thesis in the field of Chemical Oceanography: Marine iodine biogeochemistry: inorganic speciation, redox dynamics and organic complexation

Abstract

Iodine holds significant importance across various disciplines, including medicine, industrial processes, organic synthesis, paleoclimatology, atmospheric chemistry and modern climate science. The ocean, as a major surficial iodine reservoir and the primary source of this element to the atmosphere, plays a central role in global iodine cycling. Despite significant progress, key aspects of iodine cycling in the marine environment remain poorly understood. This thesis leverages recent advances in high-precision techniques, including liquid chromatography and mass spectrometry, to enhance our understanding of marine iodine biogeochemistry. Detailed analyses of the major inorganic iodine species in seawater, iodide and iodate, were conducted in the oligotrophic waters of the North Pacific and the oxygen minimum zones of the Eastern Tropical Pacific. The observed distributions reflect the impact of both in situ and ex situ processes on dissolved iodine concentrations, offering valuable insights into the prevalence and extent of anoxic conditions within oxygen minimum zones. Iodate formation rates were investigated through surface seawater incubations using iodide-129, a long-lived radioisotope, as a tracer. The experimental results underscore the pivotal role of particles in mediating redox transformations between iodide and iodate, while also emphasizing the significance of iodine species with intermediate oxidation states in these processes. Building on this observation, a significant focus of this thesis is the characterization of dissolved organic iodine in the ocean. Two innovative methodologies for identifying dissolved organic iodine compounds are presented. The first approach focuses on labelling cultures of the cyanobacterium Synechococcus with iodide-129 to generate a diagnostic isotopic pattern in resultant dissolved organic iodine complexes. The second approach employs sequential purification and isolation of a target compound from a large-volume seawater sample collected in the North Pacific. Collectively, the findings presented in this thesis significantly enhance our understanding of iodine cycling in the marine environment, offering novel insights into the distribution and composition of both inorganic and organic iodine, as well as the rates and dependencies governing iodine cycling processes. Furthermore, the methodologies introduced here pave the way for future research to elucidate the mechanisms driving iodine redox transformations in seawater, refine the marine distribution of inorganic iodine, and advance the molecular characterization of dissolved organic iodine.