Fig. 4.

Sensitivity of ${}^{34}\epsilon$–csSRR (A) and ${}^{34}\epsilon -\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]$ (B) relationships to a halving (solid curves) and a doubling (broken curves) of the default [MK_red]/[MK_ox] (= 100) and [H₂S] (= 0.1 mM) values. Fractionation resulting from the default state is shown by the black curves. Shaded envelopes in A and B show the reversibility of the steps in the sulfate reduction pathway resulting from variation of [MK_red]/[MK_ox] and [H₂S] for a range of $\left[\mathrm{S}{\mathrm{O}}_4^{2-}\right]$ and csSRRs. Values of $f_{p,r}$ range from 0.45 to 0.99 for $\mathrm{S}{\mathrm{O}}_4^{2-}$ uptake, 0.98 to ∼1 for activation, ∼0 to 0.98 for APS reduction, and 0.99 to ∼1 for $\mathrm{S}{\mathrm{O}}_3^{2-}$ reduction.