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1.
In White Oak Creek watershed in eastern Tennessee, the radionuclides60Co,90Sr, and137Cs were retained by streambed gravels on different sites:60Co was associated with manganese in the hydrous oxide coatings on rocks and minerals;90Sr occurred primarily as an exchangeable cation although small amounts were held in a nonexchangeable form; and137Cs was held very selectively, presumably by illite in shale fragments composing the sediment. The distribution of radionuclides on sediments was size dependent: the 0.85–3.35 mm fine-gravel fraction was higher in radionuclide concentration than the sand fraction. An areal survey of radionuclide concentrations on streambed gravels from throughout the watershed, located numerous contamination sources. These radionuclide concentrations, when combined with both distribution coefficients of radionuclides between gravel and water and drainage area, were used to rank subsections of the watershed by their relative contribution to the overall contamination of the watershed. For90Sr, this ranking procedure agreed with the measured discharges of subsections of the watershed which are routinely monitored. Research sponsored by the Oak Ridge National Laboratory (operated by U.S. Department of Energy under contract W-7405-eng-26 with Union Carbide Corporation. Publication No. 2017, Environmental Sciences Division, ORNL.  相似文献   

2.
The annual balance of radionuclides inflow/outflow was assessed for 137Cs and 90Sr isotopes in the Barents Sea, taking into account the atmospheric precipitation, inflow from the Norwegian and the White seas, as well as riverine discharge, liquid radioactive waste disposal (LRWD), and outflow to the adjacent seas. The original and published data for the period of 1950–2009 were analyzed. According to the multiyear dynamics (1960–2009), the inflow of 137Cs and 90Sr into the Barents Sea was significantly preconditioned by the Norwegian Sea currents; and precipitation played a major role in the 1950s, 1960s, and in 1986. Currently, the trans-border redeposition of 90Sr prevails over 137Cs redeposition in the Barents Sea, and constitutes about 99% of inflow of each element.  相似文献   

3.
The rate of chemical weathering of rocks has been determined by using uranium as a natural isotopic tracer. The concentration of uranium and 234U/238U ratio in natural waters, rocks, and soils of the Preto river basin (Bahia State, Brazil) was measured by alpha-ray spectroscopy.The activity ratio U234/U238 measured in the various samples indicates the uranium fraction which is dissolved from rocks during the weathering process. The results obtained show that 1 m of rock needs 25,000 yr to be weathered in this region under present climatic conditions.  相似文献   

4.
Iron and manganese solubility at the sediment/water interface has been studied at a water depth of 20 m in Kiel Bight, Western Baltic. By means of an in situ bell jar system enclosing 3.14 m2 sediment surface and 2094 l water a complete redox turn-over in the bottom water was simulated in an experiment lasting 99 days. The concentration of dissolved Fe in the bell jar water never exceeded 0.041 μmol · dm?3during the first 50 days of the experiment and then rose abruptly as the Eh fell from +600 to ?200 mV. The concentration of dissolved Fe under oxic and anoxic conditions seems to be limited by equilibria with solid Fe-phases (hydroxides and amorphous sulphide, respectively). In contrast to Fe, manganese was released continuously from the bottom during the first 50 days of the experiment leading to exponentially increasing manganese concentrations in the bell jar water. During this time dissolved O2 had become ready depleted and pH had dropped from 8.3 to 7.5. Contrary to iron, manganese being solubilized in reduced sediment layers can penetrate oxic strata in metastable form due to slow oxidation kinetics; when the redoxcline moves upwards Mn2+ is enriched in bottom waters. The maximum concentration of dissolved Mn under anoxic conditions is controlled by a solid phase with solubility properties similar to MnCO3 (rhodochrosite). Bottom water enrichment in dissolved Mn2+ could be traced to originate from excess solid manganese within the top 3 cm of the sediment.  相似文献   

5.
The balance between physicochemical processes, influencing vertical and temporal distributions of metal compounds in one relatively isolated anoxic environment, constitutes the objective of the present work. Ion activity product (IAP) was calculated for manganese and iron sulfides, in order to define the metal sulfide forms that control Fe and Mn solubility in the bottom waters of anoxic lagoons. Iron solubility depended on amorphous FeS formation, while manganese sulfides were a minor component in a solid solution lowering its solid-phase activity. A theoretical physicochemical model was developed for the iron speciation, based on experimental pH and redox potential data. A very good match was achieved for the measured and the theoretical total dissolved iron, at all depths. The dominance of oxidant iron species Fe(OH) 3 ? in the surface waters and their sequence by FeSH+ and FeSaq in the deeper layers brings out the influence of physicochemical parameters (dissolved oxygen, sulfide, pH and Eh) in vertical distribution of dissolved metal species, in anoxic/hypoxic basins. Based on these findings, we can conclude that the distribution of manganese and iron is of special interest, not only because these are the indicators of redox conditions but also for the role of their oxidized/reduced forms in the formation of the biogeochemical structure of redox zone.  相似文献   

6.
Controlled laboratory experiments have been used to study the diagenetic chemistry of 239,240Pu 137Cs, and 55Fe. Experiments using Buzzards Bay sediments in small tanks show that sulfate reduction is accompanied by the production of large pore water concentration gradients of alkalinity, phosphate, ammonia and dissolved organic carbon and the formation of subsurface maxima in Fe and Mn. These pore water profiles demonstrate that bacterially-mediated processes of organic matter degradation and redox reactions can be simulated in the laboratory.A vertical profile of 55Fe in pore waters is reported for the first time: it follows the profile of stable Fe and as such has a large (200 dpm/100 kg) subsurface maximum between 2–4 cm depth. Comparison of 55Fe/Fe ratios in sediments and pore waters shows that there is preferential solubilization of 55Fe over stable Fe.The pore water activities of 239,240Pu show no gradients within the large uncertainties of the counting statistics, but are two to four times higher than Buzzards Bay seawater (0.05 dpm/100 kg).The activity of 137Cs in the pore water profile is constant (40 dpm/100 kg) within the large counting uncertainties and is twice that of Buzzards Bay seawater. Cs-137 does not appear to be involved in diagenetic chemistry but may increase in pore waters as a result of ion exchange reactions.Flux estimates based on the pore water data show that remobilization and transport of 239,240 Pu in coastal sediments are not significant processes while the transport of l37Cs may be.  相似文献   

7.
The Sr isotope stratigraphy of the biogenic apatite was used to determine the age of pelagic sediments in the Brazil Basin (Station 1541) that contain ferromanganese micronodules, nodules, and coatings on the weathered volcanic rocks. The age of sediments at horizons 0–5 and 86–90 cm was estimated at 24.1 ± 0.2 Ma and 24.8 ± 0.2 Ma, respectively. The average sedimentation rate in the Late Oligocene was about 13 mm/ka. The hydrogenous Fe–Mn nodule on the sediment surface with the Mn/Fe value of 1.05–1.95 was formed at a rate of 1.2–2.4 mm/Ma, which is 1000 times lower than the growth rate of buried nodule (Mn/Fe 0.4) at depth of 83 cm. Diagenesis provoked changes in the mineral composition of the buried nodule (asbolane-buserite partially replaced by goethite), leading to the loss of a part of Mn, Ni, Li, and Tl but accumulation of trace elements linked with iron oxyhydroxides (Ce, Th, Be, As, and V) were retained. The composition of manganese micronodules at two studied depths in sediments evolved in the course of two stages of ore formation: related to the oxic and suboxic diagenesis. The Sr isotopic composition in manganese micronodules from both horizons do not differ from that of dissolved Sr in the ocean water. The 143Nd/144Nd ratio, which reflects the Nd isotopic composition in the paleocean during the micronodule formation, varies in manganese micronodules from different horizons and is constant in different size fractions.  相似文献   

8.
Activity profiles of excess 234Th, excess 210Pb, 232Th, 230Th, 234U and 238U, and 228/232Th ratios determined in eight box cores of sediment from six sites in central Puget Sound provide new insights into the dynamic nature of solid phase mixing in surface sediments, the exchange of 228Ra and other soluble species across the sediment-water interface, and the cycling of U, Th and 210Pb in this coastal zone.Comparison of excess 234Th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central Puget Sound water column.Surface sediment horizons with excess 234Th have no excess 228Th which might be used to ascertain sediment accumulation rates over the past decade. Instead, deficiencies of 228Th due to loss of soluble 228Ra from pore water to the overlying water persist to 20–30 cm, revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of Puget Sound.Solid phase U isotope concentrations tend to increase by up to a factor of two with depth in sediments, as a result of dissolved U being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing. 232Th and 230Th activities and 230/232Th ratios are constant with depth in sediments, indicating constant detrital phase compositions and essentially no authigenic 230Th. Steady state 210Pb depositional activities in and fluxes to Puget Sound sediments average only about onehalf those for sediments of the open Washington coast north of the Columbia River mouth, primarily because of a much lower supply of dissolved 210Pb in sea waters adverting into Puget Sound.Excess 234Th profiles in sediments reveal much more detail about the depth dependency, dynamic nature and recent history of solid phase mixing processes than excess 210Pb profiles. At least six of eight 234Th profiles show that mixing within the 210Pb-defined surface mixed layer is depth dependent. In three profiles, 234Th-derived mixing rates are fastest several centimeters below the sediment-water interface, indicating greater macro-benthic activity at these depths. Depth dependent mixing coefficients derived from the best fit of a four layer, advection-diffusion-decay model to the 234Th data are consistent with 210Pb profiles determined for the same sediments, strongly suggesting that 234Th and 210Pb are mixed equivalently and in a multilayered manner.  相似文献   

9.
《Applied Geochemistry》2004,19(7):1137-1155
Within the Lower Peninsula of Michigan, groundwaters from the Marshall Formation (Mississippian) contain As derived from As-rich pyrites, often exceeding the World Heath Organization drinking water limit of 10 μg/L. Many Michigan watersheds, established on top of Pleistocene glacial drift derived from erosion of the underlying Marshall Formation, also have waters with elevated As. The Huron River watershed in southeastern Lower Michigan is a well characterized hydrogeochemical system of glacial drift deposits, proximate to the Marshall Fm. subcrop, which hosts carbonate-rich groundwaters, streams, and wetlands (fens), and well-developed soil profiles. Aqueous and solid phase geochemistry was determined for soils, soil waters, surface waters (streams and fens) and groundwaters from glacial drift aquifers to better understand the hydrogeologic and chemical controls on As mobility. Soil profiles established on the glacial drift exhibit enrichment in both Fe and As in the oxyhydroxide-rich zone of accumulation. The amounts of Fe and As present as oxyhydroxides are comparable to those reported from bulk Marshall Fm. core samples by previous workers. However, the As host in core samples is largely unaltered pyrite and arsenopyrite. This suggests that the transformation of Fe sulfides to Fe oxyhydroxides largely retains As and Fe at the oxidative weathering site. Groundwaters have the highest As values of all the waters sampled, and many were at or above the World Health limit. Most groundwaters are anaerobic, within the zones of Fe3+ and As(V) reduction. Although reduction of Fe(III) oxyhydroxides is the probable source of As, there is no correlation between As and Fe concentrations. The As/Fe mole ratios in drift groundwaters are about an order of magnitude greater than those in soil profiles, suggesting that As is more mobile than Fe. This is consistent with the dominance of As(III) in these groundwaters and with the partitioning of Fe2+ into carbonate cements. Soil waters have very low As and Fe contents, consistent with the stability of oxyhydroxides under oxidizing vadose conditions. When CO2 charged groundwaters discharge in streams and fens, dissolved As is effectively removed by adsorption onto Fe-oxides or carbonate marls. Although Fe does not display conservative behavior with As in groundwaters, a strong positive correlation exists between As and Sr concentrations. As water–rock interactions proceed, the As/Fe and Sr/Ca ratios would be expected to increase because both As and Sr behave as incompatible elements. Comparisons with groundwater chemistries from other drift-hosted aquifers proximate to the Marshall sandstone are consistent with these relations. Thus, the Sr content of carbonate-rich groundwaters may provide useful constraints on the occurrence, origin and evolution of dissolved As in such systems.  相似文献   

10.
The 87Sr/86Sr ratios of evaporitic carbonates and sulphates from Miocene sediment cored in the Mediterranean Sea show a depletion of 87Sr when compared to the isotopic composition of the Miocene contemporaneous marine strontium: 0.70803 versus 0.70936. The arrival into the evaporitic environment of strontium brought by continental waters can explain this difference. The variation of the 87Sr/86Sr ratios is, nevertheless, noticeable only when the influence of the continental waters is already well marked. This is proved when one compares the results obtained with strontium, to the results of isotopic analysis made on oxygen, carbon, sulphur and hydrogen taken from the same samples.  相似文献   

11.
The watershed in the southern Jiangxi Province (Jiangxi Province is called simply Gan) (SGW) and the watershed in the central Guizhou Province (Guizhou Province is called simply Qian) (CQW) are two subtropical watersheds of the Yangtze River in China. Both watersheds have similar latitudes and climate, but distinct differences in basin lithology. These similarities and differences provide a good natural laboratory in which to investigate weathering processes and Sr end-members in river waters. This work aims to identify and contrast the sources, fluxes and controls on Sr isotopic composition in the river waters of these two areas. Results showed that the 87Sr/86Sr in the SGW waters ranged from 0.716501 to 0.724931, with dissolved Sr averaging 27 μg l− 1. Rhyolites and granites are two major sources for the dissolved Sr. The SGW waters receive 42% of their Sr from silicates weathering, 32% from carbonates and 3.2% from evaporites. 87Sr/86Sr in the CQW waters has a lesser variation from 0.707694 to 0.710039, but higher Sr contents (average of 208 μg l− 1). Dolomite, limestone and dolomitic limestone are major sources of Sr in the waters. The CQW waters receive 69% of their Sr from carbonates, 1.7% from silicates and 0.9% from evaporites. The chemical erosion rate and Sr flux in the CQW are 122 t km− 2 a− 1 and 0.079 t km− 2 a− 1, respectively, which are higher than those of the SGW (56 t km− 2 a− 1 and 0.021 t km− 2 a− 1, respectively). These data suggest that the intensive carbonates weathering occurred in the karstic area in the upper-reach of the Yangtze River exert great influence on the high Sr concentration and low Sr isotopic ratios in the River.  相似文献   

12.
The decay constant 87Rb has been redetermined by measuring the amount of radiogenic 87Sr produced over a period of 19 years, in 20 g samples of purified RbClO4, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO3)2. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of 1.419(±0.012) × 10?11 yr?1 (2σ). [τ12 = 4.89(±0.04) × 1010yr.]  相似文献   

13.
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14.
Forty-one metric tons of the mineral wollastonite (CaSiO3) was applied to an 11.8 hectare watershed at the Hubbard Brook Experimental Forest (HBEF; White Mountains, New Hampshire, USA) with the goal of restoring the Ca estimated to have been depleted from the soil exchange complex by acid deposition. This experiment provided an opportunity to gain qualitative information on whole watershed hydrologic flow paths by studying the response of stream water chemistry to the addition of Ca. Because the Ca/Sr and 87Sr/86Sr ratios of wollastonite strongly contrast that of other Ca sources in the watershed, the wollastonite-derived Ca can be identified and its amount estimated in various ecosystem components. Stream water chemistry at the HBEF varies seasonally due to shifts in the proportion of base flow and interflow. Prior to the wollastonite application, seasonal variations in 87Sr/86Sr ratios indicated that 87Sr/86Sr was higher during base flow than interflow, due largely to greater amounts of biotite weathering along deeper flow paths. After the application, Ca/Sr and 87Sr/86Sr changed markedly as the high Ca/Sr and low 87Sr/86Sr wollastonite dissolved and mixed with stream water. The Ca addition provided information on the response times of various flow paths and ion exchange processes to Ca addition in this small upland watershed. During the first year after the addition, wollastonite applied to the near stream zone dissolved and was partially immobilized by cation exchange sites in the hyporheic zone. In the second and third years after the addition we infer that much of this Ca and Sr was subsequently desorbed from the hyporheic zone and was exported from the watershed in stream flow. In the fourth through ninth years after the addition, Ca and Sr from wollastonite that had dissolved in upland soils was transported to the stream by interflow during wet periods when the ground water table was elevated. Between years three and nine the minimum annual Ca/Sr ratio (in late summer base flow) increased, providing evidence that Ca and Sr had increasingly infiltrated to the deepest flow paths. Strong seasonal variations in Ca/Sr and 87Sr/86Sr ratios of stream water resulted from the wollastonite addition to upland forest soils, and these ratios have become sensitive to changing flow paths during the annual cycle. Most notably, high flow events now produce large excursions in stream geochemistry toward the high Ca/Sr and low 87Sr/86Sr ratios of wollastonite. Nine years after the application we estimate that ∼360 kg of Ca from wollastonite has been exported from the watershed in stream flow. The rate of export of Ca from wollastonite dissolution has stabilized at about 11 kg of Ca per year, which accounts for ∼30% of the dissolved Ca in the stream water. Given that 19 metric tons of Ca were applied to the watershed, and assuming this current rate of loss, it should take over 1000 years for this added Ca to be transported from the watershed.  相似文献   

15.
《Applied Geochemistry》2003,18(1):117-125
This paper describes the results of a study that was conducted to determine the relationship between hydrogeochemical composition and 87Sr/86Sr isotope ratios of the Mt. Vulture spring waters. Forty samples of spring waters were collected from local outcrops of Quaternary volcanites. Physico-chemical parameters were measured in the field and analyses completed for major and minor elements and 87Sr/86Sr isotopic ratios. A range of water types was distinguished varying from alkaline-earth bicarbonate waters, reflecting less intense water–rock interaction processes to alkali bicarbonate waters, probably representing interaction with volcanic rocks of Mt. Vulture and marine evaporites. The average 87Sr/86Sr isotope ratios suggest at least 3 different sources. However, some samples have average Sr isotope ratios (0.70704–0.70778) well above those of the volcanites. These ratios imply interaction with other rocks having higher 87Sr/86Sr ratios, probably Triassic evaporites, which is substantiated by their higher content of Na, SO4 and Cl. The Sr isotope ratios for some samples (e.g. Toka and Traficante) are intermediate between the value for the Vulture volcanites and that for the local Mesozoic rocks. The salt content of these samples also lies between the value for waters interacting solely with the volcanites and the value measured in the more saline samples. These waters are thus assumed to result from the mixing of waters circulating in volcanic rocks with waters presumably interacting with the sedimentary bedrock (marine evaporites).  相似文献   

16.
《Applied Geochemistry》2002,17(3):285-300
Strontium and particularly 87Sr/86Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and 87Sr/86Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and 87Sr/86Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, 87Sr/86Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, 87Sr/86Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members.  相似文献   

17.
《Applied Geochemistry》2000,15(9):1345-1367
Rare Earth Elements (REEs), and Sr and Nd isotope distributions, have been studied in mineralized waters from the Massif Central (France). The CO2-rich springs are characterized by a neutral pH (6–7) associated with total dissolved solids (TDS) from 1 to 7 g l−1. The waters result from the mixing of very mineralized water pools, thought to have equilibrated at a temperature of around 200°C with superficial waters. These two mineral water pools evidenced by Sr isotopes and dissolved REEs could reflect 2 different stages of water–rock interaction and an equilibrium with different mineral assemblages.The concentrations of individual dissolved REEs and total dissolved REEs (ΣREE), in the mineral waters examined, vary over several orders of magnitude but are not dependent on the main parameters of the waters (TDS, T°C, pH, Total Organic C). The dissolved REE concentrations presented as upper continental crust normalized patterns show HREE enrichment in most of the samples. The time evolution of REE patterns does not show significant fluctuations except in 1 borehole, located in the Limagne d’Allier area, which was sampled on 16 occasions over an 18 month period. Ten samples are HREE-enriched, whereas 6 samples show flat patterns.The aqueous speciation of REEs shows that CO2−3 complexes dominate (>80%) over the free metal, F−, SO2−4 and HCO−3 complexes. The detailed speciation demonstrates that the fractionation of REEs (i.e. the HREE enrichment) in CO2-rich and pH neutral fluids is due essentially to the predominance of the CO2−3 complexes.The Sr isotopic composition of the mineral waters in the Massif Central shows different mixing processes; in the Cézallier area at least 3 end-member water types exist. The most dilute end-member is likely to originate as poorly mineralized waters with minimal groundwater circulation. Two other mineralized end-members are identified, although the link between the geographical location of spring outflow and the mixing proportion between the 2 end-members is not systematic. The range in ϵNd(0) for mineralized waters in the Massif Central correlates well with that of the known parent rocks except for 4 springs. One way to explain the ϵNd(0) in these instances is a contribution from drainage of volcanic rocks. The isotopic systematics help to constrain the hydrogeological models for this area.  相似文献   

18.
The Sr isotopic composition of ‘seawater’, as measured on carbonate rocks, shows a composite pattern during geologic history. All known Archaean data are compatible with contemporaneous upper mantle 87Sr/86Sr values. This is followed by a strong increase in the radiogenic component during the 2.5–2.1 b.y. period, a less pronounced increase during the remaining portion of the Proterozoic and a decrease during the Phanerozoic. The trend closely resembles the K2O/Na2O secular variations in composition of igneous and sedimentary rocks (Engelet al, Bull. Geol. Soc. Amer. 85, 843–858, 1974) and probably reflects the fractionation state of the contemporary crust. The data are compatible with recent suggestions of three major tectonic regimes during geologic history: greenstone belts during the Archaean, mobile belts during the Proterozoic and plate tectonics during the Phanerozoic. They also indicate that continental crust during the Archaean contributed only subordinate Sr into the meteoric cycle.  相似文献   

19.
Rare earth element (REE) and strontium isotope data (87Sr/86Sr) are presented for hydromagnesite and surface waters that were collected from Dujiali Lake in central Qinghai-Tibet Plateau (QTP), China. The goal of this study is to constrain the solute sources of hydromagnesite deposits in Dujiali Lake. All lake waters from the area exhibit a slight LREE enrichment (average [La/Sm]PAAS = 1.36), clear Eu anomalies (average [Eu/Eu*]PAAS = 1.31), and nearly no Ce anomalies. The recharge waters show a flat pattern (average [La/Sm]PAAS = 1.007), clear Eu anomalies (average [Eu/Eu*] PAAS = 1.83), and nearly no Ce anomalies (average [Ce/Ce*]PAAS = 1.016). The REE+Y data of the surface waters indicate the dissolution of ultramafic rock at depth and change in the hydrogeochemical characteristics through fluid-rock interaction. These data also indicate a significant contribution of paleo-groundwater to the formation of hydromagnesite, which most likely acquired REE and Sr signatures from the interaction with ultramafic rocks. The 87Sr/86Sr data provide additional insight into the geochemical evolution of waters of the Dujiali Lake indicating that the source of Sr in the hydromagnesite does not directly derive from surface water and may have been influenced by both Mg-rich hydrothermal fluids and meteoric water. Additionally, speciation modeling predicts that carbonate complexes are the most abundant dissolved REE species in surface water. This study provides new insights into the origins of hydromagnesite deposits in Dujiali Lake, and contributes to the understanding of hydromagnesite formation in similar modern and ancient environments on Earth.  相似文献   

20.
87Sr86Sr measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest 87Sr/86Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low 87Sr86Sr in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate.  相似文献   

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