Award: OCE-1235248

This project was part of a larger study investigating ocean chemistry that extended far across the Southern Pacific Ocean, from the coast of Peru to the islands of Tahiti. The remoteness of the area meant that this region had not been well studied previously but our investigation, part of the international GEOTRACES program was able to provide new insights into ocean chemistry as a whole. Our emphasis, in particular, was on trace chemical compositions in the oceans – these are defined as chemicals that are only present in very low concentrations (less than one part in a million) in seawater. While the entire survey revealed many processes, a particular highlight from the Western half of the section (the entire sampling cruise in Fall 2013 lasted nearly 2 months at sea) was that hydrothermal venting (underwater hot springs) located along an underwater volcanic chain called the East Pacific Rise, spewed chemicals upward into the overlying water column which we could trace, westward, for more than 4000km. All across the Pacific, water samples were collected at 34 different depths at 36 different stations and each sample was filtered so that both dissolved and particulate trace metal concentrations could be determined. In our project, we focussed upon the concentrations of trace elements in the particulate material found in the deep part of the ocean (deeper than 1000km) and this included the long-range hydrothermal plume. Other colleagues involved in the project showed, while we were out at sea, that dissolved iron and manganese (two metals that are most enriched in hydrothermal fluids where they exit the seafloor) could be traced all across the section as far west as Tahiti. What our project showed, which was even more of a surprise, was that particles rich in these same metals also persist in the water column all the way across the section with iron only sinking slowly and manganese apparently not sinking at all. Previously, it had been expected that iron in particular would precipitate to form mineral particles that would sink rapidly to the seabed to generate halos of metal rich sediments closer to the vent sites. While that process does happen, too, what our new results showed is that an important proportion of the iron released from hydrothermal vents can be bound up with organic matter in the overlying plumes. Then, apparently because of the reduced density of these iron-organic complexes, the iron can avoid deposition and be transported long distances into the ocean interior. In turn, this opens up the possibility that this iron may persist in the ocean long enough to well up into the surface ocean, particularly toward Antarctica in the Southern Ocean. There, iron – which acts as an essential micro-nutrient for life – is in short supply and acts as the limiting ingredient for biological productivity. Our research has helped pose a new hypothesis, therefore, that hydrothermally sourced iron released from hot springs on the deep ocean floor may help drive a significant proportion of the biological productivity (and associated uptake of carbon dioxide from the overlying atmosphere) across much of the surface of the Southern Ocean. As well as being significant for our own planet, awareness of these new processes that we had not previously been aware of is also helping shape planning for investigating the geochemical cycles (and perhaps also the functioning of life?) on other newly-revealed ocean worlds in our outer solar system, including Jupiter?s moon Europa and Saturn?s moon Enceladus where it is argued that evidence for seafloor hydrothermal venting has also now been found. Our project provided important training and research opportunities for an early career female scientist and our exciting first results have been published in the high impact journal Nature Geosciences. Last Modified: 03/27/2017 Submitted by: Chris German