河海大学Bi Zhu等人近日取得一项新成果。他们报道了塔里木盆地早寒武世序列氮同位素地层学——氮循环的空间变异性及其对古海洋氧化还原条件的指示意义。2024年4月25日出版的《地球化学学报》杂志发表了这项成果。

早寒武纪是一个以非凡的生物创新和海水动态氧化还原条件为特征的关键时期。古沉积物氮同位素特征对揭示早寒武世海洋氧化还原状态演化路径，和海洋水柱生物地球化学氮循环具有重要意义。关于这一主题的现有研究主要集中在华南，对其他大陆边缘的探索有限，使得相互矛盾的假设未经检验。

本文对塔里木盆地西北部的石尔里克剖面(内斜坡)和塔东2井(深陆架/盆地)的下寒武统序列分别进行了配对的δ¹⁵N和δ¹³g分析。数据显示，在文献中被称为黑色岩系的黑色燧石-页岩单元中，不同氮循环的作用发生了明显的变化。其下部振荡的δ¹⁵N值表明NH4+和反硝化/厌氧氨氧化交替厌氧同化。这可能是由于浅层不稳定的趋化跃层，与海侵期间深层缺氧水的上升流和侵入相一致。上升流强度减弱，好氧氮循环占主导地位，δ¹⁵N值为±2‰。

另一方面，塔洞2井西山布拉克组和西大山组的δ15N特征表明，寒武系富统期至4期，重氮营养体是主要的氮源。这两个研究区间虽然时间不相等，但至少在于图斯黑色岩系沉积期间表现出不同的氮循环状态。研究剖面氮循环在空间上的差异与于图斯黑色岩系沉积时的氧化还原层状海洋相一致。海水中NO₃⁻库的建立和好氧氮循环主要局限于近岸环境，而以N₂固定为主的厌氧氮循环是近岸环境中主要的氮吸收途径。

附：英文原文

Title: 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

Author: Zhu, Bi, Li, Xuefeng, Ge, Lu, Chen, Yongquan

Issue&Volume: 2024-04-25

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 δ13Corg 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.

DOI: 10.1007/s11631-024-00681-7

Source: https://link.springer.com/article/10.1007/s11631-024-00681-7