biotic geochemical and environmental changes through the early sheinwoodian wenlock silurian carbon isotope excursion escie buttington quarry wales abstract palaeontological sedimentological and geochemical data particularly from la icpms analyses of pyrite from a 12 4 m section through the lower sheinwoodian trewern brook mudstone formation of buttington quarry wales are combined to reveal the environmental changes taking place during the early wenlock epoch of the silurian during the early part of the escie early sheinwoodian carbon isotope excursion the uppermost cyrtograptus murchisoni through to lower monograptus riccartonensis biozones show increased nutrient levels and peaks in palynomorph and graptolite abundance at this time graptolite diversity globally was declining dramatically possibly associated with metal particularly zinc concentrations attaining toxic levels and probably with the base of the euphotic zone being higher in the water column resulting from blooms of phytoplankton causing those graptolites that specialized in feeding on phytoplankton that were outcompeted in this diminished volume of illuminated surface waters to decline in numbers and eventually become extinct correlation of the graptolite and conodont biozonations enables the levels of the main stratigraphically higher ireviken event datum points to be proposed for the buttington section and this in turn allows events recognised elsewhere intervals of malformed palynomorphs and re organization of polychaete faunas together with an oceanic anoxic event to be correlated with the lower part of the studied buttington section the commencement of the oae identified on gotland at ireviken event datum point 4 appears to correlate rather well with a rapid transition marking the top of the butterley mudstone member at buttington from heavily bioturbated mudstones to laminated graptolitic mudstones surprisingly the most rapid change in 13ccarb values within the buttington section from 0 6 at 8 19 8 25 m to 2 09 at 8 50 8 55 m is not associated with any major faunal palynological abundance or geochemical changes these are all coincident with the initial rise in 13ccarb values up to about the 5 m level in the buttington section another unexplained feature of the graptolite record is a sudden change from monograptus dominated to pristiograptus dominated assemblages about half way through the studied section

a strong seasonal dependence in the martian hydrogen exosphere hubble space telescope hst and mars express mex observed unexpected rapid changes in the martian hydrogen exosphere involving a decrease in scattered lyman intensity in fall 2007 solar longitude ls 331 345 these changes detected were speculated to be a combination of seasonal variation and or dust storms and lower atmospheric dynamics here we present hst observations of mars in 2014 over a broad range of heliocentric distances and seasons ls 138 232 which indicate a factor of 3 5 change in martian lyman brightness associated with a factor 5 4 variation of hydrogen escape flux in the absence of global dust storms and significant solar variability we thus conclude that seasonal effects have a strong influence on the hydrogen exosphere which in turn has major implications for the processes that control water supply to the martian upper atmosphere and the history of water escape from mars

mars water isotope d h history in the strata of the north polar cap inferences about the water cycle abstract a time varying stable isotope model for the d h history of mars water cycle is developed with variable atmosphere space loss rate ground and ice cap flux rates it considers coupled ground reservoirs and traces d h in the air and reservoirs secularly and over obliquity cycles the various flux rates are prescribed time variables that simulate surface flux and solar driven space loss rates predicted bulk averages for the ice cap ground ice reservoirs and atmosphere span the observed ranges reported by mumma et al mumma m j novak r e disanti m a bonev b dello russo n magee sauer k 2003 the martian atmosphere conference reports of sixth international conference on mars atmosphere no 3186 when the dominant obliquity cycle variations are scaled so that the model delivers present seasonal variations the present long term bulk d h average for the ice cap is 2 7 equivalent to 1700 in d wrt smow the obliquity driven d h cycle in the ice cap s layers varies between 3 and 6 the smaller more accessible reservoirs have larger bulk averages with the smallest being able to reach d h values over 9 within 10 5 years small hypothetical solar activity driven variations in the escape rate to space and in the fractionation constant krasnopolsky v a feldman p d 2001 science 294 1914 1917 for the escape process can produce a solar wiggle whose d h amplitude can reach 0 1 d amplitude of 100 because of the temporal variability a single modern measured atmospheric d h ratio at a particular ls cannot tell very much about the total water inventory of mars a bulk average for the northern ice cap and better still a dated vertical profile of d h from the ice cap would however go a long way towards illuminating the modern water history of mars the age and stability of the northern ice cap and the d h history locked in the layering is discussed an ice cap that is very young and exchanges its mass through the atmosphere often will necessarily have a large d h

a new index for the atlantic meridional overturning circulation at 26 n abstractthe atlantic meridional overturning circulation amoc has received considerable attention motivated by its major role in the global climate system observations of amoc strength at 26 n made by the rapid climate change rapid array provide the best current estimate of the state of the amoc the period 2004 11 when rapid amoc is available is too short to assess decadal variability of the amoc this modeling study introduces a new amoc index called amocsv at 26 n that combines the florida straits transport the ekman transport and the southward geostrophic sverdrup transport the main hypothesis in this study is that the upper midocean geostrophic transport calculated using the rapid array is also wind driven and can be approximated by the geostrophic sverdrup transport at interannual and longer time scales this index is expected to reflect variations in the amoc at interannual to decadal time scales this estimate of the surface branch of the amoc can be constructed as long as reliable measu

biochemostratigraphy of the eramosa formation in southwestern ontario canada the lithostratigraphic term eramosa was introduced in ontario more than a century ago to include a distinctive package of thin to medium bedded black to medium brown dolostones that make up key cuesta faces and railway roadcuts along the eramosa river in the city of guelph southwestern ontario canada this stratigraphic unit makes up part of a stacked carbonate succession that constitutes one of the most economically significant paleozoic sedimentary rocks in ontario the strata assigned to the eramosa have a complex history of lithostratigraphic study and the relative age regional lithostratigraphic relationships and varied depositional environments of the eramosa were poorly understood this research which combines conodont biostratigraphy and carbonate carbon 13ccarb isotope stratigraphy to generate a detailed chronostratigraphic framework for the eramosa formation in southwestern ontario is part of a regional scale surface and subsurface mapping initiative of the silurian strata by the ontario geological survey dolostone samples from wiarton and the city of guelph ontario yielded three biostratigraphically important conodonts ozarkodina sagitta rhenana kockelella walliseri and kockelella ortus ortus the carbonate carbon 13ccarb isotope data from the studied intervals record the descending limb of the sheinwoodian ireviken positive 13ccarb isotope excursion including a distinctive positive shift in 13ccarb isotope values typical of records of the descending limb of the excursion in laurentia and baltica aldridgeodus minimus was also recovered from these conodont faunas and co occurs with kockelella walliseri below the last occurrence of ozarkodina sagitta rhenana lower kockelella walliseri zone suggesting that the range of aldridgeodus minimus should be extended lower into the sheinwoodian stage

ordovician and silurian global geography presidential address delivered 3 may 2000 the main palaeocontinents during the early ordovician were gondwana laurentia baltica and siberia and a brief survey is made of their limits in the ordovician and silurian in particular gondwana by far the largest continent is analysed as including a core of south america antarctica africa australia and peninsular india and also the marginal terranes all of which show faunal links with but which may not have been attached to the core of avalonia ibero armorica and other european fragments turkey arabia and terranes from the far east including south china sibumasu and parts of australasia changes within the benthic faunas reveal that baltica whilst isolated in the early ordovician became united with avalonia by the end ordovician and the two with laurentia by the mid silurian to form the new supercontinent of laurussia island arcs had distinctive faunas during the ordovician in particular those in the iapetus between laurentia baltica and gondwana and the huge kipchak arc which ran from baltica to siberia as the period progressed the iapetus arcs became subducted beneath or accreted to their neighbouring cratons and the kipchak arc gradually collapsed to form the core of the new kazakhstania terrane gondwana drifted over the south pole and this movement is reflected in the cratonic benthic faunas particularly brachiopods and trilobites which in the early ordovician had formed a cline between the high latitude faunas of north africa and the equatorial faunas of the far east and australia but by the devonian lived in much warmer seas in southern europe and cooler waters in antarctica the faunas also reflect the global palaeoclimates which were warm in the early to mid ordovician and mid to late silurian but which were much colder in the half million year period of the late ordovician and early silurian particularly in the latest ordovician hirnantian ice age with its attendant widespread hirnantia brachiopod fauna the relative closeness of the chief palaeocontinents by the early silurian enabled brachiopod and trilobite larvae to cross the narrower oceans enabling a relatively cosmopolitan benthic fauna to be established over much of the globe apart from the cooler water higher latitude clarkeia fauna to the south and the tuvaella fauna to the north

integrated silurian conodont and carbonate carbon isotope stratigraphy of the east central appalachian basin abstract the eastern and southeastern margins of the appalachian basin contain extensive exposures of telychian through pridoli upper silurian strata litho and biostratigraphic analyses of these strata over the last century provided a general means for their chronostratigraphic correlation to global series and stage boundaries however a rarity of zonally diagnostic fauna limited precise correlation both within and beyond the appalachian basin recent conodont biostratigraphic studies substantially revised the chronostratigraphic position of these strata by more than a series in some cases much of the interval however remained biostratigraphically unzoned and the precise position of series and stage boundaries remained tentative to improve the chronostratigraphic correlation of these strata to global series and stage boundaries we provide new integrated conodont and high resolution 13ccarb chemostratigraphic analyses from two outcrops located in west virginia and virginia spanning the upper rose hill through tonoloway formations upper telychian through pridoli these new data show that the silurian carbon isotope record for this portion of the appalachian basin is shifted to more negative 13ccarb values than typical global data however the overall isotopic records appear correlative at least within the appalachian basin thereby providing a means for future correlation within the basin for intervals in which biostratigraphy is traditionally lacking these coupled data demonstrate that the ireviken and mulde excursions are contained within our sections the high amplitude lau excursion was however not observed and the present biostratigraphic data suggest that a majority of the ludlow series was either not recorded or was later removed by erosion these coupled analyses therefore indicate that the ludlow pridoli boundary resides within the wills creek formation and we propose that the overlying isotopic record presented here records the late ludlow carbon isotope excursion llcie and silale low previously identified only within the baltic basin

an observational overview of dusty deep convection in martian dust storms abstractdeep convection as used in meteorology refers to the rapid ascent of air parcels in earth s troposphere driven by the buoyancy generated by phase change in water deep convection undergir

western boundary currents in the atmosphere of mars western boundary currents wbcs are an intensification of north south flow adjacent to an eastward facing meridional boundary although most familiar in the oceans where the gulf stream is the best known example wbcs also occur in the earth s troposphere the main example being the east african jet1 which is thought to play an important role in the asiatic monsoon here we identify boundary currents in a different geophysical context a numerical simulation of the atmosphere of mars in our simulation wbcs exist in association with significant cross equatorial flow and the presence of equatorial martian topography which has vertical scale far exceeding terrestrial relief2 the intensity and width of these currents depend on model parameters notably the surface drag from a comparison of our results with other martian models we suggest that wbcs have already been simulated although they were not previously identified as such3 the available observational evidence appears to be consistent with the presence of martian wbcs which may be important in the generation of global and great dust storms

mountain ranges favour vigorous atlantic meridional overturning 1 we use a global ocean atmosphere general circulation model oagcm to show that the major mountain ranges of the world have a significant role in maintenance of the atlantic meridional overturning circulation amoc a simulation with mountains has a maximum amoc of 18 sv 1 sv 106 m3 s 1 compared with 0 sv for a simulation without mountains atlantic heat transport at 25 n is 1 1 pw with mountains compared to 0 2 pw without the difference in amoc is due to major changes in surface heat and freshwater fw fluxes over the atlantic in the pacific changed surface fluxes lead to a meridional overturning circulation of 10 sv our results suggest that the effects of mountains on the large scale atmospheric circulation is to force the ocean towards a state with a vigorous amoc and with no overturning in the pacific

a rapid decrease of the hydrogen corona of mars mars is believed to have lost much of its surface water 3 5 billion years ago but the amounts that escaped into space and remain frozen in the crust today are not well known hydrogen atoms in the extended martian atmosphere some of which escape the planet s gravity can be imaged through scattered solar uv radiation hubble space telescope hst images of the ultraviolet h ly emission now indicate that the coronal h density steadily decreased by a factor of roughly 40 over 4 weeks a far greater variation than had been expected the leading candidate cause is a decrease in the source rate of water molecules from the lower atmosphere consistent with seasonal changes and a recent global dust storm this implies that the rate of escape of martian hydrogen and thereby water into space is strongly dependent on the lower atmospheric water content and distribution

the effects of orography on midlatitude northern hemisphere dry climates abstract the role of mountains in maintaining extensive midlatitude arid regions in the northern hemisphere was investigated using simulations from the gfdl global climate model with and without orography in the integration with mountains dry climates were simulated over central asia and the interior of north america in good agreement with the observed climate in contrast moist climates were simulated in the same regions in the integration without mountains during all season but summer large amplitude stationary waves occur in response to the tibetan plateau and rocky mountains the midlatitude dry regions are located upstream of the troughs of these waves where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited in summer this mechanism contributes to the dryness of interior north america as a stationary wave trough remains east of the rockies but is not effective in eurasia due to seasonal changes in the atmospheric circulation the dryness

oceans on mars an assessment of the observational evidence and possible fate the edges of the northern plains and interpreted to be shorelines remaining from these former standing bodies of water we examine the elevations and geologic relations along these contacts in detail and find little support for their interpretation as shorelines some contacts are clearly of volcanic origin and all have significant variations in elevation better support for the former presence of water over large parts of the northern plains is provided by the vastitas borealis formation vbf most of the post noachian fill within the northern basin is ridged plains of lower hesperian age interpreted to be volcanic in origin overlying the ridged plains is the vbf a thin veneer of material of upper hesperian age the vbf may have been deposited from large floods support for this interpretation is the similarity in age between the outflow channels and the vbf the presence of the vbf at the lower ends of the outflow channels and identification of numerous features in the outcrop areas of the vbf that are suggestive of basal melting of an ice sheet to cover all the area over which the vbf is exposed would require 2 3 10 7 km 3 of water spread over the entire surface of mars this volume is equal to a global layer global equivalent layer or gel 156 m deep we find no support for the larger estimates of ocean volumes that range up to 3 10 8 km 3 and which imply comparable amounts of water per unit area as are currently present on the surface of the earth under present climatic conditions on mars an ocean would freeze in a geologically short time period 10 4 years then would sublimate away at rates strongly dependent on the presence or absence of debris on the ice surface the present vbf is interpreted as a sublimation residue from the ponded outflow channel effluents the fate of a volume of water thought to have been emplaced by the outflow channels 2 3 10 7 km 3 is largely accounted for by the presence of other existing reservoirs on the planet an approximately 20 30 m gel of water is estimated to be in the present polar caps and a 50 m gel may have escaped to space since the hesperian leaving 80 m gel unaccounted for this amount may be partly trapped in other volatile rich deposits on the surface and a significant amount could have reentered the groundwater system by south polar basal melting and been progressively cold trapped at the base of a growing cryosphere on the basis of our assessment of the hesperian aged deposits we predict that testing of the clifford and parker 2001 hypothesis that a noachian aged ocean covered up to one third of the surface of mars will be made very difficult by the enhanced degradation rates in the noachian and subsequent geological events in the northern lowlands index terms 6225 planetology solar system objects mars 4267 oceanography general paleoceanography 5407 planetary sciences atmospheres evolution 5470 planetary sciences surface materials and properties 1635 global change oceans 4203 keywords mars oceans channels shorelines hesperian

mountain ranges climate and weathering do orogens strengthen or weaken the silicate weathering carbon sink abstract the role of mountains in the geological evolution of the carbon cycle has been intensively debated for the last decades mountains are thought to increase the local physical erosion which in turns promotes silicate weathering organic carbon transport and burial and release of sulfuric acid by dissolution of sulfides in this contribution we explore the impact of mountain ranges on silicate weathering mountains modify the global pattern of atmospheric circulation as well as the local erosion conditions using an ipcc class climate model we first estimate the climatic impact of mountains by comparing the present day climate with the climate when all the continents are assumed to be flat we then use these climate output to calculate weathering changes when mountains are present or absent using standard expression for physical erosion and a 1d vertical model for rock weathering we found that large scale climate changes and enhanced rock supply by erosion due to mountain uplift have opposite effect with similar orders of magnitude a thorough testing of the weathering model parameters by data model comparison shows that best fit parameterizations lead to a decrease of weathering rate in the absence of mountain by about 20 however we demonstrate that solutions predicting an increase in weathering in the absence of mountain cannot be excluded a clear discrimination between the solutions predicting an increase or a decrease in global weathering is pending on the improvement of the existing global databases for silicate weathering nevertheless imposing a constant and homogeneous erosion rate for models without relief we found that weathering decrease becomes unequivocal for very low erosion rates below 10 t km2 yr we conclude that further monitoring of continental silicate weathering should be performed with a spatial distribution allowing to discriminate between the various continental landscapes mountains plains

carbon isotope δ13ccarb stratigraphy of the lower middle ordovician tremadocian darriwilian in the great basin western united states implications for global correlation abstract new stable carbon isotope data 13ccarb from lower middle ordovician tremadocian to darriwilian carbonate mudstone and wackestone rocks of the pogonip group are presented from two sections in the great basin region usa shingle pass east central nevada and the ibex area western utah the pogonip group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the late cambrian furongian to middle ordovician darriwilian the shingle pass and ibex area sections have been previously studied for their conodont biostratigraphy and contain a north american midcontinent conodont fauna that range from the cordylodus intermedius zone uppermost cambrian to the phragmodus polonicus zone darriwilian the 13c trend has four distinct characteristics recognized in both great basin sections 1 a drop in 13c from 1 at the base of the ordovician tremadocian to 0 7 2 a 1 to 2 positive 13c shift in the uppermost rossodus manitouensis zone during the late tremadocian 3 a gradual 13c increase from 2 to ca 0 during the end of the early ordovician floian and 4 a steady 13c decrease from 0 to 4 to 5 during middle ordovician dapingian darriwilian in the lower ordovician 13c trends reported here from the great basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies instead these lower ordovician isotope data most likely reflect primary seawater chemistry and changes in 13c on a global scale this interpretation is supported by the excellent correlation of 13c in the lower ordovician to other 13c trends reported from the sections in the argentine precordillera la silla and san juan formations and in western newfoundland st george and table head groups these correlations using 13c are consistent with published biostratigraphic data and provide an integrated and high resolution chemo biostratigraphic framework for the lower ordovician sedimentary record of the laurentian margin the middle ordovician portion of the 13c curves in the great basin represented by the kanosh and lehman formations shows significant isotopic depletion relative to the section in argentina thus although there is some indication that minima and maxima in the middle ordovician curves can be correlated the great basin sections show clear evidence of overprinting by local variables related to both diagenesis dolomitization and platform restriction

unexpected variability of martian hydrogen escape mars today is much drier than the earth though they likely began with similar relative amounts of water one potential cause for this discrepancy is hydrogen loss to space which may have removed a large fraction of mars initial water here we demonstrate an order of magnitude change in the martian hydrogen escape rate in 2007 inconsistent with established models for the source of escaping hydrogen we analyze 121 6 nm hydrogen lyman airglow observations made by the ultraviolet spectrometer on the mars express spacecraft over the second half of 2007 the enhanced escape rates we observe may be due to lower atmospheric heating and overturn during the 2007 mars year 28 global dust storm suggesting that hydrogen escape from mars during dust storms may dominate loss of the planet s water inventory this scenario has major implications for reconstructing the total amount of water lost to space over martian history

zonally symmetric response to atmospheric tidal forcing in the dusty martian atmosphere abstract we have computed a steady state zonally symmetric response of an atmospheric circulation to the combined efffects of the very large zonal mean diabatic heating and thermotidal forcing thought to exist in the dusty martian atmosphere during one of its episodic global dust storms the zonal mean components of the tidal flux convergences of momentum and heat are computed using an existing classical atmospheric tidal model constrained by the surface pressure observations at the two viking lander sites on mars the zonally symmetric response to the computed tidal flux convergences and to the zonal mean heating of the airborne dust is then computed by a nearly inviscid two dimensional 2 d model previously used to study the mars hadley circulation tidally induced easterly forces at southern summer solstice can extend the core of the descending branch of the zonally symmetric circulation from 50 to 55 n which is consistent with viking lander 2 surface pressure observations but not far enough polewa

the carbon isotope and sequence stratigraphic record of the sheinwoodian and lower homerian stages silurian of the midland platform uk abstract new high resolution 13 c carb isotope data are presented through the sheinwoodian and lower homerian stages of the wenlock series silurian from sections in shropshire herefordshire and the west midlands uk this data identifies the well known early sheinwoodian positive carbon isotope excursion escie the minor mid escie negative shift and an early homerian positive carbon isotope excursion ehcie in the region of 1 4 as developed upon the midland platform and in close proximity to the sheinwoodian and homerian gssps these carbon isotope excursions have been correlated against graptolite conodont and chitinozoan frameworks from across the midland platform and alongside details of sedimentology and faunas have allowed for the establishment of a sequence stratigraphic framework these integrated data sets reveal that the shallow water barr buildwas woolhope limestones reflect approximately synchronous deposition across the midland platform in that they all contain the escie and five widely traceable depositional cycles moreover biostratigraphic and sequence stratigraphic frameworks from across the midland platform avalonia confirm that both the escie and the five cycles are synchronous with records from other palaeocontinents baltica and laurentia and indicate that the early sheinwoodian graptolite zones as established in the type area are in need of review

investigating the role of the tibetan plateau in the formation of atlantic meridional overturning circulation abstractthe tibetan plateau tp over the eurasian continent has significant effects on both regional and global climate it can even affect the remote atlantic meridional overturning circulation

mountain uplift and surface temperature changes mountain uplift significantly affects both sea surface temperature sst and land surface temperature this was studied using a coupled atmosphere mixed layer ocean model with and without mountains the global mean sst dropped 1 4 c with mountain uplift mainly due to increased lower tropospheric clouds in the subtropical eastern pacific increased clouds hinder solar radiation into the ocean and lower sst the increased frequency of stratus incidence is related to subtropical anticyclones intensified by a strong temperature contrast between the continent and ocean in mountainous regions land surface temperature drops due to the lapse rate effect when this effect is eliminated the continental interior does not become as cool with mountain uplift because clouds become fewer and the surface drier due to decreased moisture transport southern asia becomes cooler because monsoon induced precipitation wets the ground and increases clouds our result showed greater northern high latitude temperature changes than the previous study indicating the importance of cloud related feedback in paleoclimate modeling

silurian carbonate high energy deposits of potential tsunami origin distinguishing lateral redeposition and time averaging using carbon isotope chemostratigraphy abstract stable carbon isotope curves are used as a precise stratigraphic tool in the paleozoic even though they are commonly based on shallow water carbonate record characterized by low stratigraphic completeness identification of episodes of large scale redeposition and erosion may improve 13ccarb based correlations here a series of at least three episodes of high energy onshore redeposition are described from the makarivka member new unit of the ustya formation from the homerian middle silurian of podolia ukraine the makarivka member is emplaced within a tidal flat succession its most prominent part is divided into a lower polymictic conglomerate of sand to boulder sized clasts representing a range of subtidal facies and an upper heterolithic unit composed of grainstone and mudstone laminae the aim of the study is to identify the mechanism of deposition of the allochthonous conglomeratic material in this member based on analogies with recent tsunami deposits the conglomerate is interpreted to reflect the strongest landward directed current in the tsunami run up phase and the heterolith alternating high density landward currents stagnant intervals allowing mud and land derived debris to settle and backwash flows the tsunamite was deposited during an interval of decreasing isotopic values of the mulde excursion a global 13c excursion reaching 5 2 in the studied sections clast redeposition in an interval characterized by rapidly changing 13ccarb offers the opportunity to evaluate the degree of temporal and spatial averaging caused by the tsunami the clasts in the polymictic conglomerate show scattered 13ccarb values 0 3 to 2 1 compared to homogenous 1 3 to 1 6 values in the matrix the presence of clasts characterized by low 13ccarb values is explained by their decrease with bathymetry rather than erosion of pre excursion strata whereas high values characterize material entrained from the sea floor and strata directly underlying the tsunamite close 1 3 and 1 5 average 13ccarb values suggest that the matrix of the conglomerate is potentially a product of clast grinding

is the gulf stream responsible for europe s mild winters is the transport of heat northward by the gulf stream and north atlantic drift and its subsequent release into the midlatitude westerlies the reason why europe s winters are so much milder than those of eastern north america and other places at the same latitude here it is shown that the principal cause of this temperature difference is advection by the mean winds south westerlies bring warm maritime air into europe and north westerlies bring frigid continental air into north eastern north america further analysis of the ocean surface heat budget shows that the majority of the heat released during winter from the ocean to the atmosphere is accounted for by the seasonal release of heat previously absorbed and not by ocean heat flux convergence therefore the existence of the winter temperature contrast between western europe and eastern north america does not require a dynamical ocean two experiments with an atmospheric general circulation model coupled to an ocean mixed layer confirm this conclusion the difference in winter temperatures across the north atlantic and the difference between western europe and western north america is essentially the same in these models whether or not the movement of heat by the ocean is accounted for in an additional experiment with no mountains the flow across the ocean is more zonal western europe is cooled the trough east of the rockies is weakened and the cold of north eastern north america is ameliorated in all experiments the west coast of europe is warmer than the west coast of north america at the same latitude whether or not ocean heat transport is accounted for in summary the deviations from zonal symmetry of winter temperatures in the northern hemisphere are fundamentally caused by the atmospheric circulation interacting with the oceanic mixed layer copyright 2002 royal meteorological society

improved general circulation models of the martian atmosphere from the surface to above 80 km we describe a set of two new generation general circulation models of the martian atmosphere derived from the models we originally developed in the early 1990s the two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations the vertical resolution near the surface has been refined and the vertical domain has been extended to above 80 km these changes are accompanied by the inclusion of state of the art parameterizations to better simulate the dynamical and physical processes near the surface boundary layer scheme subgrid scale topography parameterization etc and at high altitude gravity wave drag in addition radiative transfer calculations and the representation of polar processes have been significantly improved we present some examples of zonal mean fields from simulations using the model at several seasons one relatively novel aspect previously introduced by wilson 1997 is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi global angular momentum conserving hadley cell which has no terrestrial equivalent within such a cell the coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km this winter polar warming had been observed but not properly modeled until recently in fact thermal inversions are generally predicted above one and often both poles around 60 70 km however the mars middle atmosphere above 40 km is found to be very model sensitive and thus difficult to simulate accurately in the absence of observations

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

high resolution late silurian correlations between gotland sweden and the broken river region ne australia lithologies conodonts and isotopes high resolution correlations partly with a precision better than 10 ka are achieved between late ludfordian sequences on gotland sweden on baltica and a section cog through the coral gardens formation along the broken river northeastern australia on gondwana despite these sections having been on different palaeocontinents facing different oceans the interval is characterised by rapid very large faunal isotopic and lithologic changes lithologies are remarkably similar in the two areas consisting of in order from below marls with thin limestone beds flaggy limestones oncoidal crinoidal limestone oncoidal marls terrigenous clastics silty clay mudstone sandstone oolite and cliff gorge forming limestones further independent correlation dates several of the lithological changes as coeval the 813 c excursion in whole rock carbonates is one of the three largest ones known during the phanerozoic the increase in delta c 13 is very similar in the two areas from below 1 parts per thousand to c 9 parts per thousand vpdb though two gotland samples yielded 9 71 parts per thousand and 10 54 parts per thousand the corresponding cog interval included fewer carbonate layers than needed to definitely exclude that this difference is due to a lack of suitable rocks for sampling compared with the lithologies and the delta c 13 curves the conodont faunas display some divergence but key taxa permit precise correlations for much of the studied interval the conodont assemblages change stepwise from very diverse in the polygnathoides siluricus zone to a low diversity fauna dominated markedly by a single taxon upper icriodontid subzone returning abruptly to a comparatively diverse ozarkodina snajdri zone fauna a method resembling graphic correlation in some respects is used to propose a similarly high resolution correlation through a longer interval for future testing c 2006 elsevier b v all rights reserved less

testing the limits of paleozoic chronostratigraphic correlation via high resolution 500 k y integrated conodont graptolite and carbon isotope δ13ccarb biochemostratigraphy across the llandovery wenlock silurian boundary is a unified phanerozoic time scale achievable the resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration by virtue of deep ocean cores and astrochronology the cenozoic and mesozoic time scales carry error bars of a few thousand years k y to a few hundred k y in contrast most of the paleozoic time scale carries error bars of plus or minus a few million years m y and chronostratigraphic control better than 1 m y is considered high resolution the general lack of paleozoic abyssal sediments and paucity of orbitally tuned paleozoic data series combined with the relative incompleteness of the paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the paleozoic time scale to the level achieved during the mesozoic and cenozoic was viewed as impractical impossible or both here we utilize integrated graptolite conodont and carbonate carbon isotope delta c 13 carb data from three paleocontinents baltica avalonia and laurentia to demonstrate chronostratigraphic control for upper llandovery through middle wenlock telychiansheinwoodian similar to 436 426 ma strata with a resolution of a few hundred k y the interval surrounding the base of the wenlock series can now be correlated globally with precision approaching 100 k y but some intervals e g uppermost telychian and upper shein woodian are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and or extinction or carbon isotopic features to delineate such small time slices although producing such resolution during the paleozoic presents an array of challenges unique to the era we have begun to demonstrate that erecting a paleozoic time scale comparable to that of younger eras is achievable less

continental island from the upper silurian ludfordian stage of inner mongolia implications for eustasy and paleogeography an unconformity between the silurian xibiehe formation and ordovician igneous rocks marks the perimeter of a small paleoisland near bater obo in north central inner mongolia 180 km northwest of the provincial capital of hohhot the stratigraphic position of the lower part of the xibiehe formation is correlated by means of conodonts with the upper part of the ancoradella ploeckensis zone in the basal ludfordian stage corresponds to mid ludlovian epoch ca 421 ma elongate in plan 610 m 200 m the exhumed diorite core rises 30 m above the lowest elevations of surrounding silurian strata paleoshores along the principal axis of the inlier delineate contrasting facies robust stromatoporoids are in growth position within silty limestones some directly encrusting the unconformity surface of the sheltered southeast margin a basal conglomerate of diorite cobbles and boulders characterizes the high energy northwest margin the depositional constraints and timing of transgressive facies associated with this continental paleoisland have implications for the eustatic and paleogeographic history of the parent sino korean plate burial of the island corresponds to the beginning of a global rise in sea level that peaked in late ludlovian time our interpretation of windward and leeward facies requires an approximate 90 clockwise rotation of the parent plate to accommodate the dominant pattern of low latitude trade winds and storms

impact of mountains on tropical circulation in two earth system models abstracttwo state of the art earth system models esms were used in an idealized experiment to explore the role of mountains in shaping earth s climate system similar to previous studies removing mountains from both esms results in the winds becoming more zonal and weaker indian and asian monsoon circulations however there are also broad changes to the walker circulation and el nino southern oscillation enso without orography convection moves across the entire equatorial indo pacific basin on interannual time scales enso has a stronger amplitude lower frequency and increased regularity a wider equatorial wind zone and changes to equatorial wind stress curl result in a colder cold tongue and a steeper equatorial thermocline across the pacific basin during la nina years anomalies associated with enso warm events are larger without mountains and have greater impact on the mean tropical climate than when mountains are present without mountains the centennial mean pacific walker circulation wea

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

solar flux variability of mars exosphere densities and temperatures 1 using densities derived from precise orbit determination of the mars global surveyor mgs spacecraft from 1999 to mid 2005 the response of mars exosphere to long term solar change is established and compared to that of earth and venus at mars exosphere temperatures weighted towards high latitude southern hemisphere daytime conditions change only 36 50 as much as those at earth as solar activity increases from solar minimum to solar maximum whereas the response at venus is one fifth that at mars general circulation models suggest that this difference may be strongly influenced by adiabatic cooling associated with the thermosphere general circulation however other processes such as differences in co2 cooling rates may also be playing a role

a model for co isotopic signatures of evolving ground ice in the cold dry environments of earth and mars abstract the basics of the model rego are summarized including diffusive water vapor transport ice content adsorption and the isotopic ratios of the various water species inclusion of a temperature and stress strain solution is also described as are the effects of cracking when the tensile breaking strengths are exceeded cracking is viewed as a multi scale phenomena whereby the larger cracks are produced by chaining together smaller ones after some validation the model is applied to the upper university valley antarctic dry valleys data and to the phoenix landing site at lat 69n on mars for the university valley the model when forced by measured diurnal temperature cycles and known relative humidity reproduces some of the complex the ice content and isotope structure found in the upper university valley ground ice cores the model takes a few hundred years to fill the pores just under the ice table the excess ice observed in the university valley cores can be produced by thermal cracking combined with the diurnal temperature cycle time taken to achieve the observed excess ice content is of order 10 4 years there is limited subsurface data for the phoenix site but there are measured ice table depths surface thermal properties and met variables there is enough to do diurnal and seasonal runs of the rego model as with the university valley the upper few tens of centimeters icing are controlled by the diurnal temperatures largely in the warmer part of the year the observed ubiquitous pore full ice right at the ice table could be produced from a dry soil in about 10 3 a and the nearly pure ice patches could be produced by the cracking diffusion mechanism in about million years if shear stresses are included emplacement of ice down to 10 m is possible using the seasonal temperature cycle and cracking excess ice in the upper meter could produced by the model in about 6 10 6 a if the temperature at the phoenix site was 10 20 deg warmer

investigating the role of the tibetan plateau in enso variability abstractthe role of the tibetan plateau tp in el nino southern oscillation enso variability is investigated using coupled model experiments with different topography setups removing the tp res

early mars serpentinization derived ch4 reservoirs h2 induced warming and paleopressure evolution ch4 has been observed on mars both by remote sensing and in situ during the past 15 yr it could have been produced by early mars serpentinization processes that could also explain the observed martian remanent magnetic field assuming a cold early mars a cryosphere could trap such ch4 as clathrates in stable form at depth the maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 x 10 19 to 2 x 10 20 moles of methane we estimate how large amounts of serpentinization derived ch4 stored in the cryosphere have been released into the atmosphere during the noachian and the early hesperian due to rapid clathrate dissociation and photochemical conversion of ch4 to h 2 these episodes of massive ch4 release may have resulted in transient h 2 rich atmospheres at typical levels of 10 20 in a background 1 2 bar co2 atmosphere the collision induced heating effect of h 2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point we show how local and rapid destabilization of the cryosphere can be induced by large events such as the hellas basin or tharsis bulge formation and lead to such releases our results show that the early mars cryosphere had a sufficient ch4 storage capacity to have maintained h 2 rich transient atmospheres during a total time period up to several million years or tens of million years having potentially contributed to the formation of valley networks during the noachian early hesperian

impact of north atlantic freshwater forcing on the pacific meridional overturning circulation under glacial and interglacial conditions abstractthe pacific meridional overturning circulation pmoc is not well known compared to the atlantic meridional overturning circulation amoc due to its absence today however considering pm

evolution of permian conodont provincialism and its significance in global correlation and paleoclimate implication the main components of asselian through artinskian conodont faunas found around the world are basically the same and the provincialism is indicated only by less common endemic elements such as gondolelloides and new genus a henderson in north pangea sweetognathus bucaramangus around the equator and vjalovognathus in eastern gondwana provincialism is marked by differences at the species level of mesogondolella neostreptognathodus and sweetognathus during the kungurian and becomes very distinct with differences at the genus level during the guadalupian and lopingian three provinces of permian conodonts referred to as the north cool water province ncwp the equatorial warm water province ewwp and the peri gondwana cool water province gcwp are recognized the ncwp is marked by gondolelloides in the early cisuralian dominance of neostreptognathodus and no or rare sweetognathus in the late cisuralian dominance of merrillina and mesogondolella and absence of sweetognathus in the guadalupian and dominance of merrillina and mesogondolella and absence of iranognathus in the lopingian the ewwp is characterized by the absence of gondolelloides and vjalovognathus in the cisuralian abundance of sweetognathus and pseudosweetognathus in the kungurian late cisuralian jinogondolella and sweetognathus in the guadalupian and clarkina and iranognathus in the lopingian the gcwp is marked by vjalovognathus merrillina in the cisuralian vjalovognathus merrillina and mesogondolella in the guadalupian and vjalovognathus and merrillina in the lopingian mixed faunas are recognized in regions bordering between the ewwp and gcwp including western timor during the artinskian pamirs during the kungurian and the salt range during the guadalupian and lopingian three different conodont zonations are proposed one for each of the three conodont provinces four potential horizons for inter provincial correlation of permian conodonts are recognized they are in ascending order 1 the first appearance of sweetognathus whitei which is closely related to the last occurrence of carboniferous type conodonts such as streptognathodus and adetognathus 2 the first appearance of neostreptognathodus pequopensis 3 the base of the jinogondolella nankingensis zone and 4 the base of the clarkina postbitteri iranognathus erwini zone the spatial and temporal distribution pattern of permian conodonts suggests that temperature is the primary controlling factor evolution of permian conodont provincialism reveals a glaciation during the asselian and sakmarian a global warming during the artinskian a climate cooling in north pangea during the kungurian a continuation of kungurian climate trends in the guadalupian a relatively minor warming during the wuchiapingian a returned cooling in the changhsingian and lower and middle griesbachian and a global warming in the late griesbachian which ended the permian conodont lineage

middle palaeozoic extinction events faunal and isotopic data abstract at least 9 and possibly as many as 12 extinction events of global or near global impact can be discriminated in the mid palaeozoic earliest silurian to early carboniferous on the basis of brachiopod coral conodont and ammonoid data and the history of carbonate build ups isotopic data from whole rock samples are presented for three of these events based on australian carbonate sequences constrained by conodont data these data represent the initial phase of a more extensive investigation using c o s sr and nd isotopic signatures derived from conodonts articulate and inarticulate brachiopod shell and fish remains from numerous australian and european sequences the aim of the project is to identify isotopic responses to extinction events and address causes for these changes in limestone sections analysed so far variations in carbon isotope compositions on a whole rock scale are most marked at horizons that can be correlated with times of significant reduction in biomass and diversity this is despite the fact that the whole rocks are in fact multicomponent systems with respect to carbon in part arising from diagenetic rearrangements of carbon distribution within the scale of the whole rock thus the carbon isotope data do provide evidence on a whole rock scale for fundamental changes in the global carbon cycle that are correlative with extinction events it is however unlikely that the magnitude of the isotopic shift will be precisely documented from such whole rock analyses the oxygen isotope results are less obviously related to the times of carbon isotope excursion this contrast in apparent resilience to post depositional modification of pristine isotope compositions exemplifies the extreme caution needed in evaluating isotopic data from carbonates

high resolution event stratigraphy hires and the quantification of stratigraphic uncertainty silurian examples of the quest for precision in stratigraphy abstract the ability to resolve the stratigraphic record of earth history is critical to understanding past global change events and the range of natural variability within the global paleoclimate the geosphere hydrosphere atmosphere biosphere system operates on geologically very short time scales and we must be able to resolve the stratigraphic record of past events on similar time scales if we are to understand the cause and effect relationships operating within the system global resolution 100 kyr is commonly available when evaluating events from the cenozoic era but such resolution is less common for older portions of earth history the introduction of high resolution event stratigraphy hires more than two decades ago revolutionized the ability to resolve the mesozoic and cenozoic stratigraphic records of global change and a similar revolution is currently underway for the paleozoic as well here we review the application of the principles of hires to the paleozoic stratigraphic record with examples from the silurian system discuss the quantification of chronostratigraphic uncertainty and highlight the current deep time earth history revolution that has the potential to open the entire phanerozoic record to the evaluation of short time scale global climate dynamics

the atlantic pacific seesaw abstract a global oceanic teleconnection of salinity meridional overturning circulation moc and climate of the north atlantic and north pacific is proposed simulations with a global climate model show that an extraction of freshwater from the pacific results not only in an increase of salinity there but also in a decrease of salinity in the atlantic as a result a pacific moc develops while the atlantic moc collapses without freshwater perturbation in the atlantic similarly an input of freshwater to the atlantic leads not only to a decrease of salinity there but also to an increase of salinity in the pacific the atlantic moc collapses whereas the pacific moc develops without freshwater perturbation in the pacific the mechanism behind this antiphase atlantic pacific relationship is the positive feedback between ocean circulation and salinity contrasts originally proposed by stommel to operate between low and high latitudes here the authors show that the same mechanism operates on the atlan

late early silurian wenlockian general circulation model generated upwelling graptolitic black shales and organic rich source rocks an accident of plate tectonics through an evolutionary process of plate tectonics the wenlockian physical world was composed of an oceanic northern hemisphere and a mainly continental southern hemisphere dominated by the giant continent of gondwana the high latitude position of gondwana placed much of its extensive margin in the middle latitudes laurentia and baltica occupied an equatorial position siberia and kazakh were to the north the global paleoclimate was created and driven by the diverse paleogeography of the two hemispheres in the model the northern hemisphere is dominated by strong zonality in all seasons in contrast the continental southern hemisphere reacts to wide temperature ranges of summer heating and winter cooling of gondwana the extensively flooded 28000 km margin of gondwana occupied the zone of the southern hemisphere westerlies these westerlies produced intense surface upwelling and by extension of present analogies high primary biologic productivity gondwana s northern margin extended far enough toward the equator that significant winter sea ice did not form in the southern hemisphere this allowed a vertically stratified sea to develop as well as probable dysaerobic and anoxic conditions a high organic flux of marine plankton accumulated on the sea floor where it was subsequently buried and preserved a strong positive correlation exists between upwelling generated by a general circulation model gcm on gondwana s northern margin and the distribution of southern hemisphere basins with source rocks containing types i and ii kerogens

ocean and atmosphere feedbacks affecting amoc hysteresis in a gcm theories suggest that the atlantic meridional overturning circulation amoc can exhibit a hysteresis where for a given input of fresh water into the north atlantic there are two possible states one with a strong overturning in the north atlantic on and the other with a reverse atlantic cell off a previous study showed hysteresis of the amoc for the first time in a coupled general circulation model hawkins et al in geophys res lett doi 10 1029 2011gl047208 2011 in this study we show that the hysteresis found by hawkins et al 2011 is sensitive to the method with which the fresh water input is compensated if this compensation is applied throughout the volume of the global ocean rather than at the surface the region of hysteresis is narrower and the off states are very different when the compensation is applied at the surface a strong pacific overturning cell and a strong atlantic reverse cell develops when the compensation is applied throughout the volume there is little change in the pacific and only a weak atlantic reverse cell develops we investigate the mechanisms behind the transitions between the on and off states in the two experiments and find that the difference in hysteresis is due to the different off states we find that the development of the pacific overturning cell results in greater atmospheric moisture transport into the north atlantic and also is likely responsible for a stronger atlantic reverse cell these both act to stabilize the off state of the atlantic overturning

orographic control of storm zones on mars low pressure cyclones and their accompanying frontal systems in the middle latitudes of the earth s troposphere develop travel eastwards and decay preferentially within latitudinally and longitudinally confined geographical regions known as storm zones1 these zones can be readily identified in the circulation statistics of terrestrial weather systems2 mars like the earth is a rapidly rotating solid planet and has a seasonally varying shallow atmosphere large scale orography and in a broadly defined context continental structures3 although there are also important differences between the two planets travelling weather systems do exist on mars3 this raises the question of whether storm zones also occur there and if so by what mechanisms here we report the results of numerical simulations of global atmospheric circulation patterns on mars we find that storm zones can exist during the northern winter and that continental scale orography rather than surface thermal contrasts is the main factor determining the development of these zones storm zones on mars should play an important role in the martian climate cycle4 5 by influencing the transport of for example heat momentum water vapour and atmospheric dust3 6 8 towards the poles

ocean gyres and abrupt change in the thermohaline circulation a conceptual analysis the implications are investigated of representing ocean gyre circulations by a diffusion term in the stommel and rooth box models of the thermohaline circulation thc in one and two hemispheres respectively the approach includes mostly analytical solution and study of the bifurcation structure but also numerical integration and feedback analysis sufficient diffusion gyre strength eliminates multiple equilibria from either model highlighting the need for accurate gyre circulation strength in general circulation models gcms when considering the potential for abrupt climate change associated with thc shutdown with diffusion steady state flow strength in the rooth model depends on freshwater forcing i e implied atmospheric water vapor transport in both hemispheres not only on that in the upwelling hemisphere as in the nondiffusive case with asymmetric freshwater forcing two solutions strong stable and weak unstable are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle node bifurcation if diffusion is sufficiently strong to prevent a subcritical hopf bifurcation first followed by flow reversal thus the bifurcation structure with respect to freshwater forcing of the diffusive rooth model of two hemisphere thc is similar to that of the stommel model of single hemisphere thc albeit with a very different dynamical interpretation gyre circulations stabilize high latitude sinking in the stommel model in the rooth model gyre circulations only stabilize highlatitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere by an amount that increases with diffusion the values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with gcms suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric thc and also for the potential of the atlantic thc to undergo abrupt change

gssps global stratigraphy and correlation procedures used to define an international chronostratigraphic stage boundary and to locate and ratify a global boundary stratotype section and point gssp are outlined a majority of current gssps use biostratigraphic data as primary markers with no reference to any physico chemical markers despite the international subcommission on stratigraphic classification issc suggestion that such markers should be included if possible it is argued that such definitions will not produce the high precision phanerozoic time scale necessary to understand such phenomena as pre pleistocene ice ages and global climate change it is strongly recommended that all gssps should have physico chemical markers as an integral part of their guiding criteria and where such markers cannot be found the gssp should be relocated the methods and approach embodied in oceanic stratigraphy coring logging analysing and archiving of drill sites by numerous experts using a wide range of methods could usefully serve as a scientific model for the analysis and archiving of gssps all of which are on the present day continents the incorporation of many more stratigraphic sections into gssp studies the application of physico chemical methods and the replacement of old u pb dates by newer ca tims u pb dates together with the use of constrained optimization conop programs that produce a calendar of events from many sections should lead to much more precise timescales for pre cenozoic time than are currently available

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

the silurian of gotland sweden facies interpretation based on stable isotopes in brachiopod shells the silurian of gotland sweden consists of 440 m of carbonate deposits repeatedly uniform sequences of micritic limestones and marls are interrupted by complex structured reefs and by adjacent platform sediments generally the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the purpose of this study is a facies interpretation based on both field observations and stable isotope measurements of brachiopod shells approximately 700 samples from stratigraphically arranged localities in different facies areas have been investigated the carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic sequence lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than to paleotemperature carbon isotope ratios are believed to have been connected to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions consequently the facies succession on gotland results from global paleoclimatic conditions changes in terrigenous input due to different rates of weathering and freshwater runoff rather than sea level fluctuations control the carbonate formation of the silurian on gotland

studies of the 2018 mars year 34 planet encircling dust storm mars planet encircling or global dust storms are an iconic and enigmatic feature of the red planet occurring every few mars years on average they are a stochastic process in the otherwise largel

carbon and oxygen isotopic composition of silurian brachiopods gotland sweden palaeoceanographic implications abstract one hundred and seventy two non luminescent brachiopod shells from the silurian sequence of gotland were investigated for their carbon and oxygen isotopic composition diagenetic alteration of the shell calcite was excluded by means of petrographic trace element and cathodoluminescence analysis oxygen and carbon isotopic signals vary considerably within the uppermost llandovery to the ludlow average 18o and 13c values of well defined stratigraphic units range from 2 4 to 5 9 and 1 to 8 respectively we observed parallel trends in the carbon and oxygen isotope curves with the most prominent shifts occurring in the lower wenlock upper wenlock and middle ludlow average 18o values 2 4 to 2 9 recorded in the middle ludlow are among the highest 18o values reported for the early palaeozoic the recorded 18o variations cannot solely be explained by changes in sea surface temperature and salinity the variations in the isotopic signals correlate negatively with changes in sea level heavy carbon and oxygen signals coincide with sea level lowstands whereas light signals are recorded during sea level highstands sea level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves inducing oxygen deficiency in the basins alumshale facies the low 18o values are attributed to 18o depletion in surface waters owing to the storage of 18o enriched saline waters in the basins low 13c values are explained by a low surface water productivity since nutrients were not supplied to the photic zone but rather stored in the deeper ocean during sea level lowstands colder waters that formed in higher latitudes may have represented the main intermediate water source a better ventilation of the basins is documented by the deposition of well aerated sediments whereas the high 18o values are explained by the change in oceanic circulation the high 13c values are attributed to an enhanced primary productivity and possibly to an enhanced burial of corg the model discussed here may provide an attractive scenario for accounting for the recorded short term variations in the isotopic signals without the necessity of introducing an isotopic correction factor for silurian sea water

mars volatile evolution implications of the recent measurement of 17o in water from the snc meteorites escape of oxygen to space on mars is capable of producing a fractionation of oxygen isotopes in the remaining oxygen which is off of the fractionation line produced by the usual mass dependent exchange processes the magnitude of fractionation which can occur can be larger than that which was measured in oxygen in water derived from the snc meteorites thought to have come from mars in conjunction with the apparent lack of fractionation of 18o in the atmosphere this result suggests that less than 25 of the oxygen in the martian climate system has been lost over geologic time other mechanisms also might be able to produce such a fractionation so that this conclusion is not unique however it is consistent with other lines of evidence there is no requirement for an initial volatile inventory with an isotopic composition different from that of the bulk planet as might be the case for a late veneer of volatile rich material

u pb zircon age constraints on the timing and duration of wenlock silurian paleocommunity collapse and recovery during the big crisis high precision isotope dilution u pb zircon dating was conducted on three volcanic ash fall bentonite samples from the swedish island of gotland and on a fourth bentonite from the west midlands england zircons from the ireviken grotlingbo djupvik gotland and wren s nest hill 15 west midlands bentonites yielded weighted mean pb 206 u 238 ages of 431 83 0 23 0 67 ma 428 45 035 0 73 ma 428 06 0 2110 66 ma and 427 86 032 0 71 ma respectively analytical total uncertainties these biostratigraphically well controlled age dates effectively bracket the wenlock epoch of the silurian period and provide control for the duration of one of the major paleozoic biotic events and associated perturbations to the global carbon cycle the big crisis or lundgreni event graptolites the niulde event conodonts the mulde excursion carbon isotopes these new data suggest an older and shorter duration for the recalibration of the wenlock series and demonstrate that the cascade of biological and chemical events that took place during the big crisis happened on time scales of tens to hundreds of thousands of years less

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

stable carbon isotope stratigraphy in the ordovician of south china abstract about 500 samples from 11 ordovician sections in south china have been analysed for their 13 c carb composition four sections are located in the yangtze platform region the remaining seven sections in the jiangnan slope region the sections span more or less the entire ordovician but the sample resolution in its lower part tremadocian to dapingian is lower than in its upper part darriwilian to hirnantian overall the data show a good correlation with the published global isotope curve at the cambrian ordovician boundary positive values are observed in the succeeding tremadocian the values decrease and remain negative into the early floian in the late floian the values increase to slightly positive the correlation of dapingian rocks between the yangtze and the jiangnan regions is somewhat problematic which might be due to high siliciclastic input in the yangtze section in the darriwilian a small positive excursion is observed in the yangtze platform area however it is missing in the jiangnan slope area this excursion probably correlates with the mid darriwilian excursion mdice reported from baltoscandia both chemostratigraphy and conodont biostratigraphy indicate a possible gap between the shihtzupu formation and the pagoda formation i e most of the sandbian is probably not represented in the upper yangtze region the earliest katian is characterised by a pronounced positive excursion correlating with the well known global guttenberg excursion gice in the late katian at least one additional positive excursion was determined the precise biostratigraphy however is not clear and therefore it is not known which of the known four to five remaining katian 13 c excursions identified in baltica correlate s with the one s observed in the present study and further biostratigraphic study is required the lower part of the hirnantian excursion hice is probably not preserved with only small remnants of the hice observed near the base of the kuanyinchiao bed

properties of plasma waves observed upstream from mars we describe a new method to analyze the properties of plasma waves and apply it to observations made upstream from mars by the mars atmosphere and volatile evolution maven mission the slow meas

sequences cycles and basin dynamics in the silurian of the appalachian foreland basin abstract field stratigraphic analysis of the early and medial silurian strata of ontario new york and pennsylvania permits recognition of six major sequences reflecting 3rd order cycles in the niagaran llandoverian ludlovian of new york and adjacent regions these sequences correspond approximately to the medina lower clinton middle clinton upper clinton two sequences and lockport groups each is bounded by widespread unconformities produced by the interplay of eustatic sea level drop and local tectonic uplift in general erosional sequence boundaries and transgressive surfaces are merged with no record of the lowstand wedge each sequence is subdivisible into two to five sub sequences that are marked by sharp though generally non erosional basal discontinuities investigation of the clinton and lockport groups corroborates earlier suggestions that carbonate and siliciclassic tongues extended basinward from opposite margins synchronously in response to sea level drops analysis of the thickness and basinward extent of the progradational tongues reveals the existence of multi scale sedimentary cycles sequences and component sub sequences are correlative basinwide and are synchronous within the resolution of bio and event stratigraphy major drops in relative sea level are represented by the sequence bounding discontinutiies these surfaces are overlain by siliciclastic wedges near terrigenous source areas or winnowed carbonate pack and grainstones on the tectonically passive margin overlying highstand deposits typically shales calcareous mudstones and limestones are separated from lowstand or transgressive deposits by thin condensed intervals commonly marked by phosphatic or glauconitic material and or surfaces of maximum sediment starvation the relative highstand deposits of sequences and sub sequences are developed in relatively deep water facies and are at most slightly progradational hence most sequences and sub sequences are sharply bounded roughly upward deepening to slightly shallowing successions of strata in turn sub sequences are further divisible into widespread minor parasequences and parasequence sets probably corresponding to 6th and 5th order cycles respectively the smaller scale units 0 2 5 mthick commonly display an upward shallowing motif the nested relationship of these cycles supports the hierarchical model of transgressive regressive allocycles proposed by busch and rollins 1984 niagaran cycles discussed herein are equivalent to third fourth and fifth order cycles and are traceable circumbasinally depocenters of successive sequences and sub sequences display a pattern of eastward westward eastward basin axis migration through the silurian during early llandoverian the taconic queenston deltaic complex began to subside and the basin axis depocenter and eastern shoreline shifted eastward approximately 200 km over a 6 million year period as the basin axis migrated eastward the western basin ramp forebulge was repeatedly upwarded and eroded resulting in the generation of regionally angular unconformities at the bases of sequences ii iv and vi the pattern of unconformity greatest to the west suggests rise of the algonquin arch concomitant with eastward basin axis migration during the late llandoverian to early wenlockian the algonquin arch subsided slightly concurrently with a reversal in direction of basin axis migration during deposition of upper clinton and lockport vernon strata sequences v vi the basin axis shifted approximately 300 km back to the west eastward migration of the basin axis corresponds with a time of tectonic quiescence and probable thrust load relaxation the abrupt reversal of migration in the late llandoverian may be a signal of renewed thrusting in the hinterland at the onset of the salinic disturbance

paleobiogeography high resolution stratigraphy and the future of paleozoic biostratigraphy fine scale diachroneity of the wenlock silurian conodont kockelella walliseri the wenlock epoch of the silurian period has become one of the chronostratigraphically best constrained intervals of the paleozoic the integration of multiple chronostratigraphic tools such as conodont and graptolite biostratigraphy sequence stratigraphy and delta c 13 carb chemostratigraphy has greatly improved global chronostratigraphic correlation and portions of the wenlock can now be correlated with precision better than 100 kyr additionally such detailed and integrated chronostratigraphy provides an opportunity to evaluate the fidelity of individual chronostratigraphic tools here we use conodont biostratigraphy sequence stratigraphy and carbon isotope delta c 13 carb chemostratigraphy to demonstrate that the conodont kockelella walliseri an important guide fossil for middle and upper sheinwoodian strata lower stage of the wenlock series first appears at least one full stratigraphic sequence lower in laurentia than in baltica rather than serving as a demonstration of the unreliability of conodont biostratigraphy this example serves to demonstrate the promise of high resolution paleozoic stratigraphy the temporal difference between the two first occurrences was likely less than 1 million years and although it is conceptually understood that speciation and colonization must have been non instantaneous events paleozoic paleobiogeographic variability on such short timescales tens to hundreds of kyr traditionally has been ignored or considered to be of little practical importance the expansion of high resolution paleozoic stratigraphy in the future will require robust biostratigraphic zonations that embrace the integration of multiple chronostratigraphic tools as well as the paleobiogeographic variability in ranges that they will inevitably demonstrate in addition a better understanding of the paleobiogeographic migration histories of marine organisms will provide a unique tool for future paleozoic paleoceanography and paleobiology research c 2010 elsevier b v all rights reserved less

integrated δ13ccarb conodont and graptolite biochemostratigraphy of the silurian from the illinois basin and stratigraphic revision of the bainbridge group

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

deuteronomy a puzzle of deuterium and oxygen on mars mars is covered by channels that were likely formed by liquid water sometime in its past among the very few available clues to a wetter and warmer ancient climate on mars are the isotopic signatures left by processes that have modified its atmosphere over time in their research commentary yung and kass discuss two sets of results in the same issue krasnopolsky et al have used the hubble space telescope to detect the presence of deuterium in the upper atmosphere of mars and farquhar et al report measurements of the martian meteorite alh84001 showing that the oxygen isotopes in the carbonate are fractionated with respect to silicate minerals the findings suggest the presence of chemical processes operating in mars atmosphere that have yet to be fully understood

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events

aeronian llandovery silurian conodonts from the densmore creek phosphate bed and the budd road phosphate bed clinton group western new york state conodonts from the densmore creek phosphate bed at the base of the maplewood shale in irondequoit new york include panderodus unicostatus pranognathus tenuis and icriodella discreta the taxa ozarkodina cf oz hassi pranognathus tenuis oulodus panuarensis panderodus unicostatus panderodus recurvatus and pseudooneotodus beckmanni were recovered from the overlying budd road phosphate bed at the base of the reynales limestone near lockport new york the specimens from both beds are indicative of the pranognathus tenuis zone which correlates to the lower to middle aeronian and indicate that the budd road phosphate bed at lockport was deposited in a more offshore environment than the densmore creek phosphate bed at irondequoit

the lower williamson shale silurian of new york a biostratigraphical enigma the lower williamson shale at tryon park rochester new york state is unique in the co occurrence of stimulograptus clintonensis hall previously recorded only from the middle telychian and pterospathodus amorphognathoides amorphognathoides walliser previously recorded only from the upper telychian cyrtograptus lapworthi biozone and higher these two co occurring fossils are clearly providing contradictory ages for the lower williamson shale the incorporation of chitinozoan data from both the williamson and willowvale shales does not support unequivocally either the graptolite or conodont derived biostratigraphy and thus the age of the lower williamson shale remains problematical