Natural age dispersion arising from the analysis of broken crystals, part I. Theoretical basis and implications for the apatite (U-Th)/He thermochronometer

Over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite (U-Th)/He thermochronometry and it is now standard practice, and generally seen as best practice, to analyse single grain aliquots. These individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite. This is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains, and evidence of freshly broken ends when grains are viewed using a scanning electron microscope. This matters because if the 4He distribution within the whole grain is not homogeneous, because of partial loss due to thermal diffusion for example, then the fragments will all yield ages different from each other and from the whole grain age. Here we use a numerical model with a finite cylinder geometry to approximate 4He ingrowth and thermal diffusion within hexagonal prismatic apatite crystals. This is used to quantify the amount and patterns of inherent, natural age dispersion that arises from analysing broken crystals. A series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios. In addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets. The results indicate that natural dispersion of a set of single fragment ages (defined as the range divided by the mean) arising from fragmentation alone varies from c. 7% even for rapid (c. 10 ∘C/Ma), monotonic cooling to over 50% for protracted, complex histories that cause significant diffusional loss of 4He. The magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius, and is of a similar magnitude to dispersion expected from differences in absolute grain size alone (spherical equivalent radii of 40 to 150 μm). This source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses (c. 6%) and standard deviations on multiple grain analyses from a single sample (c. 10-20%). Where there is a significant difference in the U and Th concentration of individual grains (eU), the effect of radiation damage accumulation on 4He diffusivity (assessed using the RDAAM model of Flowers et al. (2009)) is the primary cause of dispersion for samples that have experienced a protracted thermal history, and can cause dispersion in excess of 100% for realistic ranges of eU conentration (i.e. 5-100 ppm). Expected natural dispersion arising from the combined effects of reasonable variations in grain size (radii 40-125 μm), eU concentration (5-150 ppm) and fragmentation would typically exceed 100% for complex thermal histories. In addition to adding a significant component of natural dispersion to analyses, the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration (eU). Considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic. An important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4He distribution within the whole grains, thus providing an important source of information about the thermal history of the sample. We suggest that if the primary focus of a study is to extract the thermal history information from (U-Th)/He analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages, not minimise it, and should aim to analyse circa 20-30 grains from each sample. The key observations and conclusions drawn here are directly applicable to other thermochronometers, such as the apatite, rutile and titanite U-Pb systems, where the diffusion domain is approximated by the physical grain size

Electrical writing, deleting, reading, and moving of magnetic skyrmioniums in a racetrack device

A magnetic skyrmionium (also called 2$\pi$-skyrmion) can be understood as a skyrmion - a topologically non-trivial magnetic whirl - which is situated in the center of a second skyrmion with reversed magnetization. Here, we propose a new optoelectrical writing and deleting mechanism for skyrmioniums in thin films, as well as a reading mechanism based on the topological Hall voltage. Furthermore, we point out advantages for utilizing skyrmioniums as carriers of information in comparison to skyrmions with respect to the current-driven motion. We simulate all four constituents of an operating skyrmionium-based racetrack storage device: creation, motion, detection and deletion of bits. The existence of a skyrmionium is thereby interpreted as a '1' and its absence as a '0' bit.Comment: This is a post-peer-review, pre-copyedit version of an article published in Scientific Reports. The final authenticated version is available online at [DOI

Concave Switching in Single and Multihop Networks

Switched queueing networks model wireless networks, input queued switches and numerous other networked communications systems. For single-hop networks, we consider a {($\alpha,g$)-switch policy} which combines the MaxWeight policies with bandwidth sharing networks -- a further well studied model of Internet congestion. We prove the maximum stability property for this class of randomized policies. Thus these policies have the same first order behavior as the MaxWeight policies. However, for multihop networks some of these generalized polices address a number of critical weakness of the MaxWeight/BackPressure policies. For multihop networks with fixed routing, we consider the Proportional Scheduler (or (1,log)-policy). In this setting, the BackPressure policy is maximum stable, but must maintain a queue for every route-destination, which typically grows rapidly with a network's size. However, this proportionally fair policy only needs to maintain a queue for each outgoing link, which is typically bounded in number. As is common with Internet routing, by maintaining per-link queueing each node only needs to know the next hop for each packet and not its entire route. Further, in contrast to BackPressure, the Proportional Scheduler does not compare downstream queue lengths to determine weights, only local link information is required. This leads to greater potential for decomposed implementations of the policy. Through a reduction argument and an entropy argument, we demonstrate that, whilst maintaining substantially less queueing overhead, the Proportional Scheduler achieves maximum throughput stability.Comment: 28 page

Cosmology on a Mesh

An adaptive multi grid approach to simulating the formation of structure from collisionless dark matter is described. MLAPM (Multi-Level Adaptive Particle Mesh) is one of the most efficient serial codes available on the cosmological 'market' today. As part of Swinburne University's role in the development of the Square Kilometer Array, we are implementing hydrodynamics, feedback, and radiative transfer within the MLAPM adaptive mesh, in order to simulate baryonic processes relevant to the interstellar and intergalactic media at high redshift. We will outline our progress to date in applying the existing MLAPM to a study of the decay of satellite galaxies within massive host potentials.Comment: 3 pages, 2 figures, to appear in the proceedings of "The IGM/Galaxy Connection - The Distribution of Baryons at z=0", ed. M. Putman & J. Rosenber

Emergence of the Shackleton Range from beneath the Antarctic Ice Sheet due to glacial erosion

This paper explores the long-term evolution of a subglacial fjord landscape in the Shackleton Range, Antarctica. We propose that prolonged ice-sheet erosion across a passive continental margin caused troughs to deepen and lower the surrounding ice-sheet surface, leaving adjacent mountains exposed. Geomorphological evidence suggests a change in the direction of regional ice flow accompanied emergence. Simple calculations suggest that isostatic compensation caused by the deepening of bounding ice-stream troughs lowered the ice-sheet surface relative to the mountains by ~800m. Use of multiple cosmogenic isotopes on bedrock and erratics (26Al, 10Be, 21Ne) provides evidence that overriding of the massif and the deepening of the adjacent troughs occurred earlier than the Quaternary. Perhaps this occurred in the mid-Miocene, as elsewhere in East Antarctica in the McMurdo Dry Valleys and the Lambert basin. The implication is that glacial erosion instigates feedback that can change ice-sheet thickness, extent, and direction of flow. Indeed, as the subglacial troughs evolve over millions of years, they increase topographic relief; and this changes the dynamics of the ice sheet. © 2013 Elsevier B.V

Comparative Analysis of Molecular Clouds in M31, M33 and the Milky Way

We present BIMA observations of a 2\arcmin field in the northeastern spiral arm of M31. In this region we find six giant molecular clouds that have a mean diameter of 57$\pm$13 pc, a mean velocity width of 6.5$\pm$1.2 \kms, and a mean molecular mass of 3.0 $\pm$ 1.6 $\times$ 10$^5$\Msun. The peak brightness temperature of these clouds ranges from 1.6--4.2 K. We compare these clouds to clouds in M33 observed by \citet{wilson90} using the OVRO millimeter array, and some cloud complexes in the Milky Way observed by \cite{dame01} using the CfA 1.2m telescope. In order to properly compare the single dish data to the spatially filtered interferometric data, we project several well-known Milky Way complexes to the distance of Andromeda and simulate their observation with the BIMA interferometer. We compare the simulated Milky Way clouds with the M31 and M33 data using the same cloud identification and analysis technique and find no significant differences in the cloud properties in all three galaxies. Thus we conclude that previous claims of differences in the molecular cloud properties between these galaxies may have been due to differences in the choice of cloud identification techniques. With the upcoming CARMA array, individual molecular clouds may be studied in a variety of nearby galaxies. With ALMA, comprehensive GMC studies will be feasible at least as far as the Virgo cluster. With these data, comparative studies of molecular clouds across galactic disks of all types and between different galaxy disks will be possible. Our results emphasize that interferometric observations combined with the use of a consistent cloud identification and analysis technique will be essential for such forthcoming studies that will compare GMCs in the Local Group galaxies to galaxies in the Virgo cluster.Comment: Accepted for Publication in the Astrophysical Journa

Inner Molecular Rings in Barred Galaxies: BIMA SONG CO Observations

Although inner star-forming rings are common in optical images of barred spiral galaxies, observational evidence for the accompanying molecular gas has been scarce. In this paper we present images of molecular inner rings, traced using the CO (1-0) emission line, from the Berkeley-Illinois-Maryland-Association Survey of Nearby Galaxies (BIMA SONG). We detect inner ring CO emission from all five SONG barred galaxies classified as inner ring (type (r)). We also examine the seven SONG barred galaxies classified as inner spiral (type (s)); in one of these, NGC 3627, we find morphological and kinematic evidence for a molecular inner ring. Inner ring galaxies have been classified as such based on optical images, which emphasize recent star formation. We consider the possibility that there may exist inner rings in which star formation efficiency is not enhanced. However, we find that in NGC 3627 the inner ring star formation efficiency is enhanced relative to most other regions in that galaxy. We note that the SONG (r) galaxies have a paucity of CO and H alpha emission interior to the inner ring (except near the nucleus), while NGC 3627 has relatively bright bar CO and H alpha emission; we suggest that galaxies with inner rings such as NGC 3627 may be misclassified if there are significant amounts of gas and star formation in the bar.Comment: To be published in the Astrophysical Journal, July 2002 A version of the paper with full resolution figures is available at: http://www.astro.umd.edu/~mregan/ms.ps.g

Crop expansion and conservation priorities in tropical countries

Expansion of cropland in tropical countries is one of the principal causes of biodiversity loss, and threatens to undermine progress towards meeting the Aichi Biodiversity Targets. To understand this threat better, we analysed data on crop distribution and expansion in 128 tropical countries, assessed changes in area of the main crops and mapped overlaps between conservation priorities and cultivation potential. Rice was the single crop grown over the largest area, especially in tropical forest biomes. Cropland in tropical countries expanded by c. 48,000 km2 per year from 1999–2008. The countries which added the greatest area of new cropland were Nigeria, Indonesia, Ethiopia, Sudan and Brazil. Soybeans and maize are the crops which expanded most in absolute area. Other crops with large increases included rice, sorghum, oil palm, beans, sugar cane, cow peas, wheat and cassava. Areas of high cultivation potential—while bearing in mind that political and socio-economic conditions can be as influential as biophysical ones—may be vulnerable to conversion in the future. These include some priority areas for biodiversity conservation in tropical countries (e.g., Frontier Forests and High Biodiversity Wilderness Areas), which have previously been identified as having ‘low vulnerability’, in particular in central Africa and northern Australia. There are also many other smaller areas which are important for biodiversity and which have high cultivation potential (e.g., in the fringes of the Amazon basin, in the Paraguayan Chaco, and in the savanna woodlands of the Sahel and East Africa). We highlight the urgent need for more effective sustainability standards and policies addressing both production and consumption of tropical commodities, including robust land-use planning in agricultural frontiers, establishment of new protected areas or REDD+ projects in places agriculture has not yet reached, and reduction or elimination of incentives for land-demanding bioenergy feedstock

How Well Can We Infer the Properties of the Solar Acoustic Sources

Measurements of the p-mode line asymmetry in the solar oscillation velocity power spectrum have been used on several occasions to infer the properties of the acoustic sources. These inferences are based on the assumption that, unlike the observed intensity signal, the velocity signal does not contain a nonresonant (background) component that is correlated with the p-mode signal. Line asymmetry measurements have also been used to draw inferences on the nature of the correlated background signal that is present in intensity observations. By simultaneously modeling the observed velocity and intensity power spectra and the intensity-velocity cross spectrum, we enforce strict observational constraints on the properties of the fitting model. We find that in order to accurately describe the observed data, we have to include a correlated background component in both our models for the V and I signals at low frequencies. Our results also show that we cannot uniquely determine the acoustic source depth for low-frequency waves or the detailed properties of the correlated background signals. It appears that further physical and/or observational constraints are needed before we can obtain this information