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Publication Detail
Interpretation of the Nitrogen isotopic composition of Precambrian sedimentary rocks: assumptions and perspectives
Abstract
With the surge of interest in understanding the evolution of environments and ecosystems on the early Earth, many proxies are being applied to ancient sedimentary rocks. The isotope composition of nitrogen recorded in sedimentary rocks (δ15Nsed) is one of these proxies. Nitrogen isotopes are now routinely used as a tracer of the global and regional marine N-biogeochemical cycle during the Cenozoic and are increasingly being applied to the ancient rock record, including the Precambrian. The objectives of this review are (i) to articulate guidelines for using δ15Nsed as a proxy for the past marine global N-biogeochemical cycle with an emphasis on the Precambrian and (ii) to develop a broad framework for interpreting the Precambrian δ15Nsed record. Based on the isotopic pattern displayed by the present day N-biogeochemical cycle, significant δ15Nsed spatial variability is expected for most of the ocean redox structures envisaged for the Precambrian. Furthermore, fundamentally different N-cycling processes may give rise to only subtly different δ15Nsed signatures, which themselves may be masked or accentuated by post-depositional processes. Consequently, δ15Nsed profiles from individual basins alone are insufficient for inferring behavior of the global nitrogen cycle. Rather, δ15Nsed distributions based on data from multiple basins are essential. Furthermore, in order to interpret δ15Nsed data from Precambrian sedimentary rocks with more confidence, several avenues of focused research are required. The effects of diagenesis and metamorphism and their manifestation in the δ15N compositions of both bulk and kerogen records need to be better understood. Much more data are required in order to apply statistical approaches to interpreting δ15Nsed variability within given geological time intervals. Finally, numerical modeling of the δ15Nsed distributions expected from different redox scenarios envisaged for the Precambrian environment is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record. In spite of the intrinsic complexity of the δ15Nsed proxy and the great deal of work still required to realize its full potential, the available Precambrian δ15Nsed record shows several intuitive features within the context of the inferred evolution of the marine N-biogeochemical cycle. The δ15Nsed distributions are roughly similar in shape and amplitude to that of the present ocean, with a change in mode from +1 to +3‰ at ca. 2.7 Ga and transient excursions to 15N-enriched values at 2.7 and 1.9 Ga. Fundamental shifts in global marine N-cycling, perhaps related to stepwise oxygenation of the surface environment, are inferred, highlighting the potential of nitrogen isotopes to reveal clues about the evolution of early Earth.
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