The marine geochemistry of iron and iron isotopes

Abstract

This thesis addressed questions about the Fe cycle by measuring detailed profiles and transects of Fe species in the ocean and also by exploring the use of a new tracer of Fe, Fe isotopic fractionation. In the subtropical and tropical Atlantic Ocean, transects and profiles are presented for dissolved Fe (<0.4 μm), soluble Fe (<0.02 μm), and colloidal Fe (0.02 to 0.4 μm). Surface dissolved Fe distributions reflect atmospheric deposition trends with colloidal Fe following dust deposition more strongly than the soluble fraction of Fe. Observed surface maxima and shallow minima in dissolved Fe were always due to variations in the colloidal Fe fraction. Deep-water dissolved and colloidal Fe concentrations vary with water mass source, age, and transport path. Elevated dissolved Fe concentrations (>1 nmol/kg) were associated with an oxygen minimum zone in the tropical Atlantic at 10°N, 45°W. Fractionation of iron isotopes could be an effective tool to investigate the geochemistr of iron. Trace metal clean plankton tows, river samples, aerosol leachates, and porewater samples were measured for their iron isotopic composition using a GV Instruments IsoProbe Multi-collector ICPMS. The Fe isotopic composition of plankton tow samples vared by over 4% (in 56Fe/54Fe). North Pacific plankton tow samples had isotopically lighter Fe isotopic compositions 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. The main channel river dissolved Fe samples and suspended loads were isotopically similar (≈ -0.2 to -0.45% relative to igneous rocks). The isotopically heaviest sample collected was dissolved Fe from an organic rich tributary, the Negro River (+0.16%). In contrast, the suspended load from the Negro River was isotopically light (-1 %). The isotopically lightest sample from the Amazon region was shelf porewater (-1.4%). 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 observed range in the Fe isotopic composition of the natural samples including biological and aqueous samples demonstrates that significant and useful fractionation is associated with Fe biogeochemistry in the environment.