global teleconnections in response to a shutdown of the atlantic meridional overturning circulation abstract the global response to a shutdown of the atlantic meridional overturning circulation amoc is investigated by conducting a water hosing experiment with a coupled ocean atmosphere general circulation model in the model the addition of freshwater in the subpolar north atlantic shuts off the amoc the intense cooling in the extratropical north atlantic induces a widespread response over the global ocean in the tropical atlantic a sea surface temperature sst dipole forms with cooling north and warming on and south of the equator this tropical dipole is most pronounced in june december displacing the atlantic intertropical convergence zone southward in the tropical pacific a sst dipole forms in boreal spring in response to the intensified northeast trades across central america and triggering the development of an el nino like warming that peaks on the equator in boreal fall in the extratropical north pacific a basinwide cooling of 1 c takes place with a general westward increase in in

paleomagnetism and 40ar 39ar geochronology of the high grade metamorphic rocks of the jequie block sao francisco craton atlantica ur and beyond abstract paleomagnetic magnetic anisotropy data and 40 ar 39 ar ages are presented for high grade metamorphic rocks of the jequie block sao francisco craton jequie charnockites and enderbites gneisses from the eastern border of the block present northern and steep downward directions carried by ti poor titanomagnetite with high unblocking temperatures 550 600 c a mean direction for 12 sites of enderbites from the eastern sector yielded a magnetic component a d m 47 0 i m 75 7 95 6 3 k 48 8 with a corresponding paleomagnetic pole at 339 6 e 5 4 n a 95 11 2 sites sampled on other metamorphic rocks including granulite facies tonalites and dacites yield different magnetic components anisotropy of magnetic susceptibility measured for all sampling sites shows a high degree of anisotropy p 1 121 1 881 with ne or nw trending magnetic lineations and vertical magnetic foliations these data were used to correct the mean site directions for all components while the other components presented larger scatter after anisotropy correction the component a shows a slightly tighter clustering of magnetic directions d m 61 2 i m 76 5 95 5 4 k 66 2 giving a new anisotropy corrected paleomagnetic pole at 342 1 e 0 5 n a 95 9 6 40 ar 39 ar plateau ages of 2035 4 ma hornblende and 1876 4 ma biotite obtained for one of the samples with component a imply a very low cooling rate of 1 4 c ma for these rocks based on these ages and corrected unblocking temperatures of the magnetic component a we argue that the characteristic magnetization of the jequie metamorphic rocks was acquired by high temperature thermo chemical processes during regional cooling of the adjacent itabuna salvador curaca belt between 2089 and 1985 ma pole age likely at 1 99 ga comparison of this pole with available paleomagnetic poles from south america and africa suggests that neither atlantica nor ur have ever existed and dispersed continental fragments dominated the paleogeography at ca 2 0 ga ago

towards columbia paleomagnetism of 1980 1960 ma surumu volcanic rocks northern amazonian craton abstract we present a paleomagnetic study on the well dated 1980 1960 ma u pb volcanic rocks from the surumu group northern roraima state amazonian craton after thermal and af demagnetization characteristic northwestern remanent magnetization chrm directions with downward inclinations were isolated mean d m 298 6 i m 39 4 95 10 1 n 20 k 11 4 which yielded a paleomagnetic pole sg located at 234 8 e 27 4 n a 95 9 8 magnetic mineralogy experiments show that chrm is carried by high hc and high unblocking temperature ti poor titanomagnetite and or hematite the well dated pole sg contributes to better define the apparent polar wander apw path for the guiana shield during the paleoproterozoic 2070 1960 ma comparison of the amazonian and west african craton s apw paths for the same time interval suggests that these cratonic blocks were linked together at about 1980 1960 ma ago in a paleogeography along which the guri guiana shield and sassandra west africa craton shear zones were aligned as previously proposed

integrated geologic and geophysical interpretation of the buraco da velha copper deposit rondonia brazil a basis for exploring in related environments abstractthe buraco da velha copper deposit lies at the northern limit of the parecis basin in the colorado graben rondonia brazil new geophysical data indicate the presence of a magnetic source below and to the north of the buraco da velha deposit where it corresponds with high gamma ray u th and k counts we have studied the source of the magnetic anomaly and delineated it through derivative transforms euler deconvolution maximin total magnetization direction analysis staged inverse modeling th k ratio and radiometric ternary imaging the magnetic anomaly is elongated in the east west direction and measures 23 6 km and the top of the source is estimated to lie at depths mostly between 50 and 100 m based on the magnetic model we indirectly estimate a jurassic to cretaceous age which is compatible with the 180 80 ma range dated for the copper mineralization gamma ray data are consistent with the hydrothermal alteration in the sedimentary cover and with the presence of an intrusion in the s

strandflat development and quaternary shorelines on tiree and coll scottish hebrides the islands of tiree and coll in the scottish inner hebrides are dominated by staircases of glaciated rock platform surfaces in lewisian gneiss that are interpreted here as areas of strandflat the rock platform surfaces exhibit differential glacio isostatic uplift from east to west and are considered to indicate at least four separate periods of strandflat formation during prolonged intervals of quaternary cold climate it is suggested that many of the well known areas of high rock platform located farther east in the scottish inner hebrides represent eroded remnants of the strandflat surfaces described here the existence of the tiree and coll strandflat surfaces in conjunction with their formation during periods of quaternary glaciation implies that they were produced during periods when mainland scotland was ice covered yet when the western margin of the ice sheet was normally located east of these islands a prominent late glacial raised shoreline also occurs on both islands and declines in altitude to the west from 22 m in northeastern coll to 9 m in western tiree it is believed that this shoreline whose regional gradient is 0 39 m km 1 may represent part of the main wester ross shoreline that was formed in the northwest highlands of scotland during a period of glacial advance that interrupted the decay of the last late devensian ice sheet the general similarity in altitude between the late glacial shoreline features and rock platform surfaces implies that during each period of quaternary strandflat formation relative sea level returned to the same approximate position this would appear to indicate that the glacio isostatic and glacio eustatic components affecting the positions of relative sea level in scotland have remained in approximate equilibrium throughout the duration of the quaternary

sand drift and soil formation along an exposed north atlantic coastline 14 000 years of diverse geomorphological climatic and human impacts this paper sets out the history of aeolian deposition and soil formation over the last 14 15 000 calendar years along the exposed and gale prone atlantic coastlines of the southern isles of the outer hebrides of scotland detailed palaeoenvironmental interpretation and chronological control have been provided by field mapping soil micromorphological analyses and radiocarbon and optically stimulated luminescence osl dating programmes two dominant types of aeolian sands are described in this region quartzose sands are shown to originate mainly as periglacial wind blown sands derived from glacial and periglacial deposits of late devensian age which were formerly exposed on the present continental shelf this aeolian deposition did however continue into the early holocene with re working perhaps by slope wash taking place as recently as the 4th millenniumbp quartzose sands mantle valley floors and hillslopes by the end of the devensian and early holocene a variety of soil types with extensive horizonation had developed upon them including podzolic brown earths peaty gleys and peat soils radiocarbon and osl dating demonstrate that the carbonate sands of the machair sand sheets which characterize the present atlantic shorelines began to accumulate very early for a european shoreline from approximately 8700 calendar years ago on benbecula and north uist whereas further south deposition began later osl dating suggests that non sequences and diastems are common in greater part these machair sand sheets are best interpreted as a composite of separate slivers of sand reflecting small scale and local episodes of sand drift rather than distinct layers of region wide distribution which might reflect past major storms or significant palaeoclimatic fluctuations the new evidence presented here provides little support for the following explanatory geomorphic or stratigraphic models which have previously sought to explain the development of these coastal machair landscapes continuous development layer cake or step wise prograding coastal barrier qualified support is given for the explanations that emphasize i the long term spasmodic input and re cycling of sands and ii that the present smoothed landscapes and distinctive machair grassland vegetation are essentially recent in age and reflect the interactions of natural and human processes in a stormy environment characterized by re working widespread layers of thick organic materials within the carbonate sands in the study area which have previously been regarded as dune palaeosols dating to the bronze age and iron age are suggested by soil micromorphological analysis to be in significant part anthropogenic in origin they are organically enriched layers or midden soils the consequence of human activities notably the disposal of organic wastes and other forms of soil modification during times of significant settlement in these coastal sands these periods of relatively intense human activity are therefore shown to be associated at least locally with periods of soil formation and stability not universal instability and dune drift along these shorelines in general these extensive layers of midden soil do not necessarily imply periods of decreased storminess relatively widespread soil formation in these coastal dunes did however take place in the period between 400 and 100 years ago essentially within the period of increased storminess associated with the little ice age it is suggested that at some point s within the following periods of time broadly defined in calendar yearsbp notable sand drift affecting the quartzose sands the carbonate sands or both took place within the study area 13 700 13 000 11 000 10 500 9000 8300 7500 7000 6900 6400 5800 4200 one of the most notable episodes of drift in the outer hebrides as well as elsewhere on the coast of north west europe 3800 3300 1700 1300 600 200 with individual great storms being recorded in the 17th and 18th centuries and 100 present day limited drift associated with machair stratification

evidence of temperature depression and hydrological variations in the eastern sierra nevada during the younger dryas stade sediment records from two lakes in the east central sierra nevada california provide evidence of cooling and hydrological shifts during the younger dryas stade yd 12 900 11 500 cal yr bp a chironomid transfer function suggests that lake water temperatures were depressed by 2 c to 4 c relative to maximum temperatures during the preceding bolling allerod interstade ba 14 500 12 900 cal yr bp diatom and stable isotope records suggest dry conditions during the latter part of the ba interstade and development of relatively moist conditions during the initiation of the yd stade with a reversion to drier conditions later in the yd these paleohydrological inferences correlate with similar timed changes detected in the adjacent great basin vegetation response during the yd stade includes the development of more open and xeric vegetation toward the end of the yd the new records support linkages between the north atlantic the north pacific and widespread yd cooling in western north america but they also suggest complex hydrological influences shifting hydrological conditions and relatively muted vegetation changes may explain the previous lack of evidence for the yd stade in the sierra nevada and the discordance in some paleohydrological and glacial records of the yd stade from the western united states

climatic backdrop to the terminal pleistocene extinction of north american mammals north american terminal pleistocene mammal extinctions are the subject of a long running scientifi c debate although the role of climate has fi gured centrally we lack clear knowledge of the timing and nature of terminal pleistocene climate variability herein we document lengthy terminal pleistocene drought in the southwestern united states usa using 13 c and 234 u effective moisture proxy data in speleothem calcite stalagmite fs2 from fort stanton cave new mexico supplemented with age data from pool basin shelfstone speleothems from the big room in carlsbad cavern this terminal pleistocene drought defi ned by a sharp rise in both 13 c and 234 u values began just before 14 5 k y ago and lasted at least until 12 9 k y ago when it was briefl y and only mildly interrupted by the younger dryas the timing and length of this drought 1500 yr match the northern hemisphere bolling allerod oscillation preserved in greenland ice cores and exhibited in the 18 o record of stalagmite fs2 rapid transition from cool moist late glacial to warm dry holocenelike climatic conditions was likely unfavorable to many species of pleistocene mammals in the southwestern usa a climate induced extinction implies that this last glacial cycle and its termination were more extreme than previous glacial cycles and or glacial terminations

the 1420 ma indiavai mafic intrusion sw amazonian craton paleomagnetic results and implications for the columbia supercontinent abstract the configuration and the timing of assembly and break up of columbia are still matter of debate in order to improve our knowledge about the mesoproterozoic evolution of columbia a paleomagnetic study was carried out on the 1420 ma indiavai mafic intrusive rocks that crosscut the polycyclic proterozoic basement of the sw amazonian craton in southwestern mato grosso state brazil alternating field and thermal demagnetization revealed south southwest chrm directions with downward inclinations for sixteen analyzed sites these directions are probably carried by sd psd magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests including thermomagnetic data hysteresis data and the progressive acquisition of isothermal remanent magnetization different stable magnetization components isolated in host rocks from the basement 10 km nw away to the indiavai intrusion further support the primary origin of the chrm a mean of the site mean directions was calculated at dm 209 8 im 50 7 95 8 0 k 22 1 which yielded a paleomagnetic pole located at 249 7 e 57 0 s a95 8 6 the similarity of this pole with the recently published 1420 ma pole from the nova guarita dykes in northern mato grosso state suggests a similar tectonic framework for these two sites located 600 km apart implying the bulk rigidity of the rondonian san ignacio crust at that time furthermore these data provide new insights on the tectonic significance of the 1100 1000 ma nova brasil ndia belt a major ew feature that cuts across the basement rocks of this province which can now be interpreted as intracratonic in contrast to previous interpretation from a global perspective a new mesoproterozoic paleogeography of columbia has been proposed based on comparison of these 1420 ma poles and a 1780 ma pole from amazonia with other paleomagnetic poles of similar age from baltica and laurentia a reconstruction in agreement with geological correlations

holocene relative sea level changes in harris outer hebrides scotland uk evidence for relative sea level changes during the middle and late holocene is examined from two locations on the atlantic coast of harris outer hebrides scotland using morphological mapping and survey stratigraphical grain size and diatom analysis and radiocarbon dating the earliest event identified is a marine flood which occurred after 7982 8348 cal a 7370 80 14c a bp when the sea crossed a threshold lying at 0 08 m ordnance datum newlyn od 2 17 m mean high water springs mhws before withdrawing this could have been due to a storm or to the holocene storegga slide tsunami by 6407 6122 cal a 5500 60 14c a bp relative sea levels had begun to fall from a sandflat surface with an indicated mhws level of between 0 08 and 1 96 m 2 01 to 4 05 m this fall reached between 0 30 and 2 35 m 2 39 to 4 44 m after 5841 5050 cal a 4760 130 14c a bp but was succeeded by a relative sea level rise which reached between 0 54 and 1 57 m 1 55 to 3 66 m by 5450 4861 cal a 4500 100 14c a bp this rise continued possibly with an interruption until a second sandflat surface was reached between 2 34 and 0 26 m 0 25 to 2 35 m between 2952 3375 cal a 3000 80 14c a and 1948 2325 cal a 2130 70 14c a bp before present levels were reached the regressive episode from the earliest sandflat is correlated with the abandonment of the main postglacial shoreline it is maintained that the fluctuations in relative sea level recorded can be correlated with similar events elsewhere on the periphery of the glacio isostatic centre and may therefore reflect secular changes in nearshore sea surface levels despite published evidence from trim lines of differential ice sheet loading across the area no evidence of variations in uplift between the locations concerned could be found copyright 2009 john wiley sons ltd

modeling the accumulation and dynamics of beaches on shore platforms a mathematical model was used to study the development and dynamics of beaches on shore platforms the model was based on assumptions that the gradient of the beachface at its seaward edge must be greater than platform gradient beach sediment tends to move landwards and beach occurrence and extent depends upon the amount of sediment available equilibrium beachface gradient was related to breaker height wave period and sediment grain size three grain sizes were used representing fine and coarse sand and pebble two deep water wave sets representing storm wave and west coast swell wave environments and two tidal ranges of 9 1 and 3 3 m runs were made using linear concave and convex platform profiles with gradients ranging between about 0 6 and 8 because platform and beachface gradients are normally quite similar shore platforms are unable to store much sediment for a linear platform with a lower gradient than the beachface sediment first accumulates at the cliff base and then depending on the sediment available extends increasingly seawards beach development first occurs on the gently sloping seaward portions of concave platforms and on the more landward portions of convex profiles the amount of available material partly determines the landward and seaward extent of these beaches respectively although it is also controlled by beachface and platform gradients beaches on shore platforms become thinner and more gently sloping during storms and they may disappear completely depending on such factors as sediment grain size platform gradient the amount of sediment that can be stored during calm conditions wave characteristics and the duration of the storm the response of beaches on rocky coasts to rising sea level will be different from those on sedimentary coasts and will depend in part on the morphology of the rocky foundation

last millennium climate and its variability in ccsm4 abstractan overview of a simulation referred to as the last millennium lm simulation of the community climate system model version 4 ccsm4 is presented the ccsm4 lm simulation reproduces many large scale climate patterns suggested by historical and proxy data records with northern hemisphere nh and southern hemisphere sh surface temperatures cooling to the early 1800s common era by 0 5 c nh and 0 3 c sh followed by warming to the present high latitudes of both hemispheres show polar amplification of the cooling from the medieval climate anomaly mca to the little ice age lia associated with sea ice increases the lm simulation does not reproduce la nina like cooling in the eastern pacific ocean during the mca relative to the lia as has been suggested by proxy reconstructions still dry medieval conditions over the southwestern and central united states are simulated in agreement with proxy indicators for these regions strong global cooling is associated with large volcanic erup

latest pleistocene to holocene hydroclimates from lake elsinore california abstract the hydroclimate of the southwestern united states us region changed abruptly during the latest pleistocene as the continental ice sheets over north america retreated from their most southerly extent to investigate the nature of this change we present a new record from lake elsinore located 36 km inland from the pacific ocean in southern california and evaluate it in the context of records across the coastal and interior southwest united states including northwest mexico the sediment core recovered from lake elsinore provides a continuous sequence with multi decadal resolution spanning 19 9 ka bp sedimentological and geochemical analyses reveal hydrologic variability in particular sand and carbonate components indicate abrupt changes at the oldest dryas od bolling allerod ba and younger dryas yd transitions consistent with the timing in greenland hydrogen isotope analyses of the c28 n alkanoic acids from plant leaf waxes dwax reveal a long term trend toward less negative values across 19 9 ka bp dwax values during the od suggest a north pacific moisture source for precipitation consistent with the dipping westerlies hypothesis we find no isotopic evidence for the north american monsoon reaching as far west as lake elsinore therefore we infer that wet dry changes in the coastal southwest were expressed through winter season precipitation consistent with modern climatology comparing lake elsinore to other southwest records notably cave of bells and fort stanton we find coincident timing of the major transitions od to ba ba to yd and hydrologic responses during the od and ba the hydrologic response however varied during the yd consistent with a dipole between the coastal and interior southwest the coherent pattern of hydrologic responses across the interior southwest us and northwest mexico during the od wet the ba drier and yd wet follows changes in the atlantic meridional overturning circulation presumably via its combined influence on north pacific winter storm tracks and the extent magnitude of the north american monsoon in contrast lake elsinore and the coastal southwest experiences a deglacial drying trend punctuated by abrupt change at the od to ba and ba to yd transitions this trend tracks rising greenhouse gases through the deglacial with an apparent southward shift in westerly moisture sources adjusting to the retreating ice sheet

a new understanding of the provinces of the amazon craton based on integration of field mapping and u pb and sm nd geochronology abstract new conventional and sensitive high resolution ion microprobe zircon u pb dating has led to a new understanding of the subdivision and evolution of the amazon craton during precambrian time with major improvements and changes made to the previous rb sr based model the interpretation of u pb and sm nd isotopic data identifies eight main precambrian tectonic provinces in the craton with ages ranging from 3 1 to 0 99 ga some of the provinces were generated by accretional arc related processes carajas transamazonic tapajos parima and rondonia juruena and others by recycling of continental crust central amazon rio negro and sunsas the exposed archean crust is restricted to the east carajas and south amapa in brazil and north imataca in venezuela of the craton indicating that the amazon craton is largely a proterozoic crust the carajas imataca 3 10 2 53 ga and transamazonian 2 25 2 00 ga provinces are composed predominantly of granite greenstone terranes the tapajos parima 2 10 1 87 ga and rondonia juruena 1 75 1 47 ga provinces represent new crust added as orogenic belts while the rio negro 1 86 1 52 ga and sunsas 1 33 0 99 ga provinces originated mainly by magmatic tectonic recycling of the above two orogenic belts the only zone with a prominent northeast trend is the poorly known k mudku shear belt characterized by a 1 20 ga shear zone which deforms the rocks of at least three different provinces rio negro tapajos parima and transamazonic the central amazon province comprises mostly orosirian volcano plutonic rocks uatuma magmatism and is a terrane in which the exposed crustal structure and deformation are pluton related the sm nd tdm model ages and nd suggest that the central amazon province was generated by the partial melting of archean continental crust carajas province perhaps related to underplating that began at the end of the tapajos parima orogeny 1 88 1 86 ga

is barrier island morphology a function of tidal and wave regime abstract classification of barrier island morphology stems from the seminal work of m o hayes and others which linked island shape to tidal range and wave height and defined coastal energy regimes i e wave dominated mixed energy tide dominated if true this general relationship represents a process based framework to link modern and ancient systems and is key for determining paleomorphodynamic relationships here we present a new semi global database of barrier islands and spits n 702 shape parameters aspect circularity and roundness are used to quantify island boundary shape and assess potential correlation with coastal energy regime using global wave and tide models in adopting the original energy classification as originally put forth i e wave dominated wave influenced mixed tide influenced mixed tide dominated results show that wave dominated islands have statistically different mean shape values from those in the mixed energy fields but the two mixed energy designations are not distinct from each other furthermore each energy regime field contains a wide range of island shapes with no clear trends present linear regression modeling shows that tidal range and wave height account for

new insights into martian dust distribution and water ice cloud microphysics 1 in this paper we use a direct method which combines microphysics and photometric simulations of the martian atmosphere this approach allows us to reproduce vertical profiles of the light scattered by the haze at the limb of mars subsequent results are compared to a multiple color image provided by the viking orbiter camera the free parameters of our study are related to vertical transport and dust size distribution a sensitivity study has been conducted encompassing a relevant range of parameters in order to obtain the most satisfying multispectral modeled profile with respect to the one inferred from data except for an unlikely distribution of submicron particles reff 0 2 m no consistent fit can be obtained this implies that size distributions of dust suggested by previous studies are not in agreement with the present analysis this apparent discrepancy can be resolved by adding a separate peak of submicron particles to the size distribution previously extracted from viking lander images only in that case can spectral and vertical structures of haze be successfully matched while a bimodal distribution would change the current picture of martian dust such distribution is a common representation of soil derived aerosol size function in earth deserts we also attempted to derive information from the cloud shown in the viking limb image our estimates suggest that cloud particle effective radius is around 1 2 1 8 m while the cloud visible opacity is 0 02

interhemispheric atlantic seesaw response during the last deglaciation the asynchronous relationship between millennial scale temperature changes over greenland and antarctica during the last glacial period has led to the notion of a bipolar seesaw which acts to redistribute heat depending on the state of meridional overturning circulation within the atlantic ocean here we present new records from the south atlantic that show rapid changes during the last deglaciation that were instantaneous within dating uncertainty and of opposite sign to those observed in the north atlantic our results demonstrate a direct link between the abrupt changes associated with variations in the atlantic meridional overturning circulation and the more gradual adjustments characteristic of the southern ocean these results emphasize the importance of the southern ocean for the development and transmission of millennial scale climate variability and highlight its role in deglacial climate change and the associated rise in atmospheric carbon dioxide

the barrier island system abstract barrier islands are part of a major coastal system composed of six interactive sedimentary environments the six environments are also elements needed to impose the designation barrier island on littoral sand bodies the elements are 1 mainland 2 backbarrier lagoon 3 inlet and inlet deltas 4 barrier island 5 barrier platform and 6 shoreface the morphodynamic and sedimentary evolution of each element affects adjacent environments as well as the entire system the interactive elements are defined and described by their morphologic sedimentologic and stratigraphic relationships to the barrier island and the barrier island system the stratigraphic relationships of the various elements of the system may be a valuable tool for paleogeographic reconstruction and sand body mapping

changing amounts and sources of moisture in the u s southwest since the last glacial maximum in response to global climate change abstract the u s southwest has a limited water supply and is predicted to become drier in the 21st century an improved understanding of factors controlling moisture sources and availability is aided by reconstruction of past responses to global climate change new stable isotope and growth rate records for a central texas speleothem indicate a strong influence of gulf of mexico gom moisture and increased precipitation from 15 5 to 13 5 ka which includes the majority of the bolling allerod warming ba 14 7 12 9 ka coeval speleothem records from 900 and 1200 km to the west allow reconstruction of regional moisture sources and atmospheric circulation the combined isotope and growth rate time series indicates 1 increased gom moisture input during the majority of the ba producing greater precipitation in texas and new mexico and 2 a retreat of gom moisture during younger dryas cooling 12 9 11 5 ka reducing precipitation these results portray how late pleistocene atmospheric circulation and moisture distribution in this region responded to global changes providing information to improve models of future climate

stratigraphy and internal structure of wind dominated barrier islands dune and machair of the outer hebrides scotland stratigraphy and internal structure of wind dominated barrier islands

tectonic implications of the 1419 ma nova guarita mafic intrusives paleomagnetic pole amazonian craton on the longevity of nuna abstract the nuna supercontinent was probably assembled in the paleoproterozoic but its paleogeography and the timing for its demise are still a matter of debate a paleomagnetic and geochronological study carried out on the mesoproterozoic nova guarita dyke swarm northern mato grosso state sw amazonian craton provides additional constraints on the duration of this supercontinent paleomagnetic af and thermal treatment revealed south southwest northeast magnetic directions with downward upward inclinations for 19 analyzed sites these directions are carried by psd magnetite with high unblocking temperatures as indicated by additional magnetic tests including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence in selected samples a positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component a mean site direction was calculated at d m 220 5 i m 45 9 95 6 5 k 27 7 which yielded a paleomagnetic pole located at 245 9 e 47 9 s a 95 7 0 40 ar 39 ar dating carried out on biotites from four analyzed dykes yielded well defined plateau ages with a mean of 1418 5 3 5 ma the nova guarita pole precludes a long lived nuna configuration in which laurentia baltica north china and amazonia formed a long and continuous block as previously proposed for the paleoproterozoic it is nevertheless fully compatible with a samba amazonia baltica link at mesoproterozoic times

zircon u pb ages of rocks from the rio apa cratonic terrane mato grosso do sul brazil new insights for its connection with the amazonian craton in pre gondwana times abstract this paper presents 14 zircon u pb determinations shrimp and la mc icp ms for key geological units from the rio apa cratonic terrane ract which is considered the southernmost exposed part of the amazonian craton in southwestern brazil the zircon u pb ages and geological data indicate that the ract was formed by the accretion of magmatic arcs in a continental margin active from 1950 to 1720 ma the ract is composed of three major terranes western eastern and southeastern terranes with distinct evolution histories the western terrane presents orthogneisses and granites formed at 1950 1940 ma and subduction related granites and volcanic rocks formed at 1900 1880 ma and 1840 1830 ma these basement rocks are covered by a greenschist facies metavolcano sedimentary succession rio naicata formation with basal volcanic rocks formed at 1813 18 ma a gabbronoritic dyke of the rio perdido suite hosted by the rio naitaca formation yields an age of 1589 44 ma the eastern and southeastern terranes present deformed leucogranites generated within the intervals 1780 1720 ma and 1810 1790 ma respectively both terranes are covered by a metavolcano sedimentary succession alto terere formation dominated by barrovian type amphibolite facies metamorphic assemblages suggestive of a collisional event available 40 ar 39 ar data hornblende muscovite and biotite indicate that the proto ract evolved to a collisional orogen between 1310 and 1270 ma and behaved as a cratonic mass after 1270 ma preceding the assembly of rodinia the available data allow us to interpret the ract as a part of the ventuari tapajos province of the amazonian craton which was fragmented and dispersed as a microcontinent it was subsequently reincorporated into the sw amazonian craton along the sunsas belt as an allochthonous terrane in a global perspective the tectono magmatic events of the ract are consistent with a long lived accretionary orogen possibly related to an active margin of columbia

north pacific climate response to freshwater forcing in the subarctic north atlantic oceanic and atmospheric pathways abstract sudden changes of the atlantic meridional overturning circulation amoc are believed to have caused large abrupt climate changes over many parts of the globe during the last glacial and deglacial period this study investigates the mechanisms by which a large freshwater input to the subarctic north atlantic and an attendant rapid weakening of the amoc influence north pacific climate by analyzing four different ocean atmosphere coupled general circulation models gcms under present day or preindustrial boundary conditions when the coupled gcms are forced with a 1 sv sv 106 m3 s 1 freshwater flux anomaly in the subarctic north atlantic the amoc nearly shuts down and the north atlantic cools significantly the south atlantic warms slightly shifting the atlantic intertropical convergence zone southward in addition to this atlantic ocean atmosphere response all of the models exhibit cooling of the north pacific especially along the oceanic frontal zone consistent with paleoclimate recons

last glacial maximum and holocene climate in ccsm3 abstract the climate sensitivity of the community climate system model version 3 ccsm3 is studied for two past climate forcings the last glacial maximum lgm and the mid holocene the lgm approximately 21 000 yr ago is a glacial period with large changes in the greenhouse gases sea level and ice sheets the mid holocene approximately 6000 yr ago occurred during the current interglacial with primary changes in the seasonal solar irradiance the lgm ccsm3 simulation has a global cooling of 4 5 c compared to preindustrial pi conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at lgm tropical sea surface temperature sst cools by 1 7 c and tropical land temperature cools by 2 6 c on average simulations with the ccsm3 slab ocean model suggest that about half of the global cooling is explained by the reduced lgm concentration of atmospheric co2 50 of present day concentrations there is an increase in the antarctic circumpolar current an

palaeomagnetism of precambrian dyke swarms in the north china shield the 1 8 ga lip event and crustal consolidation in late palaeoproterozoic times abstract the north china shield ncs is cut by a laterally extensive dyke swarm emplaced at 1 78 1 76 ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by 1 85 ga palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with ne to n declinations and rare opposite polarity dykes with ne shallow magnetic declination a1 d i 36 1 recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations a2 d i 8 2 in lower grade metamorphic terranes to the south contact tests indicate a primary cooling related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at 1 85 ga a temporal migration a2 a1 is implied by the higher crustal level and earlier acquisition of the former and the deeper source and later acquisition of the latter a third population of dyke magnetisations a3 d i 18 43 is distributed towards steeper inclinations and close to the mesozoic recent palaeofield these are either partial or complete overprints of a1 a2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra or are much younger dykes of mesozoic tertiary age a minority fourth later precambrian but presently undated dual polarity population has a magnetisation 11 dykes d i 108 7 with contact tests indicating a primary cooling related origin the 1 78 1 76 ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major large igneous province lip related events the new definition of a 1 83 1 76 ga apw swathe from the north china shield permits a comparison with other shields and yields a constraint to continental configurations during the late palaeoproterozoic a quasi integral reconstruction of palaeopangaea is tested here and supported by conformity of predominantly of uplift related palaeopoles from the 1 90 1 70 ga tectono thermal belts and from sw ne trending apw implied by the distribution of poles from the 1 80 ga igneous suites including the lip events this trend incorporates the a2 a1 migration and the granulite terrane cooling polar swathe from north china the reconstruction indicates that continental crust consolidated in palaeoproterozoic times by accretion of 2 3 1 7 ga orogenic belts around a hemispheric and crescent shape core already established by late archaean times the north china shield is interpreted to have bordered the western cratonic margin of the indian shield in a proximity supported by correlation of geological features and suggested by a number of previous workers the central orogenic zone of the north china shield characterised by tectono thermal activity prior to 1 85 ga was then contiguous with a comparable zone running through the centre of the indian shield and continuing into the capricorn belt of western australia the 1 78 1 76 ga dykes in north china continue into dyke swarms in the south india shield and may have been sourced in a plume related lip focussed near the continental margin in the xiong er aulacogen

40ar 39ar thermochronometry of the imataca complex venezuela the imataca complex of venezuela an archean metamorphic belt located on the northern margin of the amazonian craton underwent upper amphibolite to granulite facies metamorphism during the 2 0 ga trans amazonian orogeny 40ar 39ar plateau dates on hornblende separates from the imataca complex vary from 1972 to 1760 ma and record post trans amazonian orogeny cooling biotite and feldspar pairs which were separated from the same samples or from samples collected at the same exposure and should record identical thermal histories yield distinct results the biotite 40ar 39ar spectra are primarily plateau profiles with dates ranging from 2044 to 1254 ma whereas perthitic potassium feldspar yield stair step 40ar 39ar spectra and integrated dates ranging from 1395 to 1166 ma the internally discordant perthite spectra appear to reflect cooling at rates of 0 1 0 2 c ma 1 consistent with the cooling rates estimated from the blocking temperature and age differences between biotite feldspar pairs from the same sample two plagioclase samples yield saddle shaped 40ar 39ar spectra with 1235 and 1221 ma minimum dates these internally discordant spectra are caused by degassing of non atmospheric trapped ar from the plagioclase and of radiogenic ar sited in k rich inclusion phases the 2 0 1 7 ga biotite dates are confined to the margins of the imataca complex and record the waning stages of trans amazonian orogeny uplift and cooling the 1 4 1 2 ga biotite dates which are confined to the center of the imataca complex are correlated with their 200 350 c ar blocking temperatures as estimated from their average grain size and fe fe mg composition the 1 3 1 1 ga feldspar ages are correlated with their 200 300 c ar blocking temperatures as determined from analysis of the 40ar 39ar step heating data the extremely slow cooling rates implied by the biotite feldspar pairs suggest that between 1 7 and 1 1 ga the imataca complex cooled isobarically at intermediate crustal levels all ar mineral systems closed by 1 1 ga perhaps reflecting a renewed phase of uplift associated with the mild tectonism of the nickorie episode

out of the tropics the pacific great basin lakes and late pleistocene water cycle in the western united states the water cycle in the western united states changed dramatically over glacial cycles in the past 20 000 years higher precipitation caused desert lakes to form which have since dried out higher glacial precipitation has been hypothesized to result from a southward shift of pacific winter storm tracks we compared pacific ocean data to lake levels from the interior west and found that great basin lake high stands are older than coastal wet periods at the same latitude westerly storms were not the source of high precipitation instead air masses from the tropical pacific were transported northward bringing more precipitation into the great basin when coastal california was still dry the changing climate during the deglaciation altered precipitation source regions and strongly affected the regional water cycle

rollover drowning and discontinuous retreat distinct modes of barrier response to sea level rise arising from a simple morphodynamic model we construct a simple morphodynamic model to investigate the long term dynamic evolution of a coastal barrier system experiencing sea level rise using a simplified barrier geometry the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux with barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response the modeled barrier system demonstrates four primary behaviors height drowning width drowning constant landward retreat and a periodic retreat height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea level rise on the other hand width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash driven landward migration during periodic barrier retreat the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens this periodic retreat which occurs even with a constant sea level rise rate arises when overwash rates and shoreface response rates are large and of similar magnitude we explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to the maximum overwash flux and the shoreface response rate barrier response can be particularly sensitive to the sea level rise rate and back barrier lagoon slope overall our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea level rise rate sediment supply or back barrier geometry

can barrier islands survive sea level rise quantifying the relative role of tidal inlets and overwash deposition barrier island response to sea level rise depends on their ability to transgress and move sediment onto and behind the barrier either through flood tidal delta deposition or via overwash our understanding of these processes over decadal or longer timescales however is limited here we use a recently developed barrier island model brie to better understand the interplay between tidal dynamics overwash fluxes and sea level rise on barrier coasts and barrier island stratigraphy model results suggest that in microtidal environments with large alongshore sediment transport fluxes tidal inlets are ephemeral and migrate rapidly these conditions lead to effective deposition of flood tidal deltas and allow inlets to constitute most of the landward sediment flux whether barrier islands can survive sea level rise depends on the combined landward sediment flux from overwash and flood tidal delta deposition likely making barrier islands with artificially stabilized inlets via jetty construction or maintenance dredging more vulnerable to sea level rise

sediment supply from shoreface to dunes linking sediment transport measurements and long term morphological evolution a significant problem in current coastal research is the understanding of linkages between morphological phenomena occurring on different temporal and spatial scales morphodynamic processes in the nearshore typically exhibit nonlinear behaviour and consequently phenomena which occur on short temporal event scales as e g observed during field experiments have generally been difficult to upscale to provide an understanding of the long term behaviour of the coast on the time scale of seasons or decades this paper documents a persistent onshore migration of nearshore bar features over a period of 3 decades using a set of cross shore profile surveys from a barrier on the danish north sea coast the observed behaviour is consistent with experimental data obtained during weeklong field campaigns when sediment transport and bar migration were found to be landward directed during high energy dissipative conditions hydrodynamic and sediment transport processes leading to onshore bar migration and eventually welding of the bars are documented to provide an explanation for the observed long term evolutionary trends in addition bar welding and the resulting intertidal bar formation represent an onshore sediment storage which is subsequently available for aeolian transport observations on aeolian sediment transport indicate that transport rates and sediment supply to the foredunes critically depend on the presence of dry sand sources as represented by well drained intertidal bars measurements of long term accretion rates in foredunes indicate that volumes deposited on the crest and lee slope of the dunes are in reasonable correspondence with volumes supplied to the subaerial beach through the process of bar welding hence the data suggest that a persistent onshore directed sediment supply occurs from the shoreface across the surf and intertidal zones and into the dunes of the investigated barrier

barrier islands on bedrock a new landform type demonstrating the role of antecedent topography on barrier form and evolution barrier islands commonly occur in coastal plain and deltaic settings and a variety of behavioral and stratigraphic models exist in this paper a 90 km long barrier island system in the outer hebrides of scotland is identified for the first time uniquely among barrier islands the system rests directly on a planar bedrock gneiss surface a strandflat this bedrock surface has a profound influence on the morphology of the barrier island chain and exerts the dominant influence on barrier island form and evolution during transgression topographical irregularities on the bedrock surface control both plan and profile morphology of the barrier islands under sea level rise by barrier migration sediment is redistributed onshore and alongshore in a strongly three dimensional manner determined by topographic variability in the bedrock the barrier islands represent an endpoint in transgressive barrier island geomorphology that is controlled by the bedrock surface they are at the upper end of the recognized tidal amplitude for barrier islands the offshore wave regime is very high energy they lack a shoreface and neither ravinement surface nor transgressive sand sheet are developed sediment supply is dominated by offshore derived skeletal carbonate

u pb and sm nd evidence for 1 76 1 77 ga magmatism in the moriru region mato grosso brazil implications for province boundaries in the sw amazon craton abstract the moriru region situated in the sw amazon craton mato grosso brazil is composed of two main domains 1 an area of undeformed bimodal volcanic rocks and 2 an area of deformed granitic gneissic migmatite rocks that also include a small occurrence of slightly deformed rhyolite the undeformed domain is composed of felsic to mafic flows and volcanoclastic sediments the felsic flows are essentially explosive deposits classified as ignimbrites however flows lava occurs interlayed the compositions of ignimbrites and associated felsic lava are rhyodacite to dacite the mafic volcanics are phaneritic fine grained and are often strongly hydrothermally altered unaltered samples have tholeiitic to transitional compositions and have within plate petrologic and geochemical characteristics the felsic volcanics are enriched in light ree show a negative eu anomaly and are depleted in heavy ree the trace elements indicate a post collisional environment u pb data on single zircon grains from the volcanic sequence yield ages between 1770 6 ma and 1778 6 ma end t values range from 1 4 to 1 5 t 1 75 ga and the t dm model ages range from 2 02 to 2 16 ga rocks from the deformed domain are calc alkaline to transitional and have metaluminous to peraluminous compositions the metaluminous granites are hornblende monzogranites and the peraluminous rocks are biotite granites and granodiorite both groups plot in post tectonic fields of trace element diagrams u pb data on single zircon grains from the deformed granite gneiss migmatite domain range from 1662 13 ma to 1803 6 ma but most cluster about a mean of 1763 ma 1759 3 ma to 1767 2 ma end t values t 1 75 ga vary from 3 4 to 3 0 and the t dm model ages range from 1 94 to 2 28 ga the approximately 1775 ma volcano plutonic rocks in the area probably resulted from partial melting of slightly older continental lithosphere and may have formed during an extensional event about 1 78 1 76 ga the plutonic rocks seem to have a similar source and may be intrusive into the volcanic sequence despite their more deformed nature these ages are similar to those proposed for older parts of the 1 80 1 55 ga rio negro juruena province and younger than 1 9 1 8 ga as proposed for the ventuari tapajos province sm nd isotopic data are consistent with formation of these rocks from ca 1 9 2 2 ga continental lithosphere it is not yet necessary to relocate the proposed boundary between the rio negro juruena and ventuari tapajos provinces since provinces should be defined by the ages of the main accretionary assemblages and not the ages of younger intrusive suites more studies are needed to refine these province boundaries

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

some key issues in reconstructions of proterozoic supercontinents abstract supercontinents containing most of the earth s continental crust are considered to have existed at least twice in proterozoic time the younger one rodinia formed at 1 0 ga by accretion and collision of fragments produced by breakup of the older supercontinent columbia which was assembled by global scale 2 0 1 8 ga collisional events little consensus has been reached regarding configurations of these supercontinents because of some unresolved issues concerning continental fits one of these issues concerns how siberia was related to laurentia previous reconstructions that consider the aldan shield of siberia as a continuation of the wyoming province of laurentia have been largely abandoned in favor of models connecting siberia to northern laurentia but it remains controversial which part of siberia is contiguous with northern laurentia also at issue is the western laurentia australia east antarctica connection most rodinia reconstructions place australia together with east antarctica adjacent to either western canada the sweat hypothesis or the western united states the auswus hypothesis however recent studies combining paleomagnetic and isotopic age data have called into question the validity of sweat auswus and other variants another issue is the position of north china in rodinia columbia limited paleomagnetic data seem to be consistent with the paleo mesoproterozoic north china siberia baltica connection whereas geological data support the recently proposed archean to mesoproterozoic north china india connection controversial issues have also been raised about the timing and history of the amalgamation and fragmentation of south america and west africa both geological and paleomagnetic data suggest that south america sao francisco and amazonia cratons and west africa congo and west african cratons coalesced into a single landmass along the 2 1 2 0 ga transamazonian eburnean orogens however whether they were divorced and then re married to form part of gondwana or remained largely coherent from their amalgamation at 2 1 2 0 ga until their incorporation into gondwana is unclear also little known is the position of amazonia west africa in the proposed supercontinents with some workers believing that they existed as a separate landmass whereas others place amazonia west africa adjacent to baltica in summary although geological and paleomagnetic data are supportive of the existence of proterozoic supercontinents rodinia and columbia they are insufficient to determine their exact geometries

the long life of samba connection in columbia a paleomagnetic study of the 1535 ma mucajai complex northern amazonian craton brazil abstract in recent years there has been a significant increase in the paleomagnetic data of the amazonian craton with important geodynamic and paleogeographic implications for the paleo mesoproterozoic columbia supercontinent a k a nuna hudsoland despite recent increase of paleomagnetic data for several other cratons in columbia its longevity and the geodynamic processes that resulted in its formation are not well known a paleomagnetic study was performed on rocks from the 1535 ma amg anorthosite mangerite rapakivi granite mucajai complex located in the roraima state brazil in the northern portion of the amazonian craton the guiana shield thermal and af treatments revealed northwestern southeastern directions with upward downward inclinations for samples from twelve sites this characteristic remanent magnetization is mainly carried by ti poor magnetite and in a lesser amount by hematite site mean directions were combined with previous results obtained for three other sites from the mucajai complex producing the dual polarity mean direction dm 132 2 im 35 4 n 15 95 12 7 k 10 0 and a paleomagnetic pole located at 0 1 e 38 2 s a95 12 6 k 10 2 the mucajai pole favours the samba south america baltica link in a configuration formed by amazonia and baltica in columbia also there is geological and paleomagnetic evidence that the juxtaposition of baltica and laurentia at 1 76 1 26 ga forms the core of columbia the present paleomagnetic data predict a long life 1 78 1 43 ga samba connection forming part of the core of the supercontinent

steering of westerly storms over western north america at the last glacial maximum the last glacial maximum hydroclimate over western north america differed from the modern climate a proxy model comparison suggests that the glacial storm track was squeezed and steered by atmospheric high pressure systems

nearly synchronous climate change in the northern hemisphere during the last glacial termination the climate of the north atlantic region underwent a series of abrupt cold warm oscillations when the ice sheets of the northern hemisphere retreated during the last glacial termination 17 7 11 5kyr ago evidence for these oscillations which are recorded in european terrestrial sediments as the oldest dryas bolling older dryas allerod younger dryas vegetational sequence1 2 has been found in greenland ice cores3 4 the geographical extent of many of these oscillations is not well known5 6 but the last major cold event the younger dryas seems to have been global in extent7 8 9 10 here we present evidence of four major oscillations in the hydrological balance of the owens basin california that occurred during the last glacial termination dry events in western north america occurred at approximately the same time as cold events recorded in greenland ice with transitions between climate regimes in the two regions taking place within a few hundred years of each other our observations thus support recent climate simulations which indicate that cooling of the north atlantic ocean results in cooling of the north pacific ocean11 which in turn leads to a drier climate in western north america12

influence of inherited geologic framework on barrier shoreface morphology and dynamics abstract passive margin coastlines with limited sand supplies such as much of the u s atlantic margin are significantly influenced by the geologic framework of older stratigraphic units that occur beneath and seaward of the shoreface many u s east coast barrier islands are perched barriers in which the underlying pre modern sediments determine the morphology of the shoreface and strongly influence modern beach dynamics and composition perched barriers consist of varible layers of beach sand on top of older eroding stratigraphic units with highly variable compositions and geometries along many parts of the coastal system stratigraphically controlled bathymetric features on the inner shelf modify waves and currents and thereby effect patterns of sediment erosion transport and deposition on the adjacent shoreface it is essential to understand this geologic framework before attempting to model the large scale behavior of these types of coastal systems in north carolina most shoreline features are controlled by the pre holocene stratigraphic framework of the shoreface the beaches are perched on top of pre existing pleistocene tertiary and cretaceous sediments the surficial geology of the coastal zone is subdivided into two distinct provinces resulting in different stratigraphic controls of the shoreface north of cape lookout the geological framework consists of a quaternary sequence that fills a regional depositional basin called the albemarle embayment the coastal zone south of cape lookout is dominated by tertiary and cretaceous units that crop out across the coastal plain and continental shelf with very thin quaternary units only locally preserved superimposed upon this regional stratigraphy is an ancient drainage system resulting in a series of fluvial valleys filled with younger coastal sediments separated by large interfluve areas of older stratigraphic units this results in a coastal system in which the shoreface is either nonheadland or headland dominated respectively headland dominated shorefaces are further divided into subaerial and submarine categories nonheadland dominated shorefaces are further divided into those influenced primarily by transgressive or regressive processes or channel dominated depositional processes i e inlet migration or stream valley fill examples of each of these six types of shorefaces are presented to demonstrate the control that the geologic framework exhibits on shoreface morphologies and processes

mesoscale geomorphic change on low energy barrier islands in chesapeake bay u s a this paper presents an analysis of decadal mesoscale geomorphic change on sandy barrier islands in thefetch limited environment of chesapeake bay low energy barrier islands exist in two settings on the fringe of marshes and in open water and this analysis shows the various types of barrier island to be genetically related barrier islands that face the dominant wind and wave direction e or w retreat via barrier translation preserving the barrier island volume those that exist in re entrants are dominated by longshore transport processes are strongly affected by sediment supply and are subject to disintegration marsh fringe barrier islands are perched on or draped over the surface of the underlying marsh they migrate landwards via barrier translation during periodic high water events accompanied by large waves hurricanes and northeasters the underlying marsh surface erodes under all water levels and the rate of retreat of the barrier island and underlying marsh may take place at different rates leading to various configurations from perched barrier islands several metres landward of themarsh edge to barrier islands that have a sandy shoreface extending into the subtidal zone the coastal configuration during landward retreat of marsh fringe barrier islands is subject to strong local control exerted by the underyling marsh topography as erosion ofmarsh promontories occurs and marsh creeks are intersected and bypassed the configuration is subject to rapid change periodic sediment influxes cause spits to develop at re entrants in themarsh the spits are initiated as extensions of adjacentmarsh fringe barrier islands but as the sediment volumeis finite the initial drift aligned spits become sediment starved and begin to develop a series of swash aligned cells as they strive formorphodynamic equilibrium the individual cells are stretched until breaches form in the barrier islands creating inlets with tidal deltas at this stage the low energy barrier islands closely resemble open ocean barrier islands continued reworking leads towidening of the inletswith consequent loss of constriction of tidal flow the tidal deltas are thus no longermaintained and ultimately the island systemdisintegrates through inlet widening and is transformed to subtidal shoals barrier islands at various stages in this evolutionary cycle can be observed around the bay mid bay barrier islands are affected by wave processes from both sides this helps maintain the barrier island form and enables barrier islands to persist as sediment is exchanged between both sides of the island rates of barrier island translation are extremely high up to 30 m year over a 12 year period this is attributed to the lowvolume of sand which facilitates complete rollover in short periods accelerated sea level rise is likely to hasten the translation rates ofmarsh fringe barrier islands the rapid disintegration ofmost spits compared to the persistence of marsh fringe barrier islands points to a reliance on the marsh as a stabilising point if the marshes are overstepped by rising sea level as appears to be happening the complete disintegration of the barrier islands is highly likely

last glacial maximum and holocene lake levels of owens lake eastern california usa abstract stratigraphic investigations of fluvio deltaic and lacustrine sediments exposed in stream cuts quarry walls and deep trenches east of the sierra nevada in owens valley near lone pine california have enabled the reconstruction of pluvial owens lake level oscillations age control for these sediments is from 22 radiocarbon 14 c dates and the identification and stratigraphic correlation of a tephra which when plotted as a function of age versus altitude define numerous oscillations in the level of pluvial owens lake during the latest pleistocene and early holocene we have constructed a lake level altitude curve for the time interval 27 000 cal yr bp to present that is based on the integration of this new stratigraphic analysis with published surface stratigraphic data and subsurface core data pluvial owens lake regressed from its latest pleistocene highstands from 27 000 to 15 300 cal yr bp as recorded by 15 m of down cutting of the sill from the altitudes of 1160 to 1145 m by 11 600 cal yr bp the lake had dropped 45 m from the 1145 m sill this lowstand was followed by an early holocene transgression that attained a highstand near 1135 m before dropping to 1120 m at 7860 7650 cal yr bp that had not been recognized in earlier studies the lake then lowered another 30 m to shallow and near desiccation levels between 6850 and 4300 cal yr bp fluvial cut and fill relations north of lone pine and well preserved shoreline features at 1108 m indicate a minor lake level rise after 4300 cal yr bp followed by alkaline and shallow conditions during the latest holocene the new latest quaternary lake level record of pluvial owens lake offers insight to the hydrologic balance along the east side of the southern sierra nevada and will assist regional paleoclimatic models for the western basin and range

a speleothem record of younger dryas cooling klamath mountains oregon usa awell dated d 18 o record in a stalagmite from a cave in the klamath mountains oregon with a sampling interval of 50 yr indicates that the climate of this region cooled essentially synchronously with younger dryas climate change elsewhere in the northern hemisphere the d 18 o record also indicates significant century scale temperature variability during the early holocene the d 13 c record suggests increasing biomass over the cave through the last deglaciation with century scale variability but with little detectable response of vegetation to younger dryas cooling d 2005 university of washington all rights reserved

variable winter moisture in the southwestern united states linked to rapid glacial climate shifts the last glacial period was characterized by large rapid climate fluctuations an analysis of a speleothem from new mexico shows that the coldest conditions over greenland coincide with increased winter precipitation in the southwestern united states which can be attributed to a southward displacement of the polar jet stream and the north american storm track

depositional controls on a hypertidal barrier spit system architecture and evolution pointe du banc spit north western france physical stratigraphy architecture and evolution of barrier systems in hypertidal environments tidal range above 6 m are understudied and depositional controls are poorly understood compared with wave dominated barrier systems and barrier spits in microtidal mesotidal and macrotidal settings based on vibracores ground penetrating radar radiocarbon and optically stimulated luminescence age data the formation and evolution of the hypertidal barrier system of pointe du banc north western france has been reconstructed the study shows that the barrier spit has a complex composite sedimentary architecture consisting of wave dominated and tide dominated sedimentary bodies a morpho sedimentary model is presented that demonstrates how barrier spit progradation resulting from littoral drift was the main manner of spit elongation whereas sediment convergence caused by landward migration of swash bars and seaward migration of tidal dunes caused the spit terminus to grow in height and width these results suggest that long term accretion rates varied considerably in response to changes in sediment supply variations in storminess together with the large scale topography of the coast controlled the sediment supply and thus the evolution of the barrier system during latest holocene despite architectural complexity hypertidal barrier systems preserve records of past climate changes

a late paleoproterozoic key pole for the fennoscandian shield a paleomagnetic study of the keuruu diabase dykes central finland abstract we present an updated paleomagnetic pole and a new isotope age for the late paleoproterozoic svecofennian diabase dykes from keuruu central finland the paleomagnetic results reveal a dual polarity remanent magnetization with asymmetry which we explain by an unremoved secondary component contaminating both normal n and reversed r vectors the chrm is carried by magnetite most of which are primary grains crystallized from basaltic magma the primary nature of the chrm is further supported by positive baked contact tests an r polarity dyke shows a u pb concordia age of 1870 9 ma zircons from another dyke of n polarity reveal a mean pb pb age of 1867 8 ma the combined n and r mean chrm direction is d 340 4 i 35 2 95 5 8 k 31 3 n 21 dykes yielding a paleomagnetic pole of plat 45 7 plon 230 9 a 95 5 5 this positions the fennoscandian shield at low latitudes 19 4 and aligns with superior craton in a way that would naturally proceed into the 1 8 1 2 ga nena north europe north america configuration paleosecular variation analysis of n polarity dykes is consistent with the low latitude position of baltica

barrier island evolution middle atlantic shelf u s a part i shoreface dynamics abstract in part i of this paper near bottom fluid velocity and sediment concentration measurements from the shoreface of a middle atlantic barrier long island coast are analyzed to provide insight into the dynamics of erosional shoreface retreat in part ii these data are combined with observations of coastal stratigraphy in order to construct a model for the evolution and behavior of middle atlantic barriers wave motions on the long island shoreface tend to drive sediment onshore calculations of the onshore sediment flux driven by asymmetrical shoaling waves show that for the wave states commonly encountered on the middle atlantic shelf values become strongly positive landward of approximately 10 m water depth upper shoreface but drop to very low values shortly seaward of that isobath on the lower shoreface and inner shelf floor fair weather wave current interaction tends to cause a landward creep of sediment even though the wave orbital component of the near bottom velocity field is nearly symmetrical this movement occurs because the wave approach direction generally lies within 90 of the direction up coast or down coast of the alongshore wind driven current therefore fluid shear stress acting on the sediments tends to be reinforced during the shoreward stroke of the wave orbital motion and partially cancelled during the seaward stroke during major storms the rate of sediment transport increases by at least an order of magnitude and the role of wind driven currents becomes important much more sediment is resuspended by the wave orbital current component because storm waves are more powerful than fair weather swells but the wind driven flow component is also much stronger and is now available for transporting the resuspended sediment storm flows over the shoreface occur in distinct dynamic zones surf zone friction dominated zone transition zone geostrophic zone the zones are defined by dynamical considerations and expand or contract with the intensity of the causative wind stress during peak flow events the characteristic length scales of the friction dominated and geostrophic zones tend to correspond with the morphologic zones of the shoreface and inner shelf the storm intensified wind driven alongshore flows of the lower shoreface are frequently jet like in nature and may be upwelling or downwelling wind driven coastal flows with a strong downwelling component occur during most storms on the long island coast and are particularly important to the coastal sand budget during these events sand is entrained by the storm intensified upper shoreface circulation system of wave driven alongshore currents and rip currents and is fed into the main wind driven coastal current because of the offshore component of bottom flow sand is swept down the lower shoreface and onto the adjacent inner shelf fair weather processes may be unable to return storm deposited sand to the beach from such an offshore position the shoreface transport regime of the long island barrier coast thus consists of long periods of time months during which sand moves slowly toward the beach punctuated by short intense periods hours or days during which sand is transferred from the shoreface to the adjacent inner shelf the long term sense of movement of the shoreface whether retreating the long island case or prograding must depend on the loss or gain of sand by shoreface with respect to the inner shelf the data shows that the coastal sand budget is controlled not only by the upper shoreface cycle withdrawl of sand from the beach prism storage in the breakpoint bar and its subsequent return to the beach but by a cycle of larger spatial and temporal scale in which sand is exchanged between the shoreface as a whole and the adjacent inner shelf

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

u pb geochronology of the 2 0 ga itapecerica graphite rich supracrustal succession in the sao francisco craton tectonic matches with the north china craton and paleogeographic inferences abstract la icpms u pb isotopic studies are conducted for key samples from the itapecerica graphite rich supracrustal succession and the crystalline basement in the southern sao francisco craton sfc the detrital zircon analyses in paragneiss indicated a maximum deposition age of 2080 ma the age spectra in the relative probability diagrams suggested the precursor basin was essentially fed with detritus from the accretionary complexes of the mineiro belt 2470 2000 ma which is the most preserved segment of the minas orogen the basement rocks yielded zircon u pb crystallization ages between 2753 81 and 2679 61 ma whereas the metamorphic overprint gave 2069 84 ma one nearby charnockitic gneiss yielded a sm nd garnet whole rock linear array with an age of 2086 19 ma the youngest ages estimate the timing of the regional prograde granulite facies metamorphism that also overprinted the itapecerica rocks given our upper intercept zircon u pb ages of 1997 20 ma and 1971 40 ma in paragneiss and quartzite respectively these ages also constrained the timing of the conversion of carbonaceous material into graphite in the itapecerica mine the second largest graphite mine in brazil the high grade metamorphism was associated with the final docking of the mineiro belt against the neoarchean foreland the minas orogen has geologic matches with the jiao liao ji orogen located in the present eastern margin of the north china craton where khondalites uht granulites anatectic charnockites as well as large graphite deposits are documented such as in the 1 97 1 88 ga korean arc the tectonic similarities between the two cratons support a relatively close position at the time frame the dynamic scenario considers broadly coeval accretionary collisional belts along the active margins of archean blocks the progressive closure of the paleoproterozoic ocean and final docking of the sao francisco congo paleocontinent against the active continental margin of the north china craton the work model allows new insights into the paleogeography of the core of supercontinent columbia at 1 90 ga and the subsequent 1 78 1 50 ga intraplate activity

geomorphic response type model for barrier coastlines a regional perspective based on quantitative documentation of historical changes in shoreline position between 1847 and 1991 eight geomorphic response types were established for classifying megascale changes along barrier coastlines 1 lateral movement 2 advance 3 dynamic equilibrium 4 retreat 5 in place narrowing 6 landward rollover 7 breakup and 8 rotational instability long term decades to centuries monitoring of shoreline position over a spatial scale of 10 to 100 km provides a scientific basis for documenting process response relationships that shape regional coastal morphodynamics although megascale shoreline change studies often are lacking this type of information is critical for developing realistic research and management strategies regarding form process relationships in coastal depositional systems the spatial distribution of geomorphic response types is delineated along the barrier coastlines of louisiana mississippi and southern georgia northern florida at megascale the rate of relative sea level rise along these barrier coastlines appears to be one of the major factors controlling the occurrence of geomorphic response types however sediment supply exerts significant influence on shoreline response as well

a review of the geochronology of the amazonian craton tectonic implications abstract the available geochronological data for the amazonian craton permit delineation of its main age provinces and their respective tectonic development demarcation of the boundaries of each age province is primarily based on rb sr whole rock isochrons supported by k ar determinations on minerals some u pb results on zircons pb pb whole rock isochrons and sm nd model ages complement these data in key areas at the center of the amazonian craton lies the archean age central amazonian province bounded in its northern part by the maroni itacaiunas mobile belt which developed during early proterozoic times the western and southern margins of the archean core are bordered by three tectonic provinces 1 the rio negro juruena magmatic arc 2 the rondonian mobile belt and 3 the sunsas mobile belt each belt borders the western margin of the preceding belt and each records a successively younger middle proterozoic orogeny within the central amazonian province 2 7 ga age determinations have been obtained from the serra dos carajas area although the maroni itacaiunas belt includes several reworked archean nuclei rock units formed during the 2 2 1 9 ga trans amazonian orogeny predominate the isotopic evidence suggests that a mantle derived component constituted a significant portion of the crust accreted at that time the rio negro juruena province is of mid proterozoic age 1 75 1 50 ga and includes almost entirely mantle derived material juxtaposed with the pre existent continental mass along a series of successive magmatic arcs the rondonian and sunsas provinces both exhibit older sialic basements 1 5 ga the former is characterized by important tectonomagmatic events in the 1 45 1 25 ga interval whereas the latter records tectonic reworking at 1 1 0 9 ga finally the amazonian craton is bordered to the east by the late proterozoic paraguai araguaia or araguaia tocantins fold belt

review of electromagnetic induction for mapping barrier island framework geology abstract the geologic framework controls on modern barrier island transgression and the relationship of these controls to subsurface structure hydrology and island geomorphology are not well understood recent evidence suggests that alongshore variations in pre holocene geology of barrier islands modify nearshore hydrodynamic processes and sediment transport ultimately affecting how barrier islands will respond to relative sea level rise explorations of holocene barrier island geology are usually based on cores to supplement bathymetric onshore offshore seismic and or ground penetrating radar gpr surveys the advantages and limitations of these methods with respect to barrier island investigations are briefly described in this review alternative near surface geophysical methods including electromagnetic induction emi sensors are increasingly being used for coastal research because they are non invasive provide continuous subsurface information across a variety of sub environments and are capable of characterizing large areas in a short time although these emi sensors have shown promise in coastal applications a number of issues primarily related to subsurface hydrology need to be addressed to fully assess the limitations of this technique this paper reviews the theory methodology and applications of emi in support of geologic framework studies with particular reference to barrier islands resolution of these issues will allow emi sensors to complement and offer significant advantages over traditional methods in support of an improved understanding of large scale barrier island evolution

moisture variability in the southwestern united states linked to abrupt glacial climate change the atmospheric response to millennial scale circulation changes in the north atlantic ocean during the last glacial period has been difficult to constrain cave deposits from southwestern north america reveal that atmospheric moisture in this region increased in response to slowdowns of the atlantic meridional overturning circulation

stalagmite records of hydroclimate in central california during termination 1 abstract the last deglaciation is marked by large and abrupt hydroclimatic changes in the great basin and the american southwest however comparatively little is known about how hydroclimate varied on the western side of the sierra nevada we present new evidence for abrupt changes in precipitation amount in the central sierra nevada during the last deglaciation our new record from mclean s cave overlaps with a previously published record from nearby moaning cave oster et al 2009 and extends the record of hydroclimatic change in the region through heinrich stadial 1 mclean s cave speleothem 18o 13c mg ca and sr ca indicate a shift to drier conditions at the beginning of heinrich stadial 1 from 16 1 to 17 5 ka followed by wetter conditions at the end of heinrich stadial 1 when the majority of great basin lakes reached their deglacial highstands during the last deglaciation coincident shifts in the moaning and mclean s cave proxy records indicate drier conditions in the western sierra nevada during millennial scale intervals of warming at high latitudes such as the bolling and allerod and wetter conditions during cooler intervals such as the older and younger dryas thus the sierra nevada speleothem records are consistent with other regional records that document increased winter rainfall during millennial scale cold periods of the last deglaciation however regional differences exist in the hydroclimatic response to the younger dryas with central californian and interior southwestern sites indicating an increase in winter storms that is not apparent in southern california dynamic simulations of atmospheric circulation indicate that wetter conditions during the younger dryas relative to the bolling at mclean s cave may have resulted from a stronger storm track in the north pacific at the latitude of northern california

large arctic temperature change at the wisconsin holocene glacial transition analysis of borehole temperature and greenland ice sheet project ii ice core isotopic composition reveals that the warming from average glacial conditions to the holocene in central greenland was large approximately 15 c this is at least three times the coincident temperature change in the tropics and mid latitudes the coldest periods of the last glacial were probably 21 c colder than at present over the greenland ice sheet

late pleistocene california droughts during deglaciation and arctic warming abstract recent studies document the synchronous nature of shifts in north atlantic regional climate the intensity of the east asian monsoon and productivity and precipitation in the cariaco basin during the last glacial and deglacial period yet questions remain as to what climate mechanisms influenced continental regions far removed from the north atlantic and beyond the direct influence of the inter tropical convergence zone here we present u series calibrated stable isotopic and trace element time series for a speleothem from moaning cave on the western slope of the central sierra nevada california that documents changes in precipitation that are approximately coeval with greenland temperature changes for the period 16 5 to 8 8 ka from 16 5 to 10 6 ka the moaning cave stalagmite proxies record drier and possibly warmer conditions signified by elevated 18 o 13 c mg sr and ba and more radiogenic 87 sr 86 sr during northern hemisphere warm periods bolling early and late allerod and wetter and possibly colder conditions during northern hemisphere cool periods older dryas inter allerod cold period and younger dryas moaning cave stable isotope records indicate that wet conditions persisted in this area well beyond 11 5 ka suggesting the effects of the younger dryas event may have been longer lived in the western sierra nevada than in greenland however a shifting drip center and corresponding change in seepage water routing may have influenced the trace element records between 10 6 and 9 6 ka linkages between northern high latitude climate and precipitation in the sierra nevada suggested here could indicate that under conditions of continued global warming this drought prone region may experience a reduction in pacific sourced moisture

geochronology of late pleistocene to holocene speleothemsfrom central texas implications for regional paleoclimate a detailed chronology for four stalagmites from three central texas caves separated by as much as 130 km provides a 71 000 yr record of temporal changes in hydrology and climate mass spectrometric 238 u230 th and 235 u231 pa analyses have yielded 53 ages the accuracy of the ages and the closedsystem behavior of the speleothems are indicated by interlaboratory comparisons concordance of 230 th and 231 pa ages and the result that all ages are in correct stratigraphic order over the past 71 000 yr the stalagmites have similar growth histories with alternating periods of relatively rapid and slow growth the growth rates vary over more than two orders of magnitude and there were three periods of rapid growth 71 60 ka 39 33 ka and 24 12 ka these growth rate shifts correspond in part with global glacial interglacial climatic shifts paleontological evidence indicates that around the last glacial maximum 20 ka climate in central texas was cooler and wetter than at present this wetter interval corresponds with the most recent period of increased growth rates in the speleothems which is consistent with conditions necessary for speleothem growth the temporal shift in wetness has been proposed to result

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

younger dryas cooling and the greenland climate response to co2 greenland ice core 18o temperature reconstructions suggest a dramatic cooling during the younger dryas yd 12 9 11 7 ka with temperatures being as cold as the earlier oldest dryas od 18 0 14 6 ka despite an approximately 50 ppm rise in atmospheric co2 such yd cooling implies a muted greenland climate response to atmospheric co2 contrary to physical predictions of an enhanced high latitude response to future increases in co2 here we show that north atlantic sea surface temperature reconstructions as well as transient climate model simulations suggest that the yd over greenland should be substantially warmer than the od by approximately 5 c in response to increased atmospheric co2 additional experiments with an isotope enabled model suggest that the apparent yd temperature reconstruction derived from the ice core 18o record is likely an artifact of an altered temperature 18o relationship due to changing deglacial atmospheric circulation our results thus suggest that greenland climate was warmer during the yd relative to the od in response to rising atmospheric co2 consistent with sea surface temperature reconstructions and physical predictions and has a sensitivity approximately twice that found in climate models for current climate due to an enhanced albedo feedback during the last deglaciation

prolonged wet period in the southwestern united states through the younger dryas the younger dryas was one of the more dramatic climatic transitions ever recorded how these types of climatic shifts are expressed in continental interiors is of primary scientific interest and of vital societal concern here we present a speleothem based absolutely dated record using uranium series data of climate change for the southwestern united states from growth chronology of multiple speleothems the stalagmite growth represents the onset of wetter climate 12 500 yr b p soon after the start of the younger dryas the wetter climate persisted a millennium beyond the termination of the younger dryas this wet cycle is likely related to a more southern positioning of the polar jet stream in response to cooler northern hemisphere climate the end of the wet period coincides with the peak of the holocene summer insolation maximum ca 10 500 yr b p the allerod prior to the younger dryas which corresponds to clovis occupation in the southwestern united states was drier in comparison and seems in line with a climatic contribution to megafauna extinction

insights from a synthesis of old and new climate proxy data from the pyramid and winnemucca lake basins for the period 48 to 11 5 cal ka abstract a synthesis of old and new paleoclimatic data from the pyramid and winnemucca lake basins indicates that between 48 0 and 11 5 103 calibrated years bp hereafter ka the climate of the western great basin was to a degree linked with the climate of the north atlantic paleomagnetic secular variation psv records from pyramid lake core plc08 1 were tied to the gisp2 ice core record via psv matches to north atlantic sediment cores whose isotopic and or carbonate records could be linked to the gisp2 18o record relatively dry intervals in the western great basin were associated with cold heinrich events and relatively wet intervals were associated with warm dansgaard oeschger do oscillations the association of western great basin dry events with north atlantic cold events and vice versa switched sometime after the laurentide ice sheet lis reached its maximum extent for example the lahontan highstand which culminated at 15 5 ka and a period of elevated lake level between 13 1 and 11 7 ka were associated with cold north atlantic conditions the latter period with the youngest dryas event relatively dry periods were associated with the bolling and allerod warm events a large percentage of the lis may have been lost to the north atlantic during heinrich events 1 and 2 and may have resulted in the repositioning of the polar jet stream over north america the trego hot springs wono carson sink and marble bluff tephras found in core plc08 1 have been assigned gisp2 calendar ages of respectively 29 9 33 7 34 1 and 43 2 ka given its unique trace element chemistry the carson sink bed is the same as wilson creek ash 15 in the mono lake basin this implies that the mono lake magnetic excursion occurred at approximately 34 ka and it is not the laschamp magnetic excursion the entrance of the first americans into the northern great basin is dated to approximately 14 4 ka a time when the climate was relatively dry evidence for human occupation of the great basin is lacking for the next 1100 years y i e the oldest western stemmed point site in the great basin dates to 13 3 ka two hypotheses are suggested for this cultural hiatus 1 the climate had deteriorated to the point that people vacated the great basin or 2 people moved to basin bottom wetlands that persisted during the dry period and then the subsequent younger dryas wet event erased the archaeological evidence deposited around the low elevation wetland sites

paleoproterozoic 2155 1970 ma evolution of the guiana shield transamazonian event in the light of new paleomagnetic data from french guiana abstract we present a comprehensive paleomagnetic study on paleoproterozoic 2173 2060 ma plutonic and metamorphic rocks from french guiana representative of the full range of the main transamazonian tectonothermal steps twenty seven groups of directions and poles were obtained from combination of 102 sites 613 samples based on age constraint similar lithology and or geographical proximity paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases this is also reinforced by positive field tests baked contact and reversal tests recent u pb and pb pb on zircon and complementary 40ar 39ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages estimated magnetic ages based on amphibole dating as a proxy enable us to construct a guiana shield apparent polar wander path for the 2155 1970 ma period it is also possible to present paleolatidudinal evolution and continental drift rates related to specific transamazonian tectonic regimes french guiana and probably the guiana shield were located at the equator from ca 2155 to 2130 ma during the meso rhyacian d1 magmatic accretion phase related to subduction of eorhyacian oceanic crust after closure of the eorhyacian ocean and collision of west african and amazonian plates the guiana shield moved the first evolution towards 60 latitude occurs after 2080 ma during the neorhyacian d2a post collisional sinistral transcurrent phase during the late rhyacian d2b phase up to 2050 ma the guiana shield reaches the pole and starts to move to lower latitudes on an opposite meridian by the orosirian d2c phase from ca 2050 to 1970 ma the guiana shield reaches the equator based on the amphibole 40ar 39ar dates we estimate the continental drift between 12 and 16 cm y for the meso to late rhyacian period followed by a lower rate between 9 and 14 cm y up to orosirian time this study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100 200 ma time period our results confirm the possibility of apwp construction on paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi method radiometric dating

columbia revisited paleomagnetic results from the 1790 ma colider volcanics sw amazonian craton brazil abstract in an attempt to improve our understanding of the paleoproterozoic geodynamic evolution a paleomagnetic study was performed on 10 sites of acid volcanic rocks of the colider suite southwestern amazonian craton these rocks have a well dated zircon u pb mean age of 1789 7 ma alternating field and thermal demagnetization revealed northern southern directions with moderate to high upward downward inclinations rock magnetism experiments and magnetic mineralogy show that this characteristic magnetization is carried by ti poor magnetite or by hematite that replaces magnetite by late magmatic deuteric alteration both magnetite and hematite carry the same characteristic component the mean direction dm 183 0 im 53 5 n 10 95 9 8 k 25 2 yielded a paleomagnetic pole located at 298 8 e 63 3 s 95 10 2 k 23 6 which is classified with a quality factor q 5 paleogeographic reconstructions using this pole and other reliable paleoproterozoic poles suggest that laurentia baltica north china craton and amazonian craton were located in laterally contiguous positions forming a large continental mass at 1790 ma ago this is reinforced by geological evidence which support the existence of the supercontinent columbia in paleoproterozoic times

bedrock controls on barrier island development west central florida coast abstract fifty vibracores and 130 km of high resolution seismic profiles were taken in the lagoons and nearshore area of northern pinellas county florida to determine the role of antecedent topography upon barrier island location origin and development east west seismic profiles show a terrace with 1 1 5 m of relief located along the seaward trend of the barrier islands between 4 and 6 m msl this antecedent topography is composed of a relatively erosion resistant miocene limestone and the terrace is usually present as an increase in slope a contour map of the pre holocene surface shows that this subsurface feature is continuous along the entire study area and is mimicked by the opposing orientations of honeymoon and caladesi islands the correspondence of the modern barrier system to the antecedent topography indicates that 1 the islands formed on the terrace or that 2 landward migration of the barriers was stopped by grounding on this feature stratigraphic analyses of lagoonal and nearshore environments were conducted in order to establish the overall response of the northern pinellas coastal system to the late holocene sea level rise this analysis indicates a basically transgressive holocene sequence with lagoonal sediments occurring seaward of the islands underlying the lagoonal deposits in all areas is a 1 m thick relict pleistocene unit that has been flooded reworked and vegetated by intertidal to subtidal organisms a model of development for these barriers is proposed which incorporates an early transgressive history followed by a recent progradational phase the progradational phase was probably initiated by the late holocene slowdown in sea level rise and an increased sediment supply as the barrier islands migrated over a relict sediment source the antecedent topography thus served as a stabilization point for landward barrier migration and seaward progradation the antecedent topographic control of barrier island location described by this model is dependent upon the coincident decline in the rate of sea level rise and an increased sediment supply

proterozoic palaeomagne tism and single continent plate tectonics summary proterozoic 2700 570 ma palaeomagnetic poles from the major shields conform to a single apparent polar wander path on a contiguous reconstruction which brings into alignment both with each other and with the long axis of the primary continental crust tectonic and magmatic features ranging from late archaean 2900 ma to late proterozoic 1100 ma age this discovery implies that the continental lithosphere behaved on a megatectonic scale as a single coherent plate during this era subject to internal deformation but not breaking up in a brittle fashion until late precambrian times peripheral parts of the supercontinent were redistributed over a short interval at ca 1100 ma but the continental crust was not dismembered and widely dispersed until 570 530 ma this latter episode correlates with the proterozoic phanerozoic transition and defines the onset of multi continent plate tectonics the proterozoic apw record comprises nine large loops with periods ranging from 270 to 110ma and averaging 200ma between 2150 and 1000 ma the loops had a similar signature and their resultant effect was to return the continental lithosphere repeatedly to a stable position with its centre at the geographic pole probably by the goldreich toomre mechanism this periodicity correlates with theoretical and experimental studies of mantle convection predicting avalanche type behaviour due to build up of instability in the surface boundary layer with cycles of 50 200 ma the consolidation of the continental lithosphere defines several phases in the history of mantle convection 1 between 3500 and 2800 ma the sialic scum resulting from differentiation of a thickened and subducting lithosphere was accumulated into the primary body by a large scale mantle convection system 2 this operated simultaneously with a subcontinental lithosphere small scale 50 100 km convection system showing a transition from bimodal to cylindrical roll mode represented by the later greenstone belts 2900 2200 ma and straight belts ca 2800 2400 ma 3 a transition to larger scale episodic subcontinental convection is represented by the proterozoic mobile belts 2200 1100 ma 4 a modification of the major suboceanic lithosphere convection is suggested by reorganization of the peripheral shields at 1100 ma and 5 a subdivision of this major system resulted in continental dispersal during lower cambrian times