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- ItemWhat happened at the end of the mid-Pleistocene transition in the Southern Hemisphere? Insights from western Tasmania, Australia(International Union for Quaternary Research (INQUA), 2019-07-30) Fletcher, MS; Lisé-Pronovost, A; Mallett, T; Mariani, M; Cooley, S; May, JH; Gadd, PS; Herries, A; Blaauw, M; Heijnis, H; Hodgson, DA; Pedro, JBThe current southward shift in the southern westerlies that is stripping southern Australia of rainfall is unprecedented over the past 12 kyrs years at least, and is due to the effects of both the anthropogenic hole in the ozone layer and greenhouse gas-driven global warming. Predictions of future climate suggest the Earth is moving in to a “super-interglacial” (peak warming) because of anthropogenic greenhouse gas release. “Super-interglacials”, which are warmer than today, are uncommon in the geological record. A recent increase in the frequency of these peak warming events since ca. 450 ka (the end of the mid-Pleistocene transition; MPT) is associated with a 7° latitude southward shift of the southern westerlies and an increase in atmospheric CO2 that warmed the atmosphere - a stark similarity to current trends. Here, we present multi-proxy analyses of two closely spaced (<50 km) sediment cores that comprise a complete sequence from the mid-Pleistocene to the present from western Tasmania, Australia - in the mid-latitudes of the Southern Hemisphere. Lake Selina is a modern-day lake with a continuous ca. 230 kyr sediment sequence, while Darwin Crater is a palaeo-lake within a meteorite impact crater that formed at ca. 816 ka and which completely in-filled during MIS5 (ca. 120 ka). We report on petrophysical whole core logging, lithological core description, spectrophotometry, grain size, natural gamma ray, paleo- and rock-magnetism, loss-on-ignition, pollen analyses and micro-XRF geochemisrty. The composite record is unique in the Australian sector of the Southern Hemisphere and we discuss the data in the context of the global and regional changes that occurred at the end of MPT, paying particular attention to impact of the shift toward warmer super-interglacials, the large-scale southward shift of the southern westerlies and higher atmospheric CO2 content that occurred at ca. 450 ka (MIS11 to present).
- ItemEnergy optimisation of propagation-based phase-contrast computed tomography: a quantitative image quality assessment(SPIE, 2022-04-04) Lim, B; Lewis, S; Arhatari, BD; Nesterets, YI; Mayo, SC; Fox, J; Thomposon, D; Kumar, B; Häusermann, D; Maksimenko, A; Hall, C; Dimmock, M; Lockie, D; Rickard, M; Giannoitti, N; Peele, AG; Quiney, HM; Gureyev, TE; Brennan, PC; Taba, STPurpose: This study aims at establishing the optimum x-ray energy for synchrotron acquired propagation-based computed tomography (PB-CT) images to obtain highest radiological image quality of breast mastectomy samples. It also examines the correlation between objective physical measures of image quality with subjective human observer scores to model factors impacting visual determinants of image quality. Approach: Thirty mastectomy samples were scanned at Australian Synchrotron’s Imaging and Medical Beamline. Samples were scanned at energies of 26, 28, 30, 32, 34, and 60 keV at a standard dose of 4mGy. Objective physical measures of image quality were assessed using signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), SNR/resolution (SNR/res), CNR/resolution (CNR/res) and visibility. Additional calculations for each measure were performed against reference absorption-based computer tomography (AB-CT) images scanned at 32 keV and 4mGy. This included differences in SNR (dSNR), CNR (dCNR), SNR/res (dSNR/res), CNR/res (dCNR/res), and visibility (dVis). Physical measures of image quality were also compared with visual grading analysis data to determine a correlation between observer scores and objective metrics. Results: For dSNR, dCNR, dSNR/res, dCNR/res, and dVis, a statistically significant difference was found between the energy levels. The peak x-ray energy for dSNR and dSNR/res was 60 keV. For dCNR and dCNR/res 34 keV produced the highest measure compared to 28 keV for dVis. Visibility and CNR correlate to 56.8% of observer scores. Conclusion: The optimal x-ray energy differs for different objective measures of image quality with 30-34 keV providing optimum image quality for breast PB-CT. Visibility and CNR correlate highest to medical imaging expert scores. © (2022) Society of Photo-Optical Instrumentation Engineers (SPIE).
- ItemProbing protein structures in solution by molecular dynamics simulation and small-angle x-ray scattering(International Union of Crystallography, 2021-08-14) Yang, HC; Lin, SW; Ge, YC; Huang, MY; Yang, CH; Liu, WM; Duff, AP; Wu, CM; Lan, YK; Su, AC; Yeh, YQ; Jeng, US; Chou, PTLore of chemical biology guides us that drug discovery of protein binding relies on either optimize the active site complexity of lock and key or induced-fit with conformation selection dynamics; yet, the latter that often-coupled protein interior transport dynamics was much harder to study due to its lack of strong interactions in transient states.[1-2] This study starts to make progress in using in-situ operando X-ray and neutron contrast variation techniques to depict the landscape of protein binding substrate dynamics in solution. We herein demonstrate, for the first time, the 3-D dynamical structures of hydrated CYP450 protein exterior surfaces to interior buried heme site by a distributed connection of channels that direct the reactant in and out. Using CYP450s of prostacyclin synthase (PGIS) and thromboxane synthase (TXAS) as prototypes we have unveiled the unique dynamics of P450 functional channels in/out the haem site, which drive a variety of water molecules motion, water density change and pre-organization toward the heme active site and hence harness the substrate binding selectivity. The result is able to clarify how these two proteins catalyze the same substrate of prostaglandin H2 by entirely different regio-chemical-selective pathways. © The Authors
- ItemLate Pleistocene deglaciation history of the SW British Isles: new evidence from Lundy and the outer Bristol Channel(International Union for Quaternary Research (INQUA), 2019-07-27) Rolfe, C; Hughes, PD; Brown, T; Bateman, M; Gibbard, P; Fink, DThe SW British Isles was glaciated by the British-Irish Ice Sheet during the Late Pleistocene. Glacial deposits are widely preserved in the Bristol Channel and are revealed in boreholes and geophysical data. Lundy, an island in the Outer Bristol Channel, displays unique terrestrial evidence of glaciation in the Outer Bristol Channel. The evidence is dominated by erosional evidence of ice-scouring, although there are wide spreads of erratic cobbles and pebbles several of which contain evidence of striae and faceting. In addition, subsurface thick silt-clay deposits are common on the island with clay mineralogy indicating limited weathering, in contrast to occasional pockets of strongly weathered granite (growan). These deposits are interpreted as subglacial in origin. Offshore of Lundy, especially to the E and NE, geophysical data reveals the presence of thick glacial deposits and dense fields of boulders. This suggests that an ice limit occurred between Lundy and the mainland. This allowed the drainage of the proto-Severn to drain between this ice limit and the mainland in Devon, and the course of this palaeochannel can be traced in bathmetry and geophysical survey data. Periglacial activity after deglaciation is recorded by the presence of coversands, loess, ventifacts, scree slopes and associated colluvial deposits. Luminescence ages from colluvial sands on Lundy date to 24-26 ka indicating that this island was deglaciated by marine isotope stage (MIS) 2. This is consistent with deglaciation and exposure of the island in MIS 3 following glaciation in MIS 4 or possibly earlier. Previously obtained exposure ages from ice-scoured bedrock surfaces have been recalculated using the most recent production rate models and support this interpretation. In addition, a new programme of cosmogenic exposure dating of glacially-transported boulders and displaced tors will further test the previously obtained exposure age data from glaciated bedrock surfaces. In contrast to other weathered granite surfaces in the British Isles, Lundy has not experienced a prolonged weathering history through multiple glacial cycles. Whilst the precise timing of deglaciation is undergoing further testing, the current evidence overwhelmingly indicates that Lundy was eroded by an ice sheet during the Late Pleistocene. Morphometric analysis of weathering pits in the ice-moulded granite bedrock on Lundy supports this exposure history as do the OSL ages from sands overlying the bedrock surfaces. Evidence from elsewhere around the Celtic Sea indicates that the glaciation of Lundy most likely occurred in MIS 4 with ice retreating in MIS 3. However, during MIS 2 ice reached to the continental shelf in the Celtic Sea. This apparent paradox may be explained by a highly mobile Irish Sea Ice Stream which changed its configuration in response to fluctuating marine incursions in MIS 3 before exploiting the exposed continental shelf to the southwest during MIS 2. ©The Authors.
- ItemAchieving the ultimate sensitivity in Accelerator Mass Spectrometry of high mass isotopes(Australian National University, 2019-09-09) Hotchkis, MAC; Child, DP; Williams, ML; Wallner, A; Froehlich, MB; Koll, DThe VEGA AMS system at ANSTO, based on a 1MV tandem accelerator, was custom-designed to achieve the highest possible sensitivity for high mass isotopes. It incorporates multiple medium-resolving power analysing elements: one magnetic element for the injected negative ions, followed by magnetic, electrostatic and second magnetic elements for positive ions after acceleration. This design, with mass and energy resolving powers in the range 500 to 1000, separates isotopes and suppresses backgrounds that may originate from a variety of ion species. The gas stripper in the high-voltage terminal is key both to system efficiency and to background suppression. Helium gas stripping is used, providing around 40% ion yield to the most abundant charge state (3+). The stripper pressure must be sufficient to break up all molecules while minimising the scattering angle of the ions as they undergo charge-changing collisions. Our recent work has demonstrated that the need for production of negative molecular ions in AMS of actinides is not such a barrier to high efficiency: the VEGA sputter ion source can achieve greater than 1% efficiency for production of plutonium oxide negative ions and so overall sensitivity to a few hundred atoms in a sample is possible. We are involved in a number of projects requiring high sensitivity and low backgrounds. Examples include the detection of 244Pu of extraterrestrial origin in deep oceanic ferromanganese crusts; radioecology of plutonium in the environment of former nuclear test sites; detection of nuclear signatures for nuclear safeguards and forensics; use of Pu in global fallout as a chrono-marker in environmental studies; measurement of platinum-group-element isotope ratios in meteorites; evaluation of the radio-purity of materials for use in dark matter searches. Each of these projects presents their own particular challenges. In some cases, sensitivity is limited by background from scattered ions of species other than the one of interest. In other situations, cross-contamination between samples, in the sample prep lab or ion source, limits sensitivity. Other projects or previous uses of laboratories may leave residual contamination. For stable and very long-lived species, such as PGEs and major uranium isotopes, the ubiquity of those species at low levels in almost all materials sets limits. © The Authors.