Editorial: The marine iodine cycle, past, present and future

Abstract

Editorial on the Research TopicThe marine iodine cycle, past, present and future Iodine is a redox-active element that exists in multiple oxidation states and phases in the oceans, and is taken up and transformed by living organisms. The dominant forms in seawater are the dissolved anions iodide (I -) and iodate (IO 3 -), along with smaller fractions of dissolved organic iodine (DOI), and particulate iodine (Chance et al., 2014). It plays an important role in atmospheric chemistry, impacting air quality and climate. Reaction with iodide-iodine at the ocean surface is a major sink for tropospheric ozone, a pollutant gas, and the main driver of the sea-air iodine flux. Understanding the distribution and drivers of marine iodine speciation is necessary to accurately quantify sea-air iodine fluxes and the marine ozone sink. Another key motivation for understanding the modern marine iodine cycle is the use of iodate-iodine abundance in ancient carbonate minerals as a proxy for oxygenation in the paleo-ocean (Lu et al., 2010). Refinement of this proxy to be more quantitative requires an improved understanding of the marine iodine cycle and how it responds to changes in redox conditions. Finally, iodine species, including anthropogenic radioisotopes, are also proposed as tracers of water masses and sedimentary inputs.In this Research Topic, we bring together ten articles from the diverse research communities interested in the marine iodine cycle, including paleoceanographers, atmospheric chemists, and biogeochemists. The physical chemistry underpinning iodine's chemical speciation and transformations in the ocean is reviewed by Luther; this paper provides a theoretical basis for the field observations presented in this Research Topic.Three observational papers report present-day iodine speciation in the Pacific, Atlantic and Indian Oceans. New profiles of iodide and iodate concentrations from the vicinity of station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the subtropical North Pacific (Sţreangăet al., 2023) and station BATS (Bermuda Atlantic Time-series Study) in the Atlantic (Schnur et al., 2024) are in good agreement with observations made more than 30 years earlier (Campos et al., 1996), suggesting long term temporal stability in the distribution of iodine species. In addition, these two papers both use radiotracers as a powerful tool to probe iodine transformations. By incubating seawater spiked with 129 I, Sţreangăet al. find evidence for an intermediate iodine pool that is rapidly converted to iodate, and Schnur et al. are able to place an upper limit of <2.99 nM day -1 on the iodide Frontiers in Marine Science frontiersin.org 01