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Changes in hydroclimate and vegetation in the São Francisco river drainage basin during the last 45000 years

Ferreira, Jaqueline Quirino

Biblioteca Digital de Teses e Dissertações da USP; Universidade de São Paulo; Instituto de Geociências 2021-03-01

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  • Título:
    Changes in hydroclimate and vegetation in the São Francisco river drainage basin during the last 45000 years
  • Autor: Ferreira, Jaqueline Quirino
  • Orientador: Chiessi, Cristiano Mazur
  • Assuntos: América Do Sul; Vegetação; Quaternário; N-Alcanos De Cadeia Longa; Forçante Orbital; Orbital Forcing; Long Chain N-Alkanes; Quaternary; South America; Vegetation
  • Notas: Dissertação (Mestrado)
  • Descrição: The São Francisco River Drainage Basin is the larger hydrographic system in eastern South America. This basin has a huge ecological importance due to its extension and because it hosts a large portion of Cerrado (a type of tropical savanna), as well as Caatinga and, subordinately, Atlantic Forest. Despite that, the main factors that controlled changes in the biomes during the last tens of thousands of years and were, at least partially, responsible for their current distribution are still largely unknown. This is due to the small number of long paleoenvironmental reconstructions that integrate large areas of the São Francisco River basin. Here, we used the stable hydrogen (\'delta\'D) and carbon (\'delta\' 13C) isotope composition of longchain n-alkanes from a marine sediment core to reconstruct past hydroclimate and vegetation, respectively, in an independent way. Long-chain n-alkanes are plant-wax biomarkers present in the wax coating of leaves. \'delta\'D values of plant-waxes register the isotope composition of meteoric water, which is, in turn, controlled by the amount effect in the tropics. Hence, the \'delta\'D of plant-waxes can be used to reconstruct past precipitation.\'delta\' 13C values of plant-waxes allows the differentiation between vegetation types using the C3 and C4 photosynthetic pathways. Since tropical forest taxa mostly use the C3 metabolisms, while tropical savanna taxa predominantly use the C4 metabolism, this method allows to study potential shifts in the dominant vegetation type. To do so, we analyzed marine sediment core M125-95-3 collected from the western tropical South Atlantic (10.94ºS, 36.20ºW, 1897 m water depth, 1040 cm core length), near the mouth of the São Francisco River, spanning the last 45 kyr. On top of millennial-scale changes, \'delta\' 13C data indicate for the first time a marked obliquity control over the proportion of trees (C3) versus grasses (C4). During periods of maximum (minimum) obliquity, trees (grasses) reached maximum coverage. Importantly, our \'delta\'D record does not indicate orbital-scale changes in precipitation. We suggest that maximum (minimum) obliquity decreased (increased) the length of the dry season allowing (hampering) the development of trees. Periods of maximum (minimum) obliquity increased (decreased) the intra-hemispheric insolation gradient during austral winter, strengthening the austral Hadley circulation and the southeastern trade winds. Both processes slightly increased winter precipitation over the São Francisco River Drainage Basin, decreasing the length of the dry season. Our results suggest that the dry season length is a governing factor in the long-term control of tree density in tropical savannas.
  • DOI: 10.11606/D.44.2021.tde-09042021-081711
  • Editor: Biblioteca Digital de Teses e Dissertações da USP; Universidade de São Paulo; Instituto de Geociências
  • Data de criação/publicação: 2021-03-01
  • Formato: Adobe PDF
  • Idioma: Inglês
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  • Realização: ABCD-USP USP   Logos de Redes Sociais: Freepik

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