Studies on filtration rate in four species of suspension feeding bivalves

The filtration rates of four kinds of bivalves that were cultivated dominantly around Xiamen sea area were measured by using a laboratory flowing system. The experimental results were shown below: (1) Filtration rates were measured in the range of 54 - 74.8 ml/ (g·min) among the four bivalves , sequencing descently Saccostrea cucullata > Sinonovacula constricta > Mytilus viridis > Rudi-tapes philippinarum. (2) The relationship between filtration rates on individual size showed a negative exponential function (FR = aW~b, FR' = aW~(b-1)), with b - 1 =- 0.435 6 and -0.392. (3) Filtration rates on Skeletonema costatum were much higher than on Alexandrium tamarensis and Scripp-silla trochoidea inS. cucullata and R. philippinarum. (4)FR'on algal densities was also shown a negative function (FR' = aD~(b-1)), with b -1 =- 0.143 and -0.215 2 in S. cucullata and R. philippinarum , respectively.     

Recovery of the seabed following marine aggregate dredging on the Hastings Shingle Bank off the southeast coast of England

The aim of this study was to investigate the effect of dredging intensity on the physical and biological recovery times of the seabed following marine aggregate dredging. Two areas of seabed, previously subject to, respectively, relatively high and lower levels of dredging intensity, were identified on the Hastings Shingle Bank. Two reference areas were also selected for comparative purposes. All four sites were monitored annually over the period 2001–2004, using a combination of acoustic, video and grab sampling techniques. Since the site was last dredged in 1996, this was intended to provide a sequence of data 5–8 years after cessation of dredging. However, an unexpected resumption of dredging within the high intensity site, during 2002 and 2003, allowed an additional assessment of the immediate effects and aftermath of renewed dredging at the seabed. The early stages of recovery could then be assessed after dredging ceased in 2003. Results from both dredged sites provide a useful insight into the early and latter stages of physical and biological recovery. A comparison of recent and historic dredge track features provided evidence of track erosion. However, tracks were still visible 8 years after the cessation of dredging. Within the high dredging intensity site, recolonisation was relatively rapid after the cessation of dredging in 2003. Rather than indicating a full recovery, we suggest that this initial ‘colonization community’ may enter a transition phase before eventually reaching equilibrium. This hypothesis is supported by results from the low intensity site, where biological recovery was judged to have taken 7 years. Further monitoring is needed in order to test this. An alternative explanation is that the rapid recovery may be explained by the settlement of large numbers of Sabellaria spinulosa. As the resumption of dredging within the high intensity site limited our assessment of longer-term recovery it is not yet possible to assume that a 7-year biological recovery period will be applicable to other, more intensively dredged areas at this or more distant locations.     

Timing and pace of ice-sheet withdrawal across the marine–terrestrial transition west of Ireland during the last glaciation

Understanding the pace and drivers of marine-based ice-sheet retreat relies upon the integration of numerical ice-sheet models with observations from contemporary polar ice sheets and well-constrained palaeo-glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine-geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice-marginal landforms, to reconstruct the timing and rate of ice-sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine- to a terrestrially-based ice-sheet margin. Seafloor bathymetric data in the form of moraines and grounding-zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid-shelf where it formed a large grounding-zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice-free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice-sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio-isostatically induced, high relative sea level. Net rates of ice-sheet retreat across the shelf were slow (62–19 m a⁻¹) and reduced (8 m a⁻¹) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine-based sectors of ice sheets.     

The braided Milk River, northern Montana, fails the Leopold–Wolman discharge-gradient test

The Milk River, the northernmost tributary to the Missouri–Mississippi River system, exhibits an anomalous sand-bed braiding reach in an otherwise meandering system. Shortly after leaving Alberta and entering Montana the river suddenly changes to braiding and maintains this pattern for 47 km before entering Fresno Reservoir. Measured stream gradient and bankfull discharge in the braiding reach severely fail the Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] slope–discharge test for differentiating channel patterns. While channel slope has long been regarded as one of the primary variables associated with braiding, our data from the sand-bed Milk River do not support this relationship. Instead, the data show that the braiding reach has a lower channel slope (0.00047) than the meandering reach (0.00055). Coupled with a constant discharge the unit length stream power is comparable between the two reaches. At the morphologic transition between meandering and braiding, a dramatic reduction in channel bank strength occurs where the sampled silt–clay content declines from 65% in the meandering reach to 18% in the braiding. This enables channel widening which is reflected in a 60% reduction in unit area stream power in the braiding reach. Thus, sediment transport capacity declines and channel bars are deposited. During waning flows, these bars are dissected, producing a braiding morphology. We suggest that for sand-bed braiding rivers the silt–clay percentage in the channel banks may be more important than slope. A review of the original Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] dataset, and many subsequent analyses, reveals that most braided rivers studied were gravel-bed. As a result, causal variables associated with braiding in sand-bed environments may need a thorough evaluation.     

Stability of composite river banks

The stability of a river bank depends on the balance of forces, motive and resistive, associated with the most critical mechanism of failure. Many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size, geometry and structure of the bank, the engineering properties of the bank material, the hydraulics of flow in the adjacent channel and climatic conditions. Rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt/clay. Bank erosion occurs by fluvial entrainment of material from the lower, cohesionless bank at a much higher rate than material from the upper, cohesive bank. This leads to undermining that produces cantilevers of cohesive material. Upper bank retreat takes place predominantly by the failure of these cantilevers. Three mechanisms of failure have been identified: shear, beam and tensile failure. The stability of a cantilever may be analysed using static equilibrium and beam theory, and dimensionless charts for cantilever stability constructed. Application of the charts requires only a few simple measurements of cantilever geometry and soil properties. In this analysis the effects of cracks and fissures in the soil must be taken into account. These cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface. Following mechanical failure, blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue. Hence, the rate of bank retreat is fluvially controlled, even though the mechanism of failure of the upper bank is not directly fluvial in nature. This cycle of bank erosion: undermining, cantilever failure and fluvial scour of the toe, operates over several flood events and has important implications for river engineering, channel changes, and the movement of sediment through fluvial systems.     

Insights about the structure and development of Zhongsha Bank in the South China Sea from integrated geophysical modelling

ABSTRACT

We construct a complete density transection based on the velocity structures across the Zhongsha Bank in the South China Sea. Gravity modelling of the lateral density contrasts between tectonic units helps us to determine the structural attributes and boundaries between continental blocks and deep basins. The configuration of the continent–ocean boundary (COB) around the Zhongsha Bank is mapped based on the gravity/magnetic anomaly and crustal structures. A low-density mantle is found beneath the Zhongsha Bank and the oceanic basins, and this mantle is associated with the high heat-flow background. The COB orientation is northeast-east in the north of the bank, with faulted linear structures. In further southeast, where there is a more intact crust, the COB orientation changed to north-northeast. The reconstructed density model and gravity/magnetic map indicate that the Zhongsha Bank is conjugated with the Liyue Bank by a rifted basin, where the crust had experienced localized deformation before the seafloor spreading. Because of the insufficient magmatism in the oceanic basin, the spreading ridge propagates into the weakened continental lithosphere between the two continental blocks, thus completely separating the Zhongsha Bank from the Liyue Bank. Seafloor spreading ridge jumps within the South China Sea may also be affected by the heterogeneous lithosphere beneath the continental blocks and oceanic basins.     

Fluvial process and morphology of the Brahmaputra River in Assam, India

The Brahmaputra River finds its origin in the Chema Yundung glacier of Tibet and flows through India and Bangladesh. The slope of the river decreases suddenly in front of the Himalayas and results in the deposition of sediment and a braided channel pattern. It flows through Assam, India, along a valley comprising its own Recent alluvium. In Assam the basin receives 300 cm mean annual rainfall, 66–85% of which occurs in the monsoon period from June through September. Mean annual discharge at Pandu for 1955–1990 is 16,682.24 m³ s^{− 1}. Average monthly discharge is highest in July (19%) and lowest in February (2%). Most hydrographs exhibit multiple flood peaks occurring at different times from June to September. The mean annual suspended sediment load is 402 million tons and average monthly sediment discharge is highest in June (19.05%) and lowest in January (1.02%). The bed load at Pandu was found to be 5–15% of the total load of the river. Three kinds of major geomorphic units are found in the basin. The river bed of the Brahmaputra shows four topographic levels, with increasing height and vegetation. The single first order primary channels of this braided river split into two or more smaller second order channels separated by bars and islands. The second order channels are of three kinds. The maximum length and width of the bars in the area under study are 18.43 km and 6.17 km, respectively. The Brahmaputra channel is characterised by mid-channel bars, side bars, tributary mouth bars and unit bars. The geometry of meandering tributary rivers shows that the relationship between meander wavelength and bend radius is most linear. The Brahmaputra had been undergoing overall aggradation by about 16 cm during 1971 to 1979. The channel of the Brahmaputra River has been migrating because of channel widening and avulsion. The meandering tributaries change because of neck cut-off and progressive shifting at the meander bends. The braiding index of the Brahmaputra has been increasing from 6.11 in 1912–1928 to 8.33 in 1996. During the twentieth century, the total amount of bank area lost from erosion was 868 km². Maximum rate of shift of the north bank to south resulting in erosion was 227.5 m/year and maximum rate of shift of the south bank to north resulting in accretion was 331.56 m/year. Shear failure of upper bank and liquefaction of clayey-silt materials are two main causes of bank erosion.     

沿江岸线资源地理信息系统的设计及实现

利用成熟的GIS技术、数据库技术、网络技术，建设岸线资源管理信息系统来作为沿江开发过程中对岸线资源进行统一管理的工具，根据沿江岸线资源分布的特点，合理地组织、布局生产力，建立工业区、城市群和产业带，提高沿江开发过程中决策的科学性，从而促进沿江开发区内乃至江苏经济持续快速健康发展。因此，对沿江岸线资源管理信息系统的设计及实现进行了详细描述和介绍。     

SOCIAL AND ENVIRONMENTAL IMPACTS OF WORLD BANK/IMF-FUNDED ECONOMIC RESTRUCTURING IN BOLIVIA: AN ANALYSIS OF ENRON AND SHELL'S HYDROCARBONS PROJECTS

With the spectacular financial collapse of Enron in 2001, Enron and Shell's Rio San Miguel‐Cuiabá gas pipeline gained international notoriety for degrading the last, most intact dry tropical forest in the world, Bolivia's Chiquitano forest. The paper uses specific case studies, including the case of the Cuiabá pipeline, to examine how economic restructuring sponsored by the World Bank, International Monetary Fund (IMF) and Inter‐American Development Bank (IDB) facilitated the entrance of multinational oil corporations that have caused significant social and environmental impacts in Bolivia. The paper explores the influence of international financial institutions and multinational oil corporations on Bolivian state institutions, particularly their ability to regulate and address impacts caused by developments in the hydrocarbons sector. It concludes that neoliberal policies, which resulted in partial privatisation of the state oil company and in expanded control over natural resources by multinational corporations (MNCs), were detrimental to sensitive ecosystems and indigenous inhabitants.     

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.