spatial zonation limits magnesite dissolution in porous media abstract we investigate how mineral spatial distribution in porous media affects their dissolution rates specifically we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration v v was held constant the largest difference was observed between a mixed column containing uniformly distributed magnesite and a one zone column containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid flow parallel one zone column the columns were flushed with acid water at a ph of 4 0 at flow velocities of 3 6 or 0 36 m d breakthrough data show that the rate of magnesite dissolution is 1 6 2 times slower in the one zone column compared to the mixed column this extent of rate limitation is much larger than what was observed in our previous work 14 for a similar one zone column where the magnesite was packed in a layer aligned perpendicular to flow flow transverse one zone column two dimensional reactive transport modeling with crunchflow revealed that ion activity product iap and local dissolution rates at the grid block scale 0 1 cm vary by orders of magnitude much of the central magnesite zone in the one zone flow parallel column is characterized by close or equal to equilibrium conditions with iap k eq 0 1 two important surface areas are defined to understand the observed rates the effective surface area a e reflects the magnesite that effectively dissolves under far from equilibrium conditions iap k eq a i reflects the effective magnesite surface that lies along the quartz magnesite interface modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of a e and a i under the conditions examined in this work the value of a e varies from 2 to 67 of the total magnesite bet surface area column scale bulk rates r mgco 3 b in units of mol s vary linearly with a e and a i using a e to normalize rates we calculate a rate constant 10 9 56 mol m 2 s that is very close to the value of 10 10 0 mol m 2 s under well mixed conditions at the grid block scale this implies that the laboratory field rate discrepancy can potentially be caused by differences in the effective surface area if we know the effective surface area of dissolution we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems in this work approximately 60 70 of the a e is at the magnesite quartz interface this implies that in some field systems where the detailed information that we have for our columns is not available the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution chemical heterogeneity on dissolution

implications of subduction and subduction zone migration of the paleo pacific plate beneath eastern north china based on distribution geochronology and geochemistry of late mesozoic volcanic rocks several major volcanic zones are distributed across the eastern north china craton from northwest to southeast the greater xing an range jibei liaoxi xishan and songliao basins and the yanji huanghua and ludong volcanic zones the huanghua depression within the bohai bay basin was filled by middle late mesozoic volcanic rocks and abundant cenozoic alkaline basalts zircon la icp ms and shrimp u pb dating show that basic intermediate volcanic rocks were extruded in the early cretaceous of 118 8 1 0 ma weighted mean 206pb 238u age before late cretaceous acid lavas at 71 5 2 6 ma an inherited zircon from andesite has a paleoprotoerozoic core crystallization age of 2 424 22 ma 206pb 207pb age indicating that the basement of the bohai bay basin is part of the north china craton early cretaceous basic and intermediate lavas are characterized by strong enrichments in lree and lile and depletions in hree and hfse indicating a volcanic arc origin related to oceanic subduction depletion in zr only occurs in basic and intermediate volcanic rocks while depletions in sr and ti exist only in acid samples indicating that the acid series is not genetically related to the basic intermediate series formation ages and geochemical features indicate that the late cretaceous acid lavas are products of crustal remelting in an extensional regime combined information from all these volcanic zones shows that subduction related volcanic rocks were generated in the jibei liaoxi and xishan volcanic zones during the early jurassic about 60 ma earlier than their analogues extruded in the huanghua and ludong volcanic zones during the early cretaceous this younging trend also exists in the youngest extension related volcanism in each of these zones early cretaceous asthenosphere derived alkaline basalts in the northwest and late cretaceous in the southeast a tectonic model of northwestward subduction and continuous oceanward retreat of the paleo pacific plate is proposed to explain the migration pattern of both arc related and post subduction extension related volcanic rocks as the subduction zone continuously migrated active continental margin and backarc regimes successively played their roles in different parts of north china during the late mesozoic j1 k2

kinetically limited weathering at low denudation rates in semiarid climatic conditions biogeochemical cycling within the critical zone depends on the interactions between minerals and fluids controlling chemical weathering and physical erosion rates in this study we explore the role of water availability in controlling soil chemical weathering in semiarid climatic conditions weathering rates and intensities were evaluated for nine soil profiles located on convex ridge crests of three mountain ranges in the spanish betic cordillera we combine a geochemical mass balance with 10be cosmogenic nuclides to constrain chemical weathering intensities and long term denudation rates as such this study presents new data on chemical weathering and 10be derived denudation for understudied semiarid climate systems in the betic cordillera chemical weathering intensities are relatively low 5 to 30 of the total denudation of the soil and negatively correlated with the magnitude of the water deficit in soils chemical mass losses are inversely related to denudation rates 14 109 mm kyr and positively to soil thickness 14 58 cm these results are consistent with kinetic limitation of chemical weathering rates a worldwide compilation of chemical weathering data suggests that soil water balance may regulate the coupling between chemical weathering and physical erosion by modulating soil solute fluxes therefore future landscape evolution models that seek to link chemical weathering and physical erosion should include soil water flux as an essential driver of weathering

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

a spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady state denudation abstract spheroidal weathering a common mechanism that initiates the transformation of bedrock to saprolite creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets in the spheroidally weathering rio blanco quartz diorite puerto rico diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides a positive v of reaction results in the build up of elastic strain energy in the rock formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy the rate of spheroidal weathering is thus a function of the concentration of reactants the reaction rate the rate of transport and the mechanical properties of the rock substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the rio icacos bedrock 2 3 cm and a time interval between fractures 200 300 a based on an assumption of steady state denudation at the measured rate of 0 01 cm a averaged over times longer than this interval the rate of advance of the bedrock saprolite interface during spheroidal weathering the weathering advance rate is constant with time assuming that the oxygen concentration at the bedrock saprolite interface varies with the thickness of soil saprolite yields predictive equations for how weathering advance rate and steady state saprolite soil thickness depend upon atmospheric oxygen levels and upon denudation rate the denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite in our model for spheroidal weathering of the rio blanco quartz diorite fractures occur every 250 yr ferric oxide is fully depleted over a four rindlet set in 1000 yr and saprolitization is completed in 5000 yr in the zone containing 20 rindlets spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures but for the same kinetics and transport parameters the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the rio blanco system the spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time

an adakitic pluton on kyushu island southwest japan arc abstract an adakitic pluton is designated the shiraishino granodiorite ca 15 km2 in outcrop area on kyushu island southwestern japanese arc sr y and rare earth element concentrations and crystallization characteristics of the pluton are similar to those of adakites geochemical and tectonic considerations show that the pluton was derived from partial melting of a subducted slab with minor contamination of high 87sr 86sr material and subsequent fractional crystallization of plagioclase hornblende compiled sr and y data indicate that most cretaceous granitic rocks of southwestern japan generated by partial melting of mafic lower crust adakitic plutons derived by slab melting occur rarely and have older ages it is concluded that plutonism in the southwestern japanese arc during cretaceous time was began with the emplacement of small adakitic intrusions and progressed to more abandant granitic rocks of the type derived from the mafic lower crust

a theoretical model coupling chemical weathering rates with denudation rates uplift of the himalayas has been proposed to have locally accelerated chemical weathering thus leading to enhanced co2 sequestration and global cooling this hypothesis assumes that rapid erosion exposes fresh highly reactive minerals at earth s surface empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results where the nature of the relationship is dependent on the weathering regime of the sampled landscapes we derive a quantitative model that defines this relationship across the range of weathering regimes from supply limited to kinetically limited conditions the model matches trends in field data collected by others and reconciles apparently conflicting results the model also demonstrates that as erosion rates increase potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith we conclude that the relationship between weathering and erosion is one of diminishing returns in which increases in erosion rate lead to progressively smaller increases in weathering rate indeed at the highest erosion rates weathering rates may decline the ability therefore of accelerated uplift and erosion to stimulate greater co2 sequestration may be significant in landscapes eroding at rates of 10 102 t km 2 yr 1 however where erosion rates are greater than 102 t km 2 yr 1 increases in denudation may not be matched by increases in chemical weathering finally our results suggest that watersheds with regolith thicknesses of 0 5 m will yield the greatest solute fluxes

investigating the complex interface where bedrock transforms to regolith abstract the interface where bedrock transforms to regolith is not planar but rather has a roughness that varies with the scale of observation the complexity of this surface is manifested in both element depth and fragment size depth distributions and may sometimes be related to the longitudinal profiles of watershed streams the fractal nature of the bedrock regolith interface means that the interface has a thickness which is 20 m in two ridgetop examples from pennsylvania and puerto rico such weathering thicknesses modeled as a function of one dimensional fluid flow are affected by the balance between rates of weathering and erosion one dimensional models are consistent with weathering advance rates that vary with equilibrium solubility and porefluid velocities and not reaction kinetics however fluid flow is not strictly downward and one dimensional permeability of regolith changes as particle size and bulk density changes with depth thus both downward and lateral flow occurs especially at reaction fronts where reactions change permeability the rate of weathering advance is therefore affected by the 3 dimensional distribution of reaction zones that affect permeability across the watershed quantitative models of such phenomena over a range of spatial and temporal scales are needed

high pmetamorphism pattern of induced flow in the mantle wedge and the link with plutonism in paired metamorphic belts the convex form of subduction stage pressure temperature p t paths up to c 2 0 gpa implies the sambagawa high p metamorphic belt japan formed a few million years before ridge subduction additional compilation of p t conditions for higher p sambagawa rocks c 2 0 2 5 gpa reveals that the thermal profile along the slab surface shows a remarkable high t ward warping at c 2 0 gpa c 65 km previous thermal models indicate that this warping corresponds to the onset of induced mantle flow towards the subducting slab if a normal thickness continental crust of c 30 km was present this implies the hangingwall region between 30 and 65 km depth was occupied by serpentinized wedge mantle isolated from large scale mantle flow subsequent arrival of the spreading ridge reheating and dehydration of the serpentinized wedge probably supplied the water necessary for causing granitic magmatism in the ryoke high t metamorphic belt which is paired with the sambagawa belt

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

a reactive transport model for examining tectonic and climatic controls on chemical weathering and atmospheric co2 consumption in granitic regolith we developed a 1d reactive transport model for examining how tectonic and climatic parameters namely uplift rate water seepage velocity and temperature control chemical weathering and atmospheric co2 consumption rates during regolith development the model consists of mass conservation equations describing how mineral and solute concentrations change temporally and spatially vertically during weathering of granite containing 30 plagioclase an20 and up to 3 accessory calcite the equations are coupled by volumetric weathering rates which depend on mineral abundances intrinsic dissolution rate constants and surface areas as well as the departure of pore water from thermodynamic equilibrium we numerically solve a non steady state non dimensional version of the model by eliminating the need to prescribe regolith thickness non dimensionalization introduces two key scaling parameters that drive variations in the model results the rock residence time r the time for rock to travel upward through the model domain and the water residence time w the time for water to travel downward through the model domain we use the model to examine fundamental properties of chemical weathering especially reaction front propagation with the primary aim of characterizing weathering regimes and identifying factors that maximize the dissolution of plagioclase because only silicate weathering regulates atmospheric co2 levels over geological timescales weathering regimes are commonly classified as transport or supply limited versus weathering or kinetically limited but as outgrowths of geomorphology these terms primarily refer to the role of r in regulating regolith thickness our findings suggest that the paradigm should be expanded to include kinetic controls determined by w both regimes can experience far from and near equilibrium mineral dissolution however transport limited regimes generally have lower w r ratios compared to kinetically limited regimes under transport limitation thick to thin reaction fronts propagate downward indefinitely yielding extensive mineral depletion deeply weathered regolith and high ratios of silicate to carbonate weathering under kinetic limitation thin to non existent reaction fronts evolve to steady state yielding minimal mineral depletion no regolith development and low ratios of silicate to carbonate weathering kinetically limited regimes typifying tectonically active mountain ranges have low long term atmospheric co2 consumption rates transport limited regimes typifying tectonically stable cratons also have low long term atmospheric co2 consumption rates but because much of the earth s surface is transport limited we infer that such environments have the greatest impact on the global atmospheric co2 consumption flux the model output leads us to hypothesize that the maximum contribution should occur for extensive regions of silicate bedrock consolidated in warm wet climates because the combination of tectonic stability high temperatures and rapid seepage velocities accelerates reaction front propagation facilitates calcite depletion and sustains deep regolith development over long timescales our findings point to the importance of internal climate feedbacks for stabilizing long term atmospheric co2 levels

strong climate and tectonic control on plagioclase weathering in granitic terrain abstract investigations to understand linkages among climate erosion and weathering are central to quantifying landscape evolution we approach these linkages through synthesis of regolith data for granitic terrain compiled with respect to climate geochemistry and denudation rates for low sloping upland profiles focusing on na as a proxy for plagioclase weathering we quantified regolith na depletion na mass loss and the relative partitioning of denudation to physical and chemical contributions the depth and magnitude of regolith na depletion increased continuously with increasing water availability except for locations with mean annual temperature

evidence for significant clockwise rotations of the korean peninsula during cretaceous abstract in an effort to evaluate the cretaceous magnetostratigraphy for the korean peninsula and to establish the tectonic coherence of its various elements we collected paleomagnetic data from 121 samples from 20 sites within the chilgok formation 108 3 109 9 ma in the gyeongsang basin together with previously published data we evaluate the results from a total of 163 sites in the basin we combine our age model with results from recent stratigraphic paleomagnetic and radiometric geochemical studies in this study we found that two distinct declination shifts decrease with younging direction indicating two clockwise rotational events of the korean peninsula with respect to the eurasia continent the earlier event took place during 130 100 ma phase i newly termed goguryeo disturbance and a later one during 80 50 ma phase iii belonging to bulguksa orogeny the mean rotation rate in the interval from 115 2 to 103 8 ma phase i is about 0 74 ma while the rate from 90 9 to 79 8 ma phase ii is 0 19 ma based on paleolatitude change during phase i we infer that the korean peninsula eastern part of the sino korea block migrated southward about 300 km after the complete amalgamation of the sino korea block into the eurasian continent resulting in n s compression within the korean peninsula and manchuria large scale strike slip faults e g tan lu fault okcheon boundary fault were probably rejuvenated in the sino korea block during phase i

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

coupling chemical weathering with soil production across soil mantled landscapes soil covered upland landscapes constitute a critical part of the habitable world our understanding of how they evolve as a function of different climatic tectonic and geological regimes is important across a wide range of disciplines and depends in part on understanding the links between chemical and physical weathering processes extensive previous work has shown that soil production rates decrease with increasing soil column thickness but chemical weathering rates were not measured here we examine a granitic soil mantled hillslope at point reyes california where soil production rates were determined using in situ produced cosmogenic nuclides 10be and 26al and we quantify the extent as well as the rates of chemical weathering of the saprolite from beneath soil from across the landscape we collected saprolite samples from the base of soil pits and analysed them for abrasion ph as well as for major and trace elements by x ray fluorescence spectroscopy and for clay mineralogy by x ray diffraction spectroscopy our results show for the first time that chemical weathering rates decrease with increasing soil thickness and account for 13 to 51 per cent of total denudation we also show that spatial variation in chemical weathering appears to be topographically controlled weathering rate decreases with slope across the divergent ridge and increases with upslope contributing area in the convergent swale furthermore to determine the best measure for the extent of saprolite weathering we compared four different chemical weathering indices the vogt ratio the chemical index of alteration cia parker s index and the silicon aluminium ratio with saprolite ph measurements of the cia were the most closely correlated with saprolite ph showing that weathering intensity decreases linearly with an increase in saprolite ph from 4 7 to almost 7 data presented here are among the first to couple directly rates of soil production and chemical weathering with how topography is likely to control weathering at a hillslope scale copyright 2006 john wiley sons ltd

controls on rock weathering rates by reaction induced hierarchical fracturing abstract weathering of rocks takes place through complex processes involving an intimate coupling between mechanical disintegration of the rocks and interactions between the exposed rock surface the hydrosphere and the biosphere it is still poorly understood how this coupling affects weathering rates and patterns produced by weathering here we show that the observed spheroidal weathering patterns in basaltic intrusions dolerites in the karoo basin in south africa are created through volume expanding reactions which set up stresses that not only lead to spalling of spheroidal layers of rock but also drive a large scale hierarchical fracturing process by which the dolerite is continuously undergoing domain division the fracture processes generates new reactive surface area in a self accelerating manner thus providing a first order control on the total weathering rate

influence of interaction between small asperities on various types of slow earthquakes in a 3 d simulation for a subduction plate boundary recently the occurrence of slow earthquakes such as low frequency earthquakes and very low frequency earthquakes have been recognized at depths of about 30 km in southwest japan and cascadia these slow earthquakes occur sometimes in isolation and sometimes break into chain reaction producing tremor that migrates at a speed of about 5 15 km day and suggesting a strong interaction among nearby small asperities in this study we formulate a 3 d subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km our simulation succeeds in representing various types of slow earthquakes including low frequency earthquakes and rapid slip velocity in the same asperity and indicates that interaction between asperities may cause the very low frequency earthquakes our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters which suggests that the interactions also explain the observed migration of slow earthquakes

regolith production rates calculated with uranium series isotopes at susquehanna shale hills critical zone observatory abstract in the critical zone where rocks and life interact bedrock equilibrates to earth surface conditions transforming to regolith the factors that control the rates and mechanisms of formation of regolith defined here as material that can be augered are still not fully understood to quantify regolith formation rates on shale lithology we measured uranium series u series isotopes 238 u 234 u and 230 th in three weathering profiles along a planar hillslope at the susquehanna shale hills observatory ssho in central pennsylvania all regolith samples show significant u series disequilibrium 234 u 238 u and 230 th 238 u activity ratios range from 0 934 to 1 072 and from 0 903 to 1 096 respectively these values display depth trends that are consistent with fractionation of u series isotopes during chemical weathering and element transport i e the relative mobility decreases in the order 234 u 238 u 230 th the activity ratios observed in the regolith samples are explained by i loss of u series isotopes during water rock interactions and ii re deposition of u series isotopes downslope loss of u and th initiates in the meter thick zone of bedrock that cannot be augered but that nonetheless consists of up to 40 clay silt sand inferred to have lost k mg al and fe apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m myr to 17 m myr with increasing regolith thickness from the ridge top to the valley floor with increasing distance from the ridge top toward the valley apparent equivalent regolith residence times increase from 7 kyr to 40 kyr given that the ssho experienced peri glacial climate 15 kyr ago and has a catchment wide averaged erosion rate of 15 m myr as inferred from cosmogenic 10 be we conclude that the hillslope retains regolith formed before the peri glacial period and is not at geomorphologic steady state both chemical weathering reactions of clay minerals and translocation of fine particles colloids are shown to contribute to mass loss of u and th from the regolith consistent with major element data at ssho this research documents a case study where u series isotopes are used to constrain the time scales of chemical weathering and regolith production rates regolith production rates at the ssho should be useful as a reference value for future work at other weathering localities

models of transport and reaction describing weathering of fractured rock with mobile and immobile water we propose models for calculating the rate of chemical weathering of minerals as a function of depth in the weathering zone a macropore network is assumed to be responsible for the transport of mobile water which removes soluble weathering products at the interface of that network and the matrix conditions of infrequent rainfall a and of very frequent rainfall b are separately modeled but both lead to a volumetric weathering rate with the general form 0exp z where the amplitude 0 and the equilibration length depend on the pore geometry of regolith and on parameters describing transport across the macropore matrix interface a or mineral dissolution b this is obtained with no assumption of steady state for regolith evolution extrapolating these end members into intermediate conditions the model is consistent with the exponential decay of regolith production rate versus depth reported by several authors and yields in a variety of regolith types in the range from centimeters to meters the velocity of the regolith bedrock interface also shows an exponential decay with weathering zone thickness and is enhanced by bedrock fractures when compared to models of unfractured bedrock when the residence time of fluid in the weathering zone f is large the bulk weathering rate rb is inversely proportional to this time and for small f rb reverts to the laboratory rate application to compiled data from several sites leads to estimates of a dissolution rate constant and specific surface area consistent with those of albite

erosional dynamics flexural isostasy and long lived escarpments a numerical modeling study erosional escarpments are common features of high elevation rifted continents fission track data suggest that these escarpments form by base level lowering and or marginal uplift during rifting followed by lateral retreat of an erosion front across tens to hundreds of kilometers previous modeling studies have shown that this characteristic pattern of denudation can have a profound impact upon marginal isostatic uplift and the evolution of offshore sedimentary basins yet at present there is only a rudimentary understanding of the geomorphic mechanisms capable of driving such prolonged escarpment retreat in this study we present a nonlinear two dimensional landscape evolution model that is used to assess the necessary and sufficient conditions for long term retreat of a rift generated escarpment the model represents topography as a grid of cells with drainage networks evolving as water flows across the grid in the direction of steepest descent the model accounts for sediment production by weathering fluvial sediment transport bedrock channel erosion and hillslope sediment transport by diffusive mechanisms and by mass failure numerical experiments presented explore the effects of different combinations of erosion processes and of dynamic coupling between denudation and flexural isostatic uplift model results suggest that the necessary and sufficient conditions for long term escarpment retreat are 1 incising bedrock channels in which the erosion rate increases with increasing drainage area so that the channels steepen and propagate headward 2 a low rate of sediment production relative to sediment transport efficiency which promotes relief generating processes over diffusive ones 3 high continental elevation which allows greater freedom for fluvial dissection and 4 any process including flexural isostatic uplift that helps to maintain a drainage divide near an escarpment crest flexural isostatic uplift also facilitates escarpment retreat by elevating topography in the vicinity of an eroding escarpment thereby increasing channel gradients and accelerating erosion which in turn generates additional isostatic uplift of all the above conditions high continental elevation is common to most rift margin escarpments and may ultimately be the most important factor

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

late orogenic post orogenic and anorogenic granites distinction by major element and trace element chemistry and possible origins granites classified into four categories based solely on tectonics of occurrence and associated rock types also have compositional characteristics that are consistent within groups and different am

thickness of the chemical weathering zone and implications for erosional and climatic drivers of weathering and for carbon cycle feedbacks quantitative understanding of variability in weathering fluxes on the modern earth is limited because little is known about where the most important weathering reactions take place this is partly because the locus of weathering is difficult to measure empirically inverse analysis of a parametric model presented here provides first order constraints on variability in the thickness of the zone of active weathering results suggest that the effective thickness of the weathering zone varies relatively little across several orders of magnitude of denudation rate at low to moderate denudation rates reactions in soils may dominate weathering fluxes at the catchment scale but the contribution from soil weathering decreases at higher denudation rates consequently increased erosion leads to higher weathering fluxes sustained by progressively greater contributions from weathering in bedrock the effect of climate temperature and runoff on weathering fluxes is apparently weaker at low denudation rates than at high denudation rates such that erosion and potentially associated bedrock weathering may be important for maintaining climate stabilizing feedbacks in earth s carbon cycle

using a reactive transport model to elucidate differences between laboratory and field dissolution rates in regolith the reactive transport model flotran was used to forward model weathering profiles developed on granitic outwash alluvium over 40 3000 ka from the merced california usa chronosequence as well as deep granitic regolith developed over 800 ka near davis run virginia usa baseline model predictions that used laboratory rate constants km measured fluid flow velocities v and bet volumetric surface areas for the parent material a o m were not consistent with measured profiles of plagioclase potassium feldspar and quartz reaction fronts predicted by the baseline model are deeper and thinner than the observed consistent with faster rates of reaction in the model reaction front depth in the model depended mostly upon saturated versus unsaturated hydrologic flow conditions rate constants controlling precipitation of secondary minerals and the average fluid flow velocity va unsaturated hydrologic flow conditions relatively open with respect to co2 g resulted in the prediction of deeper reaction fronts and significant differences in the separation between plagioclase and potassium feldspar reaction fronts compared to saturated hydrologic flow relatively closed with respect to co2 g under saturated or unsaturated flow conditions the rate constant that controls precipitation rates of secondary minerals must be reduced relative to laboratory rate constants to match observed reaction front depths and measured pore water chemistry additionally to match the observed reaction front depths va was set lower than the measured value v for three of the four profiles the reaction front gradients in mineralogy and pore fluid chemistry could only be modeled accurately by adjusting values of the product kma o m by assuming km values were constrained by laboratory data field observations were modeled successfully with tst like rate equations by dividing measured values of a o m by factors from 50 to 1700 alternately with sigmoidal or alinhibition rate models this adjustment factor ranges from 5 to 170 best fit models of the wetter hydrologically saturated davis run profile required a smaller adjustment to a o m than the drier hydrologically unsaturated merced profiles we attributed the need for large adjustments in va and a o m necessary for the merced models to more complex hydrologic flow that decreased the reactive surface area in contact with bulk flow water e g dead end pore spaces containing fluids that are near or at chemical equilibrium thus rate models from the laboratory can successfully predict weathering over millions of years but work is needed to understand how to incorporate changes in what controls the relationship between reactive surface area and hydrologic flow

basalt weathering across scales abstract weathering of silicate minerals impacts many geological and ecological processes for example the weathering of basalt contributes significantly to consumption of atmospheric carbon dioxide co2 and must be included in global calculations of such consumption over geological timeframes here we compare weathering advance rates for basalt wd where d and indicate the scale at which the rate is determined and surface area measured respectively from the laboratory to the watershed scales data collected at the laboratory weathering rind soil profile and watershed scales show that weathering advance rate of basalt is a fractal property that can be described by a fractal dimension dr 2 3 by combining the fractal description of rates with an arrhenius relationship for basalt weathering we derive the following equation to predict weathering advance rates at any spatial scale from weathering advance rates measured at the bet scale w d k 0 a d r 2 e e a r t here k0 is the pre exponential factor 1 29 107 mm3 mm 2 yr 1 ea is the activation energy 70 kj mol 1 and a is a spatial constant related to the scale of measurement of bet surface area 10 7 mm the term a d r 2 is the roughness the roughness fractal dimension can be conceptualized as a factor related to both the thickness of the reaction front and the specific surface area within the reaction front however the above equation can also be written in terms of a surface fractal dimension and the hypothetical average grain radius these fractal dimensions provide insight into reaction front geometry and should vary with lithology once the surface area discrepancy has been accounted for using this method we find a one to two order of magnitude range in weathering advance rates measured at any scale or temperature that can be attributed to factors such as changes in erosional regime parent lithology mechanism climate composition of reacting fluid and biological activity our scaled equation when used to predict global basalt co2 consumption based upon global lithologic maps yields an uptake flux 1 75 1013 mol co2 yr 1 within the predicted error of fluxes estimated based upon riverine measurements

soil transport driven by biological processes over millennial time scales downslope soil transport in the absence of overland flow has been attributed to numerous mechanisms including particle by particle creep and disturbances associated with biological activity process stochasticity and difficulties associated with field measurement have obscured the characterization of relevant long term soil transport rates and mechanisms in a series of incised fluvial terraces along the charwell river south island new zealand we documented vertical profiles of tephra concentration and topographic derivatives along a hillslope transect to quantify soil transport processes along the undissected hilltop we observed a thin primary tephra layer ca 22 6 ka within loess deposits 80 cm below the landscape surface in the downslope direction the depth to the highly concentrated tephra layer decreases coincident with an increase in hillslope convexity which is proportional to landscape lowering rate if soil flux varies linearly with hillslope gradient exhumation of the tephra layer results from landscape lowering due to disturbance driven soil transport approximately 20 m downslope of the interfluve the depth to the tephra layer declines to 40 50 cm peak tephra concentrations decrease by a factor of 4 and tephra is distributed uniformly within the upper 40 cm of soil the transition from a thin highly concentrated tephra layer at depth to less concentrated widely distributed tephra in the upper soil may result from soil mixing and transport by biological disturbances along our transect the depth to this transition is 50 cm coincident with the rooting depth of podocarp and nothofagus trees that populated the region during much of the holocene our observations can be used to calibrate the linear transport model but more important they suggest that over geomorphic time scales stochastic bioturbation may generate a well mixed and mobile soil layer the depth of which is primarily determined by flora characteristics

competition between erosion and reaction kinetics in controlling silicate weathering rates article i nfo weathering rates of minerals may be limited by the reaction kinetics of the constitutive minerals or the supply of fresh minerals delivered to the near surface by denudation we use a model of denudation and reaction kinetics to analyze the relative importance of each in moderating silicate weathering fluxes in different erosional environments for different commonly occurring silicate minerals we find that minerals that reside in the near surface weathering zone for far longer than the reaction takes to complete as is the case with slow denudation produce weathering fluxes that respond most strongly to changes in denudation rate conversely when transit through the weathering zone is rapid relative to a given mineral s weathering timescale the weathering responds most vigorously to changes in the reaction kinetics based on field and laboratory measured values of silicate weathering kinetic constants observed weathering zone thicknesses in relation to denudation rates and denudation rates inferred from topography it appears that fresh mineral supply may play a sub equal or dominant role in moderating silicate weathering fluxes for four mineral phases considered if correct this suggests that the concentration of atmospheric co2 which is regulated by silicate weathering over geologic timescales may depend on those factors that control long term erosion rates across earth s surface

stochastic processes of soil production and transport erosion rates topographic variation and cosmogenic nuclides in the oregon coast range landscapes in areas of active uplift and erosion can only remain soil mantled if the local production of soil equals or exceeds the local erosion rate the soil production rate varies with soil depth hence local variation in soil depth may provide clues about spatial variation in erosion rates if uplift and the consequent erosion rates are sufficiently uniform in space and time then there will be tendency toward equilibrium landforms shaped by the erosional processes soil mantle thickness would adjust such that soil production matched the erosion previous work in the oregon coast range suggested that there may be a tendency locally toward equilibrium between hillslope erosion and sediment yield here results from a new methodology based on cosmogenic radionuclide accumulation in bedrock minerals at the base of the soil column are reported we quantify how soil production varies with soil thickness in the southern oregon coast range and explore further the issue of landscape equilibrium apparent soil production is determined to be an inverse exponential function of soil depth with a maximum inferred production rate of 268 m ma 1 occurring under zero soil depth this rate depends however on the degree of weathering of the underlying bedrock the stochastic and large scale nature of soil production by biogenic processes leads to large temporal and spatial variations in soil depth the spatial variation of soil depth neither supports nor rejects equilibrium morphology our observed catchment averaged erosion rate of 117 m ma 1 is however similar to that estimated for the region by others and to soil production rates under thin and intermediate soils typical for the steep ridges we suggest that portions of the oregon coast range may be eroding at roughly the same rate but that local competition between drainage networks and episodic erosional events leads to landforms that are out of equilibrium locally and have a spatially varying soil mantle copyright 2001 john wiley sons ltd

quasigeostrophic dynamics of the western boundary current abstract a local model is used to investigate the dynamics of the western boundary current in a midlatitude wind driven gyre this current is important for the gyre as a whole and its local instability is correlated with structural changes of the separated eastward jet and the interior gyres in particular the eastward jet can be disrupted and broadened in the regime with a strong local instability in the western boundary current such a regime occurs with a no slip lateral boundary condition alternatively in the absence of local instability the eastward jet is narrow and penetrates farther in the basin interior this behavior is typical with free slip boundary condition both the linear stability and nonlinear time dependent behavior of the western boundary current are analyzed for a wide range of parameters the current loses stability at moderate reynolds numbers and the stability threshold strongly depends upon the vertical stratification profile the nonlinear time dependent flow contains wel

a mathematical model for steady state regolith production at constant erosion rate it has been hypothesized that many soil profiles reach a steady state thickness in this work such profiles were simulated using a one dimensional model of reaction with advective and diffusive solute transport a model rock is considered consisting of albite that weathers to kaolinite in the presence of chemically inert quartz the model yields three different steady state regimes of weathering at the lowest erosion rates a local equilibrium regime is established where albite is completely depleted in the weathering zone this regime is equivalent to the transport limited regime described in the literature with an increase in erosion rate transition and kinetic regimes are established in the transition regime both albite and kaolinite are present in the weathering zone but albite does not persist to the soil air interface in the weathering limited regime here called the kinetic regime albite persists to the soil air interface the steady state thickness of regolith decreases with increasing erosion rate in the local equilibrium and transition regimes but in the kinetic regime this thickness is independent of erosion rate analytical expressions derived from the model are used to show that regolith production rates decrease exponentially with regolith thickness the steady state regolith thickness increases with the darcy velocity of the pore fluid and in the local equilibrium regime may vary markedly with small variations in this velocity and erosion rate in the weathering limited regime the temperature dependences for chemical weathering rates are related to the activation energy for the rate constant for mineral reaction and to the h of dissolution while for local equilibrium regimes they are related to the h only the model illustrates how geochemical and geomorphological observations are related for a simple compositional system the insights provided will be useful in interpreting natural regolith profiles copyright 2010 john wiley sons ltd

weathering and erosion of fractured bedrock systems we explore the contribution of fractures joints in controlling the rate of weathering advance for a low porosity rock by using methods of homogenization to create averaged weathering equations the rate of advance of the weathering front can be expressed as the same rate observed in non fractured media or in an individual block divided by the volume fraction of non fractured blocks in the fractured parent material in the model the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material the low porosity rock weathers by reaction transport processes as observed in field systems the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non fractured system because the volume fraction of blocks is 1 the increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density and to the increase in exposure of surface of low porosity rock to reaction transport for constant fracture aperture the weathering advance rate increases when the fracture spacing decreases equations describing weathering advance rate are summarized in the list of selected equations if erosion is imposed at a constant rate the weathering systems with fracture bounded bedrock blocks attain a steady state in the erosional transport limited regime bedrock blocks no longer emerge at the air regolith boundary because they weather away in the weathering limited or kinetic regime blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate for convex hillslopes the block size exposed at the surface increases downslope this model can explain observations of exhumed rocks weathering in the luquillo mountains of puerto rico published 2017 this article is a u s government work and is in the public domain in the usa

an initial continental margin plutonism cretaceous older ryoke granitoids southwest japan the emplacement of the older ryoke granitoids took place in the mid cretaceous period in the outermost side of the inner zone of southwest japan which was located at the eastern margin of the eurasian continent before the opening of the japan sea the ryoke belt constitutes a long segment of the cretaceous to palaeogene felsic magmatic belt of pacific asia the older ryoke granitoids represent its initial magmatism related to the subduction of an oceanic plate underneath the eurasian plate in that time their magmas were generated in the lower to middle crust beneath the ryoke belt in the subduction regime they were emplaced during the ryoke regional metamorphism and converted to orthogneisses

solute transfer in the unsaturated zone groundwater continuum of a headwater catchment this study deals with solute transfer in the vertical continuum between the unsaturated zone and shallow groundwater of a weathered granite aquifer in the kerbernez headwater catchment of western france the objectives are i to determine the mechanisms responsible for solute transfer in the unsaturated and water table fluctuation zones of the aquifer and ii to analyse the implications of these results on solute transfer times at the catchment scale an experimental site located in the plateau area of the catchment was equipped with 6 tensiometers 18 ceramic cups at depths from 0 25 to 2 5 m and 7 piezometers from 3 to 20 m measurements of hydraulic head and water sampling were carried out over a period of 2 5 years in the unsaturated zone 0 2 m the water table fluctuation zone 2 9 m and the permanently saturated zone 9 m two tracer experiments were carried out by applying two pulses of water one enriched with deuterium and the other with bromide natural chloride concentrations as well as deuterium and bromide concentrations were analysed from solution samples from the artificial tracer concentrations two porosity compartments can be identified and partly quantified 1 the slow mobile porosity 36 of the bulk volume accounting for the slow piston flow transfer 2 3 m per year and 2 the rapid mobile porosity which transfers small quantities of bromide at a rate of 19 cm h1 down to the water table natural chloride concentrations are characterised by a high temporal variability in the water table fluctuation zone whereas the concentrations remain steady in time in the permanently saturated zone 42 mg l1 at 20 m depth the temporal variability is related to the water table fluctuations and follows the same pattern each hydrological year i e low concentrations during rising water table followed by a progressive increase in concentrations during the periods of high piezometric level and water table recession this pattern is explained in terms of the two mobile porosity compartments and groundwater hydraulics based on these findings we propose a conceptual model of solute transfer along the hillslope of a headwater catchment we conclude that mixing in the water table fluctuation zone could occur at two spatial scales firstly at the pore scale with mixing of waters in slow mobile and rapid mobile porosity and secondly at the scale of the hillslope the mixing at this latter scale would appear to result from differences of flow path geometry and velocity between the unsaturated zone and the groundwater

modelling chemical depletion profiles in regolith abstract chemical or mineralogical profiles in regolith display reaction fronts that document depletion of leachable elements or minerals a generalized equation employing lumped parameters was derived to model such ubiquitously observed patterns c c 0 c 0 c x 0 c x 0 exp ini k x 1 here c cx 0 and co are the concentrations of an element at a given depth x at the top of the reaction front or in parent respectively ini is the roughness of the dissolving mineral in the parent and k is a lumped kinetic parameter this kinetic parameter is an inverse function of the porefluid advective velocity and a direct function of the dissolution rate constant times mineral surface area per unit volume regolith this model equation fits profiles of concentration versus depth for albite in seven weathering systems and is consistent with the interpretation that the surface area m2 mineral m 3 bulk regolith varies linearly with the concentration of the dissolving mineral across the front dissolution rate constants can be calculated from the lumped fit parameters for these profiles using observed values of weathering advance rate the proton driving force the geometric surface area per unit volume regolith and parent concentration of albite these calculated values of the dissolution rate constant compare favorably to literature values the model equation useful for reaction fronts in both steady state erosional and quasi stationary non erosional systems incorporates the variation of reaction affinity using ph as a master variable use of this model equation to fit depletion fronts for soils highlights the importance of buffering of ph in the soil system furthermore the equation should allow better understanding of the effects of important environmental variables on weathering rates

geochemical characteristics and variations of the ryoke granitoids southwest japan petrogenetic implications for the plutonic rocks of a magmatic arc abstract geochemical characteristics and variations of the ryoke granitoids southwest japan have been elucidated based on more than 70 analyses from the mikawa and kinki districts for major and trace elements including ree the ryoke granitoids are the products of a continental margin arc magmatism of pacific asia during late cretaceous period and represent its magmatic front they are classified into the older and younger groups according to their intrusive ages and textural and petrographic characteristics the older ryoke granitoids were emplaced at ca 20 km depth during 100 85 ma and deformed and recrystallized together with the host ryoke metamorphic rocks the younger ryoke granitoids are post tectonic plutons emplaced at ca 15 km depth 85 75 ma and accompanied by the formation of contact aureoles the majority belongs to i type and ilmenite series granitoids the granitoids of both groups are geochemically quite similar calcic alkali lime index 65 very low in fe 3 fe 2 ratio and both metaluminous and peraluminous they have low rb

the illusion of diffusion field evidence for depth dependent sediment transport soil covered upland landscapes are common in much of the habitable world and our understanding of their evolution as a function of different climatic tectonic and geologic regimes is important across a wide range of disciplines erosion laws direct quantitative study of the processes shaping earth s surface and form the basis of landscape evolution modeling but are based on limited field data here we use in situ produced cosmogenic 1 0 be and 2 6 al concentrations from granitic saprolite to quantify an exponential decline in soil production with increasing soil thickness for a new field site in point reyes california results are similar to soil production functions from two different previously studied field sites and are used with extensive measurements of soil thickness to quantify depth integrated sediment transport flux plots of calculated sediment fluxes against the product of soil depth and hillslope gradient provide the first field based evidence that soil transport is a nonlinear depth dependent function data from all sites suggest that the widely used linear diffusion equation is only appropriate for shallow gradient convex up regions while the depth dependent transport law is more broadly applicable quantifying both the mobile soil thickness and landscape morphology is therefore critical to understanding how landscapes evolve

the convenient fiction of steady state soil thickness abstract steady state regolith or soil thickness ssst whereby surface removals are approximately balanced by production of new soil by bedrock weathering is a common assumption in most models of hillslope and landscape evolution ssst is also a fundamental assumption in the use of cosmogenic radionuclides crn to estimate erosion and weathering rates the steady state concept is based on feedbacks between soil thickness and weathering at the base of the regolith such that sometimes after an optimal or threshold thickness is achieved thicker soils lead to lower weathering rates and vice versa ssst is thus only applicable to soils formed chiefly from weathering of the underlying bedrock where sufficient time has elapsed for regolith accumulation and where effects of processes other than weathering and surface removals on thickness are negligible even within this domain the widespread occurrence of deep weathering profiles regolith stripping and inherited regolith features makes ssst problematic as a conceptual model for pedogenesis or weathering profile development the ratio of soil thickness to total weathering profile thickness is proposed as a simple index of steady state steady state profiles formed on weathered bedrock should exhibit ratios close to unity data from the cumberland plateau region of eastern kentucky show that soil weathering profile ratios in shallow 20 m and generally significantly greater than soil thicknesses in the region suggesting ratios convenient fiction facilitating the use of some models and tools the potential pitfalls arise from the possibility that the utility of ssst as a convenient fiction in some contexts may be mistaken for a realistic representation of the dynamics of pedogenesis and weathering profile evolution