Fig. 2.

Silicified organic matter preserved in the 3.45-Ga SPF stromatolites. (A–E) Transmitted light photomicrographs showing the organic-rich regions analyzed by NanoSIMS. Scale bars, 20 μm. (F) NanoSIMS ion maps of ¹²C and ³⁴S (red square in E). The coregistration between the abundance of ¹²C and ³⁴S in the ion images indicates that measured δ³⁴S values derive from sulfur incorporated into organic matter. The organic material is finely dispersed within the chert, forming aggregates that rarely exceed a size of 3 × 3 μm.