Dahab stream sediments, southeastern Sinai, Egypt: a potential source of gold, magnetite and zircon   总被引：1，自引：0，他引：1  

A. A. Surour  A. A. El-Kammar  E. H. Arafa  H. M. Korany 《Journal of Geochemical Exploration》2003,77(1):25-43

The stream sediments of Dahab area, southeastern Sinai, Egypt, were studied for their content of economic minerals. These sediments are immature as indicated by poor sorting and other mechanical parameters. They are derived from Precambrian basement rocks, which are mostly represented by granitic rocks in addition to lesser amounts of volcanics and gabbros. The mineralogical investigation revealed that these sediments contain considerable amounts of placer gold, Fe–Ti oxides and zircon.The concentrated Fe–Ti oxides comprise homogeneous magnetite and ilmenite in addition to ilmeno-magnetite, hemo-ilmenite and rutile–hematite intergrowths. Isodynamic separation of some raw samples of SIZE=1 mm revealed that up to 15.12% magnetic minerals can be recovered. Zircon shows remarkable variations in morphology, colour, chemistry and provenance. U-poor and U-rich varieties of zircon were discriminated containing UO₂ in the ranges of 0.04–1.19 and 3.05–3.68 wt.%, respectively. REE-bearing minerals comprise monazite, allanite and La-cerianite.On mineralogical basis, the present work suggests that Dahab stream sediments represent a promising target for further geochemical exploration for precious metals, especially gold. Fire assay data indicate that placer gold in the studied sediments sometimes reaches 15.34 g/t. Narrow gullies and valleys cutting the basement manifest the development and preservation of gold in this arid environment. Background concentration of gold and variation in lithology suggest multiple source of the metal in the investigated sediments.  相似文献   

Nutrient enrichment does not affect diet selection by a tropical shredder species in a mesocosm experiment     

《Limnologica》2021

The decomposition of plant litter is a fundamental ecological process in small forest streams. Litter decomposition is mostly controlled by litter characteristics and environmental conditions, with shredders playing a critical role. The aim of this study was to evaluate the effect of leaf species (Maprounea guianensis and Inga laurina, which have contrasting physical and chemical characteristics) and water nutrient enrichment (three levels) on leaf litter chemical characteristics and fungal biomass, and subsequent litter preference and consumption by Phylloicus sp. (a typical shredder in tropical streams). Maprounea guianensis leaves had lower lignin and nitrogen (N) concentrations, higher polyphenols concentration and lower lignin:N ratio than I. laurina leaves. Phosphorus concentrations were higher for both leaf species incubated at the highest water nutrient level. Fungal biomass was higher on M. guianensis than on I. laurina leaves, but it did not differ among nutrient levels. Relative consumption rates were higher when shredders fed on M. guianensis than on I. laurina leaves, due to the lower lignin:N ratio and higher fungal biomass of M. guianensis. Consumption rates on M. guianensis leaves were higher for those exposed to low water nutrient levels than for those exposed to moderate water nutrient levels. Feeding preferences by shredders were not affected by leaf species or nutrient level. The low carbon quality on I. laurina leaves makes it a less attractive substrate for microbial decomposers and a less palatable resource for shredders. Changes in litter input characteristics may be more important than short-term nutrient enrichment of stream water on shredder performance and ecosystem functioning.  相似文献   

Observations of the Alaskan Stream near Samalga Pass and its connection to the Bering Sea: 2001–2004     

《Deep Sea Research Part I: Oceanographic Research Papers》2014

Year-long moorings were deployed across the Alaskan Stream near Samalga Pass (169°W) on two occasions, first in 2001–2002 (5 moorings) and again in 2003–2004 (3 moorings). Currents were measured throughout the water column, and temperature and salinity were measured at selected depths. Satellite altimetry and satellite-tracked drifters revealed a well defined Alaskan Stream, with the largest near-surface average speeds (>60 cm s⁻¹) and highest eddy kinetic energy just upstream from the mooring sites. Excluding periods when large eddies disrupted the flow, transport in the Alaskan Stream ranged from 10 to 30×10⁶ m³ s⁻¹. The estimated mean transport in 2001–2002 was 19×10⁶ m³ s⁻¹, and in 2003–2004 was 21×10⁶ m³ s⁻¹. Large (diameter>200 km), anti-cyclonic eddies were not uncommon in the vicinity of Samalga Pass (14 times in 20 year period, 1992–2012). Although there were no such eddies observed during the period 2000–2003, one of the largest ever recorded eddies occurred in spring 2004. In addition, smaller eddies occurred on several occasions. Eddies disrupted the flow, shifting the Alaskan Stream farther off shore and were clearly evident in both the satellite imagery and the mooring data. Other energetic events, which were less evident in the satellite records, but clearly evident in the mooring measurements, also disrupted the flow. In addition to the moorings in the Alaskan Stream, pressure gauges were placed in Samalga Pass and a single mooring measuring currents was placed in the Aleutian North Slope Current (ANSC) in the Bering Sea. The alongshore, near-surface flow measured at the moorings deployed on the 1000-m isobaths in the Alaskan Stream and the ANSC were significantly correlated with the bottom pressure time series. In addition, at periods longer than 14 days, the bottom pressure measured at the mooring sites in Samalga Pass was significantly correlated with the sea surface height measured by the satellites. The eddy kinetic energies measured from the satellites and from moorings were also significantly correlated.  相似文献   

Formational history of the Wicklow Trough: a marine-transgressed tunnel valley revealing ice flow velocity and retreat rates for the largest ice stream draining the late-Devensian British–Irish Ice Sheet     

Mark Coughlan  Zsuzsanna TÓth  Katrien J. J. Van Landeghem  Stephen Mccarron  Andrew J. Wheeler 《第四纪科学杂志》2020,35(7):907-919

The Wicklow Trough is one of several Irish Sea bathymetric deeps, yet unusually isolated from the main depression, the Western Trough. Its formation has been described as proglacial or subglacial, linked to the Irish Sea Ice Stream (ISIS) during the Last Glacial Maximum. The evolution of the Wicklow Trough and neighbouring deeps, therefore, help us to understand ISIS dynamics, when it was the main ice stream draining the former British–Irish Ice Sheet. The morphology and sub-seabed stratigraphy of the 18 km long and 2 km wide Wicklow Trough is described here from new multibeam echosounder data, 60 km of sparker seismic profiles and five sediment cores. At a maximum water depth of 82 m, the deep consists of four overdeepened sections. The heterogeneous glacial sediments in the Trough overlay bedrock, with indications of flank mass-wasting and subglacial bedforms on its floor. The evidence strongly suggests that the Wicklow Trough is a tunnel valley formed by time-transgressive erosional processes, with pressurised meltwater as the dominant agent during gradual or slow ice sheet retreat. Its location may be fault-controlled, and the northern end of the Wicklow Trough could mark a transition from rapid to slow grounded ice margin retreat, which could be tested with modelling.  相似文献   

El Ni?o 事件期间南海环流的异常特征     

邢延松  程国胜  舒业强  王东晓 《海洋与湖沼》2012,43(2):201-209

基于CORA再分析资料对南海环流的季节特征和其受ElNio事件的响应进行了分析。结果表明,冬季整个南海海区表现为一个大的气旋式环流,夏季南海北部是气旋式环流,南部是一个反气旋式环流。通过对南海海区异常流场进行MV-EOF分解,分析其前两个模态,其空间型主要体现了南海环流冬季和夏季的特征,对应的时间系数与Nio3.4_NDJ指数有很好的相关性。通过分析南海环流在1986—2008年间ElNio年份的异常流场和异常流函数场,证明了MV-EOF分解后得到的联合时间系数所反映各阶段南海环流的季节特征与ElNio事件有相关性,即在8月[0],南海南部异常流函数场表现为反气旋式环流,北部为气旋式环流,南海夏季环流被增强,且ElNio事件时间尺度越长,北部的气旋式异常流场的影响范围就越大;在12月[0],南海除了东南部外,其余整个海区异常流函数场主要表现为反气旋式环流,冬季环流被减弱;在8月[+1],南海夏季流场强度都被削弱了。  相似文献   

Profile convexities in bedrock and alluvial streams     

Jonathan D. Phillips  J. David Lutz 《Geomorphology》2008,102(3-4):554-566

Longitudinal profiles of bedrock streams in central Kentucky, and of coastal plain streams in southeast Texas, were analyzed to determine the extent to which they exhibit smoothly concave profiles and to relate profile convexities to environmental controls. None of the Kentucky streams have smoothly concave profiles. Because all observed knickpoints are associated with vertical joints, if they are migrating it either occurs rapidly between vertical joints, or migrating knickpoints become stalled at structural features. These streams have been adjusting to downcutting of the Kentucky River for at least 1.3 Ma, suggesting that the time required to produce a concave profile is long compared to the typical timescale of environmental change. A graded concave longitudinal profile is not a reasonable prediction or benchmark condition for these streams. The characteristic profile forms of the Kentucky River gorge area are contingent on a particular combination of lithology, structure, hydrologic regime, and geomorphic history, and therefore do not represent any general type of equilibrium state. Few stream profiles in SE Texas conform to the ideal of the smoothly, strongly concave profile. Major convexities are caused by inherited topography, geologic controls, recent and contemporary geomorphic processes, and anthropic effects. Both the legacy of Quaternary environmental change and ongoing changes make it unlikely that consistent boundary conditions will exist for long. Further, the few exceptions within the study area–i.e., strongly and smoothly concave longitudinal profiles–suggest that ample time has occurred for strongly concave profiles to develop and that such profiles do not necessarily represent any mutual adjustments between slope, transport capacity, and sediment supply. The simplest explanation of any tendency toward concavity is related to basic constraints on channel steepness associated with geomechanical stability and minimum slopes necessary to convey flow. This constrained gradient concept (CGC) can explain the general tendency toward concavity in channels of sufficient size, with minimal lithological constraints and with sufficient time for adjustment. Unlike grade- or equilibrium-based theories, the CGC results in interpretations of convex or low-concavity profiles or reaches in terms of local environmental constraints and geomorphic histories rather than as “disequilibrium” features.  相似文献   

Stream Interactions and Interplanetary Coronal Mass Ejections at 0.72 AU     

L. K. Jian  C. T. Russell  J. G. Luhmann  R. M. Skoug  J. T. Steinberg 《Solar physics》2008,249(1):85-101

We present comprehensive surveys of 203 stream interaction regions (SIRs) and 124 interplanetary CMEs (ICMEs) during 1979 – 1988 using Pioneer Venus Orbiter (PVO) in situ solar-wind observations at 0.72 AU and examine the solar-cycle variations of the occurrence rate, shock association rate, duration, width, maximum total perpendicular pressure (P _t), maximum dynamic pressure, maximum magnetic field intensity, and maximum velocity change of these two large-scale solar-wind structures. The medians, averages, and histogram distributions of these parameters are also reported. Furthermore, we sort ICMEs into three groups based on the temporal profiles of P _t, and we investigate the variations of the fractional occurrence rate of three groups of ICMEs with solar activity. We find that the fractional occurrence rate of magnetic-cloud-like ICMEs declined with solar activity, consistent with our former 1-AU results. This study at 0.72 AU provides a point of comparison in the inner heliosphere for examining the radial evolution of SIRs and ICMEs. The width of SIRs and ICMEs increases by 0.04 and 0.1 AU, respectively, and the maximum P _t decreases to about 1/3 from Venus to Earth orbit. In addition, our work establishes the statistical properties of the solar-wind conditions at 0.72 AU that control the solar-wind interaction with Venus and its atmosphere loss by related processes. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.  相似文献   

Stream turbidity responses to storm events in a pristine rainforest watershed on the Coral Coast of southern Fiji     

《国际泥沙研究》2016,(4):279-290

On the Coral Coast of Viti Levu Island in Fiji, inadequate knowledge of suspended sediment delivery patterns in small pristine coastal watersheds hinders any future assessment of accelerated erosion in disturbed areas nearby. This study adopts a rainfall–stream turbidity monitoring approach in the Votua Creek, which drains a small, steep but minimally-disturbed coastal rainforest catchment. Storm rainfall characteristics, stream depth and water turbidity were continuously monitored over one complete Fiji wet season from October 2009 to April 2010. The aim was to evaluate whether these parameters provide sufficient information to illustrate basic features of storm–sediment transport responses, in the case of limited stream gauging and very simple sediment rating curves. This is important because Pacific Island nations like Fiji do not have the resources to initiate long-term gauging and sediment sampling pro-grammes across numerous small catchments.
A significant power function demonstrates that turbidity (T) is a suitable proxy for total suspended solids (TSS) for turbidity measurements above 5 NTU, with TSS?0.930T1.111 (r?0.98, Po0.001). Over the study period, 10 individual storms 11.2–120.1 mm in size produced a‘significant turbidity response’ (STR) in the Votua Creek. Rainfall parameters (totals and intensities) showed positive linear relationships (r ? 0.72–0.94) with stream turbidity parameters (mean, maximum, duration), whilst relationships of similar strength (r ? 0.76–0.98) were also derived between stream flow depth and turbidity. This implies that for small rainforest watersheds in Fiji, rainfall parameters offer no substantial disadvantage over flow as predictors of stream sediment responses to major storms. Event-based analysis revealed that negative (anticlockwise) hysteresis is a typical flow–turbidity pattern for STR events. Negative hysteresis is produced when secondary episodes of renewed (heavy) rainfall occur after maximum intensity, in the later phase of storm events. Tropical Cyclone Mick in December 2009 generated the largest flood and the greatest turbidity response (Tmax ? 1021 NTU, Tmean ? 207 NTU). This concurs with earlier work confirming that tropical cyclones are the most important events for sediment transport in Fiji stream networks.  相似文献   

Impact of eddies on ocean diapycnal mixing in Gulf Stream region     

LI Min  XIE LingLing  YANG QingXuan  TIAN JiWei 《中国科学:地球科学(英文版)》2014,57(6):1407-1414

Based on high-resolution,Array for Real-time Geostrophic Oceanography(Argo)profiles and Sea Level Anomaly(SLA)data,this study statistically analyzes and compares turbulent diapycnal mixing profiles inside and outside mesoscale eddies in the Gulf Stream region.The result indicates that average diapycnal diffusivity at 300–540 m depths in anticyclonic eddies reaches4.0×10-5 m2 s-1.This is significantly higher than the 1.6×10-5 m2 s-1 outside eddies and 0.8×10-5 m2 s-1 in cyclonic eddies.Probabilities of diapycnal diffusivity greater than 10-4 m2 s-1 within anticyclonic and cyclonic eddies and outside eddies are29%,5%and 12%,respectively.However,magnitudes of average diapycnal diffusivity at 540–900 m depths in these three cases are of the same order,10-5 m2 s-1.Twenty-four of a total 38 anticyclonic eddies had enhanced mixing in the ocean interior,and 22 were observed during or shortly after strong winds.The coincidence between enhanced mixing and strong wind stress indicates that more wind-induced,near-inertial wave energy propagates downward in anticyclonic eddies.The deeper part of 12 profiles(below 540 m)in anticyclonic eddies had vertical overturns with Thorpe scale exceeding 5 m,among which three profiles had overturns reaching 20 m.Enhanced mixing may have occurred in deep layers of some profiles,although it was not evident in average conditions.  相似文献   

Seasonal hydrology of Port Fitzroy,Great Barrier Island,New Zealand   总被引：1，自引：1，他引：0  

R. W. Hickman 《新西兰海洋与淡水研究杂志》2013,47(2):231-240

At 2‐monthly intervals between March 1974 and January 1975 seawater temperature and salinity (at 0 m and 5 m depths) and turbidity (as Secchi disc visibility) were measured in Port Fitzroy and adjacent bays on Great Barrier Island (36°11'S, 175°21'E). A seasonal pattern was evident in all three parameters. Variation between depths and between high and low tides was generally only slight, although differences of up to 3.6°c and 12.58‰ were recorded occasionally. Maximum temperatures (22–23°c) occurred in January, with minima (13–14°c) in July. Salinities were generally between 35.0 and 35.6‰, but values as low as 22.25‰ at the surface were recorded after heavy rainfall. Secehi disc visibility measurements of turbidity varied between 2.0 and 12.5 m, but were usually 5–9 m.

A graded series of four generalised areas was distinguished, ranging from sheltered, almost estuarine inner bays, to exposed open water with properties similar to outer Hauraki Gulf water.

The circulation through Port Fitzroy comprises the main exchange of water through the northern entrance channel, causing a southerly movement of water on the flood tide and a northerly movement on the ebb, together with a small compensatory exchange through the narrow southern entrance.  相似文献