X-ray Absorption Spectroscopy to Unravel Fe Speciation in Soil and Sediment Cores from Redox-Dynamic Marine and Freshwater Coastal Environments

Coastal terrestrial-aquatic interfaces (TAIs) represent highly dynamic subsurface environments that play an important role in global biogeochemical cycles and ecosystem functions [1]. These systems harbor spatial and temporal redox variations known to impact the geochemical behavior of redox sensitive elements such as iron (Fe) and sulfur (S), that are directly implicated in the biogeochemical cycling of carbon, nutrients, and contaminants [2].

The Coastal Observations, Mechanisms, and Predictions Across Systems and Scales-Field, Measurements, and Experiments (COMPASS-FME) pilot study aims, in part, to better understand Fe biogeochemistry in the Chesapeake Bay and Lake Erie regions, chosen as model systems to represent marine and freshwater coastal environments, respectively. Using X-ray Absorption Spectroscopy, depth-dependent Fe oxidation state and molecular environment was determined within sediment/soil cores collected along upland to shoreline gradients. Results from linear combination fitting of x-ray absorption near edge spectroscopy (XANES) data show that Fe occurs mainly in its oxidized form, Fe(III), in the cores collected from upland regions at Lake Erie and Chesapeake Bay. In the sediment/soil cores collected closer to the shorelines (e.g., transition zones and coastal wetlands), Fe is partitioned between Fe(III) and Fe(II), with increasing proportions of Fe(II) from intermittently flooded to permanently flooded locations. Extended x-ray absorption fine structure (EXAFS) spectroscopy suggests that Fe(III) occurs as Fe-(hydr)oxide and Fe-bearing phyllosilicate minerals at both sites. By contrast, the Fe(II)-bearing species at Lake Erie and Chesapeake Bay differ. In particular, we observed the presence of Fe sulfides in the Chesapeake Bay sediments, indicating a sulfur-dominated redox chemistry, while Fe(II) is mainly present in Fe-bearing phyllosilicate minerals in the samples collected at Lake Erie sites. Complemented with geochemical and hydrological information, these results contribute to a better understanding of the mechanisms governing Fe speciation and redox cycling in marine and freshwater coastal systems.

[1] Ward et al. Nature Communications, 2020, 11,1:2458.

[2] Kappler et al. Nature Reviews Microbiology, 2021, 19, 6.