Fig. 1: Lithologic and geochemical data for the studied section.

Data are listed in Supplementary Data 1, and include Hg concentrations (ppb), total organic carbon (TOC, wt%), Hg/TOC ratios for samples with TOC > 0.2 wt% (ppb/wt%), Hg isotope values (δ²⁰²Hg, Δ¹⁹⁹Hg, and Δ²⁰⁰Hg, in ‰), and pyrite S-MIF data (Δ³⁶S/Δ³³S; from ref. ²⁶). Uncertainties on Hg isotope data correspond to the larger value of either the measurement uncertainty of replicate digests of MESS-2 or the uncertainty of repeated measurements of UM-Almadén. Uncertainty on Δ³⁶S/Δ³³S was computed from the larger of the internal or external uncertainties associated with the raw Δ³³S and Δ³⁶S data (as previously reported²⁶). Analytical uncertainties for additional data are encompassed within each individual data point. The gray shaded area indicates the interval where changes in Δ³⁶S/Δ³³S correspond to changes in Hg-MIF values; the vertical purple line illustrates the background Archean Δ³⁶S/Δ³³S ratio; vertical dashed lines also illustrate 0‰ in Δ¹⁹⁹Hg and Δ²⁰⁰Hg.