Constraints on the Archean atmospheric oxygen and sulfur cycle from mass-independent sulfur records from Anshan-Benxi BIFs, Liaoning Province, China

Abstract

The Archean atmospheric oxygen concentration and sulfur cycle was long debated. The banded iron formation (BIF) is a special type of the sedimentary formation, which has truly recorded the atmospheric and oceanic conditions at that time. In this study, the composition of multiple sulfur isotope (δ 34S/δ 33S/δ 32S) for sulfides bedded in the Archean (∼2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of Δ33S varies from −0.89‰ to +1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero Δ33S values indicate that the Archean sulfur cycles are different from what it is today, which have been deeply influenced by gas phase photochemical reactions. Algoma-type BIFs which are closely related to the volcanic activity have negative Δ33S value, however, Superior-type BIFs which are far away from the volcanic center have positive Δ33S value. The δ 34S varies in a large range from −22.0‰ to +11.8‰, which indicates that the bacteria reduction activity has already existed at that time, and that the oceanic sulfate concentration has at least reached 1 mmol/L in local areas. Combined with the contemporaneous existence of the hematite, magnetite and the occurrence and preservation of the sulfur MIF, it can be inferred that the Archean atmospheric oxygen level must be at 10−2–10−3 of the present atmospheric level (PAL).