Biogeochemical Response of Multiple Iron Redox Oscillations: Laboratory and Field Investigations

Abstract

Iron (Fe) exerts strong control over environmental biogeochemistry. As the fourth most abundant element, Fe is present in nearly all earth environments, where it plays important roles in governing the transformation and movement of organic and inorganic constituents, and in microbial respiration. Consequently, the body of work on Fe biogeochemistry is vast. This study is specifically concerned with the dynamic changes in the oxidation state of Fe (i.e., redox cycling) and their impact on the inorganic, organic and microbial components in soil. I constructed a special apparatus to fluctuate redox potential on soil slurries while concurrently sampling a wide range of biogeochemical variables (pH, redox potential, major and trace elements, CO2 release, DNA community composition charges, etc.). Previous research has documented redox fluctuations along a climate gradient in Hawaii and a primary goal of this dissertation was to reconstruct these redox fluctuations, subjected to experimental constraints afforded by a laboratory setting, with minimal disruption to the biogeochemical processes controlling Fe redox cycling. By recasting the spatial and temporal characteristics of in situ Fe redox cycling in the laboratory, I was able to form testable hypotheses regarding the importance of Fe redox oscillations to soil mineral transformations, colloid composition/dynamics and microbial community structure. A second goal of this dissertation was to explore the utility of Fe isotopic composition for providing information on soil weathering processes along age and climate gradients at the field scale in Hawaii. This portion of the study tested emerging theories of Fe isotope fractionation during mineral dissolution using well-characterized sequences in soil weathering intensity.The principal findings of the laboratory redox fluctuation experiments are that Fe redox oscillations: (1) trigger an increase in the crystallinity of Fe-oxides; (2) mobilize colloids containing refractory elements (e.g., Zr, Nb, U, etc.); (3) reveal redox sensitive rare earth element (REE) anomalies in the aqueous phase; and (4) induce changes in the microbial community favoring microbes capable of growth under both oxic and anoxic conditions. The principal finding of the Fe isotope measurements is that isotopic composition is directly related to weathering intensity in the field, consistent with theoretical predictions.