Constraining the rise of oxygen with oxygen isotopes

Abstract

After permanent atmospheric oxygenation, anomalous sulfur isotope compositions were lost from sedimentary rocks, demonstrating that atmospheric chemistry ceded its control of Earth’s surficial sulfur cycle to weathering. However, mixed signals of anoxia and oxygenation in the sulfur isotope record between 2.5 to 2.3 billion years (Ga) ago require independent clarification, for example via oxygen isotopes in sulfate. Here we show <2.31\u2009Ga sedimentary barium sulfates (barites) from the Turee Creek Basin, W. Australia with positive sulfur isotope anomalies of ∆33S up to\u2009+\u20091.55‰ and low δ18O down to −19.5‰. The unequivocal origin of this combination of signals is sulfide oxidation in meteoric water. Geochemical and sedimentary evidence suggests that these S-isotope anomalies were transferred from the paleo-continent under an oxygenated atmosphere. Our findings indicate that incipient oxidative continental weathering, ca. 2.8–2.5\u2009Ga or earlier, may be diagnosed with such a combination of low δ18O and high ∆33S in sulfates. The loss of anomalous sulfur isotope compositions from sedimentary rocks has been considered a symptom of permanent atmospheric oxygenation. Here the authors show sulfur and oxygen isotope evidence from <\u20092.31\u2009Ga sedimentary barium sulphates (barites) from the Turee Creek Basin, W. Australia, demonstrating the influence of local non-atmospheric processes on anomalous sulfur isotope signals.