Showing papers by "Australian National University published in 1992"

I- and S-type granites in the Lachlan Fold Belt

Abstract: Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth's surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe3+/Fe2+, and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks.S-type granites have higher δ18O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks.Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.

Remnants of ≥3800 Ma crust in the Chinese part of the Sino-Korean craton

Abstract: Ion microprobe U-Pb analyses of zircons from the Sino-Korean craton have identified remnants of ≥3800 Ma crust at two localities—near Caozhuang east of Beijing, and near Anshan, northeast China. Near Caozhuang, the detrital zircons in an Archean metaquartzite are 3550 Ma or older; about one-fourth have ages between 3800 and 3850 Ma. The ages for detrital zircons with concordant U-Pb ages form a polymodal distribution, interpreted to show that the detritus that formed the quartzite was derived from a terrane containing early Archean rocks of several ages. Near Anshan, sheared gneiss is present in a complex containing ca. 3300 and 3000 Ma granites. Some of the zircons in this gneiss are concordant with a weighted mean 207 Pb/ 206 Pb age of 3804 ±5 Ma (2 σ), interpreted to be the age of the protolith of the gneiss.

Zircon U-Pb ages for the Early Cambrian time-scale

Abstract: Single zircons from two Early Cambrian volcanic horizons have been analysed using the SHRIMP ion microprobe. Full details of the analytical procedures and data reduction are given. Zircons from tuff within the Lie de Vin Formation, near Tiout, Morocco, show little spread in U-Pb age and have a mean value of 521 ± 7 Ma (2σ). Those from a bentonite within unit 5 of the Meishucun section near Kunming, southern China, show relatively dispersed U-Pb ages, revealing the presence of both detrital or xenocrystic grains as well as areas within grains that have lost radiogenic Pb. The main population has a mean age of 525 ± 7 Ma, but a mean 207 Pb/ 206 Pb age of 539 ± 34 Ma which is a maximum estimate for the bentonite age. These results conflict with previous Rb-Sr whole rock ages of c. 580 Ma for overlying Cambrian shales at Meishucun, and c. 570 Ma for Atdabanian shales from the E. Yangtse Gorges area.

Synchronism of the Siberian Traps and the Permian-Triassic Boundary

Abstract: Uranium-lead ages from an ion probe were taken for zircons from the ore-bearing Noril9sk I intrusion that is comagmatic with, and intrusive to, the Siberian Traps. These values match, within an experimental error of ±4 million years, the dates for zircons extracted from a tuff at the Permian-Triassic (P-Tr) boundary. The results are consistent with the hypothesis that the P-Tr extinction was caused by the Siberian basaltic flood volcanism. It is likely that the eruption of these magmas was accompanied by the injection of large amounts of sulfur dioxide into the upper atmosphere, which may have led to global cooling and to expansion of the polar ice cap. The P-Tr extinction event may have been caused by a combination of acid rain and global cooling as well as rapid and extreme changes in sea level resulting from expansion of the polar ice cap.

A Cointegration Analysis of Treasury Bill Yields

Abstract: This paper shows that yields to maturity of U.S. Treasury bills are cointegrated and that, during periods when the Federal Reserve specifically targeted short-term interest rates, the spreads between yields of different maturity define the cointegrating vectors. This cointegrating relationship implies that a single nonstationary common factor underlies the time-series behavior of each yield to maturity and that risk premia are stationary. An error-correction model that uses spreads as the error-correction terms is unstable over the Federal Reserve's policy regime changes, but a model using post 1982 data is stable and is shown to be useful for forecasting changes in yields. Copyright 1992 by MIT Press.

Uncertainty, sustainability and change☆

Abstract: This article offers an exploration of connections between sustainability, risk and uncertainty Global environmental change and human sustainability are characterized as the challenge of managing change in dynamic systems riddled with uncertainty A number of disciplines and intellectual traditions, including systems thinking, risk and ecology, are surveyed briefly as sources to inform an approach to this challenge Approaches to managing risk and uncertainty are discussed, a typology of resilience constructed, and an approach to sustainability defined The discussion is based on the three imperatives of constant change, everpresent uncertainty and ignorance, and an increasingly stressed interdependency between humans and the biosphere

Deformational Mass Transport and Invasive Processes in Soil Evolution

Abstract: Soils are differentiated vertically by coupled chemical, mechanical, and biological transport processes. Soil properties vary with depth, depending on the subsurface stresses, the extent of mixing, and the balance between mass removal in solution or suspension and mass accumulation near the surface. Channels left by decayed roots and burrowing animals allow organic and inorganic detritus and precipitates to move through the soil from above. Accumulation occurs at depths where small pores restrict further passage. Consecutive phases of translocation and root growth stir the soil; these processes constitute an invasive dilatational process that leads to positive cumulative strains. In contrast, below the depth of root penetration and mass additions, mineral dissolution by descending organic acids leads to internal collapse under overburden load. This softened and condensed precursor horizon is transformed into soil by biological activity, which stirs and expands the evolving residuum by invasion by roots and macropore networks that allows mixing of materials from above.

Low conductances for CO2 diffusion from stomata to the sites of carboxylation in leaves of woody species

Abstract: Concurrent measurements of leaf gas exchange and on-line 13C discrimination were used to evaluate the CO2 conductance to diffusion from the stomatal cavity to the sites of carboxylation within the chloroplast (internal conductance; gi). When photon irradiance was varied it appeared that gi and/or the discrimination accompanying carboxylation also varied. Despite this problem, gi, was estimated for leaves of peach (Prunus persica), grapefruit (Citrus paradisi), lemon (C. limon) and macadamia (Macadamia integrifolia) at saturating photon irradiance. Estimates for leaves of C. paradisi, C. limon and M. integrifolia were considerably lower than those previously reported for well-nourished herbaceous plants and ranged from 1.1 to2.2μmol CO2 m−2 s−1 Pa−1, whilst P. persica had a mean value of 3.5 μmol CO2 m−2 s−1 Pa−1. At an ambient CO2 partial pressure of 33Pa, estimates of chloroplastic partial pressure of CO2 (Cc) using measurements of CO2 assimilation rate (A) and calculated values of gi, and of partial pressure of CO2 in the stomatal cavity (Cst) were as low as 11.2 Pa for C. limon and as high as 17.8Pa for peach. In vivo maximum rubisco activities (Vmax) were also determined from estimates of Cc. This calculation showed that for a given leaf nitrogen concentration (area basis) C. paradisi and C. limon leaves had a lower Vmax than P. persica, with C. paradisi and C. limon estimated to have only 10% of leaf nitrogen present as rubisco. Therefore, low CO2 assimilation rates despite high leaf nitrogen concentrations in leaves of the evergreen species examined were explained not only by a low Cc but also by a relatively low proportion of leaf nitrogen being used for photosynthesis. We also show that simple one-dimensional equations describing the relationship between leaf internal conductance from stomatal cavities to the sites of carboxylation and carbon isotope discrimination (Δ) can lead to errors in the estimate of gi. Potential effects of heterogeneity in stomatal aperture on carbon isotope discrimination may be particularly important and may lead to a dependence of gi upon CO2 assimilation rate. It is shown that for any concurrent measurement of A and Δ, the estimate of Cc is an overestimate of the correct photosynthetic capacity-weighted value, but this error is probably less than 1.0 Pa.

On the emergence of plate tectonics

Abstract: At present, young oceanic lithosphere is positively buoyant, and it does not become negatively buoyant until it is older than about 20 m.y. If, in the past, the mantle was hotter, the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater. On the other hand, the hotter mantle would have had a lower viscosity and convected faster, so the average age of oceanic plates at subduction would have been less than the present 100 m.y. At some time in the past, average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone. It is estimated here that this condition occurred when the mantle was only about 50 °C hotter than at present and as recently as 0.9-1.4 b.y. ago. Transformation of basaltic crust to eclogite might have enhanced the vigor of the plates, but possibly only modestly and intermittently. In earlier times, plate tectonics could have operated, but more slowly, so that it could not by itself have accomplished the necessary rate of heat removal from the earth. A different tectonic mode must also have operated, perhaps subcrustal delamination or dripping. Plate tectonics would have gradually taken over from the earlier mode. Plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer, and so they could have operated in conjunction with both regimes of the upper boundary layer.

Mantle plumes and continental tectonics.

Abstract: Mantle plumes and plate tectonics, the result of two distinct modes of convection within the Earth, operate largely independently. Although plumes are secondary in terms of heat transport, they have probably played an important role in continental geology. A new plume starts with a large spherical head that can cause uplift and flood basalt volcanism, and may be responsible for regional-scale metamorphism or crustal melting and varying amounts of crustal extension. Plume heads are followed by narrow tails that give rise to the familiar hot-spot tracks. The cumulative effect of processes associated with tail volcanism may also significantly affect continental crust.

Direct evidence for human use of plants 28,000 years ago: starch residues on stone artefacts from the northern Solomon Islands

Abstract: The excavation of Kilu Cave and the discovery of a Pleistocene prehistory for the Solomon Islands have already been reported in ANTIQUITY by Wickler & Spriggs (62: 703–6). Residue analysis of stone artefacts from the site now provides the earliest direct evidence for the prehistoric use of root vegetables, in the form of starch grains and crystalline raphides identifiable to genus. The direct microscopic identification of starch grains opens new avenues for the study of the plant component of human diets in the distant past.

On the regularity of solutions to the Yamabe equation and the existence of smooth hyperboloidal initial data for Einstein's field equations

Abstract: The regularity of the solutions to the Yamabe Problem is considered in the case of conformally compact manifolds and negative scalar curvature. The existence of smooth hyperboloidal initial data for Einstein's field equations is demonstrated.

Reduction of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Content by Antisense RNA Reduces Photosynthesis in Transgenic Tobacco Plants

Abstract: A complementary DNA for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was cloned from tobacco (Nicotiana tabacum) and fused in the antisense orientation to the cauliflower mosaic virus 35S promoter. This antisense gene was introduced into the tobacco genome, and the resulting transgenic plants were analyzed to assess the effect of the antisense RNA on Rubisco activity and photosynthesis. The mean content of extractable Rubisco activity from the leaves of 10 antisense plants was 18% of the mean level of activity of control plants. The soluble protein content of the leaves of anti-small subunit plants was reduced by the amount equivalent to the reduction in Rubisco. There was little change in phosphoribulokinase activity, electron transport, and chlorophyll content, indicating that the loss of Rubisco did not affect these other components of photosynthesis. However, there was a significant reduction in carbonic anhydrase activity. The rate of CO2 assimilation measured at 1000 micromoles quanta per square meter per second, 350 microbars CO2, and 25°C was reduced by 63% (mean value) in the antisense plants and was limited by Rubisco activity over a wide range of intercellular CO2 partial pressures (pi). In control leaves, Rubisco activity only limited the rate of CO2 assimilation below a pi of 400 microbars. Despite the decrease in photosynthesis, there was no reduction in stomatal conductance in the antisense plants, and the stomata still responded to changes in pi. The unchanged conductance and lower CO2 assimilation resulted in a higher pi, which was reflected in greater carbon isotope discrimination in the leaves of the antisense plants. These results suggest that stomatal function is independent of total leaf Rubisco activity.

Evidence From 12S Ribosomal RNA Sequences That Onychophorans Are Modified Arthropods

Abstract: The evolutionary relationships of the onychophorans (velvet worms) and the monophyly of the arthropods have generated considerable debate. Cladistic analyses of 12S ribosomal RNA sequences indicate that arthropods are monophyletic and include the onychophorans. Maximum parsimony analyses and monophyly testing within arthropods indicate that myriapods (millipedes and centipedes) form a sister group to all other assemblages, whereas crustaceans (shrimps and lobsters) plus hexapods (insects and allied groups) form a well-supported monophyletic group. Parsimony analysis further suggests that onychophorans form a sister group to chelicerates (spiders and scorpions) and crustaceans plus hexapods, but this relationship is not well supported by monophyly testing. These relationships conflict with current hypotheses of evolutionary pathways within arthropods.

The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia

Abstract: The ore-element associations of granite-related ore deposits in the eastern Australian Palaeozoic fold belts can be related to the inferred relative oxidation state, halogen content and degree of fractional crystallisation within the associated granite suites Sn mineralisation is associated with both S- and I-type granites that are reduced and have undergone fractional crystallisation Cu and Au are associated with magnetite- and/or sphene-bearing, oxidised, intermediate I-type suites Mo is associated with similar granites that are more fractionated and oxidised W is associated with a variety of granite types and shows little dependence on inferred magma redox state The observed ore deposit-granite type distribution in eastern Australia, and the behaviour of ore elements during fractionation, is consistent with models of ore element sequestering by sulphides and Fe-Ti phases (eg pyrrhotite, ilmenite, sphene, magnetite) whose stability is nominally fO2-dependent Fractional crystallisation acts to amplify this process through the progressive removal of compatible elements and the concentration of incompatible elements into decreasing melt volumes The halogen content is also important S-type granites are poorer in Cl than I-types Cl decreases and F increases in both S- and I-type granites with fractional crystallisation Low Cl contents combined with low magma fO2 in themselves seem to provide an adequate explanation for the rarity of Mo, Cu, Pb and Zn type mineralisation with S-type granites Although such properties of granite suites seem adequately to predict the associated ore-element assemblage to be expected in associated mineral deposits, additional factors determine whether or not there is associated economic mineralisation

Origins of genes: "big bang" or continuous creation?

Abstract: Many protein families are common to all cellular organisms, indicating that many genes have ancient origins. Genetic variation is mostly attributed to processes such as mutation, duplication, and rearrangement of ancient modules. Thus it is widely assumed that much of present-day genetic diversity can be traced by common ancestry to a molecular "big bang." A rarely considered alternative is that proteins may arise continuously de novo. One mechanism of generating different coding sequences is by "overprinting," in which an existing nucleotide sequence is translated de novo in a different reading frame or from noncoding open reading frames. The clearest evidence for overprinting is provided when the original gene function is retained, as in overlapping genes. Analysis of their phylogenies indicates which are the original genes and which are their informationally novel partners. We report here the phylogenetic relationships of overlapping coding sequences from steroid-related receptor genes and from tymovirus, luteovirus, and lentivirus genomes. For each pair of overlapping coding sequences, one is confined to a single lineage, whereas the other is more widespread. This suggests that the phylogenetically restricted coding sequence arose only in the progenitor of that lineage by translating an out-of-frame sequence to yield the new polypeptide. The production of novel exons by alternative splicing in thyroid receptor and lentivirus genes suggests that introns can be a valuable evolutionary source for overprinting. New genes and their products may drive major evolutionary changes.

Smoothed cross-validation

Abstract: For bandwidth selection of a kernel density estimator, a generalization of the widely studied least squares cross-validation method is considered. The essential idea is to do a particular type of “presmoothing” of the data. This is seen to be essentially the same as using the smoothed bootstrap estimate of the mean integrated squared error. Analysis reveals that a rather large amount of presmoothing yields excellent asymptotic performance. The rate of convergence to the optimum is known to be best possible under a wide range of smoothness conditions. The method is more appealing than other selectors with this property, because its motivation is not heavily dependent on precise asymptotic analysis, and because its form is simple and intuitive. Theory is also given for choice of the amount of presmoothing, and this is used to derive a data-based method for this choice.

Direct measurement of depletion and structural forces in a micellar system.

Abstract: The force as a function of separation is measured between two mica surfaces coated with adsorbed bilayers of cetyltrimethylammonium bromide (CTAB) and immersed in aqueous ionic micellar solutions of CTAB. At low separations double-layer forces are observed and arise solely from the dissociated counterions and free CTA + . At large separations, the repulsion is reduced and a secondary minimum in the interaction potential deepens when the micellar concentration is increased. Finally, at higher concentrations, structural erects superimpose an oscillatory force profile at further separations

Neutron emission as a probe of fusion-fission and quasifission dynamics.

Abstract: Pre-scission and post-scission neutron yields have been measured as a function of projectile mass, compound nucleus fissility, and fission mass split and total kinetic energy (TKE) for 27 fusion-fission and quasifission reactions induced by beams of $^{16,18}\mathrm{O}$, $^{40}\mathrm{Ar}$, and $^{64}\mathrm{Ni}$. A new method of interpretation of experimental pre-scission neutron multiplicities ${\ensuremath{ u}}_{\mathrm{pre}}$ and mean kinetic energies ${\mathrm{\ensuremath{\varepsilon}}}_{\ensuremath{ u}}$ allows the extraction of fission time scales with much less uncertainty than previously, all fusion-fission results being consistent with a dynamical time scale of (35\ifmmode\pm\else\textpm\fi{}15)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ s for symmetric fission. All reactions show that ${\ensuremath{ u}}_{\mathrm{pre}}$ falls quite rapidly with increasing mass asymmetry; evidence is presented that for fusion-fission reactions this is partly due to a reduction of the dynamical fission time scale with mass asymmetry. For quasifission, the data indicate that the pre-scission multiplicity and mean neutron kinetic energy are very sensitive to the final mass asymmetry, but that the time scale is virtually independent of mass asymmetry. It is concluded that for fusion-fission there is no dependence of ${\ensuremath{ u}}_{\mathrm{pre}}$ on TKE, while for $^{64}\mathrm{induced}$ quasifission reactions, a strong increase of ${\ensuremath{ u}}_{\mathrm{pre}}$ with decreasing TKE is observed. This is probably largely caused by neutron emission during the acceleration time of the fission fragments in these fast reactions. Interpretation of post-scission multiplicities in terms of fragment excitation energies leads to deduced time scales consistent with those determined from the pre-scission data.

Solidification and Morphology of Submarine Lavas: A Dependence on Extrusion Rate

Abstract: The results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity. The experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter, the ratio of the time taken for the surface to solidify, and a time scale for lateral flow. For submarine basalts a solution of the cooling problem (which is dominated by conduction in the lava but convective heat transfer in the water) and estimates of lava viscosities place this parameter within the empirically determined 'pillowing' regime over a wide range of extrusion rates. This results is consistent with the observation that pillow basalts are the most common products of submarine eruptions. Smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusion of basaltic magma. Still higher eruption rates in regions of low topographic relief may produce submarine lava lakes. Minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type.

Distinctive Responses of Ribulose-1,5-Bisphosphate Carboxylase and Carbonic Anhydrase in Wheat Leaves to Nitrogen Nutrition and their Possible Relationships to CO2-Transfer Resistance

Abstract: The amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total chlorophyll (Chl), and total leaf nitrogen were measured in fully expanded, young leaves of wheat (Triticum aestivum L.), rice (Oryza sativa L.), spinach (Spinacia oleracea L.), bean (Phaseolus vulgaris L.), and pea (Pisum sativum L.). In addition, the activities of whole-chain electron transport and carbonic anhydrase were measured. All plants were grown hydroponically at different nitrogen concentrations. Although a greater than proportional increase in Rubisco content relative to leaf nitrogen content and Chl was found with increasing nitrogen supply for rice, spinach, bean, and pea, the ratio of Rubisco to total leaf nitrogen or Chl in wheat was essentially independent of nitrogen treatment. In addition, the ratio of Rubisco to electron transport activities remained constant only in wheat. Nevertheless, gas-exchange analysis showed that the in vivo balance between the capacities of Rubisco and electron transport in wheat, rice, and spinach remained almost constant, irrespective of nitrogen treatment. The in vitro carbonic anhydrase activity in wheat was very low and strongly responsive to increasing nitrogen content. Such a response was not found for the other C3 plants examined, which had 10- to 30-fold higher carbonic anhydrase activity than wheat at any leaf-nitrogen content. These distinctive responses of carbonic anhydrase activity in wheat were discussed in relation to CO2-transfer resistance and the in vivo balance between the capacities of Rubisco and electron transport.

A new approach to the discretization of continuous-time controllers

Abstract: A new method for discretizing continuous-time controllers is derived, using principles of controller approximation. It focuses on the closed-loop use of the discrete-time controller. The resulting approximation criterion is a measure for stability of the control system, provides an upper bound on the sampling time for which stability can be guaranteed, and because it is based on continuous-time controller approximation it indicates the cost of discretization in terms of performance degradation. The discrete-time controller is obtained through minimization of this criterion, which can be performed with standard software used in H/sub infinity / controller design. >

Studies of the degradation and protection of wood surfaces.

Abstract: Large weight losses occurred in thin veneers of radiata pine (P. radiata) during natural weathering due mainly to loss of lignin and hemicelluloses. This was demonstrated by direct chemical analysis and by spectrocopic techniques. Treatment of veneers with dilute aqueous solutions of the recognized surface stabilizing compound chromium trioxide was found to dramatically restrict weight loss during weathering, but similar applications of ferric compound were less effective (Evans, Schmalzl 1989). In order to rationalize these protective effects FTIR internal reflectance spectroscopy was used to study chemical changes taking place at the wood surface upon treatment and during weathering. The spectra obtained provide direct evidence for the modification and stabilization of the lignin aromatic system with aqueous chromium trioxide and to a lesser extent with ferric salts. It is postulated that photostable lignin complexes are formed. The implications of these findings for the development of improved surface stabilizing compounds for wood are discussed briefly.