LAUSR

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 towards 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 approx. 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 afterwards. Mössbauer spectroscopy showed that the relative abundance of metastable iron-sulphur mineral phases (FeSx) increased during initial incubation and decreased after perturbation. We show that Fe2+ mobilization in marine sediments is stimulated by physical disturbances impacting the Fe redox distribution. Our study suggests that in addition to microbial Fe(III) and abiotic Fe(III) reduction, including Fe(III) photoreduction, physical mixing processes also provide sediments and the inhabiting microbial community with Fe2+.