Increase of Oxygen in the Earth's Atmosphere and Hydrosphere between −2.5 and −2.4 Ga B.P.

Abstract

Currently, two competing concepts that both are based on (amongst others) the Fe-Ti systematics of palaeosols, try to describe the evolution of the oxygen content of the Precambrian Earth's atmosphere. One model favours a rapid and significant increase of atmospheric oxygen levels during the Palaeoproterozoic (summarized in Holland, 1994), whereas another suggests that only minor changes of less than _ 50% occurred throughout Earth's history (summarized in Ohmoto, 1997). Although the former is currently more widely accepted than the latter, it suffers from two shortcomings: (i) the validity of the conclusions derived from the palaeosol record rely heavily on the assumption of absence of organic matter from Archaean and Palaeoproterozoic weathering profiles, and (ii) the time of the postulated increase of atmospheric oxygen is rather poorly constrained due to large uncertainties of some palaeosol ages; nevertheless, however, the rapid increase is suggested to have occurred between c. 2.2 and 1.9 Ga B.P. (Holland, 1994). Thus, there is need for (i) other redox-indicators that are able to provide at least qualitative evidence for a rather constant or an increasing redox-level of the Precambrian atmosphere-hydrosphere system, but which are rather insensitive to the presence or absence of organic matter, and (ii) evidence from samples that originate from well-dated profiles with a (for Precambrian standards) reasonably good time resolution. Hence, we studied the distribution of Mn and rareearth elements (REEs) in only slightly metamorphosed chemical platform sediments from the Kuruman and Griquatown Iron-Formations (IFs) and the Hotazel Manganese-Formation (MnF), and in palaeoweathering products in the Transvaal Supergroup, South Africa. While the only abundant primary oxide minerals in the Kuruman and Griquatown IFs are Fe(III) oxides, the Hotazel MnF is the oldest largescale deposit of Mn(III) oxides in the geological record. The results bear clear evidence for a rapid increase of the oxygen content of both, the Earth's ocean and atmosphere between -2.5 and -2.4 Ga B.P.