pollen distribution in marine sediments on the continental margin off northern california abstract pollen analysis of 84 samples from surface sediment of the continental margin off northern california between 38 and 39 n and 123 and 124 w shows that the composition of pollen assemblages in this part of the north pacific ocean reflects the regional vegetation of the russian river and other local drainage basins of the northern coast of california systematic trends in the quantitative and qualitative variations in these marine pollen assemblages show that fluvial and marine transport are important factors in determining the nature of pollen assemblages offshore these conclusions are supported by the analyses of pollen in rivers of the north coast of california and in particulate matter collected in a mid slope sediment trap

the elkhorn mountains revisited new data for the late cretaceous paleomagnetic field of north america the late cretaceous portion of north america s apparent polar wander apw path is poorly known due to a dearth of suitable post 85 ma reference poles a paleomagnetic re investigation of elkhorn mountains volcanics 80 ma was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of apw during this time interval correlates with known late cretaceous tectonic events a total of 249 paleomagnetic samples 30 sites were collected from three separate areas within the elkhorn mountains using both alternating field and thermal demagnetization a characteristic direction of magnetization was isolate at 15 sites a95 15 which passes a fold test at the 95 confidence level the mean pole calculated from site virtual geomagnetic poles is located at 80 3 n 189 5 e n 15 a95 9 6 k 16 8 interestingly the elkhorn mountains pole is not concordant with the cretaceous stillstand poles but is nearly coincident with the paleocene reference pole and the recently published pole for the adel mountains dated at 76 ma these new data suggest the onset of apw which brought the pole from its cretaceous position to near its early tertiary position occurred prior to 80 ma and the transition was exceedingly rapid the onset of this rapid apw now appears to coincide more closely in time with many of the significant late cretaceous tectonic events e g the laramide orogeny however since the elkhorn mountains volcanics are floored by a major east directed thrust and have undergone crustal scale deformation the data should be viewed with caution until more results are available for this time interval

formation of interior basins associated with curved faults in alaska the positions of certain nonmarine sedimentary basins in central and southern alaska are spatially related to the geometry of adjacent curved faults these interior basins occur along the northern convex side of these fault systems which change trend from northwest in canada and eastern alaska to east west in central alaska and to northeast in western alaska the basins probably initiated in early to middle tertiary time and deposition within all basins continued through the quaternary broadly synchronous with tertiary strike slip displacements along the tintina denali and border ranges fault systems we examine the spatial and temporal relationship between basins and faults by using a mechanical model of curved strike slip faults based on boundary elements we relate slip along the curved faults to tertiary pacific north american plate motions by using principal stress directions inferred from the plate motion directions locations of the predicted mean stress reductions around the fault bends caused by fault slip correlate well with positions of the interior basins our results suggest that basin subsidence and location were controlled primarily by the geometry of adjacent curved fault systems and the state of stress in alaska during the tertiary mechanical interaction between curved alaskan fault systems increases the tendency for basin subsidence stresses inferred from early tertiary plate motion vectors are not oriented correctly to drive right lateral slip along kaltag farewell and parallel faults in western alaska furthermore these stress states would have produced left lateral displacements for which there is no evidence along these western faults in their present orientations we find that a combination of slightly rotated fault geometry and low friction along the faults appears to facilitate right lateral slip along the faults in western alaska our results show that deformation well inside the continent can be explained in terms of the pacific north american plate motions thus tertiary plate motions have resulted in a broad zone of fault slip uplift and subsidence in the nonrigid north american plate producing several large basins

global vegetation dynamics and latitudinal temperature gradients during the mid to late miocene 15 97 5 33 ma a 617 site palaeobotanical dataset for the mid to late miocene is presented this dataset is internally consistent and provides a comprehensive overview of vegetational change from 15 97 to 5 33 ma the palaeobotanical dataset has been translated into the biome4 classification scheme to enable direct comparison with climate model outputs the vegetation change throughout the langhian seravallian tortonian and messinian is discussed the data shows that the langhian which includes the end of the mid miocene climatic optimum represents a world significantly warmer than today the high northern latitudes were characterised by cool temperate forests the mid latitudes by warm temperate mixed forests the tropics by tropical evergreen broadleaf forests and antarctica by tundra shrub vegetation cool temperate mixed forest existed during the seravallian in the high northern latitudes a reduction in warm temperate mixed forests in the mid latitudes and a loss of tundra on antarctica was noted tortonian vegetation distribution indicates that further cooling had occurred since the seravallian the major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm temperate mixed forests by the messinian this cooling trend had eliminated warm temperate mixed forests from the western usa and australia and had formed mid latitude deserts despite the cooling trend throughout the mid to late miocene the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre industrial conditions the latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the langhian would have been significantly shallower than at present and has gradually but asymmetrically become more modern towards the end of the miocene first the southern hemisphere distribution of bioclimatic zones became more modern probably due to the climatic effects of a fully glaciated antarctica the northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the miocene and slowly became more modern by the messinian

ballast brook and beaufort formations late tertiary on northern banks island arctic canada abstract excellent exposures of late tertiary terrestrial sediments occur in the ballast brook region of northwestern banks island canadian arctic archipelago these sediments contain abundant concentrations of organic detritus as well as autochthonous peats which can be traced for some kilometres all of these deposits were previously assigned to the beaufort formation in this paper the deposits formerly grouped as the beaufort formation are subdivided into two formations the newly named ballast brook formation and the beaufort formation the ballast brook formation is characterized by alluvial sand silt clay and peat with compressed altered wood paleobotanical evidence indicates an early to mid miocene age with mid miocene being the most probable this formation is believed to overlie the eureka sound group and is in erosional contact with the overlying beaufort formation plant macrofossils from the ballast brook formation include a number of taxa now found in northern hardwood and southern boreal forests while the prominent peat which characterizes the formation contains a number of taxa that now grow or could be expected to grow in a cypress swamp type of environment for example the conifer glyptostrobus is well represented in the peat the beaufort formation at ballast brook consists of alternating sands and gravels and silt in the lower part it is characterized by an abundance of unaltered wood and other plant materials paleobotanical evidence indicates an early pliocene to early late pliocene age i e approx 5 3 ma like the ballast brook formation the beaufort formation contains an abundance of fossils of conifers but in contrast with the ballast brook formation nearly all of them belong to pinaceae a group well represented in the present boreal forest the beaufort formation also contains insect fossils which are very rare in the ballast brook formation together the insect and plant macrofossils suggest an environment similar to but richer in species some extinet others now found only in asia than present boreal forest and taiga the ballast brook formation flora resembles the flora from mary sachs gravel formerly beaufort formation on southwestern banks island and the flora from the west river beds on the northwest territories mainland it also is similar to a 40 ar 39 ar dated mid miocene flora from east central alaska and to the mamontova gora mid miocene flora from russia

climatic comparison of miocene pollen floras from northern east china and south central alaska usa abstract the shanwang area of shandong china lat 36 5 n and the nenana coal field southern central alaska lat 63 9 n have very different vegetation and climate types today however during the late early to early middle miocene pollen floras of the two areas were rather similar though they represent different subregions of deciduous forest vegetation more than half the genera and families of the two floras are held in common and some trees with modern asian affinities appear in both fossil floras pollen and leaf data imply that during the first half of the miocene the fossil floras of south central alaska were closely related to those in northeastern asia a summer wet climate existed at both fossil locations the shanwang flora records a mean annual temperature mat about like that of the present about 13 c but winters were warmer probably above zero in contrast we estimate that the alaskan suntrana flora records a mat of 7 9 c about 10 12 c warmer than now winters average probably higher than 5 to 11 c were considerably warmer and summers average probably 16 18 c were somewhat warmer the difference in mat between sites 27 in latitude apart suggests a latitudinal temperature gradient of about or less than 0 3 c per degree latitude by comparison the average latitudinal temperature gradient for the northern hemisphere between latitude 35 and 65 is about 0 7 c at least twice as steep as that for the early to middle miocene interval at these sites the comparatively low temperature gradient is a quantitative estimate of warmth in the north polar region in the early neogene

holocene vegetation and climatic change in the peace river district canada a sediment core from a pond on the alberta plateau in the peace river district of british columbia was studied using pollen analysis and radiocarbon dating percentage and influx diagrams were produced and radiocarbon dates were corrected to calendar years to calculate the sedimentation rate the 231 cm core terminated in clay and a basal date of 7250 120 years bp was obtained several thousand years after the recession of glacial lake peace the formation of the pond is interpreted as resulting from a climatic change probably a transition from the peak of the hypsithermal zone 1 from 7250 to 5500 years bp is interpreted as representing a seasonal slough with upland vegetation percentages consistent with a boreal forest at about 5500 years bp a permanent pond with surrounding sedge wetlands was formed vegetation has been essentially modern during the last 3100 years measurements of spruce grains suggest the presence of black and white spruce throughout the pollen record the formation of perma

paleosols associated with miocene basalts porcupine river northeastern alaska implications for regional paleoclimates abstract paleosols of middle miocene age consisting of wood peat organic matter rich mineral soil and weathered basalt are described from the upper ramparts of the porcupine river in northeastern alaska a highly acidic soil representing a floodplain wetland swamp forest environment underlies the basalts of the upper ramparts and is composed of 1 m of compressed peat and interbedded fluvial sediments wetland soils from between basalt flows were also observed in depressional sites on basalt surfaces palynological investigations indicate that these wetland peat profiles developed in association with temperate mixed hardwood conifer flora correlative to the early middle miocene seldovian floristic stage previously defined for southcentral alaska three well preserved types of basalt saprolites residual soils were identified in the study area depending on the drainage condition at the time of formation the greatest degree of basalt weathering occurred under apparent well drained conditions where a red clay rich soil with a solum depth greater than 1 m was observed between basalt flows the colour of this well drained paleosol and the presence of iron oxides including possible hematite is interpreted to reflect soil development under warm temperate climatic conditions

palynology and geochronology of eocene beds from cheslatta falls and nazko areas central british columbia early tertiary lavas and sediments were collected from two areas west and south of prince george british columbia and processed for k ar dating and palynoassemblages samples from cheslatta falls gave k ar dates of 36 5 and 37 7 ma and yielded a palynoassemblage very similar to that from the jackson group in alabama and mississippi this late eocene assemblage is interpreted as having developed in a humid subtropical environment prior to a climatic cooling that we believe should be assigned to early oligocene time the tertiary lavas and sediments from the nazko area west of quesnel gave three middle eocene dates and over 60 species of palynomorphs these correlate with other rocks and palynoassemblages of kamloops group equivalents that occur southward across the international boundary and north to latitude 55 n the paleoclimate was wet and in the range of very warm temperate to humid subtropical

geologic evolution of the yukon river implications for placer gold abstract the greatest placer potential in northwest canada lies in preglacial drainage systems the largest placer gold deposits are associated with pliocene pre glacial fluvial gravels of the dawson range in particular klondike plateau of west central yukon the abundance and or the lack of gold concentration in this area were controlled by the ability of the streams to aggrade in response to changes caused by extensional faulting in the tintina trench differential uplift and regional denudation gold bearing gravels in the northern and eastern slopes of the dawson range are found in low order tributaries of the pre glacial south flowing yukon river glaciation played an important role in masking and changing the direction of drainage and creating new channels in this region the earliest known drainage changes relate to late miocene formation of the tintina trench and early pliocene uplift of the st elias mountains prior to these events the yukon river drained south into the gulf of alaska initial regional glaciation 2 6 2 9 ma diverted the southward drainage toward the northwest becoming part of the kwikhpak river in alaska the present yukon river drainage basin is about 27 5 larger than the ancestral kwikhpak river basin the evolution of the river is described here in three stages 1 drainage in pre late miocene 2 drainage in early pliocene and 3 drainage after earliest pliocene glaciation

sequence stratigraphy biotic change 87sr 86sr record paleoclimatic history and sedimentation rate change across a regional late cenozoic unconformity in arctic canada eustasy tectonics and climate contributed to a remarkable miocene pliocene regional unconformity in the beaufort mackenzie area of arctic canada the unconformity extends from beneath deep basin turbidites on the continental rise upslope across an erosional paleocontinental shelf onto the cratonic margin as a regional paleosurface peneplain in the mackenzie delta area and into pediment surfaces cut into the orogenic highlands of the richardson mountains the unconformity was initiated by shelf exposure during latest messinian or earliest pliocene eustatic lowstand and was accentuated by tectonic uplift from the culmination of a major late miocene compressional pulse on the basin margin palynomorph benthic foraminiferal strontium isotopic paleomagnetic and radiometric data document the climatic and chronological events surrounding the unconformity a widespread hardground k 59 limestone occurs at the unconformity and caps the late miocene akpak sequence the hardground yields the benthic fora

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

identification of neogene woods from alaska yukon abstract woods collected from early middle miocene to late pliocene localities at high latitudes more than 60 n of alaska and canada were examined microscopically to determine taxonomic affinities all were conifers and most are pinaceae features useful in the identification of conifers are reviewed including characteristics of growth rings resin canals cross field pitting ray tracheids longitudinal and ray parenchyma helical thickenings and cell sizes some of the problems in feature recognition in fossil conifer woods due to compression and fungal and microbial degradation are discussed although relatively few wood samples were examined these samples provide information consistent with that derived from the pollen leaf and fruit and seed record obtained from the same deposits and provide additional documentation for a cooling of climate and changes in the flora and vegetation from the early middle miocene to the late pliocene in the early middle miocene samples from northeastern alaska there are woods of taxodiaceae and soft pines pinus spp with growth rings greater than 1 cm wide in the late pliocene there are woods of spruce picea and larch larix with growth rings less than 1 mm wide

helicosporous fungi and early eocene pollen eureka sound group axel heiberg island northwest territories two types of helicosporous fungal spores were recovered from a sample from the iceberg bay formation eureka sound group at strand fiord axel heiberg island these spores appear to be identical to those produced by the recent genera helicoon helicodendron and helicosporium the associated pollen flora indicates an early eocene age for the sample and interval from which it was collected the pollen and spores suggest that swamp forest vegetation dominated by trees of taxodiaceae cupressaceae and growing in a temperate climate was present at or close to the site of deposition

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

alluvial history of the porcupine river alaska role of glacial lake overflow from northwest canada the stratigraphy and morphology of alluvial terraces in the lower porcupine valley permit the definition of twelve river stages each marked by distinctive surface characteristics sediment composition and regional gradient terraces that exhibit characteristics suggestive of extremely high discharge such as coarse bouldery braided gravel surfaces and intense scouring formed at times when the porcupine river at the ramparts acted as an overflow outlet for glacial lakes in northern yukon territory which had been impounded by the lauren tide ice sheet terraces capped by sediment suggestive of relatively low discharge meandering streams and which were strongly affected by coleen river drainage probably formed when glacial lake overflow did not occur ten radiocarbon dates on alluvial sediments from the lower porcupine river range from greater than 35 000 to 2 350 55 yr b p when combined with geomorphologic interpretations on terraces in alaska and with radiocarbon dates from northern yukon territory these dates suggest repeated glaciolacustrine innundations of the old crow and bluefish basins during wisconsinan time

paleoenvironmental reconstruction of a middle miocene forest from the western canadian arctic abstract abundant fossil plant remains are preserved in deposits of middle miocene age of the ballast brook formation on banks island northwest territories canada intact seed cones logs and stumps are preserved in situ as mummified remains and present an opportunity to reconstruct the composition structure and productivity of a pinaceae dominated forest that once grew north of the arctic circle paleolatitude ca 74 n we mapped and measured 78 tree stumps exposed in three dimensions on a 0 12 ha peat layer an analysis of the wood anatomy and seed cones indicates that a haploxylon pine dominated this lowland forest although species of both picea and glyptostrobus were growing in the swamp as well stump diameters ranged from 7 cm to 108 cm average 42 cm we utilized allometric relationships to predict tree heights based on the stump diameters in the fossil forest our results indicate that the average tree height of this miocene forest may have been as large as 21 m we used stump diameter data and predicted tree height to calculate the parabolic stem volume and stem biomass for the exposed area of fossil forest stem biomass assuming an average wood density of 410 kg m 3 may have been as great as 259 mg ha 1 or as little at 202 mg ha 1 the annual ring width of the stem wood sampled in the field was 1 26 mm for pinus 1 20 mm for picea and 0 21 mm for glyptostrobus based on these growth rates our estimate of biomass sequestered aboveground as wood is at most 3 8 mg ha 1 y 1 based on our estimates these lowland forest communities were of moderate biomass and productivity typical of modern cool temperate forests in north america

late cenozoic uplift of denali and its relation to relative plate motion and fault morphology apatite fission track analysis of samples that cover a 4 kilometer vertical section from the western flank of denali mount mckinley north america9s highest mountain suggests that the mountain massif was formed by rapid uplift 1 kilometer per million years beginning 6 million years ago ma uplift was a result of the morphology of the denali fault and a change in motion of the pacific plate with respect to north america at 5 ma which created opposing tangential vectors of relative movement along the fault and forced the intervening crustal blocks upward

miocene and pliocene lacustrine and fluvial sequences upper ramparts and canyon village porcupine river east central alaska abstract cenozoic strata exposed along the porcupine river between the upper ramparts and canyon village alaska can be divided into five unconformity bounded units sequences which are lower and middle miocene unit a the white sandy fluvial sequence with peat beds middle miocene unit b the basalt sequence part b1 is basalt and part b2 is organic rich sedimentary beds upper miocene unit c mudrock dominated lake sequence late miocene or pliocene to pleistocene unit d terrace gravels detrital organic matter and associated sediments and holocene unit e mixed sand and gravel rich sediment and other sedimentary material including peat and eolian silt the sequence unit a of lower and middle miocene fluvial deposits formed in streams and on flood plains just before the inception of local volanism fossil pollen from unit a suggests conifer dominated regional forests and cool temperate climates peat beds and lake deposits from unit b contain pollen that indicates a warmer temperate climate coinciding with the middle miocene thermal maximum the lake deposits unit c downstream from the basalts accumulated in a small basin which resulted from a hydrologic system that was dammed in the late miocene but breached soon thereafter the lower part of the terrace gravels unit d expresses breaching of the dammed hydrologic system of unit c the porcupine river became a major tributary of the yukon river in late pleistocene time when laurentide ice blocked drainage from the yukon interior basins causing meltwater to spill over the low divide separating it from the porcupine river drainage initiating erosion and capture of the yukon interior basins

palynology paleoclimatology and correlation of middle miocene beds from porcupine river locality 90 1 alaska abstract beds in the upper ramparts canyon of the porcupine river alaska 67 20 n 141 20 w yielded a flora rich in pollen of hardwood genera now found in the temperate climates of north america and asia the beds are overlain or enclosed by two basalt flows which were dated to 15 2 0 1 ma by the 40 ar 39 ar method fixing the period of the greatest abundance of warm loving genera to the early part of the middle miocene the assemblage is the most northern middle miocene flora known in alaska organic bed 1 underlies the basalt and is older than 15 2 ma but is of early to middle miocene age the pollen assemblage from organic bed 1 is dominated by conifer pollen from the pine and redwood cypress yew families with rare occurrences of temperate hardwoods organic bed 2 is a forest floor containing redwood trees in life position engulfed by the lowest basalt flow a pine log has growth rings up to 1 cm thick organic beds 3 and 4 comprise lacustrine sediment and peat between the two basalt flows their palynoflora contain conifers and hardwood genera of which about 40 have modern temperate climatic affinities hickory katsura walnut sweet gum wingnut basswood and elm pollen are consistently present and beech and oak alone make up about 20 of the pollen assemblage a warm high latitude climate is indicated for all of the organic beds but organic bed 3 was deposited under a time of peak warmth climate data derived by comparison with modern east asian vegetation suggest that at the time of deposition of organic bed 3 the mean annual temperature mat was ca 9 c the warm month mean temperature wmmt was 20 c and the cold month mean temperature cmmt was ca 2 c in contrast the modern mat for the region is 8 6 c wmmt is 12 6 c and cmmt is 28 c organic beds 3 and 4 correlate to rocks of the middle miocene late seldovian stage of cook inlet and also probably correlate to and more precisely date the lower third of the suntrana formation in the alaska range beds at unalaklect part of the upper mackenzie bay sequence in the beaufort mackenzie basin and the mary sachs gravel of banks island this suggests that forests with significant percentages of temperate deciduous angiosperms existed between latitudes 60 and 72 n during the early middle miocene

on the resolution of density anomalies in the earth s mantle using spectral fitting of normal mode data summary the resolution of the 3 d density structure in the earth s deep interior has long eluded geoscientists high quality data from digital seismic instruments emplaced this decade have renewed interest in the measurement of low frequency earth normal modes with the goal of extracting heterogeneous density structure here we perform a series of synthetic experiments aimed at investigating the resolution of lateral variations in the mantle from normal mode spectral data contamination effects between seismic velocities and density are examined in two ways 1 by using resolution matrices computed from data kernels and 2 by inverting synthetic spectra computed from realistic input earth models the first type of experiment assumes that the non linearity of the inverse problem is weak no such assumptions are necessary in the second type of tests which nevertheless concur in their results with the former these synthetic tests indicate that density structure retrieved from presently available normal mode data is not reliable contamination of seismic velocity structure into the density model space produces patterns that resemble those obtained from inversions of real data the resulting density models appear to be dependent on a combination of the starting velocity models and the model parametrization

ventilation of the miocene arctic ocean an idealized model study a model study of an idealized early miocene arctic ocean has been undertaken the work is motivated by the first drill core retrieved from the lomonosov ridge in the central arctic ocean which sug

the gang ranch big bar area south central british columbia stratigraphy geochronology and palynology of the tertiary beds and their relationship to the fraser fault tertiary rocks in the gang ranch big bar area south central british columbia consist of 1 early or middle eocene 50 ma lavas breccias and tuffs capped by a mappable unit of conglomerate a

late cenozoic paleomagnetic record of duck hawk bluffs banks island canadian arctic archipelago detailed paleomagnetic investigations have been completed on unconsolidated sediments from duck hawk bluffs on banks island in the canadian arctic archipelago that record some of the oldest late cenozoic glacial and nonglacial events in canada the preglacial worth point formation the overlying duck hawk bluffs formation including marine and glacial deposits laid down during the banks glaciation and the lower part of the interglacial morgan bluffs formation have magnetically reversed directions and therefore are of matuyama age 790 ka upper morgan bluffs formation organic beds and deposits of the younger thomsen glaciation cape collinson interglaciation and amundsen glaciation are normally magnetized and therefore of brunhes age 790 ka the brunhes matuyama boundary is recorded in the upper portion of the morgan bluffs formation its precise position within the interglacial sequence can be identified since the sediments document the gradual change from reversely inclined directions to norma

pliocene terrace gravels of the ancestral yukon river near circle alaska palynology paleobotany paleoenvironmental reconstruction and regional correlation abstract gravels deposited by the ancestral yukon river are preserved in terrace remnants on the margins of the yukon river valley near the village of circle in east central alaska plant fossils recovered from sandy silt lenses within these gravels include cones and needles of picea and larix and a variety of seeds seed types include several taxa which no longer grow in alaska such as epipremnum prunus and weigela pollen types recovered from these deposits represent tree and shrub taxa that grow in interior alaska today such as picea larix betula and alnus as well as several taxa that no longer grow in interior alaska today such as pinus tsuga abies and corylus pollen of herb taxa identified include gramineae cyperaceae caryophyllaceae compositae polemonium and epilobium the fossil flora from the gravels near circle are similar and probably age equivalent to the flora recovered from the nenana gravel in the alaska range 250 km to the south palynological and tectonic evidence summarized in this paper now suggests that the nenana gravel was deposited during the early and middle pliocene the presence of plant fossils of tsuga abies pinus weigela and prunus suggests that the mean annual temperature mat of eastern interior alaska during the early and middle pliocene was perhaps 7 9 c warmer and less continental than today s mat of 6 4 c

using probabilistic seismic tomography to test mantle velocity density relationships we use a neighborhood algorithm to explore the fit to long period seismic data of a wide variety of long wavelength mantle models this approach to the global tomographic inverse problem yields probability distributions for seismic velocities density and related properties as functions of depth such distributions can be robust even when individual models are not and allow us to test several assumptions about the earth that have long been enforced a priori in inversions in particular we are able to test the paradigm ofdeep mantle heterogeneity that is dominantly thermal in origin producing velocity and density perturbations that are well correlated and have relative amplitudes given by nlnb nlnvs 6 0 5 our distributions show that such relationships are unlikely and even though the results are consistent with recent best fitting models from damped seismic inversions they demonstrate that many specific properties ofsuch models are not robust the data clearly f density perturbations that are poorly or negatively correlated with velocity heterogeneity and have amplitudes several times larger yielding nlnb nlnvs s 1 0 than damped inversions allow these characteristics are most pronounced in the upper mantle transition zone and the base ofthe lower mantle suggesting layered convection the negative density velocity correlations f at these depths imply dominantly chemical heterogeneity while the likelihood ofrelatively high amplitude density variations suggests that variable iron content is an important component ofthis heterogeneity these results which we show to be consistent with independent gravity constraints represent a profound change in the interpretation of seismic constraints in addition the distributions show that even though best fitting density models from recent inversions or our sampling are consistent with the data most specific properties of such models are not robust this implies that it is more appropriate to use seismic model distributions rather than individual models to make geodynamic and geochemical inferences

a long pollen sequence of neogene age alaska range abstract a thick sediment sequence 590 m of neogene age exposed in the nenana coal field alaska at a latitude of 63 5 n provides an extensive pollen and spore record spanning most of the miocene and probably part of the early pliocene the sediments of the upper usibelli group miocene and overlying nenana gravel pliocene are the basis of this study the flora from both leaves and pollen of the usibelli group connotes a vegetation similar to the mixed northern hardwood forest of northeastern united states and the northern part of east china northern korea and southern hokkaido the warmest pollen and leaf assemblages of the miocene occur in the suntrana formation in the lower third of the unit and these correlate very well with flora of the upper seldovian stage as the reference section on cook inlet which is of late early to early middle miocene age shifts in forest composition from suntrana formation sediments upper seldovian stage into the overlying sediments of the lignite creek and grubstake formations connote a cooling with minor changes of vegetation and climate these cooler phases are correlative with the homerian and clamgulchian stages of cook inlet mean annual temperature mat estimate based on fossil pollen and leaves from the lower suntrana formation is ca 9 c and summers were consistently warm and wet in the cooler phases of the late miocene the mat probably dropped to about 5 c

modeling the miocene climatic optimum part i land and atmosphere abstractthis study presents results from the community climate system model 3 ccsm3 forced with early to middle miocene 20 14 ma vegetation topography bathymetry and modern co2 a decrease in the meridional temperature gradient of 6 5 c and an increase in global mean temperature of 1 5 c are modeled in comparison with a control simulation forced with modern boundary conditions seasonal poleward displacements of the subtropical jet streams and storm tracks compared to the control simulation are associated with changes in hadley circulation and significant cooling of the polar stratosphere consistent with previously predicted effects of global warming energy budget calculations indicate that reduced albedo and topography were responsible for miocene warmth in the high latitude northern hemisphere while a combination of increased ocean heat transport and reduced albedo was responsible for relative warmth in the high latitude southern hemisphere compared to the present model data analysis suggest

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

age constraints on neogene sedimentary beds upper ramparts half way pillar and canyon village sites porcupine river east central alaska abstract 40 ar 39 ar ages of volcanic rocks are used to provide numerical constraints on the age of middle and upper miocene sedimentary strata collected along the porcupine river intercalated sedimentary rocks north of latitude 67 10 n in the porcupine terrane of east central alaska contain a rich record of plant fossils the fossils are valuable indicators of this interior region s paleoclimate during the time of their deposition integration of the 40 ar 39 ar results with paleomagnetic and sedimentological data allows for refinements in estimating the timing of deposition and duration of selected sedimentary intervals 40 ar 39 ar plateau age spectra from whole rock basalt samples collected along the upper ramparts and near half way pillar on the porcupine river range from 15 7 0 1 ma at site 90 6 to 14 4 0 1 ma at site 90 2 with exception of the youngest basalt flow at site 90 2 all of the samples are of reversed magnetic polarity and all 40 ar 39 ar age spectrum results are consistent with the deposition of the entire stratigraphic section during a single interval of reversed magnetic polarity the youngest flow at site 90 2 was emplaced during an interval of normal polarity with age paleomagnetic and sedimentological data the ages of the middle miocene sedimentary rocks between the basalt flows at sites 90 1 and 90 2 can be assigned to an interval within the limits of analytical precision of 15 2 0 1 ma thus the sediments were deposited during the peak of the middle miocene thermal maximum sediments in the upper parts of sites 90 1 and 90 2 were probably deposited during cooling from the middle miocene thermal maximum 40 ar 39 ar results of plagioclase and biotite from a single tephra collected at sites 90 7 and 90 8 along the canyon village section of the porcupine river indicate an age of 6 57 0 02 ma for its time of eruption and deposition these results together with sedimentological and paleomagnetic data suggest that all of the upper miocene lacustrine sedimentary rocks at these sites were deposited during a single interval of reversed magnetic polarity and may represent a duration of only about 40 000 years the age of this tephra corresponds with a late late miocene warm climatic interval the results from the upper ramparts and half way pillar sites are used to estimate a minimum interval of continental flood basalt activity of 1 1 1 5 million years and to set limits for the timing and duration of tertiary extensional tectonic activity in the porcupine terrane our data indicate that the oroclinal flexure that formed before the deposition of the basalts at the eastern end of the brooks range was created prior to 15 7 0 1 ma