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821.
Donald R. Whitehead Stephen T. Jackson Mark C. Sheehan Barbara W. Leyden 《Quaternary Research》1982,17(2):241-257
The Christensen Mastodon Site, located in central Indiana, contains a rich assemblage of vertebrates (including mastodon, caribou, and giant beaver), invertebrates, and plant macrofossils in situ in lake and bog sediments of late-glacial age. Studies of pollen and plant macrofossils suggest the existence of open, white spruce-dominated boreal forests from >; 14,000 yr B.P. to ca. 13,000 yr B.P. The regional decline of spruce, local occurrence of black spruce, white spruce, and larch, immigration of many hardwood taxa (e.g., ash, oak, elm), and the initiation of bog development are recorded beginning about 13,000 yr B.P. Recent reconstructions of late-glacial and early postglacial vegetational changes provide a context for understanding the disappearance of mastodons. The dramatic and rapid restriction of boreal forests along the retreating ice margin from 11,000 to 9000 yr B.P. may have caused a substantial reduction of mastodon populations. A diminished population would be more susceptible to small-scale, stochastic events such as short-term extremes of weather, outbreaks of disease, or predation pressure from paleoindian hunters. 相似文献
822.
Mark A.S. McMenamin David J. Blunt Keith A. Kvenvolden Scott E. Miller Leslie F. Marcus Richard R. Pardi 《Quaternary Research》1982,18(2):174-183
Low aspartic acid d:l ratios and modern collagenlike concentration values indicate that amino acids in bones from the Rancho La Brea asphalt deposit, Los Angeles, California are better preserved than amino acids in bones of equivalent age that have not been preserved in asphalt. Amino acids were recovered from 10 Rancho La Brea bone samples which range in age from less than 200 to greater than 36,000 yr. The calibrated rates of aspartic acid racemization range from 2.1 to 5.0 × 10?6yr?1. Although this wide range of rate constants decreases the level of confidence for age estimates, use of the larger rate constant of 5.0 × 10?6yr?1 provides minimum age estimates which fit the known stratigraphic and chronologic records of the Rancho La Brea deposits. 相似文献
823.
Replicate groups of larvae from each of three female grass shrimp,Palaemonetes pugio, collected at each of six sites within Chesapeake Bay were reared in the laboratory to metamorphosis at 25°C, 25 ppt salinity. They were provided with approximately 300Artemia nauplii/larva/day. Sites were selected to provide both suspected Kepone® contaminated and non-contamined shrimp populations. A sample of eggs, adult females with and without eggs, and newly hatched larvae from each site were analyzed by Virginia Institute of Marine Science for Kepone as were postlarvae obtained through laboratory rearings. Females and eggs from the James River had highest concentrations of Kepone (0.63 and 0.47 ppm, respectively) with samples from the Lafayette River at the mouth of the James River having the second highest levels (0.04 and 0.4 ppm). Concentrations in ovigerous females and eggs in the populations most distant from the James (mouth of Potomac and upper Potomac River) were at or below the limits of detectability (0.001 to 0.015 ppm). In laboratory reared postlarvae from each site, concentrations of Kepone were undetectable. Despite these apparent differences in Kepone concentrations in the various populations we found no significant differences in larval survival, larval duration, or length of post-larvae attributable to site of origin. Highly significant differences in larval duration and size of post-larvae were attributable to parental variation. 相似文献
824.
825.
Mark A. Flock 《Quaternary Research》1983,20(2):165-176
The Savanna Terrace, composed of alternating red and gray clayey sediments of late Wisconsinan age, can be found in five states along the upper Mississippi valley from Pepin County, Wisconsin, to Jackson County, Illinois. The terrace is the highest glaciofluvial-lacustrine deposit without a loess cover in the upper Mississippi valley. Chemical, physical, and mineralogical data show that two different sources provided sediment. The red clay is believed to have come from Lake Superior sources, while the gray clay is believed to have come from sources farther west. Large-scale flood events from glacial Lakes Agassiz, Grantsburg, and Superior were probably the main contributors of the sediments. The red clay in the terrace is similar in composition to red glaciolacustrine sediment found in eastern and northern Wisconsin. It also is mineralogically similar to the Hinckley Sandstone and the Fond du Lac Formation, which occur under and around Lake Superior. Radiocarbon dates obtained from the lower Illinois valley indicate that the terrace sediments were deposited sometime between about 13,100 and 9500 yr ago. Soils developed on the terrace are variable in their physical, chemical, and mineralogical properties, which reflect the composition of the clayey sediments. 相似文献
826.
Mark F. Deering Eileen T. Mohr Bruce F. Sypniewski Ernest H. Carlson 《Journal of Geochemical Exploration》1983,19(1-3)
A cluster of minor Mississippi Valley-type deposits occurs in northwestern Ohio. The district, which forms a northeasterly trending belt that cuts across the Findlay Arch, extends from the Indiana border to the Lake Erie Islands. The minerals of the deposits — chiefly celestite, fluorite and sphalerite with lesser amounts of barite and galena — show variation in both geographic and stratigraphic distribution. Dolomites of Middle Silurian to Middle Devonian age, which are the host rocks, also form an important aquifer system. The deposits are of interest because they might be indicators of economic mineralization at depth.Through a reconnaissance study, one-hundred ground-water samples from shallow wells (less than 50 m deep) were collected across an area of approximately 19 000 km2. Recharge takes place in the southern part of the area while ground-water flow is northward towards Lake Erie. The majority of the samples are high in sulfate with the source being evaporites within the carbonate sequence. The remainder of the water samples are rich in bicarbonate. Trend surface maps for the major constituents indicate that the ground-water chemistry for the region is established chiefly by the lithology and the flow system. Trend surface maps for F−, Sr and Ba reflect the geographic distribution of the minerals in the deposits. Correlations are weakened, however, due to the influence of geochemical barriers such as SO42− on Ba and Sr, and Ca on F−. The map for Pb follows the trends of the major constituents instead of the mineralization.In a detailed study across northwestern Sandusky County, which lies near the center of the district, 46 samples were collected in an area of 78 km2. Trend surface maps for Ca, Mg, SO42− and total dissolved solids reflect the chemistry of the bedrock and display concentrations that increase along the local flow path. Maps for F−, Sr and Ba correlate with mineralization in the vicinity, the first of these displaying a local trend and the last two correlating with regional trends. 相似文献
827.
Bonacci S Browne MA Dissanayake A Hagger JA Corsi I Focardi S Galloway TS 《Marine pollution bulletin》2004,49(5-6):445-455
Marine environments are continuously being threatened by a large number of xenobiotics from anthropogenic sources. Even in sparsely populated and relatively pristine areas, such as Antarctica, hazardous chemicals can pose a serious environmental problem. The main aims of the present study were to (1) validate and optimize an analytical technique utilizing a microtitre-plate photometer to ascertain background levels of esterase activities in the Antarctic bivalve Adamussium colbecki, (2) carry out in situ monitoring of esterase activities to assess any potential environmental impacts of the Italian Scientific Antarctic Base "Terra Nova Bay" on the surrounding marine area. Results showed the presence of organophosphorous-sensitive cholinesterase (ChE) and carboxylesterase (CbE) activities in the gills of A. colbecki and optimal assay conditions were comparable with those found for bivalve species from temperate areas. A higher sensitivity of ChE versus acetylthiocholine activity in A. colbecki to chlorpyrifos compared to species from temperate areas may also be inferred. The in situ study indicated no differences in the environmental quality of the three study sites located around the Italian Base. 相似文献
828.
Inputs of dissolved and particulate 226Ra to lakes and implications for 210Pb dating recent sediments 总被引:8,自引:6,他引:2
Gamma spectroscopy was used to measure radioisotope (210Pb, 226Ra, 137Cs) activities in sediment cores from 20 lakes and a wetland in Florida, USA. Nine profiles display relatively low (<5 dpm g–1) and constant 226Ra activities, whereas 12 show high (>5 dpm g–1) and variable 226Ra activities. In the latter group, most display up-core increases in activity. Upper sediments from two lakes (Round and Rowell) possess very high (>20 dpm g–1) 226Ra activities that exceed total 210Pb activities, clearly illustrating disequlibrium between 226Ra and supported 210Pb. Supported 210Pb activity is generally thought to come from in situ, 226Ra-containing detrital mineral particles, and is typically assumed to be in secular equilibrium with 226Ra activity. Since 1966, Round Lake has been augmented hydrologically with 226Ra-rich (6.2 dpm L–1) groundwater pumped from the local deep aquifer. Adsorption of dissolved 226Ra to recent Round Lake sediments probably accounts for the high measured 226Ra activities and the pronounced disequilibrium between 226Ra and supported 210Pb in topmost deposits. We suspect that many Florida waterbodies receive some 226Ra-rich runoff and seepage from groundwater pumped for irrigation, residential use, industrial applications, and mining. This may account for up-core increases in 226Ra activity measured in sediment cores from some Florida lakes. Significant groundwater pumping began within the last century, and there has been insufficient time for supported 210Pb to come into equilibrium with adsorbed 226Ra in uppermost deposits. Input of 226Ra-rich groundwater to lakes may occur in any geographic region where local bedrock contains 238U and its daughters. When dissolved 226Ra adsorbs to recent sediments, it complicates accurate estimation of supported 210Pb activity, and confounds calculation of unsupported 210Pb activity that is used in dating models. 相似文献
829.
Microbial mercury (Hg) methylation and methylmercury (MeHg) degradation processes were examined using radiolabled model Hg
compounds in San Francisco Bay-Delta surface sediments during three seasonal periods: late winter, spring, and fall. Strong
seasonal and spatial differences were evident for both processes. MeHg production rates were positively correlated with microbial
sulfate reduction rates during late winter only. MeHg production potential was also greatest during this period and decreased
during spring and fall. This temporal trend was related both to an increase in gross MeHg degradation, driven by increasing
temperature, and to a build-up in pore water sulfide and solid phase reduced sulfur driven by increased sulfate reduction
during the warmer seasons. MeHg production decreased sharply with depth at two of three sites, both of which exhibited a corresponding
increase in reduced sulfur compounds with depth. One site that was comparatively oxidized and alkaline exhibited little propensity
for net MeHg production. These results support the hypothesis that net MeHg production is greatest when and where gross MeHg
degradation rates are low and dissolved and solid phase reduced sulfur concentrations are low. 相似文献
830.
Combining Sample and Census Data in Small Area Estimates: Iterative Proportional Fitting with Standard Software* 总被引:1,自引:0,他引:1
The combination of detailed sample data with less detailed but fully enumerated marginal subtotals is the focus of a wide range of research. In this article we advocate careful modeling of sample data, followed by Iterative Proportional Fitting (IPF). The modeling aims to estimate accurately the interaction or odds ratios of complex tables, which is information not contained in the marginal subtotals. IPF ensures consistency with the subtotals. We advance this work in three practical ways. First, we show that detailed small‐area estimates of both counts and proportional distributions usually gain accuracy by combining data for larger areas containing the small areas, and we illustrate the multilevel framework to achieve these estimates. Second, we find that a general classification or socioeconomic typology of the small areas is even more associated with the within‐area interactions than is membership of the larger area. Third, we show how the Statistical Package for the Social Sciences (SPSS) can be used for IPF in any number of dimensions and with any structure of constraining marginal subtotals. Throughout, we use an example taken from the 1991 U.K. Census. These data allow us to evaluate various methods combining 100 percent tabulations and the Samples of Anonymised Records. 相似文献