基于时间平均的海底沉积物声速预测

在海底沉积物声速预测中,把不同海域的物理性质完全不同的沉积物试验数据拟合出一个统一的方程存在不足,不但数据过于离散,而且方程中参量的物理意义不明确。借鉴Wyllie等建立的时间平均方程的思路,基于声传播过程中路程、时间和声速之间的基本关系,引入了表征固液双相之间的堆垒方式和耦合状态对声传播路径影响的耦合系数,建立了沉积物声速预测模型。将鹿回头外海、南海南部和北部的沉积物测量数据进行线性回归分析,分别得出适用于不同海域的沉积物声速预测模型,拟合的复相关系数较大,偏差较小,证明该模型能够反映声速随孔隙度的变化规律,且各参数物理意义明确,具有一定的研究和理论探索意义。     

Marine geology of St. George’s Bay, Newfoundland, as interpreted from multibeam bathymetry and back-scatter data

 Multibeam bathymetric data collected in St. George’s Bay, Newfoundland, show glacial and postglacial landforms in a 100-m-deep basin offshore from a barrier. Back-scatter data provide information on sediment lithology. Features undetected by previous conventional surveys include large bedforms on the surface of a barrier platform and submarine fans on its flanks. The data demonstrate that sediment transport processes are more complex than was previously believed. Multibeam bathymetric images are the sea-floor equivalents of air photographs. They can be used effectively to plan conventional acoustic surveys. Received: 16 February 1996 / Revision received: 4 September 1996     

基于遗传BP神经网络的海底沉积物声速预报

在海底沉积物声速预报中,针对传统经验公式存在预测精度差、适用范围窄、缺乏物理意义等问题,在已有BP神经网络预测的基础上,运用遗传算法优化其初始权值和阈值的方法,构建出基于含水量、孔隙度的声速预报模型。将南沙海域采集得到的海底沉积物样品分为两部分,抽取120组涵盖陆架、陆坡、海槽等地貌单元的样品作为训练数据,另外剩余6组作为测试数据。经试验对比后发现,在对本区域进行声速预报时,宜采用遗传算法优化的BP神经网络,其要优于传统的单参数、双参数回归拟合预报方法和国内外其他学者所得到的经验公式。此种预报方法具有一定的科学依据和广泛的应用前景,可在今后为建立明确、统一的声速预报模型提供参考。     

The reflection of the present distribution of the primary production in the bottom sediments of the Sea of Okhotsk

Using the authors’ data obtained during expeditions to the Sea of Okhotsk during the last decade, the primary phytoplankton production and the distribution of organic carbon and chlorophyll a degradation products (chlorin) in the bottom sediments were studied. Using the authors’ and published data, the spatial distribution of the production and paleoproduction indicators was plotted. The ratios of the chlorin and C_org content in the sediments was considered, and the correlation between these parameters was revealed. It was shown that the average annual primary production of phytoplankton and the paleoproduction indices were maximum in the coastal and upwelling zones and decreased towards the sea’s center. A quantitative correlation was found between the distribution of the present primary production in the photic layer and that of the rates of the accumulation of organic matter buried within the surface sediments. As a result, it was shown that the content of chlorin and C_org in the marine bottom sediments may be used to reconstruct the paleoproduction variability of the past.     

The Marine VHR 2.5-D Seismic Brute Stack Cube as a Feasible Tool for Low Budget Investigation and Research

Between 1997 and 1999 several marine seismic surveys were carried out in Kiel Bay aimed towards the development of a three-dimensional acquisition and interpretation technique for small scale subsurface structures using high-frequency sources and multichannel streamers. The data set was recently revisited by the author and reprocessed to obtain a multichannel stacked seismic data cube. Nominal hydrophone positions are deduced by determining offsets from first arrival times and estimating the hydrophone positions under consideration of the ships track. Processing towards a ‘seismic cube’ mainly comprised CMP sorting, constant velocity NMO correction and stacking. The resulting VHR 3-D seismic ‘brute stack cube’ reveals rich structural details. The fluvial Pleistocene channel system already documented in an earlier publication was tracked further to the north. It is situated below a flat cover of gas-bearing Holocene sediments, which locally constitute the seafloor. This till-horizon is superimposed on a second till layer showing strong topographic variations. Seismic signal phase and shielding effects indicate the possible presence of gas in these formations. This case history demonstrates that the VHR 3-D seismic method is a feasible tool for low budget investigation and research.     

Pleistocene surface water temperatures in the Benguela upwelling area

Analysis of carbonate microfossils (planktonic foraminifers and nannoplankton) in the DSDP Hole 362 Quaternary section made it possible to specify its zonal subdivision (almost all zones of Gartner’s high-resolution nannofossil scale are recognized), establish depositional environments, and restore past surface water temperatures. The latter appeared to be several degrees lower than their present-day values, which is evident from the anomalously high share of the subpolar species Neogloboquadrina pachyderma sin. that constitutes 97% of the fossil assemblage in Lower Pleistocene sediments. It is shown that the Benguela upwelling existed throughout the entire Pleistocene, being less intense in the Late Pleistocene and Holocene.     

Late quaternary seismic stratigraphy and active faults of the Gulf of İzmit (NE Marmara Sea)

The Upper Quaternary seismic stratigraphy and active faults of the Gulf of İzmit were investigated by means of high-resolution shallow seismic profiling data in the source region of 1999 İzmit earthquake. High-resolution seismic reflection data correlated with borehole data indicate that the stratigraphy of İzmit Bay consists of three distinct depositional sequences formed in response to middle Pleistocene-Holocene sea-level changes. Reflector R, separating the pre-Holocene sequences (1 and 2) from the Holocene sequence (3), represents an erosional unconformity produced by the subaerial fluvial erosion of the continental shelves at the time of the last glacial maximum. Occasional, anomalous reflections (acoustic turbidity) observed within the Holocene sequence are interpreted as gas accumulations. The maximum thickness of the Holocene sediments is found to be about 25 m. The isopach map of Holocene sediment implies that the thickness of the Holocene decreases from the east towards the central and western basins of İzmit Bay. Two distinct fault systems are interpreted in İzmit Bay. The main fault system extending roughly in an E-W direction along the Gulf of İzmit is an active right lateral strike slip fault with a normal component. The secondary faults are normal faults striking in different directions and these are identified as being both active and inactive. In addition, prominent compressive features are identified in the seismic cross-sections of some profiles acquired to the east of Hersek Peninsula where the focal mechanisms of the aftershocks of the 1999 İzmit earthquake also reveal predominantly reverse faulting mechanisms, as identified by a local dense seismic network.     

Controls on organic carbon distribution in sediments from the eastern Arabian Sea Margin

 Sediment cores from the upper continental slope of the eastern Arabian Sea have high organic carbon (OC), CaCO₃, and sand content at the top. The values decrease with increasing depth in the Holocene and Upper Pleistocene. Topographic highs show highest OC and lower CaCO₃ in the Holocene clayey sediments and vice versa in the Pleistocene sandy sediments. The OC is immature and marine or a mixture of both marine and terrestrial in the Holocene sediments and is mostly terrestrial and/or reworked marine in the Pleistocene sediments. Productivity is the main controlling factor for the organic carbon enrichment. Texture and reworking also influence the organic carbon variations. Received: 29 May 1996/Revision received: 10 January 1997     

莱州湾东部滨海水域第四纪沉积及古地理特征

本文通过对６个钻孔和４６１ｋｍ浅层地震剖机测量获得的测年、微古、孢粉、藻类、古地磁以及岩持征和地震反射界面等资料的综合分析，将本区第四系划分为中更新统、上更新统和全新统，对各时期的沉积进行了较详细的阐述，并对不同时期的沉积相特征和古地理环境的变化进行了初步地揭示。     

The coastal profile in the eastern Gulf of Finland: The results of a survey and the reconstruction of the evolution in the Late Holocene

The studies carried out by the Karpinskii All-Russia Research Institute of Geology using side-scan profiling, echo sounding, and surface sediment sampling allowed revealing the detailed structure of the underwater coastal slope in the eastern Gulf of Finland. In particular, a submarine sand terrace was found at depths of 4–5 m. An attempt at the reconstruction of the coastal evolution over the period of the Late Holocene was made using mathematical modeling in order to explain the observed morphology of the submarine coastal slope. The key assumption of the concept suggested is that, at the earlier stage, the tectonic processes played the main role, while, at the later stage, the sea-level changes were of greater importance. The tectonic block comprising the investigated area of the Gulf of Finland at first rapidly increased and then it stabilized and was influenced by the sea level’s rise. These processes resulted in the formation of a series of terraces. The earlier of those are now located on dry land, while the later terraces are observed on the submarine slope. Within the concept proposed, the coastal evolution in the Late Holocene appears as a process of the gradual erosion of the above-water terraces and the formation of new underwater terraces. During the transgressive phases, the rate of the coast’s recession reached 0.5 m year⁻¹, while decreasing by a factor of two during the intermediate stages. The submarine terrace developed over the period of 3.2–1.2 thousand years ago, and it extended in equal measure due to the coast’s recession and the material’s accumulation near its external edge. During that period, the coast retreated by approximately 500 m, while the average accumulation rate could have been as high as 0.7 m³ m⁻¹ year⁻¹.     

Preliminary investigation of the surficial sediments in the Cap-Breton Canyon (southwest France) and the surrounding continental shelf

A total of 120 grab samples of the surficial sediments in the Cap-Breton submarine canyon and surrounding continental shelf were collected and analyzed by grain-size sieving. A Q-mode Factor Analysis was made on the grain-size data in order to define the most meaningful facies types. Four distinct lithological facies were found to exist: silt and clay, very fine sand, fine sand, and coarse sand. Comparison with previous work and a ¹⁴C date on the silt and clay facies showed that the facies are not contemporaneous. The sands and coarse sands on the shelf were emplaced during the pre-Würm and Würm regressions, and later probably reworked during the Holocene (Flandrian) transgression. The silty clays found in the canyon and on the shelf to the south are younger and represent sediments brought in as suspended load by the Adour and other nearby rivers during the Holocene (Flandrian) transgression.     

Millennium scale environment changes of the Okhotsk Sea during last 80 kyr and their phase relationship with global climate changes

The paper presents the records of several paleoproductivity proxies (PP) (biogenic opal and Ba (Si_bio, Ba_bio), organic carbon (C_org) and carbonate Ca_carb, chlorin and benthic foraminifera abundance (BFA)) in sediments of the Okhotsk Sea for the last 80 kyr with a resolution of ∼100–300 years. The sediment age model was based on the AMS ¹⁴C data, records of benthic foraminifera δ¹⁸O, paleointensity of the Earth’s geomagnetic field and magnetic susceptibility. PP values demonstrate series of severe prolonged productivity drops in the Okhotsk Sea followed by a sharp increase during the last glaciation. On the basis of quantitative estimations of the paleoproductivity in the Okhotsk Sea during the cold MIS 2 and warm Holocene (Gorbarenko and Goldberg, 2005), it is suggested that the millennium scale relationship in productivity-climate changes of this basin was similar: an increase in the sea’s productivity was related with regional climate warming and vice-versa. The PP records of productivity/climate oscillations in the Okhotsk Sea during MIS 2–4 occurred contemporaneously with the North Atlantic cold Heinrich events (HE) and Greenland Dansgaard-Oyeshger interstadial (DOI). Observed successions of prolonged climate cooling events followed by rapid, abrupt warming are similar to climate and environmental oscillations in the N. Atlantic and Greenland, that confirms the millennium-scale climate changes synchronicities in the Northern Hemisphere including the far NW Pacific, the hydrology and climate conditions of which are close to those of the Okhotsk Sea. Synchronism of the N. Hemisphere severe cooling (including the Okhotsk Sea) with the Antarctic warming suggests that mechanisms of the “seesaw” effect (Blunier and Brook, 2001) in the low latitude heat redistribution between high latitudes of both hemispheres were more complicated than direct NADW formation forcing and encompasses the global atmospheric reorganization. Within the PP used a closer connection in the Okhotsk Sea millennium oscillations was observed for the C_org, BFA and chlorin; Ba_bio increases more closely to DOI. Si_bio variability does not show any evident correlation with productivity changes.     

冲绳海槽中段近4万年来沉积物属性和来源

对冲绳海槽中段DGKS9603孔134个样品(孔深0~481cm),测试了18种常、微量元素,对该柱样的29个样品(孔深0~488cm)进行了REE测试。对采自长江三角洲的CYm柱样和黄河三角洲的S3柱样中的晚更新统的5个样品进行了常、微量元素和REE测试。分析表明,冲绳海槽中段在全新世、末次盛冰期和末次间冰期的沉积物均以陆源物质为主,生物沉积为次。末次盛冰期以陆源硅质碎屑沉积的大量增加为特征;全新世生物沉积量有较大的增加;末次间冰期介于上述两者之间,但较全新世更富陆源物质。冲绳海槽的物源演化受末次冰期的影响,具有阶段性。对比冲绳海槽与长江和黄河物源的地球化学参数,包括REE、稀有元素、惰性常、微量元素,判识出末次盛冰期冲绳海槽的陆源物质,具有长江源的物质属性,沉积物可能主要来自古长江。     

Modern sediments and sediment accumulation rates on the narrow shelf off central Vietnam,South China Sea

The narrow shelf along the coast of central Vietnam is seasonally supplied by large amounts of sediment from the adjacent mountainous hinterland following monsoonal precipitation. This study examines the fate of these sediments, and their accumulation rates along two transects across the shelf, based on analyses of radionuclides (²¹⁰Pb, ¹³⁷Cs), sediment texture and structure, as well as carbonate content. The inner shelf is covered by sands, and probably serves as bypass zone for fine sediments transported offshore. Sediment characteristics suggest that the transport to the mid and outer shelf is related to flood events. Averaged over the last century, the ²¹⁰Pb-based mud mass accumulation rates on the mid and outer shelf vary between 0.25 g cm ⁻² and 0.56 g cm ⁻² year ⁻¹ (corresponding to linear sediment accumulation rates of 0.20–0.47 cm year ⁻¹). Along with high excess ²¹⁰Pb inventories, these high accumulation rates suggest a significant sediment depocentre on the mid shelf. The ²¹⁰Pb-derived sediment accumulation rates were found to be several times higher than ¹⁴C-derived rates previously reported for the Holocene, at the same location on the outer shelf. This is probably due to the incompleteness of the Holocene record, and an overestimation of the modern rate. Another explanation would be increased erosion within the rivers’ drainage basins, due to 20th century deforestation. This hypothesis is supported by the difference between recent (less sand, more lithic grains in the sand fraction) and older sediments. In terms of modern sedimentation processes and rates, the central Vietnam shelf, although being a part of a narrow passive continental margin, is similar to active flood-dominated continental margins.     

The redox profile of the bottom sediments in the Ob River’s mouth area

By the data of cruise 54 of the R/V Akademik Mstislav Keldysh (September 2007), the results of geochemical studies of the redox processes in the bottom sediments of the Ob River’s near-mouth area as applied to the redox indicator elements such as manganese, iron, and sulfur are presented. The characteristics of the bottom sediments and the distribution of these elements in them testify not only to the significant role of the mixing processes in the formation of the geochemical profile of the bottom sediments in the estuary but also to the role of the postsedimentation (diagenetic) processes.     

Do inverted depositional sequences and allochthonous foraminifers in sediments along the Coast of Kachchh, NW India, indicate palaeostorm and/or tsunami effects?

The study of 134 subsurface sediment samples from three cores collected along the Coast of Kachchh, off Gujarat Province, NW India, shows the presence of 151 foraminifer species. Q-mode cluster analysis on foraminifer data for each core and radiocarbon dating at ten different downcore depths reveal an inverted sequence, with fine-grained sediments and small foraminifers sandwiched between normal detrital sediments at water depths of 10–20 m. It is postulated that ∼8,000 years B.P., sediments ranging in age from ∼10,000 to ∼12,000 years B.P. were eroded from deeper offshore deposits by storm/tsunami(s), and were subsequently transported and redeposited in shallow regions, resulting in an inverted sequence, followed by a normal detrital sequence deposited between ∼8,000 and ∼7,000 years B.P. A second, similar event resulted in the deposition of ∼10,000 year old sediments over a normally deposited, ∼7,000 year old sequence. The second inverted sequence was subsequently overlain by a normal sequence. It is hypothesised that the erosion and transportation of fine-grained sediments from deeper water, and their deposition in shallower water against the rule of gravity, are the result of storm/tsunami influence.     

Sea-level changes and related depositional environments on the southern Marmara shelf

Petrographic data obtained from 182 surface sediment samples together with the available bathymetric data are used to investigate the effects of the last major sea-level changes on shelf evolution in the southern Sea of Marmara. Grain-size analysis reveals the presence of at least three belts or zones which are rich in coarse-grained (sand and gravel) sediments. These coarse-grained belts which are up to 45 km long, 15 km wide, show up to 20 m of relief and are presently found at 40–80 m (average 60 m) water depths. Based on microscopic examination and residual analysis, the sediments from these belts are interpreted as indicators of high-energy shallow waters where detrital siliciclastics, with some benthic contribution, accumulated. The presence of a 62–65 m deep sill in the Çanakkale Strait and the consideration of sea-level curves would suggest that the Marmara shelves must have been subaerially exposed down to −65 m water depths for about 10,000 yrs (22,000–12,000 yrs B.P.), sufficient time to modify former shelf topographies and form such bottom relief. While difficult to date, we believe that coarse-grained belts found on the southern Marmara shelf must be relict (i.e., former shorelines, beaches) and their formation is largely related to low stands of sea-level during the Late Pleistocene regression and early Holocene transgression. However, the available high-resolution seismic profiling data suggest that the neotectonism in this seismically active Sea of Marmara plays an important role to explain the raise of these older shorelines to their present levels on the sea-floor.     

Physical and Acoustic Properties of Gas-bearing Sediments in Jinhae Bay,the South Sea of Korea

High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. The sediment in Jinhae Bay is mostly homogenous mud deposited after the Holocene transgression. Along with the 410 km of chirp seismic profiling, five piston core samples were collected on the track lines.

Gassy sediments are common and occur widely in the bay. Core samples were analyzed for sediment texture, physical properties (porosity, water content, bulk density, and grain density), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. X-radiograph image analysis was also performed to observe the shape of degassing cracks. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, compressional wave velocity dramatically decreases from average 1480 to 1380～739 m/s for the cores that penetrate the gas-bearing zones. This is probably due to degassying cracks that developed by escaping gases and free gas bubbles that are still trapped in the cores. Electrical resistivity is the only geotechnical property that increases in the gas-bearing zone where compressional wave velocity abruptly decreases. This indicates the possibility of using both electrical resistivity as an index variable as well as to compressional wave velocity to identify gassy sediment microstructure because there are little changes in texture and composition of sediment.     

Peculiarities of the East Scotia ridge’s geochronology

The first map of the acoustic basement and the new map of chrons C1-C5E for the region of the East Scotia mid-ocean ridge have been made. The analysis of the maps and the calculations have indicated that the sea-floor spreading at the ridge’s flanks started in its southeast in the interval of chrons C5Er-6An (18.52–20.17 Ma BP). The maximal spreading rate (5.3 cm/year) was in the interval of chrons C5Bn-C5Br (14.78–15.97 Ma BP). Then, the spreading rate was slow and increased again from 3–6 Ma BP until the present. The spreading in the last 1–2 Ma was accompanied by the propagating of the axes southwards in the E1, E2, and E4 segments and northwards in the E8 and E9 segments.