Atmospheric Inputs of Iron to the Ocean Surface

Iron is an essential component of many enzyme systems in marine phytoplankton, but the solubility of iron in seawater is extremely low. This can make it difficult for phytoplankton to acquire sufficient iron and in some ocean areas it is believed that lack of iron limits phytoplankton growth, although other nutrients are in plentiful supply. Experiments in the equatorial Pacific (IRONEX I & II) and Southern Oceans (SOIREE) have shown that deliberate release of iron into these areas can lead to increased phytoplankton productivity.

In comparison to the oceans, the continents are extremely iron-rich. Large quantities of iron are washed into the oceans from rivers, but because of its low solubility, almost all of this iron is precipitated very close to the coast and is not available to phytoplankton. Dust in the atmosphere, on the other hand, can be transported very great distances before it finally falls back to the Earth's surface and can therefore supply iron directly to remote ocean areas. The image on the left, from the SeaWifs satellite, shows a huge dust cloud being carried from the Saharan region of north Africa, out over the Atlantic Ocean in February 2000. Images of other spectacular dust storms can be found on the NASA Earth Observatory website.

Some of the questions we are working on include:

HOW MUCH OF THE IRON IN ATMOSPHERIC DUST IS ACTUALLY SOLUBLE IN SEAWATER? Iron makes up a small fraction of Saharan dust (5% or less) and only a very small fraction of this iron dissolves when the dust enters seawater (perhaps as little as 0.1% in some cases) . Most of the iron in the dust therefore falls into deep water without any benefit to the phytoplankton community.

WHAT EFFECT DOES THE ATMOSPHERE HAVE ON DUST PARTICLES BEFORE THEY REACH THE SEA? Dust particles may pass through clouds several times before they land on the ocean surface. Clouds are very acidic and the acid may alter the chemical properties of the dust particles and enhance the solubility of the iron contained within them.

HOW DOES THE PHYSICAL MEANS BY WHICH DUST ENTERS THE SEA AFFECT THE AMOUNT OF IRON THAT IS SOLUBLE? Deposition in rain is much more efficient than dry deposition of dust. Rainwater is also slightly acidic so this can increase iron solubility. There may also be natural iron-binding ligands present in rainwater which can further enhance iron solubility.

HOW DO ATMOSPHERIC INPUTS OF IRON RELATE TO THOSE OF OTHER NUTRIENTS, AND WHAT EFFECT MIGHT THESE COMBINED INPUTS HAVE ON MARINE PHYTOPLANKTON? Mineral dust is well known as an important source of iron, but other nutrients (especially fixed nitrogen) are also present in the atmosphere. The balance between inputs of different nutrients may have various impacts on different phytoplankton types. Human activity has dramatically increased the amount of fixed nitrogen in the atmosphere, and this continues to increase the input of nitrogen to the oceans.

We have been collecting samples for our work along long transect cruises of the Atlantic Ocean, as well as during several localised studies off the coast of West Africa. These cruises allow us to sample the dust-laden air moving off the Sahara, air impacted by European and North American urban / industrial emissions and much cleaner air in the open South Atlantic.