US GEOTRACES GP17-OCE: Mass balance constraints on in situ and ex situ drivers of open ocean iodine cycling and paleo proxy applications

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The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. This project would add a study of iodine cycling to a U.S. GEOTRACES expedition to the South Pacific and Southern Ocean. Iodine geochemistry is widely applied as a tracer of oxygen availability and carbon cycling in ancient seawater and is a key regulator of tropospheric ozone. However, fundamental constraints on the role of local biogeochemical versus regional physical mixing processes in governing large-scale distribution of iodine (in the form of iodide and iodate species) in the ocean are lacking. This study would be the first investigation aimed at constraining the relative impact of these important processes on iodine across an ocean basin. The project would support graduate and undergraduate students. The lead investigator and the graduate student would participate in science communication activities to Greater Lansing K-12 students through the Michigan State University Museum.This project proposes to utilize the GEOTRACES GP17-OCE meridional Pacific transect from Tahiti to Chile to conduct the first basin-scale mass balance of iodine, quantifying biogeochemical and physical mixing processes governing iodine cycling evolution across key gradients. Specific objectives include: (A) determine the relative contributions of vertical mixing and productivity and nutrient variations toward maintaining euphotic, meridional iodine speciation gradients; (B) budget the roles of water mass mixing and iodine redox transformations during the downwelling and circulation of high latitude near-surface water masses, most prominently the Antarctic Intermediate Water. The two proposed mass balance studies include iodine speciation measurements from vertical profiles along targeted intervals of the transect and computational constraints on water mass abundances via an Optimum Multi-Parameter Analysis. Importantly, the project is uniquely poised for success because the GP17-OCE transect captures gradients in and already prioritizes tracers for vertical/horizontal mixing and water mass age. Together, these parameters will place first order constraints on the role of primary production in driving iodine redox transformations in oxic, euphotic waters and quantify rates of change across the proposed basin-scale spatiotemporal gradients associated with biogeochemical and physical mixing mechanisms.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.