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N2 in deep subsurface fracture fluids of the Canadian Shield: Source and possible recycling processes
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- Author(s) / Creator(s)
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In addition to high concentrations of CH4 and H2, abundant dissolved N2 is found in subsurface fracture fluids in Precambrian cratons around the world. These fracture fluids have hydrogeological isolation times on order of thousands to millions and even billions of years. Assessing the sources and sinks of N2 and related (bio)geochemical processes that drive the nitrogen cycle in these long isolated systems can shed insights into the nitrogen cycle on early Earth with implications for other planets and moons. In this study, we collected dissolved gas samples from deep subsurface fracture fluids at seven sites (Kidd Creek, LaRonde, Nickel Rim, Fraser, Copper Cliff South, Thompson, and Birchtree) in the Canadian Shield. Multiple gas components (e.g., H2, O2 and Ar) were integrated with δ15NN2 values to characterize the N2 signatures. Results show that the dissolved N2 in deep subsurface fracture fluids from the Canadian Shield sites are more 15N-enriched than those from the Fennoscandian Shield and the Witwatersrand Basin in the Kaapvaal Craton. The nitrogen isotopic signatures of the Canadian Shield samples coupled with their hydrogeological framework indicate the N2 was sourced from fixed ammonium in silicate minerals in host rocks and was generated by metamorphic devolatilization. Modeling of nitrogen devolatilization from host rocks supports this interpretation, but also suggests that a second process, likely abiotic N2 reduction, is required to account for the observed 15N enrichment in the N2 samples from the Canadian Shield. A 10-year monitoring study for one of the boreholes, at 2.4 km of the Kidd Creek Observatory, shows a steady decrease in δ15NN2 values with time, which coincides with the temporal isotopic evolution of some other gas components in this borehole. Although it cannot be confirmed at this time, this isotopic shift in N2 may be potentially attributed to microbial processes (e.g., anaerobic oxidation of ammonium). Nevertheless, the large 15N enrichments for the majority of the samples in this study suggest that the nitrogen cycle in the deep saline fracture fluids in the Canadian Shield is dominated by abiotic processes. This is in contrast to the nitrogen cycles in the subsurface fracture fluids in the Fennoscandian Shield and the Witwatersrand Basin, which have been shown to be strongly affected by extant microbial ecosystems discovered in those fracture waters.
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- Date created
- 2021-12-20
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- Type of Item
- Article (Draft / Submitted)