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Sedimentary recycling and chemical weathering: a silicon and lithium isotopes perspective

Abstract : At the Earth surface, the interplay between the water cycle and the rock cycle leads to transport of materials from the continents to the sea. In the ocean, detrital sediments are thought to react with seawater through a set of hypothesized, but yet-under constrained, processes called “reverse weathering” which consume cations, form authigenic clays, and liberate CO2. Over long-time scales, sediments are transformed into sedimentary rocks. Under the action of tectonic uplift these sedimentary rocks are "recycled" back to the Earth surface. This thesis investigates these processes through (1) a set of laboratory experiments; (2) the close examination of detrital sediments deposited in environments varying from alluvial to marine at an Eocene delta; and (3) the role of sedimentary rocks in erosion and weathering, using the geochemistry of river sediments from 42 world rivers. These objectives have been achieved using "non-traditional" isotope systems: Si and Li. First, our experiments show that detrital clays exhibit a strong capacity to adsorb Li and fractionate seawater Li isotopes (d7Limin/fluid ~ 20‰). Second, the field data show that seawater composition (especially d7Li) can be modified by interacting with detrital sediments over geological time scales, and that deltaic “reverse weathering"-type processes result in a coupling between the Li, Si, and Fe cycles. Third, Si isotopes suggest that the recycling of sedimentary rocks has a major influence on the composition of sediments currently transported to the ocean, and that this possibly results in a long-term weakening of the capacity of continental silicate weathering to consume atmospheric CO2.
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Xu (yvon) Zhang. Sedimentary recycling and chemical weathering: a silicon and lithium isotopes perspective. Geochemistry. Institut de Physique du Globe de Paris, 2018. English. ⟨tel-03070776⟩

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