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The effects of water content on mantle circulation, geochemical cycles and surface observables

Chotalia, Kiran Rashmikant; (2020) The effects of water content on mantle circulation, geochemical cycles and surface observables. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

Water is one of the key components for life and yet its role in Earth evolution is not well understood and its interaction with the mantle even less so. There is evidence of mantle water from mid-ocean ridge basalts, ocean island basalts and diamond inclusions that indicate local hydration. However, how much water resides in the mantle today is completely unknown. I use a parametrised mantle convection model to understand the interaction of the surface ocean and mantle reservoir by tracking degassing of water to the surface and regassing of the mantle at subduction zones. This thesis aims to examine the effects of viscosity laws and mixing on thermal evolution and the water cycle. I also use present and past surface exposure of topography to constrain the behaviour of the water cycle and assess how much water the mantle resides in the mantle today. I show that a temperature-dependent viscosity law best matches present day average mantle temperature compared to water-dependent laws that promote excessive cooling. With this viscosity law, I take into account a mixing time in the water cycle where it takes a finite amount of time for subducted water to be degassed at a mid-ocean ridge. Results show that mixing effectively traps water at depth forming a more hydrous mantle and a smaller surface ocean. This delay also induces a period of net degassing and the associate fluctuations in the water cycle translate to changes in sea level >10m. Finally, I calculate the relationship between surface ocean mass and sea level, using the Earth's hypsometry. I use present and past surface exposure and present day estimates for the degassing flux to show that no more than 1 ocean of water can reside in the mantle today.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The effects of water content on mantle circulation, geochemical cycles and surface observables
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2020. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
Keywords: mantle convection, deep water cycle, sea level, viscosity, mantle mixing
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10093095
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