Sea ice and biological production variability reconstructed in the Adélie Basin, East Antarctica, during the late Holocene

第3回極域科学シンポジウム　横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月）、27日（火） ２階ラウン

Pressure induced Metallization of Fordite SnNb₂O₆ from First Principles

From DFT based calculations establishing energy-volume equations of state, a proposition of high pressure candidate structures with reduced volumes, namely columbite and trirutile are proposed for fordite mineral SnNb₂O₆. The key effect is the destabilization of divalent tin in fordite towards tetravalent state upon compression. This is helped with smaller size tetravalent tin. The remarkable electronic structure change is the transformation from semi-conducting fordite to metallic high pressure forms thanks to the electron transfer from Sn to Nb and O as quantified from charge density analyses.Keywords: High pressure, Fordite, DFT, Density of states, Equation of statesKeywords: High pressure, Fordite, DFT, Density of states, Equation of state

The past : a compass for future earth

Antarctic sea ice impacts on the ocean-atmosphere heat and gas fluxes, the formation of deep and intermediate waters, the nutrient distribution and primary productivity, the so-called &#8216;biological carbon pump&#8217;, one of the most active in the global ocean. In this study, we explore the link between sea ice dynamic, biological production and nutrient cycling during the late Holocene (the last 2,000 yrs) in the Adélie Basin, East Antarctica, from the well-dated sediments of the Ocean Drilling Program (ODP) Site U1357. This archive, composed from ~32 meters of seasonal to annual laminated diatomaceous sequences, allows reconstructions at an unprecedented time resolution (5-10 yrs). Our study combines records of diatom census counts and diatom-specific biomarkers (a ratio (D/T) of di- and tri-unsaturated Highly Branched Isoprenoid lipids (HBI)) as indicators of sea ice and biological production changes, XRF data as markers for terrigenous inputs and bulk nitrogen isotopes (d15N) and d15N on chlorins as proxies for reconstructing nitrogen cycle. The diatom and HBI records reveal five distinct periods. From 0 to 350 yrs AD, decreasing occurrences of sea ice-related diatom species (e.g. Fragilariopsis curta + F. cylindrus) together with low D/T values and increasing open ocean diatom species (large centrics, Chaetoceros Resting Spores (CRS)) document a progressive decline of sea ice presence during the year (>9 months per year) with spring melting occurring earlier in the year and autumn sea ice formation appearing later. In contrast, between 350 and 750 yrs AD, high production of open ocean diatom species and low low D/T values and sea ice related species indicate a short duration of sea ice cover (~10 months per year) is illustrated by a pronounced increase of sea ice-associated diatom species and high D/T values. Between ~1400 and 1850 yrs AD, seasonal sea ice strongly declines (<~7 months per year) as a result of early spring melting (increasing CRS production) and late autumn waxing (high occurrences of Thalassiosira antarctica). Longer growing seasons promoted a substantial development of phytoplankton communities (especially large centric diatoms) that conducted to lower D/T values. Consistent with diatom and HBI reconstructions, XRF data show higher Fe/Al and Zr/Al ratios values during inferred warmer periods and lower ratio values during inferred cooler and icier periods, thus supporting a strong impact of the sea ice seasonal cycle on glacial runoffs. The link between sea ice conditions, biological production and nutrient cycling is still being explored and we will discuss its relationship by combining all the cited records cited above with the d15N records that we are currently generated. Based on our results, we find that sea ice dynamic and associated diatom production in the Adélie Basin revealed an opposite climatic trend than that identified in the Northern Hemisphere for the last 2000 years. The 'Little Ice Age' (1400-1850 yrs AD) or the 'Dark Ages' (400-750 yrs AD) corresponded to warmer climate conditions in the Adélie Basin, while the 'Roman Warm Period' (0-350 yrs AD) or the 'Medieval Warm Period' (900-1200 yrs AD) were associated to colder conditions. We therefore emphasize that Northern and Southern Hemisphere climate evolved in anti-phase seesaw pattern during the late Holocene

LYACOLORE: synthetic datasets for current and future Lyman-alpha forest BAO surveys

The statistical power of Lyman-α forest Baryon Acoustic Oscillation (BAO) measurements is set to increase significantly in the coming years as new instruments such as the Dark Energy Spectroscopic Instrument deliver progressively more constraining data. Generating mock datasets for such measurements will be important for validating analysis pipelines and evaluating the effects of systematics. With such studies in mind, we present LyaCoLoRe: a package for producing synthetic Lyman-α forest survey datasets for BAO analyses. LyaCoLoRe transforms initial Gaussian random field skewers into skewers of transmitted flux fraction via a number of fast approximations. In this work we explain the methods of producing mock datasets used in LyaCoLoRe, and then measure correlation functions on a suite of realisations of such data. We demonstrate that we are able to recover the correct BAO signal, as well as large-scale bias parameters similar to literature values. Finally, we briefly describe methods to add further astrophysical effects to our skewers—high column density systems and metal absorbers—which act as potential complications for BAO analyses

Temporal controls on silicic acid utilisation along the West Antarctic Peninsula

The impact of climatic change along the Antarctica Peninsula has been widely debated in light of atmospheric/oceanic warming and increases in glacial melt over the past half century. Particular concern exists over the impact of these changes on marine ecosystems, not only on primary producers but also on higher trophic levels. Here we present a record detailing the historical controls on the biogeochemical cycling of silicic acid [Si(OH)4] on the west Antarctica Peninsula margin, a region in which the modern phytoplankton environment is constrained by seasonal sea-ice. We demonstrate that Si(OH)4 cycling through the Holocene alternates between being primarily regulated by sea-ice or glacial discharge from the surrounding grounded ice-sheet. With further climate-driven change and melting forecast for the 21st Century, our findings document the potential for biogeochemical cycling and multi-trophic interactions along the peninsula to be increasingly regulated by glacial discharge, altering food-web interactions

Overview of the DESI Legacy Imaging Surveys

The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing–Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project

Electronic and Magnetic Structures of New Interstitial Boron Sub-Oxides B12O2:X (X = B, C, N, O)

The boron-rich boron sub-oxide rhombohedral B6O considered in B12O2 full formulation has a large O-O spacing of ~3 &Aring; and a central vacant position that can receive interstitial atoms X, forming a central O-X-O alignment in the dodecaboron cage as observed in well-known triatomic B12 compounds as B12{C-C-C}, B12{N-B-N}, etc. Plane wave density functional theory (DFT) based calculations of unrestricted geometry relaxation of B12{O-X-O}, X = B, C, N, and O let one identify new ternary sub-oxides, all found cohesive while showing different d(X-O) distances ranging from d(B-O) = 1.95 &Aring; down to d(O-O) = 1.73 &Aring; with intermediate d(C-O) = 1.88 &Aring;. The different magnitudes were assigned to the chemical affinities of X-inserts versus host oxygen with the increasing development of X-O bonding along the series with larger cohesive B12{O-O-O}. From the atom projected charge density, B presents none, while significant magnitudes are shown on C and N, the latter developing bonding with terminal oxygen atoms especially N. The presence of unpaired valence electrons leaves nonbonding charge density on X = C, N interstitial compounds, which, besides the relative isolation of the central C and N lead to the onset of magnetic moments: M(C) = 1.9 &mu;B, and M(N) = 1 &mu;B in a ferromagnetic ground state. Atom-resolved assessments are provided with the magnetic charge density and electron localization function electron localization function (ELF) projections on one hand and the site and spin projected density of states and the chemical bonding based on the overlap integral Sij within the COOP criterion, on the other hand

Novel (super)hard SiCN from crystal chemistry and first principles

The purpose of this work is to predict the existence of novel equiatomic SiCN based on tetragonal C6 structure (“glitter”), the elementary building unit being the 1,4 cyclohexadiene molecule comprising both tetrahedral (sp3) and trigonal (sp2) carbons. From crystal chemistry rationale the structural transformations of C6 to SiC2 and then to the ternary SiCN were fully relaxed to the ground states using first principles DFT-based calculation. Like early proposed C6 and SiC2, SiCN was found bonding and structurally stable from the elastic properties on one hand and dynamically stable from the phonons, on the other hand. The Vickers hardness of SiCN is close to that of cubic silicon carbide, a conventional superabrasive, whereas hardness of tetragonal SiC2 is slightly lower. Besides the abrasive properties, the electronic band structure indicates metal-like behavior of SiCN thus suggesting the potential for heat dissipation in operating conditions

Crystal and chemical anisotropy effects in AE2Zn2N2, (AE = Ca, Sr, Ba) from ab initio

Crystal and chemical anisotropy effects in AE 2 ZnN 2 , (AE = Ca, Sr, Ba) from ab initio. Abstract: In the tetragonal ternary nitrides AE 2 ZnN 2 (AE = Ca, Sr, Ba), Zn has a rare linear coordination with nitrogen (N–Zn–N) along the c axis and N is located in an AE 5 Zn–like octahedron. Such features lead to original anisotropic crystal and chemical bond effects addressed herein with DFT-GGA based methods accounting for the cohesive energies, the energy volume equations of state EOS, the elastic constants and the properties of chemical bonding. Along AE = Ca, Sr, Ba an increasing overall compressibility under hydrostatic pressure is inferred from the respective EOS's, but the elastic constants C ij exhibit highly anisotropic features with large magnitudes of C 33 along the tetragonal c axis versus smaller C 11 = C 22 along a, b. Decreasing C 33 -C 11 along the title compounds signals decreasing anisotropy. The three ternaries are calculated as small gap insulators in agreement with experimental results of conductivity. The chemical bonding characterizes strong Zn-N bonds versus weaker AE–N thus reflecting the crystal anisotropy.