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The flux of iron and iron isotopes from San Pedro Basin sediments

John, Seth G. and Mendez, Jeffery and Moffett, James and Adkins, Jess (2012) The flux of iron and iron isotopes from San Pedro Basin sediments. Geochimica et Cosmochimica Acta, 93 . pp. 14-29. ISSN 0016-7037.

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Iron is an important nutrient in the ocean, but the different sources and sinks of iron are not well constrained. Here, we use measurements of Fe concentration and Fe stable isotope ratios to evaluate the importance of reducing continental margins as a source of Fe to the open ocean. Dissolved iron concentration ([Fe]) and iron stable isotope ratios (δ^(56)Fe) were measured in the San Pedro and Santa Barbara basins. Dissolved δ^(56)Fe ranges from −1.82‰ to 0.00‰ in the San Pedro Basin and from −3.45‰ to −0.29‰ in the Santa Barbara Basin, and in both basins the lowest δ^(56)Fe values and highest Fe concentrations are found at the bottom of the basin reflecting the input of isotopically light Fe from reducing sediment porewaters. In the San Pedro Basin, we are also able to fingerprint an advective source of Fe from shallow continental shelves next to the basin and the atmospheric deposition of Fe into surface waters. A one-dimensional model of the Fe isotope cycle has been constructed for the deep silled San Pedro Basin. By fitting model output to data, values of several important iron cycle parameters are predicted including a flux of Fe from sediment porewaters into the water column of 0.32–1.14 μmol m^(−2) d^(−1), a first-order dissolved Fe precipitation rate constant of 0.0018–0.0053 d^(−1), a flux δ56Fe of −2.4‰, and an isotope effect for Fe precipitation of Δδ^(56)Fe_(particulate-dissolved) = −0.8‰. Applying our model-predicted Fe cycle parameters to the global ocean suggests that continental margins contribute 4–12% of world ocean dissolved Fe and make the ocean’s Fe lighter by −0.08‰ to −0.26‰. The dramatically negative δ^(56)Fe signature seen in the water column of the San Pedro and Santa Barbara basins demonstrate the utility of Fe isotopes as a tracer for continental margin Fe input from reducing sediments to the oceans, while the isotopic fractionation observed during loss of Fe from the dissolved phase suggests that this signature will be modified by subsequent reactions. Our modeling provides an initial framework for testing how these signals are transmitted into the open ocean.

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Adkins, Jess0000-0002-3174-5190
Additional Information:© 2012 Elsevier Ltd. Received 20 May 2011; accepted in revised form 3 June 2012; available online 23 June 2012. Associate editor: Mark Rehkamper.
Record Number:CaltechAUTHORS:20121101-110258784
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Official Citation:Seth G. John, Jeffery Mendez, James Moffett, Jess Adkins, The flux of iron and iron isotopes from San Pedro Basin sediments, Geochimica et Cosmochimica Acta, Volume 93, 15 September 2012, Pages 14-29, ISSN 0016-7037, 10.1016/j.gca.2012.06.003. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35242
Deposited By: Ruth Sustaita
Deposited On:01 Nov 2012 18:22
Last Modified:24 Feb 2020 10:30

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