A study of the chemistry of deep-sea hydrothermal vents involving researchers at the National Oceanography Centre, Southampton reveals how scorching hot vent fluids become depleted in isotopically light iron as they rise through the water column and mix with much colder seawater. Such studies will potentially help reveal the relative importance of different sources of iron in the deep ocean.
Iron is an essential micronutrient for phytoplankton, the microscopic marine algae that dominate biological production in the sunlit surface waters. A major aim of oceanography - the science of the oceans - is therefore to understand and quantify sources of dissolved iron in the oceans, and how this iron is utilised and cycled within the marine system. A particular challenge is to identify sources of iron in the deep ocean, which are as yet relatively poorly characterized.
Iron isotopes could be a powerful tool for this purpose, allowing scientists to estimate the relative importance of different iron sources and their contribution to the dissolved iron budget. However, before this can be done, it is important to understand just how iron isotopes are fractionated as iron enters the ocean.
Deep-sea hydrothermal vents are one important source of iron in the deep ocean. A team including Dr Sarah Bennett, Professor Peter Statham and Professor Christopher German of NOCS have investigated the iron inputs from a deep-sea basalt-hosted hydrothermal system and the processes by which the iron isotopes are fractionated as the chemical cocktail in hot (~ 400 degrees Celsius), acidic vent fluids mixes with cold, oxygen-rich seawater.
Their study site was an area of recent volcanic eruptions on the Mid-Atlantic Ridge - the chain of mountains that runs down the middle of the Atlantic Ocean. Previous investigations by remotely operated vehicle (ROV) had revealed three separate hydrothermal vent systems, named Turtle Pits, Red Lion and (rather sinisterly) Comfortless Cove.
"We are really excited by the research," said Professor Statham: "We are among the first to measure iron isotopes in deep-sea vents directly, and we are really learning a lot about the chemistry of the vents and their importance as sources of deep-ocean iron. Also, temperature measurements at the Turtle Pits hydrothermal vent system yielded one of the highest vent fluid temperatures yet recorded from the deep ocean - 407 degrees Celsius."
The new research was conducted during research cruises aboard the RRS Charles Darwin and FS Meteor, during which the researchers used sampling equipment mounted on the ROV Quest to collect vent fluids directly from hydrothermal chimneys as well as from a buoyant plume as it rose up through the water column.
The researchers found that 25% of the iron in particles within the plume was precipitated as insoluble iron sulphides, and that the iron isotopes were fractionated during this process, with the iron sulphide fraction being slightly enriched with isotopically light iron. At least some of the insoluble iron sulphide precipitates are lost from the vent plume by preferential settling as it rises and mixes with colder seawater, leading to the removal of isotopically light iron. The net effect of this and other fractionation processes involving hydroxide formation is to leave the remaining dissolved Fe slightly isotopically heavier than the original vent fluid.
"This is important," said Professor Statham, "as this heavier isotopic signature could help us and other scientists to trace hydrothermally sourced dissolved iron throughout the deep ocean."
The work was supported by the Natural Environment Research Council, WHOI Deep Ocean Exploration Institute, the Frank and Lisina Hoch Endowed Fund, NSF-OCE, the Kristin Bruhn Foundation and the Challenger Society for Marine Science.
The researchers are: Sarah Bennett (NOCS; now at the University of Southern California), Olivier Rouxel (Woods Hole Oceanographic Institution, WHOI), Katja Schmidt (Jacobs University Bremen), Dieter Garbe-Schönberg (University of Kiel), Peter Statham (NOCS) and Christopher German (NOCS; now at WHOI).
Reference: Bennett, S. A. et al. Iron isotope fractionation in a buoyant hydrothermal plume, 5°S Mid-Atlantic Ridge. Geochimica et Cosmochimica Acta 73, 5619-5634 (2009).