The Niger (I) carbonaceous chondrite and implications for the origin of aggregates and isolated olivine grains in C2 chondrites     

Christian Desnoyers 《Earth and Planetary Science Letters》1980,47(2):223-234

The origin of olivine grains in C2 carbonaceous chondrites is a controversial topic: directly condensed material or detrital remnants of preexisting chondrules? This study shows that the Niger C2 meteorite is similar to Murchison but reveals several interesting features in relation to the origin of the olivine. Microprobe analysis of olivine (Si, Fe, Mg, Ca, Mn, Cr), glass and nickel-iron inclusions within the grains, and Fe-S-O phase as well as the relationships between the olivine grains in the aggregates, between the grains and the interstitial phyllosilicate matrix, between the inclusions and their host olivine grains, and the morphology of some aggregates all show that two populations of olivine coexist, probably crystallized from chondrule melts rather than by direct condensation from a solar nebula gas. The characteristics of the nickel-iron inclusions within the olivine suggest a magmatic chondrule-making stage from previously condensed materials.  相似文献   

Northern Hess Rise extended by multi-beam sonar     

N. Christian Smoot 《Tectonophysics》1982,89(4):T27-T32

The bathymetry for three of the ridges north of the Hess Rise is presented from U.S. Naval Oceanographic Office SASS surveys. These ridges strike in directions not parallel or perpendicular to any other features in the North Pacific Ocean. They were previously uncharted and presumably undetected. They probably predate the Emperor Guyots and are contemporaries of the Musician Seamounts.  相似文献   

Productivity of algal epiphytes in a Georgia salt marsh: effect of inundation frequency and implications for total marsh productivity     

R. Christian Jones 《Estuaries and Coasts》1980,3(4):315-317

Primary production by algal epiphytes of dead Spartina alterniflora shoots in a Georgia salt marsh was measured using the ¹⁴C technique. A 2³ factorial design was used to quantify the effects of light intensity and inundation frequency (stem height) on carbon fixation at two sites along a salt marsh creek. Algae inundated daily fixed carbon more rapidly than those which had dried for several days, but this may have been the results of greater biomass on more frequently immersed stems. This result corroborates studies showing desiccation is not always a severe stress for intertidal algae. Similarity of epiphyte algal productivity to that of salt marsh benthic diatoms suggests that, given adequate substrate, the epiphytes may be an important source of primary production during some seasons of the year.  相似文献   

The Earth. Its origin, structure and evolution: M.W. McElhinny (Editor). Academic Press, London-New York- San Francisco, 1979, 597 pp., $ 7     

Hans Christian Larsen  David Bridgwater 《Tectonophysics》1980,68(3-4)

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Carbon isotope geochemistry of the 3.7 × 10⁹-yr-old Isua sediments,West Greenland: implications for the Archaean carbon and oxygen cycles     

Manfred Schidlowski  Peter W.U Appel  Rudolf Eichmann  Christian E Junge 《Geochimica et cosmochimica acta》1979,43(2):189-199

One hundred and twenty-four carbonate samples from the meta-sedimentary sequence of the 3.7 × 10⁹ yr old Isua supracrustal belt (W-Greenland) have yielded a δ¹³C_carb average of ?2.5 ± 1.7%. vs PDB and a δ¹⁸O_carb average of +13.0 ± 2.5%. vs SMOW. The oxygen mean comes fairly close to the averages of other early Precambrian carbonates. The carbon average, however, is some 2%. more negative than those of younger marine carbonates. In terms of a simple terrestrial ¹³C mass balance, if δ¹³C_carb values are original sedimentary values, this more negative δ¹³C average would imply a considerably smaller $\text{C}{}_{\mathrm{org}}\text{C}{}_{\mathrm{carb}}$ ratio in the sedimentary shell during Isua times, and would thus support the concept of a gradual buildup of a sedimentary reservoir of organic carbon during the early history of the Earth. Since, however, the Isua supracrustal rocks have experienced amphibolite-grade metamorphism, which in other areas has been shown to lower δ¹³C_carb values, it is most likely that the original values of these rocks were approx 0%.. This indicates that C_orx and C_carb were present in the ancient carbon reservoir in about ‘modern’ proportions. Unless this early stabilization of the terrestrial carbon cycle in terms of a constant partitioning of carbon between the reduced and oxidized species is shown to have been caused by some inorganic geochemical process, a considerably earlier start of chemical evolution and spontaneous generation of life must be considered than is presently accepted.  相似文献   

Coastal and near-shore changes correlated with die-back in eel-grass (Zostera Marina,L.)     

Christian Christiansen  Henning Christoffersen  JØrgen Dalsgaard  Per NØrnberg 《Sedimentary Geology》1981,28(3):163-173

The die-back of eel-grass (Zostera marina, L.) is found to have played an important role with regard to near-shore sedimentation and coastal changes.In a natural harbour at Kyholm, Denmark a hiatus is found between silty sediments that date from about 4000 BC and overlying modern sandy sediments. It is suggested that this is a consequence of the die-back of eel-grass in the nineteen-thirties, resulting in mobilisation and disturbance of nearshore sediments and the shoreward movement of mobilised sandy material into the harbour. The coastal morphology of Kyholm was relatively stable from 1802 to 1933, but between 1933 and 1978 there have been two periods of drastic progradation correlated with die-back of eel-grass.  相似文献   

Combining a rainfall–runoff model and a regionalization approach for flood and water resource assessment in the western Po Valley,Italy     

Alessio Cislaghi  Daniele Masseroni  Christian Massari  Stefania Camici  Luca Brocca 《水文科学杂志》2020,65(3):348-370

ABSTRACT

Selecting the best structure and parameterization of rainfall–runoff models is not straightforward and depends on a broad number of factors. In this study, the “Modello Idrologico Semi-Distribuito in continuo” (MISDc) was tested on 63 mountainous catchments in the western Po Valley (Italy) and the optimal model parameters were regionalized using different strategies. The model performance was evaluated through several indexes analysing hydrological regime, high-flow condition and flow–duration curve (FDC). In general, MISDc provides a good fit behaviour with a Kling-Gupta Efficiency index greater than 0.5 for 100% and 84% of cases for calibration and validation, respectively. Concerning the regionalization, spatial proximity approach is the most accurate solution obtaining satisfactory performance. Lastly, the predicted FDCs showed an excellent similarity with the observed ones. Results encourage to apply MISDc over the study area for flood forecasting and for assessing water resources availability thanks to the modest computational efforts and data requirements.  相似文献   

Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget   总被引：2，自引：0，他引：2  

Manfred Schidlowski  Rudolf Eichmann  Christian E. Junge 《Precambrian Research》1975,2(1):1-69

Carbon isotope values of 260 Precambrian limestones and dolomites (most of them being substantially unaltered) have yielded an overall mean of $\text{δ}{}^{13}\text{C}\text{= +0.4 ± 2.7‰}$ vs. PDB; the corresponding oxygen values average at $\text{δ}{}^{13}\text{O = +20.0 ± 4.2‰}$ vs. SMOW. Like the overall mean, the δ ¹³C values furnished by individual carbonate occurrences are, as a rule, fairly “modern” and almost constant as from the very beginning of the sedimentary record. A remarkable exception are the “heavy” dolomites of the Middle Precambrian Lomagundi Group, Rhodesia, with $\text{δ}{}^{13}\text{C}\text{= +9.4 ± 2.0‰}$ vs. PDB. As a result of our measurements, the sporadic occurrence in the geological past of anomalously heavy carbonates seems to be established.The approximate constancy around zero per mill of the δ ¹³C values of marine carbonates through geologic time would imply a corresponding constancy of the relative proportion of organic carbon in the total sedimentary carbon reservoir since about 3.3 · 10⁹ y ago (with C_org/C_total ? 0.2). Utilizing this ratio and current models for the accumulation of the sedimentary mass as a function of time, we get a reasonable approximation for the absolute quantity of organic carbon buried in sediments and, accordingly, of photosynthetic oxygen released. Within the constraints of our model (based on a terrestrial degassing constant λ = 1.16 · 10^?9 y^?1) close to 80% of the amount of oxygen contained in the present oxygen budget should have been released prior to 3 · 10⁹ y ago. Since geological evidence indicates an O₂-deficient environment during the Early and most parts of the Middle Precambrian, there is reason to believe that the distribution of this oxygen between the “bound” and the “molecular” reservoir was different from that of today (with effective O₂-consuming reactions bringing about an instantaneous transfer to the crust of any molecular oxygen released). Accordingly, the amount of C_org in the ancient sedimentary reservoir as derived from our isotope data is just a measure of the gross amount of photosynthetic oxygen produced, withholding any information as to how this oxygen was partitioned between the principal geochemical reservoirs. As a whole, the carbon isotope data accrued provide evidence of an extremely early origin of life on Earth since the impact of organic carbon on the geochemical carbon cycle can be traced back to almost 3.5 · 10⁹y.  相似文献   

Fractionnement des éléments en trace Li,Rb, Ba,Sr dans les séries andésitiques et shoshonitiques du Pérou. Comparaison avec d'autres zones orogéniques     

Claude Dupuy  Dr. Christian Lefèvre 《Contributions to Mineralogy and Petrology》1974,46(2):147-157

In the calc-alkaline rocks of Peru, the rubidium content and the K/Ba, Rb/Sr ratios increase, while the strontium content and the K/Rb, Ba/Rb ratios decrease according to the distance of the trench. It is suggested that theses variations are related to a fractionation of plagioclase and contamination with differential migration of mobile elements. A comparative study between andesites of various localities shows in Peru an enrichment in Rb, Sr and Ba and a disparition of the Rb-Ba correlation; this last observation looks characteristic of continental margin andesites. The shoshonitic rocks differ from the andesites by a high content of Sr and Ba. The genetic relationships between these two kinds of rocks are discussed. It appears that the shoshonites are originated from a different magmatic source.  相似文献   

Buchbesprechungen     

Christian Zimmermann  K. Wuhrmann  HR. Bürgi  Peter Perret  K. Klingler 《Aquatic Sciences - Research Across Boundaries》1973,35(1):196-200

Ohne Zusammenfassung  相似文献