Iron isotope fractionation in subterranean estuaries
Rouxel, Olivier J.
Sholkovitz, Edward R.
Charette, Matthew A.
Edwards, Katrina J.
MetadataShow full item record
Dissolved Fe concentrations in subterranean estuaries, like their river-seawater counterparts, are strongly controlled by non-conservative behavior during mixing of groundwater and seawater in coastal aquifers. Previous studies at a subterranean estuary of Waquoit Bay on Cape Cod, USA demonstrate extensive precipitation of groundwater-borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands. Waquoit Bay is thus an excellent natural laboratory to assess the mechanisms of Fe-isotope fractionation in redoxstratified environments and determine potential Fe-isotope signatures of groundwater sources to coastal seawater. Here, we report Fe isotope compositions of iron-coated sands and porewaters beneath the intertidal zone of Waquoit Bay. The distribution of pore water Fe shows two distinct sources of Fe: one residing in the upward rising plume of Fe-rich groundwater and the second in the salt-wedge zone of pore water. The groundwater source has high Fe(II) concentration consistent with anoxic conditions and yield δ56Fe values between 0.3 and –1.3‰. In contrast, sediment porewaters occurring in the mixing zone of the subterranean estuary have very low δ56Fe values down to –5‰. These low δ56Fe values reflect Fe-redox cycling and result from the preferential retention of heavy Fe-isotopes onto newly formed Fe-oxyhydroxides. Analysis of Feoxides precipitated onto subsurface sands in two cores from the subterranean estuary revealed strong δ56Fe and Fe concentration gradients over less than 2m, yielding an overall range of δ56Fe values between –2 and 1.5‰. The relationship between Fe concentration and δ56Fe of Fe-rich sands can be modeled by the progressive precipitation of Fe-oxides along fluid flow through the subterranean estuary. These results demonstrate that large-scale Fe isotope fractionation (up to 5‰) can occur in subterranean estuaries, which could lead to coastal seawater characterized by very low δ56Fe values relative to river values.
Author Posting. © Elsevier B.V., 2008. 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 72 (2008): 3413-3430, doi:10.1016/j.gca.2008.05.001.
Showing items related by title, author, creator and subject.
Rastetter, Edward B.; Kwiatkowski, Bonnie L.; McKane, Robert B. (2005-01-07)To facilitate the simulation of isotope dynamics in ecosystems, we developed software to model changes in the isotopic signatures of the stocks of an element using the output from any parent model that specifies the stocks ...
Scheer, Edward K. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1982-05)Seismic refraction experiments have been used extensively in the past thirty five years in investigations of the structure of the oceanic crust. The longer range of the refraction or wide angle reflection technique, on ...
Peltzer, Edward T.; Alford, Jame B.; Gagosian, Robert B. (Woods Hole Oceanographic Institution, 1984-04)A procedure is described for the collection of remote marine aerosol samples by high-volume filtration, cascade impaction, dry fallout collection and rain. Samples were analyzed quantitatively for five classes of naturally ...