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

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

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

the erosion response to quaternary climate change quantified using uranium isotopes and in situ produced cosmogenic nuclides abstract studying how catchment erosion has responded to past climate change can help us better understand not only how landscape evolution operates but also predict the consequences of future climate change on soil resource availability recent years have seen the development of tools that allow a quantitative assessment of past changes in catchment erosion this work reviews the principles of the application of in situ produced cosmogenic nuclides and uranium isotopes to quantifying past erosion rates results highlight the role of periglacial processes and mass wasting in dictating how catchment erosion responds to climatic variability at the 10 kyr scale at the million year scale it is more difficult to untangle the role of climate and tectonics a strong coupling exists at the 10 kyr to 100 kyr scales between climatic cycles and the transfer time of regolith from source to sink this coupling reflects changes in sediment source that are either set by changes in vegetation cover at the catchment scale or by the storage of sediments on continental shelves at a larger scale although further analytical developments are required for these tools to reach their full potential existing works suggest that in the near future they will provide unprecedented quantitative insights on how soil and fluvial systems adapt to external perturbations climatic tectonic and or anthropic

the delicate balance between soil production and erosion and its role on landscape evolution abstract the diversity in landscapes at the earth s surface is the result amongst other things of the balance or imbalance between soil production and erosion while erosion rates are well constrained it is only recently that we have been able to quantify rates of soil production uranium series isotopes have been useful to provide such estimates independently of erosion rates in this study new u series isotope are presented data from weathering profiles developed over andesitic parent rock in puerto rico and granitic bedrock in southeastern australia the site in australia is located on a highland plateau neighbouring a retreating escarpment where soil production rates between 10 and 50 mm kyr have been determined the results show that production rates are invariant in these two regions of australia with values between 15 and 25 mm kyr for the new site andesitic soils show much faster rates about 200 mm kyr overall soil production rates determined with u series isotopes range between 10 and 200 mm kyr this is comparable to erosion rates in soil mantled landscapes but faster than erosion in cratonic areas and slower than in alpine regions and cultivated areas this suggests that soil mantled landscapes maintain soil because they can there is a balance between production and erosion similarly thick weathering profiles develop in cratonic areas because despite slow erosion rates soil production is still significant bare landscapes in alpine regions are probably the result of the inability of soil production to catch up with fast erosion rates although this needs testing by u series isotope studies of these regions finally the range of production rates is up to several orders of magnitude lower than erosion rates in cultivated areas demonstrating quantitatively the fast depletion of soil resources with common agricultural practices

the 1975 1977 crisis of la soufriere de guadeloupe f w i a still born magmatic eruption on july 8 1976 eruptive activity broke out at la soufriere de guadeloupe f w i after about one year of increasing seismic activity seismic activity continued to increase until august 1976 reaching more than 1500 events a 200 fold increase over the preceding quiet period of a few years and an energy output of about 1017 ergs in a day a total of 26 major phreatic eruptions similar to the july 8 outburst took place during an eight months period the steam blasts that characterized the eruptions gave rise to particle and sometimes block charged plumes that deposited an estimated 106 m3 of solids the h2o rich gases emitted during the blasts presumably contained other gases h2s so2 co2 that were partly adsorbed on solid particles all material was erupted at temperatures of the order of 100 to 200 c the observation of vertical migration of earthquake foci in less than a few hours and over about 6 km depth and of abnormal variations of the geomagnetic field indicate a deep energy source for the phreatic eruptions a small proportion of the gases adsorbed on solid particles had a magmatic origin however most of the steam and the tephra seemed to originate from superficial levels of a hydrothermal system similar phreatic eruptions have occurred several times in recorded history in the case of la soufriere the origin of the phreatic eruptions is best described by an abnormal energy input versus steady state from a crustal magma chamber the occurrence of truly magmatic eruptions is presumably inhibited by an extensive hydrothermal system the abrupt release of more power from the magma chamber could have resulted in an explosive pyroclastic eruption substantial improvement of the guadeloupe volcano observatory has followed the 1975 1977 crisis permanent telemetered geophysical networks and regular geochemical observations have provided a five year data base of the volcano behavior in its noneruptive state which can be compared to crisis situations

evidence for a new shallow magma intrusion at la soufriere of guadeloupe lesser antilles insights from long term geochemical monitoring of halogen rich hydrothermal fluids more than three decades of geochemical monitoring of hot springs and fumaroles of la soufriere of guadeloupe allows the construction of a working model of the shallow hydrothermal system this system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption 1530 ad and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions because it is confined into the low volume highly compartmented and partially sealed upper edifice structure the hydrothermal system is highly reactive to perturbations in the volcanic activity input of deep magmatic fluids the edifice structure sealing and fracturing and meteorology wet tropical regime the current unrest which began with a mild reactivation of fumarolic activity in 1990 increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome in 1997 seismic activity increased further and was accompanied by a sudden high flux hcl rich gas from summit fumaroles we focus on the interpretation of the time series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens f cl br and i this extensive geochemical time series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater scrubbing it is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series t co2 and total s content in fumaroles t and cl in hot springs and the relative fractionations between f cl br and i in both fluids coupling 35 years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976 1977 crisis the contrasting behaviours of s and cl bearing species in fumarolic gas and in thermal springs suggests that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since 1992 although it might still be evolving the characteristics of this new intrusion indicate that it hasalready reached a magnitude similar to the intrusion that was emplaced during the 1976 1977 eruptive crisis the assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurization of the hydrothermal system on the likelihood of renewed phreatic explosive activity moreover the role of hydrothermal pressurization on the basal friction along low strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse at this stage enhanced monitoring research and data analysis is required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution

the significance of e w structural trends for the alice springs orogeny in the charters towers province north queensland e w structural and igneous trends in the charters towers province are highly anomalous within the overall n s trending tasman orogen of eastern australia and resulted from a sequence of n s shortening events a succession of e w trending fias foliation intersection inflexion axis preserved within porphyroblasts dated at 474 7 7 2 413 13 and 381 1 8 1 ma correspond with adjacent granite crystallization ages and e w trending fia ages in the greenvale province to the nw the magnetic anomaly map of australia reveals that this e w trending portion of the tasman orogen links with and truncates several earlier formed orogens the e w trend of the charters towers province resulted from overprinting of early palaeozoic n s trending structures in the tasman orogen by younger e w ones during the alice springs orogeny 450 300 ma resolving the e w pluton shape and distribution of cambrian to devonian magmatic activity plus the truncational nature of aeromagnetic data from central australia to the east coast

constraining the recent history of the perennially ice covered lake bonney east antarctica using he kr and xe concentrations abstract lake bonney is a perennially ice covered lake in the mcmurdo dry valleys mdvs that has long been studied in order to provide constraints on the paleoclimate of west antarctica the lake is divided into two lobes west lake bonney wlb and east lake bonney elb that are separated by a narrow ridge the two lobes currently receive surface melt water during austral summers from glacier fed ephemeral streams and this meltwater enters the lake via a narrow ring or moat of liquid water that forms around the lake during summer the west lobe also receives water from direct input of melt water from taylor glacier and saline water from irregular subglacial discharge here we combine previously published he data from lake bonney with new kr and xe concentration data to examine the signatures of water recharge via the seasonal moat and these data are used to constrain a model for he kr and xe transport within both wlb and elb over about the last 5000 6000 yrs a detailed numerical simulation is presented that combines diffusive transport of noble gases within the stratified water column of lake bonney along with ice ablation at the top of the ice cover partitioning of noble gases between water and ice plus exchange of noble gases between wlb and elb results strongly suggest that open moats have only operated for about 2 3 centuries within the last millennium these results are corroborated by the high concentration of he especially within wlb which points to a history of ice cover with no open moats operating for both lobes for at least about 5 millennia in addition the distribution of he kr and xe suggest that a significant rise of the water level of lake bonney associated with a warmer period may have been interrupted by a roughly 4 5 century long cold period during which the moats were not large enough to allow air saturated water into the lake with this cold period ending about one century ago in addition during this cold period there is evidence for a thickening of the ice cover in the last century lake bonney levels have been rising pointing either to more extended austral summers or elevated summer temperatures a rise that is compatible with the heavy noble gas record in the upper portion of the lake a preliminary analysis that includes all stable noble gases heavier than he suggests that glacial melt water gmw directly from taylor glacier below the ice cover may have been a significant input into the deepest parts of wlb but not elb

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

ore controls in the charters towers goldfield ne australia constraints from geological geophysical and numerical analyses the approach taken in this paper namely synthesising a wealth of previous information with new data and a genetic model in combination with integrated numerical analyses led to new insights into the geological controls on the localisation of auriferous veins and residual prospectivity of the charters towers goldfield ne australia the method also has implications for the assessment of other mature goldfields worldwide despite a number of different ore controls having operated within the charters towers goldfield the controlling factors can be linked to a single genetic model for orogenic granitoid hosted lode gold mineralisation in a brittle deformation regime d4 of ne sw to nne ssw shortening under conditions of supralithostatic fluid pressure and low stress difference spatial autocorrelation results suggest district scale alignment of the auriferous veins parallel to and overlapping with the ese wnw to e w striking charters towers ravenswood lineament a major crustal boundary in the basement to the ravenswood batholith at the camp scale auriferous veins have abundance and proximity relationships with nw se nnw sse ne sw and ene wsw oriented lineaments suggesting that structures that controlled gold deposition in one camp did not necessarily control mineralisation in other camps fractal dimensions obtained with the box counting method range from 1 02 to 1 10 whereas veins in the charters towers city camp are characterised by a significantly higher fractal dimension of 1 28 this discrepancy may be taken to imply that most or all outcropping and near surface deposits within the charters towers city camp have been found and that new discoveries are more likely to occur at greater levels of depth or outside the boundaries of this camp the new understanding has implications for the assessment of the residual prospectivity of the charters towers goldfield where large areas of prospective rock types and structures e g approximately 40 of the charters towers ravenswood lineament are hidden under cover this parameter space was inaccessible to the historic prospectors and has received relatively little attention from recent explorers the following steps are suggested for the development of a targeting strategy for lode gold exploration in areas of the goldfield under cover 1 identify from geological and geophysical data the ene wsw 15 and nnw sse 15 striking structures and geological boundaries within a 20 km wide corridor parallel to and centred upon the charters towers ravenswood lineament the potential control on gold deposit distribution at the regional to district scale 2 interpret from geological and geophysical data the distribution of pre middle devonian granitoids within these areas that are the preferred host rocks of the payable gold deposits 3 deduce from geophysical data the ene wsw 15 and nnw sse 15 striking structures that cut or bound the intrusions identified in step 2 4 locate segments along the structures identified in step 3 that deviate most from the geometry of a straight line e g potential bends or splays and or intersect other structures or geological contacts or both and 5 define and rank potential targets within the prospective areas identified in step 4 and systematically test the best ones

helium isotope systematics of volcanic gases and thermal waters of guadeloupe island lesser antilles abstract the island of guadeloupe is located in the middle of the 850 km long lesser antilles island arc present day volcanic and geothermal activity is concentrated in two systems both located in the southwestern part of the island basse terre the la soufriere volcanic complex and the bouillante hydrothermal system some 20 km to the northwest of the volcano we report here the largest isotopic data set for helium isotopes in hydrothermal gases and waters from both systems acquired between 1980 and 2012 3 he 4 he ratios in the fumarolic gases of la soufriere volcano have been quite homogeneous and stable over the last thirty years the average ratio of 8 2 0 2 r a confirms that the volcano is tapping a morb like mantle source in contrast the nearby bouillante geothermal system displays a much lower 3 he 4 he ratio 4 5 0 1 r a he c elemental and isotopic relationships show that both systems are actually fed by the same magmatic source and that their marked difference in 3 he 4 he results from the 4 he contamination of the bouillante deep aquifer by the surrounding wallrock this conclusion is strengthened by the spatial distribution of 3 he 4 he ratios which shows that la soufriere fumaroles and the bouillante geothermal system are the two end members of a spatial trend of decreasing 3 he 4 he ratio with distance from la soufriere summit dome implying an increasing addition of radiogenic 4 he from the host rocks away from the present day active volcanic edifice

the evolution of weathering profiles through time new insights from uranium series isotopes the sustainability of soil is a major issue for society in principle the evolution of soil resources can be constrained by comparing the rates of soil erosion and production cosmogenic isotopes provide one measurement of soil erosion rates they can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production this implies that the evolution of soil resources thinning thickening or constant since soil thickness is assumed to be constant with time here we utilise an independent method to estimate soil production rates using uranium series u series isotopes the study of a site in temperate australia undisturbed by human activity shows that soil production rates inferred from u series isotopes are similar to erosion rates derived from beryllium 10 10be measurements implying that at this site there is no net accumulation or loss of soil saprolite production rates the migration rate of the weathering front into the bedrock are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady state this study demonstrates that the combination of u series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles preliminary observations suggest that the rate of bedrock weathering i e saprolite production in temperate australia is of the same order of magnitude as that inferred for laterites in tropical climates this may suggest that for thick weathering profiles although the extent of weathering strongly differs between temperate and tropical climates the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions

zn isotope compositions of the thermal spring waters of la soufriere volcano guadeloupe island abstract to trace the sources and pathways of zn in hydrothermal systems the zn isotope compositions of seventeen water samples from eight thermal springs and six gas samples from two fumaroles from la soufriere an active volcano on guadeloupe island french west indies fwi were analyzed using a method adapted for purifying zn from fe and so4 enriched thermal solutions the fumaroles are enriched in zn 100 to 8000 times compared to the local bedrock and have isotopic compositions 66zn values from 0 21 to 0 35 similar to or slightly higher than fresh andesite 0 21 the enrichment of zn in the thermal springs compared with the surface waters shows that zn behaves as a soluble element during hydrothermal alteration but is significantly less mobile than na the 66zn values of most of the spring waters are relatively constant approximately 0 70 indicating that the thermal springs from la soufriere are enriched in heavy isotopes i e 66zn compared to the host rocks from 0 14 to 0 42 only three thermal springs have lower 66zn values as low as 0 43 while the zn in the fumaroles is essentially derived from magma degassing which is consistent with a previous study on merapi volcano toutain et al 2008 we show that the zn in the thermal springs is mainly derived from water rock interactions the 66zn enriched isotopic signature in most of the spring waters can be explained qualitatively by the precipitation at depth of sulfide minerals that preferentially incorporate the light isotopes this agrees with the isotopic fractionation that was recently calculated for aqueous complexes of zn the few thermal springs with lower 66zn values also have low zn concentrations indicating the preferential scavenging of heavy zn isotopes in the hydrothermal conduits this study shows that unlike chemical weathering under surface conditions hydrothermal alteration at high temperatures significantly fractionates zn isotopes and enriches thermal waters in heavy zn isotopes e g 66zn continental hydrothermal systems therefore constitute a source of heavy zn isotopes to the oceans this should be taken into account in the global oceanic budget of zn

how to resolve the controls on mesothermal vein systems in a goldfield characterized by sparse kinematic information and fault reactivation a structural and graphical approach current mine exposures of auriferous veins in the charters towers goldfield ctgf are characterized by a scarcity of kinematic indicators furthermore the dip direction of exposed and historically mined orebodies largely coincides with the orientation of pre existing discontinuities consequently the reactivation of earlier structures during gold deposition is strongly indicated both factors complicate the structural analysis of the auriferous vein systems as slip on earlier faults likely occurred before new rock failure and evolution of fault networks reflecting the stress field present at the time of gold deposition despite access to drillcore open pits and underground workings extracted data were limited to ten occurrences of wear groove striations and three reverse separations of dykes and a pegmatite cut by sulphide bearing quartz veins because of the scarcity of kinematic information the structural analysis focused on the geometry of veins and ore shoots also applying basic graphical and kinematic methods

rapid regolith formation over volcanic bedrock and implications for landscape evolution abstract the ability to quantify how fast weathering profiles develop is crucial to assessing soil resource depletion and quantifying how landscapes evolve over millennia uranium series isotopes can be used to determine the age of the weathering front throughout a profile and to infer estimates of regolith production rates because the abundance of u series isotopes in a weathering profile is a function of chemical weathering and time this technique is applied to a weathering profile in puerto rico developed over a volcaniclastic bedrock u series isotope compositions are modelled revealing that it takes 40 60 kyr to develop an 18 m thick profile this is used to estimate an average regolith production rate of 334 46 mm kyr this value is higher by a factor of up to 30 when compared to production rates estimated for weathering profiles developed over granitic or shale lithologies this quantitatively underpins the lithological control on rates of regolith production in a neighbouring watershed but over a granitic bedrock production rates are only 30 40 mm kyr moreover by comparing these results to a compilation of soil erosion rates it is clear that landscapes are controlled by the balance or imbalance between regolith production and erosion soil mantled landscapes are the result of a relative balance between production and erosion whereas in cratonic areas thicker weathering profiles are generated because erosion fails to match regolith production rates

electrical tomography of la soufriere of guadeloupe volcano field experiments 1d inversion and qualitative interpretation the soufriere of guadeloupe lava dome consists of several 3d domains of relatively unaltered massive andesite separated by major radial fractures that reach at least up to about half of the dome s height more than 20 000 geo electrical measurements made on the top and the flanks of the lava dome are used to construct the first geophysical image of the internal structure of this active volcano the main features of the apparent conductivity structure of the lava dome are high conductivity regions associated with the strongly altered material of present and fossil hydrothermal zones local 1d inversions of data subsets belonging to regions where the geological structure is supposed laterally invariant confirm the structures observed in the pseudo sections the relatively stable units lie above a basal inclined layer of highly conductive geological materials interpreted to be more altered and thus characterised by reduced internal friction and probably increased pore pressure past and current hydrothermal fluid circulation through these fractures has promoted rock alteration and particularly along the base of the dome that likely contributes to the instability of parts of the edifice

regolith evolution on the millennial timescale from combined u th ra isotopes and in situ cosmogenic 10be analysis in a weathering profile strengbach catchment france abstract u th ra disequilibria cosmogenic in situ 10be concentrations and major and trace element concentrations have been analyzed in a 2 m deep weathering profile sampled at the summit of the granitic strengbach catchment france the data have been used to independently estimate both the long term regolith production and denudation rates and the weathering and erosion rates modeling of the 238u 234u 230th 226ra disequilibrium variations in the lower part of the profile yields a regolith production rate of 12 4 mm kyr 30 10 t km2 yr while modeling of the high resolution 10be concentration profile leads to an exposure age of 19 7 2 2 kyr an inherited concentration of 15 000 1 000 at g in quartz and a mean denudation rate of 22 10 mm kyr 37 15 t km2 yr the consistency between production and denudation rates suggests that on a millennial timescale the regolith mass balance at the summit of the catchment is close to a steady state even if the watershed may have been impacted by quaternary climatic changes and by recent anthropogenic perturbations e g 20th century acid rain and recent afforestation efforts the results also indicate that physical erosion is likely the dominant long term process of regolith denudation in the catchment furthermore the comparison of the long term production and denudation rates and of weathering and erosion rates determined from the depth profile analyses with the current weathering and erosion rates estimated at the outlet of the watershed based on monitoring of the water chemistry and sediment fluxes suggests that physical erosion may have varied more than the chemical weathering flux during the last 150 kyr although very few other sites with u series in situ 10be and stream monitoring data are available for comparison the current data suggest that 1 the mass balance steady state of regolith might be commonly achieved in soil mantled landscapes and 2 physical erosion has varied much more than chemical weathering in mid mountain catchments over the last 10 150 kyr these results highlight the importance of the combined analysis of u series nuclides and in situ 10be in the same weathering profile for the determination of key geomorphic parameters which are important to constraining landscape stability and the responses of landscapes to natural or anthropogenic forcing

helium 3 emission related to volcanic activity the helium 3 helium 4 ratio in bubbling gases from ten hot springs located around mount ontake an active volcano in central japan ranges from 1 71 r sub atm 1 71 times the atmospheric ratio of 1 40 x 10 sup 6 to 6 15 r sub atm the value of the ratio decreases with distance from the central cone of the volcano such a tendency may be a characteristic of helium 3 emission in volcanic areas and suggests more primitive helium 3 is carried with fluid flowing through a conduit during volcanic activity 6 references 1 figure 1 table

linking geomorphology weathering and cation availability in the luquillo mountains of puerto rico abstract we explored the links between geomorphology weathering and cation availability across the most common forest types and geologic provinces in the el yunque national forest puerto rico our goal was to connect the controls on primary mineral supply to soil with stocks of plant available cations we established a network of 216 soil profiles comprising toposequences ridges slopes and valleys in three forest types colorado palm and tabonuco on two parent rock types quartz diorite and volcaniclastic in a factorial design the landscape underlain by quartz diorite is divided into lower slopes that are eroding rapidly in response to renewed uplift in the last 5 ma and those above a regional knickpoint where remnant landforms are eroding more slowly quartz diorite derived soils 0 80 cm below the knickpoint have lost significantly less calcium ca magnesium mg and potassium k relative to parent material 80 65 and 57 respectively than those above it which have lost 95 81 and 83 p 90 of all base cations relative to parent material while geology and geomorphology exert the most control on the degree of soil weathering as measured by total soil cation loss relative to parent material exchangeable cations are influenced by geologic biologic and meteoric factors tabonuco forests have significantly higher exchangeable cation stocks 0 80 cm than colorado forests even in the 500 700 m elevation zone where the two forest types are not correlated with elevation p 0 02 0 005 and 0 002 for ca mg and k respectively forest type explains 30 of the variance in exchangeable mg and k stocks but only 10 of the variance in exchangeable ca quartz diorite derived soils below the knickpoint have 2 more exchangeable ca than those above it and within that rapidly eroding region sites closest in elevation to the knickpoint have 2 more again the isotopic composition of bulk soil and exchangeable strontium suggests that the high ca stocks below and near the knickpoint are mostly derived from primary mineral weathering whereas above the knickpoint exchangeable ca is supplied by atmospheric deposition these data demonstrate that regional uplift lithology weathering atmospheric inputs and forest communities combine to shape cation availability across the luquillo landscape and we hypothesize that similar interactions are important in other montane tropical forests

noble gas isotopes in hydrothermal volcanic fluids of la soufriere volcano guadeloupe lesser antilles arc article the ascending magmatic component in la soufriere volcano guadeloupe lesser antilles was investigated by measuring noble gas concentrations and isotopic ratios in thermal springs and fumaroles a clear magmat ic helium signal 4 he 3 he 89 000 r ra 8 equivalent to the morb value is recorded in most fluids 3 he fluxes in both fumarole and springs have been estimated and related to 3 he content in the magma chamber in order to explain the 3 he flux measured at the surface we conclude that the magma chamber must be reg ularly fed by fresh magma batches using our new results and data from literature we propose that the his torical activity of la soufriere volcano can be explained by both abnormal energy inputs from new batches of magma in the chamber and cycles of clogging opening of the hydrothermal system we propose a new sce nario for the origin of the 1976 1977 crisis whereby a fresh batch of magma could have been emplaced prior to 1968 possibly between 1959 and 1962 in the magma chamber the resulting heat flux is not stored in the different aquifers but preferentially evacuated through fractures reactivated or created during the 1956 phre atic eruption only when the self sealing of the hydrothermal system is sufficiently developed can pressure and temperature within the aquifers rapidly increase to trigger a crisis

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

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

new boron isotopic evidence for sedimentary and magmatic fluid influence in the shallow hydrothermal vent system of milos island aegean sea greece abstract magmatic sources may contribute a significant amount of volatiles in geothermal springs however their role is poorly understood in submarine hydrothermal systems worldwide in this study new results of b and 11 b in 41 hydrothermal vent waters collected from the shallow hydrothermal system of milos island in the aegean sea were combined with previously published data from other tectonic settings and laboratory experiments to quantify the effects of phase separation fluid sediment interaction and magmatic contribution two cl extreme solutions were identified high cl waters cl as high as 2000 mm and low cl waters cl 3 mol mol and extremely low 11 b 1 4 6 3 except for the waters with mg content of near the seawater value and 11 b 10 3 17 4 these high cl waters with high b cl and low 11 b plot close to the vent waters in sediment hosted hydrothermal system i e okinawa trough or fumarole condensates from on land volcanoes implying b addition from sediment or magmatic fluids plays an important role this is in agreement with fluid sediment interactions resulting in the observed b and 11 b as well as previously reported br i cl ratios supporting a scenario of slab derived fluid addition with elevated b 11 b rich and low br cl and i cl which is derived from the dehydration of subducted sediments the slab fluid becomes subsequently mixed with the parent magma of milos the deep brine reservoir is partially affected by injections of magmatic fluid gases during degassing the results presented here are crucial for deciphering the evolution of the brine reservoirs involved in phase separation fluid sediment interaction and magmatic contribution in the deep reaction zone of the milos hydrothermal system they also have implications in the understanding of the formation of metallic vein mineralization

he ne ar and c isotope systematics of geothermal emanations in the lesser antilles islands arc the authors present he ne ar and c isotope analyses of hydrothermal brines and gases from fumaroles hot springs mofettes and hydrothermal exploration drillings on the major islands of the lesser antilles arc the origin of hydrothermal brines which have been analyzed also for o and h isotopes is essentially meteoric hydrothermal air corrected isotope compositions of helium 2 2 rc ra sup 3 he sup 4 he 8 6 rc ra and carbon minus 20 delta sup 13 c sub pdb 0 5 are variable and require a variety of crustal and magmatic sources the diversity of delta sup 13 c sub pdb and sup 3 he co sub 2 ratios within individual volcanic centers suggests that crustal sources e g limestone contaminate magmatic co sub 2 en route from high level magma reservoirs depth 15 km to the surface a similar contamination may be found for magmatic helium on distal springs the sup 3 he sup 4 he signature of summit fumaroles thought to reflect the sup 3 he sup 4 he signature of high level magmas shows a remarkable systematic variation along the arc in addition there is a correlation throughout the arc between published sr pb and nd isotope signatures of lavas and the sup 3 he sup 4 he signatures of summit fumaroles on the northern islands nevis montserrat guadeloupe and dominica summitmore fumaroles have the n morb signature sup 3 he sup 4 he 8 1 r ra and the isotope signature of lavas is not dissimilar from comparable intra oceanic arc tholeiites elsewhere variable enrichments in radiogenic sr and pb have been reported for lavas of individual volcanic centers of the southern islands martinique st lucia and grenada and summit fumaroles on these centers match these variations by variable radiogenic he enrichments i e lower sup 3 he sup 4 he ratios this correlation suggests that radiogenic sr and pb enrichments of lavas and low sup 3 he sup 4 he signatures on summit fumaroles have a common origin i e a terrigenous contaminant derived from the orinoco depositionary fan crustal assimilation is thought to decouple the he isotope system from any other radiogenic isotope system and therefore the authors argue that the observed correlation of he sr pb and nd isotope systems is related to a terrigenous contaminant derived from subducted sediments support for this scenario also comes from the matching of low sup 3 he sup 4 he ratios and tectonic features of the forearc thought to favor the subduction of forearc sediments the present study offers a first clue that under suitable conditions crustal helium from oceanic sediments might be subducted to the depth of arc magma sources and possibly even recycled into the deeper mantle less

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

paleozoic terranes of eastern australia and the drift history of gondwana abstract critical assessment of paleozoic paleomagnetic results from australia shows that paleopoles from locations on the main craton and in the various terranes of the tasman fold belt of eastern australia follow the same path since 400 ma for the lachlan and thomson superterranes but not until 250 ma or younger for the new england superterrane most of the paleopoles from the tasman fold belt are derived from the lolworth ravenswood terrane of the thomson superterrane and the molong monaro terrane of the lachlan superterrane consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic australia by the late early devonian the lolworth ravenswood terrane is interpreted to have undergone a 90 clockwise rotation between 425 and 380 ma although the tamworth terrane of the western new england superterrane is thought to have amalgamated with the lachlan superterrane by the late carboniferous geological syntheses suggest that movements between these regions may have persisted until the middle triassic this view is supported by the available paleomagnetic data with these constraints an apparent polar wander path for gondwana during the paleozoic has been constructed after review of the gondwana paleomagnetic data the drift history of gondwana with respect to laurentia and baltica during the paleozoic is shown in a series of paleogeographic maps

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

mobility of u series nuclides during basalt weathering an example from the deccan traps india abstract u series disequilibria have been measured by tims and mc icp ms in dissolved phases and suspended sediments of the main basaltic rivers from the deccan trap region india for dissolved phases 234u 238u 230th 238u and 226ra 238u range between 1 11 and 1 28 between 0 03 10 2 and 1 35 10 2 and between 0 02 and 0 16 respectively for suspended particles 234u 238u 230th 238u and 226ra 238u range from 0 95 to 1 1 from 1 23 to 1 59 and from 0 1 to 0 47 respectively 230th 238u and 234u 238u systems show direct links with indices of weathering the data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady state because present day physical erosion rates are higher than expected 100 400 mm ka versus 40 mm ka for a steady state process for the narmada and the tapti basins models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40 90 ka in contrast with basins located in northern latitudes erosion in the deccan basins does not appear to have been significantly disturbed during the last glaciation 234u 238u disequilibria in the dissolved phase of large rivers world wide are also explored and two key factors are highlighted in this study the dissolution of carbonates releasing dissolved u at secular equilibrium and the silicate weathering rate estimated at the scale of the watershed in particular a positive correlation is found between dissolved 234u 238u and basaltic weathering rates for the deccan that could reflect a key role of physical erosion since for silicate monolithological basins chemical erosion rates are primarily controlled by physical erosion rates for basins with mixed lithology dissolved organic matter also plays a key role in the preferential release of 234u from silicate minerals 234u 238u ratios in rivers therefore reflect a mixture between u released by carbonate dissolution and 234u preferentially released from silicate minerals located in high physical erosion and peat areas

short lived u and th isotope distribution in a tropical laterite derived from granite pitinga river basin amazonia brazil application to assessment of weathering rate we have analyzed samples of a 15 m thick profile weathered from the madeira granite located in the pitinga basin river north of manaus in the state of amazonia brazil this profile consists essentially of a yellow red saprolite covered by a soil u and th concentrations are particularly high in the granite 20 and 80 g g respectively normalized element to th concentrations indicate that th is most resistant to chemical weathering except to some extent in the top soil higher concentrations in the saprolite compared to the granite comprise a relative enrichment resulting from a loss of mass the saprolites are initially generated by a descending weathering front which alters the granite to a yellow red saprolite a second front close to the top turns the saprolite into a soil weathering has led to leaching of u the 234u 238u and 230th 238u isotopic ratios are in radioactive disequilibrium numerous nodules are present and apparently started to form at the base of the saprolite these nodules achieve more developed form during their relative ascent until they are reached by the descending top front where they undergo dissolution the th and pb are concentrated in the nodules close to the top front the u being more mobile is strongly leached by the first front and most of the remainder freed by the second engages in a descending flux which supplies the underlying saprolite using the data an attempt is made to model the isotopic distribution in the profile we conclude that the first front has descended at a rate of 5 cm 1000 yt and that the time needed to create the saprolite must have been around 300 000 yr

the effects of lithology on trace element and ree behavior during tropical weathering abstract the thick regolith developed in the humid tropics represents an endmember of critical zone evolution where shallow and deep biogeochemical cycles can be decoupled in terms of the predominant source of trace elements atmospheric input at the surface weathering at depth and of the processes that control their cycling to investigate the influence of lithology on trace element behavior and in this potential decoupling we studied two deep 9 3 and 7 5 m highly leached ridgetop regolith profiles at the luquillo critical zone observatory puerto rico these profiles have comparable internal degree of weathering topography and external vegetation climate characteristics but differ in their underlying bedrock andesitic volcaniclastic and granitic at these two sites we analyzed a large suite of trace elements and used the rare earth elements and yttrium rey as tracers of critical zone processes because they are fractionated by the chemical reactions involved in weathering and pedogenesis e g sorption dissolution colloidal transport and by redox fluctuations we found that both regolith profiles show atmospheric inputs of trace elements at the surface and evidence of bedrock dissolution at depth as expected we also found noticeable differences in the re distribution of trace elements and rey within the profiles indicative of different geochemical environments with depth and lithology in the volcaniclastic profile trace element and rey behavior is controlled mainly by redox mediated sorption desorption reactions whereas ph controlled dissolution precipitation and sorption reactions predominate in the granitic profile the most noticeable difference between the two regolith profiles is in the long term redox conditions inferred from redox sensitive elements and ce anomaly variations which are more variable and stratified in the volcaniclastic profile and change gradually with depth in the granitic profile the contrasting redox conditions and the different sources of elements dust vs bedrock produce a decoupling between the surface and deep geochemical environments of the volcaniclastic regolith the difference in redox conditions between the two lithologies likely stems from the finer grain size and higher clay content of the volcaniclastic regolith

a review of the metallogeny and tectonics of the lachlan orogen future mineral exploration within eastern australia will be enhanced by resolving the tectonic evolution of the lachlan orogen to establish the spatial and temporal terrane distribution of the various mineral deposits the lachlan orogen from north eastern tasmania through to central and eastern new south wales is host to a number of major mineral deposit styles including orogenic gold e g stawell ballarat bendigo volcanic hosted massive sulphide e g woodlawn currawong sediment hosted cu au e g cobar basin deposits porphyry au cu e g cadia parkes cowal and granite related sn e g ardlethan beechworth each of these mineral deposit styles is a sensitive and diagnostic indicator of the prevalent tectonic environment during their formation in this review we briefly summarise the deposit to large scale factors that define the diverse metallogenic evolution of the lachlan orogen this overview is intended to set the scene for subsequent specialist papers published in this thematic issue on the metallogeny and tectonics of the lachlan orogen in south east australia

reply to comment on electrical tomography of la soufriere of guadeloupe volcano field experiments 1d inversion and qualitative interpretation by n linde and a revil we thank linde and revil for their comment 1 hereafter referred to as l r which gives us the opportunity to provide more details about our methodology and discuss our approach for interpreting electrical resistivity data acquired over complex geological structures we observe that l r mainly argue on a speculative basis and suggest that traditional smoothness constrained 2d inversion could represent a better option in the present reply we bring more precise arguments and address the main

u th ra variations in himalayan river sediments gandak river india weathering fractionation and or grain size sorting abstract understanding the origin of u th ra variations in the ganga river sediments is a prerequisite for correctly using u series nuclides to constrain the sediment transport times in himalayan rivers for this purpose u th and ra concentrations along with 238 u 234 u 230 th 226 ra radioactive disequilibria were analyzed in bank bedload and suspended sediments from the gandak river one of the main tributaries of the ganga river the data confirm that u and th budgets of the himalayan sediments are significantly influenced by minor resistant minerals such as zircon garnet and ti bearing minerals the dissolution of which required the use of a high pressure acid digestion process most importantly the results indicate that the variations in 238 u 232 th and 230 th 232 th activity ratios and 238 u 234 u 230 th 226 ra disequilibria in sediments along the river alluvial plain mainly reflect modifications in the mineralogical and grain size compositions rather than the degree of weathering during transport the 238 u 232 th and 230 th 232 th activity ratios in the bank and bed sediments are related to variations in the minor primary minerals strongly enriched in u and th i e zircon ree bearing minerals and ti bearing minerals whereas the activity ratios in the suspended load are related to variations in the proportions of clay fe oxyhydroxides and the silt sand fraction which contains u and th bearing minor minerals the data also indicate that 238 u 234 u 230 th 226 ra disequilibria are strongly influenced by secondary mineral phases the 230 th budget is likely mainly controlled by fe oxyhydroxides and the 226 ra budget is likely mainly controlled by clay minerals therefore the variations in the 238 u 234 u 230 th 232 th system in the sediments of the gandak river cannot simply be interpreted as the result of fractionation due to chemical transformation of the bulk sediment during its transport within the alluvial plain and or the result of radioactive decay consequently they cannot be used to infer long sediment transport times within the gandak plain 10 100 ka as previously proposed such analytical and interpretative artifacts are certainly not specific to the present study on the gandak basin these issues will certainly be encountered anytime this technique is applied to alluvial systems in which the u and th budgets of the sediments are influenced by heavy minerals that can be sorted during the transport of sediments within the plain

impact of hydrologically driven hillslope erosion and landslide occurrence on soil organic carbon dynamics in tropical watersheds the dynamics of soil organic carbon soc in tropical forests play an important role in the global carbon c cycle past attempts to quantify the net c exchange with the atmosphere in regional and global budgets do not systematically account for dynamic feedbacks among linked hydrological geomorphological and biogeochemical processes which control the fate of soc here we quantify effects of geomorphic perturbations on soc oxidation and accumulation in two adjacent wet tropical forest watersheds underlain by contrasting lithology volcaniclastic rock and quartz diorite in the luquillo critical zone observatory this study uses the spatially explicit and physically based model of soc dynamics tribs eco triangulated irregular network based real time integrated basin simulator erosion and carbon oxidation and measurements of soc profiles and oxidation rates our results suggest that hillslope erosion at the two watersheds may drive c sequestration or co2 release to the atmosphere depending on the forest type and land use the net erosion induced c exchange with the atmosphere was controlled by the spatial distribution of forest types the two watersheds were characterized by significant erosion and dynamic replacement of upland soc stocks results suggest that the landscape underlain by volcaniclastic rock has reached a state close to geomorphic equilibrium and the landscape underlain by quartz diorite is characterized by greater rates of denudation these findings highlight the importance of the spatially explicit and physical representation of c erosion driven by local variation in lithological and geomorphological characteristics and in forest cover this article is protected by copyright all rights reserved

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

deep seismic images and the tectonic framework of early rifting in the otway basin australian southern margin abstract the otway basin is one of a number of basins formed along australia s southern continental margin during the late jurassic early cretaceous rifting of australia from antarctica it lies on a transitional area of extended lithosphere between the margin to the west where continental separation took place close to the australian craton and the region to the east where separation failed to develop through bass strait but instead switched to the south of tasmania seven agso regional deep seismic profiles recorded during 1992 across onshore parts of the basin provide images of synrift basin bounding faults that dip predominantly towards the continent basin sequences in half graben rift segments at two way times greater than 4 s and mid crustal detachments with ramp and flat geometry at mid crustal levels when combined with industry seismic data trend variations on faults indicate that extensional strain direction varied along the rift system probably locally controlled by pre existing palaeozoic geology but consistent with general north south lithospheric extension basement highs between some of the rift segments are interpreted as accommodation or transfer zones multiple basin bounding faults in some places mirror features within basement and may indicate a component of local strike slip during early rifting the syn rift segments form part of the first stage early cretaceous failed rift system in the otway bass strait region crustal thickness is interpreted to be 31 km 10 3 10 5 s twt near the northern basin margin thinning to about 25 km 9 s twt 40 reduction in thickness of palaeozoic crust seaward of the tartwaup fault zone this fault zone and the timboon fault system farther east are interpreted to be headwall faults landward limit of a late cretaceous second stage rifting system along a lower plate margin which ultimately separated australia from antarctica

climatic and vegetation control on sediment dynamics during the last glacial cycle as climate is changing rapidly there is an increasing need to understand how water and soil resources respond to climate change soil and sediment dynamics are sensitive to several external factors such as climate vegetation type and distribution human activity and tectonic activity however the relationship between erosion and changes in these factors is difficult to constrain with current available approaches here we show that uranium isotopes in sediments from river paleochannels can be used to reconstruct variations in the residence time of sediments in a catchment over the past 100 k y we find that sediment residence times increase by an order of magnitude during interglacials compared to glacial periods this is interpreted as a change in sediment stores in the landscape that are tapped by catchment erosion young upland soils during glacial periods reworking of old alluvial sediments during interglacials a direct correlation is found between the sediment residence time and climatic parameters sea surface temperature atmospheric carbon dioxide content and paleorainfall estimates suggesting that during a glacial cycle sediment dynamics closely follow variations in climate however this relationship is not simple because there is no correlation between sediment residence time and paleodischarge estimates because sediment residence time variations correlate with changes in vegetation inferred from pollen data it is hypothesized that the influence of climate on erosion over a glacial cycle may be indirect and operates via the influence of climate on the type of plant ecosystems within a catchment if verified elsewhere this conclusion would emphasize the important role of biology in the physical evolution of earth s surface here observed over a 100 k y time scale

a model for episodic degassing of an andesitic magma intrusion 1 episodic magmatic degassing has been observed at numerous volcanoes especially those of intermediate composition it can span timescales from years to decades here we propose a physical model for the degassing of a shallow magma intrusion to explain this phenomenon the magma cools by convection which leads to melt crystallization volatile exsolution and magma overpressure when the pressure reaches a critical value wall rocks fracture and the exsolved gas escapes the intrusion then returns to the initial lithostatic pressure and a new cooling crystallization degassing cycle occurs a series of such cycles leads to episodic degassing the trend and timescale of the degassing process are mainly governed by magma cooling two degassing regimes are exhibited an early phase with a high frequency of gas pulses and a later phase with a lower gas pulse frequency the transition between these two regimes is caused by the viscosity increase when the magma crystallinity exceeds the crystal percolation threshold we find that the time to this transition is dependent on magma volume to a first approximation where observations are available from sustained geochemical surveillance the model provides constraints on key aspects of the subsurface magmatic system with estimation of the volume of an intrusion and tensile strength of the surrounding rocks it therefore represents a relevant tool for volcanic surveillance and hazard assessment

remote and in situ plume measurements of acid gas release from la soufriere volcano guadeloupe this paper presents the first remote measurements of la soufriere gas emissions since the fumarolic and seismic reactivation in 1992 the chemical composition of the plumes has been measured from may 2003 to september 2004 using an open path fourier transform infrared op ftir spectrometer up to 15 m downwind the south crater hcl is clearly detected concentration between 2 4 and 12 ppmv whereas so2 and h2s generally remain below the detection limit of the op ftir direct measurements of so2 and h2s near the south crater with a lancom iii analyzer show a fast decrease of their concentrations with the distance calculated cl s mass ratios are high from 9 4 1 7 at 15 m from the vent to 2 8 0 6 at 140 m the enrichment in hcl of the gas emitted at la soufriere observed since 1998 corresponds to the degassing of a magma enriched in cl and depleted in s this result agrees with isotopic measurements which suggest a magmatic origin of the gases readjustments inside the volcanic system may have taken place during the seismic activity beginning in 1992 and enhance the transfer of magmatic gases to the summit

the volcanic activity of la soufriere of guadeloupe lesser antilles structural and tectonic implications abstract among observations made on la soufriere volcano guadeloupe to monitor its its activity are temperature measurements temperature of fumaroles hot springs ground surface and water inside two boreholes 80 and 90 m deep those measurements proved to be of prime importance in understanding mechanisms working in the volcano and governing its activity they lead with other observations such as self potential measurements and geochemical analysis of fumaroles and hot springs to a possible interpretation of phreatic activity rather different from the one retained earlier which was essentially based on seismic data in this model the magma reservoir is only in a stable state and in particular supplies heat at a constant rate to the overlying geological structures the cycle of volcanic activity and period of quiescence is interpreted as due to a cycle of clogging and reopening of the system of fractures transferring heat from a lower aquifer to an upper one and eventually to the atmosphere the model is discussed in regard of other geophysical observations in particular seismicity some inferences are made concerning a new phase of activity and suitable observations to predicit it

u series isotopes and destructive plate margin magma genesis in the lesser antilles high precision tims230th232th isotope ratios are presented from a suite of lavas which span the length of the lesser antilles island arc compositions vary from low k tholeiitic lavas with low87sr86sr in the north to high k calc alkaline to alkalic lavas with higher87sr86sr in the south the northern lavas are characterised by 230th238u 1 a three component model is required for magma genesis in the lesser antilles sediments from the subducting slab melt and variably enrich the mantle wedge while the subducting oceanic crust dehydrates releasing fluids enriched in u ba sr and k but with negligible th and low87sr86sr which promote partial melting in the wedge ta zr increases with87sr86sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment mass balance calculations are consistent with a relatively constant fluid flux except for u and little variation in the degree of partial melting 10 15 along the arc and they suggest that the sediment contribution to the arc lavas increases from 2 in the north to 15 in the south in the northern parts of the arc the addition of u by these fluids overprints any increases inth u that might have been caused by partial melting and results in lavas with 230th238u 1 m yr 1 which imply channelled flow through the mantle wedge

chlorine isotopes of thermal springs in arc volcanoes for tracing shallow magmatic activity the evaluation of the status of shallow magma body i e from the final intrusion stage to quiescence and back to activity one of the key parameters that trigger and sustain volcanic eruptions has been challenging in modern volcanology among volatile tracers chlorine cl uniquely exsolves at shallow depths and is highly hydrophilic consequently cl enrichment in volcanic gases and thermal springs has been proposed as a sign for shallow magmatic activities however such enrichment could also result from numerous other processes e g water evaporation dissolution of old chloride mineral deposits seawater contamination that are unrelated to magmatic activity here based on stable isotope compositions of chloride and dissolved inorganic carbon as well as previous published 3he 4he data obtained in thermal springs from two recently erupted volcanoes la soufriere in guadeloupe and montagne pelee in martinique in the lesser antilles arc we show that the magmatic cl efficiently trapped in thermal springs displays negative 37cl values 0 65 0 65 consistent with a slab derived origin but distinct from the isotope compositions of chloride in surface reservoirs e g seawater local meteoric waters rivers and cold springs displaying common 37cl values of around 0 using this 37cl difference as an index of magmatic cl we further examined thermal spring samples including a 30 year archive from two thermal springs in guadeloupe covering samples from its last eruption in 1976 1977 to 2008 and an island wide sampling event in martinique in 2008 to trace the evolution of magmatic cl in the volcanic hydrothermal systems over time the results show that magmatic cl can be rapidly flushed out of the hydrothermal systems within 30 to 80 years after the eruption much quicker than other volatile tracers such as co2 and noble gases which can exsolve at greater depths and constantly migrate to the surface because arc volcanoes often have well developed hydrothermal systems where magmatic cl is easily transferred to the surface following its exsolution from shallow magma body we suggest that 37cl has great potential to be a unique proxy to monitor the cessation and revival of infrequent arc volcanoes particularly at centennial time scales

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

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

assessing the effect of sequential extraction on the uranium series isotopic composition of a basaltic weathering profile abstract soil sustainability implies maintaining the balance between soil erosion and production while it is known how to assess soil erosion only recently we have been able to estimate rates of soil and saprolite namely regolith production using uranium series isotopes this method assesses the time elapsed since rock forming minerals start fractionating the u series isotopes in this study we assess a sample pre treatment protocol that has the potential to improve the method used to estimate regolith production rates we propose that removal or partial removal of secondary phases precipitated from solution during pedogenesis solution derived phases and organic matter from regolith may improve the accuracy of this method this is tested using sequential extraction followed by etching as sample pre treatment here we assess their effect on the u series isotopic composition of regolith and infer whether they minimize the presence of solution derived phases and organic matter we applied sequential extraction and etching to a basaltic weathering profile bedrock saprolite and soil and compared the u series isotopic composition before and after treatment we also measured major elements concentrations and assessed mineralogy the bedrock was in secular equilibrium and sequential extraction resulted in unchanged 234u 238u activity ratios while increased 230th 238u in contrast etching resulted in increased 234u 238u and 230th 238u activity ratios which is attributed to the removal of primary minerals relative to the untreated bedrock the untreated saprolite showed no changes in u and th concentrations and activity ratios we hypothesise that during the conversion of bedrock into saprolite u and th budgets are unaffected moreover major element and mineralogical analyses suggest that during this process rock forming minerals are converted into secondary phases clays we hypothesise that during this conversion the u series isotopes are not fractionated therefore the removal of these secondary phases is not necessary relative to the saprolite the soil showed gains of u and th 234u 238u 1 and 230th 238u 1 to minimize the presence of solution derived phases and organic matter in basaltic weathering profiles we suggest that only soil samples should undergo sequential extraction because only these are significantly affected by solution derived phases and organic matter additionally our experiments show the existence of fractionation processes that are often overlooked in u series isotopic studies i e implantation of 234u and 230th recoiled from u rich mineral i e glass into adjacent u poor phases e g pyroxene and feldspar

mercury gas emissions from la soufriere volcano guadeloupe island lesser antilles abstract quantifying mercury hg emissions from active volcanoes is of particular interest for better constraining the global cycle and environmental impact of this highly toxic element here we report on the abundance of total gaseous tgm hg 0 g hg ii g and particulate hg p mercury in the summit gas emissions of la soufriere andesitic volcano guadeloupe island lesser antilles where enhanced degassing of mixed hydrothermal magmatic volatiles has been occurring since 1992 from the southern summit crater we demonstrate that hg in volcanic plume occurs predominantly as gaseous mercury with a mean tgm hg p mass ratio of 63 combining the mean tgm h 2 s mass ratio of the volcanic plume 3 2 10 6 measured close to the source vent with the h 2 s plume flux 0 7 t d 1 determined simultaneously allows us to estimate a gaseous mercury emission rate of 0 8 kg yr 1 from la soufriere summit dome somewhat lower tgm s tot mass ratio in fumarolic gases from the source vent 4 4 10 7 suggests that plume chemical composition is not well represented by the emission source fumaroles due to chemical processes prior to or upon discharge current mercury emission from la soufriere volcano represents a very small contribution to the estimated global volcanic budget for this element

inert gas abundances in basalts of the lesser antilles island arcs possible implications for their volcanic evolution abstract measurements of noble gas element abundances in igneous rock samples from the lesser antilles island arc laia show an enrichment of ne kr and xe relative to ar when compared to the atmospheric inert gas composition the element concentrations vary as a function of time and a cyclic variation appears to occur in the pre pliocene stage of arc development the pattern of elemental abundances closely resembles that of ocean floor basalts suggesting that 1 the origin of the inert gases in these samples is related to the subduction of oceanic lithosphere responsible for the formation of the laia and 2 the subduction process does not modify the noble gas patterns of the materials being subducted there is no evidence of a change in concentration or composition of noble gases occurring at ca 9 ma the time when it has been suggested the laia shifted to form the western younger volcanic caribbees however a major hiatus in the noble gas evolution is recognised at ca 16 ma

denudation rates determined from the accumulation of in situ produced 10be in the luquillo experimental forest puerto rico we present a simple method for estimation of long term mean denudation rates using in situ produced cosmogenic 10be in fluvial sediments procedures are discussed to account for the effects of soil bioturbation mass wasting and attenuation of cosmic rays by biomass and by local topography our analyses of 10be in quartz from bedrock outcrops soils mass wasting sites and riverine sediment from the icacos river basin in the luquillo experimental forest puerto rico are used to characterize denudation for major landform elements in that basin the 10be concentration of a discharge weighted average of size classes of river sediment corresponds to a long term average denudation of 43 m ma 1 consistent with mass balance results

nutrient cycling in a tropical montane rainforest under a supply limited weathering regime traced by elemental mass balances and mg stable isotopes abstract forest ecosystem nutrition and rock weathering are intimately linked through the supply of mineral nutrients and recycling re utilisation of nutrients released from plant litter during mineralisation here we report on novel applications of mg stable isotopes in combination with mass balance calculations to quantify nutrient supply from bedrock atmospheric wet and dry deposition and recycling in a tropical ecosystem characterised by a supply limited weathering regime in the central highlands of sri lanka we measured mg stable isotopes radiogenic sr isotope ratios and element concentrations in the different compartments of the earth s critical zone bedrock saprolite soil soil pore water stream water and vegetation as tracers for element mass fluxes and pathways we find that in the clay rich saprolite mg isotopes are fractionated towards isotopically heavy mg during secondary mineral formation soil pore water and stream water are the complementary isotopically light reservoirs the mg isotope composition of above ground vegetation is on average identical to soil pore water based on these observations we suggest that a tight near surface mineral nutrient loop between plants and soil sustains the tropical forest ecosystem this loop is disconnected from the rock weathering nutrient source in the saprolite beneath soil external atmospheric wet and dry inputs mainly mg and ca strongly contribute to the plant available nutrient budget that the nutrient depleted regolith does not supply these findings based on mg stable isotopes agree with independently obtained information on bio element p k ca mg si recycling this nutrient supply setting contrasts with that reported for the southern sierra nevada usa where a kinetically limited weathering regime prevails our study illustrates the application of new flux estimators based on elemental and metal stable isotope measurements across a range of climate and weathering regimes

mobilization and re distribution of major and trace elements during extreme weathering of basalt in hainan island south china major trace and organic elements of a laterite profile developed on neogene basalts in northern hainan island south china were reported in this paper the aim of which was to investigate element mobilization and re distribution during extreme weathering the results indicate that most of the elements have been mobilized and transferred downwards along the profile by aqueous solution organic matter om can significantly improve the transport of insoluble elements among all the elements th is the least mobile as for the general conservative elements during incipient chemical weathering such as fe ti zr hf nb and ta the removals are up to 20 40 in the upper profile however these elements behave as conservatively as th in the lower profile in the middle profile oxic environment occurs accompanied with significant om decomposition the mn and ce transferred downward are readily oxidized into insoluble mn iv and ce iv and precipitate in the oxic front important om decomposition decreases the capacity of transfer of insoluble elements in aqueous solution consequently al significantly precipitates in the oxic front and rees with the exception of ce precipitate largely in the omdepleted layers co and u are also concentrated in the oxic front in association with mn and ce respectively however cr shows a negative correlation with mn because its response to redox condition changes is reversed from that of mn mn oxides hydroxides fe oxides hydroxides and secondary phosphate minerals other than clay minerals are potential hosts for rees except for ce in the profile rees with high concentrations in the profile seem closely associated with mn oxides hydroxides remarkable highly correlated ce and gd anomalies are observed in the profile ce anomalies are caused by ce precipitation in the oxic environment and successive decomposition of organic matter gd anomalies are likely to have resulted from lower stability constants of gd om complexes compared to those of neighboring rees the overall elemental behaviors in this profile suggest that organic matter plays a very important role in the mobilization and re distribution of the elements during extreme weathering

u series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains the case of the himalayan rivers 238u 234u 230th radioactive disequilibria were analyzed in suspended sediments collected at different depths from the ganges river and one of its main tributaries the narayani gandak river results associated with bedload sediment data suggest that uranium series u series disequilibria in river sediments of the ganges basin vary with grain size and sampling location the range of observed u series disequilibria is explained by a mixing model between a coarse grained sediment end member represented by bedload and bank sediments and a fine grained end member that both originate from himalaya but undergo different transfer histories within the plain the coarse grained sediment end member transits slowly i e several 100 s ky in the plain whereas the fine grained sediment end member is transferred much faster 20 25 ky as indicated by the absence of significant variations in th isotope composition of the fine grained sediment end members these results show that u series isotopes can be used to quantify the various transfer times of river sediments of different sizes and infer that there can be an order of magnitude of difference or more between the transfer time of suspended and bedload sediments this underlines that a good knowledge of the proportion of suspended vs bedload sediments transported in the river is required to accurately assess how fast erosion products are transferred in a catchment and how fast a catchment is likely to respond to external forcing factors

uranium series isotopes in colloids and suspended sediments timescale for sediment production and transport in the murray darling river system abstract we have measured 238u 234u 230th radioactive disequilibria in five size fractions of colloids and suspended sediments for four rivers from the murray darling river basin se australia continuous compositional trends of composition are observed for 234u 238u and 230th 238u activity ratios over the range of size fractions studied they are explained by the variable contribution of two components detrital alumino silicates and organic matter using relationships between activity ratios and the loss on ignition it is possible to estimate the u series isotopic composition of both detrital and organic end members which provide the best estimate for the composition of the true detrital phase i e controlled only by the removal of elements through weathering and the true dissolved phase i e colloid free respectively when performing a single filtration at 0 2 m it appears that size fractions and

steam and gas emission rate from la soufriere volcano guadeloupe lesser antilles implications for the magmatic supply during degassing unrest abstract since its last magmatic eruption in 1530 ad la soufriere andesitic volcano in guadeloupe has displayed intense hydrothermal activity and six phreatic eruptive crises here we report on the first direct quantification of gas plume emissions from its summit vents which gradually intensified during the past 20 years gas fluxes were determined in march 2006 and march 2012 by measuring the horizontal and vertical distributions of volcanic gas concentrations in the air diluted plume and scaling to the speed of plume transport fluxes in 2006 combine real time measurements of volcanic h2s concentrations and plume parameters with the composition of the hot 108 5 c fumarolic fluid at exit fluxes in 2012 result from multigas analysis of h2s h2o co2 so2 and h2 concentrations combined with thermal imaging of the plume geometry and dynamics measurements were not only focused on the most active south crater sc vent but also targeted tarissan crater and other reactivating vents we first demonstrate that all vents are fed by a common h2o rich 97 98 mol fluid end member emitted almost unmodified at sc but affected by secondary shallow alterations at other vents daily fluxes in 2012 averaged 200 tons of h2o 15 tons of co2 4 tons of h2s and 1 ton of hcl increased by a factor 3 compared to 2006 even though modest such fluxes make la soufriere the second most important volcanic gas emitter in the lesser antilles arc after soufriere hills of montserrat taking account of other hydrothermal manifestations hot springs and diffuse soil degassing the summit fumarolic activity is shown to contribute most of the bulk volatile and heat budget of the volcano the hydrothermal heat output 8 mw exceeds by orders of magnitude the contemporaneous seismic energy release isotopic evidences support that la soufriere hydrothermal emissions are sustained by a variable but continuous heat and gas supply from a magma reservoir confined at 6 7 km depth by using petro geochemical data for la soufriere magma s and their dissolved volatile content and assuming a magmatic derivation of sulfur we estimate that the volcanic gas fluxes measured in 2012 could result from the underground release of magmatic gas exsolved from 1400 m3 d 1 of basaltic melt feeding the system at depth we recommend that fumarolic gas flux at la soufriere becomes regularly measured in the future in order to carefully monitor the temporal evolution of that magmatic supply

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

effects of a tectonically triggered wave of incision on riverine exports and soil mineralogy in the luquillo mountains of puerto rico we document the long term response of a tropical mountain stream to tectonics and show how this response exerts a first order spatial control on the delivery of weatherable minerals to forest soils in its catchment these minerals in turn affect nutrient availability and biogeochemical cycles the luquillo mountains is a rainforest covered isolated massif on puerto rico island instead of displaying typical concave up long profiles of most mountain streams the rivers draining the southern flank of these mountains display a systematic downstream increase in gradient using concentrations of in situ produced cosmogenic 10be in quartz from river sediments we find that the downstream steepening is due to the propagation of an erosion wave along the rivers from the coastal plain upstream toward the headwaters decreased 10be concentration along the steepened reaches knickpoint faces or knickzones results from faster denudation of the valley sides around the steepened reaches the upper portions of the steepened reaches knickpoint lips cluster in elevation around the altitude of a flat lying dissected surface traceable around the luquillo mountains inspection of similar platforms over other parts of puerto rico and in the surrounding seas suggests that the dissected surface is an uplifted shore platform formed in the early pliocene 4 my ago upstream of the knickpoints rivers possess alluvial reaches still graded to this uplifted platform they represent relict profiles of the rivers that once drained an island that was surrounded by the now uplifted platform we conclude that the knickpoints initiated when the platform started to rise above the caribbean sea the knickpoints then started propagating upstream defining the front of a slow moving wave of erosion 10be derived catchment scale denudation rates measured above and below the knickpoints indicate a 30 210 increase in denudation associated to the passage of the knickpoints mineralogical analysis of the soils show that soils upstream of the knickpoints overlie deeply depleted saprolite and only contain recalcitrant minerals whereas soils downstream of the knickpoints experience an input of fresh weatherable minerals this influx considerably increases nutrient availability porder et al 2015 in a forest where atmospheric inputs are otherwise the main source of nutrients mcdowell et al 1990 zarin and johnson 1995 pett ridge 2009

resolving the integral connection between pedogenesis and landscape evolution abstract soil is central in the terrestrial ecosystem linking and providing feedback responses to the other components i e water atmosphere and vegetation however the role of soil in landscape evolution is usually not well acknowledged in modeling landscape evolution soil is only treated as a residue of weathering that is transported and redistributed along the hillslope weathering is considered as a process that produces clays and generates unconsolidated materials available for erosion while pedology has been debating the form of qualitative factorial models for 75 years models for soil water heat solute gas and chemical reactions in a profile have matured as soils are distributed continuously in three dimensions across landscapes the profile models need to consider lateral fluxes this review outlines the role of soil in landscape modeling first we review the role of soil in the current landscape evolution models we then review data and models on soil weathering rates and transport processes we discuss soil profile models that simulate soil formation processes and combined soil landscape evolution models finally we discuss how the models can be tested and validated in the real world and suggest how both soil scientists and landscape modelers can work together to address the grand challenges in modeling earth surface processes

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

lack of bedrock grain size influence on the soil production rate abstract our study deals with the part played by bedrock grain size on soil formation rates u and th series disequilibria were measured in two soil profiles developed from two different facies of the same bedrock i e fine and coarse grain size granites in the geomorphically flat landscape of the experimental breuil chenue forest site morvan france the u and th series disequilibria of soil layers and the inferred soil formation rate 1 2 mm ky 1 are nearly identical along the two profiles despite differences in bedrock grain size variable weathering states and a significant redistribution of u and th from the uppermost soil layers this indicates that the soil production rate is more affected by regional geomorphology than by the underlying bedrock texture such a production rate inferred from residual soil minerals integrated over the age of the soil is consistent with the flat and slowly eroding geomorphic landscape of the study site it also compares well to the rate inferred from dissolved solutes integrated over the shorter time scale of solute transport from granitic and basaltic watersheds under similar climates however it is significantly lower than the denudation or soil formation rates previously reported from either cosmogenic isotope or u series measurements from similar climates and lithologies our results highlight the particularly low soil production rates of flat terrains in temperate climates moreover they provide evidence that the reactions of mineral weathering actually take place in horizons deeper than 1 m while a chemical steady state of both concentrations and u series disequilibria is established in the upper most soil layers i e above 70 cm depth in such cases the use of soil surface horizons for determining weathering rates is precluded and illustrates the need to focus instead on the deepest soil horizons

carbon and helium isotopes in thermal springs of la soufriere volcano guadeloupe lesser antilles implications for volcanological monitoring abstract la soufriere guadeloupe lesser antilles is one of the most active and dangerous volcanoes of the lesser antilles arc the renewal of the volcanic activity at la soufriere requires replenishment of the volatile poor andesitic stored magma with more primitive volatile rich basaltic magma this event will trigger early release of large quantities of highly volatile species like co 2 and helium in the hydrothermal system to use these precursors first their baseline abundances and their behaviour in the hydrothermal system must be known during the current phase of dormancy in this study we have combined a previous systematic investigation of noble gases with carbon data of hydrothermal fluids flowing out of la soufriere the results for dissolved gases reveal that an intense interaction between rising magmatic volatiles and groundwaters currently takes place at la soufriere we show that the pristine magmatic composition with a 13 c co2 value of 3 19 0 03 a 3 he 4 he ratio of 8 1 0 2 r a and a co 2 3 he ratio of 1 03 0 18 10 10 may vary during the ascent of magmatic fluids due to either fractionation or mixing processes with meteoric water and or an organic component here we provide a baseline for co 2 and helium isotopes at each monitored spring and describe the superficial processes affecting both elemental and isotopic ratios

a new scenario for the last magmatic eruption of la soufriere of guadeloupe lesser antilles in 1530 a d evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products la soufriere of guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest in the last 15 000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption we propose a new age of 1530 a d and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates and new field and geochemical data this eruption is the only magmatic eruption likely to have occurred in guadeloupe during the last 1400 years the eruption mainly involved an andesitic magma which in the first phase of the eruption partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub plinian phase with scoria fallout and column collapse pyroclastic flows followed by near vent pyroclastic scoria fountains the eruption ended with growth of a lava dome our revised interpretation of the last magmatic eruption of la soufriere constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992

soil formation rates on silicate parent material in alpine environments different approaches different results abstract high mountain soils develop in particularly sensitive environments consequently deciphering and predicting what drives the rates of soil formation in such environments are a major challenge in terms of soil production or formation from chemical weathering the predominating perception for high mountain soils and cold environments is often that the chemical weathering portion of soil development is temperature inhibited often to the point of non occurrence several concepts exist to determine long term rates of soil formation and development we present three different approaches 1 quantification of soil formation from minimally eroded soils of known age using chronosequences known surface age and soil thickness sast 2 determination of soil residence times srt and production rates through chemical weathering using un stable isotopes e g 230 th 234 u activity ratios and 3 a steady state approach using cosmogenic isotopes e g 10 be for each method data from different climate zones and particularly from high mountains alpine environment are compared the sast and steady state approach give quite similar results for alpine environments european alps and the wind river range rocky mountains usa using the srt approach soil formation rates in mountain areas but having a temperate climate do not differ greatly from the sast and steady state approaches independent of the chosen approach the results seem moderately comparable soil formation rates in high mountain areas alpine climate range from very low to extremely high values and show a clear decreasing tendency with time very young soils have up to 3 4 orders of magnitude higher rates of development than old soils 10 5 to 10 6 yr this apparently is a result of kinetic limits on weathering in regions having young surfaces and supply limits to weathering on old surfaces due to the requirement for chemical weathering to occur soil production rates cannot be infinitely high consequently a speed limit must exist in the literature this limit has been set at about 320 to 450 t km 2 a our results from the sast approach show however that in alpine areas soil formation easily reaches rates of up to 800 2000 t km 2 a these data are consistent with previous studies in mountain regions demonstrating that particularly young soils intensively weather even under continuous seasonal snowpack and thus that the concept of temperature controlled soil development soil forming intervals in alpine regions must be reconsidered

mean bedrock to saprolite conversion and erosion rates during mountain growth and decline abstract weathering and associated atmospheric co2 consumption are thought to increase during the erosion of uplifting mountain ranges but the effect of enhanced erosion on weathering is still the subject of active debate we explore the possibility that erosion heterogeneity in uplifting landscapes significantly impacts the temporal relationships among mean uplift erosion and weathering using a 3d landscape evolution model applied to a synthetic surface with different uplift and climate scenarios although we do not strictly simulate the weathering outflux of the mountain we analyze the weathering response through the evolution of the mountain mean saprolite production rate and compare it to the mountain mean erosion rate through time the parametrical analysis shows that the temporal relationship between the mean erosion and saprolite production rates depends mainly on the ratio of the maximum saprolite production rate and the uplift rate wm u we explore two end members 1 when wm u 1 which corresponds to mountain ranges under a hot and humid climate the mean erosion and saprolite production rates vary at the same rate during the uplift and after once the uplift is stopped when the uplift is stopped the mean saprolite production increases and then decreases locally at different times this heterogeneity induces an overall decrease in the mean saprolite production rate 2 when wm u

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

tasman orogenic system a model for its subdivision and growth history based on gravity and magnetic anomalies the basement of the tasman orogenic system can be subdivided using gravity and magnetic anomalies into 11 geophysical domains the unity of each domain is given by subparallel trends which show the direction of the dominant tectonic structure in the nonsedimentary part of the upper crust basement crosscutting trends at domain boundaries indicate the relative ages of the domain cratonization these relative ages are consistent with the dated ages of first deformation and they show that cratonization occurred in numerous episodes and resulted in an apparent growth of the tasman orogenic system from west to east between the cambrian and the carboniferous each domain can be subdivided into bands of different character in the main area of basement outcrop the southern part of the lachlan orogen the position of the boundaries of these domains and bands agree with boundaries based on the composition of granites structure and strontium isotope ratios over the larger part of the tasman orogenic system where basement is under cover these geophysically defined domains and bands are our best estimate of basement subdivision and history near domain boundaries there are characteristic associations of gravity and magnetic anomalies due to reworking of the margin of the older crust by deformation intrusion and metamorphism differences between domains in mean upper crustal density and magnetization and the common occurrence of volcanic bands ophiolite serpentinite and deep elongate sedimentary troughs caused by crustal extension the domains can be divided into a sedimentary basement type formed by crustal extension and sedimentation or by the addition of an accretionary wedge and a volcanic arc basement type added by the accretion

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

architecture of the deep critical zone in the rio icacos watershed luquillo critical zone observatory puerto rico inferred from drilling and ground penetrating radar gpr

sediment residence times in catchments draining to the gulf of carpentaria northern australia inferred by uranium comminution dating abstract uranium u isotopes are useful for constraining the timescales of weathering and erosion processes the 234u 238u activity ratio parentheses denote activity ratio of fine grained detrital minerals is proposed to record the time elapsed since mineral grains were reduced to