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Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation

Bristow, Thomas F. and Kennedy, Martin J. and Derkowski, Arkadiusz and Droser, Mary L. and Jiang, Ganqing and Creaser, Robert A. (2009) Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation. Proceedings of the National Academy of Sciences of the United States of America, 106 (32). pp. 13190-13195. ISSN 0027-8424. PMCID PMC2726375.

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Assemblages of clay minerals are routinely used as proxies for paleoclimatic change and paleoenvironmental conditions in Phanerozoic rocks. However, this tool is rarely applied in older sedimentary units. In this paper, the clay mineralogy of the Doushantuo Formation in South China is documented, providing constraints on depositional conditions of the Ediacaran Yangtze platform that host the earliest animal fossils in the geological record. In multiple sections from the Yangtze Gorges area, trioctahedral smectite (saponite) and its diagenetic products (mixed-layer chlorite/smectite, corrensite, and chlorite) are the dominant clays through the lower 80 m of the formation and constitute up to 30 wt% of the bulk rock. Saponite is interpreted as an in situ early diagenetic phase that formed in alkaline conditions (pH ≥ 9). The absence of saponite in stratigraphically equivalent basin sections, 200–400 km to the south, indicates that alkaline conditions were localized in a nonmarine basin near the Yangtze Gorges region. This interpretation is consistent with crustal abundances of redox-sensitive trace elements in saponitic mudstones deposited under anoxic conditions, as well as a 10‰ difference in the carbon isotope record between Yangtze Gorges and basin sections. Our findings suggest that nonmarine environments may have been hospitable for the fauna preserved in the Yangtze Gorges, which includes the oldest examples of animal embryo fossils and acanthomorphic acritarchs.

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Bristow, Thomas F.0000-0001-6725-0555
Additional Information:Copyright ©2009 by the National Academy of Sciences. Edited by L. Paul Knauth, Arizona State University, Tempe, AZ, and accepted by the Editorial Board June 22, 2009 (received for review February 4, 2009). Published online before print July 29, 2009, doi: 10.1073/pnas.0901080106 We thank T. Lyons for discussions and use of laboratory facilities; S. H. Zhang and H. C. Wu for field assistance and logistical support; D. Mrofka and K. Morrison for laboratory assistance; K. Gray, N. J. Butterfield, S. Xiao, R. Kleeberg, R. Raiswell, S. Severmann, C. Scott, B. Jones, and D. Pevear for advice and comments; and R. Ferrell and numerous anonymous reviewers for constructive reviews of this manuscript. This work was supported by Geological Society of America student research grant (to T.F.B.), National Aeronautics and Space Administration Exobiology Grant NNG04GJ42G, and National Science Foundation Division of Earth Sciences Grants 0345207 and 0345642 (to M.J.K.). Author contributions: T.F.B. and M.J.K. designed research; T.F.B., A.D., G.J., and R.A.C. performed research; M.J.K. and R.A.C. contributed new reagents/analytic tools; T.F.B., M.J.K., A.D., and M.L.D. analyzed data; and T.F.B. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. L.P.K. is a guest editor invited by the Editorial Board. This article contains supporting information online at
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Geological Society of AmericaUNSPECIFIED
Subject Keywords:clays; saponite; Neoproterozoic; nonmarine; early animals
Issue or Number:32
PubMed Central ID:PMC2726375
Record Number:CaltechAUTHORS:20090828-231031572
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:15438
Deposited By: George Porter
Deposited On:08 Sep 2009 19:26
Last Modified:03 Oct 2019 00:57

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