uranium isotopic evidence for groundwater chemical evolution and flow patterns in the eastern snake river plain aquifer idaho the isotopic composition and concentration of uranium and strontium in groundwater combined with solute concentration data provide important details regarding groundwater geochemical evolution and flow pathways in the eastern snake river plain aquifer the study was conducted in the vicinity of the idaho national engineering and environmental laboratory ineel idaho which has a long history of storing and disposing of radioactive waste some of which has entered the aquifer uranium concentrations in ineel groundwater range from 0 3 to 3 6 ppb and 234 u 238 u atomic ratios range between 0 000085 and 0 000168 activity ratios of 1 5 to 3 1 all of the samples have natural 235 u 238 u ratios and 236 u was not detected thus the trends delineated by the 234 u 238 u ratios reflect natural variations in the aquifer groundwater nearest the valleys that provides focused recharge to the snake river plain aquifer from the northwest has high 234 u 238 u ratios when compared to values of regional groundwater flowing southwestward in the aquifer mixing of these water masses can account for the intermediate uranium isotope ratios of some of the samples however water rock interaction must also be invoked to account for the observed trends in isotopic data uranium and 87 sr 86 sr isotope ratios are positively correlated and define a trend toward isotope ratios of the aquifer host rock these relations indicate that dissolution and or ion exchange are more important than alpha recoil or selective leaching in controlling 234 u 238 u ratios as a result 234 u 238 u ratios decrease along flow pathways toward the secular equilibrium values of the aquifer host rock uranium and strontium isotopic modification can be explained by incongruent dissolution of the host basalt lateral distributions of 234 u 238 u ratios indicate elongate zones of high 234 u 238 u ratios extending southward from the mouths of birch creek and the little lost river these elongate zones are interpreted as preferential flow paths two isolated pockets of groundwater located in the central and western parts of the study area have lower 234 u 238 u ratios than the adjacent aquifer water both of these zones are interpreted to contain stagnant waters that are relatively isolated from flow in the regional aquifer due to lower permeability physical and chemical evidence strongly suggests that the stagnant zones are dominated by water from the big lost river that infiltrated via flood control ponds spreading areas playas and the riverbed

precise ages of the reunion event and huckleberry ridge excursion episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core the reunion event is one of the earliest recognized periods of normal polarity within the reversed matuyama chron named for the site at which it was first discovered on reunion island it has since purportedly been found globally in both volcanic rocks and sediments and thus has become a key chronostratigraphic marker however geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration but also the number of reunion events new 40ar 39ar ages from eight reunion island lavas in three distinct sections are indistinguishable from one another thereby placing the event at 2 200 0 007 0 010 ma2 200 0 007 0 010 ma 2 2 analytical total uncertainty note this format is used throughout the paper the paleomagnetic behavior recorded at two of the island sites shows that the opposite normal polarity was reached and sustained for a period during which several lava flows were erupted whether this can be classified as a very short subchron bounded by a rapid set of back to back reversals or as a special case of a geomagnetic excursion is unclear hence we choose to continue labeling the dynamo activity recorded by these reunion island lavas as an event this event preceded a 38 kyr 38 kyr period of normal polarity that we name the feni subchron after its locality of discovery at odp site 981 the feni subchron was succeeded by the huckleberry ridge excursion for which 40ar 39ar sanidine and u pb zircon ages of 2 077 0 001 0 003 ma2 077 0 001 0 003 ma and 2 084 0 012 0 013 ma2 084 0 012 0 013 ma respectively from member b of the huckleberry ridge tuff in idaho are in agreement these findings suggest that the full normal polarity recorded on reunion island is a singular brief period of unstable field behavior at the onset of a 125 kyr 125 kyr bundling of dynamo instabilities from 2 20 to 2 07 ma disturbances to the axial dipole component of earth s magnetic field during this period and by analogy similar periods of temporally clustered excursions during the early and late portions of the brunhes chron may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner core or core mantle boundaries a proposition that might be tested by future numerical dynamo simulations

paleomagnetic study of juvenile basaltic andesite clasts from andean pyroclastic density current deposits abstract additional paleomagnetic data are necessary to improve geomagnetic models of secular variation during the holocene especially from the southern hemisphere in most of the andean volcanoes from ecuador to the chilean central volcanic zone very well dated lava flows are rare in contrast andesitic to basaltic pyroclastic density current pdc deposits commonly contain charcoal facilitating their age determination with 14c in this study we present the magnetic properties and the paleomagnetic results obtained from three pdc deposits of basaltic to andesitic composition one is the 2006 pdc deposit from the ecuadorian tungurahua volcano and the two others are well dated pdc deposits from chilean volcanoes osorno and villarica although most paleomagnetic studies in pyroclastic rocks deal with the estimation of emplacement temperatures from bulk deposits or accessory and accidental non juvenile clasts we show that juvenile clasts embedded in pdc deposits provide well grouped paleomagnetic directions indicating post emplacement in situ cooling below curie points moreover the rapid cooling of the juvenile clasts yields an abundance of single domain titanomagnetite grains providing low unblocking temperatures and a reliable material for paleointensity determination

paleomagnetic determination of emplacement temperatures of pyroclastic deposits an under utilized tool paleomagnetic data from lithic clasts collected from mt st helens usa volcan lascar chile volcan de colima mexico and vesuvius italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to highlight the usefulness of the paleomagnetic method for determining emplacement temperatures at mt st helens the temperature of the deposits tdep at three sites from the june 12 1980 eruption was found to be 532 c 509 c and 510 570 c respectively one site emplaced on july 22 1980 was emplaced at 577 c these new paleomagnetic temperatures are in good agreement with previously published direct temperature measurements and paleomagnetic estimates lithic clasts from pyroclastic deposits from the 1993 eruption of lascar were fully remagnetized above the respective curie temperatures which yielded a minimum tdep of 397 c samples were also collected from deposits thought to be pyroclastics from the 1913 2004 and 2005 eruptions of colima at colima the sampled clasts were emplaced cold this is consistent with the sampled clasts being from lahar deposits which are common in the area and illustrates the usefulness of the paleomagnetic method for distinguishing different types of deposit tdep of the lower section of the lithic rich pyroclastic flow lrpf from the 472 a d deposits of vesuvius was 280 340 c this is in agreement with other recently published paleomagnetic measurements in contrast the upper section of the lrpf was emplaced at higher temperatures with tdep 520 c this temperature difference is inferred to be the result of different sources of lithic clasts between the upper and lower sections with the upper section containing a greater proportion of vent derived material that was initially hot our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature estimation

magnetic properties in an ash flow tuff with continuous grain size variation a natural reference for magnetic particle granulometry the tiva canyon tuff contains dispersed nanoscale fe ti oxide grains with a narrow magnetic grain size distribution making it an ideal material in which to identify and study grain size sensitive magnetic behavior in rocks a detailed magnetic characterization was performed on samples from the basal 5 m of the tuff the magnetic materials in this basal section consist primarily of low impurity magnetite in the form of elongated submicron grains exsolved from volcanic glass magnetic properties studied include bulk magnetic susceptibility frequency dependent and temperature dependent magnetic susceptibility anhysteretic remanence acquisition and hysteresis properties the combined data constitute a distinct magnetic signature at each stratigraphic level in the section corresponding to different grain size distributions the inferred magnetic domain state changes progressively upward from superparamagnetic grains near the base to particles with pseudo single domain or metastable single domain characteristics near the top of the sampled section direct observations of magnetic grain size confirm that distinct transitions in room temperature magnetic susceptibility and remanence probably denote the limits of stable single domain behavior in the section these results provide a unique example of grain size dependent magnetic properties in noninteracting particle assemblages over three decades of grain size including close approximations of ideal stoner wohlfarth assemblages and may be considered a useful reference for future rock magnetic studies involving grain size sensitive properties

semi analytical solutions for vertically and laterally loaded piles in multilayered soil deposits in geotechnical practice it is of considerable importance to assess the behavior of vertical lateral coupled loading piles in multilayered soil deposits this study deals with a semi analytical formulation for the performance of a pile suffering from combined vertical and lateral loads the emphasis is on quantifying the mobilization of the subgrade reaction provided by the layered soil stratums in the proposed method subgrade reactions correlated with both the accumulative axial load transfer and side resistances depending upon the pile soil interaction are abstracted as a series of nonlinear springs in both vertical and lateral directions on account of this an alternative transfer matrix method is applied to characterize the pile reaction along the depth under the identified boundary conditions atop the pile meanwhile the condition of static equilibrium between two specific pile segments located at the border of soil layers is also essential on this basis validation of the solution is conducted by comparing with observations from experiments and predictions obtained from other existing methods in addition the influence of properties in shallow soil layer and the vertical load on the lateral response of the pile is also discussed the results indicate a reduction in the lateral displacement and the maximum bending moment within the pile with the increase in the shallow soil stiffness but a growth with the increase in vertical load due to the p effect

facies stratigraphy and relative sea level history upper cretaceous eutaw formation central and eastern alabama abstract the eutaw formation 130 to 400 ft thick rests disconformably on incised valleys a type 1 unconformity occuring at the top of the underlying non marine tuscaloosa formation cenomanian early turonian the eutaw is topped by an erosional discontinuity which has slight relief and a discontinuous conglomeratic lag in the outcrop and shallow subsurface the eutaw formation has four main paralic and nearshore facies which are arranged in two discontinuity bounded genetic packages of facies or parasequences both of which developed in a single eustatic cycle the eutaw facies are carbonaceous and ostried rich clayey silts back barrier planar trough and low angle cross bedded medium fine sands barrier island fossiliferous bioturbated fine sands lower shoreface and calcareous clays silts and sands inner shelf the intraformational facies discontinuity is likely a parasequence boundary developed at maximum eustatic high stand in latest santonian

the kamikatsura event and the matuyama brunhes reversal recorded in lavas from tjornes peninsula northern iceland abstract we report paleomagnetic results from four overlapping stratigraphic sections 55 flows through the lava pile in tjornes peninsula north iceland the initial aim of our study was to check the existence of intermediate directions previously interpreted as belonging to the matuyama brunhes geomagnetic reversal mbr and complete this record with thellier thellier paleointensity determinations and 40ar 39ar radioisotopic dating the directional results corroborate the findings by kristjansson et al 1988 a sequence of reversed transitional normal and transitional polarity r t n t is found in each section the polarity change is characterized by a jump from reversed virtual geomagnetic poles vgps to a quasi cluster of transitional vgps located over china followed by a second jump to a cluster of normal polarity before moving again towards intermediate polarities reversely magnetized flows exhibit a magnetic mineralogy well suited for paleointensity determination of these 25 flows 20 yielded paleointensity estimates of good quality unfortunately the systematic presence of coarse multidomain md titanomagnetite in the transitional and normal polarity flows hampered paleointensity estimates for these flows the virtual dipole moments vdm calculated for the reversed polarity flows vary from 4 9 to 7 0 1022 am2 with an arithmetic mean value of 5 5 0 8 1022 am2 this value is identical to the mean vdm obtained for the 0 3 5 ma time window and thus strengthens the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0 3 ma 40ar 39ar isochron ages were obtained with the incremental heating technique on groundmass separates or phenocryst poor whole rock samples six of the transitionally magnetized lavas yielded isochron ages that are indistinguishable from one another at the 95 confidence level the weighted average age is 862 51 ka in the upper part of this volcanic sequence the normally magnetized flows yielded a weighted average age of 787 40 ka this finding supports the conclusion that a geomagnetic excursion within the matuyama chron is recorded in the lower part of the tjornes volcanic sequence whereas only the latest portion of the mbr is recorded by flows in the upper part of the lava pile this conclusion is supported via comparison of the vgps obtained from the lavas with those predicted by a geomagnetic field model

a model of isotope fractionation in reacting geochemical systems the authors present a numerical technique that predicts how the stable isotopes sup 2 h sup 13 c sup 18 o and sup 34 s fractionate among solvent aqueous species minerals and gases over the course of a geochemical reaction process this model is based on mass balance techniques similar to those already presented in the literature but differs from previous techniques in that it allows minerals to be segregated form isotopic exchange instead of remaining in isotopic equilibrium such an approach allows us to simulate the fractionation of isotopes between rock and fluid resulting solely from mineral dissolution and precipitation the technique was tested by modeling isotopic fractionation during several reaction processes including 1 dolomitization of limestone by a migrating pore fluid 2 diagenetic alteration of the permian lyons sandstone in the denver basin and 3 hydrothermal alteration of the okanagan batholith in southern british columbia the results of calculations in which minerals are segregated from isotopic exchange compare well to isotopic trends observed in nature but differ markedly from calculations that assume isotopic equilibrium 54 refs 4 figs 3 tabs

the nature of magnetic single domains in volcanic glasses of the kbs tuff magnetic and optical variations in volcanic glasses of the east african rift have been investigated by means of experimental and analytical study the results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature origin and significance of fine grained magnetic minerals and on the magnetic petrology of glassy rocks the samples that we have studied in detail glass shards from the kbs tuff of northern kenya contain a superparamagnetic precipitate of microcrystals 2 to 10 nm in size the precipitate is likely to have formed at elevated temperature during or subsequent to eruption about 1 9 m y ago we have studied these microcrystals by means of transmission electron microscopy tem ac and dc magnetic methods electron spin resonance esr mossbauer spectroscopy and optical spectroscopy our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism s responsible for the optical and magnetic behavior of these glasses the particle size determination for the larger of these microcrystals has been achieved using tem imaging and mossbauer spectroscopy the mineralogy is nonstoichiometric magnetite based on mossbauer x ray diffraction and electron diffraction results the ratio of saturation remanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single domain theory for a uniaxial anisotropy the origin of which may lie with stress or surface effects we interpret magnetic and esr results to infer the possible existence of microcrystals or amorphous clusters of fe atoms 1 nm or smaller in kbs glasses that appear homogeneous in high resolution tem images results from optical absorption experiments show that the color variations in kbs shards arise due to optical absorption by the precipitate a result consistent with classical electromagnetic theory

improvements in procedure and paleointensity selection criteria picrit 03 for thellier and thellier determinations application to hawaiian basaltic long cores abstract in this paper we first describe the paleointensity furnaces and the procedures specifically designed to minimize oxidation of the samples during thellier and thellier paleointensity experiments routinely used at gif we also propose some improvements to the set of criteria commonly used to select the results of the thellier experiments in order to further increase the reliability of paleointensity determination finally to be included in databases the new set of paleointensity criteria picrit 03 paleointensity criteria in 2003 is discussed and the results obtained from two long hawaiian basaltic cores the scientific observation hole 4 soh 4 and the hawaiian scientific drilling project 1 hsdp1 are used to illustrate its effectiveness some of the results are re evaluated and a new composite record of absolute paleointensity and inclination spanning 420 kyr is presented the inclination values obtained from the two cores combined support the hypothesis of persistent shallow inclinations at hawaii in intensity the upper 100 kyr are covered by the data from soh 4 with 309 reliable determinations out of 766 examined obtained from 143 units which belong to the kilauea volcano at the top of this core the 341 reliable data covering the 100 420 kyr interval are from 149 units of core hsdp1 belonging to the mauna kea volcano the interval 320 to 420 kyr is particularly detailed because of high extrusion rate of the volcano the use of more stringent criteria decreases the number of determinations considered as reliable in previous publications but it improves their accuracy and robustness

a reliable absolute palaeointensity determination obtained from a non ideal recorder reliable measurements of the intensity of the palaeomagnetic field are notoriously difficult to obtain the approach generally taken is to produce multiple estimates per rock unit and assume that those which meet certain minimum standards of technical quality will combine to produce an accurate mean however here using results produced from a package of 20th century basaltic lava flows from mount etna in sicily we demonstrate that this approach can fail in this case the application of typical sets of selection criteria actually introduces bias into the mean determination that we measure we demonstrate that this is caused by two types of non ideal behaviour acting in combination the first is a result of the multidomain grains that the samples contain and the second is caused by differences in the natural and laboratory cooling rates we discuss means of avoiding these sources of error in future palaeointensity studies performed on ancient rocks we also develop a new more general reliability criterion which is effective here and which we argue should be applied wherever possible in future palaeointensity studies in conjunction with standard criteria it requires two distinct types of materials which both produce some good quality palaeointensity measurements and uses their range of overlap to constrain the true palaeointensity both the application of this criterion and general reliability considerations require that future palaeointensity studies should measure many samples per cooling unit and that these should be as diverse in terms of their rock magnetic properties and cooling histories as possible

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

paleointensity on volcanic glass of varying hydration states abstract we have characterised the magnetic properties of variably hydrated volcanic glasses that were collected from rhyolitic deposits at blahnukur torfajokull iceland the glasses span the range from fresh obsidians to highly fractured perlites that contain 2 wt water lava hydration plays a key role in the formation of perlite and hence these rocks are ideal to study hydration effects on remanence carriers and reliability of the paleomagnetic record the total volatile content of the different samples was determined as a proxy for the degree of perlitisation hydration it was found that coercivity of remanence saturation magnetisation and saturation of remanence decrease with increasing hydration i e that magnetic remanence carriers get lost and that magnetic stability is reduced additionally thermal demagnetisation of a three component isothermal remanence revealed that mainly the high coercive material is destroyed within the more strongly hydrated samples while lower coercive material seems to be less affected grain sizes of all but one samples are in the pseudo single domain range the one exception shows multi domain characteristics it was impossible to unambiguously identify the remanence carriers but titanomagnetites are most likely responsible for the lower coercivity component while hemoilmenites possibly represent the higher one a modified thellier method was used to determine paleointensity values as most of the samples are hydrated it is not astonishing that the overall paleointensity data is not of very high quality however it is important to note that there are hydrated samples with well defined arai diagrams although seemingly of high quality these paleointensity values are incorrect as there is a trend towards lower paleointensity values observed with increasing perlitisation we attempted to test for magnetic anisotropy and cooling rate dependency but this was hampered by alteration during the experiments however we argue that both anisotropy and cooling rate dependency are unlikely to be responsible for the observed trend in paleointensity thus even well defined paleointensity values can be erroneous when obtained from hydrated glass this emphasises the need for unaltered samples and additional attention during paleointensity determinations

single domain chemical thermochemical and thermal remanences in a basaltic rock summary tiny basaltic samples containing finely grained titanomagnetite with curie temperature less than 100 c were heated in air in weak field 25 100 t at temperatures between 400 c and 560 c for times as long as 32 hr oxyexsolution of titanomagnetite resulted in the crystallization of interacting single domain particles with curie point close to 540 c and the concomitant development of one of two types chemical remanence depending upon thermal treatment isothermal chemical remanence crm or thermochemical remanence tcrm the latter acquired under the combined effects of chemical change and temperature decrease crm and tcrm acquired under various conditions were subjected to thellier type experiments all these treatments were carried out using a vibrating sample thermomagnetometer allowing the continuous recording of magnetization and a very precise temperature control all crm trm and tcrm trm plots were found to be linear over almost the entire trm blocking temperature range whether ptrm checks are positive or not an apparent strength of the acquisition field of crm or tcrm could thus be obtained and divided by the actually applied field to obtain a ratio r which is representative of the crm trm or tcrm trm ratios over most of the unblocking blocking temperature spectrum for crm it is found that r is less than 1 and increases rapidly with acquisition temperature 0 36 0 07 at 400 c 0 66 0 02 at 450 c and 0 90 0 02 at 500 c in qualitative agreement with expressions derived from the theory of non interacting single domain grains thus very large underestimate of geomagnetic field palaeostrength can occur when a natural crm is not recognized as such and is believed to be a natural trm palaeointensity data obtained from geological material prone to the development of secondary minerals as for example baked contacts and volcanic glasses have therefore to be considered with caution in an attempt to mimic deuteric oxyexsolution in cooling magma tcrm was imparted from 560 c to 400 c at a cooling rate of 0 1 c min in contrast to the crm case r is found to slightly exceed 1 1 11 0 05 according to theoretical considerations this finding suggests that in practice tcrm approximates to trm

discrete single domain and pseudo single domain titanomagnetite particles in silicic glass of an ash flow tuff single domain and pseudo single domain titanium poor magnetite grains are present in silicic vitrophyre glass of an ash flow tuff from western nevada conventional transmission electron microscopy and analytical electron microscopy corroborate inferences made solely from rock magnetic and remanence data that the thermoremanent magnetization in the vitrophyre is carried by such grains the magnetite grains vary in size shape and distribution throughout the glass they probably crystal lized during violent eruption and welding of the ash flow at temperatures well in excess of 600 c

rapid magma evolution constrained by zircon petrochronology and 40ar 39ar sanidine ages for the huckleberry ridge tuff yellowstone usa understanding the time scales of magmatic differentiation storage and eruption of large volume silicic magmas is a primary goal of igneous petrology within the huckleberry ridge tuff hrt idaho usa representing the earliest and largest caldera forming eruption associated with yellowstone volcanic activity zircon morphological zoning patterns coupled to strongly correlated changes in ti in zircon thermometry and trace element indicators of progressive differentiation provide a proxy record for the evolution of the hrt member b magma body tandem in situ and isotope dilution u pb dating of single zircon crystals demonstrates an absence of pre pleistocene xenocrysts but reveals the presence of antecrysts recycled from pre caldera rhyolites in the hrt magma the petrochronologic interpretation of autocrystic zircon thermal chemical and temporal characteristics suggests that hrt member b differentiated over 10 k y prior to eruption at 2 0794 0 0046 ma as defined by new astronomically calibrated single crystal total fusion 40 ar 39 ar sanidine analyses this refined eruption age demonstrates that the transitional polarity preserved by hrt member b does not record the reunion subchron but rather a separate younger geomagnetic event our novel approach places the thermal and chemical regime of silicic magmas within a temporal context and demonstrates the rapid evolution of a large volume of silicic magma

paleointensities for 10 22 ka from volcanic rocks in japan and new zealand abstract eight volcanic rocks from japan and new zealand in the age range 10 50 ka b p were studied using the thellier method paleointensity experiment and yielded five successful results for 10 22 ka samples from the shikotsu kuttara and daisen volcanoes in japan were taken from pyroclastic flows both welded and non welded and a pumice fall deposit one of the two new zealand rocks is a rhyolite lava from the taupo volcanic zone four sites out of seven located on rocks that are not usually subjected to thellier s experiment yielded successful results which indicates that volcanic products such as pyroclastic flows and rhyolite lavas are as good a material as basalt and andesite lavas for application of the paleointensity experiment one paleointensity from new zealand at 9 9 ka is very large at about 100 t and provides support for the paleointensity high found in europe and japan 1 2 the other four paleointensities for 14 22 ka are smaller than the present day value and this agrees with the idea of a broad paleointensity minimum for the period 10 50 ka that was suggested by mcelhinny and senanayake 3 it is also noted that the variation curve of absolute paleointensity from the volcanic rocks is reasonably similar to the relative paleointensity curves from marine and lake sediments which were summarized by tauxe 4

magnetic grain size variations through an ash flow sheet influence on magnetic properties and implications for cooling history rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks provide a basis for interpretation of aeromagnetic data over volcanic terranes and yield insights into the depositional and cooling histories of ash flow sheets a rhyolitic ash flow sheet the miocene aged tiva canyon member of the paintbrush tuff contains both titanomagnetite phenocrysts present in the magma prior to eruption and cubic fe oxide microcrystals that grew after emplacement systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the nevada test site natural remanent magnetization varies from less than 1 10 4 to more than 8 10 4 a m3 kg 1 and in phase magnetic susceptibility varies from less than 1 10 6 to more than 10 10 6 m3 kg 1 the microcrystals which include both magnetite and maghemite have curie points and maximum unblocking temperatures between 580 c and 640 c rock magnetic data including in phase and quadrature magnetic susceptibilities as well as hysteresis parameters demonstrate that these microcrystals are of superparamagnetic and single domain sizes titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15 m thick zones at the top and bottom within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet the decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break possibly represented by a concentration of pumice laboratory heating produces changes in magnetic properties which vary complexly from sample to sample such changes include 1 decrease in saturation magnetization 2 both increase and decrease in in phase and quadrature magnetic susceptibilities and 3 shifts in coercivity spectra the complex sample to sample variation in response to heating is attributed to differences in initial grain size distributions

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

thermochemical remanence acquisition in single domain particle ensembles a case for possible overestimation of the geomagnetic paleointensity 1 single domain sd particle ensembles are the best studied systems in rock magnetism yet even for this simple system a complete theoretical treatment of the acquisition of thermochemical remanent magnetization tcrm is missing on the basis of the sd theory of chemical remanent magnetization it is generally taken for granted that tcrm acquisition must be less efficient than acquisition of a thermoremanent magnetization trm and that consequently the paleofield intensity is inevitably underestimated when a tcrm is interpreted as a trm here it is shown that this conclusion holds only when a continuous chemical change e g grain growth takes place during remanence blocking in cases where an initial trm at a later time and at lower temperature is modified by chemical processes the final tcrm can result in considerable paleofield overestimation when interpreted as a trm this is demonstrated for two different scenarios first partial grain dissolution after trm acquisition and second low temperature oxidation these mechanisms are not unlikely to occur in nature and numerical experiments indicate that both would lead to perfectly straight arai plots which makes such sd tcrms indistinguishable from true trms in the thellier experiment

toward an optimal geomagnetic field intensity determination technique 1 paleointensity determinations based on double heating techniques in field zero field cooling zero field in field cooling and two in field steps with opposite laboratory fields are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates to investigate this premise we have developed a simple mathematical model we find that both the zero field first and in field first methods have a strong angular dependence on the laboratory field parallel orthogonal and anti parallel while the two in field steps method is independent of the direction of the laboratory produced field contrary to common practice each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met even with marginal 10 tails of partial thermoremanence our calculations suggest that the zero field first method with the laboratory produced field anti parallel to the natural remanence nrm is the most robust paleointensity determination technique when the intensity of the lab induced field is smaller than ancient field however the zero field first method with the laboratory field parallel to the nrm is the optimum approach when the intensity of the lab induced field is larger than the ancient field by far the best approach however is to alternatethe infield zerofield iz steps with zerofield infield zi steps

paleointensity of the earth s magnetic field during the laschamp excursion and its geomagnetic implications the reversed paleomagnetic direction of the laschamp and olby flows represents a specific feature of the geomagnetic field this is supported by paleomagnetic evidence showing that the same anomalous direction was recorded at several distinct sites including scoria of the laschamp volcano to examine this anomalous geomagnetic fluctuation we studied the paleointensity of the laschamp and olby flows using the thellier method twenty five samples were selected for the paleointensity experiments and from seven we obtained reliable results because the paleointensity results of the olby and laschamp flows as well as laschamp scoria are very similar they can be represented by a single mean paleointensity f 7 7 t considering that this low paleointensity is less than 1 6 of the present geomagnetic field and is more characteristic of transitional behavior our results suggest that the paleomagnetic directions of the laschamp and olby flows were not acquired during a stable reversed polarity interval a more likely explanation is that the laschamp excursion represents an unsuccessful or aborted reversal

displacement ductility capacity assessment for a fixed head pile in cohesionless soil abstractthis study develops assessment formulas for the displacement ductility capacity of a fixed head pile in cohesionless soils the derivations of the formulas are based on an ideal model of a semiinfinite nonlinear pile in a gibson soil whose subgrade reaction modulus increases linearly with depth to link the displacement ductility capacity of the pile with the pile soil system parameters the parameters in the formulas include the sectional overstrength ratio and curvature ductility capacity as well as a modification factor for considering soil nonlinearity the modification factor is a function of the displacement ratio of the pile s ultimate displacement to the effective soil yield displacement which is constructed through a number of numerical pushover analyses the format of the formulas is similar to that for a pile in cohesive soils for the case of linear soil the formulas provide a lower bound estimate of pile ductility capacity the displacement ductility capacity of a pile in cohesion

geomagnetic paleointensity deduced for the last 300 kyr from unzen volcano japan and the dipolar nature of the iceland basin excursion abstract we have conducted paleointensity measurements of lavas and pyroclastic flows from unzen volcano japan they have been shown in a previous study to contain a record of brunhes chron secular variation with one flow recording a transitional direction senbongi geomagnetic excursion 191 17 ka various rock magnetic analyses indicate that the main magnetic carriers of samples are ti poor titanomagnetites which have mixtures of single domain sd and or pseudo sd and multi domain grain sizes the low temperature demagnetization ltd double heating test dht shaw method yielded 209 successful results from 56 sites giving 27 acceptable site mean paleointensities for the past 5 300 kyr when excluding the senbongi excursion data and two other data points the data give an average virtual dipole moment vdm of 7 69 10 22 a m 2 with a standard deviation of 2 51 10 22 a m 2 n 24 this is consistent with the selected 80 site mean thellier paleointensity data from the latest paleointensity database with an average vdm of 7 64 10 22 a m 2 and a standard deviation of 2 56 10 22 a m 2 paleointensity measurements on samples that record the senbongi excursion which is claimed to correlate with the iceland basin excursion provide a value of 7 74 9 37 t using the ltd dht shaw method this corresponds to an average standard deviation vdm of 2 19 10 22 a m 2 0 21 10 22 a m 2 when calibrating sedimentary records that capture the iceland basin excursion by averaging the ltd dht shaw vdms obtained from the pre and post excursional period the records give consistent vdm values with the volcanic senbongi record during the excursion this indicates that variation of the geomagnetic field direction and intensity during the iceland basin excursion can be well represented by a simple dipolar variation

some aspects of magnetic viscosity in subaerial and submarine volcanic rocks summary the magnetic viscosity of 334 upper tertiary and pre bruhnes quaternary volcanic rocks from the massif central france and the steens mountain oregon and of 40 basaltic cores from dsdp leg 37 has been investigated thellier s viscosity index follows a log normal distribution with mean values equal to 6 and 3 5 per cent for subaerial and submarine rocks respectively for subaerial rocks the average intensity of the viscous remanent magnetization vrm acquired in the earth s field since the beginning of the bruhnes polarity epoch t 0 7 myr is estimated to be equal to one quarter of the average intensity of the primary remanence alternating field demagnetization of vrms acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t neel s diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard vrm is carried by single domain grains and soft vrm carried by multidomain particles single domain particles carrying hard vrm in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold resulting from high temperature oxidation of titanomagnetite soft vrm is carried by low curie point homogeneous titanomagnetite unlike his single domain theory neel s multidomain theory of magnetic viscosity does not account quantitatively for the resistance of vrm to alternating fields

palaeointensity study of quaternary east eifel phonolitic rocks germany summary compared to the other volcanic rocks of the quaternary east eifel volcanic field germany the phonolites stand out because of very low susceptibilities and intensities of natural remanent magnetization geomagnetic palaeointensities have been studied by means of the thellier method for seven of these rock units while five of them are lava flows or domes the other two have been deposited at temperatures of about 500 c as zeolitized tuffs results of rock magnetic investigations indicate that the magnetic carriers are ti poor titanomagnetites or magnetite heating above 200 300 c often leads to an alteration of the magnetic carriers of the rocks in the low temperature range well defined lineari segments in the succession of the thellier experiment data points are found for most of the lava samples as well as for the zeolitized tuffs because of inconsistencies of within unit results and the presence of viscous magnetization components the palaeointensities of only two rock units are regarded as reliable the obtained values are about 30pt as compared to the present day field of 48 pt

assessment of the usefulness of lithic clasts from pyroclastic deposits for paleointensity determination 1 paleomagnetic and rock magnetic measurements were carried out on lithic clasts found within pyroclastic deposits to assess their potential for paleointensity determinations the use of multiple lithologies in a single paleointensity determination would provide confidence that the result is not biased by alteration within one lithology lithic clasts were sampled from three historically active volcanoes lascar in the chilean andes mt st helens united states and vesuvius italy at lascar triple heating paleointensity experiments allow development of new selection criteria for lithic clasts found within pyroclastic deposits using these criteria the lascar data yield a mean paleointensity of 24 3 1 3 t 1 n 26 which agrees well with the expected value of 24 0 t this indicates that pyroclastic rocks have promise for paleointensity determinations pyroclastics however still suffer from the range of problems associated with conventional paleointensity experiments on lava flows samples from mt st helens are strongly affected by multidomain md behavior which results in all samples failing to pass the paleointensity selection criteria at vesuvius md grains magnetic interactions and chemical remanent magnetizations contributed to failure of all paleointensity experiments rock magnetic analyses allow identification of the causes of failure of the paleointensity experiments however in this study they have not provided adequate preselection criteria for identifying pyroclastics that are suitable for paleointensity determination

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

geochemistry and mineralogy of fumarolic deposits valley of ten thousand smokes alaska bulk chemical and mineralogical evolution of dacite rich protolith assrracr samples from a fossil fumarole originating in the 1912 ash flow tuffin the valley of ten thousand smokes have been analyzed to ascertain chemical changes resulting from high temperature fumarolic alteration and subsequent cooling and weathering of the protolith samples of the underlying dominantly leached dacite rich portion of the ash flow tuff adjacent to the fumarolic conduit and samples of encrusted fallout from the shallow part of the fossil fumarole were interpreted using the isocon method of grant 1986 the results show that relative to unaltered l9l2 dacite chosen as a standard composition for the protolith in this fossil fumarole mass was conserved during the alteration reactions for most of the system but mass gains of l4 2do o were determined for three samples in the leached ash flow tuff relative to unaltered dacite protolith significant enrichments occurred in sor loi hro cl f zn pb cu sn cr ni as sb au br in various parts of the fossil fumarole some of these were during the high temperature part of the alteration and some were during cooling processes when acid alteration becomes prominent the rees indicate some depletion in highly altered samples relative to dacite protolith and differential mobility of eu2 relative to trivalent rees this is manifested by positive eu anomalies in ree patterns normalized against ree in the dacite protolith mineral phases introduced in the alteration assemblages include alunite reflecting high so activity hydrated aluminum hydroxy fluoride a ralstonite like phase and fluorite reflecting high f activity smectite magnetite hematite and goethite reflecting oxidation and hydration reactions opal and a portion of the a cristobalite reflect sio mobility however the abundance of a cristobalite is formed from pumice leached during hightemperature vapor phase processes and devitrification of the altered glass

observing thermomagnetic stability of nonideal magnetite particles good paleomagnetic recorders the thermomagnetic behavior of remanence induced magnetite fe3o4 particles in the pseudo single domain psd size range 0 1 10 m which dominate the magnetic signature of many rock lithologies is investigated using off axis electron holography construction of magnetic induction maps allowed for the visualization of the vortex domain state in an individual fe3o4 grain 200 nm in diameter as a function of temperature acquisition of a series of electron holograms at 100 c intervals during in situ heating up to 700 c demonstrates the vortex state of the fe3o4 grain in this instance remains thermally stable close to its unblocking temperature and exhibits a similar in plane remanent state upon cooling i e the particle is effectively behaving like a uniaxial single domain particle to temperatures near tc such particles are thought to be robust magnetic recorders it is suggested that evidence for psd behavior should therefore not preclude paleomagnetic investigation

geochemistry of groundwater in cretaceous sediments of the southeastern coastal plain of eastern mississippi and western alabama geochemical samples of waters along two hydrologic flow paths in four upper cretaceous aquifers of northeastern mississippi and western alabama indicate similar geochemical evolution of their respective waters the waters of the coker gordo and eutaw mcshan aquifers noncalcareous sands increase downgradient in dissolved solids and ph and are dominated by sodium and bicarbonate ions which generally result from a calcite dissolution cation exchange process increases in dissolved iron from oxidation reduction reactions followed by decreases in total inorganic carbon from siderite precipitation occur along the flow paths as the total inorganic carbon increases carbon 13 13c generally is enriched in the moving waters indicating the addition of a predominantly heavy source of carbon most likely dissolving calcite in the coker aquifer 13c values in the waters become more negative downgradient resulting from lignite oxidation followed by 13c values becoming more positive resulting from dissolving calcite and perhaps some mixing with brines in northeastern mississippi the ripley aquifer a calcareous sand initially contains calcium bicarbonate dominated water that evolves to a sodium bicarbonate dominated water downgradient primarily from the calcite dissolution cation exchange process feldspar hydrolysis to kaolinite dominates aluminosilicate reactions in the upgradient parts of the aquifers authigenesis of smectite clay may be occurring in the deeper downgradient parts of the aquifers

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

chlorine 36 dating of very old groundwater 1 the great artesian basin australia chlorine 36 has many advantages as a dating tool for very old groundwater these advantages include a suitable half life 3 01 x 10 sup 5 years simple geochemistry conservative behavior in groundwater and a general absence of subsurface sources at levels comparable to the atmospheric input recent advances in tandem accelerator mass spectrometry have permitted the analysis of sup 36 cl at low abundance expected following residence in the subsurface for 10 sup 6 years or more in order to test the suitability of sup 36 cl for dating very old groundwater the sup 36 cl cl ratios of 26 groundwater samples from the great artesian basin of australia have been measured groundwater ages calculated from the sup 36 cl data compare favorably with ages computed independently from hydrodynamic simulations

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

paleointensity measurements of pyroclastic flow deposits co born with widespread tephras in kyushu island japan we report the results of paleointensity determination with welded pyroclastic flow deposits co born with widespread tephras derived from kikai k ah 7 3 ka aira at 28 ka and aso aso 1 270 ka aso 2 140 ka aso 3 120 ka aso 4 90 ka calderas in kyushu island japan since the widespread tephras are commonly used as good key layers their ages and oxygen isotope stages have been well established thermal demagnetization and several rock magnetic experiments showed that the pyroclastic flow samples had stable remanent magnetizations and are suitable for paleointensity measurements using thelliers method we obtained reliable paleointensities from six eruptive units and calculated corresponding virtual dipole moment vdm values ranging from 2 7 to 9 7 10 22 am 2 vdms for the youngest units k ah and at are consistent with the global trend of increasing dipole moment for the past 40 kyr aso 4 3 and 1 yield moderate vdms similar to the mean paleointensity for the past 1 myr and especially the results from aso 4 and 3 show good agreement with the vdm variation between 70 and 130 ka obtained from la reunion island j geophys res 96 1991 1981 earth planet sci lett 140 1996 173 a low vdm value and an abnormal vgp position obtained from aso 2 suggest a possibility that aso 2 records an excursion like event occurred around 140 ka between blake and jamaica events 2002 elsevier science b v all rights reserved

curie temperatures of titanomagnetite in ignimbrites effects of emplacement temperatures cooling rates exsolution and cation ordering pumices ashes and tuffs from mt st helens and from novarupta contain two principal forms of titanomagnetite homogeneous grains with curie temperatures in the range 350 500 c and oxyexsolved grains with similar bulk composition containing ilmenite lamellae and having curie temperatures above 500 c thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the curie temperature of the homogeneous phase the exsolved grains are most common where emplacement temperatures exceeded 600 c and in laboratory experiments heating to over 600 c in air causes the homogeneous titanomagnetites to oxyexsolve rapidly where emplacement temperatures were lower curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales and thermomagnetic curves are generally irreversible with lower curie temperatures measured during cooling but little or no change is observed in room temperature susceptibility we interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites although we cannot conclusively rule out an alternative interpretation involving fine scale subsolvus unmixing short range ordering within the octahedral sites may play a key role in the observed phenomena changes in the curie temperature have important implications for the acquisition stabilization and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites

magnetism and magnetic mineralogy of ash flow tuffs from yucca mountain nevada the magnetic susceptibility and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement spatial variations in these properties therefore provide a potentially useful tool for geological studies maxima and minima in measured along profiles at yucca mountain nevada identify persistent magnetic marker horizons within vitric portions of the tiva canyon and underlying topopah spring members of the paintbrush tuff the correlatable variations of with vertical position in these units exist over the 8 km of strike examined the observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of fe ti oxide phenocrysts and the presence shape size and mineralogy of magnetic fe oxide microcrystals that precipitated at high temperature after emplacement of each sheet vitrophyre near the top of the topopah spring member has a distinctive maximum in due to titanomagnetite phenocrysts susceptibility in the lower portion of the overlying tiva canyon member reaches maximum values of about 1 3 10 2 si near the base of the member beneath the columnarly jointed vitrophyre due to the presence of abundant superparamagnetic cubic fe oxide these elongate microcrystals are several tens of nm long and are typically deformed and tapered up section in the tiva canyon member the susceptibility decreases going to a minimum within the columnar vitrophyre at this stratigraphic level the precipitated fe oxides are larger and carry intense nrm of up to 30 a m is only 3 10 3 the whisker like microcrystals of fe oxide which typically nucleated on what appears to be chain silicate grew as needles up to 25 m in length the size variations of the precipitated fe oxides which were established using transmission electron microscopy tem and petrographic observation are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization several interpretations of the shape anisotropy of the precipitated feoxide are possible including growth by a dislocation mechanism additionally the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field susceptibility variations as established at the outcrop as well as in the borehole offer a potentially useful tool for stratigraphic correlation of ash flow sheets

reactive transport of trace elements and isotopes in the eutaw coastal plain aquifer alabama 1 we integrate groundwater geochemistry mineralogy and numerical modeling techniques to study the reactive transport of heavy metals and isotopes in the eutaw coastal plain aquifer alabama geochemical data show that the elevated concentrations of fe mn and sr can be correlated with high ph and alkalinity these geochemical correlations suggest that that elevated metal concentrations may be derived from bacterial iron and manganese reduction geochemical modeling of bacterial fe iii and mn iv reduction shows that the biotransformation of iron and manganese minerals could control mobility and concentrations of fe and mn in coastal plain aquifers petrographic sem and edax studies of sediments show the formation of biogenic siderite rhodochrosite and pyrite in sediments associated with fe and mn rich groundwater rhodochrosite and siderite occur together as spheroids 1 0 mm in which rhodochrosite forms the center and siderite forms an outer rind which is consistent with the redox sequence of mineral precipitation predicted by the geochemical modeling the low 13c ratios of siderite and rhodochrosite 14 4 to 15 4 pdb and groundwater dic 20 6 to 14 1 pdb imply carbon of biogenic origin higher dic 13c levels are found to be correlated with elevated fe and mn concentrations and high ph values of groundwater this unexpected result implies novel carbon isotopic fractionation processes associated with bacterial fe iii reduction we used 36cl cl ratios of groundwater and isotope transport modeling to calculate the residence time of regional groundwater in the eutaw aquifer the calculation considering the natural decay only would yield 36cl levels that are significantly higher than field data suggesting that a significant mixing with cl rich older groundwater at depth is an important reason for substantial 36cl depletion along flow path

absolute geomagnetic paleointensity after the cretaceous normal superchron and just prior to the cretaceous tertiary transition 1 we report a detailed rock magnetic and thellier paleointensity study from autlan volcanic succession in western mexico the whole rock k ar age obtained for these lavas 67 4 1 2 ma together with the magnetostratigraphy results indicates that the lava flows were erupted during chron 30n just prior to cretaceous tertiary transition thirty nine samples from eight cooling units were preselected for paleointensity experiments on the basis of their low viscosity index stable remanent magnetization and close to reversible continuous thermomagnetic curves high quality determinations were obtained for 18 individual samples from four independent lava flows the mean virtual dipole moment vdm obtained in this study is 4 9 0 6 x 10 22 a m 2 which is significantly lower than the present geomagnetic field strength but in reasonably good accord with other comparable quality determinations between 35 and 80 ma our data are not significantly different from almost all published paleointensities between 5 and 90 ma moreover rigorously selected worldwide paleointensity data in the same interval better fit trend 2 of heller et al 2002 a careful examination of high quality paleointensity data from cretaceous indicates that there is no simple relation between magnitude of earth s magnetic field and reversal rate

nature and origin of authigenic rhodochrosite and siderite from the paleozoic aquifer northeast mississippi u s a abstract botryoidal rhodochrosite siderite and pyrite are the principal authigenic phases in fractured chert comprising the paleozoic aquifer of northeast mississippi locally alternating bands of near end member composition rhodochrosite and siderite occur in veinlets up to 2 mm wide in the fractured chert carbon isotope data for the carbonates 13c values ranging from 9 6 to 11 3 pdb indicate that both inorganic and organic c were incorporated into the authigenic phases the paleozoic aquifer is hydraulically connected with the overlying upper cretaceous coastal plain strata groundwater in the coastal plain units is undersaturated with respect to calcite and has relatively high fe and mn contents the latter apparently caused by the dissolution of fe mn oxyhydroxides the natural downward recharge of these waters into the paleozoic aquifer causes calcite dissolution in limestone at the base of the chert which in turn leads to the precipitation of rhodochrosite and siderite additionally pyrite is precipitated as so4 reduction occurs in the same zone rhodochrosite has not been previously documented as an authigenic phase in a fresh water aquifer system suggesting that the geochemical environment necessary for its precipitation is not common the alternating bands of rhodochrosite and siderite observed in this study coupled with available thermodynamic data indicate that the groundwater a mn2 afe2 ratio must have fluctuated over time variations in the ratio are best explained by variable eh conditions at the site where fe mn oxyhydroxide dissolution occurs

thermoremanent behavior in synthetic samples containing natural oxyexsolved titanomagnetite understanding earth s geodynamo provides us a window into the evolution of the earth s core which requires accurate data about how its strength varies with time classic thellier style paleointensity experiments assume that studied specimens contain only noninteracting single domain sd magnetic particles interacting grains commonly occur in volcanic rocks but are generally assumed to behave like equivalently sized sd grains multidomain md grains can cause erroneous pi estimates or cause thellier style experiments to fail entirely synthetic specimens containing naturally formed magnetite with md grains and oxyexsolved titanomagnetite closely packed sd grains were subjected to various partial thermoremanent magnetization ptrm experiments which tested nonideal behavior as a function of ptrm acquisition and loss inequality thermal history and repeated heating steps for all grain sizes and domain states ptrmc heating and cooling in a nonzero field gives larger values compared to ptrmb heating in a zero field and cooling in a nonzero field by 5 5 oxyexsolved grains appear prone to the same concave up nonideal arai plots commonly observed in md specimens which also has potential implications for the multiple specimen domain state corrected protocol repeated heatings cause additive deviations from ideality with relatively small impacts on arai plot curvature for both grain types experiments with higher initial demagnetization temperatures had lower curvatures with the most sd like behavior occurring in the uppermost 20 urn x wiley 15252027 media ggge21571 ggge21571 math 0001c of the un blocking temperature range samples containing mixtures of magnetic domain sizes are likely to behave less ideally at lower temperatures but become more ideal with increasing temperature as the nonideal grains unblock

depth of the ash flow deposit in the valley of ten thousand smokes katmai national park alaska in anticipation of a drill hole seismic refraction and gravity profiles acquired more than 20 years ago have been reinterpreted in light of new density data to determine the thickness of the ash flow deposit in the valley of the thousand smokes vtts the data show the ash flow is at least 170m thick and most likely welded in its lower portion its average density is 1800 kg m 3 seismic velocities range from 0 6 to 1 km sec for the deposit s unwelded 30 to 50m thick top section and 1 8 to 2 5 km sec for the lower welded section the data confirm the first realistic estimate of the volume of the ash flow of about 11 km by curtis 1968 based on a geomorphic analysis of stream profiles

testing determinations of absolute paleointensity from the 1955 and 1960 hawaiian flows abstract a new exhaustive study of the flows erupted in 1955 and 1960 from the kilauea volcano big island of hawaii has been conducted in order to constrain the dispersion inherent to the data of absolute paleointensity reported so far and ultimately to determine what are the most critical parameters to obtain suitable determinations the mean directions derived from both flows agree with the expected field within less than 2 paleointensity experiments have been performed using a double heating thellier procedure completed by systematic ptrm checks almost all samples display a linear nrm trm straight line but only 20 of the samples have provided suitable results less than 10 away from the expected field in the other cases the deviation from the expected paleointensity is related to the quality of the ptrm checks which thus provides a threshold value for acceptance of ptrm checks all successful determinations less than 10 away from the expected value have a maximum drat selkin and tauxe 2000 that never exceeds 5 while the mean value of all deviations is lower than 3 5 another interesting feature is the existence of a link between the unblocking temperature spectra and the accuracy of the paleofield which points out the origin of difficulties in the presence of a wide distribution of grain sizes and or the existence of several magnetic phases the results also indicate that magnetite with a narrow range of high unblocking temperatures and little variability in grain size is a favorable situation to obtain suitable determinations with a high success rate these observations may well explain the dispersion inherent to the previous studies of the hawaiian flows

palaeomagnetism of the upper volcanic supergroup southern part of the sierra madre occidental mexico ash flow tuffs or ignimbrites have been recently proposed to be a good material for palaeointensity determination in this paper we present a multidisciplinary study combining geochronology petrology rock magnetism and palaeomagnetism carried out on oligocene to early miocene ignimbrites and related flows from the southern part of the sierra madre occidental two new 40ar 39ar ages were determined for ignimbrites 20 4 0 2 ma the youngest age obtained so far in this area and 29 2 0 5 ma density measurements as a proxy for welding proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites after alternating field and thermal demagnetizations the mean palaeomagnetic pole lat 66 8 n long 180 5 e kappa 142 a95 6 3 calculated for the period 28 31 ma is in close agreement with our only miocene determination comparison with the north america synthetic apparent polar path indicates a net counter clockwise vertical axis rotation of about 10 4 compared to stable north america which occurred likely during the last extensional episode in the late miocene ca 12 9 ma palaeointensity estimates obtained with the thellier coe method are mainly questionable and should not be used for global interpretation therefore these ignimbrites are not a viable material for reliable palaeointensity determinations

simple estimation for ductility capacity of a fixed head pile in cohesive soils displacement ductility capacity of a pile is an important concern when its ductile behavior is taken as a mechanism of energy absorption in seismic design existing methods to assess the ductility capacity of a pile have not considered all major influencing factors such as the overstrength ratio of the pile section this study uses a simple ideal model to link the displacement ductility capacity with parameters in the pile soil system for a fixed head pile in cohesive soils based on the simple model an expression about the displacement ductility capacity is firstly derived for a pile in linear soils and then modified to consider the possible softening of subgrade stiffness for a pile in nonlinear soils through reduction of the expressions derived a set of simple equations are proposed to estimate the ductility capacity of a pile in linear and nonlinear soils for a pile in linear soils its ductility capacity can be computed only from the sectional property of the pile for a pile in nonlinear soils

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

18 o 16 o studies of fossil fissure fumaroles from the valley of ten thousand smokes alaska at three sample sites where there are good exposures of the upper 15 m of the 1912 ash flow sheet in the valley of ten thousand smokes vtts alaska 18o 16o studies indicate that fumarolic activity produced a very wide range of 18o values 0 1 to 12 6 n 32 in the groundmass adjacent to fossil fissure fumaroles this contrasts sharply with the uniformity of 18o in the groundmass away from fumarolic conduits 18o 5 9 to 7 1 n 7 and in all of the feldspar phenocrysts 18o 6 11 to 7 5 1 for 11 samples from this study and hildreth 1987 independent of whether these were collected from fossil fumaroles or from unaltered tuff only one sample contained feldspars that were even slightly 18o enriched relative to the others cloudy plagioclase 18o 8 45 and this sample also contained the most 180 enriched groundmass of any of those analyzed 18o 12 6 this preservation of primary magmatic 18o values in the vtts feldspar phenocrysts is clearly a consequence of the extremely short time span i e 1912 to 1923 of vigorous high temperature fumarolic activity in the 1912 ash flow sheet these 18o l6o systematica are strikingly similar to those discovered in the 2 8 ma intracaldera chegem tuff gazis et al 1996 and in the fossil fumaroles in the outflow sheet of the 0 76 ma bishop tuff holt and taylor 1998 thus confirming that a similar type of fumarolic meteoric hydro thermal activity occurred above the zone of intense welding in all three of these ash flow tuffs this is particularly important because it provides a direct linkage between the older tuffs and the actual observations at the vtts of steam chemistry water rock interaction circulation geometry flow velocities and fumarolic temperatures up to 645 c the 18o l6o effects in the vtts can all be explained in terms of a two stage history a an early 10 to 15 year long high temperature 450 c fumarolic 18o depletion event groundmass 18o 0 1 to 4 8 and b a subsequent much longer lived low temperature 150 c 18o enrichment episode groundmass as high as 18o 12 6 steam in these low temperature fumaroles probably passed through various parts of the same hydrothermal system associated with the earlier higher temperature fumarolic activity and a weakened form of this low temperature hydrothermal circulation continues to the present day keith et al 1992 lowell and keith 1991 this low temperature 18o 16o exchange probably occurred in combination with mineralogical alteration of both the groundmass and the calcium rich portions of feldspar phenocrysts during the waning 150 c stages of fumarolic activity spilde et al 1993 the slight 18o enrichment of apparently pristine transparent feldspar phenocrysts 18o 7 51 in one of the 18o depleted meteoric hydrothermally altered fumarolic samples whole rock 18o 4 8 probably indicates that this sample was incipiently altered at low temperatures as fumarolic activity waned and thus may have had a whole rock 18o value much lower than 4 87 prior to 1923

possible mechanisms causing failure of thellier palaeointensity experiments in some basalts summary the normally magnetized zone of the jurassic lesotho basalts although providing apparently quite reliable palaeofield directions kosterov perrin 1996 shows anomalous behaviour when studied in vacuum using the thellier palaeointensity method typically the slope of the natural remanent magnetization thermoremanent magnetization nrm trm curves is very steep at intermediate temperatures 200 to 400 460 c in order to elucidate the reasons for such an anomalous behaviour six representative samples from a total of 74 studied using this method were subjected to a variety of analyses these experiments indicate that the magnetic properties are dominated by pseudo single domain psd magnetite grains some 1 mm in size resulting from high temperature oxidation of titanomagnetite laboratory heatings in vacuum up to the curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k similarly the k t curves in vacuum are with a single exception rather reproducible since the laboratory trms yield almost ideal nrm trm plots the anomalous nrm trm plot is presumably due to some peculiarity of the natural trm the partial trm ptrm acquisition capacity in the moderate temperature range cooling from 200 to 20 c is generally very strongly reduced after heating to 270 c which indicates that some magnetic alteration has already occurred at these temperatures hysteresis measurements between room temperature and the curie temperature t c show that some small less than 10 per cent but significant irreversible changes in hysteresis characteristics also occur during heating in particular the coercive force h c0 at room temperature is typically reduced after heating at a moderate temperature 175 c but increases after treatments at 475 c and more pronouncedly at 580 c the saturation magnetization j s0 remains unchanged except for a very small decrease less than 5 per cent occurring in some samples after the two latter treatments these changes are most clearly seen on h c t j s t bilogarithmic plots which show that the moderate temperature change in coercivity can extend up to 200 250 c thus hysteresis measurements as a function of temperature over a promising tool for sample pre selection for thellier experiments alternating field demagnetization and cycling of ptrms at liquid nitrogen temperature suggest that the blocking mechanism is largely multidomain like near room temperature but becomes less so as the curie point is approached the main reason for the failure of the thellier experiments is the loss of a fraction of the nrm natural trm at temperatures apparently lower than the blocking temperatures in nature it is suggested that this anomalous behaviour results from the reorganization of the domain structure of the psd grains during heating this transformation which seems to be triggered by the coercivity decrease observed at very moderate temperatures can reduce the nrm intensity without requiring any correlated ptrm acquisition

some additional parameters to estimate domain state from isothermal magnetization measurements abstract domain state and mixing analysis based on isothermal magnetization measurements is often restricted to the use of the day plot in order to make more information from simple rock magnetic measurements accessible for the quantitative analysis of domain state four additional parameters are proposed two of them a hysteresis shape parameter and a coercivity ratio can be directly calculated from standard measurements of the hysteresis loop and backfield curve the other two a transient energy dissipation ratio and the viscosity of isothermal remanent magnetization require additional measurements which however do not increase measurement time noticeably the theoretical background of the above parameters is discussed their ability to discern domain state is shown for a small set of synthetic magnetite and titanomagnetite samples and in a case study of a transect from the rim to the center of a dredged mid ocean ridge basalt

theoretical solutions of laterally loaded fixed head piles in elastoplastic soil considering pile head flexural yielding group piles with a cap under lateral loading behave like fixed head piles because of the rotational restraint of the pile cap they are susceptible to flexural yielding at the pile head due to high

fe oxide microcrystals in welded tuff from southern nevada origin of remanence carriers by precipitation in volcanic glass although it is widely recognized that remanent magnetism in ash flow tuffs is carried by fine grained fe oxides the origin mineralogy and significance of such magnetic carriers are not well understood the authors have obtained transmission electron microscope images of distinctive fe oxide microcrystals in rhyolitic samples located 3 8 7 6 and 18 5 m above the base of a 110 m thick section of the miocene tiva canyon member of the paintbrush tuff the fe oxide microcrystals are lath shaped and increase in size from 20 x 140 nm in the lowermost sample near base of the member to 120 x 800 nm in the uppermost sample within the flow interior microcrystals in this size range are within or close to the range of single domain grain size for magnetite electron diffraction and analytical x ray data indicate that the microcrystals in the lower two samples are cubic fe oxides magnetite maghemite with less than 10 mol ti end member and that those in the uppermost sample are manganiferous hematite systematic variations in magnetic properties are consistent with the observed variations in size and mineralogy of the microcrystals these microcrystals are morphologically distinct from grains that the authors interpret to be fragments of phenocrysts the morphology and spatial distribution ofmore the microcrystals as well as their increase in grain size from the rapidly cooled base of the ash flow sheet into the flow interior are consistent with an origin by nucleation and growth from volcanic glass at elevated temperature subsequent to emplacement less

effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments titanomagnetite fe3 xtixo4 0 x 1 is a common naturally occurring magnetic mineral critical to many paleomagnetic studies underlying most interpretations is the assumption that lacking chemical alteration curie temperature tc remains constant however recent work has demonstrated that tc of many natural titanomagnetites varies strongly as a function of thermal history independent of chemical alteration this is inferred to arise from reordering of cations and or vacancies in the crystal structure and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments because changes take place at t tclose will have their original tb spectrum truncated at t tclose predicted behavior during thellier type paleointensity experiments results in only modest deviations in nrm lost or ptrm gained from the non reordering case much larger deviations are predicted for ptrm checks compared to paleointensity results from titanomagnetite bearing pyroclastic deposits modeled non ideal behavior occurs in the same temperature intervals but is much more systematic reordering is likely one contributing factor to failure of paleointensity experiments this article is protected by copyright all rights reserved

geochemical and microbiological zonation of the middendorf aquifer south carolina we consider the nature and origin of the chemical and microbiological zonation of the cretaceous middendorf aquifer in the coastal plain of south carolina in light of the chemical evolution of groundwater along a flow path there some types of microbial activity most notably aerobic respiration and iron reduction lead to a net generation of acid as co2 but reaction of groundwater with minerals in the aquifer consumes the acid driving up ph and controlling the evolution of the groundwater s major ion chemistry the thermodynamic energy available to the various functional groups of microbes except the autotrophic hydrogen oxidizing methanogens is everywhere along the flow path sufficient to drive microbial growth the concentrations of acetate formate and lactate do not vary systematically from zone to zone along the flow path as might be expected if each zone were dominated by a single functional group of microbes the overall rate of microbial respiration in the aquifer appears to be limited by the rate of the initial fermentation of organic matter we found no compelling evidence indicating that one functional group of microbes excludes others from any of the aquifer s zones sulfate reduction for example may be non existent in the high iron zone associated with iron reduction or may account for up to 90 of the respiration occurring there zonation of the groundwater into high iron and low iron facies may result from a minor change along the aquifer in the balance between the activities of iron and sulfate reducers

multi level consistency tests in paleointensity determinations from the welded tuffs of the aso pyroclastic flow deposits abstract we have performed multi level consistency tests in an ltd dht shaw paleointensity study on the welded tuffs of the aso pyroclastic flow deposits aso 1 aso 2 aso 3 and aso 4 in order to obtain reliable mean paleointensities from each unit for a unit level consistency test we collected samples at three or more sites in each unit to enhance a site level consistency test we conducted paleointensity experiments on both whole rock and glass samples from the same sites thermal demagnetization curves of the samples indicate that the main magnetic carrier is titanium poor titanomagnetite to pure magnetite hysteresis parameters are different among the sites of the same units also differences in unblocking spectra and hysteresis parameters are recognized between the whole rock and glass samples from the same sites therefore these samples of various properties are expected to work effectively for multi level consistency test twenty nine out of 65 whole rock samples and 10 out of 31 glass samples gave accepted paleointensity results at the site level consistency of paleointensity estimates is observed with the exception of one site two paleointensity estimates from glass samples of a site of aso 4 are inconsistent and are excluded from further analysis at the unit level consistency or inconsistency among the sites is observed for individual units for aso 1 inconsistency in paleointensity estimates among the sites is observed and we can recognize more reliable results from two sites for aso 2 consistency in paleointensity estimates from glass and whole rock samples supports the acceptance of a site for aso 3 all paleointensity estimates from two sites are thought to be reliable because consistency between the site means of the two sites as well as site level consistency between the glass and whole rock samples for each site is recognized for aso 4 some difference among the sites is observed but there is no obvious reason to exclude the results of particular sites overall both the site level and unit level consistency checks work effectively in these samples which confirm the reliability of the paleointensity estimates or highlights the erroneous ones on the basis of the reliable paleointensity estimates unit mean values are obtained as 24 9 2 2 1 t n 9 for aso 1 22 9 1 6 t n 5 for aso 2 28 3 3 4 t n 10 for aso 3 and 40 1 6 2 t n 7 for aso 4

alabama gulf coast groundwaters 4he and 14c as groundwater dating tools analyses of 4 he and 1 4 c in groundwaters from a miocene quartz aquifer on the alabama gulf coast show the usefulness of 4 he for dating these holocene groundwaters in this aquifer system of low alkalinities and low phs radiocarbon ages can be used without model correction the groundwaters studied ranged from 42 3 to 95 5 modern carbon and yielded uncorrected ages of 375 6790 yr radiogenic 4 he ages ranged from 50 7500 yr

palaeointensity study of the hawaiian 1960 lava implications for possible causes of erroneously high intensities summary it is known that the hawaiian 1960 lava sometimes yields higher palaeointensities than the igrf 36 2 t in order to clarify the causes we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava according to various rock magnetic analyses they are classified into three groups with degrees of the deuteric oxidation in coe s version of the thellier method a significantly larger mean palaeointensity of 49 0 9 6 t n 17 was observed similar to previous studies these palaeointensities showed a dependence on the oxidation indices which can be explained by thermochemical remanent magnetization tcrm acquisition during the natural cooling stage of the lava although it is generally difficult to distinguish the tcrm from thermoremanent magnetization trm a lowtemperature demagnetization ltd treatment on nrm and trm in the thellier experiment seems to allow us to detect the influence of the tcrm on nrm another possible tool for tcrm detection is the double heating technique dht of the shaw method combined with ltd ltd dht shaw method it gave a better average of 39 4 7 9 t n 9 though the success rate was much lower most of the inappropriate results were screened by a combination of the double heating test the arm correction and the ltd treatment if two outliers are further excluded the average is improved to be 35 7 3 3 t n 7 which agrees well with the expected intensity the results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow

cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows 1 cooling rate effects on the intensity of thermoremanent magnetization trm have been well documented in ceramics in that case laboratory cooling is generally more rapid than the initial cooling leading to an overestimate of the paleofield by 5 10 in thellier type paleointensity experiments the reverse scenario however has never been tested we examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass sbg samples from 13 sites at 17 30 18 30 s on the east pacific rise absolute cooling rates determined by relaxation geospeedometry at five of these sites range from 10 to 330 c min 1 at the glass transition 650 c over the dominant range of remanence blocking temperatures 200 400 c the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the thellier experiment these results suggest that while the cooling rate effect might introduce some within site scatter it should not result in a systematic bias in paleointensity from sbg paleointensity estimates from the 15 sites range from 29 to 59 t with an average standard error of 1 t comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent indistinguishable from present day and the oldest is at least 500 and probably several thousand years old these age estimates are consistent with available radiometric ages and geologic observations

revised ages for tuffs of the yellowstone plateau volcanic field assignment of the huckleberry ridge tuff to a new geomagnetic polarity event 40 ar 39 ar ages were determined on the three major ash flow tuffs of the yellowstone plateau volcanic field in the region of yellowstone national park in order to improve the precision of previously determined ages total fusion and incremental heating ages of sanidine yielded the following mean ages huckleberry ridge tuff 2 059 0 004 ma mesa falls tuff 1 285 0 004 ma and lava creek tuff 0 639 0 002 ma the huckleberry ridge tuff has a transitional magnetic direction and has previously been related to the reunion normal polarity subchron dating of the reunion event has been reviewed and its ages have been normalized to a common value for mineral standards the age of the huckleberry ridge tuff is significantly younger than lava flows of the reunion event on re union island supporting other evidence for a normal polarity event younger than the reunion event