δ15N

In geochemistry, hydrology, paleoclimatology and paleoceanography δ¹⁵N (pronounced "delta fifteen n") or delta-N-15 is a measure of the ratio of stable isotopes ¹⁵N:¹⁴N.

Formulas

Two very similar expressions for δ¹⁵N are in wide use in hydrology. Both have the form $1000{\frac {s-a}{a}}$ ‰ (‰ = permil or parts per thousand) where s and a are the relative abundances of ¹⁵N in respectively the sample and the atmosphere. The difference is whether the relative abundance is with respect to all the nitrogen, i.e. ¹⁴N plus ¹⁵N, or just to ¹⁴N. Since the atmosphere is 99.6337% ¹⁴N and 0.3663% ¹⁵N, a is 0.003663 in the former case and 0.003663/0.996337 = 0.003676 in the latter. However s varies similarly; for example if in the sample ¹⁵N is 0.385% and ¹⁴N is 99.615%, s is 0.003850 in the former case and 0.00385/0.99615 = 0.003865 in the latter. The value of $1000{\frac {s-a}{a}}$ is then 51.05‰ in the former case and 51.38‰ in the latter, an insignificant difference in practice given the typical range of -20 to 80 for δ¹⁵N.

Applications

One use of ¹⁵N is as a tracer to determine the path taken by fertilizers applied to anything from pots to landscapes. Fertilizer enriched in ¹⁵N to an extent significantly different from that prevailing in the soil (which may be different from the atmospheric standard a) is applied at a point and other points are then monitored for variations in δ¹⁵N.

Another application is to assessment of human waste water discharge into bodies of water. The abundance of ¹⁵N is greater in human waste water than in natural water sources. Hence δ¹⁵N in benthic sediment gives an indication of the contribution of human waste to the total nitrogen in the sediment. Sediment cores analyzed for δ¹⁵N yield an historical record of such waste, with older samples at greater depths.

See also

• δ¹³C
• δ¹⁸O
• δ³⁴S
• Isotopic signature
• Isotope analysis
• Isotope geochemistry