排序方式： 共有25条查询结果，搜索用时 15 毫秒
1.
Ivanka Stefanova Nadja Ognjanova-Rumenova Wolfgang Hofmann Brigitta Ammann 《Journal of Paleolimnology》2003,30(1):95-111
Diatoms, Cladocera, and chironomids preserved in the sediments of Lake Dalgoto were studied to reconstruct the history of the lake ecosystem in the context of the vegetation history as represented by the pollen stratigraphy. Younger Dryas silty sediments at the base of the core are characterized by low diversity of aquatic organisms. The transition to the Holocene is indicated by a sharp change from silt to clay-gyttja. The migration and expansion of trees at lower elevations between 10200 and 8500 14C-yr BP, along with higher diversities and concentrations of aquatic organisms and the decreased proportion of north-alpine diatoms, point to rapidly rising summer temperatures. After 6500 14C-yr BP the expansion of Pinus mugo in the catchment coincides with signs of natural eutrophication as recorded by an increase of planktonic diatoms. In the late Holocene (4000–0 14C-yr BP) Pinus peuce and Abies are reduced and Picea expands. Cereal grains and disturbance indicators suggest late-Holocene human modification of the vegetation. 相似文献
2.
T. N. Krishnamurti L. Stefanova L. Watson S. Pattnaik 《Pure and Applied Geophysics》2007,164(8-9):1429-1441
This paper addresses two avenues for gaining insight into the hurricane intensity issue—the angular momentum approach and
the scale interaction approach. In the angular momentum framework, the torques acting on a parcel's angular momentum are considered
along an inflowing trajectory in order to construct the angular momentum budget. These torques are separable into three components:
The pressure torque, the surface friction torque, and the cloud torque. All torques are found to diminish the angular momentum
of an inflowing parcel, with the cloud torques having the most important role. In the scale interaction approach, energy exchanges
among different scales within a hurricane are considered as a means of understanding hurricane intensity. It is found that
the majority of kinetic energy contribution to the hurricane scales originates from potential-to-kinetic in-scale energy conversions.
The contribution of mean-wave interactions in the kinetic energy varies with distance from the center and with the life stage
of a storm. In the early stages, as the disorganized convection becomes organized on the hurricane scales, upscale energy
transfers (i.e., from small to large scale) are found to take place in the outer radii of the storm. In a mature storm, the kinetic energy
transfers are downscale, except for the inner radii. 相似文献
3.
The geochemistry of trace elements in the underground and open-pit mine of the Goze Delchev subbituminous coal deposit have been studied. The coals in both mines are highly enriched in W, Ge and Be, and at less extent in As, Mn and Y as compared with the world-wide Clarkes for subbituminous coals. Ni and Ti are also enhanced in the underground coals, and Zr, Cr and Mo in the open-pit mine coals.Characteristic for the trace element contents in the deposit is a regular variation with depth. The following patterns were distinguished for profile I: a — the element content decreases from the bottom to the top of the bed paralleling ash distribution (Fe, Co, As, Sb, V, Y, Mo, Cs, REE, Hf, Ta, Th, P and Au); b — Ge and W are enriched in the near-bottom and near-top coals; c — in the middle part of the bed the content of K and Rb is maximal, while that of U is slightly enriched; d — Ba content decreases from the top to the bottom of the bed. In profile II, W and Be contents decrease from the bottom to the top. The near-bottom, and especially the near-roof samples of profile IV are highly enriched in Ge, while for W the highest is the content of the near-bottom sample.Ge, Be, As, Mn, Cl and Br are mainly organically associated. The organic affiliation is still strong for Co, B, Sr, Ba, Sb, U, Th, Mo, La, Ce, Sm, Tb and Yb in the underground coals, and Fe, Co, Na, W, Sr, Y and Ag in the coals from the open-pit mine. K, Rb, Ti, Zr, Hf and Ta are of dominant inorganic affinity. The chalcophile and siderophile elements correlate positively with Fe and each other and may be bound partly with pyrite or other sulphides and iron containing minerals.Compared statistically by the t-criteria, the elements Na, Li, Cu, Zn, Pb, Cr, Ni, Co, Mo, Fe and Be are of higher content in the open-pit mine. Tungsten is the only element of higher concentration in the underground mine. The contents of Ge, As, Sr, V, Mn, Y, Zr and P are not statistically different in both mines.It was supposed that there were multiple sources of the trace elements in the deposit. The source of the highly enriched elements (W, Ge, Be, and As) most probably were the thermal waters in the source area. The contemporary mineral springs are of high content of these elements. Another source were the hosting Mesta volcanic rocks, which are enriched in Sb, Mo, Hf, U, Th, As, Li and Rb. Some of the volcanics were hydrothermally altered and enriched or depleted of many elements. Thus, the hydrothermal solutions were also suppliers of elements for the coals. It is obvious that the contents, distribution and paragenesis, of the trace elements in both Goze Delchev coals reflect the geochemical specialization of the source area, including rocks, paleo- and contemporary thermal waters. 相似文献
4.
R. S. Ross A. Chakraborty A. Chen L. Stefanova S. Sirdas T. N. Krishnamurti 《Meteorology and Atmospheric Physics》2007,98(3-4):137-174
Summary Climate variations in the Caribbean, largely manifest in rainfall activity, have important consequences for the large-scale
water budget, natural vegetation, and land use in the region. The wet and dry seasons will be defined, and the important roles
played by the El Ni?o-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) in modulating the rainfall during
these seasons will be discussed.
The seasonal climate forecasts in this paper are made by 13 state of the art coupled atmosphere-ocean general circulation
models (CGCMs) and by the Florida State University Synthetic Superensemble (FSUSSE), whose forecasts are obtained by a weighted
combination of the individual CGCM forecasts based on a training period. The success of the models in simulating the observed
1989–2001 climatology of the various forecast parameters will be examined and linked to the models’ success in predicting
the seasonal climate for individual years. Seasonal forecasts will be examined for precipitation, sea-surface temperature (SST), 2-meter air temperature, and
850 hPa u- and v-wind components during the period 1989–2001. Evaluation metrics include root mean square (RMS) error and Brier skill score.
It will be shown that the FSUSSE is superior to the individual CGCMs and their ensemble mean both in simulating the 1989–2001
climatology for the various parameters and in predicting the seasonal climate of the various parameters for individual years.
The seasonal climate forecasts of the FSUSSE and of the ensemble mean of the 13 state of the art CGCMs will be evaluated for
years (during the period 1989–2001) that have particular ENSO and NAO signals that are known to influence Caribbean weather,
particularly the rainfall. It will be shown that the FSUSSE provides superior forecasts of rainfall, SST, 2-meter air temperature,
and 850 hPa u- and v-wind components during dry summers that are modulated by negative SOI and/or positive NAO indices. Such summers have become
a feature of a twenty-year pattern of drought in the Caribbean region. The results presented in this paper will show that
the FSUSSE is a valuable tool for forecasting rainfall and other atmospheric and oceanic variables during such periods of
drought. 相似文献
5.
Young-Kwon Lim Lydia B. Stefanova Steven C. Chan Siegfried D. Schubert James J. O��Brien 《Climate Dynamics》2011,37(5-6):1061-1080
This study assesses the regional-scale summer precipitation produced by the dynamical downscaling of analyzed large-scale fields. The main goal of this study is to investigate how much the regional model adds smaller scale precipitation information that the large-scale fields do not resolve. The modeling region for this study covers the southeastern United States (Florida, Georgia, Alabama, South Carolina, and North Carolina) where the summer climate is subtropical in nature, with a heavy influence of regional-scale convection. The coarse resolution (2.5° latitude/longitude) large-scale atmospheric variables from the National Center for Environmental Prediction (NCEP)/DOE reanalysis (R2) are downscaled using the NCEP/Environmental Climate Prediction Center regional spectral model (RSM) to produce precipitation at 20?km resolution for 16 summer seasons (1990?C2005). The RSM produces realistic details in the regional summer precipitation at 20?km resolution. Compared to R2, the RSM-produced monthly precipitation shows better agreement with observations. There is a reduced wet bias and a more realistic spatial pattern of the precipitation climatology compared with the interpolated R2 values. The root mean square errors of the monthly R2 precipitation are reduced over 93% (1,697) of all the grid points in the five states (1,821). The temporal correlation also improves over 92% (1,675) of all grid points such that the domain-averaged correlation increases from 0.38 (R2) to 0.55 (RSM). The RSM accurately reproduces the first two observed eigenmodes, compared with the R2 product for which the second mode is not properly reproduced. The spatial patterns for wet versus dry summer years are also successfully simulated in RSM. For shorter time scales, the RSM resolves heavy rainfall events and their frequency better than R2. Correlation and categorical classification (above/near/below average) for the monthly frequency of heavy precipitation days is also significantly improved by the RSM. 相似文献
6.
Jean-Robert Disnar Maya Stefanova Jean-Gabriel Bréhéret Jean-Jacques Macaire 《Organic Geochemistry》2011,42(9):1089-1098
The Sarliève marsh in the Limagne plain, in the heart of the French Massif Central, functioned as an endorheic lake during the Late Glacial and the Early and Middle Holocene. During the Late Boreal and the Atlantic it experienced drastic lowering of the water level as a result of dry and warm climatic episodes. Then, pre-evaporitic conditions triggered the deposition of sediment rich in organic matter (OM) and in carbonates including dolomite. Fifty-one samples from a ca. 1.8 m sediment core section covering the period were analysed using Rock–Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS). Throughout the interval, the OM content remained notable to high [up to 13.35% total organic carbon (TOC)] and of good quality as indicated by low oxygen index (OI) values (<200 mg CO2 g−1 TOC) and high hydrogen index (HI) values (160–660 mg HC g−1) which, as a rule, increased with increasing TOC content.In contrast to the acid fractions, which sometimes contained notable proportions of n-C16 or n-C18 fatty acids (FAs; analysed as the methyl esters, FAMEs), the neutral fractions were almost devoid of low molecular weight compounds. The latter were probably biodegraded during early diagenesis. Conversely, high molecular weight compounds were abundant in both fractions and were dominated by n-alkanols, n-alkanes, steroids and hopanoids in the neutrals and even numbered FAs and hopanoids in the acid fractions. The hopanoids were dominated by regular bishomohopanoids, accompanied by 2-methylated bishomohopanoids, as well as by unidentified bishomohopanoids with methylation in either the D or E ring. These distributions, typical for bacteria, provide support for previous hypotheses on the contribution of microorganisms to the studied record and for providing conditions for the precipitation and growth of dolomite and other pristine carbonate minerals [Bréheret, J.G., Fourmont, A., Macaire, J.J., Négrel, Ph., 2008. Microbially mediated carbonates in the Holocene lacustrine deposits of the Marais de Sarliève (French Massif Central) testify to the evolution of a restricted environment. Sedimentology 55, 557–578]. The preservation of the compounds was probably ensured by persistent reducing conditions during diagenesis, despite variable climatic conditions and related changes in lake level and sedimentation rate. Comparable distributions of hopanoic acids and hopanols, as well as roughly parallel variation with depth in the corresponding components in both fractions, strongly suggest that all the hopanoids derive from the same microbial precursors, slight shifts in the acid/alcohol ratio being governed by limited changes in redox conditions during early diagenesis. 相似文献
7.
V. Stefanova A. V. Volkov T. Serafimovskii A. A. Sidorov 《Doklady Earth Sciences》2013,451(2):818-823
Mineralogical-geochemical studies have shown that gold from the Plavica high sulfidation epithermal deposit (Republic of Macedonia) contains an admixture of Fe and Cu, that the morphology of gold indicates the proximity of the primary source, and that the chemical composition of gold is independent of its morphology. The composition of gold and its morphology are the direct searching criteria, indicating the genetic type of mineralization. Pan sampling and the subsequent study of the typomorphism of gold may serve as an effective method for searching for epithermal high sulfidation deposits. Pan sampling allows us to distinguish and study in detail the chemical composition and morphology of native gold from deposits of this type. 相似文献
8.
9.
Dylan J. Blumentritt Herbert E. WrightJr. Vania Stefanova 《Journal of Paleolimnology》2009,41(4):545-562
Study of Lake Pepin and Lake St. Croix began more than a century ago, but new information has permitted a closer look at the
geologic history of these two riverine lakes located on the upper Mississippi River system. Drainages from large proglacial
lakes Agassiz and Duluth at the end of the last glaciation helped shape the current valleys. As high-discharge outlet waters
receded, tributary streams deposited fans of sediment in the incised river valleys. These tributary fans dammed the main river,
forming riverine lakes. Lake Pepin was previously thought to be a single long continuous lake, extending for 80 km from its
dam at the Chippewa River fan all the way up to St. Paul, with an arm extending up the St. Croix valley. Recent borings taken
at bridge and dam locations show more than a single section of lake sediments, indicating a more complex history. The Minnesota
and Mississippi Rivers did not always follow their current paths. Valleys cut into bedrock but now buried by glacial sediment
indicate former river courses, with the most recent of these from the last interglacial period marked at the surface by chains
of lakes. The morphology of the Mississippi valley bottom, and thus the morphology of Lake Pepin as it filled the valley,
is reflect in part by the existence of these old valleys but also by the presence of glacial outwash terraces and the alluvial
fans of tributary streams. A sediment core taken in Lake Pepin near Lake City had a piece of wood in gravels just below lake
sediments that dated to 10.3 ka cal. BP, indicating that the lake formed as the Chippewa River fan grew shortly after the
floodwaters of Lakes Agassiz and Duluth receded. Data from new borings indicate small lakes were dammed behind several tributary
fans in the Mississippi River valley between the modern Lake Pepin and St. Paul. One tributary lake, here called Early Lake
Vermillion, may have hydraulically dammed the St. Croix River, creating an incipient Lake St. Croix. The tributary fans from
the Vermillion River, the Cannon River, and the Chippewa River all served to segment the main river valley into a series of
riverine lakes. Later the growth of the Chippewa fan surpassed that of the Vermillion and Cannon fans to create a single large
lake, here called late Lake Pepin, which extended upstream to St. Paul. Sediment cores taken from Lake Pepin did not have
significant organic matter to develop a chronology from radiocarbon dating. Rather, magnetic features were matched with those
from a Lake St. Croix core, which did have a known radiocarbon chronology. The Pepin delta migration rate was then estimated
by projecting the elevations of the top of the buried lake sediments to the dated Lake Pepin core, using an estimated slope
of 10 cm/km, the current slope of Lake Pepin sediment surface. By these approximations, the Lake Pepin delta prograded past
Hastings 6.0 ka cal BP and Red Wing 1.4 ka cal BP.
This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this
special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue. 相似文献
10.