constraints on mantle anisotropy beneath precambrian north america from a transportable teleseismic experiment data from the apt89 transportable teleseismic experiment was examined to characterize the variations in shear wave splitting along a 1500 km traverse in north america see silver et al 1993 this issue three geologic terranes were sampled the western superior province of the canadian shield the trans hudson orogen and the wyoming craton the primary goal was to determine the extent to which the anisotropy was controlled by archean and proterozoic tectonic episodes of the large suite of teleseismic events recorded 5 were suitable for determining the fast polarization direction and delay time t from the phases sks and skks the resulting observations reveal two important features first the values of display good geologic coherence stations along the northern part of the array in the exposed shield reveal a consistent ene direction for that is generally parallel to exposed geologic structures within the part of the superior province buried beneath phanerozoic cover rotates to a ne sw azimuth as do geophysical indicators of geologic fabric moving onto the trans hudson changes abruptly to a more ew azimuth the stations within the wyoming craton suggest values with a more northerly direction although the waveforms from these stations are more complex second t which ranges between 0 40s and 1 75s shows systematic variations across the traverse the largest delay times approaching 2 s are found in a 250 km wide band from red lake ontario to the u s canada border t is reduced to about 1 s or less both north and south of this region for the stations with detectable splitting we find an inverse relationship between s isotropic station delays derived from the portable data set and t which is most easily explained by the anisotropy being primarily localized in the fast presumably lithospheric mantle the splitting observations overall suggest that mantle anisotropy along this traverse is dominated by precambrian fossil anisotropy that is preserved in the subcontinental lithosphere

a review of holocene solar linked climatic variation on centennial to millennial timescales physical processes interpretative frameworks and a new multiple cross wavelet transform algorithm article i nfo wereportontheexistenceandnature of holocene solar andclimaticvariationson centennialto millennial time scales we introduce a new solar activity proxy based on nitrate no3 concentration from the talos dome ice core east antarctica we also use a new algorithm for computing multiple cross wavelet spectra in time fre quency space that is generalized for multiple time series beyond two our results provide a new interpretive framework for relating holocene solaractivity variations on centennial to millennial timescales to co varying cli mate proxies drawn from a widespread area around the globe climatic proxies used represent variation in the north atlantic ocean western pacific warm pool southern ocean and the east asian monsoon regions our wavelet analysis identifies fundamental solar modes at 2300 yr hallstattzeit 1000 yr eddy and 500 yr unnamed periodicities leaves open the possibility that the 1500 1800 yr cycle may either be fundamental or derived and identifies intermediary derived cycles at 700 yr and 300 yr that may mark rectified responses of the atlantic thermohaline circulation to external solar modulation and pacing dating uncertainties suggest that the 1500 yr and 1800 yr cycles described in the literature may represent either the same or two separate cycles but in either case and irrespective too of whether it is a fundamental or derived mode in the sense of dima and lohmann 2009 the 1500 1800 yr periodicity is widely represented in a large number of paleoclimate proxy records it is obviously premature to reject possible links between changing solar activity at these multiple scales and the variations that are commonly observed in paleoclimatic records

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

millennial band climate spectrum resolved and linked to centennial scale solar cycles abstract spectral comparison of the hulu cave grip and gisp2 18o records suggests that the single dominant period centered on 1470 yr in the gisp2 spectrum the so called 1500 yr cycle is an artifact of the age model the 230th dated hulu cave record indicates a millennial band spectrum with a weak spectral peak near this period flanked by two stronger peaks centered on 1667 and 1190 yr similar to the grip 18o record all three millennial band cycles may derive from heterodynes of centennial band solar cycles as observed in the holocene record of atmospheric 14c production this suggests a link between centennial scale solar variability and abrupt climate change at the millennial time scale

contributions of la nina and el nino to middle holocene drought and late holocene moisture in the american southwest eolian landforms in estancia basin central new mexico record two episodes of extreme drought and low groundwater levels during the middle holocene 7000 5400 1 4 c yr b p followed by a rise in the water table throughthe late holocene blowouts and associated lunettes formed when water levels fell below the desiccated floor of pluvial lake estancia allowing widespread deflation elevation of the water table in the basin is regulated by a balance between evaporation from playas that occupy the blowouts and recharge of aquifers in the adjacent manzano mountains isotopic analyses of modern precipitation and groundwater reveal that recharge originates primarily as winter moisture which is amplified during el ninos and diminished during la ninas thus changes in the elevation of a reconstructed holocene water table may reflect systematic changes in el nino southern oscillation enso circulation

sediment transport and shear stress partitioning in a vegetated flow vegetation is a common feature in natural coastal and riverine water ways interacting with both the water flow and sediment transport however the physical processes governing these interactions are still poorly understood which makes it difficult to predict sediment transport and morphodynamics in a vegetated environment we performed a simple experiment to study how sediment transport responds to the presence of flexible single blade vegetation and how this response is influenced by the vegetation density we found that the skin friction and sediment transport are reduced in a plant patch and that this effect is larger for denser vegetation we then evaluated several methods to calculate the skin friction in a vegetated flow which is the key to sediment transport prediction among these the inversion of bed load transport formulas and the einstein and banks 1950 methods appeared to produce the most reasonable values of the skin friction finally we suggest using the parameter which is the ratio of the skin friction computed by these methods to the total bed shear stress to make more realistic sediment transport predictions in morphodynamic models

geomorphic expression of abrupt climate change in southwestern north america at the glacial termination eolian and subaqueous landforms composed of gypsum sand provide geomorphic evidence for a wet episode at the termination of glacial climate in southwestern north america drying of pluvial lake estancia central new mexico occurred after ca 12 000 14c yr b p thereafter eolian landforms on the old lake floor constructed of gypsum sand were overridden by rising lake water modified by subaqueous processes and organized into beach ridges along the lake s eastern shore preservation of preexisting eolian landforms in the shallow lake suggests abupt changes in lake level and climate available radiocarbon ages suggest that the final highstand recorded by beach ridges may have developed during the younger dryas yd stade the beach ridges provide information about lake surface area which was 45 of the lake area reached during the maximum highstands of the late pleistocene a similar proportional response has been reported for yd climate changes outside the north atlantic region

seismic evidence for preservation of the archean uchi granite greenstone belt by crustal scale extension abstract in the westernmost superior province of canada the east west alignment of granite greenstone belts and the adjacent highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2 5 ga ago with the belts thrust against one another by east west oriented subduction zones here we present seismic reflection data which demonstrate that in this region the present juxtaposition of the uchi granite greenstone belt and the north caribou gneiss terrane occurred along a late southeast dipping extensional shear zone that extends from the surface into the lower crust the preservation of the uchi belt and probably the english river metasedimentary belt is directly related to their dropping along extensional shear zones which limited subsequent erosion the relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust nor preserved by vertical crustal movements as might occur in the absence of plate tectonics extension may have been associated with the emplacement of mantle derived magmas at 2700 ma which has been linked to slab break off or lithospheric delamination making the extension approximately coeval with local gold mineralisation since crustal scale faults can facilitate the circulation of gold bearing fluids we suggest that greenstone rocks preserved in the hanging walls of syn to post accretion extensional shear zones may preferentially host archean lode gold deposits in the westernmost superior province our seismic observations imply that some of the late structures in the well developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes

evidence from western north america for rapid shifts in climate during the last glacial maximum the estancia basin in southwestern united states contains evidence for strong and rapid pulsations in the supply of moisture brought into the region during the last ice age the pulses were recorded during episodes of stream discharge that spread plumes of fresh water laden with quartz sand over the saline lake the largest pulses in stream discharge lasted only a few decades were organized into cycles that were spaced approximately 200 to 250 and 2000 years apart and were of sufficient magnitude to freshen and maintain the lake at its maximum recorded elevation

solar forcing of holocene climate new insights from a speleothem record southwestern united states holocene climate change has likely had a profound infl uence on ecosystems and culture a link between solar forcing and holocene climate such as the asian monsoon has been shown for some regions although no mechanism for this relationship has been suggested here we present the fi rst high resolution complete holocene climate record for the north american monsoon region of the southwestern united states southwest in order to address the nature and causes of holocene climate change we show that periods of increased solar radiation correlate with decreased rainfall the opposite to that observed in the asian monsoon and suggest that a solar link to holocene climate is through changes in the walker circulation and the pacifi c decadal oscillation and el nino southern oscillation systems of the tropical pacifi c ocean given the link between increased warming and aridity in the southwest additional warming due to greenhouse forcing could potentially lead to persistent hyperarid conditions similar to those seen in our record during periods of high solar activity

climate history of the southwestern united states based on estancia basin hydrologic variability from 69 to 10 ka abstract newly acquired u th dates on gypsum sands contained within pleistocene lake sediments allow us to extend the record of climatic change in the estancia basin of central new mexico back to 69 ka from a previous limit of 29 ka using a hydrologic balance index hbi that reflects variations in sediment mineralogy and bedding characteristics we find strong similarities to speleothem oxygen isotopic records for the southwestern united states as well as to isotopic variations in greenland ice cores intervals of wetter climate in the estancia basin generally appear to correlate to colder conditions in the north atlantic and conversely the existence of a pronounced dry period from 18 1 to 17 ka matches in timing the extrapolar climate reversal of asmerom et al 2017 at fort stanton cave in southern new mexico and confirms a different origin for this stadial like event furthermore the new u th dates support the existing estancia basin interpretation of a dry bolling allerod followed by a wet younger dryas a result consistent with speleothem and wet meadow evidence from new mexico and arizona asmerom et al 2010 2017 polyak et al 2004 hall et al 2012 wagner et al 2010 but in conflict with an interpretation of speleothem evidence from central texas feng et al 2014

timing and extent of late quaternary paleolakes in the trans pecos closed basin west texas and south central new mexico abstract the trans pecos closed basin is a hydrographically closed region covering 20 000 km 2 centered on salt basin 160 km east of el paso texas geomorphic and limnetic evidence have been used to identify four major highstands for lake king during the last glacial maximum lgm additional geomorphic features from a second recently identified paleolake lake sacramento have been found in the beargrass subbasin a nested subbasin approximately 75 km northwest of salt basin radiocarbon ages of the organic material in lake king sediments date four abrupt climate changes and rapid lacustrine transgressions during the lgm with a quasi periodicity of 2000 yr geomorphic evidence in the beargrass subbasin identifies lake cycles contemporaneous with those in lake king the dates for these transgressions correlate with the dates of freshening events identified by researchers in paleolake basins elsewhere in new mexico the quasi periodicity of the events approximates that of dansgaard oeschger events identified from greenland ice cores the contemporaneity of the trans pecos transgressions with transgressive events in other basins in the region suggests that paleolakes in the region were in phase with respect to abrupt climate changes during the latter stages of the lgm

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

heat flow in the nipigon arm of the keweenawan rift northwestern ontario canada 1 in the archean superior province the nipigon embayment in the area of lake nipigon north of lake superior is covered by midproterozoic sediments intruded by keweenawan diabase sills it has been interpreted as a failed arm of the ca 1100 ma keweenawan rift six new heat flow values in this area show that the region of low heat flow associated with the keweenawan rift in lake superior extends northwards along the western margin of the nipigon embayment the average heat flow in the nipigon area 39 5 mwm 2 is only slightly lower than

a continuous high resolution record of late pleistocene climate variability from the estancia basin new mexico lake sediments that accumulated in the estancia basin central new mexico provide a detailed record of submillennial latest pleistocene climatic oscillations sedimentologic biologic and geochemical proxies for changes in salinity and lake level were used to identify episodes of wet and dry climate that occurred between 24 and 12 k y b p as dated by 14c by means of accelerator mass spectrometry from shoreline and basin center deposits these dates determine the timing and duration of the episodes of wet and dry climate the lake expanded and contracted repeatedly during the last glacial maximum and fluctuated near its highest elevation several times during the interval ca 20 to 15 k y b p a pronounced lowstand lasting 1 k y occurred between ca 15 and 14 k y b p followed by two more highstands of the lake between ca 14 and 12 5 k y b p desiccation of the perennial lake after 12 k y b p was followed by a final poorly dated highstand at ca 10 k y b p the record of lake level fluctuations from the estancia basin provides a high resolution record of changes in water budget in a basin the comparatively simple physiography and hydrogeology of which ensured a direct response to the strong climatic fluctuations that occurred during the last glacial maximum and termination

a high resolution record of climate vegetation and fire in the mixed conifer forest of northern colorado usa high resolution pollen charcoal 13c total organic carbon toc and magnetic susceptibility data from sediment cores from a montane lake in northern colorado record variations in vegetation fire history and sedimentation since 14 5 ka 1 ka 1000 cal yr b p this record shows warm conditions during the bolling allerod and the coldest conditions in this area during the younger dryas event warming occurred throughout the early and middle holocene lasting until ca 5 ka when the warmest and wettest summer conditions were recorded progressive climate cooling and enhanced winter precipitation are then observed until present day these long term climatic trends correlate to changes in summer insolation charcoal accumulation rates char increased along with the arboreal vegetation from minima in the late glacial period to maxima during the early and middle holocene suggesting that charcoal influx was also controlled by climate and vegetation toc and 13c show a progressive increase and a decrease trend during the late pleistocene and holocene respectively related to changes in vegetation and productivity in the lake major peaks in the char record correspond with peaks in magnetic susceptibility indicating enhanced fire induced erosion and sedimentation millennial and centennial scale changes are also observed throughout the different proxy records they exhibit strong correlations with climate records from distant regions including greenland and the north atlantic providing evidence for global teleconnections among regional climates a solar climate connection is suggested by prominent ca 225 and 390 yr cycles which may correlate with the 208 yr suess and 400 yr solar cycles

global plate velocities from the global positioning system we have analyzed 204 days of global positioning system gps data from the global gps network spanning january 1991 through march 1996 on the basis of these gps coordinate solutions we have estimated velocities for 38 sites mostly located on the interiors of the africa antarctica australia eurasia nazca north america pacific and south america plates the uncertainties of the horizontal velocity components range from 1 2 to 5 0 mm yr with the exception of sites on the pacific and nazca plates the gps velocities agree with absolute plate model predictions within 95 confidence for most of the sites in north america antarctica and eurasia the agreement is better than 2 mm yr we find no persuasive evidence for significant vertical motions 3 standard deviations except at four sites three of these four were sites constrained to geodetic reference frame velocities the gps velocities were then used to estimate angular velocities for eight tectonic plates absolute angular velocities derived from the gps data agree with the no net rotation nnr nuvel 1a model within 95 confidence except for the pacific plate our pole of rotation for the pacific plate lies 11 5 west of the nnr nuvel 1a pole with an angular speed 10 faster our relative angular velocities agree with nuvel 1a except for some involving the pacific plate while our pacific north america angular velocity differs significantly from nuvel 1a our model and nuvel 1a predict very small differences in relative motion along the pacific north america plate boundary itself our pacific australia and pacific eurasia angular velocities are significantly faster than nuvel 1a predicting more rapid convergence at these two plate boundaries along the east pacific rise our pacific nazca angular velocity agrees in both rate and azimuth with nuvel 1a

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

lithoprobe new perspectives on crustal evolution lithoprobe is canada s national collaborative multidisciplinary earth science research program directed toward an enhanced understanding of how the north american continent evolved research in i

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

late holocene palaeoenvironments of southern crimea soils soil climate relationship and human impact occupying 4 of crimea s territory the sub mediterranean landscapes of southern crimea stand out for their distinct soil climatic conditions and record of human activity this paper presents the r

chronology of expansion and contraction of four great basin lake systems during the past 35 000 years abstract during the past 35 000 years lake bonneville lake russell and lake searles underwent a major period of lake level change the lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15 000 and 13 500 yr b p the rise of lake lahontan was gradual but not continuous in part because of topographic constraints intrabasin spill lake lahontan also had an oscillation in lake level at 15 500 yr b p radiocarbon age estimations for materials that were deposited in the lake basins indicate that lake bonneville rose more or less gradually from 32 000 yr b p and had major oscillations in level between 23 000 and 21 000 yr b p and between 15 250 and 14 500 yr b p lake russell and lake searles had several major oscillations in lake level between 35 000 and 14 000 yr b p the timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states all four lakes may have had nearly synchronous recessions between about 14 000 and 13 500 yr b p after the recessions the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11 500 and 10 000 yr b p these data provide circumstantial evidence that the younger dryas event affected climate on at least a hemispheric scale during the holocene the four lakes remained at low levels and small oscillations in lake level occurred an important aspect of the lake level data is the accompanying expansion of lake surface area at the time of the last highstand lake bonneville and lake lahontan had surface areas about 10 times larger than their mean historical reconstructed areas whereas lake russell and lake searles had surface areas about 5 times larger than their mean historical reconstructed areas differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream or they may have resulted from lake atmosphere feedback processes

evaluating escarpment evolution and bedrock erosion rates in the western guadalupe mountains west texas and new mexico abstract the guadalupe mountains tower above surrounding plains and basins in western texas and southeast new mexico although this mountain range is extensively studied for the permian stratigraphy related to the petroleum reservoirs in the adjacent delaware basin little is known about the geomorphic history the western escarpment in particular rises abruptly from the salt basin to the west of the range and guadalupe peak rises 1600 m above the surrounding landscape this study investigates how geologic processes create and maintain the steep profile of the escarpment on the western boundary of the guadalupe mountain range in situ produced cosmogenic nuclides were used to quantify erosion rates across the steep western front of the range to assess how tectonic climatic and lithologic forces influence the escarpment profile erosion rates range from 1 to 73 m myr faster erosion rates in the southern area of the escarpment coincide with more frequently mapped faults higher elevations at the top of the rim and locations with colder temperatures and higher precipitation topographic profiles across the western escarpment surfaces indicate greater retreat from the original positions of fault surfaces at the north end of the mountain range faster erosion rates in the mountain range are linked to greater uplift and more recent activity in the southern end of the guadalupe mountains

obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle teleseismic shear wave splitting and magnetotelluric experiments across the grenville front between the archean craton and the proterozoic grenville province in the regions of the pontiac subprovince and northwestern grenville province canada show a consistent obliquity between the polarization direction of the fast split shear wave and the most electrically conductive direction mt in the upper mantle transcurrent shear zones at all well recorded stations is nearly n103 e and mt is approximately n80 e the obliquity may be considered a potential kinematic indicator because the seismic and electrical anisotropies are thought to be controlled by lattice preferred and shape preferred orientations of mantle minerals mainly olivine respectively the dextral movement sense of the transcurrent shear zones in the mantle inferred from the observed obliquity is consistent with that inferred from surface geology of the crustal shear zones this consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region

millennial to century scale variability in gulf of mexico holocene climate records of the intertropical convergence zone itcz related to orbital forcing the d 18 o of the surface dwelling planktic foraminifer globigerinoides ruber show negative excursions between 14 and 10 2 ka radiocarbon years that reflect influx of meltwater into the western gom during melting of the laurentide ice sheet the relative abundance of the planktic foraminifer globigerinoides sacculifer is related to transport of caribbean water into the gom maximum transport of caribbean surface waters and moisture into the gom associated with a northward migration of the average position of the itcz occurs between about 6 5 and 4 5 ka in addition abundance variations of g sacculifer show century scale variability throughout most of the holocene the gom record is consistent with records from other areas suggesting that century scale variability is a pervasive feature of holocene climate the frequency of several cycles in the climate records is similar to cycles identified in proxy records of solar variability indicating that at least some of the century scale climate variability during the holocene is due to external solar forcing index terms 4267 oceanography general paleoceanography 9604 information related to geologic time cenozoic 3030 marine geology and geophysics micropaleontology

shear wave splitting observations in the lower great lakes region evidence for regional anisotropic domains and keel modified asthenospheric flow 1 fast axis directions obtained from shear wave splitting analysis of core refracted shear waves reveal several distinct anisotropic domains in the lower great lakes region we used data from 27 broadband seismograph stations extending across a low velocity anomaly in the lithospheric mantle observed splitting times vary from 0 4 to 1 4 s with a mean of 0 7 s a subset of the splitting vectors across a failed rift are oriented parallel to one arm of the rift zone most others show a coherent pattern of fast directions close to the direction of plate motion but oblique to surface tectonic belts within the area of the low velocity anomaly our observations are most simply explained by single layer anisotropy induced by asthenospheric flow spatial variability in the direction and magnitude of splitting are consistent with a flow regime influenced by basal topology of the lithospheric keel

shear stress at the base of shield lithosphere 1 one of the basic unresolved issues regarding plate tectonics is the degree of coupling between surface plates and convecting mantle below are the plates effectively decoupled from the mantle flow field by a low viscosity asthenosphere or are they strongly coupled to mantle flow while these two views are essentially incompatible they both do a reasonably good job of predicting the motions of the surface plates and cannot therefore be distinguished on this basis the significant distinguishing feature for these models is the magnitude of basal shear stress that is applied to the base of the plate while it is difficult to measure this stress directly it is possible in principle to measure the corresponding deformation of the plate through observations of seismic anisotropy and to infer stress here we focus on the canadian shield for which we expect strong plate mantle interaction we show that seismic anisotropy can be used to constrain the magnitude of the stress level applied to the base of the plate and to document the level of interaction between tectonic plates and the mantle below

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

wetter or colder during the last glacial maximum revisiting the pluvial lake question in southwestern north america abstract well preserved shorelines in estancia basin and a relatively simple hydrologic setting have prompted several inquiries into the basin s hydrologic balance for the purpose of estimating regional precipitation during the late pleistocene estimates have ranged from 86 to 150 of modern the disparity largely the result of assumptions about past temperatures in this study we use an array of models for surface water runoff groundwater flow and lake energy balance to examine previously proposed scenarios for late pleistocene climate constraints imposed by geologic evidence of past lake levels indicate that precipitation for the last glacial maximum lgm may have doubled relative to modern values during brief episodes of colder and wetter climate and that annual runoff was as much as 15 of annual precipitation during these episodes

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

decadal to millennial scale solar forcing of last glacial maximum climate in the estancia basin of central new mexico abstract lacustrine sediments from the estancia basin of central new mexico reveal decadal to millennial oscillations in the volume of lake estancia during last glacial maximum lgm time lgm sediments consist of authigenic carbonates detrital clastics delivered to the lake in stream flow pulses and evaporites that precipitated in mudflats exposed during lake lowstands and were subsequently blown into the lake variations in sediment mineralogy thus reflect changes in hydrologic balance and were quantified using rietveld analysis of x ray diffraction traces radiocarbon dates on ostracode valve calcite allowed the construction of mineralogical time series for the interval 23 600 to 18 300 ka which were subjected to spectral analysis using redfit schulz and mudelsee 2002 dominant periods of 900 375 and 265 yr are similar to cycles in holocene 14 c production reported for a variety of tree ring records suggesting that the lake estancia sediments record variations in solar activity during lgm time a prominent spectral peak with a period of 88 yr appears to reflect the solar gleissberg cycle and may help along with the 265 yr cycle to explain an ongoing mystery about how lake estancia was able to undergo abrupt expansions without overflowing its drainage basin

a detailed east asian monsoon history surrounding the mystery interval derived from three chinese speleothem records the mystery interval mi 17 5 14 5 ka was the first stage of the last deglaciation a key interval for understanding mechanisms of glacial interglacial cycles to elucidate possible causes of the mi here we present three high resolution precisely dated oxygen isotope records of stalagmites from qingtian and hulu caves in china reflecting changes in the east asian summer monsoon easm then based on well established chronologies using precise 230th dates and annual band counting results the two cave 18o profiles of 7 yr resolution match well at decadal timescales both of the two cave records document an abrupt weakening 2 of 18o rise within 20 yr in the easm at 16 1 ka coinciding with the transition of the two phased mi reconstructed from new mexico s lake estancia our results indicate that the maximum southward displacement of the intertropical convergence zone and associated southward shift of polar jet stream may generate this two phase feature of the mi during that time we also discover a linear relationship among decreasing easm intensity rising atmospheric co2 and weakening atlantic meridional overturning circulation between the mi and younger dryas episodes suggesting a strong coupling of atmospheric oceanic circulations in response to the millennial scale forcing which in turn regulates global climate changes and carbon cycles

lithospheric variations across the superior province ontario canada evidence from tomography and shear wave splitting 1 the superior province of the canadian shield is the largest contiguous region of the archean crust a combination of data from multiple experiments is used to obtain shear wave splitting parameters and a three dimensional tomographic velocity model beneath a large portion of the superior corresponding approximately to the province of ontario shear wave splits are obtained at 24 sites across the superior displaying strong averaging 1 34 s ene wsw splitting at stations west of 86 w and weaker 0 67 s e w splitting in the east the fast direction is subparallel to both absolute plate motion and tectonic belt boundaries the recovered tomographic velocity model shows a major boundary oriented nnw sse separating high velocities in the western superior ws from low velocities in the east and coinciding with the divide between weak and strong shear wave splits other features include a 200 km thick low velocity anomaly corresponding to the nipigon embayment a 1 0 ga failed rift branch and a linear low velocity anomaly in the east attributed to the great meteor hot spot track the nipigon anomaly in the western portion of the model is probably in situ while the great meteor track is displaced from crustal features associated with the hot spot we interpret this displacement as evidence that the eastern lithosphere has been deformed by basal drag while the western lithosphere has remained stable and propose that the east west lithospheric boundary we have detected represents a change in mechanical properties between stronger higher velocity western material with consistent anisotropic fabric and weaker eastern material with more variable fabric

simulation of the climate of 18 000 years bp results for the north american north atlantic european sector and comparison with the geologic record of north america abstract the large ice sheets in north america and europe and the extensive sea ice cover in the north atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and in turn the location and intensity of the surface winds and jet streams a general circulation model was used to simulate the january and july patterns of temperature precipitation and wind for 18 ka bp boundary conditions for the model consisting of ice sheet location and height sea ice location and sea surface temperature were prescribed from climap 1981 the model results are illustrated and described for the north american north atlantic european sector the jet stream splits around the north american ice sheet and the southern branch strengthens considerably compared to present over the southern portion of the united states the sea ice margin of the north atlantic and the southern edge of the european ice sheet geologic evidence principally from north america of wind temperature and moisture conditions is assessed from sand dune and loess records estimates of snowline depression pollen records and lake level studies the geologic evidence is generally compatible with the model simulation

millennial and centennial scale vegetation and climate changes during the late pleistocene and holocene from northern new mexico usa abstract high resolution pollen and magnetic susceptibility ms data from a sediment core from an alpine bog 3100 m in the sangre de cristo mountains new mexico record variations in treeline elevation and in sedimentation for the last 14 ka cal yr bp strong correspondence between the paleovegetation and the ms profile suggests that both records can be used to reconstruct past climatic conditions the coldest conditions in this area occurred at the end of the late pleistocene during the pinedale glaciation and the younger dryas chron a general warming trend took place during the early holocene lasting until 5 6 ka when the warmest conditions occurred a progressive climate cooling is then observed until today millennial and centennial scale changes are also observed throughout the vegetation and ms records the higher amplitude millennial scale cold events appear to correlate with the timing and duration of episodes of enhanced sea ice drift in the north atlantic and the lower amplitude centennial scale cold events may correlate with periods of lower solar activity a solar climate connection is suggested from these records by a prominent ca 200 yr cycle in tree pollen abundance which may correlate with the 208 yr suess solar cycle this study shows an immediate response of the vegetation treeline to climate change at millennial and centennial scales probably related to variations in summer insolation and solar activity during the late pleistocene and holocene such high resolution studies are very important in order to predict future climate change and particularly in very arid areas where human activity and economies are strongly influenced by climate