soil profiles as indicators of mineral weathering rates and organic interactions for a pennsylvania diabase abstract basaltic bedrock dissolves quickly and its weathering rate is therefore important towards controlling the composition of natural waters soil formation and co 2 concentrations in the atmosphere despite its importance however few reports of basalt or diabase and gabbro weathering rates exist in the literature and most have been measured in laboratory dissolution experiments or based on watershed studies here using elemental profiles measured through regolith on a jurassic diabase dike in south central pennsylvania we calculate time integrated log dissolution rates mol m 2 s 1 of the primary minerals plagioclase 14 9 s 1 and augite 14 8 and of smectite 17 6 a secondary clay mineral formed in the soil characteristic patterns in elemental profiles are consistent with preserved signatures of corestone formation elemental and mineral signatures of the soils relative to the parent rock are compared to predictions from citrate containing basalt column dissolution experiments depletion of apatite and of al fe mn ti p y ni cr sc v ga cu zn and la are observed in the upper meter of the profile relative to the parent rock

rates of weathering rind formation on costa rican basalt weathering rind thicknesses were measured on 200 basaltic clasts collected from three regionally extensive alluvial fill terraces qt 1 qt 2 and qt 3 preserved along the pacific coast of costa rica mass balance calculations suggest that conversion of unweathered basaltic core minerals plagioclase and augite to authigenic minerals in the porous rind kaolinite allophane gibbsite fe oxyhydroxides is iso volumetric and ti and zr are relatively immobile the hierarchy of cation mobility ca na k mg si al fe p is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering anorthite albite hypersthene orthoclase apatite alteration profiles across the cm thick rinds document dissolution of plagioclase and augite and the growth of kaolinite with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age the rate of weathering rind advance is evaluated using a diffusion limited model which predicts a parabolic rate law for weathering rind thickness rr as a function of time t rr t and an interface limited model which predicts a linear rate law for weathering rind thickness as a function of time rr kappt in these rate laws is a diffusion parameter and kapp is an apparent rate constant the rate of advance is best fit by the interface model terrace exposures are confined to the lower reaches of streams draining the pacific slope near the coast where the stream gradient is less than 3 m km and terrace deposition is influenced by eustatic sea level fluctuations geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest assigning the most weathered regionally extensive terrace qt 1 mean rind thickness 6 9 0 6cm to oxygen isotope stage ois 7 ca 240 ka and assuming that at time 0 rind thickness 0 it is inferred that terrace qt 2 rr 2 9 0 1 cm is coincident with stage 5e ca 125 ka and that qt 3 rr 0 9 0 1 cm is consistent with ois 3 ca 37 ka these assignments yield a value of kapp of 8 6 10 13 cm s 1 r 2 0 99 only this value satisfies both the existing age controls and yields ages coincident with sea level maxima using this value elemental weathering release fluxes across a weathering rind from qt 2 range from 6 0 10 9 mol si m 2 s 1 to 2 5 10 11 mol km 2 s 1 the rate of rind advance for the costa rican terraces is 2 8 10 7 my r 1 basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface controlled weathering with an apparent activation energy of about 50 kj mol 1 rates of rind formation in costa rica are an order of magnitude slower than reported for global averages of soil formation rates copyright 2004 elsevier ltd

changing fluvial environments across the permian triassic boundary in the karoo basin south africa and possible causes of tetrapod extinctions abstract the karoo supergroup is a 12 km thick succession of sedimentary rocks that accumulated in a large intracratonic retroarc foreland basin in southwestern gondwana the strata record 100 million years of almost continuous sediment accumulation from the permo carboniferous 300 ma through to the early jurassic 190 ma under a range of climatic regimes and within several tectonically controlled sub basins alluvial sediments dominate the succession from the late permian onwards fossils of synapsid reptiles and early dinosaurs are sufficiently common to be used in a ten fold biostratigraphic subdivision of these strata the permian triassic boundary in the karoo succession is marked by a major extinction of the herbivorous dicynodonts which co incides with a rapid change in fluvial facies this study uses field observations of the sedimentary facies palaeosols and in situ fossils of well exposed p tr boundary sequences in the southern karoo basin to provide evidence of environmental changes that may have caused the tetrapod extinctions in the main karoo basin below the p tr boundary sequence the sediments comprise drab greenish grey mudrocks with a few thick 3 12 m multistoried laterally accreted fine grained sandstone bodies the mudrocks host hydromorphic palaeosols with palustrine carbonate horizons and mostly disarticulated skeletal remains of dicynodon assemblage zone fauna above a transition zone of approximately 20 m thickness these strata change into reddish brown mudrocks with numerous thin 0 5 5 m ribbon sandstone bodies and more continuous sheet sandstones calcic palaeosols with rhizocretions nodular horizons and pervasive calcareous encrustation of articulated skeletons of lystrosaurus and associated fauna of the lystrosaurus assemblage zone within a 20 m thick interval of the transitional strata fossils of dicynodon and lystrosaurus occur together the facies transition that co incides with the latest permian dicynodont extinction is interpreted as a change in fluvial style from meandering to low sinuosity channels with general drying of the floodplain habitats and redistribution of vegetation belts the horsetails in particular became significantly reduced in areal distribution the changes in fluvial style in the karoo basin at the end of the permian were triggered by a pulse of thrusting in the southerly source area dated at 247 ma 2 ma which brought about rapid progradation of a large sandy braided fan system the katberg sandstone fm into the central parts of the basin a concomitant basinward migration of depositional environments allowed the lystrosaurus communities that were adapted to dry floodplains to colonise the entire basin to the detriment of most of the wet floodplain dicynodon fauna that lived there

constancy of silicate mineral weathering rate ratios between natural and experimental weathering implications for hydrologic control of differences in absolute rates abstract inverse models which apportion watershed effluxes geochemical mass balance over estimated mineral surface area give weathering rates for individual silicate minerals e g feldspar 1 3 orders of magnitude slower than laboratory rates comparisons of rates calculated from several recent watershed geochemical mass balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data thus the coefficient which corrects the weathering rate for any mineral from a laboratory data set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets the correction factor linking experimental and natural rates must be independent of composition physical e g hydrologic controls rather than compositional or chemical controls cause the difference between weathering rates in nature and the laboratory because flow in natural weathering profiles is spatially heterogeneous not all of the potentially available surface in natural systems actually participates in reactions with pore fluids

late palaeozoic crustal block rotations within the gondwana sector of pangea abstract from the late carboniferous to the onset of the triassic gondwana was transected by several major shear systems the location of which was controlled by weak zones joining the ancient cratons these shear systems subdivided gondwana into mega blocks each of which experienced rotational motions at different angular speeds collisions between gondwana and laurussia in the north and between gondwana and the palaeo pacific plate in the west resulted in a counterclockwise rotation for the gondwana mega blocks during the latest carboniferous and early permian these rotational motions led to the opening of fault controlled basins along shear zones in the interior of gondwana and a 10 000 km long transtensional basin system partly inboard of the convergent margin of the supercontinent southward propagation of the collision zone between gondwana and the palaeo pacific plate during the late permian and early triassic caused the mega blocks within gondwana to rotate clockwise except for the tethys sea margin of gondwana these motions increased the compressional stresses within the interior of the supercontinent resulting in crustal uplift and the formation of the gondwanan and new england fold belts the primary driving mechanism of the rotational plate motions was collisional tectonics during periods of plate boundary reorganisation temporal changes in the motions of the mega blocks are believed to be linked to the greater relevance of the mantle convection system during the mesozoic and which culminated in the breakup of the supercontinent

early diagenetic laumontite in the lower triassic manda beds of the ruhuhu basin southern tanzania abstract porosity permeability values of sandstones of the lower triassic manda beds have been severely reduced by authigenic laumontite the formation of laumontite took place preferentially in bioturbated arkosic sandstones both as interstitial cement and as replacement nodules r0 values on coal implying maximum temperature of 75 c and disruption of laumontite cement by roottubes suggest that laumontite formed during early diagenesis the manda beds were deposited in climatically controlled upward fining and desiccating cyclothems of a fluvio deltaic depositional regime homoaxial quartz overgrowth which is the first diagenetic phase was effected during modification of initially low ph pore waters to more alkaline conditions this event was followed by the formation of laumo tite reflecting a low pco2 and a change in ph values from acid to alkaline in regions of higher pco2 calcite cement was formed in preference to laumontite frequently laumontite was also displaced by a later calcite phase

importance of environment on the order of mineral weathering in olivine basalts hawaii examination of weathering rinds from lava flows on hawaii with backscatter electron microscopy and electron micro probe analysis reveals that olivine weathers more slowly than adjacent clinopyroxene and plagioclase in environments with a paucity of organic acids yet when weathering rinds are in contact with abundant organic acids secreted by lichens olivine weathers before clinopyroxene and plagioclase weathers last the exception to goldich s widely accepted mineral stability series in organic poor environments runs counter to a thermodynamics explanation for the order of mineral weathering and illustrates the importance of the biogeochemical environment

clay mineralogy of weathering rinds and possible implications concerning the sources of clay minerals in soils weathering rinds on volcanic clasts in quaternary deposits in the western united states contain only very fine grained and poorly crystalline clay minerals rinds were sampled from soils containing well developed argillic b horizons in deposits about 105 yr old or more the clay size fraction of the rinds is dominated by allophane and iron hydroxy oxides whereas the b horizons contain abundant well crystallized clay minerals the contrast between the clay mineralogy of the weathering rinds in which weathering is isolated from other soil processes and that of the associated soil matrices suggests a need to reassess assumptions concerning the rates at which clay minerals form and the sources of clay minerals in argillic b horizons this study suggests that crystalline clay minerals form more slowly in weathering rinds than is generally assumed for soil environments and that the weathering of primary minerals may not be the dominant source of crystalline clay minerals in middle to late pleistocene soils

the effect of curvature on weathering rind formation evidence from uranium series isotopes in basaltic andesite weathering clasts in guadeloupe abstract to quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions we analyzed uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from basse terre island in guadeloupe u th and ti elemental profiles reveal that th and ti behave conservatively during rind formation but that u is added from an external source to the rind in the rind weathering reactions include dissolution of primary minerals such as pyroxene plagioclase and glass matrix as well as formation of fe oxyhydroxides gibbsite and minor kaolinite rare earth element ree profiles reveal a significant eu negative anomaly formed during clast weathering consistent with plagioclase dissolution significant porosity forms in the rind mostly due to plagioclase dissolution the new porosity is inferred to allow influx of soil water carrying externally derived dissolved u due to this influx u precipitates along with newly formed clay minerals and oxyhydroxides in the rind the conservative behavior of th and the continuous addition of u into the rind adequately explain the observed systematic trends of 238 u 232 th and 230 th 232 th activity ratios in the rind rind formation rates determined from the measured u series activity ratios with an open system u addition model increase by a factor of 1 3 0 18 0 24 mm kyr from a low curvature to a high curvature section 0 018 0 12 mm 1 of the core rind boundary revealing that curvature affects rates of rind formation as expected for diffusion limited rind formation u series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale the weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300 2200

from unweathered core to regolith in a single weathering andesitic clast rates and trends of in situ chemical weathering on a tropical volcanic island basse terre island french guadeloupe abstract we conduct x ray microprobe chemical and u series isotope analyses on an oriented weathering clast collected from the regolith of a weathered quaternary volcanoclastic debris flow on basse terre island french guadeloupe the sample consists of an unweathered basaltic andesite core surrounded by a weathering rind and an indurated crust that separates the rind from the overlying soil matrix u th disequilibria dating indicates that rind age increases away from the core rind boundary to a maximum of 66 ka this translates to a rind advance rate of 0 2 mm kyr 1 broadly consistent with rind advance rates calculated elsewhere on basse terre island the overlying indurated crust is 72 ka indicating a possible minimum duration of the rind formation elemental variations are constrained by a bulk chemical analysis along a vertical transects from the core to the overlying soil matrix and parallel electron microprobe analyses the hierarchy of elemental loss across the core rind boundary varies in the order ca na mg k mn si al ti 0 p fe consistent with the relative loss of phases in the clast from plagioclase glass pyroxene apatite ilmenite the abrupt

conditions of subaerial weathering of basalts in the neoarchean and paleoproterozoic abstract precambrian weathering profiles or regoliths were formed on the earth s surface in contact with meteoritic water this signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering the aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation we have demonstrated that in a wide range of modeling conditions weathering profiles of basalts consist mostly of clay minerals smectites and illite carbonate minerals may also be deposited within the profile at the initial stage of weathering which are later dissolved during a further process the time necessary for complete dissolution of primary minerals and crystallization of authigenic minerals is between 100 and 140 kyr modeling results indicate that neoarchean and paleoproterozoic 2 8 2 0 ga weathering profiles were formed under an atmosphere with pco2 not much higher than 25 pal and climatic conditions close to those found in modern times weathering under co2 rich pco2 1 and 10 bar and ch4 rich atmospheres as well as high temperature 50 c and 75 c conditions do not produce compounds corresponding to the analyzed samples

chemical weathering of silicate rocks in karelia region and kola peninsula nw russia assessing the effect of rock composition wetlands and vegetation this study is aimed at assessing the effect of factors including lithology forest peatland coverage dissolved organic carbon and vegetation on chemical fluxes and concentrations of major elements in rivers the mean annual element concentrations and dissolved fluxes of acid and basic rock dominated watersheds of the karelia region and kola peninsula nw russia have been estimated from the chemical composition of river water samples collected in the summers of 1996 2000 and from annual discharge and chemical composition data of 16 watersheds collected from 1961 to 1975 the mean annual flux corrected for atmospheric input varies from 0 5 to 5 6 t km2 y and is dominated by alkali metals for the kola alkaline rocks and by calcium for the karelian granito gneisses the pure granitic and pure basaltic river chemical denudation rates in the baltic shield are 0 33 and 2 3 t km2 y respectively the cationic and silica fluxes of both granitic and basaltic watersheds are comparable with those of other boreal regions having similar runoff compositions and climate the cationic flux of the rivers is positively correlated with the basic rock fraction of the watershed whereas the dissolved organic carbon flux increases with the increasing peatland fraction the chemical composition of the river water is controlled by bedrock dissolution during dry periods and by plant litter degradation and washout of organic debris from the topsoil horizon during wet periods taking account the chemical composition of the plant litter and the biological turnover intensity it was estimated that the plant litter degradation in kola and karelia regions provides 10 40 of total annual dissolved riverine element flux

calcretes of the santa maria supersequence middle triassic rio grande do sul brazil classification genesis and paleoclimatic implications article i nfo keywords calcretes paleosol paleoclimate triassic carbonate concretions are common features in arid and semi arid climates they can be formed under pedogenic conditions developing complex profiles with or without biogenic contribution or in groundwater conditions forming thickmassive bedsof carbonate togetherwith otherdata calcretescanbeusedas toolsforpaleoclimatic and paleoenvironmental interpretation in southern brazil the triassic santa maria supersequence sequences i and ii includes four types of calcretes with distinct modes of formation a precipitation of carbonate in the oscillation zone of the phreatic level with posterior overlap of vadose and phreatic diagenetic processes b pre cipitation of pedogenic carbonate forming crusts inthe paleosol c precipitation of carbonate forming concentric concretions in the stable phreatic level and d preservation of root casts due to processes of precipitation in the oscillation zone of the phreatic level the presence of silica minerals and barite in the concretions is attributed to seasonal changesinwatertable level due tothehighevaporation rate insemi aridclimates the mostcompatible process of formation of these calcretes is the per ascensum model integration of calcrete data with the sedimen tological and paleontological framework allowed a refinement of paleoclimatic data suggesting that despite the scarcity of plant fossils the middle triassic in southern brazil was vegetated with plants

a porosity related diffusion model of weathering rind development abstract a model of weathering rind growth is proposed based on the study of andesite cobbles sampled from a modern floodplain and four fluvial terraces 0 20 130 290 and 660 ka in central japan the times elapsed since each of the above ages can be assumed to be the weathering periods the 0 ka rocks are not visibly weathered the 20 ka rocks have an alteration layer with white or brown colours and a mean thickness of 0 7 mm on their surface the 130 290 and 660 ka rocks have brown weathering rinds with average thicknesses of 3 6 mm based on the measurements of physical and mechanical properties throughout from the rock surface to the interior weathering rinds are divided into two bands the outer brown band is characterized by ferric hydroxides which experienced both oxidization and leaching the inner band is formed by leaching alone of alkalis alkaline earths the inner border of this band is located at the position where vickers hardness numbers start decreasing with decreasing depth the thickness of each band was determined from its physical and mechanical properties the results show that brown band thickness l i is a function of time however total thickness of the two bands i e total weathering rind thickness l i ii is also controlled by initial rock porosity n their relationships are represented by diffusive equations l i dt 1 2 0 0283 t 1 2 and l i ii dt 1 2 0 0431 exp 0 4287 n t 1 2 these findings indicate that rind characterization and original rock porosity should be taken into consideration when weathering rind thickness is used as a dating tool

element mobilization and redistribution under extreme tropical weathering of basalts from the hainan island south china abstract chemical weathering of rocks has substantial influence on the global geochemical cycle in this paper the geochemical profile of a well developed basalt weathering profile 15 m thick including soil saprolite semi weathered rock and fresh basalt on the island of hainan south china was presented the soil and saprolite samples from this profile are characterized by high al2o3 and fe2o3 concentrations up to 32 3 and 28 5 respectively the mineral assemblage is dominated by kaolinite fe oxides hydroxides and gibbsite or boehmite indicating extensive desilicate and ferrallitic weathering the acidic and organic rich environment in the soil horizon may have promoted elemental remobilization and leaching the strongest sio2 depletion and al2o3 enrichment at about 2 4 m deep indicate that the main kaolinite hydrolysis and gibbsite formation occurred near the soil saprolite interface the mild sr reconcentration at about 3 9 m and 7 1 m deep may be attributed to secondary carbonate precipitation mn oxides hydroxides precipitated at 6 1 m deep accompanied by the strongest enrichment of ba and co uranium is mildly enriched in the middle part about 7 1 m and 9 1 m deep of the weathering profile and the enrichment may have been caused by the decomposition of uranyl carbonates or the accumulation of zircon immobile element i e zr hf nb ta th and ti distributions at different depths are mainly controlled by secondary fe oxides hydroxides and follow the stability sequence of nb ta th zr hf ti the limited thickness 15 cm of the semi weathered basalt horizon at the rock regolith interface 15 28 m deep suggests that plagioclase and pyroxene are readily altered to kaolinite smectite and fe oxides under tropical climate the marked enrichment of transitional metals such as cu zn ni and sc along the rock regolith interface may have associated mainly with increasing ph values as well as the dissolution of primary apatite and formation of secondary phosphates our findings highlight the importance of secondary phosphates in the redistribution of transition metals and in the possible mg cu and ni isotopic fractionation under extreme weathering of basalt in tropic climate

sequence stratigraphy of continental triassic strata of southernmost brazil a contribution to southwestern gondwana palaeogeography and palaeoclimate abstract the continental triassic succession of southernmost brazil comprises two second order depositional sequences the sanga do cabral early triassic and the santa maria middle to late triassic supersequences the first one includes ephemeral low sinuosity fluvial deposits developed on a low gradient plain based on fossil tetrapods especially procolophonids an upper induan age is estimated for this sequence facies association of the santa maria supersequence indicates low sinuosity fluvial rivers deltas and lakes this supersequence can be further subdivided into three third order sequences age provided by palaeovertebrate biostratigraphic data as follows santa maria 1 ladinian santa maria 2 carnian to early norian and santa maria 3 probably raethian or early jurassic sequences the gondwanides paroxysms i and ii in the sierra de la ventana cape fold belt are directly related to the development of both supersequences the source area of the sanga do cabral supersequence was located to the south it consisted of an uplifted peripheral bulge situated landward of the retro foreland system from where older sedimentary rocks were eroded the source area of the santa maria supersequence was also positioned southwards and related to the uplifted sul rio grandense and uruguayan shields the santa maria supersequence stratigraphic architecture is comparable to the triassic rift basins of western argentina diagenesis facies and palaeontology of the studied succession suggest a dominantly semiarid climate during the triassic

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

basalt weathering rates on earth and the duration of liquid water on the plains of gusev crater mars where martian rocks have been exposed to liquid water chemistry versus depth profi les could elucidate both martian climate history and potential for life the persistence of primary minerals in weathered profi les constrains the exposure time to liquid water on earth mineral persistence times range from 10 k y olivine to 250 k y glass to 1 m y pyroxene to 5 m y plagioclase such persistence times suggest mineral persistence minima on mars however martian solutions may have been more acidic than on earth relative mineral weathering rates observed for basalt in svalbard norway and costa rica demonstrate that laboratory ph trends can be used to estimate exposure to liquid water both qualitatively mineral absence or presence and quantitatively using reactive transport models qualitatively if the martian solution ph 2 glass should persist longer than olivine therefore persistence of glass may be a ph indicator with evidence for the ph of weathering the reactive transport code crunchflow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profi le for the profi le measured on the surface of the exposed martian rock known as humphrey in gusev crater the calculated exposure time is 22 k y which is a minimum due to physical erosion if correct this estimate is consistent with short term episodic alteration accompanied by ongoing surface erosion more of these depth profi les should be measured to illuminate the weathering history of mars

petrophysical properties of volcanic rocks and impacts of hydrothermal alteration in the guadeloupe archipelago west indies abstract the results of the geotref project presented here aim to develop high enthalpy geothermal energy in fractured reservoirs they focus on active volcanic islands especially the basse terre island guadeloupe archipelago as part of this project we measured several petrophysical properties to better understand how fluids flow in the hydrothermal system provide a set of parameters to modelers and build datasets for future interpretation of well logs the thermo physical rock properties of bulk and grain density porosity permeability compressional wave velocity thermal conductivity and diffusivity specific heat capacity of outcropping volcanic rocks from the guadeloupe archipelago lesser antilles were measured and a dataset was built according to field observations samples were grouped into three lithological classes and then separated by their degree of alteration according to macroscopic aspects and optical microscopy results geochemical data were obtained and magnetic minerals identified by combining anisotropy of magnetic susceptibility and variation of magnetic susceptibility with temperature the lithotypes investigated were lava flows debris flows and pyroclastics gathering ash pumice and scoria deposits for lavas and debris flows three degrees of alteration were defined fresh slightly to moderately altered and highly hydrothermalized only fresh pyroclastics were studied because the altered state was not outcropping typically samples are composed of andesite and belong to tholeiitic and calc alkaline series lava porosity consists of high density cooling joints and vesicles of different shapes whereas pores in debris flows and pyroclastics are located both between and within particles median porosities and permeabilities are clearly distinct for lavas 4 and 6 10 18 m2 debris flows 33 and 4 10 14 m2 and pyroclastics 34 and 8 10 13 m2 the porosity difference implies a thermal conductivity twice to three times higher for lavas 1 68 w m 1 k 1 specific heat capacity of all lithothypes is similar 0 75 0 97 kj kg 1 k 1 advancing hydrothermal alteration removes magnetic minerals and transforms initial rocks the original plagioclases and pyroxenes in neoformed chlorite white micas quartz epidote and clay minerals are replaced this modifies the shape of the pore network furthermore hydrothermal alteration results in a reduction of pore throat diameter it tends to decrease petrophysical differences between lavas and debris flows increasing the porosity and permeability of effusive rocks while the opposite effects are observed for debris flows

basalt weathering in an arctic mars analog site abstract the martian surface has undergone chemical and physical weathering in the past and these processes may continue intermittently today to explore whether martian rocks are likely to retain features indicative of weathering we investigated how basaltic material weathers on earth specifically we investigated weathering of a quaternary aged basaltic flow at the sverrefjell volcano in svalbard above the arctic circle this flow weathered since deglaciation under cold dry

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

late cretaceous and cenozoic intra plate compressional deformations in the alpine foreland a geodynamic model abstract the mesozoic grabens and wrench induced troughs of western and central europe developed in response to triassic early cretaceous intra plate tensional stresses which affected the arctic north atlantic and the tethys borderlands during the rifting phase preceding the opening of the respective oceans this resulted in the fragmentation of the continental crust of the alpine foreland during the late cretaceous and tertiary alpine orogeny collision related compressive stresses exerted on the foreland induced the reactivation of these fracture systems causing the inversion of mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present alpine deformation front the structural style of these inverted basins and basement uplifts is indicative of compressional and transpressional deformations the total amount of crustal shortening associated with these deformations is unlikely to exceed a few tens of kilometres these displacements require a coupling between the foreland and the orogen at the alpine a subduction zones and within the foreland a decoupling at intra crustal levels between the crust and mantle and or at deeper lithospheric levels the example of the alpine foreland indicates that compressive stresses inducing major deformations can be transmitted from a collisional plate boundary through continental lithosphere over distances of up to 1300 km circumstantial evidence suggests that collision related compressive stresses not large enough to give rise to significant deformations can be transmitted through continental lithosphere over even greater distances

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

late palaeozoic deformation in central africa a result of distant collision seismic reflection profiles from the cuvette centrale central basin of zaire show that a major contractional deformation affected a part of central africa during the late permian and early triassic periods at the same time other regions of central africa experienced extensional and strike slip deformation and associated rift formation the coexistence of such varied styles of tectonic activity over a large continental area resembles the cenozoic tectonics of central asia and northwest europe the intracontinental deformation seen in these latter areas is attributed to collisional processes at a distant continental margin1 3 by analogy we argue here that deformation of central africa in the late palaeozoic period was a direct result of collisional processes some distance away at the southern margin of gondwana

erosion of deccan traps determined by river geochemistry impact on the global climate and the 87sr 86sr ratio of seawater abstract the impact of the deccan traps on chemical weathering and atmospheric co2 consumption on earth is evaluated based on the study of major elements strontium and 87sr 86sr isotopic ratios of the main rivers flowing through the traps using a numerical model which describes the coupled evolution of the chemical cycles of carbon alkalinity and strontium and allows one to compute the variations in atmospheric pco2 mean global temperature and the 87sr 86sr isotopic ratio of seawater in response to deccan trap emplacement the results suggest that the rate of chemical weathering of deccan traps 21 63 t km2 yr and associated atmospheric co2 consumption 0 58 2 54 106 mol c km2 yr are relatively high compared to those linked to other basaltic regions our results on the deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of co2 consumption during weathering of basalts according to the relationship f r f c 0 exp ea r 1 t 1 298 where f is the specific co2 consumption rate mol km2 yr rf is runoff mm yr c0 is a constant 1764 mol l ea represents an apparent activation energy for basalt weathering with a value of 42 3 kj mol determined in the present study r is the gas constant and t is the absolute temperature k modelling results show that emplacement and weathering of deccan traps basalts played an important role in the geochemical cycles of carbon and strontium in particular the traps led to a change in weathering rate of both carbonates and silicates in carbonate deposition on seafloor in sr isotopic composition of the riverine flux and hence a change in marine sr isotopic composition as a result deccan traps emplacement was responsible for a strong increase of atmospheric pco2 by 1050 ppmv followed by a new steady state pco2 lower than that in pre deccan times by 57 ppmv implying that pre industrial atmospheric pco2 would have been 20 higher in the absence of deccan basalts pco2 evolution was accompanied by a rapid warming of 4 c followed after 1 myr by a global cooling of 0 55 c during the warming phase continental silicate weathering is increased globally since weathering of continental silicate rocks provides radiogenic sr to the ocean the model predicts a peak in the 87sr 86sr ratio of seawater following the deccan traps emplacement the amplitude and duration of this spike in the sr isotopic signal are comparable to those observed at the cretaceous tertiary boundary the results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on earth

a reactive transport model for weathering rind formation on basalt abstract saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution here we investigate the chemical and physical transformations that occur during weathering by studying small scale saprolites in the form of weathering rinds which form on rock in soil or saprolite and grow in thickness without physical disturbance with time we compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in costa rica to diffusion reaction simulations of rind formation using the fully coupled reactive transport model crunchflow the four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include 1 the mineralogy of weathering products 2 weathering rind thickness 3 the coincidence of plagioclase and augite reaction fronts and 4 the thickness of the zones of mineral reaction i e reaction fronts four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations to fit the model to all four criteria however it was necessary to 1 treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9 and 2 treat mineral surface area as a fitting factor this latter approach was presumably necessary because the mineral water surface area of the connected accessible porosity in the costa rica samples is much less than the total porosity navarre sitchler et al 2009 the model fit surface area here termed reacting surface area was much smaller than the bet measured surface area determined for powdered basaltic material in the parent basalt reacting surface area and diffusivity are low due to low pore connectivity and early weathering is therefore transport controlled however as pore connectivity increases as a result of weathering the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics the transition point between transport and kinetic control appears to be related to a critical porosity 9 at which pore connectivity is high enough to allow rapid transport based on these simulations we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface and that this porosity increases because of mineral dissolution following a rate that is largely surface reaction controlled

basin fill evolution and paleotectonic patterns along the samfrau geosyncline the sauce grande basin ventana foldbelt argentina and karoo basin cape foldbelt south africa revisited as integral parts of du toit s 1927 samfrau geosyncline the sauce grande basin ventana foldbelt argentina and karoo basin cape foldbelt south africa share similar paleoclimatic paleogeographic and paleotectonic aspects related to the late paleozoic tectono magmatic activity along the panthalassan continental margin of gondwanaland late carboniferou earliest permian glacial deposits were deposited in the sauce grande sauce grande formation and karoo dwyka formation basins and falkland malvinas islands lafonia formation during an initial sag phase of extension the pre breakup position of the falkland malvinas islands on the easternmost part of the karoo basin immediately east of the coast of south africa is supported by recent paleomagnetic data lithofacies associations paleoice flow directions and age similarities between the dwyka and the lafonia glacial sequences the desintegration of the gondwanan ice sheet gis triggered widespread transgressions reflected in the stratigraphic record by the presence of inter basinally correlatable open marine fine grained deposits piedra azul formation in the sauce grande basin prince albert formation in the karoo basin and port sussex formation in the falkland islands capping glacial marine sediments these early postglacial transgressive deposits characterised by fossils of the eurydesma fauna and glossopteris flora represent the maximum flooding of the basins cratonward foreland subsidence was triggered by the san rafael orogeny ca 270 ma in argentina and propogated along the gondwanan margin this subsidence phase generated sufficient space to accommodate thick synorogenic sequences derived from the orogenic flanks of the sauce grande and karoo basins compositionally the initial extensional phase of these basins was characterized by quartz rich craton derived detritus and was followed by a compressional foreland phase characterized by a paleocurrent reversal and dominance of arc foldbelt derived material in the sauce grande basin tuffs are interbedded in the upper half of the synorogenic foldbelt derived tunas formation early early late permian likewise the first widespread appearance of tuffs in the karoo basin is in the whitehill formation of late early permian 260 ma age silicic volcanism along the andes and patagonia choiyoi magmatic province peaked between the late early permian and late permian a link between these volcanics and the consanguineous airborne tuffs present in the sauce grande and karoo basins is suggested on the basis of their similar compositions and ages

glaciers show direct linkage between erosion rate and chemical weathering fluxes field studies suggest a linkage between high physical erosion rates and rates of chemical denudation mechanical erosion by temperate glaciers is commonly an order of magnitude higher than in mountainous fluvial catchments leading to an expectation that chemical weathering fluxes should also be high from glacierized basins yet solute fluxes from glaciers are not found to be higher than non glacierized catchments application of a model of silicate weathering from glaciers based on mineral surface area production and mineralogy shows that solute fluxes are consistent with the low temperatures dilute water chemistry and high mineral surface area production in these environments low temperatures reduce silicate weathering rates this effect explains the difference between silica fluxes from glaciers and from non glacierized basins as in laboratory flow through reactors glacial solute flux should depend on surface area production and mineral weathering rate constants the surface area production is significant a typical glacial erosion rate and grain size distribution produces on the order of 104 km2 of mineral surface area per square kilometer per year this new surface area is highly reactive because mineral weathering rates decline with surface age application of the reactor model yields results largely consistent with measured solute fluxes for the example of bench glacier alaska the model underpredicts potassium fluxes probably due to accelerated initial dissolution of biotite strained by abrasion the success of the model in predicting other silicate weathering fluxes reflects the far from equilibrium conditions in glacial runoff such that mineral weathering rate constants are not limited by saturation state in a small data set from glacial catchments both annual silica fluxes and mean concentrations increase with water discharge this suggests that mineral surface area increases with water discharge from glaciers an effect plausibly linked to erosion rates those glaciers for which both erosion rate and silica flux data are available support the idea that production of new reactive mineral surface area by glacial erosion drives silicate weathering fluxes

controls on rind thickness on basaltic andesite clasts weathering in guadeloupe abstract a clast of low porosity basaltic andesite collected from the b horizon of a soil developed on a late quaternary volcaniclastic debris flow in the bras david watershed on basse terre island guadeloupe exhibits weathering like that observed in many weathered clasts of similar composition in other tropical locations specifically elemental profiles measured across the core rind interface document that primary minerals and glass weather to fe oxyhydroxides gibbsite and minor kaolinite in the rind the earliest reaction identified in the core is oxidation of fe in pyroxene but the earliest reaction that creates significant porosity is plagioclase dissolution elemental loss varies in the order ca na k mg si al fe p ti consistent with the relative reactivity of phases in the clast from plagioclase pyroxene glass apatite ilmenite the rind surrounds a core of unaltered material that is more spherical than the original clast the distance from the core rind boundary to a visually prominent rind layer l was measured as a proxy for the rind thickness at 36 locations on a slab cut vertically through the nominal center of the clast this distance averaged 24 4 3 1 mm maximum and minimum values for l 35 8 and 20 6 mm were observed where curvature of the core rind boundary is greatest 0 12 mm 1 and smallest 0 018 mm 1 respectively extrapolating from other rinds in other locations the rate of rind formation is estimated to vary by a factor of about 2 from 4 to 7 10 14 m s 1 from low to high curvature the observation of a higher rate of rind formation for a higher curvature interface is consistent with a diffusion limited model for weathering rind formation the diffusion limited model predicts that like rind thickness values of the thickness of the reaction front h for a given reaction defined as the zone over which a parent mineral such as plagioclase completely weathers to rind material should also increase with curvature values of h were quantified as a function of interface curvature using bulk chemical analysis 500

columnar jointed bentonite below a doleritic sill tideswell dale derbyshire uk formation during prograde contact metamorphism columnar jointed tuffs bentonites are located below the lower carboniferous tideswell dale sill derbyshire in an abandoned quarry there are three zones of prismatic joints columnar joints 60 cm thick and mean column width of 4 1 cm and massive conchoidally fractured rock sequentially from the dolerite contact downwards the rocks are very fine grained 1 10 m and consist mainly of sanidine interlayered illite smectite and hematite with minor quartz apatite montmorillonite anatase and detrital minerals high k2o and fe2o3 low na2o and low mgo are interpreted as due to alteration of felsic volcanic ash the bentonite was contact metamorphosed by the sill to temperatures of c 300 c the progression of fracture geometries thermal considerations and application of theories of column formation indicate that columns formed downwards away from the sill due to prograde contact metamorphism of the originally clay rich bentonite the formation of columnar fracture networks by shrinkage due to igneous heating may have implications for the effectiveness of bentonite as a barrier for disposal of high level radioactive waste regional dimensions include documenting widespread felsic explosive volcanism and inferring a triassic oxidation event from palaeomagnetic data

reconciling chemical weathering rates across scales application of uranium series isotope systematics in volcanic weathering clasts from basse terre island french guadeloupe abstract to elucidate the factors that contribute to the orders of magnitude difference generally observed in apparent weathering rates across scales we utilized a u series isotopic technique to directly determine the duration and rates of chemical weathering recorded in weathering clasts in this study we systematically compiled 15 individual volcanic weathering clasts collected from eight andesitic watersheds along a gradient of mean annual precipitation ranging from 1600 mm to 6400 mm on the tropical volcanic basse terre island of french guadeloupe we measured u series isotope compositions to quantify weathering advance rates along the core rind transects in these weathering clasts total rind formation ages the length of time it took for these weathering clasts to form from andesitic rock fragments or volcanic clasts in the soil zones range from 60 kyr to 300 kyr as expected for each individual core rind weathering transect the rind ages generally increase almost linearly with distance away from the core rind boundary the derived clast weathering rates range from 0 08 0 04 to 0 34 0 01 mm kyr 1 and the rates exhibit a strong positive correlation r 2 0 74 with the annual runoff values of these watersheds this correlation documents the first direct evidence that weathering over geological timescales on the island is controlled by the amount of precipitation and corroborates the relationship that was previously reported for the riverine weathering fluxes at basse terre island in guadeloupe but measured with much shorter time scales the clast scale rates inferred for the characteristic length scale of mm are compared to the watershed scale rates of basse terre island length scale of km constantly higher rates are observed at the watershed scales than the clast scale and the discrepancy equals 1800 400 times despite changes in watershed characteristics such as watershed size relief and runoff values the discrepancy is attributed to the fractal nature of the roughness and surface area across scales for example the presence of undulations and fractures e g roughness at the bedrock saprolite contact of the watershed which is generally not assessed in the watershed surface area estimates is most likely the main contributor to the constant rate difference of 1800 times

tectonics and sedimentation in southern south america during triassic abstract triassic continental sedimentation in southern south america is related to the gondwanides orogenesis directly there were two distinct phases of sedimentation 1 the wide alluvial basins during early triassic and 2 the restricted extensional basins during middle and late triassic the main early triassic deposits are the sanga do cabral supersequence southernmost brazil and uruguay and the talampaya and tarjados sequences western argentina these deposits represent poorly channelized braided rivers with source areas in the gondwanides units under compressional tectonic setting the main middle and late triassic strata are rifts in the back arc setting such as the ischigualasto and cuyo basins western argentina and in the intracontinental setting santa maria supersequence southernmost brazil these basins are mainly composed of alluvial lacustrine and deltaic deposits with a great pyroclastic contribution in the case of the basins of argentina two rift sequences were identified in the western argentina basins composed of syn rift alluvial deposition followed by lake transgression the sag phase is marked by the progradation of deltaic and high sinuosity rivers the southernmost brazil deposits were divided into three rift sequences composed of low sinuosity rivers followed by lake transgression the progradation of deltaic and fluvial systems marks the sag phase the accommodation space generation within the back arc rifts was high if compared to the intracontinental coeval basin due to higher base level and higher tectonic subsidence rates the development of deep organic matter rich lakes suggests a more humid palaeoclimate in the back arc region than in the intracontinental setting where playa lakes dominated the back arc rifts are related to sinistral transtensional stresses in the western branch of gondwanides region the intracontinental rifts the southernmost brazil deposits and coeval rift basins of southern africa were generated by transtensional stresses within a dextral shear zone the shear is related to stress transmission from the southern branch of the gonwanides where oblique compression occurred

using a mathematical model of a weathering clast to explore the effects of curvature on weathering abstract for the first time we show that rind thicknesses developed on surfaces of a clast with different values of curvature can be used to estimate the duration of clast weathering to obtain an analytical expression for the velocity of the curvilinear weathering front on a clast of arbitrary shape we approximate our previous multi mineral reactive diffusion model and explore a simplified 2 d model numerically and analytically our analysis documents that with increasing curvature of the weathering front the mathematical description of weathering advance is equivalent to that derived for advection as the dominant solute transport mechanism even for the case where transport is occurring by diffusion only specifically for a curvilinear weathering front with constant curvature k

using the chemical composition of carbonate rocks on mars as a record of secondary interaction with liquid water in the search for habitable environments on mars sites that have been altered by liquid water represent an important target quickly dissolving and precipitating carbonates found in multiple locations on mars represent a key potential mineral indicator of locations with a history of aqueous alteration and therefore possible habitability recent liquid surface water on mars may differ significantly from the fluids either sedimentary hydrothermal or pedogenic that formed carbonate minerals on mars interactions of carbonate rocks with such recent surface water may therefore cause changes in chemistry and mineralogy with depth that form weathering profiles recording the conditions of alteration to test the prediction that martian carbonates may record a signature of aqueous alteration on mars we performed reactive transport modeling of putative carbonate outcrops on mars using the reactive transport code crunchflow model inputs included two parent rocks chosen based on the carbonate rich rock at gusev crater comanche one consisting of 60 magnesite 20 siderite and 20 porosity and one consisting of 50 magnesite 20 siderite 9 calcite 1 6 rhodochrosite and 20 porosity reacting solutions consisted of four scenarios acidic sulfate containing solutions perhaps resulting from an acid vapor scenario dilute solutions resulting from martian analog basalt dissolution concentrated solutions resulting from modeled evaporation of the dilute solutions and solutions based on the results of the phoenix lander modeling was performed under low oxidation and high oxidation conditions based on the modeling results a carbonate rock would become enriched in siderite upon interaction with acidic water under low oxidation conditions the same rock would become enriched in calcite after interaction with ca enriched solutions a carbonate rock reacting under more oxidizing conditions would preferentially dissolve siderite leaving a surface enriched in magnesite and ferric oxide therefore the mg rich carbonates on mars are consistent with possible interaction with liquid water under oxidizing conditions similar to those measured at the phoenix landing site future detailed chemical and mineralogical profiles through weathered rock into the unaltered interior of carbonate rocks may therefore allow the quantitative interpretation of the aqueous history and possible habitability of locations on mars

sequence stratigraphy of nonmarine jurassic cretaceous rocks central cordilleran foreland basin system an analysis of the upper jurassic lower cretaceous morrison and cedar mountain formations of utah and colorado has resulted in a general sequence stratigraphic model for nonmarine rocks in this model nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level these fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized internally nonmarine depositional sequences comprise three systems tracts degradational transitional and aggradational which are analogous to the lowstand transgressive and highstand systems tracts of marine depositional sequences degradational systems tracts overlie sequence bounding unconformities and consist of relatively coarse grained low sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread thin sheets above shallow erosion surfaces transitional systems tracts represent an increase in basin accommodation development following degradational systems tract deposition they are marked by the transition from laterally continuous low sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon like meandering and anastomosing channel sandstones aggradational systems tracts are characterized by meandering anastomosing channel sandstones and abundant fine grained overbank and lacustrine deposits the upper jurassic lower cretaceous nonmarine rocks of the study area contain three depositional sequences the first of these the uj 1 sequence consists primarily of an aggradational systems tract overlain by a sequence bounding unconformity however the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine highstand systems tract the upper morrison sequence uj 2 consists of degradational transitional and aggradational systems tracts above the uj 2 sequence are a sequence bounding unconformity and degradational and transitional systems tracts of the lk 1 sequence represented by the buckhorn conglomerate the buckhorn is overlain by a sequence bounding unconformity and transitional aggradational systems tracts of the lk 2 sequence that is composed of the upper part of the cedar mountain formation the upper jurassic lower cretaceous sequences in utah and colorado can be traced regionally and correlated with nonmarine depositional sequences in central utah and sequences that contain nonmarine transitional and marine rocks in central wyoming these sequences were deposited in the back bulge forebulge and distal foredeep depozones of the late jurassic early cretaceous foreland basin system

u pb age constraints on dinosaur rise from south brazil abstract triassic dinosaurs from south brazil are considered as some of the oldest known worldwide but their precise correlation to other early dinosaur occurrences have proven difficult the problem of such long distance faunal correlations has been exacerbated by reliance on an equivocal tetrapod biostratigraphy and the lack of palynomorphs and other age bearing microfossils here we present new high precision u pb zircon geochronology ca id tims method from two classical dinosaur bearing fossil sites in south brazil a weighted mean 206pb 238u date of 233 23 0 73 ma 2 complete uncertainty from the santa maria formation and a single zircon 206pb 238u date of 225 42 0 37 ma representing the maximum age of the caturrita formation the new age results combined with available geochronology from other triassic dinosauromorph occurrences allow establishing a chronostratigraphic framework for the late triassic appearance of dinosaurs in western pangea within this context the major macroevolutionary events associated with the rise of dinosaurs can be recognized and their possible links to coeval paleoenvironmental and biotic shifts can be examined

changes in the rate of release of ca and mg and normative mineralogy due to weathering along a 5300 year chronosequence of boreal forest soils changes in the normative mineralogy and chemical weathering rates elemental loss of ca and mg from the uppermost mineral soil horizon were determined at five forested sites in the boreal zone forming a chronosequence the sites were all located on the same glaciofluvial esker deposit well sorted sand over a distance of 27 km in the coastal region of the gulf of bothnia in finland and had estimated ages from 339 to 5276 years because of isostatic land uplift the region is continually emerging from the sea the rates of which are known allowing the age of current surfaces to be dated soil profile development progressed from gleyic arenosols youngest and nearest the coast through haplic arenosols to haplic podzols 3 oldest and furthest away from the coast elemental concentrations of soil 2 mm were determined using xrf and icp ms from a hot aqua regia digest normative mineralogy was determined using these data and a stoichiometric mineral distribution model weathering elemental loss rates were calculated using xrf concentrations and normative quartz as the conservative reference component quartz was increasingly enriched in the uppermost horizon with soil age commensurate with the loss of easily weathered minerals using quartz as the conservative component the mass residual fraction remaining in the uppermost soil horizon of the oldest soil was 0 27 0 35 and 0 57 respectively for biotite hornblende and anorthite which are important sources of ca and mg b e horizon ratios of hydrous aluminosilicate contents increased from 1 to 14 with soil age clearly indicating the increase in degree of podzolisation the losses of ca and mg increased with soil age but at a decreasing rate ca and mg losses for the youngest soil were respectively 3 3 and 1 8 mol m 2 corresponding values for the oldest soil were 10 6 and 10 5 mean rates of loss for the youngest soil were 10 and 5 mmol m 2 yr 1 for ca and mg respectively and corresponding values for the oldest soil were 2 mmol m 2 yr 1 for both ca and mg the loss of these base cations from the soil will have resulted in a lower acid neutralizing capacity and increased sensitivity of these soils to acidic deposition and represents a reduction in the soil reserve of important plant nutrients

slow advance of the weathering front during deep supply limited saprolite formation in the tropical highlands of sri lanka abstract silicate weathering initiated by major mineralogical transformations at the base of ten meters of clay rich saprolite generates the exceptionally low weathering flux found in streams draining the crystalline rocks of the mountainous and humid tropical highlands of sri lanka this conclusion is reached from a thorough investigation of the mineralogical chemical and sr isotope compositions of samples within a regolith profile extending 10 m from surface soil through the weathering front in charnockite bedrock a high grade metamorphic rock corestones formed at the weathering front as well as from the chemical composition of the dissolved loads in nearby streams weatherable minerals and soluble elements are fully depleted at the top of the profile showing that the system is supply limited such that weathering fluxes are controlled directly by the supply of fresh minerals we determine the weathering rates using two independent means 1 in situ produced cosmogenic nuclides in surface soil and creek sediments in the close vicinity of the regolith combined with immobile element mass balance across the regolith and 2 river dissolved loads silicate weathering rates determined from both approaches range from 16 to 36 t km 2 y 1 corresponding to a weathering front advance rate of 6 14 mm ky 1 these rates agree across the 10 1 to 10 4 y time scales over which our rate metrics integrate suggesting that the weathering system operates at steady state within error these rates are furthermore compatible with those obtained by modeling the advance rate of the weathering front from chemical gradients and mineral dissolution rates the silicate weathering flux out of the weathering profile measured on small creeks amounts to 84 of the profile s export flux the remaining 16 is contributed by non silicate atmospheric derived input the silicate weathering flux as measured by dissolved loads in large catchments amounts to ca 50 of the total dissolved flux the remainder being contributed by dust rain and weathering of local marble bands spheroidal weathering is the key processes of converting the fresh bedrock into saprolite at the weathering front the mineralogical composition of weathering rinds shows that the sequence of mineral decomposition is pyroxene plagioclase biotite k feldspar observable biotite alteration does not appear to initiate spheroidal weathering within corestones therefore we infer that other processes than biotite oxidation like pyroxene oxidation clay formation from pyroxene and plagioclase decomposition the development of secondary porosity by plagioclase dissolution or even microbiologic processes at depth enable the coupling between slow advance of the weathering front and slow erosion at the surface the comparison to tectonically more active tropical landscapes lets us conclude that the combination of hard rock with tightly interlocked mineral grains and slow erosion in the absence of tectonically induced landscape rejuvenation lead to these exceptionally low weathering rates

weathering rind formation in buried terrace cobbles during periods of up to 300ka abstract weathering rinds formed in mesozoic sandstone and basalt cobbles buried in terrace deposits for up to 300 ka have been investigated the aim was to determine the formation process and elemental mass balances during rind development the ages of terraces distributed in the western part of fukui prefecture central japan have been determined as 50 ka 120 ka and 300 ka based on a tephro stratigraphic method detailed investigations across the weathering rinds consisting of microscopic observations porosity measurements and mineralogical and geochemical analyses using x ray diffractometry xrd x ray fluorescence xrf secondary x ray analytical microscopy sxam scanning electron microanalyser sem and electron probe microanalysis epma have been carried out the results revealed that the fe concentrations in the weathering rind of a basalt cobble slightly decreased from the cobble s surface rim towards the unweathered core in contrast in a sandstone cobble formed under the same environmental conditions over the same period of time there is an fe rich layer at some distance below the cobble s surface elemental mass balances across the rinds were determined by using open system mass balance i j calculations and show that the fe was precipitated as fe oxyhydroxides in the basalt cobbles although fe was slightly removed from the rims in sandstone cobbles on the other hand fe migrated along a fe concentration gradient by diffusion and precipitated as fe oxyhydroxide minerals to form the weathering rinds presumably precipitation was due to the relatively higher ph conditions caused by mineral dissolution within the pores principally involving calcite but probably also silicates including feldspar the detailed characterization of the weathering rinds revealed the influence of lithology on the accumulation and dissolution of fe oxyhydroxides causing weathering rinds with different characteristics to develop in different kinds of buried cobbles under the same conditions relatively large climatic changes in the study area did not cause discernable variations in the mean formation rates of the studied rinds which were in the order of 10 8 m a for both basalt and sandstone cobbles these rates are 1 2 orders of magnitude slower than those reported for tropical areas elsewhere most probably due to the lower rainfall in the studied area

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

strengthening western gondwanan correlations a brazilian dicynodont synapsida anomodontia in the middle triassic of namibia article i nfo terrestrial middle triassic strata occur throughout continental africa and are particularly well exposed in south africa tanzania zambia and namibia the youngest age for all these african deposits is widely accept ed as early middle triassic anisian fossils collected recently from the uppermost strata of the upper omingonde formation in namibia highlighted the presence of chiniquodon a carnivorous cynodont previ ously only found in ladinian carnian aged rocks of south america in addition work in progress indicates that a large archosaur originally reported as erythrosuchus also discovered from levels close to the top of this unit is in fact a rauisuchian a group of archosaurs well known from ladinian carnian beds of southern south america here we present the first record of the tuskless dicynodont stahleckeria potens from the top of the upper omingonde formation in central namibia this taxon was up until now only known from the ladinian dinodontosaurus assemblage zone of the santa maria formation in southern brazil thus compel ling evidence for a ladinian age for the upper levels of the upper omingonde formation is provided by two therapsid and one archosaur taxa the tetrapod fauna of the upper omingonde formation partially fills the gap of the well documented hiatus ladinian gap prevalent throughout the karoo basins of south and cen tral africa the presence of the same therapsid taxa in the namibian waterberg basin and the parana basin of brazil during middle triassic suggests that these basins were biogeographically linked through a series of interconnecting lowlands with no major ecological climatic and or physical barriers

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

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

deformation related alteration of basaltic clasts during deep burial in sedimentary basins abstract the once deeply buried conglomerates of the solund basin contain up to 30 basaltic clasts together with up to 20 altered peridotite clasts both derived from the adjacent solund stavfjord ophiolite complex ssoc the basaltic clasts range in size from 2 to 30 cm across and display centimeter thick hematite reddened zones four types which form an alteration related evolutionary sequence can be distinguished type 1 consists of a green core with a 1 2 cm thick red zone along its rim type 2 shows in addition a perturbation of the red zone into the center of the clasts in type 3 numerous compartments with green cores surrounded by red zones build up the clast type 4 displays diffuse red zones and patches typically with a circular form that resembles liesegang s structures this evolutionary sequence relates to a progressive deformation evident from fractures faults and shear zones hematite reddened zones form along clast boundaries and along fractures that are oriented at high angle to the boundaries of the clasts the boundary parallel zones migrate towards the center of the clasts and the fracture parallel zones migrate away from the fractures these fracturing and deformation are coupled with chemical and mineralogical changes responsible of the changes in color of the alteration zones compared to the basaltic rocks of the ssoc the basaltic clasts of the solund basin contain higher contents of k 2 o volatiles loi and mgo lower contents of cao and are more oxidized with an average fe 3 fe 3 fe 2 ratio of 0 43 0 10 as compared to 0 30 0 07 in the ssoc basalts the replacement of magmatic clinopyroxene by chlorite induces the decrease in cao the replacement of ilmenite by titanite provides iron to form hematite in the red zones the red zones progressively replace the green core and are at their external boundary in turn replaced by mg riched green alteration zones characterized by a high content of zoned amphibole a decrease in cao and an increase in mgo towards the rim of the basaltic clasts is contrary to the chemical evolution of the peridotite clasts where the mgo is depleted and the cao increases with increasing degree of alteration this suggests that clasts of different lithologies exchanged components during the alteration process to explain these observations we use a model including internal stresses through a diffusion reaction process and external stresses through sediment loading and tectonic forces the mineralogy of the altered basaltic clasts chlorite epidote albite actinolite corresponds to low grade metamorphic conditions in accordance with or slightly higher than the peak temperature of 230 320 c previously calculated for this basin the study shows that the final compaction of multicomponent clast systems takes place through an interaction between mineral reactions metasomatism and deformation which differs from the compaction of monocomponent systems that occurs dominantly through pressure solution

dynamic of particulate and dissolved organic carbon in small volcanic mountainous tropical watersheds abstract in the tropical zone small watersheds are affected by intense meteorological events these events play an important role in the erosion of soils and therefore on the associated organic carbon fluxes to the ocean we studied the geochemistry of three small watersheds around the basse terre volcanic island french west indies fwi during a four years period by measuring dissolved organic carbon doc particulate organic carbon poc and dissolved inorganic carbon dic concentrations the mean annual yields ranged between 8 1 15 8 tc km 2 yr 1 1 9 8 6 tc km 2 yr 1 and 8 1 25 5 tc km 2 yr 1 for dic doc and poc respectively floods and extreme floods i e extremely high discharge associated to extreme meteorological events such as cyclones or tropical storms account for 42 6 of the yearly water flux and represent 54 5 of the annual doc flux and more than 85 of the annual poc flux the dic flux occurs essentially during low water levels with 75 of the annual flux during low water levels and floods the dissolved carbon is exported in majority under the inorganic form dic doc 2 6 2 1 while during extreme floods the dissolved carbon transported is mostly organic dic doc 0 7 0 2 the partial residence time taking into account only the physical processes erosion and transport in guadeloupean soils has been estimated between 381 and 1000 years these relatively short times could be linked to the intensity of meteorological events rather than the frequency of meteorological events the total export of organic carbon coming from small tropical and volcanic mountainous rivers is estimated at 2 4 0 6 mtc yr 1 for doc and at 5 9 2 4 mtc yr 1 for poc emphasizing that these carbon fluxes are significant and should be included in global carbon budgets in addition the quality of terrestrial organic matter poc doc and c n ratios arriving to the ocean is different from the one of large river origin these inputs are responsible for a fast transport of terrestrial organic matter to the ocean but their effect on regional and global carbon budget is still a matter of debate

controls on chemical weathering on a mountainous volcanic tropical island guadeloupe french west indies abstract guadeloupe island is a natural laboratory ideally suited to the study of biogeochemical processes in tropical and mountainous volcanic environments the island s east west rainfall gradient 1200 8000 mm yr is superimposed on a north south age gradient 2 7 ma to present providing a unique opportunity to investigate the influence of rainfall and rock age on the chemical weathering of volcanic terrains taking advantage of this configuration we present the first temporal survey 2007 2013 of the geochemical composition of the dissolved load of rain and river waters in guadeloupe our data demonstrate that the chemical composition of river water is influenced by rainfall abundance hydrothermal alteration from active or fossilized volcanic systems and interactions between water and minerals during chemical weathering processes the contribution of rain to the overall chemical balance is especially significant in the older northern part of the island where the ferralitic soils are base cation depleted between 15 and 65 of the ca or mg riverine budgets comes from atmospheric deposits highlighting the major role of rainfall in the geochemical budgets of small tropical and mountainous watersheds the river water dataset indicates that different chemical weathering processes dominate the budget depending on the age of the local bedrock in the younger southern part of the island a pool of easily weatherable andesitic minerals from the bedrock dominates the contribution from this pool decreases significantly to 5 15 wt of the bulk soil towards the older terrains in the north the northern rivers are characterized by low ca mg ratios 0 5 1 0 intermediate between those of fresh rocks 1 7 3 3 and soil 0 1 weathering in the northern part of the island is therefore dominated by the dissolution of depleted secondary minerals into soils the ca mg ratio of the river water increases from north to south eventually reaching values similar to those of the bedrocks arguing for congruent dissolution of the youngest volcanic rocks the magnesium isotopic composition of river water 26mg reflects inputs from both rain and weathering processes in southern and central rivers the mg isotopic value of waters after correction for rain inputs 26mgwea is systematically depleted in heavy isotopes mean value of 0 34 relative to that of the bedrock 0 24 to 0 15 in the north the 26mgwea of the river water 0 09 is heavier than that of the andesitic bedrock possibly reflecting the dissolution of 26mg rich secondary minerals ferralitic soil measured around 0 13 the robustness of 26mg and ca mg as proxies of the degree of soil weathering should be investigated further via more detailed sampling campaigns in the future by combining high frequency monitoring of river discharge with measurements of concentration discharge relationships for a wide range of chemical elements we estimate the mean annual chemical weathering fluxes for three rivers that belong to the obsera critical zone observatory fluxes vary from 51 7 to 91 8 t km2 yr north to south along the bedrock age gradient and are among the highest recorded in volcanic tropical regions flash floods can explain 21 31 of the annual chemical weathering fluxes the results highlight the importance of monitoring rivers over periods of several years in order to obtain accurate estimates of chemical exports in tropical and mountainous environments

subduction mega shear systems and late palaeozoic basin development in the african segment of gondwana basins within the african sector of gondwana contain a late palaeozoic to early mesozoic gondwana sequence unconformably overlying precambrian basement in the interior and mid palaeozoic strata along the palaeo pacific margin small sea board pacific basins form an exception in having a carboniferous to early permian fill overlying devonian metasediments and intrusives the late palaeozoic geographic and tectonic changes in the region followed four well defined consecutive events which can also be traced outside the study area during the late devonian to early carboniferous period up to 330 ma accretion of microplates along the patagonian margin of gondwana resulted in the evolution of the pacific basins thermal uplift of the gondwana crust and extensive erosion causing a break in the stratigraphic record characterised the period between 300 and 330 ma at the end of this period the gondwana ice sheet was well established over the uplands the period 260 300 ma evidenced the release of the gondwana heat and thermal subsidence caused widespread basin formation late carboniferous transpressive strike slip basins e g sierra australes colorado karoo falklands ellsworth central transantarctic mountains in which thick glacial deposits accumulated formed inboard of the palaeo pacific margin in the continental interior the formation of zambesi type rift and extensional strike slip basins were controlled by large mega shear systems whereas rare intracratonic thermal subsidence basins formed locally in the late permian the tectonic regime changed to compressional largely due to northwest directed subduction along the palaeo pacific margin the orogenic cycle between 240 and 260 ma resulted in the formation of the gondwana fold belt and overall north south crustal shortening with strike slip motions and regional uplift within the interior the gondwana fold belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dextral transpressive motions associated with the orogenic cycle was the formation of mega shear systems one of which falklands east africa tethys shear split the supercontinent in the permo triassic into a west and an east gondwana by a slight clockwise rotation of east gondwana a supradetachment basin formed along the tethyan margin and northward displacement of madagascar west falkland and the gondwana fold belt occurred relative to a southward motion of africa

chemical weathering in a tropical watershed luquillo mountains puerto rico i long term versus short term weathering fluxes abstract the pristine rio icacos watershed in the luquillo mountains in eastern puerto rico has the fastest documented weathering rate of silicate rocks on the earth s surface a regolith propagation rate of 58 m ma 1 calculated from iso volumetric saprolite formation from quartz diorite is comparable to the estimated denudation rate 25 50 ma 1 but is an order of magnitude faster than the global average weathering rate 6 ma 1 weathering occurs in two distinct environments plagioclase and hornblende react at the saprock interface and biotite and quartz weather in the overlying thick saprolitic regolith these environments produce distinctly different water chemistries with k mg and si increasing linearly with depth in saprolite porewaters and with stream waters dominated by ca na and si such differences are atypical of less intense weathering in temperate watersheds porewater chemistry in the shallow regolith is controlled by closed system recycling of inorganic nutrients such as k long term elemental fluxes through the regolith e g si 1 7 10 8 moles m 2 s 1 are calculated from mass losses based on changes in porosity and chemistry between the regolith and bedrock and from the age of the regolith surface 200 ma mass losses attributed to solute fluxes are determined using a step wise infiltration model which calculates mineral inputs to the shallow and deep saprolite porewaters and to stream water pressure heads decrease with depth in the shallow regolith 2 03 m h2o m 1 indicating that both increasing capillary tension and graviometric potential control porewater infiltration interpolation of experimental hydraulic conductivities produces an infiltration rate of 1 m yr 1 at average field moisture saturation which is comparable with libr tracer tests and with base discharge from the watershed short term weathering fluxes calculated from solute chemistries and infiltration rates e g si 1 4 10 8 moles m 2 s 1 are compared to watershed flux rates e g si 2 7 10 8 moles m 2 s 1 consistency between three independently determined sets of weathering fluxes imply that possible changes in precipitation temperature and vegetation over the last several hundred thousand years have not significantly impacted weathering rates in the luquillo mountains of puerto rico this has important ramifications for tropical environments and global climate change

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

uplift history of a transform continental margin revealed by the stratigraphic record the case of the agulhas transform margin along the southern african plateau the south and southeast coast of southern africa from 28 s to 33 s forms a high elevated transform passive margin bounded to the east by the agulhas falkland fracture zone affz we analysed the stratigraphic record of the outeniqua and durban thekwini basins located on the african side of the affz to determine the evolution of these margins from the rifting stage to present day the goal was to reconstruct the strike slip evolution of the agulhas margin and the uplift of the inland high elevation south african plateau the agulhas transform passive margin results from four successive stages 1 rifting stage from late triassic to early cretaceous 200 134 ma punctuated by three successive rifting episodes related to the gondwana breakup 2 wrench stage 134 131 ma evidenced by strike and dip slip deformations increasing toward the affz 3 active transform margin stage 131 92 ma during which the falkland malvinas plateau drifts away along the affz with an uplift of the northeastern part of the outeniqua basin progressively migrating toward the west 4 thermal subsidence stage 92 0 ma marked by a major change in the configuration of the margin onset of the shelf break passive margin morphology two main periods of uplift were documented during the thermal subsidence stage of the agulhas margin 1 a 92 ma short lived margin scale uplift followed by a second one at 76 ma located along the outeniqua basin and 2 a long lasting uplift from 40 to 15 ma limited to the durban thekwini basin this suggests that the south african plateau is an old upper cretaceous relief 90 70 ma reactivated during late eocene to early miocene times 40 15 ma

silicon isotopes and the tracing of desilication in volcanic soil weathering sequences guadeloupe abstract silicon si stable isotopes have the potential to become a useful weathering proxy given that light si isotopes are preferentially incorporated into secondary clay minerals here we investigate how si depletion in soils and associated clay mineralogy influence the si isotope fractionation associated with clay mineral formation we report 30 si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from guadeloupe that were formed under contrasting climatic conditions strongly desilicated soils containing kaolinite that formed in wet areas high precipitation are compared with less desilicated soils containing smectite formed in drier conditions low precipitation clay fractions are isotopically lighter than the parent andesite 30 si 0 23 and increasingly lighter with si depletion in soils which supports the view that the si isotope composition in secondary clay fractions is controlled by the degree of soil desilication it is shown that the si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for si rich secondary clay minerals such as smectite and larger for si poor secondary clay minerals such as kaolinite this study provides new insights to better define si isotopes as a proxy for environmental conditions for clay neoformation

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

meandering subtropical jet and precipitation over summertime east asia and the northwestern pacific abstractit has been revealed that in summertime precipitation is enhanced to the south of the upper level tropopausal potential vorticity contours which are accompanied by instantaneous jets over the eastern coastal region of china to the northwestern pacific it is frequently exhibited as precipitation bands ranging in size from over a thousand to several thousands of kilometers long in this study an analysis was conducted to quantify the relationship depending on the phase of upper level disturbances with composite analysis it is shown that the enhancement along the contours occurs at all phases it occurs not only to the east but also to the west of the upper level troughs although it is weaker the midtropospheric distributions of upwelling and the q vector convergence are collocated with the precipitation enhancement suggesting the importance of dynamical induction by geostrophic flow at all phases the effects of upper level disturbances and low level jets lljs with a southerly component

towards an understanding of climate proxy formation in the chew bahir basin southern ethiopian rift abstract deciphering paleoclimate from lake sediments is a challenge due to the complex relationship between climate parameters and sediment composition here we show the links between potassium k concentrations in the sediments of the chew bahir basin in the southern ethiopian rift and fluctuations in the catchment precipitation evaporation balance our micro x ray fluorescence and x ray diffraction results suggest that the most likely process linking climate with potassium concentrations is the authigenic illitization of smectites during episodes of higher alkalinity and salinity in the closed basin lake due to a drier climate whole rock and clay size fraction analyses suggest that illitization of the chew bahir clay minerals with increasing evaporation is enhanced by octahedral al to mg substitution in the clay minerals with the resulting layer charge increase facilitating potassium fixation linking mineralogy with geochemistry shows the links between hydroclimatic control process and formation of the chew bahir k patterns in the context of well known and widely documented eastern african climate fluctuations over the last 45 000 years these results indicate characteristic mineral alteration patterns associated with orbitally controlled wet dry cycles such as the african humid period 15 5 ka or high latitude controlled climate events such as the younger dryas 12 8 11 6 ka chronozone determining the impact of authigenic mineral alteration on the chew bahir records enables the interpretation of the previously established xrf derived aridity proxy k and provides a better paleohydrological understanding of complex climate proxy formation

chemical denudation rates of the western canadian orogenic belt the stikine terrane we report in this paper major element and sr isotopic ratio analyses for 19 rivers draining the stikine province in the western canadian cordillera dissolved solutes suspended sediments and river sands were analyzed the aim of the paper is to calculate chemical denudation rates for this uplifted accretionary prism mainly dominated by volcanics and volcanic derived sedimentary rocks the major chemical features of the stikine rivers are their dilute character and the excess of ca mg and sr in the dissolved load with regards to na molar ca na ratios from 2 to 14 mean value of 5 4 are observed although the region is dominated by silicate rocks suspended sediments show ca na ratios close to 0 4 isotopically most of the rivers investigated display lower sr isotopic ratios in the dissolved load compared to the suspended load which is very unusual to account for the excess of cations three different hypotheses are tested by an inversion method allowing us to determine the origin of each major solute the first scenario assumes that the excess of ca and mg is due to the dissolution of unradiogenic carbonates the second scenario supposes that the excess of base cations is derived from the preferential dissolution of ca and mg rich minerals such as amphiboles in the third scenario all solutes are assumed to derived from silicate weathering the cationic weathering rates of silicates although imprecise are in the order of 2 5 tons km2 year for the two first hypotheses if all solutes are taken into account denudation rates are between 9 and 15 tons km2 year on a global scale these numbers are among the lowest found for rivers draining basaltic rocks they are however found to be consistent with the global law for chemical weathering rates established for basaltic rocks showing that chemical denudation rate are linearly correlated to runoff and depend on temperature according to an arrhenius law the rivers draining the complex assemblage of volcanic and volcanic derived rocks in the subduction collision geological context of the stikine province thus respond similarly to the other volcanic provinces of the world this does not seem to be true for associated co2 consumption rates the abundance of sulfate ions in the dissolved load possibly derived from oxidative weathering of sulfides may indeed provide a non negligible part of the protons necessary for chemical weathering within canada the stikine geologic province is one whose silicates weather at a high rate the order of chemical weatherabilty of canadian geological province from the lowest to the highest is established as follows granites shales of the rockies volcanics of the western cordillera shales of the interior platform carbonates

remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs an example from the izu bonin arc

spheroidal weathering of granite porphyry with well developed columnar joints by oxidation iron precipitation and rindlet exfoliation spheroidal weathering one of the important rock weathering styles has been attributed to chemical weathering by the water from joint surfaces and mechanical aspects of the weathering have not been well addressed we made an investigation on spheroidal weathering of miocene granite porphyry with well developed columnar joints and found that this spheroidal weathering proceeds through chemical processes and accompanying mechanical processes the investigation of the textures physical properties mineralogy and chemistry of the porphyry revealed the presence of a brown band on the surface margins of corestones representing the oxidation of pyrite and chlorite and the precipitation of iron hydroxides and the consequent generation of micro cracks within the band during weathering oxidation progresses inwards from joints that surround the rindlets including both high angle columnar and low angle planar joints and causes rounding of the unweathered interior portion of the rock microscopic observations of the brown band embedded with fluorescent resin show that pores are first filled with iron hydroxides and that micro cracks then form parallel to the oxidation front in the outer portion of the brown band iron hydroxide precipitation increases the p wave velocity in the brown band while micro crack formation decreases the tensile strength of the rock where the brown band has thickened to 6 cm the micro cracks are connected to one another to create continuous cracks which separate the rindlets from the corestone micro crack formation parallel to the corestone surface may be attributed to compressive stresses generated by small amounts of volumetric expansion due to the precipitation of iron hydroxides in the brown band earth surface is under oxidizing environments so that precipitation of iron hydroxides commonly occurs the spheroidal weathering in this paper is a typical example of the combination of chemical and mechanical processes under such environments copyright 2016 john wiley sons ltd

quantifying chemical weathering rates along a precipitation gradient on basse terre island french guadeloupe new insight from u series isotopes in weathering rinds abstract inside soil and saprolite rock fragments can form weathering clasts alteration rinds surrounding an unweathered core and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates recently uranium series u series disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds to further investigate whether the u series isotope technique can document differences in long term weathering rates as a function of precipitation we conducted a new weathering rind study on tropical volcanic basse terre island in the lesser antilles archipelago in this study for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient electron microprobe emp point measurements bulk major element contents and u series isotope compositions were determined in two weathering clasts from the deshaies watershed with mean annual precipitation map 1800 mm and temperature mat 23 c on these clasts five core rind transects were measured for locations with different curvature high medium and low of the rind core boundary results reveal that during rind formation the fraction of elemental loss decreases in the order ca na k mg si al zr ti fe such observations are consistent with the sequence of reactions after the initiation of weathering specifically glass matrix and primary minerals plagioclase pyroxene weather to produce fe oxyhydroxides gibbsite and minor kaolinite uranium shows addition profiles in the rind due to the infiltration of u containing soil pore water into the rind as dissolved u phases u is then incorporated into the rind as fe al oxides precipitate such processes lead to significant u series isotope disequilibria in the rinds this is the first time that multiple weathering clasts from the same watershed were analyzed for u series isotope disequlibrian and show consistent results the u series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a u series mass balance model the weathering advance rates generally decreased with decreasing curvature 0 17 0 10 mm kyr for high curvature 0 12 0 05 mm kyr for medium curvature and 0 11 0 04 0 08 0 03 0 06 0 03 mm kyr for low curvature locations the observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models i e that the curvature of the rind core boundary controls the porosity creation and weathering advance rates at the clast scale at the watershed scale the new weathering advance rates derived on the low curvature transects for the relatively dry deshaies watershed average rate of 0 08 mm kyr map 1800 mm and mat 23 c are 60 slower than the rind formation rates previously determined in the much wetter bras david watershed 0 18 mm kyr low curvature transect map 3400 mm and mat 23 c also on basse terre island thus a doubling of map roughly correlates with a doubling of weathering advance rate the new rind study highlights the effect of precipitation on weathering rates over a time scale of 100 kyr weathering rinds are thus a suitable system for investigating long term chemical weathering across environmental gradients complementing short term riverine solute fluxes

influence of atmospheric deposits and secondary minerals on li isotopes budget in a highly weathered catchment guadeloupe lesser antilles abstract to better constrain li dynamics in the tropics we sampled critical zone compartments of a small forested andesitic catchment in guadeloupe soils parent rock atmospheric dust plants soil solutions stream and rain waters the aims of this study are to identify the origin of li in the different compartments and to better characterize the behavior of li and its isotopes during water rock interaction in a highly cation depleted soil the li isotope signature 7li of throughfall samples varies between 11 2 and 26 4 as this is lower than the seawater signature 31 and vegetation does not fractionate li isotopes our data indicate that saharan dust 0 7 significantly contributes to the throughfall signature li isotope composition measured in a 12 5 m deep soil profile varies from 3 9 near the surface to 13 5 at 11 m depth compared to unweathered andesite 5 the deep soil signature is in agreement with preferential incorporation of light li into secondary minerals in the top soil however our results also emphasized that atmospheric deposition wet and dry is a main source of li to the soil the decreasing 7li with increasing depth is consistent with a vertical gradient of incorporation of heavy atmospheric li this input being maximal near the surface at the catchment scale throughfall and total atmospheric inputs sea salts saharan dust provide 12 1 and 23 9 g li yr 1 respectively to the quiock creek catchment these fluxes represent 34 and 67 respectively of li exported at the outlet indicating that atmospheric deposition is one of the main li inputs to the critical zone li concentration and isotopic mass balance at the catchment scale indicate that in addition to atmospheric deposition secondary mineral phase dissolution is a major solute source and that andesite no longer participates in significant production of li

permo triassic anatexis continental rifting and the disassembly of western pangaea crustal anatectites are frequently observed along ocean continent active margins although their origins are disputed with interpretations varying between rift related and collisional we report geochemical isotopic and geochronological data that define an 1500 km long belt of s type meta granites along the andes of colombia and ecuador which formed during 275 223 ma these are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon u pb dates ranging between 240 and 223 ma a model is presented which places these rocks within a compressive permian arc setting that existed during the amalgamation of westernmost pangaea anatexis and mafic intrusion during 240 223 ma are interpreted to have occurred during continental rifting which culminated in the formation of oceanic crust and initiated the break up of western pangaea compression during 275 240 ma generated small volumes of crustal melting rifting during 240 225 ma was characterized by basaltic underplating the intrusion of tholeiitic basalts and a peak in crustal melting tholeiitic intrusions during 225 216 ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust and we assign this period to the onset of continental drift dissected ophiolitic sequences in northern colombia yield zircon u pb dates of 216 ma the permo triassic margin of ecuador and colombia exhibits close temporal faunal and geochemical similarities with various crustal blocks that form the basement to parts of mexico and thus these may represent the relict conjugate margin to nw gondwana the magmatic record of the early disassembly of pangaea spans 20 ma 240 216 ma and the duration of rifting and rift drift transition is similar to that documented in cretaceous tertiary rift settings such as the west iberia newfoundland conjugate margins and the taupo lau havre system where rifting and continental disassembly also occurred over periods lasting 20 ma

permo triassic fluvial systems in the southeastern karoo basin south africa abstract a prominent clastic wedge of latest permian to early triassic age katberg sandstone prograded northwestwards into the main karoo basin from a southerly source area as a sedimentary response to renewed tectonism associated with the cape fold belt proximal to distal relationships within this clastic wedge and its relationships with underlying and overlying formations reveal a variety of stream types which reflect changing tectonic and climatic conditions towards the close of permian times the depositional area of the southeastern karoo basin was crossed by meandering river channels cutting through earlier formed floodplain deposits balfour formation tectonic rejuvenation of the source area in early triassic times led to steeper gradients and a sharp increase in the supply of coarser grained detritus as a result alluvial fans developed in areas adjacent to the source terrane and the river channels became braided depositing only sands katberg sandstone with muds and silts being carried down into the most distal parts of the floodplain burgersdorp formation subsequent denudation of the source area gradually reduced slopes and allowed the source ward shift of the distal facies until it eventually overstepped the earlier formed braided stream deposits accompanying the changes in tectonic conditions was a change in climatic which also influenced stream type the late permian to early triassic period records a general change to warmer climatic conditions following the widespread glaciation of the early to middle permian thus the lowest beaufort group sediments were probably deposited in warm temperate to humid conditions with later deposits being laid down under an increasingly arid regime

mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences guadeloupe magnesium mg stable isotopes are increasingly used as a weathering proxy in soils and rivers but the impact of the mineralogy of secondary phases on isotopefractionation remains obscure a better understanding of the behaviour of mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone here we investigate isotopic variations in 26mg in bulk soils and clay fractions relative to their parent andesite in three soilweatheringsequences from guadeloupe formed under contrasting climatic conditions soils formed in drier conditions low precipitation contain smectite whereas soils formed under wet conditions high rainfall are characterized by halloysite and fe oxides or kaolinite all clay fractions have mg isotopic compositions 26mg 0 41 to 0 10 similar to or heavier than their parent andesite 26mg 0 47 supporting the preferential incorporation of heavy mg isotopes in secondary mg bearing clay minerals with the first direct measurements on clay fractions soils with lighter mg isotope compositions have greater quantities of exchangeable mg the data support a contribution from sea spray to the exchangeable mg pool correlated to the soilweathering degree this study highlights for the first time that the soil 26mg not only depend on 26mg of the parent rock and on any fractionation that might occur but also on the mg retention on the exchange complex which could in turn be controlled by external inputs such as sea spray

chemical weathering and cation loss in a base poor watershed weathering of minerals in soils provides important plant nutrients and consumes acidity yet mineral weathering is difficult to observe and quantify we derived present day weathering flux estimates for soil minerals in a forested watershed in central new hampshire using a geochemical mass balance the cone pond watershed is characterized by low alkalinity surface waters and acidic spodosols developed on a thin mantle of locally derived till making it susceptible to continued inputs of acid precipitation weathering reactions were developed on the basis of observed patterns of mineral abundance in the soil profile and measured mineral chemistry the dissolution of approximately 171 mol ha 1 yr 1 of plagioclase feldspar is the predominant weathering reaction at cone pond weathering fluxes of hornblende biotite chlorite and potassium feldspar ranged from 3 to 20 mol ha 1 yr 1 an order of magnitude lower when normalized to their abundances in the soil however hornblende and chlorite had the fastest weathering rates as measured by cation release chemical weathering mainly of plagioclase resulted in the neutralization of only 52 of incoming acidity furthermore silicate weathering could only account for the release of about 53 mol ha 1 yr 1 of ca compared to the observed net output stream loss minus bulk precipitation input of 75 mol ha 1 yr 1 on the basis of these observations and a companion study we conclude that current acid inputs exceed the ability of cone pond soils to neutralize hydrogen ion and that depletion of approximately 22 53 mol ha 1 yr 1 of ca from labile soil pools is occurring in this area the rate of ca depletion is one third to one eighth of the rate estimated for a more base rich watershed nearby

geochemistry of the eocene clastic sediments suonahu formation in the north qiangtang basin tibet implications for paleoclimate conditions provenance and tectonic setting the geochemistry of the clastic sedimentary units of the suonahu formation from the qd17 well in the northern qiangtang basin tibet was studied using various chemical analyses sio2 al2o3 ratios