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Accueil du site → Doctorat → Royaume-Uni → 2022 → Reconstructing the timing and location of North-West Saharan rainfall with spatially distributed secondary carbonates

University of Oxford (2022)

Reconstructing the timing and location of North-West Saharan rainfall with spatially distributed secondary carbonates

Couper, H

Titre : Reconstructing the timing and location of North-West Saharan rainfall with spatially distributed secondary carbonates

Past periods of increased rainfall in the northwest Sahara

Auteur : Couper, H

Université de soutenance : University of Oxford

Grade : Doctor of Philosophy (PhD) 2022

Résumé
There is significant evidence that the presently hyper-arid Sahara belt stretching across North Africa has experienced periods of increased rainfall and surface moisture coverage at numerous times during the middle to late Pleistocene. During these times, there was increased vegetation coverage, greater mega-fauna distributions and more extensive human occupation. However, our understanding of precisely where and when rainfall increased is currently limited due to a paucity of high-resolution paleoclimate data. Furthermore, present records are over-extrapolated in their interpretations of moisture source and the spatial distribution of rainfall. In this thesis, periods of past in- creased rainfall are assessed in the central and northern Sahara using the formation age, stable isotope compositions and trace element concentrations of radiometrically dated secondary carbonate deposits. Stalagmites from the High- and Anti-Atlas Mountains provide evidence that sub-tropical rainfall occurs in response to increasing North-South inter-hemispheric temperature anomalies during the mid-Holocene African Humid Pe- riod, rather than as the direct result of a northwards extension of the West African Monsoon. Subsequently, older periods of Saharan-, High- and Anti-Atlas speleothem formation throughout the middle to late Pleistocene identifies periods of increased northern Sahara rainfall during a number of major northern hemisphere interglacial and interstadials. In the central Sahara, chronology from flowstone-like secondary cal- cite samples provides the first evidence of carbonate deposition from this region formed from direct, local rainfall rather than as a result of increased surface moisture avail- ability. Calcite formation during Marine Isotope Stage 11 suggests a consistent tropical source of rainfall in the central and northern Sahara. This thesis presents the north- west Sahara as a unique and climatically significant Saharan region, from which the activation of perennial southward flowing river systems was critical for human migration and development. Furthermore, this work better characterises the minimum geographic extent of increased north-west Saharan rainfall and identifies the need to move away from overly-generalised “Green Sahara” interpretations of Saharan climate.

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Page publiée le 10 janvier 2023