U–Pb systematics of the McClure Mountain syenite: thermochronological constraints on the age of the 40Ar/39Ar standard MMhb

Recent advances in U–Pb geochronology allow unprecedented levels of precision in the determination of geological ages. However, increased precision has also illuminated the importance of understanding subtle sources of open-system behavior such as Pb-loss, inheritance, intermediate daughter product disequilibria, and the accuracy of the model assumptions for initial Pb. Deconvolution of these effects allows a much richer understanding of the power and limitations of U–Pb geochronology and thermochronology. In this study, we report high-precision ID-TIMS U–Pb data from zircon, baddelleyite, titanite and apatite from the McClure Mountain syenite, from which the ⁴⁰Ar/³⁹Ar hornblende standard MMhb is derived. We find that excess ²⁰⁶Pb in zircon due to inclusions of high-Th minerals and elevated Th/U in titanite and apatite jeopardize the utility of the ²³⁸U–²⁰⁶Pb system in this rock. Strongly air-abraded zircons give dates that are younger than chemical-abraded zircons, which yield a statistically robust ²⁰⁷Pb/²³⁵U date of 523.98±0.12 Ma that is interpreted as the crystallization age. We explore the best method of Pb_c correction in titanite and apatite by analyzing the U–Pb isotopes of K-feldspar and using 2-D and 3-D regression methods—the latter of which yields the best results in each case. However, the calculated compositions of Pb_c for titanite, apatite and K-feldspar are different, implying that using a single Pb_c correction for multiple U–Pb thermochronometers may be inaccurate. The U–Pb thermochronological results are used to predict a closure time for Ar in hornblende of 522.98±1.00 Ma. Widely cited K–Ar and ⁴⁰Ar/³⁹Ar dates overlap with the U–Pb date, and relatively large errors make it impossible to verify whether U–Pb dates are systematically ≤1% older than K–Ar and ⁴⁰Ar/³⁹Ar dates.     

Topographic, geologic and cavity effects on the harmonic content of tidal strain

Summary. The combined effects of topography, geology and cavities upon the measurements of tidal strains are considered. A theoretical model is proposed in which the topography and geology are modelled with finite elements, and the cavity (assumed to be elliptical) modelled using the analytical theory. The three effects are then superimposed to form a total model. Previous experimental results obtained for the solid tidal strain in Eastern Australia (Blair & Sydenham) are compared with the present theoretical model and reasonable agreement is found assuming values of the Love numbers for the Gutenberg—Bullen A earth model in which h = 0.61 and l = 0.08 (Farrell). Further experimental results are presented for two strain meters, only one of which is not significantly influenced by the cavity. The relative difference, in harmonic content between the recordings of both strain meters is compared with that predicted by the theoretical model.     

The Frequency Content of Ground Vibration

A detailed examination of the frequency content of ground vibration is presented in terms of the so-called instantaneous frequency and time-frequency analysis. These techniques, and others, are applied to the surface vibration from a mass blast that triggered a large seismic event and to an earthquake vibration measured in the walls of a massive dam. As an associated issue it is shown that the relationship between peak levels of acceleration and velocity also reveals information on the frequency content of ground vibration. It is demonstrated quite clearly that the popular zero-crossing method cannot be used to obtain the frequency associated with the peak vibration level. In fact it is a false notion that one particular frequency can be associated with the peak level except for the ideal (and impractical) case of a single sinusoid. Realistically, there is a distribution of frequencies associated with the peak level, and a technique of sliding filters is suggested in order to examine the dependence of the peak level upon this frequency content. In light of the sliding filter approach, a new frequency dependent criterion for allowable levels of vibration is presented. This criterion is a completely continuous and well-defined function of frequency and so is more realistic than the current criteria which are only piecewise continuous and based upon an ill-defined frequency. The new criterion is applied to vibration data obtained from quarries and underground operations and is also applied to a model of resonant vibrations in urban dwellings.     

Comparison of physical and chemical properties of maceral groups separated by density dradient centrifugation

Eight coals have been selected for study of the physical and chemical properties of the maceral group . The density-gradient centrifugation technique was employed to affect maceral group separation. The maximum reflectance method of Ting and Lo was used for estimating the reflectance of very small coal particles. The reflectograms were used to characterize the separated maceral fractions. The density, elemental composition, reflectance, NMR parameters of aromaticity, protonated aromatic carbon content and ƒ_stuggereda^stuggeredH of the maceral groups were compared. Variations in the aromatic structure of the maceral groups are discussed as well as the observation that ƒ_stuggereda and ƒ_stuggereda^stuggeredH change with particle density.     

Basin and petroleum system modelling of the East Coast Basin,New Zealand: a test of overpressure scenarios in a convergent margin

In the East Coast Basin (ECB), an active convergent margin of the North Island, New Zealand, the smectite‐rich Eocene Wanstead Formation forms an effective regional seal, creating high overpressure in the underlying Cretaceous through Palaeocene units due to disequilibrium compaction. This study examines the evolution of pore pressure and porosity in Hawke Bay of the ECB based on stepwise structural reconstruction of a stratigraphic and structural framework derived from interpretation of a regional two‐dimensional seismic line. This framework is incorporated into a basin and petroleum system model to predict the generation, distribution, and dissipation of overpressure, and examine the influence of faults, erosion, structural thickening, and seal effectiveness of the Wanstead Formation on pore pressure evolution. We find that natural hydraulic fracturing is likely occurring in sub‐Wanstead source rocks, which makes it a favourable setting for potential shale gas plays. We use poroelastic modelling to investigate the impact of horizontal bulk shortening due to tectonic compression on pore pressure and the relative order of principal stresses. We find that shortening modestly increases pore pressure. When 5% or greater shortening occurs, the horizontal stress may approach and exceed vertical stress in the last 4 Myr of the basin's history. Shortening impacts both the magnitude and relative order of principal stresses through geological time. Due to the overpressured nature of the basin, we suggest that subtle changes in stress regime are responsible for the significant changes in structural deformational styles observed, enabling compressional, extensional, and strike‐slip fault regimes to all occur during the tectonic history and, at times, simultaneously.     

ASCA observations of deep ROSAT fields — II. The 2–10 keV AGN luminosity function

The explosion mechanism associated with thermonuclear supernovae (SNIa) is still a matter of debate. There is a wide agreement that high amounts of radioactive nuclei are produced during these events and they are expected to be strong γ-ray emitters. In the past, several authors have investigated the use of this γ-ray emission as a diagnostic tool. In this paper we have performed a complete study of the γ-ray spectra associated with all the different scenarios currently proposed. This includes detonation, delayed detonation, deflagration and the off-centre detonation. We have performed accurate simulations for this complete set of models in order to determine the most promising spectral features that could be used to discriminate among the different models. Our study is not limited to qualitative arguments. Instead, we have quantified the differences among the spectra and established distance limits for their detection. The calculations have been performed considering the best current response estimations of the SPI and IBIS instruments aboard INTEGRAL in such a way that our results can be used as a guideline to evaluate the capabilities of INTEGRAL in the study of Type Ia supernovae. For the purpose of completeness we have also investigated the nuclear excitation and spallation reactions as a possible secondary source of γ-rays present in some supernova scenarios. We conclude that this mechanism can be neglected because of its small contribution.     

Quantifying and simulating stormwater runoff in watersheds

We developed the Stormwater Runoff Modeling System (SWARM) based on curve number and unit hydrograph methods of the U.S. Department of Agriculture, Natural Resources Conservation Service. SWARM models single events, targets watersheds fitting easily within hydrologic units with 12‐digit codes, and has been calibrated for low‐gradient topography of the Southeast coastal plain. We established protocols; made changes related to peak rate factors, travel time formulas, curve numbers, and the initial abstraction ratio; and then tested the output with multi‐site validation using U.S. Geological Survey measurements of discharge and rainfall. Validation results from both undeveloped and developed watersheds support the robustness of our system in quantifying and simulating runoff: rainfall to runoff differences between measured and simulated volumes ranged from 3 to 11%; r² for hydrograph curves ranged from 0.82 to 0.98. SWARM can be a useful tool for scientific research and for coastal resource management and decision making in the Southeast coastal plain specifically and also may be applied to other areas by recalibrating parameters and modifying calculation templates. Copyright © 2012 John Wiley & Sons, Ltd.     

Early diagenetic remineralization of sedimentary organic C in the Gulf of Papua deltaic complex (Papua New Guinea): Net loss of terrestrial C and diagenetic fractionation of C isotopes

Oceania supplies ∼40% of the global riverine flux of organic carbon, approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems. The Gulf of Papua, on the south coast of the large island of New Guinea, is one such deltaic clinoform complex. It receives ∼4 Mt yr⁻¹ particulate terrestrial organic carbon with initial particle C_org loading ∼0.7 mg m⁻². C_org loading is reduced to ∼0.3 mg m⁻² in the topset-upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus, and high net sediment accumulation rates of 1-4 cm yr⁻¹. Carbon isotopic analyses (δ¹³C, Δ¹⁴C) of ΣCO₂ and C_org demonstrate rapid (<100 yr) remineralization of both terrestrial (δ¹³C <−28.6) and marine C_org (δ¹³C ∼−20.5) ranging in average age from modern (bomb) (Δ¹⁴C ∼60) to ∼1000 yr (Δ¹⁴C ∼−140). Efficient and rapid remineralization in the topset-upper foreset zone is promoted by frequent physical reworking, bioturbation, exposure, and reoxidation of deposits. The seafloor in these regions, particularly <20 m, apparently functions as a periodically mixed, suboxic batch reactor dominated by microbial biomass. Although terrestrial sources can be the primary metabolic substrates at inshore sites, relatively young marine C_org often preferentially dominates pore water ΣCO₂ relative to bulk C_org in the upper foreset. Thus a small quantity of young, rapidly recycled marine organic material is often superimposed on a generally older, less reactive terrestrial background. Whereas the pore water ΣCO₂ reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of C_org from particles in the topset-upper foreset zone (i.e. recycled marine C_org leaves little residue). Preferential utilization of C_org subpools and diagenetic fractionation of C isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics. Early diagenetic fractionation of C isotopes relative to the bulk sedimentary C_org composition can produce changes in ¹⁴C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ¹⁴C of pore water ΣCO₂ averaged ∼ 300‰ greater than C_org sediment between ∼1-3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary C_org compromises simple application of ¹⁴C for determination of sediment accumulation rates in diagenetically reactive deposits.     

GHR1 Zircon – A New Eocene Natural Reference Material for Microbeam U‐Pb Geochronology and Hf Isotopic Analysis of Zircon

We present multitechnique U‐Pb geochronology and Hf isotopic data from zircon separated from rapakivi biotite granite within the Eocene Golden Horn batholith in Washington, USA. A weighted mean of twenty‐five Th‐corrected ²⁰⁶Pb/²³⁸U zircon dates produced at two independent laboratories using chemical abrasion‐isotope dilution‐thermal ionisation mass spectrometry (CA‐ID‐TIMS) is 48.106 ± 0.023 Ma (2s analytical including tracer uncertainties, MSWD = 1.53) and is our recommended date for GHR1 zircon. Microbeam ²⁰⁶Pb/²³⁸U dates from laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ion mass spectrometry (SIMS) laboratories are reproducible and in agreement with the CA‐ID‐TIMS date to within < 1.5%. Solution multi‐collector ICP‐MS (MC‐ICP‐MS) measurements of Hf isotopes from chemically purified aliquots of GHR1 yield a mean ¹⁷⁶Hf/¹⁷⁷Hf of 0.283050 ± 17 (2s, n = 10), corresponding to a εHf₀ of +9.3. Hafnium isotopic measurements from two LA‐ICP‐MS laboratories are in agreement with the solution MC‐ICP‐MS value. The reproducibility of ²⁰⁶Pb/²³⁸U and ¹⁷⁶Hf/¹⁷⁷Hf ratios from GHR1 zircon across a variety of measurement techniques demonstrates their homogeneity in most grains. Additionally, the effectively limitless reserves of GHR1 material from an accessible exposure suggest that GHR1 can provide a useful reference material for U‐Pb geochronology of Cenozoic zircon and Hf isotopic measurements of zircon with radiogenic ¹⁷⁶Hf/¹⁷⁷Hf.     

A Coupled Groundwater–Surface Water Modeling Framework for Simulating Transition Zone Processes

There is an identified need for fully representing groundwater–surface water transition zone (i.e., the sediment zone that connects groundwater and surface water) processes in modeling fate and transport of contaminants to assist with management of contaminated sediments. Most existing groundwater and surface water fate and transport models are not dynamically linked and do not consider transition zone processes such as bioturbation and deposition and erosion of sediments. An interface module is developed herein to holistically simulate the fate and transport by coupling two commonly used models, Environmental Fluid Dynamics Code (EFDC) and SEAWAT, to simulate surface water and groundwater hydrodynamics, while providing an enhanced representation of the processes in the transition zone. Transition zone and surface water contaminant processes were represented through an enhanced version of the EFDC model, AQFATE. AQFATE also includes SEDZLJ, a state‐of‐the‐science surface water sediment transport model. The modeling framework was tested on a published test problem and applied to evaluate field‐scale two‐ and three‐dimensional contaminant transport. The model accurately simulated concentrations of salinity from a published test case. For the field‐scale applications, the model showed excellent mass balance closure for the transition zone and provided accurate simulations of all transition zone processes represented in the modeling framework. The model predictions for the two‐dimensional field case were consistent with site‐specific observations of contaminant migration. This modeling framework represents advancement in the simulation of transition zone processes and can help inform risk assessment at sites where contaminant sources from upland areas have the potential to impact sediments and surface water.