TitleSedimentary iron-phosphorus cycling under contrasting redox conditions in a eutrophic estuary
Publication TypeJournal Article
AuthorsKraal P., Burton E., Rose A., Kocar BD, Lockhart R., Grice K., Bush R., Tan E., Webb S.
JournalChemical Geology
Keywordsbaltic sea, biogeochemistry, continental-margin sediments marine-sediments, diagenesis, Eutrophication, iron and phosphorus speciation, key role, lake-sediments, minerals, nutrient release, organic-matter, phosphate, rich sediments, x-ray spectroscopy

Phosphorus (P) is often a limiting nutrient within freshwater and estuarine systems, thus excess inputs of P from anthropogenic activities (dominantly agriculture) can induce eutrophication in receiving water bodies. The sequestration of P within estuarine sediments is controlled by sorption and precipitation processes, which are impacted by local redox conditions and burial environment. Despite the global spread of oxygen depletion in coastal marine systems, P burial under reducing conditions remains poorly understood. We investigated P cycling in relation to iron (Fe) redox chemistry in sediments from the eutrophic Peel-Harvey Estuary in Western Australia, using a combination of porewater analysis, sequential chemical P extractions, synchrotron-based micro-scale X-ray fluorescence mapping and Fe K-edge X-ray absorption spectroscopy, and PO43- sorption experiments. The sediments represented redox regimes varying from strongly reducing, organic-rich sediments with little or no reactive Fe(III) (oxyhydr) oxides to oxygenated sediments that were enriched in reactive Fe(III) phases. Organic P and Fe-associated P were the main P burial phases, and the latter was quantitatively important even in sediments with an overall strongly reducing character. We attribute this to adsorption of P onto micro-scale Fe(III) oxyhydroxide enrichments and/or Fe-bearing clay minerals. The organic-rich sediments showed a strong decline in P contents with depth; P was released from organic matter and Fe phases but apatite precipitation was apparently inhibited in these sediments. Despite greater and stronger PO43- sorption capacity, the oxic sediments contained relatively little P due to a lack of the primary P source in marine sediments: organic matter. Our results provide detailed insight into P burial in dynamic estuarine sediments and show that micro-scale spectroscopic analyses greatly advance our understanding of P sequestration processes. (C) 2014 Elsevier B.V. All rights reserved.

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    <p><span>Professor Benjamin Kocar</span><br><span>Ralph M. Parsons Laboratory for Environmental Science and Engineering</span><br><span>Massachusetts Institute of Technology</span><br><span>15 Vassar Street</span><br><span>Cambridge, MA 02139</span></p>
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    Our group measures and deciphers biogeochemical processes responsible for controlling the fate and cycling of nutrients, contaminants, and trace gases in soils, sediments, and natural waters. These biotic and biotic reactions are often chemically intertwined and transpire within complex systems containing aqueous solutions, mineral assemblages, gases, and microorganisms.

    Further, they are often linked with other physical and (geo)chemical processes, including hydraulic transport and photochemical reaction pathways. We are particularly interested in these “coupled” processes, since they often dominate pathways controlling the cycling of elements in natural and engineered systems. We perform laboratory experiments to understand the importance of specific (coupled) processes, and use variety of analytical tools, including conventional and synchrotron-based techniques, to understand mechanisms at the molecular scale. A goal of our group is to understand how these reactions “scale-up”, and this is accomplished by linking molecular-scale reactions and processes with observed field measurements using computational tools such as reactive transport modeling. <

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