Regional water temperature characteristics of lakes subjected to climate change     

Midhat Hondzo  Heinz G. Stefan 《Climatic change》1993,24(3):187-211

A deterministic, validated, one-dimensional, unsteady-state lake water quality model was linked to a daily weather data base to simulate daily water temperature profiles in lakes over a period of twenty-five (1955–79) years. Twenty seven classes of lakes which are characteristic for the north-central U.S. were investigated. Output from a global climate model (GISS) was used to modify the weather data base to account for a doubling of atmospheric CO₂. The simulations predict that, after climate change, epilimnetic temperatures will be higher but increase less than air temperature, hypolimnetic temperatures in seasonally stratified dimictic lakes will be largely unchanged or even lower than at present, evaporative water loss will be increased by as much as 300 mm for the season, onset of stratification will occur earlier and overturn later in the season, and overall lake stability will become greater in spring and summer.  相似文献   

Reducing monitoring gaps at the aquifer–river interface by modelling groundwater–surface water exchange flow patterns     

Matthias Munz  Stefan Krause  Christina Tecklenburg  Andrew Binley 《水文研究》2011,25(23):3547-3562

This study investigates spatial patterns and temporal dynamics of aquifer–river exchange flow at a reach of the River Leith, UK. Observations of sub‐channel vertical hydraulic gradients at the field site indicate the dominance of groundwater up‐welling into the river and the absence of groundwater recharge from surface water. However, observed hydraulic heads do not provide information on potential surface water infiltration into the top 0–15 cm of the streambed as these depths are not covered by the existing experimental infrastructure. In order to evaluate whether surface water infiltration is likely to occur outside the ‘window of detection’, i.e. the shallow streambed, a numerical groundwater model is used to simulate hydrological exchanges between the aquifer and the river. Transient simulations of the successfully validated model (Nash and Sutcliff efficiency of 0·91) suggest that surface water infiltration is marginal and that the possibility of significant volumes of surface water infiltrating into non‐monitored shallow streambed sediments can be excluded for the simulation period. Furthermore, the simulation results show that with increasing head differences between river and aquifer towards the end of the simulation period, the impact of streambed topography and hydraulic conductivity on spatial patterns of exchange flow rates decreases. A set of peak flow scenarios with altered groundwater‐surface water head gradients is simulated in order to quantify the potential for surface water infiltration during characteristic winter flow conditions following the observation period. The results indicate that, particularly at the beginning of peak flow conditions, head gradients are likely to cause substantial increase in surface water infiltration into the streambed. The study highlights the potential for the improvement of process understanding of hyporheic exchange flow patterns at the stream reach scale by simulating aquifer‐river exchange fluxes with a standard numerical groundwater model and a simple but robust model structure and parameterization. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Imaging the cosmic matter distribution using gravitational lensing of pre-galactic H i     

Stefan Hilbert  R. Benton Metcalf  S. D. M. White 《Monthly notices of the Royal Astronomical Society》2007,382(4):1494-1502

21-cm emission from neutral hydrogen during and before the epoch of cosmic reionization is gravitationally lensed by material at all lower redshifts. Low-frequency radio observations of this emission can be used to reconstruct the projected mass distribution of foreground material, both light and dark. We compare the potential imaging capabilities of such 21-cm lensing with those of future galaxy lensing surveys. We use the Millennium Simulation to simulate large-area maps of the lensing convergence with the noise, resolution and redshift-weighting achievable with a variety of idealized observation programmes. We find that the signal-to-noise ratio of 21-cm lens maps can far exceed that of any map made using galaxy lensing. If the irreducible noise limit can be reached with a sufficiently large radio telescope, the projected convergence map provides a high-fidelity image of the true matter distribution, allowing the dark matter haloes of individual galaxies to be viewed directly, and giving a wealth of statistical and morphological information about the relative distributions of mass and light. For instrumental designs like that planned for the Square Kilometre Array, high-fidelity mass imaging may be possible near the resolution limit of the core array of the telescope.  相似文献   

Impact of lateral transport on organic proxies in the Southern Ocean     

Jung-Hyun Kim  Xavier Crosta  Stefan Schouten  Jaap S. Sinninghe Damsté 《Quaternary Research》2009,71(2):246-124

Lateral transport of fine-grained organic carbon particles can complicate the interpretation of paleoclimate records based on organic proxies. Here we investigated the effect of lateral transport on newly developed temperature and soil organic matter proxies, TEX₈₆ and BIT index, respectively, in core MD88-769 recovered from the South East Indian Ridge. Our results show that TEX₈₆ and BIT records in comparison to diatom and foraminifera records were representative for more local climate changes while alkenones and n-alkanes originated from distant areas by oceanic and atmospheric transport, respectively. This suggests that TEX₈₆ and BIT paleoclimate records are primarily influenced by local conditions and less subjected to long-distance lateral transport than other organic proxies in the Southern Ocean.  相似文献   

Occurrence and biomarker potential of 23-methyl steroids in diatoms and sediments     

Sebastiaan W. Rampen  Stefan Schouten  Ellen C. Hopmans  Ben Abbas  Anna A.M. Noordeloos  Jan A.J. Geenevasen  J. Michael Moldowan  Peter Denisevich  Jaap S. Sinninghe Damsté 《Organic Geochemistry》2009,40(2):219-228

23-Methyl sterols have been reported to be synthesized by a few marine algae, but unambiguous identification of 23-methyl steroids in sediments and petroleum is lacking. We report the presence of 23-methylcholesta-5,22E-dien-3β-ol in 14 out of 106 diatom cultures, thereby showing that diatoms, together with dinoflagellates, may be an important environmental source for such steroids. Synthesis of authentic 23-methylcholestanes showed that their mass spectra are identical to those of 24-methylcholestanes, but that they elute earlier on apolar stationary phases during gas chromatography (GC) analysis. Co-injection of the authentic standards with sediment extracts revealed the presence of these compounds in the Skole unit of the Oligocene Menelite Formation, the Miocene Monterey Formation and the Messinian Vena del Gesso Formation. In addition, we tentatively identified 23,24-dimethyl-27-norcholestanes in some of these sediments. Molecular clock calculations suggest that diatoms are a possible source for 23-methyl steroids in sediments and petroleum from the late Jurassic onwards.  相似文献   

Effects of long term oxic degradation on the U37K′, TEX86 and BIT organic proxies     

Carme Huguet  Jung-Hyun Kim  Gert J. de Lange  Jaap S. Sinninghe Damsté  Stefan Schouten 《Organic Geochemistry》2009,40(12):1188-1194

One of the primary prerequisites for the application of organic proxies is that they should not be substantially affected by diagenesis. However, studies have shown that oxic degradation of biomarker lipids can affect their relative distribution. We tested the diagenetic stability of the $U_{37}^{K^{\prime }}$ and TEX₈₆ palaeothermometers upon long term oxygen exposure. For this purpose, we studied the distributions of alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in different sections of turbidites at the Madeira Abyssal Plain (MAP) that experienced different degrees of oxygen exposure. Sediments were deposited anoxically on the shelf and then transported by turbidity currents to the MAP, which has oxic bottom water. This resulted in partial degradation of the turbidite organic matter as a result of long term exposure to oxic bottom water. Concentrations of GDGTs and alkenones were reduced by one to two orders of magnitude in the oxidized parts of the turbidites compared to the unoxidized parts, indicating substantial degradation. High-resolution analysis of the Pleistocene F-turbidite showed that the $U_{37}^{K^{\prime }}$ index of long chain alkenones increased only slightly (0.01, corresponding to <0.5 °C) in the oxidized part of the turbidite, suggesting minor preferential degradation of the C_37:3 alkenone, in agreement with previous studies. TEX₈₆ values showed a small increase (0.02, corresponding to ～2 °C) in the F-turbidite, like $U_{37}^{K^{\prime }}$, while for other Pliocene/Miocene turbidites it either remained unchanged or decreased substantially (up to 0.06, corresponding to ～6 °C). Previous observations showed that the BIT index, a proxy for the contribution of soil organic matter to total organic carbon, was always substantially higher in the oxidized part in all the turbidites, as a result of preferential degradation of marine-derived GDGTs. This relative increase in soil-derived GDGTs affects TEX₈₆, as the isoprenoid GDGT distribution on the continent can be quite different from that in the marine environment. Our results indicate that the organic proxies are affected by long term oxic degradation to different extents; this should be taken into account when applying these proxies in palaeoceanographic studies of sediments which have been exposed to prolonged oxic degradation.  相似文献   

激光剥蚀-等离子体质谱技术及其在地球化学宇宙化学和环境研究中的应用   总被引：1，自引：0，他引：1  

JOCHUM Klaus Peter  STOLL Brigitte  FRIEDRICH Jon M  AMINI Marghaleray  BECKER Stefan  DüCKING Marc  EBEL Denton S  ENZWEILER Jacinta  胡明月  KUZMIN Dmitry  MERTZ KRAUS Regina  MüLLER Werner E G  REGNERY Julia  SOBOLEV Alexander  王晓红  詹秀春 《岩矿测试》2009,28(1):53-68

激光剥蚀-等离子体质谱(LA-ICPMS)已成为地球化学、宇宙化学和环境研究领域元素和同位素原位分析最重要的技术之一。文章介绍了多种类型的质谱仪及其使用的激光器。用途最广的LA-ICPMS仪器之一是单接收器扇形磁场质谱仪,配有Nd:YAG激光剥蚀系统(激光波长分为193 nm和213 nm两种),MPI Mainz实验室使用的就是这套系统,文章对此作一详细介绍。文中阐述了数据优化技术及其多种校正过程;介绍LA-ICPMS在痕量元素和同位素分析领域的一些应用,包括参考物质的研制,Hawaiian玄武岩、Martian陨石、生物骨针和珊瑚虫中痕量元素分析及熔融包裹体和富钙-铝碳质球粒陨石中的铅和锶同位素测量。  相似文献   

Early MIS 3 glacial lake evolution, ice-marginal retreat pattern and climate at Sokli (northeastern Fennoscandia)     

Karin F. Helmens  Jan Risberg  Krister N. Jansson  Jan Weckstrm  Annika Berntsson  Pivi Kaislahti Tillman  Peter W. Johansson  Stefan Wastegrd 《Quaternary Science Reviews》2009,28(19-20):1880-1894

Lake development at Sokli, northern Finland, is traced through the analysis of diatoms and other siliceous micro-fossils in a 2-meter thick minerogenic, laminated clay-silt deposit dated to the early part of Marine Isotope Stage (MIS) 3. Fluctuating water levels and changes in lake extent depicted by the siliceous micro-fossil record, together with lithology, suggest that an important part of the sediment sequence was deposited in a glacial lake. The proxy-based glacial lake evolution is tested using a Digital Elevation Model and geomorphologic evidence including eskers dated to the early MIS 3 Tulppio Interstadial at Sokli. Despite the apparent ice-dammed nature of the lake, the sediment is relatively rich in fossils and there are limited signs of re-deposition of older fossil material. The siliceous micro-fossil record together with data from other proxies previously analysed in the same sediment samples provides a coherent picture of past environmental changes around the Sokli site. This is most probably due to the sheltered position of the coring-site in a lake embayment. Quantitative climate reconstructions based on the diatom record show mean July air temperatures as high as present-day values at Sokli, and the temperature ranges indicated by the diatom record are in agreement with temperature reconstructions based on chironomids. The position of Sokli in the northeastern portion of the central area of the Scandinavian glaciations and the northern retreat pattern implies that an important part of eastern Fennoscandia was deglaciated during the early MIS 3 warming event.  相似文献   

Comparison of groundwater recharge estimation methods for the semi-arid Nyamandhlovu area, Zimbabwe   总被引：1，自引：0，他引：1  

Tenant Sibanda  Johannes C. Nonner  Stefan Uhlenbrook 《Hydrogeology Journal》2009,17(6):1427-1441

The Nyamandhlovu aquifer is the main water resource in the semi-arid Umguza district in Matebeleland North Province in Zimbabwe. The rapid increase in water demand in the city of Bulawayo has prompted the need to quantify the available groundwater resources for sustainable utilization. Groundwater recharge estimation methods and results were compared: chloride mass balance method (19–62 mm/year); water-table fluctuation method (2–50 mm/year); Darcian flownet computations (16–28 mm/year); ¹⁴C age dating (22–25 mm/year); and groundwater modeling (11–26 mm/year). The flownet computational and modeling methods provided better estimates for aerial recharge than the other methods. Based on groundwater modeling, a final estimate for recharge (from precipitation) on the order of 15–20 mm/year is believed to be realistic, assuming that part of the recharge water transpires from the water table by deep-rooted vegetation. This recharge estimate (2.7–3.6% of the annual precipitation of 555 mm/year) compares well with the results of other researchers. The advantages/disadvantages of each recharge method in terms of ease of application, accuracy, and costs are discussed. The groundwater model was also used to quantify the total recharge of the Nyamandhlovu aquifer system (20?×?10⁶–25?×?10⁶ m³/year). Groundwater abstractions exceeding 17?×?10⁶ m³/year could cause ecological damage, affecting, for instance, the deep-rooted vegetation in the area.  相似文献   

最早的栉水母动物化石——华南伊迪 卡拉纪的“八臂仙母虫”   总被引：2，自引：1，他引：1  

唐烽  尹崇玉  Stefan Bengtson  刘鹏举  王自强  陈寿铭  高林志 《地球学报》2009,30(4):543-553

华南伊迪卡拉系陡山沱组保存的磷酸盐化球状化石中有一部分被认为是最早的后生动物胚胎化石, 但由于缺乏成年或幼年过渡期的化石证据, 对这些最早微化石的动物属性还一直存在争议。最近在贵州江口县翁会地区伊迪卡拉(震旦)系陡山沱组的页岩中发现一个与“庙河生物群”相似的宏体碳质压膜化石群, 其中包含一个盘状的宏体化石新类型——八臂仙母虫(Eoandromeda octobrachiata, Tang et al. 2008), 具有八条螺旋状辐射的旋臂, 封闭在一个包膜内。这种八辐射结构在八射珊瑚和栉水母动物中都可见到, 但同时具有的螺旋特性与现生的所有类群都无法对比。在贵州陡山沱组的“瓮安生物群”中发现的球状胚胎化石同样具有螺旋性状, 尽管还没有直接的个体发育证据, 但我们认为新属种可能代表这些螺旋状胚胎化石的成年实体化石。最新报道的南澳大利亚Rawnsley石英岩层中也发现相似的印痕化石, 被归为同一类别, 表明华南的“庙河生物群”与澳大利亚的“伊迪卡拉生物群”时代相当。翁会化石库中少量出露的三辐射化石与俄罗斯“白海生物群”的典型分子—Albumares和Anfesta可以对比。新发现的倾斜保存的八臂仙母虫化石和碳质条带呈子午线状排列的核桃状化石, 可能是该类化石归属栉水母类的有力证据, 这一发现推测可将栉水母类的最早化石记录从“澄江生物群”的早寒武世推前大约3千万年。  相似文献