Iron in the Ocean: Laboratory Experiments of Iron Geochemistry in the Presence of Marine Particles

Author: Beck, Anna Rose

Year: 2012

Degree: Dissertation (Ph.D.)

Advisor: Adkins, Jess F.

Committee Members: Leadbetter, Jared R.; Blake, Geoffrey A.; Berelson, William M.; Adkins, Jess F.

Option: Environmental Science and Engineering

DOI: 10.7907/0CAB-KF69

Abstract

Iron (Fe) is an important micronutrient for primary productivity in the ocean. The Fe cycle in the ocean is relatively unconstrained, especially when it comes to quantifying sources and sinks related to exchange with particulate matter. This thesis attempts to constrain some of the kinetic and equilibrium particle interactions with Fe bound to the siderophore desferrioxamine B (DFB). Out of five inorganic particle types investigated, ferrihydrite, goethite, opal, foraminifera, and montmorillonite, ferrihydrite has the largest, extended impact on dissolved FeDFB. From experimental and modeling results, ferrihydrite has two primary exchange pathways, absorption, with a rate of 4 ± 2 x 10-4 /(mg/L) per day, and dissolution, with a rate of 0.015 ± 0.01 per day. Uptake appears irreversible and follows a colloidal pumping model. Isotopic fractionation is also the greatest in the presence of ferrihydrite with signals up to +1‰ or higher with excess ligand. Dry montmorillonite has the biggest initial impact on FeDFB, resulting in a nearly instantaneous equilibrium and little isotopic fractionation. Goethite, opal, and foraminifera all have a minimal impact on FeDFB and show slight enriched isotopic fractionation, +0.15‰, in the presence of large particle concentrations. DFB seems to induce heavy Fe desorption or dissolution, while particle uptake seems to favor transfer of lighter Fe. These isotopic and kinetic parameters are important constraints on the ability of particles to control dissolved Fe, since they fall through the water column faster than equilibrium will be obtained.

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