late cenozoic arctic ocean sea ice and terrestrial paleoclimate sea otter remains found in deposits of two marine transgressions bigbendian and fishcreekian of the alaskan arctic coastal plain which occurred between 2 4 and 3 ma suggest that during these two events the southern limit of seasonal sea ice was at least 1600 km farther north than at present in alaskan waters perennial sea ice must have been severely restricted or absent and winters were warmer than at present during these two sea level highstands paleomagnetic faunal and palynological data indicate that the later transgression fishcreekian occurred during the early part of the matuyama reversed polarity chron amino acid diagenesis in fossil mollusks suggests that since the later transgression the effective diagenetic temperature edt in the deposits has been about 16 c which is about 7 c colder than modern values and slightly colder than the edt calculated for the past 125 ka such a low edt suggests that permafrost and perennial sea ice have been present nearly continuously since this transgression permafrost probably was absent however during the earlier bigbendian transgression permafrost and extensive perennial sea ice may have been initiated during the late stages of climatic cooling that spanned the gauss normal matuyama reversed polarity chron boundary and led into the first major late cenozoic glaciation of the northern hemisphere

thermal and compositional evolution of the martian mantle effects of water abstract we present numerical models of the thermochemical evolution of the mantle of mars with particular attention paid to the effects of water with a two dimensional anelastic compressible convection algorithm in combination with a parameterized model of composition and thermoelastic properties we consider how water in nominally anhydrous minerals may have influenced the melting and rheology of the martian mantle and how it may have been redistributed by melting melt extraction and convection over a period of 4 gy the strongest effect of water is found to be a marked increase in convective vigor due to the lower viscosity which leads to faster cooling of the mantle but nonetheless enhances melt production modeling results are compared with observations and independent inferences from spacecraft observations and martian meteorite analyses some models with radionuclide concentrations from wanke and dreibus 1994 but with initial mantle water concentrations between their estimate of 36 ppm by weight and 10 times this value combined with 50 degassing of crust forming material partly match observed crustal concentrations of k th and water in order to account for the observed chemical variation in crustal rocks however the formation and survival of ancient mantle heterogeneities appears to have been necessary water loss from the mantle since the end of the magma ocean stage has been minor elevated water contents notwithstanding the depth variation of viscosity in the sublithospheric mantle of a given model is likely minor depths to the curie temperature at the end of the noachian range from 40 to 60 km for magnetite and hematite in the models and present day total crustal thicknesses lie between 75 and 100 km the depth to the brittle ductile transition increases from values around 30 km in the early stages of the models to 170 240 km at present an estimate of total core entropy renders the existence of a basal perovskite ferropericlase layer unlikely the absence of stable superplumes in all of our preferred models suggests that the volcanic provinces of tharsis and elysium were formed by a succession of more short lived events such as transient plumes from specific regions at the core mantle boundary

mixing of heterogeneities in the mantle effect of viscosity differences the effect of viscosity variations associated with large scale chemical heterogeneities on mixing in the mantle is studied for a model two dimensional problem low viscosity regions are rapidly deformed develop long tendrils and are entrained towards regions of surface divergence very viscous regions are only slowly stretched thus geochemical reservoirs can persist relatively undisturbed for long periods of time if they are o 10 100 times more viscous than the surrounding mantle high viscosity ratio blobs have a tendency to aggregate leading to the formation of large scale heterogeneities from smaller ones

correlated lightning mapping array and radar observations of the initial stages of three sequentially triggered florida lightning discharges 1 correlated lightning mapping array and vertical scan radar images are presented for three rocket and wire triggered lightning flashes that occurred sequentially within 17 min in the presence of a decaying multicellular convective storm system over north central florida the initial stage is of each flash propagated generally vertically to the altitude of the 0 c melting level about 5 km and then subsequently propagated for many kilometers horizontally along the melting level contour radar images suggest that the propagation paths of the is channels below and above the melting level were heavily influenced by precipitation gradients flash uf 11 24 exhibited a 12 6 km unbranched is channel the longest unbranched channel observed in the study by a factor of three during flash uf 11 25 119 ms following the cessation of the measured is current at ground and prior to the first return stroke a natural cloud to ground discharge perhaps induced by the is initiated between 2 5 and 4 km altitude and struck ground 5 to 7 km from the launching facility the is of flash uf 11 26 propagated upward through a descending precipitation packet and apparently induced a naturally appearing bi level intracloud discharge via an upward negative leader that initiated within the is breakdown region 3 5 km from the launching facility the upward negative leader propagated from 5 6 to 9 3 km altitude in a time of 11 ms the electrical current measured at ground during the is of flash uf 11 26 exhibited a 57 ms polarity reversal transferring 19 c of positive charge to ground

comparing e field changes aloft to lightning mapping data 1 a newly developed balloon borne instrument contains electric and magnetic sensors for determining how lightning alters electric field vectors relative to a coordinate system fixed with the earth by combining data from this instrument with results from the compact lightning mapping array lma at langmuir laboratory in central new mexico and the national lightning detection network nldn charge transported to ground by several strokes in a cloud to ground cg lightning flash can be quantified as the flash progresses the locations of the charge centers drained by successive strokes are seen to move further from the ground strike point using this new instrument and two different models to map lma source points onto charge centers the charge transported in an intracloud ic flash is also estimated details of the instrument design and data analysis are also presented

onset of hudson strait heinrich events in the eastern north atlantic at the end of the middle pleistocene transition 640 ka 1 heinrich events are well documented for the last glaciation but little is known about their occurrence in older glacial periods of the pleistocene here we report scanning xrf and bulk carbonate 18o results from integrated ocean drilling program site u1308 reoccupation of deep sea drilling project site 609 that are used to develop proxy records of ice rafted detritus ird for the last 1 4 ma ca sr is used as an indicator of ird layers that are rich in detrital carbonate i e heinrich layers whereas si sr reflects layers that are poor in biogenic carbonate and relatively rich in detrital silicate minerals a pronounced change occurred in the composition and frequency of ird at 640 ka during marine isotope stage mis 16 coinciding with the end of the middle pleistocene transition at this time hudson strait heinrich layers suddenly appeared in the sedimentary record of site u1308 and the dominant period of the si sr proxy shifted from 41 ka prior to 640 ka to 100 ka afterward the onset of heinrich layers during mis 16 represents either the initiation of surging of the laurentide ice sheet lis off hudson strait or the first time icebergs produced by this process survived the transport to site u1308 we speculate that ice volume i e thickness and duration surpassed a critical threshold during mis 16 and activated the dynamical processes responsible for lis instability in the region of hudson strait we also observe a strong coupling between ird proxies and benthic 13c variation at site u1308 throughout the pleistocene supporting a link between iceberg discharge and weakening of thermohaline circulation in the north atlantic

live stained deep sea benthic foraminiferans in the western weddell sea trends in abundance diversity and taxonomic composition along a depth transect entire live foraminiferal assemblages i e including soft walled species were analysed using replicate subcores 3 45 cm2 surface area 0 1 cm layer 63 m fraction from multiple corer samples collected along a transect 1100 5000 m water depth of the continental slope and rise and adjacent abyssal plain to the east of the antarctic peninsula foraminiferans usually accounted for 61 83 of all meiofaunal organisms mean densities were highest 576 individuals indiv 10 cm 2 at 2100 m water depth and declined to 240 indiv 10 cm 2 at 5000 m although there was considerable variability between replicates at some stations e g 304 333 1090 indiv 10 cm 2 at 2100 m foraminiferan and metazoan meiofaunal densities were broadly coherent across the depth range sampled a total of 158 live foraminiferal species was recognized assemblages were most diverse on the lower slope with species numbers peaking at 3100 m and the fisher diversity index at 3100 4100 m monothalamous taxa increased in relative abundance from 8 at 1100 m to 33 at 4975 m and there was a corresponding decrease in the proportion of calcareous taxa from 65 to 16 on the continental slope 1100 3100 m the most abundant hard walled species were epistominella exigua adercotryma glomeratum alabaminella weddellensis and stetsonia hovarthi in the deeper samples 4100 5000 m a glomeratum became the dominant hard walled species important soft walled species included tinogullmia riemanni and bathyallogromia weddellensis in some samples from 1100 and 2100 m more than a third of the live assemblage was hidden within phytodetrital aggregates species such as e exigua a weddellensis and t riemanni were concentrated within these microhabitats many of the weddell sea species are typical bathyal and abyssal forms well known from the north atlantic and elsewhere the phytodetrital assemblages are strikingly similar to those reported from abyssal sites in the north atlantic our observations suggest that there are close faunal links between the deep water faunas of the weddell sea and those of other oceanic regions perhaps mediated by thermohaline circulation

constraints on the pleistocene chronology of sediments from the lomonosov ridge 1 despite its importance in the global climate system age calibrated marine geologic records reflecting the evolution of glacial cycles through the pleistocene are largely absent from the central arctic ocean this is especially true for sediments older than 200 ka three sites cored during the integrated ocean drilling program s expedition 302 the arctic coring expedition acex provide a 27 m continuous sedimentary section from the lomonosov ridge in the central arctic ocean two key biostratigraphic datums and constraints from the magnetic inclination data are used to anchor the chronology of these sediments back to the base of the cobb mountain subchron 1215 ka beyond 1215 ka two best fitting geomagnetic models are used to investigate the nature of cyclostratigraphic change within this chronology we show that bulk and mineral magnetic properties of the sediments vary on predicted milankovitch frequencies these cyclic variations record glacial and interglacial modes of sediment deposition on the lomonosov ridge as evident in studies of ice rafted debris and stable isotopic and faunal assemblages for the last two glacial cycles and were used to tune the age model potential errors which largely arise from uncertainties in the nature of downhole paleomagnetic variability and the choice of a tuning target are handled by defining an error envelope that is based on the best fitting cyclostratigraphic and geomagnetic solutions

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

variability of sea surface temperature and sea ice cover in the fram strait over the last two millennia abstract a sediment core located on the west spitzbergen margin in the fram strait 78 54 931 n 6 46 005 e water depth 1497 m was analyzed for its dinocyst content in order to reconstruct hydroclimatic variations of the last 2500 years the relative abundance of dinocyst taxa and principal component analysis show a major transition at about 300 cal years bp it is characterized by the disappearance of thermophilic taxa spiniferites mirabilis hyperacanthus and impagidinium sphaericum and the increase of polar subpolar taxa impagidinium pallidum and pentapharsodinium dalei sea surface temperature sst estimates suggest warmer conditions than present anomaly 2 c averaging at 7 c in summer until 300 cal years bp although cooling pulses are recorded around 1700 1500 1200 and 800 cal years bp the last 300 years were marked by a cooling from 7 6 to 3 5 c and sea ice cover increasing up to 7 months yr the results demonstrate that the fram strait area is sensitive to hydroclimatic variations notably with respect to sea ice and ssts which are linked to the relative strength of northward flow of north atlantic waters to the east and southward outflow of cold and fresh waters from the arctic ocean based on our data the warmest part of our record around 1320 cal years bp is the only interval of the last 2500 years that provides a possible analogue for the modern post ad 2000 interval which is characterized by sea ice free conditions

biostratigraphy and late cenozoic paleoceanography of the arctic ocean foraminiferal lithostratigraphic and isotopic evidence detailed studies of benthonic foraminifera stable isotopes and lithofacies in cores from the southeastern alpha ridge central arctic ocean reveal some new aspects of arctic ocean paleoceanography high ratios of benthonic to planktonic foraminifera are found in most of the quaternary sediment units and ratios of 1 1 appear to characterize the arctic deep water sediments benthonic foraminifera in the carbonate mud unit m show a succession of calcareous species reflecting increased influx of norwegian sea bottom water to the arctic ocean during the past 0 4 m y foraminiferal and lithological data indicate less uniform sedimentation during a warmer interval from 0 4 to 0 6 ma when most of the silty lutite unit l was deposited at the cesar site lower pleistocene units j to i contain less limestone and more dolomite and they contain a uniform faunal assemblage with low numbers of calcareous foraminifera upper pliocene units h to ab contain rare limestone and relatively large amounts of do lomite and quartz sand middle to upper pliocene units ab to a3 are marked by abundant sand sized ferromanganese coated particles which in many cases have a silt nucleus hence much of the coarse sand in these units does not indicate increased ice rafting the pliocene sediments mostly contain a low diversity assemblage of agglutinated foraminifera but a mixed calcareous arenaceous fauna occurs in a short interval above the matuyama gauss boundary 2 4 ma stable isotopic curves occur within sequences which broadly correspond to stages 1 9 of the global record below stage 9 the record is discontinuous strong vertical mixing apparently prevailed during most of the pliocene and early pleistocene then decreased during the past 0 4 m y owing to damping by a perennial ice cover isotopic and foraminiferal data however suggest that an interval of perennial sea ice also occurred during the late pliocene at the time of the earliest glacial event recorded in the north atlantic

three dimensional imaging of upward positive leaders in triggered lightning using vhf broadband digital interferometers 1 upward positive leaders upls in two artificially initiated lightning flashes were imaged in three dimensions using vhf broadband digital interferometers and a high speed video camera with time synchronized channel base current measurements locatable vhf sources of the two upls began at 1 1 km and 1 5 km a few milliseconds after the upl inception and ascended to 2 4 km and 3 7 km respectively with average 3 d speeds on the order of 106 ms 1 the initial stage currents for both flashes were unusually large and had peak values of 6 ka and 18 ka vhf sources associated with positive leader propagation were located when the average current was higher than 3 ka and had significant pulse activity the source altitudes and channel base currents suggest that there might have been a region of significant negative charge at altitudes from 2 to 4 km which is below the freezing level of typical thunderstorms in florida

cenozoic record of elongate cylindrical deep sea benthic foraminifera in the north atlantic and equatorial pacific oceans abstract in the late pliocene middle pleistocene a group of 95 species of elongate cylindrical deep sea lower bathyal abyssal benthic foraminifera became extinct this extinction group ext gp belonging to three families all the stilostomellidae and pleurostomellidae some of the nodosariidae was a major component 20 70 of deep sea foraminiferal assemblages in the middle cenozoic and subsequently declined in abundance and species richness before finally disappearing almost completely during the mid pleistocene climatic transition mpt so what caused these declines and extinction in this study 127 ext gp species are identified from eight cenozoic bathyal and abyssal sequences in the north atlantic and equatorial pacific oceans most species are long ranging with 80 originating in the eocene or earlier the greatest abundance and diversity of the ext gp was in the warm oceanic conditions of the middle eocene early oligocene the group was subjected to significant changes in the composition of the faunal dominants and slightly enhanced species turnover during and soon after the rapid eocene oligocene cooling event declines in the relative abundance and flux of the ext gp together with enhanced species loss occurred during middle late miocene cooling particularly at abyssal sites the overall number of ext gp species present began declining earlier at mid abyssal depths in middle miocene than at upper abyssal in late pliocene early pleistocene and then lower bathyal depths in mpt by far the most significant ext gp declines in abundance and species loss occurred during the more severe glacial stages of the late pliocene middle pleistocene clearly the decline and extinction of this group of deep sea foraminifera was related to the function of their specialized apertures and the stepwise cooling of global climate and deep water we infer that the apertural modifications may be related to the method of food collection or processing and that the extinctions may have resulted from the decline or loss of their specific phytoplankton or prokaryote food source that was more directly impacted than the foraminifera by the cooling temperatures

convective mixing in the mantle the role of viscosity differences summary the fate of the subducted oceanic crust has been a subject of controversy since the hypothesis of plate tectonics was first proposed there is now considerable evidence to support the mantle mixing hypothesis according to which the subducted oceanic lithosphere is heated and softened and then becomes entrained in the convective flows in the mantle normal strains stretch and thin the oceanic crust until diffusion processes complete the homogenization since solid state diffusion processes are effective only on the scale of centimetres then 6 km thick oceanic crust must be stretched by a factor of lo5 or more before losing its identity direct observational evidence for this hypothesis comes from studies of high temperature peridotite massifs in these massifs pyroxenite layers with a basaltic composition are embedded in a lherzolitic matrix the pyroxenite layers with thicknesses varying from centimetres to a few metres may be identified with the stretched subducted oceanic crust with the aim of improving our understanding of the role of convective mixing we have carried out a study of the influence of viscosity differences on the stretching process we have obtained equations that describe the stretching of a symmetrical two dimensional body of highly viscous fluid embedded in a normal strain flow these can be solved in closed form when the body is elliptical and asymptotically for a general shape we find that for mantle conditions viscosity differences are unlikely to inhibit the stretching of the subducted oceanic crust significantly

aminostratigraphic correlations and paleotemperature implications pliocene pleistocene high sea level deposits northwestern alaska abstract multiple periods of late pliocene and pleistocene high sea level are recorded by surficial deposits along the coastal plains of northwestern alaska analyses of the extent of amino acid epimerization in fossil molluscan shells from the nome coastal plain of the northern bering sea coast and from the alaskan arctic coastal plain of the chukchi and beaufort sea coasts allow recognition of at least five intervals of higher than present relative sea level three late pliocene transgressions are represented at nome by the complex and protracted beringian transgression and on the arctic coastal plain by the colvillian bigbendian and fishcreekian transgressions these were followed by a lengthy period of non marine deposition during the early pleistocene when sea level did not reach above its present position a middle pleistocene high sea level event is represented at nome by the anvilian transgression and on the arctic coastal plain by the wainwrightian transgression anvilian deposits at the type locality are considerably younger than previously thought perhaps as young as oxygen isotope stage 11 410 000 bp finally the last interglacial pelukian transgression is represented discontinuously along the shores of northwestern alaska amino acid epimerization data together with previous paleomagnetic measurements radiometric age determinations and paleontologic evidence provide geochronological constraints on the sequence of marine deposits they form the basis of regional correlations and offer a means of evaluating the post depositional thermal history of the high sea level deposits provisional correlations between marine units at nome and the artic coastal plain indicate that the temperature difference that separates the two sites today had existed by about 3 0 ma since that time the effective diagenetic temperature was lowered by about 3 4 c at both sites and the mean annual temperature was lowered considerably more this temperature decrease was largely accomplished by the close of the fishcreekian beringian iii transgression ca 2 5 2 1 ma since then intervals of warm temperature must have been extremely brief these data suggest that the steep latitudinal temperature gradient and the frigid temperatures that characterize the high latitudes of alaska today are ancient features of arctic climate

expanded record of quaternary oceanographic change amerasian arctic ocean four sediment cores collected from the northwind and mendeleyev ridges arctic ocean from 1089 m to 1909 m water depth provide an oceanographic record extending back into the matuyama reversed polarity chron benthic foraminiferal analyses show four prominent assemblage zones bolivina arctica cassidulina teretis bulimina aculeata and oridorsalis tener from the upper matuyama reversed polarity chronozone through the brunhes normal polarity chronozone these assemblage zones represent depth dependent benthic foraminiferal biofacies changes associated with oceanographic events that occurred in the amerasian basin at 780 and 300 ka and indicate oceanographic influence from the north atlantic recognition of these benthic assemblage zones in arctic cores from the alpha ridge indicates that the benthic foraminiferal zonations in intermediate to deep water 1000 m arctic cores may be more useful than preexisting lithostratigraphic zonations and should provide important information pertaining to the quaternary paleoceanographic evolution of the arctic ocean

arctic north american seasonal temperatures from the latest miocene to the early pleistocene based on mutual climatic range analysis of fossil beetle assemblages late tertiary and early quaternary fossil beds in the arctic regions of north america have yielded abundant well preserved remains of plants and arthropods documenting the existence of coniferous forests in the high arctic latitudes nearly all of the beetle coleoptera specimens in these fossil assemblages represent extant species we have applied the mutual climatic range mcr method of paleotemperature analysis to fossil beetle assemblages from 11 sites to estimate mean summer t max and winter t min temperatures we found that arctic t min values during the latest miocene and pliocene were substantially warmer than they are today the mcr estimates therefore support the scenario derived from the paleobotanical data namely that arctic pliocene climates were far less continental several pliocene age assemblages from the high arctic yielded t max estimates 9 10 c warmer than modern values at the sites this is the same degree of warming required to allow coniferous forests to grow in the high arctic by 3 million years bp a cooling trend is marked in the paleobotanical and fossil beetle evidence from alaska all assemblages dating between 5 7 and 2 million years bp yielded calibrated t max values between 12 4 and 13 8 c regardless of location thus the insect fossil data support the theory that there was far less latitudinal gradation in temperatures during the late pliocene than there is today our reconstructions suggest regional climatic cooling especially winter temperatures began by at least 2 million years bp

manganese rich brown layers in arctic ocean sediments composition formation mechanisms and diagenetic overprint we present inorganic geochemical analyses of pore waters and sediments of two late quaternary sediment cores from the western arctic ocean southern mendeleev ridge rv polarstern expedition ark xxiii 3 focussing on the composition and origin of distinct brown colored mn rich sediment layers carbonate enrichments occur in association with these layers as peaks in ca al mg al sr al and sr mg suggesting enhanced input of both ice rafted and biogenic carbonate for the first time we show that the mn rich layers layers are also consistently enriched in the scavenged trace metals co cu mo and ni distinct bioturbation patterns specifically well defined brown burrows into the underlying sediments suggest these metal enrichments formed close to the sediment water interface the geochemical signature of these metal and carbonate rich layers most probably documents formation under warmer climate conditions with an intensified continental hydrological cycle and only seasonal sea ice cover both rivers and sea ice delivered trace metals to the arctic ocean while enhanced seasonal productivity exported reactive organic matter to the sea floor the coeval deposition of organic matter mn oxyhydr oxides and trace metals triggered intense diagenetic mn cycling at the sediment water interface these processes resulted in the formation of mn and trace metal enrichments and the degradation of labile organic matter with the onset of cooler conditions reduced riverine runoff and or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter resulting in deposition of grayish yellowish metal poor sediments oxygen depletion of arctic bottom waters under these cooler conditions is not supported by our data and did not cause the sedimentary mn distribution while the original composition and texture of the brown layers resulted from specific climatic conditions and corresponding diagenetic processes pore water data show that diagenetic mn redistribution is still affecting the organic poor deeper sediments given persistent steady state conditions purely authigenic mn rich brown layers may form while others may be partly or completely dissolved the degree of diagenetic mn redistribution largely depends on the depositional environment the mn and organic matter availability and apparently affected the co mo ratios of mn rich layers thus brown arctic layers are not necessarily synchronous features and should not be correlated across the arctic ocean without additional age control

high speed video observation of stepwise propagation of a natural upward positive leader using a high speed video camera operated at 150 000 frames per second we have documented the upward propagation of an initial positive leader from a 325 m meteorological tower in beijing the stepwise development of the upward positive leader was clearly revealed during its self sustained propagation toward the cloud base providing the first optical evidence for the stepping processes of a positive leader the leader developed with definitive intermittent pauses and re establishment with abrupt jump of the leader top obvious brush like corona zone emitting outward from the leader top was identified in the frame of stepping and the luminosity waves propagated downward along the already formed channel from the leader top immediately following the stepping indicating that a current wave was generated at the leader top and subsequently traveled down the channel the channel luminosity decreased during the leader pause stage with the corona zone dimmed correspondingly the positive leader experienced an average interstep interval of 61 7 s ranging between 30 s and 120 s and an average 2 d speed of 8 1 104 m s the transient speed of the step jump was estimated to be larger than 7 3 105 m s with an average step length of 4 9 m the need of sufficient positive charge accumulation at the leader top could be the main cause for the stepping characteristics

convection in a two layer mantle with a strongly temperature dependent viscosity recent geochemical and isotopic data support the hypothesis that the earth s mantle convects in two separate layers with an interface at the 650 km seismic discontinuity this paper studies the implications of this hypothesis with regard to the thermal boundary layers that are expected to develop adjacent to the interface between the convecting layers we are particularly concerned with a possible decrease in the mantle viscosity across this interface in order to study the structure of the thermal boundary layers in a fluid with a strongly temperature dependent viscosity we utilize an approximate method developed by howard 1966 if the upper and lower mantle have the same viscosity law we find that stratified convection requires a lower mantle viscosity which is several orders of magnitude lower than the upper mantle viscosity this is not consistent with studies of postglacial rebound it is possible to obtain a viscosity law for the lower mantle that gives a viscosity nearly equal to that of the upper mantle and reasonable temperatures however the required law is not consistent with the currently available experimental data on lower mantle materials therefore we conclude that thermal arguments favor whole mantle convection

recent calcium carbonate dissolution in the barents sea paleoceanographic applications abstract studies of benthic foraminifera in surface sediments of the arctic epicontinental barents sea show several indications of calcium carbonate dissolution low values of calcium carbonate correspond to low ratios of calcareous agglutinating foraminifera planktic benthic foraminifera high ratios of living dead foraminifera corroded calcareous foraminifera and high numbers of exposed organic linings of foraminifera dissolution increases eastwards and northwards in the study area we suggest that the dissolution is caused by the presence of dense cold saline and co 2 rich bottom water which is linked to sea ice production and the position of the oceanic polar front we also think calcium carbonate dissolution in this area indicates a sink for atmospheric co 2

stretching and mixing of viscous blobs in earth s mantle the stretching and mixing of blobs more viscous than the surrounding mantle is an important issue the subducted lithospheric material at depth might be more viscous than the surrounding mantle and its ability to mix affects its properties as a geochemical reservoir in order to study the effect of the viscosity ratio viscosity of the blob to the viscosity of the surrounding mantle on the deformation we perform three types of two dimensional numerical simulations a pure shear simulation a simulation of steady state convection and a simulation of time dependent convection in a pure shear flow the inverse of the rate of deformation of a viscous blob is to a first approximation a linear function of the ratio ar where ar is the aspect ratio of the blob for a viscous blob in a steady convection cell a periodic deformation is superimposed on the long term deformation the period corresponds to an overturn time along the streamline followed by the blob the strains of this periodic deformation obey the laws found for pure shear in a time dependent flow more representative of the mantle flow we find that the average distance a between two tracers situated within a viscous blob obeys the relationship ln a a0 at 1 2 and that it is independent of the aspect ratio of the blob these two results are well explained by a simple model of successive strains in arbitrary directions the viscosity contrast controls the mixing efficiency and it has an effect much larger than predicted by earlier studies blobs of matter more viscous than the average mantle can therefore persist unmixed in the mantle over earth s history

quaternary history of sea ice in the western arctic ocean based on foraminifera sediment cores from the northwind ridge western arctic ocean including uniquely preserved calcareous microfossils provide the first continuous proxy record of sea ice in the arctic ocean encompassing more than half of the quaternary the cores were investigated for foraminiferal assemblages along with coarse grain size and bulk chemical composition by combination of glacial cycles and unique events reflected in the stratigraphy the age of the foraminiferal record was estimated as ca 1 5 ma foraminiferal abundances diversity and composition of benthic assemblages especially phytodetritus and polar species were used as proxies for sea ice conditions foraminiferal assemblage zone 2 in the lower pleistocene indicates diminished mostly seasonal sea ice probably facilitated by enhanced inflow of pacific waters a gradual decrease in ice free season with episodes of abrupt ice expansion is interpreted for the mid pleistocene transition consistent with climatic cooling and ice sheet growth in the northern hemisphere a principal faunal and sedimentary turnover occurred near the earlyemiddle pleistocene boundary ca 0 75 ma with mostly perennial sea ice indicated by the overlying assemblage zone 1 two steps of further increase in sea ice coverage are inferred from foraminiferal assemblage changes in the glacial pleistocene by ca 0 4 and 0 24 ma possibly related to hemispheric mid brunhes event and laurentide ice sheet growth respectively these results suggest that year round ice in the western arctic was a norm for the last several 100 ka in contrast to rapidly disappearing summer ice today

arctic planktic foraminiferal assemblages implications for subsurface temperature reconstructions abstract earlier reconstructions of the inflow of atlantic water to arctic and subarctic oceans based on foraminiferal proxy data have been obstructed by uncertain quantitative reconstructions of sea surface and subsurface temperatures in this study surface sediment samples with undisturbed sediment water interface from polar north atlantic and barents sea were retrieved and prepared at the size fractions from 100 m to 1000 m the foraminiferal analyses show that neogloboquadrina pachyderma constitutes 96 99 of the fauna in arctic and polar surface water masses the same result is obtained when investigating the 150 m size fraction however in arctic areas influenced by atlantic water additional faunal information is obtained when using the smaller 100 m size fraction in these areas n pachyderma is reduced to about 50 and the relatively small species turborotalita quinqueloba becomes very frequent this also applies to the coastal water masses which are dominated by neogloboquadrina incompta and globigerinita uvula transfer functions using the current dataset based on the 100 m size fraction were developed and assessed different statistical models were tested using both seasonal and annual temperature data from 0 m 10 m 50 m and 100 m water depth the most precise reconstructions of subsurface temperatures were found when using summer temperatures from the 100 m depth level the transfer function was tested on holocene foraminiferal records and compared to previous reconstructions the results show that our new transfer function based on the 100 m fraction generally yields lower temperatures at both 10 and 100 m water depth than earlier reconstructions e g hald et al 2007 this could be due to the increased number of samples containing both small species and or the presence of more small specimens representing cold conditions

quaternary paleoceanography of the central arctic based on integrated ocean drilling program arctic coring expedition 302 foraminiferal assemblages the integrated ocean drilling program iodp arctic coring expedition acex hole 4c from the lomonosov ridge in the central arctic ocean recovered a continuous 18 m record of quaternary foraminifera yielding evidence for seasonally ice free interglacials during the matuyama progressive development of large glacials during the mid pleistocene transition mpt 1 2 0 9 ma and the onset of high amplitude 100 ka orbital cycles 500 ka foraminiferal preservation in sediments from the arctic is influenced by primary sea ice organic input and other environmental conditions and secondary factors syndepositional long term pore water dissolution taking these into account the acex 4c record shows distinct maxima in agglutinated foraminiferal abundance corresponding to several interglacials and deglacials between marine isotope stages mis 13 37 and although less precise dating is available for older sediments these trends appear to continue through the matuyama the mpt is characterized by nearly barren intervals during major glacials mis 12 16 and 22 24 and faunal turnover mis 12 24 abundant calcareous planktonic mainly neogloboquadrina pachyderma sin and benthic foraminifers occur mainly in interglacial intervals during the brunhes and very rarely in the matuyama a distinct faunal transition from calcareous to agglutinated foraminifers 200 300 ka in acex 4c is comparable to that found in arctic sediments from the lomonosov alpha and northwind ridges and the morris jesup rise down core disappearance of calcareous taxa is probably related to either reduced sea ice cover prior to the last few 100 ka cycles pore water dissolution or both

plio pleistocene trends in ice rafted debris on the lomonosov ridge although more than 700 sediment cores exist from the arctic ocean the plio pleistocene evolution of the basin and its marginal seas remains virtually unknown this is largely due the shallow penetration of most of these records and difficulties associated with deriving chronologies for the recovered material the integrated ocean drilling program s iodp expedition 302 arctic coring expedition acex recovered 197 m of neogene quaternary sediment from the circumpolar regions of the lomonosov ridge as detailed analyses of this material emerge research is beginning to formulate a long term picture of paleoceanographic changes in the central arctic ocean this paper reviews the acex plio pleistocene age model identifies uncertainties and addresses ways in which these may be eliminated within the established stratigraphic framework a notable reduction in the abundance of ice rafted debris ird occurs in the early part of the pleistocene and persists until marine isotope stage 6 mis 6 therefore while global oceanographic proxies indicate the gradual growth of terrestrial ice sheets during this time ird delivery to the central arctic ocean remained comparatively low and stable within the resolution of existing data the pleistocene reduction in ird is synchronous with predicted changes in both the inflow of north atlantic and pacific waters which in modern times are known to exert a strong influence on sea ice stability

self reversal and apparent magnetic excursions in arctic sediments the arctic oceans have been fertile ground for the recording of apparent excursions of the geomagnetic field implying that the high latitude field had unusual characteristics at least over the last 1 2 myrs alternating field demagnetization of the natural remanent magnetization nrm of core hly0503 6jpc from the mendeleev ridge arctic ocean implies the presence of primary magnetizations with negative inclination apparently recording excursions in sediments deposited during the brunhes chron thermal demagnetization on the other hand indicates the presence of multiple often anti parallel magnetization components with negative inclination components having blocking temperatures predominantly but not entirely below 350 c thermo magnetic tests x ray diffraction xrd and scanning electron microscopy sem indicate that the negative inclination components are carried by titanomaghemite presumably formed by seafloor oxidation of titanomagnetite the titanomaghemite apparently carries a chemical remanent magnetization crm that is partially self reversed relative to the detrital remanent magnetization drm carried by the host titanomagnetite the partial self reversal could have been accomplished by ionic ordering during oxidation thereby changing the balance of the magnetic moments in the ferrimagnetic sublattices

northeastern atlantic benthic foraminifera during the last 45 000 years changes in productivity seen from the bottom up we studied benthic foraminifera from the last 45 kyr in the 63 mu m size fraction in biogeochemical ocean flux studies bofs cores 5k 50 degrees 41 3 n 21 degrees 51 9 w depth 3547 m and 14k 58 degrees 37 2 n 19 degrees 26 2 w depth 1756 m at a time resolution of several hundreds to a thousand years the deepest site showed the largest fluctuations in faunal composition species richness and benthic foraminiferal accumulation rates the fluctuations resulted from changes in abundance of epistominella exigua and alabaminella weddellensis in the present oceans these species bloom opportunistically when a spring plankton bloom results in seasonal deposition of phytodetritus on the seafloor the phytodetritus species had very low relative abundances and accumulation rates during the last glacial maximum a strong increase in absolute and relative abundance of e exigua and a weddellensis during deglaciation paralleled the decrease in abundance of the polar planktonic foraminifer neogloboquadrina pachyderma s and the increase in abundance of warmer water planktonic species such as globigerina bulloides this strong increase in relative abundance of the phytodetritus species and the coeval increase in benthic foraminiferal accumulation rate were thus probably caused by an increase in the deposition of phytodetritus to the seafloor and thus probably of surface productivity when the polar front retreated to higher latitudes the abundance of phytodetritus species decreased during the younger dryas but not to the low levels of fully glacial conditions during heinrich events periods of excessive melt water formation and ice rafting benthic accumulation rates were very low as were the absolute and relative abundances of the phytodetritus species supporting suggestions that surface productivity was very low during these events in both cores pullenia and cassidulina species were common during isotope stages 2 3 and 4 as were bolivinid buliminid and uvigerinid species high relative abundances of these species have been interpreted as indicative either of sluggish deep water circulation or of high organic carbon fluxes to the seafloor in our cores relative abundances of these species are negatively correlated with benthic foraminiferal accumulation rates and we can thus not interpret them as indicative of increased productivity during glacials the percentage of these low oxygen species calculated on a phytodetritus species free basis decreased slightly at deglaciation at 5k but not at 14k this suggests that decreased production of north atlantic deep water during the last glacial might have slightly affected benthic foraminiferal faunas in the eastern north atlantic at 3547 m depth but not at 1756 m in conclusion major changes in deep sea benthic foraminiferal faunas over the last 45 000 years in our cores from the northeastern atlantic were the result of changes in surface water productivity not of changes in deep water circulation productivity was lower during the glacial probably because of extensive ice cover

thermal and compositional evolution of the martian mantle effects of phase transitions and melting abstract we present new numerical models for the thermochemical evolution of the mantle of mars over the past 4 gy specifically we have developed a parameterized model of composition and thermoelastic properties of mantle material and combined it with a two dimensional anelastic compressible convection and melting algorithm in a spherical annulus geometry these models include a detailed treatment of the effects of solid solid phase transitions and of compositional changes that accompany generation and removal of mantle partial melt during magmatism among other questions we examine if a perovskite ferropericlase layer pv fp exists at the base of the martian mantle if long lived plumes can explain the volcanic provinces under which circumstances mars may still be volcanically active and how iron and radionuclide rich mars is results of the models are compared with geophysical and chemical observations from spacecraft and information from martian meteorites most models yield crustal thicknesses between 75 and 90 km ancient depths for the curie temperatures of possible magnetic carriers that include the entire crust and mechanical lithosphere thicknesses that increased from less than 100 km in the noachian to 200 250 km now generally models with a large core mg 0 75 and radionuclide contents derived from those suggested by wanke and dreibus chemistry and accretion history of mars philos trans r soc lond a 349 285 293 1994 tend to explain observations best but none of the models reproduces the full range of concentrations found in martian meteorites thus implicitly supporting the idea that ancient chemical heterogeneities survive in the mantle only a subset of the models develops a pattern of mantle convection that evolves towards two or three large long lived plumes and it takes at least 2 gy before this stage is reached whereas models with a larger number of plumes or with plumes that are less stationary are more common as tharsis and probably elysium are older than 2 5 3 gy either model assumptions more complex than those made in this study or a dynamical mode different from low degree convection with long lived large plumes is required to explain these major volcanic provinces the effect of the mid mantle phase transitions on mantle dynamics is rather weak so that whole mantle convection prevails however a basal pv fp layer would form a dynamically separate unit and decouple the mantle and core to some extent

origin of apparent magnetic excursions in deep sea sediments from mendeleev alpha ridge arctic ocean 1 arctic deep sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere and the lack of tight age control usually provides excessive freedom in the labeling of arctic excursions fortuitous variations in sedimentation rate have been invoked to explain the amplified excursions alternating field demagnetization of natural remanent magnetization nrm of sediment cores 08jpc 10jpc 11jpc and 13jpc recovered by the healy oden trans arctic expedition in august 2005 hotrax05 to the mendeleev alpha ridge yields apparent magnetic excursions in sediments deposited in the brunhes chron thermal demagnetization of the nrm however implies multiple magnetization components with negative inclination components usually unblocked below 350 c analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another magnetic experiments conducted under high and low temperatures show features that are characteristic for titano magnetite and titanomaghemite presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for x ray energy dispersive spectroscopy eds and by high resolution x ray diffraction xrd it is unlikely that anomalously thick intervals of negative inclination in these brunhes aged sediments are caused by unusual behavior of the magnetic field in the arctic area we therefore attribute low and negative nrm inclinations in these cores to partially self reversed chemical remanent magnetizations apparently carried by titanomaghemite and acquired during the oxidation of detrital titano magnetite grains the high ti contents and high oxidation states indicated by eds and xrd data provide the conditions required for partial self reversal by ionic reordering during diagenetic maghemitization and this process appears to have affected all hotrax05 cores collected from the mendeleev alpha ridge

variability of sea ice conditions in the fram strait over the past 30 000 years sea ice is a critical component of the climate and oceanographic system in the north atlantic ocean a biomarker record reveals millennial scale and glacial interglacial fluctuations in sea ice coverage in the northernmost atlantic ocean over the past 30 000 years

last global extinction in the deep sea during the mid pleistocene climate transition 1 twenty percent 19 genera 95 species of cosmopolitan deep sea 500 4000 m benthic foraminiferal species became extinct during the late pliocene middle pleistocene 3 0 12 ma with the peak of extinctions 76 species occurring during the mid pleistocene climate transition mpt 1 2 0 55 ma one whole family stilostomellidae 30 species was wiped out and a second pleurostomellidae 29 species was decimated with just one species possibly surviving through to the present our studies at 21 deep sea core sites show widespread pulsed declines in abundance and diversity of the extinction group species during more extreme glacials with partial interglacial recoveries these declines started in the late pliocene in southern sourced deep water masses antarctic bottom water circumpolar deep water and extending into intermediate waters antarctic intermediate water north atlantic deep water in the mpt with the youngest declines in sites farthest downstream from high latitude source areas for intermediate waters we infer that the unusual apertural types that were targeted by this extinction period were adaptations for a specific kind of food source and that it was probably the demise of this microbial food that resulted in the foraminiferal extinctions we hypothesize that it may have been increased cold and oxygenation of the southern sourced deep water masses that impacted on this deep water microbial food source during major late pliocene and early pleistocene glacials when antarctic ice was substantially expanded the food source in intermediate water was not impacted until major glacials in the mpt when there were significant expansion of polar sea ice in both hemispheres and major changes in the source areas temperature and oxygenation of global intermediate waters

benthic foraminiferal biodiversity response to a changing arctic palaeoclimate in the last 24 000 years investigations on the benthic foraminiferal fauna of sediment cores from the eurasian basin arctic ocean continental margin reveal well correlated biodiversity maxima and minima during the last 24 kyrs the temporal variable biodiversities observed at the core sites on a large scale reflect the palaeoclimatic evolution of the high northern latitudes herein sediments deposited during interglacials and interstadials reveal highest species richness whereas glacials and stadials are documented by lower species numbers in high resolution core ps2837 different periodicities in species richness in sediments from the late weichselian and holocene can be detected biodiversity periodicities of 1 57 kyrs and 0 76 kyrs characterize sediments of the late weichselian whereas with the retreat of glacial ice sheets and shorter seasonal ice coverage holocene sediments reveal shorter periodicities of 1 16 kyrs and 0 54 kyrs with the establishment of modern hydrographic conditions at about 4000 bp 4 kyrs a significant increase in the amplitudes of species richness can be observed although sediments deposited during warmings reveal highest species richness we doubt that water temperature determines the abundance and distribution of most benthic foraminifera in the arctic ocean we rather suggest that the temporal variabilities in species richness reflect changes in the availability of food which in the arctic ocean mainly depends on extend and duration of seasonal sea ice retreat in the study area the latter one besides the seasonal varying insolation is essentially determined by the advection rates and temperature of atlantic water entering the arctic ocean via the west spitsbergen current because both flowrate and temperature of the west spitsbergen current were increased during interstadials and interglacials a predominantly indirect influence of palaeotemperature on the biodiversity of benthic foraminifera is suggested yet rare species like atlantic species may indeed have a temperature sensitive metabolism 2007 elsevier b v all rights reserved

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

holocene cooling culminates in sea ice oscillations in fram strait a reconstruction of holocene sea ice conditions in the fram strait provides insight into the palaeoenvironmental and palaeoceanographic development of this climate sensitive area during the past 8500 years bp organic geochemical analyses of sediment cores from eastern and western fram strait enable the identification of variations in the ice coverage that can be linked to changes in the oceanic and atmospheric circulation system by means of the sea ice proxy ip25 phytoplankton derived biomarkers and ice rafted detritus ird increasing sea ice occurrences are traced along the western continental margin of spitsbergen throughout the holocene which supports previous palaeoenvironmental reconstructions that document a general cooling a further significant ice advance during the neoglacial is accompanied by distinct sea ice fluctuations which point to short term perturbations in either the atlantic water advection or arctic water outflow at this site at the continental shelf of east greenland the general holocene cooling however seems to be less pronounced and sea ice conditions remained rather stable here a major neoglacial increase in sea ice coverage did not occur before 1000 years bp phytoplankton ip25 indices pip25 index are used for more explicit sea ice estimates and display a mid holocene shift from a minor sea ice coverage to stable ice margin conditions in eastern fram strait while the inner east greenland shelf experienced less severe to marginal sea ice occurrences throughout the entire holocene highlights biomarker and ird data give insight into holocene sea ice conditions in fram strait we find increasing sea ice coverage off west spitsbergen throughout the holocene oceanic atmospheric variability caused neoglacial sea ice fluctuations ice conditions along east greenland shelf remain rather stable until 1000 years bp

arctic ocean deep sea record of northern eurasian ice sheet history the sediment composition of deep sea cores from the central arctic ocean the fram strait and the yermak plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200 000 years layers with high amounts of coarse terrigenous ice rafted debris ird and often high contents of smectite were deposited during extensive glaciations in northern eurasia when ice sheets reached the northern continental margins of the barents and kara seas and discharged icebergs into the arctic ocean intercalated layers with relatively low ird and smectite contents but abundant planktic foraminifers in the coarse fraction were deposited during periods of atlantic water inflow to the arctic ocean and seasonally open waters leads in a sea ice cover with only few icebergs in the arctic ocean high ird contents in the sediments reflect the presence of ice sheets on the kara and barents seas shelves and the hinterland during the entire oxygen isotope stage 6 ca 190 130 ka in substage 5b ca 90 80 ka at the stage boundary 5 4 around 75 ka and in late stage 4 early stage 3 ca 65 50 ka these results are in excellent correlation with those from recent field work in northern scandinavia european russia siberia and on the shelves relatively low amounts of ird in central arctic ocean sediments from the late weichselian glacial maximum ca 24 18 ka correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the arctic ocean the synchroneity of freshwater events and ird input suggests a common source strongest events were associated with deglaciations of the barents and kara seas after the ice sheets had blocked the outflow of large rivers for several millennia the outflow of freshwater from large ice dammed lakes occurred at ca 130 80 75 and 52 ka freshwater events in the central arctic ocean during the last deglaciation ca 18 ka were relatively small compared to the previous events this indicates that during most of the late weichselian glacial maximum a river outflow from northern siberia to the arctic ocean was possible atlantic water inflow to the arctic ocean and seasonally open waters in the ice leads occurred during the interglacials ofoxygen isotope stage 1 and substage 5e during several interstadials stage 3 substages 5a and 5c and to a lesser degree within stadials and glacials stages 2 4 and 6 with the exception ofthe interglacials these periods were times ofstrong ice growth on the continents as revealed by terrestrial data the coincidence suggests that open waters in the arctic ocean and the nordic seas were an important moisture source in addition to more southerly sources which fostered the growth of ice sheets on northern eurasia

environmental change in eastern greenland during the last 1300 years evidence from foraminifera and lithofacies in nansen fjord 68 n lithofacies and benthic foraminiferal analyses of two sediment cores bs1191 k13b and k14 from nansen fjord eastern greenland show evidence of changing oceanographic and sea ice conditions from

high resolution paleoproductivity fluctuations during the past 24 kyr as indicated by benthic foraminifera in the marginal arctic ocean abstract analyses of benthic foraminifera in sediment cores taken at about 1000 m water depth at the yermak plateau and the barents sea slope adjacent to the position of the ice sheet edge during the last glacial maximum show that paleoproductivity was reduced to about a third of its present level during peak glacial stadials these reduced values are still higher than values for modern permanently ice covered regions suggesting that the core locations were at least partially ice free even during stadials paleoproductivity at the core locations was higher than or equal to that of today during initial deglaciations and warm substages peak paleoproductivity occurred in samples with low salinity surface waters as indicated by oxygen isotope values of planktonic foraminifera and slightly after increased abundance of atlantic species suggesting that enhanced advection of warmer waters from the atlantic supported the initial ice sheet retreat during the holocene climatic optimum productivity was much less than at present on the yermak plateau high at the barents sea site perhaps because of increased advection of atlantic water to the latter site and heavier ice coverage at the former after this optimum paleoproductivity at both sites was similar with slightly lower values during cold periods such as the little ice age

manganese and color cycles in arctic ocean sediments constrain pleistocene chronology sequential variations in manganese mn content and color of deepsea sediments retrieved from the lomonosov ridge 87 n in the central arctic ocean apparently mimic low latitude 18 o glacial interglacial cyclicity thereby providing stratigraphic information that together with biostratigraphic data permit the construction of a detailed chronological model correlation of this mn and color chronology to established apparent brunhes age estimates of geomagnetic excursions reveals a remarkable fit between these two independently derived time scales the mn and color cycles probably provide paleoenvironmental information about material fluxes in the arctic ocean over the past 1 m y we suggest that the primary source for the observed manganese variations in our sediment core is northern siberia which has extensive peat bogs and boreal forests these siberian source areas could operate in an off and on mode tuned to pleistocene glacial and interglacial periods contrasts in ventilation of arctic ocean waters during interglacial glacial cycles probably could also enhance the observed mn and color variability

leader return stroke like processes in the initial stage of rocket triggered lightning 1 linear streak film video current and electric field records from nine triggered lightning flashes are analyzed to examine the process of cutoff and reestablishment of current during the initial stage of rocket triggered lightning all of the data were acquired at the international center for lightning research and testing at camp blanding florida in 2002 and 2003 it is shown that in some rocket triggered lightning events the process of current cutoff and reestablishment during the initial stage is similar to a leader return stroke sequence although the currents in this process are typically an order of magnitude smaller 1 ka or so than those in a triggered or natural lightning subsequent stroke 10 15 ka the events were separated into two groups based on observed characteristics with the duration of the current cutoff interval being the primary differentiating characteristic in some cases two or three failed attempts at current reestablishment prior to the successful resumption of current flow in the channel were observed currents associated with the unsuccessful attempts were typically an order of magnitude smaller 100 a or so than in the process which finally reestablished the current

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

isotopic evolution of the mantle a model we investigate a model of mantle evolution which acknowledges the importances in early earth history of large degrees of partial melting and crystal fractionation rather than as in recent models small degrees of partial melting operating on primary mantle the major source regions are in the upper mantle and evolve from a primitive mantle melt by crystal fractionation and the formation of chemically distinct cumulate layers the rb sr and sm nd ratios in melts are relatively unfractionated in processes involving large 15 degrees of partial melting the ratios are even less fractionated in orthocumulates forming from these melts small degrees of partial melting or removal of residual fluids after nearly complete crystalization of cumulate layers are the processes that lead to significant fractionation the importance of these processes increases as the upper mantle cools the observation of nearly primitive isotopic ratios does not necessarily imply the presence of ancient primitive undifferential reservoirs the transfer of intercumulus fluids from a crystalizing cumulate layer gives time dependent rb sr and sm nd ratios in both the donor and receiving reservoirs since the late stage fluids become more enriched as crystalization proceeds enrichment in such a system is progressive with time time integrated nd sm and rb sr ratios therefore need not depart much from primitive ratios even though current values for lile concentration rb sr nd sm etc may be large the time constant for lile enrichment appears to be 2 gyr about the same as required for the u pb system and the same as the characteristics time associated with mantle cooling the mantle reservoirs may be much more ancient than generally assumed

late quaternary paleoceanography and paleo sea ice conditions in the mackenzie trough and canyon beaufort seathis article is one of a series of papers published in this special issue on the theme polar climate stability network the mackenzie trough provides a high resolution signal for paleoceanography as a result of high sedimentation rates at the mouth of the mackenzie river three cores were collected along a transect covering a depth range of 58 671 m and the time period of the last 11 500 cal bp prior to the last 10 000 cal bp the distal core is characterized by laminated sediment and a foraminiferal fauna of arctic bottom water calcareous species and abundant planktic foraminifera suggesting little freshwater runoff and or perennial sea ice cover this occurs at a similar time as laminated sediments from the west of this site which have been suggested to be part of the lake agassiz flood outburst and or cold period if this outburst occurred the very positive oxygen isotope values from our core pc3 3 0 ppm indicate that it did not flow through the mackenzie trough after 9000 cal bp the faunas in the three cores differ because of timing and different water depths however it is possible to see a progression

living benthic foraminifers from the central arctic ocean faunal composition standing stock and diversity abstract fifty short sediment cores collected with a multiple corer and five box cores from the central arctic ocean were analysed to study the ecology and distribution of benthic foraminifers to work out living faunal associations standing stock and diversity separate analyses of living rose bengal stained and dead foraminifers were carried out for the sediment surface the size fractions between 63 and 125 m and 125 m were counted separately to allow comparison with former arctic studies and with studies from the adjacent norwegian greenland sea barents sea and the north atlantic ocean benthic foraminiferal associations are mainly controlled by the availability of food and competition for food while water mass characteristics bottom current activity substrate composition and water depth are of minor importance off spitsbergen in seasonally ice free areas high primary production rates are reflected by high standing stocks high diversities and foraminiferal associations 125 m that are similar to those of the norwegian greenland sea generally in seasonally ice free areas standing stock and diversity increase with increasing food supply in the central arctic ocean the oligotrophic permanently ice covered areas are dominated by epibenthic species the limited food availability is reflected by very low standing stocks and low diversities most of these foraminiferal associations do not correspond to those of the norwegian greenland sea the dominant associations include simple agglutinated species such as sorosphaerae placopsilinellae komokiacea and aschemonellae as well as small calcareous species such as stetsonia horvathi and epistominella arctica those of the foraminiferal species that usually thrive under seasonally ice free conditions in middle bathyal to lower bathyal water depth are found under permanently ice covered conditions in water depths about 1000 m shallower if present at all

convective mixing and the fine structure of mantle heterogeneity a simple theoretical model is presented for the statistical distribution of small scale passive heterogeneity in a convecting mantle the important new variables used to characterize the anomaly field are the autocorrelation function r and its fourier transform the energy spectrum e the anomaly variance the sample variance i e the anomaly variance as filtered by the sampling process and the mixing time scale are expressed in terms of e equations governing the time evolution of r and e are derived starting with the transport diffusion equation the energy spectrum equation is solved for deformation models with homogeneous normal and shear strain components the predictions from these models are compared with results of numerical experiments on mixing of slab shaped heterogeneities by benard convection both the numerical experiments and the theory indicate that compositional heterogeneity subjected to convective flow becomes drawn into thin sheets oriented subparallel to the transport direction re mixing of subducted slabs produces a laminated mantle the energy spectrum of the laminate is highly anisotropic where the transport direction is predominantly horizontal the energy at high wave numbers is in the vertical the low wave number energy is in the horizontal plane the shape of the energy spectrum contains information on mean mantle strain rates age of the anomalies and solid state diffusion rates

stability of a chemically layered upper mantle the possibility that the upper mantle at depths less than 670 km is chemically as well as mineralogically layered has been extensively discussed one idea posits that sublithospheric upper mantle d 400 km is dominantly harzburgitic and of low intrinsic density compared with majoritic and clinopyroxene rich piclogite which occupies the seismic transition region at depths between 400 670 km the gravitational stability of the harzburgite over piclogite arrangement light above heavy when heated from below is investigated here in order to better understand the dynamics of mixing the calculations neglect the effects of plates compressibility viscous dissipation phase change and radiogenic heating and focus on the role played by 0 the intrinsic density difference between the layers during mixing at fixed rayleigh number the dimensionless parameters of this problem include the thermal rayleigh number based on the heat flux q0 into the basal piclogitic layers rq gq0d4k the ratio of chemical to thermal buoyancy r 0 k q0d and the thickness ratio of the two layers here g d k 0 represent the expansitivity gravity total depth thermal diffusivity thermal conductivity kinematic viscosity isothermal difference in density between the two layers i e the intrinsic density difference and density of the piclogitic bottom layer respectively a constant viscosity newtonian rheology is assumed for the sublithospheric upper mantle between 100 670 km three measures of the extent and thoroughness of mixing are used to quantify mixing these include the variance of the compositional field the two point spatial correlation function for composition and the average composition within each layer the spatial correlation enables one to define a dominant length scale characteristic of the size of the chemical anomalies l the adimensional variance sometimes called the mixing intensity may be used to define a mixing time simulations at fixed rq but with 0 0 2 4 8 have been carried out for periods of time equivalent to the age of the earth the critical r that separates well mixed states from poorly mixed ones is r 15 for rq 2 105 for nominal upper mantle parameters this implies a critical intrinsic density difference 0 3 the style of mixing is grossly different depending on whether 0 is less than or greater than the critical value plume penetration with rapid changes in the average size of chemical heterogeneities is the dominant mechanism at low 0 whereas for high 0 viscous entrainment and the stretching of tendrils along the layer interface is the dominant mixing style for density ratios near the critical value the fraction of fertile easily fused peridotite within the dominantly harzburgitic upper mantle above the top of the transition region varies quasiperiodically with period 0 6 ga roughly equal to the supercontinent cycle time intermittent periods of increased lower layer transport across the top of the transition zone may correlate with spikes in the volumetric rate of magma generation due to decompression melting of ascending fertile peridotite

biostratigraphic and paleoclimatic significance of a new pliocene foraminiferal fauna from the central arctic ocean abstract a pliocene benthic foraminiferal fauna containing a previously unknown species association was found in the basal section of a piston core collected from the crest of northwind ridge nwr in the central arctic ocean the fauna is dominated by epistominella exigua cassidulina reniforme eponides tumidulus cibicides scaldisiensis lagena spp cassidulina teretis eponides weddellensis bolivina arctica and patellina corrugata the presence of cibicides scaldisiensis in the assemblage and the occurrence of cibicides grossus higher in the core are indicative of an early pliocene age the morphologically distinctive species cibicidoides sp 795 of mcneil in press which occurs in the nwr core sample was previously known only from oligocene through miocene deposits in the beaufort mackenzie basin of arctic canada ehrenbergina sp a and cibicidoides aff c sp 795 also present in the core are new and endemic to the arctic late miocene and early pliocene these species and possibly others are survivors of the late miocene messinian sea level crisis which caused a significant faunal turnover in the arctic ocean the predominantly calcareous assemblage indicates deposition above the calcium carbonate compensation depth in an upper bathyal environment paleogeographic affinities for the bulk of the assemblage indicate probable connections between the arctic and the north atlantic oceans but the endemic species identify environmental differences or partial isolation of the western arctic ocean the species association suggests a cold but milder paleoclimate than that which existed during pleistocene glacial intervals

paleoproductivity the benthic planktonic ratio in foraminifera as a productivity index abstract when the effects of differential dissolution are excluded the benthic to planktonic ratio in deep sea foraminifera can be read as a productivity index for pelagic conditions we use the bz index which is the product of the benthic planktonic ratio in percent and the depth of deposition which controls supply of organic matter for hemipelagic conditions we use bz d which is the product of the bz value and a factor for proximity to the shore square root of distance from land which controls coastal productivity anomalous values readily stand out from the normal populations which cluster around typical values of bz or bz d off northwest africa wherever calcite dissolution can be excluded as a factor because of presence of aragonite these anomalies are due to lateral supply of redeposited organic matter

contrasting glacial interglacial regimes in the western arctic ocean as exemplified by a sedimentary record from the mendeleev ridge distinct cyclicity in lithology and microfaunal distribution in sediment cores from the mendeleev ridge in the western arctic ocean water depths ca 1 5 km reflects contrasting glacial interglacial sedimentary patterns we conclude that during major glaciations extremely thick pack ice or ice shelves covered the western arctic ocean and its circulation was restricted in comparison with interglacial modern type conditions glacier collapse events are marked in sediment cores by increased contents of ice rafted debris notably by spikes of detrital carbonates and iron oxide grains from the canadian arctic archipelago composition of foraminiferal calcite n 18 o and n 13 c also shows strong cyclicity indicating changes in freshwater balance and or ventilation rates of the arctic ocean light stable isotopic spikes characterize deglacial events such as the last deglaciation at ca 12 14 c kyr bp the prolonged period with low n 18 o and n 13 c values and elevated contents of iron oxide grains from the canadian archipelago in the lower part of the mendeleev ridge record is interpreted to signify the pooling of freshwater in the amerasia basin possibly in relation to an extended glaciation in arctic north america unique benthic foraminiferal events provide a means for an independent stratigraphic correlation of sedimentary records from the mendeleev ridge and other mid depth locations throughout the arctic ocean such as the northwind and lomonosov ridges this correlation demonstrates the disparity of existing age models and underscores the need to establish a definitive chronostratigraphy for arctic ocean sediments 2003 elsevier b v all rights reserved

temperature and precipitation history of the arctic as the planet cooled from peak warmth in the early cenozoic extensive northern hemisphere ice sheets developed by 2 6 ma ago leading to changes in the circulation of both the atmosphere and oceans from not vert similar2 6 to not vert similar1 0 ma ago ice sheets came and went about every 41 ka in pace with cycles in the tilt of earth s axis but for the past 700 ka glacial cycles have been longer lasting not vert similar100 ka separated by brief warm interglaciations when sea level and ice volumes were close to present the cause of the shift from 41 ka to 100 ka glacial cycles is still debated during the penultimate interglaciation not vert similar130 to not vert similar120 ka ago solar energy in summer in the arctic was greater than at any time subsequently as a consequence arctic summers were not vert similar5 c warmer than at present and almost all glaciers melted completely except for the greenland ice sheet and even it was reduced in size substantially from its present extent with the loss of land ice sea level was about 5 m higher than present with the extra melt coming from both greenland and antarctica as well as small glaciers the last glacial maximum lgm peaked not vert similar21 ka ago when mean annual temperatures over parts of the arctic were as much as 20 c lower than at present ice recession was well underway 16 ka ago and most of the northern hemisphere ice sheets had melted by 6 ka ago solar energy reached a summer maximum 9 higher than at present not vert similar11 ka ago and has been decreasing since then primarily in response to the precession of the equinoxes the extra energy elevated early holocene summer temperatures throughout the arctic 1 3 c above 20th century averages enough to completely melt many small glaciers throughout the arctic although the greenland ice sheet was only slightly smaller than at present early holocene summer sea ice limits were substantially smaller than their 20th century average and the flow of atlantic water into the arctic ocean was substantially greater as summer solar energy decreased in the second half of the holocene glaciers re established or advanced sea ice expanded and the flow of warm atlantic water into the arctic ocean diminished late holocene cooling reached its nadir during the little ice age about 1250 1850 ad when sun blocking volcanic eruptions and perhaps other causes added to the orbital cooling allowing most arctic glaciers to reach their maximum holocene extent during the warming of the past century glaciers have receded throughout the arctic terrestrial ecosystems have advanced northward and perennial arctic ocean sea ice has diminished here we review the proxies that allow reconstruction of quaternary climates and the feedbacks that amplify climate change across the arctic we provide an overview of the evolution of climate from the hot house of the early cenozoic through its transition to the ice house of the quaternary with special emphasis on the anomalous warmth of the middle pliocene early quaternary warm times the mid pleistocene transition warm interglaciations of marine isotope stages 11 5e and 1 the stage 3 interstadial and the peak cold of the last glacial maximum

a benthic foraminiferal proxy of pulsed organic matter paleofluxes in the temperate open ocean of the modern northeast atlantic the spring bloom of phytoplankton leads to a seasonal pulse of detrital organic material phytodetritus to the ocean floor opportunistic benthic foraminifera rapidly colonise this food resource producing large numbers of individuals whose tests are ultimately added to the sediment one of these taxa epistominella exigua shows periodic peaks in abundance in the fossil record at many open ocean sites previously such peaks have been commonly interpreted to result from changes in physicochemical properties of bottom water mass and thus deep sea circulation the main purpose of this paper is to propose thate exigua may be used as a proxy of pulsed organic matter inputs to the deep ocean and therefore as an indicator of relative changes in productivity

the depth dependency of planktonic benthic foraminiferal ratios constraints and applications abstract study of the ratio between planktonic and benthic foraminifera in a great number of areas shows that variation of this ratio with depth can be described using organic matter flux equations essentially the share of benthic foraminifera in the total association is inversely proportional to depth as is the amount of organic matter resulting from primary productivity reaching the sea bottom variation in the p b ratio from area to area appears to be strongly dependent on the amount of inbenthic living foraminifera if the ratios are corrected for this the regressions between the proportion of planktonic foraminifera p and depth appear to be near identical in the gulf of mexico the gulf of california the west coast of the usa and the adriatic sea this regression can be described by depth e 3 58718 0 03534 p a study of the behaviour of this function in nine cores from the adriatic upper quaternary reveals that the palaeodepth reconstructions are not influenced by fluctuations in productivity and temperature but that they are significantly influenced by redeposition of fine grained deposits

paleomagnetism of quaternary sediments from lomonosov ridge and yermak plateau implications for age models in the arctic ocean abstract inclination patterns of natural remanent magnetization nrm in quaternary sediment cores from the arctic ocean have been widely used for stratigraphic correlation and the construction of age models however shallow and negative nrm inclinations in sediments deposited during the brunhes chron in the arctic ocean appear to have a partly diagenetic origin rock magnetic and mineralogical studies demonstrate the presence of titanomagnetite and titanomaghemite thermal demagnetization of the nrm indicates that shallow and negative inclination components are largely unblocked below 300 c consistent with a titanomaghemite remanence carrier following earlier studies on the mendeleev alpha ridge shallow and negative nrm inclination intervals in cores from the lomonosov ridge and yermak plateau are attributed to partial self reversed chemical remanent magnetization crm carried by titanomaghemite formed during seafloor oxidation of host detrital titanomagnetite grains distortion of paleomagnetic records due to seafloor maghemitization appears to be especially important in the perennially ice covered western mendeleev alpha ridge and central arctic ocean lomonosov ridge and to a lesser extent near the ice edge yermak plateau on the yermak plateau magnetic grain size parameters mimic the global benthic oxygen isotope record back to at least marine isotope stage 6 implying that magnetic grain size is sensitive to glacial interglacial changes in bottom current velocity and or detrital provenance

changes in arctic ocean paleoproductivity and hydrography during the last 145 kyr the benthic foraminiferal record the benthic foraminiferal record of two sediment cores at 900 and 2500 m water depth in the marginal arctic ocean reflects fluctuations in paleoproductivity ice sheet extent and atlantic water inflow highest paleoproductivity is observed in interglacial periods and at the termination of interstadials to stadials within oxygen isotope substages ois 6 3 5 3 5 1 3 1 and termination ia stable and high paleoproductivity at the ice edge during terminations is indicated by the melonis zaandami benthic foraminiferal association periods of glacial ice sheet advance or retreat from the core site are dominated by benthic foraminiferal associations related to seasonal organic carbon flux temperate saline atlantic water entered the arctic ocean within ois 6 3 5 5 and the holocene coincident paleoproductivity maxima are related to extended seasonal ice retreat during glacial periods of stagnating atlantic water advection unusual low paleoproductivity values are observed in the arctic ocean indicating increasing ice coverage deep water was poorly ventilated during these periods and the deeper site became exposed to corrosive bottom waters

a 600 ka arctic sea ice record from mendeleev ridge based on ostracodes arctic paleoceanography and sea ice history were reconstructed from epipelagic and benthic ostracodes from a sediment core hly0503 06jpc 800 m water depth located on the mendeleev ridge western arctic ocean the calcareous microfaunal record ostracodes and foraminifers covers several glacial interglacial cycles back to estimated marine isotope stage 13 mis 13 500 ka with an average sedimentation rate of 0 5 cm ka for most of the stratigraphy mis 5 13 results based on ostracode assemblages and an unusual planktic foraminiferal assemblage in mis 11 dominated by a temperate water species turborotalita egelida show that extreme interglacial warmth high surface ocean productivity and possibly open ocean convection characterized mis 11 and mis 13 400 and 500 ka respectively a major shift in western arctic ocean environments toward perennial sea ice occurred after mis 11 based on the distribution of an ice dwelling ostracode acetabulastoma arcticum spectral analyses of the ostracode assemblages indicate sea ice and mid depth ocean circulation in western arctic ocean varied primarily at precessional 22 ka and obliquity 40 ka frequencies

major pleistocene stages in a carbon perspective the south china sea record and its global comparison 1 carbon isotope sequences at ocean drilling program site 1143 south china sea reveal a long term cyclicity of 500 kyr that is superimposed on the glacial cycles and is present in long 13c sequences from all oceans the quaternary 13c record is punctuated by four 13c maximum events 13cmax i which began in marine isotope stage mis 3 around 50 60 kyr ago 13cmax ii mis 13 0 47 0 53 ma 13cmax iii mis 27 29 0 97 1 04 ma and 13cmax vi mis 53 57 1 55 1 65 ma as the same cyclicity is also found in carbonate curves the 13cmax events must denote major reorganization in the carbon reservoir of the global ocean they also are associated with major changes in glacial cyclicity such as the mid brunhes event following 13cmax ii and the mid pleistocene revolution following 13cmax iii which in turn were associated with expansion of the ice sheets from a carbon perspective therefore the quaternary period has passed through three major stages and each appears to represent a further step in ice cap development

the 41 kyr world milankovitch s other unsolved mystery 1 for most of the northern hemisphere ice ages from 3 0 to 0 8 m y global ice volume varied predominantly at the 41 000 year period of earth s orbital obliquity however summer or summer caloric half year insolation at high latitudes which is widely believed to be the major influence on high latitude climate and ice volume is dominated by the 23 000 year precessional period thus the geologic record poses a challenge to our understanding of climate dynamics here we propose that variations in the insolation gradient between high and low latitudes control high latitude climate and ice volume during the late pliocene and early pleistocene the differential heating between high and low latitudes driven by obliquity controls the atmospheric meridional flux of heat moisture and latent energy which may exert the dominant control on high latitude climate on milankovitch timescales in the two dimensional zonal energy balance models typically used to study the long term evolution of climate the meridional atmospheric moisture flux is usually kept fixed the hypothesis that insolation gradients control the poleward energy fluxes precipitation and ice volume at high latitudes has never been directly examined within the context of an ice sheet model in light of what we know about modern energy fluxes and their relative influence on high latitude climate this possibility should be examined

atmospheric carbon dioxide concentration across the mid pleistocene transition the dominant period of pleistocene glacial cycles changed during the mid pleistocene from 40 000 years to 100 000 years for as yet unknown reasons here we present a 2 1 million year record of sea surface partial pressure of co2 pco2 based on boron isotopes in planktic foraminifer shells which suggests that the atmospheric partial pressure of co2 pco2 was relatively stable before the mid pleistocene climate transition glacial pco2 was approximately 31 microatmospheres higher before the transition more than 1 million years ago but interglacial pco2 was similar to that of late pleistocene interglacial cycles 450 000 years ago these estimates are consistent with a close linkage between atmospheric co2 concentration and global climate but the lack of a gradual decrease in interglacial pco2 does not support the suggestion that a long term drawdown of atmospheric co2 was the main cause of the climate transition

significantly warmer arctic surface temperatures during the pliocene indicated by multiple independent proxies temperatures in the arctic have increased by an astounding 1 c in response to anthropogenic forcing over the past 20 years and are expected to rise further in the coming decades the pliocene 2 6 5 3 ma is of particular interest as an analog for future warming because global temperatures were signifi cantly warmer than today for a sustained period of time with continental confi gurations similar to present here we estimate mean annual temperature mat based upon three independent proxies from an early pliocene peat deposit in the canadian high arctic our proxies including oxygen isotopes and annual ring widths mat 0 5 1 9 c coexistence of paleovegetation mat 0 4 4 1 c and bacterial tetraether composition in paleosols mat 0 6 5 0 c yield estimates that are statistically indistinguishable the consensus among these proxies suggests that arctic temperatures were 19 c warmer during the pliocene than at present while atmospheric co 2 concentrations were 390 ppmv these elevated arctic pliocene temperatures result in a greatly reduced and asymmetrical latitudinal temperature gradient that is probably the result of increased poleward heat transport and decreased albedo these results indicate that arctic temperatures may be exceedingly sensitive to anthropogenic co 2 emissions