Chromium isotope, REE and redox-sensitive trace element chemostratigraphy across the late Neoproterozoic Ghaub glaciation, Otavi Group, Namibia A.S. Rodler a,b,⇑ , R. Frei a,b , C. Gaucher b,c , G.J.B. Germs d a Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark b Nordic Center for Earth Evolution (NordCEE), Denmark c Departamento de Geología, Facultad de Ciencias, Universidad de la República, UDeLaR, Iguá 4225, 11400 Montevideo, Uruguay d Department of Geology, University of the Free State, Bloemfontein, South Africa article info Article history: Received 20 March 2016 Revised 5 October 2016 Accepted 7 October 2016 Available online 8 October 2016 Keywords: Neoproterozoic ocean redox changes Oxidative weathering Detrital contamination abstract Chromium isotopes constitute a powerful paleoenvironmental tracer recording fluctuations of atmo- spheric oxygenation and continental weathering thus facilitating the reconstruction of the redox state of ancient seawater. We use the d 53 Cr signature coupled with REE+Y patterns and redox-sensitive trace elements to monitor environmental changes recorded by marine carbonates of the Otavi Group, Namibia. These carbonates were deposited in a platform and foreslope setting in subtropical latitudes during the Neoproterozoic and comprise the transition from a marine depositional setting through glacia- tion into a postglacial environment in four stages. Preglacial carbonates (stage 1) yield positively fraction- ated d 53 Cr values, increased U and Mn concentrations, indicative of mobilization during oxidative terrestrial weathering and stabilization in oxic surface waters. Carbonates deposited just before the Ghaub diamictites (stage 2) record d 53 Cr values (>+0.4‰) comparable to modern seawater and negative Ce anomalies (0.7) characteristic for oxygenated seawater. We interpret this as a pulse of intense oxida- tive weathering shortly before the advance of the glaciers. Marginal shale contamination persists in car- bonates of both sections and is slightly elevated during the glacial aftermath; Cr is vulnerable towards detrital contamination. Early postglacial cap dolostones (stage 3) were influenced by enhanced detrital contamination potentially supplied by freshwater particulate load, which was then drastically reduced in the overlying postglacial limestones in the upper Maieberg Fm (stage 4) where near-preglacial d 53 Cr values are reached again. REE+Y patterns along with Eu and Ce anomalies record a transformation from a marine, slightly anoxic and stratified water column with distal hydrothermal influence to a freshwater- influenced depositional environment with decreased hydrothermal activity and fluctuating oxic surface water conditions after glacial retreat. Here, we demonstrate that carbonate d 53 Cr signatures are sensitive to changes in continental weathering balanced between detrital contamination and oxidative weathering on land and are capable of tracing fluctuating redox conditions prior and after one of the major syn- Marinoan glaciations. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction The Neoproterozoic was a period in Earth’s history with signif- icant increases in atmospheric O 2 concentration accompanied by changes in ocean redox conditions from anoxic to oxic conditions in the shallow and perhaps deep seawater (e.g. Canfield et al., 2008; Johnston et al., 2010; Li et al., 2010; Lyons et al., 2014). Ancient paleoenvironmental conditions, particularly compositional variation in seawater through time, can be constrained by system- atic differences in rare earth elements and yttrium (REE+Y) distri- butions of marine carbonates (e.g. Kamber and Webb, 2001; Frimmel, 2009 and references therein). REE concentrations in sea- water are controlled by different input sources (e.g. terrestrial weathering-related input, hydrothermal input) and scavenging processes. Marine REE signatures can provide information on changes in input source flux and oxygenation, thereby on geo- chemical processes and changes in continental weathering; and more locally, for example, on ocean circulation, stratification and depositional conditions (e.g. Kamber and Webb, 2001; Nothdurft http://dx.doi.org/10.1016/j.precamres.2016.10.007 0301-9268/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark. E-mail address: alexandra.rodler@reflex.at (A.S. Rodler). Precambrian Research 286 (2016) 234–249 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres