High‐pressure metamorphism in Taiwan: from oceanic subduction to arc‐continent collision?

Abstract The Taiwan orogen has been the focus of a number of models of mountain building processes, but little attention has been paid to high‐pressure (HP) metamorphic rocks that are found as exotic blocks intermingled within the deepest units of the mountain belt. In this study, we re‐appraise from updated petrological and thermodynamic databases the physical conditions of HP metamorphism in Taiwan, and we combine our findings with available geochronological data to estimate the thermal history of these rocks. Our results indicate that peak metamorphic conditions of ～550 °C and 10–12 kbar have been followed by a rapid isothermal decompression, with exhumation possibly as rapid as burial. These units have subsequently been stored at a pressure of ～3 kbar for ～4–5 Myr, before their final exhumation, probably facilitated by the accretion of passive margin sequences during the Late Cenozoic collision. Therefore, HP units in Taiwan maintain a record of processes at depth from the early stages of oceanic subduction to the present arc‐continent collision.     

Nonlinear Analysis of Local Site Effects on Seismic Ground Response in the Bam Earthquake

The influence of local geologic and soil conditions on the intensity of ground shaking is addressed in this study. The amplification of the ground motion due to local site effects resulted in severe damage to dwellings in the Bam area during the 2003 Bam Earthquake. A unique set of strong motion acceleration recordings was obtained at the Bam accelerograph station. Although the highest peak ground acceleration recorded was the vertical component (nearly 1 g), the longitudinal component (fault-parallel motion) clearly had the largest maximum velocity as well as maximum ground displacement. Subsurface geotechnical and geophysical (down-hole) data in two different sites have been obtained and used to estimate the local site condition on earthquake ground motion in the area. The ground response analyses have been conducted considering the nonlinear behavior of the soil deposits using both equivalent linear and nonlinear approaches. The fully nonlinear method embodied in FLAC was used to evaluate the nonlinear soil properties on earthquake wave propagation through the soil layer, and compare with the response from the equivalent linear approach. It is shown that thick alluvium deposits amplified the ground motion and resulted in significant damage in residential buildings in the earthquake stricken region. The comparison of results indicated similar response spectra of the motions for both equivalent and nonlinear analyses, showing peaks in the period range of 0.3–1.5 s. However, the amplification levels of nonlinear analysis were less than the equivalent linear method especially in long periods. The observed response spectra are shown to be above the NEHRP building code design requirements, especially at high frequencies.     

THE ELECTRICAL CURRENT PATTERN INDUCED BY AN OSCILLATING MAGNETIC DIPOLE IN A SEMI INFINITE CONDUCTIVE THIN PLATE*

In a paper by Koefoed and Kegge (1968), which was based on previous work of Wesley (1958), the electrical current pattern has been derived that is induced by an oscillating magnetic dipole in a semi-infinite thin plate of infinitesimal resistivity. In the present paper, the range of validity of the assumptions, on which the work of Wesley is based, is subjected to a theoretical analysis. It is found that the decisive factor for the validity of Wesley's derivation is the quotient of the square of the penetration depth of the electrical current over the product of the thickness of the plate and a distance that is indicative of the size of the current loops in the plate. Wesley's derivation is shown to be valid only when this factor is negligible. It is also shown that in this condition the imaginary component of the anomaly must be negligible. Model experiments are described in which the electrical current pattern is studied also in the range in which the derivation of Wesley is not valid. The procedure used in these model experiments was to measure the tangential component of the magnetic field strength very close to the metal plate that simulated the conductive dyke. In order to express the results of the measurements in terms of the imaginary to real ratio, these results are compared with an interpretation graph for field measurements that was published by Hedström and Parasnis (1958). It is found that the current pattern in the plate is essentially the same as that which follows from Wesley's derivation, provided that the imaginary to real ratio is less than one third. The measurements do not permit to draw conclusions regarding the current pattern in the plate in conditions that correspond to larger values of the imaginary to real ratio.     

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The expansion of clusters of particles under the action of atmospheric macroturbulence

Summary Making use of a 500 mb numerical forecast some problems of atmospheric large-scale diffusion are investigated. The expansion of various sets of clusters of particles is compared with the predictions based on the similarity theory of homogeneous turbulence. The spreading rates were in the beginning of the period of the forecast in a good agreement with the theoretical prediction, but later on they fell to the values considerable smaller than the predicted ones. The reasons of such behaviour are discussed. Furthermore the difference in zonal and meridional expansion of the clusters is demonstrated, the zonal expansion rates being more than one order of magnitude greater than the meridional ones.This work is part of the research on atmospheric macroturbulence, which has been sponsored partly by the Cambridge Research Laboratories, O.A.R. through the European Office, Aerospace Research, USAF.     

Isotopic variation of fishes in freshwater and estuarine zones of a large subtropical coastal lagoon

We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine spatial variability in isotopic composition and vertical trophic structure across freshwater and estuarine habitats. Nitrogen isotope ratios indicated a smooth gradation in trophic positions among species, with most fishes occupying the secondary and tertiary consumer level. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from −24.7‰ to −17.8‰) were found in freshwater, whereas more enriched signatures (from −19.1‰ to −12.3‰) were obtained within the estuarine zone downstream. Based on our survey of the C₃ and C₄ plants and isotopic values for phytoplankton and benthic microalgae reported for ecosystems elsewhere, we hypothesized that the observed δ¹³C differences in the fish assemblage between freshwater and estuarine sites is due to a shift from assimilating organic matter ultimately derived from C₃ freshwater marsh vegetation and phytoplankton at the freshwater site (δ¹³C ranging from −25‰ to −19‰), to C₄ salt-marsh (e.g. Spartina) and widgeon grass (Ruppia maritima), benthic microalgae and marine phytoplankton at the estuarine sites (from −18‰ to −12‰). Our results suggested that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had depleted δ¹³C values that were characteristic of the upper lagoon. These results suggest that spatial foodweb subsidies can occur within the lagoon.     

Biogeochemical impacts due to mesoscale eddy activity in the Sargasso Sea as measured at the Bermuda Atlantic Time-series Study (BATS)

A comparison of monthly biogeochemical measurements made from 1993 to 1995, combined with hydrography and satellite altimetry, was used to assess the impacts of nine eddy events on primary productivity and particle flux in the Sargasso Sea. Measurements of primary production, thorium-234 flux, nitrate+nitrite, and photosynthetic pigments made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site were used. During the 3 years of this study, four out of six high thorium-234 flux events occurred during the passage of an eddy. Primary production nearly as high as the spring bloom maximum was observed in two mode-water eddies (May 1993 and July 1995). The 1994 spring bloom at BATS was suppressed by the passage of an anticyclone. Distinct phytoplankton community shifts were observed in mode-water eddies, which had an increased percentage of diatoms and dinoflagellates, and in cyclones, which had an increased percentage of Synechococcus. These variations in species composition within mode-water eddies and cyclones may be associated with the ages of the sampled eddies, and/or differences in physical, chemical, and biological factors in these two distinct eddy types. In general, eddies that were 1–2 months old elicited a large biological response; eddies that were 3 months old may show a biological response and were accompanied by high thorium flux; eddies that were 4 months old or older did not show a biological response or high thorium flux. A conceptual model depicting temporal changes during eddy upwelling, maturation, and decay can explain the observations in all seven upwelling eddies present in the time-series investigated herein.     

Glucuronidation in the polar bear (Ursus maritimus)

Polar bears bioaccumulate lipophilic pollutants, including polychlorinated biphenyls (PCBs), into their bodies from their exclusive diet of marine organisms. Hydroxylated PCB metabolites (OH-PCBs) have been found in plasma, presumably due to CYP-dependent biotransformation of PCBs in liver. Little is known about the phase 2 metabolism of hydroxylated xenobiotics in polar bears. The objective of this study was to examine UDP-glucuronosyltransferase (UGT) activity with OH-PCBs and a hydroxylated polycyclic aromatic hydrocarbon, 3-hydroxy-benzo(a)pyrene (3-OH-BaP), in polar bear liver. Samples of frozen polar bear liver were used to prepare microsomes. UGT activity with 3-OH-BaP in Brij-treated microsomes, measured by a fluorescence assay, was readily measurable with protein concentrations in assay tubes of up to 10 g/ml, but dropped off very sharply at higher protein concentrations. The apparent Km for 3-OH-BaP was 1.71 +/- 0.04 microM, and Vmax 1.26 +/- 0.16 nmol/min/mg protein (mean +/- SD, n=3). UGT activities with a model tetrachloro-OH-PCB (4'-OH-CB72) and a model hexachloro-OH-PCB (4'-OH-CB159) were assayed with [14-C]-UDPGA and separation of the [14-C]-glucuronide by ion-pair extraction and thin-layer chromatography. [14-C]-glucuronide conjugates were readily formed by polar bear liver microsomes in the absence of added substrate, apparently from contaminants present in liver. This phenomenon was not observed using hepatic microsomes from laboratory-held catfish. Glucuronidation efficiency was much higher with 4'-OH-CB72 (Km 7.3 microM; Vmax 1.55 nmol/min/mg) than 4'-OH-CB159 (Km 16.1 microM; Vmax 0.46 nmol/min/mg). The identities of the aglycones present in polar bear liver are not known, but could include OH-PCBs or hydroxylated metabolites of other persistent organic pollutants. This study demonstrates that UGT with high activity for 3-OH-BaP and other substrates is present in polar bear liver.     

Observed foundation behaviour of concrete gravity platforms installed in the North Sea 1973–1978

Available measurements of foundation behaviour of the 13 concrete gravity platforms installed in the North Sea in the years 1973–1978 are presented. The measurements cover both the installation phase and the early part of the operation period. For the installation phase, measurements of dowel and skirt penetration resistance, base contact stresses, tilt, piping and erosion, are presented. For the operation period, the presentation includes measurements of settlements, pore water pressures, base contact stresses, permanent lateral displacements, cyclic displacements, dynamic behaviour and erosion. The measurements are used to back calculate soil properties and to evaluate the adequacy of the design procedures which have been used. The observations give valuable experience with respect to the foundation performance of offshore gravity platforms.