Paleoenvironmental reconstruction of a Neoarchean oxygen oasis

Abstract

The Neoarchean-Paleoproterozoic Transvaal Supergroup in South Africa contains the Campbellrand-Malmani carbonate platform (CMCP), which was deposited in shallow seawater between ~2.58 to 2.50 billion years ago, about 200 million years before the rise of atmospheric oxygen (Great Oxidation Event - GOE). The platform is a prominent candidate for (isotope-) geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow seawater preceding the GOE. Thus, the CMCP might represent an Archean ‘oxygen oasis’ in an otherwise anoxic environment. Carbon, molybdenum, and iron isotope compositions of CMCP sediments support the presence of molecular oxygen in the shallow-marine system and emphasize the utility of Ca-Mg carbonates as proxies for trace metal systematics in the aqueous environment. However, this study also shows that special environmental and depositional conditions were necessary to induce this development, in particular the formation of the rimmed margin and the restriction of the platform interior from the open ocean. In this restricted environment, oxygen production by aerobe photosynthesis could have increased relative to oxygen consumption by reducing species and induced an increasing oxidation of the shallow-marine environment over time.