tectonic control on 10 be derived erosion rates in the garhwal himalaya india erosion in the himalaya is responsible for one of the greatest mass redistributions on earth and has fueled models of feedback loops between climate and tectonics although the general trends of erosion across the himalaya are reasonably well known the relative importance of factors controlling erosion is less well constrained here we present 25 10 be derived catchment averaged erosion rates from the yamuna catchment in the garhwal himalaya northern india tributary erosion rates range between 0 1 and 0 5 mm yr 1 in the lesser himalaya and 1 and 2 mm yr 1 in the high himalaya despite uniform hillslope angles the erosion rate data correlate with catchment averaged values of 5 km radius relief channel steepness indices and specific stream power but to varying degrees of nonlinearity similar nonlinear relationships and coefficients of determination suggest that topographic steepness is the major control on the spatial variability of erosion and that twofold to threefold differences in annual runoff are of minor importance in this area instead the spatial distribution of erosion in the study area is consistent with a tectonic model in which the rock uplift pattern is largely controlled by the shortening rate and the geometry of the main himalayan thrust fault mht our data support a shallow dip of the mht underneath the lesser himalaya followed by a midcrustal ramp underneath the high himalaya as indicated by geophysical data finally analysis of sample results from larger main stem rivers indicates significant variability of 10 be derived erosion rates possibly related to nonproportional sediment supply from different tributaries and incomplete mixing in main stem channels

a high resolution paleointensity stack of the past 14 to 68 ka from black sea sediments abstract detailed paleo and mineral magnetic analyses of a sediment composite record from the southeastern black sea yielded a high resolution well dated paleointensity record though hampered by some larger hiatuses in some cores and contaminated by diagenetically formed greigite the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period relative variations of paleointensity inferred from the sediments magnetisations were converted into a record of the virtual axial dipole moment vadm lowest vadm values are linked with the laschamp 0 50 10 22 a m 2 at 41 0 ka the norwegian greenland sea 1 5 10 22 a m 2 at 64 5 ka and the mono lake 3 0 10 22 a m 2 at 34 5 ka geomagnetic excursions the fully reversed field during the laschamp excursion exhibits a vadm of 2 0 10 22 a m 2 which is more than 25 of the present day axial dipole moment 7 628 10 22 a m 2 rates of change calculated from the black sea vadm record also give some information on how to assess the global decay of the present day geomagnetic field which is significantly enhanced in the area of the south atlantic anomaly comparison with provided c 14 and 10 be records confirm partly in the very detail the non linear anti correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the earth s upper atmosphere however discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge

modeling mountain building and the seismic cycle in the himalaya of nepal a host of information is now available regarding the geological and thermal structure as well as deformation rate across the himalaya of central nepal these data are reconciled in a two dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes over geological timescale 5 ma the 20 mm yr estimated shortening rate across the range is accommodated by localized thrust faulting along the main himalayan thrust fault mht the mht reaches the surface along the foothills where it is called the main frontal thrust fault mft the mht flattens beneath the lesser himalaya and forms a midcrustal ramp at the front of the higher himalaya consistent with the river incision and the anticlinal structure of the lesser himalaya farther northward the mht roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust increasing the coulomb stress beneath the front of the higher himalaya where most of the microseismic activity clusters negligible deformation of the hanging wall requires a low apparent friction coefficient less than 0 3 on the flat portion of the mht on the ramp might be as high as 0 6 sensitivity tests show that a rather compliant quartz rich rheology and a high radioactive heat production in the upper crust of 2 5 w m 3 is required erosion affects the thermal structure and interplays with crustal deformation a dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure topography and deformation field using a linear diffusion model of erosion we constrain the value of the mass diffusivity coefficient to 0 5 1 6 l0 4 m 2 yr this study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period

atmospheric radiocarbon during the younger dryas production ventilation or both a new reconstruction of past atmospheric v 14 c v 14 catm based on polish lake varved sediments has suggested that previous v 14 catm values e g from the cariaco basin record for the beginning of the younger dryas cold event yd are overestimates and that the v 14 catm rise at the yd onset could only be due to changes in atmospheric 14 c production pc 14 this result would have profound climatic implications for the yd is a paradigm example of abrupt climate change which is usually thought to have been triggered by a reduction in the northward heat flux by the atlantic thermohaline circulation here we examine results from a large number of simulations 300 based on a zonally averaged ocean circulation model to constrain the effect on v 14 catm of pc 14 changes during the yd as reconstructed from a greenland ice core record of 10 be flux our results suggest that the scatter in the lake data set is too large to exclude the probable change in deep ocean ventilation at the onset of the yd by contrast the model fit to the higher v 14 catm levels throughout the yd detected in the marine record is generally better when a substantial decrease in deep ocean ventilation is simulated the early v 14 catm drawdown that initiated during the first half of the yd however could entirely be due to production changes if this was the case the drawdown would not reflect an increasing formation of north atlantic intermediate water or southern ocean water as previously suggested the rapid v 14 catm rise at the yd onset documented in the marine record however remains unexplained fl 2001 elsevier science b v all rights reserved

geomagnetic and mineral magnetic characterization of the anthropocene geomagnetic and mineral magnetic data provide geological indices that are both independent of human impact i e geomagnetic and respond to human induced environmental impact i e mineral magnetic we provide the first discussion of such magnetic events for help in defining the anthropocene within the holocene a potential geomagnetic marker for the anthropocene is the low dipole latitude at c 2700 cal a bp which is associated with distinct palaeosecular variation features in northerly mid to high latitude sites mineral magnetic records from lake and marine sediments identify major deforestation and soil delivery events from catchment systems in many parts of the world during the last 4000 years in europe clusters of these events occur around both 2600 cal a bp and ad 1100 the former coinciding with a low in geomagnetic field dipole latitude and peak intensity mineral magnetic records in peats and lake sediments can reflect particulate pollution from fossil fuel burning the expansion of major coal burning began c ad 1800 in western europe and eastern north america but around ad 1900 this expanded due to more widely distributed coal use and this event is the most clear mineral magnetic marker for the base of the anthropocene

mountain building in taiwan a thermokinematic model the taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening however a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge we propose here a new thermokinematic model of the taiwan mountain belt reconciling existing kinematic thermometric and thermochronological constraints in this model shortening across the orogen is absorbed by slip on the most frontal faults of the foothills crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge tananao complex tc where the uplift rate is estimated to 6 3 mm a 1 and beneath the westernmost internal region of the orogen hsueshan range units hr where the uplift rate is estimated to 4 2 mm a 1 our model suggests that the tc units experienced a synchronous evolution along strike despite the southward propagation of the collision it also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures exhumation of the hr units increases northward but has not yet reached an exhumational steady state presently frontal accretion accounts for less than 10 of the incoming flux of material into the orogen although there is indication that it was contributing substantially more 80 before 4 ma the incoming flux of material accreted beneath the tc significantly increased 1 5 ma ago our results also suggest that the flux of material accreted to the orogen corresponds to the top 7 km of the upper crust of the underthrust chinese margin this indicates that a significant amount 76 of the underthrust material has been subducted into the mantle probably because of the increase in density associated with metamorphism we also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field by combining available geological data on the thermal and kinematic evolution of the wedge our study sheds new light onto mountain building processes in taiwan and allows for reappraising the initial structural architecture of the passive margin

loess stratigraphy using palaeomagnetism application to the poiana ciresului archaeological site romania abstract palaeomagnetic investigations were carried out at the archaeological site of poiana ciresului romania to check whether the loess records a reliable palaeomagnetic signal and to compare this signal to reference data the sediments are composed of loess and loess like sediments pedogenic alterations and a pronounced cultural layer occur in the section the archaeology bearing sedimentary geoarchive spans the time interval from ca 20 to ca 55 ka and was dated mainly by 14c and luminescence techniques the record of geomagnetic palaeosecular variation from this european loess site is discussed the presented palaeomagnetic record bears analogies to data from lac du bouchet france and is a reliable high resolution record mean sedimentation rates are ca 20 cm ky of palaeosecular variation the results imply that european loess geoarchives may record palaeosecular variation features even if loess is pedogenically altered the palaeosecular variation pattern of loess is here employed as a stratigraphic tool

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

magnetostratigraphic results from the eastern arctic ocean ams 14c ages and relative palaeointensity data of the mono lake and laschamp geomagnetic reversal excursions summary a new high resolution magnetostratigraphic record from the eastern arctic ocean has yielded further evidence for the existence of the laschamp excursion 37 35 ka the mono lake excursion 27 25 5 ka and possibly another very short excursion 22 ka inferred from steep negative inclinations ages are based on nine ams accelerator mass spectrometry 14c dates oxygen isotope stratigraphy and correlation with odp site 983 estimates of relative palaeointensity variations for the time interval from 80 to 10 ka have revealed that the documented geomagnetic excursions are linked to large fluctuations of the relative palaeointensity the lowest values were obtained for the two excursions and the normal reversed n r and reversed normal r n transitions of the laschamp polarity excursion which itself is characterized by a slight increase of relative palaeointensity during its reversed state the results are in general agreement with palaeointensity studies from other regions indicating that these fluctuations could be global phenomena and that the geomagnetic field of the brunhes chron was very variable in amplitude as well as in geometry the new result is one of the rare records comprising large directional as well as large relative palaeointensity variations

chlorine 36 fallout in the summit greenland ice core project ice core the natural cosmogenic radioisotope 36cl has been measured in the greenland ice core project ice core from summit greenland generally a good correlation between 36cl and 18o was found even in glacial periods with strong 18o oscillations the 36cl flux is calculated assuming an exponential relationship between accumulation rate and 18o a prominent excursion is detected between 35 and 40 kyr b p which coincides with a similar 10be peak measured in the same samples this sheds light on possible causes of this excursion in the lowest part of the ice core the flux decreases smoothly this can be interpreted as radioactive decay of 36cl t1 2 300 kyr

interseismic strain accumulation on the himalayan crustal ramp nepal the departement of mines and geology has been monitoring the seismicity of the central himalayas of nepal since 1985 intense microseismicity and frequent medium size earthquakes ml 4 tend to cluster beneath the topographic front of the higher himalaya this 10 20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data both microseismic and geodetic data indicate strain accumulation on a mid crustal ramp that had been previously inferred from geological and geophysical evidence this ramp connects a flat decollement under the lesser and sub himalaya with a deeper decollement under the higher himalaya and probably acts as a geometric asperity where strain and stress build up during the interseismic period the large himalayan earthquakes could nucleate there and probably activate the whole flat and ramp system up to the blind thrusts of the sub himalaya

a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10be in globally stacked deep sea sediments the reconstruction of geomagnetic field intensity variations during the last 200 kyr from paleomagnetic data is at present the subject of numerous studies and major debate there is currently no generally accepted record here we present a global stacked record of 230thex normalized 10be deposition in marine sediments representing relative variations in 10be production rate which are translated into field intensity variations the record shows major periods during which the field intensity was between 10 and 40 of the present day value namely 30 42 60 75 85 110 and 180 192 kyr b p our results are compared to independently derived paleomagnetic studies and th u calibrations of 14c dates on corals during most of the observed period the geomagnetic field intensity was weaker than today resulting in an overall 30 reduced value for the last 200 kyr

erosion on a line abstract an erosion model is proposed to calculate erosion rates for plane strain models in which the earth s surface is represented on a line the fundamentals of river erosion networks are captured by two principles hack s law which describes the drainage area structure of river network and a stream power erosion law which describes the rate of incision of a river for a simple morphology of parallel transverse rivers with rectangular drainage basins this allows the earth s surface to be parameterized by two heights the trunk stream channel height and the interfluvial ridge height the resulting expressions are solved for the simple cases of constant uplift rate and a constant mean slope as occurs in critical wedge problems in the latter case the uplift rate is variable and changes in space so that the trunk channel elevation and the interfluvial ridge elevation average to maintain a constant mean slope a general numerical solution is presented for application to any numerical model with arbitrary surface velocity variable rock erodibility and precipitation this algorithm is coupled to a plane strain plastic deformation model to demonstrate the utility of the model

late brunhes polarity excursions mono lake laschamp iceland basin and pringle falls recorded at odp site 919 irminger basin abstract component natural remanent magnetizations derived from u channel and 1 cm3 discrete samples from odp site 919 irminger basin indicate the existence of four intervals of negative inclinations in the upper brunhes chronozone according to the age model based on planktic oxygen isotope data these excursional intervals occur in sediments deposited during the following time intervals 32 34 ka 39 41 ka 180 188 ka and 205 225 ka these time intervals correspond to polarity excursions detected elsewhere known as mono lake laschamp iceland basin and pringle falls the isotope based age model is supported by the normalized remanence paleointensity record that can be correlated to other calibrated paleointensity records for the 0 500 ka interval such as that from odp site 983 for the intervals associated with the mono lake and laschamp excursions virtual geomagnetic poles vgps reach equatorial latitudes and mid southerly latitudes respectively for intervals associated with the iceland basin and pringle falls excursions repeated excursions of vgps to high southerly latitudes indicate rapid directional swings rather than a single short lived polarity reversal the directional instability associated with polarity excursions is not often recorded probably due to smoothing of the sedimentary record by the process of detrital remanence drm acquisition

regionally consistent western north america paleomagnetic directions from 15 to 35 ka assessing chronology and uncertainty with paleosecular variation psv stratigraphy abstract new paleomagnetic secular variation psv data from fish lake utah usa along with previously published regional records allow us to build an independently dated western north america psv stack wnam17 from about 35 to 15 ka that quantifies dating and paleomagnetic uncertainties in february 2014 we recovered a composite 11 m long sediment record from fish lake utah that retrieved glacially influenced and post glacial sediments computed tomography ct scans and magnetic susceptibility were used to build a precise depth scale while radiocarbon dating of terrestrial macrofossils and tephrostratigraphy provide tight age control glacially influenced sediments have well defined characteristic remanent magnetizations with all maximum angular deviation values

chlorine 36 evidence for the mono lake event in the summit grip ice core abstract a distinct peak has been discovered in the 36 cl data from the grip ice core between the dansgaard oeschger d o events 6 and 7 at approximately 32 kyr bp this peak can be attributed to a minimum of the geomagnetic dipole field associated with the mono lake event since the 36 cl peak reflects a higher production rate of all cosmogenic radionuclides it has an impact on the 14 c dating of the last ice age furthermore it provides an additional time marker similar to a peak found earlier corresponding to the laschamp event at approximately 39 kyr bp

beryllium 10 in the greenland ice core project ice core at summit greenland concentrations of the cosmogenic isotope 10be have been measured in more than 1350 samples from the greenland ice core project grip ice core drilled at summit greenland although a dust associated component of 10be retained by 0 45 m filters in some of the samples complicates the interpretations the results confirm that the first order origin of 10be concentration variations is changes in precipitation rate associated with different climate regimes this effect is seen not only between glacial and interglacial periods but also during the shorter dansgaard oeschger interstadials by contrast the 10be data do not support the interpretation of rapidly varying accumulation i e climate during the last interglacial they can however be used to help place limits on the origin of the ice in these events after taking into account variable snow accumulation effects variations in the 10be flux are observed probably caused by solar and geomagnetic modulation but possibly also by primary cosmic ray variations the most dramatic is a 10be peak 40 000 years ago similar to that found in the vostok ice core thus permitting a very precise correlation between climate records from arctic and antarctic ice cores the 36cl 10be ratio considering either total or only ice associated 10be shows significant variability over the whole core depth thus confirming the difficulty in using this parameter for dating ice cores

chlorine 36 dating of deep groundwater from northern sahara summary the continental intercalaire from sahara is one of the largest confined aquifers in the world in the northern part of the basin the depth of the aquifer ranges between 400 and 1800 m and the main flow direction is from west to east chlorine 36 analyses were performed on groundwater samples mainly collected along this flowpath they show a wide range of 36 cl contents from 8 to 99 10 15 at at 1 expressed as 36 cl cl atomic ratio in order to evaluate the epigene production measurements were also performed on chloride extracted by leaching from a soil profile near the recharge zone this showed that the contribution of epigene production remains limited a range of 36 cl cl ratios from 116 to 133 10 15 was adopted for groundwater recharging the aquifer including both meteoric production and epigene production for most of the samples collected along the main flowpath a significant decay effect was observed an attempt of dating was made by using an equation that takes into account radioactive decay of the meteoric epigene input deep production and chloride dissolution within the aquifer the calculated 36 cl residence time varies from 16 to 500 ka for the minimum ages and from 25 to 1200 ka for the maximum ages an increase of residence time is observed up to about 500 km from the recharge zone this corresponds to mean flow velocities of 0 2 0 5 m a 1 and to an average permeability of 1 1 0 6 10 5 m s 1 at the scale of several hundreds of kilometres

are chinese loess deposits essentially continuous 1 we conducted a paleomagnetic examination of the last glacial loess of three representative profiles along an east west transect in the central loess plateau in order to assess the continuity of chinese loess the results show that the xifeng and luochuan profiles record the laschamp geomagnetic excursion but with different morphologies together with the published results from weinan southern plateau our results suggest that sedimentation of the last glacial loess in the central southern plateau was continuous at the time scale equivalent to the duration of the laschamp excursion 2 kyr but probably episodic at finer time scales 2 kyr no geomagnetic excursion was found at the yichuan profile near the yellow river valley where loess accumulation may be strongly affected by local environmental changes and thus may have been discontinuous both site location and time scale therefore need to be considered when considering continuity of chinese loess

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

earth s magnetic field is probably not reversing the geomagnetic field has been decaying at a rate of 5 per century from at least 1840 with indirect observations suggesting a decay since 1600 or even earlier this has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion we have derived a model of the geomagnetic field spanning 30 50 ka constructed to study the behavior of the two most recent excursions the laschamp and mono lake centered at 41 and 34 ka respectively here we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field at earlier times centered at 49 and 46 ka the field is comparable to today s field with an intensity structure similar to today s south atlantic anomaly saa however neither of these saa like fields develop into an excursion or reversal this suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal the saa like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production despite the global dipole field not weakening significantly in our model during this time this agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

erosional control on the dynamics of low convergence rate continental plateau margins s u m m a r y over the last decade thermomechanical models have revealed the control of both tectonics and erosion on the morphology of continental plateaus margins however unravelling the specific effects of these two coupled processes has been difficult in practice here to assess the control of erosion we investigate the dynamics of the eastern and the northern borders of the tibetan plateau which are characterized by a low convergence rate and a steep topographic escarpment adjacent to the sichuan and tarim basins respectively thermomechanical modelling of continental lithosphere coupled with fluvial denudation reveals that important crustal deformation with large scale horizontal displacements can occur without any convergence as a response to mass transfer due to gravitational collapse and to erosional unloading these processes are sensitive to crustal structure geothermal gradient as well as surface erosion at a timescale of several million years our results suggest that this denudation triggered deformation exerts a primary control on the evolution of those plateau margins by counterbalancing the mass removal due to erosion and stabilizing the topographic escarpment this finding supports a possible explanation for the morphology of the longmen shan eastern tibet in which the paradoxical combination of persistent high topographic gradients close to the foreland and low convergence rates can be related to the influence of erosion on deformation patterns

understanding erosion rates in the himalayan orogen a case study from the arun valley understanding the rates and pattern of erosion is a key aspect of deciphering the impacts of climate and tectonics on landscape evolution denudation rates derived from terrestrial cosmogenic nuclides tcns are commonly used to quantify erosion and bridge tectonic myr and climatic up to several kiloyears time scales however how the processes of erosion in active orogens are ultimately reflected in 10be tcn samples remains a topic of discussion we investigate this problem in the arun valley of eastern nepal with 34 new 10be derived catchment mean denudation rates the arun valley is characterized by steep north south gradients in topography and climate locally denudation rates increase northward from 0 2 mm yr 1 to 1 5 mm yr 1 in tributary samples while main stem samples appear to increase downstream from 0 2 mm yr 1 at the border with tibet to 0 91 mm yr 1 in the foreland denudation rates most strongly correlate with normalized channel steepness r2 0 67 which has been commonly interpreted to indicate tectonic activity significant downstream decrease of 10be concentration in the main stem arun suggests that upstream sediment grains are fining to the point that they are operationally excluded from the processed sample this results in 10be concentrations and denudation rates that do not uniformly represent the upstream catchment area we observe strong impacts on 10be concentrations from local nonfluvial geomorphic processes such as glaciation and landsliding coinciding with areas of peak rainfall rates pointing toward climatic modulation of predominantly tectonically driven denudation rates

three dimensional numerical simulations of crustal systems undergoing orogeny and subjected to surface processes as several modeling studies indicate the structural expression and dynamic behavior of orogenic mountain belts are dictated not only by their rheological properties or by far field tectonic motion but also by the efficiency of erosion and sedimentation acting on its surface until recently numerical investigations have been mainly limited to 2 d studies because of the high computational cost required by 3 d models here we have efficiently coupled the landscape evolution model cascade with the 3 d thermomechanically coupled tectonics code fantom details of the coupling algorithms between both codes are given we present results of numerical experiments designed to study the response of viscous plastic crustal materials subjected to convergence and to surface processes including both erosion and sedimentation in particular we focus on the equilibration of both the tectonic structures and on the surface morphology of the orogen we show that increasing the efficiency of fluvial erosion increases the frontal thrust angle which in turn decreases the width of the orogen in addition the maximum summit elevation of the orogen during transient evolution is significantly higher in those models showcasing surface processes than those that do not this illustrates the strong coupling between tectonics and surface processes we also demonstrate that an along strike gradient of erosion efficiency can have a major impact upon the landscape morphology and the tectonic structure and deformation of the orogen in both the across strike and along strike directions overall our results suggest that surface processes by enhancing localization of deformation can act as a positive forcing to topographic building

rapid climatic variations during marine isotopic stage 3 magnetic analysis of sediments from nordic seas and north atlantic abstract the bulk magnetic parameters of seven deep sea cores distributed from the nordic seas 67 n to the north atlantic as far south as the bermuda rise 33 n exhibit short term variations which correlate with rapid climatic changes during marine isotopic stage 3 mis3 the magnetic mineralogy is uniformly dominated by well sorted low ti content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals because the magnetic minerals predominantly originate from one common source area the nordic basaltic province these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition these results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep sea circulation the latter was active reduced during the interstadials stadials and heinrich events transporting the magnetic particles from the norwegian sea into the north atlantic ocean along a path similar to the present path of the nadw it is tentatively suggested that the faeroe shetland channel and the denmark strait were the only two active paths for the overflow water during mis3 the presence of magnetic oscillations in the bermuda rise core in phase with those from the north atlantic indicates that the activity of the southern newfoundland basin gyre was linked to that of the nadw during mis3

ar ar ages from transitionally magnetized lavas on la palma canary islands and the geomagnetic instability timescale 1 a detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of barranco de los tilos on the island of la palma canary islands reveals a complex temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events the matuyama brunhes m b reversal is recorded in five transitionally magnetized lava flows from the north tn section the isochrons obtained from three of the lower four m b lavas are defined by 14 incremental heating experiments that together with a previous age determination yielded a weighted mean of 798 4 6 2 ka all uncertainties 2s in addition a 780 3 10 3 ka isochron was determined for the overlying transitionally magnetized flow indicating that it was erupted during a distinctly younger portion of the transition near the base of the south ts section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole vgp positions in the southwest indian ocean between latitudes 56 s and 65 s suggesting instability of the geomagnetic field beyond that of typical secular variation 40 ar 39 ar isochrons from three of these flows defined by 11 separate incremental heating experiments gave a weighted mean of 822 2 8 7 ka this anomalous field behavior recorded 24 11 kyr prior to the m b reversal may coincide with an event featured in several marine sediment records directly above two normal polarity flows 40 ar 39 ar isochrons of 751 9 18 1 ka and 675 0 15 7 ka are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude vgps spanning 23 60 n these flows are then capped by a single flow possessing normal polarity based on 12 incremental heating experiments 40 ar 39 ar isochrons of five of these nine lavas along with the uppermost flow gave a weighted mean age of 580 2 7 8 ka for this period of transitional to normal field behavior from these same transitional lavas quidelleur et al 1999 reported three unspiked k ar ages with a weighted mean of 602 24 ka and proposed a new event called the la palma excursion however the 40 ar 39 ar age presented here is three times more precise than the k ar age and is indistinguishable at the 95 confidence level from the 40 ar 39 ar age of a lava from the snake river plain idaho that originally defined the big lost event transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the big lost event recorded at la palma was indeed global in extent rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process index terms 1560 geomagnetism and paleomagnetism time variations secular and long term 1520 geomagnetism and paleomagnetism magnetostratigraphy 1035 geochemistry geochronology 1513 geomagnetism and paleomagnetism geomagnetic excursions keywords argonargon dating lavas excursion paleomagnetism reversal

competing effects of mountain uplift and landslide erosion over earthquake cycles large earthquakes can construct mountainous topography by inducing rock uplift but also erode mountains by causing landslides observations following the 2008 wenchuan earthquake show that landslide volumes in some cases match seismically induced uplift raising questions about how the actions of individual earthquakes accumulate to build topography here we model the two dimensional surface displacement field generated over a full earthquake cycle accounting for coseismic deformation postseismic relaxation landslide erosion and erosion induced isostatic compensation we explore the related volume balance across different seismotectonic and topographic conditions and revisit the wenchuan case in this context the ratio between landslide erosion and uplift is most sensitive to parameters determining landslide volumes particularly earthquake magnitude m w seismic energy source depth and failure susceptibility as well as the seismological factor responsible for triggering landslides and is moderately sensitive to the effective elastic thickness of lithosphere t e for a specified magnitude more erosive events higher tend to occur at shallower depth in thicker t e lithosphere and in steeper more landslide prone landscapes for given landscape and seismotectonic conditions the volumes of both landslides and uplift to first order positively scale with mw and seismic moment m o however higher m w earthquakes generate lower landslide and uplift volumes per unit m o suggesting lower efficiency in the use of seismic energy to drive topographic change with our model we calculate the long term average seismic volume balance for the eastern tibetan region and find that the net topographic effect of earthquakes in this region tends to be constructive rather than erosive overall destructive events are rare when considering processes over the full earthquake cycle although they are more likely if only considering the coseismic volume budget as was the case for the 2008 wenchuan earthquake where landsliding substantially offset coseismic uplift irrespective of the net budget our results suggest that the erosive power of earthquakes plays an important role in mountain belt evolution including by influencing structures and spatial patterns of deformation for example affecting the wavelength of topography

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

dominance of tectonics over climate in himalayan denudation landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography no consensus currently exists concerning their respective infl uences to address this question denudation rates at centennial to millennial time scales were deduced from 10be concentrations in detrital sediments derived from 30 small basins 10 600 km2 in an 200 km wide region in central nepal along a northward strike perpendicular transect average denudation rates sharply increase from 0 5 mm yr in the lesser himalayas to 1 mm yr when crossing the physiographic transition and then accelerate to 2 3 mm yr on the southern flank of the high peaks in the greater himalayas despite a more than five fold increase in denudation rate between the southern and northern parts of this transect the corresponding areas display similar precipitation rates the primary parameter that presents a signifi cant co variation with denudation is the long term rockuplift rate that is interpreted to result from the ramp fl at transition along the main himalayan thrust we propose that in this rapidly uplifting mountain range landscapes adjust quickly to changing climatic conditions such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes in this particular context variations in precipitation appear to have only a second order role in modulating the denudation signal that is primarily set by the background rock uplift rate 2014 geological society of america

an ice core perspective on the age of the matuyama brunhes boundary abstract two intervals of enhanced 10be flux thought to be associated with periods of low dipole intensity and identified as the matuyama brunhes transition and a precursor event have been observed in the bottom section of the epica dome c ice core the peaks span 764 776 ka and 788 798 ka on the new edc3 chronology with a stated absolute age uncertainty of 6 ka 2 this chronology uses orbital tuning of atmospheric oxygen 18 18oatm to correct for anomalies in ice flow in the bottom 500 m of the core an additional 28 18oatm data points have been measured to improve resolution and verify the accuracy of the tuning and the stated timescale uncertainty both the dating of the increased 10be and that relative to climatic records are compared to paleointensity records found in orbitally tuned marine sediments the mid point of the 10be peak associated with the m b is approximately 10 ka younger than the age determined radioisotopically from lavas with transitional orientations taking into account recent revisions to the 40ar 39ar dating standard and improved precision climatic constraints on the edc3 agescale make an error of this magnitude in the ice chronology implausible this age difference however is consistent with recent modeling suggesting that directional changes are spatially asynchronous and may precede the dipole intensity minimum in some locations although formally less precise than the published age from astrochronologically dated marine sediments ice core ages are potentially more accurate because they are not subject to lock in depth uncertainties

changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10be and 14c records the variations in atmospheric radiocarbon c 14 concentration during the last 50 000 years can be attributed to changes in the c 14 production rate due to changes in solar activity the geomagnetic field and or interstellar galactic cosmic ray flux and to changes in the global carbon cycle the relative contributions of these processes is the subject of current debate although the discrepancies between the various reconstructions of the past atmospheric radiocarbon concentration increase with age the relatively good agreement over the last 25 000 years allows a quantitative discussion of the causes of the observed c 14 variations for this period using be 10 measurements from greenland summit ice cores we show that in addition to solar and geomagnetically induced production rate changes significant changes in the carbon cycle have to be considered to explain the measured c 14 concentrations there is evidence that these changes are connected to 1 global deglaciation and 2 climate changes in the north atlantic region on centennial to millennial time scales related to changes in the ocean circulation differences between be 10 and geomagnetic field records however suggest that uncertainties of about 20 still exist in determinations of past changes in the c 14 production rate c 2004 elsevier b v all rights reserved less

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

spatial distribution of denudation in eastern tibet and regressive erosion of plateau margins article i nfo the longmen shan range is one of the major topographic and structural features of the eastern margin of the tibetan plateau with an impressive topographic gradient and low convergence rates across the range this region has raised important questions concerning the dynamics of plateau margin settings such as the long term mechanisms of topographic evolution the investigation of the distribution in space and time of denudation can provide critical insight into such dynamics and shed light on still unresolved controversies for that purpose we present a new dataset that documents the intensity and distribution of denudation processes across this plateau margin through field survey of fluvial incision markers quantitative geomorphology and cosmogenic nuclide derived basin wide erosion rates erosion is b0 5 mm year in the frontal region of the longmen shan and between 0 5 and 1 mm year further west with a gradual decrease when reaching the northern headwaters of the min jiang watershed adjacent to the beginning of the tibetan plateau the spatial distribution of denudation inferred from the various methods we use suggests that most of the differential uplift in the central longmen shan is accommodated by the beichuan fault and frontal structures located in the foothills the denudation pattern seems also to reflect the large scale propagation of erosion from the sichuan basin toward the plateau this suggests that the longmen shan range is submitted to the combined influences of slow thrusting activity on the frontal structures and progressive westward regressive erosion as a probable response to a pulse of uplift of the tibetan plateau eastern margin that

decoupling of erosion and precipitation in the himalayas the hypothesis that abrupt spatial gradients in erosion can cause high strain rates in active orogens has been supported by numerical models that couple erosional processes with lithospheric deformation via gravitational feedbacks1 2 3 most such models invoke a stream power rule in which either increased discharge or steeper channel slopes cause higher erosion rates spatial variations in precipitation and slopes are therefore predicted to correlate with gradients in both erosion rates and crustal strain here we combine observations from a meteorological network across the greater himalaya nepal along with estimates of erosion rates at geologic timescales greater than 100 000 yr from low temperature thermochronometry across a zone of about 20 km length spanning the himalayan crest and encompassing a more than fivefold difference in monsoon precipitation significant spatial variations in geologic erosion rates are not detectable decreased rainfall is not balanced by steeper channels instead additional factors that influence river incision rates such as channel width and sediment concentrations must compensate for decreasing precipitation overall spatially constant erosion is a response to uniform upward tectonic transport of greater himalayan rock above a crustal ramp

dynamics of the earth magnetic field in the 10 75 kyr period comprising the laschamp and mono lake excursions new results from the french chaine des puys in a global perspective abstract we report here on a new paleomagnetic directions and intensities and coupled k ar and 40 ar 39 ar analysis of 35 different flows emplaced in the chaine des puys during the 75 to 10 kyr interval which contains the mono lake and laschamp excursions there is a remarkable agreement between the new set of absolute volcanic intensities and published sedimentary glopis 75 and cosmogenic 10 be and 36 cl records the laschamp and mono lake excursions are clearly revealed by a very significant intensity drop at 41 2 1 6 ka and 34 2 1 2 ka respectively the duration of the laschamp excursion is 1500 yr and about 640 yr when the drop of paleointensity or the directional change are considered respectively the intensity drop at the mono lake is twice as short in the 7 ka interval separating the two excursions the field intensity recovers to almost non transitional values the rate of decrease of the field intensity during these excursions attains 18 nt yr for the laschamp and even greater value 33 nt yr for the mono lake this figure is for the laschamp excursion similar to the present field intensity decrease in the last two centuries so that one may wonder whether such a high rate of change may be characteristic of an impending geomagnetic event reversal or excursion we suggest that the name auckland excursion should be used for the present day called mono lake

solar activity during the last 1000 yr inferred from radionuclide records identification of the causes of past climate change requires detailed knowledge of one of the most important natural factors solar forcing prior to the period of direct solar observations radionuclide abundances in natural archives provide the best known proxies for changes in solar activity here we present two independent reconstructions of changes in solar activity during the last 1000 yr which are inferred from be 10 and c 14 records we analyse the tree ring c 14 data shcal intca104 from 1000 to 1510 ad and annual data from 1511 to 1950 ad and four be 10 records from greenland ice cores camp century grip milcent and dye3 together with two be 10 records from antarctic ice cores dome concordia and south pole in general the be 10 and c 14 records exhibit good agreement that allows us to obtain reliable estimates of past solar magnetic modulation of the radionuclide production rates differences between be 10 records from antarctica and greenland indicate that climatic changes have influenced the deposition of be 10 during some periods of the last 1000 yr the radionuclide based reconstructions of past changes in solar activity do not always agree with the sunspot record which indicates that the coupling between those proxies is not as close as has been sometimes assumed the tree ring c 14 record and be 10 from antarctica indicate that recent solar activity is high but not exceptional with respect to the last 1000 yr c 2006 elsevier ltd all rights reserved less

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

controls on the patterns of topography and erosion rate in a critical orogen 1 we solve for the steady state patterns of erosion rate and topography in a critical wedge to understand the mutual adjustment of tectonics erosion and topography in the absence and presence of spatial variations in precipitation rate we consider steady state systems in which tectonics favors a critical topographic form assumed to be the mean elevation across the range and in which surface erosion by rivers and hillslopes operates on a two dimensional landscape we find that 1 a nonuniform critical topographic form implies a nonuniform pattern of ridge valley relief and hence a nonuniform pattern of erosion rate and 2 when the system is forced by local variations in precipitation rate maintenance of the critical topographic form requires a local response of rock uplift that greatly dampens changes in topography we apply these concepts to the western side of the olympic mountains of washington state where mean elevation ridge valley relief and precipitation rates increase from the coast to the topographic crest of the range we find that the main control on the erosion rate pattern is the pattern in mean elevation and the amount of precipitation in contrast the pattern of precipitation is only a minor control as a whole our work demonstrates an approach for developing the theoretical context that is necessary for interpreting spatial associations between patterns in topography precipitation and erosion in natural orogens

numerical modelling of erosion processes in the himalayas of nepal effects of spatial variations of rock strength and precipitation the interplay between tectonics and erosion has a predominant control on the evolution of the morphology of mountain belts here we investigate the modalities of deformation in central nepal on a c 100 ka time scale in response to tectonic and external forcings through the use of a finite element thermomechanical model coupled with an integrative denudation formulation that accounts for fluvial incision and hillslope landsliding we study the complex coupling existing between tectonics and erosion with special emphasis on the influences of rock strength and rainfall distributions our results underline the key role played by lithologic variations in the elevation of both rivers and mean topography we show that the location of the main frontal thrust is mainly controlled by the low erodability of the unconsolidated sandstone in the siwaliks hills as previously suspected burbank et al 2003 our simulations demonstrate that the pattern of uplift in nepal is mainly dependent on both erodability and fault geometry rather than on rainfall distribution

saw tooth pattern of north atlantic current speed during dansgaard oeschger cycles revealed by the magnetic grain size of reykjanes ridge sediments at 59 n detailed mineral rock magnetic measurements including high resolution magnetic hysteresis loops were carried out on three sediment cores recovered from the reykjanes ridge in the north atlantic supported by physical grain size analyses titanomagnetite grain size variations form a proxy of the speed of near bottom currents and reveal a cyclic saw tooth pattern between dansgaard oeschger cycles 8 and 5 the magnetic grain size data suggest that these dansgaard oeschger cycles were characterized by gradual intensification of the near bottom current speed while other paleoceanographic proxies indicate escalating iceberg discharge and declining sea surface temperatures these observations are contrary to the frequently assumed and strictly applied positive relationship between high latitude warmth and north atlantic deep water formation within the confines of temporal control however it would appear that peaks in iceberg discharge and slow near bottom current speeds were synchronous the magnetic hysteresis data also demonstrate that magnetic concentration parameters such as magnetic susceptibility do not necessarily provide direct evidence of paleocirculation less

age of the mono lake excursion and associated tephra abstract the mono lake excursion mle is an important time marker that has been found in lake and marine sediments across much of the northern hemisphere dating of this event at its type locality the mono basin of california has yielded controversial results with the most recent effort concluding that the mle may actually be the laschamp excursion earth planet sci lett 197 2002 151 we show that a volcanic tephra ash 15 that occurs near the midpoint of the mle has a date not corrected for reservoir effect of 28 620 300 14c yr bp 32 400 gisp2 yr bp in the pyramid lake basin of nevada given the location of ash 15 and the duration of the mle in the mono basin the event occurred between 31 500 and 33 300 gisp2 yr bp an age range consistent with the position and age of the uppermost of two paleointensity minima in the napis 75 stack that has been associated with the mle philos trans r soc london ser a 358 2000 1009 the lower paleointensity minimum in the napis 75 stack is considered to be the laschamp excursion philos trans r soc london ser a 358 2000 1009

geomagnetic paleointensity and environmental record from labrador sea core md95 2024 global marine sediment and ice core chronostratigraphy for the last 110 kyr piston core md95 2024 from the labrador rise provides a continuous record of rapidly deposited detrital layers denoting laurentide ice sheet lis instability the core also provides a high resolution record of geomagnetic paleointensity that is consistent with but at higher temporal resolution than previous labrador sea records correlation to the greenland summit ice cores grip gisp2 is achieved by assuming that labrador sea detrital layers correspond to cold stadials in the ice cores this allows a gisp2 official chronology to be placed on md95 2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes 10 be and 36 cl in greenland ice cores synchronous millennial scale variability observed from the md95 2024 paleointensity

sediment flux from a mountain belt derived by landslide mapping in humid uplands landsliding is the dominant mass wasting process in the western southern alps of new zealand landslides are scale invariant and have a power law magnitude frequency distribution independent studies from other regions suggest that this is a general property of landsliding this observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution it is particularly useful when estimating regional geomorphic rates because it constrains the frequency and overall significance of extreme events which cannot otherwise be evaluated by integrating the complete response of the system we estimate the regional denudation rate due to landsliding to be 9 4 mm yr 1 sediment discharge from the western southern alps is dominated by landslide derived material

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

quantifying sand provenance and erosion marsyandi river nepal himalaya we use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the marsyandi basin of the central himalaya a privileged natural laboratory in which a series of multidisciplinary geomorphological sedimentological geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic climatic and sedimentary processes in high relief orogenic belts although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the marsyandi river uncertainties are caused by potential compositional variation between the monsoon and post monsoon season estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin ourresults support focusederosion of the southern tectonically lower part of the greater himalayain the hangingwall of the mct zone where thesummer monsoon reachesitspeak intensity up to 5 m a andsediment yieldsanderosion ratesreach14 100 3400t km 2 and 5 1 1 2 mm a erosion rates sharply decrease southward in low relief lesser himalayan units 1 6 0 6 mm a and also progressivelydecreasenorthwardsinthehigh altitude tectonically upperpartofthegreaterhimalaya whererainfalldecreasesrapidly to b2 m a even areas of extreme topography such as the manaslu granite are characterized by relatively low erosion rates 2 4 0 9 mm a because precipitations become too scarce to feed significant ice flux and glacial activity monsoonal rainfall decreases further tob0 5 m a in the tethys himalayan zone farther north where erosion rates are 1 mm a coupling between erosion and peak monsoonal rainfall along the southern front of the greater himalaya is consistent with both channel flow models of tectonic extrusion

evidence for large century time scale changes in solar activity in the past 32 kyr based on in situ cosmogenic 14c in ice at summit greenland abstract we present results of estimates of cosmic ray flux in greenland at the summit 3200 m a s l 72 6 n 38 5 w during the past 32 kyr we derive these estimates based on concentrations of in situ cosmogenic 14 c produced in ice crystals based on the secular equilibrium concentration of in situ produced 14 c in quartz in terrestrial rocks we find that on century time scales the cosmic ray production rate of 14 c at the summit was close to its estimated long term average production rate except during 3 periods i during 8500 9500 yr b p and 27 000 32 000 yr b p when the production rate was higher by about a factor of 2 and ii during 12 000 16 000 yr b p when the production rate was lower by a factor of 1 5 the observed variation in cosmic ray flux at the polar site is best attributed to changes in solar activity resulting in variable modulation of terrestrial cosmic ray flux changes in the geomagnetic field in the past do not affect the cosmic ray flux at polar latitudes likewise climate changes do not affect the in situ 14 c record in ice during the first two epochs the solar activity must have been very low as during maunder minimum virtually no sunspots resulting in essentially no modulation of the cosmic ray flux by the solar plasma during the low cosmic ray flux epoch 12 000 16 000 yr b p the observed decrease in cosmic ray flux corresponds to high solar activity as seen in 1958 sun spot number 190 we discuss the proxy evidence from tree ring and sediment based records of atmospheric 14 c 12 c ratios during the three epochs these records have been used as a measure of changes in cosmic ray flux and solar activity in the past however since they are also appreciably affected by climatic changes a comparison of the two records is potentially valuable for delineating the nature of past changes in solar activity and large scale ocean circulation and air sea exchange

high resolution paleomagnetic secular variations and relative paleointensity since the late pleistocene in southern south america abstract paleomagnetic inclination declination and relative paleointensity were reconstructed from the sediments of laguna potrok aike in the framework of the international continental scientific drilling program icdp potrok aike maar lake sediment archive drilling project pasado here we present the u channel based full vector paleomagnetic field reconstruction since 51 2 ka cal bp the relative paleointensity proxy rpi was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the chrm interval nrm arm 10 40 mt a grain size influence on the rpi was removed using a correction based on the linear relationship between the rpi and the median destructive field of the natural remanent magnetisation mdf nrm the new record is compared with other lacustrine and marine records and stacks from the mid to high latitudes of the southern hemisphere revealing consistent millennial scale variability the identification of the laschamp and possibly the mono lake geomagnetic excursions and a direction swing possibly associated to the hilina pali excursion at 20 ka cal bp nonetheless a global scale comparison with other high resolution records located on the opposite side of the earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern south america at 46 ka cal bp

absolute palaeointensity study of the mono lake excursion recorded by new zealand basalts abstract one of the geomagnetic excursions recorded in basalts of the auckland volcanic field new zealand has recently been correlated unequivocally with the mono lake excursion making it the only confirmed record from the southern hemisphere this record is also exceptional in occurring in five separate monogenetic basaltic volcanoes absolute palaeointensity determinations using the microwave technique based on a comprehensive suite of samples of the auckland basalts recording the excursion show that the geomagnetic field in new zealand at the time of the mono lake excursion was reduced to about 14 t i e to 30 of its normal value this result confirms previous estimates based on more limited sampling in addition it provides a comparison of the microwave and ldt dht shaw methods of measuring palaeointensity which give results that are statistically indistinguishable the palaeointensities determined from the five different volcanoes are also indistinguishable though the palaeodirection data suggest that a very small segment of the vgp path may have been recorded this study confirms the reliability of palaeointensity and palaeodirection determinations from these particular new zealand basalts which together with their definitive 40ar 39ar ages establishes this record of the mono lake excursion as one of the best documented consequently there is significant potential in searching for records of this excursion elsewhere in the pacific region for use as a stratigraphic marker in studies of recent volcanism and palaeoclimate reconstructions

oxygen 18 16 variability in greenland snow and ice with 10 3 to 105 year time resolution the 3 km long greenland ice sheet project 2 gisp2 ice core presents a 100 000 year detailed oxygen isotope profile covering almost a full glacial interglacial cycle measuranents of isotopic fluctuations in snow frost and atmospheric water vapor samples collected during summer field seasons up to 20 0 are compatible with the large and abrupt 80 160 changes observed in accumulated tim snow pit 15 80 profiles from the gisp2 summit area however show rapid smoothing of the 180 160 signal near the surface beyond about 2 m depth the smoothedi5180 signal is fairly well preserved and can be interpreted in terms of average local weather conditions and climate the longer climate fluctuations also have regional and often global significance in the older part of the record corresponding to marine isotope stages mis 5a to 5d the effect of orbital climate forcing via the 19 and 23 kyr precession cycles and the 41 kyr obliquity cycle is obvious from the end ofmis 5a at about 75 000 years b p till the end of the glacial at the younger dryas preboreal transition at 11 650 years b p the o180 160 record shows frequent rapid switches between intermediate interstadial and low stadial values fourier spectra of the oscillations that are superimposed on the orbitally induced changes contain a strong periodicity at 1 5 kyr a broad peak at 4 0 kyr and additional shorter periods detailed comparison of the gisp2 180 160 record with the vostok antarctica 15d record pacific ocean foraminiferal 180 160 grande pile france tree pollen and insolation indicates that a counterpart to many of the rapid 180 160 fluctuations of gisp2 can be found in the other records and that the gisp2 isotopic changes clearly are the local expression of climate changes of worldwide extent correlation of events on the independent gisp2 and specmap time scales for the interval 10 000 50 000 years b p shows excellent chronometric agreement except possibly for the event labeled 3 1 the glacial to interglacial transition evidently started simultaneously in the arctic and the antarctic but its development and its expression in greenland isotopes was later suppressed by the influence of meltwater especially from the barents sea ice sheet on deep water formation and ocean circulation meltwaters from different ice sheets bordering the north atlantic also influenced ocean circulation during the bolling allerod interstadial complex and the younger dryas and led to a distinct development of european climate and greenland 180 160 values the holocene interval with long term stable mean isotopic values contains several fluauations with periods from years to millennia dominant is a 6 3 year oscillation with amplitude up to 3 to 4 0 periodicities of 11 and 210 years also found in the solar modulated records of the cosmogenic isotopes 1obe and 14c suggest solar processes as the cause of these cycles depression of180 160 values cooling by volcanic eruptions is observed in stacked gisp215 80 records but the effect is small and not likely to trigger major climate changes

geomagnetic modulation of the 36cl flux in the grip ice core greenland geomagnetic field strength is expected to affect the production rate of cosmogenic isotopes such as beryllium 10 carbon 14 or chlorine 36 chlorine 36 data from the greenland ice core project grip ice core agree well with a production rate calculation based on a paleomagnetic reconstruction for the past 100 000 years over both long and short term variations a chlorine 36 peak at 38 000 years ago previously found in the beryllium 10 record from the vostok ice core can be explained by a period of low geomagnetic field intensity

the laschamp geomagnetic dipole low expressed as a cosmogenic 10 be atmospheric overproduction at 41 ka authigenic 10be 9be ratiomeasurementswere performed at high resolution along a portuguese margin deep sea core 37 48 n 10 09w spanning the 20 50 ka time interval in order to reconstruct variations in atmospheric cosmogenic 10be production rates and derive the related geomagnetic dipole moment modulation a complementary approach consisting in 230thxs normalized 10be deposition rate determination on selected samples confirmed the reliability of the authigenic 10be 9be record this study constitutes the first successful comparison of the two widely used normalization techniques of 10be concentrations for both methods the presented results herein evidence a factor of 2 cosmogenic nuclide overproduction linked to a minimum dipole moment associated with the laschamp excursion the latter is stratigraphically constrained beneath heinrich event 4 its age is estimated at 41 ka on the basis of direct correlation between the series of rapid paleoclimatic events recorded in the portuguese margin sediments and in the greenland ice sheet and is confirmed by calibrated radiocarbon dating carried out on the same sediments the remarkable agreement between the authigenic 10be 9be and the greenland ice cores 10be deposition rate records attests to their global significance this new authigenic 10be 9be record has been combined with that previously obtained at the same site to produce a stacked record that is calibrated using absolute values of virtual dipole moment determined on lava flows this provides a reconstruction of dipole geomagnetic moment variations over the 20 50 ka interval independent frompaleomagnetically constrainedmethods that documents the laschamp dipole low but fails to express any dipole low related to t e mono lake excursion this high resolution record responds to the necessity of supplementing the knowledge of the atmospheric 14c variations in the 30 45 ka interval during which the 14c calibration curve suffers from a lack of accurate data and during which a discrepancy of about 5500 yr between the 14c and u th ages is due to the laschamp geomagnetic dipole low such new high resolution datasets from records obtained from different latitudes will be required to make significant advances in understanding the causes of atmospheric 14c variations

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

southern ocean control on the extent of denitrification in the southeast pacific over the last 70 ka abstract temporal changes in oceanic denitrification the bacterial reduction of nitrate under suboxic conditions highlight the potential importance of n inventory changes and the production of n 2 o on the climate system at the same time the cause of the globally observed variation in denitrification remains unclear high resolution benthic foraminiferal oxygen isotope and bulk sediment nitrogen isotope records from odp site 1234 on the chile margin record integrated denitrification changes within the peru chile upwelling system over the last 70 ka denitrification changes in the southeast pacific are coherent with antarctic climate changes recorded by the byrd ice core 18 o record and lead northern hemisphere climate events the southern hemisphere character of the chile margin 15 n record suggests that episodes of reduced denitrification in the se pacific represent times when more oxygen was supplied as the result of changes in the ventilation and preformed nutrient content of subantarctic mode water samw which forms in the subantarctic zone of the southern ocean and feeds into the low latitude thermocline

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

mechanical analysis of controls on strain partitioning in the himalayas of central nepal we present a mechanical analysis of the problem of slip partitioning between the major thrust systems in a collisional range we focus on two structures in the himalayas of central nepal the main himalayan thrust mht and the main central thrust mct we use finite element modeling to test the influence of various parameters such as friction coefficients and surface processes and we investigate how they affect the distribution of deformation between these two faults we observe that reproduction of the late quaternary kinematic pattern across the range with our model requires strict conditions on the friction coefficients such that the mht is very weak whereas the mct is significantly stronger the most important parameter that controls slip partitioning appears to be the dip angle of the mct with a gentler or steeper mct promoting or inhibiting slip respectively we also show that transient loading and unloading through focused glacial erosion in the higher part of the range can unclamp the mct and allow a significant increase in slip rates the results of this mechanical sensitivity investigation have important implications for the dynamics of the himalayan wedge and point toward along strike structural variations as a first order control on slip partitioning

paleoclimatic context of geomagnetic dipole lows and excursions in the brunhes clue for an orbital influence on the geodynamo the hypothesis of an influence of the astronomical precession on the geodynamo energy budget was recently reappraised by theoreticians paleomagnetic indications of such an influence remain controversial because reconstructions of paleointensity variations from sediments are suspected to be contaminated by lithological paleoclimatically induced influences three sets of complementary indicators are however available 1 records of the direction of magnetization in sediments 2 records of magnetic anomalies of the deep sea floor basalts and 3 records of production variations of cosmogenic isotopes from sediment and ice cores these records confirm the genuine geomagnetic origin of paleointensity lows and their narrow link with excursions or short reversals recorded in various materials and often dated by radiometric methods the analysis of these time series and their comparison with 18o records of the paleoclimate suggest that such globally recorded geomagnetic dipole lows have preferentially occurred in the context of interglacial or transitional paleoclimates at the time of low or decreasing obliquity the dominant periods extracted with the complex continuous wavelet transform technique range from 40 to 125 ka further suggesting a link with orbital parameters these results encourage future efforts of research to improve the precision the resolution and the dating of the time series of geomagnetic dipole low in order to better decipher orbital signatures and understand their origin an important implication of this topic is that the next geomagnetic dipole low should be related with the present interglacial

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

tree rings and ice cores reveal c 14 calibration uncertainties during the younger dryas the younger dryas interval during the last glacial termination was an abrupt return to glacial like conditions punctuating the transition to a warmer interglacial climate despite recent advances in the layer counting of ice core records of the termination the timing and length of the younger dryas remain controversial also a steep rise in the concentration of atmospheric radiocarbon at the onset of the interval recorded primarily in the cariaco basin has been difficult to reconcile with simulations of the younger dryas carbon cycle here we discuss a radiocarbon chronology from a tree ring record covering the late glacial period that has not been absolutely dated we correlate the chronology to ice core timescales using the common cosmic production signal in tree ring 14c and ice core 10be concentrations the results of this correlation suggest that the cariaco record may be biased by changes in the concentration of radiocarbon in the upper ocean during the early phase of the younger dryas climate reversal in the cariaco basin this bias in the marine record may also affect the accuracy of a widely used radiocarbon calibration curve over this interval our tree ring based radiocarbon record is easily reconciled with simulated production rates and carbon cycle changes associated with reduced ocean ventilation during the younger dryas attaching a floating tree ring chronology to ice core records that cover the abrupt younger dryas cold interval during the last glacial termination provides a better estimate of the onset and duration of the radiocarbon anomaly the chronology suggests that marine records may be biased by changes in the concentration of radiocarbon in the ocean which may affect the accuracy of a popular radiocarbon calibration program during this interval

seven thousand year duration for a geomagnetic excursion constrained by 230thxs 1 the duration of geomagnetic excursions may provide important information about the geodynamo it has proven difficult however to assess the exact duration of both excursions and reversals we combine high resolution paleomagnetic records from marine sediments with measurements of the 230thxs flux to the seafloor in order to better constrain the duration of the iceland basin excursion ibe we find the duration of the ibe to be 7 8 kyr implying that it is considerably longer than previous estimates for the laschamp excursion 2 kyr and more similar in duration to that of full polarity reversals there may not be therefore a characteristic timescale for transitional field configurations that distinguishes excursions from reversals

variations in the geomagnetic dipole moment during the holocene and the past 50 kyr all absolute paleointensity data published in peer reviewed journals were recently compiled in the geomacia50 database based on the information in geomagia50 we reconstruct variations in the geomagnetic dipole moment over the past 50 kyr with a focus on the holocene period a running window approach is used to determine the axial dipole moment that provides the optimal least squares fit to the paleointensity data whereas associated error estimates are constrained using a bootstrap procedure we subsequently compare the reconstruction from this study with previous reconstructions of the geomagnetic dipole moment including those based on cosmogenic radionuclides be 10 and c 14 this comparison generally lends support to the axial dipole moments obtained in this study our reconstruction shows that the evolution of the dipole moment was highly dynamic and the recently observed rates of change 5 per century do not appear unique we observe no apparent link between the occurrence of archeomagnetic jerks and changes in the geomagnetic dipole moment suggesting that archeomagnetic jerks most likely represent drastic changes in the orientation of the geomagnetic dipole axis or periods characterized by large secular variation of the non dipole field this study also shows that the holocene geomagnetic dipole moment was high compared to that of the preceding similar to 40 kyr and that similar to 4 10 22 am 2 appears to represent a critical threshold below which geomagnetic excursions and reversals occur c 2008 elsevier b v all rights reserved less

gravity anomalies crustal structure and thermo mechanical support of the himalaya of central nepal summary we use two gravity profiles that we measured across central nepal in conjunction with existing data to constrain the mechanical behaviour and the petrological structure of the lithosphere in the himalayan collision zone the data show 1 overcompensation of the foreland and undercompensation of the higher himalaya as expected from the flexural support of the range 2 a steep gravity gradient of the order of 1 3 mgal km x1 beneath the higher himalaya suggesting a locally steeper moho and 3 a 10 km wide hinge in southern tibet we compare these data with a 2 d mechanical model in which the indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland the model assumes brittle coulomb failure and nonlinear ductile flow that depends on local temperature which is computed from a steadystate thermal model the computed moho fits seismological constraints and is consistent with the main trends in the observed bouguer anomaly it predicts an equivalent elastic thickness of 40 50 km in the foreland the flexural rigidity decreases northwards due to thermal and flexural weakening resulting in a steeper moho dip beneath the high range residuals at short wavelengths over distances of 20 30 km are interpreted in terms of 1 sediment compaction in the foreland dr 150 kg m x3 between the lower and middle siwaliks 2 the contact between the tertiary molasse and the meta sediments of the lesser himalaya at the mbt dr 220 kg m x3 and 3 the palung granite intrusion in the lesser himalaya dr 80 kg m x3 finally if petrological transformations expected from the local p t are assumed a gravity signature of the order of 250 mgal is predicted north of the lesser himalaya essentially due to eclogitization of the lower crust which is inconsistent with the gravity data we conclude that eclogitization of the indian crust does not take place as expected from a steady state local equilibrium assumption we show however that eclogitization might actually occur beneath southern tibet where it could explain the hinge observed in the gravity data we suspect that these eclogites are subducted with the indian lithosphere

analytic solution of the mean elevation of a watershed dominated by fluvial incision and hillslope landslides n active mountainous landscapes dominated by detachment limited fluvial incision and hillslope landslides the scaling relations that characterize both river profiles and planimetric organization of the river network can be linked to provide an analytic solution to describe the mean elevation of a watershed this new analytical approach directly links mountain elevation precipitation bedrock erodibility and uplift rate and yields results that are consistent with those obtained through classic 2 d surface process model when applied to a growing linear mountain range the approach provides an approximate but much faster solution for 2 d or 3 d numerical models of crustal deformation that explicitly include surface processes

the role of landslides in mountain range evolution abstract we review the role of landslides in current concepts of the topographic development of mountain ranges we find that many studies in this field address basin or orogen scale competition between rock uplift and fluvial bedrock erosion hillslopes in general and bedrock landslides in particular are often assumed to respond rapidly to incision and development of the fluvial drainage network this leads to a one sided view of the geomorphic coupling between hillslopes and rivers that emphasizes the fluvial control of hillslopes but ignores the alternative view that landslides can affect the fluvial network there is growing evidence that landslides are a dominant source of sediment in mountain belts and that they exert a direct geomorphic control on fluvial processes landslides can influence the river network in a variety of ways from determining basin area and drainage divide positions to setting streamwise variations in sediment load and calibre the geomorphic legacy of large landslides on hillslope and channel morphologies may persist for up to 104 yr adding considerable variability to fluvial erosion and sedimentation patterns over these timescales we identify a number of questions for future research and conclude that a better understanding and quantification of the geomorphic feedbacks between landslides and river channels builds an important link between short term 105 yr landscape evolution models

paleomagnetic correlations between scandinavian ice sheet fluctuations and greenland dansgaard oeschger events 45 000 25 000 yr b p abstract two paleomagnetic excursions the skjong correlated with the laschamp about 41 000 gisp2 yr b p and the valderhaug correlated with the mono lake about 34 000 gisp2 yr b p have been identified in stratigraphic superposition in laminated clay deposited in ice dammed lakes in three large caves in western norway during both periods the margin of the scandinavian ice sheet advanced and reached the continental shelf beyond the outermost coastline the mild 4000 yr long alesund interstade when the coast and probably much of the hinterland were ice free separated the two glacial advances the two paleomagnetic excursions have also been indirectly identified as increased fluxes of 36cl and 10be in the grip ice core greenland this article presents a correlation between ice margin fluctuations of the scandinavian ice sheet and the stratigraphy of grip gisp cores using the paleomagnetic excursions and the 36cl and 10be peaks and thus circumventing the application of different dates or time scales some of the fluctuations of the scandinavian ice sheet were of the allerod younger dryas type in the sense that its margin retreated during mild interstades on greenland and readvanced during cold stades however some fluctuations were apparently not in phase with the greenland climate

geomagnetic field behavior during the iceland basin and laschamp geomagnetic excursions a simple transitional field geometry we present four new records of the iceland basin excursion ibe and five new records of the laschamp excursion le obtained from rapidly deposited marine sediments in the north atlantic ocean the nordic seas the gulf of mexico the south china sea and the southern indian ocean marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records the virtual geomagnetic pole vgp paths of the four ibe records are all similar the vgps move southward over europe and africa reaching the southern hemisphere three reach antarctica and then move to more eastern longitudes before returning northward over australia and east asia describing a large counterclockwise loop the same vgp pattern is observed in other published records the vgp paths observed for the le are similar to those of the ibe however they loop clockwise instead of counterclockwise despite the different sense of looping the marked similarity among the paths for the two excursions suggests that a similar relatively simple geometry dominated the transitional field during both the ibe and the le similar dynamo mechanisms must therefore have been active in the earth s core for both excursions the duration of the excursions is estimated at 2 000 years which supports the suggestion that a difference exists between the mechanisms for excursions and reversals however the coincidence of the longitudinal bands for vgps associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions

numerical modeling of mountain building interplay between erosion law and crustal rheology 1 coupling between erosion and tectonics is thought to play a determinant role in orogenic evolution here we investigate the interplay in this coupling between the assumed erosion law and the crustal rheology at the margin of a collisional plateau like the himalaya of central nepal lithospheric deformation is calculated over a time scale of 100 kyr by a 2d finite element model that incorporates the rheological layering of the crust and the main features of the convergence across the range for the upper boundary condition two surface processes were tested a linear diffusion model and a 1d1 2 integrative model including fluvial incision along the fluvial network and hillslope erosion by landsliding model results and their sensitivity to the chosen combinations of erosion law and crustal properties are discussed in light of the constraining geologic and geomorphologic observations in contrast with the conclusions of cattin and avouac 2000 where a compliant quartz rich crustal rheology and diffusion law were required we combine a composite quartz diabase rheology for the crust with fluvial incision erosion law to account for erosion and elevation profiles across the himalaya of central nepal more generally it is proposed that because of the interplay between the dominant denudation conditions and the rheology of the crust both well documented erosion rates and processes can provide significant constraints on crustal properties within an active orogen

evidence for warm wet heinrich events in florida analyses of pollen and plant macrofossils from a new core spanning the past 60 000 years from lake tulane florida show a strong antiphase relationship in temperature between florida and the north atlantic region during the pleistocene oak scrub and prairie phases were coeval with long intense dansgaard oeschger interstadials warm periods that initiated bond cycles pine phases were coeval with the north atlantic long stadials cold periods that ended bond cycles and were terminated by heinrich events lake levels were higher during pine phases and climate was wetter however climate in florida was also warmer during these phases which were cold periods in the north atlantic perhaps diminution of thermohaline circulation before and during heinrich events reduced northward heat transport and retained warmth in the subtropical atlantic and gulf of mexico

relative paleointensity rpi in the latest pleistocene 10 45 ka and implications for deglacial atmospheric radiocarbon abstract we report magnetic properties and relative paleointensity rpi proxies from a suite of 10 conventional piston cores and kasten cores from the sw iberian margin collected during cruise jc089 of the rss james cook in august 2013 mean sedimentation rates are in the 10 20 cm kyr range age models were acquired by correlation of ca ti and zr sr xrf core scanning data to l reflectance from the cariaco basin that is in turn tied to the greenland ice core chronology the natural remanent magnetization nrm is represented by a single magnetization component carried by a low coercivity mineral magnetite although reflectance and bulk magnetic properties indicate the presence of a high coercivity hematitic magnetic phase possibly from eolian dust the presence of fine grained hematite means that the sediments are not ideal for rpi studies however the detrital hematite does not appear to contribute to the nrm or anhysteretic remanent magnetization arm in order to test the usefulness of the rpi data we construct a stack of 12 rpi records from the sw iberian margin for the 0 45 ka interval and compare it with a stack of 12 globally distributed marine and lake records chosen on the basis of mean sedimentation rates 15 cm kyr and superior age models the two stacks are similar but different from published rpi stacks particularly for the 10 30 ka interval and imply a virtual axial dipole moment vadm high at 15 18 ka followed by a drop in field strength from 15 to 13 ka a revised vadm estimate calculated from greenland 10be ice core flux using a contemporary age model is remarkably consistent with the new overall rpi stack based on iberian margin and global rpi records the elevated atmospheric 14c levels of the last ice age cannot however be fully explained by this rpi stack although relative changes such as the long term drop in atmospheric 14c from 30 to 15 ka are reproduced supporting the hypothesis of a combined influence of production rate and ocean ventilation on 14c during the last ice age

on the interrelationship between temporal trends in delta 13 c delta 18 o and delta 34 s in the world ocean the phenomena of i inverse correlation between the oceanic carbon and sulfur isotopic curves and ii covariation between the oceanic carbon and oxygen isotopic curves at all their major excursions appear as paradoxes in the current paradigm of global biogeochemical cycles these phenomena however are fully explicable by our model proposing that the ocean alternates between two general modes stagnant stratified and net autotrophic overfed ocean on the one hand and vigorously mixed and net heterotrophic hungry ocean on the other this model is in fact strongly supported by the carbon isotopic evidence the directions of change in the isotopic ratios of carbon oxygen and sulfur should be different in the lower anoxic box of a stratified ocean than in the upper oxic box whereas ocean destratification and mixing of the two boxes should lead to coeval shifts in the oceanic isotopic curves of these elements the model has far reaching implications for i the causal explanation of both secul

mono lake excursion recorded in sediment of the santa clara valley california 1 two intervals recording anomalous paleomagnetic inclinations were encountered in the top 40 meters of research drill hole ccoc in the santa clara valley california the younger of these two intervals has an age of 28 090 330 radiocarbon years b p calibrated age 32 8 ka this age is in excellent agreement with the latest estimate for the mono lake excursion at the type locality and confirms that the excursion has been recorded by sediment in the san francisco bay region the age of an anomalous inclination change below the mono lake excursion was not directly determined but estimates of sedimentation rates indicate that the geomagnetic behavior it represents most likely occurred during the mono lake laschamp time interval 45 28 ka if true it may represent one of several recurring fluctuations of magnetic inclination during an interval of a weak geomagnetic dipole behavior noted in other studies in the region

evidence for enhanced 10be deposition in mediterranean sediments 35 kyr bp in this letter we report the profiles 160 samples of 10be 9be zn in a mediterranean sediment core spanning the last 60 kyr we show the existence of a 10be peak whose absolute age is estimated to be 34 3 kyr bp based on its stratigraphic position between two tephra layers originating from two volcanic eruptions campanian ignimbrite and citara dated by k ar 40ar 39ar and 14c methods this peak is similar to the increase in the 10be concentration observed in vostok and dome c ice cores at the same age the increase with approximately similar amplitude in both ice and sediment cores at different latitudes and hemispheres cannot be explained by changes in the archives the present result supports the view of a well defined global enhancement of the 10be flux related to an increase of the 10be production rate and not to a redistribution of the 10be fallout between different latitudes

the relationship between 10be and geomagnetic field strength records in central north atlantic sediments during the last 80 ka abstract a profile of leached 10 be 9 be has been measured in an 80 ka sediment core from the northeast atlantic ocean this profile shows a significant inverse correlation with the palaeomagnetic intensity profile inferred from nrm arm measurements in the same core particularly between 40 and 50 ka where a rapid decrease in field intensity is accompanied by a rapid increase in 10 be 9 be the extent to which this increase in 10 be 9 be at 40 ka is associated with a 10 be peak seen in polar ice cores at about the same time remains speculative