Iron isotopes cycling in ferruginous and anoxic Lake Pavin from water column to sediment

Iron isotopes are an emerging biogeochemical paleoproxy that can improve our understanding of the Fe cycle in early Earth's ocean and early microbial evolution. Sediments deposited during the Precambrian record a large range of d56Fe values, with an exceptional negative excursion (down to -3.5‰) between 2.9 and 2.3 Ga, a transition period believed to be marked by stratified redox ocean basins. The origin of this negative excursion is still debated but may be linked to a unique period of water column Fe cycling or a time of enhanced microbial Fe reduction in Fe-rich sediments. Lake Pavin is a stratified aquatic system characterized by permanent anoxic and ferruginous deep water topped by oxic shallow water, and can thus be regarded as a modern analog for Archean ocean. In the present work, we have studied Lake Pavin Fe isotope cycling along a profile in the water column down to the sediment. Four sediment cores were drilled and analyzed (one in the oxic zone, three in the anoxic zone). The data demonstrate that iron isotope variability in Lac Pavin sediments is tied to water column cycling—foremost the oxidation of dissolved ferrous iron. In this light, the enhanced iron isotope variability in the Archean may record a unique stage in Earth's history after photosynthesis had evolved but while surface oxygen was kept at a low level by the delivery of iron and other reduced phases.