a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

late quaternary zonation of vegetation in the eastern grand canyon fossil assemblages from 53 packrat middens indicate which plant species were dominant during the last 24 000 years in the eastern grand canyon past vegetational patterns show associations that cannot be attributed to simple elevational displacement of the modern zones a model emphasizing a latitudinal shift of climatic values is proposed

integrated stratigraphy of the paleocene eocene thermal maximum in the new jersey coastal plain toward understanding the effects of global warming in a shelf environment 1 in the new jersey coastal plain a silty to clayey sedimentary unit the marlboro formation represents deposition during the paleocene eocene thermal maximum petm this interval is remarkably different from the glauconitic sands and silts of the underlying paleocene vincentown and overlying eocene manasquan formation we integrate new and published stable isotope biostratigraphic lithostratigraphic and ecostratigraphic records constructing a detailed time frame for the petm along a depth gradient at core sites clayton wilson lake ancora and bass river updip to downdip the onset of the petm marked by the base of the carbon isotope excursion cie is within the gradual transition from glauconitic silty sands to silty clay and represented fully at the updip sites wilson lake and clayton the cie core interval is expanded at the updip sites but truncated the cie core is complete at the bass river and ancora sites where the early part of the recovery is present most complete at ancora the extent to which the petm is expressed in the sediments is highly variable between sites with a significant unconformity at the base of the overlying lower eocene sediments our regional correlation framework provides an improved age model allowing better understanding of the progression of environmental changes during the petm high resolution benthic foraminiferal data document the change from a sediment starved shelf setting to a tropical river dominated mud belt system during the petm probably due to intensification of the hydrologic cycle the excellent preservation of foraminifera during the petm and the lack of severe benthic extinction suggest there was no extreme ocean acidification in shelf settings

ir anomalies and other elemental markers near the palaeocene eocene boundary in a flysch sequence from the western tethys slovenia ir abundance anomalies platinum group elements pge enrichment and increased concentrations of meteoritic ni fe co and nonmeteoritic sb as zn and cu elements were found in the palaeocene eocene p e boundary interval in a flysch sequence from the western tethys goriska brda section w slovenia this records one of the most important calcareous deep benthic extinctions in the history of the earth although the observed geochemical patterns could indicate complex sources for these metals such as weathering of the continental and oceanic crust volcanic processes as well as diagenetic mobilization and redistribution we cannot preclude the highly speculative possibility that the observed ir abundances may also indicate extraterrestrial contamination

antipodal hotspot pairs on the earth the results of statistical analyses performed on three published hotspot distributions suggest that significantly more hotspots occur as nearly antipodal pairs than is anticipated from a random distribution or from their association with geoid highs and divergent plate margins the observed number of antipodal hotspot pairs depends on the maximum allowable deviation from exact antipodality at a maximum deviation of 700 km 26 to 37 of hotspots form antipodal pairs in the published lists examined here significantly more than would be expected from the general hotspot distribution two possible mechanisms that might create such a distribution include 1 symmetry in the generation of mantle plumes and 2 melting related to antipodal focusing of seismic energy from large body impacts

correlation between isotope records in marine and continental carbon reservoirs near the palaeocene eocene boundary changes in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land the carbon isotope compositions of marine carbonate sediments from the late palaeocene vary considerably exhibiting a sudden decrease close to the palaeocene eocene boundary which coincides with deep sea benthic extinctions1 and with changes in ocean circulation here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel in using palaeosol carbonates to reconstruct the co2 content of the ancient atmosphere2 isotope shifts of this sort will have to be taken into account the sharp decrease in 13c 12c ratios in the late palaeocene provides a datum for precise correlation of marine and continental records and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals

fossil packrat middens and the tandem accelerator mass spectrometer analyses of well preserved plant remains from ancient packrat neotoma middens have yielded much information on the history of vegetation fauna and climate from the more arid portions of north america over the past 40 000 years1 2 in most of the modern deserts woodland or forest communities were present during the last glacial age the wisconsin radiocarbon dating of packrat midden assemblages from a single cave or from several nearby sites in a homogeneous local area can yield a detailed local vegetation chronology for many thousands of years from within 30 50 m on the rocky slopes in front of the dry rock shelters3 4 the development of radiocarbon dating using the tandem accelerator mass spectrometer tams allows direct dating of very small samples here we present 20 examples of tams radiocarbon dates on packrat midden materials to illustrate their usefulness in testing anomalous mixtures of species in ice age communities understanding the details of plant migrations and refining wisconsin and holocene time boundaries based on range changes of important plant species table 1 fig 1

stratigraphic and climatic implications of clay mineral changes around the paleocene eocene boundary of the northeastern us margin abstract kaolinite usually is present in relatively small amounts in most upper paleocene and lower eocene neritic deposits of the northern us atlantic coastal plain however there is a short period less than 200 000 k y in the latest paleocene upper part of calcareous nannoplankton zone np 9 when kaolinite dominated clay mineral suites replaced the usual illite smectite dominated suites during this time of global biotic and lithologic changes kaolinite increased from less than 5 of the clay mineral suite to peak proportions of 50 60 of the suite and then returned to less than 5 in uppermost paleocene lowermost eocene strata this kaolinite pulse is present at numerous localities from southern virginia to new jersey these sites represent both inner and middle neritic depositional environments and reflect input from several river drainage systems thus it is inferred that kaolinite rich source areas were widespread in the northeastern us during the latest paleocene erosion of these source areas contributed the kaolinite that was transported and widely dispersed into shelf environments of the salisbury embayment the kaolinite increase which occurred during a time of relatively high sea level probably is the result of intensified weathering due to increased temperature and precipitation the southern extent of the kaolinite pulse is uncertain in that uppermost paleocene beds have not been identified in the southern atlantic coastal plain the late paleocene kaolinite pulse that consists of an increase to peak kaolinite levels followed by a decrease can be used for detailed correlation between more upbasin and more downbasin sections in the salisbury embayment correlations show that more upbasin paleocene eocene boundary sections are erosionally truncated they have varying portions of the kaolinite increase and if present at all discontinuous portions of the subsequent kaolinite decrease as these truncated sections are disconformably overlain by lower eocene strata rapid erosional removal of large parts of the most kaolinite rich p e boundary clay deposits occurred by early eocene time erosion of the kaolinite rich p e boundary beds was enhanced during times of sea level fall when kaolinite rich sediments were redeposited to produce kaolinite spikes in basal beds of lower and middle eocene sequences that have little or no kaolinite elsewhere in the sequence in contrast more downbasin sites document only the upper decreasing part of the kaolinite pulse the absence of strata documenting the earlier kaolinite increase is attributed to slow sedimentation condensed interval as a result of a significant sea level rise that ponded most sediments in shallower waters combined with the probable subsequent erosional removal of these thin downbasin deposits by oceanic currents

geochemistry of cenozoic microtektites and clinopyroxene bearing spherules abstract we have determined the major and trace element compositions of 176 individual microtektites spherules from the australasian ivory coast and north american microtektite and clinopyroxene bearing cpx spherule layers trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis inaa and major element compositions were determined using energy dispersive x ray edx analysis in combination with a scanning electron microscope sem in addition petrographic data were obtained for the cpx spherules using the sem and edx this is the first trace element study of individual australasian microtektites and the data revealed the presence of a previously unrecognized group of australasian microtektites with high contents of ni up to 471 ppm in previous studies the high mg hmg australasian microtektites were thought to be related to the hmg australasian tektites but our trace element data suggest that the high ni hni australasian microtektites rather than the high mg microtektites are related to the high mg australasian tektites we find that cenozoic microtektites spherules from a given layer can be distinguished from microtektites spherules from other layers as a group but it is not always possible to determine which layer an individual microtektite spherule came from based only on trace element compositions the cpx spherules and most of the microtektites have cr co and ni contents that are higher than the average contents of these elements in the upper continental crust suggesting the presence of a meteoritic component the highest cr co and ni contents are found in the cpx spherules and low si cpx related microtektites unetched to slightly etched cpx spherules have ni cr and ni co ratios that generally lie along mixing curves between the average upper continental crust and chondrites the best fit appears to be with an ll chondrite the moderately to heavily etched cpx spherules have values that lie off the mixing curves in a direction that suggests ni loss probably as a result of solution of a ni rich phase olivine the ni rich australasian microtektites also have ni values that lie close to mixing curves between the average upper continental crust and chondrites however both the cpx spherules and hni australasian microtektites appear to have ir and to a lesser extent au contents that are much too low to have ni ir ratios similar to chondritic values we have no explanation for the low ir and au contents except to speculate that they may be the result of a complex fractionation process the ivory coast and north american microtektites do not have high enough siderophile element contents to reach any firm conclusions regarding the presence of or nature of a meteoritic component in them trace element compositions are consistent with derivation of the cenozoic microtektite spherule layers from upper continental crust the normal australasian microtektites appear to have been derived from a graywacke or lithic arenite with a range in clay and quartz content the source rock for the high mg australasian microtektites is not known but the hmg microtektites do not appear to be normal australasian microtektites that were simply contaminated by meteorites or ultramafic rocks the average ivory coast microtektite composition can be matched with a mixture of target rocks at the bosumtwi crater the average composition of the north american microtektites suggests an arkosic source rock but with graywacke and quartz rich end members however we could not match the composition of the north american microtektites with lithologies in impact breccias recovered from the chesapeake bay impact structure that is believed to be the source crater likewise we could not match the composition of the cpx spherules with mixtures of basement rocks and overlying sedimentary deposits for which compositional data are available at the popigai impact crater that may be the source crater for the cpx spherules this may be because the cpx spherules were derived in large part from clastic surface rocks sandstones and shales for which no compositional data are available

middle and late wisconsin paleobotanic and paleoclimatic records from the southern colorado plateau usa the colorado plateau is a distinct physiographic province in western north america which presently straddles the transition between summer wet and summer dry climatic regimes to the south and northwest respectively in addition to climate the diversity of environments and plant communities on the colorado plateau has resulted from extreme topographic diversity desert lowlands as low as 360 m elevation are surrounded by forested plateaus and even higher peaks greater than 3800 m elevation this environmental diversity provides a unique opportunity to study the history of biotic communities in an arid region of north america although the colorado plateau harbours numerous potential sites the paleoecological record of the plateau is poorly known potential deposits for analysis include packrat middens alluvial and cave sites at lower elevations and lake bog and wetland sites at higher elevations forty six sites have been analysed across the nearly 337 000 km2 region of which 27 contain records that span marine oxygen isotope stage is 2 data with is 3 information coming from only 12 sites most is 2 and 3 sites are clustered along the lowland regions of the colorado river corridor and the uplands of the mogollon rim area we compiled selected data from long paleoecological records to examine patterns of vegetation and climate change across the southern colorado plateau for the middle and late wisconsin during the middle wisconsin mixed conifers covered middle elevations presently dominated by ponderosa pine pinus ponderosa and juniper juniperus woodland grew at elevations today covered by blackbrush coleogyne and sagebrush artemisia desert during the late wisconsin boreal conifers primarily engelmann spruce picea engelmannii replaced the mixed conifer association estimates of mean annual temperatures mat during is 3 were at least 3 4 c cooler than today whereas is 2 mat estimates are at least 5 c colder our investigation of millennial scale climatic variability within the region provided equivocal results the packrat midden sequence could not distinguish vegetation changes that might be associated with heinrich events in the north atlantic from the lake records however many heinrich events were associated with generally drier intervals often with elevated sagebrush pollen concentrations future paleoecological investigations should concentrate on the northern colorado plateau as well as the eastern and western margins additional sites along with closer spaced sampling in regions already studied will be important in determining the history of important climatic phenomena such as the timing of the arizona monsoon

distinguishing shocked from tectonically deformed quartz by the use of the sem and chemical etching abstract multiple sets of crystallographically oriented planar deformation features pdfs are generated by high strain rate shock waves at pressures of 12 gpa in naturally shocked quartz samples on surfaces pdfs appear as narrow 50 500 nm lamellae filled with amorphosed quartz diaplectic glass which can be etched with hydrofluoric acid or with hydrothermal alkaline solutions in contrast slow strain rate tectonic deformation pressure produces wider semi linear and widely spaced arrays of dislocation loops that are not glass filled etching samples with hf before examination in a scanning electron microscope sem allows for unambiguous visual distinction between glass filled pdfs and glass free tectonic deformation arrays in quartz this etching also reveals the internal pillaring often characteristic of shock induced pdfs this technique is useful for easily distinguishing between shock and tectonic deformation in quartz but does not replace optical techniques for characterizing the shock features

vegetation dynamics during the last deglaciation southeastern great basin u s a abstract vegetation changes in the southeastern great basin between 17 500 and 8000 yr b p can be reconstructed from plant macrofossil assemblages in ancient packrat neotoma spp middens vegetation instability after ca 16 000 yr b p involved a dispoportionate number of local immigrations arrivals relative to apparent local extinctions departures at a site in modern woodland less than 50 of the arrivals were of transients species that subsequently went locally extinct but at a low elevation desert site almost all arrivals before 10 000 yr b p were transient species thus by the close of the last glacial age higher elevations supported vegetation that resembled present woodland while lower elevations supported desertscrub that bore little resemblance to modern thermophilous desertscrub differences in the pace of plant community modernization in different elevational zones can be attributed to migrational lag woodland species survived the last glacial age at low elevations nearby and began arriving at sites within current woodland before 11 700 yr b p warm desert plants were displaced far to the south and began arriving after 9500 yr b p moreover their staggered arrival times suggest that low winter temperatures did not inhibit their migration during the early holocene these data suggest that biotic factors particularly variable dispersal distances and consequent migrational lag did affect deglacial vegetation change

late holocene vegetation changes in greenwater valley mojave desert california abstract small scale late holocene vegetation changes were determined from a series of 13 modern and fossil packrat middens collected from a site in the greenwater valley northern mojave desert california although the site is above the modern lower limit of coleogyne ramosissima black brush macrofossils of this shrub are only present in samples younger than 270 yr b p in order to measure changes more subtle than presence vs absence macrofossil concentrations were quantified and principal components and factor analyses were used to distinguish midden plant assemblages both the presence absence data and the statistical analyses suggest a downward shift of 50 to 100 m for coleogyne blackbrush communities between 1435 and 1795 a d

periodicity of the earth s magnetic reversals reversals of the earth s magnetic field may occur with a certain regularity using observed percentages of normal and reversed polarity during different time intervals negi and tiwari1 detected several significant periodicities notably one of 32 34 myr raup2 used the dates of individual reversals to propose a 30 myr period in the frequency of reversals although mazaud et al 3 obtained a periodicity of only 15 myr like negi and tiwari s1 other detected short periods the 15 myr period may be harmonically related to a basic 30 myr period4 it has also been suggested that these are accidental periodicities arising in a short record5 6 or are harmonics of the record length itself7 in fact all of the cited studies have used different data different record lengths and different methods of time series analysis raup8 has consequently retracted his original claim here i present a much fuller analyis of the reversal record and show that a statistically significant period of 30 myr does formally exist in spite of the cited differences

50 000 years of vegetation and climate history on the colorado plateau utah and arizona usa sixty packrat middens were collected in canyonlands and grand canyon national parks and these series include sites north of areas that produced previous detailed series from the colorado plateau the exceptionally long time series obtained from each of three sites n48 000 14 cy r bp to present include some of the oldest middens yet discovered most middens contain a typical late wisconsinan glaciation mixture of mesic and xeric taxa evidence that plant species responded to climate change by range adjustments of elevational distribution based on individual criteria differences in elevational range from today for trees and shrubs ranged from no apparent change to as much as 1200 m difference the oldest middens from canyonlands np however differ in containing strictly xeric assemblages including middens incorporating needles of arizona single leaf pinyon far north of its current distribution similar aged middens from the eastern end of grand canyon np contain plants more typical of glacial climates but also contain fossils of one seed juniper near its current northern limit in arizona holocene middens reveal the development of modern vegetation assemblages on the colorado plateau recording departures of mesic taxa from low elevation sites and the arrival of modern dominant components much later published by university of washington

revised global model of thermosphere winds using satellite and ground based observations thermospheric wind data obtained from the atmosphere explorer e and dynamics explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground based incoherent scatter radar and fabry perot optical interferometers to generate a revision hwm90 of the hwm87 empirical model and extend its applicability to 100 km comparison of the various data sets with the aid of the model shows in general remarkable agreement particularly at mid and low latitudes the ground based data allow modeling of seasonal diurnal variations which are most distinct at mid latitudes while solar activity variations are now included they are found to be small and not always very clearly delineated by the current data they are most obvious at the higher latitudes the model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of fabry perot observations

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

characteristics of shocked quartz in late eocene impact ejecta from massignano ancona italy clues to shock conditions and source crater the microstructure and optical properties of shocked quartz discovered in a distal ejecta horizon of the stratigraphically continuous upper eocene section at massignano italy have been investigated in detail the microstructural observations confirm the impact origin of the shocked quartz grains and reveal that the optically visible planar lamellae are well preserved amorphous to partially recrystallized planar deformation features refractive index measurements of shocked quartz and analysis of the orientation and thickness of planar deformation features indicate shock pressures of 10 to 26 gpa at ambient preshock temperature altogether the characteristics of shocked quartz from massignano imply a nonporous quartz rich near surface rock as its source popigai russia is favored as the source crater because of the presence of compact quartzites in its preimpact cover

observations of thermospheric horizontal neutral winds at watson lake yukon territory λ 65 n fabry perot interferometer observations of the thermospheric o i 6300 a emission have been conducted from an airglow observatory at a dark field site in the southeastern yukon territory canada for the period november 1991 to april 1993 the experiment operated in an unattended remote fashion has resulted in a substantial data set from which mean neutral winds have been determined dependent upon geomagnetic activity the nocturnal location of the site is either equatorward of the auroral oval or within the oval boundaries the data set is rich enough to permit hourly binning of neutral winds based upon the kp geomagnetic disturbance index as well as the season for cases of low geomagnetic activity the averaged vector horizontal neutral wind exhibits the characteristics of a midlatitude site displaying antisunward pressure gradient driven winds as the geomagnetic activity rises the late afternoon and evening winds slowly rotate sunward in an anticlockwise direction initially remaining near 100 m s in speed but eventually increasing to 300 m s for kp 5 for the higher levels of activity the observed neutral wind flow pattern resembles a higher latitude polar cap pattern characterized by ion drag forcing of thermospheric neutral gases in addition rotational coriolis forcing on the dusk side enhances the ion drag forcing resulting in dusk winds which trace out the clockwise dusk cell plasma flow on the dawn side the neutral winds also rotate in an anticlockwise direction as the strength of geomagnetic disturbances increase since the site is located at a transition latitude between the midlatitude and the polar cap the data set provides a sensitive test for general circulation models which attempt to parameterize the contribution of magnetospheric processes a comparison with the vector spherical harmonic vsh model indicates several regions of poor correspondence for december solstice conditions but reasonable agreement for the vernal equinox

the magnetic reversal record is not periodic statistical analysis of one version of the magnetic reversal record suggests that the frequency of reversals has changed cyclically with a period of 30 myr starting at least 165 myr ago but factors other than periodicity affect the analysis used and here i show that there is no compelling evidence for periodicity in the reversal record the results of essentially identical analyses that have been applied to other geolocial data should also be re evaluated

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

an empirical model of the earth s horizontal wind fields hwm07 1 the new horizontal wind model hwm07 provides a statistical representation of the horizontal wind fields of the earth s atmosphere from the ground to the exosphere 0 500 km it represents over 50 years of satellite rocket and ground based wind measurements via a compact fortran 90 subroutine the computer model is a function of geographic location altitude day of the year solar local time and geomagnetic activity it includes representations of the zonal mean circulation stationary planetary waves migrating tides and the seasonal modulation thereof hwm07 is composed of two components a quiet time component for the background state described in this paper and a geomagnetic storm time component dwm07 described in a companion paper

latest paleocene lithologic and biotic events in neritic deposits of southwestern new jersey in the southwestern new jersey coastal plain four drill holes contain continuous neritic sedimentation across the paleocene eocene boundary calcareous nannofossil zone np 9 np 10 boundary significant lithologic and biotic changes occur in these strata near the top of the paleocene global warming increased precipitation and other oceanographic and climatic events that have been recognized in high latitude deep oceanic deposits of the latest paleocene also influenced mid latitude shallow marine and terrestrial environments of the western north atlantic the diverse well preserved calcareous nannofossil flora that is present throughout the entire new jersey boundary section accurately places these events within the uppermost part of the upper paleocene zone np 9 several rapid but gradational changes occur within a 1 1 m interval near the top of zone np 9 the changes include 1 a change in lithology from glauconitic quartz sand to clay 2 a change in clay mineral suites from illite smectite dominated to kaolinite dominated 3 a change in benthic foraminiferal assemblages to a lower diversity fauna suggestive of low oxygen environments 4 a significant increase in planktonic foraminiferal abundance and 5 an increased species turnover rate in marine calcareous nannofossils pollen was sparse in the new jersey drill holes but terrestrial sporomorph species in virginia exhibit increased turnover rates at a correlative level foraminiferal assemblages and lithology indicate that relative sea level rose in new jersey at the same time as these late paleocene events occurred in late biochron np 9 the higher sea levels influenced sediment type and absolute abundance of planktonic foraminifers in the deposits above the initial increase of kaolinite in the upper part of zone np 9 the kaolinite percentage continues to increase and the maximum kaolinite value occurs in the uppermost part of zone np 9 there are few changes in either the sediments or the biota precisely at the zone np 9 np 10 boundary in new jersey the clay rich deposits with a high kaolinite clay mineral suite the lowered diversity benthic foraminiferal assemblages the abundant planktonic foraminiferal specimens and the calcareous nannofossil assemblages continued essentially unchanged into the earliest eocene zone np 10 within the lower part of zone np 10 the kaolinite percentage decreased to very low values

the apectodinium acme and terrestrial discharge during the paleocene eocene thermal maximum new palynological geochemical and calcareous nannoplankton observations at tawanui new zealand manifestations of profound perturbations in biogeochemical systems during the paleocene eocene thermal maximum petm include a prominent global negative 13c and a pronounced increase in the relative abundance of dinoflagellate cysts dinocysts assigned to the genus apectodinium while motile representatives of apectodinium were most likely thermophilic and heterotrophic the underlying causes of this dinoflagellate response are not well understood here we provide new insight by examining the palynology chemistry and calcareous nannoplankton across the petm in a continental slope section at tawanui new zealand across the petm marked changes in the relative abundance of apectodinium vary antithetically with significant changes in the 13c of carbonate and organic matter in general the high relative abundance of apectodinium relates to enhanced concentrations of dinocysts signifying a bloom of apectodinium in surface waters during the petm changes in apectodinium and 13c records correspond to variations in many other parameters including a smaller negative shift in bulk carbonate 13c than expected increased terrestrial palynomorphs elevated toc and c n ratios lower carbonate contents higher sio2 and al2o3 contents and lower si al ratios all of these variations can be explained by an increase in delivery of terrigenous material to the continental margin a peak in the relative abundance of glaphyrocysta dinocysts at the onset of the petm may indicate greater down slope transport of neritic material changes in calcareous nannoplankton abundances suggest increased nutrient availability in surface waters during the petm the combined results show that apectodinium dominated assemblages global perturbations in carbon isotopes and enhanced terrigenous delivery closely correspond in time at tawanui a sudden and massive carbon injection to the ocean atmosphere system may have enhanced weathering and increased terrigenous inputs to continental margins during the petm we further suggest that these inputs caused the apectodinium acme by elevating primary productivity in marginal seas

on the duration of the paleocene eocene thermal maximum petm 1 the paleocene eocene thermal maximum petm is one of the best known examples of a transient climate perturbation associated with a brief but intense interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean atmosphere system one key to quantifying the mass of carbon released identifying the source s and understanding the ultimate fate of this carbon is to develop high resolution age models two independent strategies have been employed cycle stratigraphy and analysis of extraterrestrial helium heet both of which were first tested on ocean drilling program odp site 690 these two methods are in agreement for the onset of the petm and initial recovery or the clay layer main body but seem to differ in the final recovery phase of the event above the clay layer where the carbonate contents rise and carbon isotope values return toward background values here we present a state of the art age model for the petm derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during odp leg 208 walvis ridge southeastern atlantic integrated with published records from site 690 weddell sea southern ocean odp leg 113 during leg 208 five paleocene eocene p e boundary sections sites 1262 to 1267 were recovered in multiple holes over a depth transect of more than 2200 m at the walvis ridge yielding the first stratigraphically complete p e deep sea sequence with moderate to relatively high sedimentation rates 1 to 3 cm ka where a is years a detailed chronology was developed with nondestructive x ray fluorescence xrf core scanning records on the scale of precession cycles with a total duration of the petm now estimated to be 170 ka the revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and heet

impact induced melting and the development of large igneous provinces abstract we use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater this mechanism may generate sufficient quantity of melt to auto obliterate the crater melting would initiate almost instantaneously but the effects of such massive mantle melting may trigger long lived mantle up welling that could potentially resemble a mantle hotspot decompression melting is well understood it is the main method advocated by geophysicists for melting on earth whether caused by thinned lithosphere or hot rising mantle plumes the energy released is largely derived from gravitational energy and is outside but additive to the conventional calculations of impact modelling where energy is derived solely from the kinetic energy of the impacting projectile be it comet or asteroid the empirical correlation between total melt volume and crater size will no longer apply but instead there will be a discontinuity above some threshold size depending primarily on the thermal structure of the lithosphere we estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces 106 km3 similar melting of the mantle beneath an oceanic impact was also modelled by roddy et al int j impact eng 5 1987 525 the mantle melts will have plume like geochemical signatures and rapid mixing of melts from sub horizontal sub crater reservoirs is likely direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record we suggest that the end permian siberian traps should be reconsidered as the result of a major impact at 250 ma auto obliteration by volcanism of all craters larger than 200 km would explain their anomalous absence on earth compared with other terrestrial planets in the solar system

latest on the absolute age of the paleocene eocene thermal maximum petm new insights from exact stratigraphic position of key ash layers 19 and 17 abstract we constructed a precise early eocene orbital cyclostratigraphy for dsdp site 550 leg 80 goban spur north atlantic utilizing precession related cycles as represented in a high resolution x ray fluorescence based barium core log based on counting of those cycles we constrain the exact timing of two volcanic ash layers in site 550 which correlate to ashes 19 and 17 of the fur formation in denmark the ashes relative to the onset of the paleocene eocene thermal maximum petm are offset by 862 kyr and 672 kyr respectively when combined with published absolute ages for ash 17 the absolute age for the onset of the petm is consistent with astronomically calibrated ages using the current absolute age of 28 02 ma for the fish canyon tuff fct standard for calibrating the absolute age of ash 17 is consistent with tuning option 2 in the astronomically calibrated paleocene time scale of westerhold et al 2008 westerhold t rohl u raffi i fornaciari e monechi s reale v bowles j and evans h f 2008 astronomical calibration of the paleocene time palaeogeography palaeoclimatology palaeoecology v 257 p 377 403 using the recently recalibrated absolute age of 28 201 ma for the fct standard is consistent with tuning option 3 in the astronomically calibrated paleocene time scale the new results do not support the existence of any additional 405 kyr cycle in the early paleocene astronomically tuned time scale

the last pluvial climatic episodes in the deserts of southwestern north america long term variations in the effective moisture of deserts are linked both to glacial interglacial climatic cycles and through the milankovitch theory to orbitally induced changes in solar radiation intensity1 4 for southwestern north america comparison of palaeobotanical data with model predictions of palaeoclimate reveals two distinct pluvial regimes during the late wisconsin and early holocene 23 8 kyr bp southward displacement of the aleutian low pressure centre and strengthened westerlies from 40 to 30 n were important features of the full glacial 18 kyr bp pluvial maximum increased cyclonic storms and weakened monsoons led to a winter precipitation regime a feature seen both in the fossil record5 6 and climate models7 8 dominantly zonal flow during this period contrasts with the latest wisconsin and early holocene 12 8 kyr bp when meridional circulation transported maritime tropical air into the desert interior we now show that plant macrofossil assemblages from packrat neotoma spp middens9 in the mojave desert provide evidence for increased temperatures and summer rainfall during this period10 12 the anomalously late persistence to 8 kyr bp of woodland in low latitude low elevation deserts of the south west13 is attributed to enhanced monsoonal circulation3

severity of ocean acidification following the end cretaceous asteroid impact ammonites went extinct at the time of the end cretaceous asteroid impact as did more than 90 of species of calcium carbonate shelled plankton coccolithophores and foraminifera comparable groups not possessing calcium carbonate shells were less severely affected raising the possibility that ocean acidification as a side effect of the collision might have been responsible for the apparent selectivity of the extinctions we investigated whether ocean acidification could have caused the disappearance of the calcifying organisms in a first detailed modelling study we simulated several possible mechanisms from impact to seawater acidification our results suggest that acidification was most probably not the cause of the extinctions

late quaternary environments and biogeography in the great basin abstract plant and animal remains found in packrat neotoma spp middens and cave fill from the eastern and southern great basin region reveal the presence of subalpine conifers and boreal mammals at relatively low elevations during the late wisconsin limber pine pinus flexilis and bristlecone pine p longaeva were important in the late pleistocene plant communities throughout this region spruce picea cf engelmannii and common juniper juniperus communis were present in some of the more northerly localities and douglas fir pseudotsuga menziesii and white fir abies concolor were present in southern and eastern localities single needle pinyon pine pinus monophylla common across this region today was apparently not present north of the sheep range of southern nevada during the late wisconsin pikas ochotona cf princeps small boreal mammals present in only a few great basin mountain ranges today were common throughout the region heather voles phenacomys cf intermedius have been found in two cave fill deposits in nevada though they are unknown in the great basin today limber and bristlecone pines are generally restricted to rocky substrates in modern subalpine habitats in the great basin and this may also have been the case when these plants grew at lower elevations during the late wisconsin subalpine conifers were present on the rock outcrops sampled by the packrat middens but shrub communities perhaps dominated by sagebrush artemisia spp may have been present on alluvial valley bottom substrates forested habitats would thus have been isolated habitat islands as they are today boreal small mammals including pikas and heather voles were able to colonize the great basin mountain ranges during the late pleistocene we suggest that these mammals were able to survive in the intervening valley bottoms under a cool summer climatic regime and that continuous forest or woodland corridors were not necessary for migration

the hex experiment determination of the neutral wind field from 120 to 185 km altitude near a stable premidnight auroral arc by triangulating the drift of rocket deployed chemical trails 1 on 25 march 2003 horizontal e region experiment hex released trimethyl aluminum trails from two rockets launched northward from poker flat research range near fairbanks to map the vertical wind field near a stable premidnight auroral arc system they deployed three trails of trimethyl aluminum chemical puffs whose subsequent motion traced the prevailing wind field this motion was determined using triangulation from four ground observation sites position and speed accuracies were estimated to be 1 km and a few meters per second respectively the first rocket followed a novel flat trajectory it released a nearly horizontal trail of length 200 km at an average altitude of 145 km the second rocket was launched 19 min later and released two trails between 125 and 175 km altitude along a conventional steep trajectory all puffs between 130 and 175 km altitude drifted geomagnetic westward almost exactly parallel to the aurora from prior observations and modeling we had expected to observe convective upwelling near the arcs we did not vertical winds were essentially downward throughout the horizontal trail with speeds between 0 and 20 m s 1 although an abatement of downward flow was observed 40 km equatorward of the arcs these data alone do not establish a causal relationship between the abatement and the arcs vertical speeds of 20 m s 1 are relatively modest however because the observed wind field would entrain air parcels in flow parallel to the arc system even vertical speeds around 15 m s 1 could displace individual air parcels by several scale heights if they occurred all along the arcs

volcanism impact and mass extinctions incredible or credible coincidences massive continental volcanism and or bolide impacts are considered by many authors to have caused three major mass extinction events during the last 300 million years the end permian end cretaceous and end triassic extinctions however reevaluation of the frequency of bolide impacts and plume related flood basalt provinces indicates that both types of event occur much more frequently than mass extinctions and so in isolation may not be responsible for the largest extinctions furthermore the kill mechanisms associated with either flood basalts or impacts do not appear to be sufficiently powerful to cause worldwide collapse of ecosystems leading to the largest mass extinctions contemporaneous flood basalts and bolide impact may be prerequisites for the largest mass extinctions we present a statistical analysis of the probability of coincidence between volcanism and impact and show that three random coincidences of these events in the last 300 m y are likely no causal relationship between impact and volcanism is necessary the lesser mass extinctions on the other hand may not require juxtaposition of two such catastrophic events such coincidences occurring on more than three occasions during the last 300 m y become increasingly unlikely d 2004 elsevier b v all rights reserved

a biogenic origin for anomalous fine grained magnetic material at the paleocene eocene boundary at wilson lake new jersey 1 the paleocene eocene thermal maximum which occurred 55 5 ma was caused by a massive release of carbon as indicated by an 3 negative carbon isotope excursion recorded in the marine atmospheric and terrestrial reservoirs one suggested source for the carbon a cometary impactor is based on the sudden appearance and high concentration of single domain sd magnetite in paleocene eocene p e boundary cores from the north atlantic continental margin we evaluate the potential sources of sd magnetite at the p e boundary by presenting new magnetic hysteresis low temperature magnetic remanence and transmission electron microscopy data from the north atlantic coastal ocean our results show a similar increase in sd material but demonstrate that the magnetic material has a biogenic origin these findings indicate that the high concentrations of sd magnetite immediately above the p e boundary are the result of unusual accumulations and or preservation of magnetotactic bacteria such bacteria typically occupy the oxic anoxic transition zone near the sediment water interface or in the water column the high abundances of sd magnetite in sediments from across the shelf may be an artifact of nonsteady state redox conditions and exceptional preservation of sd magnetite it may also indicate that the oxic anoxic redox boundary shifted into the water column the latter explanation implies transient eutrophy of the coastal ocean in this region most likely due to seasonally enhanced runoff and increased stratification and nutrient loading

time scale controversy accurate orbital calibration of the early paleogene 1 timing is crucial to understanding the causes and consequences of events in earth history the calibration of geological time relies heavily on the accuracy of radioisotopic and astronomical dating uncertainties in the computations of earth s orbital parameters and in radioisotopic dating have hampered the construction of a reliable astronomically calibrated time scale beyond 40 ma attempts to construct a robust astronomically tuned time scale for the early paleogene by integrating radioisotopic and astronomical dating are only partially consistent here using the new la2010 and la2011 orbital solutions we present the first accurate astronomically calibrated time scale for the early paleogene 47 65 ma uniquely based on astronomical tuning and thus independent of the radioisotopic determination of the fish canyon standard comparison with geological data confirms the stability of the new la2011 solution back to 54 ma subsequent anchoring of floating chronologies to the la2011 solution using the very long eccentricity nodes provides an absolute age of 55 530 0 05 ma for the onset of the paleocene eocene thermal maximum petm 54 850 0 05 ma for the early eocene ash 17 and 65 250 0 06 ma for the k pg boundary the new astrochronology presented here indicates that the intercalibration and synchronization of u pb and 40ar 39ar radioisotopic geochronology is much more challenging than previously thought

a possible k t boundary bolide impact site offshore near bombay and triggering of rapid deccan volcanism abstract the temporal coincidence of a major biological mass extinction including dinosaurs the well known iridium excess anomaly at the cretaceous tertiary k t boundary and the eruption of deccan flood basalts at about 65 ma has aroused global interest among geologists and biologists it is widely debated whether the mass extinction and iridium anomaly are due to an asteroid impact or the massive outpouring of extensive deccan volcanism an oval shaped unusual positive gravity anomaly 10 000 km 2 in area near bombay has attracted our attention during a search for an impact site near deccan basalts a detailed gravity interpretation indicates the presence of a fossil conduit structure of 12 km height extending from a shallow crust mantle boundary at 18 km to an approximate depth of 6 km from the surface the conduit structure with a maximum diameter of about 35 km at its base may originate from cracking of a weak pre deccan trap shallow upwarped mantle the structure may have been caused by a bolide impact which triggered the eruption of massive flood basalts deccan traps on the western margin of the fast moving indian plate an impact in this locality can explain the sudden detachment of the arcuate seychelles block from india as well as the large scale reorganisation of plate boundaries in the indian ocean our hypothesis of impact triggered volcanism at 65 ma advocates a bimodal cause for the mass extinction at the k t boundary extraordinary geothermal and structural conditions of the nearby region are also discussed as circumstantial evidence to support the twin cause mechanism by weakened features and the presence of partial melt at subcrustal depth

evidence for periodicity and nonlinearity in a high resolution fossil record of long term evolution the application of new signal analysis techniques provides increased insight into the study of the fossil record and processes of evolution the fossil record of 622 planktic foraminifera contains data from 200 stratigraphic stages of the past 127 m y time series analyses wavelet and fourier transform of the planktic foraminifera fossil record were used to discern periodic components in long term evolution the correlation function analysis was used to distinguish between random and deterministic behavior of the fossil record the analyses show that stationary 30 m y periodicity and complex deterministic patterns occur in the long term planktic foraminifera evolution in particular in the extinction record our results suggest that the occurrence of intense diversity fluctuations with 3 10 m y periodicity after major extinction events may be attributed to nonlinear self organized evolutionary response to the availability of new ecospace this coupled nonlinear periodic scenario may explain the repetitive appearance of similar morphotypes in 30 m y intervals

impacts and glacio eustasy plate tectonic episodes geomagnetic reversals a concept to facilitate detection of impact events abstract it is suggested that the cretaceous tertiary c t impact destabilised and then profoundly changed the climatic structure of the earth for cenozoic times by establishing freezing conditions at the poles via the c t dust veil and the subsequent h 2 o greenhouse evaporation polar snow fields ice caps see level fall tp1 it is also suggested that the c t impact affected the earth s mantle shock induced phase transitions and via chain reactions also the core mantle boundary this mantle effect could possibly explain the laramian plate tectonic episode 65 ma and reversal t29 r n which occurred 150 000 years after the c t boundary as long term impact effects the combination of these and the multitude of associated events c f faunal breaks appear to correlate episodically in the geological past it is proposed that these repetitions can be used as a potential approach to narrow the search for impact traces also more examples of correlations between impact events and their possible effects are needed to ensure that the implications of the c t impact are generally valid

impacts do not initiate volcanic eruptions eruptions close to the crater many papers on meteorite impact suggest that large impacts can induce volcanic eruptions through decompression melting of the underlying rocks we perform numerical simulations of the impact of an asteroid with a diameter of 20 km striking at 15 km s 1 into a target with a near surface temperature gradient of 13 k km 1 cold case or 30 k km 1 hot case the impact creates a 250 300 km diameter crater with 10 000 km3 of impact melt however the crater collapses almost flat and the pressure field returns almost to the initial lithostat even an impact this large cannot raise mantle material above the peridotite solidus by decompression statistical considerations also suggest that impacts cannot be the common initiator of large igneous provinces any time in post heavy bombardment earth history

effect of a single large impact on the coupled atmosphere interior evolution of venus abstract we investigate the effect of a single large impact either during the late veneer or late heavy bombardment on the evolution of the mantle and atmosphere of venus we use a coupled interior exterior numerical code based on stagyy developed in gillmann and tackley gillmann c tackley p j 2014 j geophys res 119 1189 1217 single vertical impacts are simulated as instantaneous events affecting both the atmosphere and mantle of the planet by i eroding the atmosphere causing atmospheric escape and ii depositing energy in the crust and mantle of the planet the main impactor parameters include timing size mass velocity and efficiency of energy deposition we observe that impact erosion of the atmosphere is a minor effect compared to melting and degassing triggered by energy deposition in the mantle and crust we are able to produce viable pathways that are consistent with present day venus especially considering large late veneer impacts small collisions

packrat middens from canyon de chelly northeastern arizona paleoecological and archaeological implications abstract in western north america pollen data from highland lakes are often used to reconstruct vegetation on the adjacent lowlands plant macrofossils and pollen from packrat middens now provide a means to evaluate such reconstructions on the basis of pollen diagrams from the chuska mountains h e wright jr a m bent b s hansen and l j maher jr 1973 geological society of america bulletin 84 1155 1180 arrived at conservative estimates for late pleistocene depression of highland conifers in their interpretation a proposed slight depression of 500 m for lower tree line precluded expansion of pinus ponderosa into elevations now in desertscrub instead it was suggested that pinyon pine and artemisia occupied the lowland plateaus packrat midden records on either side of the chuskas fail to verify this model early holocene middens from chaco canyon new mexico and a terminal pleistocene midden from canyon de chelly arizona show that blue spruce limber pine douglas fir dwarf juniper and rocky mountain juniper expanded at least down to 1770 m elevation neither colorado pinyon nor ponderosa pine was found as macrofossils in the middens artemisia pollen percentages are high in the terminal pleistocene midden as they are in the chuska mountain pollen sequence suggesting regional dominance by sagebrush steppe of 38 taxa identified only 3 are shared by middens dated 11 900 and 3120 yr b p from canyon de chelly indicating a nearly complete turnover in the flora between the late pleistocene and late holocene although corn was previously thought to have been introduced to the colorado plateaus after 2200 yr b p the midden dated 3120 yr b p contains pollen of corn and other indicators of incipient agriculture

evidence for a rapid release of carbon at the paleocene eocene thermal maximum the paleocene eocene thermal maximum petm and associated carbon isotope excursion cie are often touted as the best geologic analog for the current anthropogenic rise in pco2 however a causal mechanism for the petm cie remains unidentified because of large uncertainties in the duration of the cie s onset here we report on a sequence of rhythmic sedimentary couplets comprising the paleocene eocene marlboro clay salisbury embayment these couplets have corresponding 18o cycles that imply a climatic origin seasonal insolation is the only regular climate cycle that can plausibly account for 18o amplitudes and layer counts high resolution stable isotope records show 3 5 13c decrease over 13 couplets defining the cie onset which requires a large instantaneous release of 13c depleted carbon during the cie a clear 13c gradient developed on the shelf with the largest excursions in shallowest waters indicating atmospheric 13c decreased by 20 our observations and revised release rate are consistent with an atmospheric perturbation of 3 000 gigatons of carbon gtc

reconstruction of past desert vegetation along the colorado river using packrat middens abstract the paleoecological reconstruction of pleistocene deserts along the colorado river of western north america is attempted using data from fossil packrat middens the colorado river drainage is set into a physiographic context along a gradient from the hyperarid colorado desert to the moist high elevations on the colorado plateau the pleistocene and modern distributions of individual plant species along this corridor are compared emphasizing records from the picacho peak area of the colorado desert and the eastern grand canyon in general plant species are now distributed 700 900 m higher in elevation and 400 700 km further up river than they were during the late wisconsin however some plant species have not conformed to the general pattern emphasizing the individualistic nature of species distributions in time

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

late quaternary environmental changes in the upper las vegas valley nevada abstract five stratigraphic units and five soils of late pleistocene to holocene age crop out in dissected badlands on corn creek flat 30 km northwest of las vegas nevada and at tule springs nearer to las vegas the record is dominantly fluvial but contains evidence of several moister marsh forming periods the oldest unit b dates perhaps to the middle wisconsin and the more widespread unit d falls between 30 000 and 15 000 yr b p unit d therefore correlates with pluvial maximum lacustrine deposits elsewhere in the great basin standing water was not of sufficient depth or extent during either period to form lake strandlines between 14 000 and 7200 yr b p unit e standing surface water gradually decreased a trend also apparent in great basin pluvial lake chronologies during the same period groundwater carbonate cementation and burrowing by cicadas cicadae accompany the moist phase units after 7200 yr b p increased wind action decreased biotic activity and at least 25 m of water table lowering accompanied widespread erosion of older fine grained deposits based on pack rat midden and pollen evidence this coincides with major vegetation changes in the valley from sagebrush dominated steppe to lower mohave desertscrub

a statistically significant long term characteristic time scale of test size variation of calcareous trochospiral benthic foraminifera ctbf during the past 120 m y 1 geo marine records are faithful recorders of the earth s evolutionary history and hence are most suitable for identifying long term cyclic patterns although a remarkable synchronism among calcareous trochospiral benthic foraminifera ctbf test size variation ocean productivity dissolved oxygen and global climate exists global geospectroscopy is essential for understanding the wider ramification of significant physical linkages of the various terrestrial and extra terrestrial processes earlier claims of a quasi periodicity of 26 36 m y in various geo bio ocean atmospheric records sparked a major controversy here we examine spectral characteristics of the latest available time series representing fluctuations in test size of ctbf for the past 120 m y using the multi taper method mtm and maximum entropy method mem of spectral analyses the analyses of test size variations of ctbf time series reveal hitherto unidentified a statistically significant and stable harmonic component of 35 5 m y against a robust red background noise further wavelet spectral analysis of this record exhibits a strong stationary power in the above frequency band suggesting temporal persistence of spectral peak through the entire evolutionary period this finding renders credible support to the concept of cyclic geo marine processes and will provide stimulus for further quantitative research into the coupling of marine geo bio chemical cycles

late pleistocene vegetation of kings canyon sierra nevada california seven packrat midden samples make possible a comparison between the modern and late pleistocene vegetation in kings canyon on the western side of the southern sierra nevada one modern sample contains macrofossils and pollen derived from the present day oak chaparral vegetation macrofossils from the six late pleistocene samples record a mixed coniferous forest dominated by the xerophytic conifers juniperus occidentalis pinus cf ponderosa and p monophylla the pollen spectra of these pleistocene middens are dominated by pinus sp taxodiaceae cupressaceae taxaceae tct and artemisia sp mesophytic conifers are represented by low macrofossil concentrations sequoiadendron giganteum is represented by a few pollen grains in the full glacial edaphic control and snow dispersal are the most likely causes of these mixed assemblages the dominant macrofossils record a more xeric plant community than those that now occur on similar substrates at higher elevations or latitudes in the sierra nevada these assemblages suggest that late wisconsin climates were cold with mean annual precipitation not necessarily greater than modern values this conclusion supports a model of low summer ablation allowing for the persistence of the glaciers at higher elevations during the late wisconsin the records in these middens also suggest that s giganteum grew at lower elevations along the western side of the range and that p monophylla was more widely distributed in cismontane california during the pleistocene

oxygen isotope composition of annually banded modern and mid holocene travertine and evidence of paleomonsoon floods grand canyon arizona usa abstract holocene and modern travertine formed in spring fed havasu creek of the grand canyon arizona was studied to determine the factors governing its oxygen isotope composition analysis of substrate grown travertine indicates that calculated calcite formation temperatures compare favorably with measured water temperatures and include silt rich laminae deposited by monsoon driven floods ancient spring pool travertine is dated by u series at 7380 110 yr and consists of 14 travertine silt couplets of probable annual deposition one hundred eighty high resolution 18o analyses of this mid holocene sample average 11 0 pdb the average value for modern travertine is 0 5 lower perhaps because mid holocene temperature was higher or there was proportionally greater summer recharge 18o cyclicity in the mid holocene travertine has average amplitude of 1 9 0 5 pdb slightly less than the inferred modern day annual temperature range of havasu creek the annual temperature range might have been reduced during the 14 yr interval compared to present although other non temperature factors could account for the muted annual variation silt rich laminae within isotopically lower calcite in the modern and mid holocene travertine verifies the seasonal resolution of both samples and suggests that similar temperature precipitation conditions as well as monsoon generated summer floods prevailed in the mid holocene as they do throughout the grand canyon region today

a record of rapid holocene climate change preserved in hyrax middens from southwestern africa the discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central namib desert africa is providing unprecedented insight into the region s environmental history high resolution stable carbon and nitrogen isotope records spanning 0 11 700 cal calibrated yr b p indicate phases of relatively humid conditions from 8700 7500 6900 6700 5600 4900 and 4200 3500 cal yr b p with a period of marked aridity occurring from 3500 until ca 300 cal yr b p transitions between these phases appear to have occurred very rapidly often within 200 years of particular importance are 1 the observed relationship between regional aridifi cation and the decline in northern hemisphere insolation across the holocene and 2 the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing together these elements call for a fundamental reexamination of the role of orbital forcing on tropical african systems and a reconsideration of what factors drive climate change in the region the quality and resolution of these data far surpass any other evidence available from the region and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern africa

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

major episodes of geologic change correlations time structure and possible causes abstract published data sets of major geologic events of the past 250myr extinction events sea level lows continental flood basalt eruptions mountain building events abrupt changes in sea floor spreading ocean anoxic and blackshale events and the largest evaporite deposits have been synthesized with estimated errors these events show evidence for a statistically significant periodic component with an underlying periodicity formally equal to 26 6 myr and a recent maximum close to the present time the cycle may not be strictly periodic but a periodicity of 30myr is robust to probable errors in dating of the geologic events the intervals of geologic change seem to involve jumps in sea floor spreading associated with episodic continental rifting volcanism enhanced orogeny global sea level changes and fluctuations in climate the period may represent a purely internal earth pulsation but evidence of planetesimal impacts at several extinction boundaries and a possible underlying cycle of 28 36 myr in crater ages suggests that highly energetic impacts may be affecting global tectonics a cyclic increase in the flux of planetesimals might result from the passage of the solar system through the central plane of the milky way galaxy an event with a periodicity and mean phasing similar to that detected in the geologic changes

dwm07 global empirical model of upper thermospheric storm induced disturbance winds 1 we present a global empirical disturbance wind model dwm07 that represents average geospace storm induced perturbations of upper thermospheric 200 600 km altitude neutral winds dwm07 depends on the following three parameters magnetic latitude magnetic local time and the 3 h kp geomagnetic activity index the latitude and local time dependences are represented by vector spherical harmonic functions up to degree 10 in latitude and order 3 in local time and the kp dependence is represented by quadratic b splines dwm07 is the storm time thermospheric component of the new horizontal wind model hwm07 which is described in a companion paper dwm07 is based on data from the wind imaging interferometer on board the upper atmosphere research satellite the wind and temperature spectrometer on board dynamics explorer 2 and seven ground based fabry perot interferometers the perturbation winds derived from the three data sets are in good mutual agreement under most conditions and the model captures most of the climatological variations evident in the data

auroral zone thermospheric dynamics 1 averages forty four nights of thermospheric neutral wind and temperature measurements were obtained with a ground based fabry perot spectrometer at college alaska 65 invariant latitude mostly during high to moderate levels of solar activity the spectrometer scanned the o i 15 867 k 6300 a emission line yielding line profile measurements that originate from an altitude range between 170 and 280 km the observed average wind direction is to the northwest in the early evening changing to the south near midnight and then to the southeast in the morning the overall wind pattern generally persists but the wind speed increases with increasing geomagnetic activity a change in the zonal wind direction occurs earlier in magnetic local time with increasing magnetic activity the meridional wind pattern also shifts equatorward with the auroral oval during increasing magnetic activity consequently the average wind pattern in the north during low geomagnetic activity is rather similar to the moderate activity average in the south and the moderate geomagnetic activity average wind pattern in the north is similar to the high geomagnetic activity average in the south the observed average thermospheric temperature is found to be governed by geomagnetic activity and the previous day s 10 7 cm solar flux

volcanism around k t boundary time its role in an impact scenario for k t extinction events abstract a widespread debate on the cretaceous tertiary k t boundary 65 1 ma extinctions has largely revolved around an impact or a volcanic cause evidence for both events is strengthened by recent work impact triggered deccan volcanism or independent impact and mantle plume events have been suggested this review examines evidence for both events and suggests that they overlapped in time but with globally uneven effects the combined effects produced generally inhospitable k t environments leading to mass extinctions although some life forms flourished afterwards volcanic contributions to the k t boundary environment probably include increased platinum group element pge and acid rain fall out particularly from large scale mantle upwellings e g deccan cameroon and coral sea events a previously proposed post k t greenland starting plume apparently already existed in the cretaceous k t features such as the iridium ir anomaly most likely include longer term volcanic as well as sudden meteoritic impact contributions volcanic contributions were greatest from southern hemisphere sources whereas meteorite impacts featured in the northern hemisphere

tektites and microtektites key facts and inferences abstract tektites are generally small black rounded silicate glass bodies found scattered over several widely separated areas of the earth s surface called strewn fields four major tektite strewn fields are known 1 australasian 2 ivory coast 3 czechoslovakian and 4 north american tektites from these four strewn fields have been dated at 0 7 1 15 and 34 ma respectively most tektites are splash forms i e spheres teardrops dumbbells etc but some have undergone a second period of melting to produce ablated forms tektites with a blocky layered appearance are called muong nong types microtektites microscopic tektites belonging to the australasian ivory coast and north american strewn fields have been found in deep sea sediments the geographic occurrence and abundance of microtektites at each site have been used to determine the size and mass of each strewn field the australasian ivory coast and north american strewn fields contain 100 million 20 million and 1 billion tonnes of glass respectively although tektites resemble obsidian they can be distinguished from terrestrial volcanic glasses by their composition and petrography tektites for example are drier than volcanic glasses and have higher feo fe 2 o 3 ratios in addition tektites unlike volcanic glasses contain lechatelierite particles sio 2 glass and are devoid of primary crystallites although some authors favor a lunar volcanic origin most of the compositional and petrographic data indicate a terrestrial impact origin the major trace and isotopic compositions of tektites indicate that the parent material was a terrestrial sedimentary deposit and the presence of lechatelierite coesite and shocked relict inclusions favors an impact origin furthermore two tektite strewn fields have been associated with known impact craters and tektites from one strewn field have been found associated with impact ejecta unlike the cretaceous tertiary boundary layer microtektite layers are not associated with mass extinctions or iridium anomalies

gauss s theorem mass deficiency at chicxulub crater yucatan mexico and the extinction of the dinosaurs using gauss s theorem we estimated the mass deficiencies of the chicxulub impact structure yucatan mexico from its gravity anomaly the mass deficiency obtained from the residual gravity anomaly map ranges between 1 06 1016 and 1 67 1016 kg because the gravity anomaly has approximately radial symmetry we also estimated the mass deficiency from selected profiles in this way we obtained slightly lower mass deficiency 6 16 1015 to 1 35 1016 kg the central gravity high which is supposed to be associated with the central structural high has a mean excess mass of 1 93 1014 kg by assuming a mean density contrast of 100 kg m3 between the country rock and the sedimentary and brecciated rocks we estimated the equivalent total mass 1 60 1017 to 4 34 1017 kg and volume 6 16 1013 to 1 67 1014 m3 of breccias and sedimentary rocks responsible for the gravity anomaly these figures represent lower bounds on the mass and volume ejected from the impact crater they represent estimates m

seasonal dependence of northern high latitude upper thermospheric winds a quiet time climatological study based on ground based and space based measurements this paper investigates the large scale seasonal dependence of geomagnetically quiet time northern high latitude f region thermospheric winds by combining extensive observations from eight ground based optical remote sensing and three space based optical remote sensing and in situ instruments to provide a comprehensive picture of the wind morphology data are assimilated into a seasonal empirical vector wind model as a function of season latitude and local time in magnetic coordinates the model accurately represents the behavior of the constituent data sets there is good general agreement among the various data sets but there are some major offsets between goce and the other data sets especially on the duskside the assimilated wind patterns exhibit a strong and large duskside anticyclonic circulation cell sharp latitudinal gradients in the duskside auroral zone strong antisunward winds in the polar cap and a weaker tendency toward a dawnside cyclonic circulation cell the high latitude wind system shows a progressive intensification of wind patterns from winter to equinox to summer the latitudinal extent of the duskside circulation cell does not depend strongly on season zonal winds show a mainly diurnal variation two extrema around polar and middle latitudes and semidiurnal variation four extrema at auroral latitudes meridional winds are primarily diurnal at all high latitudes the strength of zonal winds channeling through the auroral zone on the duskside is strongest in the summer season the vorticity of the wind pattern increases from winter to summer whereas divergence is maximum in equinox in all three seasons divergence is weaker than vorticity

osmium tungsten and chromium isotopes in sediments and in ni rich spinel at the k t boundary signature of a chondritic impactor it is now established that a large extraterrestrial object hit the earth at the end of the cretaceous period about 65 ma ago we have investigated re os hf w and mn cr isotope systems in sediments from the cretaceous and the paleogene in order to characterize the type of impactor within the cretaceous tertiary k t boundary layer extraterrestrial material is mixed with terrestrial material causing a dilution of the extraterrestrial isotope signature that is difficult to quantify a phase essentially composed of ni rich spinel formed in the atmosphere mainly from melted projectile material is likely to contain the extraterrestrial isotopic signature of the impactor we show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor and that w and cr isotopes confirm that the projectile was a carbonaceous chondrite

global climatic forcing of deep sea benthic foraminiferal test size during the past 120 m y stratigraphic distribution of the maximum diameter of the largest deep water 1500 m water depths calcareous trochospiral benthic foraminiferal tests in samples representing the past 120 m y shows that average values of the maximum diameter have doubled since early cretaceous time fluctuations in test size show six minima and six maxima it is significant that the fluctuations correspond to changes in global climate and in deep water dissolved oxygen conditions this latter synchronism indicates that deep sea foraminiferal test size was likely controlled by fluctuations in the level of dissolved oxygen or related factors in the deep ocean caused by global climatic changes

basaltic explosive volcanism but no comet impact at the paleocene eocene boundary high resolution chemical and isotopic records from egypt spain and denmark in the search for a triggering mechanism for the paleocene eocene p e boundary event 55 ma centimeter resolution chemical e g ir os pt and isotope e g os o 187 os 188 he 3 he 4 sr 87 sr 86 records across this boundary have been established for six uplifted marine sections in egypt spain and denmark the sections studied represent some of the stratigraphically most complete records across the onset of the carbon isotopic excursion cie and associated benthic foraminifera extinctions that mark the paleocene eocene boundary high sensitivity analyses failed to uncover evidence of extraterrestrial element or isotope enrichments in the six sections refuting the hypothesis of a major comet impact at the boundary preliminary searches for other impact indicative features such as spherules or shocked quartz also gave negative results in the danish section studied three basaltic ir rich ash layers occur at the paleocene eocene boundary but no similar ashes were found in egypt or spain the three ashes represent the earliest known manifestation of an unusual i to 2 million year long phase of explosive basaltic volcanism in the faero greenland region this volcanism is synchronous with major flood basalt effusions in east greenland and is associated with prominent paleogeographic changes in the high latitude north atlantic region discharge of mantle derived os to seawater during this volcanism may explain a small decrease in os o 187 os o 188 ratio at the paleocene eocene boundary in the zumaya section in spain the environmental perturbations at the paleocene eocene boundary appear to have been triggered by basaltic volcanism but any model for the detailed causal relation remains speculative c 2004 elsevier b v all rights reserved less

spatial evolution of frictional heating and the predicted thermospheric wind effects in the vicinity of an auroral arc measured with the sondrestrom incoherent scatter radar and the reimei satellite 1 simultaneous observation with the sondrestrom incoherent scatter is radar and the reimei satellite was conducted on 3 october 2007 the objective was to measure horizontal patterns of the ionospheric structure in the vicinity of an auroral arc the is radar was scanned azimuthally with a fixed elevation angle and the satellite narrow view camera 557 7 nm was directed downward taking pictures every 0 12 s a stationary auroral arc was captured at 05 17 ut 02 54 mlt with both instruments simultaneously gross ionospheric features measured with the is radar around the arc were in good agreement with the expected magnetosphere ionosphere current system of particular interest was the horizontal pattern in the ion speed and temperature in the f region the ion speed within the arc was close to zero by contrast the larger ion speed 350 400 m s 1 on the poleward side was parallel to the arc and almost no horizontal shear within about 20 km width perpendicular to the arc this area was separated into two parts by the ion temperature one was associated with clear enhancements in excess of 1200 k and another was with more moderate enhancements less than 1000 k the widths of the two areas were approximately 10 km each the horizontal shear seen in the ion temperature suggested the presence of a narrow thermospheric wind shear of about 10 km width this paper suggests that the possible cause for the thermospheric wind shear was ion drag associated with localized soft particle precipitation or f region ionization

spatial structure in the thermospheric horizontal wind above poker flat alaska during solar minimum a new all sky imaging wavelength scanning fabry perot spectrometer was used to record high resolution r 200 000 spectra of the 630 nm thermospheric optical emission above poker flat alaska these spectra were used to derive spatially resolved maps of the horizontal wind vector at approximately 250 km altitude we describe the procedure used to infer vector winds from line of sight doppler shifts along with its limitations we present the time evolution of the vector wind fields obtained from this method for 6 nights of observation five of the 6 nights contained periods when we inferred the existence of significant curvature divergence or shear in the thermospheric wind across our instrument s 1000 km diameter field of view the sixth night exhibited little spatial structure and is shown for comparison we compare these results with a generic solar minimum winter time run of the national center for atmospheric research s thermosphere ionosphere and electrodynamics general circulation model while agreement was good at the start and end of the night considerable differences were found in the late evening and midnight sectors some possible origins for these discrepancies are proposed in particular we suggest that the f region horizontal wind may be deflected by upwelling vertical winds which are in turn driven by e region heating in the auroral zone we note that both the instrument used and our high latitude implementation of the analysis procedure are new experimental techniques thus the data presented here should be regarded as preliminary and if possible be validated by comparison with results from other techniques

geomagnetic reversals from impacts on the earth the impact of a large extraterrestrial object on the earth can produce a geomagnetic reversal through the following mechanism dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age the redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the earth if the sea level change is sufficiently large 10 meters and rapid in a few hundred years then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo the new convective cells that subsequently form distort and tangle the previous field reducing the dipole component near to zero while increasing the energy in multipole components eventually a dipole is rebuilt by dynamo action and the event is seen either as a geomagnetic reversal or as an excursion sudden climate changes from other causes such as volcanic eruptions could also trigger reversals this mechanism may not be the sole cause of geomagnetic reversals but it can account for the rapid drop of the dipole component preceding a reversal the predominance of multipole components during a transition the associations of microtektites temperature drops and extinctions with reversals and the possible correlation between peaks in the geomagnetic reversal rate and the times of mass extinctions the model may also account for the long term changes in the average rate of reversals we make several testable predictions

the extraterrestrial impact evidence at the palaeocene eocene boundary and sequence of environmental change on the continental shelf we have identified clear evidence of an extraterrestrial impact within the onset of the carbon isotope excursion cie that defines the palaeocene eocene p e boundary hyperthermal event approx 56 ma from several sites on the eastern atlantic coastal plain and offshore we review and update the state of the evidence for an impact at the p e boundary including a k ar cooling age of the ejecta that is indistinguishable from the depositional age at the p e which establishes the ejecta horizon as an isochronous stratigraphic indicator at the p e immediately above the ejecta peak at the base of the coastal plain marlboro clay unit we identify a sharp increase in charcoal abundance coincident with the previously observed dramatic increase in magnetic nanoparticles of soil pyrogenic origin we therefore revisit the observed sequence of events through the p e boundary on the western atlantic coastal plain showing that an extraterrestrial impact led to wildfires landscape denudation and deposition of the thick marlboro clay whose base coincides with the spherule horizon and cie onset the sr ca ratio of the spherules indicates that the carbon responsible for the onset may be vaporized caco 3 target rock mixed with isotopically light carbon from the impactor or elsewhere crucially we do not argue that the impact was responsible for the full manifestation of the cie observed globally onset to recovery approx 170 kyr rather that a rapid onset was triggered by the impact and followed by additional carbon from other processes such as the eruption of the north atlantic igneous province such a scenario agrees well with recent modelling work though it should be revisited more explicitly this article is part of a discussion meeting issue hyperthermals rapid and extreme global warming in our geological past

upper eocene tektite and impact ejecta layer on the continental slope off new jersey during leg 150 of the ocean drilling project odp two sites 903c and 904a were cored that have sediments of the same biostratigraphic age as the upper eocene tektite bearing ejecta layer at deep sea drilling project dsdp site 612 core 45x from odp site 904a 4 km north of site 612 contains a 5 cm thick tektite bearing ejecta layer and core 56 from site 903c 8 km north northwest of site 904 contains a 2 cm thick layer of impact ejecta without any tektite or impact glass shocked quartz and feldspar grains with multiple sets of planar deformation features pdfs and abundant coesite bearing grains are present at both sites the major oxide contents trace element compositions and rare earth element ree patterns of the site 904 tektites are similar to those of the site 612 tektites and to north american tektites especially bediasites the sr and nd values for one composite tektite sample from site 904 fall within the range previously obtained for the site 612 tektites which defines a linear trend that if extrapolated would intersect the values obtained for north american tektites the water contents of eight tektite fragments from site 904 range from 0 017 to 0 098 wt and thus are somewhat higher than is typical for tektites the heavy mineral assemblages of the 63 125 m size fractions from the ejecta layers at sites 612 903 and 904 are all similar therefore we conclude that the ejecta layer at all three sites is from the same impact event and that the tektites at sites 904 and 612 belong to the north american tektite strewn field clinopyroxene bearing cpx spherules occur below or in the lower part of the main ejecta layer at all three sites at all three sites the cpx spherules have been partly or completely replaced with pyrite that preserved the original crystalline textures site 612 903 and 904 cpx spherules are similar to those found in the caribbean sea gulf of mexico central equatorial pacific western equatorial pacific and eastern indian ocean the cpx event appears to have preceded the north american tektite event by 10 15 ka or less the fining upward sequence at all three sites and concentration of the denser unmelted impact ejecta at the top of the tektite layer at sites 612 and 904 suggest that the tektite bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass geographic variations in thickness of the tektite bearing ejecta layer the lack of carbonate clasts in the ejecta layer and the low cao content of the tektite glass suggest that the ejecta including the tektite glass were derived from the chesapeake bay structure rather than from the toms canyon structure a sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera radiolaria sponges and fish as well as the planktic foraminifera

the significance of multiple causes and coincidence in the geological record from clam clusters to cretaceous catastrophe specific causes of unusual events recorded in the geological record are commonly difficult to distinguish and isolate in some instances event strata contain features that cannot be explained by a single causal mechanism unicausal hypotheses when applied to complex problems can lead to the misidentification misinterpretation and force fitting of observations great expectations syndrome the close timing or temporal overlap of significant events although statistically improbable on short time scales becomes possible on long time scales event coincidence may occur on a wide range of scales from local to global on the local scale a multiple event interpretation is offered for both the concentration and clustering of bivalves at specific levels within the upper cretaceous bearpaw formation of southern alberta for this example the relative timing of fluctuations in benthic substrate texture oxygen concentration abundance of planktotrophic larvae and degree of sea floor scouring was crucial

siderophile rich magnetic spheroids from the cretaceous tertiary boundary in umbria italy the magnetic fraction of some cretaceous tertiary k t boundary sediments sampled at two locations in italy contains a spheroidal component which has retained some relict mineral textures and possibly mineral chemistry from the k t event the spheroids contain small crystals 1 50 m of magnetite in a highly altered groundmass these crystals are commonly skeletal indicating formation by rapid growth from a high temperature liquid variable concentrations of ni mg and al differing by at least a factor of 5 in the magnetite may also reflect a relict chemistry these spheroids contain higher concentrations of ir and probably other siderophiles than the bulk sediment and may have been an important carrier for these elements at 65 myr this discovery supports evidence for a major accretionary event at the end of the cretaceous

geochemical evidence for suppression of pelagic marine productivity at the cretaceous tertiary boundary the normal biologically productive ocean is characterized by a gradient of the 13c 12c ratio from surface to deep waters here we present stable isotope data from planktonic and benthic micro fossils across the cretaceous tertiary boundary in the north pacific which reveal a rapid and complete breakdown in this biologically mediated gradient the fluxes of barium a proxy for organic carbon and caco3 also decrease significantly at the time of the major marine plankton extinctions the implied substantial reduction in oceanic primary productivity persisted for 0 5 myr before the carbon isotope gradient was gradually re established in addition the stable isotope and preservational data indicate that environmental change including cooling began at least 200 kyr before the cretaceous tertiary boundary and a peak warming of 3 c occurred 600 kyr after the boundary event

late eocene north american microtektites and clinopyroxene bearing spherules approximately 35 m y ago two events separated by only 10 000 20 000 years each spread roughly 1 billion metric tons of silicate spherules over large areas of the earth s surface the older event produced mostly clinopyroxene bearing spherules cpx spherules the cpx spherules have been found in the caribbean sea gulf of mexico equatorial pacific and eastern equatorial indian ocean the cpx spherule layer is associated with an iridium anomaly and appears to coincide with the extinction of several species of radiolaria the composition and widespread distribution of the cpx spherules indicate that they were formed by an impact event the younger layer of spherules consists of microtektites belonging to the north american tektite strewn field based on age and elemental and isotopic composition the north american microtektites have been found in the western equatorial atlantic on barbados in the caribbean sea and in the gulf of mexico the north american microtektite layer is not synchronous with an iridium anomaly nor does it appear to be associated with any major changes in the microfossils neither the cpx spherule event nor the north american tektite event appears to be associated with a climatic change but no detailed studies have been made across either layer excluding the cpx layer which contains mostly crystal bearing spherules the evidence at the present time indicates that there is only one late eocene microtektite layer and it is part of the north american strewn field

linking river flood dynamics to hyperpycnal plume deposits experiments theory and geological implications turbid river plumes entering ocean or lake water of lesser density i e hyperpycnal plumes can plunge to form turbidity currents providing an important link between terrestrial sediment sources and marine depositional sinks a leading hypothesis suggests that hyperpycnal plume deposits accurately record the rising and falling discharge of a flooding river in terms of sediment size grading bedform sequence and deposit thickness which if correct has significant implications for unraveling river dynamics reservoir potential and earth history from marine event beds herein we present one of the first experimental flume studies aimed at testing this hypothesis results indicate that depth averaged hyperpycnal plume velocities can be uncorrelated or even anti correlated with river discharge at certain seabed locations because of translation of the plunge point resulting from temporal variations in discharge and sediment concentration through the duration of a river flood an advection length scale of settling sediment is found to be an important control on hyperpycnal plume deposits where coarse sediment sand is most likely to record multiple flow accelerations and decelerations related to plunge point translation even for a river flood with a single peaked hydrograph in contrast fine sediment mud is relatively insensitive to local plunge point dynamics and is most likely to preserve directly rising and falling river discharge finally it was found that the necessary fluvial sediment concentration to form a plunging plume can be much larger than the concentration typically used assuming density equivalence because of deposition upstream of the plunge point

winds in the high latitude lower thermosphere dependence on the interplanetary magnetic field 1 wind observations in the summertime lower thermosphere at high southern latitudes measured by the wind imaging interferometer windii on the upper atmosphere research satellite are statistically analyzed in magnetic coordinates and correlated with the interplanetary magnetic field imf to determine influences of imf dependent ionospheric convection on the winds effects are clearly detectable down to 105 km altitude above 125 km the wind patterns show considerable similarity with ionospheric convection patterns and the speed of the averaged neutral wind in the polar cap often exceeds 300 m s the correlation between the imf bz component and the diurnal harmonic of the winds is generally best when the imf is averaged over the preceding 1 4 5 hours the magnetic zonal mean zonal wind below 120 km correlates best with the imf by component when the latter is averaged over approximately the preceding 20 hours the wind has a significantly stronger rotational than divergent component around and above 120 km a dusk side anticyclonic wind vortex is prominent consistent with earlier findings around 140 km and higher the dusk side vortex intensifies for negative bz but around 120 km it is the dawn side cyclonic vortex that responds more strongly to bz variations the dependence of the wind on the imf is nonlinear especially with respect to imf bz for positive bz the difference winds are largely confined to the polar cap while for negative bz the difference winds extend to subauroral latitudes a significant correlation between the diurnal bz dependent neutral and convection velocity components exists above 108 km when the convection velocity is suitably rotated in magnetic local time mlt with respect to the wind the rotation that maximizes the correlation ranges from 1 5 hours at 130 km wind preceding convection to nearly 6 hours at 108 km wind lagging convection the rotated diurnal bz dependent wind pattern projects onto the diurnal bz dependent ionospheric convection pattern with about 60 the amplitude of the latter above 125 km decreasing to about 17 at 108 km on timescales of 20 hours a by dependent magnetic zonal mean zonal wind generally exists with maximum wind speeds at 80 magnetic latitude typically 10 m s at 105 km increasing to about 60 m s at 123 km and 80 m s at 200 km in the southern hemisphere the wind is cyclonic when the time averaged by is positive and anticyclonic when by is negative the wind direction is expected to be opposite in the northern hemisphere

fossil insect evidence for late quaternary climatic change in the big bend region chihuahuan desert texas abstract a series of 50 packrat midden assemblages from the big bend region of the chihuahuan desert ranging in age from 36 000 yr b p to recent yielded abundant diverse arthropod faunas the mesic nature of regional wisconsin age climates is substantiated by the fauna from 30 000 12 000 yr b p especially during the middle wisconsin 30 000 20 000 yr b p late wisconsin faunas contained grassland species which are confined today to cooler moister regions following 12 000 yr b p most of these temperate species were replaced either by desert species or by more cosmopolitan taxa marking the climatic shift from late wisconsin to postglacial time insects indicative of more severe aridity are first recorded at about 6000 yr b p but some temperate species persisted until about 2500 yr b p after this only desert dwellers are recorded

biogeographic implications of a packrat midden sequence from the sacramento mountains south central new mexico abstract thirteen packrat neotoma spp and two porcupine erethizon dorsatum middens from 1555 to 1690 m elevation from the sacramento mountains new mexico provide an 18 000 yr vegetation record in the northern chiuahuan desert the vegetation sequence is a mesic wisconsin fullglacial 18 000 16 000 yr b p pinyon juniper oak woodland a xeric early holocene ca 11 000 8000 yr b p juniper oak woodland a middle holocene ca 8000 4000 yr b p desert grassland and a late holocene ca 4000 yr b p to present chihuahuan desertscrub the frequency of spring freezes and summer droughts in the late wisconsin probably set the northern limits of pinus edulis and juniperus monosperma at about 34 n or 6 south of today s limit rising summer tempratures in the early holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope at this time pinyon juniper woodland and pine forest dominated by pinus ponderosa probably began their spectacular holocene expansions to the north continued warming in the middle holocene led to very warm summers with strong monsoons relatively dry cold winters and widespread desert grasslands desertscrub communities in the northern chihuahuan desert did not develop until the late holocene when the biseasonal rainfall shifted slightly back toward the winter catastrophic winter freezes decreased and droughts in all seasons increased the creosote bush desertscrub corridor across the continental divide between the chihuahuan and sonoran deserts was probably connected for the first time since the last interglaciation

constraints on the numerical age of the paleocene eocene boundary here we present combined radio isotopic dating u pb zircon and cyclostratigraphic analysis of the carbon isotope excursion at the paleocene eocene p e boundary in spitsbergen to determine the numerical age of the boundary incorporating the total uncertainty from both radio isotopic and cyclostratigraphic datasets gives an age ranging from 55 728 55 964 ma within error of a recently proposed astronomical age of 55 93 ma combined with the assumption that the paleocene epoch spans twenty five 405 kyr cycles our new age for the boundary suggests an age of 66 ma for the cretaceous paleogene k pg boundary furthermore our p e boundary age is consistent with the hypothesis that the onset of the paleocene eocene thermal maximum petm at the boundary occurred on the falling limb of a 405 kyr cycle suggesting the event was initiated by a different mechanism to that which triggered the other early eocene hyperthermals

nivation landforms in the western great basin and their paleoclimatic significance abstract more than 10 000 nivation landforms occur in the higher mountain ranges of the western great basin they range from small subtle hollows with head scarps a few meters high and a few tens of meters long to broad clearly defined terraces as much as 220 m wide bounded by bold steeply sloping head scarps as much as 30 m high and 1600 m long distribution of these nivation hollows is strongly influenced by elevation slope orientation local relief and substrate lithology about 95 occur between 2200 and 3000 m elevation and nearly 80 are situated on north northwest to east northeast facing slopes they occur mainly in areas of moderately sloping terrain and moderate local relief and they are preferentially developed on relatively incompetent substrates including terrigenous sedimentary deposits volcanic and metavolcanic rocks of intermediate composition and deeply weathered granitoid rocks nearly all of these nivation hollows are relict they are most abundant near areas of late pleistocene glaciation but rarely occur within such areas most are veneered with colluvium and are well vegetated and many hollows in the mono basin area are veneered with volcanic ash at least 700 yr old distribution of nivation hollows suggests that 1 the full glacial nivation threshold altitude nta rose from north to south at 190 m per degree of latitude subparallel to and approximately 740 m lower than the full glacial equilibrium line altitude ela and about 1370 m lower than the estimated modern ela 2 the difference between the full glacial and modern elas indicates an approximate 7 c full glacial mean annual temperature depression throughout the great basin and 3 the full glacial mean annual temperature at the nta is estimated to have been approximately 0 to 1 c assuming little change in accumulation season precipitation

utopia and hellas basins mars twins separated at birth 1 using topography and gravity data as constraints we formulate spherical harmonic thin elastic shell models to determine the subsurface structure of the hellas and utopia basins for hellas we show that our model is consistent with the elastic thickness results of mcgovern et al 2002 2004 the thin elastic lithosphere at the time of formation implies that hellas is close to isostatic since utopia formed earlier we argue that an isostatic assumption is justified for the utopia basin before it was filled from this supposition we derive a system of equations that allows us to solve for the amount of fill the prefill topography and the amount of flexure due to the fill within the utopia basin an analysis of the parameter space shows that the fill density and the amount of fill is strongly dependent on the elastic thickness at the time of infilling a thinner elastic lithosphere favors a denser fill while a thicker lithosphere will allow for less dense material likewise larger crustal thickness values lead to smaller fill density values the presence of quasi circular depressions interpreted as impact craters within the utopia basin indicates that the majority of the material within utopia was deposited prior to 4 04 4 11 ga the early timing for the deposition combined with the heat imparted by the basin forming event argues for a thinner lithosphere which could in turn suggest fill densities that are more consistent with a volcanic load than with pure sediment or ice rich material these results are supported using an alternative method of determining the amount of fill and flexure within utopia this model assumes that hellas and utopia were initially identical and that the only difference in their subsequent evolution was the addition of material in the utopia basin the volume of material needed to fill utopia is immense on the order of 50 million km3 or more the high density obtained for the fill requires that it contain a large igneous component the source of which is problematic relaxing the isostatic assumption to a reasonable degree perturbs the density bound only slightly

release of methane from a volcanic basin as a mechanism for initial eocene global warming a 200 000 yr interval of extreme global warming marked the start of the eocene epoch about 55 million years ago negative carbon and oxygen isotope excursions in marine and terrestrial sediments show that this event was linked to a massive and rapid approximately 10 000 yr input of isotopically depleted carbon it has been suggested previously that extensive melting of gas hydrates buried in marine sediments may represent the carbon source and has caused the global climate change large scale hydrate melting however requires a hitherto unknown triggering mechanism here we present evidence for the presence of thousands of hydrothermal vent complexes identified on seismic reflection profiles from the voring and more basins in the norwegian sea we propose that intrusion of voluminous mantle derived melts in carbon rich sedimentary strata in the northeast atlantic may have caused an explosive release of methane transported to the ocean or atmosphere through the vent complexes close to the palaeocene eocene boundary similar volcanic and metamorphic processes may explain climate events associated with other large igneous provinces such as the siberian traps approximately 250 million years ago and the karoo igneous province approximately 183 million years ago

the age of the popigai impact event and its relation to events at the eocene oligocene boundary ages ranging from the late cretaceous 65 myr to the oligocene 29 myr have been reported for the 100 km diameter popigai impact structure1 2 on the anabar shield central siberia these ages3 4 5 6 7 8 overlap the timing of several possible impact related features including the cretaceous tertiary and eocene oligocene stratigraphic boundaries the north american tektites9 and the recently reported occurrences of an iridium anomaly and shocked quartz in late eocene deposits in northern italy10 here we report age determinations of several popigai impact melt rocks using the 40ar 39ar step heating technique to constrain the age of the impact event our results are consistent with a late eocene impact age of 35 7 0 2 myr 2 coincident in time with the impact deposits found in italy as this age is also similar to that of the north american tektites which have been associated with the chesapeake bay impact structure in the eastern united states11 12 13 there seem to have been at least two large and essentially contemporaneous impacts during the late eocene