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“Weathering and nutrient export from subglacial environments: A novel stable isotope approach”

Periodic Reporting for period 1 - ICE-OTOPE (“Weathering and nutrient export from subglacial environments: A novel stable isotope approach”)

Reporting period: 2015-11-02 to 2017-11-01

Glaciated catchments are one of the most vulnerable environments to climate change, and ice-sheet retreat is presently taking place at an unprecedented level. Processes occurring at the glacier bed (the subglacial environment) turn bedrock into a cocktail of potentially bioavailable elements such as iron (Fe). The addition of Fe in a bioavailable form to the oceans can result in plankton blooms which drawdown atmospheric carbon dioxide. Therefore the release of bioavailable Fe from glacial settings has the potential to act as a fertilizer of Fe-poor circumpolar seas. As global temperatures rise and ice-sheets retreat, it is critical to constrain the roles that subglacial processes play in controlling such elements in biogeochemical cycles.

Predicting the impact of future ice-sheet and glacier behavior depends upon a clear understanding of how glacier dynamics relate not only to climatic driving forces but also to subglacial processes. The overarching goal of ICE-OTOPE is to investigate the subglacial conditions that control elemental release through a study of glacial outflows which vary fundamentally in size, hydrology and bedrock composition. To achieve this goal I proposed to use a novel multidisciplinary framework utilizing cutting-edge geochemical techniques, fieldwork and geochemical modeling to investigate the chemistry of subglacial systems. Through ICE-OTOPE new constraints on element cycling in glacial systems have been produced which can be used to enhance our understanding of the key processes that have governed the Earth’s chemical evolution and biogeochemical cycling.
A large sample set of subglacial sediments and waters was initially generated through extensive fieldwork and archived samples (Figure 1), which came from glaciers of different sizes (ice sheet and icefield), bedrock types (e.g. carbonate, silicate and ancient silicate) and climates (rainforest, high altitude and Arctic). Each sample underwent rigorous geochemical measurements and high-precision analyses to ascertain the controls on elemental distributions between sediments and waters and how these evolved downstream. The large geochemical data sets produced were incorporated into computer modeling programs to determine the elemental speciation and redox environments within each subglacial system, and in the case of the Greenland fieldwork samples, how these parameters changed over the course of a melt seasons, and how they varied downstream and into the oceans. Geochemical and isotopic method development was also at the core of this project. State-of-the-art analytical machines have been utilized throughout ICE-OTOPE to maximize the data quality and precision so that the highest quality research can be produced.

A particular emphasis was placed on the element Fe, Fe-isotopes, and the coupled sulfur-Fe biogeochemical cycle. Using the analytical-modeling techniques above I was able to broadly determine in what conditions Fe is released in different subglacial systems around the Arctic, how much of the Fe is bioavailable and its fate downstream in glacial rivers. One of the key findings is that the concentration of Fe (and other elements) from glaciers draining different regions of the Greenland Ice Sheet (and other regions of the Arctic) span orders of magnitude. Therefore a huge amount of care must be taken when calculating elemental and nutrient fluxes from regions such as the Greenland Ice Sheet into the proximal oceans.
The advent of next-generation MC-ICMPS mass-spectrometers permits isotope measurements at unprecedented levels of precision, opening up a new 'toolbox' of geochemical tracers. Such machines have been utilized throughout this project to develop isotopic analyses of new sample types, and to maximize the data quality and precision. The empirical geochemical data produced here can be used by climate and ocean modelers who can use this data to calibrate, test and build more reliable and robust models of glacier dynamics and the impacts of glacier change in our changing climate and oceans. The dataset collected here can be also directly incorporated to estimate how fluxes from glaciated environments influence biogeochemical processes in wider regions: Observational datasets are crucial for validating species selection protocols, but are presently limited, particularly in Polar Regions.

A comprehensive assessment of weathering products (i.e. the dissolved and suspended elemental fluxes) from recently deglaciated landscapes is severely lacking from the literature. It is still unknown whether warming temperatures will lead to a net increase in biological productivity. This fundamental question must be addressed before further assessment of the impact of climate change on chemical weathering and associated CO2 drawdown. By characterizing the weathering products of many different glacial systems, the data generated by ICE-OTOPE will continue to fill these missing gaps to provide crucial information and constraints on the chemical weathering processes in subglacial systems and their exported products, and thus climate evolution.
Close up Dr Stevenson at the Kangaarsarsuup Sermia ready to sample the (sub)glacial water
The Kangaarsarsuup Sermia fieldsite in West Greenland
ICE-OTOPE sample sites. Map modified from