Figure 7: Comparison of clumped isotope compositions with mixing-related temperature anomalies.

(a) Diagram depicting the fractional contribution of methane-derived carbon (f_methane) based on δ¹³C_carb values and a range of methane carbon isotope compositions (δ_meth). Here, f_methane is calculated assuming that carbon is derived from methane and seawater (with δ¹³C=0). Ranges of f_methane arise from variable δ¹³C_carb and hypothetical δ_meth values. Notice how modern and most ancient seep carbonate δ¹³C_carb values overlap with those required for a 50:50 mixture (that is, f_methane=0.5). (b) Results of mixing between two environmentally constrained end-members as calculated using the approach of Defliese et al.⁶⁵. Grey envelope and curves correspond to the potential range of values resulting from mixing in Norwegian Sea carbonates. Black envelope and curves correspond to all others. See text for information concerning end-member isotope compositions. Notice that mixing cannot account for the clumped isotope and temperature anomalies recorded in cold seep carbonates, both the Tripati et al.⁴⁰ and Tang et al.⁴¹ calibration-derived temperatures included. Note that dolomites of the Eel River Basin are not included in this diagram. This is because positive δ¹³C_carb values indicate formation in the zone of methanogenesis and therefore potential mixing cannot be independently assessed as with the other samples.