Estimating nutrient budgets in tropical estuaries subject to episodic flows

Most Australian estuaries are subject to riverine discharge regimes that are highly episodic. This characteristic poses difficulties for estimating nutrient budgets of such systems based on sampling regimes that do not resolve the discharge variation and the changes in nutrient distributions that they cause. This paper presents a method for calculating nutrient budgets in estuaries having episodic hydrology. The method utilises a simple hydrodynamic transport model that is calibrated using measured salinities and which is used to describe the transport properties of the estuary as they respond to river discharge. Using this transport model, the temporal variation in nutrient concentrations within the estuary can be resolved between sampling surveys even when the discharge events are of short duration. An inverse method is then applied to calculate internal fluxes of nutrients from measurements obtained on successive sampling surveys. The approach is demonstrated through an application to the Fitzroy Estuary in Queensland, Australia.     

Using computational intelligence techniques to model subglacial water systems

Measurements of water pressure beneath Trapridge Glacier, Yukon Territory, Canada show that the basal water system is highly heterogeneous. Three types of behaviour were recorded: pressure records which are strongly correlated, records which are strongly anticorrelated, and records which alternate between strong correlation and strong anticorrelation. We take the pressure in bore-holes that are connected to the evacuation route for basal water as the forcing, and the other pressures as the response to this forcing. Previous work (Murray and Clarke 1995) has shown that these relationships can be modelled using low-order nonlinear differential equations optimized by inversion. However, despite optimizing the model parameters we cannot be sure that the final model forms are themselves optimal. Computational intelligence techniques provide alternative methods for fitting models and are robust to missing or noisy data, applicable to non-smooth models, and attempt to derive optimal model forms as well as optimal model parameters. Four computational intelligence techniques have been used and the results compared with the more conventional mathematical model. These methods were genetic programming, artificial neural networks, fuzzy logic and self-organizing maps. We compare each technique and offer an evaluation of their suitability for modelling the pressure data. The evaluation criteria are threefold: (1) goodness of fit and an ability to predict subsequent data under different surface weather conditions; (2) interpretability, and the extent and significance of any new insights offered into the physics of the glacier; (3) computation time. The results suggest that the suitability of the computational intelligence techniques to model these data increases with the complexity of the system to be modelled.     

Design principles for origin-destination flow maps

Origin-destination flow maps are often difficult to read due to overlapping flows. Cartographers have developed design principles in manual cartography for origin-destination flow maps to reduce overlaps and increase readability. These design principles are identified and documented using a quantitative content analysis of 97 geographic origin-destination flow maps without branching or merging flows. The effectiveness of selected design principles is verified in a user study with 215 participants. Findings show that (a) curved flows are more effective than straight flows, (b) arrows indicate direction more effectively than tapered line widths, and (c) flows between nodes are more effective than flows between areas. These findings, combined with results from user studies in graph drawing, conclude that effective and efficient origin-destination flow maps should be designed according to the following design principles: overlaps between flows are minimized; symmetric flows are preferred to asymmetric flows; longer flows are curved more than shorter or peripheral flows; acute angles between crossing flows are avoided; sharp bends in flow lines are avoided; flows do not pass under unconnected nodes; flows are radially distributed around nodes; flow direction is indicated with arrowheads; and flow width is scaled with represented quantity.     

Use of high spectral resolution remote sensing to determine leaf palatability of eucalypt trees for folivorous marsupials

In this study we assess the feasibility of remotely measuring canopy biochemistry, and thus the potential for conducting large-scale mapping of habitat quality. A number of studies have found nutrient composition of eucalypt foliage to be a major determinant of the distribution of folivorous marsupials. More recently it has been demonstrated that a specific group of secondary plant chemicals, the diformylphloroglucinols (DFPs), are the most important feeding deterrents, and are thus vital determinants of habitat quality. We report on the use of laboratory spectroscopy to attempt to identify one such DFP, sideroxylonal-A, in the foliage of Eucalyptus melliodora, one of the few eucalypt species browsed by folivorous marsupials. Reflectance spectra were obtained for freeze-dried, ground leaves using near infrared spectroscopy (NIRS) and for both oven-dried and fresh whole leaves using a laboratory-based (FieldSpec) spectroradiometer. Modified partial least squares (MPLS) regression was used to develop calibration equations for sideroxylonal-A concentration based on the reflectance spectra transformed as both the first and second difference of absorbance (Log 1/R). The predictive ability of the calibration equations was assessed using the standard error of calibration statistic (SECV). Coefficients of determination (r²) were highest for the ground leaf spectra (0.98), followed by the fresh leaf and dry leaf spectra (0.94 and 0.87, respectively). When applied to independent validation sub-sets, sideroxylonal-A was most accurately predicted from the ground leaf spectra (r² = 0.94), followed by the dry leaf and fresh leaf spectra (0.72 and 0.53, respectively). Two spectral regions, centred on 674 nm and 1394 nm, were found to be highly correlated with sideroxylonal-A concentration for each of the three spectral data sets studied. Results from this study suggest that calibration equations derived from modified partial least squares regression may be used to predict sideroxylonal-A concentration, and hence leaf palatability, of Eucalyptus melliodora trees, thereby indicating that the remote estimation of habitat quality of eucalypt forests for marsupial folivores is feasible.     

The petrology of the Las Canadas volcanoes,Tenerife, Canary islands

Tenerife is the largest of the seven Tertiary to Recent volcanic islands that make up the Canary Archipelago. The island is composed of volcanics belonging to the basanitetrachyte-phonolite assemblage that characterises many Atlantic islands. The most voluminous development of intermediate and salic volcanics has been in the centre of the island where the Las Canadas volcanoes arose upon a basement shield composed mainly of basanite and ankaramite flows, tuffs and agglomerates. The initial post-shield activity built the Vilaflor volcanic complex (Lower and Upper Canadas Series) that originally covered much of the underlying shield volcanics. A vast collapse of the complex, probably during post-Pleistocene times, in the centre of the island has left a large semi-circular wall, and provides an excellent vertical section through the complex. Quaternary volcanism within the collapsed area has built the twin, central-type volcanoes, Viejo and Teide, both of which have attendant satellite vents. That part of the Vilaflor Complex exposed in Las Canadas, together with the Viejo and Teide volcanoes, comprise the Las Canadas volcanoes.Four distinct rock types can be recognised in these volcanoes, basanite, trachybasanite, plagioclase phonolite, and phonolite. Each rock type can be recognised chemically and mineralogically, but there is essentially a gradational series from basanite to phonolite that includes both aphyric and glomerophyric rocks. The volcanics are strongly undersaturated and sodic, and some of the phonolites are mildly peralkaline. Variations in degree of undersaturation, and trace element abundances indicate a number of cycles of activity which would be consistent with the known field relations.Forsteritic olivine occurs in the basanites and trachybasanites but is not a stable phase in the more salic volcanics. Clinopyroxene is ubiquitous, varying in composition from titanaugite in the basanites to slightly sodic augite in the phonolites. Strongly sodic pyroxene is restricted to the groundmass of the microcrystalline phonolites along with aenigmatite and a kataphoritic amphibole. Plagioclase is found only in the groundmass of the basanites, but andesine and potash-oligoclase are common phenocryst minerals in the trachybasanites and plagioclase phonolites respectively, whereas the characteristic feldspar of the phonolites is anorthoclase.The relatively smooth curves of major and trace element variation, the presence of accumulative volcanics at all stages of differentiation, zoning of the mineral phases, and the clustering of the phonolites around the low temperature trough in Petrogeny's Residua System, all indicate that the descent from basanite to phonolite has resulted from fractional crystallisation of a basanite parent magma. The trend of pyroxene crystallisation, and the fairly constant FeO/Fe₂O₃ ratio during fractionation indicate crystallisation under low PO₂ conditions.     

Post-peak,fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block,southern India

The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH₂O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.     

Rate of growth of the preserved North American continental crust: Evidence from Hf and O isotopes in Mississippi detrital zircons

Detrital zircons from the Mississippi River have been analyzed for U-Th-Pb, Lu-Hf and O isotopes to constrain the rate of growth of the preserved North American continental crust. One hundred and forty two concordant zircon U/Pb dates on grains mounted in epoxy, obtained by Excimer laser ablation ICP-MS method, resolved six major periods of zircon crystallization: 0-0.25, 0.3-0.6, 0.95-1.25, 1.3-1.5, 1.65-1.95 and 2.5-3.0 Ga. These age ranges match the ages of the recognized lithotectonic units of the North American continent in the hinterland of the Mississippi River. Ninety-six zircons mounted on tape, which show no age zonation and were within 7.5% of concordance, were selected to represent the six U/Pb age time intervals and analyzed for Lu-Hf and O isotope by laser ablation MC-ICP-MS and SHRIMP II, respectively. The δ¹⁸O values of the zircons show a small step increase in the maximum δ¹⁸O values at the Archean-Proterozoic boundary from 7.5‰ in the Archean to 9.5‰, and rarely 13‰, in the Proterozoic and Phanerozoic. However, the average value of δ¹⁸O in zircons changes little with time, showing that the increase in the maximum δ¹⁸O values between 2.5 and 2.0 Ga, which can be attributed to an increase in the sediment content of the source regions of younger granitoids, is largely balanced by an increase in zircons with anomalously low δ¹⁸O, which can be attributed to hydrothermally altered crust in the granitoid source region.εHf_i values for the zircons range from 13.1 to −26.9. Zircons derived from juvenile crust, which we define as having mantle δ¹⁸O (4.5-6.5‰) and lying within error of the Hf depleted mantle growth curve, are rare or absent in the Mississippi basin. The overwhelming majority of zircons crystallized from melted pre-existing continental crust, or mantle-derived magmas that were contaminated by continental crust. The average time difference between primitive crust formation and remelting for each of the recognized lithotectonic time intervals, which is defined as crustal incubation time in this study, is 890 ± 460 Myr. There is also a suggestion that the crustal incubation time increases with decreasing age in the Mississippi basin, which is consistent with the declining role of radioactive heat production in the lower crust with time.The average Hf model age (1.94 Ga), weighted by fraction of zircons in the river load is in reasonable agreement with the Nd model age (1.7 Ga) for the Mississippi River. However, if the zircons are weighted by the area of North America covered by the six recognized periods of zircon crystallization the average model age is 2.35 Ga, which compares favorably with an area weighted Nd model age of 2.36 Ga. Our preferred approach is to use the measured O isotope values to constrain variations in the ¹⁷⁶Lu/¹⁷⁷Hf ratio of the granitic source region from which the zircons crystallized, making the assumption that zircons with mantle-like O isotopic ratios have higher ¹⁷⁶Lu/¹⁷⁷Hf than zircons with higher O isotope values. This method gives an average Hf model age of 2.53 Ga, which is 180 Myr older than the constant ¹⁷⁶Lu/¹⁷⁷Hf calculation.The area weighted zircon Hf model ages show two distinct periods of crust formation for the North American continent, 1.6-2.2 and 2.9-3.4 Ga. At least 50% of the preserved North American continental crust was extracted from the mantle by 2.9 Ga and 90% by 1.6 Ga. Two similar periods of crustal growth are also recognized in Gondwana (Hawkesworth C. J. and Kemp A. I. S. (2006) Using hafnium and oxygen isotopes in zircons to unravel the record of crustal evolution. Chem. Geol.226, 144-162.), suggesting that these may be periods of global continental crustal growth. However, we stress that more data from other continents are required before the hypothesis of episodic global continental growth can be accepted with confidence.     

Iron(III) accumulations in inland saline waterways,Hunter Valley,Australia: Mineralogy,micromorphology and pore-water geochemistry

Discharge of Fe(II)-rich groundwaters into surface-waters results in the accumulation of Fe(III)-minerals in salinized sand-bed waterways of the Hunter Valley, Australia. The objective of this study was to characterise the mineralogy, micromorphology and pore-water geochemistry of these Fe(III) accumulations. Pore-waters had a circumneutral pH (6.2–7.2), were sub-oxic to oxic (Eh 59–453 mV), and had dissolved Fe(II) concentrations up to 81.6 mg L⁻¹. X-ray diffraction (XRD) on natural and acid-ammonium-oxalate (AAO) extracted samples indicated a dominance of 2-line ferrihydrite in most samples, with lesser amounts of goethite, lepidocrocite, quartz, and alumino-silicate clays. The majority of Fe in the samples was bound in the AAO extractable fraction (Fe^Ox) relative to the Na-dithionite extractable fraction (Fe^Di), with generally high Fe^Ox:Fe^Di ratios (0.52–0.92). The presence of nano-crystalline 2-line ferrihydrite (Fe₅HO₃·4H₂O) with lesser amounts of goethite (α-FeOOH) was confirmed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). In addition, it was found that lepidocrocite (γ-FeOOH), which occurred as nanoparticles as little as ∼5 lattice spacings thick perpendicular to the (0 2 0) lattice plane, was also present in the studied Fe(III) deposits. Overall, the results highlight the complex variability in the crystallinity and particle-size of Fe(III)-minerals which form via oxidation of Fe(II)-rich groundwaters in sand-bed streams. This variability may be attributed to: (1) divergent precipitation conditions influencing the Fe(II) oxidation rate and the associated supply and hydrolysis of the Fe(III) ion, (2) the effect of interfering compounds, and (3) the influence of bacteria, especially Leptothrix ochracea.