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111.
Ice-shell thickness and ocean depth are calculated for steady state models of tidal dissipation in Europa's ice shell using the present-day values of the orbital elements. The tidal dissipation rate is obtained using a viscoelastic Maxwell rheology for the ice, the viscosity of which has been varied over a wide range, and is found to strongly increase if an (inviscid) internal ocean is present. To determine steady state values, the tidal dissipation rate is equated to the heat-transfer rate through the ice shell calculated from a parameterized model of convective heat transfer or from a thermal conduction model, if the ice layer is found to be stable against convection. Although high dissipation rates and heat fluxes of up to 300 mWm−2 are, in principle, possible for Europa, these values are unrealistic because the states for which they are obtained are thermodynamically unstable. Equilibrium models have surface heat flows around 20 mWm−2 and ice-layer thicknesses around 30 km, which is significantly less than the total thickness of the H2O-layer. These results support models of Europa with ice shells a few tens of kilometers thick and around 100-km-thick subsurface oceans. 相似文献
112.
Stabilisation of soil organic matter by interactions with minerals as revealed by mineral dissolution and oxidative degradation 总被引:4,自引:0,他引:4
Karin Eusterhues Cornelia Rumpel Markus Kleber Ingrid Kgel-Knabner 《Organic Geochemistry》2003,34(12):1591-1600
Soil organic matter is known to contain a stable fraction with an old radiocarbon age. Size and stabilisation processes leading to the formation of this old soil carbon pool are still unclear. Our study aims to differentiate old organic matter from young and labile carbon compounds in two acid forest soils (dystric cambisol, haplic podzol). To identify such fractions soil samples were exposed to oxidation with Na2S2O8 and to dissolution by hydrofluoric acid (HF). A negative correlation between 14C activity and carbon release after dissolution of the mineral matrix by HF indicates a strong association of stabilised carbon compounds with the mineral phase. A negative correlation between the 14C activity and the relative proportion of carbon resistant to oxidation by Na2S2O8 shows that young carbon is removed preferentially by this treatment. The fraction remaining after oxidation represents a certain stabilised, long residence time carbon pool. This old fraction comprises between 1 and 30% of the total soil organic carbon in the surface horizons, but reaches up to 80% in the sub-surface horizons. Old OC is mainly stabilised by organo-mineral associations with clay minerals and/or iron oxides, whereas intercalation in clay minerals was not found to be important. 相似文献
113.
Evidence for a nitrogen flux directly derived from the European subcontinental mantle in the Western Eger Rift, Central Europe 总被引:1,自引:0,他引:1
In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO2-rich (>99 vol.%) and have δ13C values of −1.95 and −4.29‰, respectively. With distance from the center of CO2 degassing (Bublak) the δ13C values decrease, most likely due to physico-chemical fractionation of CO2 between gaseous and aqueous phases rather than to admixture of organic/biogenic CO2. The δ15N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by 3He/4He ratios decreasing from 5.9 Ra in the center (Bublak) to 0.8 Ra in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest 3He/4He ratio (5.9 Ra) is accompanied by the highest 40Ar/36Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ15N value of ≈−4.5 ± 1.0‰ represents the first free gas δ15N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the 3He/4He ratios, the δ15N values are equal for ESCM and MORB, respectively. 相似文献
114.
Recent studies of pan sediment stratigraphy in southern Africa have indicated the potential of geophysical and geochemical analyses for elucidating the evolution of pans and the environmental conditions under which formation took place. We have investigated three cores, of 1.8 to 2.6 m in length, from Lebatse Pan in southeast Kalahari, Botswana, and compare the results with the few other similar studies available. Through the sedimentological characteristics of Lebatse we identify cycles of deposition, chemical precipitation and post-depositional alteration under shifting environmental conditions from c . 60,000 years ago to the present. Four periods are separated by depositional hiatuses, marking dry conditions during which deflation of the pan cannot be excluded. During the two oldest periods the sequence of finely laminated, magnesia-rich sediments suggests deposition in a permanent shallow waterbody under evaporative conditions, causing chemical precipitation of dolomite from groundwater. Less evaporative conditions are inferred for the third period of deposition between hiatuses H2 and H1. The youngest period, 14 C-dated to 29,000 years BP, is characterised by water-logged conditions and the development of reed beds. It is believed to have been laid down under fluctuating water levels. The cycles of deposition exhibited in the pan cores appear synchronous with wetter periods in the Southern Kalahari, indicated by the limited evidence currently available. 相似文献
115.
116.
Roland Schmidt Christian Kamenik Richard Tessadri Karin Anne Koinig 《Journal of Paleolimnology》2006,35(3):491-505
Major and trace elements, minerals, and grain-size were analysed from the early to mid-Holocene (12 to 4 ky BP) period of
a sediment core from the Alpine lake Oberer Landschitzsee (ObLAN, 2076 m a.s.l.), which is located on predominantly crystalline
bedrock on the southern slopes of the Austrian Central Alps. Geochemistry and mineralogy were compared with diatom-inferred
(Di-) ‘date of autumn mixing’ (Amix), DOC, pH, and selected indicator pollen species from the same sediment core. Principal components analysis (PCA) indicated
a positive correlation between processes triggered by temperature and precipitation (e.g., lake mixing, DOC). PCA grouped
indicators of physical weathering and enhanced catchment run-off (sand, quartz, feldspar), elements of weathering (e.g., Ti,
Rb, Mn) under dryer conditions (clay to silt fractions), and elements that probably were related to changes in redox conditions
(Cu, Fe, S, Zn). The duration and height of the snow-pack played an important role in this high-alpine environment, affecting
weathering, erosion, pH, and lake stratification. Low Alnus viridis pollen abundance, together with markers for increased elements of erosion, indicated extensive snow-pack. Changes in S coupled
with As, and elements indicating increased weathering, reflected climate oscillations. LOI was affected by productivity and
erosion. High (late) Di–Amix coupled with increased Di-DOC indicated prolonged summers with increased productivity. Cold and wet (snow-rich) phases and
subsequent melting caused low pH and a decoupling of the significant linear correlation between sedimentary Ca and Di-pH.
Weathering and leaching during climate deteriorations opposed the long-term trend in a loss of cations and forced in-lake
alkalinity generation during the following lake warming. Overall, the multi-proxy study indicated complex climate-driven processes
within different time-scales (long-term trends, climate oscillations, seasonality). The climate oscillations within 12–5 ky
BP corresponded well with the cool and wet phases known from central Europe suggesting a dominant common Atlantic climate
impact. When Mediterranean climate established between 5 and 4 ky BP, its influence on the southern slopes of the Alps increased. 相似文献
117.
118.
Diatoms from the genus Achnanthidium are abundant in rivers, streams, and springs of the Appalachian Mountains. They inhabit clean and polluted waters, including those affected by acid mine drainage. The identification of Achnanthidium taxa is difficult due to their small cell size and insufficient information in the diatom floras. We studied the taxonomy and ecology of Achnanthidium in Appalachian rivers by analyzing a data set of benthic diatom samples and corresponding water chemistry data collected during several water-quality surveys from 181 sampling sites. Ten species were identified using scanning electron and light microscopy: A. alpestre (Lowe & Kociolek) Lowe & Kociolek, A. atomus (Hustedt) Monnier, Lange-Bertalot, & Ector, A. deflexum (Reimer) Kingston, A. duthii (Sreenivasa) Edlund, A. eutrophilum (Lange-Bertalot) Lange-Bertalot, A. cf. gracillimum (Meister) Lange-Bertalot, A. cf. latecephalum Kobayasi, A. minutissimum (Kützing) Czarnecki (sensu lato), A. reimeri (Camburn) comb. nov., and A. rivulare Potapova & Ponader. The distribution of common taxa in relation to water chemistry was studied by fitting non-parametric regression models (generalized additive models, GAM, and non-parametric multiplicative regression models, NPMR) to species relative abundances. Studied Achnanthidium species differed considerably in their responses to water chemistry. These results suggest that species-level identifications will lead to more accurate bioassessments. 相似文献
119.
Rain-fed agriculture is threatened by an increased frequency of droughts worldwide thereby putting millions of livelihoods at risk especially in sub-Saharan Africa. This makes drought preparedness critical. In this study, we sought to establish whether maize yield can be predicted using the number of dry dekads that occur at specific maize growth stages for purposes of yield early warning. The dry dekads were derived from remotely sensed Vegetation Condition Index calculated from the SPOT NDVI time series ranging from 1998 to 2013. Regression between dry dekads and maize yield show a negative linear relationship for four growing seasons (2010–2013) and indicates that dry dekads at both the vegetative and reproductive stages are important for predicting maize yield. Results suggest that early warning alert could be given using dry dekads that occur at the vegetative stage, while those at the reproductive stage can be used to give better yield estimate later on. 相似文献
120.
Karin Şeşetyan Laurentiu Danciu Mine B. Demircioğlu Tümsa Domenico Giardini Mustafa Erdik Sinan Akkar Levent Gülen Mehdi Zare Shota Adamia Anooshiravan Ansari Avetis Arakelyan Ayşegül Askan Mher Avanesyan Hektor Babayan Tamaz Chelidze Raffi Durgaryan Ata Elias Hossein Hamzehloo Khaled Hessami Doğan Kalafat Özkan Kale Arkady Karakhanyan Muhammad Asif Khan Tahir Mammadli Mahmood Al-Qaryouti Mohammad Sayab Nino Tsereteli Murat Utkucu Otar Varazanashvili Muhammad Waseem Hilal Yalçın Mustafa Tolga Yılmaz 《Bulletin of Earthquake Engineering》2018,16(8):3535-3566
The Earthquake Model of Middle East (EMME) Project aimed to develop regional scale seismic hazard and risk models uniformly throughout a region extending from the Eastern Mediterranean in the west to the Himalayas in the east and from the Gulf of Oman in the south to the Greater Caucasus in the North; a region which has been continuously devastated by large earthquakes throughout the history. The 2014 Seismic Hazard Model of Middle East (EMME-SHM14) was developed with the contribution of several institutions from ten countries. The present paper summarizes the efforts towards building a homogeneous seismic hazard model of the region and highlights some of the main results of this model. An important aim of the project was to transparently communicate the data and methods used and to obtain reproducible results. By doing so, the use of the model and results will be accessible by a wide community, further support the mitigation of seismic risks in the region and facilitate future improvements to the seismic hazard model. To this end all data, results and methods used are made available through the web-portal of the European Facilities for Earthquake Hazard and Risk (www.efehr.org). 相似文献