time scales of sedimentary transfer and weathering processes from u series nuclides clues from the himalayan rivers abstract in order to define time scales of erosion and sedimentary transfer in the himalaya 238 u 234 u 230 th disequilibria have been analysed in river bank sediments and in bedloads collected along the kali gandaki river one of the main nepalese rivers and in the ghaghara and gandak rivers two major plain tributaries of the ganges the th activity ratios and u th ratios in river sediments of the two ganges tributaries constantly decrease from upstream to downstream this is related to the maturation of sediments by weathering during their transfer to the plain the u series data allow to calculate a transfer time for the sediments in the alluvial gangetic plain from the chain front to the confluence with the ganges of about 100 kyr for both rivers the kali gandaki river sediment data highlight a decrease of both the th isotopic and u th ratios which is explained by a mixing between two sources with similar u th ratios but having suffered a different u th fractionation history interpretation of the u series data in the frame of this scenario gives long time scales of weathering of several 100 s kyr for the himalayan terranes the results imply that himalayan bedrocks are submitted to a long in situ stage of weathering before their erosion and transfer into the rivers in addition occurrence of similar u sr signatures in dissolved i e

uranium series isotopes in river materials insights into the timescales of erosion and sediment transport abstract the uranium series isotope signatures of the suspended and dissolved load of rivers have emerged as an important tool for understanding the processes of erosion and chemical weathering at the scale of a watershed these signatures are a function of both time and weathering induced fractionation between the different nuclides provided appropriate models can be developed they can be used to constrain the residence time of river sediment this chronometer is triggered as the bedrock starts weathering and the inferred timescale encompasses the residence time in the weathering profile storage in temporary sediment deposits e g floodplain and transport in the river this approach has been applied to various catchments over the past five years showing that river sediments can reside in a watershed for timescales ranging from a few hundreds of years iceland to several hundreds of thousands of years lowlands of the amazon various factors control how long sediment resides in the watershed the longest residence times are observed on stable cratons unaffected by glacial cycles or more generally climate variability and human disturbance shorter residence times are observed in active orogens andes or fast eroding recently glaciated catchments iceland in several cases the residence time of suspended sediments also corresponds to the time since the last major climate change the u series isotope composition of rivers can also be used to predict the river sediment yield assuming steady state erosion is reached by comparing this estimate with the modern sediment yield obtained by multi year sediment gauging it is clear that steady state is seldom reached this can be explained by climate variability and or human disturbance steady state is reached in those catchments where sediment transport is rapid iceland or where the region has been unaffected by climate change and or human disturbance u series are thus becoming an important tool to study the dynamics of erosion

igneous petrogenesis and tectonic setting of plutonic and volcanic rocks of the cordillera de talamanca costa rica panama central american arc

time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

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

the ancient shorelines of lanai hawaii revisited abstract three ancient shorelines the mahana kaluakapo and manele have been described at elevations of 365 190 and 170 m in the kaluakapo crater lanai hawaii bernice p bishop mus bull 237 1978 1 stearns original observation of a fossil bearing outcrop at 326 m was interpreted as an ancient shoreline subsequently moore and moore 1984 1988 science 226 1984 1312 geol soc am spec pap 229 1988 101 argued that stearns fossil evidence represented the highest inundation of tsunami waves associated with the collapse of the flanks of the hawaiian islands chain the giant wave hypothesis gwh subsequently stearns fossil bearing swale site has never been observed geological field studies of kaluakapo crater were conducted to clarify the nature of the highest coral bearing outcrops preserved on lanai these field studies found no support for a shoreline at 326 m however the field studies did document a shingle terrace with numerous coral clasts at 190 m elevation we find in situ fossil bearing marine deposits at 170 m elevation within kaluakapo crater furthermore we observe the undisturbed nature of marine deposits at 190 m the preservation of fine grained materials at 170 m the absence of in situ deposits with a biological component between 200 and 365 m the dearth of rounded boulders lacking weathering rinds between 200 and 365 m and the lack of post depositional burial of the terrace deposit despite loose debris being present on the high ground surrounding the 190 m terrace these observations lead us to believe that erosion by the hypothesized giant waves did not take place our observations are inconsistent with the giant wave tsunami inundation of the kaluakapo crater area thus we concur with stearns 1938 1978 geol soc am bull 49 1938 615 bernice p bishop mus bull 237 1978 1 jones 1993 jones a t 1993 elevated fossil coral deposits in the hawaiian islands a measure of island uplift in the quaternary phd dissertation department of oceanography university of hawaii 1 274 and grigg and jones 1997 mar geol 141 1997 11 that the coral bearing deposits on lanai represent ancient shorelines and reflect a history of uplift of the island associated with lithospheric deformation of the sea floor around the hawaiian hot spot

the role of biological weathering in periglacial areas a study of weathering rinds in south iceland the importance of biogeochemical processes in the morphogenic system of periglacial environments has been underestimated for decades weathering rind studies help to characterise and quantify biogeochemical weathering over time experiments using fungi strains isolated from basaltic boulders allow one to decipher biological weathering a historical approach using heterochronous moraines provides the opportunity to follow the life of weathering rinds and helps to build a chronology of dominant processes of weathering during the first century of exposure it appears that exfoliation of weathering rinds commonly observed on boulders is the result of a combination of both biogeochemical and physical processes

quaternary glaciers and climate on cerro chirripo costa rica abstract glacial lake sediments and glacial geomorphology in valle de las morrenas a glacial trough on the north face of cerro chirripo costa rica provide evidence on high altitude pleistocene conditions in central america the most recent glacier in the valley chirripo stage i receded very rapidly near the end of the younger dryas chronozone radiocarbon dates on basal organic sediments from lakes beneath upper middle and lower limits of that glacier fall close together and two sigma calibrated ages overlap for the period 9700 9600 cal yr b p earliest datable transition sediments from the central lake date to 12 360 11 230 cal yr b p larger older moraines and associated trimlines allowed reconstruction of three paleoglaciers chirripo stages ii iii and iv computer analysis of hypsometry using published tropical glacier vertical mass balance profiles yields elas of 3506 3523 3515 3537 and 3418 3509 m respectively chirripo ii ela estimate positions applied to chirripo i yield an ela of 3538 3546 m we infer minimal temperature depressions of 7 4 8 0 c for the chirripo i iv stages modeling the behavior of modern tropical glaciers yields basinwide net accumulation estimates of 440 620 550 830 and 960 1760 mm yr 1 for the chirripo ii iii and iv stages

relative dating of terraces of the owens river northern owens valley california and correlation with moraines of the sierra nevada abstract in the northern owens valley the owens river has cut and preserved three terraces uplifted above its modern floodplain we have analyzed soils and used other relative dating techniques in order to differentiate the terrace surfaces to correlate from the terraces to moraines of the eastern sierra nevada and to test the effects of relief and climatic differences on relative dating results most of the relative dating techniques attempted were successful in differentiating the terraces of the owens river sequence in addition weathering rinds soil descriptions and an early holocene 14 c date for the youngest terrace support correlation of the three terrace levels with the three post sherwin age glacial stages tioga tahoe and pre tahoe found in nearby canyons of the sierra nevada other relative dating techniques that are commonly employed however suggest distinct differences between the terrace and moraine sequences these differences probably reflect the difference in relief such that slow erosion of the moraine crests may have retarded soil development relative to the flat terrace surfaces and altered surface weathering processes use of relative dating analyses to correlate between dissimilar geomorphic systems requires caution but use of a broad range of techniques and absolute age calibration may make correlation possible

uranium colloidal transport and origin of the 234u 238u fractionation in surface waters new insights from mount cameroon in order to evaluate the spatial variability of the 234u 238u fractionation and its origin in surface waters 234u 238u activity ratios a r and trace element concentrations were measured in 34 streams and springs draining mount cameroon volcano in addition tangential ultrafiltration techniques were applied to 10 of the samples to assess the role of colloids on the transport of the uranium isotopes u concentration variations in the different ultrafiltered fractions indicate that inorganic colloids can significantly influence the transport of this chemical element even in diluted and organic poor waters such influence which may be related to the chemical and physical nature of colloids does induce fractionations between u and sr along with filter size 234u 238u a r variations among the different ultrafiltered fractions of a sample remain small or even negligible only two samples exhibit a significant isotopic difference between colloid associated and solute u colloids of these two samples seem to correspond to a phase which is distinct from the dissolved and particulate loads of the waters at the scale of the volcano the 234u 238u a r in surface waters is large ranging from 1 038 to 1 397 small 234u 238u fractionations between 1 038 and about 1 10 are attributed to surface and meteoric weathering of the bedrock by contrast the highest 234u 238u fractionations up to 1 397 are associated with groundwater contributions thus variations in the 234u 238u a r could correspond to the water source and not to the surface weathering processes consequently 234u 238u disequilibrium may represent a powerful tool for detecting and quantifying the influence of groundwater inputs on the chemical fluxes carried by surface waters

a fluvial record of long term steady state uplift and erosion across the cascadia forearc high western washington state six late quaternary river terraces preserved along the clearwater river in northwestern washington state provide a 140 ka record of long term incision and uplift across the western side of the cascadia forearc high terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio fluvial deposits and the global eustatic curve the terraces overlie flat bedrock surfaces called straths which represent uplifted segments of the river channel bedrock incision is measured by the height of a strath relative to the adjacent modern river channel the straths along the clearwater show an upstream increase in bedrock incision ranging from 0 at the coast to a maximum of 110 m in the headwaters the incision at any point along the profile increases systematically with strath age the calculated incision rates range from 10 ky the clearwater river valley has maintained a steady state profile defined by a long term balance in the rates of incision and rock uplift upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast toward the central part of the range these results are consistent with other evidence indicating a long term steady state balance between the accretionary influx and the erosional outflux for this part of the cascadia subduction wedge since 14 ma these results help show how terrace deposits form in tectonically active landscapes the dominantly fluvial clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages in turn the straths were buried during the transition to interglacial times because of increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well the fluvial system shows strong forcing by the glacial climate cycle even so the river appears to have returned to the same valley profile during each cycle of strath cutting thus bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle 100 ky but appears to be relatively steady when averaged over longer time scales a simple kinematic model is used to examine how uplift of the cl our analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating if accretion occurred entirely at the front of the wedge the present west coast should be moving to the northeast at 3 m ky relative to a fixed puget sound this prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the clearwater valley profile in this case the long term margin perpendicular shortening would account for 20 to 35 percent of the geodetically measured northeast southwest shortening across the olympic mountains

in situ weathering rind erosion abstract the use of cosmogenic nuclide dating methods place in doubt the long term future of weathering rinds wrs as a chronometric tool why estimate ages when radiometric control is possible this paper presents evidence that wrs can provide invaluable clues about what particular sample would provide the most accurate cosmogenic age by avoiding inheritance of cosmogenic nuclides and by avoiding boulders undergoing spalling the key to this new use requires testing the occurrence and nature of wr erosion back scattered electron microscopy reveals that ignimbrite andesite basalt and granitic clasts experience wr erosion in all subsurface and surface contexts thus far studied in arizona california hawaii oregon and washington to understand the magnitude of wr erosion we measured wr thicknesses all while controlling the cosmogenic and surface stability ages of clasts by chlorine 36 and rock varnish microlamination dating methods our data reveal that traditional methodologies of sampling cobbles remove two thirds of the true thickness of weathering rinds calling into question many beliefs including the notion that clay minerals are not produced during weathering rind formation comparisons of optical and electron microscope measurements support concerns that measurement of wrs by color changes while useful as a pedagogical tool creates serious biases in underestimating the true dynamics of wrs in the end we found that concordance of wr and rock coating age trends indicates the ideal boulder for cosmogenic nuclide dating offsets reveal either ongoing boulder erosion invalidating zero erosion model ages or a potential problem with inheritance of cosmogenic nuclides

a new explanation for extreme 234u238u disequilibria in a dolomitic aquifer high 234u238u activity ratios are found in the shallow groundwater of the phreatic transvaal dolomite aquifer the aquifer is uranium poor while the waters are oxygen rich and young tritium and 14c are used to show that the disequilibrium increases the longer the water resides in contact with the dolomite host the 228ra and 222rn isotopes were used as supporting evidence that ion exchange between the 238u series nuclides in the water and the carbonate wall rock is necessary in assisting in the fractionation process the wolkberg cave speleothems preserve a record of the uranium isotopic fractionations that evolved as water flowed through the aquifer extremely variable and elevated 234u238u ratios of 2 12 are characteristic individual caverns may exhibit very large 234u238u variability in their drip water and in derivative carbonate precipitates speleothems deeper chambers where water has spent a longer time in contact with the aquifer tend to exhibit a greater degree of uranium isotopic fractionation in their speleothems the recorded fractionation process has been an ongoing one from at least the middle late pleistocene to the present the speleothem study supports the contention that ion exchange processes that produce sufficient enrichment of uranium on carbonate can provide ideal conditions for generating uranium isotopic fractionation in the water of carbonate aquifers in the absence of redox changes and within a uranium poor environment

depth to pedogenic carbonate horizon as a paleoprecipitation indicator an analysis of 1168 modern soil profiles from the u s natural resources conservation service yields no correlation between mean annual precipitation and depth to the top of the carbonate horizon r 2 0 03 p 0 001 parent material and soil texture both play negligible roles in this regression when combined with similar published studies n 1481 r 2 improves slightly r 2 0 31 p 0 001 caution is therefore advised in using this or any previously published regression for inferring paleoprecipitation from paleosols however in the combined data set carbonate horizon bearing soils correlate with a mean annual precipitation of 760 mm p 0 05 thus the presence vs absence of pedogenic calcium carbonate is a good indicator of paleoprecipitation above or below this value

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

234u 238u activity ratios in freshwaters as tracers of hydrological processes the strengbach watershed vosges france abstract 234u 238u and 87sr 86sr isotopic ratios as well as major and trace u ba sr rb element concentrations were analyzed in dissolved loads of the strengbach stream vosges france in order to constrain the sources of u isotopes 234u 238u to river waters the variations of the 234u 238u activity ratios along the stream indicate a clear dependence between the 234u 238u activity ratios of the surface waters and the different types of rocks forming the watershed with near equilibrium values for the waters draining granites and high 234u 238u disequilibria 1 4 for those flowing over carbonates the high 234u 238u disequilibria are related to a supply of 234u enriched groundwaters located within the carbonate rocks the 234u 238u activity ratios of the waters at the outlet of the catchment collecting the sources of the stream on the granitic lithology decrease from 1 02 to 0 96 when the discharge of the stream increases such a tendency requires mixing between a water body enriched in 234u which weathered the granitic bed rock at secular equilibrium and a water with a 234u 238u activity ratio below unity representing a mobilization of u from material that has already been weathered comparison of the geochemical characteristics of waters collected during the year and those collected during a flood event reveals the involvement of two different weathered end members depending on the hydrological conditions during the year the dissolved u transported by the river originates from bed rock and deep horizons of the weathering profile whereas a significant part of u during the flood event is supplied by superficial horizons of soils probably complexed by organic colloids these results outline the potential of 234u 238u activity ratios to investigate hydrological processes and emphasize that their use as tracers of rock weathering should be made cautiously

u th ra fractionation during weathering and river transport the potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960 s e g rosholt et al 1966 hansen and stout 1968 this interest results from the dual property of the nuclides of the u and th radioactive series 1 to be fractionated during water rock interactions and 2 to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters therefore the study of radioactive disequilibria in surface environments should help to bring information about the nature the intensity but also the time scale of the water rock interactions produced during weathering and related chemical transfers these different properties have justified many of the studies on u th series in weathering profiles and river waters rosholt 1982 scott 1982 and osmond and ivanovich 1992 gave a synthesis of the studies of u series in weathering and surface hydrology up to the 1980 s and the 1990 s respectively here we present the main directions taken in these domains over the last decade they were partly stimulated by the analytical developments made in the measurement of the medium half life nuclides of 238u series i e 234u 230th 226ra in the mid 1980 s namely the use of thermal ionisation mass spectrometry tims e g chen et al 1986 edwards et al 1987 cohen et al 1991 chabaux 1993 chabaux et al 1994 and more recently the use of mc icp ms turner et al 2001 robinson et al 2002 for u and th isotope analysis details of these developments are given in this volume goldstein and stirling 2003 compared to the radioactive counting methods previously used the new techniques permit 1 a reduction by one order of magnitude or more of the size of the sample required for

paleosol evidence for quaternary uplift and for climate and ecosystem changes in the cordillera de talamanca costa rica abstract a noncalcareous volcanic paleosol succession from the upper montane rain forest of the cordillera de talamanca costa rica lat 9 41 03 3 n long 83 53 45 9 w 2507 m elev provides evidence for changing quaternary elevation climate and ecosystems basal deposits crb horizons are saprolitized porphyritic andesite enriched in si al and fe and depleted in base cations which record an earliest period of very intense weathering forming a leached residuum of smectite kaolinite and fe oxides or oxyhydroxides on a mature paleolandscape probably in tropical lowlands paleo saprolite deposits are sharply overlain by interbedded lapilli tuffs and two paleosols with dark colored organic c rich 2 3 wt ab horizons one containing abundant grass pollen interpreted as mollic epipedons of grassland soils andic mollisol like paleosols with 13c values 24 5 pdb that record a former c3 grassland ecosystem possible pedogenic carbonate btkb horizons with all carbonate removed by overprinting by younger pedogenic processes provide evidence of former drier and drained conditions uppermost buried soils mainly btb horizons are non calcareous clay rich andic ultisol like paleosols that record pedogenesis under climates similar to current conditions with highly weathered clay mineral suites ams dating of charcoal from the upper btb paleosol constrains a latest glacial age 36 640 650 14c yr bp ages for two additional deeper samples were 43 48 ka for the 2btb paleosol uplift rates of the cordillera de talamanca are estimated as 1 km 1 ma consequently the described paleosols provide evidence for 1 initial lowland pedogenesis perhaps at 2 3 ma 2 regional uplift and major unconformity volcanism and development of a grassland ecosystem that has 13c values similar to those measured in modern high elevation paramo sites in the cordillera 24 4 to 26 3 pdb but formed mollic epipedons and pedogenic carbonates which do not characterize modern paramo soils followed by 3 continued late pleistocene volcanism and pedogenesis in environments similar to current oak dominated montane rain forest conditions the paleosol succession provides a valuable record of dynamic climate ecosystem change and uplift history in the cordillera de talamanca that can complement and extend existing bog and lake sediment records

the relationship between riverine u series disequilibria and erosion rates in a basaltic terrain u series isotopes have been measured in the dissolved phase suspended load and bedload of the main rivers draining basaltic catchments in iceland for the dissolved phase 234u 238u and 238u 230th range between 1 08 and 2 2 and 7 4 and 516 respectively for the suspended load and bedload 234u 238u and 238u 230th range from 0 97 to 1 09 and from 0 93 to 1 05 respectively chemical erosion rates calculated from dissolved major elements range between 13 and 333 t km 2 yr 1 physical erosion rates have also been estimated from existing data and range between 21 and 4864 t km2 yr with an average of 519 t km 2 yr 1 u series disequilibria indicate that weathering in iceland operates at close to steady state conditions a model of continuous weathering indicates a maximum weathering timescale of 10 kyr with an average rate of uranium release into water of 1 6 10 4 yr 1 which is significant when compared to granitic terrains located at similar latitudes and to tropical basaltic terrains all river waters display 234u 238u greater than secular equilibrium consistent with the effects of alpha recoil the same dissolved phase 234u 238u exhibit a negative trend with physical erosion rates explained by the dominant effect of close to congruent chemical weathering of hyaloclastites in the younger basaltic terrains therefore chemical erosion rate and mineral weathering susceptibility play a major role in determining 234u 238u disequilibria in basaltic river waters comparison of global data for river basins in which weathering was recently strongly limited indicates a negative correlation between silicate weathering rates estimated with major elements and the age of weathering estimated with u series disequilibria this strongly suggests a key role of time and soil thickness on the chemical erosion of silicates

evaluation of pleistocene groundwater flow through fractured tuffs using a u series disequilibrium approach pahute mesa nevada usa abstract groundwater flow through fractured felsic tuffs and lavas at the nevada national security site represents the most likely mechanism for transport of radionuclides away from underground nuclear tests at pahute mesa to help evaluate fracture flow and matrix water exchange we have determined u series isotopic compositions on more than 40 drill core samples from 5 boreholes that represent discrete fracture surfaces breccia zones and interiors of unfractured core the u series approach relies on the disruption of radioactive secular equilibrium between isotopes in the uranium series decay chain due to preferential mobilization of 234 u relative to 238 u and u relative to th samples from discrete fractures were obtained by milling fracture surfaces containing thin secondary mineral coatings of clays silica fe mn oxyhydroxides and zeolite intact core interiors and breccia fragments were sampled in bulk in addition profiles of rock matrix extending 15 to 44 mm away from several fractures that show evidence of recent flow were analyzed to investigate the extent of fracture matrix water exchange samples of rock matrix have 234 u 238 u and 230 th 238 u activity ratios ar closest to radioactive secular equilibrium indicating only small amounts of groundwater penetrated unfractured matrix greater u mobility was observed in welded tuff matrix with elevated porosity and in zeolitized bedded tuff samples of brecciated core were also in secular equilibrium implying a lack of long range hydraulic connectivity in these cases samples of discrete fracture surfaces typically but not always were in radioactive disequilibrium many fractures had isotopic compositions plotting near the 230 th 234 u 1 1 line indicating a steady state balance between u input and removal along with radioactive decay numerical simulations of u series isotope evolution indicate that 0 5 to 1 million years are required to reach steady state compositions once attained disequilibrium 234 u 238 u and 230 th 238 u ar values can be maintained indefinitely as long as hydrological and geochemical processes remain stable therefore many pahute mesa fractures represent stable hydrologic pathways over million year timescales a smaller number of samples have non steady state compositions indicating transient conditions in the last several hundred thousand years in these cases u mobility is dominated by overall gains rather than losses of u

effect of porosity on the increase in weathering rind thicknesses of andesite gravel abstract factors affecting the increase in weathering rind thickness i e weathering rates were investigated using andesite gravel in fluvial terrace deposits with a known emergence time the time between the age of each terrace formation 20 130 290 and 660 ka and the present was assumed to be the weathering period three zones were classified from measurements of the vickers hardness number vhn across weathering rinds from the rock surface to the inner fresh part 1 zone i the outermost brown layer which has low vhn values of 10 to 80 gf m2 2 zone ii the inner gray layer which has increasing vhn values 100 to 500 gf m2 with increasing depth 3 the innermost fresh part which has large vhn values of 500 gf m2 in this paper zones i and i ii are defined as the brown layer and the weathering rind respectively diffusion theory was applied to the relationship between the thickness of the brown layer or the weathering rind and time weathering periods the results show that the thickness of the brown layer depends mainly on time whereas the thickness of the weathering rind is controlled by the porosity of the host rock as well as time consequently rocks with higher porosity have thicker weathering rinds than brown layers

u isotope ratios as tracers of groundwater inputs into surface waters example of the upper rhine hydrosystem major elements trace elements and sr and u isotopes were analyzed in the dissolved load of water samples collected in the main reservoirs of the rhine graben i e the rhine river its main tributaries and the shallow and deep ground waters in several different aquifers our results confirm that the variations of major elements trace elements especially sr and rb and sr isotope ratios of the water samples are mainly correlated to the lithological variations of the watershed the atmospheric inputs can be considered as a minor cause for the chemical variability of the surface waters in the rhine hydrosystem except for a few chemical elements in the mountain streams human activity in particular the pollution by the exploitation of the alsace potash mines mainly affects the na and cl concentrations of the rhine and the south alluvial groundwaters but remains negligible for the other water samples the specific study of the lauter stream located in the lower triassic sandstones buntsandstein sequences demonstrates the importance of groundwater influences onto the chemical composition of river waters up to 50 of the total dissolved load tds of this stream is supplied by deep waters this contribution significantly modifies the chemical composition of the stream especially the 238u 234u radioactive disequilibrium which in turn can be used as a specific tracer of deep water inputs into surface waters and could be useful to distinguish in the dissolved load of rivers surface weathering fluxes from deeper ones

refractory element mobility in volcanic soils refractory trace element concentrations in strongly weathered hawaiian soils ranging in age from 20 to 4100 ka are highly elevated over parent rock values due to extensive mass loss of more soluble major elements during pedogenesis nb and ta exhibit virtually no mobility soil nb ta ratios are within the range of fresh bedrock even when soil nb concentrations are residually enriched by a factor of 10 in contrast al zr and hf are depleted relative to nb in surface soil horizons but are enriched at depth clearly indicating mobility of these elements variations in th nb ratios in soil profiles indicate significant th mobility within the soil column however mass balance calculations require that accretion of th enriched asian dust has resulted in a net increase in th in some soils soils developed on a 150 ka rainfall gradient show that the mobility and loss of zr increase with mean annual precipitation

formation process of the trace fossil phymatoderma granulata in the lower jurassic black shale posidonia shale southern germany and its paleoecological implications abstract the ichnogenus phymatoderma is a subhorizontal branching burrow system consisting of radiating tunnels filled with fecal pellets this ichnogenus has been interpreted as a product of a deposit feeding animal but the question of whether the phymatoderma producer was a subsurface deposit feeder or a surface deposit feeder is still a topic of controversy herein i present evidence based on carbon isotope analyses for the formation of the trace fossil phymatoderma granulata occurring in the lower toarcian black shale in southern germany carbon isotope ratios of organic carbon 13corg in the pelletal infill of p granulata the surrounding black shale and the overlying gray mudstone are 26 64 0 42 2 28 49 0 44 2 and 26 27 0 13 2 respectively the difference between the pelletal infill and overlying mudstone in terms of 13corg is much smaller than that between the fillings and black shale therefore these data clearly indicate that the phymatoderma producer ingested the surface sediments and subsequently excreted fecal pellets into the subsurface sediments such a surface deposit feeding style would be an especially effective means of absorbing nutrients because surface sediments contain much fresh organic material whereas organic matter in subsurface deposits consists mostly of refractory material that is poorly utilized by most marine benthos

erosion timescales derived from u decay series measurements in rivers the relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades the principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates in this study 234u 238u 230th 238u and 226ra 238u were measured by tims and by mc icp ms in the dissolved and suspended loads of rivers from the mackenzie basin northwest territories canada the data show a complementary nature between 234u 238u 230th 238u in the dissolved and suspended loads while 226ra is characterized by a more complex behavior modeling of fractionation of u series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed 9 28 10 ka as well as the rates of release of u series nuclides the results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the mackenzie river

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

formation of weathering rinds on andesite to explore the mechanisms of weathering rind formation rock properties of an andesite cobble were investigated the andesite cobble studied in this paper was taken from a fluvial terrace which formed around 290 ka bp in the nasuno ga hara area central japan the sampled cobble has a double weathering rind comprising an outer brown zone brown band and an inner pale grey zone white band with thicknesses of 3 mm and 7 mm respectively the boundary between the outer brown band and the inner white band is clearly defined whereas the boundary between the white band and the grey fresh looking interior of the rock is not so clearly defined mineralogical chemical chromatic and mechanical properties of the cobble were investigated in the laboratory the outer zone the brown band is characterized by high brittleness high feo fe2o3 content and low contents of cao mgo na2o and k2o the oxidation of iron accounts for the brown colour of the rind this outer zone is considered to have experienced both oxidation and dissolution the inner zone the white band is characterized by hardness reduction and the dissolution of alkali alkaline earth metals towards the surface of the cobble this inner band is considered to have experienced only dissolution the formation of the weathering rinds can be explained by changes in water movement directions in terrace deposits alkali alkaline earth metals dissolve when water is supplied to a cobble from the surrounding matrix subsequently the dissolved metals move out of the cobble accompanied by outward water movement when the subsurface water table is low the concentrations of cao mgo na2o k2o and al2o3 therefore decrease in both weathering bands but iron tends to remain as ferric hydroxides in the outer brown band processes of dissolution and oxidation have produced the brown band copyright 2001 john wiley sons ltd

late quaternary glaciation of costa rica the highest peaks of costa rica 9 15 9 36 n were glaciated during the late quaternary attesting to a significantly cooler climate in central america than today during the last local glacial maximum llgm an ice cap 35 km2 in extent covered the highest peaks of the cordillera de talamanca around cerro chirripo and around cerros de la muerte and kamuk 5 and 2 km2 of ice existed respectively in chirripo national park three moraine groups define three glacial stages the talamanca stage extends to 3040 m and represents the last local glacial maximum chirripo stage moraines reach elevations of 3300 m and talari stage moraines formed at 3300 and 3420 m former equilibrium line altitudes elas were 3500 m 3600 m and 3620 m respectively cirque floor elevations of 3260 m at cerro kamuk suggest a lower pre llgm ela radiosonde data from san jose place the modern 0 c isotherm at 5000 m which suggests a late quaternary ela depression of 1500 m which is consistent with a temperature reduction at chirripo of 8 9 c during the llgm some of this cooling may be attributable to a steeper lapse rate associated with a drier atmosphere possibly due to a more restricted position of the intertropical convergence zone decreased yearly convection or more frequent incursion of polar air masses

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

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

isotopic tracing of the dissolved u fluxes of himalayan rivers implications for present and past u budgets of the ganges brahmaputra system u activity ratios have been measured in the dissolved loads of selected rivers from the himalayan range in central nepal and from the bangladesh as well as in some rain waters a few european and asian rivers have also been analyzed for their u activity ratios the data confirm the negligible effect of rainwater on the budget of dissolved u in river waters the results also indicate that rivers on each himalayan structural unit have homogeneous and specific u isotope compositions i 234u 238u activity ratios slightly lower than unity in the dissolved load of the streams draining the tethyan sedimentary series tss ii values slightly higher than unity for waters from the high himalaya crystalline hhc and the lesser himalaya lh iii systematically higher 234u 238u activity ratios for waters from the siwaliks thus u activity ratios in association with sr isotopic ratios can be used to trace the sources of dissolved fluxes carried by these rivers coupling of u with sr isotope data shows 1 that the u carried by the dissolved load of the himalayan rivers mainly originates from u rich lithologies of the tss in the northern formations of the tibetan plateau and 2 that the elemental u and sr fluxes carried by the himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the himalayan chain rivers flowing on the indian plain define a different trend from that of the himalayan rivers in the u sr isotopic diagram indicating the contribution of a specific floodplain component to the u and sr budgets of the ganges and the brahmaputra the influence of this component remains limited to 10 to 15 percent for the u flux but can contribute 35 to 55 of the sr flux the variations of the sr and u fluxes of the ganges brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high altitude terrains during glacial episodes this scenario would significantly decrease the dissolved u flux of the ganges brahmaputra river system and increase its u activity ratio such a climatic dependence of the himalayan u flux could induce a periodic variation of the mean u activity ratio of the world rivers on glacial interglacial timescales

the chemical weathering regime of karkevagge arctic alpine sweden abstract karkevagge is a valley located in swedish lapland at approximately 68 n and represents an arctic alpine landscape it is a presently periglacial glaciated trough incised into essentially horizontal metamorphic rocks some of which are presumably pyrite rich a set of coordinated studies was undertaken to investigate the nature of chemical weathering and pedogenesis in the valley and upon the abutting ridges august 1996 water quality measures reveal considerable spatial variation in solute totals with the highest total dissolved solute abundances being correlated with high sulfate abundances ridge crest soils exhibited poor horizonation but more extensive development of secondary clay minerals developed in situ than was found in valley flank and valley bottom soils valley soils exhibited multiple thin horizons many of which were buried and are taken to reflect great paraglacial and periglacial instability favorable microenvironments in the valley permit significant development of spodosols coarse debris along and across the valley bears both weathering rinds and rock coatings rock coatings in the valley include several types of iron films sulfate crusts carbonate skins and heavy metal skins karkevagge represents a mild arctic environment which does not preclude substantial chemical weathering in locations where abundant pyrite rich bedrock and water coincide this weathering follows pathways which are eminently expectable given that weathering of the pyrite rich rock permits generation of sulfuric acid which in turn weathers muscovite mica and calcite in local schists and marble respectively zones of intense chemical weathering also generate clearly visible deposits of gypsum and iron sulfate deposits such as jarosite not all arctic and or alpine environments are likely to be so active chemically but the results from karkevagge clearly show that dismissal of chemical weathering in cold regions on the basis of presumed first principles is erroneous consequently chemical weathering in such environments merits substantially more attention than it has hitherto received

isotopic studies of the upper and middle rio grande part 2 salt loads and human impacts in south new mexico and west texas abstract increasing groundwater and soil salinity is a threat to the land and water resources in arid regions global warming will likely increase salinity of dryland river systems in order to characterize salt loading into the semi arid portion of the rio grande in south new mexico and west texas we sampled seasonally 2009 2011 the river agricultural drains and saline groundwater in addition to major element chemistry these samples were analyzed for sulfur and oxygen isotope compositions 34s and 18o of dissolved so4 and in some cases for nitrogen and oxygen isotope compositions 15n and 18o of dissolved no3 uranium isotopes 234u 238u activity ratio were also measured for selected samples the natural inflow of basinal brines groundwater 34s of 8 to 11 in the semi arid rio grande study area was minor in the investigated seasons and could not be detected by the 34s mass balance however we did find localized increases of 34s 2 to 5 in the rio grande that were attributable to salt loads from the intersections of agricultural drains with the water table of a natural salt flat and associated evaporative brine 34s of 12 in the shallow subsurface in the areas with higher water use for land irrigation the 34s of the river and drain water was relatively consistent from 0 to 2 compared to the 18o from 2 to 6 most likely this resulted from application of s rich fertilizers e g ammonium sulfates elemental s sulfuric acid with low 34s 2 to 4 and high 18o 9 to 16 additionally we observed considerably lower 18o so4 in the rio grande and agricultural drains

u series disequilibria in minerals from gandak river sediments himalaya abstract the impact of mineralogical sorting on u th ra variations observed for the ganga sediments in the himalayan alluvial plain due to transport and deposition processes highlights the importance to better understand the u series isotopic system in himalayan minerals for this purpose u th and ra concentrations along with 238u 234u 230th 226ra radioactive disequilibria were measured in several pure mineral fractions i e monazite zircon titanite ilmenite rutile garnet magnetite quartz biotite and muscovite separated from one bank sediment of the gandak river ganga basin the data obtained through a double digestion protocol in high pressure and high temperature vessels confirm that u and th concentrations and isotopic ratios of the sediments are mainly influenced by a few minor mineral species that are very enriched in u and th namely monazite titanite and epidote as well as major minerals with lower u and th concentrations such as quartz more importantly the data clearly indicate that similarly to whole rock samples the u th ra systematics of individual minerals have likely recorded complex u th ra fractionations in particular the 226ra enrichment observed in the biotite and clay fractions likely results from the affinity of ra for phyllosilicate minerals while the 230th and to a lesser extent the 234u enrichment observed for the other minerals might result from a nuclide adsorption process on the fe oxide coatings present on the grains or directly on the mineral surface the linear trend outlined by the minerals phases in the 226ra 230th vs 230th 234u diagram suggests that 230th is probably mobilized from zircons the only fraction presenting 230th 234u

uranium thorium chronometry of weathering rinds rock alteration rate and paleo isotopic record of weathering fluids abstract the potential of 238u 234u 230th chronometry for constraining the formation rate of weathering rinds developed on fresh rocks is assessed by analyzing a weathering rind on a basaltic clast from a 125 kyr old costa rican alluvial terrace eighteen subsamples were collected from one section of the clast by drilling cores 4 mm in diameter and 5 mm depth along two transects straddling the core rind boundary variations of major and trace element concentrations along the two transects point out a intense loss of alkaline and alkaline earth elements b conservative behaviour of elements such as zr hf and th and c external input of u into the rind without any evidence of u loss during basalt weathering in addition variations in u concentrations along the transects show that the main u th fractionation process associated with the weathering of the basaltic clast is an external input of u without addition nor loss of th in the basalt rind transition zone and that once deposited u is immobile in the weathering rind in the frame of this interpretation scheme a weathering rate of 0 5 0 2 mm kyr can be calculated for the studied clast which is consistent with geological and isotopic evidence constraining the depositional ages of the terraces in addition the variations in the 234u 238u ratio along the analysed transects as well as the increase in sr isotopic ratios within the weathering rind are best explained by temporal variation of the u activity ratios and sr isotope ratios of the soil solutions brought into the rind this work highlights how well detailed u th chronological studies of weathering rinds can 1 constrain the formation rates of weathering systems and 2 record the time variation of isotopic composition of weathering fluids