li and δ7li in himalayan rivers proxies for silicate weathering this paper presents the first systematic survey of lithium and its isotopes in the dissolved load and suspended and bed sediments of tributaries of the ganges both before and after the monsoon locations were chosen in order to cover catchments draining both silicates and carbonates at high 2000 4000 m and low 550 1300 m altitudes modelling of the dissolved composition shows that the li ca ratio of the silicate endmember in himalayan rivers is at least an order of magnitude higher than that of the carbonate endmember most of the dissolved li 90 is derived from silicates even in carbonate dominated catchments while the sr isotope composition of the dissolved load reflects that of the bedrock the main control on its li isotope composition is fractionation during weathering fractionation between the dissolved and suspended load in silicate dominated catchments is greatest at high altitude and lower at low altitude where weathering is more intense tributaries draining silicates have lower dissolved 7li values by 2 3 to 4 2 following the monsoon when weathering is more intense because of higher runoff and elevated temperatures our data suggest that riverine li fluxes largely reflect silicate weathering rates while riverine 7li varies with weathering intensity as rivers presently contribute 50 of the li input to the oceans seawater li concentrations and 7li show potential as proxies for global silicate weathering processes

silicate weathering rates decoupled from the 87sr 86sr ratio of the dissolved load during himalayan erosion this study establishes that carbonate weathering dominates the supply of sr with elevated 87sr 86sr ratios in the bhote kosi sun kosi river system central nepal a major himalayan tributary to the ganges the dissolved load of the bhote kosi sun kosi displays a rapid increase in the 87sr 86sr ratio in its lower section downstream of the main central thrust a feature common to other himalayan tributaries of the ganges mass balance analysis of the contributing sources to the dissolved load identifies the weathering of palaeoproterozoic impure carbonates comprised largely of dolomite and mica from the lesser himalaya as the most significant lithological control upon elevated 87sr 86sr ratios in situ laser ablation analysis of the dolomite component yields 87sr 86sr ratios of 0 93 to 1 11 indistinguishable from bulk rock this confirms the presence of a highly soluble source of radiogenic strontium and suggests that diffusive exchange during a protracted metamorphic history has homogenised sr isotopes between silicate and carbonate phases within the assemblage weathering of the carbonate component of such rocks has yielded unusually high 87sr 86sr at significant sr fluxes in the dissolved load approximately 60 of the impact of the bhote kosi sun kosi on the marine 87sr 86sr ratio can be traced to weathering of this carbonate since similar lithologies outcrop within the catchments of most himalayan tributaries that feed the ganges and brahmaputra rivers the presence of such radiogenic carbonates undermines the inference that the impact of these rivers on sr isotopes in seawater is indicative of high silicate weathering rates that result from the uplift of tibet and the himalaya

the relationship between riverine lithium isotope composition and silicate weathering rates in iceland abstract this study presents lithium isotope and elemental data for the dissolved phase and suspended and bedload sediments of the major icelandic rivers for the dissolved phase 7li values range between 10 1 and 23 8 while river sediments display lower and much more homogeneous values 7li 3 1 4 8 close to the composition of unweathered mid ocean ridge basalt morb high 7li values are associated with high k li na li and mg li ratios in waters draining mainly old and weathered basalt catchments whereas low 7li rivers are located in younger parts of the island simple mixing between precipitation li rich hydrothermal springs and basalt weathering is unable to explain the entire range of 7li values instead a simple model of li uptake by secondary minerals associated with clay water li isotope fractionation 7li ranging from 1 to 7 5 can explain both water and sediment 7li values a negative correlation is observed between basalt chemical erosion rates and 7li measured in icelandic rivers and an empirical law is inferred comparison with literature data suggests that this relationship may be applicable at a more global scale and if confirmed could be of particular use for estimating the evolution of continental weathering preserved in marine sedimentary records however more data are now needed for rivers draining silicates typical of the continental crust in order to refine large scale modelling

riverine dissolved lithium isotopic signatures in low relief central africa and their link to weathering regimes the isotopic composition of dissolved lithium 7li near the congo river mouth varied from 14 to 22 in 2010 and was negatively correlated to discharge from the relationship between dissolved 7li and strontium isotopes we suggest that this large variation is due to mixing of waters from two contrasting continental weathering regimes one end member high 7li 25 represents waters sourced from active lateritic soils covering the periphery of the basin li highly sequestered into secondary mineral products and another representing blackwater rivers low 7li 5 7 derived from the swampy central depression where high organic matter content in water leads to congruent dissolution of the tertiary sedimentary bedrock this suggests that the lithium isotopic signature of tropical low relief surfaces is not unique and traces the long term large scale vertical motions of the continental crust that control geomorphological settings this evolution should be recorded in the oceanic secular 7li curve

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

insights into post magmatic metasomatism and li circulation in granitic systems from phosphate minerals of the nanping no 31 pegmatite se china abstract phosphate minerals are key indicators for understanding the processes of diagenesis and mineralization in granitic systems more importantly these minerals provide constraints on the post magmatic behaviour of li in pegmatite systems that remain to be explored although li from early crystallized li phosphates is known to leach into hydrothermal fluids in this study we performed a detailed petrographic and compositional analysis of phosphate minerals from the nanping no 31 pegmatite in southeastern of china which provides new evidence of li circulation in granitic systems primary phosphate minerals evolved from xenotime and monazite to mn rich fluorapatite and then to triphylite and montebrasite with large amounts of montebrasite in the intermediate zones of the pegmatite reflecting the increasing activities of both li and p in peraluminous granitic melts montebrasite experienced stages of both high temperature 360 273 c and low temperature 273 100 c hydrothermal alteration the high temperature hydrothermal alteration of montebrasite by fe and mg rich fluids resulted in the formation of a series of fe mg bearing phosphates the replacement of montebrasite under low temperature alteration generated amounts of ca sr ba bearing phosphate minerals muscovite and akdalaite al2o3 4 h2o the formation of micro networks of akdalaite from montebrasite indicate the low mobility of al in hydrothermal fluids a four stage scenario of post magmatic li transport in the nanping no 31 pegmatite is proposed 1 li derived from the breakdown of primary montebrasite was locally recrystallized and produced secondary montebrasite mtb 1 2 hydrothermal alteration by fe and mg bearing fluids leached li from montebrasite to form secondary triphylite and simferite 3 li derived from the alteration of montebrasite by sr ca rich fluids was involved in the formation of secondary palermoite and bertossaite and 4 the replacement of secondary triphylite montebrasite palermoite and bertossaite by later phases apatites ludlamite anapaite augelite and fine grained muscovite reflect the leaching of li back into hydrothermal fluids the re enrichment of li during the post magmatic stage most likely increased the solubility of ta in the hydrothermal fluids of the granitic pegmatite

hydrothermal water rock interaction and the redistribution of li b and cl in the taupo volcanic zone new zealand abstract lithium b and cl occur as trace elements in the dominant protoliths rhyolite andesite and greywacke underlying high temperature hydrothermal systems in the taupo volcanic zone tvz the fluid mobility of li decreases substantially at 200 c 20 c with the pervasive deposition of quartz chlorite regardless of protolith and altering fluid compositions boron and cl are more fluid mobile than li at all temperatures with mobility affected by the rock type composition of circulating aqueous solutions and to a limited extent by incorporation of the trace elements in hydrothermal minerals uptake of b by clays decreases its mobility at 320 c occur compared to volcanic rocks the mobility of b and cl in greywackes is very low and thus the b cl signature of greywacke in hydrothermal aqueous solutions of the tvz is overwhelmed by the effects of rhyolite and andesite dissolution temperature dependent transitions from mg fe clays and phyllosilicates to k bearing illitic clays tend to decrease the mg li ratios in aqueous solutions at 200 c

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

lithium isotopes and implications on chemical weathering in the catchment of lake donggi cona northeastern tibetan plateau abstract this study presents lithium li isotope ratios 7 li for rocks sediments suspended particulate material and dissolved li from the lake donggi cona catchment located on the northeastern tibetan plateau china the average 7 li 1 9 of the bedrocks is estimated from local loess 7 li values decrease progressively within the sediment cascade from loess to river and lake floor sediments the lake floor sediments average at 0 7 the difference between bedrock and lake sediments reflects the preferential fractionation of dissolved 6 li into clay minerals mostly illite in the weathering zone and grain size sorting during fluvial sediment transport the 7 li values of stream and lake water samples range from 13 6 to 20 8 whereas thermal waters fall between 5 9 and 11 6 the 7 li values of lake water samples are close to 17 and reflect mixing of waters from two perennial inflows and thermal waters dissolved li in streams represents an integrated isotopic signal derived from soil solutions in the weathering zone an apparent isotopic fractionation of 17 8 1 6 sec sol 0 982 between secondary minerals and solution was determined an inflow that drains a sub catchment in the north carries a high proportion of thermal waters despite of the high proportion of admixed thermal waters with high li concentrations and low 7 li this stream has the highest 7 li values of about 21 this is consistent with admixing of thermal waters to solutions in the weathering zone and subsequent fractionation by preferential uptake of isotopically light dissolved li into secondary phases based on li isotope ratios of the dissolved and solid export flux from the weathering zone we calculated that around five times more li is exported in particles than dissolved in streams an average 7 li value of about 17 of most streams and the lake is reflecting a low weathering intensity and chemical weathering rate of about 4 t km 2 a low weathering rates and an erosion dominated weathering system are consistent with moderate precipitations the cold climate and the high relief of the study area

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

evolution of cenozoic seawater lithium isotopes coupling of global denudation regime and shifting seawater sinks abstract the li isotopic record of seawater shows a dramatic increase of 9 over the past 60 million years here we use a model to explore what may have caused this change we focus particularly on considering how changes in the reverse weathering sinks that remove li from seawater can contribute to explain the observed increase our interpretation is based on dividing the oceanic sink which preferentially removes light li into two components i removal into marine authigenic clays in sediments at low temperatures with associated high fractionation factors and ii removal into altered oceanic basalt at higher temperatures and resulting lower fractionation factors we suggest that increases in the flux of degraded continental material delivered to the oceans over the past 60 ma could have increased removal of li into sedimentary authigenic clays versus altered basalt because altered basalt is associated with a smaller isotopic fractionation an increasing portion of the lower temperature authigenic clay associated sink could contribute to the rise of the seawater li isotope value this effect would moderate the extent to which the isotopic value of continental inputs must have changed in order to explain the seawater record over the cenozoic nonetheless unless the magnitude of fractionation during removal differs significantly from current understanding substantial change in the li 7 of inputs from continental weathering must have occurred our modeling suggests that dissolved riverine fluxes in the early eocene were characterized by li 7 of 0 to 13 with best estimates of 6 6 12 6 these values imply increases over the past 60 myrs of between 10 and 24 and we view a 13 increase as a likely scenario these changes would have been accompanied by increases in both the dissolved li flux from continental weathering and the removal flux from seawater into marine authigenic clays increases in li 7 of continental input are consistent with a change in the global denudation regime as a result of increasing continental erosion rate through the cenozoic changes in denudation may have meant increasing climate sensitivity of weathering over time but do not require globally supply limited and thus entirely climate insensitive weathering in the early cenozoic

a reactive transport model for examining tectonic and climatic controls on chemical weathering and atmospheric co2 consumption in granitic regolith we developed a 1d reactive transport model for examining how tectonic and climatic parameters namely uplift rate water seepage velocity and temperature control chemical weathering and atmospheric co2 consumption rates during regolith development the model consists of mass conservation equations describing how mineral and solute concentrations change temporally and spatially vertically during weathering of granite containing 30 plagioclase an20 and up to 3 accessory calcite the equations are coupled by volumetric weathering rates which depend on mineral abundances intrinsic dissolution rate constants and surface areas as well as the departure of pore water from thermodynamic equilibrium we numerically solve a non steady state non dimensional version of the model by eliminating the need to prescribe regolith thickness non dimensionalization introduces two key scaling parameters that drive variations in the model results the rock residence time r the time for rock to travel upward through the model domain and the water residence time w the time for water to travel downward through the model domain we use the model to examine fundamental properties of chemical weathering especially reaction front propagation with the primary aim of characterizing weathering regimes and identifying factors that maximize the dissolution of plagioclase because only silicate weathering regulates atmospheric co2 levels over geological timescales weathering regimes are commonly classified as transport or supply limited versus weathering or kinetically limited but as outgrowths of geomorphology these terms primarily refer to the role of r in regulating regolith thickness our findings suggest that the paradigm should be expanded to include kinetic controls determined by w both regimes can experience far from and near equilibrium mineral dissolution however transport limited regimes generally have lower w r ratios compared to kinetically limited regimes under transport limitation thick to thin reaction fronts propagate downward indefinitely yielding extensive mineral depletion deeply weathered regolith and high ratios of silicate to carbonate weathering under kinetic limitation thin to non existent reaction fronts evolve to steady state yielding minimal mineral depletion no regolith development and low ratios of silicate to carbonate weathering kinetically limited regimes typifying tectonically active mountain ranges have low long term atmospheric co2 consumption rates transport limited regimes typifying tectonically stable cratons also have low long term atmospheric co2 consumption rates but because much of the earth s surface is transport limited we infer that such environments have the greatest impact on the global atmospheric co2 consumption flux the model output leads us to hypothesize that the maximum contribution should occur for extensive regions of silicate bedrock consolidated in warm wet climates because the combination of tectonic stability high temperatures and rapid seepage velocities accelerates reaction front propagation facilitates calcite depletion and sustains deep regolith development over long timescales our findings point to the importance of internal climate feedbacks for stabilizing long term atmospheric co2 levels

paleomagnetic study of the juiz de fora complex se brazil implications for gondwana the juiz de fora complex is mainly composed of granulites and granodioritic migmatite gneisses and is a cratonic basement of the ribeira belt paleomagnetic analysis on samples from 64 sites widely distributed along the alem paraiba dextral shear zone se brazil rio de janeiro state yielded a northeastern steep downward inclination direction dm 40 4 im 75 4 a95 6 0 k 20 1 for 30 sites the corresponding paleomagnetic pole rb is situated at 335 2 e 0 6 s a95 10 0 k 7 9 rock magnetism indicates that both titano magnetite and titanohematite are the main magnetic minerals responsible for this direction anisotropy of low field magnetic susceptibility ams measurements were used to correct the chrm directions and consequently its corresponding paleomagnetic pole this correction yielded a new mean chrm dm 2 9 im 75 4 a95 6 4 k 17 9 whose paleomagnetic pole rbc is located at 320 1 e 4 2 n a95 10 3 k 7 5 both mean chrm and paleomagnetic pole obtained from uncorrected and corrected data are statistically different at the 95 confidence circle geological and geochronological data suggest that the age of the juiz de fora complex pole is probably between 535 500 ma and paleomagnetic results permit further constraint on these ages to the interval 520 500 ma by comparison with high quality paleomagnetic poles in the 560 500 ma gondwana apw path

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 li isotope response to mountain uplift silicate weathering is a key process by which co2 is removed from the atmosphere it has been proposed that mountain uplift caused an increase in silicate weathering and led to the long term cenozoic cooling trend although this hypothesis remains controversial lithium isotopes are tracers of silicate weathering processes and may allow this hypothesis to be tested recent studies have demonstrated that the li isotope ratio in seawater increased during the period of himalayan uplift starting ca 45 ma but the relationship between uplift and the li isotope ratio of river waters has not been tested here we examine li isotope ratios in rivers draining catchments with variable uplift rates from south island new zealand a negative trend between 7li and uplift shows that areas of rapid uplift have low 7li whereas flatter floodplain areas have high 7li combined with u activity ratios the data suggest that primary silicates are transported to floodplains where 7li and 234u 238u are driven to high values due to preferential uptake of 6li by secondary minerals and long fluid mineral contact times that enrich waters in 234u in contrast in mountainous areas fresh primary mineral surfaces are continuously provided driving 7li and 234u 238u low this trend is opposite to that expected if the increase in cenozoic 7li in the oceans is driven directly by mountain uplift these data suggest that the increase in seawater 7li reflects the formation of floodplains and the increased formation of secondary minerals rather than weathering of mountain belts

weathering dynamics reflected by the response of riverine uranium isotope disequilibrium to changes in denudation rate abstract the ratio between the activity of dissolved 234 u and 238 u in river water 234 u 238 u riv shows promise for tracing weathering processes it has been suggested that preferential 234 u release generates a high 234 u 238 u riv under low weathering intensity while congruent weathering as expected for high weathering intensity would release uranium with a secular equilibrium 234 u 238 u riv of 1 however weathering intensity depends on the combination of exposure age of weathering interfaces and dissolution rate complicating the use of 234 u 238 u riv as a tool in weathering studies this work attempts to resolve the weathering dynamics reflected by 234 u 238 u riv using catchment scale denudation rate d as a first order approximation for the age of weathering interfaces a global dataset n 653 including 132 new measurements from chinese catchments indicates changing responses of 234 u 238 u riv to d higher values 1 5 and larger variability of 234 u 238 u riv are observed in catchments both with high 3 mm yr and low d 234 u 238 u riv are associated with catchments of medium to high d 0 3 2 mm yr the observed pattern can be explained by a model that involves competing uranium release from active and inactive weathering interfaces in response to a shifting weathering regime from kinetically limited to supply limited 234 u 238 u riv decreases with decreasing d under a kinetically limited weathering regime due to the increasing contribution of congruent weathering relative to the preferential dissolution of 234 u from radioactively damaged sites as d decreases in contrast 234 u 238 u riv increases with decreasing d under a supply limited weathering regime because the minerals that remain under these conditions continue to release 234 u to solution by recoil while contributing little to the dissolved 238 u because of their slow dissolution rate a much deeper depth of the weathering interface is suggested for catchments with high d due to higher weathering contribution from deep fractures relative to the poorly developed surface regolith a young age of 10 kyrs is constrained by the model for complete dissolution of exposed weathering active minerals which implies a rapid shifting from kinetically limited to supply limited weathering regime and limited influence of weathering on the calculation of comminution age based on 234 u 238 u in sediments a site specific dissolution rate that is consistent with those estimated from weathering profiles is also constrained which suggests that the discrepancy between field and laboratory weathering rate may largely be controlled by the difference in chemical affinity associated with stagnant micro pores low permeability and high rock fluid ratios rather than the passivation of weathering interfaces

behaviour of lithium and its isotopes during weathering in the mackenzie basin canada we report li isotopic compositions for river waters and suspended sediments of about 40 rivers sampled within the mackenzie river basin in northwestern canada the aim of this study is to characterize the behaviour of li and its isotopes during weathering at the scale of a large mixed lithology basin the mackenzie river waters display systematically heavier li isotopic compositions relative to source rocks and suspended sediments the range in delta li 7 is larger in dissolved load from 9 3 parts per thousand to 29 0 parts per thousand compared to suspended sediments from 1 7 parts per thousand to 3 2 parts per thousand which are not significantly different from delta li 7 values in bedrocks our study shows that dissolved li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to na the highest enrichment of li 7 in the dissolved load is reported when li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution this counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different li isotopic compositions within the mackenzie river basin the incipient weathering regime characterizing the rocky mountains and the shield areas produces li 7 enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating li isotopes the second weathering regime is found in the lowland area and produces the lower delta li 7 waters but still enriched in li 7 compared to bedrocks and the most li depleted waters compared to na fractionation factors suggest that the incorporation of li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers the correlation of boron and lithium concentrations found in the dissolved load of the mackenzie rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater in the shield and rocky mountains li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for li 7 appear although the weathering mechanisms operating in the mackenzie basin need to be characterized more precisely the li isotope data reported here clearly show the control of li isotopes by the weathering intensity the spatial diversity of weathering regimes resulting from a complex combination of factors such as topography geology climate and hydrology explains in fine the spatial distribution of li isotopic ratios in the large drainage basin of the mackenzie river there is no simple relationship between li isotopic composition and chemical denudation fluxes in the mackenzie river basin

temporal record of lithium in seawater a tracer for silicate weathering abstract this paper presents multi species records of the li ca ratio and li isotopic composition 7 li of planktonic foraminifera from the pacific and atlantic oceans for the past 18 ma the li ca record is corrected for interspecific offsets determined from recent holocene foraminifera interspecific offsets in 7 li are not significant despite different diagenetic histories the records produced from both oceans are remarkably consistent corrected planktonic foraminiferal li ca ratios range from 6 3 to 10 9 mol mol while planktonic foraminiferal 7 li ranges from 25 to 31 both records are interpreted in terms of long term changes in seawater li ca and 7 li enabling issues related to higher resolution variability in li ca and 7 li to be ignored by assuming that the hydrothermal flux of li into the oceans and the flux of li removed from the oceans during low temperature uptake by marine basalts and sediments have not changed significantly since 18 ma and using published records for the seawater calcium concentration the seawater li ca and 7 li records can be used to estimate global average river 7 li and li fluxes our records indicate that the river flux of dissolved li decreased between 16 and 8 ma while the 7 li value of the river input increased these data imply that both silicate weathering rates and weathering intensity decreased over this interval which may have been responsible for putative increases in levels of atmospheric co 2 in contrast the riverine flux of li has increased since 8 ma while its 7 li value has increased this implies that the silicate weathering rate has increased while weathering intensity has decreased since that time

chemical weathering outputs from the flood plain of the ganga abstract transport of sediment across riverine flood plains contributes a significant but poorly constrained fraction of the total chemical weathering fluxes from rapidly eroding mountain belts which has important implications for chemical fluxes to the oceans and the impact of orogens on long term climate we report water and bedload chemical analyses from the ganges flood plain a major transit reservoir of sediment from the himalayan orogen our data comprise six major southern tributaries to the ganga 31 additional analyses of major rivers from the himalayan front in nepal 79 samples of the ganga collected close to the mouth below the farakka barrage every two weeks over three years and 67 water and 8 bedload samples from tributaries confined to the ganga flood plain the flood plain tributaries are characterised by a shallow 18o d array compared to the meteoric water line with a low dexcess from evaporative loss from the flood plain which is mirrored in the higher dexcess of the mountain rivers in nepal the stable isotope data confirms that the waters in the flood plain tributaries are dominantly derived from flood plain rainfall and not by redistribution of waters from the mountains the flood plain tributaries are chemically distinct from the major himalayan rivers they can be divided into two groups tributaries from a small area around the kosi river have 87sr 86sr ratios 0 75 and molar na ca ratios as high as 6 tributaries from the rest of the flood plain have 87sr 86sr ratios 0 74 and most have na ca ratios

lithium isotope evidence for enhanced weathering during oceanic anoxic event 2 the ocean anoxic event 2 oae2 about 93 5 million years ago was marked by high atmospheric co2 concentration rapid global warming and marine anoxia and euxinia the event lasted for about 440 000 years and led to habitat loss and mass extinction the marine anoxia is thought to be linked to enhanced biological productivity but it is unclear what triggered the increased production and what allowed the subsequent rapid climate recovery here we use lithium isotope measurements from carbonates spanning the interval including oae2 to assess the role of silicate weathering we find the lightest values of the li isotope ratio 7li during oae2 indicating high levels of weathering and therefore atmospheric co2 removal which we attribute to an enhanced hydrological cycle we use a geochemical model to simulate the evolution of 7li and the ca sr and os isotope tracers our simulations suggest a scenario in which the eruption of a large igneous province led to high atmospheric co2 concentrations and rapid global warming which initiated oae2 the simulated warming was accompanied by a roughly 200 000 year pulse of accelerated weathering of mafic silicate rocks which removed co2 from the atmosphere the weathering also delivered nutrients to the oceans that stimulated primary productivity we suggest that this process together with the burial of organic carbon allowed the rapid recovery and stabilization from the greenhouse state

a global ge isotope budget we present measurements of ge isotope composition and ancillary data for samples of river water low and high temperature hydrothermal fluids and seawater the dissolved 74ge composition of analyzed rivers ranges from 2 0 to 5 6 which is significantly heavier than previously determined values for silicate rocks 74ge 0 4 0 7 escoube et al geostand geoanal res 36 2 2012 from which dissolved ge is primarily derived an observed negative correlation between riverine ge si and 74ge signatures suggests that the primary 74ge fractionation mechanism during rock weathering is the preferential incorporation of light isotopes into secondary weathering products high temperature 150 c hydrothermal fluids analyzed in this study have 74ge of 0 7 1 6 most likely fractionated during fluid equilibration with quartz in the reaction zone low temperature 25 63 c hydrothermal fluids are heavier 74ge between 2 9 and 4 1 and most likely fractionated during ge precipitation with hydrothermal clays seawater from the open ocean has a 74gesw value of 3 2 0 4 and is indistinguishable among the different ocean basins at the current level of precision this value should be regulated over time by the isotopic balance of ge sources and sinks and a new compilation of these fluxes is presented along with their estimated isotopic compositions assuming steady state non opal ge sequestration during sediment authigenesis likely involves isotopic fractionation 74gesolid solution that is 0 6 1 8

silicate weathering in the ganges alluvial plain the ganges is one of the world s largest rivers and lies at the heart of a body of literature that investigates the interaction between mountain orogeny weathering and global climate change three regions can be recognised in the ganges basin with the himalayan orogeny to the north and the plateaus of peninsular india to the south together delimiting the ganges alluvial plain despite constituting approximately 80 of the basin weathering processes in the peninsula and alluvial plain have received little attention here we present an analysis of 51 water samples along a transect of the alluvial plain including all major tributaries we focus on the geochemistry of silicon and its isotopes area normalised dissolved si yields are approximately twice as high in rivers of himalaya origin than the plain and peninsular tributaries 82 51 and 32 kmol sio2 km 2 yr 1 respectively such dissolved si fluxes are not widely used as weathering rate indicators because a large but variable fraction of the dsi mobilised during the initial weathering process is retained in secondary clay minerals however the silicon isotopic composition of dissolved si expressed as delta si 30 varies from 0 8 parts per thousand in the ganges mainstem at the himalaya front to 3 0 parts per thousand in alluvial plain streams and appears to be controlled by weathering congruency i e by the degree of incorporation of si into secondary phases the higher delta si 30 values therefore reflect decreasing weathering congruency in the lowland river catchments this is exploited to quantify the degree of removal using a rayleigh isotope mass balance model and consequently derive initial silica mobilisation rates of 200 150 and 107 kmol sio2 km 2 yr 1 for the himalaya peninsular india and the alluvial plain respectively because the non himalayan regions dominate the catchment area the majority of initial silica mobilisation from primary minerals occurs in the alluvial plain and peninsular catchment 41 and 34 respectively c 2015 the authors published by elsevier b v less

the stable calcium isotopic composition of rivers draining basaltic catchments in iceland calcium isotopic compositions 44 42ca were measured in icelandic rivers draining a range of catchment types the 44 42ca values of the rivers ranged from 0 45 to 0 67 which in all cases was higher than the 44 42ca value of basaltic rock standards 0 42 0 03 a single explanation was unable to satisfactorily explain the 44 42ca values of all rivers rather it was found that the rivers formed three distinct groups based on the extent of glacial coverage in each catchment the ca isotopic composition of rivers draining catchments with less than 10 glacial cover could be explained by the mixing of water sources basalt derived solutes meltwater taken to represent meteorological precipitation inputs and hydrothermal water however fractionation of 44 42ca in these catchments cannot unequivocally be ruled out in catchments with greater than 22 glacial cover ca isotopic compositions could not be explained by a mixture of water sources and instead reflected a fractionation process most likely the precipitation of ca bearing secondary minerals or the adsorption ion exchange of ca onto mineral surfaces the fractionation factor for this process was calculated to be 0 9999 the third group of rivers with partially glaciated 10 21 catchments grouped with glaciated catchments with respect to their sr geochemistry and with non glaciated catchments with respect to their ca geochemistry the difference in the controls of ca isotope fractionation between glaciated and unglaciated catchments was attributed to different weathering regimes

seawater 234 u 238 u during the last 800 thousand years abstract constraining the history of seawater 234u 238u is important because this ratio is used to assess the validity of u th ages and because it provides information about the past rate of physical weathering on the continents this study makes use of u rich slope sediments from the bahamas in an attempt to reconstruct seawater 234u 238u for the last 800 kyr for the last 360 kyr u th dating of these sediments provides ages and initial 234u 238u values sixty seven samples largely from marine highstands have initial 234u 238u which scatter somewhat about the modern seawater value 1 145 but neither this scatter nor the average value increases with age of sample these data contrast with published coral data and suggest that seawater 234u 238u has remained within 15 of the modern value for the last 360 kyr this confirms the rejection of coral u th ages where the initial 234u 238u is significantly different from modern seawater data from older highstands dated with 18o stratigraphy or by the presence of the brunhes matuyama b m reversal at 780 kyr allow seawater 234u 238u to be assessed prior to the range of the 230th chronometer unfortunately diagenetic scatter in the data between the b m reversal and 360 kyr is rather large probably relating to low u concentrations for these samples but there is no indication of a trend in seawater 234u 238u with age high u samples from close to the b m reversal show less diagenetic scatter and an initial 234u 238u that averages 1 102 this lower value can be explained by lower seawater 234u 238u at the time of the b m reversal or by progressive loss of 234u from the sediment by recoil a simple box model is presented to illustrate the response of seawater 234u 238u to variations in riverine input such as might be caused by changes in continental weathering comparison of the bahamas 234u 238u data with model results indicates that riverine 234u 238u has not varied by more than 65 for any 100 kyr period during the last 360 kyr it also indicates that the ratio of physical to chemical weathering on the continents has not been higher than at present for any extended period during the last 800 kyr

assessing bulk carbonates as archives for seawater li isotope ratios abstract silicate weathering is a primary control on the carbon cycle and therefore long term climate tracing silicate weathering in the geological record has been a challenge for decades with a number of proxies proposed and their limits determined recently lithium isotopes in marine carbonates have emerged as a potential tracer bulk carbonates are increasingly being used as a li isotope archive though with limited tests thus far of the robustness of this approach in the modern ocean as the bulk composition of marine pelagic carbonates has changed through time and geographically assessing the fidelity of bulk carbonate as proxy carrier is fundamental to address the impact of compositional variability in bulk carbonate on li isotopes we examine 27 bahamian aragonitic bulk carbonates and 16 atlantic largely calcitic core top sediment samples two core tops only have trace

lithium isotope systematics in a forested granitic catchment strengbach vosges mountains france abstract over the last decade it has become apparent that li isotopes may be a good proxy to trace silicate weathering however the exact mechanisms which drive the behaviour of li isotopes in surface environments are not totally understood and there is a need to better calibrate and characterize this proxy in this study we analysed the li concentrations and isotopic compositions in the various surface reservoirs soils rocks waters and plants of a small forested granitic catchment located in the vosges mountains strengbach catchment france ohge http ohge u strasbg fr li fluxes were calculated in both soil profiles and at the basin scale and it was found that even in this forested basin atmospheric inputs and litter fall represented a minor flux compared to input derived from the weathering of rocks and soil minerals which together represent a minimum of 70 of dissolved li li isotope ratios in soil pore waters show large depth dependent variations average dissolved 7 li decreases from 1 1 to 14 4 between 0 and 30 cm but is 30 7 at 60 cm this range of li isotopic compositions is very large and it encompasses almost the entire range of terrestrial li isotope compositions that have been previously reported we interpret these variations to result from both the dissolution and precipitation of secondary phases large isotopic variations were also measured in the springs and stream waters with 7 li varying from 5 3 to 19 6 7 li increases from the top to the bottom of the basin and also covaries with discharge at the outlet these variations are interpreted to reflect isotopic fractionations occurring during secondary phase precipitation along the water pathway through the rocks we suggest that the dissolved 7 li increases with increasing residence time of waters through the rocks and so with increasing time of interaction between waters and solids a dissolution precipitation model was used to fit the dissolved li isotopic compositions it was found that the isotopic compositions of springs and stream waters are explicable by an isotopic fractionation of 5 to 14 best fit 10 8 in agreement with li incorporation into clay in soil solutions it was found that isotopic fractionation during secondary precipitation is larger at least 23 suggesting a major role for different secondary phases such as iron oxides that maybe incorporate li with a higher isotope fractionation

climatic and tectonic controls on chemical weathering in the new zealand southern alps abstract climatic and tectonic controls on the relative abundance of solutes in streams draining the new zealand southern alps were investigated by analyzing the elemental and sr isotope geochemistry of stream waters bedload sediment and hydrothermal calcite veins the average relative molar abundance of major cations and si in all stream waters follows the order ca2 50 si 22 na 17 mg2 6 k 5 for major anions the relative molar abundance is hco3 89 so42 7 cl 4 weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters but nearly all streams have a carbonate dominated ca2 and hco3 mass balance stream water ca sr and 87sr 86sr ratios vary from 0 173 to 0 439 mol nmol and from 0 7078 to 0 7114 respectively consistent with the ionic budget these ratios lie solely within the range of values measured for bedload carbonate ca sr 0 178 to 0 886 mol nmol 87sr 86sr 0 7081 to 0 7118 and hydrothermal calcite veins ca sr 0 491 to 3 33 mol nmol 87sr 86sr 0 7076 to 0 7097 streams draining regions in the southern alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain 10 more ca2 and 30 more sr2 from carbonate weathering compared to streams draining regions in drier more stable landscapes similarly streams draining glaciated watersheds contain 25 more sr2 from carbonate weathering compared to streams draining non glaciated watersheds the highest abundance of carbonate derived solutes in the most physically active regions of the southern alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock which contains trace amounts of calcite estimated to weather 350 times faster than plagioclase in this environment in contrast regions in the southern alps experiencing lower rates of uplift and erosion have a greater abundance of silicate versus carbonate derived cations these findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters using the southern alps as a model for assessing the role of active tectonics in geochemical cycles this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater

lithium isotope fractionation in the ganges brahmaputra floodplain and implications for groundwater impact on seawater isotopic composition abstract lithium isotopes are a promising proxy for reconstructing past weathering processes but unraveling the seawater record requires a comprehensive understanding of the magnitude and isotopic composition of li fluxes to the oceans and of how these change over time little information is available on the role of floodplain sediments and groundwater systems in setting the li isotope signature of the dissolved flux delivered from the continents to the oceans here we investigate the li dissolved fluxes of river waters and groundwaters in the ganges brahmaputra floodplain the data suggest that a maximum of 3 1 10 8 and 1 5 10 8 moles li yr are carried to the bay of bengal by ganges brahmaputra rivers and groundwaters respectively the riverine flux has a significantly heavier li isotope composition average 7li 26 than the groundwater flux average 7li 16 and increases downstream across the floodplain 7li in both river waters and shallow groundwater can be explained by li scavenging by quaternary floodplain sediments following a rayleigh fractionation process with preferential removal of 6li on the other hand deep groundwaters 40 m contributing to submarine groundwater discharge to the bay of bengal are enriched in 6li at depth likely due to the dissolution of floodplain sediments releasing li with a light isotope composition similarly low 7li has been reported in other large sedimentary aquifers the deep groundwater values are close to the average isotope composition of the global li inputs to the ocean 15 so groundwater submarine discharge has only a minor influence on the assessment of the modern li isotope budget of the ocean our analysis further suggests that groundwater discharge of li has probably played at most a small and secondary role in past changes in the isotope composition of the total continental flux of li to the ocean

the effect of hydrothermal spring weathering processes and primary productivity on lithium isotopes lake myvatn iceland lithium isotopes are rapidly becoming one of the most useful tracers of silicate weathering processes but little is known on their behaviour in groundwaters and hydrothermal springs and how these sources might influence the weathering signal in surface waters this study presents lithium isotope compositions 7li for cold groundwaters 3 7 c and hydrothermal springs that were at geothermal temperatures 200 300 c but have cooled during transport 17 44 c both represent an important source of water and nutrients for lake myvatn iceland we also present a time series from the laxa river which is the single outflow from the lake the 7li values in the input springs to lake myvatn are highly variable 5 27 and correlate inversely with temperature and total dissolved solids these co variations imply that even in such waters the processes controlling 7li variations during weathering still operate that is the ratio of primary rock dissolution to secondary mineral formation where the latter preferentially incorporates 6li with a temperature dependent fractionation factor in high temperature geothermal waters 300 c secondary mineral formation is inhibited and has a low fractionation factor leading to little 7li fractionation even in waters that have cooled considerably over several months from their geothermal temperatures fractionation is still low and 7li values are similar to those reported from waters measured at 350 c in contrast cooler groundwaters promote relatively high proportions of clay formation which scavenge dissolved solids including 6li the time series on the laxa river the single outflow from lake myvatn shows little 7li variation with time over the 12 month sampling period 17 21 demonstrating that in contrast to tracers such as si isotopes li isotopes are unaffected by the significant seasonal phytoplankton blooms that occur in the lake thus these results clearly illustrate that li isotopes are ideally suited to constrain silicate weathering processes because fractionation by secondary mineral formation operates even when groundwater and hydrothermal inputs are significant and because li isotopes are demonstrably unaffected by phytoplankton or plant growth

lithium and its isotopes in major world rivers implications for weathering and the oceanic budget abstract the outstanding problem in the lithium geochemical cycle is the lack of an isotopic mass balance in the ocean the 6li compositions of fresh basalts 4 the hydrothermal fluids derived from them average 9 and seawater significantly heavier at 32 are well understood but only very sparse river input data are available for li mass balance calculations in an attempt to rectify the situation we have measured the lithium concentrations and isotopic compositions of major world rivers draining representative geological terrains this helps both to constrain the river endmember and to understand the behavior of lithium isotopes in the continental weathering environment fluvial isotopic compositions display a very large range 6 0 to 32 2 there is no definitive relationship between 6li and lithology but in general rivers draining marine evaporites are 20 to 22 carbonates 26 to 32 black shales 26 shields 6 6 to 19 and mixed siliceous terrains 6 to 28 the flow weighted mean concentration of the measured rivers is 215 nm at 23 this updated riverine 6li value responsible for 30 of the global riverine discharge does not solve the isotopic imbalance if the measured li concentrations and isotopic compositions are representative of all rivers the presence of a yet unidentified sink with a higher fractionation factor 1 023 than determined for low temperature basalt alteration 1 019 is required for an isotopic steady state of li in the ocean authigenic clays are a possible candidate as clays are known to be enriched in both lithium and in the light isotope preferentially alternatively the hydrothermal flux must be much less than half of that estimated by the 3he inventory and the oceanic budgets for 87sr 86sr and mg the relationship of 6li to the major ions and 87sr 86sr suggests that the important processes affecting river dissolved lithium isotopic compositions are fractionation between solution and secondary minerals and thus the intensity of weathering

relationship between mechanical erosion and atmospheric co2 consumption in the new zealand southern alps to examine the influence of mountain uplift on the long term carbon cycle we used geochemical hydrologic and suspended load data for 12 streams draining the new zealand southern alps to quantify rates of erosion weathering and atmospheric co 2 consumption rapid uplift in the western southern alps elevates mechanical erosion rates by a factor of 13 relative to those on the tectonically stable eastern side 125 10 8 vs 9 4 10 8 g km 2 yr respectively similarly the average chemical weathering rate is 5 times higher on the western compared to eastern side of the mountain range 9 8 10 7 vs 2 0 10 7 g km 2 yr respectively however because the proportion of stream water ca 2 and mg 2 from carbonate weathering increases as the rate of mechanical erosion increases the long term atmospheric co 2 consumption rate on the western side is 2 times higher than that on the eastern side 14 10 4 vs 6 9 10 4 mol km 2 yr respectively and only 1 5 times higher than the global mean value 9 10 4 mol km 2 yr data for major world rivers including himalayan rivers provide a consistent interpretation regarding the relationship between mechanical erosion intensity and the ratio of silicate to carbonate weathering thus we conclude that mountain building increases atmospheric co 2 consumption rates by only a factor of 2 which is much lower than previous estimates

experimental determination of li isotope behaviour during basalt weathering silicate weathering is the primary control of atmospheric co2 concentrations on multiple timescales however tracing this process has proven difficult lithium isotopes are a promising tracer of silicate weathering this study has reacted basalt sand with natural river water for 9 months in closed experiments in order to examine the behaviour of li isotopes during weathering aqueous li concentrations decrease by a factor of 10 with time and 7li increases by 19 implying that li is being taken up into secondary phases that prefer 6li mass balance using various selective leaches of the exchangeable and secondary mineral fractions suggest that 12 16 of li is adsorbed and the remainder is removed into neoformed secondary minerals the exchangeable fractionation factors have a 7liexch soln 11 6 to 11 9 while the secondary minerals impose 7lisecmin soln 22 5 to 23 9 overall the experiment can be modelled with a rayleigh fractionation factor of 0 991 similar to that found for natural basaltic rivers the mobility of li relative to the carbon cycle critical cations of ca and mg changes with time but rapidly evolves within one month to remarkably similar mobilities amongst these three elements this evolution shows a linear relationship with 7li largely due to a co variation between aqueous li and 7li suggesting that li isotopes have the potential to be used as a tracer of ca and mg mobility during basaltic weathering and ultimately co2 drawdown

competition between erosion and reaction kinetics in controlling silicate weathering rates article i nfo weathering rates of minerals may be limited by the reaction kinetics of the constitutive minerals or the supply of fresh minerals delivered to the near surface by denudation we use a model of denudation and reaction kinetics to analyze the relative importance of each in moderating silicate weathering fluxes in different erosional environments for different commonly occurring silicate minerals we find that minerals that reside in the near surface weathering zone for far longer than the reaction takes to complete as is the case with slow denudation produce weathering fluxes that respond most strongly to changes in denudation rate conversely when transit through the weathering zone is rapid relative to a given mineral s weathering timescale the weathering responds most vigorously to changes in the reaction kinetics based on field and laboratory measured values of silicate weathering kinetic constants observed weathering zone thicknesses in relation to denudation rates and denudation rates inferred from topography it appears that fresh mineral supply may play a sub equal or dominant role in moderating silicate weathering fluxes for four mineral phases considered if correct this suggests that the concentration of atmospheric co2 which is regulated by silicate weathering over geologic timescales may depend on those factors that control long term erosion rates across earth s surface

riverine li isotope fractionation in the amazon river basin controlled by the weathering regimes we report li isotope composition 7li 7li of river borne dissolved and solid material in the largest river system on earth the amazon river basin to characterize li isotope fractionation arl a continental scale the 7li 7li in the dissolved load 1 2 to 32 is fractionated toward heavy values compared to the inferred bedrock 1 to 5 and the suspended sediments 6 8 to 0 5 as a result of the preferential incorporation of 66li into secondary minerals during weathering despite having very contrasted weathering and erosion regimes both andean headwaters and lowland rivers share similar ranges of dissolved 7li 7li 1 2 to 18 correlations between dissolved 7li 7li and li na and li mg ratios suggest that the proportion of li incorporated in secondary minerals during weathering act as the main control on the 7lidiss 7lidiss across the entire amazon basin a batch steady state fractionation model for andean and lowland rivers satisfactorily reproduces these variations with a fractionation factor between weathering products and dissolved load sec dis sec dis of 0 983 two types of supply limited weathering regimes can be identified for the lowlands clearwaters with dominant incorporation of li in secondary minerals and black waters e g rio negro where dissolution of secondary minerals enhanced by organic matter produces low 7li 7li apart from the black waters the 7li 7li of andean and lowland rivers is negatively correlated to the denudation rates with the lowest 7li 7li corresponding to the rivers having the highest denudation rates in contrast the main tributaries draining both the andes and the lowlands have higher 7li 7li compared to other rivers we propose that part of the dissolved li derived from weathering in the andes is re incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved 7li 7li along the course of these rivers unlike other rivers the dissolved 7li 7li in the main tributaries is best described by a rayleigh fractionation model with a fractionation factor sec dis sec dis of 0 991 altogether the control imposed by residence time in the weathering zone and floodplain processes results in i a non linear correlation between dissolved 7li 7li and the weathering intensity defined as w d and ii a positive relationship between the dissolved li flux and the denudation rate these results have important implications for the understanding of past ocean 7li 7li and its use as a paleo weathering proxy

lithium isotope fractionation during continental weathering processes a comprehensive understanding of lithium isotope fractionation during terrestrial weathering is necessary in order to use lithium isotopes to trace chemical cycles climatic changes and igneous processes this study investigates lithium isotope fractionation in two laboratory experiments and by analyses of natural basalt weathering products partial dissolution of basalts in the laboratory does not result in fractionation of lithium isotopes but similar dissolution of a granite sample causes significant fractionation this may reflect dissolution of secondary minerals from the granite or differences in the li isotope composition of primary minerals in this more evolved igneous rock significant li isotope fractionation was also observed during sorption onto mineral surfaces in the laboratory although this was highly dependent on sample mineralogy no fractionation accompanies the outer sphere physisorption of li to smectite surfaces some fractionation accompanies sorption onto ferrihydrite and significant fractionation with 0 986 is seen during inner sphere chemisorption to gibbsite surfaces repeat experiments with varying amounts of sorption demonstrate that li remains in an exchangeable site on the gibbsite surface the extent of fractionation onto gibbsite observed in this study 13 is about half that required in order for clay surface removal to balance the ocean li isotope budget isotopic fractionation of li was found to occur on a 300 yr timescale in both cold and warm natural environments a minimally altered rock surface from iceland was found to be two lighter in 7li than the sample interior probably due to the preferential incorporation of 6li in clay or oxide rich alteration products on the sample surface soil samples from hawaii also demonstrate li fractionation during weathering in this environment rainwater 7li 10 contributes a significant flux of isotopically heavy li to the developing soil despite this soils have similar li isotope compositions to the primary basalt 7li 4 indicating that 6li is preferentially retained during weathering this conclusion is supported by isotopically heavy river water on hawaii 7li 22 and by the correlation between li concentration and 7li in the soil the lab and field based measurements of this study clearly demonstrate that lithium fractionates during weathering potentially providing information about the weathering environment now and in the past

lithium magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in iceland this study presents lithium magnesium and silicon isotope ratios from pore waters and soils from a well characterised histic andosol in south west iceland the soil 7li composition ranges between values slightly lighter than basalt to those that are much heavier 1 1 to 26 8 and are possibly influenced by sea salt in contrast precipitation corrected dissolved pore water 7li values 1 8 10 0 appear to reflect preferential adsorption of 6li onto secondary minerals where allophane supersaturation results in high 7li values conversely low 7li together with high li are probably due to destabilisation of allophane at low ph and thus desorption of li when compared to icelandic river values it would appear that soil pore waters reflect an intermediate isotope composition between basalts and river waters precipitation corrected pore water mg isotope ratios 26mg range between 0 46 and 0 12 and correlate with the amount of heavy mg adsorbed onto the soil exchange complex silicon isotopes in the soils are isotopically lighter 30si 0 91 to 0 53 than basalt 0 29 whereas pore waters are heavier 0 13 to 1 03 soil 30si values show a clear evolution between unweathered basalt and a hypothetical isotopically light endmember representing secondary minerals dissolved si isotopes also respond to chemical weathering processes and show that isotopically heavy 30si corresponds to high cation fluxes and high secondary mineral formation however comparison of all these proposed isotopic weathering tracers suggests that they respond differently to the same chemical weathering conditions this indicates a differing behaviour during secondary mineral neoformation or adsorption depending on whether the incorporated element is a major or trace constituent in turn this behaviour can potentially yield important information on secondary mineral behaviour and destabilisation and thus on the chemical weathering processes

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

direct evidence of the feedback between climate and weathering long term climate moderation is commonly attributed to chemical weathering the higher the temperature and precipitation the faster the weathering rate weathering releases divalent cations to the

downstream and seasonal changes of lithium isotope ratios in the ganges brahmaputra river system the li isotope ratio 7li is expected to be a useful tracer of silicate weathering in river and groundwater systems which is an important contributor to the seawater compositional changes that accompany the evolution of the earth s surface environment to obtain accurate estimates of continental li fluxes to the ocean we determined 7li values of dissolved li in the lower ganges brahmaputra river system in both the dry and rainy seasons and in deep groundwater in the bengal basin dissolved li and 7li values in the lower reaches of the rivers 0 04 0 66 mol kg 1 and 19 1 to 34 2 respectively were predominantly derived from silicate weathering as is the case in the upper parts of these rivers we observed large changes in 7li over a distance of more than 1000 km downstream that were due mainly to rayleigh type removal of li from river water extremely high li concentrations 1 15 1 67 mol kg 1 and low 7li values 5 1 to 11 6 in groundwater samples indicate congruent isotope leaching and dissolution of silicate minerals in the deep aquifer where the water residence time is long in the rainy season li concentrations and 7li values were lower than in the dry season owing to the shorter residence time of river water and the substantial input of local subsurface flow through lowland alluvium these results suggest that accurate estimation of continental li fluxes to the ocean should take account of downstream and seasonal changes as well as aquifer depth variations in 7li values

influence of grass roots on shear strength of pyroclastic soils this paper investigates the effects of indigenous vegetation on the shear strength of loose pyroclastic soils of the campania region southern italy these soils are frequently affected by shallow

lithium isotope behaviour during weathering in the ganges alluvial plain the ganges river system is responsible for the transportation of a large flux of dissolved materials derived from himalayan weathering to the oceans silicate weathering driven cooling resulting from uplift of the himalayas has been proposed to be a key player in cenozoic climate variation this study has analysed li isotope 7li ratios from over 50 ganges river waters and sediments in order to trace silicate weathering processes sediments have 7li of 0 identical to bulk continental crust however suspended sediment depth profiles do not display variations associated with grain size that have been observed in other large river systems dissolved 7li are low 11 in the ganges headwaters but reach a constant value of 21 1 6 within a relatively short distance downstream which is then maintained for almost 2000 km to the ganges mouth given that li isotopes are controlled by the ratio of primary mineral dissolution to secondary mineral formation this suggests that the ganges floodplain is at steady state in terms of these processes for most of its length low 7li in the mountainous regions suggest silicate weathering is therefore at its most congruent where uplift and fresh silicate exposure rates are high however there is no correlation between 7li and the silicate weathering rate in these rivers suggesting that li isotopes cannot be used as a weathering rate tracer although they do inform on weathering congruency and intensity the close to constant 7li values for the final 2000 km of ganges flow also suggest that once the size of the alluvial plain reached more than 500 km the flow distance after which riverine 7li stops varying the ganges exerted little influence on the changing cenozoic seawater 7li because riverine 7li attained a near steady state composition

riverine behaviour of uranium and lithium isotopes in an actively glaciated basaltic terrain this study presents u and li isotope and major and trace element data for the dissolved load suspended particulates and bedload for icelandic rivers draining predominantly basaltic catchments physical erosion rates range from 920 to 2084 t km2 yr with the higher values associated with glacier fed rivers chemical erosion rates range from 45 to 91 t km2 yr with lower rates being associated with glacier fed rivers uranium activity ratios 234u 238u are close to secular equilibrium in the suspended and bedloads but all dissolved load samples show values greater than unity ranging from 1 13 to 2 41 the highest 234u 238u values are found in glacier fed rivers and can be attributed to recoil effects as grinding by glaciers locally enhances rates of physical weathering activity ratios in glacial rivers decrease with distance from the glacial source due to input from non glacial tributaries which have high levels of dissolved uranium and lower activity ratios in contrast in non glacial rivers uranium activity ratios increase with distance downstream due to continued weathering in soils and of bedrock the 7li value of the suspended load is always lower than that of the bedload due to preferential retention of 6li in secondary minerals during weathering in turn the 7li value of the dissolved load is always greater than that of the bedload ranging from 17 0 to 43 7 7li for the dissolved load decreases with increasing levels of dissolved silicon and the saturation index of secondary minerals which suggests that 7li decreases with increasing chemical weathering there is no correlation between 7li values and uranium activity ratios for the dissolved load for non glacial rivers but for glacier fed rivers there is an increase in 7li with increasing 234u 238u suggesting that where physical comminution of mineral grains by glaciers is high chemical weathering is suppressed and vice versa

tracing weathering regimes using the lithium isotope composition of detrital sediments lithium li isotopes are a promising tracer of chemical weathering processes for both modern and ancient times in order to improve the use of li isotopes in the sedimentary record here we calibrate the relationship between weathering intensity and detrital li isotope composition 7li using the fine fraction of modern large river sediments through independent estimates for sediment provenance to calculate the li isotope signature of the rock from which the sediments derive through weathering we show that source rock variability in particular the relative contribution of sedimentary versus igneous rocks must be corrected for before using li isotopes as a weathering proxy we also show that for rivers draining mountain ranges the contribution to river sediments of particles derived from sedimentary rocks is correlated to their li al ratio making it possible to use li contents to estimate the average source rock composition once corrected for bedrock variability the li isotope signature defines a negative relationship with the weathering intensity ratio between silicate weathering rate and total denudation rate with highest li isotope fractionation for the highest weathering intensity altogether we propose a set of new relationships between weathering erosion provenance and li isotopes that can be used to quantify present day and paleo weathering using detrital sediment