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1.
海底硫化物矿体的三维展布是当前大洋调查急需解决的关键问题,在陆地上电磁方法一直是解决矿产资源三维问题的重要手段之一。2011年我国自主研发了调查海底多金属硫化物的瞬变电磁设备,并于2011年6月在中国大洋第22航次第5航段中的大西洋中脊“贝利珠”热液区( 13.2°S、14.4°W)进行了试验,取得了我国首批海底热液区的瞬变电磁资料。本文对这批资料进行初步分析,阐述一种新的海底硫化物探测方法及其试用结果,得到的视电阻率断面与热液区的实际有很好的对应关系,从2条测线的实测结果来看,设备的探测深度达到海底面以下50~100 m。在本次试用过程中,较好验证了自主研发的深海瞬变电磁设备、获得的数据以及探测深度的可靠性。  相似文献   

2.
Abstract

Palar basin is located between Pennar and Cauvery sedimentary basins of East coast of India in Bay of Bengal, northeast Indian Ocean. Sea floor drill (Wire-line Autonomous Coring System – WACS) with operational capability of up to 3000?m water depth was developed to collect long cores from deep sea floor for geotechnical and ocean resource assessment studies. During the drilling operation it encountered Nummulitic coralline limestone of Lower Eocene age at 18 meters below the seafloor (mbsf) at 850?m water depth indicating carbonated platform presence for the first time at the study region. Bathymetry contour from Naval Hydrography Chart and General Bathymetric Chart of the Oceans (GEBCO) has revealed the presence of shallow mounds from 50 to 200?m depth closure contour near the sampling site at 850?m water depth which might be a submerged carbonated structure. Since, Nummulites are shallow water dwelling fauna (<20?m depth) but its occurrence at 18 mbsf in 850?m water depth is recorded because of the advancement in technology tool for long core sampling by means of sea floor drill.  相似文献   

3.
瞬变电磁法是深海多金属硫化物勘探的有效手段。海底多金属硫化物中高品位的金属组分会引起极强的激电效应,且对瞬变电磁响应产生显著影响。本文通过实验室测量和数值模拟对深海多金属硫化物的激电效应进行了探讨和分析。首先对西南印度洋中脊热液区的岩矿石样品进行了较为系统的电性测试,典型硫化物的复电阻率在频率域有最大达160 mrad相位移动,时间域与频率域的测量结果表明,极化率参数可以很好地区分硫化矿物与围岩。利用Cole-Cole模型对实测复电阻率进行解释,得到复电阻率的特征参数,分析各参数与块状硫化物组分和结构的关系,并根据极化率参数对典型硫化物进行了分类。将典型硫化物的激电参数用于计算层状介质的瞬变电磁响应,计算结果表明,在海底多金属硫化物矿床瞬变电磁法响应的最佳观测时窗内可同时观测到激电效应的影响。虽然在采集时窗晚期瞬变响应发生畸变,但在信号接收早期,激电效应能有效增强瞬变电磁法对深海多金属硫化物的探测能力,为瞬变电磁实测数据提供解释依据。  相似文献   

4.
An electromagnetic sounding system has been developed to map the shallow electrical conductivity structure of the deep sea floor. The instrument consists of a magnetic source and several colinear magnetic receivers forming an array which is towed along the seafloor. The source generates a time varying magnetic field; the shape of the resulting magnetic field waveform at the receivers depends on the electrical conductivity below the seafloor between the receivers and the source. The instrument can be towed systematically over a study area under acoustic transponder or GPS navigation to construct a map of the electrical conductivity. Towing speeds of greater than 1 m s–1 (2 knots) can be achieved without adversely effecting data quality. The instrument is sufficiently robust to survive continual contact with thinly sedimented, abrasive basalt. We present the first results from a deployment in August, 1990 near the Cleft Segment of the Juan de Fuca Ridge along an 8 km track to the west of the spreading center. Unforeseen problems with the instrument restricted the utility of the measurements for constructing detailed vertical conductivity profiles, but the measurements were adequate to determine an average conductivity in the upper 25 m, at more than 70 stations. The conductivity was found to vary from 0.1 to 0.4 S/m along the track.  相似文献   

5.
A distinct porcellanite layer from the Southwest Indian Ridge intercalated in Pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data. This bed was sampled by two piston cores at a water depth of 2615 m. The 3–5 cm thick porcellanite layer appears in the cores at a depth of 6.03 m (Core PS2089-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its characteristic physical properties the porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped with echosounding. The physical index properties, wet bulk density and electrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of porcellanite. The porcellanite has the special characteristic to affect a positive anomaly in resistivity but not in susceptibility. Most marine sediments, in contrast, show a positive correlation of magnetic susceptibility versus electrical resistivity; therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the porcellanite bed. Images from the X-ray CT survey indicate that the porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed.  相似文献   

6.
Abstract

Mineralogical, geochemical and S-isotopic studies were carried out on seafloor massive sulfides (SMS) and hydrothermally altered rocks from the mafic-hosted TaiJi field (including TaiJi-1 and TaiJi-2 sites), which is located on the margin of a nontransform discontinuity (NTD) on the Southern Mid-Atlantic Ridge (SMAR). The main hydrothermal precipitates of TaiJi-1 were massive sulfides, while TaiJi-2 produced a large amount of semi-massive sulfides. Significant rock alteration is an important feature of this field, and large amounts of clay minerals (chlorite) occurred in the semi-massive sulfides. Geochemically, notable negative correlations were identified between rock forming elements (Mg) and major hydrothermal metal elements such as Zn?+?Cu and Co in semisulfides. Such mineralogical and geochemical characteristics, together with low Co/Ni ratio (similar to the surrounding rocks) of the semi-massive sulfides and the unique REE features of some altered rocks, tell us that the prominent mineralization mechanism for these semi-massive sulfides was probably related to rock mineralization. Thus, TaiJi-2 appears to differ significantly from the TaiJi-1 in terms of sulfide mineralization mechanism. Moreover, the TaiJi sulfides are remarkable for large δ34S variation (7.2–15.3%). We suggest that seawater corrosion after sulfides precipitation were responsible for 34S enrichment in sulfides. Furthermore, our work sheds some light on the comparison between TaiJi and other NTD-related fields along the Mid-Atlantic Ridge. We argue that rock mineralization may represent an important mineralization type in NTDs.  相似文献   

7.
Acouso-physical properties of sea floor sediments in the southeast offshore sea area of Hainan Island on the northern continental shelf of the South China Sea are analyzed. In many cruises, conductivity-temperature-depth measurements of seawater, measurements of shallow stratum and side-scan sonar have been made. Acoustic parameters, basic sedimentary parameters, physical-mechanical parameters and 14C age, etc., have been measured. The sediment elastic parameters, including Young's modulus, bulk modulus, constrained modulus, rigidity modulus, Poisson's ratio, Lames constant, etc., have been calculated. Results show that the compression wave velocity of the seafloor sediment in the sea area ranges from 1474–1700 m/s, and there are high and low sound velocity sediment types in the different sea areas; the shear wave velocity is 150–600 m/s; at 100 kHz the sediment sound attenuation is 35–260 dB/m, the sediment density is 1.4–2.0 g/cm3; the sediment porosity is 42–88%. Sound field parameters and describing sound reciprocity between sea and seafloor are described.  相似文献   

8.
A novel autonomous free-fall lander vehicle, with a capability down to 6000 m, was deployed off Cape Verde for studies on bioluminescence in the deep sea. The system was equipped with a high-sensitivity Intensified Silicon Intensified Target (ISIT) video camera, a programmable control-recording unit and an acoustic current meter with depth and temperature sensors. The ISIT lander was used in three modes: (1) free falling at 34 m min−1, with the camera looking downwards at a mesh screen, recording impacts of luminescent organisms to obtain a vertical profile down to the abyssal sea floor, sampling at >100 l s−1; (2) rotating, with the lander on the sea floor and the camera orienting to the bottom current using a servo-controlled turntable, impacts of luminescent organisms carried by the bottom current onto a mesh screen mounted 0.5 m in front of the camera were recorded to estimate abundance in the benthic boundary layer; (3) baited, with the camera focused on a bait placed on the sea floor.Profiles recorded abundance of luminescent organisms as 26.7 m−3 at 500–999 m depth, decreasing to 1.6 m−3 at 2000–2499 m and 0.5 m−3 between 2500 m and the sea floor at 4046 m, with no further detectable significant change with depth. Rotator measurements at a 0.5 m height above the sea floor gave a mean abundance of 0.47 m−3 in the benthic boundary layer at 4046 m and of 2.04 m−3 at 3200 m. Thirty five minutes after the bait was placed on the sea floor at 3200 m, bioluminescent fauna apparently arrived at the bait and produced luminescent displays at a rate of 2 min−1. Moving, flashing light sources were observed and luminescent material was released into the bottom current.  相似文献   

9.
High-resolution, side-looking sonar data collected near the seafloor (100 m altitude) provide important structural and topographic information for defining the geological history and current tectonic framework of seafloor terrains. DSL-120 kHz sonar data collected in the rift valley of the Lucky Strike segment of the Mid-Atlantic Ridge near 37° N provide the ability to quantitatively assess the effective resolution limits of both the sidescan imagery and the computed phase-bathymetry of this sonar system. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are <1 m, statistical analysis of DSL-120 sonar data collected from the Lucky Strike segment indicates that the effective spatial resolution of features is 1–2 m for sidescan imagery and 4 m for phase-bathymetry in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparison of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10–30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is 20 times smaller than that of hull-mounted multibeam at these seafloor depths (2000 m). Comparison of DSL-120 bathymetry with itself on crossing lines indicates that residual depth values are ±20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better navigation and less complex seafloor terrain had residual depth values half those of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly beneath the tow vehicle and also towards the swath edges.Variations in sidescan amplitude observed across the rift valley and on Lucky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct changes in sonar backscatter amplitude were also observed between areas covered with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydrothermal vent chimneys or mounds taller than 2 m and greater than 9 m2 in surface area, can easily be resolved and mapped using this system. These features at Lucky Strike have been confirmed visually using the submersible Alvin, the remotely operated vehicle Jason, and the towed optical/acoustic mapping system Argo II.  相似文献   

10.
Multi-scale reflection seismic data, from deep-penetration to high-resolution, have been analyzed and integrated with near-surface geophysical and geochemical data to investigate the structures and gas hydrate system of the Formosa Ridge offshore of southwestern Taiwan. In 2007, dense and large chemosynthetic communities were discovered on top of the Formosa Ridge at water depth of 1125 m by the ROV Hyper-Dolphin. A continuous and strong BSR has been observed on seismic profiles from 300 to 500 ms two-way-travel-time below the seafloor of this ridge. Sedimentary strata of the Formosa Ridge are generally flat lying which suggests that this ridge was formed by submarine erosion processes of down-slope canyon development. In addition, some sediment waves and mass wasting features are present on the ridge. Beneath the cold seep site, a vertical blanking zone, or seismic chimney, is clearly observed on seismic profiles, and it is interpreted to be a fluid conduit. A thick low velocity zone beneath BSR suggests the presence of a gas reservoir there. This “gas reservoir” is shallower than the surrounding canyon floors along the ridge; therefore as warm methane-rich fluids inside the ridge migrate upward, sulfate carried by cold sea water can flow into the fluid system from both flanks of the ridge. This process may drive a fluid circulation system and the active cold seep site which emits both hydrogen sulfide and methane to feed the chemosynthetic communities.  相似文献   

11.
A mound related to a cold vent in a columnar seismic blanking zone (CSBZ) was formed around site UBGH1-10 in the central Ulleung Basin (2077 m water depth), East Sea, Korea. The mound is 300–400 m wide and 2–3 m high according to multi-beam bathymetry, 2–7 kHz sub-bottom profiler data, and multi-channel reflection seismic data. Seafloor topography and characteristics were investigated using a remotely operated vehicle (ROV) around site UBGH1-10, which is located near the northern part of the mound. The origin of the mound was investigated through lithology, mineralogy, hydrate occurrence, and sedimentary features using dive cores, piston cores, and a deep-drilling core. The CSBZ extends to ∼265 ms two-way traveltime (TWT) below the seafloor within a mass-transport deposit (MTD) unit. Gas hydrate was entirely contained 6–141 m below the seafloor (mbsf) within hemipelagic deposits intercalated with a fine-grained turbidite (HTD) unit, characteristically associated with high resistivity values at site UBGH1-10. The hydrate is commonly characterized by veins, nodules, and massive types, and is found within muddy sediments as a fracture-filling type. Methane has been produced by microbial reduction of CO2, as indicated by C1/C2+, δ13CCH4, and δD4CH analyses. The bowl-shaped hydrate cap revealed at 20–45 ms TWT below the seafloor has very high resistivity and high salinity, suggesting rapid and recent gas hydrate formation. The origin of the sediment mound is interpreted as a topographic high formed by the expansion associated with the formation of the gas hydrate cap above the CSBZ. The lower sedimentation rate of the mound sediments may be due to local enhancement of bottom currents by topographic effects. In addition, no evidence of gas bubbles, chemosynthetic communities, or bacterial mats was observed in the mound, suggesting an inactive cold vent.  相似文献   

12.
Characteristics of two natural gas seepages in the North Sea   总被引：1，自引：0，他引：1  
Two occurrences of active gas seepages are described from the North Sea. The southernmost one, situated above a salt diapir in Norwegian block , has been studied and sampled by use of a remotely operated vehicle (ROV). This seepage consists of about 120 single seeps located within a diameter of 100 m. It is estimated to produce 24 m3 of methane gas per day (at ambient pressure, 75 m water depth). Isotope values of the methane gas and higher hydrocarbon gases in the surrounding seafloor sediments, show that their origin is from a deep seated, thermogenic source. No typical gas-induced erosion features are found on the seafloor at this location, probably due to the lack of very fine grained material.The second occurrence is located in U.K. block (Geoteam, 1984), where the seepage is associated with a very large pockmark depression, measuring 17 m in depth and 700×450 m in width. This depression represents an eroded fine grained sediment volume of 7.105 cubic metres. No detailed inspection or sampling of the gas has been performed here. However seismic reflection anomalies are seen on airgun seismic records at various levels down to a depth of at least 1100 m below seafloor. The seeping gas, possibly mixed with liquids, at this location is therefore also expected to be of a thermogenic origin.  相似文献   

13.
Concentrations of minerals on the sea floor around New Zealand occur in a manner which makes them economically significant as future mineral resources.

Three major environments of interest are beaches, the continental shelf, and the adjacent deep‐sea floor. New Zealand's west coast beaches are well known as mineral resources containing large quantities of iron and titanium ores. Similar concentrations representing fossil beaches are also known from the continental shelf. The deep‐sea floor adjacent to the continental shelf is formed around New Zealand by the New Zealand Plateau, an extensive submarine platform in 500–1,500 m. Terrigenous sedimentation is negligible in this environment where, as a result, pure calcareous oozes are common. Vigorous bottom currents and suitable reducing micro‐environments encourage glauconite formation. In the past, possibly from warmer waters of the early and mid Tertiary, phosphates were precipitated from seawater to form phosphorite nodules, a potential resource of phosphates. During late Tertiary or Quaternary, volcanicity at the Antipodes Islands and on the Macquarie Ridge resulted in the formation of manganese deposits. Manganese minerals also occur in bulk on the floor of the Southwestern Pacific Basin away from the New Zealand Plateau.

The origin, bulk, and significance of these deposits are discussed.  相似文献   

14.
Mud volcanoes, mud cones, and mud ridges have been identified on the inner portion of the crestal area, and possibly on the inner escarpment, of the Mediterranean Ridge accretionary complex. Four areas containing one or more mud diapirs have been investigated through bathymetric profiling, single channel seismic reflection profiling, heat flow measurements, and coring. A sequence of events is identified in the evolution of the mud diapirs: initially the expulsion on the seafloor of gasrich mud produces a seafloor depression outlined in the seismic record by downward dip of the host sediment reflectors towards the mud conduit; subsequent eruptions of fluid mud may create a flat topped mud volcano with step-like profile; finally, the intrusion of viscous mud produces a mud cone.The origin of the diapirs is deep within the Mediterranean Ridge. Although a minimum depth of about 400 m below the seafloor has been computed from the hydrostatic balance between the diapiric sediments and the host sediments, a maximum depth, suggested by geometric considerations, ranges between 5.3 and 7 km. The presence of thermogenic gas in the diapiric sediments suggests a better constrained origin depth of at least 2.2 km.The heat flow measured within the Olimpi mud diapir field and along a transect orthogonal to the diapiric field is low, ranging between 16 ± 5 and 41 ± 6 mW m–2. Due to the presence of gas, the thermal conductivity of the diapiric sediments is lower than that of the host hemipelagic oozes (0.6–0.9 and 1.0–1.15 W m–1 K–1 respectively).We consider the distribution of mud diapirs to be controlled by the presence of tectonic features such as reverse faults or thrusts (inner escarpment) that develop where the thickness of the Late Miocene evaporites appears to be minimum. An upward migration through time of the position of the décollement within the stratigraphic column from the Upper Oligocene (diapiric sediments) to the Upper Miocene (present position) is identified.  相似文献   

15.
文中设计基于可编程逻辑控制器(PLC)的测控平台、伺服加压控制、步进推动控制、多传感器测量、触摸屏交互等功能的温压可控声学测量系统,实现海底沉积物样品的孔隙水自动可控加压,通过声学测量功能单元测量海底沉积物在各个压力下的声速,模拟海底沉积物处于大陆坡2 000 m以浅海底表层任何深度变化时的声学特性测量,得到海底沉积物声速-压力特性,为校正实验室测量数据还原到海底原位测量数据提供一种方法。  相似文献   

16.
海南岛东南外海海底沉积物特征及其声学物理性质研究   总被引：3，自引：1，他引：2  
分析研究了南海北部大陆架西南缘的海南岛东南外海海底沉积物声学物理特性,在多个航次中进行了海底沉积层取样、海水CTD测量、浅地层及旁侧声呐扫测等工作.在实验室里对沉积物样品进行声学参数、沉积学基本参数、物理力学参数和14C年龄测试等分析.根据多尔特曼公式求解出弹性模量、体积弹性模量、压缩系数、切变模量、泊松比和拉梅常数等六项沉积物弹性参数.分析结果表明在该海区海底沉积物的压缩波速为1.474~1.700 m/s,在不同的海区内有高低声速两类性质的沉积物分布;沉积物的切变波速为150~600 m/s;沉积物在100 kHz的声衰减为35~260 dB/m;沉积物的密度为1.4~2.0 g/cm3;沉积物的孔隙度为42%~88%.  相似文献   

17.
Joseph  E. John  Toh  H.  Fujimoto  H.  Iyengar  R.V.  Singh  B.P.  Utada  H.  Segawa  J. 《Marine Geophysical Researches》2000,21(1-2):1-21
Seafloor magnetometer array experiments were conducted in the Bay of Bengal to delineate the subsurface conductivity structure in the close vicinity of the 85°E Ridge and Ninety East Ridge (NER), and also to study the upper mantle conductivity structure of the Bay of Bengal. The seafloor experiments were conducted in three phases. Array 1991 consisted of five seafloor stations across the 85°E Ridge along 14°N latitude with a land reference station at Selam (SLM). Array 1992 also consisted of five seafloor stations across 85°E Ridge along 12°N latitude. Here we used the data from Annamalainagar Magnetic Obervatory (ANN) as land reference data. Array 1995 consisted of four seafloor stations across the NER along 9°N latitude with land reference station at Tirunelveli (TIR). OBM-S4 magnetometers were used for seafloor measurements. The geomagnetic Depth Sounding (GDS) method was used to investigate the subsurface lateral conductivity contrasts. The vertical gradient sounding (VGS) method was used to deliniate the depth-resistivity structure of the oceanic crust and upper mantle. 1-D inversion of the VGS responses were conducted and obtained a 3-layer depth-resistivity model. The top layer has a resistivity of 150–500 m and a thickness of about 15–50 km. The second layer is highly resistive (2000–9000 m) followed by a very low resistive (0.1–50 m) layer at a depth of about 250–450 km. The 3-component magnetic field variations and the observed induction arrows indicated that the electromagnetic induction process in the Bay of Bengal is complex. We made an attempt to solve this problem numerically and followed two approaches, namely (1) thin-sheet modelling and (2) 3-D forward modelling. These model calculations jointly show that the observed induction arrows could be explained in terms of shallow subsurface features such as deep-sea fans of Bay of Bengal, the resistive 85°E Ridge and the sea water column above the seafloor stations. VGS and 3-D forward model responses agree fairly well and provided depth-resistivity profile as a resistive oceanic crust and upper mantle underlained by a very low resistive zone at a depth of about 250–400 km. This depth-range to the low resistive zone coincide with the seismic low velocity zone of the northeastern Indian Ocean derived from the seismic tomography. Thus we propose an electrical conductivity structure for the oceanic crust and upper mantle of the Bay of Bengal.  相似文献   

18.
The dissolved methane (CH4) plume rising from the crater of the blowout well 22/4b in the Central North Sea was mapped during stratified water column conditions. Geochemical surveys were conducted close to the seafloor at 80.3 m water depth, below the thermocline (61.1 m), and in the mixed surface layer (13.2 m) using membrane inlet mass spectrometry (MIMS) in combination with a towed CTD. Seawater was continuously transferred from the respective depth levels of the CTD to the MIMS by using an inline submersible pump. Close to the seafloor a well-defined CH4 plume extended from the bubble release site ∼460 m towards the southwest. Along this distance CH4 concentrations decreased from a maximum of 7872 nmol l−1 to less than 250 nmol l−1. Below the thermocline the well-defined CH4 plume shape encountered at the seafloor was distorted and filaments were observed that extended towards the west and southwest in relation to current direction. Where the core of the bubble plume intersected this depth layer, footprints of high CH4 concentrations of up to 17,900 nmol l−1 were observed. In the mixed surface layer the CH4 distribution with a maximum of up to 3654 nmol l−1 was confined to a small patch of ∼60 m in diameter. The determination of the water column CH4 inventories revealed that CH4 transfer across the thermocline was strongly impeded as only ∼3% of the total water column inventory was located in the mixed surface layer. Best estimate of the CH4 seabed release from the blowout was 1751 tons yr−1. The fate of the trapped CH4 (∼97%) that does not immediately reach the atmosphere remains speculative. In wintertime, when the water column becomes well mixed as well as during storm events newly released CH4 and the trapped CH4 pool can be transported rapidly to the sea surface and emitted into the atmosphere.  相似文献   

19.
The sea floor of Fram Strait, the over 2500 m deep passage between the Arctic Ocean and the Norwegian-Greenland Sea, is part of a complex transform zone between the Knipovich mid-oceanic ridge of the Norwegian-Greenland Sea and the Nansen-Gakkel Ridge of the Arctic Ocean. Because linear magnetic anomalies formed by sea-floor spreading have not been found, the precise location of the boundary between the Eurasian and the North American plate is unknown in this region. Systematic surveying of Fram Strait with SEABEAM and high resolution seismic profiling began in 1984 and continued in 1985 and 1987, providing detailed morphology of the Fram Strait sea floor and permitting better definition of its morphotectonics. The 1984 survey presented in this paper provided a complete set of bathymetric data from the southernmost section of the Svalbard Transform, including the Molloy Fracture Zone, connecting the Knipovich Ridge to the Molloy Ridge; and the Molloy Deep, a nodal basin formed at the intersection of the Molloy Transform Fault and the Molloy Ridge. This nodal basin has a revised maximum depth of 5607 m water depth at 79°8.5N and 2°47E.  相似文献   

20.
On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ13C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ13C (up to –5.2‰) and low δ18O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by 13C-rich CO2 and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.  相似文献   

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