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Marine snow and epipelagic suspensoids in the Reda carbonates and a pronounced mid-Ludfordian (Silurian) CIE in the axis of the Baltic Basin (Poland)

Kozłowski Wojciech

Acta Geologica Polonica

2020

Vol. 70, no. 4

529--567

The mid-Ludfordian pronounced, positive carbon isotope excursion (CIE), coincident with the Lau/kozlowskii extinction event, has been widely studied so far in shallow-water, carbonate successions, whereas its deep-water record remains insufficiently known. The aim of this research is to reconstruct the sedimentary environments and the palaeoredox conditions in the axial part of the Baltic-Podolian Basin during the event. For these purposes, the Pasłęk IG-1 core section has been examined using microfacies analysis, framboid pyrite diameter and carbon isotope measurements. The prelude to the event records an increased influx of detrital dolomite interpreted as eolian dust, coupled with a pronounced decrease in the diameter of the pyrite framboids, indicating persistent euxinic conditions across the event. The event climax is recorded as the Reda Member and consists of calcisiltites, composed of calcite microcrystals (‘sparoids’), which are interpreted as suspensoids induced by phytoplankton blooms in the hipersaturation conditions present in the epipelagic layer of the basin. Both the prelude and climax facies show lamination, interpreted as having resulted from periodical settling of marine snow, combined with hydraulic sorting within a ‘benthic flocculent layer’, which additionally may be responsible for a low organic matter preservation rate due to methanogenic decomposition. Contrary to the observed basinward CIE decline in the benthic carbonates in the basin, the Reda Member records an extremely positive CIE (up to 8.25‰). Given the pelagic origin of the sparoids, the CIE seems to record surface-water carbon isotope ratios. This points to a large carbon isotope gradient and kinetic fractionation between surface and bottom waters during the mid-Ludfordian event in a strongly stratified basin. The Reda facies-isotope anomaly is regarded as undoubtedly globally triggered, but amplified by the stratified and euxinic conditions in the partly isolated, Baltic-Podolian basin. Hence, the common interpretation of the basin record as representative for the global ocean needs to be treated with great caution.

Osady eoliczne (wydmowe) w suchym i półsuchym regionie płaskowyżu Alashan (MongoliaWewnętrzna, Chiny)

Kenig K.

Przegląd Geologiczny

2012

Vol. 60, nr 5

267--272

In northern China, sandy and rocky deserts are located in arid regions west of the Helan Mountains (Helan Shan), while the areas of fixed sand dunes stretch in semi-arid regions east of the Helan Mountains. Helan Mountains, trending nearly NS in the northern part of China's Inner Mongolia Autonomous Region, are a natural border between the dry climate to the west and the semi-dry climate to the east. In the Badain Jaran Desert (part of the Alashan Desert), there are the largest dunes in China, and possibly the largest in the world. They attain a height of up to 460 m in the southeastern part of the desert. Due to their size, they are called megadunes by the Chinese researchers. They are accompanied by numerous smaller dunes, developed in various forms, which cover their slopes. As a result, megadune complexes form, reaching 10 km in length. The general direction of the dunes is NE-SW. In their lower parts, they are covered with sparse xerophytic vegetation, mainly Artemisia and Ephedra. In interdunal depressions of the Badain Jaran Desert, there are numerous (140) permanent, relatively shallow and drying-up lakes, overgrowing with vegetation. Their average depths are 2 m, rarely reaching 16 m mainly in the northern part. The water in the lakes is highly alkaline, with pH values of up to 10.5. Sometimes, the accumulation of salt is so substantial that allows it to be extracted. The groundwater in the immediate surroundings of dunes shows a much lower alkalinity and the average pH is 7.8. To the east of the Helan Mountains, there are several deserts of generally smaller areas and specific environmental nature. This is a sandy land of Hunshandake. Dunes and dune fields are lower and less well developed morphologically, reaching an average of 5-10 m in height. They are mostly stabilized completely or partially by vegetation. The main trend of dunes here is WNW-ESE. Locally, shallow lakes occur in interdunal areas. Studies of mineral and chemical composition of silt material are important for the considerations of the source and direction of silt transport in Asia. One of the diagnostic components of detrital minerals is dolomite. The newest studies of silts recently deposited in Beijing provide information about the wind-transportation direction. It appears that there is a fundamental difference in the mineral composition of silts originating from the dust storm (in 2006) and silts from the entire spring period. Such conclusions are also confirmed by the content of rare earths elements in the 2006 dust storm sediments from Beijing and, comparatively, from other deserts as potential source areas of silts (Yang et al., 2007b). Isotopic investigations of Nd and Sr in aeolian sediments of northern China confirm such a possibility. An important problem for the areas located in northern China is the process of desertification. Noticeable differences have occurred already during the last three decades. They consist in changes in the distribution of movable dunes, which is relative to the humidity and temperature. It is believed that the climate may be the main factor causing desertification in Hunsheandake. In other regions of China, there is another significant factor: human activity, especially shepherding that causes the process of desertification and environmental degradation.