Lithium isotope behaviour during basalt weathering experiments amended with organic acids

Pogge von Strandmann, Philip A.E.; Liu, Xianyi; Liu, Chun-Yao; Wilson, David J.; Hammond, Samantha J.; Tarbuck, Gary; Aristilde, Ludmilla; Krause, Alexander J. and Fraser, Wesley T. (2022). Lithium isotope behaviour during basalt weathering experiments amended with organic acids. Geochimica et Cosmochimica Acta, 328 pp. 37–57.



Lithium (Li) isotopes are a tracer of silicate weathering processes, but how they react to different components of organic and plant-assisted weathering is poorly known. To examine the effect of organic acids compared to a strong mineral acid (HCl) on Li isotope behaviour, basalt-water weathering experiments were amended with different organic acids (glycine, malic acid, cinnamic acid, and humic acid; 0.01 M). The presence of the different acids significantly affects the behaviour of dissolved elemental concentrations (such as Mg, Fe, and Al), both by increasing primary rock dissolution and hindering rates of secondary mineral formation. However, the behaviour of Li isotopes appears unaffected, with all experiments following an almost identical trend of δ7Li versus Li/Na. This observation was consistent with a single fractionation factor during the uptake of Li into secondary minerals, yet both calculated saturation states and leaching experiments on the reacted solids indicated that Li was removed into multiple phases, suggesting that the bulk combined fractionation factor barely varied. Of the Li lost from solution in the organic experiments, we estimated that on average 76% went into neoformed clays, 16% into oxides/oxyhydroxides, and 10% into the exchangeable fraction. The fractionations observed for each phase were Δ7Liexch-soln = −12.7 ± 1.7‰, Δ7Liox-soln = −26.7 ± 0.4‰, and Δ7Liclay-soln = −21.6 ± 3.3‰. These fractionations were identical, within error, to those from experiments with organic-free water, implying that the Li isotope behaviour was unaffected by the presence of organic acids in the weathering reaction. This result has interesting consequences for the interpretation of Li isotopes in terms of plant-assisted weathering and the geological record of terrestrialisation. In particular, it appears to imply that seawater Li isotope records can be expected to resolve the integrated effect of plants on weathering fluxes or weathering congruence, rather than being sensitive to specific organic-mediated weathering mechanisms.

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