msec data sets record glacially driven cyclicity examples from the arrow canyon mississippian pennsylvanian gssp and associated sections abstract here we report magnetic susceptibility ms analyses from three mainly marine sections the mississippian pennsylvanian mid carboniferous global boundary stratotype section and point gssp in arrow canyon se nevada and two secondary sections nearby one in arrow canyon 200 m distant and the second in battleship wash 2 km south of the gssp all three sections are easily correlated using the magnetosusceptibility event and cyclostratigraphy method msec cyclicity is clearly apparent in all three sections and time series analysis independently verifies the ms zonation developed from the smoothed data set the periods determined in the three sections support the argument that deposition of these mid carboniferous rocks resulted from climate controlled glacio eustatic fluctuations that were driven by gondwana glaciation given the published average thicknesses of t r cycles 50 m and the estimates for timing of glacial interglacial cyclicity during the mid carboniferous we assign the strong ft peak observed for the three ms data sets a value of 400 000 years corresponding to the milankovitch e1 eccentricity band we then calculate a floating point time scale fpts for the sampled sequence the fpts results indicate that 1 sediment accumulation rates bracketing the mid carboniferous boundary averaged 0 7 cm 1000 years and 2 were relatively constant for these reference sections

magnetic susceptibility as carrier of a climatic signal in chalk abstract the composition and the magnetic properties of the insoluble residue of maastrichtian chalk from the north sea have been investigated in order to get a better understanding of the cyclic signals in chalk carbonates and the depositional processes that have contributed to the pattern the insoluble residue of the maastrichtian north sea chalk contains quartz dolomite pyrite organic material ferrimagnetic minerals and clay minerals smectite illites the concentration and in particular the type of clay minerals determine the variations in the magnetic properties of the insoluble residue and the bulk magnetic susceptibility of the chalk the existence of a detrital component in the insoluble residue suggests that the cyclic variations in the bulk magnetic susceptibility and consequently in the composition of the chalk are controlled by changes in the runoff from land into the basin

a tale of shales the relative roles of production decomposition and dilution in the accumulation of organic rich strata middle upper devonian appalachian basin abstract a new consensus on the processes responsible for organic carbon burial in ancient epeiric seas has emerged more firmly grounded in the uniformitarian framework of modern oceanography and biogeochemistry this consensus recognizes the interdependent roles of sedimentation primary production and microbial metabolism in favor of earlier end member models e g production vs preservation in this study one of the classic stratigraphic sequences upon which the preservation end member was based is re interpreted in light of this new consensus the study employs an extensive new sedimentological biogeochemical database from cores drilled in western new york the database spans over 500 m and 15 my of devonian deposition in the appalachian basin and provides a framework for comparative study of organic matter burial the major conclusions are 1 few organic rich units were deposited under pervasive anoxic sulfidic water columns 2 establishment and breakdown of seasonal thermoclines on annual or longer timescales were the predominant mode of stratification and 3 under such conditions remineralization of bio limiting nutrients may have played a key role in organic matter burial by creating a eutrophication pump this pump may have augmented an already rising nutrient inventory such that productivity levels exceeded the threshold required for development of suboxic to anoxic conditions in sediments and episodically in bottom waters a final conclusion asserts that the master variable for organic matter accumulation was relative sea level change which exerted influence on clastic dilution preservation and production processes sea level rise events led to sediment starvation and organic carbon concentration in distal basin sediments as well as to decreased effectiveness of seasonal mixing and thus longer build up intervals for remineralized nutrients episodic mixing of nutrient enriched bottom waters led to enhanced production ultimately increased clastic sediment delivery and water column mixing during relative sea level fall diluted surface sediment organic content such that respiratory demand could be met by increased oxygen supply thus terminating deposition of strata enriched in organic carbon

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

cyclostratigraphic calibration of the frasnian late devonian time scale western alberta canada until now the duration of the frasnian stage has remained very poorly constrained hampering a detailed understanding of sedimentation processes and environmental and evolutionary change in this study timeseries analyses of high resolution 10 20 k y magnetic susceptibility data identify sixteen 405 k y eccentricity cycles in the magnetic susceptibility stratigraphy of the frasnian late devonian derived from carbonate platform and surrounding slope and basin deposits in western alberta canada previous studies demonstrated the generally consistent pattern of magnetic susceptibility change across the alberta basin and thus demonstrated the utility of magnetic susceptibility stratigraphy as a refi ned regional correlation tool compared to biostratigraphy in the present study we show that the magnetic susceptibility stratigraphy of the frasnian interval in western alberta has been signifi cantly infl uenced by astro nomical forcing using the sixteen 405 k y eccentricity cycles as a geochronometer we constructed a frasnian astronomical time scale this time scale indicates a duration of 6 5 0 4 m y for the frasnian calibrating this duration to the best available devonian chronology the absolute age of the givetian frasnian boundary is recalculated to 383 6 3 0 ma and the age of the frasnian famennian boundary is recalculated to 376 7 3 0 ma these new absolute ages take into account the astronomically derived duration of the frasnian but they also yield a narrowing of the error margins of the absolute ages by several hundreds of thousands of years

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

widespread chemical remagnetization orogenic fluids or burial diagenesis of clays paleomagnetic rock magnetic and geochemical results from mesozoic carbonates in the vocontian trough in southeast france support a hypothesized acquisition of a chemical remanent magnetization crm during burial diagenesis of smectite and are inconsistent with an orogenic fluid remagnetization mechanism the geographic and vertical distribution of a widespread crm which resides in magnetite is parallel to the degree of burial diagenesis of clays the crm is absent only where there is no evidence for clay diagenesis the crm is poorly developed where significant smectite is still present and is well developed with higher natural and anhysteretic remanent intensities where smectite has completely altered to illite strontium isotope results do not support alteration by orogenic type fluids a commonly invoked mechanism for similar crms the results suggest that the burial diagenesis of clays is a viable remagnetization mechanism for limestones in the vocontian trough

magnetic susceptibility evolution and sedimentary environments on carbonate platform sediments and atolls comparison of the frasnian from belgium and alberta canada article i nfo magnetic susceptibility ms measurements on carbonate rocks are considered as a proxy for impurities delivered to the carbonate environments in the absence of strong climatic or tectonic variations bulk ms values have been linked to sea level variations because sea level fall increases clastic supply and therefore increases in magnetic mineral deposition in this paper we explore the relationship between the average magnitude of bulk ms with shallowing up sequences and facies evolution in different devonian carbonate complexes similarities and differences between these parameters have been scrutinized in carbonate attached platform and detached platforms mounds and or atolls from belgium and canada in the carbonate attached platforms from belgium and canada the ms patterns are directly related to depositional environment mean ms values increase from the most distal towards the most proximal facies and towards the top of the majority of fourth order shallowing up sequences these trends are in agreement with theoretical background ms increases with regression

magnetosusceptibility event and cyclostratigraphy method applied to marine rocks detrital input versus carbonate productivity magnetic susceptibility data from marine rocks can be used for global correlation due to synchronous variations in global erosion we show here that the magnetic susceptibility signature found in two forms resides mainly in paramagnetic and other detrital constituents in most marine rocks the first form is a short term low magnitude high frequency cyclic climate signature that is often useful for regional correlation the second form is a longer term higher magnitude low frequency signature resulting from transgressive and regressive events that can be used for global correlation fluctuations in detrital input due to eustatic based erosion are the primary cause of events these fluctuations are driven by large scale processes such as global orogenic cycles however variations in carbonate productivity cannot be ruled out when explaining the low magnitude climate driven cyclicity also observed in magnetic susceptibility data sets

subglacial bedform geomorphology of the irish ice sheet reveals major configuration changes during growth and decay the belated realisation that ribbed rogen moraines form such an integral part of irish geomorphology and the piecemeal approach to previous drumlin mapping is probably responsible for the highly contrasting views of palaeoflow patterns of the irish ice sheet using a high resolution 25 m digital elevation model we present morphological maps of a large part 100 100 km of the so called drumlin belt of north central ireland the landforms comprise mostly ribbed moraine much larger than found elsewhere up to 16 km in length which in places are superimposed on each other contrary to most prior assessments we find the bedform record to contain numerous and overlapping episodes of bed formation ribbed moraine drumlins and crag and tails that provide a palimpsest record of changing flow geometries these demonstrate an ice sheet with a centre of mass and flow geometry that changed during growth and decay using distinctive flow patterns and relative age relationships between them we reconstruct ice sheet evolution into four phases during a single glacial cycle in phase 1 early in the glacial cycle scottish and local ice coalesced to form a northeast centred irish ice sheet as it grew its centre of mass migrated southwards culminating in a major n s divide positioned down the east of ireland phase 2 ca last glacial maximum during retreat the centre of mass migrated at least 120 km northwards and became established in northwest ireland and at this point a dramatic bedforming event produced one of the world s largest and most contiguous ribbed moraine fields phase 3 final deglaciation is thought to be by fragmentation into many topographically controlled minor ice caps phase 4 rather than any dramatic or unexpected behaviour the reconstructed phases indicate a relatively predictable pattern of ice sheet growth and decay with changes in centres of mass and does not require major readvances or ice stream events copyright 2001 john wiley sons ltd

magnetostratigraphic results from impact crater lake el gygytgyn northeastern siberia a 300 kyr long high resolution terrestrial palaeoclimatic record from the arctic summary a 12 7 m long sedimentary record recovered from lake el gygytgyn located in a meteorite impact crater created 3 6 ma in late cretaceous igneous rocks on chukotka peninsula northeast siberia has been analysed for its palaeo and rock magnetic properties continuous high resolution 1 mm measurements of magnetic susceptibility yielded successions of pronounced lows and highs analyses of the rock magnetic properties by low and high temperature runs of magnetic susceptibility determination of hysteresis parameters as well as irm acquisition experiments yielded a dominance of psd pseudo single domain magnetite in intervals of high magnetic susceptibility whereas due to selective magnetite dissolution associated with anoxic lake water and or pore water conditions during times of enhanced deposition of organic matter haematite dominates within low susceptibility intervals in terms of mass percentage here however magnetic properties are still dominated by magnetite five ams accelerator mass spectromentry 14 c ages eight irsl infrared stimulated luminescense ages together with preliminary pollen data suggest that variations in magnetite content reflect climatic variability of the last 300 ka with low high susceptibilities representing cold warm climates this pattern caused by a complex system of deposition preservation or decomposition of organic matter and or magnetic minerals in turn can be correlated in detail to global climate archives such as the oxygen isotope records from greenland ice cores and marine sediments respectively thus the sedimentary sequence recovered from lake el gygytgyn represents the longest continuous terrestrial climate record now available from the arctic

the late frasnian kellwasser horizons of the harz mountains germany two oxygen deficient periods resulting from different mechanisms in the harz mountains germany three late devonian sections aeketal huhnertal and kellwassertal were analyzed for major and trace element concentration the inorganic geochemical data were used to determine the environmental conditions associated with the deposition of the two late frasnian black kellwasser horizons in a submarine rise environment low ti al and zr al values and slight enrichment in nutrient and organic matter related trace elements e g ba cu and ni suggest that the kellwasser horizons were deposited during two periods of minimum detrital input but relatively elevated primary production enrichments in u v and mo in both black kellwasser horizons and values of redox indices including u th v cr and ni co indicate oxygen restricted conditions prevailed during the late frasnian times under oxygen depleted conditions reductive diagenesis caused precipitation of iron sulphides such as pyrite that are observed with sem however variations in major and trace element concentrations are not similar in the two kellwasser horizons particularly regarding redox markers the lower kellwasser horizon seems to be more o2 depleted than the upper kellwasser horizon in some german sections on the basis of these results we propose that the two kellwasser horizons are not caused by identical conditions the lower kellwasser horizon results from increased primary productivity enhanced by land derived nutrient loading and which triggered the anoxic conditions recorded in drowned platform setting in contrast the upper kellwasser horizon results from the onset of oxygen impoverished bottom water in the deepest part of the ocean due to episodic water stratification during maximum frasnian flooding oxygen depleted water may have impinged shallower environment during the linguiformis transgressive pulse nutrients would have been released from organic matter decay under reducing conditions and brought up to the photic zone during episodic water mixing

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

astronomical calibration of the paleocene time the first complete cyclic sedimentary successions for the early paleogene from drilling multiple holes have been retrieved during two odp expeditions leg 198 shatsky rise nw pacific ocean and leg 208 walvis ridge se atlantic ocean these new records allow us to construct a comprehensive astronomically calibrated stratigraphic framework with an unprecedented accuracy for both the atlantic and the pacific oceans covering the entire paleocene epoch based on the identification of the stable long eccentricity cycle 405 kyr high resolution x ray fluorescence xrf core scanner and non destructive core logging data from sites 1209 through 1211 leg 198 and sites 1262 1267 leg 208 are the basis for such a robust chronostratigraphy former investigated marine odp sites 1001 and 1051 and land based e g zumaia sections have been integrated as well the high fidelity chronology is the prerequisite for deciphering mechanisms in relation to prominent transient climatic events as well as completely new insights into greenhouse climate variability in the early paleogene we demonstrate that the paleocene epoch covers 24 long eccentricity cycles we also show that no definite absolute age datums for the k pg boundary or the paleocene eocene thermal maximum petm can be provided by now because of still existing uncertainties in orbital solutions and radiometric dating however we provide two options for tuning of the paleocene which are only offset by 405 kyr our orbitally calibrated integrated leg 208 magnetostratigraphy is used to revise the geomagnetic polarity time scale gpts for chron c29 to c25 we established a high resolution calcareous nannofossil biostratigraphy for the south atlantic which allows a much more detailed relative scaling of stages with biozones the re evaluation of the south atlantic spreading rate model features higher frequent oscillations in spreading rates for magnetochron c28r c27n and c26n

cross basin variations in magnetic susceptibility influenced by changing sea level paleogeography and paleoclimate upper devonian western canada sedimentary basin abstract upper devonian rocks in western canada were deposited as carbonate platform and surrounding slope and basinal rocks during a second order sea level cycle characterized by long term climatic warming that ended with cooling associated with the frasnian famennian mass extinction event in this contribution we assess the hypothesis that high resolution magnetic susceptibility ms data records cross basin variations that are intimately tied to sea level and associated climatic change the depositional environment and the paleogeographic location of siliciclastic sediment sources four main trends are apparent in the ms data 1 the magnitude of the ms signature increases from west to east across western canada 2 platform facies generally have low ms signatures compared with off platform facies 3 the overall ms signature increases and then decreases throughout the stratigraphic profile largely in response to changes in depositional setting i e item 2 4 ms is generally high during third and fourth order sea level lowstand and early transgression and lower during late transgression and highstand during the late middle givetian to early late frasnian devonian siliciclastic sediments silt and fine sand size sourced from the west alberta arch and peace river arch contributed to relatively high ms signatures in facies deposited proximal to these source areas other important detrital sediment sources were the ellesmerian orogenic belt in northern canada and the laurussian continental landmass to the east these latter sources supplied fine grained sediments that were dispersed basinwide and resulted in a nearly order of magnitude increase in ms values to the east this trend and fine grain size implies that some detrital sediment was delivered from the east as eolian dust from the old red continent by the early frasnian the west alberta arch was submerged and by middle to late frasnian time the peace river arch was largely covered during the late middle frasnian a pronounced rise in ms signatures can be tied directly to deposition of the clay and locally silt rich mount hawk formation during the late middle to late frasnian carbonate ramp facies prograded from east to west resulting in seaward migration of the shoreline and increased siliciclastic input to the ramp slope that influenced the pronounced increase in ms signatures to the east by the late frasnian cross basin ms profiles record uniformly low susceptibilities indicating that high ms siliciclastic detritus was either bypassed seaward was swamped by carbonate input or was dominated by diamagnetic quartz individual ms events can be accurately correlated across much of the basin at a resolution that is higher than single conodont zones despite the local influences on variability of the signature variations in the ms signature can thus be explained partly by depositional environment platforms with low ms basins with higher ms and partly by third and fourth order sea level and associated climatic changes fourth order changes in sea level are routinely interpreted to result from variations in paleoclimate in the milankovitch band and recent work indicates that third order sea level changes may be linked to longer term modulation of milankovitch band orbital variations the increase in ms during prolonged frasnian warming thus appears to be directly linked to paleoclimatic change and comparison of the ms signature with more direct measures like oxygen isotopes would serve as a test of its utility as a paleoclimate proxy

magnetic fabric variations in mesozoic black shales northern siberia russia possible paleomagnetic implications abstract a 28 m long section situated on the coast of the arctic ocean russia 74 n 113 e was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the jurassic cretaceous boundary the section consists predominantly of marine black shales with abundant siderite concretions and several distinct siderite cemented layers low field magnetic susceptibility k ranges from 8 10 5 to 2 10 3 si and is predominantly controlled by the paramagnetic minerals i e iron bearing chlorites micas and siderite the siderite bearing samples possess the highest magnetic susceptibility usually one order of magnitude higher than the neighboring rock the intensity of the natural remanent magnetization m0 varies between 1 10 5 and 6 10 3 a m several samples possessing extremely high values of m0 were found there is no apparent correlation between the high k and high m0 values on the contrary the samples with relatively high m0 values possess average magnetic susceptibility and vice versa according to the low field anisotropy of magnetic susceptibility ams three different groups of samples can be distinguished in the siderite bearing samples i an inverse magnetic fabric is observed i e the maximum and minimum principal susceptibility directions are interchanged and the magnetic fabric has a distinctly prolate shape triaxial fabric samples ii showing an intermediate magnetic fabric are always characterized by high m0 values it seems probable that the magnetic fabric is controlled by the preferred orientation of paramagnetic phyllosilicates e g chlorite and mica and by some ferromagnetic mineral with anomalous orientation in relation to the bedding plane oblate fabric samples iii are characterized by a bedding controlled magnetic fabric and by moderate magnetic susceptibility and m0 values the magnetic fabric is controlled by the preferred orientation of phyllosilicate minerals and to a minor extent by a ferrimagnetic fraction most probably detrital magnetite considering the magnetic fabric together with paleomagnetic component analyses the siderite bearing and the high nrm samples about 15 of samples were excluded from further magnetostratigraphic research

magnetism of chinese loess deposits summary the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene and ascertained a maximum loess age of about 1 2 myr a new collection of nearly 500 samples from a natural outcrop and a borehole section near lochuan lat 35 8 n long 109 2 e shaanxi province has been dated by magnetic stratigraphy thermal cleaning of the natural remanent magnetization nrm removes a strong secondary component of viscous origin along the present geomagnetic field which resides largely in magnetite the characteristic nrm component is due to haematite which is thought to be of chemical origin rhythmical intensity variations of nrm and initial susceptibility depend on the loess lithology and may reflect climatic changes during loess deposition the palaeomagnetic results are consistent between the two sections and yield a clearly defined magnetic polarity zonation the brunhes matuyama boundary and the jaramillo subchron have been positively identified in both outcrops at exactly the same stratigraphic level the olduvai subchron has been found in the borehole section which records the entire loess sequence most probably the formation of chinese loess began shortly after the matuyama gauss polarity transition therefore a late pliocene age of about 2 4 myr is assigned to the oldest loess sediments measured

stratigraphic and oxygen isotope evidence for my scale glaciation driving eustasy in the early middle devonian greenhouse world article i nfo million year scale 3rd order depositional sequences few 10s to 100 m thick are common in the devonian marine record and are correlated between sedimentary basins and across widely separated continents implying a eustatic origin tectonically driven changes in mid ocean ridge spreading rates lengths are too slow to account for the calculated 3rd order rise fall rates and typical orbital frequencies 20 400 ky are too fast we analyzed oxygen isotopes from conodont apatite to evaluate if early middle devonian 3rd order sea level changes were controlled by paleoclimatically driven ice volume glacio eustasy and seawater temperature change thermo eustasy two successive depositional sequences in the western u s nevada and czech republic prague basin were sampled the 18 o values range from 17 8

astronomical calibration of the early oxfordian vocontian and paris basins france consequences of revising the late jurassic time scale abstract magnetic susceptibility ms variations record earth s orbital parameters in the clays and marls of the early oxfordian in the paris and vocontian basins of france climatically driven weathering of surrounding emergent areas and the resulting detrital input to the basins is a significant source of the ms signal ms proves to be an effective tool for deciphering the orbital forcing signal in these sediments and for assessing the chronology of these formations the 405 kyr orbital eccentricity cycle is clearly visible in the ms signal and consequently is a valuable geochronometer for this portion of the jurassic time scale astronomical calibration of the mariae ammonite zone basal oxfordian stage indicates a duration of 2 2 myrs whereas current time scales assign only 0 6 myrs to this biozone however the late jurassic time scale has large uncertainties in stage boundary ages 4 myrs hence of interval durations these results could lead to significant revisions in the m sequence magnetic anomaly block model and greatly improve the resolution of the jurassic time scale

allocyclic control on late devonian buildup development southern canadian rocky mountains abstract parasequence stacking patterns in upper devonian fairholme group reef complexes of the southern canadian rocky mountains suggest that deposition and buildup stratigraphy were controlled by superimposed short term and long term fluctuations in relative sea level correlation of these stacking patterns from one reef complex to another reveals a hierarchy of fifth order and third order sea level changes the flume platform and overlying upper cairn biostrome consist of meter scale shallowing upward parasequences these are interpreted to have been deposited during short term high frequency fifth order oscillations of relative sea level superimposed on this high frequency cyclicity are larger broadly shallowing upward trends in which dominantly subtidal meter scale parasequences or subtidal noncyclic intervals gradually sometimes abruptly pass upward into peritidal parasequences of comparable thickness these sequences are regionally correlative and are the product of third order driving mechanisms of extrabasinal origin collectively the flume upper cairn and overlying peechee members represent a single depositional sequence sensu vail et al 1977 the various order of cyclicity are best developed in the eastern main ranges where greater syndepositional subsidence allowed for increased sediment accumulation the development of entirely subtidal parasequences and the regional correlation of larger scale groups of vertically stacked subtidal and peritidal parasequences suggest a syndepositional allocyclic control on buildup development rather than an autocyclic one however large uncertainties in the calculated durations of both the high frequency parasequences and the larger scale sequences and assumptions inherent in these calculations obscure the evidence for possible milankovitch control on the development of the cairn formation instead late devonian differential subsidence related to the formation of the antler flysch basin is invoked to explain the regionally correlative large scale groupings of parasequences

variations of terrestrial input and marine productivity in the southern ocean 48 s during the last two deglaciations various biomarkers n alkanes n alcohols and sterols have been studied in a piston core tsp 2pc taken from the southern ocean to reconstruct the paleoenvironmental changes in the subantarctic region for the last two deglaciations mass accumulation rates of terrestrial higher molecular weight n alkanes and n alcohols and marine dinosterol and brassicasterol biomarkers increased significantly at the last two glacials and stayed low during interglacial peaks early holocene and the eemian these records indicate that the enhanced atmospheric transport of continental materials and the increased marine biological productivity were synchronously linked in the southern ocean at the last two glacials this suggests that increased glacial dust inputs have relieved iron limitation in the subantarctic southern ocean these two processes however were not linked at the cooling phase from the eemian to marine isotope stage mis 5d during this period paleoproductivity may have been influenced by the latitudinal migration of the high production zone associated with the antarctic polar front

magnetostratigraphy susceptibility of the frasnian famennian boundary magnetosusceptibility event and cyclostratigraphy msec is used to establish a non polarity based magnetostratigraphy susceptibility mss between the stratotype region for the frasnian famennian f f boundary sequence in the montagne noire of southern france the eastern rheinisches schiefergebirge of germany belgium the eastern anti atlas of morocco and the arbuckle mountains of southern oklahoma usa despite differences of depositional environment the mss is remarkably consistent and can be described in the context of a hierarchy of magnetozones that allow the extension of correlation away from the montagne noire reference section the nature of the controls on the influx of iron into the marine system produces a natural hierarchy of at least seven orders or magnetozones designated msz1 msz2 msz3 msz4 msz5 msz6 and msz7 these are characterized in terms of the magnitude of their duration the mss reference section of choice for the f f boundary is the well known trench c at la serre lsc in the northeastern montagne noire of southern france the f f boundary is located at the lower boundary of magnetozone la serre i 3b at la serre as well as in all other sections studied msz4 and msz5 magnetozones are used to establish intra and inter regional correlation between the reference sequence and the other regions the base of the upper kellwasser event is as variable within the mss zonation as it is within the biostratigraphic zonation as is the position of the lower kellwasser event

climatically sensitive eolian and hemipelagic deposition in the cariaco basin venezuela over the past 578 000 years results from al ti and k al al ti and k al ratios in bulk sediment are used to interpret wind blown and hemipelagic sources of deposition to a 578 kyr record in the cariaco basin venezuela ocean drilling program site 1002 graphical and cross spectral analyses indicate that these ratios vary extremely closely with planktonic foraminiferal 18o with both ratios being significantly higher during interglacials and lower during glacials k al indicates that during glacials the lower sea level that results in relative basin isolation increases the relative proportion of kaolinite derived from local rivers draining the relatively humid venezuelan margin al ti decreases during glacials suggesting greater proportions of eolian rutile sourced from the northern sahara due to increased wind strength and or aridity this interpretation is consistent with previous studies of the chemistry and mineralogy of saharan derived eolian matter in the caribbean and with a mass balance determining the effect of changes in eolian rutile accumulation on the bulk sedimentary al ti ratio

microbial carbonates as indicators of environmental change and biotic crises in carbonate systems examples from the late devonian alberta basin canada microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the proterozoic we report here the varied roles that microbial carbonates played in late devonian carbonate platforms in the alberta basin canada we recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the frasnian famennian mass extinction detailed sequence stratigraphic analysis of two isolated platforms in the canadian rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation aggradation and backstepping and renewed progradation related to rates of second order and third order sea level change and basin infill the carbonate system was reorganized following annihilation of many carbonate producing biota during the frasnian famennian mass extinction microbial carbonates figure prominently in both frasnian platform development and the famennian recovery of the carbonate system following the frasnian famennian mass extinction during transgressive platform phases deeper water facies with a crinoid rugose coral and microbe dominated biota abruptly overlie prograding stromatoporoid framestones microbial carbonates consist of abundant girvinellid oncoids that nucleated around other fossil clasts the temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply microbes played a significant role in highstand reef margin facies where renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral stromatoporoid mounds microbially laminated carbonates also form stromatolitic mats in peritidal shallowing upward cycles and repetitive stromatolitic intervals commonly indicate sea level lowstands associated with the development of sequence boundaries stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats an end frasnian sea level lowstand exposed the isolated carbonate platforms transgression and mass extinction characterize the frasnian famennian boundary event and microbial carbonates occur at this horizon in western canada stromatolites oncoids and large scale microbial thrombolites appear to be

a climate driven model and development of a floating point time scale for the entire middle devonian givetian stage a test using magnetostratigraphy susceptibility as a climate proxy abstract here we propose an approach to establish a preliminary age chronology for complete stages within the phanerozoic by applying cyclostratigraphic methods that employ high resolution data sets this requires use of geochemical or geophysical data known to serve as a cyclic climate proxy to demonstrate the method we use the magnetostratigraphic susceptibility technique as the basis for fitting a climate model to the givetian stage of the middle devonian system we show a milankovitch eccentricity climate zonation for the givetian that is pinned to time series analyses from outcrop samples from its upper givetian frasnian france and lower eifelian givetian morocco stage boundaries using these data sets we construct a uniform cyclicity model designed to conform to a 405 kyr cyclicity with a duration corresponding to the published duration for the givetian of 4 4 myr kaufmann 2006 to this model we fit two well established conodont zonation schemes thus allowing time estimates for conodont ranges for the givetian indicating a range from 1 8 myr to 100 kyr before extinction of an individual conodont species we then test and adjust the model using independent data sets from the eastern united states adjustments to the initial model yield a duration of 28 climate half cycles zones for the givetian stage and an increased age to 5 6 myr these zones allow high correlation among sections to better resolve timing of major bio events and provide a floating point time scale to which absolute ages can be applied

magnetostratigraphy susceptibility of the upper ordovician kope formation northern kentucky abstract as a test of the utility of magnetic susceptibility ms measurements for correlation among lithified marine sequences samples were collected from well studied shale and limestone outcrops of the upper ordovician kope formation northern kentucky the bulk initial low field ms of these samples is compared among two litho and biostratigraphically correlated sections containing the same beds the results of this comparison demonstrated an excellent correlation among equivalent beds thermomagnetic susceptibility analysis supports previous xrd work indicating that illite is primary and is the main paramagnetic mineral responsible for the ms variations observed in this study a composite of ms results for a sequence of 31 named shale limestone couplets is used to build a composite section of kope sequences for the area these ms variations permit division of the various sections into a set of 28 ms cycles within the kope formation covering 50 m of section these cycles suggest a systematic control by climate on the influx of detrital material that formed these sedimentary sequences and support other work indicating kope climate cyclicity

covariance of c and o isotopes with magnetic susceptibility as a result of burial diagenesis of sandstones and carbonates an example from the lower devonian la vid group cantabrian zone nw spain the burial diagenesis of sandstones limestones and dolostones of the lower devonian la vid group in the cantabrian zone nw spain reveal a covariance of carbon and oxygen isotope values with magnetic susceptibility also strontium isotopes and to a minor degree fe follow this trend the main carriers of the magnetic susceptibility appear to be diagenetic fe carbonates i e siderite ferroan dolomite and ankerite which occur as cements in primary and secondary voids as well as in fractures in some layers especially at the top of the succession there occurs additionally secondary fe chlorite and pyrite the fe carbonates were formed during upward migration of a reducing iron bearing petroliferous fluid that was depleted in 13c and carried radiogenic sr similar geochemical covariance and or correlations can be expected in other sedimentary successions affected by the migration of petroliferous formation fluids

high resolution magnetic susceptibility and geochemistry for the cenomanian turonian boundary gssp with correlation to time equivalent core abstract the upper cretaceous cenomanian turonian c t boundary represents the most significant thermal climatic event in the mesozoic and it has been well studied here we present a relatively new approach for providing both independent climate based cyclostratigraphic analysis and correlation among c t sections we have collected samples for bulk initial low field magnetic susceptibility ms measurement and high resolution inorganic geochemical analyses from two c t sequences 1 the well defined c t global boundary stratotype section and point gssp located at lake pueblo colorado and deposited in the upper cretaceous western interior seaway of north america and 2 the well studied marlstone and limestone usgs 1 portland core drilled at portland colorado 40 km to the west of the c t gssp comparing the lithostratigraphy chemostratigraphy and magnetostratigraphic susceptibility developed from magnetic susceptibility ms data indicates that the usgs 1 portland core closely compares with the c t gssp sequence using any of the three measures demonstrating that intervals collected and sampled from these two sequences are highly correlated a preliminary magnetostratigraphic susceptibility zonation for the c t boundary is presented to which other ms work can be compared fourier transform ft analysis of the ms data from the gssp when compared with previous ft work using geochemical analysis of portland core samples shows close similarities to the gssp with milankovitch eccentricity obliquity and precession bands well defined in both data sets using this result we assign relative ages to the intervals sampled and evaluate changes in absolute sediment accumulation rates through the c t boundary interval we show that while sediment accumulation rate is relatively high in the upper cenomanian a change toward lower accumulation rates in the gssp section at the c t boundary is due to reduced productivity following the c t extinction events that resulted from global warming associated with oceanic anoxic event 2 oae2 that began in the uppermost cenomanian

climate response to orbital forcing across the oligocene miocene boundary spectral analyses of an uninterrupted 5 5 million year my long chronology of late oligocene early miocene climate and ocean carbon chemistry from two deep sea cores recovered in the western equatorial atlantic reveal variance concentrated at all milankovitch frequencies exceptional spectral power in climate is recorded at the 406 thousand year ky period eccentricity band over a 3 4 million year period 20 to 23 4 my ago ma as well as in the 125 and 95 ky bands over a 1 3 million year period 21 7 to 23 0 ma of suspected low greenhouse gas levels moreover a major transient glaciation at the epoch boundary approximately 23 ma mi 1 corresponds with a rare orbital congruence involving obliquity and eccentricity the anomaly which consists of low amplitude variance in obliquity a node and a minimum in eccentricity results in an extended period approximately 200 ky of low seasonality orbits favorable to ice sheet expansion on antarctica

geophysical records of dispersed weathering products on the frasnian carbonate platform and early famennian ramps in moravia czech republic proxies for eustasy and palaeoclimate abstract an exceptionally high sea level in frasnian times early late devonian changed many peneplained cratonised and tectonically undisturbed continental margins of tropical belts into carbonate platforms which became covered by thick banks of limestone the stages marked by very pure carbonates developed during partial sea level highstands since when the flooding reached its maximum the shoreline was distant and conditions for reef growth were good an increased extension of emerged flat coastal areas was coupled with an opposite situation when oscillating sea level fell to its momentary minimum and the accommodation space available for accumulation of carbonate material was low subaerially exposed surfaces were subject to weathering and the products were dispersed by aeolian or aquatic means to form layers when these weathering materials were trapped in carbonates this overall scenario relating to the trapping of weathering products in pure carbonates was tested at geographically isolated platform with a stable and only slowly subsiding basement eastern moravia czech republic the amounts of weathering products dispersed in limestone were documented using the combined methods of ray logging ray spectrometry and magnetic susceptibility measuring i e according to their radioactivity k th and u and magnetism fe six third order frasnian eustatic cycles have been inferred from the data with an estimated period of 1 ma the late frasnian and especially the frasnian famennian f f transition brought the total destruction or at least a strong overlay on this hypothetical background setting the record of strong sea level fluctuations in the f f interval is shown in strong oscillations and offsets related to pronounced hiatuses and flooding surfaces such rapid sea level fluctuation is typical for glacioeustatic control although the present estimates of the maximum amplitudes are larger than realisable by glaciation events and may be explained in combination with tectonoeustatic models comparison of sections between the centre and periphery of the platform substantiates the concept that oscillating late frasnian shorelines were rapidly prograding very low sea level stages of early famennian times changed the depositionary systems from extensive platforms to narrow carbonate ramps which were positioned out and down on the extinct reef bank slopes the central parts of the eastern moravian and also other frasnian platforms in the world were not normally covered by f f sediments the transgressive onlaps above any crystalline highs were not observed and therefore the drowned frasnian reefs on their periphery although emphasised by many geologists may reflect sliding or tectonic subsidence on the margins of platform blocks rather than eustatic rise of sea level

late devonian glacial deposits from the eastern united states signal an end of the mid paleozoic warm period abstract a late devonian polymictic diamictite extends for more than 400 km from northeastern pennsylvania across western maryland and into east central west virginia the matrix supported unbedded locally sheared diamictite contains subangular to rounded clasts up to 2 m in diameter the mostly rounded clasts are both locally derived and exotic some exhibit striations faceting and polish the diamictite commonly is overlain by laminated siltstone mudstone facies associations laminites the laminites contain isolated clasts ranging in size from sand and pebbles to boulders some of which are striated the diamictite laminite sequence is capped by massive coarse grained pebbly sandstone that is trough cross bedded a stratigraphic change from red calcic paleo vertisols in strata below the diamictite to non calcic paleo spodosols and coal beds at and above the diamictite interval suggests that the climate became much wetter during deposition of the diamictite the diamictite deposit is contemporaneous with regressive facies that reflect fluvial incision during the late devonian of the appalachian basin these deposits record a late devonian episode of climatic cooling so extreme that it produced glaciation in the appalachian basin evidence for this episode of climatic cooling is preserved as the interpreted glacial deposits of diamictite overlain by glaciolacustrine varves containing dropstones and capped by sandstone interpreted as braided stream outwash the appalachian glacigenic deposits are contemporaneous with glacial deposits in south america and suggest that late devonian climatic cooling was global this period of dramatic global cooling may represent the end of the mid paleozoic warm interval that began in the middle silurian

bypass margins basin restricted wedges and platform to basin correlation upper devonian canadian rocky mountains implications for sequence stratigraphy of carbonate platform systems abstract carbonate platforms can commonly keep up with relative sea level rise because of high rates of sediment accumulation and platform aggradation surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment if accumulation rates in basinal settings lag behind those of the platform a bypass or erosional margin can develop under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered we report here the results of high resolution stratigraphic analyses of two upper devonian isolated carbonate platforms in western alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences the slope and basin sequences surrounding the miette and ancient wall platforms consist of basin restricted onlapping wedges of fine grained background sediment deposited dominantly from suspension and coarse grained platform derived sediment redeposited by a variety of gravity flow mechanisms sequence boundaries are identified within the redeposited carbonate intervals identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry mineralogy and clast content of redeposited carbonate units highstand carbonates contain sheet like debris flows and turbidites with abundant slope derived clasts and background facies with high total carbonate content lowstand carbonates contain sheet like and channelized debris flows and turbidites with abundant platform derived clasts and background facies with low carbonate content and locally high amounts of organic carbon transgressive facies are dominated by initially carbonate poor and organic rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content these patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes lowstand redeposited carbonates indicate platform and platform margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions high siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine grained carbonate sediment successive stages of platform development at miette and ancient wall were controlled by accommodation changes driven by relative sea level fluctuations backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development greater subsidence at ancient wall fostered the development of a steeper bypass margin and different slope evolution compared to miette slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform to basin gradient and facilitated regressive platform progradation in conventional siliciclastic sequence stratigraphy basin restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin wedge shaped basinal units along the miette and ancient wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries the results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences interpretation of carbonate basin restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy platform to basin correlation and the volumetric partitioning of sediments deposited in different systems tracts

upper devonian carbonate platform correlations and sea level variations recorded in magnetic susceptibility abstract this paper concerns the analysis of four outcrops in the frasnian upper devonian of belgium we compare the results of a precise sedimentological analysis with magnetic susceptibility ms data this comparison allows us to improve stratigraphic correlations and to test the relationship between magnetic susceptibility and sea level changes considering the sedimentological study different microfacies have been identified from the external belt dominated by relatively argillaceous open marine facies with crinoids to the biostromal and the lagoonal belt dominated by algae rich muddy facies fourth and third order sequences have also been identified and are probably related to sea level variations magnetic susceptibility data provides us very good fourth order correlations and the link between magnetic susceptibility and different sedimentological parameters is obvious more precisely ms appears to be related to fourth and third order sequences and to microfacies

frasnian upper devonian integrated facies analysis magnetic susceptibility and sea level fluctuations in the nw algerian sahara changes in the palaeoenvironment are investigated in two representative frasnian sections of the nw algerian sahara integrating sedimentology and magnetic susceptibility ms the ben zireg section is characterized by condensed and ferruginous calcareous deposits in the south marhouma section the sedimentation rate is high dominated by muddy nodular limestones with several hypoxic shale intervals in both sections sediments were mostly emplaced on pelagic outer ramps below the limit of storm wave base evolving through time from proximal to distal setting investigations of the temporal evolution of facies and ms data permit a first estimate of the local sea level trends in nw algeria these trends match the overall long term rise of sea level recognized worldwide from frasnian zone 5 upwards noteable positive excursions of the sea level curve related to the semichatovae transgression as well as to the late frasnian transgression prior to the late kellwasser event can be established in this area although the sharp regression of sea level at the upper kellwasser level can be confirmed from our data no particular trend is depicted at the transition of conodont zones frasnian zones 12 13 where the presence of the lower kellwasser level has not yet been clearly recognized

the magneto susceptibility event and cyclostratigraphy msec method used in geological correlation of devonian rocks from anti atlas morocco magneto susceptibility event and cyclostratigraphy msec is used here as a means of establishing sensitive chronostratigraphic markers and chronohorizons useful for high resolution correlation msec is a composite of the magnetic susceptibility ms record of marine strata and the coeval biostratigraphic record and similar to chronozones zones based on msec data have boundaries that are isochronous ms a measure of the concentration of magnetic grains in sediments proxies for the ratio of lithogenic to biogenic components controls on the detrital input of lithogenic material include eustasy and climate however induced and sea floor basin subsidence the development of an initial msec composite reference curve for the middle upper devonian of the anti atlas region of southern morocco is presented and discussed in the context of global sea level transgressive and regressive cycles msec trends composed of increasing ms magnitudes correlate well with episodes of regression whereas trends of decreasing ms magnitudes correlate with episodes of transgression

tectonic controls of phanerozoic sedimentary rock cycling the cyclic nature of the phanerozoic sedimentary rock distribution carbon sulphur coupling and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors the distribution of preserved sedimentary mass in terms of rock mass remaining u geologic age shows a minimum c 300 350 ma ago which separates two subcycles of erosion and deposition of sedimentary rocks the older subcycle was interrupted because of the major continental collisions of the devonian and late carboniferous these collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses leading to a decline in the probability of destruction and an increase in half life of these older sediments a strong correlation exists between the long term cyclicity in the phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs this correlation is related to two principal tectonic modes of the phanerozoic oscillatory and submergent it is postulated that the submergent mode of active plate convergence obduction and subduction of sediments large ridge volume and high sea level gave rise to low erosion and sedimentation rates less restricted environments of carbonate deposition and relatively high atmospheric co 2 levels high temperatures resulting from an increased rate of production of co 2 from diagenetic and metamorphic reactions at subduction zones as sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir as sea level fell reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors these reservoir transfers are consistent with secular changes observed in the distribution of 13 c and 34 s in sedimentary materials during the phanerozoic petrographic examination of phanerozoic oolite formations shows that ooids with preserved calcitic relict textures are characteristic of pre carboniferous carbonate rocks whereas ooids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age these changes in ooid mineralogy may be interpreted as reflecting changes in co 2 levels of the ocean atmosphere system consistent with the above tectonic considerations atmospheric co 2 levels were higher prior to carboniferous time favouring formation of calcitic ooids and skeletal parts after the carboniferous co 2 levels fell and aragonite and mg calcites of greater than 8 mol mg increased in abundance as precipitates

feedback between aeolian dust climate and atmospheric co2 in glacial time 1 enhanced aeolian supply of iron to the biota of the southern ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric co2 declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in co2 but the reasons behind the dust decline itself are as yet unknown here we show that the dust record from the vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of patagonian sources of aeolian material and the efficiency with which it is transported through the atmosphere we suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that in turn climate is responsive to the dust due to iron fertilization this positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states characterized by distinct co2 concentrations recognition of this glacial subcycle may help to account for the timing and sequence of events at the terminations

high resolution chronostratigraphy from downhole susceptibility logging tuned by palaeoclimatic orbital frequencies abstract high resolution magnetic records have been obtained by downhole measurement with the geological high resolution magnetic logging tool ghmt in a well drilled in the north sea for hydrocarbon exploration this tool enables a continuous measurement of the polarity of the magnetic remanence and of the magnetic susceptibility throughout the drilled formations a high resolution magnetic chronostratigraphy between 2 6 and 1 6 ma is deduced from the tie points given by the downhole magnetostratigraphy and from a detailed spectral analysis of climatically influenced variations of susceptibility the orbital frequencies identified by spectral analysis of the magnetic susceptibility in the depth domain allow a refined estimation of the sedimentation rate and determination of a high resolution chronology of the sequence characteristic features of the onset of northern hemisphere glaciation and its fluctuations during the late pliocene are identified through this analysis of the susceptibility record our analysis indicates that massive discharge events of non pelagic deposits similar but with a higher amplitude to heinrich events that are observed during the last glaciation occurred between 2 5 and 2 0 ma and were probably related to instabilities of the laurentide ice sheet the relative power of the orbital forcing factors is compared with that documented at other north atlantic ocean sites comparison with an insolation model does not indicate direct correlation but suggests a strong influence of the northern hemisphere ice cover and its local variations

global correlation using magnetic susceptibility data from lower devonian rocks magnetic susceptibility data from marine rocks can be used for global correlation because of synchronous variations in global erosion we show here correlations between magnetic susceptibility data from two lower devonian biostratigraphically well defined sections one in northwestern spain and one from the anti atlas region of morocco using the moroccan outcrop section we correlate to cuttings from a bolivian well with minimal biostratigraphic control using the magnetic susceptibility data we identify the lower devonian pragian emsian and lower upper emsian stage boundaries in the bolivian well samples

influences of oceanic rheostats and amplifiers on atmospheric co2 content during the late quaternary abstract although a clear explanation has yet to emerge for the observed decline in atmospheric co2 content during glacial episodes numerous hypotheses have been offered selected examples are reviewed or evaluated here a strengthened biological pump resulting from intensified upwelling at low latitudes was suggested early on as a likely direct cause of the glacial co2 drawdown two problems have since arisen with this hypothesis first on some upwelling continental margins for example off california oregon northwestern mexico peru and parts of nw africa organic matter accumulation decreased in some areas markedly during the last glacial maximum lgm second upwelled cool co2 rich water degasses carbon dioxide in warm regions and for a key area such as the modern equatorial pacific to become a net sink for atmospheric co2 rather than a source as it is today the rate of downward organic carbon export must exceed the rate of degassing recent sedimentary carbon and nitrogen isotopic data suggest however that the equatorial pacific has remained a strong co2 source for at least the last 40 000 yr and probably much longer although the direct extraction of carbon from surface waters and burial in the sediments in productive regions now appears unlikely to provide the sought for explanation indirect effects related to export productivity may hold the key these can modulate the chemical character of the ocean so as to increase its uptake of co2 a rheostat effect or they could increase indirectly the nutrient inventory of the sea an amplifier effect for example it is now recognized that denitrification was greatly reduced during glacial maxima in all three principal oxygen minimum zones in the oceans i e the subtropical north and south pacific and the arabian sea the implication is that nitrate the limiting nutrient in the modern ocean must have been more abundant during glacial periods and that this surfeit would have supported increased export production in the meso or oligotrophic areas that are today nitrate limited dust may represent another rheostat increases in atmospheric turbidity during glacials are clearly recorded by ice and marine sediment cores in both hemispheres in addition to fostering enhanced export production by adding needed iron to nitrate rich areas it has been suggested recently that increased dust inputs to nitrate depleted regions during glacials might have encouraged growth of iron hungry n2 fixing cyanobacteria thus alleviating nitrate limitation no sedimentary 15 n data yet exist to support this hypothesis but it remains viable finally regional changes in physical hydrography may have played a major and hitherto underestimated role for example in the glacial southern ocean south of the polar front recent nitrogen isotope and other data imply that the upper water column was well stratified during the lgm by limiting upwelling this would have reduced the ocean atmosphere co2 leak in the area this could have made a significant contribution to the pco2 drawdown

oxygen isotope evolution of biogenic calcite and apatite during the middle and late devonian oxygen isotope ratios of well preserved brachiopod calcite and conodont apatite were used to reconstruct the palaeotemperature history of the middle and late devonian by assuming an oxygen isotopic composition of 1 v smow for devonian seawater the oxygen isotope values of eifelian and early givetian brachiopods and conodonts give average palaeotemperatures ranging from 22 to 25 c late givetian and frasnian palaeotemperatures calculated from 18o values of conodont apatite are close to 25 c in the early frasnian and increase to 32 c in the latest frasnian and early famennian oxygen isotope ratios of late givetian and frasnian brachiopods are significantly lower than equilibrium values calculated from conodont apatite 18o values and give unrealistically warm temperatures ranging from 30 to 40 c diagenetic recrystallization of shell calcite different habitats of conodonts and brachiopods as well as non equilibrium fractionation processes during the precipitation of brachiopod calcite cannot explain the 18o depletion of brachiopod calcite moreover the 18o depletion of brachiopod calcite with respect to equilibrium 18o values calculated from conodont apatite is too large to be explained by a change in seawater ph that might have influenced the oxygen isotopic composition of brachiopod calcite the realistic palaeotemperatures derived from 18o apatite may suggest that biogenic apatite records the oxygen isotopic composition and palaeotemperature of palaeozoic oceans more faithfully than brachiopod calcite and do not support the hypothesis that the 18o 16o ratio of devonian seawater was significantly different from that of the modern ocean

estimating ancient sediment fluxes original phanerozoic sediment flux rates can be approximated by fitting an exponential decay curve to the observed massjage distribution of sediments then multiplying the ratio of the observed mass to the exponential decay curve mass by the zero age flux rate predicted by the exponential decay curve applied to the massjage distribution of existing phanerozoic sediments the reconstruction method indicates that original global sediment flux rates varied cyclically by a factor of up to three this implies significant changes in the rates of uplift and mountain building and global plate tectonic activity

conodont biostratigraphy and magnetic susceptibility of upper devonian chattanooga shale eastern united states evidence for episodic deposition and disconformities abstract recognition of stratigraphic hiatuses in fine grained siliciclastic sedimentary rocks can be challenging but is feasible using high resolution biostratigraphic and chemostratigraphic data within a regional correlation framework in this case study of the upper devonian chattanooga shale in the dupont ghs drillcore from the western margin of the nashville dome the upper devonian chattanooga shale comprises several depositional units separated by intraformational hiatuses these features are developed within a 13 3 m interval consisting of 4 2 m of frasnian strata and 9 1 m of famennian strata that unconformably overlie the sellersburg formation three frasnian seven famennian and one tournaisian conodont biozones are recognized the frasnian famennian boundary is on a disconformity to the northeast in the southern illinois basin the new albany shale in the bcc drillcore consists of 35 m of givetian and frasnian strata and 30 m of famennian strata that conformably overlie the sellersburg formation one givetian four frasnian and three of the four lowest famennian conodont zones are recognized the frasnian famennian boundary is conformable and constrained to within a 5 cm interval bulk magnetic susceptibility ms shows a long term increase through most of the frasnian shorter term ms trends were observed in association with depositional pulses linked to global sea level rises and highstand system tracts characterized by total organic carbon toc maxima eight trends were resolved in the dghs core and six in the bcc the high frequency shifts in ms likely represent milankovitch band sea level and depositional cycles at a scale that cannot be resolved based on the condensed and irregular nature of the depositional packages gamma ray counts in the dghs peak in association with the eight depositional cycles with a toc peak at the base of each cycle

saharan dust transport and high latitude glacial climatic variability the alboran sea record millennial to submillennial marine oscillations that are linked with the north atlantic s heinrich events and dansgaard oeschger cycles have been reported recently from the alboran sea revealing a close ocean atmosphere coupling in the mediterranean region we present a high resolution record of lithogenic fraction variability along images core md 95 2043 from the alboran sea that we use to infer fluctuations of fluvial and eolian inputs to the core site during periods of rapid climate change between 28 000 and 48 000 cal yr b p comparison with geochemical and pollen records from the same core enables end member compositions to be determined and to document fluctuations of fluvial and eolian inputs on millennial and faster timescales our data document increases in northward saharan dust transports during periods of strengthened atmospheric circulation in high northern latitudes from this we derive two atmospheric scenarios which are linked with the intensity of meridional atmospheric pressure gradients in the north atlantic region

upper devonian buildup margin and slope development in the southern canadian rocky mountains the geometries of upper devonian mid frasnian carbonate buildup margins of the peechee member southesk formation in the southern canadian rocky mountains reflect the influence of rising sea level punctuated by stillstands most peechee buildup margins in the cordillera retrograded whereas some aggraded or prograded in response to these sea level changes and variations in the relative rates of adjacent basin filling sedimentation peechee margin accumulation and adjacent basin deposition is best explained by a reciprocal model of sedimentation thick tongues tens of meters of stromatoporoid rudstones and bioclastic grainstones aggraded vertically or retreated during periods of sea level rise as sea level decelerated carbonate production was restricted and outbuilding occurred basinal onlapping sedimentation of the perdrix formation muds increased during the ensuing stillstand of sea level subsequently a renewed episode of relative sea level rise caused build up margin retrogradation this pattern of sedimentation driven by episodic fluctuations of relative sea level occurred repeatedly during peechee time a broad scale shallowing upward trend culminated in a progressive increase up section in the abundance of submarine erosion surfaces in the eastern main ranges and paleocaliche horizons in the front ranges southwesterly paleocurrents are inferred to have influenced peechee buildup margin geometries and adjacent basin sedimentation retrograded windward margins formed in response to active currents and breaking waves along the northeastern edge of the fairholme and southesk cairn reef complexes leeward buildup margins protected from the southwest directed currents exhibit prograding geometries these characteristics are exemplified by the ancient wall complex where the windward buildup margins to the northwest and southeast are vertically aggrading whereas the coeval peechee buildup margin to the southwest prograded 2 km onlapping dysaerobic waters during upper cairn and peechee time inhibited the accumulation of forereef skeletal organisms and served as an additional control on buildup margin geometry

new perspectives on orbitally forced stratigraphy this survey of the current status of research into earth s orbitally forced paleoclimatic record summarizes recent developments in the theory of earth s orbital parameters and reviews how various techniques of data collection and analysis have fared in the search and recovery of orbital signals in ancient stratigraphy the emerging significance of the quasi periodicity of earth s orbital variations as a prin cipal tool in the analysis of orbitally forced stratigraphy is discussed in detail five case studies are presented that illustrate new directions in research a time series analysis of discontinuous strata b measurement of ultra high resolution stratigraphic signals c new perspectives on the 100 kyr pleistocene glaciation problem d strati graphic evidence for solar system resonance modes and e evaluating phanerozoic length of day from orbitally forced stratigraphy the study of how earth s orbital parameters came to be recorded in ancient stra tigraphy has recently moved into a new phase of scientific inquiry and discovery notable advances in celestial mechanics and in the analysis of stratigraphic data have given rise to new ways of solving old problems and have uncovered new problems for the first time new tools have facilitated the recovery of extended high resolution orbital signals from stratigraphy advanced analytical techniques have now been fully incorporated into the field an updated theory of earth s orbital parameters provides an accurate ephemeris for the past 16 million years for precision correlation between earth s orbital parameters and orbitally forced stratigraphy some orbital eccentricity modes can be used for orbital stratal cal ibrations to several hundred million years ago these breakthroughs have transformed the study of orbitally forced stratigra phy into an applied field that addresses long standing problems in disciplines as far ranging as geochronology geodynamics and astrodynamics orbital signals provide the framework for a high resolution orbital chronostratigraphy as far back

mercury s global color mosaic an update from messenger s orbital observations abstract we report an update to the photometric correction used to produce global color mosaics of mercury derived from an analysis of photometric observations acquired during the orbital phase of messenger s primary mission comparisons between versions of the color mosaic produced with photometric corrections derived from flyby and orbital data indicate that areas imaged at high incidence and emission angles 50 are better standardized to a common illumination and viewing geometry with the orbit derived corrections seams between images taken at very different illumination geometries however are still present at visually detectable levels further improvements to the photometric correction await updates to the radiometric calibration that will enable data retrieval over a larger range of photometric angles