Date of Award

1-1-2013

Document Type

Open Access Thesis

Department

Marine Science

First Advisor

Seth G John

Abstract

Iron (Fe) is an essential nutrient for life. However its low solubility in the seawater limits primary productivity in many regions of the world's oceans. Dissolved phase Fe (<0.4μm) has traditionally been considered the most biologically accessible form, but, recent studies suggest the particulate phase (>0.4μm) may contain an important, labile reservoir of Fe that may also be available to phytoplankton. Although valuable, Fe concentrations of particles alone do not provide complete evidence about the sources of particulate Fe to the ocean, or the extent particulate iron may be accessed by phytoplankton. Fe isotopes may help to reveal the biogeochemical cycling of particulate Fe. This study aims to develop a chemical leaching technique that accesses the labile reservoir of particulate Fe and the isotope signatures associated with this reservoir. A comparative study of thirty-six different chemical leaches was performed on a marine sediment reference material, MESS-3 and sediment trap samples from the Cariaco Basin, Venezuela. The combinations included three different acids (25% acetic acid, 0.01M HCl, and 0.5M HCl), various redox conditions (0.02M hydroxylamine hydrochloride or untreated), three temperatures (25°C, 60°C, 90°C), and three time points (10 minutes, 2 hours, 24 hours). Leached Fe concentrations were determined by high-resolution inductively coupled plasma-mass spectrometery (ICP-MS) and varied from 1mg/g to 35mg/g, with longer treatment times, stronger acids, and hotter temperatures generally associated with an increase in leached Fe. δ56Fe in these leachates were determined by Neptune, mulit-collector ICP-MS and varied from -1.3 / to +0.2 /. Regardless of acidic leaching method used, there was a very similar relationship between the amount of Fe leached from the particles and the δ56Fe of this iron. Isotopically lighter δ56Fe values were associated with smaller amounts of leached Fe whereas isotopically heavier δ56Fe values were associated with larger amounts of leached Fe. However, a leach of 0.1M oxalate-0.05M EDTA at pH 8 did not show this relationship. Therefore the oxalate-EDTA leach was determined to be the leach best suited to extract the labile fraction of particulate Fe and the iron isotope signatures associated with this fraction. This leach was applied to three stations (the coast of Bermuda, over the TAG hydrothermal plume, and the coast of Maurtiania) along the US GEOTRACES North Atlantic transect. Labile iron in these profiles was found to be isotopically light in comparison to crustal values and remained relatively light throughout the water column. Labile particulate iron concentrations increased near the coast of Mauritania and even lighter δ56Fe values of these particles are associated with reductive dissolution from the continental shelf and at depth from the nephloid layer off Mauritania

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