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1.
《Applied Geochemistry》1996,11(4):583-587
An isotope study (δ18O, δ2-H) of 62 samples of waters from both the interior of the Nerja cave (Málaga, Southern Spain) and the exterior sinkholes and springs at the site was carried out. Rainfall water in the study area was also studied. The mean value of seepage waters can be considered as the mean isotopic value of the precipitation waters of the area. The origin and aquifers of these different types of waters has also been studied, obtaining a clear meteoric origin both for the seepage waters and the waters from the wells and springs, but the aquifer of the latter is different, showing more negative isotopic values. This suggests that they could have mixed with waters from other aquifers which are either deeper or originate at other altitudes. 相似文献
2.
Tiziano Boschetti Lorenzo Toscani Orfan Shouakar-Stash Paola Iacumin Giampiero Venturelli Claudio Mucchino Shaun K. Frape 《Aquatic Geochemistry》2011,17(1):71-108
The salt waters from the Emilia-Romagna sector of the Northern Apennine Foredeep have been investigated using major and trace
element and stable isotope (δ2H, δ18O, δ37Cl, δ81Br and 87Sr/86Sr ratio). Ca, Mg, Na, K, Sr, Li, B, I, Br and SO4 vs. Cl diagrams suggest the subaerial evaporation of seawater beyond gypsum and before halite precipitation as primary process
to explain the brine’s salinity, whereas saline to brackish waters were formed by mixing of evaporated seawater and water
of meteoric origin. A diagenetic end-member may be a third component for mud volcanoes and some brackish waters. Salinization
by dissolution of (Triassic) evaporites has been detected only in samples from the Tuscan side of the Apennines and/or interacting
with the Tuscan Nappe. In comparison with the seawater evaporation path, Ca–Sr enrichment and Na–K–Mg depletion of the foredeep
waters reveal the presence of secondary processes such as dolomitization–chloritization, zeolitization–albitization and illitization.
Sulfate concentration, formerly buffered by gypsum-anhydrite deposition, is heavily lowered by bacterial and locally by thermochemical
reduction during burial diagenesis. From an isotopic point of view, data of the water molecule confirm mixing between seawater
and meteoric end-members. Local 18O-shift up to +11‰ at Salsomaggiore is related to water–rock interaction at high temperature (≈150°C) as confirmed by chemical
(Mg, Li, Ca distribution) and isotopic (SO4–H2O) geothermometers. 37Cl/35Cl and 81Br/79Br ratios corroborate the marine origin of the brines and evidence the diffusion of halogens from the deepest and most saline
aquifers toward the surface. The 87Sr/86Sr ratio suggests a Miocene origin of Sr and rule out the hypothesis of a Triassic provenance of the dissolved components
for the analyzed waters issuing from the Emilia-Romagna sector of the foredeep. Waters issuing from the Tuscan side of the
Apennines and from the Marche sector of the foredeep show higher 87Sr/86Sr ratios because of the interaction with siliciclastic rocks. 相似文献
3.
The isotopic composition of groundwater sources of the Sinai Desert was surveyed. The results are characterized by a large spread in the oxygen-18 and deuterium abundances, compared to equivalent systems from less arid climates. The variability reflects differences in the altitude at which precipitation occurred, the evaporation from stagnant surface waters prior to their infiltration into the ground and admixtures of waters which are not of meteoric or recent origin. It is difficult to distinguish between water sources recharged by direct infiltration and others recharged through the intermediary of flood waters, on the basis of their isotope composition. The isotopio composition enables a clearcut distinction, however, between paleowaters and more recently recharged groundwaters. Among the conclusions: paleowaters play a central role in the deep aquifers of desert areas; direct rain recharge to aquifers is widespread; surface waters which have undergone extensive evaporation contribute their water to local perched aquifers which are found along their route. 相似文献
4.
I. B. Goni 《Hydrogeology Journal》2006,14(5):742-752
The southwestern Chad basin is a semi-arid region with annual rainfall that is generally less than 500 mm and over 2,000 mm of evapotranspiration. Surface water in rivers is seasonal, and therefore groundwater is the perennial source of water supply for domestic and other purposes. Stable isotope has been measured for rainwater, surface water and groundwater samples in this region. The stable isotope data have been used to understand the inter-relationships between the rainwater, surface water, shallow and deep groundwater of this region. This is being used in a qualitative sense to demonstrate present day recharge to the groundwater. Stable isotope in rainwater for the region has an average value of –4‰ δ18O and –20‰ δ2H. Surface water samples from rivers and Lake Chad fall on the evaporation line of this average value. The Upper Zone aquifer water samples show stable isotope signal with a wide range of values indicating the complex character of the aquifer Zone with three distinguishable units. The wide range of values is attributable to waters from individual unit and/or mixture of waters of different units. The Middle and Lower aquifers Zones’ waters show similar stable isotopes values, probably indicating similarity in timing and/or mechanism of recharge. These are palaeowaters probably recharged under a climate that is different from today. The Upper Zone aquifer is presently being recharged as some of its waters show stable isotope compositions similar to those of average rainfall waters of the region. 相似文献
5.
The chemical and hydrodynamic characteristics of groundwater in deltaic regions are strongly influenced by the complex stratigraphy of these areas, caused by the continuously varying depositional environments associated with their recent hydrographic evolution. As a case study, the eastern sector of the Po River plain, northern Italy, has been investigated to understand the quality of the available groundwater resources. Based on the analysis of hydrochemical and isotopic data, the recharge characteristics, the groundwater residence time and the aquifer vulnerability are defined. The results show significant qualitative degradation of the unconfined aquifer due to the shallow depth to water, while in the underlying confined aquifer, a hydrochemical facies of Ca–HCO3 type prevails. The spatial variation and relationship between oxygen-18 and deuterium determine: firstly, hydraulic separation of the two hydrogeological units; secondly, direct infiltration of local precipitation to the unconfined aquifer; thirdly, the occurrence of waters originating in the Alps and locally from the Apennines, pervading the confined aquifer. The tritium results suggest local mixing between the superficial waters and the confined aquifer, occurring along the palaeo-river channels. This increases the pollution vulnerability of the confined hydrogeological unit within the plain, which is the only natural groundwater resource exploited for water supply. 相似文献
6.
为了进一步全面理解和探索青藏高原水文水循环过程,采用同位素方法并结合气象资料对青藏高原北麓河区域2011年6~12月降水和河水稳定同位素时空特征进行分析。探讨了北麓河降水同位素与日平均气温、降水量之间的相互关系,同时也对比分析了北麓河降水和河水的同位素变化特征。结果表明:北麓河降水同位素在整个观测期内总体受温度控制,但存在季节变化,其中6~9月降水同位素受到温度和降水量效应的共同控制,9月以后则主要受温度的影响。河水同位素与降水同位素相似的变化特征,体现了降水补给特征,另外降水量也能够影响河水同位素变化:降水量小则降水对其影响较小,反之则大。与北麓河降水线相比,河水δ18O~δD关系的斜率和截距偏大,揭示该区域河水除了受大气降水的补给外,还受到区域水体内循环和蒸发分馏作用的影响。 相似文献
7.
The isotopic composition of meteoric water in Sicily, Italy was investigated from May 2004 until June 2006. Samples were sampled monthly from a network of 50 rain gauges. During the same period 580 groundwater samples were collected from springs and wells to obtain insight into the isotopic composition of the water circulating in the main aquifers of the area. The mean weighted precipitation values were used to define the weighted local meteoric water line for five different sectors of Sicily. The use of Geographical Information System tools, coupled with isotopic vertical gradients, allowed designing an isotopic contour map of precipitation in Sicily. The defined meteoric compositions were highly consistent with most of the groundwater samples in each sector. However, in some areas fractionation processes occurring during and after rainfall slightly modify the isotopic composition of the groundwater. The obtained data set defines the present day isotopic composition of meteoric water in the central Mediterranean area and provides baseline values for future climatic and/or isotope-based hydrology studies. 相似文献
8.
9.
In recent years, a series of important progresses have been made in the aspect of magnesium isotopes behavior in weathering processes. These progresses are not only favorable to understand the change of the magnesium isotopic compositions in rivers, but also establish the foundation to further reveal the magnesium isotope geochemical cycle. The magnesium in rivers is both magnesium sink for weathering and magnesium source for the ocean. The Mg isotopic compositions in rivers are dominated by the magnesium sources and Mg isotope fractionations processes. The sources of magnesium in rivers originate mainly from draining rocks, as well as less contribution from the eolian deposition, groundwater, plant debris, and precipitation. The Mg isotope fractionations in rivers are mainly related to precipitation and dissolution of carbonate minerals, silicate mineral hydrolysis, adsorption on mineral or colloidal matter surface, and plant uptake. Generally, the contribution of carbonate minerals dissolution or precipitation is equal to add or reduce magnesium from carbonate endmember, which has a remarkably negative δ26Mg value. Based on the fact that most clay minerals are rich in 26Mg during nature silicate mineral hydrolysis, then it is possible to infer that residual weathering products enrich in 26Mg. However, there is no significant Mg isotope fractionation causing by the adsorption on mineral or colloidal matter surface during river water migration. For the plant uptake, the root prefers to have 26Mg, leading the plant itself rich in heavier Mg isotopic composition. In addition, formation of secondary minerals in rivers could also reflect the changes of chemical parameters in rivers (such as major elements, CO2 solubility, pH, etc.). Hence, Mg isotopic composition in rivers and associated isotope fractionations are not only the basis for the application of magnesium isotope to trace surface material cycle, but also have important significance for the further understanding the geochemical cycle of magnesium isotopes. 相似文献
10.
Paleowaters in Silurian-Devonian carbonate aquifers: Geochemical evolution of groundwater in the Great Lakes region since the Late Pleistocene 总被引：2，自引：0，他引：2
Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from −10 to −2‰, 87Sr/86Sr ratios range from 0.7080 to 0.7090, and δ34S-SO4 values range from +10 to 30‰. A few waters have elevated δ13CDIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ18O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, −15 to −13‰ δ18O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ18O values within the range of Holocene precipitation: −11 to −8‰ and −7 to −4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ13CDIC = −14.7 to−11.1‰ and 87Sr/86Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins. 相似文献
11.
Chemosynthetic carbonates, identified by isotopic, palaeoecological and sedimentological features, are concentrated in middle-late Miocene satellite and foredeep deposits of the northern Apennines. Chemoherms in the foredeep are hosted in thick pelitic intervals, probably deposited in intrabasinal structural highs, which are entirely or partly involved in large slumps, in many cases associated with extrabasinal slides. Sediment textures in carbonates and in the enclosing foredeep pelitic sediments indicate a link between hydrocarbon-fluid venting, sediment deformation and mobilisation, and tectonics. The intensity and style of fluid release phases directly influenced chemoherm typology, and also determined overpressure conditions in low shear strength pelitic sediments, favouring sediment mobilisation and influencing slope instability, which widely affected the Apennine foredeep. Chemosynthetic carbonates are associated with sites of tectonically fractured and compressed sediments in the Apennine foredeep-thrust belt system, thus indicating a relation with the tectonic loading of the Apennine thrust-sheets, which favoured fluid expulsion along forerunner faults. Possible gas hydrate contributions to fluid expulsion processes are discussed, based on sediment textures compared with modern vent areas. Finally, sediment instability may have facilitated a large amount of fluid escape, thus stopping carbonate precipitation. 相似文献
12.
Identifying the recharge sources and age of groundwater in the Songnen Plain (Northeast China) using environmental isotopes 总被引：5，自引：0，他引：5
The recharge sources and groundwater age in the Songnen Plain, Northeast China, were confirmed using environmental isotopes. The isotopic signatures of the unconfined aquifers in the southeast elevated plain and the north and west piedmont, cluster along local meteoric water lines (LMWLs) with a slope of about 5. The signature of source water was obtained by the intersection of these LMWLs with the regional meteoric water line (RMWL). This finding provides evidence that the recharge water for these areas originate from the Changbai Mountains and the Low and High Hingan Mountains, respectively. Groundwater in the unconfined aquifer in the low plain yields a LMWL with a slope of 4.4; its nitrate concentration indicates the admixture of irrigation return flow. The δ-values of the unconfined aquifer in the east elevated plain plot along the RMWL, reflecting recharge by local precipitation. The mean residence time of groundwater in these aquifers is less than 50?years. However, the 14C age of the groundwater in the confined Quaternary aquifer ranges from modern to 19,500?years, and in the Tertiary confined aquifer from 3,100 to 24,900?years. Modern groundwater is mainly recharged to the Quaternary confined aquifer on the piedmont by local precipitation and lateral subsurface flow. 相似文献
13.
Natural gas production from the Marcellus Shale formation has significantly changed energy landscape in recent years. Accidental release, including spills, leakage, and seepage of the Marcellus Shale flow back and produced waters can impose risks on natural water resources. With many competing processes during the reactive transport of chemical species, it is not clear what processes are dominant and govern the impacts of accidental release of Marcellus Shale waters (MSW) into natural waters. Here we carry out numerical experiments to explore this largely unexploited aspect using cations from MSW as tracers with a focus on abiotic interactions between cations released from MSW and natural water systems. Reactive transport models were set up using characteristics of natural water systems (aquifers and rivers) in Bradford County, Pennsylvania. Results show that in clay-rich sandstone aquifers, ion exchange plays a key role in determining the maximum concentration and the time scale of released cations in receiving natural waters. In contrast, mineral dissolution and precipitation play a relatively minor role. The relative time scales of recovery τrr, a dimensionless number defined as the ratio of the time needed to return to background concentrations over the residence time of natural waters, vary between 5 and 10 for Na, Ca, and Mg, and between 10 and 20 for Sr and Ba. In rivers and sand and gravel aquifers with negligible clay, τrr values are close to 1 because cations are flushed out at approximately one residence time. These values can be used as first order estimates of time scales of released MSW in natural water systems. This work emphasizes the importance of clay content and suggests that it is more likely to detect contamination in clay-rich geological formations. This work highlights the use of reactive transport modeling in understanding natural attenuation, guiding monitoring, and predicting impacts of contamination for risk assessment. 相似文献
14.
Hydrogeochemical and isotopic evidence of groundwater evolution and recharge in aquifers in Beijing Plain, China 总被引：2，自引：1，他引：1
An investigation was conducted in Beijing to identify the groundwater evolution and recharge in the quaternary aquifers. Water samples were collected from precipitation, rivers, wells, and springs for hydrochemical and isotopic measurements. The recharge and the origin of groundwater and its residence time were further studied. The groundwater in the upper aquifer is characterized by Ca-Mg-HCO3 type in the upstream area and Na-HCO3 type in the downstream area of the groundwater flow field. The groundwater in the lower aquifer is mainly characterized by Ca-Mg-HCO3 type in the upstream area and Ca-Na-Mg-HCO3 and Na-Ca-Mg-HCO3 type in the downstream area. The δD and δ18O in precipitation are linearly correlated, which is similar to WMWL. The δD and δ18O values of river, well and spring water are within the same ranges as those found in the alluvial fan zone, and lay slightly above or below LMWL. The δD and δ18O values have a decreasing trend generally following the precipitation → surface water → shallow groundwater → spring water → deep groundwater direction. There is evidence of enrichment of heavy isotopes in groundwater due to evaporation. Tritium values of unconfined groundwater give evidence for ongoing recharge in modern times with mean residence times <50 a. It shows a clear renewal evolution along the groundwater flow paths and represents modern recharge locally from precipitation and surface water to the shallow aquifers (<150 m). In contrast, according to 14C ages in the confined aquifers and residence time of groundwater flow lines, the deep groundwater is approximately or older than 10 ka, and was recharged during a period when the climate was wetter and colder mainly from the piedmont surrounding the plain. The groundwater exploitation is considered to be “mined unsustainably” because more water is withdrawn than it is replenished. 相似文献
15.
Samples of suspended matter were collected at different locations, seasons, depths and lateral profiles in the Amazon River and three of its main tributaries, the Madeira, the Solimões and the Negro rivers. Their iron isotope compositions were studied in order to understand the iron cycle and investigate the level of isotopic homogeneity at the river cross-section scale. Samples from four depth profiles and three lateral profiles analyzed show suspended matter δ57Fe values (relative to IRMM-14) between −0.501 ± 0.075‰ and 0.196 ± 0.083‰ (2SE). Samples from the Negro River, a blackwater river, yield the negative values. Samples from other stations (whitewater rivers, the Madeira, the Solimões and the Amazon) show positive values, which are indistinguishable from the average composition of the continental crust (δ57FeIRMM-14 ∼ 0.1‰). Individual analyses of the depth and lateral profiles show no significant variation in iron isotope signatures, indicating that, in contrast to certain chemical or other isotopic tracers, one individual subsurface sample is representative of river deeper waters. This also suggests that, instead of providing detailed information on the riverine iron cycling, iron isotopes of particulate matter in rivers will rather yield a general picture of the iron sources. 相似文献
16.
Major element concentrations, stable (δ18O and δ2H) and radiogenic (3H, 14C) isotopes determined in groundwater provided useful initial tracers for understanding the processes that control groundwater mineralization and identifying recharge sources in semi-arid Cherichira basin (central Tunisia).Chemical data based on the chemistry of several major ions has revealed that the main sources of salinity in the groundwaters are related to the water–rock interaction such as the dissolution of evaporitic and carbonate minerals and some reactions with silicate and feldspar minerals.The stable isotope compositions provide evidence that groundwaters are derived from recent recharge. The δ18O and δ2H relationships implied rapid infiltration during recharge to both the Oligocene and Quaternary aquifers, with only limited evaporation occurring in the Quaternary aquifer.Chemical and isotopic signatures of the reservoir waters show large seasonal evolution and differ clearly from those of groundwaters.Tritium data support the existence of recent recharge in Quaternary groundwaters. But, the low tritium values in Oligocene groundwaters are justified by the existence of clay lenses which limit the infiltration of meteoric water in the unsaturated zone and prolong the groundwater residence time.Carbon-14 activities confirm that groundwaters are recharged from the surface runoff coming from precipitation. 相似文献
17.
《Applied Geochemistry》2000,15(1):51-65
The Po Valley brines represent the base level of the Quaternary aquifer located in a thick clay-sands sedimentary sequence. Geochemistry indicates that these are marine waters, evaporated to the stage of gypsum precipitation and trapped at the bottom of the basin in the late Messinian. Most of the groundwater samples collected from different springs and wells in the plain result from a mixture of these Na–Cl brines and shallow groundwaters laterally recharged by the Alpine and Apennine chains.Natural outflows of brackish waters are associated with major tectonic features. Mud volcanoes, located in the eastern sector of the Po plain, are constantly monitored as sudden chemical changes are significant precursors of seismic activity. In the western sector, calcite-filled veins isotopically record different degrees of water-rock interaction. These are outcropping fossil conduits, where mixing between shallow groundwaters and deep seated brines has occurred. The temporal continuity of the hydrological circuits allows the reconstruction of past and present groundwater circulation patterns.This paper summarises and integrates the geochemical data produced over many years in order to obtain a regional picture of brine origins and the natural mechanisms of groundwater flow. 相似文献
18.
We report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in δ7Li is larger in dissolved load (from +9.3‰ to +29.0‰) compared to suspended sediments (from −1.7‰ to +3.2‰), which are not significantly different from δ7Li values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of 7Li in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces 7Li enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower δ7Li waters (but still enriched in 7Li compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for 7Li appear.Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basin. 相似文献
19.
Lithium isotope systematics in a forested granitic catchment (Strengbach, Vosges Mountains, France) 总被引：1，自引：0，他引：1
Emmanuel Lemarchand François Chabaux Marie-Claire Pierret 《Geochimica et cosmochimica acta》2010,74(16):4612-7724
Over the last decade it has become apparent that Li isotopes may be a good proxy to trace silicate weathering. However, the exact mechanisms which drive the behaviour of Li isotopes in surface environments are not totally understood and there is a need to better calibrate and characterize this proxy. In this study, we analysed the Li concentrations and isotopic compositions in the various surface reservoirs (soils, rocks, waters and plants) of a small forested granitic catchment located in the Vosges Mountains (Strengbach catchment, France, OHGE http://ohge.u-strasbg.fr). Li fluxes were calculated in both soil profiles and at the basin scale and it was found that even in this forested basin, atmospheric inputs and litter fall represented a minor flux compared to input derived from the weathering of rocks and soil minerals (which together represent a minimum of 70% of dissolved Li). Li isotope ratios in soil pore waters show large depth dependent variations. Average dissolved δ7Li decreases from −1.1‰ to −14.4‰ between 0 and −30 cm, but is +30.7‰ at −60 cm. This range of Li isotopic compositions is very large and it encompasses almost the entire range of terrestrial Li isotope compositions that have been previously reported. We interpret these variations to result from both the dissolution and precipitation of secondary phases. Large isotopic variations were also measured in the springs and stream waters, with δ7Li varying from +5.3‰ to +19.6‰. δ7Li increases from the top to the bottom of the basin and also covaries with discharge at the outlet. These variations are interpreted to reflect isotopic fractionations occurring during secondary phase precipitation along the water pathway through the rocks. We suggest that the dissolved δ7Li increases with increasing residence time of waters through the rocks, and so with increasing time of interaction between waters and solids. A dissolution precipitation model was used to fit the dissolved Li isotopic compositions. It was found that the isotopic compositions of springs and stream waters are explicable by an isotopic fractionation of −5‰ to −14‰ (best fit −10.8‰), in agreement with Li incorporation into clay. In soil solutions, it was found that isotopic fractionation during secondary precipitation is larger (at least −23‰), suggesting a major role for different secondary phases, such as iron oxides that maybe incorporate Li with a higher isotope fractionation. 相似文献
20.
We synthesize a new fluvial terrace chronostratigraphy of the Bidente and Musone Rivers cast within a broader European framework, which forms the basis of a terrace genesis and river incision model for the northern Apennines, Italy. Our model, supported by terrace long profiles, correlation to Po foreland sediments, 15 new radiocarbon dates, and published numeric and relative stratigraphic ages, highlights how drainage basin substrate drives concurrent formation of strath terraces in the Bidente basin and fill terraces in the Musone basin. Quaternary climate change paces the formative geomorphic processes through unsteady discharges of water and sediment. In the weathering-limited setting represented by the Bidente basin, siliciclastic detritus carves broad strath surfaces during glacial climates that are preserved as terraces as the river incises during the transition to an interglacial climate. In contrast, the transport-limited and carbonate detritus dominated Musone basin sees valleys deeply buried by aggradation during glacial climates followed by river incision during the transition to an interglacial climate. Incision of these rivers over the past ～1 million years has been both unsteady and non-uniform. These and all Po-Adriatic draining rivers are proximal to a base level defined by mean sea level and have little room for increasing their longitudinal profile concavities through incision, particularly in their lower reaches despite periodic glacio-eustatic drawdowns. As a result, the observed incision is best explained by rock uplift associated with active local fault or fold growth embedded in the actively thickening and uplifting Apennine foreland. 相似文献