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Doctoral thesis
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English

Multi-scale quantitative sedimentology of Upper Pleistocene coastal dunes from the Northern Bahamas and Western Australia

ContributorsVimpere, Lucasorcid
Imprimatur date2021-11-18
Defense date2021-11-12
Abstract

In the age of global climate change and the ensuing growing concern about future implications for the planet, scientists are working in a joint effort to better understand Earth systems. Coastal dunes present numerous benefits for the 40% of the global population living in coastal areas and for their ecosystems but also represent a potential desertification threat if reactivated. The understanding of the triggering factors for dune instability is thus capital to better predict their future behaviour and adapt coastal management programs. The project of this thesis originates in the controversy around some coastal “chevrons” described in the Bahamas. These U-shape landforms date from the Last Interglacial, stage of the Quaternary that was characterised by a higher than present global average temperature (+ 1-2 °C) and a higher sea level (+ 6-9 m.a.s.l.). The discussion on their genesis opposes a dramatic interpretation of these “chevrons”, linked with superstorms related to warmer and unstable climate conditions, to a more classic one suggesting drier and windier conditions in the Caribbean allowing the dunes formation. Considering the lack of clarity around the definition and the use of the term “chevron”, I first reviewed the literature mentioning them. The term “chevrons” has been used independently to describe coastal parabolic dunes in the Bahamas, Madagascar, and Western Australia where they have been correlated to wind, giant storms, and tsunamis caused by asteroid impacts. In each of these countries, a debate arose among geoscientists on the depositional processes and the validity of the terminology. This review provides with a complete and comprehensive summary of the problematic and demonstrates than “chevrons” are coastal parabolic dunes of aeolian origin. I then further investigated the connection between the coastal parabolic dunes in the Bahamas and the particular climate conditions prevailing in the region during the Last Interglacial. First, the study of the morphology and sedimentary structures coupled with the grain size analysis of the dunes confirmed the conclusions reached in the review. The stratigraphic study then correlated the deposition of the dunes with the southward displacements of the Intertropical Convergence Zone, which lead to a deterioration of the vegetation cover under a drier and windier climate. Dunes formed during a short sea level drop during the ii highstand of the Last Interglacial and the ensuing regression when these conditions prevailed, following periods of significant carbonate production on the platforms. The link between coastal aeolianites, climate conditions, and sea level variations was again investigated in Western Australia where the carbonate platform geometry is different. The formation of transverse aeolianites along the western coast of the Shark Bay region was correlated to a drier and windier climate caused by northward displacement of the Intertropical Convergence Zone, and more generally by a northward shift of the atmospheric cells. These changes in atmospheric circulation and the associated variations in ocean circulation were correlated to glacial periods whilst aeolianite-capping paleosols were correlated to interglacials stages. Following the conclusions reached on coastal aeolianites and the climate under which they form relative to the geometry of the platform, I extended the investigation to continental parabolic dunes. Their activity during the pre-industrial and the mid. Holocene was investigated because climate conditions are well constrained for these periods. The Intertropical Convergence Zone moved north during the mid. Holocene, which resulted in the aridification of the presently-humid areas and the reactivation of continental parabolic dunes in these regions. Inversely the presently-dry regions were wetter, and dunes stabilised by vegetation. The link between dune activity, atmospheric circulation, and the related climate provides valuable information to constrain potential future climate change and define the regions the most at risk for desertification through parabolic dunes migration. All together, these results highlight the causal relationship between parabolic dunes or coastal aeolianites and global atmospheric circulation. Further efforts should be directed towards a better understanding of the general trends of aeolian dunes activity related to climate. This would serve both future climate predictions and palaeoclimate interpretations, two important areas that are intricately intertwined.

eng
Keywords
  • Dune
  • Aeolian
  • Climate
  • Quaternary
  • Bahamas
  • Australia
  • Carbonate
  • Coastal sedimentology
  • Stratigraphy
Citation (ISO format)
VIMPERE, Lucas. Multi-scale quantitative sedimentology of Upper Pleistocene coastal dunes from the Northern Bahamas and Western Australia. 2021. doi: 10.13097/archive-ouverte/unige:157371
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Creation11/29/2021 9:41:00 AM
First validation11/29/2021 9:41:00 AM
Update time03/16/2023 2:09:38 AM
Status update03/16/2023 2:09:37 AM
Last indexation01/29/2024 11:11:10 PM
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