The uptake of atmospheric carbon dioxide by phytoplankton in the surface oceans represents one of the main natural processes that regulate the accumulation of this potent greenhouse gas in the atmosphere. To predict the impact of the accumulation of carbon dioxide into the atmosphere on the global carbon cycle, it is necessary to predict the carbon fixation process on a global scale. Carbon fixation, however, does not respond linearly to the input of atmospheric carbon dioxide, as phytoplankton growth is limited by the supply of the essential and poorly soluble trace element iron to the surface waters. The atmosphere and continental margin sediments represent the main sources of iron to the open oceans. Yet, the output of iron from sediments has only been sparsely measured and its chemical form poorly characterized, such that its residence time in marine waters is unknown. To establish whether the flux of iron from sediments has important implications on primary production, possibly rivaling atmospheric inputs, it is necessary to demonstrate that ferric iron originating in sediments is under the form of stable iron species with potential for a high residence time in the water column. The overall objective of this project was to test the hypotheses that iron fluxing from continental margin sediments is truly dissolved under the form of organic-Fe(III) complexes and that the magnitude of the iron flux is influenced by the redox conditions in the overlying waters, the composition of the complexes, and the biogeochemical processes in the underlying sediments. To test these hypotheses, the flux and speciation of dissolved Fe(III) were quantified in the sediments of the Carolina depocenter off Cape Lookout, a site that represents the conditions of most passive continental margins with low upwelling and negligible riverine inputs. As contrasting environments, the flux and speciation of dissolved Fe(III) were quantified in continental margin sediments exposed to the input of organic carbon and iron minerals from two major rivers in the United States and France: the Mississippi delta in the Northern Gulf of Mexico and the Rhône River delta in the Mediterranean Sea. The biogeochemical processes regulating the production and flux of iron as a function of the redox regime of the environment were determined using in situ measurements and state-of-the-art analytical techniques. These studies revealed that respiration processes in the sediment not surprisingly decrease significantly from estuaries to the continental shelf and upper-slope of the continental margin off Cape Lookout. In turn, the intensity of benthic respiration increases and peaks on the mid-slope, then decreases deeper along the lower slope. Sulfate reduction dominates anaerobic respiration processes in coastal and near-coast shelf sediments, but iron reduction rapidly takes over in the far-shelf and more importantly in the upper- and mid-slope sediments. Interestingly, the upward flux of both dissolved Fe(III) and Fe(II) across the sediment column peaks simultaneously with the increase in the intensity of carbon mineralization processes on the mid-slope. Mid-slopes are regions of high resuspension, suggesting they may act as a significant source of iron to the overlying waters. Benthic flux measurements point toward the relative importance of microbial iron and sulfate reduction and their role on benthic exchange processes in continental margin sediments. The flux of dissolved iron from passive continental margin sediments is generally lower than that of deltaic sediments exposed to large riverine inputs in contrast to what was hypothesized. Despite the potential for sulfate reduction to immobilize iron in the sediment of continental margin sediments exposed to large riverine inputs, the concentration of iron oxides in these sediments is high enough to release a significant fraction of iron from the sediment into the overlying waters. Simultaneously, intense sulfate and iron reduction in deltaic sediments produce dissolved inorganic carbon and alkalinity that may diffuse into the overlying waters and enhance the buffering capacity of coastal waters. Overall, these findings demonstrate that continental margin sediments exposed to riverine inputs are highly dynamic and driven by the input of organic and inorganic riverine particles during episodic floods. These environments represent major sources of iron the overlying waters that may promote primary production in continental shelf and the open ocean. On the other hand, the benthic flux of iron from continental margin sediments not exposed to upwelling or riverine inputs is much lower and mainly occurs in lower slopes, where sediment resuspension may be significant. This project also included a significant educational impact with the training of four graduate students, six undergraduate scientists and engineers, and a technician in state-of-the-art in situ oceanographic instrumentation and analytical techniques to investigate diagenetic processes in marine environments. Finally, a 24-hour long cruise with a class of senior undergraduates was conducted as part of their capstone project course to expose them to oceanography.

Last Modified: 06/05/2018
Modified by: Martial Taillefert