Hydrothermal vents at the bottom of the ocean are the link between the Earth’s interior and the deep-sea. In this study, researchers at the University of Delaware performed field work at three locations on the slow spreading center of the mid-Atlantic Ridge (figure 1) to determine the importance of nanoparticulate pyrite (also known as fool’s gold) as a source of iron to the deep ocean from black smoker hydrothermal vents. Iron is an essential element for the growth of bacteria and phytoplankton, which support the food chain; thus, understanding the flux and transport of iron in the ocean is of critical importance to carbon processes. Pyrite is an iron disulfide, which contains iron and sulfur in the ratio of one atom of iron to two atoms of sulfur (FeS₂), and nanoparticle size is operationally defined from 1 to 200 nanometers (a diameter of 100 nanometers is 1000 times smaller than the diameter of the average hair on our heads). See the left portion of figure 2 for a representative example of a transmission electron micrograph of pyrite nanoparticles. The three mid-Atlantic Ridge locations had total hydrogen sulfide to iron content ranging from 1:20 to 1:1. Up to 5% of the filtered iron from these vents was emitted as nanoparticulate pyrite. This study showed that nanoparticulate pyrite is a widespread component of hydrothermal vent emissions, making these nanoparticles an important source of iron to the world’s oceans. Metals such as copper and zinc were also detected in many of the pyrite nanoparticles. All of these phases were found below the orifice of each of the hydrothermal vent studied and within the first 1.5 meters above their orifices. The data from this study can support the dissolved iron anomaly recently discovered that is 1000 kilometers from and within 1000 meters above the mid-Atlantic Ridge hydrothermal vents.

Other nanoparticulate phases were also identified in this project, and these included nanoparticulate elemental sulfur and iron silicate material, which were found within the first 1.5 meters above the orifice. We documented that the area above the vent orifice is a dynamic reaction zone as the molecular oxygen in the cold bottom waters of the ocean mix with the hot sulfide and iron rich vent waters; the greater than 300 degrees Celsius hot waters cool to less than 20 degrees Celsius in this zone. During this mixing, molecular oxygen reacts with reduced dissolved iron to form oxidized iron, which in turn reacts with the sulfide to reform reduced iron and nanoparticulate elemental sulfur. This is an example of a catalytic cycle with oxygen, iron and hydrogen sulfide. The nanoparticulate pyrite still forms in this reaction zone, and appears unreactive with molecular oxygen as dissolved reduced iron reacts with molecular oxygen within seconds.

This study also had a significant laboratory component, which included synthesizing nanoparticulate pyrite and then reacting the synthesized material with molecular oxygen in seawater at various temperatures to understand its reactivity and possible stability in seawater. The left portion of figure 2 shows representative transmission electron micrographs of nanoparticulate pyrite from the hydrothermal vents and in the right portion of figure 2 from our laboratory syntheses using the reduced starting materials that are detected at the vents.

This work described the detailed chemical kinetics of the reaction of synthesized nanoparticulate pyrite with molecular oxygen in seawater over a variety of temperatures including at the two degrees Celsius temperature of bottom ocean water. The reaction is dependent on the molecular oxygen concentration, which can vary at different hydrothermal vent systems around the world. We found that comp...

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