Response of nitrogen isotopes to paleo-environment and organic carbon accumulation in a Late Paleozoic alkaline lake, Junggar Basin

Abstract

The geochemistry of nitrogen (N) in ancient lacustrine systems, especially in alkaline conditions, is poorly understood. To fill the knowledge gap, we investigated alkaline lacustrine sediments of the Upper Paleozoic Fengcheng Formation (ca. 300 Ma) in the Mahu Sag, Junggar Basin, northwestern China. Based on the general characteristics and stratigraphic variations in N isotopes, their relationship to the paleo-environmental setting and type/accumulation of organic matter were evaluated. The lacustrine sediments can be divided into alkaline (δ15N > 10–12‰) and circum-neutral (δ15N < 10–12‰) groups in general. The geochemical N cycle was mainly controlled by the pH of the lake water, and ammonia volatilization triggered N isotopic fractionation. In contrast, N isotopes are not significantly affected by the salinity and redox state of the water mass. The δ15N, particularly for the alkaline group, exhibit a correlation with the type of organism present (i.e., the algae/bacteria ratio), and also the hydrocarbon-generation potential and type of organic matter. This reflects the response of lake water pH to the types of organisms present and accumulation of organic matter. The N isotopic compositions of lacustrine sediments, and the factors that control this, are more complex than in the marine system, and are dependent on the biomass and paleo-environmental setting.