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71.
Aqueous reactivity of phytoliths and plant litter: Physico-chemical constraints on terrestrial biogeochemical cycle of silicon 总被引:1,自引:0,他引:1
F. Fraysse F. Cantais O.S. Pokrovsky J. Schott J.D. Meunier 《Journal of Geochemical Exploration》2006,88(1-3):202
Quantification of silicon recycling by plants is hampered by the lack of physico-chemical data on reactivity of natural phytoliths and plant litter. In this study, we used an experimental approach for determining the silica release rates of phytoliths from tropical and temperate plants (bamboos, horsetails). Results are compared with litter degradation of horsetails and pine needles. Silica release rates suggest that the reactivity of phytolith surface does not depend on topology and geometry of local structures, and does not support the existence of preferential dissolution sites on surface. Litter degradation results suggest that the silica release rate is independent of cellulose hydrolysis that implies the presence of phytoliths in an “inorganic” pool not complexed with organic matter. 相似文献
72.
B.B. Kochnev B.G. Pokrovsky A.B. Kuznetsov V.V. Marusin 《Russian Geology and Geophysics》2018,59(6):585-605
We propose a detailed δ13C curve for the Vendian and Lower Cambrian (Tommotian) strata of the central Siberian Platform. Two positive carbon isotope excursions identified near the base of the Yuryakh Formation (up to 5.5‰) and in the lower Bilir Formation (up to 5‰) are assigned to the lowermost and middle Tommotian, respectively. This correlation is supported by paleontological data, specific 87Sr/86Sr values (0.70845-0.70856), and similar C isotope record in coeval Early Cambrian basins. The documented minor vertical oscillations (a few meters) of these isotope excursions relative to the formation boundaries in remote boreholes is presumably caused by the spatiotemporal migration of facies. A high-amplitude negative δ13C excursion (-8 to -11‰) in the upper Nepa Regional Stage putatively corresponds to the global Shuram-Wonoka negative carbon isotope excursion (Middle Ediacaran). Carbonates of the lower Nepa Regional Stage (Besyuryakh Formation) demonstrate positive δ13C values (up to 5‰) and minimum 87Sr/86Sr ratios of 0.70796-0.70832. The C and Sr isotope record of the Nepa Regional Stage provides its robust correlation with the Dal’nyaya Taiga and Zhuya Groups of the Patom Foredeep. Micropaleontological data herein reported and glacial diamictites documented at the base of the Vendian sedimentary cover both in the central Patom Foredeep and on its periphery suggest a full stratigraphic volume of the Ediacaran System in the most stratigraphically complete sections of the central Siberian Platform. 相似文献
73.
B. G. Pokrovsky N. M. Chumakov V. A. Melezhik M. I. Bujakaite 《Lithology and Mineral Resources》2010,45(6):577-592
The characteristic feature of many Upper Neoproterozoic glacial sequences is their “cap carbonates” (CC) resting without visible
unconformity upon glaciogenic diamictites. Such an unusual association, peculiar structures and textures, and negative δ13C values (approximately −4 ± 2‰) that are atypical of marine carbonates provoked long debates about the nature of these carbonates,
which play an important role in the Snowball Earth hypothesis. According to this hypothesis, the Earth was entirely covered
by ice during large-scale glaciations, and CC accumulation was related to the global change in geochemical processes. In this
work, we discuss data on the chemical and isotopic (C, O, Sr) compositions of CCs, which overlie glacial sediments of the
Nichatka and Bol’shoi Patom formations accumulated in different parts of the Neoproterozoic Patom paleobasin (Central Siberia).
High concentrations of Fe (up to 6400 ppm), Mn (2320 ppm), and radiogenic Sr (87Sr/86Sr0 up to 0.7172) established in CCs indicates a strong influence of the continental flow. Extraordinary Snowball Earth conditions
are not necessary for the accumulation of these rocks, geochemical and sedimentological properties of which may be explained
by the discharge of thawing waters into partly or completely isolated near-glacier basin, their intermittent freezing, and/or
washout of “frozen” carbonates from the surface of thawing glaciers. The peculiar thin-laminated texture of CC may be related
to seasonal processes of climatic cycles. They were accumulated in the course of general (relatively long-term) depletion
of the atmosphere and hydrosphere in 13C, which has nothing to do with the CC formation as a specific type of carbonate sediments. Amplitude and duration of the
negative δ13C excursion in carbonates associated with the Lower Vendian glacial sediments (665–635 Ma) are appreciably lower than the
negative anomaly in rocks of the Zhuya Group that likely correspond to the Shuram-Vonoka Event (∼560−580 Ma ago), which probably
marks the crucial point in the Precambrian deglaciation: mass destabilization of methane hydrates and degradation of the Early
Vendian psychrosphere in oceans. 相似文献
74.
V. P. Shevchenko V. B. Korobov A. P. Lisitzin A. S. Aleshinskaya O. Yu. Bogdanova N. V. Goryunova I. V. Grishchenko O. M. Dara N. N. Zavernina E. I. Kurteeva E. A. Novichkova O. S. Pokrovsky F. V. Sapozhnikov 《Doklady Earth Sciences》2010,431(2):497-501
The descent of a large quantity of dust responsible for bright colors of atmospheric precipitation in the temperate, subpolar, and polar zones of the northern hemisphere is rarely observed [1–5]. In the twentieth century and in the beginning of the twenty-first century in the northern part of European Russia, such events had not been registered right up to March 25–26, 2008. At that time in some parts of the Arkhangelsk region, Komi Republic, and Nenets Autonomous Area, atmospheric precipitation as moist snow and rain responsible for sand and saffron colors of ice crust formation on the snow surface was observed. Thus, due to detailed mineralogical, geochemical, pollen, diatom, and meteorological investigations, it was established that the main source of the yellow dust is the semidesert and steppe regions of the Northwest Kazakhstan, and the Volgograd and Astrakhan regions, Kalmykia. 相似文献
75.
O.S. Pokrovsky A. Feurtet-Mazel R.E. Martinez S. Morin M. Baudrimont T. Duong M. Coste 《Applied Geochemistry》2010
This study addresses the interaction of Cd with natural biofilms of periphytic diatoms grown during different seasons in metal-contaminated and metal-non-contaminated streams, along a tributary of the Lot River, France. Specifically, it aims to test whether the biofilms from contaminated sites have developed a protective mechanism due to high Cd exposure. Towards this goal, reversible adsorption experiments on untreated biofilms were performed in 0.01 M NaNO3 with a pH ranging from 2 to 8, Cd concentration from 0.5 to 10,000 μg/L and exposure time from 1 to 24 h. Two types of experiments, pH-dependent adsorption edge and constant-pH “Langmuirian”-type isotherms were conducted. Results were adequately modeled using a Linear Programming Model. It was found that the adsorption capacities of natural biofilm consortia with respect to Cd do not depend on season and are not directly linked to the growth environment. The biofilms grown in non-contaminated (4.6 ppb Cd in solid) and contaminated (570 ppb Cd in solid) settings exhibit similar adsorption capacities in the Cd concentration range in solution of 100–10,000 μg/L but quite different capacities at low Cd concentration (0.5–100 μg/L); unexpectedly, the non-contaminated biofilm adsorbs approximately 10 times more Cd than the contaminated one. It is therefore possible that the strong low-abundant ligands (for example, phosphoryl or sulfhydryls) are already metal-saturated on surfaces of biofilm grown in the contaminated site whereas these sites are still available for metal adsorption in samples grown in non-contaminated sites. 相似文献