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.
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