Investigating biological nitrogen cycling in lacustrine systems by FT-ICR-MS analysis of nitrogen-containing compounds in petroleum

Abstract

Biological nitrogen cycling plays an important role in bio, environmental and petroleum geochemistry. Until now most research has focused on marine environments using nitrogen isotopes, and in contrast, nitrogen cycling in lacustrine environments remains poorly understood. To fill this knowledge gap, we present a case study of crude oils from Permian brackish and saline alkaline lacustrine rocks in the Junggar Basin, northwestern China. To complement the large variation in nitrogen isotope values in lacustrine sediments, we analyzed the pyrrolic N-containing compounds of crude oil using negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS). Our results show that pyrrolic compounds derived from biological sources are more abundant in oils from brackish lacustrine settings (i.e., the Jimusar Sag oils of the Junggar Basin) than in oils from alkaline lacustrine settings (i.e., the Mahu Sag oils of the Junggar Basin), which is mainly due to greater higher plant contributions to the latter. Higher plants provide abundant alkaloids and chlorophyll that are the biological precursors of pyrrolic N-containing compounds. In addition, anoxic, sulfate-type, brackish lacustrine settings provide abundant ferrous iron and sulfide that promote N fixation. This study provides the first insights into biological N cycling in brackish and alkaline lacustrine systems, which has significance for fossil fuel resources and their environmental impacts. Our data suggest that the N cycling in lacustrine systems might have an important role in entire Earth system N budget.