synchronizing terrestrial and marine records of environmental change across the eocene oligocene transition records of terrestrial environmental change indicate that continental cooling and or aridification may have predated the greenhouse icehouse climate shift at the eocene oligocene transition eot by ca 600 kyr in north america marine terrestrial environmental change asynchronicity is inferred from a direct comparison between the astronomically tuned marine eot record and published 40ar 39ar geochronology of volcanic tuffs from the white river group wrg sampled at flagstaff rim wyoming and toadstool geologic park nebraska which are type sections for the chadronian and orellan north american land mammal ages we present a new age model for the wrg underpinned by high precision 206pb 238u zircon dates from 15 volcanic tuffs including six tuffs previously dated using the 40ar 39ar technique weighted mean zircon 206pb 238u dates from this study are up to 1 0 myr younger than published anorthoclase and biotite 40ar 39ar data calibrated relative to fish canyon sanidine at 28 201 ma giving consideration to the complexities strengths and limitations associated with both the 40ar 39ar and 206pb 238u datasets our interpretation is that the recalculated 40ar 39ar dates are anomalously old and the 206pb 238u zircon dates more accurately constrain deposition 206pb 238u calibrated age depth models were developed in order to facilitate a robust intercomparison between marine and terrestrial archives of environmental change and indicate that i early orellan terrestrial cooling recorded at toadstool geologic park was synchronous with the onset of early oligocene antarctic glaciation and ii the last appearance datums of key chadronian mammal taxa are diachronous by ca 0 7 myr between central wyoming and nw nebraska

rapid stepwise onset of antarctic glaciation and deeper calcite compensation in the pacific ocean the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth at present this depth is 4 500 m with some variation between and within ocean basins the calcite compensation depth is linked to ocean acidity which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate1 geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the eocene oligocene boundary 34 million years ago 2 when significant permanent ice sheets first appeared on antarctica3 4 5 6 but the relationship between these two events is poorly understood here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical pacific ocean that cover the eocene oligocene boundary we find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40 000 years each synchronous with the stepwise onset of antarctic ice sheet growth the glaciation was initiated after climatic preconditioning7 by an interval when the earth s orbit of the sun favoured cool summers the changes in oxygen isotope composition across the eocene oligocene boundary are too large to be explained by antarctic ice sheet growth alone and must therefore also indicate contemporaneous global cooling and or northern hemisphere glaciation

evidence for an earliest oligocene ice sheet on the antarctic peninsula there is growing consensus that development of a semipermanent ice sheet on antarctica began at or near the eocene oligocene e o boundary beyond ice rafted debris in oceanic settings however direct evidence for a substantial ice sheet at this time has been limited and thus far restricted to east antarctica it is unclear where glacier ice first accumulated and how extensive it was on the antarctic continent in the earliest oligocene sediments at the top of the eocene marine shelf section on seymour island antarctic peninsula include glacial marine deposits and a lodgment till with clasts derived from a variety of rock units on the peninsula dinoflagellate biostratigraphy and strontium isotope stratigraphy indicate an age at or very close to the e o boundary glacier ice extending to sea level in the northern peninsula at this time suggests the presence of a regionally extensive west antarctica ice sheet and thus an even more dramatic response to the forcing factors that facilitated high latitude ice expansion in the earliest oligocene

stable isotope paleoecology of white river ungulates during the eocene oligocene climate transition in northwestern nebraska abstract we use the carbon and oxygen stable isotope content of mammalian tooth enamel to evaluate ancient biomes habitat partitioning and water use among 12 ungulate taxa from the white river group in nebraska the white river group spans an interval of climate change from the upper eocene to the lower oligocene when global climate cooled in a heterogeneous fashion we test whether changes in biome structure or mammalian ecology occurred during this interval we find that mean 13c values for both eocene and oligocene faunas are consistent with dry open biomes e g woodland savanna or woody scrubland a significant increase in mean faunal 13c values 0 4 occurs in the lower oligocene primarily from the loss of individuals feeding in wetter denser parts of the ecosystem in the upper eocene megacerops sp brontothere and perchoerus probus peccary have low mean 13c values consistent with feeding in wetter denser habitats presumably riparian whereas significantly higher mean values in the cursorial horse mesohippus spp and the clawed oreodont agriochoerus antiquus are consistent with feeding in more open drier habitats the extinctions of megacerops sp and trigonias sp a rhinocerotid with relatively low 13c values along with a decrease in the abundance of p probus and other taxa suggests reduction in the extent of wet habitats in the oligocene the rare tapir colodon sp is the only taxon sampled with a mean 13c value low enough to indicate consistent feeding in a wet habitat in the oligocene however range through taxa occupy the same relative positions in isotopic space in both the eocene and oligocene faunas suggesting stability in drier habitats a significant increase also occurs in mean faunal 18o values 1 0 in the oligocene primarily from the loss or reduction in abundance of water dependent species variance in 18o values increases significantly in the oligocene which is consistent with greater water stress in plants from drier conditions oxygen values increase in a antiquus 0 9 the most aridity sensitive species in both faunas as expected for drier conditions but values in another aridity sensitive taxon mesohippus spp are virtually unchanged together 13c and 18o enamel values generally suggest a shift to drier conditions in the early oligocene although 13c values yielded more consistent results the lack of a large negative shift in 18o values expected with a decrease in mean annual temperature suggests that any temperature changes in the early oligocene were relatively small in the continental interior of north america

accuracy and precision of the late eocene early oligocene geomagnetic polarity time scale an accurate and precise geomagnetic polarity time scale is crucial to the development of a chronologic framework in which to test paleoclimatic and paleoenvironmental interpretations of marine and terrestrial records of the eocene oligocene transition eot the magnetic polarity patterns of relatively continuous marine and terrestrial records of the eot have been dated using both radioisotopic techniques and astronomical tuning both of which can achieve a precision approaching 30 k y for much of the paleogene however the age of magnetic reversals between chrons c12n and c16n 2n has proved difficult to calibrate with discrepancies of up to 250 k y between radio isotopically dated and astronomically tuned marine successions rising to 600 k y for comparisons with the 206pb 238u dated terrestrial record of the white river group in north america in this study we reevaluate the magnetic polarity pattern of the flagstaff rim and toadstool geologic park records of the white river group c12n c16n 2n our interpretation of the flagstaff rim polarity record differs significantly from earlier studies identifying a previously unreported normal polarity zone correlated to c15n which eliminates discrepancies between the wrg and the 206pb 238 dated marine record of the rupelian global stratotype section and point in the italian umbria marche basin however residual discrepancies persist between u pb dated and astronomically tuned records of the eot even when stratigraphic and systematic uncertainties associated with each locality and dating method are taken into account which suggests that the uncertainties associated with astronomically tuned records of the eot may have been underestimated

magnetostratigraphy of eocene oligocene boundary sections in italy no evidence for short subchrons within chrons 12r and 13r abstract the marine magnetic polarity record shows many small scale anomalies that could be due to short normal polarity subchrons within chrons 12r and 13r new examination of published magnetostratigraphic results from a section near gubbio shows that suspected short chrons in this section are probably not real the eocene oligocene boundary in the stratotype section at massignano lies just above a tentatively identified normal magnetozone that might correlate with a possible short normal subchron within chron 13r in a new investigation of the weathered marlstones in this section acquisition and thermal demagnetization of irm reveal a magnetic mineralogy dominated by magnetite with minor amounts of maghemite and hematite vector analysis of the natural remanent magnetization discloses a complex structure in some samples at least two and up to four components were isolated the magnetostratigraphy based on a magnetite component correlates well to the main features of the chron 13n 16n polarity sequence no short polarity subchrons are found although the resolution is inadequate to exclude the possibility that they exist in particular we did not find a short normal polarity magnetozone previously located close to the eocene oligocene boundary the results of this study favour the interpretation that low amplitude magnetic anomalies represent geomagnetic intensity fluctuations rather than unresolved short polarity intervals

metrology and traceability of u pb isotope dilution geochronology earthtime tracer calibration part i abstract mixed 235u 233u 205pb 202pb tracers for u pb isotope dilution isotope ratio mass spectrometry have been prepared under the auspices of the earthtime initiative the methods and results for the preparation and calibration of the u pb ratio and isotopic abundances are given and the various sources of uncertainty are discussed and quantified the accuracy of the earthtime u pb tracer isotopic composition can be traced back to si units via a series of assay and isotopic composition reference materials combined with the experiments described herein the parameters used in calculating u pb ratios and inferentially u pb dates have correlated uncertainties that result in a total uncertainty contribution to 206pb 238u dates of

oligocene calibration of the magnetic polarity time scale magnetostratigraphic studies of the oligocene white river group in wyoming colorado nebraska and the dakotas yield a radiometrically dated polarity stratigraphy that provides mid tertiary calibration points for the magnetic polarity time scale anomaly 12 13 reversal is bracketed by dates of 32 4 and 34 6 m y in best agreement with the time scale of labrecque and colleagues the magnetostratigraphy also helps calibrate the oligocene north american land mammal ages and allows correlation with the european marine microfossil zonation this correlation suggests that the age of the eocene oligocene boundary is 37 0 m y contrary to younger dates obtained from glauconites and microtektites

the heartbeat of the oligocene climate system a 13 million year continuous record of oligocene climate from the equatorial pacific reveals a pronounced heartbeat in the global carbon cycle and periodicity of glaciations this heartbeat consists of 405 000 127 000 and 96 000 year eccentricity cycles and 1 2 million year obliquity cycles in periodically recurring glacial and carbon cycle events that climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle solar forcing and glacial events box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass the pronounced 405 000 year eccentricity cycle is amplified by the long residence time of carbon in the oceans

high precision zircon u pb geochronology of astronomically dated volcanic ash beds from the mediterranean miocene several orbitally tuned miocene sedimentary sequences around the mediterranean contain abundant intercalated volcanic ash beds these sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology we present a large data set n 16n 16 n 166n 166 of zircon u pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims techniques for ash beds from an almost continuous orbitally tuned messinian to langhian 6 2 15 4 ma sedimentary sequence exposed along the adriatic coast south of ancona italy we use this unique data set to evaluate 1 the accuracy of zircon u pb dates 2 the significance of initial intermediate daughter product disequilibria for zircon u pb geochronology of young rocks 3 the effect of prolonged pre eruption zircon crystallization and zircon recycling on u pb derived ash bed deposition ages and 4 discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the geologic time scale

non antipodal directions in magnetostratigraphy an overprint bias s u m m a r y paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal as one would expect for an axial geocentric dipole model specifically in magnetostratigraphic studies of tertiary sediments which include large data sets of paleomagnetic directions of both polarities it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions we investigate the conventional explanation for such asymmetry which involves the presence of a persistent partially unremoved present day field magnetization in the studied rocks both plate motion history and inclination shallowing which we evaluate in detail can play an important role in the paleomagnetic record our analysis shows that alternatively it is plausible that the inclination anomaly is due to a contribution of an axial octupole in accordance with observations of the recent 0 5 ma geomagnetic field recorded in lava flows

the upper tortonian lower messinian at monte dei corvi northern apennines italy completing a mediterranean reference section for the tortonian stage abstract a high resolution integrated stratigraphy including biostratigraphy magnetostratigraphy tephrostratigraphy and cyclostratigraphy is presented for the upper tortonian and lower messinian upper miocene at monte dei corvi numerical age control comes from a combination of magnetobiostratigrapic dating and astronomical tuning while radioisotope dating of ash layers produced less reliable ages the deep marine succession is characterized by large scale non repetitive stratigraphic changes allowing the discrimination between a lower brownish rossini and euxinic shale interval the sedimentary cycles which occur superimposed on these large scale changes are related to astronomical climate forcing astronomical tuning of the sedimentary cycles provides not only absolute ages for biostratigraphic events reversal boundaries and volcanic ash layers but also for the observed non cyclic changes these can be linked to environmental and geodynamic changes in the mediterranean region preceding the messinian salinity crisis the beginning of the brownish interval marked by tripartite cycles at 8 017 ma coincides with tectonic re arrangements in the mediterranean such as compression in the betics and opening of the rifian corridor at 7 616 ma the change to the calcareous rossini interval can be linked to tectonic uplift in the northern apennines and the deepening of the rifian corridor while the betic corridor became restricted coinciding with the end of evaporite deposition in the eastern betics the change to euxinic shales at 7 168 ma reflects basin wide restriction marked by diatomite and sapropel formation in the mediterranean basin related to the severely constricted betic and rifian corridors the occurrence of a tripartite construction of cycles in the euxinic shales at 6 719 ma coincides with diatomite formation in the eastern mediterranean uplift and shallowing in the northern apennines and further constriction of the atlantic gateways we conclude that late tortonian early messinian tectonic and environmental events occur remarkably synchronously in the mediterranean basin and most likely result from a combination of geodynamic processes and eccentricity controlled climatic changes in combination with the upper serravallian and lower tortonian interval which includes the formal tortonian global stratotype section and point gssp the monte dei corvi is the only complete section in the mediterranean covering the entire tortonian stage we therefore propose the monte dei corvi section as tortonian reference section particularly for the mediterranean region in addition it may serve as a unit stratotype for the tortonian

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

a complete high resolution paleocene benthic stable isotope record for the central pacific odp site 1209 1 we have compiled the first stratigraphically continuous high resolution benthic foraminiferal stable isotope record for the paleocene from a single site utilizing cores recovered at pacific odp site 1209 the long term trend in the benthic isotope record suggests a close coupling of volcanic co2 input and deep sea warming over the short term the record is characterized by slow excursions with a pronounced periodic beat related to the short 100 kyr and long 405 kyr eccentricity cycle the phase relationship between the benthic isotope record and eccentricity is similar to patterns documented for the oligocene and miocene confirming the role of orbital forcing as the pace maker for paleoclimatic variability on milankovitch time scales in addition the record documents an unusual transient warming of 2 c coeval with a 0 6 carbon isotope excursion and a decrease in carbonate content at 61 75 ma this event which bears some resemblance to eocene hyperthermals marks the onset of a long term decline in 13c the timing indicates it might be related to the initiation of volcanism along greenland margin

completing the neogene geological time scale between 8 5 and 12 5 ma a much improved high resolution magnetostratigraphy is presented for the interval between 8 5 and 12 5 ma in the deep marine astronomically tuned monte dei corvi beach section in northern italy this section contains the formally designated boundary between the middle and upper miocene defined by the tortonian gssp the natural remanent magnetization is carried by an unusual low temperature component of primary origin according to interpretations of isothermal remanent magnetization irm acquisition curves the palaeomagnetic signal is most probably carried by iron sulphides such as greigite or pyrrhotite the resultant magnetostratigraphy is reliable and can be straightforwardly calibrated to the astronomically tuned neogene time scale atnts2004 showing that the section ranges from chron c5an 2n up to c4 2r 1 the inferred correlation of the tortonian gssp to the older part of chron c5r 2n is confirmed thereby guaranteeing global correlation potential in addition the monte dei corvi beach section has been re tuned to the new numerical solution la2004 for the interval between 8 5 and 12 9 ma this exercise shows that the characteristic cycle patterns are not consistent with a single solution with fixed values for dynamical ellipticity and tidal dissipation for the younger part of the section sedimentary cycles show an excellent fit with intricate details of precession obliquity interference in the nominal la2004 1 1 solution however this solution produces serious mismatches between the sedimentary cycles and the precession obliquity interference in the interval older than 10 2 ma for this interval changes in tidal dissipation value while keeping dynamical ellipticity constant at its present day value are necessary to maintain a perfect fit a detailed visual comparison reveals a very good to excellent fit with la2004 1 1 2 and to lesser extent with la2004 1 0 5 however la2004 1 1 2 is the preferred tuning target for the moment in the interval older than 10 2 ma the astronomical re tuning on precessional scale to la2004 1 1 yielded slightly different astronomical ages for each basic cycle and hence also for the calcareous plankton events and magnetic reversal boundaries with uncertainties ranging within a few thousand years the age of the tortonian gssp turns into 11 625 ma instead of the previously published 11 608 ma our ages should replace existing ages in the astronomically tuned neogene time scale atnts2004

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

on the duration of magnetochrons c24r and c25n and the timing of early eocene global warming events implications from the ocean drilling program leg 208 walvis ridge depth transect 1 five sections drilled in multiple holes over a depth transect of more than 2200 m at the walvis ridge se atlantic during ocean drilling program odp leg 208 resulted in the first complete early paleogene deep sea record here we present high resolution stratigraphic records spanning a 4 3 million yearlong interval of the late paleocene to early eocene this interval includes the paleocene eocene thermal maximum petm as well as the eocene thermal maximum etm 2 event a detailed chronology was developed with nondestructive x ray fluorescence xrf core scanning records and shipboard color data these records were used to refine the shipboard derived spliced composite depth for each site and with a record from odp site 1051 were then used to establish a continuous time series over this interval extensive spectral analysis reveals that the early paleogene sedimentary cyclicity is dominated by precession modulated by the short 100 kyr and long 405 kyr eccentricity cycles counting of precession related cycles at multiple sites results in revised estimates for the duration of magnetochrons c24r and c25n direct comparison between the amplitude modulation of the precession component derived from xrf data and recent models of earth s orbital eccentricity suggests that the onset of the petm and etm2 are related to a 100 kyr eccentricity maximum both events are approximately a quarter of a period offset from a maximum in the 405 kyr eccentricity cycle with the major difference that the petm is lagging and etm2 is leading a 405 kyr eccentricity maximum absolute age estimates for the petm etm2 and the magnetochron boundaries that are consistent with recalibrated radiometric ages and recent models of earth s orbital eccentricity cannot be precisely determined at present because of too large uncertainties in these methods nevertheless we provide two possible tuning options which demonstrate the potential for the development of a cyclostratigraphic framework based on the stable 405 kyr eccentricity cycle for the entire paleogene

evaluation of the astronomical time scale for the paleocene and earliest eocene abstract the astronomical tuned time scale is rapidly extended into the paleogene but due to the existence of an eocene gap different tuning options had to be presented for the paleocene these options differ both in number and tuning of 405 kyr eccentricity related cycles and are only partially consistent with recalculated 40 ar 39 ar constraints for the cretaceous paleogene k pg and paleocene eocene p e boundaries in this paper we evaluate the cyclostratigraphic interpretation of records from odp leg 198 and 208 sites and the zumaia section to solve the problem of the different tuning options we found that the interval between the k pg boundary and the early late paleocene biotic event elpe comprises 17 instead of 16 405 kyr eccentricity related cycles as previously proposed while the entire paleocene contains 25 405 kyr cycles starting from 40 ar 39 ar age constraints for the k pg boundary a new tuning to 405 kyr eccentricity is presented for the paleocene and earliest eocene which results in ages of 66 0 and 56 0 ma for the k pg and p e boundaries respectively this tuning introduces considerable differences in age for a number of nannofossil events at odp sites 1209 and 1262 in the interval between 61 and 63 ma but eliminates large and abrupt changes in the seafloor spreading rate the tuning seems further consistent with recalculated 40 ar 39 ar ages for ash layer 17 of early eocene age however despite this apparent consistency with existing radio isotopic constraints an alternative 405 kyr younger or less likely older tuning cannot be excluded at this stage

integrated stratigraphy of the oligocene pelagic sequence in the umbria marche basin northeastern apennines italy a potential global stratotype section and point gssp for the rupelian chattian boundary the oligocene represents an important time period from a wide range of perspectives and includes significant climatic and eustatic variations the pelagic succession of the umbria marche apennines central italy includes a complete and continuous sequence of marly limestones and marls with volcaniclastic layers that enable us to construct an integrated stratigraphic framework for this time period we present here a synthesis of detailed biostratigraphic magnetostratigraphic and chemostratigraphic studies along with geochronologic results from several biotite rich volcaniclastic layers which provide the means for an accurate and precise radiometric calibration of the oligocene time scale from this study the interpolated ages for the rupelian chattian stage boundary located in the upper half of chron 10n at meter level 188 in the monte cagnero section and corresponding to the o4 o5 planktonic foraminiferal zonal boundary are 28 36 ma paleomagnetic interpolation 28 27 0 1 ma direct radioisotopic dating and 27 99 ma astrochronological interpolation these ages appear to be slightly younger than those reported in recent chronostratigraphic time scale compilations the monte cagnero section is a potential candidate for defining the chattian global stratotype section and point gssp and some reliable criteria are here proposed for marking the rupelian chattian boundary according to international union of geological sciences iugs recommendations

astronomical forcing in late eocene marine sediments we use an x ray fluorescence xrf core scanner to obtain records of elemental concentrations in sediment cores from ocean drilling program odp leg 171b site 1052 blake nose atlantic margin of northern florida this record spans the middle to late eocene as indicated by bio and magnetostratigraphy and displays cyclicity that can be attributed to the orbital forcing of a combination of climate ocean circulation or productivity we use the xrf counts of iron and calcium as a proxy of the relative contribution from calcium carbonate and terrestrial material to construct a new composite depth record this new composite depth record provides the basis to extend the astronomically calibrated geological time scale into the middle eocene and results in revised estimates for the age and duration of magnetochrons c16 through c18 in addition we find an apparent change in the dominance of orbitally driven changes in obliquity and climatic precession at around 36 7 ma on our new time scale long term amplitude modulation patterns of eccentricity and obliquity in the data do not seem to match the current astronomical model any more suggesting the possibility of new constraints on astronomical calculations

paleopedology of the chadron formation of northwestern nebraska implications for paleoclimatic change in the north american midcontinent across the eocene oligocene boundary abstract the big cottonwood creek member is a newly recognized lithostratigraphic unit of the chadron formation of northwestern nebraska i measured 23 superimposed paleosol profiles within 29 m of strata exposed 0 4 km 0 25 mile east of the type section of the big cottonwood creek member at toadstool park in northwestern nebraska and recognized four main types of paleosols the paleosols are all well drained moderately alkaline to alkaline oxidized and range from weakly developed azonal ustifluvents to moderately and well developed paleustalfs and argiustolls with argillic and petrocalcic horizons based on changes in soil structure horizons root traces and geochemical trends these paleosols record a gradual change from humid forested conditions of the late eocene to more seasonal semi arid open range conditions leading up to the global climatic deterioration event of the early oligocene at 33 2 ma this interpretation is supported by associated changes in sedimentology fluvial architecture floras and faunas over this period of time the section at toadstool park is significant in that it establishes a link between the big badlands of south dakota and correlative deposits of wyoming the contact of the chadron and brule formations in the big badlands of south dakota is marked by an unconformity that is in part correlative to the earliest oligocene sediments that elsewhere record the global climatic deterioration event at 33 2 ma the paleosols of the big cottonwood creek member of northwestern nebraska fill in a significant part of this gap and record paleoclimatic conditions of the late eocene leading up to the global climatic deterioration event of the early oligocene using the latest lithostratigraphic biostratigraphic magnetostratigraphic and tephrostratigraphic data for the white river group this change to more arid conditions in the midcontinent appears to have progressed from west to east over time

chronology causes and progression of the messinian salinity crisis the messinian salinity crisis is widely regarded as one of the most dramatic episodes of oceanic change of the past 20 or so million years 1 3 earliest explanations were that extremely thick evaporites were deposited in a deep and desiccated mediterranean basin that had been repeatedly isolated from the atlantic ocean1 2 but elucidation of the causes of the isolation whether driven largely by glacio eustatic or tectonic processes have been hampered by the absence of an accurate time frame here we present an astronomically calibrated chronology for the mediterranean messinian age based on an integrated high resolution stratigraphy and tuning of sedimentary cycle patterns to variations in the earth s orbital parameters we show that the onset of the messinian salinity crisis is synchronous over the entire mediterranean basin dated at 5 96 0 02 million years ago isolation from the atlantic ocean was established between 5 59 and 5 33 million years ago causing a large fall in mediterranean water level followed by erosion 5 59 5 50 million years ago and deposition 5 50 5 33 million years ago of non marine sediments in a large lago mare lake sea basin cyclic evaporite deposition is almost entirely related to circum mediterranean climate changes driven by changes in the earth s precession and not to obliquity induced glacio eustatic sea level changes we argue in favour of a dominantly tectonic origin for the messinian salinity crisis although its exact timing may well have been controlled by the 400 kyr component of the earth s eccentricity cycle

eocene oligocene ocean de acidification linked to antarctic glaciation by sea level fall one of the most dramatic perturbations to the earth system during the past 100 million years was the rapid onset of antarctic glaciation near the eocene oligocene boundary some 34 million years ago this climate transition was accompanied by a deepening of the calcite compensation depth the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear merico et al use a global biogeochemical box model to test competing hypotheses put forward to explain the eocene oligocene transition they find that only shelf to deep sea carbonate partitioning can explain the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor this work sheds new light on the mechanisms linking glaciation and ocean acidity change during this important climate transition a global biogeochemical box model is used to test competing hypotheses put forward to explain the eocene oligocene transition it is found that only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor this work sheds new light on the mechanisms linking glaciation and ocean acidity change across the climate transition of the cenozoic one of the most dramatic perturbations to the earth system during the past 100 million years was the rapid onset of antarctic glaciation near the eocene oligocene epoch boundary1 2 3 34 million years ago this climate transition was accompanied3 by a deepening of the calcite compensation depth the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution changes in the global carbon cycle4 rather than changes in continental configuration5 have recently been proposed as the most likely root cause of antarctic glaciation but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear here we use a global biogeochemical box model to test competing hypotheses put forward to explain the eocene oligocene transition we find that of the candidate hypotheses only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor in our simulations glacioeustatic sea level fall associated with the growth of antarctic ice sheets permanently reduces global calcium carbonate accumulation on the continental shelves leading to an increase in pelagic burial via permanent deepening of the calcite compensation depth at the same time fresh limestones are exposed to erosion thus temporarily increasing global river inputs of dissolved carbonate and increasing seawater 13c our work sheds new light on the mechanisms linking glaciation and ocean acidity change across arguably the most important climate transition of the cenozoic era

magnetostratigraphy of the white river group and its implications for oligocene geochronology abstract magnetostratigraphic studies of the oligocene white river group in wyoming colorodo nebraska and the dakotas have yielded a radiometrically dated polarity stratigraphy they provided a mid tertiary calibration point for the marine magnetic polarity timescale an unusually long interval of reversed polarity in the flagstaff rim section natrona co wyoming is bracketed by k ar dates biotite of 32 4 ma slightly above its top and 34 6 ma at its base corrected for new decay constants it probably corresponds to the long reversed interval between marine magnetic anomalies 12 and 13 also the magnetostratigraphies of 17 other fossiliferous sections of chadronian whitneyan oligocene rocks have been correlated with anomalies 9 12 on this basis the boundaries of the oligocene north american land mammal ages are chadronian orellan mid anomaly 11 12 reversal about 32 4 ma orellan whitneyan mid anomaly 10 111 reversal about 30 7 ma whitneyan arikareenan base anomaly 9 about 29 0 ma these dates are in good agreement with recent estimates based on mammalian biochronology and with a corrected radiometric date of 28 7 0 7 ma at the base of the arikareean

large temperature drop across the eocene oligocene transition in central north america the eocene oligocene transition about 33 5 million years ago was a major global climate event the end of the eocene was unusually warm with no significant ice on antarctica but the oligocene saw the arrival of a permanent antarctic ice sheet two papers this week relate to the continental effects of this global change dupont nivet et al examined sedimentary records from the tibetan plateau and find a drop in atmospheric water which caused cooling and aridification coincident with antarctic cooling previous studies attributed this phenomenon to the rapid uplift of the tibetan plateau but this new work suggests that regional tibetan climate was influenced by global events in an unrelated paper on the same climate transition zanazzi et al explore the cooling in north america at the time using stable isotope measurements from fossil teeth and bones to create a proxy temperature record they find a large drop in mean annual temperature of 8 2 c a greater fall than seen in the oceans this continental transition may explain why many cold blooded reptiles and amphibians became extinct whereas mammals able to regulate their body temperature escaped relatively unscathed an analysis of fossil tooth enamel and fossil bones is used to derive a continental temperature record for the eocene oligocene transition and reports a large drop in mean annual temperature over roughly 400 000 years this cooling explains the decline in cold blooded terrestrial animals whereas most mammals in the region were unaffected the eocene oligocene transition towards a cool climate 33 5 million years ago was one of the most pronounced climate events during the cenozoic era1 the marine record of this transition has been extensively studied however significantly less research has focused on continental climate change at the time yielding partly inconsistent results on the magnitude and timing of the changes2 3 4 5 6 7 8 here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high resolution about 40 000 years continental temperature record for the eocene oligocene transition we find a large drop in mean annual temperature of 8 2 3 1 c over about 400 000 years the possibility of a small increase in temperature seasonality and no resolvable change in aridity across the transition the large change in mean annual temperature exceeding changes in sea surface temperatures at comparable latitudes9 10 and possibly delayed in time with respect to marine changes by up to 400 000 years explains the faunal turnover for gastropods amphibians and reptiles whereas most mammals in the region were unaffected our results are in agreement with modelling studies that attribute the climate cooling at the eocene oligocene transition to a significant drop in atmospheric carbon dioxide concentrations

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

astronomic calibration of the late eocene early oligocene massignano section central italy we present an astronomically tuned polarity timescale for the late eocene early oligocene based on a cyclic limestone marls sedimentary succession exposed at massignano italy gssp for the eocene oligocene boundary here we applied spectral analysis techniques to the high resolution magnetic susceptibility record the results provide clear evidence of a regular and cyclic signal that allows us to discard a stochastic mechanism as controlling these fluctuations conversely these outcomes reveal the presence of stable and notable periodicities that are in tune with those associated with orbital forcing through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record we refine the late eocene portion of the gpts through astronomical tuning of the rhythmic sedimentary alternations to recent astronomically calculated variations of earth s orbit we recalibrated the polarity boundaries ages and the main stratigraphic events recognized throughout the sedimentary record among these an age of 33 714 ma is proposed for the e o boundary

dynamics and coastal effects of tsunamis generated by asteroids impacting the black sea two dimensional 2d and one dimensional 1d shallow water models are used to evaluate the seashore effects of the tsunami generated by an asteroid impacting the western regions of the black sea about 208 km from the coast the tsunami s speed and the effects on the coastal regions depend on many factors among which the most important is asteroid size the tsunami generated by a 250 m asteroid reaches the nearest land location in 24 min and needs about 2 h to arrive at all black sea coast the run up value is about 7 m high on the turkish and crimean coasts in the western black sea regions the wave height is two or three times smaller the run up values strongly depend on bathymetry and topography peculiarities the run in value in case of the tsunami generated by a 1 000 m asteroid is up to eight times larger than in case of a 250 m impactor depending on location the results reported herein are upper limit values in case of the 250 m asteroid the real wave amplitude may be up to two times smaller the uncertainty factor decreases in case of larger asteroids ways of diminishing the social consequences are briefly discussed

chadronian early oligocene magnetostratigraphy of eastern wyoming implications for the age of the eocene oligocene boundary new magnetostratigraphic studies have been conducted on early oligocene sections at flagstaff rim and ledge creek natrona county wyoming and the dilts ranch converse county wyoming thermal demagnetization has considerably improved the polarity interpretation of flagstaff rim since af demagnetization had failed to remove a diagenetic normal overprint the polarity pattern in these sections appears to span the interval from chron c10r to chron c15 of the magnetic polarity timescale 31 to 37 5 m y based on k ar dates from flagstaff rim this additional more complete magnetostratigraphic evidence shows clearly that the eocene oligocene boundary must lie between 36 and 37 m y in age

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

confirmation of the astronomical calibration of the magnetic polarity timescale from sea floor spreading rates the dating of the history of reversals of the earth s magnetic field since the jurassic period has been based primarily on radiometric dating of a small number of reversals with most ages of individual reversals interpolated or extrapolated on the basis of the widths of marine magnetic anomalies recently however pliocene and pleistocene reversals have been dated by an entirely independent method which relies on correlating climate proxies such as oxygen isotope ratios in sediments with calculated variations of the earth s orbit and inclination1 4 the dates obtained from this astronomical calibration are 3 8 older than radiometrically calibrated ages published between 1979 and 1989 see for example refs 5 and 6 although recent dating by the 40ar 39ar method tends to confirm the astronomical calibration7 10 at least for the past 3 3 myr here i use sea floor spreading rates determined from high precision plate rotation solutions for five plate pairs to test the astronomically calibrated timescale over the past 5 32 myr the results suggest that the errors in the astronomical calibration are no greater than 0 02 myr and also show that spreading rate can remain constant for several million years even for fast moving plates

observations of pressure jump lines in the midwest 10 12 august 1976 abstract strong thunderstorm activity over iowa on two successive afternoons was the apparent source of pressure jump lines pjl s which moved sse at 50 km h 1 through the nocturnal boundary layer and were detected by national weather service nws stations as far away as paducah kentucky rainshowers and thunder were reported at many nws stations as the pjl s passed the regional air monitoring system rams network at st louis provided detailed information on the pjl s arrival there was indicated by an abrupt pressure rise of 1 5 mb a near reversal of the surface flow and a vertical displacement of 750 m extending through the lower 4 km of the atmosphere the passage of each pjl was coincident with the turbulent collapse of the nocturnal jet the observations of the pjl events seem indicative of an internal bore and are similar to those of the morning glory seen in northern australia we speculate that the bore originates from a late afternoon convergence produced by thunderstorm outflow and oppos

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research