early jurassic late pliensbachian co2 concentrations based on stomatal analysis of fossil conifer leaves from eastern australia the stomatal index a measure of stomatal density of an extinct australian early jurassic araucariacean conifer species allocladus helgei jansson is used to reconstruct the atmospheric carbon dioxide concentration pco 2 in the early jurassic the fossil leaves are preserved in a single bed palynologically dated to late pliensbachian similar to 185 183 mya atmospheric pco 2 is estimated from the ratios between the stomatal index of a helgei and the stomatal indices of three modern analogs nearest living equivalent plants co2 concentration in the range of similar to 750 975 ppm was calibrated from the fossil material with a best estimated mean of similar to 900 ppm the new average pco 2 determined for the late pliensbachian is thus similar to although similar to 10 lower than previously inferred minimum concentrations of similar to 1000 based on data from the northern hemisphere but may help constrain pco 2 during this period our results are the first pco 2 estimates produced using jurassic leaves from the southern hemisphere and show that i paleo atmospheric pco 2 estimates are consistent at a global scale though more investigations of southern hemisphere material are required and ii the stomatal proxy method can now be used without the context of relative change in pco 2 when applying the correct methodology c 2014 the authors published by elsevier b v on behalf of international association for gondwana research less

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

complex anisotropy in d beneath the eastern pacific from sks skks splitting discrepancies article i nfo although observations of seismic anisotropy in the lowermost mantle are abundant the mechanism which generates anisotropy in the d layer is not well understood most observational constraints on d anisotropy come from the splitting of shear waves that propagate nearly horizontally through the lowermost mantle however anisotropy in the d layer can also give rise to discrepancies in shear wave splitting for sks and skks phases for the same event station pair as these phases sample different regions of the lowermost mantle here i report observations of strongly discrepant sks skks splitting observed at broadband stations in western mexico and california in particular strong skks splitting with fast polarization directions near 60 and delay times up to 3 s is observed for a group of raypaths that sample a region of the d layer beneath the eastern pacific ocean a comparison of skks splitting with sks splitting observed from the same events as well as with sks splitting observed at a variety of backazimuths at each station suggests that the anomalous anisotropic structure is in the lowermost mantle my preferred model for the unusual anisotropic geometry in this region is large shear deformation in d at the edge of a region where slab material impinges upon the core mantle boundary resulting in lattice preferred orientation of lower mantle minerals

a high resolution14c dated sediment sequence from southwest sweden age comparisons between different components of the sediment a macrofossil rich glaciomarine marine lacustrine sediment from a soft water lake in southwestern sweden has provided an opportunity to 14c date different components of its sediments bulk sediment dates are 100 to 500 yr older than fragile terrestrial macrofossils of corresponding levels with a mean age difference of ca 300 yr this is explained by the presence of old and reworked organic material in the sediment five age comparisons between terrestrial macrofossils and periostraca of marine bivalves probably arctica islandica of allerod age give a mean age difference of 380 yr i e slightly greater than the present reservoir age of 340 30 yr for the swedish west coast this difference is roughly the same as for the marine macroalga desmarestia aculeata a date from shell carbonate fragments of mytilus edulis yields an age that is more than 1000 yr older than corresponding periostraca and terrestrial macrofossils and 500 1000 yr older than the age of the supposed deglaciation of the site altogether this indicates a larger marine reservoir effect during the allerod than at present 1998 john wiley sons ltd

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

timing of the antarctic cold reversal and the atmospheric co2increase with respect to the younger dryas event the transition from the last glacial to the holocene is a key period for understanding the mechanisms of global climate change ice cores from the large polar ice sheets provide a wealth of information with good time resolution for this period however interactions between the two hemispheres can only be investigated if ice core records from greenland and antarctica can be synchronised accurately and reliably the atmospheric methane concentration shows large and very fast changes during this period these variations are well suited for a synchronisation of the age scales of ice cores from greenland and antarctica here we confirm the proposed lead of the antarctic cold reversal on the younger dryas cold event the antarctic cooling precedes the younger dryas by at least 1 8 kyr this suggests that northern and southern hemispheres were in anti phase during the younger dryas cold event a further result of the synchronisation is that the long term glacial interglacial increase of atmospheric co 2 was not interrupted during the younger dryas event and that atmospheric co 2 changes are not necessarily dominated by changes in the north atlantic circulation

ash layers from iceland in the greenland grip ice core correlated with oceanic and land sediments four previously known ash layers ash zones i and ii saksunarvatn and the settlement layer all originating in iceland have been identified in the central greenland ice core grip this correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash this establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately a precise connection with dates obtained by14c beyond the range of dendrochronology is established which provides an excellent confirmation of230th 234u dates from corals four additional icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

stomatal proxy record of co2 concentrations from the last termination suggests an important role for co2 at climate change transitions a new stomatal proxy based record of co2 concentrations co2 based on betula nana dwarf birch leaves from the hasseldala port sedimentary sequence in south eastern sweden is presented the record is of high chronological resolution and spans most of greenland interstadial 1 gi 1a to 1c allerod pollen zone greenland stadial 1 gs 1 younger dryas pollen zone and the very beginning of the holocene preboreal pollen zone the record clearly demonstrates that i co2 were significantly higher than usually reported for the last termination and ii the overall pattern of co2 evolution through the studied time period is fairly dynamic with significant abrupt fluctuations in co2 when the climate moved from interstadial to stadial state and vice versa a new loss on ignition chemical record used here as a proxy for temperature lends independent support to the hasseldala port co2 record the large amplitude fluctuations around the climate change transitions may indicate unstable climates and that tippingpoint situations were involved in last termination climate evolution the scenario presented here is in contrast to co2 records reconstructed from air bubbles trapped in ice which indicate lower concentrations and a gradual linear increase of co2 through time the prevalent explanation for the main climate forcer during the last termination being ocean circulation patterns needs to re examined and a larger role for atmospheric co2 considered

impact of oceanic reorganizations on the ocean carbon cycle and atmospheric carbon dioxide content a zonally averaged circulation biogeochemical ocean model is used to explore how the distribution of po4 and 13c in the major basins and the atmospheric pco2 respond to rapid changes in the thermohaline circulation thc different evolutions of the atlantic thc are simulated by applying surface freshwater pulses typical for example of heinrich events and the last deglaciation in the model when the thc completely collapses po4 increases 0 5 mmol m 3 and 13c decreases 1 because of a longer residence time of waters at the surface these surface anomalies are propagated to the bottom north atlantic when the thc resumes when the thermohaline overturning is only partly reduced and at shallower depths changes in bottom waters are accompanied by a po4 decrease and 13c increase at intermediate levels in the mid latitude atlantic this results in enhanced vertical gradients of these properties consistent with chemical and isotopic reconstructions for the last glacial maximum finally the atmospheric pco2 increases during the cold period in the north atlantic when the thc is reduced with an amplitude 7 30 atm and timescale 10 to 1 2 10 yr depending on the intensity of the thc change this is qualitatively consistent with recent data from an antarctic ice core documenting a pco2 increase during the younger dryas and after heinrich events 4 and 5

tectonic implications of post 30 ma pacific and north american relative plate motions the pacific plate moved northwest relative to north america since 42 ma the rapid half rate of pacific farallon spreading allowed the ridge to approach the continent at about 29 ma extinct spreading ridges that occur offshore along 65 of the margin lonsdale 1991 document that fragments of the subducted farallon slab became captured by the pacific plate and assumed its motion prior to the actual subduction of the spreading ridge this plate capture process can be used to explain much of the post 29 ma cordilleran north america extension strike slip and the inland jump of oceanic spreading in the gulf of california the pacific and north american contact zone lengthened with each successive plate capture event underpinning the parts of western north america directly inland with a strong plate undergoing pacific relative motion we suggest that much of the post 29 ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent the plate capture hypothesis is distinctly different from theories involving shallow slab gaps kinematic problems associated with shallow slab gap models cause us to question them this conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the californian margin and the documented reduction of slab pull forces and density associated with young subducting slabs thermal and rheologic modeling suggests that coastal california was a strong zone at all depths allowing it to be firmly linked to pacific motion our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer

tertiary metamorphic core complexes in sonora northwestern mexico several ranges encompassing more than 35 000 km 2 of sonora mexico contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed prominent northwest trending fractures dikes and normal faults are orthogonal to the lineation most kinematic indicators in lower plate mylonitic rocks record top to the southwest sense of shear upper plate stratigraphic sequences include mesozoic supracrustal rocks tertiary volcanic and sedimentary rocks and allochthonous precambrian basement tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults previously published u pb and k at ages from lower plate granitic orthogneisses upper plate volcanic sequences and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late oligocene early miocene partitioning of extensional strain in sonora was influenced by pre tertiary crustal structure the belt of core complexes developed across two contrasting blocks of continental crust separated by the n60ow striking mojave sonora megashear portions of the southern papago block northeast of the megashear consisting of jurassic magmatic arc rocks and upper jurassic cretaceous siliciclastic and carbonate strata resting upon a concealed tectonically fragmented precambrian basement were especially susceptible to crustal attenuation some core complexes of the southern papago block occur within zones trending northwest that may coincide with late jurassic lineaments in the caborca block southwest of the megashear core complex related rocks and structures have not been identified where surface exposures of middle proterozoic basement and overlying upper proterozoic paleozoic platform strata are common however extensional mylonitic fabrics are locally developed along the margins of a tertiary two mica granite batholith core complexes on both sides of the megashear appear to be preferentially developed where tertiary granites have intruded regions of crust with basement disrupted by pre tertiary structures sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the united states and canadian cordillera the timing of mid crustal extension in sonora 25 18 ma is contemporaneous with the timing of core complex development in arizona nevada and utah extension occurred later in these areas than in the pacific northwest british columbia region but earlier than in the

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

shear wave splitting observations at the regions of northern baja california and southern basin and range in mexico 1 recent installation of 3 component broadband digital stations around the gulf of california allowed us to make shear wave splitting observation from records of 73 sks and skks phases from 30 events recorded at 12 stations since january of 2001 stations in the southern basin and range province yield upper mantle fast shear wave direction from ne sw to e w consistent with both local direction of miocene extension and north american absolute plate motion the shear at the pacific north american plate limit seems to control tightly the anisotropy pattern of the closest station ne70 the anisotropic pattern along the peninsular range east of the former trench between the north american plate and the now subducted farallon plate is almost uniform and is interpreted as asthenospheric flow induced by the sinking fragment of the farallon plate although frozen anisotropy associated with in situ captured fragment of this plate cannot be ruled out

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

seismic anisotropy observations in the mexicali valley baja california mexico a study of seismic anisotropy was performed using data from earthquakes of the mexicali valley the investigated region encompasses the cerro prieto geothermal field cpgf one of the most important fields in the world the results showed that at most of our stations the average polarization directions of the fast s waves range from n14 w to n17 e a n s polarization direction was obtained for the whole area by averaging the polarization directions from all stations used in terms of the eda hypothesis this average trend agrees with the postulated state of stress for southern california and with fault plane solutions for events of the mexicali valley notorious deviations from the n s global trend were found southeast of the cpgf with polarization trends between n25 e and n67 e and in the geothermal field with polarization directions between n7 w and n14 w the polarization results for these zones indicated stress conditions that are different from the more regional stress pattern the delay times that were measured between the fast and slow shear waves reached values of up to 0 6 sec with a mean value of 0 35 sec consistent with our polarization results the larger delay times 0 2 0 6 sec were found in the cpgf smaller or null values were observed at the periphery of the study area no temporal trends in the delay times were apparent as shown by data from the two stations that recorded the larger number of events overall we conclude that the splitting effects of this study result from a shallow anisotropy volume the splitting results are thus interpreted as caused by the preferred orientation of vertical fluid filled microcracks aligned in a direction that is parallel to the regional stress field the stronger splitting effects that were observed in the area of the cpgf were found consistent with the geothermal reservoir that is embedded in the sedimentary cover of the zone at depths of 1 to 4 5 km from the surface we thus believe that such marked splitting effects have a direct relation with the reservoir of the cpgf

surface wave tomography of the gulf of california 1 we measured interstation fundamental mode rayleigh wave phase velocities using data from the nars baja seismic network located around the gulf of california a region average shear velocity model and a set of azimuthally anisotropic phase velocity maps are obtained from these data the average shear velocity structure shows a strong low velocity zone underlying a thin lid and the data are suggestive of low velocities down into the transition zone the phase velocity maps display signatures of sedimentary layers crustal thickness variations upwelling under the plate boundary and the presence of the subducted farallon microplate remnants beneath the gulf the upper mantle features inferred from this study provide new seismic evidence on the tectonic evolution of the region

seismic anisotropy in northern and central gulf of california region mexico from teleseismic receiver functions and new evidence of possible plate capture 1 we present anisotropic models for the shallow lithosphere in the northern and central gulf of california obtained from receiver function rf analysis this region has undergone intense plate reorganization over the past 25 ma that led to the transfer of the plate boundary east of the modern baja california peninsula posterior to the cease of the subduction of the farallon plate beneath the north american plate our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary we calculate rfs from teleseismic p waves recorded by the network of autonomously recording seismographs nars baja array and we stack them in 20 wide back azimuthal bins to enhance coherent information these rfs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping moho therefore we interpret the conspicuous rfs energy of the transverse component as convincing evidence of anisotropy we present forward models obtained by fitting the radial and transverse rfs with synthetic seismograms calculated with a reflectivity code our analysis suggests substantial variations in the structure of the shallow lithosphere along the baja california peninsula and the presence of stalled fragments of the extinct farallon plate beneath its central part the results at station ne81 in sonora are interpreted as frozen crustal anisotropy inherited from miocene basin and range extension consistent with previous observations of the upper mantle anisotropy beneath this station

evidence for the occurrence of vedde ash in sweden radiocarbon and calendar age estimates a tephra layer of rhyolitic composition has been recorded in sediments from lake madtjarn southwestern sweden geochemical analyses have shown that the tephra is identical to the rhyolitic component of the middle younger dryas vedde ash a series of ams radiocarbon measurements places the radiocarbon age of the tephra within a 14c plateau at 10 400 10 300 14c y bp based on a linear younger dryas sedimentation rate and assumptions about the apparent synchroneity of changes in lake sediments tree rings and ice core records the calendar year age of the tephra concentrations is estimated at 12 045 11 975 yr bp which accords well with the age of the equivalent tephra in the grip core 1998 john wiley sons ltd

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

crustal thickness of the peninsular ranges and gulf extensional province in the californias we estimate crustal thickness along an east west transect of the baja california peninsula and gulf of california mexico and investigate its relationship to surface elevation and crustal extension we derive moho depth estimates from p to s converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at 318n latitude depth to the moho is 33 63 km near the pacific coast of baja california and increases gradually toward the east reaching a maximum depth of 40 64 km beneath the western part of the peninsular ranges batholith the crust then thins rapidly under the topographically high eastern peninsular ranges and across the main gulf escarpment crustal thickness is 15 18 62 km within and on the margins of the gulf of california the moho shallowing beneath the eastern peninsular ranges represents an average apparent westward dip of 258 this range of moho depths within the peninsula ranges as well as the sharp east west gradient in depth in the eastern part of the range is in agreement with earlier observations from north of the international border the moho depth variations do not correlate with topography of the eastern batholith these findings suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that a local airy crustal root does not support the highest elevations we suggest that moho shallowing under the eastern peninsular ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the gulf extensional province that commenced in the late cenozoic support of the eastern peninsular ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and or upper mantle

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

stomatal evidence for a decline in atmospheric co2 concentration during the younger dryas stadial a comparison with antarctic ice core records a recent high resolution record of late glacial co2 change from dome concordia in antarctica reveals a trend of increasing co2 across the younger dryas stadial gs 1 these results are in good agreement with previous antarctic ice core records however they contrast markedly with a proxy co2 record based on the stomatal approach to co2 reconstruction which records a ca 70 ppm mean co2 decline at the onset of gs 1 to address these apparent discrepancies we tested the validity of the stomatal based co2 reconstructions from kru akenes by obtaining further proxy co2 records based on a similar approach using fossil leaves from two independent lakes in atlantic canada our late glacial co2 reconstructions reveal an abrupt ca 77 ppm decrease in atmospheric co2 at the onset of the younger dryas stadial which lagged climatic cooling by ca 130 yr furthermore the trends recorded in the most accurate high resolution ice core record of co2 from dome concordia can be reproduced from our stomatal based co2 records when time averaged by the mean age distribution of air contained within dome concordia ice 200 to 550 yr if correct our results indicate an abrupt drawdown of atmospheric co2 within two centuries at the onset of gs 1 suggesting that some re evaluation of the behaviour of atmospheric co2 sinks and sources during times of rapid climatic change such as the late glacial may be required copyright 2002 john wiley sons ltd journal of quaternary science

rayleigh wave dispersion measurements reveal low velocity zones beneath the new crust in the gulf of california rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the gulf of california low velocity zones lvzs are found on axis between 26 and 50 km depth beneath the guaymas basin but mostly off axis under the other rift basins with the largest feature underlying the ballenas transform fault we interpret the broadly distributed lvzs as regions of partial melting in a solid mantle matrix the pathway for melt migration and focusing is more complex than an axis centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments we also consider the existence of solid lower continental crust in the gulf north of the guaymas basin where the association of the lvzs with asthenospheric upwelling suggests lateral flow assisted by a heat source these results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift

routing of meltwater from the laurentide ice sheet during the younger dryas cold episode rooth1 proposed that the younger dryas cold episode which chilled the north atlantic region from 11 000 to 10 000 yr bp was initiated by a diversion of meltwater from the mississippi drainage to the st lawrence drainage system the link between these events is postulated to be a turnoff during the younger dryas cold episode of the north atlantic s conveyor belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the north atlantic region2 this turnoff is attributed to a reduction in surface water salinity and hence also in density of the waters in the region where north atlantic deep water nadw now forms here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from orca basin core en32 pc4 which reveal a significant reduction in meltwater flow through the mississippi river to the gulf of mexico from about 11 200 to 10 000 radiocarbon years ago this finding is consistent with the record for lake agassiz which indicates that the meltwater from the southwestern margin of the laurentide ice sheet was diverted to the northern atlantic ocean through the st lawrence valley during the interval from 11 000 to 10 000 years before present yr bp

simulation of abrupt climate change induced by freshwater input to the north atlantic ocean temperature records from greenland ice cores1 2 suggest that large and abrupt changes of north atlantic climate occurred frequently during both glacial and postglacial periods one example is the younger dryas cold event broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the atlantic ocean which were caused by the release of large amounts of melt water from continental ice sheets here we describe an attempt to explore this intriguing phenomenon using a coupled ocean atmosphere model in response to a massive surface flux of fresh water to the northern north atlantic of the model the thermohaline circulation weakens abruptly intensifies and weakens again followed by a gradual recovery generating episodes that resemble the abrupt changes of the ocean atmosphere system recorded in ice and deep sea cores4 the associated change of surface air temperature is particularly large in the northern north atlantic ocean and its neighbourhood but is relatively small in the rest of the world

late glacial and early holocene variations in atmospheric co2 concentration indicated by high resolution stomatal index data data from ice cores suggest that late glacial and early holocene atmospheric co2 variations were rather conservative the most important change being a gradual younger dryas increase by contrast palaeo co2 records based on the inverse relationship between co2 partial pressure and stomatal frequency of terrestrial plant leaves reflect a more dynamic co2 evolution including an abrupt decrease at the allerod younger dryas transition here we present a late glacial and early holocene co2 record based on stomatal index data from leaves preserved in the sediments of a small lake in southwestern sweden three independent records constructed from stomatal index data of salix polaris salix herbacea and betula nana leaves were combined to form a high resolution co2 reconstruction for the period 12 800 10 800 cal yr bp atmospheric co2 concentrations were found to have decreased rapidly from c 260 ppmv to 210 215 ppmv within 200 years during the allerod gi 1 younger dryas gs 1 transition after 100 200 years co2 concentration started to gradually increase to 270 290 ppmv at the end of the younger dryas stadial gs 1 co2 concentrations were relatively stable during the early holocene except for a short lived period of lower 240 250 ppmv values c 11350 11200 cal yr bp this late glacial and early holocene co2 evolution partly resembles previous stomatal based co2 reconstructions and the overall trend is almost identical to that seen in ice core records the amplitude of change is however markedly higher in the swedish stomatal based record compared to the ice cores this difference may partly be accounted for by the inherent smoothing of ice core co2 records caused by diffusion but a major part of the difference in amplitude between ice core and stomatal based records still remains to be explained based on our reconstruction atmospheric co2 may have played an important role in climate dynamics during the last deglaciation c 2003 elsevier b v all rights reserved less

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

century scale changes of atmospheric co2 during the last interglacial the crucial role of atmospheric co2 in glacial interglacial transitions is demonstrated by recent ice core studies that highlight the importance of accurate co2 records for our understanding of quaternary climate dynamics previous estimates of co2 levels during the last interglacial stage the eemian relied on measurements on air trapped in the antarctic vostok ice core due to uncertainties associated with in situ chemical alteration and gas diffusion there is a need for independent estimates of past co2 levels here we report 33 eemian co2 estimates based on the stomatal index of salix herbacea l leaves preserved in greenland sediments we reconstruct eemian co2 levels centered on 250 280 parts per million by volume ppmv in general agreement with ice core data two deviating lower stomatal estimates may reflect diffusional smoothing of vostok co2 data and indicate century scale eemian co2 variability less

shear wave splitting along a nascent plate boundary the north anatolian fault zone summary the north anatolian fault zone nafz is a transform structure that constitutes the boundary between the anatolian plate to the south and the eurasia plate to the north we analysed the properties of the upper mantle strain field and mantle anisotropy in the vicinity of nafz via splitting of sks and skks phases we used data from the north anatolian fault naf passive seismic experiment this is the first study that analyses the upper mantle anisotropy in this region and our results indicate that the observed upper mantle strain field is uniform underneath the array with consistent ne sw polarization directions for fast split waves the measured lag times between the arrivals of the fast and slow split waves varies from 0 5 to 1 6 s for the study area smaller lag times are obtained consistently in the eastern part of the array however we do not observe any significant variation in either the polarization directions or the delay times across the plate boundary nafz the uniformity of the fast polarization directions throughout the study area and the strength of anisotropy favour an asthenospheric source for the anisotropy the regional tectonic framework favours a sw direction of asthenospheric flow due to the forces acting on the upper mantle exerted by the slab roll back taking place along the aegean and the cyprean subduction zones

synchronous records of pco2 and δ14c suggest rapid ocean derived pco2 fluctuations at the onset of younger dryas just before the onset of the younger dryas yd cold event several stomatal proxy based pco2 records have shown a sharp increase in atmospheric co2 concentration pco2 of between ca 50 and 100 ppm followed by a rapid decrease of similar or even larger magnitude here we compare one of these records a high resolution pco2 record from southern sweden with the intcal13 record of radiocarbon d 14 c the two records show broadly synchronous fluctuations at the yd onset specifically the intcal13 record documents decreasing d 14 c just before the yd onset when pco2 peaks consistent with a source of old co2 from the deep ocean we propose that this fluctuation occurred due to a major ocean flushing event the cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor we confirm that the earth system can produce such large multidecadal timescale fluctuations in pco2 through simulating an artificial ocean flushing event with the genie earth system model we suggest that sharp transitions of pco2 may have remained undetected so far in ice cores due to inter firn gas exchange and time averaging the stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change

the preboreal oscillation around the nordic seas terrestrial and lacustrine responses the occurrence of an early preboreal climatic cooling oscillation pbo in lacustrine and glacial records from northwest europe iceland and greenland is reviewed and documented the often subtle response of the proxy records to this oscillation in combination with its short duration make it difficult to detect owing to its chronostratigraphic position between the 10 000 9900 and 9600 9500 14 c plateaux c 11 300 11 150 calendar yr bp it is also difficult to 14 c date with precision we find that the vegetation response to the pbo varies between sites and regions in contrast to the pioneer vegetation in iceland and southern sweden the expanding birch pine forest in germany denmark was more susceptible to deteriorating growing conditions the combined lacustrine tree ring and glacial records imply that the pbo was characterised by cool and humid conditions throughout northwestern and central europe this is documented by vegetation changes decreased aquatic production increased soil erosion increased 2 h and 13 c content in tree rings readvances or stillstands of the ice sheet in norway and finland and ingression of brackish water into the baltic icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous preboreal period but not as extreme as during the younger dryas greenland records suggest that the early preboreal was characterised by ice readvances as an effect of cool climate and increased precipitation in relation to the younger dryas it was not until the end of the pbo that climate was warm enough to melt the land based ice sheet this preboreal oscillation found on both sides of the nordic seas is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the nordic seas which is implied by a simultaneous and distinct rise in the atmospheric 14 c 12 c ratio a slow down of the thermohaline circulation may tempor arily have pushed the polar front further south 1997 by john wiley sons ltd preboreal oscillation thermohaline circulation lacustrine records glacial records chronology

geodynamics of the gulf of california from surface wave tomography the gulf of california which forms part of the pacific north american plate boundary is an ideal place to investigate upper mantle dynamics in a continental rifting area with 19 seismic stations located around the gulf the nars baja experiment 2002 2008 was designed to image its crustal and mantle structure here we present results of a tomographic inversion of love and rayleigh interstation phase velocity measurements for a radially anisotropic shear velocity model of the gulf of california this study confirms the overall low shear wave velocities in the upper 200km of the mantle found in other rayleigh wave studies and the presence of a positive shear wave velocity anomaly at depths of roughly 80 160km beneath the central gulf zhang et al 2009 in addition we find that the horizontal shear velocity vsh is generally higher than the vertical shear velocity vsv for the northern gulf however there is strong indication for vsv vsh in the 40 60km depth range this region also has anomalously low shear wave velocities down to 100km depth combining these observations we suggest that the low velocity anomaly and the negative radial anisotropy vsh vsv beneath the northern gulf are related to vertical flow of low velocity material in an area of a slab window either by the vertical alignment of olivine crystals or by a fabric of vertically oriented sheets of melt the high velocity anomaly beneath the central gulf is interpreted as a remnant slab fragment which inhibits vertical flow from the deeper mantle our tomographic results indicate that the formation of the gulf of california cannot be explained by simple models of crustal extension or dynamic upwelling instead its structure and geodynamics are caused by the cessation of subduction by stalled microplates in the central and southern gulf and the presence of a slab window in the north 2012 elsevier b v

natural and anthropogenic changes in atmospheric co2 over the last 1000 years from air in antarctic ice and firn a record of atmospheric co2 mixing ratios from 1006 a d to 1978 a d has been produced by analysing the air enclosed in three ice cores from law dome antarctica the enclosed air has unparalleled age resolution and extends into recent decades because of the high rate of snow accumulation at the ice core sites the co2 data overlap with the record from direct atmospheric measurements for up to 20 years the effects of diffusion in the firn on the co2 mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close off depth the uncertainty of the ice core co2 mixing ratios is 1 2 ppm 1 preindustrial co2 mixing ratios were in the range 275 284 ppm with the lower levels during 1550 1800 a d probably as a result of colder global climate natural co2 variations of this magnitude make it inappropriate to refer to a single preindustrial co2 level major co2 growth occurred over the industrial period except during 1935 1945 a d when co2 mixing ratios stabilized or decreased slightly probably as a result of natural variations of the carbon cycle on a decadal timescale

subduction control on the curie isotherm around the pacific north america plate boundary in northwestern mexico gulf of california preliminary results abstract spectral analysis of namag magnetic data enabled us to estimate depths to the curie isotherm along the pacific north america plate boundary the estimated depths for the gulf of california and southern baja california peninsula correlate quite well with the depths to the moho available from previous studies based on receiver function and seismic refraction in northernmost baja california and southwestern usa no correlation is observed between depths to the curie isotherm and depths to the mantle obtained from receiver function analysis depths to the curie isotherm are shallower along the coastal plains of sonora and sinaloa differences between receiver function based mantle depths and curie depths are equally large however there is more consistency with depths established by seismic refraction studies four zones with shallow curie isotherm are identified in southern gulf of california gs1 gs2 gs3 and gs4 gs1 is located at the gulf of california mouth the alarcon rise is located between gs1 and gs2 pescaderos basin is located northwest of gs2 gs3 stretches from the carmen basin to the farallon basin gs4 extends from tiburon island up to northern guaymas basin northernmost shallow curie depth zone gn1 comprises upper and lower delfin basins as well as the consag and wagner basins a northern prolongation also covers pinacate volcanic center the southern shallow curie isotherm zones are associated with lithospheric asthenospheric material upwelling zones gn1 also includes a convective component related to the younger magmatism of the pinacate volcanic field deeper curie depths are found at southern baja california peninsula interpreted as associated with thicker continental or stretched crust it is possible that the crust is underlain or underplated by magnetic oceanic crust as suggested by satellite magnetic anomalies centered at the magdalena shelf a contrasting pattern features northern baja california deep curie zones to the west contrasting with relative shallower curie isotherm areas to the east the prolongation towards the peninsula of the extinct shirley transform fracture zone roughly separates the two domains observed along the peninsula and suggests that the curie isotherm along the peninsula is controlled by differences subduction of the guadalupe and magdalena microplates major differences pointed out between northern and southern gulf of california might indicate that subduction also played a key role at the beginning of the opening of the gulf of california however currently the curie pattern is controlled by the rifting processes shallow curie zones correlating quite well with the rift basins and the lithosphere asthenosphere upwelling material

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

synchronized terrestrial atmospheric deglacial records around the north atlantic on the basis of synchronization of three carbon 14 c 14 dated lacustrine sequences from sweden with tree ring and ice core records the absolute age of the younger dryas preboreal climatic shift was determined to be 11 450 to 11 390 80 years before the present a 150 year long cooling in the early preboreal associated with rising delta 14 c values is evident in all records and indicates an ocean ventilation change this cooling is similar to earlier deglacial coolings and box model calculations suggest that they all may have been the result of increased freshwater forcing that inhibited the strength of the north atlantic heat conveyor although the younger dryas may have begun as an anomalous meltwater event

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

fossil leaves effective bioindicators of ancient co2 levels 1 past variations in the concentration of the greenhouse gas co2 are thought to have played a major role in controlling earth s climate on prequaternary and quaternary timescales to identify the contribution of co2 to past climatic change requires accurate quantification of its content in the ancient atmosphere and a number of proxies have been developed for this purpose for a review see royer et al 2001a for the late quaternary there is the unique opportunity to measure directly the composition of fossil air samples trapped in bubbles preserved in the polar ice sheets results from antarctic ice cores reveal that the glacialinterglacial changes characterizing quaternary climate were accompanied by variations in the atmospheric concentration of co2 petit et al 1999 although detection of phase relations between isotope derived temperature estimates and trace gas concentration values is hampered by the difference in age of ice and air from the same ice sample it is believed that co2 lags glacial interglacial temperature change and has acted as an amplifier of orbitally forced changes in temperature petit et al 1999 shackleton 2000 mudelsee 2001

extensive and rapid changes in seawater chemistry during the phanerozoic evidence from br contents in basal halite abstract variation of seawater chemical composition during the phanerozoic is still a contentious subject br concentration in primary marine basal halites was used to indicate variations in seawater composition during the past 550 ma evaporation experiments and thermodynamic modelling suggests that the partition coefficient of br in halite depends on the composition of the seawater a combination of this correlation with the time dependent models for ocean chemistry of hardie leads to a prediction of br in basal halite during the phanerozoic this prediction is in good agreement with data from different ancient evaporite basins the br data give evidence for rapid and massive changes in seawater chemistry during the phanerozoic and are thus a first independent test for the hardie model

the relationship between upper mantle anisotropic structures beneath california transpression and absolute plate motions we calculated sks splitting parameters for the california integrated seismic network in southern california we also estimated splitting in the upper 100 km using azimuthal anisotropy determined from surface waves the inferred splitting from surface waves in the mantle lithosphere is small on average 0 2 s compared with sks splitting 1 5 s and obtains a maximum value 0 5 s in the transpressive region of the big bend south of and aligned with the san andreas fault saf in contrast the sks splitting is approximately e w and is relatively uniform spatially either side of the big bend of the saf these differences suggest that most of the sks splitting is generated much deeper down to 300 400 km than previously thought probably in the asthenosphere fast directions align with absolute plate motions apm in northern and southeastern california but not in southwestern california we interpret the parallelism with apm as indicating the sks anisotropy is caused by cumulative drag of the asthenosphere by the overlying plates the discrepancy in southwestern california arises from the diffuse boundary there compared to the north where relative plate motion has concentrated near the saf system in southern california the relative motion originated offshore in the borderlands and gradually transitioned onshore to the saf system this has given rise to smaller displacement across the saf 160 180 km compared with central and northern california 400 500 km thus in southwestern california the inherited anisotropy from prior north american apm has not yet been overprinted by pacific apm

s wave polarization inversion of the 15 october 1979 23 19 imperial valley aftershock evidence for anisotropy and a simple source mechanism s polarizations from the 16 near source strong motion stations of the 15 october 1979 23 19 imperial valley aftershock have been analyzed for constraining the source mechanism they show a significant delay of the east component relative to the north component at most stations assuming that this is an effect of anisotropy with a horizontal symmetry axis in the upper 4 km of sediments where the rays are dominantly vertical the measured fast s polarizations are mainly oriented between n0 e and n25 e and show a mean delay of 0 25 s between slow and fast s this is consistent with an hexagonal anisotropy due to vertical cracks induced by the regional compressional stress field and parallel to it with a mean fluid filled crack density of 0 1 to 0 2 the records once corrected for the splitting effect show s polarizations stable in time which have been used to estimate the fault mechanism parameters this is done by applying a new inversion method which is based on a model probability estimate the error function is related to the polarization misfits and the associated joint probability function of the data and parameter is computed in the whole model space the inversion of the corrected s polarizations provides a unique and well constrained solution for the imperial valley aftershock mechanism strike 145 5 dip 85 10 slip 155 15 which is compatible with the available p polarities and is quite similar to the main shock mechanism

crustal thickness estimates for baja california sonora and sinaloa mexico using disperse surface waves 1 dispersed surface waves of regional events recorded at network of autonomously recording seismographs nars baja and red sismica de banda ancha resban networks located over the baja california peninsula sonora and sinaloa mexico were used to estimate shear wave elastic models and crustal thickness we analyzed fundamental modes of surface waves with period between 10 and 40 s multiple filter analysis and the inversion method described by herrmann and ammon 2003 was used crustal thickness estimates for the peninsular ranges of northern baja california agree with those obtained by previous studies in the peninsular ranges of northern baja california we analyzed dispersion of surface waves with northwest southeast travel paths along the east and west sides of the baja california peninsula as well as a northwest southeast travel path along the western sides of the mexican states of sonora and sinaloa it was found that the crustal structure east of the baja california peninsula is similar to the structure of sonora and sinaloa the correlation between those two structures suggests dextral offset of the order of 275 25 km if we consider baja california peninsula as a rigid body moving toward the northwest relative to the north america plate this displacement between the structures is in agreement with the displacement determined by the dating of miocene deposits located in san felipe on the baja california peninsula pacific plate and isle tiburon located west of sonora north america plate

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

isotopic events in the grip ice core a stratotype for the late pleistocene an event stratigraphy for the last termination based on the stratotype of the grip ice core record has been outlined for the north atlantic region it is suggested that such an approach to stratigraphic subdivision may o er a more satisfactory alternative to conventional stratigraphical procedures for those parts of the recent quaternary record that are characterised by rapid and or short term climatic uctuations 1999 elsevier science ltd all rights reserved

a review of the geodynamic evolution of flat slab subduction in mexico peru and chile abstract subducting plates around the globe display a large variability in terms of slab geometry including regions where smooth and little variation in subduction parameters is observed while the vast majority of subduction slabs plunge into the mantle at different but positive dip angles the end member case of flat slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle by employing a comparative assessment for the mexican peruvian and chilean flat slab subduction zones we find a series of parameters that apparently facilitate slab flattening among them trench roll back as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important however we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones in order to unravel the origin of flat slab subduction it is probably necessary a numerical approach that considers also the influence of surrounding plates and their corresponding geometries on 3d subduction dynamics

a holocene co2 record from the stomatal index of subfossil salix herbacea l leaves from northern sweden a stomatal based method of palaeo co2 estimation has been applied to a temporally detailed sequence of leaves from a high latitude lake 68 n in northern sweden spanning the last 9000 years the resulting atmospheric co2 reconstruction documents the onset of a gradual increase c 5000 years before present indicating that the carbon cycle has not been in steady state over this time stable carbon isotope 13c measurements of the subfossil leaves constrain the interpretation of the inferred changes in the operation of the global carbon cycle the 13c data reveal no marked or systematic shifts towards more negative values indicating that the co2 addition to the atmosphere 5000 1000 yr bp may have been predominantly of oceanic origin

co 2 record in the byrd ice core 50 000 5 000 years bp the analysis of air in polar ice cores revealed 30 lower co2 values during glacial periods than during interglacial periods at present this is confirmed by results from six different ice cores two from greenland and four from antarctica1 5 in all cores the co2 change coincides with the change in the isotopic composition of the ice expressed as either the 18o or d ratio both are indicators for the mean annual surface temperature6 to investigate the relationship between atmospheric co2 concentration changes with changes in climate the atmospheric co2 concentration during and at the end of the last glaciation has to be known in detail to achieve this we have studied a great number of samples from the deep ice core from byrd station westantarctica drilled in 1968 these measurements allow us to reconstruct the atmospheric co02 concentration in the time period 50 000 15 000 yr bp in great detail

active tectonics of northeastern sonora mexico southern basin and range province and the 3 may 1887 mw 7 4 earthquake north south striking and west dipping basin and range province normal faults form the western edge of the sierra madre occidental plateau in northeastern sonora these faults and associated half grabens extend over a distance of more than 300 km between the san bernardino basin in the north and the sahuaripa basin in the south an earthquake in 1887 ruptured three neighboring segments of this major fault zone our field mapping in this region indicates that the surface rupture of the 1887 earthquake extends farther to the south and is considerably longer 101 4 km end to end length than previously reported a compilation of the seismicity in the epicentral region of the 1887 earthquake shows the epicenters to be distributed in well defined clusters at the northern end of the 1887 surface rupture in the step overs between the three rupture segments on a neighboring fault in the west fronteras fault and on fault segments farther south along the same fault zone granados region the distribution of seismicity correlates well with calculated changes in coulomb failure stress resulting from the 1887 earthquake

rapid late glacial atmospheric co2 changes reconstructed from the stomatal density record of fossil leaves the younger dryas stadial 11 000 10 000 yr bp was an abrupt return to a glacial climate during the termination of the last glaciation we have reconstructed atmospheric co2 concentrations from a high resolution sequence of fossil salix herbacea leaves through this climatic oscillation from krakenes western norway using the relationship between leaf stomatal density and atmospheric co2 concentration high allerod co2 values median 273 ppmv decreased rapidly during 130 200 14c years of the late allerod to ca 210 ppmv at the start of the younger dryas they then increased steadily through the younger dryas reaching typical interglacial values once more ca 275 ppmv in the holocene the rapid late allerod decrease in co2 concentration preceded the younger dryas temperature drop possibly by several decades this striking pattern of changes has not so far been recorded unambiguously in temporally coarse measurements of atmospheric co2 from ice cores our observed late glacial co2 changes have implications for global modelling of the ocean atmosphere biosphere over the last glacial interglacial transition

timing and east west correlation of south swedish ice marginal lines during the late weichselian abstract the retreat of the late weichselian ice sheet over the southern part of sweden is marked along the southwest coast by distinct marginal moraine ridges their timing can directly and indirectly be assessed based on a number of radiocarbon dates and pollen stratigraphic investigations on lake sediment sequences adjacent to the ice marginal lines along the southeastern side of the peninsula the ice recession has been reconstructed based on a combination of clay varve chronology pollen and radiocarbon stratigraphy a morphological correlation of ice marginal lines between the west and east coast is problematic since the distinct west coast moraines cannot be followed through the central part of the peninsula towards the east coast this paper is an attempt to reconstruct an age equivalent west east extension of the ice recession lines on the basis of existing data sets for our correlation we use calibrated radiocarbon ages for ice marginal deposits on the west coast and compare these with a partly radiocarbon dated clay varve chronology on the east coast we conclude that the two oldest moraines on the west coast formed at 18 000 16 000 and 15 400 14 500 cal yr bp respectively during the following rapid deglaciation which may have coincided with the beginning of the bolling pollen zone large parts of southernmost sweden became ice free except for higher elevated areas where stagnant ice remained for another 400 500 yr a best guess is that the formation of the next younger ice marginal lines may have occurred at 14 400 14 200 14 200 and 13 400 cal yr bp and during the younger dryas cold event

forest re growth on medieval farmland after the black death pandemic implications for atmospheric co2 levels well dated pollen assemblages from an organic rich infill of an oxbow lake of the river roer southeastern netherlands provide a high resolution reconstruction of regional vegetation development and land use for the period between ad 1000 and 1500 regional effects of the mid 14th century plague pandemic known as the black death are reflected by a period of significant agricultural regression between ad 1350 and 1440 concomitant re growth of forest indicates the existence of a terrestrial carbon sink following the black death pandemic a direct temporal correlation of the reconstructed changes in land cover with a proxy record of atmospheric co2 mixing ratios based on stomatal frequency analysis of quercus robur leaf remains suggests the coupling of long term co2 trends of the 13th 15th centuries and coeval trends in regional forest density during the period of maximum reforestation between ad 1400 and 1440 co2 levels seem to be relatively low but the onset of a co2 decline may predate the spread of the black death in europe 2005 elsevier b v all rights reserved

modelling the concentration of atmospheric co2 during the younger dryas climate event the younger dryas yd dated between 12 7 11 6 ky bp in the grip ice core central greenland is a distinct cold period in the north atlantic region during the last deglaciation a popular but controversial hypothesis to explain the cooling is a reduction of the atlantic thermohaline circulation thc and associated northward heat flux as triggered by glacial meltwater recently a ch4 based synchronization of grip 18o and byrd co2 records west antarctica indicated that the concentration of atmospheric co2 coatm2 rose steadily during the yd suggesting a minor influence of the thc on coatm2 at that time here we show that the coatm2 change in a zonally averaged circulation biogeochemistry ocean model when thc is collapsed by freshwater flux anomaly is consistent with the byrd record cooling in the north atlantic has a small effect on coatm2 in this model because it is spatially limited and compensated by far field changes such as a warming in the southern ocean the modelled southern ocean warming is in agreement with the anti phase evolution of isotopic temperature records from grip northern hemisphere and from byrd and vostok east antarctica during the yd 13c depletion and po4 enrichment are predicted at depth in the north atlantic but not in the southern ocean this could explain a part of the controversy about the intensity of the thc during the yd potential weaknesses in our interpretation of the byrd co2 record in terms of thc changes are discussed

an event stratigraphy for the last termination in the north atlantic region based on the greenland ice core record a proposal by the intimate group it is suggested that the grip greenland ice core should constitute the stratotype for the last termination based on the oxygen isotope signal in that core a new event stratigraphy spanning the time interval from ca 22 0 to 11 5 k grip yr bp ca 19 0 10 0 k 14 c yr bp is proposed for the north atlantic region this covers the period from the last glacial maximum through termination 1 of the deep ocean record to the pleistocene holocene boundary and encompasses the last glacial late glacial of the traditional northwest european stratigraphy the isotopic record for this period is divided into two stadial episodes greenland stadials 1 gs 1 and 2 gs 2 and two interstadial events greenland interstadials 1 gi 1 and 2 gi 2 in addition gi 1 and gs 2 are further subdivided into shorter episodes the event stratigraphy is equally applicable to ice core marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon dated chronostratigraphy that has been used hitherto 1998 john wiley sons ltd

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

reconstructing past atmospheric co2 concentration based on ice core analyses open questions due to in situ production of co2 in the ice analysis of air extracted from bubbles of polar ice showed the anthropogenic increase of atmospheric co2 concentration during the past few hundred years preindustrial concentration 280 ppmv and unexpectedly that the concentration also increased due to natural causes from 200 to 280 ppmv during the transition from the last glacial epoch to the holocene however more detailed reconstructions based on ice core analyses suggested that some of the additional observed variations were actually due to the modification of co2 concentration in the bubbles by chemical reactions between impurities in the ice detailed analyses of acidity the carbonate concentration the concentration of oxidation agents like hydrogen peroxide h2o2 and organic compounds e g formaldehyde hcho along short intervals of ice cores representing only a few annual layers allowed us to investigate these chemical reactions the records are not conclusive but they indicate that the oxidation of organic compounds is at least as important as acid carbonate reactions ice containing a low mean carbonate and a low h2o2 concentration as well as a small scattering of co2 results obtained on several adjacent samples representing a few annual layers at most is at present the best guarantee for reliable results of the atmospheric co2 concentration

stress orientation and anisotropy based on shear wave splitting observations in the cerro prieto fault area baja california mexico digital seismograms continuously recorded from 1988 to 1992 by two stations of the resnom seismic network in northern baja california mexico were used to search for probable shear wave anisotropic characteristics in the region of the cerro prieto fault shear wave splitting was identified in many of the three component records analyzed we measured the polarization direction of the leadings wave inside thes wave window as well as the delay times between fast and slow phases on those records displaying shear wave splitting for station cpx which is nearest the imperial valley region to the north the preferred polarization direction found in this study azimuth 180 10 coincides with the direction of the regional maximum compressive stress determined for the region this polarization direction can be interpreted in terms of the extensive dilatancy anisotropy model as the effect of vertical parallel aligned cracks the preferred polarization direction measured at lmx however gives an azimuth of 45 5 thus it appears that faults and fractures aligned oblique to the main tectonic trend have a greater influence on the anisotropic characteristics of the crust south of cerro prieto volcano than that of the regional stress field time delays between slow and fasts waves observed at cpx appear constant from 1988 to 1992 while delays measured at lmx for the same interval indicate a small increase with time which cannot be attributed to azimuthal variations of paths

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

early holocene atmospheric co2 concentrations wagner et al 1 1 present a high resolution record of leaf stomatal index si from which they reconstruct the concentration of atmospheric co2 in the early holocene 10 070 to 9 38014c yr bp 11 300 to 10 600 cal yr bp 1 1 2 2 the resulting record shows a co2 concentration of

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

microstructural and seismic properties of the upper mantle underneath a rifted continental terrane baja california an example of sub crustal mechanical asthenosphere the gulf of california rift is a young and active plate boundary that links the san andreas strike slip fault system in california to the oceanic spreading system of the east pacific rise the xenolith bearing lavas of the san quintin volcanic area provide lower crust and upper mantle samples from beneath baja california peninsula the microstructures crystallographic preferred orientations cpo and petrology of the san quintin xenoliths suggest that the continental lithosphere in this region has undergone several stages of deformation recrystallisation and melt rock interaction melt rock interactions have led to enrichment in olivine while fine grained microstructures suggest intense deformation in an active shear zone in the shallow upper mantle in this study we highlight the effect of the fine grained mylonitic shear zone development in the upper mantle as an important process of weakening of continental lithosphere the results of the microstructural study show a reduction in cpo strength with increasing grain size reduction most cpos are consistent with dominant slip on the 0kl 100 system as a consequence corresponding seismic anisotropies decrease for both p and s waves with increasing grain size reduction the shallow crystallographic fabric can be related to active shear zones which accommodate the relative motion between the northern baja terrane and the pacific plate estimates of the strain rate stress and viscosity indicate that the shallow mantle beneath northern baja is thermally and chemically lithospheric but mechanically has similar viscosity as the asthenosphere the northern baja terrane is an interesting case of continental crust lying directly on low viscosity upper mantle 2012 elsevier b v all rights reserved

frequency dependent shear wave splitting and heterogeneous anisotropic structure beneath the gulf of california region abstract the gulf of california region has undergone a major evolution over the past 15 ma since subduction of the now extinct farallon plate ceased and transform motion began leading to the initiation of rifting and the opening of the gulf of california 5 ma ago the character of dynamic processes in the upper mantle beneath the region remains poorly understood but constraints on seismic anisotropy in the upper mantle can shed light on contemporary mantle flow processes in order to characterize more fully the anisotropic structure of the upper mantle beneath the region i present measurements of sks splitting at 14 broadband stations of the nars baja array at periods between 8 and 50 s the measured splitting parameters t at nars baja stations exhibit a high degree of complexity with dramatic lateral variations significant backazimuthal variations at individual stations and a large number of well constrained null measurements over a range of backazimuths the dependence of splitting parameters on frequency is evaluated by applying a series of bandpass filters to high signal to noise sks arrivals i find that while fast directions are generally insensitive to the sks frequency content delay times are frequency dependent at several stations the measurements presented in this study indicate that the anisotropic structure of the upper mantle beneath the gulf of california region is complex and exhibits a high degree of both lateral and vertical heterogeneity the observed splitting is consistent with a scenario in which a large scale reorganization of the mantle flow field has been inhibited by the presence of a stalled fragment of oceanic lithosphere associated with farallon subduction the pattern of mantle flow appears to be controlled by more localized processes likely including small scale buoyant upwelling associated with partial melting beneath the central part of the gulf of california rift