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  • 1
    Publikationsdatum: 2018
    Beschreibung: Abstract The replacement of the late Precambrian Ediacaran biota by morphologically disparate animals at the beginning of the Phanerozoic was a key event in the history of life on Earth, the mechanisms and the time‐scales of which are not entirely understood. A composite section in Namibia providing biostratigraphic and chemostratigraphic data bracketed by radiometric dating constrains the Ediacaran–Cambrian boundary to 538.6–538.8 Ma, more than 2 Ma younger than previously assumed. The U–Pb‐CA‐ID TIMS zircon ages demonstrate an ultrashort time frame for the LAD of the Ediacaran biota to the FAD of a complex, burrowing Phanerozoic biota represented by trace fossils to a 410 ka time window of 538.99 ± 0.21 Ma to 538.58 ± 0.19 Ma. The extremely short duration of the faunal transition from Ediacaran to Cambrian biota within less than 410 ka supports models of ecological cascades that followed the evolutionary breakthrough of increased mobility at the beginning of the Phanerozoic.
    Print ISSN: 0954-4879
    Digitale ISSN: 1365-3121
    Thema: Geologie und Paläontologie
    Publiziert von Wiley
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
    Publikationsdatum: 2011-11-29
    Beschreibung: Constraining variations in marine N2-fixation over glacial-interglacial timescales is crucial for determining the role of the marine nitrogen cycle in modifying ocean productivity and climate, yet paleo-records from N2-fixation regions are sparse. Here we present new nitrogen isotope (δ15N) records of bulk sediment and foraminifera test-bound (FB) nitrogen extending back to the last ice age from the oligotrophic Gulf of Mexico (GOM). Previous studies indicate a substantial terrestrial input during the last ice age and early deglacial, for which we attempt to correct the bulk sediment δ15N using its observed relationship with the C/N ratio. Both corrected bulk and FB-δ15N reveal a substantial glacial-to-Holocene decrease of δ15N toward Holocene values of around 2.5 ‰, similar to observations from the Caribbean. This δ15N change is most likely due to a glacial-to-Holocene increase in regional N2-fixation. A deglacial peak in the FB-δ15N of thermocline dwelling foraminifera Orbulina universa probably reflects a whole ocean increase in the δ15N of nitrate during deglaciation. The δ15N of the surface dwelling foraminifera Globigerinoides ruber and the corrected bulk δ15N show little sign of this deglacial peak, both decreasing from last glacial values much earlier than does the δ15N of O. universa; this may indicate that G. ruber and bulk N reflect the euphotic zone signal of an early local increase in N2-fixation. Our results add to the evidence that, during the last ice age, the larger iron input from dust did not lead to enhanced N2-fixation in this region. Rather, the glacial-to-Holocene decrease in δ15N is best explained by a response of N2-fixation within the Atlantic to the deglacial increase in global ocean denitrification.
    Print ISSN: 0883-8305
    Digitale ISSN: 1944-9186
    Thema: Geologie und Paläontologie
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
    Publikationsdatum: 2018
    Beschreibung: This study presents a multi‐proxy record from Lake Kotokel in the Baikal region at decadal‐to‐multidecadal resolution and provides a reconstruction of terrestrial and aquatic environments in the area during a 2000‐year interval of globally harsh climate often referred to as the Last Glacial Maximum (LGM). The studied lake is situated near the eastern shoreline of Lake Baikal, in a climatically sensitive zone that hosts boreal taiga and cold deciduous forests, cold steppe associations typical for northern Mongolia, and mountain tundra vegetation. The results provide a detailed picture of the period in focus, indicating (i) a driest phase (c. 24.0–23.4 cal. ka BP) with low precipitation, high summer evaporation, and low lake levels, (ii) a transitional interval of unstable conditions (c. 23.4–22.6 cal. ka BP), and (iii) a phase (c. 22.6–22.0 cal. ka BP) of relatively high precipitation (and moisture availability) and relatively high lake levels. One hotly debated issue in late Quaternary research is regional summer thermal conditions during the LGM. Our chironomid‐based reconstruction suggests at least 3.5 °C higher than present summer temperatures between c. 22.6 and 22.0 cal. ka BP, which are well in line with warmer and wetter conditions in the North Atlantic region inferred from Greenland ice‐cores. Overall, it appears that environments in central Eurasia during the LGM were affected by much colder than present winter temperatures and higher than present summer temperatures, although the effects of temperature oscillations were strongly influenced by changes in humidity.
    Print ISSN: 0300-9483
    Digitale ISSN: 1502-3885
    Thema: Geographie , Geologie und Paläontologie
    Publiziert von Wiley
    Standort Signatur Erwartet Verfügbarkeit
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