PRESERVATION OF ANCIENT SEDIMENTARY NITROGEN ISOTOPE RECORDS

A subject of considerable interest is the response of the N cycle to changes in redox state and oxygenation of Earth’s atmosphere. The evolution of the N cycle from operation in an essentially neutral-reduced state to one which includes oxidized states occurs with N transfer steps which are accompanied by strong isotopic fractionation. The changes in the isotopic composition of the dissolved forms of N can be used to infer the redox state of the ocean at the time of organic matter formation, assuming all available N was assimilated. Critical to these studies is the understanding that there have been no subsequent changes in the isotopic composition of sedimentary N, from early diagenesis to possible metamorphism. Studies of N isotopes in bulk sediment and different N-bearing phases from the late Archean South African Traansvaal and late Paleoproterozoic Animkie Basin are considered in the framework of metamorphic N isotope evolution in the more recently metamorphosed marine sediment terrane of the Catalina Schist. The influence on N isotopes in sedimentary systems by emplacement of volcanic complexes such as the Bushveld and Duluth complex, often necessary in preserving underlying ancient sedimentary systems, is addressed. Analysis of a sequence of cores across the Animikie Basin reveal that the central core, located closest to the Duluth Complex has kerogen delta N-15 as low as -5 per mil and bulk delta N-15 of +11 per mil indicating that simple loss of N-14 from organic matter cannot adequately explain the distribution of N isotopes. Similar discrepancies between bulk, silicate-bound and organic N are found in the Late Archean sequences. Loss of N-14 and condensation in organic molecules along a reaction front can account for these observations and highlight the necessity of detailed preservation studies before conclusions about paleoenvironmental conditions can be drawn.