Stable nitrogen and oxygen isotope fractionation during precipitation of nitrate salt from saturated solutions

• Published in: Rapid Communications in Mass Spectrometry
• Main Authors: Welsh, Hannah, Gueorguieva, Gloria‐Alexandra, Kounaves, Samuel, Amundson, Ronald
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2020
• Subjects: Antarctic; Antarc*
• Online Access: http://dx.doi.org/10.1002/rcm.8905; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Frcm.8905; https://onlinelibrary.wiley.com/doi/pdf/10.1002/rcm.8905; https://onlinelibrary.wiley.com/doi/full-xml/10.1002/rcm.8905

Rationale Nitrate is an oxyanion similar to CO 3 − and thus should undergo stable N and O isotope fractionation during dissolution or precipitation. This process should dominate abiotic soil nitrate processes in hyperarid regions of Earth and possibly Mars. Methods The N and O isotope fractionations during the precipitation of nitrate salt from saturated solutions at ~20°C were determined by two methods: rapid precipitation by antisolvent crystallization and slow uninhibited precipitation in a desiccator. In the antisolvent crystallization procedure, increasing volumes of acetone were added to samples of saturated sodium and strontium nitrate solutions to instantaneously precipitate nitrate salt. In the slow procedure (requiring weeks), slow evaporative water loss drove the process. Results There was little difference between the two procedures. Using a Rayleigh model, the calculated N fractionation ( 15 ε product–residual ) ranged from 1.69‰ to 2.77‰, whereas for O, the 18 ε product–residual values were between 1.27‰ and 4.61‰. The N isotope fractionation between NO 3 − and the metal solid is similar to that between C in dissolved CO 3 −2 and carbonates. We found that O versus N isotope plots of soil nitrate in a cold/dry Antarctic chronosequence had slopes similar to those from the experiments, revealing abiotic transport. In the Atacama Desert, where the soil nitrates are a mix of biological and tropospheric nitrate, there is an inverse relationship between soil N and O isotopes. These two relationships were compared with the isotope composition of nitrate from Martian meteorite EETA79001. Conclusions While the N and O isotope composition of the Martian nitrate is remarkably similar to that of the present Atacama Desert, the interpretation of the slope of the O versus N isotopes remains ambiguous due to the limited number of samples. Additional NO 3 samples from Martian meteorites are needed to address the question of abiotic versus biotic alteration of NO 3 − on Mars.