A study led by a team of isotope geochemists and glaciologists from University of Bristol shows the importance of subglacial weathering on the export of silica from glacial systems and how differences in rock weathering can alter the geochemical signature of the silica that is released.
Glaciers and ice sheets have been shown to play an important role in nutrient cycles, including through the export of silica. Silica is a key nutrient for some types of algae, such as diatoms, which form the basis of many aquatic ecosystems and are responsible for carbon storage on a global scale.
In the study published in Geochimica et Cosmochimica Acta (accessible version Hatton-et-al-2019), the researchers measured meltwaters from two Greenlandic glaciers – Leverett Glacier (Southwest Greenland) and Kiattuut Sermiat (South Greenland). These glaciers vary in size (~600km2 compared to 36km2) and the rocks beneath each weather in different ways. By measuring silica concentrations, silicon isotope composition and major ion concentrations of the meltwaters, the team have shed light on the subglacial processes occurring and have produced a conceptual model of how the two systems differ.
They wanted to determine:
- If the distinct isotopic signature previously measured in Greenland was consistent in a different glacial catchment;
- What processes are causing this distinct isotopic signature;
- To produce a new conceptual model of the subglacial system by considering the changes in this isotopic signal over the melt season.
The study found that both Greenlandic catchments have an isotopic signature that is distinct from non-glacial rivers, despite their size differences. But there are differences between the catchments, which appear to correspond to differences in the weathering processes that are occurring under the glaciers themselves.
By combining the silicon isotope composition measurements with major ion concentrations, the team found that the two catchments have contrasting weathering regimes. Leverett Glacier is larger and so waters are likely to spend a longer time under the glacier, resulting in enhanced silicate mineral weathering. Whereas Kiattuut Sermiat is much smaller and acts more like a small alpine glacier, dominated by carbonate weathering over the melt season. The difference in weathering regime may impact the isotopic signature, as silicate weathering will lead to the formation of new, distinct solids, which will can react and dissolve – given enough time.
The study also considers the importance of the high rates of physical erosion in these systems, which results in finely crushed rocks, also influencing the geochemistry of meltwaters.
There is still a lot of work to be done to continue improving our understanding of glacial weathering and how they impact the nutrients exported in meltwaters. However, this work builds on work done recently by Dr Jon Hawkings and team, which shows the impact of past ice sheets on the global silicon cycle.
The research team are continuing to work on other glacial systems to investigate whether the patterns seen in Greenland are consistent globally. By combining further field studies with laboratory experiments, they hope to build on this study and provide the community with a better constrained silicon isotopic composition of glacial rivers.
This work is part of the European Research Council Funded project ICY-LAB, led by Dr Kate Hendry, which aims to provide insight into nutrient cycling, biomineralization, and the taxonomy and biogeography of siliceous organisms in an ecologically important region near Greenland. It also works towards the Leverhulme trust funded project led by Professor Jemma Wadham, exploring the role of sub-ice weathering on the silica cycle. These projects will consider how these glacial meltwaters travel through fjord systems, to predict the fluxes of glacial silica exported into the open ocean.
Blog post written by Jade Hatton, January 2019