The earliest Cambrian record of animals and ocean geochemical change

“… if my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.… To the question why we do not find recordsof these vast primordial periods, I can give no  satisfactory answer.”

(On the Origin of Species Darwin, 1859, p. 306).

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 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

 

Beginning around 542 million years ago, a profusion of animals with shells and skeletons began to appear in the fossil record. So many life forms appeared during this time that it is often referred to as the “Cambrian Explosion.”

The Cambrian diversification of animals was long thought to have begun with an explo- sive 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 diver- sification was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tom- motian) records of δ13CCaCO3 variability from Siberia, Mongolia, and China onto a Moroc- can U/Pb–δ13CCaCO3 age model constrained by five U/Pb ages from interbedded volcanic ashes. The δ13CCaCO3 correlations ignore fos- sil 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 appear- ances 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 hydro- thermal flux of Sr.

 

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