Soluble iron and global climate

The environment that humans inhabit is better described as the Earth System, for a good reason. Every part of our planet, the living and the seemingly inert, from the core to the outermost atmosphere, is and always has been interacting with all the others in one way or another. Earth-logs aims to express that, as does my recently revised and now free book Stepping Stones. The vagaries of the Earth’s climate present good examples, the most obvious being the role of chemistry in the form of atmospheric greenhouse gases, especially carbon dioxide, and their interaction with other parts of the Earth System.

Carbon and oxygen atoms that make up CO2 are also present in dissolved form in rain, freshwater and the oceans as the dissolved gas itself, carbonic acid (H2CO­3­) and the soluble bicarbonate ion HCO3, in proportions that depend on water temperature and acidity (pH). Those forms make the oceans an extremely large ‘sink’ for carbon; i.e. CO2 in dissolved form is removed from the atmospheric greenhouse effect. In the short term, there is a rough balance because water bodies also emit CO2, particularly when they heat up.

Phytoplankton bloom in the Channel off SW England (Landsat image)

Carbon dioxide enters more resilient forms through the marine part of the biosphere, at the base of which is photosynthesising phytoplankton. Photosynthesisers ‘sequester’ CO2 from the oceans as various carbohydrates in their soft tissue. Some of them use bicarbonate ions to form calcium carbonate in shells or tests. Once the organisms die both their soft and hard parts may end up buried in ocean-floor sediments: a longer-term sink. How much carbon is buried in these two forms depends on whether bacteria break down the soft tissues by oxidation and on the acidity of water that tends to dissolve the carbonate. Both processes ultimately yield dissolved CO2 that returns to the atmosphere.

Even the simplest phytoplankton cannot live on carbon dioxide and water alone: they need nutrients. The most familiar to any gardener are nitrogen, phosphorus and potassium. These are mainly supplied in runoff from the continents; although marine upwellings supply large amounts where deep ocean water is forced to the surface. Large tracts in the central parts of the oceans are, in effect, marine deserts whose biological productivity is very low. Surprisingly this is not because of severe shortages of N, P and K. This is because a key nutrient, albeit a minor one, is missing; dissolved iron that phytoplankton and ocean fertility in general depend on. This was discovered in the 1970s by US oceanographer John Martin. Just how important iron is to fertility of the oceans and to global climate emerged from studies of ice cores from the Antarctic ice sheet. Air bubbles in the myriad annual layers reveal that their CO2 content falls with each change in oxygen isotopes related to the periodic build up of polar ice caps during cold periods. The greenhouse effect diminished as a result during each stadial, for the simple reason that up to a third of all atmospheric carbon dioxide – about 200 billion tonnes – was withdrawn. The clearest of these are at the last glacial maximum and during the rapid build up glacial ice between 70 and 60 thousand years ago; a time of low sea level when a major ‘out-of-Africa’ human migration took place. A possible candidate for achieving this could have been massively increased ocean fertility and the burial of dead phytoplankton and their shells.

Analyses of Antarctic ice cores record fluctuations in atmospheric CO2 trapped in bubbles during the last ice age (top) and how iron-rich dust deposition onto the ice increased hugely during two major cold periods (bottom) – the last glacial maximum (35 to 18 ka) and between 70 and 60 ka. (Credit, Stoll; Fig. 1)

During stadials the ice cores also reveal that a great deal more dust found its way from the continents to the polar ice sheets. Analysing the dusty layers showed that to have included lots of iron. Falling into the cold ocean-surface waters around the polar regions would have added this crucial nutrient to a medium already rich in CO2 – the colder water is the more gas it will dissolve. These distant oceans bloomed with phytoplankton, speeding up the sequestration of carbon into ocean-floor sediments. Iron may have triggered a biological pump of gargantuan proportions that amplified ice-age cooling. Today the remotest parts of the world’s oceans are starved of iron so the pump only functions in a few places where iron is supplied by rivers or upwellings of deep ocean water

The marine biosphere is clearly a very important active component in the Earth’s climate subsystem. Climate’s continually changing interactions with the rest of the Earth System make climate change hugely complex. It is difficult to predict but growing understanding of its past behaviour is helpful. The late John Martin’s hypothesis of the effects on climate of changing iron concentrations in surface ocean water has a corollary: the stronger the biological pump the more oxygen in deep water must be used up in bacterial decay of descending organic matter. Indeed it was as recent estimates of the degree of oxygenation in ocean-sediment layers correlate with changes in climate that they also reveal.

So, would deliberate iron-fertilisation of polar oceans help draw down greenhouse warming? When several small patches of the Southern Ocean were injected with a few tonnes of dissolved iron they did indeed respond with phytoplankton blooms. However, it is impossible to tell if that had any effect on the atmosphere. ‘Going for broke’ with a massive fertilisation of this kind has been proposed, but this ventures dp into the political swamp that currently surrounds global warming and the wider environment. It is becoming possible to model such a strategy by using the data from the experiments and from ice cores, and early results seem to confirm the role of iron and the biological pump in CO2 sequestration by suggesting that half the known draw-down during ice ages can be explained in this way.

Based on a review by: Heather Stoll in February 2020. (30 years of the iron hypothesis of ice ages. Nature, v. 578, p. 370-371; DOI: 10.1038/d41586-020-00393-x}