Iron-containing nanoparticles emitted from deep sea hydrothermal vents are increasingly recognized as an important source of iron in modern Earth oceans, and could be on icy ocean moons as well. In particular, nanoparticles emanating from vents have large transport potentials and can record the (bio-)geochemical conditions at their point of formation, suggesting that iron-bearing particles may have untapped “biosignature” potential to act as tracers of life-conducive conditions on extraterrestrial ocean worlds (e.g. Enceladus). However, our knowledge of particle alteration pathways limits our ability to use particles as geochemical tracers. On Earth, iron speciation has been investigated previously in nanoparticles collected from hydrothermal vents, but the relationship between nanoparticle aggregation and oxidation must be better constrained to inform hydrothermal iron export models. In Enceladus-like conditions nanoparticle aggregation and the oxidation rates of iron containing complexes and minerals are not well understood, but likely influence particle transport away from the vent source and therefore potential for ejection onto ice shell surfaces.<br/><br/>To constrain iron transport from vent to surface waters on Earth and Enceladus, we synthesize pyrite nanoparticles and subject them to a high-throughput screening framework. We identified pyrite as the most common iron particulate phase in Earth hydrothermal vent systems. We investigate the effects of seawater composition, atmospheric oxidation state, and the presence/absence of organic ligands on pyrite nanoparticle aggregation and oxidation rates. We use a suite of analytical techniques (i.e. ferrozine method, laser particle size/dynamic light scattering analyses, and electron microscopy). All experiments are performed in parallel under conditions representative of Earth’s modern ocean <i>and</i> more alkaline ocean worlds (e.g. Enceladus). Preliminary results indicate that the presence of surfactants during hydrothermal synthesis of pyrite particles affects sulfur availability for pyrite formation, leading to excess, easily-oxidized iron. Laboratory results are interpreted in the context of natural samples to help constrain the dynamic processes affecting iron export from hydrothermal vents on Earth. Results may also inform future searches for particles that trace hydrothermal activity on icy ocean moons in the outer solar system.

Constraining effects of ocean chemistry on nanopyrite chemical and morphological evolution in order to predict long-range iron transport

ACS Spring 2022

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