『Abstract
　Most of the organic nitrogen (Norg) on Earth is disseminated in crustal sediments and rocks in the form of fossil nitrogen-containing organic matter. The chemical speciation of fossil Norg within the overall molecular structure of organic matter changes with time and heating during burial. Progressive thermal evolution of organic matter involves phases of enhanced elimination of Norg and ultimately produces graphite containing only traces of nitrogen. Long-term chemical and thermal instability makes the chemical speciation of Norg a valuable tracer to constrain the history of sub-surface metamorphism and to shed light on the subsurface biogeochemical nitrogen cycle and its participating organic and inorganic nitrogen pools. This study documents the evolutionary path of Norg speciation, transformation and elimination before and during metamorphism and advocates the use of X-ray photoelectron spectroscopy (XPS) to monitor changes in Norg speciation as a diagnostic tool for organic metamorphism. Our multidisciplinary evidence from XPS, stable isotopes, traditional quantitative coal analyses, and other analytical approaches shows that at the metamorphic onset Norg is dominantly present as pyrrolic and pyridinic nitrogen. The relative abundance of nitrogen substituting for carbon in condensed, partially aromatic systems (where N is covalently bonded to three C atoms) increases exponentially with increasing metamorphic grade, at the expense of pyridinic and pyrrolic nitrogen. At the same time, much Norg is eliminated without significant nitrogen isotope fractionation. The apparent absence of Rayleigh-type nitrogen isotopic fractionation suggests that direct thermal loss of nitrogen from an organic matrix does not serve as a major pathway for Norg elimination. Instead, we propose that hot H, O-containing fluids or some of their components gradually penetrate into the carbonaceous matrix and eliminate Norg along a progressing reaction front, without causing nitrogen isotope fractionation in the residual Norg in the unreacted core of the carbonaceous matrix. Before the reaction front can reach the core, an increasing part of core Norg chemically stabilizes in the form of nitrogen atoms substituting for carbon in condensed, partially aromatic systems forming graphite-like structural domains with delocalized π-electron systems (nitrogen atoms substituting for “graphitic” carbon in natural metamorphic organic matter). Thus, this nitrogen species with a conservative isotopic composition is the dominant form of residual nitrogen at higher metamorphic grade.』

1. Introduction
2. Materials and methods
　2.1. Simple origin and vitrinite reflectance
　2.2. Elemental, isotopic, and thermal analyses
　2.3. X-ray photoelectron spectroscopy
3. Results and discussion
　3.1. Elemental, rank, and isotopic characterization
　3.2. Deconvolution of the N ls XPS spectrum: sub-peaks of inorganic and organic N
　　3.2.1. Inorganic nitrogen: N ls XPS of ammonium intercalated in clay
　　3.2.2. Sub-peak N-Q1: organic N-C3
　　3.2.3. Sub-peaks N-Q2 and N-X: uncertain Norg assignments
　　3.2.4. Sub-peak N-6: contribution from pyridinic nitrogen
　　3.2.5. Sub-peak N-5: pyrrolic and pyridinic rings with or without oxygen-containing substituents
　3.3. Organic nitrogen loss during low-grade metamorphism
　3.4. Mechanism for organic nitrogen loss without isotope fractionation
　3.5. Increase of N-C3 substituting for carbon in condensed, partially aromatic systems, at the expense of edge-located organic nitrogen
　3.6. The role of H, O-containing hot fluids during transformation of organic nitrogen
　3.7. Proposed model of progressive metamorphism of organic nitrogen
4. Conclusions
Acknowledgments
References