Date of Award

1-1-2010

Document Type

Campus Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

Chemistry

First Advisor

Benjamin S. Twining

Second Advisor

Timothy J. Shaw

Abstract

Recent years have seen an increased occurrence of large icebergs in the Southern Ocean originating from Antarctic ice shelves. These free-drifting icebergs may serve as an important source of Fe to surrounding waters. I measured the concentrations and speciation of dissolved Fe via flow injection-chemiluminescence and cathodic stripping voltammetry (CSV) in the waters surrounding several icebergs during cruises to the Scotia and Weddell Seas in June 2008 and March 2009. Surface dissolved Fe (dFe) concentrations varied from 0.70 to 2.65 nM and were elevated up to 60% at some stationswaters, regardless of distance to the iceberg. Significantly higher Fe(II) (up to 727 pM, 50% of dissolved Fe) was observed at the face of iceberg, possibly due to direct and indirect Fe(II) sources associated with the iceberg. Fe(II) concentrations were found to be significantly correlated with ambient light levels during the 2009 austral fall cruise, implicating the importance of photochemical reduction source. Electrochemical measurements revealed that over 98% of dissolved Fe was bound to organic ligand, and concentrations of total Fe-binding ligands ranged from 0.98 nM to 3.07 nM, with conditional stability constants ( ) ranging from 11.5 to 13.1. During the fall cruise, the highest concentrations of ligands were observed within 1 km to iceberg, presumably due to enhanced grazing activity near the iceberg; while no enrichment of Fe-binding ligands was seen during the 2008 cruise, presumably due to low biological activity in the winter.

Grazing experiments were also conducted using the marine diatom Thalassiosira pseudonana as prey and the heterotrophic dinoflagellate Oxyrrhis marina as grazer to investigate the potential production of Fe-binding ligands via grazing processes. Ligand samples were analyzed using voltammetry. The concentration of dissolved Fe increased from 0.87 nM to 7.67 nM by the end of the grazing period (36 h), suggesting intracellular Fe of phytoplankton cells was released during the grazing process. Observed increase in dissolve Fe concentration of treatment bottle at the end of 12 h grazing was comparable to expected values based on consumption of T. pseudonana and its cellular Fe quota. Dissolved Fe concentration of treatment at the end of 24 h and 36 h grazing was less than those expected from cellular Fe calculation, indicating O. marin a retained some of the Fe or that a portionof the remineralized Fe was reaccumulated by T. pseudonana or O.marina. Fe-binding ligand was detected at 12 and 24 h with a concentration of 4.34 and 5.62 nM. The conditional stability constant of ligand ( ) was around 11.2, comparable with typical porphyrin-like intracellular ligand such as phaeophytin. These results will help us to better understand the source of Fe-binding ligands in the ocean.

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