Voltammetric (micro)electrodes for the in situ study of Fe²⁺ oxidation kinetics in hot springs and S₂O₃ ^2- production at hydrothermal vents

We have used solid-state Au/Hg voltammetric electrodes to understand redox and biogeochemical processes in hot spring and deep sea hydrothermal environments. These electrodes are non-specific and have the capability of measuring simultaneously a suite of chemical species including several of the principal redox species involved in early diagenesis (O₂, Mn ²⁺, Fe²⁺, H₂S/HS^-, and I ^-) as well as some Fe species (FeS and Fe³⁺) and sulfur species (S_x ^2- and S₂O₃ ^2-). Here we demonstrate how in situ data obtained in complex environments can be used to study specific iron and sulfur reactions and processes at (sub)millimeter to centimeter resolution and over short time scales. Examples include the oxidation of Fe²⁺ by O₂ produced by cyanobacterial mats in Yellowstone National Park hot springs and the formation of S₂O₃ ^2- in diffuse flow waters from the hydrothermal vents at Lau Basin. In one example, profiles of redox species in cyanobacterial mats from Yellowstone National Park hot springs show that in the light dissolved Fe²⁺ is completely removed from the source waters as cyanobacterial mats produce O₂ and oxidize the Fe²⁺. Performing kinetic experiments in the dark and light at the depth of maximum O₂ production indicates that the decay of Fe ²⁺ follows a zero order rate law consistent with photosynthesis as the source of O₂. These dynamic environments show how kinetic data can be obtained in situ and be used to understand the interactions between biology and chemistry. We know of no other analytical technique that can provide this information in both clear and turbid waters on the time scales (seconds) observed.