Figure 3: Variation of stable isotope composition within different calcite and pyrite crystals.

Transects of SIMS analyses are shown in back-scattered SEM images above, and isotopic compositions corresponding to these analyses below. Growth direction of calcite is from left to right. (a) Calcite with episodic methanogenesis-related signature (positive δ¹³C, blue symbol). This is the dominant appearance of methanogenesis-related calcite, related to δ¹⁸O with marine-influenced signature followed by lighter δ¹³C and δ¹⁸O. (b–e) AOM-related calcite (green symbols) with typical associated increase in δ¹⁸O values from the earlier growth zone (indicative of increased marine influence). (d,e) AOM-related calcite succeeded by calcite with significantly heavier δ¹³C and lighter δ¹⁸O (fresh water, with large glacial component, especially in e). (f) δ³⁴S evolution with growth from core to rim in pyrite from three different fractures. The symbol sizes are generally larger than the analytical uncertainties (except for δ¹⁸O, where error bars are shown). Scale bars (a) 300 μm, (b) 200 μm, (c) 200 μm, (d) 100 μm, (e) 200 μm, (f) 50 μm.