Evidence - Natural Iron Fertilisation of the Oceans

In the geological record (ice-core analysis), low levels of atmospheric carbon dioxide correlate to ice ages, while contemporary observations show natural iron fertilisation produces phytoplankton blooms. These may be linked.

Natural Dust Events

In April 2001, two deep-diving "Carbon Explorer Floats" were launched into the North Pacific Ocean to measure particulate carbon at various depths.

2 days later, a large dust storm originating in the Gobi Desert dumped tonnes of dust into the sea. Subsequently, significant increases in 'particulate carbon' were recorded, while satellites recorded an almost-doubling of chlorophyll. [1]

This natural event produced results similar to those of the Soiree and SOFeX experiments, which themselves had been designed to mimic natural dust-fertilisation. [2]

A decade earlier (1991), the Mount Pinatubo eruption was followed by a slowing-down of Earth's (post-Industrial-Revolution, continual) atmospheric carbon dioxide increase. One (unproven) theory suggests Pinatubo’s ejecta, which contained millions of tonnes of iron, produced rapid ocean fertilisation, subsequent phytoplankton blooms, and consequent slow-down of atmospheric carbon-dioxide–increase. [3]

A New Zealand team are currently (2002-2005) studying whether Australian outback dust causes fertilisation of the Southern Oceans. [4]

The Upwelling Hypothesis

Whilst dust storms are typically credited for the climate-changing iron-enrichment in the geological record, some contend that consequent phytoplankton blooms would be too short-lived to have a significant effect.

The "Upwelling Iron Hypothesis" credits rivers instead, which carry large amounts of eroded material directly to the deep ocean floor. Alternatively, natural dust deposited into oceans in the Northern Hemisphere (e.g. from the Sahara Desert) may sink and be transported to the Southern Ocean by deep-sea currents. Vigorous ocean-circulation causes upwelling, which brings it to the surface.

A similar situation possibly exists today in the HNLC area of the eastern equatorial Pacific.

Furthermore; some sediment cores from glacial times indicate the geochemical signature of a mid-ocean ridge [5], while recent exploration in New Zealand has found mineral venting from volcanic arcs, just 220 metres below the surface. This is commercially exploitable, and is a probable, continual, source of iron to surface waters. [6]

Not only may upwelling iron be a more consistent source than dust-storms, but it may have a longer residence time than surface-sources (dust or experimental). This would explain the short-term success of experiments to date.

Natural sources of ocean iron

• air-borne terrigenous iron as dust
• air-borne magmatic iron from volcanic eruptions
• upwelling terrigenous iron from river-deposits or distant dust-storms
• upwelling magmatic iron from ocean ridges and submarine volcanic arcs.

SeaWiFS image acquired on April 18, 2001, showing the continuing development of the highly reflective phytoplankton bloom in the Gulf of California

References:

[1] "Asian dust storm causes plankton to bloom in the North Pacific" by Paul Preuss. Berkeley Lab Research News (press release). 24th October 2002

http://www.lbl.gov/Science-Articles/Archive/ESD-Gobi-plankton-Bishop.html

[2] Experimental results, within this site

Experimental Ocean Iron Fertilisation Experiments.htm

[3] "Increase of carbon dioxide concentrations slowed during 1992". Greenpeace Climate Impact Database. Based on a unlocated reference dated Autumn 1993.

http://archive.greenpeace.org/climate/database/records/zgpz0369.html

[4] "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/

[5] "Iron from below, not above" Lisa M. Pinsker. GeoTimes, March 2002

http://www.agiweb.org/geotimes/mar02/NN_iron.html

[6] "New Zealand Scientists Revealing more of the Oceans' Secrets" May 17, 2002 New Zealand Press association (via Lexis Nexis)

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