Serpentinized peridotites as a possible source for oceanic magnetic anomalies

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.     

Rock magnetic properties of the fine-grained sediment on the outer shelf of the East China Sea: implication for provenance

The rock magnetic properties of surface sediments from the fine-grained depositional area on the outer shelf of the East China Sea, an area surrounded by sands, were investigated with a view to providing information on the sediment provenance. Multiple magnetic parameters such as magnetic susceptibility, anhysteretic remanent magnetization, saturation remanent magnetization (SIRM), coercivities of SIRM, and S ratios (relative abundance of low-coercivity magnetic minerals) were measured for all 179 surface samples, and partial representative samples were examined for their magnetic hysteresis parameters, temperature dependence of magnetic susceptibility and X-ray diffraction spectra. Our research indicates that the magnetic mineralogy is dominated by magnetite with a small amount of hematite and is primarily of a pseudo-single-domain to multidomain nature with a detrital origin. The magnetic concentration in the study area generally decreases to the east or southeast accompanied by magnetic particle fining to the east or northeast. The geographic pattern of magnetic properties is most reasonably explained by a major source of sediment jointly from the erosion of the old Huanghe River deposit and the discharge of the Changjiang River. The rock magnetic data facilitate understanding of the transport mechanism of fine-grained sediments in the outer shelf of the East China Sea.     

Comparison between the magnetic and petrological characteristics of the peridotites from the gorringe ridge and the peridotites from the mid-ocean ridges

The purpose of our work was to obtain the most possible detailed information about the composition, concentration, and structural features of the magnetic minerals contained in the rock to reveal the differences in the magnetic properties of the peridotites under various circumstances of the mantle magmatism and different conditions of metamorphism. To do this, we examined and analyzed the magnetic and petrographic characteristics of four collections of oceanic and alpinotype spinel peridotites. The main object for comparing the magnetic characteristics was the Gorringe ridge, which lies in the eastern part of the Atlantic Ocean. The peridotite samples from the Gorringe ridge differ from the other collections in many magnetic parameters: I _n , χ, Q, I _rs /I _s , H _c , H _cr , and H _m . The principal question of our work was to clarify the nature of the Earth’s crust where the Gorringe ridge formed. This subject was studied many times in the literature, but the researchers did not reach a common opinion. In accordance with our data, the spinel peridotites from the Gorringe ridge represent a subcontinental lithosphere mantle of the Iberian continental margin. During the metamorphism, the formation of magnetite occurred in the peridotites of the Gorringe ridge in several stages and had a regressive character. Our investigations explain the results of the analysis of the anomalous magnetic field over the Gorringe ridge, which is characterized by sharp roughness and high intensity of the local signchanging anomalies.     

A 23 Myr magnetostratigraphic time framework for Site 1148, ODP Leg 184 in South China Sea and its geological implications

The Ocean Drilling Program (ODP) Leg 184 recovered a continuous sedimentary sequence since Miocene at Site 1148 in the northern South China Sea (SCS), which provided a unique record for better understanding the tectonic, environmental and climatic evolution of the SCS. In this study, we report a detailed paleomagnetic study on 881 discrete samples from the upper 460 m cores of Hole 1148A through stepwise alternating field demagnetization (AF) or thermal demagnetization (TD). Rock magnetic results from isothermal remanent magnetization (IRM) acquisition, temperature dependence magnetic susceptibility (κ-T) and hysteresis loop parameters of representative samples indicate that the main magnetic mineral is low coercivity pseudo-single domain magnetite. The characteristic remanent magnetization (ChRM) with moderate downward or upward inclinations was identified after removing a low-stability drilling-induced remanence with steep downward inclinations. A new 23 Myr magnetostratigraphic time framework was established by correlating the magnetic polarity sequence to the Geomagnetic Polarity Time Scale 2012 (GPTS 2012; Ogg, 2012) in combination with biostratigraphic data. The mean sediment accumulation rates for lithostratigraphic Units V and IV, Unit III and lower Unit II, upper Unit II, Unit IB and Unit IA are calculated as ∼20.8 m/Myr, ∼15–17 m/Myr, 24.18 m/Myr, 17.07 m/Myr and 49.01 m/Myr, respectively.     

Evaluating the timing of hydrocarbon generation in the Devonian Duvernay Formation: paleomagnetic,rock magnetic and geochemical evidence

The physical and chemical changes associated with the thermal maturation of organic-rich shale have affected the paleomagnetic and rock magnetic characteristics of the Devonian Duvernay Formation in the Western Canada Sedimentary Basin. This formation has several lithofacies that correspond to deposition in platform, slope and deeper water settings under varied redox conditions. Shale, laminated mudstone and some massive mudstone facies show evidence of magnetic changes associated with maturation but wackestone, packstone and some massive mudstone facies appear to be unaffected by the process. Rock magnetic evidence suggests that thermal maturation induces a change in the magnetization carrier from magnetite and hematite to solely magnetite.The packstone and wackestone facies commonly show a reversed characteristic magnetization with a paleopole at 194°E, 70°N (A₉₅=13.2) of Late Cretaceous-age. Shale and laminated mudstone facies in immature areas of the basin have inclination-only characteristic remanent magnetization (ChRM) means that range from 55 to 67 °C, requiring a pre-Cretaceous magnetization age. Shale and laminated mudstone facies in mature areas of the basin have a much steeper ChRM in direction ranging from 77 to 83 °C. Their very steep nature suggests that step demagnetization has not completely removed a drilling-induced remanence in some wells.     

A detailed geochemical and rock magnetic study of dredged basalt from the Sheba Ridge,Gulf of Aden

We report the results of petrological, geochemical and rock magnetic studies of basalt dredged from the eastern end of the west Sheba Ridge during cruise 11/1979 of R. R. S. Shackleton to the Western Gulf of Aden. The ridge is part of the Red Sea-Gulf of Aden spreading axis and the basalts are olivine tholeiites. The abundances of some elements are characteristic of normal MORB (mid-ocean ridge basalt) but other elemental abundances suggest affinites with transitional-type MORB.The observed magnetic properties are interpreted in terms of the composition, concentration and microstructure of the magnetic mineral fraction by recourse to the available data on synthetic analogues. The analysis has been carried out in greater detail than has been attempted in previous magneto-petrological studies. It appears that submarine weathering of the magnetic minerals (maghemitization) brings about not only the expected change in composition but also a fall in concentration of the magnetic fraction. This could result from the removal-of-iron oxidation mechanism operating in the submarine environment. It is also found that the fall in remanence with increasing degree of maghemitization is not explicable in terms of the change in composition and concentration of the magnetic minerals. A further influence—probably microstructural change—significantly reduces the remanence intensity.     

Factors influencing the magnitude of phytoplankton primary production in a high-energy surf zone

The main factors influencing phytoplankton primary production in the surf zone of the Sundays River Beach, Algoa Bay have been characterized. These factors include cell concentration, chlorophyll concentration, irradiance, temperature and salinity. Good relationships have been obtained between cell concentration, chlorophyll concentration and primary production. The P-I curves showed dependence on temperature with a linear regression between temperature and I_k values. Light saturation was shown to occur between 300 and 510 μmol m⁻² s⁻¹ at normal field temperatures. T_max and T_min were found to be 34°C and 0°C, respectively; P_max was 25°C. Salinity had a marked effect on primary production with S_max occurring at 60 ppt and an extrapolated S_min at 0 ppt. P_max was found to occur at 30 ppt.     

Gulf of Mexico sediment sources and sediment transport trends from magnetic susceptibility measurements of surface samples

The magnetic properties from 200 trigger core-top and Van Veen grab sediment samples recovered from throughout the Gulf of Mexico have been analyzed and used to characterize sediment source and flow pattern distributions. Magnetic parameters included are anhysteretic remanent magnetism (ARM) and magnetic susceptibility (MS) measurements. Results from these measurements are compared to previously determined calcium carbonate percentages, and clay and hematite influx trajectories into the Gulf of Mexico for the same samples reported by Balsam and Beeson [Balsam, W.L. and Beeson, J.P., 2003. Sea-floor sediment distribution in the Gulf of Mexico, Deep-Sea Res. I, 50, 1421–1444.]. The ARM results give an estimate of magnetic grain size distributions, and by analogy, grain size distributions in general, whereas MS patterns show high detrital sediment accumulation zones within the Gulf. The dominant influx of modern high susceptibility sediment into the Gulf of Mexico appears to originate from the Red River, flow into Atchafalaya River Basin and out into the Gulf from Atchafalaya Bay, with significant additional contributions from the Mississippi River through the Southwest Pass of the Mississippi River Delta. This material then moves across the continental shelf and down through the Mississippi Canyon into the deep Gulf where it is redistributed at depths > 3600 m. The eastern shelf margins in the Gulf, offshore from Alabama and Florida, are accumulating calcite- or quartz-rich medium to fine-grained sediment that has a very low or diamagnetic MS signature. From the Louisiana to Texas Gulf coast margins, MS is moderate to high, suggesting a river influx of magnetic constituents from the volcanic fields in New Mexico, and from igneous and metamorphic sources in the Mississippi Basin. Offshore from western Mexico, the MS is high to moderate, but the Yucatan Shelf margin is characterized by low to diamagnetic MS values due to sediment dominated by calcite sands and oozes, a trend that continues to the east onto the West Florida Shelf. Additional measurements of samples collected in association with sites characterized by hydrocarbon seepage exhibit anomalously low MS values. The samples from the lower shelf and slope areas are typified by iron reduction by bacterial organisms in these samples. These results produce anomalous localized lows in the MS trends observed.     

Extension of the Narmada — Son lineament on the continental margin off Saurashtra,Western India as obtained from magnetic measurements

Magnetic total intensity values and bathymetric data collected on the continental margin off Saurashtra were, used to prepare magnetic anomalies and bathymetric contour maps. The magnetic anomalies are considered to have been caused by the Deccan Trap flood basalts which underlie the Tertiary sediments. Interpretation of the magnetic data using two-dimensional modelling method suggests that the magnetic basement is block faulted and deepens in steps from less than 1.0 km in the north to about 8.0 km towards the southern portion of the study area. The WNW-ESE trending faults identified in the present study extend across the Saurashtra continental margin between Porbandar and Veraval and appear to represent a major linear tectonic feature. The relationship of these fault lineaments with the regional tectonic framework have been discussed to indicate that they conform better as the northern boundary faults of the Narmada rift graben on the continental margin off Saurashtra.     

Rock magnetic records of the sediments of the eastern Arabian Sea: Evidence for late Quaternary climatic change

Rock-magnetic measurements along with grain size, acid-insoluble residue (AIR), organic carbon (OC), CaCO₃ and δ¹⁸O of the planktonic foraminifers of the sediments were determined for 15 gravity cores recovered from the western continental margin of India. Magnetic susceptibility (MS) values in the surficial sediments reflect the land-derived input and, in general, are the highest in terrigenous sediment-dominated sections of the cores off Saurashtra–Ratnagiri, followed by the sediments off Indus–Gulf of Kachchh and then Mangalore–Cape Comorin.

The down-core variations in mineral magnetic parameters reveal that the glacial sediments off the Indus are characterized by low MS values/S-ratios associated with high AIR-content, low OC/CaCO₃ contents and relatively high δ¹⁸O values, while those off SW India are characterized by low MS values/high S-ratio% associated with low AIR content, and relatively high OC, CaCO₃ and δ¹⁸O values. Conversely, the Early Holocene sediments of all cores are characterized by high MS values/S-ratio% associated with high AIR content, low OC, CaCO₃ contents and gradually decreased δ¹⁸O values. These results imply that during the Last Glacial Maximum (LGM), the cores off northwestern India received abundant continental supply leading to the predominance of eolian/fluvial sedimentation. In the SW region the influence of hinterland flux is less evident during this period, but convective mixing associated with the NE monsoon resulted in increased productivity. During the early Holocene intense SW monsoon conditions resulted in high precipitation on land, which in turn contributed increased AIR content/MS values in the continental margin sediments. A shallow water core off Kochi further suggests that the intense SW monsoon conditions prevailed until about 5 ka. The late Holocene organic-rich sediments of the SW margin of India were, however, subjected to early diagenesis at different intervals in the cores. Therefore, caution is needed when interpreting regional climatic change from down-core changes in sediment magnetic properties.     

Grain size and depositional environment as predictors of permeability in coastal marine sands

More than half of the surface sediments covering the continental shelves are sandy, which may permit substantial sub-seafloor pore water advection. Knowledge of sediment permeability is required for quantifying advection and associated solute transport, but studies of marine sediments typically report grain size analyses rather than permeability. Here data from 23 studies were examined to determine the range in permeabilities reported for sublittoral marine sands and to assess the utility of permeability–grain size relationships in this setting. In the resulting database, the permeability of small (∼30 cm) undisturbed cores collected from the sea floor all fell between 2 × 10⁻¹² and 4 × 10⁻¹⁰ m², a range where advective transport induced by wave and current action should be pervasive. The range in grain size was very similar for near-shore (<10 m water depth) and continental shelf samples (>10 m water depth), but the permeability of the continental shelf samples was consistently lower for the same median grain size. Empirical permeability–grain size relationships generated a poor fit (r² = 0.35) for the aggregate data, but separate relationships for near-shore and continental shelf samples were significantly better, r² = 0.66 and 0.77, respectively. Permeability–grain size relationships thus may be useful for sublittoral sands, but a larger database needs to be accumulated before reliable fit parameters and variability can be predicted. Thus it is recommended that permeability be routinely determined when characterizing sedimentological properties of marine sand deposits. Concurrent determinations of sediment bulk density and porosity may further improve estimates of permeability.     

冲绳海槽中部火山事件的环境磁学记录

对冲绳海槽中部DH180岩心沉积物进行了AMS14C测年、磁化率、非磁滞剩余磁化强度和饱和等温剩余磁化强度等试验,在含火山物质高的两段层位各磁学参数发生了异常变化。在57.5~82.5和212.5~252.5 cm深度处磁化率、非磁滞剩余磁化强度和饱和等温剩余磁化强度表现出增大的变化趋势,饱和度则出现相对减小的变化趋势。根据磁学参数异常变化的位置和测年数据,对所记录的两次火山事件发生年代进行了计算,它们分别为距今约12.6和6.2 ka,最近一次火山喷发的年代与K-Ah火山喷发的年代基本一致。     

Semi‐diurnal tides in Cook Strait

The phases and amplitudes of the M₂ and S₂ constituents of tidal elevation in Coiok Strait may be adequately described as an open mouth reflection of tidal waves advancing from the east and west coasts of New Zealand and reflecting near the latitudes of Titahi Bay and Cape Campbell (a distance of approximately 60 km). Other reflection conditions give amplitude and phase distributions different from those observed. Best fits to the observed phases and amplitudes for the M₂ tide elevation arise from non‐rotational one‐dimensional solutions which allow for the bathymetry. If rotation is introduced in the form of Kelvin Wave solutions, the result is a two‐dimensional phase‐distribution pattern qualitatively similar to that observed, with the change in phase more rapid and the tidal amplitude smaller on the eastern than on the western side of Cook Strait.

The solution for the S₂ tide also fits closest to the observed tidal elevations for the non‐rotational one dimensional solution, but the introduction of rotation leads to a two‐dimensional phase‐distribution pattern qualitatively similar to that observed. The ratio of the amplitude of the wave (£) advancing from the northwest into Cook Strait to that (A) advancing from the southeast is substantially greater for the S₂ (B/A = 11) than for the M₂ (B/A = 2) tidal constituent and leads to the region of most rapid phase change for S₂ being shifted further to the south than that for M₂.     

Oceanic and Continental Mantle Fragments in Ophiolites of the Northwestern Pacific Margins: Composition,Age, and Genesis of Sakhalin Peridotites

The paper presents the results from a study of original and published data on the chemical composition and age of mantle peridotites from Sakhalin Island ophiolites. The material and genetic proximity of peridotites from the Berezovsky and Shelting plutons, on the one hand, and mélange zone serpentinites, on the other, have been established. In composition and Fe₂O₃ and MgO variations, Sakhalin peridotites differ radically from those of the Northeast Asia ophiolite complexes (Krasnogorsky Massif, Karaginsky Island Massif, etc.), which are fragments of Pacific Plate mantle. Conversely, Sakhalin peridotites have a subcontinental genesis and are compositionally close to xenoliths of lherzolites from Hankai Microcontinent mantle (southern Sikhote-Alin). The rythmics of alternation in compression and expansion at the margin of the Asian continent in the last 180 million years, caused by cyclical changes in the Pacific spreading rate, have been considered. According to data obtained by U-Pb dating of zircons, the formation of the Berezovsky Massif took place 169–154 Ma ago during Jurassic expansion of the continental margin. Matching age and composition data demonstrate that the Sakhalin ophiolites formed within the marginal sea basin during riftinduced destruction of the periphery of the Hankai Craton. The assumed tectonic setting was close to that reconstructed for the Jurassic Josephine ophiolites of the California margin of the North American continent. The continental genesis of the studied ophiolites agrees with the age and tectonic mode of ophiolite formation for Sakhalin Island.     

Rifting to Spreading Process along the Northern Continental Margin of the South China Sea

Understanding the development from syn-rift to spreading in the South China Sea (SCS) is important in elucidating the western Pacific's tectonic evolution because the SCS is a major tectonic constituent of the many marginal seas in the region. This paper describes research examining the transition from rifting to spreading along the northern margin of the SCS, made possible by the amalgamation of newly acquired and existing geophysical data. The northernmost SCS was surveyed as part of a joint Japan-China cooperative project (JCCP) in two phases in 1993 and 1994. The purpose of the investigation was to reveal seismic and magnetic characteristics of the transitional zone between continental crust and the abyssal basin. Compilation of marine gravity and geomagnetic data of the South China Sea clarify structural characteristics of its rifted continental and convergent margins, both past and present. Total and three component magnetic data clearly indicate the magnetic lineations of the oceanic basin and the magnetic characteristics of its varied margins. The analyses of magnetic, gravity and seismic data and other geophysical and geological information from the SCS led up to the following results: (1) N-S direction seafloor spreading started from early Eocene. There were at least four separate evolutional stages. Directions and rates of the spreading are fluctuating and unstable and spreading continued from 32 to 17 Ma. (2) The apparent difference in the present tectonism of the eastern and western parts of Continent Ocean Boundary (COB) implies that in the east of the continental breakup is governed by a strike slip faulting. (3) The seismic high velocity layer in the lower crust seems to be underplated beneath the stretched continental crust. (4) Magnetic anomaly of the continental margin area seems to be rooted in the uppermost sediment and upper part of lower crust based on the tertiary volcanism. (5) Magnetic quiet zone (MQZ) anomaly in the continental margin area coincides with COB. (6) The non-magnetic or very weakly magnetized layer is probably responsible for MQZ. One of the causes of demagnetization of the layer is due to hydrothermal alteration while high temperature mantle materials being underplated. Another explanation is that horizontal sequences of basalt each with flip-flop magnetization polarity cancel out to the resultant magnetic field on the surface. We are currently developing a synthetic database system containing datasets of seismicity, potential field data, crustal and thermal structures, and other geophysical data to facilitate the study of past, contemporary and future changes in the deep sea environment around Japan; i.e. trench, trough, subduction zones, marginal basins and island arcs. Several special characteristics are an object-oriented approach to the collection and multi-faceted studies of global data from a variety of sources.     

Estimation of mean grain size of seafloor sediments using neural network

The feasibility of an artificial neural network based approach is investigated to estimate the values of mean grain size of seafloor sediments using four dominant echo features, extracted from acoustic backscatter data. The acoustic backscatter data were collected using a dual-frequency (33 and 210 kHz) single-beam, normal-incidence echo sounder at twenty locations in the central part of the western continental shelf of India. Statistically significant correlations are observed between the estimated average values of mean grain size of sediments and the ground-truth data at both the frequencies. The results indicate that once a multi-layer perceptron model is trained with back-propagation algorithm, the values of mean grain size can reasonably be estimated in an experimental area. The study also revealed that the consistency among the estimated values of mean grain size at different acoustic frequencies is considerably improved with the neural network based method as compared to that with a model-based approach.     

New Bathymetry and Magnetic Lineations Identifications in the Northernmost South China Sea and their Tectonic Implications

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.     

Occurrence and environmental implications of magnetic iron sulfides in stiff muds from the continental shelf of the East China Sea

Magnetic properties were measured on 370 vibrocores obtained from the outer continental shelf of the East China Sea, with the aim of reconstructing environmental changes during the late Quaternary. High SIRM/χ values (>25 kA m⁻¹) found in stiff muds of late Pleistocene age suggest the presence of magnetic iron sulfides, especially greigite. Framboidal aggregates of greigite were further identified by scanning electron microscopy (SEM) and attached energy dispersive X-ray (EDAX) analyses, as well as powder X-ray diffraction (XRD) analyses of the magnetic separates. The occurrence of magnetic iron sulfides indicates the complex oxidation–reduction of the stiff muds, resulting from exposure and inundation during the last glacial maximum (LGM) and the post-glacial transgression, respectively.     

长江口表层沉积物矿物磁性分区特征及其沉积环境指示意义

本研究旨在揭示现代长江口不同沉积环境铁磁性矿物的分布差异,寻找有效识别河口-陆架沉积环境的磁学指标,以便更好地将环境磁学应用于河口古环境研究。在长江口及邻近陆架的6个沉积环境:汊道、拦门沙、三角洲前缘斜坡、前三角洲、前三角洲-陆架过渡区和残留砂区,采集表层沉积物样品,进行粒度和磁性测量。结果显示,χ和SIRM在汊道和拦门沙呈现显著高值,HIRM、χfd%、χARM、χARM/χ和χARM/SIRM在前三角洲和前缘斜坡呈现显著高值,反映了陆源物质输运距离和河口沉积动力对磁性矿物分布的控制作用。因此,参数组合HIRM、χARM、χARM/χ和χARM/SIRM可用于识别全新世地层前缘斜坡和前三角洲-陆架;参数组合χ、SIRM和S-20mT可尝试用于识别汊道和拦门沙环境。     

Reduced magnetization produced by increased methane flux at a gas hydrate vent

Magnetic susceptibility measurements on near-surface sediment cores from the North Cascadia accretionary sedimentary prism show that seismic blanking or wipe-out zones in the upper few hundred metres of sediments are associated with a prominent low magnetic susceptibility signature. Seismic blanking and low magnetization are both attributed to high upward methane flux within a vent zone, as evidenced by the presence of massive gas hydrate within the cores. Sedimentological analysis of these cores also reveals the presence of authigenic pyrite within the areas of magnetic susceptibility lows. This phenomenon is suspected to be produced by the reducing environment associated with the high upward methane flux and increased bacterial activity within the topmost sediments, resulting in diagenesis of highly magnetic detrital minerals such as magnetite into nearly non-magnetic pyrite. These low magnetic susceptibility zones may produce magnetic anomalies with a magnitude of 10–35 nT near the seafloor. Such anomalies might be detected using high-resolution near-bottom magnetometers to provide a means of mapping zones of methane venting.