Crustal magnetization and temperature at depth beneath the Yilgarn block, Western Australia inferred from Magsat data

Variations in crustal magnetization along a seismic section across the Archean Yilgarn block of Western Australia inferred from Magsat data are interpreted as a subtle thermal effect arising from variations in depth to the Curie isotherm. The isotherm lies deep within the mantle of the eastern part of the province, but transects the crust-mantle transition and rises well into the crust on the western side. The model is consistent with heat flow variations along the section line. The mean crustal magnetization implied by the model is approximately 2 A/m. The temperature variation implied by the model is consistent with the hypothesis that the crust-mantle transition seen seismically corresponds to the mafic granulite-eclogite phase transition within a zone of igneous crustal underplating.     

A comparison of the toluene distillation and vacuum/heat methods for extracting soil water for stable isotopic analysis

Hanford Loam, from Richland, Washington, was used as a test soil to determine the precision, accuracy and nature of two methods to extract soil water for stable isotopic analysis: azeotropic distillation using toluene, and simple heating under vacuum. The soil was oven dried, rehydrated with water of known stable isotopic compositions, and the introduced water was then extracted.

Compared with the introduced water, initial aliquots of evolved water taken during a toluene extraction were as much as 30 ‰ more depleted in D and 2.7 ‰ more depleted in ¹⁸O, whereas final aliquots were as much as 40 ‰ more enriched in D and 14.3 ‰ more enriched in ¹⁸O. Initial aliquots collected during the vacuum/heat extraction were as much as 64 ‰ more depleted in D and 8.4 ‰ more depleted in ¹⁸O than was the introduced water, whereas the final aliquots were as much as 139 ‰ more enriched in D, and 20.8 ‰ more enriched in ¹⁸O. Neither method appears quantitative; however, the difference in stable isotopic composition between the first and last aliquots of water extracted by the toluene method is less than that from the vacuum/heat method. This is attributed to the smaller fractionation factors involved with the higher average temperatures of distillation of the toluene. The average stable isotopic compositions of the extracted water varied from that of the introduced water by up to 1.4 ‰ in δD and 4.2 ‰ in δ¹⁸O with the toluene method, and by 11.0 ‰ in δD and 1.8 ‰ in δ¹⁸O for the vacuum/heat method.

The lack of accuracy of the extraction methods is thought to be due to isotopic fractionation associated with water being weakly bound (not released below 110°C) in the soil. The isotopic effect of this heat-labile water is larger at low water contents (3.6 and 5.2% water by weight) as the water bound in the soil is a commensurately larger fraction of the total. With larger soilwater contents the small volume of water bound with an associated fractionation is not enough to affect the remaining unbound introduced soil water. Pretreatment of the soil to equilibrate the heat-labile water to the test water produced good results for the toluene distillation but not the vacuum/heat extraction method.

Vapors collected over the soils also show stable isotopic variations related to soilwater content. These vapors also appear to be in closer equilibrium with the free water, as extracted by the toluene method, than with the originally introduced water; thus, the soil vapors do not appear to be isotopically affected by the heat-labile water.

The toluene method appears to be better for extracting soil water for stable isotopic analysis because it allows more precise temperature control and excludes the extraction of heat-labile water which is isotopically fractionated. The bound nature of this heat-labile water limits association with the hydrologically active soil water; thus, the exclusion of this water from the soil water attained by toluene distillation may be advantageous. However, the azeotropic nature of toluene distillation affords no benefit and the extraction procedure must continue to completion.     

A method for the measurement of bedload sediment transport and passive faunal transport on intertidal sandflats

A simple and inexpensive sampler to measure bedload sediment transport in shallow subtidal or intertidal areas is described. The cylindrical sub-sediment trap with an aspect ratio of 20 (height: diameter) is an improvement over conventional bedload samplers which are difficult to use in shallow areas or fail to collect the biological material associated with bedload. Traps deployed on a low-energy intertidal sandflat for six months provided daily estimates of bedload transport (quartz grains: 0.001–40 kg m^?1 d^?1), passive infaunal transport (e.g., the bivalveMya arenaria, max: 800 ind m^?1 d^?1), and organic detrital flux (e.g., macrophyte fragments, max: 400 g dry wt m^?1 d^?1). Bedload rates estimated with traps were compared to predictions from a numerical bedload model to evaluate the trap’s collection and retention efficiency. A significant linear regression between observed (trap) and predicted (model) rates (r²=0.65, p<0.001, n=97) indicated that the traps were useful for the measurement of high- and low-frequency variability in bedload transport. Potential applications of the traps in benthic oceanography include recruitment and recolonization studies.     

On the thickness and the14C age of the mixed layer in deep-sea carbonates

Published¹⁴C stratigraphies from three cores, and¹⁴C dates from four additional cores suggest that the simple mixing model of Berger and Heath (1968) can be used to simulate the effects of benthic mixing in deep-sea carbonates. Mixing depths calculated under the constraints of this model are remarkably constant (81/2 ± 1cm). Discrepancies between calculations and observations exist and may be due mainly to differential mixing (effects of size and shape) in combination with changes in sedimentation rates, and to lumpy mixing (effects of displacement of blebs or “noise”).     

Criticism of generalised models for the magmatic evolution of arc-trench systems

Recent geological and petrological results from the Lesser Antilles island arc and Papua New Guinea, and from other regions of arc-trench-type volcanism, provide notable exceptions to the spatial, volumetric, and temporal relationships claimed for generalised arc models. For example, many alkalic and shoshonitic associations do not appear to be developed over the deepest parts of downgoing slabs, and there are now several well-documented exceptions to the K₂O/SiO₂/depth-to-Benioff-zone relationship. Moreover, the temporal sequence of early tholeiitic → middle calcalkalic → late shoshonitic/alkalic is not well substantiated, although shoshonitic rocks do appear to be developed most commonly in regions with a long history of plate interactions. Exceptions to the generalised arc model are symptomatic of the need to look for the unique features of individual island arcs, rather than just similarities between different ones, so that the major factors controlling arc evolution may be determined.     

A moated knoll in the Canary Passage

Summary A 100-fathoms high one-mile-wide elongate knoll approximately ten nautical miles long lies in 650 fms (1200 m) depth in the Canary Passage. The long axis is parallel to the axis of the passage. The knoll is flanked by a moat 5 to 20 fms deep which in turn is surrounded by a gentle arch or ridge of similar height. Although similar moats have been interpreted as a result of downward isostatic adjustments of islands and seamounts, this moat is considered the product of bottom scour by ocean currents.

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Über eine von einem Graben umgebene Kuppe in der Canary Passage

Zusammenfassung Eine 100 fathoms hohe, 1 Seemeile breite längliche Kuppe, ungefähr 10 Seemeilen lang, liegt in 650 fathoms (1200 m) Tiefe in der Canary Passage. Ihre Längsachse verläuft parallel zur Axe der Passage. Die Kuppe ist von einem 5 bis 20 fathoms tiefen Graben umgeben, welcher seinerseits von einer sanften Erhebung oder Wall gleichbleibender Höhe gesäumt wird. Obgleich ähnliche Gräben als Ergebnis abwärts gerichteten isostatischen Ausgleichs von Inseln undseamounts gedeutet werden, hält man diesen Graben für eine Folgeerscheinung der Erosion durch Meeresströmungen.

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Sur un récif submergé entouré d'un fossé dans le «Canary Passage»

Résumé Dans le «Canary Passage» il se trouve un récif submergé oblong, haute de 100 fathoms, ayant 1 mille marin de large et d'environ 10 milles marins de long. Il est situé à 650 fathoms (1200 mètres) de profondeur. Son axe longitudinal est parallèle à l'axe du «Canary Passage». Un fossé ayant 5 à 20 fathoms de profondeur longe le récif. Ce fossé est de son tour entouré d'un bombement ou d'une dorsale faible d'égal hauteur. Bien qu'on ait regardé de pareils fossés comme étant le produit d'une adaptation d'isostasie des îles et desseamounts, on suppose que ce fossé est dû à l'érosion du fond causée par les courants océaniques.

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This study was supported in part by the Office of Naval Research under Contract Nonr-266(48) and in part by The Bell Telephone Laboratories.Lamont Geological Observatory Contribution No. 707.     

Early spreading history of the Indian Ocean between India and Australia

A revised model of seafloor spreading between India and Australia from the inception of spreading 125 m.y. to the change to a new system at 90 m.y. stems from the wider recognition of the M-series of magnetic anomalies off the southwestern margin of Australia, from a revised pole of opening between Australia and Antarctica, and by the extension in the central Wharton Basin of the Late Cretaceous set of magnetic anomalies back to 34. The phase of spreading represented by the later anomalies has been extended back to 90 m.y. in order to give a resolved pole that describes the rotation of India from Australia consistent with the M-series anomalies, DSDP site ages, and fracture zone trends. An abandoned spreading ridge in the Cuvier Abyssal Plain indicates a ridge jump within the first ten million years of spreading. Elsewhere, two kinds of ridge jump (one to the continental margin of Australia or India, the other by propagation of the spreading ridge into adjacent compartments thereby causing them to fuse), are postulated to account for other observations.     

The 57Fe Mössbauer parameters of pyrite and marcasite with different provenances

Eighteen pyrite and twelve marcasite samples which have different provenances have been investigated to determine the systematics of the influence of mineralogical and geological factors on the ⁵⁷Fe Mössbauer spectra at 298 K. The following results have been obtained: there is no ambiguity in distinguishing single phase pyrite from single phase marcasite by means of ⁵⁷Fe Mössbauer spectroscopy at 298 K. At 298 K the average electric quadrupole splitting, 〈ΔE_Q〉, and average isomer shift, 〈δ〉, with respect to Fe metal, are 0.6110 ± 0.0030 mm s^?1 and 0.313 ± 0.008 mm s^?1, respectively, for the 18 pyrites; 〈ΔE_Q〉 = 0.5030 ± 0.0070 mm s^?1 and 〈δ〉 = 0.2770 ± 0.0020 mm s^?1 for the 12 marcasites. At 77 K, ΔE_Q is 0.624 mm s^?1 for pyrite and 0.508 mm s^?1 for marcasite. In distinguishing pyrites from marcasites, spectra obtained at 77 K are not warranted.The Mössbauer parameters of pyrite and marcasite exhibit appreciable variations, which bear no simple relationship to the geological environment in which they occur but appear to be selectively influenced by impurities, especially arsenic, in the pyrite lattice. Quantitative and qualitative determinations of pyrite/marcasite mechanical mixtures are straightforward at 298 K and 77 K but do require least-squares computer fittings and are limited to accuracies ranging from ±5 to ±15 per cent by uncertainties in the parameter values of the pure phases. The methodology and results of this investigation are directly applicable to coals for which the presence and relative amounts of pyrite and marcasite could be of considerable genetic significance.