No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Isotopic ratios and release rates of Sr measured from weathering feldspars
... Sr is an important trace constituent of various rock-forming minerals, and Sr isotopic geochemistry has been used extensively to address a wealth of earth science issues in rock weathering. For example, in natural water bodies, the specific mineral sources of dissolved Sr in catchment waters are studied via the 87 Sr/ 86 Sr ratio [12][13][14][15][16][17][18]. In addition, many attempts have been made to characterize groundwater-rock systems and to trace groundwater pathways in complex hydrogeological settings [19][20][21][22][23][24]. ...
... Sr isotopes have been used to study material sources of soils and terrestrial deposits [13,[26][27], nutrient cation cycling in ecosystems [28], and significant tectonism [29][30] because it is generally assumed that Sr isotopes are infrequently fractionated by near-surface chemical, physical, and biological processes [31]. However, each mineral is likely to have a distinct 87 Sr/ 86 Sr ratio [32], and the difference in weathering rates of minerals leads to the variation of Sr isotope composition in weathering products [12,33]. Therefore, Sr isotopes have been used as a tracer for explaining the weathering process [17,34]. ...
... Accordingly, the 87 Sr/ 86 Sr ratios in K-bearing minerals such as biotite, K-feldspar, and illite are usually very high because 87 Sr is derived from 87 Rb by beta decay. Therefore, decomposition of K-bearing minerals in rocks might lead to decreased 87 Sr/ 86 Sr ratios in the residual weathering products relative to their parent rocks. [61,[63][64][65]. ...
The isotope ratios of Sr are useful tracers for studying parent material sources, weathering processes, and biogeochemical cycling. Mineralogical and geochemical investigations of two lateritic weathering covers, in an area close to the Tropic of Cancer (Guangxi Province, southern China), were undertaken to study the regional weathering processes and Sr isotopic sources. We found that weathering and decomposition of Rb- and Sr-bearing minerals change the Sr isotopic composition in weathering products (lateritic soils). Weathering of illite lowered the ⁸⁷Sr/⁸⁶Sr ratio whereas dissolving and leaching of carbonate minerals increased the ⁸⁷Sr/⁸⁶Sr ratio. An Fe nodular horizon is widely developed on the top of the weathering covers in the studied area and it differs from the lateritic soil horizon in mineral composition, construction, and elemental concentration. Furthermore, both Fe2O3 and P2O5 (concentrations) are negatively correlated with the ⁸⁷Sr/⁸⁶Sr ratios, suggesting fixation of apatite by Fe oxides is a controlling factor of the Sr isotopic composition in the Fe nodular horizon. The ⁸⁷Sr/⁸⁶Sr and Nb/Sr ratios imply the contents and proportions of Fe nodules and clay are critical in controlling the changes of Sr isotopic composition in the Fe nodular horizon. The two stages of the weathering process of carbonate rocks are revealed by the⁸⁷Sr/⁸⁶Sr versus Nb/Sr diagram. The ⁸⁷Sr/⁸⁶Sr and Rb/Sr ratios suggest that Sr isotopes in the weathering covers within the studied area are derived mainly from parent rock weathering and that the contributions from allothogenic Sr isotopes are limited. A comparison of Sr isotopic composition signatures in the weathering covers of the studied area and Guizhou Province provided insight into the Sr isotopic source and paleogeographic evolution of southern China. From the Permian to the Triassic, the continental fragment sources of the South China sedimentary basin changed significantly. In the Permian, Southern China presented the paleogeographic pattern that the north was higher (in elevation) than the south.
... One way in which chemical weathering rates have been estimated is by using the marine 87 Sr/ 86 Sr record as a proxy (e.g., Brass, 1976;Burke et al., 1982;Palmer and Elderfield, 1985;Wadleigh et al., 1985;Richter and DePaolo, 1988;Raymo et al., 1988;Hodell et al., 1990;Capo and DePaolo, 1990;Richter et al., 1992;Raymo and Ruddiman, 1992;Harris, 1995;McCauley and DePaolo, 1997;Veizer et al., 1999). There has, however, been little effort to establish a quantitative relationship between the kinetics of mineral dissolution and the release of Sr to solution (Brantley et al., 1998;Taylor et al., 2000). Developing a more rigorous relationship between Sr release and silicate weathering rates should allow for improved interpretations of variations in the marine 87 Sr/ 86 Sr ratio in terms of fluctuations in average global weathering rates. ...
... Once less than 5% variation in the Si concentration of successive output solutions was observed, the experiments were considered to be in steady state. As has been reported in previous studies (Casey et al., 1991;Stillings and Brantley, 1995;Brantley et al., 1998), Al, Ca, Na, and Sr are all preferentially released relative to Si during the initial phases of plagioclase dissolution experiments. After the first ϳ750 h of the experiment stoichiometric dissolution was observed. ...
... The Sr release rates from our experiments were intermediate between those measured by Brantley et al. (1998) for bytownite and albite. This agrees with the overall trend towards faster dissolution rates for more calcic plagioclase (Oxburgh et al., 1994;Stillings and Brantley, 1995;Welch and Ullman, 1996). ...
Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturation state of weathering solutions in column reactors. During the first 750 hours, rapid, nonstoichiometric dissolution was observed. Once steady state had been reached, both the overall dissolution and Sr release became stoichiometric. Under steady state conditions that were far from being in equilibrium with the labradorite, we measured the log of the overall labradorite dissolution rate (mol mineral/m²/s) to be −10.6 ± 0.1 while the Sr release rate was −13.2 ± 0.1 (mol Sr/m²/s). The isotopic ratio of the output solutions did not vary with time as both the early ⁸⁷Sr/⁸⁶Sr ratios and the later, steady state ratios were all essentially the same as that of the bulk labradorite (0.704671).
... Several workers have demonstrated that changes in the 87 Sr/ 86 Sr of Himalayan runoff have in fact occurred during the late Cenozoic (Derry and France-Lanord, 1996;Quade et al., 1997;Chesley et al., 2000;English et al., 2000). Possible causes of such a change include the following: (1) a shift in the balance of lithologies being weathered due to, for instance, the effects of uplift on the exposure of different lithologies to chemical weathering (Harris, 1995;Quade et al., 1997;English et al., 2000); (2) a change in the intensity of continental chemical weathering (Derry and France-Lanord, 1996;Zachos et al., 1999); and (3) development of a continental weathering regime that selectively weathers trace components of the bedrock with relatively high 87 Sr/ 86 Sr ratios (such as biotite; Blum et al., 1994Blum et al., , 1998Blum and Erel, 1995), or results in nonstoichiometric release of 87 Sr/ 86 Sr from minerals (Brantley et al., 1998). ...
... Glaciation could also have produced a change in continental weathering regime. Active subglacial crushing and grinding of bedrock continually renew the supply of trace components of bedrock and generate ultrafine particles with damaged surfaces that are highly reactive (Blum and Erel, 1995;Anderson et al., 1997Anderson et al., , 2000Brantley et al., 1998). Such reactive trace components include minerals, such as biotite, that have very high 87 Sr/ 86 Sr (Blum and Erel, 1997). ...
... It is less likely in major subglacial channels, where dilute waters acquire much of their solute from suspended sediment particles with surfaces that may have been depleted in cations during a previous phase of weathering . Such surfaces are likely to have equilibrated with the meltwaters and to dissolve stoichiometrically (Brantley et al., 1998). This could explain the enhanced K-silicate contribution to the Sr load of the more concentrated meltwaters that are derived primarily from a distributed subglacial drainage system. ...
The relationship between subglacial chemical weathering processes and the Sr isotope composition of runoff from Robertson Glacier, Alberta, Canada, is investigated. This glacier rests on predominantly carbonate bedrock of Upper Devonian age, but silicate minerals are also present. The provenance of solute in meltwaters is found to vary systematically with solute concentration and, by inference, subglacial water residence time. In dilute waters, the principal process of solute acquisition is calcite dissolution fueled by protons derived from the dissolution of CO2 and subsequent dissociation of carbonic acid. At higher solute concentrations, dolomite dissolution coupled to sulfide oxidation is more important. Sr concentration is found to increase with total solute concentration in two separate meltwater streams draining from the glacier, but ⁸⁷Sr/⁸⁶Sr only increases in the eastern melt stream. Carbonate and K-feldspar sources are shown to dominate the Sr content of the western stream, irrespective of concentration. They also dominate the Sr content of the eastern stream at low and intermediate concentrations, but at higher concentrations, muscovite (with high ⁸⁷Sr/⁸⁶Sr) is also an important Sr source. This reflects the outcrop of muscovite-bearing lithologies in the catchment of the eastern stream and an increase in the rate of weathering of K-silicates relative to that of carbonates as more concentrated solutions approach saturation with respect to carbonates. Nonstoichiometric release of ⁸⁷Sr/⁸⁶Sr and preferential release of Sr over K from freshly ground K-silicate surfaces may also occur. This may help to explain the radiogenic nature of runoff from distributed subglacial drainage systems, which are characterized by long water:rock contact times and water flow through environments in which crushing and grinding of bedrock are active processes.
... Therefore, temporal profiles of Nd isotope ratios of the lithogenic fractions can be used as robust proxy for the provenance studies. In contrast to εNd values, 87 Sr/ 86 Sr ratios of the deposited sediments are additionally influenced by various processes such as grain size sorting (Meyer et al., 2011;Tutken et al., 2002), and weathering intensity variations in the catchment area (Bain & Bacon, 1994;Brantley et al., 1998;Bullen et al., 1997). The temporal profiles of clay content and 87 Sr/ 86 Sr ratios of U1445A show no remarkable similarity ( Figure S1), suggesting a lack of significant influence of clay content (grain size) on 87 Sr/ 86 Sr ratios. ...
... In other words, variation exceeding 0.0099 in 87 Sr/ 86 Sr ratios can be attributed to an external shift in the sediment provenance. On the other hand, weathering intensity variations leading to differential dissolution kinetics of Sr bearing minerals such as Ca-plagioclase, K-Feldspar, and Biotite, etc. of the source rocks (Brantley et al., 1998;Bullen et al., 1997) result in change in the 87 Sr/ 86 Sr ratios. The influence of weathering intensity variations on 87 Sr/ 86 Sr ratios is suggested by a good correlation (Figure 5b) recorded between Rb/Sr and 87 Sr/ 86 Sr ratios (Chen et al., 1999;Derry & France-Lanord, 1996;Hu et al., 2018 Kroon et al., 1991;Tripathi et al., 2017). ...
Strontium and Neodymium isotopic analysis of the lithogenic fractions of the hole U1445A (International Ocean Discovery Program-353) collected from the deep-water end of the Mahanadi basin (western Bay of Bengal) was carried out to evaluate the sediment sources as well as to understand the link between the temporal variations in the Sr-Nd isotope ratios and climatic and tectonic forcings during the last 6 Ma. The Sr-Nd isotopic compositions along with Fe/Al ratios and clay mineralogy show tell-tale signature of Ganga-Brahmaputra sedimentation (Bengal fan) in the study area. The significant temporal fluctuations observed in the Sr-Nd isotopic compositions suggest variation in the relative sediment contribution by the Ganga and Brahmaputra river systems, which in turn are controlled by monsoon intensity variations coupled with orographic effect, glaciation, and multiple tectonic activities. The results show the marked influence of glacial-interglacials on the relative sediment contribution by Ganga and Brahmaputra rivers during 0–1.8 Ma, while the sedimentation during the 1.8–6 Ma time window was likely influenced by both climatic and tectonic forcings such as uplift of the Shillong plateau, eastern syntaxis development of Indo-Burma wedge, reorganization of Brahmaputra river system. The present study is the first high resolution (122 samples within 6 Ma) Sr-Nd isotope ratio analysis from the Bengal Fan. The study has brought to light multiple isotope fluctuations linked to climatic and tectonic forcings. The work also establishes dominant control of Ganga-Brahmaputra sedimentation along the deep water of the Indian east coast.
... For comparison, we compiled 87 Sr/ 86 Sr ratios from feldspar and clay in Fig. 5. Plagioclase (including feldspar, albite, anorthite, bytownite, and microcline) has very high strontium content (from several hundred to over a thousand ppm) with 87 Sr/ 86 Sr ratios higher but close to seawater and pore fluid values (0.70347-0.99590; Fig. 5) (Heier and Compston 1969, Clauer 1981, Brantley et al. 1998, White et al. 2001. Clay minerals formed from the weathering of volcanic glass and basalt have 87 Sr/ 86 Sr ratios close to or lower than the pore fluid ratios (0.70370-0.70730; ...
... To compare the strontium concentration from the different media, we assumed the abundance of feldspar, clay, and volcanic glass is 10 wt% of dry sediments and porosity is 50%. The ratio of feldspar (microcline) can be as high 0.99590(Brantley et al. 1998) which is not included in the plot. Our model estimates higher 87 Sr/ 86 Sr ratios for VA when compared to the volcanic glass. ...
Despite the important role that volcanogenic aluminosilicate (VA) alteration has on elemental cycles in marine sediments, there is no mechanism to arrive at a global assessment of this process. To quantify the VA alteration rates from Japan, New Zealand (NZ), and Costa Rica, we developed a mass balance approach that is constrained by the strontium concentration and ⁸⁷Sr/⁸⁶Sr ratios in pore fluid, authigenic carbonates, and VA. We derived VA alteration rates ranging from 10¹ to 10³ nmole Sr/m³ bulk sediment/yr with the highest rate obtained for Tuaheni, NZ (Site GeoB80202), which has the youngest sediment. We showed that ⁸⁷Sr/⁸⁶Sr ratios of VA derived from this mass balance approach are significantly higher than the reported ratios from volcanic glass samples, indicating a concomitant role of volcanogenic feldspar dissolution and/or authigenic clay formation. Most of the strontium released during VA alteration is precipitated as authigenic carbonate, with important implications for carbon inventories. The VA alteration rates derived from this approach can also be used to quantify the release of other critical elements, such as release of iron that can stimulate formation of Fe‑carbonates and/or fuel microbial activity at depth.
... On the other hand, some rock leachate samples (GW17 3-4 m and 6-7 m) show higher Sr isotope ratios than those of plagioclase and more similar to those of biotite than to those of feldspars ( Figs. 6 and 8). Thus, the rock leachates point to (preferential) cation release from feldspars and biotite dissolution in an initial phase of weathering (Brantley et al., 1998;Stillings et al., 1996; see discussion below). ...
... However, the lower Sr isotope ratios and higher Ca/Na ratios of the deep groundwater group II, i.e. the new wells-group, could in part be attributed to a preferential release of non-radiogenic Sr yielding a Sr isotope ratio lower than that of the bulk mineral in an initial stage of weathering as described for microcline and albite by Brantley et al. (1998). Similar to Sr, high values of Ca and Na can be observed early in dissolution (Stillings et al., 1996). ...
From a biogeochemical perspective, catchments can be regarded as reactors that transform the input of various substances via precipitation, deposition, or human activities as they pass through soils and aquifers towards receiving streams. Understanding and modeling the variability of solute concentration in catchment waters require the identification of prevailing processes, determining their respective contributions to the observed transformation of substances, their interplay with hydrological processes, and the determination of anthropogenic impacts. However, numerous biogeochemical processes often interact in a highly non-linear way and vary on temporal and spatial scales, resulting in temporally and spatially varying water chemistry in catchments. This is particularly true for riparian wetlands. Processes in this catchment area often superimpose the influence of the hill slope (and largest) area of the catchment on surface water quality. Accordingly, the first part of this thesis (Study 1 and 2), focuses on the temporal and spatial variability of biogeochemical processes at the catchment scale. Therefore, the first aim was to identify the prevailing biogeochemical processes which affect the quality of catchment waters in two forested granitic catchments. Based on these results, (i) the long-term behavior of these processes was determined (Study 1) and (ii) hot spots of these processes at the catchment scale along different flow paths were identified (Study 2). The second part (Study 3) focuses on the interplay between hydrological and biogeochemical processes in a riparian wetland, with the aim of systematically tracing back the temporal patterns of stream water chemistry to different biogeochemical processes and antecedent hydrological boundary conditions in the wetland. The third part (Study 4 and 5) focuses on weathering processes with the goal (i) of identifying the mineralogical sources of the groundwater’s buffer capacity against acid atmospheric deposition in a forested granitic catchment and (ii) determining the mineralogical sources of the high cation loads in surface water, induced by intensive agricultural activities in two agricultural granitic catchments. To reach these aims, multivariate statistical methods of dimensionality reduction (linear Principal Component Analysis, non-linear Isometric Feature Mapping), a low-pass filtering of time-series, a Cluster analysis, and major and trace element ratios and strontium isotopes were used. A small number of biogeochemical process bundles explained 94% and 89% of the variance of the data set in Study 1 and 2, respectively. In Study 1, redox and topsoil processes, road salt and sulfate contamination were identified as predominating processes influencing water chemistry in the respective catchments. Low-pass filtered time series of component scores revealed a different long-term behavior at different sampling sites in both catchments, which could be traced back to the fraction of wetland area in the respective subcatchments as well as by the varying thickness of the regolith. Study 2 revealed that the upper 1 m topsoil layer could be considered as a biogeochemical hot spot for redox processes, acid-induced podsolization, and weathering processes along different flow paths. Up to 97% of the biogeochemical transformation of the chemical composition of soil solution, groundwater and stream water in the Lehstenbach catchment was restricted to this soil layer representing less than 2% of the catchment’s regolith. Wetland stream water, mobilized in the topsoil layer being considered a biogeochemical hot spot, showed a highly dynamic temporal pattern of component scores. Study 3 revealed four different types of wetland stream water chemical status, depending on the interplay between discharge dynamics, biological activity, and the water table position in the wetland. The sequence of different stream water types roughly followed a seasonal pattern, albeit being heavily modified by the respective hydrological boundary conditions for different years. Extended periods of low groundwater level in the second half of the growing season drastically changed the chemical boundary conditions, becoming evident in a drastic reoxidation of reduced species like sulfides and corresponding effects. Weathering processes are one of the predominating biogeochemical process bundles influencing water chemistry in forested catchments. Study 4 showed that the mineralogical sources of the groundwater’s buffer capacity against acid atmospheric deposition were dominated by the release of base cations from apatite dissolution, preferential cation release from feldspars and biotite, and feldspars weathering. In Study 5, determining the mineralogical sources of the high cation loads in surface water induced by intensive agricultural activities revealed a dominant manure contribution in the topsoil, and enhanced mineral dissolution (plagioclase and biotite) by fertilizer application in subsoils, becoming the unique source of base cations in the saprolite. Stream water chemistry differed from that of soil water, suggesting that stream water chemistry was dominated by elements issued from enhanced mineral and rock weathering. Soil acidification induced by agriculture allows the mobilization of cations stored in soil layers, enhances the rock weathering and accelerates plagioclase dissolution, which can highly influence stream water quality. Numerous biogeochemical, hydrological, and anthropogenic processes were found to interact with each other, mostly with non-linear patterns, influencing catchment water chemistry. The integral approach used in this thesis would be a useful prerequisite to develop accurate and parsimonious models commonly used for water management purposes by distinguishing between short- term and long-term shifts, reducing the number of processes to the predominating ones ultimately to be included in the model, focusing on hot spots and including spatial patterns where necessary and appropriate.
... K-bearing feldspars might also be a source, but only a few of the southern-most catchments host rocks sufficiently metamorphosed to contain them. Moreover, release of Sr from feldspars is thought to be dominated by plagioclase (Brantley et al., 1998). Comparison of our measurements of the 87 Sr/ 86 Sr of the dissolved load in landslide seepage and river water with published measurements of 87 Sr/ 86 Sr in solid materials should allow us to qualitatively assess which mineral sources are most relevant; we can then test whether an estimate of silicate weathering made from the 87 Sr/ 86 Sr values is a fair approximation of the dissolution observed through major element concentrations. ...
... Values below 0.79 were only found in streams without landslides, while X carb in large rivers ranges from 0.79 to 0.96. These values tend to increase with increasing TDS, and Sr À13.1 @ pH 5.0 Brantley et al. (1998) are highest in seepage from landslides, with values always above 0.95. Some of the rivers draining glaciated catchments have high X carb fractions, up to 0.96, but they lack the high TDS concentrations found in landslide seepage. ...
Long-term estimates of the dissolution of silicate rock are generally derived from a range of isotopic proxies, such as radiogenic strontium isotope ratio (⁸⁷Sr/⁸⁶Sr), which are preserved in sediment archives. For these systems to fairly represent silicate weathering, the changes in isotopic ratios in terrestrial surface waters should correspond to changes in the overall silicate dissolution. This assumes that the silicate mineral phases that act as sources of a given isotope dissolve at a rate that is proportional to the overall silicate weathering. Bedrock landsliding exhumes large quantities of fresh rock for weathering in transient storage, and rapid weathering in these deposits is controlled primarily by dissolution of the most reactive phases. In this study, we test the hypothesis that preferential weathering of these labile minerals can decouple the dissolution of strontium sources from the actual silicate weathering rates in the rapidly eroding Western Southern Alps (WSA) of New Zealand. We find that rapid dissolution of relatively radiogenic calcite and biotite in landslides leads to high local fluxes in strontium with isotopic ratios that offer no clear discrimination between sources. These higher fluxes of radiogenic strontium are in contrast to silicate weathering rates in landslides that are not systematically elevated. On a mountain belt scale, radiogenic strontium fluxes are not coupled to volumes of recent landslides in large (>100km²) catchments, but silicate weathering fluxes are. Such decoupling is likely due first to the broad variability in the strontium content of carbonate minerals, and second to the combination radiogenic strontium released from both biotite and carbonate in recent landlides. This study supports previous work suggesting the limited utility of strontium isotopes as a system to study silicate weathering in the WSA. Crucially however, in settings where bedrock landsliding is a dominant erosive process there is potential for both random and systematic bias in isotope proxies if the most reactive phases exposed for dissolution by landslides disproportionately contribute to the proxy of choice. This clearly suggests that the isotopic composition of marine Sr is a proxy for periods of rapid mountain uplift and erosion rather than for the associated enhanced silicate weathering.
... The 87 Sr/ 86 Sr ratio of the global riverine input largely reflects three-component mixing between young mafic rocks (basalt), old felsic rocks (granites), and marine carbonates. In the current icehouse climatic state, riverine Sr concentrations and isotopic ratios may also be impacted by substrate age and variable chemical weathering rates following the retreat of major continental ice sheets and alpine glaciers (Taylor and Blum, 1995;Blum, 1997;Blum and Erel, 1997;Brantley et al., 1998;Millot et al., 2002;White and Brantley, 2003;Vance et al., 2009;von Blanckenburg et al., 2015;Mokadem et al., 2015). A detailed study of the Fraser River Basin, Canada demonstrates the variability and complexity of Sr concentrations and 87 Sr/ 86 Sr ratios within a single large river system (Voss et al., 2014). ...
... The global riverine (dissolved and particulate) and SGD 87 Sr/ 86 Sr ratios likely varied throughout the Phanerozoic given variations in the type and age of bedrock exposed to weathering (Wallmann, 2004;Peucker-Ehrenbrink et al., 2010;Beck et al., 2013;Voss et al., 2014), as well as the time dependent nature of weathering reactions (Taylor and Blum, 1995;Blum, 1997;Blum and Erel, 1997;Brantley et al., 1998;White and Brantley, 2003). Therefore, assuming that ancient Sr inputs have the same isotopic composition as modern inputs introduces uncertainty into model calculations. ...
While uniformitarianism underlies many aspects of the Earth sciences, modeling geochemical cycles throughout Earth history requires critically assessing the reliability and constancy of projecting modern fluxes and their isotope ratios into deep geologic time. In this study, we develop an iterative technique for modeling the radiogenic strontium isotope composition (R87 = ⁸⁷Sr/⁸⁶Sr) of seawater (sw) during the Phanerozoic utilizing as a constraint the composite record of McArthur et al. (2012) (McArthur, J. M., Howarth, R. J., & Shields, G. A. (2012). Strontium isotope stratigraphy. In F. M. Gradstein, J. G. Ogg, M. Schmitz, & G. Ogg (Eds.), The Geologic Time Scale 2012 (pp. 127–144): Elsevier). The model yields estimates for the Sr isotope composition of the riverine, submarine groundwater discharge, and diagenetic input fluxes. We use the Late Jurassic-Early Cretaceous segment of the marine Sr isotope record as a case study to demonstrate the efficacy of the new method. To begin, ⁸⁷Sr/⁸⁶Sr ratios are calculated for a combined riverine and submarine groundwater discharge flux (tw = total weathering) using a prescribed time-variable R87,sw determined from the LOWESS FIT 5 of McArthur et al. (2012). The calculated R87,tw values are checked against the range of R87,sw for the time interval of interest, as well as older carbonates available for weathering, to determine the reasonableness of the estimates. From these estimates, different prescribed R87,tw values are tested to identify the best match with target seawater ⁸⁷Sr/⁸⁶Sr ratios and the range of R87,sw. For model runs focusing on long time intervals (≳ 10 Myr), a single prescribed R87,tw value does not produce a reasonable match to the Sr isotope record. Testing the frequency of variation in R87,tw indicates that during the Early Cretaceous, R87,tw varied at least every 5 Myr, if not more frequently. The minimum frequency at which R87,tw changes likely varied throughout the Phanerozoic, as it depends on the variability of R87,sw and residence time of Sr. Model results also demonstrate the significance of estimating variation in the total hydrothermal (high and low temperature) input flux of Sr along with seawater Sr concentrations in the geologic past, which are key for properly estimating R87,tw.
... Strontium (Sr) isotopes ( 87 Sr/ 86 Sr) are widely used as environmental tracers to unravel weathering processes and soil formation (e.g., Bain and Bacon, 1994;Probst et al., 2000); water-rock interactions (e.g., Shand et al., 2009); to quantify the relative contributions of in-situ weathering and atmospheric inputs to soil and vegetation (Capo et al., 1998); and for radiocarbon dating of groundwater (Bishop et al., 1994). While natural processes such as evaporation, precipitation, and biological uptake do not change the strontium isotopic ratio in water, this ratio can be changed when water interacts with soils and rocks, due to feldspars dissolution (Brantley et al., 1998). ...
... As measured in the dissolution experiment, this 3.5 times lower Sr in oxidized tuff dispersed an amount of Sr in the solution more than 5 times higher than fresh tuff. This is due to the dissolution of feldspar minerals favored by oxidation processes (Brantley et al., 1998). ...
Determination of adsorption coefficient of strontium (Sr), one of the major elements of low and intermediate level radioactive wastes, is an important step to assess the performance of a waste repository. The adsorption coefficient of strontium on rhyolitic type pumice tuff, already selected as a potential host rock for radioactive waste repositories in Japan, was investigated with batch adsorption experiments. Some parts of the pumice tuff were oxidized and this redox effect was compared with fresh tuff. The distribution coefficient yielded negative values ranging from -0.3 to -9.6 mL/g at low initial concentration of 10(-6) and 10(-7) mol/dm(3), because a higher strontium concentration over the initial one was found in the aqueous phase at equilibrium. Batch dissolution experiment revealed that pumice tuff releases considerable amount of strontium, 8.27 x 10(-7) and 4.76 x 10(-6) mol/dm(3), respectively for fresh and oxidized tuff. Incorporating the dissolved strontium concentration in equilibrium, major changes in distribution coefficient values were found for the oxidized tuff. Considering the dissolution effect is thus necessary, especially for redox zone solid, to properly evaluate the distribution coefficient of easily soluble nuclides when a lower initial concentration is used. To incorporate the dissolution effect, a slight modification of the existing equation was made to determine the distribution coefficient.
... carbonates. An enhanced global input of acid may increase the silicate fraction relative to carbonates because the rate of silicate dissolution increases faster as pH decreases (below pH 6) than does the rate of calcite dissolution (e.g., [32] ...
... The pH dependence of 87 Sr/ 86 Sr for terrestrial runoff is not well understood. Individual minerals weather at different rates as a function of pH (e.g., [32]), and contain different concentrations and isotopic ratios of Sr. Assuming global acid deposition to both oceans and continents, 1 x 10 17 eq delivered to the oceans implies 4 x 10 16 eq to the continents and 1.3 x 10 14 moles of Sr. From Fig. 1 the foram data then require 87 Sr/ 86 Sr = .740, substantially higher than the modern runoff value of ~.7119 and Bickle's silicate endmember of .7178. ...
We focus here on geochemically constraining the production of acid during the KT impact. The potential importance of acid formation at the Cretaceous/Tertiary (K/T) boundary has been discussed by many authors (e.g., [2–7]). Fossil evidence (i.e., the survival of some species of calcareous plankton) for the lack of acidification of ocean surface waters has been cited by D’Hondt et al. [6], thus placing an important constraint on the total acid deposited in the oceans. Global surface cooling due to a decrease in solar insolation by an ejectaderived stratospheric dust layer, an idea originally proposed by Alvarez et al. [8], may have occurred on a decadal timescale as stratospheric SO2 was slowly converted to sulfuric acid aerosol [9–11]. Evaporite deposits at Chicxulub (e.g., [12]) provided the SO2 via devolatilization of anhydrite and gypsum. Numerical model estimates of the amount of SO2 liberated, based on laboratory hyper-velocity impact studies of anhydrite [13, 14], predict ~ 1 × 1016−1 × 1017 moles S were released [7]. Assuming complete oxidation of SO2 to sulfuric acid, this corresponds to ~12−20 × 1016 equivalents (eq) of acid. Zahnle [5] has estimated that ∼ 1 × 1015 moles of nitric acid were produced by shock heating of air during bolide and ejecta passage through the atmosphere, making nitric acid only 1–10 % of total strong acid production. Morgan et al. [15], by determining the diameter of the transient impact crater at Chicxulub, estimated that the bolide was the equivalent of the impact of a 12 km asteroid and that ~1 × 1016 mol SO2 were produced by the event.
... Rose (1991) and Kalinowski et al. (1998) showed that prehnite and epidote weather faster than albite under the same experimental conditions (pH, T). Moreover, some studies (Brantley et al., 1998;Taylor et al., 2000a) attributed the noncongruent dissolution of plagioclases in laboratory experiments to the preferential dissolution of trace phases (zeolite, calcite) at the beginning of the experiments. This is consistent with the preferential weathering of phases associated with feldspars, such as prehnite and epidote, whose kinetics of dissolution are not well known. ...
... With time, the 87 Sr/ 86 Sr ratio of biotite will decrease leading to less radiogenic Sr export and thus lower 87 Sr/ 86 Sr ratio in solution. Moreover, many studies in literature emphasize the discrepancies between 87 Sr/ 86 Sr ratios released by weathering and bulk 87 Sr/ 86 Sr ratios of minerals (Blum and Erel, 1997;Bullen et al., 1997;Brantley et al., 1998). ...
... Rose (1991) and Kalinowski et al. (1998) showed that prehnite and epidote weather faster than albite under the same experimental conditions (pH, T). Moreover, some studies (Brantley et al., 1998;Taylor et al., 2000a) attributed the noncongruent dissolution of plagioclases in laboratory experiments to the preferential dissolution of trace phases (zeolite, calcite) at the beginning of the experiments. This is consistent with the preferential weathering of phases associated with feldspars, such as prehnite and epidote, whose kinetics of dissolution are not well known. ...
... With time, the 87 Sr/ 86 Sr ratio of biotite will decrease leading to less radiogenic Sr export and thus lower 87 Sr/ 86 Sr ratio in solution. Moreover, many studies in literature emphasize the discrepancies between 87 Sr/ 86 Sr ratios released by weathering and bulk 87 Sr/ 86 Sr ratios of minerals (Blum and Erel, 1997;Bullen et al., 1997;Brantley et al., 1998). ...
... This is due in large part to the fact that mineral dissolution controls the release of both nutrient and toxic elements to soils and to the hydrocycle, and ultimately exerts an important control on the composition of the oceans and atmosphere. In recent years, a few studies have integrated the measurements of Sr and Pb isotopes in laboratory dissolution experiments yielding new information on (1) the importance of accessory mineral inclusions in what were previously considered "pure" mineral starting materials, (2) the differential rate of release of cations from contrasting mineralogic sites within single minerals, and (3) the relative rates of weathering of minerals in multi-mineral dissolution experiments (Bullen et al., 1997(Bullen et al., , 1998Brantley et al., 1998;White et al., 1999;Taylor et al., 2000a,b;Harlavan and Erel, 2002;Blum and Erel, 2003). ...
... Similarly, Brantley et al. (1998) observed that during the dissolution of bytownite, microcline, and albite, Sr release early in the experiments was neither stoichiometric nor isotopically the same as the bulk mineral and could be attributed to the dissolution of mineral inclusions in the feldspars and/or to leaching of cations from mineral defect sites. Once the dissolution of these minerals reaches a steady state, however, the 87 Sr/ 86 Sr ratio released to solution becomes equal to the bulk material. ...
Flow-through dissolution experiments were carried out on crushed granitoid rock (the Elat Granite) and three mineral separates (plagioclase, perthite, and biotite + chlorite) from this rock at pH 1 and 25°C. Major element concentrations were combined with Pb and Sr isotopic analyses of starting materials and output solutions and together enabled us to elucidate several important mechanisms related to granitoid rock weathering. We observed an initial stage of rock dissolution (
... Such heterogeneities could explain release patterns for some trace elements. For example, strontium has been observed to be released nonstoichiometrically during feldspar dissolution ( Brantley et al., 1998;Taylor et al., 2000). In one case, the nonstoichiometry was attributed to defects or accessory phases ( Brantley et al., 1998). ...
... For example, strontium has been observed to be released nonstoichiometrically during feldspar dissolution ( Brantley et al., 1998;Taylor et al., 2000). In one case, the nonstoichiometry was attributed to defects or accessory phases ( Brantley et al., 1998). During the nonstoichiometric stage of dissolution in that study, released Sr was enriched in 87 Sr compared to the bulk mineral. ...
... On the other hand, some rock leachate samples (GW17 3–4 m and 6–7 m) show higher Sr isotope ratios than those of plagioclase and more similar to those of biotite than to those of feldspars (Figs. 6 and 8). Thus, the rock leachates point to (preferential) cation release from feldspars and biotite dissolution in an initial phase of weathering (Brantley et al., 1998; Stillings et al., 1996; see discussion below). 5.3.2. ...
... This seems to hold for the waters in the catchment as well (pH 5–7), although the saturation state becomes pH dependent below pH 7. However, the lower Sr isotope ratios and higher Ca/Na ratios of the deep groundwater group II, i.e. the new wells-group, could in part be attributed to a preferential release of non-radiogenic Sr yielding a Sr isotope ratio lower than that of the bulk mineral in an initial stage of weathering as described for microcline and albite by Brantley et al. (1998). Similar to Sr, high values of Ca and Na can be observed early in dissolution (Stillings et al., 1996). ...
The role of different minerals in base cation release and thus the increase of buffering capacity of groundwater against acid deposition is controversially discussed in the literature. The ⁸⁷Sr/⁸⁶Sr ratios and base cation concentration were investigated in whole rock leachates, mineral separates, precipitation, soil solution, groundwater and stream water samples in the Lehstenbach catchment (Germany) to identify the weathering sequence of the granite bedrock. Three different approaches were followed in parallel. It was assumed that the contribution of different minerals to base cation supply of the groundwater with increasing weathering intensity would be observed by investigating (1) unweathered rock leachates, deep groundwater and shallow groundwater, (2) groundwater samples from new groundwater wells, reflecting the initial weathering of the drilled bedrock, and groundwater from wells that were drilled in 1988, (3) stream water during baseflow, dominated by deep groundwater, and stream water during high flow, being predominantly shallow groundwater. Whereas the first approach yielded consistent patterns, there was some evidence that groundwater from the new wells initially reflected contamination by the filter gravel rather than cation release in an initial stage of weathering. Time series samples of stream water and groundwater solute concentrations and isotope ratios turned out to reflect varying fractions of soil water and precipitation water at baseflow and high flow conditions rather than varying contributions of different minerals that prevail at different stages of granite weathering.
... flow system) or lack of detailed understanding of Sr geochemistry of for example interactions with solid materials. Over the past few years, detailed field-based studies of weathering (Bain and Bacon, 1994; Bullen et al., 1997; Hogan and Blum, 2003; Négrel, 2006; Shand et al., 2007) have been augmented by laboratory (Brantley et al., 1998) and modelling (Fritz et al., 1992; Johnson and DePaolo, 1997) studies, and both have vastly improved understanding of Sr isotope behaviour; modifying paradigms due to realisation of more realistic complex behaviour and heterogeneous systems. Strontium isotopes are a powerful tool in the geochemists toolbox for constraining and testing geochemical and hydrological models. ...
... Strontium isotopes have also proved useful in helping to quantify silicate vs. carbonate weathering fluxes (Blum et al., 1998; Chamberlain et al., 2005 ). Experimental studies are helping to understand the complexities of Sr behaviour and mineral dissolution kinetics (Brantley et al., 1998; Taylor et al., 2000) and they are proving valuable in correcting 14 C ages (Bishop et al., 1994; Johnson and DePaolo, 1996). ...
Strontium isotopes (87Sr/86Sr) are routinely measured in hydrochemical studies to determine sources and mixing relationships. They have proved particularly useful in determining weathering processes and quantifying end-member mixing processes. A number of routine case studies are presented which highlight that Sr isotopes represent a powerful tool in the geochemists toolbox helping to constrain weathering reactions, weathering rates, flow pathways and mixing scenarios. Differences in methodologies for determining the weathering component in natural environments, inherent differences in weathering rates of different minerals, and mineral heterogeneity often cause difficulties in defining the weathering component of different catchments or aquifer systems. Nevertheless, Sr isotopes are useful when combined with other hydrochemical data, to constrain models of water–rock interaction and mixing as well as geochemical processes such as ion-exchange. This paper presents a summary of recent work by the authors in constraining the sources of waters and weathering processes in surface catchments and aquifers, and indicates cases where Sr isotopes alone are insufficient to solve hydrological problems.
... Many earlier laboratory and field studies have provided evidence for the dependence of silicate mineral dissolution rates on climate and tectonics (e.g., Velbel, 1993;Chen and Brantley, 1997;Murphy et al., 1998;Brantley et al., 1998;White et al., 1999;West et al., 2005;West, 2012;Dere et al., 2013;Li et al., 2016). The climatic and geomorphic characteristics of studied catchments can hence be used to further investigate the mechanisms controlling the size-dependent 87 Sr/ 86 Sr decoupling in modern river sediments worldwide. ...
Radiogenic strontium isotopes are routinely used in provenance studies, but their application to sediments is often complicated by various grain size and weathering effects, which can influence measured ⁸⁷Sr/⁸⁶Sr ratios. Here, we report Sr isotopic data for a large number of sediment samples (n = 61) from the world's largest rivers and other river catchments draining particular geological and climatic settings; using both clay-rich (<4 μm) and silt-size (4-63 μm) detrital fractions to re-examine the factors controlling their distribution in sediments. In agreement with previous studies, the detrital material transported by world rivers defines general inverse Nd-Sr isotope relationships, which provide further empirical evidence for the utility of radiogenic Sr isotopes in sediment provenance studies. In a novel departure, however, we show that the ⁸⁷Sr/⁸⁶Sr difference between paired clay- and silt-size fractions (Δ⁸⁷Sr/⁸⁶Sr Clay-Silt) relates to the degree of chemical alteration of river sediments, as inferred from various relationships with weathering indices, such as the CIA, WIP and Na2O/Al2O3. The weathering dependence of Δ⁸⁷Sr/⁸⁶Sr Clay-Silt appears to be mainly controlled by temperature. In sub-Arctic and temperate regions, river sediments systematically display positive Δ⁸⁷Sr/⁸⁶Sr Clay-Silt indicative of preferential alteration of biotite in soils. In contrast, in sub-tropical watersheds characterized by mean annual temperatures >20 °C, intense feldspar weathering leads to the preferential incorporation of unradiogenic Sr into secondary clay minerals; a process which results in negative Δ⁸⁷Sr/⁸⁶Sr Clay-Silt values. In addition to climate forcing, the degree of size-dependent Sr isotope decoupling is also shown to be dependent on the type of weathering regime in watersheds, being more pronounced in low-elevation environments (< 2000 m), where transport-limited conditions and the presence of thick soil sequences can be associated with intense silicate weathering, than in high mountain regions (> 4000 m) dominated by kinetically-limited weathering regimes.
While further studies will be required to test the validity of these conclusions at the local scale of weathering profiles, these findings suggest that combined Sr isotopic analyses of separate size fractions could be used as a new weathering proxy in sediment records, ideally complementing the conventional use of radiogenic Sr isotopes as provenance tracers. Finally, our results are also used to re-assess the mean Sr flux and ⁸⁷Sr/⁸⁶Sr composition of the suspended sediment exported to the ocean yearly, yielding a global flux-weighted average of 0.7160, identical to that proposed earlier in the seminal study of Goldstein and Jacobsen (1988).
... Gypsum dissolution could also partly explain the increased Ca/K ratios. Dissolution of amphibole, pyroxene, calcic-plagioclase, fluorite and scheelite are enhanced under acidic conditions, contributing Ca, Mg, Fe and F − into solution (Schott et al., 1981;Franke and Teschner-Steinhardt, 1994;Brantley et al., 1998;Wong et al., 2003) as shown in Fig. 2. Although scheelite is weathered at this low pH, W which is the prominent constituent was barely detected in solution (Fig. 2). ...
Interpretation of geochemical data based primarily on elemental concentrations often lead to ambiguous results due to multiple potential sources including mineral weathering, atmospheric input, biological cycling, mineral precipitation and exchange processes. The ⁸⁷Sr/⁸⁶Sr ratio is however not fractionated by these processes. In this study, Sr isotope (⁸⁷Sr/⁸⁶Sr) ratios have been coupled with chemical data of Sr and Rb-bearing minerals, tailings and leachates (water-soluble) to gain insight into the geochemical processes occurring within the Yxsjöberg Cu-W mine tailings, Sweden. The tailings have been exposed to oxidizing conditions resulting in three geochemical zones namely (i) oxidized, (ii) transition and (iii) unoxidized zones. Leachates from the oxidized zone are acidic (pH = 3.6–4.5) and contain elevated concentrations of metals (e.g. Fe, Cu and Zn) and SO4. The low pH has also led to subsequent weathering of most silicates, releasing Al, Ca, Mg and Na into solution. The ⁸⁷Sr/⁸⁶Sr ratio in the tailings ranges from 0.84787 to 1.26640 in the oxidized zone, 0.92660–1.06788 in the transition zone, whilst the unoxidized zone has values between 0.76452 and 1.05169. For the leachates, the ⁸⁷Sr/⁸⁶Sr ratio ranges from 2.44479 to 5.87552 in the oxidized zone, 1.37404–1.68844 in the transition zone and 1.03697–2.16340 in the unoxidized zone. Mixing (between mineral weathering and atmospheric sources) was identified as the major process regulating the Sr composition of the tailings and leachates. The highly radiogenic signatures of the leachates in the oxidized zone suggests weathering of biotite, K-feldspar and muscovite. Despite the very radiogenic signatures in the oxidized zone, increments in Ca/K ratios, Be, Ce, Tl, Al, Fe and SO4 concentrations in the water-soluble phase were recorded in its lower parts which suggests the dissolution of amphibole, pyroxene, plagioclase, fluorite, gypsum, Al and Fe –(oxy) hydroxides as well as cation exchange by clay minerals. Presence of clay minerals has led to the partial retainment of radiogenic ⁸⁷Sr/⁸⁶Sr resulting in increased ⁸⁷Sr/⁸⁶Sr in the solid tailings material at these depths. The ⁸⁷Sr/⁸⁶Sr ratios of the water-soluble phase in the transition zone is similar to that of helvine and could indicate its dissolution. In the upper part of the oxidized zone, the ⁸⁷Sr/⁸⁶Sr ratios and trends of Be, Ca, SO4, Tl and Zn in the water-soluble phase suggest the dissolution of gypsum which precipitated from a leachate with the isotopic signature of helvine. In the lower part of the unoxidized zone, elevated concentrations of W were recorded suggesting scheelite weathering. But the ⁸⁷Sr/⁸⁶Sr ratios are higher than that expected from dissolution of scheelite and indicates additional processes. Possible sources include biotite weathering and groundwater. This study reveals that when interpreting geochemical processes in mine waste environments, ⁸⁷Sr/⁸⁶Sr should be considered in addition to chemical constituents, as this isotopic tracer offers better insights into discriminating between different solute sources.
... White et al. (2008) investigated the weathering profile of marine terraces in coastal California, which indicated that plagioclase has a higher dissolution rate than K-feldspar. The cation release of plagioclase follows the order of Sr 1 Ca 1 Na (Brantley et al. 1998), with less mobility of K and Rb in Kfeldspar. We believe that this reasonable result should be applied to feldspar weathering; however, in studying fine sediments it is essential to consider other minerals, such as carbonates and silicates like illite, chlorite, pyroxene, hornblende, feldspar, and so on. ...
In studies of paleoclimate change, variations in major and minor elements are often used as proxies for chemical weathering in soil or lake sediments. To prove the suitability of chemical weathering proxies and further discuss the impact of mineral dissolution on element migrations, various parent rocks in Taiwan were chosen to react with sulfuric acid for discussing mineral dissolution rankings and element leaching during chemical weathering processes. The parent rocks include granite, andesite, actinolite schist, slate, quartz sandstone, calcium sandstone, and mudstone. Experimental results show that dissolution rates are mainly controlled by mineral structures and the chemical compositions of rocks. The ranking of rock dissolution rates is as follows: mudstone 1 slate 1 actinolite schist 1 quartz sandstone > granite. Dissolution rates of andesite and calcium sandstone cannot be compared due to secondary minerals being produced in experiments. Moreover, the dissolution sequences of minerals are of the following order: calcite > chlorite > muscovite and illite > augite > hornblende ≈ feldspar > quartz. This result is consistent with the stability of silicate polymerization. Most notable among the results was that the dissolution rates of clay minerals were faster than those of feldspar. Element migration ranking in dissolution processes is as follows: Mg > Ca ≈ Rb > K > Na > Sr > Al > Zr > Fe ≈ Ti. The dissolution rates of univalent elements follow the sequence of bonding forces; however, those of divalent elements are less ordered. This is due to divalent elements being rich in different mineral phases. Therefore, the study suggests that univalent elements are more suitable as chemical weathering proxy candidates. Moreover, whether ratios of Rb/Sr and K/Rb or chemical index of alteration values in sediments are suitable proxies of paleorainfall intensity is discussed in detail.
... In principle, the release of 87 Sr/ 86 Sr due to weathering of silicate rocks depends on the individual weathering behaviour of solids with distinct Sr isotope ratios (e.g. Brantley et al., 1998;Bain and Bacon, 1994;Cartwright et al., 2007;Shand et al., 2009). For young basalt areas weathering of minerals produces insignificant differences in Sr isotope ratios in water-rock interactions (e.g. ...
Hydrochemical and isotopic data are used to examine the decisive processes controlling groundwater chemistry and to reveal sources of groundwater. Groundwater from Axum area shows distinctive chemical and isotopic variation suggesting spatial diversity of the aquifer and different geochemical evolution. Individual compositions appear to be controlled by the type of host rock and intensity of water-rock interaction. Weathering of silicates and to lesser extent of calcite containing veins as well as ion-exchange explains most of the observed spatial variability in groundwater composition. The δ 2HVSMOW and δ 18 OVSMOW values of the groundwaters indicate a meteoric origin. The δ 13 CVPDB values of dissolved inorganic carbon (DIC) range from -12 to +1% revealing magmatic CO 2 and soil CO 2 as predominant sources for DIC.
... These elemental differences reflect heterogeneous weathering of plagioclase (Fig. 3C). Experimental studies have documented the initial non-stoichiometric release of Sr > Ca > Na from plagioclase due to preferential leaching of surfaces and exsolution lamellae (Brantley et al., 1998). ...
A soil chronosequence developed on marine terraces along coastal California, exhibits deeper and more intensively weathered mineral profiles with increasing age (65 to 226 kyrs). Feldspar concentrations generally increase linearly with terrace depth. The slope or weathering gradient is defined by the ratio of the weathering rate and the velocity at which the profile penetrates into the regolith.A spread sheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations are dominated by exponential changes in mineral surface to volume and at high residual feldspar concentrations by the approach to thermodynamic saturation.These parameters, in addition to the kinetic rate constant, are of secondary importance to the fluid flux qh which controls the weathering velocity and solute fluxes from the profile.
... Jeden z reálnych procesov, ktorý mohol spôsobiť otvorenie radiačného systému v hornine, t.j. zmenu pomeru Rb/Sr, je vplyv fluíd, v princípe chemické zvetrávanie kyslého plagioklasu (Eggleton et al., 1987;Su & Fullagar, 1995;Clow et al., 1997;Brantley et al., 1998). Strata obyčajného, nerádiogénneho stroncia z plagioklasu spôsobuje nárast 87 Rb/ 86 Sr pomeru, t.j. ...
Minerals separated from diaphtoritic rocks of the Selec block of the Považský Inovec crystalline complex were dated by 40Ar/39Ar method (coarse-grained muscovites), and accessory apatites, coarse-grained muscovites and whole rocks were also analyzed by Rb-Sr method. Obtained 40Ar/39Ar muscovite plateau ages and calculated Rb-Sr ages for two point accessory apatite - coarse-grained muscovite pairs are identical (307-310 Ma) and are considered as age of diaphtoresis in the Považský Inovec Mts. crystalline complex. Both isotopic systems in minerals registered no metamorphic Alpine overprint. But in whole rocks, increasing of Rb/Sr ratio is documented that can be explained by loss of ordinary strontium from acid plagioclases during low temperature alteration under significant Alpine tectonics influence.
... Instead, the decrease in 87 Sr/ 86 Sr and increase in Sr/Na (and Ca/Na) over Period 2 is suggestive of the increased weathering of Ca and Sr rich reactive phases, which are comparatively unradiogenic (Brantley et al., 1998). Mineral phases such as apatite and epidote contain relatively high concentrations of Sr and Ca relative to other minerals, have low 87 Sr/ 86 Sr ratios compared to bulk rock and occur in the bedrock and river sediment samples (Tables 2 and 3). ...
Chemical and isotope data (ε40ε40Ca, δ44/42δ44/42Ca, 87Sr/86Sr, δ18δ18O) of river water samples were collected twice daily for 28 days in 2009 from the outlet river of Leverett Glacier, West Greenland. The water chemistry data was combined with detailed geochemical analysis and petrography of bulk rock, mineral separates and sediment samples in order to constrain the mineral weathering sources to the river. The average isotopic compositions measured in the river, with 2SD of all the values measured, were ε40ε40Ca=+4.0±1.4, δ44/42δ44/42Ca= +0.60±0.10‰ and 87Sr/86Sr=0.74243±0.00327. Based on changes in bulk meltwater discharge, the hydrochemical data was divided into three hydrological periods. The first period was marked by the tail-end of an outburst event and was characterised by water with decreasing suspended sediment concentrations (SSC), ion concentrations and pH. During the second hydrological period, discharge increased whilst 87Sr/86Sr decreased from 0.74550 to 0.74164. Based on binary mixing diagrams using 87Sr/86Sr with Na/Sr, Ca/Sr and ε40ε40Ca, this is interpreted to reflect an increase in reactive mineral weathering, in particular epidote, as the water residence time decreases. The decrease in water residence time is a result of the evolution from a distributed (long water residence time) to a channelised (short water residence time) subglacial drainage network. The third hydrological period was defined as the period when overall discharge was decreasing. This hydrological period was marked by prominent diurnal cycles in discharge. During this period, significant correlations between δ44/42δ44/42Ca and SSC and δ18δ18O were observed which are suggestive of fractionation during adsorption. This study demonstrates the potential of radiogenic Ca to both identify temporally changing mineral sources in conjunction with 87Sr/86Sr values and to separate source and fractionation effects in δ44/42δ44/42Ca values.
... In a study of granite fluid interaction and Sr availability from different minerals, Zuddas et al. (1995) stated that the dissolved Sr content and the 87 Sr/ 86 Sr ratios in groundwaters depend on the dissolution of the minerals from the granite, such as plagioclase, biotite, K-feldspar. Based upon dissolution of feldspars under controlled laboratory conditions Brantley et al. (1998) suggested that interpretation of 87 Sr/ 86 Sr ratios of catchments should be carefully analyzed, not only on the basis of whole-rock isotopic composition, but also on the basis of relative dissolution rates of the representative minerals. Those authors also stated that plagioclase dissolves at relatively fast rates and can release significant Sr into solution. ...
Strontium isotopes and other geochemical signatures are used to determine the relationships between CO2-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have 87Sr/86Sr isotopic ratios between 0.716713 and 0.728035. 87Sr/86Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO2-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have 87Sr/86Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO2-rich thermal and mineral waters and the granitic rocks. The mean 87Sr/86Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.
... Non stoichiometric dissolution of minerals, including with respect to isotopes is well known, particularly for glacial catchments (e.g., Anderson et al., 1997;Brantley et al., 1998). For example, in addition to the rapid dissolution of biotite relative to other minerals, the preferential release of 87 Sr from the interlayer sites of biotite has been observed (Erel et al., 2004). ...
Seasonal changes in river chemistry offer the potential to assess how weathering processes respond to changing meteorological parameters and ultimately how chemical weathering might respond to climatic parameters. Systematic seasonal variations in magnesium isotope ratios (the 26Mg/24Mg ratio expressed as δ26Mg in per mil units) are reported in stream waters from a mono-lithological granitic, weathering-limited, first order catchment from the Swiss Alps (Damma glacier). Rain, ground, and pore-waters, in addition to plants, rocks, mineral separates and soil are also reported. The concentration response of the river waters is attenuated compared to the large changes in discharge. However, the systematic trends in the isotope data imply that either the source of the Mg changes in a systematic manner, or that the process by which Mg is released into solution changes as a function of discharge.
... Similarly, Sr isotopes in rivers can be studied to investigate mineral weathering on a watershed scale. Previous experimental studies have shown that Sr and Ca release may not be stoichiometric over an initial period (Brantley et al., 1998;Taylor et al., 2000), but if soils are old enough to have passed this transitional time, chemical dissolution will not fractionate Sr/Ca ratios or 87 Sr/ 86 Sr ratios between the infiltrating fluids and the corresponding solid minerals. Recent studies have used 87 Sr/ 86 Sr ratios in weathering profiles as a tracer to distinguish Sr inputs derived from soil weathering and the atmosphere (Graustein and Armstrong, 1983;Dijkstra et al., 2003), to estimate Ca weathering rates in natural terrestrial ecosystems (Miller et al., 1993;Clow et al., 1997), and to identify the sources of base cations and evaluate mineral weathering intensity and elemental fluxes (e.g., Wadleigh et al., 1985;Yang et al., 1996;Bullen et al., 1997;Galy et al., 1999;Jacobson et al., 2002;Pett-Ridge et al., 2009a). ...
Michigan soils have developed on thick glacial-drift deposits that include different proportions of ground granite and gneiss from the Canadian Shield region (with radiogenic Sr) and carbonate sedimentary rocks from within the Michigan Basin (with non-radiogenic Sr). This study of the Cheboygan, Huron and Kalamazoo watersheds shows how Sr-isotope and Sr/Ca ratios in soil waters, ground waters, and soils reflect relative weathering intensities of the dominant minerals in Michigan soils, including carbonates from the Michigan Basin, and amphibole, plagioclase and K-feldspar derived from the Canadian Shield.
... Waters with different residence times have the potential to acquire different solute compositions. Laboratory experiments have shown that the initial stages of weathering are incongruent, resulting in the preferential release of Ca, Mg, K and Sr relative to Si and, to a certain extent, Na (Acker and Bricker, 1992; Brantley et al., 1998; White et al., 1999). Under high discharge conditions (short residence time), the stream water chemistry is likely to reflect incongruent weathering processes, whereas water with a longer residence time is expected reflect more congruent weathering. ...
... Non stoichiometric dissolution of minerals, including with respect to isotopes is well known, particularly for glacial catchments (e.g., Anderson et al., 1997;Brantley et al., 1998). For example, in addition to the rapid dissolution of biotite relative to other minerals, the preferential release of 87 Sr from the interlayer sites of biotite has been observed ( Erel et al., 2004). ...
Weathering processes revealed by seasonal variations in stable isotope ratios in rivers: The example of Mg
... This can be accounted for by a preferential dissolution of the plagioclase feldspars, which are the most weatherable minerals and which produces mainly non-radiogenic strontium, lowering the 87 Sr/ 86 Sr ratio of the HCl leachate. Plagioclase is indeed known to dissolve at relative fast rate and can release significant Sr, even when it is present in relatively low concentrations (Brantley et al., 1998). The 87 Sr/ 86 Sr ratios of the HCl treatment (dsimulated weatheringT) are therefore significantly lower than total soil digests and reflect the differential weatherability of the soil minerals. ...
The two main sources of Ca in forest ecosystem are the mineral weathering release and atmospheric inputs. We use the 87Sr/86Sr isotopic ratio (Sr is a proxy for Ca) to determine the Ca contribution from rain input in two forest ecosystems (beech stands) growing on soils formed from parent materials with distinct total Ca contents and contrasted isotopic ratios: Pleistocene loess in Central Belgium (leached brown soil) with present-day 87Sr/86Sr =0.72788 and Lower Devonian shales and sandstones in Ardennes (ochreous brown earth) with 87Sr/86Sr = 0.76913. The 87Sr/86Sr ratios and the Ca and Sr contents were measured in rainwater, vegetation (beech wood growth rings and leaves) and main soil horizons (total, labile and HCl 0.1 M soluble forms). The relative contributions of atmospheric input and soil mineral weathering to vegetation were calculated using mixing equations. Calculations based on the Sr isotope ratios of rainwater (endmember 1; 87Sr/86Sr close to seawater: 0.7090), labile soil fraction (endmember 2; 87Sr/86Sr: 0.71332 to 0.71785) and beech wood (mixing compartment) indicate that about 50 % (Central Belgium) to 35 % (Ardennes) of Ca uptake originate from atmospheric inputs. The choice of the appropriate 87Sr/86Sr ratio for the weathering endmember is however critical. The isotopic composition of the mineral source is theoretically determined by the mineralogical composition of the soil and the relative weatherability of the Sr-bearing minerals. Due to soil processes (weathering and clay illuviation), the distribution of minerals in both soil profiles is not homogeneous and varies from horizon to horizon. Which horizons are relevant and which kind of soil extract (labile soil fraction, acid soluble fraction, total soil,...) should be selected for isotopic measurement of weathering endmember, is therefore questionable. The different ways of estimation are discussed. Quantitative mineralogical reconstitutions of soil horizons and isotopic data indicate preferential weathering of plagioclase (high Sr content with low 87Sr/86Sr) rather than mica or K-feldspar (high 87Sr/86Sr). Our results emphasize the importance of the Ca atmospheric contribution to the tree mineral nutrition in these forest ecosystems. It is plausible that acid depositions associated with decreasing input of atmospheric cations ("acid rains") could increase the depletion of soil available cation pool at a short-time scale.
... In a Sr isotopic composition vs Sr concentration diagram, mixing between end-members is represented by curves; however, applying a normalization to the Sr concentration transforms the curves to straight lines that are easier to interpret (Faure, 1986).Figure 5-200, 205 and 206), because it is here that the greatest difference in Sr signature between the samples and the agricultural end-member is found. Franklyn et al. (1991), Zuddas et al. (1995) and Brantley et al. (1998 have demonstrated that the Sr isotopic composition in water is controlled by the chemical weathering of Sr-bearing minerals of the drained rock. Among the main minerals that constitute granitic rock, and that also contain a significant amount of Rb (and/or Sr), are apatite, feldspar (plagioclase and potassium feldspar) and mica (biotite and muscovite). ...
In a geochemical characterization of surface waters draining granites of the Vienne district, France, isotopic signatures strictly linked to water—rock interactions were determined by applying a double correction for atmospheric and anthropogenic inputs; the former based on the conservative behaviour of Cl and the latter based on the Sr concentrations and isotopic signatures, which are the only variables with clearly identifiable characteristics. The double-correction concept was validated by comparing the results with the theoretical Sr isotopic signature of water in equilibrium with the parent rock. The weathering model is based on the preferential dissolution of the three main Sr-bearing mineral phases: plagioclase, potassium feldspar and biotite. The Sr isotopic signatures of the waters are in good agreement with the calculated ones. The relative scattering observed for waters draining the same plutonic intrusion can be explained by water—rock interactions within different layers of the weathering profile.
... In addition, Sr isotopes, as recorded in marine carbonates (Burke et al., 1982; Veizer et al., 1998), represent one of the most important proxies for global weathering rates in the geologic past (Brass, 1976; Palmer and Elderfield, 1985; Wadleigh et al., 1985; Raymo et al., 1988; Richter and DePaolo, 1988; Raymo and Ruddiman, 1992; Richter et al., 1992; Harris, 1995; McCauley and DePaolo, 1997). Although Sr isotopes are gaining widespread use as a tracer of chemical weathering, there are little data that relate the behavior of Sr during weathering to mineral dissolution kinetics (Brantley et al., 1998). A more quantitative understanding of the kinetics and isotopic variation of Sr released during weathering should improve our ability to interpret Sr isotopes as a tracer of chemical weather- ing. ...
The dissolution kinetics and isotopic composition of Sr released during the weathering of biotite and phlogopite were measured under experimental conditions at pH ∼ 3 and 25°C. Although the overall release of Sr was essentially stoichiometric, the 87Sr/86Sr ratios of weathering solutions were generally higher than the reacting mineral. The initial phase of dissolution was characterized by the simultaneous rapid release of Sr from the sheet silicate and from trace amounts of calcite inclusions. During the initial phase, the 87Sr/86Sr ratios of the output solutions were lower than either the bulk mineral or the later, steady-state ratios. After the early stages of the experiments, calcite dissolution became limited by the rate at which it was exposed to the weathering solution and the 87Sr/86Sr of the output solutions increased to values above those of the biotite and phlogopite (after correction for the calcite inclusions). Differences in the location of Sr and Rb within the mica structure may cause the Sr isotopic ratio of the effluent to increase above the mineral ratio. The log values of the steady-state Sr release rate constants (moles per meter per second) were determined to be −15.1 for biotite and −15.9 for phlogopite under our experimental conditions. Cation release rates indicate that different sites within the sheet silicate structure reacted at different rates. Interlayer cations were generally released more rapidly than octahedral cations, which reacted slightly faster than the tetrahedral cations. Interlayer potassium release was considered to be controlled by diffusion with a moving boundary condition. Diffusion coefficients calculated from this model were 3.5 × 10−19 cm2/s for biotite and 1.9 × 10−18 cm2/s for phlogopite. Iron oxidation appeared to limit the overall biotite dissolution rate. Iron-free phlogopite reacted nearly twice as fast as biotite. The log values of the overall dissolution rates (moles per meter per second) were determined to be −11.6 for biotite and −11.2 for phlogopite. Cation release during phlogopite dissolution was also much less stoichiometric than during biotite weathering.
... Waters with different residence times have the potential to acquire different solute compositions. Laboratory experiments have shown that the initial stages of weathering are incongruent, resulting in the preferential release of Ca, Mg, K and Sr relative to Si and, to a certain extent, Na (Acker and Bricker, 1992;Brantley et al., 1998;White et al., 1999). Under high discharge conditions (short residence time), the stream water chemistry is likely to reflect incongruent weathering processes, whereas water with a longer residence time is expected reflect more congruent weathering. ...
The temporal and spatial controls of stream water chemistry in a small, granitic, glacial catchment were investigated in conjunction with high-resolution hydrological and meteorological measurements. Significant systematic seasonal and diurnal variations were observed in the stream water chemistry, which were not caused by the mixing of water draining different lithologies. A hydrological model (ALPINE3D, Lehning et al., 2006) was used to calculate the relative contributions of the principal water sources, snow melt and ice melt, throughout the period of this study. Pronounced seasonal minima in δ 18 O and 87 Sr/ 86 Sr were attributed to spring snow melt. Clear changes in X/Si ratios (X = Ca, Mg, K, Na, Sr) between summer and winter were observed. These changes are interpreted to reflect seasonal changes in the average residence time of water in the sub-glacial drainage network with short residence times in summer, when the discharge was greatest, leading to high X/Si ratios, and long residence times in winter, when the discharge was lowest, resulting in low X/Si ratios. This study shows that the time dependent stoichiometry of cation to Si ratios in glacial stream water (and likely all catchments) strongly depends on the hydrological state of the catchment at the time of sampling. Annual fluxes based on spot samples varied by a factor of six depending on the time of year in which the sample was collected, highlighting the importance of long-term catchment monitoring. An improved understanding of the spatial and temporal controls acting on stream water chemistry will allow silicate weathering processes to be more precisely quantified.
... This can be accounted for by a preferential dissolution of the plagioclase feldspars, which are the most weatherable minerals and which produces mainly non-radiogenic strontium, lowering the 87 Sr/ 86 Sr ratio of the HCl leachate. Plagioclase is indeed known to dissolve at relative fast rate and can release significant Sr, even when it is present in relatively low concentrations (Brantley et al., 1998). The 87 Sr/ 86 Sr ratios of the HCl treatment (dsimulated weatheringT) are therefore significantly lower than total soil digests and reflect the differential weatherability of the soil minerals. ...
The two main sources of Ca in forest ecosystems are mineral weathering release and atmospheric inputs. We use the 87Sr/86Sr isotopic ratio (Sr is used as a proxy for Ca) to determine the Ca contribution from atmospheric deposition input in two forest ecosystems (beech stands) growing on soils formed from parent materials with contrasting total Ca contents and isotopic ratios: Pleistocene loess in Central Belgium (acid leached soil) and Lower Devonian shales in the Ardenne Massif of High Belgium (ochreous brown earth). The 87Sr/86Sr ratio and the Ca and Sr contents were measured in bulk precipitation, in vegetation (beechwood growth rings and leaves) and in main soil horizons (total, acid-extractable and labile pools). The relative contributions of atmospheric input and soil mineral weathering to vegetation were calculated using mixing equations.
... However, it is likely that although the source of strontium in BR is silicate minerals, the 87 Sr/ 86 Sr released during silicate weathering in BR does not match that of strontium in the source mineral(s). Several laboratory studies of potassium-silicate weathering have shown that the 87 Sr/ 86 Sr of weathering solutions can be either higher or lower than that of the minerals being weathered, and that the 87 Sr/ 86 Sr can change over time (Brantley et al., 1998; Taylor et al., 2000). ...
Solute and runoff fluxes from two adjacent alpine streams (one glacial and one non-glacial) were investigated to determine how the inorganic solute chemistry of runoff responded to seasonal and interannual changes in runoff sources and volume, and to differences in physical catchment properties. Intercatchment differences in solute composition were primarily controlled by differences in catchment geology and the presence of soils, whereas differences in total solute fluxes were largely dependent on specific discharge. The glacial stream catchment had higher chemical denudation rates due to the high rates of flushing (higher specific discharge). The non-glacial Bow River had higher overall concentrations of solutes despite the greater prevalence of more resistant lithologies in this catchment. This is likely the result of both longer average water–rock contact times, and a greater supply of protons from organic soils and/or pyrite oxidation. Increases in snowpack depth/snowmelt runoff reduced the retention of nitrate in the Bow River catchment (i.e. increased nitrate export), probably by reducing net biological uptake, or by reducing the proportion of runoff that had contact with biologically active soil horizons that tend to remove nitrate.
... Either mechanism is consistent with the expected differences in activation energies associated with the different sites occupied by the trace element Sr and the major stoichiometric constituents K and Na in the biotite and feldspar structures. Brantley et al. (1998) previously noted preferential release of Sr having incongruent 87 Sr/ 86 Sr ratios relative to bulk sample compositions in the early stages of experimental dissolution of bytownite, albite, and microcline. On the other hand, retention of radiogenic Sr as well as Mg and Ca in biotite by an exchange mechanism provides an explanation for the similar behavior of these alkaline earths (Fig. 6). ...
The effects of climatic temperature variations (5–35°C) on chemical weathering are investigated both experimentally using flow-through columns containing fresh and weathered granitoid rocks and for natural granitoid weathering in watersheds based on annual solute discharge. Although experimental Na and Si effluent concentrations are significantly higher in the fresh relative to the weathered granitoids, the proportional increases in concentration with increasing temperature are similar. Si and Na exhibit comparable average apparent activation energies (Ea) of 56 and 61 kJ/mol, respectively, which are similar to those reported for experimental feldspar dissolution measured over larger temperature ranges. A coupled temperature–precipitation model, using an expanded database for solute discharge fluxes from a global distribution of 86 granitoid watersheds, produces an apparent activation energy for Si (51 kJ/mol), which is also comparable to those derived from the experimental study. This correlation reinforces evidence that temperature does significantly impact natural silicate weathering rates.
... The fluvial geochemistry of the rivers of Eastern Siberia neither consistently stoichiometric nor constant and that the ratios in the effluent are different from bulk mineral ratios (Brantley et al., 1998). At this point it is not clear how much relevance this has to natural weathering regimes, as the experiments were carried out at an extreme pH of 3. ...
The conventional view of the climatic influence on weathering is that weathering rates are strongly temperature-dependent due to the near-exponential relationship (Clausius-Clapeyron) between temperature and the saturation vapor pressure of water, and hence precipitation and runoff. This is a central theme in the Earth “thermostat” model, i.e., weathering of aluminosilicate rocks on continents acts through the greenhouse effect as a negative feedback on atmospheric CO2. However, there is very little direct field evidence to support this hypothesis. To remedy the lack of systematic geochemical data for cold high latitude rivers as compared to the tropics, large, pristine drainages of Eastern Siberia have been studied. Here, data from basement terrains of the Siberian Craton are reported. The low Si to total cation ratios suggest a superficially weathered system. The total dissolved solids flux of 0.39 × 106 mol/km2/yr and the CO2 uptake flux of 149 × 103 mol/km2/yr are similar to those of the tropical cratonic systems and the collisional/accretionary zone of northeastern Siberia, but about a factor of 3 lower than for the orogenic zones of the western Americas at both low and high latitudes. The lack of systematic climatic effects on the solute and CO2 fluxes is ascribed to the unique non-glacial frost shattering processes which continuously expose fresh rock surfaces and, thus, overcome the effect of temperature inhibition on high-latitude shields and to the lateritic cover that seals in the weathering front away from the weathering agents on the tropical shields. No primary climatic effects on weathering rates on the present Earth were detected.
... These elemental differences reflect heterogeneous weathering of plagioclase (Fig. 3C). Experimental studies have documented the initial non-stoichiometric release of Sr > Ca > Na from plagioclase due to preferential leaching of surfaces and exsolution lamellae (Brantley et al., 1998). ...
The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow argillic horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America.
... The intersect denotes that the source component is more radiogenic than that of plagioclase (0.7083) and less radiogenic than K-feldspar (0.7160) but does not correspond to a specific mineral phase. This component may represent exchangeable Sr in addition to incongruent mineral weathering as shown for progressive changes in 87 Sr/ 86 Sr ratios for biotite (Bullen et al., 1997) and K-feldspar (Brantley et al., 1998) during natural and experimental weathering. ...
Differential rates of plagioclase and K-feldspar weathering commonly observed in bedrock and soil environments are examined in terms of chemical kinetic and solubility controls and hydrologic permeability. For the Panola regolith, in the Georgia Piedmont Province of southeastern United States, petrographic observations, coupled with elemental balances and 87Sr/86Sr ratios, indicate that plagioclase is being converted to kaolinite at depths > 6 m in the granitic bedrock. K-feldspar remains pristine in the bedrock but subsequently weathers to kaolinite at the overlying saprolite. In contrast, both plagioclase and K-feldspar remain stable in granitic bedrocks elsewhere in Piedmont Province, such as Davis Run, Virginia, where feldspars weather concurrently in an overlying thick saprolite sequence. Kinetic rate constants, mineral surface areas, and secondary hydraulic conductivities are fitted to feldspar losses with depth in the Panola and Davis Run regoliths using a time-depth computer spreadsheet model. The primary hydraulic conductivities, describing the rates of meteoric water penetration into the pristine granites, are assumed to be equal to the propagation rates of weathering fronts, which, based on cosmogenic isotope dating, are 7 m/106 yr for the Panola regolith and 4 m/106 yr for the Davis Run regolith. Best fits in the calculations indicate that the kinetic rate constants for plagioclase in both regoliths are factors of two to three times faster than K-feldspar, which is in agreement with experimental findings. However, the range for plagioclase and K-feldspar rates (kr = 1.5 × 10−17 to 2.8 × 10−16 mol m−2 s−1) is three to four orders of magnitude lower than for that for experimental feldspar dissolution rates and are among the slowest yet recorded for natural feldspar weathering. Such slow rates are attributed to the relatively old geomorphic ages of the Panola and Davis Run regoliths, implying that mineral surface reactivity decreases significantly with time.Differential feldspar weathering in the low-permeability Panola bedrock environment is more dependent on relative feldspar solubilities than on differences in kinetic reaction rates. Such weathering is very sensitive to primary and secondary hydraulic conductivities (qp and qs), which control both the fluid volumes passing through the regolith and the thermodynamic saturation of the feldspars. Bedrock permeability is primarily intragranular and is created by internal weathering of networks of interconnected plagioclase phenocrysts. Saprolite permeability is principally intergranular and is the result of dissolution of silicate phases during isovolumetric weathering. A secondary to primary hydraulic conductivity ratio of qs/qp = 150 in the Panola bedrock results in kinetically controlled plagioclase dissolution but thermodynamically inhibited K-feldspar reaction. This result is in accord with calculated chemical saturation states for groundwater sampled in the Panola Granite. In contrast, greater secondary conductivities in the Davis Run saprolite, qs/qp = 800, produces both kinetically controlled plagioclase and K-feldspar dissolution. Faster plagioclase reaction, leading to bedrock weathering in the Panola Granite but not at Davis Run, is attributed to a higher anorthite component of the plagioclase and a wetter and warmer climate. In addition, the Panola Granite has an abnormally high content of disseminated calcite, the dissolution of which precedes the plagioclase weathering front, thus creating additional secondary permeability.
... Sr is also produced from K-feldspar and from Ca-feldspar (Négrel, 2006), but it has a larger susceptibility to weathering than Ba (Land et al., 2000). In contrast to Ba, after release Sr may form complex carbonate salts together with mainly Ca during reprecipitation (Brantley et al., 1998). However, both Ba and Sr form no important minerals on their own (Puchelt, 1972;Stueber, 1978) and have comparably little importance for vegetation (biogenic coefficients 0.04 and 0.06, Perel'man, 1977). ...
The last glacial–interglacial loess–paleosol sequences of Serbia and Ukraine provide a good climate reconstruction potential for this part of Europe. Four loess sections distributed over an area with present-day moist to semi-arid climates were studied. In addition to traditional paleoclimate proxies, such as grain-size distribution as proxy for wind-strength and source aridity, and magnetic susceptibility as proxy for precipitation (and temperature), pedo-chemical elemental ratios, obtained by selective sediment leaching procedures, function as sensitive recorders of variations in weathering intensity due to changes in precipitation and temperature. Especially, the earth alkaline element ratios of Ba/Sr and Sr/Ca are subject to strong variations as a result of carbonate dissolution/precipitation and variable cation exchange capacity on clay minerals along the elemental sequence of Ca–Sr–Ba. As shown by the close similarity to the magnetic susceptibility records in the different loess sections, these ratios reflect the effects of weathering intensity as a result of precipitation changes. The fact that Ba/Sr unequivocally shows higher values in both soils and paleosols in the four studied sections over the present-day climate gradient, singles this elemental ratio out as an interesting paleoclimate proxy.
... In contrast, Bullen et al. (1997) found that declining strontium isotope ratios in a 3 Ma soil chronosequence were best attributed to leaching of radiogenic strontium from K-feldspar, and were unrelated to biotite. Another possible source of changing 87 Sr/ 86 Sr with age in the proglacial streams is nonstoichiometric dissolution of strontium-bearing minerals (Fairchild and Killawee, 1995; Brantley et al., 1998). We were unable to separate minerals from these fine-grained rocks and sediments to characterize the strontium isotopic composition of individual phases. ...
Chemical denudation rates and strontium isotope ratios in streams vary substantially and systematically in the foreland of the retreating Bench Glacier in south-central Alaska. To study weathering of young glacier sediments, we sampled 12 streams draining a chronosequence of till and moraine soils derived from Cretaceous metagraywacke–metapelite bedrock. Both sediment age and vegetation cover increase with distance from the glacier. Cation denudation rates decline with increasing distance from the glacier, whereas silica denudation rates increase. Carbonate dissolution and sulfide oxidation account for roughly 90% of the solute flux from the youngest sediments. Biotite alteration accounts for 5–11% of the solute flux; its peak contribution is found in the glacier outlet stream. Silicate weathering is the dominant reaction only in the oldest sediments. In a laboratory dissolution experiment using fresh glacial sediment, carbonate dissolution dominated the solute flux during the first 700 hours, paralleling the behavior of young sediments in the field. In contrast to trends in the field, the silica flux did not increase after the carbonate was exhausted from the reactor. A possible reason for this difference is that establishment of vegetation causes an increase in silicate weathering. The 87Sr/86Sr ratio in the glacier outlet stream is greater than that in proglacial streams and in bulk rock, due to a greater contribution of biotite weathering in the outlet than in proglacial streams. Strontium isotope ratios decline with sediment exposure age in the proglacial streams, and are consistent with a carbonate source. Because the dominant weathering reactions in the young sediments are of carbonate and sulfide rather than silicate minerals, weathering at the glacier margin is not an important long-term sink for atmospheric CO2.
... For Sr analysis, 20 to 100 mg samples were digested in HF and HNO 3 as described above. Strontium was separated utilizing Sr Spec resin (Eichrome Industries) using methods described in Brantley et al. (1998) and Chesley et al. (2002). Samples were dried and then loaded on tantalum filaments with Ta gel to enhance ionization and run on a VG sector 54 multi-collector thermal ionization mass spectrometer. ...
Geochemical and sedimentological data in a continuous 60,000-year sediment core record from the Kalya horst region of central Lake Tanganyika provide a detailed history of paleoclimate-mediated weathering and overflow events from upstream Lake Kivu. Univariate (elemental profiles), bivariate (elemental ratios) and multivariate analyses of chemical trends show variations between the dry Late Pleistocene (32–18 ka cal yr BP) and the wetter conditions that both preceded and post-date that interval. This record places important new constraints on the timing of aridity in East Africa during the high-latitude Last Glacial Maximum (LGM) based on significant decreases in magnetic susceptibility and soluble cation concentrations coinciding with biogenic silica. Elemental indicators in the oldest portion of the sedimentary record (60–50 ka cal yr BP) characterize this interval as a comparatively wet period, similar to modern conditions. Our record demonstrates that the ensuing transition toward arid conditions in tropical Africa during high-latitude glaciation was a two staged event with intermediate levels of aridity occurring from 50–32 ka cal yr BP followed by intense aridity from 32–18 ka cal yr BP.The initiation of inflow from upstream Lake Kivu into Lake Tanganyika is evidenced at 10.6 ka cal yr BP through its influence on both elemental profiles (Mg, Ca) and through its effect on 87Sr/86Sr. Increases in elemental (Mg, Ca, Sr) concentrations coincide with the timing of the Lake Kivu overflow. Metal geochemistry suggests that the overflow from Lake Kivu into Lake Tanganyika may have ceased between 8 and 6 ka cal yr BP, suggesting a period of Middle Holocene aridity in East Africa.
The fluxes of metal cations and isotopes released by weathering of silicate rocks are crucial and a prerequisite for constraining geochemical fluxes to rivers and oceans. This study presents mineral and elemental compositions along with ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr data from a basaltic weathering regolith on Hainan Island, South China to elucidate Sr isotope fractionation and weathering fluxes. The ⁸⁷Sr/⁸⁶Sr ratios vary from 0.703936 to 0.706338 as a result of differential weathering of the minerals. δ⁸⁸Sr values in the weathering regolith range from −0.29 to 0.37‰, with the majority of the weathering regolith having lower δ⁸⁸Sr values than the parent rock. Sr is leached into the soil solution during plagioclase decomposition, while ⁸⁶Sr is preferentially adsorbed on the surface of secondary minerals. As weathering progresses, smectite decomposes and kaolinite desorbs under weakly acidic conditions, releasing the previously adsorbed ⁸⁶Sr into the soil solution. The differential weathering of kaolinite and smectite controls the δ⁸⁸Sr values of the weathering regolith, with pH being an important determinant of isotope fractionation. Furthermore, Sr elemental fluxes (SrFlux) and Sr isotopic fluxes (δ⁸⁸SrFlux) of this weathering regolith were calculated using a mass balance model, yielding mean values of 0.20 (mg cm⁻³ Myr⁻¹) and 0.052 (‰ (mg cm⁻³ Myr⁻¹)), respectively. The δ⁸⁸SrFlux exhibits a nonlinear positive correlation with the Chemical Index of Alteration (CIA), indicating that enhanced weathering leads to significant stable Sr isotope fractionation at CIA values below 95%. Our research promotes the understanding of Sr recycling and the fractionation behavior of stable Sr isotopes during consecutive weathering.
The Messejana–Plasencia great dyke (MPGD) is a Late Triassic tholeiitic gabbro intrusion related to the Central Atlantic Magmatic Province. Its large outcrop extent (~530 km), combined with its prolongation below the Duero basin (additional 100 km), makes it one of the world’s largest dykes known. We have studied felsic granophyric bodies appearing in its northernmost segment at different scales, from mm-sized (interstitial micrographic pockets) to felsic dykes of up to 10 m thick and 1.5 km long, intruding within the gabbros. Significant differences exist in the mineral and whole-rock composition of gabbros and granophyres, including the Sr–Nd isotopic ratios. The chemical variation in the gabbros is coherent with fractionation of olivine, clinopyroxene and plagioclase at depth. However, the presence of a compositional gap between gabbros and granophyres (absence of intermediate compositions) and the formation of these late-stage intergranular felsic melts within the gabbro mesostasis suggest that they could be derived by liquid immiscibility. The Sr–Nd isotopic heterogeneity in the MPGD gabbros and the presence of zircons with Variscan ages (~286 Ma), inherited from granulitic rocks, indicate that the mafic magmas experienced some degree of lower crust assimilation during fractionation close to the Moho depth. On the contrary, the scarce xenocrystic Variscan zircon crystals found in a granophyric dyke within the MPGD gabbro display similar textures and ages (~299 Ma) to those of the country rock granites and point to contamination at a different crustal level.
Water, bedrock, and saprolite samples from the Neuse and Cape Fear River basins, North Carolina, were analyzed for ⁸⁷Sr/⁸⁶Sr and [Sr] to evaluate the control of exposed bedrock on fluvial Sr isotopic compositions and the influence of geology on Sr delivered to the ocean. The ⁸⁷Sr/⁸⁶Sr and [Sr] of the two rivers start low and rise downstream, eventually approximating recent ocean isotopic compositions before entering their estuaries. Groundwater samples from the headwaters have ⁸⁷Sr/⁸⁶Sr ratios that are lower than expected from the dominant exposed bedrock. The isotopic compositions of bedrock and saprolite samples vary predictably with rock type and age and show no correlation with degree of weathering. The data indicate that dominant surficial bedrock is not the primary source of Sr to the headwaters of the rivers. Rather, mafic dikes that focus groundwater flow and are more easily weathered than their silicic hosts impact the ⁸⁷Sr/⁸⁶Sr of the waters more than their limited exposure might suggest. Furthermore, the Sr isotopic composition of the water delivered to the marine environment is buffered by groundwater from coastal plain sedimentary rocks, leaving no evidence of upstream geology. The data suggest that rock type and structure exert significant control on the Sr isotope geochemistry of groundwater that enters into streams. Considering the global fluvial Sr budget, these results emphasize that, in some settings, (1) rock exposure area can be a poor indication of the geologic influence on Sr isotopic compositions of surface waters, and (2) downstream Sr isotopic compositions may not reflect upstream geology.
What have isotopes added to our understanding of the kinetics of water-rock interaction processes at the surface of the Earth? Since the development of routine techniques of mass spectrometry, an impressive number of studies have used isotopes to constrain water-rock interaction processes. A number of these initial studies were not purposefully designed to determine the kinetics of water-rock interaction, but they demonstrated that isotopic kinetic effects were both sufficiently large and specific so that they could be used to constrain the kinetics of water-rock interactions. Here we provide a brief synthesis of the different approaches where isotopes have been applied to constrain the rates and timescales of water-rock interaction. We have adopted a very broad definition of the word “kinetics” for this review and have not limited the studies to reaction rates of particular reactions. Because most of the reactions occurring at the Earth’s surface are occurring at low temperatures, they do not proceed rapidly enough to reach thermodynamic equilibrium; therefore, we face the problem of determining the rates at which they proceed. Physical processes, such as climate variability, glacier dynamics, landsliding and tectonic processes often proceed more rapidly than chemical weathering reactions. For example, mineral weathering and the subsequent formation of secondary minerals in soils occurs over timescales on the order of tens to hundreds of thousands of years, while the characteristic timescales of climate variability are much shorter. Isotopic approaches have proven to be very powerful in constraining rates of water-rock interaction in natural systems, where physical and chemical processes are inevitably coupled.
Hydrothermal systems are a common feature of active collisional orogens. These systems can have a profound effect on the Sr fluxes and Sr isotopic composition of riverine waters through the direct input of Sr from hot springs and groundwaters and the chemical weathering of hydrothermal calcite. In an effort to quantify the role of calcite weathering we present an analytical model that examines the relationship between rock uplift and steady-state chemical weathering of granites and greywackes that contain trace amounts (
The Lebowa Granite Suite of the Bushveld Complex is a differentiated sheeted intrusion, between 1.5 and 3.5km thick, with
an areal extent of some 30 000km2. The granite suite and the overlying Rooiberg Group felsic volcanics are near coeval with the mafic Layered Suite all being
dated within 2057±3 Ma. Despite this narrow time frame of emplacement ages, a compilation of age dates ranges from ~2.06 to
~1.05 Ga and reported 87Sr/86Sr are often high and with large uncertainties. Because of hydrothermal alteration, a study of chemical and isotopic variations
of the whole rocks is equivocal. Since fluorite has a low Rb/Sr ratio, and is stable at low temperatures, the back correction
of the 87Sr/86Sr value has minimum uncertainty and the initial 87Sr/86Sr ratio (Ro) of the magma or fluid from which it crystallised can be determined. In this study Rb, Sr, Nd and Sm analyses of fluorite
from a number of different types of occurrences are reported. In addition, strontium isotope analyses were made on ‘primitive’
granite from a vein cross-cutting the layered mafic suite in the eastern Bushveld and compared to data obtained for an ‘evolved’
feldspar from the Zaaiplaats granite.
We recognise two distinct processes and events: a primary magmatic process that occurred at 2057±3 Ma and took place in a
very short time span (<1my). Hydrothermal fluid was not retained and a much later secondary hydrothermal process possibly
related to the marginal tectonism of the Kaapvaal craton in the Limpopo belt, which may have been a longer-lived event.
The primary fluorite in stockworks, volcanic pipes and disseminated in some granites originated from late magmatic fluid that
also infiltrated and engulfed country rock (eg Buffalo). These fluorites have variable but low 87Rb/86Sr, low Ro and moderate Sm and Nd (>50 and >100ppm respectively). The samples from the Vergenoeg pipe have accessory fluorite with low
Sr concentrations (32 to 70ppm) and Ro in the range 0.7166 to 0.7225. Fluorites from Buffalo, Zwartkloof and Spoedwel show a wide range of Sr content and Ro, from 39 to 231ppm and 0.727 to 0.794 respectively, resulting from mixing of a granitic fluid with assorted country rocks.
These late magmatic fluids generated large fluorite bodies such as Vergenoeg and had a significant autometasomatic imprint
as indicated by chloritisation of biotite such as in the Steelpoort Park granite veins. For the primary fluorite, most samples
fall on an array indicating an age of ~2.05 Ga. but with a large scatter.
A different and later hydrothermal fluid produced a totally different generation of fluorite and calcite in open vugs and
pegmatites at Zaaiplaats and Buffalo. These secondary fluorites display very high Ro, have low abundances of Sm and Nd with Sm varying from 30.97 to 0.17ppm and Nd ranging from 69.59 to 1.22ppm. In particular,
different coloured fluorite from one pegmatitic pod show an extreme range of Ro varying from ~0.7199 to ~0.8518, with even higher Ro in the calcite.
This article firstly discusses the general techniques for measurement of dissolution and precipitation rates of rock-forming silicates and carbonates, and then, seven of these nine factors are discussed sequentially. Full discussions of the last two phenomena, transport and biologic processes, are not included. Throughout the article, it also discusses how surface complexation models can now be used to understand the reactive sites on mineral surfaces. Nonetheless, the intent here is mostly to emphasize simpler rate equations that emphasize solution chemistry instead of surface chemistry because these are often used in reactive transport codes. Techniques to predict the rates of mineral dissolution for mineral compositions where data are unavailable are summarized.
The applications of radiogenic isotopes to investigate chemical and physical erosion processes, particularly in river basins of the Himalaya, have led to interesting inferences on the relationship between tectonics, weathering and climate. The chemical weathering studies rely more on Sr isotopes because of their widely different ratios in various end members, their uniform distribution in the oceans and the availability of continuous and robust record of marine ⁸⁷Sr/⁸⁶Sr through much of the geological past. The record for the Cenozoic shows steady increase in ⁸⁷Sr/⁸⁶Sr; one of the hypotheses suggested to explain this is enhanced continental silicate weathering due to the uplift of the Himalaya. This hypothesis linking tectonics-weathering-climate, based on ⁸⁷Sr/⁸⁶Sr as an index of silicate weathering, however, is being challenged by the recent observations that there are a variety of carbonates in the river basins of the Himalaya with ⁸⁷Sr/⁸⁶Sr similar to that of silicates which have the potential to contribute significantly to the high ⁸⁷Sr/⁸⁶Sr of rivers such as the Ganga-Brahmaputra. Further, the non-stochiometric release of Sr isotopes during chemical weathering of minerals and rocks, the imbalance of Sr isotope budget in the oceans and temporal variations in riverine fluxes due to impact of glaciations all have compounded the problem.
The Sr isotopic systematics in the weathering profiles of biotite granite and granite porphyry in southern Jiangxi Province
were investigated. The results showed that during the chemical weathering of granites, remarked fractionation occurred between
Rb and Sr. During the early stages of chemical weathering of granites, the released Sr/Si and Sr/Ca ratios are larger than
those of the parent rocks, and the leaching rate of Sr is higher than those of Si, Ca, K, Rb, etc. Dynamic variations in relative
weathering rates of the main Sr-contributing minerals led to fluctuation with time in87Sr/86Sr ratios of inherent and released Sr in the weathering crust of granite. Successive weathering of biotite, plagioclase and
K-feldspar made87Sr/86Sr ratios in the weathering residues show such a fluctuation trend as to decrease first, increase, and then decrease again
till they maintain stable. This work further indicates that when Sr isotopes are used to trace biogeochemical processes on
both the catchment and global scales, one must seriously take account of the preferential release of Sr from dissolving solid
phase and the fluctuation of87Sr/86Sr ratios caused by the variations of relative weathering rates of Sr-contributing minerals.
ResearchGate has not been able to resolve any references for this publication.