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11.
Jacqueline Michel 《Geochimica et cosmochimica acta》1984,48(6):1249-1255
Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total 232Th was gradually removed during weathering. Although the whole-rock activity ratios were in equilibrium, there were large excesses of 228Th in the leachable fraction of both parent rock () and partially weathered saprolite (), due to alpha recoil and release of daughter 228Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and , indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and were in equilibrium in both the whole-rock and leachable fractions. 230Th was deficient relative to 234U and 226Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction. 相似文献
12.
A Spatial Variant of the Basic Reproduction Number for the New Orleans Yellow Fever Epidemic of 1878
Andrew Curtis Jacqueline W. Mills Jason K. Blackburn 《The Professional geographer》2007,59(4):492-502
A spatial variant of the basic reproduction number (R0), here defined as the number of subsequent deaths attributed to an initial mortality, can be used to identify geographic variation within an epidemic. A spatial R0 was calculated at the neighborhood level, here defined by a 50‐m buffer surrounding an index case, for mortality data from the 1878 yellow fever epidemic of New Orleans. The highest number of secondary mortalities linked to a neighborhood index case was twelve, with a further eighty‐seven extrapolated morbidity cases. Results also highlight the importance of multideath residences and cultural contacts in neighborhood‐level disease spread. 相似文献
13.
Bernard Charlier Øyvind Skår Are Korneliussen Jean-Clair Duchesne Jacqueline Vander Auwera 《Contributions to Mineralogy and Petrology》2007,154(2):119-134
Major and trace element XRF and in situ LA-ICP-MS analyses of ilmenite in the Tellnes ilmenite deposit, Rogaland Anorthosite
Province, SW Norway, constrains a two stage fractional crystallization model of a ferrodioritic Fe-Ti-P rich melt. Stage 1
is characterized by ilmenite-plagioclase cumulates, partly stored in the lower part of the ore body (Lower Central Zone, LCZ),
and stage 2 by ilmenite-plagioclase-orthopyroxene-olivine cumulates (Upper Central Zone, UCZ). The concentration of V and
Cr in ilmenite, corrected for the trapped liquid effect, (1) defines the cotectic proportion of ilmenite to be 17.5 wt% during
stage 1, and (2) implies an increase of D
VIlm during stage 2, most likely related to a shift in fO2. The proportion of 17.5 wt% is lower than the modal proportion of ilmenite (ca. 50 wt%) in the ore body, implying accumulation
of ilmenite and flotation of plagioclase. The fraction of residual liquid left after crystallization of Tellnes cumulates
is estimated at 0.6 and the flotation of plagioclase at 26 wt% of the initial melt mass. The increasing content of intercumulus
magnetite with stratigraphic height, from 0 to ca. 3 wt%, results from differentiation of the trapped liquid towards magnetite
saturation. The MgO content of ilmenite (1.4–4.4 wt%) is much lower than the expected cumulus composition. It shows extensive
postcumulus re-equilibration with trapped liquid and ferromagnesian silicates, correlated with distance to the host anorthosite.
The Zr content of ilmenite, provided by in situ analyses, is low (<114 ppm) and uncorrelated with stratigraphy or Cr content.
The data demonstrate that zircon coronas observed around ilmenite formed by subsolidus exsolution of ZrO2 from ilmenite. The U-Pb zircon age of 920 ± 3 Ma probably records this exsolution process.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
14.
The influence of various cloud parameters and the interactions with the ground albedo and the solar zenith angle have been studied by means of model simulations. The radiative transfer model suitable for a cloudy atmosphere as well as for a clear atmosphere has been developed on the basis of the Discrete Ordinate Method. This study leads to a general understanding for cloudy atmospheres: in the presence of a uniform cloud, the cloud scattering is dominant to molecular and aerosol scattering, and it is also wavelength-independent; the ratio of transmitted irradiance in a cloudy atmosphere to that in the background clear atmosphere is independent of cloud height and solar zenith angle. That’s to say, the radiation downwelling out of a cloud is quite isotropic; it decreases approximately exponentially with the cloud optical depth at a rate related to the ground albedo; the reflected irradiance at the top of the atmosphere is dependent on cloud optical depth as well as on solar zenith angle, but not on ground albedo for clouds of not very thin optical depth. 相似文献
15.
Calibration of diatoms along a nutrient gradient in Florida Everglades Water Conservation Area-2A, USA 总被引:2,自引:0,他引:2
Sherri R. Cooper Jacqueline Huvane Panchabi Vaithiyanathan Curtis J. Richardson 《Journal of Paleolimnology》1999,22(4):413-437
The relationship between diatom taxa preserved in surface soils and environmental variables at 31 sites in Water Conservation Area 2A (WCA-2A) of the Florida Everglades was explored using multivariate analyses. Surface soils were collected along a phosphorus (P) gradient and analyzed for diatoms, total P, % nitrogen (N), %carbon (C), calcium (Ca), and biogenic silica (BSi). Phosphorus varied from 315-1781 g g-1, and was not found to be correlated with the other geochemical variables. Canonical correspondence analysis (CCA) was used to examine which environmental variables correlated most closely with the distributions in diatom taxa. Canonical correspondence analysis with forward selection, constrained and partial CCA, and Monte Carlo permutation tests of significance show the most significant changes in diatom assemblages along the P gradient (p < 0.01), with additional species differences correlated with soil C, N, Ca, and BSi.Weighted-averaging (WA) regression and calibration models of diatom assemblages to P and BSi were developed. The diatom-based inference model for soil [P] had a high apparent r2 (0.86) with RMSEboot = 218 g g-1. Indicator diatom species identified by assessing species WA optima and WA tolerance to [P], such as Nitzschia amphibia and N. palea for high [P] (~1300-1400 g-1) and Achnanthes minutissima var. scotica and Mastogloia smithii for low [P] (~400-600 g g-1), may be useful as monitoring tools for eutrophication in WCA-2A as well as other areas of the Everglades. Diatom assemblages analyzed by cluster analysis were related to location within WCA-2A, and dominant taxa within clusters are discussed in relation to the geochemical variables measured as well as hydrology and pH. Diversity of diatom assemblages and a Disturbance Index based on diatom data are discussed in relation to the historically P-limited Everglades ecosystem. Diatom assemblages should be very useful for reconstructions of [P] through time in the Florida Everglades, provided diatoms are well preserved in soil cores. 相似文献
16.
Willard S. Moore Kenneth W. Bruland Jacqueline Michel 《Earth and Planetary Science Letters》1981,53(3):391-399
Samples from the MANOP Santa Barbara Basin sediment trap intercomparison were analyzed for the isotopes of uranium, thorium, radium, lead, and polonium. All of the traps showed approximately the same compositions and isotopic ratios, indicating that they trapped similar materials. The234Th flux via falling particles was very close to the flux predicted from the production and scavenging rates of234Th from the water column. The210Pb content of the trapped particles and the surface sediments were the same, however, the measured flux of210Pb was seven times greater than the predicted flux. Predicted and measured fluxes of228Th and210Po were similarly out of balance. To explain this apparent inconsistency, we suggest (as others have done) that the Santa Barbara Basin is an area where scavenging from the water column is intensified and where sediments deposited initially on the margins may be physically remobilized on a short time scale. These two effects increase the apparent area from which the basin derives the longer-lived isotopes but does not increase significantly the supply of the short-lived234Th. 相似文献
17.
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19.
Jacqueline Birnie 《第四纪科学杂志》1990,5(3):171-187
The Holocene environmental history of South Georgia is important because of the island's location in the Southern Westerlies in an oceanic zone of the world devoid of high resolution terrestrial records. This is the first attempt to interpret a palaeoenvironmental record from lake sediments in South Georgia. It is based on a wide variety of analyses undertaken on cores from two lakes. Both are in the same, unglaciated, drainage basin, but one is at 80 m above sea-level and near the altitudinal limit for vegetation growth, whereas the other is at 25 m and within the zone of continuous vegetation cover. Results from both lakes indicate shifts of vegetation boundaries, which, together with evidence for changing biotic productivity within the lakes themselves, are interpreted as indicating climatic changes. Radiocarbon dates on the main changes identify a climatic optimum, beginning before 5620 ± 290 14C yr BP, and ending at around 4815 ± 330 14C yr BP, when conditions in the upper part of the catchment were more conducive to plant growth than they are today. The record obtained from the lower lake was shorter, but indicates two periods of harsher climate relative to the present since 4000 yr BP. This interpretation of the lake evidence agrees with other dated evidence of environmental change from peat sections, glacial stratigraphy and geomorphology in South Georgia. Together the work allows an overall reconstruction of environmental change in the Holocene. 相似文献
20.
Frido Welker Elza Duijm Kristiaan J. van der Gaag Bas van Geel Peter de Knijff Jacqueline van Leeuwen Dick Mol Johannes van der Plicht Niels Raes Jelle Reumer Barbara Gravendeel 《Quaternary Research》2014
Humans colonized the Balearic Islands 5–4 ka ago. They arrived in a uniquely adapted ecosystem with the Balearic mountain goat Myotragus balearicus (Bovidae, Antilopinae, Caprini) as the only large mammal. This mammal went extinct rapidly after human arrival. Several hypotheses have been proposed to explain the extinction of M. balearicus. For the present study ancient DNA analysis (Sanger sequencing, Roche-454, Ion Torrent), and pollen and macrofossil analyses were performed on preserved coprolites from M. balearicus, providing information on its diet and paleo-environment. The information retrieved shows that M. balearicus was heavily dependent on the Balearic box species Buxus balearica during at least part of the year, and that it was most probably a browser. Hindcast ecological niche modelling of B. balearica shows that local distribution of this plant species was affected by climate changes. This suggests that the extinction of M. balearicus can be related to the decline and regional extinction of a plant species that formed a major component of its diet. The vegetation change is thought to be caused by increased aridity occurring throughout the Mediterranean. Previous hypotheses relating the extinction of M. balearicus directly to the arrival of humans on the islands must therefore be adjusted. 相似文献