The Early Tertiary chemical remagnetization in the Bakjisan Syncline, Korea: Its geotectonic implications

The Bakjisan Syncline is located in the northwestern part of the Taebaeksan Basin, Korea. New paleomagnetic data for the Upper Carboniferous–Lower Triassic Pyeongan Supergroup from the Pyeongchang area on the west limb of the Bakjisan Syncline have been obtained, and synthesized and compared with previous data from the Jeongseon area on the east limb of the syncline. A total of 350 specimens were collected from 21 sites to clarify the relationship between the spatial distribution of remagnetized areas and the thrust system in the Taebaeksan Basin. The characteristic remanent magnetization (ChRM) isolated from all samples was a remagnetized component acquired after tilting of the strata and carried by various magnetic minerals (magnetite, hematite and pyrrhotite). From rock magnetic studies, electron microscope observations and XRD analyses, the pervasive remagnetization is interpreted to be associated mainly with a fluid-mediated chemical remanent magnetization (CRM). This is consistent with the results of previous work in adjacent areas. The paleomagnetic pole position (88.3°E, 83.9°N, A₉₅ = 4.9°) from the Pyeongan Supergroup in the Bakjisan Syncline indicates that the timing of the remagnetization event is Early Tertiary times (i.e. Paleocene to Eocene) by comparison with reliable paleopoles from the Korean Peninsula. Early Tertiary CRMs are also reported from previous studies of an adjacent region within the northwestern part of the Taebaeksan Basin. In contrast, a primary remanent magnetization was reported in the southeastern part of the Taebaeksan Basin. This implies that the major thrust system (the Gakdong thrust) which separates the two regions has caused them to experience substantially different geologic histories since deposition of the strata. Since many thrusts with NS trend are observed in the northwestern part of the Taebaeksan Basin compared with the southeastern region, it appears that the remagnetizing fluids pervasively penetrated the northwestern part of the basin by utilizing the already well-developed thrust system.     

Site-specific success of three transplanting methods and the effect of planting time on the establishment of Zostera marina transplants

Eelgrass, Zostera marina, is the most abundant seagrass species on the coast of Korea, but most large eelgrass meadows have disappeared since the 1970s due to human-induced disturbance. To restore disturbed seagrass habitats in die-off areas, seagrass transplanting has been attempted using various methods. Here, we assessed the feasibility and efficiency of seagrass transplanting methods depending on the sediment type at the planting site. Additionally, the effect of planting time on the establishment of transplant was examined to determine the optimal season for planting. We conducted an eelgrass transplanting experiment from fall 2003 to summer 2004 at three sites with different sediment types using three planting techniques. The staple method resulted in the highest transplant survival rate (77.1-93.8%) at all three sites, but was labor intensive. Transplanting Eelgrass Remotely with Frame Systems (TERFS) method also resulted in relatively a high survival rate (58.7-69.0%) at all sites. The shell method is a newer eelgrass transplanting method in which oyster shells are used as an anchoring device, and does not require SCUBA diving for subtidal transplanting. The shell method resulted in high survival rates in muddy (81.3%) and silty sediments (76.5%), but remarkably low survival rate in sandy sediments (5.0%). The TERFS, and shell methods reduced underwater labor; thus, these methods is suitable for large-scale seagrass restoration. Eelgrass transplants planted in summer had exhibited significant mortality due to high summer water temperatures. Although transplants planted in fall to spring had relatively high survival rates, transplanting and collection of vegetative shoots are difficult in winter and spring. Therefore, fall was suggested as the most effective transplanting season off the coast of Korea.     

Accounting for high-order correlations in probabilistic characterization of environmental variables,and evaluation

Probabilistic characterization of environmental variables or data typically involves distributional fitting. Correlations, when present in variables or data, can considerably complicate the fitting process. In this work, effects of high-order correlations on distributional fitting were examined, and how they are technically accounted for was described using two multi-dimensional formulation methods: maximum entropy (ME) and Koehler–Symanowski (KS). The ME method formulates a least-biased distribution by maximizing its entropy, and the KS method uses a formulation that conserves specified marginal distributions. Two bivariate environmental data sets, ambient particulate matter and water quality, were chosen for illustration and discussion. Three metrics (log-likelihood function, root-mean-square error, and bivariate Kolmogorov–Smirnov statistic) were used to evaluate distributional fit. Bootstrap confidence intervals were also employed to help inspect the degree of agreement between distributional and sample moments. It is shown that both methods are capable of fitting the data well and have the potential for practical use. The KS distributions were found to be of good quality, and using the maximum likelihood method for the parameter estimation of a KS distribution is computationally efficient.     

Wavenumber of the guided wave supported by a thin resistive layer in marine controlled‐source electromagnetics

This paper discusses the asymptotic behaviour of the electromagnetic fields received on the sea‐bed (target response), as well as the fields distributed inside a thin resistive target, generated by a horizontal electric dipole above the sea‐bed in marine controlled‐source electromagnetics for hydrocarbon exploration. It is found that the guided wave supported by a thin resistive target can be expressed as a single‐mode exponential function. A simple closed‐form expression is derived to relate the single‐mode wavenumber of the guided wave to the model parameters: the resistivity and thickness of the target layer, the sea‐bed resistivity and the frequency. When the air‐wave is removed, the guided wave is dominant among the fields received on the sea‐bed at far offset. Hence the wavenumber of the guided wave can be calculated from the fields measured on the sea‐bed. The closed‐form expression can then be used to invert the target property from the calculated wavenumber and hence, can be considered as a hydrocarbon indicator.     

An englacial image and water pathways of the Fourcade glacier on King George Island, Antarctic Peninsula, inferred from ground-penetrating radar

The distribution of small fractures and water content of the Fourcade glacier on King George Island, Antarctica, was investigated in November 2006 and December 2007 by two ground-based (470-and 490-m-long profiles) and one helicopter-borne (470-m-long profile) ground-penetrating radar (GPR) surveys using 50-, 100-, and 500-MHz antennas. Radar images in the pre-migrated GPR sections are characterized by a smooth ice surface and irregular bed topography, numerous diffraction hyperbolas in the ice and at the g...     

Evaluation of seismic site factors in the Mississippi Embayment. I. Estimation of dynamic properties

Deep deposits of the Mississippi Embayment, overlying the New Madrid Seismic Zone, present unique challenges for the estimation of local site effects on propagated ground motion. Limited information is available on the dynamic properties of these deposits. This paper develops generalized depth dependent modulus degradation and damping curves specific to the embayment deposits. Depth dependent small strain damping is estimated using weak motion recordings during the Enola earthquake, 2001. Modulus degradation and damping curves are further constrained using limited laboratory test data of embayment soils at low confining pressure. At high confining pressures embayment specific data is unavailable and test data from outside the embayment is used in developing the dynamic properties. The representative modulus degradation and damping curves are used in non-linear and equivalent linear one-dimensional site response analyses. A companion paper describes a large-scale probabilistic seismic hazard analysis study in the Mississippi Embayment that integrates non-linear site effects.     

Computer Simulation of Coastal Acoustic Tomography by a Two-Dimensional Vortex Model

Computer simulation of coastal acoustic tomography with four, five, seven and nine stations was applied to two-dimensional vortex fields of horizontal domain 5 km × 5 km. Travel time data obtained in reciprocal directions between all pairs of acoustic stations were analyzed to reconstruct the vortex fields by the stochastic inverse method, reduced to the damped least squares method. The weighting factor appearing in the inverse analysis was determined by applying the L-curve method, in which a point making both the size of estimated error ‖y - Ex‖ and solution ‖x‖ as small as possible in a balance is specified as an optimum. The performance of this method was examined using two-dimensional vortex models which have different wavenumber spectra and adding the random error of different levels to the travel time difference data y. This study suggests that in the selection of the optimal weighting factor the horizontal section of the simulated tidal vortex fields can be well reconstructed by the coastal acoustic tomography system composed of five to seven acoustic stations located in the periphery of the vortex fields. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamics of ecosystems and anthropogenic drivers in the Yellow Sea Large Marine Ecosystem

<正>In the developed world, governance of marine ecological environment is the important part of the national ecological and economic outcome. The Yellow Sea is one of large marine ecosystems in the seas of East Asia, which is an extension of one of the largest continental shelf areas, and forms a huge but shallow sediment body in its south area which is geographically unique in the world. As a region with the most fragile natural environment, unparalleled global ecological significance and...     

Winter distribution of diatom assemblages along the coastline of R. O. Korea in 2010

Diatoms are a globally successful and eukaryotic photosynthetic organism with an ornamented silica external wall. The relationship between their valve morphology and habitat means that diatoms can be used as bioindicators to characterize the aquatic environment. To estimate the differential distribution and diversity of diatom assemblages along the coastal line, we collected phytoplankton samples from 114 coastal sites of waters of R. O. Korea. We applied the unweighted pair-group technique usin...     

Distribution and Acoustic Characteristics of Shallow Gas in the Korea Strait Shelf Mud off SE Korea

Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acoustic turbidity, seepages with plumes in the water column, and seafloor depressions. The acoustic blanking, characterized by strong, consistent top reflection and wipeout below, is most dominant. The seaward edge of the acoustic blanking zone generally coincides with the 100-m water-depth contour, suggesting that the water depth (the pressure) may control the distribution of shallow gas. The acoustic turbidity, characterized by diffuse top reflection, is a dark smear, partially blanking the data below. The seepages with plumes, characterized by vertical smearing and disturbed seafloor, are seen only along the shallowest, landward edge of the acoustic blanking zone. This may suggest that the decreased gas solubility at shallow water depths, caused by the lowered pressure, increases the volume of free gas in the sediments, facilitating the gas escape. The seafloor depressions, interpreted as pockmarks, are accompanied by cone-shaped acoustic masking, which is probably the reflection from a narrow vent of gas. The gas-related acoustic anomalies appear to occur mostly in the upper, recent mud of the KSSM. Neither permeable beds nor faults, which can act as vertical migration pathways for deep thermogenic gas, are evident in the recent mud. We interpret that the bacterial degradation of organic matter in situ is the main source for the gas in the KSSM. The upwelling off SE Korea may be an important source for the increased organic matter in the area.