Contents lists available at ScienceDirect Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha Neoproterozoic marine carbonates and their paleoceanographic significance Ashleigh van Smeerdijk Hood a,b, ⁎ , Malcolm William Wallace b a Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA b School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia ARTICLE INFO Editor: Dr. T.M. Cronin Keywords: Carbonate sedimentology Seawater Mg/Ca Marine cements Carbonate mineralogy Precambrian seawater chemistry Dolomite ABSTRACT The primary mineralogy of marine carbonate precipitates has been a crucial factor in constraining the major element composition of ancient oceans. Secular changes in Phanerozoic marine chemistry, including Mg/Ca, have been well-documented using the original carbonate mineralogy of ooids, marine cements and biominerals. However, the history of Precambrian seawater chemistry is not as well constrained, partially due to the pre- valence of dolomitisation in the Precambrian geological record. The Neoproterozoic (~ 1000 Ma to ~ 541 Ma) record of primary carbonate mineralogy is documented here using a combination of literature data and new analysis of marine carbonate precipitates from the Otavi Fold Belt, Namibia, the Death Valley succession, USA and the Adelaide Fold Belt, Australia. These data suggest that the last ~ 460 million years of the Proterozoic were dominated by aragonite and high-Mg calcite precipitation in shallow marine settings. In contrast, low-Mg calcite has only been recognised in a small number of formations. In addition to aragonite and calcite precipitation, marine dolomite precipitation was widespread in Neoproterozoic oceans, including mimetic (syn-sedimentary) dolomitisation and primary dolomite marine cementation. The combination of marine aragonite, high Mg-calcite and dolomite precipitation during the Neoproterozoic suggests extremely high seawater Mg/Ca conditions re- lative to Phanerozoic oceans. Marine dolomite precipitation may also be linked to widespread marine anoxia during this time. 1. Introduction Variation in primary marine carbonate mineralogy is an important long-term process involving marine chemistry, climate and biogeo- chemical cycling on the Earth. Early analysis of the composition and distribution of marine carbonate precipitates represented a major ad- vance in our understanding of the evolution of the major ion compo- sition of ancient seawater (e.g. Sandberg, 1975, 1983; Mackenzie and Pigott, 1981; Tucker, 1992; Stanley and Hardie, 1998; Hardie, 1996, 2003). Additionally, as some carbonate minerals such as aragonite are particularly susceptible to dissolution and replacement during diagen- esis, these processes profoundly affect the preservation of marine geo- chemical signatures. The links between Phanerozoic seawater, carbo- nate precipitation and the preservation of marine geochemistry are relatively well-constrained. However, Precambrian seawater chemistry, carbonate mineralogy and diagenetic processes are quite poorly un- derstood, despite the reliance on C-isotope chemostratigraphy through much of this record, particularly in the Neoproterozoic. A more com- prehensive record and synthesis of the link between carbonate miner- alogy and seawater conditions in the Neoproterozoic is necessary for future sedimentological, paleo-environmental modelling and geochemical work. A broad consensus has emerged whereby the carbonate precipitates of Phanerozoic seawater have been dominantly calcite or aragonite (Hardie, 1996). Oceans with high Mg/Ca molar ratios (i.e. > 2, Hardie, 1996) preferentially precipitate aragonite, and oceans with low sulfate concentrations may be dominated by aragonite at even lower Mg/Ca conditions (~0.3 at 5 mM SO 4 , Bots et al., 2011). These “aragonite seas” have been linked to relatively slow rates of sea-floor spreading and cold climate conditions (e.g. Sandberg, 1983; Stanley and Hardie, 1998; Higgins and Schrag, 2015). Conversely, fast spreading rates lead to a drawdown of Mg and produce “calcite seas”, associated with Mg/ Ca of less than one (Wilkinson and Algeo, 1989; or < 0.3, Bots et al., 2011), low-Mg calcite precipitation and greenhouse climates. These broad-scale Mg/Ca conditions in Phanerozoic seawater have now been further constrained using fluid inclusion work, abiotic and biotic car- bonate mineralogy and evaporite compositions from the geological record (e.g. Dickson, 2004; Demicco et al., 2005; Ries, 2010; Lowenstein et al., 2014). Determining Precambrian primary carbonate mineralogy is com- plicated by almost ubiquitous dolomitisation, which although often mimetic (preserving the original textures, e.g. Tucker, 1983; Corsetti https://doi.org/10.1016/j.gloplacha.2017.11.006 Received 20 July 2017; Received in revised form 1 November 2017; Accepted 3 November 2017 ⁎ Corresponding author at: School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia. E-mail address: hood.a@unimelb.edu.au (A.v.S. Hood). Global and Planetary Change 160 (2018) 28–45 Available online 14 November 2017 0921-8181/ © 2017 Elsevier B.V. All rights reserved. T