Iron is a limiting nutrient, and it is generally accepted that the atmosphere and continental margin sediments are the main source of iron to the oceans. Interestingly, most investigations have focused on the distribution and speciation of iron in the water column. The flux of iron from sediments has only been sparsely measured and only in upwelling zones because of the likelihood that these environments provide a source of iron to surface waters. To establish whether the flux of iron from sediments has important implications for primary productivity, possibly rivalling atmospheric inputs, it is necessary to demonstrate that a flux of iron occurs from a variety of continental margin sediments, including in areas where upwelling is not significant.
In this study, researchers at the Georgia Institute of Technology will test three hypotheses to determine whether the flux of iron from continental margin sediments is significant: (1) A significant concentration of iron is produced at the sediment-water interface of continental margins where upwelling is not important. (2) Soluble organic Fe(III) complexes constitute the most important fraction of the iron flux. (3) The flux of iron is indirectly regulated by the organic matter content of the sediment and the intensity of bioturbation which control the extent of iron and sulfate reduction. To test these hypotheses, the flux and speciation of dissolved Fe(III) will be quantified across a gradient in organic content and bioturbation intensity in the sediments of the Carolina depocenter using in situ measurements and state-of-the-art voltammetric techniques. In addition, the biogeochemical processes regulating the flux of iron across the sediment-water interface will be determined using in situ measurements and sediment incubations. This study will assess the importance of the iron flux from continental margin sediments exposed to fully oxygenated waters in zones where upwelling is not significant. Studying these processes in sediments of different organic composition and bioturbation intensity will demonstrate what geochemical conditions induce a flux of iron.
Broader Impacts: This study will potentially modify the underlying paradigm that upwelling and low oxygen levels are required to generate an iron flux to the overlying waters and establish whether continental margin sediments should be considered an important source of iron, possibly rivaling atmospheric inputs to fuel primary productivity in the open ocean. This study will also expand our previous work and provide capability for autonomous deep-sea in situ profiling of most terminal electron acceptors and reduced metabolites at a relatively low cost. This instrumentation will be compact and adaptable to any benthic lander, such that deployments in deep-sea sediments on cruises of opportunities could be performed. In situ measurements become a necessary component of oceanographic research, and two Ph.D. students will develop expertise in state-of-the-art voltammetric technology, in situ deployments, and kinetic modeling. Finally, this project will have an important educational component with efforts to integrate undergraduate engineers and scientists in oceanographic research, provide research experience to a teacher every summer and give her the opportunity to bring a new prospective into the classroom, and continue taking up to 20 students to sea each year.
