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Preservation of Fe(II) by Carbon-Rich Matrices in Hydrothermal Plumes Print
Wednesday, 29 April 2009 00:00

Despite the considerable amount of iron that enters the oceans from the continents and from hydrothermal vents at mid-ocean ridges on the seafloor, there are large regions of the global ocean where iron availability is so low that it limits life. Oceanographers have long explained this anomaly by assuming that the iron in the sea is primarily incorporated as Fe(III) into inorganic minerals that lack both the mobility to circulate over long distance and bioavailability to sea life as an essential nutrient. Now, a collaboration led by researchers from the Woods Hole Oceanographic Institution has reported that the hydrothermal plumes emerging from the vents actually contain iron in both Fe(II) and Fe(III) oxidation states associated with organic material from nearby flora and fauna. The collaboration suggests that the organic matrices prevent oxidation and precipitation of the Fe(II), perhaps increasing both its circulation through the world’s oceans and its bioavailability as a deep-sea nutrient.

Fertilizing the Ocean
with Iron

As important to aquatic life as nitrogen is to terrestrial life, iron is an essential nutrient for the marine food web that sustains the fish in the world’s oceans and, ultimately, many humans as well. It turns out that suitable iron is, in fact, often in short supply because most of the considerable iron that rivers flush and winds blow into the sea and of the comparable amount that emerges from undersea hydrothermal vents—fissures in the Earth’s surface from which geothermically heated water surges—is thought to transform into insoluble minerals like iron oxide (otherwise known as rust) that marine organisms like plankton cannot digest. The result: iron remains a limiting nutrient in large parts of the oceans.

Using a combination of x-ray imaging and spectroscopy (spectromicroscopy) of particles obtained from the plumes of matter that stream from hydrothermal vents, Toner et al. recently discovered that some of the iron in the plumes can bind to carbon-based organic particles. While it is not known how much of this bound iron makes its way around the oceans, the carbon somehow protects the iron from oxidation, so that it is much more easily processed by living organisms. The potential for a “natural iron fertilization mechanism” that could be applied to making the oceans more productive is yet one more example of how much we have to learn about the oceans.

The cycling of iron throughout the oceans has been an area of intense research for the last two decades, including studies of the chemistry and mineralogy of very small particles streaming from hydrothermal vents in the global mid-ocean ridge (MOR) system, a volcanic chain 55,000 to 60,000 km in length that crosses the floor of all the Earth’s major ocean basins. It has long been assumed that insoluble mineral particles, which stay very close to the site of venting, form from iron oxidation and precipitation reactions in hydrothermal plumes. However, the details of iron mineral formation within plumes are poorly understood, in part because plume particles are a heterogeneous mixture of many different materials with exceedingly small particle size. Moreover, recently it has been proposed that some of the iron makes it way into organic particles, where it escapes mineral formation and may enter the open-ocean.

Fluids from hydrothermal vents, such as the Tica vent shown here, contain about one million times more iron than regular ocean water. While it was thought that the iron pumped out immediately forms mineralized particles when it mixes with seawater, it has now been found that some of the iron remains in a form that is bioavailable to organisms in the ocean. (Photo: Olivier Rouxel © Woods Hole Oceanographic Institution)

To understand what happens to iron once it issues from the seafloor from MOR vents, in particular whether some of it is found in organic carbon-rich matrices, collaboration members turned to the ALS to analyze hydrothermal particles captured in sediment traps as the particles rained out from the non-buoyant plume above the Tica vent west of Mexico and south of Panama. They examined the distribution and chemical speciation of iron and carbon within small (less than 10-µm diameter) plume particles from the Tica vent by scanning transmission x-ray microscopy (STXM) and near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy at Beamlines 5.3.2, and 11.0.1 and the mineralogy of the particles by micro-focused x-ray diffraction and x-ray absorption spectroscopy at Beamline 10.3.2.

iron ocean

The research team collected hydrothermal vent particles using sediment traps on the seafloor about 100 m from the Tica vent in the Eastern Pacific Rise. (Photo: Breea Govenar © Woods Hole Oceanographic Institution)

With these maps of the particles on micrometer and nanometer scales, a first for hydrothermal plume systems, they found unexpected forms of iron in association with unanticipated stringy organic materials among plume particles—iron that managed to escape precipitation as an iron mineral by associating with particulate organic matter. By weight, the samples contained an average of 6.7% particulate organic carbon, which likely came from biological debris. Some of the organic carbon was in the form of polymer-like matrices that appear to be composed of lipids, polysaccharides, and proteins.

ocean

Spectromicroscopy of iron and carbon in a Tica vent particle. Top: STXM images recorded at the Fe 2p3/2 (707.6 eV) and the C 1s (300 eV) edges. Bottom: Fe(III), Fe(II), and carbon distribution maps derived from STXM images above and below the relevant absorption edges. Scale bars are 1 µm. The STXM images and the Fe(II, III) maps demonstrate that the Tica vent particles aggregate and that these aggregates are mixtures of Fe(II) and Fe(III); they also indicate that Fe(II) and to a lesser extent Fe(III) are co-located with the background organic matrix.

The findings are important because the organic-bound iron may be more biologically available than iron minerals and has the potential to be transported off the MOR axis as fluffy colloidal particles, although exactly how the Fe(II)-laden carbon particles might interact with the ocean food web is still to be determined. Moreover, that the observed role for organic matter for the source of reactive surfaces in hydrothermal plumes stands in stark contrast to the existing inorganic paradigm for hydrothermal plumes demonstrates how much there is still to learn about iron and carbon in the dynamic and globally widespread marine environment.

iron ocean

Spectromicroscopy of a Tica vent particle showing unexpectedly high organic carbon content. Left: STXM image record at the C 1s edge and the carbon distribution map derived from images above and below the edge. Right: NEXAFS spectrum obtained from the areas outlined in white in the carbon map and reference spectra show that the biopolymer-like matrix observed in the vent particle is composed of labile lipids, polysaccharides, and proteins. Scale bars are 1 µm.Together, the images and spectra indicate that the particulate organic carbon is chemically heterogeneous at the nanometer scale with composition consistent with a mixture of organic compounds.


Research conducted by B.M. Toner (Woods Hole Oceanographic Institution and University of Minnesota); S.C. Fakra and M.A. Marcus (ALS); S.J. Manganini, C.M. Santelli, O. Rouxel, and C.R. German (Woods Hole Oceanographic Institution); J.W. Moffett (University of Southern California); and K.J. Edwards (Woods Hole Oceanographic Institution and University of Southern California).

Research Funding: National Science Foundation, National Aeronautics and Space Administration, and National Research Council. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: B.M. Toner, S.C. Fakra, S.J. Manganini, C.M. Santelli, M.A. Marcus, J.W. Moffett, O. Rouxel, C.R. German, and K.J. Edwards, “Preservation of iron(II) by carbon-rich matrices in a hydrothermal plume,” Nature Geoscience 2, 197 (2009).

ALSNews Vol. 297, April 29, 2009

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