The earliest Cambrian record of animals and ocean geochemical change
Adam C. Maloof1,†, Susannah M. Porter2, John L. Moore2, Frank Ö. Dudás3, Samuel A. Bowring3, John A. Higgins1, David A. Fike4 and Michael P. Eddy1
+Author Affiliations
1Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA
2Department of Earth Science, University of California, Santa Barbara, California 93106, USA
3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA
†E-mail: maloof@princeton.edu
Abstract
The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversification was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tommotian) records of δ13CCaCO3variability from Siberia, Mongolia, and China onto a Moroccan U/Pb–δ13CCaCO3age model constrained by five U/Pb ages from interbedded volcanic ashes. The δ13CCaCO3 correlations ignore fossil tie points, so we assume synchroneity in δ13C trends rather than synchroneity in first appearances of animal taxa. We present new δ13Corg, 87Sr/86Sr, uranium, and vanadium data from the same carbonate samples that define the Moroccan δ13CCaCO3 curve. The result is a new absolute time line for first appearances of skeletal animals and for changes in the carbon, strontium, and redox chemistry of the ocean during the Nemakit-Daldynian and Tommotian ages at the beginning of the Cambrian. The time line suggests that the diversification of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil first appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540–538 Ma), a larger pulse in the mid- to late Nemakit-Daldynian (ca. 534–530 Ma), and a moderate pulse in the Tommotian (ca. 524–522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydrothermal flux of Sr.
Received 4 June 2010.
Accepted 3 August 2010.
© 2010 Geological Society of America
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