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21.
Mjelde Rolf Fjellanger Jan Petter Digranes Per Kodaira Shuichi Shimamura Hideki Shiobara Hajime 《Marine Geophysical Researches》1997,19(1):81-96
A 140 km long wide-angle seismic profile has been acquired by use of 6 Ocean Bottom Seismographs across the Jan Mayen Ridge, North Atlantic. The profile was acquired twice; once with a traditionally tuned standard source and secondly with a somewhat smaller source tuned on the first bubble pulse. Analysis of the frequency content of the data reveals that the single-bubble source within the 10-15 Hz frequency range generates a signal with a level about 5 db above that of the standard source. These differences can partly be related to differences in airgun depth. The higher output level for these frequencies enables the single-bubble source to resolve intra-crustal structures with a higher degree of certainty, when compared to the data acquired by use of the standard source array. The standard source seems to generate slightly more energy for frequencies around 6 Hz, probably due to the use of a large 1200 in/sup3 gun in this array. These low frequencies a re of importance for mapping of lower crustal and upper mantle structures, and it is recommended that this is taken into account when seismic sources for mapping of deep crustal and upper mantle structures are designed. 相似文献
22.
Lodolo Emanuele Coren Franco Schreider Anatoly A. Ceccone Giulio 《Marine Geophysical Researches》1997,19(5):439-450
The southwestern part of the Scotia Sea, at the corner of the Shackleton Fracture Zone with the South Scotia Ridge has been investigated, combining marine magnetic profiles, multichannel seismic reflection data, and satellite-derived gravity anomaly data. From the integrated analysis of data, we identified the presence of the oldest part of the crust in this sector, which tentative age is older than anomaly C10 (28.7 Ma). The area is surrounded by structural features clearly imaged by seismic data, which correspond to gravity lows in the satellite-derived map, and presents a rhomboid-shaped geometry. Along its southern boundary, structural features related to convergence and possible incipient subduction beneath the continental South Scotia Ridge have been evidenced from the seismic profile. We interpret this area, now located at the edge of the south-western Scotia Sea, as a relict of ocean-like crust formed during an earlier, possibly diffuse and disorganized episode of spreading at the first onset of the Drake Passage opening. The successive episode of organized seafloor spreading responsible for the opening of the Drake Passage that definitively separated southern South America from the Antarctic Peninsula, instigated ridge-push forces that can account for the subduction-related structures found along the western part of the South Scotia Ridge. This seafloor accretion phase occurred from 27 to about 10 Ma, when spreading stopped in the western Scotia Sea Ridge, as resulted from the identification of the marine magnetic anomalies. 相似文献
23.
24.
K. G. Robertson 《Marine Geophysical Researches》1990,12(1-2):3-8
Accurate navigation forms an essential part of all research at sea and the deep ocean imposes it's own unique problems. This chapter discusses several of the techniques in current use on the research vessels of the Natural Environment Research Council (NERC), concentrating on those systems which provide global navigation facilities, as opposed to the more localised, coastal aids. Whilst most of the systems rely on surface propagation of radio waves, the use of acoustics and sea-bed mapping instruments constitute accurate alternatives for some sub-sea applications. 相似文献
25.
The results of the two- and three-dimensional magnetic inversions performed on data located between 20°–24° N on the Mid-Atlantic Ridge indicate the crustal magnetization has decayed exponentially for the last 10 Ma, and that this decay has been fairly symmetric about the ridge axis. After removal of the mean temporal decay, the residual field is characterized by more positive magnetizations at the second-order discontinuities, regardless of initial magnetization direction. A model that involves the preferential emplacement of serpentinized lithologies near the discontinuities is proposed to explain this correlation. The temporal detrending method also indicates that several ridge-parallel depressions located on the flanks of the ridge axis are regions of more positive magnetizations. These bathymetric depressions may mark the locations of detachment faulting that occurred during amagmatic periods of extension. The general symmetry of the crustal magnetization about the ridge axis does not support the occurrence of continuous detachment faulting proposed to correspond to the inner and outer corners of ridge axis discontinuities. 相似文献
26.
资料显示,在整个台湾以东海区内磁异常几乎全为负磁异常,磁异常分块现象明显。通过对本海区的磁力资料进行分析和研究,表明本海区的地磁异常表现为明显的条带状异常特征。由于受板块差异性运动的影响,以加瓜海脊为界,东西两侧具有不同的磁条带方向,在加瓜海脊以东,磁条带为NW-SE向(120°),而在加瓜海脊以西则为近东西向(80°)。加瓜脊以西地区的扩张时间为45~38Ma,相当于19~16号磁条带;而加瓜脊以东地区的扩张时间为40~35Ma,相当于17~13号磁条带。加瓜脊以西的磁条带相对于以东的磁条带要老,由此推断出加瓜脊以西相对于东侧发生了北向位移。 相似文献
27.
On the basis of maps of sea level anomalies data set from October 1992 to January 2004, pronounced low frequency variations with periods of about 500 d are detected in the area near 20°N from 160°W to 130°E. A linear two-layer model is employed to explain the mechanism. It is found that the first-mode long baroclinic Rossby waves at 20°N in the northwest Pacific propagate westward in the form of free waves at a speed of about 10.3 cm/s. This confirms that the observed low frequency variabilities appear as baroclinic Rossby waves. It further shows that these low frequency variabilities around 20°N in the northwest Pacific can potentially be predicted with a lead up to 900 d. 相似文献
28.
We use a synthetic data experiment to assess the accuracy of ocean tides estimated from satellite altimetry data, with emphasis on the impact of the phase-locked internal tide, which has a surface expression of several centimeters near its sites of genesis. Previous tidal estimates have regarded this signal as a random measurement error; however, it is deterministic and not scale-separated from the barotropic (surface) tide around complex bathymetric features. The synthetic data experiments show that the internal tide has a negligible impact on the barotropic tidal fields inferred under these circumstances, and the barotropic dissipation (a quadratic functional of the tidal fields) is in good agreement with the energetics of the three-dimensional regional primitive equations model which is the source of the synthetic data. 相似文献
29.
Jože Kotnik Milena Horvat Emmanuel Tessier Nives Ogrinc Mathilde Monperrus David Amouroux Vesna Fajon Darija Gibičar Suzana Žižek Francesca Sprovieri Nicola Pirrone 《Marine Chemistry》2007
Mercury speciation and its distribution in surface and deep waters of the Mediterranean Sea were studied during two oceanographic cruises on board the Italian research vessel URANIA in summer 2003 and spring 2004 as part of the Med Oceaneor and MERCYMS projects. The study included deep water profiles of dissolved gaseous Hg (DGM), reactive Hg (RHg), total Hg (THg), monomethyl Hg (MeHg) and dimethyl Hg (DMeHg) in open ocean waters. Average concentrations of measured Hg species were characterized by seasonal and spatial variations. Overall average THg concentrations ranged between 0.41 and 2.65 pM (1.32 ± 0.48 pM) and were comparable to those obtained in previous studies of the Mediterranean Sea. A significant fraction of Hg was present as “reactive” Hg (average 0.33 ± 0.32 pM). Dissolved gaseous Hg (DGM), which consists mainly of Hg0, represents a considerable proportion of THg (average 20%, 0.23 ± 0.11 pM). The portion of DGM typically increased towards the bottom, especially in areas with strong tectonic activity (Alboran Sea, Strait of Sicily, Tyrrhenian Sea), indicating its geotectonic origin. No dimethyl Hg was found in surface waters down to the depth of 40 m. Below this depth, its average concentration was 2.67 ± 2.9 fM. Dissolved fractions of total Hg and MeHg were measured in filtered water samples and were 0.68 ± 0.43 pM and 0.29 ± 0.17 pM for THg and MeHg respectively. The fraction of Hg as MeHg was in average 43%, which is relatively high compared to other ocean environments. The concentrations reported in this study are among the lowest found in marine environments and the quality of analytical methods are of key importance. Speciation of Hg in sea water is of crucial importance as THg concentrations alone do not give adequate data for understanding Hg sources and cycling in marine environments. For example, photoinduced transformations are important for the presence of reactive and elemental mercury in the surface layers, biologically mediated reactions are important for the production/degradation of MeHg and DGM in the photic zones of the water column, and the data for DGM in deep sea indicate the natural sources of Hg in geotectonicaly active areas of the Mediterranean Sea. 相似文献
30.
Shigenao Maruyama Koutaro Tsubaki Keisuke Taira Seigo Sakai 《Journal of Oceanography》2004,60(3):563-568
Deep seawater in the ocean contains a great deal of nutrients. Stommel et al. have proposed the notion of a “perpetual salt fountain” (Stommel et al., 1956). They noted the possibility of a permanent upwelling of deep seawater with no additional external energy source. If
we can cause deep seawater to upwell extensively, we can achieve an ocean farm. We have succeeded in measuring the upwelling
velocity by an experiment in the Mariana Trench area using a special measurement system. A 0.3 m diameter, 280 m long soft
pipe made of PVC sheet was used in the experiment. The measured data, a verification experiment, and numerical simulation
results, gave an estimate of upwelling velocity of 212 m/day.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献