Nitrogen isotope ratios for

Calvert, S. E., Nielsen, B., and Fontugne, M. R. (1992). Evidence from nitrogen isotope ratios for enhanced productivity during formation of eastern Mediterranean sapropels. Nature 359(6392), 223-225. 

Nitrogen is mainly taken in by humans in the form of proteins, so there is no possibility for ambiguity here nitrogen isotopic ratios must reflect those of the protein in the diet. 

The results of heat and water stress experiments (Table 12.3) show that these factors had no significant effects on nitrogen isotope ratios of bone collagen or hair. The mean collagen-diet difference (A Nco-d) values of the water-restricted litters ranged from 2.6 %o for group 10 on diet A (36°, water ad... 

Bogaard, A., Heaton, T. H. E., Poulton P. and Merbach, I. (2007) The impact of manuring on nitrogen isotope ratios in cereals archaeological implications for reconstruction of diet and crop management practices. Journal of Archaeological Science 34, 335 343. 

The anomalous micro-composition of the Martian atmosphere with regard to nitrogen, argon, neon, krypton and xenon has also been compared with trapped gases for the Martian meteorite collection (12 in total). The isotope ratios for... 

Altabet, M.A., and R. Francois. 1994. Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization. Global Biogeochemical Cycles 8(1) 103-116. 

Now, GC-IRMS can be used to measure the nitrogen isotopic composition of individual compounds [657]. Measurement of nitrogen isotope ratios was described by Merritt and Hayes [639], who modified a GC-C-IRMS system by including a reduction reactor (Cu wire) between the combustion furnace and the IRMS, for reduction of nitrogen oxides and removal of oxygen. Preston and Slater [658] have described a less complex approach which provides useful data at lower precision. Similar approaches have been described by Brand et al. [657] and Metges et al. [659]. More recently Macko et al. [660] have described a procedure, which permits GC-IRMS determination of 15N/14N ratios in nanomole quantities of amino acid enantiomers with precision of 0.3-0.4%o. A key step was optimization of the acylation step with minimal nitrogen isotope fractionation [660]. 

Unfortunately, there are no existing direct records of past nitrate concentration one must rely instead on proxy evidence. We focus here on the most specific and frequently used proxy for studying the history of the ocean N cycle, the nitrogen isotopic ratio of organic matter preserved in marine sediments. 

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