Olivier Rouxel’s team is exploring metal biogeochemical cycles in the oceans to understand past and present mechanisms of metal transfer and their links with biological and geological processes.
Metals in the deep sea environment
Many elements are present in a dissolved form in seawater and their concentrations depend on a complex interplay between biological and geological processes. Year after year, they can accumulate on the ocean bottom where they form marine deposits. Metals and metalloids (a class of elements that includes germanium, antimony and arsenic) occur at remarkably high levels in polymetallic nodules, ferromanganese crusts and in sulfide deposits around hydrothermal vents. The study of these deposits provides information on metal biogeochemical cycles in the present-day ocean and in that of the past. Olivier Rouxel investigates these metallifereous deposits using novel isotopic analytical techniques. After a few years at Woods Hole Oceanographic Institution, he took the International Chair position at Europole Mer Centre of excellence 4 in 2009. He explains: “We have several scientific questions, but the common point of interest in our work is the study of metal transfer and sources in the ocean, using isotopic measurement methods.”
What becomes of iron in the ocean?
The isotopic composition of metals and metalloids in seawater and marine deposits can tell us about their origin, and the processes that led to their deposition. The main element followed by Olivier Rouxel and his team is iron, as it is essential to the development of phytoplankton and many biochemical reactions, and is considered as a regulator of ocean productivity and climate change. Olivier Rouxel is working to identify the sources of oceanic iron: rivers, the atmosphere, marine sediments, etc. In the deep open ocean, hydrothermal vents are also a source of iron. He is exploring the fate of this dissolved iron released from hydrothermal vents and its potential impact on deep-ocean isotopic budgets.“We are also able to trace the fate of iron. We can determine whether it was originally in the mineral form or if, during its journey, for example from the coast to the open ocean, it has been integrated into marine organisms or modified through redox reactions.”
Metallifereous deposits of the seafloor
The researcher and his team are also interested in understanding how metallifereous deposits form. “Polymetallic nodules are often studied for economic purposes, but we still don’t know how they grow, and whether biological organisms are involved in their formation.”
A PhD student has made the first investigation of nickel isotopes to trace nickel sources and enrichment pathways in these deposits. The team is also developing models of element transport, in particular during the transfer of metals from deep-sea hydrothermal vents or from sediments. For example, observations are being made of the changes in hydrothermal particulates in the deep sea due to surface eddies. Such surface events are found to affect the transport around hydrothermal vents, and movement of animal larvae in the deep sea.
The story of the ancient oceans
The Chair’s team is leading a similar study on ancient sedimentary rocks from South Africa, North America and Australia, as preserved rocks of Precambrian age provide information about the ancient oceans. It is generally accepted that when the ocean and atmosphere had low concentrations of oxygen, the formation and preservation of banded iron formations was favored in the open sea. The appearance and disappearance of microorganisms may also be linked to the iron cycle. Indeed, Olivier Rouxel has also found evidence for a rapid change in the oceanic iron cycle since the beginning of the rise of atmospheric oxygen, which started 2.3 billion years ago. The phosphorus cycle also seems to have played a role in a later oxygen increase, as the enrichment of waters in phosphorous - due to global thawing about 700 million years ago - may be responsible for the increase in marine biological productivity that led to oxygen enrichment of the oceans and the rise of animals.
The deep-sea hydrothermal biosphere
Finally, the team works on the bacteria and archebacteria that live near hydrothermal sources and obtain their energy from metabolic reactions with iron or sulfur species such as hydrogen sulfide, and sulfate. A PhD student, financially supported by Europole Mer, is comparing the sulfur and iron isotopic composition of the metabolic products of thermophiles in order to understand the sources and fate of these elements in their environment. “We really appreciated the collaboration with the IUEM and IFREMER microbiology team,” says Olivier Rouxel. “And the spectrometry platform, common to Ifremer and IUEM, is a state-of-the-art infrastructure that was essential in our research.”
Developing new methods
Thanks to this innovative equipment, the team was the first to develop methods for nickel, germanium and antimony isotopic measurements and methods to study very low concentrations of sulfur and iron in aqueous samples. “In addition to available infrastructures, the Europole Mer Chair gave me independence, as the financial support is secured for 3 years and includes the possibility for several PhD and post-doc positions. This helped a great deal in allowing us to perform high quality research.” The Europole Mer Chair was a first step toward Olivier Rouxel’s return to research in France. After three years in this position, he continues his work at the Ifremer center in Brest.