Nitrogen isotopes of marine oils in the Tarim Basin, China: Implications for the origin of organic matters and the paleoenvironment

Abstract

Nitrogen isotope compositions (δ¹⁵N) of sedimentary rocks are usually used to reconstruct the paleoenvironment and nitrogen (N) biogeochemical cycle. The δ¹⁵N values of crude oils inherit the characteristics of relevant source rocks and can well reflect the information of hydrocarbon-forming organisms and environment in ancient water column. However, studies on the δ¹⁵N of crude oils are limited due to the low N content. In this study, a new efficient method is applied to the marine oils from the Bashituo (BST) and Halahatang (HLHT) areas of the Tarim Basin to obtain the nitrogenous components (i.e., nonhydrocarbons and asphaltenes) for the achievement of N concentration. The carbon and nitrogen isotopes of these components and the biomarkers of oils were measured. The δ¹⁵N values in asphaltenes (δ¹⁵N_Asp) are significantly heavier than those in nonhydrocarbons (δ¹⁵N_NSOs) in these oils, which are attributed to the potential directional N transfer and kinetic isotope fractionation during the thermal evolution of organic matters (OM). The δ¹⁵N_Asp values have significant correlations with OM origin associated parameters and weak correlations with environmental parameters, suggesting that the difference in δ¹⁵N_Asp values is mainly resulted from biological source rather than redox conditions. The δ¹⁵N_NSOs values have a closer relationship with the redox condition than biological characteristics, indicating that they have a good response to paleoenvironmental variation in the water column, which is not completely overprinted by the difference of OM origin. Different redox conditions give rise to distinct nitrogen cycles, resulting in various δ¹⁵N values. Anammox occurs in the water column of the Early Cambrian dominated by physically stratified conditions with significant isotope fractionation, resulting in relatively heavier δ¹⁵N of OM in the BST area. In the Middle–Late Ordovician period, the limited suboxic zone leads to an insignificant positive bias of δ¹⁵N caused by partial denitrification in the HLHT oils. The evaluation of δ¹⁵N in nitrogenous fractions enables a more comprehensive reconstruction of N cycle for ancient oceans.