Isotope study on organic nitrogen of Westphalian anthracites from the western middle field of Pennsylvania (USA) and from the Bramsche massif (Germany)

The objective of this study was to examine an aspect of the thermal cycling of organic nitrogen in sediments and metasediments. The cycling of organic nitrogen is important because sedimentary organic matter is a shuttle of n itrogen from the atmosphere to the lower crust and thermal decomposition of organic matter is a critical step in the recycling of nitrogen between the different nitrogen pools. Abundance and isotopic composition of organic ni trogen were determined in the particular case of two low sulfur Westphalian anthracites series from Pennsylvania and Bramsche Massif. They represent g ood examples of Euramerica coals spanning the whole range of anthracitizati on in single fields. Gold cell experimental simulation of the denitrogenati on process was conducted at moderate pressure to show that both suites make ideal metamorphic profiles without any shift due to change of facies or to hydrothermal disturbance. During anthracitization, organic nitrogen conten t decreases rapidly while organic nitrogen isotopic composition does not ch ange with rank increase. The preservation of the isotopic signature implies that organic nitrogen isotopes could be used as indicators for the paleoec ological and paleodepositional history reconstruction of the basins. The st riking contrast between the rapid and sharp decrease of nitrogen organic co ntent and the invariance of its isotopic composition during the whole anthr acitization suggests that ammonia is an important product of the denitrogen ation process. (C) 1998 Elsevier Science Ltd. All rights reserved.