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Minna V?liranta Jan Weckstr?m Susanna Siitonen Heikki Sepp? Jyri Alkio Sari Juutinen Eeva-Stiina Tuittila 《Journal of Paleolimnology》2011,45(3):339-352
We studied multiple variables in a sediment core from Lake Kipojärvi, northern Finland, to investigate Holocene ecosystem changes in relation to catchment characteristics and known climate variations. We focused on a forested catchment because previous paleolimnological studies conducted in Fennoscandia focused mainly on subarctic lakes within a range of shifting treeline(s). Data on aquatic macrophytes, diatoms, Cladocera, C:N ratio, organic matter (LOI) and regional vegetation (pollen), revealed a three-phase limnological development. The early Holocene, species-rich, mesotrophic lake was transformed into an oligotrophic, species-poor aquatic ecosystem by the early middle Holocene, ca. 7,500 cal years BP, earlier than has generally been reported. The transition involved considerable changes in aquatic macrophytes. Changes in the Cladocera and diatom communities appear to have been linked to aquatic macrophyte development, which in turn, was probably regulated by catchment development and hydrology, and a consequent decrease in nutrient input from the catchment. During the more humid late Holocene, surface flow from the catchment probably increased, but the lake??s nutrient status remained oligotrophic. Possible reasons for low nutrient concentration in the late Holocene include: 1) slower biogeochemical cycling due to cooler climate, 2) a new hydrologic outlet and associated shorter water-retention times, and 3) accelerated peatland development in the catchment that affected water flow patterns and nutrient cycling. 相似文献
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Jouni I. Peltoniemi Jukka Piironen Jyri Nrnen Juha Suomalainen Risto Kuittinen Lauri Markelin Eija Honkavaara 《ISPRS Journal of Photogrammetry and Remote Sensing》2007,62(6):434-446
The Sjökulla test site is used for testing and calibrating aerial images. The permanent test field is made of four types of gravel (dark gabbro, grey granite, red granite, white limestone) in two sizes (diameters 8–16 mm and 4–8 mm) set in various patterns. The bidirectional reflection properties of the targets together with their temporal changes must be known in order to carry out radiometric and spectral evaluation and calibration.The bidirectional reflectance distribution functions (BRF) of the gravel have been measured several times in the test fields using portable field goniospectrometers belonging Finnish Geodetic Institute (FGI), and once using the European Goniometic Facility (EGO) of the Joint Research Centre (JRC) at Ispra, Italy. Detailed BRFs have been obtained, showing features typical to particulate media, e.g. a small bowl shape, strong backscattering, and smooth wavelength dependence. Temporal range measurements over several years show that the black gabbro and red granite are fairly stable, while the grey granite has changed somewhat over the years and the white limestone has experienced dramatic darkening effects, requiring action to be taken.The measured BRF data have increased the usability of the test field considerably. The results are also useful in the development and validation of scattering models for particulate media. The site has proved to be a good test bench for goniospectrometric instruments, too. 相似文献
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In the analysis of X-ray fluorescence spectra from planetary surfaces, it is traditionally assumed that the observed surface is a plane-parallel, smooth, and homogeneous medium. The spectral and spatial resolutions of the instruments that have been used to measure X-ray emission from planetary surfaces to date have been such that this has been a reasonable assumption, but a new generation of X-ray spectrometers will provide enhanced spectral and spatial resolutions when compared with previous instrumentation. In light of these improvements in performance, it is important to assess how the requirements on the methodology of analysis of spectra may change when the surface is considered as a regolith. At other wavelengths, varying physical properties of planetary regoliths, such as the packing density, are known to have an effect on the observed signal as a function of viewing geometry. In this paper, the results from laboratory X-ray fluorescence measurements of regolith analogue materials at different viewing geometries are presented. Characteristic properties of the regolith such as particle sizes and packing density are found to affect the measured elemental line ratios. A semiempirical function is introduced as a tool for fitting fluorescent line intensity dependences as a function of viewing geometry. The importance of the results is discussed and recommendations are made for the future analysis of planetary X-ray fluorescence data. 相似文献
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