Abstract
Abstract. Lithium (Li) isotopes in marine carbonates have considerable potential as a proxy to constrain past changes in silicate weathering fluxes and improve our understanding of Earth's climate. To date the majority of Li isotope studies on marine carbonates have focussed on calcium carbonates. The determination of the Li isotope fractionation between dolomite and a dolomitizing fluid would allow us to extend investigations to deep times (i.e. Precambrian) when dolostones were the most abundant marine carbonate archives. Dolostones often contain a significant proportion of detrital silicate material, which dominates the Li budget; thus, pretreatment needs to be designed so that only the isotope composition of the carbonate-associated Li is measured. This study aims to serve two main goals: (1) to determine the Li isotope fractionation between Ca–Mg carbonates and solution, and (2) to develop a method for leaching the carbonate-associated Li out of dolostone while not affecting the Li contained within the detrital portion of the rock. We synthesized Ca–Mg carbonates at high temperatures (150 to 220 ∘C) and measured the Li isotope composition (δ7Li) of the precipitated solids and their respective reactive solutions. The relationship of the Li isotope fractionation factor with temperature was obtained: 103lnαprec-sol=-(2.56±0.27)106(1)/T2+(5.8±1.3) Competitive nucleation and growth between dolomite and magnesite were observed during the experiments; however, there was no notable effect of their relative proportion on the apparent Li isotope fractionation. We found that Li isotope fractionation between the precipitated solid and solution is higher for Ca–Mg carbonates than for Ca carbonates. If the temperature of a precipitating solution is known or can be estimated independently, the above equation could be used in conjunction with the Li isotope composition of dolostones to derive the composition of the solution and hence make inferences about the past Li cycle. In addition, we also conducted leaching experiments on a Neoproterozoic dolostone and a Holocene coral. Results show that leaching with 0.05 M hydrochloric acid (HCl) or 0.5 % acetic acid (HAc) at room temperature for 60 min releases Li from the carbonate fraction without a significant contribution of Li from the siliciclastic detrital component. These experimental and analytical developments provide a basis for the use of Li isotopes in dolostones as a palaeo-environmental proxy, which will contribute to further advance our understanding of the evolution of Earth's surface environments.
Highlights
Lithium isotopes in marine carbonates have emerged as a powerful proxy to help understand the evolution of the ocean chemistry, past silicate weathering fluxes and their links to global climate
Marriott et al (2004a) suggested that Li isotope fractionation probably occurs at equilibrium even at lower temperatures for several reasons: (i) kinetic fractionation would probably be much greater than that observed, and would require boundary layer processes or the presence of a back-reaction, for which there is no evidence; (ii) observed isotopic fractionation between calcite and growth solution, as well as between Ca–Mg carbonate and growth solution, are consistent with ab initio calculations for equilibrium fractionation (Kazuyo et al, 2001); and (iii) lithium isotope fractionation between calcite and growth solution is relatively constant across a wide range of concentrations of Li incorporated in calcite
The Li isotopic composition of the precipitated solid is isotopically lighter than the reactive solution, which is similar to previous experiments on calcium carbonates (Marriott et al, 2004b, a)
Summary
Lithium isotopes in marine carbonates have emerged as a powerful proxy to help understand the evolution of the ocean chemistry, past silicate weathering fluxes and their links to global climate. Taylor et al.: Lithium isotopes in dolostone as a palaeo-environmental proxy bated topics such as, the evolution of Earth’s climate during the Cenozoic (Misra and Froelich, 2012; Li et al, 2014; Wanner et al, 2014; Vigier and Goddéris, 2015; Hathorne and James, 2006), oceanic anoxic events (Pogge von Strandmann et al, 2013; Lechler et al, 2015) and Palaeozoic glaciation (Pogge von Strandmann et al, 2017). Postdepositional alteration can play an important role in the formation of dolomite (Geske et al, 2012; Burns et al, 2000), the application of Li isotopes to marine dolostone could help to extend our understanding of the geochemical evolution of ancient dolomitizing solutions, in early Earth geological history (i.e. Precambrian)
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