排序方式： 共有9条查询结果，搜索用时 31 毫秒
1
1.
Two Hercynian duplexes are developed in Viséan limestones in the Basse Normandie quarry. The lower duplex is completely exposed in a subvertical quarry wall; the partially exposed upper duplex lies immediately above the lower duplex. The duplexes are both located in the footwall of the Hydrequent thrust which emplaced Devonian clastic sediments above the Viséan limestones. The lower duplex exposes all the internal thrusts, a reference bed of chalky limestone, the roof and floor thrusts, and the duplex tip. The duplex has been graphically restored to its pre-deformation geometry by line-length and area balancing and its resultant geometry is close to the model of Boyer & Elliott. The lower duplex shortened by two different mechanisms, an initial phase of layer-parallel shortening which produced no cleavage, followed by thrust imbrication. The average contraction of the front portion of the duplex was ?49% (natural strain) of which ?27% is layer-parallel shortening and ?22% is thrust imbrication. However, locally the bulk shortening increases from zero at the duplex tip to over ?120% in a down-dip direction. The area balancing provides the most accurate estimates of bulk shortening; line-length balance calculations give minimum estimates only. An area balance on the whole of the lower duplex gives a bulk shortening of ?84%. An area balance of the upper duplex yields an average contraction of ?75% and the total contraction produced by both duplexes is ?92%. 相似文献
2.
Collen JD Baker JA Dunbar RB Rieser U Gardner JP Garton DW Christiansen KJ 《Marine pollution bulletin》2011,62(2):251-257
Anthropogenic lead (Pb) inputs to the atmosphere increased greatly over the past century and now dominate Pb supply to the oceans. However, the Pb content of sediments across the equatorial Pacific region is relatively unknown, and data exist only for deep sea sites where Pb deposition lags surface water inputs by up to a century. Here we present ICP-MS analyses of Pb of a core from a lagoon at Palmyra Atoll, northern Line Islands, that spans approximately the past 160 years. The non-bioturbated sediments of the euxinic lagoon, coupled with rapid rates of deposition, provide a unique fine-scale record of atmospheric Pb supply at a remote Pacific location. These first observations of historic Pb sedimentation in an atoll lagoon reveal a 63-fold increase in Pb flux to sediments during the past century and correlate directly with the North American consumption of leaded gasoline that began in 1926. 相似文献
3.
4.
Brown trout (Salmo trutta) are known to have effects on multiple trophic levels in New Zealand streams, but their impacts on lower trophic levels are less well understood within lentic systems. We examined the effects of brown trout removal using rotenone on zooplankton and phytoplankton community composition in the Upper Karori Reservoir, New Zealand. Significant shifts were observed in zooplankton and phytoplankton composition following removal of brown trout from the reservoir. Shifts in zooplankton community composition did not occur immediately following trout removal (February), but instead followed the likely timing of galaxiid spawning (July). The removal of brown trout likely resulted in reduced predation pressure on galaxiids. A major change occurred in the zooplankton community with the dominance shifting from larger crustaceans to smaller rotifers, indicating an increased predation pressure from the larval native galaxiid. A delayed response in zooplankton community composition change indicates rotenone was not the direct cause of this. A major shift in phytoplankton community composition occurred immediately following trout removal. This was not consistent with the trophic cascade hypothesis of reduced grazing pressure from larger zooplankton due to increased galaxiid predation as a result of brown trout removal. 相似文献
5.
The recognition of littoral sediment cells has proven valuable to coastal zone management in many areas of the world because it emphasises that sediment transport and deposition relate to physical processes rather than to administrative boundaries and that sedimentary units are self-contained. To date, cells identified for shoreline management planning are often extensive, extending for tens to hundreds of km. We suggest that the concept is also applicable to the management of atoll coastlines which, although spatially very small overall, consist of isolated segments with efficient intervening sediment sinks. As an example, Palmyra Atoll, a protected wildlife refuge in the northern Line Islands, is undergoing natural major changes resulting from sediment-related processes. Defining sediment cells as integral parts of management units here will aid decision-making necessary for the protection of specific areas, potentially giving savings in research effort and remedial costs and minimising disruption to other sensitive areas of the reserve. Similar benefits would apply to the management of the coastlines of other atolls. 相似文献
6.
The ‘pipe rock’ lithofacies has been much maligned by many generations of undergraduates as being dull. We attempt herein to demonstrate some of the subtleties of the pipe‐rock ecosystem and show that ichnology can be a most useful tool to stratigraphers, sedimentologists and petroleum geologists. 相似文献
7.
This study investigated how an industrial tuna fishery functions in terms of procedures, practices, governance and finance in the context of Ghana, West Africa. Tuna is Ghana’s biggest seafood export, contributing significantly to the domestic fisheries sector. A case-study approach was used to analyse relevant social and economic factors at the local scale to better understand how the global seafood industry operates in a low-income country. A value-chain framework was adopted to assess market structures, sales pathways and revenue distribution. We also investigated the role of actors engaged in the industry using secondary data, interviews, questionnaires and participant observations. The results revealed a changing organisational structure in tuna production, moving from bait-boat fishing with smaller companies to large-scale purse-seine fishing backed by consolidated Asian seafood companies. Production was found to depend significantly on local female intermediaries for access to funds through prefinancing arrangements. Considerable illegalities were identified within the value chain, highlighting the need for improved partnership and licensing negotiations, and for low-cost marine control and surveillance tools. As vertical integration increases within the industry, the influence of corporations as keystone actors becomes evident for the future social and ecological sustainability of the industry. 相似文献
8.
岩浆侵入体对有机质生烃(成熟)作用的影响及数值模拟 总被引：3，自引：1，他引：2
火山作用带来的热源对有机质的成烃进程和成烃量产生了重要影响,这种热效应一方面可以加速烃源岩的成熟和成烃作用,另一方面可以破坏先期形成的油藏。目前国内外对火山作用的生烃热效应方面的研究主要在热成熟度参数变化描述及岩浆侵入体引起的围岩热蚀变程度的讨论。火山作用对成烃/成藏的利弊,影响的时/空范围、程度的定量研究尚未报道。本次在前人研究基础上,建立了描述岩浆侵入体散热的热传导模型,进一步结合EasyRo%模型和前人报道的实测Ro数据进行了模型检验。通过调整模型的参数和岩浆侵入体厚度、位置及个数等进行了火山作用对有机质成熟作用及生烃影响的研究。模拟结果表明:岩浆侵入体的热作用范围是有限的,但是不同地质情况,影响的范围广度也不相同,对于相同厚度的侵入体,初始温度越高,作用范围越广,但是一般X/D3(X/D代表离接触面的距离与侵入体厚度的比值)。对于不同厚度侵入体而言,在侵入体热作用影响的范围内,侵入体越厚,热作用的范围越广。同时数值模拟结果也表明岩浆侵入体的存在可以改变烃源岩生烃期。 相似文献
9.
Copepods (Class Crustacea, Order Copepoda) are rarely included in paleoecological studies of lakes because they lack long-lasting exoskeletal remains. We describe the remains of eggs (egg shells) from Hesperodiaptomus copepods that are well preserved and abundant in alpine lake sediments. We demonstrate that the egg shells are the remains of Hesperodiaptomus eggs based on (i) the similar size and morphology of egg shells collected from sediments and those produced from the hatching of eggs obtained from laboratory-maintained Hesperodiaptomus, and (ii) the finding that diapausing eggs collected from lake sediments and hatched in the laboratory produced copepod nauplii that were morphologically indistinguishable from those hatched from eggs produced by laboratory-maintained Hesperodiaptomus. Egg shells were approximately two orders of magnitude more abundant in sediment cores than were viable diapausing eggs, making egg shells superior to viable diapausing eggs for quantifying the historical presence and abundance of Hesperodiaptomus. These results have important implications for alpine lake restoration as egg shells can be used to identify lakes in which Hesperodiaptomus was eliminated by fish introductions but has failed to return after fish eradication, lakes in which the pre-disturbance conditions are likely to be restored only by reintroductions of this important taxon. 相似文献
1