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

uranium series dating of pedogenic silica and carbonate crater flat nevada a 230th 234u 238u dating study on pedogenic silica carbonate clast rinds and matrix laminae from alluvium in crater flat nevada was conducted using small sample thermal ionization mass spectrometry tims analyses on a large suite of samples though the 232th content of these soils is not particularly low mostly 0 1 9 ppm the high u content of the silica component mostly 4 26 ppm makes them particularly suitable for 230th u dating on single 10 to 200 mg totally digested samples using tims we observed that 1 both micro within rind and macro stratigraphic mappable deposit order of the 230th u ages were preserved in all cases 2 back calculated initial 234u 238u fall in a restricted range typically 1 67 0 19 so that 234u 238u ages with errors of about 100 kyr 2 could be reliably determined for the oldest 400 to 1000 ka rinds and 3 though 13 of the samples were 350 ka only three showed evidence for an open system history even though the sensitivity of such old samples to isotopic disruption is very high an attempt to use leach residue techniques to separate pedogenic from detrital u and th failed yielding corrupt 230th u ages we conclude that 230th u ages determined from totally dissolved multiple sub mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples chemical enhancement of 238u 232th or isochrons

insight into factors controlling formation rates of pedogenic carbonates a combined geochemical and isotopic approach in dryland soils of the us southwest abstract natural accumulation of pedogenic carbonates has been well documented but few studies have focused on carbonate formation in agricultural drylands this study aims to determine accumulation rates of pedogenic carbonates in intensively irrigated soils and to define key linkages between flood irrigation salt loading and soil atmosphere co 2 exchange in cultivated drylands of the southwestern united states we used a combination of elemental chemistry cao soil organic and inorganic carbon contents mineralogy and u series 238 u 234 u 230 th disequilibrium dating technique to investigate calcium sources ages and formation rates of pedogenic carbonates study sites include an irrigated alfalfa field near el paso in western texas and a non irrigated natural dryland site on the usda jornada experimental range of southern new mexico our results showed that large amounts of dissolved calcium and inorganic carbon along with other soluble elements were loaded onto agricultural fields through irrigation waters in el paso tx while dust and rainfall were important for salt loading in natural soils of the jornada u series activity ratios 234 u 238 u and 230 th 238 u in bulk soils suggested eolian deposits added u and modified u isotopes in shallow soils at both the irrigated and natural sites mobility of 234 u within the soil profile is related to leaching of u and by inference other soluble ions and carbonate accumulation at depth the u series dating technique in pedogenic carbonates revealed the presence of much younger carbonates at the irrigated site compared to the natural site pedogenic carbonate formation rates in the irrigated soils were also much higher than those in the non irrigated soils likely a result of influxes from dissolved ca and inorganic carbon in water used for irrigation this study demonstrates the potential for agricultural expansion and land use change in drylands to increase rate of pedogenic carbonate accumulation such changes may have important implications to global carbon cycling since drylands are forecast to become the most expansive terrestrial biome by mid century and dryland agriculture is expanding quickly

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

u sr isotopic speedometer fluid flow and chemical weathering rates in aquifers abstract both chemical weathering rates and fluid flow are difficult to measure in natural systems however these parameters are critical for understanding the hydrochemical evolution of aquifers predicting the fate and transport of contaminants and for water resources water quality considerations 87 sr 86 sr and 234 u 238 u activity ratios are sensitive indicators of water rock interaction and thus provide a means of quantifying both flow and reactivity the 87 sr 86 sr values in ground waters are controlled by the ratio of the dissolution rate to the flow rate similarly the 234 u 238 u ratio of natural ground waters is a balance between the flow rate and the dissolution of solids and recoil loss of 234 u from the solids by coupling these two isotope systems it is possible to constrain both the long term ca 100 s to 1000 s of years flow rate and bulk dissolution rate along the flow path previous estimates of the ratio of the dissolution rate to the infiltration flux from sr isotopes 87 sr 86 sr are combined with a model for 234 u 238 u to constrain the infiltration flux and dissolution rate for a 70 m deep vadose zone core from hanford washington the coupled model for both 234 u 238 u ratios and the 87 sr 86 sr data suggests an infiltration flux of 5 2 mm yr and bulk silicate dissolution rates between 10 15 7 and 10 16 5 mol m 2 s the process of recoil enrichment while primarily responsible for the observed variation in 234 u 238 u of natural systems is difficult to quantify however the rate of this process in natural systems affects the interpretation of most u series data models for quantifying the recoil loss fraction based on geometric predictions surface area constraints and chemical methods are also presented the agreement between the chemical and theoretical methods such as direct measurement of 234 u 238 u of the small grain size fraction and geometric calculations for that size fraction is quite good

234u 238u evidence for local recharge and patterns of ground water flow in the vicinity of yucca mountain nevada usa uranium concentrations and 234u 238u ratios in saturated zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of yucca mountain a potential high level radioactive waste disposal site the u data were obtained by thermal ionization mass spectrometry on more than 280 samples from the death valley regional flow system large variations in both u concentrations commonly 0 6 10 g l 1 and 234u 238u activity ratios commonly 1 5 6 are present on both local and regional scales however ground water with 234u 238u activity ratios from 7 up to 8 06 is restricted largely to samples from yucca mountain data from ground water in the tertiary volcanic and quaternary alluvial aquifers at and adjacent to yucca mountain plot in 3 distinct fields of reciprocal u concentration versus 234u 238u activity ratio correlated to different geographic areas ground water to the west of yucca mountain has large u concentrations and moderate 234u 238u whereas ground water to the east in the fortymile flow system has similar 234u 238u but distinctly smaller u concentrations ground water beneath the central part of yucca mountain has intermediate u concentrations but distinctive 234u 238u activity ratios of about 7 8 perched water from the lower part of the unsaturated zone at yucca mountain has similarly large values of 234u 238u these u data imply that the tertiary volcanic aquifer beneath the central part of yucca mountain is isolated from north south regional flow the similarity of 234u 238u in both saturated and unsaturated zone ground water at yucca mountain further indicates that saturated zone ground water beneath yucca mountain is dominated by local recharge rather than regional flow the distinctive 234u 238u signatures also provide a natural tracer of downgradient flow elevated 234u 238u in ground water from two water supply wells east of yucca mountain are interpreted as the result of induced flow from 40 a of ground water withdrawal elevated 234u 238u in a borehole south of yucca mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic tertiary volcanic aquifer where it becomes diluted by regional flow in the fortymile system

rise and fall of late pleistocene pluvial lakes in response to reduced evaporation and precipitation evidence from lake surprise california widespread late pleistocene lake systems of the basin and range province indicate substantially greater moisture availability during glacial periods relative to modern times but the climatic factors that drive changes in lake levels are poorly constrained to better constrain these climatic forcing factors we present a new lacustrine paleoclimate record and precipitation estimates for lake surprise a closed basin lake in northeastern california we combine a detailed analysis of lake hydrography and constitutive relationships describing the water balance to determine the influence of precipitation evaporation temperature and seasonal insolation on past lake levels at its maximum extent during the last deglaciation lake surprise covered 1366 km 2 36 of the terminally draining surprise valley watershed using paired radiocarbon and 230 th u analyses we dated shoreline tufa deposits from wave cut lake terraces in surprise valley california to determine the hydrography of the most recent lake cycle this new lake hydrograph places the highest lake level 176 m above the present day playa at 15 19 0 18 calibrated ka 14 c age this significantly postdates the last glacial maximum lgm when lake surprise stood at only moderate levels 65 99 m above modern playa similar to nearby lake lahontan to evaluate the climatic factors associated with lake level changes we use an oxygen isotope mass balance model combined with an analysis of predictions from the paleoclimate model intercomparison project 3 pmip3 climate model ensemble our isotope mass balance model predicts minimal precipitation increases of only 2 18 during the lgm relative to modern compared to an 75 increase in precipitation during the 15 19 ka highstand lgm pmip3 climate model simulations corroborate these findings simulating an average precipitation increase of only 6 5 relative to modern accompanied by a 28 decrease in total evaporation driven by a 7 c decrease in mean annual temperature lgm pmip3 climate model simulations also suggest a seasonal decoupling of runoff and precipitation with peak runoff shifting to the late spring early summer from the late winter early spring our coupled analyses suggest that moderate lake levels during the lgm were a result of reduced evaporation driven by reduced summer insolation and temperatures not by increased precipitation reduced evaporation primed basin and range lake systems particularly smaller isolated basins such as surprise valley to respond rapidly to increased precipitation during late heinrich stadial 1 hs1 post lgm highstands were potentially driven by increased rainfall during hs1 brought by latitudinally extensive and strengthened midlatitude westerly storm tracks the effects of which are recorded in the region9s lacustrine and glacial records these results suggest that seasonal insolation and reduced temperatures have been underinvestigated as long term drivers of moisture availability in the western united states

estimating millennial scale rates of dust incorporation into eroding hillslope regolith using cosmogenic nuclides and immobile weathering tracers 1 dust fluxes are of wide interest because of the effects of dust on climate oceanic primary productivity terrestrial biogeochemical cycles and regolith composition estimating long term dust deposition rates however can be difficult especially in steep eroding terrain here we present a geochemical mass balance method for estimating long term average rates of dust incorporation into regolith on steadily eroding hillslopes this method requires measurements of the local regolith production rate and the concentrations of two immobile elements in the regolith its parent rock and dust dust incorporation rates inferred with this method are averaged over the long timescale of regolith residence on the hillslope typically 103 105 years and thus may serve as long term averages against which modern day dust fluxes may be compared we apply this model to 17 field sites in the south fork of the salmon river in the idaho batholith where rock and regolith compositions imply that mafic rich material has been added to the otherwise granitic regolith we suggest that the most likely source of this mafic material is dust sourced from the same glacial outburst flood sediments that generated the palouse loess on the columbia plateau and we use the published composition of these sediments to infer dust incorporation rates of 3 13 t km 2 yr 1 at these sites comparable to modern day dust fluxes elsewhere in the western united states

chemical weathering of a marine terrace chronosequence santa cruz california i interpreting rates and controls based on soil concentration depth profiles abstract the spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal california are interpreted in terms of chemical weathering rates and processes in regoliths up to 15 m deep and 226 kyrs old quartz normalized mass transfer coefficients indicate non stoichiometric preferential release of sr ca na from plagioclase along with lesser amounts of k rb and ba derived from k feldspar smectite weathering results in the loss of mg and concurrent incorporation of al and fe into secondary kaolinite and fe oxides in shallow argillic horizons elemental losses from weathering of the santa cruz terraces fall within the range of those for other marine terraces along the pacific coast of north america residual amounts of plagioclase and k feldspar decrease with terrace depth and increasing age the gradient of the weathering profile bs is defined by the ratio of the weathering rate r to the velocity at which the profile penetrates into the protolith a spreadsheet calculator further refines profile geometries demonstrating that the non linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface to volume ratios and at high residual feldspar concentrations at greater depth by the approach to thermodynamic saturation these parameters are of secondary importance to the fluid flux qh which in thermodynamically saturated pore water controls the weathering velocity and mineral losses from the profiles long term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute cl balances qh 0 025 0 17 m yr 1 during saturation controlled and solute limited weathering the greater loss of plagioclase relative to k feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics the steady state weathering rate under such conditions is defined as r q h m sol m total 1 s v b s the product of qh and the ratio of solubilized to solid state feldspar msat mtotal define the weathering velocity the weathering gradient bs reflects the kinetic rate of reaction where sv is the volumetric surface area of the residual feldspar both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates r 5 14 10 16 mol m 2 s 1 which agree with those reported for other environments of comparable climate and age weathering dependent concentration profiles are commonly described in literature the present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved

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

vadose zone infiltration rate at hanford washington inferred from sr isotope measurements i sr isotope ratios were measured in the pore water acid extracts and sediments of a 70 m vadose zone core to obtain estimates of the long term infiltration flux for a site in the hanford doe complex in eastern washington state the 8 7 sr 8 6 sr values for the pore waters decrease systematically with depth from a high value of 0 721 near the surface toward the bulk sediment average value of 0 711 estimates of the bulk weathering rate combined with sr isotopic data were used to constrain the long term century to millenial scale natural diffuse infiltration flux for the site given both steady state and nonsteady state conditions the models suggest that the infiltration flux for the site is 7 3 mm yr the method shows potential for providing long term in situ estimates of infiltration rates for deep heterogeneous vadose zones

assessing the role of climate on uranium and lithium isotope behaviour in rivers draining a basaltic terrain this study presents uranium u and lithium li isotope and major and trace element data for the dissolved load suspended particles and bedload from rivers draining dominantly basaltic catchments on the island of sao miguel in the azores archipelago uranium activity ratios are at secular equilibrium in the bedload but are higher for suspended material suggesting sorption of uranium from the solution onto particle surfaces the 234u 238u of the riverine dissolved phase varies between 1 02 and 1 86 and lies on a mixing trend between the values expected from chemical weathering of basalt high u 234u 238u at secular equilibrium and an endmember with low u and high 234u 238u which probably reflects recoil effects associated with physical weathering and or soil formation lithium isotope ratios are consistently lighter in suspended material than the corresponding bedload suggesting preferential retention of 6li in the suspended load in turn the 7li of the dissolved load is always isotopically heavier ranging from 5 9 to 36 2 although the lightest values are affected by hydrothermal input the lithium isotopic composition of the dissolved load reflects the balance of primary mineral dissolution to secondary mineral formation comparison of the behaviour of u and li isotopes between sao miguel iceland and other basaltic terrains suggests that whilst u isotopes are dominated by weathering regimes dependent on factors such as weathering rates temperature runoff and climate neither weathering intensity nor climate exerts a direct influence on li isotope behaviour rather the formation of secondary minerals which is indirectly controlled by climate weathering primary mineralogy and biology appears to play the dominant role

potential effects of alpha recoil on uranium series dating of calcrete abstract evaluation of paleosol ages in the vicinity of yucca mountain nevada at the time the site of a proposed high level nuclear waste repository is important for fault displacement hazard assessment uranium series isotope data were obtained for surface and subsurface calcrete samples from trenches and boreholes in midway valley nevada adjacent to yucca mountain 230 th u ages of 33 surface samples range from 1 3 to 423 thousand years ka and the back calculated 234 u 238 u initial activity ratios ar are relatively constant with a mean value of 1 54 0 15 1 which is consistent with the closed system behavior subsurface calcrete samples are too old to be dated by the 230 th u method u pb data for post pedogenic botryoidal opal from a subsurface calcrete sample show that these subsurface calcrete samples are older than 1 65 million years ma old enough to have attained secular equilibrium had their u th systems remained closed however subsurface calcrete samples show u series disequilibrium indicating open system behavior of 238 u daughter isotopes in contrast with the surface calcrete where open system behavior is not evident data for 21 subsurface calcrete samples yielded calculable 234 u 238 u model ages ranging from 130 to 1875 ka assuming an initial ar of 1 54 0 15 the mean value calculated for the surface calcrete samples a simple model describing continuous recoil loss predicts that the 234 u 238 u and 230 th 238 u ars reach steady state values 2 ma after calcrete formation potential effects of open system behavior on 230 th u ages and initial 234 u 238 u ars for younger surface calcrete were estimated using data for old subsurface calcrete samples with the 234 u loss and assuming that the total time of water rock interaction is the only difference between these soils the difference between the conventional closed system and open system ages may exceed errors of the calculated conventional ages for samples older than 250 ka but is negligible for younger soils

pedothem carbonates reveal anomalous north american atmospheric circulation 70 000 55 000 years ago our understanding of climatic conditions and therefore forcing factors in north america during the past two glacial cycles is limited in part by the scarcity of long well dated continuous paleoclimate records here we present the first to our knowledge continuous millennial resolution paleoclimate proxy record derived from millimeter thick pedogenic carbonate clast coatings pedothems which are widely distributed in semiarid to arid regions worldwide our new multiisotope pedothem record from the wind river basin in wyoming confirms a previously hypothesized period of increased transport of gulf of mexico moisture northward into the continental interior from 70 000 to 55 000 years ago based on oxygen and carbon isotopes determined by ion microprobe and uranium isotopes and u th dating by laser ablation inductively coupled plasma mass spectrometry this pronounced meridional moisture transport which contrasts with the dominant zonal transport of pacific moisture into the north american interior by westerly winds before and after 70 000 55 000 years ago may have resulted from a persistent anticyclone developed above the north american ice sheet during marine isotope stage 4 we conclude that pedothems when analyzed using microanalytical techniques can provide high resolution paleoclimate records that may open new avenues into understanding past terrestrial climates in regions where paleoclimate records are not otherwise available when pedothem paleoclimate records are combined with existing records they will add complimentary soil based perspectives on paleoclimate conditions

an open system model for u series age determinations of fossil corals the source of excess 234u in fossil corals and its relationship to u series age determinations has been an outstanding problem in geochronology for more than 20 years with increasing numbers of u series isotope measurements in corals and significant improvements in analytical precision through mass spectrometry it is increasingly apparent that a substantial fraction of observed isotope ratios cannot be reasonably explained by closed system decay moreover observations of a positive correlation between 234u 238u and 230th 238u ratios in corals from the same terrace are difficult to explain however the decay of dissolved uranium and recoil mobilization of uranium daughters produce particle reactive 234th and 230th and the coupled addition of these th isotopes could simultaneously increase coral 234u 238u and 230th 238u here we present a quantitative model based on decay dependent redistribution of 234th and 230th permitting calculation of open system coral ages these equations provide a general solution to the recoil redistribution problem applicable to any alpha decay series while measured isotope ratios of corals from the three youngest stratigraphically defined barbados terraces are inconsistent with closed system decay they fall in broadly linear arrays agreeing with model predictions isotopic arrays of older barbados corals and corals from terraces around the world are also consistent with model predictions suggesting the open system model is generally applicable corals with extreme isotopic compositions that are impossible to produce by closed system decay are consistent with the limited range of isotopic compositions predicted by the model at ages older than 600 ka for corals from a single terrace 234th and 230th redistribution appears to be a source of systematic conventional age error even for corals with slightly elevated 234u however open system ages are consistent even for corals with extremely elevated 234u for the youngest three barbados terraces mean open system terrace ages are consistent with mean conventional terrace ages calculated from pristine samples if the most accurate conventional ages are from corals with an initial 234u 238u identical to modern seawater then the open system model will improve the accuracy of coral u series age determinations and dramatically increase the number of reliable ages

genesis of humus b horizons in hydromorphic humus podzols a distinctive podzol b horizon characterized by accumulated humus low iron content lateral uniformity in appearance and composition and frequently hard cementation develops at the top of the water table in quartz sands on coastal plains in the subtropics and tropics soils with such b horizons are separated from freely drained podzols in many classifications such as groundwater podzols1 humus podzols2 hydromorphic humic podzols3 aquods and humods4 because of a distinctive morphology and possible differences in genesis however while an association between the humus rich b horizon bh and the underlying water table has long been recognized most explanations offered for the mode of formation of the bh have been essentially similar to those for bhir iron rich bh horizons in freely drained podzols these do not provide a satisfactory explanation of the mechanism whereby organic matter is precipitated in an environment that is virtually free of sesquidoxes we suggest here that such bh horizons may form as the result of coprecipitation in the b horizon between organic rich surface waters and aluminium from the groundwater further such a mechanism can lead to the development of organic cemented hardpans commonly called coffee rock3 5 that are so frequently associated with hydromorphic podzols in both temperate and tropical regions3

geochemical estimates of paleorecharge in the pasco basin evaluation of the chloride mass balance technique the pasco basin in southeastern washington state provides a unique hydrogeologic setting for evaluating the chloride mass balance technique for estimating recharge this basin was affected by late pleistocene catastrophic floods when glacial dams in western montana and northern idaho were breached it is estimated that multiple missoula floods occurred between 13 000 and 15 000 years b p and reached a high water elevation of 350 m these floods removed accumulated chloride from the sediment profile effectively resetting the chloride mass balance clock at the beginning of the holocene the rate of chloride accumulation qcl in the sediments was determined by two methods and compared the first method measured qcl by dividing the calculated natural fallout of 36cl by a measured ratio of 36cl cl in the pore water while the second method used the total mass of chloride in the profile divided by the length of time that atmospheric chloride had accumulated since the last flood although the two methods are based on different approaches they showed close agreement in laboratory studies the sediment to water ratio for chloride extraction was sensitive to the grain size of the sediments low extraction ratios in silt loam sediments led to significant underestimation of pore water chloride concentration br cl ratios were useful for distinguishing nonatmospheric e g rock sources of chloride field studies showed little spatial variability in estimated recharge at a given site within the basin but showed significant topographic control on recharge rates in this semiarid environment an extension of the conventional chloride mass balance model was used to evaluate chloride profiles under transient time varying annual precipitation conditions this model was inverted to determine the paleorecharge history for a given soil chloride profile and the parameters of the root extraction model required to estimate paleoprecipitation

from oxisols to spodosols and histosols evolution of the soil mantles in the rio negro basin amazonia abstract the upper rio negro basin under a constantly humid equatorial climate is a low altitude peneplain of more than 165 000 km 2 prevailing soils are oxisols and spodosols and their distribution is usually related to the lithology of the parent materials spodosols being generally associated with sandy deposits after exploratory surveys in this extensive region six major soil geomorphic units have been identified in which selected toposequences have been studied by means of micromorphological chemical and mineralogical analyses detailed field analysis of the horizonation of the soil mantle has been carried out in three sequences consisting of oxisols ultisols and spodosols results show that sharp transitions within distances of less than one hundred meters separate the oxisols from the spodosols however the arrangement of the horizons in the soil mantle and similarities in micromorphological features chemical composition and mineral components between the adjacent horizons are evidences that genetic relationships link contiguous profiles the authors propose an alternative explanation for this soil distribution based on the lateral transformation of oxisols and ultisols into spodosols and on the lateral evolution of the giant spodosols into histosols peat and waterlogged ultisols interpretative models of landscape evolution as consequence of soil evolution are thus proposed

geochemistry of speleothem records from southern illinois development of 234u 238u as a proxy for paleoprecipitation natural waters universally show fractionation of uranium series u series parent daughter pairs with the disequilibrium between 234 u and 238 u 234 u 238 u commonly used as a tracer of groundwater flow because speleothems provide a temporal record of geochemical variations in groundwater precipitating calcite 234 u 238 u variations in speleothems provide a unique method of investigating water rock interaction processes over millennium time scales we present high precision thermal ionization mass spectrometric tims u series analyses of speleothems and drip waters from fogelpole cave in southern illinois data from all speleothems from the cave show an inverse correlation between 234 u 238 u and u concentration following the pattern observed in groundwaters globally within a 65 cm long stalagmite concordant 234 u 238 u 230 th and 235 u 231 pa ages for 5 samples indicate accurate chronology from 78 5 ka to 30 ka notably 234 u 238 u o which differs from most speleothems by having 234 u 238 u ob1 positively correlates with speleothem growth rate we generalize this to the observation that speleothems globally show 234 u 238 u o deviating farther from secular equilibrium at lower growth rates and approaching secular equilibrium at higher grow rates based on the fogelpole observations we suggest that groundwater 234 u 238 u is controlled by the u oxidation state the u concentration of the water and the fluid velocity a transport model whereby u series nuclides react and exchange with mineral surfaces can reproduce the observed trend between growth rate and 234 u 238 u o based on this result we suggest that 234 u 238 u o in speleothems may record changes in hydrologic flux with time and thus could provide a useful proxy for long term records of

control of insolation on stalagmite growth rainfall and migration of the tropical rain belt in northern namibia over the last 100 kyr as suggested by a rare mis 5b 5c stalagmite from dante cave abstract dating by the 230th method indicates that stalagmite danf from dante cave in northern namibia in the summer rainfall zone srz of southern africa formed about 92 ka at the boundary of marine isotope stages mis 5b and 5c in this dominantly semiarid zone stalagmite growth may be evidence of a relatively wetter period and trends in danf s mineralogy and its c o and u isotope ratios further support formation in a period wetter than those before or after its formation danf s time of formation at 92 ka like other wet periods in the srz of southern africa over the most recent 100 kyr coincides with a maximum in january insolation at 30 s that brought the srz and more generally the tropical rain belt farther south than before or after

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

geochemical and isotopic sr u monitoring of soil solutions from the strengbach catchment vosges mountains france evidence for recent weathering evolution abstract this study presents the results of a geochemical and isotope u and sr study of soil solutions from two experimental sites in the strengbach catchment one under spruce trees and the other under beech trees the soil solution data combined with the data from soil and vegetation samples indicate that the dissolved u filtered to 0 45 m in the soil solutions is primarily of lithogenic origin implying that the colloidal u is secondarily associated with organic matter or organo metallic complexes present in the soil solutions the uranium activity ratios and sr isotope ratios of soil solutions are used to decipher the nature of the minerals that control the lithogenic contribution to the soil solutions a comparison of soil solution data collected over the period 2004 2006 with those collected over the period 1993 2010 indicates a decrease in the ca and k exported by the soil solutions between 1998 and 2004 at the spruce site and a significantly smaller decrease at the beech site in addition the sr isotope ratios of the soil solutions vary significantly between these two periods at the spruce site but not or much less at the beech site such observations suggest that the forest ecosystem at the spruce site is in a transient state associated with recent changes in weathering reactions the data also suggest that the comparison of u and sr isotopes in soil solutions and plant samples could be a relevant method for determining whether a forest ecosystem is in a steady or transient state

limited hydrologic response to pleistocene climate change in deep vadose zones yucca mountain nevada abstract understanding the movement of water through thick vadose zones especially on time scales encompassing long term climate change is increasingly important as societies utilize semi arid environments for both water resources and sites viewed as favorable for long term disposal or storage of hazardous waste hydrologic responses to pleistocene climate change within a deep vadose zone in the eastern mojave desert at yucca mountain nevada were evaluated by uranium series dating of finely layered hyalitic opal using secondary ion mass spectrometry opal is present within cm thick secondary hydrogenic mineral crusts coating floors of lithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface uranium concentrations in opal fluctuate systematically between 5 and 550 g g age calibrated profiles of uranium concentration correlate with regional climate records over the last 300 000 years and produce time series spectral peaks that have distinct periodicities of 100 and 41 ka consistent with planetary orbital parameters these results indicate that the chemical compositions of percolating solutions varied in response to near surface climate driven processes however slow micrometers per thousand years relatively uniform growth rates of secondary opal and calcite deposition spanning several glacial interglacial climate cycles imply that water fluxes in the deep vadose zone remained low and generally buffered from the large fluctuations in available surface moisture during different climates

quantifying closed basin lake temperature and hydrology by inversion of oxygen isotope and trace element paleoclimate records lake systems are important paleoclimate archives that preserve ecosystem and hydrologic responses to critical periods in earth history such as carbon cycle perturbations and glacial interglacial cycles geochemical measurements of biogenic carbonate for example 18o 13c 87sr 86sr li u sr and mg are indicators of hydrologic variability in lake systems throughout the geologic record in this study we present a new closed basin lake modeling approach hybim the hydrologic balance inverse model that employs a system of total differential equations and uses the measured 18o sr ca and mg ca of biogenic carbonate to determine changes in temperature runoff and lake evaporation using equally spaced time steps these equations are simultaneously solved to constrain the hydrologic parameters of the lake as recorded in biogenic carbonate we use a monte carlo approach to account for uncertainty in the input parameters such as 18o temperature relationships partition coefficient uncertainty and watershed solute chemistry for illustrative purposes we apply the model to two ostracod valve datasets covering different timescales 1 the cretaceous songliao basin northeast china and 2 holocene lake miragoane haiti modern water measurements of water isotopes and cation concentrations from each location are required as model inputs we compare our modeling results with author interpretations and geologic observations the modeling approach presented in this study can be applied to other closed basin lake records can be modified for other calcifying species for example gastropods or mollusks or with calibration to inorganic lacustrine carbonate in addition this approach holds promise for extension with additional proxy measurements that is d u ca or li ca and changing source area on tectonic timescales using proxies that reflect changing source lithology that is sr and pb isotopes future incorporation of age model uncertainty in the monte carlo approach will also provide utility by quantifying temporal uncertainty on the hydrologic response recorded by lake sediments

application of a soil water balance model to evaluate the influence of holocene climate change on calcic soils mojave desert california u s a abstract we used a process based soil water balance model to simulate the downward flux of soil water under varied conditions of climate vegetation and soil texture to determine the potential impact of episodic periods of wetter pluvial climate during the holocene on calcic soils in the mojave desert that have a bimodal distribution of carbonate daily weather data associated with a relatively wet climate years with extreme increases in annual rainfall 33 cm yr and dry climate historic average annual rainfall 15 cm yr was used to simulate the affects of pleistocene and holocene climate change on soil water balance linkages among atmospheric circulation patterns regional increases in precipitation and historic flooding in the mojave desert california suggest that historic wet years provide an analog for wetter climates that occurred during the last glacial period latest pleistocene and episodically during holocene periods of pluvial activity modeling results indicate that soil water balance for dry and wet years strongly corresponds with the upper and lower zones of carbonate accumulation respectively soil water only reached the lower zone of carbonate during a wet year when extreme increases in winter spring storm activity resulted in a significant increase in precipitation and the downward flux of soil water the linkage between increases in frontal storm activity and pluvial events suggests that the shallow zone of the bimodal distribution of carbonate is a result of periods of significant decreases in winter and spring rainfall and not primarily due to increases in holocene temperature or the development of clay rich horizons calculation of carbonate solubility and accumulation rates suggests that the bimodal distribution of carbonates in soils may have also been impacted by episodic periods of extreme increases in precipitation associated with perennial lakes during the holocene results suggests that much of the carbonate in the upper 75 cm of pleistocene soils may have accumulated during the late holocene rather than throughout the entire holocene

quaternary soils and dust deposition in southern nevada and california eoliandustconstitutesmuchofthepedogenic material in late pleistocene and holocene soils of many arid regions of the world comparisonofthecompositionsand influx rates of modern dust with the eolian component of dated soils at 24 sites in southern nevada and california yields informationon 1 thecompositionandinflux rate of dust in late pleistocene and holocene soils 2 paleoclimate and its effects on the genesis of aridic soils especially with regard to dustfall events 3 the timing and relative contribution of dust from playa sources versus alluvial sources and 4 the effects of accumulation of dust in soil horizons the 2mmfractionsofaandbhorizons of soils formed on gravelly alluvial fan deposits in the study area are similar to moderndustingrainsize contentofcaco3and salt majoroxides andclaymineralogy thus they are interpreted to consist largely of dust the major oxide compositions of the shallow soil horizons are nearly identical to that of the modern dust but the compositions of progressively deeper horizons approachthatoftheparentmaterial theclay mineralogyofmoderndustatagivensiteis similar to that of the av horizons of nearby holocene soils but is commonly different from the mineralogies of deeper soil horizonsandoftheavhorizonsofnearbypleistocenesoils theseresultsareinterpretedto indicate that dust both accumulates and is transformed in av horizons with time changes in soil accumulation rates provide insights into the interplay of paleoclimate dustsupply andsoil formingprocesses modern dust deposition rates are more than large enough to account for middle and late holocene soil accumulation rates at nearly all sites however the early holocene soil accumulation rates in areas near late pleistocene pluvial lakes are much higher than modern rates and clearly indicate a dust deflation and deposition event that caused rapid formation offine grained shallow soil horizons on uppermost pleistocene and lower holocene deposits we interpret late pleistocene soil accumulation rates to indicate that dust deposition rates were low during this period but that increased effective moisture during the late wisconsinan favored translocation of clay andcaco3fromnearthesurfacetodeeper inthesoilprofile pre latepleistocenerates are very low in most areas mainly due to a pedogenic threshold that was crossed when accumulations of silt clay and caco3 began to inhibit the downward transport of eolian material but in part due to erosion

the behavior of u and th series nuclides in groundwater groundwater has long been an active area of research driven by its importance both as a societal resource and as a component in the global hydrological cycle key issues in groundwater research include inferring rates of transport of chemical constituents determining the ages of groundwater and tracing water masses using chemical fingerprints while information on the trace elements pertinent to these topics can be obtained from aquifer tests using experimentally introduced tracers and from laboratory experiments on aquifer materials these studies are necessarily limited in time and space regional studies of aquifers can focus on greater scales and time periods but must contend with greater complexities and variations in this regard the isotopic systematics of the naturally occurring radionuclides in the u and th decay series have been invaluable in investigating aquifer behavior of u th and ra these nuclides are present in all groundwaters and are each represented by several isotopes with very different half lives so that processes occurring over a range of time scales can be studied table 1 within the host aquifer minerals the radionuclides in each decay series are generally expected to be in secular equilibrium and so have equal activities see bourdon et al 2003 in contrast these nuclides exhibit strong relative fractionations within the surrounding groundwaters that reflect contrasting behavior during release into the water and during interaction with the surrounding host aquifer rocks radionuclide data can be used within the framework of models of the processes involved to obtain quantitative assessments of radionuclide release from aquifer rocks and groundwater migration rates the isotopic variations that are generated also have the potential for providing fingerprints for groundwaters from specific aquifer environments and have even been explored as a means for calculating groundwater ages view this table table 1 radionuclides important in groundwater studies the highly fractionated nature of the

rates and processes of soil development on quaternary terraces in cajon pass california field and laboratory analyses of soils on 11 well dated fluvial terraces spanning the past 0 5 m y demonstrate that a threshold governs changes in several morphological and chemical characteristics of increasingly older soils correlations with respect to time among iron species soil morphology and soil silt and clay demonstrate that the chronosequence at cajon pass reflects primarily an evolutionary largely time dependent trend and does not reflect differences in external factors such as climate most of the soil development on holocene terraces of the cajon pass area is due to physical incorporation of eolian dust and organic material into initially very permeable gravels this process decreases soil permeability and is conducive for an increase in the magnitude of chemical weathering latest pleistocene and older pleistocene soils have developed clay and authigenic iron oxide rich b horizons at the expense of organic matter rich a horizons and color b horizons as the extent of chemical weathering has increased this conversion of the soil from a noncolloidal system to a much more colloidal system takes place over a relatively short period of time 4 000 yr and is herein defined as a type of pedologic threshold in the cajon pass area the attainment of the threshold and subsequent development of the argillic b horizon of soils on latest pleistocene terraces occurred during the holocene thus the absence of argillic horizons in soils on holocene terraces is attributable to simply their younger age rather than to the pleistocene to holocene climatic change the threshold is a function of several variables including influx rate of eolian dust and initial soil permeability therefore the time required to attain the threshold will vary in chronose quences characterized by geomorphic or geographic settings that are different from conditions found in cajon pass

rates and processes of soil evolution on uplifted marine terraces northern california time dependent changes in soil development have been evaluated for two flights of uplifted marine terraces with similar site characteristics and a range in terrace ages 3 9 ka to 330 ka in northern california six pedogenic properties of soils developed in unconsolidated deposits overlying bedrock marine platforms have systematic time dependent trends but each property differs in rate of change and in the time period over which its changes are useful as relative age dating or correlation tools changes in accumulated mass of organic carbon and ph are useful properties only for the youngest soils and probably for soils less than 10 20 ka in this climatic region soils on terraces older than 29 ka have nearly similar amounts of mass of organic carbon with values of about 3 6 by weight for the upper meter of the soil profile soil reaction ph in the top 20 cm of the soil profile decreases at a rapidly declining rate reaching and maintaining a value of 5 by 39 40 ka total mass of accumulated clay is an excellent indicator of relative soil age during all stages of soil development the rate of accumulation of clay is about 0 4 g cm2 soil column per thousand years so that pedons on terraces with inferred ages of about 124 ka have accumulated at least 50 g cm2 soil column of clay systematic and linear increases in accumulated mass of several forms of pedogenic iron make them very useful for chronological purposed for at least 240 ky of soil development crystalline forms of iron enable greater distinction among young soils than amorphous forms which have negligible accumulations until at least 40 ka the rate of accumulation of crystalline forms of iron fed feo is about 0 02 g cm2 soil column per thousand years so that pedons on terraces with inferred ages of about 124 ka have accumulated about 3 g cm2 soil column of crystalline forms of pedogenic iron changes with time in accumulated mass of amorphous forms of pedogenic aluminum alo are negligible within the first 29 ka then reflect a rapid increase in rate of accumulation until at least 124 ka two pedons from the same terrace inferred age 124 ka contain strikingly different amounts of pedogenic alo however and suggest that alo may be an unreliable indicator of the degree of soil development in addition to mass values of different soil substances maximum percent values were calculated soils developed on marine terraces older than 29 ka have at least 10 maximum clay older than 100 ka at least 30 maximum clay and older than 240 ka at least 40 50 maximum clay soils on terraces younger than 29 ka have less than 1 maximum fed and soils older than 240 ka at have at least 4 maximum fed these easily obtained values provide textural and chemical boundaries for separating marine terraces younger or older than about 29 100 and 240 ka in this region but are useful only as very general tools for correlation maximum percent clay is a more reliable indicator of soil age than maximum percent fed as the latter is greatly influenced by local groundwater flux and percent values of fed have a high degree of scatter

the accretion of aeolian dust in soils of the san juan mountains colorado usa 1 recent observations suggest a contemporary aeolian dust flux of at least 5 10 g m 2 yr 1 to high elevation ecosystems of the san juan mountains of southwestern colorado to better quantify the influence of dust on san juan soil geochemistry we used sr and nd isotopic mixing models to estimate the total mass of accreted dust in soils of two alpine basins underlain by bedrocks of different geochemical composition in order to minimize the potentially confounding effects caused by transient soil pools of sr and nd we implemented a sequential leaching procedure that isolates the residual mineral fraction of soils and their putative parent materials including local saprolite and exogenous dust inputs using this approach we calculated masses of accreted dust in soils which were similar across the two isotopic tracers and differing local geologies long term rates of dust accretion were estimated to be slightly higher than contemporary rates of dust deposition we conclude that dust inputs comprise from 10 to 40 of the total soil mass in these ecosystems our observations suggest that dust inputs have exerted a primary control on soil development in the san juan mountains and have likely influenced the physical and chemical characteristics of soils in this region

u series chronology of lacustrine deposits in death valley california uranium series dating on a 186 m core dv93 1 drilled from badwater basin in death valley california and on calcareous tufas from nearby strandlines shows that lake manly the lake that periodically flooded death valley during the late pleistocene experienced large fluctuations in depth and chemistry over the last 200 000 yr death valley has been occupied by a long standing deep lake perennial shallow saline lakes and a desiccated salt pan similar to the modern valley floor the average sedimentation rate of about 1 mm yr for core dv93 1 was punctuated by episodes of more rapid accumulation of halite arid conditions similar to the modern conditions prevailed during the entire holocene and between 120 000 and 60 000 yr b p from 35 000 yr b p to the beginning of the holocene a perennial saline lake existed over 70 m at its deepest a much deeper and longer lasting perennial lake manly existed from about 185 000 to 128 000 yr b p with water depths reaching about 175 m if not 330 m this lake had two significant dry excursions of 102 103yr duration about 166 000 and 146 000 yr b p and it began to shrink to the point of halite precipitation between 128 000 and 120 000 yr b p the two perennial lake periods correspond to marine oxygen isotopic stages ois 2 and 6 based on the shoreline tufa ages we do not rule out the possible existence 200 000 yr ago of yet a third perennial lake comparable in size to the ois 6 lake the234u 238u data suggest that u in tufa owes its origin mainly to ca rich springs fed by groundwater that emanated along lake shorelines in southern death valley and that an increase of this spring water input relative to the river water input apparently occurred during ois 6

weathering rates from top to bottom in a carbonate environment abstract this study investigates u series sr isotopes major and trace elements in a chalk aquifer system located in eastern france soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil the unsaturated zone and the saturated zone of the aquifer interstitial water was extracted from soils and rocks by a centrifugation technique u series offer a powerful tool to calculate weathering rates because the relative mobility of the u and th isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches as expected the data show very large mobile element depletion in the soil with large 230 th excess relative to 238 u while the rocks show more limited but not insignificant mobile element depletion the u series data have been used to constrain weathering rates based on a 1 d reactive transport model weathering rates in the near surface are about 10 100 times faster than at depth however when integrated over the depth of the cores including the unsaturated and the saturated zones this underground weathering represents more than 30 of the total weathering flux assuming congruent dissolution of carbonates the 234 u 238 u ratios in interstitial water are consistent with solid samples showing 234 u depletion near the surface and an excess 234 u at depth a leaching experiment performed on chalk shows that the excess 234 u in natural waters percolating through carbonate rocks results both from preferential 234 u leaching and direct recoil in the interstitial water a new approach was used to derive the recoil ejection factor based on bet measurements and the fractal dimension of chalk surface consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates

diagenesis of speleothems and its effect on the accuracy of 230th u ages abstract speleothems can be precisely dated by u series disequilibrium methods one basic assumption of the 230th u dating method is that the system remains closed and u and th isotopes are neither lost nor added after deposition for stalagmites this requirement is usually fulfilled due to their protected environment in caves however undersaturated drip water may lead to diagenesis of speleothem caco3 and altered 230th u ages here we present a detailed petrographic investigation using thin sections and 230th u ages of a stalagmite from the riesenberghohle northern germany which grew during marine isotope stages 5e c and a the occurrence of a mosaic fabric in large parts of the stalagmite in combination with several age inversions is a clear indication for diagenetic alteration we discuss the potential diagenetic processes and quantitatively model their influence on the 230th u ages post depositional u loss leads to increasing 230th 238u activity ratios and thus apparently older ages in contrast post depositional addition of u results in decreased 230th 238u activity ratios and apparently younger ages comparison with our modelling results suggests that diagenesis occurred early after deposition initial and post depositional detrital contamination probably played a minor role for the observed age inversions

evaluating u series tools for weathering rate and duration on a soil sequence of known ages four soil profiles of known age 40 250 600 and 3000 ka from the merced soil chronosequence in california were analysed for u and th isotopes and for major and trace element compositions to test the assumption that leaching of u series isotopes is a first order process with an invariant rate constant as frequently assumed when applying this isotopic tool to determine weathering duration and sediment residence time average 230 th 238 u values for each profile decrease from a high of 1 28 in the youngest soil 40 ka to 1 09 0 99 and 0 98 in the 250 ka 600 ka and 3000 ka soils respectively 234 u 238 u values also show variation across the 4 soils the simultaneous variation in 230 th 238 u and 234 u 238 u cannot be explained by a first order kinetic model with invariant rate constants and instead requires that leaching coefficients for the u series isotopes change with time the observed data is consistent with a two stage model that reflects an initial period of fast leaching of u and th from a readily weathered soil pool such as grain boundaries or defects followed by slow leaching from a more resistant soil pool application of such a two stage model may lead to significantly shorter calculated timescales of weathering than those obtained by the widely applied single stage model of u series weathering which assumes no change in leaching coefficients over time the results of this study have wide implications for published and future use of u series tools in weathering studies

chemical weathering rates of a soil chronosequence on granitic alluvium iii hydrochemical evolution and contemporary solute fluxes and rates although long term changes in solid state compositions of soil chronosequences have been extensively investigated this study presents the first detailed description of the concurrent hydrochemical evolution and contemporary weathering rates in such sequences the most direct linkage between weathering and hydrology over 3 million years of soil development in the merced chronosequence in central california relates decreasing permeability and increasing hydrologic heterogeneity to the development of secondary argillic horizons and silica duripans in a highly permeable younger soil 40 kyr old pore water solutes reflect seasonal to decadal scale variations in rainfall and evapotranspiration et this climate signal is strongly damped in less permeable older soils 250 to 600 kyr old where solutes increasingly reflect weathering inputs modified by heterogeneous flow elemental balances in the soils are described in terms of solid state exchange and pore water reservoirs and input output fluxes from precipitation et biomass solute discharge and weathering solute mineral nutrients are strongly dependent on biomass variations as evidenced by an apparent negative k weathering flux reflecting aggradation by grassland plants the ratios of solute na to other base cations progressively increase with soil age discharge fluxes of na and si when integrated over geologic time are comparable to solid state mass losses in the soils implying similar past weathering conditions similarities in solute and sorbed ca mg ratios reflect short term equilibrium with the exchange reservoir long term consistency in solute ratios when contrasted against progressive decreases in solid state ca mg requires an additional ca source probably from dry deposition amorphous silica precipitates from thermodynamically saturated pore waters during periods of high evapotranspiration and result in the formation of duripans in the oldest soils the degree of feldspar and secondary gibbsite and kaolinite saturation varies both spatially and temporally due to the seasonality of plant respired co2 and a decrease in organically complexed al in deeper pore waters k feldspar is in equilibrium and plagioclase is about an order of magnitude undersaturated hydrologic heterogeneity produces a range of weathering gradients that are constrained by solute distributions and matrix and macropore flow regimes plagioclase weathering rates based on precipitation corrected na gradients vary between 3 and 7 10 16 mol m 2 s 1 these rates are similar to previously determined solid state rates but are several orders of magnitude slower than for experimental plagioclase dissolution indicating strong inhibitions to natural weathering partly due to near equilibrium weathering reactions

the mechanism of 234u238u activity ratio enhancement in karstic limestone groundwater departamento de petrologia e metalogenia universidade estadual paulista campus de rio claro unesp sao paulo

the north american hydrologic cycle through the last deglaciation abstract while the climate evolution of north america during the last deglaciation has received considerable attention few detailed model data comparisons of the deglacial hydroclimate have been conducted at the continental scale here we use a transient climate simulation of the last deglaciation and a synthesis of hydroclimate proxies from across the continent to broadly assess the moisture budget and its evolution including the primary components and mechanisms for changes and evaluate the level of agreement between model and data in order to inform areas of major discrepancy for attention in future work at the broadest scale the simulation indicates that north america was wetter than modern through much of the last deglaciation with the exception of the pacific northwest this was principally due to increased moisture convergence by the mean flow over the west of the continent and by transient eddies over the east we find that the simulation demonstrates considerable skill in comparison to proxy records in the coastal southwest and pacific northwest with limited success in other regions the highest disagreement occurs in the north central region where the model and data disagree on the sign of changes in addition we find that the areas of agreement coincide with regions whose hydroclimate is dominated by winter precipitation while regions of disagreement are those where summer precipitation is predominant and where the model suggests dynamical changes are important these results illustrate the impact of the ice sheet on the atmospheric dynamics and therefore the importance of its accurate reconstruction as well as that of model resolution our analysis and synthesis provide context for future transient climate simulations and suggest numerous areas of priority for future research

pedogenesis on the uplands of the diamantina plateau minas gerais brazil a chemical and micropedological study abstract little is known about the soil distribution on quartzitic uplands of brazil the diamantina plateau where deeply weathered oxisols latosols and shallow sandy soils occur side by side represents part of the higher and oldest planation on the quartzitic uplands of minas gerais state we studied representative soils from the diamantina plateau to help understand the pedogenesis and landscape evolution of this part of minas gerais we found evidence of a lithological and tectonic control on the distribution of soils in the landscape polygenetic oxisols acrustox occur at the top of the landscape at altitudes of 1300 m or more resting uncomformably on quartzitic saprocks xanthic types or mafic rocks rhodic types they change downslope to in situ quartzpsamments and haplorthods in the b horizon of the xanthic acrustox quartz grains smaller than 0 3 mm are surrounded by gibbsitic plasma forming subspherical microaggregates microaggregates in the bw horizon of all upland oxisols display inherited features of intense biological activity regardless of the parent material or texture the nutrient poor catena of xanthic acrustox ustic quartzpsamments oxyaquic haplorthods represents a sequence of increasing hydromorphic conditions downwards in which clay minerals are destroyed in low ph and ferrolysis in the poorly drained soils downslope low ph and eh induce the movement of reduced fe 2 and organic matter om bound al and fe both per descendum and through lateral flow downslope in the bottom micromorphological features and the chemistry of organic matter indicated that some bsh horizons represent buried a horizons subjected to post burial podzolization corroborating morphological field data this indicates the pertinence of the study of humic fractions in defining the bhs spodic horizons of brazilian spodosols

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

open system u series ages of corals from a subsiding reef in new caledonia implications for sea level changes and subsidence rate abstract on the amedee islet 4 drill cores were recovered from the barrier reef of western new caledonia the coral reef is slowly subsiding and is thus percolated by sea water during sea level highstands the cores sample a 10 m thick holocene reef overlying a 24 m thick reef of marine isotope stage mis 5 5 which in turn overlies older reef material from mis 7 5 and beyond 234 u 238 u and 230 th 238 u ratios and 232 th were determined by thermal ionization mass spectrometry on aragonitic coral samples that were carefully investigated using x ray diffraction and scanning electron microscopy the petrographic study shows an increasing coral weathering with growing coral age that causes different degree of u series open system behavior and 232 th accumulation holocene corals exhibit a small degree of early diagenesis and yield 230 th 238 u ages according to the holocene sea level rise from 8200 years to 5000 years bp corals from the last interglacial section have experienced more frequent replacement of aragonite fibers and minor dissolution and u series open system behavior is evident to estimate the impact of recoil processes and alteration on the u series system two models by villemant and feuillet b villemant n feuillet dating open systems by the 238 u 234 u 230 th method application to quaternary reef terraces earth and planetary science letters 210 2003 105 118 and thompson et al w g thompson m w spiegelman s l goldstein r c speed an open system model for u series age determinations of fossil corals earth and planetary science letters 210 2003 365 381 have been tested these models yield identical ages within uncertainty which are in agreement to the sea level history of the past 250 000 years as long as physico chemical alteration and re crystallization is small consequently we were able to estimate the subsidence rate from the subsidence observed between the end of mis 5 5 and the early holocene which is 0 16 0 04 m per 1000 years in addition by using this subsidence rate and the sea level highstand of 6 3 m during mis 5 5 we can estimate the sea level highstand at 244 000 mis 7 5 to be between 2 m to 2 m respectively corals from underneath the marine isotope stage 7 5 sequence show more significant dissolution increasing re crystallization elevated 232 th and excess 234 u and 230 th here the villemant and feuillet b villemant n feuillet dating open systems by the 238 u 234 u 230 th method application to quaternary reef terraces earth and planetary science letters 210 2003 105 118 model fails to predict ages but the thompson et al w g thompson m w spiegelman s l goldstein r c speed an open system model for u series age determinations of fossil corals earth and planetary science letters 210 2003 365 381 model was applicable predicting ages between 355 000 to 430 000 years i e coral growth potentially during mis 9 or 11

uranium series comminution ages of continental sediments case study of a pleistocene alluvial fan obtaining quantitative information about the timescales associated with sediment transport storage and deposition in continental settings is important but challenging the uranium series comminution age method potentially provides a universal approach for direct dating of quaternary detrital sediments and can also provide estimates of the sediment transport and storage timescales the word comminution means to reduce to powder reflecting the start of the comminution age clock as reduction of lithic parent material below a critical grain size threshold of 50 mu m to test the comminution age method as a means to date continental sediments we applied the method to drill core samples of the glacially derived kings river fan alluvial deposits in central california sediments from the 45 m core have independently estimated depositional ages of up to 800 ka based on paleomagnetism and correlations to nearby dated sediments we characterized sequentially leached core samples both bulk sediment and grain size separates for u nd and sr isotopes grain size surface texture and mineralogy in accordance with the comminution age model where 234u is partially lost from small sediment grains due to alpha recoil we found that 234u 238u activity ratios generally decrease with age depth and specific surface area with depletions of up to 9percent relativemore to radioactive equilibrium the resulting calculated comminution ages are reasonable although they do not exactly match age estimates from previous studies and also depend on assumptions about 234u loss rates the results indicate that the method may be a significant addition to the sparse set of available tools for dating detrital continental sediments following further refinement improving the accuracy of the method requires more advanced models or measurements for both the recoil loss factor fa and weathering effects we discuss several independent methods for obtaining fa on individual samples that may be useful for future studies less

in situ radionuclide transport and preferential groundwater flows at ineel idaho decay series disequilibrium studies uranium and thorium decay series disequilibria in groundwater occur as a result of water rock interactions and they provide site specific natural analog information for assessment of in situ long term migration of radionuclides in the far field of a nuclear waste disposal site in this study a mass balance model was used to relate the decay series radionuclide distributions among solution sorbed and solid phases in an aquifer system to processes of water transport sorption desorption dissolution precipitation radioactive ingrowth decay and recoil isotopes of u 238u 234u th 232th 230th 228th 234th ra 226ra 228ra 224ra and rn 222rn were measured in 23 groundwater samples collected from a basaltic aquifer at the idaho national engineering and environmental laboratory ineel idaho the results show that groundwater activities of th and ra isotopes are 2 4 orders lower than those of their u progenitors which average 1 35 0 40 dpm 238u l with 234u 238u ratios of 1 6 3 0 222rn activities range from 20 to 500 dpm l modeling of the observed disequilibria places the following constraints on the time scale of radionuclide migration and water rock interaction at ineel 1 time for sorption is minutes for ra and th time for desorption is days for ra and years for th and time for precipitation is days for th years for ra and centuries for u 2 retardation factors due to sorption average 106 for 232th 104 for 226ra and 103 for 238u 3 dissolution rates of rocks are 70 to 800 mg l y 4 ages of groundwater range from 10 to 100 years contours of groundwater age as well as spatial patterns of radionuclide disequilibria delineate two north south preferential flow pathways and two stagnated locales relatively high rates of dissolution and precipitation and recoil of 222rn occur near the groundwater recharging sites as well as in the major flow pathways decay of the sorbed parent radionuclides e g 226ra and 228ra on micro fracture surfaces constitutes an important source of their daughter 222rn and 228th activities in groundwater

230th u dating of surficial deposits using the ion microprobe shrimp rg a microstratigraphic perspective abstract we used the sensitive high resolution ion microprobe reverse geometry shrimp rg to date pedogenic opal using the 230 th u system due to the high spatial resolution of an ion microprobe typically 30 m regions of pure opal within a sample can be targeted and detrital material can be avoided in addition because the technique is non destructive the sample can be preserved for other types of analyses including electron microprobe or other stable isotope or trace element ion microprobe measurements the technique is limited to material with u concentrations greater than 50 ppm however the high spatial resolution small sample requirements and the ability to avoid detrital material make this technique a suitable technique for dating many pleistocene deposits formed in semi arid environments to determine the versatility of the method samples from several different deposits were analyzed including silica rich pebble coatings from pedogenic carbonate horizons a siliceous sinter deposit and opaline silica deposited as a spring mound u concentrations for 30 m diameter spots ranged from 50 to 1000 ppm in these types of materials the 230 th 232 th activity ratios also ranged from 100 to 10 6 eliminating the need for detrital th corrections that reduce the precision of traditional u th ages for many milligram and larger sized samples in pedogenic material layers of high u opal ca 500 ppm are commonly juxtaposed next to layers of calcite with much lower u concentrations 1 2 ppm if these types of samples are not analyzed using a technique with the appropriate spatial resolution the ages may be strongly biased towards the age of the opal comparison with standard tims thermal ionization mass spectrometry measurements from separate microdrilled samples suggests that although the analytical precision of the ion microprobe shrimp rg measurements is less than tims the high spatial resolution results in better accuracy in the age determination for finely layered or complex deposits the ion microprobe approach also may be useful for pre screening samples to determine the age and degree of post depositional alteration analyzing finely layered samples or samples with complex growth histories and obtaining simultaneous measurements of trace elements

isotopic tracing sr nd u and hf of continental and marine aerosols in an 18th century section of the dye 3 ice core greenland determining the sources and pathways of atmospheric mineral dust and marine aerosols remains a difficult problem in order to address this problem the radiogenic isotopic composition of the dust and soluble ice components of greenland ice has been determined and used as a source tracer for mineral dust and marine aerosols sr and nd isotopic composition was measured with a yearly to seasonal resolution on both the bulk dust and soluble fractions of a section of the dye 3 1988 ice core from greenland in the age range of 1786 1793 a d hf isotopic composition was also measured for three of the dust samples as a complementary tracer of dust origin the first direct measurement of hf in paleo atmospheric dust measured nd and sr isotopic composition of the dust corrected for carbonate contributions are compared to literature potential source area psa data 6 e units and are decoupled from dust composition indicating that the nd composition of seawater is preserved in the ice the end of the ice suggests variable mixing of aerosols from arctic sea salts with another more radiogenic source during transport

middle pleistocene formation of the rio grande gorge san luis valley south central colorado and north central new mexico usa process timing and downstream implications abstract the rio grande is the fourth longest river in north america extending over 3 000 km from the rocky mountains to the gulf of mexico the pleistocene evolution of this river from individual subbasins into a coalesced fluvial system has been long debated herein we constrain the middle pleistocene evolution of the northernmost and largest rio grande basin the san luis basin and the timing of incision of the rio grande gorge based on new geologic mapping 3he surface exposure dating and u series dating on pedogenic carbonates 3he dating of shoreline deposits of closed basin lake alamosa and fluvially scoured bedrock along the rio grande gorge rim indicate the san luis basin was connected with southerly rio grande basins and progressively incised since 400 ka waning tectonic activity coupled with the hydrologic response to intensified middle pleistocene glaciation drove southward spillover of lake alamosa 400 ka expanding the river s drainage basin by nearly 24 000 km2 and adding recharge areas in the high altitude glaciated san juan tusas and sangre de cristo mountains geologic barriers to streamflow on the taos plateau were breached and basin tributary drainages were integrated and inset into their present canyons 250 200 ka with approximately 120 m of incision during 400 200 ka previous work demonstrates that southerly streamflow through basins south of the slb terminated into a large rift related bolson complex until the middle pleistocene and most axial rio grande river incision has occurred since 630 ka we propose that middle pleistocene integration of the slb was the major geomorphic event that eroded plio pleistocene tectonic and volcanic barriers and re established flow of the rio grande from the rocky mountains to the gulf of mexico

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

influence of eolian deposition and rainfall amounts on the u isotopic composition of soil water and soil minerals 230th u dating of secondary deposits such as speleothems and pedogenic carbonate and opal provides information about the original 234u 238u composition of the waters the minerals precipitated from as a result these types of deposits often display temporal variations in initial 234u 238u that could reflect environmental variability past variations in soil water 234u 238u may reflect changes in the flux of water through the soil weathering rates or dust inputs in order to evaluate these potential influences we analyzed modern soil water chemistry and u isotopic compositions of fluids and solids from three soil sequences in nevada modern soil pore waters from diamond valley newark valley and fish lake valley nevada show significant 234u 238u differences between sites as a result of differing soil parent material newark valley pore waters are more heavily influenced by carbonate weathering and display much more enriched 234u 238u values than diamond and fish lake valley pore waters which are dominated by silicate weathering newark valley and diamond valley although located very near to each other have significantly different pore water 234u 238u values suggesting that deposition of distantly sourced dust does not exert a dominant influence on pore water 234u 238u at these sites pore water 234u 238u values also vary systematically along a rainfall gradient at fish lake valley where sites with lower rainfall display higher soil water 234u 238u values the results of this modern soil water chemistry study were used to parameterize two models that investigate the relative influence of dust content recoil and weathering on pore water 234u 238u in the upper layer of the soil and at depth under changing infiltration rates these models suggest that while the composition of soluble dust entering a soil profile is an important consideration the pore water 234u 238u at depth and thus the initial 234u 238u of minerals that precipitate from pore waters should be highly sensitive to changes in the infiltration flux through the soils as infiltration flux is closely tied to rainfall the initial 234u 238u of soil minerals at these sites should provide information about past rainfall variability

dating fluvial terraces by 230th u on pedogenic carbonate wind river basin wyoming abstract reliable and precise ages of quaternary pedogenic carbonate can be obtained with 230th u dating by thermal ionization mass spectrometry applied to carefully selected milligram size samples datable carbonate can form within a few thousand years of surface stabilization allowing ages of quaternary deposits and surfaces to be closely estimated pedogenic carbonate clast rinds from gravels of glacio fluvial terraces in the wind river basin have median concentrations of 14 ppm u and 0 07 ppm 232th with median 230th 232th 270 making them well suited for 230th u dating horizons as thin as 0 5 mm were sampled from polished slabs to reduce averaging of long 105 yr and sometimes visibly discontinuous depositional histories dense translucent samples with finite 230th u ages preserve within rind stratigraphic order in all cases ages for terraces wr4 167 000 6 400 yr and wr2 55 000 8600 yr indicate a mean incision rate of 0 26 0 05 m per thousand years for the wind river over the past glacial cycle slower than inferred from cosmogenic nuclide dating terrace wr3 which formed penecontemporaneously with the final maximum glacial advance of the penultimate rocky mountain bull lake glaciation has an age of 150 000 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum

an evaluation of paired δ18o and 234u 238u 0 in opal as a tool for paleoclimate reconstruction in semi arid environments abstract authigenic opal deposits are widespread in semi arid and arid environments and often contain high uranium concentrations that facilitate high precision dating by u series and u pb geochronology however despite their ubiquity and utility for placing temporal constraints on surficial processes the potential for authigenic opal deposits to be used as archives of past climate variability remains largely unexplored compared with more frequently used authigenic carbonate deposits here we investigate the potential for authigenic opal deposits to provide records of paleoclimate and paleohydrologic variability through the development of a combined record of uranium 234 u 238 u 0 and oxygen 18 o isotope variability using in situ ion microprobe methods on opal deposits from pinnacles national park california that precipitated from approximately 16 to 3 ka pinnacles opal 234 u 238 u 0 suggests a moderate decrease in net infiltration from the early to middle holocene coincident with the timing of increasing aridity inferred from sierra nevada lake records while pinnacles opal 18 o indicates a relative increase in subtropical storms at the same time a shift to more north pacific sourced storms commenced after 5 ka coincident with the easing of aridity in northern california and increased storminess in the sierra nevada as indicated by lake sediment records these results suggest that useful insights about past hydrologic change can be gleaned from authigenic opal deposits and that these deposits have the potential to enhance the spatial and temporal coverage of paleoclimate reconstructions in regions where other archives might be scarce however further study of the influence of water in opal on 18 o variability and additional in situ analysis of 18 o variability on longer lived opal deposits with uniform growth histories are necessary to verify the controls on this potential proxy similarly further investigation into short term and spatial variability in 234 u 238 u sw will improve qualitative and quantitative applications of 234 u 238 u 0 in authigenic opal deposits enhancing our ability to use this promising new paleoclimate archive

sediment transport time measured with u series isotopes results from odp north atlantic drift site 984 abstract high precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition the approach is demonstrated on fine grained late pleistocene deep sea sediments from ocean drilling program site 984a on the bjorn drift in the north atlantic the sediments are siliciclastic with up to 30 carbonate and dated by 18o of benthic foraminifera nd and sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods volcanic rocks from iceland and a distal continental source during glacial periods an unexpected finding is that the 234u 238u ratios of the silicate portion of the sediment isolated by leaching with hydrochloric acid are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance the 234u depletions are inferred to be due to recoil loss of 234th and are used to calculate comminution ages of the sediment the time elapsed between the generation of the small 50 m sediment grains in the source areas by comminution of bedrock and the time of deposition on the seafloor transport times the difference between comminution ages and depositional ages vary from less than 10 ky to about 300 to 400 ky for the site 984a sediments long transport times may reflect prior storage in soils on continental shelves or elsewhere on the seafloor transport time may also be a measure of bottom current strength during the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from iceland that is rapidly transported to the site of deposition the comminution age approach could be used to date quaternary non marine sediments soils and atmospheric dust and may be enhanced by concomitant measurement of 226ra 230th 230th 234u and cosmogenic nuclides

petrography strontium barium and uranium concentrations and strontium and uranium isotope ratios in speleothems as palaeoclimatic proxies soreq cave israel the reconstruction of the palaeoclimate of the eastern mediterranean region for the last 60 ka bp is based on the 18o and 13c variations of speleothems from soreq cave israel climatic conditions during most of the time interval between 60 and 17 ka bp the period equivalent to the last glacial were relatively cold and dry while they were warmer and wetter from 17 ka bp to the present at 17 ka bp there was a major climatic change with a sharp increase in annual rainfall and temperature and a very wet period occurring between 8 5 and 7 0 ka bp during the colder and drier period large detritus free preferentially oriented calcite crystals were deposited from slow moving water as a result of a sharp change in the hydrological regime at 17 ka bp fast moving water started entrainment of the soil and carrying detrital material into the cave and the calcite crystals deposited became small and anhedral coinciding with the petrographic and isotopic changes a sharp drop occurred in the concentration

importance of atmospheric inputs and fe oxides in controlling soil uranium budgets and behavior along a hawaiian chronosequence abstract a long term budget of uranium calculated for a chronosequence of hawaiian soils demonstrates the importance of mineral aerosol deposition as a geochemical pathway a series of sequential extractions on the same soils shows a strong association between fe oxides and uranium especially the amorphous and poorly crystalline fe oxides present in intermediate age soils 20 and 150 ka despite intense leaching conditions soil u contents were found to increase between 20 fold and 60 fold with age along the chronosequence from 9 to 17 g cm 2 at 0 3 ka sites to 344 543 g cm 2 at the 4100 ka site an open system transport function comparing u to an index least mobile element nb was used to calculate u gains or losses from the soils relative to the input from parent material weathering results show small losses or small gains 25 of u occurred in soils 150 ka and younger while gains of up to 531 u occurred in soils older than 150 ka highlighting the importance of mineral aerosol dust deposition at the older sites in the 4100 ka soil the ratio of dust derived u inputs to parent material derived u inputs is approximately 6 sequential extractions show that between 0 1 and 1 2 of the total u in the soils was extracted with ammonium acetate indicating that very little u exists as exchangeable cations a second extraction employing citrate dithionate and hydroxylamine hydrochloride was performed to extract u associated with fe oxides average percent occurrences for u in the fe oxide extraction for each profile are 7 at 0 3 ka 53 at 20 ka 40 at 150 ka and 17 at 4100 ka illustrating an increase with age to 20 ka followed by a subsequent decline this parallels the trend observed in amorphous fe oxide content suggesting that adsorption on amorphous fe oxides is the dominant control on u in the intermediate age soils

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

a late pliocene to middle pleistocene pluvial lake in fish lake valley nevada and california the question of whether a pluvial lake existed in fish lake valley nevada and california has been debated for more than 100 yr new stratigraphic evidence indicates that a lake did exist in this valley at intervals during late pliocene to middle pleistocene time this lake may have drained northward or it may have been periodically contiguous with a pluvial lake to the north in columbus salt marsh proof of the existence of this lake informally named pluvial lake rennie is derived from three principal outcrops of shallow water deposits two outcrops of deep water deposits and several drilling logs the deposits contain beds of silicic tephra which provide age control on the basis of thickness grain size major oxide chemistry of glass shards and paleomagnetism three of the shallow water deposits including deltaic beach and siliceous hot spring sediments consist mainly of bishop ash derived from the 0 77 ma eruption of the long valley caldera a fourth shallow water deposit is associated with 1 ma glass mountain tephra beds the exposed deep water deposits consist of green claystone siltstone and fine grained sandstone containing tephra derived from the eruptions of the 2 1 ma tuff of taylor canyon and the 2 0 ma huckleberry ridge tuff the drilling logs record numerous thick beds of clay and sandy clay inferred to be deep water lacustrine deposits pluvial lake rennie fluctuated in size and depth beginning prior to 2 ma and continuing until sometime alter 0 77 ma at about 0 77 ma the lake had a highstand at an elevation of 1 460 m covered an area of 400 500 km2 and had a maximum depth of 250 m the lake level dropped just after the eruption of the bishop ash but the lake may have persisted at a lower level until 0 5 ma no large long lived lake existed in fish lake valley in late pleistocene time probably due to the increasing rain shadow effect caused by the relative uplift of the white mountains and sierra nevada in the pleistocene these results indicate that the late middle to late pleistocene history of pluvial lake rennie is similar to that of lake tecopa but is quite different from those of lake lahontan and searles lake

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

experimental evidence for 234u 238u fractionation during granite weathering with implications for 234u 238u in natural waters abstract the daughter to parent 234u 238u activity ratio in natural waters is often out of secular radioactive equilibrium the major reason for this disequilibrium is related to the energetic decay of 238u and differential release of 234u relative to 238u this disequilibrium originates from 1 preferential release of more loosely bound 234u from damaged mineral lattice sites or 2 direct recoil of 234th into surrounding media from near mineral surface boundaries however it is unclear which of the two mechanisms is most important in nature to better quantify the effects of preferential release of 234u two continuous laboratory granite leaching experiments conducted over 1100 h were performed the leachates were characterized by declining u concentrations with time and 234u 238u initially greater than unity up to 1 15 which changed to below unity during leaching 0 95 the early elevated 234u 238u suggests that additional 234u is released into solution by preferential release of 234u from mineral phases however the excess 234u constitutes a finite pool of easy leachable 234u and the 234u 238u values become lower than unity when this pool is used up a model based on first order kinetics dissolution rates and preferential release of 234u from damaged lattice sites was developed and is able to quantitatively predict the observed pattern of 234u 238u values and u concentrations for the two granite leaching experiments extending the modeling to longer time scales more comparable to natural systems shows that the production of waters with high 234u 238u ratios can be achieved in two distinct regimes 1 slow weathering where the rate of directly recoiled 234u near mineral surfaces into waters is high 2 fast weathering where the role of incipient chemical weathering and preferential release of loosely bound 234u are important the model is able to explain apparent opposite correlations between physical erosion rates and 234u 238u in waters and it provides a new framework that will be useful for examining weathering regimes their timescales and their coupling with physical erosion

uranium isotopes in soils as a proxy for past infiltration and precipitation across the western united states the intermittent presence of large pleistocene lakes in the southwestern interior of north america a region that is now a semi arid desert suggests repeated oscillations between profoundly different climatic conditions the origin of these shifts is still unresolved due to inconsistencies in existing climate proxy data for example pollen lake levels and oxygen isotopes in speleothems to resolve the inconsistencies in the water balance over the last 10 to 60 kyr we use uranium isotopic variations in secondary soil minerals to quantify net infiltration and precipitation along a north south transect in western north america we show that winter infiltration increased by 30 to 100 percent and precipitation by a lesser amount in the valleys of the great basin and mojave deserts between 60 and 26 ka this increase in infiltration and precipitation preceded the last glacial maximum lgm and the timing of most lake highstands in the region by 5 to 10 kyr respectively suggesting a possible last precipitation maximum lpm that coincided with a minimum in winter insolation subsequent decreases in infiltration and precipitation after the lgm can be reconciled with the timing of lake highstands if colder summer temperatures due to a minimum in summer insolation reduced lake evaporation the soil records combined with a range of proxy data suggest that seasonal insolation is the long term driver for large shifts in both precipitation and surface water variability in the region

mid holocene drying of the u s great basin recorded in nevada speleothems lake level records point to dramatic changes in great basin water balance over the last 25 ka but the timing and pace of holocene drying in the region remains poorly documented here we present stable isotope and trace metal data from two lehman caves nv speleothems that provide a well dated record of latest pleistocene to mid holocene hydroclimate in the u s great basin together the stalagmites span the interval between 16 4 ka and 3 8 ka with a hiatus from 15 0 ka to 12 7 ka mg ca and d 13 c covary throughout the records consistent with control by the extent of degassing and prior calcite precipitation pcp measurements of modern cave and soil waters support pcp as the primary control on drip water trace element composition we therefore interpret mg ca and d 13 ca s reflecting infiltration rates with higher values corresponding to drier periods both mg ca and d 13 c indicate a wet period at the beginning of the record 12 7e8 2 ka followed by pronounced drying after 8 2 ka this mid holocene drying is consistent with records from around the western united states including a new compilation of great basin lake level records the strong temporal correspondence with the collapse of the laurentide ice sheet over hudson bay suggests that this drying may have been triggered by northward movement of the winter storm track as a result of ice sheet retreat however we cannot rule out an alternative hypothesis that wet early holocene conditions are related to equatorial pacific sea surface temperature regardless our results suggest that great basin water balance in the early holocene was driven by factors other than orbital changes

surface ages and weathering rates from 10be meteoric and 10be 9be insights from differential mass balance and reactive transport modeling abstract to reconstruct the dynamic evolution of weathered material at the earth s surface requires techniques that record the fluxes of material into and out of the weathering zone cosmogenic isotopes formed in situ within the earth s surface or in the atmosphere meteoric are important chronometers for both time and fluxes meteoric 10 be has been used to evaluate surface ages soil residence times and rates of sediment transport however the meteoric 10 be dating tool relies on complete retention of 10 be on mineral surfaces within the weathered regolith an assumption that will not be valid where soil ph is low or mineral surface area available for sorption is lacking here we evaluate whether the stable isotope of 9 be that is released upon weathering reactive be can be used to correct for 10 be mobility this approach relies on the inventory of reactive 9 be in the weathering zone as a measure of the mass depletion whereas the 10 be inventory is a function of age to explore the spatial and temporal relationship between the be isotopes we use a multi component reactive transport model framework to evaluate a set of differential mass balance equations for the 10 be 9 be composition of the bulk solid and sorbed reservoirs on non eroding surfaces we first find that the 10 be 9 be reac is inversely proportional to the long term chemical weathering rate even in cases of incomplete be retention second we find that coupling 10 be and 9 be to provide a tool for surface age dating 10 be is complex because incomplete retention affects each isotope to a different extent beryllium 10 is supplied at the surface of soil where sorption onto secondary minerals is more extensive while 9 be is released from primary minerals at the weathering front where secondary minerals are less abundant if 9 be loss is less than about 15 we find 9 be inventories can be used to reliably correct 10 be inventories to within 10 the error associated with the correction also increases with increasing age and thus the correction is more favorable for younger surfaces over longer timescales the error on the age introduced by the loss correction using 9 be is magnified whereas the error associated with uncorrected ages decreases because the 10 be is more strongly retained as chemical weathering progresses we also find that the reactive transport model confirms a set of simple mass balance descriptions providing guidance for future data collection collectively the modeling results suggest that measurement of be isotopes in weathered regolith provides a unique approach to determine both surface ages and chemical weathering rates

the transport of u and th series nuclides in sandy confined aquifers a comprehensive evaluation of the transport of u th ra and rn nuclides of the 238 u and 232 th decay series in an unconfined sandy aquifer long island ny was conducted groundwater data are compared with results of a theoretical transport model of weathering of aquifer grains and interaction with surface coatings to establish relationships between the concentrations of the radionuclide activities in the water and flow line distance the data provide estimates for geochemical parameters including weathering rates and chemical reactivities in both the vadose zone and the aquifer a theoretical treatment of the transport is presented that considers the reaction between the water and a reactive surface layer it is found that a model with chemical exchange between the surface layer and the water for all species is not valid and that the effects of saturation and irreversible precipitation of th is required the water table shows a relatively wide range in u activities the only element in the u th series for which vadose zone input is significant in the aquifer high weathering of u and recoil inputs of 234 u to the water occur in the upper 3 m of the vadose zone while lower weathering and removal of u from the water occur below the deeper aquifer has variable 238 u activities that can be accounted for by input from the vadose zone and is not a result of non conservative behavior the isotopic composition of u is shown to be directly related to the recoil rate relative to the weathering rate the wide range of 238 u in the aquifer waters is a reflection of diverse vadose zone inputs showing that dispersive mixing is not a dominant effect the higher values of 234 u in the aquifer reflect the recoil weathering input ratios from within the aquifer where the weathering rate is lower than the vadose zone both high u activities and high 234 u values cannot be obtained in the vadose zone or within reasonable flow distances in the aquifer radium isotopes are found to be in exchange equilibrium with the surface layer 224 ra 228 ra and 226 ra have comparable activities throughout the aquifer in the vadose zone the dominant input of ra to groundwater is weathering and recoil as found elsewhere the 222 rn in the water is a large fraction 5 of the rn produced in the aquifer rock this cannot be due to ra precipitation onto surface coatings in the aquifer as supported by present weathering with th in exchange equilibrium with the surface layer it is found that th is saturated in the waters under oxidizing conditions so that the weathering input is irreversibly precipitating onto surfaces however it is shown that under somewhat reducing conditions th activities are much higher and the th u ratio in the solution is approximately that of the rock we propose that under oxidizing conditions the source of rn is a surface coating enriched in 232 th and 230 th this th was precipitated in an earlier phase during rapid dissolution of readily weathered phases that contain 10 of the u th inventory of the rock with the associated u carried away in solution therefore the previously precipitated 230 th and 232 th produce daughter nuclides in the surface coating which are the dominant contributors of ra and rn to the ground water in particular rn is provided by very efficient losses by diffusion or recoil from the surface coating this then does not require recent large recoil losses from the parent rock or the presence of nanopores in the rock the first data of both long lived 232 th and short lived 234 th and 228 th in ground water is reported the th isotope activities indicate that desorption kinetics are slow and provide the first estimate based on field data of the th desorption rate from an aquifer surface the mean residence time of th in the surface coating is 3000 y while in the water it is 1 h ra is in partition equilibrium with the aquifer surface layer however the strong fixation of th on surface coatings is very susceptible to changes in oxidation state as is shown by a comparison of two adjacent aquifers this makes it difficult to define with certainty the retentive characteristics in natural systems in general it is shown that the distributions of naturally occurring nuclides can be used to calculate values for transport parameters that are applicable to the transport of anthropogenic nuclides

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

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

radiometric isotopic and petrographic evidence of changing interglacials over the past 550 000 years from six stalagmites from the serra do courel in the cordillera cantabrica of northwestern spain abstract seventy u th ages from six stalagmites from the cordillera cantabrica in northwestern spain indicate deposition during interglacial but not glacial stages over the last 550 kyr stable isotope data and petrographic observations suggest that within each interglacial conditions typically progressed from drier to wetter consideration of the most precise u th ages suggests that stalagmite deposition occurred in times more predictable from european pollen records than from the global marine isotope record for example during the last interglacial ages suggest deposition in an interval better characterized as the eemian as opposed to mis 5e and during the st germain i or mis 5c interval the same holds for mis 7 where ages suggest deposition during mis 7a and mis 7c but not mis 7e these age ranges for interglacials consistently later than those suggested by the benthic marine record are compatible with warm conditions maintained by thermohaline circulation that simultaneously supplied the moisture for growing ice sheets of the subsequent glaciation trends both within and across these six stalagmites suggest that cooler temperatures were associated with wetter conditions both during interglacials and in conditions intermediate between interglacials and glacials in northern spain in contrast other researchers results from a stalagmite from the last glacial maximum and subsequent deglaciation suggest colder temperatures were associated with drier conditions the two trends combine to suggest wettest conditions in times intermediate between interglacials and glacials compatible with a model in which modern climatic belts from dry arctic to wet european to dry marginal saharan have moved over northern spain in the cycle from glacial to interglacial times extrapolation to a warmer future would suggest drier conditions in the northern iberian peninsula than those of today

lignite oxidation under the influence of glacially derived groundwater the pyropissite deposits of zeitz weisenfels germany abstract the middle eocene lignites of the zeitz weisenfels coal mining area in central germany hosted one of the most economically successful lithotypes of lignite deposits utilised in the paraffin industry of the 19th century i e the pyropissite deposits however due to their economic significance these rare lithotypes were almost completely mined out such that presently only a few remnant deposits are known apart from the zeitz weisenfels coal mining district other pyropissite deposits were also encountered in other lignite mines for instance in the hessian and subhercynian basins pyropissite is a whitish and bitumen rich variety of soft brown coal lignite lithotypes dominated by a homogeneous matrix with loosely embedded organic and inorganic components fresh exposure of pyropissite at grana was logged and studied petrographically and geochemically along with samples obtained from archival collections results were compared with those from the 19th and 20th century to derive general conclusions regarding origin and formation of pyropissite microscopically the main constituent of this special lignite lithotype is amorphous to detrital in appearance according to w schneider the term xanthinite is applied to this component which forms the groundmass of the pyropissite it contains high proportions of liptinitic substances with contributions of formless huminitic material and minor amounts of mineral matter geochemically pyropissite is characterised by an increased h c atomic ratio and a very high content of bitumen in terms of toluene soluble components as well as low temperature carbonisation tar the obtained data revealed that the extraordinarily high content of liptinite and the paucity of huminitic material of the pyropissite deposits of the zeitz weisenfels coal mining area are not related to primary depositional processes but rather are the result of dissipation of high condensed huminites and the consequent enrichment of liptinitic substances the mechanism involves oxidation by groundwater and is related to glaciation processes operating during the pleistocence such glacially derived processes also account for both vertical and horizontal distributions of the pyropissite thus recent research activities may also serve as an innovative inspiration to fundamental understanding of the formation of lignites with extremely high liptinite content