Measurement of geologic nitrogen using mass spectrometry, colourimetry, and a newly adapted fluorometry technique

Abstract

Abstract. Long viewed as a mostly noble, atmospheric species, recent work demonstrates that nitrogen in fact cycles throughout the Earth system, including the atmosphere, biosphere, oceans, and solid Earth. Despite this new-found behaviour, more thorough investigation of N in geologic materials is limited due to its low concentration (1 to 10 s ppm) and difficulty in analysis. In addition, N can exist in multiple species (NO3−, NH4+, N2, organic-N), and determining which species is actually quantified can be difficult. In rocks and minerals, NH4+ is the most stable form of N over geologic time scales. As such, techniques designed to measure NH4+ can be particularly useful. We measured a number of geochemical rock standards using three different techniques: mass spectrometry, colourimetry, and fluorometry. The fluorometry approach is a novel adaptation of a technique commonly used in biologic science, applied herein to geologic NH4+. Briefly, NH4+ can be quantified by HF-dissolution, neutralization, addition of a fluorescing reagent, and analysis on a standard fluorometer. We reproduce published values for several rock standards (BCR-2, BHVO-2, and G-2), especially if an additional distillation step is performed. While it is difficult to assess quality of each method, due to lack of international geologic N standards, fluorometry appears better suited to analyzing mineral-bound NH4+ than mass spectrometry, and is a simpler, quicker alternative to colourimetry. To demonstrate a potential application of fluorometry, we calculated a continental crust N budget based on new measurements. We used glacial tills as a proxy for upper crust and analyzed several poorly constrained rock types (volcanics, mid-crustal xenoliths) to determine that the continental crust contains ∼ 2 × 1018 kg N. This estimate is consistent with recent budget estimates, and shows that fluorometry is appropriate for large-scale questions where high sample throughput is helpful. Lastly, we report the first δ15N values of six rock standards: BCR-2 (1.05 ± 0.4 ‰), BHVO-2 (−0.3 ± 0.2 ‰), G-2 (1.23 ± 1.32 ‰), LKSD-4 (3.59 ± 0.1 ‰), Till-4 (6.33 ± 0.1 ‰), and SY-4 (2.13 ± 0.5 ‰). The need for international geologic N standards is crucial for further investigation of the Earth system N cycle, and we suggest that existing rock standards may be suited to this need.