Articles | Volume 15, issue 4
Biogeosciences, 15, 1243–1271, 2018
Biogeosciences, 15, 1243–1271, 2018

Research article 02 Mar 2018

Research article | 02 Mar 2018

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

Tom Jilbert1,2, Eero Asmala1,2,3, Christian Schröder4, Rosa Tiihonen1,2, Jukka-Pekka Myllykangas1,2, Joonas J. Virtasalo5, Aarno Kotilainen5, Pasi Peltola6, Päivi Ekholm7, and Susanna Hietanen1,2 Tom Jilbert et al.
  • 1Department of Environmental Sciences, Faculty of Biological and Environmental Sciences, University of Helsinki, P.O. Box 65, 00014 University of Helsinki, Finland
  • 2Tvärminne Zoological Station, University of Helsinki, J.A. Palménintie 260, 10900 Hanko, Finland
  • 3Department of Bioscience – Applied Marine Ecology and Modelling, Aarhus University, Frederiksborgvej 399, 4000 Roskilde, Denmark
  • 4Biological and Environmental Sciences, Faculty of Natural Sciences, University of Stirling, Stirling FK9 4LA, Scotland, UK
  • 5Marine Geology, Geological Survey of Finland (GTK), P.O. Box 96, 02151 Espoo, Finland
  • 6Boliden Rönnskär, 932 81 Skelleftehamn, Sweden
  • 7Department of Food and Environmental Sciences, P.O. Box 66, 00014 University of Helsinki, Finland

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. Mössbauer 57Fe 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.

Short summary
Iron is a common dissolved element in river water, recognizable by its orange-brown colour. Here we show that when rivers reach the ocean much of this iron settles to the sediments by a process known as flocculation. The iron is then used by microbes in coastal sediments, which are important hotspots in the global carbon cycle.
Final-revised paper