Glacial intervals are characterized by low levels of atmospheric CO2. A new explanation for that connection invokes nutrient export from the Southern Ocean to warmer waters at such times.
In the late 1980s came news of a spectacular discovery: that temperatures at Earth's surface and levels of CO2 in the atmosphere fell and rose in tandem during glacial– interglacial cycles1. Since then, Earth scientists have been busy seeking explanations for the connection and much recent thinking has centred on oceanic controls on CO2. Writing in Geophysical Research Letters2 and Global Biogeochemical Cycles3, respectively, Brzezinski et al. and Matsumoto et al. take this line of investigation a step forward. Together the papers provide a new hypothesis, elegantly supported by a set of palaeoceanographic evidence and biogeochemical modelling, that could help account for much of the reduction in atmospheric CO2 during glacial periods.
For some years, Milankovitch cycles — minute variations in solar radiation reaching the Earth due to changes in Earth's orbit — were taken as an explanation for the pacing of the oscillations between glacial and interglacial intervals. But accounting for the large temperature shifts involved required further amplification within the Earth's climate system. Once analyses of air trapped in ice cores1 revealed that CO2 had fallen and risen in tune with the glacial–interglacial rhythm, the finger pointed to the greenhouse effect exerted by changes in CO2 as the amplifier.
Certain parts of the world's oceans have an especially large influence on climate, the Southern Ocean — the waters around Antarctica south of the polar front — being one of them. Oceanographers have long recognized that reduced leakage of CO2 from ocean to atmosphere in such areas could contribute significantly to the lower levels of atmospheric CO2 in glacial periods. In today's Southern Ocean, CO2-charged, nutrient-rich deep waters reach the surface and release CO2 to the atmosphere. This leak could effectively be plugged by an increase in biological production in the surface waters through fuller use of the available nutrients by phytoplankton, primarily diatoms4. Increased production would transfer more CO2 back to the ocean depths in the form of sinking organic particles.
Today, nutrients — particularly nitrate — that reach the sunlit surface ocean around Antarctica remain underused by phytoplankton because of a lack of the iron needed by their photosynthetic apparatus5. But during glacial periods, the larger amounts of dust in the atmosphere should have increased the supply of iron, potentially alleviating the deficiency, increasing nutrient use and yielding a net transfer of CO2 back to deep waters. Isolating the deep waters from the atmosphere by capping them with less-dense water at the surface (stratification) would have the same effect. With these ideas in mind, palaeoceanographers have been studying Southern Ocean sediments laid down in glacial times for clues to higher nutrient utilization and biological productivity. But the results have been puzzling.
Stable isotope ratios of nitrogen and silicon in organic and diatom remains, respectively, tell us about the nutrient status during the geological past. This is because diatoms' use of nitrate (NO3–) and silicic acid (Si(OH)4) favours the uptake of the lighter isotopes, 14N and 29Si. Diatoms become progressively enriched in the heavier isotopes, 15N and 30Si, as the nutrients are depleted, and increased nutrient use should be reflected in their sedimentary remains as higher δ15N and δ30Si. As shown in Fig. 1, sediment cores from the Southern Ocean have higher δ15N values in the glacial intervals, indicating increased NO3– uptake compared to that in interglacials. But the δ30Si trends are quite the opposite: Si(OH)4 use appears to have been lower during the glacials. Other arguments that invoke, for example, increased ocean stratification, could account for the higher δ15N but they fail to explain lower δ30Si values6. How can we reconcile the two different pictures of nutrient status painted by these two forms of proxy data?
On the basis of experimental results, Brzezinski et al.2 point out that the addition of iron dramatically alters the uptake ratio of NO3– and Si(OH)4 by diatoms from as much as 4:1 to about 1:1. Given that ratios of these nutrients in the Southern Ocean today are 2:1, uptake with a ratio of 1:1 under iron-replete conditions, as might have occurred during glacial periods, would result in higher use of NO3– but relative underutilization of Si(OH)4 — a view that is consistent with the isotope data. Given this revised interpretation of the sediment records, did the Southern Ocean contribute to the drop in atmospheric CO2 during glacial periods? Brzezinski et al.2 argue that it did, but not as a direct effect of CO2 uptake in Antarctic surface waters. Rather, as Matsumoto et al.3 show in their modelling study, when ocean circulation patterns are taken into account, the consequences of this peculiar nutrient biogeochemistry are manifest further afield.
The waters of the modern Southern Ocean penetrate north as far as the subtropics, mainly as subsurface flow. If the flow paths were the same during glacials, then the Antarctic region would have supplied the low-latitude ocean with water high in Si(OH)4 and low in NO3–. This would have pushed the phytoplankton community in these regions from domination by CaCO3-secreting coccolithophores to domination by opal-secreting diatoms. Brzezinski et al. believe, and Matsumoto et al. demonstrate with their model, that such an ecological shift would have had a double effect on reducing glacial CO2. First, diatoms have higher sinking rates than coccolithophores: their dominance would have resulted in the more efficient export of particulate organic matter to the depths, thus removing CO2 from the surface waters. Second, the lowered ratios of CaCO3 to organic carbon in sinking particles would also have lowered levels of atmospheric CO2, because the resulting excess CO3– (alkalinity) in surface waters would have sequestered CO2 by converting it to bicarbonate.
But did such a shift towards increased primary production by diatoms actually occur at low latitudes in glacial times? The answer is unclear, because the telling indicator, accumulation of opal in sediments of glacial age, is not uniformly high in these regions. Increases are indeed seen in some areas7. Declines are evident elsewhere, however, such as in the eastern tropical Pacific8. This geographical variability may point to local changes in nutrient inputs as the main determinant of opal production.
The work of Brzezinski et al.2 and Matsumoto et al.3 resolves the conflict generated by the proxy evidence for the nutrient status of the Southern Ocean at various times. But assessing the effects of their proposed mechanism on atmospheric levels of CO2 during glacials, as well as the effects of other ideas that have been put forward9,10, will require more evidence. We need a more synoptic view of past shifts in nutrient ratios, and a better understanding of the ecological response to such changes. Progress is being made, but the story isn't over yet.
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