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11.
Replication of results is a basic tenet of science, but in palaeoecology this is very time-consuming and the ‘signal’ is subject to ‘noise’. The derivation of proxy-climate signals from ombrotrophic peat was carried out originally using samples from open peat faces where the stratigraphic relationships could be easily observed. Now that such sections are rare and often degraded there is a need to demonstrate that data can be replicated from core profiles. Ten short cores taken from two adjacent bogs have been analysed for macrofossils and show a coherent series of changes, which are also similar to previous profiles from the same sites. It is concluded that variation between profiles is slight and less than observations of present vegetation mosaics might suggest. Recommendations for a standard approach to fieldwork on raised bogs that emphasises the utility of subfossil pool layers are proposed and the need for a secure chronology is stressed. © 1998 John Wiley & Sons, Ltd. 相似文献
12.
We present a new approach to enhancing weak prestack reflection signals without sacrificing higher frequencies. As a first step, we employ known multidimensional local stacking to obtain an approximate ‘model of the signal’. Guided by phase spectra from this model, we can detect very weak signals and make them visible and coherent by ‘repairing’ corrupted phase of original data. Both presented approaches – phase substitution and phase sign corrections – show good performance on complex synthetic and field data suffering from severe near-surface scattering where conventional processing methods are rendered ineffective. The methods are mathematically formulated as a special case of time-frequency masking (common in speech processing) combined with the signal model from local stacking. This powerful combination opens the avenue for a completely new family of approaches for multi-channel seismic processing that can address seismic processing of land data with nodes and single sensors in the desert environment. 相似文献
13.
Molecular analysis of cyanobacterial mat communities indicated that cyanobacteria, ammonia-oxidizing Archaea (AOA), and ammonia-oxidizing bacteria (AOB) coexist in those systems, competing for ammonium; this situation would imply competitive exclusion. We attempted to model how ammonia utilization niche partitioning occurs, and how ammonium levels can influence the interaction between those groups in a one-dimensional diffusionlimited system using Michaelis-Menten kinetics to describe ammonium consumption by each of those three groups. In our model, AOAs were able to dominate ammonium uptake by the community under most circumstances, except for unrealistically high (millimolar) levels of ammonium, where AOBs gained advantage. Cyanobacteria were unable to effectively compete for ammonium with either AOBs or AOAs throughout the mat at all ammonium concentrations and cell counts, suggesting that the presence of AOAs or AOBs forces cyanobacteria into nitrogen fixation mode. Such interaction can make cyanobacterial mats a net nitrogen source, as well as provide a carbon-independent energy transfer pathway from primary producers to the rest of the ecosystem. 相似文献
14.
D. V. Ionov M. A. Kshevetskaya Yu. M. Timofeev A. V. Poberovskii 《Izvestiya Atmospheric and Oceanic Physics》2013,49(5):519-529
Atmospheric NO2 content data obtained from regular ground-based measurements of solar IR radiation in the St. Petersburg region using a spectrometer with a high spectral resolution are analyzed. The absorption spectra of the NO2 multiplet in the vicinity of ~2915 cm?1 allow one to obtain data on variations in the stratospheric total content of NO2 in 2009–2011. The accuracy of these data is estimated from their comparison with data obtained from independent ground-based and satellite measurements. The parameters of the seasonal cycle of the stratospheric content of NO2 are estimated. The body of data accumulated during these measurements in the IR region made it possible to isolate the component of a daytime photochemical increase in the stratospheric content of NO2 and estimate its rate. 相似文献
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Akira Mizuta Hideaki Takabe Jave O. Kane Bruce A. Remington Dmitri D. Ryutov Marc W. Pound 《Astrophysics and Space Science》2005,295(1-2):197-202
A sample of 54 6.7-GHz methanol maser sources was monitored at HartRAO for 4 years, and 11 12.2-GHz methanol masers for 3 years. The majority of the maser features display a significant degree of variability but with a wide range of timescales and behaviors. Some maser features remained unvarying throughout the monitoring programme, while others showed sporadic flares or sudden drops in flux density. Yet another group show quasi-periodic and periodic variations. In some cases the maser features dropped below the detection limit for a significant length of time before increasing in intensity and reappearing. 相似文献
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Feldspar-bearing lherzolite xenoliths in alkali basalts from Hamar-Daban,southern Baikal region,Russia 总被引:4,自引:0,他引:4
D. A. Ionov Suzanne Y. O'Reilly Igor V. Ashchepkov 《Contributions to Mineralogy and Petrology》1995,122(1-2):174-190
Lherzolite xenoliths in Miocene to Pleistocene basalts from five sites in the Hamar-Daban range in southern Siberia provide
sampling of the mantle close to the axis of the Baikal rift. These anhydrous spinel lherzolites commonly have foliated fabrics
and spongy rims around clinopyroxene, and many contain accessory feldspar. The feldspar occurs in reaction zones adjacent
to spinel and orthopyroxene (where it appears to have been formed by the reaction: spl+opx+cpx+fluid →fs+ol) and less commonly
as thin, irregular veins. The feldspars have variable compositions but are generally alkali-rich; their K2O content ranges from 0.3 to 11.2% and is much higher than in plagioclase from orogenic lherzolites (usually <0.1% K2O). The temperature range for the Hamar-Daban xenolith suite (950–1010° C) is more restricted than for spinel peridotite xenoliths
from other occurrences in the Baikal area. The feldspar-bearing lherzolites yield equilibration temperatures similar to or
slightly lower than feldspar-free ones. The majority of the Hamar-Daban lherzolites are fertile and clinopyroxene-rich, as
for most other occurrences in the Baikal region. Trace element compositions of selected xenoliths and their clinopyroxenes
were determined by ICP-MS, INAA and proton microprobe. Feldspar-bearing xenoliths are enriched in alkalies indicating that
feldspar formation is associated with addition of material and is not simply due to isochemical phase changes. Most xenoliths
and their clinopyroxenes studied are depleted in light REE and have contents of Sr, Zr and Y common for fertile or moderately
depleted mantle peridotites. Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of clinopyroxenes
indicate long-term depletion in incompatible elements similar to unmetasomatised xenoliths from other occurrences south and
east of Lake Baikal. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enrichment in alkalies
appear to be recent phenomena related to infiltration of an alkali-rich, H2O-poor fluid into spinel peridotites.
Received: 20 March 1995 / Accepted: 26 June 1995 相似文献
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20.
Quantitative plant macrofossil and colorimetric humification analyses have been used to reconstruct proxy-climate from two paired ombrotrophic bogs in northern England (Coom Rigg Moss and Felecia Moss). Detrended correspondence analysis was used to transform the raw floral data into indices of mire surface wetness. The chronology of each peat profile was determined by radiocarbon assay, supported by pollen correlations. Palaeoclimatic reconstructions have been made by linking documented historical changes in climate, and other proxy-climate records, to those inferred from the sites investigated in the study region. Both sites contain a sensitive palaeoclimatic record, as ten periods of increased effective precipitation have been detected between ca. cal. ad 1770–1800, ad 1400–1470, ad 1110–1260, ad 920–1060, ad 550–670, ad 210–360, 30 bc to ad 80, 180–130 bc, 590–520 bc and 760–710 bc. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献