Simulating the lateral transport of ichthyoplankton in the Potomac estuary

Previous simulations of potential ichthyoplankton entrainment by power generating stations on the Potomac estuary have not included the influence of lateral transport in distributing eggs and larvae over the nursery area. Therefore, two-dimensional, vertically-averaged hydrodynamic and kinematic models of passive organism transport were developed to represent advective and dispersive processes near the proposed Douglas Point Nuclear Generating Station. Although the more refined model did not substantially alter the estimate of ichthyoplankton entrainment, it did reveal that lateral inhomogeneities in hydrodynamics could engender severalfold differences in entrainment probabilities on opposite sides of the estuary. Models of higher resolution and greater biological detail did not project greater total entrainment by the Douglas Point plant, because the volume of nontidal flow past the site was large in comparison to the proposed rate of cooling water withdrawal.     

Multivariate statistical estimates of Holocene vegetation and climate change,forest-tundra transition zone,NWT, Canada

Newly derived fossil pollen data were obtained from four sites along a transect from the boreal forest limit into tundra in the eastern Northwest Territories. Multivariate statistical analyses were employed to interpret the pollen assemblages. Transfer functions were constructed between the pollen data and climatic data, and paleoclimatic estimates were derived. The objective nature of the reconstructions provides an independent verification of the general outlines of the chronology of tree-line movements during the mid- and late-Holocene as established in previous paleosol and pollen studies. Boreal forest extended to approximately 62°N, associated with mean July temperatures 1 to 3°C above modern means, from at least 5500 to 3700 yr B.P. Although a major episode of southward displacement of tree line at about 3700 yr B.P. is recorded, later events are not clearly represented. Considerations of the statistics, the time scales, and the nature of the pollen rain suggest only conservative interpretations of the results are possible. It is suggested that the pollen sites may have been sensitive recorders of regional vegetation change only when they were near the ecotone, corresponding to a climatic threshold.     

Pb and Sr isotopes in volcanic rocks from the Aleutian Islands and Pribilof Islands,Alaska

The Pb and Sr isotope ratios of Plio-Pleistocene volcanic rocks from the Aleutian volcanic arc are used as tracers of the lithospheric subduction process at the converging Pacific and Bering plates. Aleutian arc lavas do not have the same Pb isotopic compositions as volcanic rocks of the subducted Pacific ocean crust or the nearby Pribilof Islands, but appear to contain an ‘old continental crustal component’ with high ²⁰⁷Pb/²⁰⁴Pb ratio, as has been found in some other volcanic arcs.⁸⁷Sr/⁸⁶Sr ratios in the Aleutian volcanic arc rocks average 0.70322, slightly higher than fresh volcanic rocks from normal ridge segments, but within the range of values from ‘Icelandic’ ridge segments, oceanic islands and the Pribolof Islands. The Pb and Sr isotopic compositions of Aleutian lavas show a positive correlation and the range of values does not change for volcanoes distributed along strike in the arc, even though the crustal type in the hanging wall of the Benioff zone changes from oceanic in the west to continental in the east. Since the basement of the continental arc segment is older than the basement of the oceanic segment, and probably has a different isotopic character, the constancy of isotopic ratios along the arc argues against contamination by wall rocks of the type exposed in the arc.A sufficient explanation for the isotopic data is the mixture of several per cent of continent-derived sediment with melt derived from the underthrust oceanic crust and overlying mantle. This small amount of contaminant is difficult to document by geophysical observations. Such a model implies extensive recycling of Ba, Pb, K and Rb through volcanism at convergent plate margins like the Aleutians.     

Griffith Taylor Lecture,Geographical Society of New South Wales, 2004: Australia and the ‘State of Nature/Native’

This account sets itself against the binarism of physical versus cultural determinism that drives population–environment debate over people's place on the Australian continent. In reading back into colonial times, it evokes ‘our’ unresolved sense of relation to nature on a continent that has pressed awkward buttons since the time the botanist on Cook's Endeavour, Joseph Banks, first mused over the apparent contradiction on the east coast of human presence and uncultivated land. Australia's ‘state of nature’, including its inhabitants, elicited tensions in Anglo-Celtic peoples’ ambiguous relation with nature that have never gone away or been resolved. Indeed, I argue, against the background of what was considered in eighteenth-century Enlightenment thought to be a distinctly human separation from, and capacity to rise above, nature, that the Australian state of nature/native precipitated a crisis in prevailing ideas of ‘the human’. As consternation grew into the nineteenth century about the very capacity of Aboriginal people for improvement, and particularly for cultivation, I argue they challenged the basis upon which the unity of humankind had been assumed in Enlightenment thought. They could not be comprehended, according to the prevailing conception of racial difference, as a mere variety of the human. The radical idea of innate human difference may thus be understood as arising out of this incomprehension, as an attempt to account for the ontologically inexplicable difference of the Australian Aborigine. This lecture's excursion into the origins of ‘race’ is by way of highlighting the uncertainty of the divide between the human and non-human worlds. Imagining an entangled world of living things may help to craft fresh lines of debate about modes of environmental belonging and becoming on this continent that overcome the stale binaries and blind spots of Australia's population–environment debate.     

The nature of molecular cloud material in interplanetary dust

Eight interplanetary dust particles (IDPs) exhibiting a wide range of H and N isotopic anomalies have been studied by transmission electron microscopy, x-ray absorption near-edge structure spectroscopy, and Fourier-transform infrared spectroscopy. These anomalies are believed to have originated during chemical reactions in a cold molecular cloud that was the precursor to the Solar System. The chemical and mineralogical studies reported here thus constitute direct studies of preserved molecular cloud materials. The H and N isotopic anomalies are hosted by different hydrocarbons that reside in the abundant carbonaceous matrix of the IDPs. Infrared measurements constrain the major deuterium (D) host in the D-enriched IDPs to thermally labile aliphatic hydrocarbon groups attached to macromolecular material. Much of the large variation observed in D/H in this suite of IDPs reflects the variable loss of this labile component during atmospheric entry heating. IDPs with elevated ¹⁵N/¹⁴N ratios contain N in the form of amine (-NH₂) functional groups that are likely attached to other molecules such as aromatic hydrocarbons. The host of the N isotopic anomalies is not as readily lost during entry heating as the D-rich material. Infrared analysis shows that while the organic matter in primitive anhydrous IDPs is similar to that observed in acid residues of primitive chondritic meteorites, the measured aromatic:aliphatic ratio is markedly lower in the IDPs.     

A molecular orbital study of bonding in sulfate molecules: Implications for sulfate crystal structures

Molecular orbital calculations have been completed on sulfate monomers and a dimer in a determination of minimum-energy geometries and electron density distributions. SO bond lengths calculated for the monomer and dimer correlate linearly with the fractional s-characters of the bonds, as observed for sulfate groups in crystals. With increasing coordination number of S, the bonded radii of S and O, as determined from electron density maps, increase at the same rate. This is at variance with the assumption that the radius of the oxide ion is nearly constant and that bond length variations arise primarily from changes in cation radii. The dimer shows a relatively large change in energy as its SOS angle is deformed from its minimum-energy value (125.6°) to 180°, in conformity with the small variation among observed angles. This is in contrast to the wide variation of bridging angles observed for silicate and phosphate dimers in crystals and molecules, and may imply that significant differences should be expected in the behavior of sulfates with respect to polymorphism and glass formation. The reaction energy of SO₃ + H₂O → H₂SO₄, calculated with second-order Møller-Plesset perturbation theory, agrees with the experimental value. Other properties of H₂SO₄ are also calculated and compared with experimental observations and previous calculations.     

Botryococcus—A planktonic green alga, the source of petroleum through the ages: Transmission electron microscopical studies of oil shales and petroleum source rocks

The hydrocarbon secreting alga Botryococcus has been identified in organic remains of sediments ranging from Precambrian to Recent, and is believed to have been a major source material for petroleum generation throughout the geological time. In some petroleum source rocks of Lower Palaeozoic and Precambrian age, identification of the alga is only possible by electron microscopy. Transmission electron microscopy (TEM) has been used in the present study to identify microstructures of the algal remains in a range of oil shales and petroleum source rocks. It has been established that Botryococcus is the predominant alga in the Kukersite oil shale of Estonia. Similarly, the alga has been shown to be a major contributor to petroleum source rocks in Cambrian and Precambrian sedimentary basins in Australia. TEM has been applied to observations of Botryococcus in torbanites and to products from simulated maturation experiments on torbanite. A comparison with algal remains from Cambrian and Precambrian sediments ranging from undermature to overmature, enabled the distinction of organic matter in various stages of oil generation. Maturation/thermal effects on alginite have been established by reflectance and fluorescence, and compared with experimental results.     

The effects of survey frequency on estimates of scour and fill in a braided river model

An erratum has been published for this article in Earth Surface Processes and Landforms 27(7) 2002, 795. Estimates of scour and fill in rivers that are derived by differencing topographic surfaces are known to be negatively biased by local compensation of scour and fill between surveys but the magnitude of bias is not well known. This study examines the effect of survey frequency on volumes of scour and fill over a period of active channel braiding in a small‐scale river model. A 100 min, high temporal resolution time series of digital elevation models is artificially coarsened by selectively removing models. The resulting four overlapping time series have survey intervals of 10 min, 20 min, 50 min and 100 min. Cumulative scour and fill volumes for the 100 min period are compared between the four series. It is concluded that the decay in measured volumes of scour and fill with increased survey interval can be described using inverse functions. Cumulative scour–fill volumes are approximately 420 per cent greater over the study period for 10 min survey intervals than for a 100 min interval. After the 100 min period of competent flow, nearly 65 per cent of the channel area experienced significant compensation of scour and fill. Several compensation mechanisms were identified in association with braided channel kinetics, including lateral channel migration, the migration of bed forms, and channel avulsion. It is demonstrated that by negatively biasing scour, fill and net estimates, this error significantly affects morphological approaches to the estimation of bed load sediment transport. Copyright © 2002 John Wiley & Sons, Ltd.     

安第斯埃达克岩 :三种成因模式

在安第斯埃达克岩是常见的。由这种岩石的痕量元素特征揭示的高压组成矿物 ,表明其有三种可能的成因。这三种成因按相对重要性依次为 :安第斯地壳的构造加厚 ;弧前壳的俯冲 -侵蚀 ;年轻洋壳的俯冲。在过去 30 Ma,每种成因模式的埃达克岩在智利 -阿根廷安第斯地区均有喷发。它们的产状与智利 -阿根廷边缘快速变化的构造背景中特定构造条件和事件相吻合。埃达克岩在过去安第斯边缘的产状可望成为矿化的优异构造标志 ,也可用于指导找矿