Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58-2.50Ga-old Transvaal Supergroup carbonate platform, South Africa

Suemeyya Eroglu, Ronny Schoenberg, Martin Wille, Nicolas Beukes, Heinrich Taubald

Research output: Contribution to journalArticlepeer-review

46 Citations (Scopus)

Abstract

The Neoarchean-Paleoproterozoic Transvaal Supergroup in South Africa contains the well-preserved stromatolitic Campbellrand-Malmani carbonate platform, which was deposited in shallow seawater shortly before the 2.40-2.32Ga Great Oxidation Event (GOE). This platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for (isotope-) geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the GOE. Mo isotopes in sedimentary archives are widely used as a proxy for redox-changes in modern and ancient environments and recent evidence suggests that oxy-molybdate (MoO42-) is directly transferred from ocean water to inorganic carbonates with negligible fractionation, thus reflecting oceanic Mo isotope signatures.In this study we analyzed major and trace element compositions as well as Mo isotopic compositions of carbonate and mudstone samples from the KMF-5 drill core. Geochemical indicators, such as Fe and Mn concentrations and Fe-to-Mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform. Heavy δ30Si values of silicified carbonates between 0.53 and 2.35‰ point to Si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint. This assessment is supported by the frequent observation of rip-up structures of silica (chert) layers within the entire sedimentary succession. The δ98Mo values of whole rock samples throughout the Malmani-Campbellrand platform range between -0.82 and +1.40‰, similar to values reported for deeper slope carbonates from the Griqualand West area, but variations are independent from lithology or depositional water depth. These large variations in δ98Mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere-ocean system at that time, in agreement with earlier Mo isotopic studies on Campbellrand carbonates and shales. A similar range in δ98Mo values for carbonates between +0.40 and +0.87‰, however, was also found on the hand specimen scale, indicating the danger of a sample bias on the Mo isotopic stratigraphy of this carbonate platform. Results of previously unpublished adsorption experiments of Mo on CaCO3 clearly indicate that the Mo inventory of Malmani-Campbellrand carbonates was not only influenced by primary adsorption from seawater, but to a much larger degree by secondary processes during early diagenesis, which also affected the Mo isotopic composition of the samples on a local scale. Our results indicate that Mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification, and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through Earth's history. Nevertheless, we interpret our heavy Mo isotopic signatures from carbonates as a minimum value for Neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic Mo reservoir at that time.

Original languageEnglish
Pages (from-to)27-46
Number of pages20
JournalPrecambrian Research
Volume266
DOIs
Publication statusPublished - 1 Sept 2015

Keywords

  • Carbonate platform
  • Great Oxidation Event (GOE)
  • Molybdenum (Mo)
  • Neoarchean seawater
  • Stromatolites

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

Fingerprint

Dive into the research topics of 'Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58-2.50Ga-old Transvaal Supergroup carbonate platform, South Africa'. Together they form a unique fingerprint.

Cite this