排序方式： 共有9条查询结果，搜索用时 15 毫秒
1
1.
The replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high-magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low-magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods. Using this criterion, an original low-magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high-magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high-magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty-seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes. Ferroan calcites contain up to 6 mol % FeCO3 and up to 5 mol % MgCO3. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0-03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite. Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate-reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ18C and δ18O values, and low Mg concentrations point more to a meteoric-phreatic origin of many ferroan calcite occurrences. 相似文献
2.
DETLEV K. RICHTER 《Sedimentology》1972,19(3-4):211-218
3.
Development of Arctic sea-ice organisms under graded snow cover 总被引：5，自引：0，他引：5
In 1988, the short-term response of sea-ice organisms to manipulated changes in snow cover (no snow cover, natural snow cover, natural snow cover + black foil) was investigated in one ice floe located in the East Greenland Current northwest of Svalbard over a period of three weeks. Autotrophic organisms (flagellates and diatoms) were concentrated in the lowermost 30 cm of the floe. In the field without snow cover, the highest diatom concentrations were observed, consisting nearly entirely of pennate forms, together with a maximum bacterial abundance. The community of larger protozoa and smaller metazoa was dominated by ciliates. Under natural conditions the flora consisted of both flagellates and diatoms, while turbellaria were the dominating animals. In the darkened field, the organism concentrations decreased with time. The results indicate that brine drainage, induced by changes in ice temperature, can reduce concentrations of ice organisms over short time scales. 相似文献
4.
Moonmilk represents a conspicuous but controversially discussed precipitate of cave settings. Here, new electron backscatter diffraction microscopic and petrographic evidence on the origin of moonmilk calcite is presented. Calcite fibres in a moonmilk mat from the walls of an active cave (Tunnel‐Cave) in Devonian massive limestones in the northern part of the Rhenish Massif (Germany) show orientations of the crystallographic c‐axis independent of the morphological fibre orientation. This observation and the morphology of the fibres are in agreement with microbially induced, as opposed to abiogenic, calcite precipitation. Carbon‐isotope data are higher (1·9 to 3·3‰) than those commonly measured regionally in speleothem calcite (stalagmites, stalactites and flowstones), an observation attributed to kinetic effects. In combination, these findings add independent evidence to the complex interplay of inorganic and bio‐induced carbonate precipitation in cave environments. 相似文献
5.
Quantitative aspects of Mn-activated cathodoluminescence of natural and synthetic aragonite 总被引：1，自引：0，他引：1
The cathodoluminescence analyses of the trigonal carbonates calcite and dolomite have been applied intensively in sedimentary petrology for a long time and the properties of these minerals are well-known, but much less attention has been paid to aragonite. In this study, the cathodoluminescence behaviour and the trace element composition of natural and synthetic aragonite have been studied employing trace element analyses (proton induced X-ray emission) and luminescence spectroscopy. Aragonite doped with Mn2+ has been synthesized in a NH4 + –Mg2+ –Ca2+ –Cl− solution in contact with a CO2 –H2 O–NH3 atmosphere. The low effective distribution coefficients indicate a rapid growth of the crystals of millimetre size which occurred within hours or days. The natural aragonite samples contain Mn, Fe and Sr in different concentrations. The Mn-bearing aragonites exhibit a bright green luminescence which is caused by a strong emission band at 575 nm with a half-width of about 84 nm. The luminescence intensity shows a strong positive correlation with Mn in aragonite when Fe and Mn do not exceed 2000 p.p.m. The intensity is depressed if the concentration of these elements exceeds the critical values. In the shell of a recent Unio sp., the luminescence intensity deviates from the linear correlation, although the trace element contents are not too high; this is probably an effect of quenching by organic material between the crystallites of the biogenic aragonite. 相似文献
6.
A combination of cathodoluminescence (CL) microscopy with digital image analysis and spectroscopic analysis allows the detailed investigation of the commonly occurring colour shift from initial to final CL‐colours in detrital quartz with increasing irradiation. A method for the quantification of the colour shift is presented and applied to different quartz sands: unlithified sands of the Lower Cretaceous Kuhfeld Formation from Ottenstein (W Germany) and Holocene sands from the barrier island of Wangerooge (N Germany). The colour shift depends on the defect structure of the quartz grains. The defect structure is determined by the physico‐chemical conditions during crystallization and therefore can help trace the source of quartz grains. A total of 25 different types of quartz grains in the Ottenstein‐sands indicates a highly variable southern provenance. Many of the grains have recycled quartz cement overgrowths indicating an at least second cycle origin. In contrast, only 16 different quartz types have been observed in the Wangerooge‐sands reflecting a less varied plutonic to metamorphic Scandinavian provenance. The study demonstrates the potential of quantified CL initial colour and colour shift in assessing the provenance of quartz‐rich sands and sandstones. 相似文献
7.
8.
The Na/CL ratio in rainwater and the seasalt chloride cycle 总被引：1，自引：0，他引：1
DETLEV MÖLLER 《Tellus. Series B, Chemical and physical meteorology》1990,42(3):254-262
9.
1