大陆坡脚确定原则与方法

大陆坡脚是大陆边缘的一个重要地形特征,是沿海国扩展其大陆架权利和划定其200海里以外大陆架外部界限的基础,也是大陆架界限委员会审议沿海国划界案时特别关注的重要技术参数。《联合国海洋法公约》第76条大陆架制度的制定源于典型的被动大陆边缘。但由于全球大陆边缘的多样性和复杂性,特别是后期构造活动、沉积作用对大陆边缘的改造与影响,海底地形地貌异常复杂多变,导致大陆坡脚的识别非常困难。加上各沿海国为获得最大范围的外大陆架,对大陆坡脚的相关规定进行有利于自己的解释,使得大陆坡脚的确定成了外大陆架划界中一个颇具争议的热点问题。本文基于对《联合国海洋法公约》和《大陆架界限委员会科学和技术准则》对大陆坡脚的规定,结合不同类型大陆边缘的地质特征和各沿海国划界实践,对陆坡基部区的确定、坡度变化最大之点的选取以及相反证明规则的适用性等问题进行了探讨。     

Surface current and vertical thermal structure on the continental slope in Tosa Bay

In order to specify a vertical thermal structure related to surface current variation on the continental slope in Tosa Bay, Japan, we analyzed monthly regular hydrographic measurements in the years 1991–2004. Subsurface temperature below 200 m on the slope was found to vary synchronously with the vertical displacement of the main thermocline around 200 m. It is shown that the vertical-averaged temperature below 200 m is significantly correlated with an along-isobath/southwestward surface current velocity on the slope. This correlation indicates that when a strong (weak) southwestward surface current is observed, temperature below 200 m decreases (increases) simultaneously, that is, isotherms below the 200 m are displaced upward (downward) together with the main thermocline. Moreover, when the strong southwestward flow is detected, across-isobath isotherms around 200 m slope upward toward the offshore direction. Furthermore, it is suggested that as the Kuroshio axis moves offshore south of the bay, the southwestward flow tends to be weakened by the combined effect of other Kuroshio parameters such as transport and stream width as well as the Kuroshio axis position. As a result, it is inferred that the correlation between the surface current and subsurface temperature can be interpreted in terms of the formation and decay of an anticlockwise circulation interacting with a cold eddy.     

Oceanic exchange variability across the continental slope west coast,South Island,New Zealand

Current observations taken at depths between 630 and 830 m from the west coast South Island continental slope exhibit one‐ to four‐weekly periodicities superimposed on the semi‐diurnal tide. These variable flows at 630 m and otter large, longer‐timescale events have a significart onshore/offshore component of flow which leads to a similar transport of alongshore momentum and heat.     

南海中北部中新世陆坡凹陷沉积充填与古陆坡形态演变

对中德合作“南海地球科学联合研究”和“中国边缘海的形成演化及重大资源的关键问题”的有关航次获得的地震资料进行层序地层和沉积相解释,并结合前人的研究成果,对南海中北部陆坡凹陷的构造和沉积特征进行了研究.研究结果表明,南海中北部陆坡区凹陷的构造演化经历了裂陷期、裂陷-坳陷过渡期和坳陷期三个阶段,沉积环境经历了河流-湖泊、浅海和深海的演化过程,不同演化阶段形成不同的地形形态.南海中北部地形演变可分为三个阶段,即早第三纪、早中新世-中中新世中期和中中新世中期-现在,其中早中新世中期-中中新世中期的沉积充填使陆坡形态发生了重要的转变,这次转变基本上奠定了现代意义上的陆架-陆坡-海盆的格局.     

南沙群岛西南大陆斜坡海域浮游动物的垂直分布

浮游动物是海洋生态系统物质和能量转移的关键环节,也是渔业资源的重要饵料,其种类组成和数量分布均有明显的垂直分层现象。利用2011年4月在南沙群岛西南大陆斜坡海域开展的18个站点90份样品的调查数据,对该海域浮游动物的垂直分布进行了研究,表明:(1)该海域浮游动物种类组成丰富,共出现18个类群580种(类);(2)浮游动物种类组成垂直变化明显,特定水层出现的种类数占总种数的43.6%,各水层均出现的种类仅占总种数的15.8%;(3)优势种组成复杂,垂直变化明显,单一种类的优势度不高;(4)浮游动物平均密度和湿重生物量分别为206.27ind/m3、94.03mg/m3,密度和生物量均以0~2m层和30~75m层较高,沿水深梯度的变化呈明显的双峰型;(5)浮游动物数量的垂直变化主要受温跃层影响,温跃层内浮游动物数量最高,温跃层上方和下方的水层内数量较低;(6)南沙西南大陆斜坡区浮游动物生产力水平较高,表明该海域渔业资源有一定的开发潜力。     

Diversity patterns in macrobenthos across a continental slope in the NE Atlantic

Different estimates were used to assess the diversity of the total macrofauna and its major taxonomic groups separately from a broad bathymetric range at a site in the NE Atlantic. In the Goban Spur region, a transect was sampled from the shelf to the abyssal plain over a depth range from 200 to 4500 m and in the Porcupine Sea Bight two stations were sampled (at 3670 m and 4115 m). Species diversity (the number of species per number of individuals) increased with increasing water depth, both when expressed as Hurlbert's E(S_n) and as Shannon's H′log e. The expected number of species in a 100-individual sample E(S₁₀₀) of total macrofauna increased from 30 on the shelf to 68 on the abyssal plain. Evenness (the proportional abundance of species), estimated with Shannon's J′, also increased with water depth from 0.66 to 0.91, whereas dominance (Simpson's D) decreased from 0.09 to 0.01. Species richness (the number of species per unit of area), however, showed a parabolic pattern with a peak at the upper slope. The largest number of species was found at the slope station at 1425 m (232 species within 0.66 m²). It is argued that species richness is not a synonym of species diversity, but that species richness depends both on species density (which decreases with increasing water depth) and on species diversity. Across the whole bathymetric range (200 to 4500 m) a total of 696 species within 8327 specimens in a total sampled area of 4.12 m² were counted, yielding mean values of 12 individuals per species and 169 species per m². Different communities were found to exist on the shelf, slope and abyss. It is suggested that this could have been caused by different selection processes. Differences in life-history strategies and organic-matter supply could (at least partly) explain the different community structures and diversity patterns found along the depth gradient.     

南海珠江口盆地陆坡稳定性评价

In nature, a slope stability is determined by the ratio of a sliding resistance to a slide force. The slide force of a marine deep-water continental slope is mainly affected by sediment mechanics properties, a topography, and a marine seismic. However, the sliding resistance is mainly affected by sedimentary patterns and a sedimentary stress history. Both of these are different from case to case, and their impact can be addressed when the data are organized in a geographic information system（GIS）. The study area on the continental slope in Zhujiang River Mouth Basin in South China Sea provides an excellent opportunity to apply GIS spatial analysis technology for the evaluation of the slope stability. In this area, a continental slope topography and a three-dimension（3-D） topography mapping show a sea-floor morphology and the distribution of a slope steepness in good detail, and the sediment analysis of seabed samples and an indoor appraisal reveals the variability of a sediment density near the sea-floor surface. On the basis of the results of nine geotechnical studies of submarine study areas, it has worked out that an equivalent cyclic shear stress ratio is roughly between 0.158 and 0.933, which is mainly depending on the initial water content of sediment. A regional density, slope and level of anticipated seismic shaking information are combined in a GIS framework to yield a map that illustrates a continental slope stability zoning under the influencing factors in Zhujiang River Mouth Basin in the South China Sea. The continental slope stability evaluation can contribute to north resources development in the South China Sea, the marine functional zoning, the marine engineering construction and adjust measures to local conditions, at the same time also can provide references for other deep-water slope stability analysis.     

Large-scale seafloor stability evaluation of the northern continental slope of South China Sea

Abstract

With the continuous expansion of energy demand, the deep-water continental slope in the northern South China Sea has become one of the significant offshore oil and gas exploration regions. The frequent occurrence of marine geological hazards in this region, especially submarine landslides, can cause serious damage to engineering facilities. However, there have been few studies on the stability of the northern continental slope of the South China Sea; these studies mainly focused on a specific submarine slope or small-range evaluation, resulting in a lack of large-scale and quantitative understanding. Hence, considering the variation in the physical and mechanical properties of marine soils with depth, formulas for calculating the safety factor of submarine slopes by an infinite sliding model are established, and the factors affecting slope stability such as soil properties, slope gradient and horizontal seismic action are systematically investigated. Using GIS techniques, the terrain slope gradients and a historical seismic database of the northern South China Sea are obtained. Combined with soil mechanical parameters, a regional stability evaluation of the northern continental slope is carried out. Furthermore, the distribution of risk zones is given. On the whole, under strong seismic action, large-scale submarine slope instability occurs and must be highly considered when assessing risk. This achievement is of great significance to engineering sites, route selection and engineering risk assessment.     

南海北部陆坡中尺度涡的演化及传播规律分析

The continental slope in the northern South China Sea(SCS) is rich in mesoscale eddies which play an important role in transport and retention of nutrients and biota. In this study, we investigate the statistical properties of eddy distributions and propagation in a period of 24 years between 1993 and 2016 by using the altimeter data. A total of 147 eddies are found in the continental slope region(CSR), including 70 cyclonic eddies(CEs) and 77 anticyclonic eddies(ACEs). For those eddies that appear in the CSR, the surrounding areas of Dongsha Islands(DS) and southwest of Taiwan(SWT) are considered as the primary sources, where eddies generated contribute more than 60% of the total. According to the spatial distribution of eddy relative vorticity, eddies are weakening as propagating westward. Although both CEs and ACEs roughly propagate along the slope isobaths, there are discrepancies between CEs and ACEs. The ACEs move slightly faster in the zonal direction, while the CEs tend to cross the isobaths with large bottom depth change. The ACEs generally move further into the basin areas after leaving the CSR while CEs remain around the CSR. The eddy propagation on the continental slope is likely to be associated with mean flow at a certain degree because the eddy trajectories have notable seasonal signals that are consistent with the seasonal cycle of geostrophic current. The results indicate that the eddy translation speed is statistically consistent with geostrophic velocity in both magnitude and direction.     

南海东北部陆坡区中上层浮游动物的垂直分布*

根据2016年3月和9月南海东北部陆坡区浮游动物垂直分层采样, 比较浮游动物种类组成、丰度和生物量的垂直分布和季节变化, 探讨影响其差异的原因。文中共鉴定浮游动物225种, 其中3月和9月分别出现150种和169种; 桡足类种数达132种, 其次是毛颚类和管水母类, 各18种; 浮游动物种数在50~100m水层最高, 一般随水深增加而减少。浮游动物丰度和生物量主要集中在0~100m, 二者在100m以深水层随深度增加而降低, 而水柱生物量在100~1000m占总水柱生物量的60%以上。浮游动物优势种季节和垂直变化明显, 3月近海种如普通波水蚤(Undinula vulgaris)和微刺哲水蚤(Canthocalanus pauper)等在100m以上水层丰度较高; 9月外海种如达氏筛哲水蚤(Cosmocalanus darwinii)和黄角光水蚤(Lucicutia flavicornis)等在100m以浅水层相对于3月丰度增加; 隆线似哲水蚤(Calanoides carinatus)在3月丰度和平均体长高于9月, 并且体长较大者主要分布在深层。浮游动物可分0~100m、100~400m和400~1000m三个群落, 因不同水层种类组成和丰度差异引起。浮游动物丰度和生物量的垂直变化与温度、叶绿素a质量浓度等因子呈显著正相关。南海东北部陆坡浮游动物季节和垂直变化受季风、沿岸流和中尺度涡的影响。     

Seasonal variations in faunal distribution and activity across the continental slope of the goban spur area (NE Atlantic)

Density, biomass and community structure of macrofauna were estimated together with several sediment characteristics at seven stations ranging from 208 m to 4460 m water depth along the OMEX transect in the Goban Spur area (NE Atlantic) during three seasons (October 1993, May 1994, and August 1995). Median grain size decreased with increasing water depth and showed no differences between the seasons. The percentages of organic carbon and total nitrogen were highest at mid-slope depths (1000 to 1500 m), and were significantly higher in August at the upper part of the slope to a depth of 1500 m. The C:N ratio in the surface layer amounted to 7 to 8 in May, 10 to 12 in August and 14 to 17 in October at all stations (except the deepest at 4460 m, where it was 11 in May and August), indicating arrival of fresh phytodetritus in May, and therefore seasonality in food input to the benthos. Densities of macrofauna decreased exponentially with increasing water depth. Significantly higher densities of macrofauna were found in May at the upper part of the slope to a depth of 1500 m. These differences were mainly due to high numbers of postlarvae of echinoids at the shallowest station and ophiuroids at the deeper stations. Biomass values also decreased with increasing water depth, but biomass was relatively high at the 1000 m station and low at 1500 m, due to relatively high and low mean weights of the individual macrofaunal specimens. No significant differences in biomass were found between the seasons. Respiration was high (15 to 20 mgC·m⁻²·d⁻¹) in May at the upper part of the slope to a depth of 1000 m and low (1–3 mg C·m⁻²·d⁻¹) at the deeper part. At the shallowest stations to a depth of 1000 m respiration was highest in May, at the mid-slope stations (1400–2200 m) it was highest in August, whereas the deepest stations (3600 to 4500 m) did not show any differences in respiration rates. In conclusion; seasonal variation in organic input is reflected in denstiy, community structure and activity of the macrofauna along the continental slope in the NE Atlantic.     

Macrobenthic polychaete assemblages of the continental shelf and upper slope off the west coast of the South Island,New Zealand

Polychaete assemblages are described from replicate box‐core samples collected in summer 1983 at 18 stations on the continental shelf and upper slope (28–943 m) off the west coast of the South Island, New Zealand, south‐eastern Tasman Sea (c. 41–43°S, 169–172°E). Three main station groupings were identified by multivariate analysis: (1) inner shelf sandy stations characterised by Prionospio australiensis, Aricidea (Acesta) sp., Magelona cf. dakini, Paraprionospio aff. pinnata, Aglaophamus sp., Heteromastus filiformis, and Magelona sp.; (2) middle to outer shelf muddy stations characterised by Levinsenia cf. gracilis, Prionospio australiensis, Paraprionospio coora, Aglaophamus verrilli, and Auchenoplax mesos; and (3) upper slope sandy mud or mud stations characterised by Prionospio ehlersi. A combination of water depth and sediment clay content provided the best correlation with the biotic pattern. Spionidae was the most abundant family (49% of polychaete individuals), which may reflect the scope for opportunistic species in a shelf environment characterised by a high input of terrigenous sediment and episodic upwelling.     

南海北部陆坡深水区浅层天然气藏特征

南海北部陆坡深水区的浅层天然气藏是一种伴随天然气水合物的新型油气藏, 具有埋藏浅、规模大的特点, 其埋藏深度一般小于300m。浅层天然气藏由深部裂解气沿断裂上升被天然气水合物封盖而形成, 识别似海底反射(BSR)是寻找浅层天然气藏有效方法。浅层天然气藏的气源主要有热解气、生物气和混合气, 陆坡张性断裂是气体运移的主要通道, 水合物下部的砂层是浅层天然气藏的主要储集层, 水合物层则是封盖层。从南海发现的天然气水合物分布特征看, 浅层天然气藏在陆坡深水区广泛分布且气藏厚度大, 潜在资源量非常可观, 是一种新型的开采成本相对低廉的油气藏。     

Distribution and expression of gas seeps in a gas hydrate province of the northeastern Sakhalin continental slope, Sea of Okhotsk

Multidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the northeastern Sakhalin continental slope (NESS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2007 (CHAOS and SSGH projects). One hundred sixty-one gas seeps were detected in a 2000 km² area of the NESS (between 53°45′N and 54°45′N). Active gas seeps in a gas hydrate province on the NESS were evident from features in the water column, on the seafloor, and in the subsurface: well-defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater, and gas hydrates in sediment near the seafloor. These expressions were generally spatially related; a gas flare would be associated with a seepage structure (mound), below which a gas chimney was present. The spatial distribution of gas seeps on the NESS is controlled by four types of geological structures: faults, the shelf break, seafloor canyons, and submarine slides. Gas chimneys that produced enhanced reflection on high-resolution seismic profiles are interpreted as active pathways for upward gas migration to the seafloor. The chimneys and gas flares are good indicators of active seepage.     

南海北部陆坡30 ka以来的沉积环境演变

南海是西太地区最大的边缘海,汇集了周边陆地大量碎屑物质。这些陆源碎屑通过复杂的洋流系统经陆坡大量的输送到南海深海海盆中,使陆坡成为研究深海沉积物源汇体系不可缺少的重要环节。但陆坡区域水深变化大,洋流体系复杂,加之冰期间冰期海平面升降和季风的变化,使陆坡沉积环境一直成为研究的难点。为了研究陆坡沉积环境的演变过程,本文选用了南海北部陆坡中部和底部的两个重力柱开展元素地球化学方面的研究,探讨陆坡区域近三万年以来的沉积环境特征。研究发现海平面和季风是影响区域沉积环境的两个重要因素：（1）海平面变化是控制陆坡陆源物质/深海钙质碎屑变化的主要因素;（2）研究区域地层发育有“碳酸盐稀释事件”与东亚夏季风在全新世初期（11.5～8.5 kaBP）增强有关。     

Dissolved silica concentrations and reactions in pore waters from continental slope sediments offshore from Cape Hatteras, North Carolina, USA

The pore water concentrations of dissolved silica in sediment cores from the continental slope offshore from Cape Hatteras, North Carolina, varied from 150 to almost 700 μ,M with depth in the top 40 cm of sediment. Sediment cores from 630 to 2010 m depth had very similar profiles of dissolved silica in their pore waters, even though these cores came from regions greatly different in slope, topography, sedimentation rate, and abundance of benthic macrofauna. Cores from 474 to 525 m were more variable, both with respect to pore water dissolved silica profiles, and with respect to sediment texture. Experiments indicate that both the rate of dissolution of silica and the saturation concentration decrease as sediment depth below the sediment-seawater interface increases. These data are consistent with depletion of a reactive silica phase in surface sediment, which may be radiolarian tests, or the alteration of biogenic silica to a less reactive form over time. Experimental results suggest that the pore water dissolved silica concentration in sediments below the top few centimeters may be higher than the sediments could now achieve. The flux of dissolved silica out of these sediments is estimated to be 15 μmoles cm⁻² yr⁻¹.     

SeaMARC II study of a giant submarine slump on the northern Chile continental slope

Abstract

A giant submarine slump, encompassing a 91‐km by 26‐km block, occurring on the continental slope offshore Iquique, Chile, was identified during a SeaMARC II survey. Utilizing SeaMARC II side‐scan imagery, bathymetry, and seismic reflection data, five morphostructural zones of the slump were identified: the fissured zone, scar zone, tensional depression, central block, and front zone. The fissured zone was developed on the crown of the slump; the scar zone is characterized by scars with the crescent‐shaped slip surfaces and throws ranging from 200 m to 50 m. The tensional depression zone is marked by an area voided by mass slumping, while the central block morphology was formed by uplift. The front zone is comprised of both compressional and tensional subzones. The compressional subzone is characterized by a relative topographic low, on the middle slope, whereas the extensional subzone is characterized by a convex pattern of alternated ridges and hollows, which may represent the debris of the slump on the lower slope. The formation of the slump was strongly influenced by the subduction of the Nazca plate beneath the Chile continental margin, which resulted in the subsidence of the continental slope with a resultant increase in the slope gradient and pore‐water pressure in the sedimentary layers. Slump formation was further facilitated by the development of a complex fault system associated with the subduction and by the triggering effect of earthquakes in the area.     

Diversity patterns along and across the Chilean margin: a continental slope encompassing oxygen gradients and methane seep benthic habitats

In the present study we review datasets available for the Chilean margin to assess the relationship between environmental (or habitat) heterogeneity and benthic diversity. Several factors, such as the presence of different water masses, including the oxygen-deficient Equatorial Sub-surface Waters (ESSW) at the continental shelf and upper slope, and the Antarctic Intermediate Waters (AIW) at mid slope depths appear to control the bathymetric distribution of benthic communities. The presence of methane seeps and an extended oxygen minimum zone (OMZ) add complexity to the benthic distribution patterns observed. All these factors generate environmental heterogeneity, which is predicted to affect the diversity patterns both along and across the Chilean continental margin. The response to these factors differs among different faunal size groups: meio-, macro-, and megafauna. Physiological adaptations to oxygen deficiency and constraints related to body size of each group seem to explain the larger-scale patterns observed, while sediment/habitat heterogeneity ( e.g. at water mass boundaries, hardgrounds, biogeochemical patchiness, sediment organic content, grain size) may influence the local fauna diversity patterns.     

陆坡非旋转重力羽状流的数值模拟

借助MITgcm模式使用二维非静力近似在水平方向采用不等间距网格,模拟了陆坡非旋转重力羽状流的流动。模拟显示陆坡上的重力羽状流的运动比较复杂。通过数值实验,发现改变地形或调整冷源强度都会影响沿陆坡下沉的低温高密度水的羽状流形态,潮汐亦会对重力羽状流有一定的影响。经计算发现在陆坡处,Richardson数小于1/4,宜发生Kelvin-Helmholtz不稳定性,并由卷挟导致环境流体与高密度流体混合,沿着斜坡加速下滑。     

陆架斜坡对内孤立波的动力响应特性试验研究

针对内孤立波在行进过程中遇到海底斜坡会对海底产生力的作用,不同坡度斜坡对内孤立波的动力响应应该存在差异。本文通过水槽中制造内波,对不同角度的斜坡对内孤立波的动力响应过程进行了研究。结果表明,内孤立波通过陆架斜坡上方,会造成斜坡沉积物超孔隙水压力的积累;在相同振幅条件下,缓坡沉积物动力响应的幅度比陡坡沉积物大;随着振幅的增加,缓坡发生动力破坏程度大于陡坡;在斜坡沉积物稳定性受到破坏之前,超孔隙水压力的积累和释放同时存在,内孤立波振幅的增大会加剧超孔隙水压力的释放。该结果对于斜坡沉积物在内孤立波作用下失稳破坏的动力学研究和斜坡稳定性分析将起到指导作用。