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Warren P.H.,University of California at Los Angeles | Rubin A.E.,University of California at Los Angeles | Isa J.,University of California at Los Angeles | Brittenham S.,4860 Glenbrook Drive | And 3 more authors.
Geochimica et Cosmochimica Acta | Year: 2013

Northwest Africa 6693 is a new type of achondrite, with a unique combination of oxygen-isotopic composition (low Δ17O: -1.08‰; also δ17O=1.19‰) and FeO-rich, low mg bulk composition. A mode (in vol%) shows 70% pyroxene, 16% olivine and 13% feldspar, along with 0.6% Cr-spinel, and 0.4% NiFe metal (awaruite). Its coarse-poikilitic texture, with pigeonite oikocrysts up to 14mm, as well as the subchondritic MgO/SiO2 of the rock's bulk composition, indicate origin as an igneous cumulate. The cumulus phases included pigeonite and olivine, and the parent magma was probably also saturated with feldspar, which occurs mainly as anhedral, yet optically continuous, grains intergrown with the pyroxene. The mafic silicates are uniformly ferroan: pigeonite near En57Wo3.2 and olivine near Fo49. The feldspar is uniformly albitic, near Ab92, except for a single tiny grain of Ab57Or43. However, the albite features diverse K/Ca (Or/An) ratios: ranging from consistently ∼0.46 in one end of the oblong NWA 6693 stone, to 5.2 in an olivine-rich enclave that consists mostly of micrographic olivine-feldspar intergrowth. Also, siderophile and incompatible element data show heterogeneity among samples from different regions of this large cumulate. The rock was probably neither an orthocumulate nor an adcumulate, and the proportion of " trapped liquid" probably varied from place to place. After initial crystallization, a shock event caused very minor brecciation, and pervasively mobilized linear-arcuate trails of microinclusions (minute oxides, mostly) and bubbles. A minor proportion of additional melt was formed within, and/or infiltrated into, the rock and formed discrete overgrowth mantles, recognizable based on unusual scarcity of microinclusions, on some pyroxenes. Final cooling, based on mineral-equilibration temperatures, occurred at a moderate rate by intrusive-igneous standards.Olivine, metal, and sulfide phases are all very Ni-rich (e.g., olivine NiO averages 0.77wt%). Evidently the partitioning of NiO into the parent melt was extraordinarily high, which suggests a commensurately high oxygen fugacity. The V/(Al+Cr) ratio within spinel suggests that fO2 was IW+2. The bulk-rock composition features strong depletions in sulfur and chalcophile elements, but nonvolatile lithophile elements are only subtly fractionated from chondritic. Even siderophile element concentrations are near-chondritic; Co, Ni, Ir and Os are all at 0.7-1.0×CI chondrites, and Au at 0.55×CI. The limited fractionation of the siderophile elements may reflect igneous processing in a parent body so pervasively oxidizing that FeNi metal did not play its usual planetary role as agent for efficient sequestration of siderophile elements. More generally, the limited fractionation among nonchalcophile, nonvolatile elements suggests that the parent melt was not produced by extensive fractional crystallization; i.e., the high FeO and low mg of NWA 6693 were probably in large measure already properties of the original primary magma, produced from an extremely oxidized (FeO-rich) variety of primitive material. NWA 6693 indicates that high oxygen fugacity inherited from oxidized-chondritic building blocks may persist within small bodies, despite melting extensive enough to engender igneous cumulates. © 2012 Elsevier Ltd. Source

Arndt J.E.,Alfred Wegener Institute for Polar and Marine Research | Schenke H.W.,Alfred Wegener Institute for Polar and Marine Research | Jakobsson M.,University of Stockholm | Nitsche F.O.,Lamont Doherty Earth Observatory | And 12 more authors.
Geophysical Research Letters | Year: 2013

The International Bathymetric Chart of the Southern Ocean (IBCSO) Version 1.0 is a new digital bathymetric model (DBM) portraying the seafloor of the circum-Antarctic waters south of 60°S. IBCSO is a regional mapping project of the General Bathymetric Chart of the Oceans (GEBCO). The IBCSO Version 1.0 DBM has been compiled from all available bathymetric data collectively gathered by more than 30 institutions from 15 countries. These data include multibeam and single-beam echo soundings, digitized depths from nautical charts, regional bathymetric gridded compilations, and predicted bathymetry. Specific gridding techniques were applied to compile the DBM from the bathymetric data of different origin, spatial distribution, resolution, and quality. The IBCSO Version 1.0 DBM has a resolution of 500 × 500 m, based on a polar stereographic projection, and is publicly available together with a digital chart for printing from the project website (www.ibcso.org) and at http://dx.doi.org/10.1594/PANGAEA.805736. Key Points The first regional bathymetric compilation covering the entire Southern Ocean A new keystone dataset for Antarctic research The southern equivalent to IBCAO is now released ©2013. American Geophysical Union. All Rights Reserved. Source

Deng S.,Rockefeller University | Deng S.,Sun Yat Sen University | Xu J.,Rockefeller University | Xu J.,Wilmar Biotechnology Research and Development Center Shanghai Co. | And 6 more authors.
Plant Journal | Year: 2015

Transposable elements (TEs) and repetitive sequences are ubiquitously present in eukaryotic genomes which are in general epigenetically silenced by DNA methylation and/or histone 3 lysine 9 methylation (H3K9me). RNA-directed DNA methylation (RdDM) is the major pathway that initiates de novo DNA methylation in Arabidopsis and sets up a self-reinforcing silencing loop between DNA methylation and H3K9me. However, a key issue is the requirement of a basal level transcript from the target loci to initiate the RNA-based silencing. How the heterochromatic silenced loci are transcribed remains largely unknown. Here, we show that JMJ24, a JmjC domain-containing protein counteracts H3K9me to promote basal level transcription of endogenous silenced loci in Arabidopsis. JMJ24 functionally resembles the fission yeast JmjC protein Epe1. The transcript promoted by JMJ24 is, at least in part, processed to small RNA to initiate the RdDM. Genome-wide transcriptome profiling indicates that transcript levels of TEs are more likely regulated by JMJ24, compared with protein-coding genes. Our data suggest that JMJ24 plays a conserved role in promoting basal level transcription of endogenous silenced loci to reinforce the silencing. We also provide evidence of a physical association between JMJ24 and RNA-dependent RNA polymerase 2 (RDR2), which represents an evolved property of the RNA silencing pathway. Significance Statement We reported here that JMJ24 physically interacts with RDR2 and is required for the basal level transcription of endogenous silenced loci to initiate the RdDM of target gene in Arabidopsis. Our results suggest JmjC-domain containing protein plays a conserved role in promoting the transcription of heterochromatic silenced loci to reinforce silencing state. © 2015 The Authors. © 2015 John Wiley & Sons Ltd. Source

Hulbe C.L.,University of Otago | Scambos T.A.,University of Colorado at Boulder | Lee C.-K.,Korean Polar Research Institute | Bohlander J.,University of Colorado at Boulder | Haran T.,University of Colorado at Boulder
Earth and Planetary Science Letters | Year: 2013

Comparison of surface velocities measured during the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS, 1973 to 1978) and velocities measured via feature tracking between two Moderate-resolution Imaging Spectroradiometer (MODIS) mosaics (compiled from 2003/4 and 2008/9 images) reveals widespread slowing and minor areas of acceleration in the Ross Ice Shelf (RIS) over the approximately 30 year interval. The largest changes (-13ma -2) occur near the Whillans and Mercer Ice Streams grounding line in the southernmost part of the ice shelf. Speed has increased over the interval (up to 5ma -2) between the MacAyeal Ice Stream grounding line and the shelf front, and along the eastern shelf front. Changes in ice thickness computed using ICESat laser altimetry are used together with a well-tested model of the ice shelf to investigate underlying causes of change in the flow of the ice shelf over time. The observed transients represent a combination of recent forcings and ongoing response to ice stream discharge variations over the past millennium. While evidence of older events may be present, the modern signal is dominated by shorter time scale events, including the stagnation of Kamb Ice Stream about 160 years ago, recent changes in basal drag on the Whillans Ice Stream ice plain and, perhaps, iceberg calving. Details in embayment geometry, for example the shallow sea floor below Crary Ice Rise, modulate the spatial pattern of ice shelf response to boundary condition perturbations. © 2013 Elsevier B.V. Source

Feng S.,University of Nebraska - Lincoln | Ho C.-H.,Seoul National University | Hu Q.,University of Nebraska - Lincoln | Oglesby R.J.,University of Nebraska - Lincoln | And 2 more authors.
Climate Dynamics | Year: 2012

The ecosystems in the Arctic region are known to be very sensitive to climate changes. The accelerated warming for the past several decades has profoundly influenced the lives of the native populations and ecosystems in the Arctic. Given that the Köppen-Trewartha (K-T) climate classification is based on reliable variations of land-surface types (especially vegetation), this study used the K-T scheme to evaluate climate changes and their impact on vegetation for the Arctic (north of 50°N) by analyzing observations as well as model simulations for the period 1900-2099. The models include 16 fully coupled global climate models from the Intergovernmental Panel on Climate Change Fourth Assessment. By the end of this century, the annual-mean surface temperature averaged over Arctic land regions is projected to increase by 3. 1, 4. 6 and 5. 3°C under the Special Report on Emissions Scenario (SRES) B1, A1b, and A2 emission scenarios, respectively. Increasing temperature favors a northward expansion of temperate climate (i. e., Dc and Do in the K-T classification) and boreal oceanic climate (i. e., Eo) types into areas previously covered by boreal continental climate (i. e., Ec) and tundra; and tundra into areas occupied by permanent ice. The tundra region is projected to shrink by -1. 86 × 106 km2 (-33. 0%) in B1, -2. 4 × 106 km2 (-42. 6%) in A1b, and -2. 5 × 106 km2 (-44. 2%) in A2 scenarios by the end of this century. The Ec climate type retreats at least 5° poleward of its present location, resulting in -18. 9, -30. 2, and -37. 1% declines in areal coverage under the B1, A1b and A2 scenarios, respectively. The temperate climate types (Dc and Do) advance and take over the area previously covered by Ec. The area covered by Dc climate expands by 4. 61 × 106 km2 (84. 6%) in B1, 6. 88 × 106 km2 (126. 4%) in A1b, and 8. 16 × 106 km2 (149. 6%) in A2 scenarios. The projected redistributions of K-T climate types also differ regionally. In northern Europe and Alaska, the warming may cause more rapid expansion of temperate climate types. Overall, the climate types in 25, 39. 1, and 45% of the entire Arctic region are projected to change by the end of this century under the B1, A1b, and A2 scenarios, respectively. Because the K-T climate classification was constructed on the basis of vegetation types, and each K-T climate type is closely associated with certain prevalent vegetation species, the projected large shift in climate types suggests extensive broad-scale redistribution of prevalent ecoregions in the Arctic. © 2011 Springer-Verlag. Source

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