SHRIMP 4-S isotope systematics of two pyrite generations in the 3.49 Ga Dresser Formation

The 3.49 Ga Dresser Formation has been considered to host evidence of the earliest microbes metabolising sulfur species on Earth. However, previous bulk analyses and in situ measurements conclude disparate metabolisms based on opposite Δ³³S¹. This study first established the generations of pyrite growth, and then measured the multiple sulfur isotopes in situ using Sensitive High Resolution Ion MicroProbe-Stable Isotope analyses. Two main generations of pyrite were revealed based on core-rim textures and multiple sulfur isotopic compositions: Δ³³S-positive Generation One (G1) and δ³⁴S- and Δ³³S-negative Generation Two (G2). In the chert-barite unit, the diluted Δ³³S-positive and Δ³³S-negative photochemical products were mainly sequestered in G1 and barite, respectively. G2 were formed via the sulfide pathway with sulfur derived from sulfate reduction and magmatic H₂S. The δ³⁴S-Δ³³S-Δ³⁶S¹ systematics suggests an abiological origin for G1, and thermochemical and possible (minor) microbial sulfate reduction for G2.