The influence of sea-water inundation on coupled iron and sulfur cycling in a coastal freshwater wetland

Schoepfer, Valerie Anne, "The influence of seawater inundation on coupled iron and sulfur cycling in a coastal freshwater wetland" Coastal freshwater wetland chemistry is rapidly changing due to increased saltwater inundation, a consequence of global change. Seasonal inundation introduces sulfate, which biologically reduces to sulfide via microbial metabolism. Sulfide binds with reduced iron producing iron sulfide (FeS), recognizable in wetland soils by its characteristic black color. Iron sulfur dynamics are complex in wetlands, more so in wetlands under the threat of salt water inundation. The objective of this study is to document iron and sulfate reduction rates in a coastal freshwater wetland undergoing seasonal salt water inundation. A secondary objective is to document formation of iron sulfide complexes using the acid volatile sulfide (AVS), chromium reducible sulfide (CRS), and Indicator of Reduction in Soils (IRIS) techniques. Decreasing soil moisture correlated with increasing iron and sulfate reduction rates. Soil chloride did not predict rates despite being a direct indicator of inundation extent. Relationships were stronger at the surface, as the site experienced surface water inundation, rather than groundwater intrusion. AVS and CRS responded similarly to soil moisture, however CRS was more strongly correlated to soil chloride. IRIS plates document size, heterogeneity and concentration of FeS complexes in situ. Concentrations increased from June to July and remained steady, as FeS complexes were possibly transformed into recalcitrant forms and were not captured on IRIS plates. The IRIS technique could not predict SRR, AVS or CRS formation. However, IRIS plates document 3 heterogenerity of complexes within the sediment, an important feature not addressed by established techniques. The Timberlake wetland sees saltwater influx each summer due to decreased precipitation and increased evapotranspiration, driven by global change. Increased sulfate has the potential to transform this wetland into a net sulfidic system, however iron buffers this transformation, along with physical drying. At the current pH and redox potential of the wetland, iron is in the aqueous Fe 2+ form. Iron is not limited, and sulfide resulting from inundation is likely to be bound, buffering the wetland against any change in chemical state.