| Subject: |
Iron isotopes cycling in ferruginous and anoxic Lake Pavin (France) from water column to sediment |
| Speaker: |
Dr. Vincent Busigny
Institut de physique du globe de Paris, Université Paris 7 |
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| Time: |
3:00 pm, July 20, 2012 |
| Location: |
SKLEG Lecture Hall |
| Abstract: |
Fe 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 unique stratified aquatic system characterized by permanent anoxic and ferruginous deep water (from 60 to 92 m depth) topped by oxic shallow water (from 0 to 60 m), 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 in order to bring new insights into the record of ancient rocks. Four sediment cores were drilled and analyzed: (1) in the oxic zone, (2) at the oxic-anoxic boundary, (3) under the peak of H2S production from SO42- reduction and (4) at the bottom of the lake. In the water column, dissolved Fe concentration increases with depth from 2 µM at the oxic-anoxic boundary to 1200 µM at the lake bottom, with d56Fe increase from -1.67 to +0.31‰. The very negative d56Fe of the oxic-anoxic boundary reflects the residue of Fe oxidation and precipitation. The d56Fe increase with depth is interpreted as a coupling between (1) diffusion of Fe enriched in heavy isotopes from the bottom of the lake towards the oxic-anoxic boundary and (2) a combination of Fe reduction of downgoing Fe(III) particles and ferrous-ferric iron interations. Analyses of bulk sediment show d56Fe values close to the purported detrital source (i.e. basalts with d56Fe ~ 0.20‰). In contrast, Fe sequential extraction in sediments shows a significant variation in d56Fe. Iron isotope mass balance calculation in both water column and pore waters of the sediment cores indicate that Fe isotope variability recorded in iron sulfides reflects mostly Fe chemistry in the water column rather than isotopic fractionation during diagenetic processes or sulfide precipitation in the sediment. |
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