湖泊化学碳酸盐与生物碳酸盐氧同位素组成差异的环境意义

湖泊生物碳酸盐形成于湖水底层,化学碳酸盐形成于湖水表层;处于湖积物同一层位的两种成因类型的碳酸盐氧同位素组成(δ18O)差异可能反映了二者形成时的环境差异,如湖泊不同深度的水温差异、碳酸盐质生物如介形类小生境水体δ18O值、两种成因碳酸盐形成的时间先后、湖泊表层水体蒸发情况等。在进行碳酸盐δ18O环境意义分析时,应分开测试生物碳酸盐和化学碳酸盐δ18O。     

青海湖介壳与无机碳酸盐氧同位素组成差异的环境意义

青海湖Q14B沉积物柱芯560~415 cm(约14.0~10.5 ka B.P.)段,介壳1δ8O高于无机碳酸盐1δ8O可能反映了此时青海湖表层水温高于底层水温;介壳1δ8O与无机碳酸盐1δ8O之间的较小差值可能揭示了此时青海湖水位很浅,气候干冷;介壳1δ8O变幅大于无机碳酸盐1δ8O变幅,则可能源于此时青海湖水位大幅度波动导致的底层水温变幅超过表层,以及无机碳酸盐1δ8O测自全碳酸盐所致。在利用湖泊碳酸盐1δ8O进行气候及环境变化研究时,有必要分别测试不同种属介形虫及不同无机碳酸盐矿物的同位素值。     

黄旗海介形类及其壳体稳定同位素环境记录

利用黄旗海冬季坚固的冰封面作为稳固的司钻平台 ,在湖泊中央成功地获取了高质量的湖底连续沉积岩芯。对其中长 1 1m的HQH4岩芯中的介形类化石进行了属种古生态和壳体稳定同位素研究。结果显示 ,在 1 0 2 0 0aB .P .前后黄旗海进入了稳定的湖泊阶段。在此之前 ,介形类的丰度极低。介形类的最大丰度出现在 1 0 2 0 0 -680 0aB .P .,反映了早全新世期间黄旗海较高的生物生产率。在此期间 ,介壳的δ18O值的变化范围较大 ,反映了黄旗海水体氧同位素组成在全新世早期的变幅较大。从 680 0aB .P .起 ,介形类的丰度突然大幅度降低 ,介壳δ18O值随后降低 ,反映了中全新世黄旗海湖水变深 ,湖底还原性显著增强。从 3 2 0 0aB .P .起 ,湖泊水位显著下降 ,湖底氧化条件明显增强 ,介形类的丰度继续下降 ,直到大约 1 3 0 0aB .P .才出现回升     

湖泊自生碳酸盐氧同位素环境记录研究：（二）问题

对前人提出的湖泊自生碳酸盐氧同位素组成（δ18Oc）多种环境解释模型作了评述,指出了各种模型存在的局限性。提出在利用δ18Oc变化进行气候和环境变化研究时,需注意气温对湖泊δ18Oc的影响机制,湖水不同深度层的水温差异对化学沉积和介壳碳酸盐δ18Oc的影响δ18Oc和湖泊水位、降水δ18Oc、湖水盐度的关系,碎屑和自生碳酸盐矿物的分离,湖泊不同碳酸盐矿物间δ18Oc差值的确定,单矿物同位素组成的测试,不同前处理方法对δ18Oc测定的影响,以及碳酸盐沉积时的不平衡效应等问题。     

湖泊碳酸盐氧同位素记录的古温度定量研究

同位素地质温度计的概念一经提出,便被应用于海洋古水温恢复。该方法被部分学者运用到湖泊领域,利用湖泊碳酸盐氧同位素记录,对古湖水温做定量研究,进而定量反演古气温。在利用湖泊碳酸盐氧同位素记录进行古温度定量研究时,需要考虑如下因素:气温对湖泊碳酸盐氧同位素值变化的影响机制;介形虫Sr/Ca与古盐度的关系;湖泊沉积碳酸盐成因及种类鉴定等等,做了相应评述。     

湖泊碳酸盐记录的古水温定量研究进展

湖泊古水温乃至古气温的定量反演,可以促进过去全球变化研究由定性走向定量。目前利用湖泊碳酸盐对湖泊古水温进行重建主要有三种方式,一是运用同位素地质温度计原理,利用水温与湖泊碳酸盐氧同位素值和湖水氧同位素值三者之间的函数关系,对湖泊古水温做定量研究;二是通过测定介壳[Mg2+]/[Ca2+]进而重建古水温;三是通过测定湖泊流域范围内现代水文气象参数及一些相关的同位素资料直接建立起湖泊自生碳酸盐氧同位素值与温度间的统计模型。各种方法有不同的适用条件和适用范围及其一定的局限性,对此做了归纳和评述。     

湖相介形虫古生态学在环境变化研究中的应用

湖泊的水文状况和水化学条件 ,特别是盐度、离子组成、温度及深度等不仅对介形虫种属的分类、组合、丰度及分异度 ,而且对介形虫壳体的大小、形态、结构、壳饰及厚度也起着重要的控制作用。湖相介形虫的古生态学 ,可以提供重要的环境变化信息 ,近年来在环境变化研究中得到越来越广泛的应用。为了满足高分辨率环境变化 ,特别是定量研究的要求 ,必须了解介形虫的种属分类知识 ,掌握盐度和离子组成对介形虫种属变化的影响 ;必须了解介形虫种属的生命历史和生态消长的过程 ,积累介形虫种属的生态资料 ;通过定期收集野外介壳和水样分别进行种属鉴定和化学分析 ,了解湖水的盐度和温度对介壳生态特征的影响。     

青藏高原兹格塘错沉积物介形虫壳体同位素

兹格塘错是位于藏北高原腹地的封闭型湖泊,其变化过程可直接反映区域气候变化。对一支深727 cm沉积岩芯中介形虫进行了壳体同位素分析,发现介形虫壳体氧同位素变化主要取决于壳体形成时的湖水氧同位素组成和湖水温度的变化;封闭湖泊湖水的氧同位素组成主要取决于降水和蒸发比值的变化。兹格塘错介形虫壳体碳、氧同位素变化特征表明两者可能受控于不同的环境因子。介形虫壳体同位素记录反映了全新世气候环境变化的整体特征,同时对晚第四纪特征气候事件都有明确记录,包括8.2 ka冷事件、Roman暖期、中世纪暖期以及小冰期等,在响应全球气候变化的过程中又记录了区域间的差异响应。     

湖泊无机碳酸盐矿物氧同位素组成差值及其单矿物同位素组成的确定

湖积物中不同无机碳酸盐矿物常常混杂在一起,其氧同位素组成(δ18O)差异会影响碳酸盐δ18O环境信息提取的可靠性。不同矿物之间δ18O差值明显且幅度不一。20~25℃时生成的白云石比共生的方解石富集18O可能为0‰~9‰不等,亦或方解石比白云石可能更富集18O达0‰~12.3‰。常温状态,相同条件下形成的文石δ18O值较方解石高出0‰~1‰,或者方解石较文石δ18O值高出0‰~4.47‰。镁方解石中MgCO3的mol百分含量每增加1%,其δ18O值相对于纯方解石δ18O值增加0.06‰~0.17‰。在利用碳酸盐δ18O进行气候及环境研究时,不能根据某种差值进行校正,而应进行单矿物测试。由此,对不同无机碳酸盐矿物的分离及同位素测试、推算方法进行了归纳和述评。     

高原湖泊中萝卜螺属壳体研究现状及展望

湖泊的古水文及古水化学重建是湖泊研究领域最具挑战性的工作之一。在湖泊、河湖相及高湖面沉积物中,萝卜螺属壳体化石广泛分布;而且现生萝卜螺属亦广泛地分布于全球的湖泊及河流。这些生物碳酸盐(文石)壳体成为一种潜在的和高分辨率的环境信息记录载体。近年来,萝卜螺属的生境及其壳体的稳定碳氧同位素和元素已经逐步被用于了解青藏高原及其他地区的古水文、古水化学和古气候的信息。然而,在萝卜螺属壳体如何记录其宿生水体的古水文、古水化学等信息,以及如何基于萝卜螺属壳体化石重建古环境等方面,仍有许多科学问题有待探索。本文在前人研究的基础上,侧重在作为环境信息载体的萝卜螺属的分类体系、生境研究及其应用和壳体指标(δ¹³C_shell,δ¹⁸O_shell,⁸⁷Sr/⁸⁶Sr,Sr/Ca和Mg/Ca)特征及其在环境重建中的应用等方面进行总结和展望。     

Stable isotope compositions of recent Dreissena polymorpha (Pallas) shells: paleoenvironmental implications

Stable carbon and oxygen isotope studies are among the major proxies in investigations of recent and ancient freshwater environments. Mollusc shells are one the most frequently studied carbonates. In the present paper, stable carbon and oxygen isotope compositions of Dreissena polymorpha (Pallas) shells, regarded as the most aggressive freshwater invader worldwide, is compared with the stable isotope composition of ambient water. Macrophytes with modern D. polymorpha shells attached were sampled twice, in June and August 2011, from four transects established within the littoral of Lake Lednica (western Poland). The macrophytes were sampled between 0.5 and 7 m of depth at each site from the restricted area of the lake bottom. In order to avoid the influence of ontogeny on the results obtained the stable isotope compositions of shells of equal or nearly equal sizes within one population were compared. A significant spread was observed in the stable isotope signatures in the D. polymorpha shells, particularly in the δ¹³C values derived from one population. The spread in δ¹³C and δ¹⁸O values was observed in both juvenile and adult shells; however, it increased with age. It is suggested that stable isotope investigations of D. polymorpha shells should not be performed on single shells, as the isotope values will not be representative of the coeval individuals within population. While the shells of D. polymorpha were close to oxygen isotope equilibrium with the ambient water, they were characterised by a 1.5–2 ‰ depletion in ¹³C relative to δ¹³C_DIC. Both the spread in δ¹³C values in the shells and the ¹³C depletion observed in the shells are interpreted as resulting from a strong metabolic influence on shell composition. Because the offset observed between dissolved inorganic carbon (DIC) and shells is relatively constant, the stable carbon isotope composition of D. polymorpha shells may reflect environmental conditions and thus may be used as a palaeolimnological proxy.     

Sclerochronological oxygen and carbon isotope ratios in Radix (Gastropoda) shells indicate changes of glacial meltwater flux and temperature since 4,200 cal yr BP at Lake Karakul,eastern Pamirs (Tajikistan)

We report δ¹⁸O and δ¹³C values of 21 fossil shells from the aquatic gastropod Radix from a sediment core taken in the eastern basin of Lake Karakul, Tajikistan (38.86–39.16°N, 73.26–73.56°E, 3,928 m above sea level) and covering the last 4,200 cal yr BP. The lake is surrounded by many palaeoshorelines evidencing former lake-level changes, most likely triggered by changes in meltwater flux. This hypothesis was tested by interpreting the isotope ratios of Radix shells together with δ¹⁸O values of Ostracoda and of authigenic aragonite. The mean δ¹⁸O values of Radix and Ostracoda fall along the same long-term trend indicating a change in the isotopic composition of precipitation, which contributed to the glaciers in the catchment as snow and finally as melt water to the lake. The sclerochronological δ¹⁸O and δ¹³C patterns in Radix shells provide seasonal weather information, which is discussed in context with previously proposed climatic changes during the last 4,200 cal yr BP. The period between ~4,200 and 3,000 cal yr BP was characterized by stepwise glacier advance in the catchment most likely due to a precipitation surplus. Subsequently the climate remained relatively cold but the lake level fluctuated, as indicated by ostracod shell isotope data. From ~1,800 cal yr BP the sclerochronological patterns provide evidence for increasing melt water flux and transport of allochthonous carbon into the lake, most likely due to an accelerated glacier retreat. The period around 1,500 cal yr BP was characterized by strong warming, increasing meltwater flux, glacier retreat and an increasing lake level. Warm conditions continued until ~500 cal yr ΒP probably representing the end of the Medieval Warm Period. A short relatively cold (dry?) period and a lower lake level are assumed for ~350 cal yr BP, possibly an analogue to the Maunder Minimum cooling in the North Atlantic region. Our results show that the lake system is complex, and that changes were triggered by external forcing and feedbacks. The similarity of δ¹⁸O values in Radix and ostracod shells demonstrates that both archives provide complementary information.     

Bias of ostracod stable isotope data caused by drying of sieve residues from water

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.     

Climatic and anthropogenic influence on the stable isotope record from bulk carbonates and ostracodes in Lake Neuchâ, Switzerland, during the last two millennia

Lake Neuchåtel is a medium sized, hard-water lake, lacking varved sediments, situated in the western Swiss Lowlands at the foot of the Jura Mountains. Stable isotope data (18O and 13C) from both bulk carbonate and ostracode calcite in an 81 cm long, radiocarbon-dated sediment core represent the last 1500 years of Lake Neuchåtel's environmental history. Comparison between this isotopic and other palaeolimnologic data (mineralogical, geochemical, palynological, etc.) helps to differentiate between anthropogenic and natural factors most recently affecting the lake. An increase in lacustrine productivity (450–650AD ca), inferred from the positive trend in 13C values of bulk carbonate, is related to medieval forest clearances and the associated nutrient budget changes. A negative trend in both the bulk carbonate and ostracode calcite 18O values between approximately 1300 and 1500AD, is tentatively interpreted as due to a cooling in mean air temperature at the transition from the Medieval Warm Period to the Little Ice Age. Negative trends in bulk carbonate 18O and 13C values through the uppermost sediments, which have no equivalent in ostracode calcite isotopic values, are concomitant with the recent onset of eutrophication in the lake. Isotopic disequilibrium during calcite precipitation, probably due to kinetic factors in periods of high productivity is postulated as the mechanism to explain the associated negative isotopic trends, although the effect of a shift of the calcite precipitation towards the warmer months cannot be excluded.     

Stable isotopes and trace elements in modern ostracod shells: implications for reconstructing past environments on the Tibetan Plateau,China

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 δ¹⁸O_shell and δ¹³C_shell 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/Ca_shell 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/Ca_shell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Ca_shell 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.     

Late Holocene hydrologic changes in northern New Zealand inferred from stable isotope values of aquatic cellulose in sediments from Lake Pupuke

Isotopic records of aquatic cellulose are becoming increasingly important for palaeohydrological reconstructions, but widespread application of this climate proxy is hampered by minerogenic contamination that affects oxygen isotope measures in cellulose. Few records of isotopes in aquatic cellulose are available from palaeoclimate archives in the Southern Hemisphere. In this study, we used a new bulk cellulose extraction method and determined the oxygen (δ¹⁸O) and carbon (δ¹³C) isotope values in cellulose from a Holocene lake sediment core segment (7.2–1.1 cal ka BP) from Lake Pupuke, Auckland, New Zealand. Isotope values from modern, potential sources of sedimentary cellulose revealed the aquatic origin of the cellulose extracted from the core, and hence enabled inference of past lake water δ¹⁸O values from the δ¹⁸O of measured cellulose in the core. A shift to a more positive water balance in the lake was identified around 2.8 cal ka BP by a decrease in inferred lake water δ¹⁸O values. At that time, greater epilimnetic primary productivity is indicated by the higher δ¹³C values of sedimentary cellulose. Greater divergence between the δ¹³C values of cellulose and bulk organic matter suggests stronger stratification of the lake, likely caused by greater freshwater input. We discuss a possible link to a solar minimum that occurred at that time.     

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: VII. Carbonate isotope geochemistry as a record of riverine runoff

Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a ¹⁸O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a ¹⁸O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the ¹⁸O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the ¹⁸O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell ¹⁸O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell ¹⁸O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the ¹³C of shells against predicted values based solely on the ¹³C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle.