Melting icebergs are an inspiring reminder of nature’s immense beauty, yet also a poignant symbol of the effects of anthropogenic climate change. Icebergs in the Southern Ocean transfer iron, an essential micronutrient for all life, from glaciers to the open ocean, where it is in short supply. This iceberg iron supply can result in phytoplankton blooms visibly associated with iceberg tracks, and it is thought that icebergs can be a particularly effective iron source as they deliver iron to the open ocean. Yet quantifying the size of the iceberg fertilization effect, and what it means for marine biogeochemical cycles and ocean productivity, is a formidable research challenge.

Credit: Mie Hylstofte Sichlau Winding

Iron rapidly becomes associated with sinking particles in the ocean, rendering it unavailable to support life at the surface. It may therefore matter exactly when and where iron is delivered to a marine ecosystem. Thus, the fertilizing potential of iron may differ between sources comparing, for example, atmospheric dust with icebergs and with the shape, size and mineral state of the iron. To quantify how efficient different iron sources are for phytoplankton, Boyd et al. (J. Geophys. Res. Ocean. 117, C06009; 2012) conducted an elegant study that compared the utilization of different iron sources by phytoplankton in the Southern Ocean. Surprisingly, of all the studied sources, the supply to utilization ratio for icebergs was one of the lowest. One critical problem in determining the supply to utilization ratio is simply quantifying the supply term; that is, how much iron is contained in ice? We know that icebergs themselves are highly heterogeneous, with most iron concentrated in dirty smears and particle-rich layers that are visibly blackened. In my own work, I decided to investigate the variability of iron contained within icebergs from the same source. Working in a single catchment in Svalbard, we found that iron concentrations in icebergs varied over three orders of magnitude (M. J. Hopwood et al. Geochem. Perspect. Lett. 3, 200–209; 2017). Determining a mean iceberg iron content by direct measurements is thereby impractical without huge datasets, which are logistically difficult to collect when your targets are immense blocks of ice that roll in frigid waters without warning, or care, for what rolls with them. Yet there are alternatives to measuring iron itself, as any tracer of terrestrial or sediment input around icebergs would serve to approximate the oceanic iron input from icebergs.

With increasing discharge around Greenland and Antarctica, there is an obvious need to assess how marine ecosystems will respond to changing iceberg input and distribution. Better constraints on iron delivery by icebergs and the biological activity supported by this iron, as pioneered by Boyd et al., will help us to understand the ecosystem impacts of icebergs.