Elsevier

Precambrian Research

Volume 286, November 2016, Pages 234-249
Precambrian Research

Chromium isotope, REE and redox-sensitive trace element chemostratigraphy across the late Neoproterozoic Ghaub glaciation, Otavi Group, Namibia

https://doi.org/10.1016/j.precamres.2016.10.007Get rights and content

Highlights

  • We explore redox conditions across the late Neoproterozoic Ghaub glaciation.

  • REE+Y, δ53Cr and redox tracers follow changes in water column chemistry in 4 stages.

  • δ53Cr & Ce anomalies show peaks in oxidative weathering shortly before glacial onset.

  • Vulnerability to detrital contamination necessitates corrections of the Cr signal.

Abstract

Chromium isotopes constitute a powerful paleoenvironmental tracer recording fluctuations of atmospheric oxygenation and continental weathering thus facilitating the reconstruction of the redox state of ancient seawater. We use the δ53Cr 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 glaciation into a postglacial environment in four stages. Preglacial carbonates (stage 1) yield positively fractionated δ53Cr 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 δ53Cr 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 oxidative weathering shortly before the advance of the glaciers. Marginal shale contamination persists in carbonates 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 δ53Cr 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 δ53Cr 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.

Introduction

The Neoproterozoic was a period in Earth’s history with significant increases in atmospheric O2 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 systematic differences in rare earth elements and yttrium (REE+Y) distributions of marine carbonates (e.g. Kamber and Webb, 2001, Frimmel, 2009 and references therein). REE concentrations in seawater 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 geochemical 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 et al., 2004, and references therein). Shale-normalized (modern) marine REE+Y patterns are characterized by light REE (LREE) depletion (Elderfield et al., 1990, Bolhar and Van Kranendonk, 2007), negative Ce anomalies (e.g. Bau and Dulski, 1996), positive La and elevated Y/Ho ratios (Alibo and Nozaki, 1999, Nozaki and Alibo, 2003). Coherent chemical behavior, coupled with small, systematic changes in chemical properties of REEs render them unique tracers of fundamental processes that govern their cycling in the ocean; for example, Eu reflects the input from hydrothermal vents, whereas Ce can be a useful proxy for the degree of seawater oxygenation (e.g. Bau, 1991, German et al., 1995, Nozaki et al., 1999, Bolhar and Van Kranendonk, 2007, Halverson et al., 2010). Negative Ce anomalies are characteristic for modern oxic aquatic systems and are generally more strongly developed in seawater compared to river water (Lawrence et al., 2006, and references therein). Riverine shale-normalized REE+Y patterns, are diverse, display L-, M- or HREE enrichments (e.g. Elderfield and Greaves, 1982, Elderfield et al., 1990), positive La anomalies (Lawrence and Kamber, 2006) and processes involved remain poorly constrained.

The terrestrial mobilization of Cr and Mn are closely linked (Richard and Bourg, 1991, Garnier et al., 2013), connecting oxidative Cr(VI) mobilization to the mobilization of reduced Mn(II). The oxidation of Cr(III) to Cr(VI) is accompanied by isotopic fractionation enriching the resulting mobilized Cr(VI) in the heavier isotope 53Cr whereas the residual soil (Berger and Frei, 2014, Paulukat et al., 2015, D’Arcy et al., 2016) would remain isotopically lighter, similar to δ53Cr values reported for paleosols (Crowe et al., 2013, Frei and Polat, 2013, Babechuk et al., 2016). The isotopically heavy Cr(VI) is then transported via rivers to the open ocean, where oxygenated water bodies stabilize it. The lack of significant isotope effects during adsorption of Cr(VI) onto particles (Ellis et al., 2004) potentially allows linking a terrestrial, riverine-transported δ53Cr signal (Frei et al., 2014, Paulukat et al., 2015, D’Arcy et al., 2016) to the seawater Cr isotope composition and to Cr coprecipitating with shallow-marine carbonates (Frei et al., 2011, Bonnand et al., 2013 ). There, marginal to no effects on Cr isotope fractionation between ambient seawater and marine abiogenic carbonates are expected (Bonnand et al., 2013, Rodler et al., 2015), which further supports a link between the Cr isotope composition of (ancient) marine carbonates to weathering processes of the contemporaneous continental surface (Gilleaudeau et al., 2016). However, the heterogeneity of δ53Cr in modern oceans with depth, water-mass source and mixing (Scheiderich et al., 2015) and the recently reported fractionation factor for chromate removal into shallow marine carbonate sediments compared to the modern Caribbean Sea (Holmden et al., 2016) suggest a possibly more complex internal redox cycling of Cr in the ocean and a negative offset for carbonate δ53Cr signatures, respectively. Other redox-sensitive trace elements recorded in chemical sediments, e.g. Mn and U – both indirectly linked to bioproductivity, can also be used to infer the oxygenation of seawater (e.g. Pufahl and Hiatt, 2012).

Here, we report the Cr isotope composition of late Neoproterozoic marine carbonates of the Otavi Group in northern Namibia deposited before and after the end-Cryogenian Ghaub glaciation (635.6 ± 1.2 Ma, Hoffmann et al., 2004), coupled with REE+Y and other trace element variations, to delineate paleo-redox conditions in the pre- to post-Ghaub marine environment. We emphasize that Neoproterozoic climatic variations were probably linked to significant changes in atmospheric oxygenation.

Section snippets

Regional geology and studied sections

The Otavi Group is a 2–4 km thick platform carbonate succession (Hoffman and Halverson, 2008, and references therein) that was deposited at subtropical latitudes (Li et al., 2008) over a ∼200 Ma period (Hoffmann et al., 2004) during Cryogenian and early Ediacaran age (ca. 770 to ca. 580 Ma; Halverson et al., 2005). The region was situated on the (present) south-western promontory of the Congo Craton during a period of tectonic subsidence (Hoffman, 2011a) and was bounded to the west and south by

Sample preparation and elemental analyses

Carbonate rocks with fresh surfaces and without pervasive veining or visual siliciclastic components were sampled. Glacial diamictite samples were cut to reduce the contamination of the matrix carbonates by dropstones before they were powdered. Powders of the samples were prepared from approximately one-centimeter-thick slices of the fresh hand specimens and were crushed and subsequently milled in an automatic agate mortar (Fritsch pulverisette, type 02.102). All geochemical analyses were

Mineralogical composition

Field pictures and thin section photomicrographs of platform samples (KLB) are shown in Fig. 3. The mineralogical composition of some representative samples deduced from XRD and quantified by Rietveld refinement are listed in the supplementary material (SM, Table 1). XRD results and photomicrographs of the unstained part of the thin sections reveal a transition in the carbonates from pure dolostones with relatively little quartz, microcline, muscovite and apatite (Gauss Fm) to a

Evaluation of detrital contamination, diagenesis and dolomitization

Carbonate REE+Y patterns are relatively robust towards mobilization during diagenesis, metamorphism and surface weathering (e.g. Banner et al., 1988, Zhong and Mucci, 1995, Nothdurft et al., 2004, Bolhar and Van Kranendonk, 2007) and are considered to be largely controlled by the source of the REEs (Frimmel, 2009, Frimmel, 2010). Although, factors controlling Ce(IV) scavenging in seawater as well as post-depositional alteration can result in negative Ce anomalies (Shields and Stille, 2001, and

Conclusions

The REE+Y patterns of the studied carbonates change from build-up to retreat of the Ghaub glaciation from a marine depositional setting through glaciation and into a postglacial depositional environment in four stages. The transitions between these stages are more apparent for carbonates of the platform section, which was perhaps further removed from a river delta. The underlying tendency to positive Eu anomalies throughout both carbonate successions is consistent with previous reports of Otavi

Acknowledgements

Thanks to G.I.C. Schneider, Director of the Geological Survey of Namibia, for permission and support for our research activities in Namibia, and to W. Hegenberger and S. Rosing for their support during fieldwork in Namibia in 2011. Thanks to T. Leeper for TIMS maintenance, to T. Larsen for keeping the Cr laboratories in perfect shape, to H. Almind and T. Balic-Zunic for help with XRD analyses and Rietveld refinements, to A.R. Voegelin for help with evaluating the thin sections at the University

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