Exploring Mg-evaporites and sabkha dolomite as archives for seawater Mg isotope composition

Abstract

Recent studies of seawater compositions of some ‘non-traditional’ stable isotope systems, such as 26Mg/24Mg (reported as δ26Mg), have uncovered great potential to enhance our understanding of the past Earth. However, differences between existing oceanic records, and the scarcity of such record data, currently limit this approach. Thus, new archives for these isotope compositions, independent of the commonly-used carbonate archives, are required. Marine evaporites have been widely used to decipher the chemical and ‘traditional’ isotope compositions (such as 87Sr/86Sr, S isotopes, etc.) history of past oceans. In several studies [1, 2, 3], we investigate the Mg isotope composition of different evaporite minerals and brines, presenting two examples: an experimental study of marine Mg-K (potash) salts and an in-situ study of pore-water and sediment from a modern sabkha environment.The δ26Mg value of marine-derived brines and precipitating Mg-salts during the evaporation path of seawater were determined experimentally, up to a degree of evaporation (DE) of ca. 500. The sequence of Mg-salts included epsomite, kainite, carnallite, kieserite, and bischofite. We identify a mineral-dependent Mg isotope fractionation in both directions (i.e., some minerals are enriched in 26Mg relative to the brine, whereas others are depleted in 26Mg). Due to the precipitation of multi-mineral assemblages having opposite fractionations, the δ26Mg value of the brines changed only slightly throughout the evaporation path, despite considerable Mg removal.The Mg concentrations and δ26Mg values of all pore-water samples extracted from the sabkha sediments are elevated relative to modern seawater and the closest evaporitic lagoon. Evaporation (DE range between 6.5 and 11), mixing, and Mg loss into dolomite are the three processes that determine the Mg concentration. Dolomite formation and mixing with ‘fresh’ lagoon water determine the δ26Mg values of pore-water. This shows that, in such evaporitic environments, the evaporitic minerals may precipitate from an already altered solution (with δ26Mg different than seawater), leading to a somewhat more complicated interpretation of the ancient record.Based on these data, we suggest that by taking into account the complexity of evaporitic systems, the δ26Mg values of evaporites preserved in the geological record may be used to 1) quantify geochemical processes that fractionate Mg-isotopes within the basin, such as dolomite formation; and 2) complete the secular variations curve of the marine δ26Mg record using well-established evaporitic sequences. [1] Shalev N., Lazar B., Halicz L., and Gavrieli I. (2021), The Mg isotope signature of marine Mg-evaporites. Geochimica et Cosmochimica Acta, 301, 30-47, https://doi.org/10.1016/j.gca.2021.02.032.[2] Shalev N., Bontognali T.R.R. and Vance D. (2021), Sabkha Dolomite as an Archive for the Magnesium Isotope Composition of Seawater. Geology, 49 (3), 253–257, https://doi.org/10.1130/G47973.1.[3] Shalev N., Lazar B., Köbberich M., Halicz L., and Gavrieli I. (2018), The chemical evolution of brine and Mg-K-salts along the course of extreme evaporation of seawater - An experimental study. Geochimica et Cosmochimica Acta, 241, 164-179, https://doi.org/10.1016/j.gca.2018.09.003.