首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Stable oxygen isotope measurements on fossil chironomid head capsules from lake sediments show that these chitinous remains can be used to reconstruct past lake water δ18O and, indirectly, past climate change. We examined the impact of chemical pretreatment procedures on the chemical and stable oxygen isotope composition, and morphology of chironomid cuticles. Use of alkali, acids, and sodium chlorite alters the chemical composition and the morphological structure of chironomid cuticles by selective removal of chitin or proteins. Gas chromatograms of pyrolyzates show that NaClO2 causes deproteination, whereas the combined use of HCl and HF results in partial chitin removal. Head capsules pretreated with KOH contained both chitin- and protein-derived moieties, although the concentration of protein was reduced, especially after KOH treatment at high concentration (28%) and temperature (100°C). Scanning electron microscopy confirmed that a proteinaceous matrix is still present in modern and fossil head capsules after KOH treatment. This matrix, however, is largely absent in head capsules pretreated with NaClO2. A change in the proportion of chitin and proteins in our samples was associated with differences in chironomid δ18O values. Our results suggest that deproteination results in a relative increase of chironomid δ18O, whereas removal of chitin leads to decreased δ18O values. We therefore discourage the use of acids or prolonged (≥1 h) exposure to hot alkali (70°C) prior to chironomid δ18O analysis. Chitin purification by sodium chlorite causes significant weight loss, which may preclude down-core chironomid δ18O measurements. Caution and standardization are required when pretreating samples for chironomid δ18O analysis to ensure reliable, comparable, and reproducible results.  相似文献   

2.
We used a series of experiments to determine whether stable carbon isotope analysis of modern and fossil larval head capsules of chironomids allowed identification of their dietary carbon source. Our main focus was to assess whether carbon from naturally 13C-depleted methane-oxidizing bacteria (MOB) can be traced in chironomid cuticles using stable carbon isotope analysis. We first showed that a minimum sample weight of ~20 μg was required for our equipment to determine head capsule δ13C with a standard deviation of 0.5‰. Such a small minimum sample weight allows taxon-specific δ13C analyses at a precision sufficient to differentiate whether head capsules consist mainly of carbon derived from MOB or from other food sources commonly encountered in lake ecosystems. We then tested the effect of different chemical pre-treatments that are commonly used for sediment processing on δ13C measurements on head capsules. Processing with 10% KOH (2 h), 10% HCl (2 h), or 40% HF (18 h) showed no detectable effect on δ13C, whereas a combination of boiling, accelerated solvent extraction and heavy chemical oxidation resulted in a small (0.2‰) but statistically significant decrease in δ13C values. Using culturing experiments with MOB grown on 13C-labelled methane, we demonstrated that methanogenic carbon is transferred not only into the larval tissue, but also into chironomid head capsules. Taxon-specific δ13C of fossil chironomid head capsules from different lake sediments was analyzed. δ13C of head capsules generally ranged from −28 to −25.8‰, but in some instances we observed δ13C values as low as −36.9 to −31.5‰, suggesting that carbon from MOB is traceable in fossil and subfossil chironomid remains. We demonstrate that stable carbon isotope analyses of fossil chironomid head capsules can give insights into dietary links and carbon cycling in benthic food webs in the past and that the method has the potential to reconstruct the importance of MOB in the palaeo-diet of chironomid larvae and, indirectly, to infer past changes in methane flux at the sediment water interface in lakes.  相似文献   

3.
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   

4.
Several techniques are available to examine the isotopic composition of historic lake waters, providing data that can subsequently be used to examine environmental changes. A recently-developed technique is the stable oxygen isotope analysis of subfossil chironomid (Diptera: Chironomidae) head capsules (mostly chitin) preserved in lake sediments. This technique involves a high Temperature Conversion Elemental Analyzer (TC/EA), which has been a relatively recent addition to the suite of online peripherals for analyzing the stable isotopic composition of organic samples. The highly precise and accurate 18O/16O and D/H measurements obtainable using the TC/EA with samples in the microgram range make this instrumentation suitable for studying geochemical and biological processes. Preparation of organic samples for isotopic analysis typically requires first weighing each sample into silver/tin capsules. These capsules can introduce oxygen and hydrogen contamination (a “blank effect”), which is especially problematic for analysis of small organic samples (e.g. less than 100 μg). Here we tested tin and silver capsules from two manufacturers and a range of sizes to assess contamination to small organic samples on the TC/EA. We also assessed how a method for cleaning silver capsules affected our analysis of commercial chitin. In general, capsules made of silver have less detectible oxygen than those made of tin, and capsules from the two manufacturers varied in their detectible oxygen. There was no detectable H contamination from silver capsules. In addition to our empirical findings, we present a model demonstrating the influence that contaminant oxygen can have on the δ18O of small organic samples. Sample mass becomes an important issue for such analyses. In light of our findings, we recommend a minimum sample mass ≥50 μg (approximately 120 whole chironomid head capsules) on a TC/EA-IRMS (Deltaplus XP system). Finally, we present a detailed protocol for preparing and transferring chironomid head capsules into silver capsules that minimizes the influence of contaminant oxygen. This protocol provides the paleo-community with another potential method for reconstructing paleoenvironments.  相似文献   

5.
A paleoenvironmental perspective of temperature change is paramount to understanding the significance of recent warming in the Arctic. Late Quaternary sediments from many arctic lakes provide environmental archives with decadal resolution, but reconstructions are hampered by the relative insensitivity of many traditional proxies to temperature. Here, we show that the δ18O of head capsules of chironomid larvae are equilibrated with the δ18O of lakewaters in which they live. In suitable lakes, lakewater δ18O is controlled by the δ18O of local precipitation, which is strongly correlated to mean annual air temperature (MAT). From this correlation, chironomid δ18O can be used to examine past changes in MAT. We illustrate the potential of this novel approach to paleothermometry with examples from two arctic lakes that reveal strong regional paleoclimatic gradients in the early Holocene.  相似文献   

6.
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   

7.
Ten meters of lacustrine deposits retrieved from Lake Pergusa (Sicily, southern Italy) were investigated through stable isotope composition (carbon and oxygen) of authigenic carbonate (calcareous muds) and freshwater shells. The core chronology was established through three AMS dates, and by correlation with a previously dated nearby core. Stable isotope data show that the lake water evolution was mainly dominated by evaporation. Between ca. 20 and 28 ka the recovered sediments have very high δ18O values, likely corresponding to very dry climatic conditions. The observed rapid oscillations in the δ18O of the recovered sediments during this period also suggest important climatic fluctuations. More humid conditions dominated during the Holocene period, with the wettest interval occurring between ca. 9000 and 3000 years BP. Late Holocene sediments represent a substantial return to drier conditions. The available pollen data from a nearby core substantially confirm this general climatic trend during the Holocene. The positive correlation between δ13C of the calcareous muds and carbonate content suggests that biological activity played a key role in the carbon isotope evolution of dissolved inorganic carbon. However, a clear climatic signal is not evident from the δ13C record.  相似文献   

8.
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   

9.
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   

10.
西北干旱地区大气降水δ18O的特征及水汽来源   总被引：7，自引：1，他引：6  
In order to reveal the characteristics and climatic controls on the stable isotopic composition of precipitation over Arid Northwestern China, eight stations have been selected from Chinese Network of Isotopes in Precipitation (CHNIP). During the year 2005 and 2006, monthly precipitation samples have been collected and analyzed for the composition of δD and δ18O. The established local meteoric water line δD=7.42 δ18O+1.38, based on the 95 ob-tained monthly composite samples, could be treated as isotopic input function across the region. The deviations of slope and intercept from the Global Meteoric Water Line indicated the specific regional meteorological conditions. The monthly δ18O values were characterized by a positive correlation with surface air temperature (δ18O (‰) =0.33 T (℃)-13.12). The amount effect visualized during summer period (δ18O (‰) = -0.04P (mm)-3.44) though not appeared at a whole yearly-scale. Spatial distributions of δ18O have properly portrayed the atmospheric circulation background in each month over Arid Northwestern China. The quan-titative simulation of δ18O, which involved a Rayleigh fractionation and a kinetic fractionation, demonstrated that the latter one was the dominating function of condensation of raindrops. Furthermore, the raindrop suffered a re-evaporation during falling processes, and the pre-cipitation vapor might have been mixed with a quantity of local recycled water vapor. Multiple linear regression equations and a δ18O-T relation have been gained by using meteorological parameters and δ18O data to evaluate physical controls on the long-term data. The estab-lished δ18O-T relation, which has been based on the present-day precipitation, could be considered as a first step of quantitatively reconstructing the historical environmental climate.  相似文献   

11.
Reconstructing climate change quantitatively over millennial timescales is crucial for understanding the processes that affect the climate system. One of the best methods for producing high resolution, low error, quantitative summer air temperature reconstructions is through chironomid analyses. We analysed over 50 lakes from NW and W Iceland covering a range of environmental gradients in order to test whether the distribution of the Icelandic chironomid fauna was driven by summer temperature, or whether other environmental factors were more dominant. A range of analyses showed the main environmental controls on chironomid communities to be substrate (identified through loss-on-ignition and carbon content) and mean July air temperature, although other factors such as lake depth and lake area were also important. The nature of the Icelandic landscape, with numerous volcanic centres (many of which are covered by ice caps) that produce large quantities of ash, means that relative lake carbon content and summer air temperature do not co-vary, as they often do in other chironomid datasets within the Arctic as well as more temperate environments. As the chironomid–environment relationships are thus different in Iceland compared to other chironomid training sets, we suggest that using an Icelandic model is most appropriate for reconstructing past environmental change from fossil Icelandic datasets. Analogue matching of Icelandic fossil chironomid datasets with the Icelandic training set and another European chironomid training set support this assertion. Analyses of a range of chironomid-inferred temperature transfer functions suggest the best to be a two component WA-PLS model with r 2 jack = 0.66 and RMSEP = 1.095°C. Using this model, chironomid-inferred temperature reconstructions of early Holocene Icelandic sequences show the magnitude of temperature change compared to contemporary temperatures to be similar to other NW European chironomid sequences, suggesting that the predictive power of the model is good.  相似文献   

12.
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   

13.
A 70-year history of precipitation δ18O record has been retrieved using an ice core drilled from a plat portion of the firn area in the Guoqu Glacier (33o34′37.8″ N, 91o10′35.3″ E, 5720 m a.s.l.) on Mt. Geladaindong (the source region of Yangtze River) during October and November, 2005. Based on the seasonality of δ18O records and the significant positive relationships between monsoon/non-monsoon δ18O values and summer/spring air temperature from the nearby meteorological stations, the history of summer and spring air temperature have been reconstructed for the last 70 years. The results show that both summer and spring air temperature variations present similar trends during the last 70 years. Regression analysis indicates that the slope of the temperature-δ18O relationship is 1.3℃/‰ for non-monsoon δ18O values and spring air temperature, and 0.4℃/‰ for monsoon δ18O values and summer air temperature. Variation of air temperature recorded in the ice core is consistent with that in the Northern Hemisphere (NH), however, the warming trend in the Geladaindong region is more intense than that in the NH, reflecting a higher sensitivity to global warming in the high elevation regions. In addition, warming trend is greater in spring than in summer.  相似文献   

14.
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:
  相似文献   

15.
The mass transport of nutrients by migratory animals can markedly alter the biogeochemistry and ecology of recipient ecosystems, particularly in nutrient-poor regions such as the Arctic. However, the role of biovectors in the global cycling of nutrients is often overlooked. Here we investigate nitrogen dynamics in two seabird-affected ponds in the Canadian High Arctic. The ponds lie at the base of a large seabird colony and have been greatly enriched in nutrients due to the input of guano and other wastes. Using sediment cores that span the last ~200 years, we measured stable isotopes of nitrogen (δ15N) in bulk sediments as well from the subfossil remains of chironomid (Diptera) head capsules and Daphnia ephippia. The bulk-sediment samples from our seabird-affected ponds had elevated δ15N values relative to seabird-free sites elsewhere in the Arctic. In general, the chironomid δ15N profiles roughly paralleled those of bulk sediments in both study ponds, while the Daphnia profile remained relatively stable in contrast to the considerable variation recorded in the bulk sediments and chironomids. Interestingly, no apparent pattern emerged among δ15N values recorded in the bulk sediments, chironomids, and Daphnia between the two study ponds. The stability recorded in the δ15N profiles from bulk sediments relative to the more variable invertebrate profiles point towards the complexity of nitrogen uptake by chironomids and Daphnia at these sites. These data suggest that the bulk sediments are integrating the different fractions of the overall δ15N pool and thus may be most appropriate for reconstructing overall trends in lake trophic status.  相似文献   

16.
乌鲁木齐河流域不同水体中的氧稳定同位素   总被引：1，自引：0，他引：1  
The variations of the stable oxygen isotope in different water mediums in Urumqi River Basin, China, are analyzed. The stable oxygen isotope in precipitation has marked temperature effect either under synoptic or seasonal scale at the head of Urumqi River. The linear regression equations of δ^18O against temperature are δ^18O=-0.94T-12.38 and δ^18O=1.29T-13.05 under the two time scales, respectively. The relatively large δ^18O/temperature slopes show the strong sensitivity of δ^18O in precipitation to temperature variation at the head of Urumqi River. According to the analyses on the δ^18O in precipitation sampled at three stations with different altitudes along Urumqi River, altitude effect is notable in the drainage basin. The δ^18O/altitude gradients have distinct differences: the gradient from Urumqi to Yuejinqiao is merely -0.054‰/hm, but -0.192‰/hm from Yuejinqiao to Daxigou, almost increasing by 2.6 times over the former. No altitude effect is found in surface firn the east branch of Glacier No. 1 at the head of Urumqi River, showing that precipitation in the glacier is from the cloud cluster with the same condensation level. Influenced by strong ablation and evaporation, the δ^18O in surface firn increases with increasing altitude sometimes. Survey has found that the δ^18O in meltwater at the terminus of Glacier No. 1 and in stream water at Total Control have the similar change trend with the former all smaller than the latter, which displays the different runoff recharges, and all mirror the regime of temperature in the same term basicallv.  相似文献   

17.
  相似文献   

18.
We conducted multi-proxy geochemical analyses (including measurements of organic carbon, nitrogen and sulphur stable isotope composition, and carbonate carbon and oxygen isotope composition) on a 13.5 m sediment core from Lake Bliden, Denmark, which provide a record of shifting hydrological conditions for the past 6,700 years. The early part of the stratigraphic record (6,700–5,740 cal year BP) was wet, based on δ18Ocarb and lithology, and corresponds to the Holocene Thermal Maximum. Shifts in primarily δ18Ocarb indicate dry conditions prevailed from 5,740 to 2,800 cal year BP, although this was interrupted by very wet conditions from 5,300 to 5,150, 4,300 to 4,050 and 3,700 to 3,450 cal year BP. The timing of the latter two moist intervals is consistent with other Scandinavian paleoclimatic records. Dry conditions at Lake Bliden between 3,450 and 2,800 cal year BP is consistent with other paleolimnological records from southern Sweden but contrasts with records in central Sweden, possibly suggesting a more northerly trajectory of prevailing westerlies carrying moisture from the North Atlantic at this time. Overall, fluctuating moisture conditions at Lake Bliden appear to be strongly linked to changing sea surface temperatures in the Greenland, Iceland and Norwegian seas. After 2,800 cal year BP, sedimentology, magnetic susceptibility, δ13CORG, δ13Ccarb and δ18Ocarb indicate a major reduction on water level, which caused the depositional setting at the coring site to shift from the profundal to littoral zone. The Roman Warm Period (2,200–1,500 cal year BP) appears dry based on enriched δ18Ocarb values. Possible effects of human disturbance in the watershed after 820 cal year BP complicate attempts to interpret the stratigraphic record although tentative interpretation of the δ18Ocarb, magnetic susceptibility, δ13CORG, δ13Ccarb and δ18Ocarb records suggest that the Medieval Warm Period was dry and the Little Ice Age was wet.  相似文献   

19.
We present oxygen and carbon isotope ratios and the morphological structure of the cultured freshwater pearl mussel (Hyriopsis sp., Unionidae) shell and pearl. The number of first-order fluctuations of δ18O of the outer shell layer along the maximum growth axis was consistent with the number of cultured years. The dominant factor controlling annual δ18O fluctuations was water temperature with a minor contribution from the variation in δ18O of ambient water, especially during the rainy season. The δ13C values were approximately constant throughout the life of the mussel, suggesting that the contributions of body size to δ13C of the shell were minor. We observed nine distinct disturbance rings on the outer surface of the shell. Five rings coincided with the five winter peaks of the δ18O profile, indicating winter growth cessation below approximately 10°C, probably because of either inactive growth at low water temperatures or reproduction. Summer disturbance rings were not observed in all years. Moreover, some summer rings showed discontinuity in the inner structure. These findings suggest that summer growth cessation may be caused by occasional events such as heavy rains, as the decrease of dissolved oxygen concentration. The δ18O profile and shell structures indicated that shell aragonite was precipitated at close to equilibrium conditions with respect to the oxygen isotope composition of the ambient water. Hyriopsis sp. shells can potentially be used for reconstruction of past hydrologic conditions. The δ18O of a pearl indicated that calcification occurred over a temperature range of at least 13–23°C. The optimal temperature for pearl calcification in this species is lower than that for marine pearl calcification.  相似文献   

20.
A surface-sediment survey of pigments in 100 lakes in the Scandes Mountains, northern Sweden, was combined with a reconstruction of Holocene sedimentary pigments from Lake Seukokjaure to assess the major factors regulating phototrophic communities, and how these controls may have changed during the period from the deglaciation (~9700 cal. years BP) to the present. The study area covers a pronounced gradient of temperature and precipitation, and encompasses the subarctic tree line, an important ecotonal boundary in this region. Lake Seukokjaure is located in a presently treeless basin close to the modern tree line. The spatial survey of sedimentary pigments was analyzed using principle components analysis (PCA) and redundancy analysis (RDA). PCA explained 73–83% of variance in pigment abundance and composition, whereas RDA explained 22–32% of variation in fossil assemblages. Dissolved organic carbon (DOC) content of lake water, sediment δ13C, maximum lake depth, elevation and lake-water conductivity were all identified as environmental variables with significant association with pigment abundances in the spatial survey, although phototrophic communities of lakes situated in different vegetation zones (alpine, birch, conifer/birch) were incompletely distinguished by the ordinations. In the RDAs, the primary pigment variability occurred along a production gradient that was correlated negatively to water-column DOC content and δ13C signature of sediments. This pattern suggested that the important controls of primary production were light regime and terrestrial supplies of 13C-depleted carbon. In contrast, depth, elevation and conductivity were found to be more important for the differentiation of the phototrophic community composition. Application of these spatial survey results to the Holocene sediment record of Lake Seukokjaure demonstrated the importance of DOC for the temporal development of the lake, from an early state of high production to a period of slight oligotrophication. In general, the algal changes were regulated by the interaction of DOC and conductivity, although transitions in the phototrophic community during the late Holocene were less easily interpreted. Terrestrial vegetation development thus appears to be of utmost importance for the regulation of primary production in oligotrophic alpine and subarctic lakes and climate impacts on lakes, whereas other basin-specific factors may control the ontogeny of algal community composition.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号