Iceberg iron could soften impact of climate change
18 December 2008
Tiny particles of iron released by melting icebergs could reduce the effects of human greenhouse gas emissions, according to new research.
A paper published in Geochemical Transactions describes how scientists discovered nano-sized particles of iron in samples taken from icebergs in the Southern Ocean, using a high-resolution electron microscope.
These iron particles - just a few nanometres wide - are in a 'bio-available' form - around five to ten per cent of their mass can potentially be absorbed and used by living things. When the icebergs melt they will release their iron content into the ocean, where it could act as a fertiliser contributing to plankton growth, and so causing the plankton to absorb more carbon dioxide.
'Most of the sediment carried in icebergs is pretty uninteresting and inert, just crushed rock,' says Professor Rob Raiswell, a geochemist at the University of Leeds and the paper's lead author. 'But high-resolution microscopy showed the presence of iron nanoparticles, and this could have a significant impact on estimates of the supply of bio-available iron in the Southern Ocean,' he adds.
Lack of iron is the limiting factor for growth in these plankton - 'At the moment, if you increase the amount of iron available, you get more productivity', explains Raiswell.
Scientists had assumed that all the bio-available iron present in the Southern Ocean came from dust carried on the wind. But this newly-discovered source may provide around as much iron again as dust does - and the supply may be set to increase in the coming years.
Climate change is causing more icebergs to split off glaciers and move out into the open ocean. If Raiswell is right, this will mean more nutrients become available to marine microorganisms in the Southern Ocean, leading to more production and more absorption of carbon.
The net increase in long-term CO2 absorption could be equivalent to a few per cent, maybe as much as ten per cent, of annual human emissions. This estimate takes into account the fact that most of the CO2 absorbed by plankton quickly makes its way back to the atmosphere, with only around ten per cent sinking to the seabed to be trapped for long periods.
The process won't be a panacea for climate change, Raiswell acknowledges, but he says that 'it may buy us a little bit more time - it may slow things down.'
Oceanographers think that the Southern Ocean's total capacity to absorb carbon dioxide if iron was not a limiting factor could be around 100 petagrammes per year - about 25 per cent of total human emissions to date.
Raiswell carefully hacked the samples from sediment-bearing layers inside icebergs, using specially-cleaned equipment to avoid the risk of contamination. He took the samples while on a research cruise on HMS Endurance, the Royal Navy's icebreaker. Other samples came from icebergs that had grounded near Seymour Island in the Southern Ocean.
Co-author Professor Liane Benning, also at Leeds, found the tiny particles of bio-available iron in the samples using an electron microscope.
Earlier research had suggested that there are hotspots of biological activity around melting icebergs in the area, but this is the first time a likely mechanism has been identified for this phenomenon. 'What's new here is that we've found the active ingredient,' comments Raiswell.
He now plans to return to the Antarctic, visiting the British Antarctic Survey's Rothera base and taking many more samples from icebergs that have grounded on the shoreline nearby.
It's dangerous to get near icebergs at sea, as they can roll over and crush anything beneath them suddenly and without warning. Working with beached icebergs avoids this risk. The research should illuminate one of the key uncertainties of the field at present - just how much sediment the average iceberg contains. Scientists have only a rough idea at present from a few observations in the Arctic and Antarctic.
Raiswell with colleagues from Bristol and Southampton also hopes to use Autosub, the unmanned autonomous submersible developed by the National Oceanography Centre in Southampton, to inspect the area underneath icebergs and monitor the properties of the water around them. And he plans further lab work to shed light on how these iron nanoparticles form and influence the growth of microorganisms.
The research was financed by the Leverhulme Trust and the Natural Environment Research Council.
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