“BASS 300 PARACOM”: a “model” underwaterparametric communication system

Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s^-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency     

Going underground: in search of Carboniferous coal forests

The development of coal forests during the Carboniferous is one of the best-known episodes in the history of life. Although often reconstructed as steamy tropical rainforests, these ancient ecosystems were a far cry from anything we might encounter in the Amazon today. Bizarre giant club-mosses, horsetails and tree ferns were the dominant plants, not flowering trees as in modern rainforests. At their height, coal forests stretched all the way from Kansas to Kazakhstan, spanning the entire breadth of tropical Pangaea. Most of what we know of their biodiversity and ecology has been quite literally mined out of the ground through two centuries of hard labour. Without coal mining, our knowledge would be greatly impoverished. Over the past few years, we've been exploring underground coal mines in the United States, where entire forested landscapes have been preserved intact over huge areas. Never before have geologists had the opportunity to walk out through mile upon mile of fossilized forest. In this feature article, we describe some of our recent explorations and attempt to shed new light on these old fossils.     

Finite-element analysis of time-dependent large-deformation problems

An updated Lagrangian finite-element formulation has been developed for time-dependent problems of soil consolidation involving finite deformations. Large plastic strains as well as rotations occur in such problems and nominal stress measures are introduced in the formulation to redefine stresses. This leads to corrective terms for equilibrium and yield violations in addition to geometric stiffening terms in the governing integral equations. The soil is considered to be either a linear elastic or an elastoplastic, critical-state material. Some simple numerical examples are studied to validate the formulation, followed by a detailed analysis of the problem of penetration of a pile into soil. The results of this problem are viewed with emphasis on the physical interpretation and practical significance.     

Biological and granulometric controls on sedimentary organic matter of an intertidal mudflat

A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter.     

Rare earth, major and trace elements in Jurassic manganese shales of the Northern Calcareous Alps: hydrothermal versus hydrogenous origin of stratiform manganese deposits

Summary ?The rare earth, major and trace element geochemistry of Jurassic deep marine manganese shales allow insight into their environment of deposition. We present data of 24 samples from the Northern Calcareous Alps (Eastern Alps), collected from Late Jurassic strata of the Tennengebirge and from Early Jurassic strata of the Karwendel Mts. Whereas major and trace element geochemistry is controlled by the detrital input, rare earth element data and the relationship between total organic carbon and (pyritic) sulfur can be used to constrain the origin of the manganese accumulation. The data of this study are compatible with a hydrogenous precipitation of manganese by strongly varying redox-conditions at the sediment–water interface of a slope basin. Received December 21, 2001; revised version accepted February 18, 2002     

Tephrochronology and tephrostratigraphy of two Pleistocene continental fossiliferous successions from central Italy

The stratigraphical context of two Middle Pleistocene fossiliferous palaeosols from Central Italy (Abruzzo and Tuscany) have been studied. Small mammals and molluscs occur in both palaeosols, which are covered by tephra layers that were analysed using an interdisciplinary approach. Application of fission‐track dating to apatites separated from the Case Picconetto tephra (Pescara, Abruzzo), yielded an age of 0.48 ± 0.04 Ma, indistinguishable from those previously determined for the Campani Quarry (Lower Valdarno, Tuscany) (0.46 ± 0.05 Ma and 0.48 ± 0.05 Ma). Geochemical and petrographic investigations indicate that these tephra originated from different volcanoes, the Alban Hills Volcanic Complex and the Vico Volcano (Latium) respectively. Small mammal and mollusc assemblages indicate different palaeoclimatic and palaeoenvironmental conditions for the Case Picconetto and Campani Quarry palaeosols. Warm and humid conditions can be inferred for the Campani Quarry site, whereas open and cold conditions can be inferred for Case Picconetto. On the basis of faunal data, fission‐track dates and attribution of tephra to specific volcanic eruptions, we suggest a correlation of these faunas with marine oxygen isotope stage 14 (Case Picconetto) and with marine oxygen isotope stage 11 (Campani Quarry), respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

Modelling of deformation response and chemo‐mechanical coupling in chalk

A constitutive relation is derived for describing the mechanical response of chalk. The approach is based on a phenomenological framework which employs chemo‐plasticity. The properties of the material are assumed to be affected by the physico‐chemical processes that occur through the interaction between the skeleton and the pore fluid. The underlying mechanism is discussed by invoking a micromechanical analysis. The performance of the framework is illustrated by examining the evolution of mechanical characteristics in the presence of different pore fluids. Copyright © 2006 John Wiley & Sons, Ltd.