HR: 0800h
AN: H31C-0513    [Abstracts]
TI: Iron Isotope Composition of River Water During Estuarine Mixing: Case of North River (Massachusetts, USA)
AU: * Escoube, R
EM: rescoube@whoi.edu
AF: Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, MS#25, Woods Hole, MA 02543, United States
AU: * Escoube, R
EM: rescoube@whoi.edu
AF: LCABIE, U. Pau et Pays de l Adour, CNRS UMR 525, Helioparc 2 av President Pierre Angot, Pau, 64053, France
AU: Rouxel, O
EM: orouxel@whoi.edu
AF: Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, MS#25, Woods Hole, MA 02543, United States
AU: Sholkovitz, E
EM: esholkovitz@whoi.edu
AF: Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, MS#25, Woods Hole, MA 02543, United States
AU: Donard, O
EM: olivier.donard@univ-pau.fr
AF: LCABIE, U. Pau et Pays de l Adour, CNRS UMR 525, Helioparc 2 av President Pierre Angot, Pau, 64053, France
AB: Iron has recently been regarded as a regulator of ocean productivity and global climate change due to its key role as a major micronutrient in biological processes. To improve our understanding of the oceanic iron cycle, we need to better characterize iron sources and determine the processes that control the fractionation of Fe isotopes between continental run-off and the ocean. Recent studies (e.g. Fantle and DePaolo, 2004 and Bergquist and Boyle, 2006) have demonstrated that rivers present an isotopically light Fe source to the oceans. Since the input of dissolved iron from river water is generally controlled by flocculation processes occurring during estuarine mixing, it is important to investigate potential fractionation of Fe-isotopes during this process. In this study, we investigated the influence of the flocculation of Fe-rich colloids on the iron isotopic composition of estuarine waters. In October 2006, 16 samples were collected along a salinity gradient from the fresh water to the ocean in the North River Estuary (MA, USA). Samples were filtered at 0.22μm and the two fractions (dissolved and particles) were analyzed for iron isotopic composition using high-resolution MC-ICP-MS after chemical purification. Dissolved iron results show positive δ56Fe values (with an average of 0.43‰) relative to the IRMM- 14 standard and do not display any relationships with salinity or percentage of Fe colloid flocculation. These results suggest that riverine Fe-isotope composition are not affected by flocculation processes and yield a fractionation factor of less than 0.1‰. The iron isotopic composition of the particles suspended in fresh water is characterized by more negative δ56Fe values than for dissolve Fe and correlate with the percentage of Fe flocculation. δ56Fe values increased from -0.09‰ at no flocculation to ~0.1‰ at the flocculation maximum which reflect mixing effects between river-borne particles and newly precipitated colloids. Since the process of flocculation produce minimal Fe-isotope fractionation in the dissolve Fe pool, we suggest that the iron isotopic composition of fresh water is preserved during estuarine mixing and that the value of the global riverine source into the ocean can be assimilated to the fresh water values. However, this study also suggests that δ56Fe composition of rivers is not unique and may be characterized by more positive δ56Fe values (up to 0.3‰) relative to the crust than previously reported.
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4870 Stable isotopes (0454, 1041)
SC: Hydrology [H]
MN: 2007 Fall Meeting