Review articleInterpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives

Nitrogen isotope compositions in sedimentary rocks (d15Nsed) are routinely used for reconstructing Cenozoic N-biogeochemical cycling and are also being increasingly applied to understanding the evolution of ancient environments. Here we review the existing knowledge and rationale behind the use of d15Nsed as a proxy for the Precambrian N-biogeochemical cycle with the aims of (i) identifying the major uncertainties that affect analyses and interpretation of nitrogen isotopes in ancient sedimentary rocks, (ii) developing a framework for interpreting the Precambrian d15Nsed record, (iii) testing this framework against a database of Precambrian d15Nsed values compiled from the literature, and (iv) identifying avenues of focused research that should increase confidence in interpreting Precambrian d15Nsed data. This review highlights the intrinsic complexity of the d15Nsed proxy and the significant effort that remains to realize its potential. Specifically, it is crucial to gain a better understanding of how and when diagenesis and metamorphism affect the d15N of bulk and kerogen-bound nitrogen. Ultimately, more data are required to apply statistics to interpreting d15Nsed variability within given geological time intervals. Finally, numerical modeling of the d15Nsed variability expected in different environments under varying redox scenarios is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record. In spite of the challenges facing the application of this proxy to the Precambrian, the existing d15Nsed record shows several features possibly related to the stepwise oxygenation of the surface environment, underlining the potential for nitrogen isotopes to reveal clues about the evolution of early Earth.