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1.
A small dataset comprising all temperature available data from reliable Horner plots from the Danish Central Graben was examined. Temperatures obtained by extrapolation using standard Horner plots were determined to be lower than true formation temperatures, as interpreted from DST data. Excellent agreement between true formation temperatures and Horner plot temperatures was achieved when the Horner plot temperatures (THP) were corrected upward by an amount proportional to the slope (A) of the Horner plot using the equation
where the temperatures and the slope are in degrees Celsius. The standard deviation of the error in the corrected Horner plot temperatures was 2.1°C, indicating that this method is consistent. Further studies using larger numbers of Horner plots from a variety of geographic areas should be carried out to test and refine the hypotheses presented here. Efforts also should be made to understand the causes of variability in slopes of Horner plots. 相似文献
2.
Oxygen isotope analysis of the adult ostracod Eucypris mareotica cultured at controlled temperatures (10, 15, and 19°C) was used to measure isotopic fractionation during shell calcification.
The ostracod shells that precipitated at experimental temperatures are almost in isotopic equilibrium with the culture water
as compared to the oxygen isotope fractionation of inorganic carbonates. Moreover, they had almost constant offsets from equilibrium
based on the oxygen isotope fractionation of inorganic carbonates. The δ18O values of ostracod shells from the 10°C cultures were higher than those of the 15 and 19°C cultures by 1.6 and 2.7‰, respectively.
The observed fractionations are shown by the regression equations:
*20c 10° \textC:d 1 8 \textO\textostracod = 1. 1 7+ 0. 5 7d 1 8 \textO\textwater 1 5° \textC:d 1 8 \textO\textostracod = - 0. 4 8+ 0. 6d 1 8 \textO\textwater 1 9° \textC:d 1 8 \textO\textostracod = - 1. 6+ 0. 6d 1 8 \textO\textwater \begin{array}{*{20}c} { 10^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = 1. 1 7+ 0. 5 7\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 5^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 0. 4 8+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 9^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 1. 6+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ \end{array} The fractionation factors (α) are slightly lower for the 15 and 19°C cultures, but slightly higher for the 10°C culture, as compared to inorganic carbonates
(O’Neil et al. in J Chem Phys 51:5547–5558, 1969). The oxygen fractionation factors of E. mareotica are very close to those of synthetic calcite formed in isotopic equilibrium. The ‘vital offsets’ of valve-δ18O for E. mareotica is so small that we can neglect its effect when using the δ18O of E. mareotica living in lake waters with high pH and salinity to reconstruct the paleoenvironment. The paleotemperature or paleoisotopic
composition of lake water interpreted from a core of lacustrine sediment may be closer to the true values when the δ18O data for E. mareotica are used. 相似文献
3.
The analysis of chironomid taxa and environmental datasets from 46 New Zealand lakes identified temperature (February mean air temperature) and lake production (chlorophyll a (Chl a)) as the main drivers of chironomid distribution. Temperature was the strongest driver of chironomid distribution and consequently produced the most robust inference models. We present two possible temperature transfer functions from this dataset. The most robust model (weighted averaging-partial least squares (WA-PLS), n = 36) was based on a dataset with the most productive (Chl a > 10 μg l−1) lakes removed. This model produced a coefficient of determination () of 0.77, and a root mean squared error of prediction (RMSEPjack) of 1.31°C. The Chl a transfer function (partial least squares (PLS), n = 37) was far less reliable, with an of 0.49 and an RMSEPjack of 0.46 Log10μg l−1. Both of these transfer functions could be improved by a revision of the taxonomy for the New Zealand chironomid taxa, particularly the genus Chironomus. The Chironomus morphotype was common in high altitude, cool, oligotrophic lakes and lowland, warm, eutrophic lakes. This could reflect the widespread distribution of one eurythermic species, or the collective distribution of a number of different Chironomus species with more limited tolerances. The Chl a transfer function could also be improved by inputting mean Chl a values into the inference model rather than the spot measurements that were available for this study. 相似文献
4.
The purpose of this project was to develop and test a methodology for determining the likelihood that mineral resource location
records from two nationwide mineral resource information databases represent the same site. The long-term goal is to create
a comprehensive database by merging the Mineral Resource Data System (MRDS) of the U.S. Geological Survey, and the Mineral
Availability System/Mineral Industry Location System (MAS/MILS) of the U.S. Bureau of Mines (now part of the Geological Survey).
Part of that process involves linking records for the same site from each database. Match probabilities were estimated using
a logistic regression of mineral resource location attributes, derived from known matched (cross-referenced) and known unmatched
randomly sampled mineral site pairs from within the conterminous United States (n=10,000). Model accuracy was assessed using a randomly sampled test dataset, not used in logistic model development (n=4,000). Probability distributions were similar between the development and test datasets. The overall agreement beyond chance
was good for the test data set
using the kappa statistic. Classification accuracy was 89.6% for known matched site pairs and 84.0% for known unmatched site
pairs based on a probability threshold of 0.50 for a match. Distributions of attributes were similar between the development
and test datasets. This classification method is a viable approach for estimating match probabilities between database records. 相似文献
5.
The structure and function of many Korean ecosystems have been rapidly modified since the 1960s when industrialization of the nation began. Ulsan City was the first in Korea to develop into a major industrial complex. To assess anthropogenic impacts on ecosystems surrounding Ulsan, sediment cores were collected from Mujechi-neup (bog) and Sanggae reservoir of Ulsan, and these cores were 210Pb dated using the CRS model. Physical and chemical characteristics and pollen were analyzed, and the rates of sediment accumulation were calculated. Unsupported 210Pb inventories in Mujechi-neup and the Sanggae reservoir were 18.04 and 16.53 pCi cm–2, and the corresponding 210Pb fluxes were 0.56 and 0.52 pCi cm–2 yr–1, respectively. The overall accumulation rate of dry matter was 0.26 kg m–2 yr–1 since 1852 (14 cm in depth) in Mujechi-neup. In the Sanggae reservoir, the accumulation rates of dry matter were increased from 2.1 in 1965 to 6.0 kg m–2 yr–1 in 1999. Pollen analysis revealed that three pollen zones existed in Mujechi-neup; a Pinus pollen-dominated zone from 0 to 5 cm in depth (1974 year), an Alnus pollen-dominated zone from 5 to 15 cm in depth (1827 year), and a Quercus pollen-dominated zone below 15 cm in depth. The shift from an Alnus dominated zone to a Pinus dominated zone was related to the Korean War between 1951 and 1953 and reforestation activities in the 1970s. In the Sanggae reservoir, there was an increase of Humulus pollen since 1996, an increase of Ambrosia pollen since the 1960s and the decrease of Graminae pollen since 1993 with the expansion of the industrial area. Similar to Mujechi-neup, the Sanggae reservoir also consists of three pollen zones: a Pinus and Typha pollen-dominated zone from 0 to 10 cm in depth (since 1993), a Graminae pollen-dominated zone from 10 to 22 cm in depth (between 1947 and 1993), and a Pinus pollen-dominated zone below 22 cm in depth (before 1947). The increase of Typha and Humulus in the 1990s indicates an increase in the inflow of nutrients into the wetlands. Also, pollen analysis revealed that Ambrosia was introduced in the 1960s during industrialization of the area. The sediment was composed of humic peat in Mujechi-neup and was composed of clay in the Sanggae reservoir. High LOI, P, and low C/N ratio between the depths of 5 to 7 cm in the Sanggae reservoir indicates an increased input of P and N. As a result, the growth of Typha was at its maximum. The total Pb content in Mujechi-neup has been increasing since the 1870s, and its accumulation in the Sanggae reservoir has been increasing since the 1960s. Thus, the history of local-scale disturbances and human activities in the watershed was reconstructed through paleoecological studies in Ulsan. 相似文献
6.
Nutrient over-enrichment of estuarine environments is increasing globally. However, it is difficult to determine the eutrophication trend in estuaries over long periods of time because long-term monitoring records are scarce and do not permit the identification of baseline environmental conditions. In this study, preliminary diatom based transfer functions for the inference of total phosphorus (TP) and total nitrogen (TN) in east-Australian sub-tropical estuaries were developed to address the deficiency in knowledge relating to historical estuary water quality trends. The transfer functions were created from a calibration set consisting of water quality and associated surface sediment diatom assemblage data from fifty-two sub-tropical estuaries in New South Wales and Queensland, Australia. Following data screening processes, Canonical Correspondence Analysis confirmed that TP and TN both explained significant, independent variation in the diatom assemblages. Variance partitioning, however, indicated that the TP was confounded with and may receive some strength from TN. WA and WA-PLS 2 component models for TP that included all calibration set sites yielded statistically weak results based on the jack-knifed r 2 scores $ \left( {r_{\text{jack}}^{{^{ 2} }} \, = 0.22\;{\text{and}}\;0. 2 2 {\text{ respectively}}} \right) $ . Removal from the calibration set of 12 sites that had all PO4, NH4, NO2, and NOx concentrations below detection limit resulted in a substantial improvement in WA-PLS 2 component TP model scores $ \left( {r_{\text{jack}}^{{^{ 2} }} \; = \;\,0.69} \right) $ , indicating that this model is statistically robust, and thus suitable for down core nutrient reconstructions. Caution, however, is required when developing diatom based inference models in Australian estuaries as nutrient cycling processes may have the potential to influence diatom based transfer functions. The model reported on here provides a foundation for reconstructing nutrient histories in eastern Australian sub-tropical estuaries in the absence of monitoring data. 相似文献
7.
Subfossil zooplankton assemblages (Cladocera 22 taxa, Rotifera 1 taxon) were identified from the surface sediments of 36 shallow (median depth = 0.7 m) Danish coastal brackish lakes differing in epilimnic salinity (SAL, range 0.2–17.4), summer-mean total phosphorus (TP, 27–327 g l–1) and total nitrogen (TN, 0.850–2.629 mg l–1), as well as in submerged macrophyte coverage and planktivorous fish density (PL-CPUE). Cladoceran species richness declined significantly with increasing SAL, TP and TN, while no significant correlation was found to either PL-CPUE, macrophyte coverage or lake surface area. Bonferroni-adjusted forward selection within canonical correspondence analysis (CCA) showed that 22.1% of the variation in zooplankton data was explained by PL-CPUE, SAL and TP uniquely; each variable explaining an almost equally significant amount of variation in the zooplankton data. Predictive models to infer PL-CPUE, SAL and TP were developed using variance weighted-averaging (WA) procedures. Almost similar values of boot-strapped coefficient of determination (r2boot-strapped 0.22–0.38) were produced by the WA inference models of PL-CPUE, SAL and TP, while the inference models of TP produced the lowest boot-strapped root-mean-squared-error of prediction (RMSEPboot-strapped 0.29–0.36 log(TP + 1), g l–1). Yet, zooplankton TP and SAL optima (WA) were strongly correlated (r2 = 0.46), while PL-CPUE optima (WA) were independent of both TP and SAL optima, indicating that only the PL-CPUE inference models are suitable for making reconstructions. 相似文献
8.
Stable carbon isotope ratios in lake and swamp sediments as a proxy for prehistoric forest clearance and crop cultivation in the Neotropics 总被引：2，自引：0，他引：2
Close correspondence between stable carbon isotope ratios ( 13 C), pollen, and charcoal profiles in sediment cores from Laguna Zoncho and Machita swamp, Costa Rica, shows that prehistoric forest clearance and crop cultivation can be detected in the stable carbon isotope ratios of total organic carbon ( 13C
TOC
). Analyses of δ 13C
TOC
complement evidence from pollen, charcoal, and phytoliths and provide a proxy that is sensitive to the intensity and/or proximity to core sites of prehistoric forest clearance and agriculture in watersheds. Stable carbon isotope analyses are particularly useful in situations in which other evidence of forest clearance and agriculture is limited. 相似文献
9.
Brent B. Wolfe Thomas W. D. Edwards Ramon Aravena Glen M. MacDonald 《Journal of Paleolimnology》1996,15(2):171-181
Analysis of 18Ocellulose, 13Corganic matter, and 13Ccellulose at about 100 year intervals from organic matter deposited in Toronto Lake, Northwest Territories, Canada, revealed an 8000-year history of rapid, post-glacial hydrologic change at the treeline zone. Several mid-Holocene phases of enriched 13Corg and 13Ccell, caused by elevated lake productivity, declining [CO2(aq)], and closed basin conditions, were abruptly terminated by intervals of open hydrology recorded by sharply depleted 18Ocell. Two of these events, at 5000 and 4500 BP, are correlated with increased total organic content and Picea mariana pollen concentration, which indicate that high levels of productivity were also accompanied by northern treeline advances. A third treeline advance at about 2500 BP is also marked by an apparent outflow event from Toronto Lake, but this was not associated with 13Corg/cell enrichment in the sediment record because rapid and substantial lake water renewal probably prevented productivity-driven enrichment of the dissolved inorganic carbon and replenished the CO2(aq) supply to thriving phytoplankton. However, high sediment organic content during this period suggests increased productivity. Increases in the inflow:evaporation ratio at about 6500 and 3500 BP were also sufficient to cause Toronto Lake to overflow but the prevailing climate during these periods apparently did not favour appreciable northward treeline migration or changes in lake productivity.This is the 14th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers 相似文献
10.
Yuehan Lu Philip A. Meyers Brian J. Eadie John A. Robbins 《Journal of Paleolimnology》2010,43(2):261-272
A ~106-cm sediment core from the eastern basin of Lake Erie was examined to investigate biogeochemical processes in this large
lake during its cultural eutrophication over the last century. We measured stable carbon isotopes of total organic carbon
and calcium carbonate (δ13CTOC and
d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} ) as well as the concentrations of total organic carbon (TOC) and calcium carbonate (CaCO3). δ13CTOC and TOC show a strong positive correlation throughout the core and record changes in phytoplankton productivity and nutrient
loading. CaCO3 and TOC concentrations display a negative correlation throughout the core, suggesting that CaCO3 concentrations are controlled primarily by decomposition of TOC in the hypolimnion and the sediments, although temperature
and invasive mussels are also potential controlling factors.
d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values show a positive correlation with δ13CTOC between 1909 and 1969, indicating phytoplankton productivity was the primary control for
d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values during eutrophication. However, a negative correlation between
d13 \textC\textCaCO 3 \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} and δ13CTOC from 1970 to 2002 suggests that these two proxies tracked different aspects of the carbon cycle in the lake in more recent
times. The cause for the negative correlation is not yet known, but it is perhaps associated with temperature variations and
seasonal differences in productivity. 相似文献
11.
J. Platt Bradbury 《Journal of Paleolimnology》1991,6(3):205-255
Lacustrine diatoms are diverse, well preserved and abundant in cores of lake sediment to 334 m depth near the town of Tulelake, Siskiyou County, northern California. The cores have been dated by radiometric, tephrochronologic and paleomagnetic techniques, which indicate a basal age of about 3 million years (Ma) and a nearly continuous depositional record for the Tule Lake basin for the last 3 million years (My). Fossil diatoms document the late Cenozoic paleolimnologic and paleoclimatic history for the northwestern edge of the Basin and Range Province. During the last 3 My, Tule Lake was typically a relatively deep, extensive lake. The Pliocene is characterized by a diatom flora dominated by Aulacoseira solida suggesting more abundant summer precipitation and warmer winters. Increases in Fragilaria at 2.4 Ma and between 2.0 and 1.7 Ma imply cooler summers that correlate to glacial environments recorded elsewhere in the world.
Stephanodiscus niagarae and Fragilaria species dominate the Pleistocene. Benthic diatoms of alkalineenriched, saline waters occur with S. niagarae between 100 and 40 m depth (0.90–0.14 Ma). Tephrochronology indicates slow deposition and possible hiatuses between about 0.6 and 0.2 Ma. Overall, the Pleistocene diatom flora reflects cooler and sometimes drier climates, especially after major glaciations began 0.85 Ma. The chronology of even-numbered oxygen isotope stages approximately matches fluctuations in the abundance in Fragilaria species since 1 Ma, suggesting that glacial periods at Tule Lake were expressed by relatively cool summers with enhanced effective moisture. Interglacial periods are represented by variable mixtures of freshwater planktonic and benthic alkaline diatom assemblages that suggest seasonal environments with winter-spring precipitation and summer moisture deficits.Glacial-interglacial environments since 150 ka were distinct from, and more variable than, those occurring earlier. The last full glacial period was very dry. Aulacoseira ambigua characterizes the late glacial and early Holocene record of Tule Lake. Its distribution indicates that warmer and wetter climates began about 15 ka in this part of the Great Basin.Fluctuations in diatom concentration suggests a 41000-yr. cycle between 3.0 and 2.5 Ma and 100000-yr. cycles after 1.0 Ma. In the late Pliocene and early Pleistocene, Aulacoseira solida percentages wax and wane in an approximately 400000-yr. cycle. The apparent response of Tule Lake diatom communities to orbitally induced insolation cycles underscores the importance of this record for the study of late Cenozoic paleoclimate change.The diatom stratigraphy records the evolution and local extinction of several species that may be biochronologically important. Stephanodiscus niagarae first appeared and became common in the Tule Lake record shortly after 1.8 Ma. Stephanodiscus carconensis disappeared about 1.8 Ma, while Aulacoseira solida is rare in the core after about 1.35 Ma. Cyclotella elgeri, a diatom characteristic of some outcrops referred to the Yonna Formation (Pliocene), is common only near the base of the core at an age of about 3 Ma. Detection of local extinctions is complicated by reworking of distinctive species from Pliocene diatomites surrounding Tule Lake.A new species, Aulacoseira paucistriata, is described from Pliocene lake deposits in the Klamath Basin. 相似文献
12.
The distribution of fractures and its dependence on lithology and petrophysical properties of rock in the Asmari Formation were examined using three wells data of one of the largest oil fields of southwestern Iran. Fractures were measured on cut cores. Mineral content and petrophysical data were obtained through thin section study and core plug measurement respectively. Influence of mineral composition and petrophysical property of rocks on fracture density was explored statistically. Increasing quartz (sand) and anhydrite content of rocks decrease and dolomite increases the threshold of fracture densities, however no significant relation was observed between calcite content of rock and fracture density. Increasing porosity and permeability of rock decrease the threshold of fracture density in some of the defined lithology groups. There are significant differences between the lithology groups in terms of fracture density, although the results in the three wells are not the same. In whole data, the highest fracture density can be observed in dolostone. Limestone and impure carbonates hold broader spaced fractures and sandstones display the least fracture density. The average fracture densities in the wells are strictly different. These differences are the result of the structural position of the wells and also the trend of the well and fractures. The distribution of fractures in most lithology groups can be explained by the function: , where F is relative frequency, D is fracture density and a, b, and c are constants. 相似文献
13.
Philip R. Kaufmann John M. Faustini David P. Larsen Mostafa A. Shirazi 《Geomorphology》2008,99(1-4):150-170
Quantitative regional assessments of streambed sedimentation and its likely causes are hampered because field investigations typically lack the requisite sample size, measurements, or precision for sound geomorphic and statistical interpretation. We adapted an index of relative bed stability (RBS) for data calculated from a national stream survey field protocol to enable general evaluation of bed stability and anthropogenic sedimentation in synoptic ecological surveys. RBS is the ratio of bed surface geometric mean particle diameter (Dgm) divided by estimated critical diameter (Dcbf) at bankfull flow, based on a modified Shield's criterion for incipient motion. Application of RBS to adequately depict bed stability in complex natural streams, however, has been limited because typical calculations of RBS do not explicitly account for reductions in bed shear stress that result from channel form roughness. We modified the index (RBS) to incorporate the reduction in bed shear stress available for sediment transport that results from the hydraulic resistance of large wood and longitudinal irregularities in channel dimensions (“form roughness”). Based on dimensional analysis, we derived an adjustment to bankfull shear stress by multiplying the bankfull hydraulic radius (Rbf) by the one-third power of the ratio of particle-derived resistance to total hydraulic resistance (Cp/Ct)1/3, where both resistances are empirically based calculations. We computed Cp using a Keulegan equation relating resistance to relative submergence of bed particles. We then derived an empirical equation to predict reach-scale hydraulic resistance Ct from thalweg mean depth, thalweg mean residual depth, and large wood volume based on field dye transit studies, in which total hydraulic resistance Ct was measured over a wide range of natural stream channel complexity, including manipulation of large wood volumes. We tested our estimates of Ct and RBS by applying them to data from a summer low flow probability sample of 104 wadeable stream reaches in the Coastal Ecoregion of Oregon and Washington, USA. Stream discharges calculated using these Ct estimates compared favorably with velocity–area measurements of discharge during summer low flow, and with the range of 1 to 2-year recurrence floods (scaled by drainage area) at U.S.Geological Survey gauged sites in the same region. Log [RBS] ranged from − 4.2 to + 0.98 in the survey region. Dgm ranged from silt to boulders, while estimated bankfull critical diameter, Dcbf, ranged from very fine gravel to large boulders. The median value of Dcbf (adjusted for form roughness influences) averaged 40% (inter quartile range 28 to 59%) of the unadjusted estimate Dcbf. Log[RBS] was consistently negatively related to human disturbances likely to produce excess sediment inputs or hydrologic alteration. Log [RBS] ranged from − 1.9 to + 0.5 in the streams within the lower quartile of human disturbance in their basin and riparian areas and was substantially lower (− 4.2 to − 1.1) in streams within the upper quartile of human disturbance. The synoptic survey methods and designs we used appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. Although the RBS concept also shows promise for evaluating sediment and bed stability in individual streams, our approach is relatively coarse, so site-specific assessments using these rapid field methods might prudently be confined to identifying severe cases of sedimentation or channel alteration. Greater confidence to discern subtle differences in site-specific assessments could be gained by calculating RBS using more precise field measurements of channel slope, bed particle size and bankfull dimensions, and by refining our adjustments for energy loss from channel form roughness. 相似文献
14.
Harris DeVerle Zurcher Lukas Stanley Michael Marlow Josef Pan Guocheng 《Natural Resources Research》2003,12(4):241-255
This study compares the performance of favorability mappings by weights of evidence (WOE), probabilistic neural networks (PNN), logistic regression (LR), and discriminant analysis (DA). Comparisons are made by an objective measure of performance that is based on statistical decision theory. The study further emphasizes out-of-sample inference, and quantifies the extent to which outcome is influenced by optimum variable discretization with classification and regression trees (CARTS).Favorability mapping methodologies are evaluated systematically across three case studies with contrasting scale and geologic information:
Estimated favorabilities for all cells then are represented by computed percent correct classification, and expected loss of optimum decision.The deposit-scale Carlin study reveals that the performances of the various methods from lowest to highest expected decision loss are: PNN, nonparametric DA, binary PNN (WOE variables), LR, and WOE. Moreover, the study indicates that approximately 40% of the increase in expected decision loss using WOE instead of PNN is the result of information loss from variable discretization. The remaining increases in losses using WOE are the result of its lesser inferential power than PNN. The district-scale Alamos study shows that the lowest expected decision loss is not by PNN, but by canonical DA. CARTS discretization improves greatly the performance of WOE. However, PNN and DA perform better than WOE. Unlike findings from the Alamos and Carlin studies, results from the regional-scale Nevada study indicate that decision losses by LR and DA are lower than those by WOE or PNN. Moreover, decision losses by CARTS-based canonical DA are noticeably the lowest of all, including those by LR and DA using the original variables. 相似文献
15.
Horizontal and vertical zones of influence for root systems of four Mojave Desert shrubs 总被引：3，自引：0，他引：3
Horizontal and vertical zones of influence for root systems of four Mojave Desert shrubs were characterized using 32P as a nutrient tracer. Larrea tridentata's horizontal zone of influence was sparse near the plant's stem base, with a maximum probability of accessing 32P (Pmax) of 41%. However, its horizontal zone of influence extended beyond 5 m, and the distance from the stem base at which the probability of accessing 32P was half Pmax (L503 m) was significantly greater than the other three shrubs. Ambrosia dumosa's zone of influence was dense near the plant's stem base (Pmax78%), but was rare at distances >2 m (L501 m). Zones of influence for Lycium andersonii and Lycium pallidum were intermediate between those of L. tridentata and A. dumosa. For vertical zones of influence, L. tridentata was more likely to obtain 32P from 5 m soil depths than A. dumosa, but L. pallidum was not significantly different from either A. dumosa or L. tridentata. Horizontal zones of influence did not change with treatments that altered soil water and nitrogen availability, but vertical zones of influence increased with a flood irrigation treatment that increased water availability to 5 m soil depth. These differences among species likely reflect compromises between their shoot growth strategies and their need to acquire spatially and temporally limited soil resources, especially through competitive interactions. 相似文献
16.
Christian Bigler Evastina Grahn Isabelle Larocque Adam Jeziorski Roland Hall 《Journal of Paleolimnology》2003,29(1):13-29
We assess Holocene environmental change at alpine Lake Njulla(68°22N, 18°42E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujávri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden. 相似文献
17.
Stable isotopes and trace elements in modern ostracod shells: implications for reconstructing past environments on the Tibetan Plateau,China 总被引：1，自引：0，他引：1
Nicole Börner Bart De Baere Lailah Gifty Akita Roger Francois Klaus Peter Jochum Peter Frenzel Liping Zhu Antje Schwalb 《Journal of Paleolimnology》2017,58(2):191-211
Stable isotopes and trace elements in ostracod shells have been used widely in paleolimnological investigations of past lake hydrochemistry and climate because they provide insights into past water balance and solute evolution of lakes. Regional differences in lake characteristics and species-specific element fractionation, however, do not permit generalization of results from other regions or ostracod species to the southern Tibetan Plateau, in part because most common taxa from the southern Tibetan Plateau are endemic to the area. This study evaluated relations between present-day environmental conditions and the geochemical composition of modern ostracod shells from the southern Tibetan Plateau, to assess the suitability of using shell chemistry to infer hydrological conditions. We studied nine lakes and their catchments, located along a west–east transect in the south-central part of the Tibetan Plateau. Stable oxygen and carbon isotope values and trace element concentrations in recent shells from the four most abundant ostracod species (Leucocytherella sinensis, ?Leucocythere dorsotuberosa, Limnocythere inopinata, Tonnacypris gyirongensis) were measured, together with hydrochemical properties of host waters at the time of sampling. Results revealed significant between-species differences in stable isotope fractionation and trace element incorporation into shell calcite. Stable oxygen and carbon isotope values of ostracod shells were correlated significantly with the stable isotope composition of the respective water body \( \left( {\updelta^{18} {\text{O}}_{{{\text{H}}_{ 2} {\text{O}}}} \,{\text{and }}\updelta^{13} {\text{C}}_{{{\text{H}}_{ 2} {\text{O}}}} } \right) \), reflecting salinity and productivity, respectively. Offsets between δ18Oshell and δ13Cshell and inorganic calcite, the latter representing isotopic equilibrium, suggest shell formation of T. gyirongensis during spring melt. L. sinensis reproduces throughout the monsoon season until September and shows several consecutive generations, and L. inopinata molts to the adult stage after the monsoon season in August/September. The influence of pore water δ13C was displayed by L. inopinata, suggesting shell calcification within the sediment. Mg/Cashell is primarily influenced by water Mg/Ca ratios and salinity and confirms the use of this shell ratio as a proxy for precipitation-evaporation balance and lake level. In addition, Sr/Ca and Ba/Ca can be used to infer changes in salinity, at least in closed-basin lakes with calcite saturation. Observed effects of water Sr/Ca and salinity on Sr/Ca incorporation are biased by the presence of aragonite precipitation in the lakes, which removes bioavailable Sr from the host water, resulting in low Sr/Cashell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Cashell ratios were unaffected by water temperature. Positive correlations among Fe/Ca, Mn/Ca and U/Ca in ostracod shells, and their negative correlation with δ13C, which reflects organic matter decay, show the potential to infer changes in redox conditions that can be used to reconstruct past oxygen supply to bottom waters and thus past water-circulation changes within lakes. The intensity of microbial activity, associated with organic matter decomposition, can be inferred from U/Ca ratios in ostracod shells. These findings highlight the value of fossil ostracod records in lake deposits for inferring paleoenvironmental conditions on the southern Tibetan Plateau. 相似文献
18.
19.
Wind tunnel experiments for ‘Raindrop Detachment and Wind-Driven Transport’ (RD–WDT) process were conducted under improved lateral jetting induced by wind velocities of 6.4, 10, and 12 m s− 1 at nozzle operating pressures of 75, 100, and 150 kPa. Wind-driven rainfalls were also incident on the windward and leeward slopes of 4° and 9° to have a broad variation in the angle of incidence. The objective of this experimental set-up was to distinguish the roles of both impact components of obliquely striking wind-driven raindrops on RD and wind on WDT. Raindrop impact components and reference horizontal wind were quantified by normal (Etz) and horizontal (Etx) kinetic energy fluxes and wind shear velocity (u), respectively, to physically model the process of RD–WDT. The results showed, at each level of u, differential sand transport rates by RD–WDT (qm(RD–WDT)) occurred depending on the magnitude of raindrop impact components, and qm(RD–WDT) increased as the relative contribution of Etz increased. Although Etx was more correlated with qm(RD–WDT) than Etz, the extreme increases in Etx at the expense of Etz brought about no increases but decreases in qm(RD–WDT). An RD–WDT model was built under the process of examining the discrete effects of Etz and Etx on RD together with u and resulted in a better coefficient of determination (R2 = 0.89) than only total kinetic energy (Et) did alone with u (R2 = 0.84). In this study, Etx was strongly related to u and not to Etz, which was the principal difference from the previous rainsplash studies, which relied on the compensatory lateral jet development by the compressive pressure build-up at the raindrop–soil interface. Including Etx in the RD–WDT model both separated the distinct role of each raindrop impact component in RD and improved the performance of u in WDT by better distinguishing its interaction with Etx, which was not explicitly separated in previous models of RD–WDT. 相似文献
20.
The sidewall effects of a wind tunnel on aeolian sand transport were investigated experimentally. A wind tunnel was used to conduct the experiments with a given channel height of 120 cm and varying widths (B) of 40, 60, 80, 100 and 120 cm. Both vertical profiles of wind velocity and sand mass flux were measured at different locations across the test section. The results show that the wind velocity with saltation first increases and then decreases to a minimum, from the sidewall to the central line of the wind tunnel. The discrepancy among wind velocities at different locations of the transverse section decreases with decreasing tunnel width. The wind friction velocity across the wind tunnel floor, with the exception of the region closest to the sidewalls, does not deviate strongly in wide wind tunnels from that along the central line, whereas it does vary in narrow tunnels. The sand mass fluxes, with the exception of some near-bed regions, are larger along the central line of the wind tunnel than they are at the quarter width location from the sidewall. Unlikely previously reported results, the dimensionless sand transport rate, Qg / (ρu3) (where Q is the total sand transport rate, g is the gravitational acceleration constant, ρ is the air density, and u is the wind friction velocity), first decreases and then increases with the dimensionless friction velocity, u / ut (where ut is the threshold friction velocity). The above differences may be attributed to the sidewall effects of the wind tunnel. A dimensionless parameter, FB = u / (gB)1/2, is defined to reflect the sidewall effects on aeolian sand transport. The flows with FB of 0.33 or less may be free from the sidewall effects of the wind tunnel and can ensure accurate saltation tunnel simulation. 相似文献