geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...

hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...

paleoclimatology Paleoclimatology (American and British English spelling differences, British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the ...

and

paleoceanography Paleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation and biological productivity. Paleoceanographic studies using environment models ...

, ''δ''¹⁵N (pronounced "delta fifteen n") or delta-N-15 is a measure of the ratio of the two

stable isotope The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

s of

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

, ¹⁵N: ¹⁴N.

Formulas

Two very similar expressions for are in wide use in

. Both have the form

1000\fraca

‰ (‰ = 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\fraca

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 .

Applications

The ratio of ¹⁵N to ¹⁴N is of relevance because in most biological contexts, ¹⁴N is preferentially uptaken as the lighter isotope. As a result, samples enriched in ¹⁵N can often be introduced through a non-biological context. 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 . Another application is the 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 in benthic sediment gives an indication of the contribution of human waste to the total nitrogen in the sediment. Sediment cores analyzed for yield an historical record of such waste, with older samples at greater depths. is also used to measure food chain length and the trophic level of a given organism; high values are positively correlated with higher trophic levels; likewise, organisms low on the food chain generally exhibit lower values. Higher values in

apex predator An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic lev ...

s generally indicate longer food chains.

Formulas

Applications

References

See also