Freshwater Colloquium: Microscopy helps reveal how nanoparticulate iron from deep sea hydrothermal vents disperses into open ocean waters

Dr. Sarick Matzen is a soil and environmental geochemist specializing in better understanding trace metal cycling in environmental systems. He is an assistant professor in the Department of Earth and Environmental Sciences at the University of Illinois – Chicago. He received a BA in Environmental Science from Hampshire College (Amherst, MA), a Ph.D. in Environmental Science, Policy, and Management at the University of California, Berkeley, and conducted NASA and NSF-funded postdoctoral work at the University of Minnesota. He draws on diverse research experiences, including investigating nuclear waste disposal at Lawrence Livermore National Lab, community-based soil remediation on urban farms in California, cycling of limiting nutrients in Earth’s oceans, and habitability of extraterrestrial ocean worlds, to determine how the chemical form of contaminants and nutrients explains their landscape-scale transport.

Iron (Fe) is a necessary but often limiting nutrient for life on Earth. Supply by sediments and dust deposition are typically considered the main sources of iron to Earth’s oceans, but recent advances suggest hydrothermal vents might also be an important source of bio-essential iron over the length scales of ocean basins. Dissolved iron concentrations in hydrothermal fluids are a million times those of surrounding ocean water. Most iron (>90%) precipitates close to vent sources, but the international GEOTRACES program revealed signatures of hydrothermally-derived iron transported across deep ocean basins worldwide. However, it remains unclear how this iron persists in the water column rather than being sequestered into sinking particles. Critical processes constraining the export of hydrothermal iron to open ocean waters occur within the first ~100 km of plume evolution.

To this end, we compare hydrothermal plume particulate matter collected from the first 100 km of plumes in the low-sulfur, high-oxygen Rainbow vent system (Mid Atlantic Ridge) and high-sulfur, low-oxygen Endeavour vent field (Juan de Fuca Ridge, North Pacific). We use a novel multimodal approach combining bulk to nano-scale synchrotron-based methods and electron microscopy. We show that plume chemistry affects the mineral phase of iron, with iron(II) phases persisting longer in the high-sulfur, low-oxygen Endeavour plume. Yet suspended particle morphology–nanoparticulate iron embedded in carbon matrices–is similar in both plumes. We explore carbon matrix chemistry and potential links to vent microbial communities. Co-located iron and carbon within marine particles drives export of iron from hydrothermal vents to open ocean waters and is a signature of nutrient-rich oceanic hydrothermal activity globally.