Efficiency of phosphorus accumulation by plankton,periphyton developed on submerged artificial substrata and metaphyton: in-situ observation in two shallow ponds

Phosphorus overenrichment of shallow ponds prevailing in wetlands leads to their eutrophication causing the collapse of those vulnerable habitats. The potential...     

Using a modern analogue to interpret depositional position in ancient fluvial-tidal channels: Example from the McMurray Formation,Canada

The fluvial–tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FTT generally consists of three broad zones: (1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial–tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FTT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley (“A” Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial–tidal channels.Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north.Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the “A” Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FTT zone appears to have extended for several hundred kilometers from south to north.Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems.     

拉萨近半个世纪降水的变化特征

利用拉萨1952-2005年逐月降水量,≥0.1 mm、1.0 mm、5.0 mm和10.0 mm年降水日数,分析了近半个世纪拉萨年、季降水量及年降水日数的年际和年代际变化.结果发现:近半个世纪以来,拉萨年降水量表现为前30年呈不显著的减少趋势,减幅为17.8 mm/10 a;季降水量除夏季呈不显著的减少趋势外,其它各季均表现为增加趋势,以秋季增幅最大;≥0.1 mm、≥1.0 mm和≥5.0 mm年降水日数表现为不同程度的增加趋势.近25年≥10.0 mm的年降水日数呈极显著的增加趋势.20世纪50年代至80年代夏季降水量表现为逐年代减少趋势,秋季降水量则呈逐年代增加趋势,而冬季降水量为负距平.各等级年降水日数20世纪80年代偏少,90年代偏多.年降水量异常偏旱年主要出现在20世纪80年代,50年代和60年代的初期各出现一次异常偏涝年,70年代从未出现过异常年份.14年振荡周期可能是影响年降水量的主导周期.     

Aeolian sediment provenance and transport in the upper and middle reaches of the Yarlung Zangbo River,Tibet Plateau

Frequent and serious aeolian disasters occur in the upper and middle reaches of the Yarlung Zangbo River, which runs through the high-elevation Tibet Plateau. Sediment geochemical characteristics can be used as a proxy to identify the sediment's provenance. To determine the provenance of aeolian sediments in the river's basin, we analysed major and trace element contents from surface samples and local clastic rocks throughout the basin. We found that the major and trace elements differed between the middle reaches, upper reaches and regions south of the river. Major element contents were similar in the upper and middle reaches, but trace elements differed. Al₂O₃, MgO and Na₂O concentrations increased from the upper reaches to the lower reaches, and in the lower reaches, MgO and Na₂O were enriched compared with the crustal average. The similarities between samples in the lower part of the upper reaches and those in the middle reaches indicated that sediment transported by fluvial systems from the upper reaches were first deposited in the wide valleys near Xigaze, where they formed a large area of shifting sand. These deposits were then transported by the wind to the river's middle reaches, where they formed a large area of shifting sand. When we compared aeolian sediment in the middle reaches with the local clastic rocks, they appeared to be unrelated. The difference between sediments south of the river and those in the middle reaches means that the southern sediments were not transported to the middle reaches. Therefore, the aeolian sediment in the middle reaches of the Yarlung Zangbo River mainly came from the lower part of the upper reaches, not from the local clastic rocks.