Diatom-based interpretation of sediment banding in an urbanized lake

Sediment stratigraphy and diatom succession were studied in an 80.5-cm core taken from the deepest part of Third Sister Lake, a small kettle hole in a recently urbanized landscape of southeastern Michigan. Alternating light clay and dark organic bands documented sporadic inputs of clay from outside the basin during rain events, rather than annual laminations. Urban construction activity also disrupted the inflow stream bed and facilitated transport of clay into the lake to generate non-rhythmic banding in the lake's deep hole. Diatom analysis revealed dramatic changes in predominant taxa with sediment depth verifying the non-annual nature of the sediment bands. Observation of halophilic diatom taxa also documented effects of human activity such as road salting on this small, urban lake.     

A note on water and sediment temperature in Lake Sasykkul, a small saline lake in the eastern Pamirs, Tadzikstan

Temperatures recorded in August 1985 in the water column and sediments of Lake Sasykkul, a small mountainous saline lake in Tadzikstan, are noted. The lake was weakly heliothermal and the underlying permafrost was depressed.     

Records of aquatic pollen and sediment properties as indicators of late-Quaternary Alaskan lake levels

We investigated whether techniques developed to evaluate qualitative lake-level changes in the temperate zone can be used in sub-arctic and arctic Alaska. We focused on aquatic pollen records and sediment properties (loss-on-ignition and magnetic susceptibility) from centrally-located sediment-surface samples and cores, as these are the most commonly reported data in the literature. Modern aquatic pollen values are generally low (< 5%) and may be zero, even in lakes with abundant aquatic macrophytes. Greater diversity and higher values of aquatic pollen are likely at depths < 5 m, but pollen is found in depths up to 15 m. It is absent at depths > 20 m. Spores of Isoetes and Equisetum and Pediastrum cell-nets, when present, tend to be widely distributed, even in deep water. At Birch Lake, interior Alaska, trends in aquatic taxa and sediment characteristics for the last ca. 12,000 ¹⁴C yrs recorded in a single, deep-water core reflect the same water-level changes as do transect-based lake-level reconstructions - if modern distributional characteristics of pollen and spores are taken into account. The lake rose from extremely low levels at ca. 12,000 ¹⁴C yr B.P. After a period of fluctuation, it rose to a relatively high level by ca. 8000 ¹⁴C yr B.P. and then stabilized. A preliminary survey of aquatic pollen trends from other lake-sediment records suggests that the period ca. 11,000-8000 ¹⁴C yr B.P. may have seen relatively low lake levels in north-western and interior Alaska and high levels thereafter. Changes in aquatic pollen and sediments are evident in north-eastern interior lakes at the same time, but they are more difficult to interpret. Aquatic pollen productivity in Alaskan lakes may partly depend on factors other than water depth (e.g. temperature, pH, nutrient status, or length of the ice-free season). An Alaska-wide reconstruction of late-Quaternary lake levels based on extant single-core data would be best done after further study of contributing factors that may control sediment properties and aquatic pollen distribution.     

Coring artifacts and contaminant inventories in lake sediment

Sediments of Lake 382, Experimental Lakes Area, Canada, were sampled at six sites using a 5-cm Wildco KB core sampler (KB), a similar device incorporating a ball check valve (BC), and a 0.2 m by 1.2 m flat-faced aluminum freeze core sampler (FC). Cores were sectioned at 1-cm intervals to a depth of 15 cm. Contaminant (²¹⁰Pb and ¹³⁷Cs) concentrations (Bq g^-1) were measured by gamma spectroscopy, and inventories (Bq cm^-2) were calculated following standard methods. Sediments collected using FC, BC and KB had similar contaminant concentrations, however, cores collected by FC and BC had lower estimated inventories than KB cores. Differences between estimates appear to be caused by differences in the water content (WC) of core material. Laboratory studies confirm that FC sediments have higher WC than tube-cored sediment. We hypothesize that ice crystal formation increases the WC of freeze cores, resulting in lower contaminant inventories. Loss of surficial sediment caused by a bow wave may have a similar effect on BC samples. We conclude that KB core gear is appropriate for sampling sediments to measure contaminant concentrations and inventories in recently deposited sediments.     

A method for the extraction of carbonaceous particles from lake sediment

The methods found in the literature for the extraction of carbonaceous particles from lake sediment are discussed. The technique used by Griffin & Goldberg (1975) on Lake Michigan sediments was improved by modifying the procedure to halve the extraction time and reduce the risk of fragmentation. This method was then applied to a sediment core taken from Loch Tinker in Western Scotland, which had previously been analysed using the Renberg & Wik method. Although the basic trends for both methods are the same, the new method is found to be more sensitive to low particle numbers and more accurate, due to a more efficient extraction and a higher magnification for microscope counting.     

Field calibration and performance of sediment traps in a eutrophic holomictic lake

In wind-stressed and unstratified Lough Neagh low aspect ratio sediment traps, which allow for internal resuspension, collected sediment at a rate close to the natural sediment accumulation rate inferred from dated sediment cores. High aspect ratio traps grossly overestimated the natural rate of sediment accumulation as did, but to a lesser extent, burial rate measurements on an artificial stratigraphic marker. The former type of trap deployed over long exposure periods is therefore recommended as providing the best estimate of net downward particle flux in lakes such as Lough Neagh. However, it is emphasized that all sediment trap types, when operated in shallow turbulent lakes, will to a greater or lesser extent be contaminated by secondary or redeposited material and so will not provide a direct measure of primary sedimentation. Use of tube traps in lakes such as Lough Neagh should not, however, be discounted since they can provide a record of the quality of sedimenting material through time.     

Recent lake level variations in Lake Haubi, central Tanzania, interpreted from pollen and sediment studies

Variations in pollen assemblages and in physical and chemical composition of a dated sediment record from the small Lake Haubi in north central Tanzania, reveal lake level fluctuations since the late 19th century. Lake Haubi changed from a seasonally inundated swamp to a lake in the beginning of the 20th century. With the exception of 1942-44, when it dried out completely, it remained water filled until 1994 when it again turned into a swamp. The lake level fluctuations in Lake Haubi are largely in phase with fluctuations of the larger East African lakes levels during the 20th century, and are therefore interpreted as being mainly controlled by regional climatic fluctuations. However, the initial formation of Lake Haubi at the turn of the century was likely due to local catchment specific causes, e.g. changes in land use, as the rapid increase in the water level at this time does not correspond to other lake level records from the region.     

Influence of the regional climate variations on lake changes of Zabuye,Dangqiong Co and Bankog Co salt lakes in Tibet

Journal of Geographical Sciences - The lake hydrological and meteorological data of the Tibetan Plateau are not rich. This research reports the observed climatic data and measured water levels of...     

Suspended sediment transport and deposition in a high arctic meromictic lake

A study of sedimentary processes and sediment yields in a high arctic meromictic lake (Lake C2, Taconite Inlet, Northern Ellesemere Island, Canada) was undertaken from May 1990 through August 1992 to understand the links between climatic controls, hydrology, and the laminated sediment record preserved in the lake. Understanding the relationships between processes and the sediment record is critical for interpreting the climatic significance of the laminated sediments in a region where high resolution climate proxy records are quite limited.Sediment transport to Lake C2 is dominated by fluvial processes. During the early part of the melt season slushflows transport sediment to the lake surface. Subsequently, suspended sediment is delivered to the lake by the main inlet stream and distributed lakeward by a plume emanating the main inlet channel. Due to the strong density stratification of the water column the plume distributes sediment downlake by overflows and interflows in the epilimnion. In general, overflows are generated by lower discharge events whereas interflows penetrate to the halocline during high discharge with increased suspended sediment concentration.Sediment trap analysis demonstrates that suspended sediment transport and deposition responds to diurnal through annual changes in stream discharge. Seasonal and annual sediment trap yields agree with average accumulation rates determined from varve thickness measurements and cumulative suspended sediment discharge from the main inlet stream indicating a close link between climatological, hydrological, and sedimentological controls and varve deposition.This is the fourth in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley.     

山地丘陵坡地逆温效应对果树避冻的影响

通过对3个县8个山坡地的考察,探索山地丘陵坡地逆温效应对果树避冻的影响。试验对三年来冬季观测到日最低温度（td）进行拟合处理,运用趋势线分析探究温差（△td）与相对坡位（G）之间的总体趋势,采用相关回归方差分析方法对△td与G进行差异显著性检验。结果表明：△td与G相关达到极其显著水平。对于高差（△H）〈50m的山坡,晴夜最低气温随高度的增加而增加,越近坡顶,最低气温越高;对于76m≤△H〈150m的山坡,在该山坡相对坡位0．63以上,晴夜最低气温常出现最高,逆温效应明显。总高差不足150m山坡最大逆温效应出现的坡位G与山坡总高差成反比,即随着△H的增加,最大逆温效应出现的坡位G值逐渐变小。利用这些结果进行果树科学合理建园,发展山区特色果业,对不耐寒果树安全避冻意义重大。     

Spatial uniformity of sediment accumulation in varved lake sediments in northern Sweden

As part of an investigation aimed at assessing the potential of northern Swedish varved lake sediments for fine-resolution reconstruction of past climatic conditions, the following questions were addressed; how representative is a single core, does the appearance of varves change as the sediment ages, and how can varve thickness and within-varve structures be digitized? Analyses of replicate cores from Kassjön, using sample sequences of seasonal, annual and centennial time resolution, show that spatial variability of sediment accumulation in the varved sediment is very low. Comparisons of intensity curves from image analysis of freeze cores of recent sediments from Lake Nylandssjön, sampled in 1980 and 1985, indicate that the varves acquire their appearance at the sediment-water interface during sedimentation and that the varve structures are preserved during diagenetic processes. Measurement of varve thicknesses with a tree-ring microscope and with image analysis gave similar results. However, with image analysis, within-varve structures such as colour variations and thicknesses of seasonal layers, can also be recorded, increasing the possibilities for palaeolimnological and palaeoclimatic inferences.     

近40 年可可西里地区湖泊时空变化特征

以可可西里地区1970s 地形图和1990s、2000-2011 年Landsat TM/ETM+遥感影像为基础,通过数字化和影像解译获取研究区83 个面积大于10 km²湖泊变化数据,并对湖泊变化成因进行了分析。研究结果表明:1) 1970s 初期至2011 年,可可西里地区湖泊经历了“先萎缩后扩张”的变化过程,其中1970s-1990s 期间湖泊面积普遍减小,1990s-2000 年湖泊出现扩张,并在2000 年恢复到1970s 湖泊规模,2000 年之后湖泊面积急剧增大。2) 2000-2011 年间,可可西里地区不同规模等级湖泊整体呈扩张趋势,但表现出一定的区域差异性。面积呈增加趋势的湖泊数量最多,亦分布最广,一些湖泊由于扩张迅速出现湖泊合并或湖水外泄情况;面积呈减少趋势或波动起伏的湖泊数量较少,零散分布在研究区中部和南部,湖泊动态变化与其自身补给条件或与下游湖泊(河道) 存在水力联系有关。3) 在研究时段内,降水增多、蒸发减少是可可西里地区湖泊扩大的主要原因,而气候变暖引起的冰川融水增加、冻土水分释放是次要原因。