Make your hyporheic bed: tracing silicon supply to glacial waters from the pro-glacial landscape

Glacial waters are important suppliers of silicon (Si) to the ocean (e.g., Hendry et al., 2019), where they play a key role for biological production in nutrient-limited waters (Hopwood et al., 2020). The Si is sourced from i) sub- and supra-glacial systems and ii) the pro-glacial setting (the land in-front of the glacier). Climate change is a) driving glacier retreat which is increasing the proglacial area that waters are transported through before entering the ocean, and b) altering hydrological flow paths which is changing diurnal and seasonal water contributions to the glacial foreground. The impact of these changes on meltwater-proglacial connectivity and the supply of Si to glacial waters and ultimately the ocean is only poorly understood. The aim of this study is to assess the impact of different hydrological and climate conditions on glacial meltwater flow through the glacier foreground. To achieve this aim, silicon isotopes will be used to trace meltwater-foreground connectivity in a laboratory-simulated hyporheic zone.

The hyporheic zone (= zone of sediments and pore space below and alongside stream beds, where exchange of ground and stream waters takes place) is a key source and transport pathway of Si to pro-glacial stream waters (Gooseff et al., 2002). Water flow through the hyporheic zone is responsive to changing hydrological and climatological conditions (Wondzell et al., 2008). It follows that Si in the hyporheic zone of pro-glacial rivers can be used as a natural laboratory to determine the impact of climate change on glacial-foreground connectivity. Silicon isotopes are the key tool because in a closed system, subject to freeze-thaw cycles, silicon isotope fractionation occurs during amorphous silica precipitation (Oelze et al., 2015), for example during periods of low flow and low temperatures in the hyporheic zone of a pro-glacial stream (Hirst et al., 2020). In contrast, an open system under warmer temperatures creates conditions that do not favour amorphous silica precipitation and silicon isotope fractionation. This could be analogous to a period of high flow and higher temperatures in the hyporheic zone of a proglacial stream (Hirst et al., 2020). Here we propose to harness this behaviour and measure the silicon isotope composition of pore waters in a laboratory-simulated hyporheic zone subject to different temperatures and water flow rates.

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Image Captions

A glacial meltwater stream (Denali National Park, Alaska. Photo Credit: C Hirst)


Laboratory-simulated hyporheic beds will be built in litre-sized plastic containers order to test the response of silicon isotopes to changing temperature and flow rate conditions in a controlled environment. In detail, i) containers will be filled with glacial sediment, ii) ultra-pure water containing a silicon standard will be added to the containers, iii) water will be pumped and circulated through the container/sediment at different rates, iv) the container will be subject to different temperature conditions (e.g., different numbers of freeze-thaw cycles).

Temperature, pH and conductivity will be measured at routine intervals in a transect across the laboratory-simulated hyporheic zone. In addition, a flow rate sensor (data every 10 seconds) will record changes in flow rate through the sediments. The simulated pore waters and river waters will be filtered at 0.22 µm and ultrafiltered at 1 kDa (~ 1.3 nm). Major cations (including Si) will be analysed on each filtered fraction using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) at the Department of Earth Sciences, Durham University. Silicon isotopes will be analysed on each filtered fraction using a Multi Collector-Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS, Neptune™) at the Department of Earth Sciences, Durham. The mineralogy of the glacial sediments will be analysed using X-Ray Diffraction at the Earth and Life Institute, Université Catholique de Louvain (Belgium).

Silicon isotope analysis will be coupled with temperature and flow rate sensor data collected from the laboratory experiment to identify periods when the hyporheic zone is a closed system and periods when the hyporheic zone is an open system. There is also the possibility for fieldwork in Ny Ålesund, Svalbard, to collect river and hyporheic zone samples to compare with laboratory experiment results.

Project Timeline

Year 1

Literature review, experiment preparation and tests, introduction to Si isotope analysis

Year 2

Experiments performed, Si isotope column chemistry, Si isotope analysis using MC-ICPMS, XRD analysis of glacial sediment

Year 3

Coupling Si isotope data to water temperature and flow rate parameters, manuscript writing, conference attendance (e.g., Goldschmidt)

Year 3.5

Manuscript and thesis writing. Thesis submission and examination

& Skills

This project will provide a fantastic platform to gain multidisciplinary training, including in experiment development, isotope geochemistry and hydrology. The candidate will work closely with the supervisors to gain directly from their expertise. Dr Catherine Hirst will oversee the direction, development and progress of the project and will provide expertise in experiment design and isotope geochemistry. In addition, Dr Catherine Hirst will be a close mentor for the candidate’s career development. Dr Kate Hendry and Professor Sophie Opfergelt will provide a wealth of knowledge on silicon behavior in glacial and permafrost systems. Dr Julia Knapp will provide key insights into hyporheic zone functioning and hydrology, Dr Geoff Nowell will provide important expertise on isotope geochemistry – the candidate will have tailored training on all instruments and become a proficient user of the multi collector-inductively coupled plasma mass spectrometer. Dr Claus-Dieter Hillenbrand will provide expertise in glacial sedimentology. The candidate will additionally have access to extensive IAPETUS2-cohort and NERC training workshops.

The candidate will join vibrant research communities at Durham University, the British Antarctic Survey and Université Catholique de Louvain, where they will be welcomed and encouraged to network with colleagues and their collaborators. The candidate will have the opportunity to present their results to at least one international research conference. Furthermore, they will be encouraged to disseminate their result to the general public at school and community events. Together, these interactions will provide the candidate with opportunities to learn and practice skills in dissemination, networking and different styles of research communication.

References & further reading

Hendry, K.R., Huvenne, V.A., Robinson, L.F., Annett, A., Badger, M., Jacobel, A.W., Ng, H.C., Opher, J., Pickering, R.A., Taylor, M.L. and Bates, S.L., 2019. The biogeochemical impact of glacial meltwater from Southwest Greenland. Progress in oceanography, 176, p.102126.

Hopwood, M.J., Carroll, D., Dunse, T., Hodson, A., Holding, J.M., Iriarte, J.L., Ribeiro, S., Achterberg, E.P., Cantoni, C., Carlson, D.F. and Chierici, M., 2020. How does glacier discharge affect marine biogeochemistry and primary production in the Arctic?. The Cryosphere, 14(4), pp.1347-1383.

Gooseff, M.N., McKnight, D.M., Lyons, W.B. and Blum, A.E., 2002. Weathering reactions and hyporheic exchange controls on stream water chemistry in a glacial meltwater stream in the McMurdo Dry Valleys. Water Resources Research, 38(12), pp.15-1.

Wondzell, S.M. and Swanson, F.J., 1999. Floods, channel change, and the hyporheic zone. Water Resources Research, 35(2), pp.555-567.

Oelze, M., von Blanckenburg, F., Bouchez, J., Hoellen, D. and Dietzel, M., 2015. The effect of Al on Si isotope fractionation investigated by silica precipitation experiments. Chemical Geology, 397, pp.94-105.

Hirst, C., Opfergelt, S., Gaspard, F., Hendry, K.R., Hatton, J.E., Welch, S., McKnight, D.M. and Berry Lyons, W., 2020. Silicon isotopes reveal a non-glacial source of silicon to Crescent Stream, McMurdo Dry Valleys, Antarctica. Frontiers in Earth Science, 8, p.229.

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