Nitrogen isotope chemostratigraphy of the Ediacaran and Early Cambrian platform sequence at Three Gorges, South China
Nitrogen isotope chemostratigraphy of the Ediacaran and Early Cambrian platform sequence at Three Gorges, South China
复制标题
华南三峡埃迪卡拉系和早寒武世台地序列的氮同位素化学地层学
DOI：
10.1016/j.gr.2013.06.002
复制
发表时间：
2014-04-01
影响因子：
6.1
通讯作者：
Ueno, Yuichiro
中科院分区：
文献类型：
--
作者：
Kikumoto, Ryohei;Tahata, Miyuki;Ueno, Yuichiro
The appearance of multicellular animals and subsequent radiation during the Ediacaran/Cambrian transition may have significantly changed the oceanic ecosystem. Nitrogen cycling is essential for primary productivity and thus its connection to animal evolution is important for understanding the co-evolution of the Earth's environment and life. Here, we first report on coupled organic carbon and nitrogen isotope chemostratigraphy from the entire Ediacaran to Early Cambrian period by using drill core samples from the Yangtze Platform, South China. The results show that delta N-15(TN) values were high (similar to+6%.) until middle Ediacaran, gradually dropping down to -1%. at the earliest Cambrian, then rising back to +4% in the end of the Early Cambrian. Organic carbon and nitrogen contents widely varied with a relatively constant C/N ratio in each stratigraphic unit, and do not apparently control the carbon and nitrogen isotopic trends. These observations suggest that the delta N-15(TN) and C/N trends mainly reflect secular changes in nitrogen cycling in the Yangtze Platform. Onset of the observed negative N isotope excursion coincided with a global carbon isotope excursion event (Shuram excursion). Before the Shuram event, the high delta N-15 probably reflects denitrification in a nitrate-limited oceanic condition. Also, degradation of dissolved and particulate organic matter could be an additional mechanism for the N-15-enrichment, and may have been significant when the ocean was rich in organic matter. At the time of the Shuram event, both delta C-13(carb) and delta N-15(TN) values were dropped probably due to massive re-mineralization of organic matter. This scenario is supported by an anomalously low C/N ratio, implying that enhanced respiration resulted in selective loss of carbon as CO2 with recycled organic nitrogen. After the Shuram event, the N-15 value continued to decrease despite that delta C-13(carb) rose back to +4%0. The continued delta N-15 drop appears to have coincided with a decreasing phosphorus content in carbonate. This suggests that ocean oxygenation may have generated a more nitrate-rich condition with respect to phosphorus as a limiting nutrient. Similar to the Shuram event, another negative delta C-13(carb) event in the Canglanpuan stage of the Early Cambrian is also characterized by carbon isotopic decoupling as well as the low C/N ratio. The results strongly support that the two stages of the decoupled negative delta C-13(carb) excursions reflect a disappearance of a large organic carbon pool in the ocean. The two events appear to relate with the appearance of new metazoan taxa with novel feeding strategies, suggesting a link between ocean oxygenation, nutrient cycling and the appearance and adaptation of metazoans. The nitrogen isotope geochemistry is very useful to understand the link between the environmental, ecological and biological evolutions. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
