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101.
102.
Maria Helena Bezerra Maia de Hollanda Mrcio Martins Pimentel Emanuel Ferraz Jardim de S 《Journal of South American Earth Sciences》2003,15(8):885-900
Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/YbCN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (Nd from −9.3–−20.1) isotopic compositions. TDM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (206Pb/204Pb=16–17.3, 207Pb/204Pb=15.1–15.6, 208Pb/204Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust. 相似文献
103.
Robert Corthsy Maria Helena Leite Denis E. Gill Bernard Gaudin 《Engineering Geology》2003,69(3-4):381-397
The objective of this paper is to demonstrate that accurate and reliable in situ stress measurements can be performed in soft rocks. A quick overview of the mechanical behaviour of soft rocks is presented. After reviewing stress measurement techniques that have been used in soft rocks, the modified doorstopper technique is presented and the advantages it has over other techniques in soft rocks are underlined. Results from laboratory simulations in controlled conditions show that the technique is reliable and accurate. It is then shown, through field applications in a potash mine in Brazil, in an underwater tunnel in shales in Canada and in an exploratory drift in molassic rocks of the French Alps, how the technique yields results that can sometimes be confirmed by field observations. 相似文献
104.
Methylmercury (MeHg) and inorganic mercury (Hginorg) were evaluated in the water of a Brazilian estuary, with two size classes of plankton and seven fish species of different feeding habits. Water partition coefficients (PCs) in microplankton were fourfold higher for MeHg than for Hginorg; and water PCs in mesoplankton were 26 times higher for MeHg than Hginorg. Difference between microplankton and mesoplankton MeHg bioaccumulation factor (BAF) was higher (0.60 log units) than Hginorg BAF (0.24 log units), indicating that trophic transfer of MeHg between planktonic organisms is more efficient than Hginorg transference. MeHg concentrations, proportion of mercury as MeHg and its biotransference factors (BTFs) in the microplankton, mesoplankton and fish increased with increasing trophic level while biotic concentrations of Hginorg and proportion of mercury as Hginorg decreased thus indicating that MeHg was indeed the biomagnified species of mercury. MeHg reflected the vertical trophic guilds distribution, due to the fact that the top predator fish presented the highest concentration (0.77 μg g−1 d.w.), followed by the less voracious species (0.43 μg g−1 d.w.); while planktivorous fish presented the lowest concentrations (0.044 μg g−1 d.w.). Hginorg did not present the same behavior. Results suggest that feeding habits and trophic guild are important parameters, influencing biotransference and biomagnification processes. 相似文献
105.
Helena Mouriño Maria Isabel Barão 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(4):499-511
In this paper we consider a Linear Regression Model with a design matrix that fits the periodic structure of a time series.
As a consequence, the residuals are very often autocorrelated. The main problem is that residual autocorrelation does not
necessarily entail error autocorrelation. To analyse the effects of selecting different formulations to accommodate the autocorrelation
in the residuals, we consider two seemingly different ways to deal with this problem: the Linear Regression Model with the
error terms following an Autoregressive Stationary Process and the Partial Adjustment Model. We study the equivalence between
the two formulations. We go over the problem of estimating the parameters and, especially, of making inferences in this framework.
After parameter estimation, we analyse the adequacy of the models. We demonstrate that the issue of selecting the most appropriate
model to capture the autocorrelation in the residuals is, in this context, a kind of an artefact since the main results concerning
the fitted values and forecasting features are the same. These modelling procedures are applied to the Portuguese coastal
upwelling data and we compare the estimated models. 相似文献
106.
107.
Early Holocene dune field development in Dalarna,central Sweden: A geomorphological and geophysical case study 下载免费PDF全文
Bonäsheden, Sweden's largest continuous dune field, situated in the county of Dalarna, central Sweden, has been investigated using LiDAR (light detection and ranging) remote sensing, ground penetrating radar as well as by field observations and luminescence dating. The use of LiDAR in conjunction with geographic information system (GIS) software proved to be efficient in mapping the inactive dune field and classifying the dune morphology, especially when slope raster images were used. The dunes have formed mostly by winds from the northwest (NW) and are of a transverse type. Still other dune types, such as parabolic dunes, and transverse dunes with a deviating orientation are present. Also, there seems to be different generations of dunes, suggesting a complex palaeowind environment with a change from predominantly north‐westerly winds to more westerly winds. Luminescence dating finally allows us to have an absolute chronology of the development of the Bonäsheden dune field, revealing formation of the dune field closely following the de‐glaciation of this part of Sweden (c. 10.5 ka). The well preserved transverse shape of the majority of the dunes suggests rapid stabilization by vegetation, although sand drift still seems to have been active on a noticeable scale for at least 1500 years and also, occasionally and patchy, as coversand deposition during the Late Holocene. A simple model is proposed for the dune field development of Bonäsheden based on our findings. This model is a useful addition since the majority of present day dune field models focus on the formation of parabolic dunes or large unvegetated dune fields. Our results suggest that most models cannot adequately simulate the formation of such small dune fields as that of Bonäsheden, with apparently rapidly fixated transverse dunes in a previously glaciated, now vegetated area. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
108.
Felix Martin Hofmann Helena Alexanderson Philippe Schoeneich Jordan R. Mertes Laëtitia Lanni 《Boreas: An International Journal of Quaternary Research》2019,48(4):1019-1041
Only a few chronological constraints on Lateglacial and Early Holocene glacier variability in the westernmost Alps have hitherto been obtained. In this paper, moraines of two palaeoglaciers in the southern Écrins massif were mapped. The chronology of the stabilization of selected moraines was established through the use of 10Be cosmic ray exposure (CRE) dating. The equilibrium line altitude (ELA) during moraine deposition was reconstructed assuming an accumulation area ratio (AAR) of 0.67. Ten pre‐Little Ice Age (LIA) ice‐marginal positions of the Rougnoux palaeoglacier were identified and seven of these have been dated. The 10Be CRE age of a boulder on the lowermost sampled moraine indicates that the landform may have been first formed during a period of stable glaciers at around 16.2±1.7 ka (kiloyears before AD 2017) or that the sampled boulder experienced pre‐exposure to secondary cosmic radiation. The moraine was re‐occupied or, alternatively, shaped somewhat before 12.2±0.6 ka when the ELA was lowered by 230 m relative to the LIA ELA. At least six periods of stable ice margins occurred thereafter when the ELA was 220–160 m lower than during the LIA. The innermost dated moraine stabilized at or before 10.9±0.7 ka. Three 10Be CRE ages from a moraine of the Prelles palaeoglacier indicate a period of stationary ice margins at or before 10.9±0.6 ka when the ELA was lowered by 160 m with respect to the end of the LIA. The presented 10Be CRE ages are in good agreement with those of moraines that have been attributed to the Egesen stadial. Assuming unchanged precipitation, summer temperature in the southern Écrins massif at ~12 ka must have been at least 2 °C lower relative to the LIA. 相似文献
109.
PI Suhr Willumsen Helena L. Filipsson Maja Reinholdsson Conny Lenz 《Boreas: An International Journal of Quaternary Research》2013,42(1):210-223
Early to late Holocene sediments from core F80, Fårö Deep, Baltic Sea, are investigated for their palynomorph composition and dinoflagellate cyst record to map variations in sea‐surface‐water salinity and palaeoproductivity during the past 6000 years. The F80 palynomorph assemblages are subdivided into four Assemblage Zones (AZs) named A to D. The transition from the stratigraphically oldest AZ A to B reflects a marked increase in palaeoproductivity and a gradual increase in surface‐water salinity over the ~1500 years between the Initial Littorina (former Mastogloia Sea Stage) and Littorina Sea Stage. A period with maximum sea‐surface salinity is recorded within the overlying AZ C from 7200 to 5200 cal. a BP, where the process length of Operculodinium centrocarpum indicates that average salinities were probably the highest (~15–17 versus 7.5 psu today) since the last glaciation. The change from AZ C to D correlates with a shift from laminated to non‐laminated sediments, and the dinoflagellate cyst assemblages suggest that the surface‐ and the deep‐water environment altered from c. 5250 cal. a BP, with less productivity in the surface water and more oxygenated conditions in the deep water. Here we demonstrate that past regional changes in surface salinity, primary productivity and deep‐water oxygenation status in the Baltic Sea can be traced by mapping overall palynomorph composition, dinoflagellate cyst assemblages and variations in the process length of O. centrocarpum in relation to periods of laminated/non‐laminated sedimentation and proportion of organic‐matter in the sediments. An understanding of past productivity changes is particularly important to better understand present‐day environmental changes within the Baltic Sea region. 相似文献
110.
OSL dating and luminescence characteristics of aeolian deposits and their source material in Dalarna,central Sweden 下载免费PDF全文
Helena Alexanderson Martin Bernhardson 《Boreas: An International Journal of Quaternary Research》2016,45(4):876-893
Aeolian dunes, coversands, glacifluvial deltas and paraglacial mass‐movement deposits in Bonäsheden and Skattungheden in Dalarna, central Sweden were dated by optically stimulated luminescence (OSL) on quartz. The OSL ages confirm that the majority of the dunes started forming directly after deglaciation, as suggested by previous investigators. Dune formation seems to have lasted at least 1500 years, until c. 9 ka. Four younger episodes of sand drift resulting in coversand deposition were dated by OSL and/or radiocarbon to 4100±200 a, 1569–1412 cal. a BP, 970±60 a and 150±10 a. The youngest one is related to changes in land use, while the older ones may be due to regional climatic changes. The dated quartz had excellent luminescence characteristics; the luminescence was bright and dominated by the fast signal component, which made it possible to successfully analyse even untreated material, so‐called range‐finder dating. These characteristics are most likely inherited from one of the source rocks of the Quaternary deposits – the Mesoproterozoic Dala sandstone, which we show to also have very good luminescence properties. 相似文献