A nano-scale perspective on a novel pathway for preserving hydrothermal ferrous iron

Hydrothermal vent systems are likely important but poorly constrained sources of (bio-essential) trace metals such as iron (Fe) to the global ocean. Key to understanding the eventual fate and biogeochemical impact of hydrothermal iron is insight into early transformation mechanisms that convert dissolved and soluble ferrous Fe(II) into insoluble ferric precipitates that are not bio-available and settle rapidly. Furthermore, interaction with other bio-essential elements, such as phosphorus, further increases the potential impact of hydrothermal iron.

In this seminar, I present recent results from the Royal NIOZ and Utrecht University joined I-NANO project (https://www.nioz.nl/en/research/projects/4543-32) aiming to determine the early transformation mechanisms of hydrothermal sourced Fe from the Rainbow (36°-33°N) hydrothermal vent field at the Mid-Atlantic Ridge. To investigate the chemical and crystallographic speciation of Fe nanoparticle aggregates, we employed a direct sampling approach that bypasses conventional techniques such as filtration and resuspension. Instead, small amounts of plume fluid were immediately drop-cast onto transmission electron microscopy (TEM) grids and plunge-frozen, preserving dissolved compounds and nanocolloids through vitrification. Using an array of microscopic and spectroscopic techniques, combined with machine learning, allowed detailed characterization of the Fe nanocolloids down to the nano-scale and provided insight into their early (trans)formation and bioavailability.

Our findings shed completely new light on the transport and persistence of vent-derived reduced iron phases, highlighting the role of ferric coatings in protecting nano-scale iron sulfides and challenging the previously proposed importance of complexation with organic matter. Overall, we provide new perspectives on the early (trans)formation processes of vent-derived iron, its interaction with other essential elements, and, eventually, its impact on ocean chemistry.