Different Modes Of Formation Research Articles

Occurrence of thin, metal-rich layers in deep-sea sediments: a geochemical characterization of copper remobilization

Distinctly coloured trace metal enriched fronts with a six-fold Cu-enrichement occur in the interbedded pelagic-turbiditic sediments from the Madeira Abyssal Plain. To decipher their mode of formation, two sequential extraction techniques, an acid-gradient and a “Tessier-type” scheme, were applied. Two different front types could be distinguished: Type-1 fronts are mostly black and enriched in Co, Zn, Ni and Cu, and associated with the oxidation fronts that developed after deposition of turbidites. Type-2 fronts are purple, extremely enriched in Cu, and associated with sulphur. These fronts are sometimes found in relation with a relict oxidation front, but often show no clear relation with lithology. Two different modes of formation are postulated. (1) The transition-metal enrichment in the type-1 fronts is typical for a manganese-oxide association often found in marine sediments. After the dissolution of manganese-oxide during sub-oxic diagenesis, the transition metals were liberated and subsequently relocated and precipitated in response to a redox-gradient. (2) The CuS association in type-2 fronts is also formed during sub-oxic diagenesis. The main process leading to the formation of this CuS enrichment is the mobilization of inherited CuS compounds from organic-rich turbidites. The accumulation of these compounds is possibly related to redox changes at relict oxidation fronts and to gradual, probablly slowly evolving, redox changes that occur at present.

Origin, evolution, and distribution of shoreface sand ridges, Atlantic inner shelf, U.S.A.

A computer mapping system was employed to document the location of 259 shoreface-attached and detached sand ridges in water <20 m deep and the temporal and spatial distribution of 309 historical and active tidal inlets along the U.S. Atlantic coast (Montauk Point, New York, to Miami Beach, Florida). This database was compiled through the analysis of over 600 historical maps, 50 bathymetric charts, and other published data. A genetic relationship between the location of certain historical and active tidal inlets and shoreface-attached sand ridges is documented. It is inferred that ebb-tidal deltas provided the initial sand source for the development of many shoreface-attached sand ridges. Although shoreface-attached sand ridges appear to have several different modes of formation, a two-step process for the development of most shoreface-attached and detached sand ridges along U.S. Atlantic barrier island and cape coastlines is proposed: (1) sand is deposited as ebb-tidal deltas or river deltas along the lower shoreface and/or inner continental shelf prior to or during transgression, followed by (2) further transgression, which reworks the deltaic sand bodies into linear sand ridges at the base of the shoreface by shelf processes. The best-developed shoreface sand ridge fields along the U.S. Atlantic shelf lie adjacent to shorelines characterized by all of the following: (1) transgression, (2) mixed energy, wave-dominated barrier islands, and (3) laterally migrating tidal inlet systems. Tidal inlet systems are natural sediment sinks that capture sand carried by longshore sediment transport. Ebb-tidal delta deposits associated with these migrating tidal inlets provide the initial sand body for the development of shoreface-attached sand ridges. The oblique orientation and linear form of shoreface-attached sand ridges appear to be a function of shoreline transgression, lateral inlet migration, and wave reworking of ebb-tidal delta deposits concentrated along an ebb-tidal delta retreat path. Shelf processes act as modifying agents in the evolution of sand ridges during and after ebb-tidal delta deposition. In general, shoreface-attached sand ridges are poorly developed or absent along eroding headlands, spits, and barrier island shorelines characterized by naturally stabilized tidal inlets. These latter inlets and shoreline types are dominated or influenced by one or all of the following: (1) antecedent topography, (2) higher tidal ranges, (3) larger tidal prisms, (4) lower wave energies, or (5) finer grain sizes. The coupling of shoreline and shallow marine sedimentary processes during a transgression is critical to the origin, evolution, and distribution of shoreface sand ridges in the study area. Modern shoreface-attached sand ridges are also known to occur in different coastal and shelf settings where large amounts of sediment were supplied to the shoreface and inner shelf during, or immediately before, transgression. Commonly observed vertical and lateral interrelationships of Holocene shoreface-attached sand ridges and tidal inlets or distributary channels have important ramifications for the development of shelf sandstone facies models. In addition, the geometric relationship documented in this study provides predictive petroleum and hard mineral exploration models of the spatial and temporal distribution of shoreface and shelf sand ridges.

Coastal sand dunes as geomorphological systems

SynopsisThe importance of aeolian deposition is clearly shown by the size and bulk of coastal dunes. Sand dunes occur where there is a large supply of sand, a wind to move it, and a place in which it can accumulate. A dune classification is presented which takes into account the origin, internal geometry and surface geomorphology of coastal dunes. Since the main element that distinguishes coastal dunes from desert dunes is vegetation, the relative amount of vegetation may be used as a typology. Four dune types are distinguished: vegetated dunes, parabolic dunes, medanos (i.e. large sand hills devoid of vegetation), and artificially-inseminated dunes. Vegetated and medano dunes are the end members, with parabolic dunes in between. Parabolic dunes are “anchored” by vegetation, but the centre of the dunes has migrated in the down-wind direction. The artificially-inseminated dunes are formed by vegetation plantings, fencing, or other artificial means, but with natural sand accumulation around these obstacles. These now account for a very substantial portion of the world's coastal dunes.The role of wind and its relation to the internal geometry (i.e. cross-bed dip and direction) are discussed in some detail, with examples from Brazil, U.S. east and south coasts, Israel and China. Differences in these characteristics are detailed, and related to the different modes of formation of the four dune types.The role of the wind in transporting and depositing sand in coastal areas is being quantified through both wind tunnel and field transport measurements. An example from a field study along the coast of Israel illustrates the differences between desert and coastal dune transport, where the role of vegetation and beach topography must be taken into account. From the Israeli study, and others, it appears that transport in coastal dunes is reduced by one-third to one-half of that in deserts, due to these factors.Coastal dunes provide a useful, and often necessary, buffer against storm waves and the presently rising sea level. They form primarily through vertical sand accretion trapped by the sensitive dune vegetation. Provided that sufficient space exists between the high tide line and developed areas, planned dunes can be easily formed with the aid of plantings and fencing.

Evidence for primitive nebular components in chondrules from the Chainpur chondrite

The least equilibrated ordinary chrondrites contain chondrules which have experienced little change since the time of their formation in the early solar system. These chondrules are excellent indicators of the physical and chemical nature of the solar nebula. We separated 36 chondrules from the Chainpur (LL3.4) chondrite and analyzed each for 20 elements and petrographic properties. Sampling biases were minimized as far as possible. Chondrules seem to have formed through the melting of random mixtures of grains comprising a limited number of nebular components. The identity of these components can be deduced from chondrule compositions. The dominant components appear to be: 1) a mixture of metal and sulfide with composition similar to whole-rock metal and sulfide; 2) refractory (Ir-rich) metal; 3) refractory, olivine-rich silicates; 4) low-temperature, pyroxene-rich silicates, and, possibly, 5) a component containing the more volatile lithophiles. Most of the textural types of chondrules formed from the same set of precursor components. In some cases chondrules having different textures are almost identical in composition. A few, unusual chondrule types seem to mainly consist of uncommon nebular components, possibly indicating different modes of formation. Etching experiments confirm that chondrule rims are enriched in metal, troilite and moderately volatile elements relative to the bulk chondrules. However, a large fraction of the volatiles remains in the unetched interior.

ChemInform Abstract: TRANSTHALLATION OF ANISOLE AND CHLOROBENZENE BY PHENYLTHALLIUM(III) BIS(TRIFLUOROACETATE)

AbstractPhenyl‐Tl‐bis‐[trifluoracetat] (II) reagiert mit einem Überschuß an Anisol unter Bildung der drei isomeren Anisylphenyl‐Tl(III)‐trifluoracetate (I) und bildet bei Gegenwart eines Trifluoressigsäure‐Überschusses die isomeren Anisyl‐Tl(III)‐bis‐ [trifluoracetate] (III).

Transthallation of anisole and chlorobenzene by phenylthallium(III) bis(trifluoroacetate)

AbstractPhenylthallium(III) bis(trifluoroacetate) (PhTlX2) reacts with excess of anisole at 70‐80°, yielding mainly the three isomeric anisylphenylthallium(III) compounds (o‐, m‐, p‐AnPhTlX). Monoanisylthallium(III) derivatives (o‐, m‐, p‐AnTlX2) are obtained only in the presence of excess of HX. The slow transthallation reaction of chlorobenzene requiring the presence of HX leads to the o‐, m‐, p‐ClC6H4TlX2 isomers rather than to diarylthallium(III) compounds. The three isomeric anisylphenylthallium(III) compounds (o‐, m‐, p‐AnPhTlX) readily undergo exchange and isomerization reactions with anisole in the presence ‐ or more slowly in the absence ‐ of HX. Similar processes were observed with chlorobenzene but only in the presence of HX. (p‐Chlorophenyl)phenylthallium(III) trifluoroacetate is much less reactive, both towards anisole and chlorobenzene. With long reaction times the systems approach equilibrium distributions, which are approximately the same as those found for the prolonged direct thallation of the aromatic compounds. Tentative mechanistic interpretations include three different modes of formation of arylthallium bonds and draw a parallel between thallation, mercuration and plumbylation.

Formation of iridium silicides from Ir thin films on Si substrates

The formation of iridium silicides from the interaction of iridium films with single-crystal silicon substrates has been studied from 350 to 1000 °C. Three distinct phases, IrSi, IrSi1.75(?), and IrSi3, were identified. Different modes of formation were observed and investigated. IrSi and IrSi1.75 form in layers parallel to the substrate at temperatures from 350 to 900 °C. The growth of IrSi3 from nuclei that spread laterally occurs at about 1000 °C, where possible the kinetics were systematically studies.

Endocytosis and the adaptive acid hydrolases in Tetrahymena pyriformis GL.

Endocytosis of yeast cells by Tetrahymena pyriformis GL for a period of 2.5 h produced changes in cellular acid hydrolases. Acid phosphatase, acid deoxyribonuclease and acid proteinase activities were markedly increased, whereas there was a decrease in acid ribonuclease activity and little change in α-glucosidase activity. These alterations do not appear to be due to any alteration in the rates of secretion of these enzymes into the milieu. Evidence is presented that the cellular enzyme increases found upon endocytosis of yeast reflect changes in lysosomal enzymes, because it was shown that the acid phosphatase activity increase resulted in an increased amount of latent enzyme within the cell. The results also support the idea that there are at least 3 distinct populations of lysosomes, in addition to phagolysosomes, present in Tetrahymena pyriformis GL, with different modes of formation. There appears to be a large excess of lysosomes, uncombined with phagosomes, present in these fed cells since latency averaged 66% in broken-cell preparations which contained very few intact phagolysosomes. The phagolysosomal acid phophatase activity cannot account for more than 34% of that present in the cell. The endocytosis of yeast in the presence of growth medium resulted in a marked drop in the rate of cell division as compared to cells growing in the growth medium alone. The results are discussed.

The isotopic geochemistry of speleothems—I. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators

The effects of different modes of formation on the isotopic composition of speleothems has been examined to delineate the conditions under which they may be used to give palaeo-climatic data. The modes of deposition considered were: equilibrium loss of CO 2, kinetic loss of CO 2, and evaporation of water. If isotopic equilibrium is maintained between HCO 3 − and CO 2(aq) the calcite precipitated will be in isotopic equilibrium with the water and variations in 18O 16O will depend on climate alone, variations in 13C 12C will be independent of climate. This will occur if the loss of CO 2 from solution is slow. If loss of CO 2 is rapid a kinetic fractionation will occur between HCO 3 − and CO 2(aq) and the calcite precipitated will show a simultaneous enrichment in 13C and 18O. If evaporation of water occurs the calcite precipitated will be enriched in 18O and will be close to 13C and 14C equilibrium with the cave atmosphere. Speleothems deposited under conditions of kinetic loss of CO 2 or evaporation of water cannot be used to give palaeoclimate data.

A model of a folded zone

At the Laboratory of Tectonophysics of the Geological Faculty of the Moscow State University, experiments were conducted which imitated the mechanism of formation of folding characteristic of strongly disturbed regions, i.e., folding of general crumpling. In accordance with the data obtained when studying natural objects, all the experiments were based on the conception that folding was formed as the result of local horizontal compression of strata by forces originating in the course of vertical displacements of the earth's crust material and the loss of its gravity equilibrium, as well as during vertical movements of separate blocks of the earth's crust. The material for the models was taken with due allowance for the requirements of the theory of physical similarity. At first the experiments were conducted which gave a model of separate sides of the mechanism of folding of general crumpling: the formation of folding as the result of gravitational spreading of uplifted blocks, of flow of strata along the slopes of uplifts, of the change of direction of the vertical movements of the blocks and of the pushing aside action by the cores of injection. A sample was then made which combined all the enumerated particular mechanisms with the supposed mechanical consequences of the processes of granitisation and metamorphism. As a result a model of the whole typical folded zone was constructed, showing different modes of formation of folds in different parts of a cross-section through a folded region.

Chemical behaviour of tin fission products

The oxidation states of 27-hr 121Sn, 127-day 123Sn, 9·7-day 125Sn, 2·1-hr 127Sn, and 60-min 128Sn formed from thermal-neutron fission of 235U have been investigated in irradiated aqueous solutions of uranium and in aqueous solutions formed by dissolving irradiated uranium salts. In the irradiated solutions 121Sn is formed predominantly ( ∼ 2 3 ) in the +4 oxidation state, whereas the other isotopes are formed predominantly ( ∼ 2 3 ) in the +2 state. The difference is attributed to different modes of formation since 121Sn is known to be formed ⩾ 88 per cent by beta decay and 125Sn ⩾ 70 per cent independently. The similarity in the fractions of tin (II) for 123Sn, 125Sn, 127Sn, and 128Sn is taken as evidence that all of these isotopes have similar and large fractional independent yields. This deduction supports suggestions that the stability of the 50-proton structure enhances the primary formation of tin isotopes in fission. Similar, but smaller, differences between the fractions of tin (II) for 121Sn and for the other isotopes were observed for solutions of irradiated uranium (IV) salts. However, only tin (IV) was observed in solutions of irradiated uranium (VI) salts. Mechanisms are discussed which account qualitatively for the predominance of tin (IV) from beta decay and of tin (II) from fission in aqueous solutions.

The Dual Mechanism of Latent-Image Formation In Photographic Emulsions*

AbstractUnsensitized sliver-bromide emulsions are shown to have greater low-intensity reciprocity failure for exposures to blue light than to minus-blue light. This difference of reciprocity-failure behaviour Increases with groIn size but decreases or vanishes on spectral sensitization of the emulsions. It is shown that it is necessary to distinguish the (unction of chemical sensitization at high intensities from that at law intensities of exposure, The results are Interpreted on the basis of two different modes of formation of latent Image.

Nickel in tektites by activation analysis

The nickel abundances of most of the major occurrences of tektites, as well as several meteorite impact glasses and obsidians, have been determined using neutron activation analysis. Iron abundances have been determined in the same specimens and Ni/Fe abundance ratios tabulated. These ratios, determined for the first time on the Libyan Desert glass, the Ivory Coast tektites, and the bediasites, indicate all these specimens fall within the known range of this ratio for the other tektite occurrences. On the basis of this ratio, major differences are observed among specimens from the indochinite tektite occurrence, but no significant differences are observed among specimens from three separate philippinite sites. The differences in the Ni/Fe ratios for tektites and known meteorite impact glasses suggest different modes of formation.