[OANNES Foro] 30 years of the iron hypothesis of ice ages

[Mario Cabrejos]

In 1990, an oceanographer who had never worked on climate science proposed
that ice-age cooling has been amplified by increased concentrations of iron
in the sea - and instigated an explosion of research. 

30 years of the iron hypothesis of ice ages

 <javascript:;> Heather Stoll  

Nature 578, 370-371 (2020)

doi: 10.1038/d41586-020-00393-x

17 February 2020

https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefin
g&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_c
9dfd39373-bfd1efd8bc-42095803

Thirty years ago this month, John Martin proposed a solution to one of the
biggest mysteries of Earth's climate system: how was nearly one-third of the
carbon dioxide in the atmosphere (about 200 gigatonnes of carbon) drawn into
the ocean as the planet entered the most recent ice age, then stored for
tens of thousands of years, and released again as the ice sheets melted?
These large natural cycles in atmospheric CO2 levels (Fig. 1a) were revealed
in 1987 by an analysis of ancient air bubbles trapped in the first long ice
cores taken from the Antarctic ice sheet
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR1> 1. Martin recognized that iron was a
key ingredient that could have transformed the surface ocean during glacial
times. His landmark iron hypothesis
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR2> 2,
<https://doi.org/10.1029/PA005i001p00001> published in Paleoceanography,
described a feedback mechanism linking climatic changes to iron supply,
ocean fertility and carbon storage in the deep ocean.



Figure 1 | The anti-correlated data that inspired the iron hypothesis. a,
Measurements of air bubbles trapped in cores drilled from the Antarctic ice
sheet show that atmospheric levels of carbon dioxide were significantly
lower during the coldest periods (shaded regions) than during modern times
(data from ref. 16; CO2 concentrations are shown in parts per million by
volume; p.p.m.v.). b, The ice-core records also reveal that more iron was
transported to the Southern Ocean in wind-blown dust during the coldest
periods than during warmer times (data from ref. 17; iron flux is measured
in micrograms per square metre per year). In 1990, Martin
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR2> 2 hypothesized that the increased
levels of iron in the Southern Ocean during the coldest periods fertilized
the growth of photosynthetic microorganisms in the surface Southern Ocean,
which therefore produced more biomass from CO2. This, in turn, would have
increased the strength of the biological pump, a mechanism that sequesters
some of the biomass (and the carbon within it) in the deep ocean. Martin
proposed that the stronger biological pump explains why so much atmospheric
CO2 is drawn into the ocean during cold times.

Two hundred gigatonnes is a lot of carbon to periodically withdraw from and
release to the atmosphere. In the 1980s, a handful of models (see ref. 3,
for example) had shown that an increase in biomass production in polar ocean
regions was the most effective process for removing so much atmospheric
carbon. Photosynthetic organisms in the surface ocean convert CO2 from the
atmosphere into biomass, much of which is subsequently broken down into CO2
again by other organisms and returned to the atmosphere. But part of the
biomass sinks into the deep ocean, which therefore effectively serves as a
large storage reservoir of dissolved CO2. This mechanism of CO2 removal is
called the biological pump.

However, biomass production requires not only CO2, but also other nutrients
to build lipids, proteins and enzymes. Researchers were struggling to
ascertain how the ocean's abundance of key nutrients, such as nitrates or
phosphates, might have increased during glacial times to fuel a stronger
biological pump.

Martin argued that iron is another nutrient that limits the biological pump.
He suggested that the modern marine ecosystem of the Southern Ocean around
Antarctica is starved of iron, and therefore relatively low in biomass,
despite having abundant nitrates and phosphates. But during glacial times,
strong winds over cold, sparsely vegetated continents could have transported
large amounts of iron-bearing dust into this ocean (Fig. 1b). Martin
reasoned that this dust could have fertilized marine ecosystems and
strengthened the biological pump, so that more carbon was transferred into
the deep ocean, lowering atmospheric CO2 levels.

Around the time of publication, evidence for high dust delivery during
glacial periods had just emerged from studies of deep Antarctic ice cores
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR4> 4. But there were no reliable
measurements of dissolved iron in the Southern Ocean that could confirm that
its surface waters are iron-starved in modern times, or data supporting the
proposal that delivery of iron-rich dust would make a difference to ocean
productivity. It was clear, however, that large patches of the world's ocean
had much lower quantities of biomass than would be expected on the basis of
the concentrations of key nutrients such as nitrates and phosphates. But
many researchers argued that this was due to natural overgrazing of algae by
herbivores
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR5> 5.

The idea that modern algal growth is limited by iron availability had, in
fact, been proposed
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR6> 6 in the 1930s, but had been
incorrectly discounted by oceanographers - who had measured plenty of iron
in seawater samples collected from the waters around their iron ships
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR7> 7. Martin was one of the first
oceanographers to implement painstaking procedures to avoid the
contamination of samples and to determine that iron concentrations in the
north Pacific Ocean were extremely low
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR7> 7, certainly low enough to curtail
biomass production.

Despite the initial scepticism that greeted the iron hypothesis, 12 separate
experiments
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR8> 8 were carried out between 1993 and
2005 in which around 300-3,000 kilograms of dissolved iron were injected
into small patches of the Southern Ocean, the equatorial Pacific Ocean and
the north Pacific. The biomass of algae increased wherever iron was added,
as biological production surged.

Unfortunately, Martin died mere months before the first of these
experiments, and did not witness the ocean-scale confirmation of his
hypothesis, nor the internationally coordinated campaign to measure iron
geochemistry throughout the world's oceans
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR9> 9 - which confirmed iron limitation
and revealed the intricate strategies used by marine ecosystems to acquire
and recycle iron
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR10> 10.

Earth scientists also tried to test the iron hypothesis computationally
using simple ocean models. They used the changes in the dust-accumulation
rate recorded in ice cores as input to simulate changes in iron delivery to
the Southern Ocean, and data from the experimental iron fertilizations to
calculate how this iron could affect algal growth and the biological pump.
Such models could reproduce the timing and magnitude of about half of the
observed decrease in atmospheric CO2 levels during glacial periods
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR11> 11. Iron fertilization is therefore
clearly an important process that causes atmospheric changes, but might not
be the only one.

Finding data to prove that biological production had been higher during
glacials was a harder task - after all, the ecosystem during the most recent
glacial period (about 20,000 years ago) is long dead. One possible solution
was to extract cores from sediments piled on the sea floor, to see whether
the mineral skeletons of algae accumulated faster during glacial times than
in the modern era. However, the results were often ambiguous
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR12> 12, for several reasons: many algae
don't produce a preservable skeleton; numerous factors determine what
proportion of biological remains is preserved on the sea floor; and the
location of biological production changes through time as ocean fronts and
sea-ice positions migrate.

Fortunately, Martin
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR2> 2 and others
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR13> 13 had anticipated an alternative,
global-scale test of the biological pump during glacial times. If more
biomass reached the deep ocean during glacials, then deep-sea microorganisms
would use up more oxygen as they consumed it, decreasing the concentration
of oxygen in deep waters. Evidence of deep-ocean oxygen depletion would
therefore be indicative of a strong biological pump.

Martin recognized that the presence of certain microfossils in glacial-age
sediments meant that the deep ocean had not become completely devoid of
oxygen during glacials. But although this evidence crudely constrained
estimates of the degree to which iron fertilization might have enhanced
productivity during glacials, it could not be used to determine whether
levels of deep-ocean oxygen were lower than during modern times. Since then,
analysis of more-sensitive geochemical records indicates that the oxygen
concentration in bottom waters did decrease during glacial times
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR14> 14. This provides the strongest
confirmation yet of the large-scale accumulation of carbon in the deep ocean
during glacial periods owing to a stronger biological pump.

Slower rates of mixing between the deep and shallow oceans could also have
enhanced the biological pump during glacials. The latest generation of
climate models in which the ocean and atmosphere are coupled can test the
contribution of the multiple processes that could have resulted in a
reduction in bottom-water oxygen levels. Such models indicate that mixing
rates can account for only half of the observed deep-ocean storage of CO2
during the glacial period, and that iron fertilization of the Southern Ocean
is the major cause of the extra CO2 storage observed
<https://www.nature.com/articles/d41586-020-00393-x?utm_source=Nature+Briefi
ng&utm_campaign=bfd1efd8bc-briefing-dy-20200218&utm_medium=email&utm_term=0_
c9dfd39373-bfd1efd8bc-42095803#ref-CR15> 15.

Martin concluded his paper by saying that iron availability "appears to have
been a player" in strengthening the biological pump during glacial cycles,
but that the size of its role remained to be determined. Thirty years later,
the evidence convincingly shows that iron fertilization of the Southern
Ocean was indeed a leading actor in this global-climate feedback.

 

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