HR: 13:40h
AN: V53B-01    [Abstracts]
TI: Iron Isotopes in the Amazon River System: Weathering and Transport Signatures
AU: * Bergquist, B A
EM: bardot@mit.edu
AF: MIT, E34, 42-44 Carleton Street, Cambridge, MA 02139 United States
AU: Boyle, E A
EM: eaboyle@mit.edu
AF: MIT, E34, 42-44 Carleton Street, Cambridge, MA 02139 United States
AB: Fractionation of iron isotopes will be an effective tool to investigate and quantify the environmental geochemistry of iron. Initial studies of stable iron isotopes show measurable fractionation in both field samples and laboratory studies spanning over 4$\permil$ in the 56/54 ratio. In this study, 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. This system uses a hexapole collision cell to reduce molecular interferences and improve transmission. A plankton tow collected in a low salinity Amazon River plume in the open ocean had a $\delta$$^{56}$Fe value of -0.34$\permil$ relative to igneous rocks and a Fe:C ratio of $\sim$ 600 $\mu$mol/mol. It was inferred from the high Fe:C ratio that a majority of the Fe collected in the plankton tow was extracellular Fe and that the $\delta$$^{56}$Fe might reflect the composition of particles and Fe attached to the surface of the plankton. In order to investigate the source of Fe to the Amazon plume water, samples were collected from the Amazon River and region including filtered river water, suspended sediment, and a shelf porewater. River water-seawater mixing experiments were also performed to assess whether Fe flocculation in estuaries affects isotopic composition of the dissolved flux to the ocean. The overall Fe isotopic variation observed in the Amazon River system was 1.5$\permil$. The Fe from the dissolved and suspended loads of two main channel river sites was isotopically similar ($\sim$ -0.2 to -0.45$\permil$). The most depleted sample was the Amazon shelf porewater (-1.4$\permil$). The isotopically heaviest sample collected was the dissolved Fe from an organic rich tributary, the Negro River, in the Amazon River system (+0.16$\permil$). Although the Negro River dissolved phase was isotopically heavy relative to igneous rock, its suspended sediment Fe was isotopically light (-1$\permil$). The signature of the Negro was not observed downstream near the mouth. The variability in Fe isotopic composition from different types of river tributaries draining distinct weathering terrains suggests that Fe isotopes may reflect the degree or type of weathering or overall balance of Fe in a drainage basin. Based on river water-seawater mixing experiments, the $\delta$$^{56}$Fe signal of the Amazon River may be modified in the estuary when $>$90% of the Fe flocculates upon mixing with ocean water. In the river water-seawater mixing experiments, the Fe that flocculated was isotopically heavy compared with the riverine dissolved Fe by 0.19 $\pm$ 0.12 $\permil$ (2$\sigma$). This observation suggests that the dissolved Fe that is transported to the ocean is isotopically lighter ($\sim$ -1$\permil$ or lighter) than the river endmember. However, neither the proposed isotopically light Fe from the modified riverine input nor from shelf porewater matches the Amazon plume plankton tow iron isotopic composition. Processes in the euphotic zone (biological cycling/export, scavenging) may modify the riverine or shelf Fe input to the ocean by preferentially removing isotopically light Fe. The above studies of Fe isotope fractionation demonstrate that aqueous and biological samples in the environment have a measurable range in iron isotopic composition, and that these signals might be useful in tracking Fe pathways.
DE: 4805 Biogeochemical cycles (1615)
DE: 4875 Trace elements
DE: 3670 Minor and trace element composition
DE: 1030 Geochemical cycles (0330)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting