UC San Diego

Iron (Fe) is an essential micronutrient for marine primary producers, yet its low oceanic concentrations and complex aqueous chemistry make it difficult to measure. Current analytical techniques for assessing the ocean Fe system are rarely able to provide information about Fe-speciation at ambient seawater physico-chemical conditions. The chalcogenide glass Fe3+ ion-selective electrode (Fe-ISE) has emerged as a promising tool for more in-situ oceanographic Fe analyses given its ability for rapid, passive measurements of Fe activity (aFe3+), a parameter directly related to Fe lability and bioavailability. Work in this dissertation addresses the considerations for oceanographic implementation of the Fe-ISE, building from controlled laboratory experiments which establish fundamental sensor response characteristics, to in-situ measurements of seawater aFe3+ during several field expeditions.In Chapter 1, an improved continuous flow analyzer (CFA) incorporating in-line pH and temperature sensors is utilized to study Fe-ISE response in well-characterized, artificial media. A solution-independent temperature coefficient is determined for the sensor system. Potentiometric calibrations are carried out using previously studied media to reexamine their applicability to seawater analyses. Synchrotron radiation experiments indicate a potential-generating mechanism in metal-chelating solutions which involves the ligand-mediated diffusion of Fe ions out of the chalcogenide membrane, and suggest that previous estimates of cross- selectivity are not relevant in seawater.In Chapter 2, Fe-ISE acidimetric titrations of modified seawater media are used to validate values of aFe3+ determined by different aqueous speciation models. We present an optimized seawater-based Fe-ISE calibration system and discuss relevant aFe3+ modelling considerations. We conduct acidimetric titrations of natural seawater to show that the Fe-ISE is capable of resolving pH-dependent changes in Fe-speciation pertinent to ocean acidification and microbial uptake.In Chapter 3, the Fe-ISE CFA is incorporated into shipboard systems for continuous measurements off the Californian and Oregon coasts. Parallel hydrographic sensor measurements, as well as dissolved Fe and Fe-binding ligand analyses, suggest that surface trends in aFe3+ appear to be driven by different physico-chemical factors than aFe3+ gradients associated with depth. The Fe-ISE offers a novel, uniquely in-situ lens into the complex dynamics of ocean Fe chemistry.