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.
This study is focused on the endorheic Uyni-Coipasa Basin located in the southern Bolivian Altiplano. Stratigraphical and fossil diatom studies based on a detailed radiocarbon chrnology revealed six phases in water-level changes and paleosalinity variations. At 15,430±80 yr B.P., lacustrine conditions settled in the southern Bolivian Altiplano. A saline lake, characterized by benthic meso-metasaline species, reached +4 m altitude above the present bottom of the basin. After 15,430±80 yr B.P., the level rapidly rose to +27 m, as suggested by a tychoplanktonic mesosaline flora. Between 14,500 years and 13,000 years, finely lanminated sediments at +32 m contained successively a dominance of epiphytic mesosaline to hypersaline species and tychoplanktonic oligosaline diatoms, indicating weak fluctuations in water-level and salinity. At 13,000 years, strong changes in the diatom flora occurred; epiphytic oligo-hypersaline diatoms were replaced by planktonic meso-polysaline species. They indicate a deep salt lake (the lake reached +100 m). After 12,000 years, the lake level abruptly dropped, as suggested by fluviatile sediments with a benthic mesopolysaline diatom flora. The main lake was replaced by shallow saline ponds. A wet pulse occurred at 11,400 years, characterized by low water level (+7 m) and high salinity. This lacustrine phase remained until 10,400 yr B.P. These data indicate changes in Precipitation minus Evaporation (P-E). Our regional interpretations are based on a comparison with teh available data on the northern (Lake Titicaca) and southern (Lipez are) Bolivian Altiplano and on the northern Chilean Altiplano (Atacama Desert).  相似文献   

2.
Using the principles of electroosmosis, a subsampling technique for clay-rich sediments is introduced that produces undisturbed thinsections from sediment cores. The fragile sample remains in the same disposable box throughout the process, preventing collapse of its structure during subsampling, manipulation and impregnation, as well as other potential problems.  相似文献   

3.
Oxygen isotopes and geochemistry from lake sediments are commonly used as proxies of past hydrologic and climatic conditions, but the importance of present-day hydrologic processes in controlling these proxies are sometimes not well established and understood. Here we use present-day hydrochemical data from 13 lakes in a hydrologically connected lake chain in the northern Great Plains (NGP) to investigate isotopic and solute evolution along a hydrologic gradient. The 18O and 2H of water from the chain of lakes, when plotted in 2H - 18O space, form a line with a slope of 5.9, indicating that these waters fall on an evaporation trend. However, 10 of the 13 lakes are isotopically similar (18O = –6 ± 1 VSMOW) and show no correlation with salinity (which ranges from 1 to 65). The lack of correlation implies that the isotopic composition of various source waters rather than in-lake evaporation is the main control of the 18O of the lakes. Groundwater, an important input in the water budget of this chain of lakes, has a lower 18O value (–16.7 in 1998) than that of mean annual precipitation (–11) owing to selective recharge from snow melt. For the lakes in this chain with salinity < 15, the water Mg/Ca ratios are strongly correlated with salinity, whereas Sr/Ca is not. The poor correlation between Sr/Ca and salinity results from uptake of Sr by endogenic aragonite. These new results indicate that 18O records may not be interpreted simply in term of climate in the NGP, and that local hydrology needs to be adequately investigated before a meaningful interpretation of sedimentary records can be reached.  相似文献   

4.
Lake Chen Co, situated at 90°33–39E, 28°53–59N with a lake level of 4420 m asl, is an enclosed lake with 148 km2 of catchment area and 40 km2 of lake surface. It is mainly supplied by glacier melt water either from surface inflow or groundwater. Atmospheric precipitation is mainly concentrated in June–September. A 216-cm long lake sediment core was obtained at a site with 8 m of water depth, 800 m from the lakeshore and 1.5% of the bottom slope in this lake. The sediment core was taken by a piston sampler and was sliced with an interval of 1 cm each. 210Pb dating measurement suggested that the average sedimentary rate was 0.16 cm yr–1, which also was confirmed by 137Cs peak occurrence. Magnetic analyses included low-frequency dependent susceptibility (LF), susceptibility of anhysteretic remanent magnetism (ARM), the saturation isothermal remanent magnetism (SIRM), the isothermal remanent magnetism (IRM) reverse and Soft and Hard contents were performed for the sediment core. Results showed that LF was an index for reflecting the environmental conditions, but was not sufficient to reveal details of magnetic features. This had been proved by measurements of IRM Reverse percentage and Soft and Hard magnetic minerals values. The log(SIRM/LF) had much more information to reveal environmental changes. The ARM/LF might be more sensitive to the local environmental conditions because it was well able to indicate the grain-size variations of magnetic particles. In the past ca. 1400 years, the warm stages were ca. 620–740 AD, 1120–1370 AD and since ca. 1900 AD. After an intensively cold stage during ca. 1550–1690 AD, a cold-humid stage from ca. 1690–1900 AD and a warm-dry stage since ca. 1900 AD followed. Among these stages, the warmest one occurred in ca. 1120–1370 AD and the coldest stage was between ca. 1550 and 1690 AD. This result might be compared with many other research results from lake cores, ice cores and the Chinese historical documents.  相似文献   

5.
We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS 14C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to 9000 14C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by 9000 14C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (9000 to 6800 14C yr BP), pollen data suggest moist conditions, but high 18O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lake's water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in 18O values and associated change in ostracod species at 6800 14C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning 5780 14C yr BP may indicate early human disturbance. By 2800 14C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning 1100 to 1000 14C yr BP, following the collapse of Classic Maya civilization.  相似文献   

6.
In this article we describe the basic framework of the computerized geologic mapping system cigma. The system, whic is based on a mathematical formulation of geologic concepts, consists of the following six subsystems: (1) input of geologic data set; (2) inference of stratigraphic sequence; (3) construction of logical models of geologic structures; (4) determination of three-dimensional geologic boundary surfaces; (5) construction of three-dimensional solid model of geologic structures; and (6) graphical presentation. Geologic structures are summarized in several tables called logical models of geologic structures. Each model is constructed automatically from input data on structural relations between geologic bodies. The model interprets the data automatically to create data files necessary to determine the shapes of geologic boundaries; it also provides a threedimensional solid model of geologic structures referring to the shapes of boundaries. As a prototype, we introduce two types of contacts corresponding to conformity and unconformity into the logical model and show that it is possible to draw a geologic map automatically. More complex geologic structures can be introduced into the geologic mapping system through further formulation of geologic structures.  相似文献   

7.
Several waterbodies occupied the tectonic depressions along the Dead Sea transform during the NeogeneQuaternary. The earliest of these water bodies was the marine Sedom lagoon, which produced the SedomDead Sea brine. After the disconnection of the Sedom lagoon from the open sea several lakes were developed in the Dead Sea basinJordan Valley. Lake Amora (Samra) that existed from early to late Pleistocene, Lake Lisan (~ 70–15 kyr B.P.), and the Holocene Dead Sea. The lacustrine water bodies in the Dead Sea basin behave as amplifier lakes whose size and depth reflect the changing climatic conditions in the region. Lake level and limnological conditions of Lake Amora are not yet known, nevertheless, the lake probably extended over a large part of the Dead Sea basin-Jordan Valley. Lake Lisan level changed between ~ 330 and ~ 150 meters below sea level (m b.s.l.). Its maximum elevation was reached at ~ 27–23 kyr B.P. during marine isotope stage 2. Its minimum elevation was reached at ~ 47–43 kyr during marine isotope stage 3. Lake Lisan began to recede at ~ 17–15 kyr B.P. and at 12–11 kyr B.P. the postLisan water body declined to its minimum level. During most of the Holocene the lake (paleoDead Sea) stabilized at ~ 400 m b.s.l.The limnological evolution of water bodies in the Dead Sea basin reflects the climatic conditions in the region during the late Pleistocene, which fluctuated between wetter and drier periods. During Lisan time these fluctuations appear to be modulated by the cold and warm cycles, respectively in the northern Hemisphere. This relation is less obvious in the postLisan water body, where the strongest lake drop appears to occur during the Younger Dryas cold event.  相似文献   

8.
The Cenozoic continental sedimentary basins in eastern China are rich in lacustrine source rocks. Based on their paleogeographic location and fossils, these basins can be grouped into inland and near-shore basins. Before the collision of India and Eurasia about 45 million years ago, the inland basins were dominated by arid climates that were unfavorable for the substantial preservation of oil source rocks. In contrast, the contemporary near-shore basins experienced alternating arid and humid climates, probably induced by sea level changes, which produced conditions that favoured the formation and preservation of oil source rocks. With the rise of the Tibetan Plateau, the Asian monsoon was initiated or significantly intensified in the Late Eocene. This, in turn, changed the arid climates in the inland basins to humid or to alternating arid and humid, providing ideal conditions for the formation and preservation of lacustrine source rocks in the inland basins. The evidence suggests that Tibetan uplift played a crucial role in the generation and preservation of the Cenozoic lacustrine source rocks in eastern China.This is the third paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.  相似文献   

9.
In this paper, we describe new fuzzy models for predictive mineral potential mapping: (1) a knowledge-driven fuzzy model that uses a logistic membership function for deriving fuzzy membership values of input evidential maps and (2) a data-driven model, which uses a piecewise linear function based on quantified spatial associations between a set of evidential evidence features and a set of known mineral deposits for deriving fuzzy membership values of input evidential maps. We also describe a graphical defuzzification procedure for the interpretation of output fuzzy favorability maps. The models are demonstrated for mapping base metal deposit potential in an area in the south-central part of the Aravalli metallogenic province in the state of Rajasthan, western India. The data-driven and knowledge-driven models described in this paper predict potentially mineralized zones, which occupy less than 10% of the study area and contain at least 83% of the model and validation base metal deposits. A cross-validation of the favorability map derived from using one of the models with the favorability map derived from using the other model indicates a remarkable similarity in their results. Both models therefore are useful for predicting favorable zones to guide further exploration work.  相似文献   

10.
Lake Bonneville marl provides a stratigraphic record of lake history preserved in its carbonate minerals and stable isotopes. We have analyzed the marl in shallow cores taken at three localities in the Bonneville basin. Chronology for the cores is provided by dated volcanic ashes, ostracode biostratigraphy, and a distinctive lithologic unit believed to have been deposited during and immediately after the Bonneville Flood.A core taken at Monument Point at the north shore of Great Salt Lake encompasses virtually the entire Bonneville lake cycle, including the 26.5 ka Thiokol basaltic ash at the base and deposits representing the overflowing stage at the Provo shoreline at the top of the core. Two cores from the Old River Bed area near the threshold between the Sevier basin and the Great Salt Lake basin (the main body of Lake Bonneville) represent deposition from the end of the Stansbury oscillation ( 20 ka) to post-Provo time ( 13 ka), and one core from near Sunstone Knoll in the Sevier basin provides a nearly complete record of the period when Lake Bonneville flooded the Sevier basin (20–13 ka).In all cores, percent calcium carbonate, the aragonite to calcite ratio, and percent sand were measured at approximately 2-cm intervals, and 18O and 13C were determined in one core from the Old River Bed area. The transgressive period from about 20 ka to 15 ka is represented in all cores, but the general trends and the details of the records are different, probably as a result of water chemistry and water balance differences between the main body and the Sevier basin because they were fed by different rivers and had different hypsometries. The Old River Bed marl sections are intermediate in position and composition between the Monument Point and Sunstone Knoll sections. Variations in marl composition at the Old River Bed, which are correlated with lake-level changes, were probably caused by changes in the relative proportions of water from the two basins, which were caused by shifts in water balance in the lake.This is the second paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.  相似文献   

11.
Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (12–9 ka), 18O of ostracodes ranged from –16 to –5 (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative 18O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the 18O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.18O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from –4 at 8 ka to –11 at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (–20 SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The 18O of this inorganic calcite abruptly shifts to higher values (–6) after 4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After 2 ka, the 18O of the Mg-calcite fluctuates between –13 and –7, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The 13C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.This is the sixth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.  相似文献   

12.
New sediment core data from a unique slow-sedimentation rate site in Lake Tanganyika contain a much longer and continuous record of limnological response to climate change than have been previously observed in equatorial regions of central Africa. The new core site was first located through an extensive seismic reflection survey over the Kavala Island Ridge (KIR), a sedimented basement high that separates the Kigoma and Kalemie Basins in Lake Tanganyika.Proxy analyses of paleoclimate response carried out on core T97-52V include paleomagnetic and index properties, TOC and isotopic analyses of organic carbon, and diatom and biogenic silica analyses. A robust age model based on 11 radiocarbon (AMS) dates indicates a linear, continuous sedimentation rate nearly an order of magnitude slower here compared to other core sites around the lake. This age model indicates continuous sedimentation over the past 79 k yr, and a basal age in excess of 100 k yr.The results of the proxy analyses for the past 20 k yr are comparable to previous studies focused on that interval in Lake Tanganyika, and show that the lake was about 350 m lower than present at the Last Glacial Maximum (LGM). Repetitive peaks in TOC and corresponding drops in 13C over the past 79 k yr indicate periods of high productivity and mixing above the T97-52V core site, probably due to cooler and perhaps windier conditions. From 80 through 58 k yr the 13C values are relatively negative (–26 to –28 l) suggesting predominance of algal contributions to bottom sediments at this site during this time. Following this interval there is a shift to higher values of 13C, indicating a possible shift to C-4 pathway-dominated grassland-type vegetation in the catchment, and indicating cooler, dryer conditions from 55 k yr through the LGM. Two seismic sequence boundaries are observed at shallow stratigraphic levels in the seismic reflection data, and the upper boundary correlates to a major discontinuity near the base of T97-52V. We interpret these discontinuities to reflect major, prolonged drops in lake level below the core site (393 m), with the lower boundary correlating to marine oxygen isotope Stage 6. This suggests that the previous glacial period was considerably cooler and more arid in the equatorial tropics than was the last glacial period.  相似文献   

13.
With the purpose of studying the vegetation and climatic changes in the last millenia of the Coastal Plain of Rio Grande do Sul, Brazil, a palynological study was made of the sediments of the northern part of Lagoa dos Patos lagoon. Twenty-four samples from a 2.26 m core taken at a depth of 7.70 m (30° 50 50 S and 50° 59 05 W) were collected.The analyses revealed marine transgression at 5170±120 years B.P., giving rise to local vegetation consisting chiefly of xerophytes and halophytes. Vegetation characteristic of a humid environment was present along adjacent portions of the Coastal Plain at this time. Transgression increased at about 4080±110 years B.P., when the greatest level of tidewater was reached. This coincided with the beginning of forest vegetation development along the inner portions of the Coastal Plain. These data suggest that marine transgression may have been a consequence of higher temperatures and more humidity. After 4000 years B.P., regression occurred, resulting in fresh waters characteristics in the northern portion of the lagoon. The development of forest vegetation began at this time.This is the fourth in a series of papers published in this issue on Paleolimnology in Southern South America. Dr. C. A. Fernández served as guest editor for these papers.  相似文献   

14.
Systematic variability occurs between the oxygen isotopic composition of lake water sampled in mid-summer 1993 and cellulose extracted from surficial sediments of a suite of lakes spanning the forest-tundra transition near Noril'sk, Russia. Some tundra and all forest-tundra lakes show greater deviation from expected cellulose-water isotopic separation than forest lakes, apparently because of greater sensitivity to 18O-depleted snowmelt contributions. Cellulose derived from aquatic plants naturally integrates fluctuations in lake water 18O, providing a signal that is inherently more representative of average thaw season lake water 18O than the measure of instantaneous 18O obtained from an individual sample of lake water. Thus, indiscriminate use of empirical cellulose-water relations derived from calibration samples could lead to erroneous assessment of paleohydrology from the oxygen-isotope stratigraphy of sediment cores from arctic lakes. However, deviation from the expected cellulose-water fractionation is a source of lake-specific hydrologic information useful for qualifying paleoenvironmental interpretations and possibly constraining non-isotopic methods that rely on surface-sediment calibrations.  相似文献   

15.
Criteria for removing training set lakes and taxa in chironomidbased inference models, due to low abundances, have largely been ad hoc. We used an anoxia inference model and a hypolimnetic oxygen model from southcentral Ontario to determine what effect subfossil head capsule abundance and taxa deletion criteria have on fossil inference statistics. Results from six training set lakes suggest that a minimum abundance of 40–50 head capsules is sufficient for use in inference models, however more diverse samples likely require more than 50 head capsules. Taxa deletion criteria substantially improved the predictive ability of inference models (lowered the root mean squared error of prediction (RMSEP)). The common practice of including taxa with only 2% abundance in at least two lakes was one of the deletion criteria that much improved inference models. Similar deletion criteria, such as 2% in at least 3 lakes and 3% in at least 1 lake, produced comparable improvements (up to 18% reduction in RMSEP).  相似文献   

16.
We investigated paleolimnological records from a series of river deltas around the northeastern rim of Lake Tanganyika, East Africa (Tanzania and Burundi) in order to understand the history of anthropogenic activity in the lakes catchment over the last several centuries, and to determine the impact of these activities on the biodiversity of littoral and sublittoral lake communities. Sediment pollution caused by increased rates of soil erosion in deforested watersheds has caused significant changes in aquatic communities along much of the lakes shoreline. We analyzed the effects of sediment discharge on biodiversity around six deltas or delta complexes on the east coast of Lake Tanganyika: the Lubulungu River delta, Kabesi River delta, Nyasanga/Kahama River deltas, and Mwamgongo River delta in Tanzania; and the Nyamuseni River delta and Karonge/Kirasa River deltas in Burundi. Collectively, these deltas and their associated rivers were chosen to represent a spectrum of drainage-basin sizes and disturbance levels. By comparing deltas that are similar in watershed attributes (other than disturbance levels), our goal was to explore a series of historical experiments at the watershed scale, with which we could more clearly evaluate hypotheses of land use or other effects on nearshore ecosystems. Here we discuss these deltas, their geologic and physiographic characteristics, and the field procedures used for coring and sampling the deltas, and various indicators of anthropogenic impact.  相似文献   

17.
Increased accuracy in measuring temporal variations in the Earth's gravity field allow inprinciple the use of gravity observations to deduce subsurface water-mass changes. This canbe with respect to a small area, or as a larger spatial average of water mass change usinggravity observations from low-altitude satellites, such as the forthcoming GRACE mission.At both scales, there is a need to validate gravity-based estimates against field recordings ofactual subsurface water-mass variations. In practice, this could prove difficult because thespatial integral of all water-storage change components can be subject to considerable fieldmeasurement error. An alternative approach to the validation process is proposed by whichsuitable geological formations are utilized as giant weighing devices to directly measure area-integratedwater-mass changes. The existence of such natural geological weighing lysimetersis demonstrated using observations from a replicated experimental site in New Zealand. Sitesof this type could be used to verify water-storage change estimates derived from sensitiveground surface gravity instrumentation. In addition, geological lysimeters could be used tomake local checks on the accuracy of any estimated regional water-mass time series, whichis proposed for satellite calibration. The land area weighed by a geological lysimeter increaseswith formation depth and it is speculated that recordings made at oil well depth may allowdirect monitoring of subsurface water mass changes at the regional scale.  相似文献   

18.
The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have 13C and 18O values as high as +16.5 and +9.2 respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.The high 13C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in 13C of dissolved inorganic carbon on the order of +15 is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m-2 d-1. The 13C records from other studies of ostracode valves have values approaching, but not exceeding about +14 suggesting a limiting value to 13C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon.Values of 18O in ostracodal calcite are quite variable (–4 to +9) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the 18O variability.  相似文献   

19.
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   

20.
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.  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