『Abstract
　Deep (＞0.8 km depth) fracture water with residence time estimates on the order of several Ma from the Witwatersrand Basin, South Africa contains up to 40μM of NO3^-, up to 50 mM N2 (90 times air saturation at surface) and 1 to -400μM NH3/NH4⁺. To determine whether the oxidized N species were introduced by mining activity, by recharge of paleometeoric water, or by subsurface geochemical processes, we undertook N and O isotopic analyses of N species from fracture water, mining water, pore water, fluid inclusion leachate and whole rock cores.
　The NO2^-, NO3^- and NH3/NH4⁺ concentrations of the pore water and fluid inclusion leachate recovered from the low porosity quartzite, shale and metavolcanic units were 〜10⁴ times that of the fracture water. The δ¹⁵N-NO3^- and δ¹⁸O-NO3^- of the pore water and fluid inclusion leachate, however, overlapped that of the fracture water with the δ¹⁵N-NO3^- ranging from 2 to 7‰ and the δ¹⁸O-NO3^- ranging from 20 to 50‰. The δ¹⁵N-NO3^- of the mining water ranged from 0 to 16‰ and its δ¹⁸O-NO3^- from 0 to 14‰ making the mining water NO3^- isotopically distinct from that of the fracture, pore and fluid inclusion water. The δ¹⁵N-N2 of the fracture water and the δ¹⁵N-N from the cores ranged from -5 to 10‰ and overlapped the δ¹⁵N-NO3^-. The δ¹⁵N-NH4⁺ of the fracture water and pore water NH3/NH4⁺ ranged from -15 to 4‰ Although the NO3^- concentrations in the pore water and fluid inclusions were high, mass balance calculations indicate that NO3^- accounts for ≦10% of the total rock N, whereas NH3/NH4⁺ trapped in fluid inclusions or NH4⁺ present in phyllosilicates account for ≧90% of the total N.
　Based on these findings, the fluid inclusion NO3^- appears to be the source of the pore water and fracture water NO3^- rather than paleometeoric recharge or mining contamination. Irradiation experiments indicate that radiolytic oxidation of NH3 to NO3^- can explain the fluid inclusion NO3^- concentrations and, perhaps, its isotopic composition, but only if the NO3^- did not attain isotopic equilibrium with the hydrothermal fluid 2 billion years ago. The δ¹⁵N-N, δ¹⁵N-N2 and δ¹⁵N-NH4⁺ suggest that the reduction of N2 to NH4⁺ also must have occurred in the Witwatersrand Basin in order to explain the abundance of NH4⁺ throughout the strata. Although the depleted NO3^- concentrations in the fracture water relative to the pore water are consistent with microbial NO3^- reduction, further analyses will be required to determine the relative importance of biological processes in the subsurface N cycle and whether a complete subsurface N cycle exists.

Keywords: N isotopes; Deep; Subsurface microbial ecosystems; Radiolysis; N cycle』

1. Introduction
2. Geologic setting
3. Materials and methods
　3.1. Core collection
　3.2. Fracture water sample collection
　3.3. Pore water, fluid inclusion, exchangeable NH4⁺ extractions
　3.4. Dissolved N species analyses
　3.5. NO3^- isotopic measurements
　3.6. Solid N and C isotopic analyses and total U analyses
　3.7. Nitrogen isotopic analyses of NH4⁺
　3.8. N2 compositional and isotopic analyses
　3.9. Irradiation experiments
　3.10. Irradiation models
4. Results and discussion
　4.1. Evaluating potential mining contamination
　4.2. Radiolytic generation of NO2^- and NO3^-
　4.3. The origin of the δ¹⁸O-NO3^-
　4.4. The NH3 and N2 sources
5. Conclusion
Acknowledgments
Appendix A. Supplementary data
References