late ordovician turinian chatfieldian carbon isotope excursions and their stratigraphic and paleoceanographic significance five positive carbon isotope excursions are reported from platteville decorah strata in the upper mississippi valley all occur in subtidal carbonate strata and are recognized in the mifflin grand detour quimbys mill spechts ferry and guttenberg intervals the positive carbon isotope excursions are developed in a platteville decorah succession in which background 13c values increase upward from about 2 at the base to about 0 vienna pee dee belemnite vpdb at the top a regional north south 13c gradient with lighter values to the north and heavier values to the south is also noted peak excursion 13c values of up to 2 75 are reported from the quimbys mill excursion and up to 2 6 from the guttenberg excursion although there are considerable local changes in the magnitudes of these events the quimbys mill spechts ferry and guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities and completely starve out in deeper more offshore areas closely spaced chemostratigraphic profiles of these sculpted pyrite impregnated hardground surfaces show that they are associated with very abrupt centimeter scale negative 13c shifts of up to several per mil possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events

high resolution surface and subsurface sequence stratigraphy of late middle to late ordovician late mohawkian cincinnatian foreland basin rocks kentucky and virginia the late middle to late ordovician late mohawkian to cincinnatian supersequence in kentucky and virginia was deposited in a tectonically active foreland basin during a transition from the ice free early ordovician to the glacial late ordovician this sequence was deposited in less than 12 m y is 250 500 m thick and is composed of four large third order sequences each 40 150 m thick that are regionally correlative in outcrops cores and gamma ray logs smaller scale third order parasequence sets up to 20 m thick and component parasequences 1 8 m thick make up the larger sequences and are only locally correlative subtidal dominated parasequences comprise the basal part of each sequence whereas shallower subtidal or peritidal dominated parasequences compose upper parts of sequences each large third order sequence is asymmetric and marked by lowstand transgressive and highstand systems tracts with unique lithologic and gamma ray log response characteristics ramp margin wedges rmw are poorly developed and consist of marine siltstone or grainstone packstone sheets extending into deep ramp settings these sheets have high gamma ray values in their base and lower blockier gamma ray responses in their tops transgressive systems tracts tst are thin composed of high energy locally phosphatic grainstone copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 7 1996 revised manuscript received may 27 1997 final acceptance june 16 1997 2department of geology virginia tech blacksburg virginia 24061 0420 present address department of earth atmospheric and planetary sciences massachusetts institute of technology cambridge massachusetts 02139 3department of geology virginia tech blacksburg virginia 24061 0420 this study was supported by nsf grant ear 9316057 to j f read a grant from mobil oil company and grants in aid of research from the geological society of america and appalachian basin industrial associates to mike pope we thank john haynes brian keith and rick major for thorough and insightful reviews of an earlier version of this manuscript

distal orogenic effects on peripheral bulge sedimentation middle and upper ordovician of the nashville dome abstract a major switch in depositional style in the ordovician carbonates of the nashville dome corresponds closely with the onset of the late middle ordovician taconic orogeny this time marks a shift from tropical type to temperate type carbonates the initiation of widespread major phosphate deposition the introduction of large amounts of terrigenous silt and clay the occurrence of widespread seismically induced soft sediment deformation and a change from a low energy flat topped carbonate shelf to a high energy doubly dipping carbonate ramp soft sediment deformation and the introduction of siliciclastics are direct effects of the taconic orogeny the switch from tropical type to temperate type carbonates the initiation of phosphate deposition and the switch in carbonate ramp are lar ely oceanographic effects triggered by the orogeny in particular phosphate deposition and the switch to temperate type limestones appears to have been driven by upwelling along the eastern side of the nashville dome within the newly deepened taconic foreland basin a fourfold decrease in the rate of relative sea level rise occurred on the nashville dome nearly 3 m y following the onset of thrusting and foreland basin initiation subsidence rates were constant before and after this decrease and no evidence of a change in subsidence rates is seen to coincide with the onset of thrusting the slowing of subsidence may reflect viscoelastic uplift of the nashville dome but the abrupt change from one constant subsidence rate to another is not predicted by existing foreland basin models

global upper ordovician correlation by means of δ13c chemostratigraphy implications of the discovery of the guttenberg δ13c excursion gice in malaysia apart from a single study of the early katian 13 c chemostratigraphy in two regions in china no investigations of the sandbian and katian chemostratigraphy have been published from anywhere in asia a recent study of the conodont biostratigraphy of the classical ordovician succession on langkawi islands peninsular malaysia showed the presence there of strata coeval with those having the guttenberg carbon excursion gice on the yangtze platform in an effort to establish for the first time the presence of this widespread 13 c excursion in southern asia a series of samples from the upper part of the kaki bukit formation was isotopically analysed this resulted in the discovery of a conspicuous 13 c excursion with peak values of 2 above the baseline values the excursion is located just above the baltoniodus alobatus subzone and near the level of the first appearance of hamarodus europaeus hence the same stratigraphic position as the gice on the yangtze platform using the gice the malaysian study interval is closely correlated with the gice intervals at three localities representing an approximately 23 000 km long transect from malaysia across baltoscandia to central north america this shows the usefulness of 13 c chemostratigraphy to clarify previously obscure stratigraphic relationships between geographically very widely separated localities

evidence of a tectonically driven sequence succession in the middle ordovician taconic foredeep the late mohawkian through early cincinnatian ordovician trenton group carbonates and coeval siliciclastic succession of the central mohawk valley new york state were deposited in a foreland basin during the taconic orogeny development of this facies mosaic records changes in stacking patterns that took place during episodic basin deepening and simultaneous carbonate ramp shallowing along a ramp to basin transect located on the distal slope of the foreland basin differential rates of subsidence across the region produced accommodation rates that increased by an order of magnitude from west to east high rates of siliciclastic sediment supply and even higher subsidence rates resulted in a thick deep water succession in the eastern part of the study region the coeval trenton group succession in the west formed under conditions of moderate sediment supply and lower subsidence rate resulting in a net shallowing upward succession these relationships suggest that active margin tectonism was the proximate cause of the mohawk valley facies architecture

apatite phenocryst compositions demonstrate a miscorrelation between the millbrig and kinnekulle k bentonites of north america and scandinavia the ordovician millbrig k bentonite of north america and the kinnekulle big bed of scandinavia have been considered correlative thus representing one of the largest volcanic eruptions of the phanerozoic whether these two k bentonites correlate has implications for the paleogeographic reconstructions of laurentia and baltica transoceanic biostratigraphic correlations of graptolites and conodonts and the possibly global occurrence of the guttenberg carbon isotope excursion trace element analyses of apatite phenocrysts have proven to be good discriminators for other ordovician k bentonites and it is shown here on the basis of significantly different mg cl mn fe ce and y concentrations in apatite phenocrysts from the kinnekulle and millbrig k bentonites that the beds must be viewed as representing two or more different eruptions vertical subsampling of several layers from each bed suggests that both beds are likely composed of multiple components but none of the components coincide with one another these results indicate that the millbrig and kinnekulle beds cannot both be derived from a single ultra plinian eruption as previously proposed

utility and limitations of apatite phenocryst chemistry for continent scale correlation of ordovician k bentonites abstract two prominent ordovician altered volcanic layers the deicke and the millbrig k bentonites crop out over a wide area of north america these beds have been correlated over selected portions of north america but have not been successfully correlated throughout the entire mississippi valley southern appalachians and new york ontario region a dual approach that used trace element compositions and sr isotopes in apatite phenocrysts was applied to deicke and millbrig candidates from this large region major ca p f and minor mg mn cl fe sr si concentrations within apatite phenocrysts were determined by electron microprobe analysis mg mn and cl proved to be the most diagnostic elements to distinguish apatite between the deicke and millbrig k bentonites these three elements have a restricted range in concentration within deicke apatite whereas they vary significantly within millbrig apatite and define three major clusters two of the millbrig clusters appear to be mostly limited to t

volcanic degassing of argon and helium and the history of crustal production on venus we develop a new methodology linking 40ar and 4he degassing to crustal production on venus in order to examine two different scenarios for the history of magmatism on that planet both consistent with recent analyses of the cratering record of the venus surface the first scenario includes episodic global resurfacing events and modest levels of magmatism between such events while the second scenario invokes different rates of steady magmatism before and after a given transition time our degassing models include distinct mantle crustal and atmospheric reservoirs diffusive transfer of 40ar and 4he from the crust to the atmosphere is also taken into account the 40ar abundance in the present atmosphere reflects the integrated degassing and magmatism over most of planetary history in contrast helium escapes from the planetary atmosphere within a characteristic residence time of 200 myr to 1 8 gyr so the present atmospheric 4he abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first scenario and to the two steady rates of magmatism and the transition time under the second unfortunately large uncertainties in mineral partition coefficients and in the ar and he mixing ratios in the lower atmosphere of venus presently prevent the use of our degassing models to distinguish among crustal formation history models we therefore explore the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements that will most strongly constrain crustal production history as an important step in the development of the ar degassing model the k budget in the bulk silicate fraction of venus is re examined on the basis of venera and vega ray measurements of k u and th concentrations in surface materials for u and th concentrations in the bulk silicate planet of 18 29 ppb and 64 94 ppb respectively the degree of mantle melting that formed the surface materials at the venera 9 and 10 and vega 1 and 2 landing sites is calculated to lie between 0 02 and 0 16 the degree of melting calculated for materials at each landing site is combined with measurements of surface k concentration to estimate the k concentration in the bulk silicate portion of the planet at between 100 and 300 ppm these results suggest that venus and earth have similar heat production

immobile and mobile elements during the transition of volcanic ash to bentonite an example from the early palaeozoic sedimentary section of the baltic basin abstract in order to check the immobility and mobility of elements during conversion of acidic volcanic glass to bentonites in normal marine environments we studied the composition of three altered volcanic ash layers from the palaeozoic of the baltoscandian region correlated through different facies regular changes in element concentrations in accordance with loss and gain of material during the transformation of volcanic ash indicate that al nb ti zr sn pt ta hf and th were generally immobile and can be used for the interpretation of source magma and correlation of ash layers cd behaves similarly with immobile elements and this can be explained with preservation only of the immobile portion of cd that is fixed in phenocrysts in bentonites in shales during the formation of kaolinite the data indicate small scale mobility of al and cd in lime muds where k feldspar forms from volcanic ash ta hf and th reveal some small scale mobility these slightly mobile elements must be used with caution for interpretation of thin ash layers with thicknesses of

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

upper ordovician mohawkian carbon isotope δ13c stratigraphy in eastern and central north america regional expression of a perturbation of the global carbon cycle abstract the guttenberg carbon isotope excursion gice documented from eastern north america demonstrates the effects that regional geochemically distinct water masses upwelling and ocean circulation have on the carbon isotope record from carbonate platforms late turinian chatfieldian carbonates from oklahoma kentucky virginia and west virginia record a positive carbon isotope excursion 3 0 the gice excursion the gice excursion has relations to established biostratigraphy beginning in the north american midcontinent phragmodus undatus conodont zone and continuing through the plectodina tenuis zone sequence and event stratigraphy previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the gice excursion where geochemically distinct water masses are defined for the upper ordovician a major eustatic sea level rise before the gice promoted a greater exchange of open ocean waters onto the carbonate platform of laurentia however restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent iapetus ocean were still apparent local variations documented in the gice excursion are directly related to upwelling of nutrient rich isotopically light waters increased primary productivity and the subsequent organic carbon production and burial

40ar 39ar and nd sr isotopic characteristics of mid ordovician north american k bentonites a test of early paleozoic laurentia gondwana interactions highly altered tephra layers k bentonites of mid ordovician age are widespread throughout the midcontinent region of the united states and the appalachian taconic foreland basin the numerous ash layers their wide areal extent and their large volume provide evidence that extensive volcanic activity accompanied taconic orogenesis the mineralogical and geochemical characteristics of these widespread ordovician tephra layers can be used to constrain the nature of their source region and thereby help test recent hypotheses that gondwana collided with laurentia in mid ordovician time initial 143nd 144nd and 87sr 86sr ratios of two k bentonites that occur traced throughout the midcontinent are 0 51183 0 51187 and 0 7101 0 7120 respectively isotopic ratios for k bentonites within the taconic basin are 0 51178 0 51201 and 0 70564 0 70905 the isotopic composition of the tephra layers and the ubiquitous presence of proterozoic zircon xenocrysts within the k bentonites is consistent with an origin by crustal anatexis the cooling history of the source is constrained by 842 996 ma 40ar 39ar apparent ages of hornblende xenocrysts contained within two taconic basin k bentonites a possible south american source region of the k bentonites is the arequipa massif of peru this region contains early paleozoic arc related rocks many of which contain mid proterozoic zircon xenocrysts and early proterozoic gneisses which were affected by grenville age granulite grade metamorphism gneisses within the adirondack highlands of the grenville province have similar end 450 ma values and hornblende cooling ages to the k bentonites consistent with this region as a source caradocian k bentonites in sweden have sr isotopic ratios similar to the north american beds if the swedish and north american k bentonites were produced from the same source then baltica and laurentia were proximal in ordovician time precluding a gondwana laurentia link if caradocian k bentonites with north american characteristics are found in western south america support for a laurentia gondwana link would be established if such k bentonites cannot be found considerable doubt would be cast on this model

new martian paradigms several interrelated paradigms concerning the nature and the evolution of mars are discussed these include an impact origin of the northern lowland mars as a water rich planet former oceans in the northern lowlands a warm wet climate on early mars and the possibility of ancient life on mars locations on mars suggested for the search of possible ancient life include the heavily cratered terrain dissected by valley networks possible lacustrine deposits in valles marineris the oxidizing layer below the surface sites along the fringes of the hypothetical northern circumpolar ocean and in evaporite deposits

rhyolitic glass in ordovician k bentonites a new stratigraphic tool fresh volcanic glasses in the form of melt inclusions within quartz phenocrysts are commonly present in paleozoic k bentonites from new york state iowa kentucky pennsylvania newfoundland and quebec because these glasses are compositionally distinct from one layerto another their geochemistry can be used to define chronostratigraphic horizons in new york state the k bentonites occur within flat lying calcareous black shales of the middle ordovician utica formation this glass has survived 1 because it has been sealed within a host crystal that is stable under most diagenetic conditions and 2 because of the modest burial depths the ordovician strata in this region to demonstrate the potential of these volcanic glasses for stratigraphic correlations isochronous surfaces have been established among three localities separated by 35 km the immaculate preservation of these ordovician glasses bodes well for the general application of this approach to younger and perhaps even older strata where geologic conditions have favored the survival of glass inclusions

trace element chemistry of individual apatite phenocrysts as a tool for fingerprinting altered volcanic ash beds assessing interbed and intrabed variation at local and regional scales trace element analyses of groups of single apatite microphenocrysts separated from k bentonites define the intrabed homogeneity and interbed variability of apatites in a set of k bentonites from the gaspe peninsula and the regional scale homogeneity of the tioga b k bentonite by analyzing many single grains from each bed it is possible to quantify the homogeneity of beds for statistical comparisons of similarity and differences this approach leads directly to tests that can be used to chemically correlate or distinguish k bentonite beds the results from multiple k bentonite beds from gaspe demonstrate that many of them may be distinguished from one another the results from the tioga b k bentonite demonstrate that there is little variation in apatite chemistry over distances of several hundred kilometers this technique is advantageous for use in k bentonite correlation because it provides a direct test of sample contamination as well as the necessary data on variance of each sample allowing for robust statistical analysis

quantitative biostratigraphy achieving finer resolution in global correlation abstract the fossil record preserves a wide range of events that might be used to build timescales and correlate strata from place to place the events include the originations and extinctions of species the occurrence of distinctive faunal assemblages magnetic field reversals changes in ocean chemistry and volcanic ash falls the fundamental task is to determine the global sequence of all these events modern computer algorithms can build high resolution timescales by sequencing and calibrating thousands of such events from hundreds of localities each successful sequencing algorithm can be understood in terms of a simple two dimensional visual device graphic correlation permutable sequences permutable matrices and slotting devices are each suited to a different data problem such as contradictory evidence of sequence a lack of information about sequence or uncertain correspondence of events algorithms based upon permutable sequences and evolutionary programming heuristics have the flexibili

a tephrochronologic method based on apatite trace element chemistry geochemical correlation of ash fall beds with conventional tephrochronologic methods is not feasible when original glass composition is altered thus alternative correlation methods may be required initial studies of heavily altered paleozoic tephra k bentonites have suggested the potential for employing trace element concentrations in apatite as ash fall bed discriminators to further test the utility of apatite trace element tephrochronology we analyzed apatite phenocrysts from unaltered volcanic rocks with an electron microprobe nine samples from rocks erupted during the quaternary and one sample from a rock erupted during the paleogene the resulting apatite trace element data provide unique bed discriminators despite within crystal variability each of the volcanic rocks studied possesses unique trends in mg cl mn fe ce and y concentrations in apatite the results from this study establish an important tephrochronologic method that can be applied to nearly all portions of the phanerozoic stratigraphic record and greatly assist development of an advanced timescale in addition to establishing a fingerprint for a particular eruption apatite chemistry provides useful information about the source magma

comparative sequence stratigraphy of two classic upper ordovician successions trenton shelf new york ontario and lexington platform kentucky ohio implications for eustasy and local tectonism in eastern laurentia abstract comparison of the classic upper ordovician mohawkian to lower cincinnatian caradoc to lower ashgill black river and trenton groups in new york state southern ontario with the tyrone lexington formations exposed in the jessamine dome southern cincinnati arch in north central kentucky and southern ohio reveals striking similarities previous emphasis on complex local facies mosaic has obscured widespread regional patterns biostratigraphy and k bentonites provide broad constraints on inter regional correlations however an allostratigraphic approach permits higher resolution correlations and a partial test of eustatic vs strictly local tectonic models to explain stratigraphic patterns upper mohawkian to lower cincinnatian 455 449 ma depositional sequences previously recognized for the jessamine dome and nashville dome areas are correlated between the two main study areas and further refined chatfieldian rocklandian to shermanian or cobourgian of traditional terminology sequences m5 and m6 of previous workers are interpreted to be composite sequences and are each subdivided into three smaller scale sequences which also have counterparts in the new york ontario strata in turn these correlations indicate at least partial allocyclic control on sedimentary cycles complex lateral variations within depositional sequences especially in the late shermanian to edenian indicate that tectonically controlled patterns of basinal subsidence and uplift of crustal blocks perhaps reflecting forebulge migration exerted a strong influence on the local facies and motif of depositional sequences these tectonic features however did not obliterate the underlying allocyclic pattern indeed high resolution sequence stratigraphy enables detailed resolution of shifting patterns of minor uplift and subsidence

late middle ordovician environmental change and extinction harbinger of the late ordovician or continuation of cambrian patterns positive excursions in carbon isotope compositions of carbonate 3 and organic carbon 4 6 from the late middle ordovician middle caradocian of the midcontinent and the eastern united states indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric p co 2 precipitating global cooling although not necessarily ice sheet formation these climatic changes were associated with regional orogenic uplift a relative rise in sea level changes in epeiric sea circulation patterns and carbonate platform destruction that led to regional extinction of marine benthos the combination of sea level rise changing ocean circulation and extinction in the middle caradocian is similar to the suite of environmental changes described for cambrian biomere boundaries suggesting shared causes for these events in contrast middle caradocian environmental changes are markedly different from the environmental patterns associated with the late ordovician mass extinction despite the evidence for long term cooling from the middle to the late ordovician

sensitivity of the epeiric sea carbon isotope record to local scale carbon cycle processes tales from the mohawkian sea ancient marine carbon isotope records derived from epeiric sea carbonates can bear a strong influence of local scale carbon cycling processes across the late ordovician mohawkian sea of eastern laurentia the carbon isotope compositions d 13 c of contemporaneous sediments range from 2x in the interior of the sea to 2 5x along the margin here we use a box model to explore the relative importance of carbon fluxes within the mohawkian sea from a mass balance perspective in the model the mohawkian sea carbon reservoir is coupled to but distinct from the ocean and atmosphere carbon reservoirs at initial steady state conditions the residence time of carbon in the mohawkian sea is less than 10 years we find that to produce model d 13 c values representative of the range in d 13 c across the mohawkian sea we must divide the sea into an inner region and an outer region and limit dissolved inorganic carbon dic exchange between the two to 15 of the initial steady state unrestricted exchange rate it is also necessary to supply low d 13 c carbon 29x to the inner region of the sea at a rate of 17 4 10 12 mol c yr 1 the required flux of low d 13 c carbon could not have been sustained over the long term by remineralization of organic carbon stored in sediments of the inner region of the mohawkian sea and the geographic d 13 c pattern is unlikely to have been generated by upwelling along the sebree trough rather the low d 13 c flux might have been derived from bryophytedominated terrestrial ecosystems suggesting an important role for terrestrial carbon cycling prior to the advent of vascular plants aninterestingfeatureofthemodelisthattheisotopiccompositionsofboththeepeiric seaandthelargerocean atmospheresystem are sensitive to changes in exchange rates within the epeiric sea this has important implications for interpreting global d 13 c

solubility and partitioning of ne ar kr and xe in minerals and synthetic basaltic melts we have measured the solubilities of ne ar kr and xe in natural samples of anorthite diopside forsterite spinel and in synthetic basaltic melts the minerals and melts represent equilibrium pairs in the fo an di sio2 system samples were suspended in individual crucibles in a one bar flowing mixed noble gas atmosphere at 1300 or 1332 c for times ranging from 7 18 days the solubilities in the minerals increase with increasing noble gas atomic number and typical solubility values are surprisingly high samples of a particular mineral i e anorthite that came from different localities yield distinctly different and reproducible solubility values this indicates that properties intrinsic to individual samples can influence solubility noble gases are likely to be sited in lattice vacancy defects in contrast the solubilities in the melts decrease with increasing atomic number our data overlap the low end of the range defined previously for natural basalts however the dynamic range of the values from ne to xe is not as great as for the natural melts solubilities correlate well with melt molar volume but poorly with density the partition coefficients increase with increasing noble gas atomic number for all mineral melt pairs such trends imply that the terrestrial planet atmospheres were not derived from partial melting of chondritic source material significant fractionation of kr relative to xe is not observed ruling out an origin for earth s missing xe via magmatic fractionation partition coefficient absolute values are frequently near or greater than unity for example the ranges for five diopside measurements are the following dnel 0 013 0 37 darl 0 15 0 84 dkrl 0 31 2 4 and dxel 3 2 47 this indicates that magmatic transport is not an efficient mechanism for degassing the terrestrial planets our results favor a catastrophic origin for the atmospheres

isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans implications for secular curves isotopic analysis of conodonts and their host limestones sampled between two regionally extensive altered volcanic ash layers in eastern laurentia shows that a 454 ma epeiric sea maintained large lateral differences in nd and c isotope compositions this is consistent with inferred temperature salinity defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments because the majority of old marine fossils and sedimentary rocks are known from epeiric seas some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses rather than global scale changes in the state of the earth ocean system

evidence for a widespread carbon isotopic event associated with late middle ordovician sedimentological and faunal changes in estonia an episode of remarkable biotic climatic sea level and facies changes took place during the late viruan caradoc epoch in the baltoscandian area we studied the carbon isotopic composition of carbonate sediments from this period data on the stable carbon isotopic composition of whole rock carbonates from three south estonian core sections together with those on ostracode distribution are presented in two core sections a positive 13 c shift of 2 was revealed in the upper part of the keila stage mid caradoc the comparison of isotope and ostracode data in the sections suggested the occurrence of a gap of late keilan age in the marginal area of the basin equivalent to the north estonian confacies belt the beginning of the excursion approximately coincided with climatic and sea level changes in the shallow shelf area the peak of the late keilan excursion preceded the biotic crises and maximum black shale accumulation in the baltoscandian palaeobasin an approximately synchronous carbon isotopic event has been reported from north america referring to a possible global oceanographic event in the caradoc epoch although the positive carbon isotopic excursion and related environmental events of the late keilan age have some unique features they show more similarities to the end ordovician and silurian events characterized by oceanic change from stratified state to thermohaline circulation state than to warm anoxic events related to eustatic sea level rise

source and maintenance of the argon atmospheres of mercury and the moon we propose that argon 40 measured in the lunar atmosphere and that in mercury s atmosphere is due to current diffusion into connected pore space within the crust higher temperatures at mercury along with more rapid loss from the atmosphere will lead to a similar or smaller column abundance of argon at mercury than at the moon given the same crustal abundance of potassium because the noble gas abundance in the mercurian atmosphere represents current effusion it is a direct measure of the crustal potassium abundance we assume a fractal distribution of distance to a connected pore space with the shortest distance increasing with depth given this rock size distribution we show that the diffusive flux is not a unique function of temperature even though the diffusion coefficient is an exponential function of temperature the flux to the surface is fairly insensitive to the temperature

paired δ13ccarb and δ13corg records of upper ordovician sandbian katian carbonates in north america and china implications for paleoceanographic change late sandbian early katian marine carbonates from two sections in north america and one in south china were analyzed for paired 13ccarb and 13corg and revealed similar 13ccarb but varying trends in 13corg stratigraphy while all sections recorded a well known positive 13ccarb shift these 13corg records are the first through the guttenberg 13ccarb excursion gice in china oklahoma and west virginia in south china a positive excursion in 13corg is associated with the latter part of the positive 13ccarb excursion and closely resembles trends from iowa and pennsylvania that have been interpreted to reflect a lowering of atmospheric pco2 levels the 13corg trends from oklahoma and west virginia differ significantly from what is observed in china this likely indicates that local changes in nutrient cycling and phytoplankton growth rates were the dominant control on 12c fractionation in oklahoma and west virginia masking a possible global signal of lowered atmospheric pco2 levels observed in china

techniques for assessing spatial heterogeneity of carbonate δ13c values implications for craton wide isotope gradients the sedimentary record of carbonate carbon isotopes 13ccarb provides one of the best methods for correlating marine strata and understanding the long term evolution of the global carbon cycle this work focuses on the late ordovician guttenberg isotopic carbon excursion a ca 2 5 positive 13ccarb excursion that is found in strata globally substantial variability in the apparent magnitude and stratigraphic morphology of the guttenberg excursion at different localities has hampered high resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle this work investigates the magnitude spatial scale and sources of isotopic variability of the guttenberg excursion in two sections from missouri usa centimetre scale isotope transects revealed variations in 13ccarb and 18ocarb greater than 2 across individual beds linear 13ccarb to 18ocarb mixing lines together with petrographic and elemental abundance data demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components these patterns facilitated objective sample screening to determine the least altered data a 18ocarb filter based on empirical 18ocarb values of well preserved carbonate mudstones allowed further sample discrimination the resulting least altered 13ccarb profile improves the understanding of regional as well as continental scale stratigraphic relations in this interval correlations with other laurentian sections strongly suggest that i small scale variability in guttenberg excursion 13ccarb values may result in part from local diagenetic overprinting ii peak guttenberg excursion 13ccarb values of the midcontinent are not distinct from their taconic equivalents and iii no primary continental scale spatial gradient in 13ccarb for example arising from chemically distinct aquafacies is required during guttenberg excursion time this study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for 13ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry

origin and tectonic setting of ordovician bentonites in north america isotopic and age constraints the large volume of ordovician bentonites in the eastern and central united states indicates that they originated from a major volcanic system the location and paleotectonic setting of this volcanic source is unknown nd and sr isotopic data were collected from eight samples of primary apatite phenocrysts contained in four ordovician bentonites and u pb data were collected from one bentonite to determine the origin of this extensive volcanism the range in initial isotopic ratios for the four bentonites is nd 3 5 to 5 2 87 sr 86 sr 0 71005 to 0 71200 the age of the deicke bentonite is 457 1 1 0 2 determined by a concordant u pb zircon fraction five other zircon fractions from the deicke bentonite contain an inherited lead component lines drawn through the concordant point and the extremes of the discordant points have calculated upper intercepts with concordia of 1 090 ma and 1 540 ma the nd and sr isotopic data and the u pb age data are consistent with the generation of the volcanic ash by anatexis of evolved continental crust an evolved source region with volcanics of the same age and isotopic composition as the north american bentonites has not been found this source region however may be the same as the source of ordovician bentonites in sweden because the ages of both groups of bentonites are identical the 87 sr 86 sr ratios overlap and zircon phenocrysts from both groups of bentonites contain an inherited pb component an appropriate tectonic setting for the generation of both groups of bentonites was a continental margin volcanic arc or an extensional regime associated with such an arc

tectonic implications of radiogenic noble gases in planetary atmospheres abstract studies of the radiogenic isotopes 4 he and 40 ar in planetary atmospheres provide important constraints on tectonic processes within planetary interiors for mantle k u and th u mass ratios r k u 12 700 and r th u 3 8 and a urey number ur 0 7 the 40 ar in the earth s atmosphere can be obtained by complete outgassing of the crust and upper mantle and a 7 outgassing of the lower mantle or by a 23 to 33 outgassing of the whole mantle and crust the flux of 4 he from the mantle can be derived by complete steady state outgassing of the upper mantle or by partial outgassing 9 of the entire mantle the 40 ar in the venus atmosphere can originate from the outgassing of only the upper 6 km of an enriched crust 8 5 of the entire mantle or 28 of the upper mantle the low concentration of 36 ar in the earth s atmosphere relative to venus strongly supports the collision hypothesis for the origin of the moon such a major collision would have eroded a large fraction of the earth s atmosphere the 40 ar on mars requires that only the upper 640 m of the crust be outgassed however the presence of large volcanoes on mars indicates greater degassing thus supporting the hypothesis of past erosion of the martian atmosphere by collisions with planetesimals and large meteorites the 40 ar flux through the lunar atmosphere requires the outgassing of the upper 9 km of the lunar crust

correlation of north american ordovician bentonites by using apatite chemistry middle ordovician bentonites in the central and southeastern united states have been correlated on the basis of rare earth element ree concentrations of primary apatite phenocrysts ratios of light to heavy ree such as la tb or ce yb produce the best separation of the four studied bentonites the deicke bentonite has been correlated over a distance of more than 1000 km from southern minnesota to central tennessee and the millbrig bentonite has been correlated over 750 km from central minnesota to central missouri the successful discrimination of closely spaced bentonites over a long distance demonstrates that apatite chemistry is a powerful correlation tool on a continental scale

local controls on carbon cycling in the ordovician midcontinent region of north america with implications for carbon isotope secular curves abstract the carbon isotope record from ancient epicontinental seas may contain much more of a local scale carbon cycling signal than is generally appreciated a unique opportunity exists to examine this issue in the case of the late ordovician mohawkian sea of eastern laurentia where the millbrig k bentonite stratigraphic framework has been used to delineate a time slice at 454 ma extending over 1 500 000 km2 of the eastern united states across the time slice carbonate and organic carbon 13c vary by 4 5 and 7 5 respectively a spatial variation that is as large as temporal secular changes in epeiric sea 13c that have been reported in the past these new data are considered in the context of geographic variations in lithological biological and other geochemical sediment characteristics collectively these sediment properties distinguish regions of the mohawkian sea which likely differed in terms of the nature and relative importance of carbon cycling processes water column depth and structure and barriers to free exchange of water across the mohawkian sea may have been overarching factors in the development of these regions raising the possibility that changes in circulation patterns such as those caused by sea level change played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses if the observed effects of local carbon cycling on the distribution of mohawkian sea 13c were commonplace in ancient epicontinental marine environments it would imply that local scale carbon cycling may have left a nontrivial imprint on epeiric sea records of secular variations in 13c in addition to the imprint left by changes in the global carbon cycle this may have contributed to the broad scatter in 13c values observed in the paleozoic portion of the global carbon isotope secular curve

correlation of ordovician mohawkian k bentonites in the upper mississippi valley using apatite chemistry implications for stratigraphic interpretation of the mixed carbonate siliciclastic decorah formation abstract four ordovician k bentonites have been chemically fingerprinted using trace element content of apatite phenocrysts contained within the altered ash layers trace element analysis was performed using electron microprobe on individual crystals the bentonites include the deicke millbrig elkport and dickeyville beds these beds lie within the decorah formation of the northern mississippi valley outcrop area all four k bentonite beds are believed to have a taconic source whereas volcanic ash was transported by wind and deposited in the ordovician north america epeiric sea the decorah is a marine unit shaly to the north and west and carbonate rich to the south and east thirteen k bentonite samples were collected from six localities only one locality dickeyville wi contains all four beds in succession one hundred seventy one apatite crystals were handpicked and analyzed for major minor and trace elements using an electron microprobe each crystal was analyzed at three to six separate spots to ascertain compositional variation the most discriminating and diagnostic elements were mg and mn plotting mg vs mn content produces data clusters characteristic for each k bentonite clusters show only minimal change between localities allowing individual k bentonites to be identified and used for stratigraphic correlation time slices provided by the k bentonite horizons show that the decorah formation is composed of reciprocal wedges stratigraphic sequences of shale and carbonate shale was deposited in deeper water within the hollandale embayment but pinched out southeastward onto the flank of the wisconsin dome carbonates were formed largely in shoal water on the arch prograding northwestward and shaling out downramp into the embayment

did intense volcanism trigger the first late ordovician icehouse oxygen isotopes measured on late ordovician conodonts from minnesota and kentucky united states were studied to reconstruct the paleotemperature history during late sandbian to katian mohawkian cincinnatian time this time interval was characterized by intense volcanism as shown by the prominent deicke millbrig and other k bentonite beds a prominent carbon isotope excursion guttenberg 13 c excursion gice postdates the millbrig volcanic eruptions and has been interpreted to reflect a drawdown of atmospheric carbon dioxide and climatic cooling the oxygen isotope record in conodont apatite contradicts this earlier interpretation an increase in 18 o of 1 5 vienna standard mean ocean water just above the deicke k bentonite suggests an abrupt and short lived cooling that possibly initiated a first short term glacial episode well before the major hirnantian glaciation the decrease in 18 o immediately after the mega eruptions indicates warming before the gice and no cooling is shown in the gice interval the coincidence of the deicke mega eruption with a cooling event suggests that this major volcanic event had a profound effect on late ordovician late mohawkian climate

discovery of the ordovician millbrig k bentonite bed in the trenton group of new york state implications for regional correlation and sequence stratigraphy in eastern north america the ordovician chatfieldian millbrig k bentonite bed is a key stratigraphic marker horizon that is regionally synchronous over much of eastern and central north america this prominent marker is an independent source of correlation among the major chronostratigraphic and sequence stratigraphic units in this region the general stratigraphic position of the millbrig k bentonite has suggested to some authors that it is identical with the hounsfield k bentonite at dexter new york the traditional type area of the middle ordovician or mohawkian series in north america but previously available geochemical and biostratigraphical evidence has been insufficient to confirm this correlation analyses of apatites and melt inclusions in quartz phenocrysts from the millbrig k bentonite at eight localities in kentucky missouri virginia and pennsylvania and the hounsfield k bentonite at its type locality at dexter new york using high precision electron microprobe analysis techniques shows that the millbrig and the hounsfield have identical apatite and melt inclusion chemistry indicating a geochemical correlation this correlation is supported by conodont biostratigraphy and 13c isotope chemostratigraphy the new data demonstrate that the millbrig k bentonite and therefore the base of the chatfieldian stage of the north american mohawkian series by definition lies very close to the base of the traditional rocklandian stage of new york furthermore the millbrig k bentonite bed lies in close proximity to the base of the taconic supersequence over much of the midcontinent region and in particular lies just below the m5 sequence boundary recognized in kentucky and tennessee our results permit extension of the chatfieldian sequences into new york state and southern ontario and contributes to the resolution of the long standing uncertainty about the position of the base of the trenton group in ontario furthermore viewed in a broad context of regional stratigraphic relations we conclude that our results suggest that the persistent correlation difficulties reflect diachronous effects of widespread changes in oceanographic circulation patterns that emerged during taconic orogeny finally regional differences in the timing and character of sequence bounding surfaces and facies similarities summarized here suggest that the causes of relative sea level change during the chatfieldian may have been primarily tectonoeustatic mechanisms

possible late middle ordovician organic carbon isotope excursion evidence from ordovician oils and hydrocarbon source rocks mid continent and east central united states oils generated by middle ordovician rocks are found throughout the mid continent and east central regions of the united states gas chromatographic characteristics of these oils include a relatively high abundance of n alkanes with carbon numbers less than 20 a strong predominance of odd numbered n alkanes between c10 and c20 and relatively small amounts of branched and cyclic alkanes saturated and aromatic hydrocarbon fractions of 43 ordovician oils from the anadarko ardmore forest city illinois michigan salina sedgwick and williston basins and the iowa shelf demonstrate a wide range in carbon isotope composition dgr13csat 24 9 pmil to 33 9 pmil dgr13carom 24 3 pmil to 33 7 pmil saturated and aromatic hydrocarbons extracted from late middle ordovician shales 17 core samples show ranges in dgr13c similar to that of the oils the wide ranges in dgr13c for oils and rock extracts reflect a major positive excursion s 6 9 per pmil in organic matter dgr13c in late middle ordovician rocks this excursion has at least a regional significance in that it can be documented in sections 480 mi 770 km apart in south central kansas and eastern iowa the distance may be as much as 930 mi 1 500 km if the carbon isotope variations observed in michigan basin ordovician oils and in organic matter from late middle ordovician rocks in southwestern ontario are related to the same carbon isotope excursion organic matter dgr13c in core samples from south central kansas and eastern iowa is not directly related to variations in quantity or quality of organic matter or maceral compositi n the positive excursion in organic matter dgr13c is a possible result of increased organic matter productivity and or preservation the parallel shifts in organic and carbonate dgr13c in core samples from 1 e m greene well washington county iowa imply changes in the isotope composition of the ocean atmosphere carbon reservoir differences in the magnitude of the carbon isotope shifts between organic matter 8 8 pmil and carbonate 4 2 pmil in this core suggest a decrease either locally or regionally in available dissolved co2 possibly a result of high organic matter productivity and or limited circulation in the late middle ordovician seas

large magnitude middle ordovician volcanic ash falls in north america and europe dimensions emplacement and post emplacement characteristics abstract middle ordovician k bentonites represent some of the largest known fallout ash deposits in the phanerozoic era they cover minimally 2 2 106 km2 in eastern north america and 6 9 105 km2 in northwestern europe and represents the coeval accumulation of plinian and co ignimbrite ash on both laurentia and baltica during the closure of the iapetus ocean the three most widespread beds are the deicke and millbrig k bentonites in north america and the kinnekulle k bentonite in northwestern europe the vents were located near the laurentian margin of iapetus on an arc or microplate undergoing collision with laurentia the volume of ash preserved in the stratigraphie record converted to dense rock equivalent dre of silicic magma is minimally estimated to be 943 km3 for the deicke 1509 km3 for the millbrig and 972 km3 for the kinnekulle the millbrig and kinnekulle beds are coeval and possibly equivalent yielding a combined dre volume of nearly 2500 km3 some unknown but probably large amount of additional ash fell into oceanic regions of the iapetus but these areas became subducted and the ash is not preserved in the geologic record the symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of ordovician paleogeography based on grain size measurements and thickness area 1 2 plots we conclude the three beds were each formed from co ignimbrite or possibly phreatoplinian eruption columns analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4 dissolved water at the time of the eruption this water provided the explosive energy during the initial gas thrust phase the implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles forming a deposit composed largely of single crystals and glassy dust conversion of the ash to k bentonite resulted in a mass loss of approximately 35 mostly in the form of si with lesser amounts of na and k

partition of noble gases between olivine and basalt melt abstract the distribution coefficients ki of noble gases were determined for synthesized olivine basalt melt pairs seven samples bh series were synthesized at 1370 1300 c under a one atmospheric pressure 105 pa noble gas mixture and four samples hpp and hpg series at 1360 1050 c at high pressures 0 2 1 5 gpa the two different experiments gave consistent results with each other some bh series olivine samples were apparently contaminated with gas rich inclusions or the glass phase and the effects of such interferences on the calculated distribution coefficients were evaluated the possible ranges of the distribution coefficients of noble gases thus obtained are khe 0 07 kne 0 006 0 08 kar 0 05 0 15 kkr 0 15 and kxe 0 3 these show remarkable features compared to solid element distribution coefficients the distribution coefficients of noble gases are rather insensitive to or even positively correlated with the atomic size of noble gases the striking contrast between the partition of noble gases and that of solid elements may be attributed to the electrical neutrality of noble gases and the increasing electronic polarizability from he to xe

nature and regional significance of unconformities associated with the middle ordovician hagan k bentonite complex in the north american midcontinent stratal patterns of the middle ordovician hagan k bentonite complex and associated rocks show that the black river trenton unconformity in the north american midcontinent formed through the complex interplay of eustasy sediment accumulation rates siliciclastic influx bathymetry seawater chemistry and perhaps local tectonic uplift the unconformity is diachronous and is an amalgamated surface that resulted from local late turinian lowstand exposure followed by regional early chatfieldian transgressive drowning and sediment starvation the duration of the unconformity is greatest in southern wisconsin northern illinois and northern indiana where the deicke and millbrig k bentonite beds converge at the unconformity on the basis of published isotopic ages for the deicke and millbrig beds it is possible that in these regions erosion and nondeposition spanned a period of as much as 3 2 m y two broad coeval depositional settings are recognized within the north american midcontinent during early chatfieldian time 1 an inner shelf subtidal facies of fossiliferous shale spechts ferry shale member and ion shale member of the decorah formation and argillaceous lime mudstone and skeletal wackestone guttenberg and kings lake limestone members extended from the canadian shield and transcontinental arch southeastward through minnesota wisconsin iowa and missouri 2 a seaward relatively deep subtidal sediment starved middle shelf extended eastward from the mississippi valley region to the taconian foreland basins in the central and southern appalachians and southward through the pericratonic arkoma and black warrior basins in the inner shelf region the black river trenton unconformity is a composite of at least two prominent hardground omission surfaces one at the top of the castlewood and carimona limestone members and the other at the top of the guttenberg and kings lake limestone members both merging to a single surface in the middle shelf region the inner and middle shelves redeveloped later in approximately the same regions during devonian and mississippian time

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

thorium uranium ratio of the earth through the study of komatiites and ophiolites remnants of the old mantle we review the evolution of the lead system in the depleted mantle through time using the 230th232th vs 208pb 206pb diagram 208pb 206pb radiogenic lead ratio we constrain the th u ratio of the bulk silicate earth to 4 2 this value is also in full agreement with ratios inferred from the study of the various types of meteorites using this value in the 208pb 206pb 143nd144nd diagram bulk earth plots on the oceanic basalt correlation line as previously guessed a comparison with sr and nd data shows that the th u fractionation correlates with that of rb sr and nd sm whereas the u pb fractionation is independent of other ratios

high resolution sequence stratigraphy of a mixed carbonate siliciclastic cratonic ramp upper ordovician kentucky ohio usa insights into the relative influence of eustasy and tectonics through analysis of facies gradients abstract detailed facies analysis and event stratigraphy of an upper ordovician rocklandian edenian cratonic ramp succession in eastern north america yields insights into eustatically driven sequence architecture and localized tectonic instability seven predominantly subtidal mixed carbonate siliciclastic depositional sequences 3rd order are identified and correlated across the length of a 275 km ramp to basin profile within the larger depositional sequences 3rd order at least two smaller orders 4th and 5th of cyclicity are recognizable three systems tracts occur within each sequence transgressive tst highstand hst regressive rst and are considered in terms of their component parasequences 5th order generally tsts are composed of skeletal grainstone rudstone facies hsts are dominated by shaly nodular wacke packstone facies and rsts are mostly calcarenite facies systems tracts sequence boundaries and their correlative conformities maximum flooding surfaces and forced regression surfaces were traced from shallow shelf to basinal settings this high resolution framework also provides insight into the timing of tectonic fluctuations on this cratonic ramp during the taconic orogeny and documents the relative influence of tectonism on lateral facies distributions and eustatically derived cyclicity

precise timing of the late ordovician sandbian super eruptions and associated environmental biological and climatological events two of the largest known eruptions in the phanerozoic produced the ordovician millbrig k bentonite of north america and the kinnekulle k bentonite of scandinavia which have been previously suggested to be coeval the millbrig k bentonite from kentucky usa and the kinnekulle k bentonite from bornholm denmark yielded chemical abrasion thermal ionization mass spectrometry u pb zircon dates of 452 86 0 29 and 454 41 0 17 ma 2 analytical uncertainty respectively thus showing significant age differences contrary to what is generally held these data and four additional newly dated k bentonites directly establish the first radioisotopic age constraints for the ordovician katian sandbian global stage boundary refine global stratigraphic correlations date associated chemostratigraphic events and suggest an alternative volcanic climate hypothesis for the late ordovician supplementary material u pb radioisotopic data table and analytical methods are available at www geolsoc org uk sup18636

stratigraphic correlations using trace elements in apatite from late ordovician sandbian katian k bentonites of eastern north america the early late ordovician sedimentary rocks of eastern north america contain a relatively large number 100 of widespread heavily altered tephra layers k bentonites these beds represent an intense period of subaerial volcanism that occurred from ca 455 to 449 ma the sedimentary rocks that contain these k bentonites display complex regional lithostratigraphic relationships ranging from clastic foreland basin facies to cratonic carbonate platform facies accurate correlation of these ancient ash fall beds is essential for testing chronostratigraphic hypotheses that attempt to connect these different tectono sedimentary provinces despite the relatively thorough study of a few of these k bentonites over the past several decades the full stratigraphic potential of these beds has yet to be realized to test the utility of the apatite trace element k bentonite correlation method on a larger scale we studied over 200 k bentonite samples from the mohawkian stage of eastern north america and statistically compared our results with previous studies on the same suites of k bentonites electron microprobe epma and laser ablation inductively coupled plasma mass spectrometry la icp ms results show that apatite trace element data provide unique bed discriminators each of the k bentonite layers exhibits unique and reproducible trends in mg cl mn fe ce y and other trace element concentrations in apatite statistical evaluation of results from our apatite analyses suggests correlations for 12 k bentonite beds providing a significant improvement in stratigraphic resolution the stratigraphic relations indicated by these new k bentonite fingerprints provide a rigorous means by which to evaluate some previous interpretations of biostratigraphic chemostratigraphic and sequence stratigraphic studies in eastern north america

correlating tephras and cryptotephras using glass compositional analyses and numerical and statistical methods review and evaluation abstract we define tephras and cryptotephras and their components mainly ash sized particles of glass crystals in distal deposits and summarize the basis of tephrochronology as a chronostratigraphic correlational and dating tool for palaeoenvironmental geological and archaeological research we then document and appraise recent advances in analytical methods used to determine the major minor and trace elements of individual glass shards from tephra or cryptotephra deposits to aid their correlation and application protocols developed recently for the electron probe microanalysis of major elements in individual glass shards help to improve data quality and standardize reporting procedures a narrow electron beam diameter 3 5 m can now be used to analyze smaller glass shards than previously attainable reliable analyses of microshards defined here as glass shards in part 2 of the review we describe critically assess and recommend ways in which tephras or cryptotephras can be correlated in conjunction with other information using numerical or statistical analyses of compositional data statistical methods provide a less subjective means of dealing with analytical data pertaining to tephra components usually glass or crystals phenocrysts than heuristic alternatives they enable a better understanding of relationships among the data from multiple viewpoints to be developed and help quantify the degree of uncertainty in establishing correlations in common with other scientific hypothesis testing it is easier to infer using such analysis that two or more tephras are different rather than the same adding stratigraphic chronological spatial or palaeoenvironmental data i e multiple criteria is usually necessary and allows for more robust correlations to be made a two stage approach is useful the first focussed on differences in the mean composition of samples or their range which can be visualised graphically via scatterplot matrices or bivariate plots coupled with the use of statistical tools such as distance measures similarity coefficients hierarchical cluster analysis informed by distance measures or similarity or cophenetic coefficients and principal components analysis pca some statistical methods cluster analysis discriminant analysis are referred to as machine learning in the computing literature the second stage examines sample variance and the degree of compositional similarity so that sample equivalence or otherwise can be established on a statistical basis this stage may involve discriminant function analysis dfa support vector machines svms canonical variates analysis cva and anova or manova or its two sample special case the hotelling two sample t2 test randomization tests can be used where distributional assumptions such as multivariate normality underlying parametric tests are doubtful compositional data may be transformed and scaled before being subjected to multivariate statistical procedures including calculation of distance matrices hierarchical cluster analysis and pca such transformations may make the assumption of multivariate normality more appropriate a sequential procedure using mahalanobis distance and the hotelling two sample t2 test is illustrated using glass major element data from trachytic to phonolitic kenyan tephras all these methods require a broad range of high quality compositional data which can be used to compare unknowns with reference training sets that are sufficiently complete to account for all possible correlatives including tephras with heterogeneous glasses that contain multiple compositional groups currently incomplete databases are tending to limit correlation efficacy the development of an open online global database to facilitate progress towards integrated high quality tephrostratigraphic frameworks for different regions is encouraged

applying carbon isotope stratigraphy using well cuttings for high resolution chemostratigraphic correlation of the subsurface basin scale correlations in the subsurface generally rely on lithostratigraphic information synthesized from wireline logs and in some cases well cuttings and cores however lithostratigraphic boundaries are often diachronous and as such the correlations based upon them may not provide reliable timelines in this paper we use data from well cuttings and a core to generate chronostratigraphic logs of late ordovician strata spanning the black river group trenton group and utica shale across the subsurface of new york state although particular values may be impacted by primary variability in local dissolved inorganic carbon reservoirs and or secondary diagenetic alteration it is possible to identify spatially and stratigraphically coherent patterns in which can be used to effectively correlate time equivalent strata on a basin wide or even global scale including across lithologies e g between limestone and calcareous shale the present study emphasizes the use of well cuttings as these are commonly collected during drilling and can provide the maximum lateral resolution for subsurface correlation parallel geochemical percent carbonate and total organic carbon and isotopic and data are used to understand the origin of stratigraphic and spatial variability in the signal and to identify diagenetic alteration stratigraphically coherent trends across new york were used to identify six isotopically distinct packages of time equivalent strata within these formations pairing chemostratigraphic and lithostratigraphic data improves our ability to document the diachronous nature of lithologic contacts including the base of the utica shale which is progressively younger moving west through new york

carbon cycling across the southern margin of laurentia during the late ordovician abstract the guttenberg isotope carbon excursion gice a positive carbon isotope excursion that occurs near the base of the katian stage is thought to be a global event possibly related to late ordovician cooling documenting how much regional and global variability exists in carbon isotopic trends prior to and during the gice is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle paleoclimate and chemostratigraphic correlation during the ordovician to investigate carbon isotopic trends along the southern margin of the laurentian carbonate platform during the late ordovician we measured bulk carbonate 13 c carb and 18 o carb as well as organic carbon 13 c org values from four locations in alabama these sections are excellent study sites because they are well exposed contain the regionally well correlated deicke and millbrig k bentonites and are in a region where 13 c trends have not been studied carbonate 13 c carb results from one section i e fort payne in northeastern alabama record a 1 5 positive excursion above the millbrig k bentonite we interpret the 1 5 positive excursion as a primary feature and identify it as the gice the characteristic gice excursion is absent in the other study sections and we attribute this absence to the presence of unconformities either non depositional or erosional in these sections we find neither evidence for sea level control on 13 c trends nor conclusive evidence for 13 c dic gradients along the southeast margin of laurentia s epeiric sea combined with the proximity to the iapetus ocean these results suggest that carbon isotopic trends in alabama are a reliable proxy for open ocean conditions therefore despite the absence of gice excursion in all but one of our study sections increasing 13 c values in alabama prior to the gice provides evidence for a steady 13 c enrichment of the global surficial carbon reservoir consistent with a drawdown of atmospheric co 2 during the early late ordovician

sr ca u ca and δ18o records in recent massive corals from bermuda relationships with sea surface temperature abstract high resolution records of sr ca u ca ratios and 18o have been obtained in two recent colonies of massive corals diploria labyrinthiformis from bermuda the three geochemical proxies display regular seasonal variations and are well correlated with each other however some important discrepancies are observed between the two colonies the average seasonal variations of the three geochemical proxies are lower by almost 50 in one of the profiles where the seasonal oscillation also displays a strong asymmetry with narrower summer maxima different calculations are discussed for calibrating sr ca and 18o with sea surface temperatures sst we show that the method using only temperature minima and maxima is slightly more accurate and is also more reliable our results from bermuda corals confirm that the temperature dependency of sr ca u ca and 18o is species dependent as previously shown by others and that vital effects are clearly involved in the geochemical incorporation of trace elements in the coral skeleton finally a simple model involving seasonal variation of the growth rate and a kinetic fractionation related to growth rate is presented to explain the differences between the two bermuda colonies it is shown that these factors may prevent corals from recording the complete temperature seasonality and could be the cause for the discrepancies observed between profiles resulting in significant biases of the sst reconstructions

reconstructing late ordovician carbon cycle variations abstract the role of carbon dioxide in regulating climate during the early paleozoic when severe glaciations occurred during a putative greenhouse world remains unclear here we present the first molecular carbon isotope proxy based estimates for late ordovician early katian pco2 levels and explore the limitations of applying this approach to the reconstruction of paleozoic pco2 carbon isotope profiles from three sites in laurentia iowa ontario and pennsylvania and one site in baltica estonia exhibit overall low isotope fractionation between organic and inorganic carbon during photosynthesis ep and these values declined during the early katian carbonate carbon isotope excursion or guttenberg carbon isotope excursion gice algal ep values are sensitive to changes in co2 concentrations algae cell morphologies and cell growth rates to constrain these factors we present molecular evidence that a decrease in the relative abundance of cyanobacteria and a change in the eukaryotic algae community co occurred with the gice regardless of local biotic or oceanographic influences a decline in ep values indicates photosynthesis was sensitive to carbon concentrations and via analogy with modern taxa constrains pco2 to below 8 pre industrial levels pil or about half of previous estimates in addition the global positive carbon isotope excursions expressed in a wide variety of sedimentary materials carbonate bulk organic matter n alkanes acyclic and cyclic isoprenoid hydrocarbons provide compelling evidence for perturbation of the global carbon cycle and this was likely associated with a decrease in pco2 approximately 10 million years prior to the hirnantian glaciations isotopic records from deeper water settings suggest a complex interplay of carbon sources and sinks with pco2 increasing prior to and during the early stages of the gice and then decreasing when organic carbon burial outpaced increased volcanic inputs