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1.
While palaeohydrological changes in non-outlet lakes provide a key proxy indicator of past climatic fluctuations, for lake systems which have been chemically insensitive, it is necessary to use indicators of water depth rather than salinity to reconstruct their hydro- climatic histories. A study of diatoms in the modern sediments of Sidi Ali, a non-outlet lake in the Middle Atlas of Morocco, has shown a statistically significant correlation between water depth and the ratio of planktonic to littoral diatoms. This relationship is used to calibrate fossil diatom assemblages from a lake sediment core from the same lake to provide a quantitative index of water levels over the pastc. 6500 years. Palaeoecological evidence suggests that climatically induced hydrological variations have dominated the bulk of the mid-late Holocene lake sediment record, with significant human-induced catchment disturbance only occurring during the twentieth century. The pattern of water depth fluctuations suggests that the response time of the regional groundwater system to climatic forcing is <100 years.This is the third 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   

2.
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   

3.
Diatom assemblages in surficial sediments, sediment cores, sediment traps, and inflowing streams of perennially ice-covered Lake Hoare, South Victorialand, Antarctica were examined to determine the distribution of diatom taxa, and to ascertain if diatom species composition has changed over time. Lake Hoare is a closed-basin lake with an area of 1.8 km2, maximum depth of 34 m, and mean depth of 14 m, although lake level has been rising at a rate of 0.09 m yr-1 in recent decades. The lake has an unusual regime of sediment deposition: coarse grained sediments accumulate on the ice surface and are deposited episodically on the lake bottom. Benthic microbial mats are covered in situ by the coarse episodic deposits, and the new surfaces are recolonized. Ice cover prevents wind-induced mixing, creating the unique depositional environment in which sediment cores record the history of a particular site, rather than a lake-wide integration. Shallow-water (<1 m) diatom assemblages (Stauroneis anceps, Navicula molesta, Diadesmis contenta var. parallela, Navicula peraustralis) were distinct from mid-depth (4–16 m) assemblages (Diadesmis contenta, Luticola muticopsis fo. reducta, Stauroneis anceps, Diadesmis contenta var. parallela, Luticola murrayi) and deep-water (26–31 m) assemblages (Luticola murrayi, Luticola muticopsis fo. reducta, Navicula molesta). Analysis of a sediment core (30 cm long, from 11 m water depth) from Lake Hoare revealed two abrupt changes in diatom assemblages. The upper section of the sediment core contained the greatest biomass of benthic microbial mat, as well as the greatest total abundance and diversity of diatoms. Relative abundances of diatoms in this section are similar to the surficial samples from mid-depths. An intermediate zone contained less organic material and lower densities of diatoms. The bottom section of core contained the least amount of microbial mat and organic material, and the lowest density of diatoms. The dominant process influencing species composition and abundance of diatom assemblages in the benthic microbial mats is episodic deposition of coarse sediment from the ice surface.  相似文献   

4.
Transects of surface sediment samples were taken in 4 lakes from the Sylvania Wilderness Area, Upper Peninsula of Michigan. These surface samples were compared with diatom samples from a core taken in the Northwest basin of Crooked Lake, also from the Sylvania Wilderness Area. Weighted Averaging calibration was used to reconstruct lake depths in Crooked Lake using the diatom microfossils from the core and the surface samples to infer past lake depth. During the early Holocene the lake was dominated by planktonic species and diatom-inferred water depth was large – approx. 13 m. At about 6700 BP inferred water depth was 2 m and samples were dominated by Fragilaria construens var. venter – a species characteristic of shallow parts of the surface sample transects. From 6700 to 5000 BP reconstructed water level was at its shallowest. From 5000 to 3000 BP it increased. This rise in water level was marked by increasing abundances of Aulacoseira ambigua and occurred at the same time increasing percentages of hemlock pollen indicate increasing available moisture. Modern water depth was reached about 3000 BP. The water level changes at Crooked Lake are consistent with regional climate changes in the Upper Midwest during the Holocene. The lake was shallowest during the mid-Holocene warm period documented by other investigators. It deepened as the Midwestern climate became cooler and wetter during the late Holocene.  相似文献   

5.
Aquatic invertebrates are intrinsically capable of rapid and sensitive response to changes in their lacustrine habitat. Fossil invertebrate assemblages preserved in the sediments of a climate-sensitive lake can thus produce high-resolution proxy records of past climate. In shallow lakes, however, a potential conflict exists between the sensitivity of biota to frequent habitat change in their fluctuating environment and the increased probability of disturbance of selected proxy records by bioturbation and physical mixing of sediments. I investigated this problem with tropical-African aquatic invertebrate faunas in a paleolimnological sensitivity study that incorporates both the response of biota to short-term habitat change and the taphonomic integrity of fossil assemblages in a small, shallow, and hydrologically closed lacustrine basin. Analysis of chironomid, cladoceran, and ostracode remains in a210Pb-dated short core from Lake Oloidien (Kenya) indicates that habitat changes accompanying the late 19th- and 20th-century fluctuations in lake level (Z max range: 3 to 18 m) and salinity (conductivity range:c. 400 toc. 2000–4000 µS cm–1) were sufficient to drastically alter the composition of local benthic and planktonic invertebrate faunas. This response remained relatively unaffected by taphonomic phenomena during its incorporation into the sediment record. Results indicate that tropical-African aquatic invertebrate faunas in suitable climate-sensitive lakes are a valuable tool to resolve paleoclimatic fluctuations on a timescale of decades.This paper was presented at the VI Palaeolimnology Symposium, held at Canberra in April 1993. Dr. Mark Brenner guest edited this contribution.  相似文献   

6.
A model for the distribution of sulfur in the sediments of a lake in the Adirondack Mountains, New York, is developed to gain insight into the timing and magnitude of anthropogenic increases in sulfur loading. Surficial sulfur concentrations are about 3500 µg g–1, increase to 6000–7000 µg g–1 at a depth of about 6 cm, and then decrease downcore to background levels of about 2500 µg g–1. Sulfate concentrations are about 60–80 µM in the overlying water and decrease rapidly below the sediment-water interface. This reflects assimilatory reduction and/or dissimilatory reduction of sulfate and incorporation into the inorganic and/or organic solid-phase fractions of the sediment. A mathematical model is constructed assuming sulfur is incorporated into sediments by burial of detrital organic matter and by diffusion of sulfate from the overlying water with subsequent fixation in the sediment. Several historical scenarios of atmospheric sulfate loading rates were examined as model boundary conditions. Model results are compared with observed sedimentary sulfur profiles. The observed sediment sulfur profiles are best described using model boundary conditions showing increases in lake water sulfate concentrations since about 1940. Assuming as much as a decade of retention of sulfate within the terrestrial portion of the ecosystem, this suggests that significant increases in sulfate deposition rates began sometime after 1930 in this relatively remote area.  相似文献   

7.
Surface lake sediment was recovered from 57 lakes along an elevation gradient in the central, eastern Sierra Nevada of California. The surface sediment was analysed for subfossil chironomid remains in order to assess the modern distribution of chironomids in the region. The lakes sampled for the calibration dataset were between 2.0 and 40.0 m in depth, spanned an altitudinal gradient of 1360 m and a surface water temperature gradient of approximately 14 °C. Redundancy analysis (RDA) identified that five of the measured environmental variables – surface water temperature, elevation, depth, strontium, particulate organic carbon – accounted for a statistically significant amount of the variance in chironomid community composition. Quantitative transfer functions, based on weighted-averaging (WA), partial least squares (PLS) and weighted-averaging partial least squares (WA-PLS), were developed to estimate surface water temperature from the chironomid assemblages. The best model was a WA model with classical deshrinking, which had a relatively high coefficient of determination (r2 = 0.73), low root mean square error of prediction (RMSEP = 1.2 °C) and a low maximum bias (0.90 °C). The results from this study suggest that robust quantitative estimates of past surface water temperature can be derived from the application of these models to fossil chironomid assemblages preserved in late-Quaternary lake sediment in this region.  相似文献   

8.
During recent years, numerous studies dealing with Holocene lake level fluctuations have been conducted in Finnish Lapland. However, no quantification of lake level variations exists to date. Here, we applied a recently developed modern cladocera – lake depth transfer model to subfossil cladocerans analysed from three small and shallow (< 6 m) kettle-hole lakes in northwestern Finnish Lapland to provide estimates of the amplitudes of long-term lake-level changes in the region. The quantitative inferences were compared to pollen, charcoal and geochemical records from one of the study sites. The lake levels were inferred to be high during the early Holocene; they faced marked reduction up to 4–6 m in the mid-Holocene (≈7000–4000 cal yr BP), and rose again during the latter part of the Holocene. There is some indication of lowered lake levels around 1500 cal yr BP, but interpretation of such small-scale changes is hazardous due to large prediction errors in the initial cladoceran model. The overall pattern of the Holocene lake level variation generally followed the regional changes in climate humidity as reconstructed in previous studies by means of other sedimentary proxy indicators, such as pollen and oxygen isotopic compositions. We postulate that changes in winter precipitation may have had a greater influence on lake-levels than variations in summer precipitation or evaporation.  相似文献   

9.
Water level fluctuations affect the size of the pelagic zone relative to the size of littoral habitats, and thus may influence the relative abundance of remains from planktonic and littoral cladocerans in sediment. The application of this planktonic/littoral ratio for the reconstruction of past water level changes is discussed using examples of: (1) surficial profundal sediments from lakes of different water depths; (2) Holocene variation in a profundal sediment core; (3) horizontal variation in surficial sediments within a lake; and (4) long term variation in an inshore sediment core. The latter seemed to be the most promising application of this ratio. Maximum effects of water depth changes on the lake fauna are expected in the littoral zone. It is, however, difficult to read this effect directly from subfossil cladoceran and chironomid assemblages from inshore sediments as shown by a sediment profile from a site exposed to a long term decrease of water depth.  相似文献   

10.
Organic geochemical record of environmental changes in Lake Dianchi,China   总被引：1，自引：0，他引：1  
In order to investigate the natural ecosystem of Lake Dianchi and to assess its anthropogenic impacts, a stratigraphic study of bulk and molecular compositions of organic matter was conducted using a 63-cm long sediment core. The results show that two apparent environmental changes occurred during the evolution of Lake Dianchi: (1) the first change occurred in the 43–63 cm sediment depth, and was revealed by the amount and the composition of organic matter in the stage. Natural changes were possibly major factors responsible for triggering the environmental change, but the influence of human activities could not be excluded. Subsequently, the lake entered into a relatively stable and oligotrophic stage, which maintained until 20-cm sediment depth. (2) Eutrophication started in the upper 20 cm depth. Human activities became a major factor influencing environmental changes in this stage. Vertical profiles of various organic geochemical variables in the upper 20-cm sediments show evidence that primary productivity of the lake increased progressively and that the lake started eutrophic. Especially in the uppermost 10 cm, notable excursions to less negative δ13Corg and δ15Ntotal and high TOC concentrations have recorded an abrupt change in the lacustrine environment, suggesting that the lake entered a hypereutrophic stage. In addition, enhancement of αβ-hopanes reflects the contribution of fossil fuels to the lake sediments.  相似文献   

11.
We recovered a sediment core (DL04) from the depocenter of Dali Lake in central-eastern Inner Mongolia. The upper 8.5 m were analyzed at 1-cm intervals for grain-size distribution to partition the grain-size components and provide a high-resolution proxy record of Holocene lake level changes. Partitioning of three to six components, C1, C2, C3 through C6 from fine to coarse modes within the individual polymodal distributions, into overlapping lognormal distributions, was accomplished utilizing the method of lognormal distribution function fitting. Genetic analyses of the grain-size components suggest that two major components, C2 and C3, interpreted as offshore-suspension fine and medium-to-coarse silt, can serve as sediment proxies for past changes in the level of Dali Lake. Lower modal sizes of both C2 and C3 and greater C3 and lower C2 percentages reflect higher lake stands. The proxy data from DL04 core sediments span the last 12,000 years and indicate that Dali Lake experienced five stages during the Holocene. During the interval ca. 11,500–9,800 cal year BP, lake level was unstable, with drastic rises and falls. Following that interval, the lake level was marked by high stands between ca. 9,800 and 7,100 cal year BP. During the period from ca. 7,100 to 3,650 cal year BP, lake level maintained generally low stands, but displayed a slight tendency to rise. Subsequently, the lake level continued rising, but exhibited high-frequency, high-amplitude fluctuations until ca. 1,800 cal years ago. Since ca. 1,800 cal year BP, the lake has displayed a gradual lowering trend with frequent fluctuations.  相似文献   

12.
Sediment aggregates (“sedimentary pellets”) within the sedimentary record of Lake A (83°00′ N, 75°30′ W), Ellesmere Island, Canada, are used to construct a 1000 year proxy record of ice-cover extent and dynamics on this perennially ice-covered, High Arctic lake. These pellets are interpreted to form during fall or early winter when littoral sediment adheres to ice forming around the lake’s periphery or during summer through the development of anchor ice. The sediment likely collects in ice interstices and is concentrated in the upper ice layers through summer surface ice melt and winter basal ice growth. The pellets remain frozen in the ice until a summer or series of summers with reduced ice cover allows for their deposition across the lake basin. Sedimentary pellet frequency within multiple sediment cores is used to develop a chronology of ice-cover fluctuations. This proxy ice-cover record is largely corroborated by a record of unusual sedimentation in Lake A involving iron-rich, dark-orange to red laminae overlying more diffuse laminae with a lighter hue. This sediment sequence is hypothesized to represent years with reduced ice cover through increased chemocline ventilation and iron deposition. During the past millennium, the most notable period of inferred reduced ice cover is ca. 1891 AD to present. Another period of ice cover mobility is suggested ca. 1582–1774 AD, while persistent ice cover is inferred during the 1800s and prior to 1582 AD. The proxy ice-cover record corresponds well with most regional melt-season proxy temperature and paleoecological records, especially during the 1800s and 1900s.
Jessica D. TomkinsEmail:
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13.
生物硅在东南极湖泊古初级生产力研究中的应用   总被引：1，自引：0，他引：1       下载免费PDF全文
本文对南极中山站区的莫愁湖和大明湖沉积物中生物硅、有机质等环境指标进行了分析,并在对比研究的基础上,探讨了生物硅在重建东南极湖泊沉积生态环境记录研究中的应用.研究结果表明,大量保存较好的硅藻出现在东南极淡水湖泊沉积物中,生物硅含量在深度剖面上表现出显著波动,与有机质等生物地球化学指标具有一致的垂向变化趋势,表明它们具有...  相似文献   

14.
Hala Lake is located in the Qilian Mountains, Qinghai Province, China, at 4,078?m a.s.l. Its sediments contain an archive of climate and hydrologic changes during the Late Quaternary, as it is located close to the area influenced by the East-Asian summer monsoon and westerly-driven air masses. Sedimentation patterns and depositional conditions within the lake were investigated using eight sediment cores from different water depths, and this information was used to evaluate the feasibility of using a single core to reconstruct past climate and hydrological conditions. Long core H7, from the center of the lake (65?m water depth) and core H8 from a western, near-shore location (20?m water depth), were compared in detail using sediment composition and geochemical data (X-ray fluorescence, loss-on-ignition and CNS analysis). Age models were constructed using 17 AMS radiocarbon dates and indicate negligible reservoir error for sediments from the lake center and?~1,000?year errors for the near-shore sediment core. Cores H1?CH5 and HHLS21-1 revealed a sediment succession from sand and silty clay to laminated clay on the southern side of the lake. Undisturbed, finely laminated sediments were found at water depths???15?m. Core H5 (2.5?m long), from 31?m water depth, yielded abundant green algal mats mixed with clayey lake deposits and was difficult to interpret. Algae occurred between 25 and 32?m water depth and influenced the dissolved oxygen content of the stratified lake. Comparison of cores H7 and H8 yielded prominent mismatches for different time periods, which may, in part, be attributed to internal lacustrine processes, independent of climate influence. We thus conclude that data from a single sediment core may lead to different climate inferences. Common shifts among proxy data, however, showed that major climate shifts, of regional to global significance, can be tracked and allow reconstruction of lake level changes over the last 24,000?years. Results indicate advance of glaciers into the lake basin during the LGM, at which time the lake experienced lowest levels, 25?C50?m below present stage. Stepwise refilling began at ca. 16 kyr BP and reached the ?25?m level during the B?lling/Aller?d warm phase, ca. 13.5 kyr BP. A desiccation episode falls within the Younger Dryas, followed by a substantial lake level rise during the first millennium of the Holocene, a result of climate warming, which promoted glacier melt. By ca. 7.6 kyr BP, the lake reached a stable high stand similar to the present level, which persisted until ca. 6 kyr BP. Disturbed sediments in core H7 indicate a single mass flow that was most likely triggered by a major seismic event?~8.5 kyr BP. Subsequent lake development remains unclear as a consequence of data mismatches, but may indicate a general trend to deteriorating conditions and lake level lowstands at ca. 5.0?C4.2, 2.0 and 0.5 kyr BP.  相似文献   

15.
We analyzed lake sediment deposits and local hydrometric records to assess the potential for developing a high-resolution record of sediment delivery from the Rock Lake catchment, situated in the non-glacierized Front Ranges of the Rocky Mountains, Canada. Rhythmic couplets of silt–clay characterized the clastic sediments recovered from the deep central basin of the lake. Contemporary sediment yield to Rock Lake (10.7 ± 1.8 Mg km−2 year−1) is comparable to other studied Canadian Cordillera lakes that have sedimentary lithologies and absence of glacier cover, but distinct rhythmic deposition is relatively unique to this basin. Spatial patterns of deposition within the lake were assessed by correlating rhythmites between multiple sediment cores and by sub-bottom, acoustic profiling. Bracketed dates for a spatially continuous sequence of eight thick rhythmites were established by correlating laminations between core samples collected more than 30 years apart. We identified a consistent pattern between the rhythmite and hydrometric data series between 1975 and 2006 and determined that specific flooding events caused by summer rainstorms are associated with each of the eight thick rhythmites. We observed a good relationship between rhythmite thickness and total flood volume that exceeded a threshold discharge. Acoustic profiling showed that the lake could be a good candidate for longer-term proxy development. We discuss how some of the methods used in this study could benefit ongoing paleoenvironmental assessments based on lacustrine rhythmite series.  相似文献   

16.
Chironomids have been used extensively for reconstructing past temperatures from the late glacial chronozone but far less work has focused on their use as temperature proxies throughout the Holocene, and little work has been undertaken within the UK. Northern England does have many detailed palaeoclimate records, although the majority of these are reconstructions from ombrotrophic peat bogs, which yield a combined temperature and precipitation proxy record. A lake sediment core from Talkin Tarn, dating back 6000 years, was therefore analysed for chironomid remains in an attempt to produce a Holocene temperature reconstruction. Although chironomids have been shown to respond to air temperature by many modern training sets, it is also known that they can respond to other environmental factors. Pollen and loss-on-ignition analyses were therefore undertaken to ascertain whether the lake had been subjected to major environmental changes. Some anthropogenic changes in land use were detected, which may have affected the lake water chemistry and sediments, but they seem to have had little direct impact on the chironomid fauna for the majority of the record. Part of the geology of the catchment is limestone, which suggests that the lake may be buffered against any changes in pH. A chironomid-inferred mean July temperature transfer function from a Norwegian training set was applied to the chironomid data and produced a reconstruction with significant fluctuations throughout the later Holocene, which were associated with cold and warm stenotherms within the assemblages. The uppermost chironomid sample from the lake core (less than 100 years old) has a reconstructed temperature of 14.6 °C (± sample-specific error of 1.18 °C), which compares well with the contemporary mean July average of 14.8 °C. It is therefore concluded that chironomids can be used to reconstruct Holocene temperature, provided the site is well-buffered in relation to pH changes and can be shown not to have been influenced to any great extent by anthropogenic disturbance.  相似文献   

17.
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   

18.
A combined mineral magnetic and scaled chrysophyte study of lake sediments from Lake Lacawac and Lake Giles in northeastern Pennsylvania was conducted to determine the effects of land-use and sediment source changes on the variation of pH, conductivity, and alkalinity inferred from biotic changes. Ten 30–40 cm long gravity cores were collected from Lake Lacawac and three from Lake Giles. Isothermal remanent magnetizations (IRMs) were given to the lake sediments in a 1.3 T magnetic field to measure magnetic mineral concentration variations. IRM acquisition experiments were conducted to identify magnetic mineralogy. The bedrock, soils and a peat bog on the shores of Lake Lacawac were also sampled for magnetic analysis to determine possible lake sediment sources. The top 10 cm of sediment collected from Lakes Lacawac and Giles was two to four times more magnetic than deeper sediment. 210Pb dating suggests that this intensity increase commenced circa 1900. SEM images of magnetic extracts from the highly magnetic sediments indicates the presence of magnetic fly ash microspheres from fossil fuel burning electric power generation plants. The similarity in magnetic coercivity in the top 8 cm lake sediments and in the peat bog supports an atmospheric source for some of the magnetic minerals in the youngest lake sediments. The highly magnetic sediments also contain an antiferromagnetic mineral in two cores closest to Lake Lacawacs southeastern shore. This magnetic mineral is only present deep in the soil profile and would suggest erosion and significant land-use changes in the Lacawac watershed as another cause for the high magnetic intensities (concentrations) in the top 10 cm of the lake sediments. The most significant changes in the scaled chrysophyte flora occurred immediately above the 10 cm level and were used to infer a doubling of the specific conductivity between circa 1910 and 1929. These variations also support land-use changes in the Lacawac catchment at this time. A similar shift in the scaled chrysophte flora was not observed in the top of Lake Giles, however, distinct changes were found in the deeper sections of the core coupled with a smaller peak in magnetic concentration. Fourier analysis of the 210Pb-dated lake sediment magnetics indicates the presence of a 50 year period, low amplitude variation in the Lake Lacawac, Lake Giles, and Lake Waynewood (Lott et al., 1994) magnetic concentration records. After removal of the land-use/fly ash magnetic concentration peak by Gaussian filtering, the 50 year variation correlates strongly from lake to lake even though the lakes are in different watersheds separated by up to 30 km. When this magnetic variation is compared with Gaussian-filtered rainfall variations observed in New York City and Philadelphia over the past 120–250 years there is a strong correlation suggesting that magnetic concentration variations can record regional rainfall variations with an approximately 50 year period. This result indicates that magnetics could be used to document regional variations in climatic change.  相似文献   

19.
Records from lake sediment cores are critical for assessing the relative stability of climate and ecosystems over the Holocene. Duck Lake in south-central Lower Michigan, USA, was the focus of a study that identified how changes in the geochemical variables in lake sediments relate to variations in regional climate and local land use during the Holocene. More than 8.5 m of lacustrine sediment were recovered using Livingston and freeze corers and analyzed for organic carbon, inorganic (carbonate) carbon, total nitrogen, and trace metals. Repeating packages of sediment (1–10 cm thick) that grade from light (inorganic carbon-rich) to dark (organic carbon-rich) were found from the surface to a depth of about 8 m. Variations in the high-resolution gray scale data from core X-radiographs are highly correlated to the relative amount of inorganic carbon. Geochemical analyses of the upper 8.5 m of sediment revealed a wide range of values: 0.05–10.6% for inorganic carbon (i.e. 0.5–89% calcium carbonate) and 1.1–28% for organic carbon (i.e. 2.7–70% organic matter). Organic carbon to nitrogen ratios indicate that most of the sediment organic matter is produced within the lake. A core chronology based on eight AMS radiocarbon dates shows low sediment accumulation rates (0.05 cm/year) from 10,000 to 3,800 cal year BP and higher sediment accumulation rates (0.1–0.3 cm/year) from 3,800 cal year BP to present. We suggest that carbonate accumulates during relatively dry times, whereas organic matter accumulation dominates when nutrient input to the lake is enhanced by wetter climate. The Duck Lake core records a distinct low point in inorganic carbon deposition that may be related to the 8.2 ka cooling event now documented from several sites in North America. Spectral analysis of gray scale values shows significant ~200-year periodicities over the past 8,000 years, hypothesized to result from climate changes induced by solar forcing. Concentrations of trace metals (e.g. lead, iron, copper, zinc) indicate the onset of regional anthropogenic influence about 150 cal year BP.  相似文献   

20.
The main aim of this study was to reconstruct the history of water level fluctuations of a small dimictic lake during the Holocene based on the lithological and geochemical composition of sediments. We studied lithological and geochemical parameters in sediment cores extracted along a transect through Lake Juusa in southeastern Estonia. Analysis revealed that LOI550 and CaCO3 content in the accumulated sediment are dependent on water depth. At depths greater than 3 m, the relationship between organic carbon and CaCO3 is inverse, whereas no relationship exists at shallower depths. Lake-level changes are well depicted on the grain-size distribution profiles and macrofossil composition. An increase in the >‰315μm fraction appeared during the initial phases of water level fluctuations, followed by an increase in the 100–315μm fraction. The >‰36μm fraction dominated the silt fraction. Higher OC/N ratios in shallow water sediments reflect the dominance of macrophytes vegetation.  相似文献   

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