Glaciers transport desert dust to polar plankton

The coastal regions downstream from glaciers are highly productive ecosystems, one example of which is the large summer phytoplankton bloom in the Labrador Sea. One possible explanation of this high productivity is that meltwater from glaciers is supplying iron to these areas. Iron (Fe) is an essential micronutrient for marine phytoplankton, and in large areas of the world’s oceans its availability is a limiting factor for growth for these primary producers, on which the marine food web depends. Iron supply is also a critical factor in dictating the strength of the ocean ‘biological pump’, which affects concentrations of carbon dioxide in the atmosphere, and hence the global climate.

 
     


Glaciers and ice sheets are a significant source of nanoparticulate iron, but its bioavailability and therefore importance to the marine environment is not yet well understood. A team of researchers from the UK and Germany have used high-resolution imaging and spectroscopy to investigate the amount and type of iron present in glacial sediments.

 
Lead author Dr Jon Hawkings from the University of Bristol, explains: “The speciation and minerology of iron particulates appears to be particularly important in dictating bioavailability. In oxygen-rich waters, the more reactive and soluble ferrous oxidation state, Fe(II), readily oxidises to the ferric state, Fe(III), which is poorly soluble. The majority of available iron in the oceans is therefore thought to be Fe(III), but recent studies suggest that Fe(II) particulates may also play a significant role.”
 
The researchers collected samples from glaciers in Greenland and the Norwegian island archipelago of Svalbard. They also collected a dust sample from Libya as a comparison, because terrestrial dust is thought to be one of the main sources of iron to the ocean. They analysed the samples using scanning X-ray microscopy (SMX) on the I08 beamline, to determine the distribution and speciation of the iron particulates. They also performed complementary studies with high resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Together these techniques allowed the team to determine the form of nanoparticulate iron present, which indicates its solubility and bioavailability.
 
Their results demonstrated that glacially-derived iron nanoparticles have a distinct speciation that suggests high bioavailability. All the glacial samples tested had nanoparticles that contained potentially bioavailable Fe(II). The Libyan dust sample contained only Fe(III)-rich particles, indicating it is likely to be poorly bioavailable.
 
These findings improve our understanding of the importance of the particulate material being delivered to the polar oceans, particularly in regions where availability of iron limits primary production, and therefore the amount of carbon dioxide potentially taken out of the atmosphere.
 

Related publication:

Hawkings JR et alBiolabile ferrous iron bearing nanoparticles in glacial sedimentsEarth and Planetary Science Letters 493, 92-101 (2018). DOI:10.1016/j.epsl.2018.04.022