Award: OCE-1756590

Iron is an essential micronutrient for marine phytoplankton, and availability of dissolved iron (DFe) sets a limit on primary production over as much as one third of the surface ocean. The supply of DFe to ocean surface waters thus exerts important controls on the ocean-atmosphere balance of carbon dioxide, hence global climate, and the ocean ecosystem. As such, there is a need to include and accurately represent DFe in numerical models of the Earth system, in order to predict how future environmental changes will impact ocean biology and biogeochemistry. The emission of metal-rich hydrothermal fluids from the volcanically-active mid-ocean ridge (MOR) system is now recognized as an important source of DFe to deep ocean waters, some of which ultimately reaches the ocean surface. Here an important question concerns the chemical and physical conditions that appear to stabilize hydrothermally-derived DFe in the deep ocean, given that DFe tends to be lost from seawater via adsorption onto particles or formation of solid phases. Another important question concerns the significance of low-temperature/diffuse MOR hydrothermal emissions as a source of DFe to the deep ocean, relative to the better studied high-temperature hydrothermal venting. This project aimed to address these questions by collecting seawater samples immediately above sites of both low-temperature/diffuse hydrothermal emissions and high-temperature hydrothermal venting along the southern East Pacific Rise (SEPR), which is known to be an especially hydrothermally active portion of the global MOR system. During a 49-day expedition aboard research vessel Roger Revelle, sites of both low- and high-temperature hydrothermal emissions were identified along the SEPR using the autonomous underwater vehicle Sentry, and then seawater samples were collected from the overlying hydrothermal plumes using a sampling system lowered from the ship. The water-column samples were then processed and analyzed for different chemical forms and size fractions of DFe, as well as other dissolved trace metals and gases that are typically enriched in MOR hydrothermal fluids. Although integration and synthesis of the large chemical and observational data sets produced from the expedition is ongoing, two major results are (1) DFe in the hydrothermal plumes was generally dominated by iron in the colloidal (or nanoparticulate) size fraction (cFe, 0.02-0.2 ?m in size), whereas truly dissolved iron (sFe, <0.02 ?m in size) was only a minor component of the DFe pool (Fig. 1); and (2) dissolved, iron-binding organic compounds (iron-binding ligands) were typically weaker in diffuse venting environments, but had higher concentrations in those settings, relative to emissions from high-temperature vents. Together, these results suggest that both colloidal iron and iron-binding ligands are important players that act to stabilize hydrothermal DFe in the ocean interior. Broader impacts of this project have included the training and mentoring of undergraduate and graduate students, and early-career researchers. In addition, an improved understanding of hydrothermal iron inputs to the deep ocean is expected to advance our understanding of the role of iron in regulating primary production in the surface ocean. Last Modified: 07/30/2023 Submitted by: Peter N Sedwick