Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 4045-4059, doi:10.1016/j.gca.2009.04.026.
Description:
Recent studies have suggested that rivers may present an isotopically light Fe
source to the oceans. Since the input of dissolved iron from river water is generally
controlled by flocculation processes that occur during estuarine mixing, it is important to
investigate potential fractionation of Fe-isotopes during this process. In this study, we
investigate the influence of the flocculation of Fe-rich colloids on the iron isotope
composition of pristine estuarine waters and suspended particles. The samples were
collected along a salinity gradient from the fresh water to the ocean in the North River
estuary (MA, USA). Estuarine samples were filtered at 0.22 μm and the iron isotope
composition of the two fractions (dissolved and particles) were analyzed using high
resolution MC-ICP-MS after chemical purification. Dissolved iron results show positive
δ56Fe values (with an average of 0.43 ± 0.04 ‰) relative to the IRMM-14 standard and do
not display any relationships with salinity or with percentage of colloid flocculation. The
iron isotopic composition of the particles suspended in fresh water is characterized by
more negative δ56Fe values than for dissolved Fe and correlate with the percentage of Fe
flocculation. Particulate δ56Fe values vary from -0.09‰ at no flocculation to ~ 0.1‰ at
the flocculation maximum, which reflect mixing effects between river-borne particles,
lithogenic particles derived from coastal seawaters and newly precipitated colloids. Since
the process of flocculation produces minimal Fe-isotope fractionation in the dissolved Fe
pool, we suggest that the pristine iron isotope composition of fresh water is preserved
during estuarine mixing and that the value of the global riverine source into the ocean can
be identified from the fresh water values. However, this study also suggests that δ56Fe
composition of rivers can also be characterized by more positive δ56Fe values (up to 0.3
per mil) relative to the crust than previously reported. In order to improve our current
understanding of the oceanic iron isotope cycling, further work is now required to
determine the processes controlling the fractionation of Fe isotopes during continental
run-off.
Description:
This study was supported by the National Science Foundation (OCE 0550066) to O. Rouxel and Edward Sholkovitz.
Keywords:
Iron isotopes
;
Rivers
;
Estuary
;
Flocculation
;
Iron cycle
;
Colloids
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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