Environments of the Russian Plain during the Middle Valdai Briansk Interstade (33,000–24,000 yr B.P.) Indicated by Fossil Mammals and Plants     

Anastasia K. Markova  Alexandra N. Simakova  Andrei Yu. Puzachenko  Lev M. Kitaev 《Quaternary Research》2002,57(3):391

Abundant mammal (45 localities) and plant (52 sections) fossil data collected from the Russian Plain deposits of the Middle Valdai Briansk Interstade indicate a period of noticeable warming. These materials were jointly analyzed. The data were organized in the database software PARADOX and then moved to the GIS program ARC/INFO. Mathematical methods as well as traditional research methods were used for the analysis. The reconstructed biogeographical provinces illustrate the specific environmental and climatic conditions of the Briansk Interstade. Analogues of modern natural zones did not exist at this time on the Russian Plain. The Briansk landscapes reflect the moderate-cool climate of this period, when the majority of northern subarctic plant and mammal species extended their ranges considerably to the south to the Central Russian Plain. At the same time, steppe species penetrated farther north and west. This combination shows that the continuous forest zone was destroyed during the Briansk Inter-stade. Five biogeographical provinces have been reconstructed from the north to the south for the Russian Plain during the Briansk warming on the basis of joint analysis of mammalian and botanical data.  相似文献   

Amplitude of ENSO cycles in the Santa Barbara Basin,off California,during the past 15 000 years     

Alexandra J. Nederbragt  Jürgen Thurow 《第四纪科学杂志》2005,20(5):447-456

Varve thickness time series from ODP Site 893 in the Santa Barbara Basin (off California) were analysed to determine variation in the strength of El Niño/Southern Oscillation (ENSO) cycles during the past 15 000 yr. Mean varve thickness and variance changed over time, with thicker varves before ～8000 yr BP indicative of wetter than modern climates. A 100‐yr running standardisation was applied to correct for non‐stationarity. The contribution of ENSO‐scale variability was then estimated as the amplitude of 3–8 year bandpassed data. Results show multidecadal‐ to centennial‐scale modulation of the amplitude. On average, however, the amplitude of ENSO scale variability remained constant throughout the past 15 000 yr. We therefore conclude that, although the expression of ENSO cycles may have changed during the Holocene, there is no indication for a significant change in amplitude of interannual variability. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

New hydrothermal ore field at 16°38′ N of the Mid-Atlantic Ridge     

V. E. Bel’tenev  A. A. Shagin  V. F. Markov  I. I. Rozhdestvenskaya  L. I. Lazareva  I. P. Fedorov  G. A. Cherkashev  T. V. Stepanova  I. M. Poroshina  V. V. Shilov 《Doklady Earth Sciences》2006,408(1):530-534

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Experimental study of the formation of inverted-V structures and their stratification using AUREOL-3 observations     

O. Luízar  M. V. Stepanova  J. M. Bosqued  E. E. Antonova  R. A. Kovrazhkin 《Annales Geophysicae》2000,18(11):1399-1411

Multiple inverted-V structures are commonly observed on the same auroral zone crossing by a lowaltitude orbiting satellite. Such structures appear grouped and apparently result from an ionospheric and/or magnetospheric mechanism of stratification. More than two years of AUREOL-3 satellite observations were analyzed to study their properties and their formation in the framework of the ionosphere-magnetosphere coupling model proposed by Tverskoy. This model predicts some natural periodicity in the electrostatic potential profile (and subsequently in the field-aligned current profiles) that could account for oscillations experimentally observed in the auroral zone, such as successive inverted-Vs. Experimental results obtained during quiet or moderately active periods demonstrate that the number of structures observed within a given event is well described by a scaling parameter provided by the hot plasma stratification theory and expressed in terms of the field-aligned current density, the total width of the current band, the plasma sheet ion temperature, and the height-integrated Pedersen conductivity of the ionosphere. The latitudinal width, in the order of 100/200 km at ionospheric altitudes, is relatively independent of the current density, and is determined not only by the existence of a potential difference above the inverted-Vs, but also by basic oscillations of the ionosphere-magnetosphere coupling system predicted by Tverskoy. The large number of cases studied by the AUREOL-3 satellite provides reliable statistical trends which permits the validation of the model and the inference that the multiple structures currently observed can be related directly to oscillations of the magnetospheric potential (or the pressure gradients) on a scale of 1000/2000 km in the near-Earth plasma sheet. These oscillations arise in the Tverskoy model and may naturally result when the initial pressure gradients needed to generate a large-scale field-aligned current have a sufficiently wide equatorial scale, of about 1 R_E or more.  相似文献   

Noble metals in sulfide assemblages from deep sectors of the active TAG mound (Mid-Atlantic Ridge, 26°08′ N)     

N. N. Mozgova  Yu. S. Borodaev  T. V. Stepanova  G. A. Cherkashev  E. A. Zhirnov 《Lithology and Mineral Resources》2000,35(1):1-18

Primary sulfides from cores of holes 957M, 957C, and 957H drilled during the ODP Leg 158 on the active hydrothermal TAG mound (Mid-Atlantic Ridge, 26°08′ N) were examined for the concentration of several elements. Based on 262 microprobe analyses, it has been established that the sulfides are characterized by an extremely heterogeneous distribution of noble metals (Au, Ag, Pt, and Pd) and several associated elements (Hg, Co, and Se). Noble metals are arranged in the following order in terms of decreasing abundance, i.e., concentration level above the detection limit (the number of analyses containing the specific element is given in parentheses): Au (65), Ag (46), Pt (21), and Pd (traces). The associated trace elements make up the following series: Co (202), Hg (132), and Se (49). Main carriers of the “invisible” portion of noble metals are represented by pyrite (Au, Hg), marcasite and pyrite (Ag, Co), sphalerite and chalcopyrite (Pt, Pd), and chalcopyrite (Se). The noble metal distribution in sulfides reveals a lateral zonality: the maximal concentration and abundance of Au in chalcopyrite (or Pt and Ag in chalcopyrite and pyrite) increase from the periphery (Hole 957H) to the center (holes 957C and 957M) of the hydrothermal mound, while the Au distribution in pyrite displays a reversed pattern. The Co distribution increases with depth. This work discusses the vertical zonality in the distribution of elements mentioned above and their response to the evolution of ore genesis.  相似文献   

Marine organic geochemistry of the Eastern Mediterranean: 1. Aliphatic and polyaromatic hydrocarbons in Cretan Sea surficial sediments   总被引：1，自引：0，他引：1  

Alexandra Gogou  Ioanna Bouloubassi  Euripides G. Stephanou   《Marine Chemistry》2000,68(4)

As part of a lipid biogeochemical study, aliphatic and polyaromatic hydrocarbons were determined in surficial sediments from the Cretan Sea (South Aegean Sea) in the Eastern Mediterranean. Total concentrations of both aliphatic (AHC) and polyaromatic (PAH) hydrocarbons were low (562–5697 and 14.6–158.5 ng/g, respectively) with respect to other coastal sediments worldwide and compare with concentrations found in open sea areas. The composition of AHC was dominated by unresolved complex mixture (UCM) indicating the presence of petroleum-related hydrocarbon inputs as confirmed by the detection of specific α,β-hopanes. PAH consisted mainly of pyrolytic four- to five-ring compounds. UCM and PAH amounts revealed that Cretan Sea receives low supply of anthropogenic material compared to NW Mediterranean. The spatial distributions of AHC and PAH indicated that urban run-off and transport from the continental self are the major input pathway of anthropogenic and biogenic hydrocarbons from terrestrial sources in the near shore area, whereas atmospheric transport might be the significant source of hydrocarbons in the deep area.  相似文献   

Modeling defects and plasticity in MgSiO3 post-perovskite: Part 2—screw and edge [100] dislocations     

Alexandra M. Goryaeva  Philippe Carrez  Patrick Cordier 《Physics and Chemistry of Minerals》2015,42(10):793-803

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Fluvial transport of suspended sediment and organic carbon during flood events in a large agricultural catchment in southwest France     

Chantha Oeurng  Sabine Sauvage  Alexandra Coynel  Eric Maneux  Henri Etcheber  José‐Miguel Sánchez‐Pérez 《水文研究》2011,25(15):2365-2378

Water draining from a large agricultural catchment of 1 110 km² in southwest France was sampled over an 18‐month period to determine the temporal variability in suspended sediment (SS) and dissolved (DOC) and particulate organic carbon (POC) transport during flood events, with quantification of fluxes and controlling factors, and to analyze the relationships between discharge and SS, DOC and POC. A total of 15 flood events were analyzed, providing extensive data on SS, POC and DOC during floods. There was high variability in SS, POC and DOC transport during different seasonal floods, with SS varying by event from 513 to 41 750 t; POC from 12 to 748 t and DOC from 9 to 218 t. Overall, 76 and 62% of total fluxes of POC and DOC occurred within 22% of the study period. POC and DOC export from the Save catchment amounted to 3090 t and 1240 t, equivalent to 1·8 t km^?2 y^?1 and 0·7 t km^?2 y^?1, respectively. Statistical analyses showed that total precipitation, flood discharge and total water yield were the major factors controlling SS, POC and DOC transport from the catchment. The relationships between SS, POC and DOC and discharge over temporal flood events resulted in different hysteresis patterns, which were used to deduce dissolved and particulate origins. In both clockwise and anticlockwise hysteresis, POC mainly followed the same patterns as discharge and SS. The DOC‐discharge relationship was mainly characterized by alternating clockwise and anticlockwise hysteresis due to dilution effects of water originating from different sources in the whole catchment. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Revisiting the Gow Lake impact structure,Saskatchewan, Canada     

Gordon R. Osinski  Adam B. Coulter  Roberta L. Flemming  Alexandra Ozaruk  Annemarie E. Pickersgill  Alaura C. Singleton 《Meteoritics & planetary science》2023,58(6):775-788

The ~5 km diameter Gow Lake impact structure formed in the Canadian Shield of northern Saskatchewan approximately 197 Myr ago. This structure has not been studied in detail since its discovery during a regional gravity survey in the early 1970s. We report here on field observations from a 2011 expedition that, when combined with subsequent laboratory studies, have revealed a wealth of new information about this poorly studied Canadian impact structure. Initially considered to be a prototypical central peak (i.e., a complex) impact structure, our observations demonstrate that Gow Lake is actually a transitional impact structure, making it one of only two identified on Earth. Despite its age, a well-preserved sequence of crater-fill impactites is preserved on Calder Island in the middle of Gow Lake. From the base upward, this stratigraphy is parautochthonous target rock, lithic impact breccia, clast-rich impact melt rock, red clast-poor impact melt rock, and green clast-poor impact melt rocks. Discontinuous lenses of impact melt-bearing breccia also occur near the top of the red impact melt rocks and in the uppermost green impact melt rocks. The vitric particles in these breccias display irregular and contorted outlines. This, together with their setting within crater-fill melt rocks, is indicative of an origin as flows within the transient cavity and not an airborne mode of origin. Following impact, a hydrothermal system was initiated, which resulted in alteration of the crater-fill impactites. Major alteration phases are nontronite clay, K-feldspar, and quartz.  相似文献   

Effects of climate change and urban development on the distribution and conservation of vegetation in a Mediterranean type ecosystem     

Bray J. Beltrán  Janet Franklin  Alexandra D. Syphard  Helen M. Regan  Lorraine E. Flint  Alan L. Flint 《International journal of geographical information science》2014,28(8):1561-1589

Climate and land-use changes are projected to threaten biodiversity over this century. However, few studies have considered the spatial and temporal overlap of these threats to evaluate how ongoing land-use change could affect species ranges projected to shift outside conservation areas. We evaluated climate change and urban development effects on vegetation distribution in the Southwest ecoregion, California Floristic Province, USA. We also evaluated how well a conservation network protects suitable habitat for rare plant species under these change projections and identified primary sources of uncertainty. We used consensus-based maps from three species distribution models (SDMs) to project current and future suitable habitat for 19 species representing different functional types (defined by fire-response – obligate seeders, resprouting shrubs – and life forms – herbs, subshrubs), and range sizes (large/common, small/rare). We used one spatially explicit urban growth projection; two climate models, emission scenarios, and probability thresholds applied to SDMs; and high-resolution (90 m) environmental data. We projected that suitable habitat could disappear for 4 species and decrease for 15 by 2080. Averaged centroids of suitable habitat (all species) were projected to shift tens (up to hundreds) of kilometers. Herbs showed a small-projected response to climate change, while obligate seeders could suffer the greatest losses. Several rare species could lose suitable habitat inside conservation areas while increasing area outside. We concluded that (i) climate change is more important than urban development for vegetation habitat loss in this ecoregion through 2080 due to diminishing amounts of undeveloped private land in this region; (ii) the existing conservation plan, while extensive, may be inadequate to protect plant diversity under projected patterns of climate change and urban development, (iii) regional assessments of the dynamics of the drivers of biodiversity change based on high-resolution environmental data and consensus predictive mapping, such as this study, are necessary to identify the species expected to be the most vulnerable and to meaningfully inform regional-scale conservation.  相似文献