The Great Oxidation Event preceded a Paleoproterozoic "snowball Earth"

Abstract

The inability to resolve the exact temporalrelationship between two pivotal events in Earth history, the Paleoproterozoic Great Oxidation Event (GOE) and the first ‘snowball Earth’ global glaciation, has precluded assessing causality between changing atmospheric composition and ancient climate change. Here we present temporally resolved quadruple sulfurisotope measurements (δ34S, ∆33S and ∆36S)from the Paleoproterozoic Seidorechka and Polisarka Sedimentary Formations on the Fennoscandian Shield, NW Russia, that address this issue. Sulfides in the former preserve evidence of mass-independent fractionation of sulfur isotopes(S-MIF) falling within uncertainty of the Archean Reference Array with a ∆36S/∆33Sslope of -1.8 and have small negative ∆33S values, whereas in the latter mass-dependent fractionation of sulfur isotopes (S-MDF) is evident, with a ∆36S/∆33S slope of -8.8. These trends, combined with geochronological constraints, place the S-MIF/S-MDF transition, the key indicator of the GOE, between 2501.5± 1.7 Ma and 2434 ± 6.6 Ma. These are the tightest temporal and stratigraphic constraints yet for the S-MIF/S-MDF transition and show that its timing in Fennoscandia is consistent with the S-MIF/S-MDF transition in North America and South Africa. Further, the glacigenic part of the Polisarka Formation occurs 60 m above the sedimentary succession containing S-MDF signals. Hence, our findings confirm unambiguously that the S-MIF/S-MDF transition preceded the Paleoproterozoic ‘snowball Earth’. Resolution of this temporal relationship constrains cause-and-effect drivers of Earth’s oxygenation, specifically ruling out conceptual models in which global glaciation precedes or causes the evolution of oxygenic photosynthesis.