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Positive cerium anomaly in the Doushantuo cap carbonates from the Yangtze platform, South China: Implications for intermediate water column manganous conditions in the aftermath of the Marinoan glaciation

Positive cerium anomaly in the Doushantuo cap carbonates from the Yangtze platform, South China: Implications for intermediate water column manganous conditions in the aftermath of the Marinoan glaciation
Positive cerium anomaly in the Doushantuo cap carbonates from the Yangtze platform, South China: Implications for intermediate water column manganous conditions in the aftermath of the Marinoan glaciation

Oxygenation exerted an important control on the emergence and diversification of metazoans in the aftermath of the Marinoan glaciation. However, the relationship between the oceanic dissolved O2 (DO) levels and early metazoan evolution remains equivocal. To provide a temporal and spatial reconstruction of the redox conditions for this critical interval, this study investigates the early Ediacaran cap carbonate deposits within intra shelf facies at three localities in the Three Gorges area, the Yangtze platform, South China. Trace and rare earth elements are sequentially extracted from the carbonate fraction for pristine temporal seawater signals. The Member II dolostones of the Duoshantuo Formation, which overlies the cap carbonates, show a negative Ce anomaly. No Ce anomaly is observed in the lower units of the cap carbonates. In contrast, positive Ce anomalies (Ce/Ce* > 1.3) in the uppermost part of the cap carbonates are observed in all studied sections. These positive Ce anomalies are accompanied by high Mn/Fe ratios (ranging from 5.04 to 14.96) and slightly positive MREE anomalies, suggesting that a Fe-Mn-(oxyhydro) oxide coparticipation occurred during the cap carbonate deposition. The positive Ce anomalies may result from the reductive dissolution of the Ce-enriched Mn-(oxyhydro) oxides across a Mn(IV)/Mn(II) redoxcline, in a distinct manganous water wedge that was sandwiched between well-oxygenated and anoxic ferruginous deep water layers. This wedge may have exhibited low oxygen conditions, with approximately 10 µM DO (dissolved oxygen), in comparison to the >90 µM DO expected in an oxic setting and 0 µM DO in an anoxic setting. Therefore, the presence of a positive Ce anomaly in the uppermost part of the cap carbonates indicates a new intermediate manganous condition and further elucidates the water column redox structure that existed following the end of the Marinoan glaciation. Early Ediacaran metazoans were possibly restricted to fully oxygenated conditions and were absent in the manganous or ferruginous-rich environments.

Marinoan cap carbonates, Mn-(oxyhydr) oxides, Positive Ce anomaly, Three Gorges area, Yangtze platform
0301-9268
93-110
Wu, He Pin
b293a650-462b-4754-ad8f-6de34aac2ae3
Jiang, Shao Yong
dc929006-69ac-4366-bf2b-dc97527602d3
Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Wei, Hai Zhen
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Yang, Jing Hong
5f5bfdb4-3c78-4784-a1a7-249225bcf07a
Wu, He Pin
b293a650-462b-4754-ad8f-6de34aac2ae3
Jiang, Shao Yong
dc929006-69ac-4366-bf2b-dc97527602d3
Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Wei, Hai Zhen
60e6342b-b6d2-4ac6-a4d5-a1dec963b1a7
Yang, Jing Hong
5f5bfdb4-3c78-4784-a1a7-249225bcf07a

Wu, He Pin, Jiang, Shao Yong, Palmer, Martin R., Wei, Hai Zhen and Yang, Jing Hong (2019) Positive cerium anomaly in the Doushantuo cap carbonates from the Yangtze platform, South China: Implications for intermediate water column manganous conditions in the aftermath of the Marinoan glaciation. Precambrian Research, 320, 93-110. (doi:10.1016/j.precamres.2018.10.019).

Record type: Article

Abstract

Oxygenation exerted an important control on the emergence and diversification of metazoans in the aftermath of the Marinoan glaciation. However, the relationship between the oceanic dissolved O2 (DO) levels and early metazoan evolution remains equivocal. To provide a temporal and spatial reconstruction of the redox conditions for this critical interval, this study investigates the early Ediacaran cap carbonate deposits within intra shelf facies at three localities in the Three Gorges area, the Yangtze platform, South China. Trace and rare earth elements are sequentially extracted from the carbonate fraction for pristine temporal seawater signals. The Member II dolostones of the Duoshantuo Formation, which overlies the cap carbonates, show a negative Ce anomaly. No Ce anomaly is observed in the lower units of the cap carbonates. In contrast, positive Ce anomalies (Ce/Ce* > 1.3) in the uppermost part of the cap carbonates are observed in all studied sections. These positive Ce anomalies are accompanied by high Mn/Fe ratios (ranging from 5.04 to 14.96) and slightly positive MREE anomalies, suggesting that a Fe-Mn-(oxyhydro) oxide coparticipation occurred during the cap carbonate deposition. The positive Ce anomalies may result from the reductive dissolution of the Ce-enriched Mn-(oxyhydro) oxides across a Mn(IV)/Mn(II) redoxcline, in a distinct manganous water wedge that was sandwiched between well-oxygenated and anoxic ferruginous deep water layers. This wedge may have exhibited low oxygen conditions, with approximately 10 µM DO (dissolved oxygen), in comparison to the >90 µM DO expected in an oxic setting and 0 µM DO in an anoxic setting. Therefore, the presence of a positive Ce anomaly in the uppermost part of the cap carbonates indicates a new intermediate manganous condition and further elucidates the water column redox structure that existed following the end of the Marinoan glaciation. Early Ediacaran metazoans were possibly restricted to fully oxygenated conditions and were absent in the manganous or ferruginous-rich environments.

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Wu_et_al_2018 - Accepted Manuscript
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Accepted/In Press date: 30 October 2018
e-pub ahead of print date: 2 November 2018
Published date: 1 January 2019
Keywords: Marinoan cap carbonates, Mn-(oxyhydr) oxides, Positive Ce anomaly, Three Gorges area, Yangtze platform

Identifiers

Local EPrints ID: 426893
URI: http://eprints.soton.ac.uk/id/eprint/426893
ISSN: 0301-9268
PURE UUID: 43df2c70-f621-4195-a053-bf5291fe30cd

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Date deposited: 14 Dec 2018 17:30
Last modified: 07 Oct 2020 06:36

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Contributors

Author: He Pin Wu
Author: Shao Yong Jiang
Author: Hai Zhen Wei
Author: Jing Hong Yang

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