Composition Of Lake Water Research Articles

The influence of authigenic clay formation on the mineralogy and stable isotopic record of lacustrine carbonates

The mineralogical, compositional and stable isotopic variability of lacustrine carbonates are frequently used as proxies for ancient paleoenvironmental change in continental settings, under the assumption that precipitated carbonates reflect conditions and chemistry of ancient lake waters. In some saline and alkaline lake systems, however, authigenic clay minerals, forming at or near the sediment water interface, are a major sedimentary component. Often these clays are rich in Mg, influencing the geochemical budget of lake waters, and are therefore expected to influence the properties of contemporaneous authigenic carbonate precipitates (which may also contain Mg). This paper documents evidence for a systematic feedback between clay mineral and carbonate authigenesis through multiple precessionally driven, m-scale sedimentary cycles in lacustrine oil-shale deposits of the Eocene Green River Formation from the Uinta Basin (NE Utah). In the studied section, authigenic, Mg-rich, trioctahedral smectite content varies cyclically between 9 and 39wt.%. The highest concentrations occur in oil-shales and calcareous mudstones deposited during high lake level intervals that favored sedimentary condensation, lengthening the time available for clay diagenesis and reducing dilution by other siliciclastic phases. An inverse relation between dolomite percentage of carbonate and trioctahedral smectite abundance suggests the Mg uptake during clay authigenesis provides a first order control on carbonate mineralogy that better explains carbonate mineralogical trends than the possible alternative controls of (1) variable Mg/Ca ratios in lake water and (2) degree of microbial activity in sediments. We also observe that cyclical change in carbonate mineralogy, believed to be induced by clay authigenesis, also causes isotopic covariation between δ13CPDB and δ18OPDB of bulk sediments because of differences in the equilibrium fractionation factors of dolomite and calcite (∼2‰ and ∼2.6%, respectively). This provides an alternative mechanism for the common pattern of isotopic covariation, which is typically attributed to the effect of simultaneous changes in water balance and biological activity on the carbon and oxygen isotopic composition of lake waters. These findings may help improve paleoenvironmental reconstructions based on lacustrine carbonate records by adding to the factors known to influence the mineralogical, compositional and stable isotopic signals recorded by lacustrine carbonates.

Variation in the moisture regime of northeastern interior Alaska and possible linkages to the Aleutian Low: inferences from a late-Holocene δ18O record

Analyses of oxygen and carbon isotopes in bulk carbonate and Chara-stem encrustations, X-ray diffraction, and sediment composition from Keche Lake offer new information on climatic change over the past ~3,400 years in northeastern interior Alaska. The δ18O and δD values of water samples from the lake and its inlet streams suggest that evaporation plays an important role in determining the isotopic composition of Keche Lake water at present. However, evaporative enrichment does not appear to be a major driver of the pronounced fluctuations in the bulk-carbonate δ18O record on the basis of comparison with Chara-δ18O values. The δ18O values of bulk carbonate in the Keche Lake sediments vary by up to 10 ‰ over the past 3,400 years, with maximum values of −12 ‰ around 3,400 cal BP and between 2,100 and 1,500 cal BP. High δ18O peaks are associated with sediments dominated by quartz, feldspar, and clay minerals suggesting the influence of detrital carbonate. Multi-millennial patterns of δ18O variation at Keche Lake appear to be linked with changes in watershed and sediment-depositional processes, which may be driven by varying moisture abundance associated with the position of the Aleutian Low (AL). The increasing trend of carbonate δ18O from 3,400 to ~2,100 cal BP probably reflects the increasing importance of a westerly AL, and the high frequency of δ18O spikes ~2,100–1,500 cal BP may have resulted from the prevalence of a westerly AL position. Predominance of a westerly AL likely increased snowfall and winter temperature in the region. Such conditions would have promoted soil erosion and thermokarst activity during spring snowmelt, resulting in episodic large influxes of detrital carbonate to Keche Lake and elevating bulk-carbonate δ18O. Over the past 1,500 years, bulk-carbonate δ18O remained relatively high at Keche Lake but variation was much less pronounced than before. A broad δ18O peak centered ~400 cal BP may be related to enhanced winter moisture during the Little Ice Age, although our chronology is inadequate for a rigorous assessment of this interpretation. This study contributes a new δ18O record and offers additional information on past moisture-regime shifts associated with changing atmospheric-circulation patterns.

Chemical composition of modern and fossil Hippopotamid teeth and implications for paleoenvironmental reconstructions and enamel formation – Part 1: Major and minor element variation

Abstract. Bioapatite in mammalian teeth is readily preserved in continental sediments and represents a very important archive for reconstructions of environment and climate evolution. This project provides a comprehensive data base of major, minor and trace element and isotope tracers for tooth apatite using a variety of microanalytical techniques. The aim is to identify specific sedimentary environments and to improve our understanding on the interaction between internal metabolic processes during tooth formation and external nutritional control and secondary alteration effects. Here, we use the electron microprobe to determine the major and minor element contents of fossil and modern molar enamel, cement and dentin from Hippopotamids. Most of the studied specimens are from different ecosystems in Eastern Africa, representing modern and fossil lacustrine (Lake Kikorongo, Lake Albert, and Lake Malawi) and modern fluvial environments of the Nile River system. Secondary alteration effects - in particular FeO, MnO, SO3 and F concentrations – are 2 to 10 times higher in fossil than in modern enamel; the secondary enrichment of these components in fossil dentin and cement is even higher. In modern and fossil enamel, along sections perpendicular to the enamel-dentin junction (EDJ) or along cervix-apex profiles, P2O5 and CaO contents and the CaO/P2O5 ratios are very constant (StdDev ∼1%). Linear regression analysis reveals tight control of the MgO (R2∼0.6), Na2O and Cl variation (for both R2>0.84) along EDJ-outer enamel rim profiles, despite large concentration variations (40% to 300%) across the enamel. These minor elements show well defined distribution patterns in enamel, similar in all specimens regardless of their age and origin, as the concentration of MgO and Na2O decrease from the enamel-dentin junction (EDJ) towards the outer rim, whereas Cl displays the opposite trend. Fossil enamel from Hippopotamids which lived in the saline Lake Kikorongo have a much higher MgO/Na2O ratio (∼1.11) than those from the Neogene fossils of Lake Albert (MgO/Na2O∼0.4), which was a large fresh water lake like those in the western Branch of the East African Rift System today. Similarly, the MgO/Na2O ratio in modern enamel from the White Nile River (∼0.36), which has a Precambrian catchment of dominantly granites and gneisses and passes through several saline zones, is higher than that from the Blue Nile River, whose catchment is the Neogene volcanic Ethiopian Highland (MgO/Na2O∼0.22). Thus, particularly MgO/Na2O might be a sensitive fingerprint for environments where river and lake water have suffered strong evaporation. Enamel formation in mammals takes place at successive mineralization fronts within a confined chamber where ion and molecule transport is controlled by the surrounding enamel organ. During the secretion and maturation phases the epithelium generates different fluid composition, which in principle, should determine the final composition of enamel apatite. This is supported by co-linear relationships between MgO, Cl and Na2O which can be interpreted as binary mixing lines. However, if maturation starts after secretion is completed, the observed element distribution can only be explained by equilibration of existing and addition of new apatite during maturation. It appears the initial enamel crystallites precipitating during secretion and the newly formed bioapatite crystals during maturation equilibrate with a continuously evolving fluid. During crystallization of bioapatite the enamel fluid becomes continuously depleted in MgO and Na2O, but enriched in Cl which results in the formation of MgO, and Na2O-rich, but Cl-poor bioapatite near the EDJ and MgO- and Na2O-poor, but Cl-rich bioapatite at the outer enamel rim. The linkage between lake and river water compositions, bioavailability of elements for plants, animal nutrition and tooth formation is complex and multifaceted. The quality and limits of the MgO/Na2O and other proxies have to be established with systematic investigations relating chemical distribution patterns to sedimentary environment and to growth structures developing as secretion and maturation proceed during tooth formation.

The stable oxygen isotopic composition of ostracoda <i>Limnocythere inopinata</i> Bird and its possible response to water salinity in Lake Qinghai

介形类壳体的氧同位素组成已成为恢复湖区古气候/古环境的有效代用指标,而其氧同位素分馏及环境意义目前还缺乏研究.为评价青海湖介形类壳体氧同位素分馏及其环境意义,在青海湖进行系统的表层沉积物和水样的采集,测定表层沉积物中的意外湖花介(Limnocythere inopinata Bird)和相应水样的氧同位素组成.结果表明:意外湖花介壳体的氧同位素组成主要受控于水体的氧同位素组成,除温度的影响外,还可能受到盐度的影响,随着盐度变化,该种壳体与水体之间的同位素分馏呈减小趋势,而两者的氧同位素组成均呈偏正趋势.青海湖意外湖花介壳体的氧同位素组成在一定程度上可能间接反映水体盐度的变化,值得进行更深入的研究.;The oxygen isotopic composition of ostracod shells has been used as a useful indicator in plaeolimnological research. But studies on the oxygen isotopic fractionation of ostracod shells and its environmental significance were still not enough. In this study, surface sediments and co-existing water were systematically collected at different sites in Lake Qinghai. The oxygen isotopic composition of lake water and ostracod shells of Limnocythere inopinata Bird selected from the surface sediments were measured to test the oxygen isotopic fractionation of ostracod shells and evaluate its environmental significance in Lake Qinghai. Our results show that the oxygen isotopic composition of L. inopinata is mainly controlled by the water isotope and the oxygen isotopic fractionation between ostracod shells and water is not only affected by temperature but also maybe by water salinity. The oxygen isotopic fractionation factor of ostracod shells reduces with increasing water salinity, eliminating the effect of temperatures. However, the oxygen isotopic compositions of both ostracod shells and water become enriched with increasing salinity in Lake Qinghai. The oxygen isotopic composition of L. inopinata may be used to indicate the variability of salinity in Lake Qinghai, and further works need to be done in future.

Stable oxygen isotopes in chironomid and cladoceran remains as indicators for lake‐water δ18O

An understanding of modern relationships between the stable oxygen isotope composition (δ18O) of lake water and aquatic invertebrates is essential for the interpretation of paleoclimate records based on δ18O of organic remains of these organisms. We analyzed δ18O of lake water and invertebrate remains, including head capsules of chironomid larvae and resting eggs (ephippia) of planktonic Cladocera, in surface sediments from 31 large, deep, and stratified lakes along a latitudinal transect through Europe. The δ18O values measured for both lake water and aquatic invertebrate remains were compared to estimated δ18O in precipitation. A strong linear relationship between mean annual air temperature and δ18O of precipitation was observed along the north–south transect (r = 0.97), whereas the relationship between precipitation δ18O and lake‐water δ18O was weaker (r = 0.80). A strong positive correlation was observed between δ18O in lake water and aquatic invertebrates (r = 0.95 and 0.94 for chironomids and cladocerans, respectively). Although slopes of linear regressions between lake‐water δ18O and δ18O of both aquatic invertebrate groups are similar, a systematic offset between the absolute δ18O values of chironomids and cladocerans was observed; chironomids were on average 2.4‰ heavier than Cladocera. We attribute this offset to differences in ecology, metabolism, and/or behavior between benthic chironomid larvae and planktonic Cladocera. δ18O records based on subfossil chironomid and cladoceran remains have the potential to quantitatively characterize past lake‐water δ18O and, indirectly, past climatic changes.

An isotope mass balance model for the correlation of freshwater bivalve shell (Unio pictorum) carbonate δ18O to climatic conditions and water δ18O in Lake Balaton (Hungary)

The oxygen isotope composition of bivalve shells (δ18Oshell) can potentially record environmental variability of shallow lakes and therefore it has been extensively used as a proxy in the reconstruction of past climate and environmental conditions. As δ18Oshell reflects - besides the water temperature - the oxygen isotope composition of lake water (δ18OL), it is required to interpret the quality and impact of parameters influencing the δ18OL. Using the isotope mass balance model, I tested the hypothesis that Balaton lake water δ18O variability can be described as a result of the combined effects of three main climatic parameters such as river runoff, precipitation and evaporation. I calculated δ18OL time series for the period 1999-2008 for the whole water body at Siófok (eastern part of Lake Balaton, Hungary) based on measured precipitation, inflow and evaporation amount and measured inflow, precipitation δ18O and calculated vapour δ18O data. The comparison of the modelled δ18OL time series to measured surface δ18OL data revealed that δ18O of Balaton water is sensitive for variation of climatic parameters. This variability is most striking at the surface, while according to the results of the model, the whole water body itself is less sensitive. Monthly differences suggest that generally during summer the whole water body is mixed up, while moderate isotope stratification (0.3-0.7‰ difference between surface and whole water body) can be assumed in early spring and autumn. Predictions of shell δ18O values were made using the measured surface water δ18O data and the modelled δ18O values for the whole water body. High-resolution sampling was conducted on two Unio pictorum shells covering the period of 2001-2008, and both predictions were compared to measured shell δ18O records. The results showed that the prediction for the whole water body gives a better fit to the measured shell δ18O, suggesting that the whole water body better describes the isotope variability of shell carbonate. As a practical application of the isotope mass balance model was used to present the effect of precipitation and evaporation on δ18Oshell. The relationship between intra-shell δ18O-variability and precipitation amount (precipitation/evaporation ratio) were determined, which allows the quantified prediction the impact of meteorological parameters affecting the oxygen isotope composition of shell carbonate.

Physicochemical simulation of the evolution of small lakes in a cold climate

The paper presents a generalized algorithm for the simulation of multiyear cycles in variations of the chemical composition of lake waters with regard for the seasonal specifics of hydrogeochemical processes. Data were obtained on the behavior of the hydrogeological system during a time span of 500–1000 years. Each of the simulated model cycles involved a successively alternating “summer-winter” time periods. Terrestrial exchange fluxes between reservoirs, groundwater inflow, falls of atmospheric precipitate, and the evaporation of lake water were taken into account for summer periods, whereas winter conditions were simulated as corresponding to the development of the ice phase, the absence of water exchange fluxes, a change from oxidizing to reducing conditions, and the burial of solid phases in the sediments. The results of our physicochemical simulations with the use of data on the composition of natural hydrogeological systems are in good agreement with natural observations and make it possible to realistically predict the evolution of small lakes in the Ol’khon area.

Holocene monsoon climate documented by oxygen and carbon isotopes from lake sediments and peat bogs in China: a review and synthesis

There has been much recent debate about Holocene climate variation in the monsoon region of China, especially the temporal pattern of variations in precipitation, the time-transgressive nature of the Holocene precipitation maximum, and the extent to which variations in regions influenced by the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM) have been synchronous. We summarize and compare carbonate oxygen-isotope records (δ 18O carb) from ten lakes within the present-day ISM region. We discuss their paleoclimate significance considering the present-day moisture source, isotopic composition of precipitation and the hydrological setting. The δ 18O carb records are controlled mainly by the isotopic composition of lake water, which in turn is a function of regional Precipitation/Evaporation (P/E) balance and the proportion of precipitation that is monsoon-derived. We normalized the δ 18O carb data and used these records to generate an integrated moisture index. This index, along with oxygen-isotope records from speleothems and carbon-isotope records (δ 13C org) from peats within the monsoon region, suggests that Holocene climate was broadly synchronous across the monsoon region and, within the limits of accuracy of the existing age models, provides no strong evidence for previously-proposed anti-phasing of the ISM and the EASM. Stable-isotope records from lake sediments and peat bogs have excellent potential for providing high-quality paleoclimate data for monsoon Asia, and complement high-resolution speleothem sequences, which are only found in certain localities.

Reconstruction of solution chemistry evolution based on the sedimentary record of salt lakes in the Olkhon region

Using data on the chemical composition of lake waters in the Olkhon region, we calculated the under- and oversaturation of modern chloride solutions of Lake Tsagan-Tyrm. These data are a basis for the determination of the composition of suspension and bottom sediments. The thermodynamically calculated solubility of carbonates (inverse problem) discovered in the evaporite lacustrine sediments gave an insight into the main regularities of the chemical evolution of ancient brines and the formation conditions of various magnesian calcites. The calculations confirmed that an increase in solution mineralization leads to the deposition of high-Mg calcites, which is also observed in the bottom sediments of other lakes under climate aridization and temperature decrease.

Calculating density of water in geochemical lake stratification models

To model chemically stratified lakes numerically, chemical transformations must be reflected in the density function. In this contribution, partial molal volumes are used to calculate density from the chemical composition of lake water. Such values have been evaluated for cations and anions separately, to facilitate an easy implementation into geochemical stratification models for lakes. Coefficients for temperature dependence and variation for higher ionic strengths were evaluated from previously published data. An algorithm RHOMV to calculate density with a second order approximation for temperature dependence and ionic strength dependence is proposed. The accuracy is tested for seawater composition. We conclude that this approach delivers a representation of density based on the actual chemical composition of the lake water, which is accurate enough for most limnological purposes. The implementation of RHOMV into geochemical stratification models facilitates the numerical tackling of pressing questions, such as meromixis or double diffusive features or altered circulation patterns of lakes due to changing climate or change of use.

Comparisons between the chemical compositions of lake water, inflowing river water, and lake sediment in Nam Co, central Tibetan Plateau, China and their controlling mechanisms

The chemical composition of lake water and inflowing river water was investigated in Nam Co (lake), Tibetan Plateau, in September 2005. Lake water samples (n=76) were collected at different depths along a south–north transect of the eastern part of the lake while water samples were collected from 69 rivers flowing into the lake; a sediment core was collected at 64m midway along the water survey transect to investigate salt precipitation. Na+ and Ca2+ were the dominant cations, accounting for 76.2% and 60.6% of the lake and river water cations, respectively while HCO3− was the dominant anion accounting for 70.8% and 93.4% of lake and river anions, respectively. CaCO3 precipitation from the water column decreased Ca2+ in the lake water, with the relative proportion of other ions increasing significantly. Evaporation–crystallization processes largely control Nam Co lake-water chemistry, while rock weathering is the dominant processes influencing the chemistry of river water; carbonate and silicate weathering are the major sources of ions in these rivers.

Lithium isotopic composition of the McMurdo Dry Valleys aquatic systems

The McMurdo Dry Valleys are the largest ice-free area in Antarctica and include several terminal lake basins. These include Lakes Hoare, Fryxell, Bonney, and Vanda. The hydrology of the lakes is controlled by the inputs of glacial meltwater and loss by sublimation of ice covers. Despite the extremely dilute inflow waters, the lakes range in salinity from hypersaline to fresh due in large part to their landscape position and climatic history. Although the aqueous geochemistry of the region has been studied extensively over the past four decades, the ultimate source of solutes to the lakes is still in question. Atmospheric input/precipitation, dissolution of salts in sediments, chemical weathering, remnant seawater, and groundwater inputs have been suggested as the most likely sources of solutes to dry valley waters. We have examined the lithium isotopic composition of glacial snow, stream water, and lake water to better understand the role of these sources in determining the overall chemistry of the lakes. The data suggest that the dominant sources of lithium to the lakes are the dissolution of aeolian dust of crustal origin on the glacial surface, marine aerosols, dissolution of salts and chemical weathering in the stream channels, and ancient seawater. The streams in the valleys show mixing of seawater (and/or marine aerosol) with an isotopically lighter silicate dust end-member as seen in glacial snow. Our data reveal the following sources of lithium to the lakes: streamwater in Lake Hoare; seawater in the monimolimnia (i.e. the dense bottom water in a permanently stratified lake) of Lakes Bonney and Fryxell, a diffusional mixture of Onyx River water and a deep-water brine; hypersaline groundwater discharge in Don Juan Pond. This work confirms, yet provides a more conclusive link than, previous findings between various salt sources and the role they play in solute accumulation in this extreme environment.

Reconstructing past atmospheric circulation changes using oxygen isotopes in lake sediments from Sweden

Abstract. Here we use lake sediment studies from Sweden to illustrate how Holocene-aged oxygen isotope records from lakes located in different hydrological settings, can provide information about climate change. In particular changes in precipitation, atmospheric circulation and water balance. We highlight the importance of understanding the present lake hydrology, and the relationship between climate variables and the oxygen isotopic composition of precipitation ( δ18Op) and lake waters (δ18Olakewater) for interpretation of the oxygen isotopic record from the sediments (δ18O). Both precipitation reconstructions from Northern Sweden and water balance reconstructions from South and Central Sweden show that the atmospheric circulation changed from zonal to a more meridional airflow over the Holocene. Superimposed on this Holocene trend are δ18Op minima resembling intervals of the negative phase of the North Atlantic Oscillation (NAO), thus suggesting that the climate of Northern Europe is strongly influenced by atmospheric and oceanic circulation changes over the North Atlantic.

Stable isotope fingerprint of open-water evaporation losses and effective drainage area fluctuations in a subarctic shield watershed

Stable isotopes of water, oxygen-18 and deuterium, were measured at biweekly to monthly intervals during the open-water season in a small, headwater lake (Pocket Lake, 4.8 ha) near Yellowknife Northwest Territories, and concurrently in a nearby string-of-lakes watershed (Baker Creek, 137 km2) situated in the subarctic Precambrian Shield region. As measured in water samples collected over a 12 year period (1997–2008), the levels of evaporative isotopic enrichment in both lake and watershed outflow were differentially offset, and seasonal variations were found in both to be driven by variations in open-water evaporation. Systematic differences measured in the magnitude of the offset between the lake and watershed outflow are interpreted as being caused by changes in the effective drainage area contributing to runoff. Based on the observed and extremely consistent relationship between isotopic compositions of lake water and watershed outflow (r2 = 0.849, p < 0.001) we extend the analysis of open-water evaporation losses and effective drainage areas back to 1991 when less-frequent water sampling at the sites commenced. This 18-year record serves to demonstrate for the first time the expected variability in the evaporation and transpiration partitioning, upper limits on the effective drainage area, and isotopic signals transferred downstream in a typical shield drainage system within the Mackenzie Basin.

Uranium isotopes as proxies of the environmental history of the Lake Baikal watershed (East Siberia) during the past 150 ka

A new experimental method for the determination of the isotopic composition of authigenic uranium in Lake Baikal (Siberia) sediments has supplied a record of ( 234U/ 238U) aut ratios during the past 150 ka with a resolution of ca. 200 years. Reconstruction of the ( 234U/ 238U) aut activity ratio as a proxy for lake water composition shows that the uranium isotope disequilibrium has not stayed constant throughout this period: it varied greatly in response to regional climate. The 234U/ 238U activity ratio in Lake Baikal paleowaters was inferred to be high during interglacial time (∼ 1.85–2 during MIS 5.5 and the Holocene; ∼ 1.4–1.7 during MIS 5.1 and 5.3) and dropped to ∼ 1.16 ± 0.01 during glacial time. The variability in the deduced 234U/ 238U ratios of Lake Baikal waters of the past may have been caused by the climatically-modulated redistribution of contributions of riverine discharges from different provinces of the very large catchment of the lake. Aridity during ice ages was more pronounced in the south of eastern Siberia and in northern Mongolia relative to that in the northern regions of the Lake Baikal watershed. The southern contribution of riverine input into the lake was essentially absent during glacial periods. Records of some elements (Ca/K, Sr/Rb, Si/Al, La/Th, U/Th, CIW) from the bottom sediments of Lake Baikal during the last 0.5 Myr with a resolution of ∼ 2 kyr have also been obtained. Coupling these data with records of weathering indices shows that not only riverine waters but also terrigenous material delivered into Lake Baikal from different regions of the very large watershed during interglacial and glacial times. Glacial erosion delivered fine siliciclastic material (“glacial milk”) mainly from the nearest mountain glaciers situated in the northwestern region of Lake Baikal while during warmer periods terrigenous material was sourced from the surrounding river valleys. A conceptual model for the simulation of the fluvial discharge gradient between the northern and southern provinces of the Lake Baikal watershed is proposed and a first-order simulation of past changes in the contribution of the Selenga River to the lake water budget is given.

Water ecosystems affected by human impact within the protected area of the Tatra National Park (Poland)

Water ecosystems affected by human impact within the protected area of the Tatra National Park (Poland) The subjects of the study were two high-mountain lakes, Morskie Oko and Czarny Staw pod Rysami, in the Tatra range of the Carpathian Mountains located within the protected area of the Tatra National Park. This paper presents results of the analytical study of the lake water composition and compares this with relevant data from earlier years. The impact of the following factors was distinguished and discussed: atmospheric deposition (dry and wet), migration of pollutants with runoff water supplying both reservoirs, tourism (hiking, climbing), and the influence of mountain hostels. The resistance and sensitivity of the lakes to human impact is described in terms of water quality changes and characteristics of the catchments and the region.

Oxygen isotope fractionation in three freshwater ostracod species from early Holocene lacustrine tufa in northern Estonia

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.

Hydrogen and oxygen isotopic compositions of lake water in the western United States

Lake-water isotopes can be used to track moisture regimes and water sources at present and in the geologic record. However, the effects of seasonal drought and the seasonal distribution of precipitation on lake-water isotopes are not well documented. To improve our understanding of lake-water isotopes, we analyzed the δD and δ 18 O values of water from a hundred lakes in the western United States across a broad range of seasonal precipitation regimes. Our results show that the isotopic composition of lake-water inputs is correlated with the isotopic composition of annual precipitation. In areas associated with the summer monsoon in northern New Mexico and southern Colorado, lake-water inputs are skewed toward summer precipitation. These results contrast with published western U.S. river-water isotopic data, which are biased toward winter precipitation, and the paradigm that lakes represent the annual moisture surplus. From the lake-water input compositions, on plots of oxygen versus hydrogen isotopes, evaporative enrichment of individual lakes follows regionally coherent evaporation trends (0.96 2

Water and sediment chemistry of Lake Pumayum Co, South Tibet, China: implications for interpreting sediment carbonate

A combination of water and sediment chemistry was used to investigate carbonate produc- tion and preservation in Lake Pumayum Co (altitude 5,030 m a.s.l.), south Tibet, China. We compared the chemical composition of lake water in various parts of the lake with that of input rivers and found that the loss of Ca 2? results from calcite sedimentation induced by evaporation and biogenic precipitation. This is sup- ported by evaporation data from the catchment and d 18 O measurements on water. Results suggest that CaCO3 is the predominant carbonate in this lake. There is a positive correlation in the sediments among concentrations of total inorganic carbon (TIC), Ca, total organic carbon (TOC), and total nitrogen, con- firming that most carbonates in sediment are endogenic. The Jiaqu River is the largest inflow to Lake Pumayum Co and has a strong influence on both lake water chemistry and sediment composition. The river and lake bathymetry influence carbonate sedi- mentation by affecting water flow velocity and grow- ing conditions for macrophytes. Different carbon contents and relationships between TIC and TOC in the two long cores from different depths in the lake reveal that hypolimnetic conditions also influence carbonate precipitation and preservation.

Oxygen isotope fractionation in the ostracod Eucypris mareotica: results from a culture experiment and implications for paleoclimate reconstruction

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: \( \begin{array}{*{20}c} { 10^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = 1. 1 7+ 0. 5 7\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 5^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 0. 4 8+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 9^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 1. 6+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ \end{array} \)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.