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

expansion of ice marginal lakes at the vatnajokull ice cap iceland from 1999 to 2009 abstract outlet glaciers from the vatnajokull ice cap in iceland europe s largest ice cap have been retreating since the end of the little ice age only interrupted by short intervals of advances the net mass balance has been negative since 1995 and ice marginal lakes in front of the outlets have been expanding since then the switch from land based glacier termini to lake terminating glaciers is an eye catching environmental change the heat capacity of the ice marginal lakes and the change into calving glacier fronts accelerates ice loss from vatnajokull landsat 7etm satellite imagery from 1999 to 2009 reveals high resolution snapshots of the lake development during the last decade ice marginal lakes are growing at many outlet glaciers draining the southern sector of vatnajokull this is most pronounced at the large brei amerkurjokull glacier where the jokulsarlon lake expanded from 15 km 2 in 2000 to more than 21 km 2 in 2009 the lakes act as traps for sediment that was earlier transported across outwash plains toward the atlantic coast hence the glacier retreat and sediment trapping enhance coastline retreat and river incision in the outwash plains similar changes although at a much larger scale occurred in many past glaciated regions during the last glacial termination thus the lakes in front of vatnajokull provide important modern analogues for processes that likely impacted global climate in the past

erosional and climatic effects on long term chemical weathering rates in granitic landscapes spanning diverse climate regimes abstract we used cosmogenic nuclide and geochemical mass balance methods to measure long term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes our 42 study sites encompass widely varying climatic and erosional regimes with mean annual temperatures ranging from 2 to 25 c average precipitation ranging from 22 to 420 cm year 1 and denudation rates ranging from 23 to 755 t km 2 year 1 long term chemical weathering rates range from 0 to 173 t km 2 year 1 in several cases exceeding the highest granitic weathering rates on record from previous work chemical weathering rates are highest at the sites with rapid denudation rates consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock a simple empirical relationship based on temperature precipitation and long term denudation rates explains 89 95 of the variation in long term weathering rates across our network of sites our analysis shows that for a given precipitation and temperature chemical weathering rates increase proportionally with fresh material supply rates we refer to this as supply limited weathering in which fresh material is chemically depleted to roughly the same degree regardless of its rate of supply from breakdown of rock the temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter comparisons of catchment mass balance data from the field our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought at least in actively eroding unglaciated terrain similar to our study sites to the extent that chemical weathering rates are supply limited in mountainous landscapes factors that regulate rates of mineral supply from erosion such as tectonic uplift may lead to significant fluctuations in global climate over the long term

calcium isotope ratios in the world s largest rivers a constraint on the maximum imbalance of oceanic calcium fluxes 1 the oceanic mass balance of calcium ca is defined by a balance between the inputs rivers and hydrothermal and outputs bulk carbonate of ca large rivers were analyzed for ca isotope ratios 44ca 42ca expressed as ca to investigate the source and cycling of riverine ca and to add an isotopic mass balance constraint to the oceanic budget of ca the new data account for approximately one third of the total ca supplied to the oceans by rivers inter sample and seasonal variability was assessed by analyzing more than one sample for many rivers the range in the ca of large rivers at high water stand is extremely narrow at 0 27 variations in ca do not correlate with proxies for carbonate silicate or evaporite derived ca and are more likely related either to inherent variability in the lithological sources of ca or to process related fractionation the spread in riverine ca overlaps with the spread in marine limestone ca consistent with most riverine ca coming from the recycling of limestones the ca isotope composition of continental runoff has an average ca value of 0 38 0 04 identical to recent 5 m yr bulk carbonate ooze 0 33 0 13 2s d isotopic mass balance constrains that the input and output fluxes of ca to and from the oceans are balanced to within 15 over time scales similar to the residence time of ca in the oceans 1 m yr a greater imbalance between the fluxes would result in a detectable difference between the ca value of bulk carbonate and the riverine input at the current level of uncertainty the input and output fluxes could be imbalanced over much shorter time scales such as glacial interglacial cycles in which case the ocean carbonate system will not yet have responded because of the long residence time of ca the maximum current flux imbalance of 15 would be sufficient to account for the total variations in ca concentration over the tertiary such an interpretation is not unique but is the simplest interpretation given the similarity between the input and output isotopic compositions and rules out hypotheses of extreme imbalance in the recent global biogeochemical cycle of ca

lithium isotope evidence for enhanced silicate weathering during oae 1a early aptian selli event an abrupt rise in temperature forced by a massive input of co2 into the atmosphere is commonly invoked as the main trigger for oceanic anoxic events oaes global warming initiated a cascade of palaeoenvironmental perturbations starting with increased continental weathering and an accelerated hydrological cycle that delivered higher loads of nutrients to coastal areas stimulating biological productivity the end result was widespread anoxia and deposition of black shales the hallmarks of oaes in order to assess the role of weathering as both an oae initiator and terminator via co2 sequestration during the early aptian oae 1a selli event 120 ma the isotopic ratio of lithium isotopes was analysed in three sections of shallow marine carbonates from the pacific and tethyan realms and one basinal pelagic section from the tethyan domain because the isotopic composition of lithium in seawater is largely controlled by continental silicate weathering and high and low temperature alteration of basaltic material a shift to lighter 7li values is expected to characterize oaes the studied sections illustrate this phenomenon 7li values decrease to a minimum coincident with the negative carbon isotope excursion that effectively records the onset of oae 1a a second negative 7li excursion occurs coeval with the minimum in strontium isotopes after the event the striking similarity to the strontium isotope record argues for a common driver the formation and destruction weathering of an oceanic lip could account for the parallel trend in both isotope systems the double spike in lithium isotopes is probably related to a change in weathering congruencies such a chemostratigraphy is consistent with the hypothesis that an increase in silicate weathering in conjunction with organic carbon burial led to drawdown of atmospheric co2 during the early aptian oae 1a

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 evidence for a fractionated reservoir of ca and mg on the continents implications for the oceanic ca cycle analysis of river water rock travertine and soil from the high altitude negligible vegetation setting of the southern tibetan plateau demonstrates that ca and mg isotope ratios are fractionated during weathering dissolved ca in the two rivers studied is derived primarily by limestone dissolution 44 42ca in the rivers averages 0 43 and is statistically distinct from limestone at 0 31 the range in 44 42ca in these small rivers is 0 43 equivalent to the entire range in 44 42ca recorded in marine carbonate over the last 80 ma precipitation of isotopically light travertine with a 44 42ca of 0 21 enriches solute ca in heavy isotopes the mg isotope composition of the rivers is intermediate between limestone and silicate rock averaging 1 5 silicate soil has a 26mg of 0 03 heavier than silicate rock by 0 5 these fractionations in the soil create a companion groundwater reservoir of heavy ca and light mg seasonal variations in ca and mg isotope ratios in the dissolved load are small but define an array which can be modelled as a mixture between a fractionated groundwater reservoir and surface runoff reflecting the isotopic composition of the lithology fractionation of ca during the weathering of the continents is of importance to the global cycle of ca the riverine input of ca to the oceans dominated by carbonate weathering is controlled not only by the composition of the primary continental crust but also by the size and composition of a fractionated reservoir on the continents the impact on the oceanic cycle of ca depends on the relative residence times of dissolved ca in the ocean and the storage time of fractionated ca

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 isotopes as tracers of ecosystem processes theory and methods abstract the strontium sr isotope method can be a powerful tool in studies of chemical weathering and soil genesis cation provenance and mobility and the chronostratigraphic correlation of marine sediments it is a sensitive geochemical tracer applicable to large scale ecosystem studies as well as to centimeter scaled examination of cation mobility within a soil profile the 87sr 86sr ratios of natural materials reflect the sources of strontium available during their formation isotopically distinct inputs from precipitation dryfall soil parent material and surface or groundwater allow determination of the relative proportions of those materials entering or leaving an ecosystem the isotopic compositions of labile soil exchange complex and soil solution strontium and sr in vegetation reflect the sources of cations available to plants strontium isotopes can be used to track the biogeochemical cycling of nutrient cations such as calcium the extent of cation contributions from in situ weathering and external additions to soil from dust and rain can also be resolved with this method in this paper we review the geochemistry and isotopic systematics of strontium and discuss the use of this method as a tracer of earth surface processes

chemical weathering of basalts southwest iceland effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean river ground and peat water from a basaltic catchment area laxa in kjos southwest iceland were sampled and analyzed for major element concentrations to define the effect of glassy versus crystalline basalt the formation of secondary minerals and runoff on fluxes of dissolved elements to the ocean the proportions of the dissolved solids in the water samples derived from precipitation air and rock weathering were estimated the amounts of na mg ca k and s originating from precipitation were calculated from the respective solute cl marine ratios and the estimated aqueous cl originated from precipitation which was calculated using measured b content of the waters and b cl molal marine and rock ratios the rock contribution from weathering ranged from 22 to 46 percent of the total dissolved solids of the waters iron and al showed low mobility compared to na this was primarily due to consumption of these elements by ferrihydroxides allophane imogolite and clay minerals also si and to a lesser degree ca show apparent slightly lower mobility than na these results are in good agreement with these phases being dominant in the soils of the study area wada and others 1992 and the main secondary minerals of basaltic glass in iceland crovisier and others 1992 reaction progress calculations support these findings according to a model interaction of basaltic glass and meteoric water al fe and si are consumed by amorphous fe and al hydroxides and imogolite and allophane at low reaction progress with increasing reaction progress ca fe mg smectite became the primary secondary minerals limiting the mobility of ca fe mg si and al these findings suggest that the weathering in the laxa in the kjos catchment area has evolved into the beginning of the smectite weathering stage and that it is dominated by a relatively low reaction progress and a high water to rock ratio in the present study the effect of runoff and rock crystallinity was quantified for dissolved elemental fluxes temperature lithology other than rock crystallinity and rock age were similar in all of the catchment areas in this study silica ca f s al k mg and b had a very similar dependence on runoff on the other hand na fluxes were found to be less dependent on runoff and fe fluxes were found to be independent of runoff basaltic glass dissolves faster than fully crystallized basaltic rocks gislason and eugster 1987a therefore increased glass content of the primary rocks is thought to increase the leaching and elemental fluxes at constant runoff however because some of the solutes particularly al and fe were highly influenced or even controlled by the formation of secondary minerals this increasing dissolution with increasing glass content was hidden on the other hand basaltic glass was observed to enhance the fluxes of more mobile elements like na si ca f and s by a factor of 2 to 5 at a constant runoff of 200 cm yr constant vegetative cover and 0 to 100 percent glass content of the rocks the k and mg fluxes were found to be independent of rock crystallinity however runoff alone cannot explain the fluxes of these elements and it seems that vegetative cover and seasonal variations in biomass activities influence the k and mg fluxes

climatic control of the surge periodicity of an icelandic outlet glacier surging outlet glaciers are important in draining large ice caps but the mechanisms controlling surge periodicities are poorly known we investigated a sediment sequence from the glacier fed lake logurinn in eastern iceland and our unique annually resolved data based on sedimentary varves imply that eyjabakkajokull an outlet glacier of vatnajokull began surging about 2200 cal a bp before 1950 ad approximately 1700 cal a bp the glacier started to surge at a uniform 34 to 38 year periodicity that prevailed until the coldest part of the little ice age when the periodicity almost halved to 21 23 years since the late 1800s the surge periodicity of eyjabakkajokull has returned to a longer period of 35 40 years we suggest that surge periodicities of eyjabakkajokull are forced by climatically driven mass balance changes which may be a common forcing factor for similar surge type outlet glaciers copyright c 2011 john wiley sons ltd less

long time variation of soil co2 fluxes at the summit crater of vulcano italy here we report the first continuous data of geochemical parameters acquired directly from the active summit crater of vulcano this approach provides a means to better investigate deep geochemical processes associated with the degassing system of vulcano island in particular we report on soil co2 fluxes from the upper part of vulcano a closed conduit volcano from september 2007 to october 2010 large variations in the soil co2 and plume so2 fluxes order of magnitude coinciding with other discontinuous geochemical parameters co2 concentrations in fumarole gas and physical parameters increase of shallow seismic activity and fumarole temperatures have been recorded the results from this work suggest new prospects for strengthening geochemical monitoring of volcanic activity and for improving the constraints in the construction of a geochemical model this being a necessary condition to better understand the functioning of volcanic systems

the influence of weathering processes on riverine magnesium isotopes in a basaltic terrain this study presents major trace element and mg isotope data for the dissolved load and suspended particulates of icelandic rivers draining dominantly basaltic catchments including both glacier fed and direct runoff rivers these samples provide the opportunity to understand the behaviour of mg isotopes during chemical weathering where variations due to lithology are not extant given the significant role of mg in the carbon cycle such variations may provide important information on the regulation of earth s climate hydrothermal waters groundwater precipitation glacial ice basalt glass olivine and representative soils have also been analysed the dissolved load shows a wide range of 26mg compositions compared to the parent basaltic glass 26mg 0 29 ranging from 0 96 to 0 64 while precipitation and hydrothermal waters possess 26mg values of 0 83 and 0 85 respectively with lower mg concentrations than the dissolved load biomass activity in vegetation and organic material in soils and rivers colloids appear to have little effect on the mg isotope compositions rather the data suggest that mg elemental and isotopic variations are largely controlled by the formation and stability of secondary phases in response to differing hydrological conditions in some samples seawater in the form of direct precipitation or glacial runoff also appears to be an important source of mg glacier fed rivers groundwaters and some direct runoff rivers with a high ph have higher 26mg than basalt which is most likely due to the incorporation of light mg isotopes in secondary minerals in contrast those direct runoff rivers which have a relatively low ph have low 26mg relative to basalt consistent with preferential incorporation of heavy mg isotopes into secondary phases although it is not possible to rule out some contribution from precipitation riverine suspended particulates are depleted in mobile elements and have 26mg compositions values both higher and lower than unweathered basalt in the glacier fed and direct runoff rivers where the 26mg of the dissolved phase is heavy due to the formation of secondary phases the suspended load is light because it contains more of those phases the opposite is true for the remainder of the direct runoff rivers which have low ph this could be due to dissolution of secondary minerals enriched in light mg which are unstable at low ph or the formation of new secondary phases

calcium and magnesium isotope systematics in rivers draining the himalaya tibetan plateau region lithological or fractionation control abstract in rivers draining the himalaya tibetan plateau region the 26mg 24mg ratio has a range of 2 and the 44ca 42ca ratio has a range of 0 6 the average 26 mg values of tributaries from each of the main lithotectonic units tethyan sedimentary series tss high himalayan crystalline series hhcs and lesser himalayan series lhs are within 2 standard deviation analytical uncertainty 0 14 the consistency of average riverine 26 mg values is in contrast to the main rock types limestone dolostone and silicate which range in their average 26 mg values by more than 2 tributaries draining the dolostones of the lhs differ in their 44 42 ca values compared to tributaries from the tss and hhcs the chemistry of these river waters is strongly influenced by dolostone solute mg ca close to unity and both 26 mg 1 31 and 44 42 ca 0 64 values are within analytical uncertainty of the lhs dolostone these are the most elevated 44 42 ca values in rivers and rock reported so far demonstrating that both riverine and bedrock 44 42 ca values may show greater variability than previously thought although rivers draining tss limestone have the lowest 26 mg and 44 42 ca values at 1 41 and 0 42 respectively both are offset to higher values compared to bedrock tss limestone the average 26 mg value of rivers draining mainly silicate rock of the hhcs is 1 25 lower by 0 63 than the average silicate rock these differences are consistent with a fractionation of 26 mg values during silicate weathering given that the proportion of mg exported from the himalaya as solute mg is small the difference in 26mg 24mg ratios between silicate rock and solute mg reflects the 26mg 24mg isotopic fractionation factor silicate dissolved mg between silicate and dissolved mg during incongruent silicate weathering the value of silicate dissolved mg of 0 99937 implies that in the tss solute mg is primarily derived from silicate weathering whereas the source of ca is overwhelmingly derived from carbonate weathering the average 44 42 ca value in hhcs rivers is within uncertainty of silicate rock at 0 39 the widespread hot springs of the high himalaya have an average 26 mg value of 0 46 and an average 44 42 ca value of 0 5 distinct from riverine values for 26 mg but similar to riverine 44 42 ca values although rivers draining each major rock type have 44 42 ca and 26 mg values in part inherited from bedrock there is no correlation with proxies for carbonate or silicate lithology such as na ca ratios suggesting that ca and mg are in part recycled however in spite of the vast contrast in vegetation density between the arid tibetan plateau and the tropical lesser himalaya the isotopic fractionation factor for ca and mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of ca and mg in these catchments the discrepancy between solute and solid ca and mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of ca and mg bound as minerals in rock and released as cations in rivers the fractionation of ca and mg isotope ratios may prove useful for tracing mechanisms of chemical alteration ca isotope ratios of solute riverine ca show a greater variability than previously acknowledged the variability of ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global ca cycling if past variations in oceanic ca isotope ratios are to be of use in constraining the past carbon cycle

elucidating modern geochemical cycles at local regional and global scales using calcium isotopes abstract in earth s surface environment calcium ca is an important mobile metal that is actively and passively transported in solution and within organic and mineral phases being cycled and recycled during various biogeochemical processes with the development of modern mass spectrometric techniques small variations in the stable and radiogenic isotopic compositions of ca can be measured revealing insight in these complex biogeochemical cycles and tracing and quantifying components across a range of spatial and temporal scales similar to other more routine isotope systems more than three decades of work reveal systematic variations in the partitioning of ca isotopes due to both abiotic and biological processes an overview of processes that fractionate ca isotopes at local regional and global scales is outlined here we present detailed examples of instances in which ca isotopes have provided unique insight into the functioning of earth surface processes and the cycling of ca at multiple scales future studies should target questions for which ca isotopic analysis provide unique insight and when combined with other isotope and trace element multi proxy studies better constrain the system of interest at the same time we challenge the scientific community to explore new frontiers including polar regions and other extreme environments

positive correlation between li and mg isotope ratios in the river waters of the mackenzie basin challenges the interpretation of apparent isotopic fractionation during weathering during chemical weathering magnesium mg is released by the dissolution of both carbonate and silicate sources the degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs or cycling by a series of processes associated with weathering is poorly constrained in the river waters of the mackenzie basin canada the mg content is high and mg isotope ratios 26mg 24mg expressed as mg 26 show in excess of one per mil variability part of this variability is attributed to the 3 range in the carbonate and silicate rocks drained despite this inherent lithological control on river water mg 26 values there is also evidence for a fractionation control a linear positive covariation between lithium 7li 6li expressed as li 7 and mg isotope ratios in the river waters of the mackenzie basin is reported this covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with mg isotope ratios

modelling weathering processes at the catchment scale the witch numerical model a numerical model of chemical weathering in soil horizons and underlying bedrock witch has been coupled to a numerical model of water and carbon cycles in forest ecosystems aspects to simulate the concentration of major species within the soil horizons and the stream of the strengbach granitic watershed located in the vosges mountains france for the first time simulations of solute concentrations in soil layers and in the catchment river have been performed on a seasonal basis the model is able to reproduce the concentrations of most major species within the soil horizons as well as catching the first order seasonal fluctuations of aqueous calcium magnesium and silica concentrations however the witch model underestimates concentrations of mg2 and silica at the spring of the catchment stream and significantly underestimates ca2 concentration the deficit in calculated calcium can be compensated for by dissolution of trace apatite disseminated in the bedrock however the resulting increased ca2 release yields important smectite precipitation in the deepest model layer in contact with the bedrock and subsequent removal of large amount of silica and magnesium from solution in contrast the model accurately accounts for the concentrations of major species ca mg and silica measured in the catchment stream when precipitation of clay minerals is not allowed the model underestimation of mg2 and h4sio4 concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is overestimated and that more soluble poorly crystallized and amorphous materials may be forming in agreement with observations on other watersheds draining granitic rocks this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the strengbach watershed

silicon isotope variations accompanying basalt weathering in iceland abstract this study presents the si isotope compositions of the dissolved phase of twenty rivers from across iceland the high resolution nu plasma 1700 mc icp ms was used to provide interference free 30 si 28 si ratios with an average limiting precision of 0 04 30 si the si isotope composition ranges from 0 08 to 1 46 30 si averaging 0 63 0 38 1 sd this is only slightly lighter than the recently reported mean of 0 84 0 19 for high precision measurements of swiss rivers but is significantly lighter than compositions reported for some other rivers the si isotope composition varies slightly with the amount of glacial cover which provides evidence that the continental si input to the oceans may be climatically controlled and thus likely to vary over glacial interglacial cycles after correction for the dissolution of trace amounts of calcite in one region a correlation is found between 30 si and ca si this is consistent with the measured basaltic composition and indicates isotopic fractionation during si removal from the dissolved phase by calculating the fraction of si that is removed from the dissolved phase into secondary phases it is possible to use the riverine si flux si denudation rate to calculate the initial dissolution rate of si within these catchments from this study we estimate that the total amount of si being dissolved by primary weathering of iceland is 3 million tons per yr however only 50 of this si is released into the rivers and contributes to denudation using the si isotope composition and the si fraction removed it is possible to model the si isotope fractionation as either a rayleigh type fractionation or a steady state system both yield similar fractionation factors of about 1 5 30 si implying only one underlying fractionation process that is controlled by external parameters such as runoff furthermore rivers that follow a steady state fractionation model are associated with higher si dissolution rates and lower 30 si values indicating an inverse relationship between weathering rates and dissolved riverine si isotope composition

ca isotope constraints on chemical weathering processes evidence from headwater in the changjiang river china abstract this study aims to clarify the relationship between chemical weathering of rocks and the carbon budget of rivers and better understand the weathering mechanisms of plateau watersheds we chose to study the jinsha river which originates from the tibetan plateau and also is in the upper reaches of the changjiang river analysis of hydrochemistry radiogenic strontium isotope and stable calcium isotopes were conducted of the jinsha river water samples which were collected along its mainstream and main tributaries in the summer the results show that the water chemistry of the mainstream waters is dominated by evaporite weathering which have low 87sr 86sr values 0 7098 0 7108 and wide range of sr contents 2 70 9 35 mol l in contrast tributaries of the jinsha river have higher 87sr 86sr 0 7090 0 7157 and lower sr contents 1 mol l moreover the ca isotopic compositions in the mainstream 0 87 1 11 are heavier than the tributaries 0 68 0 88 and could not be fully explained by the conventional mixing of different sources we suggest that secondary carbonate precipitation fractionates ca isotopes in the jinsha river and fractionation factors are between 0 99935 and 0 99963 at least 66 of ca was removed in the mainstream of the jinsha river through secondary mineral precipitation and the average value is 35 in the tributaries the results highlight that evaporite weathering results in more carbonate precipitation influencing ca transportation and cycling in the riverine system constrained by stable ca isotopic compositions and water chemistry

constraining seasonal active layer dynamics and chemical weathering reactions occurring in north slope alaskan watersheds with major ion and isotope δ34sso4 δ13cdic 87sr 86sr δ44 40ca and δ44 42ca measurements abstract rising air temperatures in the arctic may destabilize a large pool of organic carbon stored in permafrost thereby causing a positive feedback to global climate warming permafrost thaw could also deepen hydrologic flow paths and expose previously frozen rock and mineral fragments to chemical weathering future shifts in the inorganic solute geochemistry of arctic rivers may signal changes in soil processes that also affect organic carbon storage tracing permafrost thaw with dissolved riverine loads requires understanding the spatial and seasonal variation of chemical weathering reactions and other biogeochemical phenomena that affect elemental mass transport to help identify connections between mineral weathering and active layer processes we studied the major ion and isotope 34 s so4 13 c dic 87 sr 86 sr 44 40 ca and 44 42 ca geochemistry of five streams draining the north slope of alaska continuous permafrost underlies all streams but the atigun river roche moutonnee creek and trevor creek primarily drain bare bedrock outcrops in the brooks range while the upper kuparuk river and imnavait creek primarily drain tundra in total we collected 546 water samples spanning the spring freshet through fall freeze up in 2009 and 2010 we also analyzed snow rock sediment soil and plant samples major ion ratios and 13 c dic values point to the overall dominance of carbonate weathering by carbonic and sulfuric acids with additional influences from atmospheric deposition plant decay sulfate salt dissolution and silicate weathering by carbonic acid 13 c dic values may also reflect partial equilibration with soil and atmospheric co 2 all streams display large seasonal variations in major ion ratios and 13 c dic values that are consistent with progressive deepening of the seasonally thawed zone over the summer in the mountain watersheds carbonate weathering dominates during the spring and summer while sulfate salt primarily caso 4 and mgso 4 dissolution dominates during the fall riverine 34 s so4 values reveal that the sulfate salts are secondary precipitates we propose a conceptual model where cryoconcentration in soils during the late fall and winter causes secondary mineral formation at depth and re exposure during subsequent thaw seasons produces the observed geochemical signals in rivers the tundra streams lack definitive evidence for sulfate salt dissolution presumably because thick peat soils limit the exposure and weathering of underlying glacial sediment where the salts are expected to form and dissolve appearance of a sulfate salt dissolution signal in tundra streams may correlate with future permafrost degradation carbonate weathering dominates riverine 87 sr 86 sr ratios but the compositional heterogeneity of bedrock limits interpretation of the data all rivers have higher 44 40 ca values compared to bedrock likely due to plant uptake of lighter ca isotopes in the tundra watersheds freshet 44 40 ca values were 0 10 0 20 lower than summer and fall values these trends likely reflect contributions from plant decay as comparison between 44 40 ca and 44 42 ca values suggests that all isotopic variation is mass dependent with minimal radiogenic 40 ca inputs from the weathering of old silicate minerals with high k ca ratios

riverine silicon isotope variations in glaciated basaltic terrains implications for the si delivery to the ocean over glacial interglacial intervals marine primary production is dominated by diatoms and these are dependent upon the riverine delivery of silicon si to the ocean in paleoreconstruction of silicic acid utilisation by diatoms it is assumed that the isotopic composition of the si that is delivered from the continent to the oceans remains constant in this study it is shown that glacier fed icelandic rivers differ from those directly draining basaltic catchments in their dissolved si isotope compositions lighter values 30si 0 17 0 18 are associated with the high physical erosion rates in glacial rivers and heavier values 30si 0 97 0 31 are associated with lower physical erosion rates and enhanced formation of secondary minerals in direct runoff rivers the si isotopic compositions correlate with those of li and provide evidence of a climatic dependence that is likely to have led to glacial interglacial differences based on existing 30si measurements from diatoms in a sediment record from the southern ocean the interpretation of changes in si utilisation between the last glacial maximum lgm and the early holocene is revisited taking into account changing isotopic compositions of the river water delivered to the ocean over glacial interglacial intervals during the lgm si utilisation values are higher when allowing for changing si isotope input to the ocean 59 5 than when a constant si isotope input is assumed 42 47 5 this reduces but does not eliminate the difference relative to the holocene 88 5 therefore changes in si isotope delivery to the ocean need to be taken into account in the precise reconstruction of ocean si utilisation and primary productivity over glacial interglacial timescales

chemical weathering in granitic environments abstract factors controlling chemical weathering in granitic environments are deduced using an extensive database from the literature based on a transition state theory model we evaluated the effects on chemical weathering of runoff temperature and ph the dependence between temperature and runoff and chemical weathering is complicated by other factors such as the presence depth of soil cover soil cover favors chemical weathering when it is thin and rich in weatherable minerals in contrast it reduces chemical weathering when is thick and poor in primary minerals processes that increase the contact time and the surface of the water rock interactions such as physical denudation increase chemical weathering rates in granitic environments the effect of temperature can be quantified assuming an apparent activation energy value of 48 7 kj mol to describe chemical weathering of silica in granitic crystalline environment under high runoff conditions

surges of glaciers in iceland surges are common in all the major ice caps in iceland and historical reports of surge occurrence go back several centuries data collection and regular obser vationover thelast several decadeshave permitteda detailed descriptionof several surges fromwhich it is possible to generalize onthe nature of surging in icelandic glaciers com bining the historical records of glacier front variations and recent field research we sum marize the geographic distribution of surging glaciers their subglacial topography and geology the frequency andduration of surges changes in glacier surface geometry during the surge cycle and measured velocity changes comparedto calculatedbalance velocities wenotetheindicatorsofsurgeonsetanddescribechangesinice waterandsedimentfluxes duringa surge surgesaccomplishasignificantfractionofthetotalmasstransportthrough the main outlet glaciers of ice caps in iceland and have important implications for their hydrology ouranalysisof the data suggeststhat surge type glaciersin icelandarecharac terizedbygentlysloping surfacesandthatthey movetoo slowlytoremain inbalance given their accumulation rate surge frequency is neither regular nor clearly related to glacier size or massbalance steeply sloping glaciers whetherhard or soft bedded seemto move sufficiently rapidly to keep inbalance with the annual accumulation

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

stable calcium isotopic composition of meteorites and rocky planets abstract new measurements of mass dependent calcium isotope effects in meteorites lunar and terrestrial samples show that earth moon mars and differentiated asteroids e g 4 vesta and the angrite and aubrite parent bodies are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution 0 07 sd for the 44 ca 40 ca ratio in contrast enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes ca 0 5 and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes ca 0 5 the calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes the isotopic variations probably survive from the earliest stages of nebular condensation and indicate that condensation occurred under non equilibrium undercooled nebular gas conditions some of this early high temperature calcium isotope heterogeneity is recorded by refractory inclusions niederer and papanastassiou 1984 and survived in planetesimals but virtually none of it survived through terrestrial planet accretion the new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets enstatite and carbonaceous chondrites are not the enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction

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

processes controlling sr in surface and ground waters of tertiary tholeiitic flood basalts in northern iceland strontium concentrations of 253 natural water samples from skagafjordur a tertiary tholeiitic flood basalt region in northern iceland range between 0 10 and 28 ppb surface environments rivers lakes and peat soil waters include the whole range of observed sr concentrations whereas the sr concentrations of ground waters are in most cases 3 5 ppb concentrations of sr derived from basalt dissolution i e rock derived sr in waters of rivers and lakes exhibit a near linear correlation with the concentration of rock derived ca with a median molar ca sr ratio of 1350 this systematic correlation suggests that ca and sr concentrations are controlled by weathering processes i e the extent of dissolution of the basalt the relative mobility of sr during weathering in skagafjordur is approximately half that of ca which is consistent with observed relative mobilities of these elements elsewhere in iceland and in other basaltic regions peat soil waters commonly have lower concentrations of sr and higher ca concentrations than rivers and lakes and molar ratios of rock derived ca to sr in peat soil waters exhibit no systematic pattern in several cases calculated concentrations of rock derived sr in peat soil waters yield negative values suggesting a mineralogic sink for sr in these waters the low sr concentrations in cold and thermal ground waters 3 5 ppb suggest mineralogic control over sr in the ground water systems precipitation of secondary sr minerals such as strontianite and celestite is ruled out as the ground waters are understaturated with respect to these minerals ground waters are characterized by high ca sr molar ratios 5000 compared to bedrock ca sr ratio of 730 suggesting that sr is being preferentially incorporated relative to ca into secondary minerals the secondary minerals present in the bedrock in skagafjordur that can preferentially incorporate sr include zeolites such as heulandite chabazite and thomsonite and smectite ion exchange calculations demonstrate that activities of sr2 and ca2 in ground water solutions in skagafjordur are consistent with ion exchange equilibria between these waters and heulandite from other tertiary basalts in iceland suggesting that this mineral may play an important role in controlling the concentration of sr in the skagafjordur ground waters incorporation of sr into calcite cannot explain the observed high ca sr ratios of the skagafjordur ground waters because calcite when precipitating only admits limited amounts of sr aragonite is not considered a likely candidate either because it has only very slight preference for sr over ca and ground waters above 40 c are undersaturated with respect to this phase however predicted sr content of calcite in equilibrium with the skagafjordur ground waters 0 5 83 ppm sr is in good agreement with measured sr content of this mineral in tertiary basalts elsewhere in iceland 0 1 63 ppm suggesting that the skagafjordur ground waters can be used as analogues for tertiary crustal solutions involved in the zeolite facies metamorphism of the icelandic crust

the erosion and suspended matter seawater interaction during and after the 1996 outburst flood from the vatnajokull glacier iceland abstract the gjalp subglacial eruption 1996 within the vatnajokull glacier iceland triggered a catastrophic outburst flood bringing at least 180 million tonnes of suspended solids to the sea in only 42 h this amounts to 1 of the total annual global river suspended flux to the oceans the specific bet surface area of the suspended solids was measured to be 11 8 18 9 m2 g translating to the average total bet surface area of 2 8 10 9 km2 providing enormous potential for adsorption desorption and precipitation dissolution fluxes at the suspended solids ocean water interface altered basalt glass was the major constituent of the suspended matter 80 secondary minerals such as zeolites and calcite amounted to 11 but only 5 was fresh volcanic glass the suspended grains were generally rounded the glass carried by the flood is different in chemical composition from the glass produced by the gjalp eruption the gjalp material has higher feototal tio2 and tio2 p2o5 ratios than the suspended glass in the flood waters the majority of the flood samples match the composition of the volcanic system down stream from the eruption site the large amount of altered material in the flood and its chemical composition suggests erosion conforming to a 2 m deep 1000 m wide and 50 000 m long channel in less than 42 h the behaviour of 28 elements on the surface of the suspended solids exposed to seawater was quantified by experiments in the laboratory the altered basaltic glass dissolved in seawater as recorded by the si release from the glass the dissolved concentrations of na ca si ba cd co cu hg mn ni and total dissolved inorganic n increased considerably when the suspended solids come into contact with the seawater but the concentrations of mg k s sr fe pb and zn decreased the experimental seawater solutions were supersaturated with respect to calcite mg montmorillonite and amorphous iron hydroxide the rate of release mol m2 s of si mn ba co ni and cd decreased continuously during the one week exposure to sweater after one week the logarithm of the dissolution rate of the altered basaltic glass was 11 9 to 11 6 si mole m2 sec significantly lower than the steady state rates for fresh basaltic glass at similar conditions calculated one day desorbed dissolution suspended material fluxes are greater than the integrated dissolved flood fluxes for mn ba ni co and cd but the si dissolved food flux was greater than the one day desorbed dissolved suspended material flux

analysis of the wallowa baker terrane boundary implications for tectonic accretion in the blue mountains province northeastern oregon the baker terrane exposed in the blue mountains province of northeastern oregon is a long lived ancient late paleozoic early mesozoic accretionary complex with an associated forearc this composite terrane lies between the partially coeval wallowa and olds ferry island arc terranes the northern margin of the baker terrane is a broad zone 25 km wide of fault bounded imbricated slabs and slices of meta igneous and metasedimentary rocks faulted into chert argillite melange of the elkhorn ridge argillite metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 ma and have low initial 87 sr 86 sr ratios 0 7033 0 7034 and positive initial e nd values 7 7 to 8 5 in contrast siliceous argillites from the chert argillite melange have initial 87 sr 86 sr values ranging from 0 7073 to 0 7094 and initial e nd values between 4 7 and 7 8 we interpret this broad imbricate fault zone as a fundamental tectonic boundary that separates the distal wallowa island arc terrane from the baker accretionary complex terrane we propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex providing an on land ancient analogue to the actualistic arc arc collisional zone developed along the margins of the molucca sea of the central equatorial indo pacific region

calcium isotopes in a proglacial weathering environment damma glacier switzerland the biogeochemical cycling and isotopic fractionation of calcium during the initial stages of weathering were investigated in an alpine soil chronosequence damma glacier switzerland this site has a homogeneous silicate lithology and minimal biological impacts due to sparse vegetation cover calcium isotopic compositions obtained by tims using a 43ca 46ca double spike were measured in the main ca pools during this very early stage of weathering the young soils which have formed 44 42ca 0 44 44 42ca 0 44 were indistinguishable to the rocks from which they were derived 44 42ca 0 44 44 42ca 0 44 and stream water 44 42ca 0 48 44 42ca 0 48 was also within error of the average rock this lack of variation indicates that the dissolution of the bulk silicate rock does not strongly fractionate ca isotopes the only ca pool which was strongly fractionated from bulk rock was vegetation which exhibited an enrichment of light ca isotopes significant ca isotope fractionation between bulk rock and the dissolved flux of ca is likely to only occur where the ca biogeochemical cycle is dominated by secondary processes such as biological cycling adsorption and secondary mineral precipitation

calcium isotope constraints on the uptake and sources of ca2 in a base poor forest a new concept of combining stable δ44 42ca and radiogenic eca signals abstract plant available reserves of major base cations ca 2 and mg 2 decreased markedly in soils over the past century thus posing a potential threat to forest ecosystem health trees are thought to obtain dissolved ca 2 ions mainly from an easily accessible soil water reservoir also termed the exchangeable cation pool the status of ca reserves in this soil pool is sensitive to anthropogenic perturbations such as soil acidification induced by acid rain and or excessive timber harvesting here we show that in a base poor forest of the northeastern usa i e wachusett mountain massachusetts the exchangeable ca pool of deeper mineral soils has a unique isotope signature that is significantly enriched in the radiogenic 40 ca due to the dissolution of k rich silicate minerals such as biotite using a simple isotope mass balance and assuming that the input of ca from biotite has a e ca signature of 16 the results of our calculation indicate that the weathering of biotite may supply a sizeable fraction up to 25 of ca 2 ions into the exchangeable cation pool of deeper mineral soils importantly samples of local vegetation i e woody tissues of red oak show no detectable excess of the radiogenic 40 ca and based on our model the upper limit of a possible biotite derived ca contribution in vegetation is estimated at 5 we also found no evidence of the radiogenic 40 ca signal in the samples of forest floor and the uppermost organic rich soils 0 15 cm depth which in turn suggest that over the long term development of the forest and its organic matter accumulation the vegetation growth must have also relied primarily on the non radiogenic ca sources based on our experimental data such sources may include i wet atmospheric deposition ii the organically complexed ca in topsoil horizons and iii chemical weathering and or fungal mediated dissolution of apatite and ca rich plagioclase hence our stable and radiogenic ca isotope data indicate that the studied base poor forest is able to bypass the exchangeable cation pool of deeper i e below 15 cm mineral soils and still manages to meet its nutritional requirements with respect to ca another important implication of this study is that the organically complexed ca in the topsoil horizon 0 15 cm depth has to be tightly bound to the ion exchange sites otherwise the large radiogenic 40 ca signatures present in the exchangeable cation pool of deep mineral soils would be swamped by the downward gravitational flux of non radiogenic ca from the decaying organic matter and litterfall hence the limited mobility of the organically complexed ca in soils and its tight biological cycling could explain the lack of a significant impact of vegetation on the ca isotope systematics observed in large rivers

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

ca isotope cycling in a forested ecosystem reports of large ca isotope fractionations between trees and soils prompted this study of a boreal forest ecosystem near la ronge saskatchewan to improve understanding of this phenomenon the results on five tree species black spruce trembling aspen white spruce jack pine balsam poplar confirm that nutrient ca uptake by plants favors the light isotopes thus driving residual ca in plant available soil pools towards enrichment in the heavy isotopes substantial within tree fraction occurs in tissues formed along the transpiration stream with low 44ca values in fine roots 2 mm intermediate values in stemwood and high values in foliage separation factors between different plant tissues are similar between species but the initial fractionation step in the tips of the fine roots is species specific and or sensitive to the local soil environment soil water 44ca values appear to increase with depth to at least 35 cm below the top of the forest floor which is close to the deepest level of fine roots the heavy plant fractionated signature of ca in the finely rooted upper soils filters downward where it is retained on ion exchange sites leached into groundwater and discharged into surface waters the relationship between ca uptake by tree fine roots and the pattern of 44ca enrichment with soil depth was modeled for two ca pools the forest floor litter and the underlying upper b mineral soil six study plots were investigated along two hillside toposequences trending upwards from a first order stream we used allometric equations describing the ca distribution in boreal tree species to calculate weighted average 44ca values for the stands in each plot and estimate ca uptake rates the 44ca value of precipitation was measured and soil weathering signatures deduced by acid leaching of lower b mineral soils steady state equations were used to derive a set of model ca fluxes and fractionation factors for each plot the model reproduces the increase in 44ca with depth found in forest floor and upper b soil waters transient model runs show that the forest ca cycle is sensitive to changes in plant ca uptake rate such as would occur during ontogeny or disturbance accordingly secular records of 44ca in tree ring cellulose have the potential to monitor changes in the forest ca cycle through time thus providing a new tool for evaluating natural and anthropogenic impacts on forest health another model run shows that by changing the size of the isotope fractionation factor and adjusting for differences in forest productivity that the range in ca isotope fractionation in forested ecosystems reported in the literature thus far is reproduced as a quantitative tool the ca cycling model produces a reasonable set of relative ca fluxes for the la ronge site consistent with environment canada s measurements for wet deposition in the region and simulated ca release from soil mineral weathering using the profile model but the sensitivity of the model is limited by the small range of fractionation observed in this boreal shield setting of 1 which limits accuracy if the model were applied to a site with a greater range in 44ca values among the principal ca fluxes it is capable of producing robust and reliable estimations of ca fluxes that are otherwise difficult to measure in forested ecosystems

the rapid resetting of the ca isotopic signatures of calcite at ambient temperature during its congruent dissolution precipitation and at equilibrium abstract this study provides direct experimental evidence of the resetting of the calcium ca isotope signatures of calcite in the presence of an aqueous fluid during its congruent dissolution precipitation and at equilibrium at ambient temperatures over week long timescales batch reactor experiments were performed at 25 c in aqueous nacl solutions air or co2 gas mixtures were bubbled through this fluid to fix ph during congruent calcite dissolution the fluid became enriched in isotopically heavy ca and the ca isotope composition continued to become heavier after the fluid attained bulk chemical equilibrium with the mineral the 44 42ca composition of the fluid was up to 0 8 higher than the dissolving calcite at the end of the dissolution experiments calcite precipitation was provoked by increasing the reactor fluid ph after chemical equilibrium had been attained via dissolution rayleigh isotope fractionation effects were observed immediately after the ph was increased and rapid calcite precipitation occurred however isotopic exchange continued after the system chemically equilibrated eradicating this rayleigh signal taken together these observations 1 confirm dynamic mineral fluid equilibrium i e dissolution and precipitation occur at equal non zero rates at equilibrium and 2 indicate that isotopic compositions of calcite can readily equilibrate even when this mineral is in bulk chemical equilibrium with its coexisting fluid this latter observation suggests the preservation of paleo environmental isotopic signatures in calcite may require a combination of the isolation of the fluid mineral system from external chemical input and or the existence of a yet to be defined calcite dissolution precipitation inhibition mechanism

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

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

volcanic soils of iceland abstract the parent materials of all icelandic soils are volcanic in origin and most of the soils are classified as andosols iceland has the largest area of andosols in europe and may have 5 of all andosols in the world a new classification scheme based on the world reference base world reference base for soil resources world soil resources reports 84 fao rome is used to provide an overview of icelandic volcanic soils the data were acquired as part of a recently initiated effort to build a database for icelandic soils but are also partly based on data from the eu cost 622 action a literature review and unpublished icelandic research the main soil types in iceland are histosols histic andosols gleyic andosols brown andosols vitrisols and leptosols in the icelandic language a single term is used to refer to each soil type this system reflects the dominant influence of eolian deposition and drainage conditions on the formation of histosols and andosols the parent materials of icelandic soils are mixtures of tephra layers and eolian sediments consisting mostly of volcanic glass the steady flux of eolian sediments from unstable sandy deserts constantly recharges the surface with basaltic tephra materials the histosols 20 c have appreciable amounts of andic materials because of the eolian deposition the andosols histic gleyic and brown andosols show characteristic properties of soils dominated by andic materials with allophane contents of up to 30 ferrihydrite commonly accounts for 5 15 of the soil the contents of both allophane and ferrihydrite generally decrease with increasing organic c content of soil horizons as organic matter inhibits allophane formation and iron tends to become reduced and lost in poorly drained locations the vitrisols are soils of desert surfaces with low organic content but they usually meet criteria for andic materials with 30 glass and al 1 2fe o 0 4 cryoturbation is intense in iceland resulting in a range of geomorphic surface features the cryoturbation is related to both climatic factors and the andic soil properties including high water retention and rapid hydraulic conductivity

ca cycling and isotopic fluxes in forested ecosystems in hawaii 1 biogeochemical processes fractionate ca isotopes in plants and soils along a 4 million year developmental sequence in the hawaiian islands we observed that plants preferentially take up 40ca relative to 44ca and that biological fractionation and changes in the relative contributions from volcanic and marine sources produce a significant increase in 44ca in soil exchangeable pools our results imply moderate fluxes enriched in 44ca from strongly nutrient depleted old soils in contrast with high 40ca fluxes in young and little weathered environments in addition biological fractionation controls divergent geochemical pathways of ca and sr in the plant soil system while ca depletes progressively with increasing soil age sr ca ratios increase systematically sr isotope ratios provide a valuable tracer for provenance studies of alkaline earth elements in forested ecosystems but its usefulness is limited when deciphering biogeochemical processes involved in the terrestrial ca cycle ca isotopes in combination with sr ca ratios reveal more complex processes involved in the biogeochemistry of ca and sr

non biological fractionation of stable ca isotopes in soils of the atacama desert chile abstract we measured ca stable isotope ratios 44 40 ca in an ancient 2 my hyperarid soil where the primary source of mobile ca is atmospheric deposition most of the ca in the upper meter of this soil 3 5 kmol m 2 is present as sulfates 2 5 kmol m 2 and to a lesser extent carbonates 0 4 kmol m 2 in aqueous extracts of variably hydrated calcium sulfate minerals 44 40 ca e values vs bulk earth increase with depth 1 4 m from a minimum of 1 91 to a maximum of 0 59 the trend in carbonate 44 40 ca in the top six horizons resembles that of sulfate 44 40 ca but with values 0 1 0 6 higher the range of observed ca isotope values in this soil is about half that of 44 40 ca values observed on earth linear correlation among 44 40 ca 34 s and 18 o values indicates either a a simultaneous change in atmospheric input values for all three elements over time or b isotopic fractionation of all three elements during downward transport we present evidence that the latter is the primary cause of the isotopic variation that we observe sulfate 34 s values are positively correlated with sulfate 18 o values r 2 0 78 and negatively correlated with sulfate 44 40 ca e values r 2 0 70 if constant fractionation and conservation of mass with downward transport are assumed these relationships indicate a 44 40 ca fractionation factor of 0 4 in caso 4 the overall depth trend in ca isotopes is reproduced by a model of isotopic fractionation during downward ca transport that considers small and infrequent but regularly recurring rainfall events near surface low ca isotope values are reproduced by a rayleigh model derived from measured ca concentrations and the ca fractionation factor predicted by the relationship with s isotopes this indicates that the primary mechanism of stable isotope fractionation in caso 4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events

meteoric water basalt interactions ii a field study in n e iceland the compositions of rain snow melt spring and geothermal waters from the rift zone of n e iceland can be explained by seaspray addition chemical fractionation at the seawater air interface burning of fossil fuel farming activities purification by partial melting of snow and ice dissolution of basalts and buffering by alteration minerals the dissolution of the rocks appears to be incongruent during solute acquisition spring compositions move through the stability fields of kaolinite and smectite to the laumontite and illite fields all but four of the springs are undersaturated with respect to calcite silica concentrations are compatible with the solubility of basaltic glass the reactions reflected in the spring waters appear to have taken place sealed off from atmospheric co2 after initial saturation the geothermal waters which are recharged by the spring waters are depleted in mg and ca but enriched in carbon and sulfur with respect to dissolution of primary rocks expressions are derived relating dissolution rates of rocks age of groundwaters physical properties of groundwaters and mass transfer the characteristic rock particle radii in the cold water aquifers range from 0 2 to 2 cm and the characteristic crack openings are of the order 0 04 to 0 4 cm using laboratory studies on the icelandic lavas as a guide the residence times of the cold waters in the aquifers can be estimated at 60 days to 4 years the average active surface area of the aquifers enclosing 1000 g of spring water is of the order of 0 6 to 6 m2 and these 1000 g of water have reacted with 0 1 to 1 g of basaltic rocks the same mass of thermal water has interacted with 100 to 300 g of unaltered rocks

isotope fractionation during ca exchange on clay minerals in a marine environment abstract in order to interpret marine porewater profiles it is mandatory to understand the behavior of calcium ca and its isotopes during cation exchange in marine sediments it has been proposed that the exchange of adsorbed ca 2 for ammonium which is a product of organic matter decomposition results in the releases of light ca from clay minerals into the porewater teichert et al 2009 in order to investigate the effect of ammonium on ca isotope fractionation experiments with clay mineral separates illites montmorillonite kaolinite and natural marine sediments from the north atlantic integrated ocean drilling project site u1306a in artificial seawater were carried out at different temperatures 4 14 21 c and ammonium concentrations 100 140 180 mm the results of the adsorption experiments carried out in artificial seawater show that during adsorption of ca 2 the light ca isotopes are favored over the heavier ca isotopes this effect is most prominent for the illite samples 1000ln 0 82 to 1 15 and kaolinite sample 1000ln 1 23 to 2 76 whereas montmorillonite and the natural marine sediments show smaller degrees of fractionation from the fluid in the range of 0 46 to 0 06 determination of the desorbed ca 2 is based on the adsorbed ca 2 left on the exchanger and reveals that the desorbed ca 2 has a significantly different isotopic signature from the surrounding fluid in general the degree of ca isotope fractionation is dependent on the ammonium concentration and does not show significant influence of temperature modeling the ca 2 desorption induced by ammonium adsorption demonstrates that according to the prevailing mineralogy and porewater sediment ratio desorbed ca 2 has the potential to shift the porewater isotopy by up to 2 5 and needs to be considered when interpreting ca isotope porewater profiles

the major ion δ44 40ca δ44 42ca and δ26 24mg geochemistry of granite weathering at ph 1 and t 25 c power law processes and the relative reactivity of minerals abstract we dissolved boulder creek granodiorite in a plug flow reactor for 5794 h at ph 1 and t 25 c the primary purpose of the experiment was to identify controls on dissolved 44 40ca 44 42ca and 26 24mg values during granite weathering herein we also examine the origin of ca and mg isotopic variability among minerals composing the boulder creek granodiorite and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output nine ca bearing minerals display an 8 80 range of 44 40ca values and a 0 51 range of 44 42ca values three mg bearing minerals display a 1 53 range of 26 24mg values these ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks most of the 44 40ca variability reflects 40ca enrichment in k feldspar and to a lesser extent biotite due to the radioactive decay of 40k over the 1 7 ga age of the rock whereas the entire range of 44 42ca values reflects mass dependent isotope fractionation during igneous differentiation and crystallization the range of 26 24mg values may represent either fractionation during the chloritization of biotite or interaction of the boulder creek granodiorite with mg rich metamorphic fluids having low 26 24mg values the elemental and isotopic composition of the reactor output varied substantially during the experiment we synthesize the mineralogical and fluid data using coupled mass conservation equations solved at non steady state model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations while surface area normalized dissolution rate constants were time invariant specific surface area increased as a power law function of time through positive feedbacks between mechanical disaggregation chemical dissolution and mineral depletion variations in dissolved 44 40ca 44 42ca and 26 24mg values reflect conservative mixing rather than fractionation apatite and calcite initially control 44 40ca and 44 42ca values followed by biotite titanite epidote hornblende and plagioclase the release of radiogenic 40ca clearly defines the period where biotite dissolution dominates the brucite layer of chlorite initially controls 26 24mg values followed by biotite the tot layer of chlorite and hornblende through direct isotopic tracking these results demonstrate that trace minerals such as apatite and calcite in the case of ca and brucite in the case of mg dominate elemental release during the incipient stages of granite weathering the results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages the ca and mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies

lithium isotopic composition of submarine basalts implications for the lithium cycle in the oceans abstract we have measured the li isotope composition of young pristine basalts from active ocean ridge crests of progressively older basalts along a dredging transect and a limited number of hydrothermally altered basalts the data significantly extend our limited knowledge of the isotopic abundance ratio of li in geological material fresh mid ocean ridge basalts have 6li values of between 3 4 and 4 7 relative to isotope standard l svec during low temperature weathering on the seafloor the isotopic composition of the rock becomes increasingly heavier due to addition of seawater li 6li 32 3 the oldest 46 m y and most altered rock studied has an isotopic composition of 14 a linear relationship exists between 6li and the inverse of li concentration suggesting that li in weathered basalts can be regarded as a two component mixture of basaltic li and seawater derived li that has been incorporated in alteration minerals most likely secondary clays such as smectite and phillipsite the inferred li isotopic composition of the alteration endmember indicates an apparent isotopic fractionation factor of 1 019 relative to the seawater source thus li uptake by secondary minerals from the low temperature weathering process and by analogy incorporation in similar authigenic minerals in marine sediments provides a mechanism for preferential removal of the lighter li isotope from ocean water however isotopic fractionation due to authigenic clay formation alone cannot account for the isotopic difference between seawater and its principal sources unless the hydrothermal flux is comparable to the river flux alternatively more important sinks of6li must exist if the steady state isotopic composition of li in ocean water is to be maintained

silicate versus carbonate weathering in iceland new insights from ca isotopes to trace sources of ca in icelandic rivers we report elemental and ca isotope data for rivers high and low temperature groundwater basalt hydrothermal calcite including iceland spar and stilbite and heulandite which are two types of zeolites commonly formed during low grade metamorphism of basalt in agreement with previous research we find that rivers have higher 44 40ca values than basalt with a maximum difference of 0 40 this difference may reflect isotope fractionation in the weathering zone i e preferential uptake of 40ca during clay mineral formation adsorption and other geochemical processes that cycle ca however calcite 44 40ca values are also up to 0 40 higher than bedrock values and on a diagram of 44 40ca versus sr ca nearly all waters plot within a plausible mixing domain bounded by the measured compositions of basalt and calcite with glacial rivers plotting closer to calcite than non glacial rivers calcite and heulandite form during hydrothermal alteration of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface because heulandite 44 40ca values are 1 2 lower than basalt we suggest that 40ca uptake by heudlandite explains the relatively high 44 40ca values of calcite and that calcite weathering in turn elevates riverine 44 40ca values high mechanical erosion rates are known to facilitate the exposure and weathering of calcite which explains the isotopic contrast between glacial and non glacial watersheds using a mixing model we find that calcite weathering provides 0 65 of the ca in non glacial rivers and 25 90 of the ca in glacial rivers with silicate weathering providing the remainder icelandic hydrothermal calcite contains mantle carbon noting that zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions and assuming that hydrothermal calcite in other basaltic settings also contains mantle carbon we suggest that the contribution of basalt weathering to long term co2 drawdown and climate regulation may be less significant than previously realized

corrigendum to riverine silicon isotope variations in glaciated basaltic terrains implications for the si delivery to the ocean over glacial interglacial intervals earth planet sci lett 369 370 2013 211 219

calcium isotope δ 44 40 ca fractionation along hydrothermal pathways logatchev field mid atlantic ridge 14 45 n we investigate the logatchev hydrothermal field at the mid atlantic ridge 14 degrees 45 n to constrain the calcium isotope hydrothermal flux into the ocean during the transformation of seawater to a hydrothermal solution the ca concentration of pristine seawater ca sw increases from about 10 mm to about 32 mm in the hydrothermal fluid endmember ca hydend and thereby adopts a delta ca 44 40 hydend of 0 95 0 07 parts per thousand relative to seawater sw and a sr 87 sr 86 isotope ratio of 0 7034 4 we demonstrate that delta ca 44 40 hydend is higher than that of the bedrock at the logatchev field from mass balance calculations we deduce a delta ca 44 40 of 1 17 0 04 parts per thousand sw for the host rocks in the reaction zone and 1 45 0 057 parts per thousand sw for the isotopic composition of the entire hydrothermal cell of the logatchev field the values are isotopically lighter than the currently assumed delta ca 44 40 for bulk earth of 0 92 0 187 parts per thousand sw skulan j depaolo d j and owens t l 1997 biological control of calcium isotopic abundances in the global calcium cycle geochim cosmochim acta 61 12 2505 2510 and challenge previous assumptions of no ca isotope fractionation between hydrothermal fluid and the oceanic crust zhu p and macdougall j d 1998 calcium isotopes in the marine environment and the oceanic calcium cycle geochim cosmochim acta 62 10 1691 1698 schmitt a d chabeaux f and stille p 2003 the calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance earth planet sci lett 6731 1 16 here we propose that ca isotope fractionation along the fluid flow pathway of the logatchev field occurs during the precipitation of anhydrite two anhydrite samples from the logatchev hydrothermal field show an average fractionation of about delta 44 4c ca 0 57 parts per thousand relative to their assumed parental solutions ca isotope ratios in aragonites from carbonate veins from odp drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average delta ca 44 40 of 1 54 0 08 parts per thousand sw the relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine ca isotope budget in particular with regard to ocean crust alteration c 2008 elsevier ltd all rights reserved

dissolution of subglacial volcanic glasses from iceland laboratory study and modelling abstract subglacial hyaloclastites from iceland with ages ranging from 2 ka to 2 2 ma have been studied from a mineralogical and geochemical standpoint the chemical composition of palagonite alteration crust formed on the surface of the glass is almost identical with that of the clayey material filling the intergranular spaces of the rock the clayey material is made up of two particle populations the first is si mg and ca rich with a smectite structure while the second is amorphous fe ti and al rich and has a smectite like morphology it is suggested that these two types of particles can be formed simultaneously in the same solution such that it is not necessary to explain their existence by local or temporary equilibria the mineralogical sequences observed in natural samples were reproduced using the geochemical computer code dissol the geochemical mass balances calculated with dissol also fit quite well with those calculated from icelandic samples illustrating the predictive capability of such a calculation code and gives us confidence in applying a similar approach to nuclear waste form glass problems

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

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

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

the impact of water rock interaction and vegetation on calcium isotope fractionation in soil and stream waters of a small forested catchment the strengbach case this study aims to constrain the factors controlling the calcium isotopic compositions in surface waters especially the respective role of vegetation and water rock interactions on ca isotope fractionation in a continental forested ecosystem the approach is to follow changes in space and time of the isotopic composition and concentration of ca along its pathway through the hydro geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls percolating soil solutions and springs the study is focused on the strengbach catchment a small forested watershed located in the northeast of france in the vosges mountains the 44 40ca values of springs brooks and stream waters from the catchment are comparable to those of continental rivers and fluctuate between 0 17 and 0 87 soil solutions however are significantly depleted in lighter isotopes 44 40ca 1 00 1 47 whereas vegetation is strongly enriched 44 40ca 0 48 to 0 19 these results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions with depletion in light calcium in the soil solutions from deeper parts of the soil compartments due to preferential 40ca uptake by the plants rootsystem however mass balance calculations require the contribution of an additional ca flux into the soil solutions most probably associated with water rock interactions the stream waters are marked by a seasonal variation of their 44 40ca with low 44 40ca in winter and high 44 40ca in spring summer and autumn for some springs nourishing the streamlet a decrease of the 44 40ca value is observed when the discharge of the spring increases with in addition a clear covariation between the 44 40ca and corresponding h4sio4 concentrations high 44 40ca values and low h4sio4 concentrations at high discharge low 44 40ca values and high h4sio4 concentrations at low discharge these data imply that during dry periods and low water flow rate the source waters carry a ca isotopic signature from alteration of soil minerals whereas during wet periods and high flow rates admixture of significant quantities of 40ca depleted waters vegetation induced signal from uppermost soil horizons controls the isotopic composition of the source waters this study clearly emphasizes the potential of ca isotopes as tracers of biogeochemical processes at the water rock vegetation interface in a small forested catchment

calcium isotopes in the global biogeochemical ca cycle implications for development of a ca isotope proxy abstract at the earth s surface calcium ca is a critical element at a variety of scales it is soluble in natural waters a biological nutrient and a major constituent of the dominant mineral sink for carbon in the ocean caco3 there is a 4 variation in the ca isotopic composition 44ca 40ca expressed as 44ca of various ca reservoirs on earth suggesting ca isotopes as a promising tracer of ca cycling in both the present and the past fifteen years of high precision ca isotope measurements has revealed much about the behavior of ca isotopes in the earth surface environment but there remain fundamental questions concerning how ca isotopes are used to elucidate the marine and terrestrial ca cycles the current work presents a data compilation of over 70 published ca isotope studies totaling over 2600 measurements presented on a common delta scale that includes data on rivers and groundwater dust soils and soil pore fluids vegetation rainwater silicate minerals rocks and authigenic marine minerals carbonates sulfates and phosphates both modern and ancient the data compilation suggests that 1 there is a significant difference between carbonate 0 60 and silicate 44ca 0 94 2 riverine 44ca 0 88 does not simply reflect the compiled carbonate 44ca and 3 terrestrial vegetation exhibits the largest range of ca isotopic compositions 3 5 in the terrestrial setting we discuss these observations in the context of the global ca cycle exploring the extent to which seawater 44ca variability is feasible and how we can achieve accurate reconstructions of seawater 44ca over geologic time scales the current study presents simple mass balance models that quantify the leverage of inputs to change the ca isotopic composition of the ocean as this directly impacts the manner in which ca isotopes are interpreted although ca fractionates isotopically in the modern system during continental cycling the 44ca range of riverine inputs to the ocean is considerably smaller than the variability observed in putative seawater proxies such as nannofossil ooze and marine barite in the terrestrial realm plants exhibit a wide 44ca range and there is evidence that ca fluxes via biomass degradation are significant at the catchment scale we therefore assess the ability of the continental biosphere to influence riverine and consequently seawater 44ca a steady state biosphere has little leverage to alter riverine 44ca except in cases where the 44ca of the recycling flux is isotopically distinct from the 44ca of the uptake flux a non steady state biosphere can substantially impact both soil and riverine 44ca driving exchangeable ca either heavier or lighter depending on the magnitude of the recycling flux relative to the uptake flux based on estimates of the size of the global biosphere 1 5 1015 mol ca we suggest a decaying biosphere has the potential to impact riverine 44ca by tenths of a permil over time scales in the marine realm we evaluate the effect of a variable fractionation factor accompanying global removal of ca from the ocean on seawater 44ca and suggest methods by which such a mechanism can be recognized in the rock record experimental data suggest that there is considerable leverage