Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives
Abstract
Nitrogen isotope compositions in sedimentary rocks (d(15)N(sed)) 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 d(15)N(sed) 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 d(15)N(sed) record, (iii) testing this framework against a database of Precambrian d(15)N(sed) values compiled from the literature, and (iv) identifying avenues of focused research that should increase confidence in interpreting Precambrian d(15)N(sed) data. This review highlights the intrinsic complexity of the d(15)N(sed) 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 d(15)N of bulk and kerogen-bound nitrogen. Ultimately, more data are required to apply statistics to interpreting d(15)N(sed) variability within given geological time intervals. Finally, numerical modeling of the d(15)N(sed) 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 d(15)N(sed) 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.