论晋北石炭二叠系紫色高岭石粘土的成因

晋北地区石炭二叠系煤层中与四号及九号煤伴生的含有机质的软质耐火粘土,即高岭石粘上,俗称紫矸或紫木节。对紫矸的成因过去有两种看法:(1)原生沉积的;(2)后期风化残积“由煤变来的”。笔者认为除这两个因素同时存在外,还与成岩后生作用中有机酸的淋滤和高岭石的重结晶、有序化等有关,尤其是后期风化表生作用使有机质氧化淋滤去SiO_2形成一水软铝石、褪色、溶蚀多孔、质地疏松,形成今日所见紫色高岭石粘土。     

Potential implications of differential preservation of testate amoeba shells for paleoenvironmental reconstruction in peatlands

Testate amoebae are now commonly used in paleoenvironmental studies but little is known of their taphonomy. There is some experimental evidence for differential preservation of some testate amoeba shell types over others, but it is unclear what, if any impact this has on palaeoenvironmental reconstruction. To investigate this issue we looked at palaeoecological evidence for the preservation of different shell types. We then investigated the possible impact of selective preservation on quantitative palaeoenvironmental inference. We first used existing palaeoecological data sets to assess the vertical patterns of relative abundance in four testate amoeba shell types: (1) shells made of secreted biosilica plates (idiosomes, e.g. Euglypha), (2) idiosomes with thick organic coating (Assulina), (3) proteinaceous shells (e.g. Hyalosphenia), (4) shells built from recycled organic or mineral particles (xenosomes) (e.g. Difflugia, Centropyxis). In three diagrams a clear pattern of decay was only observed for the idiosome type. In order to assess the implications of differential preservation of testate amoeba taxa for paleoenvironmental reconstruction we then carried out simulations using three existing transfer functions and a wide range of scenarios, downweighting different test categories to represent the impact of selective test decomposition. Simulation results showed that downweighting generally reduced overall model performance. However downweighting a shell type only produced a consistent directional bias in inferred water table depth where that shell type is both dominant and shows a clear preference along the ecological gradient. Applying a scenario derived from previous experimental work did not lead to significant difference in inferred water table. Our results show that differential shell preservation has little impact on paleohydrological reconstruction from Sphagnum-dominated peatlands. By contrast, for the minerotrophic peatlands data-set loss of idiosome tests leads to consistent underestimation of water table depth. However there are few studies from fens and it is possible that idiosome tests are not always dominant, and/or that differential decomposition is less marked than in Sphagnum peatlands. Further work is clearly needed to assess the potential of testate amoebae for paleoecological studies of minerotrophic peatlands.     

Chironomid-based Lateglacial climatic reconstruction for southeast Scotland

This paper presents the first chironomid-based climatic reconstruction for the UK Lateglacial, obtained from a lake basin in southeast Scotland, Whitrig Bog. Comparison of chironomid, lithological, geochemical, and pollen data reveals a marked vegetation lag behind the warming following deglaciation; warm-water chironomid taxa (e.g. chironomus) had replaced cold-water chironomid taxa (e.g. Paracladius) prior to the arrival of shrubs and trees and when local soils were still poorly developed. In addition to clear evidence for the Younger Dryas Stadial, chironomid data also reveal two short-term cold episodes that punctuate the Lateglacial Interstadial, both of which are also reflected in pollen and lithological data as reversals from open birch scrub/woodland to open herbaceous tundra coinciding with inwash of minerogenic matter into the basin. © 1997 John Wiley & Sons, Ltd.     

Periphyton and sphagnicolous protists of dystrophic bog lakes (Brandenburg, Germany): I. Annual cycles, distribution and comparison to other lakes

The periphyton of acid, dystrophic bogs was investigated. Glass-slides were exposed in four different bog lakes about 100 km north of Berlin, Germany, in order to examine the species composition and the annual cycle of the protist community. The annual cycles of the periphyton in bog lakes were comparable to those in other mesotrophic lakes: During the winter months small heterotrophic flagellates (HF) dominated the periphyton community. Besides that bacterivorous ciliates were present as typical pioneer species. At the end of the ice covering, almost exclusively unicellular green algae colonized the slides. In May high abundances of ciliates and choanoflagellates were observed. At the same time, a maximum of species was reached. From August to October peritrich ciliates significantly contributed to the periphyton. Their number decreased when early frosts occurred.

The four investigated bog lakes exhibit very similar species compositions, almost all of the species were present in each of the four lakes. Differences were only distinct concerning the dominance of some protistan groups. The largest deflections, however, occurred not between two lakes, but between two habitats within one lake: The upper 30 cm of the Sphagnum mat comprise two very different protistan communities, determined by a strong vertical zoning. Due to the different light and nutrient conditions, the slides of the upper region exhibited a denser colonization, mainly consisting of autotrophic cryptomonads and vagile ciliates. In contrast to this, the slides of the deeper region were mainly colonized by heterotrophic flagellates and sessile peritrich ciliates.     

Periphyton and sphagnicolous protists of dystrophic bog lakes (Brandenburg, Germany): II. Characteristic species and trophy of the lakes

Glass-slides were exposed in four different bog lakes located in great forest areas in Northern Germany, about 100 km north of Berlin, in order to examine the protist community of these dystrophic, acid lakes. Species composition and succession of the periphyton were studied during one year. The four investigated bog lakes exhibit similar species compositions, most of the species were present in each of the four lakes. The periphyton was composed of a great variety of protist species. Specimens of all major flagellated protists and of most of the ciliate taxa were found on the slides. Many of these species are ubiquitous. Some species, however, like the flagellates Spongomonas sacculus, Rhipidodendron huxleyi, Cyathobodo sp., and Pseudodendromonas vlkii as well as the ciliates Ophrydium hyalinum, Leptopharynx costatus, Uroleptus caudatus, and Platyophrya sphagni are found more frequently in acid waters. A regular and common occurrence of these species partly characterizes the acid Sphagnum bogs. Besides that, common species like Bodo spp., Cryptomonas spp., Goniomonas truncata, Chilodonella uncinata, Vorticella sp., and Cyrtolophosis mucicola are represented in the periphyton. Examinations of the testate amoebae in the Sphagnum mats show a very different species composition compared to earlier studies.

The investigated bog lakes revealed mesotrophic conditions. In the upper regions of the Sphagnum mats and in the center of the larger bog lakes, oligotrophic conditions still prevail. The lower regions of the Sphagnum mat and the smaller lakes, however, are influenced by the partly degradation of plants and the run-off water from surrounding land. In comparison to earlier studies, the species composition indicates a beginning eutrophication of the bog lakes. A direct anthropogenic impact can be excluded.     

The magnitude of error in conventional bulk-sediment radiocarbon dates from central North America

Although the carbon-reservoir problem with bulk-sediment radiocarbon dates from lakes has long been recognized, many synoptic studies continue to use chronologies derived from such dates. For four sites in central North America, we evaluate chronologies based on conventional radiocarbon dates from bulk sediment versus chronologies based on accelerator mass spectrometry (AMS) radiocarbon dates from terrestrial plant macrofossils. The carbon-reservoir error varies among sites and temporally at individual sites from 0 to 8000 yr. An error of 500–2000 yr is common. This error has important implications for the resolution of precise event chronologies.     

A procedure to identify natural arsenic sources,applied in an affected area in North Rhine-Westphalia,Germany

The aim of the study was to identify the geogenic source for elevated arsenic (As) concentrations recently discovered in soils of the Heubach plain, North Rhine-Westphalia, Germany. Therefore, a catalogue of conditions that an As source has to fulfil in order to be considered as geogenic was formulated, including the source’s linkage to the sink, its mineralogy, As total content, As mobilization potential, groundwater redox conditions and As output. Accordingly, mineralogical, geo- and hydrochemical investigations were carried out, including X-ray diffractometry, microscopy, ICP-OES and AAS analysis and a sequential extraction procedure. Paleo bog iron ores (PBIOs) of Tertiary age, occurring within unconsolidated sands (Haltern-layers, Santonian–Lower Campanian), and glauconitic marlstones (Dülmen-layers, Lower Campanian) were examined. Results indicate that output from the PBIOs is responsible for the elevated As levels. Accounting their diverse mineralogy, five types of PBIOs were defined. Type-dependent, they at least partly fulfil all of the formulated requirements. The relations and behaviour of As sources and sinks in space and time could be clarified for the area of interest. The approach presented in this paper may offer a tool for identifying natural As sources worldwide.     

Holocene tephra horizons at Klocka Bog,west‐central Sweden: aspects of reproducibility in subarctic peat deposits

This paper presents one of the most extensive Holocene tephra records found to date in Scandinavia. Microtephra horizons originating from Icelandic eruptions were recorded in two ca. 2 m thick peat profiles at Klocka Bog in west‐central Sweden. Five of the microtephra horizons were geochemically correlated to the Askja‐1875, Hekla‐3, Kebister, Hekla‐4 and Lairg A tephras respectively. Radiocarbon‐based dating of these tephras broadly agree with previously published ages from Iceland, Sweden, Germany and the British Isles. The identification of the Lairg A tephra demonstrates a more widespread distribution than previously thought, extending the usefulness of Icelandic Holocene tephrochronology further north into west‐central Scandinavia. Long‐lasting snow cover and seasonal wind distribution in the lower stratosphere are suggested as factors that may be responsible for fragmentary tephra deposition patterns in peat deposits of subarctic Scandinavia. Copyright © 2004 John Wiley & Sons, Ltd.