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61.
Bottom trawling causes physical disturbance to sediments particularly in shelf areas. The disturbance due to trawling is most
significant in deeper areas with softer sediments where levels of natural disturbance due to wave and tidal action are low.
In heavily fished areas, trawls may impact the same area of seabed more than four times per year. A single pass of a beam
trawl, the heaviest gear routinely used in shelf sea fisheries, can kill 5–65% of the resident fauna and mix the top few cm
of sediment. We expect that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly
affected by trawling activity because the physical effects of trawling are equivalent to those of an extreme bioturbator,
and yet, unlike bioturbating macrofauna, trawling does not directly contribute to community metabolism. We used an existing
box-model of a generalised soft sediment system to examine the effects of trawling disturbance on carbon mineralisation and
chemical concentrations. We contrasted the effects of a natural scenario, where bioturbation is a function of macrobenthos
biomass, with an anthropogenic impact scenario where physical disturbance results from trawling rather than the action of
bioturbating macrofauna. Simulation results suggest that the effects of low levels of trawling disturbance will be similar
to those of natural bioturbators but that high levels of trawling disturbance prevent the modelled system from reaching equilibrium
due to large carbon fluxes between oxic and anoxic carbon compartments. The presence of macrobenthos in the natural disturbance
scenario allowed sediment chemical storage and fluxes to reach equilibrium. This is because the macrobenthos are important
carbon consumers in the system whose presence reduces the magnitude of available carbon fluxes. In soft sediment systems,
where the level physical disturbance due to waves and tides is low, model results suggest that intensive trawling disturbance
could cause large fluctuations in benthic chemical fluxes and storage. 相似文献
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Two-well tracer tests are often conducted to investigate subsurface solute transport in the field. Analyzing breakthrough curves in extraction and monitoring wells using numerical methods is nontrivial due to highly nonuniform flow conditions. We extended approximate analytical solutions for the advection-dispersion equation for an injection-extraction well doublet in a homogeneous confined aquifer under steady-state flow conditions for equal injection and extraction rates with no transverse dispersion and negligible ambient flow, and implemented the solutions in Microsoft Excel using Visual Basic for Application (VBA). Functions were implemented to calculate concentrations in extraction and monitoring wells at any location due to a step or pulse injection. Type curves for a step injection were compared with those calculated by numerically integrating the solution for a pulse injection. The results from the two approaches are similar when the dispersivity is small. As the dispersivity increases, the latter was found to be more accurate but requires more computing time. The code was verified by comparing the results with published-type curves and applied to analyze data from the literature. The method can be used as a first approximation for two-well tracer test design and data analysis, and to check accuracy of numerical solutions. The code and example files are publicly available. 相似文献
69.
龙王尖金矿地球化学异常特征及找矿标志 总被引：1，自引：0，他引：1
在广泛地研究了与金矿有关的组份分布的基础上,确定了该矿床赋有高含量的Ａｕ、Ａｇ、Ａｓ、Ｓｂ、Ｈｇ、Ｐｂ、Ｚｎ、Ｃｏ、Ｎｉ和低含量的Ｃｕ、Ｍｏ、Ｂａ、Ｂｉ、Ｂ、Ｍｎ,讨论了这些指示元素在土壤和岩石地球化学异常中的特征,初步建立了该金矿的地球化学找矿标志,确认该矿床为断裂破碎蚀变岩型金矿。 相似文献
70.
黄铁矿型铜多金属矿床是与海底火山喷发—沉积建造有关的火山岩矿床,矿化赋存于一套微量元素总体含量水平较高的海相中酸性细碧角斑岩系中。赋矿地层、岩性具有以Cu,Pb,Zn为主,伴生Sb,Ba,Ag,As,Bi,Hg,Cd等多元素的特征组合,这些元素在成矿区域上形成大范围的地球化学异常,其主体异常对应于矿田。矿区大比例尺的岩石地球化学测量显示,这些指示元素的清晰的原生异常相互交替叠置于矿床内矿化富集部位,水平分带不甚明显,而垂直分带清晰。具有不同指示意义的元素组合于矿化的不同部位聚集而出现的分带,是用于评价矿化剥蚀程度及其成矿远景的重要地球化学参量 相似文献