Correcting for variation in catchability: maturity-related catchability variation in Icelandic cod     

Thorarinsson  Kristjan; Johannesson  Gardar 《ICES Journal of Marine Science》1997,54(5):787-796

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Controls on natural levée development in the Columbia River, British Columbia, Canada     

MANUEL FILGUEIRA-RIVERA  NORMAN D. SMITH  RUDY L. SLINGERLAND† 《Sedimentology》2007,54(4):905-919

Natural levées of the Columbia River near Golden, British Columbia, were investigated to identify the mechanisms that control levée development and morphology. Topographic profiles of 12 levée pairs were surveyed, and measurements of water-surface elevation, flow velocity, flow direction and turbidity were obtained during an average magnitude flood (1·2 years recurrence interval). Sedimentation rates and grain-size distributions were measured from sediment traps placed along levée-to-floodbasin transects. Results show that water and sediment exchange between the channel and floodbasin was mainly by advection. During flooding, local floodbasins behave more as efficient water pathways than water storage features, resulting in down-valley floodbasin flows capable of limiting basinward growth of levées. Levée shape results primarily from two independent factors: (1) maximum channel water stage, which limits levée height; and (2) floodbasin hydraulics, which control width. In the Columbia River, the competence of floodbasin flows results in relatively narrow and steep levées. Natural levées grow under two general conditions of deposition as governed by flood-stage elevation relative to levée-crest elevation: front loading and back loading. During large floods when crests are inundated, front loading preferentially aggrades the proximal portions of levées with sediment directly from the channel, thus increasing levée slope. During average or below-average floods when many levée crests are not overtopped, back loading preferentially aggrades the distal levée areas and floodbasin floor, reducing levée slope. In the study area, a balance between front and back loading sustains these narrow and steep levée shapes for long periods, reflecting an equilibrium between hydraulic regime, floodplain morphology and deposition.  相似文献   

Phytoplankton response to climate changes in Lake Baikal during the Holocene and Kazantsevo Interglacials assessed from sedimentary pigments     

Susanne Fietz  Andreas Nicklisch  Hedi Oberhänsli 《Journal of Paleolimnology》2007,37(2):177-203

Lake Baikal, an ancient pristine lake in Siberia, has accumulated sediment deposits that span 25 million years. These deposits have the potential to provide a long-term record of climate changes and their interaction with the ecology of the lake. In order to investigate whether sedimentary phytoplankton pigments could be used to reconstruct past changes in total phytoplankton abundance and productivity, we compared the spatial variability in sedimentary pigment distributions in Holocene cores from three separate regions of the lake; Vidrino in the south, Posolski on Selenga Delta and Continent Ridge in the north. Furthermore, we present the first continuous sedimentary pigment and organic carbon sequence of the Kazantsevo interglacial (roughly a time equivalent to the European Eemian, and Marine Isotopic Stage MIS5e) at a resolution of approximately 150 years. Results of the spatial study showed marked differences in the sediment pigment deposition. Lowest chlorophyll a plus its degradation products versus organic carbon ratios (Chlas/TOC) indicating lowest production, but highest variability with time (indicating strongest climatic oscillations) were found at Continent Ridge. The study of sedimentary pigments deposited during the last two interglacial periods at Continent Ridge showed Chlas/TOC ratios 50–1000 times higher during the Kazantsevo Interglacial compared to the glacial periods, whereas the TOC content was only five times higher, thus indicating the significance of the Chlas/TOC ratio for the reconstruction of the phytoplankton abundance and productivity. Strong oscillations occurred during the Kazantsevo Interglacial within centennial time scales. Chlorophyllb plus its degradation products provided additional information on the past development of Chlorophyceae. Highest Chlas/TOC ratios were found during the early Holocene at approximately 9 kyr BP. Indications of short phytoplankton production maxima were also found during the late Atlantic (6 kyr BP) and at the Subboreal/Subatlantic transition (3 kyr BP). From this we conclude that sedimentary chlorophyll a is a reliable indicator of phytoplanktonic response to climate changes and may serve for␣validation of future climate scenarios in continental regions.  相似文献   

云南省旅游资源综合评价方法     

张晓明  黄玉 《云南地质》2003,22(2):211-216

本文采用层次分析法，从风景旅游区的旅游资源条件、地理环境条件、旅游条件三个方面对全省已列级的旅游资源进行综合评价对比，从而为全省旅游区的开发建设提供科学依据。  相似文献   

Morphologic changes in the Paraná River channel (Argentina) in the light of the climate variability during the 20th century     

Mario L. Amsler  Carlos G. Ramonell  Horacio A. Toniolo   《Geomorphology》2005,70(3-4):257

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In situ anaerobic degradation of petroleum alkanes in marine sediments: preliminary results     

Delphine Massias  Vincent Grossi  Jean-Claude Bertrand 《Comptes Rendus Geoscience》2003,335(5):435-439

The degradation of acyclic petroleum hydrocarbons was studied during a 24-month experiment in Mediterranean coastal sediments (Gulf of Fos). Sediment cores entirely contaminated with oil (Arabian Light Crude Oil) were incubated in situ. The use of conservative tracers of sediment's particles reworking (luminophores) allowed the distinction of the reworked layer from the anoxic deeper sediments. Using the 17<em>αem>,21<em>βem>C₃₀ hopane (C₃₀H) as an inert internal reference, we could demonstrate that, after 24 months of experiment, acyclic petroleum hydrocarbons can be degraded under natural anaerobic conditions. The reactivity of individual alkanes appeared to depend on their chemical structure. <em>To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).em><em>To cite this article: D. Massias et al., C. R. Geoscience 335 (2003).em>  相似文献   

Palaeovalleys and fault-controlled depocentres in the Late-Ordovician glacial record of the Murzuq Basin (central Libya)     

Jean-François Ghienne  Max Deynoux  Giänreto Manatschal  Jean-Loup Rubino 《Comptes Rendus Geoscience》2003,335(15):1091-1100

In the Gargaf area, the stratigraphic architecture of the Late-Ordovician glacial drift results from successive glacial erosion events, with the location of the main glacial valleys partly controlled by inherited Panafrican structural trends, and by the existence of glacio-isostatically induced fault-related depocentres. Four laterally discontinuous, depositional units correspond to the filling of palaeovalleys. Each of the corresponding basal bounding surfaces was incised during a major ice front advance, reaching at least the northern Gargaf (>28°S). The bulk of the glacial record is made up of fluvial to shallow-marine sediments deposited in relatively distal glacial environments. Each unit, which recorded a glacial-interglacial climatic cycle, can be used for correlation throughout the Murzuq Basin and even at the scale of the North Gondwana platform. <em>To cite this article: J.-F. Ghienne et al., C. R. Geoscience 335 (2003).em>  相似文献   

Tectonics and sedimentation in the Curitiba Basin, south of Brazil     

Eduardo Salamuni  Hans Dirk Ebert  Mauricio da Silva Borges  Yociteru Hasui  Joo Batista Sena Costa  Riad Salamuni 《Journal of South American Earth Sciences》2003,15(8):901-910

The Curitiba Basin, Paran&#xe1;, lies parallel to the west side of the Serra do Mar range and is part of a continental rift near the Atlantic coast of southeastern Brazil. It bears unconsolidated and poorly consolidated sediments divided in two formations: the lower Guabirotuba Formation and the overlying Tinguis Formation, both developed over Precambrian basement. Field observations, water well drill cores, and interpretations of satellite images lead to the inference that regional tectonic processes were responsible for the origin of the Basin in the continental rift context and for morphotecatonic evolution through block tilting, dissection, and erosion. The structural framework of the sediments and the basement is characterized by NE–SW-trending normal faults (extensional tectonic D₁ event) reactivated by NE–SW-trending strike–slip and reverse oblique faults (younger transtensional tectonic D_2′ to transpressional tectonic D_2″ event). This tectonic event, which started in the Paleogene and controlled the basin geometry, began as a halfgraben and was later reactivated as a pull-apart basin. D₂ is a neotectonic event that controls the current morphostructures. The Basin is connected to the structural rearrangement of the South American platform, which underwent a generalized extensional or trantensional process and, in late Oligocene, changed to a compressional to transpressional regime.  相似文献   

Phase‐shifts in shear stress as an explanation for the maintenance of pool–riffle sequences     

Scott N. Wilkinson  Robert J. Keller  Ian D. Rutherfurd 《地球表面变化过程与地形》2004,29(6):737-753

The stability of the pool–rif?e sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of pool–rif?e sequence bars during high ?ows, offsetting gradual scour of rif?es and deposition in pools during low ?ows. Despite several attempts, reversal has rarely been recorded in ?eld measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase‐shifted with respect to the pool–rif?e sequence bedform pro?le, so that maximum shear stress occurs upstream of rif?e crests at high ?ow, and downstream at low ?ow. Such phase‐shifts produce gradients of shear stress that explain rif?e deposition, and pool scour, at high ?ow, in accord with sediment continuity. The proposal is supported by results of a one‐dimensional hydraulic model applied to the surveyed bathymetry of a pool–rif?e sequence in a straight reach of a gravel‐bed river. In the sequence studied, the upstream phase‐shift in shear stress at high ?ow was associated with variations in channel width, with width minima occurring upstream of rif?e crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of rif?e crests. Assuming that the width variation is itself the result of ?ow de?ection by rif?e crests at low ?ow, and associated bank‐toe scour downstream, low and high ?ow can be seen to have complementary roles in maintaining alluvial pool–rif?e sequences. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

河南安阳汤参1井数字化氡与模拟水氡观测资料的对比研究          下载免费PDF全文

胡凤英  王志敏  谢健健  胡卫建  黄卫东  王建军  刘鹏 《地震工程学报》2004,26(4):377-379

对河南汤参1井的数字化氡与模拟水氡同期观测资料的对比结果表明：二相关性差；映震能力上数字化观测的地震前兆异常反映比模拟观测明显。  相似文献