A Comparison of Sediment Varves (1950–2003 AD) in Two Adjacent Lakes in Northern Sweden

Koltjärnen and Nylandssjön are two closely situated lakes (<2 km apart) in northern Sweden. During the past century, distinct varved sediments have formed in these lakes. Nylandssjön has two varved, deep basins. Since lake and catchment characteristics superficially appear very similar for the two lakes and they are exposed to the same climate, one would expect the sediment varves to be similar. This investigation compares the varves in the two deep basins of Nylandssjön, and in the two lakes. The comparison of basins of Nylandssjön shows that varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1970–2003). The grey-scale curves are only clearly similar in about 50% of the varves. In the between-lake comparison varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1950–1996). However, the annual accumulation rates of dry mass, minerogenic matter and biogenic silica differ between the lakes, as well as within-varve successions in grey-scale. A general explanation to the differences is that the prerequisites for varve formation are not totally similar because of differences in catchment size, catchment- to-lake material fluxes, lake productivity and land-use influence. This study illustrates the complex relationships that exist between a lake, its catchment, in-lake productivity and formation of sediment varves. As a consequence, we must not apply a too simplistic view of the potential of varves as past climatic indicators, especially if the lakes are affected by land-use.     

Lake Sediment Core Records of Sulphur Accumulation and Sulphur Isotopic Composition in Central Ontario, Canada Lakes

Stable isotopic compositions and concentrations of total sedimentary sulphur (S) were determined in cores from 6 lakes in the acid-sensitive Muskoka-Haliburton region of south-central Ontario. The isotopic composition of S in deep sediment (> ~ 20 cm) was approximately constant in all lakes, and indicated a pre-industrial δ ³⁴S value between +4.0 and +5.3‰, which is similar to current bulk deposition. Similarly, total S concentrations in deep sediment were relatively low (1.9–5 mg S g⁻¹ dwt) and approximately constant with depth within cores. All lakes exhibited up-core increases in total S and decreases in δ ³⁴S at a depth corresponding to the beginning of industrialization in the Great Lakes region ( ~ 1900), resulting in a generally reciprocal depth pattern between total S concentration and δ ³⁴S ratios. While initial shifts in total S and δ ³⁴S were likely due to enhanced SO₄ reduction of newly available anthropogenic SO₄, both the magnitude and pattern of up-core S enrichment and shifts in δ ³⁴S varied greatly among lakes, and did not match changes in S deposition post 1900. Differences between lakes in total S and δ ³⁴S were not related to any single hydrologic (e.g., residence time) or physical (e.g., catchment-area-to-lake area ratio) lake characteristic. This work indicates that sediment cores do not provide consistent records of changes in post-industrial S deposition in this region, likely due to redox-related mobility of S in upper sediment.     

Seasonal Diatom Variability and Paleolimnological Inferences – A Case Study

The seasonality of physical, chemical, and biological water variables is a major characteristic of temperate, dimictic lakes. Yet, few investigations have considered the potential information that is encoded in seasonal dynamics with respect to the paleolimnological record. We used a one-year sequence of diatoms obtained from sediment traps and water samples, as well as the sedimentary diatom record covering the past ca. 1000 years in Bates Pond, Connecticut (USA), to investigate which variables influence the seasonal distribution of diatoms and how this can be used for the interpretation of the fossil record. The seasonal patterns in diatom assemblages were related to stratification and, to a lesser extent, to nitrate, silica, and phosphorus. During mixing periods in spring and autumn, both planktonic and benthic species were collected in the traps, while few lightly silicified, spindle-shaped planktonic diatoms dominated during thermal stratification in summer. Changes in fossil diatom assemblages reflected human activity in the watershed after European settlement and subsequent recovery in the 20th century. A long-term trend in diatom assemblage change initiated before European settlement was probably related to increased length of mixing periods during the Little Ice Age, indicated by the increase of taxa that presently grow during mixing periods and by application of a preliminary seasonal temperature model. We argue that the analysis of seasonal diatom dynamics in temperate lakes may provide important information for the refinement of paleolimnological interpretations. However, investigations of several lakes and years would be desirable in order to establish a more robust seasonal data set for the enhancement of paleolimnological interpretations.     

A chronological assessment of Lake Okeechobee (Florida) sediments using multiple dating markers

Reconstructing recent limnological history often relies on lead-210 dating to accurately ascribe a chronology to a sediment profile. In Lake Okeechobee, Florida, a large, shallow subtropical lake that may experience severe mixing, multiple dating methods are required to confirm that conformable sedimentation has been preserved and that the assumptions of the ²¹⁰Pb method are satisfied. This study uses stratigraphic profiles of heavy metals, ¹³⁷Cs, PCBs and pollen as independent dating markers to validate the sediment chronology as determined by ²¹⁰Pb for three cores from the central mud zone of the lake. Unsupported ²¹⁰Pb and most dating markers show distinct concentration/depth profiles, suggesting that the sediments have not been severely mixed for at least the last 75 years. Onset and maximum activity of the radioisotope ¹³⁷Cs in the cores coincides with the ²¹⁰Pb-dated interval of 1945–1970. This agrees well with the known timing of atmospheric deposition of ¹³⁷Cs that resulted from above-ground nuclear testing during late 1940s until 1963. Sediment core profiles of atmospherically deposited metals such as Zn and Pb, which reflect regional increases during industrialization and decreases after regulation in the 1970s, exhibit expected concentration increases and peaks coinciding within 5–15 years of the predicted ²¹⁰Pb dates. Uranium, a contaminant in some phosphate fertilizers, shows large concentration increases at core depths dated to be about 1950 by ²¹⁰Pb, matching the intensification of agriculture after WWII. PCBs, which are expected to peak in the early 1970s, were measured in one core, and the observed peak corresponds to a ²¹⁰Pb date of about 1960. Pollen makers were unable to verify specific events, but increases in disturbance taxa and declines in native types correspond generally with the expected dates assigned by ²¹⁰Pb dating. Conformity between the ²¹⁰Pb defined dates and independent markers of < ±15 years confirm that Lake Okeechobee sediments do preserve a sequential and reliable stratigraphic history of the lake, useful for reconstructing past limnological conditions.     

Environmental Changes at the Desert Margin: An Assessment of Recent Paleolimnological Records in Lake Qarun, Middle Egypt

Lake Qarun has been profoundly affected by a combination of human activities and climatic changes during the past 5000 years. Instrumental records available for the 20th century show that during most of this period both lake water level and salinity increased and that by the late 1980s lake water salinity was approximately that of seawater. Sediment cores (c. 1 m long) were collected from this shallow (Z_max 8.4 m) saline lake in 1998 and the master core (QARU1) was used to examine the potential of paleolimnology for reconstructing the recent environmental history of the site. According to ¹³⁷Cs and ²¹⁰Pb radio-assay, the recent sediment accumulation rate in QARU1 was around 5 mm year⁻¹ during the latter half of the 20th century but radionuclide levels were low. Spheroidal carbonaceous particles (SCPs) were present in the upper c. 30 cm of QARU1 and indicates contamination by low level particulate pollution, probably beginning around 1950. The record of exotic pollen (Casuarina) indicated that sediment at 51–52 cm depth dated to around 1930. Otherwise the pollen spectra indicated a strongly disturbed landscape with high ruderals and increased tree planting particularly since c. 1950. Diatom records were strongly affected by taphonomic processes including reworking and differential preservation but typical marine diatoms increased after the 1920s. Instrumental records show that the lake became more saline at this time. Freshwater taxa were present at approximately similar abundances throughout the core. This distribution probably reflected a combination of processes. Reworking of ancient freshwater diatomites is one likely source for freshwater diatoms in QARU1 but some taxa must also be contributed via the freshwater inflows. Overall, the diatom stratigraphy indicated increasingly salinity since the 1920s but provided no firm evidence of lake eutrophication. Diatom inferred salinity reconstructions were in only partial agreement with instrumental records but inferred for the lower section of the core (pre 20th century to the 1960s) accord with measured water salinity values. Surficial sediments of Lake Qarun contain environmental change records for the 20th century period but high sediment accumulation rate and pollen reflect the high degree of human disturbance in the region. Because of poor preservation and evidence of reworking, the relationships between diatom records and past water quality changes require careful interpretation, especially in the upper section of the core. Nevertheless, early to mid 20th century measurements of increasing lake water salinity are well supported by sediment records, a change that is probably linked to ingress of saline ground water     

华南地区晚二叠和晚三叠世煤中稀土元素特征研究

在华南地区贵州六盘水、江西乐平、露庆和湖北黄石矿区晚二叠和晚三叠世煤层的稀土元素ICP—MS分析数据的基础上，系统研究了研究区煤中稀土元素的含萤分布特征、赋存特征、配分模式以及地质控制因素。含量分布表明晚二叠世稀土元素含量总体高于晚三叠世；煤中稀土元素与灰分成正相关关系，部分煤样品由于有富稀土元素的重矿物组合的存在而出现稀土元素的异常高值；同一矿区、同一时代煤中稀土元素的配分模式具有一定的相似性，而不同地区同一时代和同一地区不同时代煤中稀土元素的配分模式具有较大差异；煤的沉积环境，煤中无机矿物组成以及海水的影响是控制煤中稀土元素含量和配分模式的主要地质因素。     

淮北煤田孙庄勘探区煤层气赋存规律及影响因素分析

通过对淮北煤田孙庄勘探区地质构造、沉积特征、煤层煤质及煤层气赋存特征的分析，研究认为有利于本区煤层气赋存的主要因素是较大的埋藏深度、较高的壳质组＋镜质组、较高的变质程度、较大的煤层厚度、低透气性的盖层、较缓的地层倾角和封闭性较好的构造条件，推测煤田深部可能具有一定工业价值的煤层气资源。     

Ammonium regeneration in intertidal sediments of the Yangtze estuary during air-exposure

The influences of exposure to the atmosphere on ammonium cycle in the intertidal surface sediments were in situ studied with a geochemical approach at a typical station in the Yangtze Estuary during three tidal cycles in September 2003. During an about 8-h emersion period of each diurnal tide, six high-resolution vertical profiles of adsorbed and dissolved ammonium were measured. It was observed that both adsorbed and dissolved ammonium generally had an increasing trend in sediment cores during the exposure. The rate of ammonium regeneration in sediments was estimated using the accumulation amount of ammonium including adsorbed and soluble fractions during the daytime emersion. The calculation result showed that there was relatively high ammonification rate (˜500 nmol N cm⁻³ day⁻¹), which reflected that organic nitrogen in sediments was quickly decomposed with a residence time of ˜52.7 days. Due to the dramatic temperature difference observed in sediment profiles, free convection was considered an important mechanism of regulating the efflux of produced ammonium into overlying waters. The total estimated amount of regenerated ammonium was ˜1.35×10⁵ t N year⁻¹ in the intertidal flat of the Yangtze Estuary, which occupied 7.6% of the total inorganic nitrogen annually transported to the estuarine ecosystem.     

基于GIS/MIS/OA的短周期工业原料林经营模式研究

提出了短周期工业原料林经营模式的概念,探讨了短周期工业原料林经营模式的体系结构、子系统功能和主要林业数表类型,说明其与GIS/MIS/OA系统集成的必然趋势,并以岳阳纸业股份公司的纸浆原料林为例进行了实例分析。     

一种实现依比例街区自动合并的算法

针对依比例街区的图形特征，介绍了一种实现街区自动合并的有效算法，即利用数学形态学和模式识别对栅格数据进行追踪，然后提取街区轮廓线，编辑后就得到最终合并的街区。