Iron diagenesis in the Indian River Lagoon subterranean estuary, Florida (USA)

Iron (Fe) is an important element for a variety of ecological and diagenetic processes, such as the microbial reduction of Fe-oxides and associated remineralization of organic carbon (OC) and production of carbon dioxide (CO2). This link between Fe and C makes it important to understand Fe diagenesis in the subterranean estuary, where submarine groundwater discharge (SGD) influences Fe reaction pathways. Submarine groundwater discharge consists of terrestrial freshwater from coastal aquifer (terrestrial SGD) and exchange of seawater across the sediment-water interface (marine SGD). To better understand Fe diagenesis in the subterranean estuary, dissolved and solid Fe, OC, and S concentrations were measured in the subterranean estuary of Indian River Lagoon, Florida. In this study area Fe-oxide coated terrestrial sediments are overlain by OC and S-rich marine sediments. Offshore thickening of these marine sediments reflects sealevel rise at the rate of 3 mm/year. Dissolved Fe maxima occur in the terrestrial sediments and Fe minima and sulfide maxima occur in the marine sediments near the sediment-water interface. Dissolved Fe maxima increase by three orders of magnitude from 1 muM nearshore to 300 muM the freshwater-saltwater boundary, where terrestrial SGD ceases to flow. Reactive-transport modeling shows Fe-oxide reduction rates do not change across the seepage face, thus the elevated Fe concentrations at the freshwater-saltwater boundary result from slow flow rates. The freshwater-saltwater boundary moved landward during the three years of sampling as a result of a decrease in effective precipitation (precipitation minus potential evapotranspiration) from 40 cm/year to -10cm/year. The landward shift decreased the width of the seepage face by 5 m and annual flux of dissolved Fe and remineralized C by 20%. The delta56Fe values in porewater were lighter than corresponding terrestrial sediments as a result of dissimilatory reduction of Fe-oxides, but porewater delta 56Fe values were heavier than corresponding marine sediments because of precipitation of Fe-sulfides. In the Indian River Lagoon study area, Fe diagenesis is controlled by the combination of hydrology of the subterranean estuary, its shoreward movement with sealevel rise, and distribution of OC. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html)...