relation between depositional environment and the elemental composition of early diagenetic siderite early diagenetic siderites from marine and fresh water depositional environments are characterized by distinctive compositional trends siderite from fresh water environments is often relatively pure i e greater than 90 mol feco sub 3 and commonly attains end member composition siderite from marine environments however is always extremely impure and has extensive substitution of mg up to 41 mol and to a lesser extent ca up to 15 mol for fe in the siderite lattice in addition marine siderite generally contains less mn and has a higher mg ca ratio than fresh water siderite this compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters inasmuch as early marine pore waters generally have a higher mg sup 2 ca sup 2 ratio and contain less mn sup 2 and fe sup 2 and more ca sup 2 and mg sup 2 than meteoric waters

meteoric sphaerosiderite lines and their use for paleohydrology and paleoclimatology sphaerosiderite a morphologically distinct millimeter scale spherulitic siderite feco 3 forms predominantly in wetland soils and sediments and is common in the geologic record ancient sphaerosiderites are found in paleosol horizons within coal bearing stratigraphic intervals and like their modern counterparts are interpreted as having formed in water saturated environments here we report on sphaerosiderites from four different stratigraphic units each of which has highly variable 13 c and relatively stable 18 o compositions the unique isotopic trends are analogous to well documented meteoric calcite lines which we define here as meteoric sphaerosiderite lines meteoric sphaerosiderite lines provide a new means of constraining ground water 18 o and thus allow evaluation of paleohydrology and paleoclimate in humid

chemical weathering indices applied to weathering profiles developed on heterogeneous felsic metamorphic parent rocks abstract chemical weathering indices are commonly used for characterizing weathering profiles by incorporating bulk major element oxide chemistry into a single metric for each sample generally on homogeneous parent rocks weathering indices change systematically with depth however the weathering of heterogeneous metamorphic rocks confounds the relationship between weathering index and depth in this paper we evaluate previously defined chemical weathering indices for their suitability in characterizing weathering profiles developed on felsic heterogeneous metasedimentary bedrock from coweeta hydrologic laboratory north carolina usa among the weathering indices evaluated here the weathering index of parker wip is the most appropriate for application to weathering profiles on heterogeneous and homogeneous parent rock because the wip includes only the highly mobile alkali and alkaline earth elements in its formulation it yields values that differ greatly from those of the parent rock in addition the wip allows for aluminum mobility unlike other weathering indices these characteristics combine to make the weathering index of parker the most applicable index for studying the weathering of heterogeneous metasedimentary rocks however the wip should be applied judiciously as alkali and alkaline earth metals may be readily depleted during weathering in addition to reflecting weathering the chemical index of alteration cia chemical index of weathering ciw plagioclase index of alteration pia and vogt s residual index v may be sensitive to subtle geochemical changes such as hydrothermal alteration along a fault and or alteration at the water table weathering indices that include iron in their calculations were not included in this study because they do not distinguish between ferric and ferrous iron the oxidation state and distribution of iron in a weathering profile is influenced by the proportion of oxidation states in the pre weathered metamorphic rock and interaction with groundwater during weathering

influence of host rock texture on the morphology of carbonate concretions in a meteoric diagenetic environment abstract the integrated stratigraphic petrographic geochemical study of quaternary marine terraces that have undergone well constrained sea level and water table oscillation s over geologically short time periods can provide important clues on diagenetic processes at the marine continental interface the punta di i cani marine terrace quaternary saint florent corsica unconformably overlies the well cemented low permeability arenites of the miocene monte s angelo formation and is erosively overlain by unlithified high permeability holocene continental sediments the terrace is composed of a retrogradational fining upward succession from fluvial pebbly sandstone to beach and shoreface deposits the entire thickness of the terrace hosts concretions made of low mg calcite concretion geometry is linked to the sedimentological facies of the host sediment i massive fluvial pebbly sandstone at the base of the terrace contains knobby subhorizontal concretions somewhat resembling turbidite flute casts ii very low angle cross bedded well sorted beach deposits contain pipy subhorizontal concretions iii trough crossbedded upper shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae locally obliterated by a patchy isotropic cementation overall the degree of cementation decreases from the bottom to the top of the terrace cement chemistry and morphology as well as stable carbon and oxygen isotopic signatures indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast elongate concretions are invariably parallel to the topographic gradient consistent with the notion that the growth of concretions was controlled by advection smectitic vadose cement is locally present its crosscutting relationships with calcite cement point to periodic oscillations of the water table the results of this study i show that elongate concretions can be used as interstitial paleoflow indicators and ii suggest that concretion geometry in an advection dominated diagenetic environment can depend on the small scale permeability distribution as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies

azimuthal flows in the earth s core and changes in length of day at millennial timescales summary variations in the length of day lod reconstructed from ancient records of eclipses contain an oscillating component with a periodicity of roughly 1500 yr a part of the time dependent variations observed in archaeomagnetic field models consist of episodic eastward and westward motions with a similar periodicity using standard inversion techniques we obtain time dependent azimuthal flows at the surface of the core that can explain parts of the archaeomagnetic secular variation and with amplitude and characteristic timescale of variation consistent to explain the lod signal on this basis we argue that exchanges of angular momentum between core flows and the mantle are responsible for the observed millennial oscillations in the earth s rotation rate however we do not obtain a detailed match between the observed changes in lod and the prediction based on simple flow profiles inside the core we argue that this is due to the presence of time dependent azimuthal flows that have an important shear in the direction of the rotation axis such flows are interpreted as oscillations in thermal and magnetic winds

formation of siderite mg calcite iron sulphide concretions in intertidal marsh and sandflat sediments north norfolk england concretions cemented mainly by siderite mg calcite and iron monosulphide are common in late holocene marsh and sandflat sediments on parts of the north norfolk coast field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate reducing bacteria are active 13c values ranging from 3 to 11 8 mean 5 9 0 suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter 18o values ranged from 6 4 to 0 8 mean 1 0 suggesting that carbonate precipitated in porewaters ranging from pure sea water to sea water diluted with meteoric water chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation some concretions have formed around fragments of wood or metal but others contain no apparent nucleus in field experiments siderite fes and mg calcite were precipitated around several different nuclei within a period of six months we suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron

steady surficial core motions an alternate method a new method for deriving steady surficial core motions from geomagnetic field models is presented and applied the original method determines the steady velocity field at the top of a frozen flux core which best fits in both the spatial and temporal linear least squares sense a model of the large scale geomagnetic secular variation sv at the base of a source free mantle voorhies 1986 the new method is based upon the same physical assumptions but fits sv at earth s surface instead of the core mantle boundary cmb the newly derived flow differs somewhat from prior solutions and shows but slight westward drift however other key global properties are virtually unchanged over a 15 year interval the new method provides a better fit to the time varying geomagnetic field at earth s surface than does the old

mineralization and element redistribution in the chah gheib ni laterite ore zone bavanat zagros belt iran abstract nickel laterites are supergene deposits derived from the weathering of serpentinized ultramafic rocks and they commonly accounts for over 70 of world land based ni resources serpentinized ultramafic rocks outcrop extensively in iran as result of collision between afro arabian and iranian micro plate during the late cretaceous two ni bearing laterite belts developed on the ultramafic rocks of the neyriz ophiolite have been recently discovered in the bavanat region south iran the chah gheib deposit as the biggest ni bearing laterite in the bavanat area hosts a total estimated reserve mineralization of about 102 000 tons of ore at 0 92 ni and 180 000 tons at 1 63 cr this study focuses on elemental fractionation and mineralogical control of major trace and rare earth elements in a selected profile from the chah gheib ni laterite in order to assess the conditions of ore formation and its paleo environmental significance four distinct horizons including saprolite lower limonite upper limonite and lateritic duricrust have been recognized from the bottom to the top of the studied profile iron oxy hydroxides are the dominant phases found in the laterite profile overall fe and other first raw transition metals i e cr ni v co cu and zn show abundances higher than those of the bedrock the main fe and cr carriers in the laterite deposit are fe oxy hydroxides and chromite respectively although authigenic ni and cr bearing palygorskite is also common throughout the profile elemental mobility estimated using th as immobile element showed that the major oxides are largely depleted with the exception of fe2o3 and cao among trace elements cr is the element resulting strongly enriched in those levels where large chromite crystals occurs while v co ni and zn are enriched only in some levels of the upper limonite horizon and in the lateritic duricrust likely as adsorbed cations onto iron oxy hydroxides and as a consequence of an alkaline ph environment alkaline ph conditions can also explain the cu depletion observed throughout the profile such a condition during lateritization appears further supported by both the occurrence of carbonates and palygorskite and by the distribution of the ba sr ratio carbonates and palygorskite as well as secondary quartz require dry climate to form and alternating wet dry climate may allow lateritization this study suggests that the chah gheib laterite formed in a subtropical climate possibly during the late cretaceous late paleocene period in a roughly 25 35 latitude range promoting si and mg mobilization during the wet season and carbonates quartz and mg rich silicate precipitation during the drier periods

weathering of charnockites and sediment production in the catchment area of the cauvery river southern india abstract in the cauvery catchment area charnockites are making high hills compared to gneisses in situ static weathering of a charnockitic boulder shows lesser extent of chemical weathering relative to that developed into a profile this difference is seen both in clay mineral composition their ree patterns and in the a cn k and a cnk fm triangular plots based on the chemical changes relative to ti three groups of elements are recognized ba si k na sr ca and mg show increasing depletion as the weathering intensity increases in listed order al remained relatively immobile whereas fe followed by mn cr ni ree and p show a variable extent of enrichment with increasing chemical weathering the sediment occurring on the foothills of the charnockite hills have a very uniform composition and texture without any depositional features reworking of these sediments in a first order stream has caused only minor compositional changes by the local segregation of heavy minerals on the channel bed and by the removal of small amount of clay materials as suspended load the ree pattern of the reworked channel bed sand is nearly identical to that of the channel wall sediments but with lower by 25 abundance although the rocks are 2500 ma and now located in tropical latitudes and probably had experienced several periods of warm and wet climate the soils and the sediments derived from them are chemically very immature suggesting their exposure to denudational processes very recently by tectonic uplift

environments of formation and controls on spatial distribution of calcite cementation in plio pleistocene fluvial deposits new mexico u s a abstract the sierra ladrones formation is the uppermost syn rift sedimentary unit of the rio grande rift in central new mexico in the vicinity of the study site it was deposited in a variety of fluvial subenvironments portions of the formation are strongly cemented by calcite forming resistant ledges in otherwise poorly consolidated sediment the calcite exhibits a wide range of macroscopic and microscopic textures and structures which formed in different hydrologic environments including vadose nonpedogenic vadose pedogenic and phreatic phreatic cements are characterized by sparite with uniform disseminated isopachous poikilotopic and drusy textures vadose nonpedogenic by micrite with meniscus textures and vadose pedogenic by textures similar to those in the vadose nonpedogenic environment plus macroscopic features such as root structures destruction of primary sedimentary textures and a red hue many of the cements are of mixed vadose phreatic origin with vadose micrite formation preceding phreatic sparite the calcite is near end member in composition with only minor substitution of trace elements sr mg mn and fe for ca in the calcite structure vadose micrite cements generally have a higher trace element content than phreatic sparite perhaps as a result of more rapid precipitation in the vadose zone carbon isotope values range from 4 7 to 3 5 pdb and 18o values range from 10 3 to 7 2 pdb cements that are dominantly vadose have a wider range of 13c values and are more 13c depleted overall than phreatic cements oxygen values are similar for both vadose and phreatic cements at large scales i e several meters to decameters vertical dimension calcite is present preferentially in coarse grained sediments that had high pre cementation saturated permeabilities relative to uncemented finer grained strata at smaller scales i e several meters or less vertical dimension however there is no clear relationship between degree of cementation and pre cementation permeability at these scales calcite cement is present preferentially in areas that contain early vadose calcite this suggests that at large scales cementation was controlled by pre cementation permeability perhaps due to a greater flux of ca2 and or hco3 in permeable horizons whereas at smaller scales within high permeability units the distribution of cementation was controlled by other factors such as the presence of nucleating material

a comparative study of gabbro and granite weathering in the piedmont of the southeastern u s a gabbros and granites weather in quite different fashions granites develop extensive weathering rind systems 2 m thick in which plagioclase is altered to a gibbsite kaolinite assemblage in rindlets adjacent to the fresh rock contact in the outer rindlets the clay fraction is dominated by kaolinite and dioctahedral vermiculite not surprisingly granitic weathering rind systems display relative depletions of si na ca rb and sr and relative enrichments of sesquioxides in contrast gabbro weathering rind systems are characterized by many thin 1 cm rindlets whose alteration products are dominated by limonitization of ferromagnesian minerals apart from a decrease in feofe2o3 ratios in the outer rindlets there is little relative depletion or enrichment of major elements in gabbro weathering rinds as compared to their fresh rock cores this contradiction in goldich s stability series is explained if weathering rinds are produced faster on gabbros than in granite outcrops thus the low degree of mineral alteration in gabbro rindlets reflects a shorter time of residence for minerals in this system compared to rindlets on granitic outcrops biotite and perthitic microcline were separated from a fresh granite and its attendant weathering rind system during alteration the albitic blebs of the perthite are dissolved leaving no trace of residual clay the result is a porous microcline whose composition is equivalent to the microcline matrix in the unweathered perthite progressive alteration of biotite results in oxidation of octahedrally coordinated fe2 with concomitant loss of k this results in the formation of dioctahedral vermiculites in outer rindlet samples of granite

can downwelling at the top of the earth s core be detected in the geomagnetic secular variation abstract it has been argued based on recent seismic and mineral physics studies that the top of earth s liquid outer core is stably stratified here i analyze persistent geomagnetic secular variation features on the core mantle boundary to examine whether a kinematic signature of core fluid upwelling downwelling can be detected i focus on regions of intense high latitude geomagnetic flux patches that may be maintained by fluid downwelling in order to identify persistent patterns the radial field and its secular variation are stacked in the flux patch moving reference frame these stacked images are compared with forward solutions to the radial induction equation based on idealized field flow models clear advective secular variation signature below north america indicates that these intense flux patches may exhibit significant mobility stretching signature in the form of persistent positive secular variation correlated with the intense flux patch below the southern indian ocean may be considered as regional scale geomagnetic evidence for whole core convection although pure toroidal flow cannot be outruled

rinded iron oxide concretions hallmarks of altered siderite masses of both early and late diagenetic origin iron bearing concretions are valuable records of oxidation states of subsurface waters but the first concretions to form can be altered drastically during later diagenetic events distinctive concretions composed of heavy rinds of iron oxide that surround iron poor mud rich cores are common along bases of fluvial cross bed sets of the cretaceous dakota formation nebraska usa concretion rinds thicken inward and cores contain 46 to 89 void space millimetre scale spherosiderites are abundant in palaeosols that developed in floodplain facies evolution of rinded concretions began when intraformational clasts were eroded from sideritic soils transported abraded and deposited in river channels alteration of siderite and formation of rinds occurred much later perhaps in the quaternary when sandstone pore waters became oxic dakota concretions are analogous to rattlestones in pleistocene fluvial channels of the netherlands and their rinded structure is analogous to that of iron rich concretions in the aeolian navajo sandstone of utah in all three deposits rinded concretions formed when pre existing siderite cemented concretions were oxidized within a sand matrix unlike fluvial examples siderite in the navajo sandstone was autochthonous and of late diagenetic origin having precipitated from carbon dioxide and methane enriched waters moving through folded and jointed strata iron rich rinds formed in all these strata because concretion interiors remained anaerobic even as oxygen accumulated in the pore waters of their surrounding permeable matrix iron oxide first precipitated at redox boundaries at concretion perimeters and formed an inward thickening rind acid generated by the oxidation reaction drove siderite dissolution to completion creating the iron poor core iron oxide rinds are indicators of the former presence of siderite a mineral that forms only under reducing conditions during either early or late diagenesis siderite is vulnerable to complete oxidation upon exposure so the distinctive rinded concretions are valuable clues that aid in deciphering diagenetic histories and for recognizing methanic floodplain palaeoenvironments and wet palaeoclimate

influence of soil pedological properties on termite mound stability this study investigated the influence of soil properties on the density and shape of epigeous fungus growing termite nests in a dry deciduous forest in karnataka india in this environment odontotermes obesus produces cathedral shaped mounds their density shape height and volume and soil physicochemical properties were analyzed in ferralsol and vertisol environments no significant difference was observed in o obesus mound density n 2 7 mound ha 1 on average in the vertisol and ferralsol areas this study also showed that o obesus has a limited effect on soil physical properties no differences in soil particle size ph or the c n ratio and base saturation were measured whereas the c and n contents were reduced and cec was higher in termite nest soils in both environments clay mineralogical composition was also measured and showed the presence of higher amounts of smectite clays in termite nest soils which thus explained the increasing cec despite the reduced c and n content however the main difference was the shape of the termite mounds the degradation of the nests created a hillock of eroded soil at the base of termite mounds in the vertisol while only a thin layer of eroded soil was observed in the ferralsol the increased degradation of termite mounds in the vertisol is explained by the presence of smectites 2 1 swelling clays which confer macroscopic swelling and shrinking characteristics to the soil soil shrinkage during the dry season leads to the formation of deep cracks in the termite mounds that allow rain to rapidly penetrate inside the mound wall and then breakdown unstable aggregates in conclusion it appears that despite a similar abundance termite mound properties depend to a large extent on the soil properties of their environments

sediments in palaeo notches potential proxy records for palaeoclimatic changes in antarctica abstract many climatic proxy materials such as ice cores coastal and lacustrine sediments and abandoned penguin rookeries have been used to study palaeoclimatic changes and the palaeoenvironmental evolution of antarctica in this paper we report another geological record well preserved sedimentary sequences in the palaeo notches in marine terraces of fildes peninsula king george island based upon grain size distribution and microtexture geochemical element compositions and radiocarbon dating we identified the sediments in one palaeo notch as lacustrine sediments most likely deposited in a small palaeo lake from these data we inferred that between 4000 and 3000 yr b p a moderate chemical weathering occurred and the climate was warmer and more humid than present in good agreement with the findings from other climatic proxy materials we believe that the sediments in palaeo notches provide another reliable proxy material for studying the palaeoclimate and the palaeoenvironment in antarctica

numerical models of a dense layer at the base of the mantle and implications for the geodynamics of d to investigate the dynamics of the mantle s d layer we explore numerical models of mantle convection which feature a dense basal boundary layer we use a double diffusive finite element convection scheme and vary thermal and chemical rayleigh numbers and properties including viscosity thermal conductivity and internal heating for isoviscous models with heating only from below the thermal rayleigh number ra is set at either 106 or 107 the negative chemical buoyancy of a dense layer produces a stable boundary layer when the ratio of chemical to thermal buoyancy the buoyancy number b is around 1 for b 0 5 the layer is unstable while b 0 6 may produce a layer which remains stable for long periods depending on other factors such as rayleigh number layer thickness and thermal diffusivity of the layer at high enough ra small scale convection can occur within the layer using ra 107 we look at changes in layer thickness from 100 to 300 km at two values of b 0 6 and 1 convection within the layer takes place most easily when the layer has a greater initial thickness for b 0 6 the layer is not always continuous along the lower boundary so convection within the layer does not occur unless the initial thickness is fairly high 300 km increasing the thermal diffusivity of the layer to simulate enrichment in metals enhances heat conduction across the layer and can suppress internal convection within d enhanced conduction also leads to higher plume temperatures including internal heating mainly increases the overall heat flow through the mantle leading to higher surface heat flux finally we examine models with temperature dependent viscosity and pressure dependent viscosity using rayleigh numbers in the range of 107 convection within the dense layer is enhanced by the relative reduction in viscosity our models exhibit only a modest decrease in viscosity an order of magnitude but illustrate how a dense low viscosity layer interacts with cold viscous downwellings

simultaneous stochastic inversion for geomagnetic main field and secular variation 2 1820 1980 we apply the method of simultaneous stochastic inversion for the geomagnetic main field and secular variation to data from the period 1820 1980 producing two time dependent field models one for the period 1900 1980 the other for 1820 1900 we introduce several adaptations of our original formalism of the inverse problem we choose to represent the time dependency of the field by an expansion in chebychev polynomials we apply an alternative regularization condition in the time domain and we adopt a different method from that used in our earlier study for solving the resulting system of equations whereas our earlier study used only twentieth century observatory data we consider almost all the available data from the time interval 1820 1980 we examine the effects of the crustal field on our observations and find that existing methods of accounting for these fields as sources of random noise are inadequate in two circumstances the first is sequences of measurements made at one particular site for example at permanent magnetic observatories in which case the crustal field produces temporal correlation of the errors in the observations we devise a method for accounting for these correlations the other is measurements made at satellite altitude the crustal field producing spatial correlation of the errors we show how using a very simple statistical model of the crustal field these correlations can be calculated rather than attempting to account directly for these correlations which would require without further assumptions massive computation we instead limit our data selection to points sufficiently well separated in space that the effect of the crustal field is minimal we are able to fit the data to within our estimates of the noise in the data yet the resulting models have lower power in the secular variation than previously postulated by some authors many of the features in the field at the core mantle boundary described in recent work by bloxham and gubbins are seen most clearly indeed this time varying representation of the field reinforces many of their earlier conclusions

biosignatures link microorganisms to iron mineralization in a paleoaquifer concretions preferentially cemented masses within sediments and sedimentary rocks are records of sediment diagenesis and tracers of pore water chemistry for over a century rinded spheroidal structures that exhibit an fe iii oxide rich exterior and fe poor core have been described as oxidation products of fe ii carbonate concretions however mechanisms governing fe iii oxide precipitation within these structures remain an enigma here we present chemical and morphological evidence of microbial biosignatures in association with fe iii oxides in the fe iii oxide rich rind of spheroidal concretions collected from the jurassic navajo sandstone southwest united states implicating a microbial role in fe biomineralization the amount of total organic carbon in the exterior fe iii oxides exceeded measured values in the friable interior the mean 13 c value of organic carbon from the fe iii oxide cemented exterior 13 c of 20 55 is consistent with a biogenic signature from autotrophic bacteria scanning electron micrographs reveal microstructures consistent with bacterial size and morphology including a twisted stalk morphotype that resembled an fe ii oxidizing microorganism gallionella sp nanoscale associations of fe o c and n with bacterial morphotypes demonstrate microorganisms associated with fe iii oxides together these results indicate that autotrophic microorganisms were present during fe iii oxide precipitation and present microbial catalysis as a mechanism of fe iii oxide concretion formation microbial biosignatures in rinded fe iii oxide rich concretions within an exhumed quaternary aquifer has broad implications for detection of life within the geological record on earth as well as other fe rich rocky planets such as mars where both fe ii carbonate and fe iii oxide rich concretions have been identified

ree geochemistry of recent clastic sediments from the kaveri floodplains southern india implication to source area weathering and sedimentary processes abstract the kaveri river drains through archean gneissic and charnockitic terranes in south india the rare earth element ree chemistry of the floodplain sediments along with their major and trace element composition in different size fractions are discussed we observe a strong coherence between ree and tio2 contents in the sediments which is suggestive of strong control by titanite or its weathered products on the ree chemistry ree concentrations also show a general increase with decreasing size of bulk sediments different size fractions and of light and heavy mineral fractions this suggests the presence of ree in the fe oxy hydroxide surface coatings on clastic grains formed by the weathering of hornblende and titanite in the granodioritic to dioritic protoliths we also interpret the data as showing that ree patterns of clastic sediments notably eu anomaly are affected by the degree of source rock weathering as well as by the fluvial processes of sorting and mineral differentiation therefore for provenance study sediments having all granulometric grades such as the floodplain sediments are preferable because they most closely represent the source particularly when the source region suffered little chemical weathering in the present case the sediments were derived from the high standing hills of archean charnockites because of their recent uplift and physical denudation

morphologic clues to the origins of iron oxide cemented spheroids boxworks and pipelike concretions navajo sandstone of south central utah u s a abstractconcretions cemented by iron oxide are abundant and diverse in the jurassic navajo sandstone of southern utah some of these structures are considered terrestrial analogs for concretions imaged on mars the navajo concretions can be spheroidal pipelike or tabular with multicompartmented boxworks these iron oxide concretions typically display a rinded structure dense sandstone rinds cemented by iron oxide surround pale iron poor sandstone cores within a single structure the iron rich rinds may be single or multiple pseudomorphs of siderite are present in local residual iron rich cores of boxworks workers in the late nineteenth through mid twentieth centuries many of whom found evidence for siderite precusors concluded that many spherical rinded iron oxide cemented concretions were formed by centripetal precipitation of iron oxide inward from the perimeter of the concretion in contrast the walls of pipelike concretions of iron oxide grew centrifugally outward we interpret the navajo

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

rare earth element mobility in the oceanic lower sheeted dyke complex evidence from geochemical data and leaching experiments hydrothermally altered rocks from the base metal depleted reaction zone in the lower sheeted dyke complex odp hole 504b show significant losses of rare earth elements ree in extensively altered domains alteration patches while very similar in major element composition and alteration mineralogy the alteration patches have up to 50 lower ree concentrations than the surrounding weakly altered diabases in order to test if there is a dependence of ree mobility on the alteration history we carried out leaching experiments on a weakly altered diabase and an adjacent alteration patch the leaching experiments are used as a probe to identify the hosting phases of major and trace elements before and after alteration they are not designed to simulate natural alteration processes major element compositions of the leaching solutions show that small amounts 9 to 14 of the rocks and similar proportions of plagioclase amphibole and chlorite were dissolved from both rock types in contrast to the major elements up to 100 of the ree could be leached from the strongly recrystallised alteration patch after 20 h all leaching solutions were light ree enriched and had negative eu anomalies compared to the host rock geochemical modelling suggests that mainly interstitial material formerly late stage magmatic liquids and probably some apatite are the main ree sources in the rocks affected during experimental leaching dissolution of plagioclase amphibole clinopyroxene and chlorite cannot explain the ree compositions of the leachates the higher ree leachability of the heavily altered rock suggests that the extensive hydrothermal alteration resulted in a complete redistribution of the ree from lattices of the magmatic minerals to the grain boundaries of secondary minerals this can explain the marked increase in ree accessibility during ongoing alteration which finally led to ree depletions in the highly altered rock we propose that late stage interstitial material provides the most important ree source of rocks in the oceanic crust this interpretation is in contrast to a model by klinkhammer et al klinkhammer g p elderfield h edmond j m mitra a 1994 geochemical implications of rare earth element patterns in hydrothermal fluids from mid ocean ridges geochim cosmochim acta vol 58 pp 5105 5113 who suggested that the similarity in the ree patterns of plagioclase and that of hydrothermal vent fluids is a result of chemical exchange between plagioclase and hot fluids in the ocean crust our leaching results indicate that although dramatic dissolution of plagioclase occurred it did not control the ree patterns of the leachates moreover ree patterns inferred for fluids that mobilised ree from the highly altered rock are different from those of vent fluids we therefore propose that the characteristic ree pattern of vent fluids may result from differences in aqueous chloride complex stabilities sorption controlled ree fractionation during fluid migration and mineral precipitation in the discharge zone

time average and time dependent parts of core flow abstract we infer fluid motions below the core mantle boundary by inverting geomagnetic secular variation data over a 150 years assuming helical geostrophic flow we obtain snapshot images of core flow at 5 year intervals which we combine to give time average and time dependent parts of the motion over this time interval the most prominent time average flow structure is a large anti cyclonic vortex in the southern hemisphere beneath the atlantic and indian oceans the time average zonal core flow outside the inner core tangent cylinder is significantly westward in the southern hemisphere but nearly zero in the northern hemisphere westward polar vortices occur inside the tangent cylinder in both hemispheres particularly in the north in terms of mantle versus core origins mantle driving appears to be responsible for the mid latitude asymmetry in the zonal core flow whereas core driving appears to be responsible for the flow at high latitudes we also compare changes in the core s angular momentum calculated from our time dependent core flow with changes in the mantle s angular momentum derived from decade scale length of day variations and find adequate agreement a fit of our time dependent core flow to a torsional oscillations model yields dominant periods of 110 and 53 years

relationship between oriented calcite concretions and permeability correlation structure in an alluvial aquifer sierra ladrones formation new mexico abstract fluvial deposits of the sierra ladrones formation pliocene pleistocene contain abundant elongate calcite concretions that are interpreted to have precipitated in the saturated zone parallel to the direction of groundwater flow the orientation of these concretions is coincident with the orientation of the permeability correlation structure as determined from architectural element mapping and permeameter measurements in cases where the paleo groundwater flow closely corresponds with the direction of fluvial deposition measurement of concretion orientations in alluvial rocks and sediments may provide a means of rapidly estimating the major axis of the permeability correlation structure over large areas

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

geomagnetic evidence for fluid upwelling at the core mantle boundary summary previous studies both geomagnetic and seismic have been unable to show conclusively whether or not there is fluid upwelling at the core mantle boundary here a new method is developed in which an attempt is made to invert geomagnetic secular variation data measured at the earth s surface for a frozen flux purely toroidal core mantle boundary cmb velocity field under the assumption that the mantle is electrically insulating and flux is frozen in at the cmb these data have previously been inverted for the core mantle boundary radial secular variation from which the appropriate fit between model and data is known two different main field models were used to assess the effect of uncertainty in its radial component at the cmb the conclusions were the same in both cases frozen flux purely toroidal motions provide a poor fit a statistical test allows very firm rejection of the hypothesis that the residuals are not significantly larger whereas there is no statistical difference between the residuals of inversions for radial secular variation and frozen flux velocity fields at the cmb if upwelling and down welling is included the inherent non uniqueness in the velocity field obtained is not of concern since only their statistical properties are utilized and no physical significance is attached to the flows obtained

surface deformation caused by pressure changes in the fluid core pressure load love numbers are presented for calculating the mantle deformation induced by the variation of the pressure field at the core mantle boundary cmb we find that the cmb geostrophic pressure fields derived from frozen flux core surface flow estimates at epochs 1965 and 1975 produce a relative radial velocity rrv field in the range of 3mm decade with uplift near the equator and subsidence near the poles the contribution of this mechanism to the change in the length of day i o d is small about 2 3 x 10 2 ms decade the contribution to the time variation of the ellipticity coefficient j 2 is more important about 1 3 x x 10 11 yr

the structure of the core mantle boundary from diffracted waves diffracted p and s waves pd sd traveling around the core mantle boundary cmb of the earth give us information about the velocity structure and therefore the thermochemistry of d the base of the earth s mantle by examining pd and sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle by comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify d average velocities using these averaged velocities with a thermochemical modeling of lower mantle minerals using a birch mumaghan equation of state we have been able to make chemical and physical inferences as to the causes of lateral variations at the cmb examinations found significant lateral heterogeneity at the base of the mantle amounting to 4 for both p and s velocities these velocities did not always vary in parallel and the poisson ratio varied regionally by almost 6 the most unusual region of the cmb found was under indonesia where velocities 3 slower than the preliminary reference earth models were found adjacent to a region of faster than average velocities these regions currently correspond to areas of core up welling and down welling respectively found by voorhies 1986 which if mostly held in place by core mantle coupling might cause a flux of heat and iron into the mantle making the anomaly both thermally and chemically derived at the cmb under the northern pacific rim the fastest shear velocities were found but the same region yielded slower than average p velocities while the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection perhaps continuing down from the north pacific subduction zones the presence of slow p velocities suggests additional complications our thermochemical modeling suggests that the d poisson ratio is very sensitive to variations in the silicate oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region

the geologic context of wonderstone a complex outcrop scaled pattern of iron oxide cement abstract although siderite is a widespread early diagenetic mineral in fluvial systems it is unstable in oxidizing environments and destroyed in permeable rocks that experience uplift and exhumation the products of siderite oxidation however mm to cm scale rhombs concretions and complex bands of iron oxide cement are widespread in the rock record of fluvial systems the fluvial channels of the shinarump member of the chinle formation in southern utah and northern arizona u s a provide an excellent suite of examples of diagenetic features produced by triassic and neogene oxidation of early diagenetic siderite these diagenetic features also provide direct evidence of the level of the water table during deposition of the shinarump member large in situ discoidal concretions containing preserved siderite are present in shinarump floodplain siltstones rip up clasts derived from the siltstones developed iron oxide rinds during late stage near surface oxidation these two structures show that floodplain silts contained abundant organic matter and methanic pore water groundwater recharging through these silts carried reducing water through underlying sand bodies and discharged into active channels degassing of co2 and methanogenesis caused rhombic crystals of siderite to precipitate in channel sands during these wet intervals some of this siderite may have been oxidized during dry intervals when groundwater circulation reversed but most siderite in the channel sands was preserved until the shinarump was exhumed during the neogene as oxygenated near surface water entered joints in the lithified shinarump colonies of iron oxidizing microbes living in the phreatic zone occupied redox boundaries and used the rhombic crystals of siderite in the sandstone and the spherulitic siderite in transported siltstone intraclasts as their sources of energy and carbon the ferrous iron released from dissolving siderite within the intraclasts was oxidized at the siltstone sandstone contact generating rinded concretions similar to those in the cretaceous dakota formation complex banding known as wonderstone was produced in the channel sandstones from oxidation of the rhombic siderite the pattern is a combination of liesegang bands and microbially mediated cements the preserved rhombs are pseudomorphs after siderite crystals that were either oxidized during triassic dry intervals or escaped neogene microbial oxidation in the phreatic zone only to be oxidized abiotically in the vadose zone microbes are likely oxidizing shinarump siderite a few kilometers down dip of outcrops with exposed wonderstone at such locations the shinarump is in contact with overlying water saturated quaternary alluvium

constraints on magnetic energy and mantle conductivity from the forced nutations of the earth observations of the earth s nutations are used to explore the possible presence of a conducting layer at the base of the mantle the existence of such a layer is suggested by recent experiments on iron and silicate mixtures at high pressures and temperatures knittle and jeanloz 1986 1989 which indicate that iron may react with silicate minerals to produce metallic compounds nutations provide an ideal tool for investigating the existence of such a layer because they are sensitive to the electrical properties within a thin magnetic boundary layer along the core mantle boundary cmb evidence in favor of a conducting layer is found in the effects of ohmic dissipation which cause the amplitude of the earth s nutation to be out of phase with tidal forcings although out of phase components in the earth s nutations are conventionally attributed to the dissipative influences of ocean tides and mantle anelasticity these effects appear incapable of explaining nutation amplitudes determined from very long baseline interferometry vlbi observations the additional effects of ohmic dissipation enhanced by the presence of a thin conducting layer at the cmb are sufficient to reconcile theory and observation the largest mismatch between observations and theory taking account of ocean tides and mantle anelasticity is a discrepancy of 0 39 0 04 milliarcseconds in the out of phase component of the retrograde annual nutation this discrepancy can be eliminated if the lowermost 200 m of the mantle has a conductivity of 5 105s m 1 and the magnetic energy in the spherical harmonic components of the field at the cmb in degrees l 12 is approximately 4 times greater than the energy inferred in degrees l 12 from surface observations unfortunately the vlbi observations do not uniquely resolve the mantle conductivity and magnetic energy so that trade offs exist between these two parameters nevertheless a high mantle conductivity is favored on the grounds that the alternative involving high levels of magnetic energy yields an excessive amount of ohmic heating other interpretations of the vlbi discrepancy are also discussed one such alternative involves viscous dissipation at the cmb requiring a kinematic viscosity of 0 1m2s 1

relating the spatial variability of chemical weathering and erosion to geological and topographical zones abstract the relationship between the spatial variability of soil elements weathering and erosion rates can be very complex topography abruptly changing elevation gradients and slopes of varying geological composition exert an important influence on weathering trajectories and erosion this complex interconnectivity is only rarely addressed in weathering studies the main objectives of this investigation are therefore to relate weathering and erosion to various and geomorphic units in a dry alpine and threshold landscape having steep slopes kan catchment tehran province iran a number of common weathering indices were tested using a genetic algorithm the best indices are the k na ti ratio the wip weathering index according to parker and the pi product index according to ruxton however the recently suggested 4si m r2 and m f wmin systems discriminate weathering trends more accurately our results show that these soils have a low to moderate weathering stage weathering trajectories point to the active formation of kaolinite and oxyhydroxides river sediments are slightly more weathered and contain the most weathered topsoils and suggest erosion in the catchment is predominantly due to topsoil removal chemical weathering is influenced by the dominant geomorphic units unconsolidated deposits of the quaternary and solid bedrock and landform features combination of altitude and slope aspect does not appear to have a significant impact on weathering denudation erosion rates were determined using meteoric 10be since the fluvial sediments did not contain enough quartz for in situ 10be analyses this procedure has higher uncertainties due to difficulties in estimating the depositional flux of meteoric 10be due to tectonic uplift the erosion rates are very high to maintain a soil layer soil production rates must be also high although annual precipitation is relatively low slope and soil dynamics seem particularly high which leads to a fast turnover of the soil material and maintains the weathering intensity at a rather low to intermediate level

the steady part of the secular variation of the earth s magnetic field the secular variation of the earth s magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle adequate observations to map the magnetic field at the core mantle boundary extend back over three centuries providing a model of the secular variation at the core mantle boundary here we consider how best to analyze this time dependent part of the field we propose that the first step should be to seek the steady core flow that best fits the field maps isolating what we call the steady secular variation the part of the secular variation explained by steady core flow to calculate steady core flow over long time periods we introduce an adaptation of our earlier method of calculating the flow in order to achieve greater numerical stability we perform this procedure for the periods 1840 1990 and 1690 1840 and find that well over 90 of the variance of the time depedent field can be explained by simple steady core flow the core flows obtained for the two intervals are broadly similar to each other and to flows determined over much shorter recent intervals although we can explain a large part of the signal with these flows none of them provides an adequate explanation of the secular variation in particular the fit to the original observations from which the field models were derived is inadequate we examine the residual secular variation the part of the signal that remains we argue that it is dominated by the effects of nonsteady flow and magnetic diffusion we do not find any compelling evidence for torsional oscillations as the dominant ingredient of this nonsteady part of the secular variation

fluid flow near the surface of earth s outer core maps of the fluid flow at the core surface are important for a number of reasons foremost they may provide some insight into the workings of the geodynamo and may place useful constraints on geodynamo models from the flow the force balance at the top of the core can at least in part be deduced the flow can provide short term predictions of the secular variation the flow is important in understanding changes in the length of day and constraints on lateral temperature variations and topography at the core mantle boundary may be derived unlike the case of mantle convection only very small lateral variations in core density are required to drive the flow these density variations are too small by several orders of magnitude to be imaged seismically so instead we use the geomagnetic secular variation to infer the flow despite considerable recent progress in mapping the core flow substantial differences exist between maps produced by different researchers here we examine the possible underlying reasons for these differences paying particular attention to the inherent problems of nonuniqueness we focus on the aspects of the flow which do seem to be well determined and discuss their geophysical implications

paleohydrologic and stratigraphic significance of crayfish burrows in continental deposits examples from several paleocene laramide basins in the rocky mountains abstract paleocene crayfish burrows are present locally in great abundance in the greater green river hanna wind river and piceance basins of wyoming and colorado in the washakie sub basin of the greater green river basin burrows are found in 1 lenticular cross bedded sandstones of fluvial channel origin 2 massive sandstones and mudrocks of floodplain origin that surround and interfinger with the cross bedded sandstones and 3 thin sandy ironstone beds of overbank and crevasse splay origin the burrows which represent crayfish activity in proximal to distal floodplain settings are assigned to camborygma symplokonomos c eumekenomos and c litonomos on the basis of their architectural morphologies c araioklados is not present crayfish burrows are part of pedogenically modified channel and overbank deposits in all the basins studied burrows are abundant only in non coal bearing rocks the presence of elongate burrows indicates that the landscape was imperfectly drained on a seasonal basis and the burrow depths suggest the paleo water table was 1 4 m below the paleosurface swampy conditions prevailed in areas where carbonaceous shales and coals formed and thus crayfish did not construct deep burrows the crayfish burrows and associated lithologies suggest a warm humid wet seasonal climate in the eastern washakie basin the vertical and lateral distribution of burrows and their abundance reflect changes in the paleohydrologic regime over space and time burrow depth and overall abundance decreases towards local basin lowlands where they are rare the paleohydrologic regime coupled with a progressively less seasonal climate through time allowed the formation of the cherokee coal zone in the late paleocene in an otherwise lithologically homogeneous non variegated deposits stratigraphically significant surfaces may be indicated only by the tops of burrowed intervals and by other types of paleosols the tops of burrowed intervals represent paleosurfaces of varying environmental stability on which there are different frequency and magnitude of sedimentation events immature cumulative paleosols characterized by less intensely burrowed beds or bedsets record higher sedimentation rates more intensely burrowed beds or bedsets reflect the activity of many generations of crayfish hence the tops of these layers record longer durations of exposure and pedogenic modification with infrequent or no sedimentation paleosols represented by burrowed intervals in conjunction with sandstone and mudrock stacking patterns can be used to identify significant stratigraphic surfaces such as parasequences and sequence boundaries in fully continental environments

the axial dipole strength and flow in the outer core from 1590 to 1833 although extensive measurements of the geomagnetic field were made no direct measurements of its intensity are available field models for this period either make use of indirect palaeomagnetic intensity determinations or assume a similar change in dipole strength to that seen presently here we examine the impact of different choices of field model strength on models of the core surface flow derived from the geomagnetic secular variation we find that for a reasonable range of possible changes in the field strength the variability that this unknown imposes on the flow structure is small compared with other choices in the modelling strategy such as the degree to which the geomagnetic models are fit the smoothness of the flows and a priori information included to reduce non uniqueness in the flow calculation the flows predict variations in core angular momentum of similar magnitude to those seen in recent times these variations are larger than seen in models from geodetic data but do not seem inconsistent with the geodetic data themselves we therefore suggest that it is reasonable to study core flows back at least until 1650 thereby doubling the time interval for which flows can be considered to constrain the behaviour of the geodynamo

distribution of rare earth elements and oxyhydroxide phases within a weathered felsic igneous profile in hong kong abstract variation in type and abundance of oxyhydroxide phases and the fractionation of rare earth elements ree within a weathered felsic pyroclastic profile in hong kong were investigated in order to assess the key factors and micro environmental conditions controlling their distribution during weathering at rock and mineral scales three types of oxyhydroxide phases fe mn and ce rich were recognized based on their most abundant elements the fe rich types appeared in all samples at different stages of weathering the mn rich types however were limited to samples at advanced stages of weathering while the ce rich types were found solely in samples from oxyhydroxide rich zones fractionation of ree likely started at the early stages of weathering but became evident only at the advanced stages characteristics of ree patterns especially the direction negative or positive and extent of ce anomalies correlated very well with the appearance and abundance of mn oxyhydroxides and were attributed to the co existence of distinct micro environmental conditions within the profile these conditions resulted from two major competing processes leaching and fixation and oxidation precipitation and reduction dissolution which operated simultaneously within the profile at any given stage of weathering

major element ree and other trace element behavior in amphibolite weathering under semiarid conditions in southern india abstract a body of komatiitic amphibolite an enclave within the archean high grade orthogneisses in southern india shows mild chemical weathering under semiarid conditions along fractures chemical weathering has advanced chemical index of alteration documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm cia 53 end document cia of fresh documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage

accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation summary we use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core we find that the contribution of radial magnetic diffusion to the secular variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters the correlation between the two diffusive terms is interpreted in terms of the variation in the strength of poloidal flow along a columnar flow tube the amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the secular variation at earth like conditions we then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic secular variation data we find that including magnetic diffusion does not change dramatically the global flow but some significant local variations appear in the non frozen flux core flow models termed diffusive the hemispherical dichotomy between the active atlantic and quiet pacific is weaker a cyclonic vortex below north america emerges and the vortex below asia is stronger our results have several important geophysical implications first our diffusive flow models contain some flow activity at low latitudes in the pacific suggesting a local balance between magnetic field advection and diffusion in that region second the cyclone below north america in our diffusive flows reconciles the difference between mantle driven thermal wind predictions and frozen flux core flow models and is consistent with the prominent intense magnetic flux patch below north america in geomagnetic field models finally we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic secular variation than usually assumed

weathering of gneissic rocks in the upper reaches of cauvery river south india implications to neotectonics of the region in the upper reaches of the cauvery catchment area in southern india the archean peninsular gneisses exhibit a greater degree of weathering in the field relative to the massive charnockites and granites although the gneisses have been physically broken down along foliation planes fractures and compositional discontinuities secondary mineral such as chlorite smectite kaolinite and fe oxyhydroxide formation is insignificant consequently there is little chemical change even up to the stage of saprolite development only in the regolith there is some loss of ca na sr ba mg and sio2 other elements such as fe mn al cr ni and ree all show enrichment relative to tio2 in areas which have developed profiles by in situ weathering the chondrite normalized ree patterns however show little change excepting a slight flattening of the patterns because of greater hree mobility during weathering clay minerals also have flattened ree patterns because of relative hree enrichment the depletion and the enrichment of different elements appear to be related to mild leaching of primary mafic minerals by meteoric water rather than by mineral breakdown this and the extent of variation in chemical index of alteration values suggest that that the gneisses have suffered only incipient chemical weathering for the extent of physical weathering undergone by them it is likely that this contrasting weathering extents of these 2500 ma rocks is due to their exposure to surface geological processes only recently because of ongoing periodic uplift and physical erosion of this region

geomagnetic field analysis v determining steady core surface flows directly from geomagnetic observations summary geomagnetic secular variation is a result of inductive effects of fluid motion at the top of the earth s liquid core which can be mapped using the frozen flux approximation and inverting mathematical models of the secular variation nonuniqueness may be removed by assuming the flow to be stationary the use of an intermediate mathematical model of secular variation makes comparison of the derived models with observation unsatisfactory and here we devise a new method inverting the geomagnetic measurements directly to obtain steady core motion the method is applied to observatory annual mean data for the 30yr period 1960 1990 to produce six separate models of steady flow three that are stationary over 10yr intervals 1960 1970 1970 1980 and 1980 1990 two 20 yr models 1960 1980 and 1970 1990 and one 30yr model 1960 1990 the method is not restricted to continuous time series or linear x y z data the 10 yr models fit the weighted data with misfits of 1 0 1 3 the 20 yr models fit less well and the 30yr model has a misfit of 1 7 when constrained to have the same norm as the 10yr models all models appear to fit the central decade with a gross misfit close to unity suggesting that changes in core motion may be associated with the two geomagnetic jerks that occurred around 1969 and 1978 furthermore close inspection of the fits at individual observatories shows that steady core motions with this norm cannot produce the secular accelerations required at a large number of high quality observatories steady flows can predict magnetic fields that follow the trend at most observatories but cannot follow the detailed time variations of the past three

slow chemical weathering in a semiarid climate changes in the mineralogy and geochemistry of subaerial lava flows in the deschutes river basin central oregon abstract the progression of mineral alteration and chemical changes of thirteen pairs of fresh and weathered surficial lava samples from the deschutes river basin of central oregon were studied to understand incipient weathering in a semiarid climate samples range from basalt to dacite in composition and quaternary to late miocene in age only the tertiary samples showed significant chemical weathering based on element mobility and mineral alteration the slight climatic variations between tertiary outcrop locations map 38 74 cm yr and mat of 7 2 7 6 c did not correlate with degree of weathering the susceptibility sequence of phases was determined to be glass olivine plagioclase orthopyroxene clinopyroxene fe ti oxides the older weathered samples have experienced significant leaching of mobile cations e g ca mg sr resulting in a lower calculated weathering index the tertiary weathered samples are enriched in ree particularly lree which is attributed to a lree phosphate found in association with allophane clay mineralization in plagioclase these samples also have a negative ce anomaly likely due to its tetravalent state in oxidizing conditions and subsequent lack of mobility in weathering solutions although there is a clear petrographic and chemical distinction between fresh and weathered lavas 2 ma the distinction between fresh and weathered samples is not evident in the quaternary lavas significant element mobility between sample pairs is closely correlated to the breakdown of plagioclase and orthopyroxene and the subsequent secondary mineralization of clays and or lree rich phosphates with the exception of plagioclase all the mineral phases in the deschutes lavas have persistence times or stability of at least one and possibly several orders of magnitude greater than the same phases in soil chronosequences or weathering rinds in more temperate and humid climates as a result elemental release rates and silicate fluxes are much lower under semiarid conditions and their role in the global carbon cycle should be examined more closely

simple models of fluid flow at the core surface derived from geomagnetic field models summary we examine the problem of determining the fluid flow at the core surface given a model of the temporal variation of the magnetic field seeking to fit the field over a finite time interval the flow is assumed steady over the time interval and we adopt the frozen flux approximation we produce a range of solutions of varying complexity and subject to different constraints estimate the errors on the solutions and assess their consistency with the underlying assumptions we find both direct evidence for ageostrophic flow in the form of flow across the geographical equator and indirect evidence in the form of poorer fits to the field when the flow is contained to be geostrophic on account of this evidence we do not adopt the geostrophic approximation however we find that unconstrained flows are poorly determined and consider the various alternatives for producing better determined solutions we choose to restrict our attention to determining the large scale toroidal part of the flow and find a simple pattern of flow which is nearly symmetric about the equator from a sequence of maps of the flow over 5 year time intervals back to 1915 we are still able to identify many of the basic aspects of this simple pattern but changes in the flow pattern are discernible although our models of the flow explain a large proportion of the signal typically 95 per cent of the variance the fits which we obtain are not as good as might be expected failure of the underlying assumptions together with our decision to map only the large scale toroidal part of the flow explains the poorer than expected fit

weathering assessment in the achala batholith of the sierra de comechingones cordoba central argentina i granite regolith fractionation abstract weathering assessment in a weathering limited environment is a difficult task because a significant proportion of the regolith has been removed by tectonically induced denudation in such an environment the remnants of the altered mineral debris must be studied with care in order to attain a meaningful picture of the rate and intensity of ongoing weathering a monolithologic i e granite small 2 km2 and mountainous 1500 m elevation drainage basin in the sierras de comechingones 31 54 07 s 64 45 28 w 31 53 11 s 64 44 16 w cordoba central argentina was selected as a pilot area to survey the nature of weathering in a weathering limited erosional setting and a semiarid climatic regime a relatively thin layered coarse grained regolith and scattered sediment i e fine grained regolith that had accumulated in valleys and topographic depressions were analyzed the most abundantly identified clay mineral in the regolith is illite followed by kaolinite smectite smectite seems to be the only clay mineral clearly associated with weathering petrographic observations and geochemical analyses supported by statistical tests define chemical weathering as incipient petrography indicates that plagioclase and biotite are the main mineral phases affected by alteration which is more intense in the fine grained regolith coarse and fine grained regoliths are chemically similar among them and with the country rock with statistically significant losses in the fine fraction of mgo mno and p2o5 depletions of trace elements and ree which are best explained by sorting than by actual rock alteration are not statistically significant ternary diagrams reveal that the masses of al2o3 cao na2o and k2o were not significantly altered during weathering and transportation and that regolith samples correspond to a coarse residue relatively enriched in feldspars and quartz where the mud fraction with high clay mineral contents has been removed from the drainage basin by high energy processes

steady core flow in an azimuthally drifting reference frame summary flows at the top of the earth s core generating the observed geomagnetic secular variation sv can be deduced in the frozen flux approximation with various nonuniqueness reducing assumptions such as tangential geostrophy and steadiness steady flows are attractive because they require only a small number of parameters to explain the gross features of the sv however they are unable to reproduce the fine detail contained in the sv and cannot be used to explain observed decadal changes in the length of day here we find flows steady in a local reference frame within the core but the frame is allowed to rotate relative to the mantle previous investigations have studied steady drift with respect to the mantle introducing just a single extra parameter into the calculation here we also allow the drift to vary with time we then minimize a linear combination of the fit to time varying coefficients expressing the sv a measure of the complexity of the flow and the drift acceleration the resulting non linear inverse problem is solved in a two stage iterative process for a given drift we solve for the best fitting steady flow and then adjust the drift to improve the fit we seek solutions for the intervals 1900 1980 and 1840 1990 over both epochs allowing the reference frame of a steady flow to drift gives a strikingly improved fit for flows with a relatively high misfit the frame drift is westwards at a rate similar to the observed westward drift rate of the geomagnetic field at the earth s surface approximately 0 2u yr x1 requiring a tighter fit and hence a more complex flow gives rise to two solutions one with a westward frame drift with respect to the mantle the other eastward and the relative drift rates gradually increase to a maximum of 0 9u yr x1 as the misfit decreases flows in the mantle reference frame are similar to those deduced previously by any of the nonuniqueness reducing assumptions however in the drifting frame the flows are almost completely dominated by the drift between the two reference frames although the time dependence of the drift is weak and results in only a small additional misfit reduction over a uniform drift by assuming that the variation in drift reflects variation in solid body rotation of the whole core it can explain decadal length of day changes almost as well as and prior to 1900 perhaps better than fully time dependent tangentially geostrophic flows with vastly more free parameters we examine the significance of these models in terms of large scale wave motion in the core

chemical weathering in the drainage basin of a tropical watershed nsimi zoetele site cameroon comparison between organic poor and organic rich waters abstract this study deals with the weathering processes operating at the scale of a small catchment nsimi zoetele cameroon and is focused on the role of organic colloids on mineral weathering and transport of elements in natural waters samples of river spring and groundwaters from nsimi zoetele were filtered through membranes of decreasing pore size 0 22 m 0 025 m or 300 000 da 5000 da to separate colloidal fractions from the truly dissolved one major and trace elements and dissolved organic carbon doc were analysed in each fraction two kinds of waters can be distinguished in the catchment clear and coloured waters clear waters exhibit low concentrations of major and trace elements and doc elements are carried in these solutions in a true dissolved form except al and rare earth elements rees by contrast the higher abundances of al fe and trace elements in coloured waters are controlled by the colloidal fraction thermodynamic equilibrium calculations show that clear waters are in equilibrium with kaolinite and iron oxi hydroxide which are major minerals in the weathered soil for coloured waters the aqueous speciation of ca mg cu fe al la and th was calculated taking into account the complexes with humic acids speciation calculations for cu fe al la th show a strong complexation with humic acids in good agreement with the results of the filtration experiments by contrast although filtration experiments show a strong control of major cations by organic matter for example 75 for ca speciation calculations reveal that their complexes with humic ligands do not exceed a few percent of total dissolved elements this discrepancy is explained as an artefact induced by the organic colloids and occurring during the filtration procedure finally both filtration experiments and speciation calculations show that organic matter plays an important role in natural doc rich waters organic acids increase significantly the dissolution rates of silicates and oxi hydroxides and thus the amounts of solutes and of complexed elements leaving the catchment

steady flows at the top of earth s core derived from geomagnetic field models select models of the main geomagnetic field and its secular variation are extrapolated to the base of an insulating mantle and used to estimate the adjacent fluid motion of a perfectly conducting outer core the assumption of steady motion provides formally unique solutions and is tested along with that of no upwelling the hypothesis of no upwelling is found to be substantially worse than that of steady motion although the actual motion is not thought to be steady the large scale secular variation at the top of the core can be adequately described by a large scale combined toroidal poloidal circulation which is steady for intervals of at least a decade or two the derived flows include a bulk westward drift but are complicated by superimposed jets gyres and surface divergence indicative of vigorous vertical motion at depth the circulation pattern and key global properties including rms speed upwelling and westward drift are found to be fairly insensitive to variations in modeling parameters

mean electromotive force generated by asymmetric fluid flow near the surface of earth s outer core the phi component of the mean electromotive force etf generated by asymmetric flow of fluid just beneath the core mantle boundary cmb is obtained using a geomagnetic field model this analysis is based on the supposition that the axisymmetric part of fluid flow beneath the cmb is tangentially geostrophic and toroidal for all the epochs studied the computed phi component is stronger in the southern hemisphere than that in the northern hemisphere assuming a linear relationship between etf and the azimuthally averaged magnetic field aamf the only nonzero off diagonal components of the pseudotensor relating etf to aamf are estimated as functions of colatitude and the physical implications of the results are discussed 14 refs

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

does secular variation involve motions in the deep core abstract the time evolution or secular variation of different components of the geomagnetic field at many observatories presents a broken line pattern along time invervals whose length is typically a few tens of years the secular variation is linear or the field is parabolic with a high accuracy sudden changes of slope take place at the ends of these intervals identified as magnetic jerks we show that it is difficult to make these characteristics compatible with the hypothesis that the secular variation is generated by a flow involving only a thin say of the order of 10 km layer at the top of the core indeed diffusion would then prohibit the observed broken line pattern and its linear parts unless rather unlikely assumptions are made about the time behaviour of the flow

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

early cenozoic drainage reorganization of the united states western interior gulf of mexico sediment routing system continental scale drainages host the world s largest rivers and offshore sediment accumulations many of which contain significant petroleum reserves rate of sediment supply in these settings may be a signal of external controls e g tectonics on landscape evolution yet deciphering these controls remains a major challenge in interpreting the ancient stratigraphic record integration of new and published detrital zircon u pb ages from the united states rocky mountain region and gulf of mexico gom sedimentary basin demonstrates profound changes in the u s continental drainage divide that controlled the rate of sediment delivery to the northern gom during paleocene eocene time sedimentation rate increased dramatically during deposition of the lower wilcox group reaching approximately three times the cenozoic average accompanied by pronounced shoreline regression and delivery of a large volume of sand to the basin floor we hypothesize that this increase in sediment delivery to the gom resulted from drainage capture of a significant portion of the sevier laramide structural province 900 000 km2 that included the headwaters of the california and idaho rivers capture of the california river drainage may have occurred in the vicinity of the hanna basin of eastern wyoming that previously emptied northward into a shallow seaway but was subsequently diverted southward to the rockdale delta which accumulated within the houston embayment during the time of deposition of the lower wilcox group detrital zircon u pb ages from wilcox samples within the rockdale delta show a remarkable similarity with contemporaneous laramide synorogenic units including enrichment in detritus derived from the cordilleran arc and basement terranes of western north america relative to older and younger units in the houston embayment a subsequent order of magnitude decline in sedimentation rate to the gom can be partly attributed to well documented drainage closure 800 000 km2 that accompanied lake formation in interior laramide basins ca 53 51 8 ma our results demonstrate that tectonically induced drainage migration in the high relief segments of continental scale drainages can have a pronounced effect on the rate of sediment transfer to continental margins

mapping the fluid flow and shear near the core surface using the radial and horizontal components of the magnetic field summary we examine the problem of determining the fluid flow and the shear the radial derivative of the flow at the core surface given a model of the temporal variation of the magnetic field whereas most previous work has focused on determining only the flow which requires only the use of the radial component of the magnetic field here in addition we determine the shear for which we must use the horizontal component of the magnetic field estimates of the jump in the value of the horizontal magnetic field bh across the boundary layer between the top of the free stream and the base of the mantle are small and suggest that to a high level of accuracy the mantle valuks of bh can be used at the top of the core except in the special case of an insulating mantle only the horizontal poloidal field is known at the core mantle boundary and supplies one extra equation for the determination of velocity and shear we show how the matrix elements relating the coefficients of the spectral expansion of the flow and shear are related to the geomagnetic secular variation coefficients in closed form we examine the uniqueness of the resulting inverse problem and show that one part of the non uniqueness from which the shear suffers is particularly easy to describe it takes the same form as that which affects the flow namely a toroidal ambiguity in the field u b however certain uniqueness theorems can be derived we extend the steady motions theorem of voorhies backus 1985 and the geostrophic motions theorem of hills 1979 and backus lemouel 1986 to the determination of the flow and shear and derive closely analogous results uniqueness in the steady case depends on the value of the same discriminant as the velocity and in the geostrophic case the shear can be determined uniquely in the same areas as can the velocity i e outside certain ambiguous patches for the geostrophic regime the lateral density or temperature variations at the top of the core can be found in a self consistent manner we apply our method to the temporal evolution of the field over the period 1960 1980 and produce solutions for each of the assumptions of unconstrained steady motions geostrophic motions and purely toroidal motions we find that the form of the flow changes very little from solutions based only on the radial induction equation and that the shear is weak and aligned with the flow with a sense such that the strength of the flow decreases with depth with a length scale for linear decay of half the core radius this suggests that the flow near the core surface is indicative of whole core flow rather than a flow confined to a layer near the core surface

active tectonics at wheeler ridge southern san joaquin valley california wheeler ridge is an east west trending anticline that is actively deforming on the upper plate of the pleito wheeler ridge thrust fault system holocene and late pleistocene deformation is demonstrated at the eastern end of the anticline where salt creek crosses the anticlinal axis uplift tilting and faulting associated with the eastward growth of the anticline are documented by geomorphic surfaces that are higher and older to the west faulting and associated folding is propagating eastward as indicated by increases in both the degree of surface dissection and the degree of soil development from east to west distinct topographic areas having distinct degrees of surface dissection bounded by tear faults suggest that faulting and folding have propagated eastward in discrete segments numerical dates indicate 1 the anticline is propagating eastward at a rate of about 30 mm yr about 10 times the rate of uplift 2 folding was initiated about 400 ka 3 a prominent wind gap was formed during q3 time about 60 ka when an antecedent stream was defeated forcing the stream east around the nose of the fold today drainage through the ridge is by way of two antecedent streams water gaps east of the wind gap and 4 the rate of uplift at the easternmost and youngest past 1 k y part of the fold is at least 3 mm yr investigations of the tectonic geomorphology of wheeler ridge support the hypothesis that climatic perturbations are primarily responsible for producing geomorphic surfaces such as alluvial fan segments and river terraces tectonic perturbations deform the surfaces

lateral temperature variations at the core mantle boundary deduced from the magnetic field recent studies of the secular variation of the earth s magnetic field over periods of a few centuries have suggested that the pattern of fluid motion near the surface of earth s outer core may be strongly influenced by lateral temperature variations in the lowermost mantle in this paper we introduce a self consistent method for finding the temperature variations near the core surface by assuming that the dynamical balance there is geostrophic and that lateral density variations there are thermal in origin the calculation is accomplished by using the radial and horizontal components of the magnetic field at the core mantle boundary to calculate the fluid flow and its radial derivative near the core surface which are related to the pattern of lateral temperature variations by the thermal wind equation as expected these lateral temperature variations are very small we find some agreement between this pattern and the pattern of topography of the core mantle boundary but are unable to answer conclusively to what extent core surface motions are controlled by the mantle rather than being determined by processes in the core

core mantle interactions abstract recent studies of the magnetic field at the core mantle boundary have revealed fixed sites of either static magnetic features or persistent secular variation this suggests part of the magnetic field behaviour is controlled by the mantle the most plausible mechanism for core mantle interaction is thermal coupling although topography may also be significant the magnetic sites coincide with anomalies in lower mantle seismic velocity as determined from tomography and density as determined by flow models of mantle convection constrained by tomography and the geoid some magnetic features coincide with subduction zones particularly those near indonesia they may be caused by bumps on the core mantle boundary beneath trenches paleomagnetic pole positions suggest the magnetic behaviour has persisted for at least 5 myr as would be expected if it were controlled from the mantle

distinguishing syntectonic unconformity types to enhance analysis of growth strata an example from the cretaceous southeastern nevada u s a abstract two syntectonic unconformity types are recognized in nonmarine upper cretaceous growth strata adjacent to the willow tank thrust in southeastern nevada u s a unconformities with larger angular discordance 6 90 traditional type developed when uplift outpaced sediment accumulation more subtle unconformities with less discordance 2 5 subtle type developed when sediment accumulation nearly kept pace with uplift traditional type unconformities are the focus of most studies of growth strata because the discordances are obvious in the field by contrast subtle type unconformities are more difficult to identify subtle type unconformities are mappable surfaces of erosion or nondeposition located within progressive flattening up stratigraphic successions and are associated with clustering of large scale 0 5 4 m soft sediment deformation features possible seismites better developed or clustered paleosols greater reworking and or grain size increase increasing sediment supply in the presence of positive net accommodation allows syntectonic deposits to aggrade above a growing structure despite no change in uplift rate aggradation across a structure is punctuated when uplift is slightly higher than sedimentation producing subtle discordances subtle type with evidence of slope destabilization seismicity or longer residence time instead of the traditional type angular unconformities identification of unconformity types in growth strata can document additional phases of uplift particularly for intervals where sediments aggraded above an active structure because of higher sediment supply and regional subsidence or sea level rise although these syntectonic unconformity types are identified in nonmarine compressional growth strata they are likely present in many other depositional and structural settings

geochronology and mammalian biostratigraphy of middle and upper paleocene continental strata bighorn basin wyoming the bighorn basin of northwestern wyoming preserves one of the most complete records of middle paleocene to lower eocene continental biota the geochronology of this important interval depends partly on numerical calibration of the geomagnetic polarity time scale gpts but the middle and late paleocene parts of the gpts have been poorly constrained radioisotopically a new volcanic ash from the northern bighorn basin provides the first radioisotopic age for upper middle paleocene strata upper selandian and is an important calibration point for the base of polarity chron c26n and the selandian thanetian stage boundary in the gpts sanidine from the ash has a weighted mean age of 59 00 0 30 2 ma based on twenty three 40 ar 39 ar bulk laser fusion analyses the ash corroborates the revised age estimate for the base of chron c26n in the most recent geomagnetic polarity time scale gpts 04 and confirms that c26n is older than depicted in the 1995 gpts we place paleocene mammalian biozones in the bighorn basin into a refined geochronologic framework these biozones are the primary basis for biostratigraphic correlation of middle and upper paleocene continental deposits in north america new paleomagnetic data provide synchronous tie points for correlation among three stratigraphic sections in the northern bighorn basin and to the gpts paleomagnetic correlation study of new fossil material review of known faunas and refined stratigra phy enable us to reevaluate several biozones we define three new zones based on first occurrences of taxa the phenacolemur ti 4b probathyopsis ti 5a revised and copecion cf 3 revised zones the phenacolemur zone includes some faunas previously placed in the plesiadapis churchilli and plesiadapis fodinatus zones ti 4 and ti 5a respectively the probathyopsis zone replaces the p fodinatus zone and the copecion zone replaces the phenacodus ectocion acme zone cf 3 seven of the biozones considered here are subdivisions of the tiffanian land mammalage thetiffanianwasoriginallytypifiedbythemasonpocketfaunainsouthwest ern colorado but it is now much better known from faunas in the northern bighorn basin biostratigraphic evidence and new paleomagnetic data from the bighorn basin indicate that mason pocket is considerably older than was previously recognized and occurs in chron c26r rather than c25r the geochronologic framework presented here helps to constrain the temporal ranges of species that occur in the middle and late tiffanian and it allows the bighorn basin record to be better compared to other faunal floral and paleoclimate records both regionally and globally

constraining electromagnetic core mantle coupling abstract electromagnetic core mantle coupling is one candidate for explaining length of day lod variations on decadal time scales this coupling has traditionally been calculated from models of core surface flow u but core flow inversions are not unique unfortunately the main torque contribution depends on the toroidal part r of the vector field u b r which is left undetermined by the radial induction equation that links core flows to magnetic field data under the frozen flux hypothesis b r is the radial magnetic field at the core surface we have developed a new method that bypasses flow inversions using the vector field u b r directly we estimate its toroidal part by imposing that u b r vanishes where b r 0 the calculation of the electromagnetic torque is also hindered by the uncertainty in the conductivity structure of the lower mantle however we conclude that the conductivity of the lower mantle has to be much larger than current estimates derived from diamond anvil cell experiments to make the electromagnetic torque significant the region beneath africa and the mid atlantic where there is the most significant b r 0 curve apart from the magnetic equator is particularly important here the field is relatively well constrained and this is the single region that contributes most to the torque following holme holme r 1998a electromagnetic core mantle coupling 1 explaining decadal changes in the length of day geophys j int 132 167 180 we show nevertheless that small changes to suffice to recover the torque lod required to explain lod variations provided that the lower mantle conductivity is high enough we find a value similar to holme holme r 1998b electromagnetic core mantle coupling 2 probing deep mantle conductance in gunis m wysession m e knittel e buffett b a eds the core mantle boundary region agu pp 139 151 for the minimum conductance 10 8 s however we were not able to simultaneously recover lod and constrain u to be tangentially geostrophic

interactions of a paleocene river a rising fold and early diagenetic concretions abstract the relative rates of sediment accumulation erosion and structural uplift determine whether a growing fold develops positive topographic relief is beveled by antecedent streams or is buried under thick growth strata when folds rise in subsiding basins upward convergent flow of groundwater through the permeable growth strata that underlie antecedent streams enhances the flux of ions required for concretion growth early diagenetic concretions that grow in such alluvial strata may constitute the only clasts larger than sand size available for transport when antecedent streams become erosive the first reworked concretions deposited by these streams should accurately mark the transition from aggradation to erosion as folds rise into the paths of streams in this situation the ability to differentiate reworked from in situ concretions is crucial the west vergent simpson ridge anticline a n s trending thick skinned laramide structure in east central wyoming separates the larger hanna basin from the carbon basin near the north nose of this anticline in situ iron oxide rich concretions are abundant in folded paleocene strata ferris formation and just to the east large reworked iron rich concretions are abundant in younger more gently dipping conglomerates in the basal hanna formation of the backlimb smaller reworked concretions are also present near the base of the hanna formation at least 7 km south of the anticlinal nose and just east of the fold9s axis at the anticlinal nose in situ non reworked concretions up to 3 m 1 m 1 m are abundant at the top of an 7 km thick sequence of sandstones and siltstones that constitute the late cretaceous early paleocene ferris formation reworked concretions are absent in the strata hosting these in situ concretions but reworked concretionary clasts up to 2 m in diameter are present in exposures of conglomerates in the lowermost hanna formation that lie just above the in situ ferris concretions and southeast of the anticlinal nose these early diagenetic concretions were originally cemented by siderite feco 3 oxidation of some small rinded siderite cemented clasts took place after their fluvial transport into the hanna formation but abundant angular un rinded iron oxide cemented clasts indicate that many large in situ siderite concretions had resided in the vadose zone before they were entrained the distribution of reworked concretions and the orientations of crossbeds show that antecedent hanna streams eroded a swath at least 5 km wide across the rising structure these streams transported ferris formation concretions southeastward into the carbon basin and deposited them in a conglomeratic sandstone body in the hanna formation large calcite cemented concretions many with a pipe like morphology then grew within hanna crossbeds in many cases these in situ concretions enclose transported iron rich concretions but there is no evidence any calcite cemented concretions were reworked the nw se alignment of the pipes record southeastward flow of groundwater and thus also provide evidence together with the orientation of the crossbeds of the original continuity of the hanna formation across the anticline reworked concretions reveal the interplay of deposition diagenesis and erosion due to convergent groundwater flow over growing anticlines early diagenetic concretions both in situ and as reworked clasts are especially likely to be found in growth strata

equatorially dominated magnetic field change at the surface of earth s core slow temporal variations in earth s magnetic field originate in the liquid outer core we analyzed the evolution of nonaxisymmetric magnetic flux at the core surface over the past 400 years we found that the most robust feature is westward motion at 17 kilometers per year in a belt concentrated around the equator beneath the atlantic hemisphere surprisingly this motion is dominated by a single wavenumber and persists throughout the observation period this phenomenon could be produced by an equatorial jet of core fluid by hydromagnetic wave propagation or by a combination of both discrimination between these mechanisms would provide useful constraints on the dynamics of earth s core

a model for sedimentary compaction of a viscous medium and its application to inner core growth summary a detailed study of the physics of a 1 d sedimentary compaction of a viscous medium was carried out both numerically and analytically for columnar and self gravitating spherical cases in view of applying it to the inner core growth process of the earth the effects of sedimentation rate and surface porosity upon the porosity profile were investigated it was found that the porosity profile differs depending on whether or not the sedimentation rate is larger than the darcy velocity velocity of the solid matrix when the fluid flows by buoyancy alone when the sedimentation rate is larger than the darcy velocity a thick constant porosity layer develops at the surface and below it the porosity decreases gradually towards the bottom when the sedimentation rate is smaller than the darcy velocity the porosity profile is characterized by a mushy layer at the top where the fluid is expelled by the deformation of the solid underlain by a thick layer of constant porosity termed the residual porosity such a porosity profile can be understood as the propagation of a half sided solitary wave the study was extended further for the self gravitating spherical case formation of an unstable porosity structure and the appearance of solitary waves were discovered for the case of monotonically decreasing sedimentation rate given the size of the sphere formed by sedimentary compaction according to the magnitude of the ratio of sedimentation rate to darcy velocity three types of porosity structure which differ in force balance and the typical length scale required for porosity decrease were discovered one such structure is where a low porosity layer forms at the top accompanied by solitary waves beneath it indicating that a crust like region can develop at the surface of the inner core

redox front penetration in the fractured toki granite central japan an analogue for redox reactions and redox buffering in fractured crystalline host rocks for repositories of long lived radioactive waste abstract redox buffering is one important factor to be considered when assessing the barrier function of potential host rocks for a deep geological repository for long lived radioactive waste if such a repository is to be sited in fractured crystalline host rock it must be demonstrated that waste will be emplaced deeper than the maximum depth to which oxidizing waters can penetrate from the earth s surface via fractures during the assessment timeframe typically 1 ma an analogue for penetration of such oxidizing water occurs in the cretaceous toki granite of central japan here a deep redox front is developed along water conducting fractures at a depth of 210 m below the ground surface detailed petrographical studies and geochemical analyses were carried out on drill core specimens of this redox front the aim was to determine the buffering processes and behavior of major and minor elements including rare earth elements rees during redox front development the results are compared with analytical data from an oxidized zone found along shallow fractures up to 20 m from the surface in the same granitic rock in order to understand differences in elemental migration according to the depth below the ground surface of redox front formation geochemical analyses by xrf and icp ms of the oxidized zone at 210 m depth reveal clear changes in fe iii fe ii ratios and ca depletion across the front while fe concentrations vary little in contrast the redox front identified along shallow fractures shows strong enrichments of fe mn and trace elements in the oxidized zone compared with the fresh rock matrix the difference can be ascribed to the changing eh and ph of groundwater as it flows downwards in the granite due to reactions with rock forming minerals in particular feldspar dissolution these observations give important insights into the processes that control the rates of redox front penetration in fractured crystalline rock the findings of the study can be used to help build confidence among stakeholders that radioactive waste would be emplaced in such rocks at greater depth than that to which oxidizing water is likely to penetrate in future

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

a laboratory model for solidification of earth s core abstract to better understand the influence of rotating convection in the outer core on the solidification of the inner core we have constructed a laboratory model for solidification of earth s core the model consists of a 15 cm radius hemispherical acrylic tank concentric with a 5 cm radius hemispherical aluminum heat exchanger that serves as the incipient inner core onto which we freeze ice from salt water long exposure photographs of neutrally buoyant particles in illuminated planes suggest reduction of flow parallel to the rotation axis thermistors in the tank near the heat exchanger show that in experiments with rotation the temperature near the pole is lower than near the equator unlike for control experiments without rotation or with a polymer that increases the fluid viscosity the photographs and thermistors suggest that our observation that ice grows faster near the pole than near the equator for experiments with rotation is a result of colder water not readily convecting away from the pole because of the reversal of the thermal gradient we expect faster equatorial solidification in the earth s core such anisotropy in solidification has been suggested as a cause of inner core elastic and attenuation anisotropy though the plausibility of this suggestion will depend on the core nusselt number and the slope of the liquidus and the effects of post solidification deformation previous experiments on hexagonal close packed alloys such as sea ice and zinc tin have shown that fluid flow in the melt can result in a solidification texture transverse to the solidification direction with the texture depending on the nature of the flow a comparison of the visualized flow and the texture of columnar ice crystals in thin sections from these experiments confirms flow induced transverse textures this suggests that the convective pattern at the base of the outer core is recorded in the texture of the inner core and that outer core convection might contribute to the complexity in the seismically inferred pattern of anisotropy in the earth s inner core

geochemical and mineralogical characteristics of reey occurrences in the mocambo granitic massif tin bearing a type granite central brazil and its potential for ion adsorption type reey mineralization abstract the mocambo granitic massif is an a type granite located in the northeastern part of goias state near the city of monte alegre de goias it belongs to the pedra branca suite which is itself part of the goias tin province a group of stanniferous granites that exhibit similar geochemical features including to a greater or lesser degree of rare earth elements enrichment the parent rock in the mocambo granitic massif is a coarse porphyritic pink biotite granite thin section observations coupled with electron microprobe analyses reveal the presence of several ree bearing minerals allanite fergusonite ce fergusonite y zircon and apatite as accessory minerals and xenotime y bastnasite ce thorite monazite ce and hingganite y as secondary hydrothermal minerals fergusonite and hingganite are identified in this massif for the first time this work investigates the forms of occurrence and enrichment of rare earth elements in the mocambo granitic massif in the parent rock as well as in greisen saprolite and soil with emphasis placed on the rare earth elements adsorbed on clay minerals and the entire process of transport and fractionation of these elements from the fresh rock until adsorption the results show enrichment of rare earth elements with the exception of ce in soils in comparison to rock especially in the clay fraction moreover they suggest that these elements are mostly adsorbed on clay minerals mostly kaolinite and illite which are identified in the x ray patterns the surface oxidation of ce3 to ce4 on surfaces and the probable formation of ce oxides cause this element to present a smaller quantity of exchangeable ions the presence of cerianite grains in one superficial soil sample shows that ce is mostly found in the ce4 state in the soil profile the mocambo granitic massif displays substantial enrichment in reey 555 1229 ppm for the parental granite and 569 ppm for the greisen and the exchangeable reey concentrations in the soils horizons more than 600 ppm in particular representing a potential target for an ion adsorption type rare earth elements deposit

follow the water connecting a co2 reservoir and bleached sandstone to iron rich concretions in the navajo sandstone of south central utah usa concretions with thick iron oxide cemented rinds and lightly cemented iron poor sandstone cores are abundant within the navajo sandstone near the southeast fl ank of the escalante anticline utah united states previous workers suggested the spheroidal concretions as analogs for martian blueberries hematite concretions and linked the origin of concretions and bleaching of the navajo sandstone to the buoyant rise of hydrocarbons toward anticlinal crests we measured azimuths of 163 pipe like pipy joint associated concretions and those of 58 comet tails that extend from some spherical concretions both data sets have vector magnitudes 90 and subparallel southeast directed resultant vectors our data indicate that groundwater transported and deposited iron over a fl ow path that spanned more than 100 km from the anticlinal crest to the colorado river buoyant fl uids could not have transported iron from bleached rock to sites of concretion growth however because iron moved downdip we propose that iron was transported southeastward by reducing groundwater that fl owed through a co 2 and methane charged reservoir within the escalante anticline iron rich carbonates precipitated during co 2 degassing after methane was fl ushed from the aquifer microaerophiles oxidized fe 2 in concretions dissolving the carbonates and forming the iron oxide rich rinds and iron poor cores these concretions defi ne the fl ow direction and geochemical evolution of a paleoaquifer this approach could be used to follow the water on other planets

siderite concretions indicators of early diagenesis in the gammon shale cretaceous the upper cretaceous gammon shale in the northern great plains contains abundant siderite concretions that formed during the early compactional history of the fine grained marine sediments the relative depth of siderite precipitation can be estimated on the basis of textural mineralogical and isotopic evidence siderite concretions that formed at shallow depths in the sediment column contain a large volume percent carbonate 75 85 preserve uncompacted sediment textures and have oxygen isotope ratios similar to those of carbonates precipitated in equilibrium with seawater delta 18 o nearly equal e 0 to 2 per thousand pdb in contrast lower volume percent carbonate values 55 75 and relative decreases in 18 o 16 o values characterize concretions that formed longer after deposition and or at greater depths in the accumulating sediments both volume percent carbonate and delta 18 o values are strongly correlated with independently determined sediment accumulation rates concretions in rapidly deposited sediments formed at shallow depths 200 m later in the burial history the gammon served as both source rock and reservoir for accumulations of biogenic methane and like the siderite concretions the methane was generated during early diagenesis both siderite precipitation and methane accumulation require interstitial pore waters that have sufficiently high bicarbonate activities and that are depleted in dissolved sulfur and free oxygen because of chemical requirements siderite did not precipitate until after the onset of methane generation therefore if the depth of siderite formation is known the minimum depth for the onset of methane accumulation can be inferred the presence of abundant pyrite in slowly accumulated gammon sediments suggests by analogy to data from recent sediments that the depth of siderite precipitation and methane accumulation was controlled by the sedimentation rate in slowly accumulating sediments most organic matter was probably consumed by aerobic organisms and sulfate reducing anaerobic bacteria whereas in areas of more rapid deposition large quantities of organic matter were available for methane generation