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A community-based geological reconstruction of Antarctic ice sheet deglaciation since the last glacial maximum
Get PDFA robust understanding of Antarctic Ice Sheet deglacial history since the Last Glacial Maximum is important in order to constrain ice sheet and glacial-isostatic adjustment models, and to explore the forcing mechanisms responsible for ice sheet retreat. Such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and Sub-Antarctic islands. Here, we report a new synthesis of those datasets, based on an accompanying series of reviews of the geological data, organised by sector. We present a series of timeslice maps for 20 ka, 15 ka, 10 ka and 5 ka, including grounding line position and ice sheet thickness changes, along with a clear assessment of levels of confidence. The reconstruction shows that the Antarctic Ice sheet did not everywhere reach the continental shelf edge at its maximum, that initial retreat was asynchronous, and that the spatial pattern of deglaciation was highly variable, particularly on the inner shelf. The deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small Antarctic contribution to meltwater pulse 1a. We discuss key areas of uncertainty both around the continent and by time interval, and we highlight potential priorities for future work. The synthesis is intended to be a resource for the modelling and glacial geological community. © 2014 The Authors
FeCr2S4 in magnetic fields: possible evidence for a multiferroic ground state
No full textWe report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K
Equatorial Pacific coral geochemical records show recent weakening of the Walker Circulation
Get PDFEquatorial Pacific ocean-atmosphere interactions affect climate globally, and a key component of the coupled system is the Walker Circulation, which is driven by sea surface temperature (SST) gradients across the equatorial Pacific. There is conflicting evidence as to whether the SST gradient and Walker Circulation have strengthened or weakened over the late twentieth century. We present new records of SST and sea surface salinity (SSS) spanning 1959–2010 based on paired measurements of Sr/Ca and δ18O in a massive Porites coral from Butaritari atoll in the Gilbert Islands, Republic of Kiribati, in the central western equatorial Pacific. The records show 2–7 year variability correlated with the El Niño–Southern Oscillation (ENSO) and corresponding shifts in the extent of the Indo-Pacific Warm Pool, and decadal-scale signals related to the Pacific Decadal Oscillation and the Pacific Warm Pool Index. In addition, the Butaritari coral records reveal a small but significant increase in SST (0.39°C) from 1959 to 2010 with no accompanying change in SSS, a trend that persists even when ENSO variability is removed. In contrast, larger increases in SST and SSS are evident in coral records from the equatorial Pacific Line Islands, located east of Butaritari. Taken together, the equatorial Pacific coral records suggest an overall reduction in the east-west SST and SSS gradient over the last several decades, and a recent weakening of the Walker Circulation.
© 2014, American Geophysical Union. All Rights Reserved
Magnetic properties of uncultivated magnetotactic bacteria and their contribution to a stratified estuary iron cycle
Get PDFOf the two nanocrystal (magnetosome) compositions biosynthesized by magnetotactic bacteria (MTB), the magnetic properties of magnetite magnetosomes have been extensively studied using widely available cultures, while those of greigite magnetosomes remain poorly known. Here we have collected uncultivated magnetite- and greigite-producing MTB to determine their magnetic coercivity distribution and ferromagnetic resonance (FMR) spectra and to assess the MTB-associated iron flux. We find that compared with magnetite-producing MTB cultures, FMR spectra of uncultivated MTB are characterized by a wider empirical parameter range, thus complicating the use of FMR for fossilized magnetosome (magnetofossil) detection. Furthermore, in stark contrast to putative Neogene greigite magnetofossil records, the coercivity distributions for greigite-producing MTB are fundamentally left-skewed with a lower median. Lastly, a comparison between the MTB-associated iron flux in the investigated estuary and the pyritic-Fe flux in the Black Sea suggests MTB play an important, but heretofore overlooked role in euxinic marine system iron cycle.© 2014, Rights Managed by Nature Publishing Group
Multi-Responsive Photo- and Chemo-Electrical Single-Molecule Switches
No full textIncorporating molecular switches as the active components in nanoscale electrical devices represents a current challenge in molecular electronics. It demands key requirements that need to be simultaneously addressed including fast responses to external stimuli and stable attachment of the molecules to the electrodes while mimicking the operation of conventional electronic components. Here, we report a single-molecule switching device that responds electrically to optical and chemical stimuli. A light pointer or a chemical signal can rapidly and reversibly induce the isomerization of bifunctional spiropyran derivatives in the bulk reservoir and, consequently, switch the electrical conductivity of the single-molecule device between a low and a high level. The spiropyran derivatives employed are chemically functionalized such that they can respond in fast but practical time scales. The unique multistimuli response and the synthetic versatility to control the switching schemes of this single-molecule device suggest spiropyran derivatives as key candidates for molecular circuitry. © 2014, American Chemical Society
The importance of large benthic foraminifera to reef island sediment budget and dynamics at Raine Island, northern Great Barrier Reef
No full textLow-lying reef islands are among the most vulnerable environments on earth to anthropogenic-induced climate change and sea-level rise over the next century because they are low, composed of unconsolidated sediment that is able to be mobilised by waves and currents, and depend on sediments supplied by reef organisms that are particularly sensitive to environmental changes (e.g. ocean temperatures and chemistry). Therefore, the spatial and temporal links between active carbonate production and island formation and dynamics are fundamental to predicting future island resilience, yet remain poorly quantified. In this paper we present results of a detailed geomorphological and sedimentological study of a reef and sand cay on the northern Great Barrier Reef. We provide an empirical investigation of the temporal linkages between sediment production and reef island development using a large collection of single grain AMS 14C dates. Large benthic foraminifera (LBF) are the single most important contributor to contemporary island sand mass (47%; ranging from 36% to 63%) at Raine Island, reflecting rapid rates of sediment production and delivery. Standing stock data reveal extremely high production rates on the reef (1.8 kg m− 2 yr− 1), while AMS 14C dates of single LBF tests indicate rapid rates of sediment transferral across the reef. We also demonstrate that age is statistically related to preservation and taphonomic grade (severely abraded tests > moderately abraded tests > pristine tests). We construct a contemporary reef and island sediment budget model for Raine Island that shows that LBF (Baculogypsina, Marginopora and Amphistegina) contribute 55% of the sediment produced on the reef annually, of which a large proportion (54%) contribute to the net annual accretion of the island. The tight temporal coupling between LBF growth and island sediment supply combined with the sensitivity of LBF to bleaching and ocean acidification suggests that islands dominated by LBF are likely to be very sensitive to short and long term climate change projections. Potential outcomes of this work relate to improving the understanding of the future change dynamics of reef islands in response to climate change. © 2014, Elsevier B.V
Peatlands in eastern Australia? Sedimentology and age structure of Temperate Highland Peat Swamps on Sandstone (THPSS) in the Southern Highlands and Blue Mountains of NSW, Australia
No full textTemperate Highland Peat Swamps on Sandstone (THPSS) are a form of topogenous mire found on the plateau areas of eastern Australia. They are well recognised for their ecological value, but our understanding of their geomorphic structure, function and evolution remains limited. Across 19 sites, the valley fills of THPSS comprise sequences of mineral-rich sand and loam deposits. Basal sand and gravel sedimTOPOGRAPHYents have low organic content and low carbon to nitrogen (C:N) ratios (a measure of peat formation) and are dated between 15.3 and 9 kyr cal. BP, with outliers back to 44 kyr cal. BP. These units reflect mineral-sediment trapping and accumulation on the valley floor. The transition to ‘swamp’ conditions through paludification occurred between 15.2 and 10.3 kyr cal. BP in some systems, and between 7.6 and 1.4 kyr cal. BP in other, adjacent valleys. These ‘swamp’ sediments comprise a package of units, progressing upward from fine cohesive sands, through assemblages of alternating organic sands to surface organic fines. These beds vary in texture from loams to sands and have a range of organic matter content (from 7.6% to 79.9%) and C:N ratios (from 15 to 58). The surface organic fines at 0 and 100 cm depth range in age from 13.1 to 0.7 kyr cal. BP. The composition and age structure of the valley fill suggest a mix of allogenic and autogenic controls are responsible for the formation of these swamps, but a regional model of THPSS evolution is emerging. Given these ‘peatlands’ have formed under a climate that experiences significant inter-annual variability in rainfall, conditions for peat formation are localised and not directly equivalent to those documented internationally. © 2016, SAGE Publications
Solution speciation of plutonium and Americium at an Australian legacy radioactive waste disposal site
No full textDuring the 1960s, radioactive waste containing small amounts of plutonium (Pu) and americium (Am) was disposed in shallow trenches at the Little Forest Burial Ground (LFBG), located near the southern suburbs of Sydney, Australia. Because of periodic saturation and overflowing of the former disposal trenches, Pu and Am have been transferred from the buried wastes into the surrounding surface soils. The presence of readily detected amounts of Pu and Am in the trench waters provides a unique opportunity to study their aqueous speciation under environmentally relevant conditions. This study aims to comprehensively investigate the chemical speciation of Pu and Am in the trench water by combining fluoride coprecipitation, solvent extraction, particle size fractionation, and thermochemical modeling. The predominant oxidation states of dissolved Pu and Am species were found to be Pu(IV) and Am(III), and large proportions of both actinides (Pu, 97.7%; Am, 86.8%) were associated with mobile colloids in the submicron size range. On the basis of this information, possible management options are assessed. © 2014, American Chemical Society
Atmospheric transport and chemistry of trace gases in LMDz5B:
Get PDFRepresentation of atmospheric transport is a major source of error in the estimation of greenhouse gas sources and sinks by inverse modelling. Here we assess the impact
on trace gas mole fractions of the new physical parameterizations
recently implemented in the atmospheric global climate
model LMDz to improve vertical diffusion, mesoscale
mixing by thermal plumes in the planetary boundary layer
(PBL), and deep convection in the troposphere. At the same
time, the horizontal and vertical resolution of the model used
in the inverse system has been increased. The aim of this paper
is to evaluate the impact of these developments on the
representation of trace gas transport and chemistry, and to
anticipate the implications for inversions of greenhouse gas
emissions using such an updated model. © Author(s, 2015
Hydrogen Bonding of O-Ethylxanthate Compounds and Neutron Structural Determination of C–H···S Interactions
No full textA range of ethylxanthate (EtXn) salts, containing either protic or aprotic cations (guanidinium (1), methylammonium (2), dimethylammonium (3), trimethylammonium (4), tetramethylammonium (5), tetraethylammonium (6), and tetrapropylammonium (7)), have been synthesised and structurally characterised. The cations in these compounds differ in their degree of hydrogen-bonding ability, i.e. the number of donor groups, with significant structural consequences. Compounds 1–4 contain cations that are able to form N–H···S hydrogen bonds, with six, three, two, and one donor groups in 1–4 respectively. The number of donor atoms affects greatly the dimensionality of the hydrogen-bonding networks in the solid state. The structure of 1 has a 3-D hydrogen-bonding network, 2 and 3 form 2-D sheets and 1-D chains respectively, whereas the lone NH donor group in 4 has strong hydrogen bonding only within a discrete cation–anion pair. The tetraalkylammonium salts 5–7 have no strong hydrogen bonding, with only C–H···S and C–H···O interactions possible. To determine unambiguously the presence of such interactions, single-crystal Laue neutron diffraction data were obtained for compound 5, providing a fully anisotropic model, which can be used to rationalise potential close interactions in the other structures. The neutron structure of 5 confirms the existence of C–H···S hydrogen bonds, with the H···S distance falling well within the sum of the van der Waals radii of the atoms. The close-packing in 5–7 is mediated solely through these weak interactions, with the size of the cations influencing the structures. © CSIRO 1996-202