The Marine Geochemistry of Iron and Iron Isotopes

Bridget Bergquist, Ph.D., 2004
Edward Boyle, Advisor

This thesis addressed questions about the Fe cycle by measuring detailed profiles and transects of Fe species and also by exploring the use of a new tracer of Fe, Fe isotopic fractionation. Dissolved Fe concentrations (<0.4 mm) reflect atmospheric deposition trends in surface waters, water mass source, age, and transport path. Most of the variation in dissolved Fe is due to variations in colloidal Fe (0.02 to 0.4 mm). Trace metal clean plankton tows, river samples, aerosol leachates, and porewater samples were measured for their Fe isotopic composition. A range of 4‰ (in 56Fe/54Fe) was observed in the isotopic composition of plankton tow samples. Plankton samples from the North Pacific had isotopically lighter Fe than samples from the Atlantic. The overall isotopic range observed in the Amazon River system was 1.5‰, with variability observed for different types of tributaries and isotopically light Fe observed in shelf porewaters. In river water-seawater mixing experiments, the Fe isotopic signal of dissolved Fe of river water was modified by flocculation of isotopically heavy Fe. The range in the Fe isotopic composition of the natural samples including biological and aqueous samples demonstrates that useful is associated with Fe biogeochemistry in the environment.