Theory of the piezomagnetic effect in titanomagnetite-bearing rocks

Summary The stress sensitivitiesS _x andS _R of susceptibility and remanence for titanomagnetite-bearing rocks are calculated in terms of magnetostriction constants ₁₀₀ and ₁₁₁ and anisotropy constantsK ₁,K ₂ of the magnetic minerals.S _x andS _R are represented by quite different algebraic expressions but happen to have comparable numerical values over the whole range of titanomagnetite compositions. Both increase strongly with titanium content. This leads to more optimistic calculations of tectonomagnetic effects than with the previously assumed stress sensitivity for pure magnetite.     

Tertiary geodynamics of Sakhalin (NW Pacific) from anisotropy of magnetic susceptibility fabrics and paleomagnetic data

Sakhalin has been affected by several phases of Cretaceous and Tertiary deformation due to the complex interaction of plates in the northwest Pacific region. A detailed understanding of the strain is important because it will provide constraints on plate-scale processes that control the formation and deformation of marginal sedimentary basins. Anisotropy of magnetic susceptibility (AMS) data were obtained from fine-grained mudstones and siltstones from 22 localities in Sakhalin in order to provide information concerning tectonic strain. AMS data reliably record ancient strain tensor orientations before significant deformation of the sediments occurred. Paleomagnetically determined vertical-axis rotations of crustal rocks allow rotation of the fabrics back to their original orientation. Results from southwest Sakhalin indicate a N035°E-directed net tectonic transport from the mid-Paleocene to the early Miocene, which is consistent with the present-day relative motion between the Okhotsk Sea and Eurasian plates. Reconstruction of early–late Miocene AMS fabrics in east Sakhalin indicates a tectonic transport direction of N040°E. In west Sakhalin, the transport direction appears to have remained relatively consistent from the Oligocene to the late Miocene, but it has a different attitude of N080°E. This suggests local deflection of the stress and strain fields, which was probably associated with opening of the northern Tatar Strait. A northward-directed tectonic transport is observed in Miocene sediments in southeast Sakhalin, mid-Eocene sediments in east Sakhalin, and in Late Cretaceous rocks of west and northern Sakhalin, which may be associated with northwestward motion and subduction of the Pacific Plate in the Tertiary period. The boundaries of the separate regions defined by the AMS data are consistent with present-day plate models and, therefore, provide meaningful constraints on the tectonic evolution of Sakhalin.     

An experimental study of rare earth element partitioning between a shergottite melt and pigeonite: implications for the oxygen fugacity of the mart ian interior

A series of experiments on a synthetic, pigeonite-saturated, basaltic shergottite were carried out to constrain the variation of D(Eu/Gd)_{pigeonite/melt} and D(Eu/Sm)_{pigeonite/melt} with oxygen fugacity (f_O2). The experiments have been run under both dry and hydrous conditions. The shergottite was doped with 0.1, 0.5, and 1.0 wt.% Eu, Gd, and Sm oxides in different experiments and was equilibrated at liquidus conditions for 24 hours. D(Eu/Gd)_{pigeonite/melt} in dry melts ranges from 0.156 ± 0.014 (f_O2 = IW − 1) to 0.630 ± 0.102 (IW + 3.5). D(Eu/Sm)_{pigeonite/melt} in dry melts ranges from 0.279 ± 0.021 (IW − 1) to 1.114 ± 0.072 (IW + 3.5). Due to difficulties with low-f_O2 experiments, hydrous distribution coefficients were measured, but were not used in the calibration of the Eu-oxybarometers. These two Eu-oxybarometers provide an accurate way to measure f_O2 recorded during pigeonite crystallization, thereby yielding a record of f_O2 during the earliest period of Martian meteorite parent magma crystallization history.Using this new calibration, Martian meteorite pigeonite cores record f_O2 values of IW − 0.6 (±0.3) (QUE94201) to IW + 1.9 (±0.6) (Shergotty). These new values differ in magnitude, but not trend, from previously published data. The pigeonite Eu-oxybarometer yields an f_O2 range in the basaltic shergottites of 2 to 3 orders of magnitude. Several processes have been proposed to explain the origin of this f_O2 range, the majority of which rely on assimilation of an oxidized source. A potential correlation between this new pigeonite data and recent Fe-Ti oxide data, however, is consistent with intrinsic f_O2 differences in the magma source region being responsible for the measured f_O2 variations. This implies that the Martian meteorite source region, the mantle or lithosphere, may be heterogeneous in nature. However, the process of assimilation cannot be completely ruled out in that an assimilation event that took place before crystallization commenced would result in the overprinting of the source region f_O2 signature.     

High-resolution coral records of rare earth elements in coastal seawater: biogeochemical cycling and a new environmental proxy

In this study we have used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to produce a high resolution coral record of rare earth elements (REE), Mn and Ba from coastal Porites corals from the Great Barrier Reef of Australia. Validation of the LA-ICP-MS technique indicated that the method provides accurate and reproducible (RSD = 13-18%) analysis of low concentration REE in corals (∼1 to 100 ppb). The REE composition in coral samples was found to closely reflect that of the surrounding seawater and distribution coefficients of ∼1-2 indicated minimal fractionation of the series during incorporation into coral carbonate. To explore the idea that coral records of REE can be used to investigate dissolved seawater composition, we analyzed two coastal corals representing a total of ∼30 yr of growth, including a 10-yr overlapping period. Comparable results were obtained from the two samples, particularly in terms of elemental ratios (Nd/Yb) and the Ce anomaly. Based on this evidence and results from the determination of distribution coefficients, we suggest that useful records of seawater REE composition can be obtained from coral carbonates. When compared to the REE composition of a mid shelf coral, coastal corals showed a significant terrestrial influence, characterized by higher REE concentrations (greater than 10 times) and light REE enrichment. The REE composition of coastal seawater inferred from the coral record was dependent on seasonal factors and the influence of flood waters. REE fractionation displayed a strong seasonal cycle that correlated closely with Mn concentration. We suggest that higher Nd/Yb ratios and higher Mn concentrations in summer result from scavenging of heavy REE by particulate organic ligands and Mn reductive dissolution respectively, both processes displaying higher rates during periods of high primary productivity. The Ce anomaly also displayed a strong seasonal cycle showing an enhanced anomaly during summer and during flood events. This is consistent with the Ce anomaly being primarily controlled by the abundance of Ce oxidizing bacteria. Based on these arguments, we suggest that the coral record of dissolved REE and Mn may be regarded as a useful proxy for biological activity in coastal seawater.     

Historical and pooled flood frequency analysis for the River Tay at Perth, Scotland

Improved estimates of UK flood risk during a period of increased climatic variability place challenges on existing methods that rely on short instrumental records. This paper examines the value of using historical data (both documentary and epigraphic) to augment existing gauged records for the River Tay at Perth as part of a multi-method approach to assessing flood risk. Single station and pooled methods are compared with flood risk estimates based on an augmented historical series (1815–2000) using the Generalized Logistic and Generalized Pareto distributions. The value of using an even longer, but less reliable, extended historical series (1210–2000) is also examined. It is recommended that modelling flood risk for return periods >100 years should incorporate historical data, where available, and that a multi-method approach using a high threshold Generalized Pareto distribution can also add confidence in flood risk estimates for return periods <100 years based on standard methods.     

Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (²³⁰Th/²³²Th) (≥1.06) with low (²³⁸U/²³⁰Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (²³⁰Th/²³²Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (²³⁸U/²³⁰Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion ⁴⁰Ar/³⁹Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (²³⁰Th/²³²Th)₀ ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano. Received: 29 May 1996 / Accepted: 24 November 1997     

Study of (010)[101] and (001)[110]/2 dislocations in K-feldspars by HRTEM and modelling

Dislocations in K-feldspars were studied by high resolution transmission electron microscopy (HRTEM) and HRTEM images were further submitted to a filtering in order to improve their interpretation. (010)[101] dislocations appear to be dissociated with (001)[001]/2 planar defect, whereas (001)[110]/2 dislocations are perfect. Structural models of planar defects in (010) and (001) planes were investigated. The energy estimation of these models was performed using the Keating potential. The structural analysis agreed with the experimental result in that dislocations can be dissociated in the (010) plane, whereas they cannot be dissociated in the (001) plane.     

Enhancing the credibility of ecology: Is interdisciplinary research for land use planning useful?

Interdisciplinary approaches involving inputs from both the natural and social sciences represent one operational and social response to the complexity of today's environmental and land use problems. The resulting amalgam can be useful to both science and society, given the right mix of ingredients. Important steps in the planning and conduct of interdisciplinary research include problem indentification; definition of a precise scientific theme derived from the social demand; choice of relevant disciplines; selection of study area and spatial scale; ensuring that the project is adaptable to changing circumstances; obtaining the involvement of scientists, planners and local people; developing a continuum of actions from basic and applied research through to training and information diffusion; early and explicit definition of the criteria for evaluation. Though interdisciplinary approaches have shown their worth, they founder easily. Obstacles include the behavioural and psychological characteristics of individual scientists, differences in scientific method, and bottlenecks of a more administrative, institutional, political and financial nature.The views expressed are those of the authors and not necessarily those of the authors' employers.     

Enhancing the credibility of ecology: Can research be made more comparable and predictive?

A sounder theoretical basis for extrapolation and prediction, typologies of what is and what is not comparable, and representative natural areas for long-term field research are three geographically-based ingredients of comparative research in ecology. Two types of conceptual model allow assessment of the effects of phylogenesis and environment on ecosystem structure and functioning. The first is based on fixing of genetic variables and study of biological performance along a gradient of environmental conditions — an approach of sequential analysis of environmental gradients. The second is based on fixing of environmental variables and comparison of performance of different phylogenetic stocks in non-contiguous geographical areas — an approach entailing matching of disjunct ecological analogues. The two approaches are important complements in examining hypotheses of convergent and divergent evolution, in helping to define what really can be compared in comparative ecological research, and in contributing to ecology becoming a more predictive and credible science.