The origin of rotational remanent magnetization

Summary. Rotational remanent magnetization, RRM, is the magnetization acquired when a sample is rotated during alternating field demagnetization. Although the existence of RRM has been well documented in different laboratories, until now no physical mechanism explaining its origin has been given. We propose that the RRM originates from thermal fluctuations biased by a precessional torque associated with the alternating field. Our theory is consistent with the observation that no directional preference exists in the experimental situation until the sample is rotated relative to the alternating field. Moreover, our theory predicts that the combined sample rotation and precession will produce a RRM that switches direction when the frequency of sample rotation increases from any value below the frequency of the alternating field to any value above that frequency as observed in experiments. Although no precise theory is given for the intensity of RRM, the model presented here can qualitatively explain previous intensity observations.     

Trace metal composition of and accumulation rates of sediments in the Upper Gulf of Thailand

Sediment cores and grab samples were collected in the Upper Gulf of Thailand to determine sedimentation rates and to determine if metal concentrations reflect anthropogenic inputs.Accumulation rates of sediments in the Upper Gulf measured using the ²¹⁰Pb method, appear to vary from ca. 4 to 11 mm yr^?1. Sediment budgets suggest that little of the sediment delivered to the Upper Gulf by the major rivers is ultimately transported to the Lower Gulf. Metal concentrations in Upper Gulf sediments appear to be dominantly controlled by natural inputs.     

A comparative isotopic study of the Lady Loretta zinc-lead-silver deposit

Measurements of the ³⁴S/³²S ratios in sulphides from the slightly metamorphosed Lady Loretta deposit show the sphalerite and galena to be in isotopic equilibrium. Pyrite in immediate association with these sulphides is not isotopically related. A similar distribution of sulphur isotopes had previously been noted in the even less altered McArthur deposit, for which a dual sulphur source was postulated. This fresh isotopic evidence from Lady Loretta now suggests that such a genesis for stratiform sulphides from the Proterozoic is not uncommon.     

The metallic microstructures and thermal histories of severely reheated chondrites

The metallographic structures of eight severely reheated chondrites (Farmington, Ramsdorf, Orvinio, Wickenburg, Lubbock, Rose City, Arapahoe and Tadjera) have been studied by optical, scanning electron microscope and electron microprobe techniques. Unreheated chondrites and experimentally heat treated chondritic material have also been examined.The following metallographic characteristics can be used to estimate the post-shock residual temperature of reheated chondrites: melted appearance of metal-troilite, presence of martensite, P enrichment of the metal, and the averaging of central metal grain compositions. Metallographic characteristics used to estimate the cooling rates of the severely reheated chondrites are the Ni content of troilite, the Ni gradients in metal grain rims, and the presence of secondary kamacite and phosphides. Farmington, Ramsdorf, Orvinio, Lubbock, Rose City and several of the heat treatment specimens have substantial P in solution in the metal grains ( > 0.1 wt%). P enrichment is apparently caused by reduction of phosphates upon severe reheating and partial melting of metal-troilite areas in chondritic meteorites.The eight severely reheated chondrites studied showed evidence of reheating to temperatures ranging from ～ 950°C to ～ 1250°C. Ramsdorf has the highest reheating temperature (1200–1250°C) and the fastest cooling rate ～ 100°C/day. Wickenberg has the lowest reheating temperature (950–1000°C) and the slowest cooling rate, ～ 1°C/100yrs. Cooling rate estimates correspond to post-reheating burial depths of less than 1 to ～ 1000m.     

Developing Governance Principles for Public Natural Resources

Wildlife governance principles (WGPs) identify desirable governance characteristics for wildlife conservation in the United States (US). The types of institutional, ecological, and socio-cultural challenges that WGPs are designed to address also affect governance of public natural resources other than wildlife and in places other than the US. This raises the possibility that a similar set of governance principles might help natural resource professionals working in other resource contexts address the particular challenges they face. We describe the process by which we developed WGPs and offer seven practically oriented questions to help natural resource professionals ascertain whether a similar set of principles could improve governance in their context. In some contexts, minor modification of WGPs might be appropriate; in others, the process by which we developed WGPs could serve as a blueprint for formulating appropriate principles.     

The forward problem of electromagnetic induction: accurate finite-difference approximations for two-dimensional discrete boundaries with arbitrary geometry

We describe finite-difference approximations to the equations of 2-D electromagnetic induction that permit discrete boundaries to have arbitrary geometrical relationships to the nodes. This allows finite-difference modelling with the flexibility normally ascribed to finite-element modelling. Accuracy is demonstrated by comparison with finite-element computations. We also show that related approximations lead to substantially improved accuracy in regions of steep, but not discontinuous, conductivity gradient.