Iron Redox Biogeochemistry Across Coastal Terrestrial-Aquatic Interfaces: Field Study in the Great Lakes and Chesapeake Bay Regions


Lucie Stetten1* (, Maxim Boyanov1,2, Edward O’Loughlin1, Donnie Day4, Khadijah Homolka3, Anya Hopple5, Matthew Kovach4, Roberta Peixoto4, Fausto Machado Silva4, Nathan McDowell3, Patrick Megonigal5, Stephanie Wilson5, Allison Myers-Pigg3,4, Opal Otenburg3, Nick Ward3, Michael Weintraub3,4, Kenneth Kemner1, Vanessa Bailey3


1Biosciences Division, Argonne National Laboratory, Lemont, IL; 2Bulgarian Academy of Sciences, Sofia, Bulgaria; 3Pacific Northwest National Laboratory, Richland, WA; 4University of Toledo, Toledo, OH; 5Smithsonian Environmental Research Center, Edgewater, MD



Coastal terrestrial-aquatic interfaces (TAIs) are highly dynamic environments with spatial and seasonal hydrological fluctuations that impact the geochemical behavior of redox sensitive elements such as iron (Fe) and sulfur, as well as the biogeochemical cycling of carbon and nutrients. The Coastal Observations, Mechanisms, and Predictions Across Systems and Scales–Field, Measurements, and Experiments (COMPASS-FME) pilot study aims to better understand the mechanisms that control the structure, function, and evolution of ecosystem across the coastal TAI. As part of this project, this study aims to better understand Fe redox biogeochemistry across coastal sites in the Great Lakes and the Mid-Atlantic regions of the United States, chosen as model systems to represent freshwater and saltwater coastal environments, respectively. Using X-Ray absorption spectroscopy (XAS), the oxidation state and molecular environment of Fe was determined with depth in undisturbed soil cores collected across upland to shoreline gradients from coastal locations along the Western Lake Erie Basin and the Chesapeake Bay. XAS results show that Fe occurs mainly in its oxidized form, Fe(III), in cores collected from upland locations in both the Lake Erie and Chesapeake Bay regions. In soil cores collected closer to the shoreline (i.e., forest transition zones and coastal wetlands), Fe is partitioned between Fe(III) and its reduced form, Fe(II), with Fe(II) proportions increasing from the intermittently flooded to the permanently flooded locations. Extended X-Ray absorption fine structure spectroscopy suggests that Fe(III) occurs as Fe-(hydr)oxide and Fe-bearing phyllosilicate minerals in both the Lake Erie and Chesapeake Bay regions. Thus, it appears that factors affecting Fe chemistry are similar in the two regions. In contrast, researchers observed the presence of Fe sulfides together with Fe-bearing phyllosilicate minerals in the Chesapeake Bay wetland soils, indicating a sulfur-driven redox chemistry, whereas Fe(II) is mainly observed in Fe-bearing phyllosilicate minerals at the Lake Erie sites. Complemented with mineralogical and porewater chemistry information, these results contribute to a better understanding of Fe redox cycling in saltwater and freshwater coastal systems.