Nitrogen isotope stratigraphy of the Early Cambrian successions in the Tarim Basin: Spatial variability of nitrogen cycling and its implication for paleo-oceanic redox conditions

Abstract

The Early Cambrian represents a critical time period characterized by extraordinary biological innovations and dynamic redox conditions in seawaters. Nitrogen isotopic signatures of ancient sediments have the potential to elucidate the evolutionary path of marine redox states and the biogeochemical nitrogen cycle within the water column of the Early Cambrian ocean. While existing research on this topic has predominantly focused on South China, the exploration of other continental margins has been limited, leaving contradictory hypotheses untested. In this study, paired δ¹⁵N and δ¹³C_org analyses were performed on the Lower Cambrian successions from the Shiairike section (inner ramp) and Well Tadong 2 (deep shelf/basin) in the northwestern and eastern Tarim Basin, respectively. Our data from the Shiairike section reveal a discernible shift in the operation of different nitrogen cycles for the black chert-shale unit, also referred to as the black rock series in Chinese literature, of the Yurtus Formation (Fortunian stage to lower Stage 3). Oscillating δ¹⁵N values for its lower part are suggestive of alternating anaerobic assimilation of NH₄⁺ and denitrification/anammox. This is likely attributed to a shallow, unstable chemocline consistent with the upwelling and incursion of deep, anoxic waters during a major transgression. In contrast, aerobic nitrogen cycling, indicated by positive δ¹⁵N values of  >2‰, dominated the upper part alongside a reduction in upwelling intensity. On the other hand, the δ¹⁵N signatures of Xishanbulake and Xidashan Formations of Well Tadong 2, which encompass a time interval from the Cambrian Fortunian Age to Age 4, are indicative of N₂ fixation by diazotrophs as the major nitrogen source. The two studied intervals, although not time-equivalent, exhibit separated states of nitrogen cycling at least during the deposition of the Yurtus black rock series. The spatially different nitrogen cycling of the studied sections is compatible with a redox-stratified ocean during the deposition of the Yurtus black rock series. The build-up of a NO₃⁻ reservoir and aerobic nitrogen cycling in seawater was largely restricted to near-shore settings whereas anaerobic nitrogen cycling dominated by N₂ fixation served as the main nitrogen uptake pathway in off-shore settings.