the palisades is a key reference site for the middle pleistocene of eastern beringia new evidence from paleomagnetics and regional tephrostratigraphy abstract the palisades in central alaska is one of the most prominent exposures of quaternary sediments on the yukon river perennially frozen silt and sand at the palisades are presently thought to preserve paleoenvironmental records from the holocene to marine isotope stage mis 8 and beneath a major unconformity the earliest pleistocene 2 ma we present new paleomagnetic and tephrochronologic constraints that substantially revise the age of the sediments at the palisades we describe 15 new tephra beds including five beds below the prominent pal tephra that correlate to known tephra with independent age control from other sites in eastern beringia these five known tephra include chester bluff tephra which is present in east central alaska and the yukon and the newly named alyeska pipeline and taylor highway tephra from central alaska all are constrained to the middle pleistocene paleomagnetic transects from the base of the bluff to the mis 5e forest bed yield normal polarity with the exception of a brief reversal event between old crow tephra 124 10 ka and the mis 5e forest bed that is likely the first documentation of the blake paleomagnetic event in alaskan loess the detailed tephrostratigraphy and paleomagnetic data collectively suggest that most of the sedimentary record at the palisades is middle pleistocene in age the palisades thus preserves a rare record of late to middle pleistocene paleoenvironments with multiple regionally distributed tephra beds

old crow tephra a new late pleistocene stratigraphic marker across north central alaska and western yukon territory old crow tephra is the first extensive pleistocene tephra unit to be documented in the northwestern part of north america it has a calc alkaline dacitic composition with abundant pyroxene plagioclase and fe ti oxides and minor hornblende biotite apatite and zircon thin clear bubble wall fragments are the dominant type of glass shard this tephra can be recognized by its glass and phenocryst compositions as determined by x ray fluorescence microprobe and instrumental neutron activation techniques it has an age between the limits of 60 000 and 120 000 yr set by 14c and fission track measurements respectively old crow tephra has been recognized in the koyukuk basin and fairbanks region of alaska and in the old crow lowlands of the northern yukon territory some 600 km to the east northeast the source vent is unknown but these occurrences considered in relation to the distant locations of potential quaternary volcanic sources demonstrate the widespread distribution of this tephra and underscore its importance as a regional stratigraphic marker

nd and pb isotope time series from atlantic ferromanganese crusts implications for changes in provenance and paleocirculation over the last 8 myr high resolution nd and pb isotope time series for the last 8 myr are reported for four atlantic ferromanganese crusts dated by 10be 9be chronology these are compared to new high resolution and high precision pb isotope time series and recently published nd isotope time series for two previously studied crusts from the nw atlantic ocean these records allow a more detailed examination of atlantic deepwater variability over the time period of intensification of northern hemisphere glaciation nhg changes in the pb isotope time series started after 3 ma but were most dramatic over the last 1 8 myr coinciding with changes of pb isotopes in the arctic ocean this latter change post dates the intensification of nhg at 3 1 to 2 5 ma and reflects an increase in the input of material eroded from the archean shield of canada and greenland shifts in nd and pb isotope compositions in a crust from the blake plateau occur before 5 ma and most prominent at 8 ma suggest that water masses from either the pacific or southern ocean influenced the isotope composition of this crust the relatively high end values around 8 ma recorded by the blake plateau crust are explained by a contribution of eastward flowing pacific water through the panama gateway into the caribbean sea this high end signal decreased between 8 and 5 ma suggesting that the supply of pacific water into the caribbean became restricted this is earlier than the caribbean seawater salinity increase at 4 2 ma deduced from 18o data and may indicate that there was only a surface water connection between the caribbean and pacific between 5 and 4 2 ma the closure of the panama gateway to intermediate and deep water exchange 200 m depth apparently occurred much earlier than the intensification of northern hemisphere glaciation at 3 1 2 5 ma and cannot therefore have been a direct cause of this climatic change but may as recently argued only have been a necessary precondition

chronostratigraphy of odp 181 site 1121 sediment core southwest pacific ocean using 10be 9be dating of entrapped ferromanganese nodules a 10be 9be based chronostratigraphy has been determined for odp 181 site 1121 sediment core recovered from the foot of the campbell plateau southwest pacific ocean this core was drilled through the campbell skin drift in ca 4500 m water depth on the mid western margin of the extensive campbell nodule field beneath the flow of the major cold water deep western boundary current dwbc in the absence of detailed biostratigraphy beryllium isotopes have provided essential time information to allow palaeo environmental interpretation to be undertaken on the upper 7 m of the core measured 10be 9be ratios of sediment and of ferromanganese nodules entrapped in the sediment decrease systematically with depth in the core in accordance with radioactive decay however the 10be 9be data diverge from ca 3 m below the seafloor mbsf to the top of the core giving rise to several possible geochronological models the preferred model assumes that the measured 10be 9be ratios of the nodule rims reflect initial 10be 9be ratios equivalent to contemporary seawater and that these can be used to derive the true age of the sediment where the nodules occur the nodule rim ages can be then used to interpret the sediment 10be 9be data which indicate an overall age to ca 7 mbsf of ca 17 5 ma the derived chronology is consistent with diatom biostratigraphy which indicates an age of 2 2 3 6 ma at 1 mbsf calculated sedimentation rates range from 8 to 95 cm m y 1 with an overall rate to 7 mbsf of ca 39 cm m y 1 the lowest rates generally coincide with the occurrence of entrapped nodules and reflect periods of increased bottom current flow causing net sediment loss growth rates of individual nodules decrease towards the top of the sediment core similar to the observed decrease in growth rate from core to rim of seafloor nodules from the campbell nodule field this may be related to an overall increase in the vigour of the dwbc from ca 10 ma to the present

using 234u 238u to assess diffusion rates of isotope tracers in ferromanganese crusts abstract ferromanganese crusts record seawater isotope history for several elements useful for paleoceanography interpreting crust isotope profiles however requires an assessment of the rate of diffusion of each element within the crust we address this issue using u and th isotope profiles coupled with concentration measurements of some trace elements whose isotopes are often measured in crusts a depth profile of 230thxs 234u for a north atlantic crust alv539 yields a growth rate of 3 05 mm myr consistent with an existing 10be 9be record but the growth rate implied by a 234u 238u profile in the same crust is 6 times faster due to diffusion of u in the crust the difference between the observed 234u 238u profile and that expected from 230thxs 234u enables the effective diffusivity of u to be calculated as 1 2 10 6 cm2 year recalculated literature data indicate that other crusts have diffusivities which range down to 4 7 10 8 cm2 yr this variation is to be expected given the range of porosity observed for ferromanganese crusts concentration measurements of various elements in the surface layer of the crust provide relative distribution coefficients for each element between seawater and crust these distribution coefficients enable the effective diffusivity of each element to be calculated by comparison with the diffusivity of u such diffusivities are assessed for crust alv539 and give a first indication of the expected preservation of isotope signals in ferromanganese crusts li os and sr have diffusivities of 2 10 4 3 10 5 and 2 10 5 cm2 year respectively and diffusion is expected to be sufficiently fast that crusts will not accurately preserve past seawater compositions th nd pb and be have diffusivities less than 10 9 cm2 year and are highly immobile in crusts a reassuring result for th and be crust chronologies and for nd and pb isotope tracing of past water masses using crusts hf has a diffusivity of 3 10 8 in alv539 fast enough to perturb but not destroy its isotope history for studies of hf in this and other crusts diffusion should therefore be considered measuring u isotope profiles provides a potential way of assessing such diffusion for hf or for other elements with relatively low distribution coefficients into ferromanganese crusts

tephrochronology magnetostratigraphy and mammalian faunas of middle and early pleistocene sediments at two sites on the old crow river northern yukon territory canada abstract alluvial and lacustrine sediments exposed beneath late pleistocene glaciolacustrine silt and clay at two sites along the old crow river northern yukon territory are rich in fossils and contain tephra beds surprise creek tephra szt occurs in the lower part of the alluvial sequence at crh47 and little timber tephra ltt is present near the base of the exposure at crh94 surprise creek tephra has a glass fission track age of 0 17 0 07 ma and little timber tephra is 1 37 0 12 ma all sediments at crh47 have a normal remanent magnetic polarity and those near ltt at crh94 have a reversed polarity in agreement with the geomagnetic time scale small mammal remains from sediments near ltt support an early pleistocene age but the chronology is not so clear at crh47 because of the large error associated with the szt age determination tephrochronological and paleomagnetic considerations point to an mis 7 age for the interglacial beds just below szt at crh47 and at chester bluffs in east central alaska but mammalian fossils recovered from sediments close to szt suggest a late irvingtonian age therefore older than mis 7 further studies are needed to resolve this problem

a late middle pleistocene marine isotope stage 6 vegetated surface buried by old crow tephra at the palisades interior alaska abstract a 40 cm thick primary bed of old crow tephra 131 11 ka an important stratigraphic marker in eastern beringia directly overlies a vegetated surface at palisades west on the yukon river in central alaska analyses of insect bryophyte and vascular plant macrofossils from the buried surface and underlying organic rich silt suggest the local presence of an aquatic environment and mesic shrub tundra at the time of tephra deposition autochthonous plant and insect macrofossils from peat directly overlying old crow tephra suggest similar aquatic habitats and hydric to mesic tundra environments though pollen counts indicate a substantial herbaceous component to the regional tundra vegetation trace amounts of arboreal pollen in sediments associated with the tephra probably reflect reworking from older deposits rather than the local presence of trees the revised glass fission track age for old crow tephra places its deposition closer to the time of the last interglaciation than earlier age determinations but stratigraphy and paleoecology of sites with old crow tephra indicate a late marine isotope stage 6 age regional permafrost degradation and associated thaw slumping are responsible for the close stratigraphic and paleoecological relations between old crow tephra and last interglacial deposits at some sites in eastern beringia

variations in the fine scale composition of a central pacific ferromanganese crust paleoceanographic implications a 47 to 60 mm thick fe mn crust from horizon guyot water depth 1800 1780 m central pacific was used to evaluate the potential of crusts as recorders of neogene paleoceanographic and paleoclimatic conditions the chemical composition was determined by microprobe for 16 elements from a polished thin section three analyses were made per millimeter and averaged to give the composition of each millimeter the age of the crust was determined by measuring the strontium isotope composition of the crust and comparing it with the tertiary seawater curve the crust represents 18 5 m y of growth of fe and mn oxyhydroxides the crust is composed of alternating botryoidal and laminated layers the botryoidal layers formed during the same time intervals that widespread neogene deep sea hiatuses were forming in bottom sediments the botryoidal layers represent growth during times of intensified deepwater flow whereas the laminated intervals represent more quiescent conditions the correspondence between the botryoidal layers and the neogene hiatuses is so strong that we were able to choose a variable growth rate model over a constant growth rate model for the crust chemical changes in the crust take two forms the first is represented by broad changes in the composition defined chiefly by fourth order polynomial fits to the chemical profiles of each element with depth in the crust the second is high frequency changes in composition the broader changes occurred primarily at about 15 11 5 7 4 6 4 5 2 and 4 6 ma which may correlate with major changes in paleoceanographic circulation and development of ice caps at the poles the periods of the high frequency changes may reflect climatic changes that resulted from orbital forcing these high frequency changes may correspond to the high order eccentricity periods of 3 47 2 04 and 1 31 m y

late cretaceous and cenozoic paleoceanography from north east atlantic ferromanganese crust microstratigraphy abstract oceanic hydrogenetic ferromanganese fe mn crusts precipitate directly from ambient seawater over millions of years their very slow growth rates and physio chemical properties mean that they adsorb numerous elements from seawater as such they provide condensed records of seawater evolution through time that can be used for paleoceanographic reconstruction here we present the results of a high resolution stratigraphic textural and geochemical investigation of a core sample obtained from a fe mn crust pavement located on the summit of tropic seamount in the tropical north east atlantic ocean a number of observations and interpretations are proposed within the context of a well constrained age model spanning the last 75 2 myr this core has textural stratigraphic coherence with pacific fe mn crusts formed since the late cretaceous highlighting that global oceanic and climatic phenomena exert first order controls on fe mn crust development all major hiatuses observed in the fe mn crusts are contemporaneous with erosion events occurring throughout the atlantic ocean high resolution geochemical data indicate that there is variability in the composition of fe mn crusts at the cm to m scale the dominant factors controlling this include major oceanographic events mineral textures and micro topography

the kamikatsura event in the gold hill loess alaska 1 as part of a broader geological study of loess palaesol sequences in alaska the paleomagnetism of a section at gold hill near fairbanks has been investigated samples were collected at 5 cm intervals through a 5 m loess paleosol loess sequence detailed alternating field demagnetization of the natural remanent magnetization yields excellent results average mad 2 the brunhes matuyama b m polarity transition is present at the level of the paleosol which was formedduringoxygenisotopestage19 asystematicperturbation in both declination and inclination occurs 1 0 1 5 m belowtheb mboundary thecorrespondingvirtualgeomagnetic poles define a track centred on the 60 w meridian from high southerly to equatorial latitudes and back again this is in very good agreement with results from a sequence of lava flows on the island of maui that are regarded as a record of the kamikatsura event the alaskan results thus confirm the reality of this geomagnetic feature and simultaneously provide a firm chronological control point for ongoing geological investigations mineral magnetic experiments indicate that the remanence of the gold hill loess is dominated by magnetite and or maghemite with no subsequent chemical overprinting by hematite and or goethite citation evans m e b j l jensen v a kravchinsky and d g froese 2011 the kamikatsura event in the gold hill loess alaska geophys res lett 38 l13302 doi 10 1029 2011gl047793

river fluxes to the sea from the oceanʼs 10be 9be ratio the ratio of the meteoric cosmogenic radionuclide 10be to the stable isotope 9be is proposed here to be a flux proxy of terrigenous input into the oceans the ocean s dissolved 10be 9be is set by 1 the flux of meteoric 10be produced in the atmosphere 2 the denudational flux of the rivers discharging into a given ocean basin 3 the fraction of 9be that is released from primary minerals during weathering meaning the 9be transported by rivers in either the dissolved form or adsorbed onto sedimentary particles and incorporated into secondary oxides and 4 the fraction of riverine 10be and 9be actually released into seawater using published 10be 9be data of rivers for which independent denudation rate estimates exist we first find that the global average fraction of 9be released during weathering into river waters and their particulate load is 20 and does not depend on denudation rate we then evaluate this quantitative proxy for terrigenous inputs by using published dissolved seawater be isotope data and a compilation of global river loads we find that the measured global average oceanic dissolved 10be 9be ratio of about 0 9 10 7 is satisfied by the mass balance if only about 6 of the dissolved and adsorbed riverine be is eventually released to the open ocean after escaping the coastal zone when we establish this mass balance for individual ocean basins good agreement results between 10be 9be ratios predicted from known river basin denudation rates and measured ocean 10be 9be ratios only in the south atlantic and the south pacific the 10be 9be ratio is dominated by advected be and in these basins the ratio is a proxy for ocean circulation as the seawater 10be 9be ratio is faithfully recorded in marine chemical precipitates the 10be 9be ratio extracted from authigenic sediments can now serve to estimate relative changes in terrigenous input into the oceans back through time on a global and on an ocean basin scale

the intav intercomparison of electron beam microanalysis of glass by tephrochronology laboratories results and recommendations abstract the international focus group on tephrochronology and volcanism intav of the international union for quaternary research inqua has conducted an intercomparison of tephrochronology laboratories with electron beam microanalytical data on volcanic glasses submitted from 27 instruments at 24 institutions in 9 countries this assessment includes most active tephrochronology laboratories and represents the largest intercomparison exercise yet conducted by the tephrochronology community the intercomparison was motivated by the desire to assess the quality of data currently being produced and to stimulate improvements in analytical protocols and data reporting that will increase the efficacy of tephra fingerprinting and correlation participating laboratories were each supplied with a mount containing three samples for analysis 1 rhyolitic lipari obsidian id3506 2 phonolitic sheep track tephra from mt edziza british columbia canada and 3 basaltic laki 1783 a d tephra a fourth sample rhyolitic old crow tephra was also distributed most laboratories submitted extensive details of their analytical procedures in addition to their analytical results most used some combination of defocused or rastered beam and modest beam current to reduce alkali element migration approximately two thirds reported that they routinely analyze one or more secondary standards to evaluate data quality and instrument performance despite substantial variety in procedures and calibration standards most mean concentrations compare favorably between laboratories and with other data typically four or fewer data contributions had means for a given element on a given sample that differed by more than 2 standard deviations from the overall means obtaining accurate na 2 o concentrations for the phonolitic tephra proved to be a challenge for many laboratories only one half of the data sets had means within 1 standard deviation of the 8 2 wt na 2 o value obtained by other methods one mean is higher and 14 are lower three of the data set means fall below 7 wt na 2 o most submissions had relative precision better than 1 5 for the major elements for low abundance elements the precision varied substantially with relative standard deviations as small as 10 and as large as 110 because of the strong response to this project the tephrochronology community now has a large comparative data set derived from common reference materials that will facilitate improvements in accuracy and precision and which can enable improved use of published data produced by the participating laboratories finally recommendations are provided for improving accuracy precision and reporting of electron beam microanalytical data from glasses

thallium isotope evidence for a permanent increase in marine organic carbon export in the early eocene article the first high resolution thallium tl isotope records in two ferromanganese crusts fe mn crusts cd29 and d11 from the pacific ocean are presented the crusts record pronounced but systematic changes in 205 tl 203 tl that are unlikely to reflect diagenetic overprinting or changes in isotope fractionation between seawater and fe mn crusts it appears more likely that the fe mn crusts track the tl isotope composition of seawater over time the present day oceanic residence time of tl is estimated to be about 20 000 yr such that the isotopic composition should reflect ocean wide events new and published os isotope data are used to construct age models for these crusts that are consistent with each other and significantly different from previous age models application of these age models reveals that the tl isotope composition of seawater changed systematically between 55 ma and 45 ma using a simple box model it is shown that the present day tl isotope composition of seawater depends almost exclusively on the ratio between the two principal output fluxes of marine tl these fluxes are the rate of removal of tl from seawater via scavenging by authigenic fe mn oxyhydroxide precipitation and the uptake rate of tl during low temperature alteration of oceanic crust it is highly unlikely that the latter has changed greatly therefore assuming that the marine tl budget has also not changed significantly during the cenozoic the low 205 tl

identification of last interglacial deposits in eastern beringia a cautionary note from the palisades interior alaska last interglacial sediments in unglaciated alaska and yukon eastern beringia are commonly identified by palaeoe cological indicators and stratigraphic position 2 5 m above the regionally prominent old crow tephra 124 10 ka we demonstrate that this approach can yield erroneous age assignments using data from a new exposure at the palisades a site in interior alaska with numerous exposures of last interglacial sediments tephrochronology stratigraphy plant macrofossils pollen and fossil insects from a prominent wood rich organic silt unit are all consistent with a last interglacial age assignment however six 14 c dates on plant and insect macrofossils from the organic silt range from non finite to 4 0 14 c ka bp indicating that the organic silt instead represents a holocene deposit with a mixed age assemblage of organic material in contrast wood samples from presumed last interglacial organic rich sediments elsewhere at the palisades in a similar stratigraphic position with respect to old crow tephra yield non finite 14 c ages given that local permafrost thaw since the last interglaciation may facilitate reworking of older sediments into new stratigraphic positions minimum constraining ages based on 14 c dating or other methods should supplement age assignments for last interglacial sediments in eastern beringia that are based on palaeoecology and stratigraphic association with old crow tephra copyright 2011 john wiley sons ltd

late quaternary tephrostratigraphy ahklun mountains sw alaska radiocarbon dated sediment cores from six lakes in the ahklun mountains south western alaska were used to interpolate the ages of late quaternary tephra beds ranging in age from 25 4 to 0 4 ka the lakes are located downwind of the aleutian arc and alaska peninsula volcanoes in the northern bristol bay area between 159 and 161 w at around 60 n sedimentation rate age models for each lake were based on a published spline fit procedure that uses monte carlo simulation to determine age model uncertainty in all 62 14c ages were used to construct the six age models including 23 ages presented here for the first time the age model from lone spruce pond is based on 18 ages and is currently the best resolved holocene age model available from the region with an average 2 age uncertainty of about 109 years over the past 14 5 ka the sedimentary sequence from lone spruce pond contains seven tephra beds more than previously found in any other lake in the area of the 26 radiocarbon dated tephra beds at the six lakes and from a soil pit seven are correlated between two or more sites based on their ages the major element geochemistry of glass shards from most of these tephra beds supports the age based correlations the remaining tephra beds appear to be present at only one site based on their unique geochemistry or age the 5 8 ka tephra is similar to the widespread aniakchak tephra 3 7 0 2 1 ka but can be distinguished conclusively based on its trace element geochemistry the 3 1 and 0 4 ka tephras have glass major and trace element geochemical compositions indistinguishable from prominent aniakchak tephra and might represent redeposited beds only two tephra beds are found in all lakes the aniakchak tephra 3 7 0 2 ka and tephra b 6 1 0 3 ka the tephra beds can be used as chronostratigraphic markers for other sedimentary sequences in the region including cores from cascade and sunday lakes which were previously undated and were analyzed in this study to correlate with the new regional tephrostratigraphy copyright 2012 john wiley sons ltd

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

a catalogue of late cenozoic tephra beds in the klondike goldfields and adjacent areas yukon territory preece s j westgate j a froese d g pearce n j g perkins w t 2011 a catlogue of late cenozioc tephra beds in the klondike goldfields and adjacent areas yukon canadian journal of earth sciences 48 10 1386 1418

revision of the tephrostratigraphy of the lower sixtymile river area yukon territory canada five distinctive tephra beds are preserved in fine grained sediments on a prominent terrace strath along the lower sixtymile river of the yukon territory commencing with the oldest unit they are ta sm3 gold run hollis 2 and flat creek tephra beds the uppermost three tephra beds have rhyolitic compositions the other two are bimodal with a much more basic glass phase three glass fission track age determinations point to deposition during the early middle pleistocene we correct an error in an earlier study by clarifying that flat creek tephra is younger than gold run tephra ta tephra is older

permafrost response to last interglacial warming field evidence from non glaciated yukon and alaska abstract we present stratigraphic observations from three sites in eastern beringia ch ijee s bluff in northern yukon and nearby exposures on the old crow river the palisades on the yukon river in alaska and placer mining exposures at thistle creek in west central yukon which provide insight into the response of permafrost to regional warming during the last interglaciation chronology is based on the presence of old crow tephra an important regional stratigraphic marker that dates to late marine isotope stage 6 supplemented by paleoecology and non finite 14c ages on wood rich organic silts old crow tephra overlies several relict ice wedges at the palisades and thistle creek indicating that permafrost at these sites did not thaw completely during the last interglaciation prominent deposits of last interglacial wood rich organic silt are present at multiple sites in eastern beringia and probably represent accumulations of reworked forest vegetation due to thaw slumping or deposition into thermokarst ponds or depressions consistent stratigraphic relations between these deposits old crow tephra and ice wedge pseudomorphs at our three study sites and at least six other sites in eastern beringia suggest that thaw of shallow permafrost was widespread during the last interglaciation limited stratigraphic evidence suggests that thaw was probably on the order of meters rather than 10s of meters the ubiquity of shallow permafrost degradation during the last interglaciation suggests that current ground warming may foreshadow widespread near surface thaw under even modest future warming scenarios however the persistence of relict pre last interglacial ice wedges highlights the potential for the regional antiquity of discontinuous permafrost and provides compelling field evidence for the long term resilience of deep permafrost during sustained periods of warmer than present climate

diffuse flow hydrothermal manganese mineralization along the active mariana and southern izu bonin arc system western pacific 1 abundant ferromanganese oxides were collected along 1200 km of the active izu bonin mariana arc system chemical compositions and mineralogy show that samples were collected from two deposit types fe mn crusts of mixed hydrogenetic hydrothermal origin and hydrothermal mn oxide deposits this paper addresses only the second type mn oxides cement volcaniclastic and biogenic sandstone and breccia layers mn sandstone and form discrete dense stratabound layers along bedding planes and within beds stratabound mn the mn oxide was deposited within coarse grained sediments from diffuse flow systems where precipitation occurred below the seafloor deposits were exposed at the seabed by faulting mass wasting and erosion scanning electron microscopy and microprobe analyses indicate the presence of both amorphous and crystalline 10 a and 7 a manganate minerals the fundamental chemical difference being high water contents in the amorphous mn oxides alternation of amorphous and crystalline laminae occurs in many samples which likely resulted from initial rapid precipitation of amorphous mn oxides from waxing pulses of hydrothermal fluids followed by precipitation of slow forming crystallites during waning stages the chemical composition is characteristic of a hydrothermal origin including strong fractionation between fe mean 0 9 wt and mn mean 48 wt for the stratabound mn generally low trace metal contents and very low rare earth element and platinum group element contents however mo cd zn cu ni and co occur in high concentrations in some samples and may be good indicator elements for proximity to the heat source or to massive sulfide deposits for the mn sandstones fe mean 8 4 and mn 12 4 are not significantly fractionated because of high fe contents in the volcaniclastic material however the proportion of hydrothermal fe nondetrital fe to total fe is remarkably constant 49 58 for all the sample groups regardless of the degree of mn mineralization factor analyses indicate various mixtures of two dominant components hydrothermal mn oxide for the stratabound mn and detrital aluminosilicate for the mn cemented sandstone and two minor components hydrothermal fe oxyhydroxide and biocarbonate biosilica our conceptual model shows that mn mineralization was produced by hydrothermal convection cells within arc volcanoes and sedimentary prisms that occur along the flanks and within calderas the main source of hydrothermal fluid was seawater that penetrated through fractures faults and permeable volcanic edifices the fluids were heated by magma enriched in metals by leaching of basement rocks and sediments and mixed with magmatic fluids and gases dikes and sills may have been another source of heat that drove small scale circulation within sedimentary prisms

correlation of co concentration and growth rate a method for age determination of ferromanganese crusts abstract based on the analysis and comparison of metal content and growth rate determination by the 10be method an empirical law has been developed to calculate the growth rate evaluating after 520 chemical analyses we have shown that the flux of co into ferromanganese seamount crusts has been constant over the entire growth period of all crusts investigated the amount of co supplied per unit of time and seabottom area is nearly constant in the oceanic water column the layered texture of ferromanganese crusts reflects the change of phases and hydrogenetically precipitated amounts as well as their different growth rates the most important indicator revealing the growth rate is the co enrichment of each crust layer the higher the growth rate the lower the co concentration therefore it is possible to recalculate the age and growth rate of each ferromanganese crust layer by g mm ma 1 1 28 co 0 24 based on the intense phosphorite impregnation of the lower part of the ferromanganese crust which has grown within the time period 18 12 ma so called older crust two additional effects have to be taken into account a the intense phosphorite impregnation causing dilution and b the chemical dissolution of the ferromanganese crusts caused by the phosphate rich pore water both reduce the co concentration and the relating co contents co x and co x have thus to be compensated a co x co x m mn co x mn co b b co x co x 1 0 05 p 1 where co x double corrected co concentration of the layer x of the older crust generation co x corrected co concentration of the layer x of the older crust generation concerning the phosphate dilution co x m measured co concentration of the layer x of the older crust generation mn co x mn co ratio measured of the layer x of the older crust generation mn co b mn co ratio measured of the boundary layer between younger and older crust generation and p difference between the apatite fraction of the older crust of the layer x and the average value of the younger crust generation this simple cobalt calculation model makes it possible to recalculate the growth rate and age of each simple layer of ferromanganese crusts

formation age of the dual structure and environmental change recorded in hydrogenetic ferromanganese crusts from northwest and central pacific seamounts abstract thick hydrogenetic ferromanganese fe mn crusts from the northwest and central pacific seamounts often show a distinct dual structure composed of a typical hydrogenetic porous friable upper part of fe mn oxides layer 1 and the underlying dense hard phosphatized growth generation of fe mn oxides layer 2 in this study layer 2 always appears above the substrate rock and composes the lower part of the crust it is never found as the upper crust layer in contact with seawater the chemical composition of layer 2 clearly differs from the younger layer 1 hydrogenetic fe mn oxides and is depleted in fe al ti and co and enriched in ni cu and zn relative to layer 1 the be isotope age models of the crusts were refined with paleomagnetic and paleontological information and applied to selected crust samples the age model indicates fairly continuous growth from the substrate to the surface and fairly constant growth rates during the past 17 ma the growth rate from the miocene to the present has varied by a factor of two about 2 4 mm myr in layer 1 while layer 2 has similar but more variable growth rates than layer 1 the calculated age for the base of layer 1 and possibly the age of termination of phosphatization is never younger than the late miocene the age seems to vary with water depth shallower water crusts between 991 and 1575 m showing a younger age of about 10 ma whereas the deeper water 2262 m crusts have extrapolated ages for the base of layer 1 of be 17 1 2 5 ma this trend indicates that phosphatization took place in a less oxidizing environment during growth of layer 2 followed by a weakened oxygen minimum zone or intensified aabw during growth of layer 1

ultrafine scale magnetostratigraphy of marine ferromanganese crust hydrogenetic ferromanganese crusts are iron manganese oxide chemical precipitates on the seafloor that grow over periods of tens of millions of years their secular records of chemical mineralogical and textural variations are archives of deep sea environmental changes however environmental reconstruction requires reliable high resolution age dating earlier chronological methods using radiochemical and stable isotopes provided age models for ferromanganese crusts but have limitations on the millimeter scale for example the reliability of 10 be 9 be chronometry commonly considered the most reliable technique depends on the assumption that the production and preservation of 10 be are constant and requires accurate knowledge of the 10 be half life to overcome these limitations we applied an alternative chronometric technique magnetostratigraphy to a 50 mm thick hydrogenetic ferromanganese crust d96 m4 from the northwest pacific submillimeter scale magnetic stripes originating from approximately oppositely magnetized regions oriented parallel to bedding were clearly recognized on thin sections of the crust using a high resolution magnetometry technique called scanning squid superconducting quantum interference device microscopy by correlating the boundaries of the magnetic stripes with known geomagnetic reversals we determined an average growth rate of 5 1 0 2 mm m y which is within 16 of that deduced from the 10 be 9 be method 6 0 0 2 mm m y this is the finest scale magnetostratigraphic study of a geologic sample to date ultrafine scale magnetostratigraphy using squid microscopy is a powerful new chronological tool for estimating ages and growth rates for hydrogenetic ferromanganese crusts it provides chronological constraints with the accuracy promised by the astronomically calibrated magnetostratigraphic time scale 1 40 k y

the variegated vt tephra a new regional marker for middle to late marine isotope stage 5 across yukon and alaska abstract the variegated vt tephra likely sourced from the eastern aleutian arc has a geographic distribution that places it amongst the most widespread tephra beds in eastern beringia first identified in the fairbanks area of interior alaska it has been identified at eight additional sites ranging from togiak bay in southwestern alaska to the klondike area of west central yukon correlation of these occurrences is established through the equivalence of glass major and trace element geochemistry fe ti oxide geochemistry stratigraphy and independent age data in yukon and alaska vt tephra has a minimum bulk tephra volume estimate of 32 km3 previous age estimates for vt tephra have varied ranging from a glass fission track age of 125 30 ka to a weighted mean thermoluminescence tl age of 77 8 4 1 ka from bracketing ages on loess a new infrared stimulated luminescence irsl age of 106 10 ka paleoenvironmental data and several tl and irsl ages from togiak bay suggest that the time of deposition is more likely between these previous age estimates post marine isotope stage mis 5e but underlying a prominent soil likely associated with mis 5c placing it within late mis 5d

a summary of brunhes paleomagnetic field variability recorded in ocean drilling program cores the ocean drilling program odp has recovered many long sediment sequences from around the world that contain medium to high resolution paleomagnetic records of brunhes age these records have provided an important new global view of geomagnetic field variability during stable magnetic dipole polarity that could not be recovered by conventional terrestrial or deep sea piston coring the odp paleomagnetic records of directional variability can be routinely recovered and used regionally as a very highresolution relative chronostratigraphic tool 200 yr resolution odp paleomagnetic records of brunhes age relative paleointensity have dramatically improved our understanding of global geomagnetic field intensity these records document the global scale pattern of paleointensity and demonstrate its use as a high resolution relative chronostratigraphic tool that under ideal conditions could have a precision of 500 yr odp paleomagnetic records and associated oxygen isotope chronostratigraphies have also greatly improved our understanding of the number and ages of brunhes geomagnetic field excursions and their relationship to normal directional secular variation and paleointensity variability although significant progress has been made though odp paleomagnetic studies of brunhes aged sediments it is clear that more work is needed to further resolve and define the detailed global pattern of brunhes psv and its relationship to excursions and true polarity reversals

compositional variation and provenance of late cenozoic distal tephra beds fairbanks area alaska abstract thin fine grained discontinuous tephra beds from interior alaska can be divided into two compositional groups by their mineralogy and major and trace element glass geochemistry the source vents of these beds are unknown but are believed to be in the aleutian arc alaskan peninsula and wrangell volcanic field type i tephra has evolved trace element profiles while type ii is primitive type ii contains abundant hydrous minerals that are absent or rare in type i trace elements were used to compare the fairbanks tephra beds to volcanic rocks in the potential source areas there are very few published glass trace element analyses from these regions this necessitates comparing whole rock with glass analyses for this reason incompatible elements particularly rare earth elements are most useful whole rock analyses and petrography of lavas throughout the aleutian arc alaskan peninsula and the wrangell volcanic field are similar to type i tephra lavas and intrusive rocks similar to type ii tephra are rare and occur at one volcano in the alaskan peninsula and at three volcanoes in the wrangell volcanic field

sequential leaching of marine ferromanganese precipitates genetic implications abstract sequential leaching experiments were carried out on twenty one hydrogenetic crust samples from different locations in the central pacific and the results are compared with four crust and nodule samples of different genetic origin fe vernadite mno2 is the most important minor and trace metal bearing mineral phase and shows high concentrations of co ni cd ti ba zn and cu which are present in seawater mainly as hydrated and labile complexed cations elements forming carbonate and hydroxide complexes and oxyanions in seawater like pb mo v and minor proportions of cu and zn are bound in the fe00h fraction the ca carbonate phase and the residual fraction which consists of aluminosilicates and crystalline oxides show only minor heavy metal associations e g cu and zn only a small part of the high ti concentrations in hydrogenetic crusts is of detrital origin ti mainly forms a hydrogenetic phase probably consisting of ti02 2h20 intergrown with the amorphous fe00h phase analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements the most notable change was observed for pb which during phosphatization of crusts is transferred from the mn fe oxide fraction to the apatite fraction a colloid chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed in the first stage mn2 rich water from the oxygen minimum zone is mixed with oxygen rich deep water and oxidized mn iv and other metals like fe ti al and si form oxide and hydroxide colloids these form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species in stage two the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations incorporating the sorbed heavy metals into the mineral phases

tectonic and paleoceanographic conditions during the formation of ferromanganese nodules from the northern south china sea based on the high resolution geochemistry mineralogy and isotopes abstract deep water ferromanganese fe mn nodules cumulate trace elements from the underlying sediment pore water and seawater during the concentric mineral layers growth process over millions of years they can record long term tectonic movements and paleoceanographic changes to systematically ascertain paleoceanographic evolution of the deep south china sea scs since the last 3 2 ma we comprehensively investigated microstructure mineralogy geochemical element concentrations as well as sr nd and pb isotopic compositions in the fe mn nodules nearby the dongsha islands in the northern scs results indicate that these deposits were formed in a continental marginal environment by a mixture of diagenetic and hydrogenetic processes overall the studied nodules can be divided into three stages with different microstructures and chemical compositions which indicate different formation environments stage 1a is the innermost and oldest layer with a porous mottled texture and has the lowest mn fe ratio cu ni and zn which implies comparatively high oxidization conditions in contrast the mn mineralogy of stage 2 shows conditions similar to those of buserite formed under less oxidative conditions between stage 1a and 2 stage 1b represents a mixture of stage 1a and 2 and has inhomogeneous growth structures based on the nd and sr isotopic composition of the detrital phases and pb isotopes three potential terrigenous source regions including south china luzon and taiwan that discharge into the northern scs from 3 2 ma to 1 06 ma after 1 06 ma the terrigenous fraction in the fe mn deposits on the northern slope of the scs is a two end members mixture of fluvial input from taiwan islands and eolian dusts from the chinese loess our study indicates that scs bottom water variation occurred at 3 2 ma 2 1 ma and 1 06 ma respectively which could be attributed to global cooling and the influence of luzon arc forearc uplift

effects of phosphatization on the geochemical and mineralogical composition of marine ferromanganese crusts most hydrogenetic ferromanganese crusts in the central pacific consist of two growth generations a phosphatized older growth generation that is impregnated with carbonate fluorapatite cfa and a younger non phosphatized growth generation geochemical and mineralogical investigations of non phosphatized and phosphatized crust layers revealed significant differences that are not simply dilution effects by cfa certain elements are depleted in old crusts compared to young crusts in the order si fe al ti co mn pb others like ni zn cu y and rees rare earth elements are mostly enriched suboxic conditions in the phosphate rich water of the oxygen minimum zone that infiltrated the crusts caused a partial redissolution of crust phases resulting in mobilization of associated elements a partial recrystallization of more stable phases and a secondary input of nutrient type elements the frequent occurrence of todorokite in phosphatized crust layers indicates that mn mobilized from the vernadite crust phase was partially recrystallized as todorokite which is more stable under suboxic conditions incorporating ni cu and zn into the crystal lattice but rejecting co other metals like pb y and rees probably form stable phosphate phases in phosphatized crusts our investigations emphasize that under the influence of phosphatization diagenetic remobilization and reorganization in fe mn crusts took place in contrast to non phosphatized crusts the older phosphatized crust generation no longer represents a complete primary crust precipitate therefore young non phosphatized and old phosphatized crust generations should be considered as two different sample types with respect to geochemical composition mineralogy and evolution we also suggest that most methods for age determination like the 87sr86sr isotopic dating method or co chronometry cannot be used for phosphatized crusts without caution

terrace stratigraphy in the tunica hills of louisiana abstract the age and number of important fossil producing late pleistocene terraces in the tunica hills have recently become quite controversial one hypothesis holds that only a single loess mantled farmdalian terrace flanks the streams in this area the other maintains that there are two terraces the youngest is considered essentially holocene in age while the older is sangamon in age radiometric and stratigraphic evidence collected for this study indicates that there are two terraces the youngest is late woodfordian to holocene in age while the other is farmdalian

characterization identity distribution and source of late cenozoic tephra beds in the klondike district of the yukon canada a large number of distal silicic tephra beds have been preserved in the late cenozoic deposits of the klondike region yukon territory forty one tephra samples representing twelve distinctive beds are detailed in this study they range in composition from basaltic andesite to high silica rhyolite and were deposited during the late pliocene to late wisconsinan time interval seven tephra beds are derived from volcanoes in the wrangell volcanic field and four come from the more distant eastern aleutian arc alaska peninsula region but the source of the single andesitic tephra is unknown the widespread and well known old crow and sheep creek tephra beds have been identified in the klondike district but all the other tephra units are characterized in detail for the first time the ages of most tephra beds are poorly constrained but will undoubtedly become better known with the application of recently developed glass fission track methods hence prospects are favourable for the eventual development

scanning squid microscopy of a ferromanganese crust from the northwestern pacific submillimeter scale magnetostratigraphy as a new tool for age determination and mapping of environmental magnetic parameters ferromanganese crusts record long term deep sea environmental changes thus providing their reliable high resolution age models is important we applied a magnetostratigraphic technique to estimate the growth rate of a ferromanganese crust using scanning squid superconducting quantum interference device microscope ssm ssm is designed to map the magnetic field across thins sections with sub millimeter resolution the crust sample was taken from the takuyo daigo seamount northwestern pacific and recorded a limited supply of dust and sediment from continents after drift correction and removal of spike noises the magnetic field values were stacked within the areas of high signal to noise ratios by correlating the obtained profiles with a standard geomagnetic polarity timescale we obtained an average growth rate of 3 37 0 06 mm m y which is consistent with that obtained by 10be 9be geochronology 2 93 0 15 mm m y s ratio mapping shows low values after 3 ma associated with voids between columnar structures

effects of ocean chemistry sea level and climate on the formation of primary sedimentary manganese ore deposits the mineralogy geochemistry and in some cases the mode of origin of many phanerozoic sedimentary manganese orebodies are fairly well known but the effects of the important variables represented by changing ocean chemistry sea level and climate on ore formation are only recently being considered the high degree of mobility of manganese particularly through redox processes insures that it can change phase during weathering transport deposition and diagenesis during chemical weathering manganese is solubilized in acid reducing conditions and carried in surface and subsurface waters to the coastal zone where many of the exploitable deposits can form in slightly reducing carbonate ores to oxidizing conditions oxide ores rainfall controls ground water acidity through fostering vegetation growth major deposits at groote eylandt australia and the extensive deposits of the paratethys seaway in eastern europe and the soviet union accumulated as a result of large scale delivery of dissolved mn to restricted black shale basins during marine transgressions the deposits show compositional zoning and display evidence of having been deposited in shallow marine environments following accumulation of a dilute ore solution in the basin interiors manganese was precipitated when the environments of the basin became oxidized apparently during the early stages of marine regressions during the phanerozoic the concentrations of dissolved oxygen and carbon in the oceans varied significantly in response to large changes in the burial rate of organic carbon when the rate was high there was a drawdown of atmospheric co 2 which led to global reverse greenhouse cooling later upon oxidation of sea floor organic matter co 2 was released and greenhouse warming eventuated manganese is now known to have precipitated rapidly as crusts in the oceans late in the glacial and early in the interglacial stages of the quaternary probably as a result of extensive sea floor oxidation accompanying the release of co 2 this same set of processes operating over geologic time may have played a significant role first in generating large reservoirs of dissolved manganese in intracratonic basins and in the oceans and second by providing the metal for the formation of ore deposits when conditions changed

an extensive middle to late pleistocene tephrochronologic record from east central alaska abstract chester bluff cb in yukon charley rivers national preserve ycnp alaska contains an extensive middle to late pleistocene tephra record within thick loess deposited on a terrace the regionally prominent old crow oc tephra 140 10 ka gi tephra 560 80 ka of fairbanks the preido hill prh tephra of the klondike and vt tephra 77 8 4 1 ka of both the klondike and fairbanks were found at cb the presence of these tephra beds will aid in correlating disparate paleoenvironmental records across eastern beringia there are 15 new tephra beds 11 from the wrangell volcanic field wvf and or hayes volcano one from the aleutian arc alaska peninsula aaap and three unclassified beds cb is normally magnetized indicating the entire sequence is of bruhnes age

age of sheep creek tephra pleistocene in central alaska from thermoluminescence dating of bracketing loess the age of the sheep creek tephra sct a widespread marker ash bed in eastern alaska and western yukon territory has been ambiguous and controversial we have obtained three reliable thermoluminescence age estimates from bracketing loess near fairbanks that imply a deposition age of about 190 000 20 000 yr for sct three of six loess samples near and closely bracketing the sct beds near fairbanks yielded younger age estimates 117 000 and 135 000 yr most likely based on field aspects because of reworking and contamination by translocated grains the new reliable age assignment of 190 000 yr confirms independent stratigraphic evidence of a pre last interglaciation age and stratigraphic evidence from one site upper eva creek that sct is older than the more widespread 140 000 yr old old crow tephra the sct age also has implications for regional correlations of glacial and nonglacial deposits in particular it supports the stratigraphic and geomorphic interpretation that the delta glaciation in the east central alaska range and the reid glaciation in western yukon territory are older than the last interglaciation isotope substage 5e

10 be dating of a manganese crust from central north pacific and implications for ocean palaeocirculation one of the most promising applications of the new 10be detection technique using accelerator mass spectrometry is the determination of growth rates of mn nodules and crusts1 5 because the half life of cosmogenic 10be 1 5 myr is of the same order as the time needed for a few millimetres of nodule material to grow 10be is a useful tool for unravelling such evolution during the late tertiary when most nodules are assumed to have started their growth6 mass spectrometric measurement of a 10be profile across the manganese crust va 13 2 from the central pacific using a tandem van de gr a ff accelerator yields growth rates of 2 7 and 4 8 mm myr 1 for the layers of the crust accumulated between recent and 6 myr bp and between 6 and 11 myr respectively using these measurements as well as 230th dating we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them these ages are 0 12 2 9 3 4 5 7 6 7 7 9 10 12 and 13 16 myr bp all of these boundaries apparently coincide with the times reported for quaternary and late tertiary palaeoclimatic events suggesting that the crust growth has been strongly influenced by palaeoclimate

evolution of a deep water ferromanganese nodule in the south china sea in response to pacific deep water circulation and continental weathering during the plio pleistocene abstract the south china sea scs is connected to the west pacific through a deep channel in the luzon strait thus the scs deep water is sensitive to the evolution of pacific ocean circulation which significantly influences the global climate system geochemical data pb isotope and redox sensitive elements data and magnetic data were determined for a fe mn nodule obtained from jiaolong seamount in the central scs these records reflect interactions between changes in ice sheets deep pacific circulation and weathering inputs to the deep scs during the pliocene and quaternary our results show that the scs deep water environment can be divided into three major stages stages 1 3 stage 1 4 8 1 4 ma was characterized by a well oxygenated pacific deep water pdw and lower dust inputs then moderate stable deep water ventilation and greater inputs of asian dust occurred during stage 2 1 4 0 9 ma during stage 3

evidence of quaternary climatic variations in a sequence of loess and related deposits at birch creek alaska implications for the stage 5 climatic chronology abstract a 45 m outcrop of quaternary sediments on birch creek near circle alaska reveals a record of fluctuating environmental conditions that probably spans several glacial interglacial cycles from base to top the deposits are forested floodplain warm colluvium with ice wedges cold forest soil warm loess cold paleosol containing old crow tephra oct cool to warm loess cold lacustrine very warm loess cold and modern forest soil warm resolution of the paleoclimatic history associated with the oct event is critical to understanding the nature of stage 5 in the western north american arctic application of recent age estimates for the oct tephra ca 140 000 yr bp to the birch creek section would indicate that either i the tephra paleosol dates from the 6 5 5 transition a strongly developed glacial interval occurred within stage 5 and the overlying very warm interval occurred in 5 3 or 5 1 or ii the tephra was deposited during a non milankovitch warming event late in stage 6 a paleoclimate chronology provides an alternative interpretation iii in which the tephra paleosol corresponds to stage 6 or even stage 7 the overlying loess to stage 6 and the lake sediments to all or part of stage 5 but the oct is older than 140 000 yr bp chronologies ii and iii imply a very warm beginning to stage 5 consistent with paleoclimate model simulations and data from other regions

late pliocene to late pleistocene environments preserved at the palisades site central yukon river alaska abstract the palisades site is an extensive silt loam bluff complex on the central yukon river preserving a nearly continuous record of the last 2 myr volcanic ash deposits present include the old crow oct 140 000 yr sheep creek sct 190 000 yr pa 2 02 myr ec ca 2 myr and mining camp ca 2 myr tephras two new tephras pal and pau are geochemically similar to the pa and ec tephras and appear to be comagmatic the pa tephra occurs in ice wedge casts and solifluction deposits marking the oldest occurrence of permafrost in central alaska three buried forest horizons are present in association with dated tephras the uppermost forest bed occurs immediately above the oct the middle forest horizon occurs below the sct the lowest forest bed occurs between the ec and the pa tephras and correlates with the dawson cut forest bed plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central alaska today large mammal fossils are all from common late pleistocene taxa those recovered in situ came from a single horizon radiocarbon dated to ca 27 000 14 c yr b p the incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments

late pliocene dawson cut forest bed and new tephrochronological findings in the gold hill loess east central alaska thick loess deposits in the fairbanks region of interior alaska are a rich source of information on past climates and environments during the late cenozoic and the numerous distal silicic tephra beds preserved within them offer the potential for good chronological control the dawson cut forest bed lies in the lower part of this loess cover plant macrofossils consist of picea glauca picea mariana betula papyrifera and populus balsamifera but no pinus these fossils together with the abundance and size of spruce remains pollen tree ring characteristics and 13 c values of spruce wood demonstrate that the boreal forest represented by the dawson cut forest bed was similar to the modern boreal forest of central alaska warming conditions during the early part of the dawson cut interglaciation initiated thawing of permafrost and melting of ground ice as evidenced in the presence of ice wedge casts and major erosion of the lower gold hill loess tephrochronological magnetostratigraphic and glass fission track dating studies in the fairbanks area and at the palisades site on the yukon river in central alaska suggest an age for the dawson cut forest bed of ca 2 ma hence the northern boreal forest of northwestern north america as we know it today has a long history that probably extends back to at least 2 ma

continuous growth of hydrogenetic ferromanganese crusts since 17 myr ago on takuyo daigo seamount nw pacific at water depths of 800 5500 m ferromanganese crusts cover all outcrops on takuyo daigo seamount traversed during remotely operated underwater vehicle rov dives except in places covered by foraminifera sand takuyo daigo is a cretaceous seamount located in the northwest pacific ocean geological and bathymetric mapping provide the framework for this study chemical and mineralogical analyses of the hydrogenetic ferromanganese crusts show temporal and spatial variations typical of those found in previous studies outcrops from 800 to 5500 m water depths are covered with ferromanganese crusts up to 105 mm thick beryllium isotope dating shows that the crusts have apparently been growing continuously at all water depths even through the modern oxygen minimum zone omz contrary to some earlier models for deposition growth rates vary from 2 3 to 3 5 mm myr with fe or mn fluxes of 0 07 0 11 g cm2 myr since the early middle miocene co mn ratios decrease with water depth while fe mn and other metallic elements increase or show no change based on the analysis of the uppermost crust surface this is probably because co is the most abundant redox sensitive element derived from seawater that occurs in crusts

constancy of oceanic deposition of 10 be as recorded in manganese crusts recent developments in the use of nuclear accelerators as high energy mass spectrometers1 2 has renewed interest in the study of cosmogenic 10be half life 1 5 myr as a geophysical time tracer the accelerator techniques enable us to measure 10be with a sensitivity of 107 atoms or less the potential age dating span with 10be is extended back to at least 15 myr bp providing that the 10be production and deposition rates remain constant or nearly constant over that time period a knowledge of the long term constancy in the 10be deposition rate is thus very important we report here the first detailed measurements of 10be and 9be in two ferromanganese oxide crusts from the sea floor of the equatorial atlantic sample no k 9 21 and the north pacific schw 1d each representing 10 myr of accumulation history we have found in these crusts that the deposition of 10be and 10be 9be during the past 7 9 myr has been constant averaged over time intervals of 1 myr the variation is within 6 before that time both crusts show similar significant deviations the 7 9 myr demarcation may be related to the reported late miocene global abyssal circulation change in the ocean3

two major cenozoic episodes of phosphogenesis recorded in equatorial pacific seamount deposits seamount phosphorites have been recognized since the 1950s but this is the first study to provide an in depth exploration of the origin and history of these widespread deposits representative samples from equatorial pacific cretaceous seamounts were analyzed for chemical mineralogical and stable isotope compositions the phosphorites occur in a wide variety of forms but most commonly carbonate fluorapatite cfa replaced middle eocene and older carbonate sediment in a deep water environment 1000 m element ratios distinguish seamount phosphorites from continental margin plateau and insular phosphorites uranium and thorium contents are low and total rare earth element ree contents are generally high ree ratios and shale normalized patterns demonstrate that the rees and host cfa were derived from seawater strontium isotopic compositions compared with inferred cenozoic seawater curves define two major episodes of cenozoic phosphatization late eocene early oligocene 39 34 ma and late oligocene early miocene 27 21 ma three minor events are also indicated the major episodes occurred at times of climate transition the first from a nonglacial to glacial earth and the second from a predominantly glacial to warm earth the paleoceanographic conditions that existed at those times initiated and sustained development of phosphorite by accumulation of dissolved phosphorus in the deep sea during relatively stable climatic conditions when oceanic circulation was sluggish fluctuations in climate sealevel and upwelling that accompanied the climate transitions may have driven cycles of enrichment and depletion of the deep sea phosphorus reservoir as temperature gradients in the oceans increased antarctic glaciation expanded and oceanic circulation and upwelling intensified expansion and intensification of the oxygen minimum zone may have increased the capacity for midwater storage of phosphorus supplied by dynamic upwelling around seamounts however the bottom waters never became anoxic during the phosphogenic episodes fluctuations in the ccd and lysocline co2 fluxes and changes in bottom water circulation and temperatures may have bathed the seamount carbonates in more corrosive waters which coupled with increased supplies of dissolved phosphorus promoted replacement processes the late eocene early oligocene phosphogenic episode recorded in seamount deposits is not matched by large phosphorite deposits in the geologic record whereas the late oligocene early miocene episode and middle miocene event are matched by large deposits distributed globally the seamount phosphorites are exposed at the surface of the seamounts and have been for most of the neogene and oligocene the phosphorites do not show signs of etching that would indicate substantial undersaturation of seawater phosphate with respect to cfa mass balance calculations indicate that about 5 4 19 1012 g of p2o5 are locked up in equatorial pacific seamount phosphorites that amount is equivalent to about 2 7 years of the present annual input from rivers

paleomagnetic study of ferromanganese crusts recovered from the northwest pacific testing the applicability of the magnetostratigraphic method to estimate growth rate abstract previous studies have shown that ferromanganese crusts in the pacific ocean commonly record paleomagnetic reversals and that the reversal patterns can be used to estimate growth rates in order to investigate the applicability of the magnetostratigraphic method we conducted paleomagnetic measurements of crust samples recovered from five locations in the northwest pacific a series of thin slices with thicknesses of 0 5 1 0 mm was prepared for each sample and a paleomagnetic polarity was determined for each slice in all five samples we found a consistent reversal pattern of n1 r1 n2 r2 n3 from the surface to the inner part of the crust in three samples another polarity interval r3 was recognized below the n3 section of the crust these data suggest that ferromanganese crusts in the northwest pacific recorded paleomagnetic reversals and that reversal patterns can be used for ocean scale correlations the magnetostratigraphic method suggests constant growth rates of 1 49 2 54 3 56 and 3 67 mm ma for four samples three of which are consistent with those estimated using 10 be 9 be dating at the 2 standard deviation level

submarine hydrothermal manganese deposits from the ogasawara bonin arc off the japan islands abstract numerous submarine hydrothermal sulfide and oxide deposits have been found in the mid ocean ridge systems but until now no activity has been found in an island arc system the geological survey of japan gsj has started a geological geophysical and geochemical investigation program in the bonin mariana arc since 1984 aiming at evaluation of submarine hydrothermal metal deposits and understanding of their geological background during the reconnaissance survey by r v hakurei maru in june 1984 thick hydrothermal manganese crusts were recovered from three seamounts of the bonin arc at a depth of ca 1000 m the chemical and mineralogical features and characteristic structure and occurrence of associated calcareous fossils and nontronite claystones suggest the quaternary low temperature hydrothermal activity forming well crystallized todorokite pyrolusite goethite and nontronite deposits thin hydrogenous ferromanganese encrustations on the hydrothermal crusts from a guyot of the northern arc indicate that the recent very active hydrothermal activities around this area may be declining or already finished whereas the fresh hydrothermal manganese crusts from the kaikata seamount suggest recent hydrothermal activity

recurring middle pleistocene outburst floods in east central alaska abstract recurring glacial outburst floods from the yukon tanana upland are inferred from sediments exposed along the yukon river near the mouth of charley river in east central alaska deposits range from imbricate gravel and granules indicating flow locally extending up the yukon valley to more distal sediments consisting of at least 10 couplets of planar sands granules and climbing ripples with up valley paleocurrent indicators overlain by massive silt an interglacial organic silt occurring within the sequence indicates at least two flood events are associated with an earlier glaciation and at least three flood events are associated with a later glaciation which postdates the organic silt a minimum age for the floods is provided by a glass fission track age of 560 000 80 000 yr on the gi tephra which occurs 8 m above the flood beds a maximum age of 780 000 yr for the floods is based on normal magnetic polarity of the sediments these age constraints allow us to correlate the flood events to the early middle pleistocene and further the outburst floods indicate extensive glaciation of the yukon tanana upland during the early middle pleistocene likely representing the most extensive pleistocene glaciation of the area

40ar 39ar dating links albuquerque volcanoes to the pringle falls excursion and the geomagnetic instability time scale abstract incremental heating of basaltic lava groundmass from the albuquerque volcanoes new mexico and plagioclase from ash d at pringle falls oregon yield 40ar 39ar isochron ages of 218 14 and 211 13 ka respectively sediments in which ash d was deposited and the eight lava flows of the albuquerque volcanoes display excursional paleomagnetic data with virtual geomagnetic poles vgps in the southern hemisphere and together with the statistically indistinguishable 40ar 39ar dates establish that both sites record the pringle falls excursion this excursion is also recorded by the 227 8 ka mamaku ignimbrite new zealand and by high deposition rate sediments at odp site 919 in the north atlantic ocean that are dated astrochronologically at 209 207 ka we propose that the names albuquerque and jamaica excursion be abandoned and that a radioisotopic age of 211 13 ka be adopted for the pringle falls excursion which is one of five globally expressed well documented excursions in marine sediment cores or dated by 40ar 39ar methods that took place from 220 to 30 ka together with at least five other well dated excursions between 730 and 520 ka some ten excursions define the geomagnetic instability time scale gits for the bruhnes chron eight of these excursions have been dated using 40ar 39ar methods if the temporal clustering of excursions in the gits is not a sampling artefact this suggests that the geodynamo is not intrinsically unstable but rather that instability in the flow pattern of the outer core fluid flow causes the main dipole field to be considerably weakened such that it becomes unstable with a 200 to 300 ka recurrence interval excursions are an important part of dynamo behavior over geologic time scales that in addition to reversals of the main dipole field need to be fully considered when assessing whether theoretical and numerical simulations of the dynamo produce earth like results

evolution of nd and pb isotopes in central pacific seawater from ferromanganese crusts abstract hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts by analysing nd pb and be isotope profiles within crusts it is possible to reconstruct seawater tracer histories depth profiles of 10be 9be ratios in three pacific ferromanganese crusts have been used to obtain growth rates which are between 1 4 and 3 8 mm ma nd and pb isotopes provide intact records of isotopic variations in pacific seawater over the last 20 ma or more there were only small changes in pb isotope composition in the last 20 ma this indicates a constant pb composition for the erosional sources and suggests further that erosional nd inputs may have been uniform too end values vary considerably with time and most probably reflect changes in ocean circulation the end values of the crusts not only vary as a function of age but also as a function of water depth from 25 to 0 ma crust va13 2 from 4 8 km water depth has a similar pattern of end variation to the two shallower crusts from 1 8 and 2 3 km but about 1 0 to 1 5 units more negative this suggests that end stratification in pacific seawater as demonstrated for the present day has been maintained for at least 20 ma each crust shows a decrease in end from 3 5 ma to the present which is interpreted in terms of an increase in the nadw component present in the pacific from 10 to 3 5 ma ago the crusts show an increase in end this suggests a decreasing role for a deep water source with end less than circum pacific sources in this regard the panamanian gateway restriction from 10 ma with final closure at 3 5 ma may have played an important role in reducing access of atlantic derived nd to the pacific

old crow tephra across eastern beringia a single cataclysmic eruption at the close of marine isotope stage 6 abstract old crow tephra is the largest and most widespread quaternary eruption presently known in eastern beringia its major and trace element geochemistry fe ti oxides and stratigraphic and paleoecological context indicate that it is the result of a single cataclysmic eruption the proximal region may well have experienced tephra fallout from small eruptions just prior to or after the old crow event but there is no evidence to indicate that the distal area was affected we recalculate the glass fission track age at 124 10 ka which coupled with stratigraphic and paleoenvironmental reconstructions indicates that deposition occurred prior to development of the last interglacial boreal forest which suggests a latest marine isotope stage mis 6 age the bulk tephra volume erupted is estimated by three different approaches that are in broad agreement at 200 km 3 but this result must be considered as tentative given the poor controls on definition of isopachs over such a large area the source caldera although presently unrecognized is located in the eastern aleutian arc possibly at or near the emmons lake volcanic center

isotopic compositions of ce nd and sr in ferromanganese nodules from the pacific and atlantic oceans the baltic and barents seas and the gulf of bothnia abstract ferromanganese nodules from the pacific and atlantic oceans the barents and baltic seas and the gulf of bothnia were analyzed for the isotopic compositions of ce nd and sr and the abundances of ree ba and sr ree patterns of barents baltic and bothnian samples show no ce anomaly or even a negative one in contrast to the positive anomaly observed for the pacific and atlantic samples moreover the baltic and bothnian samples have distinctly low end values average end values of the four regions are as follows pacific 5 barents 10 atlantic 11 and baltic inclusive of gulf of bothnia 19 the characteristic low end values of the baltic samples are indicative of the influence of precambrian rocks from the baltic shield of particular interest is the feature of the ce isotopic composition that ece values of the samples from the pacific are negative and those from the other three regions positive this novel finding might suggest a difference in sources of ce between the pacific and other regions these results demonstrate that ce isotopic ratios can be a useful tracer in marine geochemistry in combination with isotopic compositions of nd and sr

a 3 m y record of pliocene pleistocene loess in interior alaska many distal tephra beds exist in the thick fossiliferous loess deposits near fairbanks interior alaska isothermal plateau fission track ages determined on glass shards from tephra beds in conjunction with tephrostratigraphic and magnetostratigraphic techniques indicate that loess deposition began in the late pliocene an antiquity previously unsuspected hence there is the opportunity now to reconstruct a detailed well dated record of environmental changes in interior alaska during the past 3 m y

distribution stratigraphy petrochemistry and palaeomagnetism of the late pleistocene old crow tephra in alaska and the yukon revision accepted january l i 1985 the late quaternary old crow tephra is a two pyroxene calc alkaline dacite whose known areal extent is broadly delimited by a triangle with apices at the seward peninsula the wrangell mountains and the old crow basin in the northern yukon everywhere the tephra is fine grained mdu 5 33 2 0 32 and moderately to poorly sorted 6u 1 33 0 29 samples from the extremities of the fall out zone have similarity coefficients of 0 95 a palaeomagnetic excursion is recorded in sediments just below old crow tephra in the fairbanks area and at imuruk lake on the seward peninsula a short full reversal of the magnetic field has been preserved at the corresponding stratigraphic level near old crow in the northern yukon chronological controls based on i4c and fission track dates and sedimentation rates show that these palaeornagnetic features are almost certainly partial records of the complex geomagnetic blake event which occurred 100 120 ka ago new petrochemical data demonstrate that the source of old crow tephra is in the eastern aleutian arc

old crow tephra found at the palisades of the yukon alaska abstract a 20 cm thick volcanic ash layer at the palisades along the yukon river is geochemically correlated with the old crow tephra a buried organic and wood rich layer above the old crow tephra probably records the last interglaciation the recognition of the old crow tephra provides the first chronologic information on the age of the thick sequence of pleistocene sediments at the palisades an important ice age mammal site in central alaska and a key locakity for the reconstruction of the history of the yukon river and the last interglaciation in central alaska

magnetostratigraphy of hydrogenetic manganese crusts from northwestern pacific seamounts abstract fine scale dating is crucial to understanding the growth and environments of formation of marine manganese deposits the paleomagnetic method of dating of manganese nodules and crusts has been attempted but with no success so far we measured remanent magnetization rm on up to 75 mm thick hydrogenetic crusts at intervals of 2 5 mm after mineralogical and chemical examination careful separation and alternative field demagnetization stable sharp rm and well correlated polarity change patterns between four samples made it possible to identify the pliocene quaternary magnetic chrons in them these data show that the three crusts have grown continuously at rates of 14 17 mm m y since the early pliocene this is three or four times faster than those for central pacific seamount hydrogenetic crusts estimates of growth rate based on determination of co flux are within this range magnetostratigraphy therefore appears to be a powerful method for dating hydrogenetic manganese crusts when supported by conventional dating methods these results encourage finer scale investigations which promise more detailed paleoceanographic reconstruction

new age for the skalamaelifell excursion and identification of a global geomagnetic event in the late brunhes chron article ar 39 ar geochronology paleomagnetism sixteen lava flows sampled from isolated hills including skalamaelifell hill on the reykjanes peninsula in southwest iceland record excursional geomagnetic field directions that correspond to virtual geomagnetic poles vgps grouped at 12 s 250 e in the eastern pacific ocean the vgps are similar to those obtained in earlier studies of these lavas for which the name skalamaelifell excursion was previously assigned based on unspiked k ar dating of nine samples from six hills it has been accepted for more than two decades that the transitionally magnetized lavas reflect the 41 ka laschamp excursion however nine new 40 ar 39 ar incremental heating experiments on four of the transitionally magnetized lavas give an age of 91 13 ka moreover 238 u 230 th isochrons determined for two of these lava flows yield a weighted mean age of 96 10 ka 2 the weighted meanof the

magmatic differentiation at an island arc caldera okmok volcano aleutian islands alaska okmok volcano is situated on oceanic crust in the central aleutian arc and experienced large 15 km caldera forming eruptions at 12 000 years bp and 2050 years bp each caldera forming eruption began with a small plinian rhyodacite event followed by the emplacement of a dominantly andesitic ash flow unit whereas effusive interandpost caldera lavashavebeenmorebasaltic phenocryst assemblages are composed ofolivine pyroxene plagioclase fe ti oxides and indicate crystallization at1000 11008cat 0 1 0 2gpa in thepresence of 0 4 h2o the erupted products follow a tholeiitic evolutionary trend and calculated liquid compositions range from 52 to 68 wt sio2 with 0 8 3 3 wt k2o major and trace element models suggest that the more evolved magmas were produced by 50 60 in situ fractional crystallization around the margins of the shallow magma chamber oxygen and strontium isotope data o 4 4 4 9o sr sr 0 7032 0 7034 indicate interaction with a hydrothermally altered crustal component which led to elevated thorium isotope ratios in some caldera forming magmas this compromises the use of uranium thorium disequilibria th u 1 4 0 849 0 964 to constrain the time scales of magma differentiation but instead suggests that the age of the hydrothermal system is 100 ka modelling of the diffusion ofstrontiuminplagioclase indicates thatmanyevolved crystal rims formed less than 200 years prior to eruption this addition of rim material probably reflects the remobilization of crystals from the chamber margins following replenishment basaltic recharge led to the expansion of the magma chamber which was responsible for the most recent caldera forming event

gold run tephra a middle pleistocene stratigraphic and paleoenvironmental marker across west central yukon territory canada gold run tephra has been found at thistle creek sixtymile river and the klondike goldfields of west central yukon canada it is a hornblende bearing rhyolitic tephra with thicknesses of up to 10 cm at each site suggesting a widespread distribution across interior alaska and the yukon territory given the long distance to the nearest volcanic centre old crow flat creek and ta tephra beds are stratigraphically associated with gold run tephra at our study sites and have distinctive compositions gold run tephra is not accommodated by the current classification scheme for late cenozoic distal tephra beds in alaska and the yukon territory a scheme based on the physical and chemical attributes so that its provenance is unknown an early middle pleistocene age is supported by a glass fission track age of 0 74 0 06 ma a normal remanent magnetic polarity for the enclosing loess and the presence of a late irvingtonian faunal assemblage in the associated organic rich silts at thistle creek hence gol

old crow tephra 140 10 ka constrains penultimate reid glaciation in central yukon territory abstract old crow tephra 140 10 ka is closely associated with deposits from the penultimate reid glaciation along the pelly river in central yukon territory the elevation of the tephra above present base level and association with deglacial sediments suggests a close minimum age estimate confirming a mis 6 age for the reid glaciation this is in contrast to a mis 4 age for the penultimate gladstone glaciation in southwest yukon this study demonstrates that the assumed synchrony of the penultimate advance of the cordilleran ice sheet has little support and regional controls on the mass balance of source regions of the ice sheet are more important than previously considered