Climate, krill, iron, and the Southern Ocean

Half a year of humanity's fossil carbon release might be bound in an iron-nourished Southern Ocean each year, but what would the climate effect be?

Humanity causes 36.7 gigatons of carbon release into air each year, from land use, cement-making and fossil fuel burning. But if future krill trawlers went to sea, not empty, but full of iron, half that carbon could be fixed by sealife in the Southern Ocean, where the krill are caught per year. But what would happen to the carbon thus caught? Would it fall to the bottom? Would it anaerobically become methane, or induce nitrous oxide release, both worsening the climate crisis, or would it remain in sediments, halfway solving our greenhouse gas problems?

I understand China hopes to increase it's annual Antarctic krill catch 7-fold, to 2 million tons wet weight[8]. This krill catch would, at 28 micrograms iron per gram of krill dry weight[3], and at 20%dry weight:wet weight[5] contain 11.2 tons of iron.

The projected Chinese krill catch, ten-fold existing catches, would be from wild standing stock of ~0.5 billion tons[2], which might contain 2,800 tons Fe[3,4]. Since 1/4 of the krill's range's Fe[1] is in the krill, there could be a total krill range iron content of 11,200 tons. So taking 11.2 tons/year from this would reduce total Fe in range by ~0.1%.

Let's say those trawlers carried 2 million tons of ferrous sulfate heptahydrate from China's iron works to the Southern Ocean's' krill pastures in otherwise empty holds, and spread it evenly as they caught their krill each year. This would contain, at an Fe:S:O4:H14:O7 ratio of 56:32:64:14:112, about 1/5th Fe, or 0.4 million tons Fe.

With krill containing 1gram Fe for every 355 grams P and at Redfield's ratio of 106C:16N:1P , every ton of Fe ending up in krill would temporarily bind 1 x 355 x 106 = approximately 37,630 tons of carbon. So (37,630 x 0.4 million tons x 1/4 of the range's iron being in krill [1], the year's iron supplied by the fleet could fix ~3.8 billion tons of atmospheric carbon temporarily, in living krill, with an additional 11.3 billion tons of carbon in phytoplankton or dissolved. Summed up, 15 gigatons of carbon per year might be fixed, which is about half the fossil fuel carbon emitted per year.

But would all that iron be taken up by Southern Ocean sealife? The Southern Ocean is 20.3 million square kilometers[6], about 4% of Earth's total surface area of 510 millionsq. km. This ocean's productivity is iron limited; It is a part of the 
1/5th of the world's oceanic area limited by iron. Experimental addition of iron
to the Southern Ocean resulted in vastly increased photosynthesis[7]. I'm still 
pursuing the effects on sealife.

But if half the carbon temporarily fixed became methane and a fifth of that returned as methane to the atmosphere, with 72 times the warming potential effect of CO2 over two decades (1/2  x 1/5 x 72 =7.2), the carbon as methane would increase heat trapped by about seven times. We need to know the fate of that carbon.


1    NicoiS 2010 'Southern Ocean fertilization by...'
2    https://en.wikipedia.org/wiki/Antarctic_krill
3    LocarninaSJP 1995 'Trace element concentrations in Antarctic Krill, Euphausia superba'
     Locarnina reports Fe content of 28 micrograms/gram fresh weight, and 9.94 milligrams P per gram, for an P:Fe ration of 355P:1Fe.
4    Partly derived from an estimate within JenningsS, KaiserH, ReynoldsJD; _Marine Fisheries Biology_, John Wiley & Sons 2009 :34 "..if krill wet weight is 10% carbon(Morrill et al 1988, Ikeda & Kirkwood 1989)..."
5   Approximate average from Table 1, RaymontJEG 1971 'The biochemical composition of Euphausia superba'
6    06/30/15 https://www.cia.gov/library/publications/the-world-factbook/geos/oo.html
7    BarberRT 'SOFeX: Southern Ocean Iron Experiments. An Overview of the Biological Responses.
8    http://usa.chinadaily.com.cn/epaper/2015-03/04/content_19716649.htm