u th ages obtained by mass spectrometry in corals from barbados sea level during the past 130 000 years the study of the sea level record during the last glacial cycle has primarily proceeded indirectly by means of oxygen isotope measurements on foraminifera from deep sea sediments1 2 the direct approach of dating sea level indicators stagnated during the past decade mainly because the samples required to complete our knowledge of the past glaciations are below the present day sea level3 7 using the 14c ages of acropora palmata samples collected by drilling offshore the island of barbados fairbanks8 presented the first detailed chronology for the last deglaciation this radiocarbon chronology is limited to the past 30 kyr because of the short half life of 14c 5 730 yr we must therefore rely on other dating methods to obtain information for the whole last glacial cycle during the past four years 230th 234u dating of corals by thermal ionization mass spectrometry has been shown to be significantly more precise and accurate than the classical counting method9 14 we have used this technique to measure u th ages in coral samples from the barbados collection and from subaerially exposed outcrops see also ref 15 here we present results bearing on the sea level record for the past 130 000 years we conclude that the last deglaciation started 3 000 years earlier than previously thought and confirm that there were two surges in melt water8 at about 11 kyr and 14 kyr bp before present

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

paleogeographic and paleoclimatic significance of diatoms from middle pleistocene marine and glaciomarine deposits on baldwin peninsula northwestern alaska abstract a newly revised diatom biostratigraphic scheme for the north pacific ocean based on studies of the diatom flora of the cape blossom and hotham inlet formations of baldwin peninsula northwestern alaska provides new information on the paleoceanography of kotzebue sound alaska during middle pleistocene time sediments exposed along coastal bluffs of baldwin peninsula contain subarctic north pacific zonal index species including extinct proboscia barboi pr curvirostris actinocyclus ochotensis var fossilis thalassiosira nidulus var nidulus th jouseae th gravida var fossilis and stephanopyxis dimorpha which are correlated with the middle part of the proboscia barboi zone 0 43 0 36 ma based on the distribution of the zonal diatom species and on changes in the paleoecological structure of the diatom assemblages e g warm vs cold water assemblages we conclude that the marine deposits of the cape blossom formation and lower baldwin silt member of the hotham inlet formation formed under relatively warm high sea level conditions associated with the marine oxygen isotope stage ois 11 transgression glaciomarine sedimentation middle and upper baldwin silt and selawik gravel members of the hotham inlet formation initiated during relatively warm conditions at the end of the stage 11 transgression and continued into stage 10 high latitude glacier growth during a global interglacial period was likely facilitated by warm surface waters on flooded continental shelves and limited regional sea ice cover the biostratigraphic record at baldwin peninsula provides further evidence that high latitude ice growth is favored by increased moisture supply during warm climatic intervals and inhibited by arid conditions during full glacial intervals

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr

paleoclimatic forcing of magnetic susceptibility variations in alaskan loess during the late quaternary visual matches and statistical tests suggest correlations between marine isotope curves retrodictive solar insolation at lat 65 n and magnetic susceptibility profiles through late quaternary age alaskan loess sections the susceptibility changes largely appear to reflect variability in magnetite content due to climatically controlled changes in wind intensity and competence magnetic susceptibility profiles through massive loess can provide stratigraphic context for intercalated paleosols and tephras a prominent paleosol correlated with marine isotope stage 5 occurs several metres above the old crow ash in loess sections indicating that this important tephra is older than suggested by thermoluminescence dates and may have been deposited ca 215 25 ka

crustal accretion at high temperature spreading centres rheological control of crustal thickness abstract new determinations of lateral crustal thickness variations at anomalous oceanic spreading centres such as iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides bjarnason and schmeling 2009 to understand this behaviour crustal accretion models have been carried out solving the conservation equations of mass momentum and energy with melting melt extraction and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume the dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust four accretional modes with characteristic crustal thickness variations are identified depending on the width of the volcanic emplacement zone the accretional heating rate which can be associated with the thickness of the surface layer in which magmatic emplacement takes place and the spreading rate mode 1 zero crustal thickness at the spreading axis develops for cool accretion and a wide emplacement zone mode 2 strongly or moderately crustal thickening away from the axis develops in case of warm deep reaching accretion and wide emplacement zones mode 3 nearly constant crustal thickness develops in case of warm deep reaching accretion but narrow emplacement zones dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis modes 2 or 3 may be identified with the situation in iceland mode 4 a stagnating central crustal block evolves for cool accretion and narrow emplacement this mode disappears for increasing spreading rates no accretional mode with maximum crustal thickness above the plume at the rift axis has been found the absence of mode 1 accretion with zero crust at ridge axis on earth may be an indication that in general crustal accretion is not cold and shallow the model is also applied to other hotspot ridge settings azores galapagos and suggests modes 2 3 accretion

stratigraphic context of old crow tephra holitna lowland interior southwest alaska abstract a thick deposit of old crow tephra was discovered in a bluff exposure along the middle holitna river near the kulukbuk hills 61 20 n latitude 157 10 w longitude in interior southwest alaska this locality is the southwesternmost known deposit of old crow tephra in alaska thickness and grain size data from this site support a source volcano in the eastern aleutian arc pleistocene stratigraphic sequences in the lowland are dominated by upward fining eolian sand sheet deposits and loess separated by organic silt these deposits record at least two episodes of regional glaciation and an intervening nonglacial period marine oxygen isotope stage 3 stage 5 or both old crow tephra crops out near the top of the lower upward fining eolian unit indicating that the ash erupted near the end of an interval of periglacial eolian sedimentation the sequence of eolian deposits that contain old crow tephra probably accumulated during the latter part of marine oxygen isotope stage 6 whereas the overlying eolian sequence formed during the last glaciation stage 2 this stratigraphic position is consistent with other stratigraphic contexts for the tephra and with fission track and thermoluminescence ages of ca 140 000 10 000 yr b p

vegetation and paleoclimate of the last interglacial period central alaska the last interglacial period is thought to be the last time global climate was signi cantly warmer than present new stratigraphic studies at eva creek near fairbanks alaska indicate a complex last interglacial record wherein periods of loess deposition alternated with periods of soil formation the eva forest bed appears to have formed about the time of or after deposition of the old crow tephra dated to 160 to 120 ka and is therefore correlated with the last interglacial period pollen macrofossils and soils from the eva forest bed indicate that boreal forest was the dominant vegetation and precipitation may have been greater than present around fairbanks during the peak of the last interglacial period a new compilation of last interglacial localities indicates that boreal forest was extensive over interior alaska and yukon territory boreal forest also extended beyond its present range onto the seward and baldwin peninsulas and probably migrated to higher elevations now occupied by tundra in the interior comparison of last interglacial pollen and macrofossil data with atmospheric general circulation model results shows both agreement and disagreement model results of warmer than present summers are in agreement with fossil data however numerous localities with boreal forest records are in con ict with model reconstructions of an extensive cool steppe in interior alaska and much of yukon territory during the last interglacial 2000 elsevier science ltd all rights reserved

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

chlorine 36 and 14c chronology support a limited last glacial maximum across central chukotka northeastern siberia and no beringian ice sheet abstract the pekulney mountains and adjacent tanyurer river valley are key regions for examining the nature of glaciation across much of northeast russia twelve new cosmogenic isotope ages and 14 new radiocarbon ages in concert with morphometric analyses and terrace stratigraphy constrain the timing of glaciation in this region of central chukotka the sartan glaciation last glacial maximum was limited in extent in the pekulney mountains and dates to 20 000 yr ago cosmogenic isotope ages 30 000 yr as well as non finite radiocarbon ages imply an estimated age no younger than the zyryan glaciation early wisconsinan for large sets of moraines found in the central tanyurer valley slope angles on these loess mantled ridges are less than a few degrees and crest widths are an order of magnitude greater than those found on the younger sartan moraines the most extensive moraines in the lower tanyurer valley are most subdued implying an even older probable middle pleistocene age this research provides direct field evidence against grosswald s beringian ice sheet hypothesis

influence of orbital parameters on pleistocene loess deposition in central alaska few long proxy climatic records are available from terrestrial locations the isotope records of pleistocene climate change found in marine cores reflect worldwide ice volume and temperature changes forced by variations in the earth s orbital geometry comparable terrestrial records are rare loess wind deposited silt is perhaps the closest terrestrial analogue to marine sediments in that both result from more or less continuous deposition of fine grained sediment here we examine the hypothesis that mag netic susceptibility variations through loess sections reflect climate forcing we statistically tested cross correlations between high precision orbitally tuned marine oxygen isotope curves recording the past 250 000 yr bp and magnetic susceptibility profiles through a thick loess section in central alaska highly significant p 0 0001 correlations indicate strong thick palaeosols formed in arctic loess during early parts of interglacial periods correspond ing to isotope stages 7 and 5 thinner soils formed during interstadial events corresponding to early stage 7 and stage 3 autocorrelation and time series analysis of the loess data show spectral peaks at 125 000 41 000 and 23 000 yr bp showing for the first time that terrestrial orbital periodicities influenced and are recorded in proxy climatic data from terrestrial aeolian deposits at high latitudes

two dimensional gas chromatograms as fingerprints of sour gas associated oils abstract examining a well documented case history in western canada sedimentary basin using two dimensional gas chromatography gc gc has produced a useful tool for fingerprinting thermochemical sulfate reduction tsr affected petroleum fluids application of the approach and data interpretation strategies to the northeastern jinxian sag of bohai bay basin has provided molecular evidence for tsr as a plausible source for the h2s in the gases associated with the studied oils

origin of diamondoid and sulphur compounds in the tazhong ordovician condensate tarim basin china implications for hydrocarbon exploration in deep buried strata abstract in this paper we analyse the ordovician condensate in the tazhong area of the tarim basin nw china using a new high resolution comprehensive two dimensional gas chromatography with time of flight mass spectrometry gc gc tofms results show that 4778 compounds at a signal to noise ratio of more than 100 were detected reflecting the advantage of this method in the analysis of condensate abundant adamantane series compounds and some diamantane series compounds as well as a series of sulphur containing compounds including the benzothiophene series and the dibenzothiophene series were detected for the first time in the study area combined with the results of a study on oil and gas sources and migration accumulation we address the origin of these compounds it is implied that the diamondoids were not formed in situ due to oil cracking in reservoir approximately 5500 m deep at temperature around 139 c whereas the cracking of oil to gas may have occurred in the deeper buried cambrian dolomite reservoir at a reservoir temperature of 180 220 c 8500 11 000 m depth the oil cracked gas secondary cracking product might have migrated to and accumulated in the shallower buried ordovician reservoir accompanied by abundant diamondoids the formation of sulphur containing compounds may be related to thermochemical sulphate reduction tsr which similar to the oil cracking took place in the deeper buried cambrian reservoir under high temperature tsr so 4 2 in the formation water is reduced to h 2 s and organic sulphur compounds leading to the enrichment of sulphur in crude oils e g benzothiophene and dibenzothiophene series as such it is implied that the tazhong area could potentially be explored for deep buried oil cracked gas

characterization of compounds in unresolved complex mixtures ucm of a mesoproterzoic shale by using gc gc tofms abstract unresolved complex mixtures ucm in shales have important implications for organic matter formation and diagenesis however their origin is not well understood one important reason for this is lacking the knowledge of their molecular compositions here using the comprehensive two dimensional gas chromatography gc gc coupled with time of flight mass spectrometry tofms we conduct a pilot study based on a mesoproterozoic black shale from north china detailed identification of compounds was made and a possible new origin was suggested results show that the separation of ucm hydrocarbons by a reversed phase column system polar non polar is efficient for the identification of molecular composition of ucm the ucm in the shale is composed mainly of c 11 c 27 compounds including paraffin hydrocarbons cycloalkanes and aromatics in particular 1 3 cyclic paraffins with a short chain alkyl group multi branched isoparaffins and their homologous series which are indicative of isomerization of molecules intensively this implies that the origin of the ucm might include the isomerization of organic molecules in addition to the commonly thought microbial activities this study improves the understanding of the geochemical compositions and origin of ucm in shales

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

stratigraphy and palaeoclimatic significance of late quaternary loess palaeosol sequences of the last interglacial glacial cycle in central alaska abstract loess is one of the most widespread subaerial deposits in alaska and adjacent yukon territory and may have a history that goes back 3 ma based on mineralogy and major and trace element chemistry central alaskan loess has a composition that is distinctive from other loess bodies of the world although it is quartz dominated central alaskan loess was probably derived from a variety of rock types including granites metabasalts and schists detailed stratigraphic data and pedologic criteria indicate that contrary to early studies many palaeosols are present in central alaskan loess sections the buried soils indicate that loess sedimentation was episodic or at least rates of deposition decreased to the point where pedogenesis could keep ahead of aeolian input as in china loess deposition and pedogenesis are likely competing processes and neither stops completely during either phase of the loess soil formation cycle loess deposition in central alaska took place before and probably during the last interglacial period during stadials of the mid wisconsin period during the last glacial period and during the holocene an unexpected result of our geochronological studies is that only moderate loess deposition took place during the last glacial period our studies lead us to conclude that vegetation plays a key role in loess accumulation in alaska factors favouring loess production are enhanced during glacial periods but factors that favour loess accumulation are diminished during glacial periods the most important of these is vegetation boreal forest serves as an effective loess trap but sparsely distributed herb tundra does not thus thick accumulations of loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in central alaska much of the stratigraphic diversity of north american loess including that found in the central lowlands the great plains and alaska is explained by a new model that emphasizes the relative importance of loess production factors versus loess accumulation factors

the last interglaciation in alaska stratigraphy and paleoecology of potential sites at least 20 localities in alaska contain deposits that may provide information on the last interglaciation oxygen isotope substage 5e these widely dispersed localities include river bluffs coastal bluffs and terraces elevated marine shorelines lake basins and artificial excavations most of the inferred interglacial deposits contain macrofossils or pollen that are older than the range of radiocarbon dating and commonly indicate climate as warm as or warmer than the present at a few localities evidence for deep thaw of permafrost also indicates a warm paleoclimate at eight localities the old crow tephra occurs at or below organic deposits that may represent substage 5e the tephra occurs beneath conspicuous organic deposits at fairbanks the yukon palisades and holitna lowland and directly above a peat bed at hogatza mine at birch creek halfway house ky 11 and imuruk lake the tephra occurs within a paleosol or organic deposit but other organic horizons that more likely indicate interglacial conditions occur at higher stratigraphic levels the varied stratigraphic relations of the old crow tephra suggest that it may have been deposited close to the boundary between isotope substages 6 and 5 which is dated at about 130 ka in the marine record and between 132 and 140 ka on land these age relations suggests that the tephra may have been deposited about 135 5 ka validating the recent fission track age determination of 140 10 ka for this deposit six coastal localities contain deposits of probable interglacial age and these commonly are associated with evidence for eustatic sea levels higher than those of the present beach and sublittoral sediments of the pelukian transgression occur up to 12 m asl along the northwest coast of alaska and are correlative with barrier island and lagoonal sediments on the alaskan arctic coastal plain both sets of deposits commonly contain extralimital mollusks and microfauna that indicate marine water slightly warmer than present and suggest that seasonal sea ice did not extend south of bering strait during the last interglacial as it does today farther south elevated marine terrace deposits on amchitka island contain marine invertebrates that indicate a climate warmer than at present peat horizons in coastal exposure at goose bay and coastal terraces at lituya bay contain pollen spectra that suggest forests like those of the present day and spruce macrofossils exposed on baldwin peninsula indicate boreal forest more extensive than at present sediments from several lakes in northwestern alaska may contain continuous records of the last interglaciation a major warm interval possibly isotope substage 5e has been identified in a core from squirrel lake by a peak in picea pollen that indicates forest extension beyond present limits similar pollen records are potentially available from two maars which formed in the cape espenberg area more than 125 ka terrestrial organic deposits thought to record the last interglaciation occur interstratified with marine and glaciogenic sediments in the nushagak lowland of southwest alaska and on baldwin peninsula in kotzebue sound extensive exposures along the copper and nenana rivers may also contain organic deposits that record the last interglaciation

reduced sea ice concentrations in the arctic ocean during the last interglacial period revealed by sediment cores off northern greenland 1 we present a record encompassing marine isotope stages 7 1 from a hitherto unexplored and heavily ice covered area of the arctic ocean the lomonosov ridge off the northern greenland canada continental margin using nannofossil and benthic foraminifera stratigraphy planktic foraminifera assemblages are used as a key paleoceanographic proxy and a surprisingly large variability is found for an interior arctic ocean site abundant small 63 125 m subpolar turborotalita quinqueloba occur in two sections possibly representing substages 5e last interglacial and 5a warm interstadial however the present day circulation pattern and the very distant location of high productive regions cannot explain such high abundances of subpolar specimens in the interior perennially sea ice covered arctic ocean hence our proxy record indicates that last interglacial sea ice concentrations were reduced off some areas of northern greenland canada whether this was part of a larger regional pattern or it represents the influence of polynya areas with locally increased productivity remains to be solved with respect to glacial conditions increased ice rafted debris ird deposition in the area appears to be associated with glacial stages 6 4 and late 3 stage 2 sediments including the last glacial maximum are condensed with a sparse ird content only

interstadial proboscidean from south central alaska implications for biogeography geology and archeology abstract in 1980 a large proboscidean femur probably mammuthus sp was found in situ in a bluff exposure at the mouth of the tyone river in the northwestern part of the copper river basin alaska the regional setting stratigraphy radiocarbon chronology flora and implications of the fossil locality which represents the first documented occurrence of pleistocene terrestrial mammalian fauna in southern alaska are described radiocarbon dates and stratigraphic relations at the site indicate that the sediments containing the fossil accumulated during the transition from interstadial to glacial conditions during terminal middle wisconsin time during this interval the immediate vicinity was unforested and large areas of south central alaska may have been available for faunal and possibly human habitation this documented find dated at 29 450 610 14 c yr b p extends the known range for pleistocene mammals and possibly steppe tundra conditions south ward at least 150 km and suggests that mountain passes through the alaska range to the north were ice free during the last part of the middle wisconsin interstadial

on the orientation of tensile dikes in a ridge centered plume abstract we present three dimensional numerical models of the ascent of a plume under a spreading ridge and the concomitant melt generation and crust formation using a mantle viscosity which depends on pressure temperature melt content and optionally water content from the velocity field of these convection models we compute the viscous stress tensor in the mantle assuming that melt rich channels or melt filled dikes are oriented parallel to the maximum compressional stress we calculate the orientation of such dikes in the partially molten zone of the plume head and beneath the ridge for the central part of the plume we find dike orientations parallel to the ridge in shallow parts of the plume head while they are ridge perpendicular at greater depth the boundary between these two regimes is shifted downwards for a water rich plume which dehydrates upon melting the two regimes of different dike or melt channel orientations seem to be in good agreement with seismic observations on depth dependent seismic anisotropy indicating a preference of our wet model we find a modest focusing of melt towards the spreading center beneath normal ridge in both models but also in the fully dehydrated parts of the plume head in the model where water was included those are regions where upwelling is essentially passive in contrast the plume in the water free model and the only partly dehydrated parts of the plume in the water bearing model tend to develop a dike orientation which moves melt away from the spreading center these regions are characterized by active upwelling the defocusing of melt in actively upwelling plume mantle has radial symmetry about the plume axis i e an outward orientation of the dikes is not only visible in planes perpendicular to the ridge but also along the ridge

middle pleistocene age of the nome river glaciation northwestern alaska abstract during the middle pleistocene nome river glaciation of northwestern alaska glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals the age of the nome river glaciation is constrained by laser fusion 40 ar 39 ar analyses of basaltic lava that overlies nome river drift at minnie creek central seward peninsula that average 470 000 190 000 yr 1 milligram size subsamples of the lava were dated to identify and eliminate extraneous 40ar enrichments that rendered the mean of conventional k ar dates on larger bulk samples of the same flow too old 700 000 570 000 yr while the 40 ar 39 ar analyses provide a minimum limiting age for the nome river glaciation maximum ages are provided by a provisional k ar date on a basaltic lava flow that underlies the nome river drift at nearby lave creek by paleomagnetic determinations of the drift itself at and near the type locality and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the anvilian marine transgression that underlie nome river drift on the coastal plain at nome taken together the new age data indicate that the glaciation took place between 580 000 and 280 000 yr ago the altitude of the anvilian deposits suggests that eustatic sea level during the anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression by correlation with the marine oxygen isotope record the transgression probably dates to stage 11 at 410 000 yr and the nome river glaciation is younger still analyses of floor altitudes of presumed nome river cirques indicate that the nome river regional snowline depression was at least twice that of the maximum late wisconsin the cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern alaska during the middle pleistocene

transgressions rethinking beringian glaciation abstract the purpose of this investigation is to encourage a fresh look at pleistocene beringia heretofore flooding of bering strait has been cited as the only barrier to migration with marine sea transgressions being a sea gate that closed off migration during glacial interstadials and interglaciations however the possibility exists that glacial advances were also barriers with marine ice transgressions being an ice gate that closed off migration during glacial stadials and glacial maxima this possibility proceeds from the marine ice transgression hypothesis mith which states that marine ice sheets form on the broad arctic continental shelf of northern hemisphere continents when sea ice thickens grounds and domes in shallow water and then transgresses landward as continental ice sheets and seaward as floating ice shelves hughes 1987 landward transgression is onto coastal lowlands during pleistocene glaciations a marine ice sheeet extending from spitsbergen to greenland may have transgressed the circumpolar continental landmass at its lowest and narrowest gap central beringia and calved into the pacific ocean four models of beringian glaciation are presented based on the distinction between marine glaciation and highland glaciation central beringia was glaciated only in highlands in the traditional model hopkins et al 1982 was also glaciated by a self sustaining ice shelf floating over the deep ocean basins of the bering sea in the model by grosswald and vozovik 1984 was glaciated by a marine ice sheet that covered highlands the continental shelf and supplied the ice shelf in a model for maximum pleistocene glaciation and was glaciated by a marine ice sheet in the chukchi sea that merged with highland glaciers transgressed the continental shelf of the western bering sea and calved into the southern bering sea along the edge of the continental shelf in a model for the last glaciation field tests are suggested to assess the viability of these four models the first model is already established for highland glaciation in alaska but less established in siberia the last model should be the easiest to evaluate for marine glaciation the last model limits human migration across the beringian land bridge to brief intervals between stadials and interstadials of the last glaciation cycle when both the ice gate and the sea gate were opened to human migration this model can influence the sea change now underway among quaternary scientists studying peopling of the americas based on the archaeological linguistic and ethnic diversity among native american populations

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

interglacial deposits at birch creek northeast interior alaska abstract a 12 m sequence of lake sediment and peat in a 45 m high exposure on birch creek northeast alaska contains pollen of picea betula alnus and populus and wood of picea and populus this sequence which may represent 10 000 yr of more of accumulation is beyond the limit of radiocarbon dating it lies between two units of loess the underlying loess lies above the old crow tephra recently dated at 149 000 13 000 yr b p the lake sediments probably were deposited during the last interglaciation isotope substage 5e and subsequently buried by wisconsinan loess analogs for the ancient lake may be deep long lived thaw lakes that are present in the modern landscape when birch creek is correlated with other sites across nonglaciated alaska and northwest canada there appears to be a common interglacial signal in sediments overlying the old crow tephra

luminescence chronology of late pleistocene loess paleosol and tephra sequences near fairbanks alaska abstract thermoluminescence tl and infrared stimulated luminescence irsl sediment dating methods have been applied to paleosol and tephra bearing loess sequences younger than marine oxygen isotope stage mis 7 at three important sites tl ages indicate the development of significant paleosols 75 000 and 30 000 yr ago in the loess sequence at the gold hill site relatively minor soil development occurred 70 000 and 48 000 yr ago like the 75 000 yr old soil the 30 000 yr old soil is apparently of global extent and consistent in timing with inferred warm intervals elsewhere e g greenland europe western and central china at birch hill replicate tl dating of primary loess combined with two earlier tl results from the same site and with an earlier mean fission track glass shard age of 140 000 10 000 yr for the associated old crow tephra yield a more precise numeric age of 142 300 6600 yr for this alaska yukon chronostratigraphic marker ash bed three of the tl ages at the halfway house site are difficult to interpret but combined with other evidence they indicate 1 the upper 5 6 m of loess from halfway house is not part of the gold hill loess equivalent to pre mis 5 age as long thought by t l pewe but rather is much younger 2 the regionally significant variegated tephra found in the fairbanks and klondike areas and previously thought to be older than mis 5 has an age of 77 800 4100 yr late mis 5

a 5000year record of intertidal peat stratigraphy and sea level change from northwest alaska abstract coastal environments of northwest alaska preserve a detailed record of sea level change during the past 5000 14 c yr rapid burial of intertidal marshes by storm overwash processes a short open water period and the arctic maritime climate contribute to the preservation of marine and eolian facies eustatic and storm controlled changes in sea level can be identified from interbedded sequences of marsh peat and coastal flood deposits on barrier islands and estuaries along northwest seward peninsula mean eustatic sea level has risen about 1 5 m during the last 5000 years at an average of ca 0 028 cm yr 1 based on the depth and age relationship of a suite of 23 peat horizons sampled from a 140 km long reach of coast nearshore erosion and sedimentation are controlled by secular variations in wave climate peaks in tidal amplitude and storminess correlate with transgressive sedimentary regimes and occurred during the periods 3300 1700 1200 900 and since about 400 14 c yr bp short term fluctuations in relative sea level are superimposed on the long term regional eustatic trend and record the coastal response to variable rates of sea level change during the late holocene

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

variable viscosity convection and partial melting in the continental asthenosphere convection beneath continents may be associated with the generation of partially molten zones at depth experimentally determined non newtonian temperature and pressure dependent creep laws for olivine were used to model upper mantle convection in the presence and absence of a melt phase in our models convection is chaotic and exhibits a strong time dependence using model dry and water undersaturated solidus relations partially molten regions develop at asthenospheric depths forming low viscosity regions the existence of these regions is linked to convection and consequently varies in space and time with an episodicity of 5 to 10 m y in good agreement with observations based on seismic tomography and the episodicity and volumetric abundance of surface volcanics for an average continental heat flux of about 53 3 mw m2 maximum melt fractions range between 0 and 2 dry solidus and 2 and 4 water undersaturated solidus on the basis of experimental deformation studies we employed an empirical relationship between stress and grain size to assess the grain size variations possible in the upper mantle the grain sizes obtained millimeters to centimeters are in good agreement with grain sizes observed in natural examples the smallest grain sizes occur along narrow zones of high stress within the deepest part of the lithosphere the largest grain sizes are found in the overlying lithosphere and in partially molten asthenospheric regions as our models scale well to natural observations these results appear to preclude diffusion creep n 1 as a viable upper mantle steady state deformation mechanism

middle to late pleistocene ice extents tephrochronology and paleoenvironments of the white river area southwest yukon abstract sedimentary deposits from two middle to late pleistocene glaciations and intervening non glacial intervals exposed along the white river in southwest yukon canada provide a record of environmental change for much of the past 200 000 years the study sites are beyond the marine isotope stage mis 2 glacial limit near the maximum regional extent of pleistocene glaciation non glacial deposits include up to 25 m of loess peat and gravel with paleosols pollen plant and insect macrofossils large mammal fossils and tephra beds finite and non finite radiocarbon dates and twelve different tephra beds constrain the chronology of these deposits tills correlated to mis 4 and 6 represent the penultimate and maximum pleistocene glacial limits respectively the proximity of these glacial limits to each other compared to limits in central yukon suggests precipitation conditions were more consistent in southwest yukon than in central yukon during the pleistocene conditions in mis 5e and 5a are recorded by two boreal forest beds separated by a shrub birch tundra that indicate environments as warm or warmer than present a dry treeless steppe tundra dominated by artemisia frigida upland grasses and forbs existed during the transition from late mis 3 to early mis 2 these glacial and non glacial deposits constrain the glacial limits and paleoenvironments during the middle to late pleistocene in southwest yukon

the last interglacial to glacial transition togiak bay southwestern alaska an 18 m high coastal bluff at togiak bay northwestern bristol bay southwestern alaska exposes marine lacustrine fluvial glacial volcanic and organic deposits that record the 50 000year long transition from the peak of the last interglaciation to the early wisconsin glaciation the base of the section is dominated by stratified sand and silt extending up to 4 3 m above sea level marine diatoms are present and pollen assemblages are characterized by relatively high percentages of picea alnus and betula and low percentages of poaceae and cyperaceae the marine sediment was probably deposited during the peak of marine oxygen isotope stage ois 5e an infrared stimulated luminescence irsl age of 151 000 13 000 yr from near the base of the exposure is permissive of this correlation the marine sand and silt are overlain by 0 8 m of peaty silt with diatoms that record a transition from marine to lacustrine conditions during this interval poaceae and cyperaceae dominate the pollen assemblages and picea and shrubs are nearly absent suggesting that herb tundra occupied the landscape this interval probably encompasses ois 5d on the basis of the herb tundra and an irsl age of 119 000 10 000 yr from 60 cm below the marine lacustrine transition the organic mud is overlain by 3 1 m of stratified sand and organic silt that apparently record shallowing of the lake reappearance of spruce and shrubs dois 5c and subsequent deepening of the lake dois 5b followed by aggradation of a floodplain dois 5a which was dry at the time basaltic lava buried the site thermoluminescence analyses on lava baked sediment indicate that the eruption occurred 70 000 10 000 yr ago sometime thereafter but prior to 53 600 14 c yr b p an outlet of the ahklun mountains ice cap advanced over the site and deposited 7 m of bouldery ice contact drift the sedimentary sequence contains at least four tephra beds major and trace element chemistry provide a basis for correlating two of the tephras with tephra beds at nearby sites the tephras luminescence ages and correlations with marine isotope stages provide the geochronological control to place the togiak bay section into a global context the site serves as an important new reference section for late pleistocene paleoenvironmental change in eastern beringia

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

dinoflagellate cyst evidence for warm interglacial conditions at the northern barents sea margin during marine oxygen isotope stage 5 dinoflagellate cyst analysis has been conducted on a sediment core from the northern barents sea margin eastern arctic ocean to reconstruct sea surface conditions during marine oxygen isotope stage 5 cyst concentrations and composition of assemblages display a distinct variability reflecting deglacial warming and the onset of relatively warm interglacial conditions in early stage 5 dinoflagellate cyst assemblages of the last interglacial which corresponds broadly to substage 5e are comparable with those from recent sediments of the area but have lower abundances of cold water taxa such assemblages suggest that sea surface temperatures of the last interglacial were therefore at least as warm as today or even warmer after a distinct cooling relatively stable cold conditions with minor periods of surface water warming prevailed from mid substages 5d to 5b temperatures might have been similar in substages 5a and 5e but the different compositions of assemblages indicate a stronger stratification of surface waters probably owing to a larger meltwater supply in substage 5a these results are compared with previous studies on marine carbonate and calcareous microfossil records from the fram strait and yermak plateau area which show that the general view of a cold last interglacial in the eastern arctic ocean is untenable copyright 2001 john wiley sons ltd

tsr altered oil with high abundance thiaadamantanes of a deep buried cambrian gas condensate reservoir in tarim basin abstract the first exploratory well the zs1c well with 158 545 m 3 daily gas production was discovered in 6861 6944 m deep strata of the cambrian gypsolyte layer of the tarim basin china in 2014 the discovery opens a new target for the cambrian reservoired oil and gas exploration and directly leads to large scale oil and gas exploration of the deep reservoired cambrian oil and gas fields in the basin comprehensive two dimensional gas chromatography time of flight mass spectrometry and a comprehensive two dimensional gas chromatography flame ionization detector revealed the presence of abundant adamantane compounds 2 thiaadamantanes and 2 thiadiamantanes and a large amount of sulfur containing compounds in the condensate oil the formation of organic sulfur containing compounds such as 2 thiaadamantanes is an indication of sulfur incorporation from the gypsum in the stratum into oil and gas in the course of tsr this reservoir has apparently suffered severe tsr alteration because 1 high content of h 2 s 2 h 2 s sulfur isotopes 3 co 2 carbon isotopes and others abundant data to support this findings similar sulfur isotopic composition of h 2 s oil condensate and the gypsum in the cambrian strata indicate that the produced condensate is experienced tsr alteration therefore the deep accumulated cambrian oil reservoir has experienced severe tsr alteration and accumulated natural gas and condensate contains high sulfur content

non cracked oil in ultra deep high temperature reservoirs in the tarim basin china abstract thermal stability of liquid petroleum in the subsurface is closely linked to reservoir temperature most oil accumulations occur at temperatures 150 160 c leading to the dominance of gas condensate and free gas accumulations in ultra deep high temperature reservoirs the recently drilled fuyuan 1 exploration well northern tarim basin produced high quality non cracked single phase black oil from a carbonate reservoir located at maximum depth 7711 m and temperature 172 c this is the deepest oil discovery in china to date and among the deepest in the world the oil density 0 825 g cm3 at 20 c or api gravity 40 relatively low gas oil ratio 135 m3 m3 or 758 scf bbl low variety and abundance of adamantanes as well as lack of thiaadamantanes and dibenzothiophenes indicate that the oil was expelled from a source rock at moderate thermal maturity and has not been cracked the molecular and isotopic composition of oil associated gases are consistent with this interpretation we suggest that the oil remained uncracked because the residence time at temperatures 150 160 c was relatively short

late pleistocene debris flow deposits in large glacial lakes in british columbia and alaska the movement of late wisconsin 30 000 b p ice sheet lobes counter to the direction of regional drainage ponded large lakes up to 5000 km2 in both the upper fraser river valley in british columbia and the cooper river valley in alaska along the fraser river near quesnel glaciolacustrine sediment gravity flow complexes contain thick up to 10 m and laterally extensive up to 3 km diamict units containing large rafts of poorly lithified tertiary rocks diamicts offlap and thin away from tertiary bedrock highs and were emplaced by subaqueous debris flows these flows probably evolved from slumps and slides associated with the downslope failure of unstable tertiary bedrock slopes triggered by rising lake levels a process akin to first filling slope failures around the margins of man made lakes in alaska along the copper river valley massive and stratified diamicts were also deposited by debris flows and are interbedded with thin bedded turbidites poorly sorted gravels and normally graded diamicts debris flow deposits show basal grooves internal compressional structures and rafted clasts that protrude from bed tops flows originated by slumping of rapidly deposited and underconsolidated glacial sediments around the basin margins an important factor in flow generation has been the combination of steep substrate slopes and frequent earthquake shock observations in both british columbia and alaska stress the significance of sediment gravity flow processes acting to deposit glacial sedimentary successions in large ice contact lake basins

paleoecology of two marine oxygen isotope stage 7 sites correlated by the sheep creek tephra northwestern north america abstract pollen analysis at two sites correlated by the presence of the 190 000 yr old sheep creek tephra documents fluctuations in vegetation and climate consistent with this date and indicates that the records span marine oxygen isotope stage 7 and the stage 6 7 transition dawson cut near fairbanks alaska provides a 5 2 m long pollen record of interglacial boreal forest succeeded by shrub tundra and then forest tundra ash bend stewart river central yukon provides a 9 5 m long record of interglacial boreal forest succeeded by forest tundra shrub tundra and herbaceous tundra the replacement of forest at both sites by more open or tundra vegetation indicates warm interglacial conditions giving way to cold and arid climate it is not clear whether stage 7 was warmer than the present the warm cool warm climate oscillation evident at both sites may correlate to lake baikal substages 7a 7b and 7c sheep creek tephra fell on forest tundra vegetation

late quaternary paleoenvironmental records from the chatanika river valley near fairbanks alaska abstract perennially frozen deposits are considered as excellent paleoenvironmental archives similar to lacustrine deep marine and glacier records because of the long term and good preservation of fossil records under stable permafrost conditions a permafrost tunnel in the vault creek valley chatanika river valley near fairbanks exposes a sequence of frozen deposits and ground ice that provides a comprehensive set of proxies to reconstruct the late quaternary environmental history of interior alaska the multi proxy approach includes different dating techniques radiocarbon accelerator mass spectrometry ams 14 c optically stimulated luminescence osl thorium uranium radioisotope disequilibria 230 th u as well as methods of sedimentology paleoecology hydrochemistry and stable isotope geochemistry of ground ice the studied sequence consists of 36 m thick late quaternary deposits above schistose bedrock main portions of the sequence accumulated during the early and middle wisconsin periods the lowermost unit a consists of about 9 m thick ice bonded fluvial gravels with sand and peat lenses a late sangamon mis 5a age of unit a is assumed spruce forest with birch larch and some shrubby alder dominated the vegetation high presence of sphagnum spores and cyperaceae pollen points to mires in the vault creek valley the overlying unit b consists of 10 m thick alternating fluvial gravels loess like silt and sand layers penetrated by small ice wedges osl dates support a stadial early wisconsin mis 4 age of unit b pollen and plant macrofossil data point to spruce forests with some birch interspersed with wetlands around the site the following unit c is composed of 15 m thick ice rich loess like and organic rich silt with fossil bones and large ice wedges unit c formed during the interstadial mid wisconsin mis 3 and stadial late wisconsin mis 2 as indicated by radiocarbon ages post depositional slope processes significantly deformed both ground ice and sediments of unit c pollen data show that spruce forests and wetlands dominated the area the macrofossil remains of picea larix and alnus incana ssp tenuifolia also prove the existence of boreal coniferous forests during the mid wisconsin interstadial which were replaced by treeless tundra steppe vegetation during the late wisconsin stadial unit c is discordantly overlain by the 2 m thick late holocene deposits of unit d the pollen record of unit d indicates boreal forest vegetation similar to the modern one the permafrost record from the vault creek tunnel reflects more than 90 ka of periglacial landscape dynamics triggered by fluvial and eolian accumulation and formation of ice wedge polygons and post depositional deformation by slope processes the record represents a typical wisconsin valley bottom facies in central alaska

crustal accretion and dynamic feedback on mantle melting of a ridge centred plume the iceland case abstract a major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation in the case of iceland crustal thickness as large as 20 to 40 km has been reported crustal seismic velocities are high and have to be explained by thermal or chemical effects in the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting to study this question we solve the convection equations for a ridge centred plume with non newtonian rheology including melting melt extraction associated with deposition of cold crust at the surface of the model and using a simplified approach for compaction the assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km depending on the buoyancy flux of the plume crustal thicknesses between 10 and 40 km are modelled showing characteristic dipping structures resembling the rift ward dipping basaltic layers of east and western iceland comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg s based on the observed crustal thickness of iceland our models predict a plume buoyancy flux of 1140 kg s in the second part we study the accretion of the icelandic crust by a thermo mechanical model in more detail based on the navier stokes the heat transport and the mass conservation equations including volumetric sources hot 1200 c molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions a deep widespread emplacement of dykes and sills including crustal underplating b magma chambers at shallow to mid crustal level and c surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres we identify the material from the different source regions by a marker approach varying the relative dominance of these source regions characteristic crustal structures evolve showing shallow dipping upper crustal layers with dip angles between 10 and 15 the thermal structure of the crust varies between cold crust shallow source region dominating and hot crust deep source region dominating we use observations of maximum depth of seismicity to constrain the depth of the 650 c isotherm and seismological inferences on the lower crust to constrain temperatures in that region the best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion

precise chronology of the last interglacial period 234u 230th data from fossil coral reefs in the bahamas a detailed study of 238 u 234 u 230 th ages was made for different coral species from two bahamian reefs to determine the time scale of the sea level high during the last interglacial period using recently developed mass spectrometric techniques for the measurement of u and th isotopes thirty seven coral samples were analyzed in replicate typical errors at 125 ky are 1 5 ky 2 and 3 ky time intervals appear to be well resolved this high precision permits detailed chronologic study with time resolution adequate to define stages of the reef s history these data demonstrate that the time between transgression and regression of the ocean in the last interglacial was about 12 ky after which sea level fell rapidly at more than 2 m ky the high sea level stand began possibly by 132 ky and certainly by 129 ky ago when sea level reached 6 m above present mean low sea level high sea level was sustained until 120 ky and then fell rapidly this time period covers the milankovitch insolation peak at 65 n but is not sharply defined and the sea level high lasted from about 4 ky before the insolation peak to 8 ky after it we find no evidence for a double peak of sea level rise in the last interglacial episode that is high stands at both 125 and 142 ky there appears to be some discrepancy between this precise chronology for a coral reef on a tectonically stable platform and the chronology assigned to deep sea sediments the present results show that the duration of the last interglacial led to a 12 ky period of high sea level stand which is much longer than the insolation peak the study area consists of two sizeable bank barrier coral reefs of sangamon age that crop out along the coast northwest of cockburn town on san salvador island and at devil s point on great inagua island the initial stage of reef development is not revealed at either site but the main parts of both reefs are well preserved at cockburn town the beginning of the reef was marked by colonization of acropora palmata on hardground areas and possibly other corals at 132 ky ago patch reefs at the southeastern flank of the developing reef were growing by 129 to 126 ky ago by 126 to 123 ky ago the crest of the reef was formed by the frame building coral a palmata and associated patch reefs flourished along its flanks a palmata coral heads grew upward to a mean low sea level 6 m above the present one the reef began to decline as sea level dropped at about 123 to 120 ky ago near two measured profiles at devil s point the top of a rubblestone dominated by a cervicornis and patch reef corals preserved in growth position have been beveled off and represent an ancient wave cut surface 230 th ages on corals below and above this surface indicate that it was cut at about 125 1 ky ago

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

a framework for interpreting paleoclimatic variations in eastern beringia abstract paleoclimatic variations in a particular region can be viewed as the outcome of the superimposition of the effects of a number of large scale controls a framework for understanding paleoclimatic variations in a region can be established by considering the long term history of those controls jointly with climatic modeling results that illustrate how regional climates respond to changes of individual controls the relevant controls of late quaternary paleoclimatic variations in eastern beringia include 1 the size of the ice sheet which influences both atmospheric circulation and temperature 2 insolation which influences mainly temperature and seasonality 3 carbon dioxide concentration in the atmosphere which influences temperature and 4 feedbacks involving sea ice sea surface temperatures and snow cover which reinforce the insolation effects we review the results of a number of paleoclimatic experiments with general circulation models to illustrate the potential response of eastern beringia to the variations of these controls we then present a framework for understanding paleoclimatic variations in eastern beringia by describing the likely responses of the climate of this region to the known variations in the controls in addition we discuss key times during the quaternary when particular combinations of the controls give rise to natural experiments which lend themselves to the testing of particular hypotheses our understanding of the nature of climatic variations at high latitudes should increase through examination of paleoclimatic evidence that documents the response of the region at those key times

glacial isostatic adjustment constrains dehydration stiffening beneath iceland abstract during melting in the upper mantle the preferred partitioning of water into the melt will effectively dehydrate the solid residue linear extrapolation of laboratory experiments suggests that dehydration can produce a sharp viscosity contrast increase of a factor 500 across the dry solidus in this study we show that the suggested magnitude of dehydration stiffening in a plume ridge setting is incompatible with the present glacial isostatic adjustment gia in iceland using gps observations of current gia in iceland we find that the data are best fit by a viscosity contrast over the dry solidus in the range 0 5 3 a viscosity contrast higher than 10 requires a mantle viscosity below the dry solidus lower than 4 8 10 18 pa s depending on the thickness of the dehydrated layer a viscosity contrast of 100 or more demands a mantle viscosity of 10 18 pa s or less however we show here that a non linear extrapolation of the laboratory data predicts a viscosity contrast as low as a factor 3 29 assuming conditions of constant strain rate to constant viscous dissipation rate this is compatible with our gia results and suggests that the plume ridge interaction beneath iceland is governed by a non linear rheology and controlled by a combination of kinematic and dynamic boundary conditions rather than buoyant forces alone

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

a review of current research on tl dating of loess abstract the thick loess deposits of china central europe and north america are particularly suitable for the application of the thermoluminescence tl dating method because individual grains are likely to have travelled large distances and thus been well exposed to light before deposition wind blown silt grains collected close to glacial rivers in alaska have been shown to give a zero age of not more than 2 ka demonstrating the efficiency of the zeroing mechanism differences in laboratory procedures can cause differences in the tl ages reported particularly for loess over 100 ka tl ages for interstadial soils in europe are in agreement with the very few radiocarbon dates on reliable material such as charcoal on the other hand tl dates from sites in belgium and north western germany indicate that the previously accepted chronology which was based on two radiocarbon dates on humic rich sediments is incorrect the use of the last interglacial palaeosol for assessing the tl dates on loess above and below it is limited by the uncertainty in the timing of termination of soil forming processes 107 and 71 ka are the limits suggested by the astronomically derived time scale results on deposits from china north america alaska and europe are discussed in this context

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

a test of uranium series dating of fossil tooth enamel results from tournal cave france abstract a series of well preserved mammal bones and horse teeth was analyzed from archaeological levels of tournal cave magdalenian aurignacian and mousterain to test the hypothesis that well crystallized enamel behaves more as a closed system than does whole bone the isotopic composition of bones and tooth enamels from this deposit meet criteria for confidence and gave no reasons to suspect contamination or open system behavior two samples for which231pa could be analyzed showed internal concordance with the respective230th ages in spite of the favourable isotopic criteria however comparison of the u series ages of the bones and the tooth enamel with stratigraphic position and14c control indicated the dates were not meaningful in general both bones and tooth enamels gave ages too young although some were clearly too old neither group showed any systematic increase of age with stratigraphic depth tooth enamel therefore shows no advantage over bone for u series dating for this site in tournal cave both bones and enamel are apparently open to u which is probably cycling as a consequences of post depositional groundwater movement

last interglacial arctic warmth confirms polar amplification of climate change the warmest millennia of at least the past 250 000 years occurred during the last interglaciation when global ice volumes were similar to or smaller than today and systematic variations in earth s orbital parameters aligned to produce a strong positive summer insolation anomaly throughout the northern hemisphere the average insolation during the key summer months m j j was ca 11 above present across the northern hemisphere between 130 000 and 127 000 years ago with a slightly greater anomaly 13 over the arctic greater summer insolation early penultimate deglaciation and intensification of the north atlantic drift combined to reduce arctic ocean sea ice allow expansion of boreal forest to the arctic ocean shore across vast regions reduce permafrost and melt almost all glaciers in the northern hemisphere insolation amplified by key boundary condition feedbacks collectively produced last interglacial summer temperature anomalies 4 5 c above present over most arctic lands significantly above the average northern hemisphere anomaly the last interglaciation demonstrates the strength of positive feedbacks on arctic warming and provides a potentially conservative analogue for anticipated future greenhouse warming

identification of c24 and c25 lanostanes in tertiary sulfur rich crude oils from the jinxian sag bohai bay basin northern china abstract unusual short chain lanostanes c 24 and c 25 and c 30 lanostane were identified in sulfur rich crude oils from the jinxian sag bohai bay basin northern china besides the regular steranes c 27 30 a series of 4 methyl steranes c 22 23 c 27 30 4 4 dimethyl steranes c 22 24 c 28 30 short chain steranes c 23 26 abundant pregnanes c 21 22 and androstanes c 19 20 together with sulfur containing steroids 20 thienylpregnanes and thienylandrostanes were detected in the aliphatic and branched cyclic hydrocarbon fraction of these crude oils a literature survey of some long chain sterane analogues e g a nor steranes norcholestanes c 30 steranes lanostanes and pregnanes seems to point to a sponge and or dinoflagellate source 4 methyl 4 4 dimethyl steroids and lanosterols 4 4 14 trimethyl steroids as the basic skeleton of lanostanes can be derived from methanotrophic bacteria thus a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes c 23 26 4 methyl c 22 23 and 4 4 dimethyl steranes c 22 24 as well as lanostanes c 24 25 and c 30 analogues in our oil samples generally the steroids appear to have been extensively sulfurized with sulfur substitution at the c 22 position in the side chain during the early stage of diagenesis which was readily subject to attack by bacterial degradation enzymatic cleavage and or abiotic oxidation as a consequence short chain sterane analogues e g abundant pregnanes and androstanes in this study and short chain lanostanes c 24 c 25 might later be released through cleavage of weak c s bonds at the c 22 carbon in the sulfurized steroids and lanostane sulfides finally the formation of the short chain c 24 c 25 lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir

emergent marine record and paleoclimate of the last interglaciation along the northwest alaskan coast the last interglacial high sea level stand the pelukian transgression of isotope substage 5e is recorded along the western and northern coasts of alaska by discontinuous but clearly traceable marine terraces and coastal landforms up to about 10 m altitude the stratigraphy indicates that sea level reached this altitude only once during the last interglacial cycle from the type area at nome to st lawrence island in the bering sea to the eastern limit of the beaufort sea pelukian deposits contain extralimital faunas indicating that coastal waters were warmer than present amino acid ratios in molluscs from these deposits decrease to the north toward barrow consistent with the modern regional temperature gradient fossil assemblages at nome and st lawrence island suggest that the winter sea ice limit was north of bering strait at least 800 km north of its present position and the bering sea was perennially ice free microfauna in pelukian sediments recovered from boreholes indicate that atlantic water may have been present on the shallow beaufort shelf suggesting that the arctic ocean was not stratified and the arctic sea ice cover was not perennial for some period in coastal regions of western alaska spruce woodlands extended westward beyond their modern range and in northern alaska on the arctic coastal plain spruce groves may have entered the upper colville river basin the flaxman member of the gubik formation on the alaskan arctic coastal plain was deposited during marine isotope substage 5a and records the breakup of an intra stage 5 ice sheet over northwestern keewatin

isotope stage 5 130 74 ka in greenland a review abstract marine and glacigene deposits dated with varying degrees of certainty to isotope stage 5 130 74 ka are exposed in coastal cliffs in 18 areas and localities in northern and eastern greenland dating methods include thermoluminescence tl u th methods and correlation with the deep sea record amino acid parameters are used for correlation between localities deposits from the last interglaciation isotope substage 5e are rare and have been found only at three localities in both west and east greenland their mollusc and foraminifer faunas indicate more vigorous sea surface circulation and sea surface temperatures some degrees higher than known from the holocene during this warm period ice sheet growth began in northwest greenland the ice sheet reached its maximum for the last ice age during the succeeding phase provisionally correlated with isotope substage 5d this was followed by a new warm period with increased influx of warm subarctic water and high summer temperatures on land provisionally correlated with isotope substage 5a in east greenland the observations indicate a more complex ice sheet history the record from greenland is very similar to that obtained in adjacent coastal areas of the high arctic implying that the rapid shifts from warm to cold during isotope stage 5 were especially conducive to the formation of the embryonic north american and scandinavian ice sheets at high northern latitudes probably because the pulses of warm water were required to advect sufficient moisture into these areas to allow the growth of continental ice sheets

kinematic models for the thickness of oceanic crust at and near mid oceanic spreading centers 1 we consider analytical solutions for different cases of the one dimensional equation of mass transport as applied to an idealized mid oceanic spreading center in order to assess the conditions for the existence of thickness variations in oceanic crust and their possible magnitude models with an internally homogeneous crust suggest that in order to achieve the near constant crustal thickness of oceanic crust the zones of volcanic accretion and of deformation must be more or less identical especially if the far field plate velocity is not reached near the ridge in a jump like fashion but increases more slowly over a certain distance this indicates that accretion and strain are physically coupled if crustal accretion occurs in a narrower strip around the spreading center than does deformation there will be a maximum of thickness within the strain zone which is greater than the thickness far away from the ridge if the accretion zone is broader than the zone of deformation the near axis realms will have thinned crust throughout a discontinuity of the plate velocity across the spreading axis causes a central strip of strongly thinned crust and generally damps variations in crustal thickness an internally tripartite oceanic crust will almost unavoidably show some thickness variations near the spreading center transport by diffusive crustal flow is generally not of great importance at normal ridges but may have some significance in thickened crust near hot spots although the solutions are restricted to the steady state they show that the mere interaction of a few geometrical characteristics of the crust can produce a wide range of thickness variations in the regions near the spreading center

thermoluminescence age of old crow tephra in alaska thermoluminescence dates for the loess deposits above and below the old crow tephra at the halfway house site near fairbanks limit the age of the tephra to 86 8 ka this is in excellent agreement with other estimates already in the literature the dating was performed using additive dose procedures and optical selection of the potassium feldspar signal from a polymineral mixture of fine grains

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

on the dynamics of mid ocean ridges most extant flow models for ocean ridges suffer either from unrealistic geometry vertical conduit models or a stress singularity at the ridge axis corner flow models i present a model which avoids these difficulties by incorporating a weak plate boundary zone pbz between diverging surface plates an analytical solution for the flow of a constant viscosity asthenosphere and a coexisting melt phase is obtained in elliptic coordinates and used to predict the dynamically supported topography axial valley on the pbz the force required to pull the plates apart and the rate of melt extraction at the ridge stress in the asthenosphere increases linearly with spreading rate but decreases as the plates become thinner and the dynamics of ridge crests is controlled by the balance between these opposing effects the depth of the axial valley is controlled by the plate thinning effect and is therefore a decreasing function of spreading rate the depth of the axial valley on the mid atlantic ridge can be explained if the viscosity of the asthenosphere is 6 1019 1021 pa s and the lower surface of the plate is the 600 c isotherm the plate separation force is controlled by the spreading rate effect and therefore increases with spreading rate about five times as much force is required to maintain a fast spreading ridge 9 cm yr 1 half rate as a slow spreading ridge 1 cm yr 1 melt extraction can produce 6 km of basaltic crust if the volume fraction of melt present in the asthenosphere total degree of melting ranges from about 2 5 4 0 for a slow spreading ridge to 4 0 6 5 for a fast spreading ridge the dynamics of melt extraction does not depend significantly on the finite width of the pbz

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

geochemical characteristics of the zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the jixian sag of bohai bay basin abstract natural gas from the zhaolanzhuang field of the jizhong superdepression bohai bay basin contains the highest proportions of h 2 s 40 92 among the sour gases encountered in china the gas payzones include the eocene oligocene kongdian formation ek and the es4 member of the shahejie formation the sedimentary sequence consists of halite anhydrite carbonate sandstone and shale interbeds deposited in the evaporative brackish water lacustrine salt lake setting in the deepest part of the jinxian sag the total thickness of evaporites is more than 1000 m of which halite accounts for over 40 various organic rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window with a depth of 2500 3500 m and likely the source for the oil and sour gas in the zhaolanzhuang field the temperatures of the gas reservoirs range from 75 to 100 c too low for significant thermochemical sulfate reduction the co occurrence of abundant elemental sulfur with the sour gas and the 34 s values of the various sulfur containing compounds indicate that the h 2 s gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred

new perspectives on beringian quaternary paleogeography stratigraphy and glacial history abstract aspects of the paleogeography stratigraphy and glacial history of beringia have been greatly revised over the past 15 20 yr access to north east russia in particular has provided the opportunity to evaluate the beringia landscape as a contiguous subcontinent during the quaternary for the first time new research has made clearer the connection between tectonic forces and the submergence of the bering strait during the middle pliocene revisions in the regional stratigraphy of glacial and interglacial deposits in northwest alaska and northeast russia provide a new foundation for assessing the causes for differences in glacial ice extent through time the consensus of all field workers verifies that glacial ice throughout most of beringia was of very limited extent during the last glacial maximum the onset of regional glaciation during the waning stages of the last interglaciation is clearly out of phase with glaciation at lower latitudes despite the lack of much glacial activity during the early holocene alaska contains a rich record of late holocene glacial response to neoglacial cooling changes in the holocene environment of beringia likely had a profound affect on early inhabitants the curiosity driven vision and spirit of both david hopkins and the late troy pewe have had a profound influence on arctic paleoenvironmental research

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

age of pre late wisconsin glacial estuarine sedimentation bristol bay alaska pleistocene glacial estuarine sediment deposited in an intertidal environment of northeastern bristol bay southwestern alaska was dated using a variety of approaches including infrared stimulated and thermoluminescence irsl and tl techniques analysis of modern and14c dated holocene tide flat mud demonstrates that the bulk of sediment in this environment is reset by solar radiation thereby lending confidence to ages obtained from similar pleistocene deposits by luminescence techniques irsl seems to be especially well suited for dating with resolution on time scales of 10 000 yr the ages of tide flat mud of the nushagak formation derived from the ahklun mountains to the northwest of bristol bay and of halfmoon bay drift derived from the alaska peninsula to the southeast suggest contemporaneous glacial estuarine deposition related to independent glacial source areas about 75 000 80 000 yr ago this age is consistent with other geochronological data that indicate a pre late wisconsin and post substage 5e age including nonfinite14c ages a lack of interglacial indicators and old crow tephra 140 000 yr atop the drift normal paleomagnetic inclinations and amino acid isoleucine epimerization ratios aile ile aile ile ratios inportlandia arctica 0 052 0 003 from a marine lag horizon at south naknek beach which separates halfmoon bay drift above from older glacial estuarine drift below are only slightly higher than inmya truncata 0 041 0 007 from last interglacial pelukian deposits at nome as laboratory heating experiments show that the two genera epimerize at similar rates these data imply correlation of the marine lag at south naknek beach with pelukian deposits hence glaciers on the alaska peninsula experienced major pre late wisconsin advances both before and after the last interglaciation shells reworked into halfmoon bay drift yield aile ile ratios of 0 028 0 005 forportlandiaat second point and 0 027 0 001 forhiatella arcticaat etolin point together with assumptions about the postdepositional temperature history these ratios indicate that the shells are at least 55 000 yr and probably closer to 90 000 yr although the uncertainty in this age estimate is broad the amino acid and luminescence data converge on an age between about 75 000 and 90 000 yr late during oxygen isotope stage 5 for a major ice advance far beyond late wisconsin limits

unprecedented occurrence of novel c26 c28 21 norcholestanes and related triaromatic series in evaporitic lacustrine sediments a novel series of c26 c28 21 norcholestanes and their triaromatic analogs have been tentatively identified as major components in eogene evaporitic sediments in jinxian sag of the bohai bay basin northern china these compounds may be viewed as representatives of a new class of molecules whose biological or diagenetic origin remains to be determined their unprecedented high abundance in the evaporitic lacustrine sediments and related oils show important source and or depositional environment controls in addition to thermal maturity

late quaternary vegetational change in the kotzebue sound area northwestern alaska abstract two sediment cores from kaiyak and squirrel lakes in northwestern alaska yielded pollen records that date to ca 39 000 and 27 000 yr b p respectively between 39 000 and 14 000 yr b p the vegetation around these lakes was dominated by gramineae and cyperaceae with some salix and possibly betula nana glandulosa forming a local shrub component of the vegetation betula pollen percentages increased about 14 000 yr b p indicating the presence of a birchdominated shrub tundra alnus pollen appeared at both sites between 9000 and 8000yr b p and picea pollen mostly p mariana arrived at squirrel lake about 5000 yr b p the current foresttundra mosaic around squirrel lake was established at this time whereas shrub tundra existed near kaiyak lake throughout the holocene when compared to other pollen records from north western north america these cores 1 represent a meadow component of lowland beringian tundra between 39 000 and 14 000 yr b p 2 demonstrate an early holocene arrival of alnus in northwestern alaska that predates most other alnus horizons in northern alaska or northwestern canada and 3 show an east to west migration of picea across northern alaska from 9000 to 5000 yr b p

late quaternary pollen records from the kobuk and noatak river drainages northwestern alaska abstract pollen diagrams from joe and niliq lakes date to ca 28 000 and 14 000 yr b p respectively mesic shurb tundra grew near joe lake ca 28 000 to 26 000 yr b p with local populus populations prior to ca 27 000 yr b p shrub communities decreased as climate changed with the onset of itkillik ii glaciation 25 000 to 11 500 yr b p and graminoid dominated tundra characterized vegetation ca 18 500 to 13 500 yr b p herb tundra was replaced by shrub betula tundra near both sites ca 13 500 yr b p with local expansion of populus ca 11 000 to 10 000 yr b p and alnus ca 9000 yr b p mixed picea glauca p mariana woodland was established near joe lake ca 6000 yr b p these pollen records when combined with others from northern alaska and northwestern canada indicate 1 mesic tundra was more common in northwestern alaska than in northeastern alaska or northwestern canada during the duvanny yar glacial interval 25 000 to 14 000 yr b p 2 with deglaciation shrub betula expanded rapidly in northwestern alaska but slowly in areas farther east 3 an early postglacial thermal maximum occurred in northwestern alaska but had only limited effect on vegetation and 4 pollen patterns in northern alaska and northwestern canada suggest regional differences in late quaternary climates

continuous late quaternary proxy climate records from loess in beringia abstract loess deposits in eastern beringia contain continuous proxy records of the effects of past climatic change on terrestrial landscapes at high latitudes variations of environmental magnetism and sedimentology of high latitude loess deposits indicate that the timing and pattern of responses to local variations in wind intensity storminess and pedogenesis in eastern beringia closely resemble the pattern of global climate change during the late quaternary deduced from studies of marine and ice core records the age of paleoclimatic fluctuations permafrost features volcanic ash horizons buried forest layers and paleosols and other features of the eastern beringian loess record can be determined using a variety of quaternary dating methods tephrochronologic correlations between the loess record and the glacial history of eastern beringia indicate the delta glaciation occurred during marine isotope stage 6 several other middle and late quaternary glaciations across eastern beringia can be tephrochronologically tied to the loess record and appear to have been in phase with episodes of global cooling recorded in deep sea records

early wisconsinan mis 4 arctic ground squirrel middens and a squirrel eye view of the mammoth steppe abstract fossil arctic ground squirrel spermophilus parryii middens were recovered from ice rich loess sediments in association with sheep creek klondike and dominion creek tephras ca 80 ka exposed in west central yukon these middens provide plant and insect macrofossil evidence for a steppe tundra ecosystem during the early wisconsinan mis 4 glacial interval midden plant and insect macrofossil data are compared with those previously published for late wisconsinan middens dating to 25 2914c ka bp mis 3 2 from the region although multivariate statistical comparisons suggest differences between the relative abundances of plant macrofossils the co occurrence of steppe tundra plants and insects e g elymus trachycaulus kobresia myosuroides artemisia frigida phlox hoodii connatichela artemisiae provides evidence for successive reestablishment of the zonal steppe tundra habitats during cold stages of the late pleistocene arctic ground squirrels were well adapted to the cold arid climates steppe tundra vegetation and well drained loessal soils that characterize cold stages of late pleistocene beringia these glacial conditions enabled arctic ground squirrel populations to expand their range to the interior regions of alaska and yukon including the klondike where they are absent today arctic ground squirrels have endured numerous quaternary climate oscillations by retracting populations to disjunct interglacial refugia during warm interglacial periods e g south facing steppe slopes well drained arctic and alpine tundra areas and expanding their distribution across the mammoth steppe biome during cold arid glacial intervals

lithospheric structure and compensation mechanisms of the galapagos archipelago volcanic islands of the galapagos archipelago are the most recent subaerial expression of the galapagos hotspot these islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young 10 m y oceanic lithosphere just south of the active galapagos spreading center the 91 w fracture zone crosses the platform and creates an estimated 5 m y age discontinuity in the lithosphere major tectonic features of the galapagos include an unusually broad distribution of volcanic centers pronounced structural trends such as the nw se wolf darwin lineament wdl and a steep escarpment along the western and southern margins of the archipelago we use shipboard gravity and bathymetry data along with geosat geoid data to explain the tectonic and structural evolution of the galapagos region we model the gravity anomalies using a variety of compensation models including airy isostasy continuous elastic flexure of the lithosphere and an elastic plate with embedded weaknesses and we infer significant lithospheric strength variations across the archipelago the outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of 12 km this area includes the large shield volcanoes of fenandina and isabela islands where the lithosphere regionally supports these volcanic loads the central platform is weaker with an elastic thickness of 6 km or less and close to airy isostasy the greatest depths to the moho are located beneath eastern isabela island and the central platform thinner lithosphere in this region may account for the broad distribution of volcanoes the extended period of eruption of the central volcanoes and their reduced size the transition from strong to weak lithosphere along the southern escarpment appears to be abrupt within the resolution of our models and can be best represented by a free end or faultlike discontinuity also modeling the wdl as a lithospheric fault increases the match to the observed gravity anomalies the primary feature is the weak central platform whose existence is a logical convergence of several factors the 91 w transform age offset the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping and the formation of the central platform at a spreading ridge

last interglacial plant macrofossils and climates from ziegler reservoir snowmass village colorado usa abstract ninety plant macrofossil taxa from the ziegler reservoir fossil site near snowmass village colorado record environmental changes at high elevation 2705 m asl in the rocky mountains during the last interglacial period present day vegetation is aspen forest populus tremuloides intermixed with species of higher picea abies and lower artemisia quercus elevations stratigraphic units 4 13 contain montane forest taxa found near the site today and several species that today generally live at lower elevations within abies concolor lycopus americanus and outside colorado najas flexilis these data suggest near modern climatic conditions with slightly warmer summer and winter temperatures this montane forest period was succeeded by a shorter treeless interval unit 14 representing colder and or drier conditions in units 15 16 conifer trees reoccur but deciduous and herb taxa are lacking suggesting a return to warmer conditions although cooler than during the earlier forest period comparison of these inferred paleoclimatic changes with the site s geochronologic framework indicates that the lower interval of sustained warmth correlates with late mis 6 early 5b 138 94 ka the cold interval with mis 5b 94 87 ka and the uppermost cool assemblages with mis 5a 87 77 ka

strontium isotopic composition of pliocene and pleistocene molluscs from emerged marine deposits north american arctic high precision strontium sr isotopic measurements were obtained for 53 pliocene and pleistocene molluscan shells from emerged marine deposits around the coasts of arctic north america to test whether such data can be used for chronostratigraphic purposes 87sr 86sr ratios from sr isotopic measurements on many marine fossils from arctic ocean borderland sites are broadly consistent with their expected values based on independent age control and on a comparison with the sr isotopic evolution of seawater recorded in deep sea cores all 87sr 86sr ratios measured for shells from middle and late pleistocene deposits are consistent with expected values but only 9 of 22 ratios in shells from older deposits are consistent with independent age estimates aberrant 87sr 86sr ratios are consistently higher than expected at nome alaska and baffin island canada all 87sr 86sr ratios are higher than expected because these shells were formed along the pacific ocean and atlantic ocean margins respectively their h

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

pre late wisconsin glacial history coastal ahklun mountains southwestern alaska new amino acid thermoluminescence and 40ar 39ar results abstract new stratigraphic and geochronologic data from the togiak bay area of southwestern alaska indicate that glaciers advanced from the southern ahklun mountains at least three and as many as six times prior to the late wisconsin the oldest glaciations are represented by glacial marine sediment in coastal exposures on hagemeister island the extent of amino acid isoleucine epimerization in fossil molluscs indicates that at least one and possibly four older middle pleistocene glacial intervals are represented with age estimates spanning 500 280 ka and averaging 400 100 ka the youngest glacial marine drift on hagemeister island may correlate with the eruption of the togiak tuya a new 40 ar 39 ar age on basalt that overlies pillow lava indicates that the volcano erupted through glacial ice at least 300 m thick 263 22 ka the youngest drift in the region overlies the old crow tephra 140 10 ka and a 70 10 ka basaltic lava flow dated by thermoluminescence analysis of underlying baked sediment the drift delimits flat piedmont lobes that spread out onto the continental shelf and terminated 100 km from their source areas during the early wisconsin sensu lato the glacial geologic evidence suggests that major expansions of glaciers were out of phase with global ice volume

the last interglacial stage definitions and marine highstand north america and eurasia abstract delineation of the boundary between the last interglacial lig and the last wisconsinan glacial stage in north america represents a critical yet unresolved issue subdivisions of the late pleistocene are based on oxygen isotope ice cover and pollen stratigraphic data boundaries defined by isotope chronology hinge on complex interrelationships between 18o in foraminifer tests ice volumes stored on land and coeval sea level position in the absence of adequate pollen stratigraphic documentation pleistocene subdivision boundaries were harder to establish in north america than in europe time transgressive pollen zones revealed increased lengths of the climatically floristically defined lig from the european subarctic to the mediterranean conflicting definitions of sangamon as representing only the last interglacial of minimum ice cover and higher temperatures or broadly defined sensu lato also incorporating early part of the last wisconsinan glacial stage persist in the north american literature the exclusively interglacial age of the sangamon geosol originally used in dating the sangamonian stage proved untenable designation of an eowisconsinan interval corresponding to susbtages mis 5d a also lacks merit despite climate and vegetation related discrepancies pollen and coastal deposit based comparisons between europe and north america during mis 5 and the holocene are useful in establishing the climate history of the north american sangamonian and subsequent early wisconsinan substages an overarching mis 5 cooling trend represented by scattered subarctic and high mountain ice accumulation events followed the mis 5e eemian sangamonian temperature peak adoption of the general european practice that asymmetrically splits mis 5 into a short mis 5e interglacial and a long early wisconsinan glacial mis 5d a interval is preferred in north america as well subdivisions in the normalized 18o curve that serve as the chronological framework and the wealth of european pollen data support this approach while multiple pre sangamon pleistocene marine paralic intervals do occur on the nw gulf coast all pre sangamon pleistocene marine and brackish inshore deposits had been removed by erosion in the ne coastal plain a single inshore nearshore marine sediment and highstand interval is well documented in this region the lig highstand sequence correlates with varied eemian marine and paralic mis 5e deposits encountered along northern and western european siberian and additional shores apart from reliably dated sangamonian s florida coral reefs identification and dating of lig highstand deposits remain highly problematical in se atlantic shore terraces

last interglacial age of the eva forest bed central alaska from thermoluminescence dating of bracketing loess abstract the eva forest bed in the yukon tanana upland of east central alaska represents a frozen buried boreal forest this bed consists of well preserved peat lenses sticks roots logs as well as rooted and unrooted stumps of trees where preserved the eva forest bed is overlain by the goldstream formation and underlain by gold hill loess almost all exposures have a prominent unconformity at the top of the gold hill loess even where the forest bed is not preserved attempts have been made over the last 50 yr to determine the age of this bed numerically we report here details of thermoluminescence tl dating of 14 samples of loess from above and below this unconformity in the fairbanks area together with knowledge about the climatic indicators from the eva forest bed these tl age estimates 1 indicate that the most probable age of the eva forest bed is 125 ka the time of the warmest part of oxygen isotope stage 5 and 2 confirm stratigraphic inferences from tephra beds in the upper gold hill loess that erosion has removed variable thicknesses of the original loess complex just prior to the development of the eva forest bed

origin of sulfur rich oils and h2s in tertiary lacustrine sections of the jinxian sag bohai bay basin china abstract very high s oils up to 14 7 with h 2 s contents of up to 92 in the associated gas have been found in the tertiary in the jinxian sag bohai bay basin pr china several oil samples were analyzed for c and s stable isotopes and biomarkers to try to understand the origin of these unusual oil samples the high s oils occur in relatively shallow reservoirs in the northern part of the jinxian sag in anhydrite rich reservoirs and are characteristic of oils derived from s rich source rocks deposited in an enclosed and productive stratified hypersaline water body in contrast low s oils as low as 0 03 in the southern part of the jinxian sag occur in tertiary lacustrine reservoirs with minimal anhydrite these southern oils were probably derived from less s rich source rocks deposited under a relatively open and freshwater to brackish lake environment that had larger amounts of higher plant inputs the extremely high s oil samples 10 underwent biodegradation of normal alkanes resulting in a degree of concentration of s in the residual petroleum although isoprenoid alkanes remain showing that biodegradation was not extreme interestingly the high s oils occur in h 2 s rich reservoirs h 2 s up to 92 by volume where the h 2 s was derived from bacterial so 4 reduction most likely in the source rock prior to migration three oils in the jinxian sag have 34 s values from 0 3 to 16 2 and the oil with the highest s content shows the lightest 34 s value this 34 s value for that oil is close to the 34 s value for h 2 s 0 it is possible that h 2 s was incorporated into functionalized compounds within the residual petroleum during biodegradation at depth in the reservoir thus accounting for the very high concentrations of s in petroleum

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

tephrochronology and paleoclimatology of the last interglacial glacial cycle recorded in alaskan loess deposits abstract alaskan loess deposits contain long paleoclimatic records analogous to those found in marine and lacustrine sediments and in ice sheets the sedimentology and geophysical properties of alaskan loess were influenced by climatically forced changes in wind intensity storminess temperature and precipitation loess deposits also contain evidence of episodic permafrost and paleosol formation explosive volcanic eruptions paleoecologic changes in high latitude regions quaternary fossils and early human sites and artifacts the old crow tephra ca 140 000 10 000 bp dates the transition between full glacial conditions and the last interglaciation in central alaska climatic amelioration in alaska apparently began prior to astronomically forced increases in insolation a finding consistent with the well dated devil s hole record however time series analysis of loess proxy climate data also demonstrates the effects of orbital precession and obliquity on alaskan climate change numerous short term climate events are recorded in alaskan loess deposits including a climate reversal corresponding to the younger dryas