Ediacaran carbon isotope anomaly records shallow marine event, not entire ocean

説明

The Ediacaran is one of the most important periods in the history of life when multicellular animals firstly appeared on the earth. However, we still poorly understand the relationship between the abrupt biological evolution and environmental change. Ediacaran sections record the largest d13C arnomaly through the Earth’s history, named Shuram excursion [1,? 2] . The observed excursion may reflect extensive remineralization of large organic matter in the Ediacaran ocean [2, ? 3], or merely the result of diagenetic alteration [4]. However, marine sediments must have been globally preconditioned in a unique way, to allow ordinary and local process to produce an extraordinary and widespread response [5]. We analyzed carbon and nitrogen isotopes by using drill core samples from four different depositional settings in South China: shallow marine Three Gorges and Weng’an sections, and deeper Tianping and Shiduping sections. The new results of deeper sections show high carbon isotope ratio and no negative excursion in spites of high spacial resolution. Weng’an section in shalow shelf also shows only smaller negative excursion (>-4?) compare to that of Three Gorges section in another shallow marine setting. Thus, the limited appearance of the negative d13C excursion in shallow marine settings may suggest that extensive remineralization took place only in shallow part of the organic-carbon-rich Ediacaran ocean. Alternatively, shallow section preferentially suffered from diagenetic alteration possibly in response to sea-level fall, though the regressive trend is not evident. On the other hand, d15NTN in Three Gorges shows long-term gradual decrease from the Shuram excursion to early Cambrian, which may rather reflect change in long-term oceanic signal.

identifier:oai:t2r2.star.titech.ac.jp:50194195

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