Citation: Lin, S.-S.; You, C.-F.; Chung, C.-H.; Huang, K.-F.; Zhou, C. Mg and Sr Isotopes in Cap Dolostone: Implications for Oceanic Mixing after a Neoproterozoic Snowball Earth Event. Water 2023, 15, 2688. https://doi.org/10.3390/w15152688 Academic Editor: Maurizio Barbieri Received: 28 June 2023 Revised: 20 July 2023 Accepted: 21 July 2023 Published: 25 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Mg and Sr Isotopes in Cap Dolostone: Implications for Oceanic Mixing after a Neoproterozoic Snowball Earth Event Shiau-Shiun Lin 1 , Chen-Feng You 1, * , Chuan-Hsiung Chung 1 , Kuo-Fang Huang 2 and Chuanming Zhou 3 1 Department of Earth Sciences, National Cheng Kung University, Tainan 701, Taiwan; ben82916@gmail.com (S.-S.L.); bear_chung@mail.ncku.edu.tw (C.-H.C.) 2 Institute of Earth Sciences, Academic Sinica, Nankang, Taipei 115, Taiwan; kfhuang@earth.sinica.edu.tw 3 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; cmzhou@nigpas.ac.cn * Correspondence: cfy20@mail.ncku.edu.tw Abstract: The snowball Earth (SBE) describes a state of the Earth’s climate with global or near-global ice cover. The cap dolostone at the base of the Ediacaran successions serves as useful archives for studying environmental change during the Marinoan Snowball Earth deglaciation in Neoproterozoic. The characteristic compositions in dolomite provide critical information on continental weathering and coastal water mixing after glacial retreat. However, valid methods for pristine dolomite sepa- ration remain challenging. In this study, four selected cap dolostone samples from the base of the Ediacaran Lantian Formation were used for establishing a new 3-step leaching method, to remove the secondary calcite and other impurities before determination of δ 26 Mg and 87 Sr/ 86 Sr in dolomite. Non-destructive Raman, X-ray diffractometer (XRD) and scanning electron microscopy (SEM) were used to examine the distribution of dolomite and minor calcite/silicate in each sample. Micro-drill powders before each extraction procedure were examined in weight loss and mineralogical composi- tions, as well as the chemicals in the leaching solutions. Potential diagenetic artifacts were evaluated using Sr/Ca, Mn/Sr, 87 Sr/ 86 Sr and δ 26 Mg in solutions. By applying a simple two-end member mixing between the seawater and the silicate sources (R 2 = 0.48, n = 23), the down-core variations of δ 26 Mg and 87 Sr/ 86 Sr in cap dolostone can be used to gain a better understand of the temporal weathering intensity changes, as well as the coastal oceanic mixing processes, after the Marinoan deglaciation. Keywords: Snowball Earth; cap dolostone; δ 26 Mg and 87 Sr/ 86 Sr; Shiyu 1. Introduction The Snowball Earth (SBE) in Neoproterozoic era, 1000~539 Ma, is one of the most unrest periods in the early Earth. Particularly, the global glaciations from ~720 to ~635 Ma, covered nearly the entire Earth surface [1,2], evident found in the equatorial regions [3]. The SBE events changed not only the ocean, continent and atmosphere environments [4], but also impacted the early life evolution history. The Marinoan diamictite has been one of the most attractive subjects; however, its δ 13 C remains contentious in timing and sedimentation enviroments. Melezhik et al. [5] and Alene et al. [6] studied the cap dolostone deposition and proposed processes of biotic productivity, gas discharge and weathering may influence the δ 13 C compositions in dolostone. The characteristic compositions in dolomite will provide critical information on continental weathering and coastal water mixing after glacial retreat [7]. One of the main features of the ocean circulation in the SBE aftermath is the stratification that develops as a consequence of the freshwater inflow during the deglaciation [8]. The SBE hypothesis and the gas-hydrate destabilization (GHD) hypothesis are two major models for the Neoproterozoic global glaciation events [4], and they are opposite models. The deglaciation scenes described in SBE model involve complicated geological and biological processes, including volatile released in mid-ocean ridges, gas Water 2023, 15, 2688. https://doi.org/10.3390/w15152688 https://www.mdpi.com/journal/water