Some <3710 - >3695 Ma dolomitic rocks (with seawater-like REE+Y signatures and dolomite marine-like δ13CPDB = +1‰) with relicts of stromatolite and sedimentary structures (cross-bedding and tempestite breccias) within the Isua supracrustal belt (Greenland), provide a unique window into Earth’s early environment. In situ ion microprobe 4-isotope sulfur analyses of rare pyrites in these dolomitic rocks show more dispersion in δ34SVCDT (total +1 to +8‰) than Δ33SVCDT (≈-1 to -1.8‰ for all but one sample). Signature of Archean atmospheric photolysis with negative Δ33S is preserved in the measured pyrites but the distinctive Δ36S/Δ33S correlation observed on those samples (Δ36SVCDT ≈ -6.85 Δ33SVCDT) indicates a mass-dependent isotopic fraction effect superimposed to the primary mass-independent sulfur signature. The sulfur isotope data is incompatible with a carbonate metasomatic (non-sedimentary) origin for these rocks. Instead, we explain the observed sulfur isotopic signature as the result of microbial sulfate reduction under low-abundance seawater sulfate (negative Δ33S) in a sedimentary environment, with the importance being that this extends evidence of this metabolic pathway to c. 250 million years earlier than before. In contrast, 3695 Ma banded iron formations stratigraphically succeeding the dolostones (Whitehouse et al., 2005; Papineau and Mojzsis, 2006) and siliceous rocks at a c. 3700 Ma unconformity show a different sulfur isotope signature, with a mixture between volcanic sulfur (no MIF signature) and photolytic elemental sulfur (positive Δ33S). This demonstrates in this case, the retention of different S-isotope signatures through superimposed tectonothermal events.

Reference(s)

Papineau, D and Mojzsis, SJ 2006, Mass-independent fractionation of sulfur isotopes in sulphides from the pre-3770 Ma Isua Supracrustal Belt, West Greenland.: Geobiology, v. 4, p. 227–238. 

Whitehouse, MJ, Kamber, BS, Fedo, CM and Lepland, A 2005, The importance of combined Pb and S isotope data from early Archaean rocks, southwest Greenland, for the interpretation of S-isotope signatures: Chemical Geology, v. 222, p. 112–131