Frozen iron: how Antarctic glaciers feed the Southern Ocean’s hidden fertiliser
Nov 26, 2025
Iron is an essential micronutrient for phytoplankton, tiny ocean plants that underpin marine food webs and help regulate Earth’s climate by drawing down carbon. Before this work, scientists knew that Antarctic continental shelves supply iron to the Southern Ocean, but much of that iron comes in particulate (solid) form, often as Fe(III). Fe(III) is iron in its oxidised form, carrying three positive charges, and it is the less easily used version of iron for living things compared to Fe(II). What was poorly understood was how glacier meltwater contributes iron(II)-rich particles, and whether those iron(II) particles survive long enough (in the oxidising ocean surface) to be used by phytoplankton.
An international team of researchers tackled this gap. Their work, published in Nature Communications, shows that Antarctic glaciers export particles rich in iron(II) stabilised by organic carbon, which slows down oxidation, delivering more bioavailable iron to surface waters. They used X-ray microscopy at the I08 beamline to map co-localisation of iron and organic carbon phases in particles from meltwater.
- The left panel shows the Fe optical difference map, where black indicates no Fe and white indicates the highest optical density of Fe. The right panel provides colourised red-green composite maps of Fe (green) and C (red) SXM images. Brighter colours indicate a higher abundance of Fe and C. (a, b) are images taken for samples from the northern site (King George Island), (c, d) the central site (Anvers Island) and (e, f) the southern site (Adelaide Island). The areas outlined in white in the composite images represent the areas selected for pixel co-localisation analysis in ImageJ using the coloc2 plugin. The r values represent the Pearson’s R correlation coefficient corresponding to the pixels within these areas.
The Southern Ocean is often limited in nutrients needed by phytoplankton; among those, iron is a key player. Iron can exist in oxidation states (primarily Fe(II) = ferrous, Fe(III) = ferric) that affect how easily organisms can use it. Fe(II) is more bioavailable but gets oxidised in oxygenated water, turning into Fe(III), which is less available. Past studies had mostly focused on dissolved iron or Fe(III) particulates.
In this article, the authors studied particles derived from glacial meltwaters around Antarctica, focusing on those rich in Fe(II). The material sampled included particulate matter from meltwater, shelf sediments, and surface waters beneath or adjacent to glaciers. The aim was to see whether glacier-derived Fe(II) particles survive long enough in surface ocean and whether organic carbon associated with them helps protect the Fe(II) from oxidation.
Results showed that many particles exported by glaciers are rich in Fe(II), and those particles often have organic carbon intimately mixed in. This carbon appears to slow down oxidation of the Fe(II) when particles enter oxygenated surface waters. The conclusion: glacial meltwater is an underestimated source of bioavailable iron(II) to the Southern Ocean, and its impact may increase as glaciers melt faster.
This work has significant implications for marine biogeochemistry and climate science. It suggests that as Antarctic glaciers melt (due to warming), more Fe(II)-rich particles might be exported to surface waters, potentially enhancing phytoplankton growth, which could boost carbon drawdown (the “carbon pump”). On a broader scale, the findings could influence how we understand nutrient limitation in polar oceans, how ecosystems may respond to climate change, and perhaps even guide geoengineering ideas.
To find out more about the I08 beamline, please contact the Principal Beamline Scientist: Burkhard Kaulich: burkhard.kaulich@diamond.ac.uk.
Related Publication
Jones, R. L., et al. Antarctic glaciers export carbon-stabilised iron(II)-rich particles to the surface Southern Ocean. Nature Communications, 16, 5015. (2025) DOI: 10.1038/s41467-025-59981-y
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Diamond Light Source Ltd
Diamond House
Harwell Science & Innovation Campus
Didcot
Oxfordshire
OX11 0DE
Copyright © Diamond Light Source. Diamond Light Source® and the Diamond logo are registered trademarks of Diamond Light Source Ltd
Registered in England and Wales at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom. Company number: 4375679. VAT number: 287 461 957. Economic Operators Registration and Identification (EORI) number: GB287461957003.