Origin of negative Δ33S anomalies and Δ36S–Δ33S trends in 3700 Ma Isua sulfides: Searching for evidence of Eoarchean life in dolomitic meta-sedimentary rocks

Proof of life in the oldest rock record is complex and at the limit of current scientific capabilities. However, investigations using the covariation between minor isotopes of sulfur (Δ 33 S and Δ 36 S) reveals the potential of using the sulfur isotope system to discriminate between biologically-influenced and entirely non-biological signatures. This paper reports new in-situ four sulfur isotope data measured on rare pyrite grains in Isua supracrustal belt (Greenland) 3700 Ma metamorphosed dolomitic rocks with primary sedimentary structures. The sulfide grains have negative Δ 33 S and positive Δ 36 S values indicative of photolytic sulfate as their primary sulfur source. The limited spread in δ 34 S values relative to an inferred Archean seawater sulfate reservoir, along with covariation between the minor sulfur isotopes (Δ 33 S and Δ 36 S) can be modelled by microbial sulfate reduction under conditions of low sulfate concentration (<100 μM). Potential abiogenic origins for these signatures are also considered, but they either fail to reproduce the observed sulfur isotope relationships or require reaction conditions or sulfur sources that are difficult to reconcile with the geological context of the Isua meta-dolomitic rocks. Our findings reinforce earlier interpretations that these pyrite-bearing dolomitic meta-sedimentary rocks have rare primary sedimentary structures preserved along with trace-element signatures indicative of biomediation in a marine-like environment. Collectively, these observations support microbial sulfate reduction as a coherent and geologically reasonable interpretation for the observed sulfur isotopic patterns when considered alongside the petrographic, sedimentological, and geochemical context. In this framework, the Isua sulfur isotope data would extend the documented record of microbial sulfate reduction by more than 200 million years, to ca. 3700 Ma, and provide one of the earliest geochemical constraints on the evolution of microbial sulfur metabolisms.