Geogenic nitrogen as a nutrient source to subglacial microbial ecosystems

• Main Author: Dixon, Timothy John
• Format: Thesis
• Language: English
• Published: University of Leeds 2019
• Subjects: Adventdalen; Svalbard
• Online Access: https://etheses.whiterose.ac.uk/26726/; https://etheses.whiterose.ac.uk/26726/1/Dixon_TJ_EarthandEnvironment_PhD_2019.PDF

Glacial runoff often contains nitrate far in excess of nitrogen (N) concentrations in snow. ‘Excess’ nitrate is likely produced by nitrifying bacteria in subglacial environments, yet the source of the additional N has not been identified. In this thesis, rocks are investigated as an N source to subglacial microbial ecosystems following their comminution by glacial erosion and leaching with meltwater. Weathering processes involving the release of N and micronutrients, iron (Fe) and manganese (Mn), from rock to glacial environments were evaluated using a combination of field studies in Adventdalen (Svalbard) and rock weathering experiments. Attempts were also made to constrain isotope fractionation during weathering to improve the use of stable isotopes to trace N provenance. The sedimentary geological succession underlying glaciers in the study area contained significant N (up to 0.21 wt. %), particularly in organic rich, siltstones and shales. Fe and Mn were largely found within oxide and carbonate minerals while significant pyrite bound Fe (max. 5.2 wt. %) was present in organic rich sediments. In simulated glacial weathering experiments, nitrogen was almost entirely liberated as ammonium with much of it absorbed to mineral surfaces rather than dissolved in solution. Pyrite oxidation liberated Fe and generated acidity which drove i) silicate dissolution releasing N and ii) carbonate dissolution yielding dissolved Mn. The largest N yields were associated with rocks containing abundant organic N which may have been liberated via oxidative weathering, protonation of amines and/or organic matter degradation by free radicals produced during crushing. Liberated ammonium was partly lost as ammonia gas when the solution pH exceeded 8 and also absorbed to sediment, particularly when clay minerals were abundant. Further experiments demonstrated an isotope enrichment in adsorbed ammonium of up to 10‰ δ15N relative to bedrock N, caused by fractionation from a combination of adsorption and volatilisation processes. In ...