High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone

We recorded high-resolution seismic-reflection data in the northern Gulf of Mexico to study gas and gas-hydrate distribution and their relation to seafloor slides. Gas hydrate is widely reported near the seafloor, but is described at only one deep drill site. Our data show high-reflectivity zones (HRZs) near faults, diapirs, and gas vents and interbedded within sedimentary sections at shallow depth (<1 km). The HRZs lie below the gas-hydrate-stability zone (GHSZ) as well as within the zone (less common), and they coincide with zones of shallow water-flows. Bottom simulating reflections are rare in the Gulf, and not documented in our data.We infer HRZs result largely from free gas in sandy beds, with gas hydrate within the GHSZ. Our estimates for the base BHSZ correlate reasonably with the top of HRZs in some thick well-layered basin sections, but poorly where shallow sediments are thin and strongly deformed. The equivocal correlation results from large natural variability of parameters that are used to calculate the base of the GHSZ. The HRZs may, however, be potential indicators of nearby gas hydrate. The HRZs also lie at the base of at least two large seafloor slides (e.g. up to 250 km²) that may be actively moving along decollement faults that sole within the GHSZ or close to the estimated base of the GHSZ. We suspect that water/gas flow along these and other faults such as ‘chimney’ features provide gas to permit crystallization of gas hydrate in the GHSZ. Such flows weaken sediment that slide down salt-oversteepened slopes when triggered by earthquakes.     

我国极地海底观测系统的发展与展望

极地海洋特殊环境对观测技术提出了更高的要求.随着海底观测技术的日趋成熟,极地海底观测系统也得到了一定的发展,但后勤保障的高要求制约了系统的发展,目前广泛使用的仍是潜标系统.受国家高技术研究发展计划、国家海洋公益性行业科研专项经费项目和南北极环境综合考察与评估专项等支持,我国发展了极地在线近岸海洋环境监测系统、极地潜标和极地大型海—气耦合浮标等海床基观测系统,对我国的极地海洋学观测与研究做出了积极贡献.但总体而言,我国的极地海底观测仍未形成真正意义上的网络体系.未来有望在北冰洋太平洋扇区和南大洋普里兹湾率先建立海床基观测体系,系统观测并揭示该海域的环境变化特性.     

大洋钻探对海底热液活动研究的贡献

大洋钻探为海底热液活动研究提供了大量数据资料和样品。在此基础上,通过研究流体-沉积物／岩石相互作用,热、物质通量和流体流动,分析构造对海底热液活动的控制作用,使人们对海底热液循环有所了解,并对深部热液成因石油的特征,海底热液沉积物的空间结构和物质组成,以及深部生物与海底热液活动的关系有了一些认识。未来通过实施IODP,海底热液活动研究将会取得更多新的成果。     

南海北部陆坡块体搬运沉积体系的地震响应与成因机制

块体搬运沉积体系是一种重力作用下形成的深水沉积体系,在全球大陆边缘沉积体系中扮演着重要角色.块体搬运沉积作用不仅造成危害极大的深水地质灾害,而且与海洋天然气水合物和深水油气的形成过程与富集有密切联系.根据新获得的海底地形地貌资料和地震资料,揭示了块体搬运沉积体系识别特征,明确了其分布范围.这些块体搬运沉积体系具有明显的...     

Meter-Scale Seafloor Geodetic Measurements Obtained from Repeated Multibeam Sidescan Surveys

Abstract

We calibrate a technique to use repeated multibeam sidescan surveys in the deep ocean to recover seafloor displacements greater than a few meters. Displacement measurements from seafloor patches (3?km by 20?km) on the port and starboard side of the ship are used to estimate vertical and across-track displacement. We present displacement measurements from a survey of the Ayu Trough southwest of the Marianas Trench using a 12?kHz multibeam. Vertical and across-track displacement errors for the 12?kHz multibeam sonar are typically 0–2?m with RMS uncertainties of 0.25–0.67 m in the across-track and 0.37–0.75 m in the vertical as determined by 3-way closure tests. The uncertainty of the range-averaged sound velocity is a major error source. We estimate that variations in the sound velocity profile, as quantified using expendable bathythermographs (XBTs) during data collection, contribute up to 0.3?m RMS uncertainty in the across-track direction and 1.6?m RMS uncertainty in the vertical direction.     

New insights into the genesis of volcanic-hosted massive sulfide deposits on the seafloor from numerical modeling studies

Numerical computer simulations have been used to gain insight into the evolution of marine hydrothermal systems and the formation conditions of massive sulfide deposits in ancient and modern submarine volcanic terrains. Simulation results have been used to gain a better understanding of the formation of massive sulfide ore deposits, their location, zonation, size, and occurrence in various geotectonic settings.Most hydrothermal fluid discharging at the seafloor exhibits temperatures ranging from 200 °C to about 410 °C and average fluid discharge velocities of 1 to 2 m/s in agreement with seafloor observations. Mass calculations imply that average massive sulfide deposits may form in ~ 5000 years while giant deposits take longer than 5000 years to accumulate; supergiant deposits either need much longer time to form (> 35,000 years) or at least 100 ppm of metal in solution. Results indicate that supergiant deposits may only form in certain geotectonic environments where longevity and preservation potential of the hydrothermal system are high. An additional process (mineral precipitate cap) is proposed here to explain the zinc content of massive sulfide deposits. This cap would prevent the widespread dissolution of anhydrite and the ‘wash-out’ of zinc by subsequent hydrothermal fluid discharge.     

Thirty Years of the Seafloor CORK Borehole Observatories: Development,Applications and Future Perspective

In the past 50 years, we have witnessed remarkable progress in our understanding of the Earth and ocean system, as a result of the internationally integrated deep ocean drilling programs, the Deep Sea Drilling Program (DSDP), the Ocean Drilling Program (ODP), and the Integrated Ocean Drilling Program (IODP). One of the legacies of the deep ocean drilling programs is the development and applications of the CORK, Circulation Obviation Retrofit Kit. Earth and ocean sciences have been shifting from a traditional discontinuous, expeditionary mode toward a mode of sustained in situ observations today. The seafloor CORK observatories offer Earth, ocean and life scientists new opportunities to study multiple, interrelated deep marine subsurface processes, over time scales ranging from seconds to decades. Here, we first provided a concise examination of the development history of the CORKs, then described the first installations of ODP CORKs, the evolution of different models of CORK, and finally, summarized the scientific lessons learned in the installation and operation effort of the CORKs. In the end, we offered our perspectives on using CORKs to study geological, hydrogeological, microbiological, and biogeochemical processes in the deep marine subsurface biosphere, particularly pertaining to China’s efforts in establishing and enhancing its deep-sea and deep-biosphere research and monitoring programs.     

Revised tectonic implications for the magnetic anomalies of the western Weddell Sea

Ten years after the USAC (U.S.–Argentina–Chile) Project, which was the most comprehensive aeromagnetic effort in the Antarctic Peninsula and surrounding ocean basins, questions remain regarding the kinematics of the early opening history of the Weddell Sea. Key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the Weddell Sea and merging the USAC data set with the other magnetic data sets in the region. For this purpose we reprocessed the USAC data set using a continuation between arbitrary surfaces and equivalent magnetic sources. The equivalent sources are located at a smooth crustal surface derived from the existing bathymetry/topography and depths estimated by magnetic inversions. The most critical area processed was the transition between the high altitude survey over the Antarctic Peninsula and the low altitude survey over the Weddell Sea that required downward continuation to equalize the distance to the magnetic source. This procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion.The enhancement of the Mesozoic sequence permits refining the interpretation of the seafloor-spreading anomalies. In particular, the change in shape and wavelength of an elongated positive in the central Weddell Sea suggests that it was formed during the Cretaceous Normal Polarity Interval. The older lineations in the southwestern Weddell Sea are tentatively attributed to susceptibility contrasts modeled as fracture zones. Numerical experimentation to adjust synthetic isochrons to seafloor-spreading lineations and flow lines to fracture zones yields stage poles for the opening of the Weddell Sea since 160 Ma to anomaly 34 time. The corresponding reconstructions look reasonable within the known constraints for the motions of the Antarctic and South America plates. However, closure is not attained between 160 and 118 Ma if independent published East Antarctica–Africa–South America rotations are considered. The lack of closure may be overcome by considering relative motion between the Antarctic Peninsula and East Antarctica until 118 Ma time, an important component of convergence.     

Patterns of stress at midocean ridges and their offsets due to seafloor subsidence

The effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two-dimensional (2D) horizontal stresses arising from isostatically compensated vertical loads. The topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge-parallel tensile stresses. These may predominate over the ridge-perpendicular stresses and explain observations at midocean ridge offsets such as (1) oblique normal faulting at ridge-transform intersections trending up to 60° relative to the ridge axis, and (2) nontransform offsets consisting of structures oriented at 45° relative to the ridge trend. At midocean ridge overlaps, rotation of the ridge-parallel tensile stresses favours rift propagation at more than 45° relative to the ridge trend. It is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets, and as a result large overlaps may eventually start to rotate and evolve into a microplate.