Nitrogen Traces Across the Ediacaran-Cambrian Boundary

Nitrogen isotope chemostratigraphy of the Ediacaran and Early Cambrian platform sequence at Three Gorges, South China

Authors:

1. Ryohei Kikumoto (a)
2. Miyuki Tahata (a)
3. Manabu Nishizawa (b)
4. Yusuke Sawaki (a)
5. Shigenori Maruyama (a, c)
6. Shu Degan (d)
7. Han Jiang (d)
8. Tsuyoshi Komiya (e)
9. Ken Takai (b, c)
10. Yuichiro Ueno (a, b, c)

Affiliations:

a. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152–8551, Japan

b. Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2–15 Natsushima-cho, Yokosuka 237–0061, Japan

c. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152–8551, Japan

d. Department of Geology and Key Laboratory for Continental Dynamics, Northwest University, Xi'an 710069, China

e. Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153–8902, Japan

Abstract:

The appearance of multicellular animals and subsequent radiation during the Ediacaran/Cambrian transition may have significantly changed oceanic ecosystem. Nitrogen cycling is essential for primary productivity and thus its connection to animal evolution is important for understanding co-evolution of the Earth’s environment and life. Here, we first report 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 δ15NTN values were high (~ + 6‰) until middle Ediacaran, gradually dropped down to − 1‰ at the earliest Cambrian, then rose back to + 4‰ in the end of the Early Cambrian. Organic carbon and nitrogen contents widely varied with relatively constant C/N ratio in each stratigraphic unit, and do not apparently control the carbon and nitrogen isotopic trends. These observations suggest the δ15NTN 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 δ15N probably reflects denitrification in a nitrate-limited oceanic condition. Also, degradation of dissolved and particulate organic matter could be an additional mechanism for the 15 N-enrichment, and may have been significant when the ocean was rich in organic matter. At the time of Shuram event, both δ13Ccarb and δ15NTN values were dropped probably due to massive re-mineralization of organic matter. This scenario is supported by anomalously low C/N ratio, implying enhanced respiration resulted in selective loss of carbon as CO2 with recycled organic nitrogen. After the Shuram event, δ15N value continued to decrease in spite of δ13Ccarb rose back to + 4‰. The continued δ15N drop appears to have coincided with decreasing phosphorus content in carbonate. This suggests ocean oxygenation may have generated more nitrate-rich condition with respect to phosphorus as a limiting nutrient. Similar to the Shuram event, another negative δ13Ccarb 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 δ13Ccarb excursions reflect disappearance of large organic carbon pool in the ocean. The two events appear to relate with appearance of new metazoan taxa with novel feeding strategies, suggesting the link between ocean oxygenation, nutrient cycling and appearance and adaptation of metazoan. The nitrogen isotope geochemistry is very useful to understand the link between the environmental, ecological and biological evolutions.