The Iron Isotope Fingerprints of Redox and Biogeochemical Cycling in Modern and Ancient Earth

The largest Fe isotope fractionations occur during redox changes, as well as differences in bonding, but these are expressed only in natural environments in which significant quantities of Fe may be mobilized and separated. At the circumneutral pH of most low-temperature aqueous systems, Fe²⁺aq is the most common species for mobilizing Fe, and Fe²⁺aq has low ⁵⁶Fe/⁵⁴Fe ratios relative to Fe³⁺-bearing minerals. Of the variety of abiologic and biologic processes that involve redox or bonding changes, microbial Fe³⁺ reduction produces the largest quantities of isotopically distinct Fe by several orders of magnitude relative to abiologic processes and hence plays a major role in producing Fe isotope variations on Earth. In modern Earth, the mass of Fe cycled through redox boundaries is small, but in the Archean it was much larger, reflecting juxtaposition of large inventories of Fe²⁺ and Fe³⁺. Development of photosynthesis produced large quantities of Fe³⁺ and organic carbon that fueled a major expansion in microbial Fe³⁺ reduction in the late Archean, perhaps starting as early as ∼3 Ga. The Fe isotope fingerprint of microbial Fe³⁺ reduction decreases in the sedimentary rock record between ∼2.4 and 2.2 Ga, reflecting increased bacterial sulfate reduction and a concomitant decrease in the availability of reactive iron to support microbial Fe³⁺ reduction. The temporal C, S, and Fe isotope record therefore reflects the interplay of changing microbial metabolisms over Earth's history.