N2 in deep subsurface fracture fluids of the Canadian Shield: Source and possible recycling processes

Abstract

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 cycles 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 δ 15 NN2 values to characterize the N2 signatures. Results show that the dissolved N2 in deep subsurface fracture fluids from the Canadian Shield sites are more 15 N-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 15 N 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 15 NN2 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 15 N 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.

The origin and dynamics of nitrogen in the Earth's mantle constrained by 15N15N in hydrothermal gases

“…This is an important observation; nitrogen derived from air does not have a single These data confirm that magmatic N2 formed by degassing at ~ 1200 °C has D30 ~ 0‰. The mechanism for N2 formation in crustal environments is breakdown of NH4-bearing phyllosilicates at peak metamorphic temperatures (Li et al, 2021). Gases from two deep mines in the Canadian shield have D30 values between 1.1±0.4‰ and 0.5±0.2‰ (2 σ, n=2).…”

Potential for natural hydrogen in Quebec (Canada): a first review

“…The major and minor gases measured are characteristic of a source originating from the crystalline basement according to the classification of Milkov (2022). In at least four locations in Ontario, the gases analyzed (CH 4 , C 2 H 6 , C 6 H 8 , H 2 , N 2 ) have a clear abiotic origin (Sherwood Lollar et al, 1993a;Sherwood Lollar et al, 2006;Sherwood Lollar et al, 2008;Montgomery, 1994;Li et al, 2021) and, in at least one case, dihydrogen isotopes suggest that its genesis is linked to a serpentinization process (Sherwood Lollar et al, 1993a).…”

“…Combustible gases (hydrocarbons and hydrogen) have been documented in solution in the groundwater of some gold mines in the Abitibi greenstone belt (play type A1), mainly in Ontario where hydrogen can represent between 0.06% and 57.8% of the dissolved gases but also in Quebec with values between 0.51% and 3.63%, although studies are less detailed (Fritz et al, 1987;Sherwood et al, 1988;Sherwood Lollar et al, 1993a;Sherwood Lollar et al, 1993b;Sherwood Lollar et al, 2006;Sherwood Lollar et al, 2008;Warr et al, 2019;Li et al, 2021). Nitrogen content can be as high as 83.3%, CO 2 content is less than 1% with a few exceptions, and helium content can be as high as 19.3% (Sherwood et al, 1988).…”

The application of Monte Carlo modelling to quantify in situ hydrogen and associated element production in the deep subsurface

“…To date, significant research has focused on evaluating the role of radiolytic-driven cycling of hydrogen and sulfur in supporting chemosynthetic ecosystems (e.g., Lin et al, 2005b;Lin et al, 2006;Chivian et al, 2008;D'Hondt et al, 2009;Sherwood Lollar et al, 2014;Dzaugis et al, 2016;Li et al, 2016Li et al, , 2022Telling et al, 2017;Warr et al, 2019;Bomberg et al, 2021;Sauvage et al, 2021;Nisson et al, 2023). Most recently, research has expanded this aspect to consider cycling of other associated critical bio-elements, such as nitrogen, and simple organic molecules (e.g., Silver et al, 2012;Adam et al, 2021;Li et al, 2021;Vandenborre et al, 2021;Karolytė et al, 2022), and to evaluate alternative habitability models beyond Earth on other worlds such as Mars, Enceladus, and Europa (Onstott et al, 2006;Bouquet et al, 2017;Dzaugis et al, 2018;Tarnas et al, 2018;. However, underpinning this entire research field is the fundamental requirement of accurately being able to model representative radiolytic production of elements within the system(s).…”

“…For example, recent papers re-evaluated the mechanisms generating crustal noble gases in the deep crust (Lippmann-Pipke et al, 2011;Warr et al, 2018;Warr et al, 2022), and proposed novel habitability models in the deep Earth and marine sediments based on hydrogen generation via radiolytic decomposition of water (e.g., D 'Hondt et al, 2009;Sherwood Lollar et al, 2014;Li et al, 2016;Sauvage et al, 2021). Documenting, quantifying, and refining these radiogenic and radiolytic processes with respect to their ability to potentially generate H 2 , 4 He, 40 Ar, SO 4 2-, N 2 , organic molecules and others within crustal settings on Earth and beyond has considerable recent interest with respect to the understanding of early Earth processes and prebiotic chemistry, constraining habitability models on Earth and beyond, and the formation of economic resources (e.g., 4 He) over geologic timescales (e.g., Lin et al, 2005a;Lin et al, 2005b;Onstott et al, 2006;Blair et al, 2007;Lefticariu et al, 2010;Sherwood Lollar et al, 2014;Dzaugis et al, 2016;Li et al, 2016;Bouquet et al, 2017;Altair et al, 2018;Dzaugis et al, 2018;Tarnas et al, 2018;Yi et al, 2018;National Academies of Sciences, Engineering, and Medicine, 2019;Warr et al, 2019;Adam et al, 2021;Boreham et al, 2021;Warr et al, 2021b;Li et al, 2021;Sauvage et al, 2021;Vandenborre et al, 2021;Li et al, 2022;Warr et al, 2022;Nisson et al, 2023).…”