Iron isotope fractionation in sediments of an oligotrophic freshwater lake

In situ iron (Fe) isotope compositions of pore water and solid-bound Fe phases were measured in sediments of an oligotrophic, sulfate-poor freshwater lake (Lake Tantaré, Québec, Canada). Previous work has shown that dissimilatory Fe(III) reduction (DIR) is the main Fe reduction pathway in this setting. Hence, the lake sediments provide a well-characterized, natural environment in which to assess the transferability of DIR-produced Fe isotope fractionations obtained in laboratory model systems. Iron redox cycling within the sediments produces isotopically light pore water Fe(II) (average δ56Fe=−2.1±0.6‰) and sorbed Fe(II) (average δ56Fe=−1.2±0.2‰), compared to the Fe(III) oxyhydroxide (average δ56Fe=+0.6±0.2‰) and tightly solid-bound Fe(II) (average δ56Fe=+0.3±0.2‰) sediment pools. The apparent isotope fractionation factor between aqueous Fe(II) and Fe(III) oxyhydroxides of −2.6±0.5‰ derived for Lake Tantaré sediments falls within the range reported for experimentally determined isotopic fractionations during DIR. Our results yield the first comprehensive set of Fe isotope signatures associated with microbially driven Fe redox cycling obtained directly in sediments of an oligotrophic freshwater lake.