wAbstract
@Recent studies have suggested that rivers may present in 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 0.22ƒÊm
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 ƒÂ56Fe
values (with an average of 0.43}0.04ñ) 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 ƒÂ56Fe values than for dissolved Fe and
correlate with the percentage of Fe flocculation. Particulate
ƒÂ56Fe values vary from -0.09ñ at no flocculation to
`0.1ñ 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 ƒÂ56Fe composition of rivers can also be characterized
by more positive ƒÂ56Fe values (up to 0.3ñ) 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.x
1. Introduction
2. Materials and location
3. Analytical method
@3.1. Sample filtration
@3.2. Chemical analysis
@3.3. Iron isotope analysis
4. Results
@4.1. River end-member composition
@4.2. Element behavior in North River estuary
@4.3. Determination of the flocculation factor
5. Discussion
@5.1. Fe-isotope systematics in colloidal and particulate
pools in the river end member
@5.2. Fe-isotope systematics of dissolved Fe during flocculation
process in estuaries
@5.3. Fe-isotope systematics in the particulate pool
@5.4. Implication for coastal seawater Fe sources
6. Conclusion
Acknowledgements
References