The effect of salinity on the development ofLoxothylacus panopaei larvae (Crustacea: Cirripedia: Rhizocephala)

The rhizocephalan barnacleLoxothylacus panopaei, which parasitizes the mud crabRhithropanopeus harrisii, releases its larvae as nauplii. The nauplii develop through four stages to the cypris stage; both larval forms are lecithotrophs. Parasitized crabs were acclimated in 10‰, 15‰, and 20‰ water during August and the development of releasedL. panopaei larvae was monitored in a range of salinities. After 48–60 h, the nauplii and cyprids in each experimental salinity were counted and the numbers used to calculate a developmental index. The range of salinities in which successful development to cyprids occurred (developmental index >70%) could be shifted as a consequence of the acclimating salinity.L. panopaei larvae can develop successfully over a 10–30‰ salinity range, indicating that settlement onto host crabs and virgin externae is likely to take place in nature within this range.     

Exogenic processes and the origin of the sialic crust

Large scale segregation of sialic crust from mantle material may have taken place by magmatic differentiation, partial melting or by exogenic processes. Weathering is a little considered, but verifiable process by which free silica can be generated in quantity from mafic material. In proto-Archean time acid ocean water must have been highly reactive. Even submarine weathering by such acid water could lead to large scale generation of free silica. Evidence from soil formation supports this view. Recrystallization of weathering products would take place by anatexis in the course of relatively intense recycling processes in pre-Archean and early Archean times. Much Fe and Mg in shales and carbonates would segregate downward as mafic xenoliths. The sialic fraction would be returned to shallow crustal levels by diapiric ascent and upward melting. This model is valid only if the sialic crust has grown little since Archean time. We show that “primitive” isotope ratios in later sialic rocks which may suggest gradual accretion of sialic crust throughout geologic time can also be explained by recycling mechanisms. We conclude that exogenic processes in pre-Archean times could have generated most of the sialic crust.     

Adaptive finite element analysis of cone penetration in clay

This paper presents a numerical technique for the analysis of the cone penetration test by means of the commercial finite element code ABAQUS. The von Mises yield criterion with its associated flow rule is assumed to model the plastic behaviour of elastoplastic undrained clays. An explicit finite element scheme is used to efficiently perform a large number of loading increments and to simplify the treatment of contact. An Arbitrary Langrangian–Eulerian (ALE) scheme is adopted to preserve the quality of mesh throughout the numerical simulation. A volumetric weighting algorithm adjusts the relative positions of nodes after each loading increment. This prevents mesh over distortion and allows the simulation to run continuously. The variation of the cone resistance is examined in relation to various parameters such as the in situ stress state, shaft and cone face roughness, and the material strength when steady state conditions have been reached. The trends of these variations are highlighted and compared with those found by other researchers. This technique can be extended to analyse the plastic behaviour of elastoplastic sands often modelled using either the Drucker–Prager yield criterion or a critical state model.     

Platinum-osmium isotope evolution of the Earth’s mantle: Constraints from chondrites and Os-rich alloys

Separation of a metal-rich core strongly depleted the silicate portion of the Earth in highly siderophile elements (HSE), including Pt, Re, and Os. To address the issues of how early differentiation, partial melting, and enrichment processes may have affected the relative abundances of the HSE in the upper mantle, ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os data for chondrites are compared with data for Os-rich alloys from upper mantle peridotites. Given that ¹⁸⁷Os and ¹⁸⁶Os are decay products of ¹⁸⁷Re and ¹⁹⁰Pt, respectively, these ratios can be used to constrain the long-term Re/Os and Pt/Os of mantle reservoirs in comparison to chondrites. Because of isotopic homogeneity, H-group ordinary and other equilibrated chondrites may be most suitable for defining the initial ¹⁸⁶Os/¹⁸⁸Os of the solar system. The ¹⁸⁶Os/¹⁸⁸Os ratios for five H-group ordinary chondrites range only from 0.1198384 to 0.1198408, with an average of 0.1198398 ± 0.0000016 (2σ). Using the measured Pt/Os and ¹⁸⁶Os/¹⁸⁸Os for each chondrite, the calculated initial ¹⁸⁶Os/¹⁸⁸Os at 4.567 Ga is 0.1198269 ± 0.0000014 (2σ). This is the current best estimate for the initial ¹⁸⁶Os/¹⁸⁸Os of the bulk solar system. The mantle evolution of ¹⁸⁶Os/¹⁸⁸Os can be defined via examination of mantle-derived materials with well-constrained ages and low Pt/Os. Two types of mantle-derived materials that can be used for this task are komatiites and Os-rich alloys. The alloys are particularly valuable in that they have little or no Re or Pt, thus, when formed, evolution of both ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os ceases. Previously published results for an Archean komatiite and new results for Os-rich alloys indicate that the terrestrial mantle evolved with Pt-Os isotopic systematics that were indistinguishable from the H-group ordinary and some enstatite chondrites. This corresponds to a Pt/Os of 2.0 ± 0.2 for the primitive upper mantle evolution curve. This similarity is consistent with previous arguments, based on the ¹⁸⁷Os/¹⁸⁸Os systematics and HSE abundances in the mantle, for a late veneer of materials with chondritic bulk compositions controlling the HSE budget of the upper mantle. It is very unlikely that high pressure metal-silicate segregation leading to core formation can account for the elemental and isotopic compositions of HSE in the upper mantle.     

Models of Earth's main magnetic field incorporating flux and radial vorticity constraints

We describe a new technique for implementing the constraints on magnetic fields arising from two hypotheses about the fluid core of the Earth, namely the frozen-flux hypothesis and the hypothesis that the core is in magnetostrophic force balance with negligible leakage of current into the mantle. These hypotheses lead to time-independence of the integrated flux through certain 'null-flux patches' on the core surface, and to time-independence of their radial vorticity. Although the frozen-flux hypothesis has received attention before, constraining the radial vorticity has not previously been attempted. We describe a parametrization and an algorithm for preserving topology of radial magnetic fields at the core surface while allowing morphological changes. The parametrization is a spherical triangle tesselation of the core surface. Topology with respect to a reference model (based on data from the Oersted satellite) is preserved as models at different epochs are perturbed to optimize the fit to the data; the topology preservation is achieved by the imposition of inequality constraints on the model, and the optimization at each iteration is cast as a bounded value least-squares problem. For epochs 2000, 1980, 1945, 1915 and 1882 we are able to produce models of the core field which are consistent with flux and radial vorticity conservation, thus providing no observational evidence for the failure of the underlying assumptions. These models are a step towards the production of models which are optimal for the retrieval of frozen-flux velocity fields at the core surface.     

Application of a library based method to time domain electromagnetic data for the identification of unexploded ordnance

A simple library based algorithm for the identification of unexploded ordnance is tested on time domain electromagnetic data. A library of polarization tensors is generated from data acquired on a test stand over a collection of different UXO. The objective of the algorithm is to determine which target of the library is most likely to have produced an observed data anomaly. For each target in our library a non-linear inverse problem is solved for the position and orientation that minimizes the least-squares difference between the observed data anomaly and the data predicted from each target. This technique avoids direct inversion for polarization tensor, making it feasible for cases where sensor data quality may not be sufficient to support confident estimation of model parameters. For cases where the background soil response is significant, we also invert for the t⁻¹ decay characteristic of viscous remnant magnetic soil. We present results of preliminary tests of the library technique to Geonics EM63 time domain electromagnetic data collected on a test plot seeded with UXO. These tests demonstrate an excellent ability to accurately identify isolated targets. Misidentification of single targets occur when data anomalies have low signal to noise ratios or when targets within the library have similar polarization tensors.     

Stability criteria in many-body systems

Celestial Mechanics and Dynamical Astronomy - In previous papers of this series the stability of hierarchical many-body dynamical systems has been considered in terms of parameters which are a...     

Stability criteria in many-body systems

The analytical stability criterion applicable to coplanar hierarchical three-body systems described in the first paper of this series, Walkeret al. (1980), is modified to give an exact representation ofHill-type stability in all such cases. The dependence of the stability on all orbital parameters (in the coplanar case) is taken into account. The criterion for stability is now dependant upon the participating masses, the elements of the initial osculating Keplerian orbits of the system (viz. the orbits ofm ₂ aboutm ₁ andm ₃ about the mass-centre of the (m ₁,m ₂) system) and the positions within these orbits.The behaviour of the stability of such systems is demonstrated (both analytically and numerically) with respect to certain of the parameters involved to consider effects not dealt with in the above-mentioned paper. In particular two interesting real cases of triple systems in the Solar System are discussed, namely Sun-Jupiter-Saturn and Earth-Moon-Sun. The results of the present paper are compared with those of past authors who considered the same systems.Finally some general features arising out of our analysis are discussed.