Impacts of flocculation on the distribution and diagenesis of iron in boreal estuarine sediments

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Jilbert , T S , Asmala , E , Schröder , C , Tiihonen , R M E , Myllykangas , J-P , Kotilainen , A , Virtasalo , J , Peltola , P , Ekholm , P I & Hietanen , S S 2018 , ' Impacts of flocculation on the distribution and diagenesis of iron in boreal estuarine sediments ' , Biogeosciences , vol. 15 , no. 4 , pp. 1243-1271 . https://doi.org/10.5194/bg-15-1243-2018

Title: Impacts of flocculation on the distribution and diagenesis of iron in boreal estuarine sediments
Author: Jilbert, Thomas Stephen; Asmala, Eero; Schröder, Christian; Tiihonen, Rosa Maria Elina; Myllykangas, Jukka-Pekka; Kotilainen, Aarno; Virtasalo, Joonas; Peltola, Pasi; Ekholm, Päivi Inkeri; Hietanen, Siru Susanna
Contributor: University of Helsinki, Aquatic Biogeochemistry Research Unit (ABRU)
University of Helsinki, Tvärminne Zoological Station
University of Helsinki, Environmental Sciences
University of Helsinki, Environmental Sciences
University of Helsinki, Department of Food and Nutrition
University of Helsinki, Aquatic Biogeochemistry Research Unit (ABRU)
Date: 2018-03-02
Language: eng
Number of pages: 29
Belongs to series: Biogeosciences
ISSN: 1726-4170
URI: http://hdl.handle.net/10138/233897
Abstract: Iron (Fe) plays a key role in sedimentary diagenetic processes in coastal systems, participating in various redox reactions and influencing the burial of organic carbon. Large amounts of Fe enter the marine environment from boreal river catchments associated with dissolved organic matter (DOM) and as colloidal Fe oxyhydroxides, principally ferrihydrite. However, the fate of this Fe pool in estuarine sediments has not been extensively studied. Here we show that flocculation processes along a salinity gradient in an estuary of the northern Baltic Sea efficiently transfer Fe and OM from the dissolved phase into particulate material that accumulates in the sediments. Flocculation of Fe and OM is partially decoupled. This is likely due to the presence of discrete colloidal ferrihydrite in the freshwater Fe pool, which responds differently from DOM to estuarine mixing. Further decoupling of Fe from OM occurs during sedimentation. While we observe a clear decline with distance offshore in the proportion of terrestrial material in the sedimentary particulate organic matter (POM) pool, the distribution of flocculated Fe in sediments is modulated by focusing effects. Labile Fe phases are most abundant at a deep site in the inner basin of the estuary, consistent with input from flocculation and subsequent focusing. The majority of the labile Fe pool is present as Fe (II), including both acid-volatile sulfur (AVS)-bound Fe and unsulfidized phases. The ubiquitous presence of unsulfidized Fe (II) throughout the sediment column suggests Fe (II)-OM complexes derived from reduction of flocculated Fe (III)-OM, while other Fe (II) phases are likely derived from the reduction of flocculated ferrihydrite. Depth-integrated rates of Fe (II) accumulation (AVS-Fe + unsulfidized Fe (II) + pyrite) for the period 1970-2015 are greater in the inner basin of the estuary with respect to a site further offshore, confirming higher rates of Fe reduction in near-shore areas. Mossbauer Fe-57 spectroscopy shows that refractory Fe is composed largely of superparamagnetic Fe (III), high-spin Fe (II) in silicates, and, at one station, also oxide minerals derived from past industrial activities. Our results highlight that the cycling of Fe in boreal estuarine environments is complex, and that the partial decoupling of Fe from OM during flocculation and sedimentation is key to understanding the role of Fe in sedimentary diagenetic processes in coastal areas.
Subject: 1172 Environmental sciences
1171 Geosciences
DISSOLVED ORGANIC-MATTER
NORTHERN BALTIC SEA
MEDIATED ANAEROBIC OXIDATION
REACTIVE IRON
MARINE-SEDIMENTS
MOSSBAUER-SPECTROSCOPY
HYDROGEN-SULFIDE
ARCHIPELAGO SEA
FRESH-WATER
LAND-USE
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