Iron-56 As a Tracer for Iron Isotopic Distribution in Sedimentary Rocks and Soils

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iron-56 is a stable isotope of the element iron. It has a very low atomic mass (only 3 protons) and is strongly and tightly bound in nuclei. It is the most abundant of the 24 naturally occurring metallic isotopes of iron, making it a natural tracer for iron isotopic distribution in sedimentary rocks and soils.

In contrast to the light atomic weight of 62Fe, 56Fe has a much higher binding energy and is therefore less likely to be dissociated from its matrix by mechanical forces. This makes it an ideal isotopic marker for tracking the mobility of redox active metals in terrestrial systems.

Iron is a major component of the Earth’s crust and is present in many geologically significant mineral deposits. It also oxidizes rapidly in moist air to form the reddish-to-brown oxide known as magnetite, which is found in a wide range of terrestrial soils and rocks. The light isotopic composition of terrestrial hematite and pyrite means that they can be used as tracers for the early stages of oxygenation of Earth’s atmosphere.

Unlike sulfur isotopes, which are prone to isobaric interferences, the isotopic composition of hematite and pyrite can be reliably determined in situ using Mossbauer spectrometry. Consequently, d56Fe values can be recorded in a single sample of sediment at well-defined points and depths through the soil sequence.

Despite the limitations of this method, d56Fe data from modern marine sediments show good agreement with igneous rock standards and have been used to track the transport and accumulation of Fe in sedimentary environments. In hydromorphic soils, the d56Fe signature will usually be highest in the upstream and shallower horizons, whereas it will become less intense with deeper sampling and lower horizons. Guilbaud et al. (2011) showed that in Archean sedimentary pyrite, variations in d56Fe between +1.2%0 and -0.5%0 coexist. This range is compatible with the various degrees of pyritization observed in their samples, and the negative d56Fe values in some pyrites can be linked to sulfate reduction through dissimilatory iron reduction.