Iron and Manganese in the Ocean: Investigation of Atmospheric Input by Dust and Coastal Ocean Time Series

Citation

Mendez, Jeffrey Michael (2008) Iron and Manganese in the Ocean: Investigation of Atmospheric Input by Dust and Coastal Ocean Time Series. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Q7JK-MV77. https://resolver.caltech.edu/CaltechETD:etd-05022008-220144

Abstract

Trace metals such as iron (Fe) and manganese (Mn) are essential micronutrients in the biogeochemistry of the ocean (Turner and Hunter, 2001), and dry deposition is a substantial source of both Fe and Mn to the surface ocean (Duce and Tindale, 1991; Guieu et al., 1994). Kinetic and thermodynamic values for the release of metals from dust are needed for computer models which incorporate dust as part of their ocean system. Here we investigate the thermodynamic and kinetics parameters involved in the dissolution of metals from dust in seawater. We added dust from the Sahara and the Western United States to seawater in a variety of ways to investigate the dissolution patterns of Fe and Mn. Results show different apparent thermodynamic constants for manganese (Mn) and iron (Fe). The final Mn concentrations are proportional to the added dust concentration and light intensity, and independent of initial dissolution rate. Fe concentrations in fresh seawater reach a maximum concentration of less than 2 nM. However, depletion of organic ligands lead to the precipitation of Fe oxide from solution, and the addition of siderophores enhanced both the total Fe capacity of the seawater and the rate of Fe dissolution from dust. The first order rate constant for the dissolution of dust differed by dust source and was dependent on oxalate concentration and intensity of natural UV light. We conclude that final Mn concentrations are limited by available Mn on the dust surface, while Fe concentrations are limited by the ligand concentrations in the seawater, which ultimately are determined by the biological community. Because the coastal ocean plays a significant role in global biogeochemical cycles, (Smith and Hollibaugh, 1993; Tsunogai and Noriki, 1991), we conducted a coastal ocean time series to investigate the basic modes and cycles which characterize the ocean. We found that Mn is highly dependent on seasonal rain events, with surface water concentrations observed as high as 30 nM after rain events. Fe within the coastal ocean is highly variable and can be used as a tool to track water mass movements and mixing patterns.

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