palaeoceanography the briny deep modern deep waters form in the nordic seas when high salinity surface waters cool and sink an analysis of arctic ocean sediments suggests that throughout the past fifteen million years brines created during sea ice formation controlled the sinking of water

the origin of the sierra de aracena hollows in the sierra morena huelva andalucia spain hollows in the sierra de aracena part of western sector of sierra morena region huelva spain are geoecologically unusual macroforms they are underlain by deeply weathered bedrock but have eutrophic soils with distinctive vegetation paleosols with very dark colours a predominance of smectites and large amounts of total and free iron occur on the floors on the hollows an evolutionary model is proposed for the hollows involving differential weathering during the mesozoic on plutonic and amphibolitic rocks alpine tectonic activity followed by quaternary erosion and exhumation leading to formation of erosional terraces

the palaeontology and dating of the weybourne crag an important marker horizon in the early pleistocene of the southern north sea basin abstract in the north sea basin the marine bivalve macoma balthica first appears within the early pleistocene weybourne crag which forms an important biostratigraphical datum here we review the fossil assemblages from sites of this age prompted by new discoveries from sidestrand norfolk uk the molluscan assemblages from this horizon are dominated by intertidal species with some colder deeper water taxa and a few temperate non marine species a high boreal low arctic marine environment with reduced salinities is indicated an extensive assemblage of small mammals dominated by voles includes two species mimomys hordijki and ungaromys dehmi previously unknown from the british pleistocene the assemblage can be assigned to tesakov s mammal biozone mnr1 mn17 middle villafranchian which according to current estimates corresponds to a date of 2 2 2 1 ma mis 84 79 it matches another assemblage from 61 m to 65 m in the zuurland 2 borehole in the netherlands and is similar to that from the dutch tiglian type site at tegelen although this has more temperate elements a late tiglian age is consistent with the co occurrence of the marine bivalves macoma balthica mya arenaria and the freshwater gastropod viviparus glacialis in the zuurland 2 borehole and in a north sea borehole bgs 52 02 472 a macoma balthica mya arenaria concurrent range zone is defined for this assemblage which can be traced across the north sea basin amino acid dating provides strong independent support for these correlations and indicates that the baventian cold stage post dates the bramertonian norwich crag it also confirms that early pleistocene molluscan assemblages with m balthica are younger than those without it the correlation of this marine marker horizon with mammal biozone mnr1 provides a secure link between continental and marine sequences during the early pleistocene it also provides a basis for dating events in the pre glacial fluvial drainage history and linking it to the east european mammal zonation

middle miocene tectonic boundary conditions for use in climate models utilizing general circulation models gcms for paleoclimate study requires the construction of appropriate model boundary conditions we present a middle miocene paleotopographic and paleobathymetric reconstruction geographically constrained at 15 ma for use in gcms paleotopography and paleogeography are reconstructed using a published global plate rotation model and published geological data paleobathymetry is reconstructed through application of an age depth relationship to a middle miocene global digital isochron map followed by the overlay of reconstructed sediment thickness and large igneous provinces adjustments are subsequently made to ensure our reconstruction may be utilized in gcms

kaolinitic paleosols in the south west of the iberian peninsula sierra morena region spain paleoenvironmental implications abstract this paper studies the physical and chemical characteristics of eight different soil profiles with paleohorizons rich in kaolinites and developed on different lithologies along the southern border of the hercinian iberian peninsula sierra morena region the paleohorizons of four of these soils are defined by low ph levels desaturation of the exchange complex and proportions of kaolinite of over 75 these characteristics are associated with subtropical pedogenic conditions during the pliocene period a further two soil profiles are contemporary and defined by the following features they occupy depression areas the clay fraction is dominated by kaolinites and smectites the exchange complex is saturated and the ph is above neutral values the last two soil profiles are developed on pliocene continental deposits and have more moderate pedogenic features their kaolinite levels are around 50 and their evolution is associated with shorter periods of exposure to these subtropical alteration conditions

palaeoenvironmental reconstruction and biostratigraphy with marine palynomorphs of the plio pleistocene in tjornes northern iceland abstract deposits from the tjornes peninsula in northern iceland enable the assessment of past ocean currents and the influence of the nearby island most palaeoecological studies with dinoflagellate cysts from the northern and central atlantic focus on oceanic or shelf settings and deal with outer neritic and oceanic species dinoflagellate cyst studies of marginal marine settings are scarce and represent only short time intervals the tjornes section however accommodates 1200 m of sediments that are mainly shallow marine the sediments are attributed to the lower pliocene tjornes beds and the overlying upper pliocene to pleistocene breidavik group the dinoflagellate cysts and other marine palynomorphs from 68 samples from the tjornes beds and 20 samples from the breidavik group are studied the deposits are divided into five dinoflagellate cyst assemblage zones daz and one barren interzone biz the changes in the assemblages proved to be independent of the changes in bathymetry of the sedimentary environment heterotrophic dinoflagellate cysts dominate in daz1 an abrupt impoverishment in species diversity is observed in daz2 when autotrophic species dominate the assemblage slightly preceding the entrance of pacific molluscs in the area in unit 14 of the serripes zone heterotrophic species unit 12 13 re enter the area in daz3 the marked decrease and return of the heterotrophic species does not relate to conditions of preservation but may result from the loss of nutrients the upper part of daz1 and the base of daz3 reflect major changes in the palaeoceanography and span a period during which the polar front may have moved temporarily from the area the first event situated at the top of daz1 occurred in litholog unit 4 halfway the tapes zone between circa 5 and 4 8 ma the second event at the top of litholog unit 12 around 4 5 ma has been linked to the shoaling of the central american seaway heterotrophic dinoflagellates disappear almost completely from the area during the deposition of the pleistocene breidavik group daz4 5 a transition from a heterotrophic dominated assemblage to an autotrophic dominated assemblage is observed in the horgi formation daz4a an assemblage dominated by autotrophic cold water species comparable to the present day assemblage of the area north of iceland occurs from unit 10 in the pleistocene svarthamar member on daz5 this study underscores that the distribution of temperature sensitive dinoflagellate cysts is influenced by the availability of nutrients and changes in ocean currents

a multi proxy study of pliocene sediments from ile de france north east greenland abstract a multi technique approach has been used to study a pliocene shallow water marine deposit designated the ile de france formation in north east greenland the sequence is correlated on the basis of 87sr 86sr ratios in shells and palaeomagnetic studies with the gauss normal polarity chron which is dated to between 2 60 and 3 58 ma years bp this dating is in accordance with amino acid epimerisation and evidence from dinoflagellates foraminifers and molluscs sediments marine molluscs and foraminifers show that the sequence was deposited on the inner shelf below storm wave base seawater temperatures were much higher than today as demonstrated by the occurrence of a number of southern extra limital species the same applies to air temperature and the few remains of land plants may indicate a forested upland with picea and thuja a number of extinct taxa are present including nucula jensenii that is erected as a new species

an 18 million year record of vegetation and climate change in northwestern canada and alaska tectonic and global climatic correlates abstract we reconstruct long term vegetation paleoclimatic trends spanning the last 18 million years in alaska yukon and far western northwest territories twenty one average percentage spectra for pollen and spores are assembled from eight surface subsurface sections the sections are dated independently or by correlation pollen and spore ratios indicate the direction of change in vegetation and climatic parameters growing season temperature test tree canopy density cest and paludification at study sites pest a global warm peak ca 15 ma is shown by the abundance of thermophilous taxa including fagus and quercus a temperature decline immediately following 15 ma parallels climatic reconstructions based on marine oxygen isotopes subsequent declines correlate to the messinian event and the onset of late pliocene pleistocene glaciation after 7 ma herbs and shrubs become more important elements of the palynological assemblages suggesting a more continental colder drier climate however a late pliocene warm interval is evident vegetation climatic changes during the early to late miocene show synchrony with and are most economically attributable to global events after 7 ma vegetation climate change is attributed primarily to latest miocene to pleistocene uplift of the alaska range and st elias mts the continuing influence of global climatic patterns is shown in the late pliocene warm interval despite uplift to the south the opening of the bering strait ca 3 ma may have moderated the climate in the study area

genes diversity and geologic process on the pacific coast abstract we examine the genetics of marine diversification along the west coast of north america in relation to the late neogene geology and climate of the region trophically important components of the diverse west coast fauna including kelp alcid birds e g auks puffins salmon rockfish abalone and cancer crabs appear to have radiated during peaks of upwelling primarily in the late miocene and in some cases secondarily in the pleistocene phylogeographic barriers associated with mio pliocene estuaries of the mid california coast the pliocene opening of the gulf of california tectonic and eustatic evolution of the california bight as well as the influence of pleistocene and holocene climate change on genetic structure are assessed in a geologic context comparisons to east coast and western freshwater systems as well as upwelling systems around the globe provide perspective for the survey

salinity changes in the north west pacific ocean during the late pliocene early quaternary from 2 73 ma to 2 52 ma recent research has increasingly advocated a role for the north pacific ocean in modulating global climatic changes over both the last glacial cycle and further back into the geological record here a diatom 18o record is presented from ocean drilling program site 882 over the pliocene quaternary boundary from 2 73 ma to 2 52 ma mis g6 mis 99 large changes in 18odiatom of c 4 from 2 73 ma onwards are documented to occur on a timeframe broadly coinciding with glacial interglacial cycles these changes are primarily attributed to large scale inputs of meltwater from glacials surrounding the north pacific basin and the bering sea despite these inputs and associated change in surface water salinity on the basis of existing opal and uk37 temperature data and new modelled water column densities no evidence exists to suggests a removal of the halocline stratification or a resumption of the high productivity system similar to that which prevailed prior to 2 73 ma the permanence of the halocline suggests that the region played a key role in driving global climatic changes over the early glacial interglacial cycles that followed the onset of major northern hemisphere glaciation by inhibiting deep water upwelling and ventilation of co2 to the atmosphere

geological correlations with the interior density structure of venus a model has been constructed by stipulating that venusian long wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms for convenience the latter are represented by a single airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper mantle convection is represented at a single depth by mass perturbations that drive viscous flow we invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near surface density anomalies the mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings the near surface density anomalies show no particular pattern the inversion results have been compared with a global synthesis of magellan data impact crater density does not correlate with the mantle convection pattern or the near surface density anomalies large volcanic structures are associated with upwellings planitiae are associated with downwellings coronae are anticorrelated with high amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust tesserae show no definitive relationship to the present mantle convection pattern

magnetostratigraphic biostratigraphic and stable isotope stratigraphy of an upper miocene drill core from the sale briqueterie northwestern morocco a high resolution chronology for the messinian stage we report a high resolution stable isotope carbonate magnetostratigraphic and biostratigraphic record from a 175 m drill core from the sale briqueterie which is part of the bou regreg section in northwestern morocco the sale drill core spans the interval from paleomagnetic chron c4n partim to c3r earliest gilbert which represents the time leading up to and including the isolation and desiccation of the mediterranean i e the messinian salinity crisis during chrons c3an and c3ar 6 935 to 5 894 ma the isotope and carbonate signals display quasi periodic variations with estimated periods of 40 and 100 kyr respectively we interpret the 40 kyr 18o variations as reflecting changes in global ice volume caused by obliquity induced changes 41 kyr in solar insolation in polar regions the 100 kyr carbonate variations probably represent long term modulation of the amplitude of the precessional cycle 21 kyr which is not resolved by our sampling frequency the cyclic nature of the oxygen isotope signal permits us to extend the isotope nomenclature of shackleton et al 1994a from stage tg24 in chron c3r earliest gilbert to stage c3ar 18o 18 at the base of chron c3ar 6 935 ma a major change in paleoceanographic conditions is recorded across the tortonian messinian boundary which we correlate to chron c3bn at 7 04 ma benthic foraminiferal 18o values increased by an average of 0 4 in two steps at 7 17 ma and 6 8 ma and 13c values decreased by 0 7 0 8 between 7 1 and 6 8 ma representing the late miocene carbon shift the first step in 18o values coincides with an inferred reversal in deep water circulation through the rifian corridor and the second correlates with the base of the tripoli formation and onset of crisis conditions in the mediterranean we suggest that the increase in 18o values represents at least in part an increase in global ice volume that lowered sea level and contributed to the establishment of a negative water budget in the mediterranean average 18o values remained high throughout most of chrons c3ar and c3an reaching maximum 18o values during isotope stages tg20 and 22 in chron c3r earliest gilbert the glacio eustatic falls associated with these events may have resulted in the complete isolation of the mediterranean from the world ocean shackleton et al 1994a following stage tg12 in the sale record there exists a trend toward progressively lower 18o values that may represent a series of marine transgressions that eventually reflooded the mediterranean and ended the salinity crisis

influence of brine formation on arctic ocean circulation over the past 15 million years the early oceanographic history of the arctic ocean is important in regulating and responding to climatic changes however constraints on its oceanographic history preceding the quaternary the past 1 8 myr have become available only recently because of the difficulties associated with obtaining continuous sediment records in such a hostile setting here we use the neodymium isotope compositions of two sediment cores recovered near the north pole to reconstruct over the past approx15 myr the sources contributing to arctic intermediate water a water mass found today at depths of 200 to 1 500 m we interpret high neodymium ratios for the period between 15 and 2 myr ago and for the glacial periods thereafter as indicative of weathering input from the siberian putoranan basalts into the arctic ocean arctic intermediate water was then derived from brine formation in the eurasian shelf regions with only a limited contribution of intermediate water from the north atlantic in contrast the modern circulation pattern with relatively high contributions of north atlantic intermediate water and negligible input from brine formation exhibits low neodymium isotope ratios and is typical for the interglacial periods of the past 2 myr we suggest that changes in climatic conditions and the tectonic setting were responsible for switches between these two modes

closing of the central american seaway and the ice age a critical review 1 what role did the closing of the central american seaway play in enabling continental ice sheets to wax and wane over north america and fennoscandia a summary of relevant evidence presented here permits a causal relationship between them but can be interpreted to show none the common denominator of such evidence is the approximate simultaneity of that closing with global cooling and the first major ice advance at the resolution of paleoclimate however geologic evidence from central america places only weak constraints on when interchange between pacific and caribbean water ceased the strongest evidence for when north and south america became connected by a continuous land bridge the great american exchange of vertebrates assigns climate change the causal role for an arid central american climate typical of glacial times seems necessary for animals that inhabited savannas to pass through panama paleoceanographic and geochemical studies of environments and water masses in the eastern pacific western caribbean and atlantic in general call for continual change since 6 ma or earlier if evidence concurrent with global change near 3 5 3 ma seems more widespread than for other times hypothesized connections between a closed seaway and glaciation commonly call for profoundly different north atlantic ocean circulation but simulations using general circulation models provide a spectrum of differences in circulation for open and closed seaways might much not all of the evidence implicating the closing of the seaway in global climate change in fact be a consequence of that change and blind to central america s tectonic development

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

central american paleogeography controlled pliocene arctic ocean molluscan migrations the pliocene interchange of north pacific and north atlantic marine faunas via the arctic ocean was long thought to have been a single episode of faunal exchange between the northern oceans that took place as soon as the bering strait first opened new evidence implies that there were two northern migration events instead of one and that the second phase of migration was much later than the first the migration of atlantic arctic mollusks into the bering sea and north pacific when the bering strait first opened at 4 8 5 5 ma constituted the initial phase of this interchange but the abrupt appearance of north pacific mollusks in the north atlantic at 3 6 ma postdated the first opening of the bering strait by 1 2 1 9 m y this second phase of trans arctic migration was also coeval with shoaling of the central american seaway between north and south america this late pliocene trans arctic migration of north pacific mollusks is evidence for the reversal of marine flow to northward through the bering strait which was one consequence of the reorganization of northern hemisphere ocean circulation caused by substantial closure of the central american seaway this inferred causal link between the histories of the beringian and panamanian ocean gateways is in agreement with ocean circulation models

sources of iron oxides in reddish brown soil profiles from calcarenites in southern spain abstract we studied the distribution with depth of the different forms of iron in two reddish brown rubified soils developed on hematite free calcarenites in the province of cordoba southern spain hematite appeared in significant amounts in the a and b horizons of the soils and seemed to have originated mainly from the weathering of the fe bearing clay minerals smectites because of their low contents of fe both the non clay fraction and the carbonate of the parent material contributed little to fe oxide formation the distribution with depth of goethite in the clay fractions suggested that part of the initial goethite had been lost and might have been altered to hematite

late paleocene arctic coastal climate inferred from molluscan stable and radiogenic isotope ratios abstract characterizing polar climates during past warm intervals is important for understanding greenhouse climate dynamics because high latitude surface temperatures and precipitation patterns are extremely sensitive to global climatic conditions model data comparisons of high latitude climates during past warm intervals cretaceous eocene pliocene are currently at odds specifically simulations of past warm climates produce polar regions characterized by sub freezing temperatures and significant seasonality whereas limited fossil proxy data indicate higher mean annual temperatures and low seasonality i e an equable climate we have constructed a data set to infer northern hemisphere polar marine temperatures during the late paleocene seasonal and mean annual temperature and the oxygen isotopic composition of precipitation are reconstructed for an arctic coastal setting during the thanetian 57 58 ma using fossil shell stable isotope profiles we estimate that coastal water temperatures varied between 10 and 15 c during the seasons of growth presumably spring summer and fall these findings support paleontological evidence implying northern hemisphere polar climates were seasonally warm during the late paleocene in addition estuarine fossil oxygen isotope profiles show periodic excursions to low values as low as 19 vpdb which indicate seasonal pulses of isotopically depleted freshwater

neogene glacigenic debris flows on james ross island northern antarctic peninsula and their implications for regional climate history detailed sedimentological and microtextural analyses of newly discovered late neogene diamictites and other coarse grained facies mostly sandwiched between hyaloclastite of the james ross island volcanic group and cretaceous sandstone and mudstone indicate deposition mainly by glacigenic debris flows the deposits on james ross island northern antarctic peninsula constrain the depositional setting ice bed dynamics and regional palaeoclimate the sequences on james ross island vary in age but are mainly late miocene and pliocene unlike neogene sedimentary sequences elsewhere in antarctica those on james ross island are unusually well dated by a combination of 40ar 39ar and 87sr 86sr analyses on fresh interbedded lavas and pristine bivalve molluscs respectively the sr isotopic ages of the debris flows cluster around 4 74 4 89 5 44 5 78 and 6 31 ma and probably date relatively warm periods in the northern antarctic peninsula region when the bivalves lived under ice poor or seasonally ice free conditions the bivalves are often remarkably well preserved lack adhering lithified sediment and in at least two locations are large mainly unfragmented and sometimes articulated suggesting that they were alive immediately prior to their incorporation in subaqueous debris flows at the margins of an advancing glacier these fossiliferous glacigenic debris flows signify episodes of ice expansion during relatively warm periods or interglacials of the late miocene and pliocene the james ross island glacigenic sedimentary successions attain thicknesses of up to 150 m and extend over 4 km laterally the high volume of glacigenic sediment delivery implicit in the james ross island successions indicates that a series of dynamic ice fronts crossed the region during the late miocene and pliocene epochs associated evidence in the form of clast abrasion including striations and faceting and bedrock erosion is indicative of basal sliding and subglacial sediment deformation active at the ice bed interface and wet based temperate or polythermal regimes prior to remobilisation the evidence further suggests two local ice caps on james ross island during the warm periods as well as ice overriding by the antarctic peninsula ice sheet from the west and northwest

arctic perennial ice cover over the last 14 million years knowledge of the long term history of the perennial ice is an important issue that has eluded study because the cenozoic core material needed has been unavailable until the recent arctic coring expedition acex detrital fe oxide mineral grains analyzed by microprobe from the last 14 ma 164 m of the acex composite core on the lomonosov ridge were matched to circum arctic sources with the same mineral and 12 element composition these precise source determinations and estimates of drift rates were used to determine that these sand grains could not be rafted to the acex core site in less than a year thus the perennial ice cover has existed since 14 ma except for the unlikely rapid return to seasonal ice between the average sampling interval of about 0 17 ma both north america and russia contributed significant fe grains to the acex core during the last 14 ma

stratigraphy and paleoceanography of pliocene deposits of karaginsky island eastern kamchatka u s s r abstract the lithology and paleontology of pliocene deposits from karaginsky island off eastern kamchatka peninsula u s s r are described in order to provide a paleoceanographic reconstruction of this region of the eastern north pacific ocean the stratigraphic sequence is divided into three suites the limimtevayamian the ust limimtevayamian and the tusatuvayamian which correspond to three standard pacific diatom zones the thalassiosira oestrupii neodenticula koizumi n kamtschatica and n koizumi the entire sequence is further subdivided into six members and 18 distinct beds on the basis of molluscan assemblages and three members and seven beds on the basis of benthic foraminiferal assemblages marine ostracodes are described from the limimtevayamian suite the marine invertebrate assemblages from karaginsky island provide evidence that 1 ocean water temperatures were warmer than today about 4 ma and again about 3 5 3 0 ma 2 several arctic atlantic species of molluscs and ostracodes migrated to the eastern north pacific between 4 2 3 0 ma presumably through the proto bering strait 3 many species found in the karaginsky island pliocene represent pacific taxa which migrated through the arctic into the north atlantic during the late pliocene

palynology and micropalaeontology of the pliocene pleistocene transition in outcrop from the western caspian sea azerbaijan potential links with the mediterranean black sea and the arctic ocean abstract new palynological ostracod and foraminiferal data are presented from a long outcrop section in the jeirankechmez river valley azerbaijan near the western coast of the caspian sea the interval studied includes the upper part of the pliocene productive series and overlying plio pleistocene akchagylian akchagyl and apsheronian apsheron regional stages productive series sediments were deposited in a closed fluvio lacustrine basin isolated from any marine influence the onset of akchagyl deposition is marked by a lithological change associated with a significant flooding event that at its maximum extent reached the sea of azov and into present day iran kazakhstan turkmenistan and russia at the jeirankechmez locality the lowermost beds of the akchagyl contain predominantly freshwater assemblages with very minimal marine or brackish content showing that the onset of akchagyl deposition was not a marine induced event reworked mesozoic palynomorphs occur frequently in this lowermost interval including the reworked pollen taxa aquilapollenites triprojectus that were eroded from the north or north east significant marine influence is evident ca 30 m above the base of the akchagyl in the studied outcrop marked by the cassidulina beds which contain a distinct but low diversity assemblage of foraminifera that occurs widely and can be correlated in many parts of the greater caspian region dinoflagellate cysts dinocysts in the marine interval include frequent specimens very similar to algidasphaeridium capillatum matsuoka and bujak a species only previously recorded from the northern bering sea the combined evidence from these dinocysts and foraminifera suggests that a marine i e seaway connection existed briefly between the arctic ocean and the caspian sea at the very end of the pliocene re examination of core material from the adriatic sea shows that cassidulina reniforme norvang was present in the mediterranean during and shortly after the last glacial maximum the possibility that the end pliocene marine incursion came from the mediterranean via the black sea region to the caspian sea cannot be entirely ruled out but is considered unlikely biometric analyses are applied to obtain a better understanding of the palaeoenvironmental significance of the assemblages dominated by cassidulinids an interval 300 m thick is assigned to the apsheron regional stage on the basis of predominantly brackish ostracod and dinocyst associations the dinocysts are of peri paratethyan affinity and closely resemble species first described from miocene and pliocene sediments in the pannonian and dacic basins of eastern europe many similarities exist in the microplankton records dinocysts and acritarchs between the caspian sea the black sea and central paratethys

a new age model for the pliocene pleistocene tjornes section on iceland its implication for the timing of north atlantic pacific palaeoceanographic pathways abstract the plio pleistocene outcrops of the tjornes peninsula in northern iceland present a unique section in which near shore and terrestrial deposits alternate with lava flows this section plays an important role in the correlation of climate signals from around the north atlantic but a solid age model has not been available for the entire sequence due to problematic k ar dating and incompletely recorded palaeomagnetic signal a palynological analysis with dinoflagellate cysts of 68 samples from the tjornes beds and twenty samples from the younger breidavik group was carried out in order to establish an age model for the tjornes section independent of the available k ar ages the dinoflagellate cyst record of the tjornes beds consists mainly of pliocene taxa and indicates most probably a post miocene age for the tapes and mactra zones of the tjornes beds both operculodinium tegillatum and batiacasphaera minuta which have a highest occurrence near the top of the zanclean at c 3 8 ma were present in the section up to the middle of the serripes zone reticulatosphaera actinocoronata with a highest occurrence around 4 4 ma was recorded in situ at the base of the serripes zone this suggests that the entire serripes zone was deposited between c 4 0 and 4 5 ma further the normal polarities below and above the skeifa lavas in the serripes zone were consequently linked to the nunivak and cochiti subchron respectively the entire tjornes beds were likely deposited in the early pliocene before c 4 0 ma the breidavik group is deposited much later in the quaternary from just before the olduvai subchron at c 2 2 ma the gilbert gauss transition is located between the tjornes beds and the hoskuldsvik lavas and the gauss matuyama transition between the furuvik and horgi formations the exact position of both reversals is not preserved because two appreciable hiatuses of c 600 kyr up to maximally 900 kyr occur at these locations according to the new age model the major invasion of pacific molluscs at the base of the serripes zone took place before 3 8 ma in combination with the palaeopolarity data this event can be placed in the nunivak subchron at c 4 5 ma the invasion is likely a result of the northward flow of pacific waters through the bering strait to the north atlantic which may have been caused by the shoaling of the central american seaway between 4 7 and 4 2 ma sarnthein et al 2009

the tectonic architecture of wrinkle ridges on mars abstract the geodynamic history of mars is written largely in the form of its tectonic structures a global population of compressional structures records a history of interior cooling and contraction in combination with volcanic loading and flexure of the lithosphere the most common of these structures are wrinkle ridges interpreted as having formed by folding of near surface layered volcanic materials above blind thrust faults topographic profiles of wrinkle ridges are found to dominantly fall into three classes symmetric ridges arch style ridges characterized by broad convex upward backlimbs and double ridges characterized by a broad arch and narrow ridge a monte carlo boundary element model was used to constrain the underlying geometry of faults associated with the different ridge types symmetric ridges are pop up structures formed between quasi symmetric thrust and backthrust faults in some cases above a horizontal decollement or low angle fault at depth arch style ridges formed above a dominant primary thrust fault of near constant dip with a secondary backthrust playing a lesser role in generating subtle inflections in the backlimb double ridges require two backthrust faults to localize strain within the narrow ridge at the ridge crest and to generate the secondary antiform on the backlimb in these ridges both the upper primary thrust and backthrust are contained within the layered volcanic materials while the lower portions of the primary thrust can extend deep into the crust or upper mantle a shallower depth of fault penetration in areas of thicker crust is consistent with the depth of faulting being limited by the brittle ductile transition dips vary widely but are typically steep in the upper faults and quite gentle in the lower faults thus while wrinkle ridges do provide important records of tectonic and geodynamic processes on mars as well as other bodies the associated tectonic structures are more complex and varied than previously considered

lead isotopes in north pacific deep water implications for past changes in input sources and circulation patterns the sources of non anthropogenic pb in seawater have been the subject of debate here we present pb isotope time series that indicate that the non anthropogenic pb budget of the northernmost pacific ocean has been governed by ocean circulation and riverine inputs which in turn have ultimately been controlled by tectonic processes despite the fact that the investigated locations are situated within the asian dust plume and proximal to extensive arc volcanism eolian contributions have had little impact we have obtained the first high resolution and high precision pb isotope time series of north pacific deep water from two ferromanganese crusts from the gulf of alaska in the ne pacific ocean and from the detroit seamount in the nw pacific ocean both crusts were dated applying 10be 9be ratios and yield continuous time series for the past 13 5 and 9 6 myr respectively lead isotopes show a monotonic evolution in 206pb 204pb from low values in the miocene 18 57 to high values at present day 18 84 in both crusts even though they are separated by more than 3000 km along the aleutian arc the variation exceeds the amplitude found in equatorial pacific deep water records by about three fold there also is a striking similarity in 207pb 204pb and 208pb 204pb ratios of the two crusts indicating the existence of a local circulation cell in the sub polar north pacific where efficient lateral mixing has taken place but only limited exchange in terms of pb with deep water from the equatorial pacific has occurred both crusts display well defined trends with age in pb pb isotope mixing plots which require the involvement of at least four distinct pb sources for north pacific deep water the pb isotope time series reveal that eolian supplies volcanic ash and continent derived loess have only been of minor importance for the dissolved pb budget of marginal sites in the deep north pacific over the past 6 myr the two predominant sources have been young volcanic arcs one located in the northeastern part and one located in the northwestern part of the pacific margin from where material has been eroded and delivered to the ocean most likely via riverine pathways

relative sea level variations in the chukchi region arctic ocean since the late eocene 1 relative sea level rsl variations are a result of tectonic activity changing of the water volume in ocean basins e g due to increasing decreasing of ice volume evaporation and variations in regional to global climate which influence erosional processes and material transport we present multi channel seismic data combined with dated sediment horizons from the chukchi shelf arctic ocean based on a series of prograding sequences in the upper 4 km of sediments and the method of seismic sequence stratigraphy we introduce the first rsl curve for the chukchi region beginning in the late eocene 40 ma the comparison of the chukchi rsl curve with the global rsl curve shows that rsl lowering events in the chukchi region do not correlate with global events for the eocene oligocene early miocene between the eocene oligocene and the late oligocene the chukchi rsl variations were small 100 m since the late oligocene the chukchi rsl increased until the opening of the fram strait in the early miocene we show that the chukchi rsl variations are representative for the arctic ocean and conclude that the arctic ocean was an isolated basin for the eocene oligocene early miocene

some aspects of paleoweathering in the iberian hercynian massif summary the iberian hercynian massif occupies most of the western half of the iberian perninsula and forms the basement over which mesozoic cenozoic and quaternary materials rest unconformably over it can be found remains of weathering profiles that are fossilized by marine and or continental sediments of different ages on the southwestern border of the tertiary basin of the duero and below the series attributed to the lower tertiary palaeocene lies a weathering mantle that has been displaced by the effects of alpine tectonics accordingly its remains may appear truncated and or covered by other more recent deposits the present work studies this ancient weathering mantle in the zones in which it appears developed over the slaty series of the basement in the province of salamanca central west spain the profiles chosen reveal the existence of a strong chemical weathering that involves the destruction of some primary materials fe bearing chlorites and feldspars and the formation of kaolinites and fe oxihydroxides the start of the degradation of chlorite occurred during the first weathering phases this evolved towards an expandible smectite like mineral this first phase reflects the disappearance of albite and chemical analysis points to a pronounced loss of na in the most advanced stage the chlorites and smectites have disappeared giving way to the presence of kaolinite goethite and hematites this weathering phase is associated with a strong loss of mg the main processes occurring in these profiles are monosiallitisation and ferruginisation

the ages and soils of two levels of rana surfaces in central spain abstract the geomorphology of two ranas and a specimen soil profile on each have been studied in the province of guadalajara central spain the ranas were first delineated on an aerial photograph then examined in the field for further identification and for the preparation of topographic profiles two soil profiles were described and sampled by horizons analyses were made of the samples for particle size distribution ph cation exchange relations and organic matter plus some observations on mineralogy of clay and fine sand fractions results of the study support earlier suggestions that the younger ran asurface s 1 is of donau age middle villafranchian but indicate that the second s 2 should be assigned to the middle upper pliocene lower villafranchian the soil profiles were found to be an alfisol on the younger and an ultisol on the older rana which is consistent with a greater age for the latter

pliocene palaeoceanography of the arctic ocean and subarctic seas the pliocene is important in the geological evolution of the high northern latitudes it marks the transition from restricted local to extensive regional scale glaciations on the circum arctic continents between 3 6 and 2 4 ma since the arctic ocean is an almost land locked basin tectonic activity and sea level fluctuations controlled the geometry of ocean gateways and continental drainage systems and exerted a major influence on the formation of continental ice sheets the distribution of river run off and the circulation and water mass characteristics in the arctic ocean the effect of a water mass exchange restricted to the bering and fram straits on the oceanography is unknown but modelling experiments suggest that this must have influenced the atlantic meridional overturning circulation cold conditions associated with perennial sea ice cover might have prevailed in the central arctic ocean throughout the pliocene whereas colder periods alternated with warmer seasonally ice free periods in the marginal areas the most pronounced oceanographic change occurred in the mid pliocene when the circulation through the bering strait reversed and low salinity waters increasingly flowed from the north pacific into the arctic ocean the excess freshwater supply might have facilitated sea ice formation and contributed to a decrease in the atlantic overturning circulation

post miocene cold water molluscan faunas from hokkaido northern japan abstract pliocene to recent molluscs from hokkaido are divided mainly into three faunas from oldest to youngest the takikawa honbetsu the setana and the oyashio faunas they correspond to the fundamental division of japanese cold water faunas in post miocene time biostratigraphic and radiometric data show that the takikawa honbetsu stage is assigned to ca 6 0 2 0 ma the setana stage to ca 2 0 0 6 ma and the oyashio stage to 0 6 ma present the takikawa honbetsu fauna characterized by fortipecten takahashii flourished under cool temperate to subarctic climatic conditions the setana fauna can be subdivided into two lower and upper by stratigraphic horizon and species composition the lower setana fauna including yabepecten tokunagai existed under subarctic climatic conditions the onset of the setana stage is marked in particular by cryophilic taxa such as chlamys islandicus this cooling event appears to have resulted from an intensification of glaciation during the early pleistocene the upper setana fauna is in turn characterized by mizuhopecten tokyoensis s s and lived under cool temperate climatic conditions the oyashio fauna is composed of extant cold water species such as mizuhopecten yessoensis that have been affected by the oyashio current however a molluscan fauna characterized by warm water elements lived in hokkaido for a short time in the middle pleistocene ca 0 4 ma during a warm interglacial period the faunal succession mentioned above is presumably controlled by environmental changes such as glacio eustacy climatic changes tectonic movements etc which may have occurred in the northwestern pacific during the late cenozoic time the appearance of these faunas in hokkaido probably resulted from the immigration of boreal elements from the northern regions

evidence for an early opening of the bering strait the first opening of the bering strait was an important palaeogeographical and biogeographical event for marine and terrestrial biotas in asia and north america1 2 3 and an oceanographic event of global importance4 5 6 this event however has never been precisely dated so it has not been accurately incorporated into models of global biogeography and oceanography a recent find of the bivalve mollusc astarte in southern alaska is a clear signal that the strait had opened by at least the late miocene or earliest pliocene epochs because this genus otherwise dwelled only in the arctic and north atlantic oceans at that time1 2 here we show that marine diatoms with astarte are correlative with subzone b of the neodenticula kamtschatica zone of the north pacific diatom biochronology yielding an age range of 4 8 5 5 myr refs 7 8 stratigraphic information suggests that this may be a minimum age range for the strait s first opening which evidently occurred between 4 8 and 7 3 7 4 myr ago our results contrast with past studies9 10 11 12 13 that suggested an age of 3 1 4 1 myr for the initial opening of the bering strait

new evidence for the age of bering strait abstract the earliest known opening of bering strait in signaled by the presence in southern alaskan neogene strata of the marine bivalve mollusk astarte which had dwelled throughout the cenozoic in the arctic and north atlantic oceans astarte occurs with age diagnostic marine diatoms in the middle and upper miocene bear lake formation of the alaska peninsula southwestern alaska the diverse diatom flora of more than 100 taxa contains species characteristic of subzone b of the neodenticula kamtchatica zone of the north pacific diatom biochronology which has an age range of 4 8 5 5 ma this inferred age for an early opening of bering strait predates the generally cited ages of 3 1 4 1 ma stratigraphic data suggest that the first opening of the strait may have occurred even earlier

first evidence of percids teleostei perciformes in the miocene of north america miocene sediments of southern saskatchewan have long been known for the mammalian fauna they have produced but little or nothing has been reported of the associated fish material recent examination of this material collected over half a century ago has revealed the presence of a walleye fish walleyes and other members of the family percidae had previously been thought to have arrived in north america as late as the pleistocene and only in the last few years has evidence been found demonstrating their existence in north america in the pliocene the pliocene material from arctic canada indicated that percids may have invaded north america from europe during the late tertiary via the bering isthmus the material presented here shows that in fact percids were already in more southern areas of canada about 10 million years prior to the arctic walleye

eocene gastropods of western kamchatka implications for high latitude north pacific biostratigraphy and biogeography abstract fossiliferous rocks of the snatolskaya and kovachinskaya formations comprise a middle and late eocene shallow marine record of the central part of western kamchatka gastropod assemblages of these formations contain taxa that are conspecific with those in paleogene strata of western north america and japan as well as a large percentage of endemic species analysis of the latitudinal ranges and worldwide occurrences of gastropod genera from these formations show the presence of three biogeographic components cosmopolitan north pacific and endemic no tethyan or circumtropical genera are present in these kamchatkan middle and late eocene gastropod faunas changes in the geographic distribution of north pacific gastropod assemblages through the middle and late eocene indicate that only eastern pacific tethyan taxa were subjected to latitudinal range reduction the distribution of cosmopolitan and north pacific elements did not significantly change from the middle to late eocene which suggests a relatively stable environment and climate stability during that time high latitude eocene gastropod assemblages from western kamchatka demonstrate a high level of endemism at the species level and a low level of endemism on the genus level this pattern is thought to be a result of the unrestricted migration of cosmopolitan taxa northward along the shallow marine margin of the pacific rim

new occurrences of fortipecten hallae dall 1921 mollusca bivalvia in the pliocene of the north pacific abstract the large north pacific bivalve mollusk index fossil fortipecten hallae dall 1921 is present in a well dated stratigraphic section of the milky river formation alaska peninsula southwestern alaska co occurring marine diatoms belong to the upper part of the subzone b of the neodenticula kamtschatica diatom zone of the north pacific diatom chronostratigraphy with an age range of 4 8 5 1 ma early pliocene based on coeval occurrences in northeastern kamchatka russia and synchronous changes in the two molluscan assemblages f hallae is a useful indicator of early pliocene climatic warming along the high latitude north pacific margin

the cenozoic cooling continental signals from the atlantic and pacific side of eurasia a b s t r a c t the evolution of cenozoic continental climate signals from the atlantic and pacific side of eurasia can be assessed for the first time by comparing climate records obtained for two mid latitudinal regions for the west a detailed climate record over the past 45 ma based on palaeofloras from two northern german cenozoic basins mosbrugger et al 2005 r evealed major trends and shorter term events throughout the cenozoic cooling thus testifying the close correlation of continental and marine temperature evolution as derived from oxygen isotopes zachos et al 2008 using the same methodology we analyze a total of 14 floral horizons originating from continental strata of southern primory e russia in order to study the evolution at the eastern side of the continent the primory e record spans the middle eocene to early pleistocene as the coeval record for the atlantic side it reflects major global signals of cenozoic climate change such as the temperature decline throughout the late eocene coinciding with the growth of antarctic ice sheets warming during the mid miocene climatic optimum and step wise cooling throughout the later neogene the comparison of both records reveals differing regional patterns the considerable longitudinal temperature gradient currently existing between both study areas already began to evolve during the aquitanian and was very significant during the mid miocene climatic optimum the temperature offset between east and west is likely attributable to an effective north atlantic current already operational from the late early miocene onwards bringing about mild winters and low seasonality in western europe while in primory e seasonality steadily increased from the late oligocene on the strong late pliocene decline of cold month mean temperatures recorded in primory e is supposed to coincide with the establishment of the siberian high as semi permanent structure of the northern hemisphere circulation pattern when comparing the precipitation records obtained for both study areas an unexpected co variability at the longer term in the order of 5 20 ma is noted pointing to continent wide hydrological changes the steady decline of mean annual precipitation in the primory e record beginning in the bartonian and culminating in the aquitanian coincides with an aridity increase reported from coeval chinese inland localities of the mid latitudes the seasonality patterns of rainfall point to progressive intensification of the east asian summer monsoon in primory e since the later tortonian while the post zanclean decline of the precipitation of the dry season can be related to an increasing impact of the winter monsoon 2015 elsevier b v all rights reserved

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

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

miocene ocean circulation inferred from marine carbon cycle modeling combined with benthic isotope records 1 in a modeling sensitivity study we investigate the evolution of the ocean circulation and of marine carbon isotope 13c records during the miocene about 23 5 million years ago for this purpose we ran an ocean circulation carbon cycle model of intermediate complexity large scale geostrophic hamburg ocean carbon cycle model version 2s exploring various seaway configurations our investigations confirm that the central american seaway played a decisive role in the history of the miocene ocean circulation in simulations with a deep central american seaway depth range 1 3 km typical for the early to middle miocene deep water production in the north atlantic is absent or weak while the meridional overturning circulation is dominated by water mass formation in the southern ocean deep water formation in the north atlantic begins when the central american seaway shoals to a few hundreds of meters which is typical for the late miocene our results do not support ideas that the mid miocene closing of the eastern tethys contributed to antarctic glaciation on the other hand we find some water exchange between the indian ocean and the atlantic via the eastern tethys during the early miocene our model results for the atlantic meridional overturning circulation and for atlantic 13c during the late miocene are largely independent from depth variations of the greenland scotland ridge to a large extent the evolution of miocene deep sea 13c records can be explained with large scale ocean circulation changes our model data comparison for the middle and early miocene suggests that during the early neogene the seaway effect on benthic 13c may have been superimposed by further factors such as climate regime shifts and or terrestrial carbon cycle changes

intensive kaolinization during a lateritic weathering event in south west spain mineralogical and geochemical inferences from a relict paleosol abstract this study documents the mineralogical and geochemical record of a lateritic weathering event during the pliocene in south west spain the paleoweathering profile derived from arkosic sands and comprises a white sandy clayey saprolite a red mottled clay zone overlain by a thick soft layer and a ferruginous pisolitic hardcap partially dismantled by erosive processes kaolinite quartz and degraded k feldspars are the major minerals forming the saprolite along with al goethite mottled zone as well as gibbsite and boehmite soft layer while the pisolitic duricrust is essentially composed of quartz grains embedded in a matrix of fe oxy hydroxides hematite goethite and or maghemite besides quartz the phases most resistant to alteration were a variety of heavy minerals mostly titanium oxides that are present as minor impurities in the residual kaolin deposit the dominant weathering process of kaolinization is geochemically characterized by striking losses of na k ca mg and large ion lithophile elements rb sr cs ba and u because of the almost entirely breakdown of primary minerals mainly feldspars and 2 1 phyllosilicates the ti normalized distribution patterns of trace elements also reveals remobilization and redistribution processes within the profile involving high field strength elements classically regarded as immobile like zr and hf iron showed a contrasted behaviour with depletion in the saprolite zone and strong accumulation in the pisolitic duricrust various transition trace metals cr v sc th nb and ta remained relatively immobile during the weathering and were residually concentrated in the duricrust linked to the fe oxy hydroxides the geochemical and mineralogical evolution of the paleosol studied is consistent with a laterization process evolved under humid subtropical conditions and driven by morphodynamic and hydrodynamic factors gently sloping coastal area with fluctuating water table and rapid water percolation favourable for the development of a deeply kaolinized paleosurface

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

a refined age for the earliest opening of bering strait abstract biostratigraphically and chronostratigraphically important diatoms from the milky river formation alaska peninsula southwestern alaska imply an age range of 5 4 5 5 ma for the oldest north pacific cenozoic occurrence of the marine bivalve mollusk astarte which migrated from the arctic ocean into the north pacific when bering strait first flooded the data presented here are a refinement of the age range of 4 8 5 5 ma reported earlier and imply that bering strait first opened very near the end of the miocene at 5 32 ma

early pliocene fish remains from arctic canada support a pre pleistocene dispersal of percids teleostei perciformes percid remains from pliocene deposits on ellesmere island arctic canada are identified as a species of sander similar to the walleye and sauger of north america and the pike perch of europe and western asia they are named as a new species sander teneri these remains are the most northerly percid elements found to date and suggest the palaeoen vironment was significantly warmer in the pliocene than it is currently the fossil remains show the presence in north america of the family percidae as well as the genus sander prior to the pleistocene indicating a previously proposed pleistocene immigration from europe or asia can be discounted these fossils contradict an earlier hypothesis that percids in particular sander crossed from eurasia to north america in the pleistocene instead the fossils show percids were al ready in the area by the pliocene resume des restes de percides provenant de depots pliocenes de l o le ellesmere dans l arctique canadien sont attribues aune espece de sander semblable au dorejaune et au dorenoir d amerique du nord et au sandre d europe et d asie oc cidentale le nouveau nom d espece sander teneri leur est attribue ces restes constituent les elements de percides les plus nordiques trouvesace jour et portent acroire que le paleomilieu etait sensiblement plus chaud au pliocene qu il ne l est actuellement les restes fossiles demontrent que la famille des percidesd e meme que le genre sander etaient presents en amerique du nord avant le pleistocene ce qui invalide la these anterieurement proposee d une immigration pleistocene provenant d europe ou d asie l existence de ces fossiles vient contredire une hypothese anterieure voulant que les per cides et plus particulierement sander soient arrives en amerique du nord en provenance de l eurasie au pleistocene les fossiles montrent plutot que les percidesetaient dejapresents en amerique du nord durant le pliocene traduit par la redaction

seismicity of the bering strait region evidence for a bering block newly compiled russian and u s seismological data support an independent bering block in motion relative to the north american plate this motion is likely to be driven by the westward extrusion of southwestern alaska resulting from compression in southern alaska due to subduction of the pacific plate and terrane accretion seismicity extends from central alaska through the bering strait and into chukotka in eastern chukotka several southwest trends are evident some of which continue through the koryak highlands to kamchatka the seismicity outlines the bering block which includes most of the bering sea chukchi peninsula seward peninsula and parts of western alaska focal mechanisms young basaltic volcanism and normal faults in western alaska and chukotka indicate that the bering strait is under northeast southwest extension this in conjunction with thrust faulting in the koryak highlands indicates that the bering block is rotating clockwise relative to the north american plate

mass movements on venus preliminary results from magellan cycle 1 observations mass movements on venus seen in radar images acquired by the magellan spacecraft during its first mapping cycle are easily interpreted within the scheme commonly used to classify terrestrial landslides rock slumps rock and or block slides rock avalanches debris avalanches and possibly debris flows are seen in areas of high relief and steep slope gradients and are most abundant in the tectonic troughs that crisscross much of the equatorial region of venus many classes of regolith and sediment movements are not seen such features might be too small to resolve in the 75 meter per picture element radar images or their absence may reflect the relatively thin cover of fine sediments inferred from emissivity measurements and other observations venusian landslides like those found within the valles marineris on mars tend to come from escarpments typically higher than those on earth they appear to fall between the terrestrial and martian height to length trends they are also somewhat larger using length as a surrogate for volume than terrestrial subaerial landslides but smaller than their martian counterparts good morphologic analogs can be found in terrestrial volcanic slides both subaerial and submarine oversteepening of volcanic edifices by intrusion and subsequent lateral collapse appears responsible for shaping a number of large isolated volcanos on venus faulting and seismically induced accelerations are probably responsible for the majority of non volcanic mass movements the atmosphere may participate in promoting the movement of some of the landslide debris but environmental factors e g rainfall temperature cycling do not appear to play as dominant a role as they do on earth based on the types and locations of landslides seen in the magellan data it is possible to scale the terrestrial occurrence rate to venus if venus is as seismically and volcanically active as the earth than of order one major landslide i e discernable in magellan images or 5 10 km in runout distance should occur per year which careful re examination of magellan images acquired during later mapping cycles may be able to detect