Liu, Xianyi; (2023) Investigating how the presence of land plants affected silicate weathering: evidence from Lithium isotopes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Terrestrial silicate weathering is a key component in the operation of the Earth system, because it drives one of the main CO2 sequestration pathways. The rise of plants could influence the silicate weathering rate by secreting organic acids, creating thick soils, elevating soil CO2, etc. However, there is no consensus on how plants would influence silicate weathering over long time-scales, and approaches to quantify such influences are relatively sparse, hindering the assessment of the role that plants may have played on weathering in deep time. In this project, I use a silicate weathering tracer (Li isotopes) to examine these problems, by 1) studying paleorecords of Li isotopes across the Devonian (359-420Ma) to understand the transformation of terrestrial weathering during this period, 2) conducting one-year organic-compound amended water-rock interaction experiments (compared to abiotic water-rock interaction experiments), and 3) investigating the Li isotope behaviour in soil chronosequences with different tree afforestation ages. These experiments cover a wide range of temporal and spatial ranges to study the effects of plants on weathering. The results suggests that 1) the weathering transformations during the Mid-Devonian (~380- 390Ma), as indicated by a 10‰ rise in δ 7Licarb, can be attributed to the emergence of deep-rooted vascular forests that significantly increased regolith thickness. 2) Li isotopes could be used to track rock weatherability in deep time, which can significantly help global long-term climate modelling, 3) organic compounds do not change the Li isotope fractionation factor, but might influence the fluid δ 7Li evolution pattern by changing the rock weatherability, 4) Li can be fractionated within plants and then influence pore waters, but such effects would only be significant when the Li uptake rate exceeds the endurance of plants, and The above investigations suggest that plants can influence weathering on various timescales. I further argue that to quantify the effects of plants on weathering over geological time-scales, it is necessary to consider the nutrient requirements of plants, which might depend on the growing stages of the plants, nutrient recycling etc. That is to say, the effects of plants on silicate weathering is driven by the growing motivation of plants and thus a detailed terrestrial forest ecosystem model that considering the ecological traits of the trees is needed in the future.

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