The Mesoarchaean (2.96 to 2.91 Ga) Witwatersrand-Mozaan succession of southern Africa contains multiple units that show evidence for the presence of free molecular oxygen in oceanic water columns approximately 500 million years prior to the Great Oxidation Event. The lithostratgraphically correlatable Thalu and Brixton formations of the Mozaan and West Rand groups, respectively, now yield further evidence for an oxygen-containing water column. The two formations contain multiple beds of manganese carbonate-bearing mudstone. This study documents these beds and their stratigraphy, mineralogy, petrography, whole rock geochemistry and stable carbon and oxygen isotopes from deep-level drill cores.
The manganese carbonate-bearing beds occur towards the base of upward-coarsening units, indicating deposition during higher sea levels, followed by regressions. The mudstones show sharp contacts and compaction around manganiferous carbonate concretions, suggesting early diagenetic growth of the latter. The concretions are composed of either rhodochrosite cores and kutnahorite rims, or kutnahorite cores and ankerite rims, illustrating a decrease in manganese concentration from core to rim. Relative to the surrounding mudstone, the carbonate concretions are markedly enriched in manganese. The rare earth element and yttrium contents in the concretions, normalised to shale, show heavy over light rare earth element enrichment and positive europium and yttrium anomalies, indicating precipitation from mixed marine-hydrothermal water. The carbon and oxygen isotopes of concretionary carbonates are depleted in 13C (-12.2 to -21.30/00) and 18O (-14.9 to -23.90/00) relative to Pee Dee Belemnite, respectively.
It is concluded that the main mode of deposition for iron and manganese was by precipitation likely facilitated by iron- and manganese-oxidising bacteria. The iron and manganese were sourced as dissolved Fe2+ and Mn2+ from a distal, high-temperature hydrothermal plume. The stratigraphic position of the studied units suggests that deposition occurred on the middle to outer shelf where iron oxidation and deposition proceeded far enough so that the environment could transition to manganese oxidation and deposition. These precipitates then reacted with organic carbon to form rhodochrosite that nucleated very locally to grow concretions. As the available manganese was all reduced and incorporated into the carbonates, Fe3+-oxyhydroxides were also reduced by any excess organic carbon and incorporated into the concretion rims during later stages of growth.
The main implication of the proposed model for concretion formation is that free molecular oxygen was available in the water column of the shelf for manganese-oxidising microaerophyllic chemolithoautotrophs to function. The oxygen concentration was in excess of approximately 5 μM. This concentration, along with the depositional setting of the studied units, falls within the ranges and parameters of previous studies indicating conditions in “oxygen oases” of Archaean oceans.