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  • 1
    Keywords: climate change ; Dissertation ; Pleistocene ; environmental change ; Atlantic Ocean ; Africa ; Hochschulschrift ; Paläoklimatologie ; Mittelpleistozän ; Tropenklima ; Atlantischer Ozean Süd ; Zentralafrika
    Type of Medium: Book
    Pages: 187 S , graph. Darst., Kt
    ISBN: 9057440814
    Series Statement: Geologica Ultraiectina 223
    Language: English
    Note: Zsfassung in niederländ. und dt. Sprache , Zugl.: Utrecht, Univ., Diss., 2003
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  • 2
    Online Resource
    Online Resource
    [Bremen] : [Universität Bremen, MARUM - Zentrum für Marine Umweltwissenschaften]
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (26 Seiten, 275,47 KB) , Illustrationen, Diagramme
    Language: German
    Note: Autoren und durchführende Institutionen der Teilprojekte den Berichtsblättern entnommen , Förderkennzeichen BMBF 03G0840A/B. - Verbund-Nummer 01144766 , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden , Mit deutscher und englischer Zusammenfassung
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  • 3
    Keywords: Hochschulschrift
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (108 Seiten = 5 MB) , Graphen, Karte
    Edition: Online-Ausgabe 2023
    Language: English
    Note: Zusammenfassung in deutscher und englischer Sprache
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 437 (2005), S. 1003-1006 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Past hydrological changes in Africa have been linked to various climatic processes, depending on region and timescale. Long-term precipitation changes in the regions of northern and southern Africa influenced by the monsoons are thought to have been governed by precessional variations in summer ...
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants—adapted to warm, dry conditions and low atmospheric CO2 concentrations—have been attributed to marked changes in environmental ...
    Type of Medium: Electronic Resource
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  • 6
    Publication Date: 2020-02-06
    Description: Changes in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high‐resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near‐linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.
    Type: Article , PeerReviewed
    Format: text
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  • 7
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    Nature Research
    In:  Nature Communications, 8 (1). Art.Nr. 1015.
    Publication Date: 2020-02-06
    Description: Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people. Historical records and observations of the Walker circulation over the 20th century disagree on the sign of change and therefore, longer climate records are necessary to better project tropical circulation changes in response to global warming. Here we examine proxies for thermocline depth and rainfall in the eastern tropical Indian Ocean during the globally colder Last Glacial Maximum (19–23 thousand years ago) and for the past 3000 years. We show that increased thermocline depth and rainfall indicate a stronger-than-today Walker circulation during the Last Glacial Maximum, which is supported by an ensemble of climate simulations. Our findings underscore the sensitivity of tropical circulation to temperature change and provide evidence for a further weakening of the Walker circulation in response to greenhouse warming.
    Type: Article , PeerReviewed
    Format: text
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  • 8
    Publication Date: 2020-02-06
    Description: Variations in Amazonian hydrology and forest cover have major consequences for the global carbon and hydrological cycles as well as for biodiversity. Yet, the climate and vegetation history of the lowland Amazon basin and its effect on biogeography remain debated due to the scarcity of suitable high-resolution paleoclimate records. Here, we use the isotopic composition (δD and C) of plant-waxes from a high-resolution marine sediment core collected offshore the Amazon River to reconstruct the climate and vegetation history of the integrated lowland Amazon basin for the period from 50,000 to 12,800 yr before present. Our results show that δD values from the Last Glacial Maximum were more enriched than those from Marine Isotope Stage (MIS) 3 and the present-day. We interpret this trend to reflect long-term changes in precipitation and atmospheric circulation, with overall drier conditions during the Last Glacial Maximum. Our results thus suggest a dominant glacial forcing of the climate in lowland Amazonia. In addition to previously suggested thermodynamic mechanisms of precipitation change, which are directly related to temperature, we conclude that changes in atmospheric circulation are crucial to explain the temporal evolution of Amazonian rainfall variations, as demonstrated in climate model experiments. Our vegetation reconstruction based on C values shows that the Amazon rainforest was affected by intrusions of savannah or more open vegetation types in its northern sector during Heinrich Stadials, while it was resilient to glacial drying. This suggests that biogeographic patterns in tropical South America were affected by Heinrich Stadials in addition to glacial–interglacial climate variability.
    Type: Article , PeerReviewed
    Format: text
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  • 9
    Publication Date: 2024-02-07
    Description: Despite its great ecological importance, the main factors governing tree cover in tropical savannas as well as savanna-forest boundaries are still largely unknown. Here we address this issue by investigating marine sediment records of long-chain n-alkane stable carbon (δ13Cwax) and hydrogen (δDwax) isotopes from a core collected off eastern tropical South America spanning the last ca. 45 thousand years. While δ13Cwax is a proxy for the main photosynthetic pathway of terrestrial vegetation, tracking the relative proportion of C3 (mainly trees) versus C4 (mainly grasses) plants, δDwax is a proxy for continental precipitation, tracking the intensity of rainfall. The investigated core was collected off the mouth of the São Francisco River drainage basin, a tropical savanna-dominated region with dry austral autumn, winter and spring. On top of millennial-scale changes, driven by anomalies in the amount of precipitation associated with Heinrich Stadials, we identify a marked obliquity control over the expansion and contraction of tree and grass cover. During periods of maximum (minimum) obliquity, trees (grasses) reached maximum coverage. We suggest that maximum (minimum) obliquity decreased (increased) the length of the dry season allowing (hampering) the expansion of tree-dominated vegetation. Periods of maximum obliquity induced an anomalous heating (cooling) of the summer (winter) hemisphere that in combination with a delayed response of the climate system slightly increased autumn precipitation over the São Francisco River drainage basin, through a shift of the Intertropical Convergence Zone towards or further into the anomalously heated hemisphere. We found that atmospheric CO2 concentration has only a secondary effect on tree cover. Our results underline the importance of the dry season length as a governing factor in the long-term control of tree cover in tropical landscapes.
    Type: Article , PeerReviewed
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  • 10
    Publication Date: 2023-08-10
    Description: Sequences of sapropels intercalated with hemipelagic mud are a prominent feature of most eastern Mediterranean sediments. The most recent sapropel (S 1 ), recovered in a box core from the Medina Rise in the Ionian Sea, was sampled at ultra-high resolution to evaluate the paleoceanographic conditions during its formation, to characterise its organic matter and to determine post-depositional effects of diagenesis on the geochemistry. The paleoceanographic reconstruction was based on oxygen and carbon isotope compositions of planktonic foraminifera and planktonic foraminiferal census counts to estimate paleo sea surface temperatures. Combined, these results indicate a depleted surface water salinity of about 4 psu and, thus, also surface water density, which at least weakened vertical circulation and thus, bottom water ventilation during sapropel formation. However, sapropel formation may not have been caused by depleted bottom water oxygen levels alone. Significantly increased rates of primary production during times of sapropel deposition are inferred from enhanced barium accumulation rates. The immobility and stability of barium as barite in most Mediterranean sediments makes it a valuable proxy for paleoproductivity in this case. Lipid analyses were conducted for characterisation of sapropel organic matter and estimation of possible carbon sources. Results of the extractable lipids clearly indicate a predominantly marine origin, with dinoflagellates, coccolithophorid algae, other microalgae and eubacteria as main contributing organisms. Comparison with overlying oxidised samples reveals no enrichment of terrestrial organic matter. The ratio of unsaturated long-chain-ketones is strongly affected by sapropel oxidation, so that the derived paleotemperature estimates should used with caution. Elemental concentrations were measured to recognise effects of the post-depositional oxidation front prograding into the formerly anoxic sediment. As a consequence, the present thickness of the organic-rich layer is only a fifth of its original extent. The maximum dissolution effect of diagenetic reactions cannot explain the observed depletions in carbonate content in the sapropel. Decreased carbonate production during sapropel formation is therefore concluded. Diagenetic relocation of many redox-sensitive elements has occurred. Most elements expected to be associated with organic matter or sulphide-rich sediments display maximum concentrations above or below the present sapropel layer. Similarities are observed in the geochemical behaviour of Ni, Cu, Zn, and Co; and of Mo, V, and Sb. Se exhibits a large, sharp concentration peak and is inferred to be a useful marker for the present boundary of oxic to post-oxic conditions. A model of the oxygen-flux into the sapropel interval closely resembles the observed enrichments of oxidised elements. The sapropel is thus still being oxidised.
    Type: Thesis , NonPeerReviewed
    Format: text
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