The use of 210Pb as a heavy metal tracer in the Susquehanna River system

Analysis of soil profiles and shallow ground water in the Susquehanna River basin, northeastern U.S.A., indicates that the atmospheric flux of ²¹⁰Pb is efficiently scavenged by the organic-rich horizons of the soils. This atmospherically supplied ²¹⁰Pb in soil profiles can only be lost from the system by soil erosion. Based on the annual sediment yield of the Susquehanna River and the excess ²¹⁰Pb concentration in particulate matter, a mean residence time of 2000 yr is calculated for metals similar to Pb in soil profiles.The West Branch of the Susquehanna River (WBSR) is strongly affected by acid mine drainage and is low in pH and high in dissolved ( <0.4 μm) ²¹⁰Pb, Fe and Mn. Along its course iron hydroxide is precipitating at a pH of between 4 and 4.5 and the ²¹⁰Pb supplied by the acid mine water is diminished by about 25% as a result of dilution. As the WBSR enters the Valley and Ridge Province of the Appalachians it has a ²¹⁰Pb concentration of ~ 0.2 dpm/l. At this juncture it receives a considerable influx of alkalinity from tributaries draining carbonate terranes, resulting in neutralization of the sulfuric acid and increase of the river pH to around 6.5–7. This pH adjustment is accompanied by the precipitation of Fe and Mn. Due to the slow rate of Mn removal from solution, the Mn precipitation extends a considerable distance down river from the point of acid neutralization. Analyses for ²¹⁰Pb in the river at points in or below the region of Mn precipitation show that ²¹⁰Pb is rapidly scavenged from solution onto suspended particles. From the data it is possible to calculate the removal rate of Pb from water in the presence of Fe and Mn hydroxides and other particles. At a pH of 4–4.5 Pb removal is nonexistent relative to the river flow rate, but at a pH of 6.5–7 the ²¹⁰Pb data indicate a residence time of <0.7 day for dissolved Pb.     

TRANSIENT EM RESPONSE OF A LARGE LOOP ON A LAYERED GROUND *

The voltage induced in a horizontal loop on a layered ground has been calculated for the case where the loop is excited by a step current and measurements are made during the off-cycle. The expressions derived for a uniform ground show that for large time t the induced voltage E(t) is approximately given by E(t)?— (Ibαμ/20t) (σμ²/t)^3/2 where σ is the conductivity of the ground, μ the permeability, b the loop radius, and I the amplitude of the current step. For small times the corresponding result is E(t)?—Ibμ/2t. When the ground is composed of a number of layers a numerical procedure for calculating the induced voltage is described. The calculated responses of various multilayered structures show that at short times the induced voltage is asymptotic to that produced in the case of a uniform ground of conductivity equal to the top layer. Interference effects in the top layer can lead to anomalous decay curves which may result in the underestimation of the conductivity of a buried layer.     

Distribution of gold and rhenium between nickel-iron and silicate melts: implications for the abundance of siderophile elements on the Earth and Moon

The distribution equilibrium of Au and Re between nickel-iron and basaltic melts was studied at 1400–1600°C, using radioactive tracers. Metal/silicate distribution coefficients were 1–3 orders of magnitude higher than earlier estimates, as follows. Mauna Loa basalt—Fe₁₀Ni₉₀: D_Au = 3.3 × 10⁴, D_Re = (2.4?89) × 10⁴. Gorda Ridge basalt—Fe₁₀Ni₉₀: D_Au = (18?75) × 10⁴. Synthetic lunar basalt—Fe₇₀ Ni₃₀: D_Au≥ 2 × 10⁴, D_Re ≥ 2 × 10³. The experimental ΔG₁₈₀₀^° for the distribution of Au between nickel-iron and Mauna Loa basalt is ?40 kcal/mole, compared to a calculated value of about ?110 kcal/mole for a reaction involving simple Au³⁺ ions. Presumably the difference represents stabilization of Au(III) by complex formation with ligands such as Cl^?, H₂O, etc.Gold abundances in lunar basalts are roughly consistent with the measured D_Au, but those in terrestrial basalts are two orders of magnitude too high. This discrepancy may reflect complexing by volatiles in the Earth's upper lithosphere, as well as oxidative destruction of metal in the final stages of accretion. In the absence of a metal phase, siderophile trace elements would remain trapped in the upper mantle and crust.     

Carbon and sulfur in the Swaziland sequence

Sedimentary and igneous rocks of the Swaziland Sequence of South Africa (>3,200 my. old) contain total carbon (0.09–8.28%), organic carbon (0.04–1.95%) and sulfur (9–53,000 ppm). Amounts of organic carbon are generally high in the lower part of the Sequence (based on two samples from the Theespruit Formation). Sulfur content varies widely with no obvious correlations or trends. These rocks may contain clues to the early carbon and sulfur cycles on earth.     

Gravitational microlensing: A parallel,large-data implementation

Gravitational lensing allows us to probe the structure of matter on a broad range of astronomical scales, and as light from a distant source traverses an intervening galaxy, compact matter such as planets, stars, and black holes act as individual lenses. The magnification from such microlensing results in rapid brightness fluctuations which reveal not only the properties of the lensing masses, but also the surface brightness distribution in the source. However, while the combination of deflections due to individual stars is linear, the resulting magnifications are highly non-linear, leading to significant computational challenges which currently limit the range of problems which can be tackled. This paper presents a new and novel implementation of a numerical approach to gravitational microlensing, increasing the scale of the problems that can be tackled by more than two orders of magnitude, opening up a new regime of astrophysically interesting problems.     

A bulk cloud parameterization in a Venus General Circulation Model

A condensing cloud parameterization is included in a super-rotating Venus General Circulation Model. A parameterization including condensation, evaporation and sedimentation of mono-modal sulfuric acid cloud particles is described. Saturation vapor pressure of sulfuric acid vapor is used to determine cloud formation through instantaneous condensation and destruction through evaporation, while pressure dependent viscosity of a carbon dioxide atmosphere is used to determine sedimentation rates assuming particles fall at their terminal Stokes velocity. Modifications are described to account for the large range of the Reynolds number seen in the Venus atmosphere.Two GCM experiments initialized with 10 ppm-equivalent of sulfuric acid are integrated for 30 Earth years and the results are discussed with reference to “Y” shaped cloud structures observed on Venus. The GCM is able to produce an analog of the “Y” shaped cloud structure through dynamical processes alone, with contributions from the mean westward wind, the equatorial Kelvin wave, and the mid-latitude/polar Mixed Rossby/Gravity waves. The cloud top height in the GCM decreases from equator to pole and latitudinal gradients of cloud top height are comparable to those observed by Pioneer Venus and Venus Express, and those produced in more complex microphysical models of the sulfur cycle on Venus. Differences between the modeled cloud structures and observations are described and dynamical explanations are suggested for the most prominent differences.     

Predicting suitability of cockle Austrovenus stutchburyi restoration sites using hydrodynamic models of larval dispersal

An important aspect of shellfish restoration projects is to evaluate whether potential sites are likely to be recolonised naturally once disturbances are removed and habitats are restored. Similarly, if active translocation or reseeding of juveniles or adults is undertaken, it is important to understand whether these introduced populations will be self‐maintaining in future, and whether any seed they produce are likely to be retained within the restored area. We used a combined hydrodynamic and particle tracking model to predict larval dispersion patterns for the common cockle Austrovenus stutchburyi under different hydrodynamic scenarios for seven release locations in Whangarei Harbour, New Zealand. Our results implied that sites varied substantially in their potential for self‐seeding and for exporting seed to other locations. For sites with more restricted dispersal, the model predicted that most larvae originating at these sites would settle inside the release region (68–94% for passive particle simulations), whereas relatively few larvae originating from the other release sites settled at these sites. In contrast, model larvae released from sites exhibiting high connectivity dispersed to all sub‐regions in the harbour, and export outside of the model region was high. Forthcoming field validation of these model predictions will result in better integration of hydrodynamic connectivity in whole estuary restoration programs.     

Migrating magmatism in the northern US Cordillera: in situ U–Pb geochronology of the Idaho batholith

New in situ laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe U–Pb geochronology of zircons from the Idaho batholith and spatially overlapping Challis intrusions reveals a series of discrete magmatic belts of different ages and compositions. Following the accretion of the Blue Mountains province to North America along the Salmon River suture zone, two compositionally diverse belts of metaluminous plutons formed both adjacent to the suture and well inboard of it. These were constructed from ~100 to 85 Ma and were followed by a voluminous pulse of peraluminous magmatism, forming the bulk of the Atlanta lobe and largest fraction of the batholith between ~80 and 67 Ma. Around 70 Ma, a later and more spatially restricted suite of metaluminous plutons formed around the Bitterroot lobe of the batholith. This was followed by another pulse of voluminous peraluminous magmatism in the Bitterroot lobe, lasting from ~66 to 54 Ma. The changes from low volume metaluminous to high volume peraluminous magmatism may reflect a combination of changes in the angle and segmentation of the subducting Farallon plate and over thickening of the continental lithosphere. All of these features were then cut by plutons and dikes associated with the Challis volcanic field, lasting from ~51 to 43 Ma. Inherited components are pervasive in zircons from most phases of the batholith. While Precambrian components are very common, zircons also often contain cores or mantles that are 5–20 million years older than their rims. This suggests that the early phases of the batholith were repeatedly cannibalized by subsequent magmas. This also implies that the older suites may have been originally more aerially extensive than their currently exposed forms.