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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. 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 pre-treatment 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) determining 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 that contained within the detrital portion of the rock. We synthesized Ca-Mg carbonates at high temperature (150 to 220 °C) and measured the Li isotope composition (δ7Li) of 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) × 106T2 + (5.8 ± 1.3) Competitive nucleation and growth between dolomite and magnesite were observed during the experiments, however, without notable effect of their relative proportion on the apparent Li isotope fractionation. We found that Li isotope fractionation between precipitated solid and solution is much greater for Ca-Mg carbonates than for Ca carbonates. If the seawater temperature can be estimated independently, the above equation could be used in conjunction with the Li isotope composition of dolostones to derive those of the precipitating solutions and hence make inferrals about the past oceanic Li cycle. In addition, we also conducted leaching experiments on a Neoproterozoic dolostone and a Holocene coral. Results show that leaching with 0.05M HCl or 0.5 % acetic acid at room temperature for 60 min releases Li from the carbonate fraction without 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.

Abstract. The termination of Cryogenian glaciations would have undoubtedly impacted the chemistry of Neoproterozoic oceans, with possible consequences for life; but the extent and duration of this impact are poorly constrained. In this study, we use the lithium (Li) isotope composition of Ediacaran cap dolostones from South Australia (Nuccaleena Formation) and China (Doushantuo Fm) to investigate changes in ocean chemistry that followed the Marinoan deglaciation. The effect of diagenesis was evaluated and while the Nuccaleena Fm is likely to have preserved the primary composition of cap dolostone deposition, the offset in Li isotope ratios observed for the Doushantuo Fm could possibly reflect partial overprinting by diagenetic fluids. The Li isotope composition of Ediacaran seawater was estimated and we suggest it was similar to that of late Cenozoic oceans for most of the cap dolostone deposition. Using a box model for the oceanic Li cycle, we show that at the onset of deglaciation, the supply of riverine Li to the oceans was up to 50 times the modern flux. The modelled riverine Li isotope composition suggests that continents resembled modern high-latitude regions during this time. This episode was short-lived (up to 1 Myr) and the subsequent supply of riverine Li was similar to modern conditions, both in flux and isotope composition, for the whole duration of cap dolostone deposition. These results suggest that Ediacaran oceans and continents rapidly recovered from the Marinoan glaciation to reach environmental conditions similar to the late Cenozoic. From the standpoint of the Li oceanic budget, the Ediacaran oceans in which complex lifeforms emerged may have not been that different from our modern oceans.