The distribution of nitrogen isotopes in sediments

Abstract

The results from this thesis elucidate some of the major processes involved in the sedimentary nitrogen distribution, although much research on the sedimentary nitrogen cycle still needs to be done. During early diagenesis the nitrogen isotopic composition of organic matter can be altered strongly through microbial activity (oxidation). An environmental signature, however, remains preserved in the fixed nitrogen isotope ratios. As a result, the ϐ¹⁵N(fix) values in a sediment can be successfully used as an indicator of the palaeoenvironmental setting. Thermal degradation (maturation) has only little effect on the nitrogen isotopic composition of organic matter. Oils have ϐ¹⁵N-values similar to their rocks. Therefore, ϐ¹⁵N may be useful to correlate oils with their source rocks. Recrystallization of the clay minerals upon maturation causes an increase in the fixed nitrogen content of shales. The fixed nitrogen content of the clays can be used as a maturity-indicator. In mature shales, the isotopic composition of this fixed nitrogen is similar to the ϐ¹⁵N of the decomposed organic matter.