Preliminary Constraints on Modern Marine Iodine Cycling: Calibrating a Metric for Comparing Modern and Ancient Marine Oxygen and Carbon Cycling

Sune Nielson, Geology & Geophysics
Tristan Horner, Marine Chemistry & Geochemistry

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Abstract

This proposal seeks to fund a pilot study providing the first observations and fundamental quantitative constraints on iodine redox transformations under marine conditions.  Iodine oxidizes to the oxidized species iodate at very low oxygen contents in seawater and, therefore, is thought to be a tracer of fluctuations between zero and some low oxygen level close to that necessary to host marine fauna.  Further, previous work has demonstrated the propensity of past seawater iodate availability to be tracked using carbonate sediments, and the post-doctoral fellow slated to lead the study has generated a record of its distribution through time.  As such, iodine cycling is particularly poised for application toward tracking low oxygen environments and related faunal distributions throughout Earth history.  However, despite abundant modern and past iodine cycling records, little is known about the absolute conditions and rates of iodate production.  Therefore, it is currently not possible to use iodine cycling as a quantitative tracer of modern or past marine oxygenation.  Our study will yield the first observations of the marine production of the oxidized iodine species (iodate) — the most common in the modern ocean — and provide principal constraints on the associated reaction pathways, kinetics, oxidants necessary, and environments hosting its formation.
 
We propose to perform a series of laboratory incubation experiments using sediment and seawater from Buzzards Bay, USA, to track the reaction pathways, rates, oxidants, and environments hosting iodine redox transformations.  We will use a novel approach, doping the experiments with the radioactive iodine isotope 129I and tracking its distribution relative to the stable 127I within end-member environments with varying levels of oxidants hypothesized to produce iodate.  Though this approach has not been formerly applied to iodine cycling, it is straightforward, builds on previous work, and has a high likelihood of success.  This project is specifically designed to provide pilot results for NSF Chemical Oceanography and NASA Exobiology proposals that will, respectively: (1) constrain the modern marine iodine budget and associated elemental cycles (e.g.  N, Mn, C) and; (2) provide quantitative constraints on oxygen concentrations and the abundance of OMZ-like settings in ancient oceans and their relationship with the initial evolution and later temporal distribution of complex life.