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1.
New paleomagnetic data from 11 sites in layered gabbros and lava flows from the Oman Ophiolite indicate stable, early remagnetizations and suggest that the southern portion of the ophiolite (the Wadi Tayin, Sumail, Nakhl-Rustaq and Haylayn massifs) is relatively unrotated since detachment near the paleoridge. The gabbros possess a magnetization carried by a combination of primary and secondary magnetites derived from hydrothermal alteration. Evidence from positive tilt tests, constancy of remanence directions in differing magnetic mineralogies and agreement with previous paleomagnetic data, however, suggests that this remagnetization was acquired early – analogous to the remagnetization of the V2 volcanic series. Thus, the evidence implies that the southern portion of the ophiolite has been primarily translated from the paleoridge since the time of V2 remagnetization, and 120° of clockwise rotation affecting the northern Oman Ophiolite is internal to the ophiolite, rather than a combination of internal and global rotation as previously hypothesized. Given this evidence, we propose a simplified model of a rapid, active microplate rotation of a portion of the ophiolite resulting from spreading at an EPR-type propagating ridge at a high angle to the previous spreading direction. Paleomagnetic data from this and previous studies can be well explained by a rapidly rotating microplate, similar to the kinematic evolution documented for the Juan Fernandez microplate in the modern setting.  相似文献   

2.
Nicolas  A.  Boudier  F.  Ildefonse  B.  Ball  E. 《Marine Geophysical Researches》2000,21(3-4):147-180
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3.
Event plumes form as episodic discharges of large volumes of hydrothermal solutions in response to magmatic diking/eruptive events. In consequence, event plumes represent the sudden injection of exploitable reduced chemical substrates, as well as inhibitory constituents, and likely induce successional changes in the microbial community structure and activity within event plume waters. In response to a major seismic event detected beginning 28 February 1996 at the northern Gorda Ridge, a series of three rapid response and follow-up cruises (GREAT 1, 2 and 3) were mounted over a period of three months. This report focuses on time-series measurements of manganese geomicrobial parameters in the two event plumes found in association with this seismic event.Scanning transmission electron microscopy, elemental microanalysis, and radioisotope (54Mn) uptake experiments were employed on samples collected from vertical and tow-yo casts from the three cruises. Numbers of bacteria and ratios of metal precipitating capsuled bacteria to total bacteria were greatest in the youngest (days old) plume, EP96A, found during GREAT 1; however, when normalized to the hydrothermal temperature anomaly, the greatest values were found in a second event plume, EP96B, discovered during GREAT 2 (up to 1 month old). Early capsule bacteria and particulate Mn distributions may have been influenced by entrainment of resuspended sediment, while those of the oldest (2–3 months) plume sample may have been subjected to preferential aggregation and particle settling.  相似文献   

4.
东菲律宾海柱样沉积物的磁性特征   总被引：2，自引：0，他引：2  
对东菲律宾海帕里西维拉海盆西侧F090102柱样进行了系统的古地磁和岩石磁学研究,测量了天然剩磁倾角、磁化率、饱和等温剩磁、非磁滞剩磁、-T曲线等磁学参数,进行了磁性地层的划分对比,识别出松山反极性时内的贾拉米洛和奥都维正极性亚时。对磁学参数及其有关比值的研究发现,岩心磁性矿物以低矫顽力磁铁矿为主,可能含有一定的钛磁铁矿。磁性矿物主要是陆源碎屑成因,岩心底段受到海底火山作用的影响。根据磁学参数随深度的变化将岩心分为3段,各段反映了沉积环境和古气候的变化。  相似文献   

5.
Magnetic data collected in conjunction with a Sea Beam bathymetric survey of the Mid-Atlantic Ridge south of the Kane Fracture Zone are used to constrain the spreading history of this area over the past 3 Ma. Two-dimensional forward modeling and inversion techniques are carried out, as well as a full three-dimensional inversion of the anomaly field along a 90-km-long section of the rift valley. Our results indicate that this portion of the Mid-Atlantic Ridge, known as the MARK area, consists of two distinct spreading cells separated by a small, zero-offset transform or discordant zone near 23°10′ N, The youngest crust in the median valley is characterized by a series of distinct magnetization highs which coalesce to form two NNE-trending bands of high magnetization, one on the northern ridge segment which coincides with a large constructional volcanic ridge, and one along the southern ridge segment that is associated with a string of small axial volcanos. These two magnetization highs overlap between 23° N and 23°10° N forming a non-transform offset that may be a slow spreading ridge analogue of the small ridge axis discontinuities found on the East Pacific Rise. The crustal magnetizations in this overlap zone are generally low, although an anomalous, ESE-trending magnetization high of unknown origin is also present in this area. The present-day segmentation of spreading in the MARK area was inherited from an earlier ridge-transform-ridge geometry through a series of small (∼ 10 km) eastward ridge jumps. These small ridge jumps were caused by a relocation of the neovolcanic zone within the median valley and have resulted in an overall pattern of asymmetric spreading with faster rates to the west (14 mm yr−1) than to the east (11 mm yr−1). Although the detailed magnetic survey described in this paper extends out to only 3 Ma old crust, a regional compilation of magnetic data from this area by Schoutenet al. (1985) indicates that the relative positions and dimensions of the spreading cells, and the pattern of asymmetric spreading seen in the MARK area during the past 3 Ma, have characterized this part of the Mid-Atlantic Ridge for at least the past 36 Ma.  相似文献   

6.
Swath bathymetric, sonar imagery and seismic reflection data collected during the SOPACMAPS cruise Leg 3 over segments of the Vitiaz Trench Lineament and adjacent areas provide new insights on the geometry and the stuctural evolution of this seismically inactive lineament. The Vitiaz Trench Lineament, although largely unknown, is one of the most important tectonic feature in the SW Pacific because it separates the Cretaceous crust of the Pacific Plate to the north from the Cenozoic lithosphere of the North Fiji and Lau Basins to the south. The lineament is considered to be the convergent plate boundary between the Pacific and Australian Plates during midde to late Tertiary time when the Vitiaz Arc was a continuous east-facing are from the Tonga to the Solomon Islands before the development of the North Fiji and Lau Basins. Progressive reversal and cessation of subduction from west to east in the Late Miocene-Lower Plioene have been also proposed. However, precise structures and age of initiation and cessation of deformation along the Vitiaz Trench Lineament are unknown.The lineament consists of the Vitiaz Trench and three discontinuous and elongated troughs (Alexa, Rotuma and Horne Troughs) which connect the Vitiaz Trench to the northern end of the Tonga Trench. Our survey of the Alexa and Rotuma Troughs reveals that the lineament is composed of a series of WNW-ESE and ENE-WSW trending segments in front of large volcanic massifs belonging to the Melanesian Border Plateau, a WNW trending volcanic belt of seamounts and ridges on Pacific crust. The Plateau and Pacific plate lying immediately north of the lineament have been affected by intense normal faulting, collapse, and volcanism as evidenced by a series of tilted blocks, grabens, horsts and ridges trending N 120° to N100° and N60°–70°. This tectonism includes several normal faulting episodes, the latest being very recent and possibly still active. The trend of the fault scarps and volcanic ridges parallels the different segments of the Vitiaz Trench Lineament, suggesting that tectonics and volcanism are related to crustal motion along the lineament.Although the superficial observed features are mainly extensional, they are interpreted as the result of shortening along the Vitiaz Trench Lineament. The fabric north of the lineament would result from subduction-induced normal faulting on the outer wall of the trench and the zig-zag geometry of the Vitiaz Trench Lineament might be due to collision of large volcanic edifices of the Melanesian Border Plateau with the trench, provoking trench segmentation along left-lateral ENE-WSW trending transform zones. The newly acquired bathymetric and seismic data suggest that crustal motion (tectonism associated with volcanism) continued up to recent times along the Vitiaz Trench Lineament and was active during the development of the North Fiji Basin.  相似文献   

7.
In order to determine why the sedimentation to supply ratio of nutrients in Tokyo Bay is markedly small, the nitrogen budget was investigated for 1979, when a systematic and continuous observation of flow and salinity was carried out. The data were analyzed by use of a simple advective-diffusive box model and dissolved oxygen balance in the lower layer was also examined. The calculated values of two-layer flow, settling, primary production, mineralization, denitrification, and dissolved oxygen consumption were comparable to those observed.The factors making the sedimentation to supply ratio makedly small were summarized as: 1) a strong and stable two-layer flow generated by a large freshwater supply, 2) further intensification of this two-layer flow by the northern winter monsoon, 3) coincidence of the discharge region with the supply region of nutrients caused by the transverse inclination of the interface, probably due to the earth's rotation. 4) effective discharge of nutrients from the bay due to a strong tidal flow and a possible cyclonic tidal residual circulation in the inner bay mouth, 5) incomplete consumption of nutrient salts by phytoplankton in the upper layer even in the most productive season, and 6) possible denitrification in the anaerobic bottom water in summer and in the bottom sediment itself throughout the year in the inner bay.  相似文献   

8.
Mitre Island samples, Pliocene in age, can be classified together with Anuda samples, of unknown age, as island-arc basaltic andesites with tholeiitic tendencies. The presence of morphologically unusual iron—titanium oxides and of probably xenocrystic plagioclase suggest that at least part of the observed minerals crystallized in a magma chamber underlying the island. The samples apparently represent a mixture of magma fluid, cumulate plagioclase, pyroxene, and iron—titanium oxides which were ponded in a crater lava lake where they were reheated by subsequent eruptions. Many of them show symplectic magnetite formed by high-temperature oxidation of olivine. The morphological complexity and compositional homogeneity of the iron—titanium oxides cannot be explained at present.The presence of sulfide droplets inside olivine, magnetite and ilmenite crystals suggests a formation of an immiscible sulfide liquid in the magma chamber. Droplets of this liquid were overgrown by minerals crystallizing at that time and thus protected against oxidation during and after eruption.Mitre Island is a part of a currently inactive Vitiaz island arc associated in the past with a westward subduction of Pacific plate along the Vitiaz trench. Increased difficulty in subducting the large mass of the Pacific Border Plateau under the northern Fiji Plateau apparently produced counterclockwise rotation of the Vitiaz island arc. Oblique subduction was active until a steep angle was reached between the Vitiaz trench and the motion vector of the Pacific plate. Then a strike-slip fault developed in the Vitiaz trench and the subducted plate was sheared off. Recently the strike-slip zone migrated south from the Vitiaz trench across the northern Fiji Plateau and is presently extending from Aoba Island, in the New Hebrides, northeastward toward the Pacific Border Plateau.  相似文献   

9.
In 1963, Off defined a bedform type which he described as rhythmic linear sand bodies caused by tidal currents. He figured twelve examples from around the world. Since then, the morphology and dynamics of sand transport in one of these areas, the tidal shelf seas around Great Britain, have undergone intensive study. The tidal sand ridges emerge as anomalies, in that they do not fit into the sequence of morphologic provinces which characterize the major sediment transport paths.It is suggested here that the ridge fields are analogous to the shoal-retreat massifs of the Middle Atlantic Bight in that they have been inherited from a nearshore regime during the course of the Holocene transgression. Shoal-retreat massifs are low, broad, shelf-transverse sand bodies which mark the retreat paths of coastal depocenters associated with littoral drift convergences. Two main types of shoal-retreat massifs in the Middle Atlantic Bight are: (1) estuarine shoal-retreat massifs; and (2) cape shoal-retreat massifs.Two similar classes of shoal-retreat massifs may develop in tidal shelf seas, but the mechanism is somewhat different. Class-1 tidal massifs are tidal ridge fields whose ridges were hydraulically packaged in an estuarine environment. If, upon transgression, they find themselves in a broad tidal bight which continues to funnel tidal flow, the ridges may survive for long distances out into the bight. The ridge fields of the Southern Bight of the North Sea may have undergone such an evolution.Class-2 tidal massifs occur off promontories in tidal seas that are swept by the edge waves generated by amphidromic tidal systems. Here the debris of shoreface erosion tends to be stored as shoreface-connected, tide-maintained ridges. Such ridges are also pre-adapted to survive with modification for long distances out on the associated shelf, as the water column deepens during a marine transgression.  相似文献   

10.
The Gebel Yelleg area includes a number of folds belonging to the northern Sinai Syrian Arc structures. Detailed surface structural mapping and subsurface (seismic and borehole) data show that the Gebel Yelleg structures are related to Late Cretaceous-Early Tertiary inversion of a Jurassic asymmetric (or half) graben formed during the opening of Neotethys. The inversion structures include a large (45-km long) asymmetric fold (Yelleg Anticline) with a steep flank overlying the southeastern (main) bounding fault of the inverted half graben as well as some right-stepped en echelon folds overlying the northwestern bounding fault of the half graben. The large inversion anticline is dissected by a large number of long, nearly orthogonal normal faults whereas the en echelon folds are dissected by transverse normal faults and two sets of oblique-slip faults. Inversion of the northern Sinai extensional basins is related to Africa-Eurasia convergence and was probably transpressional with a small component of dextral slip. This study shows that the magnitude of inversion in the northern Sinai fold belt decreases toward the southern boundary of the Jurassic extensional province.  相似文献   

11.
In the Ulleung Basin, East Sea, the Dok Do seamount group comprises Dok Do (Dok Island), consisting of very small islets/rocks and a large submerged volcanic edifice, and two voluminous tablemounts, Simheungtaek and Isabu. We attempted to reconstruct the evolution of these seamounts, using virtual geomagnetic poles (VGPs) determined by the least-squares and the seminorm magnetization methods, with 1,500 m upward continued magnetic anomalies. The VGPs of Dok Do with normal dipole anomaly, and of Simheungtaek with normal dipole anomaly are located near the present magnetic pole. The VGP of Isabu with normal dipole anomaly is located at low latitude, presumably due to overprints of reversals in the Tertiary, and the distortion of magnetization and structures associated with volcanism after its formation. In contrast to the tablemounts, magnetic anomalies over Dok Do are a combination of both normal polarity and reversed polarity dipoles in the northern hemisphere, indicating that Dok Do has had at least two major eruptions, one during normal and another during reversed polarity intervals. From these results, and information on the ages of the seamounts (either published radiometric ages of subaerial volcanic rocks, or ages reconstructed in terms of reported elastic thickness incorporated into an existing cooling plate model), we tentatively propose that (1) Isabu formed first, during a normal polarity interval after the opening of the East Sea had ceased; (2) this was followed by an initial and subsequent large eruption of Dok Do during a normal polarity and a reversed polarity interval after about 5 Ma; and (3) the formation of Simheungtaek occurred in between that of Isabu and Dok Do in a normal polarity interval. The pattern of normal/reversed magnetization is not inconsistent with the geomagnetic polarity timescale for at least the last 5 Ma. Nevertheless, precise ages of formation would need verification by additional geophysical/geochemical constraints. Evaluating various possible models explaining the successive formation of the Dok Do seamounts, we currently favor fracturing and volcanism related to compression-induced weakening of the extensional field from the late Miocene to Pliocene after the opening of the East Sea.  相似文献   

12.
X.H. Wang   《Ocean Modelling》2005,10(3-4):253-271
The Princeton Ocean Model was implemented to investigate the response of northern Adriatic Sea during the Bora event in January 2001 when strong wind and surface cooling was reported. The model has been run with realistic wind stress, surface heat flux and river runoffs forcings continuously from 1 January 1999 to 31 January 2001. The wind stress and surface heat flux was computed by the bulk parameterization, using the European Centre for Medium Range Weather Forecast analysis fields and the Comprehensive Ocean Atmosphere Data Set cloud data. All the freshwater sources along the Adriatic coastlines were represented by point or line source functions. Open boundary conditions in the Ionian Sea along a latitudinal boundary were nested within a large scale model of the Mediterranean Sea. The numerical study found that, before the Bora event of 13–17 January 2001, the water column of the northern Adriatic Sea was stratified by salinity, and the temperature was already cooler at the surface and over the shallower shelf region. The pre-Bora circulation of the northern Adriatic Sea was relatively weak and baroclinic with maximum surface currents occurred near the Italian coast. During the Bora event, the water column was well mixed in the most of coastal region of the northern Adriatic Sea. The atmospheric cooling produced colder water over the northern and western Adriatic Coast. The circulation of the northern Adriatic Sea was barotropic and dominantly wind driven, with maximum current speed of about 1 m s−1. The numerical study also demonstrated that the Bora event decreased the heat content of the water column with an area averaged value of 205 W m−2 over the shallow northern shelf. It was concluded that the heat budget of the northern Adriatic Sea during the Bora event was a balance between the surface heat loss, horizontal net heat inflow and resulting heat content decrease. The horizontal advection played a particularly important role in controlling the water temperature change over the shallower northern shelf.  相似文献   

13.
It is demonstrated that weakened wind mixing and strengthened water column stratification resulted in the anomalously low sea surface chlorophyll in the northern South China Sea during the 1997–1998 El Niño event. Remotely sensed sea surface temperature, wind and chlorophyll, which were validated by shipboard observations at the SouthEast Asian Time-series Study (SEATS) station (18°N, 116°E) in the northern South China Sea (SCS) provided the basis for this study. During the 1997–1998 winter at the SEATS station, the sea surface temperature was elevated by about 2 °C above the climatological mean, while the wind speed of the northeast monsoon was reduced from a climatological mean of 9.4 to 6.8 m/s. The concentration of surface chlorophyll-a dropped from 0.2 to 0.1 mg/m3. The monthly area-averaged integrated primary production estimated for the northern SCS area (112–119°E, 15–21°N) was reduced by about 40% of the normal winter value. Under the anomalously high sea surface temperature and weak monsoon, the mixed-layer depth would have been reduced from an average of 65 to 45 m and the nutrients in the mixed layer would have been reduced by half, according to observations at the SEATS station in more recent years. During the 1997–1998 El Niño event, the onset of warming in the northern SCS lagged behind that in the eastern equatorial Pacific by about 5 months and lingered for 11 months. This course of change resembled that of the western Pacific warm pool region. However, contrary to the northern SCS, the sea surface chlorophyll was enhanced in the warm pool region during the event, probably mainly because of the uplifted nutricline. Unlike the eastern equatorial Pacific, the dramatic recovery of biological production did not happen in the SCS in the summer of 1998. These distinctive biogeochemical responses reflect fundamental differences between the SCS and the equatorial Pacific in terms of upper water column dynamics.  相似文献   

14.
以盐城海岸带为研究对象,分析了Cr,Cu,Fe,Mn,Ni,V,Zn,Co,Cd 9种重金属元素在二维空间分布的异质性,以及沿着经纬度方向的梯度变化,并通过主成分分析和元素区域背景值的分析,探讨重金属元素的可能来源.结果表明:自陆向海存在着逐渐减少的格局梯度,而在区域的北部分布着重金属元素的峰值;二维分析的结果表明,重...  相似文献   

15.
Recently, as oil exploitation has become focused on deepwater slope areas, more multi-channel high resolution 2D and 3D seismic data were acquired in the deepwater part of the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data and coherence time slice, RMS and 3D visualization, a series of deepwater channels were recognized on the slope that probably developed in the late Quaternary period. These channels trend SW–NE to W–E and show bifurcations, levees, meander loops and avulsions. High Amplitude Reflections (HARs), typical for channel–levee complexes, are of only minor importance and were observed in one of the channel systems. Most of the detected channels are characterized by low-amplitude reflections, and so are different from the typical coarse-grained turbidite channels that had been discovered worldwide. The absence of well data in the study area made it difficult to determine the age and lithology of these channels. Using a neighboring drill hole and published data about such depositional systems worldwide, the lithology of these channels is likely to be dominated by mudstones with interbedded thin sandstones. These channels are formed by turbidity currents originated from the little scale mountain river of mid-Vietnam in SW direction and were probably accompanied by a relative sea level drop in the last glacial age. These channels discovered on the northern South China Sea slope are likely to be fine-grained, mud-dominant and low N:G deposits in a deepwater paleogeographic setting.  相似文献   

16.
2015—2018年, 国家自然科学基金重大研究计划“南海深海过程演变”的重点支持项目“南海东部马尼拉俯冲带深部结构探测与研究”以马尼拉俯冲带为研究重点, 从深部地球物理的角度探索南海形成演化史与运行规律。项目执行期间, 在国家基金委共享航次协助下, 先后开展和参与5次综合地球物理探测, 共投放海底地震仪(Ocean Bottom Seismometer, OBS)台站73台次, 海底电磁仪(Ocean Bottom ElectroMagnetometers, OBEM)仪器5台次, 累积放炮达13872炮, 成功获得了60台OBS数据和5台OBEM数据。同时, 取得了一系列创新性研究成果: (1)基于人工地震探测及天然地震层析成像结果, 确定南海东北部的地壳属性为受到张裂后期岩浆活动影响的减薄陆壳(12~15km), 划分了南海北部陆缘洋陆边界(Continent-Ocean Boundary, COB); (2)根据多道地震反射剖面, 划分了马尼拉俯冲带北部增生楔前缘的精细结构; (3)圈定了南海停止扩张时洋壳范围; (4)初步构建了南海与菲律宾海板块构造演化模型。本项目为重大研究计划“南海深海过程演变”核心科学问题(海底扩张的年代与过程)提供了实质性的证据, 同时为南海构造演化生命史的“骨架”提供了重要的基础数据, 具有深远的科学意义。  相似文献   

17.
New data acquired on the slow spreading Mid-Atlantic Ridge during several Legs of the Ocean Drilling Programme (ODP) give evidence that, in many places, serpentinized peridotites constitute the upper oceanic crust in the vicinity of rift valley. This discovery contradicts the classical view on the formation of oceanic crust at the ridge axis, which postulates that only basalts constitute the upper oceanic crust.The magnetic properties of 57 samples of such serpentinized peridotites, collected at five ODP sites in the Atlantic Ocean, have been analyzed in order to examine the origin and evolution of their natural remanent magnetization (NRM). All samples are characterized by NRM (average value about 3.5 A/M) comparable with NRM of altered oceanic basalts. Average Q-ratio (NRM to induced magnetization ratio) was about 2.The results reported here give evidence that serpentinization is a complex and irregular process. The local concentration of magnetite is determined by magnetostatic interaction between magnetic grains rather than volume concentration of magnetite. This local concentration, which represents the degree of serpentinization, affects the NRM value. The domain structure of magnetite grains developed during serpentinization is controlled by the degree of serpentinization.Experimental data show that original remanence of serpentinites exposed in the upper oceanic crust may contribute to the oceanic magnetic anomalies. In particular, serpentinites with magnetite of pseudodomain size represent a very probable source for magnetic anomalies. It is however unlikely that such ODP serpentinites systematically contribute to the oceanic magnetic lineations.  相似文献   

18.
Prior to extension of the lithosphere in the Eurasia Basin, the Yermak Plateau was an element of the Eurasian Arctic margin. Extension of the Barents Sea shelf culminated gradually in rifting of the continental crust with separation of this block from the continent during Chrons C25r?C26n (57.656?59.237 Ma ago) and emplacement of numerous basic dikes, which could be responsible for the formation of high-amplitude magnetic anomalies on the Yermak Plateau. The investigation included reconstruction of axes in the breakup zones along peripheral continental fragments of Spitsbergen with determination of the Euler poles and angles of rotation, which describe the kinematics of this process. It is revealed that the difference between depths of conjugate isobaths can be as large as many tens of meters, which reflects the nonuniformly scaled slide of peripheral areas of the continental crust along the plane of the crustal-penetrating fault and, correspondingly, their different subsidence during rifting.  相似文献   

19.
The seafloor spreading of the South China Sea (SCS) was previously believed to take place between ca. 32 and 15 Ma (magnetic anomaly C11 to C5c). New magnetic data acquired in the northernmost SCS however suggests the existence of E–W trending magnetic polarity reversal patterns. Magnetic modeling demonstrates that the oldest SCS oceanic crust could be Late Eocene (as old as 37 Ma, magnetic anomaly C17), with a half-spreading rate of 44 mm/yr. The new identified continent–ocean boundary (COB) in the northern SCS generally follows the base of the continental slope. The COB is also marked by the presence of a relatively low magnetization zone, corresponding to the thinned portion of the continental crust. We suggest that the northern extension of the SCS oceanic crust is terminated by an inactive NW–SE trending trench-trench transform fault, called the Luzon–Ryukyu Transform Plate Boundary (LRTPB). The LRTPB is suggested to be a left-lateral transform fault connecting the former southeast-dipping Manila Trench in the south and the northwest-dipping Ryukyu Trench in the north. The existence of the LRTPB is demonstrated by the different patterns of the magnetic anomalies as well as the different seafloor morphology and basement relief on both sides of the LRTPB. Particularly, the northwestern portion of the LRTPB is marked by a steep northeast-dipping escarpment, along which the Formosa Canyon has developed. The LRTPB probably became inactive at ca. 20 Ma while the former Manila Trench prolonged northeastwards and connected to the former Ryukyu Trench by another transform fault. This reorganization of the plate boundaries might cause the southwestern portion of the former Ryukyu Trench to become extinct and a piece of the Philippine Sea Plate was therefore trapped amongst the LRTPB, the Manila Trench and the continental margin.  相似文献   

20.
Continuous measurements between 0 and 200 m depth were performed every 2 h over two separate periods of four days at a station in the open northwestern Mediterranean Sea (Dyfamed Station) during the Dynaproc cruise in May 1995. Estimates of the daily variations in profiles of temperature, partial pressure of CO2, oxygen, chlorophyll a and nutrients were obtained. The distributions of the various physical and chemical properties were clearly different during the two time series, which were separated by a period of 11 days during which a wind event occurred. The mean daily utilization or production due to biological processes of dissolved inorganic carbon (DIC), nitrate+nitrite and oxygen were calculated along isopycnals using a vertical diffusion model. Between the surface and about 20 m depth, DIC was consumed and O2 released during the two time series while the nitrate+nitrite concentrations as well as supplies were zero. After the wind event, the O2 : C : N ratios of consumption (or production) were, on average, near the Redfield ratios, but during the first time series, the C : N utilization ratio between 20 and 35 m was two to three times that of Redfield stoichiometry and the oxygen release was low. The integrated net community production (NCP) in terms of carbon was equivalent during the two time series, whereas the chlorophyll a biomass was twice as high, on average, during the first time series but did decrease. These results imply that the production systems were different during the two periods. The first time series corresponds to a period at the end of production, due to the nutrient depletion in the euphotic layer. The formation of degradation products of the living material in dissolved organic form is probably important as indicated by the high C : N utilization ratios. The second time series corresponds to a reactivation of the primary production due to the upward shift of nutrients after the wind event.  相似文献   

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