The Mg isotope signature of marine Mg-evaporites

Netta Shalev,Boaz Lazar,Ludwik Halicz,Ittai Gavrieli

doi:10.1016/j.gca.2021.02.032

Abstract

Marine Mg-evaporites are a small oceanic sink of magnesium, precipitating only from extremely evaporated brines. The isotopic composition of Mg in seawater, δ26Mgseawater, has recently been shown to be an effective tool for reconstructing the Mg budget of the modern and past oceans. However, estimations of the Mg isotope fractionation between the Mg-evaporites and their precipitating solution are required for full quantification of the isotope effect of the evaporitic sink on δ26Mgseawater, as well as for utilizing ancient evaporitic sequences as an archive for past δ26Mgseawater. Here, we estimate the Mg isotope fractionation between Mg-evaporites and modern marine-derived brine along the course of seawater evaporation, up to degree evaporation of >200. The sequence of Mg-salts included epsomite (MgSO4·7H2O), kainite (KMgClSO4·3H2O), carnallite (KMgCl3·6H2O), kieserite (MgSO4·H2O) and bischofite (MgCl2·6H2O).The following isotope fractionation values, either negative or positive, were calculated from the isotope difference between the salt and its precipitating brine, and from the evolution of δ26Mg in the brine throughout the evaporation: Δcarnallite-brine = +1.1‰, Δepsomite-brine = +0.59‰, Δbischofite-brine = +0.33‰, Δkieserite-brine = −0.2‰ and Δkainite-brine = −1.3‰. Magnesium isotopic compositions determined on minerals from different ages in the geological record corroborate well these results. Due to precipitation of multi-mineral assemblages having isotope fractionation values of opposing signs, the δ26Mg value of the brine changes only slightly (<0.5‰) throughout the evaporation path, despite the considerable Mg removal (>50%). The isotope fractionations are shown to correlate with the number of water molecules coordinated to the Mg2+ and with Mg-O bond length in the mineral lattice.Given these isotope fractionations, it is calculated that a volume of 0.4 · 106–0.8 · 106 Km3 of a mono-mineral assemblage of kainite or carnallite needs to precipitate in order to change seawater δ26Mg by only 0.1‰. This huge volume is by far larger than the volume of these minerals known to date in the global geological record. Therefore, it is concluded that the impact of Mg-evaporites formation on δ26Mgseawater has been insignificant since the Proterozoic. The results of this study suggest that the Mg isotopic composition of Mg-evaporites preserved in the geological record of evaporitic basins may be used to: 1) quantify geochemical processes that fractionate Mg-isotopes within these basins, such as dolomitization; and 2) complete the secular variations curve of the marine δ26Mg record using basins with well-established evaporitic sequences.

Highlights

Magnesium is the third most abundant cation in the ocean and, due to its long residence time ($13 Myr; Berner and Berner, 1996) relative to the mixing time of the oceans, it is well-mixed
The concentration and isotopic composition of Mg in seawater are determined by the oceanic Mg budget, which is controlled by Mg supply from rivers, and Mg removal, mainly by precipitation of carbonate minerals and hydrothermal reactions with the volcanic oceanic crust and, to a lesser extent, by reverse weathering and precipitation of Mg-evaporites (e.g., Elderfield and Schultz, 1996; Holland, 2005; Arvidson et al, 2006)
The Dkainite-brine value obtained from the evaporation experiments, À1.30‰, is similar to the Dkainite-SW of the three Messinian natural samples, assuming they precipitated from a brine having Mg isotopic composition of modern seawater

Summary

Introduction

Magnesium is the third most abundant cation in the ocean and, due to its long residence time ($13 Myr; Berner and Berner, 1996) relative to the mixing time of the oceans, it is well-mixed. Understanding and quantifying the Mg budget of the modern and ancient oceans are important to our understanding of how fundamental Earth processes, such as weathering, volcanism and sedimentation, have changed globally throughout the geological past, and how these processes are linked to Earth’s carbon cycle and long-term climate change (e.g., Holland, 2005; Elderfield, 2010). The isotopic composition of dissolved Mg in seawater, d26Mgseawater, has recently been shown to be a reliable proxy for the reconstructions of the Mg budget of the modern and past oceans (e.g., Tipper et al, 2006; Pogge Von Strandmann et al, 2014; Higgins and Schrag, 2015; Li et al, 2015; Gothmann et al, 2017; Shalev et al, 2019; Xia et al, 2020). The scarcity of Mg isotope data from Mg-evaporites has far prevented estimations of their potential effect, as a Mg-sink, on the d26Mgseawater value, as well as their use as an archive for past d26Mgseawater

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Conclusion