The excursion of Nitrogen and Carbon isotope in the Lower Cambrian of Tarim Basin: Implication for the transformation of anaerobic and euxinic settling

Abstract

The Nitrogen (N) along with Carbon (C) isotopes, Rear earth elements (REEs) and trace elements (TEs) were detected in the lower Cambrian Yuertusi formation, Northwest Tarim Basin. Two sections of obvious isotopic and elemental cycles have been investigated, which reflects the sedimentary transformation from lower euxinic to upper anaerobic setting. The extreme negative excursion of kerogen δ13C emerged in the bottom layer silica shales. The lowest value of kerogen δ13C is up to -37‰, which is similar with the global “Base” excursion in the bottom of lower Cambrian. It could be inferred the deposition of Yuertusi formation might be occurred after the Neoproterozoic glacial. Consequently, the kerogen δ13C of upper Yuertusi formation mostly exceeded -32‰, which could inferred the rapid rise of atmosphere and oceanic temperature. Based on the typical theory of isotope fraction, the δ13C of bitumen should be lower than that of kerogen. The bitumen δ13C in the upper Yuertusi formation varies from -34‰ ∼ 31.5‰, which is lighter than the δ13C of kerogen. Nevertheless, the δ13C of bitumen in the bottom layer silica shales is heavier than the δ13C of kerogen. This phenomenon has been noticed by previous researchers. The nadir negative excursion of bulk δ15N occurred in the bottom of Yuertusi formation, which has been indicated the sedimentary environment of Euxinic photic zone. The assimilation of bioavailable nutrient N (either in form of NH4 + or NO3-) may have contributed to the persistently low δ15C values. Ammonium may have served as the dominant nutrient N source for this period. The recycled NH4+ can be used directly or was sometimes oxidized to nitrate and then quantitatively used by phytoplankton to fuel primary productivity, which finally resulted into accumulation of high TOC and negative excursion of carbon isotope in the bottom of Yuertusi formation. It could be alternative interpretation for the depositional conditions of TOC-rich shales around Ediacaran-Cambrian boundary.