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281.
Potential implications of differential preservation of testate amoeba shells for paleoenvironmental reconstruction in peatlands 总被引:1,自引:0,他引:1
Edward A. D. Mitchell Richard J. Payne Mariusz Lamentowicz 《Journal of Paleolimnology》2008,40(2):603-618
Testate amoebae are now commonly used in paleoenvironmental studies but little is known of their taphonomy. There is some
experimental evidence for differential preservation of some testate amoeba shell types over others, but it is unclear what,
if any impact this has on palaeoenvironmental reconstruction. To investigate this issue we looked at palaeoecological evidence
for the preservation of different shell types. We then investigated the possible impact of selective preservation on quantitative
palaeoenvironmental inference. We first used existing palaeoecological data sets to assess the vertical patterns of relative
abundance in four testate amoeba shell types: (1) shells made of secreted biosilica plates (idiosomes, e.g. Euglypha), (2) idiosomes with thick organic coating (Assulina), (3) proteinaceous shells (e.g. Hyalosphenia), (4) shells built from recycled organic or mineral particles (xenosomes) (e.g. Difflugia, Centropyxis). In three diagrams a clear pattern of decay was only observed for the idiosome type. In order to assess the implications
of differential preservation of testate amoeba taxa for paleoenvironmental reconstruction we then carried out simulations
using three existing transfer functions and a wide range of scenarios, downweighting different test categories to represent
the impact of selective test decomposition. Simulation results showed that downweighting generally reduced overall model performance.
However downweighting a shell type only produced a consistent directional bias in inferred water table depth where that shell
type is both dominant and shows a clear preference along the ecological gradient. Applying a scenario derived from previous
experimental work did not lead to significant difference in inferred water table. Our results show that differential shell
preservation has little impact on paleohydrological reconstruction from Sphagnum-dominated peatlands. By contrast, for the minerotrophic peatlands data-set loss of idiosome tests leads to consistent underestimation
of water table depth. However there are few studies from fens and it is possible that idiosome tests are not always dominant,
and/or that differential decomposition is less marked than in Sphagnum peatlands. Further work is clearly needed to assess the potential of testate amoebae for paleoecological studies of minerotrophic
peatlands. 相似文献
282.
Summary This paper concerns the use of airborne or ground-based Doppler radars to observe precipitating systems over complex orography.
As nearly all of the previous experiments involving Doppler radars were conducted over flat surfaces over the continents or
the oceans, new techniques are needed firstly to separate ground clutter from meteorological signal and, in the case of airborne
Doppler observations, to deduce navigational errors. Secondly, it is necessary to take the atmospheric circulation induced
by orography into account in the three-dimensional wind field analysis. Variational techniques are presented to solve these
problems.
The proposed methods are tested with simulated ground-based and airborne Doppler radar observations for analytic flows over
analytic terrains and for numerically simulated wind and reflectivity fields for the Brig event (22 September 1993) of heavy
precipitation over the southern flank of the Alps (Cosma and Richard, 1998), and with actual airborne Doppler data relative
to weak snow showers over the Rocky Mountains on 12 March 1995.
Received March 22, 1999/Revised June 1, 1999 相似文献
283.
284.
285.
Geocentre motion from the DORIS space system and laser data to the Lageos satellites: comparison with surface loading data 总被引:3,自引:0,他引:3
F. Bouillé A. Cazenave J. M. Lemoine J. F. Crétaux 《Geophysical Journal International》2000,143(1):71-82
Surface mass redistribution within the Earth system, especially in the atmosphere, oceans, continents and ice sheets, causes the position of the centre of mass to vary in a reference frame attached to the solid Earth. Space techniques are now precise enough to measure the centre of mass motion. Here we present a determination of the centre of mass coordinates at regular monthly intervals using DORIS data on SPOT‐2, SPOT‐3 and Topex–Poseidon (1993–1997) and laser data on Lageos‐1 and Lageos‐2 (1993–1996). The amplitude and phase of the space‐geodesy‐derived annual cycle for each coordinate are further compared to estimates based on surface mass redistribution at the Earth surface derived from various climatic data sources: surface pressure, soil moisture, snow depth and ocean mass variations. 相似文献
286.
The specific features of the generation and intensification of internal gravity wave structures in different atmospheric-ionospheric regions, caused by zonal local nonuniform winds (shear flows), are studied. The model of the medium has been explained and an initial closed system of equations has been obtained in order to study the linear and nonlinear dynamics of internal gravity waves (IGWs) when they interact with the geomagnetic field in a dissipative ionosphere (for the D, E, and F regions). 相似文献
287.
288.
Gabriella Sch?ll-Barna Attila Demény Gábor Serlegi Szilvia Fábián Pál Sümegi István Fórizs Bernadett Bajnóczi 《Journal of Paleolimnology》2012,47(1):87-100
Geochemical records of bivalve shells have been increasingly studied in the last decade to obtain information on climate conditions.
In this paper we present stable isotope compositions of living and prehistoric shells of freshwater mussels (Unionidae) and
their relationships with climate conditions in a shallow lake environment of Lake Balaton, West-Central Hungary. Physical
conditions and stable oxygen isotope compositions of lake water samples were monitored where living bivalves were collected.
Comparisons between seasonal variations in ambient temperature, water composition and within-shell isotopic variations indicate
that the shells of Unio pictorum do reflect local changes at high resolution and thus can be used to study past conditions. Additionally, shells covering
the last two decades were gathered at several locations along the lake in order to determine spatial and temporal variations
in the shells’ isotopic compositions as a function of weather conditions. As an application, prehistoric shells collected
in archaeological excavations were analysed in order to study past environmental variations. Climate variations during the
Late Copper Age (5460–4870 cal. yr BP) have been assumed on the basis of geomorphological and archaeozoological observations
at the site Balatonkeresztúr-Réti-dűlő (south of Lake Balaton), that suggested increasing humidity as a cause of changes in
settlement location and domestic livestock husbandry. Stable carbon and oxygen isotope compositions of prehistoric bivalve
shells were analyzed from excavations representing five archaeological subphases (Boleráz subphase, 5460–5310 cal. yr BP;
two transitional subphases around 5310 cal. yr BP; Early Classic subphase, 5310–5060 cal. yr BP; Late Classic subphase, 5040–4870 cal.
yr BP). The analyses revealed significant negative C and O isotope shifts in the transitional subphases relative to the earlier
and later subphases. The isotopic variations indicate that the local climate became relatively wet and possibly cold around
5310 cal. yr BP, then it returned to drier (and likely warmer) conditions during the Classic subphases. This interpretation
is in agreement with previous studies on climate changes related to the “5.3 ky event” in the European continental area and
the North Atlantic Region, indicating an Atlantic influence in the Carpathian Basin. 相似文献
289.
This paper presents a typology of local‐government data sharing arrangements in the US at a time when spatial data infrastructures (SDI) are moving into a second generation. In the first generation, the US National Spatial Data Infrastructure (NSDI) theoretically involved a pyramid of data integration resting on local‐government data sharing. Availability of local‐government data is the foundation for all SDI‐related data sharing in this model. However, first‐generation SDI data‐sharing activities and principles have gained only a tenuous hold in local governments. Some formalized data sharing occurs, but only infrequently in response to SDI programmes and policies. Previous research suggests that local‐government data sharing aligns with immediate organizational and practical concerns rather than state or national policies and programmes. We present research findings echoing extending these findings to show that local‐government data sharing is largely informal in nature and is undertaken to support existing governmental activities. NSDI principles remain simply irrelevant for the majority of surveyed local governments. The typology we present distinguishes four distinct types of local‐government data sharing arrangements that reflect institutional, political, and economic factors. The effectiveness of second generation, client‐service‐based SDI will be seriously constrained if the problems of local government take‐up fail to be addressed. 相似文献
290.
The determination of boundary points and their correlation are prerequisites for defining the PrecambrianCambrian boundary and the intercontinental correlation. A large number of good Precambrian-Cambrian boundary sections in South Asia not only possess the conditions for selection of a boundary stratotype, but also provide the basis for the long-distance correlation of those boundary sections.Through a detailed study of the latest data obtained from the typical sections in South China, the Lesser Himalaya Mountains and the Alborz Mountains. the locations of boundary points have been defined, and a correlation scheme is proposed, which contributes to the solution of the problem of worldwide correlation of boundary sections. 相似文献