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1.
We quantified differences in oxygen isotope fractionation among three biostratigraphically important subfossil ostracod species (Metacypris cordata, Pseudocandona rostrata and Candonopsis kingsleii) from an early Holocene freshwater tufa layer in northern Estonia. Estimated mean δ18O values are −10.05‰ for M. cordata, −9.34‰ for C. kingsleii and −8.75‰ for P. rostrata. All three species exhibit positive offset from the weighted mean annual δ18O of contemporary precipitation (−10.7‰ in δ18OV-PDB) and from the mean δ18O value of authigenic tufa carbonate (−10.64‰) in the ostracod-bearing layer. Assuming that the known oxygen isotope fractionation in P. rostrata (+2.5‰) and M. cordata (+1.5‰) has remained constant over time, the theoretical δ18OV-SMOW of the early Holocene lake water was calculated to have been between −11.52 and −11.92‰, slightly less negative than the local Ordovician groundwater (−11.7 to −12.2‰). δ18O values of the tufa carbonate differ by +0.6 to +1.0‰ from the calculated theoretical isotope composition (δ18OV-PDB) of lake water, indicating that the tufa also did not precipitate in isotopic equilibrium with ambient waters. Results show that the greater the δ18O offset from the calculated, theoretical isotope composition of lake water for an ostracod species, the lower is its preferred mean July temperature. Both our data and earlier published results on δ18O values in Holocene lacustrine carbonates and ostracods from north-eastern Europe, display pronounced decreases in δ18O with an increase in latitude of the study site. This suggests that temperature-dependent, and therefore latitude-dependent isotopic composition of meteoric waters controlled the δ18O values in lacustrine tufa and ostracods throughout the Holocene.  相似文献   

2.
Stable isotope analysis of ostracod shells is used routinely for palaeoenvironmental studies of ostracod-bearing records. Sample treatment usually involves the disaggregation of sediments and sieving; before the sieving residues were washed with water onto petri dishes and oven-dried. In our study, we compared δ18O and δ13C values of shells that were oven-dried from water and from ethanol alternatively. Large isotopic differences of up to 3‰ were determined for δ18O values, whereas differences in δ13C values were less pronounced with differences of up to 1.6‰. Stable isotope values of shells dried from water were lower for both oxygen and carbon as a result of calcite crystals precipitated on the shell surfaces during the drying process. Therefore, ostracod shells for stable isotope analysis should not be prepared by drying from water. Instead, shells should be dried from ethanol to obtain reliable stable isotope data; likewise freeze-drying is expected to provide trustworthy results.  相似文献   

3.
The apparent isotope enrichment factor εmacrophyte of submerged plants (εmacrophyte–DIC = δ13Cmacrophyte − δ13CDIC) is indicative of dissolved inorganic carbon (DIC) supply in neutral to alkaline waters and is related to variations in aquatic productivity (Papadimitriou et al. in Limnol Oceanogr 50:1084–1095, 2005). This paper aims to evaluate the usage of εmacrophyte inferred from isotopic analyses of submerged plant fossils in addition to analyses of lake carbonate as a palaeolimnological proxy for former HCO3 − concentrations. Stable carbon isotopic analysis of modern Potamogeton pectinatus leaves and its host water DIC from the Tibetan Plateau and Central Yakutia (Russia) yielded values between −23.3 and +0.4‰ and between +14.0 and +6.5‰, respectively. Values of ε Potamogeton–DIC (range −15.4 to +1.1‰) from these lakes are significantly correlated with host water HCO3 − concentration (range 78–2,200 mg/l) (r = −0.86; P < 0.001), thus allowing for the development of a transfer function. Palaeo-ε Potamogeton–ostracods values from Luanhaizi Lake on the NE Tibetan Plateau, as inferred from the stable carbon isotope measurement of fossil Potamogeton pectinatus seeds (range −24 to +2.8‰) and ostracods (range −7.8 to +7.5%) range between −14.8 and 1.6‰. Phases of assumed disequilibrium between δ13CDIC and δ13Costracods known to occur in charophyte swards (as indicated by the deposition of charophyte fossils) were excluded from the analysis of palaeo-ε. The application of the ε Potamogeton–DIC-HCO3 − transfer function yielded a median palaeo-HCO3 − -concentration of 290 mg/l. Variations in the dissolved organic carbon supply compare well with aquatic plant productivity changes and lake level variability as inferred from a multiproxy study of the same record including analyses of plant macrofossils, ostracods, carbonate and organic content.  相似文献   

4.
The utility of ostracod-based palaeoenvironmental reconstruction was evaluated using instrumental data for Lake Qarun, Egypt. The euryhaline ostracod Cyprideis torosa was the only species found in the lake’s recent sediment record. This species is known to tolerate salinity levels and water solute compositions that may prevent colonisation by other species. Oxygen and carbon isotope ratios of ostracod carbonate from lake sediments covary with changes in instrumental values for lake level and salinity for the period 1890–1974. δ13C-values correlate negatively with lake water salinity (r 2 = 0.87) and δ18O-values correlate negatively with measured lake level changes (r 2 = 0.41). Other ostracod proxy data provide qualitative information on lake level trends. Fossil assemblage data (juvenile/adult and valve/carapace ratios and valve preservation) provide information on wave energy. Ecophenotypic variation of C. torosa valves provided some useful palaeolimnological information. Sr/Ca and Mg/Ca ratios in ostracods were not found to reflect water composition, due to the uncoupling of these ratios with salinity in Lake Qarun. Overall, our results highlight the need to calibrate ostracod proxy data in modern systems prior to their use for palaeoenvironmental reconstruction.  相似文献   

5.
Stable isotopes and trace-element content of calcite ostracod valves and aragonite mollusc shells from the Pliocene lacustrine succession of Villarroya allow depicting the geochemical record of environmental changes and to compare our data to the paleoenvironmental reconstruction obtained from other proxies. The lower sequences (A and B) are characterized by relatively high isotopic and Me/Ca values in the biogenic carbonates. The recorded large variations of δ18O in these carbonates mainly reflect variations in the δ18Ow due to precipitation–evaporation processes and, to a lesser extent, variations in temperature of calcification. The δ13C data inform about changes in DIC although they are probably biased by the vital effects of the studied taxa. Minor and trace element contents in ostracod (Mg/Ca, Sr/Ca) and mollusc shells (Sr/Ca) are mainly linked to the Me/Ca of the lake water (Me/Caw), and to a lesser extent to temperature and to uptake kinetic effects. Several possible mechanisms may make the Me/Caw to vary: long periods of Ca-carbonate (calcite, aragonite) deposition after charophyte development, and different inputs for Ca and Mg to the lake due to changes in drainage area configurations through time, including the changes in saline inputs (Na-Cl type) to the lake. The stable isotopes and the calculated Sr/Caw and Mg/Caw from sequence C display lower values than those from sequences A and B. The isotopic values from biogenic carbonates of unit C indicate isotopically diluted waters in a hydrologically open lacustrine environment. Distinct δ13C and δ18O plots for molluscs from unit C reflect the different biotopes and metabolism type. For several intervals of the Villarroya succession there is no direct relationships among: (i) salinity changes inferred from invertebrate paleoecology, (ii) paleoenvironmental slices based on isotopic signatures and Me/Caw calculations (from biogenic carbonate geochemistry) and (iii) climate in the hinterland deduced from pollen data. Lakes where geochemical behaviour is constrained by sporadic saline inputs and/or relative depletion in Ca due to long periods of Ca-carbonate precipitation or biomineralization, like the Pliocene Villarroya lake was, do not show clear correlation patterns between geochemical signals and climate proxies. In these lakes only major environmental trends display unambiguous geochemical signatures, and only some main shifts in the geochemical signature profiles may be correlated with significant global and/or regional environmental changes that have been reported from other paleoenvironmental records. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

6.
Oxygen isotope analysis of the adult ostracod Eucypris mareotica cultured at controlled temperatures (10, 15, and 19°C) was used to measure isotopic fractionation during shell calcification. The ostracod shells that precipitated at experimental temperatures are almost in isotopic equilibrium with the culture water as compared to the oxygen isotope fractionation of inorganic carbonates. Moreover, they had almost constant offsets from equilibrium based on the oxygen isotope fractionation of inorganic carbonates. The δ18O values of ostracod shells from the 10°C cultures were higher than those of the 15 and 19°C cultures by 1.6 and 2.7‰, respectively. The observed fractionations are shown by the regression equations: *20c 10° \textC:d 1 8 \textO\textostracod = 1. 1 7+ 0. 5 7d 1 8 \textO\textwater 1 5° \textC:d 1 8 \textO\textostracod = - 0. 4 8+ 0. 6d 1 8 \textO\textwater 1 9° \textC:d 1 8 \textO\textostracod = - 1. 6+ 0. 6d 1 8 \textO\textwater \begin{array}{*{20}c} { 10^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = 1. 1 7+ 0. 5 7\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 5^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 0. 4 8+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 9^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 1. 6+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ \end{array} The fractionation factors (α) are slightly lower for the 15 and 19°C cultures, but slightly higher for the 10°C culture, as compared to inorganic carbonates (O’Neil et al. in J Chem Phys 51:5547–5558, 1969). The oxygen fractionation factors of E. mareotica are very close to those of synthetic calcite formed in isotopic equilibrium. The ‘vital offsets’ of valve-δ18O for E. mareotica is so small that we can neglect its effect when using the δ18O of E. mareotica living in lake waters with high pH and salinity to reconstruct the paleoenvironment. The paleotemperature or paleoisotopic composition of lake water interpreted from a core of lacustrine sediment may be closer to the true values when the δ18O data for E. mareotica are used.  相似文献   

7.
Stable isotopes and trace-element contents of ostracod (Candona neglecta) valves mostly from the Holocene portion of two assembled cores from Petit Lac (Lake Geneva, Switzerland-France) were analysed in order to depict the geochemical record of post-glacial environmental changes of this lake. Additional stable isotope and trace element data from the gastropod Bithynia tentaculata (shells and opercula) from some intervals of these cores, as well as previous data from bulk carbonate from the lower part of the studied intervals were also considered. Mg/Ca and Sr/Ca molar ratios for the Holocene lake water have been estimated from evaluations of the partitioning coefficients for Mg and Sr for C. neglecta and B. tentaculata taking into account the modern-lake water composition. This study shows an overall gentle trend to higher δ18O values in C. neglecta valves from the Boreal interval (mean −8.44‰) to the upper part of the core (mean −8.11‰). This trend is superimposed to higher frequency oscillations of stable isotope values and trace element ratios, especially through the upper Older Atlantic and the Subboreal. The overall isotopic oxygen trend includes several shifts in δ18O of about 1‰. These shifts are interpreted as major regional-global climate changes that have also been observed in other coeval δ18O and pollen records which reflect the Holocene climate variability in other European basins. Especially well-defined peaks in some episodes like Older Atlantic (~8200 yr BP), Younger Atlantic – Subboreal transition (~5600 yr BP) and early Subatlantic (~ 2500 yr BP) correspond to well-recognized events in globally-distributed records. Some of these shifts are correlated with pulses in the lake-level curve of the Lake Geneva. An erratum to this article is available at .  相似文献   

8.
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9.
We established the relationships between water chemistry changes in a pool fed by a permanent spring and seasonal variations in trace-element contents (Sr & Mg) in the shells of the ostracod species Herpetocypris intermedia, based on monthly collections of ostracod and water samples. The water chemistry of the investigated pool (Maïques, Valëncia, Spain) was dominated by calcium and bicarbonate, and showed marked seasonal variation in alkalinity, Ca2+ content, Sr/Ca and Mg/Ca ratios. Although the variability in the water chemistry was relatively low (~10% relative standard deviation over the entire period), the trace-element contents in the ostracod shells tracked the seasonal change in the water chemistry of Maïques pool. Moreover, due to the rapid renewal of H. intermedia population, this species is able torecord in its shells the evolution of the water chemistry at a monthly time scale. Our results also showed that, in the Maïques pool system, ostracod Sr/Ca and Mg/Ca ratios increased with the decrease in water salinity.To our knowledge, this is the first geochemical study of ostracods dwelling in spring environments. The results of this study may be applied to paleohydrological reconstruction using ostracods preserved in sediments deposited around springs (i.e. travertine and tufa deposits).  相似文献   

10.
A 7.6-m lake sediment core from a marl lake, Lough Inchiquin, records variation in landscape evolution from 16,800 cal yrs B.P. to 5,540 cal yrs B.P. We observe significant variations (up to 12‰) in δ 13Corg and δ 13Ccalcite values that are interpreted to reflect secular changes in lake water δ 13CDIC values that result from a regional landscape transition from barren limestone bedrock to a forested ecosystem. Lake water δ 13CDIC values are therefore influenced by two isotopically distinct sources of carbon: terrestrial organic material (−27.1 to −31.2‰VPDB) via oxidized soil organic matter and weathered limestone bedrock (+3.4‰VPDB). Isotope excursions in lacustrine sediment records are forced not only by changes in productivity but also by changes in the terrestrial environment. This has profound implications for the interpretation of paleoclimate records derived from lacustrine sediment and suggests that selection of appropriate lakes can provide records of terrestrial change where other related records are not available.  相似文献   

11.
西江河口段溶解无机碳稳定同位素组成的时空变化   总被引：3，自引：1，他引：2  
For researching the spatio-temporal variation of the stable isotopic composition of the riverine dissolved inorganic carbon（DIC）,we had carried out a survey throughout the hydrologic year during which theδ^13CDIC of the surface water and its physicochemical parameter were examined along the Xijiang River Inner Estuarine waterway from September 2006 to June 2007.There was a striking seasonal variation on the averageδ^13CDIC,as the averageδ^13CDIC in summer（-13.91‰）or autumn（-13.09‰）was much less than those in spring（-11.71‰）or winter（-12.26‰）.The riverineδ13C DIC was controlled by decomposed condition of the riverine organic matter linking the seasonal variation of the physicochemical parameter in the surface water according to the correlation analysis which indicated notable relations betweenδ^13CDIC and water temperature（p=0.000;r=-0.569）or betweenδ^13CDIC and oxide-reduction potential（p=0.000;r=0.646）.The striking positive correlation between δ^13CDIC and the sampling distance happened in the summer rainy season,while striking negative correlation happened in the spring dry season,indicating that river-sea interaction influenced water physicochemical parameters and controlled the riverine DIC property in the survey waterway.In view of the riverineδ^13CDIC decreasing for the decomposition of the terrestrial organic matter in the rainy season in summer and increasing for the briny invaded zone extending in the spring dry season along the waterway from the Makou gauging station to the Modaomen outlet,theδ^13CDIC spatio-temporal variation was closely related to the geographical environment of the Xijiang drainage basin.  相似文献   

12.
Stable isotopes (δ18O and δD) are useful tracers for investigating hydrologic and climatic variability on a variety of temporal and spatial scales. Since the early isotopic studies on mountainous glaciers in the late 1960s, a great deal of information has been generated on the isotopic composition of rainfall, snow, ice, surface waters, and lake carbonate sediments across the Tibetan Plateau. However, measurements of δ18O and δD values of lake water are scarce. Here we present a new dataset of δ18O and δD values of lake waters collected from 27 lakes across the plateau during a reconnaissance survey in summer 2009. δ18O and δD values of lake water range from −19.9 to 6.6‰ and from −153 to −16‰, respectively. The average values of δ18O and δD are −6.4 and −72‰, considerably greater than those of precipitation observed in this region. The derived Tibetan lake water line, δD = 5.2δ18O − 38.9, is significantly different from the global meteoric water line. Most of the lakes, including some freshwater lakes, contain water with negative values of d-excess (d). There is a negative correlation between d and total dissolved solids (TDS). Each of these findings indicates that evaporation-induced isotopic enrichment prevails in Tibetan lakes. Moreover, we develop an isotope modeling scheme to calculate E/P ratios for Tibetan lakes, using a combination of existing isotopic fractionation equations and the Rayleigh distillation model. We use the intersection of the local evaporation line and GMWL as a first approximation of δ18O and δD values of lake water inputs to infer an E/P ratio for each lake. Our modeling calculations reveal that although variable from lake to lake, the water budget across the plateau is positive, with an average E/P of 0.52. This is in good agreement with other observational and model data that show varying degrees of increases in lake size from satellite imagery and significant decreases in lake salinity in many lakes on the plateau over the last several decades. Together with the new isotopic dataset, the proposed modeling framework can be used to examine and quantify past changes in a lake’s hydrologic balance from the isotopic record of downcore carbonate sediments in the region.  相似文献   

13.
This study presents detailed lithostratigraphy and stable carbon and nitrogen isotopic variations in a 520-cm-long sediment core from a cirque basin in the Labsky důl Valley, Krkonoše Mountains, Czech Republic. Detailed study of the core reveals five major periods of sedimentation during the last 7600 years: silt and sand deposition during ~7.6–5.1 ka cal BP, Sphagnum peat accumulation during ~5.1–4.0 ka cal BP, sandy silt and sand during ~4.0–2.8 ka cal BP, raised peat bog during ~2.8–2.0 ka cal BP (Sphagnum peat), and sedimentation of sandy silt since ~2.0 ka cal BP. The δ13C values of the organic matter in the core vary in the range typical for C3 plants, from −24.35 to −27.68‰, whereas the δ15N values vary from −2.65 to +4.35‰. Core sections having ash contents ≥70% have δ15N > 1‰ and δ13C < −26‰, whereas those having ≤70% ash content have δ15N < 1‰ and δ13C > −26‰. Strong linear correlations are observed between δ13C and δ15N values as well as between C:N ratios and δ15N values in the horizons with ash content >10%, primarily for sand and silt horizons. On the other hand, poor correlations between δ13C and C:N ratio, as well as δ15N and C:N ratio, were observed in Sphagnum peat layers (45–125 and 185–265 cm). We conclude that the primary stable isotope variations are not preserved in the layers where significant correlation between δ15N and C:N ratio is observed. The relatively small δ13C variation in the uppermost Sphagnum peat layer suggests stable temperature during ~2.8–2.0 ka cal BP.  相似文献   

14.
Walker Lake, a hydrologically closed, saline, alkaline lake located along the western margin of the Great Basin of western United States, has experienced a 77% reduction in volume and commitment drop in lake level as a result of anthropogenic perturbations and climatic fluctuations over the last century. The history of lake-level change in Walker Lake has been recorded instrumentally since 1860. A high-resolution multi-proxy sediment core record from Walker Lake has been generated through analysis of total inorganic carbon (TIC), total organic carbon (TOC), and oxygen and carbon isotope ratios (δ18O and δ13 C) of both downcore bulk TIC and ostracods over the last 200 yr. This allows us to examine how these sediment indices respond to actual changes in this lake’s hydrologic balance at interannual to decadal timescales. In Walker Lake sediments, changes in %TIC, %TOC, and δ13C and δ18O of TIC and ostracods are all associated to varying degrees with changes in the lake’s hydrologic balance, with δ18O of the TIC fraction (δ18OTIC) being the most highly correlated and the most effective hydrologic indicator in this closed-basin lake. The δ18OTIC record from Walker Lake nearly parallels the instrumental lake-level record back to 1860. However, comparison with sporadic lake-water δ18O and dissolved inorganic carbon δ13C (δ13CDIC) results spanning the last several decades suggests that the isotopic values of downcore carbonate sediments may not be readily translated into absolute or even relative values of corresponding lake-water δ18O and δ13CDIC. Changes in the lake’s hydrologic balance usually lead to changes in isotopic composition of lake waters and downcore sediments, but not all the variations in downcore isotopic composition are necessarily caused by hydrologic changes.  相似文献   

15.
Authigenic vivianite and siderite microconcretions were found, respectively, in hemipelagic and deltaic facies of 600-m-long BDP-98 sediment section from Lake Baikal. Textural investigations of these microconcretions show that they are typically <1 mm in size, irregular in shape and composed of aggregated crystallites. Dissimilar orientation of vivianite and siderite crystallites suggests formation at different depths in the sediment; up to tens of centimeters for vivianite and tens of meters for siderite. Chemical analyses of both the vivianite and the siderite indicate cation composition characterized by minor amounts of Mn, Ca and Mn apart from the dominating Fe. Rather limited and distinctive carbon isotopic composition of the siderite, with δ 13CVPDB values between about +13 and +16‰, implies formation of the mineral in the methanogenic zone of diagenesis. Isotopic composition of oxygen in the siderite (δ 18OVPDB values between about −10 and -11‰ ) is consistent with crystallization temperature at about 10–30°C and water δ 18OSMOW values between about −10 and -16‰ . The distribution of the authigenic minerals in the section suggests changes in both sedimentary facies and climate, where vivianite formation was controlled by hemipelagic depositional conditions during the Pliocene and Quaternary, whereas siderite reflects impact of deltaic conditions during the Miocene.  相似文献   

16.
We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (18Op) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (18Oc). Comparison of the calcite oxygen isotope proxy for 18Op in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This 18Oc/temperature relationship is calculated to be 0.39/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 anomalous negative shift in calcite 18O values since the 1960s. Possible explanations for this recent 18Oc shift, as it is not related to mean annual air temperature, include changes in 18Op due to synoptic circulation patterns. In particular, the 0.8 negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 offset in 18Oc data.  相似文献   

17.
Sediment cores from Lakes Punta Laguna, Chichancanab, and Petén Itzá on the Yucatan Peninsula were used to (1) investigate “within-horizon” stable isotope variability (δ18O and δ13C) measured on multiple, single ostracod valves and gastropod shells, (2) determine the optimum number of individuals required to infer low-frequency climate changes, and (3) evaluate the potential for using intra-sample δ18O variability in ostracod and gastropod shells as a proxy measure for high-frequency climate variability. Calculated optimum sample numbers (“n”) for δ18O and δ13C in the ostracod Cytheridella ilosvayi and the gastropod Pyrgophorus coronatus vary appreciably throughout the cores in all three lakes. Variability and optimum “n” values were, in most cases, larger for C. ilosvayi than for P. coronatus for δ18O measurements, whereas there was no significant difference for δ13C measurements. This finding may be explained by differences in the ecology and life history of the two taxa as well as contrasting modes of calcification. Individual δ18O measurements on C. ilosvayi in sediments from Lake Punta Laguna show that samples from core depths that have high mean δ18O values, indicative of low effective moisture, display lower variability, whereas samples with low mean δ18O values, reflecting times of higher effective moisture, display higher variability. Relatively dry periods were thus consistently dry, whereas relatively wet periods had both wet and dry years. This interpretation of data from the cores applies to two important periods of the late Holocene, the Maya Terminal Classic period and the Little Ice Age. δ18O variability during the ancient Maya Terminal Classic Period (ca. 910–990 AD) indicates not only the driest mean conditions in the last 3,000 years, but consistently dry climate. Variability of δ13C measurements in single stratigraphic layers displayed no relationship with climate conditions inferred from δ18O measurements.  相似文献   

18.
Stable isotopes and trace elements in ostracod shells have been used widely in paleolimnological investigations of past lake hydrochemistry and climate because they provide insights into past water balance and solute evolution of lakes. Regional differences in lake characteristics and species-specific element fractionation, however, do not permit generalization of results from other regions or ostracod species to the southern Tibetan Plateau, in part because most common taxa from the southern Tibetan Plateau are endemic to the area. This study evaluated relations between present-day environmental conditions and the geochemical composition of modern ostracod shells from the southern Tibetan Plateau, to assess the suitability of using shell chemistry to infer hydrological conditions. We studied nine lakes and their catchments, located along a west–east transect in the south-central part of the Tibetan Plateau. Stable oxygen and carbon isotope values and trace element concentrations in recent shells from the four most abundant ostracod species (Leucocytherella sinensis, ?Leucocythere dorsotuberosa, Limnocythere inopinata, Tonnacypris gyirongensis) were measured, together with hydrochemical properties of host waters at the time of sampling. Results revealed significant between-species differences in stable isotope fractionation and trace element incorporation into shell calcite. Stable oxygen and carbon isotope values of ostracod shells were correlated significantly with the stable isotope composition of the respective water body \( \left( {\updelta^{18} {\text{O}}_{{{\text{H}}_{ 2} {\text{O}}}} \,{\text{and }}\updelta^{13} {\text{C}}_{{{\text{H}}_{ 2} {\text{O}}}} } \right) \), reflecting salinity and productivity, respectively. Offsets between δ18Oshell and δ13Cshell and inorganic calcite, the latter representing isotopic equilibrium, suggest shell formation of T. gyirongensis during spring melt. L. sinensis reproduces throughout the monsoon season until September and shows several consecutive generations, and L. inopinata molts to the adult stage after the monsoon season in August/September. The influence of pore water δ13C was displayed by L. inopinata, suggesting shell calcification within the sediment. Mg/Cashell is primarily influenced by water Mg/Ca ratios and salinity and confirms the use of this shell ratio as a proxy for precipitation-evaporation balance and lake level. In addition, Sr/Ca and Ba/Ca can be used to infer changes in salinity, at least in closed-basin lakes with calcite saturation. Observed effects of water Sr/Ca and salinity on Sr/Ca incorporation are biased by the presence of aragonite precipitation in the lakes, which removes bioavailable Sr from the host water, resulting in low Sr/Cashell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Cashell ratios were unaffected by water temperature. Positive correlations among Fe/Ca, Mn/Ca and U/Ca in ostracod shells, and their negative correlation with δ13C, which reflects organic matter decay, show the potential to infer changes in redox conditions that can be used to reconstruct past oxygen supply to bottom waters and thus past water-circulation changes within lakes. The intensity of microbial activity, associated with organic matter decomposition, can be inferred from U/Ca ratios in ostracod shells. These findings highlight the value of fossil ostracod records in lake deposits for inferring paleoenvironmental conditions on the southern Tibetan Plateau.  相似文献   

19.
The oxygen isotope ratios of diatoms (δ18Odiatom), and the oxygen and hydrogen isotope ratios of lake water (δW) of lakes in south Alaska provide insight into past changes in atmospheric circulation. Lake water was collected from 31 lakes along an elevation transect and diatoms were isolated from lake sediment from one lake (Mica Lake) in south Alaska. In general, δW values from coastal lakes overlap the global meteoric water line (GMWL). δW values from interior lakes do not lie on the GMWL; they fall on a local evaporation line trajectory suggesting source isotopes are depleted with respect to maritime lakes. Sediment cores were recovered from 58 m depth in Mica Lake (60.96° N, 148.15° W; 100 m asl), an evaporation-insensitive lake in the western Prince William Sound. Thirteen calibrated 14C ages on terrestrial macrofossil samples were used to construct an age-depth model for core MC-2, which spans 9910 cal years. Diatoms from 46, 0.5-cm-thick samples were isolated and analyzed for their oxygen isotope ratios. The analyses employed a newly designed, stepwise fluorination technique, which uses a CO2 laser-ablation system, coupled to a mass spectrometer, and has an external reproducibility of ±0.2‰. δ18Odiatom values from Mica Lake sediment range between 25.2 and 29.8‰. δ18Odiatom values are relatively uniform between 9.6 and 2.6 ka, but exhibit a four-fold increase in variability since 2.6 ka. High-resolution sampling and analyses of the top 100 cm of our lake cores suggest large climate variability during the last 2000 years. The 20th century shows a +4.0‰ increase of δ18Odiatom values. Shifts of δ18Odiatom values are likely not related to changes in diatom taxa or dissolution effects. Late Holocene excursions to lower δ18Odiatom values suggest a reduction of south-to-north storm trajectories delivered by meridional flow, which likely corresponds to prolonged intervals when the Aleutian Low pressure system weakened. Comparisons with isotope records of precipitation (δP) from the region support the storm-track hypothesis, and add to evidence for variability in North Pacific atmospheric circulation during the Holocene.
Zachary SharpEmail:
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20.
The Muteh gold district with nine gold deposits is located in the Sanandaj-Sirjan metamorphic zone. Gold mineralization occurs in a pre-Permian complex which mainly consists of green schists, meta-volcanics, and gneiss rocks. Shear zones are the host of gold mineralization. Gold paragenesis minerals include pyrite, chalcopyrite, pyrrhotite, and secondary minerals. Pyrites occur as pre-, syn-, and post-metamorphism minerals. To determine the source of the ore-bearing fluids, fifty samples were selected for petrographical and stable isotope studies. The mean values of 12.4‰, and −42‰ for δ18O and δD isotopes, respectively, and a mean value of 7.75‰ of calculated fractionation factors for δ18O H2O, from quartz veins indicate that metamorphic host rocks are the most important source for the fluids and gold mineralization. Three generations of pyrite can be distinguished showing a wide range of δ34S. Gold mineralization is closely associated with intense hydrothermal alteration along the ductile shear zones. The characteristics of the gold mineralization in the study area are similar to those of orogenic gold deposits elsewhere. An erratum to this article can be found at  相似文献   

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