SOIREE/EisenEx OIF paper

To accompany the previous post on ocean iron fertilization I've looked at a case study to evaluate the effectiveness of the technique.  A paper published by Bakker et al. compared the 2nd and 3rd OIF experiments in the Southern Ocean and examined the effects added iron had upon biological carbon uptake in surface waters. 

The location of the two experiment sites are illustrated below:

   


















SOIREE
The SOIREE experiment took place for 13 days in Feburary 1999 in a stable hydrographic setting.  A ship released iron and a sulphur hexafluoride tracer to create a 50km² iron-enriched patch.  This process was repeated a further 3 times on the 3rd, 5th and 7th days with the total amount of iron added totalling 1745kg, raising the total iron concentration in surface waters to between 2.5-4nM (x10 higher than before any iron addition).
Storms occurred on days 1 and 4 which mixed iron to depths of ~60-75m.  Results show a phytoplankton bloom was stimulated, and as a result CO₂ in surface waters reduced from day 4 onwards at a rate of 3.8μatm day⁻¹.  Reduction of DIC occurred evenly above 50m depth, but no change in DIC was recorded below this.
A 'top hat effect' was produced; where CO₂ reduction was most efficient in the centre of the patch, which highlights the importance of selecting a site for future experimentation; focusing on optimum areas of fertilization as the centre of the top hat.

EisenEx
The EisenEx experiment occurred for 22 days in November 2000 in a cyclonic eddy, and overall 2340kg of iron was added to surface waters on days 0, 8 and 16.  The site experienced severe storms on days 5 and 13 and resulted in deep mixing of iron.  Algal growth was limited by heavy cloud cover, and only a small reduction in surface water CO₂ was observed until the day of the first storm.  Iron was shown to have mixed to depths of 76m after 7 days and additionally the size of the patch was stronlgy enlarged.
The figure below illustrates the amount of surface water carbon is being removed:

Surface water CO₂ remained constant for several days after the storm, and a small decrease is shown after 7 days.  Days 8-12 show surface water CO₂ decreasing at a steady rate, and a storm on day 13 removes some of the surface carbon by transporting it to greater depths.  In contrast to the SOIREE experiment, EisenEx surface carbon did not decrease at a steady rate..

Climatic influences were more apparent in the EisenEx expt; with wind speed and atmospheric pressure both affecting the air-sea transfer of CO₂.  This experiment has shown there is a link between the magnitude of the air-sea gradient and the reduction of surface water CO₂ once iron fertilization has ensued, illustrating again the need for careful site selection for OIF experiments.

Total biological DIC reduction
Despite the differences in patch size, mixed layer depth and the air-sea gradient the total removal of DIC by organisms was on a comparable scale.  After 12 days the SOIREE experiment had reduced DIC by 1389 tons, and the EisenEx expt by 1433 tons respectively.

Conclusion
The experiments show differences in the removal of surface water CO₂ as a result of different mixing regimes (intensity and depth), but similarities in the amount of DIC uptaken by biological activity.  This shows the mixing regimes has little or no effect on overall biological uptake - which could also be important in terms of site selection in future experiments.  However, the EisenEx experiment shows as a result of mixing, the area was 4x more efficient at drawing down CO₂ from the atmosphere.
The experiments failed to quantify how much carbon actually left the surface ocean; and furthermore how much of this was actually sequestered.  It was suggested however that more carbon may have been stored longer-term at the EisenEx site due to it being located closer to subduction sites; where surface water sinks and mixes with deeper water, creating a greater potential for atmospheric CO₂ storage and transport from the surface.
The concluding remarks of the paper focus on how hard it is to accurately quantify carbon storage, the efficiancy of carbon removal and DIC uptake from biological activity.  Currently a suitable solution has still not been applied to solve these issues.  The experiments were useful in terms of highlighting the importance of site selection; not only in terms of wind and atmospheric pressure but also the opportunity for transport to lower depths.
I have decided I am undecided as to whether this is a good technique for fixing climate.  Probably not; although obviously it is effective in the short-term.  It has the advantage of using technology already developed but I am doubtful in terms of it's use in long term climate change mitigation and the uncertainty surrounding the quantities of carbon actually being stored.



Bakker, D.C.E. (2005), 'Iron and mixing affect biological carbon uptake in SOIREE and EisenEx, two Southern Ocean iron fertilisation experiments', Deep Sea Reasearch I: Oceanographic Research Papers, 52, 6, 1001-1019.