Nitrogen Isotopic Composition of Shungite from the Onega Structure, Russia, and the Origin of the Organic Matter

Аннотация

The paper presents first nitrogen isotope analyses in shungites sampled from the core material of boreholes and from adits in the Onega region, Russia. The samples represent all varieties of the rocks, both stratified and migrated, with concentrations of reduced carbon ranging from a few percent to almost 100%. Analyses were carried out on a mass spectrometric complex (designed at the Open University), with the simultaneous measurements of the isotopic composition and concentrations of nitrogen and carbon using the step oxidation technique. It was determined that nitrogen is released mostly when the carbon is oxidized. The individual samples differ from one another in nitrogen isotopic composition and the nitrogen/carbon ratio, which indicates that these samples contain three distinct organic components of biological origin. Variations in δ¹⁵N and C/N that correspond to mixing of organic matter of the three types have been also observed during stepped oxidation. One of them has a relatively low C/N of approximately 200 and contains isotopically light nitrogen (δ¹⁵N up to –10‰). This component is thought to be primary, produced when the shungite rocks have been formed in a biological cycle involving reduced nitrogen species and formation of chemoautolithotrophic organisms, similar to what nowadays takes places in hydrothermal systems related to volcanic activity in spreading zones in the seafloor. Another component of the organic matter, with a relatively high δ¹⁵N from 0 to +3‰ and C/N of approximately 1000, was likely derived from the primary one in the course of diagenesis and metamorphism, due to nitrogen losses, resulted in heavier isotopic composition of the residual nitrogen. Finally, during post metamorphism time, the shungites have been contaminated with organic matter brought by meteoric waters. This organic matter had a relatively high δ¹⁵N of approximately +10‰ and the lowest C/N < 50. The different oxidation temperatures of the three organic components explain the reason for the variations in δ¹⁵N and C/N during step combustion of the material.