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 109 毫秒
1.
Unconsolidated, flocculent sediments that are frequently resuspended by wind action are found in many shallow-water lakes. Collecting sediment/water interface cores in such lakes for paleolimnological study may be problematic because it is difficult to determine the depth to the water/sediment interface. Accurately determining this water depth is necessary to guarantee that a piston corer does not penetrate the sediments prior to the drive and to maximize the core length. A simple instrument constructed with inexpensive, readily available components is described. This infrared floc detector (IFD) is used to sense the increased optical density of unconsolidated sediments as the detector is lowered into a lake. The IFD, in effect, yields a precise as well as an accurate measure of water depth. The depth to the water/sediment interface can be measured with an accuracy of approximately 1 cm, provided surface waters are relatively calm.  相似文献   

2.
We present a design for a piston corer that can take a sediment core up to 1 m long, with an undisturbed sediment–water interface. The coring device possesses a tripod unit and a core-tube unit, the latter comprised of a core tube, a piston with a wire, a drive weight in which the core tube is mounted and a steel rod. The tripod stands on the sediment surface during coring, stabilizing the system and serving as an anchor point for the piston wire. A ball clamp, the critical component of the design, is mounted on top of the tripod. The steel rod runs through the ball clamp and at its lower end, holds the drive weight and the core tube. The ball clamp allows the core tube to slide downward, while the piston is held in a fixed vertical position by the wire connected to the tripod. When the corer is lifted, however, the ball clamp locks. This makes it possible to operate the corer with a single cable, because the piston is not subject to any lifting force when the corer is retrieved. The piston remains in position in the core tube even if the collected sediment core is very short. The piston corer can be deployed from a raft or a boat.  相似文献   

3.
A lightweight, percussion corer, suitable for use in remote lakes of moderate depth, is described. The operation of the corer and suggestions for securing and transporting long sediment cores are presented. In particular, the design and use of a recovery pot, which allows the retrieval of undisturbed cores from sediments of unknown depth, is detailed.  相似文献   

4.
We present a design for a large diameter piston corer, deployed from a raft that is suitable for use in shallow lakes. The piston corer, known as Big Ben, consists of a core tube, a piston on a rope and a corer head, to which rods are attached to drive the tube into the sediment. A core catcher, which aids the support of the core tube when full of sediment, has been incorporated into the design. To extrude the sediment, a framework has been designed to keep the core tube upright and stationary and a modified bottle jack is used to push the piston upwards during the extrusion process. The practical operation of the Big Ben coring system from setting up a coring platform to collecting and safely extruding a core is detailed. Finally we summarise recent experiences of deploying the corer and highlight its potential uses in the developing field of multi-proxy palaeolimnology.  相似文献   

5.
A new hammer-driven freeze corer   总被引：1，自引：0，他引：1  
A shortcoming of freeze corers has been the inability to collect long cores in stiff sediments. We describe a hammerdriven freeze corer that collects undisturbed sediment cores up to 1.3 m in length in stiff or soft lake sediments. The corer was tested in prairie, montane and alpine lakes of western Canada. For these stiff sediments, conventional (gravity-only) freeze coring methods collected cores ranging from 25–59 cm compared with 100–124 cm for our hammer-driven device. The maximum length of cores could exceed 1.3 m if a longer core barrel was constructed.  相似文献   

6.
We present a unique, versatile piston corer for recovering continuous lake and bog sediment sequences with superior accuracy and quality. The main components of the system and their function are described, with special focus on measures for obtaining long, continuous lake sediment sections up to a current maximum length of 95 m. Examples of lake sediment profiles obtained with this system from different climatic zones are presented.  相似文献   

7.
A wide variety of scientific disciplines require representative samples of benthic sediment. As a result, a large range of sampling devices have been developed, each best suited to a particular set of conditions. However, all sediment sampling devices have inherent design problems that affect the degree to which samples represent the intact sediment. These issues are summarised, and a new corer design (the LOG corer) is presented and discussed. The LOG corer is a remotely-operated light-weight gravity corer suitable for obtaining relatively undisturbed short sediment cores in soft lacustrine sediments.  相似文献   

8.
In order to compare two widely used piston-coring techniques, parallel cores were taken with both a Kullenberg and a Livingstone corer in the deepest part of Soppensee (25 m), a small eutrophic Swiss lake containing varved sediments. The cores were taken within a horizontal distance of 3 m and yield comparable stratigraphic records. Differences in millimetre-scale microstructure are attributed to primary sedimentation processes at the water/sediment interface. Sediment thin-sections, as well as sediment x-radiograph investigations, reveal no difference in microstructure that could unequivocally be attributed to one of the coring methods used. The differences in deposition are therefore thought to reflect the inherent variability of lacustrine sedimentation in Soppensee.Major differences in overall core recovery do occur, however, in organic-rich, highly porous sediments. These variations are primarily attributed to differential gas expansion. Actual sediment-accumulation rates can therefore only be correctly estimated if the sedimentary record can be constrained within a high-resolution temporal framework, e.g. by annual laminations.  相似文献   

9.
Subfossil chironomids in the surface sediments of five small and shallow Norwegian lakes were studied to determine the within-lake variability of fossil assemblages, changes in chironomid assemblages with respect to water depth, and the representativeness of single samples for the entire chironomid fauna of a lake. In each of the lakes studied, six short sediment cores in the deepest part of the lake basin and two littoral to deep-water transects of seven cores each were obtained using a gravity corer, and chironomid assemblages in the uppermost centimetre of sediment were analysed. In three of the five lakes, chironomid concentrations were highest in the deepest parts of the lake basins. In the remaining two lakes, concentrations were either very variable or, in a lake with clear indications of anoxia in the bottom waters, highest at intermediate water depth. Chironomid assemblages tended to be dominated by the same taxa within a lake basin. However, in each of the lakes studied there was a clear and statistically significant shift in chironomid assemblages with respect to water depth. The organic content of the sediments was statistically significant in explaining the variance in the chironomid assemblages only in lakes where organic matter content was closely related to water depth. Only a few chironomid taxa were restricted to the shallowest parts of the lake basins, whereas a number of chironomids were found exclusively in deep-water sediments. Chironomid head capsules of running water taxa and simuliid remains were generally found in sediments close to lake tributaries and in the deepest parts of the lake basins. Although any individual sample contained only a part of the total subfossil chironomid fauna (21–63% of the total taxa per lake), chironomids dominant in any section of the study lakes were found in most of the transect and mid-lake samples.  相似文献   

10.
The Baikal sediment box corer represents an innovation in design of the closing mechanism and involves a single, thin, and flexible stainless steel blade rather than closing jaws. With this light-weight box corer only a relatively small cross-sectional area is offered to the sediment, allowing easier penetration. The closure blade is propelled smoothly across the base of the box in a downwardly convex path by a set of constant force springs. Depending on choice of blade-release mechanisms, the corer can either be triggered immediately as the support cable slackens, after a delay of a few seconds, or immediately upon retrieval. The box corer has been used successfully to sample poorly consolidated sediments in freshwater lakes over a depth range of 8 to 1624 m. Cores of deep-water surficial sediment from Lake Baikal were tested for representativity and replicability by profiling natural and artificial radioisotopes and lithostratigraphic features.  相似文献   

11.
Nesje, A., Søgnen, K., Elgersma, A. &; Dahl, S. O. 1987. A piston corer for lake sediments. Norsk geogr. Tidsskr. Vol. 41, 123-125. Oslo. ISSN 0029-1952.

A modified piston sampler for lake sediments is described. The sampler employs the piston of the Livingstone device in conjunction with the use of a falling weight to penetrate the sampling tube into compact sediments without the use of casing. The sampler can core up to 6 metres of minerogenic and organic sediments and can easily be handled by 2–3 persons both from ice and rafts in lakes with water depths to at least 20–30 m.  相似文献   

12.
We describe an improved piston corer, the Mk II, for sampling soft sediments for ancient DNA analysis. The original Mk I model, designed to minimize contamination and successfully used in New Zealand, was subsequently deployed on Easter Island where three problems arose. Two of these problems related to sediment and water entering the core barrel and contaminating samples. The other difficulty was that plant material accumulated ahead of the piston and blocked the corer aperture. Design improvements were made to the prototype model and eliminated these problems.  相似文献   

13.
Loss-on-ignition analysis of Quaternary lake sediments provides an inexpensive and easy way to investigate past environmental changes. The mass loss on ignition at 550 °C (LOI) from lake sediment cores may vary because of temporal changes in: (1) sediment composition controlled by factors such as productivity, inorganic inputs, and decomposition; and (2) the patterns of sediment accumulation controlled by factors such as basin morphology and water level. Climatic changes can alter both. Here, modern surface samples and transects of sediment cores, collected across small (<10 ha), shallow (<4 m) lakes in the northeastern United States, show that LOI varies little (2–5%) across the deep portions of these small lakes at a given time. Large changes in LOI occur only at the transition into the littoral (shallow) zone. LOI variations in sediment cores that exceed 2–5%, therefore, appear to represent meaningful environmental changes. However, because of the many possible controls, changes in the LOI of a single core are often hard to interpret. Multiple cores increase the interpretability. At lakes studied here, similar LOI trends among several cores confirm that some LOI changes resulted from basin-wide shifts in sediment composition. Differences among cores, however, developed during the early- and mid-Holocene and indicate that the edge of the littoral zone moved towards the centers of the lakes during two periods of low lake levels, at ca. 11 000–8000 and ca. 5400–3000 cal yr B.P. The basin-wide balance of sediment sources controlled the LOI from deep-water sediments, but sedimentation patterns, which changed as lake levels changed, were also important. LOI differences among cores may therefore help identify past lake-level changes in other lakes.  相似文献   

14.
Lacustrine sediment sampling equipment, such as lightweight gravity corers, are limited in operation to relatively shallow water environments (less than 50 m). This is because the ability to control such equipment becomes difficult with increasing water depth. In addition, adverse weather conditions, wind and waves etc. can further limit the success of such operations. The deep water equipment is designed to attach to gravity corers of the Kajak Brinkhurst type, and extend the operating depth by eliminating the need for direct control from the surface. The equipment stabilizes the corer on entry into the sediment and automatically closes the top of the core tube after driving is completed. No changes are required to be made to the corer whose mechanical operation remains the same.  相似文献   

15.
A monopod and piston coring apparatus for coring water-saturated sediment is described. The lightweight apparatus can be used from either an ice platform or the ground surface and can be transported by aircraft. Using a piston, core recovery is increased to better than 90%, and depending upon the monopod height, 7 m-long cores can be obtained. Tips for coring in freezing temperatures are also given.  相似文献   

16.
High resolution sediment physical properties, measured on gravity and piston cores collected during cruises to Lake Winnipeg, include bulk density, acoustic velocity, magnetic susceptibility, shear strength and colour reflectance. The high resolution data are used here to construct complete stratigraphic (composite) sections of Lake Winnipeg sediments from a series of individual, discontinuous cores for the North and South Basins. These composite sections are used to evaluate basin-wide glacial and post-glacial depositional histories and to compare the northern and southern basin histories. In addition, these sections provide a baseline depth reference for interpretation of the biostratigraphy, paleomagnetic record and rock magnetic stratigraphy. Some of the data (density and shear strength) are also be used to estimate sediment stress history for the two major lithostratigraphic units and their variations across the basin.  相似文献   

17.
We investigated whether techniques developed to evaluate qualitative lake-level changes in the temperate zone can be used in sub-arctic and arctic Alaska. We focused on aquatic pollen records and sediment properties (loss-on-ignition and magnetic susceptibility) from centrally-located sediment-surface samples and cores, as these are the most commonly reported data in the literature. Modern aquatic pollen values are generally low (< 5%) and may be zero, even in lakes with abundant aquatic macrophytes. Greater diversity and higher values of aquatic pollen are likely at depths < 5 m, but pollen is found in depths up to 15 m. It is absent at depths > 20 m. Spores of Isoetes and Equisetum and Pediastrum cell-nets, when present, tend to be widely distributed, even in deep water. At Birch Lake, interior Alaska, trends in aquatic taxa and sediment characteristics for the last ca. 12,000 14C yrs recorded in a single, deep-water core reflect the same water-level changes as do transect-based lake-level reconstructions - if modern distributional characteristics of pollen and spores are taken into account. The lake rose from extremely low levels at ca. 12,000 14C yr B.P. After a period of fluctuation, it rose to a relatively high level by ca. 8000 14C yr B.P. and then stabilized. A preliminary survey of aquatic pollen trends from other lake-sediment records suggests that the period ca. 11,000-8000 14C yr B.P. may have seen relatively low lake levels in north-western and interior Alaska and high levels thereafter. Changes in aquatic pollen and sediments are evident in north-eastern interior lakes at the same time, but they are more difficult to interpret. Aquatic pollen productivity in Alaskan lakes may partly depend on factors other than water depth (e.g. temperature, pH, nutrient status, or length of the ice-free season). An Alaska-wide reconstruction of late-Quaternary lake levels based on extant single-core data would be best done after further study of contributing factors that may control sediment properties and aquatic pollen distribution.  相似文献   

18.
As many as 2500 interdune lakes lie within the Nebraska Sand Hills, a 50000 km stabilized sand sea. The few published data on cores from these lakes indicate they are typically underlain by less than two m of Holocene lacustrine sediments. However, three lakes in the southwestern Sand Hills, Swan, Blue, and Crescent, contain anomalously thick marsh (peat) and lacustrine (gyttja) sediments. Swan Lake basin contains as much as 8 m of peat, which was deposited between about 9000 and 3300 years ago. This peat is conformably overlain by as much as 10.5 m of gyttja. The sediment record in Blue lake, which is 3 km downgradient from Swan lake, dates back to only about 6000 years ago. Less than two m of peat, which was deposited from 6000 to 5000 years ago, is overlain by 12 m of gyttja deposited in the last 4300 years. Crescent Lake basin, one km downgradient from Blue Lake, has a similar sediment history except for a lack of known peat deposits. Recently, a 8-km long segment of a paleovalley was documented running beneath the three lakes and connecting to the head of Blue Creek Valley. Blockage of this paleovalley by dune sand during two arid intervals, one shortly before 10500 yr BP and one in the mid-Holocene, has resulted in a 25 m rise in the regional water table. This made possible the deposition of organic-rich sediment in all three lakes. Although these lakes, especially Swan, would seem ideal places to look for a nearly complete record of Holocene climatic fluctuations, the paleoclimatic record is confounded by the effect dune dams have on the water table. In Swan Lake, the abrupt conversion from marsh to lacustrine deposition 3300 years ago does not simply record the change to a wetter regional climate; it reflects the complex local hydrologic changes surrounding the emplacement and sealing of dune dams, as well as regional climate.  相似文献   

19.
In wind-stressed and unstratified Lough Neagh low aspect ratio sediment traps, which allow for internal resuspension, collected sediment at a rate close to the natural sediment accumulation rate inferred from dated sediment cores. High aspect ratio traps grossly overestimated the natural rate of sediment accumulation as did, but to a lesser extent, burial rate measurements on an artificial stratigraphic marker. The former type of trap deployed over long exposure periods is therefore recommended as providing the best estimate of net downward particle flux in lakes such as Lough Neagh. However, it is emphasized that all sediment trap types, when operated in shallow turbulent lakes, will to a greater or lesser extent be contaminated by secondary or redeposited material and so will not provide a direct measure of primary sedimentation. Use of tube traps in lakes such as Lough Neagh should not, however, be discounted since they can provide a record of the quality of sedimenting material through time.  相似文献   

20.
The effect of within-lake diatom assemblages variability on sample representativity and its subsequent impact on between-lake comparisons were addressed in three environmentally heterogeneous shallow lakes from the Argentinean Pampas. Surface sediment samples were collected from the open waters and the highly vegetated littoral areas on a seasonal basis and analyzed for diatom assemblages composition. Within-lake variability was assessed by comparing the Bray Curtis distances between original data and the Monte Carlo-simulated average assemblages composition through non-metric multidimensional scaling (NMDS). Diatom assemblages showed a high variability in composition, evidencing large dispersions of samples around the centroid in NMDS plots. Permutational multivariate analysis of variance tests signaled significant differences in average composition between the three lakes, related mainly to their differences in conductivity and depth. Representativity of original samples was assessed through principal coordinates analyses ordinations of the three lakes, being samples lying in the overlapping areas of the plot classified as poor representatives of between-lake differences. Several samples, both from littoral and open waters, were classified as poor representatives through this method. Simulation allowed us to evaluate the effect of sample replication on improving between-lake comparisons, and showed that collecting two littoral and two open-water samples allowed us to faithfully capture differences in average composition among the three lakes. Hence, the results suggest that using a single sample to estimate diatom assemblages composition in these lakes should be avoided, as it fails to capture between-lake differences, leading to biases in compositional comparisons among lakes and regions. Consequently, including multiple samples from each lake when constructing calibration sets would be the best option to obtain reliable paleoenvironmental reconstructions from single sediment cores in these environmentally heterogeneous shallow lakes.  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