Figure 3: δ²³⁸U values for U roll-front deposits at four depths.

191.0, 194.0, 196.0 and 198.0. Error bars indicate two s.d. from the mean of triplicate extractions. The yellow box at δ²³⁸U=−0.2 to −0.4‰ indicates the average U isotope signature of the Earth’s crust and is often considered as the reference to delineate between abiotic vs biotic U reduction^41,42. CRM-112A was used as the standard. δ²³⁸U values in (‰) and depth in (m-bgs).