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31.
Sr isotope and Ca/Mg/Sr chemical compositions of freshwater ostracode tests separated from a sediment core represent the last 16 ka of sedimentation in Lake Constance, Central Europe. The chemical evolution of the paleowater's dissolved load of Lake Constance was estimated by correcting the ostracode data for Ca/Mg/Sr fractionation due to biogenic calcification. Since the Late Pleistocene deglaciation, the Ca/Sr molar ratios of paleowaters increased systematically from about 100 (a near marine signature) to about 200. Ca/Mg molar ratios varied in the range of 1–25. The 87Sr/86Sr ratios indicate Late Pleistocene paleowater compositions of 0.7086–0.7091, significantly more radiogenic than present day waters (0.7085). Sr isotopes and Ca/Mg/Sr chemical data together show that weathering of Mesozoic evaporites consistently dominated the dissolved Sr load (80–90%). Carbonate and silicate weathering were less important (1–10%). Trends of Sr dissolved loads were therefore not related to Mg which was mainly mobilized by carbonate weathering. Biotite weathering was an important source of radiogenic Sr in the paleowaters. The short-term release (duration about 600–800 years) of radiogenic Sr during glacier retreat started 15.2 ka ago and was due to enhanced biotite weathering at the glacier base. Long-term release of radiogenic Sr was due to biotite weathering in glacial soils and silicate rocks, and has gradually declined since the Late Pleistocene/Holocene transition. 相似文献
32.
33.
Lorena Ramos Marta Alperin Antje Schwalb Vera Markgraf Daniel Ariztegui Gabriela Cusminsky 《Boreas: An International Journal of Quaternary Research》2019,48(3):746-760
Changes in the ostracod assemblages from two sediment cores collected from Lago Cardiel in southeastern Patagonia (49°S) reflect the main regional abrupt climatic changes over the last 15.6 cal. ka BP. Shifts in species abundance and switches in dominances suggest that these were mainly driven by variable salinity. During the Late Pleistocene, Limnocythere rionegroensis was abundant and dominant, indicating waters with high salinity and prevalence of evaporative processes. Between 12.6 and 10.8 cal. ka BP, Lago Cardiel expanded markedly and reached an Early Holocene highstand of +55 m above present lake level. A major change in ostracod assemblage in which Limnocythere patagonica appears as the dominant species in parallel with the disappearance of L. rionegroensis and Eucypris aff. cecryphalium mirrored this transitional period between the cold and dry Late Pleistocene and the humid and warm Early Holocene. Over the last 4 cal. ka BP, L. rionegroensis returned to the species assemblage and Riocypris whatleyi increased its abundance pointing towards increasing salinities. The variations in size, shape and ornamentation of L. rionegroensis and R. whatleyi fossil valves were examined using geometric morphometric techniques and further compared to those of modern Patagonian sites. Limnocythere rionegroensis specimens displayed high morphological variability during the evolution of Lago Cardiel. More specifically, the switch in reproductive mode – from sexual to parthenogenetic– and the increase in valve ornamentation around 12.7 cal. ka BP suggest that these changes were promoted by the hydrological alteration that occurred in the Late Pleistocene. This exercise provides a robust range of morphological variation for these proxies, which will be useful in further taxonomic and palaeoenvironmental studies adding more information about different factors influencing the observed morphological trends. 相似文献
34.
Karen Helen Wiltshire Alexandra Kraberg Inka Bartsch Maarten Boersma Heinz-Dieter Franke Jan Freund Christina Gebühr Gunnar Gerdts Karina Stockmann Antje Wichels 《Estuaries and Coasts》2010,33(2):295-310
The Helgoland Roads time series is one of the richest temporal marine data sets available. Running since 1962, it documents
changes for phytoplankton, salinity, Secchi disc depths and macronutrients. Uniquely, the data have been carefully quality
controlled and linked to relevant meta-data, and the pelagic time series is further augmented by zooplankton, intertidal macroalgae,
macro-zoobenthos and bacterioplankton data. Data analyses have shown changes in hydrography and biota around Helgoland. In
the late 1970s, water inflows from the south-west to the German Bight increased with a corresponding increase in flushing
rates. Salinity and annual mean temperature have also increased since 1962 and the latter by an average of 1.67°C. This has
influenced seasonal phytoplankton growth causing significant shifts in diatom densities and the numbers of large diatoms (e.
g. Coscinodiscus wailesii). Changes in zooplankton diversity have included the appearance of the ctenophore Mnemiopsis leidyi. The macroalgal community also showed an increase in green algal and a decrease in brown algal species after 1959. Over 30
benthic macrofaunal species have been newly recorded at Helgoland over the last 20 years, with a distinct shift towards southern
species. These detailed data provide the basis for long-term analyses of changes on many trophic levels at Helgoland Roads. 相似文献
35.
Computing with functions on the rotation group is a task carried out in various areas of application. When it comes to approximation,
kernel based methods are a suitable tool to handle these functions. In this paper, we present an algorithm which allows us
to evaluate linear combinations of functions on the rotation group as well as a truly fast algorithm to sum up radial functions
on the rotation group. These approaches based on nonequispaced FFTs on SO(3) take O(M+N)\mathcal{O}(M+N) arithmetic operations for M and N arbitrarily distributed source and target nodes, respectively. In this paper, we investigate a selection of radial functions
and give explicit theoretical error bounds, as well as numerical examples of approximation errors. Moreover, we provide an
application of our method, namely the kernel density estimation from electron back scattering diffraction (EBSD) data, a problem
relevant in texture analysis. 相似文献
36.
Robert Mikutta Klaus Kaiser Antje Vollmer Jon Chorover Georg Guggenberger 《Geochimica et cosmochimica acta》2010,74(7):2142-3154
Large portions of organic N (ON) in soil exist tightly associated with minerals. Mineral effects on the type of interactions, chemical composition, and stability of ON, however, are poorly understood. We investigated mineral-associated ON along a Hawaiian soil chronosequence (0.3-4100 kyr) formed in basaltic tephra under comparable climatic, topographic, and vegetation conditions. Mineral-organic associations were separated according to density (ρ > 1.6 g/cm3), characterized by X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge fine structure (NEXAFS) and analyzed for amino acid enantiomers and amino sugars. The 14C activity of mineral-bound OC was estimated by accelerator mass spectrometry. The close OC-ON relationship (r = 0.96) and XPS results suggest that ON exists incorporated in bulk mineral-bound OM and likely becomes associated with minerals as part of sorbing OM. The youngest site (0.3 kyr), with soils mainly composed of primary minerals (olivine, pyroxene, feldspar) and with little ON, contained the largest proportion of hydrolyzable amino sugars and amino acids but with a small share of acidic amino acids (aspartic acid, glutamic acid). In soils of the intermediate weathering stage (20-400 kyr), where poorly crystalline minerals and metal(hydroxide)-organic precipitates prevail, more mineral-associated ON was present, containing a smaller proportion of hydrolyzable amino sugars and amino acids due to the preferential accumulation of other OM components such as lignin-derived phenols. Acidic amino acids were more abundant, reflecting the strong association of acidic organic components with metal(hydroxide)-organic precipitates and variable-charge minerals. In the final weathering stage (1400-4100 kyr) with well-crystalline secondary Fe and Al (hydr)oxides and kaolin minerals, mineral-organic associations held less ON and were, relative to lignin phenols, depleted in hydrolyzable amino sugars and amino acids, particularly in acidic amino acids. XPS and NEXAFS analyses showed that the majority (59-78%) of the mineral-associated ON is peptide N while 18-34% was aromatic N. Amino sugar ratios and d-alanine suggest that mineral-associated ON comprises a significant portion of bacterial residues, particularly in the subsoil. With increasing 14C age, a larger portion of peptide N was non-hydrolyzable, suggesting the accumulation of refractory compounds with time. The constant d/l ratios of lysine in topsoils indicate fresh proteinous material, likely due to continuous sorption of or exchange with fresh N-containing compounds. The 14C and the d/l signature revealed a longer turnover of proteinous components strongly bound to minerals (not NaOH-NaF-extractable). This study provides evidence that interactions with minerals are important in the transformation and stabilization of soil ON. Mineral-associated ON in topsoils seems actively involved in the N cycling of the study ecosystems, accentuating N limitation at the 0.3-kyr site but increasing N availability at older sites. 相似文献
37.
Vikas C. Baranwal Antje Franke Ralph-Uwe Börner Klaus Spitzer 《Journal of Applied Geophysics》2011,75(2):363-372
We present a 2-D inversion code incorporating a damped least-squares and a minimum-model approach for plane wave electromagnetic (EM) methods using an adaptive unstructured grid finite element forward operator. Unstructured triangular grids permit efficient discretization of arbitrary 2-D model geometries and, hence, allow for modeling arbitrary topography. The inversion model is parameterized on a coarse parameter grid which constitutes a subset of the forward modeling grid. The mapping from parameter to forward modeling grid is obtained by adaptive mesh refinement. Sensitivities are determined by solving a modified sensitivity equation system arising from the derivative of the finite element equations with respect to the model parameters. Firstly, we demonstrate that surface topography may induce significant effects on the EM response and in the inversion result, and that it cannot be ignored when the scale length of topographic variations is in the order of magnitude of the skin depth. Secondly, the dependency of the inversion on the starting model is discussed for VLF and VLF-R data. Thirdly, we demonstrate the inversion of a synthetic data set obtained from a model with topography. Finally, the inversion approach is applied to field data collected in a region with undulating topography. 相似文献
38.
Liseth Pérez Peter Frenzel Mark Brenner Jaime Escobar Philipp Hoelzmann Burkhard Scharf Antje Schwalb 《Journal of Paleolimnology》2011,46(1):59-74
We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala
from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant
ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation
gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode
communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6,
~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest
climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial.
Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance
(<940 valves g−1), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm−1, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper
water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred
late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6
spp.) and abundances up to 18,115 valves g−1, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm−1, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m.
The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g−1, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene. 相似文献
39.
Michael Hein Brigitte Urban David Colin Tanner Anton Hermann Buness Mario Tucci Philipp Hoelzmann Sabine Dietel Marie Kaniecki Jonathan Schultz Thomas Kasper Hans von Suchodoletz Antje Schwalb Marcel Weiss Tobias Lauer 《地球表面变化过程与地形》2021,46(14):2884-2901
The prevailing view suggests that the Eemian interglacial on the European Plain was characterized by largely negligible geomorphic activity beyond the coastal areas. However, systematic geomorphological studies are sparse. Here we present a detailed reconstruction of Eemian to Early Weichselian landscape evolution in the vicinity of a small fingerlake on the northern margin of the Salzwedel Palaeolake in Lower Saxony (Germany). We apply a combination of seismics, sediment coring, pollen analysis and luminescence dating on a complex sequence of colluvial, paludal and lacustrine sediments. Results suggest two pronounced phases of geomorphic activity, directly before the onset and at the end of the Eemian period, with an intermediate period of pronounced landscape stability. The dynamic phases were largely driven by incomplete vegetation cover, but likely accentuated by fluvial incision in the neighbouring Elbe Valley. Furthermore, we discovered Neanderthal occupation at the lakeshore during Eemian pollen zone (PZ) E IV, which is chronologically in line with other known Eemian sites of central Europe. Our highly-resolved spatio-temporal data substantially contribute to the understanding of climate-induced geomorphic processes throughout and directly after the last interglacial period. It helps unraveling the landscape dynamics between the coastal areas to the north and the loess belt to the south. 相似文献
40.
Lacustrine ostracodes as stable isotope recorders of late-glacial and Holocene environmental dynamics and climate 总被引:3,自引:0,他引:3
Antje Schwalb 《Journal of Paleolimnology》2003,29(3):265-351
Oxygen- and carbon-isotopic signatures of benthic ostracodes from lake sediments from climate sensitive regions in the Alpine region, Central Europe, the north-central USA, the Chilean Altiplano and Patagonia, Argentina, are used to characterize lake system processes and to reconstruct climate patterns of the past 16,000 years. The case studies provide examples that highlight different aspects of the broad application of isotope stratigraphies, and provide keys for the interpretation of complex lacustrine records.The integration of stable-isotope stratigraphy, sedimentology, and ecological information from ostracode assemblages is a new tool that acquires climate information from the indirect views of climate series provided by lake sediments. This tool (1) identifies lake system characteristics, (2) confines which isotopic signatures are controlled by which processes in the lake system and/or in the catchment, and (3) defines which signatures are ultimately controlled by climate change. If sudden shifts in the isotopic composition occur concomitantly with changes between sedimentological units, then the isotopes reflect first of all changes in catchment hydrology that may be ultimately controlled by climate. Also, if ostracode 18O and 13C values show the same timing and direction of shifts, then this indicates a major change in the hydrological budget of the lake.The case studies presented here show that coupled isotopic signatures may be used to track hydrological changes related to meltwater and deglaciation, shifting rivers and ground water sources, and changes in precipitation mechanisms and patterns. Values of 18O from large lakes with short water residence time, low evaporation rates and homothermic bottom waters provide records of past temperatures of precipitation. The 13C values reflect changes in the ratio of C3:C4 plants in the catchment. They indicate shifts in modes of organic decay in the surface sediments that can be linked to a change in hydrodynamics within a lake. The 13C values also allow detection of the input of volcanically charged ground waters providing large quantities of 14C-free CO2 that hinders accurate 14C chronology. General climate trends for the sites in the Americas indicate a dry mid-Holocene punctuated by moist spells, and show a general increase in moisture during the past approximately 4000 years, interrupted by recurring droughts. This hints at an interhemispheric connection and a common driving mechanism.Environmental isotopes from high-resolution lake sediments thus provide an ideal tool to identify and characterize the regional impact and magnitude of global climate change. This tool contributes to a better understanding of regional climate change and its driving mechanisms and thus provides the type of information needed to improve climate models. Environmental isotopes provide more information than just moisture balance and airmass history if they are integrated with the detailed sedimentological and ostracode ecological evidence, and understanding for the component system. Thus environmental isotopes serve to a better understanding of the climate signal archived in lake records and represent an essential contribution to Global Change research and Earth System Science. 相似文献