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South Africa's Archean and Paleoproterozoic chemical sedimentary successions: implications for paleoenvironments and life on early Earth

Dr Bertus Smith  |  PPM Research Group and DST-NR CIMERA, Department of Geology   |  University of Johannesburg

Abstract: South Africa has a detailed and well preserved geological record spanning the Archean to Paleoproterozoic (ca. 3.5 to 2.0 billion years old). The major geological successions that span this time period, in chronological order, include: The Paleo- to Mesoarchean Barberton Greenstone Belt; the Mesoarchean Pongola and Witwatersrand Supergroups; the Meso- to Neoarchean Ventersdorp Supergroup; and the Neoarchean to Paleoproterozoic Transvaal Supergroup. All these successions contain significant sedimentary units that recorded valuable early Earth paleoenvironmental and biological information. Especially the chemical sedimentary units contain multiple lines of evidence for unique depositional environments with the possibility of a diverse prokaryotic biosphere during the Archean and Paleoproterozoic. Strong evidence for early life include well preserved algal mats and stromatolites in the Barberton Greenstone belt, Pongola Supergroup and Transvaal Supergroup. Possible microfossils have also been discovered in multiple units. Banded iron formations, that contain important direct and indirect evidence for early marine conditions and life, occur throughout most of the successions. The Great Oxidation Event (GOE), when the Earth first developed a stable ozone layer 2.4 billion years ago, is well preserved in the Transvaal Supergroup. However, the much older Pongola and Witwatersrand Supergroups have been the source of great debate around whether free oxygen had been around well before the GOE. Although well studied, more research on the early Earth sedimentary successions of South Africa is still required in order to better understand the evolution of surface environments leading up to, and following, the GOE as well as to find more convincing evidence for the presence or absence of an evolving prokaryotic biosphere.

Stromatolites in the Pongola Supergroup of South Africa. These dome-shaped structures are interpreted to have been formed by bacteria living in a shallow ocean approximately 3 billion years ago (picture by Dr Bertus Smith)