5:15 PM - 6:45 PM
[AOS15-P03] Effect of sulfate on geochemical behaviors of Fe in marine sediments during early diagenesis
Keywords:iron cycle, marine sediment, dissolved iron flux, sulfate
Iron (Fe) is an essential trace element for life and is known to be a limiting factor in primary production in areas such as the Southern Ocean (e.g., Martin et al., 1994; Moore et al., 2013). While Fe sources to the ocean include rivers, hydrothermal systems, and dust, recent research highlights the significant potential contribution from continental margin sediments (e.g., Tagliabue et al., 2016). However, because previous estimates of the dissolved ferrous iron (Fe2+) flux from sediments have been highly uncertain (e.g., Elrod et al., 2004; Dale et al., 2015), understanding of the impacts of sediment-derived Fe on marine iron cycles and ecosystems remains uncertain. In addition, the concentrations of dissolved oxygen and sulfate in seawater have varied greatly over geological time (e.g., Algeo et al., 2015; Luo et al., 2016; Ozaki et al., 2019). It seems therefore that the behavior of Fe in marine sediments has undergone significant changes throughout Earth's history. There has been, however, a lack of quantitative understanding of the behavior of Fe in sediments under oceanic redox conditions different entirely from those of the present day (e.g., anoxic, euxinic).
In this study, we utilized a vertical one-dimensional reactive-transport model of van de Velde et al. (2023) to estimate the behaviors of carbon (C), nitrogen (N), iron (Fe), manganese (Mn), and sulfur (S) during the early diagenesis in marine sediments. By conducting systematic parameter studies for the numerical model under various conditions representing both current and past marine environments, we have examined the behavior of Fe in these sediments.
The results confirmed that the primary factors influencing Fe2+ efflux from sediments are the deposition rates of iron (FeOOH), organic matter, and the dissolved oxygen concentration in bottom water. These results are consistent with those of previous studies (Dale et al., 2015; van de Velde et al., 2023). We also systematically examined the effects of bottom water nitrate (NO3-) and sulfate (SO42-) concentrations, and depositional rates of sediment on the amount of dissolved Fe2+ fluxes from sediments. We found that, Fe2+ diffusion rate from sediment to seawater, the primary mechanism for dissolved Fe2+ flux, increases, contrary to intuition, as sulfate concentration increases from early Earth levels to present values under high dissolved oxygen conditions (O2 = ~300μM). This increase is attributed to the enhancement of two different Fe2+ leaching reactions in the uppermost sedimentary layer against increasing sulfate concentrations: iron reduction by hydrogen sulfide produced by sulfate reduction of organic matter and the iron dissolution from iron sulfides (FeS) which is supplied from deeper layers through bioturbation. These insights into the early diagenetic processes in marine sediments provide a new perspective for understanding not only the current marine iron cycle but also the iron cycle in oceans during the Archean and the Proterozoic.
In this study, we utilized a vertical one-dimensional reactive-transport model of van de Velde et al. (2023) to estimate the behaviors of carbon (C), nitrogen (N), iron (Fe), manganese (Mn), and sulfur (S) during the early diagenesis in marine sediments. By conducting systematic parameter studies for the numerical model under various conditions representing both current and past marine environments, we have examined the behavior of Fe in these sediments.
The results confirmed that the primary factors influencing Fe2+ efflux from sediments are the deposition rates of iron (FeOOH), organic matter, and the dissolved oxygen concentration in bottom water. These results are consistent with those of previous studies (Dale et al., 2015; van de Velde et al., 2023). We also systematically examined the effects of bottom water nitrate (NO3-) and sulfate (SO42-) concentrations, and depositional rates of sediment on the amount of dissolved Fe2+ fluxes from sediments. We found that, Fe2+ diffusion rate from sediment to seawater, the primary mechanism for dissolved Fe2+ flux, increases, contrary to intuition, as sulfate concentration increases from early Earth levels to present values under high dissolved oxygen conditions (O2 = ~300μM). This increase is attributed to the enhancement of two different Fe2+ leaching reactions in the uppermost sedimentary layer against increasing sulfate concentrations: iron reduction by hydrogen sulfide produced by sulfate reduction of organic matter and the iron dissolution from iron sulfides (FeS) which is supplied from deeper layers through bioturbation. These insights into the early diagenetic processes in marine sediments provide a new perspective for understanding not only the current marine iron cycle but also the iron cycle in oceans during the Archean and the Proterozoic.