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Lithium isotopes in carbonate-rich marine sections: implications for diagenesis, authigenic clay formation, and the global lithium cycle
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Andrews, Elizabeth Marie
Master of Science
Date of Defense:
May 21, 2018
Matthew S Fantle, Thesis Advisor
Katherine Haines Freeman, Committee Member
Lee Kump, Committee Member
Authigenic Clay Formation
With advances in mass spectrometric techniques, lithium (Li) isotope research has rapidly expanded in the last two decades. Many studies have used Li isotopes as proxies for continental weathering intensity and to understand the global Li cycle through geologic history. However, as attempts are made to understand the Li cycle, it is essential to consider if the isotopic composition of the proxy material is a primary seawater signal or if it reflects post-depositional alteration as a result of processes such as diffusion, secondary mineral formation, or recrystallization. The goals of this research are to investigate the processes that control the isotopic composition of pore fluids by elucidating the extent of post-depositional alteration at a carbonate-rich site (IODP Site 1338) located in the Eastern Equatorial Pacific. At this site, a large Li concentration gradient in the pore fluid was observed (Palike et al., 2010b). The current study finds the concentration gradient is associated with an isotopic enrichment of the pore fluid δ7Li with depth. Such a signature is evidence that authigenic clay formation actively controls the elemental and isotopic composition of pore fluids in carbonate-rich sediments. The large gradient indicates that this site is not at diffusive steady state and over time, diffusion will smooth out the gradients in concentration and isotopic composition. Analysis and numerical modeling of carbonate-rich sediment indicate that diffusion damps the expected signal of clay formation by 2‰. Model results also predict that formation of a very small volume (~1%) of authigenic clay mineral is sufficient to explain the concentration gradient at Site 1338. If authigenic clay formation in carbonate-rich sites takes place globally, the Li removed from the global ocean would be 1.68•1010 moles Li/year, and similar to previous estimates of the flux of Li into secondary mineral phases, 2.0•1010 moles Li/year. Additionally, these processes have the potential to impact the isotopic composition of proxy material as the carbonate in this site shows evidence of interaction with the pore fluid and recrystallization.
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