摘要
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 △33S varies from -0.89‰ to +1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero △33S 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 △33S value, however, Superior-type BIFs which are far away from the volcanic center have positive △33S 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).
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^(34)S/δ^(33)S/δ^(32)S) for sulfides bedded in the Archean (～2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of Δ^(33)S varies from -0.89‰ to +1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero Δ^(33)S 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 Δ^(33)S value, however, Superior-type BIFs which are far away from the volcanic center have positive Δ^(33)S value. The δ^(34)S 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).
作者
HOU KeJun1,2, LI YanHe1,2? & WAN DeFang1,2 1 Key Laboratory of Metallogeny and Mineral Resource Assessment, Institute of Mineral Resource, Chinese Academy of Geological Sciences (CAGS), Beijing 100037, China
2 Key Laboratory of Isotope Geology, CAGS, Beijing 100037, China
基金
National Natural Science Foundation of China (Grant No. 40273008)
