The evolution of the oceanic redox state through Precambrian times

Abstract

The oceanic redox state distinctly changed during the Precambrian eon. Entirely anoxic oceans in earliest Earth history first became mildly oxygenated in some shallow marine areas during the late Archean. The areal extension of such ‘oxygen oases’ may have triggered atmospheric oxygenation during the subsequent Great Oxidation Event around 2.4 billion years ago. In the aftermath of the Great Oxidation Event and the proposed oxygen ‘overshoot’ during the following Lumagundi Jatuli Event oxygen levels decreased and remained continuously low during mid-Proterozoic times. It is assumed that complete ocean oxygenation (including the deep ocean) first developed during a second oxygenation event close to the Precambrian-Cambrian transition around 0.5 billion years ago.

In this cumulative dissertation periods of major environmental redox-changes were investigated in detail by the use of isotope measurements of several redox-sensitive elements. The transitional period before the Great Oxidation Event was examined by a Mo isotope study of late Archean black shales and iron formations of the Hamersley Basin, Australia (CHAPTER I). Both types of sediments show a temporally continuous enrichment of heavy isotopes between 2.6 and 2.5 billion years ago (increasing δ98Mo values), which requires a rising seawater δ98Mo value due to a contemporaneously upcoming sink of isotopically light Mo. Manganese oxides, which formed in oxygenated surface ocean areas represent a likely candidate for such a sink. To further verify this interpretation, 2.48 billion years old Mn-rich iron formations from the Griqualand West Basin, South Africa, were analyzed for the Mo and Fe isotopic composition (CHAPTER II). Mn enrichment is expected to result from the oxidation of dissolved Mn2+ and the subsequent burial of Mn oxides close to the oxic-anoxic boundary. The observed negative correlation of δ98Mo values and Mn concentrations confirms that the adsorption of (isotopically light) molybdate onto Mn oxides represented an important Mo burial pathway. Furthermore, also the Fe burial mechanism was possibly linked to Mn oxide formation, considering the negative correlation of δ56Fe values with Mn concentration. It is hypothesized that the uppermost water column along the chemocline became depleted in heavy Fe isotopes due to the oxidation of dissolved Fe2+ by Mn oxides. This local seawater Fe isotope signal was then preserved in Mn-rich sediments deposited closest to the oxic-anoxic boundary. Thus, multiple evidence confirms local Mn oxide formation in oxygen-rich shallow marine waters during the late Archean already before the Great Oxidation Event.

The assumed oxygenation of the deep ocean during the late Neoproterozoic would have further extended the depositional area of Mn oxides, thus also increasing the sink of isotopically light Mo. As a consequence the seawater δ98Mo value is expected to rise to modern-like high values. However, the geochemical study of Teplá Barrandian black shales from this time period draws a different picture (CHAPTER III). The S isotope and the Mo/TOC record indicates temporal restriction of the local depositional basin and accompanied depletion of dissolved sulfate and molybdate. The δ98Mo value of black shales from such restricted basins best resembles the ancient global seawater Mo δ98Mo composition (like in the modern Black Sea). Constantly low δ98Mo values in Teplá Barrandian black shales indicate the lack of Mn oxide formation in deep sea settings. This, in turn, strongly suggests continuously anoxic conditions in the deep oceanic environment at the end of the Precambrian eon.