Nitrogen isotope and trace element composition characteristics of the Lower Cambrian Niutitang Formation shale in the upper -middle Yangtze region, South China

Abstract

The Early Cambrian is a key interval in the global development of biological evolution, in which occurred many important environmental events and organic-rich sedimentary layers were deposited, which formed important hydrocarbon source rocks. The Lower Cambrian Niutitang Formation is one of the most important marine sedimentary layers in the Yangtze region, South China, and it records abundant important geological information that can be used for the reconstruction of the Lower Cambrian paleo-climates and paleo-environments.Nitrogen isotope compositions record and reflect variations in the original sedimentary environment and the redox conditions of marine ecosystems. Trace elements are also reliable indicators of sedimentary environments. In this study, we collected samples from the Lower Cambrian Niutitang Formation shale in the upper-middle Yangtze region of South China and measured the nitrogen isotope values of bulk sediments (δ15Nbulk), as well as their organic carbon isotope values (δ13Corg), total organic carbon (TOC) contents, total nitrogen (TN) contents and trace element concentrations. The δ15Nbulk values of the Niutitang shale range from 0.6‰ to 2.8‰, they show a certain positive excursion from bottom to top in the longitudinal direction, and they exhibit a significant correlation with the longitudinal variations in trace elements. These trace element characteristics suggest that the lower section of the Niutitang Formation existed in a strongly reduced sedimentary environment and was located in a hydrothermally active layer. During the deposition of the lower section of black shale, the nitrogen cycle was dominated by anaerobic nitrogen fixation; thus, its nitrogen isotope values are relatively lighter. In contrast, during the deposition of the upper section of the Niutitang Formation, the redox conditions of water column were unstable, the oxygen contents increased, and oxygenated reactions were intermittent or localized; thus producing relatively strong isotopic fractionation and causing the δ15Nbulk values in the upper section of black shale to exhibit a positive excursion. We conclude that the sedimentary environment is the key factor affecting nitrogen isotopes and that the redox conditions of the water column exhibit a significant correlation with the nitrogen isotope values.