Fig. 3.

Carbon isotope (δ¹³C) (A), TOC (B), and δ³⁴S (C) vs. Δ³⁶S/Δ³³S data from 800- to 900-m core depth in core GKF01. In each plot the data have been color-coded, differentiating the background (black) from the C-S anomaly (blue), with orange arrows illustrating its temporal evolution. Vertical red lines in each plot give the average Neoarchean Δ³⁶S/Δ³³S with a ±0.1 uncertainty envelope (vertical gray bar; ref. 8). The horizontal green line in A marks δ¹³C = −37‰, a threshold commonly used to identify methanotrophy (8, 9, 26). The horizontal gray bar in C represents the range of seawater δ³⁴S estimates derived from carbonate associated sulfate (55). Typically analytical uncertainty is encompassed within the data points, with the exception of a few Δ³⁶S/Δ³³S ratios as in Fig. 2.