Supposing that the isotopic level of d15N became heavier (say to +3%0) over a period longer than 3ka (residence of marine N), how would this be seen in old (>40Ma) global sediments? Would these be enriched by a similar level? Or only partially reflect this atmos. change? To a non-specialist (and engineer) intuitivally most nitrates etc would enter the N cycle already enriched such that the sediments should show a step-like increase (?) similar to the new atmos. standard.
Interesting question, difficult to answer. First of all, what process would cause (would have caused) a shift of +3 ‰ of 15N abundance in atmospheric N2 and where would the N2 causing this shift come from? To shift the d15N of atmospheric N2 (in any direction) one needs considerably more N2 than there is N2 present in the atmosphere.
The difference between d15N of 0 ‰ (N2 in Air) and d15N of +3 ‰ is too small to cause any significant change in the magnitude of mass discriminatory effects during, for example, N2 fixation by terrestrial plants. That is not to say the differences could not be detected (measured) but as far as terrestrial plant matter or marine POM are concerned (and any NH4+ / NO3- thus derived), any changes in d15N of these materials would be in single digit territory.
Thanks Wolfram
The process could be similar to that on Mars ie sputtering and preferential loss of 14N. I note (clipped) that at multiple geomag reversals there is often a spike in 15N (or step change) and afterwards a major cooling with biotic turnover. At carlton heights SA there is a similar shift in 15N over the Triassic-Jurassic in terrestial sediments and over a several tens of thousands of years.
The d15%0 step is about 2 (yet to be added to this version).
The inverse could take place during superchrons (and large scale seafloor spreading) with lighter mantle N2 entering the atmos and restoring the general equilibrium.
Kinetic Mass Fractionation of CO2 seems to be the best proxy for patm and agrees with levels for the flight of large creatures (permian, K and Miocene).
Hope you find something of interest.
Best
Alan
Correct me if I'm wrong but is not the case that owing to its lower mass (compared to Earth) Mars ability to hold on to atmospheric gases, especially gases of low molecular weight, is greatly reduced? As a consequence, what passes for Mars atmosphere is approx. 95% CO2 and ca. 2.5% N2.
Irrespective of the difference between Mars and Earth atmosphere-wise, the question remains regarding quantities required (of loss pf 14N2 or gain of 14N15N and 15N2) to yield a shift in d15N of atmospheric N2 if either end-point or starting point is 0 ‰.
Isotope mass balance is: +3 ‰ = f1×0 ‰ + f2×3 ‰ with f1 + f2 = 1
Conversely: 0 ‰ = f1×(-3 ‰) + f2×0 ‰
Of course, in the case of a one-off shift as a result of preferential loss of 14N2, the process would be a Rayleigh process (removal of a fraction from an amount limited reservoir). Change in d15N resulting from such a process would be described by: d15N = [(1+d15No) × (X/Xo)^e]-1
with Xo = amount of X at t = 0, d15No = d15N at t = 0; e = epsilon = enrichment factor = fractionation factor alpha - 1.
Thanks again wolfam.
I am now travelling, so find it difficult to do the math, but to get a one off shift (+) of 2%0, how much %mass of 14N would have to lost?
Mars has the lower gravity, but sputtering can strip a fair amount as suggested for O16.
Best
Alan
Depends on d15N(0) of original amount X(0), the original amount X(0) of N2 and the value of epsilon (or alpha). While I know a thing or two about stable isotopes, loss of atmospheric gases into space and related mass discrimination (an resulting isotopic fractionation) are way outside my comfort zone.
For the purpose of my work, the d15N of N2 in terrestrial atmosphere is treated as being constant (with a d15N of 0 ‰).
- Badges
- Science topic
- Similar topics
- Engineering