An iron-rich plume of water from a hydrothermal vent has been found to extend more than 4,000 kilometres through the ocean. The finding has implications for the productivity of marine algae, and therefore for climate. See Letter p.200
In sunlit surface waters of the ocean, planktonic algae convert carbon dioxide from seawater into organic matter, which subsequently settles to the deep sea and sequesters the carbon from the atmosphere. In ocean regions that govern atmospheric CO2 levels, the availability of iron — a trace nutrient — limits this primary production by algae1. Changes in iron availability have partly modulated climate variability during transitions from glacial to interglacial periods in the past2, and are expected to affect future climate. On page 200, Resing et al.3 report that a substantial amount of iron released from fissures in an abyssal mid-ocean ridge is transported thousands of kilometres by slow-moving deep-ocean currents. Using an ocean model, they show that iron from the global mid-ocean ridge is supplied to the euphotic layer (the sunlit surface region), and potentially contributes to algal growth.
In the late 1970s, scientists discovered hot, mineral-rich waters seeping from cracks in the sea floor called hydrothermal vents. Chemical analyses4 of these waters showed them to be remarkably iron-rich compared with the surrounding ocean water. Hydrothermal iron was thought for decades to make only a minor contribution to the iron budget of the global ocean, because scientists assumed that it forms a solid precipitate near the vent sites as a result of its low solubility in seawater. Subsequent observations5,6 showed, however, that some of the iron released from hydrothermal vents may be transported away from vent sites. This possibility is called the leaky vent hypothesis7.
Scientists have detected unusually high concentrations of dissolved iron distributed over horizontal distances of hundreds to thousands of kilometres in various deep ocean basins8,9,10,11. The hydrothermal origin of these iron-rich anomalies has been inferred by examining the isotopic signatures of helium within them. Helium has two stable isotopes, 3He and 4He, and hydrothermal water is enriched in 3He relative to the proportion found in atmospheric helium — there is said to be excess 3He. The presence of excess 3He therefore indicates water of hydrothermal origin10. Correlations between the anomalous dissolved iron concentrations and excess 3He have been reported10,12, but the data for helium and for dissolved iron were collected at different times, and so the hydrothermal origin of the dissolved-iron anomalies could not be confirmed.
Resing and colleagues collected hundreds of seawater samples (Fig. 1) around the southern part of a mid-ocean ridge called the East Pacific Rise, which is particularly active volcanically. They identified a remarkable plume of water containing high concentrations of dissolved iron extending more than 4,000 km downstream. The dissolved-iron concentration correlated linearly with the concentration of excess 3He, which was measured using simultaneously collected helium data. This unambiguously proves that the anomalous dissolved iron is derived from hydrothermal vents in the East Pacific Rise.
Furthermore, the authors found that the linear relationship between dissolved iron and excess 3He was maintained throughout the large plume, which indicates that the same process — inferred to be dilution with the surrounding seawater — controls the concentration of both helium and dissolved iron. Such conservative behaviour is inconsistent with the expected chemical behaviour of inorganic iron, and supports an aspect of the leaky vent hypothesis: the idea that physico-chemical stabilization enables iron to be transported long distances from hydrothermal vent sites. Moreover, from the linear relationship, the authors estimated that the amount of iron transported globally from hydrothermal vents is 3–4 gigamoles per year, which is more than 4 times higher than previous estimates.
Resing and co-workers went on to use a cutting-edge global ocean model13 to estimate the contribution of hydrothermal iron to the export of organic carbon from the euphotic layer, and found that the contribution was substantial, especially in the Southern Ocean. This has implications for the role of hydrothermal iron in past, present and future climates. For example, during glacial periods, increased deposition of iron-bearing dust onto the ocean surface is thought to have contributed to the lowering of atmospheric CO2 concentrations2. But a stable supply of hydrothermal iron over millennial timescales would have buffered short-term variations of the iron supply, and thus also of oceanic CO2 uptake, casting this theory in doubt.
Many questions need to be answered before the role of hydrothermal iron in marine biogeochemical cycles can be fully understood. One issue is that the size of global hydrothermal iron flux is highly uncertain. Resing et al. estimated the global flux using data from a single hydrothermal system, but the relationship between levels of iron and 3He is likely to vary for different hydrothermal sites, because the tectonic history and the chemical compositions of the surrounding rocks will differ. For example, the ratio of the concentration of dissolved iron to that of 3He is 80-fold higher in the southern Atlantic Ocean than in the southern Pacific Ocean10. More data are therefore needed from different sites to constrain estimates of the global hydrothermal iron flux, and the mechanisms causing variability among sites must be better understood.
Another issue concerns the mechanism by which iron is stabilized around hydrothermal vents. One possible mechanism is the formation of complexes between iron ions and organic ligand molecules5. Organic ligands are thought to be ubiquitous in seawater and to control dissolved iron concentrations by increasing iron solubility — more than 99% of dissolved iron in seawater is organically complexed14. But our knowledge of the sources and sinks of organic ligands in the ocean is still limited, and most global ocean models assume a fixed ligand concentration. The global ocean model13 used by Resing and colleagues mechanistically represents the dynamics of organic ligands in the ocean. The team could thus simulate the transport of organically complexed iron away from the hydrothermal vents. Although the model makes several assumptions, the authors' results highlight the value of mechanistic representations of ligand dynamics in such models.
Will hydrothermal iron continue to be a nutrient source for surface algae? Rapid environmental changes have been occurring in the Southern Ocean over the past few decades: increases in the levels of greenhouse gases and the depletion of stratospheric ozone have led to intensified upwelling of deep water15, whereas recent amplification of the water cycle and the melting of Antarctic glaciers has strengthened surface-water stratification16. These changes have the potential to alter exchanges between surface and deep waters, and thus the contribution of hydrothermal iron to surface biological productivity.