mass transport 2 analysis of uncertainty in prediction the first paper in this series presented a description of a stochastic modeling concept for mass transport in this paper we extend that analysis to consider a more realistic set of transport conditions in a groundwater basin with geologic layering hydraulic anisotropy spatial variations in porosity and geochemical retardation uncertainties in transport predictions can be characterized by frequency distributions formed on the time of arrival of mass at the water table on the exit location and on the quantity of mass arriving at the water table as a function of time results show that transport is highly sensitive to porous medium heterogeneities considerable uncertainty is possible in predicting the spatial and temporal distribution of mass porous medium parameters capable of changing both the magnitude and direction of advective transport are of primary importance in influencing predictive ability most important in this respect are the arrangement of units with different mean hydraulic conductivities the standard deviation and spatial continuity of hydraulic conductivity and the hydraulic anisotropy in certain geologic systems patterns of contaminant migration can be predicted with more certainty features such as layering and hydraulic anisotropism can constrain the flow of mass to specific directions thus limiting the size of the region through which mass is likely to spread depending upon the relative time scales of the release function and the transport process lack of information about the timing and concentrations added at the source may cause greater uncertainty than the heterogeneity

on the effect of crustal layering on ring fault initiation and the formation of collapse calderas collapse calderas can be attributed to subsidence of the magma chamber roof along bounding sub vertical normal faults ring faults after a decompression of the magma chamber it has previously been shown that for ring faults to initiate and thus facilitate collapse the stress field both at the surface and around the magma chamber must satisfy specific critical conditions here we present new numerical models that use a finite element method to investigate the effects of crustal layering on local stress field distribution results are compared with existing criteria for ring fault initiation different subsurface scenarios were simulated by varying the stiffness young s modulus of layers placed above the magma chamber and the host rock in which the chamber is seated we consider depressurisation of a magma chamber so as to simulate magma withdrawal results indicate that mechanical layering is a further first order variable in the rare achievement of stress conditions required for ring fault formation and may be influential in facilitating or inhibiting caldera collapse we show that for a given geometrical set up the magnitude and position of maximum tensional stress at the earth s surface are influenced by the occurrence and relative distribution of mechanically stiff or soft lithologies above the magma chamber for example tensional stress at surface may be reduced by the presence of stiff layers e g lavas or increased by soft layers e g pyroclastic units compared to generic simulations using a homogeneous background medium overall we find that the presence of mechanically soft material promotes surface fracture initiation in addition we suggest that the position of peak tensional stress at surface derived by the numerical models does not represent that related to the position of the bounding ring fault but is instead related to the position of initial tensional fractures appearing prior to the collapse faults

effects of internal structure and local stresses on fracture propagation deflection and arrest in fault zones the way that faults transport crustal fluids is important in many fields of earth sciences such as petroleum geology geothermal research volcanology seismology and hydrogeology for understanding the permeability evolution and maintenance in a fault zone its internal structure and associated local stresses and mechanical properties must be known this follows because the permeability is primarily related to fracture propagation and their linking up into interconnected clusters in the fault zone here we show that a fault zone can be regarded as an elastic inclusion with mechanical properties that differ from those of the host rock as a consequence the fault zone modifies the associated regional stress field and develops its own local stress field which normally differs significantly both as regard magnitude and orientation of the principal stresses from the regional field the local stress field together with fault rock heterogeneities and interfaces discontinuities fractures contacts determine fracture propagation deflection along discontinuities interfaces and arrest in the fault zone and thereby its permeability development we provide new data on the internal structure of fault zones in particular the fracture frequency in the damage zone as a function of distance from the fault core new numerical models show that the local stress field inside a fault zone modelled as an inclusion differ significantly from those of the host rock both as regards the magnitude and the directions of the principal stresses also when the mechanical layering of the damage zone due to variation in its fracture frequency is considered the numerical models show abrupt changes in local stresses not only between the core and the damage zone but also within the damage zone itself abrupt changes in local stresses within the fault zone generate barriers to fracture propagation and contribute to fracture deflection and or arrest also analytical solutions of the effects of material toughness the critical energy release rate of layers and their interfaces show that propagating fractures commonly become deflected into and often arrested at the interfaces generally fractures propagating from a compliant soft layer towards a stiffer one tend to become deflected and arrested at the contact between the layers whereas fractures propagating from a stiff layer towards a softer one tend to penetrate the contact thus it is normally easier for fractures to propagate from the host rock into the damage zone than vice versa similarly it is easier for fractures to propagate from the outer stiffer parts of the damage zone to the inner softer parts and from the stiff host rock to the outer damage zone than in the opposite directions these conclusions contribute to increased understanding as to how fractures propagate and become arrested within fault zones and how the fault zone thickness is confined at any particular time during its evolution

vegetation and environmental dynamics in the southern black sea region since 18 kyr bp derived from the marine core 22 gc3 abstract sediments from the black sea a region historically dominated by forests and steppe landscapes are a valuable source of detailed information on the changes in regional terrestrial and aquatic environments at decadal to millennial scales here we present multi proxy environmental records pollen dinoflagellate cysts ca ti and oxygen isotope data from the uppermost 305 cm of the core 22 gc3 42 13 53 n 36 29 55 e collected from a water depth of 838 m in the southern part of the black sea in 2007 the records span the last 18 kyr all ages are given in cal kyr bp the pollen data reveal the dominance of the artemisia steppe in the region suggesting rather dry cold environments 18 14 5 kyr bp warming humidity increase during melt water pulses 16 1 14 5 kyr bp indicated by 18o records from the 22 gc3 core sediment and from the sofular cave stalagmite is expressed in more negative 13c values from the sofular cave usually interpreted as the spreading of c3 plants the records representing the interstadial complex 14 5 12 9 kyr bp show an increase in temperature and moisture indicated by forest development increased primary productivity and reduced surface run off whereas the switch from primary terrigenous to primary authigenic ca origin occurs 500 yr later the younger dryas cooling is clearly demonstrated by more negative 13c values from the sofular cave and a reduction of pines the early holocene 11 7 8 5 kyr bp interval reveals relatively dry conditions compared to the mostly moist and warm middle holocene 8 5 5 kyr bp which is characterized by the establishment of the species rich warm mixed and temperate deciduous forests in the low elevation belt temperate deciduous beech hornbeam forests in the middle and cool conifer forest in upper mountain belt the border between the early and middle holocene in the vegetation records coincides with the opening of the mediterranean corridor at 8 3 kyr bp as indicated by a marked change in the dinocyst assemblages and in the sediment lithology changes in the pollen assemblages indicate a reduction in forest cover after 5 kyr bp which was likely caused by increased anthropogenic pressure on the regional vegetation

a geochemical method of determining dispersivity in regional groundwater systems abstract wood w w 1981 a geochemical method of determining dispersivity in regional ground water systems in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 209ndash 224 a method for determining values of dispersivity for large scale regional aquifer systems is developed using the concept of hydrochemical facies in this approach the aquifer system is viewed as a large imaginary column analysis of hydrodynamic dispersion is made by observing the relative position of solute concentration contours in space that is because the natural aquifer is so large and the flow rate so slow time has been interchanged for space and solute ratios based on concentration contours are substituted for column effluents which is commonly used in such analyses the proposed method of evaluation of dispersivity yields a true value if conservative tracers are used or homogeneous distribution of reacting materials can be assumed

propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields abstract palaeofluid transporting systems observed as networks of mineral filled veins in deeply eroded parts of extinct geothermal fields indicate that hydrofractures commonly supply fluids to geothermal fields here we examine well exposed vein networks that occur at crustal depths of around 1 5 km below the initial surface of the tertiary lava pile in north iceland the veins are located in the damage zone of a major fault zone that dissects basaltic lava flows the most common host rocks of geothermal fields in iceland the lava flows contain numerous weaknesses particularly columnar cooling joints and contacts for hydrofractures to supply fluids to geothermal fields the fractures must be able to propagate and transport fluids to the surface we explore hydrofracture pathway formation using boundary element models of hydrofractures with fluid overpressure varying linearly from 10 mpa at the fracture centre to 0 mpa at the fracture tip or the fluid front the hydrofractures propagate through a vertically jointed and horizontally layered pile of lava flows with a general rock matrix young s modulus of 1 10 10 pa and a poisson s ratio of 0 25 the joints and contacts between layers are modelled as internal springs each with a stiffness strength of 6 mpa m the location and sizes of discontinuities as well as the location of the hydrofracture tip vary between the models the results indicate that tensile stresses generated at the tip of an overpressured hydrofracture can open up horizontal and vertical discontinuities out to a considerable distance from the tip and that these discontinuities eventually link up to form the hydrofracture pathway analytical models indicate that for a hot spring of a given yield associated with a fault the dimensions of the fluid transporting part of the fault are likely to be similar for a typical normal fault and a strike slip fault also a hot spring of yield 180 l s the maximum in the low temperature fields of iceland can be supplied through a hydrofracture of aperture 3 mm and trace length 1 2 m these dimensions are very similar to those of typical veins in the studied networks buoyancy rather than excess pressure in the fluid source appears to be the primary driving force of hydrofractures in the geothermal fields of iceland

40ar 39ar dating of latest pleistocene 41 ka marine tephra in the mediterranean sea implications for global climate records abstract fifty one 40 ar 39 ar laser heating analyses of sanidine crystals in the c 13 tephra layer recovered out of a piston core raised from 3000 m depth in the tyrrhenian sea yielded apparent ages between 25 and 112 ka twenty four of these measurements define an age probability maximum and generate an isochron of 41 1 2 1 ka 2 with a 40 ar 36 ar intercept of 288 21 that is indistinguishable from the modern atmosphere however the entire population is skewed toward older apparent ages with smaller peaks at 48 ka 55 ka 67 ka and 110 ka the composition of glass shards in the c 13 tephra overlaps that of the sio 2 rich phase of the 150 km 3 campanian ignimbrite which erupted from the campi flegrei caldera 130 km northwest of the core site although the 41 1 2 1 ka isochron is 15 older than k ar and 14 c ages for the campanian ignimbrite it is indistinguishable from recent 40 ar 39 ar age determinations the campanian ignimbrite eruptions were also chemically similar to and correlated with the y 5 ash layer the most voluminous and widely dispersed late pleistocene tephra in the mediterranean region the age of the y 5 layer was constrained originally at 38 0 2 5 ka on the basis of astronomical dating of o isotope stages the 40 ar 39 ar ages astronomical dating plus chemical similarities permit correlation of the c 13 tephra layer with both the campanian ignimbrite and the y 5 tephra layer the older ages preserved in the c 13 tephra layer may reflect xenocrysts incorporated during the explosive eruption accompanying collapse of the campi flegrei caldera 41 ka the c 13 tephra layer is correlated with a 10 be peak in an adjacent marine core o isotope stage 3 12 and immediately preceded a cooling episode corresponding to heinrich event 4 thus the age of these features is constrained by radioisotope dating the 41 ka peak in 10 be deposition is a global phenomenon whose age matches that of the laschamp geomagnetic excursion thus our results provide a new tie point among several polar ice core and marine proxy records of paleoclimate during the past 50 kyr precise age information independent of radiocarbon dating can be recovered from latest pleistocene marine tephra using the 40 ar 39 ar laser heating method despite the presence of multiple components that may have been incorporated during or after its deposition

supra regional correlations of the most ancient paleosols and paleolithic layers of kostenki borschevo region russian plain abstract the archaeological site kostenki12 located on the middle don river provides a key stratigraphic profile for regional paleopedological paleoenvironmental geological and cultural sequences containing the oldest known cultural layers of the region layer v paleolithic layer iv upper paleolithic layer iii kostenki strelets culture early phase dating to the early part of mis3 or in chronometric terms to 54 42 ka kostenki12 complements kostenki14 markina gora which is a key profile for the interval 42 27 ka the new data from kostenki12 show that the east european upper paleolithic began 45 ka the stratigraphy exhibits similarities to that of borschevo5 the kostenki12 pollen diagram is correlated with 1 other pollen diagrams from kostenki borschevo region 2 the most detailed climatostratigraphical scale of the russian plain late pleistocene 3 16 o 18 o greenland gisp2 scale 4 13 c 14 c record from stalagmite at villars cave france as well as with pollen records 5 7 from 5 lake monticchio italy 6 southern black sea m72 5 25 gc1 and 7 glinde and moershoofd northern germany the results of the supra regional paleoenvironmental correlations demonstrate that the lowest paleolithic layer v and paleosol d characterized by elm dominance correlate to the second half of the optimum of the glinde interstadial at 51 48 ka corresponding to do 14 the earliest upper paleolithic layer iv and paleosol b characterized by coexistence of elm forests and wet meadows began to form during the second part of the moershoofd interstadial optimum at 46 44 ka correlating with do 12 paleosol a and layer iii kostenki strelets culture began to form after the abrupt end of the moershoofd interstadial 43 5 ka during unstable conditions according to pollen and paleozoological data steppe with horse dominance and later spruce forest tundra with reindeer dominance in paleozoological complex these correlations provide more accurate dating of the paleolithic layers and paleosols at kostenki borschevo suggesting that previously reported radiocarbon dates on units below ci tephra layer are too young but that the osl chronology is generally accurate

geometric difference between non feeder and feeder dikes feeder dikes bring magma to the surface non feeder dikes become arrested and never reach the surface the differences if any between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non feeder here we present measurements of feeder and non feeder dikes exposed from depths of 200 m to the surface in the walls of the a d 2000 caldera collapse of the miyakejima volcano japan a typical feeder thickness reaches a maximum of 2 4 m at the surface decreases rapidly to 1 m at a depth of 20 40 m and then remains constant to the bottom of the exposure by contrast a typical non feeder thickness reaches a maximum of 1 5 2 m at 15 45 m below the tip and then decreases slowly with depth to 0 5 1 m at the bottom of the exposure we propose that free surface effects and magmatic overpressure driving pressure changes during the eruption cause the overall shape of a feeder to differ from that of a non feeder

weichselian palynostratigraphy palaeovegetation and palaeoenvironment the record from lago grande di monticchio southern italy abstract the palynostratigraphic record from lago grande di monticchio s italy is presented an independent sedimentation rate based chronology provides an age of 101 530 calendar years bp for the lowermost sediments sampled this chronology enables assessment of the rapidity of vegetation changes during the last glacial contrary to widely held views the vegetation showed numerous large magnitude changes in periods of

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

an analysis of dispersion in a stratified aquifer the dispersion of a conservative solute produced as a result of vertical variations of hydraulic conductivity in a horizontal stratified aquifer of finite thickness is analyzed by applying the moment method of aris to solve the governing advection dispersion equation describing mass transport in the analysis it is assumed that the aquifer is of constant thickness and of infinite lateral extent the hydraulic conductivity is a known function of the vertical coordinate only and the flow is unidirectional parallel to the stratification the applicable aris moment equations are developed in a suitable nondimensional form analytical solutions are obtained for the zeroth and first moments and for the time derivative of the second moment of the longitudinal concentration distribution for the case of an instantaneous plane source for several idealized hydraulic conductivity profiles parabolic linear step function and cosine profiles and their even periodic extensions the analysis gives the time dependent variation of the longitudinal macrodispersivity for these idealized cases throughout the transient development of the dispersion process the results of the analysis are applied to a field measured hydraulic conductivity profile and predicted values of the longitudinal macrodispersivity are compared with field results an important conclusion from the analyses is that nonuniformities in the hydraulic conductivity profile which persist over long distances may produce rather large values of longitudinal macrodispersivity which are comparable to those observed in some aquifers and which are much larger than those predicted by some previous stochastic analyses implications of the analytical results for field dispersion problems are discussed

vegetation history and palaeoclimate of the last glacial period at lago grande di monticchio southern italy a high resolution palynological study of a 51 m core from lago grande di monticchio southern italy has provided a palaeonvironmental record for the last glacill a annual lamination based chronology supported by radiometric and tephrochronological dates provides an absolute timescale for this record that spans 76 300 years correlations are established between the pollen stratigraphy the grip ice core 18o record and foraminiferal assemblages from atlantic core v23 81 both dansgaard oeschger and heinrich events are reflected by changes in the pollen stratigraphy revised dates are estimated for heinrich events h1 h6 a quantitative palaeoclimate reconstruction based upon the pollen data provides evidence of the climate changes in southern italy associated with these and other fluctuations during the last glacial

toughness and failure of volcanic edifices abstract major volcanic edifices are of two main types stratovolcanoes composite edifices and basaltic edifices shield volcanoes these types of edifices show several remarkable differences for example while some basaltic edifices erupt very frequently many stratovolcanoes erupt very infrequently also most basaltic edifices have gentle slopes commonly 2 12 whereas stratovolcanoes have steeper slopes particularly in their uppermost parts where the slopes may reach 35 42 there have been various attempts to explain these differences but here the focus is on edifice strength or rather toughness toughness of a material is a measure of its resistance to fracture alternatively for a tough material large amounts of energy are needed to cause failure most quoted material toughness critical energy release rate values for rocks are 20 400 j m 2 for extension fractures such as dykes and 0 01 0 03 mj m 2 for shear fractures such as dip slip faults the related property of critical stress intensity or fracture toughness is for most rocks 0 5 3 mpa m 1 2 these values for material and fracture toughness refer to small essentially homogeneous laboratory rock samples here i present equations for calculating the material toughness in volcanic edifices based on the measured or estimated dimensions of associated large structures such as dykes and faults for typical dykes the calculated material toughness is 1 3 47 mj m 2 with a corresponding stress intensity of 114 690 mpa m 1 2 similarly for a 3 8 km long dip slip fault in an edifice the calculated material toughness is 2 3 mj m 2 and the corresponding stress intensity 150 mpa m 1 2 these results compare well with previous material toughness estimates for large strike slip faults and stress intensity estimates for large dykes generalising these results and using field data as well as materials science results i conclude that stratovolcanoes normally have a much higher material toughness than basaltic edifices this is primarily because of different internal structures a basaltic edifice is made of layers and contacts all of which have essentially similar mechanical properties whereas a stratovolcano is made of layers and contacts many of which have widely different properties accordingly it requires much more energy is more difficult for a fracture to propagate through a stratovolcano than through a basaltic edifice thus contrasting toughnesses may be one principal reason why these edifices differ in slope and in failure frequencies including the number of dyke fed eruptions caldera collapses and landslides

how fracture systems affect permeability development in shallow water carbonate rocks an example from the gargano peninsula italy abstract fracture networks control the permeability of many reservoirs since the fracture patterns of petroleum reservoirs in situ are difficult to study in detail field analogues are very important for understanding their fracture related permeability here we present the results of a study of the fracture system of carbonate rocks of lower cretaceous age in a quarry associated with the damage zone and fault core of a major fault zone on the gargano peninsula in south italy we measured the attitude of 1541 fractures and faults along several vertical and horizontal scan lines there are two main fracture sets one strikes between e w and ese wnw the other nne ssw a total of 675 fracture spacing measurements indicate log normal spacing distributions with an arithmetic mean fracture spacing of 0 29 m and a median of 0 15 m the data plotted on a log log plot suggest three main spacing subpopulations each of which follows approximately a power law with different fractal dimensions subpopulation 1 where the spacing ranges from 1 to 10 cm and the straight line slope d fractal dimension is 0 20 represents fractures confined to laminated carbonate mudstones multilayers that form the microbial mat deposits of a peritidal cycle subpopulation 2 where the spacing ranges from 11 to 55 cm and d is 0 77 represents fractures confined to thicker layers forming a part of a peritidal cycle the contacts of which are marked by stylolites subpopulation 3 where the spacing ranges from 56 to 243 cm and d is 2 81 represents fractures that dissect comparatively thick units of an entire peritidal cycle for the spacing the minimum coefficient of variation c v defined as standard deviation divided by the mean is 1 00 essentially randomly spaced fractures while its maximum c v is 1 62 suggesting that some fractures form clusters some clusters being denser than others the clusters composed of fractures with varying attitudes and therefore commonly intersecting are likely to contribute significantly to the overall permeability of the carbonate rock fracture aperture opening data n 324 also show a log normal size distribution with a mean opening of 1 01 cm and median of 0 29 cm log log plots indicate that a part of this data groups into two subpopulations i and ii each of which follows approximately a power law the straight line slope d the fractal dimension of subpopulation i is 0 46 whereas that of subpopulation ii is 1 49 we present boundary element models showing that laminated carbonate mudstones and their contacts modify the local stress fields so as to encourage fracture offset and commonly arrest our results also show that when a fluid driven subpopulation 2 fracture approaches subpopulation 1 fractures the induced tensile stresses may result in the opening up of many of the subpopulation 1 fractures directly above the tip of the subpopulation 2 fractures if in addition the contacts between the multilayers are weak they also tend to open up thus generating a large interconnected cluster of vertical fractures and horizontal contacts the results suggest that the tensile stresses induced by a comparatively large fluid driven subpopulation 2 fracture may contribute to the formation of an interconnected cluster of subpopulation 1 fractures and associated contacts thereby significantly increasing the permeability of the carbonate rock

length displacement scaling and fault growth abstract following an earthquake in a fault zone commonly the co seismic rupture length and the slip are measured similarly in a structural analysis of major faults the total fault length and displacement are measured when possible it is well known that typical rupture length slip ratios are generally orders of magnitude larger than typical fault length displacement ratios so far however most of the measured co seismic ruptures and faults have been from different areas and commonly hosted by rocks of widely different mechanical properties which have strong effects on these ratios here we present new results on length displacement ratios from 7 fault zones in holocene lava flows on the flanks of the volcano etna italy as well as 10 co seismic rupture length slips and compare them with fault data from iceland the displacement and slip data from etna are mostly from the same fault zones and hosted by rocks with largely the same mechanical properties for the co seismic ruptures the average length is 3657 m the average slip 0 31 m and the average length slip ratio 19 595 for the faults the average length is 6341 m the average displacement 73 m and the average length displacement ratio 130 thus the average rupture slip ratio is about 150 times larger than the average length displacement ratio we propose a model where the differences between the length slip and the length displacement ratios can be partly explained by the dynamic young s modulus of a fault zone being 101 2 times greater than its static modulus in this model the dynamic modulus controls the length slip ratios whereas the static modulus controls the length displacement ratio we suggest that the common aseismic slip in fault zones is partly related to adjustment of the short term seismogenic length slip ratios to the long term length displacement ratios fault displacement is here regarded as analogous to plastic flow in which case the long term displacement can be very large so long as sufficient shear stress concentrates in the fault

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

on the age of the laschamp geomagnetic excursion abstract the laschamp geomagnetic excursion is a critical tie point found directly in deep sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice and to providing an independent calibration of the radiocarbon calendar its timing remains poorly known previous k ar 40 ar 39 ar and u th isochron determinations from lava flows at the type locality in the massif central france vary widely are imprecise and suggest a mean age of about 46 2 2 5 ka 2 results of 6 new unspiked k ar and 13 40 ar 39 ar incremental heating experiments on subsamples from three sites on the laschamp and olby flows are concordant and give a weighted mean age of 40 4 1 1 ka 2 uncertainty including analytical sources only that is 10 younger than the previous estimates considering that the 40 k 40 ar decay constant is not known to a precision better than 2 4 the most probable radioisotopic age for the laschamp excursion is 40 4 2 0 ka 2 analytical plus decay constant uncertainties this new age for the laschamp excursion agrees precisely with that deduced from the napis 75 deep sea sediment paleointensity stack when calibrated against the gisp2 ice core chronology using the o isotopes in ice and the magnetic properties of the marine cores

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

fracture propagation and coalescence at bedding plane in layered rocks abstract fracture propagation fracture coalescence and their interactions with the bedding plane in layered rocks are of great significance when conducting a stability analysis of structures on or within the layered rock mass in this work we developed a numerical model by the inserted cohesive element ce method to simulate the evolution of fracture paths in layered rocks then validated it with modified wedge splitting tests the results indicate that the fracture penetration deflection behaviour at the bedding plane is controlled by two strength thresholds of the bedding plane 1 if the bedding plane strength threshold i the fracture directly penetrates the bedding plane and 2 if threshold i bedding plane strength threshold ii first the fracture deflects along the bedding plane without any daughter fractures or penetration and then one or two daughter fractures can form a two flawed model is used to simulate the fracture coalescence the fracture coalescence is first determined by the bedding plane s strength thresholds if bedding plane strength threshold i fracture coalescence can occur only if the second flaw is located within a certain range no coalescence occurs if the bedding plane strength

sedimentology dating and palaeoclimatic interpretation of a 76 3 ka record from lago grande di monticchio southern italy abstract detailed sedimentological and microstratigraphical investigations of lacustrine sediments from lago grande di monticchio southern italy provide a time scale based on rates of sedimentation obtained from annual laminations although not annually laminated throughout a time scale covering the last 76 3 ka is available by interpolation of sedimentation rates results of this chronology agree with radiocarbon dates and with ar ar dates on tephra layers from the same lacustrine sediment sequence occurrence of varves in combination with physical and geochemical properties are related to variations in the trophic state of the lake and to changing runoff both controlled by palaeoclimatic conditions water depth was relatively shallow with dominating bioturbation and only few varves preserved for most of the pleniglacial a marked increase in precipitation and related runoff characterises the early last glacial maximum when water depth reached a maximum and clastic varves were deposited subsequently water depth was shallower again a drastic change towards nutrient rich conditions occurred at the pleistocene holocene transition during the holocene organic varves were deposited two periods with higher organic productivity were recognised prior to the last glacial maximum around 50 ka and 73 ka these intervals are interpreted as the result of interstadial conditions the oldes of the two events shows laminations comparable to those at the glacial holocene transition the periods with increased minerogenic influx may be characterised as stadial conditions lasting from 14 40 ka and from 50 73 ka

migration of contaminants in groundwater at a landfill a case study 4 a natural gradient dispersion test abstract a natural gradient tracer test using a chloride solution with an initial injection volume of 0 7 m 3 was performed in the sandy aquifer at the borden site the solution was injected into five well points set 1 m below the water table in an uncontaminated zone situated above the contaminant plume at a location 450 m downflow from the landfill under conditions of natural groundwater flow the tracer slug was monitored for a period of 4 months by withdrawing small volume samples from points in a three dimensional array of bundle type multilevel samplers measurements of hydraulic head were obtained from a network of miniature piezometers soon after injection the tracer slug gradually split into two halves one half moving horizontally at an average velocity of 2 9 10 6 ms 1 and the other horizontally at 8 2 10 7 ms 1 although the split has been attributed to local lateral heterogeneity the nature of the heterogeneity and its influence on the hydraulic head distribution were not clearly distinguishable in the field data obtained before during or after the test the chloride patterns for each of the two halves of the tracer slug evolved into gaussian forms although the patterns demonstrated some irregularity at early time the relatively smooth gaussian forms were unexpected because the aquifer has numerous small scale heterogeneities observed in vertical cores obtained from the tracer zone and because the glaciofluvial origin of the aquifer suggests that heterogeneities are not laterally continuous simulated chloride distributions from a three dimensional analytical solution to the advection dispersion equation were fitted to the field data to obtain best fit estimates of the values of longitudinal transverse horizontal and transverse vertical dispersivity at various travel distances for each of the two halves of the tracer zone this is the first known field test that has permitted the estimation of three principal dispersion coefficients in layered media the longitudinal dispersivity was found to increase from 0 01 m at a distance of 0 75 m from the source to 0 08 m at 11 0 m the transverse horizontal dispersivity increased also to a value of 0 03 m at 11 0 m transverse vertical dispersion was very weak and was accounted for by molecular diffusion the relative lack of vertical dispersion is consistent with the shape of the plume of leachate contamination from the landfill it was concluded that the observed increase in dispersivity along the path of migration is likely caused by heterogeneities information on the dispersion controlling heterogeneities is not yet available as practical field methodologies for their identification and description have not yet been developed until such information is incorporated into mass transport models a realistic solution of the dispersion problem in heterogeneous media will remain elusive

palaeoecology of three interstadial events during oxygen isotope stages 3 and 4 a lacustrine record from lago grande di monticchio southern italy abstract three interstadial events recorded in a pollen diagram from lago grande di monticchio southern italy 40 56 40 n 15 36 e can be correlated with dansgaard oeschger events 12 14 and 20 of the grip summit greenland ice core based on a chronology of varve counts the events are dated respectively to the intervals 37 600 40 000 43 600 50 000 and 72 000 74 600 calendar yr bp the interstadials are of unequal duration the longest occupying 6400 years they appear to begin and end abruptly each is characterised by a sequence of pollen changes in the mesic forest component similarities in these sequences of mesic forest trees imply similar patterns of climatic change within each interstadial the sequences resemble that of the late glacial interstadial at monticchio although tree pollen abundance values reach as much as 60 during the interstadials there is always a substantial proportion of herb pollen tree growth may have been constrained by low co2 levels during the glacial phase of the last glacial interglacial cycle this record illustrates the potential of long continental records to present opportunities for detailed ecological and chronological analyses of sub interglacial warming events during oxygen isotope stages 3 and 4

transform faults or overlapping spreading centers oceanic ridge interactions revealed by analogue models abstract transform faults and overlapping spreading centers osc are the two most common types of interaction between oceanic ridge segments various factors have been proposed to control one interaction type or the other however measurements of several tens of transforms and osc reveal that a simple ratio between the total length of the interacting ridge segments l and their overstep s distinguishes transforms l s l s 11 scaled experiments have been performed to test the possibility that the initial configuration of neighbor ridges controls the interaction type with l s l s 12 osc always develop at earlier stages independently from any ridge parallel extension ridge parallel extension commonly observed along oceanic ridges may accommodate the increase in length of an oceanic divergent boundary during its migration and expansion enhancing the development of transforms under suitable l s conditions the matching between experimental and natural data suggests that the interaction type between oceanic ridges depends from their initial configuration and subsequently the overall growth of the divergent boundary

krotovinas pedogenic processes and stratigraphic ambiguities of the upper palaeolithic sites kostenki and borshchevo russia abstract the excavations of the upper palaeolithic sites of kostenki and borshchevo located in the middle russian plain bear several meters of loess derived colluvial deposits of the middle and late valdai which cover alluvial sediments of the don floodplain at least four cultural layers and more than three paleosol units occur within the colluvial deposits a high number of krotovinas is most obvious mainly the burrows of cricetus cricetus and lagurus lagurus which on first view seem only to disturb sediment and soil stratigraphy to disprove this assumption the present paper investigates the significance of krotovina fillings within soil research by applying micromorphological analysis the study gives insight into different filling materials soil forming processes inside abandoned and filled burrow systems and surrounding material in some cases krotovina fillings in this part of the middle russian plain can be areas of bulking and compaction of preferential paths of infiltration followed by calcium carbonate depletion and of drying followed by secondary calcification further burrows are paths for second and third generations of soil faunal activities

the climate in western europe during the last glacial interglacial cycle derived from pollen and insect remains using the pollen sequence of la grande pile xx france we review problems with the application of transfer functions in paleoclimatic reconstructions one of them is to find modern analogues for the herbaceous vegetation of the cold periods we propose a method to distinguish between steppes and tundra vegetations for which the moderns are only partial analogues of the glacial periods another method to solve these problems is based on constraining by insect remains the two methods provide coherent reconstructions the results are also compared with other paleodata there is a good correlation with the six cold heinrich events between 70 and 15 ka b p a cooling event during the eemian period marked by high percentages of taxas at about 125 ka b p needs still to be correlated with high resolution ocean and ice cores

staircase fractures in microbialites and the role of lamination related mechanical anisotropy the example of the acquasanta terme travertine deposits central italy this study describes a peculiar yet common type of fracture showing a staircase trajectory which forms in rocks with moderately weak planar anisotropies the staircase fracture trajectory is given by alternating fracture segments oriented parallel to lap or at an angle ramp with respect to the lamination layering the analyses has been accomplished on travertines which are continental microbial hydrothermal deposits having a typical poorly stratified yet strongly laminated texture in these rocks porosity and permeability have a high across lamination variability and are mostly controlled by an interconnected and locally corroded array of permeable layers fractures and faults structural analysis integrated with analytical modeling provided a conceptual model of staircase fracture localization as a function of the travertine lamination dip lamination parallel fracture segments localize within the porous laminae mostly at the interface with tight laminae ramp type fracture segments cut the lamination connecting lamination parallel segments two types of staircase fracture can be modeled the first group develops in subhorizontal to gently dipping deposits lamination dip 30 and corresponds to high energy environments major discoveries of hydrocarbon have been recently made in continental lacustrine microbial carbonates in the brazilian south atlantic margin some of which exhibit a texture similar to those usually observed in travertines understanding of the lacustrine carbonates is still at an early stage given that in modern rift settings vent related thermal travertine and nonthermal tufa carbonates are a major component the proposed conceptual model of staircase fracture localization contributes to the preparation of a model for the potential occurrence of high permeability pathways in hydrocarbon and geothermal microbial reservoirs

numerical methods in rock mechanics the purpose of this civilzone review paper is to present the techniques advances problems and likely future development directions in numerical modelling for rock mechanics and rock engineering

paleoceanographic reconstructions from planktonic foraminifera off the iberian margin temperature salinity and heinrich events a quantitative analysis of planktonic foraminifera in a core from the iberian margin allows a reconstruction of the evolution of oceanographic parameters during the last glacial cycle with a resolution of 1000 years a principal component analysis performed on 19 species allows the identification of 11 intervals characterized by increased abundances of the subpolar species the youngest six of these intervals are correlated with the last 6 heinrich events hes the five cold events older than stage 4 are dated at 81 90 110 129 and 140 ka respectively paleotemperatures reconstructed using the modern analog technique indicate 4 c decreases during all even numbered isotopic stages and stage 3 during the hes temperature decreases reach 10 c and seawater 18o anomalies reach 1 temperature and salinity reconstructions indicate that the environment of the iberian margin has been under the combined influence of global factors such as the migration of the polar front and iceberg discharge and of regional factors such as the precipitation evaporation regime on both oceanic and continental area

mechanical controls on horizontal stresses and fracture behaviour in layered rocks a numerical sensitivity analysis in layered materials the deformation style orientation confinement and 3d connectivity of natural fractures is generally impacted by changes in sedimentary facies and alternations in mechanical properties in this study we address this effect and perform a numerical sensitivity analysis mechanical properties confining pressures and interfacial frictions are varied for a three layered model to investigate and quantify the relation between contrasting material properties the principal horizontal stresses and fracture behaviour i e deformation style and orientation firstly the results show that tensile stresses develop in the stiffer layers due to the contrasting elastic parameters the magnitude of these stresses is dependent on the ratio between the elastic parameters of stiffer and softer layers i e estiff esoft and stiff soft there are no horizontal tensile stresses when applying a compressive horizontal confining pressure approx 1 5 of the applied vertical stress implementing an interfacial friction lower than 0 2 will result in decoupling of the layers resulting in slip on the layer boundaries and no tensile stresses within the stiffer layers further the acquired numerical results are in good agreement with previously conducted laboratory work finally we discuss whether the presented results can be used for better relating contrasting mechanical properties to potential fracture deformation styles and orientations in layered outcrops or subsurface reservoirs

how plate tectonics is recorded in chalk deposits along the eastern english channel in normandy france and sussex uk abstract intra plate stresses that occurred in the anglo paris basin and english channel during upper cretaceous and cenozoic times are a consequence of the convergence between eurasia and africa and the opening of the north atlantic area this geodynamic re organisation is recorded on each side of the english channel with the emergence of regional structures such as the the weald artois anticline and the reactivation of large scale strike slip faults we analyse the anglo paris basin chalk fracture system on each side of the eastern english channel using a set of 1600 meso scale fractures data collected on coastal chalk cliffs in normandy nw france and sussex uk meso scale fracture system is precisely dated using chalk lithostratigraphy correlations within the basin moreover an inversion method is used on fault slip data to evidence a paleostress chronology in the anglo paris basin three main upper cretaceous extensive events characterized by normal faults and jointing are recorded in normandy and two cenozoic compressive and extensive events with strike slip and normal faults appear in sussex paleostress records vary on each part of the eastern english channel the meso scale fracture system is thus used to better define the type of relationship between meso scale and large scale brittle deformation in the chalk during meso cenozoic a first ne sw extension is recorded in normandy in relation with local anticlines structures and related to the lower rhine graben opening a second event is a wnw ese extension of local origin in relation with the subsidence axis of the paris basin the third event is a nne ssw extension well marked in normandy and related to the activation of e w normal faults in the western approaches of the english channel this event is also recorded in sussex and reactivates locally older fractures in strike slip the oligocene n s compression e w extension related to the pyrenean tectonics and the last e w extension relative to the north sea graben opening are well recorded in sussex but not in normandy recent far field stresses developed in the nw european platform are focused on deep crustal structures like the artois hills and the cotentin areas in france these structures act as a stress barrier by protecting the normandy chalk from recent far field stresses on the contrary recent far field stresses are easily recorded by meso scale brittle deformation on the folded chalk in sussex

numerical simulation of long term storage of co2 in yanchang shale reservoir of the ordos basin in china abstract shale reservoirs have attracted increasing attention in recent years as a new and promising geological repository for carbon dioxide co 2 storage however their long term storage mechanism is unclear and the interactions between co 2 adsorption to shale and the co 2 water rock reactions have not been investigated a 2 d simplified reactive transport model is developed in this paper based on the properties of a shale reservoir in the ordos basin in china which has been identified as a promising location for shale gas production and co 2 storage the model is used to study the co 2 water rock reactions coupled with gas adsorption and to identify the long term trapping mechanisms of co 2 in shale gas results show that in short several decades to middle term hundreds of years co 2 storage in a shale reservoir 60 co 2 is trapped in the supercritical and adsorbed phases but in the long term thousands of years mineral trapping gradually plays a dominant role among other storage mechanisms the displacement of ch 4 with co 2 during injection is also analyzed results show that this process can be divided into two stages the pre displacement stage during which the adsorbed concentrations of both ch 4 and co 2 increase due to pressure build up but due to low concentration of co 2 displacement does not occur at this stage and the displacement stage during which the co 2 concentration is high enough to displace the ch 4 from the host rock also higher pressure facilitates the displacement of ch 4 by co 2 in this stage study of the interaction between gas adsorption and co 2 water rock reactions shows that by buffering the reservoir pressure during co 2 injection into the subsurface gas adsorption can impact the solubility trapping process so as to interfere with the co 2 water rock reactions furthermore this interference is intensified with reduction in co 2 concentration in the reservoir over time sensitivity analysis indicates that variations in the co 2 adsorption isotherm abundance of smectite and chlorite reaction rate and reactive surface area of smectite can significantly affect the co 2 storage by different trapping mechanisms the high co 2 adsorption capacity of shale is very beneficial for co 2 injection into a shale reservoir the amount of adsorbed co 2 is proportional to the co 2 adsorption isotherm while co 2 in other phases gas supercritical phase dissolved phase and mineral phase is inversely related to the co 2 adsorption isotherm smectite and chlorite are the key minerals that influence the co 2 water rock reactions in yanchang shale

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

linking of fractures in layered rocks implications for permeability abstract the permeability of many rocks including many reservoirs is mostly attributable to fractures that form interconnected clusters here we present the results of field studies and numerical models on the linkage of fractures using primarily fractures in carbonate rocks from the uk the numerical models focus on two fracture configurations five offset fractures in a 5 layer model and a single hydrofracture in a 3 layer model in some of the 3 layer models a weak open contact is added between the two topmost layers in the 5 layer models loading is by 5 mpa tensile stress whereas the 3 layer models the only loading is the internal fluid overpressure of 6 mpa in the hydrofracture itself located in the lowermost of the three layers for the 5 layer models tensile stresses occur between the nearby tips of the offset fracture pairs but these stresses are generally too low in the soft shale layers to initiate fractures the tensile stresses however commonly concentrate at the contacts between the shale and limestone layers and may therefore result in delamination debonding that is opening of the contacts by contrast the shear stresses between the nearby tips of the fracture pairs are often high enough to generate shear or mixed mode fractures that connect the nearby tips of the original offset fractures resulting in clusters that can conduct fluids for the 3 layer models the fracture induced tensile stresses in the uppermost layer are suppressed by a comparatively thick compliant shale layer and or the weak open contact thus no fracture formation is possible in that layer but at their contacts with the compliant layer the hydrofracture apertures increase thus a compliant layer may arrest the vertical hydrofracture propagation from its stiff host layer but at the same time increase its maximum aperture and encourage lateral fluid transport at the layer contact between the layers

the mechanical contrast between layers controls fracture containment in layered rocks abstract open natural fractures allow fluids to flow which is necessary for the production of low permeable geothermal and petroleum reservoirs these reservoirs often consist of lithological layers with significant variation in rock strength which makes it difficult to predict fracture containment within the rock succession in this paper fractures are classified as contained when they do not cross the layer interface of the adjacent layer so that formation and growth are inhibited in the adjacent layer this study concerns the impact of the differences in rock strength i e mechanical contrast between two adjacent brittle layers on the fracture containment in finely layered rocks laboratory deformation tests and 2d finite element modelling were performed on three layered samples with varying mechanical contrasts to examine the behaviour of fractures at different stress conditions fractures initiate as shear fractures in weak layers and propagate with a steeper angle tensile fracture through the adjacent stronger layer the mechanical contrast within a layered rock does not always act as a containment barrier meaning that fractures do not cross the layer interface of the adjacent layer this is due to differences in differential stress between the weakest layer and strongest layer the mechanical contrast combined with the magnitude of the confining pressure has a significant influence on fracture containment an increase in mechanical contrast and confining pressure prevents fractures from propagating into stronger layers the results contribute to predict natural fracture containment in brittle sequences at shallow depth in the subsurface

the influence of pretreatment chemistry on the radiocarbon dating of campanian ignimbrite aged charcoal from kostenki 14 russia the presence of an independently dated marker in an archaeological site offers rare opportunities for assessing the reliability of radiocarbon dates especially when these are close to the age limit of the technique two different pretreatment protocols routine aba and more rigorous abox sc were employed in the chemical preparation of the same charcoal sample from a layer closely associated to the campanian ignimbrite tephra at the russian palaeolithic site of kostenki 14 markina gora the aba treated fraction gave an age of 33 14c ka bp comparable to a previous determination from the same layer whereas the abox sc produced an older age of 35 14c ka bp this is the first radiocarbon determination of an archaeological sample to provide an age consistent with the calendar age for the ci tephra marker

comparison of 14c and luminescence chronologies at puritjarra rock shelter central australia abstract at puritjarra rock shelter a long comparative sequence permits detailed comparisons of luminescence and radiocarbon chronologies over 35 000 years both techniques produce self consistent chronologies for the puritjarra deposit but observed discrepancies between luminescence dates on unburnt sediments and 14c assays on charcoal are greater than reported elsewhere tl ages are generally older than 14c ages but dates converge at depths below 70 cm dating first significant human occupation of the rock shelter 35 000 years ago the discrepancies are not removed by calibrating 14c dates or adjusting tl ages for lower water content of sediments the 14c chronology is broadly supported by sedimentary or palaeobotanical evidence stone artefact typology and other archaeological data radiocarbon dates on intact hearths agree with those on detrital charcoal tl ages were arrived at using both total bleach and selective bleach methods and the latter agree with optical ages for the same samples incomplete bleaching of sediments during deposition can be ruled out incorporation of old material into the luminescence samples via in situ disintegration of local sandstone remains a possibility and will be an important issue to resolve as luminescence techniques are increasingly deployed to date archaeological deposits in rock shelters

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

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

the relationship between the structural orientation and the gold mineralisation of quartz sulphide veins in the navachab gold deposit namibia abstract this study uses a representative dataset of more than 500 vein samples from the navachab gold deposit containing information on the gold concentration structural orientation vein thickness alteration and host rock type to investigate small scale controls on the grade distribution in the veins in order to obtain a better understanding of the architecture of the navachab ore bodies we calculated mean gold concentrations for each vein set in different parts of the mine we then examine the preferential mineralisation of certain vein sets by setting this grade information into context to other geological parameters such as the structural geology of that particular part of the mine as well as the above mentioned factors structural orientation of the vein vein thickness alteration and host rock type our main findings are that vein sets oriented favourably to the stress field and at high angles to the bedding are better mineralised than veins sets oriented at low angles to the bedding other factors do not exert a great influence on the gold concentration in the veins we propose that the gold grade in the veins is influenced by the effectiveness of fluid flow into the veins and by time rates during vein propagation and fluid flow

geomagnetic excursions reflect an aborted polarity state abstract geomagnetic excursions represent short episodes of a few thousand years at most during which the field considerably exceeds its normal range of variability during a polarity state paleomagnetic records have now been obtained with extremely high temporal resolution which have improved our knowledge of these short events we have compiled the most detailed records of excursions that had occurred during the brunhes and matuyama chrons we show that virtual geomagnetic poles vgps of at least one record of each event are able to reach the opposite polarity in the next step we have computed different simulations of excursions during which the dipole progressively vanishes before growing back without reversing this scenario produces very few reversed directions which are only visible at some latitudes we infer that it is impossible to reach the ratio of reversed to intermediate vgps present in the paleomagnetic records if the excursions were not associated with a short period of reversed dipole field therefore excursions should be regarded as two successive reversals bracketing an aborted polarity interval we propose that the same underlying mechanisms prevail in both situations excursions or reversals and that below a certain strength the field reaches an unstable position which preludes either the achievement of a reversal or its return to the former polarity

dynamics of the laschamp geomagnetic excursion from black sea sediments abstract investigated sediment cores from the southeastern black sea provide a high resolution record from mid latitudes of the laschamp geomagnetic polarity excursion age constraints are provided by 16 ams 14c ages identification of the campanian ignimbrite tephra 39 28 0 11 ka and by detailed tuning of sedimentologic parameters of the black sea sediments to the oxygen isotope record from the greenland ngrip ice core according to the derived age model virtual geomagnetic pole vgp positions during the laschamp excursion persisted in antarctica for an estimated 440 yr making the laschamp excursion a short lived event with fully reversed polarity directions the reversed phase centred at 41 0 ka is associated with a significant field intensity recovery to 20 of the preceding strong field maximum at 50 ka recorded field reversals of the laschamp excursion lasting only an estimated 250 yr are characterized by low relative paleointensities 5 relative to 50 ka the central fully reversed phase of the laschamp excursion is bracketed by vgp excursions to the sargasso sea 41 9 ka and to the labrador sea 39 6 ka paleomagnetic results from the black sea are in excellent agreement with vgp data from the french type locality which facilitates the chronological ordering of the non superposed lavas that crop out at laschamp olby in addition vgps between 34 and 35 ka reach low northerly to equatorial latitudes during a clockwise loop inferred to be the mono lake excursion

precise dating of dansgaard oeschger climate oscillations in western europe from stalagmite data the signature of dansgaard oeschger events millennial scale abrupt climate oscillations during the last glacial period is well established in ice cores and marine records but the effects of such events in continental settings are not as clear and their absolute chronology is uncertain beyond the limit of 14 c dating and annual layer counting for marine records and ice cores respectively here we present carbon and oxygen isotope records from a stalagmite collected in southwest france which have been precisely dated using 234u 230th ratios we find rapid climate oscillations coincident with the established dansgaard oeschger events between 83 000 and 32 000 years ago in both isotope records the oxygen isotope signature is similar to a record from soreq cave israel and deep sea records indicating the large spatial scale of the climate oscillations the signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western europe an important site in human cultural evolution we also find evidence for a long phase of extremely cold climate in southwest france between 61 2 0 6 and 67 4 0 9 kyr ago