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  • 1
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    Modeling the early Paleozoic long-term climatic trend (2011)
    Geological Society of America (GSA)
    Details
    Publication Date: 2011-05-01
    Description: The early Paleozoic climate has been described as warm and equable. However, recent data based on conodont oxygen isotopic composition reveal a large, long, cooling trend through the Ordovician, followed by an abrupt cooling during the Late Ordovician glaciation. This long-term climate change is associated with a major radiation in the Earth life history. Nonetheless, the driving mechanisms for this cooling trend remain unknown. Carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change. However, the impact of the plate motion context has not been explored yet, although it might have a major impact on atmospheric CO2 levels. Simulations with a climate model coupled to a biogeochemical model (GEOCLIM) show that the atmospheric CO2 decreased from more than 20 PAL ([~]5600 ppmv) in the Furongian down to approximately 10 PAL ([~]2800 ppmv) in the Llandovery before rising again in the Early Devonian. We suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large CO2 drawdown that led to the onset of cooler to glacial conditions from the Middle Ordovician to the Llandovery. The weathering of fresh volcanic rocks is itself responsible for 33% of the Late Ordovician atmospheric CO2 decrease; the rest being related to the continent motion through the intertropical convergence zone (ITCZ). Mean annual continental temperature falls by 3{degrees}C in the Early Ordovician, reaching 13.5{degrees}C during the glacial interval, and rises to 16{degrees}C in the Early Devonian.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 2
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    Structure of the exon junction core complex with a trapped DEAD-box ATPase bound to RNA (2006)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2006-08-26
    Description: In higher eukaryotes, a multiprotein exon junction complex is deposited on spliced messenger RNAs. The complex is organized around a stable core, which serves as a binding platform for numerous factors that influence messenger RNA function. Here, we present the crystal structure of a tetrameric exon junction core complex containing the DEAD-box adenosine triphosphatase (ATPase) eukaryotic initiation factor 4AIII (eIF4AIII) bound to an ATP analog, MAGOH, Y14, a fragment of MLN51, and a polyuracil mRNA mimic. eIF4AIII interacts with the phosphate-ribose backbone of six consecutive nucleotides and prevents part of the bound RNA from being double stranded. The MAGOH and Y14 subunits lock eIF4AIII in a prehydrolysis state, and activation of the ATPase probably requires only modest conformational changes in eIF4AIII motif I.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Andersen, Christian B F -- Ballut, Lionel -- Johansen, Jesper S -- Chamieh, Hala -- Nielsen, Klaus H -- Oliveira, Cristiano L P -- Pedersen, Jan Skov -- Seraphin, Bertrand -- Le Hir, Herve -- Andersen, Gregers Rom -- New York, N.Y. -- Science. 2006 Sep 29;313(5795):1968-72. Epub 2006 Aug 24.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Molecular Biology, University of Aarhus, DK-8000 Aarhus, Denmark.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/16931718" target="_blank"〉PubMed〈/a〉
    Keywords: Adenosine Triphosphate/analogs & derivatives/metabolism ; Adenylyl Imidodiphosphate/metabolism ; Amino Acid Motifs ; Amino Acid Sequence ; Animals ; Crystallography, X-Ray ; DEAD-box RNA Helicases ; Dimerization ; Drosophila Proteins/chemistry/metabolism ; Eukaryotic Initiation Factor-4A/*chemistry/metabolism ; *Exons ; Humans ; Hydrogen Bonding ; Hydrolysis ; Models, Molecular ; Molecular Sequence Data ; Mutation ; Neoplasm Proteins/*chemistry/metabolism ; Nuclear Proteins/*chemistry/metabolism ; Nucleic Acid Conformation ; Poly U/*chemistry/metabolism ; Protein Conformation ; Protein Structure, Secondary ; Protein Structure, Tertiary ; RNA Helicases/chemistry/metabolism ; RNA, Messenger/*chemistry/metabolism ; RNA-Binding Proteins/*chemistry/metabolism
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 3
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    Snowball Earth climate dynamics and Cryogenian geology-geobiology (2017)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2017-11-09
    Description: Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO 2 was 10 2 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO 2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
    Published by American Association for the Advancement of Science (AAAS)
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