depths of 4000 5000 m in the northeast atlantic ocean exhibit pronounced subsurface maxima caused by sediment reworking by benthic infauna small scale spatial heterogeneity in bioturbation rates is indicated by large differences in tracer profiles from duplicate cores separated only by a few centimeters 210 pb and 239 240 pu activity distributions from each subcore exhibit a high degree of correlation and most tracer profiles exhibit one or more subsurface maxima one dimensional biodiffusion analogue models do not adequately simulate the principal features of this data set however an inverse conveyer belt mixing model which simulates subsurface egestion or a functionally equivalent process of surficial material which is enriched both in organic debris and radioactive tracers can reproduce the subsurface tracer maxima single event and continuous subsurface egestion models have been formulated and solved for different feeding rates and background biodiffusive fields the single event model provides a better fit to the data and in particular ensures the observed high degree of correlation between the 210 pb and 239 240 pu activity profiles regardless of the different tracer input functions the most likely candidate responsible for subsurface tracer egestion is a large infaunal worm of the phylum sipunculida which dominates the biomass below a depth of 3 cm

is burial velocity a master parameter for bioturbation abstract the intensity of biological mixing in a marine sediment is widely believed to be related to the flux of labile organic matter at the same time this flux has been shown to be strongly correlated to the burial velocity one would then expect a significant correlation between biological mixing coefficient d b and burial velocity a nonlinear regression of existing data with a power law eqns 2 and 3 of the text produces a statistically significant correlation the statistical significance of the regression in no way implies causation and in fact it accounts for only about 22 of the total variance in the dataset thus reducing its physical significance a number of additional processes contribute to the large observed variance including timing of mixing events temporal and spatial heterogeneity differences in temporal scales sampling artifacts and specific particle associations with due care the d b relationship may prove useful in defining the likely range of d b values in a particular environment a similar analysis of the tracer identified surface mixed depth shows that it is essentially independent of burial velocity with a worldwide mean of 9 8 4 5 cm depth of mixing is probably more closely related to the energetic costs of deep burrowing

a revision of ordovician series and stages from the historical type area this paper is a revision of the standard chronostratigraphy of the ordovician of the historical type area in england and wales the revision is a response to the need for more precise definitions of series and stages especially for practical international correlation in some cases this has entailed moving horizons for series bases away from those in the classical sections the scheme will be used as the standard for a forthcoming revision of the correlation chart of the british ordovician system the ordovician is divided into five series tremadoc arenig llanvirn caradoc and ashgill the llanvirn is extended to include part of the classical llandeilo series which is included within it as a stage where stage subdivisions of standard series have not yet been defined as in the tremadoc they are proposed in this paper however bancroft s fine stadial divisions of the caradoc which have been criticized for their brevity and local utility are combined into four new stages which will have wider application the former caradoc subdivisions are retained as substages for the purposes of regional correlation

combined flow model for vertical stratification sequences in shallow marine storm deposited beds abstract we develop a model to help explain the wide variety of vertical stratification sequences in shallow marine sandstones this model embodied in a set of vertical sequence cartoons predicts or accounts for vertical stratification sequences in beds of free sandstone deposited from a full range of decelerating combined flows as well as from purely unidirectional and oscillatory flows according to the model vertical stratification sequences in storm deposited sandstone beds are the outcome of the time history of the bed configuration as deposition proceeds the idealized sequences are based on the initial velocities and subsequent deceleration histories of the unidirectional and oscillatory flow components twenty qualitatively different sequences starting with plane bed are generated and an additional 26 sequences result from omitting one or more of the lowermost intervals in succession the model thus accounts for 46 distinct kinds of sequences five major simplifying assumptions are built into the model bidirectional flows linear deceleration of both unidirectional and oscillatory flow components time independent deposition rate time independent sediment size and equilibrium bed configurations owing to these assumptions the model is likely to be applicable to only a small subset of vertical stratification sequences observed in shallow marine sandstones as more information on combined flow bed configurations becomes available these assumptions could be relaxed to provide a more refined and complete representation of vertical stratification sequences since the model generates vertical sequences formed from any kind of decelerating flow the bouma sequence for deposits from purely unidirectional turbidity currents and also sequences resulting from decelerating purely oscillatory flows similar in appearance to many proposed generalized tempestite sequences are end member cases in the model the model is also extended slightly to explain the geometry of the uppermost bedding surfaces of storm generated sandstones by consideration of disequilibrium effects

cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences over the past 25 yr the science of stratigraphy has evolved to include time correlative data from vastly disparate components of the earth system not least of these is the global signal afforded by cyclostratigraphy which has recorded the evolution of earth s astronomical milankovitch forcing of insolation and the paleoclimate system fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies geochemistry mineralogy rock magnetism color etc in step with the documentation of astronomically forced paleoclimate from ever older older geologic times innovations in computational science have provided ever longer high accuracy astronomical model targets that can be used for time scale calibration the earth s orbit is affected by motions of other planets notably the orbital perihelia of venus and jupiter which impose a dominant 405 k y eccentricity cycle on earth s orbital evolution the large mass of jupiter stabilizes this cycle over hundreds of millions of years the cyclostratigraphic record of 405 k y cycles is therefore often used to correct chronologies affected by variable sedimentation earth s shape and rotation rate are influenced by tidal dissipation and climate friction these effects affect earth s precession rate through time thus a record of earth moon evolution is also embedded in cyclostratigraphy the geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high precision radioisotope dating which today has the potential to define the ages of stratigraphic horizons with 2 uncertainties at the scale of a precession cycle precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca 50 ma limit of accurate astronomical solutions and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time finally space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of mars with patterns suggestive of astronomical forcing analogous to earth

grain fabric in hummocky cross stratified storm beds genetic implications abstract a detailed analysis of grain fabric in samples from hummocky cross stratified hcs storm beds suggests that these shallow marine sandstones were deposited from waning combined oscillatory unidirectional flows particle a axis orientations in the deposits in plan view parallel to visible lamination vary widely but have a mean orientation approximately normal to unidirectional paleoflow indicators sole marks and subparallel to regional shoreline this alignment of a axes is attributed to deposition under combined flows with a multidirectional oscillatory component produced by wind induced surface gravity waves with a modal crest orientation approximately parallel to the shoreline imbrication of particles varies in both the horizontally laminated and hcs intervals of the hcs stor beds due to waxing and waning of boundary shear stress caused by the oscillatory component of the flow in the basal horizontally laminated interval of an hcs storm bed grain imbrication angles vary about a mean of approximately 13 from the horizontal dipping into the paleoflow inferred from sole marks this preferred imbrication direction is attributed to boundary shear stress influenced by the unidirectional component of the combined flow however quasi cyclic variation in imbrication angle over a broad range and periodic reversal of imbrication direction suggest that the oscillatory component was also significant during deposition of the horizontally laminated interval in hcs intervals imbrication angles vary about a mean that is parallel to visible laminae suggesting th t oscillatory flow had become predominant the sequence of structures and changing patterns of fabric through these hcs storm beds is consistent with deposition under waning combined flows erosional scour forming the sharp base and unidirectional sole marks occurred in response to combined flows with an effective unidirectional component the unidirectional component continued to influence the direction of boundary shear stress during deposition of the horizontally laminated sands but waned and became negligible relative to a strong oscillatory component during formation of hcs

hinterland directed transtensional faulting at an orogen structural front the example of the cap chat melange quebec appalachians the cap chat melange is a regional scale dismembered unit that occurs close to the quebec appalachian front the melange records early d 1 taconian and late d 3 acadian regional deformations the greater degree of disruption of the melange compared with surrounding units is due to the superimposition of d 2 structures that are rare outside the melange d 2 structures include both layer extensional features such as block disruption along shear zones and faults and layer contractional features such as folds that record the transtensional collapse of the taconian orogenic wedge along a rheologically weak deformation zone the cap chat melange is interpreted as the westernmost remnant of a zone of mechanical decoupling in the upper crust which allows normal or transtensional faulting in the inner parts of the orogen this paper presents evidence that beyond the general complexity of melange units geological observations may still contribute to unraveling their tectonic history and that transtensional reactivation of original thrust faults is a viable mechanism in the frontal parts of orogens

late ordovician early silurian ashgillian llandovery sea level curve derived from conodont community analysis anticosti island quebec abstract earlier workers have established sea level curves for the ashgillian llandovery sequence on anticosti island quebec canada based on sedimentology and on brachiopod and reef communities the complete anticosti sequence has been collected for conodonts to provide a database of over 77 600 conodonts derived from 272 samples through the 800 1100 m sequence this paper uses a statistical analysis of the database to establish the pattern of conodont communities and their interpreted water depths this allows the establishment of a eustatic sea level curve for the sequence this curve is in general agreement with those of the earlier workers but is more detailed and better constrained stratigraphically given the large sampling program the curve reflects more oscillating sea level changes for the whole sequence than those constructed earlier and different water depths for certain intervals from those established earlier the curve is valuable given that the anticosti sequence is one of the most complete in the world for this interval and its divergence from global eustatic curves for this interval is noted

environmental control on temporal and spatial differentiation of early silurian pentameride brachiopod communities anticosti island eastern canada the early silurian carbonate succession of anticosti island eastern canada contains a rich and diverse pentam eride brachiopod fauna multivariate analyses of 91 samples total 17 230 specimens confirm a high degree of temporal segregation but only moderate spatial differentiation of the virgiana pentamerus stricklandia ehlersella microcardina lia and clorinda communities most pentameride communities show a wider range of water depth than previously be lieved the v barrandei community occupied a substrate setting from lower ba2 to upper ba3 whereas the v mayvillensis community lived mainly in ba4 the p oblongus community preferred a ba3 setting comparable to the classic pentamerus community but the p palaformis community was predominantly ba4 the stricklandiid communities have a high b diversity dominated by various species of stricklandia ehlersella microcardinalia and kulumbella and occurred most commonly in ba4 but could extend to ba3 environments the clorinda community of anticosti island preferred deep and quiet water settings comparable to its widely accepted ba5 assignment elsewhere at the generic level the virgiana pentamerus and the stricklandiid communities have a largely overlapping ba range but a clear strati graphical separation thus their temporal alternations were not likely controlled by water depth water turbulence or sub

did changes in atmospheric co2 coincide with latest ordovician glacial interglacial cycles article i nfo the late ordovician hirnantian stage 444 million years ago was one of three time periods during the past half billion years in which large continental glaciers formed over earth s polar regions the effects of this glaciation were far reaching and coincided with one of the largest marine mass extinction events in earth history the cause of this ice age is uncertain and a paradoxical association with evidence for high atmospheric co2 levels has been debated precise linkages between sea level ice volume and carbon isotope 13 ccarb and 13 corg proxy records of pco2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections although correlation difficulties remain recent hirnantian biostratigraphic studies now allow for improved correlations here we show that consistent trends in both 13 ccarb and 13 corg from two well dated stratigraphic sequences in estonia and anticosti island canada coincide with changes in late ordovician hirnantian climate as inferred from sea level and the extent of ice sheets the integrated datasets are consistent with increasing pco2 levels in response to ice sheet expansion that reduced silicate weathering ultimately the time period of elevated pco2 levels is followed by geologic evidence of deglaciation

terminal ordovician carbon isotope stratigraphy and glacioeustatic sea level change across anticosti island quebec canada globally documented carbon isotope excursions provide time varying signals that can be used for high resolution stratigraphic correlation we report detailed inorganic and organic carbon isotope curves from carbonate rocks of the ellis bay and becscie formations spanning the ordovician silurian boundary on anticosti island quebec canada strata of the anticosti basin record the development of a storm dominated tropical carbonate ramp these strata host the well known hirnantian positive carbon isotope excursion which attains maximum values of 4 5 in carbonate carbon of the laframboise member or the fox point member of the becscie formation the excursion also occurs in organic carbon and 13 c carb and 13 c org values covary such that no reproducible 13 c 13 c carb 13 c org excursion is observed the most complete stratigraphic section at laframboise point in the west shows the characteristic shape of the hirnantian stage excursion at the global stratotype section and point gssp for the hirnantian stage in china and the silurian system in scotland we therefore suggest that the entire hirnantian stage on anticosti island is confined to the laframboise and lower fox point members by documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin we are able to reconstruct glacioeustatic sea level fluctuations corresponding to maximum glacial conditions associated with the end ordovician ice age the combined litho and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and becscie limestones the former transgresses from west to east and the latter progrades from east to west the sea level curve consistent with our sequence stratigraphic model indicates that glacioeustatic sea level changes and the positive carbon isotope excursion were not perfectly coupled although the start of the isotope excursion and the initial sea level drawdown were coincident the peak of the isotope excursion did not occur until after sea level had begun to rise carbon isotope values did not return to baseline until well after the anticosti ramp was reflooded the sea level 13ccarb relationship proposed here is consistent with the weathering hypothesis for the origin of the hirnantian 13 c carb excursion

were jurassic tholeiitic lavas originally widespread in southeastern canada a test of the broad terrane hypothesis two competing models have been proposed for early jurassic magmatism on the eastern north american margin the broad terrane hypothesis argues that tholeiitic lavas were extruded over a large are

a model for radionuclides in sediments influenced by mixing and compaction a model for the activity of radionuclides in sediments has been set up based on the one dimensional advection diffusion equation the model considers sedimentation mixing and compaction observed distributions of deposit feeding organisms and the nature of physical disturbances indicate that a gaussian diffusion coefficient may be appropriate two layer models with constant and gaussian mixing respectively in the upper layer are compared with measured pb 210 profiles for two cores from green bay lake michigan and it is found that the maximal diffusion coefficient d0 in the gaussian model is considerably greater than the constant d value e g d0 4 versus d 0 2 cm2 year if compaction effects are neglected the solution for the gaussian model is based on an approximate power series method compaction can be taken into account by using an exponential sedimentation rate or bulk sediment density by comparing with measured data for a lake ontario core it is seen that compaction alone can give an appreciable flattening of the activity curves near the top layers

braided fluvial to marine transition the basal lower cambrian wood canyon formation southern marble mountains mojave desert california abstract the lower cambrian wood canyon formation recognized over a 39 000 km2 belt through the southern great basin and mojave desert was examined in the southern marble mountains eastern mojave desert southeastern california there it crops out as a well exposed 90 130 m thick feldspathic sandstone nonconformably overlying proterozoic basement previous investigators have suggested a tide dominated nearshore shallow marine origin for the entire unit in the eastern mojave based on broad scale stratigraphic trends and localized fossil occurrences in the upper member of the formation in contrast to a regional approach the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata the middle and upper members of the wood canyon formation are recognized in the southern marble mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed cross stratified sandstone facies a mediumpebble conglomerate facies b planar stratified sandstone facies c sandstone and thin mudstone facies d mudstone with intercalated sandstone facies e and horizontally laminated siltstone facies f facies a d contained within the middle member comprise the bulk of wood canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced fluvial dominated braid delta complex cross stratification in the braid plain sediments displays a strongly unimodal pal ocurrent pattern suggestive of downstream migrating sinuous crested megaripples braid delta sediments however show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage a change from fluvial dominated to marine dominated processes is recorded in upper member rocks facies e and f facies e contains many of the traits characteristic of a low to mid tidal flat sequence such as coarsely interlayered bedding flaser and lenticular bedding microripple marks and locally abundant rusophycus and planolites trace fossils facies f which dominantly consists of horizontally laminated siltstone is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars

exceptionally preserved soft bodied assemblage in ordovician carbonates of anticosti island eastern canada abstract a diverse benthic fauna containing exceptionally preserved soft bodied organisms is described from the new vaureal konservat lagerstatte from the upper ordovician vaureal formation katian of anticosti island canada these fossils occur as pyritic or goethitic if weathered calcitic aggregates or sediment filled voids on micritic bedding planes of a marlstone limestone alternation originated on a shallow tropical carbonate ramp many organisms are preserved in association with their traces e g burrows trails whose changing shapes indicate increasing cohesiveness of the substrate rapid burial obrution under dysoxic bottom conditions must have favoured exceptional preservation although anatomical details of the organisms are often lacking due to recrystallization members of different groups of organisms could be interpreted based on their shape and other characteristic features including those of which represent their oldest fossil record i e acoelomorpha turbellaria nemertea nematoda beside polychaeta sipuncula ostracoda and other arthropoda in parallel with molecular data which recently have changed the phylogenetic status of some clades this soft bodied fauna and their traces not only record their appearance but add to the understanding of their body plan and behaviour the new konservat lagerstatte provides important information about a diverse benthic community in the late ordovician after the great ordovician biodiversification event gobe and represents a calibration point for comparison across the late ordovician mass extinction moreover comparable fossil lagerstatten are probably more common in shallow marine carbonates than currently known but might have been simply overlooked

a record of astronomically forced climate change in a late ordovician sandbian deep marine sequence ordos basin north china abstract the late ordovician pingliang formation on the southwestern margin of the ordos basin north china consists of rhythmic alternations of shale limestone and siliceous beds to explore the possible astronomical forcing preserved in this lithological record continuous lithological rank and magnetic susceptibility ms stratigraphic series were obtained from a 34 m thick section of the pingliang formation at guanzhuang power spectral analysis of the ms and rank series reveal 85 5 cm to 124 cm 23 cm to 38 cm and 15 cm to 27 cm thick sedimentary cycles that in ratio match that of late ordovician short eccentricity obliquity and precession astronomical cycles the power spectrum of the ms time series calibrated to interpreted short orbital eccentricity cycles aligns with spectral peaks to astronomical parameters including 95 kyr short orbital eccentricity 35 3 kyr and 30 6 kyr obliquity and 19 6 kyr and 16 3 kyr precession cycles the 15 cm to 27 cm thick limestone shale couplets mainly represent precession cycles and siliceous bed deposition may be related to both precession and obliquity forcing we propose that precession forced sea level fluctuations mainly controlled production of lime mud in a shallow marine environment and transport to the basin precession and obliquity controlled biogenic silica productivity and temperature dependent preservation of silica may have been influenced by obliquity forcing

models of depth dependent bioturbation at manop site h in the eastern equatorial pacific the vertical distribution of excess 226ra sorbed to manganese rich sediments from a box core taken at manop site h 6 34 n 92 48 w in the eastern equatorial pacific can be modeled only if the rate of bioturbation decreases rapidly downward from the seafloor within the upper few centimeters of sediment independent bioturbation rates estimated by balancing dissolved and particulate diagenetic mn fluxes are compatible with the values obtained from the radium model such results are consistent with 210pb and 230th measurements reported here and with manop lander and porewater observations that rapid organic carbon degradation in these sediments consumes oxygen within a few centimeters of the sediment seawater interface the modeled decrease in bioturbation expressed as a biodiffusion coefficient is consistent with values of more than 25 cm2 kyr at the surface falling to less than 0 4 cm2 kyr at a depth of 14 cm

lipid biomarkers record fundamental changes in the microbial community structure of tropical seas during the late ordovician hirnantian glaciation the late ordovician mass extinction was linked to climate cooling and glaciation of gondwana during the terminal ordovician hirnantian age 444 7 443 4 ma extinction patterns have been well described for many marine taxa but much less is known about marine microbial communities through this interval to elucidate the structure of microbial communities in tropical marine basins through the late ordovician we analyzed lipid biomarkers in thermally well preserved strata from the taconic foreland anticosti island canada the cincinnati arch midwestern united states and the western continental margin vinini formation nevada united states despite clear oceanographic differences lipid biomarker profiles show similarities between these three localities major shifts in biomarker distributions of anticosti island and the vinini formation mainly hopane sterane ratios record changes in the balance of bacterial versus algal primary production bacterial contributions to sedimentary organic matter were highest during warm intervals both before and after hirnantian cooling in particular 3 methylhopanes likely sourced from aerobic methanotrophic bacteria occur in high relative abundance many times the phanerozoic average across laurentia throughout most of the interval studied 3 methylhopane abundances also reveal an overall positive relationship with paleotemperature proxies implying increased methane cycling during warm intervals these results suggest that enhanced methane cycling could have provided an important positive feedback on climate during extended intervals of early paleozoic time

early silurian sea level changes global sea level fluctuated markedly during the early silurian probably as a result of the waxing and waning of ice sheets in the south american portion of gondwana the highest sea levels of the silurian are recorded by the telychian upper crispus lower griestoniensis and spiralis lower lapworthi biozones other highstands occurred in the early aeronian during the convolutus zone mid aeronian guerichi zone and late turriculatus zone early telychian and early sheinwoodian low sea levels characterized much of the argenteus and sedgwickii zones aeronian the utilis subzone late guerichi early turriculatus zones early telychian the late telychian commencing in the mid lapworthi zone and after a period of apparently only small amplitude sea level fluctuations in the late sheinwoodian and earliest homerian the mid late homerian in particular the early nassa zone facies and faunal changes in the lower silurian do not support the p and s model of jeppsson and others but are consistent with the sea level changes proposed herein mid telychian marine red beds appear to have been deposited during a minor sea level fall immediately after a period of very high sea levels rather than during a transgressive episode as previously suggested comparison of the sea level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early silurian deep water graptolitic and shallow water shelly sequences improving the precision of this correlation should be a priority for future research

microbially induced sedimentary structures a new category within the classification of primary sedimentary structures abstract cyanobacterial films and mats syndepositonally influence erosion deposition and deformation of sediments the biomass levels surface morphologies and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing growing microbial mats dredge grains from their substrate upwards whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle trap and bind suspended particles these and similar biotic physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures we distinguish structures on bedding planes leveled bedding surfaces wrinkle structures microbial mat chips erosional remnants and pockets multidirectional ripple marks and mat curls and internal bedding structures sponge pore fabrics gas domes fenestrae structures sinoidal laminae oriented grains benthic ooids biolaminites mat layer bound grain sizes we propose to place this group of microbially mediated structures as a fifth category bedding modified by microbial mats and biofilms in pettijohn and potter s 1964 existing classification of primary sedimentary structures

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

revision of the position of the ordovician silurian boundary in southern ontario regional chronostratigraphic implications of δ13c chemostratigraphy of the manitoulin formation and associated strata delta c 13 values of 142 samples from the manitoulin formation and subjacent strata collected from 14 exposures and two drill cores on manitoulin island bruce peninsula and the region south of georgian bay suggest that the manitoulin formation is latest ordovician hirnantian rather than earliest silurian in age a delta c 13 excursion identified as the hirnantian isotope carbon excursion hice which has a magnitude of nearly 2 5 parts per thousand above baseline values is present in an interval from the upper queenston formation to the lower to middle part of the manitoulin formation in most of bruce peninsula and in the area south of georgian bay whereas on manitoulin island the rice appears to be absent this indicates that a significant part of the manitoulin formation is older on the bruce peninsula and in its adjacent region than on manitoulin island the chemostratigraphically based conclusions are consistent with biostratigraphic data from conodonts and brachiopods the hirnantian delta c 13 curve from anticosti island quebec is closely similar to those of southern ontario traditionally the ordovician silurian boundary has been placed at the base of the manitoulin formation but the new results suggest that it is more likely to be at or near the base of the overlying cabot head formation these new results have major implications for the interpretation of the geologic history and marine depositional patterns of the latest ordovician of a large part of the north american midcontinent

particle mixing rates in sediments of the eastern equatorial pacific evidence from 210pb 239 240pu and 137cs distributions at manop sites abstract particle mixing rates db calculated from excess 210pb gradients in sediments of the east equatorial pacific range from 0 04 to 0 5 cm2 y with variation of a factor of 3 4 at a single site diffusion of the 236ra daughter 222rn may affect 210pb distributions under conditions of slow mixing and low 210pb flux to the seafloor as shown by a siliceous ooze clay core which contained the fallout radionuclides 239 240pu and 137cs but no excess 210pb relative to 226ra there is no clear relationship between 210pbderived mixing rates and sediment type accumulation rate or organic carbon flux to the sediments comparison of 210pb mixing rates with those calculated from 239 240pu and 137cs distributions reveals better agreement for a pulse input of the fallout radionuclides db 0 03 0 4 cm2 y than for continuous input at a constant rate db 0 1 1 6 cm2 y although the pu and 137cs data are better fit by the latter model the agreement may be fortuitous because 239 240pu and 137cs appear significantly deeper than 210pb in at least one core tracer separation could be caused by particle size selective mixing by the benthic fauna or by chemical mobilization if the fallout radionuclides are scavenged from surface waters by large organic rich particles such as fecal pellets their release and migration may result from decomposition of the carrier in surface sediments either a relatively unreactive form of pu e g oxidized pu has been released by this process or a one dimensional model is inadequate to explain its observed penetration into the sediments activity ratios of 239 240 pu 137 cs in the sediments decrease with increasing north latitude and the trend reflects higher fluxes of 239 240pu near the weapons test site at christmas island 2 n the 239 240 pu 137 cs ratios and fluxes to the sediment assuming constant input at the siliceous ooze red clay site are consistent with published sediment trap data from a nearby site thus if fallout radionuclide fluxes to the sea floor were higher in the past both 239 240pu and 137cs have been released from sinking particles

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

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

scarcity of the c30 sterane biomarker 24 n propylcholestane in lower paleozoic marine paleoenvironments abstract 24 n propylcholestane 24 npc a c30 sterane compound derived from sterol precursors which are the major sterol constituents of modern pelagophyte microalgae occurs in certain neoproterozoic rocks and oils and throughout the phanerozoic rock record this broad distribution leads 24 npc to be widely considered a reliable indicator of open to partially restricted marine depositional conditions for source rocks and oils here we report two significant hiatuses in the occurrences of 24 npc in the lower paleozoic marine rock record the first in the middle late cambrian and the second in the late ordovician early silurian transition for a range of lithofacies carbonates and siliciclastic rocks organic carbon contents both organic lean and organic rich and paleoceanographic environments shelf and deeper water marine settings and observed offshore of two paleocontinents laurentia and baltica the ordovician silurian gap is at least 9 million years and possibly up to 20 million years in duration robust older occurrences of 24 npc steranes in some neoproterozoic rocks and oils suggest that oceanographic conditions in our intervals of lower paleozoic time were unfavorable for the proliferation of pelagophyte algae as phytoplankton caution should therefore be applied when interpreting a lacustrine versus marine depositional environmental setting for source rocks and oils in these intervals of early paleozoic time using lipid biomarker assemblages

modern analogues to middle ordovician sedimentary mounds and washout depressions abstract mounds and depressions containing dense fossil accumulations occur in a bed of middle ordovician age in the upper ottawa valley canada the appearance of these relict seabed features is similar to modern features containing dense accumulations of live species in the north sea it is suggested that the middle ordovician fossil accumulations relied on a similar local upward flux of minerals and nourishment as north sea communities which live in areas where sediments contain light hydrocarbon gases if further petrological and sedimentological studies on the middle ordovician deposits could confirm such a relationship then it would be the first time relict or fossil pockmarks were found in the sedimentary column

steady state tracer dynamics in a lattice automaton model of bioturbation the intensity of biogenic sediment mixing is often expressed as a biodiffusion coefficient db quantified by fitting a diffusive model of bioturbation to vertical profiles of particle bound radioisotopes the biodiffusion coefficient often exhibits a dependence on tracer half life short lived radioisotopes e g 234th tend to yield notably larger db values than longer lived radioisotopes e g 210pb it has been hypothesized that this is a result of differential mixing of tracers by particle selective benthos this study employs a lattice automaton model of bioturbation to explore how steady state tracers with different half lives are mixed in typical marine settings every particle in the model is tagged with the same array of radioisotopes so that all tracers experienced exactly the same degree of mixing two different estimates of the mixing intensity are calculated a tracer derived db obtained in the standard way by fitting the biodiffusion model to resulting tracer profiles and a particle tracking db derived from the statistics of particle movements the latter provides a tracer independent measure of mixing for use as a reference our simulations demonstrate that an apparent db tracer dependence results from violating the underlying assumptions of the biodiffusion model breakdown of the model is rarely apparent from tracer profiles emphasizing the need to evaluate the model s criteria from biological and ecological parameters rather than relying on obvious indications of model breakdown e g subsurface maxima simulations of various marine environments coastal slope abyssal suggest that the time scales of short lived radioisotopes such as 234th and 7be are insufficient for the tracers to be used with the biodiffusion model 210pb appears an appropriate tracer for abyssal sediments while 210pb and 228th are suitable for slope and coastal sediments

the stratigraphic and geochemical imprints of late ordovician glaciation on far field neritic carbonates anticosti island eastern canada abstract the storm dominated open marine neritic carbonate facies continuously exposed in coastal cliffs and wide rocky tidal flat exposures at the western end of anticosti island form a 315 m thick expanded section spanning the ordovician silurian boundary these carbonate facies display deepening shallowing trends of different orders resulting in a hierarchical stacking of sedimentary cycles the carbonate facies within these multi order cycles represent sediments that were deposited from shoreline to deeper outer carbonate ramp 80 m in water depth and exhibit sea level changes of several tens of metres in magnitude field petrographic and geochemical evidence support a well preserved primary 18o signal in close equilibrium with the original paleoenvironmental conditions the 18o intracycle variations up to 1 0 in magnitude reveal repeated decreasing and increasing values during the transgressive and regressive portions of decametre tr cycles respectively although temperature salinity and ice volume influenced the recorded primary 18o signal orbital scale glacioeustasy ultimately controlled facies and cycle development and 18o signal during the katian immediately before the latest ordovician hirnantian glaciation the 3 5 myr long 18o signal recorded by our entire succession supports a protracted period of cooling leading into the main phase of latest ordovician hirnantian glaciation spectral analysis of the 18o record also reveals decametre variations which are associated with decametre scale transgressive regressive tr cycles possibly reflecting eccentricity orbital signals our study shows the importance of a thick expanded stratigraphic succession sampled at the highest resolution possible to decipher potential linkages between facies changes multi order stratigraphic cycles 18o signal and the earth s orbital parameters

the east siberian basin in the silurian evidence for no large scale sea level changes according to conventional ideas numerous eustatic sea level changes occurred during the phanerozoic with amplitudes 20 100 m and periods t 1 3 myr third order cycles at the same time it is widely recognized that tectonic factors have an important role in such water depth changes rapid large scale sea level changes have been suggested for the silurian for which detailed and continuous stratigraphic successions are available in the east siberian sedimentary basin v2u10 6 km 2 in size these show slow deposition at depths 95 10 m proceeded for 10 20 myr in many regions and enable the mathematical analysis of maximum eustatic sea level changes with periods of t 1 3 myr third order that could have occurred during the silurian the amplitudes of such events could not exceed v20 30 m and for some epochs could not be larger than 8 10 m at the same time rapid water depth changes of up to v100 m occurred in some other cratonic areas as the silurian had a relatively stable sea level these must be attributed to rapid crustal uplift and subsidence the absence of third order eustatic fluctuations with amplitudes v10 20 m has also been demonstrated for the cambrian and earliest ordovician it is probable that no significant sea level changes also occurred at many other epochs while rapid changes in water depth resulted from large scale vertical crustal movements fl 2001 published by elsevier science b v

stratigraphie framework for the cambrian ordovician rift and passive margin successions from southern quebec to western newfoundland the taconian humber zone stretches from western newfoundland to southern quebec the early cambrian slope succession in newfoundland is found in the curling group whereas in quebec various units were deposited during that first time slice biostratigraphic data allow correlation of the curling group with the labrador group in newfoundland and with the newly time constrained slope succession in quebec the end of the rift drift transition is marked by a sea level lowstand at the end of the early cambrian the middle cambrian to latest early ordovician passive margin history recorded five cyclic sea level fluctuations three of these cycles are recorded in the shallow marine middle to late cambrian platform port au port group and slope sediments preserved in the cow head and northern head groups in newfoundland the biostratigraphic information assists correlation with cambrian passive margin units in quebec major sea level lowstands are recognized along the continental margin in early middle late camb

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

bioturbation short lived radioisotopes and the tracer dependence of biodiffusion coefficients bioturbation refers to the mixing of sediment particles resulting from benthic faunal activity it is the dominant particle mixing process in most marine sediments and exerts an important control on diagenetic processes in models bioturbation is usually treated as a diffusive process where the biodiffusion coefficient db characterizes the biological mixing intensity biodiffusion coefficients are classically computed by fitting a diffusive model to vertical profiles of particle bound radioisotopes one peculiar observation is tracer dependence db values from short lived tracers tend to be larger than those obtained from long lived tracers from the same site recent theoretical work based on random walk theory and lattice automaton bioturbation simulations labs has suggested that this tracer dependence is simply a model artifact and has concluded that the biodiffusion model is not applicable to the short observational time scales associated with short lived radioisotopes here we have compiled a global dataset of db values obtained from different radiotracers to assess tracer dependence from a data perspective tracer dependence is significant in low mixing environments like slope and deep sea sediments but is not present in intensely mixed coastal areas tracer dependence is absent when the number of mixing events is larger than 20 or the potential length scale is greater than 0 5 cm roughly this comes down to tracer derived db values greater than 2 cm2 yr 1 this condition is met for 68 50 and 8 of published db values obtained from coastal continental slope and abyssal environments respectively these results show that short lived radioisotopes are suitable to quantify biodiffusion mixing in sedimentary environments featuring intense bioturbation

production of subtidal tubular and surficial tempestites by hurricane kate caicos platform british west indies abstract storm infilling of deep open callianassa burrows with molluscan to peloidal packstone was the dominant style of preservable sedimentation produced by the passage of hurricane kate across caicos platform british west indies on 18 november 1985 this moderate hurricane was the first to affect an actively studied portion of the carbonate platform environments of the south florida bahama region in 19 years a time during which many of our concepts of storm sedimentation have evolved prior to hurricane kate much of the shallow platform interior peloidal packstone to grapestone grainstone had a sparse seagrass cover and a highly irregular bottom caused by abundant callianassa mounds hurricane kate flattened the mounded bottom of the platform interior and caused minor smothering and erosion of the seagrass beds a thin surficial tempestite though deposited over much of the platform was difficult to identify because it lacked a coarse base instead much of the reworked surface sediment poured down the large callianassa burrow entrances and filled the vast burrow networks with either a coarse molluscan packstone or peloidal skeletal packstone storm burrow filling tubular tempestites effectively removed the coarse fraction from contributing to a surficial storm layer ten percent of the upper 75 cm of platform interior sediment consists of tubular tempestites from hurricane kate

bioturbation in shelf slope sediments off cape hatteras north carolina the use of 234th chl a and br to evaluate rates of particle and solute transport abstract biogenic particle reworking 234 th chl a chloropigment distributions and pore water irrigation rates br tracer were examined in the continental shelf break upper slope region off the north carolina cape hatteras coastline sediment cores were obtained along three primary east west transects water depth 75 800 m 36 20 n 35 50 n 35 25 n at additional shallow stations along 35 40 n and at slope stations within a region of complex topography known as the manteo lease block samples were collected during august 1994 july 1996 and august 1996 and were recovered using two shipboard techniques haps corer and box corer as well as by the deep submersible johnson sea link natural and experimental tracer distributions demonstrate that with few exceptions surface deposits throughout this region are rapidly reworked and irrigated by abundant infaunal benthos excess 234 th t 1 2 24 1 days was present at all stations with surface activities 0 0 5 cm ranging from 2 to 62 dpm cm 3 5 54 dpm g 1 average inventories of 28 21 median 24 dpm cm 2 and typical penetration depths of 5 7 cm steady state particle mixing coefficients d b estimated using excess 234 th ranged from 1 to 200 cm 2 yr 1 although the highest mixing intensities were found between 300 and 500 m water depths rates were locally variable and there was little or no evidence for any consistent attenuation with bathymetric depth in either 234 th inventories or mixing intensity estimates of d b made using chl a distributions are similar to those estimated using 234 th ranging from 36 to 110 cm 2 yr 1 added br tracer penetrated 7 cm in 24 h periods in shipboard incubated sediment cores representing rates ranging from 1 5 to 38x molecular diffusion mean 13 1 13 0 median 11x sedimentary chl a and phaeophytin a distributions below the photic zone are indicative of high input of fresh planktonic debris and rapid remineralization these inputs presumably fuel the abundant benthic fauna which rework sediments on the hatteras slope at some of the highest mixing rates yet reported

latitudinal variations in benthic processes in the abyssal equatorial pacific control by biogenic particle flux abstract the equatorial pacific forms a band of high globally significant primary production this productivity drops off steeply with distance from equatorial upwelling yielding large latitudinal gradients in biogenic particle flux to the abyssal seafloor as part of the us jgofs program we studied the translation of these particle flux gradients into the benthic ecosystem from 12 s to 9 n along 135 140 w to evaluate their control of key benthic processes and to evaluate sediment proxies of export production from overlying waters in october december 1992 the remineralization rates of organic carbon calcium carbonate and biogenic opal roughly matched the rain rates of these materials into deep sediment traps exhibiting peak values within 3 of the equator rates of bioturbation near the equator were about ten fold greater than at 9 n and appeared to exhibit substantial dependence on particulate organic carbon flux tracer time scale i e age dependent mixing and pulsed mixing from burrowing urchins organic carbon degradation within sediments near the equator was dominated by a very labile component reaction rate constant k approximately 15 per year that appeared to be derived from greenish phytodetritus accumulated on the seafloor organic carbon degradation at the highest latitudes was controlled by a less reactive component with a mean k of approximately 0 075 per year where measured megafaunal and macrofaunal abundances were strongly correlated with annual particulate organic carbon flux macrofaunal abundance in particular might potentially serve as a proxy for export production in low energy abyssal habitats sedimentary microbial biomass also was correlated with the rain rate of organic carbon but less strongly than larger biota and on shorter time scales i e approximately 100 days we conclude that the vertical flux of biogenic particlues exerts tight control on the nature and rates of benthic biological and chemical processes in the abyssal equatorial pacific and suggest that global changes in productivity on decadal or greater time scales could yield profound changes in deep sea benthic ecoystems

revisiting the appalachian structural front and offshore anticosti basin northern gulf of st lawrence canada by integrating old and new geophysical datasets abstract the anticosti basin is a large predominantly marine sedimentary basin centered within the northern gulf of st lawrence poor quality vintage seismic reflection data collected more than 25 years ago in the offshore part of the basin failed to depict its geometry however recently acquired multibeam bathymetry and magnetic data can be used in combination with high resolution seismic lines to map the paleozoic succession within the honguedo strait the upper part of the offshore sedimentary succession consists of approximately 1140 m of sediments younger than those exposed on anticosti island the offshore part of the basin includes a 40 km wide fold and fault belt sub parallel to the coastline of the gaspe peninsula the structural style is characterized by broad open synclines narrow anticlines and nw striking faults with dextral strike separations which bears similarities to that of the gaspe belt that formed mainly during the middle devonian acadian orogeny this study indicates that the appalachian deformation front is located to the north of its classically inferred location moreover it suggests that the curvature of the early paleozoic assemblages in the gulf of st lawrence is partly a late devonian feature an improved understanding of the offshore architecture of the basin may help to develop new paleogeographic reconstructions and hydrocarbon exploration models in this under explored frontier basin

field experiments on bioturbation in salt marshes bombay hook national wildlife refuge smyrna de usa implications for sea level studies the suitability of marsh sites for sea level studies was examined based on field experiments along a transect from low to high marsh bead distributions were determined both seasonally and after 7 years seasonal sediment mixing was greatest in the low marsh and in the late spring and early summer when biological activity is greatest however after an initial interval of relatively intense reworking the bead concentrations reached an approximate equilibrium profile characteristic of each marsh environment as reflected by the profiles obtained after 7 years mixed layer thickness is greatest 10 cm in the intermediate and low marsh and burial rates are rapid 3 7 11 1 mm yr 1 moreover burial rates are comparable to or even surpass longer term 30 to 150 yr radiotracer derived sediment accumulation rates and rates of local and regional sea level rise 4 mm yr 1 therefore sediment accumulation rates appear to reflect primarily sediment resuspension redeposition within the system due to bioturbation thus bioturbation may be critical to the ability of marshes to keep pace with sea level while seemingly precluding the use of low marsh for high resolution sea level studies copyright 2008 john wiley sons ltd

paleogeography of the matapedia basin in the gaspe appalachians initiation of the gaspe belt successor basin the matapedia basin consists of the uppermost ordovician lowermost silurian deep water fine grained carbonate siliciclastic rocks of the honorat garin formation and matapedia groups pabos and white head formations the lower rock assemblage of the gaspe belt in the gaspe appalachians paleogeographic maps of eight time slices from the caradocian to the llandoverian are presented to better understand the tectonosedimentary evolution of the matapedia basin deposition evolved from siliciclastic garin fm to argillaceous limestones pabos fm to limestones white head fm the overall change from terrigenous garin fm to limestone facies white head fm reflects a change in the source area paleocurrent directions and composition of sandstones indicate an orogenic source area to the south for the garin formation which is believed to be the inliers of the humber and dunnage zones in the southern gaspe and new brunswick appalachians lime muds deposited by turbidity currents coming from the nor

proliferation of early triassic wrinkle structures implications for environmental stress following the end permian mass extinction lower triassic strata in the western united states and northern italy have yielded the first reported occurrence of microbially mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the cambrian we hypothesize that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end permian mass extinction these wrinkle structures occur in siliciclastic sedimentary rocks in association with hummocky cross stratification and in some cases trace fossils such as asteriacites rhizocorallium planolites and gyrochorte that indicate a subtidal paleoenvironment wrinkle structures were found in the campil member in the werfen formation of northern italy and in the virgin limestone member of the moenkopi formation and the upper member of the thaynes formation exposed in the western united states the presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm dominated siliciclastic shelves wrinkle structures commonly formed in a variety of subtidal paleoenvironments during the proterozoic cambrian but thereafter became restricted to intertidal supratidal and deep sea environments this restriction has been attributed to the increase in infaunalization of metazoans following the cambrian radiation the proliferation of wrinkle structures in subtidal settings during the early triassic suggests that infaunal bioturbation was reduced after the end permian mass extinction and that this reduction lasted for millions of years

low temperature thermochronology of anticosti island a case study on the application of conodont u th he thermochronology to carbonate basin analysis abstract the ordovician to silurian carbonate succession preserved on anticosti island is a near continuous record of the changing depositional and tectonic history of the early paleozoic laurentian margin these strata are a prime natural laboratory to test the applicability of conodont apatite u th he thermochronology cahe for carbonate basin analysis in a setting with a well understood burial history and abundant paleotemperature constraints we report 63 cahe dates from seven samples collected from two boreholes x ray micro computed tomography xr ct was used to assess individual conodont element size volume and alpha ejection correction cahe dates are negatively correlated with effective uranium concentration eu with single conodont dates enigmatically older than the age of deposition to conodont dates as young as 2 7 0 2 ma we rationalize this dispersion as a product of several factors including diagenetic processes and pervasive conodont crystalline porosity and permeability additionally one basement and two sandstone samples were collected for apatite and zircon u th he ahe zhe and apatite fission track aft thermochronology in the precambrian basement zhe dates range from 581 7 46 5 ma to 761 2 60 9 ma n 4 and ahe dates range from 26 7 1 6 to 48 4 2 9 ma n 4 ahe dates from two lower ordovician sandstones n 8 are positively correlated with eu increasing from 13 7 0 8 ma to 105 2 6 3 ma over a range of 186 ppm eu thermal history modeling of these data suggests two plausible thermal histories both of which are novel as they imply that far field effects related to the opening of the atlantic ocean have influenced the thermal history of anticosti island ultimately these data highlight the complexities associated with and necessity of low temperature thermochronology in carbonate basins

stratigraphy of the upper ordovician upper vaureal and ellis bay formations eastern anticosti island quebec marked facies changes occur in late ordovician strata assigned to the uppermost vaureal and ellis bay formations ashgill rawtheyan hirnantian on anticosti island quebec western anticosti features shales and carbonates whereas outcrops along the eastern coast contain prominent discontinuous mixed siliciclastic carbonate units detailed section measurement along the northeast coast allows for the first time accurate definition of seven new members within this uninterrupted sequence sands present in the upper vaureal and lower ellis bay formations in the east appear to have deterred the growth of muddy bottom brachiopod communities comparable to those in the western and central regions of anticosti sand units within the upper vaureal formation contain 1 m diameter colonies of paleofavosites coeval small coral patch reefs are found in the central part of the island where sands are absent the uppermost ellis bay formation of northeast anticosti is marked by a shallow subtidal coral algal onco

polygenetic polyphase karsted hardground omission surfaces in lower silurian neritic limestones anticosti island eastern canada abstract exquisitely preserved and well exposed rocky paleoshoreline omission surfaces in lower silurian chicotte formation limestones on anticosti island quebec are interpreted to be the product of combined marine and meteoric diagenesis the different omission features include 1 planar erosional bedding tops 2 scalloped erosional surfaces 3 knobs ridges and swales at bedding contacts and 4 paleoscarps an interpretation is proposed that relates specific styles of omission surface to different diagenetic depositional processes that acted in separate terrestrial peritidal shallow neritic zones such processes were linked to fluctuations in relative sea level with specific zones of diagenesis such as 1 karst corrosion 2 peritidal shallow subtidal erosion 3 subtidal seawater flushing and cementation and 4 shallow subtidal deposition most surfaces are interpreted to have been the result of initial extensive shallow water synsedimentary lithification forming hardgrounds that were as sea level fell planed by inner neritic abrasion successively altered by exposure and subaerial corrosion only to be buried by sediments as sea level rose again this succession was repeated several times resulting in a suite of recurring polyphase omission surfaces through many meters of stratigraphic section synsedimentary cloudy marine cements are well preserved and are thus interpreted to have been calcitic originally aragonite components are rare and thought to have been dissolved just below the silurian seafloor large mollusks that survived such seafloor removal were nonetheless leached and the resultant megamolds were filled with synsedimentary calcite cement these silurian inner neritic strandline omission surfaces are temporally unique they are part of a suite of marine omission surfaces that are mostly found in early to middle paleozoic neritic carbonate sedimentary rocks these karsted hardgrounds formed during a calcite sea time of elevated marine carbonate saturation and extensive precipitation of marine cement the contemporaneous greenhouse atmosphere was supercharged with co2 leading to profound surface karst under strongly acid rain younger peritidal omission surfaces although potentially formed during aragonite sea or calcite sea times would have been subject to very different terrestrial diagenetic process with lower atmospheric pco2 values but increasingly complex biogenic soils producing dissimilar alteration features

cyclic late katian through hirnantian glacioeustasy and its control of the development of the organic rich wufeng and longmaxi shales south china abstract deposition of the organic rich shales of the wufeng and lowermost longmaxi formations late katian through hirnantian occurred in a relatively stable tectonic setting and commenced at approximately 4 5 myr prior to the end ordovician glacial maximum these organic rich shales are important because they provide a continuous sedimentary record of high frequency eustatic changes associated with ice sheet expansion and shrinkage here we carried out a high resolution cyclostratigraphic study of fe3 series and associated geochemical analysis of the wufeng and lowermost longmaxi shales in order to better understand sea level cyclicity in the late katian through hirnantian ocean the organic rich shales deposited during this interval in the ehd1 drill core yichang china record four major myr scale eustatic cycles which were associated with third order sea level fluctuations within each of these major cycles 3 4 minor eustatic cycles corresponding to 405 kyr long eccentricity astronomical cycles were recognized and representing fourth order eustatic changes the cyclostratigraphic age of this study is generally in agreement with the radiometric dates in gts2012 although the duration of the hirnantian stage is 0 5 myr longer than previously thought this study also infers glacial controls on the depositional environment of the organic rich shales as enhanced cooling triggered ice sheet expansion and concurrent sea level fall leading to higher thermohaline circulation intensity and enhanced paleoproductivity through upwelling and terrestrial influx in contrast warming intervals led to ice sheet shrinkage and concurrent deepening characterized by water column stratification and reduced nutrient supply the results provide evidence in support of continuous continental glaciation in the late katian through hirnantian

testing the foodbancs hypothesis seasonal variations in near bottom particle flux bioturbation intensity and deposit feeding based on 234th measurements abstract naturally occurring 234th 24 d half life was used on the west antarctic continental shelf to evaluate temporal variations in the flux of particulate material reaching the seabed bioturbation intensity the seasonal continuity of feeding by benthic fauna and trends in particle selection during ingestion for six common detritivores four surface deposit feeders and two subsurface deposit feeders these measurements were made at three stations during the five foodbancs cruises december 1999 march june and october 2000 and march 2001 to assess the nature of pelagic benthic coupling on the shelf and to evaluate the seabed as a potential food bank for deposit feeders when surface primary production is minimal two summer regimes were sampled march 2000 and march 2001 with the latter exhibiting a distinct 1 2 cm thick phytodetritus layer in nearly all sediment core samples at site b the 234th fluxes into the near bottom 150 170 mab sediment traps were indistinguishable for the december march 2000 march june 2000 and june october 2000 sampling intervals fluxes ranging from 170 to 280 dpm m 2 d 1 however the sediment trap 234th flux measured for the october 2000 march 2001 interval 1000 dpm m 2 d 1 was 5 fold greater than during the other three sampling periods consistent with the deposition of a phytodetritus layer the steady state 234th fluxes derived from seabed inventories at site b were 2 4 2 7 times greater than the sediment trap 234th fluxes indicating substantial scavenging of this particle reactive radiotracer in the bottom 150 m of the water column and or lateral transport near the seabed the seabed 234th inventories at the three stations showed no variation during the first four cruises but were significantly greater during cruise fb v march 2001 when the phytodetritus layer occurred based on 234th distributions in the seabed bioturbation intensities quantified using the diffusive mixing coefficient db varied from 0 5 to 97 cm2 yr 1 mixing coefficients were significantly lower during fb ii than during fb iii but no other time comparisons were statistically significant p 0 05 234th dbs showed no distinct seasonality and no correlation with either organic carbon flux into the sediment traps or 234th inventory in the seabed the four surface deposit feeders examined an echiuran worm and three holothurians protelpidia murrayi bathyplotes fuscivinculum and peniagone vignoni exhibited greater particle selection for recently deposited sediment during ingestion than the two subsurface deposit feeders studied a head down deposit feeding holothurian molpadia musculus and an irregular urchin amphipneustes lorioli all six deposit feeders contained excess 234th activity in gut sediments during all five cruises indicating sediment ingestion year round even during the austral winter the lack of seasonal variation in bioturbation intensity and the demonstration of year round feeding in deposit feeders are consistent with the hypothesis that the seafloor sediments accumulate labile organic matter produced during periods of high primary production and that deposit feeders utilize this food source as a food bank on a year round basis

local and global perspectives on carbon and nitrogen cycling during the hirnantian glaciation abstract the impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the late ordovician hirnantian glaciation three sections were studied two from nevada one proximal and one distal and one from the yukon the opportunity to compare 13 c carb 13 c org and 13 c grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater 13 c values at each stratigraphic section using the best preserved sedimentary components from the two nevada sections a lowstand sedimentary gradient of 4 was found between platform and ocean settings during the hirnantian glaciation gradients of similar magnitude have been reported from hirnantian platforms in the canadian arctic and baltic basins taking the outboard sections as the most representative of the ocean carbon pool a mean positive 13 c shift of just 2 7 0 4 is calculated which is significantly smaller than the 5 7 positive shifts recorded in epeiric platform settings the discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas it is hypothesized that increased carbonate weathering caused 13 c values of local carbon weathering fluxes to increase which in turn caused seawater 13 c values to increase in basin proximal settings in addition nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings driving sedimentary 13 c values even higher much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive a 15 n profile reconstructed from deposits of the distal vinini creek section records a 1 5 positive shift through the hirnantian glacial interval interpreted as a major late ordovician upwelling zone high organic carbon 1 wt and high hydrogen indices 400 600 make the vinini creek section a suitable candidate for the preservation of original sedimentary 15 n records to explain the results a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers it is hypothesized that in greenhouse climates the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean s expansive oxygen minimum zones this caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone yielding sedimentary 15 n values between 0 and 2 by contrast the cooler icehouse climate of the hirnantian led to increased ocean ventilation greater partitioning of atmospheric oxygen into downwelling surface waters and oxygen minimum zone shrinkage as a result fixed nitrogen levels climbed in the ocean s abyssal depths due to the lower denitrification rates this in turn allowed nitrogen with high 15 n values to recycle back into the photic zone driving sedimentary 15 n values higher during the hirnantian glaciation

late ordovician carbon isotope trend in estonia its significance in stratigraphy and environmental analysis abstract carbon isotope changes during most of late ordovician time from the mid caradoc kinnekulle k bentonite until the beginning of the silurian were investigated as the corresponding sequence of rocks is stratigraphically nearly complete in estonia an attempt was made to use it to elaborate the general pattern of carbon isotope changes in the late ordovician complications were caused by several local or regional hiatuses in the middle and late caradoc and hirnantian a total of 385 whole rock samples were studied from eight drill cores in northern and central estonia the following positive carbon isotope events were observed 1 the mid caradoc excursion peak 13c value 2 2 in the uppermost part of the keila stage also known in sweden 2 the first late caradoc excursion 1 9 in the lower part of the rakvere stage 3 the second late caradoc excursion 2 4 in the upper part of the nabala stage 4 the early ashgill excursion 2 5 in the lowermost part of the pirgu stage 5 the widely known large hirnantian excursion in estonia the peak value reaches 6 7 in the porkuni stage the study interval comprises a long 10 ma period characterized by low magnitude carbon isotope changes and a following brief 2 ma interval with large changes no obvious lithological preference for hosting the positive shifts was recorded in principle the 13c values exceeding the background values may occur in all types of rocks present in a sedimentary basin several 13c positive excursions values 1 5 to 3 in the mohawkian of north america are evidence that the minor caradoc and early ashgill 13c positive shifts in baltoscandia may have counterparts in laurentia if correctly correlated these shifts may have global significance the hirnantian excursion is usually linked to a major glacial event even if some carbon cycling mechanisms are not completely understood the environmental causes suggested for the earlier minor shifts range from global climatic and glacial events to very local changes in basin regime and sea level our study supports the primary role of climatic or climatically triggered oceanic processes

deposition rates mixing intensity and organic content in two contrasting submarine canyons the hydrographically different conditions characterising the western iberian margin ne atlantic and the gulf of lions mediterranean may play an important role in determining the biogeochemical characteristics of the sediments to investigate this we compared the nazare and cap de creus canyons and their respective adjacent open slopes in terms of the organic carbon corg contents chlorophyll a chl a concentrations c n and chl a phaeopigment ratios and also in terms of modelled mixing intensities chl a and 210pb deposition and background concentrations in sediments chlorophyll a and 210pb profiles were fitted simultaneously with a reactive transport model to estimate mixing intensity deposition and background concentrations further to account for the possibility that the decay of chl a may be lower in the deep sea than in shallow areas we estimated the model parameters with two models in one approach model 1 the temperature dependent decay rate of chl a as given by sun et al sun m y lee c aller r c 1993 laboratory studies of oxic and anoxic degradation of chlorophyll a in long island sound sediments geochimica et cosmochimica acta 57 147 157 for estuaries was used in the other approach model 2 an extra parameter was estimated to derive the chlorophyll a degradation rate an f test taking into account the different number of parameters in the models was used to single out the model that significantly fitted the data best in most cases the model parameters were best explained with model 1 indicating the empirical relationship by sun et al 1993 is a valid means to estimate the chlorophyll a degradation rate in deep sea sediments to assess the robustness with which the model parameters were estimated we provide a first application of bayesian analysis in the modelling of tracers in sediments bayesian analysis allows calculating the mean and standard deviation for each model parameter and correlations among parameters the model parameters for stations for which 210pb and chlorophyll a profiles were available were robustly fitted as evidenced by an average coefficient of variation of 0 22 corg contents chl a concentrations chl a phaeo ratios mixing intensities depositions and background concentrations of chl a and 210pb indicated that the cap de creus canyon and adjacent slope were less active in terms of organic matter accumulation and burial than the nazare canyon and respective open slope

evolution of the late ordovician plaesiomyid brachiopod lineage in laurentia multivariate analysis of 197 well preserved specimens among 11 species of late late ordovician katian and hirnantian plaesiomyid brachiopods from north america revealed two trends of morphological changes 1 increasing globosity and dorsal convexity from the early to late katian and 2 coarser but fewer ribs in species from the paleoequatorial intracratonic seas compared with species from the mid to high tropical pericratonic shelves and platforms another notable change albeit not quantified was the enlargement of cardinal process in plaesiomyids from early to late katian characterized by a predominantly trilobed cardinal process in the intracratonic species versus a bilobed cardinal process in pericratonic species increases in dorsal convexity and shell globosity are interpreted to have allowed for development of a larger lophophore improving the capacity and efficiency of filter feeding and gas exchange smaller but more frequently bifurcating ribs would have led to more numerous setae nea