Structure and composition of the consolidated mud tube of Maldane sarsi (Polychaeta: Maldanidae)

Maldanid polychaetes can be important components of marine benthic communities, playing significant roles in particle subduction or sediment irrigation. Many maldanids are known to inhabit tubes consisting of sediments consolidated by mucus; the structure and composition of these tubes, and their potential impact on benthic environments, are poorly known. We examined the three-dimensional organization of Maldane sarsi tubes, using CT scanning together with analyses of sediment grain size and concentrations of Fe, Mn, organic carbon and bacteria in tube material. M. sarsi tubes consist of stacks of individual consolidated mud disks, surrounding a dense, continuous, inner tube. The tubes of M. sarsi contained fewer fine particles than surrounding sediments, and greater concentrations of Fe, Mn, organic carbon and bacteria, especially in the inner zone. These distributions suggest that tube irrigation affects Fe and Mn oxidation and precipitation in a narrow zone surrounding M. sarsi, and that mucous secretion and potential feeding activities (the hoeing of surface sediments) lead to increases in organic carbon and bacteria in the inner, and deepest parts of the tube. The finding of relict tubes, buried at up to 15 cm depth, indicates a relatively high longevity for these structures and suggests a potential importance in biogeochemical cycling.     

Effects of hypoxia on macrobenthos of the inner shelf off Cameron,Louisiana

The effects of hypoxic bottom water, an annual event, were documented on the inner shelf off Cameron, Louisiana during the summer of 1981. Populations of most species of macrobenthos were dramatically reduced. In an area of fine sediment that was numerically dominated by polychaetous annelids, the most severely affected populations were those of tube-dwelling and surface-feeding species. Burrowing species were less influenced by the hypoxia.     

Magnetotelluric observational techniques on land

During the period since the Workshops in Sopron and Murnau we have witnessed some quite remarkable changes in the techniques employed in magnetotelluric (MT) observations. These changes have brought about significant improvements in the quality of MT data that can be gathered today. The new techniques are very likely to bring improvements in many areas beside MT, but are reviewed here in the light of the progress they have rendered possible for MT soundings on land. Three main subjects are covered in this review. The first one is concerned with the use of cryogenic or SQUID magnetometers. The new instruments are presented to the geomagnetist. An explanation of how these instruments work is given without, however, details of their operation and fabrication, and without showing that they usually have a much larger, i.e. better, signal-to-noise ratio than conventional magnetometers. Next, the method of the remote magnetic reference is examined. Originally the MT method assumed incident signals in the form of plane waves. While it is known today that incident wave amplitudes which vary linearly with distance are perfectly acceptable for MT soundings, there are many other higher order distortions of the incident signal which are not acceptable. Most of these high order or local perturbations are man-made and can seriously falsify MT data. All processes of MT data analysis try to minimize the errors caused by locally perturbed signals. But they are only partly successful, especially when the perturbations produce strong cross-power bias. The remote magnetic reference method has proved far superior in avoiding most of this cross-power as well as faulty autopower. The last section of this review deals with the use in the field of microcomputers or microprocessors. These have made it possible to process the data on the site already to such a degree that the sounding crew knows whether the sounding is proceeding satisfactorily, and can decide how best to continue the survey work. Of particular interest is digital filtering, which is especially easy with the microcomputer, and which may be used to avoid specific sources of perturbation.     

Strength Behavior of Marine Sands at Elevated Confining Stresses

A testing program was initiated to determine the stress-strain and strength behavior of two very different marine sands (a calcareous sediment from South Australia and a siliceous sediment from the United Kingdom) at elevated confining pressures. The testing matrix consisted of a series of isotropically consolidated, undrained (CIU) and drained (CID), triaxial compression tests on samples of naturally deposited calcareous and siliceous sediment and remolded calcareous sediment. It was found that the calcareous samples displayed little cemented behavior during shear. For tests conducted at pressures up to 1.5 MPa, a significant amount of particle crushing occurred in the calcareous samples but not in the siliceous samples. Particle degradation and reorientation facilitates transitions from dilative to contractive behavior with increases in confining stress. The calcareous sediment exhibited contractive behavior at confining pressures above approximately 500 kPa and the siliceous sediment remained dilative at stresses up to 1 MPa during undrained loading. Comparison with data collected by the University of Sydney (CID tests with confining pressures up to 60 MPa) showed that most of the variations in strength behavior occurred within the low stress range (up to 2 MPa) tests conducted at URI. This was evident in the friction angle data and in the reloading Young's modulus data.     

Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)

The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones.     

Demonstration of a Microbiologically Enhanced Vertical Ground Water Circulation Well Technology at a Superfund Site

A full-scale ground water circulation well (GCW) system was installed and operated to demonstrate in situ remediation of soil and ground water impacted with a mixture of chlorinated and nonchlorinated organic compounds at a Superfund site in upstate New York. System performance and applicability under site-specific conditions were evaluated based on the system's ability to meet the New York State Department of Environmental Conservation (NYSDEC) cleanup goals for target compounds in ground water and soil. Contaminants from the unsaturated zone were mobilized (volatilized) by one-way vacuum extraction, and treated via enhanced biodegradation (bioventing). In the saturated zone, contaminants were mobilized by soil flushing (solubilized) and treated by a combination of air stripping and biodegradation. An in situ aqueous phase bioreactor, and an ex situ gas phase bioreactor, were integrated into the system to enhance treatment via bioremediation. After 15 months of operation, the mass of target contaminants in soil and ground water combined had been reduced by 75%. Removal by biological mechanisms ranged from 35% to 56% of the total observed mass reduction. The in situ and the ex situ bioreactors mineralized 79% and 76%, respectively, of their target biodegradable contaminant loads. Results indicate that some mass reduction in target contaminants may have been from aerobic and aerobic processes within the circulation cell. Nonchlorinated compounds were relatively easy to mobilize (volatilize, solubilize, and/or transport) and treat when compared to chlorinated compounds. The data collected during the 15-month study indicate that remediation could be accomplished at the Sweden-3 Chapman site using the technology tested.     

Announcement Internatioal Society for Diatom Research and Nederlands-Vlaamse Kring van Diatomisten

Announcement Internatioal Society for Diatom Research and Nederlands-Vlaamse Kring van Diatomisten     

δ13C variation of soil organic matter as an indicator of vegetation change during the Holocene in central Cameroon

In order to better understand the dynamics of the forest–savanna mosaic from central Cameroon, we analyzed ¹³C and ¹⁴C profiles of six oxisols: two under forests and four under savannas. The δ¹³C soil profiles collected in the forests indicate that these environments are stable at least since the mid-Holocene, whereas the areas currently covered by savannas were formerly occupied by more forested vegetations. The ¹⁴C dating of organic matter indicate that the late extension of the savannas in central Cameroon date from the Late Holocene, starting from 4000–3500 ¹⁴C yr BP.