Geotechnical characteristics of deep-sea sediments from the North Atlantic and North Pacific oceans

Geotechnical properties from a series of deep-sea sites in the North Atlantic and North Pacific oceans are examined to evaluate overall trends and to compare with similar fine-grained soils found on land. The study areas encompass a range of sedimentary environments dominated by combinations of turbidite and pelagic deposits. Carbonate content in excess of 20% is seen to result in a decrease in liquid limit and compressibility. Vertical profiles of geotechnical properties in the North Pacific show broader changes in down-core geotechnical properties compared to the North Atlantic and reflect the effects of long-term climatic changes and seafloor spreading. Sediments in the North Atlantic indicate significant differences depending on location, which is attributed to variability in turbidite deposition, water depth, distance from sediment sources, and the effects of bottom currents. Compared to equivalent fine-grained soils on land, deep-sea sediments are generally softer, more compressible and have higher friction angles at comparable Atterberg limits. Deeper and older sediments in the North Pacific are characterized by unusually large plastic limits, which are attributed to the presence of volcanic fractions. Empirical relationships for compression index and friction angle are discussed for sediments from both oceans.     

Mean temperature-salinity,salinity-depth and temperature-depth curves for the North Atlantic and the North Pacific

All available hydrographic data are used to compute average temperature-salinity, salinity-depth and temperature-depth curves for 5° squares in the North Atlantic and North Pacific (10°S to 60°N). Both objective and subjective methods are used to filter out obviously erroneous data. Summary maps of all 5° square mean curves, and their corresponding standard deviations, are used to select regions of spatial homogeneity over which the curves are averaged. Scatter diagrams of each parameter pair are used to identify 5° squares containing a variety of water masses.The area average curves, and their standard deviations, are presented individually along with tables of the parameter values. These curves, in conjunction with the 5° square summary maps, provide a comprehensive picture of the distributions of these properties in the upper and mid-depth portions of these oceans. This well resolved picture of the water mass structure both confirms and adds new insights into the traditional concepts of these property distributions.The application of the mean temperature-salinity and salinity-depth curves to the inference of salinity from observed temperature profiles, is examined. Shallow (0/500 db) dynamic heights, calculated from observations of temperature and salinity (true dynamic height), are compared with dynamic heights computed from observed temperature profiles and salinities inferred from the mean property curves. Maps of the RMS differences between true and inferred dynamic heights (by 5° square) reveal those areas where salinities, inferred from either temperature-salinity or salinity-depth curves, yield reliable dynamic heights.     

Chlorofluorocarbons in the western North Pacific in 1993 and formation of North Pacific intermediate water

Chlorofluorocarbons (CFC-11 and CFC-12) in the intermediate water having between 26.4 and 27.2 were determined at 75 stations in the western North Pacific north of 20°N and west of 175.5°E in 1993. The intermediate water of 26.4–26.6 was almost saturated with respect to the present atmospheric CFC-11 in the zone between 35 and 45°N around the subarctic front. Furthermore, the ratios of CFC-11/CFC-12 of the water were also of those formed after 1975. These suggest that the upper intermediate water (26.4–26.6) was recently formed by cooling and sinking of the surface water not by mixing with old waters. The water below the isopycnal surface of 26.8 contained less CFCs and the area containing higher CFCs around the subarctic front was greatly reduced. However, the CFC age of the lower intermediate water (26.8–27.2) in the zone around the subarctic front was not old, suggesting that the water was formed by diapycnal mixing of the water ventilated with the atmosphere with old waters not containing appreciable CFCs, probably the Pacific Deep Water. The southward spreading rate decreased with depth and it was one sixth of its eastward spreading rate of the North Pacific Intermediate Water (NPIW).     

冬季北太平洋与北大西洋上空风暴轴特征比较与分析

采用1948—2007年NCEP/NCAR再分析资料,选取850,700,600,500,250hPa层风场(u,v)、垂直运动场(ω)、位势高度场(H)和温度场(T),对北半球冬季(12、1、2月)太平洋和大西洋上空风暴轴的特征及影响因素进行了分析和比较。结果表明,北半球冬季风暴轴最强中心位于北大西洋上空,北太平洋高空急流的强度明显大于大西洋高空西风急流的强度。通过诊断分析北太平洋与北大西洋上空风暴轴的影响因素,发现北太平洋和北大西洋上空风暴轴强度的影响因素具有明显的差异:在北太平洋上空,涡度平流随高度的变化和厚度平流项对风暴轴的作用较大西洋而言更明显,而涡动能量的向上输送对北大西洋风暴中心的作用更大。对风暴轴年际变化的分析发现,两大洋风暴轴在1980年代之后均有明显的增强现象,而在空间分布的时间演化上,两大洋风暴轴有着相反的演化:太平洋风暴轴有向西南方向移动的趋势,而大西洋风暴中心则略微向东北方向移动。     

North sea offshore structures

Existing and planned North Sea offshore structures for exploration (drillings) and structures for exploitation (production platforms) are mentioned. Introductorily the mereorological conditions, wave climate and bottom composition in the North Sea are reviewed briefly. They are determining factors for design as well as for operation. Furthermore a brief account on present and planned activities on exploration for and exploitation of oil and gas in the North Sea is given.     

Marine capacity building in North and West Africa

The North and West African coastal and marine areas discussed in this paper consist of the countries from Morocco to Libya for North Africa, and from Morocco to South Africa (up to the Cape of Good Hope) in West Africa. These marine areas consist of a wide to narrow continental shelf, estuaries, lagoons, coastal plains, isolated mountains, mangroves and wetlands. These areas are centres of productive socio-economic activities with a high population density, vast fisheries, and mineral resources. Presently, these resources are exploited with little attention for the environmental consequences. Sustainable management of the marine environment and its resources requires effective management plans and a human capacity to formulate, implement and monitor these plans. However, most of the countries still lack funds, human and infrastructural capacity for the management of their marine territories and its resources. Though there are many universities, research institutes and private organisations with a modest infrastructure for both data and information handling and the management of the marine environment, the region still lags behind in human and infrastructural capacity. Existing capacity development programmes by United Nations agencies, governments, universities, nongovernmental agencies, have so far not been very effective. A vigorous marine capacity building effort is needed to ensure the sustainable management of the marine environment in this African region. Externally funded programmes with a focus ensuring technology cooperation are needed. Such programmes must be based on mutual co-operation and participation of scientists and administrators from the region at all levels of programme formulation, implementation and assessment.     

Dissolved and particulate tin in North Atlantic seawater

A study of tin concentrations in the North Atlantic Ocean has revealed concentrations much lower than previously reported. Concentrations of dissolved tin in surface water are in the range of non-detectable (< 1.5 pM) to 16 pM. Almost all depth profiles have maxima in tin concentrations from 50 to 200 m depth. These maxima can be as high as 16 pM. In most depth profiles, dissolved tin is at or below detection limits at depths exceeding 1000 m.     

The North Atlantic nutrient stream

Western boundary currents are the locus of intense nutrient transport, or nutrient streams. The largest fraction of this transport takes place in the upper-thermocline layers, between the surface layers (where speed reaches a maximum) and the nutrient bearing strata of the subtropical gyres (where nutrient concentration is maximum). The core of the nutrient stream of the North Atlantic subtropical gyre is located slightly offshore the Gulf Stream, its density coordinate centered on the 26.5–27.3_–band, approximately constant along the axis of the stream. During late spring and summer the nutrient stream reaches the surface seasonal mixed layer at the outcropping of this isopycnal band. We argue that this must be a principal factor sustaining the seasonal high productivity of the subpolar North Atlantic Ocean. Additionally, we investigate the possibility of intermittent shear-induced diapycnal mixing in the upper-thermocline layers of the Gulf Stream, induced by frontogenesis taking place during some phase of the meanders. Here we illustrate that diapycnal mixing has a maximum at the location of the nutrient stream, being associated to observed nutrient anomalies. We suggest that diapycnal mixing associated to the passage of steep meanders brings nutrients from the nutrient stream to the shallow photic layers, and sustains intermittent (day-to-week) patchy (10–100 km) productivity over the stream itself.     

Variability of the North Pacific Current and its bifurcation

The North Pacific Current (NPC) bifurcates approaching the west coast of North America into a subpolar branch that forms the Alaska Current, and a subtropical branch that includes the California Current. The variability of this current system is discussed using numerical results from a wind-driven, reduced-gravity model. Indices of the strength of the subpolar and subtropical components of the NPC are examined based on output from multi-decadal simulations with the numerical model. This shows periods of both correlated and anti-correlated variability of the subpolar and subtropical gyres. A decomposition of the gyre transport time series indicates that the dominant mode of variability is a “breathing” mode in which the subpolar and subtropical gyres co-vary in response to fluctuations in the strength of the NPC. This finding is consistent with an analysis of dynamic height data of limited duration from the array of Argo drifting floats.The variability of the NPC is also examined using sea surface height (SSH) data from satellite altimetry over the period 1993-2005. The leading mode of SSH over the northeast Pacific dominates the variability of the NPC and is shown to be associated with in-phase variations in the transport of the subtropical and subpolar gyres. A strong correlation is found between time-dependent fluctuations in SSH across the NPC and variations in the strength of the transport of the NPC in the model. This agreement provides evidence for variability of the NPC occuring in direct response to large-scale atmospheric forcing.     

苏北全新世海进与古砂堤研究

本文探讨了苏北全新世以来的海进与古砂堤的演变地过程。结果表明，全新世以来，苏北地区曾有海进，大约距今７ｋａ左右，海进达到了高峰，里下河地区成为一片浅 海。此后，海面相对稳定，沿岸堆积作用显著，出现了西冈，中冈，东冈，新冈等古砂堤群，其成为苏北海岸变迁的重要标志。