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

seasonal variability of element fluxes in two central siberian rivers draining high latitude permafrost dominated areas in order to constrain the origin and fluxes of elements carried by rivers of high latitude permafrost dominated areas major and trace element concentrations as well as sr and u isotopic ratios were analyzed in the dissolved load of two siberian rivers kochechum and nizhnyaya tunguska regularly sampled over two hydrological cycles 2005 2007 large water volumes of both rivers were also collected in spring 2008 in order to perform size separation through dialysis experiments this study was completed by spatial sampling of the kochechum watershed carried out during summer and by a detailed analysis of the main hydrological compartments of a small watershed from element concentration variations along the hydrological cycle different periods can be marked out matching hydrological periods during winter baseflow period october to may there is a concentration increase for major soluble cations and anions by an order of magnitude the spring flood period end of may beginning of june is marked by a sharp concentration decrease for soluble elements whereas dissolved organic carbon and insoluble element concentrations strongly increase when the spring flood discharge occurs the significant increase of aluminum and iron concentrations is related to the presence of organo mineral colloids that mobilize insoluble elements the study of colloidal ree reveals the occurrence of two colloid sources successively involved over time spring colloids mainly originate from the uppermost organic rich part of soils whereas summer colloids rather come from the deep mineral horizons furthermore u and sr isotopic ratios together with soluble cation budgets in the kochechum river impose for soluble elements the existence of three distinct fluxes over the year a at the spring flood a surface flux coming from the leaching of shallow organic soil levels and containing a significant colloidal component b a subsurface flux predominant during summer and fall mainly controlled by water rock interactions within mineral soils and c a deep groundwater flux predominant during winter which enters large rivers through unfrozen permafrost paths detailed study of the kochechum watershed suggests that the contribution of this deep flux strongly depends on the depth and continuous nature of the permafrost

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

particle transport of 234u 238u in the kalix river and in the baltic sea the role of particles for u isotope transport was investigated in the kalix river watershed a particle poor fe mn rich river in northern sweden and in the baltic sea estuary particles 0 45 m are strongly enriched in u and contain 20 50 of the total riverine uranium budget and 1 of the total u in brackish waters 3 7 psu the particles have high 234 u which is close to that of dissolved u in the associated water indicating that u on particles is dominantly nondetrital and isotopically exchanges rapidly with the ambient dissolved u particles at the river mouth are dominated by nondetrital fe mn oxyhydroxides uranium and fe are strongly correlated clearly demonstrating that secondary fe oxyhydroxide is the major carrier of u in river water there is no evidence for significant association of u with mn oxyhydroxide apparent u distribution coefficients k d fe were calculated for u between the authigenic fe on particles and the solution these values appear to be relatively constant throughout the year this suggests an equilibrium between fe in solution and authigenic fe oxyhydroxides on detrital particles high values of k d fe calculated for one summer as well as high u concentrations in brackish waters can be explained by u scavenging by biogenic phases with low authigenic fe content

transport of u and th series nuclides in a baltic shield watershed and the baltic sea the transport of th ra and 210 pb from a continental source region and through an estuarine environment was investigated unlike previous studies here both short and long lived nuclide data were obtained for river inputs river water and estuarine waters the mire rich kalix river drainage basin was chosen as a typical northern shield area because this river may represent typical waters flowing into the arctic and northern seas groundwaters from bedrock and glacial tills have comparable th isotope concentrations and do not exhibit significant th isotopic shifts relative to host rocks the extensive peat deposits of the basin receive groundwater discharges and concentrate th and u but not ba and ra which cause high 230 th 232 th ratios in mire waters however mire outflows do not have a significant impact on th and ra isotopic compositions of the river overall weathering characteristics for the basin are obtained from the river data the 230 th 232 th 228 ra 226 ra and 226 ra ba river ratios are comparable to those of source rocks consistent with similar release rates of these nuclides from u th and ba bearing minerals river ratios of 230 th 238 u ar and 226 ra 238 u ar are 1 so that th and possibly ra are accumulating in the weathering regions and the weathering profile is still evolving low 228 ra 232 th ar and 226 ra 230 th ar ratios indicate that th is preferentially retained over ra river 234 th 238 u ar ratios are greater than 230 th 238 u ar ratios and suggest that in systems where river inputs are well characterized these ratios can be used to calculate th transit times through the watershed filtration data indicate that although a dominant fraction of the th is transported in the river on particles the rest is almost entirely carried by colloids the kalix river discharges into the baltic sea model calculations for the transport of th and ra isotopes in the baltic sea show that the high ratios of 234 th 238 u ar found here reflect long th residence times relative to particle scavenging of 50 d the water column 232 th budget is dominated by eolian inputs the 226 ra concentrations may be higher than those of water inflows with 60 derived from underlying sediments a greater fraction of 228 ra is derived from sediments to balance the decay of 228 ra within the water column during the 35 yr residence time of water in the baltic the baltic 228 ra 226 ra ar ratios which are relatively constant over a range of salinities are fortuitously similar to those of the river inflows

transport and exchange of u series nuclides between suspended material dissolved load and colloids in rivers draining basaltic terrains this study presents uranium and thorium concentrations and activity ratios for all riverine phases bedload suspended load dissolved load and colloids from basaltic terrains in iceland and the azores small basaltic islands such as these are thought to account for 25 of co2 consumed by global silicate weathering and for 45 of the flux of suspended material to the oceans these data indicate that u and th in the dissolved and colloidal fractions are strongly controlled by ph and to a much lesser extent by levels of dissolved organic carbon which are low in these environments at high ph basalt glass dissolution is enhanced and secondary mineral formation e g fe oxyhydroxides and allophane is suppressed resulting in high dissolved u and low colloidal u and th indicating a direct chemical weathering control on elemental abundances when the dissolved 234u 238u activity ratio is 1 3 i e when physical weathering groundwater contribution or soil formation are high there is little isotope exchange between dissolved and colloidal fractions at lower activity ratios the dissolved load and colloids have indistinguishable activity ratios suggesting that when chemical weathering rates are high secondary clay formation is also high and colloids rapidly adsorb dissolved u many of the suspended sediment samples have 234u 238u activity ratios of 1 which suggests that uptake of u onto the suspended load is important identical 230th 232th in suspended dissolved and colloidal samples suggests that th like u is exchanged or sorbed rapidly between all riverine phases this particle reactivity combined with poorly constrained contributions from groundwater and hydrothermal water and short term variations in input to soils volcanic and glacial suggests that u series nuclides in riverine material from such basaltic terrains are unlikely to reflect steady state erosion processes

strontium dissolved and particulate loads in fresh and brackish waters the baltic sea and mississippi delta a study was conducted of the isotopic composition and concentration of sr and of major elements in dissolved and suspended loads of fresh and brackish waters the purpose was to establish the contributions of different parent rocks and minerals to sr during weathering and transport and to identify the role of fe mn oxyhydroxides in the redistribution of sr in the water column during the sedimentary cycle studies were conducted on a profile across an oxic anoxic boundary in the baltic and on rivers covering behavior over an annual cycle in general the 87sr 86sr ratios differ between particulate and dissolved loads with more radiogenic sr in the particulate loads these differences are attributed to differential weathering of minerals where high rb sr minerals dominate the particulate load and low rb sr the dissolved load there is broad correlation of 87 sr 86 sr with k al in the suspended load the differences in 87 sr 86 sr between suspended and dissolved load are highly variable and are related to the fe or mn concentration on the particulates in samples with high fe al the difference becomes small a good correlation was found between sr al and fe al or mn al in the particulates both in brackish and fresh waters sr is removed from solution both in rivers and in the baltic sea whenever there is formation of fe mn oxyhydroxide particulates this precipitation greatly diminishes the difference in isotopic composition of the dissolved and suspended loads as the particles containing fe mn oxyhydroxides settle they dissolve in anoxic zones and release sr this provides a mechanism for sr redistribution in the water column sr is thus only quasi conservative in environments where fe mn oxyhydroxides form or dissolve from consideration of the isotopic differences in sr between dissolved and suspended loads it follows that the net sr input depends upon weathering characteristics of the contributing mineral phases changes in weathering mechanisms due to climate change may cause sr isotopic shifts in the marine environment

colloidal rare earth elements in a boreal river changing sources and distributions during the spring flood abstract variations in the physico chemical speciation of the rare earth elements ree have been investigated in a subarctic boreal river during an intense spring flood event using prefiltered

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

the importance of colloids and mires for the transport of uranium isotopes through the kalix river watershed and baltic sea the importance of colloids and organic deposits for the transport of uranium isotopes from continental source regions and through the estuarine environment was investigated in the mire rich kalix river drainage basin in northern sweden and the baltic sea ultrafiltration techniques were used to separate uranium and other elements associated with colloids 10 kd and 3 kd from solute uranium and provided consistent results and high recovery rates for uranium as well as for other elements from large volume samples uranium concentrations in 0 45 m filtered kalix river water samples increased by a factor of 3 from near the headwaters in the caledonides to the river mouth while major cation concentrations were relatively constant 234 u 238 u ratios were high 234 u 770 1500 throughout the basin without showing any simple pattern and required a supply of 234 u rich water throughout the kalix river a large fraction 30 90 of the uranium is carried by 10 kd colloids which is compatible with uranium complexation with humic acids no isotopic differences were found between colloid associated and solute uranium within the baltic sea about half of the uranium is removed at low salinities the proportion that is lost is equivalent to that of river derived colloid bound uranium suggesting that while solute uranium behaves conservatively during estuarine mixing colloid bound uranium is lost due to rapid flocculation of colloidal material the association of uranium with colloids therefore may be an important parameter in determining uranium estuarine behavior mire peats in the kalix river highly concentrate uranium and are potentially a significant source of recoil 234 u to the mirewaters and river waters however mirewater data clearly demonstrate that only small 234 u 238 u shifts are generated relative to inflowing groundwater a simple box model of uranium accumulation in peat and transport through the mire that is compatible with the mire data demonstrates that with efficient removal of uranium from solution only small shifts in 234 u 238 u ratios can be generated in mirewater uranium the measurements and model calculations show that mirewaters are not the primary source of the uranium in the river bedrock groundwaters with high 234 u 238 u ratios and uranium concentrations must be the dominant source of riverine uranium

the behavior of u and th series nuclides in the estuarine environment rivers carry the products of continental weathering and continuously supply the oceans with a broad range of chemical constituents this erosional signature is however uniquely moderated by biogeochemical processing within estuaries estuaries are commonly described as complex filters at land sea margins where significant transformations can occur due to strong physico chemical gradients these changes differ for different classes of elements and can vary widely depending on the geographic location u and th series nuclides include a range of elements with vastly different characteristics and behaviors within such environments and the isotopic systematics provide methods for investigating the transport of these nuclides and other analog species across estuaries and into the coastal ocean there are numerous types and definitions of estuaries see dyer 1973 in this paper it is simply considered to be a region where freshwater and seawater mix to emphasize the chemical focus of the issues involved here this region can either be contained within a river channel or extend onto the shelf with effects often extending well into an adjoining ocean basin fairbridge 1980 these are complex ecological biogeochemical and hydrodynamic systems with strong gradients in the concentrations and composition of micro and macro particulate matter as well as dissolved organic and inorganic species particle reactive radionuclides and trace elements being transported across estuaries are subject to partitioning between the dissolved colloidal and particulate phases the variable chemical nature of the u and th series radionuclides is evident in their distributions in an estuarine environment such radionuclides can effectively serve as tracers to identify the sources fate and transport of particles and colloids as well as other pollutants that behave similarly to these nuclides these nuclides also have a wide range of half lives that can be used to examine processes over various time scales for example th and ra daughter isotopes have

concentration vertical flux and remineralization of particulate uranium in seawater abstract concentrations and fluxes of particulate u were measured throughout the water column at several locations in the atlantic and pacific oceans with in situ filtration systems and sediment traps the results indicate that dissolved u is fixed to particles in surface seawater organic matter appears to be the carrier phase formation of particulate authigenic u below the surface waters could not be detected authigenic u is remineralized within the bathypelagic layers at the open ocean sites studied in the panama basin an upwelling area with high biological productivity remineralization of authigenic u in the deep water column was not observed the rate of remineralization of authigenic u in the deep sea is insufficient to produce a measurable concentration gradient between surface and deep waters within the mixing time of the oceans formation of authigenic u in the water column in areas such as the panama basin is not a significant sink for u on an ocean wide basis

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

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

identification of a novel c25 highly branched isoprenoid thiophene in sediments a novel c25 highly branched isoprenoid hbi thiophene 2 2 methylpropyl 4 9 2 6 12 trimethyltetradecyl thiophene has been identified in three sediment samples by isolation followed by characterization by 1h nmr gc ms and raney ni desulphurization this component is probably formed by incorporation of inorganic sulphur species into a specific sedimentary c25 hbi alkadiene this latter compound is probably derived from diatoms

trace metal geochemistry of organic carbon rich watercourses draining the nw german coast abstract numerous small watercourses are draining the hinterland of the nw german coast the waters mainly originate from marsh and fen areas and have yellow to deep brownish color during their flow path the waters exhibit gradients in salinity 0 2 3 ph 6 2 8 8 particulate organic carbon 5 25 and iron oxides 7 12 which alter the concentrations of most dissolved and particulate trace metals for example dissolved fe is rapidly removed from solution at increasing salinities by flocculation whereas dissolved u is removed in the very low salinity zone by fe and organic rich colloids the waters at the flood gate of neuharlingersiel where a composite sample of the entire study area is collected before the freshwater is discharged into the marine dominated tidal flat area in front of the mainland dike have the following average trace metal concentrations dissolved fe 11 m mn 4 m mo 10 m u 1 8 m v 75 m and particulate fe 7 mg kg 1 mn 1200 mg kg 1 mo 2 mg kg 1 u 1 7 mg kg 1 v 140 mg kg 1 after passing the flood gate most mo and u salinity distributions fit well to the conservative mixing line that connects the seawater of the wadden sea to the low salinity river water entering the tidal flat area in contrast dissolved fe mn and v are removed from solution especially at intermediate salinities which may be due to mixing of freshwater with seawater depleted in these elements flocculation and or scavenging by particulate matter in general processes similar to those in estuarine systems alter the geochemical signatures on transects from land to sea compared to average river water the watercourses are enriched in dissolved and particulate fe mn mo u and v due to the fast removal of dissolved fe in the salinity gradient and the high concentrations of mo and u in seawater the watercourses only form a source for mn v terrestrial organic carbon and particulate fe for the wadden sea

uranium isotopes as proxies of the environmental history of the lake baikal watershed east siberia during the past 150 ka abstract a new experimental method for the determination of the isotopic composition of authigenic uranium in lake baikal siberia sediments has supplied a record of 234u 238u aut ratios during the past 150 ka with a resolution of ca 200 years reconstruction of the 234u 238u aut activity ratio as a proxy for lake water composition shows that the uranium isotope disequilibrium has not stayed constant throughout this period it varied greatly in response to regional climate the 234u 238u activity ratio in lake baikal paleowaters was inferred to be high during interglacial time 1 85 2 during mis 5 5 and the holocene 1 4 1 7 during mis 5 1 and 5 3 and dropped to 1 16 0 01 during glacial time the variability in the deduced 234u 238u ratios of lake baikal waters of the past may have been caused by the climatically modulated redistribution of contributions of riverine discharges from different provinces of the very large catchment of the lake aridity during ice ages was more pronounced in the south of eastern siberia and in northern mongolia relative to that in the northern regions of the lake baikal watershed the southern contribution of riverine input into the lake was essentially absent during glacial periods records of some elements ca k sr rb si al la th u th ciw from the bottom sediments of lake baikal during the last 0 5 myr with a resolution of 2 kyr have also been obtained coupling these data with records of weathering indices shows that not only riverine waters but also terrigenous material delivered into lake baikal from different regions of the very large watershed during interglacial and glacial times glacial erosion delivered fine siliciclastic material glacial milk mainly from the nearest mountain glaciers situated in the northwestern region of lake baikal while during warmer periods terrigenous material was sourced from the surrounding river valleys a conceptual model for the simulation of the fluvial discharge gradient between the northern and southern provinces of the lake baikal watershed is proposed and a first order simulation of past changes in the contribution of the selenga river to the lake water budget is given

uranium series disequilibrium in river sediments and waters the significance of anomalous activity ratios abstract samples of river water suspended and bottom sediments from the fenland catchment system draining into the wash u k have been analysed for the longer lived u th series radionuclides 238u 234u 230th and 232th the aim of this work was to assess the effects of open system phase transfer on the u series disequilibria observed in the river waters and sediments and to investigate the controls exerted on this phase transfer by the hydrological regime two important aspects of the u series radionuclide mobilisation in this modern river system have been observed 1 u series radionuclides are in a continuous state of flux between river sediments and river water and 2 u sorption on sediment particles is seasonably variable sequential extraction of whole sediments revealed that the u series isotopic characteristics of the ion exchangeable organic bound carbonate amorphous fe mn oxyhydroxides and crystalline fe mn oxides are controlled to a significant degree by the acquisition of u isotopes from the aqueous phase during summer when surface run off and river discharges are generally low the u isotope acquisition by sediment particles results in anomalously high234u 238u activity ratios 1 this is of importance for studies which rely upon the isotopic and elemental separation of u series radionuclides in the weathering system i e mass balance weathering rate and sediment yield assessments

sample preparation for determination of comminution ages in lacustrine and marine sediments abstract the uranium isotope composition 234u and 238u of detrital matter has become an essential tool for evaluating the response of erosion and catchment dynamics to climate variability on geological time scales relative variability of the 234u 238u activity ratio can be used as an estimate of the time that has elapsed since physical and chemical weathering has formed grains combined u isotopes and gas adsorption surface area analyses on two sediment samples from the mediterranean sea yield comminution ages up to 5 times older after sample treatment which highlights the importance of a comprehensive removal of non detrital matter from the bulk sediment composition moreover gas sorption analysis allowed determining whether a fractal correction for calculation of the recoil fraction should be applied precise estimates of the recoil fraction are crucial for calculating the comminution ages as it governs the loss of 234u from detrital matter samples analyzed in this study display type ii isotherms suggesting a non porous or macroporous surface micro and mesopores which increase the surface area during gas adsorption analyses but do not contribute to the loss of 234u are absent thus a fractal correction to account for micro and mesopores is not required

evaluating the removal of non detrital matter from soils and sediment using uranium isotopes this study was supported by an arc discovery project grant dp1093708 and arc future fellowship ft0990447 to ad and a university of wollongong postgraduate scholarship award to anm

temporal variations in dissolved and suspended iron and manganese in the kalix river northern sweden abstract particulate and dissolved fe and mn were measured once a week from may to september 1982 in the kalix river north sweden during winter baseflow the dissolved fe concentration was 450 g l 1 early snow melt discharge increased the concentration slightly but during springflood maximum the concentration decreased to a minimum value of 60 g l 1 in mid july particulate matter in the kalix river is dominated by fe which ranges between 14 and 38 of the ashed suspended load 70 80 of the total fe load being retained by 0 45 m filters approximately 10 of particulate fe can be accounted for by a detrital phase the particulate fe al ratio showed a mean value of 6 5 which is more than ten times the ratio in mean world river particulate fe showed a peak during early snowmelt and decreased during maximum discharge similar to dissolved fe dissolved mn concentrations ranged from 1 1 to 53 4 m l 1 with a mean value of 11 2 12 4 g l 1 whereas particulate mn ranged from 3 1 ti 18 2 g l 1 with a mean concentration of 10 6 4 9 g l 1 during may and june most mn was transported in the dissolved phase whereas in july and august the particulate fraction dominated there was a drastic increase in non detrital particulate mn with a concomitant decrease in the dissolved fraction in early july with peak values in mid july and early august the decrease in dissolved mn and increase of particulate mn is probably the consequence of two processes namely addition of non detrital particulate mn to the river possibly from lakes and transformation of dissolved mn to a particulate phase within the river a combination of increased ph temperature and particulate mn triggered the precipitation of dissolved mn although the significance of each factor can not be evaluated from this study

geochemistry of manganese in the kalix river northern sweden dissolved and suspended mn in the kalix river northern sweden were measured weekly over a period of eighteen months during the same period four lakes in the kalix catchment were sampled at their outlets and in vertical profiles within the lakes together with a stream draining a series of mires with shallow lakes snow melting in mid may increased the dissolved mn concentration in the river tenfold compared with a concentration of 5 g l 1 during the winter discharge january to april we suggest that the increase was caused by mn rich mire water mixing with melting snow and being transported to the river large concentrations of dissolved mn built up in the hypolimnion of the lakes studied during the icecovered period break up of the ice and spring overturn in june increased the dissolved mn concentration tenfold in lake discharge and a concomitant peak in the dissolved mn concentration was observed in the river lake derived mn was the dominant source for mn in the river during this time suspended mn in the river was hosted mainly in detrital particles during flood in may in mid june non detrital suspended mn started to accumulate and reached a maximum in late july and early august the mnal ratio was 25 times higher during this period than during flood in may suggesting the precipitation of an mn oxyhydroxide phase the precipitation of the non detrital mn rich phase was correlated in time with increased temperature increased ph and increased concentration of suspended biogenic particles the precipitation of dissolved mn was biologically mediated sedimentation and mineralisation of the non detrital mn phase in river and lake sediments resulted in a steady increase of the dissolved mn concentration in the river water during autumn

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

temporal variations in the fractionation of the rare earth elements in a boreal river the role of colloidal particles rare earth element ree data from weekly sampling of the filtered 0 45 m and suspended particulate phase during 18 months in the kalix river northern sweden are presented together with data on colloidal particles and the solution fraction 3 kda the filtered ree concentration show large seasonal and temporal variations in the river lanthanum varied between approximately 300 and 2100 pm high ree concentration in the filter passing fraction is related to increased water discharge and there is a strong correlation between the ree concentration organic carbon al and fe physical erosion of detrital particles plays a minor role for the yearly transport of particulate ree in this boreal river system the suspended particulate fraction which is dominated by non detrital fractions accounted for only 35 of the yearly total transport of la in the river approximately 10 of the ree were transported in detrital particles during winter at spring flood in may about 30 of the lree and up to 60 of the hree where hosted in detrital particles ultrafiltration of river water during spring flood shows that colloidal particles dominate the transport of filter passing ree less than 5 of the filtered ree are found in the fraction smaller than 3 kda the colloidal fraction shows a flat to slightly lree enriched pattern whereas the solution fraction 3 kda show an hree enriched pattern compared with till in the catchment suspended particles show a lree enriched pattern data indicate that the ree are associated with two phases in the colloidal and particulate fraction an organic rich phase with associated al fe and an fe rich fe oxyhydroxide inorganic phase the ce anomaly in the suspended particulate fraction in the river shows systematic variations and can be used to interpret fractionation processes of the ree during weathering and transport there was no anomaly at maximum spring flood but during the ice covered period the anomaly became more and more negative the temporal and seasonal variations of the ce anomaly in the suspended particulate phase reflect transport of ree c al fe enriched colloids from the upper section of the till and or from mires to the river at storm events

temporal variability of uranium concentrations and 234u 238u activity ratios in the mississippi river and its tributaries abstract 234 u 238 u activity ratios and total dissolved uranium concentrations in the lower mississippi river at new orleans la have been analyzed on a bi weekly basis over 2003 2004 during this time period the range in 234 u 238 u activity ratios is approximately 17 from 1 241 to 1 473 while uranium concentrations span an even greater range of 130 0 28 to 1 33 ppb there is no correlation between uranium activity ratio and discharge and only a very weak correlation between uranium concentration and discharge in order to examine the cause of the substantial variability in the lower river concentrations and 234 u 238 u activity ratios we sampled the four major tributaries of the mississippi river ohio missouri upper mississippi and arkansas rivers each river was sampled four times each time representing a unique discharge season in general lithological and climatological parameters in the tributary sub basins exert the greatest control on lower mississippi river uranium concentration and isotope signatures these parameters may also be influenced by groundwater infiltration below the confluences of the major tributaries our temporal analysis suggests that if one were to estimate mississippi river uranium fluxes to the ocean based on single point sampling during 2003 2004 significant over or under estimations of these values would ensue our two year average mississippi river uranium flux values suggest that previous best estimates of the contribution of mississippi river uranium to the global average riverine 234 u 238 u activity ratio are too high by 36 if other major world rivers are as poorly constrained with respect to uranium systematics as the mississippi river then the global average riverine u flux and 234 u 238 u activity ratio need to be revised

molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain the application of the molybdenum mo isotope system as a proxy for determining changes in the redox state of the oceans is predicated on the assumption that the composition of continental input can be characterised from crustal rock types and remains constant however it has recently been shown that the 98 95mo composition of global rivers varies between 0 15 and 2 4 and is therefore systematically heavier than the average composition of the continental crust 0 in order to understand the processes that control mo isotope fractionation during weathering this study presents 98 95mo and mo abundance data for rivers and estuarine samples from iceland that drain predominantly basaltic terrains resolvable differences are observed in the isotopic composition of the riverine mo sources ice 98 95moice 1 8 basaltic bedrock 98 95mobedrock 0 0 and hydrothermal waters 98 95mohydrothermal 3 4 systematic changes in the dissolved mo isotope composition are also observed within river catchments with 98 95mo values increasing from 0 in glacial rivers close to the source to 1 downstream consistent with mo isotopes being fractionated during weathering analysis of other riverine phases bedload colloids and iron precipitates demonstrates that these phases preferentially incorporate light mo isotopes and remain coupled to the dissolved load during riverine transportation a 98 95mo profile through the borgarfjor eth ur estuary exhibits a predominantly conservative mixing behaviour but suggests that the release of isotopically light mo from the particulate and or colloidal phases may occur in the low salinity part of the estuarine mixing zone 2010 elsevier b v

lithium magnesium and uranium isotope behaviour in the estuarine environment of basaltic islands this study presents major and trace elements and uranium u lithium li and magnesium mg isotope data for dissolved and suspended particulate material from estuaries draining dominantly basaltic terrains in iceland borgarfjor ur and sao miguel povoacao in the azores archipelago the concentrations of dissolved sodium na calcium ca potassium k sulphate so4 mg and li and 7li 6li and 26mg 24mg isotope ratios vary directly as a function of the dissolved chloride concentration cl indicating that these species are non reactive in these estuaries however u appears to be affected either by colloid flocculation or the formation of fe oxyhydroxides in borgarfjor ur the major element composition of suspended particulate material in the estuary at borgarfjor ur shows that the detrital silicates delivered to the estuarine mixing zone are relatively unweathered accordingly the 26mg isotope composition of the suspended material is indistinguishable from that of the parent basalts however the 234u 238u activity ratios of the suspended material are significantly different from the parent basalt indicating rapid equilibration of the u isotopes between particles and seawater the li concentration and 7li isotope composition of the suspended load are also observed to increase with increasing cl consistent with ongoing weathering of detrital material and the formation of secondary minerals in seawater depending on the particle residence time in the estuary this process could remove as much as 15 25 of the global riverine input of li to the ocean

origin and diagenetic transformations of c25 and c30 highly branched isoprenoid sulphur compounds further evidence for the formation of organically bound sulphur during early diagenesis abstract a number of c25 and c30 highly branched isoprenoid hbi sulphur compounds e g thiolanes 1 oxo thiolanes thiophenes and benzo b thiophenes with 2 6 10 14 tetramethyl 7 3 methylpentyl pentadecane and 2 6 10 14 18 pentamethyl 7 3 methylpentyl nonadecane carbon skeletons were identified in sediments ranging from holocene to upper cretaceous these identifications are based on mass spectral characterisation desulphurisation and in some cases by comparison of mass spectral and relative retention time data with those of authentic standards the presence of unsaturated c25 and c30 hbi thiolanes in a recent sediment from the black sea age 3 6 10 3 a strongly supports their formation during early diagenesis the co occurrence of hbi polyenes c25 and c30 and unsaturated hbi thiolanes c25 and c30 possessing two double bonds less than the corresponding hbi polyenes in this recent sediment testifies to the formation of unsaturated hbi thiolanes by a reaction of inorganic sulphur species with double bonds of the hbi polyenes furthermore a diagenetic scheme for hbi sulphur compounds is proposed based on the identification of hbi sulphur compounds in sediment samples with different maturity levels

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

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

environmental and seasonal controls on riverine dissolved uranium in the hudson james and ungava bays region canada this study documents the spatiotemporal variability of riverine uranium contents and fluxes in rivers discharging into the hudson james and ungava bays hjub samples retrieved during a monitoring program of the koksoak great whale la grande and nelson rivers were analyzed for dissolved uranium concentration u and activity ratio 234u 238u field surveys conducted during baseflow and snowmelt in six other rivers of the hjub basin provided complementary data it is estimated that altogether the studied rivers export 3 4 105 mol year of u towards the hjub with a discharge weighted average 234u 238u of 1 27 two main factors appear to control seasonal fluctuations in dissolved u exports from hjub rivers snowmelt and anthropogenic flow control under natural hydrological regimes the dilution of u caused by snowmelt does not compensate for the associated increase in discharge and riverine u fluxes are intensified during springtime contrastingly the timing of riverine u exports is decoupled

chemical and physical weathering in south patagonian rivers a combined sr u be isotope approach abstract we investigated physical and chemical weathering in south patagonia encompassing both the tectonically active andes with alpine glaciers and the quiescent seaboard plain with arid climate chemical denudation rates determined from riverine dissolved major elements were 0 07 5 10 5 tons year 1 and the long term rates of co 2 consumption by alkaline earth silicates were 0 03 0 5 10 5 mol km 2 year 1 commensurate with the average global co 2 consumption rate 0 25 10 5 mol km 2 year 1 unradiogenic strontium isotope ratios indicated that the source of silicate weathering was volcanic sedimentary cover basin average total denudation rates based on 10 be measured in active streambed sediments ranged from 0 009 to 0 6 mm year 1 uranium series disequilibria suggested that there is significant redistribution of nuclides between the dissolved and suspended material when applying the simultaneous gain and loss model to the u series data of the suspended load sediment residence times of 10 150 ky were obtained comparison of the dissolved load based chemical denudation rate and 10 be based total denudation rate revealed that some basins are dominated by chemical and some by physical denudation

the distribution of 234u 238u activity ratios in river sediments abstract uranium u isotopes can be used to estimate the comminution age of sediments i e the time elapsed from sediment production on continents via weathering and physical erosion to deposition in the sedimentary record the calculation of this comminution age is based on measured 234u 238u activity ratios in river sediments and inferred time dependent recoil effect which leads to the preferential release of 234u from mineral lattices during erosion processes in this study we report on a large scale 234u 238u investigation of modern river sediments worldwide with the aim to determine the extent to which parameters such as grain size lithology weathering climate and geomorphology may influence the distribution of u isotopes in fine grained sediments our extensive dataset n 64 includes u isotopic measurements for many of the world s largest rivers but also rivers draining particular climatic and geological settings our results indicate that sediments collected from river basins draining mostly igneous metamorphic or volcanic rocks often display 234u 238u ratios 1 with clay size fractions

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

variability in the uranium isotopic composition of the oceans over glacial interglacial timescales uranium series mass spectrometric analyses of corals from the uplifted last glacial terraces at huon peninsula papua new guinea that grew from 50 000 years ago to 30 000 years ago show systematically low values of 234u 238u at the time of coral growth compared with modern corals when combined with coral data from other studies a systematic trend emerges indicating shifts in the 234u 238u ratio at times of major glacial interglacial transitions that involve large variations in sea levels from last glacial to holocene the rate of change in 234u is approximately 1 per thousand years the variations in the u budget of the oceans appear to be due to accumulation of excess 234u in near shore areas in anoxic and suboxic sediments in salt marshes and mangroves in estuaries and in continental margins during periods of warm climate and high sea levels these near shore areas are exposed during periods of low sea level resulting in rapid oxidation of u into highly soluble phases the subsequent release of 234u enriched uranium into the oceans occurs over a sustained period in step with rising sea levels

lipid biomarker fluxes in the arabian sea with a comparison to the equatorial pacific ocean abstract fluxes and distributions of organic carbon oc and lipid biomarkers were measured in the western arabian sea as a function of season depth and distance from the coast of oman during the us jgofs arabian sea process study in 1994 1995 a strong seasonal pattern in oc and lipid flux is related to the annual monsoon cycle in the western arabian sea with the highest fluxes measured during the southwest monsoon fluxes were greatly attenuated with depth in the water column and in surface sediments as remineralization effectively consumed particulate organic matter a comparison of water column fluxes with oc and lipid accumulation rates in sediments confirms that the water sediment interface is a hot spot for organic matter degradation biomarker compositions also varied seasonally and with distance offshore reflecting seasonal and spatial succession of their biological sources and their subsequent export through the water column degradation of oc and biomarkers was extremely efficient with only a tiny fraction

weathering geochronology by u th he dating of goethite nine samples of supergene goethite feooh from brazil and australia were selected to test the suitability of this mineral for u th he dating measured he ages ranged from 61 to 8 ma and were reproducible to better than a few percent despite very large variations in u and th in all samples with internal stratigraphy or independent age constraints the he ages corroborated the expected relationships these data demonstrate that internally consistent he ages can be obtained on goethite but do not prove quantitative 4he retention to assess possible diffusive he loss stepped heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic 3he the 3he release pattern indicates the presence of at least two diffusion domains one with high helium retentivity and the other with very low retentivity at earth surface conditions the low retentivity domain which accounts for 5 of 3he contains no natural 4he and may represent poorly crystalline or intergranular material which has lost all radiogenic 4he by diffusion in nature diffusive loss of 3he from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate so probably represents diffusion from individual micrometer size goethite crystals the 4he 3he evolution during the incremental heating experiments shows that the high retentivity domain has retained 90 95 of its radiogenic helium this degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to earth surface temperature and held for the appropriate duration considering both the high and low retentivity domains these data indicate that one of the samples retained 90 of its radiogenic 4he over 47 5 ma and the other retained 86 over 12 3 ma thus while diffusive loss corrections to supergene goethite he ages are required these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton irradiation 4he 3he method

sedimentation distribution and diagenesis of organic matter in a recent carbonate environment abu dhabi u a e abstract in the modern hypersaline carbonate lagoon and sabkha sedimentary environments of abu dhabi united arab emirates three types of organic matter originate respectively from microbial mat avicennia mangrove and halodule lagoonal seagrass the study of recent sedimentary processes and cross sections through the sabkha sediments lead to the definition of organo sedimentary facies based on geochemical and sedimentological criteria this permits the construction of an organo sedimentary sequence which expresses the holocene sedimentary record involving a transgressive and a regressive sequence the various organic facies occur in both sequences heterogeneity within the individual organic facies reflects several factors including sedimentation dynamics mineral matrix oxidation and reduction and selective organic and mineral diagenesis these parameters are discussed in terms of depositional environment and location within the organo sedimentary sequence changes in distribution quantity and preservation potential of the buried organic matter are discussed in terms of sea level changes and sedimentary accretion rates

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

dissolved suspended and colloidal fluxes of organic carbon major and trace elements in the severnaya dvina river and its tributary abstract the chemical status of major and trace elements te and organic carbon oc has been studied in the severnaya dvina and pinega rivers draining granitic moraine carbonate and sedimentary deposits of the arkhangelsk region nw russia the white sea basin of the arctic ocean sampling was performed during winter and summer baseflow seasons and during the spring flood 2007 2008 size separation procedure included on site filtration through 5 m 0 22 m 100 10 and 1 kda and dialysis through 1 kda and 10 kda pore size membranes the organic carbon concentration in truly dissolved form trace and major element concentrations in the suspended matter of the river severnaya dvina were measured during different seasons over three years of observation based on collected data we evaluate here for the first time the seasonal fluxes of major and trace elements in the suspended 0 22 m total dissolved

a novel c35 highly branched isoprenoid polyene in recent indian ocean sediments a novel c35 7 highly branched isoprenoid hbi alkene has been identified in extracts of surface sediment samples from the indian ocean by isolation followed by characterization by gc ms and hydrogenation this polyunsaturated compound might be derived from diatoms

extreme iron isotope fractionation between colloids and particles of boreal and temperate organic rich waters abstract significant and systematic variations of iron isotopic composition in surface water sample fractions obtained by frontal cascade filtration and ultrafiltration have been recorded in 1 subarctic organic rich boreal river and stream mire lake and soil solutions in northern taiga zone karelia nw russia and 2 temperate river and lake waters of the southern boreal zone central russia water samples were filtered in the field employing progressively decreasing pore size from 100 m to 1 kda followed by iron isotope analysis in all river samples there was a gradual increase of 57fe relative to irmm 14 with decreasing pore size from 0 4 0 1 at 100 m up to 4 2 0 1 at 10 kda fraction in the subarctic zone and from 0 024 0 2 at 100 m up to 1 2 0 2 at 10 kda in the temperate zone in the series of filtrates ultrafiltrates of subarctic and temperate streams and rivers the 57fe value decreases with increasing molar fe corg ratio therefore small size fe poor c rich colloids 1 10 kda and low molecular weight lmw fractions of oxygenated water exhibit strong enrichment in heavy isotope whereas high molecular weight fe rich colloids 100 kda 0 22 m and particles 1 100 m are isotopically lighter and closer to the continental crust fe isotope composition the relative enrichment of 1 10 kda ultrafiltrates in heavy isotopes suggests that low molecular weight ligands bind fe more strongly fe o c bonds than fe iii oxy hydr oxides fe o fe bonds in accord with quantum mechanics principles highly positive 57fe of the lmw fraction of labile and potentially bioavailable fe in small subarctic rivers may turn out to be a very important source of isotopically heavy fe in the arctic ocean the mechanisms involved in the production of this isotopically heavy fe may lead this tracer to become a new indicator of environmental changes occurring in the boreal zone

hydrocarbons of the hypersaline basins of shark bay western australia shark bay is an 8000 sq km shallow marine embayment on the western coast of western australia the waters exhibit a well established salinity gradient from oceanic to hypersaline the chemical assemblage of sediment hydrocarbons along the salinity gradient may be classified into two distinct chemogeographic types firstly oceanic sediments contain n alkanes and a suite of highly branched and branched cyclic c25 alkenes hypersaline sediments are characterised by a high relative abundance of a c25h50 alkene together with an analogous c20h40 alkene and its parent c20h42 alkene 2 6 10 trimethyl 7 3 methylbutyl dodecane a pair of alkanes c21h42 and c22h44 increase in concentration and relative abundance with depth the hydrocarbons of the hypersaline basins are found in only trace amounts in oceanic sediments these chemical signals are overlain by further input indicative of the immediate biotic community

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

the tracing of riverine u in arctic seawater with very precise 234u 238u measurements abstract the riverine flux of u that enters the deep oceans is not well constrained since the net losses during estuarine mixing are difficult to quantify riverine dissolved u normally has a higher 234 u 238 u activity ratio 234 u 238 u ar than the uniform value that characterizes open ocean seawater and could be used as a tracer of riverine inputs if one could resolve subtle variations in seawater composition using new mass spectrometry techniques we achieve a long term reproducibility 0 3 on 234 u 238 u ar which permits the tracing of riverine u in seawater samples from the arctic a partially restricted basin that is ideal for such a study we find that surface waters from the arctic basins carry elevated 234 u 238 u ar when compared with deep ocean seawater samples from the canada basin have a significant freshwater component and provide evidence that the mackenzie river loses 65 of its u in the mackenzie shelf estuary zone before entering the deeper basin this is in contrast to samples from the makarov basin which provide evidence that all of the freshwater input is derived from the major yenisey river alone despite the proximity of the lena and ob rivers the differing behaviour of u between the mackenzie and yenisey rivers is most likely a consequence of the strong binding of u to dissolved organic matter doc or secondary phases in these rivers the yenisey river appears to transport the majority of the doc through the shelf and into the makarov basin in contrast the mackenzie river appears to lose a significant amount of doc 50 in the estuary shelf zone which may lead to loss of associated u these findings offer a more detailed picture of the fresh riverine water flow patterns in the arctic ocean when compared to other geochemical proxies the non conservative behaviour of u in the mackenzie river through the shelf estuaries has important implications for u input into oceans and the total marine budget

the sources and transport of sr and nd isotopes in the baltic sea we have determined the concentration and isotopic composition of sr and nd in waters from the baltic sea the baltic sea is an intracontinental stratified brackish water estuarine like system and the rivers emptying into it drain a suite of terranes ranging from proterozoic archean in the north to phanerozoic in the south the sampled brackish waters range in salinity from seawater sw at 35 289 to a minimum of 2 460 at the surface in the innermost part of the gulf of bothnia the sr concentrations show generally conservative behavior indicating a simple two component mixing however small deviations 3 70 from a perfect mixing line reveal that the imprints from rivers with different sr concentrations are preserved in the blending strontium concentrations from a depth profile across the redoxcline in the baltic proper indicate that vertical particle transport alters the sr concentration in the water our estimated concentration of sr in the average freshwater input to the baltic is 0 03 ppm which is only about 0 4 of the sw concentration the sr isotopic data range from e sr sw 0 in seawater to e sr bw sw 7 8 in the least saline baltic water bw sample in the gulf of bothnia the isotopic composition of sr versus 1 sr in the baltic sea follows an almost perfect mixing line which shows that seawater sr is mixed with much more radiogenic components calculated end member values of e sr sw for each sample show that the riverine input into the gulf of bothnia has e sr sw 120 200 and 10 50 e units in the baltic proper these values are in general agreement with direct measurements of river waters in each region however the calculated values in the gulf of bothnia are lower than the measured river water input in this region which indicates the presence of less radiogenic sr presumably originating from the river waters draining the southern part of the basin which are partially transported northward and mixed with sr from the gulf of bothnia rivers the nd concentration in the baltic sea is not conservative varying between 5 and 45 ppt with the highest concentrations in the bottom waters due to vertical particulate transport a plot of e nd o in baltic water yields a good correlation with the calculated freshwater end member e sr sw the data show that it is possible to unravel the different freshwater sources into the baltic and to identify the zones of particulate removal of both non conservative species such as the ree and of quasi conservative species such as sr the use of isotopic tracers in this estuarine environment may provide a much better insight into mixing and element transport it should also be possible to trace lateral movements of freshwater inputs

algal uptake of u and some other metals implications for global geochemical cycling abstract freshwater green algae ankistrodesmus sp cultured in dilute solutions of metals chelated with edta revealed massive uptake of u examination of thames river waters and selected micro organisms of its aquatic ecosystem revealed an average dissolved u concentration of 1 46 ppb representing about twice the global mean riverine solute concentration of 0 6 ppb u suspended particulates principally algae contain u at 103 to 105 times that of their river habitat averaging 28 300 ppb u one community of the filamentous algae spirogyra sp contained intracellular crystals tentatively identified as the ca oxylate weddelite studies of algae from a variety of environments have shown large concentration factors for a wide range of elements algae from an acid u mine tailings site contain up to 2 7 u dry weight plus very high concentrations of other metals micro organisms may act as an important sink for metal solutes in the hydrosphere

uranium geochemistry on the amazon shelf evidence for uranium release from bottom sediments in amazon shelf waters as salinity increases to 36 5 10 3 dissolved uranium activities increase to a maximum of 4 60 dpm 1 1 this value is much higher than the open ocean value 2 50 dpm 1 1 indicating a source of dissolved uranium to shelf waters in addition to that supplied from open ocean and riverine waters uranium activities are much lower for surface sediments in the amazon shelf seabed mean 0 69 09 dpm g 1 than for suspended sediments in the amazon river 1 82 dpm g 1 data suggest that the loss of particulate uranium from riverine sediments and the consequent input of dissolved uranium to shelf waters is probably the result of uranium desorption from the ferric oxyhydroxide coatings on sediment particles and or uranium release by mobilization of the ferric oxyhydroxides the total flux i e riverine flux plus desorbed remobilized particulate flux of dissolved 238u from the amazon shelf about 1 2 1015 dpm yr 1 constitutes about 15 of uranium input to the world ocean commensurate to the amazon river s contribution to world river water discharge approximately 18 measurement of only the riverine flux of dissolved 238u underestimates by a factor of about 5 the flux of dissolved238u from the amazon shelf to the open ocean

an estimate of the efficiency of the iron and manganese driven dissolved inorganic phosphorus trap at an oxic euxinic water column redoxcline 1 geochemical records suggest the ocean has undergone periods of at least partial deeper ocean anoxia or euxinia two counteracting feedback loops involving redox control of the dynamics of the phytoplankton nutrient dissolved inorganic phosphorus dip might coexist helping to stabilize the redox state of the atmosphere and oceans this concept implies that during deeper ocean anoxia the dip transfer from the deep anoxic into the oxic surface ocean is uninhibited by processes taking place at the redoxcline this implicit assumption requires testing because iron fe and manganese mn dynamics at oxic anoxic water column redoxclines have the potential to form a dip trap inhibiting dip transport from anoxic deep into oxic surface waters using a time series data set of fe mn dip and dissolved oxygen distributions in the eastern gotland basin of the baltic sea we provide estimates of the efficiency of this fe and mn driven dip trap this efficiency was estimated by calculating the ratios of 1 the downward flux of dip adsorbed onto and or coprecipitated into the settling authigenic fe and mn rich particles just above the redoxcline and 2 the upward turbulent diffusive dip flux across the redoxcline depending on the assumed particle densities we find average 1 sd trapping efficiencies of 0 38 0 29 and 0 63 0 45 the efficiencies are significant in that they seem to impact cyanobacterial dynamics in the central baltic sea we discuss possible implications of the trapping mechanism for and propose two hypotheses relating to the potential importance of fe controlled dip trapping at redoxclines during ocean anoxic events

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

the importance of colloids for the behavior of uranium isotopes in the low salinity zone of a stable estuary particle mediated removal processes of u isotopes were investigated during spring flood discharge in the low salinity zone lsz up to 3 practical salinity units psu of a stable estuary a shipboard ultrafiltration cross flow filtration cff technique was used to separate particles 0 2 m and colloids between 3000 daltons 3 kd and 0 2 m from ultrafiltered water 3 kd containing dissolved species sediment traps were used to collect sinking material concentration of fe and organic c which are indicators of the major u carrier phases were used to interpret the behavior of 234 u 238 u during estuarine mixing colloids dominated the river water transport of u carrying 90 of the u on entering the estuary colloids accounted for the dominant fraction of u to about a salinity of 1 psu but only a minor fraction 1 psu there is a general correlation between u and salinity in all filtered fractions the 234 u 238 u ratios in different filtered fractions and sinking particles were generally indistinguishable at each station and showed enrichment in 234 u compared with secular equilibrium 234 u 266 567 this clearly shows that all size fractions are dominated by nondetrital u consideration of u isotope systematics across the estuary reveals that substantial u exchange must occur involving larger particles at least to 1 psu and involving colloids at least to 1 5 psu further exchange at higher salinities may also occur as the proportion of u on colloids decreases with increasing salinity this may be due to decreasing colloid concentration and increasing stabilization of uranyl carbonate complexes during mixing in the estuary the results show that although u is a soluble element that shows generally conservative mixing in estuaries removal occurs in the very low salinity zone and this zone represents a significant sink of u variation in composition and concentration of colloidal particles between different estuaries might thus be an important factor for determining the varying behavior of u between estuaries

uranium geochemistry on the amazon shelf chemical phase partitioning and cycling across a salinity gradient abstract the size distribution of u was examined in surface waters of the amazon shelf water samples were collected during a low discharge river stage across a broad salinity gradient 0 3 35 4 and fractionated by planar filtration and tangential flow ultrafiltration into 1 solution u s c 10 000 mw 0 4 m 3 dissolved u d p 0 4 m phases concentrations of colloidal u comprise up to 92 of the dissolved u fraction at the river mouth and attain highest values 0 45 g l in the productive biogenic region of the amazon shelf salinities above 20 u d and u c distributions are highly nonconservative relative to ideal dilution of river water and seawater indicating extensive removal at salinities below 10 the distribution of u s also shows some nonconservative behavior yet removal if any is minimal saltwater induced precipitation and aggregation of riverine colloidal material is most likely the dominant mechanism of u removal in the low salinity terrigenous region of the amazon shelf there is evidence of a substantial colloidal u input 245 of the riverine u c flux into surface waters above 5 such u c enrichment most likely is the result of colloidal u rich porewater diffusion advection from the seabed and fluid muds or shelf wide particle colloid disaggregation removal of solution and dissolved phase u via a colloidal intermediate and u c aggregation was examined in terms of coagulation theory the highly reactive nature of all u phases on the amazon shelf suggests that remobilization and fractionation of u may also occur in other river influenced coastal environments

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

magnesium isotope fractionation during shale weathering in the shale hills critical zone observatory accumulation of light mg isotopes in soils by clay mineral transformation abstract magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments but few have focused on the mechanisms that control mg isotope fractionation during shale weathering in this study we focus on understanding mg isotope fractionation in the shale hills catchment in central pennsylvania mg isotope ratios were measured systematically in weathering products along geochemical pathways of mg during shale weathering from bedrock to soils and soil pore water on a planar hillslope and to sediments stream water and groundwater on a valley floor significant variations of mg isotopic values were observed 26mg values 0 6 to 0 1 of stream and soil pore waters are about 0 5 to 1 lighter than the shale bedrock 26mg values of 0 4 consistent with previous observations that lighter mg isotopes are preferentially released to water during silicate weathering dissolution of the carbonate mineral ankerite depleted in the shallow soils but present in bedrock at greater depths produced higher mg2 concentrations but lower 26mg values 1 1 in groundwater 1 5 lighter than the bedrock 26mg values 0 2 to 0 4 of soil samples on the planar hillslope are either similar or up to 0 2 lighter than the bedrock hence a heavy mg isotope reservoir complementary to the lighter mg isotopes in soil pore water and stream water is missing from the residual soils on the hillslope in addition soil samples show a slight but systematic decreasing trend in 26mg values with increasing weathering duration towards the surface we suggest that the accumulation of light mg isotopes in surface soils at shale hills is due to a combined effect of i sequestration of isotopically light mg from soil water during clay dissolution precipitation reactions and ii loss of isotopically heavy particulate mg in micron sized particles from the hillslope as suspended sediments this latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation stream sediments 26mg values of 0 3 to 0 5 accumulated on the valley floor are 0 2 heavier than the bedrock and are thus consistent with that mobile particulates are the heavy mg isotope reservoir our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter mg isotopes in residual bulk soils this example also demonstrates that transport of isotopically distinct fine particles from clay rich systems could be a new and important mechanism to drive the mg isotope compositions of silicate weathering residuals this mechanism drives fractionation in an opposite direction as might be expected from previous studies i e residual soils are driven to lighter mg values and sediments become isotopically heavier

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

238u234u and232th230th in the baltic sea and in river water the concentration c of dissolved238u 234u 232th and230th in fresh and brackish waters from the baltic sea were determined using tims the brackish waters range in salinity from that of sea water sw to 2 5 c238u in oxygen saturated surface waters is well correlated with salinity and shows quasi conservative behavior as does sr samples from the redox water interface show depletion in c238u demonstrating that dissolved u is being removed by femn oxyhydroxides from a simple mixing relationship for the brackish water 234u 1000 was calculated for the fresh water source in the northern baltic a study of the kalixalven river over an annual cycle yields high 234u during spring and summer discharge and lower values during fall and winter showing that different sources contribute to the u load in the river during different seasons c232th and c230th in river water are governed by the discharge reflecting the importance of the increased abundance of small particles 0 45 m for the232th230th load at high discharge 232th 238u in river water is about 40 times less than in detrital material in the brackish water c232th drops 2 orders of magnitude in the low salinity region 5 reaching a value close to that of sea water at a salinity of 7 5 almost all of the riverine232th must be deposited in the low salinity regions of the estuary the230th 232th in river waters is about twice the equilibrium value for232th 238u 3 8 in the brackish waters 230th 232th is greater by a factor of 10 100 than both river water and sw the big increase in230th 232th in the baltic sea waters over the riverine input indicates that the th isotopes enter the estuary as a mixture of two carrier phases we infer that about 96 of232th in river water is carried by detrital particles whereas the other phase solution colloidal has a much higher232th 232th entering the estuary the detrital particles sediment out rapidly whereas the non detrital phase is removed more slowly causing a marked increase in230th 232th in the brackish water in sw 230th 232th is closer to river input and detrital material than in brackish water we conclude that in the deep sea 232th is almost exclusively dominated by windblown dust and can be used to monitor dust flux the230th excess in baltic rivers is produced in u rich 232th poor peatlands and trapped in authigenic particles and transported with the particles time scales for producing the230th excess are 2000 8000 yr this is younger than but comparable to the time of the latest deglaciation which ended some 9000 yr ago when the mires were forming these results have implications for the possible mobility of actinides stored in repositories

climatic significance of seasonal trace element and stable isotope variations in a modern freshwater tufa abstract we present a continuous 14 yr long 1985 to 1999 high resolution record of trace element mg sr ba u and stable isotope 13c 18o variations in a modern freshwater tufa from northwestern queensland australia by utilizing the temperature dependence of the 18o signal an accurate chronology was developed for the sampled profile which allowed a comparison of the chemical records with hydrological and meteorological observations as a consequence it was possible to constrain the relevant geochemical processes relating climate variables such as temperature and precipitation to their chemical proxies in the tufa record temperatures calculated from the mg concentrations of the tufa samples provide close approximations of average annual water temperature variations furthermore we demonstrate that temporal changes in mg ca water can be estimated using an empirically derived equation relating mg ca water to the sr ba ratio measured in the tufa samples by means of this relationship it is theoretically possible to determine the mg ca ratio of paleowaters and hence to derive reliable estimates of former water temperatures from the mg concentrations of fossil tufas from the study area sympathetic variations in sr ba and 13c along the sampled profile record changes in water chemistry which are most probably caused by variable amounts of calcite precipitation within the vadose zone of the karst aquifer this process is thought to be markedly subdued whenever the amount of wet season precipitation exceeds a given threshold accordingly distinct minima in sr ba and 13c are interpreted to reflect years with above average rainfall the pronounced seasonal and annual variability of the u concentration along the profile is thought to primarily record changes in the u flux from the soil to the water table we suggest that during intensive rain events u is transported to the phreatic zone by complexing organic colloids giving rise to conspicuous u maxima in the tufa after above average wet seasons this study demonstrates the potential of freshwater tufas to provide valuable information on seasonal temperature and rainfall variations if tufa deposits turn out to be reasonably resistant to secondary processes combined investigation of speleothems and tufas from the same area could become a promising approach in future research while speleothems offer continuous records of long term paleoenvironmental changes tufas could provide high resolution time windows into selected periods of the past

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

a molecular and stable carbon isotopic study of lipids in late quaternary sediments from the arabian sea the distribution of apolar and polar lipids and their stable carbon isotopic compositions were determined for a number of sediment samples from different sites in the arabian sea lipids are mainly derived from planktonic archaea and a range of algae including haptophytes eustigmatophytes and diatoms the stable carbon isotopic compositions of compounds from diatoms highly branched isoprenoids fall into distinct groups suggesting other sources beside the diatom species rhizosolenia setigera for these compounds high amounts of sterol ethers which may be derived from diatoms were also detected a recently identified triterpenoid malabaricatriene was also present in high abundance in these samples the terrestrial input of lipids consists of n alkanes and their carbon isotopic compositions show that they are derived from aeolian dust input from the arabian peninsula the isotopic compositions of c37 alkenones from two cores during the last 15 ka is relatively constant and suggests that growth conditions for haptophyte algae averaged over 500 years did not change significantly over this period

u th isotope systematics from the soreq cave israel and climatic correlations precise 230 th 234 u ages were obtained on thirty one growth laminae in speleothem samples which are self consistent with the detailed layer stratigraphy samples with low 232 th 238 u ratios give ages with analytical uncertainties of 40 years at 2 ky and 1000 years at 90 ky some growth zones with high but variable 232 th 238 u were dated by intermal isochrons this permits the determination of the initial 230 th 232 th assuming equilibrium of 232 th and 238 u series in the source of the high 232 th component this shows initial 230 th 232 th in activity units of from 1 3 to 2 9 the calculated atomic ratios of 232 th 238 u for the high 232 th component range from 1 08 to 2 4 which is well below the average crustal value speleothem materials with high 232 th 238 u are found to exhibit clear correlations of 232 th with si al and fe while 238 u correlates with sr and ba analyses of soreq cave drip waters show that the particulates in the waters have high 232 th concentrations and a 232 th 238 u ratio much lower than that found in the high 232 th component in speleothems but with 230 th 232 th 1 0 to 2 4 we infer that the trapped high th component in speleothems is from particulate matter in water with a large concentration of adsorbed u and not simply from detrital material the speleothems have only small234u excess the initial 234 u 238 u 0 show a range of 1 02 to 1 14 that was found to correlate with age over the past 25 ky the youngest samples have values in the same range as the modern drip waters there appears to be a correlation of 234 u 238 u 0 with the 18 o values there is a drop of 18 o in the time interval 20 to 15 ky which then remains relatively constant to recent times as the high 18 o values have been related to rainfall and associated climatic conditions we suggest that the 234 u 238 u in the speleothem reflects the effects of rainfall and soil weathering conditions on drip water composition and may provide a proxy for climate change