Abstract

The biogeochemical cycle of sulfur is intimately linked with those of carbon and iron through the formation of iron-sulfur complexes. Iron-sulfide minerals such as mackinawite (FeS) and greigite (Fe3S4) form under anoxic conditions in marine and lacustrine sediments. Reactive ferric iron oxide species, abundant in oxic surface sediments, can undergo reductive dissolution at the oxic-anoxic interface leading to the formation of soluble Fe(II) which can then precipitate in the form of iron sulfide species. While iron-oxides have been thoroughly explored in terms of their ability to sorb and sequester organic carbon (OC) in sediments (Lalonde et al., 2012), the role of FeS in the long-term sequestration of OC remains poorly defined. In this study, we present depth concentration profiles for dissolved OC, iron, and sulfur in the liquid-phase (pore water) along with speciation data from sequential extractions of sulfur in the solid-phase collected from sediment cores from the St. Lawrence Estuary and the Saguenay Fjord, indicating possible evidence of FeS-promoted OC sequestration in sediments, the only sink for atmospheric CO2 on a geological time scale. Additionally, we present synthetic iron-sulfide mineral sorption experiments using natural organic matter (NOM) in order to assess the importance of FeS in sedimentary OC storage. Scanning electron microscopy shows that NOM influences the size and surface area of FeS aggregates, therefore also influencing their reactivity. The effect of NOM on the reactivity of FeS towards re-oxidation was explored using a modified dithionite-citrate buffer reduction method (1) for both dry and wet synthetic FeS minerals. Characterization of the synthetically prepared FeS-NOM complexes and specific anoxic sediments was carried out using synchrotron x-ray techniques, which also show the functional groups of the NOM which preferentially bind to FeS. This study shows for the first time that FeS plays an active role in sedimentary OC preservation.