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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.
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   

4.
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   

5.
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   

6.
Optical and geochemical techniques were applied to sedimentary organic matter from the profundal area of the Eocene Lake Prinz von Hessen, which formed in a pull-apart basin on the Sprendlinger Horst, near Darmstadt, Germany. Variations in total sulphur content (S tot) and total organic carbon content (TOC), hydrogen index (HI), oxygen index (OI) and 13C values of the organic matter were used to reconstruct the lakes filling history. Following an initial rapid deepening phase, open lake conditions developed with HI reaching more than 500 mg HC/g TOC and TOC values up to 40%. The productivity of the lake was probably high and organic matter preservation was enhanced by a stratified water column. As the lake began to fill with sediment and became shallower, TOC and HI values declined, as the lake water was better oxygenated and preservation conditions declined. 13C values between –31 and –27 are controlled by the mixing of aquatic (algae and microbial mats) and terrigenous organic matter (wood, spores, pollen and cuticles). Following a rapid drop in lake level, shallow lake conditions alternated with swamp deposits (lignites) in the basin center. The organic matter preserved during this stage is strictly terrigenous in nature and experienced oxic degradation (HI 100 mg HC/g TOC). 13C values between –26 and –24 are typical for Eocene terrigenous matter. The inferred lake level fluctuations are interpreted to have been controlled by tectonic as well as climatic processes.  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.
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   

9.
Southeastern Missouri has been a major Pb mining region since 1720 AD. Missouri mines produce a Pb ore that has a distinctive elevated 206Pb/207Pb isotopic ratio (>1.30) that is easily recognized in Pb-contaminated sediments. Five 210Pb dated sediment cores from Horseshoe Lake, Madison County, Illinois were examined to reconstruct Pb-contamination of the site from southeastern Missouri mines and from a local Pb smelter located adjacent to the lake. Pb concentrations increased in the cores from 5 ppm in the early 1800s to approximately 350 ppm in the late 1940s and 1950s. Pb concentrations in recently deposited sediment range from 100 to 300 ppm depending on the location within the lake. Throughout the 1800s and early 1900s the 206Pb/207Pb ratios in the sediment cores increased indicating contamination from southeastern Missouri (mean = 1.243). After the local smelter began recycling lead-acid storage batteries in the 1950s, the 206Pb/207Pb ratio declined (mean = 1.224) suggesting contamination of Horseshoe Lake with Pb from sources elsewhere around the world. The results of this study demonstrate how isotopic ratios of Pb can be used to reconstruct historical anthropogenic contamination.  相似文献   

10.
Sediments of the marl lake Malham Tarn located in NW Englandpreserve an environmental record since 12 Ka. Eight Holocene pollen zones wereidentified, and the 13C of total organic carbon (TOC) showsthree stratigraphic divisions. The basal clay unit and overlayingsand/clay/marl unit have 13C of –24which decreases at the base of the principal marl unit to a mean value around–30, whilst the topmost black marl unit 13Cincreases to –28 at the surface. Representative samples of theseunits were selected for analysis of n-alkanes andn-fatty acids and their 13C.Samples of modern Chara and peat were analysed forcomparison. The clay unit has a minor contribution of redeposited matureorganic matter and autochthonous algae, the marl unit a high contribution ofChara, and the dark marl unit has a high contribution fromhigher plants. Compound-specific 13C revealssystematic differences between alkanes and fatty acids of different chainlength. The major shift in 13C in the short and medium chainfatty acids are probably due to the decreasing influence of carbonate rockflour as source of DIC. The major shift in 13C in the longchain n-fatty acids andn-alkanes could reflect the lower atmosphericCO2 concentration at Last Glacial. The negative shift of short chainfatty acids in organic rich dark marls reflects introduction of detrital peatinto the lake. The 13C results show a dramatic change fromdominance of autochthonous plus eroded sources up to Pollen Zone IV, then slowcolonisation of the hinterland by higher plants, followed by constantChara contributions throughout the deposition of the marl,and a further increase of higher plant material after the rise in water levelin 1791.  相似文献   

11.
Reconnaissance 18O,, D, and 87Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of 87Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the 87Sr values. Based on the mutual variations of 18O and D, the lakes can be divided into three groups. In Group 1, both 18O and D values increase from spring to fall. The 18O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.  相似文献   

12.
Ultrafine clays (<0.1 m) from six East African basins show wide chemical variability that can be conveniently described as the octahedral cation index (OCI), calculated as octahedral element ratio Mg/(Al+Fe). Values range from dioctahedral clays with OCI < 0.1 (upland soils/detrital sources) to trioctahedral clays with OCI > 4 (saline, alkaline lake deposits). The association of Mg-rich clays with saline, alkaline conditions provides a paleochemical proxy in sedimentary contexts. The octahedral chemistry of central-basin clays from paleolake Olduvai (1.79 Ma) records two lake expansions, with an intervening lake contraction, over 18,000 years. The initial lake expansion correlates well with the marine record of increased African humidity, but the subsequent drying episode does not, suggesting that some high-frequency environmental oscillations in East Africa may occur independent of obliquity and precession control. The clay record correlates well with the fossiliferous and archaeologically rich stratigraphy of the lake margin, and suggests that fossils and artifacts accumulated on the exposed lake margin over <4000 years. The chemistry of clays is a potentially powerful tool to add to multi-proxy studies in basins with alkaline waters, particularly in stratigraphic intervals that lack pollen, calcareous, or siliceous microfossils.  相似文献   

13.
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   

14.
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   

15.
The paleohydrological evolution of several high altitude, saline lakes located in the southernmost Altiplano (El Peinado and San Francisco basins, Catamarca province, NW Argentina) was reconstructed applying sedimentological, geochemical and isotopic techniques. Several playa lakes from the San Francisco basin (26° 56 S; 68° 08 W, 3800-3900 m a.s.l.) show evidence of a recent raise in the watertable that led to modern deposition of carbonate and diatomaceous muds. A 2 m - long core from El Peinado Lake (26° 29 59 S, 68°05 32 W, 3820 m a.s.l.) consists of calcitic crusts (unit 3), overlaid by an alternation of macrophyte-rich and travertine clast- rich, laminated muds (unit 2), and topped by travertine facies (unit 1). This sedimentary sequence illustrates a paleohydrological evolution from a subaerial exposure (unit 3) to a high lake stand (unit 2), and a subsequent smaller decrease in lake level (unit 1). The 13Corganic matterrecord also reflects the lake transgression between units 3 and 2. Although there is a general positive correlation between 18Ocarbonate and salinity proxies (Na, Li and B content), the large data dispersion indicates that other factors besides evaporation effects control chemical and isotopic composition of lakewater. Consequently, the oxygen isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO2 during groundwater discharge can explain the enriched 13C values for primary carbonates precipitated. The carbon budget in these high altitude, saline lakes seems to be controlled by physical rather than biological processes.The Altiplano saline lakes contain records of environmental and climatic change, although accurate 14C dating of these lacustrine sediments is hindered by the scarcity of terrestrial organic material, and the large reservoir effects. Sedimentologic evidence, a 210Pb-based chronology, and a preliminary U/Th chronology indicate a very large reservoir effect in El Peinado, likely as a result of old groundwaters and large contributions of volcanic and geothermal 14C-free CO2 to the lake system. Alternative chronologies are needed to place these paleorecords in a reliable chronological framework. A period of increased water balance in the San Francisco basin ended at about 1660 ± 82 yr B.P. (calendar yr U/Th age), and would correlates with the humid phase between 3000 and 1800 yr B.P detected in other sites of the southern Altiplano. Both, 210Pb and preliminary U/Th dating favor a younger age for the paleohydrological changes in El Peinado. The arid period reflected by subaerial exposure and low lake levels in unit 3 would have ended with a large increase in effective moisture during the late 17th century. The increased lake level during deposition of unit 2 would represent the period between AD1650 - 1900, synchronous to the Little Ice Age. This chronological framework is coherent with other regional records that show an abrupt transition from more arid to more humid conditions in the early 17th century, and a change to modern conditions in the late 19th century. Although there are local differences, the Little Ice Age stands as a significant climatic event in the Andean Altiplano.  相似文献   

16.
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   

17.
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   

18.
During the late Wisconsin, glacial flour from alpine glaciers along the east side of the Cascade Range in southern Oregon was deposited in Upper Klamath Lake. Quantitative interpretation of magnetic properties and grain-size data of cored sediments from Caledonia Marsh on the west side of the lake provides a continuous record of the flux of glacial flour spanning the last 37 000 calendar years. For modeling purposes, the lake sediments from the 13-m core were divided into three sedimentary components defined from magnetic, geochemical, petrographic, and grain-size data. The components are (1) strongly magnetic, glacial flour made up of extremely fine-grained, fresh volcanic rock particles, (2) less magnetic lithic material made up of coarser, weathered volcanic detritus, and (3) non-magnetic biogenic material (largely biogenic silica). Quantitative interpretation is possible because there has been no significant postdepositional destruction or formation of magnetic minerals, nor alteration affecting grain-size distributions. Major steps involved in the interpretation include: (1) computation of biogenic and lithic components; (2) determination of magnetic properties and grain-size distributions of the non-glacial and glacial flour end-members; (3) computation of the contents of weathered and glacial flour components for each sample; (4) development of an age model based on the mass accumulation of the non-glacial lithic component; and (5) use of the age model and glacial flour contents to compute the flux of glacial flour. Comparison of the glacial flour record from Upper Klamath Lake to mapped glacial features suggests a nearly linear relation between flux of glacial flour and the extent of nearby glaciers. At 22 ka, following an extended period during which glaciers of limited size waxed and waned, late Wisconsin (Waban) glaciers began to grow, reaching their maximum extent at 19 ka. Glaciers remained near their maximum extent for 1000 years. During this period, lake sediments were made up of 80% glacial flour. The content of glacial flour decreased as the glaciers receded, and reached undetectable levels by 14 ka.  相似文献   

19.
Destruction of cultural resources has increased dramatically in the last quarter of the 20th century as a result of two forces: first, development and construction, and second, the astonishing explosion of demand for primitive and prehistoric art. On the cover: Rock art of the Mimbres Culture at the Pony Hills site in south-west New Mexico. Similar rock art nearby has been destroyed by pistol shots or chipped out and illegally sold. Photo by Andrew Gulliford, Public History Program, Middle Tennessee State University.For the purposes of Legacy, the term cultural resource refers to both archeological and architectural resources. For archaeology, it includes, but is not limited to, traditions, lifeways, cultural and religious practices, and other institutions to which a community, neighborhood, Native American tribe, or other group ascribes cultural significance, together with any artifacts and real property associated with such elements. For architecture, it includes, but is not limited to, buildings, sites, districts, structures, or objects, landscapes, and vistas. In addition, the term encompasses historic documents and relics.  相似文献   

20.
Lead analysis of sediment cores from seven Connecticut lakes   总被引：2，自引：0，他引：2  
Lead analyses were performed on sediment cores from seven Connecticut lakes by dilute acid leaching. The absolute concentrations of lead varied from core to core, but the lead profiles were similar in all of the cores, and each depicted a point where the concentration of lead sharply increased above background levels. The background lead concentrations observed in the bottom portions of the cores ranged from 12-54 g Pb/g sediment with a mean of 30 g Pb/g, and increased to a mean maximum of 310 g Pb/g. In six of the lakes significant increases above background levels were not noted until between 1924 and 1952, suggesting that, at least for Connecticut lakes, the point in a lake sediment core where the concentration of lead began to increase was after ca. 1924, and on average about 1930. Lead concentrations declined in the surface sediments of only three of the lakes, mostly since the 1980's, and presumably in response to the phasing out of leaded gasoline. However, significant declines in lead concentrations were not observed in the remaining cores, possibly due to increased motor vehicle use within the airshed of the lakes and/or changes in the export of lead to the lakes from their watersheds.  相似文献   

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