Influence of Physical Perturbation on Fe(II) Supply in Coastal Marine Sediments

Iron (Fe) biogeochemistry in marine sediments is driven by redox transformations creating Fe(II) and Fe(III) gradients. As sediments are physically mixed by wave action or bioturbation, Fe gradients re-establish regularly. In order to identify the response of dissolved Fe(II) (Fe²⁺) and Fe mineral phases toward mixing processes, we performed voltammetric microsensor measurements, sequential Fe extractions, and Mössbauer spectroscopy of 12 h light-dark cycle incubated marine coastal sediment. Fe²⁺ decreased during 7 days of undisturbed incubation from approximately 400 to 60 μM. In the first 2-4 days of incubation, Fe²⁺ accumulated up to 100 μM in the top 2 mm due to Fe(III) photoreduction. After physical perturbation at day 7, Fe²⁺ was re-mobilized reaching concentrations of 320 μM in 30 mm depth, which decreased to below detection limit within 2 days afterward. Mössbauer spectroscopy showed that the relative abundance of metastable iron-sulfur mineral phases (FeS) increased during initial incubation and decreased together with pyrite (FeS₂) after perturbation. We show that Fe²⁺ mobilization in marine sediments is stimulated by chemical changes caused by physical disturbances impacting the Fe redox distribution. Our study suggests that, in addition to microbial and abiotic Fe(III) reduction, including Fe(III) photoreduction, physical mixing processes induce chemical changes providing sediments and the inhabiting microbial community with Fe²⁺.