Large non-mass-dependent iron isotope fractionation in an oxic-anoxic transition zone of lake sediments

Laboratory studies detected non-mass-dependent Fe-isotope fractionation during magnetotactic-bacteria-controlled iron (III) reduction, suggesting its potential as a biomineralization proxy. In nature, the preservation of the non-mass-dependent Fe-isotope signature may be difficult due to the abundance of other Fe-rich materials. Here we report a set of distinctly large non-mass-dependent Fe-isotope composition in the top 6.5 cm of the oxic-anoxic transition zone from a sediment core of Lake Aha, southwestern China. Negative ∆'57Fed-δ'56Fed and positive ∆'57Fed-[Mn] correlations support that an abundance of manganese (IV) and ongoing sulfate reduction created a zone of Fe-limited porewaters in the top 6.5 cm of the sediment where magnetotactic bacteria thrived. Non-mass-dependent Fe-isotope signatures were not detected in a sediment core taken at a nearby site in the same lake where in the oxic-anoxic transition zone porewater Fe concentrations were orders-of-magnitude higher. The discovery of non-mass-dependent Fe-isotope signatures in natural sediment offers clues to detecting similar biosignatures. Natural occurrence of large non-mass-dependent iron isotope fractionation was discovered in sediments of Lake Aha, southwestern China, where the magnetotactic bacteria thrived top centimeters are iron concentration limited in pore water due to abundant manganese (IV) and ongoing sulphate reduction.