Holocene millennial to centennial carbonate cyclicity recorded in slope sediments of the Great Bahama Bank and its climatic implications

A 38 m long sediment core (MD992201) retrieved from a water depth of 290 m from the leeward margin of the Great Bahama Bank (GBB; 25°53·49′N, 79°16·34′W) has been investigated for changes in aragonite content. The core covers the Mid to Late Holocene (the past 7230 yr). Sediment lightness (L*-values) was used as a proxy for aragonite content, based on a high linear correlation (R = 0·93) between the X-ray diffraction derived aragonite content and L*-values. The resulting time resolution of the L*-values derived aragonite content ranges from 1 yr at the base of the core to 4 yr at the top. Detailed time series analysis using Monte Carlo Singular Spectrum Analysis and spectral analysis (Lomb–Scargle Fourier transform) identifies the presence of seven signals with varying amplitudes and wavelengths that could be traced throughout the past 5500 yr. During the first ∼1600 yr of sedimentation the aragonite record is dominated by the initial flooding of the flat-topped GBB. Superimposed on a multimillennial signal, related to Holocene sea-level changes, a millennial-scale fluctuation and five quasi-periodic oscillations were detected (∼1·3–2 kyr, ∼500–600 yr, ∼380 yr, ∼260 yr, ∼200 yr and ∼100 yr period). Comparisons with other proxies (e.g. tree ring-Δ¹⁴C, ¹⁰Be and δ¹⁸O in ice cores) provides information on the origin and dynamics of the individual signals. The analysis shows that the ∼200 yr and ∼100 yr signals can be attributed to solar forcing. The ∼260 yr, ∼380 yr and the ∼500–600 yr quasi-periodic signals are found to be of climatic origin, whereas the millennial scale fluctuations remain enigmatic, although solar forcing mechanisms seem likely. The data show that variability of solar output as well as past oceanographic and atmospheric changes have modulated the Mid to Late Holocene climate, which in turn controlled sediment input variations found in the Holocene wedge leeward of the GBB. Although these periplatform sediments have a rather uniform appearance, they still contain a large variety of subtle sedimentary variations.     

Bank erodibility of shallow sandbed streams

Cross-sections were surveyed at straight reaches of 16 sandbed streams in the midwestern U.S. Two stratigraphic horizons are found in the banks at each site, an upper cohesive unit usually composed of silt and clay, and a lower unit composed of sand. Bank erosion on these rivers occurs when the upper cohesive unit is undercut by scour at bends. The overhanging cohesive block fails by toppling forward into the channel. During failure, the soil is primarily in tension rather than compression or shear. Analysis of this failure mechanism leads to a field method for measuring the tensile strength of riverbanks. Measured values of the tensile strength are not correlated with the channel geometry. Thus, the erodibility of the cohesive bank sediments does not influence the geometry of the rivers studied.     

A 2D MATHEMATICAL MODEL FOR THE BED DEFORMATION IN THE LOWER YELLOW RIVER

1 BANK EROSION IN THE LOWER YELLOW RIVER In alluvial rivers, riverbeds are always in a state of transition and development. Two kinds of deformations result for the fluvial process according to certain basic characteristics. One is longitudinal deformation that is characterized by the deformation of a riverbed in the direction of streamwise flow such as riverbed scour or deposition. The other is transverse or lateral deformation that is distinguished by the deformation of a riverb…     

TOWARDS A BETTER FORECAST FOR RESERVOIR SLOPE SLIDING

The siltation of reservoir causes many problems of management and environmental disturbances that are important to minimize. Solving these problems requires in particular a good knowledge of mud bank instabilities. In partnership with the Laboratoire National d'Hydraulique (LNH), a subsidiary of Electricite de France (E.D.F.), authors realize a study of soft soils sliding entailed by reservoir emptying. Using a geotechnical flat sliding mechanism, the theoretical formulation of the safety factor can be established when the sediment mass is submerged and emergent. The measurement campaign carried out makes it possible to obtain bathymetric and topographic surveys and a geotechnical characterization of the soil for the whole reservoir of Grangent dam, located on the Loire (France). The proposed model appears to agree satisfactorily with the instability of the zones emerged because of the emptying and with the stability of the profile after emptying. The use of this model within the framework of forecasts     

闽南-台湾浅滩渔场蓝圆鲹资源的评估

以1971—1987年的蓝圆鲹叉长组成资料,应用体长股分析法评估了闽南-台湾浅滩渔场蓝圆鲹的资源。结果表明,1971—1980年的现存资源数量和生物量分别为45292×10~5尾和9.49×10~4t,1981—1987年分别为63908×10~5尾和13.51×10~4t;1971—1980年和1981—1987年的瞬时捕捞死亡率分别为0.6347和0.4859。通过Beverton-Holt模式估算出蓝圆鲹目前的持续产量为9.58×10~4t,最佳最小捕捞叉长为135mm。     

闽南-台湾浅滩表层沉积物中底栖有孔虫分布及其环境特征

本文分析了闽南-台湾浅滩表层沉积物样品中的底栖有孔虫计有132属、328种。根据种类组成及分布特征,可将海区划分为6个动物群区,每个动物群区各有自己的环境特点。文中分析了动物群种类组成特征与其所赋存的环境之间的关系,并讨论了浅滩西南303站底栖有孔虫的分布与上升流的关系。     

闽南-台湾浅滩渔场蓝圆鲹和鲐鱼群聚空间分布的研究

根据闽南-台湾浅滩渔场灯光围网信息船1990～2002年的渔捞记录，研究该渔场蓝圆鲹和鲐鱼群聚空间分布．结果表明，中上层鱼类蓝圆鲹和鲐鱼年间数量变动具有时间同步的特征，年均网次产量显著相关．蓝圆鲹与鲐鱼群聚区分布差异表现在：蓝圆鲹分布的海区水深较鲐鱼浅，蓝圆鲹和鲐鱼分布渔区的水深，春季分别为40～660m和40～70m，夏季分别为30～50m和30～60m，秋、冬季分别为30～60m和40～60m；鲐鱼的群聚区分布较偏北、偏向台湾岛西岸，蓝圆鲹的群聚区分布较偏南、偏向台湾海峡的中心区域；蓝圆鲹与鲐鱼群聚区的时空隔离、彼此取代比较明显．蓝圆鲹与鲐鱼的群聚区存在一定的重叠，与其种间对食物的竞争有关．两鱼种所出现两种适应性趋向。增加了它们在同一生境中长期共存的可能性．     

The Best of Times and the Worst of Times: Structural Adjustment Programs and Uneven Development in Africa: The Case Of Ghana

Since 1983, Ghana has been undergoing World Bank and International Monetary Fund (IMF) sponsored Structural Adjustment Programs (SAPs). The implementation of the SAPs, it is claimed, has arrested Ghana's economy from complete collapse, resulted in consistent growth in GDP averaging 6% over the past decade, reduced inflation levels, created budget surplus, and increased export earnings. Compared to the 1970s, these are the best of times indeed. But while these SAPs‐derived improvements in the national economy have been recorded at the macro level, the benefits at the micro level are a matter of considerable debate. This study revisits the issue of socioeconomic and spatial disparities that have characterized Ghana since colonial times, emphasizing the period from 1983 when Ghana's SAPs began. It examines current patterns of socioeconomic disparities with emphasis on the distribution of, and access to, health, education, basic services, and the like. The study focuses on urban‐rural as well as interregional disparities in the country.     

Investigations of the bottom current sculpted margin of Hatton Bank, NE Atlantic

The northwest Hatton Bank margin is an ideal locality to demonstrate the interaction between bottom currents and slope configuration in controlling the distribution and morphology of bottom current deposits. The slope area investigated is isolated from any major terrigenous sediment supply and at present is influenced by the Deep Northern Boundary Current (DNBC). Swath bathymetry and high resolution acoustic data allow us to evaluate both local and regional controls on slope sedimentation and the possible mechanisms for bottom-current velocity variability across a slope setting within the NW European continental margin. The slope exhibits sculpting by bottom currents that flow in a predominantly southwest to northeast direction, and is only locally modified by slope failures. Positive relief features such as the Endymion Spur play an important role in constraining and accelerating bottom-current flow and, consequently, in redistributing sediment along the margin. We demonstrate that the size, morphology and distribution of bottom-current deposits along the slope vary as a function of the interaction between bottom currents, regional slope orientation and local seafloor topography.     

大贵州滩二叠系-三叠系界线附近锶同位素组成特征

位于贵州南部的“大贵州滩”上,二叠系—三叠系界线附近地层为一套连续的发育完整的碳酸盐岩。二叠系顶部主要为厚层生物屑泥粒灰岩,而三叠系底部则主要为钙质微生物骨架灰岩及厚层粒泥灰岩。穿过二叠系—三叠系界线的取样累计厚度81m,二叠系和三叠系大致各占一半的厚度,共取样品91件,n(87Sr)/n(86Sr)值变化在0.707269～0.707409之间,平均为0.707327,低于现代海水的锶同位素比值的平均值（0.7090737）。锶同位素比值变化曲线表明从晚二叠世末期到早三叠世初期,同位素比值虽小幅度的起伏波动频繁,但总体上没有明显变化,三叠系底部的平均值（0.707330）略高于二叠系顶部的平均值（0.707323）。大贵州滩晚二叠世末至早三叠世初锶同位素组成的总体演化趋势与扬子台地及世界其它地方同时期的锶同位素组成的演化趋势（呈急剧的上升）不同,造成这种状况的原因可能是在全球事件控制的大背景下,由于大贵州滩特殊的环境条件,陆源物质对其影响小,导致了壳源锶对锶同位素组成的制约作用减弱,因而锶同位素组成未发生明显变化。