milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

evidence suggesting that methods of rock varnish cation ratio dating are neither comparable nor consistently reliable abstract using samples from a prehistoric quarry site in the mojave desert we tested and compared the two principal methods of rock varnish cation ratio dating analysis of rock varnish scrapings r i dorn 1983 quaternary research 20 49 73 and 1989 physical geography 13 559 596 and analysis of rock varnish in situ c d harrington and j w whitney 1987 geology 15 967 970 because we found no consistent relationship between the varnish cation ratio k ca ti and the relative age of the varnish neither method could be used to cation ratio date the underlying chert artifacts moreover in situ analyses yielded systematically higher cation ratios and lower ti abundances than bulk analyses of scraped varnish or microprobe analyses of varnish in cross section our results when considered along with other recently published evidence indicate that these two methods of varnish chemical analysis and cation ratio dating may not be comparable nor consistently reliable chronometers

a modern analogue of western united states summer palaeoclimate at 6000 years before present a modern summer climate analogue of western united states 6 ka palaeoclimate based on broad scale patterns from proxy data and general circulation model simulations provides details concerning its sum mer synoptic climatology august 1955 serves as a close analogue featuring an amplified subtropical ridge aloft and anticyclonic flow near the surface over the central united states and stronger westerly flow along the canadian northwest united states border the august 1955 summer temperature pattern similar to those from general circulation model simulations exhibits positive anomalies throughout most of the continental interior the august 1955 precipitation pattern corresponds very closely with patterns of effective moisture from proxy data showing widespread aridity and some stronger monsoonal activity in the southwest the analogue suggests that the amplified subtropical ridge is a very important feature of 6 ka palaeoclimate with the circulation pattern differing from that in the early holocene implying interactions between both milankov itch and sub milankovitch forcing factors

new early jurassic paleopole from france and jurassic apparent polar wander abstract a new early jurassic paleopole for stable europe obtained from a doleritic dyke system in brittany france is given at latitude 61 n and longitude 79 e k 36 anda95 10 2 based on seven equivalent sites and more than a hundred samples demagnetized by alternating field and thermal treatments d 41 i 62 k 66 95 7 5 radiometric datings provide ages between 190 and 200 ma this result represents the first indication obtained from european igneous data for a very westerly cusp in the apparent polar wander path around 200 ma the european data also tend to support the hypothesis of a higher latitude track for the jurassic apwp as already proposed from north american data

structure and dynamics of the arizona monsoon boundary abstract the arizona monsoon boundary is defined as the boundary separating two distinctly different air masses over mexico and the adjacent pacific during the summer the structure and dynamics of this boundary are examined by cross sectional analysis using three different data sources 1 a composite cross section through the boundary constructed from the fleet numerical oceanography center fnoc analysis 2 a time height cross section constructed using radiosonde observations at the time the boundary passed through tucson in 1984 and 3 a cross section through the boundary on 22 july 1985 using high resolution fields of temperature moisture and geopotential height obtained from the vissr atmospheric sounder vas all draw cross sections showed similar structure in some respects the arizona monsoon boundary resembles a midlatitude front i e there is a distinct and relatively sharp air mass change forced almost entirely by confluence a direct ageostrophic circulation is produced by this f

holocene vegetation and climate in the puerto blanco mountains southwestern arizona abstract plant macrofossils from 21 pack rat neotoma sp middens at 535 605 m from the puerto blanco mountains southwestern arizona provide and excellent history of vegetation and climate for the last 14 120 yr b p in the sonoran desert a late wisconsin juniper joshua tree woodland gave way to a transitional early holocene desertscrub with sparse juniperus californica california juniper by 10 540 yr b p important sonoran desert plants including carnegiea gigantea saguaro and encelia farinosa brittle bush were dominants riparian trees such as acacia greggii catclaw acacia prosopis velutina velvet mesquite and cerdicium floridum blue palo verde grew on dry south facing slopes in a middle holocene sonoran desertscrub in a warm wet summer climate with frequent winter freezes modern subtropical sonoran desertscrub formed about 4000 yr b p as summer rainfall and winter freezes declined cercidium microphyllum foothills palo verde sapium biloculare mexican jumping bean olneya tesota ironwood and stenocereus thurberi organ pipe cactus became dominant as riparian trees retreated to wash habitats the inferences of a latest wisconsin early holocene summer monsoonal maximum by j e kutzbach 1983 modeling of holocene climates in late quaternary environments of the united states vol 2 the holocene h e wright ed pp 271 277 univ of minnesota press minneapolis are not supported for the southwest apparently the persistence of late wisconsin circulation patterns offset any increases in insolation

a reduced relevance of vegetation change for alluvial aggradation in arid zones a basic tenet of arid region geomorphology is that a decrease in hillslope vegetation density coupled with increases in monsoonal precipitation after a change to a warmer and drier climate is an essential factor for an increase in sediment yield and fan aggradation over the last two decades vegetative cover change is increasingly cited as a prominent factor in triggering the onset of fan aggradation in deserts of southwestern north america especially in analysis of fan deposition during the late pleistocene holocene lph climatic transition to assess this assertion we compiled paleobotanical and alluvial fan stratigraphy histories from 18 published studies broadly representing four different areas in the mojave and northern sonoran deserts instead of focusing on chronology aggregation and comparison with global or regional climate change proxies forced by orbital parameters we discriminated by altitudinal regions looking for linkages between local vegetation change and alluvial deposition results indicate that the onset of extensive alluvial fan deposition 1 began well before changes in catchment vegetative cover and 2 can occur during several possible combinations of vegetation change the ambiguous relation between vegetation change and alluvial fan aggradation indicates that vegetation had a reduced role in lph aggradation other factors such as local storm intensity and water and sediment redistribution pathways on hillslopes need to be considered in this analysis given the important role that the combined hillslope alluvial system has on arid region ecosystem functions

north american jurassic apparent polar wander the anwser from other continents abstract jurassic paleomagnetic data from north america which come essentially from the ne and sw usa outline two contradictory apparent polar wander paths apwps following besse and courtillot j geophys res 96 4029 4050 1991 we derive synthetic apwps for north america based on data from other atlantic bordering continents without resorting to the north american data themselves we discuss variations in the data bases and in the kinematics used for the reconstructions and find that a set of some 12 predicted apwps is well constrained to lie within a band of 5 half width along 75 n latitude the main uncertainties occur along the track i e longitude but not perendicular to it i e latitude we conclude that as has been suggested before most of the jurassic north american data show the effect of either partial remagnetization in a recent field or tectonic rotation and that until more reliable data become available the synthetic transferred paths from other atlantic bordering continents offer more reliable estimates of jurassic north american paleogeography and polar wander

introducing a terrestrial carbon pool in warm desert bedrock mountains southwestern usa growth of the phoenix metropolitan area led to road cut or house platform exposures of the internal bedrock material of surrounding semiarid mountain ranges similar exposures in the surrounding sonoran and mojave deserts reveal the presence of sedimentary calcium carbonate infilling the preexisting fracture matrix of the bedrock field surveys at 31 sites with bedrock fractures filled with carbonate referred to as bffc in the following text reveal an average of 0 079 0 036 mtc m2 stored in the upper 2 m of analyzed bedrock exposures backscattered electron microscopy images indicate the presence of carbonate at the micron scale not included in this estimate of carbon storage pilot radiocarbon and sr isotope analyses suggest that one of the surveyed bffc veins was flushed into the bedrock from a nonbedrock source during the wetter last glacial period in the late pleistocene

reply to comment by robert f butler steven r may and david r bazard on high latitude paleomagnetic poles from middle jurassic plutons and moat volcanics in new england

extinction patterns δ18 o trends and magnetostratigraphy from a southern high latitude cretaceous paleogene section links with deccan volcanism although abundant evidence now exists for a massive bolide impact coincident with the cretaceous paleogene k pg mass extinction event 65 5 ma the relative importance of this impact as an extinction mechanism is still the subject of debate on seymour island antarctic peninsula the lopez de bertodano formation yields one of the most expanded k pg boundary sections known using a new chronology from magnetostratigraphy and isotopic data from carbonate secreting macrofauna we present a high resolution high latitude paleotemperature record spanning this time interval we find two prominent warming events synchronous with the three main phases of deccan traps flood volcanism and the onset of the second is contemporaneous with a local extinction that pre dates the bolide impact what has been termed the k pg extinction is potentially the sum of multiple independent events at least at high latitudes

quantification of soil production and downslope creep rates from cosmogenic 10be accumulations on a hillslope profile average soil transport rates over a period of 3500 yr on a convex soil mantled hillslope have been quantified using a mass balance model that incorporates the soil concentration of the cosmogenic isotope 10 be the 10 be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient the predicted diffusion coefficient is 360 55 cm 3 yr 1 cm 1 contour length and the average rate of soil production is 0 026 0 007 cm yr within the uncertainty of this technique the data do not reject g k gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate however the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil production rate law

cretaceous extinctions multiple causes figure 1 deccan plateau basalts lava from deccan volcanism formed distinct layering credit gsfc nasa in the review the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary p schulte et al 5 march p 1214 2 the terminal cretaceous

channel initiation and the problem of landscape scale since the 1940s it has been proposed that landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape this threshold is equal to the hillslope length that is just shorter than that necessary to support a channel head a field study supports this hypothesis by showing that an empirically defined topographic threshold associated with channel head locations also defines the border between essentially smooth undissected hillslopes and the valley bottoms to which they drain this finding contradicts assertions that landscapes are scale independent and suggests that landscape response to changes in climate or land use depends on the corresponding changes in the threshold of channelization

the apparent polar wander path of the european plate in upper triassic lower jurassic times and the liassic intraplate fracturing of pangaea new palaeomagnetic constraints from nw france and sw germany summary in order to constrain the european plate motions during triassic lower jurassic times new palaeomagnetic data were acquired from various triassic liassic sediments of northeastern france and southwestern germany a carnian palaeopole 49 n 131 e k 193 a95 7 n 4 was obtained for keuper marls with consistent directions of normal and reversed polarities rhaetian sandstones also with both polarities and carnian dolomites with a magnetization of probable secondary origin provide a mean pole 51 n 112 e k 109 a95 6 n 6 more westerly than that given above the lower hettangian sampled in limestones of the xeuilley quarry near nancy shows a n r n polarity sequence with well defined anti parallel directions the mean hettangian pole is 55 n 100 e k 123 a95 11 n 3 induan sandstones from the northern vosges show two distinct overprints one consistent with the ladinian carnian magnetizations already observed in the same region the second consistent with the recalculated mean lower jurassic pole of europe five other sites with different rock types also exhibit late overprints with lower jurassic directions the mean pole calculated from these overprints is 74 n 124 e k 125 a95 6 n 6 the revised apparent polar wander path apwp presented here shows that for the mid permian to the earliest jurassic the palaeopoles of europe stayed around 50 latitude while the longitudes drifted from 185 e to 100 e the previously established upper triassic lower jurassic loop of the apwp is shifted westwards by about 25 the revised apwp involves the following plate motions after the late alleghanian tectonic phase which marked the end of the variscan orogeny central western europe rotated counterclockwise during about 70 myr up to the earliest jurassic the south north drift in latitude by 35 nearly 4000 km remained regular from 320 to 205 ma with a constant rate of 3 4cm yr 1 around 205 ma this motion ceased probably because of a collision somewhere in the north of eurasia then in the lower liassic the plate underwent a rapid counterclockwise rotation with a eulerian pole located in northeastern asia synchronous with fracturing in the atlantic and the neotethys areas it is proposed that the intraplate fracturing of pangaea was initiated by the rapid change of the megacontinent motion at the end of the trias

paleomagnetic study of the messejana plasencia dyke portugal and spain a lower jurassic paleopole for the iberian plate abstract the only iberian lower jurassic paleomagnetic pole come from the central atlantic magmatic province related messejana plasencia dyke but the age and origin of its remanence have been a matter of discussion with the aim of solving this uncertainty and to go further into a better understanding of its emplacement and other possible tectonic features a systematic paleomagnetic investigation of 40 sites 625 specimens distributed all along the 530 km of the messejana plasencia dyke has been carried out rock magnetic experiments indicate psd low ti titanomagnetite and magnetite as the minerals carrying the nrm the samples were mostly thermally demagnetized most sites exhibit a characteristic remanent component of normal polarity with the exception of two sites where samples with reversed polarities have been observed the paleomagnetic pole derived from a total of 35 valid sites is representative of the whole structure of the dyke and statistically well defined with values of pla 70 4 n plo 237 6 e k 47 9 and a 95 3 5 paleomagnetic data indicates that i there is no evidence of a cretaceous remagnetization in the dyke as it was suggested ii most of the dyke had a brief emplacement time furthermore two dyke intrusion events separated in time from it by at least 10 000 y have been detected iii the high grouping of the vgps directions suggests no important tectonic perturbations of the whole structure of the dyke since its intrusion time iv the pole derived from this study is a good quality lower jurassic paleopole for the iberian plate and v the messejana plasencia dyke paleopole for the iberian plate is also in agreement with quality selected european and north american lower jurassic paleopoles and the magnetic anomalies data sets that are available for rotate them to iberia

approaches to dynamic equilibrium in theoretical simulations of slope development the model slop3d is used to identify characteristic aspects of slope development under different conditions a series of slopes along the kall valley in the northern eifel is reproduced by a developmental sequence of model slopes the spatial and temporal variations of the components of slope development systems relief slope form weathering rates denudation rates are discussed by means of mass movement and wash denudation models the negative feedbacks between system components create a tendency towards the establishment of a dynamic equilibrium which is reached however only if the endogenic and exogenic conditions remain constant for the required relaxation time the latter is shown to be a function of slope length of the rate of uplift of the denudational process type and of the intensity of exogenic inputs e g precipitation varying rock resistance leads to the development of shield inselbergs only if the rate of denudation becomes higher than the maximum possible weathering rate of the resistant bedrock

landform characterization using geophysics recent advances applications and emerging tools abstract this paper presents an overview of the strengths and limitations of existing and emerging geophysical tools for landform studies the objectives are to discuss recent technical developments and to provide a review of relevant recent literature with a focus on propagating field methods with terrestrial applications for various methods in this category including ground penetrating radar gpr electrical resistivity er seismics and electromagnetic em induction the technical backgrounds are introduced followed by section on novel developments relevant to landform characterization for several decades gpr has been popular for characterization of the shallow subsurface and in particular sedimentary systems novel developments in gpr include the use of multi offset systems to improve signal to noise ratios and data collection efficiency amongst others and the increased use of 3d data multi electrode er systems have become popular in recent years as they allow for relatively fast and detailed mapping novel developments include time lapse monitoring of dynamic processes as well as the use of capacitively coupled systems for fast non invasive surveys em induction methods are especially popular for fast mapping of spatial variation but can also be used to obtain information on the vertical variation in subsurface electrical conductivity in recent years several examples of the use of plane wave em for characterization of landforms have been published seismic methods for landform characterization include seismic reflection and refraction techniques and the use of surface waves a recent development is the use of passive sensing approaches the use of multiple geophysical methods which can benefit from the sensitivity to different subsurface parameters is becoming more common strategies for coupled and joint inversion of complementary datasets will once more widely available benefit the geophysical study of landforms three cases studies are presented on the use of electrical and gpr methods for characterization of landforms in the range of meters to 100 s of meters in dimension in a study of polygonal patterned ground in the saginaw lowlands michigan usa electrical resistivity tomography was used to characterize differences in subsurface texture and water content associated with polygon swale topography also a sand filled thermokarst feature was identified using electrical resistivity data the second example is on the use of constant spread traversing cst for characterization of large scale glaciotectonic deformation in the ludington ridge michigan multiple cst surveys parallel to an 60 m high cliff where broad 100 m synclines and narrow clay rich anticlines are visible illustrated that at least one of the narrow structures extended inland a third case study discusses internal structures of an eolian dune on a coastal spit in new zealand both 35 and 200 mhz gpr data which clearly identified a paleosol and internal sedimentary structures of the dune were used to improve understanding of the development of the dune which may shed light on paleo wind directions

the role of true polar wander on the jurassic palaeoclimate from the late carboniferous until the middle jurassic continents were assembled in a quasi rigid supercontinent called pangea the first palaeomagnetic data of south america indicated that the continent remained stationary in similar present day latitudes during most of the mesozoic and even the palaeozoic however new palaeomagnetic data suggest that such a scenario is not likely at least for the jurassic in order to test the stationary versus the dynamic continent model we studied the jurassic apparent polar wander paths of the major continents that is eurasia africa and north america that all in all show the same shape and chronology of the tracks with respect to those from south america we thus present a master path that could be useful for the jurassic pangea one of the most remarkable features observed in the path is the change in pole positions at 197 ma early jurassic which denotes the cessation of the counter clockwise rotation of pangea and commencement of a clockwise rotation that brought about changes in palaeolatitude and orientation until the end of the early jurassic 185 ma here we analyse a number of phenomena that could have triggered the polar shift between 197 and 185 ma and conclude that true polar wander is the most likely in order to do this we used morgan s tectonophysics 94 123 139 1983 grid of hotspots and performed absolute palaeogeographical reconstructions of pangea for the late triassic and jurassic the palaeolatitudes changes that we observe from our palaeomagnetic data are very well sustained by diverse palaeoclimatic proxies derived from geological and palaeoecological data at this time of both the southern and northern hemispheres

40ar 39ar dating and geochemistry of tholeiitic magmatism related to the early opening of the central atlantic rift tholeiitic effusive and intrusive magmatism from iberia morocco algeria and mali related to the early opening of the central atlantic rift was dated by the 40ar 39ar step heating method four plateau ages ranging from 203 7 2 7 to 197 1 1 8 ma were obtained on plagioclase from dykes from the taoudenni area mali and two lava flows from morocco the messejana dyke iberia which previously yielded discrepant conventional kar dates did not furnish any 40ar 39ar plateau dates however there is a clear inverse relationship between apparent age and k ca ratio for gas fractions from a plagioclase separate proportional to the alteration degree which combined with dates obtained on amphibole biotite and pyroxene allows us to determine an age of around 200 ma for this body these data and those obtained on the foum zguid morocco and the ksi ksou algeria dykes give evidence of a brief magmatic event between 206 and 195 ma ago which affected a large area ca 2500 km long trace element modelling shows that most of these formations originated from a homogeneous enriched source material such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces e g the deccan traps and the red sea rift precisely dated by the 40ar 39ar method as well

using in situ produced cosmogenic isotopes to estimate rates of landscape evolution a review from the geomorphic perspective the application of in situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade at least 57 papers and numerous abstracts have been published since the mid 1980s when the first mass spectrometric measurements of terrestrially produced cosmogenic isotopes were made taken at face value these studies provide quantitative information about rates of landscape evolution and landform age however the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data the validity of assumptions inherent to these models and the geologic surroundings in which the samples were collected this paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted at present the establishment of high precision cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces the erosion of rock from sampled boulders the potential for isotope inheritance from previous exposure and the uncertainty of isotopic measurements uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques estimation of erosion rates at single points on the landscape gives useful small scale information extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites however because erosion rates are so poorly constrained at present even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

paleomagnetic evolution of the armorican massif in late paleozoic times in the light of overprints recorded in cadomian and paleozoic units abstract previous investigations have shown that in paleozoic outcrops of the variscides the major part of the paleomagnetic record consists of overprints after identification and dating of these overprints the magnetic record can be used to constrain the geotectonic evolution of the investigated area after the major tectonic event in order to test this hypothesis a paleomagnetic investigation was carried out on various volcanic and plutonic units from northern brittany 48 5 n 3 w emplaced and metamorphosed in the late proterozoic during the cadomian orogeny to calibrate the overprints in terms of their ages doleritic dykes intruded into the cadomian basement in the early paleozoic as well as early carboniferous intrusive and extrusive rocks were also sampled thermal demagnetization often shows multivectorial behaviour of most nrms and indicates a wide range of magnetic components characterized by various unblocking temperatures a large majority of the components isolated in cadomian units display directions that are consistent whith the results from the younger paleozoic units with exception of the d components mean vgp 7 s 152 e which are interpreted as being ordovician in age all other mean directions poles agree with results obtained from visean volcanic plutonic and metamorphic rocks of the central part of the variscan belt this means that no cadomian magnetization has survived the tectonic magmatic and metamorphic phases that have affected the cadomian basement during the paleozoic the major part of the magnetic overprints were acquired during the variscan orogeny particularly in relation to visean and westphalian magmatism the early carboniferous 340 328 ma cn 7 s 53 e co 29 s 60 e and cp 51 s 62 e components the middle carboniferous 328 310 ma b components 11 n 124 e the late carboniferous 310 300 ma a1 30 n 151 e the late permian a 51 n 154 e and the jurassic j1 overprints 72 n 107 e delineate an apparent polar wander path that can be interpreted in terms of geotectonic evolution since the late visean this evolution was the same as for the massif central the vosges the black forest and the odenwald spessart the variable inclinations of the c components suggest a northwestward tilting of the area north of saint brieuc and a possible latitudinal drift of the whole massif in late visean times the c b and b a1 segments of the apwp reveal two successive clockwise rotations by 70 80 in the latest visean and by 30 in the late namurian early westphalian the first rotation closed the rhenohercynian basin during the last rotation the variscan belt was tied to northern europe the a and j1 overprints give evidence of hydrothermal activity in northern brittany in the late permian and the early jurassic

the last pluvial climatic episodes in the deserts of southwestern north america long term variations in the effective moisture of deserts are linked both to glacial interglacial climatic cycles and through the milankovitch theory to orbitally induced changes in solar radiation intensity1 4 for southwestern north america comparison of palaeobotanical data with model predictions of palaeoclimate reveals two distinct pluvial regimes during the late wisconsin and early holocene 23 8 kyr bp southward displacement of the aleutian low pressure centre and strengthened westerlies from 40 to 30 n were important features of the full glacial 18 kyr bp pluvial maximum increased cyclonic storms and weakened monsoons led to a winter precipitation regime a feature seen both in the fossil record5 6 and climate models7 8 dominantly zonal flow during this period contrasts with the latest wisconsin and early holocene 12 8 kyr bp when meridional circulation transported maritime tropical air into the desert interior we now show that plant macrofossil assemblages from packrat neotoma spp middens9 in the mojave desert provide evidence for increased temperatures and summer rainfall during this period10 12 the anomalously late persistence to 8 kyr bp of woodland in low latitude low elevation deserts of the south west13 is attributed to enhanced monsoonal circulation3

a middle carnian to early norian 225 ma paleopole from sediments of the newark basin pennsylvania paleomagnetic study of the middle carnian stockton formation upper carnian lockatong formation and the lower part of the norian passaic formation from 24 sites along 3 traverses perpendicular to strike in the newark basin of eastern pennsylvania shows that two hematitic magnetizations can be isolated from these rocks thermal demagnetization experiments reveal a distributed unblocking temperature typically 300 to 680 c magnetization with uniformly downward and northerly directions and a high unblocking temperature 660 c and above magnetization with shallow northerly normal polarity and southerly reversed polarity directions on the basis of a correlatable magnetic polarity stratigraphy between 3 traverses spanning 40 km along strike and within site directional scatter similar to that expected from paleosecular variation the high temperature magnetization was most probably acquired at or near the time of deposition the pole calculated from the high unblocking temperature magnetization 53 6 n 101 6 e a95 4 8 is consistent with other late triassic poles and indicates a paleolatitude of 3 8 n 3 0 the lower unblocking temperature magnetization is an overprint acquired at about the same time as the jurassic newark trend igneous n2 pole magnetization

geochemical evolution of a 10 m thick intrusive body the south brenterc h diabase dyke western armorican massif france the south brenterc h 9 5 m thick diabase dyke belongs to the intermediate ti quartz normative tholeiitic magmatism which has intruded the western end of britanny france near the trias lias boundary it has been previously identified as a simple dyke resulting from a single magmatic injection the present study is based on ten samples collected along a half transverse section of this dyke geochemical analyses and some textural parameters have been used to point out two flow related differentiation processes partially masked by alteration these late magmatic mechanisms have induced subtle geochemical variations among samples which sometime counteract each other the processes pointed out here are i expulsion of phenocrysts from the borders during flow differentiation likely by the way of an hydrodynamic grain dispersive pressure and ii emplacement of a more evolved magma in the central part of the intrusion probably due to a viscosity segregation mechanism during the flow

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

comment on high latitude paleomagnetic poles from middle jurassic plutons and moat volcanics in new england and the controversy regarding jurassic apparent polar wander for north america by mickey c van fossen and dennis v kent

magmatisme fissural triasico liasique dans l ouest du massif armoricain france petrologie geochimie âge et modalites de la mise en place the dolerite dykes at the western end of britanny france which is located on the north atlantic margin were emplaced in a nw se field of faults at the trias lias time limit between 210 and 195 ma these rocks are quartz tholeiites enriched in tio2 with a trace element composition close to that of the deccan basalts the initial concentrations of ba k and rb though slightly modified by the alteration and of nb seem to have been affected by crustal contamination during the magma transfer these dykes are comparable in age and composition to the tholeiitic dykes exposed in the iberian peninsula in the moroccan atlas and in the northern appalachians they however have a different orientation furthermore these various domains differ in their postintrusion history the rifting leading to the opening of the central atlantic occurred soon after the intrusions of the dykes whereas the opening of the north atlantic began several tens of millions of years later following the direction of the armoric

low intensity of the geomagnetic field in early jurassic time from a large collection of jurassic continental tholeiites cropping out in europe and africa we selected 90 samples for paleointensity determinations the samples were carefully selected to avoid any secondary magnetizations especially viscous magnetization use of the thellier method reveals that magnetic property changes due to heating begin often at quite low temperatures but fortunately without modifying noticeably their natural remanent magnetization thermoremanent magnetization ratio twenty eight well clustered paleointensity estimates were obtained from two european dikes that were emplaced during early jurassic time the kerforne dike at brenterc h in brittany northwestern france and the messejana dike on the iberian peninsula spain and portugal virtual dipole moments calculated from both magmatic units are similar and only about one third of present day values these new data lend support to the recently postulated low dipole moment of the mesozoic geomagnetic field

climatic influences on holocene variations in soil erosion rates on a small hill in the mojave desert a detailed study of a small hill in ne mojave desert in eastern california was conducted to elucidate the effect of climate on the variations in soil erosion rates through holocene field surveys and sampling were carried out to obtain information on topography geomorphology soil and vegetation conditions seismic refraction sediment deposition and hillslope processes integration of this information allowed reconstruction of the hill topography at the end of the pleistocene deduction of the evolution of the hill from the end of the pleistocene to the present and estimation of total soil losses resulting from various hillslope processes the estimates are consistent with the premise that early holocene climate change resulted in vegetation change soil destabilization and topographic roughening current very slow hillslope transport rates e g 5 mm ky 1 by rodent burrowing a presently important transport form appear inconsistent with the inferred total soil loss rate 31 mm ky 1 packrat midden studies imply that the ne mojave desert experienced enhanced monsoonal precipitation in the early holocene presumably accentuating soil loss water erosion on one slope of the hill was simulated using water erosion prediction project wepp a process based erosion model using 4 and 6 ky of precipitation input compatible with an appropriate monsoonal climate and the present climate respectively the wepp predicted soil losses for the chosen slope were compatible with inferred soil losses identification of two time periods within the holocene with distinct erosion characteristics may provide new insight into the current state of mojave desert landform evolution

paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we first re examined the apparent polar wander curves for stable eurasia and africa since the triassic these curves were then combined together with curves of north and south america according to the kinematics of the atlantic ocean and a synthetic polar wander curve was given then most of the paleomagnetic results from the tethys mobile belt from the atlantic to the pamirs were analysed several groups of plates microplates and blocks can be seen first relatively stable regions like maghreb and sicily which have not moved much then we have a group formed by iberia sardinia italy and to a lesser extent corsica and the western and central alps for these blocks movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of africa but they are sometimes stronger to the east a major change takes place the north of the aegean sea and the ionian zone are clockwise rotated and these rotations are recent oligocene miocene for the first part pliocene to the present for the second part a major problem arises in turkey caucasus and iran paleomagnetic results indicate a position far to the south of eurasia and at the same time geological evidence is in favour of a position close to eurasia we discuss these discrepancies

paleomagnetism and potassium argon age of the messejana dike portugal and spain angular limitation to the rotation of the iberian peninsula since the middle jurassic abstract a paleomagnetic and potassium argon dating investigation has been carried out on a 530 km long dike system which transects the western iberian peninsula in a northeasterly direction the k ar age determinations were made on mineral separates exclusively they range between 160 and 200 ma and the authors suppose that this reflects the actual time interval of the intrusion in accord with previous results the paleomagnetic pole derived from 12 sites regularly distributed along the dike 71 n 236 e coincides well with other mesozoic paleomagnetic poles from the western africa a contemporaneous pole from stable europe is tentatively deduced from african and north american late triassic early jurassic poles using different reconstruction models around the north atlantic ocean the divergence between this pole and the iberian pole corresponds to the result obtained for permian poles

modelling net erosion responses to enviroclimatic changes recorded upon multisecular timescales land use change has been recognized throughout the earth as one of the most important factors influencing the occurrence of rainfall driven geomorphological processes however relating the occurrence of historical soil erosion rates is difficult because of the lack of long term research projects in river basins also complex models are not adequate to reconstruct erosion rate changes because they require significant input data not always available on long timescales given the problems with assessing sediment yield using complex erosion models the objective of this study is to explore a parsimonious scale adapted erosion model adt from the original thornes and douglas algorithms which aims at reconstruction of annual net erosion ane upon multisecular timescales as a test site the calore river basin 3015 km 2 in southern italy provides a peculiar and unique opportunity for modelling erosion responses to climate and land cover changes where input data generation and interpretation results were also supported by documented hydrogeomorphological events that occurred before and after land deforestation in this way aneadt values were reconstructed for the period 1675 2004 by using precipitation indexes complemented by recent instrumental records and by using land cover statistics from documented agrarian sources pulses of natural sedimentation in the predeforestation period have been related to vesuvius volcanic activity and changes in rainstorm frequency after deforestation the basin system became unstable with sudden fluctuations in the hydrogeomorphological regime contributing significantly to increased erosion and in turn sediment transport sequences via river drainage towards the tyrrhenian coast

iridium and dinocysts at the cretaceous tertiary boundary on seymour island antarctica implications for the k t event a single iridium anomaly 40 times background concentration correlated with the cretaceous tertiary k t boundary is present within an interval of unconsolidated glauconitic silty sands in a marine clastic sequence on seymour island antarctic peninsula dinocysts provide the basis for biostratigraphic identification of a 20 30 cm k t transitional interval calcareous microfossils have not been preserved in the glauconitic beds dinocysts also record an earliest danian transgressive event 10 cm above the transition interval the ir spike is near the base of the dinocyst transition interval and 30 cm below the flooding horizon the seymour island site located at a high latitude 63 s at the end of the cretaceous provides no compelling evidence for mass extinction at the k t boundary

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

relating soil erosion and sediment yield to geomorphic features and erosion processes at the catchment scale in the spanish pre pyrenees erosion and sediment redistribution are important processes in landscape changes in the short and long term in this study the rmmf model of soil erosion and the sedd model of sediment delivery were used to estimate annual soil loss and sediment yield in an ungauged catchment of the spanish pre pyrenees and results were interpreted in the context of the geomorphic features the estana catchment is divided into 15 endorheic sub catchments and there are 17 dolines gullies and slopes were the main erosive geomorphic elements whereas the colluvial alluvial valley floor and doline deposits were depositional elements spatially distributed maps of gross soil erosion sediment delivery ratio sdr and sediment yield sy were generated in a gis severe erosion rates 100 mg ha 1 year 1 were found in gullies whereas mean and maximum erosion rates were very high on slopes developed on keuper facies and high in soils on muschelkalk facies where crops are grown the depositional type geoforms were predicted by the models to have an erosive dynamic those results were consistent with the rates of erosion quantified by 137cs which reflects the significant role of human activities in triggering soil erosion catchment area was positively correlated with erosion rate but negatively correlated with sdr and sy the latter were negatively correlated with the proportion of the surface catchment covered with forests and scrublands the topography of the area influenced the high sdr and sy in the dolines and valley floors near the sinks intra basin stored sediment was 59 2 of the total annual eroded soil in the catchment the combination of the rmmf and sedd models was an appropriate means of assessing the effects of land uses on soil erosion and obtaining a better understanding of the processes that underlie the geomorphic changes occurring in mountainous environments of the mediterranean region

subannual stable isotope records reveal climate warming and seasonal anoxia associated with two extinction intervals across the cretaceous paleogene boundary on seymour island antarctica

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

the triassic of the thakkhola nepal ii paleolatitudes and comparison with other eastern tethyan margins of gondwana during the triassic the thakkhola region of the nepal himalaya was part of the broad continental shelf of gondwana facing a wide eastern tethys ocean this margin was continuous from arabia to northwest australia and spanned tropical and temperate latitudes a compilation of permian triassic and early jurassic paleomagnetic data from the reconstructed gondwana blocks indicates that the margin was progressively shifting northward into more tropical latitudes the thakkhola region was approximately 55 s during late permian 40 s during early triassic 30 s during middle triassic and 25 s during late triassic this paleolatitude change produced a general increase in the relative importance of carbonate deposition through the triassic on the himalaya and australian margins regional tectonics were important in governing local subsidence rates and influx of terrigenous clastics to these gondwana margins but eustatic sea level changes provide a regional and global correlation of major marine transgressions prograding margin deposits and shallowing upward successions a general mega cycle characterizes the triassic beginning with a major transgression at the base of the triassic followed by a general shallowing upward of facies during middle and late triassic and climaxing with a regression in the latest triassic