Chromium isotope, REE and redox-sensitive trace element chemostratigraphy across the late Neoproterozoic Ghaub glaciation, Otavi Group, Namibia
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
References (111)
- et al.
Rare earth elements in seawater: Particle association, shale-normalization, and Ce oxidation
Geochim. Cosmochim. Acta
(1999) - et al.
Coupled molybdenum, iron and uranium stable isotopes as oceanic paleoredox proxies during the Paleoproterozoic Shunga Event
Chem. Geol.
(2013) - et al.
Ion exchange separation of chromium from natural water matrix for stable isotope mass specrometric analysis
Chem. Geol.
(2000) Rare-earth element mobility during hydrothermal and metamorphic fluid-rock interaction and the significance of the oxidation state of europium
Chem. Geol.
(1991)- et al.
Distributions of yttrium and rare-earth elements in the Penge and Kuruman iron-formation, Transvaal Supergroup, South Africa
Precambr. Res.
(1996) - et al.
Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling
Mar. Chem.
(1997) - et al.
The fate of chromium during tropical weathering: A laterite profile from Central Madagascar
Geoderma
(2014) - et al.
A non-marine depositional setting for the northern Fortescue Group, Pilbara Craton, inferred from trace element geochemistry of stromatolitic carbonates
Precambr. Res.
(2007) - et al.
The chromium isotopic composition of seawater and marine carbonates
Earth Planet. Sci. Lett.
(2013) - et al.
The Cr/Th ratio in Precambrian pelites from the Kaapvaal Craton as an index of craton evolution
Earth Planet. Sci. Lett.
(1990)
Processes controlling the chromium isotopic composition of river water: Constraints from basaltic river chatchments
Geochim. Cosmochim. Acta
The rare earth elements in rivers, estuaries, and coastal seas and their significance to the composition of ocean waters
Geochim. Cosmochim. Acta
Chromium isotope fractionation during oxidative weathering—Implications from the study of a Paleoproterozoic (ca. 1.9 Ga) paleosol, Schreiber Beach, Ontario, Canada
Precambr. Res.
Search for traces of the late heavy bombardment on Earth—Results from high precision chromium isotopes
Earth Planet. Sci. Lett.
Chromium isotopes in carbonates – A tracer for climate change and for reconstructing the redox state of ancient seawater
Earth Planet. Sci. Lett.
Fluctuations in late Neoproterozoic atmospheric oxidation – Cr isotope chemostratigraphy and iron speciation of the late Ediacaran lower Arroyo del Soldado Group (Uruguay)
Gondwana Res.
Weathering on land and transport of chromium to the ocean in a subtropical region (Misiones, NW Argentina): A chromium stable isotope perspective
Chem. Geol.
Trace element distribution in Neoproterozoic carbonates as palaeoenvironmental indicator
Chem. Geol.
On the reliability of stable carbon isotopes for Neoproterozoic chemostratigraphic correlation
Precambr. Res.
Cr(VI) genesis and dynamics in Ferralsols developed from ultramafic rocks: The case of Niquelândia, Brazil
Geoderma
Rare earth elements in Saanich Inlet, British Columbia, a seasonally anoxic basin
Geochim. Cosmochim. Acta
Dissolved rare earth elements in the Southern Ocean: Cerium oxidation and the influence of hydrography
Geochim. Cosmochim. Acta
Syn- to late-orogenic sedimentary basins of southwestern Africa. Neoproterozoic to Early Palaeozoic evolution of southwestern Africa
Neoproterozoic chemostratigraphy
Precambr. Res.
Are basal Ediacaran (635 Ma) post-glacial “cap dolostones” diachronous?
Earth Planet. Sci. Lett.
Multiproxy constraints on alteration and primary compositions of Ediacaran deep-water carbonate rocks, Yangtze Platform, South China
Geochim. Cosmochim. Acta
Response of the Cr isotope proxy to Cretaceous Ocean Anoxic Event 2 in a pelagic carbonate succession form the Western Interior Seaway
Geochim. Cosmochim. Acta
Enigmatic carbonates of the Ombombo Subgroup, Otavi Fold Belt, Namibia: A prelude to extreme Cryogenian anoxia?
Sedimentary Geology
The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth?
Earth Planet. Sci. Lett.
An emerging picture of Neoproterozoic ocean chemistry: Insights from the Chuar Group, Grand Canyon, USA
Earth Planet. Sci. Lett.
The geochemistry of late Archaean microbial carbonate: implications for ocean chemistry and continental erosion history
Geochim. Cosmochim. Acta
A new estimate for the composition of weathered young upper continental crust form alluvial sediments, Queensland, Australia
Geochim. Cosmochim. Acta
Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change
Earth Planet. Sci. Lett.
Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism
Precambr. Res.
The behaviour of the rare earth elements during estuarine mixing – revisited
Mar. Chem.
Assembly, configuration, and break-up history of Rodinia: A synthesis
Precambr. Res.
Cerium anomaly variations in Ediacaran–earliest Cambrian carbonates from the Yangtze Gorges area, South China: Implications for oxygenation of coeval shallow seawater
Precambr. Res.
Authigenic uranium: relationship to oxygen penetration depth and organic carbon rain
Geochim. Cosmochim. Acta
Molybdenum and uranium geochemistry in continental margin sediments: Paleoproxy potential
Geochim. Cosmochim. Acta
The geochemistry of redox sensistive trace metals in sediments
Geochim. Cosmochim. Acta
Uranium diagenesis in sediments underlying bottom waters with high oxygen content
Geochim. Cosmochim. Acta
Sedimentology and chemostratigraphy of the Bwipe Neoproterozoic cap dolostones (Ghana, Volta Basin): A record of microbial activity in a peritidal environment
C.R. Geosci.
Rare earth element geochemistry of Late Devonian reefal carbonates, Canning Basin, Western Australia: confirmation of a seawater REE proxy in ancient limestones
Geochim. Cosmochim. Acta
Dissolved rare earth elements and hydrography in the Sulu Sea
Geochim. Cosmochim. Acta
The estuarine geochemistry of rare earth elements and indium in the Chao Phraya River, Thailand
Geochim. Cosmochim. Acta
Oxidative release of chromium from Archean ultramafic rocks, its transport and environmental impact – A Cr isotope perspective on the Sukinda valley ore district (Orissa, India)
Appl. Geochem.
Oxygenation of the Earth’s atmosphere–ocean system: A review of physical and chemical sedimentologic responses
Mar. Pet. Geol.
Aqueous geochemistry of chromium: A review
Water Res.
Fractionation behavior of chromium isotopes during coprecipitation with calcium carbonate: implications for their use as paleoclimatic proxy
Geochim. Cosmochim. Acta
Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China
Chem. Geol.
Cited by (48)
Chemostratigraphy of early Ediacaran carbonate rocks of the Cachoeirinha Group (Northeastern Brazil): Implications for paleonvironmental conditions and atmospheric oxygenation
2024, Journal of South American Earth SciencesRedox evolution in the subtropical Northwest Pacific across the Middle Miocene Climate Transition
2024, Journal of Asian Earth SciencesDevelopments in analytical techniques for chemostratigraphy, chronostratigraphy, and geochemical fingerprinting studies: Current status and future trends
2023, Journal of South American Earth SciencesRedox heterogeneity of the Ediacaran ocean constrained by chromium isotopes
2023, Geochimica et Cosmochimica Acta