Evidence of Natural Oceanic Iron Sources

Much of our belief that iron-fertilisation will promote cooling is the geological record, while ice-core analysis indicates low levels of atmospheric carbon dioxide correlate to ice ages.
The hypothesis is that increased volumes of phytoplankton caused this reduction in atmospheric CO₂, while contemporary observations support the theory that natural iron fertilisation produces phytoplankton blooms.

Contemporary Natural Oceanic Iron Sources

Over the past 2 decades, a number of sources and delivery methods of natural iron fertilisation of the oceans have been detected, or suspected.

Iron-Rich Dust - the Asian Dust Storm 2001

In April 2001, two deep-diving "Carbon Explorer Floats" were launched into the North Pacific Ocean, with intent to measure particulate carbon at various depths in the upper 1000m of the ocean.
Serendipitously, a large dust storm originating near the Gobi Desert in China and Mongolia was heading over Japan at the time of launch, and 2 days later, deposited a huge amount of dust into the sea. The Carbon Explorers continued to record data at various ocean levels during this time, and was able to record a natural increase in particulate carbon, while a NASA SeaWiF satellite recorded increases in chlorophyll, from across the Pacific to the Gulf of California.

http://visibleearth.nasa.gov

This natural event produced similar results to the Soiree and SOFeX experiments, which themselves had been designed to mimic natural dust-fertilisation.[1]

A team from New Zealand's Otago University are currently studying whether Australian outback dust causes fertilisation of the Southern Oceans.[2]

Volcanic fallout - Mount Pinatubo, 1991

A decade earlier, in 1991, the eruption of Mount Pinatubo was followed by a slowing-down in the increase of atmospheric carbon dioxide. (Atmospheric CO₂ has been steadily increasing since the Industrial Revolution as we burn more and more fossil fuels. The CO₂ level continued to increase in 1991, but at a slower rate than in the years preceding and following).
Although not proven, one theory suggests that Pinatubo, which erupted around 500 million tonnes of iron, caused rapid fertilisation of the oceans, and consequent phytoplankton blooms. [3]

Upwelling Iron

Despite encouraging geological and experimental results, alternative theories have been suggested which account for the fact that most of the phytoplankton blooms seen so far have been too short-term to have any significant climatic effect.

Upwelling Terrigenous Iron

The "Upwelling Iron Hypothesis" (Gabriel Filippelli and Jenifer Latimer [4]) claims that erosion of continents during times of low sea levels resulted in large amounts of terrigenous material being washed by rivers into the sea, and sinking directly to the deep ocean floor. From there, the intensified circulation of the time promoted upwelling, and brought the material to the surface.

Another source of upwelling terrigenous iron may be dust deposited in the oceans of the Northern Hemisphere from the Saharan Desert, for example. This can sink, been transported across deep oceans to the Southern Ocean, where it upwells.

Upwelling Magmatic Iron

Furthermore; some of the sediment cores analysed from glacial times indicate a mid-ocean ridge (i.e., magmatic) geochemical signature, rather than one of surface dust.
Upwelled iron may have a much longer residence time that dust or artificial additions of iron, which would explain the short-term success of experiments.

Recent research in New Zealand supports this proposal. Prior to 1999, it had been believed that mineral-rich under sea volcanic emissions were produced only by the deep 'spreading ridges', over 2km below the surface, and too deep for exploitation. Recent research has however found a source of mineral venting from volcanic arcs, only 220 meters below the surface. This is commercially exploitable - and a probable source of iron to surface waters.[5]

Upwelling Iron today

This situation may exist in the HNLC area around the Galapagos Islands of the eastern equatorial Pacific today.

From NASA's "Ocean Color Data and Resources" site:
http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/ OCDST/classic_scenes/03_classics_galapagos.html

Here, volcanic minerals supply the iron, but not as ash blown off the islands. Instead, the volcanic rocks beneath the surface and the deposited iron in the sediments around the islands, appear to be the main source of the iron that nourishes the productivity in the Galapagos plume.

Iron sources

The contemporary sources of iron-rich dust that has, or may, fertilise the oceans thus include:

• air-borne terrigenous iron as dust from the land,
• air-borne magmatic iron from volcanic eruptions,
• upwelling terrigenous iron from river-deposits and/or distant subducted dust, transported along the ocean floor,
• upwelling magmatic iron from ocean ridges and submarine volcanic arcs.

Which of these, or what combination, led to the fertilisation of the ice-age oceans, is not clear; so whether artificial iron fertilisation from the sea surface will be able to mimic this natural process, is similarly uncertain.

References:

[1] Asian dust storm causes plankton to bloom in the North Pacific Paul Preuss
http://www.lbl.gov/Science-Articles/Archive/
ESD-Gobi-plankton-Bishop.html
[2] Dust Down Under excites scientists John Gibb
http://www.odt.co.nz/cgi-bin/ getitem?date=25Oct2002&object=MOG20Q0742NG&type=html
also:
"Outback storms and ocean life - is there a link?" Royal Society of New Zealand, Press Release. September 2002.
(download .pdf from within "Marsden Fund Grants" (Sept 2002) near page end) http://www.rsnz.govt.nz/news/releases/
[3] Increase of carbon dioxide concentrations slowed during 1992
http://archive.greenpeace.org/.. climate/database/records/zgpz0369.html
[4] Iron from below, not above Lisa M. Pinsker GeoTimes, March 2002
http://www.agiweb.org/geotimes/mar02/NN_iron.html
[5] "New Zealand Scientists Revealing more of the Oceans' Secrets" May 17, 2002 New Zealand Press association (via Lexis Nexis)